Efficient Visible-Light Photosensitization of a Nonheme Fe-Molecular Catalyst by Colloidal CuInS ₂ Quantum Dots with ZnS Shells for Selective Reduction of CO ₂ to CO in Water

Photosensitization of molecular catalysts (MCs) for selective CO 2 -to-CO conversion offers a sustainable pathway toward renewable-driven circular carbon utilization but hinges on the rapid and directional transfer of redox equivalents from the photosensitizer (PS) to the catalyst. As a part of our quest in this direction, we report the photosensitization of a nonheme iron MC, ([Fe(AAP) 2 Cl 2 ]Cl) (Fe Cat), by colloidal, heavy-metal-free bare Cu-deficient CuInS 2 (CIS) and core/shell CuInS 2 /ZnS (CIS/ZnS) quantum dots (QDs) for photocatalytic CO 2 reduction to CO in water under 440 nm irradiation. The CIS/ZnS/Fe Cat assembly delivers a CO turnover number (TON Cat ) of ∼4536 over 23 h, substantially outperforming the unshelled CIS/Fe Cat system (TON Cat ∼1545). Transient absorption spectroscopy over femtosecond-to-microsecond time scales reveals that ZnS shelling decelerates the photoinduced electron transfer from the QDs to Fe Cat but more than compensates for it by suppressing charge recombination via back electron transfer to a greater extent. This and the surface curing brought about by formation of the shell are responsible for the enhanced catalytic output. Spectroelectrochemical and photochemical analyses further elucidate the sequential electron-transfer steps and CO 2 activation events associated with Fe Cat under operating conditions. Together, these results establish a tunable QD/MC architecture, enabled by shell engineering, for exclusive aqueous CO 2 -to-CO conversion and provide mechanistic design rules for maximizing interfacial charge transfer in photosensitized molecular catalysis.