Watching Visible Light-Driven CO2 Reduction on a

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Watching Visible Light-Driven CO Reduction on a Plasmonic Nanoparticle Catalyst Gayatri Kumari, Xueqiang Zhang, Dinumol Devasia, Jaeyoung Heo, and Prashant K. Jain ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b03617 • Publication Date (Web): 08 Aug 2018 Downloaded from http://pubs.acs.org on August 8, 2018

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Watching Visible Light-Driven CO2 Reduction on a Plasmonic Nanoparticle Catalyst Gayatri Kumari1, Xueqiang Zhang1, Dinumol Devasia1, Jaeyoung Heo2, and Prashant K. Jain1,3* 1

Department of Chemistry, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, United States 2

3

Department of Materials Science and Engineering, University of Illinois at UrbanaChampaign, Urbana, Illinois 61801, United States

Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, United States Corresponding Author Email: [email protected]

Abstract: Photocatalytic reduction of carbon dioxide (CO2) by visible light has the potential to mimic plant photosynthesis and facilitate the renewable production of storable fuels. Accomplishing desirable efficiency and selectivity in artificial photosynthesis requires understanding of light-driven pathways on photocatalyst surfaces. Here, we probe with single-NP spatial resolution the dynamics of a plasmonic silver (Ag) photocatalyst under operando conditions of visible light-driven CO2 reduction. In-situ surface enhanced Raman spectroscopy captures discrete adsorbates and products formed dynamically on single photocatalytic NPs, most prominent among which is a surface-adsorbed hydrocarboxyl (HOCO*) intermediate critical to further reduction of CO2 to carbon monoxide (CO) and formic acid (HCOOH). Density functional theory (DFT) simulations of the captured adsorbates reveal the mechanism by which plasmonic excitation activates physisorbed CO2 leading to the formation of HOCO*, indicating close interplay between photoexcited states and adsorbate/metal interactions. Keywords: surface enhanced Raman scattering, single-molecule, localized surface plasmon resonance, carbon fixation, artificial photosynthesis

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Recent climate agreements have ratified the need to simultaneously curtail fossil fuel usage, deploy solar energy for residential and industrial use, and sequester existing stores of atmospheric CO2. These targets are intensifying efforts to mimic the photosynthetic ability of green plants to utilize sunlight for conversion of water and CO2 into O2 and sugars, an energy source for later use. Central to artificial photosynthesis are light-absorbing photocatalysts that provide the thermodynamic driving force and kinetic acceleration needed to accomplish challenging multi-electron, multi-proton reactions involved in CO2 conversion. Since the seminal work on photocatalytic CO2 reduction reaction (CO2RR) performed on various semiconductors by Inoue et al.1 in 1979, transition metal NPs supported on semiconductors and oxides, coordination compounds, and metal organic frameworks have been explored as photocatalysts.2–5 More recently nanoparticles (NPs) of the CO2RR catalyst metals Au, Cu, and Ag have garnered attention due to their localized surface plasmon resonances (LSPRs) in the visible wavelength region.6–8 Photoexcitation of these collective electron oscillations results in the generation of hot electrons (and holes) that have been found to drive/catalyze chemical reactions9–13 on the metal NP surface, such as hydrogen dissociation,14 CO2 reduction,15,16 water splitting,17 H2 production,18 and ethylene epoxidation.19 For most photocatalytic CO2RR processes, including plasmon excitation-assisted CO2RR, the complex manner in which photoexcited states and catalytically active surfaces synergize is not fully understood at the molecular level. Abundant insights from electrocatalytic CO2RR are available,20–23 but it is still mysterious what intermediates and pathways govern CO2RR when strong light/matter interactions and photoexcited states are involved. Plasmonic control of CO2RR is possible, in principle, through engineering handles, such as photon energy, excitation density, metal composition, active facet, and presence of oxide support. But without mechanistic models, it is difficult to employ these handles in a rational manner for optimization of efficiency and product selectivity. We construct a mechanistic picture of plasmon-assisted CO2RR on Ag. Using in-situ Raman vibrational spectroscopy, we probed a plasmonic Ag NP photocatalyst in dynamic catalytic action under CO2RR conditions and captured discrete events of adsorption and product generation on individual NPs. We observed a key surface intermediate, HOCO*, that represents the very first step in the plasmonic CO2RR: the transfer of a 1e-/1H+ pair from the Ag NP to the CO2. From the observed adsorbates and DFT simulations, we postulate a reaction pathway for how plasmonic excitation leads to multi-electron reduction products 3 ACS Paragon Plus Environment

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from physisorbed CO2. The adsorption motif of the HOCO* intermediate dictates product selectivity. We first devised a spatially-resolved method to track in real-time the catalytic activity of individual NPs of a supported plasmonic Ag photocatalyst under continuous 514.5 nm excitation (Fig. 1a). The power of single-NP or single-molecule-level catalytic probing is evident in recent work.24–26 In the present study, single-NP, sub-second-resolution probing allowed us to capture stochastically formed surface adsorbates and intermediates, which are otherwise undetectable in spatially-averaged kinetic studies due to their shorter lifetimes and/or lower steady state concentrations as compared to stable products. Such single-NP catalytic probing typically utilizes fluorescence microscopy which is limited to fluorogenic probes and reactions; however, CO2 or species involved in CO2RR do not exhibit visible fluorescence. Therefore, we employed Raman vibrational scattering from adsorbates formed on the photocatalyst surface under laser excitation. Raman vibrational spectra provide a labelfree method of detection that also has the chemical specificity to identify the adsorbed species and, in some cases, its binding mode. Although, Raman scattering suffers from low crosssections, excitation of the LSPR generates a strong electric field around the NP,27,28 leading to intense surface enhanced Raman scattering (SERS) selectively from adsorbates at the NP surface.29 While NPs of all three coinage metals (Au, Ag, and Cu) have the potential for plasmonassisted CO2RR, we chose Ag NPs for maximizing the sensitivity of SERS detection. Ag NPs have a visible-spectrum LSPR that is not damped by inter-band transitions,30 unlike the case for Au and Cu. Due to the lower damping and higher quality of the LSPR,31 plasmon-excited Ag NPs support much stronger electromagnetic fields32 and orders-of-magnitude larger SERS amplification factors when compared to Au NPs and, to an even greater extent, when compared to Cu NPs.33 In recent years, the complementary technique of in-situ Fourier-transform infrared spectroscopy (FTIR) has been of considerable utility in probing the kinetics of surface chemical reactions occurring in catalytic and electrocatalytic transformations, including CO2RR processes.34 However, SERS, despite the requirement of a SERS-active substrate, has distinct advantages when it comes to capturing rare adsorbates and intermediates in catalytic processes. Whereas FTIR spectroscopy functions in attenuation mode, the scattering mode of SERS allows high signal-to-background detection and therefore, high sensitivity. The


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selective detection of species vicinal or adsorbed to the NP surface also reduces background interference from abundant species in the bulk reaction medium. With a focused incident laser beam and a sparse area distribution of NPs, single-NP-level spatial resolution is possible with SERS. In fact, SERS using Ag NPs is known to have the sensitivity to detect single molecules.35,36 In the past, SERS has been successfully employed to monitor the dynamics of ligand binding/unbinding,37 dye on/off blinking,38 and redox reactions at the surface of NPs;39 however these studies have generally been limited to SERS labels: a specific dye or chromophore (e.g., Rhodamine 6G38 and p-nitrothiophenol40) that is chemically linked to the NP surface. The work here pushes the limit of single-NP-level SERS by employing it in a truly label-free manner for probing the rich dynamics of the photocatalytic transformation of an important chemical substrate, CO2, which is not a SERS label. Surface intermediates and products of this transformation, which are multifarious and a priori unknown, are captured on-the-fly in the SERS spectra, allowing mechanistic reaction pathways to be reconstructed.

Results and Discussion The photocatalyst consisted of a low-area dispersion of Ag NPs on a cleaned glass substrate (Fig. 1c), detailed characterization of which is provided in the Supporting Information (Figs. S1-S5). The photocatalyst was subject to mild vacuum-annealing. Following this cleaning procedure, the Ag NP surfaces were ensured to be free of any citrate ligands or carbonaceous contaminants (Fig. S5). HRTEM imaging also shows that the Ag NPs themselves are quasispherical in shape, crystalline, and comprised primarily of metallic Ag (Fig. S1). X-ray photoelectron spectroscopy (XPS) shows that the surface of the Ag NPs is comprised of AgOH or adsorbed OH-, in addition to metallic Ag (Fig. S2). This partially hydroxylated condition of the Ag surface is consistent41,42 with the presence of gas-phase water in the ambient medium (relative humidity set at 30 %) and the basic environment posed by the KOH-treated glass substrate. On the substrate-supported photocatalyst, we identified well-isolated, individual NP scatterers by means of their dark-field scattering (Fig. 1c). High-resolution transmission electron microscopy (HRTEM) characterization of a representative photocatalyst sample (Fig. S1) shows that a typical scatterer can be either a single Ag NP or it can be an aggregate (dimer, trimer, …) of closely spaced Ag NPs. However, single Ag NPs have a narrow LSPR 5 ACS Paragon Plus Environment

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mode centred at 400 nm, with little to no overlap with a 514.5 nm excitation. Therefore, only aggregates are likely to be photocatalytically active and SERS responsive in our studies. As shown by electrodynamic simulations in Fig. S3 and dark-field scattering spectra of typical NP scatterers in Fig. S4, aggregates (e.g., dimers) of closely spaced Ag NPs have red-shifted LSPR modes that spectrally overlap the 514.5 nm excitation. Moreover, constructive coupling between plasmonic oscillations of individual NPs within an aggregate results in intense electric fields at the junction between the NPs. Such electric field hotspots locally enhance photochemistry and also amplify SERS cross-sections to levels necessary for sensitive detection of adsorbates and intermediates in the CO2RR process. For instance, a dimer of touching Ag NPs, a typical feature seen in TEM imaging (Fig. S1), is estimated to support under 514.5 nm excitation an electric field enhancement, (|E|/|E0|)2, of 2.78 x 104 at its inter-particle junction. The resulting peak SERS amplification factor of 7.72 x 108 is large enough to allow single-molecule-level sensitivity (Fig. S3 and Supporting Information) and comparable to past studies.43,44 The Ag photocatalyst was plasmonically excited by a continuous-wave 514.5 nm laser under a gas-phase CO2 environment inside a microfluidic reaction cell (Fig. 1a). These conditions were found to induce CO2 reduction on the surface of the Ag photocatalyst, with the counter half-reaction involving the oxidation of water adsorbed in the form of OH- under the basic conditions at the Ag surface. We also considered the role of photothermal heating in driving CO2RR. Under continuous-wave excitation, the surface of the NPs can be at a higher steadystate temperature compared to that of the surrounding bulk medium. Under the excitation conditions employed in our studies, the steady-state local temperature at the NP surface is estimated to be 17 K higher than the ambient environmental temperature of 296 K (see Supporting Information). However, the marginally elevated temperature of 313 K or 40 ºC at the NP surface is not sufficient to activate thermocatalytic CO2RR, which requires an activation energy on the order of 1 eV. In other words, the photocatalytic role of the excitation light (with a photon energy of 2.4 eV) is of critical importance for driving the reaction and the contribution of photothermal activation is marginal, at best. The CO2RR activity of each selected NP under laser excitation was monitored in real-time by SERS. Following the initiation of CO2 flow (t = 0 s), a representative photocatalytic NP showed dynamic SERS activity (waterfall plots of Figs. 1e, S8-S11 and Supporting Movie S2). A hallmark of the dynamics is the stochastic appearance-disappearance of distinct


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Raman vibrational modes, which upon analysis and assignment (supported by DFTcalculated modes as summarized in Table 1) are found to correspond to specific adsorbates and products formed in CO2RR. This dynamics is captured in a digital time-trajectory of the catalytic activity of the NP (Fig. 1h). While a larger catalogue of key adsorbates dynamically sampled across all our investigated NPs is presented later in Fig. 2, two classic examples from this representative NP are presented in Fig. 1f and 1g. One of the representative events captured was the physisorption of CO2, identified by its characteristic Fermi resonance dyad at 1263 and 1355 cm-1 (Fig. 1f).27,45,46 Unlike common SERS analytes such as Rhodamine 6G,38 CO2 is not known to offer resonant Raman enhancement. The on-the-fly detection of physisorbed CO2 is, therefore, a testimony of the sensitivity of our in-situ single-NP-level SERS detection.38 Another notable event on this NP (Fig 1g) was the detection of formic acid (HCOOH), the two-electron reduction product in CO2RR, common on Ag, discussed in detail in the next section. The discrete nature of HCOOH formation events is shown in the digital trajectory for this NP (black curve, Fig. 1h). It must be noted that for each NP, we performed a prior control experiment in air, under laser excitation. Only a broad scattering spectrum was seen with no distinct vibrational bands or time-dependent stochastic dynamics. The control experiment ensured that the chosen NP presented a clean surface and did not exhibit SERS from residual citrate ligands or photogenerated species in air. The dynamic SERS activity in CO2 gas was indeed attributable to CO2RR, further ascertained by a separate experiment with

13

CO2 (Fig. S16) wherein the

vibrational modes of the detected surface adsorbate (i.e., carbonate) exhibited the frequency shift expected for the heavier 13C-labeled isotope. NP-to-NP heterogeneity was observed, which we classified into three broad categories of behaviour. Of the 300 NPs studied (Fig. 1b), 27% showed CO2RR activity similar to that discussed above. This finding is consistent with conventional wisdom in heterogeneous catalysis that not all surface sites are catalytically active; only a fraction of all available sites exhibit catalytic turnover.47 A small fraction of the selected NPs showed spectra with only the D and G bands corresponding to graphitic carbon, but no other CO2RR activity, indicating possible poisoning of the NP surface by carbon. Under laser excitation, CO2 may have served as the source of the deposited carbon at these NPs (Fig. S7). About 68% were categorized as inactive (Fig. 1b and Fig. S6) owing potentially to a catalytically inactive surface or the


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absence of SERS hot spots.48 Results on the photocatalytic dynamics of all the active NPs are summarized in Fig. 2. One may expect that the primary event in plasmonic CO2RR is the adsorption of CO2 on the Ag NP surface. From ultrahigh vacuum single-crystal studies on metallic (unoxygenated) Ag surfaces, CO2 does not appear to chemisorb, but physisorption of CO2 is known to occur;49 however, on hydroxylated Ag surfaces, this adsorption behaviour may be modified. Several instances of adsorbed CO2 were detected in our studies, evident by the appearance of ν- and ν+ Fermi resonance modes that result from the coupling of the first overtone of the δ OCO bending mode and the symmetric νs CO stretching vibration. Across various NPs, the ν- and ν+ mode frequencies were centred around 1270 ± 10 and 1380 ± 20 cm-1 respectively (Figs. 1f and 2a).50,51 The close agreement with the Fermi resonance modes of gas phase CO2 50,51 indicate that the detected CO2 is in a physisorbed form with near-linear, undistorted geometry, as further confirmed by the DFT-computed spectrum of physisorbed CO2 on Ag (Fig. 3a) and calculations of the Fermi resonance dyad (Table S3). Multiple products of CO2RR were observed across all single NPs investigated, with the relative abundance of various species shown in Fig 2g. It must however be acknowledged that the relative abundance of a specific intermediate or product, as captured by our SERS spectra, is not only a function of the relative turnover frequency of that species, but also a function of the surface lifetime of the species and its SERS cross-section. Therefore, the relative abundance ought not to be assumed to be a direct representation of the specific activity of the Ag NP catalyst for that intermediate or product. The dominant C1 product on Ag is known to be carbon monoxide (CO).52,53 Correspondingly, we detected CO at the most number of NPs (Fig. 2g); but this correspondence may only be coincidental, as acknowledged above. An example of CO detection is shown in the SERS spectrum in Fig. 2c, where identification was made on the basis of the signature CO stretching mode at 2221 cm-1 in close agreement with literature54,55 and the DFT-calculated mode frequency for CO on Ag (Fig. 3f). Other instances of CO exhibited a stretching mode frequency in the range of 1940-2230 cm-1, which is thought to arise from its adsorption geometry varying between atop or bridged (Fig. S13a-f and Table S1).55 CO is a precursor to the formation of other C1 species such as the four-electron product formaldehyde or HCHO (Fig. S14) and the six-electron product methanol or CH3OH (Fig. S15) which were also observed, but less frequently (Fig. 2g).56,57 The propensity of Ag 8 ACS Paragon Plus Environment

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catalysts to form HCHO is low attributed to the high over-potential requirement for the step involving CO → CHO* intermediate, a critical precursor of HCHO.58,59 The propensity of Ag catalyst to form CH3OH is likewise low, consistent with the finding of only one catalytic NP that displayed CH3OH formation in the sample set of nearly 300 NPs studied. Formic acid (HCOOH) or formate (HCOO-), known to be the second most predominant product on Ag,52,53 was detected as the second most frequently observed species in SERS captures. As an example, intense fingerprint modes and twin modes (2739 and 2820 cm-1) characteristic of HCOO- are seen in the spectrum shown in Fig. 2d.20 Another example of HCOOH detection is shown in Fig. S8c. Further, CO2 is known to form carbonate and bicarbonate species on oxide or hydroxide-rich Ag surfaces.60 In our studies, we observed carbonate formation (Figs. 2e, S10 and S16) on a surface oxidized Ag NP, e.g., Ag2O + CO2 → Ag2CO3. Bicarbonate was seen in the form of strong modes at 1067 and 1436 cm-1 alongside a surface hydroxyl feature around 3750 cm-1 (Fig. 2f), indicating the reaction: AgOH + CO2 → AgHCO3.61 However, 3750 cm-1 could also be attributed to (H2)AgH, i.e., hydrogen complexed with Ag hydride.62 A competing reaction in CO2 reduction is hydrogen (H2) production from reduction of protons from adsorbed water or a hydroxylated surface. Molecular H2 was observed in possibly weakly adsorbed form in SERS spectra (Fig 2f), identified by the 4120 cm-1 band, ascribed to the H-H stretching mode.63,64 The capture of molecular hydrogen, otherwise difficult to detect at standard temperature and pressure owing to its low inherent Raman scattering cross section, illustrates the sensitivity of our methodology. Most interestingly, some of the NPs exhibited a species, which could not be assigned to any of the aforementioned adsorbates or products. A representative SERS spectrum of this species exhibited two strong bands at 1223 and 1640 cm-1 and a weak O-H stretching mode around 3200 cm-1 (Fig. 2b). Following an extensive search of potential adsorbates simulated using DFT, the intermediate was found to be a surface-adsorbed hydrocarboxy (HOCO*) intermediate (Fig. 3d, e). The assignment to the HOCO* structure could be made with high confidence due to the simplicity of the spectrum with two strong Raman vibrational modes in all observed instances of this species (Fig. S12). Depending on its binding motif or orientation, and therefore binding strength, the two modes in HOCO*, δ O-H and ν CO can vary in their vibrational frequency and relative intensity. We observed such variation across HOCO* detection events on different NPs (Fig. S12a-f and Table S1).


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HOCO is an elusive molecular radical involved in combustion and atmospheric chemistry.65 We postulate that the surface-adsorbed HOCO* captured in our experiments is a critical intermediate in the plasmonic excitation-triggered CO2 reduction. This intermediate can undergo further reduction to stable hydrocarbon products along separate reaction branches (Fig. 3i). The finding arises from detailed DFT modelling of possible adsorbate structures, energies, and reaction pathways (Figs. 3 and 4, Tables S2-S10). While considerable theoretical understanding of electrocatalytic CO2RR is available,59,66,67 our finding sheds unique light on the nature of CO2RR triggered on a photocatalyst surface by visible light excitation. Guided by the experimentally observed species, eight key adsorbates/intermediates on a prototype Ag surface were modelled by DFT (Fig 3). Upon geometry optimization of the adsorbate/metal complex, the Ag surface restructured to a stable cluster-like geometry. Adsorption-induced surface reconstruction is known in catalysis.68 Experimentally observed SERS vibrational modes are tabulated (Table 1 and Tables S3-S10) alongside Raman modes from computed spectra of adsorbate models to show the basis for all assignments made in prior discussions. Note that DFT-calculated Raman spectra provide estimates of mode frequencies that are sufficiently reliable for making assignments. However, computed Raman spectra cannot reproduce intensities of SERS bands, because the latter depend on the local field enhancement or amplification factors for the vibrational modes in question. On Ag, CO2 exhibited a physisorbed form (stable by ∆G = -0.30 eV relative to 1 atm gasphase CO2), indicated by its large distance of 3.4 Å from the Ag surface in the relaxed structure and only a slightly off-linear geometry having an OCO angle of 178.7 ͦ (Fig. 3a). Numerical calculations (Table S2) using modes at 649 & 664, 1396, and 2444 cm-1 assigned to bending, symmetric, and antisymmetric vibrations respectively yielded the Fermi resonance dyad, νc- and νc+, at 1287 and 1421 cm-1, respectively. Physisorbed, near-linear CO2 has a relatively high stability on the Ag surface (Fig. 4b) and requires further activation. CO2 conversion involves a rate-limiting transformation of the strong sp bonding to sp2 bonding. Activation of CO2 for this transformation can be achieved by chemisorption and/or charge transfer-mediated distortion of the CO2 to a bent CO2-. anion radical. From our DFT calculations of CO2δ- on Ag, the energetic cost of such activation, ∆G = 0.9-1.3 eV, is prohibitively large at 0 V potential and room temperature. However, LSPR excitation (with photon energy of 2.4 eV) can provide the needed activation.


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The LSPR excitation can trigger the activation of the physisorbed CO2 by way of interband or intraband damping within the metal. Although interband damping is not prevalent in Ag in the visible frequency region, intraband damping of the LSPR can result in the formation of an excited (hot) electron/hole pair in the Ag NP, with the electron above the Fermi level and the corresponding hole located 2.4 eV lower in energy. The hot electron can be transferred69 to the physisorbed CO2 if the electron occupies a state higher in energy than the LUMO of the physisorbed CO2. Alongside, the leftover hot hole can be transferred to the HOMO of another adsorbate such as surface OH-, provided the hole lies deeper in energy than the HOMO.

A second possible mechanism for plasmonic excitation emerges from an analysis of the molecular orbitals of physisorbed CO2. For gas phase CO2, the HOMO-LUMO gap is calculated to be 14.03 eV, in the deep-UV range. However, upon physisorption, the energy states of CO2 and the Ag admix, and the HOMO-LUMO gap is reduced to 3.5 eV (Fig. 4a). In fact, the actual HOMO-LUMO gap on the extended NP surfaces may be even lower as seen from the DFT-calculated trend of decreasing gap with increasing cluster size. The gap approaches the visible energy range (where our excitation wavelength is located) as the bulk band structure of Ag emerges (Fig. S19).70 Enabled by the spectral overlap of the LSPR excitation with the HOMO/LUMO gap of the CO2/metal complex, the LSPR can be damped

via a direct HOMO/LUMO excitation of the CO2/metal complex.71 Charge density maps show that the HOMO and LUMO states of the adsorbate/metal complex are dominated by the electronic states of Ag, but the contribution of the CO2 fragment is greater in the admixed LUMO than in the admixed HOMO. Thus, HOMO-LUMO excitation of the adsorbate/metal complex by visible light can be expected to result in electron transfer from the Ag NP to the physisorbed CO2 (Fig. 4a), amounting to charge separation across the CO2/metal interface.16 To account for such an influence of the plasmonic light excitation, we modeled a chargeseparated state of the CO2/metal complex with a -1 charge confined to CO2 and +1 charge on the Ag surface. Upon geometry relaxation, CO2 retained a partial electronic charge of -0.4 (counterbalanced by a positive charge of +0.4 on Ag). Depending on the binding motif of CO2δ- on the surface: C atom facing the surface (Fig 3b) or O atoms facing the surface (Fig 3c), the adsorbate structure, free energy, (Table S2) and computed Raman spectrum varied somewhat. But, the νs OCO mode was estimated to be in the 1200-1300 cm-1 range, a signature which was not observed in our studies. This charged CO2δ- exhibited a bent geometry, the activated form of CO2. But its high energy (∆G = 0.9-1.3 eV relative to 11 ACS Paragon Plus Environment

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physisorbed CO2) implies that the bent CO2δ- lifetime may be too low for this species to be captured in our spectra, even if it is produced transiently by plasmon-excitation induced hot electron transfer. The hot hole remaining on the Ag NP can generate a proton from adsorbed water (in the form of surface hydroxide under the basic conditions on the Ag surface). The proton (Agn-1AgO--Hδ+) can transfer from the Ag surface to CO2δ- forming the HOCO* intermediate. HOCO* can be considered to be a derivative of a bent CO2-. anion radical that is considerably stabilized by protonation. In fact, energy optimization of CO2δ- on Ag (bound to the surface by the C and one of the O atoms) in the presence of a surface adsorbed Hδ+ resulted in the formation of a surface adsorbed HOCO* intermediate (Fig. 3d, 3e) with its C atom bound to the surface but O and H atoms pointing away from the surface. The computed Raman vibrational spectrum of this intermediate matched the experimentally observed spectra (Figs. 2b and S12) with the variation in observed spectra explained by small differences in the binding orientation (Fig. 3d vs. 3e). Thus, as per this model, plasmonic excitation leads to charge separation across the CO2/metal interface, leading to CO2 activation and subsequently its reduction to the HOCO* intermediate, elementary steps of which are indicated in Fig. 3i. Given the partially hydroxylated state of the Ag surface, as determined by XPS, it is likely that a hydroxylated site on the Ag surface (Fig. S17), represented as Agn-1AgOH, serves as the proton source, while undergoing oxidation to Ag2O in the counter half-reaction: Agn-1AgOH + CO2(ads) → Agn-2Ag2O + HOCO* (1) Such a scheme is supported by observations of Ag oxide formation in the lower frequency region of our in-situ SERS spectra (Fig. S18). Thus, the first-step in the plasmonically driven CO2RR involves a 1e-/1H+ transfer from the partially hydroxylated Ag photocatalyst to the physisorbed CO2 resulting in the formation of HOCO*. Based on the finding of surface OH- as an active participant in the reaction, local heterogeneities in the degree of hydroxylation, i.e., the availability of surface hydroxide, can be thought of as a primary factor in the NP-to-NP variation observed in the CO2RR activity. NPs with little to no surface hydroxide available are likely to be inactive (about 68% of the NPs investigated), due to the lack of a proton source. HOCO* is the critical intermediate that gets converted to downstream products (Fig. 3i and 4b). Product branching appears from our model to be dictated by the binding motif of 12 ACS Paragon Plus Environment

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HOCO*. The HOCO* intermediate bound to the Ag surface via its C atom (formed from a CO2δ- adsorbate of similar geometry) can accept a e-/H+ pair cascading down the energy landscape (green arrows, Fig. 4b) to form the two-electron reduction product HCOOH (Fig. 3h).67 Other potential pathways for formation of HCOO- and HCOOH (Fig. 3g and 3h) directly from the short-lived CO2δ- adsorbate (surface bound by the O atoms) are shown by pink arrows in Fig. 4b. However, HOCO* bound to the Ag surface via an O atom (obtained from a CO2δ- adsorbate of similar geometry) was unstable, dissociating spontaneously to form surface-adsorbed CO and hydroxyl. The latter reaction is a potential pathway for plasmonically assisted CO2 conversion to the other two-electron reduction product, CO (Fig. 3f). The calculated υs CO stretching mode at 2231 cm-1 (Fig. 3f) agrees with the experimentally observed SERS mode in Fig. 2c, implying that the adsorbed CO generated experimentally is parallel to the Ag surface. These findings on photocatalytic CO2RR made here on Ag NPs could be applicable to other plasmonic metal and/or oxide photocatalysts. In particular, light excitation-mediated e-/h+ separation across the CO2/photocatalyst interface and photoinduced electron or hydride (H+ + e-) transfer to CO2 may have general relevance for CO2RR activation on photocatalysts. However, specific aspects such as which intermediates are involved, prevalence of C-C coupling, what hydrocarbon products are formed, and in what branching ratios and yields are likely to be dictated by specific physicochemical properties of the photocatalyst, such as photoabsorption characteristics, charge carrier dynamics, Fermi level and electrochemical properties, surface electronic structure, surface chemistry, and electronic interactions with adsorbates, including CO2, H2O, O2, H2, hydride, and hydroxide.

Conclusion This study shows the power of in-situ single-NP SERS spectroscopy. In particular, we demonstrate the ability of this method to capture non-resonant, non-chromophoric surface species formed dynamically in catalytic and photocatalytic reactions, advancing beyond seminal reports of single-molecule detection using SERS. The captured adsorbates and intermediates, which would have otherwise remained undetected in ensemble studies, reveal deeper insights into the molecular mechanism of a plasmonically assisted multi-electron, multi-proton conversion reaction. In particular, the discovery of the photogenerated HOCO* 13 ACS Paragon Plus Environment

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intermediate suggests a close interplay between photoexcited states and surface/adsorbate interactions that may be applicable to other plasmonic photocatalytic reactions72 besides CO2RR. The mechanistic understanding of the activity of the plasmonic catalyst reaction is being utilized for efficient, selective production of liquid fuels from CO2 and visible light. Further elucidation of the chemical/structural origin of the observed NP-to-NP variation in activity and product generation is also of importance for identifying champion photocatalysts.73


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Methods Experimental Methods: A gas flow cell was constructed for each imaging/spectroscopy experiment. Glass cover slips (24 x 60 mm2, VWR SuperSlips, No.1) and 1-mm thick glass slides (25 x 75 mm2, VWR) were cleaned by etching away the top silica surface in 2 M KOH under heat for 30 min, followed by multiple washing and sonication steps in deionized (DI) water. To 10 µL of PELCO® NanoXactTM 60-nm diameter citrate-capped Ag NPs from Ted Pella (Item no. 84050-60), 50 µL of water was added. The diluted colloid was sonicated for a few minutes and 10 µL of it was drop-casted on the cleaned cover slip followed by drying in air. This procedure has been optimized to yield a sample consisting of a wide area of immobilized, well-separated catalytic Ag NP scatterers, as shown by TEM imaging of such a sample (Fig. S1). In order to rid the Ag NPs of citrate-based ligands, the sample was annealed at 80-90 ͦC for 5 hr in vacuum. After cooling, the cover slip was incorporated into a gas flow cell. These annealing conditions, while effective at removing citrate ligands and any resulting carbonaceous products (see Fig. S5), do not cause any observable reshaping or sintering of the Ag NPs. Also, the NPs continue to be comprised of crystalline, metallic Ag following the vacuum-annealing. The flow cell was mounted onto the stage of an Olympus inverted microscope and secured using clips. In order to identify Ag NP scatterers, dark-field scattering imaging was performed by illuminating the sample with an Olympus U-LH100-3 100 W halogen lamp focussed through an Olympus U-DCW 1.2-1.4 NA oil immersion dark field condenser. The scattered light was collected by an Olympus UPlanApo 0.5-1.35 NA 100x oil immersion objective, the adjustable aperture of which was tuned to get high S/N scattering from scatterers against a dark background. Dark-field imaging allowed the identification of a scatterer that is well isolated spatially from other scatterers. The chosen scatterer was aligned with the spectrograph slit for Raman spectroscopy. Raman imaging/spectroscopy was performed with a 60x Olympus UplanApo water immersion objective. A 514.5 nm green laser (Stabilite 2017) was attenuated by neutral density filters to a power of 5 mW. The laser beam was reflected onto the sample using a fluorescence cube (Z514LP) and focussed using the 60x objective onto a single Ag NP scatterer identified in the sample. The laser power at the sample surface was ca. 3 mW and the beam diameter (fwhm) was 3.6 µm, resulting in an excitation intensity or density of 0.295 mW/µm2. Surface-enhanced Raman scattering (SERS) from the emitter was imaged on a liquid nitrogen cooled Princeton Instruments PyLoN 100B charge-coupled device (CCD). For Raman spectroscopy, an Acton SP 2300 spectrograph 15 ACS Paragon Plus Environment

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with a 300 lines/mm grating was used for dispersing the scattering signal onto the CCD. SERS spectra were collected continuously (in movie format) from the selected NP in gas phase (air, CO2, or 13CO2) with a 0.2-s acquisition time per frame using WinSpec software. For each NP selected, a 100-s long control experiment was first performed in air. If no distinct Raman bands from residual citrate or its decomposition products were seen in air, in-

situ SERS spectra were acquired (200 or 300-s long movies) in an atmosphere of pure CO2 (Airgas Inc., research grade). The gas was filled in a balloon through which it was fed into the flow cell using a syringe. The instant at which CO2 gas was injected into the flow cell was set at t = 0 s, but the emitter typically had to be refocused to correct for drift caused by CO2 injection. SERS spectra analysed and presented in the data were those collected after such refocusing, which took several seconds. For isotopic studies,

13

CO2 gas purchased from

Sigma Aldrich (catalogue # 364592) was used. SERS spectra movies were processed using ImageJ and OriginPro 2016. For most frames in a movie from a NP, the collected spectral signal is featureless and those frames were not subject to further analysis. Many other frames displayed SERS spectra, which were unassignable to an identifiable species or to a complex mixture of multiple species. A subset of movie frames, which exhibited SERS spectra assignable to a specific adsorbate or product, were subject to further analysis. Each SERS spectrum was smoothed using the 5-point, Savitzky-Golay method and the spectrum baseline was subtracted in OriginPro. SERS peak positions, obtained by fitting using the inbuilt Lorentzian function in OriginPro, were used for assignment of species. Labeled modes use the following symbology: νas-antisymmetric stretch, νs-symmetric stretch, ω-wagging, ρ-rocking, σ-scissoring, τ-twisting, and δ-bending. The results were based on a catalogue of NPs investigated in multiple experiments all performed with the same protocol. Density functional theory (DFT) simulations: DFT calculations on model adsorbates were performed using Gaussian 09.74 Adsorbate geometries were built in open-source Avogadro software.75 A two-layer Ag slab consisting of 22 atoms and exposing the Ag (121) surface was constructed using in-built crystallographic data. The chosen molecular moiety was located with angstroms of the Ag slab. The adsorbate (metal + moiety) geometry was then relaxed in the ground electronic state using the wB97XD functional. For C, H, and O atoms, the all-electron basis set, 6-311++G(d,p) with polarization and diffuse functions was employed. Outer shell electrons of Ag were described by the LANL2DZ basis set, separately from the core electrons, which were described by the LANL2 effective core potential (ECP). 16 ACS Paragon Plus Environment

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In some adsorbate models, a light-induced charge separated state was simulated by assigning a fragment-wise charge of +1 to the metal and -1 charge to the molecular moiety, but allowing the Mulliken charge to redistribute in the relaxation. Raman vibrational mode calculations were performed for the optimized adsorbate geometries at 298.15 K and 1 atm. Both harmonic and anharmonic models were tested, but no significant difference was found. Computed Raman spectra were plotted using Gauss View.76 Thermochemical parameters were extracted from the vibrational mode calculations performed at 298.15 K and 1 atm using the harmonic model. For calculations carried out in this study, the free energy of a reaction was calculated as ∆rG = ∑(εo + Gcorr)products - ∑(εo + Gcorr)reactants (2) where (εo + Gcorr) represents the sum of electronic and corrected thermal free energies. For reaction steps involving transfer of a H+/e- pair, the free energy of the pair was set as per the computational hydrogen electrode concept.59 At 0 V, the following reaction: (3) is in equilibrium at all temperatures and 1 atm H2. Therefore, the chemical potential or the free energy of H+/e- pair is half the free energy of gaseous H2. The latter was obtained from a separate DFT calculation. Chemical structures of relaxed adsorbate geometries and charge density maps for molecular orbitals (only for physisorbed CO2) were generated using Avogadro. Mulliken atomic charges for relaxed structures were viewed in Gauss View.

Acknowledgements. G.K. acknowledges Aditi Sharma for her help with substrate preparation and Sungju Yu for discussions. P.K.J. acknowledges funding support from the Arnold and Mabel O. Beckman Foundation through a Young Investigator Award and Draine and Flatau for making their DDSCAT code freely available. G.K. designed experiments, conducted sample preparation, experimentation, analysis, and modeling, and wrote manuscript. X.Z. conducted XPS studies, and TEM sample preparation. D.D performed preliminary SERS studies in aqueous media. J. H. conducted TEM imaging. P.K.J. conceived project, designed experiments, performed DDSCAT simulations, helped with analysis and interpretation, and wrote manuscript. Supporting Information. The Supporting Information containing supporting figures, supporting tables, supporting movies and movie captions, discussion of photothermal temperature rise, and estimation of SERS enhancement factors is available free of charge on the ACS Publications website. 17 ACS Paragon Plus Environment

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Figures

Figure 1. Capture of key adsorbates and intermediates formed dynamically under operando conditions of photocatalytic CO2 reduction on individual NPs. (a) Schematic of gas flow cell: Plasmonic Ag NPs of 60-nm diameter were deposited on a glass substrate and baked in vacuum for surface cleaning before the substrate was integrated into a gas flow cell. (b) Pie chart showing three different types of catalytic NPs observed and their relative prevalence: inactive NPs (red), NPs that get poisoned by formation of graphitic carbon only (purple), and catalytically active NPs (green). (c) Representative dark-field scattering micrograph showing the distribution of scatterers on the Ag NPcoated glass substrate. A discrete NP scatterer was selected by aligning it with the spectrograph slit. (d) Waterfall plot showing continuously acquired in-situ SERS spectra of the selected NP in air under plasmonic photoexcitation. In air, a broad featureless scattering spectrum that does not fluctuate with time is observed. (e) Waterfall plot of SERS spectra of the same NP in CO2 gas and plasmonic photoexcitation. In CO2, photocatalytic activity is observed in the form of dynamically varying SERS spectra. Many frames show SERS spectra with distinct vibrational bands corresponding to specific adsorbates or products. (f) SERS spectrum from a selected frame (t = 144.4 s) captures physisorbed CO2 identified by its signature Fermi resonance mode. The peak at 1600 cm-1 corresponds to the G vibrational band associated with graphitic carbon or defective graphene77 formed on the Ag surface. (g) SERS spectrum from another frame (t = 95.8 s) captures HCOOH formed by two-electron reduction of CO2. (h) Digital time-trajectories of the net activity of the selected NP in air (blue curve) and in CO2 (red curve) present the photocatalytic dynamics. Also shown is the digital time-trajectory of HCOOH formation (black curve) detected on this NP. Color scales in Fig. 1d and e represent spectral intensity.


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Figure 2. Representative SERS spectra that capture specific adsorbates or products observed in photocatalytic CO2 reduction on Ag NPs. Each spectrum corresponds to a discrete frame from a dynamic SERS movie of a single NP in CO2 gas under plasmonic excitation. Specific species captured include (a) physisorbed CO2, (b) HOCO* reaction intermediate, (c) carbon monoxide or CO, (d) formate ion or HCOO-, (e) carbonate or CO32-, (f) bicarbonate or HCO3-, adsorbed hydroxide or OH*, and H2. Species were assigned on the basis of signature Raman vibrational modes, which are labeled. (g) Histogram of prevalence by the number of catalytic NPs on which each type of species was observed. Note formic acid refers to HCOO- and HCOOH and carbonate implies both CO32- and HCO3-. Other labels have their usual meaning.


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Figure 3. DFT-computed Raman spectra of key adsorbates in CO2 reduction on a model Ag surface. For each adsorbate, the Raman spectrum with labeled vibrational modes is shown along with the DFT-optimized geometry of the adsorbate. Ag atoms are shown in silver, O atoms in red, C atoms in grey, and H atoms in white. (a) physisorbed CO2, (b) CO2δ- anion with both O atoms facing the Agδ+ surface, (c) CO2δ- anion bound to the Agδ+ surface via both a C and an O atom, (d) HOCO* intermediate bound to the Ag surface via both a C and an O atom, (e) HOCO* intermediate bound to the Ag surface via a C atom, (f) Surface-bound CO* and OH* formed from dissociation of an HOCO* intermediate with both O atoms facing the Ag surface, (g) HCOOδ- bound to the Agδ+ surface via the two O atoms, and (h) HCOOH adsorbed on the Ag surface. (i) Schematic model of a possible pathway for CO2 photoreduction on an Ag surface, showing step-wise evolution of different adsorbates or products.


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Figure 4. Proposed mechanism for photocatalytic reduction of CO2 on Ag surface, based on DFT simulations (a) Highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of admixture states of CO2 physisorbed on a model Ag surface. The Mulliken electron density map shows the presence of a higher electronic contribution from the CO2 fragment in the LUMO state, indicating that HOMO-LUMO photoexcitation of the adsorbate/metal complex can transfer charge from Ag to CO2. (b) Possible reaction pathways for photocatalytic reduction of CO2 on Ag. Starting from CO2 (marked by the box), there are two possible paths. Branch B1, elementary reaction steps of which are shown by pink arrows, culminates in product P1B1 (CO) or product P2B1 (HCOOH). Branch B2, elementary reaction steps of which are shown by green arrows, culminates in product P2B2 (HCOOH). Reaction intermediates and products are shown by their optimized geometries (Ag, silver; C, grey; O, red; H, white), for which a tabulated legend is provided. The free 21 ACS Paragon Plus Environment

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energy (eV) of the elementary reaction step, in reference to the free energy of physisorbed CO2/Ag, is indicated alongside each arrow. hυ refers to plasmonically induced charge separation.


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Tables Table 1. Comparison of experimentally observed and DFT-calculated frequencies of Raman vibrational modes of key surface species (CO2, HOCO*, CO, and HCOOδ-) observed in photocatalytic CO2 reduction on Ag NPs. The tabulated data corresponds to peaks in experimental SERS spectra shown in Fig. 2 a, b, c, & d for these species, respectively. The peaks are assigned to vibrational modes on the basis of calculated Raman spectra shown in Fig. 3 a, f, d, and g, respectively. Experimental Calculated Mode assignment frequency (cm-1) frequency (cm-1) 1270 1287 CO2 Fermi resonance 1380 1421 1223 1265 ρ OH HOCO* 1642 1675 ν C=O + ρ OH CO 2221 2231 ν CO 1158 1076 ω CH 1372 1398 ρ CH δHCOO 1453 1404 νs OCO 1600 1657 νas OCO 2730, 2820 2980 ν CH, (νs OCO + ρ CH) νs – symmetric stretch, νas – antisymmetric stretch, ω - wagging, ρ – rocking. Species


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