Elucidating the Epitaxial Growth Mechanisms of Solution-Derived BiVO 4 Thin Films Utilizing Rapid Thermal Annealing

Photoelectrochemical water splitting is regarded as a promising strategy for solar energy conversion, but the efficacy of metal oxide photoelectrodes is significantly limited by suboptimal charge transport. In this regard, epitaxial BiVO4 thin films provide a suitable research platform, enabling a deeper understanding of carrier dynamics and energy relaxation pathways. In this work, epitaxial BiVO4 thin films were synthesized through a solution-based method combined with a rapid thermal annealing system. A systematic exploration of various annealing parameters, coupled with tracking the isothermal evolutions of the epitaxial quality and film morphology, establishes a comprehensive model for converting solution-derived BiVO4 films into epitaxial layers. We show that fast heating ramp rates promote the ripening of heteroepitaxial BiVO4 grains while minimizing vanadium loss at higher processing temperatures. For BiVO4 films below 60 nm thickness, the solid-state epitaxial transformation is completed in seconds; thicker films require a higher thermal budget and thus longer annealing times for zipping the grain boundaries. In contrast to most vacuum-based deposition methods, which often struggle to prevent island formation in the growth of BiVO4, this facile chemical solution deposition approach consistently produces superior structural uniformity, improved compactness, and reduced surface roughness. These capabilities are further exemplified by preparing Mo- and Co-doped BiVO4 (001) films on yttria-stabilized zirconia (YSZ), as well as the BiVO4 (010) films grown on strontium titanate (STO). Compared to BiVO4 (001) films, BiVO4 (010) films are identified as 50% more effective in harvesting photons from the solar spectrum for the photodegradation of rhodamine B. Taken together, this work expands the availability of functional oxide thin films with a well-defined growth direction, opens avenues for studying fundamental charge transport mechanisms, and assists the design of more efficient photoelectrochemical energy conversion devices.