A combined experimental and computational study of the photocatalytic activity of titanium dioxide (TiO 2 ) for the degradation of nitrogen(II) oxide (NO) and acetaldehyde is reported. We employ five different TiO 2 photocatalysts including pure anatase and rutile samples. The experimental photonic efficiencies indicate that, under visible irradiation, the samples containing pure TiO 2 are active for the decomposition of NO but inactive for the acetaldehyde degradation. This is in accordance with our theoretical predictions, which reveal the presence of weak absorption bands in the visible region of the absorption spectra of the TiO 2 -NO complexes. We demonstrate that these bands originate from charge-transfer excitations between the pollutant and the substrate. Although a ligand-to-metal charge-transfer process is expected to predominate, we find a competing mechanism in which one electron is promoted from the valence band of the semiconductor to the virtual π * states of NO. Both experimental and theoretical results show an enhanced vis-activity of anatase TiO 2 compared to rutile.We observe from the theoretical simulations the formation of reactive monocoordinated oxygen atoms at the anatase (001) surface for moderately low concentrations of the contaminant.Based on these findings, a new mechanism for the photo-oxidation of NO is proposed.

New TiO2/WO3 films were produced by the layer-by-layer (LbL) technique and successfully applied as self-cleaning photocatalytic surfaces. The films were deposited on fluorine doped tin oxide (FTO) glass substrates from the respective metal oxide nanoparticles obtained by the sol-gel method. Thirty alternative immersions in pH = 2 TiO2 and pH = 10 WO3 sols resulted in ca. 400 nm thick films that exhibited a W(VI)/Ti(IV) molar ratio of 0.5, as determined by X-ray photoelectron spectroscopy. Scanning electron microscopy, along with atomic force images, showed that the resulting layers are constituted by aggregates of very small nanoparticles (<20 nm) and exhibited nanoporous and homogeneous morphology. The electronic and optical properties of the films were investigated by UV-vis spectrophotometry and ultraviolet photoelectron spectroscopy. The films behave as nanoscale heterojunctions, and the presence of WO3 nanoparticles caused a decrease in the optical band gap of the bilayers compared to that of pure LbL TiO2 films. The TiO2/WO3 thin films exhibited high hydrophilicity, which is enhanced after exposition to UV light, and they can efficiently oxidize gaseous acetaldehyde under UV(A) irradiation. Photonic efficiencies of ξ = 1.5% were determined for films constituted by 30 TiO2/WO3 bilayers in the presence of 1 ppm of acetaldehyde, which are ∼2 times higher than those observed for pure LbL TiO2 films. Therefore, these films can act as efficient and cost-effective layers for self-cleaning, antifogging applications.

The photocatalytic properties of titanium dioxide (TiO2 ) layers on different metal plates are investigated. The metal-semiconductor interface can be described as a Schottky contact, and is part of a depletion layer for the majority carriers in the semiconductor. Many researchers have demonstrated an increase in the photocatalytic activity, due to the formation of a metal-semiconductor contact that are obtained by deposition of small metal islands on the semiconductor. Nevertheless, the influence of a Schottky contact remains uncertain, sparking much interest in this field. The immobilization of nanoparticulate TiO2 layers by dip-coating on different metal substrates results in the formation of a Schottky contact. The recombination rate of photoinduced electron-hole pairs decreases at this interface provided that the thickness of the thin TiO2 layer has a similar magnitude to the depletion layer. The degradation of dichloroacetic acid in aqueous solution and of acetaldehyde in a gas mixture is investigated to obtain information concerning the influence of the metal work function of the back contact on the efficiency of the photocatalytic process.

The photocatalytic properties of titanium dioxide layers on metal sheets prepared via cold gas spraying have been investigated employing the ISO 22197‐2 standard test for acetaldehyde degradation in the gas phase. The contact between the TiO2 and the metal substrate can be described as a Schottky contact. A depletion region is formed at the semiconductor–metal interface leading to an enhanced separation of the photoinduced charge carriers. Thus, the lifetime of the e–/h+ pair is prolonged. The variation of process parameters, such as temperature and pressure, during the CGS preparation leads to different photocatalytic properties of the obtained TiO2 layers. Under high pressure and temperature even visible light active TiO2 coatings are prepared. The CGS technique allows the formation of thin TiO2 films without any binders exhibiting high photocatalytic activities in the same range as the pure powders employed for their preparation. Schematic diagram presenting the Schottky contact of an n‐type semiconductor and met al. Ef is the Fermi level, VB and CB are the valence and conduction band of the semiconductor, respectively. A proposed degradation pathway of acet‐aldehyde at the solid/gas interface is depicted. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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