Visible-Light-Driven Photoreduction of CO2 with H2O Vapor over Oxygen Vacancy-Rich Mn3O4–Nanocrystal/FeOOH S-Scheme Heterojunction

Constructing S-scheme heterojunctions to achieve spatial separation of oxidative and reductive centers, rapid transfer electrons, and functional interactions is a promising strategy for realizing the overall photoreaction of CO₂ and H₂O. Herein, a series of Mn₃O₄/FeOOH S-scheme photocatalysts were prepared by anchoring Mn₃O₄ nanocrystals onto 1D FeOOH by the solvothermal method. The difference in the Fermi level between FeOOH and Mn₃O₄ in the composite system and the band bending at the interface are enhanced, thereby generating a built-in internal electric field (BIEF). In situ X-ray photoelectron spectroscopy demonstrated that BIEF directs the flow of photogenerated electrons from the conductive band of FeOOH to the valence band of Mn₃O₄. As a result, without cocatalysts or sacrificial agents, the S-scheme Mn₃O₄/FeOOH heterojunction delivers a high C1 yield rate of 22.5 μmol g^–1 h^–1 under visible-light irradiation, which is ca. 11.3 times higher than that of the single counterpart Mn₃O₄. Furthermore, introducing FeOOH with oxygen vacancies can obtain an oxidative center with a high oxygen production capacity during photocatalytic water oxidation. This enhancement not only accelerates the overall reaction but also promotes the photoreduction of CO₂ by H₂O. The synergistic results achieved through the S-scheme heterojunction and oxygen vacancies make it possible to produce solar fuels through reaction involving the reduction of CO₂ with H₂O.