In situ urea-assisted self-reduction strategy for constructing oxygen-vacancy-rich chlorine-doped SnO2 with enhanced visible-light photocatalytic activity.

The rational design of defect-engineered metal oxide photocatalysts via mild and sustainable routes remains a key challenge in environmental remediation. Herein, we report a novel in situ urea-assisted self-reduction strategy for the one-pot synthesis of chlorine-doped SnO2 nanocrystals enriched with oxygen vacancies. By employing a confined dual-crucible configuration, thermal decomposition of urea at 300 °C generates a localized reducing microatmosphere, which simultaneously promotes the substitution of lattice oxygen by residual chlorine from the tin hydroxy chloride precursor and induces the formation of high-density oxygen vacancies. Comprehensive characterization reveals that the synergistic effect of chlorine doping and oxygen vacancies progressively narrows the bandgap from 3.50 to 2.76 eV, extends the photoresponse into the visible region, and significantly enhances photogenerated charge separation and interfacial charge transfer. As a result, the optimized sample achieves 97.5% degradation of methyl orange (20 mg/L) within 45 min under visible-light irradiation, with superoxide radicals identified as the predominant reactive species. This work presents a facile and scalable approach for the controlled synthesis of defect-rich metal oxides, offering new insights into the design of high-performance photocatalysts for water purification.