Defect-Driven Dynamics in Gas-Phase Photocatalytic CO ₂ Conversion to Solar Fuels Using Ti ^{3 <b>+</b>} /Ti ^{4 <b>+</b>} Containing TiO ₂ and Nonstoichiometric Ag ₂ S Nanowires

We studied CO2 photoreduction on nonstoichiometric surface photocatalysts using a comprehensive approach combining materials design, advanced spectroscopy, and Quantum Mechanics (QM) calculations. We developed a direct Z-scheme heterostructure, A-TiO2/Ag2S NWs, composed of amorphous TiO2 and nonstoichiometric Ag2S nanowires. This structure promotes defect-rich characteristics and a strong internal electric field (IEF), enhancing charge separation and minimizing electron–hole recombination. Employing in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and QM-simulated IR spectra revealed the CO2-to-CH4 conversion mechanism which involves H2COH* intermediate. Ti3+/Ti4+ and Ag+ defect environments were precisely characterized through X-ray photoelectron spectroscopy (XPS) and in situ extended X-ray absorption fine structure (EXAFS). Under concentrated solar illumination, this heterostructure achieved a CH4 production rate of 30.31 μmol/g, a 5-fold enhancement over conventional 1-sun conditions. These findings provide valuable insights into solar-driven fuel synthesis through targeted defect engineering and strategic heterostructure design.