Fabrication of TiO2/C3N4 heterostructure for enhanced photocatalytic Z-scheme overall water splitting

Z-scheme overall water splitting based on semiconductors has been extensively investigated for hydrogen fuel production from renewable resources. The Z-scheme involves two different kinds of semiconductor photocatalysts and the closely related oxidation-reduction processes. However, the two separated photocatalytic processes require good production and separation of carriers, which greatly limit the efficiency of overall water splitting. Herein, to solve the recombination problem of photogenerated charge carriers, we developed a one-step hydrothermal process to synthesize TiO2/C3N4 heterojunctions. The layered g-C3N4 provides the template and guidance for the anatase TiO2 heterogeneous nucleation with (001) facet exposed. Meanwhile, the bulk g-C3N4 turned into thicker nanosheets through the self-exfoliation with the growth of TiO2. The heterojunctions with TiO2 (001) facet exposed ensure the efficient separation of photogenerated carriers, which accordingly enhances the photocatalytic hydrogen evolution. WO3 and BiVO4 are chosen as photocatalysts for the half reaction of oxygen evolution. With suitable redox mediators i.e. I−/IO3− or Fe2+/Fe3+, the overall water splitting to H2 and O2 could be achieved. If β-Ni(OH)2, was loaded on the oxygen evolution photocatalysts, the efficiency of Z-scheme water splitting was greatly enhanced. Under the monochromatic light irradiation of LEDs, the apparent quantum efficiency (AQE) values for the Z-scheme of TiO2/C3N4, β-Ni(OH)2/WO3(PtOx) and I−/IO3− system were 4.94% and 4.01% under 365 and 405 nm, respectively. The heterojunction strategy for enhancing Z-scheme overall water splitting may give us some hints on the design of composite systems for overall water splitting.