Boosted charge separation in CoWO4/SnIn4S8 S-scheme heterojunctions toward high-performance photocatalytic carbon dioxide reduction and formaldehyde oxidation.

The fabrication of S-scheme heterojunctions with a robust internal electric field is regarded as an effective approach to improve the photocatalytic activity. Herein, a CoWO4/SnIn4S8 (abbr. CWO/SIS) S-type heterojunction was prepared applying electrospinning and hydrothermal synthesis techniques and exhibited outstanding photocatalytic properties toward carbon dioxide (CO2) reduction and formaldehyde (HCHO) oxidation. Specifically, the CO2 reduction rate of the optimal 50% CWO/SIS sample reached 1.75 μmol·g-1·h-1 with a CO selectivity of 99%. Meanwhile, the HCHO removal efficiency achieved 78% in 1 h. The reaction routes for photocatalytic CO2 reduction and HCHO degradation were probed via in situ diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) and Gibbs free energy analysis. The in situ DRIFTS results identified that the *COOH is the pivotal intermediate during the CO2 reduction process, verifying the high CO selectivity. The boosted photocatalytic activities of CWO/SIS heterojunctions mainly stemmed from the synergistic effect of efficient charge separation and improved redox capability. Furthermore, the charge migration route for the CWO/SIS S-scheme heterojunction was convincingly corroborated by employing Kelvin probe force microscopy (KPFM), density functional theory (DFT) calculations, in situ X-ray photoelectron spectroscopy (XPS) and femtosecond transient absorption spectroscopy (fs-TAS). This work provides important guidance for the construction of stable and effective S-type photocatalysts toward the energy conversion and environmental protection.