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 CO 2 and H 2 O. Herein, a series of Mn 3 O 4 /FeOOH S-scheme photocatalysts were prepared by anchoring Mn 3 O 4 nanocrystals onto 1D FeOOH by the solvothermal method. The difference in the Fermi level between FeOOH and Mn 3 O 4 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 Mn 3 O 4 . As a result, without cocatalysts or sacrificial agents, the S-scheme Mn 3 O 4 /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 Mn 3 O 4 . 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 CO 2 by H 2 O. 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 CO 2 with H 2 O.