Unprecedented solar water splitting of dendritic nanostructured Bi₂O₃ films by combined oxygen vacancy formation and Na₂MoO₄ doping

We demonstrate the synergetic effect of Na₂MoO₄-doping and vacuum-annealing on dendritic nanostructured bismuth oxide (Bi₂O₃) thin films prepared by electrodeposition for visible-light-assisted photoelectrochemical (PEC) water oxidation. After evaluating various extents of Na₂MoO₄-doping as well as vacuum-annealing temperatures, it was evidenced that both Na₂MoO₄-doping and vacuum-annealing significantly improved the efficiency and PEC water oxidation performance. Compared to the undoped Bi₂O₃ photoanode, the optimized Na₂MoO₄-doped Bi₂O₃, after vacuum-annealing, resulted in more than 25-fold enhancement in the photoanodic current density to 1.06 mA/cm² at 1.23 V_RHE under AM1.5 G illumination. The PEC enhancement is credited mainly to the increased PEC surface active sites in the Na₂MoO₄-doped vacuum annealed sample. Confirmed by combined XPS and Mott-Schottky (M − S) analysis, vacuum annealing resulted in surface oxygen vacancies that can contribute to the photocatalytic activity. Besides, Na₂MoO₄-doping resulted in reduced dimensions of the dendritic structure, revealed by FE-SEM and XRD measurements, resulting in larger surface area and, therefore, larger surface/electrolyte contact. This dual strategy (metal doping + vacuum annealing) can be generalized to assemble photoanodes of other materials used for the production of solar fuels. Our results make a valuable step towards efficient Bi₂O₃/BiVO₄ pn heterojunctions.