Article pubs.acs.org/JACS
Powering a CO2 Reduction Catalyst with Visible Light through Multiple Sub-picosecond Electron Transfers from a Quantum Dot Shichen Lian, Mohamad S. Kodaimati, Dmitriy S. Dolzhnikov, Raul Calzada, and Emily A. Weiss* Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208-3113, United States S Supporting Information *
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 (FeIIITPP → FeIITPP, and FeIITPP → FeITPP) both occur in 95% selectivity for photoexcitation of the QD sensitizer as opposed to FeTPP at 450 nm, the excitation wavelength used for all studies, Figure 1A (green). We use a large excess of QD sensitizer for catalysis in order to ensure that at most one FeTPP binds per QD, and thereby maximize the probability that photoexcited electrons all go to the same catalyst. Figure 1B shows cyclic voltammograms (CVs) of MPOcapped QDs under N2, FeTPP under N2, and FeTPP under CO2, all in DMSO. When the FeTPP sample is purged with CO2, catalytic current is observed upon reduction of Fe+ to Fe0, which confirms that the Fe0 center within FeTPP molecule is the catalytic site.13,14 The first observed reductive wave of the QDs (∼ −2.5 V) is more negative than the catalyst standard potential, which indicates that the photoinduced electrons within CuInS2 QDs used in this study have enough energy to reduce FeIII to Fe0, and thereby photosensitize FeTPP. Scheme 1 summarizes a photocatalytic mechanism for CO2 reduction by QD-sensitized FeTPP, based on the previously reported electrochemical mechanism.41 During the initial sensitization step (top blue shaded region), a QD sequentially accepts three photons, one at a time. After each photoexcitation (to form QD*), the QD donates one electron to FeTPP and one hole to the sacrificial reductant, TMPD, to regenerate its ground state. The FeIII center within 1 thereby consecutively accepts three electrons to become Fe0 (2), which binds CO2 to form the catalytically active species Fe0-C(+4)O2 (3). The first step of the reduction of CO2 is a proton-coupled electron
Figure 1. Characterization of the catalyst and sensitizer. (A) Groundstate absorption spectra of 16 μM CuInS2/ZnS core/shell QDs (black), 8 μM FeTPP purged with CO2 (red), and a mixture of the QDs and 0.5 equiv of FeTPP purged with CO2 (green); and PL spectrum of the QDs (black dash), in DMSO. The blue arrow indicates the excitation wavelength used for the catalysis, PL, and transient absorption experiments. (B) CV of 1 mM QDs under N2 (black), FeTPP under N2 (blue), and FeTPP under CO2 (red), with 50 mM tetrabutylammonium hexafluorophosphate in DMSO, a glassy carbon working electrode, an Ag wire reference electrode, and a Pt wire counter electrode. The potentials have been referenced to SCE using a ferrocene internal standard. The discontinuities in the scans are artifacts caused by switching the polarity of the electrodes (shifted below 0 V when referencing the data to Fc/Fc+).
demonstrated the efficacy of a combination of more highly reducing Cu2S nanorods with Pt nanoparticles for conversion of CO2 to CO and methane, although the 1.2 eV bandgap of Cu2S required an exceptional hole scavenger.36 Here we show that soluble, colloidal, heavy metal-free CuInS2/ZnS QDs photosensitize the catalytic conversion of CO2 to CO by a model catalyst, FeTPP, in DMSO, using visible light, Scheme 1. Upon successive photoexcitations at 450 nm, the QD donates three electronsthe first and second in
