18848
2007, 111, 18848-18852 Published on Web 12/05/2007
Shaping the Morphology of Gold Nanoparticles by CO Adsorption Keith P. McKenna* and Alexander L. Shluger Department of Physics and Astronomy and London Centre for Nanotechnology, UniVersity College London, Gower Street, London, WC1E 6BT, U.K. ReceiVed: October 16, 2007; In Final Form: NoVember 20, 2007
We demonstrate by density functional theory calculations that the morphology of a Au nanoparticle can be transformed by interactions with a CO atmosphere. Even at low pressure and at room temperature, the population of low-coordinated Au atoms is increased significantly, which affects electronic, optical, and chemical properties. The mechanism for this effect is the strong dependence of the CO adsorption energy on the coordination number of the adjacent Au atom.
The modification of the structure and properties of metallic nanoparticles (NPs) by molecules is an issue relevant to a broad range of applications in areas such as biology,1 nanoelectronics,2 catalysis,3 and gas sensing technologies.4 Molecules, present in an ambient atmosphere, for example, can be responsible for local modifications of atomic structure and overall morphology transformations by their interaction with the NP surface. Such restructuring effects, which are known for some extended surfaces,5,6 are expected to be particularly important for NPs due to their increased surface area and population of lowcoordinated atoms. As the chemical, optical, and electronic properties of metallic NPs depend sensitively upon their size, shape, composition, and atomic-scale structure, this can have important consequences for applications. The modification of the surface plasmon resonances of NPs by the adsorption of molecules has received interest for surface-enhanced spectroscopies7,8 and for applications including nanoscale chemical sensors,9,10 plasmonic waveguides,11 and biological markers.12 For heterogeneous catalysis, atomic-scale structural changes may be connected to their chemical reactivity13 and may also play a role in catalytic deactivation. It has also been demonstrated that the adsorption of molecules on noble-metal NPs can induce ferromagnetism14,15 which is localized near the surface and may be dependent on the NP morphology. It is clear that molecules, whether they play an active or a passive role in NP applications, can have both useful and detrimental effects as a consequence of their influence on NP structure. While molecule-induced structural transformations for NPs are not unexpected, it is only recently that experimental methods have been developed and applied to investigate them. Some recent examples include the application of in situ infrared reflection absorption spectroscopy and ex situ X-ray photoelectron spectroscopy to study Au NPs supported on a TiO2 substrate exposed to CO16 and the use of in situ energy-dispersive extended X-ray absorption fine-structure spectroscopy in combination with vibrational spectroscopy to study supported Pd NPs exposed to CO and NO.17 However, to date, most * To whom correspondence should be addressed. E-mail: k.mckenna@ ucl.ac.uk. Tel: +44 (0)20 7679 9932. Fax: +44 (0)20 7679 1360.
10.1021/jp710043s CCC: $37.00
theoretical models treat these NP systems as rigid objects when interacting with molecules, which, in some cases, may be misleading. This approximation is made because computationally costly first-principles calculations are often necessitated by the complex interplay of electronic and geometric factors. Theoretically understanding trends in molecule-induced structural transformations for NPs is an important and challenging problem that underpins many applications in nanotechnology. We demonstrate that by combining density functional theory (DFT) calculations together with thermodynamic models, one can understand how molecules may influence the atomic structure and properties of NPs. As a useful model system, we consider a Au NP interacting with an ambient CO atmosphere, which has important applications in a wide range of areas. In particular, Au NPs have been found to be active for lowtemperature CO oxidation, which has been the subject of considerable interest.3,13,18-22 Our investigations show that at moderate CO pressure, even at room temperature, molecular adsorption induces a transformation to a more pointed NP morphology. In the process, the concentration of low-coordinated atoms is increased significantly. The origin of this dramatic effect is the dependence of CO adsorption energies on the coordination number of the adjacent Au atom. Similar trends are expected for many noble- and transition-metal systems and different molecules, making the effect quite general. To compare with in situ infrared (IR) spectroscopy studies, we have calculated corresponding IR absorption frequencies. In the limit of zero temperature, thermodynamically favored NP structures correspond to the global minimum of potential energy. Determining these structures is an important problem, and many approaches have been applied to Au NPs, often employing empirical potentials or DFT for small clusters.23 However, at finite temperature24,25 and when interacting with molecules, the atomic configuration that is thermodynamically favored can be quite different. To address these issues, one can compare the free energy of different NP configurations at the equilibrium of adsorption and desorption of CO. Such approaches have been applied to understand the structure of surfaces;26,27 however, we are not aware of any application to NPs. © 2007 American Chemical Society
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J. Phys. Chem. C, Vol. 111, No. 51, 2007 18849
Figure 1. The optimized geometry of Au79 in the lowest-energy configuration G and four other isomers that are higher in energy (labeled C1, C2, E1, and E2). Arrows indicate possible diffusion paths, and the formation energies relative to the configuration G are shown in brackets.
To investigate the interaction of CO with Au NPs, we perform spin-polarized periodic DFT calculations using the projectoraugmented wave (PAW) method28 and the Perdew-Wang functional29 as implemented in the VASP code.30,31 The scalar relativistic PAW potentials represent Au, C, and O with 11, 4, and 6 valence electrons, respectively, and the gamma point is sampled using plane waves with energies up to 400 eV. This method has been applied previously to predict structural trends in Au NPs over a wide range of sizes.32 A cubic cell with an edge length of 21 Å is used, which is sufficiently large that interactions between periodic images are negligible. The geometry of the NPs are optimized to within a force tolerance of 0.01 eV Å-1, and IR frequencies for CO are calculated by evaluation of the dynamical matrix by the finite difference method. A neutral Au NP, 10 Å in diameter and containing 79 atoms, is considered for this study. There is consensus from a number of previous investigations, using different empirical potentials, that this particle has a compact truncated octahedron structure exposing (100) and (111) facets32-34 (labeled G in Figure 1). The NP has 60 atoms on its exterior surface (accounting for 76% of the total number of atoms) and possesses a wide variety of surface sites, from closely packed facets to low-coordinated corner atoms. Its truncated nature means that it is relatively stable; therefore, if CO has a significant effect, it is indicative of more general trends. To identify the thermodynamically favored structures in the presence of CO, four alternative NP configurations are also considered (labeled C1, C2, E1, and E2 in Figure 1). Forming these configurations may involve the diffusion of lowcoordinated atoms at the surface, for example, edge (E) or corner (C) atoms, as indicated in the figure. The time scale for dynamical changes in the morphology of NPs depends upon the temperature and the magnitude of barriers to relevant atomic diffusion processes. To give an estimation of these time scales, the barrier for the transformation, G f C1, has been calculated using the nudged elastic band method.35 We assume an that an exchange process takes place where the 6C atom (NC is shorthand for N-coordinated) occupies the site of a 7C atom,
while the 7C atom is displaced into the 3C atom position. The calculated barrier is therefore an upper estimate as more complex cooperative processes may be important in reality.36 Nevertheless, the barrier of 0.67 eV corresponds to a rate on the order of 103 Hz at room temperature (assuming a prefactor of 1013 Hz). In the presence of CO, diffusion barriers may be smaller;6 therefore, dynamical changes in the morphology of a NP can take place at room temperature over the time scale of seconds. The formation energy for a given configuration, defined as the total energy relative to configuration G, is also shown in Figure 1. To quantify the energetics of interactions between CO and Au NPs, adsorption energies have been calculated for various possible adsorption sites. The CO molecule adsorbs with the C atom closest to the Au atom (see ref 37), and the change in NP geometry upon adsorption is very localized. Figure 2a shows the dependence of the adsorption energy on the first-neighbor Au-Au coordination number, Z. The adsorption energies for sites with the same Z but on different clusters, for example, Z ) 3 in configurations C1 and E1, differ by less than 20 meV (the size of the points in Figure 2a); therefore, an average is shown. We note that the adsorption energy for 9C atoms (0.35 eV) is very similar to that calculated for an ideal Au(111) surface using a similar method38 (0.32 eV). To understand the trend of increasing adsorption energy with decreasing Z, we have analyzed the electronic structure in some detail. Chemisorption of CO on Au involves hybridization of the unoccupied 2π* orbital of the CO molecule with the d states of the NP.39 Our calculations show that d states associated with atoms of low coordination are closer to the Fermi energy than those in the bulk of the NP. This leads to an enhanced interaction upon decreasing coordination and can involve partial charge transfer from the Au NP to the CO molecule, as has been observed for small Au clusters.40 The adsorption energies for 3C and 4C sites are quite similar, which can perhaps be rationalized as both sites corresponding to adatoms, on (111) and (100) facets, respectively. We have calculated vibrational frequencies for adsorbed CO to compare with IR spectroscopy studies. Figure 2c shows that
18850 J. Phys. Chem. C, Vol. 111, No. 51, 2007
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Figure 2. (a) The dependence of the CO adsorption energy on the first-neighbor Au-Au coordination number Z. (b) Corresponding CO adsorption geometries. (c) The Au-C IR frequency as a function of Z. (d) The Au-C equilibrium separation as a function of Z. Dashed lines are a guide to the eye.
the Au-C mode is characterized by two frequencies associated with Au sites of different coordination. CO adsorbed on lowcoordinated Au atoms (3-6C) contributes to an IR band at around 370 cm-1, and higher-coordinated sites correspond to a band about 80 cm-1 lower in frequency. This change in frequency correlates with a decrease in the Au-C bond length from 2.04 to 1.95 Å (Figure 2d), and this is attributed to a transition from physisorption to chemisorption. The adsorption energy dependence (Figure 2a) does not show such a sharp transition. This is, in part, because it includes a contribution from relaxation of the NP geometry, which can vary considerably in magnitude between different sites. CO stretching modes also group into two bands depending upon Au coordination: 7-9C sites corresponding to 2170 cm-1; however, lowcoordinated sites span a wider range, 2060-2120 cm-1. These calculated frequencies may contain systematic errors due to the approximations of exchange and correlation used in the density functional; however, relative frequencies should be indicative. The ratio of the intensity of these separate bands provides a useful measure of the proportion of low-coordinated surface sites. At the equilibrium of adsorption and desorption of CO, there is an average number of molecules, NCO, adsorbed on the NP. In equilibrium, the chemical potential of the surrounding CO gas and the adsorbed CO are equal and can be determined by statistical mechanics.41 We define a separate chemical potential for CO adsorbed on Au atoms of different coordination and obtain the following expression for NCO as a function of CO pressure PCO and temperature T
NCO )
∑
Z)3,9
(
NZAu 1 +
kBT2 Λ3θrotPCO
( ))
EZ exp kBT
-1
where NZAu is the number of atoms on the NP with coordination Z, Λ is the thermal de Broglie wavelength, θrot ) 4.3 K for CO, and EZ is the CO adsorption energy on a Au atom with
coordination Z. We assume, as a first approximation, that the adsorption energies do not depend on NCO; however in general, one would expect an effect due to intermolecular interactions. Figure 3 compares the equilibrium number of adsorbed CO molecules on different NP configurations at 300 K. For PCO ∼ 10-8 bar, 3C and 4C sites are, on the average, occupied by CO. These sites are not present in configuration G, and this difference is highlighted in the inset. Edge and corner atom (7C and 6C) sites become occupied at 10-5 bar, while 9C atoms are not occupied until beyond 102 bar. The NP morphology that is thermodynamically favored is the one with the lowest Gibb’s free energy, which can be expressed in the following way
G S ) ES -
EZNZCO - NCOµCO (PCO, T) ∑ Z)3,9
(1)
where S refers to the NP configuration (e.g., S ) C1) and ES is its total energy, NZCO is the number of CO molecules adsorbed on Au atoms with coordination Z, and µCO is the chemical potential of the CO gas. In vacuum and at zero temperature, configuration G is favored by at least 0.26 eV over the other configurations considered. However, at room temperature and for pressures exceeding 1.7 µbar, configurations G and C2 have comparable free energies. The reason for this is that the energetic price to form configuration C2 is paid for by the energy gained by adsorption of CO on the 4C site. Therefore, one can expect, given complete freedom, that transformation to a NP morphology with more than one 4C atom will be more favorable. To test this possibility, we constructed a “pointed” NP possessing four 4C atoms (configuration P). This was achieved at the expense of increasing the area of a single (100) facet (see P in Figure 4). The free energy for configuration P becomes lower than that of all other configurations for PCO > 0.2 µbar. This corresponds to an equilibrium coverage of only 4 CO molecules (Figure 3); therefore, the approximation of additive adsorption energies is likely to be quite reasonable. The transformation
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J. Phys. Chem. C, Vol. 111, No. 51, 2007 18851
Figure 3. The average number of CO molecules adsorbed on different NP configurations at 300 K. The inset shows the region of pressure where CO adsorbs on 3C and 4C sites. Configuration P (Figure 4) is proposed as the equilibrium morphology at elevated CO pressure and is described in the text.
Figure 4. A phase diagram indicating the thermodynamically favored NP morphology for Au79 in a CO atmosphere. The shaded atoms are four-coordinated.
G f P significantly increases the population of 4C surface atoms, from 0 to about 7%. Using eq 1, the CO pressure required to induce the transformation can be calculated as a function of temperature. In Figure 4, the line divides the regimes of temperature and pressure in which configurations G or P are thermodynamically favored. Importantly, different electronic, optical, and chemical properties will be associated with each of these NP morphologies. The CO pressures quoted above are numerically sensitive to small changes in CO adsorption energies and, therefore, should be taken as semiquantitative. It is also known that DFT tends to overestimate the adsorption energy of molecules on many transition-metal and noble-metal surfaces due to underestimation of the singlet-triplet splitting of molecule.38,42,43 However, differences in free energy depend upon differences in adsorption
energies on different Au sites; therefore, the observed structural trends are more reliable. Our results predict the most thermodynamically favored NP morphology. We note that, in general, one should be concerned with the dissipation of energy due to molecular adsorption that may lead to nonequilibrium processes; nevertheless, the approach used here should be qualitatively correct and useful to investigate structural trends. Two very recent experimental studies provide some indirect evidence to support our results.16,17 The conclusions of both studies are that upon exposure to CO, the particles flatten to increase their surface area and, in the process, increase the number of low-coordinated atoms. Diemant et al. note the appearance of an additional IR band that is red-shifted by about 50 cm-1 at mbar CO pressure. The origin of this band cannot be attributed on the basis of IR vibrational frequency alone; however, it is in agreement with our predictions for CO adsorbed on low-coordinated Au atoms. This agreement should be taken cautiously as our model does not consider the role of the TiO2 support. However, studies on two quite different systems illustrate the generality of these effects, but they also indicate that these are difficult issues to investigate and interpret experimentally. To fully understand the nature of NP structural transformations, further experiments are needed, utilizing scanning tunneling microscopy and temperature-programmed desorption, for example, in conjunction with first-principles theoretical models of the type presented in this letter. To summarize, we have investigated how the interaction of a Au NP with a CO atmosphere can affect its structure and electronic properties using DFT methods. We find that a transformation of morphology occurs at moderate CO pressure and room temperature, which dramatically increases the proportion of low-coordinated Au atoms. A truncated Au NP corresponding to a geometric magic number was selected for this study, but in situ spectroscopy studies suggests that this result is not specific to this particular system. It is known that lowcoordinated atoms play an important role in heterogeneous catalysis,18-22 and their increased concentrations may contribute to reactivity and catalytic deactivation. Changes in morphology
18852 J. Phys. Chem. C, Vol. 111, No. 51, 2007 will also affect their charge trapping and optical properties, which is important for many applications in nanotechnology. The results demonstrate that the structural rigidity assumed in many models of molecular adsorption on metallic nanoparticles can be invalid when surface reorganization energies are comparable to molecular adsorption energies. Acknowledgment. K.P.M. is supported by EPSRC Grant GR/S80080/01. Computer time on HPCx was provided through EPSRC Grant EP/D504872/1. References and Notes (1) Niemeyer, C. M. Angew. Chem., Int. Ed. 2001, 40, 4182. (2) Hipps, K. W. Science 2001, 294, 536. (3) Meyer, R.; Lemire, C.; Shaikhutdinov, S. K.; Freund, H.-J. Gold Bull. 2004, 37, 72. (4) Elghanian, R.; Mucic, J. J.; Letsinger, R. C.; Mirkin, C. A. Science 1997, 277, 1078. (5) Chung, J. W.; Ying, S. C.; Estrup, P. J. Phys. ReV. Lett. 1986, 56, 749. (6) Loffreda, D.; Piccolo, L.; Sautet, P. Phys. ReV. B 2005, 71, 113414. (7) Haes, A. J.; Zou, S.; Zhao, J.; Schatz, G. C.; Van Duyne, R. P. J. Am. Chem. Soc. 2006, 128, 10905. (8) Laurent, G.; Felidj, N.; Grand, J.; Aubard, J.; Levi, G.; Hohenau, A.; Aussenegg, F. R.; Krenn, J. R. Phys. ReV. B 2006, 73, 245417. (9) Riboh, J. C.; Heas, A. J.; McFarland, A. D.; Youson, C. R.; Duyne, R. P. V. J. Phys. Chem. B 2003, 107, 1772. (10) McFarland, A. D.; Van Duyne, R. P. Nano Lett. 2003, 3, 1057. (11) Maier, S. A.; Atwater, H. A. J. Appl. Phys. 2005, 98, 011101. (12) Rosi, N. L.; Mirkin, C. A. Chem. ReV. 2005, 105, 1547. (13) Sanchez, A.; Abbet, S.; Heiz, U.; Schneider, W.-D.; Hakkinen, H.; Barnett, R. N.; Landman, U. J. Phys. Chem. A 1999, 103, 9573. (14) Hori, H.; Teranishi, T.; Nakae, Y.; Seino, Y.; Miyake, M.; Yamada, S. Phys. Lett. A 1999, 263, 406. (15) Yamamoto, Y.; Miura, T.; Suzuki, M.; Kawamura, N.; Miyagawa, H.; Kobayashi, K.; Teranishi, T.; Hori, H. Phys. ReV. Lett. 2004, 93, 116801. (16) Diemant, T.; Zhao, Z.; Rauscher, H.; Bansmann, J.; Behm, R. J. Top. Catal. 2007, 44, 83. (17) Newton, M. A.; Belver-Coldeira, C.; Martı´nez-Arias, A.; Ferna´ndezGarcı´a, M. Nat. Mater. 2007, 6, 528.
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