Mechanistic Study of Photocatalytic CO₂ Conversion to CH₄ by Dopant-Defect Engineered SnS₂ Thin Films

To address the issue of CO₂ amount increment in the Earth's atmosphere, various semiconductor photocatalysts have been employed to convert CO₂ into valuable products. Designing an efficient photocatalyst that can activate the CO₂ molecule with the least amount of activation energy is one of the challenging problems. In this regard, we report phosphorous ion-implanted SnS₂ thin films. Thermal evaporation followed by sulfurization and ion implantation processes were used to prepare the regulated amount of phosphorous ion-implanted 20-nm SnS₂ thick thin films. Our findings reveal that Sulfur vacancy and Doping phosphorous synergistically enhance the exciton separation and maximize their lifetime and energetically stabilize CO₂ binding sites with the lowest activation energy. The optimized phosphorous-doped SnS₂ (4.5% P with 16% S_V) thin films have a three times higher Photocatalytic CO₂ conversion rate than the pristine one, with a high selectivity of about 92% towards CH₄ formation. Because Phosphorous plays a vital role in the activation of CO₂ by serving as an active site and due to its low electronegativity, it increases the charge density of the Sn atom adjacent to it. Also, P-doping affects the charge density of the neighboring S atom by serving as a bridge to improve the charge distribution between Sn and S. Also, this change in electron density promotes electron transfer and stabilizes the key reaction intermediates *COOH and *CHO during the photocatalytic reaction. In this work, we employ In-situ near ambient pressure X-ray photoelectron spectroscopy (NAP XPS), In-situ Fourier Transform Infrared Spectroscopy (FTIR), and X-ray absorption spectroscopy (XAS) measurements combined with formation energy, Bader charge, and Gibbs free energy calculations to carefully assess the overall impact of phosphorus in the SnS₂ sample and reaction pathway determination. From the In-situ NAP XPS result we have observed the ratios of C–O/C–C, C=O/C–C, and O–C=O/C–C peak ratios are enhanced after implanting P into the SnS₂ at different reaction conditions. This implies that the stabilization of these observed intermediates plays a crucial role in achieving higher photocatalytic performance, ultimately resulting in increased CH₄ production for P-doped SnS₂ samples. In conjunction, the relative peak positions Sn 3d_5/2 and S 2p_3/2 exhibit a nearly identical change to that of the C–C peak for the pristine SnS₂. In contrast, we observed a slight relative shift for S 2p_3/2, which amounted to 0.10 eV, while larger relative shifts were detected for Sn 3d_5/2 and P 2p_3/2, measuring 0.17 and 0.25 eV, respectively. These shifts were noted in the presence of CO₂ + H₂O atmosphere under light, compared to the ultra-high vacuum (UHV) condition. It implies that P plays a significant role in charge transfer to the adsorbed CO₂, the reaction intermediates, or activating the neighboring elements. Furthermore, the In-situ Fourier Transform Infrared Spectroscopy (FTIR) result shows An absorption band of carbonate ion (CO₃^2-) is formed at 1510 cm^-1, which is the first key intermediate when CO₂ is adsorbed and activated on the surface of the photocatalyst. Notably, the peak intensities decrease after light illumination because of the CO₃^2- ion transformation to other intermediates. Most importantly, the absorption bands between 1149 cm^-1 and 1160 cm^-1 are attributed to the carboxyl (COOH*) and methoxy (CH₃O*) groups, respectively. Both peaks are generally regarded as the crucial intermediate during CO₂ reduction to CH₄. Unlike CO₃^2-, the peak intensities of these intermediates increase after light illumination, implying the undergoing reaction, which is in good agreement with the in situ NAP-XPS. We expect that our results will inspire further research on ion implantation to tailor active sites for CO₂ reduction examine its CO₂ conversion capability and study the reaction intermediate changes and enhancement under different reaction environments.