Interfacial charge transfer in 0D/2D defect-rich heterostructures for efficient solar-driven CO₂ reduction

Two-dimensional graphitic carbon nitride (g-C₃N₄) has been widely explored as a promising photocatalyst for solar CO₂ conversion. However, rapid charge recombination and low visible-light utilization are severely detrimental to photocatalytic CO₂ conversion. Zero-dimensional/two-dimensional (0D/2D) heterostructures are considered the promising materials with size tunability and enhanced charge separation efficiency for photocatalysis. Herein, a 0D/2D heterostructure of oxygen vacancy-rich TiO₂ quantum dots confined in g-C₃N₄ nanosheets (TiO_2-x/g-C₃N₄) was prepared by in-situ pyrolysis of NH₂-MIL-125 (Ti) and melamine. Charge dynamics analysis by time-resolved photoluminescence (tr-PL) and femtosecond and nanosecond pump-probed transient absorption (TA) spectra revealed that charges transfer occured from 2D-g-C₃N₄ to 0D-TiO₂ at an ultrafast subpicosecond time scale (<1 ps) through the intimate interface. The overall fast decay of the charge carriers was attributed to interfacial charge transfer, which was accompanied by recombination relaxation mediated by shallow trapped sites. Ultrafast interfacial charge transfer greatly promoted charge separation and electrons in shallow trapped sites were easily trapped by CO₂. In addition, combining with the synergetic advantage of strong visible light absorption, high CO₂ adsorption and large surface area, TiO_2-x/g-C₃N₄ exhibited a superior CO evolution rate of 77.8 μmol g⁻¹ h⁻¹, roughly 5 times that of pristine g-C₃N₄ (15.1 μmol g⁻¹ h⁻¹). This work provides in-depth insights into optimizing the heterojunction for robust solar CO₂ conversion.