Discovery of Overcoating Metal Oxides on Photoelectrode for Water Splitting by Automated Screening

We applied an automated semiconductor synthesis and screen system to discover overcoating film materials and optimize coating conditions on the BiVO₄/WO₃ composite photoelectrode to enhance stability and photocurrent. Thirteen metallic elements for overcoating oxides were examined with various coating amounts. The stability of the BiVO₄/WO₃ photoelectrode in a highly concentrated carbonate electrolyte aqueous solution was significantly improved by overcoating with Ta₂O₅ film, which was amorphous and porous when calcined at 550 °C. The photocurrent for the water oxidation reaction was only minimally inhibited by the presence of the Ta₂O₅ film on the BiVO₄/WO₃ photoelectrode

Cs-Modified WO₃ Photocatalyst Showing Efficient Solar Energy Conversion for O₂ Production and Fe (III) Ion Reduction under Visible Light

Cs-modification effects of WO₃ on photocatalytic O₂ evolution and Fe (III) ion reduction over WO₃ under visible light irradiation were investigated. WO₃ having cation-exchange ability at the surface was successfully prepared by hydrothermal and impregnation methods using cesium aqueous solutions. The photocatalytic activity of Cs-modified WO₃ was partially improved by the ion-exchange of Cs⁺ for H⁺ and Fe²⁺, and more than 10 times higher than that of WO₃ without any treatment. The optimized WO₃ showed 48 times higher quantum efficiency (19% at 420 nm) than that reported previously under visible light, and showed a high solar-to-chemical energy conversion efficiency (η_sun = 0.3%). This η_sun value is comparable to the solar-to-product energy conversion efficiencies of natural plant photosynthesis for biomass energy

Photocatalytic Reduction of Carbon Dioxide over Ag Cocatalyst-Loaded ALa₄Ti₄O₁₅ (A = Ca, Sr, and Ba) Using Water as a Reducing Reagent

Ag cocatalyst-loaded ALa₄Ti₄O₁₅ (A = Ca, Sr, and Ba) photocatalysts with 3.79–3.85 eV of band gaps and layered perovskite structures showed activities for CO₂ reduction to form CO and HCOOH by bubbling CO₂ gas into the aqueous suspension of the photocatalyst powder without any sacrificial reagents. Ag cocatalyst-loaded BaLa₄Ti₄O₁₅ was the most active photocatalyst. A liquid-phase chemical reduction method was better than impregnation and in situ photodeposition methods for the loading of the Ag cocatalyst. The Ag cocatalyst prepared by the liquid-phase chemical reduction method was loaded as fine particles with the size smaller than 10 nm on the edge of the BaLa₄Ti₄O₁₅ photocatalyst powder with a plate shape during the CO₂ reduction. CO was the main reduction product rather than H₂ even in an aqueous medium on the optimized Ag/BaLa₄Ti₄O₁₅ photocatalyst. Evolution of O₂ in a stoichiometric ratio (H₂+CO:O₂ = 2:1 in a molar ratio) indicated that water was consumed as a reducing reagent (an electron donor) for the CO₂ reduction. Thus, an uphill reaction of CO₂ reduction accompanied with water oxidation was achieved using the Ag/BaLa₄Ti₄O₁₅ photocatalyst