BiVO₄ Photoanode with NiV₂O₆ Back Contact Interfacial Layer for Improved Hole-Diffusion Length and Photoelectrochemical Water Oxidation Activity

The short hole diffusion length (HDL) and high interfacial recombination are among the main drawbacks of semiconductor-based solar energy systems. Surface passivation and introducing an interfacial layer are recognized for enhancing HDL and charge carrier separation. Herein, we introduced a facile recipe to design a pinholes-free BiVO₄ photoanode with a NiV₂O₆ back contact interfacial (BCI) layer, marking a significant advancement in the HDL and photoelectrochemical activity. The fabricated BiVO₄ photoanode with NiV₂O₆ BCI layer exhibits a 2-fold increase in the HDL compared to pristine BiVO₄. Despite this improvement, we found that the front surface recombination still hinders the water oxidation process, as revealed by photoelectrochemical (PEC) studies employing Na₂SO₃ electron donors and by intensity-modulated photocurrent spectroscopy measurements. To address this limitation, the surface of the NiV₂O₆/BiVO₄ photoanode was passivated with a cobalt phosphate electrocatalyst, resulting in a dramatic enhancement in the PEC performance. The optimized photoanode achieved a stable photocurrent density of 4.8 mA cm^-2 at 1.23 V_RHE, which is 12-fold higher than that of the pristine BiVO₄ photoanode. Density Functional Theory (DFT) simulations revealed an abrupt electrostatic potential transition at the NiV₂O₆/BiVO₄ interface with BiVO₄ being more negative than NiV₂O₆. A strong built-in electric field is thus generated at the interface and drifts photogenerated electrons toward the NiV₂O₆ BCI layer and photogenerated holes toward the BiVO₄ top layer. As a result, the back-surface recombination is minimized, and ultimately, the HDL is extended in agreement with the experimental findings.