Interfacial electron orchestration: Vacancy-engineered CQD/ NiSV heterojunctions achieve near-perfect CO2-to-CH4 photocatalysis with selectivity approaching 100 %

Photocatalytic CO 2 -to-CH 4 conversion via solar energy offers a sustainable carbon cycle pathway, yet its efficiency and selectivity are limited by the kinetically challenging eight-electron process. Herein, a sulfur vacancy-enriched NiS microflower hybrid decorated with carbon quantum dots (CQDs-X/NiS V ) is synthesized via hydrothermal method. Sulfur vacancies, induced in situ by NH₃ from thermal decomposition of NH 4 Cl and thiourea, serve as electron traps to promote carrier separation. The highly conductive CQDs facilitate rapid charge transport and enhance the CO 2 -to-CH 4 eight-electron reduction. A Schottky junction at the CQDs/NiS V interface further accelerates electron transfer from NiS V to CQDs, synergistically boosting CO₂ reduction. Experimental results reveal a positive correlation between CQD loading and sulfur vacancy concentration, elevating CH 4 yield from 0.19 to 2.47 mmol·g −1 . The optimal CQDs-100/NiS V achieves 2.47 mmol·g −1 CH 4 with 95.6 % selectivity. Notably, in diluted CO 2 atmosphere, the yield and the selectivity of CH 4 is apparently higher than that of in pure CO 2 due to the competition for photogenerated electrons between two reaction pathways: (1) CO 2 + 2e⁻ → *CO (2) *CO + 6e⁻ → CH 4 . This work provides a novel strategy for in situ generation of sulfur vacancies and a triple-strategy design (Schottky junction, CQD modification, defect engineering) for high-efficiency photocatalysts.