Structure and activity relationship of rGO–V2O5/Nb composites in the photocatalytic removal of dyes

In this study, Nb-doped V2O5/rGO composites were synthesized via the hydrothermal method, with reduced graphene oxide contents ranging from 0 to 15% to evaluate their photocatalytic performance in the degradation of methyl orange (MO), rhodamine B (RhB), and malachite green (MG) under UV light. XRD analysis with Rietveld refinement confirmed the stabilization of the orthorhombic alpha-V2O5 phase, where the presence of Nb2O5 signals and a shift in diffraction peaks evidenced lattice expansion induced by doping. Furthermore, XPS analysis revealed the coexistence of mixed oxidation states (V4+/V5+) and the formation of oxygen vacancies, which are critical for charge transfer. Morphological characterization by SEM showed a structural evolution from microplates in the rGO-free material to a porous network of interconnected nanosheets at 15% rGO. Although thermal stability improved with rGO content, the 5% rGO composite exhibited the optimal photocatalytic activity, achieving removal efficiencies exceeding 90% for RhB and MG. The degradation of malachite green was found to be strongly pH-dependent, suggesting that electrostatic interactions govern the adsorption mechanism. These results point to a synergistic effect between Nb-doping and the rGO network, promoting an efficient Z-scheme charge transfer mechanism.