Direct Z-scheme nickel vanadate/phosphotungstic acid nanocomposites for concurrent photocatalytic water purification and CO₂ reduction

The co-dependent challenges of environmental remediation and solar energy conversion necessitate the development of highly efficient, noble-metal-free photocatalytic systems, particularly those engineered to overcome rapid charge recombination while preserving high redox potential. Herein, we report the construction of an intimate PTA/Ni₃V₂O₈ (Phosphotungstic Acid/Nickel Vanadate) nanocomposite via a facile hydrothermal method, aiming to leverage the multi-electron redox properties of PTA as a highly efficient charge mediator. Structural and optical characterization confirmed the formation of a robust heterojunction with enhanced visible-light harvesting capabilities. Under simulated solar irradiation, the optimized PTA10/Ni₃V₂O₈ composite demonstrated remarkable dual-functional performance: achieving 94.4% degradation of Methylene Blue (MB) with a kinetic rate constant ~ 7.2-fold higher than that of pristine Ni₃V₂O₈. Furthermore, the composite exhibited superior CO₂ photoreduction activity, doubling the yield of valuable liquid fuels (HCOOH and HCHO) in 6 h. Detailed mechanistic analyses, encompassing photoluminescence (PL) spectroscopy and radical trapping experiments, validated the direct Z-scheme charge transfer mechanism. This pathway successfully spatialized charge carriers, confirmed by PL quenching, and preserved their potency, evidenced by the prevalence of highly oxidizing holes (h⁺ on the PTA HOMO) and potent reducing species (^•O₂^- derived from the Ni₃V₂O₈ CB). This work establishes PTA as an effective interfacial promoter for vanadate-based materials, presenting a sustainable, high-performance Z-scheme composite for advanced environmental and energy applications.