Unveiling the Role of Oxygen Vacancies in BiVO ₄ /TiO ₂ Thin Films for Photocatalytic Cr(VI) Reduction: DFT and Experimental Study

This study focuses on addressing the critical challenge of heavy metal pollution in water by developing an O V -BiVO 4 /TiO 2 heterojunction film to extend the light absorption range and enhance the photocatalytic activity of TiO 2 . The composite film was synthesized on an FTO substrate via a hydrothermal method. Structural characterization confirmed the uniform distribution of oxygen vacancy-rich BiVO 4 (O V -BiVO 4 ) particles on TiO 2 nanorods, forming an intimate heterostructure. The composite material exhibited a significantly enhanced visible-light absorption capability, with a narrowed band gap of 2.15 eV compared to 2.76 eV for pristine TiO 2 . Accordingly, the photoelectrochemical performance was remarkably improved, as evidenced by an increased photocurrent density and reduced charge transfer resistance. Under simulated solar irradiation, the O V -VT(20) film achieved a Cr(VI) reduction efficiency of 75.67%, which is 4.2 times higher than that of pristine TiO 2 (17.9%) and 2.1 times that of commercial P25-TiO 2 (36%). Under the coexistence system of Cr(VI) and the organic pollutant methylene blue (MB), the photocatalyst achieved an exceptionally high Cr(VI) removal efficiency of 97.1%. Excellent stability was also demonstrated, with 70.34% of the initial activity retained after multiple cycles. DFT calculations revealed that the introduced oxygen vacancies effectively modulate the electronic structure of BiVO 4, lowering its work function and thereby promoting the separation and migration of photogenerated charge carriers. The synergistic effect between oxygen vacancy engineering and heterojunction construction is identified as the key factor for the superior performance, offering a promising strategy for designing highly efficient and stable TiO 2 -based photocatalysts.