Light-Induced Photoactivation Driven by Oxygen Vacancy Reconfiguration in Direct Current-Sputtered SnO ₂ Thin Films

Oxygen vacancy (VO) engineering provides a powerful approach to tailor the optoelectronic properties of metal-oxide photoanodes by modulating the defect states, surface chemistry, and photoelectrochemical (PEC) activity. This study investigates light-induced photoactivation and defect-state reconfiguration in DC-sputtered SnO2 films prepared under varied O2 flow rates (Ar: O2 = 20:1, 20:2, 20:3 sccm; denoted DC–1, DC–2, and DC–3). XPS analysis reveals that as-sputtered DC–1 and DC–2 films incorporate Sn0, SnO, and SnO2 phases, while DC–3 consists primarily of SnO and SnO2. Upon illumination, DC–2 exhibits progressive photocurrent enhancement of photoactivation (∼4 times the initial), whereas DC–1 and DC–3 display an initial photocurrent spike, followed by decay. EPR analysis confirms illumination-induced VO+ formation, indicating oxygen lattice participation in O2 evolution. UV–vis and UPS characterization show illumination passivates defects, reducing the Urbach energy (EU) in DC–3 and band tail states in DC–1 while narrowing the DC–1 band gap concomitant with Sn4+ to Sn2+ reduction. The pronounced photocurrent spike and rapid decay in DC–1 are attributed to rapid Sn2+/adsorbed OH– oxidation, followed by deep–trap recombination and passivation reduced adsorbs. In contrast, progressive photoactivation of DC–2 stems from photoinduced formation of shallow donor-like states (VO+), defect passivation, and a narrow and small band gap. Although DC–3 initially achieves the highest photocurrent, it suffers decay due to the conversion of VO+ and band gap widening. PL, EPR, and XPS analyses directly link VO charge state redistribution and Sn valence changes to radiative versus nonradiative recombination pathways. Distribution of relaxation times (DRT) analysis further resolves defect-specific relaxation processes. These findings elucidate the dynamic roles of oxygen vacancies during PEC operation and establish the defect-state balance as a critical design parameter for stable, high-performance oxide photoanodes.