Article pubs.acs.org/JPCC
Graphene Oxide: An Ideal Support for Gold Nanocatalysts Ming Yang,† Miao Zhou,† Aihua Zhang,† and Chun Zhang*,†,‡ †
Department of Physics and ‡Department of Chemistry, National University of Singapore, 2 Science Drive 3, Singapore, 117542 ABSTRACT: Via ab initio calculations, we predict that compared with pristine graphene, graphene oxide (GO) is a much better candidate for support of metal (in particular, gold) nanocatalysts in terms of activity and feasibility. Using Au8 clusters supported on GO as model catalysts, we show that the reaction barrier of the catalyzed CO oxidation can be greatly reduced from around 3.0 to 0.2 eV without the need of defects or strain in the underlying support. The origin of the substantially enhanced activity of Au catalysts is the charge transfer between the GO and supported Au clusters, resulting in activated positively charged Au8 clusters. Both Langmuir−Hinshelwood and Eley−Rideal mechanisms of the reaction are investigated. We expect our theoretical predictions to stimulate new experiments in the application of GO, one of the most important chemical derivative of graphene, in nanocatalysis.
C
easy to be chemically modified. As a result, GO is believed to be a good candidate for various applications such as sensors, fieldeffect transistor (FET), and biomedical applications through chemical functionalizaitons.22 Recently, GO has also been considered for applications of catalysis. It was found in an experiment that the composition of GO and oxide nanoparticles such as TiO2 may show improved photoelectronic and photocatalytic performance.23 Another experimental work demonstrated that Co2O3 nanoparticles can be adsorbed on GO sheet with high stability and catalytic activity.24 These results triggered our interest in studying effects of GO substrate on gold nanocatalysis, which has not been discussed before. Here we report first-principle calculations on catalytic properties of Au clusters adsorbed on GO. The goal of the research is to show whether GO is a better substrate than pristine graphene for gold nanocatalysts. All calculations are based on density functional theory (DFT) by using Vienna ab initio simulation package (VASP).25,26 The frozen-core all-electron projector-augmented wave (PAW) method27 with the generalized gradient approximation (GGA) in PW91 format28 for the exchangecorrelational functional was used in all calculations. A planewave basis set with the kinetic energy cutoff of 400 eV and 5 × 5 × 1 k sampling was employed. In GO plane, the supercell was set to be (3√3 × 5) primitive graphene unit cells with 60 C and 12 O atoms, and in the direction perpendicular to the GO plane, a vacuum region of larger than 20 Å was adopted in the supercell to minimize the interaction between different slabs. In the current study, we chose the reaction of CO oxidation as the probe for catalytic activity of gold clusters supported on GO substrate because this reaction is important in chemical engineering and has been very intensively studied in nanocatalysis. The reaction barrier was calculated by constrained
atalytic activity of gold nanoclusters has attracted tremendous interest in the last several decades because of their high catalytic activity and selectivity for various chemical reactions.1−6 As for the support-induced heterogeneous gold catalysis, it has become a common knowledge that the catalytic properties of gold catalysts (clusters) are closely related to the interaction between the clusters and the underlying substrate.7−11 Metal oxides such as MgO and TiO2 are often used substrates for Au nanocatalysts.12,13 Effects of inert supports such as h-BN on reactivity of Au clusters have been discussed also.14 More recently, graphene, a single atomic layer of graphite that exhibits many exceptional electronic, mechanical, and chemical properties,15,16 was found to be a promising support for Pt nanocatalyst,17 which triggered much interest in graphene-supported metal nanocatalysis. However, the situation for graphene-supported gold catalysis is not encouraging. A number of theoretical and experimental studies have shown that because graphene itself is chemically inert the interaction between the supported gold clusters and graphene is quite weak. Consequently, the gold clusters are highly mobile on graphene,18,19 which is not desired for real applications, and also, the catalytic activity of gold clusters on graphene is not expected to be as high as that of metal oxide supported ones. In very recent studies, it was proposed that carbon-vacancy defects or mechanical strain in graphene can significantly increase the chemical reactivity of graphene and in turn greatly enhance the stability and activity of graphene-supported gold clusters.20,21 In real industrial applications, controlling the defects or strain in graphene is still a challenging issue. In current work, via high level ab initio calculations, we suggest that graphene oxide (GO) is an ideal support for gold nanocatalysts that can be used to overcome all aforementioned limitations of graphene substrate. We expect our theoretical predictions to stimulate new experiments along this direction and pave the way for future applications of graphene-supported gold nanocatalysis. GO is probably the most studied chemical derivative of graphene. GO contains reactive oxygen atoms, which make it © 2012 American Chemical Society
Received: May 31, 2012 Revised: September 28, 2012 Published: September 28, 2012 22336
dx.doi.org/10.1021/jp3053218 | J. Phys. Chem. C 2012, 116, 22336−22340
The Journal of Physical Chemistry C
Article
energy minimization method that is incorporated into VASP.29 In all calculations, the structures were fully optimized with a force convergence criteria of 0.01 eV/Å. In literature, there are quite a few different structure models for GO.22,30−32 A common fact of these models is that GO contains epoxy (mainly bridge site oxygen) or hydroxyl (−OH) groups adsorbed on graphene basal plane, of which the component, structure, and O/C ratio depend much on different oxidation processes.22 Recently, first-principle studies predicted that the GO structure with partial oxidation is thermodynamically favorable, and structures with pure oxygen and C are energetically favored under high oxygen chemical potential.30 Therefore, in our calculations, we chose the GO structure model with oxygen atoms bonded to graphene at bridge sites (the most stable adsorption sites), as shown in Figure 1a, Figure 2. (a) Three isomers of Au8 clusters in gas phase: Two planar structures P1, P2, and a 3D configuration. The most stable adsorption configurations of P1 (b), P2 (c), and 3D Au8 (d) on GO substrate. In panel d, the isosurface of electron density redistribution is also superimposed with the orange (blue) color denoting depletion (accumulation) of electrons. The electron density redistribution is defined by δρ = ρGO+Au8 − (ρGO+ρAu8). (e) Density of states (DOS) projected on the “bonded” O and Au atoms for the 3D Au8 adsorbed on GO. For comparisons, DOS of these two atoms before the adsorption are also shown. The Fermi Level is shifted to 0 eV.
energetically favored than the 3D one by 0.4 eV,20 whereas when adsorbed on GO, our calculations show that the 3D isomer is more stable than two planar ones by more than 0.4 eV, and also in this case, the 3D cluster strongly binds to the substrate with the adsorption energy of 1.7 eV. Here the adsorption energy was calculated by Ea = EAu8 + EGO − EAu8+GO, where EAu8+GO is the energy of the combined system and EAu8 and EGO are energies of the isolated gold cluster and GO substrate, respectively. The most stable configurations of planar (P1 and P2) and 3D Au8 clusters adsorbed on GO substrate are shown in Figure 2b−d, respectively. Because of the strong interaction between GO and the adsorbed 3D Au8 cluster, the Au atom bonds to the nearest O atoms at the interface with a short bond length of 2.02 Å, resulting in a tilted 3D Au8 cluster on GO, as shown in Figure 2d. In the Figure, the electron density redistribution after the adsorption of the Au8 cluster on GO substrate is also shown, from which we can see that there are “excess” electrons accumulated around “bonded” O atoms, and the “depletion” of electrons mainly occurs around “bonded” Au atoms. The Bader charge analysis35 confirmed that there are ∼0.7 electrons transferred from the Au cluster to GO substrate in this case. The partial density of states (PDOS) projected on bonded O and Au atoms (Figure 2e) shows the change of electronic structures of these atoms before and after the adsorption. The significant electron transfer from Au 5d orbital to O atoms can be clearly seen from the PDOS. Because of the charge transfer at the interface of Au8 and GO, the Au8 cluster becomes positively charged. This is also confirmed by its projected DOS. In Figure 3a, the partial density of states of Au8 (blue solid line) on GO is plotted in comparison with that of the free-standing Au8 cluster (red dash line). It can be clearly seen from this Figure that the electron transfer from Au8 to GO mainly happens near Fermi energy (the energy range inside the green box in the Figure). We then examine the chemical reactivity of this positively charged Au8
Figure 1. (a) Atomic structure of GO. (b) Variation of temperature as a function of MD time step. (c) Time-dependent averaged variation of the bond length between the O and the nearest C atom, where the variation is defined as the difference between C−O bond length and its equilibrium value. C, O, and Au atoms are in gray, red, and yellow, respectively.
making the system hybrid of sp2 and sp3 bonding. This structure is similar to the model proposed by Boukhvalov et al.,31 but with a lower O/C ratio of 1:5. The low O/C ratio (1:5) was recently verified in GO films by electron energy loss spectroscopy.32 As shown in Figure 1a, in the relaxed structure, the oxygen atoms deviated from the center of C−C bond slightly, forming inequivalent C−O bonds with the adjacent C atoms. The thermal stability of this GO structure was examined by ab initio molecular dynamics (MD) calculations. Results of MD simulations are shown in Figure 1b,c. The temperature fluctuation becomes stable after 50 time steps (in our calculations, one time step was set to 1 fs) and oscillates slightly around room temperature (Figure 1b). From Figure 1c, it can be seen that the maximum fluctuation of C−O bond length (deviation from the equilibrium value) at the temperature 300 K is 1.5 eV. Finally, we showed that the reaction barrier of the CO oxidation catalyzed by Au8 cluster supported on GO substrate is lower than 0.25 eV for both LH and ER mechanisms, clearly demonstrating the high catalytic activity of the system. All of these results make us believe that the GO is an ideal support for Au nanocatalysis.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected].
Figure 5. LH mechanism of CO oxidation catalyzed by Au8/GO. (a) Initial state of the reaction. In this case, the O2 and CO molecules are coadsorbed on Au8. The O−O bond length of the O2 molecule, d(O− O), is 1.38 Å, and the C−O bond length of CO molecule, d(C−O), is 1.16 Å. The distance between these two molecules as defined in Figure 3, d(C−O2), is ∼3.0 Å. The isosurface of electron density redistribution due to the coadsorption of O2 and CO is superimposed (the color scheme is the same as before), where we can see that electrons are transferred from Au8 to 2π* orbital of both O2 and CO molecules. The electron density redistribution here is defined as δρ = ρGO+Au8+O2+CO − (ρGO + ρAu8 + ρO2 + ρCO). (b) Transition state of the reaction: d(O−O) = 1.61, d(C−O2) = 1.7, and d(C−O) = 1.17 Å. (c) Final state with the formation of CO2. (d) Energy profile along the reaction path. S1, S2, and S3 are initial, transition, and final states, respectively. The reaction barrier is estimated to be 0.2 eV.
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This work was supported by NUS Academic Research Fund (grant nos.: R-144-000-237-133 and R-144-000-298-112). Computations were performed at the Center for Computational Science and Engineering at NUS.
■
REFERENCES
(1) Haruta, M.; Kobayashi, T.; Sano, H.; Yamada, N. Chem. Lett. 1987, 2, 405−408. (2) Hammer, B.; Norskov, J. K. Nature 1995, 376, 238−240. (3) Valden, M; Lai, X.; Goodman, D. W. Science 1998, 281, 1647− 1650. (4) Enache, D. I.; Edwards, J. K.; Landon, P.; Solsona-Espriu, B.; Carley, A. F.; Herzing, A. A.; Watanabe, M.; Kiely, C. J.; Knight, D. W.; Hutchings, G. J. Science 2006, 311, 362−365. (5) Edwards, J. K.; Solsona, B. N; Carley, A. F.; Herzing, A. A.; Kiely, C. J.; Hutchings, G. J. Science 2009, 323, 1037−1041. (6) Zope, B. N.; Hibbitts, D. D.; Neurock, M.; Davis, R. J. Science 2010, 330, 74−78. (7) Zhang, C.; Yoon, B.; Landman, U. J. Am. Chem. Soc. 2007, 129, 2229−2230. (8) Häkkinen, H.; Landman, U. J. Am. Chem. Soc. 2001, 123, 9704− 9705. (9) Lopez, N.; Nrskov, J. K. J. Am. Chem. Soc. 2002, 124, 11262− 11263. (10) Socaciu, L. D.; Hagen, J.; Bernhardt, T. M.; Wste, L.; Heiz, U.; Häkkinen, H.; Landman, U. J. Am. Chem. Soc. 2003, 125, 10437− 10445. (11) Guzman, J.; Gates, B. C. J. Am. Chem. Soc. 2004, 126, 2672− 2673. (12) Heiz, U.; Landman, U. Nanocatalysis; Springer: New York, 2006. (13) Wahlström, E.; Lopez, N.; Schaub, R.; Thostrup, P.; Rønnau, A.; Arich, C.; Lægsgaard, E.; Nørskov, J. K.; Besenbacher, F. Phys. Rev. Lett. 2003, 90, 026101−026104. (14) Gao, M.; Lyalin, A.; Taketsugu, T. J. Phys. Chem. C 2012, 116, 9054−9062. (15) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Filrsov, A. A. Science 2004, 306, 666−669. (16) Neto, A. H. C.; Guinea, F.; Peres, N. M. R.; Novoselov, K. S.; Geim, A. K. Rev. Mod. Phys. 2009, 1, 109−162. (17) Yoo, E.; Okata, T.; Akita, T.; Kohyama, M.; Nakamura, J.; Honma, I. Nano Lett. 2009, 9, 2255−2259. (18) Gan, Y. J.; Sun, L. T.; Banhart, F. Small 2008, 4, 587−591.
molecule. The energy profile along the reaction path is shown in Figure 5d, where S1, S2, and S3 denote the initial, transition, and the final stale, respectively. In this case, the reaction barrier is calculated to be 0.20 eV. The atomic configurations of the transition and the final state are given in Figure 5b,c, from which we can see that the transition state occurs when the distance between the C atom and the nearest O atom of O2 is 1.7 Å. We then calculated the second step of the reaction and confirmed that for both mechanisms the reaction barriers of the second step are very low (∼0.16 eV), as expected. Some other reaction mechanisms of oxidizing CO discussed in literature might even be more energetically favorable,10,36 whereas our results have been enough to demonstrate the high activity of Au8 adsorbed on GO. It is worth mentioning here that although the GO structure with single C−O bond is much more energetically favorable than that with double CO bond, we also performed calculations for O2 adsorption on Au8 clusters supported on GO with CO double bonds and found that in this case the O2 molecule also strongly binds to Au8 with an adsorption energy around 1.54 eV and an elongated O−O bond length of 1.37 Å, suggesting the high reactivity of Au8. In summary, via first-principle calculations, we investigated effects of GO substrate on support-induced catalytic properties of Au8 clusters. The chemical reaction of CO oxidation was chosen as a probe for the catalytic activity of the supported Au clusters. These results were compared with the case of pristine graphene support. For pristine graphene, our calculations agree with previous studies that the planar isomers of Au8 clusters are more stable than the 3D one. However, when supported on GO substrate, the 3D one is the lowest energy configuration. 22339
dx.doi.org/10.1021/jp3053218 | J. Phys. Chem. C 2012, 116, 22336−22340
The Journal of Physical Chemistry C
Article
(19) Ponomarenko, L. A.; Yang, R.; Mohiuddin, T. M.; Katsnelson, M. I.; Novoselov, K. S.; Morozov, S. V.; Zhukov, A. A.; Schedin, F.; Hill, E. W.; Geim, A. K. Phys. Rev. Lett. 2009, 102, 206603−206606. (20) Zhou, M.; Zhang, A.; Dai, Z.; Zhang, C.; Feng, Y. J. Chem. Phys. 2010, 132, 194704−194706. (21) Zhou, M.; Zhang, A.; Dai, Z.; Feng, Y.; Zhang, C. J. Phys. Chem. C 2011, 114, 16541−16546. (22) Dreyer, D. R.; Park, S.; Bielawski, C. W.; Ruoff, R. S. Chem. Soc. Rev. 2010, 39, 228−240. (23) Zhang, N.; Qiu, H.; Wang, W.; Li, Y.; Wang, X. D.; Gao, J. P. J. Mater. Chem. 2011, 21, 11080−11083. (24) Liang, Y. Y.; Li, Y. G.; Wang, H. L.; Zhou, J. G.; Wang, J.; Regier, T.; Dai, H. J. Nat. Mater. 2011, 10, 780−786. (25) Kresse, G.; Hafner, H. Phys. Rev. B 1993, 47, 558−561. (26) Kresse, G.; Hafner, H. Phys. Rev. B 1994, 48, 13115−13118. (27) Blöchl, P. E. Phys. Rev. B 1994, 50, 17953−17979. (28) Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M. A.; Singh, D. J.; Fiolhais, C. Phys. Rev. B 1992, 46, 6671−6681. (29) Reaction barriers were calculated via constrained total energy minimization, in which all degrees of freedom except the reaction coordinate were allowed to be relaxed. The reaction coordinate denotes the distance between C atom of CO and the nearest O atom of the O2 molecular. (30) Wang, L.; Sun, Y. Y.; Lee, K.; West, D.; Chen, Z. F.; Zhao, J. J.; Zhang, S. B. Phys. Rev. B 2010, 82, 161406−161409(R). (31) Boukhvalov, D. W.; Katsnelson, M. I. J. Am. Chem. Soc. 2008, 130, 10697−10701. (32) Mkhoyan, K. A.; Contryman, A. W.; Silcox, J.; Stewart, D. A.; Eda, G.; Mattevi, C.; Miller, S.; Chhowalla, M. Nano Lett. 2009, 9, 1058−1063. (33) Landman, U.; Yoon, B.; Zhang, C.; Heiz, U.; Arenz, M. Top. Catal. 2007, 44, 145−158. (34) Han, Y. K. J. Chem. Phys. 2006, 124, 024316−024318. (35) Henkelman, G.; Arnaldsson, A.; Jónsson, H. Comput. Mater. Sci. 2006, 36, 354−360. (36) Wang, F.; Zhang, D. J.; Xu, D. J.; Ding, Y. J. Phys. Chem. C 2009, 113, 18032−18039.
22340
dx.doi.org/10.1021/jp3053218 | J. Phys. Chem. C 2012, 116, 22336−22340
