Composite TiO2 films modified by CeO2 and SiO2 for the photocatalytic removal of water pollutants.

TiO2 particles of high photocatalytic activity immobilised on various substrates usually suffer from low mechanical stability. This can be overcome by the utilisation of an inorganic binder and/or incorporation in a robust hydrophobic matrix based on rare-earth metal oxides (REOs). Furthermore, intrinsic hydrophobicity of REOs may result in an increased affinity of TiO2-REOs composites to non-polar aqueous pollutants. Therefore, in the present work, three methods were used for the fabrication of composite TiO2/CeO2 films for photocatalytic removal of dye Acid Orange 7 and the herbicide monuron, as representing polar and non-polar pollutants, respectively. In the first method, the composition of a paste containing photoactive TiO2 particles and CeCl3 or Ce(NO3)3 as CeO2 precursors was optimised. This paste was deposited on glass by doctor blading. The second method consisted of the deposition of thin layers of CeO2 by spray coating over a particulate TiO2 photocatalyst layer (prepared by drop casting or electrophoresis). Both approaches lead to composite films of similar photoactivity that of the pure TiO2 layer, nevertheless films made by the first approach revealed better mechanical stability. The third method comprised of modifying a particulate TiO2 film by an overlayer based on colloidal SiO2 and tetraethoxysilane serving as binders, TiO2 particles and cerium oxide precursors at varying concentrations. It was found that such an overlayer significantly improved the mechanical properties of the resulting coating. The use of cerium acetylacetonate as a CeO2 precursor showed only a small increase in photocatalytic activity. On the other hand, deposition of SiO2/TiO2 dispersions containing CeO2 nanoparticles resulted in significant improvement in the rate of photocatalytic removal of the herbicide monuron.

Graphitic Carbon Nitride for Photocatalytic Air Treatment.

Graphitic carbon nitride (g-C3N4) is a conjugated polymer, which recently drew a lot of attention as a metal-free and UV and visible light responsive photocatalyst in the field of solar energy conversion and environmental remediation. This is due to its appealing electronic band structure, high physicochemical stability and earth-abundant nature. In the present work, bulk g-C3N4 was synthesized by thermal decomposition of melamine. This material was further exfoliated by thermal treatment. S-doped samples were prepared from thiourea or further treatment of exfoliated g-C3N4 by mesylchloride. Synthesized materials were applied for photocatalytic removal of air pollutants (acetaldehyde and NOx) according to the ISO 22197 and ISO 22197-1 methodology. The efficiency of acetaldehyde removal under UV irradiation was negligible for all g-C3N4 samples. This can be explained by the fact that g-C3N4 under irradiation does not directly form hydroxyl radicals, which are the primary oxidation species in acetaldehyde oxidation. It was proved by electron paramagnetic resonance (EPR) spectroscopy that the dominant species formed on the irradiated surface of g-C3N4 was the superoxide radical. Its production was responsible for a very high NOx removal efficiency not only under UV irradiation (which was comparable with that of TiO2), but also under visible irradiation.

Transparent Nanotubular TiO2 Photoanodes Grown Directly on FTO Substrates.

This work describes the preparation of transparent TiO2 nanotube (TNT) arrays on fluorine-doped tin oxide (FTO) substrates. An optimized electrolyte composition (0.2 mol dm-3 NH4F and 4 mol dm-3 H2O in ethylene glycol) was used for the anodization of Ti films with different thicknesses (from 100 to 1300 nm) sputtered on the FTO glass substrates. For Ti thicknesses 600 nm and higher, anodization resulted in the formation of TNT arrays with an outer nanotube diameter around 180 nm and a wall thickness around 45 nm, while for anodized Ti thicknesses of 100 nm, the produced nanotubes were not well defined. The transmittance in the visible region (λ = 500 nm) varied from 90% for the thinnest TNT array to 65% for the thickest TNT array. For the fabrication of transparent TNT arrays by anodization, the optimal Ti thickness on FTO was around 1000 nm. Such fabricated TNT arrays with a length of 2500 nm exhibit stable photocurrent densities in aqueous electrolytes (~300 µA cm-2 at potential 0.5 V vs. Ag/AgCl). The stability of the photocurrent response and a sufficient transparency (≥65%) enables the use of transparent TNT arrays in photoelectrochemical applications when the illumination from the support/semiconductor interface is a necessary condition and the transmitted light can be used for another purpose (photocathode or photochemical reaction in the electrolyte).

Photoanodes based on TiO2 and α-Fe2O3 for solar water splitting - superior role of 1D nanoarchitectures and of combined heterostructures.

Solar driven photoelectrochemical water splitting (PEC-WS) using semiconductor photoelectrodes represents a promising approach for a sustainable and environmentally friendly production of renewable energy vectors and fuel sources, such as dihydrogen (H2). In this context, titanium dioxide (TiO2) and iron oxide (hematite, α-Fe2O3) are among the most investigated candidates as photoanode materials, mainly owing to their resistance to photocorrosion, non-toxicity, natural abundance, and low production cost. Major drawbacks are, however, an inherently low electrical conductivity and a limited hole diffusion length that significantly affect the performance of TiO2 and α-Fe2O3 in PEC devices. To this regard, one-dimensional (1D) nanostructuring is typically applied as it provides several superior features such as a significant enlargement of the material surface area, extended contact between the semiconductor and the electrolyte and, most remarkably, preferential electrical transport that overall suppress charge carrier recombination and improve TiO2 and α-Fe2O3 photoelectrocatalytic properties. The present review describes various synthetic methods and modifying concepts of 1D-photoanodes (nanotubes, nanorods, nanofibers, nanowires) based on titania, hematite, and on α-Fe2O3/TiO2 heterostructures, for PEC applications. Various routes towards modification and enhancement of PEC activity of 1D photoanodes are discussed including doping, decoration with co-catalysts and heterojunction engineering. Finally, the challenges related to the optimization of charge transfer kinetics in both oxides are highlighted.

Photocatalytic behavior of nanosized TiO2 immobilized on layered double hydroxides by delamination/restacking process.

INTRODUCTION: Efficient immobilization of TiO(2) nanoparticles on the surface of Mg(2)Al-LDH nanosheets was performed by delamination/restacking process. EXPERIMENTAL PART: The structural and textural properties of as-prepared nanocomposite were deeply analyzed using different solid-state characterization techniques such as: X-ray powder diffraction, Fourier transform infrared spectroscopy, and Raman spectroscopies, chemical analysis, X-ray photoelecton spectroscopy, N(2) adsorption-desorption, and electronic microscopy. RESULTS AND DISCUSSION: The photocatalytic properties of immobilized TiO(2) nanoparticles on Mg(2)Al were investigated using the photodegradation of two model pollutants: Orange II and 4-chlorophenol, and compared with pure colloidal TiO(2) solution. CONCLUSION: It appears that Orange II photodegradation was systematically faster and more efficient than 4-chlorophenol photodegradation regardless of the medium pH. Moreover under slightly basic conditions, even if the TiO(2) photocatalytic efficiency decreases, photodegradation performed in presence of easily recovered TiO(2)/Mg(2)Al(1.5) nanocomposite gives rise to comparable or better results than pure TiO(2).