Polyoxometalate-Mediated Spatially Compartmentalized S-scheme Heterojunction Coupled with Molecular Junction for Overall CO 2 Photoreduction

Rational design of photocatalytic systems coupling the reduction of CO 2 with water oxidation is vital for carbon-neutral technologies. Herein, we propose a strategy based on the synergistic assembly of polyoxometalates (POMs) within and on a porphyrinic metal–organic framework (MOF, PCN-222). Cross-scale integration of an S-scheme heterojunction and a molecular junction within a single POM@MOF-POM-TiO 2 (TiO 2 @Ni4PCN-222) system is demonstrated. In this spatially compartmentalized platform, Ni 4 (H 2 O) 2 (PW 9 O 34 ) 2 10– (Ni4POM) confined in the MOF pores modulates the band structure and enhances the built-in electric field of the S-scheme heterojunction, promoting efficient bulk charge separation. Meanwhile, surface-exposed Ni4POM enables uniform TiO 2 deposition, forming molecular-level W–O–Ti coordination contacts that act as electron-transport bridges. Benefiting from this dual mechanism, TiO 2 @Ni4PCN-222 achieves a CO evolution rate of 32.4 μmol g –1 h –1 with 98.0% selectivity in a gas–solid CO 2 photoreduction system. In situ DRIFTS, illuminated KPFM, and QIS-XPS confirm the proposed mechanism.