Graphene quantum dots mediated charge transfer of CdSe nanocrystals for enhancing photoelectrochemical hydrogen production

We demonstrated the use of CdSe/graphene quantum dot (QD) nanoheterostructures as the photoanode for remarkable photoelectrochemical hydrogen production. By employing a delicate hydrothermal cutting approach, reduced graphene oxide (RGO) sheets with the lateral size in a desirable range can be obtained, from micrometer size (micro-RGO), to 30-100 nm (nano-RGO), and to 2-4 nm (QD-RGO). Because of the significant zigzag edge effect, nano-RGO and QD-RGO possessed well-defined band structure which enabled efficient light absorption and distinctive photoluminescence emission. Time-resolved photoluminescence spectra showed that nano-RGO and QD-RGO surpassed micro-RGO in enhancing the charge separation efficiency of CdSe. According to the cyclic voltammetry data, a type-II vectorial charge transfer model was considered for CdSe/nano-RGO and CdSe/QD-RGO nanoheterostructures, fundamentally different from the unidirectional electron transfer mechanism of CdSe/micro-RGO. Among the three CdSe/RGO samples tested, CdSe/QD-RGO achieved the highest photocurrent generation in the photoelectrochemical cell, which exceeded 5 times the value of CdSe. The incident photon-to-electron conversion efficiency (IPCE) spectra suggested that the significantly enhanced photoactivity of CdSe/QD-RGO originated from the type-II vectorial charge transfer feature, which not only promoted charge carrier separation but also improved the overall light harvesting. Furthermore, no appreciable decay of photocurrent was found for CdSe/QD-RGO after continuously used in the photoelectrochemical cell for over 2 h, revealing its substantially high stability during the water reduction process. The demonstrations from this work may facilitate the use of graphene QDs in semiconductor-based photocatalysis, in which the efficient light harvesting and high chemical inertness of graphene QDs can be well employed. (C) 2014 Elsevier B.V. All rights reserved.