Nanocrystalline brookite-type titanium(IV) oxide photocatalysts prepared by a solvothermal method: correlation between their physical properties and photocatalytic activities

Nanocrystalline brookite-type titanium(IV) oxide (TiO2) powder was synthesized by solvothermal treatment of oxobis(2,4-pentanedionato- O, O') titanium in a 1,2-ethanediol-water system in the presence of sodium acetate followed by hydrothermal treatment at 373 K in order to remove organic moieties contaminating the powder. The powder was calcined at various temperatures to change its physical properties and then used for three types of photocatalytic reaction: mineralization of acetic acid ( AcOH) in an aerated aqueous suspension of bare TiO2 powder, evolution of molecular hydrogen from 2-propanol in an aqueous suspension of in situ platinized powder and formation of molecular oxygen (O-2) from silver sulfate in a deaerated aqueous suspension of bare TiO2 powder. Dependence of the photocatalytic activities on calcination temperature (T-c) and correlations with the physical properties of brookite- type TiO2 samples were examined. In the case of mineralization of AcOH, the uncalcined brookite TiO2 sample having the largest surface area showed the highest rate of carbon dioxide evolution (R-CO2), which was equal to that of representative commercial TiO2 (Degussa P-25), and R-CO2 decreased monotonously with T-c, i.e., with decrease in surface area. On the other hand, in the case of O-2 formation, the photocatalytic activity was enhanced by calcination at a higher temperature, despite the simultaneous decrease in surface area. Overall, the effects of calcination on the photocatalytic activities for the three reaction systems strongly suggested that photocatalytic activity of brookite- type TiO2 depends on two significant factors, adsorbability and recombination probability, corresponding to the specific surface area and crystallinity, respectively, and that the balance of these two factors determines T-c dependence.