Illustration of charge transfer in graphene-coated hexagonal ZnO photocatalysts using Kelvin probe force microscopy

Yunlong Zhang; Yuzhi Zhang; Lixin Song; Yang Su; Yunfeng Guo; Lingnan Wu; Tao Zhang

doi:10.1039/c7ra12037k

Illustration of charge transfer in graphene-coated hexagonal ZnO photocatalysts using Kelvin probe force microscopy

RSC Adv. 2018 Jan 3;8(2):885-894. doi: 10.1039/c7ra12037k. eCollection 2018 Jan 2.

Authors

Yunlong Zhang^{1

2}, Yuzhi Zhang¹, Lixin Song¹, Yang Su², Yunfeng Guo^{1

3}, Lingnan Wu¹, Tao Zhang¹

Affiliations

¹ The Key Laboratory of Inorganic Coating Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences 1295 Dingxi Road Shanghai 200050 China lxsong@mail.sic.ac.cn yzzhang@mail.sic.ac.cn.
² University of Chinese Academy of Sciences Beijing 100049 China.
³ School of Materials Science and Engineering, Shanghai University No. 99 Shangda Road Shanghai 200444 China.

Abstract

A graphene coated hexagonal ZnO (HZO@Gr) with enhanced activity in photocatalysis was synthesized. However, the photoinduced charge transfer behavior and the beneficial role of graphene in promoting photocatalytic reactions have not been sufficiently investigated experimentally. In this paper, the surface potentials of the ±(0001)-polar plane of HZO (Zn-polar plane and O-polar plane), graphene, graphene/Zn-polar plane and graphene/O-polar plane were measured using Kelvin probe force microscopy (KPFM). On the basis of the KPFM results, the respective Fermi levels were calculated and the internal electric field (IEF) of HZO was confirmed. Taking the IEF of HZO into consideration, the three-dimensional band diagrams of the HZO@Gr composites in methyl blue (MB) solution in the dark and under UV-visible irradiation after equilibrium were proposed. Accordingly, it is found that there could emerge different interactions between graphene and HZO at the ±(0001)-polar plane of HZO. Furthermore, the photogenerated holes and electrons tend to migrate to opposite directions. With the participation of graphene and IEF, the composites show a decrease in possibility of charge recombination. As a result, the active groups, namely ˙OH and ˙O₂ ^- radicals, could be mainly generated at/near the O-polar plane and Zn-polar plane, respectively. This work can serve as a supplemental explanation of the charge transfer during the photocatalytic process at the polar ZnO/graphene composite surface.