Publications by Year: 2012

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.
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..
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.
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.
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.
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.
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, 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.
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.