Defect engineering and opening of the ion tracks in the swift heavy ion irradiated thin films of bismuth vanadate: impact on the oxygen evolution reaction for solar water splitting†

Swift heavy ion (SHI) irradiation (Xe ions, 150 MeV, 5 × 10⁹ to 5 × 10¹¹ ions per cm²) is utilized to engineer the defect landscape in hydrothermally synthesized BiVO₄ (BVO) thin films, aiming to understand its role in photoelectrochemical (PEC) performance toward the oxygen evolution reaction (OER). Our findings show that SHI irradiation, from individual to overlapping ion tracks, induces residual stress and amorphization in BVO, accompanied by the formation of bismuth-rich hillocks above oxygen-depleted ion tracks. While high fluence irradiation results in the irreversible reduction of PEC activity, the lower fluences (5 × 10⁹ ions per cm² and 1 × 10¹⁰ ions per cm²) induce defects that initially trap charge carriers, but over time lead to a 58.6% and 25.2% increase in the photocurrent density, respectively. Detailed post-PEC morphological analysis reveals opening of ion tracks and the formation of nanoscale holes, reaching up to 30 nm in diameter and up to 200 nm in depth. Our study establishes a link between defect creation and PEC performance in BVO thin films, paving the way for innovative approaches to its morpho-structural manipulation and nano-structuring while simultaneously contributing to the fundamental understanding of SHI-induced phenomena in BVO films.