Catalytic thermal decomposition of residual solvent on ZnO promotes defect-driven visible-light photocatalysis

Abstract

Defect engineering is a powerful strategy to activate wide-bandgap semiconductors, yet controlling the formation of specific vacancies remains challenging. Here, we introduce a general, solvent-directed approach to engineer defects, demonstrated with ZnO. By selecting methanol as the synthesis solvent and performing thermal annealing under nitrogen, the in-situ decomposition of the solvent generates a local reductive environment that selectively creates zinc-vacancy-related surface defect states (VZn-related). These defects, absent in ethanol-derived ZnO, enable sub-bandgap absorption and improved charge separation, leading to enhanced visible-light photocatalytic activity. While solvent effects on morphology and defect populations have been occasionally noted, leveraging the catalytic decomposition of the solvent itself as a design principle for controlled, vacancy-centered defect formation has not, to our knowledge, been demonstrated. Comprehensive spectroscopic analyses, including steady-state and time-resolved photoluminescence, steady-state and time-resolved electron paramagnetic resonance, and positron annihilation spectroscopy, elucidate the nature, dynamics, and photoactivity of these vacancies. Under visible-light irradiation (λ > 395 nm), methanol-derived ZnO achieves up to a twofold increase in p-nitrophenol degradation compared to untreated samples. This work establishes a simple, dopant-free, and scalable route to defect engineering via solvent selection, offering a broadly applicable strategy for activating wide-bandgap semiconductors under visible light.