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

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.