The origin of annealing atmosphere-dependent defect formation and photocathodic behaviour in BiFeO₃thin films

Bismuth ferrite (BiFeO₃) offers a unique blend of advantages—made of earth-abundant materials, capable of harnessing visible light (λ ≤ 600 nm), and the prospect of piezocatalysis due to its inherent ferroelectric behaviour. Despite these advantages, the inadequate photocatalytic performance of BiFeO₃ is attributed to charge carrier recombination/trapping processes, promoted by defects formed upon varying the synthesis parameters. This work investigates the effect of the annealing atmosphere (argon or ambient air) on structural/optoelectronic properties and photoelectrochemical water splitting. Though optical absorption and the electrochemical surface area remained virtually similar, almost four times enhancement in photocathodic current density is observed for BiFeO₃ annealed in argon compared to that annealed in ambient air. This intriguing observation is rationalised based on the mutually correlated complex defect chemistry, Fermi level position, band bending profile, and efficiency of the interfacial charge transfer process. The interaction of Bi (due to its volatile nature) with its surrounding gas during the annealing process eventually determines the overall optoelectronic properties of BiFeO₃. Results offer a simple and facile yet effective strategy to engineer defects by just controlling the annealing atmosphere without doping or complicated post-synthesis processes. Discussed mechanistic insights shed light on the vulnerable nature of defects in BiFeO₃ and tuning them rationally for enhancing the solar fuel production.