Enhanced visible light photocatalytic degradation of acetaminophen with Ag₂S-ZnO@rGO core-shell microsphere as a novel catalyst: Catalyst preparation and characterization and mechanistic catalytic experiments

Ag₂S-ZnO@rGO core-shell structured microspheres were prepared through a very simple, novel and green in-situ method using dimethyl sulfoxide as both the solvent and sulfur S source at low temperature (140 °C). The photocatalytic performance of synthesized samples was tested for degradation of acetaminophen (ACT) under visible light irradiation. ACT removal efficiency was directly depended on the GO-level in composite and the maximum photocatalytic yield was achieved for the sample with GO concentration of 0.2 g/L. Formation of Ag₂S in the nanocomposite was confirmed using XRD, XPS and UV–Vis DRS and photoluminescence (PL) measurement. XRD patterns pointed to the existence of a wurtzite structure for all samples and by silver loading, Ag₂S peaks were visualized in the XRD patterns, with increased intensity by rising the Ag content in the solution precursor. EDS and XPS data indicated the presence of sulfur and silver in the form of Ag⁺ and S²⁻. A considerable red shift in absorption edge and reduce in PL emission peak intensity was observed over the introduction of Ag in the precursor solution due to the existence of Ag₂S. The Ag₂S-ZnO@rGO composites indicated enhanced photocatalytic performance toward degradation of ACT under visible light compared to bare ZnO and ZnO@rGO. The underlying mechanism has been investigated based on the results of UV–Vis DRS, PL and reactive species scavenging experiments. The improved photocatalytic activity is mainly ascribed to better light harvesting and increased photogenerated electron-hole separation. Ag₂S play an important role in the composite due to the synergistic effect of Ag₂S and rGO, which not only improve the light harvesting but also reduce the charge recombination. The as-made catalyst indicated a great potential for recycling, suggesting the potentially large-scale applications of prepared catalyst for emerging water contaminants and wastewater treatment. The cytotoxicity of untreated and photocatalytic oxidation (PCO)-treated ACT solutions was evaluated using the cultured human embryonic kidney (HEK) cells, which revealed that ACT solution in the developed PCO system could be significantly detoxified.