Role of Alkali Metal in BiVO₄Crystal Structure for Enhancing Charge Separation and Diffusion Length for Photoelectrochemical Water Splitting

Alkali metal (Na or K) doping in BiVO4 was examined systematically for enhancing bulk charge separation and transport in addition to improving charge transfer from the surface. The alkali metal-doped BiVO4 thin film photoanodes having nanostructured porous grain surface morphology exhibited better photocurrent density than pristine BiVO4. In particular, Na:BiVO4/Fe:Ni/Co-Pi photoanode showed a significantly improved photocurrent of 3.2 ± 0.15 mA·cm-2 in 0.1 M K2HPO4 electrolyte at 1.23 VRHE under 1 sun illumination. The depth-dependent Doppler broadening spectroscopy measurements confirmed the significant reduction in Bi- and V-based defect density with Na metal doping, and this led to a higher bulk diffusion length of charge pairs (four times that of the pristine one). Na doping led to reduced surface defects resulting in improved surface charge transfer based on cyclic voltammetry experiments. The density functional theory calculations confirmed the improved performance in Na-doped BiVO4 photoanodes achieved through interband formation and reduction in the band gap.