S-Doped ZnSnO₃ Nanoparticles with Narrow Band Gaps for Photocatalytic Wastewater Treatment

Nowadays, obtaining photocatalysts with a narrow band gap that can degrade contamination under visible light has been a hot topic in the field of environmental protection. In this report, the S-doped hierarchically structured ZnSnO3 with a high S doping ratio, a narrow band gap, and a large specific surface area is synthesized via a two-step hydrothermal method. ZIF-8 was used as the Zn source to synthesize ZnSnO3 with a hollow structure for the first time. Characterization techniques to confirm doping by S in the ZnSnO3 structure include the use of X-ray photoelectron and energy-dispersive spectroscopies. Owing to the hollow-structured ZnSnO3 precursor, S-doped ZnSnO3 demonstrated a large specific surface area (up to 80.63 m2/g) that is favorable for the strong adsorption of reactants. In addition, the S-doping ratio is as high as 90%, which is much higher than that of other related work. Because of the elevated S 3p energy level, the band gap of S-doped ZnSnO3 is rapidly decreased from 3.7 to 2.4 eV, which gives S-doped ZnSnO3 a higher efficiency in the utilization of visible light. Because of the enhanced adsorption capabilities and decreased band gap, the as-synthesized nanocomposite can be used as a high-efficiency photocatalyst for wastewater treatment. About 36% rhodamine B (RhB) is absorbed by S-doped ZnSnO3 even before 350 W Xe-lamp irradiation. After being irradiated under visible light for about 80 min, the RhB is almost completely degraded (degradation efficiency ≈90%) using S-doped ZnSnO3, which is much faster than using pure ZnSnO3 or other zinc–tin oxide-based photocatalysts. In this report, detailed discussions are also given for the synthesis process of hollow-structured ZnSnO3 and the mechanism of narrowing the band gap via S doping.