Toumazatou A, Antoniadou M, Sakellis E, Tsoutsou D, Gardelis S, Romanos GE, Ioannidis N, Boukos N, Dimoulas A, Falaras P, et al. Boosting visible light harvesting and charge separation in surface modified TiO2 photonic crystal catalysts with CoOx nanoclusters. Mater. Adv. [Internet]. 2020;1:2310-2322. Publisher's VersionAbstract
Photonic crystal structuring has emerged as a promising approach to improve the utilization of solar energy by metal oxide semiconductor photocatalysts based on the combination of slow-light{,} pore interconnectivity and high surface accessibility of macroporous periodic structures with judicious compositional modifications of the materials’ properties. In this work{,} surface modification of photonic band gap engineered TiO2 inverse opals fabricated by the convective evaporation-induced co-assembly technique was performed with nanoscale Co oxides using the chemisorption–calcination-cycle method in order to explore the interplay of metal oxide heterostructuring and photonic amplification for the development of visible light-activated photonic catalysts. Fine tuning of the films’ photonic and electronic properties by controlling the inverse opal macropore size and Co oxides’ loading and composition resulted in significant enhancement of the photocatalytic activity for organics decomposition under visible light{,} exceeding that of benchmark mesoporous TiO2 films subjected to the same treatment. The underlying mechanism was related to the slow-photon-assisted light harvesting by low amounts of Co oxide nanoclusters that exert minimal effects on the inverse opal periodicity and texture{,} while enabling visible light electronic absorption and promoting charge separation via strong interfacial coupling on the nanocrystalline titania skeleton of the photonic crystals.
Apostolaki M-A, Toumazatou A, Antoniadou M, Sakellis E, Xenogiannopoulou E, Gardelis S, Boukos N, Falaras P, Dimoulas A, Likodimos V. Graphene quantum dot-TiO2 photonic crystal films for photocatalytic applications. Nanomaterials [Internet]. 2020;10:1-18. Publisher's VersionAbstract
Photonic crystal structuring has emerged as an advanced method to enhance solar light harvesting by metal oxide photocatalysts along with rational compositional modifications of the materials’ properties. In this work, surface functionalization of TiO2 photonic crystals by blue luminescent graphene quantum dots (GQDs), n–π* band at ca. 350 nm, is demonstrated as a facile, environmental benign method to promote photocatalytic activity by the combination of slow photon-assisted light trapping with GQD-TiO2 interfacial electron transfer. TiO2 inverse opal films fabricated by the co-assembly of polymer colloidal spheres with a hydrolyzed titania precursor were post-modified by impregnation in aqueous GQDs suspension without any structural distortion. Photonic band gap engineering by varying the inverse opal macropore size resulted in selective performance enhancement for both salicylic acid photocatalytic degradation and photocurrent generation under UV–VIS and visible light, when red-edge slow photons overlapped with the composite’s absorption edge, whereas stop band reflection was attenuated by the strong UVA absorbance of the GQD-TiO2 photonic films. Photoelectrochemical and photoluminescence measurements indicated that the observed improvement, which surpassed similarly modified benchmark mesoporous P25 TiO2 films, was further assisted by GQDs electron acceptor action and visible light activation to a lesser extent, leading to highly efficient photocatalytic films. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.
Loukopoulos S, Toumazatou A, Sakellis E, Xenogiannopoulou E, Boukos N, Dimoulas A, Likodimos V. Heterostructured CoOx–TiO2 mesoporous/photonic crystal bilayer films for enhanced visible-light harvesting and photocatalysis. Materials [Internet]. 2020;13:1-14. Publisher's VersionAbstract
Heterostructured bilayer films, consisting of co-assembled TiO2 photonic crystals as the bottom layer and a highly performing mesoporous P25 titania as the top layer decorated with CoOx nanoclusters, are demonstrated as highly efficient visible-light photocatalysts. Broadband visible-light activation of the bilayer films was implemented by the surface modification of both titania layers with nanoscale clusters of Co oxides relying on the chemisorption of Co acetylacetonate complexes on TiO2, followed by post-calcination. Tuning the slow photon regions of the inverse opal supporting layer to the visible-light absorption of surface CoOx oxides resulted in significant amplification of salicylic-acid photodegradation under visible and ultraviolet (UV)–visible light (Vis), outperforming benchmark P25 films of higher titania loading. This enhancement was related to the spatially separated contributions of slow photon propagation in the inverse opal support layer assisted by Bragg reflection toward the CoOx-modified mesoporous P25 top layer. This effect indicates that photonic crystals may be highly effective as both photocatalytically active and backscattering layers in multilayer photocatalytic films. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.
Likodimos V. Advanced photocatalytic materials. Materials [Internet]. 2020;13. WebsiteAbstract
Semiconductor photocatalysts have attracted a great amount of multidiscipline research due to their distinctive potential for solar-to-chemical-energy conversion applications, ranging from water and air purification to hydrogen and chemical fuel production. This unique diversity of photoinduced applications has spurred major research efforts on the rational design and development of photocatalytic materials with tailored structural, morphological, and optoelectronic properties in order to promote solar light harvesting and alleviate photogenerated electron-hole recombination and the concomitant low quantum efficiency. This book presents a collection of original research articles on advanced photocatalytic materials synthesized by novel fabrication approaches and/or appropriate modifications that improve their performance for target photocatalytic applications such as water (cyanobacterial toxins, antibiotics, phenols, and dyes) and air (NOx and volatile organic compounds) pollutant degradation, hydrogen evolution, and hydrogen peroxide production by photoelectrochemical cells. © 2020 by the authors.
Wang B, Likodimos V, Fielding AJ, Dryfe RAW. In situ Electron paramagnetic resonance spectroelectrochemical study of graphene-based supercapacitors: Comparison between chemically reduced graphene oxide and nitrogen-doped reduced graphene oxide. Carbon [Internet]. 2020;160:236-246. WebsiteAbstract
An in situ electrochemical electron paramagnetic resonance (EPR) spectroscopic study of N-doped reduced graphene oxide (N-rGO) is reported with the aim of understanding the properties of this material when employed as an electrical double-layer capacitor. N-rGO shows a capacitance of 100 F g−1 in 6 M KOH, which is twice that found for reduced graphene oxide (rGO). The temperature dependence of the rGO EPR signal revealed two different components: a narrow component, following the Curie law, was related to defects; and a broad curve with a stronger Pauli law component was attributed to the spin interaction between mobile electrons and localised π electrons trapped at a more extended aromatic structure. The N-rGO sample presented broader EPR signals, indicative of additional contributions to the resonance width. In situ EPR electrochemical spectroscopy was applied to both samples to relate changes in unpaired electron density to the enhanced capacitance. The narrow and broad components increased and diminished reversibly with potential. The potential-dependent narrow feature was related to the generated radical species from corresponding functional groups: e.g. O- and N-centred radicals. Improved capacitance seen for the N-modified basal graphene planes can be accordingly suggested to underlie the enhanced capacitance of N-rGO in basic electrolytes. © 2020 The Authors
Kontos AG, Romanos GE, Veziri CM, Gotzias A, Arfanis MK, Kouvelos E, Likodimos V, Karanikolos GN, Falaras P. Correlating vibrational properties with temperature and pressure dependent CO2 adsorption in zeolitic imidazolate frameworks. Applied Surface Science [Internet]. 2020;529. Publisher's VersionAbstract
Zeolitic imidazolate frameworks (ZIFs) feature a rigid porous structure where the interplay of pore merits and wall functionality, determined by the different imidazolate functional groups, results in superior CO2 capture ability. In this work, the vibrational properties of ZIF-68 and ZIF-69, two characteristic complex gmelinite (GME) type ZIFs comprising of benzimidazolate (bIm) and chloro-benzimidazolate (cbIm) linkers, respectively, were investigated by micro-Raman spectroscopy as a function of CO2 pressure and temperature, in combination with macroscopic adsorption experiments and extended molecular simulations, in order to explore the underlying host-guest interactions and particularly the variation of the framework lattice dynamics and flexibility to CO2 loading. The CO2 isosteric heat of adsorption (Qst) was quantitatively determined by the temperature dependence of the CO2 Fermi dyad intensity at constant pressure. ZIF-69 was consistently found to present higher Qst than ZIF-68 due to the cbIm polar functionality, in close agreement with macroscopic CO2 adsorption experiments and Monte Carlo analysis. More importantly, high CO2 uptake was found to cause significant blue shifts and enhancement of the frequency shift temperature gradients of several low-frequency Raman modes, which according to detailed polarization analysis of ZIF microcrystals, arise from free breathing vibrations of the functionalized ligands in the large ZIF pores. Low-frequency micro-Raman spectroscopy may accordingly constitute a sensitive spectroscopic tool for unveiling lattice dynamics upon CO2 sorption in ZIFs. © 2020 Elsevier B.V.
Arfanis MK, Athanasekou CP, Sakellis E, Boukos N, Ioannidis N, Likodimos V, Sygellou L, Bouroushian M, Kontos AG, Falaras P. Photocatalytic properties of copper—Modified core-shell titania nanocomposites. Journal of Photochemistry and Photobiology A: Chemistry [Internet]. 2019;370:145-155. WebsiteAbstract
To face the acute problem of water contamination, intensive scientific activity focuses on advanced photocatalytic treatment based on titanium dioxide nanomaterials. Herein, we investigate the development of innovative photocatalysts consisting of copper-modified core-shell titania nanocomposites (m–TiO2/Cu), synthesized using a smoldering combustion sol–gel route. The structural and chemical analysis (XRD, Raman, UV/vis, EDS, TEM, porosimetry, FTIR, EPR and XPS) confirmed that the mixed m–TiO2/Cu nanostructures are formed by a TiO2 core, crystalized in the anatase phase, an external nitrogen rich carbonic shell, which acted as a sensitizer and well dispersed species consisting of copper oxide and possibly plasmonic nanoparticles depending on the Cu loading. The synthesized materials were successfully employed for the photocatalytic degradation of caffeine – CA, salicylic acid – SA and hexavalent chromium – Cr(VI) emerging contaminants frequently detected in wastewaters, under UVA and solar illumination. Copper addition contributes to the photooxidation process enhancing the final degradation efficiencies of CA and SA, while preserving the performance of the reference material for the Cr(VI) photoreduction. An optimum copper loading was determined for both illumination conditions and the results are compatible with a photocatalytic mechanism implying enhanced light absorption and more effective separation of the photogenerated charge carriers. © 2018 Elsevier B.V.
Diamantopoulou A, Sakellis E, Romanos GE, Gardelis S, Ioannidis N, Boukos N, Falaras P, Likodimos V. Titania photonic crystal photocatalysts functionalized by graphene oxide nanocolloids. Applied Catalysis B: Environmental [Internet]. 2019;240:277-290. WebsiteAbstract
Photonic crystal-assisted semiconductor photocatalysis has been attracting significant attention as an advanced photon management approach that combines light harvesting with the macro/mesoporous structured materials properties permitting enhanced mass transport and high adsorption. In this work, surface functionalization of well-ordered photonic band gap engineered TiO2 inverse opal films fabricated by the convective evaporation-induced co-assembly method was performed by graphene oxide nanocolloids (nanoGO). The loading of GO nanosheets was determined by the films’ macropore size, with minimal effects on their long range periodicity and photonic properties. While nanoGO deposition reduced mesoporosity of the nanocrystalline titania walls, their surface functionality was greatly improved by the abundant oxygen groups of the GO nanosheets leading to increased pollutant adsorption. Slow photon amplification in the aqueous phase methylene blue photodegradation was identified for the unmodified TiO2 photonic films under both UV–vis and Vis illumination upon spectral overlap of the low energy edge of the inverse opal stop band (in water) with the dye electronic absorption, due to (red) slow photons localized in the titania skeleton that distinctly accelerated dye photodegradation kinetics. The photocatalytic efficiency was further improved for the nanoGO functionalized TiO2 inverse opal films via the synergetic action of interfacial electron transfer from TiO2 to the GO nanosheets. Under UV–vis light, the functionalized photonic films outperformed benchmark mesoporous Aeroxide® P25 TiO2 films where nanoGO modification, despite the enhanced dye adsorption, resulted in adverse effects in photocatalytic degradation due to pore clogging. Combination of the exceptional structural and photonic properties of TiO2 inverse opals with the high adsorption capacity and charge separation afforded by GO nanocolloids is proposed as a promising modification route for the development of efficient photocatalytic films. © 2018 Elsevier B.V.
Diamantopoulou A, Sakellis E, Gardelis S, Tsoutsou D, Glenis S, Boukos N, Dimoulas A, Likodimos V. Advanced photocatalysts based on reduced nanographene oxide-TiO2 photonic crystal films. Materials [Internet]. 2019;12. Publisher's VersionAbstract
Surface functionalization of TiO2 inverse opals by graphene oxide nanocolloids (nanoGO) presents a promising modification for the development of advanced photocatalysts that combine slow photon-assisted light harvesting, surface area, and mass transport of macroporous photonic structures with the enhanced adsorption capability, surface reactivity, and charge separation of GO nanosheets. In this work, post-thermal reduction of nanoGO-TiO2 inverse opals was investigated in order to explore the role of interfacial electron transfer vs. pollutant adsorption and improve their photocatalytic activity. Photonic band gap-engineered TiO2 inverse opals were fabricated by the coassembly technique and were functionalized by GO nanosheets and reduced under He at 200 and 500 °C. Comparative performance evaluation of the nanoGO-TiO2 films on methylene blue photodegradation under UV-VIS and visible light showed that thermal reduction at 200 °C, in synergy with slow photon effects, improved the photocatalytic reaction rate despite the loss of nanoGO and oxygen functional groups, pointing to enhanced charge separation. This was further supported by photoluminescence spectroscopy and salicylic acid UV-VIS photodegradation, where, in the absence of photonic effects, the photocatalytic activity increased, confirming that fine-tuning of interfacial coupling between TiO2 and reduced nanoGO is a key factor for the development of highly efficient photocatalytic films. © 2019 by the authors.
Papadakis D, Diamantopoulou A, Pantazopoulos PA, Palles D, Sakellis E, Boukos N, Stefanou N, Likodimos V. Nanographene oxide-TiO2 photonic films as plasmon-free substrates for surface-enhanced Raman scattering. Nanoscale [Internet]. 2019;11:21542-21553. Publisher's VersionAbstract
The development of nanostructured semiconductors with tailored morphology and electronic properties for surface-enhanced Raman scattering (SERS) has been attracting significant attention as a promising alternative to conventional coinage metal SERS substrates. In this work, functionalized TiO2 photonic crystals by graphene oxide nanocolloids (nanoGO) are demonstrated as highly sensitive, recyclable, plasmon-free SERS substrates that combine slow-photon amplification effects with the high adsorption capacity and surface reactivity of GO nanosheets. Comparative evaluation of photonic band gap engineered nanoGO-TiO2 inverse opal films was performed on methylene blue SERS detection under different laser excitations in combination with rigorous theoretical simulations of the photonic band structure. A very low detection limit of 6 × 10-7 M and an enhancement factor of 5 × 104 along with excellent self-cleaning performance and reusability could be achieved by the interplay of slow-photon effects assisted by interfacial charge transfer between the analyte and the nanoGO-TiO2 semiconducting substrate. Slow-photon management in combination with judicious engineering of chemical enhancement in photonic nanostructures is accordingly proposed as an advanced approach for the design of efficient dielectric SERS substrates. © 2019 The Royal Society of Chemistry.
Likodimos V. Photonic crystal-assisted visible light activated TiO2 photocatalysis. Applied Catalysis B: Environmental [Internet]. 2018;230:269-303. Publisher's VersionAbstract
Photonic crystals have been established as unique periodic structures to promote photon capture and control over light-matter interactions. Their application in semiconductor, mainly TiO2, photocatalysis has emerged as a promising structural modification to boost light harvesting of photocatalytic materials by means of slow photons i.e. light propagation at reduced group velocity near the edges of the photonic band gap (PBG). In this review, the latest advances in the development of TiO2 photonic crystal photocatalysts are highlighted, targeting primarily on the design, fabrication, structure-activity and performance evaluation of visible light activated (VLA) TiO2 inverse opals in the degradation of water and air pollutants as well as water splitting. Up to date work demonstrating the amplification effect of PBG engineered photonic crystals on the photocatalytic and photoelectrochemical performance under UV excitation is accordingly presented. Recent developments on the combination of enhanced light trapping, mainly via slow photons, mass transport and adsorption of macro/mesoporous inverse opals with targeted compositional and electronic modifications currently pursued to promote charge separation and visible light activation, i.e. dye sensitization, non-metal and self-doping, coupling with metallic nanoparticles and plasmonic effects, heterostructuring with narrow band gap semiconductors, quantum dots and graphene as well as the use of alternative metal oxide photocatalysts beyond TiO2 are thoroughly reviewed with respect to their potential for key improvements of the photocatalytic efficiency under visible light. Pertinent challenges and future prospects in photonic crystal-assisted VLA photocatalysts are addressed aimed at advanced photon management routes that could step up photocatalytic applications. © 2018 Elsevier B.V.
Athanasekou CP, Likodimos V, Falaras P. Recent developments of TiO2 photocatalysis involving advanced oxidation and reduction reactions in water. Journal of Environmental Chemical Engineering [Internet]. 2018;6:7386-7394. Publisher's VersionAbstract
Advanced nanostructures of titanium dioxide are intensively investigated for environmental protection. The latest developments in the field pay special attention to innovative and highly performing titania materials (anatase/rutile mixed-phase nanocomposites, anion-doped core-shell nanostructures, self-organized nanotubes, photonic crystals and their modifications with graphene oxide and metal nanoparticles) with original functionalities and tailored properties (visible light activated photocatalysts-VLA), the elucidation of the corresponding mechanisms involving interaction of light with matter at the nanoscale and resulting photoinduced electron transfer reactions. These materials are also considered as key components for the design and fabrication of devices (photocatalytic reactors) for efficient degradation and/or transformation of emerging environmental contaminants. The presence of a TiO2 photocatalyst on the asymmetric membrane surface and pores insures simultaneous pollutant retention and photodegradation, permitting continuous long-term device operation without fouling, practical absence of concentrated retentate and cost effective production of clean water. Focusing on recent investigations of our group concerning the use of innovative titania nanostructured photocatalysts, the present work attempts to explore novel trends and present perspectives of TiO2 photocatalysis inside and outside the well-established frame of advanced oxidation processes (AOPs), expanding the field borders by including advanced reduction processes (ARPs) and relating technological applications (ARTs). © 2018 Elsevier Ltd.
Diamantopoulou A, Glenis S, Zolnierkiwicz G, Guskos N, Likodimos V. Magnetism in pristine and chemically reduced graphene oxide. Journal of Applied Physics [Internet]. 2017;121. WebsiteAbstract
The evolution of magnetism for graphene oxide (GO) before and after chemical reduction was investigated by means of static magnetization and electron spin resonance (ESR) spectroscopy. Strong paramagnetism with a saturation magnetization of ∼1.2 emu/g and weak antiferromagnetic interactions were identified in pristine GO. Apart from spin-half defect centers, ESR spectroscopy indicated the excitation of high spin states, consistently with the high spin (S = 2) magnetic moments derived from the magnetization analysis, corroborating the formation of spatially "isolated" magnetic clusters in GO. A marked reduction of GO's magnetization (∼0.17 emu/g) along with an appreciable rise of diamagnetism (-2.4 × 10-6emu/g Oe) was detected after chemical reduction by sodium borohydride, reflecting the drastic removal of paramagnetic defects and the concomitant growth of sp2 domains in reduced graphene oxide (rGO). ESR revealed a large drop of the spin susceptibility for rGO, which, in addition to the main paramagnetic Curie component, showed an appreciable Pauli contribution. The latter together with the g-factor shift and the broadening of the ESR line indicated the coupling of localized spins with conduction electrons. The rGO ESR signal presented a metallic line shape, which could be analyzed in terms of two separate spectral components, a broad one that may be related to defect states strongly coupled with itinerant spins within the sp2 clusters and a narrow one due to edge/vacancy defect spins, indicative of rGO's persistent structural inhomogeneity. © 2017 Author(s).
Toumazatou A, Arfanis MK, Pantazopoulos P-A, Kontos AG, Falaras P, Stefanou N, Likodimos V. Slow-photon enhancement of dye sensitized TiO2 photocatalysis. Materials Letters [Internet]. 2017;197:123-126. Publisher's VersionAbstract
Photonic band gap engineered TiO2 inverse opals were fabricated using self-assembled polystyrene films as sacrificial templates with controlled optical properties, aimed at the identification of the slow-photon effect on dye sensitized TiO2 photocatalysis. The materials’ photocatalytic efficiency was evaluated using Raman spectroscopy, on methylene blue photodegradation following both UVA and monochromatic visible light illumination. Contrary to UVA, where no photonic effect could be traced, laser irradiation within the slow-photon energy range of the TiO2 inverse opals, resulted in a marked increase of the dye photosensitized degradation rate, outperforming not only compact nanocrystalline films but also the benchmark mesoporous Aeroxide® P25 TiO2 films. This effect provides direct evidence for the presence of slow photons that amplify the interaction of visible light with the adsorbed dye molecules on the periodically structured TiO2 film. © 2017 Elsevier B.V.
Stergiopoulos T, Kontos AG, Jiang N, Milliken D, Desilvestro H, Likodimos V, Falaras P. High boiling point solvent-based dye solar cells pass a harsh thermal ageing test. Solar Energy Materials and Solar Cells [Internet]. 2016;144:457-466. WebsiteAbstract
Dye solar cells (DSCs) have emerged as one of the most efficient third-generation photovoltaic (PV) technologies, whose commercialization is mainly hampered by the lack of sufficient long-term stability compared to conventional PV devices. In this work, it is demonstrated that solvent based DSCs using tetraglyme as a non-nitrile, high boiling point, organic solvent for the iodide/triiodide redox shuttle, can pass a harsh accelerated thermal ageing test of 3000 h light soaking followed by additional 2000 h thermal ageing at 85 °C. Electrochemical and spectroscopic analysis on thermal degradation effects revealed that a conduction band edge shift towards more negative potentials for tetraglyme-DSCs underlies the enhanced photopotential of aged cells, compensating for the thermally induced photocurrent reduction due to slight triiodide loss. The tetraglyme-based solar cells (in contrast to cells based on methoxypropionitrile-MPN) showed exceptional stability, compatible with the established IEC61646 protocol for thin film PVs, keeping ca. 90% of their initial performance under 1 sun illumination. Quite notably, the cells even increased their initial efficiency by 4% when illuminated under 0.1 sun. This is the first time in literature that such a stability record is accomplished for solvent based DSCs utilizing commercially available and cost-effective materials. © 2015 Elsevier B.V. All rights reserved.
Likodimos V, Chrysi A, Calamiotou M, Fernández-Rodríguez C, Doña-Rodríguez JM, Dionysiou DD, Falaras P. Microstructure and charge trapping assessment in highly reactive mixed phase TiO2 photocatalysts. Applied Catalysis B: Environmental [Internet]. 2016;192:242-252. Publisher's VersionAbstract
The structural-microstructural characteristics and interfacial charge transfer are key issues to the development of efficient mixed phase TiO2 photocatalysts. In this work, the interplay of lattice deformation and microstrains as well as the identification of charge trapping sites and electron transfer were investigated for a series of nanostructured titania photocatalysts by X-ray powder diffraction analysis, Raman and electron paramagnetic resonance (EPR) spectroscopy. These mixed phase nanomaterials were selected as model sol-gel TiO2 systems based on their exceptional photocatalytic performance over a wide range of hazardous water pollutants (including degradation/mineralization of phenol, 2,4-dichlorophenoxyacetic acid and imazalil) under UV light. Lattice contraction with respect to the bulk anatase together with anisotropic microstrains was identified for the smallest (11 nm) anatase nanoparticles. Both effects gradually relaxed with the increase of calcination temperature and the concomitant particle growth, with microstrains scaling linearly with the relative change of the c-axis lattice constant and the broadening of the main anatase Raman mode. The growth of anatase nanoparticles at 1023 K with minimal lattice deformation and microstrains resulted in the optimal photocatalytic efficiency, outperforming the benchmark Aeroxide® P25 catalyst. This mixed phase catalyst comprised also larger, though more strained, rutile nanocrystals than P25, and presented an additional deeper electron trapping lattice site according to light-induced EPR measurements. More importantly, electron transfer from rutile to anatase lattice traps was identified by EPR under visible light in the mixed phase photocatalyst. The improved crystal quality of the anatase nanocrystals combined with the enhanced charge separation in anatase/rutile interfaces is concluded crucial to the design of competent solar photocatalytic nanomaterials. © 2016.
Pelaez M, Falaras P, Likodimos V, O'Shea K, de la Cruz AA, Dunlop PSM, Byrne JA, Dionysiou DD. Use of selected scavengers for the determination of NF-TiO2 reactive oxygen species during the degradation of microcystin-LR under visible light irradiation. Journal of Molecular Catalysis A: Chemical [Internet]. 2016;425:183-189. Publisher's VersionAbstract
Although UV-induced TiO2 photocatalysis involves the generation of several reactive oxygen species (ROS), the formation of hydroxyl radicals is generally associated with the degradation of persistent organic contaminants in water. In this study, a variety of radical scavengers were employed to discriminate the roles of different ROS during visible light-activated (VLA) photocatalysis using nitrogen and fluorine doped TiO2 (NF-TiO2) in the degradation of the hepatotoxin, microcystin-LR (MC-LR) in water. The addition of hydroxyl radical scavengers, methanol and tert-butyl alcohol to the reaction mixture resulted in negligible inhibition of NF-TiO2 photocatalytic degradation of MC-LR at pH 3.0 and only partial inhibition at pH 5.7 under visible light. While hydroxyl radicals ([rad]OH) generally play the primary role in UV-TiO2 photocatalysis, the minimal influence of MeOH and t-BuOH on the degradation process under these experimental conditions indicates that [rad]OH are not crucial in VLA NF-TiO2 photocatalysis. However, strong inhibition was observed in VLA NF-TiO2 photocatalytic degradation of MC-LR in the presence of superoxide dismutase, benzoquinone and catalase at pH 3.0 and 5.7 indicating that O2[rad]− and H2O2 play critical roles in the degradation process. Similar degradation rates were observed in the presence of deuterium oxide, which enhances singlet oxygen mediated processes further suggesting singlet oxygen is not a key species in the degradation of MC-LR. Formic acid and cupric nitrate were added to probe the roles of the valence band holes and conduction band electrons, respectively. Under UV–vis light irradiation, almost complete inhibition of MC-LR removal is observed with NF-TiO2 in the presence of [rad]OH scavengers at pH 5.7. These results demonstrate that the solution pH plays a major role in the formation and reactivity of ROS during VLA NF-TiO2 photocatalysis. The adsorption strength of scavengers and MC-LR onto NF-TiO2 as well as the speciation of ROS as a function of pH needs to be carefully considered since they also play a major role in the efficiency of the process. These results indicate that the reduction of molecular oxygen by photo-generated electrons rather than hydroxyl radicals produced by oxidative reactions of photo-generated holes is the key factor in the VLA NF-TiO2 photocatalytic degradation of MC-LR. © 2016 Elsevier B.V.
Labropoulos A, Veziri C, Kapsi M, Pilatos G, Likodimos V, Tsapatsis M, Kanellopoulos NK, Romanos GE, Karanikolos GN. Carbon Nanotube Selective Membranes with Subnanometer, Vertically Aligned Pores, and Enhanced Gas Transport Properties. Chemistry of Materials [Internet]. 2015;27:8198-8210. WebsiteAbstract
Membranes consisting of ultrathin, oriented, single-wall carbon nanotube (SWCNT) micropores with a diameter of ∼4 Å were developed. c-Oriented AFI-type aluminophosphate (AlPO) films (AlPO4-5 and CoAPO-5), consisting of parallel channels 7.3 Å in diameter, were first fabricated by seeded growth on macroporous alumina supports, and used as templates for synthesis of CNTs inside the zeolitic channels by thermal treatment, utilizing the structure directing agent (amine) occluded in the channels as carbon source. Incorporation of CNTs inside the AFI channels altered the transport mechanism of all permeating gases tested, and imposed a substantial increase in their permeation rates, in comparison to the AlPO4-5 membrane, despite the pore size reduction due to nanotube growth. The enhancement of the permeation rates is attributed to repulsive potentials between gas molecules and occluded nanotubes, which limit adsorption strength and enhance diffusivity, coupled to the smooth SWCNT surface that enables fast diffusion through the nanotube interior. Separation ability, evaluated with respect to H2 and CO2 gases, was enhanced by using polysterene as defect-blocking medium on both AlPO and CNT/AlPO membranes and was preserved after CNT growth. © 2015 American Chemical Society.
Perganti D, Kontos AG, Stergiopoulos T, Likodimos V, Farnell J, Milliken D, Desilvestro H, Falaras P. Thermal Stressing of Dye Sensitized Solar Cells Employing Robust Redox Electrolytes. Electrochimica Acta [Internet]. 2015;179:241-249. WebsiteAbstract
Robust Dye Sensitized Solar Cells have been prepared employing liquid electrolytes using ethyl isopropyl sulfone (EiPS) high boiling point solvent. The cells were tested for their durability under harsh thermal stressing conditions of 85 °C and prolonged ageing time, 3000 h in the dark. The use of EiPS outperforms stability-wise the typical methoxypropionitrile MPN solvent, improving the cell stability from 38 to 75%. For both solvents, the physicochemical analysis infers the thermal degradation of the cell with the main changes occurring in the first 300 h of ageing. This was attributed to partial triiodide loss which reduces short circuit photocurrent and leads to formation of luminescent species in the electrolyte that affects the TiO2 surface and reduces open circuit photovoltage. The degradation effects were notably supressed by the use of the more stable EiPS solvent, where it was possible to optimize the iodine content in the redox mediator. It has been thus confirmed that iodine concentration as low as 0.05 M in the EiPS electrolyte is slightly preferable in terms of stability and device performance, comparing with higher concentrations, 0.1 and 0.15 M, respectively. © 2015 Elsevier Ltd. All rights reserved.
Athanasekou CP, Morales-Torres S, Likodimos V, Romanos GE, Pastrana-Martinez LM, Falaras P, Faria JL, Figueiredo JL, Silva AMT. Prototype composite membranes of partially reduced graphene oxide/TiO2 for photocatalytic ultrafiltration water treatment under visible light. Applied Catalysis B: Environmental [Internet]. 2014;158-159:361-372. WebsiteAbstract
A highly efficient hybrid photocatalytic/ultrafiltration process is demonstrated for water purification using visible light. The process relies on the development of partially reduced graphene oxide/TiO2 composite membranes and their incorporation into an innovative water purification device that combines membrane filtration with semiconductor photocatalysis. Composites consisting of graphene oxide sheets decorated with TiO2 nanoparticles were deposited and stabilized into the pores of ultrafiltration mono-channel monoliths using the dip-coating technique. Cross-flow and dead-end filtration experiments were sequentially conducted in dark, under UV and visible light. The membrane surface was irradiated for the elimination of two synthetic azo-dyes, methyl orange and methylene blue, from water solutions. The synergetic effects of graphene oxide on pollutant adsorption and photocatalytic degradation capacity of TiO2 were thoroughly studied, while the influence of the pore size of the monolithic substrate on the deposition morphologies was also elucidated. Moreover, the performance of the novel hybrid process was compared with that of standard nanofiltration with respect to pollutant removal efficiency and energy consumption, providing firm evidence for its economic feasibility and efficiency. © 2014 Elsevier B.V.
Kontos AG, Likodimos V, Veziri CM, Kouvelos E, Moustakas N, Karanikolos GN, Romanos GE, Falaras P. CO2 captured in zeolitic imidazolate frameworks: Raman spectroscopic analysis of uptake and host-guest interactions. ChemSusChem [Internet]. 2014;7:1696-1702. WebsiteAbstract
Zeolitic imidazolate frameworks (ZIFs) exhibit enhanced selectivity and increased CO2 uptake due to the incorporation of functional imidazolate units in their structure as well as their extensive porosity and ring flexibility. In situ Raman investigation of a representative host compound, ZIF-69, in practical CO2 pressure and temperature regimes (0-10 bar and 0-64 °C) correlates well with corresponding macroscopic CO2 sorption data and shows clear clear spectroscopic evidence of CO2 uptake. Significant positive shift of the 159 cm-1 phenyl bending mode of the benzimidazole moiety indicates weak hydrogen bonding with CO 2 in the larger cavities of the ZIF matrix. Raman spectroscopy is shown to be an easy and sensitive tool for quantifying CO2 uptake, identifying weak host-guest interactions and elucidating CO2 sorption mechanism in ZIFs. Are you Raman enough? In situ Raman investigation of the interactions of zeolitic imidazolate frameworks (ZIFs) with CO2 in practical pressure and temperature regimes (0-10 bar and 0-64 °C) correlates well with corresponding macroscopic CO2 sorption data and shows clear spectroscopic evidence of CO2 uptake (see image). Raman is found to be an easy and sensitive tool for quantifying CO2 uptake, identifying weak host-guest interactions, and elucidating CO2 sorption mechanism in ZIFs. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Likodimos V, Steriotis TA, Papageorgiou SK, Romanos GE, Marques RRN, Rocha RP, Faria JL, Pereira MFR, Figueiredo JL, Silva AMT, et al. Controlled surface functionalization of multiwall carbon nanotubes by HNO3 hydrothermal oxidation. Carbon [Internet]. 2014;69:311-326. WebsiteAbstract
Controlled surface functionalization is demonstrated by nitric acid hydrothermal oxidation on multiwall carbon nanotubes (MWCNTs). The formation and evolution of oxygen functional groups were systematically investigated as a function of the HNO3 concentration on MWCNTs with different structural and morphological characteristics, employing temperature-programmed desorption coupled with mass spectrometry, thermogravimetry and differential scanning calorimetry, Raman spectroscopy and N2 porosimetry analysis. Hydrothermal treatment provides controlled MWCNT modification by specific oxygen functionalities at amounts determined by the morphology, texture and crystallinity of the pristine materials. Hydrothermal oxidation competes well with the harsh boiling nitric acid treatment regarding the total amount of oxygen functionalities, while requiring much lower amounts of oxidizing agent and, most importantly, reducing amorphous carbon deposits on the MWCNT surface, a major drawback of aggressive liquid phase oxidation methods. Detailed pore structure analysis revealed a progressive increase of the surface area upon hydrothermal functionalization, whereas the mesopore structure varied consistently with the intrinsic MWCNT properties related to the packing of the nanotube bundles and the reduction of amorphous carbon. These advantageous features render nitric acid hydrothermal oxidation an efficient functionalization process to fine tune and optimize the surface chemistry of MWCNTs for target applications, circumventing the need for additional purification post-processing. © 2013 Elsevier Ltd. All rights reserved.
Molchan IS, Thompson GE, Lindsay R, Skeldon P, Likodimos V, Romanos GE, Falaras P, Adamova G, Iliev B, Schubert TJS. Corrosion behaviour of mild steel in 1-alkyl-3-methylimidazolium tricyanomethanide ionic liquids for CO2 capture applications. RSC Advances [Internet]. 2014;4:5300-5311. WebsiteAbstract
{The corrosion behaviour of mild steel (MS) was systematically investigated as a function of the alkyl chain length in the cation of 1-alkyl-3- methylimidazolium tricyanomethanide ([Cnmim]TCM
Zhang G, Nadagouda MN, O'Shea K, El-Sheikh SM, Ismail AA, Likodimos V, Falaras P, Dionysiou DD. Degradation of cylindrospermopsin by using polymorphic titanium dioxide under UV-Vis irradiation. Catalysis Today [Internet]. 2014;224:49-55. WebsiteAbstract
The frequent occurrence of cyanobacterial harmful blooms due to eutrophication necessitates the development of appropriate water treatment technologies for the released cyanotoxins. In this study, nanoparticles composed of anatase-brookite-rutile polymorphic titanium dioxide (PM-TiO2) were synthesized using a modified sol-gel method with a low temperature oil bath for the photocatalytic degradation of cylindrospermopsin (CYN), an important cyanotoxin that has been only little explored with respect to water treatment technologies. The physiochemical properties of synthesized PM-TiO2 nanoparticles, such as the formation of heteronanostructure (with 66% anatase, 22% brookite and 12% rutile), high surface area (207 m2/g), small particle size (∼5 nm), and bandgap (Eg = 3.0 eV), endow PM-TiO2 to be an effective photocatalyst for CYN treatment under UV-Vis irradiation. Moreover, the impacts of certain process parameters (i.e. photocatalyst loading, pH and presence of natural organic matter (NOM)) were examined and discussed in detail. It was found that the presence of NOM decreased the observed reaction rate constants by 52% and 95% compared to clean water samples when 2 and 10 mg/L NOM were applied, respectively. In particularly, when two natural water samples from East Fork Lake and Toledo Water Plant were applied as the matrices in the photocatalytic experiments, the degradation rate constants were reduced by 90% and 75%, respectively. This work demonstrates that the polymorphic TiO2 containing three phases can be effective photocatalyst for cyanotoxin treatment, however, the degradation kinetics are inhibited significantly in the presence of NOM. Therefore, pretreatment is necessary to remove NOM before the photocatalytic treatment of surface water containing CYN. © 2013 Elsevier B.V.
Han C, Andersen J, Likodimos V, Falaras P, Linkugel J, Dionysiou DD. The effect of solvent in the sol-gel synthesis of visible light-activated, sulfur-doped TiO2 nanostructured porous films for water treatment. Catalysis Today [Internet]. 2014;224:132-139. WebsiteAbstract
The effects of solvent on the synthesis of visible light-activated, sulfur-doped TiO2 (S-TiO2) films were studied. Four different polar, protic solvents, isopropanol, 1-butanol, ethanol, and methanol (iPrOH, BtOH, EtOH, and MeOH), were chosen as the solvent in four titania sol-gel preparations. The films were characterized by X-ray diffraction (XRD), high resolution transmission electron microscopy (HR-TEM), atomic force microscopy (AFM), environmental scanning electron microscopy (ESEM), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR), Ultraviolet (UV)-vis diffuse reflectance, and porosimetry. The structural, morphological, and porous characteristics of the sulfur-doped TiO2 films were correlated with solvent physical properties such as the dielectric constant (D-value) and the saturated vapor pressure. According to XPS and FT-IR, S6+/S4+ cations replaced Ti4+ ions in the lattice of TiO2, resulting in the formation of localized states within the bandgap of TiO2. The optical absorption edge for all S-TiO2 films was significantly shifted toward the visible light region. The solvent D-value has a negligible effect on the bandgap energy change and the doping states of the prepared S-TiO2 samples. S-TiO 2 films synthesized using MeOH (S-TiO2-MeOH), despite their lower Brunauer, Emmett, and Teller (BET) surface area and porosity compared to the other films, showed the highest photocatalytic activity for the degradation of the hepatotoxin microcystin-LR (MC-LR) under visible light irradiation due to their high surface roughness and large pore size. The tailor-designed structure of the S-TiO2-MeOH film contributed to the high photocatalytic degradation rates of MC-LR. The larger pore size of the S-TiO2-MeOH films allowed easier transport of MC-LR inside the porous film, while the higher film surface roughness could increase nano-interfacial interactions between MC-LR and surface active sites. These results indicate that the structural and morphological properties of S-TiO2 photocatalysts can be tailor-designed using different solvents in the sol-gel synthesis, while inducing negligible effects on the sulfur doping and the visible light activation of TiO2. Therefore, the enhancement of photocatalytic activity of S-TiO2 films can be achieved by judicious choice of the main solvent for the sol-gel method. © 2013 Elsevier B.V.
Diamantopoulou A, Glenis S, Likodimos V, Guskos N. Low temperature magnetic phase transition and interlayer coupling in double-wall carbon nanotubes. Journal of Applied Physics [Internet]. 2014;116. WebsiteAbstract
The magnetic properties of double wall carbon nanotubes (DWCNTs) were investigated using electron spin resonance (ESR) spectroscopy. An asymmetric resonance line of low intensity was identified and analyzed by the superimposition of a narrow and a broad metallic lineshape, attributed to the distinct contributions of defect spins located on the inner and outer DWCNTs shells. The spin susceptibilities of both ESR components revealed a ferromagnetic phase transition at low temperatures (T<10K) with small variation in the corresponding Curie-Weiss temperatures, approaching closely that of metallic single wall carbon nanotubes. Interlayer coupling between the DWCNT layers is suggested to effectively reduce the difference between the transition temperatures for the inner and outer shells and enhance spin-spin interactions between defect spins via the RKKY-type interaction of localized spins with conduction electrons. © 2014 AIP Publishing LLC.
Miranda SM, Romanos GE, Likodimos V, Marques RRN, Favvas EP, Katsaros FK, Stefanopoulos KL, Vilar VJP, Faria JL, Falaras P, et al. Pore structure, interface properties and photocatalytic efficiency of hydration/dehydration derived TiO2/CNT composites. Applied Catalysis B: Environmental [Internet]. 2014;147:65-81. WebsiteAbstract
Manifold advantages are foreseen by using carbon nanotubes (CNTs) as support for inorganic TiO2 nanoparticles due to the unique texture/morphology and adsorption capacity of CNTs. Synergistic effects might also result from interfacial charge transfer between the CNTs and TiO2. Effective charge transfer has the potentiality to limit electron/hole recombination and shift the TiO2 photocatalytic response to the visible range. Homogeneous mixing and intimate contact between the graphitic and TiO2 surfaces are of high importance in order to trigger synergistic effects. Thus, the existence of complementary methods to shed light on both these features is of high importance when developing TiO2/CNT composite photocatalysts. In this work, a wide variety of TiO2/CNT composites was prepared by a simple hydration/dehydration procedure, using single-wall (SWCNTs) and multi-wall (MWCNTs) carbon nanotubes, either functionalized or not, and TiO2 nanoparticles of different size. To evaluate the degree of homogeneity between the graphitic and inorganic phases, a new methodology which was based on a complex interpretation of the liquid nitrogen porosimetry (LN2) isotherms of the composites and of each phase in the composite separately was developed. Furthermore, interface interaction characteristics were elucidated by micro-Raman spectroscopy while small-angle X-ray scattering (SAXS) measurements provided insight on the surface roughness and micropore structure of the TiO2/SWCNT samples. The Raman analysis concluded to the absence of any interfacial interaction. In this context the efficiency of the prepared composites to photocatalytically oxidize caffeine was evaluated in regard to their homogeneity, as derived by the LN2 method. As expected, in the absence of synergetic effects the photocatalytic efficiency correlated well with the extent of mixing between the CNTs and TiO2 phases. The discrepancy observed for one of the samples was attributed to the existence of large micropores, a feature that was distinguishable solely by SAXS measurements. © 2013 Elsevier B.V.
Pastrana-Martínez LM, Morales-Torres S, Likodimos V, Falaras P, Figueiredo JL, Faria JL, Silva AMT. Role of oxygen functionalities on the synthesis of photocatalytically active graphene-TiO2 composites. Applied Catalysis B: Environmental [Internet]. 2014;158-159:329-340. WebsiteAbstract
Photocatalysis has gained relevance in many applications, including production of fuels, green synthesis of added value products and water detoxification. Graphene-TiO2 photocatalysts are attracting great attention, but they should be prepared adequately, protecting the carbon material from the surrounding reactive media, maximizing the contact between TiO2 and graphene, and envisaging solar applications. Hereby, graphene oxide was chemically reduced using vitamin C and glucose (environmental friendly reducing agents) as well as hydrazine, and the evolution of the graphene oxygenated surface groups was systematically analyzed (pHPZC, TPD, TG, XPS, DRUV-Vis, Raman and ATR-FTIR). These functionalities (such as epoxy and hydroxyl groups) mediate the efficient and uniform assembly of the TiO2 nanoparticles on the graphene oxide sheets, leading to highly efficient photocatalysts both under near-UV/Vis and visible light, which is of particular relevance for solar applications. © 2014 Elsevier B.V.
Khan JA, Han C, Shah NS, Khan HM, Nadagouda MN, Likodimos V, Falaras P, O'Shea K, Dionysiou DD. Ultraviolet-Visible Light-Sensitive High Surface Area Phosphorous-Fluorine-Co-Doped TiO2 Nanoparticles for the Degradation of Atrazine in Water. Environmental Engineering Science [Internet]. 2014;31:435-446. WebsiteAbstract
Conventional titanium dioxide (TiO2) materials can be activated only by ultraviolet (UV) light, which is only 4-5% of the whole solar spectrum. As a result, visible light (vis)-active TiO2-based photocatalysts have recently received significant attention in the field of TiO2 photocatalytic treatment and purification of water and air. This study reports the preparation of UV-visible light-active phosphorous (P)-doped, fluorine (F)-doped, and PF-co-doped anatase TiO2 nanoparticles via an innovative sol-gel method. Prepared nanoparticles were characterized by UV-vis diffuse reflectance spectroscopy, X-ray diffraction analysis, Raman spectroscopy, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy (FTIR), and porosimetry analysis. Synthesized materials exhibited improved structural properties, including high surface area, small crystallite size, reduced band gap energy, mesoporous structure, and high porosity. Due to doping with P and F, light absorption of TiO2 in the visible light region was efficiently enhanced with effective band gap energy of 2.70 eV. Brunauer-Emmett-Teller (BET) surface area for PF-co-doped, P-doped, F-doped, and reference TiO2 nanoparticles was 212.0, 175.0, 88.8, and 79.7 m2/g, respectively. PF-co-doped TiO2 showed the highest photocatalytic degradation of atrazine, which could be attributed to the beneficial effects including small crystallite size, high BET surface area, and light absorption in UV-visible region, induced by co-doping of TiO2 with P and F. Finally, reaction intermediates were determined, which confirms the photocatalytic degradation of atrazine using the synthesized catalysts under UV-visible light illumination. © Copyright 2014, Mary Ann Liebert, Inc. 2014.
Han C, Likodimos V, Khan JA, Nadagouda MN, Andersen J, Falaras P, Rosales-Lombardi P, Dionysiou DD. UV–visible light-activated Ag-decorated, monodisperse TiO2 aggregates for treatment of the pharmaceutical oxytetracycline. Environmental Science and Pollution Research [Internet]. 2014;21:11781-11793. WebsiteAbstract
Noble metal Ag-decorated, monodisperse TiO2 aggregates were successfully synthesized by an ionic strength-assisted, simple sol–gel method and were used for the photocatalytic degradation of the antibiotic oxytetracycline (OTC) under both UV and visible light (UV–visible light) irradiation. The synthesized samples were characterized by X-ray diffraction analysis (XRD); UV–vis diffuse reflectance spectroscopy; environmental scanning electron microscopy (ESEM); transmission electron microscopy (TEM); high-resolution TEM (HR-TEM); micro-Raman, energy-dispersive X-ray spectroscopy (EDS); and inductively coupled plasma optical emission spectrometry (ICP-OES). The results showed that the uniformity of TiO2 aggregates was finely tuned by the sol–gel method, and Ag was well decorated on the monodisperse TiO2 aggregates. The absorption of the samples in the visible light region increased with increasing Ag loading that was proportional to the amount of Ag precursor added in the solution over the tested concentration range. The Brunauer, Emmett, and Teller (The BET) surface area slightly decreased with increasing Ag loading on the TiO2 aggregates. Ag-decorated TiO2 samples demonstrated enhanced photocatalytic activity for the degradation of OTC under UV–visible light illumination compared to that of pure TiO2. The sample containing 1.9 wt% Ag showed the highest photocatalytic activity for the degradation of OTC under both UV–visible light and visible light illumination. During the experiments, the detected Ag leaching for the best TiO2-Ag photocatalyst was much lower than the National Secondary Drinking Water Regulation for Ag limit (0.1 mg L−1) issued by the US Environmental Protection Agency. © 2013, Springer-Verlag Berlin Heidelberg.
Papatryfon XL, Heliopoulos NS, Molchan IS, Zubeir LF, Bezemer ND, Arfanis MK, Kontos AG, Likodimos V, Iliev B, Romanos GE, et al. CO2 capture efficiency, corrosion properties, and ecotoxicity evaluation of amine solutions involving newly synthesized ionic liquids. Industrial and Engineering Chemistry Research [Internet]. 2014;53:12083-12102. WebsiteAbstract
The CO2 capture efficiency of nine newly synthesized ionic liquids (ILs), both in their pure states as well as in binary and ternary systems with water and amines, was investigated. The study encompassed ILs with fluorinated and tricyanomethanide anions as well as ILs that interact chemically with CO2 such as those with amino acid and acetate anions. Compared to amines, some of the novel ILs exhibited a majority of important advantages for CO2 capture such as enhanced chemical and thermal stabilities and negligible vapor pressure; the previous features counterbalance the disadvantages of lower CO2 absorption capacity and rate, making these ILs promising CO2 absorbents that could partially or totally replace amines in industrial scale processes. In addition to their ability to capture CO2, important issues including corrosivity and ecotoxicity were also examined. A thorough investigation of the capture efficiency and corrosion properties of several solvent formulations proved that some of the new ILs encourage future commercial-scale applications for appropriate conditions. ILs with a tricyanomethanide anion confirmed a beneficial effect of water addition on the CO2 absorption rate (ca. 10-fold) and capacity (ca. 4-fold) and high efficiency for corrosion inhibition, in contrast with the negative effect of water on the CO2 absorption capacity of ILs with the acetate anion. ILs with a fluorinated anion showed high corrosivity and an almost neutral effect of water on their efficiency as CO2 absorbents. ILs having amino acid anions presented a reduced toxicity and high potential to completely replace amines in solutions with water but, in parallel, showed thermal instability and degradation during CO2 capture. Tricyanomethanide anion-based ILs had a beneficial effect on the capture efficiency, toxicity, and corrosiveness of the standard amine solutions. As a consolidated output, we propose solvent formulations containing the tricyanomethanide anion-based ILs and less than 10 vol % of primary or secondary amines. These solvents exhibited the same CO2 capture performance as the 20-25 vol % standard amine solutions. The synergetic mechanisms in the capture efficiency, induced by the presence of the examined ILs, were elucidated, and the results obtained can be used as guidance for the design and development of new ILs for more efficient CO2 capture. © 2014 American Chemical Society.
Konstantakou M, Stergiopoulos T, Likodimos V, Vougioukalakis GC, Sygellou L, Kontos AG, Tserepi A, Falaras P. Influence of fluorine plasma treatment of TiO2 films on the behavior of dye solar cells employing the Co(II)/(III) redox couple. Journal of Physical Chemistry C [Internet]. 2014;118:16760-16775. WebsiteAbstract
Fluorine plasma treatment was investigated as an appropriate means for the surface modification of TiO2 thin film electrodes and the optimization of their performance as photoanodes in dye solar cells (DSCs) employing the Co(II)/(III) redox shuttle and the organic D35 sensitizer. Detailed surface and structural characterization of the titania films by contact angle measurements, atomic force microscopy, profilometry, and Raman and UV-vis spectroscopy showed that high density SF6 plasma provoked severe film densification and thus an increase of the nanoparticles packing density, leaving intact the crystallinity, particle size, and optical bandgap. Surface fluorination of the TiO2 films was also identified by X-ray photoelectron spectroscopy. The combination of the above effects resulted in the enhancement of both photocurrent and power conversion efficiency of the corresponding DSCs at moderate plasma treatment durations, while the photovoltage decreased continuously as a function of the fluorine processing time. Electrochemical impedance spectroscopy analysis revealed a marked increase of the density and distribution of trap states due to fluorine induced surface states along with a systematic downward shift of the TiO2 conduction band, probably attributed to the electrostatic coupling of intercalated Li + cations with the polar Ti-F species at the TiO2 surface, in agreement with the Voc drop. In contrast, enhanced electron injection was inferred to underlie the observed Jsc and DSC performance improvements, as surface fluorination and the concomitant film densification slightly increased electron transport while hardly affecting dye loading capacity, light harvesting efficiency, and recombination kinetics, except for the case of prolonged plasma treatment. Effective control of the detrimental side effects of fluorine species can render this kind of plasma treatment a powerful method to tune the surface and electrical properties of TiO2 films and optimize the behavior and performance of the resulting DSC devices. © 2014 American Chemical Society.
Tzialla O, Labropoulos A, Panou A, Sanopoulou M, Kouvelos E, Athanasekou C, Beltsios K, Likodimos V, Falaras P, Romanos G. Phase behavior and permeability of Alkyl-Methyl-Imidazolium Tricyanomethanide ionic liquids supported in nanoporous membranes. Separation and Purification Technology [Internet]. 2014;135:22-34. WebsiteAbstract
This work presents an investigation of the CO2 and N2 single and mixed gas phase permeability through supported ionic liquid membranes (SILMs) developed on ceramic nanoporous substrates with different pore size (1, 5 and 10 nm). ILs from the 1-alkyl-3-methylimidazolium tricyanomethanide family ([RMIM][TCM], with alkyl group, R: ethyl, butyl or octyl) were used as nanopore modifiers. These ILs exhibit high chemical and thermal stability, low viscosity and enhanced CO2 absorption capacity compared to other imidazolium based ILs. Thermal analysis of the developed SILMs unveiled a drastic liquid-to-solid transition upon confinement of the ILs into the pore channels with a size of 1 nm. The IL crystals formed inside these extremely small cavities possessed considerable thermal stability and underwent thermally induced phase transitions that differed significantly from those occurring in the unconfined bulk IL phase or in the IL phase when entrapped into the larger pore channels. The different physical state of the IL under confinement into the pores of different size resulted to significant variation of the flux properties between the developed SILM membranes. The effect of temperature on the CO2 permeability dependend strongly on the crystal thermal stability and microstructure dictated by the confinement into the nanopores. © 2014 Elsevier B.V. All rights reserved.
Labropoulos AI, Romanos GE, Kouvelos E, Falaras P, Likodimos V, Francisco M, Kroon MC, Iliev B, Adamova G, Schubert TJS. Alkyl-methylimidazolium tricyanomethanide ionic liquids under extreme confinement onto nanoporous ceramic membranes. Journal of Physical Chemistry C [Internet]. 2013;117:10114-10127. WebsiteAbstract
A method to predict the gas permeability of supported ionic liquid membranes (SILMs) was established, using as input the pore structure characteristics of asymmetric ceramic membrane supports and the physicochemical properties of the bulk ionic liquid (IL) phase. The method was applied to investigate the effect of IL nanoconfinement on the CO2 and N 2 permeability/selectivity properties of novel SILMs developed on nanofiltration (NF) membranes employing for the first time the 1-ethyl-3-methylimidazolium and the 1-butyl-3-methylimidazolium tricyanomethanide ILs as pore modifiers. The selected ILs exhibit low viscosity, which allows for faster gas solvation rates and ease of synthesis/purification that makes them attractive for large-scale production. In parallel, the use of ceramic supports instead of polymeric ones presents the advantage of operation at elevated temperatures and pressures and offers the possibility to study the "real" permeability of the confined IL phase, avoiding additional contributions from the gas diffusion through the surrounding solid matrix. The developed SILMs exhibited enhanced CO2 permeability together with high CO2/N2 separation capacity, though with distinct variations depending on the alkyl chain length of the 1-alkyl-3- methylimidazolium cation. Application of the developed methodology allowed discriminating the contribution of the NF pore structural characteristics on the SILM performance and unveiled the subtle interplay of diverse IL confinement effects on the gas permeability stemming from the specific layering of ion pairs on the nanoporous surface and the phase transition of the IL at room temperature, dictated by small variations of the IL cation size. © 2013 American Chemical Society.
Vaenas N, Bidikoudi M, Stergiopoulos T, Likodimos V, Kontos AG, Falaras P. Annealing effects on self-assembled TiO2 nanotubes and their behavior as photoelectrodes in dye-sensitized solar cells. Chemical Engineering Journal [Internet]. 2013;224:121-127. WebsiteAbstract
Self-assembled highly ordered TiO2 nanotube arrays were grown on Ti foils in NH4F ethylene glycol electrolyte under mild anodic oxidation conditions. The effect of annealing post-treatment on their structural properties was systematically investigated with respect to their electrical characteristics and photoelectrochemical performance in back-side illuminated dye-sensitized solar cells (DSCs). A variety of parameters was controlled and optimized including the annealing temperature, the heating rate and duration of the thermal treatment. The obtained results confirmed a correlation between the crystalline/structural properties of the TiO2 nanotubes and their electrical characteristics, thus revealing the close interplay of crystal size, grain boundaries and crystallite interconnectivity with the electron dynamics (transport/ recombination) governing the DSC operation and efficiency. The high importance of a barrier layer at the interface between the nanotubes and the Ti foil was also highlighted. © 2013 Elsevier B.V.
Halouzka V, Jakubec P, Kvitek L, Likodimos V, Kontos AG, Papadopoulos K, Falaras P, Hrbac J. Deposition of nanostructured Ag films on siliconwafers by electrochemical/electrophoretic deposition for electrochemical and SERS sensing. Journal of the Electrochemical Society [Internet]. 2013;160:B54-B59. WebsiteAbstract
Electrolysis of ultrapure water in a two-electrode cell with silver anode and conductive substrate (Si wafer) as a cathode leads to the formation of nanostructured silver layers deposited on cathode. In the process, the silver anode is electrochemically dissolved to silver cations, which react with water (or OH• radicals derived from water electrolysis) forming silver oxide nanoparticles, which fill the interelectrode space by electrophoretic movement, diffusion and convection induced by temperature effects of electrolysis. During the process the silver oxide nanoparticles are partially transformed into silver nanoparticles. On the cathode, silver cations and silver/silver oxide nanoparticles undergo reduction to form nanostructured silver film. The results of the present study open a new, extremely simple and ultra-low cost way to prepare nanostructured silver films on conducting and semiconducting substrates. The prepared nanosilver coated silicon substrates exhibit high performances as amperometric sensors for hydrogen peroxide and also as SERS substrates. © 2013 The Electrochemical Society.
Liu G, Han C, Pelaez M, Zhu D, Liao S, Likodimos V, Kontos AG, Falaras P, Dionysiou DD. Enhanced visible light photocatalytic activity of CN-codoped TiO 2 films for the degradation of microcystin-LR. Journal of Molecular Catalysis A: Chemical [Internet]. 2013;372:58-65. WebsiteAbstract
A sol-gel method based on the self-assembly technique with a nonionic surfactant was employed to synthesize visible-light-active CNTiO2 films with rough surface for drinking water treatment. The enhancement of photocatalytic activity of CNTiO2 films on the degradation of microcystin-LR (MC-LR) was subsequently evaluated under visible light irradiation. The films were characterized by XRD, ESEM, AFM, HRTEM, UV-vis diffuse reflectance spectroscopy (DRS), FT-IR, XPS, and porosimetry analysis. The results revealed that the physicochemical properties of the films, such as BET surface area, porosity, crystallite size and pore size distribution, could be controlled by adjusting the calcination temperature. Higher surface area, smaller crystallite size, narrow pore size distribution, and very high surface roughness (360 nm) were obtained for CN-codoped TiO2 films calcined at 400 °C. DRS showed that as-prepared CNTiO2 films exhibited higher absorption in the visible light region and a red shift in the band gap transition due to CN-doping. CNTiO2 films effectively degraded MC-LR under visible light irradiation compared to the reference film. In particular, the film calcined at 400 °C showed high mechanical stability during three consecutive cycles for MC-LR degradation. The enhancement on the photocatalytic activity of the CNTiO2 films under visible light irradiation was attributed to the synergistic effects of carbon and nitrogen doping as well as the high surface roughness of the prepared films. © 2013 Elsevier B.V.
Moustakas NG, Kontos AG, Likodimos V, Katsaros F, Boukos N, Tsoutsou D, Dimoulas A, Romanos GE, Dionysiou DD, Falaras P. Inorganic-organic core-shell titania nanoparticles for efficient visible light activated photocatalysis. Applied Catalysis B: Environmental [Internet]. 2013;130-131:14-24. WebsiteAbstract
Nanostructured modified TiO2 (m-TiO2) was synthesized using the gel combustion method based on the calcination of an acidified alkoxide solution mixed with urea. The materials were characterized by Raman, FT-IR and UV-vis diffuse reflectance spectroscopies, transmission (TEM) and scanning electron microscopies (SEM), X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR), in comparison with reference material untreated with urea (ref-TiO2). The effect of both the urea content and calcination temperature were optimized, providing the optimal absorption threshold of 2.19eV for solar light harvesting. The photocatalytic performance of the m-TiO2 powder was tested for the degradation of methylene blue (MB) azo dye under UVA (350-365nm), visible (440-460nm), and daylight (350-750nm) illumination. The hybrid inorganic/organic material shows exceptional physicochemical properties and significant photocatalytic activity, especially in the visible, attributed to sensitization of the TiO2 by a thin porous layer of carbonacious species in controlled core-shell morphology. © 2012 Elsevier B.V.
Kontos AG, Stergiopoulos T, Likodimos V, Milliken D, Desilvesto H, Tulloch G, Falaras P. Long-term thermal stability of liquid dye solar cells. Journal of Physical Chemistry C [Internet]. 2013;117:8636-8646. WebsiteAbstract
Laboratory-size dye solar cells (DSCs), based on industrially feasible materials and processes employing liquid electrolytes, have been developed. Cells based on two electrolyte solvents with different physical properties were subjected to thermal stress test at 80 C for 2000 h in the dark to monitor their long-term thermal stability. The DSCs incorporating a methoxypropionitrile (MPN)-based electrolyte presented a severe efficiency loss at 1 sun AM 1.5G of more than 70% upon thermal aging, while the solar cells using tetraglyme (TG) as a high boiling point solvent attained a promising stability with only 20% loss of performance. To better understand the above behavior, systematic experiments, including optical microscopy, linear sweep voltammetry, UV-vis absorption, electrochemical impedance, and Raman spectroscopies were conducted. Virtually no dye degradation/desorption, electrolyte decomposition, semiconductor passivation, or loss of cathode activity could be identified. For the MPN-based cells, a sharp decrease in the short-circuit photocurrent was observed at high illumination intensities following thermal stress, attributed to charge-transfer limitations due to severe triiodide loss, verified by different experimental techniques. These degradation effects were efficiently mitigated by replacing MPN with the high-boiling-point solvent in the electrolyte. © 2013 American Chemical Society.
Han C, Doepke A, Cho W, Likodimos V, de la Cruz AA, Back T, Heineman WR, Halsall HB, Shanov VN, Schulz MJ, et al. A multiwalled-carbon-nanotube-based biosensor for monitoring microcystin-LR in sources of drinking water supplies. Advanced Functional Materials [Internet]. 2013;23:1807-1816. WebsiteAbstract
A multiwalled carbon nanotube (MWCNT)-based electrochemical biosensor is developed for monitoring microcystin-LR (MC-LR), a toxic cyanobacterial toxin, in sources of drinking water supplies. The biosensor electrodes are fabricated using vertically well-aligned, dense, millimeter-long MWCNT arrays with a narrow size distribution, grown on patterned Si substrates by water-assisted chemical vapor deposition. High temperature thermal treatment (2500 °C) in an Ar atmosphere is used to enhance the crystallinity of the pristine materials, followed by electrochemical functionalization in alkaline solution to produce oxygen-containing functional groups on the MWCNT surface, thus providing the anchoring sites for linking molecules that allow the immobilization of MC-LR onto the MWCNT array electrodes. Addition of the monoclonal antibodies specific to MC-LR in the incubation solutions offers the required sensor specificity for toxin detection. The performance of the MWCNT array biosensor is evaluated using micro-Raman spectroscopy, including polarized Raman measurements, X-ray photoelectron spectroscopy, cyclic voltammetry, optical microscopy, and Faradaic electrochemical impedance spectroscopy. A linear dependence of the electron-transfer resistance on the MC-LR concentration is observed in the range of 0.05 to 20 μg L-1, which enables cyanotoxin monitoring well below the World Health Organization (WHO) provisional concentration limit of 1 μg L-1 for MC-LR in drinking water. An highly sensitive Faradaic electrochemical impedance biosensor for monitoring microcystin-LR (MC-LR) in sources of drinking water supplies is developed using millimeter-long multiwalled carbon nanotube (MWCNT) arrays grown by water-assisted chemical vapor deposition with vertical alignment. A linear sensing response shows a wide microcystin-LR concentration range that is below the World Health Organization (WHO) provisional guideline limit of 1 μg L-1 for MC-LR in drinking water. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Andreou I, Amenitsch H, Likodimos V, Falaras P, Koutsoukos PG, Leontidis E. Organized silica films generated by evaporation-induced self-assembly as hosts for iron oxide nanoparticles. Materials [Internet]. 2013;6:1467-1484. WebsiteAbstract
In this work, we prepared oriented mesoporous thin films of silica on various solid substrates using the pluronic block copolymer P123 as a template. We attempted to insert guest iron oxide (FexOy) nanoparticles into these films by two different methods: (a) by co-precipitation-where iron precursors are introduced in the synthesis sol before deposition of the silica film-and subsequent oxide production during the film calcination step; (b) by preparing and calcining the silica films first then impregnating them with the iron precursor, obtaining the iron oxide nanoparticles by a second calcination step. We have examined the structural effects of the guest nanoparticles on the silica film structures using grazing incidence X-ray scattering (GISAXS), high-resolution transmission electron spectroscopy (HRTEM), spectroscopic ellipsometry, X-ray photoelectron spectroscopy (XPS), and Raman microscopy. Formation of nanoparticles by co-precipitation may induce substantial changes in the film structure leading, in our adopted process, to the appearance of lamellar ordering in the calcination stage. On the contrary, impregnation-based approaches perturb the film structures much more weakly, but are also less efficient in filling the pores with nanoparticles. © 2013 by the authors.
Likodimos V, Han C, Pelaez M, Kontos AG, Liu G, Zhu D, Liao S, de la Cruz AA, O'Shea K, Dunlop PSM, et al. Anion-doped TiO2 nanocatalysts for water purification under visible light. Industrial and Engineering Chemistry Research [Internet]. 2013;52:13957-13964. WebsiteAbstract
Innovative sol-gel synthesis based on the self-assembling template method has been applied to synthesize mesoporous anion-doped TiO2 with N-F, S and C atoms using suitable surfactants and reagents, to improve simultaneously the structural, morphological, and electronic properties of TiO2 nanomaterials and achieve anion doping of titania with high visible light photoinduced reactivity. The incorporation of anion species in the titania structure resulted in the effective extension of TiO2 optical absorption in the visible range through the formation of intragap energy states. The anion-doped titania materials immobilized in the form of nanostructured thin films on glass substrates exhibited high photocatalytic efficiency for the degradation of the microcystin-LR (MC-LR) cyanotoxin, a hazardous water pollutant of emerging concern, under visible light irradiation. The development of these visible light-activated nanocatalysts has the potential of providing environmentally benign routes for water treatment. © 2013 American Chemical Society.
Marquez Velasco J, Kelaidis N, Xenogiannopoulou E, Raptis YS, Tsoutsou D, Tsipas P, Speliotis T, Pilatos G, Likodimos V, Falaras P, et al. Electronic band structure imaging of three layer twisted graphene on single crystal Cu(111). Applied Physics Letters [Internet]. 2013;103. WebsiteAbstract
Few layer graphene (FLG) is grown on single crystal Cu(111) by Chemical Vapor Deposition, and the electronic valence band structure is imaged by Angle-Resolved Photo-Emission Spectroscopy. It is found that graphene essentially grows polycrystalline. Three nearly ideal Dirac cones are observed along the Cu Γ ̄ K ̄ direction in k-space, attributed to the presence of ∼4°twisted three layer graphene with negligible interlayer coupling. The number of layers and the stacking order are compatible with Raman data analysis demonstrating the complementarity of the two techniques for a more accurate characterization of FLG. © 2013 AIP Publishing LLC.
Romanos GE, Zubeir LF, Likodimos V, Falaras P, Kroon MC, Iliev B, Adamova G, Schubert TJS. Enhanced CO2 capture in binary mixtures of 1-Alkyl-3- methylimidazolium tricyanomethanide ionic liquids with water. Journal of Physical Chemistry B [Internet]. 2013;117:12234-12251. WebsiteAbstract
Absorption of carbon dioxide and water in 1-butyl-3-methylimidazoliun tricyanomethanide ([C4C1im][TCM]) and 1-octyl-3- methylimidazolium tricyanomethanide ([C8C1im][TCM]) ionic liquids (ILs) was systematically investigated for the first time as a function of the H2O content by means of a gravimetric system together with in-situ Raman spectroscopy, excess molar volume (VE), and viscosity deviation measurements. Although CO2 absorption was marginally affected by water at low H2O molar fractions for both ILs, an increase of the H2O content resulted in a marked enhancement of both the CO2 solubility (ca. 4-fold) and diffusivity (ca. 10-fold) in the binary [CnC1im][TCM]/H2O systems, in contrast to the weak and/or detrimental influence of water in most physically and chemically CO2-absorbing ILs. In-situ Raman spectroscopy on the IL/CO2 systems verified that CO2 is physically absorbed in the dry ILs with no significant effect on their structural organization. A pronounced variation of distinct tricyanomethanide Raman modes was disclosed in the [CnC1im][TCM]/H2O mixtures, attesting to the gradual disruption of the anion-cation coupling by the hydrogen-bonded water molecules to the [TCM]- anions, in accordance with the positive excess molar volumes and negative viscosity deviations for the binary systems. Most importantly, CO2 absorption in the ILs/H2O mixtures at high water concentrations revealed that the [TCM]- Raman modes tend to restore their original state for the heavily hydrated ILs, in qualitative agreement with the intriguing nonmonotonous transients of CO 2 absorption kinetics unveiled by the gravimetric measurements for the hybrid solvents. A molecular exchange mechanism between CO2 in the gas phase and H2O in the liquid phase was thereby proposed to explain the enhanced CO2 absorption in the hybrid [C nC1im][TCM]//H2O solvents based on the subtle competition between the TCM-H2O and TCM-CO2 interactions, which renders these ILs very promising for CO2 separation applications. © 2013 American Chemical Society.
Romanos GE, Athanasekou CP, Likodimos V, Aloupogiannis P, Falaras P. Hybrid ultrafiltration/photocatalytic membranes for efficient water treatment. Industrial and Engineering Chemistry Research [Internet]. 2013;52:13938-13947. WebsiteAbstract
The development of hybrid materials exhibiting the simultaneous action of photocatalysis and membrane filtration can lead to improved water treatment processes. Photocatalysis has the potential to solve problems related to the fouling of membranes, the generation of toxic condensates, and the existence of very small and harmful organic pollutants in the permeate effluent. On the other hand membranes, especially the ceramic ones, are appropriate supports for the deposition of thin photocatalytic layers due to their high affinity with the photocatalyst (e.g., TiO2) and the possibility to further stabilize and activate the deposit with calcination. In addition, membranes exhibit two surfaces that come into contact with the polluted water and can be exploited for the photocatalyst deposition. Thus, with appropriate design of the membrane module it is possible to illuminate both membrane surfaces and develop very efficient photocatalytic ultrafiltration processes. Such processes must involve "double sided active photocatalytic membranes", where the pollutant undergoes two sequential photodegradation steps, the first in contact with the feed surface and the second in contact with the permeate surface of the membrane. Moreover the asymmetric pore structure of ceramic membranes assures proper mixing of the fluid and better contact with the porous photocatalytic layers. In this work double side active photocatalytic ultrafiltration (UF) membranes were developed by means of different chemical vapor deposition (CVD) techniques. Their performance in the elimination of methyl orange from water was elucidated by means of a prototype photocatalytic membrane reactor under continuous flow, applying UV irradiation on both membrane surfaces. Important aspects of membrane technology such as the evolution of water permeability and the energy consumption were compared with the standard and highly efficient nanofiltration (NF) process and the results indicated the beneficial effects of the hybrid UF/photocatalytic process. © 2013 American Chemical Society.
Vaenas N, Stergiopoulos T, Kontos AG, Likodimos V, Falaras P. Influence of controlled-charge anodization processes on the morphology of TiO2 nanotubes and their efficiency in dye-sensitized solar cells. Electrochimica Acta [Internet]. 2013;113:490-496. WebsiteAbstract
The effect of the electrochemical anodization growth process on the development of self-organized TiO2 nanotube (NT) films and their efficiency as photoelectrodes in dye sensitized solar cells (DSCs) has been comparatively investigated, by keeping constant the total anodization charge. Slow and rapid potentiostatic anodization processes were accordingly compared to the galvanostatic one, while a two step potentiostatic-galvanostatic technique was applied for the first time for the growth of TiO2 NT arrays, as a step forward in relation to the existing potentiostatic-potentiostatic (P-P) technique. Scanning electron microscopy and Raman spectroscopy verified the wide diversity in the morphological and structural characteristics of the TiO 2 NTs obtained by the different anodization modes. The novel approach of galvanostatic tube growth on a potentiostatically patterned Ti foil provided the most uniform TiO2 nanotubular films with clean top surface exempt of nanograss or cracks over extended areas. Evaluation of the TiO 2 NTs performance as photoelectrodes in DSC devices showed distinct differences of their electrical parameters that reflected finely the underlying structure/morphology variations of the different anodic oxidation conditions. Galvanostatic TiO2 NT films presented the most favorable (open-ordered) structure for DSC photoelectrodes with superior electrical performance, essentially impaired by a relatively low fill factor that requires improvement by appropriate post-treatment. Furthermore, despite the marked differences in morphology, the TiO2 NT photoelectrodes exhibited comparable overall performance (of the order of 4%), with only exception the P-P samples which presented slightly lower (about 25%) photovoltaic efficiency. These results indicate that the anodization charge is a critical factor that effectively controls the nanotubes behavior when they are used as photoelectrodes in DSCs. © 2013 Elsevier Ltd.
Bidikoudi M, Stergiopoulos T, Likodimos V, Romanos GE, Francisco M, Iliev B, Adamová G, Schubert TJS, Falaras P. Ionic liquid redox electrolytes based on binary mixtures of 1-alkyl-methylimidazolium tricyanomethanide with 1-methyl-3-propylimidazolium iodide and implication in dye-sensitized solar cells. Journal of Materials Chemistry A [Internet]. 2013;1:10474-10486. WebsiteAbstract
Innovative redox electrolytes for dye-sensitized solar cells (DSCs) were prepared using binary mixtures of 1-methyl-3-propylimidazolium iodide (MPII) with 1-alkyl-methylimidazolium tricyanomethanide, CνmimTCM (ν = 2, 4, 6, 8) ionic liquids (ILs) to lower the high viscosity of MPII. The investigation of the physicochemical properties of the IL blends as a function of temperature has shown that both density and viscosity strongly depend on the kind of the Cνmim cation in the mixture. The corresponding Raman spectra were dominated by the vibrational modes of the IL components in an additive way and confirmed the absence of any specific interaction, independent of the Cν alkyl chain length. The electrochemical properties (triiodide diffusion coefficients, specific conductivity), determined in symmetrical thin layer cells using polarization and electrochemical impedance spectroscopy (EIS) measurements, have shown that both diffusion and conductivity decreased with increasing viscosity, and further confirmed the electrolytes' compatibility with the cathode. Incorporation of the novel electrolytes in DSC devices revealed a systematic dependence of the cell photovoltaic performance on the alkyl chain length of CνmimTCM; the maximum power conversion efficiency exceeded 5 and 6.5% under 1 and 0.1 sun AM 1.5 G illumination, respectively, for the ionic liquid with the shortest alkyl chain. The solar cells were further characterized by EIS (IMPS) spectroscopy, exploring charge recombination dynamics and identifying conduction band edge shifts. Solidification of the electrolytes with silica nanoparticles, demonstrated that the ionic liquid electrolytes with long chain length (ν > 4) not only retain their efficiencies, but also exhibit a 22% efficiency enhancement, which is most pronounced for the electrolytes employing ionic liquids with the longest (hexyl- and octyl-) alkyl chains. © 2013 The Royal Society of Chemistry.
Fotiou T, Triantis TM, Kaloudis T, Pastrana-Martínez LM, Likodimos V, Falaras P, Silva AMT, Hiskia A. Photocatalytic degradation of microcystin-LR and off-odor compounds in water under UV-A and solar light with a nanostructured photocatalyst based on reduced graphene oxide-TiO2 composite. Identification of intermediate products. Industrial and Engineering Chemistry Research [Internet]. 2013;52:13991-14000. WebsiteAbstract
Microcystin-LR (MC-LR) is the most common and toxic variant of the group of microcystins (MCs) produced during the formation of harmful cyanobacterial blooms. Geosmin (GSM) and 2-methylisoborneol (MIB) may also be produced during cyanobacterial blooms and can taint water causing undesirable taste and odor. The photocatalytic degradation of MC-LR, GSM, and MIB in water under both UV-A and solar light in the presence of reduced graphene oxide-TiO2 composite (GO-TiO2) was studied. Two commercially available TiO 2 materials (Degussa P25 and Kronos) and a reference TiO2 material prepared in the laboratory (ref-TiO2) were used for comparison. Under UV-A irradiation, Degussa P25 was the most efficient photocatalyst for the degradation of all target analytes followed by GO-TiO 2, ref-TiO2, and Kronos. Under solar light irradiation GO-TiO2 presented similar photocatalytic activity to Degussa P25, followed by Kronos and ref-TiO2 which were less efficient. Intermediate products formed during the photocatalytic process with GO-TiO 2 under solar light were identified and were found to be almost identical to those observed by Degussa P25/UV-A. Assessment of the residual toxicity of MC-LR during the course of treatment with GO-TiO2 showed that toxicity is proportional only to the remaining MC-LR concentration. The photocatalytic performance of GO-TiO2 was also evaluated under solar light illumination in real surface water samples, and GO-TiO2 proved to be effective in the degradation of all target compounds. © 2013 American Chemical Society.
Katsanaki AV, Kontos AG, Maggos T, Pelaez M, Likodimos V, Pavlatou EA, Dionysiou DD, Falaras P. Photocatalytic oxidation of nitrogen oxides on N-F-doped titania thin films. Applied Catalysis B: Environmental [Internet]. 2013;140-141:619-625. WebsiteAbstract
Visible light activated nanostructured TiO2 with nitrogen and fluorine co-dopants were prepared by the surfactant assisted sol-gel method and immobilized on glass substrates by dip coating. The films were inserted inside a continuous flow photoreactor and examined for the photocatalytic oxidation of NO air pollutant with initial concentration of 200-800ppbv. The modified catalysts exhibited significant photocatalytic activity under daylight illumination, with maximum percentage of NO removal equal to 24.2% and photooxidation rate up to 0.66μgm-2s-1. The reaction rates increased proportionally to the incident light intensity whereas for the strongly absorbed UV light a deviation from linearity was observed. Mass balance during photooxidation was confirmed by determining the amount of NO3- product residues onto the photocatalyst surface. © 2013 Elsevier B.V.
Nikoleli G-P, Ibupoto ZH, Nikolelis DP, Likodimos V, Psaroudakis N, Tzamtzis N, Willander M, Hianik T. Potentiometric cholesterol biosensing application of graphene electrode with stabilized polymeric lipid membrane. Central European Journal of Chemistry [Internet]. 2013;11:1554-1561. WebsiteAbstract
A novel potentiometric cholesterol biosensor has been fabricated through the immobilization of the stabilized polymeric lipid membrane onto graphene electrode. The stabilized polymeric lipid membrane is composed of cholesterol oxidase enzyme and polymerization mixture; which holds paramount influence on the properties of the cholesterol biosensor. The presented biosensor reveals an appreciable reproducibility, good selectivity and high sensing capability with a linear slope curve of ∼64 mV per decade. The strong biocompatibility among stabilized polymeric lipid membranes and human biofluids provides the possibility to use for real blood samples and other biological applications. © 2013 Versita Warsaw and Springer-Verlag Wien.
Kontos AG, Katsanaki A, Likodimos V, Maggos T, Kim D, Vasilakos C, Dionysiou DD, Schmuki P, Falaras P. Continuous flow photocatalytic oxidation of nitrogen oxides over anodized nanotubular titania films. Chemical Engineering Journal [Internet]. 2012;179:151-157. WebsiteAbstract
Self ordered nanotubular TiO 2 films of variable morphological characteristics were grown by electrochemical anodization in ethylene glycol based electrolytes. The films were incorporated in a continuous flow reactor and were evaluated for the UV photocatalytic oxidation of nitrogen oxide (NO) gas by varying the flow rate and the pollutant concentration in the 200-1000ppbv range. After a strong initial response, the photooxidation rate decreased and reached an equilibrium state. Strong dependence of the overall NO concentration on the morphological film parameters was observed which is related to the interplay of porosity and extended surface area. Saturation of the photocatalytic performance was evidenced for nanotubular films with thickness above 14μm, pointing out the limitations imposed by the UV light penetration depth and the diffusion of the gas pollutants and intermediates into the nanotube channels. Nitrates-nitrites were identified as the principle NO x photooxidation products. © 2011 Elsevier B.V.
Romanos GE, Athanasekou CP, Katsaros FK, Kanellopoulos NK, Dionysiou DD, Likodimos V, Falaras P. Double-side active TiO 2-modified nanofiltration membranes in continuous flow photocatalytic reactors for effective water purification. Journal of Hazardous Materials [Internet]. 2012;211-212:304-316. WebsiteAbstract
A chemical vapour deposition (CVD) based innovative approach was applied with the purpose to develop composite TiO 2 photocatalytic nanofiltration (NF) membranes. The method involved pyrolytic decomposition of titanium tetraisopropoxide (TTIP) vapor and formation of TiO 2 nanoparticles through homogeneous gas phase reactions and aggregation of the produced intermediate species. The grown nanoparticles diffused and deposited on the surface of γ-alumina NF membrane tubes. The CVD reactor allowed for online monitoring of the carrier gas permeability during the treatment, providing a first insight on the pore efficiency and thickness of the formed photocatalytic layers. In addition, the thin TiO 2 deposits were developed on both membrane sides without sacrificing the high yield rates. Important innovation was also introduced in what concerns the photocatalytic performance evaluation. The membrane efficiency to photo degrade typical water pollutants, was evaluated in a continuous flow water purification device, applying UV irradiation on both membrane sides. The developed composite NF membranes were highly efficient in the decomposition of methyl orange exhibiting low adsorption-fouling tendency and high water permeability. © 2011 Elsevier B.V..
Vermisoglou E, Todorova N, Pilatos G, Romanos G, Likodimos V, Boukos N, Lei C, Markoulidis F, Lekakou C, Trapalis C. Few layer graphenes decorated with silver nanoparticles. In: ECCM 2012 - Composites at Venice, Proceedings of the 15th European Conference on Composite Materials. ; 2012. WebsiteAbstract
Graphite oxide (GO) powder was irradiated in a microwave oven and lightweight expanded graphite oxide (EGO) powder with high BET surface area 1316 m2/g was obtained. Activation of EGO was performed by impregnation in KOH solution and high temperature treatment under Ar flow, followed by annealing in vacuum (t-EGO). KOH acted more as a reducing agent diminishing the defects than as a surface modifier for high porosity. EGO and t-EGO were further decorated with Ag nanoparticles (∼40 nm) applying solar light irradiation. Along with Ag deposition the structural defects of the graphene were reduced upon photo-irradiation. It was established that among the bare graphenes the EGO exhibited the highest capacitance. From the Ag-containing composites, the KOH activated EGO acted as a supercapacitor, while the non-activated EGO as a resistant.
Stergiopoulos T, Likodimos V, Hahn R, Schmuki P, Falaras P. Front-side illuminated dye-sensitized solar cells based on bundle shaped titania nanotube membranes. Physica Status Solidi (A) Applications and Materials Science [Internet]. 2012;209:193-198. WebsiteAbstract
TiO 2 nanotubular (NT) membranes with bundle shape morphology were prepared by rapid breakdown anodization (RBA) in fluorine free aqueous electrolytes. The self-assembled nanotube-like materials were detached from the metal foil by immersion in methanolic Br 2 solution and the free-standing membranes were successfully transferred on top of transparent conductive electrodes modified by freshly prepared nanoparticulate (NP) titania films. The composite electrodes, after post-annealing at 450°C, were crystallized under the anatase polymorphism and, then, effectively sensitized by the standard N719 dye (confirmed by micro-Raman spectroscopy under Resonance conditions). The resulting photoelectrodes were incorporated in dye-sensitized solar cells (DSCs) enabling front-side illumination (FSI). The fabricated DSCs attained overall photovoltaic conversion efficiencies of the order of 4.6% under 1 sun (AM 1.5) illumination. The relatively high performance was mainly attributed to the successful packing of the two (NT and NP) layers creating a low resistance interface as well as to the favourable electron transport/recombination dynamics that govern these solar cells. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Vaenas N, Stergiopoulos T, Kontos AG, Likodimos V, Boukos N, Falaras P. Sensitizer activated solar cells based on self-organized TiO2 nanotubes. Microelectronic Engineering [Internet]. 2012;90:62-65. WebsiteAbstract
Self-ordered 1D nanostructures currently attract increasing interest due to their controlled geometry associated with unique structural and morphological characteristics. Vertically oriented TiO2 nanotubes (NTs) grown by electrochemical anodization provide a robust substrate for efficient solar-energy conversion. In this work, we report on the preparation of NTs under low applied voltage (+30 V vs. Pt) in NH4F/ethylene glycol electrolytes containing a slight amount of water. The effects of the Ti foils polishing (before anodization) and the removal of the nanograss (structural disorder created at the top surface of the NTs due to the prolonged exposure of the tubes inside the F- rich environment) on the nanotubular films properties are investigated. After optimization of the anodization conditions, homogeneous anodic films consisting of smooth and long NTs are prepared, which, when incorporated as photoelectrodes in dye-sensitized solar cells, present an overall power conversion efficiencies of about 3% under backside illumination conditions. © 2011 Elsevier B.V. All rights reserved.
Philippopoulos A, Falaras P, Chatzivasiloglou E, Igglessi-Markopoulou O, Likodimos V, Konti G-C. Synthesis and spectroscopic characterization of new heteroleptic ruthenium(II) complexes incorporating 2-(2′-pyridyl) quinoxaline and 4-carboxy-2-(2′-pyridyl)quinoline. Journal of Coordination Chemistry [Internet]. 2012;65:2535-2548. WebsiteAbstract
Starting from cis-[Ru(dcbpyH 2) 2Cl 2] (1), two new heteroleptic ruthenium(II) complexes, [Ru(dcbpyH 2) 2(L 1)](NO 3) 2 (L 1=2- (2′-pyridyl)quinoxaline (2), and [Ru(dcbpyH 2) 2(L 2)](NO 3) 2 (L 2=4- carboxy-2-(2′-pyridyl)quinoline (4); dcbpyH 2=2,2′- bipyridine-4,4′-dicarboxylic acid), were synthesized and spectroscopically characterized. During the preparation of 2 and 4, the homoleptic [Ru(dcbpyH 2) 3]Cl 2 complex (3) was isolated as a side product. Characterization includes IR and Raman spectroscopy, UV-Vis, multinuclear NMR spectroscopy, elemental, and ESI-mass spectrometric analyses. © 2012 Taylor and Francis.
Liu G, Han C, Pelaez M, Zhu D, Liao S, Likodimos V, Ioannidis N, Kontos AG, Falaras P, Dunlop PSM, et al. Synthesis, characterization and photocatalytic evaluation of visible light activated C-doped TiO 2 nanoparticles. Nanotechnology [Internet]. 2012;23. WebsiteAbstract
We have demonstrated heterogeneous photocatalytic degradation of microcystin-LR (MC-LR) by visible light activated carbon doped TiO 2 (C-TiO 2) nanoparticles, synthesized by a modified sol-gel route based on the self-assembly technique exploiting oleic acid as a pore directing agent and carbon source. The C-TiO 2 nanoparticles crystallize in anatase phase despite the low calcination temperature of 350°C and exhibit a highly porous structure that can be optimized by tuning the concentration of the oleic acid surfactant. The carbon modified nanomaterials exhibited enhanced absorption in the broad visible light region together with an apparent red shift in the optical absorption edge by 0.5eV (2.69eV), compared to the 3.18eV of reference anatase TiO 2. Carbon species were identified by x-ray photoelectron spectroscopy analysis through the formation of both Ti-C and C-O bonds, indicative of substitution of carbon for oxygen atoms and the formation of carbonates, respectively. Electron paramagnetic resonance spectroscopy revealed the formation of two carbon related paramagnetic centers in C-TiO 2, whose intensity was markedly enhanced under visible light illumination, pointing to the formation of localized states within the anatase band gap, following carbon doping. The photocatalytic activity of C-TiO 2 nanomaterials was evaluated for the degradation of MC-LR at pH 3.0 under visible light (λ>420nm) irradiation. The doped materials showed a higher MC-LR degradation rate than reference TiO 2, behavior that is attributed to the incorporation of carbon into the titania lattice. © 2012 IOP Publishing Ltd.
Athanasekou CP, Romanos GE, Katsaros FK, Kordatos K, Likodimos V, Falaras P. Very efficient composite titania membranes in hybrid ultrafiltration/photocatalysis water treatment processes. Journal of Membrane Science [Internet]. 2012;392-393:192-203. WebsiteAbstract
Composite TiO 2 photocatalytic ultrafiltration (UF) membranes were developed through chemical vapour layer-by-layer deposition (LBL/CVD) of TiO 2. The technique comprised chemisorption or physisorption of the titanium isopropoxide (TTIP) vapour and a subsequent oxidative treatment in order to promote the precursor condensation and generate new adsorption sites for the accomplishment of the successive adsorption/surface reaction steps. Both membrane sides were covered with TiO 2 photocatalyst without affecting the high water recovery efficiency. For reasons of comparison, one of the membranes was prepared through TiO 2 nanoparticle growth (NPG/CVD), a procedure extensively studied in a previous work of our group. The membrane efficiency in photo degradation of methyl orange was evaluated in an innovative continuous flow reactor, applying UV irradiation on the annular and bore surfaces. The membranes developed through the physisorption path were highly efficient in the decomposition of azo-dye pollutant, exhibiting low adsorption-fouling tendency and high water permeability. © 2011 Elsevier B.V.
Pastrana-Martínez LM, Morales-Torres S, Likodimos V, Figueiredo JL, Faria JL, Falaras P, Silva AMT. Advanced nanostructured photocatalysts based on reduced graphene oxide-TiO2 composites for degradation of diphenhydramine pharmaceutical and methyl orange dye. Applied Catalysis B: Environmental [Internet]. 2012;123-124:241-256. WebsiteAbstract
Reduced graphene oxide-TiO2 composites (GOT) were prepared by liquid phase deposition followed by post-thermal reduction at different temperatures. The composite materials were systematically evaluated as photocatalysts for the degradation of an important pharmaceutical water pollutant, diphenhydramine (DP), and an azo-dye, methyl orange (MO), under both near-UV/Vis and visible light irradiation as a function of the graphene oxide (GO) content. A marked compositional dependence of the photocatalytic activity was evidenced for DP and MO pollutants degradation and mineralization under both UV/Vis and visible light. Especially under visible light, optimum photocatalytic performance was obtained for the composites treated at 200°C comprising 3.3-4.0wt.% GO, exceeding that of the benchmark P25 (Evonik) catalyst. According to scanning electron microscopy, Raman spectroscopy, and porosimetry analysis data, this was attributed to the optimal assembly and interfacial coupling between the reduced GO sheets and TiO2 nanoparticles. Almost total degradation and significant mineralization of DP and MO pollutants (in less than 60min) was achieved under near-UV/Vis irradiation for the optimum GOT composites. However, higher GO content and calcination temperatures (350°C) led to detrimental effects due to the GO excess and the disruption of the GO-TiO2 binding. Photocatalytic experiments employing sacrificial hole and radical scavenging agents revealed that photogenerated holes are the primary active species in DP degradation for both bare TiO2 and GOT under UV/Vis irradiation, while an enhanced contribution of radical mediated DP oxidation was evidenced under visible light. These results combined with the distinct quenching of the GO photoluminescence under visible and NIR laser excitation, indicate that reduced GO acts either as electron acceptor or electron donor (sensitizer) of TiO2 under UV and visible light, respectively. Fine-tuning of the reduced GO-TiO2 interface is concluded as a very promising route to alleviate electron-hole recombination and circumvent the inherently poor light harvesting ability of TiO2 in the visible range. © 2012 Elsevier B.V.
Stergiopoulos T, Bidikoudi M, Likodimos V, Falaras P. Dye-sensitized solar cells incorporating novel Co(ii/iii) based-redox electrolytes solidified by silica nanoparticles. Journal of Materials Chemistry [Internet]. 2012;22:24430-24438. WebsiteAbstract
An innovative solidified redox electrolyte for dye-sensitized solar cells (DSCs) was prepared by incorporating amorphous silica nanoparticles in a liquid system containing the Co2+/Co3+ shuttle dissolved in methoxypropionitrile; the novel material was characterized by Raman spectroscopy and its optical properties were examined by UV-Vis spectroscopy. Electrochemical properties (diffusion, conductivity, electrolyte compatibility with the cathode) were studied in symmetrical thin layer cells using polarization and electrochemical impedance spectroscopy (EIS) measurements. Quasi-solid DSCs were fabricated by incorporating this electrolyte and attained an efficiency of about 2.6% under 1 sun (under 1000 W m-2) AM1.5G illumination, corresponding to 73% of that gained by the reference liquid electrolyte; the cell efficiency depends on the light illumination conditions, significantly increased up to more than 4% under 0.23 sun. The solar cells were further characterized by EIS and intensity modulated photocurrent spectroscopy (IMPS) to investigate charge transport and recombination dynamics, possible conduction band edge shifts, alterations of the interfacial and ohmic resistances, and variations of the diffusion rate through the photoelectrode. It was confirmed that the solidified electrolyte-based DSC presented a slightly enhanced photopotential in comparison with that of the liquid electrolyte due to a negative TiO2 conduction band edge shift upon contact with the electrolyte, accompanied with a decreased photocurrent mainly stemming from the restricted diffusion of the Co3+ species through the pores of the TiO2 photoelectrode. © The Royal Society of Chemistry 2012.
Romanos GE, Likodimos V, Marques RRN, Steriotis TA, Papageorgiou SK, Faria JL, Figueiredo JL, Silva AMT, Falaras P. Controlling and quantifying oxygen functionalities on hydrothermally and thermally treated single-wall carbon nanotubes. Journal of Physical Chemistry C [Internet]. 2011;115:8534-8546. WebsiteAbstract
The effects of hydrothermal and thermal treatments on surface oxygen functionalization of single-wall carbon nanotubes (SWNTs) were quantitatively investigated by means of water adsorption/desorption, temperature-programmed desorption (TPD), Raman spectroscopy, thermogravimetric analysis, and nitrogen porosimetry. SWNTs hydrothermally treated under mild acidic conditions were compared to highly purified reference materials heavily functionalized under aggressive reflux conditions. Water adsorption/desorption and TPD analysis were successfully combined to determine the nature, concentration, thermal stability, and acidic strength of the oxygen functional groups on the SWNT's surface. These results were correlated to Raman spectroscopy data that allowed identifying the marked evolution of the defect-activated phonon modes of SWNTs. The concomitant charge transfer effects were differentiated through the distinct variation of both first- and second-order Raman modes as a function of the amount and acidity of the surface oxygen groups as well as the SWNT's chirality. In addition, analytical investigations on thermally treated SWNTs in mild oxidative (in air) and pyrolytic conditions (under Ar) confirmed the formation of amorphous carbon that depends primarily on the acidification process, although a significant fraction of functional groups remains attached on the SWNTs' walls rather than on carboxylated carbonaceous fragments. Quenched solid density functional theory (QSDFT) analysis of the bimodal pore size distribution of the functionalized SWNTs revealed pronounced variations of the underlying microporous and mesoporous structure, associated with the diverse effects of the packing between SWNT bundles and the closer aggregation of individual carbon nanotubes upon surface oxidation and thermal treatment. The SWNTs functionalization procedure can be effectively controlled and quantified, and the optimum conditions can be defined in relation to the desired physicochemical properties and pore structure characteristics for specific applications. © 2011 American Chemical Society.
Kontos AG, Likodimos V, Vassalou E, Kapogianni I, Raptis YS, Raptis C, Falaras P. Nanostructured titania films sensitized by quantum dot chalcogenides. Nanoscale Research Letters [Internet]. 2011;6. WebsiteAbstract
The optical and structural properties of cadmium and lead sulfide nanocrystals deposited on mesoporous TiO2 substrates via the successive ionic layer adsorption and reaction method were comparatively investigated by reflectance, transmittance, micro-Raman and photoluminescence measurements. Enhanced interfacial electron transfer is evidenced upon direct growth of both CdS and PbS on TiO2 through the marked quenching of their excitonic emission. The optical absorbance of CdS/TiO2 can be tuned over a narrow spectral range. On the other side PbS/TiO2 exhibits a remarkable band gap tunability extending from the visible to the near infrared range, due to the distinct quantum size effects of PbS quantum dots. However, PbS/TiO2 suffers from severe degradation upon air exposure. Degradation effects are much less pronounced for CdS/TiO2 that is appreciably more stable, though it degrades readily upon visible light illumination. © 2011 Kontos et al.
Stergiopoulos T, Kontos AG, Likodimos V, Perganti D, Falaras P. Solvent effects at the photoelectrode/electrolyte interface of a DSC: A combined spectroscopic and photoelectrochemical study. Journal of Physical Chemistry C [Internet]. 2011;115:10236-10244. WebsiteAbstract
The behavior of organic solvents typically used in redox electrolyte media for dye-sensitized solar cells (DSCs) has been systematically investigated by resonance Raman spectroscopy in combination with electrochemical impedance (EIS), intensity modulated photovoltage (IMVS), and photocurrent (IMPS) spectroscopies. Resonance Raman spectra reveal appreciable shifts in the vibrational frequency of the dye carboxyl anchoring groups as well as intensity variations of high- and low-frequency modes of dye-redox species by varying the electrolyte solvent and the polarization bias. These results are related to the variable surface coverage of the dye-TiO2 photoelectrode by solvent molecules determined by their donor number, the concomitant change on the concentration of dye-redox couple intermediate adducts, and the dye-TiO 2 electronic coupling. EIS and IMVS reveal a marked variation of the recombination kinetics and chemical capacitance of the corresponding DSCs, providing direct evidence for the shift of the TiO2 conduction band edge and deceleration of back-reaction kinetics. This accounts well for the solvent dependence of the device opencircuit voltage, whereas more complicated effects underlie the variation of the short-circuit current density that depends on electron injection. Tuning the solvents' function in DSCs may thus result in significant improvement of the DSC photovoltaic efficiency by effectively controlling electron energetics and injected photoelectron-triiodide recombination. © 2011 American Chemical Society.
Kontos AG, Pelaez M, Likodimos V, Vaenas N, Dionysiou DD, Falaras P. Visible light induced wetting of nanostructured N-F co-doped titania films. Photochemical and Photobiological Sciences [Internet]. 2011;10:350-354. WebsiteAbstract
Nitrogen and fluorine co-doped TiO2 films have been prepared by dip coating of a modified titania sol-gel based on a nitrogen precursor and a nonionic fluorosurfactant as pore template and fluorine source. The modified NF-TiO2 films absorb in the visible spectral range, between 400-510 nm and undergo reversible hydrophilic conversion under visible light to a final contact angle of 8°, in contrast to the UV limited optical response of their undoped anatase TiO2 analogues. The phenomenon takes place at a rate slower than the corresponding one observed for the UV stimulated superhydrophilic effect. The wetting response of the N-F doped TiO2 films correlates well with the variation of their optical properties and surface morphological characteristics and most importantly with their photocatalytic activity, rendering these materials very promising for self-cleaning applications under visible light. © 2011 The Royal Society of Chemistry and Owner Societies.
Han C, Pelaez M, Likodimos V, Kontos AG, Falaras P, O'Shea K, Dionysiou DD. Innovative visible light-activated sulfur doped TiO2 films for water treatment. Applied Catalysis B: Environmental [Internet]. 2011;107:77-87. WebsiteAbstract
Visible light-activated sulfur doped TiO2 nanocrystalline films were synthesized by a sol-gel method based on the self-assembly technique with nonionic surfactant to control nanostructure and an inorganic sulfur source (i.e., H2SO4). The films were characterized by UV-vis diffuse reflectance, XRD, TEM, Raman, AFM, ESEM, XPS, FT-IR, EDX, EPR and porosimetry. The results showed that the physicochemical properties of the films, such as BET surface area, porosity, crystallite size and pore size distribution could be controlled by the calcination temperature. The highest surface area, smallest crystallite size and narrow pore size distribution were obtained for sulfur doped TiO2 films calcined at 350°C, which exhibit very smooth surface with minimal roughness (<1nm). The optical absorption edge of sulfur doped TiO2 was red shifted with indirect bandgap energy of 2.94eV. Sulfur species distributed uniformly throughout the films were identified both as S2- ions related to anionic substitutional doping of TiO2 as well as S6+/S4+ cations, attributed mainly to the presence of surface sulfate groups. EPR measurements revealed a sharp signal at g=2.004, whose intensity correlated with the sulfur content and most importantly was markedly enhanced under visible light irradiation, implying the formation of localized energy states in the TiO2 band gap due to anion doping and/or oxygen vacancies. In terms of photocatalytic activity, films calcined at 350°C were the most effective for the degradation of hepatotoxin microcystin-LR (MC-LR) under visible light irradiation, while films calcined at 400°C and 500°C degraded MC-LR to a lower extent, following the evolution of the sulfur content with calcination temperature. The photocatalytic activity of the sulfur doped TiO2 film was stable during three consecutive experiments under visible light irradiation, confirming the mechanical stability and reusability of the doped nanostructured thin film photocatalysts. © 2011 Elsevier B.V.
Chen Y, Lunsford SK, Song Y, Ju H, Falaras P, Likodimos V, Kontos AG, Dionysiou DD. Synthesis, characterization and electrochemical properties of mesoporous zirconia nanomaterials prepared by self-assembling sol-gel method with Tween 20 as a template. Chemical Engineering Journal [Internet]. 2011;170:518-524. WebsiteAbstract
Zirconium dioxide nanomaterials, including nanoparticles and their derived films, were synthesized using a new surfactant self-assembling sol-gel route, which involves complexation of zirconium alkoxide precursor with acetylacetone as a chelating agent and Tween 20 as a structural directing agent. The properties of the synthesized material were studied using XRD, SEM, SAXS, HRTEM and N2 adsorption and desorption analysis. It was found that Tween 20 can greatly increase the BET surface area and pore volume, inhibit crack formation in the derived film and retard particle growth at some extent. There exists tetragonal crystalline network with narrow pore size distribution in the ZrO2 nano-materials prepared with Tween 20 in the sol. Furthermore, a mechanism on the formation of mesopore ZrO2 structure via the Tween 20 self-assembling sol-gel process is presented. With the selection of the sono-gel carbon electrode modified by the mesoporous ZrO2 film (M-ZrO2 film) prepared with 1.10g Tween 20/ml zirconia sol, enhanced electrochemical responses (i.e., enhanced peak currents) to catechol and ascorbic acid were confirmed using cyclic voltammetry (CV). Moreover, the modified electrode demonstrated additional improved electrochemical properties, such as good reversibility (ipc/ipa≈1) and high selectivity (exceptionally resolved oxidation peaks between catechol and ascorbic acid), good stability and repeatability. © 2010 Elsevier B.V.
Mora-Seró I, Likodimos V, Giménez S, Martínez-Ferrero E, Albero J, Palomares E, Kontos AG, Falaras P, Bisquert J. Fast regeneration of CdSe quantum dots by ru dye in sensitized TiO 2 electrodes. Journal of Physical Chemistry C [Internet]. 2010;114:6755-6761. WebsiteAbstract
Interacting properties of colloidal CdSe quantum dots (QDs) and polypyridyl ruthenium dyes employed as cosensitizers of mesoporous TiO2 electrodes as well as the effect of QDs coating and anchoring mode (direct and linker adsorption) have been investigated by photoluminesce (PL), Raman, and transient absorption (TAS) spectroscopies. Direct adsorption of QDs on TiO 2 leads to a more efficient PL quenching compared to that of QDs attached by means of a molecular linker (cysteine). This fact suggests higher electron injection for the former anchoring mode. Coating of ZnS on CdSe QDs sensitized TiO2 electrodes passivates the QDs surface states and partially releases quantum confinement effects, as is observed in colloidal core-shell nanoparticles. Subsequent cosensitization with a ruthenium molecular dye dramatically quenches the PL of the QDs/TiO2 electrodes, even in the presence of ZnS coating, indicating the presence of strong photoinduced charge transfer between the CdSe QDs and the dye molecules. This is firmly supported by TAS spectroscopy on the interfacial recombination kinetics that points to the fast hole transfer from the photoexcited QDs to the dye. The regenerating action of molecular dyes for QD sensitizers can have important implications in the development of efficient photovoltaic devices based on the synergistic action of dye-QD-TiO2 heterostructures. © 2010 American Chemical Society.
Kontos AG, Katsanaki A, Maggos T, Likodimos V, Ghicov A, Kim D, Kunze J, Vasilakos C, Schmuki P, Falaras P. Photocatalytic degradation of gas pollutants on self-assembled titania nanotubes. Chemical Physics Letters [Internet]. 2010;490:58-62. WebsiteAbstract
Self-assembled titanium oxide nanotubular (NTs) arrays with vertical arrangement, tailored morphological properties and variable thickness were prepared by electrochemical anodization in ethylene glycol containing electrolytes. The NTs present significant UV photocatalysis against toluene and benzene at ppb concentrations, under normal conditions of temperature and pressure. The photocatalytic activity depends on the length of the NT arrays, showing a tendency of saturation for thickness above 2.5 μm and maximum efficiency for samples with thickness of 12 μm. The optimum NT structures outperform over standard Degussa P25 films rendering them very promising for outdoor photocatalytic applications. © 2010 Elsevier B.V. All rights reserved.
Karatasios I, Katsiotis MS, Likodimos V, Kontos AI, Papavassiliou G, Falaras P, Kilikoglou V. Photo-induced carbonation of lime-TiO2 mortars. Applied Catalysis B: Environmental [Internet]. 2010;95:78-86. WebsiteAbstract
In this work, titanium dioxide (TiO2) has been used as an additive in lime binder (Ca(OH)2), in order to benefit from its photocatalytic properties and study both the photocatalytic properties of the produced mixtures and the effect of photocatalytically produced carbon dioxide on the carbonation process of lime. TiO2 was added in three different portions (3, 6 and 10% w/w) in lime mixtures and their physicochemical and photocatalytic properties were studied and compared to those of a reference, made exclusively of lime. The photocatalytic properties of the mixtures were studied through the photo-oxidation of an organic model pollutant solution (methyl orange) to a colorless form, as well as by studying the microstructure and carbonation depth in different groups of mixtures subjected to photocatalysis cycles in the laboratory and exposed for a 2 month period in the open air (urban environment). Laboratory results confirmed that lime-TiO2 composite mixtures exhibit photocatalytic properties in both UV radiation conditions and direct exposure to sunlight. Moreover, analytical results indicated an enhanced carbonation of lime-TiO2 composites. TiO2 can be added to lime successfully and can have applications in lime-based mortars used in the conservation of architectural heritage, enhancing their performance against soiling and amenity loss. © 2009 Elsevier B.V. All rights reserved.
Likodimos V, Dionysiou DD, Falaras P. CLEAN WATER: Water detoxification using innovative photocatalysts. Reviews in Environmental Science and Biotechnology [Internet]. 2010;9:87-94. WebsiteAbstract
Environmental pollution abatement and especially the growing demand for clean water pose one of the most severe challenges worldwide. Besides the scarcity of water resources, the presence of hazardous chemicals with serious adverse health effects, even at extremely low concentrations, impose serious considerations for the quality of drinking water. The rapid evolution of nanoscale science and technology has dramatically expanded the materials' application potential towards radically new or multifunctional properties rendering nanotechnology an indispensable component in shaping modern environmental science. The nanoscale-perspective maintaining the integrity of the environment is currently the stimulus for the development of innovative and cost-effective functional materials and sustainable processes for water treatment and purification. The CLEAN WATER (EU FP7 collaborative project) aims at the development of an innovative and efficient water detoxification technology exploiting solar energy and nano-engineered titania photocatalysts in combination with nanofiltration membranes. In this approach, nanostructured titania with high UV-visible response will be synthesized and stabilized on nanotubular membranes of controlled pore size and retention efficiency as well as on carbon nanotubes exploiting their high surface area to achieve photocatalytically active nanocomposite membranes. Comparative evaluation of the UV-visible and solar light efficiency of the modified titania photocatalysts for water detoxification will be intensively investigated on various target pollutants ranging from classical water contaminants such us phenols, pesticides and azo-dyes to the extremely hazardous cyanobacterial toxins and emerging endocrine disrupting compounds in order to evaluate/optimize the materials performance and validate their competence on water treatment. Particular efforts will be devoted to the analysis and quantification of degradation products as well as their toxicity. All these will be the crucial components for the fabrication of innovative continuous flow photocatalytic-disinfection-membrane reactors for the implementation of sustainable and cost effective water treatment technologies based on nano-engineered materials. © Springer Science+Business Media B.V. 2010.
Pilatos G, Vermisoglou EC, Romanos GE, Karanikolos GN, Boukos N, Likodimos V, Kanellopoulos NK. A closer look inside nanotubes: Pore structure evaluation of anodized alumina templated carbon nanotube membranes through adsorption and permeability studies. Advanced Functional Materials [Internet]. 2010;20:2500-2510. WebsiteAbstract
Although hollow nanostructures, such as nanotubes, represent a major portion of nanoscaled materials with a tremendously large application range, a detailed evaluation of their internal characteristics still remains elusive. Transmission electron microscopy is the most common analytical technique to examine the internal configuration of these structures, yet it can only provide evidence of a minimal portion of the overall material, thus, it cannot be accurately generalized. In the present paper, in addition to electron microscopy and other spot-size analysis methods (X-ray diffraction, Raman spectroscopy, etc.), a combination of techniques including adsorption, permeability, and relative permeability are employed in order to provide important insights into various crucial details of the overall internal surface and hollow-space characteristics of carbon nanotube (CNT) arrays and membranes. The CNT arrays are fabricated using anodized alumina as a template in a flow-through chemical vapor deposition (CVD) reactor. This is the first systematic approach for investigating the internal configuration of template-based CNT arrays in detail. Key findings are made fiar the customized optimization of the resulting nanotube membranes for a variety of applications, including separations, nanofluidics and nanoreactors, biological capturing and purification, and controlled drug delivery and release. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinhelm.
Pelaez M, Falaras P, Likodimos V, Kontos AG, de la Cruz AA, Dionysiou DD. Highly efficient nanocrystalline visible light-activated TiO2 films by modified sol-gel methods for sustainable "green" applications. In: ACS National Meeting Book of Abstracts. ; 2010. WebsiteAbstract
This study describes the application of a novel chemistry method based on a modified sol-gel technique for the fabrication of nanostructured non-metal doped TiO2 photocatalytic films that can be activated under both visible and UV light. A self-assembling surfactant was employed as a pore-directing agent and a nitrogen-containing compound as doping precursor. Different surfactant ratios were explored to tailor-design the desired structural properties of TiO2 (i.e., high surface area, small crystal size) and enhanced the photocatalytic activity under visible light illumination. The films were characterized by XRD, ESEM, TEM, AFM, EPR, micro-Raman, XPS, UV-vis spectroscopy and N2 porosimetry. The photocatalytic evaluation of the films was tested for the degradation of microcystin-LR and the effects of NOM, pH, alkalinity, and dissolved oxygen under visible light irradiation were investigated. The multifunctional behavior exhibited for the non-metal doped TiO2 films can lead to important photoinduced applications including environmental protection (water disinfection, self-cleaning surfaces) and sustainable solar energy conversion to electricity (dye-sensitized solar cells).
Glenis S, Likodimos V, Guskos N, Yarmis D, Zolnierkiewicz G, Szymczyk A, Lin CL. Magnetic properties of carbon nanotube poly(ether-ester) nanocomposites. Journal of Applied Physics [Internet]. 2010;108. WebsiteAbstract
The magnetic and electronic properties of composites consisting of oxidized multiwall carbon nantubes (MWNTs) dispersed in elastomeric poly(ether-ester) segmented block copolymer have been studied by means of electron spin resonance (ESR) and dc magnetization measurements. A marked reduction in the MWNT diamagnetic response is identified, indicative of substantial hole doping related to the oxygen functional groups on the oxidized carbon nanotube's surface. Both ESR and the static magnetization reveal considerable enhancement of the spin susceptibility due to an excessive increase in the density of paramagnetic defects, which are sensitive to the dynamical polymer relaxation and thus to the MWNT-polymer interfacial coupling. © 2010 American Institute of Physics.
Pelaez M, Falaras P, Likodimos V, Kontos AG, Curell K, Stathatos E, Dionysiou DD. Structural, morphological and optical properties of nanostructured NF-TiO2 films for the photocatalytic degradation of emerging micropollutants in water under visible and solar light. In: ACS National Meeting Book of Abstracts. ; 2010. WebsiteAbstract
In this study, immobilized nitrogen and fluorine co-doped TiO2 (NF-TiO2) was synthesized employing a fluorosurfactant-based sol-gel method to tailor-design the nanoparticulate, structural and photocatalytic properties of the catalyst. Besides the co-doping of nitrogen and fluorine observed for visible light photoresponse, the nanostructure of the dip-coated films was effectively controlled by modifying the molar ratio of the fluorosurfactant. The synthesized films were evaluated for the destruction of two emerging contaminants, hepatotoxin microcystin LR (MC-LR) and herbicice Amitrole. NF-TiO2 films showed high photocatalytic activity for the degradation of both MC-LR and Amitrole compared to control experiments under both visible and solar light irradiation. Moreover, NF-TiO2 nanostructured films also exhibited high mechanical stability and no irreversible changes were observed during photocatalysis after 3 cycles under visible light. These results are promising for further development of sustainable remediation technologies for the treatment of water contaminated with MC-LR and other persistent micropollutants, based on advanced oxidation processes driven by solar light as a renewable source of energy.
Pelaez M, Falaras P, Likodimos V, Kontos AG, de la Cruz AA, O'Shea K, Dionysiou DD. Synthesis, structural characterization and evaluation of sol-gel-based NF-TiO2 films with visible light-photoactivation for the removal of microcystin-LR. Applied Catalysis B: Environmental [Internet]. 2010;99:378-387. WebsiteAbstract
This study reports on the synthesis, characterization and environmental application of immobilized nitrogen and fluorine co-doped TiO2 (NF-TiO2) photocatalyst. A fluorosurfactant-based sol-gel approach was employed to enhance the physicochemical properties and photocatalytic activity of NF-TiO2 under visible and UV light for the degradation of the hepatotoxin microcystin-LR (MC-LR). The films were characterized by XRD, environmental scanning electron microscope (ESEM), TEM, AFM, EPR, micro-Raman, X-ray photoelectron spectroscope (XPS), UV-vis spectroscopy and porosimeter analysis. The results revealed that by modifying the molar ratio of the fluorosurfactant, we could effectively control the physicochemical properties and obtain films with high BET surface area and porosity, small crystallite size and narrow pore size distribution. UV-vis spectroscopy showed an increase in the absorption capacity of NF-TiO2 in the visible light range compared to reference films. The existence of interstitial nitrogen and substitutional fluorine in the titanium dioxide (TiO2) lattice was determined by XPS. Comparative EPR measurements between the co-doped and reference samples identified distinct N spin species in NF-TiO2, with a high sensitivity to visible light irradiation. The abundance of these paramagnetic centers verifies the formation of localized intra-gap states in TiO2 and implies synergistic effects between fluorine and nitrogen dopants. Micro-Raman spectroscopy showed the growth of small amounts of brookite concomitantly with the major anatase TiO2 phase, which could promote the system's photocatalytic activity through the formation of anatase/brookite heterojunctions. Analysis of the lower frequency Eg anatase Raman mode indicated the occurrence of size effects reflecting phonon confinement in the anatase nanocrystallites as well as deviations from stoichiometry due to structural defects in the co-doped sample. NF-TiO2 films effectively degraded MC-LR under visible and UV light compared to reference film. Similar MC-LR degradation rates under visible light after three cycles revealed high mechanical stability and no irreversible changes of the film during photocatalysis. This process has the potential of providing environmentally benign routes for drinking water treatment with solar powered photocatalytic systems. © 2010 Elsevier B.V.
Stergiopoulos T, Valota A, Likodimos V, Speliotis T, Niarchos D, Skeldon P, Thompson GE, Falaras P. Dye-sensitization of self-assembled titania nanotubes prepared by galvanostatic anodization of Ti sputtered on conductive glass. Nanotechnology [Internet]. 2009;20. WebsiteAbstract
Self-organized porous TiO2 nanotubes (NTs) were prepared on conductive glass by galvanostatic anodizing of sputtered titanium in an NH 4F /glycerol electrolyte. DC magnetron sputtering at an elevated substrate temperature (500 °C) was used to deposit 650nm thick titanium films. After anodizing, NTs, 830nm long, with an average external diameter of 92nm, were grown; this gave a high conversion rate of oxide from titanium (1.9), with a 220nm thick layer of titanium, which was not oxidized, located at the base of the tubes. The NTs revealed a mainly amorphous structure, which transformed mostly to anatase upon thermal treatment in air at 450 °C. The tubes were sensitized by the N719 complex and the resultant photoelectrodes were incorporated into liquid dye solar cells (DSCs) and further tested under back-side illumination. High values of Voc (714mV) were obtained under 1 sun (AM 1.5), assigned to low dark current magnitude and large recombination resistance and electron lifetime. In addition, typical values of fill factors (of the order of 0.62) were attained, in agreement with the estimated ohmic resistance of the cells in combination with low electron transfer resistance at the platinum/electrolyte interface. The overall moderate power conversion efficiency (of the order of 0.3%) was mainly due to the low short-circuit photocurrents (Jsc = 0.68mAcm-2), which was confirmed further by the corresponding IPCE values (5.2% at 510nm). The magnitude of Jsc was attributed to absorbed light losses due to back-side illumination of the cells, the low dye loading (due to the limited thickness of anodic titania) and the high charge transfer resistance at the TiO2 /conductive substrate due to the presence of barrier layer(s) underneath the tubes. These preliminary results encourage the DSC community to explore further the galvanostatic anodizing of titanium in order to produce highly efficient porous TiO2 NTs directly on conductive glass. Current work is focusing on achieving complete anodizing of the metal substrate and full transparency for the photoelectrode in order to increase and optimize the resultant cell efficiencies. © 2009 IOP Publishing Ltd.
Konti G, Chatzivasiloglou E, Likodimos V, Kantonis G, Kontos AG, Philippopoulos AI, Falaras P. Influence of pyridine ligand nature and the corresponding ruthenium(ii) dye molecular structure on the performance of dye-sensitized solar cells. Photochemical and Photobiological Sciences [Internet]. 2009;8:726-732. WebsiteAbstract
{cis-[Ru(bpy)2(Ln)]Cl2 (n = 1, L 1 = 4-carboxy-2-(2′-pyridyl)quinoline (2); n = 2
Kontos AG, Kontos AI, Tsoukleris DS, Likodimos V, Kunze J, Schmuki P, Falaras P. Photo-induced effects on self-organized TiO2 nanotube arrays: The influence of surface morphology. Nanotechnology [Internet]. 2009;20. WebsiteAbstract
Self-organized TiO2 nanotubes with packed, vertically aligned morphology and different lateral characteristics were grown on Ti metal substrates by controlled electrochemical anodization in phosphate/HF and ethylene glycol/HF electrolytes. The wetting, photo-induced superhydrophilicity, and photocatalytic activity of the nanotubular materials were investigated under ultraviolet irradiation. The photoactivity of the TiO2 nanotube arrays was analysed in terms of their morphological characteristics that were determined by means of scanning electron microscopy and atomic force microscopy in conjunction with geometrical modelling. The wetting and the UV-induced superhydrophilicity could be accordingly modelled by the Cassie-Baxter mode arising from the large scale roughness of the nanotubular arrays in combination with the Wenzel mode due to the small scale roughness induced by ridges at the outer tube surface. The photocatalytic activity of the TiO2 nanotube arrays was further found to correlate quantitatively with the variation of the geometric roughness factor, verifying the strong impact of morphology on the photo-induced properties of the vertically oriented TiO2 tubular architecture. © IOP Publishing Ltd.
Likodimos V, Stergiopoulos T, Falaras P, Harikisun R, Desilvestro J, Tulloch G. Prolonged light and thermal stress effects on industrial dye-sensitized solar cells: A micro-raman investigation on the long-term stability of aged cells. Journal of Physical Chemistry C [Internet]. 2009;113:9412-9422. WebsiteAbstract
Micro-Raman spectroscopy is applied to investigate the long-term stability of industrial dye-sensitized solar cells under prolonged light soaking and thermal stress following continuous illumination over 6450 h at 55-60 °C. The Raman spectral characteristics of the individual cell components have been investigated using two excitation wavelengths in the visible and near-infrared range allowing us to assess the microstructure of the TiO2/conducting glass photoelectrode, the chemical bonding of the hydrophobic Ru(II)-polypyridyl dye complex on the mesoporous TiO2 film, and the electrolyte composition. Comparative ex situ resonance Raman measurements on fresh and aged cells indicate minor differences in the vibrational characteristics of the triiodide, dye molecules, and the triiodide/dye charge transfer adduct at the electrode/electrolyte interface upon aging, confirming the absence of any distinct chemical modification that could create instability. In situ Raman experiments implemented via the application of a polarization bias reveal a less pronounced potential dependence of both the electrolyte and the dye Raman response for the aged cells. These features together with the intensity reduction and broadening of the anatase Raman modes imply that the chemical stability of the cell interfaces is accompanied by a modification of the interfacial electric field on the TiO2/dye/electrolyte junction after long-term light and thermal stress.© 2009 American Chemical Society.
Kontos AI, Likodimos V, Stergiopoulos T, Tsoukleris DS, Falaras P, Rabias I, Papavassiliou G, Kim D, Kunze J, Schmuki P. Self-organized anodic TiO2 nanotube arrays functionalized by iron oxide nanoparticles. Chemistry of Materials [Internet]. 2009;21:662-672. WebsiteAbstract
Surface functionalization of self-organized Ti02 nanotube (NT) arrays produced by electrochemical anodization is implemented by dextrin-coated iron-oxide nanoparticles leading to a composite semiconductor nanostructure. The morphological and structural properties are studied by electron and atomic force microscopy, X-ray diffraction, X-ray photoelectron, and resonance micro-Raman spectroscopies revealing successful deposition of maghemite (γ-Fe2o3) nanoparticles on the nanotube walls. The nanocomposite surface simultaneously exhibits high photocatalytic activity for the degradation of model pollutants under UV irradiation at relatively low loading levels of the γ-Fe2o3 nanoparticles and light-independent wetting properties, as the initially superhydrophilic surface is converted to a moderately hydrophilic substrate, while obtaining an additional functionality through the magnetic field response of the iron-oxide component that shows appreciable magnetization anisotropy. Electrochemical impedance investigation including Mott-Schottky analysis attests to a significant improvement of the interfacial electron-transfer kinetics together with a modification of the surface chemistry for the functionalized Ti0 2 nanotubes, promoting electron-hole separation through the polyhydroxyl dextrin shell that mediates charge transfer between the constituent semiconductor oxides and validating their improved photocatalytic performance. These composite nanotubular materials offer the opportunity of advanced applications, where the unique photoinduced reactivity, the controlled wetting behavior, and the magnetic field response can be effectively combined. © 2009 American Chemical Society.
Philippopoulos AI, Tsantila N, Demopoulos CA, Raptopoulou CP, Likodimos V, Falaras P. Synthesis, characterization and crystal structure of the cis-[RhL2Cl2]Cl complex with the bifunctional ligand (L) 2-(2′-pyridyl)quinoxaline. Biological activity towards PAF (Platelet Activating Factor) induced platelet aggregation. Polyhedron [Internet]. 2009;28:3310-3316. WebsiteAbstract
The reaction of RhCl3·3H2O with the ligand L = 2-(2′-pyridyl)quinoxaline (pqo) in a 1:2 molar ratio formed the mononuclear complex cis-[RhL2Cl2]Cl (1), which has been characterized by elemental analysis, FT-IR, FT-Raman, 1H, 13C NMR, electronic absorption spectroscopy and by electrospray mass spectrometry. The molecular structure of 1 (needle like and prismatic polymorphs) in the crystal has been elucidated by single-crystal X-ray diffraction, revealing a bidentate behavior of L, while the geometry around the Rh(III) atom is that of a distorted octahedron.. Preliminary biological tests revealed that this compound inhibited PAF-induced rabbit platelet aggregation. © 2009 Elsevier Ltd. All rights reserved.
Stergiopoulos T, Ghicov A, Likodimos V, Tsoukleris DS, Kunze J, Schmuki P, Falaras P. Dye-sensitized solar cells based on thick highly ordered TiO2 nanotubes produced by controlled anodic oxidation in non-aqueous electrolytic media. Nanotechnology [Internet]. 2008;19. WebsiteAbstract
Dye-sensitized solar cells (DSSCs) were prepared using TiO2 nanotubes, grown by controlled Ti anodic oxidation in non-aqueous media. Smooth, vertically oriented TiO2 nanotube arrays, presenting a high degree of self-organization and a length of 20 μm, have been grown using ethylene glycol electrolyte containing HF. As-grown nanotubes exhibit an amorphous structure, which transforms to the anatase TiO2 crystalline phase upon post-annealing in air at 450°C. Atomic force microscopy (AFM) revealed the porous morphology together with high roughness and fractality of the surface. The annealed tubes were sensitized by the standard N719 ruthenium dye and the adsorption was characterized using resonance micro-Raman spectroscopy and adsorption-desorption measurements. The sensitized tubes were further used as active photoelectrodes after incorporation in sandwich-type DSSCs using both liquid and solidified electrolytes. The efficiencies obtained under air mass (AM) 1.5 conditions, using a back-side illumination geometry, were very promising: 0.85% using a composite polymer redox electrolyte, while the efficiency was further increased up to 1.65% using a liquid electrolyte. © IOP Publishing Ltd.
Guskos N, Likodimos V, Glenis S, Zolnierkiewicz G, Typek J, Lin CL, Biedunkiewicz A. Magnetic properties of TiCx/C nanocomposites. Journal of Non-Crystalline Solids [Internet]. 2008;354:4330-4333. WebsiteAbstract
The magnetic properties of nanocrystalline titanium carbide dispersed in a carbon matrix (TiCx/C) prepared by the non-hydrolytic sol-gel process have been studied by dc magnetization measurements. The superconducting phase of titanium carbide has been observed at low temperatures with the onset of the superconducting transition temperature Tc at about 3.5 K, superimposed on a ferromagnetic component. At T > Tc the magnetic response of TiCx/C is determined by the interplay of the ferromagnetic contribution with the paramagnetic/diamagnetic signal of the metallic system and the contribution of exchange coupled paramagnetic ions. Moreover, significant differences are observed in the magnetic response for samples of the same preparation batch, indicative of the magnetic/electronic inhomogeneity of nanocrystalline titanium carbide which is important for its practical applications.
Guskos N, Likodimos V, Glenis S, Maryniak M, Baran M, Szymczak R, Roslaniec Z, Kwiatkowska M, Petridis D. Magnetic properties of γ-Fe 2O 3/poly(ether- ester) nanocomposites. Journal of Nanoscience and Nanotechnology [Internet]. 2008;8:2127-2134. WebsiteAbstract
The magnetic properties of γ-Fe 2O 3 nanoparticles embedded in a thermoplastic elastomer poly(ether-ester) copolymer by the in situ polycondensation reaction process have been investigated by means of magnetization and ferromagnetic resonance (FMR) measurements at low filler concentrations of 0.1 and 0.3 wt% with the magnetic additive introduced in the polymer matrix in powder and solution form. The magnetic behavior of the magnetopolymeric nanocomposites indicates significant interparticle interaction effects that depend mainly on the dispersion state of the magnetic nanoparticles as well as their concentration, consistent with the variation of the particle microstructure characterized by magnetic aggregates in the nanometer and micron scale for the solution and powder dispersions, respectively. The magnetization and FMR results at different filler concentrations and dispersions show a close correspondence to the relaxation processes of the copolymer, implying the coupling of polymeric and magnetic properties. Copyright © 2008 American Scientific Publishers All rights reserved.
Likodimos V, Stergiopoulos T, Falaras P, Kunze J, Schmuki P. Phase composition, size, orientation, and antenna effects of self-assembled anodized titania nanotube arrays; a polarized micro-raman investigation. Journal of Physical Chemistry C [Internet]. 2008;112:12687-12696. WebsiteAbstract
Polarized micro-Raman spectroscopy was applied to investigate the phase composition and transformation from the amorphous to the crystalline state, size effects, and the crystallographic orientation and antenna polarization effects on self-organized anodic TiO2 nanotubes (NTs), The morphological characteristics of the NTs were tailored by electrochemical anodization in both aqueous (phosphate buffer) and organic (ethylene glycol) electrolytes as well as in perchlorate/chloride-containing electrolytes. Postgrowth thermal annealing was confirmed to reduce markedly the organic and inorganic species encapsulated in the as-grown arrays and drive the transformation of the amorphous titania to nanocrystalline anatase. Crystallite size and shape effects as well as oxygen nonstoichiometry were investigated through the variation of the low-frequency Eg anatase mode for the TiO2 NT arrays produced in different electrolyte media, and the results were compared with the predictions of the phonon confinement model for anatase nanoparticles. Polarized micro-Raman spectra identify partial orientation effects, indicating preferential (101) growth in the case of the shorter TiO2 NTs (length ≤ 1 μm), while a predominant random orientation of the anatase crystallites is found for the long tubes (length ≥ 10 μm). Polarized measurements on the cross section of free-standing NT membranes revealed significant enhancement of the Raman intensity when the polarization of the incident laser beam is parallel to the NT axis, indicating an "antenna" effect attributed to the enhanced light scattering along the cylindrical NT structure. Such an effect could result in optimum optical properties and selectively enhanced vectorial transport of electric carriers. © 2008 American Chemical Society.
Guskos N, Soldatov AG, Zolnierkiewicz G, Likodimos V, Glenis S. Spin dynamics and charge transfer in C60 · 2ferrocene studied by electron spin resonance. Journal of Non-Crystalline Solids [Internet]. 2008;354:4334-4337. WebsiteAbstract
Electron spin resonance (ESR) has been applied to study spin dynamics and charge transfer interactions in C60(Fe(C5H5)2)2. A low concentration of C60- spins is derived by ESR, suggesting the presence of weak charge transfer between C60 and ferrocene. The C60- ESR spectra display an appreciable anisotropy at T < 40 K reflecting molecular rotation freezing, while their low temperature variation (T < 10 K) implies development of antiferromagnetic spin-spin interactions. At high temperatures, the narrow ESR line of C120O- impurity spins inhibits observation of a C60- ESR. © 2008 Elsevier B.V. All rights reserved.
Likodimos V, Glenis S, Guskos N, Lin CL. Antiferromagnetic behavior in single-wall carbon nanotubes. Physical Review B - Condensed Matter and Materials Physics [Internet]. 2007;76. WebsiteAbstract
The electronic properties of single-wall carbon nanotubes (SWNTs) have been studied using dc magnetization and electron spin resonance (ESR). The dc magnetization displays a weak diamagnetic susceptibility of ≈ 10-7 emu/g. ESR measurements reveal a narrow resonance line of low intensity and metallic line shape. The spin susceptibility shows a major Pauli contribution at high temperatures (T>150 K) and an appreciable Curie component at lower temperatures. A marked drop of the spin susceptibility is observed at T<14 K, pointing to the opening of a spin gap at low temperatures. The underlying mechanism is discussed in terms of an electronic instability of the SWNTs or the occurrence of defect-mediated spin magnetism. © 2007 The American Physical Society.
Guskos N, Likodimos V, Glenis S, Typek J, Majszczyk J, Zolnierkiewicz G, Blonska-Tabero A, Lin CL. Magnetic and dielectric study of Ni2 FeV3 O11-δ. Reviews on Advanced Materials Science [Internet]. 2007;14:85-89. WebsiteAbstract
The magnetic and dielectric properties of the Ni2 FeV3 O11-6 compound synthesized using different annealing routes have been studied by means of static magnetization, electron paramagnetic resonance and dielectric spectroscopy. The temperature dependence of the magnetic susceptibility has shown marked singularities due to the coexistence of many kinds of magnetic interactions. Dielectric measurements have shown a second order phase transition to the ferroelectric state at about 200K. Dielectric loss measurements have shown an intense peak at the same temperature with activation energy of about 0.42 eV. All investigated compounds of the M-Fe-V-O (M=Ni, Zn, Mg) system have displayed a slight anomaly of the EPR parameters at about 200K. This anomaly is interpreted as arising from the competition between different magnetic subsystems preventing the formation of a long-range magnetic order. © 2007 Advanced Study Center Co. Ltd.
Guskos N, Likodimos V, Glenis S, Zolnierkiewicz G, Typek J, Szymczak R, Blonska-Tabero A. Magnetic frustration in the site ordered Mg3 Fe4 (V O4) 6 vanadate. Journal of Applied Physics [Internet]. 2007;101. WebsiteAbstract
The magnetic properties of the site ordered multicomponent vanadate Mg3 Fe4 (V O4) 6 are studied using dc magnetization and electron paramagnetic resonance (EPR) measurements. The static susceptibility shows antiferromagnetic interactions between Fe3+ spins with a Curie-Weiss temperature =-111 (1) K, while a transition to a spin-glass-like state is observed at T≈8.5 K, indicating appreciable spin frustration. EPR measurements corroborate the presence of antiferromagnetically coupled Fe3+ spins from high temperatures, while a distinct change in the temperature variation of the EPR parameters is observed at T<80 K, complying with the mean-field energy scale provided by the Curie-Weiss temperature of the system. The resulting magnetic inhomogeneity that persists despite the absence of cation disorder between magnetic and diamagnetic metal ions is attributed to the presence of small amounts of oxygen deficiency that modifies the connectivity and superexchange coupling between Fe3+ spins and leads to a disordered magnetic ground state. © 2007 American Institute of Physics.
Likodimos V, Pissas M. Phase coexistence and magnetic anisotropy in La1-x Cax Mn O3 (0. Physical Review B - Condensed Matter and Materials Physics [Internet]. 2007;76. WebsiteAbstract
We report an electron-spin-resonance (ESR) study of stoichiometric La1-x Cax Mn O3 for 0
Guskos N, Likodimos V, Typek J, Maryniak M, Grech E, Kolodziej B. Photoacoustic and EPR studies of two copper(II) complexes with spermine analogues. Reviews on Advanced Materials Science [Internet]. 2007;14:97-103. WebsiteAbstract
Two polyamine copper(II) compounds of spermine analogues: Cu[H2N(CH2)3NH (CH2)2NH(CH2)3NH2] (ClO4)2 (abbreviated as Spc323) and Cu[H2N(CH2)3NH (CH2)3NH(CH2)3NH2] (ClO4)2 (abbreviated as Spc333) have been synthesized and characterized by spectroscopic methods. The electron paramagnetic resonance (EPR) and photoacoustic spectroscopies have been used for studying CU2+ d-d transitions. The EPR spectra of copper(II) ions in both complexes have displayed a different crystal field symmetry: orthorhombic for Spc323 complex, and axial, with the hyperfine interaction, for Spc333 complex. In the visible region of electromagnetic radiation the photoacoustic absorptions from the d-d electron transitions with different energies and linewidths have been observed. Comparison of the obtained results is made with previous measurements on similar spermine compounds. © 2007 Advanced Study Center Co. Ltd.
Majszczyk J, Guskos N, Typek J, Likodimos V, Maryniak M, Roslaniec Z, Kwiatkowska M, Petridis D. The influence of low-concentration γ-Fe2O3 magnetic nanoparticles in a poly(ether-b-ester) copolymer matrix on interfacial polarization and glass transition. Journal of Non-Crystalline Solids [Internet]. 2006;352:4279-4282. WebsiteAbstract
A new poly(ester-b-ether) copolymer filled with magnetic γ-Fe2O3 nanoparticles of low concentration (0.1 and 0.3 wt%) has been synthesized, with poly(ethylene terephthalate) (PET) as a rigid segment and poly(tetraoxymethylene) (PTMO) as the flexible segment at the ratio of 50 wt%. The filler was used in the form of suspension or solid powder. The dielectric spectroscopy results for these materials, in a wide range of frequencies and temperatures, are presented and compared with similar results for a pure polymeric matrix. Two relaxation processes, α and β, have been observed clearly in all samples at about 253 K and 163 K, respectively. The α process was shifted towards the lower temperatures as the nanoparticle content increased, while the β-relaxation was unaffected by the doping. The interfacial polarization effect was observed in the samples with nanoparticles, less pronounced for the larger nanoparticles. © 2006 Elsevier B.V. All rights reserved.
Likodimos V, Pissas M. Magnetic anisotropy in the ferromagnetic insulating state of La1-x Cax Mn O3. Physical Review B - Condensed Matter and Materials Physics [Internet]. 2006;73. WebsiteAbstract
Ferromagnetic resonance (FMR) has been applied to study the variation of magnetic anisotropy and phase separation tendencies in the ferromagnetic insulating state of stoichiometric hole-doped La1-x Cax Mn O3 (0.125≤x≤0.19). A strongly anisotropic ferromagnetic phase is identified in the ferromagnetic insulating regime. The magnetic anisotropy evolves from positive uniaxial at x=0.125 and 0.15 to negative cubiclike at x=0.175 with increasing magnitude, whereas it decreases appreciably at x=0.19, which is close to the ferromagnetic metallic phase boundary. Minor contributions from ferromagnetic inhomogeneities characterized by weaker magnetic anisotropy are traced at low temperatures for x=0.125 and 0.15, while their temperature dependence implies coupling with the major anisotropic phase. A growing tendency towards the formation of spatially separate ferromagnetic regions in the paramagnetic regime is observed at x≥0.175, as the ferromagnetic metallic phase is approached. Persistent anomalies in the temperature dependence of the FMR parameters are traced concurrently with the structural transformation at 60-70 K. © 2006 The American Physical Society.
Guskos N, Likodimos V, Typek J, Zolnierkiewicz G, Szymczak R, Blonska-Tabero A. Magnetic properties of the Mg2FeV3O11-x site-disordered vanadate. Journal of Non-Crystalline Solids [Internet]. 2006;352:4179-4182. WebsiteAbstract
The magnetic properties of the Mg2FeV3O11-x ternary vanadate, characterized by disorder between diamagnetic Mg2+ and high-spin Fe3+ ions, are studied using dc magnetization and electron paramagnetic resonance (EPR). The dc susceptibility shows antiferromagnetic interactions between Fe3+ spins with a Curie-Weiss temperature of Θ = -50(1) K, followed by spin-glass-like freezing at Tf ≈ 2.8 K, suggesting significant spin frustration. Temperature-dependent EPR measurements confirm the antiferromagnetic coupling of Fe3+ spins at high temperatures, while a distinct divergence is observed at T ≈ 50 K. This behavior is associated with the formation of spin clusters providing two different energy scales for the magnetic interactions. The magnetic response of Mg2FeV3O11-x is similar to that of the Zn-analogue compound, though the observed differences of the implicated energy scales indicate that magnetic inhomogeneity depends on the extent of cation disorder.
Guskos N, Glenis S, Likodimos V, Typek J, Maryniak M, Roslaniec Z, Kwiatkowska M, Baran M, Szymczak R, Petridis D. Matrix effects on the magnetic properties of γ- Fe2 O3 nanoparticles dispersed in a multiblock copolymer. Journal of Applied Physics [Internet]. 2006;99. WebsiteAbstract
The magnetic properties of γ- Fe2 O3 ferrimagnetic nanoparticles embedded in a multiblock poly(ether-ester) copolymer have been investigated by static magnetization and ferromagnetic resonance (FMR) measurements at two different dispersion states. Significant variation of the magnetic response is identified below T≈120 K, most pronounced in the marked resonance field shift of the FMR spectra, independently of the dispersion state of the nanocomposites. This behavior correlates favorably with the dynamic relaxation of the copolymer, indicating a matrix freezing effect that is attributed to the magnetoelastic coupling of the oxide nanoparticles with the surrounding polymer. At low temperatures, the dc magnetization and FMR measurements vary considerably for the two nanocomposites, indicating essential differences in their ground state, related to the different morphology of the samples and the concomitant variation of interparticle interactions. © 2006 American Institute of Physics.
Guskos N, Likodimos V, Typek J, Zolnierkiewicz G, Blonska-Tabero A. EPR study of disorder processes in the ternary vanadate Zn2FeV3O11-x compound. Journal of Non-Crystalline Solids [Internet]. 2006;352:4250-4254. WebsiteAbstract
Two samples of the Zn2FeV3O11 compound were synthesized using different preparation routes: a solid state reaction of the ZnO-Fe2O3-V2O5 oxides and a direct reaction of the Zn2V2O7 and FeVO4 vanadates. In both cases the temperature dependences of the EPR spectra revealed a dominant contribution of antiferromagnetically coupled Fe3+ ions in the high-spin state. Specific differences in the EPR spectra of the two samples were recorded at all the investigated temperature intervals. A short range, or less likely, partial magnetic order involving a fraction of the Fe3+ distributed on the two octahedral crystallographic sites appeared to take place below 50 K. The differences observed between the EPR spectra of the two Zn2FeV3O11 samples might suggest the existence of small variations in the cation disorder phenomena of Fe3+ and Zn2+ ions or the presence of an oxygen deficiency process.
Likodimos V, Glenis S, Lin CL. Electronic properties of boron-doped multiwall carbon nanotubes studied by ESR and static magnetization. Physical Review B - Condensed Matter and Materials Physics [Internet]. 2005;72. WebsiteAbstract
The electronic properties of boron-doped multiwall carbon nanotubes (MWNTs) have been studied using static magnetization and electron spin resonance. A relatively strong ferromagnetic signal has been identified in the dc magnetization response that shows the occurrence of ferromagnetism, coexisting with the orbital and spin magnetism of the conduction electrons. The small diamagnetic susceptibility and the weak temperature variation of the g factor, which is observed in the narrow conduction-electron spin resonance (CESR), indicate a Fermi level shift of ∼0.2eV, according to the quasi-two-dimensional graphite band model. The temperature dependence of the spin susceptibility reveals considerable enhancement compared to undoped MWNTs and the presence of thermally activated behavior, complying with an increased density of states and the formation of localized impurity states close to the Fermi level. Line-shape analysis of the CESR spectra in terms of two separate spin systems implies enhanced doping inhomogeneity. © 2005 The American Physical Society.
Guskos N, Anagnostakis EA, Likodimos V, Bodziony T, Typek J, Maryniak M, Narkiewicz U, Kucharewicz I, Waplak S. Ferromagnetic resonance and ac conductivity of a polymer composite of Fe3 O4 and Fe3 C nanoparticles dispersed in a graphite matrix. Journal of Applied Physics [Internet]. 2005;97. WebsiteAbstract
Ferromagnetic resonance (FMR) and ac conductivity have been applied to study a polymer composite containing as filler a binary mixture of magnetite (Fe3 O4) and cementite (Fe3 C) nanoparticles (30-50 nm) dispersed in a diamagnetic carbon matrix, which was synthesized by the carburization of nanocrystalline iron. Ac conductivity measurements showed thermally activated behavior involving a range of activation energies and power law frequency dependence at high frequencies similar to conducting polymer composites randomly filled with metal particles. Ferromagnetic resonance measurements revealed a relatively narrow FMR line at high temperatures indicating the presence of ferromagnetic nanoparticles, where thermal fluctuations and interparticle interactions determine the FMR temperature variation. An abrupt change of the FMR spectra was observed at T<81 K (ΔT≤1 K) coinciding with a sharp anomaly resolved in the temperature derivative of the ac conductivity. This behavior is attributed to the Verwey transition of Fe3 O4 nanoparticles, where the concurrent skin depth variation unveils the FMR of large magnetite conglomerates and thus allows discriminating their contribution from relatively isolated nanoparticles. © 2005 American Institute of Physics.
Likodimos V, Pissas M. Magnetic heterogeneity in electron doped La1-xCa xMnO3 manganites studied by means of electron spin resonance. Journal of Physics Condensed Matter [Internet]. 2005;17:3903-3914. WebsiteAbstract
Electron spin resonance (ESR) has been applied to investigate the magnetic heterogeneity in electron doped La1-xCaxMnO3 (0.80≤x≤0.95). A low field ferromagnetic resonance (FMR) mode is observed for lightly doped compounds (x = 0.90,0.95), signifying the formation of ferromagnetic (FM) spin clusters within the antiferromagnetic G-type AFM phase. The anomalous temperature variations of the resonance field, linewidth and FMR intensity, as well as the observation of thermal cycling effects below TC, emphasize the non-trivial dynamics of the FM phase, which is attributed to the temperature dependent size evolution of the underlying spin clusters towards canted AFM and FM domains. For heavier electron doping (x = 0.80,0.85), distinct AFM behaviour is evinced in the vicinity of TN in the monoclinic C-type AFM phase, characterized by the absence of critical relaxation. Additional weak FMR lines are observed for x = 0.80 and 0.85, whereas a narrow superparamagnetic-like signal is detected for x = 0.95. © 2005 IOP Publishing Ltd.
Pissas M, Papavassiliou G, Devlin E, Simopoulos A, Likodimos V. Mössbauer study of 1% 57Fe doped ferromagnetic insulator La0.825Ca0.175MnO3. European Physical Journal B [Internet]. 2005;47:221-229. WebsiteAbstract
We have employed magnetization measurements, Mössbauer and ESR spectroscopic techniques, in order to study the ferromagnetic insulating (FMI) compound La1-xCaxMnO3 (x=0.175) doped with 1% 57Fe. We have used two samples; one prepared in air which has cation vacancies and a second in inert atmosphere, which is stoichiometric. An abrupt change of the experimental results is obtained, by all techniques, in the ferromagnetic insulating regime, in the temperature region of T O/O//≈60 K, where an orbital rearrangement is suggested to occur. An analysis of these findings points to an orbital rearrangement transformation. Ferromagnetic resonance reveals considerable differences between stoichiometric and cation deficient samples, indicating anisotropy of the exchange interactions in the former sample with significant temperature dependence, most pronounced in the vicinity of TO/O//. © EDP Sciences, Società Italiana di Fisica, Springer-Verlag 2005.
Guskos N, Typek J, Likodimos V, Glenis S, Dziembowska T, Wabia M, Palios G, Lin CL. Cooperative magnetic interactions in three mononuclear copper Schiff base complexes. Materials Chemistry and Physics [Internet]. 2004;83:114-119. WebsiteAbstract
The magnetic properties of three mononuclear copper complexes where the metal ion is coordinated by Schiff base ligands have been investigated by magnetic susceptibility and electron paramagnetic resonance (EPR) studies. Significant extended magnetic coupling has been observed comprising competing antiferromagnetic and ferromagnetic components, which are explained in terms of the interactions between copper ions mediated through the organic ligands. Distant substitutions in the ligand structure are suggested to affect critically the magnetic coupling leading to frustration effects on the cooperative transition that may develop under favorable structural conditions. The temperature dependence of intensity of the EPR line was studied in the temperature range 130-280K and fitted to the Curie-Weiss-type law. The consistency of magnetic and EPR results is discussed for the three investigated complexes. © 2003 Elsevier B.V. All rights reserved.
Glenis S, Likodimos V, Guskos N, Lin CL. Magnetic properties of multiwall nanotubes. Journal of Magnetism and Magnetic Materials [Internet]. 2004;272-276:1660-1661. WebsiteAbstract
The diamagnetic susceptibility of multiwall carbon nanotubes is studied as function of magnetic field and temperature. Both the low-field and the low-temperature susceptibility reveal a definite paramagnetic upturn in qualitative agreement with the Aharonov-Bohm effect. Analysis of the high-field magnetic susceptibility in terms of the orbital diamagnetism of quasi-two-dimensional graphite implies hole-doping and supports further the intrinsically divergent paramagnetic contribution of small metallic nanotubes. © 2003 Elsevier B.V. All rights reserved.
Likodimos V, Guskos N, Glenis S, Szymczak R, Bezkrovnyi A, Wabia M, Typek J, Gasiorek G, Kurzawa M, Rychlowska-Himmel I, et al. Magnetic properties of the antiferromagnetic site-disordered vanadate Zn2FeV3O11. European Physical Journal B [Internet]. 2004;38:13-18. WebsiteAbstract
The magnetic properties of the Zr2FeV3O11 vanadate, characterized by a disordered distribution of diamagnetic Zn 2+ and high-spin Fe3+ ions, are studied using magnetization and electron paramagnetic resonance (EPR) measurements. The dc susceptibility reveals antiferromagnetic interactions between Fe3 +spins with a Curie-Weiss temperature Θ = -58(1) K, followed by a transition to a frozen, spin-glass-like state at low temperature T f ≈ 2.55 K, indicating an inhomogeneous magnetic ground state. The temperature variation of the EPR parameters confirms the antiferromagnetic coupling of Fe3+ spins at high temperatures, while a distinct divergence is observed at T ≈ 55 K. This behavior is attributed to the inherent magnetic inhomogeneity of the system due to antiferromagnetic spin clusters.
Guskos N, Likodimos V, Glenis S, Typek J, Wabia M, Paschalidis DG, Tossidis I, Lin CL. Magnetic properties of rare-earth hydrazone compounds. Journal of Magnetism and Magnetic Materials [Internet]. 2004;272-276:1067-1069. WebsiteAbstract
The magnetic properties of two series of trivalent rare earth (R 3+) molecular complexes with hydrazone ligands (R=Ce, Pr, Dy, Ho, Er), adopting different crystal structures, have been investigated by static susceptibility measurements. At high temperatures, the magnetic response is dominated by the paramagnetic contribution of R3+ ions determined by the crystal-field interaction. At low temperatures (T<4K), an antiferromagnetic deviation is observed, most pronounced for the series with the heavier R3+ ions. © 2004 Elsevier B.V. All rights reserved.
Likodimos V, Glenis S, Lin CL. Static magnetic susceptibility of the RbC60 polymerized fulleride. European Physical Journal B [Internet]. 2004;41:55-60. WebsiteAbstract
The magnetic properties of the polymerized phase of RbC60 have been investigated by static magnetization measurements. The magnetic susceptibility comprises a temperature dependent part that decreases rapidly below 50 K, where the metal-insulator transition and the subsequent antiferromagnetic ordering are expected according to electron spin resonance results. The temperature variation of this contribution resembles that of quasi-one-dimensional gapped systems and shows substantial field dependence suggesting a disordered antiferromagnetic phase.
Guskos N, Papadopoulos G, Majszczyk J, Typek J, Wabia M, Likodimos V, Paschalidis DG, Tossidis IA, Aidinis K. Charge transfer and f-f transitions studied by photoacoustic spectroscopy of [R(NO3)2(PicBH)2]NO3 and [R(NO3)3(PicBH)2] complexes (R - Rare earth ion). Acta Physica Polonica A [Internet]. 2003;103:301-313. WebsiteAbstract
High-resolution photoacoustic spectra of trinitrate-bis[N-(2- pyridylmethylene)-N′-benzoyl-hydrazine]R(III), (R = La, Cc, Pr), and binitrato-bis[N-(2-pyridylmethylene)-N′-benzoyl-hydrazine]R(III) nitrate (R=Y, Nd, Eu, Yb, Tb, Gd, Ho, Dy, Er) complexes, in the visual region, were studied for powder samples. Very intense photoacoustic spectra of various shapes were attributed to the intraligand transitions of the π → π* type located mainly on the C=N group and the n → π* transitions located on the carbonyl group. The intensity of these transitions essentially depends on the type of rare earth ions. The existence of f-f electron transitions could influence the relaxation processes, which play an important role in intensity determination of the above transitions. For many investigated samples the energy levels of excited states of rare earth ions were identified (f-f electron transitions). The radiation energy for some of the rare earth ions were compared with the d-d electron transitions of certain coppcr(II) organometallic complexes, which are very important in biogenic systems. The correlation between ions with localized and extended wave functions is suggested.
Bodziony T, Typek J, Orlowski M, Majszczyk J, Wabia M, Berkowski M, Ryba-Romanowski W, Guskos N, Likodimos V, Anagnostakis EA. Magnetic resonance and dielectric studies of a nonlinear La 3Ga5.5Ta0.5O14 single crystal doped with erbium. Acta Physica Polonica A [Internet]. 2003;103:315-322. WebsiteAbstract
{Single crystal of erbium doped La3Ga5.5Ta 0.5O14 grown by the Czochralski method have been investigated by electron paramagnetic resonance and dielectric spectroscopy methods. Dielectric permittivity ε measurements performed in 90-440 K temperature range have shown negligible dispersion for 1 kHz - 1 MHz frequencies and a Curie-Weiss type behaviour with C = 47700 K and θ = -340 K. Electron paramagnetic resonance studies have revealed the presence of two different paramagnetic, monoclinic centres. The calculated g factor values are: g1 = 1.449
Likodimos V, Koutandos S, Pissas M, Papavassiliou G, Prassides K. Conduction electron spin resonance in Mg1-xAlxB2. Europhysics Letters [Internet]. 2003;61:116-121. WebsiteAbstract
Conduction electron spin resonance is employed to study the interplay of the electronic and structural properties in the normal state of Mg1-xAlxB2 alloys as a function of Al-doping for 0 ≤ x ≤ 1. The x-dependence of the spin susceptibility reveals considerable reduction of the total density of states N(EF) with increasing Al concentration, complying with theoretical predictions for a predominant filling effect of the hole σ bands by electron doping. The CESR linewidth exhibits significant broadening, especially prominent in the high-Al-content region, indicative of the presence of enhanced structural disorder, consistent with the presence of compositional fluctuations.
Guskos N, Gazda M, Typek J, Likodimos V, Wabia M, Fuks H, Sadowski W. Copper defects and conductivity in Bi-Pb-Sr-Ca-Cu-O glasses. Radiation Effects and Defects in Solids [Internet]. 2003;158:375-381. WebsiteAbstract
Glass ceramics of the composition (Bi0.8Pb0.2) 4Sr3Ca3Cu4O8 prepared by the melt quenching technique and the crystalline phases produced by the rapid thermal annealing have been studied by electrical resistivity and electron paramagnetic resonance (EPR) measurements in the temperature range from liquid helium up to room temperature. The concentration of the EPR active Cu 2+ paramagnetic centers decreases as conductivity increases for the glass ceramics and disappears alter crystallization and the growth of superconducting phases, similar to bulk high-Tc,. superconductors. The EPR spectra of both glass and crystallized ceramics after short-time annealing indicate the coexistence of Cu2+ paramagnetic ions and the exchange coupled clusters.
Guskos N, Glenis S, Likodimos V, Typek J, Fuks H, Wabia M, Szymczak R, Lin CL, Perkowska TA. Influence of water molecule coordination on the magnetic properties of polyamine copper dinitrate complexes. Journal of Applied Physics [Internet]. 2003;93:9834-9838. WebsiteAbstract
Influence of water molecule coordination on the magnetic properties of polyamine copper dinitrate complexes was studied. It was found that the magnetic coupling between Cu2+ ions strongly depends on the presence of water molecules in the crystal structure. Electron paramagnetic resonance (EPR) measurements showed considerable temperature dependence of the Lande′ splitting factors and resonance linewidths for both complexes at low temperatures.
Likodimos V, Glenis S, Guskos N, Lin L. Magnetic and electronic properties of multiwall carbon nanotubes. Physical Review B - Condensed Matter and Materials Physics [Internet]. 2003;68. WebsiteAbstract
Multiwall carbon nanotubes were studied by dc magnetization and electron spin resonance (ESR) measurements. Hole doping is inferred from both the high-field dc susceptibility and ESR parameters complying favorably with the band model of two-dimensional graphite. Paramagnetic deviations are evinced on the diamagnetic susceptibility at weak fields and low temperatures, conforming qualitatively with the Aharonov-Bohm effect on the energy gap for magnetic field parallel to the tube axis. Comparison with theoretical predictions for ensembles of carbon nanotubes reveals appreciable differences, indicative of the diverse distribution of nanotubes as well as the presence of active doping. © 2003 The American Physical Society.
Guskos N, Wabia M, Kurzawa M, Beskrovnyj A, Likodimos V, Typek J, Rychlowska-Himmel I, Blonska-Tabero A. Neutron diffraction study of Mg2FeV3O 11-δ. Radiation Effects and Defects in Solids [Internet]. 2003;158:369-374. WebsiteAbstract
The neutron diffraction patterns of Mg2FeV3O 11-δ compound at various low temperatures have been investigated. No magnetic ordering was observed in the investigated temperature range down to 10K. These materials have been formed in the triclinic space group but there are specific differences in the positions of atoms as compared to previously determined from XRD method. The iron(III) ions are distributed non-statistically with magnesium(II) ions and this could be responsible for some differences in the structure of the above sample. The method of sample preparation, in particularly thermal annealing processes could be responsible for the differences.
Likodimos V, Labardi M, Pardi L, Allegrini M, Giordano M, Arena A, Patanè S. Optical nanowriting on azobenzene side-chain polymethacrylate thin films by near-field scanning optical microscopy. Applied Physics Letters [Internet]. 2003;82:3313-3315. WebsiteAbstract
A study was performed on the optical nanowriting on azobenzene side-chain polymethacrylate thin films using near-field scanning optical microscopy (NSOM). The optical storage mechanism in azobenzene side-chain polymers is based on the isomerization cycles of the azobenzene side-chains between their cis and trans forms. The optical anisotropy was detected by coupling the red light at 690 nm with modulated polarization to the NSOM aperture.
Labardi M, Polop C, Likodimos V, Pardi L, Allegrini M, Vasco E, Zaldo C. Surface deformation and ferroelectric domain switching induced by a force microscope tip on a La-modified PbTiO3 thin film. Applied Physics Letters [Internet]. 2003;83:2028-2030. WebsiteAbstract
Surface deformation and ferroelectric domain switching induced by a force microscope tip on a La-modified PbTiO3 thin film was studied. Polarization switching of the domains comprising the deformed region demonstrated the possibility of using scanning force microscopy (SFM) as a nanoscale tool for imaging phase transitions. It was found that if loading forces are higher than a threshold value, thin film characterization becomes invasive.
Likodimos V, Labardi M, Allegrini M. Domain pattern formation and kinetics on ferroelectric surfaces under thermal cycling using scanning force microscopy. Physical Review B - Condensed Matter and Materials Physics [Internet]. 2002;66:241041-241047. WebsiteAbstract
Scanning force microscopy in the dynamic contact mode with resonance enhanced domain contrast is applied to study in situ pattern formation and relaxation kinetics of ferroelectric domains on the cleavage surface of triglycine sulfate subjected to successive annealing-cooling cycles. Substantial differences in the domain morphology and kinetics are disclosed upon cooling at temperatures close to Tc. Convolution of stripe domains with residual domain nuclei and random pinning forces due to intrinsic defects are suggested to account for variations in the domain structure and slowing down of domain kinetics.
Guskos N, Likodimos V, Glenis S, Typek J, Fuks H, Wabia M, Paschalidis DG, Tossidis D, Lin CL. Electron paramagnetic resonance and magnetic susceptibility of rare-earth hydrazone compounds. European Physical Journal B [Internet]. 2002;28:277-282. WebsiteAbstract
The magnetic properties of the rare earth molecular compounds with hydrazone ligands containing Nd3+ Gd3+ and Yb3+ have been investigated by electron paramagnetic resonance (EPR) and magnetization measurements. For the Gd-compound, partially resolved fine structure due to Gd and exchange narrowing effects at low temperatures are observed in the EPR spectra, suggesting, consistent with the EPR and dc magnetic susceptibility, weak antiferromagnetic exchange interactions. Paramagnetic behavior sustained down to low temperatures is derived for Yb3+ ions, whereas substantial ferromagnetic exchange coupling is inferred for the lighter Nd3+ ions, indicating significant variations of the exchange integrals along the lanthanide series.
Likodimos V, Pissas M. Electron spin resonance and microwave absorption study of (formula presented). Physical Review B - Condensed Matter and Materials Physics [Internet]. 2002;65:1-4. WebsiteAbstract
A comparative study of powder and bulk specimens of (formula presented) by electron spin resonance (ESR) verifies the presence of intense conduction electron spin resonance (CESR) in the normal state. A low concentration paramagnetic center stemmimg from the initial amoprhous boron powder along with traces of (formula presented) impurities are identified in the ESR spectra. Intense microwave absorption, that distorts CESR below (formula presented) is observed in fine powders implying enhanced microwave dissipation due to the viscous flux motion. © 2002 The American Physical Society.
Likodimos V, Pissas M. Electron spin resonance and microwave absorption study of MgB2. Physical Review B - Condensed Matter and Materials Physics [Internet]. 2002;65:1725071-1725074. WebsiteAbstract
A comparative study of powder and bulk specimens of MgB2 by electron spin resonance (ESR) verifies the presence of intense conduction electron spin resonance (CESR) in the normal state. A low concentration paramagnetic center stemmimg from the initial amoprhous boron powder along with traces of Fe3+ impurities are identified in the ESR spectra. Intense microwave absorption, that distorts CESR below Tc, is observed in fine powders implying enhanced microwave dissipation due to the viscous flux motion.
Guskos N, Papadopoulos GJ, Likodimos V, Patapis S, Yarmis D, Przepiera A, Przepiera K, Majszczyk J, Typek J, Wabia M, et al. Photoacoustic, EPR and electrical conductivity investigations of three synthetic mineral pigments: Hematite, goethite and magnetite. Materials Research Bulletin [Internet]. 2002;37:1051-1061. WebsiteAbstract
Three iron oxide pigments: Hematite, goethite and magnetite, have been synthesized from the copperas by-product produced as a waste material in large quantities in the titanium dioxide plant in Chemical Works "Police," Poland. These pigments have been characterized by X-ray diffractometry and X-ray fluorescence spectroscopy and it has been found that they are mono-phase systems with only a small addition of other spurious phases. The electrical resistivity measurements reveals that the ion oxide samples are semiconductor materials. Conductivity of magnetite at room temperature is over six orders of magnitude greater than that of the other two materials. Photoacoustic (PA) spectra of hematite and goethite in the visual region of the electromagnetic radiation shows a single intense absorption peak centred at 558 and 480nm, respectively, while for the magnetite a continuous spectrum of increasing amplitude with increasing wavelength of the measurement is obtained. EPR spectra for the iron(III) complexes have been registered at room temperature which indicate hematite and goethite have only one similar EPR line centred at g ∼ 2 with different linewidths, while magnetite has two very intense overlapping and broad EPR lines centred at low and high magnetic resonance fields. Additional EPR line of Mn2+ ions has been also recorded for the hematite sample. Application of these pigments is discussed in terms of economic and environmental advantages. © 2002 Elsevier Science Ltd. All rights reserved.
Lampakis D, Liarokapis E, Lykodimos V, Panagopoulos C. Pressure effects in the Raman spectra of La2-xSrxCuO4. High Pressure Research [Internet]. 2002;22:83-87. WebsiteAbstract
Micro-Raman measurements under hydrostatic pressures up to 6GPa have been carried out on high-quality La2-xSrxCuO4 polycrystalline compounds with Sr concentration up to x=0.45. The zz scattering polarization has been investigated, where two strong modes due to La/Sr and the apex oxygen, and (in the low Sr concentrations) the soft mode at ≃100cm-1 are observed. The frequency of the strong modes increases almost linearly with pressure for the Sr concentrations studied. Modifications in the increment rate dω/dp and the phonon width have been detected depending on the amount of doping. For x=0.45 a considerable increase in the width of both strong phonons with pressure was found, which must reflect a separation into phases, since this concentration is close to the solubility limit. The relative intensity of the strong phonons was investigated in connection with its correlation to the transition temperature.
Guskos N, Papadopoulos GJ, Likodimos V, Majszczyk J, Typek J, Wabia M, Grech E, Dziembowska T, Perkowska A, Aidinis K. Electronic structure of polycrystalline polyamine copper dinitrate complexes investigated by photoacoustic and electron paramagnetic resonance spectroscopy. Journal of Applied Physics [Internet]. 2001;90:1436-1441. WebsiteAbstract
Photoacoustic and electron paramagnetic resonance (EPR) spectroscopies have been applied to resolve the electronic structure in powder polycrystalline samples of three biogenic polyamine copper complexes, spermine copper dinitrate, aqua norspermine copper dinitrate, and homospermine copper dinitrate. The fine structure of the intense absorption band in the photoacoustic spectra is assigned to the d-d transitions between the crystal field split levels of copper ions, that cannot be discriminated in the UV/vis solution absorption spectra. Combination with the EPR results allows one to probe the variation of the electronic properties and bonding interaction at the copper site, consistent with the structural data for the crystalline complexes and further supports the reliability of the photoacoustic method to resolve the d-d transition band. A dominant contribution of the in-plane ligand field due to equatorial nitrogen atoms is deduced for the complexes of polyamines with copper salts. © 2001 American Institute of Physics.
Likodimos V, Labardi M, Allegrini M, Garcia N, Osipov VV. Surface charge compensation and ferroelectric domain structure of triglycine sulfate revealed by voltage-modulated scanning force microscopy. Surface Science [Internet]. 2001;490:76-84. WebsiteAbstract
The interplay between surface morphology and ferroelectric domain structure on triglycine sulfate (TGS) (010) cleavage faces is investigated by voltage-modulated scanning force microscopy in the dynamic contact mode. A resonance enhancement method is exploited to increase imaging contrast and sensitivity to slight variations of surface polarity. Evidence of electric contrast of the structural nuclei forming due to surface reconstruction of the TGS cleavage face, is provided, further supported by second harmonic measurements. Lateral growth at ambient conditions and high mobility of the surface nuclei relative to the spontaneous ferroelectric domain motion after cooling from above Tc is detected. Surface charge screening by the conductive liquid film that condenses on the cleavage surface and its interface dynamics are suggested to determine the kinetics of surface nuclei during ferroelectric domain coarsening and the formation of a zig-zag domain boundary. © 2001 Elsevier Science B.V. All rights reserved.
Likodimos V, Labardi M, Orlik XK, Pardi L, Allegrini M, Emonin S, Marti O. Thermally activated ferroelectric domain growth due to random defects. Physical Review B - Condensed Matter and Materials Physics [Internet]. 2001;63:641041-641044. WebsiteAbstract
Ferroelectric domain kinetics on cleaved triglycine sulfate, quenched at different temperatures in the ferroelectric phase, is investigated in situ by scanning force microscopy in the dynamic contact mode. Thermally activated domain growth and dynamic scaling, in accordance with theoretical predictions for quenched disorder due to random-bond defects, is inferred from the temporal evolution of the spatial correlation functions and the related characteristic length scale.
Likodimos V, Guskos N, Typek J, Wabia M. EPR study of Dy3+ ions in DyBa2Cu3O6+x. European Physical Journal B [Internet]. 2001;24:143-147. WebsiteAbstract
Concentrated polycrystalline DyBa2Cu3O6+cursive Greek chi compounds are studied by X-band EPR spectroscopy. A broad resonance line due to the highly anisotropic EPR spectrum of Dy3+ ions is identified on several specimens at low temperatures. Powder simulation of the EPR spectra complies with the ground Kramers doublet predicted by crystal field analysis of Dy3+ ions. Calculations of the second and fourth moments of the resonance lines due to the dipole-dipole interactions of Dy3+ indicate the presence of substantial exchange narrowing of the dipolar-broadened EPR linewidth.
Labardi M, Likodimos V, Allegrini M. Resonance modes of voltage-modulated Scanning Force Microscopy. Applied Physics A: Materials Science and Processing [Internet]. 2001;72:S79-S85. WebsiteAbstract
Mechanical resonance modes of the scanning force microscope (SFM) cantilever in contact conditions provide contrast enhancement in the imaging of surface charges when using voltage modulation techniques tuned to such resonances. Extensions of the method were made as regards the lateral (twisting) and frontal (buckling) modes of the cantilever, as well as the enhanced second harmonic detection of voltage-modulated response at resonance and near-resonance detection in the SFM tapping mode. As an example of application, vibration spectra and images taken on a triglycine sulfate (TGS) single crystal are discussed. © Springer-Verlag 2001.
Guskos N, Likodimos V, Wabia M, Typek J, Grybos J, Fuks H. Anomalous behavior of localized magnetic centers in (Sm,Er)123 compound. Physica B: Condensed Matter [Internet]. 2000;284-288:1381-1383. WebsiteAbstract
The EPR spectrum of the oxygen-deficient Sm0.5Er0.5Ba2Cu3O6+x mixed phase has been investigated in the temperature range of 3.3-20 K. Anomalous behavior of the EPR spectrum has been detected below 4 K that may indicate the presence of magnetic centers with singlet ground state and an excitation energy of the order of 1 K. © 2000 Elsevier Science B.V. All rights reserved.
Guskos N, Likodimos V, Los S, Kempinski W, Stankowski J, Wabia M, Typek J, Blonska-Tabero A, Tabero P, Rychlowska-Himmel I. EPR study of Ni2 FeVO6-δ. Physica B: Condensed Matter [Internet]. 2000;284-288:1456-1458. WebsiteAbstract
The temperature dependence of the EPR spectra for the oxygen deficient Ni2 FeVO6-δ compound has been investigated in the range of 50-300 K. An intense EPR line associated with high-spin iron centers or ferromagnetic clusters, is observed. The strong temperature dependence of the EPR intensity, resonance field and line width is consistent with the stepwise magnetization variation and supports the presence of successive magnetic anomalies at lower temperatures. © 2000 Elsevier Science B.V. All rights reserved.
Likodimos V, Orlik XK, Pardi L, Labardi M, Allegrini M. Dynamical studies of the ferroelectric domain structure in triglycine sulfate by voltage-modulated scanning force microscopy. Journal of Applied Physics [Internet]. 2000;87:443-451. WebsiteAbstract
Voltage-modulated scanning force microscopy has been employed to investigate the dynamics of ferroelectric domains as a function of time and temperature in triglycine sulfate (TGS) single crystals. Branching of the domain structure and nucleation of fine domain patterns by internal fields of thermal origin have been directly observed on the submicron scale. Domain coarsening after quenching TGS samples from the paraelectric phase into the ferroelectric one has been studied, revealing a nonlinear time dependence of the characteristic correlation domain length and supporting the validity of dynamical scaling law and the conservation of the total surface charge. Substantial differences in the evolution of the domain structure have been detected during cooling or heating treatment in the ferroelectric phase. Domain contrast is shown to monitor the ferroelectric phase transition through its temperature dependence, reflecting the competition between electrostatic and piezoelectric effects. © 2000 American Institute of Physics.
Guskos N, Likodimos V, Typek J, Wabia M, Fuks H. EPR spectra of Gd3+ ions and localized paramagnetic centers in La0.5Gd0.5Ba3Cu2O6+x. Physica C: Superconductivity and its Applications [Internet]. 2000;341-348:573-574. WebsiteAbstract
Oxygen deficient La0.5Gd0.5Ba2Cu3O6+x has been studied by EPR spectroscopy. The temperature dependence of EPR spectrum reveals the presence of sustained magnetic ordering of the Gd3+ sublattice and localized paramagnetic centers corresponding to copper-oxygen clusters. © 2000 Elsevier Science B.V. All rights reserved.
Labardi M, Likodimos V, Allegrini M. Force-microscopy contrast mechanisms in ferroelectric domain imaging. Physical Review B - Condensed Matter and Materials Physics [Internet]. 2000;61:14390-14398. WebsiteAbstract
Contrast mechanisms for the detection of antiparallel domains in ferroelectric materials such as triglycine sulfate are investigated in the dynamic contact electrostatic force microscope. The domain contrast exhibits strong dependence on voltage modulation frequency, which is explained with the excitation of resonant modes of the cantilever in contact conditions. Phenomena such as contrast enhancement, inversion, and nulling are detected and accounted for with a simple model for the tip-sample interaction. The model includes the effects of the viscoelastic sample response and confirms that the observed contrast features have to be attributed to the spectral response of the tip-sample junction. © 2000 The American Physical Society.
Likodimos V, Labardi M, Allegrini M. Kinetics of ferroelectric domains investigated by scanning force microscopy. Physical Review B - Condensed Matter and Materials Physics [Internet]. 2000;61:14440-14447. WebsiteAbstract
Voltage-modulated scanning force microscopy has been applied to investigate domain kinetics in triglycine sulfate single crystals, thermally quenched from the paraelectric to the ferroelectric phase. Temporal analysis of the equal-time correlation functions shows that the average domain size grows initially with a power law dependence close to that of conserved systems, whereas a crossover to the slower logarithmic growth law (Formula presented) predicted for systems with random impurities occurs in the late stage of coarsening. Domain growth is dominated by a single characteristic length scale consistent with the dynamical scaling hypothesis. The total surface charge corresponding to the domain area of the two polarization states is found to be conserved within the studied time scale, indicating that the system may be considered to behave like one with conserved order parameter. © 2000 The American Physical Society.
Guskos N, Likodimos V, Typek J, Wabia M. Localized paramagnetic centers in ErxY1-xBa3Cu2O6+y (x=1.0, 0.5) compounds. Physica C: Superconductivity and its Applications [Internet]. 2000;341-348:551-552. WebsiteAbstract
Oxygen deficient ErxY1-xBa3Cu26+y (x=1.0, 0.5) compounds are studied by EPR spectroscopy. The EPR spectra reveal the presence of high spin (S>1) paramagnetic centers, dimers and free Cu2+ ions, which appear enhanced in the mixed R phase and suppressed in the single R123 one, consistent with phase separation. © 2000 Elsevier Science B.V. All rights reserved.
Guskos N, Papadopoulos G, Likodimos V, Mair GLR, Majszczyk J, Typek J, Wabia M, Grech E, Dziembowska T, Perkowska TA. Photoacoustic detection of d-d transitions and electronic structure of three polyamine copper complexes. Journal of Physics D: Applied Physics [Internet]. 2000;33:2664-2668. WebsiteAbstract
The potential of photoacoustic spectroscopy to resolve the crystal field levels of Cu2+ ions in three polyamine copper complexes, aqua spermine copper sulfate trihydrate [NH2(CH2)3NH(CH2)4NH(CH2)3 NH2·Cu2+·H2O]·SO42-·3H2O, norspermine copper sulfate trihydrate [NH2(CH2)3NH(CH2)3NH(CH2)3 NH2·Cu2+·SO42-]·3H2O, and homospermine copper sulfate dihydrate [NH2(CH2)3 NH(CH2)2NH(CH2)3 NH2·Cu2+·SO42-]·2H2O, has been explored along with UV/VIS and electron paramagnetic resonance (EPR) spectroscopy. Intense absorption is detected in the photoacoustic spectrum corresponding to the d-d transition band weakly traced in the corresponding optical spectrum. The observed fine structure photoacoustic lines are assigned to the individual d-d transitions of Cu2+ ions, which are further exploited to determine the diverse metal-ligand interaction of the studied compound employing simple crystal field analysis.
Orlik XK, Likodimos V, Pardi L, Labardi M, Allegrini M. Scanning force microscopy study of the ferroelectric phase transition in triglycine sulfate. Applied Physics Letters [Internet]. 2000;76:1321-1323. WebsiteAbstract
Voltage-modulated scanning force microscopy is applied to study the temperature dependence of the ferroelectric domain structure of triglycine sulfate up to TC, the ferroelectric transition temperature. The polarization image contrast exhibits a power-law decrease as TC is approached, associated with the competition between Maxwell stress and the converse piezoelectric effect. Substantial differences in the static and dynamic domain patterns are detected for two samples of different aging conditions. © 2000 American Institute of Physics.
Guskos N, Likodimos V, Typek J, Wabia M, Fuks H. Cluster formation in oxygen-deficient La0.5Yb0.5Ba2Cu3Oy. Radiation Effects and Defects in Solids [Internet]. 1999;151:121-129. WebsiteAbstract
The EPR spectra of the oxygen-deficient La0.5Yb0.5Ba2Cu3Oy mixed phase has been investigated as a function of temperature. At low temperatures (T<15 K), an intense EPR line indicating the presence of magnetic clusters associated with spin-polarized clusters of copper ions through delocalized oxygen holes, has been detected. Both the EPR linewidth and g-factor of the latter EPR spectrum exhibit strong temperature dependence indicative of short-range order effects. At higher temperatures (T>30 K), the EPR spectrum reveal the contribution of Cu2+ defects and a broad EPR spectrum due to Yb3+ ions which is compared with the corresponding one in Y0.5Yb0.5Ba2Cu3Oy compounds.
Guskos N, Likodimos V, Glenis S, Patapis SK, Palilis LC, Typek J, Wabia M, Rychlowska-Himmel I. Electrical transport and EPR properties of the α, β, and γ phases of Fe2WO6. Physical Review B - Condensed Matter and Materials Physics [Internet]. 1999;60:7687-7690. WebsiteAbstract
The polymorphic modifications α, β, and γ phases of Fe2WO6 of the iron (III) tungstate have been investigated by resistivity and electron paramagnetic resonance (EPR) measurements. The high-electrical resistivity for all phases complies with the dominant antiferromagnetic component of the compounds. The EPR spectra comprise a broad exchange narrowed EPR line due to Fe3+ ions. The broadening of the EPR linewidth is explained in terms of the critical spin fluctuations near the antiferromagnetic phase transition. The shift of the resonance field and the temperature dependence of the EPR intensity imply an extended region of short-range order correlations. © 1999 The American Physical Society.
Karchava G, Guskos N, Glenis S, Likodimos V, Kekelidze N, Tsintsadze G, Euthymiou P. The influence of small doses of fast neutron irradiation on the critical temperature and EPR spectra of YBa2Cu3O7-δ high temperature superconductors. Physica C: Superconductivity and its Applications [Internet]. 1999;317-318:561-564. WebsiteAbstract
The effect of the increase of the critical temperature Tc by the irradiation of the Y123 samples with small doses of fast neutrons has been investigated by the temperature dependence of resistance and EPR studies. The results indicated that Tc and EPR signals of all specimens are increased with increasing irradiation fluences. These results are explained in terms of redistribution of oxygen, which promotes charge ordering in the superconducting CuO2 planes. © 1999 Elsevier Science B.V. All rights reserved.
Dziembowska T, Guskos N, Typek J, Szymczak R, Likodimos V, Glenis S, Lin CL, Wabia M, Jagodzinska E, Fabrycy E. Spectroscopic and magnetic properties of two di-copper(II) complexes with macrocyclic Schiff bases. Materials Research Bulletin [Internet]. 1999;34:943-954. WebsiteAbstract
Two binuclear copper(II) complexes with macrocyclic Schiff bases Cu2LI(CH3COO)2·5H2O (complex I) and Cu2LII(CH3COO)2·2H2O (complex II) were synthesized and then characterized by IR, UV, and thermogravimetric analysis (TGA) measurements. TGA was used to investigate the desolvation of lattice water molecules. IR spectra demonstrated the formation of the cyclic compound and together with chemical elemental analysis were used to propose the structure of the complexes. The UV spectra of both complexes are typical for binuclear copper(II) complexes with Robson-type ligands. Variable-temperature magnetic susceptibility measurements corroborated by EPR and low-temperature isothermal magnetization data confirmed the formation of copper dimers with antiferromagnetic exchange coupling constants of -400 and -1250 cm-1 for complexes I and II, respectively, residing outside the usual range for the phenoxide bridged Cu(II) complexes. This implies the possibility that additional superexchange paths through the macrocyclic ligand may affect the intradimer exchange interaction as well as the phenoxide oxygen bridges.
Guskos N, Likodimos V, Calamiotou M, Gantis A, Nabialek A, Szymczak H, Wabia M. Time evolution of copper defects in the mixed phase La0.5Gd0.5Ba2Cu3Oy. Radiation Effects and Defects in Solids [Internet]. 1999;151:151-157. WebsiteAbstract
XRD, magnetic and EPR studies of the mixed phase La0.5Gd0.5Ba2Cu3Oy high-Tc superconductor after prolonged (one year) `aging' at room temperature are reported. Within this time period a pronounced change of the superconducting behavior and the homogeneity of the diamagnetic shielding signal accompanied by substantial structural changes, reduction of the impurity phase BaCuO2+x and increase of the EPR spectrum's intensity of Cu2+ magnetic defects is detected. The `aging' effects are related to strain-relaxation taking place during room temperature annealing that induces the formation of Cu2+ defect centers.
Guskos N, Likodimos V, Glenis S, Nabialek A, Szymczak H, Wabia M. Time evolution of the magnetic properties of la0.5R0.5Ba2Cu3O6+x(R = rare earth) high-Tc superconductors. Physica C: Superconductivity and its Applications [Internet]. 1999;317-318:558-560. WebsiteAbstract
The magnetic properties of the mixed high-Tc superconductors Ea0.5R0.5Ba2Cu3O6+x (R - rare earth) are studied after prolonged 'aging' at room temperature (RT) for 1 year. Enhancement of the superconducting properties comprising the increase of the inhomogeneous diamagnetic signal for members of the series with the larger R-ion radius is detected after RT 'aging'. For the members with the larger R-ion radius the diamagnetic signal has been come more stronger whereas for the members with the smaller R-ion radius the time evolution of the superconducting properties remained almost the unchanged and the diamagnetic signal slightly decreases after 'aging'. The growth of the EPR intensity of localized Cu2+ centers complying with the rise of the normal-state magnetic susceptibility after prolonged storage at RT, is detected. The time evolution effects are related to slow ion diffusion processes that can be induced by stress-relaxation in the mixed Ea0.5R0.5Ba2Cu3O6+x structure, most pronounced for large R-ion radius. © 1999 Elsevier Science B.V. All rights reserved.
Guskos N, Typek J, Likodimos V, Fuks H, Wabia M, Walczak J, Lukaszczyk-Tomaszewicz E. EPR study of the Ag6S3O4 compound prepared by two methods. Applied Magnetic Resonance [Internet]. 1998;14:403-408. WebsiteAbstract
{The temperature dependence of the EPR spectra of the recently discovered Ag6S3O4 phase in the Ag-O-S system prepared by two methods, the known method of co-precipitation from aqueous solution and a new method depending on the interaction of Ag2S and Ag2SO4 solid reagents, has been investigated. No EPR spectra were observed at room temperature, while at liquid helium temperature a number of EPR spectra have been recorded, which disappeared upon increasing temperature up to liquid nitrogen temperature. The sample obtained by the co-precipitation method revealed an intense, rich EPR spectrum that has been tentatively interpreted assuming the presence of at least two different Ag2+ ion complexes, one monomer resulting in an intense anisotropic, rhombic EPR powder pattern with g1 = 1.93(1)
Likodimos V, Guskos N, Wabia M, Typek J. Exchange interactions of ions. Physical Review B - Condensed Matter and Materials Physics [Internet]. 1998;58:8244-8247. WebsiteAbstract
The EPR spectrum of (Formula presented) ions in (Formula presented) 0.5) is studied as a function of temperature. In the oxygen-deficient superconducting phase, the (Formula presented) EPR spectrum comprises a broad EPR line described by a rhombic g tensor and anisotropic linewidths along the principal axes. Analysis of the EPR linewidth provides an estimate for the effective isotropic exchange interaction, (Formula presented) between (Formula presented) ions, indicating that the magnetic ordering of the (Formula presented) sublattice is determined by the interplay between the short-range exchange interactions and the long-range dipole-dipole interactions. © 1998 The American Physical Society.
Guskos N, Typek J, Wabia M, Likodimos V, Fuks H, Rychlowska-Himmel I, Walczak J. Phase transition study in α-Fe2WO6 compound by EPR. Applied Magnetic Resonance [Internet]. 1998;14:397-402. WebsiteAbstract
The temperature dependence of the EPR spectrum for the α-phase of iron tungstate has been investigated in the temperature range of 40-260 K. At temperatures between T1 ≈ 250 K and T2 ≈ 205 K where the antiferromagnetic phase transition occurs, a relatively narrow EPR line arising from the dominant iron(III) species has emerged, gaining intensity with the temperature increase. Its linewidth temperature evolution could be described by Huber equation, with TN = 200 K, which is consistent with the peak seen in magnetic susceptibility measurements, while the corresponding g-factor shifts to higher fields reflecting the build-up of internal field emerging from increasing short-range order in the spin system. At temperatures lower than T2, a very broad and distorted EPR line with temperature dependent g-factor and linewidth has been observed reflecting the corresponding rise of the magnetic susceptibility below the antiferromagnetic phase transition, presumably arising from magnetic clusters embedded in the antiferromagnetic background. © Springer-Verlag 1998.
Likodimos V, Guskos N, Garmari-Seale H. Exchange interactions of in the ab plane for An EPR linewidth study. Physical Review B - Condensed Matter and Materials Physics [Internet]. 1998;58:14223-14226. WebsiteAbstract
The electron paramagnetic resonance (EPR) linewidth of (Formula presented) in (Formula presented) is studied as a function of Er concentration. The EPR spectrum complies with the crystal-field ground doublet, while linewidth analysis shows the presence of exchange narrowing. Comparison of the theoretical EPR linewidth with the experimental data provide evidence for the presence of nearest-neighbor exchange interactions of the order of 1 K, most probably anisotropic along the a and b crystallographic axes. © 1998 The American Physical Society.
Guskos N, Likodimos V, Patapis SK, Typek J, Wabia M, Fuks H, Gamari-Seale H, Walczak J, Rychlowska-Himmel I, Bosacka M. Magnetic, Electrical Conductivity, and EPR Investigations of a Low-Spind5System in Fe8V10W16O85. Journal of Solid State Chemistry [Internet]. 1998;137:223-230. WebsiteAbstract
The temperature dependence of the magnetic, electrical conductivity, and electron paramagnetic resonance (EPR) properties of Fe8V10W16O85has been investigated. The magnetic susceptibility measurements revealed an almost Curie-Weiss law behavior above room temperature and an additional magnetic interaction at low temperature, causing a steep rise of magnetization as the liquid helium temperature was approached. The value of the magnetic moment at high temperature,μeff=1.80μB, suggests a predominance of trivalent iron ions in a low-spin state. In the 300-4.2 K temperature range a difference between the zero field cooling (ZFC) and the field cooling (FC) modes was recorded. This irreversible behavior might be related to the presence of weakly coupled clusters. The EPR measurements revealed a broad, temperature-dependent resonance line at high temperature and two weaker lines at low temperature. The two low-temperature lines were attributed to antiferromagnetically coupled high-spin Fe3+ion clusters and to high-spin iron ions placed at sites with low symmetry of the crystal field. The broad line at high temperature was separated into two Lorentzian lines. These component lines were attributed to the two paramagnetic centers connected with the Fe3+ions involved in the magnetic structure of Fe8V10W16O85: dominant low-spin centers and a small admixture (<15%) of the high-spin centers. The line broadening and shift of the resonance field of the two component lines with decreasing temperature were studied and analyzed using a model of the EPR lines of antiferromagnets. The temperature dependence of the electrical conductivity showed a typical semiconducting-type behavior with an activation energy of 0.40 eV. The hopping mechanism of small polarons was proposed to explain the transport properties of the sample. © 1998 Academic Press.
Guskos N, Likodimos V, Typek J, Wabia M, Fuks H. Anomalous behaviour of the EPR spectra of defects in (Y,Sm)123 ceramics with oxygen deficiency at low temperatures. Materials Science Forum [Internet]. 1997;239-241:77-80. WebsiteAbstract
The temperature dependence of the EPR spectra of Y0.5Sm0.5 Ba2Cu3O6+x ceramic in tetragonal phase has been investigated at low temperatures. The complicated EPR spectra resulted from different paramagnetic centers: isolated divalent copper ions, exchange coupled copper pairs of ions, and molecular oxygen ions. The observed temperature dependence of the linewidth of exchange coupled copper ions has been interpreted in frames of a model of the collective motion of magnetic moments. The intensity of EPR spectra of molecular oxygen ions has shown critical behaviour at 3.60 K, where the intensity increased considerably.
Likodimos V, Guskos N, Gamari-Seale H, Koufoudakis A. Copper magnetic centers in oxygen deficien (R=Nd, Sm): An EPR and magnetic study. Physical Review B - Condensed Matter and Materials Physics [Internet]. 1996;54:12342-12352. WebsiteAbstract
EPR and magnetic results are reported for oxygen deficient, nonsuperconducting R(Formula presented)(Formula presented)(Formula presented) (R=Nd, Sm) compounds. The magnetic-susceptibility χ(T) and isothermal M(H) data are analyzed as the superposition of the rare-earth (Formula presented) contribution with another strongly ferromagnetic (FM) contribution arising from FM copper clusters with large total spin S. The rare-earth paramagnetic contribution in χ(T) and M(H) are calculated using the results of consistent crystal-field analysis (intermediate coupling wave functions, J-mixing effects) of (Formula presented) and (Formula presented) ions. The corresponding EPR spectra comprise an intense, almost isotropic EPR line whose intensity I(T) exhibits a ferromagnetic behavior, while (Formula presented) and the linewidth Δ(Formula presented) diverge at T<10 K indicating the presence of slowly fluctuating “internal” fields. The origin of the FM clusters is related to spin-polarized copper clusters through oxygen holes in the Cu(1) or Cu(2) layers, while the ferromagnetic interaction of the (Formula presented)(1) with the (Formula presented)(2) moments may be involved in the low-temperature (T<10 K) behavior of the EPR parameters. On increasing the oxygen deficiency, the ferromagnetic contribution is drastically reduced and more isolated (Formula presented) centers appear as shown by the corresponding EPR data. Exact simulation of the latter anisotropic EPR spectra, shows that the anisotropic linewidths Δ(Formula presented) (i=x, y, z) gradually broaden at low temperatures, while the intensity I(T) shows antiferromagnetic behavior. EPR measurements on an “aged” (Formula presented)(Formula presented)(Formula presented)(Formula presented)(Formula presented) sample revealed that the (Formula presented) EPR spectrum intensifies with time, a behavior probably related to oxygen ordering processes or to surface degradation effects. Analysis of the EPR resonance of (Formula presented) and (Formula presented) ions in combination with the absence of the corresponding EPR spectra indicate the presence of very fast spin-lattice relaxation of the rare-earth ions. © 1996 The American Physical Society.
Dziembowska T, Guskos N, Typek J, Grech E, Likodimos V, Wabia M, Fuks H, Ramos J, Palios G, Kawecka J. Physico-chemical properties of Cu(II) complexes of bis-salicylidene-1,2-phenylenediamine and bis-(3,5-dichlorosalicylidene)-1,2-phenylenediamine. Polish Journal of Chemistry [Internet]. 1996;70:1425-1434. WebsiteAbstract
The powder samples of Cu(II) complexes of bis-salicylidene-1,2-phenylenediamine (complex I) and bis-(3,5-dichlorosalicylidene)-1,2-phenylenediamine (complex II) have been prepared and investigated by magnetic, IR, UV/VIS and EPR methods. From IR similar chemical structures for both complexes have been proposed. The electronic spectra support the square-planar geometry of both complexes. Magnetic susceptibility of complex I has shown typical paramagnetic behaviour for spin S = 1/2 with some ferromagnetic interaction, but no ferromagnetic order down to 1.7 K. The observed EPR spectra for both complexes are dominated by resonances arising from coupled pairs of copper ions in the temperature range investigated.
Guskos N, Likodimos V, Koufoudakis A, Bojanowski B, Typek J, Wabia M. EPR study of Cu2+ ions in the Pr0.5RE0.5Ba2Cu3O6 + x compounds in tetragonal phase at low temperatures. Journal of Magnetism and Magnetic Materials [Internet]. 1995;140-144:1319-1320. WebsiteAbstract
Powdered samples of the Pr0.5RE0.5Ba2Cu3O6 + x compounds, where RE Yb, Ho, Tm, Er, and Y, were investigated by the EPR technique at low temperatures in the range 2.8-20 K. In the EPR spectra of these non-superconducting materials in tetragonal phase, resonance lines arising from the exchange interaction of coupled pairs of divalent copper ions and Cu2+ in the tetragonal local crystal field symmetry were recorded. Apart from the usually observed ΔMs = 1 and ΔMs = 2 transitions for the CuCu system, rarely reported singlet-triplet transitions were detected as well. The exchange parameter J has roughly the same value of about 0.12 cm-1 for all investigated samples and the zero-field splitting parameter D is of the order of 0.013 cm-1. It is suggested that formation of Cu(1)OCu(2) copper coupled pairs is responsible for the observed EPR spectrum. © 1995.
Likodimos V, Guskos N, Palios G, Koufoudakis A, Typek J, Bojanowski B, Wabia M. EPR study of localized Cu2+ paramagnetic ions and Cu2+ pairs in the oxygen-deficient PrBa2Cu3O6+x and Pr0.5R0.5Ba2Cu3O6+x (R=Y,Er) compounds. Physical Review B [Internet]. 1995;52:7682-7688. WebsiteAbstract
Low-temperature EPR measurements are presented for the PrBa2Cu3O6+x and Pr0.5R0.5Ba2Cu3O6+x (R=Y,Er) compounds with oxygen deficiency (tetragonal phase). The nuclear hyperfine interaction of the Cu2+ ions has been observed in both compounds. The origin of the Cu2+ EPR spectra has been associated with localized divalent copper ions in the Cu(1) sites. A time-induced transformation of the Cu2+ local crystal-field symmetry from orthorhombic to tetragonal, related to the oxygen distribution in the Cu(1) plane, has been detected. The EPR spectra of pairs of weakly exchange-coupled divalent copper ions were also observed. These copper pairs are suggested to occur in the Cu(1) planes and to be magnetically isolated from the antiferromagnetic CuO2 planes. © 1995 The American Physical Society.
Guskos N, Sadlowski L, Typek J, Likodimos V, Gamari-Seale H, Bojanowski B, Wabia M, Walczak J, Rychlowska-Himmel I. Magnetic and epr studies of α-, β-, and γ-fe2wo6 phases at low temperatures. Journal of Solid State Chemistry [Internet]. 1995;120:216-222. WebsiteAbstract
The polymorphic modifications α-, β-, and γ-Fe2WO6 of the iron tungstate system were studied by means of magnetic susceptibility and EPR measurements at low temperatures. Both methods revealed a significant paramagnetic contribution, probably resulting from local distortions of the antiferromagnetic bulk structure induced by a disturbed cation ordering or the presence of Fe2+ ions. The magnetic susceptibility revealed a peak at ∼260 K for all samples which can be related with an AF phase transition. The EPR spectra comprised the contribution of various isolated paramagnetic iron centers, one arising from high-spin Fe3+ ions in rhombic crystal field symmetry with E/D ≈ 1/3 and D ≈ 0.22 cm-1, an anisotropic EPR signal consistent with an S = 3/2 ground state with large zero-field splitting, and a dominant component in the g ≈ 2 region presumably arising from an S = 1/2; spin state. The latter spectra were tentatively attributed to the formation of multi-iron clusters, one of them invoking the presence of Fe2+ ions as well. For the βFe2WO6 phase an additional EPR spectrum was observed, which probably results from high-spin Fe3+ ions in a weak crystal field. © 1995 Academic Press. All rights reserved.
Guskos N, Likodimos V, Londos CA, Psycharis V, Mitros C, Koufoudakis A, Gamari-Seale H, Windsch W, Metz H. Structural, magnetic, and EPR studies of BaCuO2+x. Journal of Solid State Chemistry [Internet]. 1995;119:50-61. WebsiteAbstract
XRD, magnetic susceptibility, and EPR studies are reported for the BaCuO2+x compound in both the oxygenated and nonoxygenated phases. The XRD analysis has shown essential differences in the disordered part between the oxygenated and the nonoxygenated samples. The magnetic susceptibility measurements have shown strong ferromagnetic intracluster interactions among copper ions in the ordered part of the compound. At low temperature, antiferromagnetic intercluster interactions mediated by the disordered part predominate. EPR measurements in both phases have revealed a spectrum of Cu2+ ions, in orthorhombic local symmetry, which is attributed to copper ions in the disordered part. An intense exchange-narrowed EPR line has also been observed in both phases, which is ascribed to the copper clusters in the ordered part. The temperature dependence of the intensity and linewidth of this line is consistent with the bulk magnetic behaviour. At low temperature, the divergent behaviour of the EPR parameters indicates that antiferromagnetic ordering may occur. Additionally, it was found that the EPR spectra are essentially influenced by thermal treatment and the ordering of the oxygen atoms in the disordered part of the compound. © 1995 Academic Press Limited.
Guskos N, Likodimos V, Kuriata J, Metz H, Windsch W, Wabia M, Mitros C, Niarchos D. EPR Spectra of Polycrystalline REBaCuFeO5 (RE = Eu, Y, Yb). physica status solidi (b) [Internet]. 1994;181:K69-K72. Website
Guskos N, Likodimos V, Koufoudakis A, Mitros C, Gamari‐Seale H, Wabia M. High‐Tc Superconductor LuBa2Cu3O6 + x. physica status solidi (b) [Internet]. 1994;182:K19-K23. Website
Guskos N, Likodimos V, Palios G, Koufoudakis A, Bojanowski B, Typek J, Wabia M. Low Temperature Critical Fluctuations of the EPR Spectrum of Yb3+ Ions in the Yb0.5Sm0.5Ba2Cu3O6+x Compound in the Tetragonal Phase. physica status solidi (b) [Internet]. 1994;184:K31-K35. Website
Guskos N, Likodimos V, Patapis SK, Kuriata J, Wabia M, Sadlowski L, Koufoudakis A, Mitros C, Gamari‐Seale H, Psycharis V. Low Temperature EPR Spectra of the (Pr, RE)BaCuO Ceramics in the Orthorhombic Phase. physica status solidi (b) [Internet]. 1994;184:445-463. WebsiteAbstract
The EPR spectra of the members of the series Pr0.5RE0.5Ba2Cu3O6+x (RE = Pr, La, Nd, Sm, Eu, Gd, Dy, Ho, Y, Er, Tm, Yb, and Lu) in the orthorhombic phase are investigated at low temperature. The EPR spectra arise from two different paramagnetic centers, one from divalent copper ions in orthorhombic local crystal field symmetry and another from exchange coupled pairs of copper ions whose origin is discussed. The EPR spectra essentially depend on the time exposition. The EPR spectra of some divalent and trivalent rare earth ions are recorded. The crystal field analysis is done for the trivalent ytterbium ions. The semiconducting behaviour of the dc conductivity can be attributed to thermally activated small polaron hopping motion. Copyright © 1994 WILEY‐VCH Verlag GmbH & Co. KGaA
Triberis GP, Guskos N, Likodimos V. Small-polaron hopping conductivity in the PrBa2Cu3-xGaxO7-y system. Physica C: Superconductivity and its applications [Internet]. 1994;228:33-41. WebsiteAbstract
Based on experimental observations we propose the small-polaron hopping model to interpret the behavior of the resistivity of PrBa2Cu3-xGaxO7-y system with x = 0.00-0.60 in a wide temperature range (14-290 K), reported by other workers. percolation arguments we evaluate the maximum hopping distances, at high and low temperatures, together with other relevant parameters, and we give a unified and meaningful explanation of the transport mechanism, for different Ga concentrations and the entire temperature range. © 1994.
Guskos N, Kuriata J, Likodimos V, Sadlowski L, Koufoudakis A, Mitros C, Gamari‐Seale H, Niarchos D. EPR of Tm2+ in the Tetragonal Phase of the (Lu, Tm)—BaCuO System. physica status solidi (b) [Internet]. 1993;175:K61-K62. Website
Guskos N, Likodimos V, Londos CA, Gamari-Seale H, Koufoudakis A, Mitros C, Niarchos D. Experimental studies of the EuBa2-xEuxCu 3O7- delta compound in the tetragonal phase. Journal of Physics: Condensed Matter [Internet]. 1993;5:229-234. WebsiteAbstract
EPR and magnetic measurements were carried out on specially prepared samples of the EuBa2-xEuxCu3O 7- delta compound in the tetragonal phase, in the temperature range 100-4.2 K. The EPR spectra of Eu2+, Cu2+, coupled pairs of divalent copper ions and the superexchange interaction between Cu ions over oxygen bridges have been observed. The EPR spectrum of europium ions indicates the existence of a crystal-field splitting of the 8S7/2 ground state of Eu2+ ions with a value of the spin-Hamiltonian parameter b20 of about 14*10-2 cm -1. This parameter is analysed within the framework of Newman's superposition model. Anomalies in the temperature variation of the magnetic susceptibility are attributed to the presence of divalent and trivalent europium ions. The susceptibility of Eu3+ ions is non-zero because the excited states are thermally populated and they contribute to the result of the ground state with J=0.
Guskos N, Likodimos V, Calamiotou M, Kuriata J, Wabia M, Paraskevas SM, Windsch W, Metz H, Koufoudakis A, Mitros C, et al. Magnetic and EPR Studies of Oxygenated and Non‐Oxygenated LaBaSrCu3O6+x Compounds in the Tetragonal Phase. physica status solidi (b) [Internet]. 1993;180:491-501. WebsiteAbstract
Oxygenated and non‐oxygenated LaBaSrCu3O6+x ceramics in the tetragonal phase are studied using X‐ray, EPR, and magnetic measurements. A Meissner signal for the oxygenated sample corresponding to almost perfect diamagnetism is measured below 35 K. However, EPR data indicate that at least some parts of the sample become superconducting already below 62 K. Magnetic susceptibility of the non‐oxygenated sample exhibit a significant paramagnetic contribution attributed to Cu(1) ions ferromagneticall coupled to the Cu(2) planes. Copyright © 1993 WILEY‐VCH Verlag GmbH & Co. KGaA
Likodimos W, Guskos N, Londos CA, Paraskevas SM, Koufoudakis A, Mitros C, Gamari-Seale H, Niarchos D. EPR spectrum of Tm2+ ions in the tetragonal phase of (La, Tm)-Ba-Cu-O compound. Journal of Magnetism and Magnetic Materials [Internet]. 1992;104-107:563-564. WebsiteAbstract
EPR measurements at room temperature (RT) on the oxygen deficient (La, Tm)-Ba-Cu-O compound (tetragonal phase) revealed a line centered at g=2.895(5) with linewidth ΔH=0.015(1)T. The line is ascribed to Tm2+ ions (4f13, 2F7/2) assuming that the Γ(7) doublet is the ground state. A characteristic EPR spectrum of divalent copper ions was also observed. © 1992.
Guskos N, Likodimos V, Londos CA, Windsch W, Metz H, Koufoudakis A, Mitros C, Gamari‐Seale H, Niarchos D. Low‐Temperature Dependence of the EPR Spectra of Gd0.5RE0.5Ba2Cu3O7−δ Compounds in the Tetragonal Phase. physica status solidi (b) [Internet]. 1992;170:597-607. WebsiteAbstract
The EPR spectra of perovskite ceramics of Gd0.5RE0.5Ba2Cu3O7−δ type (RE  La, Pr, Nd, Sm, Eu, Dy, Ho, Y, Er, Tm, Yb, and Lu) in tetragonal (non‐HTC) phase are investigated as a function of temperature in the range 40 to 3 K. A signal arising from the isotropic exchange interaction of coupled pairs of Cu2+ ions is detected. The intensity of this signal is strongly dependent on the RE ions and its g‐factor varies with temperature. Magnetic measurements show the existence of ferromagnetic anomalies at these low temperature. In the orthorhombic phase, these materials exhibit a superconducting phase transition in the range of 85 to 95 K, apart from the samples with La ions where a TC ≈ 69 K is found and the samples with Prions where a superconducting state can not be observed. Copyright © 1992 WILEY‐VCH Verlag GmbH & Co. KGaA
Gamari-Seale H, Guskos N, Koufoudakis A, Kruk I, Mitros C, Likodimos V, Niarchos N, Psyharis B. A possible new magnetic phase transition in tetragonal RBa2Cu3O6+x (R Ξ Nd, Sm, Tm). Philosophical Magazine B: Physics of Condensed Matter; Statistical Mechanics, Electronic, Optical and Magnetic Properties [Internet]. 1992;65:1381-1387. WebsiteAbstract
Electron paramagnetic resonance obtained from the tetragonal RBa2Cu3O6+x (R ≡ Nd, Sm, Tm) compounds exhibit a well defined line resulting from coupled divalent Cu ions. Below 20 K the g parameter of this signal is strongly temperature dependent. At about the same temperatures there is a ferromagnetic-like transition in the magnetic susceptibility. © 1992 Taylor & Francis Ltd.
Guskos N, Calamiotou M, Londos CA, Likodimos V, Koufoudakis A, Mitros C, Gamari-Seale H, Niarchos D. Temperature dependence of the EPR spectra of EuBa2Cu3O7-δ in orthorhombic and tetragonal phases. Journal of Physics and Chemistry of Solids [Internet]. 1992;53:211-213. WebsiteAbstract
EPR spectra between 9.7 K and room temperature (RT) were taken for EuBa2Cu3O7-δ in tetragonal and orthorhombic phases. Our results indicate significant differences between the EPR spectra of Cu2+ ions in the tetragonal and orthorhombic phase. It is concluded that in the tetragonal phase, the Cu2+ complex has axial symmetry. Furthermore, at low temperatures the observed EPR signal is associated with superexchange interaction between Eu2+ ions across oxygen bridges. © 1991.
Guskos N, Londos CA, Lykodimos V, Calamiotou M, Koufoudakis A, Mitros C, Gamari-Seale H, Niarchos D. Electron paramagnetic resonance studies of the oxygenated and non-oxygenated LaBa2Cu3O7- delta compounds. Journal of Physics: Condensed Matter [Internet]. 1992;4:4261-4266. WebsiteAbstract
The EPR spectra of polycrystalline oxygenated and non-oxygenated (large oxygen deficiency) LaBa2Cu3O7- delta compounds, obtained at various temperatures, were studied and analysed. The oxygenated samples showed a transition temperature Tc at about 50 K. The EPR spectrum of the Cu2+ ions in the orthorhombic local symmetry was observed. It was found that these spectra are essentially influenced by an oxygen deficiency and thermal treatment. In the non-oxygenated samples, at a certain stage of the annealing process an EPR spectrum was detected at low temperatures. After the final anneal, this EPR spectrum showed significant changes. Changes were also observed in the corresponding XRD spectra indicating reductions in the concentrations of the CuO and BaCuO2 impurity phases. Additionally, the EPR spectrum of divalent copper ions in the axial local symmetry in the dx2-y2 ground state was observed.
Guskos N, Lyvkodimos V, Londos CA, Kondos A, Paraskevas SM, Koufoudakis A, Mitros C, Gamari-Seale H, Niarchos D, Kruk I. XRD EPR studies of ceramics Pr0.5Re0.5Ba2Cu3O7-δ in the orthorhombic and tetragonal phase. Journal of Superconductivity [Internet]. 1992;5:457-461. WebsiteAbstract
EPR, XRD, and magnetic studies are presented for Pr0.5Re0.5Ba2Cu3O7-δ compounds (Re= La, Nd, Sm, Eu, Gd, Dy, Ho, Y, Er, Tm, Yb, and Lu) in the orthorhombic and tetragonal (large oxygen deficiency) phase. For the samples with Re=Dy, Ho, Y, Er, Tm, Yb, and Lu in the orthorhombic phase, the transition to the superconducting state has been observed in the temperatures range Tc between 18 and 40 K. For the samples with Nd and Yb in the tetragonal phase, EPR spectra coming from trivalent rare earth ions have been recorded. In the nonoxygenated sample Pr0.5La0.5Ba2Cu3O7-δ the EPR spectrum arising from the non-Kramers trivalent praseodymium ion has been observed. A broad EPR line appearing in all our samples was attributed to superexchange interaction between copper ions over oxygen (O2-) bridges. Interestingly, for the Pr0.5Re0.5Ba0.5Cu3O7-δ (Re = Er and Lu) compounds in the tetragonal phase at liquid-nitrogen temperature, a nonresonant microwave absorption in low magnetic fields has been detected. © 1992 Plenum Publishing Corporation.
Guskos N, Triberis GP, Likodimos W, Kondos A, Koufoudakis A, Mitros C, Gamari‐Seale H, Niarchos D. Electron spin resonance of coupled pairs of Cu2+ ions in Pr0.5Dy0.5Ba2Cu3O7−δ and Sm0.5Gd0.5Ba2Cu3O7−δcompounds in the tetragonal phase. physica status solidi (b) [Internet]. 1991;164:K105-K108. Website
Guskos N, Likodimos W, Kontos A, Koufoudakis A, Mitros C, Gamari‐Seale H, Niarchos D. Temperatue Dependence of the EPR Spectra of PrBa2Cu3O7–δ and Pr0.5Dy0.5Ba2Cu3O7–δ Compounds in Orthorhombic and Tetragonal Phases. physica status solidi (b) [Internet]. 1991;166:K19-K24. Website
Guskos N, Triberis GP, Lykodimos V, Windsch W, Metz H, Koufoudakis A, Mitros C, Gamari‐Seale H, Niarchos D. Temperature Dependence of the EPR Spectra of Gd0.5RE0.5 Ba2Cu3O7‐δ Compounds in the Orthorhombic and Tetragonal Phases. physica status solidi (b) [Internet]. 1991;166:233-240. WebsiteAbstract
The materials Gd0.5RE0.5 Ba2Cu3O7‐δ (Re = Y, Sm, Eu, and La) are investigated in the orthorhombic (HTC) and tetragonal (non‐HTC) phases using EPR method. For both phases EPR signals associated with Gd ions are detected. In the tetragonal phase a strong temperature dependence of the intensities of this line is observed. These characteristics can be ascribed to the skin effect. A small polaron model is proposed to interpret the conduction mechanism of the tetragonal phase (semiconducting) of these samples. Copyright © 1991 WILEY‐VCH Verlag GmbH & Co. KGaA