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

Abstract

A photocatalyst which has extensively been studied so far is TiO2 with a 3.0-3.2eV band gap. Well-known photocatalysts with visible-light response are only Pt/CdS and WO3. 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 H2 and O2 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 LiTaO3, the photocatalytic activities were drastically increased. Among the tantalates, NiO/NaTaO3 showed the highest activity. Moreover, the activity of NiO/NaTaO3 was improved by La-doping. On the other hand, highly crystalline BiVO4 powders with scheelite (monoclinic) and zircon type (tetragonal) structure were selectively synthesized by an aqueous process. The BiVO4 powder with the scheelite structure showed a high photocatalytic activity for O2 evolution in the presence of sacrificial reagent (Ag+) under visible light irradiation (λ>420nm). The photocatalytic activity of the BiVO4 powder prepared by the aqueous process was much higher than that of BiVO4 prepared by a conventional solid state reaction. Zn0.957Cu0.043S (band gap: 2.5eV) and Zn0.999Ni0.001S (band gap: 2.3eV) photocatalysts showed high activities for H2 evolution from an aqueous K2SO3 and Na2S solution under visible light irradiation without co-catalysts such as Pt. ZnNb2O6, Bi2W2O9, Bi2WO6, and Bi3TiNbO9 consisting of ions with d10 and s2 configuration were also active for H2 or O2 evolution from aqueous solutions containing sacrificial reagents.