Publications by Year: 2014

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.
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, 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.
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.
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.
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
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.
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.
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.
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.
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.