Charge transfer and synergistic mechanism of polydopamine-modified oxygen-vacancy-containing TiO2 in light-driven reduction of hexavalent chromium.

Hexavalent chromium (Cr(VI)) is a widespread heavy metal pollutant in industrial wastewater that is highly carcinogenic and deleterious due to bioaccumulation. Detoxifying Cr(VI) to the less-toxic Cr(III) requires a synergistic adsorption-reduction mechanism. In this study, titanium dioxide (TiO2)-based composite photocatalysts (X-PNT) with polydopamine (PDA) and engineered oxygen vacancies (OVs) were developed to explore interfacial processes. Characterization results showed that PDA formed a bridge structure between TiO2 particles, thereby creating charge transfer pathways. The band gap width of the photocatalyst after incorporating PDA narrowed by 0.16 eV compared to pure TiO2, and the photocurrent was enhanced by 2.43 times. Density functional theory calculations showed that PDA modification improved the electronic structure and adsorption capacity of TiO2. In photocatalytic experiments, the 0.1PNT composite achieved a Cr(VI) removal efficiency of 68.88 %, with light-driven reduction contributing 60.68 %. Quenching experiments confirmed direct electron transfer as the primary reduction pathway. Two-dimensional correlation spectroscopy (2DCOS) revealed a sequential reaction pathway, suggesting that Cr(VI) removal is a multistep synergistic process. This study provides valuable insights into PDA functionalization and OV engineering, facilitating the design and development of highly efficient polymer-metal oxide composites for Cr(VI) detoxification.