Cocatalysts for photocatalysis: Comprehensive insight into interfacial charge transfer mechanism by energy band theory

Abstract

Cocatalysts have played an important role to reach high-efficiency photocatalysis with semiconductor photocatalysts. Various compositions and microstructures of inorganic and organic nanomaterials have been proposed as cocatalysts, however, consistent understanding on charge transfer mechanism at heterogenous interfaces has not been achieved yet. In this review, the existing cocatalysts are first re-sorted in terms of composition and band structure into (semi)metals, metal compounds (narrow- and wide-bandgap), nonmetals and hybrids, focusing on the underlying mechanism for charge transfer pathway at various cocatalyst–semiconductor interfaces according to energy band theory. Significantly, the polarity of Schottky contact and Ohmic contact for a semiconductor-(semi)metal heterojunction is highlighted, wherein Schottky junctions for electron transfer can be deemed as Ohmic junctions for hole transfer, and vice versa. The specific charge transfer pathway depends on the type of semiconductors, magnitude of interfacial band bending barrier, as well as that whether the metal has plasmic resonance effect. Furthermore, the function of cocatalysts (reduction- or oxidation-type) is distinguished in terms of charge transfer pathway, and consistent understanding on the roles of narrow-bandgap metal-compound cocatalysts is achieved, no matter they are deemed as semimetals or semiconductors in previous studies. Finally, the comprehensive insight into charge transfer pathways at various semiconductor-cocatalyst interfaces is obtained, which provides universal criteria to investigate carrier kinetics within heterojunction photocatalysts.