Microstructure and performance of Z-scheme photocatalyst of silver phosphate modified by MWCNTs and Cr-doped SrTiO^sub 3^ for malachite green degradation

Visible-light-driven photocatalysis as a promising technology has recently attracted great attention in environmental remediation. In this work, novel Z-scheme heterojunction photocatalysts Ag3PO4@MWCNTs@Cr:SrTiO3 with excellent visible-light-driven photocatalytic performance and photostability were successfully synthesized and characterized. The photocatalytic activity for malachite green (MG) degradation was measured, and photodegradation mechanisms were investigated using GC–MS, ESR, and radical trapping experiments. On consideration of the practical applications, the effects of coexisting ions and pH were also evaluated. Results showed that significant changes of crystal size and micro-morphology for Ag3PO4 were observed before and after the addition of MWCNTs, which transformed from a polyhedron with a diameter of about 30 μm into a spherical-like crystal with a diameter of 0.28–0.69 μm. This phenomenon was reported for the first time. The optimal catalyst with a dosage of 100 mg/g (the mass ratio of pollutant to photocatalyst) could reach 100% MG removal in 6 and 10 min under natural solar radiation and visible light irradiation, respectively. Results from the radical trapping experiments and ESR analysis confirmed that there existed the Z-scheme transfer mechanisms, and .O2− and h+ played an important role during the photocatalytic degradation. Several small molecular organic acids such as acetic acid, glyoxylic acid, fumaric acid and benzoic acid were detected by GC–MS in the photodegradation products of MG, which further indicates that photocatalytic degradation pathway involved N-demethylation, benzene removal and open-ring reactions. The performance of the photocatalyst would be inhibited under strong acid condition or at the coexistence of certain concentration of Cr6+ and Cl−. The novel addition of MWCNTs in the preparation and the changes of crystal structure and properties for the photocatalysts have the potential to promote the application of the photocatalysts for environmental remediation.