Molybdenum-Based Co-catalysts in Photocatalytic Hydrogen Production: Categories, Structures, and Roles.

Photocatalytic hydrogen production by using solar energy has attracted great interest around the world. The main challenges are the high costs of the photocatalysts and the low efficiency of photocatalytic hydrogen production. Co-catalysts, as crucial components of photocatalysts, are usually used to stimulate photoexcited electron transfer from the light absorber to the surface, and they also catalyze the proton-reduction reaction to form H2 in water. However, most co-catalysts used in photocatalytic hydrogen production are noble metals, which are expensive and contradict the low-costs demanded by industry. Therefore, the development of earth-abundant and cheap co-catalysts to replace noble metals is necessary for photocatalytic H2 production. This account highlights the performance and roles of molybdenum-based non-noble metal co-catalysts in photocatalytic hydrogen production. We developed a series of inexpensive and efficient molybdenum-based co-catalysts. We demonstrated that more H2 could be produced by loading Mo-based co-catalysts on CdS by using the co-precipitation method than by using traditional Pt/CdS same under the same photocatalytic conditions. The molybdenum-based co-catalysts were able to form heterojunctions, which served as bridges to facilitate the transport and separation of photogenerated charges; moreover, the molybdenum-based co-catalysts were able to accept and store photoexcited electrons owing to their large specific capacitance. The stored photoelectrons could then be released according to proton-reduction processes to form H2 . Furthermore, the molybdenum-based co-catalysts were found to have metastable state structures and multiple valence states, which provided more active sites and effectively catalyzed the production of H2 and inhibited the reverse reaction. The discovery of Mo-based co-catalysts with superior properties will provide guidance for the design of new co-catalysts.