Powering a CO₂ Reduction Catalyst with Visible Light through Multiple Sub-picosecond Electron Transfers from a Quantum Dot

Abstract

Photosensitization of molecular catalysts to reduce CO₂ to CO is a sustainable route to storable solar fuels. Crucial to the sensitization process is highly efficient transfer of redox equivalents from sensitizer to catalyst; in systems with molecular sensitizers, this transfer is often slow because it is gated by diffusion-limited collisions between sensitizer and catalyst. This article describes the photosensitization of a meso-tetraphenylporphyrin iron(III) chloride (FeTPP) catalyst by colloidal, heavy metal-free CuInS₂/ZnS quantum dots (QDs) to reduce CO₂ to CO using 450 nm light. The sensitization efficiency (turnover number per absorbed unit of photon energy) of the QD system is a factor of 18 greater than that of an analogous system with a fac-tris(2-phenylpyridine)iridium sensitizer. This high efficiency originates in ultrafast electron transfer between the QD and FeTPP, enabled by formation of QD/FeTPP complexes. Optical spectroscopy reveals that the electron-transfer processes primarily responsible for the first two sensitization steps (Fe^IIITPP → Fe^IITPP, and Fe^IITPP → Fe^ITPP) both occur in <200 fs.