Defect-rich sunlight-responsive SnO2 photocatalyst for methyl orange dye degradation: a step towards wastewater treatment

This study examines the potential of defect-engineered pure SnO₂ nanoparticles as efficient photocatalysts for wastewater treatment under sunlight irradiation. Two distinct SnO₂ nanosystems are synthesized through a facile and cost-effective sol–gel approach. Defects are deliberately introduced into one of the samples by altering the annealing process. After the formation of SnO₂ gel, this sample undergoes rapid cooling between two consecutive annealing stages of 2 h and 4 h at 100 ℃ under vacuum conditions respectively. In contrast, the other sample is subjected to conventional annealing in a hot air environment at 400 ℃ for 24 h. The structural and optical properties of these samples are meticulously characterized using X-ray diffraction, UV–visible spectroscopy, TEM, FTIR, BET and EPR respectively. Subsequently, the photocatalytic performance of both SnO₂ nanoparticle samples is evaluated by degrading a methyl orange solution, which serves as a model contaminant for textile industry wastewater under sunlight exposure. The defect-engineered SnO₂ sample demonstrates a remarkable improvement in photocatalytic efficiency, degrading 82.51% of methyl orange with a rate constant of 0.02886 min⁻¹ under 60 min of sunlight irradiation. In comparison, the standard SnO₂ sample degrades only 29.71% of methyl orange, with a significantly lower rate constant of 0.00575 min⁻¹ under the same irradiation conditions. This improvement is attributed to the increased number of defect sites (oxygen vacancy), surface area (59.877 m²/gm) and narrow optical bandgap (2.85 eV), which enhance light absorption and generate more active excitons, thus accelerating the degradation process. The novelty of this research lies in the successful enhancement of photocatalytic performance without the need for doping or composite formation, making it a cost-effective and scalable approach for wastewater treatment applications.