▪ Abstract  Recent research trends of the preparation and characterization of highly efficient titanium oxide–based photocatalysts are reviewed on the basis of studies done in our laboratory. Special attention is focused on the preparation and characterization of active sites of highly dispersed titanium oxide species within mesoporous matrices and on titanium oxide–based binary catalysts such as Ti/Si and Ti/B oxides and unique visible light-responsive second-generation TiO2 photocatalysts prepared by an advanced metal-ion-implantation and RF magnetron sputtering deposition method. Highly dispersed titanium oxide species within mesoporous frameworks and titanium oxide–based binary catalysts exhibit high and unique photocatalytic activities for various reactions, such as the direct decomposition of NOx. These species also have photo-induced superhydrophiric properties. It is of significance that visible light-responsive TiO2 photocatalysts can be applied to the decomposition of NOx in air and the degradation of pollutants in water, as well as to the splitting of water under UV and visible light irradiation.

A Ru complex-incorporated Ti-based MOF (Ti-MOF-Ru(tpy)2) has been synthesised by using a bis(4'-(4-carboxyphenyl)-terpyridine)Ru(ii) complex (Ru(tpy)2) as an organic linker. Ti-MOF-Ru(tpy)2 promotes photocatalytic hydrogen production from water containing a sacrificial electron donor under visible-light irradiation up to 620 nm.

A catalytically competent Cu species has been immobilized within the framework of a Zr-based metal−organic framework with bipyridine units, Zr-MOF-bpy, by a simple post synthetic modification method from CuBr2 (Zr-MOF-bpy-CuBr2) and used for the selective oxidation of cyclooctene to cyclooctene oxide. Zr-MOF-bpy was synthesized by a simple solvothermal method and was shown to have a UiO-type structure. Diffuse reflectance UV-vis and XAFS measurements have revealed that the immobilized Cu species has a square planar geometry of two N atoms and two Br atoms. Zr-MOF-bpyCuBr2 catalyzed the selective oxidation of cyclooctene to cyclooctene oxide with high activity and selectivity in the presence of tert-butyl hydroperoxide as an oxidant. In addition, the catalytic ability of Zr-MOF-bpy-CuBr2 was demonstrated to be superior to that of the corresponding homogeneous catalyst ((bpy)CuBr2). It was also confirmed that Zr-MOF-bpy-CuBr2 can be reused as a heterogeneous catalyst without significant loss of its activity and selectivity.

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Titanium−silicon binary oxide thin films of differing TiO2 content were prepared by using an ionized cluster beam (ICB) deposition method using multi-ion sources under a dry preparation process. From the results of UV−vis absorption measurements, binary oxide thin films with low TiO2 content showed much higher transmittance as compared with the pure TiO2 thin film, indicating that the Ti−oxide species of these Ti−Si binary oxide thin films exist in a highly dispersed state within the SiO2 matrices. Ti K-edge XAFS measurements revealed that such binary oxide thin films with low TiO2 content (less than 20%) showed only a sharp preedge peak attributed to the highly dispersed Ti−oxide species which are different from the peaks of anatase TiO2 thin films. Moreover, they were found to exhibit much higher and more efficient photocatalytic reactivity for the decomposition of NO into N2 and O2 under UV light irradiation than the pure TiO2 thin films.