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.