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Photocatalytically Active Superstructures of Quantum Dots and Iron Porphyrins for Reduction of CO to CO in Water 2

Shichen Lian, Mohamad S. Kodaimati, and Emily A. Weiss ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b07377 • Publication Date (Web): 03 Jan 2018 Downloaded from http://pubs.acs.org on January 4, 2018

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Photocatalytically Active Superstructures of Quantum Dots and Iron Porphyrins for Reduction of CO2 to CO in Water Shichen Lian, Mohamad S. Kodaimati, and Emily A. Weiss* Department of Chemistry, Northwestern University, 2145 Sheridan Rd., Evanston, IL 602083113 *corresponding author. Email: [email protected]

Abstract This paper describes the use of electrostatic assemblies of negatively charged colloidal CuInS2/ZnS quantum dot (QD) sensitizers and positively charged, trimethylaminofunctionalized iron tetraphenylporphyrin catalysts (FeTMA) to photoreduce CO2 to CO in water, upon illumination with 450-nm light. This system achieves a turnover number (TON) of CO (per FeTMA) of 450 after 30 h of illumination, with a selectivity of 99%. Its sensitization efficiency (TON per Joule of photons absorbed) is a factor of eleven larger than the previous record for photosensitization of an iron porphyrin catalyst for this reaction, held by a system in which both QDs and metal porphyrin were uncharged. Steady-state and time-resolved optical spectroscopy provides evidence for electrostatic assembly of QDs and FeTMA. Control of the size of the assemblies with addition of a screening counterion, K+, and a correlation between their measured size and their catalytic activity, indicates that the enhancement in performance of this system over the analogous uncharged system is due to the proximity of the FeTMA catalyst to multiple light-absorbing QDs, and the selective formation of QD-FeTMA contacts (rather than QD-QD or FeTMA-FeTMA contacts). This system therefore shows the ability to funnel photoinduced electrons to a reaction center, which is crucial for carrying out reactions that require multistep redox processes under low photon flux, and thus is an important advance in developing artificial photocatalytic systems that function in natural light. Keywords: electrostatic self-assembly, copper indium sulfide, nanocrystals, artificial photosynthesis, photo-redox catalysis

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Funneling of energy or redox equivalents to a reaction center that subsequently uses them to perform chemical reactions is a strategy regularly used in nature. In photosystems I and II, an average of ~100 light-harvesting units surround and simultaneously transport photogenerated electrons/holes to one reaction center.1 Under the very low photon flux of natural light (1015 photonscm-2s-1), even with the large extinction coefficient of the chlorophyll light harvesting molecules (109 mole-1cm2), the average time between creation of excited states has a lower limit ~0.6 s.2 If only one sensitizer were feeding redox equivalents to one reaction center, it would take an average of 2.4 s for a four-hole water oxidation reaction to occur,2 a prohibitively long time considering the instability of partially oxidized catalytic intermediates, such as the highly reactive MnVoxo species, within the oxygen-evolving complex.3,4 It is the supramolecular structure of the light harvesting complex, where the reaction center is fed by ~100 sensitizers, that decreases the time it takes to accumulate the required four redox equivalents to only 24 ms. Similarly, the efficiency of redox-driven chemical catalysis in synthetic systems is determined by (i) the availability of redox equivalents (electrons and holes with sufficient potential) to the catalyst and (ii) the ability of the catalyst to employ these redox equivalents to make or break chemical bonds selectively.5-7 In electrocatalytic systems, criterion (i) is related to the “heterogeneous” rate constant, the rate constant for injection of electrons or holes from the electrode to the surface-adsorbed catalyst.8,9 In photocatalytic systems, if the light absorber (sensitizer) and the photocatalyst are two separate species, (i) is dictated by the absorption and excited state lifetime of the sensitizer and the rate constant(s) for charge transfer from the photoexcited sensitizer to the catalyst.10,11 This rate constant is effectively increased if multiple sensitizers are electronically coupled to one catalyst, such that the same catalyst accumulates redox equivalents from several sensitizers within one catalytic cycle. Here, we demonstrate that the electrostatic self-assembly of negatively charged CuInS2/ZnS core/shell quantum dot (QD) visible-light sensitizers and positively charged, trimethylaminofunctionalized iron tetraphenylporphyrin (FeTMA) catalysts12,13 can be controlled to achieve electron-funneling superstructures that selectively reduce CO2 to CO in water. Upon illumination of optimized assemblies with 450-nm light for 30 hours, we achieved a TON of CO per FeTMA catalyst of 450, with chemical selectivity of ~99% and no observed degradation of the catalytic system. The sensitization efficiency (TON of CO per Joule of photons absorbed by the system) is a factor of 11 larger than the previous record for photosensitization of an iron porphyrin catalyst 2 ACS Paragon Plus Environment

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for this reaction, in which both the CuInS2 QDs and the porphyrins were uncharged and dissolved in DMSO14, despite the factor of ten smaller solubility of CO2 in water than in DMSO15,16. Unlike several recent reports on QD-catalyst complexes for CO2 reduction17-19, we provide evidence that photocatalytic activity is enhanced by formation of electrostatically assembled superstructures, and show that we can control this activity by tuning the size of these assemblies with addition of K+. The easily tuned surface chemistry of the colloidal QD sensitizers, and the relative insensitivity of their electronic properties to proximate charges, makes controlling their electrostatic assembly with charged catalysts easier than with molecular sensitizers (the best are Ir- or Ru-based dyes), which are difficult to functionalize without altering their redox potential and absorption spectra.20 In particular, the CuInS2 QDs used in this work are chosen because they are highly reducing (~ 2.4 V vs. SCE)14 and provide a large drive force for the proposed three-step, sequential reduction of FeTMA.21,22

Results/Discussion Evidence for Electrostatic Assembly of QD-FeTMA Complexes. The native capping ligands of CuInS2/ZnS QDs are oleate, and the QDs, as-synthesized, are only soluble in non-polar organic solvents. Addition of 3-mercaptopropionic acid (MPA) in basic water into a toluene solution of QDs exchanges the oleate ligands for MPA and solubilizes the QDs in polar solvents such as water. Since the QD surface primarily presents Zn2+ ions23 and the stability constant (pKs) for Zn-thiolate is a factor of ~35 greater than that of Zn-carboxylate24,25, upon treatment with MPA, the thiolate end (-S-) of the ligand binds to the surface of the QDs and the carboxylate end (-COO) points outward and terminates the surface of the QDs.26-29 The Supporting Information (SI) and the Methods section describe the syntheses of CuInS2/ZnS QDs and FeTMA (Figure S1), and the ligand exchange procedure in further detail. Figure 1A shows the ground-state absorption spectra of MPA-capped QDs and the QDs with 0.5 equiv. FeTMA after purging the sample with CO2. The first excitonic absorption of the QDs is centered at 420 nm (Figure 1A, red solid), which corresponds to a particle diameter of 2.5 nm.30 At 450 nm, the excitation wavelength we used for photocatalysis and optical experiments, the QDs absorb ~80% of photons and the FeTMA absorbs 20% of photons. Figure 1A (black lines) are the absorption spectra of FeTMA, either alone or in a mixture with QDs (after subtracting the 3 ACS Paragon Plus Environment

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contribution of QDs to the spectrum). Figure 1B helps us determine the origin of the bathochromic shift and change in shape of the FeTMA Soret band upon addition of QDs. This figure shows that, in the absence of QDs, these changes can be reproduced by diluting the FeTMA, which are primarily aggregated at 8 M (introducing an additional peak at 380 nm due to the dipolar coupling between monomers31,32), and primarily monomeric at 0.5 M, with a characteristic single absorption peak at 415 nm. We can therefore conclude that the FeTMA purged with CO2 (Figure 1A, black solid line) is aggregated, and that addition of QDs (Figure 1A, black dashed line) disaggregates them, at least partially. This observation is consistent with previous reports33 that QDs are capable of disassembling dye aggregates. The difference in linewidths of the spectra of both aggregated and monomeric FeTMA between panels A and B is due to the presence of CO2 in A, but not B.34 Without CO2, the primary axial ligands are chloride and water. We suspect that CO2 partially substitutes for these axial ligands, which increases the heterogeneity in the type of axial ligand and broadens the Soret band.

Figure 1. (A) Ground-state absorption spectra of 7.5 µM FeTMA only (black solid line), 15 µM MPA-capped QDs in water with (red dashed line) and without (red solid line) 7.5 µM FeTMA added, and the difference spectrum: (QD/FeTMA mixture minus QDs only), i.e., FeTMA in the presence of QDs (black dashed line). All samples are in CO2-purged water. Inset: proposed assembly mechanism for a subunit of a QD/FeTMA complex. Only one MPA ligand per QD is drawn, for clarity, but there is an average of 150 ligands per QD. (B) The Soret band of the ground-state absorption spectra of FeTMA-only in concentrations of 8 µM to 0.5 µM in Arpurged water.

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As depicted in the inset to Figure 1A, we suspect that this disaggregation is driven by binding of negatively charged carboxylate tail groups on the QDs to the positively charged trimethyl-amino groups on the FeTMA. We note that adding ammonium oxalate to a disaggregated (dilute) porphyrin solution does not cause any significant spectral shift (Figure S2A), so binding of the carboxylate to the metal center is not the cause of this shift. We then monitored the disassembly of aggregated porphyrins upon introduction of QDs through the change in characteristic absorbance of the monomeric and aggregated species to extract an equilibrium binding constant of (5.1±0.1) ×105 M-1 for the QD-FeTMA complex in CO2 saturated water (see the SI for details, Figure S2B,C). We do not believe the FeTMA is adsorbing to sulfurs on the QD surface in a “faceon” geometry through an Fe-S interaction, because that adsorption mode produces a characteristic red-shifted and sharpened porphyrin Soret band, as we observed in an uncharged, organic-soluble system.14 In contrast, the spectrum of FeTMA in the presence of QDs for our water-soluble system (Figure 1A, black dashed line) is very similar in shape to that of the FeTMA monomer in the absence of QDs (Figure 1B, pale red line). The rate of photoinduced electron transfer (eT) from the QD to the porphyrin acceptor is also diagnostic of the mode of their assembly. We have shown previously that if the QD donor is in direct contact with the Fe center of an unsubstituted Fe-tetraphenyl porphyrin (FeTPP) acceptor (an axial-ligand type interaction), eT from the QD to Fe(III)TPP, the first of three sensitization steps to reduce CO2 to CO, is sub-picosecond.14 Given that the driving force for eT from QDs to FeTMA (our catalyst in this study) is greater than that from QDs to FeTPP by ~ 0.2 eV (see Figure S3 in the SI), we expect a similar fast eT rate for this system if the QDs and porphyrin catalysts are electronically coupled in the same way. We monitor this rate by photoexciting the QDs at 450 nm with a 120-femtosecond laser pulse, and recording the dynamics of the ground-state bleach (GSB) feature within its transient absorption (TA) spectrum (Figure S4) and the dynamics of the QD’s photoluminescence (PL) decay. The dynamics of the GSB feature exclusively reflect the decay of excitonic electrons, through the intrinsic decay pathways of the QD, and here through eT to FeTMA. Figure 2A shows the TA-measured dynamics of the GSB over the time window of 250 fs to 3 ns. Figure 2B shows the time-correlated single photon counting (TCSPC)-measured dynamics of exciton decay over a longer-time window of ~500 ps to 10 µs. The SI contains the details on spectra, assignments, and fits that associated with these kinetic traces (Table S1 and S2).

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Figure 2. (A) Kinetic traces extracted from the TA spectrum of MPA-capped QDs in water with (red) and without (black) 0.5 eq. of FeTMA added, probed at 480 nm (the QD GS bleach), after excitation at 450 nm. The kinetic trace of QD+FeTMA sample (red) is scaled by 1.2 to account for the 20% attenuation due to the absorption of FeTMA at 450 nm. The initial amplitude of QD+FeTMA sample after this adjustment matches the QD only sample. (B) Decay of the PL of MPA-capped QDs in water with (red) and without (black) 0.5 eq. of FeTMA added, probed at 600 nm, after excitation at 450 nm. Multi-exponential fits of all traces, with parameters summarized in the SI (Table S1 and S2). (C) Schematic diagram of electron transfer from QDs to FeTMA with labelled time constants, measured from TA and TCSPC. 6 ACS Paragon Plus Environment

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Fits of Figures 2A,B, black traces, yield six time constants for decay of the excitonic electron in MPA-capped QDs in CO2-saturated H2O without added FeTMA, in agreement with the literature35-37: 0.5 ps and 5 ps, two ultrafast electron trapping processes to lattice and surface sites, respectively, and 1.7 ns, 5 ns, 38 ns, and 190 ns, four radiative recombination processes of either band-edge or above-band-edge electrons and holes. Upon addition of 0.5 eq. of FeTMA, the decay of the excited state of the QD on all timescales is accelerated (Figures 2A,B, red traces), and the PL of the QDs is quenched by 50% (a 100% yield of PL quenching), see the SI, Figure S5. We can assign this acceleration of excited state decay, and the associated PL quenching, to eT from QDs to FeTMA because neither hole transfer nor energy transfer from QD to FeTMA is thermodynamically favorable.14 Fits of the red traces in Figures 2A,B yield average eT time constants of 3 ps (22%), 40 ps (22%), and 800 ps (56%). We observe no sub-picosecond eT as we did in the QD-FeTPP case, so, given that there is more driving force for this eT process than there was for the QD-to-FeTPP process, these dynamics suggest that the donor-acceptor electronic coupling between the charged QD and FeTMA is smaller than that between the uncharged QD and FeTPP, and an electrostatic interaction, like that pictured in Figure 2C, is more likely than “faceon” association of the porphyrin with the QD surface via an Fe-S interaction. Despite the smaller electronic coupling than in the uncharged case, eT still proceeds quantitatively from the photoexcited QD to Fe(III)TMA to give a 100% yield for the first sensitization step of CO2 reduction. The heterogeneity of the eT dynamics (three distinct time constants) within the QD/FeTMA assemblies almost certainly originates from a distribution in the number of FeTMAs surrounding each QD, and the degree of QD-QD aggregation, which we discuss directly below. Controlling electrostatic interactions between the QDs and FeTMA controls their catalytic activity. Having presented spectral and dynamic evidence for electrostatic assembly of the QDs and FeTMA in CO2-purged water, and established that the first sensitization step for CO2 reduction to CO takes place in 100% yield in 1) geometry (our DMSO-soluble QD/FeTPP system requires a 50:1 sensitizer:catalyst ratio). Further structural characterization is needed to get a better picture of the composition and shape of a catalytically active superstructure.

Conclusions In summary, we demonstrated that the combination of negatively charged QDs and positively charged FeTMA catalysts produces highly efficient and stable catalytic mixture for the photoreduction of CO2 to CO in water, with a sensitization efficiency that is a factor of eleven greater than that of the previously best-performing photosensitized iron porphyrin system: the same QDs with FeTPP (both uncharged) in DMSO. We believe that this enhancement is due primarily to the formation of catalytically active superstructures, where one FeTMA is funneled 13 ACS Paragon Plus Environment

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photoelectrons from several QDs bound through their negatively charged ligands. This conclusion is based on spectral and dynamic evidence for an electrostatic interaction between the QD and the FeTMA, the need for an excess of QD sensitizer (30:1) to optimize the TON, and the correlation between the TON and the size of the assemblies formed in the mixture, as measured by DLS and controlled by the addition of K+. Previous work indicates that FeTMA is capable of reducing CO2 directly to CH4.21 We did not illuminate for sufficient times to explore this pathway, since it was not necessary to demonstrate our control of catalytic activity with electrostatic interactions, but it is certainly a logical nextdirection for these high-performing assemblies. The overall low quantum yield in our system is partially due to the fact that, in our best-performing catalytic mixture, only 3% of QDs have FeTMA catalysts adsorbed to the surface, as the added ratio QD:FeTMA = 30:1. The remaining loss of quantum yield is probably related to the final (out of three) sensitization steps, because the driving force for eT from QDs to FeTMA in this step is 1.4 eV lower than that of the first sensitization step. Further optimization is needed to ensure selective and specific interactions between QD chromophores and porphyrin catalysts when more porphyrins are present, and to elongate the lifetime of the doubly and triply reduced FeTMA states. In addition, we utilized triethanolamine as a sacrificial electron donor in this work, to re-establish the ground state of the QD sensitizers. p-type NiO has shown great potential as the photocathode material in photoeletrochemical (PEC) cells.47,48 A photocathode consisting of a p-type NiO substrate, CuInS2/ZnS QD absorbers, and FeTMA catalysts could operate in a PEC cell for photoreduction of CO2 without the use of sacrificial electron donors to improve the system’s potential technoeconomical value.

Methods/Experimental Synthesis of Oleate-capped CuInS2/ZnS Core/Shell QDs and Ligand Exchange Procedure with 3-mercaptoproponoic acid (MPA). We synthesized organic-soluble, oleatecapped CuInS2/ZnS core/shell QDs according to a literature procedure.14 We then performed ligand exchange procedure with MPA to produce water-soluble QDs. In a typical ligand exchange experiment, 200 equiv. MPA (relative to the amount of QDs that will be added in the following step), 1.3 equiv. tetramethylammonium hydroxide (TMOH, relative to MPA added in the previous step) in 2 mL water, and 4 mL of oleate-capped QDs in toluene were added into a 15 mL centrifuge 14 ACS Paragon Plus Environment

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tube. The centrifuge tube was shaken for several minutes to allow TMOH to deprotonate all the MPA to form thiolates, and for the thiolates to displace all the bound oleates on the surface of QDs. We then centrifuged this sample for 10 mins at 7000 rpm. The originally toluene-soluble, oleate-capped QDs were then transferred into the bottom aqueous layer and capped with MPA. We then washed the aqueous layer with chloroform to eliminate excess oleate or MPA. It is important to not use excess MPA and wash away all of the free ligands, because excess thiolate ligands will bind to the Fe center within the porphyrin and poison the catalyst. Synthesis of Fe(III) meso-Tetra(4-N,N,N- trimethylanilinium) Porphine Pentachloride (FeTMA). We synthesized FeTMA based on a literature procedure.45 Its molecular structure is confirmed by MALDI-ToF (Figure S1). Steady-State Absorption and Photoluminescence Spectroscopy. Steady-state absorption Spectra of QD and FeTMA samples were collected on a Varian Cary 5000 spectrometer in a 2 mm quartz cuvette in water, either under CO2 or Ar. All the spectra were baseline-corrected with a water only sample. The steady-state photoluminescence spectra were obtained with a Fluorolog-3 spectrofluorometer (Horiba Jobin Yvon) in a 1 cm quartz cuvette with 3-nm slit width and a right angle geometry. Time-resolved Absorption and Photoluminescence Spectroscopy. Our transient absorption spectroscopy setup and time-correlated single photon counting (TCSPC) setup are described elsewhere.49,50 TA data were acquired on samples in 2 mm quartz cuvettes with constant stirring. TCSPC data were acquired on samples in 1 cm quartz cuvettes. All samples were purged either with CO2 or Ar for 5 minutes before measurements. Electrochemical Measurements. Cyclic voltammograms of FeTMA samples (Figure S3) were obtained with a Princeton Applied Research VersaSTAT 3 potentiostat, a silver wire reference electrode, a Pt counter electrode, and a glassy carbon working electrode in anhydrous DMF, with 50 mM tetrabutylammonium hexafluorophosphate as electrolyte. All samples were purged with N2 or CO2 before measurement. Photocatalytic Setup and Gas Chromatography. All catalysis experiments were performed in air-tight vials and illuminated with a monochromic 450 nm, 4.5-mW laser pointer (Thorlabs CPS450). We sampled the headspace of these vials after illumination with a gastight syringe (Hamilton Company) through GC. All GC measurements were performed on a Shimadzu GC2014 gas chromatography equipped with a Restek ShinCarbon ST 80/100 2m 2mm ID column, Ar 15 ACS Paragon Plus Environment

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carrier gas and a thermal conductivity detector. Calibration curves for CO/H2 were established by injecting a known amount of calibration gas. The integrated intensity of the peaks that correspond to CO/H2 were used to determine the concentration of CO/H2 in the headspace of each sample.

Supporting Information GC and DLS raw data, calculation of the equilibrium binding constant between QD and FeTMA, analyses of TA and TCSPC data. This information is available free of charge via the internet at http://pubs.acs.org.

Acknowledgement This work was supported as part of the Center for Bio-Inspired Energy Science (CBES), an Energy Frontier Research Center funded by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES), under Award # DE- DE-SC0000989 (particle synthesis and optical spectroscopy), and by the National Science Foundation through Award # CHE-1664184 (catalysis experiments). This work made use of the Keck-II facility of Northwestern University’s NUANCE Center, which has received support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF ECCS-1542205); the MRSEC program (NSF DMR-1121262) at the Materials Research Center; the International Institute for Nanotechnology (IIN); the Keck Foundation; and the State of Illinois, through the IIN. The authors thank Jenna Logsdon, Prof. Yilin Wu and Prof. Michael Wasielewski for their assistance with collection of SAXS data and for the use of their GC instrument. SAXS experiments used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.

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