Unravelling the charge transfer and kinetics of BiVO4 by Co dopant and NiFeOx co-catalyst for efficient photoelectrochemical water splitting†

Abstract

Understanding the kinetic mechanism for charge transfer and reaction involved in the photoelectrochemical (PEC) process of BiVO₄-based photoanodes is key to designing better photoelectrodes in solar water splitting. Herein, we demonstrate cobalt-doping-induced CoO_x quantum dots and NiFeO_x co-catalyst on BiVO₄ (Co–BiVO₄/NiFeO_x), which was uniformly constructed on a FTO substrate with a nanoporous topological network structure composed of nanoworm-like particles. The Co–BiVO₄/NiFeO_x photoanode achieved a remarkable photocurrent density of 6.42 mA cm⁻² at 1.23 V_RHE. Multiform online PEC and operando electrochemical investigation are employed to comprehensively elucidate the kinetic processes of the BiVO₄-based photoanodes. The surface-derived CoO_x quantum dots and BiVO₄ were favourable for promoting charge separation and transfer to NiFeO_x by an enhanced built-in electric field. Theoretical calculations were employed to clarify the basic mechanism in that Co doping substitutes the Bi sites, Fe sites acted as the active sites, and the OH⁻ → OH* step was the rate-determining step in the elemental oxygen evolution reaction for Co–BiVO₄/NiFeO_x. The NiFeO_x co-catalyst, with water oxidation active sites, is beneficial for hole capture and transfer through the self-adjustable valence of Fe, thereby promoting OH⁻–h⁺ interactions. Finally, we present a synergetic mechanism that elucidates how the charge transfer dynamics and surface oxygen evolution reaction kinetics are enhanced by quantum dot binding and the catalytically active states.