An overview of co‐catalysts on metal oxides for photocatalytic water splitting

Hydrogen (H2) generation through photoelectrochemical (PEC) water splitting is a promising approach to reducing energy crises and associated environmental problems. Among currently available semiconductors, 2D nanostructured metal oxides have attracted wide interest in PEC applications because of their low-cost synthesis routes, stability in aqueous media, unique configuration and features, and favorable band edge positions. However, metal oxides still suffer from the severe recombination of photogenerated charge carriers and the low consumption of visible light. One solution is to introduce co-catalysts to enhance photocatalytic H2 generation by improving charge separation and transfer, extending light absorption, and lowering the activation energy. This review summarizes the fundamentals and recent developments of co-catalysts on metal oxides which can be classified into the following categories: metal co-catalysts, metal sulfide co-catalysts, metal hydroxide co-catalysts, metal phosphide co-catalysts, carbon-based co-catalysts, and dual co-catalysts. Charge transfer mechanisms of co-catalysts on metal oxides in photocatalytic H2 generation and present future trends for co-catalysts and strategies in enhancing PEC water splitting are also presented.