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

Abstract

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 BiVO 4 thin films provide a suitable research platform, enabling a deeper understanding of carrier dynamics and energy relaxation pathways. In this work, epitaxial BiVO 4 thin films were synthesized through a solutionbased 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 BiVO 4 films into epitaxial layers. We show that fast heating ramp rates promote the ripening of heteroepitaxial BiVO 4 grains while minimizing vanadium loss at higher processing temperatures. For BiVO 4 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 BiVO 4 , 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 BiVO 4 (001) films on yttria-stabilized zirconia (YSZ), as well as the BiVO 4 (010) films grown on strontium titanate (STO). Compared to BiVO 4 (001) films, BiVO 4 (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.