Optimization and defect engineering to boost interfacial charge transfer in a novel NiCo-LDH/CN-Cv S-scheme heterojunction modified with carbon vacancies for enhanced solar driven CO2 reduction

• Key synthesis parameters were optimized using a 2⁴ factorial design of experiments. • Carbon vacancies provided the efficient trap sites for CO 2 activation. • Synergistic s-scheme heterojunction promoted interfacial electron migration. • Optimized S-Cv 8-550-5-150 composite exhibited the highest CO evolution of 159.47 µmol g -1 h -1 . • DFT-derived Gibbs free energy profiles elucidated the CO evolution pathway. Photocatalytic CO 2 reduction to valuable fuels offers a promising pathway to carbon neutrality and global energy crisis. g-C 3 N 4 has gained extensive interest due to its broad absorption range in visible spectrum and high stability. We have successfully constructed a novel g-C 3 N 4 /NiCo-LDH (S-Cv) s-scheme heterojunction enriched with carbon vacancies, using a facile and green hydrothermal method, for enhanced CO 2 reduction. A 2 4 single-replicate factorial experimental design was utilized to assess the significance of four synthesis parameters and statistical analysis revealed that three out of four factors had significant effects on the photocatalytic activity. Consequently, the introduction of carbon vacancies in the heterostructure provided efficient active sites and modulated the electronic structure of CN-Cv. Meanwhile, coupling of NiCo-LDH with CN-Cv via s-scheme provided the built-in electric field and enhanced the interfacial transfer of photogenerated charge carriers. Morphological results indicated that the NiCo-LDH nano flowers were deposited in situ on the CN-Cv nanosheets, which resulted in enlarged surface area and improved light absorption capabilities. The optimized S-Cv-8-550-5-150 composite exhibited the highest CO evolution of 159 μmol g -1 , which was 3.9 and 7.5 times higher than the standalone CN-Cv and NiCo-LDH, respectively. The increase was attributed to the synergistic effect of carbon vacancies and systematic optimization process using factorial design of experiments. Moreover, DFT calculations also validated the directional charge transfer and reaction pathway of CO evolution. The step with the highest uphill barrier was the conversion of COOH to CO and was therefore identified as the rate determining step for photocatalytic CO₂ reduction to CO. Thus, this study provides a strong pioneer of defect engineering and statistical analysis for designing the s-scheme heterojunction photocatalysts for enhanced CO 2 reduction.