Enhanced photocatalytic Cr(Ⅵ) reduction over the Z-scheme ZnO/ZnIn2S4 composite under visible light

Abstract

In the present study, a series of ZnO/ZnIn2S4 photocatalysts with different mass ratios were synthesized for Cr(VI) removal. The surface morphology and chemical composition analysis results showed a well-distributed presence of ZnO nanoparticles on the surface of ZnIn2S4 with self-assembled microporous or mesoporous flower-like structure. The Mott−Schottky diagram indicated compatibility between the band structures of ZnO and ZnIn2S4. Moreover, the maximum Cr(VI) removal efficiency was achieved with 5% ZnO/ZnIn2S4 at pH 4.0, 1.6-fold and 14.5-fold higher than pristine ZnIn2S4 and ZnO, respectively. The pseudo-first-order kinetic model effectively simulated the Cr(VI) removal by ZnO/ZnIn2S4. The enhanced performance of ZnO/ZnIn2S4 was mainly attributed to the interaction between ZnO and ZnIn2S4, resulting in a Z-scheme heterojunction that improved visible-light absorption and charges separation efficiency. Fourier transform infrared and X-ray photoelectron spectroscopy indicated a partial reduction of Cr(VI) to Cr(III). At the same time, TR-PL tests demonstrated longer showed that the photogenerated carrier lifetimes in the compared heterojunction materials were higher than those of ZnO and ZnIn2S4. Free radical capture and electron paramagnetic resonance (EPR) experiments further confirmed that photogenerated electrons (e−) and superoxide radicals (·O2−) were the primary active species in the photoreduction of Cr(VI). These results highlight the potential of ZnO/ZnIn2S4 Z-scheme heterojunction as an efficient photocatalyst for wastewater treatment under visible light irradiation.