Article pubs.acs.org/JPCC
Au13: CO Adsorbs, Nanoparticle Responds Natalie Austin, J. Karl Johnson, and Giannis Mpourmpakis* Department of Chemical Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, United States S Supporting Information *
ABSTRACT: Nanoparticle properties are strongly correlated with their morphologies, such as shape and size. By combining density functional theory calculations with ab initio molecular dynamics simulations, we investigated the CO adsorption behavior on Au13 nanoparticles of Ih, Oh, and planar symmetries. Our results revealed a shape-specific adsorption response of the nanoparticles. Contrary to the behavior in bulk, we observe a symmetry-dependent d-band center shift on the nanoparticles with CO coverage, which affects the overall electronic stability of the nanoparticles. As a result, we observe 2D to 3D (planar to Ih) transition at high CO coverage. Because of the interactions with the adsorbed CO molecules, the 3D nanoparticles can accommodate more charge in their core than the 2D. All of these effects result in observing an unconventional, stronger CO adsorption on Ih Au13 nanoparticles that expose higher surface coordination number (CN = 6) than the peripheral atoms of the planar Au13 (peripheral CN = 3.4). This work highlights the shape effect on the adsorption behavior of small-sized Au nanoparticles (∼1 nm diameter).
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INTRODUCTION Metal nanoparticles (NPs) find tremendous applications in diverse areas of modern (nano) technology.1,2 Notably, Au NPs have a unique assortment of applications. Historically, the most traditional use of Au has been in jewelry because it does not corrode in bulk, a result of its remarkable chemical inertness. In contrast, nanoscale Au is chemically active and has been used in a large number of applications ranging from efficient catalysts for low-temperature CO oxidation3,4 to materials with unique optical properties that can be used to target cancer in medical applications.5 The physicochemical properties of metal NPs are related to their morphology (e.g., size and shape). Taking surface adsorption as an example, the adsorbate’s interactions with the NP are strongly dependent on the NP morphology. As the NP size increases, the average coordination number (CN) of the surface atoms increases with the maximum surface CN being 9 for the (111) facets of the fcc (face-centered cubic) lattice. This variation of the surface CN with NP size changes the interaction strength of the adsorbates on the NP surface.6,7 This is the primary reason why catalytic reactions such as CO oxidation on Au are structure-sensitive (activity variation vs NP size).6,8 In an effort to rationalize this effect, novel adsorption models have been recently introduced that relate the binding energy (BE) of adsorbates (CO) with the morphological characteristics of the Au NPs, such as the surface CN and angles.7 A general trend is that corner and edge sites of the NPs exhibit low coordination and are sites of high curvature. This under-coordination localizes electron density (LUMO, lowest unoccupied molecular orbital) of the metal and results in strong adsorption of CO (acting as electron donor molecule).7,9,10 In addition, the chemical environment (adsorbates) surrounding a NP can influence the morphology and thus the functionality of the NP. This phenomenon is pronounced on small metal © XXXX American Chemical Society
clusters. For example, experimental and theoretical work on charged Aun (n = 3−10 atoms) clusters have shown that the addition of CO molecules to a saturated cluster drives a stable reconstruction of the cluster.11,12 In addition, a structural change on charged and neutral Au6(CO)n (n = 4−6 CO molecules) clusters facilitates the exposure of more lower coordinated (apex sites) for additional CO adsorption.13 Thus, on the subnanoscale, adsorption behavior increases in complexity.10−15 Important questions that arise are, “What happens when the NPs become very small (clusters Oh (ΔE = +1.13 eV) > Ih (ΔE = +2.17 eV). These are fully optimized calculations by accounting for spin states up to multiplicity sextet. (The ground states are doublet for planar and Oh and sextet for Ih cluster.) The coverage effect in terms of CO BE per CO molecule as a function of the number of CO molecules on the clusters is presented in Figure 1a. CO molecules generally adsorb onto transition metals via a “push− pull” mechanism. In this mechanism the CO 5σ orbitals (HOMO) donate electrons to the d and s states of the metal, and in turn the d states of the metal back-donate to the antibonding 2π orbitals of CO.26 CO adsorbs molecularly on Au because the electron density donated to CO from the metal is not significant enough to result in a dissociative adsorption.26,27 In Figure 1b we present the cohesive energies (CEs) of the different clusters as a function of the number of CO molecules. The cohesive energy represents the average bond strength of the atoms forming the Au13COn configuration. This property allows us to understand the average bond strength of the clusters in response to changes in surface coverage. In Figure 1a the CO BEs (1−10 CO molecules) on the different Au13 clusters range from 0.8 to 1.5 eV, and in Figure 1b the cohesive energies range from 1.4 to 1.9 eV. (The negative sign is omitted in our discussion because all of the energy values are exothermic.) In general, the observed CO BE trend (absolute values) on the clusters is BE(Ih) > BE(Oh) ≥ BE(planar). An unexpected trend observed in Figure 1a is that the CO BEs on the Oh and Ih clusters increase to n = 4. For n > 4, the CO BE on the Oh and Ih clusters decreases with increasing CO coverage. This trend was not observed for the planar Au13 cluster, where the CO BE decreases with increasing coverage. The calculated CO BEs for n = 1 on all three Au13 clusters are weaker compared with CO adsorption on a 3D
We selected three geometries of the Au13 cluster: Ih, Oh, and planar. (The first two are 3D and the last is 2D.) The planar structure is reported to be the most stable in literature. (Clusters larger than 13 atoms form 3D structures.)17 The Ih exposes surface atoms of CN = 6, whereas the Oh is composed of surface atoms with CN = 5. The planar structure exhibits sites with different CNs (2, 3, and 6). We calculated the interaction of CO molecules with the clusters by gradually increasing the CO coverage and placing the CO molecules at maximal possible CO−CO distance. These calculations were performed using the BP86 functional18,19 and the LANLDZ basis set as implemented in the Gaussian 09 package.20 The combination of this method and basis set has been used to assess the energetics of metal clusters and their interactions with the adsorbates.7,21−23 The accuracy of this level of theory was further verified by comparing the CO adsorption trends and cluster structural conformations of neutral and anionic Au6(CO)n (n = 1−4 CO molecules) with those reported by Zhai et al.13,14 Our calculations revealed similar CO BE/n trends, and the optimized structures were identical to those presented by Zhai et al.13,14 (The results are presented in the Supporting Information in Table S1 and Figures S1 and S2). To assess the effect of symmetry on CO adsorption on Au13 clusters as a function of coverage, we froze the angles between the Au atoms of the clusters and allowed all bonds to relax. Our key observations are reproduced by performing full optimizations of the clusters (vide infra). The AIMD simulations were performed in both the NVE and NVT ensembles using the Vienna Ab initio Simulation Package (VASP).24,25 A time step of 1 fs was used in all simulations, which was found to conserve energy in the NVE ensemble adequately at all temperatures used in this work. Results from both ensembles gave qualitatively similar results. B
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Figure 2. (a) CO BE on the Au13 clusters as a function of their d-band center (dC). The numbers on the colored symbols represent the number of CO molecules. (b) CO HOMO, d orbital density of states, dC, and HOMO(H)−LUMO(L) orbitals of the Au13 (top) and Au13(CO)6 (bottom) systems.
cuboctahedral Pt13 (∼2.5 eV).28 The BEs of one CO molecule adsorbed on the different clusters are in agreement with the BEs presented by Amft et al. for 2D (∼0 to 1 eV) and 3D clusters (∼0.8 to 1.2 eV).29 In Figure 1b we observe linear trends between the CE of the different clusters and the number of CO molecules; however, the slopes of these lines are different, showing a shape-dependent response of the cluster’s energetics on CO coverage. This behavior, intrinsic to the shape/symmetry of the clusters, is responsible for the change in the energetic preference of the clusters (CE values) at n = 6. Specifically, the energetic preference changes from planar > Oh > Ih at zero and low CO coverage (n < 6) to Ih > Oh > planar at high CO coverage (n > 6). To understand this important observation on the energetics of the clusters we investigate the electronic properties, including the d-band center (dC) of the clusters. Nørskov’s pioneering work has shown that the BE of an adsorbate can be correlated with the dC of the metal (adsorbent).16 Consequently, we calculated the density of states (DOS) of the d orbitals and determined the dC of the clusters as detailed in ref 30. In Figure 2a we plot the CO BE as a function of dC of the clusters. The dC of the clusters has been calculated in the presence of CO on their surfaces to understand the ligand effect in altering the dC position. For example, for the nth CO molecule adsorption on a surface of the cluster (occupied by n − 1 CO molecules), we plot the BE of the nth CO molecule versus the dC of the cluster with n − 1 CO molecules on its surface. Hence, the dC value for n = 1 corresponds to the dC of the pure Au cluster. The general trend observed in Figure 2a is that the metal’s dC moves away from the Fermi level with increasing CO coverage due to ligand effects (hybridization of the metal’s d with CO’s s and p character atomic orbitals) and results in weaker adsorption of CO, with a linear trend (see lines) resembling the adsorption behavior in bulk; however, there are two key observations that demonstrate a marked deviation from the traditional behavior in bulk. Even though the dC of the Oh cluster decreases with
CO coverage, the CO BE increases at low CO coverage (n = 1−4) and decreases at higher (n > 5). In addition, the dC of the Ih structure does not change significantly for n = 1−6, resulting in a practically unchanged CO BE with coverage up to n = 6 CO molecules. To further understand this shape-specific adsorption anomaly, we plot the DOS, dC, and HOMO− LUMO levels for the clusters and the HOMO level of CO in Figure 2b. By comparing the dC values of the bare Au13 clusters of different symmetry (top figure) with those with six CO molecules adsorbed (Figure 2b, lower panel, Au13(CO)6), we observe that the dC of Ih does not change significantly (∼−8.3 to −8.5 eV, blue vertical line), whereas the dC of the Oh and planar clusters changes more noticeably (∼−7.8 to −8.5 eV for Oh and ∼−7.8 to −8.4 eV for planar; see vertical red and green lines, respectively). Additionally, we observe that the positioning of the dC of the Au13 clusters with respect to the CO HOMO follows the adsorption (BE) trend of the first CO molecule on the clusters, that is, BEIh > BEOh ≈ BEPlanar. The positioning of the dC of an individual Au atom within each of the Au13 clusters also follows the (BE) trend of the first CO adsorbed to the clusters. (See Supporting Information Figure S3.) The d DOS for Au13(CO)6 having different symmetries becomes more spread due to hybridization with sp-type orbitals of CO; this results in weaker overlap with the CO HOMO and weaker adsorption. This hybridization affects the frontier orbitals of the clusters as well. This is depicted in Figure S4 of the Supporting Information, where we plot the LUMO type orbitals of the clusters with increasing CO coverage. (The HOMO orbitals are presented in Figure S5, Supporting Information.) The highly directional and localized LUMO orbitals of the bare clusters become diffuse in response to adsorption of CO molecules. As an electron donor, the CO molecule preferentially interacts with sites of the cluster that localize LUMO orbitals.9 Thus, the loss of orbital directionality and localization results in weaker CO adsorption.7 The weak binding of CO to the clusters due to the shift in dC of the clusters can also be explained by back-donation in the push− C
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Figure 3. (a) Total natural bond orbital (NBO) charge on Au atoms as a function of the number of CO molecules and (b) charge distribution on the Ih (left), Oh (middle), and planar (right) clusters when six CO molecules are adsorbed. The color ranges from green to red with green being the positive end and red being the negative end on the color bar. The numbers in red represent the charge of the central atoms of the clusters.
planar cluster (charge spread throughout the plane). Consequently, the atoms of the 3D clusters can accept (store) more charge from their interactions with the CO molecules. This behavior explains the stronger CO binding observed on the surface of the Ih and Oh Au structures (even at high coverage) compared with the planar one. It should be noted that we investigated the adsorption of up to 10 CO molecules on these clusters due to the fact that the planar structure exhibits 10 peripheral sites (corners and edges) that can potentially bind strongly the CO molecules. (Plane atoms interact very weakly with CO.)7,31 On the basis of that, one would expect to favor adsorption on the planar structure due to the fact that the average CN of the peripheral atoms is CN = 3.4, which is much lower than the surface CN = 6 for Ih and CN = 5 for Oh structures. Despite the CN differences of the available adsorption sites on these clusters, we observed the completely opposite binding behavior due to the shape factors that we previously analyzed.7 To verify the change in the energetics of the clusters as a function of coverage, we performed full optimization calculations of the three different clusters when 6 (energetic transitional point) and 10 CO molecules (full coverage) are adsorbed on the clusters. In both cases, we observed the following two points: (a) the Oh structure relaxed to the Ih and (b) the Ih structure became energetically more favorable than the planar in the presence of the CO molecules. The fulloptimization calculations verified the (partial-optimization) observations of Figure 1b: The most energetically favorable Au13 structure is planar (2D) in the absence of CO, whereas the
pull adsorption mechanism. With increasing CO coverage, the dC of the clusters shifts away from the LUMO of CO (dC: −2.66 eV, not shown on Figure 2b). Because the d states of the metal donate electron density to the 2π antibonding orbitals of the CO molecule7,26 (thus increasing the BE), the observed shift decreases the BE of CO, as shown in Figure 2a. An important question that we have not yet answered is why the CO BE up to six CO molecules is essentially constant for the Ih and Oh structures, whereas the BE decreases linearly with increasing coverage for the planar structure. By answering this question we will be able to understand the change in the relative energetics of the clusters when six CO molecules are adsorbed on the surface. (See CE values at n = 6 of Figure 1b.) In Figure 3a we present the total charge distributed on all 13 Au atoms as a function of the number of CO molecules. CO as an electron donor molecule preferentially binds to sites of the clusters that are positively charged or “electron poor” and to sites with LUMO localization.10 According to Figure 3a, as the number of CO molecules bound on the Oh and Ih clusters increases, the total charge that the Au atoms can accommodate is −0.8 e (at n = 8). This trend completely changes in the planar cluster where the cluster can only accommodate −0.4 e charge (at n = 4). After this point the charge is practically unchanged with coverage. To identify the origin of this different charge transfer behavior between the 2D (planar) and 3D (Ih, Oh) clusters, we plot the NBO (natural bond orbital) charge distribution on each of the atoms in the clusters in Figure 3b. This plot clearly illustrates that both Oh and Ih localize more negative charge in their core atom (red colored atom) than the D
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Figure 4. (a) Average Au−Au coordination number as a function of time for an AIMD simulation at 600 K starting from a planar Au13CO10 cluster. (b) Beginning snapshot from AIMD simulation showing initial planar structure. (c) Ending snapshot from the AIMD simulation showing a welldeveloped 3D arrangement of Au atoms.
50%). This stability change of the clusters was supported by AIMD simulations, which showed a transition from 2D to 3D structures in the presence of high CO coverage. We further examined the d-band center response of the clusters of different symmetry as a function of CO coverage. Our results unraveled key deviations from the bulk behavior: The dC of each cluster (of different symmetry) responds with a unique way to the CO coverage; for example, the dC of the Ih symmetry is practically unaffected at low CO coverage compared with the important changes in the dC of the planar structure. An NBO charge analysis demonstrated that the Ih and Oh clusters can hold more charge than the planar cluster. All of these electronic structure observations result in a shapespecific adsorption response of the clusters and a 2D to 3D transition with increasing CO coverage. It should be noted that on this subnanoscale regime electronic effects dominate, resulting in stronger adsorption on clusters exposing surface sites with high CNs (Ih, surface CN = 6) compared with ones with low CNs (planar, on peripheral atoms CN = 3.4). On the contrary, shape transitions with CO coverage have been experimentally observed on Au NPs of 2−5 nm in diameter32,33 and successfully modeled with Wulff construction models that take into consideration surface energy minimization.34 Our work demonstrates the response of the electron density of NPs close to 1 nm in size under different symmetries and with increasing adsorbate coverage. Shape-specific adsorption behavior can influence structure sensitive reactions. Most importantly, understanding particle shape effects on adsorption at the nanoscale can potentially lead to the design of highly E
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(13) Zhai, H.-J.; Pan, L.-L.; Dai, B.; Kiran, B.; Li, J.; Wang, L.-S. Chemisorption-Induced Structural Changes and Transition from Chemisorption to Physisorption in Au-6(CO)(n)(−) (n=4−9). J. Phys. Chem. C 2008, 112, 11920−11928. (14) Zhai, H. J.; Kiran, B.; Dai, B.; Li, J.; Wang, L. S. Unique Co Chemisorption Properties of Gold Hexamer: Au-6(CO)(n)- (n=0−3). J. Am. Chem. Soc. 2005, 127, 12098−12106. (15) Nikbin, N.; Austin, N.; Vlachos, D. G.; Stamatakis, M.; Mpourmpakis, G. Catalysis at the Sub-Nanoscale: Complex CO Oxidation Chemistry on a Few Au Atoms. Catal. Sci. Technol. 2015, 5, 134−141. (16) Hammer, B.; Morikawa, Y.; Norskov, J. K. Co Chemisorption at Metal Surfaces and Overlayers. Phys. Rev. Lett. 1996, 76, 2141−2144. (17) Xiao, L.; Tollberg, B.; Hu, X. K.; Wang, L. C. Structural Study of Gold Clusters. J. Chem. Phys. 2006, 124, 114309. (18) Becke, A. D. Density-Functional Exchange-Energy Approximation with Correct Asymptotic-Behavior. Phys. Rev. A: At., Mol., Opt. Phys. 1988, 38, 3098−3100. (19) Perdew, J. P. Density-Functional Approximation for the Correlation-Energy of the Inhomogeneous Electron-Gas. Phys. Rev. B: Condens. Matter Mater. Phys. 1986, 33, 8822−8824. (20) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A. Gaussian 09; Gaussian, Inc.: Wallingford, CT, 2009 (21) Nikbin, N.; Mpourmpakis, G.; Vlachos, D. G. A Combined Dft and Statistical Mechanics Study for the CO Oxidation on the Au10(−1) Cluster. J. Phys. Chem. C 2011, 115, 20192−20200. (22) Hossain, D.; Pittman, C. U., Jr.; Gwaltney, S. R. Structures and Stabilities of the Metal Doped Gold Nano-Clusters: M@Au-10 (M = W, Mo, Ru, Co). J. Inorg. Organomet. Polym. Mater. 2014, 24, 241− 249. (23) Molina, B.; Soto, J. R.; Calles, A. DFT Normal Modes of Vibration of the Au-20 Cluster. Rev. Mex. Fis. 2008, 54, 314−318. (24) Kresse, G.; Furthmuller, J. Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 11169−11186. (25) Kresse, G.; Furthmuller, 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. (26) Sung, S. S.; Hoffmann, R. How Carbon-Monoxide Bonds to Metal-Surfaces. J. Am. Chem. Soc. 1985, 107, 578−584. (27) Fielicke, A.; Gruene, P.; Meijer, G.; Rayner, D. M. The Adsorption of Co on Transition Metal Clusters: A Case Study of Cluster Surface Chemistry. Surf. Sci. 2009, 603, 1427−1433. (28) Li, L.; Larsen, A. H.; Romero, N. A.; Morozov, V. A.; Glinsvad, C.; Abild-Pedersen, F.; Greeley, J.; Jacobsen, K. W.; Norskov, J. K. Investigation of Catalytic Finite-Size-Effects of Platinum Metal Clusters. J. Phys. Chem. Lett. 2013, 4, 222−226. (29) Amft, M.; Johansson, B.; Skorodumova, N. V. Influence of the Cluster Dimensionality on the Binding Behavior of Co and O-2 on Au-13. J. Chem. Phys. 2012, 136, 024312. (30) Mpourmpakis, G.; Stamatakis, M.; Herrmann, S.; Vlachos, D. G.; Andriotis, A. N. Predicting the Adsorption Behavior in Bulk from Metal Clusters. Chem. Phys. Lett. 2011, 518, 99−103. (31) Mpourmpakis, G.; Vlachos, D. G. The Effects of the Mgo Support and Alkali Doping on the Co Interaction with Au. J. Phys. Chem. C 2009, 113, 7329−7335. (32) Ueda, K.; Kawasaki, T.; Hasegawa, H.; Tanji, T.; Ichihashi, M. First Observation of Dynamic Shape Changes of a Gold Nanoparticle Catalyst under Reaction Gas Environment by Transmission Electron Microscopy. Surf. Interface Anal. 2008, 40, 1725−1727. (33) Uchiyama, T.; Yoshida, H.; Kuwauchi, Y.; Ichikawa, S.; Shimada, S.; Haruta, M.; Takeda, S. Systematic Morphology Changes of Gold Nanoparticles Supported on Ceo2 During Co Oxidation. Angew. Chem., Int. Ed. 2011, 50, 10157−10160. (34) Barmparis, G. D.; Remediakis, I. N. Dependence on CO Adsorption of the Shapes of Multifaceted Gold Nanoparticles: A
efficient nanomaterials, ranging from active nanocatalysts to targeted drug delivery nanocarriers.
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ASSOCIATED CONTENT
S Supporting Information *
HOMO−LUMO orbital illustrations, AIMD simulation results, binding energies, optimized geometries, and single Au atom dDos and dC. Movie of the simulation in Figure 4. This material is available via the Internet at The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b03459.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the Center for Simulation and Modeling at the University of Pittsburgh for computational support. This research has been supported by start-up and Central Research Development funds (CRDF) from the University of Pittsburgh. The authors would like to thank Michael G. Taylor for help on graphics preparation.
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REFERENCES
(1) Moshfegh, A. Z. Nanoparticle Catalysts. J. Phys. D: Appl. Phys. 2009, 42, 233001. (2) Bhattacharya, R.; Mukherjee, P. Biological Properties of ″Naked″ Metal Nanoparticles. Adv. Drug Delivery Rev. 2008, 60, 1289−1306. (3) Kaspar, J.; Fornasiero, P.; Hickey, N. Automotive Catalytic Converters: Current Status and Some Perspectives. Catal. Today 2003, 77, 419−449. (4) Pattrick, G.; van der Lingen, E.; Corti, C. W.; Holliday, R. J.; Thompson, D. T. The Potential for Use of Gold in Automotive Pollution Control Technologies: A Short Review. Top. Catal. 2004, 30−1, 273−279. (5) Daniel, M. C.; Astruc, D. Gold Nanoparticles: Assembly, Supramolecular Chemistry, Quantum-Size-Related Properties, and Applications toward Biology, Catalysis, and Nanotechnology. Chem. Rev. 2004, 104, 293−346. (6) Hvolbaek, B.; Janssens, T. V. W.; Clausen, B. S.; Falsig, H.; Christensen, C. H.; Norskov, J. K. Catalytic Activity of Au Nanoparticles. Nano Today 2007, 2, 14−18. (7) Mpourmpakis, G.; Andriotis, A. N.; Vlachos, D. G. Identification of Descriptors for the Co Interaction with Metal Nanoparticles. Nano Lett. 2010, 10, 1041−1045. (8) Valden, M.; Pak, S.; Lai, X.; Goodman, D. W. Structure Sensitivity of Co Oxidation over Model Au/Tio2 Catalysts. Catal. Lett. 1998, 56, 7−10. (9) Chretien, S.; Buratto, S. K.; Metiu, H. Catalysis by Very Small Au Clusters. Curr. Opin. Solid State Mater. Sci. 2007, 11, 62−75. (10) Austin, N.; Mpourmpakis, G. Understanding the Stability and Electronic and Adsorption Properties of Subnanometer Group Xi Monometallic and Bimetallic Catalysts. J. Phys. Chem. C 2014, 118, 18521−18528. (11) Fielicke, A.; von Helden, G.; Meijer, G.; Pedersen, D. B.; Simard, B.; Rayner, D. M. Gold Cluster Carbonyls: Saturated Adsorption of Co on Gold Cluster Cations, Vibrational Spectroscopy, and Implications for Their Structures. J. Am. Chem. Soc. 2005, 127, 8416−8423. (12) Fielicke, A.; von Helden, G.; Meijer, G.; Simard, B.; Rayner, D. M. Gold Cluster Carbonyls: Vibrational Spectroscopy of the Anions and the Effects of Cluster Size, Charge, and Coverage on the Co Stretching Frequency. J. Phys. Chem. B 2005, 109, 23935−23940. F
DOI: 10.1021/acs.jpcc.5b03459 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C Density Functional Theory. Phys. Rev. B: Condens. Matter Mater. Phys. 2012, 86, 085457.
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