Toxic dye removal, remediation, and mechanism with doped SnO2-based nanocomposite photocatalysts: A critical review

Abstract

Heterogeneous photo catalysis is considered to be a sustainable solution for treating organic pollutants in wastewater. Tin oxide (SnO2) has received immense attention from researchers due to its excellent photocatalytic activity, low cost, thermal stability, and resistance to photo erosion. The structural properties of SnO2, different strategies for doping of SnO2, organic degradation mechanisms, and optimization of operational parameters for enhanced photocatalysis were critically analyzed. Photocatalytic activity of pristine SnO2 was enhanced by doping with metal oxide-based semiconductor materials, metals (transitional and earth), and non-metals. Doped SnO2 exhibits higher photocatalytic efficiency than pristine SnO2 due to the enhanced charge carrier separation, reduced electron-hole pair recombination, higher surface area, and lower band-gap energy. Green synthesized TiO2 doped SnO2 exhibited reduced band gap energy of 2.8 eV, and degraded 96 % MB within 75 min under visible light irradiation. The lowest bandgap energy for transitional metal-doped SnO2 was achieved by Mn-doping on SnO2 with a bandgap of ∼2.48 eV, whereas Cu-SnO2 and pure SnO2 have bandgap energies of 3.67 eV and ∼3.75 eV, respectively. Copper chromite spinel nanoparticles (CuCr2O4) doped SnO2 with a band gap energy of 1.39 eV degraded crystal violet (CV) dye completely at neutral pH. Gadolinium (Gd) doped SnO2 particles showed the highest surface area (58 m2/g) which was almost double the pristine SnO2 particles. The degradation of organic dyes by doped-SnO2 depended on initial pH, catalyst dosage, pollutants concentration, dose, light intensity, etc. For the degradation of cationic dye (MB), approximately 50 % more degradation was found at basic pH than at acidic pH utilizing pristine SnO2 nanoparticles. On the contrary, about 20 % more degradation was found for anionic dye (Congo Red) degradation at acidic pH compared to basic pH. Moreover, optimization of catalyst dosage can result in about 50 % more degradation of pollutants. The ZnS-doped SnO2 photocatalysts have shown an increased rate constant of photocatalytic reaction by 24.5 times when the concentration was reduced from 30 mg/L to 5 mg/L. This review also assessed the future research directions to develop sustainable organic pollutants-based wastewater using SnO2.