Optimizing the performance of photocatalytic H 2 generation for ZnNb 2 O 6 synthesized by a two-step hydrothermal method.

Semiconductor-based photocatalytic H 2 generation is a promising technique and the development of efficient photocatalysts has attracted great attention. Columbite-ZnNb 2 O 6 is a wide-bandgap semiconductor capable of photocatalytic water splitting. Here we employed a two-step hydrothermal method to first dissolve Nb 2 O 5 with a highly basic aqueous solution and further react it with Zn 2+ to form nanosized ZnNb 2 O 6 . 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 2 generation at a rate of 23.6(5) μmol h -1 g -1 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 2 , Ag 2 O, and Pd/PdO, were loaded appropriately, the photocatalytic H 2 generation rate ultimately achieved 3200(100) or 680(20) μmol h -1 g -1 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-ZnNb 2 O 6 .