Polyoxometalate-loaded reduced graphene oxide-modified metal vanadate catalysts for photoredox reactions through an indirect Z-scheme mechanism

Abstract

The growing energy demand and environmental concerns have accelerated research on the emergence of photocatalysts for solar fuel generation and environmental remediation. Metal vanadates, such as silver vanadate (AV) and copper vanadate (CV), are considered promising visible-light active photocatalysts owing to their narrow bandgap and suitable band structure; however, they are limited by rapid electron–hole recombination. To overcome this limitation, amalgamation with polyoxometalate (POM)-loaded reduced graphene oxide (RGO)-based novel co-catalysts is a facile strategy to improve photocatalytic performance. Herein, metal vanadates were deposited on polyoxometalate-loaded reduced graphene oxide (RPOM) via a one-pot coprecipitation method. The developed RPOM–AV and RPOM–CV composites exhibited photocurrent densities of 223.7 and 85.8 μA cm⁻², which were 51 times and 6 times higher than those of pristine AV and CV, respectively, owing to the remarkable augmentation in the donor density after formation of composites. Moreover, the RPOM–AV composites exhibited photocatalytic Cr(VI) reduction of up to 94% in 60 minutes with a high rate constant of 0.044 min⁻¹ and 94% removal of the rose bengal dye in 120 minutes through adsorption. The RPOM–CV composites demonstrated 96% photocatalytic degradation of methylene blue dye at a rate constant of 0.011 min⁻¹. The excellent photocatalytic activity of RPOM–metal vanadate composites was attributed to the formation of an indirect Z-scheme heterojunction between metal vanadates and POM, in which RGO acted as a suitable electron-mediator, facilitated the charge transfer, boosted the separation of photogenerated charge carriers, and lowered the electron–hole recombination. The present work provides an innovative approach toward the development of polyoxometalate-based composites for wastewater remediation.