In Situ Induced Crystalline–Amorphous Heterophase Junction by K⁺ to Improve Photoelectrochemical Water Oxidation of BiVO₄

Solar water splitting is one of the most efficient technologies to produce H₂, which is a clean and renewable energy carrier. Photoanodes for water oxidation play the determining roles in solar water splitting, while its photoelectrochemical (PEC) performance is severely limited by the hole injection efficiency at the interface of semiconductor/electrolyte. To address this problem, in this research, by employing BiVO₄ as the model semiconductor for photoanodes, we develop a novel, facile, and efficient method, which simply applies K cations in the preparation process of BiVO₄ photoanodes, to in situ induce a crystalline–amorphous heterophase junction by the formation of an amorphous BiVO₄ layer (a-BiVO₄) on the surface of the crystalline BiVO₄ (c-BiVO₄) film for PEC water oxidation. The K cation is the key to stimulate the formation of the heterophase, but not incorporated in the final photoelectrodes. Without sacrificing the light absorption, the in situ formed a-BiVO₄ layer accelerates the kinetics of the hole transfer at the photoanode/electrolyte interface, leading to the significantly increased efficiency of the surface hole injection to water molecules. Consequently, the BiVO₄ photoanode with the crystalline–amorphous heterophase junction (a-BiVO₄/c-BiVO₄) exhibits almost twice the photocurrent density at 1.23 V (vs reversible hydrogen electrode) for water oxidation than the bare c-BiVO₄ ones. Such advantages from the crystalline–amorphous heterophase junction are still effective even when the a-BiVO₄/c-BiVO₄ is coated by the cocatalyst of FeOOH, reflecting its broad applications in PEC devices. We believe this study can supply an efficient and simple protocol to enhance the PEC water oxidation performance of photoanodes, and provide a new strategy for the potential large-scale application of the solar energy-conversion related devices.