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 CoOx quantum dots and NiFeOx co-catalyst on BiVO4 (Co–BiVO4/NiFeOx), which was uniformly constructed on a FTO substrate with a nanoporous topological network structure composed of nanoworm-like particles. The Co–BiVO4/NiFeOx photoanode achieved a remarkable photocurrent density of 6.42 mA cm−2 at 1.23 VRHE. Multiform online PEC and operando electrochemical investigation are employed to comprehensively elucidate the kinetic processes of the BiVO4-based photoanodes. The surface-derived CoOx quantum dots and BiVO4 were favourable for promoting charge separation and transfer to NiFeOx 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/NiFeOx. The NiFeOx 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.