Article pubs.acs.org/JPCC
Multilayer Graphynes for Lithium Ion Battery Anode Ho Jun Hwang,†,§ Jahyun Koo,†,§ Minwoo Park,† Noejung Park,‡ Yongkyung Kwon,† and Hoonkyung Lee*,† †
Division of Quantum Phases and Devices, School of Physics, Konkuk University, 120 Neungdong-ro, Gwangjin-gu, Seoul 143-701, Korea ‡ Interdisciplinary School of Green Energy and Low Dimensional Carbon Materials Center, Ulsan National Institute of Science and Technology (UNIST), Ulsan 689-798, Korea ABSTRACT: Graphynes, two-dimensional layers of sp- and sp2-bonded carbon atoms, have recently received considerable attention because of their potential as new Dirac materials. Here, focusing on their large surface area, we explore the applicability of graphynes as lithium ion battery anodes through the first-principles density functional calculations. We have found that Li potential energies are in the range suitable to be used as anodes. Furthermore, the maximum composite of Li-intercalated multilayer α- and γ-graphynes is found to be C6Li3, which corresponds to a specific capacity of 1117 mAh g−1, twice as large as the previous theoretical prediction for graphynes. The volumetric capacity of Liintercalated multilayer α- and γ-graphynes is 1364 and 1589 mAh cm−3, respectively. Both specific and volumetric capacities of Li-intercalated graphynes are significantly larger than the corresponding value of graphite, from which we conclude that multilayer graphynes can serve as high-capacity lithium ion battery anodes. window of ∼2 eV from bulk Li energy, i.e., ∼1.6−3.0 eV because the cohesive energy of bulk Li is ∼1.6 eV and the maximum binding energy of Li is ∼3 eV for an anode.2 Segregation of lithium may occur if the binding energy is less than ∼1.6 eV, and it does not work as an anode with a binding energy greater than ∼3 eV. In addition, the specific and volumetric capacities should be larger than the corresponding values of graphite and the volume change by lithium insertion should be as small as graphite. Recently, it has been reported that graphynes, two-dimensional layers of sp- and sp2-bonded carbon atoms, have intriguing electronics properties, that is, symmetric and asymmetric Dirac cones, which lead to different properties from graphenes such as electron collimation transport.8−11 Another attractive feature is that a graphyne can be considered as a sheet of one-dimensional carbon chain networks with a very large surface area because its hexagon area is much larger (approximately eight times larger) than that of graphene. From an application point of view, graphyne’s properties could allow a variety of potential applications for energy storage. For instance, it has been found that graphynes decorated with calcium or lithium atoms can be used as high-capacity hydrogen storage material because of its large surface area.12−14 Furthermore, graphynes also may have high electronic conductivity due to their metallicity. Some years ago a
1. INTRODUCTION The lithium ion battery (LIB) is one of the most essential technologies for the development of portable electronics because it has a higher energy density than other rechargeable batteries. Lithium metal anodes were used because of inherent high specific capacity of 3860 mAh g−1.1,2 However, lithium metal anodes have some concerns regarding dendrite formation and safety. Recently, graphite has been widely used as an anode material because of high diffusivity of lithium ions (high power) and high stability through lithium intercalation (small volume change). The maximum configuration of Li-intercalated graphite is one lithium atom to every six carbon atoms, which corresponds to the composite of C6Li with the specific capacity being as large as of 372 mAh g−1.3−5 Novel anode materials, which can surpass the capacity of graphite, are widely pursued to achieve higher capacity of LIBs. Silicon, germanium, and tin, which have high specific capacity of ∼4000, 1600, and 1000 mAh g−1, respectively, were suggested for new anode materials.6,7 However, they were found to have a critical issue related to crystalline expansion during insertion and extraction of lithium ions, resulting in large volume changes (∼400%). This, along with low electronic conductivity (semiconductors) and low lithium diffusivity, presents a serious obstacle in their application as anodes. In contrast, layered-structured materials are expected to resolve the volume-change issue because Li intercalation between the layers may lead to small volume change. To accomplish this, the anode materials must meet certain requirements: The binding (potential) energy of Li atoms should be in the energy © XXXX American Chemical Society
Received: September 14, 2012 Revised: March 4, 2013
A
dx.doi.org/10.1021/jp3105198 | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
Article
bottom-up approach15 to synthesize sp−sp2 carbon networks such as graphynes was proposed, and a recent report of the synthesis of graphdiyne,16 which is a sp−sp2 hybrid carbon allotrope, suggests that new carbon allotropes of graphynes could be synthesized in the immediate future. Recently, Zhang et al.17 explored the possible application of a γ-graphyne for LIB anodes by using the density functional theory (DFT) calculations. They found high Li mobility as a result of both in-plane and out-of-plane diffusion of Li ions in graphyne and Li intercalation density as high as C4Li in a multilayer γ-graphyne with AB stacking order. In this paper, we performed our own first-principles DFT calculations to make a more thorough investigation of the Li intercalation on αgraphyne as well as on γ-graphyne. While Zhang et al. considered the intercalation of a single Li atom per (either hexagonal or triangular) hollow site of a multilayer γ-graphyne, we searched for energetically the most stable structure of a Li− graphyne complex at a given Li concentration by fully relaxing all atomic degrees of freedom in our supercell. We have found stable Li-intercalated multilayer graphynes where, unlike the Zhang et al.’s result, multiple Li atoms are intercalated to one hollow site. The difference may be ascribed to the different layer stacking. The maximum composite of Li-intercalated αand γ-graphynes is found to be C6Li3, which corresponds to a specific capacity of 1117 mAh g−1, 100% higher than the highest Li intercalation density of C4Li predicted previously by Zhang et al.17 for AB-stacked multilayer γ-graphyne. The calculated volumetric capacities of C6Li3 are 1364 and 1589 mAh cm−3 for the α- and the γ-graphynes, respectively. These specific and volumetric capacities of Li-intercalated graphynes are much larger than the corresponding values of graphite (372 mAh g−1 and 818 mAh cm−3). This suggests that multilayer graphynes can serve as high-capacity LIB anodes. In addition, a recent DFT study18 on lithium storage on graphdiyne found high Li mobility between graphdiyne layers and the composite of C6Li2 for Li-dispersed single layer graphdiyne. This also shows the possible applications of multilayer graphdiyne to LIB anodes. However, a detailed study on Li intercalation to multilayer graphdiyne will be necessary to compare its applicability with that of multilayer graphyne.
consistent with the value of 0.67 eV/Li in the literature,23 confirming the reliability of our approach.
3. RESULTS AND DISCUSSION Graphynes have various phases with different local geometries consisting of hexagons, triangles, and tetragons of sp- and sp2bonded carbons. Since we have found that Li adsorption depends only on the local geometries such as hexagons and triangles, single-layer α- and γ-graphynes are chosen first as representatives for the Li adsorption study. Figure 1 shows the
Figure 1. Atomic structures of a Li atom attached on (a) 2 × 2 αgraphyne and (b) 2 × 2 γ-graphyne. The gray-colored and purplecolored dots represent carbon atoms and lithium atoms, respectively.
atomic structures of Li-dispersed α- and γ-graphynes, obtained from the calculations for a 2 × 2 supercell of graphynes. The binding energy of Li atoms on a 2 × 2 graphyne was calculated as a function of a Li concentration x, where x is defined from C6Lix. Here the binding energy of Li atoms is defined by x E bind (Li) = (EC + NE Li − ECx−Li)/N
(1)
where N is the number of attached Li atoms per 2 × 2 cell for a given x, ExC−Li is the total energy of Li-dispersed 2 × 2 graphyne with a x concentration of Li atoms, EC is the total energy of 2 × 2 isolated graphyne, and ELi is the total energy of an isolated Li atom in a vacuum. For 2 × 2 γ-graphyne, N = 8x so that C48LiN is reduced to C6Lix. The composites of Li-dispersed α- and γ-graphynes shown in Figure 1 are (a) C6Li0.188 and (b) C6Li0.125, respectively. There are several attachment sites for Li atoms on graphynes, i.e., on top of the C atoms and in the hollow sites of sp- and sp2bonded hexagons and sp-bonded triangles. For the attachment of Li atoms on α-graphyne, the most favorable adsorption site is in-plane and slightly off center of a hexagon, as shown in Figure 1a, where the distance between the Li atom and the nearest C atom is ∼2.23 Å and the Li binding energy is 2.21 eV/Li. On γgraphyne, a Li atom binds to a triangular hollow site with a binding energy of 2.69 eV/Li (see Figure 1b) and the nearest Li−C distance is 2.26 Å and the Li atom is located at a height of 0.95 Å above the graphyne sheet. This is consistent with Zhang et al.’s result.17 Li binding energy on a graphyne is greater than ELi cohesive. From this we expect that Li atoms can be dispersed on graphynes without segregation of Li. The calculated binding energies of Li atoms on graphynes are much larger than the corresponding GGA values of 1.10 eV/Li on graphene,23 1.80 eV/Li on a C60,24 and 0.34 (0.41) eV/Li on the inner (outer) shell of a (5,5) carbon nanotube.25 One can argue that graphene, isolated carbon nanotubes, and fullerenes may not be suitable for LIB anode materials because the Li binding energies enumerated above are smaller than the cohesive energy of bulk Li (ELi cohesive), whose theoretical and experimental values are 1.58
2. COMPUTATIONAL DETAILS Our calculations were performed using a first-principles method based on density functional theory19 as implemented in the Vienna ab initio simulation package (VASP) with a projectoraugmented-wave (PAW) method.20 The exchange correlation energy functional was used with the generalized gradient approximation (GGA) in the Perdew−Burke−Ernzerhof scheme,21 and the kinetic energy cutoff was set at 400 eV. Our model α-(γ-)graphyne system was a 2 × 2 hexagonal supercell containing 32 (48) C atoms. A geometrical optimization of Li-dispersed graphyne was carried out within a fixed 2 × 2 supercell obtained from the equilibrium lattice constant of the isolated graphyne until the Hellmann− Feynman force acting on each atom was less than 0.01 eV/Å. The first Brillouin zone integration was done using the Monkhorst−Pack scheme.22 A 4 × 4 × 1 k-point sampling was done for the 2 × 2 graphynes. To remove spurious interactions between image structures due to periodic calculations, a vacuum layer of 10 Å was taken in each of all nonperiodic directions. We performed some test calculations for a model system of Li-adsorbed graphene. The calculated binding energy (0.69 eV/Li) of Li on 4 × 4 graphene is B
dx.doi.org/10.1021/jp3105198 | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
Article
because Li atoms are adsorbed at the in-plane sites. These results suggest that multilayer graphynes may be used for an anode material. For anode materials, major advantages of intercalation compounds such as graphite are high Li diffusivity, small volume change by Li intercalation between the layers, long cycle life by good reversibility of Li insertion and extraction, and good scalability. Similar to graphite, multilayer graphynes could possibly form a lithium ion battery anode. We computed Li-intercalated multilayer structures based on α- or γ-graphynes. From the calculations of Li dispersion on single-layer graphynes discussed above, we predict stable Li-intercalated multilayered graphyne structures. Figure 3a shows the binding energy of Li atoms for different concentrations of x. On multilayer αgraphyne, we find that AB-like stacking is favorable to Li
and 1.63 eV, respectively. We here note that according to a theoretical study of Zhang et al.26 on carbon nanotube bundles, there are interstitial Li binding states between the nanotubes where the Li intercalation energy (∼3.0 eV/Li for C1Li0.6) is larger than the bulk Li cohesive energy. This may explain recent experimental findings1 that carbon nanotubes can serve as the anode materials. Next, we consider the stability of Li dispersion on single-layer graphynes. Since Li diffusion on single-layer graphynes readily occurs as discussed in detail in ref 17, nucleation of Li atoms could happen. Figure 2 shows the binding energy of Li atoms
Figure 2. (a, c) Calculated Li binding energy per Li atom vs concentration x. (b) Atomic structure for x = 3 on α-graphyne. (d) Atomic structure for x = 2 on γ-graphyne. The solid and empty diamonds indicate the stable and metastable configurations.
on 2 × 2 graphynes as a function of the Li concentration x, along with the optimized geometries of Li-dispersed graphynes. The x = 3 configuration for α-graphyne, in which each hexagon of somewhat distorted graphyne accommodates exactly four Li atoms, is displayed in Figure 2b. For γ-graphyne, Li adsorption on only one side of the sheet is considered because in multilayer graphynes, Li atoms are dispersed on one side of each graphyne sheet. The x = 2 configuration on a γ-graphyne is shown in Figure 2d, where Li atoms are mostly dispersed above triangles. When the Li binding energy for a given concentration x is larger than the cohesive energy (ENbind > ELi cohesive), the x-configurations of Li-dispersed graphynes are energetically more stable than the segregated phase between bulk Li and graphynes. In Figure 2a and Figure 2c, the solid dots represent the Li binding energies of the most stable configuration for a given concentration x while the empty dots correspond to those of the various local-minimum configurations found in the energy minimization process. As can be seen, the Li binding energies to a single-layer α- or γ-graphyne are larger than (or very close to) the cohesive energy for all Li concentrations (x < 3) considered here. In multilayer graphynes the Li binding energies could further increase because of the layer−layer interaction. Another attractive feature of the α-graphyne is that minimal, if any, volume change occurs in the Li insertion and extraction process
Figure 3. (a) Calculated Li binding energy per Li atom of Liintercalated AB-stacking multilayer α-graphyne as a function of concentration x (C6Lix). (b) Side view of the atomic structure for x = 3. (c) Cross-sectional view of the atomic structure for x = 3. C
dx.doi.org/10.1021/jp3105198 | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
Article
layer−layer distance in Li-intercalated graphynes shows weak dependence on the concentration x. As expected, the binding energy of Li atoms in multilayer graphynes slightly increases compared to that on single layer graphynes. As shown in Figure 4a, the Li binding energy on multilayer γgraphyne is larger than the cohesive energy of bulk Li for concentrations lower than x = 3, where Li atoms are intercalated without segregation of Li. Furthermore, we found that Li clustering occurs at high concentrations of x ≳ 3 and the layer−layer distance increases as a result of the Li clustering by ∼50% of 4.1 Å. We believe that large nucleation barrier is associated between a Li clustered configuration and an x = 3 configuration. So the maximum composite of Li-intercalated γgraphyne is expected to be C6Li3. On the other hand, Li atoms may also be intercalated on α-graphyne without segregation of Li until the concentration x reaches 3 though the Li binding energy around x ≈ 3 is slightly smaller than the cohesive energy of bulk Li because of a large nucleation barrier between a x ≈ 2.5 configuration and a Li segregated configuration. Therefore, the maximum configuration for Li-intercalated α-graphyne is also expected to be C6Li3 because of Li clustering for x > 3. This composite corresponds to the specific capacity of 1117 mAh g−1. In addition, the calculated volumetric capacities of Liintercalated multilayer α-graphyne and γ-graphyne (C6Li3) are 1364 and 1589 mAh cm−3, respectively, both of which are significantly greater than 818 mAh cm−3, the capacity of graphite.5 This demonstrates that multilayer graphynes can be used as LIB anode materials. We have discussed distinct properties of multilayer graphynes from those of graphite as a promising LIB anode material. The sp-bonded carbon atoms of a graphyne bind Li atoms with larger binding energies and higher Li gravimetric density than the sp2-bonded carbon atoms do (see Figures 2−4) because the former atoms are more reactive to Li than the latter. This is a main reason why multilayer graphynes may have larger Li capacity than graphite consisting of only sp2-bonded carbon atoms. We note that structural stability of graphynes in the process of lithium intercalation/deintercalation is critical for their practical implementations to LIB anodes, especially considering that the calculated cohesive energies of α- and γgraphynes (8.2 and 8.5 eV, respectively) are smaller than that of graphite (9.1 eV). Detailed molecular dynamics study on Li intercalation into multilayer graphynes would be necessary to answer this stability issue. On the other hand, since α-graphyne is metallic while γ-graphyne is semiconducting with a DFT bandgap of 0.471 eV,27 α-graphyne would be advantageous over γ-graphyne in terms of the requirement for the anodes, electronic conducivity. However, a more detailed study on their respective conductivities may be necessary.
intercalation. Figures 3b and Figure 3c show the side and crosssectional views of Li-intercalated α-graphyne with a x = 3 concentration, where the layer−layer distance is 3.1 Å. The Li atoms are intercalated in-plane sites of the graphyne and up to four Li atoms are accommodated within a hexagon, as shown in Figure 3c, which is the same as in the case of single layer αgraphyne. In contrast, for multilayer γ-graphyne, AA-like stacking is found to be more favorable to Li intercalation and to result in higher Li capacity than AB-like stacking which Zhang et al.17 considered in their study of Li-intercalated graphynes. Figure 4b and Figure 4c show the side and crosssectional views of Li-intercalated γ-graphyne with a x = 3 concentration, respectively. The layer−layer distance is 4.1 Å, and up to three Li atoms are accommodated within a triangle with different heights with respect to the sheet. We find that the
4. SUMMARY We performed total energy electronic structure calculations on Li-intercalated multilayer graphynes to explore their applicability as lithium ion battery anodes using first-principles density functional theory. The calculated Li potential energies are found to be in the energy range of the anodes. The specific Li capacity of multilayer graphynes can reach ∼1000 mAh g−1, much larger than that of graphite (372 mAh g−1). The volumetric capacity is ∼1500 mAh cm−3, which is also much larger than that of graphite (∼800 mAh cm−3). These results suggest that multilayer graphynes have considerable potential as promising high-capacity lithium ion battery anode materials.
Figure 4. (a) Calculated Li binding energy per Li atom of Liintercalated AA-stacking multilayer γ-graphyne as a function of concentration x (C6Lix). (b) Side view of the atomic structure for x = 3. (c) Cross-sectional view of the atomic structure for x = 3. D
dx.doi.org/10.1021/jp3105198 | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
■
Article
(16) Li, G. X.; Li, Y. L.; Liu, H. B.; Guo, Y. B.; Li, Y. J.; Zhu, D. B. Architecture of Graphdiyne Nanoscale Films. Chem. Commun. 2010, 46, 3256−3258. (17) Zhang, H.; Zhao, M.; He, X.; Wang, Z.; Zhang, X.; Liu, X. High Mobility and High Storage Capacity of Lithium in sp−sp2 Hybridized Carbon Network: The Case of Graphyne. J. Phys. Chem. C 2011, 115 (17), 8845−8850. (18) Sun, C.; Searles, D. Lithium Storage on Graphdiyne Predicted by DFT Calculations. J. Phys. Chem. C 2012, 116 (50), 26222−26226. (19) Kohn, W.; Sham, L. J. Self-Consistent Equations Including Exchange and Correlation Effects. Phys. Rev. 1965, 140, A1133− A1138. (20) Kresse, G.; Joubert, D.; Ernzerhof, M. From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method. Phys. Rev. B 1999, 59, 1758−1775. (21) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (22) Monkhorst, H. J.; Pack, J. D. Special Points for Brillonin-Zone Integrations. Phys. Rev. B 1976, 13, 5188−5192. (23) Chan, K. T.; Neaton, J. B.; Cohen, M. L. First-Principles Study of Metal Adatom Adsorption on Graphene. Phys. Rev. B 2008, 77, 235430. (24) Sun, Q.; Wang, Q.; Jena, P.; Kawazoe, Y. Clustering of Ti on a C60 Surface and Its Effect on Hydrogen Storage. J. Am. Chem. Soc. 2005, 127 (42), 14582−14583. (25) Zhou, Z.; Gao, X.; Yan, J.; Song, D.; Morinaga, M. Enhanced Lithium Absorption in Single-Walled Carbon Nanotubes by Boron Doping. J. Phys. Chem. B 2004, 108 (26), 9023−9026. (26) Zhao, J.; Buldum, A.; Han, J.; Lu, J. P. First-Principles Study of Li-Intercalated Carbon Nanotube Ropes. Phys. Rev. Lett. 2000, 85, 1706−1709. (27) Kim, B. G.; Choi, H. J. Graphyne: Hexagonal Network of Carbon with Versatile Dirac Cones. Phys. Rev. B 2012, 86, 115435.
AUTHOR INFORMATION
Corresponding Author
*Phone: +82-2-450-0451. E-mail:
[email protected]. Author Contributions §
These authors contributed equally to this work.
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This work was supported by the WCU Program (Grant R312008-000-10057-0) and by the Basic Science Research Program (Grant KRF-2012R1A1A1013124) through the National Research Foundation of Korea, funded by the Ministry of Education, Science and Technology. N.P. was supported by a grant from a Strategic Research Project (Development of Smart Prestressing System for Prestressed Concrete Bridges) funded by the Korea Institute of Construction Technology. The authors also acknowledge the support from KISTI under the Supercomputing Applications Support Program (Grant KSC2012-C2-52).
■
REFERENCES
(1) Reddy, T. B.; Hossain, S. Rechargeable Lithium Batteries (Ambient Temperature). In Handbook of Batteries, 3rd ed.; Linden, D., Reddy, T. B., Eds.; McGraw-Hill: New York, 1995. (2) Landi, B. J.; Ganter, M. J.; Cress, C. D.; DiLeo, R. A.; Raffaelle, R. P. Carbon Nanotubes for Lithium Ion Batteries. Energy Environ. Sci. 2009, 2, 638−654. (3) Dahn, J. R. Phase Diagram of LixC6. Phys. Rev. B 1991, 44, 9170− 9177. (4) Satoh, A.; Takami, N.; Ohsaki, T. Electrochemical Intercalation of Lithium into Graphitized Carbons. Solid State Ionics 1995, 80, 291− 298. (5) Stura, E.; Nicolini, C. New Nanomaterials for Light Weight Lithium Batteries. Anal. Chim. Acta 2006, 568, 57−64. (6) Chan, C. K.; Zhang, X. F.; Cui, Y. High Capacity Li Ion Battery Anodes Using Ge Nanowires. Nano Lett. 2008, 8, 307−309. (7) Meduri, P.; Pendyala, C.; Kumar, V.; Sumansekera, G. U.; Sunkara, M. K. Hybrid Tin Oxide Nanowires as Stable and High Capacity Anodes for Li-Ion Batteries. Nano Lett. 2009, 9, 612−616. (8) Idota, Y.; Kubota, T.; Matsufuji, A.; Maekawa, Y.; Miyasaka, T. Tin-Based Amorphous Oxide: A High-Capacity Lithium-Ion-Storage Material. Science 1997, 276, 1395−1397. (9) Malko, D.; Neiss, C.; Vines, F.; Gorling, A. Competition for Graphene: Graphynes with Direction-Dependent Dirac Cones. Phys. Rev. Lett. 2012, 108, 086804. (10) Coluci, V. R.; Braga, S. F.; Galvao, D. S.; Baughman, R. H. New Families of Carbon Nanotubes Based on Graphyne Motifs. Nanotechnology 2004, 15, S142−S149. (11) Kang, J.; Li, J.; Wu, F.; Li, S.-S.; Xia, J.-B. Elastic, Electronic, and Optical Properties of Two-Dimensional Graphyne Sheet. J. Phys. Chem. C 2011, 115 (42), 20466−20470. (12) Hwang, H. J.; Kwon, Y.; Lee, H. Thermodynamically Stable Calcium-Decorated Graphyne as a Hydrogen Storage Medium. J. Phys. Chem. C 2012, 116 (38), 20220−20224. (13) Guo, Y.; Jiang, K.; Xu, B.; Xia, Y.; Yin, J.; Liu, Z. Remarkable Hydrogen Storage Capacity in Li-Decorated Graphyne: Theoretical Predication. J. Phys. Chem. C 2012, 116 (26), 13837−13841. (14) Li, C.; Li, J.; Wu, F.; Li, S.-S.; Xia, J.-B.; Wang, L.-W. High Capacity Hydrogen Storage in Ca Decorated Graphyne: A FirstPrinciples Study. J. Phys. Chem. C 2011, 115 (46), 23221−23225. (15) Kehoe, J. M.; Kiley, J. H.; English, J. J.; Johnson, C. A.; Petersen, R. C.; Haley, M. M. Carbon Networks Based on Dehydrobenzoannulenes. 3. Synthesis of Graphyne Substructures. Org. Lett. 2000, 2 (7), 969−972. E
dx.doi.org/10.1021/jp3105198 | J. Phys. Chem. C XXXX, XXX, XXX−XXX
