In Situ Activation of Amorphous NiFeMo Oxide Cocatalyst To Improve the Photoelectrochemical Water Splitting Performance of BiVO₄

Amorphous NiFeMo oxides (a-NiFeMo) synthesized via a simple supersaturated coprecipitation method are explored as a cocatalyst to improve the photoelectrochemical water splitting performance of BiVO4. The performance of the composite photoelectrode is found to be further enhanced through a cyclic-voltammetry-related in situ activation process, and a photocurrent of 5.0 mA/cm2 is achieved at 1.23 VRHE with improved photoelectrochemical stability. X-ray photoelectron spectroscopy (XPS), Raman, and electron spin resonance (ESR) characterization results indicate that the surface-loaded a-NiFeMo cocatalyst undergoes a rapid surface reconstruction during the in situ activation process, which results in an incorporation of phosphate ions in the surface-loaded a-NiFeMo, along with an increased oxidation state of Ni ions and enriched oxygen vacancies. These combined effects lead to an improved oxygen evolution reaction performance and finally result in a reduced charge-transfer resistance at the solid/liquid interface, causing the interfacial charge-injection efficiency to be enormously enhanced from the original 25.1% in pure BiVO4 to 83.3% in BiVO4/a-NiFeMo after being in situ activated. Our result indicates that the rapid surface-reconstruction phenomenon shown in the amorphous materials may provide a promising strategy for designing a highly efficient cocatalyst for photoelectrodes.