From photocorrosion to photoactivity: sulfite-gated self-etching of BiVO₄ into a black Bi-O phase

Photocorrosion has long been viewed as the primary failure mode limiting BiVO₄ photoanodes. Herein, we redefine it as a programmable pathway for activation. We demonstrate that sulfite anions, beyond acting as hole scavengers, can precisely gate a light-driven self-etching/reconstruction. This converts a 1 cm² BiVO₄ photoanode into a black Bi-O phase under 5-sun illumination. Mechanistic studies reveal that sulfite oxidation generates transient Bi-O motifs, which, in concert with cleavage of the V-O framework, lead to an oxygen-vacancy-rich Bi-O network. The reconstructed black phase exhibits a spectrum-wide absorption uplift and a measurable photothermal response under high-flux excitation, consistent with enhanced non-radiative dissipation and potential plasmon-related contributions from minor Bi⁰ species. After a 12-hour in situ reconstruction, the resulting nanoporous, disordered network delivers a stable photocurrent density of over ∼12 mA cm^-2 at 1.4 V vs. RHE under 5-sun illumination. This work establishes photocorrosion not as a problem to be solved, but as an anion-directed engineering tool, opening a general avenue of "corrosion engineering" or designing high-performance oxide photoelectrodes.