Z-Scheme BiVO4/g-C3N4 Photocatalyst—With or Without an Electron Mediator?

Abstract

The hybrid system BiVO4/g-C3N4 is a prospective photocatalyst because of the favorable mutual alignment of the energy bands of both semiconductors. However, the path of the photocatalytic process is still unclear because of contradictory information in the literature on whether the mechanism of charge carrier separation at the BiVO4/g-C3N4 interface is band-to-band or Z-scheme. In this work, we clarified this issue by comparative photocatalytic studies with the use of systems without a mediator and with different kinds of mediators including Au nanoparticles, fullerene derivatives, and the Fe3+/Fe2+ redox couple. Additionally, the charge transfer dynamics at the BiVO4/g-C3N4 and BiVO4/mediator/g-C3N4 interfaces were investigated by time-resolved photoluminescence (TRPL) measurements, while the influence of the mediator on the surface recombination of the charge carriers was verified by intensity-modulated photocurrent spectroscopy (IMPS). We proved that the charge carrier separation at the BiVO4/g-C3N4 interface occurs according to the mechanism typical for a heterojunction of type II, while the incorporation of the mediator between BiVO4 and g-C3N4 leads to the Z-scheme mechanism. Moreover, a very strong synergetic effect on caffeine (CAF) degradation rate was found for the system BiVO4/Au/g-C3N4 in the presence of Fe3+ ions in the CAF solution.