Plasmonic Control of Multi-Electron Transfer and C–C Coupling in

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Plasmonic control of multi-electron transfer and C-C coupling in visible-light-driven CO reduction on Au nanoparticles 2

Sungju Yu, Andrew J. Wilson, Jaeyoung Heo, and Prashant K. Jain Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b05410 • Publication Date (Web): 06 Feb 2018 Downloaded from http://pubs.acs.org on February 7, 2018

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Plasmonic Control of Multi-Electron Transfer and C-C Coupling in Visible-Light-Driven CO2 Reduction on Au Nanoparticles Sungju Yu,† Andrew J. Wilson,† Jaeyoung Heo,‡ and Prashant K. Jain*†,§ †

Department of Chemistry, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA ‡ Department of Materials Science and Engineering, University of Illinois at UrbanaChampaign, Urbana, IL 61801, USA § Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA *Correspondence to: [email protected]

ABSTRACT: Artificial photosynthesis relies on the availability of synthetic photocatalysts that can drive CO2 reduction in the presence of water and light. From the stand-point of solar fuel production, it is desirable that these photocatalysts perform under visible light and produce energy-rich hydrocarbons from CO2 reduction. However, the multi-step nature of CO2-to-hydrocarbon conversion poses a significant kinetic bottleneck when compared to CO production and/or H2 evolution. Here, we show that plasmonic Au nanoparticle-photocatalysts can harvest visible light for multielectron, multi-proton reduction of CO2 to yield C1 (methane) and C2 (ethane) hydrocarbons. The light excitation attributes influence the C2/C1 selectivity. The observed trends in activity and selectivity follow Poisson statistics of electron harvesting. Higher photon energies and/or flux favor simultaneous harvesting of more than one electron from the photocharged Au nanoparticle catalyst, inducing C-C coupling required for C2 production. These findings elucidate the nature of plasmonic photocatalysis, which involves strong light-matter coupling, and set the stage for the controlled chemical bond formation by light excitation. KEYWORDS: hot electron, CO2RR, catalysis, artificial photosynthesis, LSPR. The conversion of CO2 captured from emissions to synthetic fuels and chemicals is a promising avenue for climate change mitigation and renewable energy production.1-5 Electrical conversion of CO2 reduction6,7 to hydrocarbons and thermocatalytic reduction8,9 of

1 Environment ACS Paragon Plus

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CO2 by H2 are two common strategies. In comparison, direct light-driven conversion of CO2 and water with the aid of a photocatalyst allows the use of abundant sunlight as a source of energy. A photocatalytic scheme may also obviate catalyst degradation or restructuring induced by electrical or thermal cycling. Moreover, the use of light excitation as a handle for control of product selectivity has scientific and technological merits. However, these promises of photocatalytic CO2 reduction remain unmet. Transition metal oxides exhibit CO2 photoreduction activity, but require ultraviolet (UV) light.10 Visible lightharvesting semiconductors, such as sulfides and selenides, are known to photocorrode.11,12 Notwithstanding these limitations, most photocatalytic schemes favor 2e−, 2H+ reduction to CO or HCOOH rather than yielding hydrocarbons, which require additional electron and proton transfer and C-C coupling steps. Even in cases where hydrocarbons are generated, it is poorly understood what light excitation conditions and mechanisms control hydrocarbon generation and how these parameters can be tuned for enhancing selectivity in favor of higher hydrocarbons. The plasmonic nanoparticle (NP) photocatalysts reported here exhibit special attributes in this regard due to their strong light-matter interaction (Figure 1a): they drive kinetically challenging multi-electron, multi-proton reduction of CO2 to C1 and C2 hydrocarbons in the presence of visible light and water. The C2/C1 selectivity is tuned systematically by variation of the attributes of plasmonic excitation. The product distribution follows a simple principle that governs the propensity of simultaneous multi-electron harvesting and light-mediated C-C coupling. Choice and design of plasmonic photocatalytic scheme for multi-electron reduction. Whereas metal NPs have been used as photosensitizers in oxide-based photocatalysis,13-15 their own photocatalytic attributes are beginning to be realized in recent years. Noble metal (Au, Ag, and Cu) NPs, in particular, display strong visible-spectrum absorption and intense light-focusing at the nanoscale, due to excitation of their localized surface plasmon resonances (LSPR).16 The collective electron oscillation induced by plasmonic excitation relaxes on the sub-picosecond timescale to yield energetic (hot) electrons. These hot electrons can vibrationally activate adsorbates at the metal surface and catalyze reactions that otherwise require high temperatures or pressures, including Suzuki coupling,17-19 H2 dissociation,20,21 water splitting,22,23 ethylene/propylene epoxidation,24-26 CO oxidation,26 and NH3 oxidation.26 Photogenerated hot carriers can also be stored on the plasmonic NP and harvested for photoredox reactions.27,28 Au NPs, under steady-state continuous-waver (CW) excitation, are


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significantly photocharged, as shown by Brus and coworkers.29 In an aqueous medium, in the presence of an effective hole scavenger, excess electrons build up on the Au NP, supported by the large double-layer capacitance of the NP in the polarizable, strongly solvating medium. We hypothesized that the strong cathodic polarization of plasmonically excited Au NPs makes them ideal photocatalysts for multi-electron reduction reactions such as CO2 conversion. Furthermore, Au is known from electrocatalytic studies to be capable of activating CO2 for further reduction30-32 and has stability against oxidation or corrosion. Colloidal Au NPs for the photocatalysis were prepared by citrate reduction. Post-synthesis, the easily photo-oxidized citrate ligands were place-exchanged with a polyvinylpyrrolidone (PVP) coating to impart the NPs with colloidal stability under hours-long photoexcitation (see Supporting Note 1 and Figure S1a,b). The PVP-coated Au NPs exhibit in their absorbance spectra a characteristic LSPR band centered at ca. 520 nm (Figure 1b) and a diameter of 11.8 ± 2.3 nm (Figure 1c and Figure S2a-d). Photocatalytic CO2 reduction was carried out on colloidal Au NPs dispersed in CO2-saturated water under irradiation of broadband visible light (λex > 400 nm, 300 mW cm−2). Hydrocarbon products generated in the closed batch reactor were monitored at periodic intervals (Figure S3a-d) by a gas chromatograph (GC) equipped with a flame ionization detector (FID). Isopropyl alcohol (IPA) was used to scavenge holes generated by plasmonic excitation of Au NPs. In this oxide/semiconductor-free photocatalytic scheme, the Au NPs serve a dual role, providing both light absorption and surface sites for adsorbate activation. Visible-light-assisted CO2 conversion to hydrocarbons. Figure 1d shows the multi-electron CO2 reduction activity of the plasmonic photocatalyst under visible light excitation (without any applied potential). The reduction yielded two hydrocarbon products: CH4 and C2H6, with turnover numbers (TONs) of ~6.8 NP−1 and ~5.6 NP−1 after 10 h of illumination, respectively. CH4 (C1) production represents an overall 8e−, 8H+ process. C2H6 (C2) production involves a 14e−, 14H+ reduction and notably, a C-C coupling step. C2H6 was confirmed to indeed be the result of C-C coupling between C1 intermediates formed from CO2 at the catalyst surface. Direct non-oxidative coupling of CH4 (CH4 + CH4 → C2H6 + H2) was not found to occur in our photocatalytic system (see Supporting Note 2 and Figure S4). Gas-phase products from photocatalytic CO2 reduction with Au NPs were also analyzed by a thermal conductivity detector (TCD) to detect species other than hydrocarbons (Figure S5). Under photocatalytic conditions (488 nm wavelength excitation at an intensity of 750 mW


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cm−2), which result in the highest product turnover, no clear evidence of O2 generation was observed (Figure S5c), suggesting that water oxidation is not prevalent under our conditions. But H2 was measured to be generated at a rate of 19.6 NP−1 h−1 (Figure S5a,b). The produced H2 is likely an outcome of the reduction of H+ generated close to the NP surface from the oxidation of IPA, the sacrificial hole scavenger. CO was not detected in this experiment (Figure S5d), indicating that the product was either not produced or produced at amounts below our detection limit. In electrochemical CO2 reduction on Au, CO is a major product and further reduction to hydrocarbons is uncommon due to the weaker binding of CO to Au.30-32 Under the electron and H+ rich conditions at the plasmonically excited NP surface, it is possible that any formed CO was reduced rapidly, via further e− and H+ transfer steps to yield the observed hydrocarbons.33-39 The CO2 reduction activity under plasmonic excitation, therefore, appears to have unique characteristics in favor of multi-electron reduction and selectivity toward hydrocarbon products. The scheme here utilizes visible light to drive the conversion; all light below the wavelength of 400 nm is filtered out. Without visible light, i.e., in a dark reaction, negligible to no hydrocarbon production was detected (see Supporting Note 4 and Figure S6a). Photothermal heating plays no role in the activity and the observed products are the outcome of a photoredox process. A control experiment without Au NPs (see Supporting Note 5 and Figure S6b) confirmed that Au NPs are indispensable to the observed activity. A control experiment in the absence of CO2, in Ar atmosphere (see Supporting Note 6 and Figure S6c) showed negligible to no hydrocarbon production, ensuring that the C1 and C2 products are from CO2 reduction and not from photoreactions of carbonaceous species leftover from colloidal synthesis. Finally, the catalyst did not exhibit measurable reaction rate without IPA, as demonstrated by a test comprised of pure water under constant illumination (see Supporting Note 7 and Figure S6d). Modulation of product branching and selectivity via tuning of light excitation attributes. A notable characteristic of the plasmonic CO2 conversion scheme is that catalytic activity and selectivity are modulated by light excitation. Photon energy (laser excitation wavelength, λex) and photon flux (light intensity) influence the activity and product branching in a systematic manner (Figures 2a-c and S7-9). The laser excitation wavelength-dependence at a fixed laser intensity of 150 mW cm−2 is shown in Figure 2a. CH4 production rate (turnover frequency or TOF) is higher at the higher photon energies, i.e., lower excitation wavelengths. On the other


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hand, C2H6 production is observed only at higher photon energies. Below these photon energies, C2 production is negligible or at sub-detection levels. A similar trend is seen as a function of increasing light intensity (λex = 488 nm; interband excitation, Figure 2c): the CH4 production rate increases near-linearly over the investigated range. On the other hand, the C2H6 production rate shows a peculiar intensity dependence. C2H6 generation is undetectable at lower intensities. However, there is a sharp onset of C2H6 generation at a light intensity of 300 mW cm−2, above which the C2H6 production rate increases with further increase in light intensity. The C2H6 selectivity mirrors this intensitydependent trend. The drastic onset of C2 production at a threshold intensity is intriguing and merits explanation. These trends elucidate the mechanism of hot-electron-driven formation of C1 and C2 hydrocarbons on the Au NP surface. The first step in CO2 reduction is the transfer of an electron to a surface-adsorbed CO2 molecule to form CO2•− (or its hydrogenated form). Due to the massive energy cost (1.9 eV) of reorganizing of the stable CO2 molecule to form the radical anion,2,40 this one-electron transfer is a rate determining step.30,31 Further e− and H+ transfer steps to form CH4 are energetically downhill and therefore not rate-limiting.33,36 Higher the laser intensity, proportionately greater is the rate of excitation and that of hot electron generation on each NP. Consequently, greater is the rate, per NP, of CO2•− formation and that of C1 production. Mechanistic understanding of hot electron-driven reaction pathways. C2 hydrocarbon generation requires the simultaneous activation of two CO2 molecules and the dimerization of the formed CO2•− (or the hydrogenated form) pair via C-C coupling on a NP,33,37,38 which is possible on the plasmonic catalyst only at high enough electron transfer rates. More elaborately, when the hot-electron-transfer rate becomes significantly high enough relative to the desorption rate kR of CO2, more than one CO2•− species can be generated on a NP within a time window defined by the residence time (τR = 1/kR) of the adsorbate. The harvesting of two electrons and the resulting formation of two CO2•− species on a NP within τR amount to a finite likelihood of the formation of a C2 intermediate. Similar to the C1 case, further electron and proton transfer steps to convert the C2 intermediate to C2H6 are likely not rate limiting. In essence, CH4 formation is rate-limited by the harvesting of one electron per NP, which is why the CH4 TOF follows a linear increase with light intensity. On the other hand, C2H6 production is rate-limited by the simultaneous (within time, τR) harvesting of two electrons


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per NP, which is why it exhibits the peculiar threshold behavior. The above-described model reproduces the experimentally observed trends (Figure 2d-f). We used Poisson statistics:

P(n;) =

e− n n!

(1)

to determine the probability of one electron (n = 1) harvested on a NP and that of two electrons (n = 2) harvested on a NP, within time, τR. In eq 1, is the average number of electrons harvested on a NP in time τR, which is obtained by multiplying τR by the experimentally measured electron harvesting rate per NP (Figure 3). The one-electron and two-electron harvesting probabilities: P(1;) and P(2;) plotted as a function of light intensity match the experimental trends in CH4 and C2H6 production rate, respectively. Most importantly, the drastic onset in C2H6 production is reproduced with a threshold intensity of 300 mW cm–2 that matches the experiment. The only adjustable parameter in this model was the surface residence time, τR, of the CO2 adsorbate, which was determined effectively to be ca. 250 s on the basis of the measured C2H6 selectivity. This effective residence time for the CO2 adsorbate is rather long and requires experimental confirmation by future adsorption kinetics studies of this photocatalytic system. The photon energy dependence is also consistent with this model. Excitation within the LSPR band (514.5 and 532 nm) corresponds to an intraband transition, which creates excited electrons with resulting holes lying within the same sp band. Recombination is rapid (100-fs timescale), resulting in a low hot efficiency and rate of electron harvesting. On the other hand, excitation at higher photon energies (457.9 nm and 488 nm) induces interband transition in Au, with the excited electrons in the sp band and resulting holes in the d band. Recombination rate is lower (1-ps timescale) than in the case of LSPR excitation and the resulting efficiency rate of electron harvesting is higher. As a result, photon energies corresponding to interband excitation show greater CO2 reduction activity. In particular, C2H6 generation is observed only under interband excitation, wherein the rate of electron harvesting is high enough to be above the threshold for simultaneous generation and dimerization of two CO2•− radical anions on a NP. This argument is supported by simulations (Figure 2d) matching the measured photon energy dependence (Figure 2a). At 488-nm (interband) excitation, there are two regimes in the intensity dependence (Figure 3, blue curve). Below 300 mW cm−2, the rate of electron harvesting, which


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corresponds only to CH4 production, shows a near-linear increase with intensity. Above 300 mW cm−2, there is an upward inflexion in the electron harvesting rate, corresponding to the onset of C2H6 production. At 514.5 nm (LSPR or intraband) excitation, where all the activity is constituted by CH4 production, the rate of electron harvesting shows a simple increase with increasing intensity (Figure 3, red curve). Conclusions. Based on the findings of this work, a mechanism for visible-light-driven CO2 conversion on the plasmonic catalyst is depicted in Figure 4a-c. Under CW plasmonic excitation, the NP is cathodically polarized (Figure 4a), making it a rich source of energetic electrons for CO2 activation. Simple Poisson statistics of electron harvesting from this photocharged NP dictate ensuing reaction pathways and product branching (Figure 4b,c). Conditions such as high photon energy and/or photon flux favor simultaneous harvesting of multiple electrons, C-C coupling reactions, and production of higher hydrocarbons (Figure 4c). The latter can become a guiding principle in plasmon-assisted solar fuel generation. The strong light-matter interaction of plasmonic catalysts imparts them special attributes: not only do these catalysts allow the use of visible light to drive kinetically challenging, multi-electron reactions, but they also enable control of mechanistic pathways and selectivity by appropriate choice of the light excitation characteristics. It is imperative to optimize further the CO2RR activity and photostability of this plasmonic catalyst system (Figure S10), a current focus of our laboratory.


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ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. Experimental methods; TEM characterization of Au nanoparticles; stability of the Au NP photocatalyst; possibility of non-oxidative coupling of CH4; detection of other products using GC-TCD; photothermal heating effect; control experiment without Au NPs; control experiment without CO2; effect of IPA; time-conversion plots for all photocatalytic reactions; and effect of PVP coating on colloidal photostability and activity.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORCID Sungju Yu: 0000-0003-0065-7486 Andrew J. Wilson: 0000-0003-3427-810X Prashant K. Jain: 0000-0002-7306-3972 Author Contributions S.Y. designed and conducted experiments, performed data analysis and modelling, and wrote paper. A.J.W. performed GC-TCD measurements and analysis. J.H. conducted TEM characterization. P.K.J. conceived project, designed experiments, developed models and analyses, and wrote paper. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This research was supported by the National Science Foundation under Grant (NSF CHE1455011). This work was conducted in part at the Frederick Seitz Materials Research Laboratory. We thank the Rauchfuss group for allowing use of their GC-TCD system. A.J.W.


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was supported by a Springborn postdoctoral fellowship.


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Figure 1. Visible-light-driven CO2 reduction to hydrocarbons using a plasmonic Au NP photocatalyst. (a) Schematic of the reaction system for Au-NP-photocatalyzed CO2 conversion to hydrocarbons under plasmonic excitation. Product turnover was monitored by gas chromatography (GC). Hydrocarbon selectivity (C2 vs. C1) was found to depend on the attributes of the light excitation such as photon energy (excitation wavelength) and photon flux (light intensity). (b) UV-vis absorbance spectrum for PVP-coated Au NPs. Vertical dashed line indicates the peak of the localized surface plasmon resonance (LSPR) band located around 520 nm. (c) Representative TEM image of Au NPs, showing NPs with diameters of 11.8 nm ± 2.3 nm (standard deviation). (d) Time course of product (CH4 and C2H6) turnover in photocatalytic CO2 reduction catalyzed by PVP-coated Au NPs under the irradiation of continuous-wave (CW) visible light (λex > 400 nm, 300 mW cm−2).


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b CH4 C2H6

0.9

0.6

0.3

c

1.2

Turnover frequency (NP‒1 h‒1)


1.2

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λex = 532 nm CH4 C2H6

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Measured selectivity for C2H6 (%)

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Figure 2. Plasmonic modulation of hydrocarbon selectivity (C2 vs. C1). (a-c) Experimentally measured turnover frequencies for CH4 and C2H6 formation (a) as a function of the laser excitation wavelength with a fixed intensity of 150 mW cm−2, (b) as a function of light intensity for intraband excitation (λex = 532 nm) of Au, and (c) light intensity for interband excitation (λex = 488 nm) of Au. Each data point in (a-c) is an average of three separate trials performed under the same conditions and the standard deviation of these measurements is indicated as the error bar in each case. (d-f) Probabilities, P(n;), of one- and two-electron transfers (d) as a function of excitation wavelength, (e) light intensity for intraband excitation of Au, and (f) light intensity for interband excitation of Au. These calculated trends reproduce the experimental ones, including the drastic onset of C2H6 generation and the threshold intensity of 300 mW cm−2. For these calculations, the surface residence time of surfaceadsorbed CO2, τR was set to 250 s, the value at which the measured selectivity P(2;)/(P(1;)+P(2;)) was reproduced by the calculations.


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e‒ harvesting rate (10 ‒3 NP‒1 s‒1)

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Figure 3. Light intensity modulation of reaction kinetics. Rate of electron harvesting (obtained from hydrocarbon generation: 8e− for each CH4 molecule and 14e− for each C2H6 molecule) as a function of light intensity for both modes of excitation: interband excitation at 488 nm and intraband excitation at 532 nm. The rate shows a super-linear rise around an intensity of 300 mW cm−2, which matches the intensity-threshold for C2H6 generation. Each data point shown in the plots is an average of measurements of three separate trials performed under the same conditions and the standard deviation of these measurements is indicated as the error bar in each case.


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Figure 4. Mechanism for plasmon-assisted CO2 reduction to C1 and C2 hydrocarbons. (a) Hot-electron-transfer process at the interface of Au and CO2; adsorption of free CO2 molecule onto a Au surface promotes hybridization of its electronic states with those of Au, narrowing its frontier orbitals and creating a reduced HOMO’-LUMO’ gap. CW illumination in the presence of the electron donor cathodically charges Au NP, resulting in a Fermi-Dirac distribution with a raised (quasi) Fermi level, EF’, at steady state. The EF’ depends on the photon energy and light intensity. A higher EF’ favors a higher rate at which hot electrons are transferred to adsorbed CO2 molecules. (b,c) Schematic representation showing how light excitation influences hydrocarbon product selectivity; (b) the plasmonic excitation of Au NPs causes a hot electron to be transferred to adsorbed CO2 to form a radical ion intermediate, CO2•− (or its hydrogenated form). After this rate-determining step (RDS for C1 generation), the formed CO2•− (or its hydrogenated form) proceeds through a cascade of hot electron and proton transfer steps to result in CH4 generation. (c) When the hot electron transfer rate is large (e.g., at high light intensity under interband excitation), more than one electron transfer can take place within the surface residence time of adsorbed CO2, resulting in the simultaneous activation of two CO2 adsorbates (RDS for C2 generation). The formed CO2•− intermediate pair can undergo C-C coupling. Subsequent transfer of a series of hot electrons and protons results in the formation of C2H6.


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Plasmonic Control of Multi-Electron Transfer and C–C Coupling in

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