Oxygen Vacancy-Induced Strong Coordination of Carbon Dots with TiO₂ for Enhanced Photocatalytic Hydrogen Production

Semiconductor nanomaterials decorated with low-cost and environmentally benign carbon dots (CDs) as cocatalysts have recently attracted increasing attention for a variety of photocatalytic reactions, driven by the main principles of circular economy and practical applications. Despite this progress, a mechanistic understanding of how semiconductor surface interacts with CDs to enhance the photogenerated charge separation still remains an open challenge. To unravel this, here, we meticulously designed two different hybrid systems: (i) CDs coupled with pristine brookite TiO₂ nanorods (CDs/P-BTi) and (ii) CDs coupled with sonoreduced brookite TiO₂ nanorods (CDs/R-BTi), synthesized via a simple hydrothermal procedure with optimized 0.5 wt % CD loading in each case. Interestingly, CDs/R-BTi shows a 10-fold higher photocatalytic H₂ production rate as compared to sonoreduced brookite TiO₂ nanorods (R-BTi). But CDs/P-BTi exhibits only 4-fold H₂ production rate enhancement as compared to pristine brookite TiO₂ nanorods (P-BTi). An in-depth mechanistic study confirms that the presence of oxygen vacancy-rich Ti³⁺ defect sites, together with the formation of strong interfacial heterojunctions, significantly enhances photogenerated charge carrier separation and their efficient migration to the respective catalytic centers in CDs/R-BTi, thereby resulting in the aforementioned higher rate enhancements. These results demonstrate that utilizing an oxygen vacancy-rich TiO₂ surface is a more effective strategy than using a pristine TiO₂ surface when CDs are used as cocatalysts for achieving high photocatalytic performance.