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TiO 2 doped by different contents of indium was prepared by the sol-gel method by using titanium(IV) tetrabutoxide and indium chloride as precursors. It was revealed that a unique chemical species, O-In-Cl x (x ) 1 or 2), existed on the surface of the indium doped TiO 2 . The surface state energy level attributed to the surface O-In-Cl x species was located at 0.3 eV below the conduction band of TiO 2 . The transition of electrons from the valence band of TiO 2 to the surface state energy level was responsive to visible light. The photogenerated carriers generated under visible light irradiation can be efficiently separated by the surface state energy level of the O-In-Cl x species and the valence band of TiO 2 to contribute to the photocatalytic reaction. Consequently, the indium doped TiO 2 showed improved photocatalytic activity for photodegradation of 4-chlorophenol compared to pure TiO 2 under visible light irradiation.

The TiO2-N-x%WO3 composite photocatalysts were prepared by introducing WO3 into nitrogen-doped TiO2. The composite catalysts present much higher photocatalytic activity than TiO2 and nitrogen-doped TiO2 under both ultraviolet and visible light irradiation. Diffuse reflectance UV-vis spectra, XPS analysis, and IR spectra show that the coordinated nitrogen species (or N-metal-O linkages) may contribute to the visible light photocatalytic activity. WO3 coupling increases the active nitrogen species and thus enhances the visible light activity of the composite photocatalysts. The superior activity of TiO2-N-x%WO3 composite photocatalysts upon UV light irradiation can be rationalized in terms of efficient charge separation and high adsorption affinity of WO3.

Titanium dioxide nanoparticles were prepared via a photoassisted sol-gel method in which ultraviolet light irradiation was used in the preparation process of TiO2 colloid. After characterization by X-ray diffraction, X-ray absorption near-edge structure (XANES) at the Ti K-edge, laser Raman spectroscopy, X-ray photoelectron spectroscopy, and transmission electron microscopy, it was found that the amorphous titania nanoparticles prepared by a photoassisted sol-gel method can be transformed into crystalline anatase phase at lower calcination temperature compared to those prepared by a conventional sol-gel method. In addition, the particle size distribution of anatase powder samples is also affected by UV illumination on the colloid. It is suggested that UV illumination can induce the formation of oxygen vacancies on the colloid and this results in the accelerated phase transition from amorphous to anatase titania.

Plasma-enhanced chemical vapor deposition (PECVD) was used in the preparation of a series of bicomponent TiO2/SnO2 particulate films. The photocatalytic activities of the films are evaluated by photodegradation of phenol in solution. When both the TiO2 and SnO2 components of the bicomponent TiO2/SnO2 catalysts are accessible to reactants at the catalyst surface, photocatalytic degradation efficiencies are improved as compared to those obtained with TiO2 films. When this condition prevails, the two components are believed to act in a cooperative manner by increasing the degree of charge carrier separation sufficient to reduce recombination, while simultaneously allowing sufficient time for photoelectrons and photoholes on the catalyst surface to form reaction intermediates (for example, the superoxide radical ion, O2 ·-, formed by reaction of O2 with photoelectrons, and the phenol radical, formed by reaction of phenol with photoholes or OH· radicals) which cooperatively participate in later stages of the degradation process.

WO3 and TiO2 colloids were synthesized by the forced hydrolysis technique, and different amounts of the TiO2 colloids were added to WO3 colloids under sonication to get stable combined TiO2 and WO3 colloids (WO3/TiO2). The experimental results indicate that the UV-light coloration of WO3 colloids can be improved greatly after the TiO2 combination. When the molar ratio of TiO2 and WO3 is about 1:40, ΔOD at 900 nm for WO3/TiO2 colloids is about 12.7 times that of WO3 colloids. The improvement effect increases with the increased concentration of TiO2 in the composite colloids, and the maximum enhancement can increase more than 50-fold. It is shown that after the combination, the recombination of photogenerated carriers is suppressed efficiently, and more electrons can be trapped within the band gap of WO3 to contribute to the coloration process.

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A new type of composite film (heterostructure) with optoelectronic properties have been prepared by coupling Sn-doped rutile TiO 2 (R-TiO 2 -Sn) and N-doped anatase TiO 2 (A-TiO 2 -N) with use of a sol-gel method. Under visible and UV light irradiation, it exhibits a higher photocatalytic activity than both R-TiO 2 -Sn and A-TiO 2 -N films due to the formation of a heterojunction at the interface, as well as the increase of total amount of photogenerated charge carriers and introduction of doping states. If R-TiO 2 -Sn is the outmost layer, moreover, the composite film shows a much higher photodegradation capability of HCHO than that when A-TiO 2 -N is the outmost layer. Our results offer a paradigm for developing optoelectronic functional materials that can be used in many fields, such as solar cells, photocatalysis, and photosynthesis.