The Importance of the Interfacial Contact: Is Reduced Graphene Oxide Always an Enhancer in Photo(Electro)Catalytic Water Oxidation?

Optimizing interfacial contact between graphene and a semiconductor has often been proposed as essential for improving their charge interactions. Herein, we fabricated bismuth vanadate-reduced graphene oxide (BiVO4/rGO) composites with tailored interfacial contact extents and revealed their disparate behavior in photoelectrochemical (PEC) and powder suspension (PS) water oxidation systems. BiVO4/rGO with a high rGO coverage on the BiVO4 surface (BiVO4/rGO HC) exhibited an 8-fold enhancement in the PEC photocurrent density with respect to neat BiVO4 at 0 V versus Ag/AgCl, while BiVO4/rGO with a low rGO coverage (BiVO4/rGO LC) gave a lesser 3-fold enhancement. In contrast, BiVO4/rGO HC delivered a detrimental effect, while BiVO4/rGO LC exhibited an enhanced performance for oxygen evolution in the PS system. The phenomenon is attributed to changes in the hydrophobicity of the BiVO4/rGO composite in conjunction with the interfacial contact configuration. A better BiVO4/rGO interfacial contact was found to improve the charge separation efficiency and charge transfer ability of the composite material, explaining the superior PEC performance of BiVO4/rGO HC. Additionally, optimization of the interfacial contact extent was revealed to further improve the energetics of the composite material, as evidenced by a Fermi level shift to a more negative potential. However, the high hydrophobicity of BiVO4/rGO HC arising from the higher rGO reduction extent triggered poor water miscibility, reducing the surface wettability and therefore hampering the photocatalytic O2 evolution activity of the sample. The study underlines water miscibility as a governing issue in the PS system.