Vlassis Likodimos

Dye-sensitized solar cells based on thick highly ordered TiO2 nanotubes produced by controlled anodic oxidation in non-aqueous electrolytic media

Dye-sensitized solar cells (DSSCs) were prepared using TiO(2) nanotubes, grown by controlled Ti anodic oxidation in non-aqueous media. Smooth, vertically oriented TiO(2) 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 TiO(2) 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.

Synthesis, characterization and photocatalytic evaluation of visible light activated C-doped TiO2 nanoparticles

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 0.5 eV (2.69 eV), compared to the 3.18 eV 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 > 420 nm) 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.

Photo-induced effects on self-organized TiO2 nanotube arrays: the influence of surface morphology

Self-organized TiO(2) 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 TiO(2) 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 TiO(2) 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 TiO(2) tubular architecture.

Double-side active TiO2-modified nanofiltration membranes in continuous flow photocatalytic reactors for effective water purification.

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.

Enhanced CO2 Capture in Binary Mixtures of 1-Alkyl-3-methylimidazolium Tricyanomethanide Ionic Liquids with Water

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 (V(E)), 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 [C(n)C1im][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 [C(n)C1im][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 CO2 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(n)C1im][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.

Dye-sensitization of self-assembled titania nanotubes prepared by galvanostatic anodization of Ti sputtered on conductive glass

Self-organized porous TiO(2) nanotubes (NTs) were prepared on conductive glass by galvanostatic anodizing of sputtered titanium in an NH(4)F /glycerol electrolyte. DC magnetron sputtering at an elevated substrate temperature (500 degrees C) was used to deposit 650 nm thick titanium films. After anodizing, NTs, 830 nm long, with an average external diameter of 92 nm, were grown; this gave a high conversion rate of oxide from titanium (1.9), with a 220 nm 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 degrees 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 V(oc) (714 mV) 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 (J(sc) = 0.68 mA cm(-2)), which was confirmed further by the corresponding IPCE values (5.2% at 510 nm). The magnitude of J(sc) 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 TiO(2)/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 TiO(2) 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.

Nanostructured titania films sensitized by quantum dot chalcogenides

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.

Use of selected scavengers for the determination of NF-TiO2 reactive oxygen species during the degradation of microcystin-LR under visible light irradiation

Although UV-induced TiO2 photocatalysis involves the generation of several reactive oxygen species (ROS), the formation of hydroxyl radicals are 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 VLA NF-TiO2 photocatalytic degradation of MCLR at pH 3.0 and only partial inhibition at pH 5.7. While hydroxyl radicals 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 hydroxyl radicals (•OH) do not play the primary role 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 O2•- and H2O2 play critical roles in the degradation process. Similar degradation rates were observed in the presence of singlet oxygen scavenger, deuterium oxide, which enhances singlet oxygen mediated processes further suggesting singlet oxygen does not play a key role in the degradation of MCLR in these system. 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 •OH scavengers at pH 5.7. These results demonstrate that solution pH plays a major role in the formation and reactivities of ROS during VLA NF-TiO2 photocatalysis. The adsorption strength of the scavengers and MCLR onto NF-TiO2 as well as the speciation of the ROS as a function of pH need to be carefully considered since they also play a key role in the efficiency of the process. These results indicate the reduction of molecular oxygen by photo-generated electrons rather than hydroxyl radicals produced by oxidative reactions of photo-generated holes play a key role in the of VLA NF-TiO2 photocatalytic degradation of MC-LR.

Neutron diffraction study of Mg2FeV3O11-δ

The neutron diffraction patterns of Mg2FeV3O11-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.

Dye-sensitized solar cells incorporating novel Co(II/III) based-redox electrolytes solidified by silica nanoparticles

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.