Activation of Ni₂V₂O₇to nonstoichiometric NiV₃O₈for solar-driven photoelectrochemical water oxidation

Hydrogen treatment is an effective strategy to introduce non-stoichiometry and improve the performance of semiconductor nanomaterials for photocatalytic applications. Herein, for the first time, we report the hydrogen treatment activation of non-photoactive nickel vanadate (NV) towards photoelectrochemical (PEC) water oxidation. The NV nanostructures are synthesized by a sonication-assisted hydrothermal method. Hydrogen-treated nickel vanadate (H-NV) nanoflakes were achieved by subjecting the NV to anneal at 350 °C for 4 h in a hydrogen atmosphere. Field emission scanning electron microscopy (FESEM), and High resolution transmission electron microscopy (HRTEM) indicates irregular network morphology for the as-synthesized NV nanostructures, calcined at 500 °C (NV-500). The XRD pattern confirms monoclinic and orthorhombic crystal phases for NV-500 and hydrogen treated nickel vanadate (H-NV-500), respectively. The defects states produced on the surface of NV-500 after high dose of electrons which implies the creation of oxygen vacancies and consistent with the XPS analysis for H-NV-500. The optical properties of H-NV-500 via UV-Vis/DRS show significant absorption in the visible regime at ? = 628 nm with an indirect bandgap value of 2.15 eV from Tauc's plot and supports by density functional theory (DFT) studies. The PEC water oxidation performance has onset potential at ~0.74 V. The photocurrent density at saturation is ~2.13 µA/cm2 for H-NV-500 photoanode. These results suggest that hydrogen treatment effectively activated NV towards PEC water oxidation process, which was initially inactive.