Optimizing the performance of photocatalytic H₂ generation for ZnNb₂O₆ synthesized by a two-step hydrothermal method

Semiconductor-based photocatalytic H₂ generation is a promising technique and the development of efficient photocatalysts has attracted great attention. Columbite-ZnNb₂O₆ is a wide-bandgap semiconductor capable of photocatalytic water splitting. Here we employed a two-step hydrothermal method to first dissolve Nb₂O₅ with a highly basic aqueous solution and further react it with Zn²⁺ to form nanosized ZnNb₂O₆. 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 H₂ 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, RuO₂, Ag₂O, and Pd/PdO, were loaded appropriately, the photocatalytic H₂ 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-ZnNb₂O₆.