Abstract:

As a higher oxidation state compound of carbon, more electrons and protons are needed to reduce CO₂. While the step-scheme (S-scheme) heterojunction driven by semiconductors performs excellently in the excitation and transport of electrons, which has strong redox ability while inhibiting electron hole recombination, and has exhibited excellent results in photocatalytic CO₂ reduction. Herein, Ag/CN was prepared by an optical deposition method, and the Ag/CN/ZnIn₂S₄ S-scheme heterojunction composite photocatalyst was prepared by a hydrothermal method. The crystal structure, morphology and elements valences of the materials were analyzed using X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and other characterization methods, and the composite of the monomer was successfully verified. According to the electron spin resonance (ESR) and ultraviolet photoelectron spectroscopy (UPS) studies, formation of the S-scheme heterojunction was observed. Based on photoelectrochemical results and photoluminescence (PL) studies, the enhanced CO₂ reduction can be attributed to the S-scheme electron transfer at the interface, which promotes charge separation. In the S-scheme electronic transmission system, CN acts as the reductive photocatalyst (RP) and ZnIn₂S₄ as the oxidative photocatalyst (OP). Owing to the difference in Fermi energy levels, the electron cloud density changes until the Fermi levels match after contact between the RP and OP. This process generates internal electric fields and band bending at the interface, facilitating hole separation and the transfer of photo-induced carriers and photogenerated electrons. The plasmonic effect of the interfacial Ag NPs of the composite was proven using UV-Vis diffuse reflectance spectroscopy (DRS). When exposed to light, Ag NPs, as reactive sites of the reaction, act as receptors for electron transport. The excitation of high-energy thermal electrons on the surface of Ag NPs leads to the generation of localized electromagnetic fields between CN and Ag NPs, which subsequently accelerates the electron transport rate on the CB of CN and enhances light absorption, thus improving the photoreduction performance of hybrid materials. Under the combined action of the S-scheme heterojunction and plasmonic effect, the interface carrier transfer efficiency can be improved. Finally, the charge transfer mechanism was analyzed. Simultaneously, the possible reaction paths of photocatalytic CO₂ reduction were explained by comparing the in situ Fourier-transform infrared spectroscopy (FT-IR) spectra of the monomer and compound. The CO₂ reduction capability of composite materials was better compared to that of monomer materials. The best yields of CO and CH₄ of ACZ-60 were 5.63 μmol·g⁻¹ and 0.23 μmol·g⁻¹, which were 6.5 and 2.1 times that of CN, respectively. Across four cycles, the CO and CH₄ yields and XRD patterns of ACZ-60 showed excellent stability. This study provides a scheme for the rational design of photocatalytic CO₂ reduction S-scheme catalysts.

Key words: S-scheme heterojunction, ZnIn₂S₄, Ag NPs, Photocatalytic CO₂ reduction, Plasmonic effect