Abstract: Excessive CO₂ emissions have led to serious environmental problems. The photocatalytic reduction of CO₂ to value-added chemicals is a promising strategy to reduce carbon emissions and alleviate the energy crisis simultaneously. Photocatalysts is crucial in the reduction process. Nanostructure engineering and heterojunction construction have been identified as prospective approaches to develop efficient photocatalysts for CO₂ reduction. Step-scheme (S-scheme) heterojunctions are novel systems composed of a reduction catalyst and an oxidation catalyst. In these systems, the charge separation at the interface between the two catalysts could be enhanced by an internal electric field directed from the reduction photocatalyst to the oxidation photocatalyst on account of their matched Fermi levels (E_f). The S-scheme transfer mode can not only efficaciously inhibit the recombination of photoinduced carriers but also accumulate electrons and holes with greater redox potential. Cu₂O and BiOI materials, as typical reduction and oxidation catalysts, are endowed with efficient visible-light absorption and favorable band position for catalyzing the coupling reaction of CO₂ reduction and H₂O oxidation. In this study, a series of S-scheme catalysts consisting of polyhedral Cu₂O-modified BiOI flakes were synthesized onto a fluorine-doped tin oxide substrate via the electrodeposition method. The structure, morphology, and surface composition of the as-obtained samples were then studied using X-ray diffractometry (XRD), X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) measurements. ¹³C/¹⁸O isotope tracer experiments indicated that the BiOI/Cu₂O composite achieved CO₂ conversion with water vapor under visible-light irradiation (λ > 400 nm). The CO, CH₄, H₂, and O₂ yields of the optimal BiOI/Cu₂O-1500 catalyst reached 53.03, 30.75, 8.49, and 82.73 μmol‧m⁻², respectively, after 11 h of visible-light illumination. The photocatalytic activity of BiOI/Cu₂O-1500 slightly decreased at the eighth cycling, but its CO, CH₄, and O₂ yields still reached 27.38, 34.08, and 75.52 μmol‧m⁻², respectively. The XPS and XRD results confirmed the excellent cycling stability of the catalysts, and analysis using the XPS core-level (CL) alignment method revealed that a staggered band structure was formed in the BiOI/Cu₂O heterojunction. The direction of the built-in electric field in the heterojunction was determined using UPS measurements, and the S-scheme mechanism of charge transfer was verified via the in situ XPS results. In addition, the production of HCO⁻₃, CO₃²⁻, HCOO⁻, and •CH₃ species during CO₂ reduction was confirmed using in situ diffuse reflectance Fourier transform spectrometry, and a possible mechanism of CO₂ conversion under water vapor was proposed. Benefiting from its S-scheme BiOI/Cu₂O heterojunction, the prepared catalyst showed improved photoinduced charge separation, and its photogenerated carriers with strong redox ability were preserved, thereby leading to enhanced photocatalytic performance.

Key words: S-scheme, Photocatalyst, CO₂ reduction, BiOI/Cu₂O, Sheet