Basic Principles for Observing the Photosensitizer Role of Graphene in the Graphene-Semiconductor Composite Photocatalyst from a Case Study on Graphene-ZnO

The graphene-ZnO (GR-ZnO) nanocomposites have been fabricated via a facile, low-temperature in situ wet chemistry process, during which the reduction of graphene oxide and intimate interfacial contact between ZnO nanoparticles and GR nanosheets are achieved simultaneously. The GR-ZnO nanocomposites exhibit visible light photoactivity toward reduction of Cr(VI) in aqueous solution under ambient conditions. The large band gaps of GR ZnO composites suggest that the visible light irradiation cannot photoexcite electrons in the valence band (VB) to conduction band (CB) of ZnO, thus ruling out the possibility that the visible light photoactivity of GR-ZnO is induced by the band gap photoexcitation of ZnO. Instead, under visible light irradiation, the GR sheet in GR-ZnO can be photoexcited from ground-state GR to excited-state GR* to generate electrons, which then inject into the CB of ZnO to make the GR-ZnO composites exhibit visible light photoactivity. Our results provide new robust proof of the role of GR as a macromolecular photosensitizer for semiconductors. Along with the fact that the photosensitization efficiency of GR is generally low, we propose that three basic principles are required to experimentally observe the exact photosensitizer role of GR in the GR-semiconductor composites under visible light. We hope that this work could boost further study on the photosensitizer role of GR and on how to improve the photosensitization efficiency of GR and inform ongoing interest in deep thinking of the microscopic charge carrier transfer pathway across the interface between GR and the semiconductor.