A comparative study on the photocatalytic activity and formation mechanism of nanostructured Cu2ZnSnS4 prepared by thermal and mechano-thermal methods

In this paper, a nanostructured Cu2ZnSnS4 (CZTS) was synthesized from elemental constituents through two simple methods: thermal and mechano-thermal. The thermal analysis was applied to determine the appropriate heat treatment conditions. Wet ball milling was employed in mechano-thermal synthesis to obtain a uniform activated powder, and the optimum conditions were investigated. The X-ray diffraction (XRD) and Raman results confirmed the formation of kesterite structure CZTS after annealing the mixture at 550 °C for 1 h in thermal synthesis, while the annealing temperature for CZTS formation is reduced to 410 °C in mechano-thermal synthesis. For mechano-thermally synthesized sample, the scanning electron microscopy (SEM) and the transmission electron microscopy (TEM) results showed the agglomerated powders with average particle sizes of 123 and 139 nm, respectively. The surface and optical properties of samples were compared through Brunauer-Emmett-Teller (BET), diffusion reflectance spectroscopy (DRS), and photoluminescence (PL) spectroscopy. DRS results demonstrated considerable absorbance over the entire range of visible light wavelengths with bandgap energies of about 1.48–1.51 eV. The photocatalytic activities of synthesized samples, degradation kinetics and mechanisms, and the effect of SnO2 phase formation have been investigated through photodegradation of methylene blue (MB) dye under visible light irradiation. The optimum mechano-thermally synthesized sample as the photocatalyst led to 94% degradation after 120 min. Mechanism explained by scavenger studies revealed that holes were the main reactive radicals responsible for the degradation process.