Photoreaction of Graphene Oxide Nanosheets in Water

Graphene oxide (GO) nanosheets have many semiconducting π-conjugated sp2 domains of various sizes within an oxygenated sp3 domain. The chemical and physical properties, therefore, can be widely tuned by adjusting the degree of oxidation through photoreaction, because the semiconducting domains act as a semiconducting photocatalyst when irradiated with light of energy exceeding the band gap of the domains. In this paper, we report new photoreactions similar to photocatalytic reactions in which H2 and CO2 are evolved from an aqueous suspension of GO nanosheets under UV irradiation; these reactions are based on the photoreactions of oxygen-containing functional groups and carbon. As a result, reduced GO (rGO) nanosheets with many holes and defects were produced. We conjecture that some carbons in the holes belong to zigzag edges, CH bonds, or both and that the rGO nanosheets therefore have ferromagnetic properties. In photoelectrochemical measurements of a GO nanosheet electrode, a large anodic photocurrent was generated by the CO2 evolution reaction and a small cathodic photocurrent was generated by reduction of oxygen-containing functional groups and the H2 evolution reaction. The mechanism of the photoreaction between GO nanosheets and water is discussed in terms of band energy levels estimated from experimental results.