Understanding the Effects of Au Morphology on CO2 Electrocatalysis

Article Cite This: J. Phys. Chem. C 2018, 122, 4274−4280

pubs.acs.org/JPCC

Understanding the Effects of Au Morphology on CO2 Electrocatalysis Seoin Back,† Min Sun Yeom,‡ and Yousung Jung*,† †

Graduate School of EEWS, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehakro, Daejeon 34141, Korea Department of Supercomputing Application, Supercomputing Service Center, Division of National Supercomputing R&D, Korea Institute of Science and Technology Information (KISTI), Daejeon 305-806, Korea

‡

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S Supporting Information *

ABSTRACT: Toward efficient CO2 electrocatalysis for CO production, nanostructured Au catalysts have been extensively investigated by the morphology control of oxygen plasma-induced Au islands, oxide-derived Au, Au nanowires (NWs), Au nanoparticles (NPs), nanoporous Au thin films, and Au needles, yet the better performance of one morphology from another is presently not wellunderstood, making a rational design difficult. Here, the effects of metal morphologies are investigated by focusing on Au NWs and NPs using density functional theory calculations. It is revealed that activity of two key undercoordinated active sites, namely, edge and corner sites, varies delicately with different local coordination environments of various NWs and NPs, and the observed activity trend is remarkably well-rationalized with a generalized coordination number. Furthermore, it is identified that the type of planes and the dihedral angle of the constituent planes are two key factors determining the catalytic activity. A general activity trend for CO2 reduction and H2 evolution with the consideration of the density of each type of sites explains why Au NWs exhibit better catalytic performance than Au NPs, as in experiments. On the basis of the theoretical understandings, atomic-level insights and design principles are provided toward efficiently catalyzing CO2 reduction using nanostructured metal catalysts.

1. INTRODUCTION The ever-increasing atmospheric CO2 concentration has been a major concern for global sustainability, and it is expected to continuously increase in future. Development of highly efficient CO2 conversion systems is thus essential to achieve a carbonneutral technology by converting waste and captured CO2 into reusable chemical fuels. Among various different approaches for converting CO2, electrochemical reduction of CO2 in an aqueous solution is a clean method because a driving force in the form of electrical energy can be supplied from renewable sources such as sunlight via photovoltaic cells for example.1,2 The main hurdle for the commercial utilization of CO2 electrochemical reduction reactions lies in the poor performance of catalysts. Optimal catalysts should be highly active, stable for long-term use, and selective toward a single target product. The simplest and one of the most useful CO2 reduction products is carbon monoxide (CO), which can be further combined with H2 to yield liquid chemicals, for example, via Fischer−Tropsch process. Among various transition metals tested for CO2 electrochemical reduction, Au, Ag, and Zn are known to produce CO with high faradaic efficiency (FE = 80%) at a potential close to −1.0 V versus the reversible hydrogen electrode (RHE).3 The remaining 20% corresponds to undesired H2 evolution reaction (HER). Therefore, to maximize the efficiency for CO2 conversion to CO, one needs to engineer the catalysts to lower the reaction overpotential and enhance the product selectivity for CO. © 2018 American Chemical Society

Toward active and selective CO2− conversion to CO, recent studies have focused on modifying the morphology of Au catalysts, for example, in the form of oxygen plasma-induced Au islands,4 oxide-derived Au,5 Au nanowires (NWs),6 Au nanoparticles (NPs),7 nanoporous Au thin films,8 and Au needles.9 As a rational catalyst design, Sun, Peterson, and coworkers systematically investigated the CO2 reduction catalytic property of Au NPs with different sizes (4−10 nm) and observed that 8 nm NPs exhibited one of the highest faradaic efficiencies for CO (an FE of 90%) at −0.67 V versus RHE. Density functional theory (DFT) calculations suggested that the high selectivity of 8 nm NPs could be originated from the maximized ratio of edge sites,7 the sites found to be more active and selective than the corner sites of NPs. On this basis, Au NWs have been proposed to increase the population of edge sites compared to NPs. Indeed, considerable improvement was observed (an FE of 94% at −0.35 V vs RHE) using NWs.6 These results can be compared with a recent detailed theoretical investigation using more realistic sizes of experimental NPs (>2 nm) in which the Au corner site is found to be the most active and selective for CO2 reduction to CO, whereas the edge site is highly reactive for H2 production.10 These contradicting results then leave a question why NWs (mostly with edge sites) are more active for CO2 reduction than NPs Received: October 22, 2017 Revised: January 24, 2018 Published: February 6, 2018 4274

DOI: 10.1021/acs.jpcc.7b10439 J. Phys. Chem. C 2018, 122, 4274−4280

Article

The Journal of Physical Chemistry C

edge sites are made up of two intersecting (100) planes yet with different local environments, for example, a dihedral angle. For h-NW, one edge site is made up of two (111) planes and another edge site is made up of (111) and (100) planes, denoted as h-NW1 and h-NW2, respectively. For NPs, we modeled cubic, icosahedral, and cuboctahedral NPs, denoted as cubic-NP, ico-NP, and cubo-NP, respectively (Figure 1). The edge sites of cubic-, ico-, and cubo-NPs are made up of two (100) surfaces, two (111) surfaces, and (111) and (100) surfaces, respectively. Note that, in addition to the edge sites of NPs, we also considered the corner sites as a comparison, which were found to be the most active and selective sites in NP catalysts.10 Because significant quantum size effects have been observed for small-sized NPs consisting of 13 to 55 atoms, we used 365-atom cubic-NP, 309-atom ico-NP, and cubo-NP (∼2 nm), which yielded well-converged binding energies and thus are sufficiently large to describe the property of NP catalysts of more realistic sizes.10,16 For NWs, we systematically tested the effect of the NW width on the binding energies, and the sizes of NWs (∼2.5 nm) were chosen so that the binding energy converges within 0.05 eV (Figure S1). The length of NWs in a periodic direction was approximately 12 Å, similar to that of the Au(211) model. As a comparison, we modeled Au(110), (100), (111), and (211) surfaces with (4 × 3), (3 × 3), (3 × 3), and (3 × 4) atom-containing surface unit cells with six, four, four, and four layers, respectively. The γ point sampling was used for NPs and NWs, and (3 × 3 × 1) Monkhorst−Pack mesh of k points for Au(110), (100), (111), and (3 × 2 × 1) for Au(211) was used. All atoms were fully relaxed for NPs (ISIF = 2 in VASP INCAR tag), and all atoms as well as the cell parameters were fully relaxed for NWs (ISIF = 3). To calculate the electronic energies of adsorbate-attached Au NWs, all Au atoms and adsorbates were fully relaxed and the cells were kept fixed to the optimized cell sizes (ISIF = 2). For slab calculations, adsorbates and the upper two layers were allowed to relax, whereas all other atoms were fixed to their optimized bulk positions. Binding energies of adsorbates were calculated as EB[*CxHyOz] = E[*CxHyOz] − E[*] − E[CxHyOz], where E[*CxHyOz], E[*], and E[CxHyOz] correspond to the electronic energies of the total system, clean slab, and adsorbates, respectively. E[C], E[H], and E[O] were referenced to the electronic energies of graphene, H2, and H2O−H2, respectively. Electronic energy of CO molecules was calculated in a large box (10 × 10 × 10 Å3). The calculated electronic energies were converted into free energies by adding zero-point energy, entropy, and heat capacity of adsorbates. Stabilization effects due to solvation were assumed to be −0.25 and −0.10 for *COOH and *CO, respectively. We also added a correction of +0.45 eV for CO2 molecules to take into account the limitation of generalized gradient approximation functional for accurately describing experimental reaction enthalpies.17 Further correction values can be found in our previous publications.10,18 To establish a potential-dependent free-energy diagram, the computational hydrogen electrode was employed,19 where the chemical potential of a proton and electron pair is assumed to be equivalent to half of that of hydrogen gas at U = 0 V versus RHE (μ(H+ + e−) = 1/2μ(H2)), and it is shifted by −eU under the external potential U, that is, μ(H+ + e−) = 1/2μ(H2) − eU. The limiting potential (UL) is then determined from the largest ΔG (ΔGMAX) among intermediate reactions using the relation UL = −ΔGMAX/e.

(with highly active corner sites). We hypothesized that perhaps the edge sites in the (211) step, NPs, and NWs have different catalytic activities, respectively, because the subtle local environments of (211), NP, and NW edge sites are clearly distinguishable. In this work, we demonstrate that the catalytic properties of various Au edge and corner sites with different local environments for CO2 electrochemical reduction to CO are indeed different, thus reconciling the previous experimental/ computational puzzle for the Au NWs being more active than NPs. While interpreting these results, we also find that the generalized coordination number (GCN) recently proposed captures these subtle effects remarkably well. Given the importance of size, morphology, and types of metal elements as crucial factors determining the catalytic properties, the present findings will provide helpful insights toward designing active nanostructure metal catalysts, which can also be extended to other catalytic reactions.

2. COMPUTATIONAL DETAILS DFT calculations were performed with the Vienna Ab initio Simulation Package (VASP) code,11,12 with revised Perdew− Burke−Ernzerhof (RPBE) exchange correlation functional,13,14 and projector augmented-wave pseudopotentials.15 The energy cutoff was set to 500 eV, and the convergence criteria for selfconsistent iteration and ionic relaxation loop were set to 10−5 eV and 0.05 eV/Å, respectively. Starting from the experimental lattice constant of Au (4.08 Å), we observed around 3% of overestimation of the lattice constants after the unit cell relaxation (4.21 Å). Although the RPBE slightly overestimates the bulk lattice constants, the relative difference in binding energies between different active sites, which is the goal of this work, will not be affected considering a uniform shift.14 Using the optimized lattice constants, NWs with square, pentagonal, and hexagonal cross sections were modeled and denoted as s-NW, p-NW, and h-NW, respectively (Figure 1), which are periodic in the z-direction. For s-NW and p-NW,

Figure 1. (Upper) Cross-sectional view of square, pentagonal, and hexagonal NWs. NWs are periodic in the z-direction. (Lower) Cubic, icosahedral, and cuboctahedral NPs. All edge sites considered for catalytic activity in this study are presented. 4275


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3. RESULTS AND DISCUSSION CO2 electrochemical reduction to CO can be decomposed into two proton−electron-transfer reactions followed by a desorption of produced *CO as follows: * + CO2 + H+ + e− → *COOH

(1)

*COOH + H+ + e− → *CO + H 2O

(2)

*CO → * + CO

(3)

NWs (s-NW, p-NW, and h-NW2) and the edge site of cubicNP bind *COOH approximately 0.20 eV more strongly than Au(211), which represents the undercoordinated edge site of the bulk Au catalysts, whereas the edge sites of ico-NPe and cubo-NPe bind *COOH similarly to Au(211). Interestingly, EB[*COOH] at the edge sites of NPs varied from 1.53 (icoNPe) to 1.47 (cubo-NPe) to 1.25 (cubic-NPe), suggesting that a proper tuning of NP shape is required to obtain a desired catalytic property because the binding strength is sensitive to the local environments even within similar edge sites. A similar behavior is also observed for the binding strength of *COOH on the NP corner sites, where EB[*COOH] at the corner sites of NPs varied from 1.33 (ico-NPc) to 1.17 (cubic-NPc) to 1.09 (cubo-NPc). Unlike a strong linear correlation between *CO and *COOH binding (R2 = 0.875), *H binding energies are less correlated with *CO binding (R2 = 0.437). Furthermore, *H binding is less sensitive to EB[*CO], whereas *COOH binding is roughly 3 times more sensitive than *H binding based on the slope of the correlation trend line. A weaker correlation between *H and *CO binding could originate from a different atomic nature and different binding sites for H (bridge or hollow sites) and CO (top site). This observation is in agreement with the previous experiments and calculations for HER and CRR.20−23 As an approximate measure of CRR activity and selectivity, the difference between the limiting potential (UL) of HER and CRR is plotted as a function of UL of CRR in Figure 3.22,24,25

where the asterisk indicates adsorbed species on the surface. It is widely perceived that CO2 reduction reaction (CRR) to CO on Au catalysts is limited by a large energy requirement for the first protonation reaction of CO2 to form adsorbed *COOH due to the weak *COOH binding on Au surfaces.17 Therefore, stronger *COOH binding would result in a higher CRR activity. The unwanted HER also consists of two proton−electrontransfer reactions: * + H+ + e− → *H

(4)

*H + H+ + e− → * + H 2

(5)

The first reductive adsorption of *H is the potentialdetermining step (PDS) of HER on Au due to the weak *H binding, but in this case, the weaker *H binding is beneficial for the overall performance of CRR catalysts because the suppressed HER would lead to higher CRR selectivity. 3.1. Activity and Selectivity for CO2 Reduction on Various Au Sites. We modeled seven edge sites using NWs with square, pentagonal, and hexagonal cross sections, denoted as s-NW, p-NW, and h-NW, respectively, and NPs with cubic, icosahedral, and cuboctahedral shapes, denoted as cubic-NP, ico-NP, and cubo-NP, respectively (Figure 1). For NPs, in addition to the edge sites, we also considered the corner sites as a comparison, which were found to be the most active and selective sites in NP catalysts.10 For clarity, subscripts “c” and “e” will be added to the corner and edge sites of NPs, respectively. These notations will be used throughout this study. In Figure 2, binding energies of *H and *COOH are plotted versus binding energy of *CO. Apparently, the edge sites of

Figure 3. UL(CRR) − UL(HER) plotted versus UL(CRR). UL(CRR) − UL(HER) is an approximate measure of the selectivity for CRR over unwanted HER, and UL(CRR) represents the activity of CRR.

More positive UL(CRR) − UL(HER) indicates more selective CO2 reduction over H2 evolution, and more positive UL(CRR) indicates more active CO2 reduction. Two points are noteworthy. First, the corner sites of Au NPs are generally the more active reaction sites for CRR than the edge sites of NPs, consistent with the previous result.10 However, we again emphasize that catalytic properties vary significantly depending on the subtle difference in NP shapes. Second, all edge sites of NWs are more active and selective than the edge sites of NPs except for cubic-NP. The origin of the behaviors will be related to the GCN in the next section. We will link our theoretical

Figure 2. Binding energies of *COOH and *H plotted vs binding energy of *CO. The full blue and red circles correspond to the edge sites of NWs and NPs, respectively, and hollow black circles indicate four representative Au facets. The black squares represent the corner sites of NPs. The dashed lines are the trend lines for the scaling relations of binding energies based on the calculated data. 4276


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The Journal of Physical Chemistry C results to the previous experimental results and discuss a strategy to design highly active and selective Au catalysts in the last section. 3.2. Descriptor of Local Environment: GCN. Recent theoretical studies have found a linear correlation between the GCN (CN) and binding energies of adsorbates (*O, *OH, and *OOH) for oxygen reduction reaction.26,27 The linear correlation could be rationalized by the tendency of undercoordinated surface atoms to bind adsorbates strongly to compensate their loss of bonding, following the bond conservation theory.28,29 The GCN is the first-order extension of usual coordination number (cn), which is the number of first-nearest neighbors. The GCN of atom i is estimated as follows: ni

CN(i) =

∑ j=1

that smaller CN facilitates CRR, whereas it does not necessarily improve HER. All values are summarized in Table S1. Figure 4 clearly demonstrates that two observations in Figure 3 originated from the different CN for different active sites. First, CN values of the corner sites of cubic- and cubo-NPs are very small (2.00 and 3.33, respectively) compared to the CN of the edge sites of both NPs and NWs (3.67−6.33), which then leads to the higher catalytic activity of the NP corner sites. Although CN of ico-NP corner sites (4.33) is not as small as CN of cubic- and cubo-NPs, it is still smaller than CN of icoNP edge site (6.33), suggesting a higher activity of ico-NP corner sites as reported previously.10 Second, CN values of the NW edge sites are smaller than those of the NP edge sites, except for cubic-NP. This observation strongly supports our previous argument that the catalytic activities of NP and NW edge sites could be different because of the different local environments.10 Interestingly, for cubic-NP and s-NW, CN values are identical and binding energies are almost the same, suggesting that their local environments are exactly the same. To understand the key factors of the local environment of active sites, we supposed that the type of plane and the dihedral angle of two planes making up the edge sites determine the CN and thus the binding strength (Table 1). For p-NW and h-

cn(j) cn max

where cn(j) is the usual coordination number of neighboring atoms of i and cnmax is the maximum number of the first-nearest neighbors in bulk (cnmax = 12 for face-centered cubic Au). We found a strong correlation between binding energies of *CO (R2 = 0.872) and CN as well as between *COOH (R2 = 0.863) and CN (Figure 4). The mean absolute error (MAE)

Table 1. Summary of CN and Dihedral Angle of Two Planes Making up the Edge Sites of NPs and NWs constituents

models

CN

dihedral angle (deg)

(100) + (100)

s-NW cubic-NP p-NW h-NW1 ico-NP h-NW2 cubo-NP

3.67 3.67 4.33 5.16 6.33 5.33 5.33

90 90 120 120 144 131 132

(111) + (111) (111) + (100)

NW1 with an identical dihedral angle but different plane constituents, more open (100) plane results in smaller CN . On the other hand, for s-NW and p-NW with different dihedral angles but the same plane constituents, smaller dihedral angle leads to smaller CN . In cases where the two variables are the same, we observed the same CN , confirming our assumption (s-NW versus cubic-NP and h-NW2 versus cubo-NP). 3.3. Link to the Experiments. The catalytic activities of experimentally relevant sites are in the decreasing order of edge site of p-NW (ΔGPDS = 0.41 eV) > corner site of ico-NP (0.48 eV) > edge site of ico-NP (0.68 eV) for CRR, whereas ΔGPDS values for HER are similar (0.29−0.33 eV). Considering the ΔGPDS of reactions and site densities of corner and edge sites of NPs and NWs, we estimated a relative activity for CRR and HER (Figure 5 and Table S2). The following three points are noticeable. First, the trend of the theoretical onset potential (or UL) (−0.41 V for NWs and −0.48 V for NPs) is in agreement with the experimental onset potential (−0.30 V for NWs and −0.37 V for NPs). Second, although the edge site density of 2 nm wide Au NWs (10.06%) is slightly smaller than that of 4 nm NPs (12.72%), the higher intrinsic CRR activity of Au NW edge sites compensates, resulting in a CRR activity which is similar to that of 4 nm NPs and three times higher than that of 8 nm NPs. Third, the site density mainly determines the HER activity because ΔGPDS values for HER are similar. Thus, 4 nm NPs with the largest site density of undercoordinated edge and

Figure 4. Binding energies of *COOH, *H, and *CO plotted vs GCN (CN ). MAE and MAX denote mean absolute error and maximum absolute error, respectively.

and the maximum absolute error (MAX) were comparable to the values reported for *OH and *OOH binding energies in the previous study.27 Interestingly, CN of the edge sites considered in this study varies significantly from 3.67 for s-NW and cubic-NP to 6.33 for ico-NP because of the different local environments, which correspond to 0.28 and 0.31 eV in binding energy differences (ΔEB) for *COOH and *CO, respectively. The differences become more significant when additionally considering the corner sites of NPs. We note in passing that CN of a grain boundary of the Au(110) surface was reported to be 4.00,30 and it is highly likely that lower CN of the grain boundary than low-index Au facets is responsible for the improved catalytic activity of grain boundary-rich oxide-derived Au.5,31 On the one hand, we observed that a correlation between *H binding and CN is rather poor (R2 = 0.254) compared to *CO and *COOH binding (Figure 4), implying 4277


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and selectivity for CO2 reduction to CO, supporting our catalyst design strategy.42 On this basis, we suggest, when dealing with metals which bind reaction intermediates weakly (Ag, Au, and Zn), that nanostructures with a smaller dihedral angle and a more open plane ((100) rather than (111)) actively and selectively catalyze CO2 reduction. Such candidates include cubic-NP, s-NW, and p-NW where the CN ranges from 2.00 to 4.33. We particularly mention that the strategy is only specific to weakly binding metals and to CRR to CO. However, given adsorbate binding energies as a descriptor for catalytic reactions and simple counting of CN, metal- and reaction-specific design strategies can be easily established.

4. CONCLUSIONS We investigated the effects of metal morphology on catalytic activities (NWs versus NPs) by focusing on the CO 2 electrochemical reduction reactions using Au. We find that the subtle local coordination environment, which can be quantified with the recently developed GCN, is mainly responsible for the observed catalytic activities of Au NPs versus NWs. The edge sites of NWs are found to be more active than the similar edge sites of NPs because of the smaller cn in NWs, which is the main origin of the observed higher performance of Au NWs than Au NPs for CRR. Importantly, we identified that the type of planes and the dihedral angle of two intersecting planes of the nanostructures are the key factors determining the local environment and catalytic activities. The present atomic-level insights provide the catalyst design strategy using metal nanostructures, and we expect that a fine tuning of metal morphology is readily applicable to other metals and reactions.

Figure 5. (A) Eight nanometer ico-NP, (B) 2 nm ico-NP, and (C) 2 nm wide and 45 nm long p-NW consisting of similar number of Au atoms. The gray, blue, and yellow balls indicate edge sites, corner sites, and facet sites, respectively. (D) Summary of density of the active sites (the corner and edge sites for NPs and NWs, respectively), free energy change of the PDS (ΔGPDS), and relative catalytic activity normalized by that of 8 nm ico-NP. The relative catalytic activity is estimated as site density × e(−ΔGPDS/kT). The ΔGPDS of 8 nm ico-NP was assumed to be equivalent to that of 2 nm ico-NP because the *COOH binding energy converges at NPs larger than 2 nm.10

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b10439. Summary of counted usual and GCNs and the calculated binding energies, comparison of the number and density of edge and corner sites for NPs and NWs with similar number of atoms, and binding energies of *CO at the edge sites of NWs with various sizes (PDF)

corner sites are most active for HER. As a whole, DFT calculations predict that 2 nm wide Au NWs catalyze CO2 reduction in a both active and selective way than Au NPs, coinciding with the experiments well.6 3.4. Catalyst Design Strategy Using Metal Nanostructures. The importance of size, morphology, and type of metal elements as crucial factors determining the catalytic properties clearly goes well beyond the Au catalysts, and the present findings will be of great help to design nanostructure catalysts for various catalytic reactions.10,32−37 On the basis of the present finding, we can thus propose a strategy to design active metal nanostructure catalysts for CO2 reduction to CO. For metals that bind reaction intermediates relatively weakly (such as Au considered here, and Ag and Zn, which are already shown to catalyze CO2 reduction well in the bulk form),3 increasing the binding strength would generally yield further increased activity because the formation of *COOH is widely known as the PDS for these metals.30,38 It should be noted that the structure-insensitive HER can be safely ruled out, and what matters most is the structure-sensitive CRR. Therefore, decreasing the CN is expected to increase the activity, which explains a remarkable activity improvement for nanostructure catalysts compared to the bulk. Experimentally, various Au NPs with controlled shapes have been synthesized, introducing a number of Au sites with low CN.39−41 Furthermore, it was reported that pentagonal Ag NWs indeed showed high activity

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Y.J.). ORCID

Seoin Back: 0000-0003-4682-0621 Yousung Jung: 0000-0003-2615-8394 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Research Foundation of Korea from the Korean Government (NRF2017R1A2B3010176), and the supercomputing resources were supported by Korea Institute of Science and Technology Information (KSC-2015-G2-008). 4278


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chemical reduction of CO2 by copper surfaces. Surf. Sci. 2011, 605, 1354−1359. (22) Shi, C.; Hansen, H. A.; Lausche, A. C.; Nørskov, J. K. Trends in electrochemical CO 2 reduction activity for open and close-packed metal surfaces. Phys. Chem. Chem. Phys. 2014, 16, 4720−4727. (23) Gomez, R.; Fernandez-Vega, A.; Feliu, J.; Aldaz, A. Hydrogen evolution on platinum single crystal surfaces: effects of irreversibly adsorbed bismuth and antimony on hydrogen adsorption and evolution on platinum (100). J. Phys. Chem. 1993, 97, 4769−4776. (24) Chan, K.; Tsai, C.; Hansen, H. A.; Nørskov, J. K. Molybdenum sulfides and selenides as possible electrocatalysts for CO2 reduction. ChemCatChem 2014, 6, 1899−1905. (25) Back, S.; Kim, J.-H.; Kim, Y.-T.; Jung, Y. Bifunctional Interface of Au and Cu for Improved CO2 Electroreduction. ACS Appl. Mater. Interfaces 2016, 8, 23022−23027. (26) Calle-Vallejo, F.; Loffreda, D.; Koper, M. T. M.; Sautet, P. Introducing structural sensitivity into adsorption−energy scaling relations by means of coordination numbers. Nat. Chem. 2015, 7, 403−410. (27) Calle-Vallejo, F.; Tymoczko, J.; Colic, V.; Vu, Q. H.; Pohl, M. D.; Morgenstern, K.; Loffreda, D.; Sautet, P.; Schuhmann, W.; Bandarenka, A. S. Finding optimal surface sites on heterogeneous catalysts by counting nearest neighbors. Science 2015, 350, 185−189. (28) Kleis, J.; Greeley, J.; Romero, N. A.; Morozov, V. A.; Falsig, H.; Larsen, A. H.; Lu, J.; Mortensen, J. J.; Dułak, M.; Thygesen, K. S. Finite size effects in chemical bonding: From small clusters to solids. Catal. Lett. 2011, 141, 1067−1071. (29) Tritsaris, G. A.; Greeley, J.; Rossmeisl, J.; Nørskov, J. K. Atomicscale modeling of particle size effects for the oxygen reduction reaction on Pt. Catal. Lett. 2011, 141, 909−913. (30) Dong, C.; Fu, J.; Liu, H.; Ling, T.; Yang, J.; Qiao, S. Z.; Du, X.W. Tuning selectivity and activity of Au catalysts for carbon dioxide electro reduction via grain boundary engineering: a DFT study. J. Mater. Chem. A 2017, 5, 7184−7190. (31) Cheng, T.; Huang, Y.; Xiao, H.; Goddard, W. A., III Predicted Structures of the Active Sites Responsible for the Improved Reduction of Carbon Dioxide by Gold Nanoparticles. J. Phys. Chem. Lett. 2017, 8, 3317−3320. (32) Huang, H.; Jia, H.; Liu, Z.; Gao, P.; Zhao, J.; Luo, Z.; Yang, J.; Zeng, J. Understanding of Strain Effects in the Electrochemical Reduction of CO2: Using Pd Nanostructures as an Ideal Platform. Angew. Chem. 2017, 129, 3648−3652. (33) Stamenkovic, V. R.; Fowler, B.; Mun, B. S.; Wang, G.; Ross, P. N.; Lucas, C. A.; Marković, N. M. Improved oxygen reduction activity on Pt3Ni (111) via increased surface site availability. Science 2007, 315, 493−497. (34) Bu, L.; Zhang, N.; Guo, S.; Zhang, X.; Li, J.; Yao, J.; Wu, T.; Lu, G.; Ma, J.-Y.; Su, D.; Huang, X. Biaxially strained PtPb/Pt core/shell nanoplate boosts oxygen reduction catalysis. Science 2016, 354, 1410− 1414. (35) Huang, X.; Zhao, Z.; Cao, L.; Chen, Y.; Zhu, E.; Lin, Z.; Li, M.; Yan, A.; Zettl, A.; Wang, Y. M. High-performance transition metal− doped Pt3Ni octahedra for oxygen reduction reaction. Science 2015, 348, 1230−1234. (36) Li, M.; Zhao, Z.; Cheng, T.; Fortunelli, A.; Chen, C.-Y.; Yu, R.; Zhang, Q.; Gu, L.; Merinov, B. V.; Lin, Z. Ultrafine jagged platinum nanowires enable ultrahigh mass activity for the oxygen reduction reaction. Science 2016, 354, 1414−1419. (37) Won, D. H.; Shin, H.; Koh, J.; Chung, J.; Lee, H. S.; Kim, H.; Woo, S. I. Highly Efficient, Selective, and Stable CO2 Electroreduction on a Hexagonal Zn Catalyst. Angew. Chem., Int. Ed. 2016, 55, 9297− 9300. (38) Peterson, A. A.; Nørskov, J. K. Activity descriptors for CO2 electroreduction to methane on transition-metal catalysts. J. Phys. Chem. Lett. 2012, 3, 251−258. (39) Vigderman, L.; Zubarev, E. R. Starfruit-shaped gold nanorods and nanowires: synthesis and SERS characterization. Langmuir 2012, 28, 9034−9040.

REFERENCES

(1) Whipple, D. T.; Kenis, P. J. A. Prospects of CO2 utilization via direct heterogeneous electrochemical reduction. J. Phys. Chem. Lett. 2010, 1, 3451−3458. (2) Jeon, H. S.; Koh, J. H.; Park, S. J.; Jee, M. S.; Ko, D.-H.; Hwang, Y. J.; Min, B. K. A monolithic and standalone solar-fuel device having comparable efficiency to photosynthesis in nature. J. Mater. Chem. A 2015, 3, 5835−5842. (3) Hori, Y. Electrochemical CO2 Reduction on Metal Electrodes. Modern Aspects of Electrochemistry; Springer, 2008; pp 89−189. (4) Koh, J. H.; Jeon, H. S.; Jee, M. S.; Nursanto, E. B.; Lee, H.; Hwang, Y. J.; Min, B. K. Oxygen plasma induced hierarchically structured gold electrocatalyst for selective reduction of carbon dioxide to carbon monoxide. J. Phys. Chem. C 2014, 119, 883−889. (5) Chen, Y.; Li, C. W.; Kanan, M. W. Aqueous CO2 reduction at very low overpotential on oxide-derived Au nanoparticles. J. Am. Chem. Soc. 2012, 134, 19969−19972. (6) Zhu, W.; Zhang, Y.-J.; Zhang, H.; Lv, H.; Li, Q.; Michalsky, R.; Peterson, A. A.; Sun, S. Active and selective conversion of CO2 to CO on ultrathin Au nanowires. J. Am. Chem. Soc. 2014, 136, 16132−16135. (7) Zhu, W.; Michalsky, R.; Metin, Ö .; Lv, H.; Guo, S.; Wright, C. J.; Sun, X.; Peterson, A. A.; Sun, S. Monodisperse Au nanoparticles for selective electrocatalytic reduction of CO2 to CO. J. Am. Chem. Soc. 2013, 135, 16833−16836. (8) Song, J. T.; Ryoo, H.; Cho, M.; Kim, J.; Kim, J.-G.; Chung, S.-Y.; Oh, J. Nanoporous Au Thin Films on Si Photoelectrodes for Selective and Efficient Photoelectrochemical CO2 Reduction. Adv. Energy Mater. 2016, 7, 161103. (9) Liu, M.; Pang, Y.; Zhang, B.; De Luna, P.; Voznyy, O.; Xu, J.; Zheng, X.; Dinh, C. T.; Fan, F.; Cao, C. Enhanced electrocatalytic CO2 reduction via field-induced reagent concentration. Nature 2016, 537, 382−386. (10) Back, S.; Yeom, M. S.; Jung, Y. Active sites of Au and Ag nanoparticle catalysts for CO2 electroreduction to CO. ACS Catal. 2015, 5, 5089−5096. (11) Kresse, G.; Joubert, D. From ultrasoft pseudopotentials to the projector augmented-wave method. Phys. Rev. B: Condens. Matter Mater. Phys. 1999, 59, 1758. (12) Kresse, G.; Furthmüller, J. Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set. Comput. Mater. Sci. 1996, 6, 15−50. (13) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized gradient approximation made simple. Phys. Rev. Lett. 1996, 77, 3865. (14) Hammer, B.; Hansen, L. B.; Nørskov, J. K. Improved adsorption energetics within density-functional theory using revised PerdewBurke-Ernzerhof functionals. Phys. Rev. B: Condens. Matter Mater. Phys. 1999, 59, 7413. (15) Blöchl, P. E. Projector augmented-wave method. Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 50, 17953. (16) Li, L.; Larsen, A. H.; Romero, N. A.; Morozov, V. A.; Glinsvad, C.; Abild-Pedersen, F.; Greeley, J.; Jacobsen, K. W.; Nørskov, J. K. Investigation of catalytic finite-size-effects of platinum metal clusters. J. Phys. Chem. Lett. 2012, 4, 222−226. (17) Peterson, A. A.; Abild-Pedersen, F.; Studt, F.; Rossmeisl, J.; Nørskov, J. K. How copper catalyzes the electroreduction of carbon dioxide into hydrocarbon fuels. Energy Environ. Sci. 2010, 3, 1311− 1315. (18) Back, S.; Kim, H.; Jung, Y. Selective heterogeneous CO2 electroreduction to methanol. ACS Catal. 2015, 5, 965−971. (19) Nørskov, J. K.; Rossmeisl, J.; Logadottir, A.; Lindqvist, L.; Kitchin, J. R.; Bligaard, T.; Jónsson, H. Origin of the overpotential for oxygen reduction at a fuel-cell cathode. J. Phys. Chem. B 2004, 108, 17886−17892. (20) Seto, K.; Iannelli, A.; Love, B.; Lipkowski, J. The influence of surface crystallography on the rate of hydrogen evolution at Pt electrodes. J. Electroanal. Chem. Interfacial Electrochem. 1987, 226, 351−360. (21) Durand, W. J.; Peterson, A. A.; Studt, F.; Abild-Pedersen, F.; Nørskov, J. K. Structure effects on the energetics of the electro4279


Article

The Journal of Physical Chemistry C (40) Sau, T. K.; Murphy, C. J. Room temperature, high-yield synthesis of multiple shapes of gold nanoparticles in aqueous solution. J. Am. Chem. Soc. 2004, 126, 8648−8649. (41) Niu, W.; Chua, Y. A. A.; Zhang, W.; Huang, H.; Lu, X. Highly symmetric gold nanostars: crystallographic control and surfaceenhanced Raman scattering property. J. Am. Chem. Soc. 2015, 137, 10460−10463. (42) Liu, S.; Wang, X.-Z.; Tao, H.; Li, T.; Liu, Q.; Xu, Z.; Fu, X.-Z.; Luo, J.-L. Ultrathin 5-Fold Twinned Sub-25 nm Silver Nanowires Enable Highly Selective Electroreduction of CO2 to CO. Nano Energy 2018, DOI: 10.1016/j.nanoen.2018.01.016.

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