Elucidating the Epitaxial Growth Mechanisms of Solution-Derived BiVO4 Thin Films Utilizing Rapid Thermal Annealing

Photoelectrochemical water splitting is regarded as a promising strategy for solar energy conversion. Nevertheless, the overall energy conversion efficiency achieved by metal oxide photoelectrodes has shown significant limitations because of their poor charge transport. In this regard, epitaxial BiVO4 thin film provides a suitable research platform for acquiring insights into the carrier dynamics and energy relaxation pathways. In this work, epitaxial BiVO4 thin films were prepared through a solution-based method combined with a rapid thermal annealing system. By systematically examining a series of annealing parameters, such as annealing temperature and heating ramp rate, and tracking the isothermal evolutions of epitaxial quality and film morphology, we establish a comprehensive model for transforming solution-derived BiVO4 films into epitaxial layers. In particular, the tendency of losing vanadium from BiVO4 at higher processing temperatures imposes an optimal annealing duration for obtaining highly compact films without pinholes. For BiVO4 films below 60 nm thickness, the solid-state epitaxial transformation is achieved in seconds; thicker films require a higher thermal budget and, thus, longer annealing times for the coarsening of heteroepitaxial grains to complete. The capability of this deposition method is exemplified by Mo- and Co-doped BiVO4 (001) films on YSZ (001), as well as the undoped BiVO4 (010) films grown on STO (001). The structural uniformities of these samples, prepared by the facile metal-organic decomposition method, are higher than the films prepared by vacuum-based techniques such as pulsed-laser deposition. We also identify the BiVO4 (010) film as more effective in harvesting photons from the solar spectrum than the BiVO4 (001) film. Taken together, this work expands the availability of functional oxide thin films with well- defined growth direction, opens avenues for studying fundamental charge transport mechanisms, and assists the design of more efficient photoelectrochemical energy conversion devices.