Optimizing the performance of photocatalytic H generation for ZnNbO synthesized by a two-step hydrothermal method.

Semiconductor-based photocatalytic H generation is a promising technique and the development of efficient photocatalysts has attracted great attention. Columbite-ZnNbO is a wide-bandgap semiconductor capable of photocatalytic water splitting. Here we employed a two-step hydrothermal method to first dissolve NbO with a highly basic aqueous solution and further react it with Zn to form nanosized ZnNbO. The reaction time plays an important role on its morphology and photocatalytic performance in water reduction. The sample synthesized through 7 days of reaction was the optimal one with an appropriate crystallinity and a large specific surface area, however the severe surficial defects prohibited its photocatalytic activity in pure water. The H generation at a rate of 23.6(5) μmol h g emerged when 20 vol% methanol was used as the hole-sacrificial agent. Most remarkably, once metal or metal oxide cocatalysts, including Pt, Au, NiO, RuO, AgO, and Pd/PdO, were loaded appropriately, the photocatalytic H generation rate ultimately achieved 3200(100) or 680(20) μmol h g with or without using methanol, respectively. Apparent quantum yields (AQYs) at 295 nm were investigated by changing the experimental parameters, and the optimal AQYs are 4.54% and 9.25% in water and methanol solution, respectively. Further post-modifications like bandgap engineering may be performed on this highly efficient nano-ZnNbO.