Heating-rate controlled defect engineering in BiVO4 thin films for enhanced photoelectrochemical water splitting

Abstract

We investigate the effect of nucleation kinetics, governed by ramping rate of annealing temperature, on the structural, chemical, and optical properties of BiVO₄ thin films and their subsequent impact on photoelectrochemical (PEC) performance, with the goal of enhancing the photocatalytic activity of BiVO₄-based photoanodes. Here, we demonstrate that controlled thermal nucleation, achieved by tuning the annealing heating rate, is an effective and scalable strategy to engineer oxygen vacancies (O_v) and modulate defect chemistry in BiVO₄ thin films. Structural and spectroscopic analyses reveal that an optimized heating rate promotes the partial reduction of V⁵⁺ to V⁴⁺, generating O_v donor states that narrow the optical band gap from 2.55 to 2.33 eV and increase carrier density. First-principles DFT calculations demonstrate that these oxygen vacancies introduce V⁴⁺ derived shallow donor states, providing theoretical validation for the experimentally observed bandgap reduction and enhanced charge transport. Atomic force microscopy (AFM) and scanning electron morphology (SEM) reveals an evolution in surface morphology with increased surface roughness and porosity, which promote electron transport and bulk conductivity. These synergistic effects result in a remarkable photocurrent density of 3.17 mA cm⁻² at 1.23 V vs. the reversible hydrogen electrode (RHE) and excellent stability for ∼4.5 hours in aqueous electrolyte. Overall, this study highlights controlled thermal nucleation as a robust strategy to tune microstructure, defect chemistry, and charge transport, paving the way for next-generation high-efficiency PEC materials.