Opportunities and Challenges of Solar-Energy-Driven Carbon Dioxide

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Opportunities and Challenges of Solar-Energy-Driven Carbon Dioxide to Fuel Conversion with Plasmonic Catalysts Sungju Yu, Andrew J. Wilson, Gayatri Kumari, Xueqiang Zhang, and Prashant K. Jain ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b00640 • Publication Date (Web): 14 Aug 2017 Downloaded from http://pubs.acs.org on August 16, 2017

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ACS Energy Letters

Opportunities and Challenges of Solar-EnergyDriven Carbon Dioxide to Fuel Conversion with Plasmonic Catalysts Sungju Yu, Andrew J. Wilson, Gayatri Kumari, Xueqiang Zhang, and Prashant K. Jain* Department of Chemistry, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA

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ABSTRACT

The ability of plasmonic nanoparticles to harness visible light can be being combined with their catalytic activity to drive photocatalytic transformations. This Perspective introduces the promise of this new class of photocatalysts for fulfilling the quest for sunlight-driven recycling of CO2 into transportable liquid fuels. We discuss the prospects and challenges of such an approach. Despite considerable advances, a selective, stable, and efficient CO2 reduction reaction (CO2RR) catalyst has been elusive. These open challenges may be addressable by the strategic utilization of plasmonic light excitation. Plasmonic catalysts have exhibited the ability to drive a rich milieu of CO2RR processes under visible light excitation. At this stage, improved mechanistic understanding and reaction control are needed. To motivate rational design of photocatalytic materials and processes by a future generation of researchers, we suggest potential pathways by which plasmonic assisted CO2RR can take place. We describe unique physical and chemical aspects of plasmonic catalysis, some of which may allow modulation of CO2RR product selectivity in favor of higher hydrocarbons. The intertwining of the photophysics of plasmon resonances and chemistry of CO2RR creates a wide-open space for fundamental inquiry and technological development. Whether the future of artificial photosynthesis is “plasmonic” will be dictated by scientific understanding and engineering advances accomplished in the coming decade.


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TOC GRAPHICS


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Fossil fuel consumption and deforestation over the past two centuries have led to the accumulation of CO2 in the atmosphere. The concentration of this greenhouse gas has now surpassed an alarming 400 ppm.1 High levels of anthropogenic CO2 emissions are at the center of adverse environmental changes including global warming and ocean acidification.2,3 Even while local and international climate accords gear up to mitigate warming and climate change, the global demand for energy continues to increase. In this scenario, the ability of green plants to use sunlight and water to fix carbon from the atmosphere and generate energy-rich sugars holds important lessons for climate technologies.4-7 Large-scale deployment of artificial systems that mimic natural photosynthesis could accomplish both CO2 sequestration, reducing atmospheric CO2 levels, and recycling of carbon back into fuels for energy consumption, thereby relieving our dependency on traditional fossil fuels. At the outset, it is straightforward to see the attraction of artificial photosynthesis. Compared to thermally or electrically driven CO2 conversion on catalysts, the use of abundant sunlight as the energy input offers a renewable, sustainable strategy. However, whether direct solar-driven reduction of CO2 to fuels is preferable to electrocatalytic CO2 conversion (where the electrical energy comes from a photovoltaic cell) is difficult to reliably assess due to a host of complex economic and technological factors at play. Nevertheless, the quest to mimic the ability of plants, which has been honed by hundreds of millions of years of evolution, offers a grand scientific challenge. Moreover, light excitation offers a rich handle for tighter control of the CO2 reduction reaction (CO2RR). In this regard, plasmonic metal nanoparticles (NPs) emerge as natural platforms for intermingling visible light harvesting with surface-catalyzed reactions. Not only do plasmonic NPs exhibit strong interaction with visible light in the form of localized surface plasmon resonances (LSPRs), but these resonances can also be tuned or engineered in myriad


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ways.8 This Perspective introduces the strategy of using plasmonic NPs as photocatalysts for solar-driven CO2RR, hereafter referred to as plasmonic CO2RR. We ask whether the unique physicochemical attributes of a plasmon-initiated or mediated process offers any advantages in terms of energy efficient CO2 conversion, high selectivity towards energy-dense hydrocarbons, and long-term catalyst stability, benchmarks that have remained elusive in electrocatalytic CO2RR. The answer to this question requires extensive investigation of the rich photochemistry initiated by plasmonic excitation and detailed study of the mechanisms and pathways at play, efforts this Perspective hopes to motivate. For the benefit of researchers interested in understanding plasmonic CO2RR and optimizing photocatalytic materials and processes, the Perspective first introduces the basic principles and reaction pathways of CO2RR from the electrochemistry and theoretical literature. These basic principles are then utilized to build a framework for photocatalyzed CO2 reduction using plasmonic NPs, motivated by examples of hot-electron-driven reactions known to be initiated by plasmonic excitation. We conclude with a discussion of mechanistic hypotheses and open questions in plasmonic CO2RR that may be at the heart of process efficiency and selectivity. Thermodynamics and kinetics of CO2RR. The central challenge in CO2RR, regardless of how the process is driven, is to convert CO2 from a non-concentrated source into a transportable liquid hydrocarbon fuel such as ethanol (C2H5OH) with high selectivity, minimum energy input, and long-term stable operation. To appreciate the nature of this challenge, it is important to understand the thermodynamics and kinetics of this multi-step, complex transformation. The standard Gibbs free energy of CO2 is estimated to be -394.4 kJ mol−1,9 which indicates the high stability of CO2 at standard conditions and its low propensity for conversion to reduction products. In electrochemical CO2RR, the rate-determining step (RDS) is thought to be the


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transfer of an electron to the CO2 molecule,10-12 which triggers its transformation from a linear, sp-bonded structure to a bent radical anion, CO2•− that approaches a sp2 structure, such as that of formate (HCOO−). After the RDS, the transient CO2•− proceeds through a cascade of protoncoupled, electron transfer steps and transforms into more thermodynamically stable molecules. Products from 2e−, 2H+ reduction include formic acid (HCOOH) or carbon monoxide (CO), 4e−, 4H+ reduction yields formaldehyde (HCHO), 6e−, 6H+ yields methanol (CH3OH), and 8e−, 8H+ reduction gives methane (CH4). Equations (1)−(5) list major product formation steps in CO2RR and their corresponding electrochemical potentials versus the standard hydrogen electrode (SHE) at pH 7 in aqueous solution.13-15 CO2 + 2H+ + 2e− → HCOOH

E0 = -0.61 V

(1)

CO2 + 2H+ + 2e− → CO + H2O

E0 = -0.53 V

(2)

CO2 + 4H+ + 4e− → HCHO + H2O

E0 = -0.48 V

(3)

CO2 + 6H+ + 6e− → CH3OH + H2O

E0 = -0.38 V

(4)

CO2 + 8H+ + 8e− → CH4 + 2H2O

E0 = -0.24 V

(5)

C2+ hydrocarbons with more than one carbon atom, e.g., ethane (C2H6) or ethanol (C2H5OH) can be formed by C-C coupling between C1 or other intermediates and products. Higher hydrocarbons are favored due to their higher energy density and/or liquid phase making transport easy. The high energetic cost of electron acceptance in the RDS is a source of the kinetic bottleneck and thereby the cause of high onset potential and energetic input in electrochemical conversion. Additionally, the competing reduction of H+ in aqueous media to H2 (E0 = -0.41 V vs SHE at pH


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7) is generally favored leading to loss of Faradaic efficiency. The ability of a catalyst surface to lower the energetic cost of the RDS, possibly by favoring a bent geometry of chemisorbed CO2,10-12 is central to achieving high CO2RR activity and energy efficiency. There are a variety of reaction pathways following CO2•− generation (e.g., Figure 1a); therefore, a distribution of hydrocarbon products is generated depending on the catalyst and process parameters such as applied potential. Reaction pathways, and thereby product selectivities, are dictated by the stability of formed adsorbates on the catalyst surface, interaction between intermediates (for C-C coupling reactions), and applied bias relative to the onset potential for individual electron and proton reactions.16-19 In the presence of strong photoexcitation, the energetics and charge of surface adsorbates can be expected to be appreciably modified, which can influence reaction kinetics, pathways, and product branching in subtle to dramatic ways.

Figure 1. (a) Multi-step conversion of CO2 to CH4. Calculated free energies (at zero external bias) are shown for two pathways, one through a CHO intermediate (blue curve) and another via the COH intermediate (red curve). Molecular species at each step are identified in the schematic. The CHO pathway is adapted here with permission from ref. 23. Copyright 2011 Elsevier. The COH pathway is adapted with permission from ref. 26. Copyright 2014 Elsevier. (b) A comparison of the activity of selected transition metal catalysts. It is found that the electrocatalytic onset potential for CO2 and CO reduction is strongly tied to the binding energy of CO on the metal surface. Dashed and solid lines constructed from blue data-points represent calculated onset potentials for elementary CO hydrogenation steps on the (111) surface of the respective metal. Red data-points indicate experimental onset potentials for CO2 reduction to C1 species on bulk metals. Theoretical data is adapted from ref. 16 with permission of the PCCP Owner Societies. Experimental data is adapted with permission from ref. 17.


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CO2RR electrocatalyts. A variety of transition metals have been found to have electrocatalytic properties for CO2RR,16-39 with Cu as the most active for hydrocarbon production. Some general principles about the choice of the metal have emerged from theoretical work. The binding energy of CO, an intermediate formed following the generation of CO2•−, on the metal surface is thought to be a critical factor influencing catalytic activity and product selectivity (Figure 1b).22-30 The *CO adsorbate is required to undergo further hydrogenation at the surface (Figure 1a) to form downstream hydrocarbons such as CH4. Therefore, the activity and selectivity for hydrocarbon production in CO2RR are closely linked to the onset potential for CO reduction. This onset potential, calculated for selected metals, exhibits a volcano relationship with respect to the surface binding strength of CO indicating a Sabatier principle at work.16 Cu has a moderate binding strength for CO. Relative to Cu, metals with weaker CO binding energies (e.g., Ag and Au) exhibit relatively high onset potentials for the hydrogenation of CO, due to competition from CO desorption. On the other hand, for metals that strongly bind CO (e.g., Pt, Ni, Pd, and Rh), there is increased likelihood of catalyst poisoning by *CO and a resulting reduction in activity. For the coinage metals, the measured onset potential for CO2 reduction to C1 species decreases (in the order, Ag > Au > Cu) with increasing CO binding strength, consistent with theoretical predictions.17 Thus, the optimal CO binding energy of Cu is an important factor in its relatively low onset potential and therefore high activity and energy efficiency for CO2 reduction to C1 species. On the basis of theoretical and experimental investigations, heterogeneous CO2RR catalysts can be classified into four groups according to their activity and selectivity: 1) Although Pt and Ni have low onset potentials for CO2 reduction, these metals, along with Fe and Ti, favor the competitive process of H2 generation due to their low overpotentials for H+ reduction, leading to


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reduced Faradaic efficiency for CO2RR,18,19 2) Heavy metals like Sn, Pb, Hg, and Cd do not sufficiently dissociate the C-O bonds of CO2, thereby primarily producing formate or formic acid,18,19,31,32 3) Au, Ag, Pd, and Zn are efficient for CO generation but not for hydrocarbon products due to a weak adsorption of the CO intermediate,18,19,33-39 and 4) Cu, with a moderate binding energy of *CO, enables hydrogenation of *CO to hydrocarbon species.18-30 Incidentally, many of the candidate CO2RR electrocatalysts (Cu, Au, and Ag) are also plasmonic, with strong visible-region LSPRs exhibited in their nanoparticulate forms. Hydrocarbon generation in CO2RR. Electrochemical CO2RR on Cu has been shown to yield myriad combinations of C1 and C2+ hydrocarbons.20-30 This richness of activity has inspired extensive efforts to understand reaction pathways for hydrocarbon production on the surface of Cu. For the simplest case of C1 species, i.e., CH4, density functional theory (DFT) studies suggest two potential pathways for the transformation of *CO to CH4 (Figure 1a),22-27 depending on the location of proton binding. Once the *CO intermediate is formed on the surface, it can be hydrogenated to formyl (*CHO) or hydroxymethylidyne (*COH) through a transfer of one electron and one proton. The *CHO and *COH intermediates have larger adsorption energies than that of *CO, allowing them to be longer-lived on the surface and be subsequently reduced to CH4 via a series of proton-assisted electron transfer steps: *CHO → *CH2O → *CH3O → CH4 (g) + *O → *OH → H2O (g or l)

(6)

*COH → *C + H2O (g or l) → *CH → *CH2 → *CH3 → CH4 (g)

(7)

Several additional pathways may be involved in the generation of C1 products of increasing complexity. For instance, it has been proposed that CH3OH is produced by the modified CHO pathway:25-27


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*CHO → *CH2O → *CH3O → CH3OH (g or l)

(8)

A comprehensive description of the majority and minority C1 products formed from the CO2RR is beyond the scope of this Perspective, but interested readers may consult recent review articles.13-15 Although the mechanisms for C1 generation on Cu are becoming lucid, there is still an open challenge to design a catalyst that targets one product with high yield. Often control of catalyst morphology, grain size, faceting, or surface modifiers/additives is employed to enhance the Faradaic efficiency for a desired product, e.g. CH4.20,21,32-39 2 Cu(100) surface Cu(111) surface Cu(211) surface

1.5

Ea (eV)

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1

0.5

0

CO + CO

CO + CHO

CO+CO

CO+CHO

CHO + CHO

COH + COH

CHO + CH2O

CH2O + CH2O

CHO+CHO COH+COH CHO+CH2O CH2O+CH2O

C-C coupling reaction

Figure 2. Activation energies for C-C coupling reactions between CO2 reduction product dimers. Energies are shown for Cu(100) (blue), Cu(111) (yellow), and Cu(211) (red) surfaces. Data for Cu(100) and Cu(111) was obtained from ref. 27. Data for Cu(211) was obtained from ref. 28. Copyright 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

As discussed before, there is a strong interest in generation of higher energy-value products such as C2 species and beyond. C2+ hydrocarbons are electrochemically produced on Cu, but with lower efficiencies due to the greater number of elementary steps (electron and proton transfers and C-C bond formation) involved and the resulting mechanistic complexity. Not only do C1 intermediates have to be stable on the surface for further transformation to C2+


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hydrocarbons, the catalyst also needs to promote C-C coupling. These coupling reactions have much slower kinetics than C-H bond formation in the hydrogenation of C1 intermediates and therefore suffer from higher energy requirements and lower efficiencies.27-29 Several mechanisms have been proposed,24-29 but the chemical identity of the intermediates at the early stages of C-C coupling is in question. For example, given that *CO is the key intermediate in CO2RR, it has been postulated that the C-C coupling reaction proceeds via dimerization of *CO with a neighboring *CO or with a hydrogenated *CO species, e.g., *CHO or *COH.24,27-29 While both reaction pathways are feasible as per computational calculations, the latter one has been predicted to have a lower activation barrier on various Cu facets (Figure 2).27-29 Other combinations of dimerization between hydrogenated *CO species or dimerization of downstream products have also been proposed as candidates in C-C coupling. A hydrogenated CO dimer, *OCCOH, was recently observed on the Cu(100) surface by Fourier transform infrared (FTIR) spectroscopy.30 Opportunities with sunlight-driven CO2 reduction. Despite extensive experimental and theoretical investigations into electrochemical CO2RR, Faradaic efficiencies, product generation rates, and yields of desired hydrocarbons continue to remain sub-optimal, even with Cu. There is a need to lower the onset potential for product generation while maintaining technologically relevant rates and finding strategies to select for a desired product. Problems with catalyst stability, restructuring, and loss of activity under electrochemical bias are common and need mitigation for the technology to be feasible. As a result of these challenges, other strategies for CO2 conversion have received attention. While a catalyst is indispensable, reduction of CO2 to fuels can be achieved with the input of thermal, electrical, and/or light energy. Of these, visible light excitation is particularly attractive


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due to the abundance of solar light hitting the Earth’s surface, obviating the need for thermal or electrical energy input and the possibility of operation at or near room temperature without the need for bias, conditions that may aid in maintenance of catalyst stability. The use of light for activating reactions also has potential benefits. In electrochemical CO2RR, CO2 activation is achieved by applying electrical energy to raise the Fermi energy, EF, of the metal catalyst until it exceeds the reduction potential of CO2 or downstream adsorbates,20-22,31-39 enabling electron flow from the metal electrode to the adsorbate. Application of a larger bias can kinetically accelerate reactions, especially needed to drive sluggish ones such as C-C coupling; but such overpotential raises the energy cost and mars efficiency. Light excitation of the catalyst/adsorbate system may serve as a means to energize electronic carriers and accelerate their delivery to adsorbates, lowering kinetic barriers and boosting rates and efficiencies.40,41 Furthermore, photoexcitation may promote alternative intermediates and mechanistic pathways for hydrocarbon formation,42 allowing tuning of product selectivity. For example, different product distributions, including substantial percentages of hydrocarbons, have been found in a range of photocatalytic CO2RR processes.42-49 Table 1. Summary of Various Metal-Based Photocatalytic Systems for CO2 Reduction.

Photocatalyst

Type

Reductant

Light source

Products

Yield (µmol g‒1 h‒1)

Ref

3770 421.2 190.1 73.3 40.8

42

2200 286

43

Au3Cu/SrTiO3/TiO2

Nanotube array

N2H4·H2O

300 W Xe lamp

CO CH4 C2H6 C2H4 C3H6

Au1Cu2/TiO2

Thin film

H2O vapor

AM1.5 lamp 1000 mW cm‒2

CH4 H2


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Au/TiO2

Thin film

H2O vapor


CH4 H2

210 49

43

Cu/TiO2

Thin film

H2O vapor


CH4 H2

280 33

43

Cu0.33Pt0.67/TiO2

Nanotube array

H2O vapor

AM1.5 lamp

CH4 C2H6 C2H4

134 24.1 12.0

44

Pt@Cu2O/TiO2

Powder

H2O vapor

200 W Xe lamp (> 320 nm)

H2 CO CH4

25 8.3 33

45

Pt/TiO2

Powder

H2O vapor

200 W Xe lamp (> 320 nm)

H2 CO CH4

66 2.2 11

45

Cu/TiO2

Powder

H2O vapor

200 W Xe lamp (> 320 nm)

H2 CO CH4

9.9 5.4 8.7

45

Au/ZnO

Powder

H2

532 nm 4×104 mW cm‒2

CO CH4

1.3 1.2

46

Pd/TiO2

Dispersed NP

H2O liquid

500 W Hg lamp (> 310 nm)

CO CH4 C2H6

0.04 0.4 0.06

47

Cu/graphene

Powder

H2O vapor

300 W Hg lamp 100 mW cm‒2

CH3OH CH3CHO

2.9 3.9

48

Au/TiO2

NP on TiO2 thin film

H2O vapor

532 nm 350 mW cm‒2

CH4

0.22

49

Au/TiO2


H2O vapor

365 nm 20 mW cm‒2

CH4

0.2

49


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Au/TiO2


Au

NP on substrate

Pt

NP on substrate

Cu

NP on substrate

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H2O vapor

254 nm 20 mW cm‒2

CH4 C2H6 HCHO CH3OH

2.3 1.6 1.4 0.9

49

H2O vapor

254 nm 20 mW cm‒2

CH4 C2H6 HCHO CH3OH

1.0 0.5 0.7 0.4

49

H2O vapor

254 nm 20 mW cm‒2

CH4 C2H6 HCHO CH3OH

0.7 0.4 0.6 0.4

49

H2O vapor

254 nm 20 mW cm‒2

CH4 C2H6 HCHO CH3OH

0.3 0.2 0.5 0.4

49

Metal-containing photocatalysts for CO2RR. Semiconducting oxide-based photocatalysts, such as TiO2, SrTiO3, and ZnO have been widely used to capture solar photons and generate energetic free charge carriers for driving CO2RR at their active surfaces.42-46 However, these materials offer limited absorption of visible light, rapid charge recombination, and consumption of electrons by competitive H2 generation.50-52 These limitations have been often alleviated by loading the oxide with noble metal NPs or clusters to serve as visible-light sensitizers, electronsinks for facilitating charge separation and concentrating energetic charge carriers on the oxide surface, and co-catalysts for enhancing rates of the desired CO2RR.42-49 Table 1 provides a summary of various composite catalysts employed for photocatalytic CO2 reduction. Most of these photocatalysts are composites of a noble metal and an oxide-based semiconductor. In some cases, a bimetallic or alloy composition (e.g., AuxCuy and CuxPty) has been employed for optimal (synergistic or bifunctional) catalytic activity.42-44 Several of these composite photocatalysts have shown the generation of C1 and/or C2+ hydrocarbons from light-mediated CO2 reduction,42-49 although some of the hydrocarbons could be products of photolysis of surface carbonaceous


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species leftover from colloidal synthesis.53,54 Nevertheless, one can easily see that many of these metal-based sensitizers involve a plasmonic metal in a nanoparticulate form, naturally because such NPs have strong light absorption in the visible region. Even though the plasmonic attribute of the metal component was not explicitly recognized in most cases, these composite catalysts are in fact early precursors of plasmonic photocatalysts. Since the metal component of these composite photocatalysts serves both as the light absorber and the catalyst, questions arise whether it is the photoexcitation of carriers in the metal that is central to the activity and whether the oxide or semiconductor is even necessary for photocatalytic function, except possibly as an inactive support. In this regard, metal-only photocatalysts – in the form of NPs for enhanced specific surface area and reduced mass transport limitations in the catalytic process – would provide simpler model systems for understanding the role of light/matter interactions in enhancing CO2RR activity and modulating selectivity. Such fundamental insight would enable rational design, whereas current photocatalysts have been found by Edisonian approaches. It would be most instructive to directly compare, without complicating heterojunction or support effects, the photocatalytic CO2RR performance of the metal catalyst with the electrochemical CO2RR counterpart. Such fundamental mechanistic investigations are not yet prevalent; however, some early work has shown promise. A report from the Cronin group showed that Cu as well as Au and Pt NPs, without any oxides, can reduce CO2 under light excitation forming a variety of products including CH4, HCHO, CH3OH, and C2H6.49 It must be noted that in this case ultraviolet (UV) light, rather than visible light, was employed and interband transitions in the metal were excited leading to excited charge carriers. Intriguingly, product yields, including the percentage of C2 species, were higher using Au and Pt as compared to Cu. This trend is in direct contrast to that


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observed in the electrochemical version. The difference in product distribution between electrocatalytic and photocatalytic CO2RR by metal NPs suggests that under light excitation CO2RR possibly proceeds through a mechanism(s) disparate from that of the electrochemical process and/or that reaction pathways and product branching are governed by factors other than the binding strength between *CO and the catalyst surface. Photoexcited states of the metal/adsorbate complex and/or charged intermediates may uniquely be involved in reaction pathways under light excitation. Energetics, pathways, activity, and selectivity may also be tuned by varying attributes of the excitation, such as photon energy, fluence, and pulse characteristics. Plasmon resonances for solar-energy-driven chemistry. Given the promise of photoexcitation for driving, enhancing, and modulating catalytic reactions, noble metal NPs are natural platforms due to their support of LSPRs, a strong form of light/matter interaction involving a resonant collective oscillation of free carriers confined to the boundaries of the NP. Not only do plasmonic NPs of Au, Ag, and Cu offer a means to absorb visible light efficiently, they can be geometrically tuned via NP size and/or shape for optimal capture of the solar spectrum. Moreover, these NPs exhibit dual function: light absorption characteristics along with adsorption and catalytic activation of a range of small molecules, such as O2, H2, and CO2. Plasmon resonant photoexcitation has special attributes when compared to light absorption by molecular sensitizers or say inter-band excitation of metals: the resonance involves a multi-electron excitation and the absorbed energy is concentrated at the NP surface in the form of intense electric fields,55-63 features that can have a dramatic influence on adsorbate activation and surface


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Figure 3. (a) The dynamics of plasmon excitation/relaxation process in a metal NP: 1) A dipolar plasmon resonance is excited by visible light. 2) Decay of the plasmonic oscillation leads to the production of hot electron-hot hole pairs via interband and intraband relaxations. 3) A Fermi-Dirac distribution of hot electrons is formed on the NP surface after electron-electron and electron-phonon collisions. 4) Electronphonon and phonon-phonon scattering thermalize NP phonons which release heat to the surrounding medium. Electron-surface or electron-adsorbate scattering, also known as chemical interface damping, is also thought to be prevalent in small NPs, but not depicted here. (b) In the presence of a hole scavenger, continuous wave (CW) visible light illumination generated a steady-state population of hot electrons on a Au NP surface, effectively charging the NP and raising its Fermi energy, resulting in a lower activation energy for electron transfer to an Fe(CN)63‒ acceptor. Reprinted with permission from ref. 40. (c) A hot electron generated following plasmonic excitation transiently populates an excited electronic state of an adsorbate, forming a transient negative ion (TNI). This process increases the distance between atomic nuclei where the TNI either overcomes an activation barrier, Ea, or decays to a ground state, depending on the vibrational energy. If the vibrational energy is sufficiently high, multiple electronic excitations of the adsorbate occur within the lifetime, τe, of the electronic state. Reprinted with permission from ref. 60. Copyright 2012 Nature Publishing Group.

chemical reactions, particularly in favor of kinetically challenging multi-electron redox reactions. Upon visible light excitation of their LSPRs, noble metal NPs have been shown to initiate or catalyze a diversity of reactions, such as H2 dissociation,55,56 water splitting,57,58 CO oxidation,59 NH3

oxidation,59

ethylene/propylene

epoxidation,59-61

and

azo-coupling

of

p-

aminothiophenol.62,63 The mechanism by which these reactions are driven by plasmonic excitation can be understood by reviewing the nature of carrier dynamics following excitation.


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Carrier dynamics following plasmon excitation. In nanostructures of metals (Figure 3a), surface plasmon oscillations of free electrons are confined by their geometric boundaries, which is why they are termed as LSPRs. When the frequency of incident light matches the resonant plasmon frequency, light is efficiently coupled to the collective electron oscillation. This process results in concentration of far-field radiation into a small volume surrounding the NP, i.e., the near-field. The plasmon oscillation then dephases and relaxes energetically through radiative or nonradiative decay channels. NP size is a handle for controlling the relative contribution of these two pathways, with larger NPs of size approaching the wavelength of light favoring radiative decay whereas non-radiative decay dominates in smaller NPs.64 Radiative decay results in emission of photons back into the far-field, i.e., light scattering, and is less preferable for light harvesting. In nonradiative decay, plasmon oscillation is dephased and damped via the excitation of electronic interband and intraband transitions within the metal. These excited electrons occupy states above the dark Fermi level of the metal, leaving behind unoccupied electronic states, i.e., holes. Metal photoexcitation and subsequent energy relaxation is conveniently described in terms of generation and recombination of such electron-hole pairs.65 The excited carriers subsequently thermalize (redistribute their energies) via electron-electron scattering on the 100-fs timescale, significantly faster than electron-phonon cooling, yielding a hot Fermi-Dirac distribution of carriers. Following this process, hot carriers relax via electron-phonon scattering on the 1-ps timescale, resulting in heating of the NP lattice. On the 100-ps timescale, the NP lattice reaches thermal equilibrium with the surrounding medium via phonon-phonon scattering. As a result, the medium undergoes a temperature rise, which has utility in thermal activation of chemical reactions. However, such photothermal activation of chemical reactions is not the most efficient use of the input photon energy.


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For harvesting a considerable fraction of the energy of hot carriers for adsorbate excitation or driving redox reactions, it is desirable to accomplish, on the sub-picosecond timescale, electron/hole separation or extraction of the hot electrons from the metal, before electron/hole recombination (electron relaxation to lower energy, unoccupied states) becomes dominant. The faster this charge separation or extraction, the “hotter” the carriers and the greater the energy harvested, boosting the rate and efficiency of electron-induced chemical processes. Metals are known for rapid carrier recombination and therefore efficient charge extraction remains a central challenge in plasmonic catalysis and one that will dictate the feasibility of the approach. Accomplishing efficient carrier extraction. A few different strategies seem to be of promise for carrier extraction in the face of the dissipative nature of carrier dynamics in a metal. Appropriately designed metal NP/semiconductor or metal NP/oxide composites can promote charge separation at a metal/oxide interface. Au/TiO2 and Au/ZnO are two popular systems.66 The presence of a junction of high quality, hybridization of states of the metal and the semiconductor mixing, and appropriate band alignments are thought to be crucial for fast, subpicosecond-scale charge separation.67-69 A metal-only, oxide-less strategy also exists where a surface adsorbate or a species in solution is involved in fast extraction of one of the carriers.5563,70-74

For instance, in our group,40,41 we have effectively utilized alcohols for extracting holes

from colloidal Au NPs irradiated by green light. The alcohol reacts rapidly with hot holes within the NP and gets oxidized, releasing protons. The hot electrons build up on the NP, assisted by Coulombic stabilization and solvation by H+ and polarizable water molecules. Under CW visible light excitation in the presence of an alcohol hole scavenger in an aqueous medium, we have measured a significant steady-state charging of the NP, by as many as 103 excess electrons stored per 12 nm Au NP. The resulting cathodic charging of the NP amounts to a considerable rise in


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the EF within the Au NP under steady illumination. The hot electrons are then transferred to electron acceptor molecules, ferricyanide, triggering their reduction to ferrocyanide. We found that the raised EF under light excitation and optimal hole scavenging led to a reduced activation barrier for the hot electron transfer (Figure 3b).40 In effect, the electrons were injected into the acceptor at an increased rate, enhancing the photon-to-chemical-conversion quantum efficiency. It is easy to recognize that the aforementioned strategy would be most effective where the charge accepting (i.e., reactant) molecules chemisorb to the surface of the metal. In this regard, the surface activity of noble metals for small molecule reactions, including those involving CO2 and related adsorbates, provides synergy to the plasmonic attributes. Chemisorption enhances the surface residence time of the charge acceptor, increasing the yield of carrier capture. Secondly, electronic hybridization of the metal and adsorbate states can promote fast, coherent electron transport without the hindrance of tunneling bottlenecks. In the ideal scenario, the charge transfer to adsorbates is irreversible ensuing reduction (hot electron-mediated) and oxidation (hot hole-mediated) half-reactions. However, even when electron transfer to the adsorbate is only transient (electron is back-scattered into the metal), catalytic activation is possible. One example of the latter scenario is where hot electrons scatter from adsorbate electronic states. Such electron-surface or electron-adsorbate scattering (also known as chemical interface damping) taking place on the sub-picosecond timescale,72,73 prior to energy-dissipating thermalization, can be effective for vibrationally activation of adsorbates. If the rate of electronic excitation is sufficiently faster than its molecular relaxation, the adsorbate can undergo multiple electronic excitations within its vibrational lifetime (Figure 3c).60,74,75 This mechanism is thought to be at the origin of the dissociation of H2 and D2 on Au NPs and in ethylene epoxidation on Ag nanocubes.55,60 The evidence for this model of multiple electronic


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transitions came from the non-linear trend of the electron transfer rate as a function of the photon flux, crossing from a linear trend to a superlinear relation above a threshold flux.60,74,75

Figure 4. Potential mechanisms for CO2RR by plasmonic photocatalysis. When CO2 is adsorbed onto the surface of a metal NP, hybridization of its electronic states with those of the metal, results in a reduction of its HOMO-LUMO energy gap from the gas phase value to a new HOMO’-LUMO’ gap corresponding to the electronic admixture of the metal/adsorbate complex (M-CO2). (a) NP charging/discharging: Under steady-state illumination, electron injection/hole removal by a reducing agent cathodically charges the NP, creating a quasi-Fermi level, EF’, that is raised above the dark Fermi level, EF. Energetic electrons occupying states above the EF’ are injected into the LUMO’ of adsorbed CO2. Higher the electronic energy “bias” relative to the LUMO’, greater is the rate of injection. (b) Chemical interface damping: LSPR excitation followed by dephasing/relaxation within the metal NP transiently produces hot electrons above the EF. A fraction of these hot electrons are scattered into the LUMO’ states of M-CO2 resulting in transient activation of CO2 or formation of CO2•− required for further transformation. (c) Direct HOMO’LUMO’ photoexcitation of the M-CO2 complex, leading to transient activation of the adsorbed CO2 required for further conversion. In all three cases, the holes are depicted to be removed by a hole scavenger or reductant.

Potential mechanistic scenarios for plasmonic CO2RR. The use of plasmonic photocatalysis for CO2RR is at an undeveloped stage. However, we motivate future attempts and advances by proposing here mechanistic scenarios, whereby the synergy between surface chemical activation and plasmonic excitation may be exploited for driving CO2RR on noble metal NP surfaces. These scenarios are not limited to CO2RR but may apply to other small-molecule reactions as well. In the gas phase, CO2 possesses a deep-UV-range energy gap between its highest occupied molecular orbital (HOMO) and its lowest unoccupied molecular orbital (LUMO). Direct


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photochemical activation of CO2 molecules in the gas phase is therefore not possible with visible light (unless a sensitizer or a multiphoton excitation are involved). However, adsorption of CO2 onto a metal surface can promote orbital hybridization. The resulting admixture (M-CO2) can have a substantially reduced HOMO’-LUMO’ energy gap that can be excited by means of visible light.75 Under this scenario, we foresee the following three potential mechanisms for CO2 activation by plasmonic excitation. A combination of these scenarios or other unanticipated mechanisms may also be operative. (1) NP charging/discharging: As previously described, steady-state plasmon-resonant excitation in the presence of a hole scavenger or electron injector cathodically charges plasmonic NPs, shifting their EF to more negative potentials with the setting up of a new steady state EF’ (Figure 4a). The EF rise can serve as a driving force for CO2RR. The plasmonic NPs can discharge by transfer of photoexcited electrons to the LUMO’ of the M-CO2 complex, forming CO2•− or a related species, for further reduction. The oxidation half-reaction involving the removal of holes by the reducing agent serves as the source of H+ for the CO2RR process. The more efficient the hole removal, the lower is the energy loss due to back-recombination of the electron and hole. Therefore, adsorption and electronic coupling of the hole scavenger to the metal surface may also be desirable. The higher the EF’ under steady-state illumination, the greater is the efficiency of electron injection into the adsorbed CO2. The degree of cathodic charging and the magnitude of the EF rise is expected to depend on the interplay of the kinetics of photoexcitation, relaxation, electron-hole recombination, and hole trapping processes.40 Such a charging/discharging mechanism can be thought of as analogous to a typical redox process in an electrochemical system, with the overpotential being supplied by solar energy.


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(2) Chemical interface damping: In this mechanism (Figure 4b), the LSPR excited in the metal NP (by photons of energy hν) undergoes dephasing or energy relaxation to transiently produce hot electrons/holes with a maximum/minimum energy of EF ± hν. A fraction of the hot electrons can coherently scatter76-78 into the unoccupied states of the CO2 adsorbate, leading to the vibrational activating of CO2 for further bond dissociation and/or formation processes, akin to recent findings.55,60. (3) Direct photoexcitation of the M-CO2 complex:79,80 In this scenario (Figure 4c), adsorbed CO2 molecules are transiently activated through the direct HOMO’-LUMO’ photoexcitation of the hybridized/admixed electronic states of the M-CO2 complex. The reduced magnitude of the HOMO’-LUMO’ gap of the M-CO2 complex allows such direct excitation by visible light, which is otherwise not possible in the gas phase. If an electron can be injected into the HOMO’ by a reducing agent within the lifetime of the excited state, a CO2•− species is effectively generated, with potential for further reduction by electron and proton transfer steps. In general, depending on the lifetimes of the photoexcited states and the kinetics of electron/hole transfer, photoinduced charge separation across the M-CO2 interface can be either irreversible (forming radical/charged intermediates such as CO2•− and H+ for a photoredox process) or transient (which may still allow for vibrational activation of the adsorbed CO2). The electronic interaction between the metal and the CO2 adsorbate and photoproduced charged/radical intermediates81-85 can govern lifetimes and kinetics. Hence, the (bulk and surface) electronic nature of the metal is likely to play a key role in the mechanism, activity, and selectivity of the catalyst. At the same time, the characteristics of the plasmonic excitation (photon energy and flux) are also likely to be of importance, because these factors dictate the


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relative rates of photoexcitation, recombination, and charge transfer, which in turn influence the rate and efficiency of adsorbate charging or activation. Need for mechanistic insights into plasmonic CO2RR. As described in this Perspective, plasmonic excitation offers a promising means to couple visible light with the catalytic function of a CO2RR metal. But we are years away from accomplishment of high efficiency and control over reaction pathways and product selectivity. The catalytic system (e.g., metallic or bimetallic composition, NP size, NP shape, presence of an oxide), reaction conditions (type and concentration of reducing agents and other additives, CO2 concentration, solution pH), and light excitation parameters (photon energy, flux, and pulse characteristics) will need design and optimization. Certainly, there is a need for screening, wherein the aforementioned parameters are changed systematically and product distributions, rates, quantum efficiencies, and performance stability are measured. However, the parameter space is prohibitively large. To guide the search for an optimal system, there is first a need to develop a comprehensive mechanistic, molecularlevel model starting from the point of plasmonic excitation to the formation of a reduction product from adsorbed CO2 on the surface of the NP. There are many gaps in this picture, which need further experimentation and theory, a mission undertaken by our laboratory and possibly several others. There are already a few basic issues that need clarity. For instance, it is a common claim that a spike in the local temperature around a plasmonically excited NP can aid thermal activation of adsorbates. There is no doubt that plasmonic excitation causes a moderate photothermal temperature rise; but under CW illumination, the steady-state local temperature at the NP surface is not significantly different, under ordinary circumstances, from the temperature of the medium.86 In such a scenario, plasmonically-assisted heating is expected to be no different from


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equilibrium heating of the bulk sample. There may be special scenarios where a local temperature rise is prevalent, however, clear evidence or characterization of selective thermal activation of adsorbates at the NP surface is needed. Another common claim is that the intense surface electromagnetic field enhances the kinetics of electron transfer or other chemical reactions. Often times, such a “field effect” is muddled with the role of energetic electrons produced by the decay of the plasmonic oscillation. It is plausible that a strong field and field gradient can influence electronic/vibrational processes, but clear-cut evidence and a physical model are needed to rationalize how an oscillating AC field of 500 THz frequency influences a chemical reaction and in what manner. As for the role of plasmonically-generated hot-electrons in catalytic activation, there is little doubt. However, there is a need to quantify the relative contributions of the different mechanisms (e.g., the three proposals in Figure 4) by which such hot-electron-assisted activation takes place. A prime question is related to the nature of the plasmon excitation. For instance, studies suggest that plasmon resonant excitation is simply one of several channels (interband or intraband excitation being other channels) for achieving strong light absorption and photoexcitation.40,87,88 However, it is not clear if there is a uniqueness associated with the excitation of a collective many-electron oscillation versus, say, a one-electron interband transition. Do collective oscillations have a role in activating adsorbates or driving the chemical reaction? This question is closely related to that about the role of the near-field. Given the fast 10-fs dephasing of the collective excitation and its rapid decay into hot electron-hole pairs, involvement of a many-body phenomenon seems unlikely at the outset, but surprises may be in store. Answers to the questions raised here are likely to need ultrafast time-resolved probing of carrier dynamics within both the metal and the adsorbate. However, insights may be available by


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systematic, carefully designed comparisons of product selectivities, rates, and quantum efficiencies under plasmon-resonant excitation vs. performance metrics under off-resonant excitation. “Normalization” of differences, e.g., in absorption cross-sections, photon energies and/or carrier relaxation times, between the two schemes needs care. The NP shape tunability of LSPRs can be exploited for designing such studies. A study of reaction rates and product distributions as a function of photon flux are also valuable for gauging the importance of multicarrier effects. Given the richness and multi-electron nature of CO2RR processes, the possibility for deepest understanding and new discoveries lay in the study of reaction pathways under plasmonic excitation. Insights about the role and uniqueness of plasmonic excitation are likely to emerge from comparisons of observed reaction intermediates, pathways, and product distributions with those for thermal or electrochemical modes of CO2RR. Comparison of pathways across various conditions of light excitation, e.g., as a function of photon energy and flux, can also be of considerable fundamental and design value. Vibrational techniques such as infrared (IR) and Raman spectroscopy are likely to be instrumental in the identification of adsorbates, reaction intermediates, and products, attributed to the ability of these techniques to probe reactions inoperando under light excitation. It is well appreciated that adsorption and reaction mechanisms in solid-state catalysis are difficult to determine due to the short lifetime of intermediates and site-to-site heterogeneities. However, single-site resolved and/or time-resolved probing may circumvent these issues. Additional to facilitating photocatalysis, plasmonic excitation results in orders-of-magnitude near-field enhancement of IR and Raman scattering signals. Such surfaceenhanced infrared absorption (SEIRA)89 and surface-enhanced Raman scattering (SERS)90,91 spectroscopy can be highly sensitive, enabling low detection limits, possibly down to single-


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molecule levels, necessary for capturing short-lived and sparsely generated reaction intermediates. Thus, plasmon excitation enables a unique drive-and-sense duality for study of photocatalytic reactions. Vibrational spectroscopies can be complemented by in-situ LSPR spectroscopy, which would allow single-NP-resolution probing of changes in the electron density and local electronic environment under plasmonic catalysis conditions.92-99 It is known that the surface structure or chemistry of the plasmonic metal NP can undergo permanent or dynamic modifications (e.g., oxidation and poisoning) by the action of light excitation, environmental species such as oxygen, or reaction intermediates. Such changes are likely to alter catalytic activity. It is therefore critical to characterize the surface and chemical state of the NP photocatalyst in-operando conditions, thereby more precisely determining the catalytic active phase. With detailed experimental data from mechanistic and kinetic investigations, theory and computations can aid understanding and development of mechanistic models. The theoretical treatment of thermal and electrocatalytic CO2RR by electronic structure methods has been exemplary. But modeling plasmon-assisted CO2RR reaction poses a special challenge due to the need to integrate light excitation, resulting electric fields and field gradients, photoexcited states, and carrier dynamics into electronic structure models of the metal surface and adsorbate. Summary and Future Outlook. The efficient conversion of CO2 to liquid fuels is a goal yet to be achieved. The thermodynamic, kinetic, and engineering challenges associated with this process merit the exploration of artificial photosynthetic schemes with metal-based photocatalysts likely to play a central role in these schemes. Noble metal NPs, in particular, allow the integration of strong visible-wavelength plasmonic excitation with surface activation of CO2, and therefore represent a novel and promising class of photocatalysts for CO2RR. Initial


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efforts with noble metals NPs (with or without semiconducting oxides) have demonstrated the potential to generate C1 and C2+ hydrocarbons from CO2 and water using solar energy. But, efficiencies are far below desirable levels, control over product selectivity is poor, and mechanistic understanding is limited. Thus, there is a wide-open space for researchers to expand these studies to a greater number of systems and process conditions so that optimal schemes can be found. A systematic library of information is needed for the development of fundamental understanding and guidance of rational design of efficient, selective, and stable plasmon-assisted CO2RR catalysts and processes. In plasmonic CO2RR, catalytic activity, reaction pathways, and selectivity are expected to depend not only on the properties of the metal and metal/adsorbate interactions, but possible to be modulated by attributes of the light excitation. Since plasmon resonances represent a strong form of light/matter interaction, reaction pathways and key intermediates in CO2RR are likely to be different under plasmonic excitation as compared to thermal and electrocatalytic CO2RR. However, whether plasmonic excitation, due to its collective nature, has any unique characteristics with potential for driving multi-electron transfer reactions in CO2RR is a question that is open and worth answering. Regardless of the answer to this question, new insights into the role of light and the close interplay of photoexcited states and surface adsorbates are expected to result from fundamental mechanistic and theoretical investigations of plasmon-assisted CO2RR. The large landscape for further exploration, the potential for fundamental discoveries into light/matter interactions and many-body phenomena, the promise of mimicking plant photosynthesis, and the technological importance of the problem of CO2 sequestration all make plasmonic CO2RR fundamentally intriguing and a ripe topic of research.


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Selected Quotes “…light excitation offers a rich handle for tighter control of the CO2 reduction reaction (CO2RR). In this regard, plasmonic metal nanoparticles (NPs) emerge as natural platforms for intermingling visible light excitation with surface-catalyzed reactions....” “…we motivate future attempts and advances by proposing here mechanistic scenarios, whereby the synergy between surface chemical activation and plasmonic excitation may be exploited for driving CO2RR on noble metal NP surfaces.” “…we are years away from accomplishment of high efficiency and control over reaction pathways and product selectivity.” “…there is first a need to develop a comprehensive, mechanistic, molecular-level model starting from the point of plasmonic excitation to the formation of a reduction product from adsorbed CO2 on the surface of the NP…” “…whether plasmonic excitation, due to its collective nature, has any unique characteristics with potential for driving multi-electron transfer reactions in CO2RR is a question that is open and worth answering.” “The large landscape for further exploration, the potential for fundamental discoveries into light/matter interactions and many-body phenomena, the promise of mimicking plant photosynthesis, and the technological importance of the problem of CO2 sequestration all make plasmonic CO2RR fundamentally intriguing and a ripe topic of research.

”


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], website: http://nanogold.org/. Notes The authors declare no competing financial interest. Biographies Sungju Yu is currently a postdoctoral research associate in the Department of Chemistry at the University of Illinois at Urbana-Champaign. He received his Ph.D. degree in Chemical Engineering at Seoul National University in 2016. His research interests are in artificial photosynthesis, light-driven charge/energy transfer processes, and synthesis of nanoparticles for solar energy conversion. Andrew J. Wilson is a Springborn postdoctoral fellow at the University of Illinois at UrbanaChampaign. He received his Ph.D. in Physical Chemistry from The University of Texas at Austin in 2015 working in the lab of Katherine A. Willets. His research interests are in photoand electocatalysis, plasmonics, optical imaging, and single molecule/nanoparticle spectroscopy. Gayatri Kumari is a postdoctoral research associate in the Department of Chemistry at the University of Illinois at Urbana-Champaign. She received her Ph.D. in 2015 from Jawaharlal Nehru Centre for Advanced Scientific Research in Bangalore, India. Her current research interests are in single-nanoparticle spectroscopy and artificial photosynthesis. Xueqiang Zhang is currently a postdoctoral research associate in the Department of Chemistry at the University of Illinois at Urbana-Champaign. He received a Ph.D. in Physical


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Chemistry from the University of Notre Dame in 2016. His research interests include the design and mechanistic understanding of heterogeneous catalysts and photocatalysts and the study of fundamental surface chemistry at vapor/solid interfaces using in-situ/operando spectroscopy. Prashant K. Jain is the I. C. Gunsalus Scholar and an Associate Professor of Chemistry and the Materials Research Laboratory at University of Illinois at Urbana-Champaign with affiliations in Physics and the Beckman Institute. His research focuses on the understanding and control of light/matter interactions on the nanoscale and the use of confined light for artificial photosynthesis and atomic-scale understanding of chemical transformations and catalytic reactions in complex solids, areas in which his lab's work has been cited 15,000 times. His full biography and research webpage can be found at http://nanogold.org. ACKNOWLEDGMENT We thank the Arnold and Mabel Beckman Foundation for funding this work through a Young Investigator Award to P.K.J. and the Springborn Foundation for a postdoctoral fellowship awarded to A.J.W.. S.Y. and P.K.J. wrote manuscript with contributions from A.J.W., G.Y., and X.Z. REFERENCES (1) Betts, R. A.; Jones, C. D.; Knight, J. R.; Keeling, R. F.; Kennedy, J. J. El Niño and a Record CO2 rise. Nat. Clim. Change 2016, 6, 806-810. (2) Maginn, E. J. What to Do with CO2. J. Phys. Chem. Lett. 2010, 1, 3478-3479. (3) Mikkelsen, M.; Jørgensen, M.; Krebs, F. C. The Teraton Challenge. A Review of Fixation and Transformation of Carbon Dioxide. Energy Environ. Sci. 2010, 3, 43-81.


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