Development of Photocatalyst Materials for Water Splitting with the Aim at Photon Energy Conversion

摘要

A photocatalyst which has extensively been studied so far is TiO_2 with a 3.0-3.2 eV band gap. Well-known photocatalysts with visible-light response are only Pt/CdS and WO_3. Thus, photocatalyst materials mainly employed have been so limited. In such a background, new photocatalysts with high activity have recently been developed. Alkali and alkaline earth tantalates have arisen as a new group of photocatalyst materials for water splitting into H_2 and O_2 under ultra-violet light irradiation. They showed the activities even without co-catalysts such as Pt, being different from titanate photocatalysts. When NiO co-catalysts were loaded on tantalate photocatalysts, except for LiTaO_3, the photocatalytic activities were drastically increased. Among the tantalates, NiO/NaTaO_3 showed the highest activity. Moreover, the activity of NiO/NaTaO_3 was improved by La-doping. On the other hand, highly crystalline BiVO_4 powders with scheelite (monoclinic) and zircon type (tetragonal) structure were selectively synthesized by an aqueous process. The BiVO_4 powder with the scheelite structure showed a high photocatalytic activity for O_2 evolution in the presence of sacrificial reagent (Ag~+) under visible light irradiation (#lambda# >420 nm). The photocatalytic activity of the BiVO_4 powder prepared by the aqueous process was much higher than that of BiVO_4 prepared by a conventional solid state reaction. Zn_(0.957)Cu_(0.043)S (band gap: 2.5 eV) and Zn_(0.999_Ni_(0.001)S (band gap: 2.3 eV) photocatalysts showed high activities for H_2 evolution from an aqueous K_2SO_3 and Na_2S solution under visible light irradiation without co-catalysts such as Pt. ZnNb_2O_6, Bi_2W_2O_9, Bi_2WO_6, and Bi_3TiNbO_9 consisting of ions with d~(10) and s~2 configuration were also active for H_2 or O_2 evolution from aqueous solutions containing sacrificial reagents.