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Role of ligand type on the geometric and electronic properties of Ag–Au bimetallic clusters
Computational and Theoretical Chemistry 1045 (2014) 35–40
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Computational and Theoretical Chemistry journal homepage: www.elsevier.com/locate/comptc
Role of ligand type on the geometric and electronic properties of Ag–Au bimetallic clusters Le Chang, Haoxiang Xu, Daojian Cheng ⇑ State Key Laboratory of Organic–Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China
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Article history: Received 31 March 2014 Received in revised form 29 June 2014 Accepted 29 June 2014 Available online 8 July 2014 Keywords: Ligand Ag–Au bimetallic clusters Geometric and electronic properties DFT calculations
a b s t r a c t The interactions of a set of typical ligands (–CN, –COOH, –CH3, –OH, –SH, –NH3, –NO, –NO2, N(CH3)2, and –PH3) with 13-atom icosahedral Ag–Au bimetallic clusters have been investigated by density functional theory (DFT) calculations. For the adsorption of all the ligands on the Ag13 cluster, the adsorption strength follows the order of Ag13–CN > Ag13–SH > Ag13–OH > Ag13–COOH > Ag13–CH3 > Ag13–NO2 > Ag13–N(CH3)2 > Ag13–NO > Ag13–PH3 > Ag13–NH3. Considering the composition effect, the adsorption strength of the ligands (X = –CN, –COOH, –CH3, and –NO2) on these clusters follows the order of Ag12Au–X > Ag13–X > Au12Ag–X > Au13–X. It is also found that the adsorption of the ligands (–NO, –N(CH3)2, and –PH3) on the Au-rich clusters can modify their geometric properties, and even the icosahedral structure of Au-rich clusters has been transformed into the more stable truncated octahedral one upon adsorption. Our results show that the geometric and electronic properties of Ag–Au bimetallic clusters can be tuned by the ligand type, which can provide useful insights for the preparation of ligand-stabilized Ag–Au bimetallic clusters by chemical reduction. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Metal clusters have been considered to be the building blocks for nanotechnology. The unique physical and chemical properties of metal clusters strongly depend on their sizes and shapes, which differ from the corresponding bulk materials. Bimetallic clusters (also called ‘‘nanoalloys’’), composed of two metallic elements, often exhibit better physical and chemical properties than their monometallic counterparts [1–3]. In addition, nanoalloys also offer new degrees of freedom for understanding their electronic and geometric properties of clusters, since their properties can be tuned by not only the size and shape, but also the composition and atomic ordering [4,5]. In recent years, Ag–Au bimetallic clusters have attracted a lot of attention for their potential applications in nanoelectronics [6], optics [7–9], catalysis [10–12], sensing [13], and biomedicine [14,15]. In general, metal clusters are prepared by the chemical reduction method [16–19], in which various capping ligands are used to control their size [20,21] and also avoid irreversible aggregation [22]. However, a lot of studies have revealed that the capping ligands can significantly affect their physicochemical properties [23]. More importantly, the type of the ligand can greatly modify ⇑ Corresponding author. Fax: +86 10 64427616. E-mail address: [email protected] (D. Cheng). http://dx.doi.org/10.1016/j.comptc.2014.06.023 2210-271X/Ó 2014 Elsevier B.V. All rights reserved.
the properties of metal clusters. For example, Shibu et al. [24] showed that the capping ligands can significantly modify the UV–vis spectrum of Au25 clusters. Udayabhaskararao et al. [25] found that the average luminescence lifetime of Ag–Au nanoalloys depends on the type of the protecting ligands. Thus, it is of great interest to comprehensively explore the effect of ligands on the physical and chemical properties of Ag–Au bimetallic clusters. In current experimental research, the complex structure of the ligand-coated metal clusters makes understanding their properties an extremely difficult task. On the other hand, theoretical methods such as density functional theory (DFT) can provide reasonable accuracy to infer the leading physical trends and to enable comparison of computational results with the experimental data. Based on DFT calculations, Malola and Häkkinen [26] found that the thiolate ligands can improve the stability of the Ag–Au bimetallic clusters. Kauffman et al. [27] found that the SC2H4Ph ligand can affect the composition of Ag–Au bimetallic clusters by DFT calculations. However, little theoretical work has been devoted to the effect of the ligand type on the electronic and geometric properties of Ag– Au bimetallic clusters. In this work, the electronic and geometric properties of 13-atom icosahedral Ag–Au bimetallic clusters upon the adsorption of a set of typical ligands (–CN, –COOH, –CH3, –OH, –SH, –NH3, –NO, –NO2, –N(CH3)2, and –PH3) are investigated by DFT calculations. The adsorption strength of the ligands on Ag–Au bimetallic clusters
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depending on the ligand type is investigated. The role of composition and geometric relaxation upon the adsorption of the ligands on Ag–Au bimetallic clusters is also studied. The paper is structured as follows. In the next section, we present the computational methodology. Section 3 describes our results and discussion, and Section 4 offers our conclusions.
Table 1 The adsorption energies (Eads in eV), distortion energies (Edis in eV), and average bond length of five Ag–Ag, Ag–Au or Au–Au bonds where ligand were adsorbed (Dcluster in Å) of the ligands (X = –CN, –SH, –OH, –COOH, –CH3, –NO2, –N(CH3)2, –NO, –PH3, and –NH3) on the Ag13, Ag12Au, Au12Ag, and Au13 clusters at the top site. S.T. corresponds to the structural transformation. Eads
Fig. 1. Snapshots (yellow atom, Au; white atom, Ag) of 13-atom Ag–Au bimetallic clusters after the local relaxation at the DFT level. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Dcluster
Ag13 Ag12Au Au12Ag Au13
3.51 3.72 2.18 2.66
0.01 0.00 1.11 1.47
2.812 2.919 2.975 2.959
–COOH
Ag13 Ag12Au Au12Ag Au13
1.92 2.26 0.69 1.30
0.01 0.06 1.11 1.48
2.936 2.937 2.919 2.977
–CH3
Ag13 Ag12Au Au12Ag Au13
1.74 2.17 0.58 1.27
0.01 0.05 1.11 1.48
2.945 2.941 2.929 2.983
–NO2
Ag13 Ag12Au Au12Ag Au13
1.48 1.46 0.16 0.36
0.01 0.00 1.10 1.46
2.920 2.906 2.958 2.953
–OH
Ag13 Ag12Au Au12Ag Au13
2.40 2.41 1.16 2.23
0.02 0.05 1.08 0.53
2.921 2.928 2.956 S.T.
–SH
Ag13 Ag12Au Au12Ag Au13
2.42 3.82 4.20 3.77
0.01 0.65 0.73 0.77
2.930 S.T. S.T. S.T.
–N(CH3)2
Ag13 Ag12Au Au12Ag Au13
1.15 1.30 1.15 1.38
0.04 0.06 0.02 0.47
3.404 2.928 S.T. S.T.
–NO
Ag13 Ag12Au Au12Ag Au13
-0.91 1.05 0.77 1.20
0.09 0.01 0.02 0.46
2.923 2.952 S.T. S.T.
–PH3
Ag13 Ag12Au Au12Ag Au13
0.69 0.83 0.97 1.32
0.09 0.07 0.03 0.48
2.892 2.891 S.T. S.T.
–NH3
Ag13 Ag12Au Au12Ag Au13
0.59 0.50 0.92 1.76
0.01 0.04 0.02 0.48
2.921 2.900 S.T. S.T.
2. Calculation details Density functional theory (DFT) calculations were performed with the spin-polarized plane-wave method implemented in the Quantum Espresso package [28]. The Perdew–Burke–Ernzerhof (PBE) functional [29] based on the generalized gradient approximation (GGA) were employed to evaluate the non-local exchange–correlation energy. In a 30 30 30 Å3 cubic supercell, the values of 40 and 400 Ry were used as the kinetic energy cutoff for wavefunctions and charge densities. The first Brillouin zone was sampled at the Gamma-point and the electronic levels were broadened though Marzari–Vanderbilt smearing technique with degauss value of 0.002 Ry [30]. The self-consistent-field calculation has convergence criteria of 106 Hartree. The lowest-energy atomic ordering of Ag–Au bimetallic clusters was calculated at the empirical EAM potential level [31], and then subjected to DFT local relaxation. In this work, four compositions (Ag13, Ag12Au, Au12Ag, and Au13) were selected as the cases of the 13-atom Ag–Au bimetallic clusters. The system of 13 atoms represents the smallest magic cluster with the icosahedral and cuboctahedral structures according to the geometric shell model. It is noticed that quantum size effects can significantly alter adsorption properties of nanoclusters. To obtain the most stable structure, we compare the total energies of Ag12Au (the Au atom on the surface) vs. Ag12Au* (the Au atom in the center), and Au12Ag (the Ag atom on the surface) vs. Au12Ag* (the Ag atom in the center). Based on DFT calculations, an energetic order of Au13 (25.90 eV) < Au12Ag (25.79 eV) < Au12Ag* (24.97 eV) < Ag12Au (21.06 eV) < Ag12Au* (20.69 eV) < Ag13 (20.50 eV) is found, indicating that the Au12Ag and Ag12Au clusters are more stable than the corresponding Au12Ag* and Ag12Au* ones, respectively, which is in good agreement with the previous work [32]. Therefore, the icosahedral (ICO) Au13, Au12Ag, Ag12Au, and Ag13 clusters are used to study the adsorption properties of the ligands, as shown in Fig. 1. It is noted that the ICO structure is not the most favorable one for the 13-atom clusters, since the cuboctahedral structure of Au12Ag is even more stable than the ICO one [33]. However, the icosahedral cluster is still a very suitable cluster model for understanding the ligand type effect on the electronic and geometric properties of 13-atom Ag–Au bimetallic clusters in
Edis
–CN
Fig. 2. The adsorption energies of different ligands (–CN, –SH, –OH, –COOH, –CH3, –NO2, –N(CH3)2, –NO, –PH3, –NH3) on the Ag13 cluster.
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this work, since it is the smallest magic cluster according to the geometric shell model. In this work, only the adsorption of the ligand on the top site of the clusters is focused on, since the adsorption of the ligand on the hollow and bridge sites are not stable after checking all the possibility. For the Au12Ag cluster, the adsorption of the ligand on the top of Ag atom is more favorable than that on the top of Au atom, so only the adsorption of the ligand on the top of Ag atom is considered. For the Ag12Au cluster, the adsorption of the ligand on the top of Au atom is more favorable than that on the top of Ag atom, thus only the adsorption of the ligand on the top of Au atom is focused on. We calculated adsorption energies according to this equation,
Eads ¼ Eclusterþligand Ecluster Eligand
ð1Þ
where Ecluster+ligand is the total energy of cluster upon ligand adsorption, Ecluster is the energy of the free cluster, and Eligand is the energy of isolated ligand in vacuum. The more negative adsorption energy, the stronger the adsorption. The distortion energy (Edis) [34] of the metal cluster is defined as the difference between the energy of the metal cluster in the configuration interacting with the ligand, and the energy of the bare cluster. The absolute value of Edis represents the distortion level. A negative value means that the cluster is changed into a more stable geometry configuration while a positive value means that the cluster remains the original structure with a relaxation upon adsorption. 3. Results and discussion 3.1. Effect of ligand type on the adsorption The adsorption energies, Eads, distortion energies, Edis, and average bond length of five Ag–Ag, Ag–Au or Au–Au bonds where
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ligand were adsorbed, Dcluster, of the ligands (X = –CN, –SH, –OH, –COOH, –CH3, –NO2, –N(CH3)2, –NO, –PH3, and –NH3) on the Ag13, Ag12Au, Au12Ag, and Au13 clusters at the top site are listed in Table 1. Fig. 2 shows a plot of the adsorption energy of the ligands (X = –CN, –SH, –OH, –COOH, –CH3, –NO2, –N(CH3)2, –NO, –PH3, and –NH3) on the Ag13 cluster. It is found that the adsorption strength follows the order of Ag13–CN > Ag13–SH > Ag13–OH > Ag13–COOH > Ag13–CH3 > Ag13–NO2 > Ag13–N(CH3)2 > Ag13–NO > Ag13–PH3 > Ag13–NH3 for the adsorption of all the ligands on the Ag13 cluster, which indicates that the adsorption strength of the ligands on the Ag13 cluster depends on the ligand type. It is also found that the –CN and –NH3 ligands on the Ag13 cluster exhibit the strongest and weakest adsorption strength, respectively. In order to gain a deeper insight of the electronic mechanisms that play a role in the adsorption properties of different ligands, we analyzed the density of states (DOS) projected on the Ag atom where ligand were adsorbed, as well as on the ligand. In addition, Ag atoms of free Ag13 cluster are plotted for comparison. Taking the ligands (–CN, –SH, –CH3, –NO) on the Ag13 cluster as the example, where the adsorption strength follows the order of Ag13–CN > Ag13–SH > Ag13–CH3 > Ag13–NO. Fig. 3 shows the PDOSs and charge redistribution plots of the ligated Ag atom and the ligands (–CN, –SH, –CH3, –NO) on the Ag13 cluster. It is found that bands of the atom of ligands below Fermi levels of the clusters are occupied, while the first band below the Fermi energy corresponds to the LUMO of the atom of ligands. It is interesting to point that the overlaps at the LUMO energy come to be weaker in the order of –CN, –SH, –CH3, –NO, which is in good agreement with the order of the adsorption strength (Ag13–CN > Ag13–SH > Ag13–CH3 > Ag13–NO). In particular, there is few overlap regions at 0.5 eV for the ligand –NO. To understand the tendency, charge redistribution induced by the Ag-ligand interaction were calculated, as shown
Fig. 3. The PDOSs and charge redistribution plots of the ligated Ag atom and the ligands (X = –CN, –SH, –CH3, –NO) on the Ag13 cluster. The black vertical lines give the Fermi energy for the Ag13–X clusters. Charge redistribution plots are inserted, in which blue and red regions represent charge accumulation and depletion, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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in Fig. 3. Note that each plot describes changes in the charge redistributions with and without the ligand. Apparently, the chargeaccumulated regions locate midway between the ligated-Ag and C atoms, and the larger regions between the ligation atoms indicate the stronger bonding. Similar to the results of PDOS, the charge transfer comes to be less in the order of –CN, –SH, –CH3, –NO, which consists with the order of the adsorption strength (Ag13–CN > Ag13–SH > Ag13–CH3 > Ag13–NO). More specifically, ¨ din charge data is 0.49, 0.40, 0.26, and the corresponding Low 0.15 eV, respectively, which agrees with the order of the adsorption strength (Ag13–CN > Ag13–SH > Ag13–CH3 > Ag13–NO). It means that the increase of charge transfer from the clusters to the ligand can strengthen the adsorption strength of the cluster upon the adsorption of the ligand. 3.2. Geometric relaxation upon adsorption
Fig. 4. Snapshots (yellow atom, Au; white atom, Ag; green atom, C; pink atom, N; red atom, O; blue atom, H; orange atom, P) of different ligands (–NO, –N(CH3)2, and –PH3) on 13-atom Ag–Au bimetallic clusters after the relaxation at the DFT level. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
The optimized geometries of clusters upon the adsorption of –N(CH3)2, –NO, and –PH3 are given in Fig. 4. It is found that the icosahedral (ICO) structure of the Au12Ag and Au13 clusters upon the adsorption of –N(CH3)2, –NO, and –PH3 has been changed into the truncated octahedral (TO) one. In contrast, ligands absorbed on the Ag13 and Ag12Au clusters have no effect on their original ICO structure. It means that the ICO structure of Au-rich clusters can be easily transformed into the more stable TO one upon adsorption. In contrast to a large energy gap (2.53 eV) between 5s and 4d orbitals in the Ag atom, the Au atom has a small energy
Fig. 5. Snapshots and the energies of the Au13 clusters during the structural transformation process from icosahedral to truncated octahedral structure upon –NO adsorption.
Fig. 6. Snapshots (yellow atom, Au; white atom, Ag; green atom, C; pink atom, N; red atom, O; blue atom, H) of the ligands (X = –CN, –COOH, –CH3, and –NO2) on Ag13, Ag12Au, Au12Ag, and Au13 clusters after the relaxation at the DFT level. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
L. Chang et al. / Computational and Theoretical Chemistry 1045 (2014) 35–40
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transition, Au13–NO is selected as the example to observe the structural transformation process upon the ligand adsorption. The snapshots and the energies of the Au13 clusters during the geometric relaxation processes are shown in Fig. 5. As shown in Fig. 5, with the structural transformation, the energies of the Au13 clusters become more negative, meaning that the TO cluster is more stable than the ICO one for the Au13 cluster. It can be concluded that the structural transformation from ICO to TO clusters upon the adsorption of ligands is attributed to that the stability of the ICO Au13 cluster is lower than that of the TO one. 3.3. Composition effect on the adsorption
Fig. 7. The adsorption energies of the ligands (X = –CN, –COOH, –CH3, and –NO2) on Ag13, Ag12Au, Au12Ag, and Au13 clusters.
gap (0.99 eV) between 6s and 5d orbitals, which results in the relativistic effects. Hence, ligands are easier to make much more contribution to the geometries of Au-rich clusters, compared with Ag12Au–X and Ag13–X clusters. In order to explore the structure transformation, the distortion energies of the clusters were calculated, as listed in Table 1. It is found that the distortion energies are relatively small for the Ag13–X and Ag12Au–X clusters upon adsorption, and high for the Au12Ag–X and Au13–X clusters (X = –N(CH3)2, –NO, and –PH3). It is found that the distortion energies are around 0.02 and 0.45 eV for the Au12Ag–X and Au13–X clusters, respectively. Apart from the distortion energy to measure the degree of structural
Fig. 6 shows the optimized geometries of Ag13, Ag12Au, Au12Ag, and Au13 clusters upon the adsorption of four different ligands (–CN, –COOH, –OH, –CH3). Obviously, these clusters remain the icosahedral structure upon the adsorption of these ligands. The adsorption energies of ligands on these clusters are listed in Table 1 and plotted in Fig. 7. It is found that the adsorption strength of the ligands (X = –CN, –COOH, –CH3, and –NO2) on these clusters follows the order of Ag12Au–X > Ag13–X > Au12Ag–X > Au13–X. As listed in Table 1, the values of the average bond lengths (Dcluster) with adsorption are larger than those without adsorption on these clusters with different composition. It means that the adsorption of ligands on these clusters may induce a structural change by elongating the metal–metal bond distances. To make a further insight for the composition effect on the adsorption of the ligand, PDOSs and charge density difference of Ag13, Ag12Au, Au12Ag, and Au13 clusters upon the adsorption of –CH3 are shown in Fig. 8. In PDOSs plot, overlap regions turn to be smaller in the order of Ag12Au–CH3, Ag13–CH3, Au12Ag–CH3 and Au13–CH3, which is in good agreement with the order of the adsorption strength (Ag12Au–CH3 > Ag13–CH3 > Au12Ag–CH3 >
Fig. 8. The PDOSs of the ligated metal atom and C atom and charge redistribution plots for the Ag12Au–CN, Au12Ag–CN, Ag13–CN, and Au13–CN systems.
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Au13–CH3). For charge density difference, ligand –CH3 induces much more charge redistribution in the Ag12Au cluster, compared with Ag13, Au12Ag, and Au13 clusters (see Fig. 8). 4. Conclusions We used DFT calculation to study the interactions of a set of typical ligands (–CN, –COOH, –CH3, –OH, –SH, –NH3, –NO, –NO2, N(CH3)2, and –PH3) with 13-atom icosahedral Ag–Au bimetallic clusters. For the Ag13 cluster upon the adsorption of the ligands, the adsorption strength follows the order of Ag13–CN > Ag13–SH > Ag13–OH > Ag13–COOH > Ag13–CH3 > Ag13–NO2 > Ag13–N(CH3)2 > Ag13–NO > Ag13–PH3 > Ag13–NH3. For the adsorption strength of the ligands on Ag13, Ag12Au, Au12Ag, and Au13 clusters, it follows the order of Ag12Au–X > Ag13–X > Au12Ag–X > Au13–X (X = –CN, –COOH, –CH3, and –NO2). It is found that the adsorption of the ligands (–NO, –N(CH3)2, and –PH3) can modify the geometric properties of Au-rich clusters, and even the icosahedral structure of Au-rich clusters has been transformed into the more stable truncated octahedral one upon adsorption. It is expected that our results can be helpful for the preparation of ligand-stabilized bimetallic clusters. Acknowledgments This work is supported by the National Natural Science Foundation of China (Nos. 21106003 and 91334203), Beijing Novel Program (No. Z12111000250000), Beijing Higher Education Young Elite Teacher Project, ‘‘Chemical Grid Project’’ of BUCT and Supercomputing Center of Chinese Academy of Sciences (SCCAS). References [1] Y. Gao, N. Shao, S. Bulusu, X.C. Zeng, Effective CO oxidation on endohedral gold-cage nanoclusters, J. Phys. Chem. C 112 (2008) 8234–8238. [2] Y. Yang, J. Shi, G. Kawamura, M. Nogami, Preparation of Au–Ag, Ag–Au core– shell bimetallic nanoparticles for surface-enhanced Raman scattering, Scripta Mater. 58 (2008) 862–865. [3] S. Pande, S.K. Ghosh, S. Praharaj, S. Panigrahi, S. Basu, S. Jana, A. Pal, T. Tsukuda, T. Pal, Synthesis of normal and inverted goldsilver coreshell architectures in b-cyclodextrin and their applications in SERS, J. Phys. Chem. C 111 (2007) 10806–10813. [4] R. Jin, H. Qian, Z. Wu, Y. Zhu, M. Zhu, A. Mohanty, N. Garg, Size focusing: a methodology for synthesizing atomically precise gold nanoclusters, J. Phys. Chem. Lett. 1 (2010) 2903–2910. [5] H. Hakkinen, Atomic and electronic structure of gold clusters: understanding flakes, cages and superatoms from simple concepts, Chem. Soc. Rev. 37 (2008) 1847–1859. [6] L.M. Demers, D.S. Ginger, S.-J. Park, Z. Li, S.-W. Chung, C.A. Mirkin, Direct patterning of modified oligonucleotides on metals and insulators by dip–pen nanolithography, Science 296 (2002) 1836–1838. [7] M.B. Cortie, A.M. McDonagh, Synthesis and optical properties of hybrid and alloy plasmonic nanoparticles, Chem. Rev. 111 (2011) 3713–3735. [8] S. Fedrigo, W. Harbich, J. Buttet, Collective dipole oscillations in small silver clusters embedded in rare-gas matrices, Phys. Rev. B 47 (1993) 10706. [9] X.c. Lo´pez Lozano, C. Mottet, H.-C. Weissker, Effect of alloying on the optical properties of Ag–Au nanoparticles, J. Phys. Chem. C 117 (2013) 3062–3068.
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Computational and Theoretical Chemistry journal homepage: www.elsevier.com/locate/comptc
Role of ligand type on the geometric and electronic properties of Ag–Au bimetallic clusters Le Chang, Haoxiang Xu, Daojian Cheng ⇑ State Key Laboratory of Organic–Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China
a r t i c l e
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Article history: Received 31 March 2014 Received in revised form 29 June 2014 Accepted 29 June 2014 Available online 8 July 2014 Keywords: Ligand Ag–Au bimetallic clusters Geometric and electronic properties DFT calculations
a b s t r a c t The interactions of a set of typical ligands (–CN, –COOH, –CH3, –OH, –SH, –NH3, –NO, –NO2, N(CH3)2, and –PH3) with 13-atom icosahedral Ag–Au bimetallic clusters have been investigated by density functional theory (DFT) calculations. For the adsorption of all the ligands on the Ag13 cluster, the adsorption strength follows the order of Ag13–CN > Ag13–SH > Ag13–OH > Ag13–COOH > Ag13–CH3 > Ag13–NO2 > Ag13–N(CH3)2 > Ag13–NO > Ag13–PH3 > Ag13–NH3. Considering the composition effect, the adsorption strength of the ligands (X = –CN, –COOH, –CH3, and –NO2) on these clusters follows the order of Ag12Au–X > Ag13–X > Au12Ag–X > Au13–X. It is also found that the adsorption of the ligands (–NO, –N(CH3)2, and –PH3) on the Au-rich clusters can modify their geometric properties, and even the icosahedral structure of Au-rich clusters has been transformed into the more stable truncated octahedral one upon adsorption. Our results show that the geometric and electronic properties of Ag–Au bimetallic clusters can be tuned by the ligand type, which can provide useful insights for the preparation of ligand-stabilized Ag–Au bimetallic clusters by chemical reduction. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Metal clusters have been considered to be the building blocks for nanotechnology. The unique physical and chemical properties of metal clusters strongly depend on their sizes and shapes, which differ from the corresponding bulk materials. Bimetallic clusters (also called ‘‘nanoalloys’’), composed of two metallic elements, often exhibit better physical and chemical properties than their monometallic counterparts [1–3]. In addition, nanoalloys also offer new degrees of freedom for understanding their electronic and geometric properties of clusters, since their properties can be tuned by not only the size and shape, but also the composition and atomic ordering [4,5]. In recent years, Ag–Au bimetallic clusters have attracted a lot of attention for their potential applications in nanoelectronics [6], optics [7–9], catalysis [10–12], sensing [13], and biomedicine [14,15]. In general, metal clusters are prepared by the chemical reduction method [16–19], in which various capping ligands are used to control their size [20,21] and also avoid irreversible aggregation [22]. However, a lot of studies have revealed that the capping ligands can significantly affect their physicochemical properties [23]. More importantly, the type of the ligand can greatly modify ⇑ Corresponding author. Fax: +86 10 64427616. E-mail address: [email protected] (D. Cheng). http://dx.doi.org/10.1016/j.comptc.2014.06.023 2210-271X/Ó 2014 Elsevier B.V. All rights reserved.
the properties of metal clusters. For example, Shibu et al. [24] showed that the capping ligands can significantly modify the UV–vis spectrum of Au25 clusters. Udayabhaskararao et al. [25] found that the average luminescence lifetime of Ag–Au nanoalloys depends on the type of the protecting ligands. Thus, it is of great interest to comprehensively explore the effect of ligands on the physical and chemical properties of Ag–Au bimetallic clusters. In current experimental research, the complex structure of the ligand-coated metal clusters makes understanding their properties an extremely difficult task. On the other hand, theoretical methods such as density functional theory (DFT) can provide reasonable accuracy to infer the leading physical trends and to enable comparison of computational results with the experimental data. Based on DFT calculations, Malola and Häkkinen [26] found that the thiolate ligands can improve the stability of the Ag–Au bimetallic clusters. Kauffman et al. [27] found that the SC2H4Ph ligand can affect the composition of Ag–Au bimetallic clusters by DFT calculations. However, little theoretical work has been devoted to the effect of the ligand type on the electronic and geometric properties of Ag– Au bimetallic clusters. In this work, the electronic and geometric properties of 13-atom icosahedral Ag–Au bimetallic clusters upon the adsorption of a set of typical ligands (–CN, –COOH, –CH3, –OH, –SH, –NH3, –NO, –NO2, –N(CH3)2, and –PH3) are investigated by DFT calculations. The adsorption strength of the ligands on Ag–Au bimetallic clusters
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depending on the ligand type is investigated. The role of composition and geometric relaxation upon the adsorption of the ligands on Ag–Au bimetallic clusters is also studied. The paper is structured as follows. In the next section, we present the computational methodology. Section 3 describes our results and discussion, and Section 4 offers our conclusions.
Table 1 The adsorption energies (Eads in eV), distortion energies (Edis in eV), and average bond length of five Ag–Ag, Ag–Au or Au–Au bonds where ligand were adsorbed (Dcluster in Å) of the ligands (X = –CN, –SH, –OH, –COOH, –CH3, –NO2, –N(CH3)2, –NO, –PH3, and –NH3) on the Ag13, Ag12Au, Au12Ag, and Au13 clusters at the top site. S.T. corresponds to the structural transformation. Eads
Fig. 1. Snapshots (yellow atom, Au; white atom, Ag) of 13-atom Ag–Au bimetallic clusters after the local relaxation at the DFT level. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Dcluster
Ag13 Ag12Au Au12Ag Au13
3.51 3.72 2.18 2.66
0.01 0.00 1.11 1.47
2.812 2.919 2.975 2.959
–COOH
Ag13 Ag12Au Au12Ag Au13
1.92 2.26 0.69 1.30
0.01 0.06 1.11 1.48
2.936 2.937 2.919 2.977
–CH3
Ag13 Ag12Au Au12Ag Au13
1.74 2.17 0.58 1.27
0.01 0.05 1.11 1.48
2.945 2.941 2.929 2.983
–NO2
Ag13 Ag12Au Au12Ag Au13
1.48 1.46 0.16 0.36
0.01 0.00 1.10 1.46
2.920 2.906 2.958 2.953
–OH
Ag13 Ag12Au Au12Ag Au13
2.40 2.41 1.16 2.23
0.02 0.05 1.08 0.53
2.921 2.928 2.956 S.T.
–SH
Ag13 Ag12Au Au12Ag Au13
2.42 3.82 4.20 3.77
0.01 0.65 0.73 0.77
2.930 S.T. S.T. S.T.
–N(CH3)2
Ag13 Ag12Au Au12Ag Au13
1.15 1.30 1.15 1.38
0.04 0.06 0.02 0.47
3.404 2.928 S.T. S.T.
–NO
Ag13 Ag12Au Au12Ag Au13
-0.91 1.05 0.77 1.20
0.09 0.01 0.02 0.46
2.923 2.952 S.T. S.T.
–PH3
Ag13 Ag12Au Au12Ag Au13
0.69 0.83 0.97 1.32
0.09 0.07 0.03 0.48
2.892 2.891 S.T. S.T.
–NH3
Ag13 Ag12Au Au12Ag Au13
0.59 0.50 0.92 1.76
0.01 0.04 0.02 0.48
2.921 2.900 S.T. S.T.
2. Calculation details Density functional theory (DFT) calculations were performed with the spin-polarized plane-wave method implemented in the Quantum Espresso package [28]. The Perdew–Burke–Ernzerhof (PBE) functional [29] based on the generalized gradient approximation (GGA) were employed to evaluate the non-local exchange–correlation energy. In a 30 30 30 Å3 cubic supercell, the values of 40 and 400 Ry were used as the kinetic energy cutoff for wavefunctions and charge densities. The first Brillouin zone was sampled at the Gamma-point and the electronic levels were broadened though Marzari–Vanderbilt smearing technique with degauss value of 0.002 Ry [30]. The self-consistent-field calculation has convergence criteria of 106 Hartree. The lowest-energy atomic ordering of Ag–Au bimetallic clusters was calculated at the empirical EAM potential level [31], and then subjected to DFT local relaxation. In this work, four compositions (Ag13, Ag12Au, Au12Ag, and Au13) were selected as the cases of the 13-atom Ag–Au bimetallic clusters. The system of 13 atoms represents the smallest magic cluster with the icosahedral and cuboctahedral structures according to the geometric shell model. It is noticed that quantum size effects can significantly alter adsorption properties of nanoclusters. To obtain the most stable structure, we compare the total energies of Ag12Au (the Au atom on the surface) vs. Ag12Au* (the Au atom in the center), and Au12Ag (the Ag atom on the surface) vs. Au12Ag* (the Ag atom in the center). Based on DFT calculations, an energetic order of Au13 (25.90 eV) < Au12Ag (25.79 eV) < Au12Ag* (24.97 eV) < Ag12Au (21.06 eV) < Ag12Au* (20.69 eV) < Ag13 (20.50 eV) is found, indicating that the Au12Ag and Ag12Au clusters are more stable than the corresponding Au12Ag* and Ag12Au* ones, respectively, which is in good agreement with the previous work [32]. Therefore, the icosahedral (ICO) Au13, Au12Ag, Ag12Au, and Ag13 clusters are used to study the adsorption properties of the ligands, as shown in Fig. 1. It is noted that the ICO structure is not the most favorable one for the 13-atom clusters, since the cuboctahedral structure of Au12Ag is even more stable than the ICO one [33]. However, the icosahedral cluster is still a very suitable cluster model for understanding the ligand type effect on the electronic and geometric properties of 13-atom Ag–Au bimetallic clusters in
Edis
–CN
Fig. 2. The adsorption energies of different ligands (–CN, –SH, –OH, –COOH, –CH3, –NO2, –N(CH3)2, –NO, –PH3, –NH3) on the Ag13 cluster.
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this work, since it is the smallest magic cluster according to the geometric shell model. In this work, only the adsorption of the ligand on the top site of the clusters is focused on, since the adsorption of the ligand on the hollow and bridge sites are not stable after checking all the possibility. For the Au12Ag cluster, the adsorption of the ligand on the top of Ag atom is more favorable than that on the top of Au atom, so only the adsorption of the ligand on the top of Ag atom is considered. For the Ag12Au cluster, the adsorption of the ligand on the top of Au atom is more favorable than that on the top of Ag atom, thus only the adsorption of the ligand on the top of Au atom is focused on. We calculated adsorption energies according to this equation,
Eads ¼ Eclusterþligand Ecluster Eligand
ð1Þ
where Ecluster+ligand is the total energy of cluster upon ligand adsorption, Ecluster is the energy of the free cluster, and Eligand is the energy of isolated ligand in vacuum. The more negative adsorption energy, the stronger the adsorption. The distortion energy (Edis) [34] of the metal cluster is defined as the difference between the energy of the metal cluster in the configuration interacting with the ligand, and the energy of the bare cluster. The absolute value of Edis represents the distortion level. A negative value means that the cluster is changed into a more stable geometry configuration while a positive value means that the cluster remains the original structure with a relaxation upon adsorption. 3. Results and discussion 3.1. Effect of ligand type on the adsorption The adsorption energies, Eads, distortion energies, Edis, and average bond length of five Ag–Ag, Ag–Au or Au–Au bonds where
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ligand were adsorbed, Dcluster, of the ligands (X = –CN, –SH, –OH, –COOH, –CH3, –NO2, –N(CH3)2, –NO, –PH3, and –NH3) on the Ag13, Ag12Au, Au12Ag, and Au13 clusters at the top site are listed in Table 1. Fig. 2 shows a plot of the adsorption energy of the ligands (X = –CN, –SH, –OH, –COOH, –CH3, –NO2, –N(CH3)2, –NO, –PH3, and –NH3) on the Ag13 cluster. It is found that the adsorption strength follows the order of Ag13–CN > Ag13–SH > Ag13–OH > Ag13–COOH > Ag13–CH3 > Ag13–NO2 > Ag13–N(CH3)2 > Ag13–NO > Ag13–PH3 > Ag13–NH3 for the adsorption of all the ligands on the Ag13 cluster, which indicates that the adsorption strength of the ligands on the Ag13 cluster depends on the ligand type. It is also found that the –CN and –NH3 ligands on the Ag13 cluster exhibit the strongest and weakest adsorption strength, respectively. In order to gain a deeper insight of the electronic mechanisms that play a role in the adsorption properties of different ligands, we analyzed the density of states (DOS) projected on the Ag atom where ligand were adsorbed, as well as on the ligand. In addition, Ag atoms of free Ag13 cluster are plotted for comparison. Taking the ligands (–CN, –SH, –CH3, –NO) on the Ag13 cluster as the example, where the adsorption strength follows the order of Ag13–CN > Ag13–SH > Ag13–CH3 > Ag13–NO. Fig. 3 shows the PDOSs and charge redistribution plots of the ligated Ag atom and the ligands (–CN, –SH, –CH3, –NO) on the Ag13 cluster. It is found that bands of the atom of ligands below Fermi levels of the clusters are occupied, while the first band below the Fermi energy corresponds to the LUMO of the atom of ligands. It is interesting to point that the overlaps at the LUMO energy come to be weaker in the order of –CN, –SH, –CH3, –NO, which is in good agreement with the order of the adsorption strength (Ag13–CN > Ag13–SH > Ag13–CH3 > Ag13–NO). In particular, there is few overlap regions at 0.5 eV for the ligand –NO. To understand the tendency, charge redistribution induced by the Ag-ligand interaction were calculated, as shown
Fig. 3. The PDOSs and charge redistribution plots of the ligated Ag atom and the ligands (X = –CN, –SH, –CH3, –NO) on the Ag13 cluster. The black vertical lines give the Fermi energy for the Ag13–X clusters. Charge redistribution plots are inserted, in which blue and red regions represent charge accumulation and depletion, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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in Fig. 3. Note that each plot describes changes in the charge redistributions with and without the ligand. Apparently, the chargeaccumulated regions locate midway between the ligated-Ag and C atoms, and the larger regions between the ligation atoms indicate the stronger bonding. Similar to the results of PDOS, the charge transfer comes to be less in the order of –CN, –SH, –CH3, –NO, which consists with the order of the adsorption strength (Ag13–CN > Ag13–SH > Ag13–CH3 > Ag13–NO). More specifically, ¨ din charge data is 0.49, 0.40, 0.26, and the corresponding Low 0.15 eV, respectively, which agrees with the order of the adsorption strength (Ag13–CN > Ag13–SH > Ag13–CH3 > Ag13–NO). It means that the increase of charge transfer from the clusters to the ligand can strengthen the adsorption strength of the cluster upon the adsorption of the ligand. 3.2. Geometric relaxation upon adsorption
Fig. 4. Snapshots (yellow atom, Au; white atom, Ag; green atom, C; pink atom, N; red atom, O; blue atom, H; orange atom, P) of different ligands (–NO, –N(CH3)2, and –PH3) on 13-atom Ag–Au bimetallic clusters after the relaxation at the DFT level. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
The optimized geometries of clusters upon the adsorption of –N(CH3)2, –NO, and –PH3 are given in Fig. 4. It is found that the icosahedral (ICO) structure of the Au12Ag and Au13 clusters upon the adsorption of –N(CH3)2, –NO, and –PH3 has been changed into the truncated octahedral (TO) one. In contrast, ligands absorbed on the Ag13 and Ag12Au clusters have no effect on their original ICO structure. It means that the ICO structure of Au-rich clusters can be easily transformed into the more stable TO one upon adsorption. In contrast to a large energy gap (2.53 eV) between 5s and 4d orbitals in the Ag atom, the Au atom has a small energy
Fig. 5. Snapshots and the energies of the Au13 clusters during the structural transformation process from icosahedral to truncated octahedral structure upon –NO adsorption.
Fig. 6. Snapshots (yellow atom, Au; white atom, Ag; green atom, C; pink atom, N; red atom, O; blue atom, H) of the ligands (X = –CN, –COOH, –CH3, and –NO2) on Ag13, Ag12Au, Au12Ag, and Au13 clusters after the relaxation at the DFT level. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
L. Chang et al. / Computational and Theoretical Chemistry 1045 (2014) 35–40
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transition, Au13–NO is selected as the example to observe the structural transformation process upon the ligand adsorption. The snapshots and the energies of the Au13 clusters during the geometric relaxation processes are shown in Fig. 5. As shown in Fig. 5, with the structural transformation, the energies of the Au13 clusters become more negative, meaning that the TO cluster is more stable than the ICO one for the Au13 cluster. It can be concluded that the structural transformation from ICO to TO clusters upon the adsorption of ligands is attributed to that the stability of the ICO Au13 cluster is lower than that of the TO one. 3.3. Composition effect on the adsorption
Fig. 7. The adsorption energies of the ligands (X = –CN, –COOH, –CH3, and –NO2) on Ag13, Ag12Au, Au12Ag, and Au13 clusters.
gap (0.99 eV) between 6s and 5d orbitals, which results in the relativistic effects. Hence, ligands are easier to make much more contribution to the geometries of Au-rich clusters, compared with Ag12Au–X and Ag13–X clusters. In order to explore the structure transformation, the distortion energies of the clusters were calculated, as listed in Table 1. It is found that the distortion energies are relatively small for the Ag13–X and Ag12Au–X clusters upon adsorption, and high for the Au12Ag–X and Au13–X clusters (X = –N(CH3)2, –NO, and –PH3). It is found that the distortion energies are around 0.02 and 0.45 eV for the Au12Ag–X and Au13–X clusters, respectively. Apart from the distortion energy to measure the degree of structural
Fig. 6 shows the optimized geometries of Ag13, Ag12Au, Au12Ag, and Au13 clusters upon the adsorption of four different ligands (–CN, –COOH, –OH, –CH3). Obviously, these clusters remain the icosahedral structure upon the adsorption of these ligands. The adsorption energies of ligands on these clusters are listed in Table 1 and plotted in Fig. 7. It is found that the adsorption strength of the ligands (X = –CN, –COOH, –CH3, and –NO2) on these clusters follows the order of Ag12Au–X > Ag13–X > Au12Ag–X > Au13–X. As listed in Table 1, the values of the average bond lengths (Dcluster) with adsorption are larger than those without adsorption on these clusters with different composition. It means that the adsorption of ligands on these clusters may induce a structural change by elongating the metal–metal bond distances. To make a further insight for the composition effect on the adsorption of the ligand, PDOSs and charge density difference of Ag13, Ag12Au, Au12Ag, and Au13 clusters upon the adsorption of –CH3 are shown in Fig. 8. In PDOSs plot, overlap regions turn to be smaller in the order of Ag12Au–CH3, Ag13–CH3, Au12Ag–CH3 and Au13–CH3, which is in good agreement with the order of the adsorption strength (Ag12Au–CH3 > Ag13–CH3 > Au12Ag–CH3 >
Fig. 8. The PDOSs of the ligated metal atom and C atom and charge redistribution plots for the Ag12Au–CN, Au12Ag–CN, Ag13–CN, and Au13–CN systems.
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Au13–CH3). For charge density difference, ligand –CH3 induces much more charge redistribution in the Ag12Au cluster, compared with Ag13, Au12Ag, and Au13 clusters (see Fig. 8). 4. Conclusions We used DFT calculation to study the interactions of a set of typical ligands (–CN, –COOH, –CH3, –OH, –SH, –NH3, –NO, –NO2, N(CH3)2, and –PH3) with 13-atom icosahedral Ag–Au bimetallic clusters. For the Ag13 cluster upon the adsorption of the ligands, the adsorption strength follows the order of Ag13–CN > Ag13–SH > Ag13–OH > Ag13–COOH > Ag13–CH3 > Ag13–NO2 > Ag13–N(CH3)2 > Ag13–NO > Ag13–PH3 > Ag13–NH3. For the adsorption strength of the ligands on Ag13, Ag12Au, Au12Ag, and Au13 clusters, it follows the order of Ag12Au–X > Ag13–X > Au12Ag–X > Au13–X (X = –CN, –COOH, –CH3, and –NO2). It is found that the adsorption of the ligands (–NO, –N(CH3)2, and –PH3) can modify the geometric properties of Au-rich clusters, and even the icosahedral structure of Au-rich clusters has been transformed into the more stable truncated octahedral one upon adsorption. It is expected that our results can be helpful for the preparation of ligand-stabilized bimetallic clusters. Acknowledgments This work is supported by the National Natural Science Foundation of China (Nos. 21106003 and 91334203), Beijing Novel Program (No. Z12111000250000), Beijing Higher Education Young Elite Teacher Project, ‘‘Chemical Grid Project’’ of BUCT and Supercomputing Center of Chinese Academy of Sciences (SCCAS). References [1] Y. Gao, N. Shao, S. Bulusu, X.C. Zeng, Effective CO oxidation on endohedral gold-cage nanoclusters, J. Phys. Chem. C 112 (2008) 8234–8238. [2] Y. Yang, J. Shi, G. Kawamura, M. Nogami, Preparation of Au–Ag, Ag–Au core– shell bimetallic nanoparticles for surface-enhanced Raman scattering, Scripta Mater. 58 (2008) 862–865. [3] S. Pande, S.K. Ghosh, S. Praharaj, S. Panigrahi, S. Basu, S. Jana, A. Pal, T. Tsukuda, T. Pal, Synthesis of normal and inverted goldsilver coreshell architectures in b-cyclodextrin and their applications in SERS, J. Phys. Chem. C 111 (2007) 10806–10813. [4] R. Jin, H. Qian, Z. Wu, Y. Zhu, M. Zhu, A. Mohanty, N. Garg, Size focusing: a methodology for synthesizing atomically precise gold nanoclusters, J. Phys. Chem. Lett. 1 (2010) 2903–2910. [5] H. Hakkinen, Atomic and electronic structure of gold clusters: understanding flakes, cages and superatoms from simple concepts, Chem. Soc. Rev. 37 (2008) 1847–1859. [6] L.M. Demers, D.S. Ginger, S.-J. Park, Z. Li, S.-W. Chung, C.A. Mirkin, Direct patterning of modified oligonucleotides on metals and insulators by dip–pen nanolithography, Science 296 (2002) 1836–1838. [7] M.B. Cortie, A.M. McDonagh, Synthesis and optical properties of hybrid and alloy plasmonic nanoparticles, Chem. Rev. 111 (2011) 3713–3735. [8] S. Fedrigo, W. Harbich, J. Buttet, Collective dipole oscillations in small silver clusters embedded in rare-gas matrices, Phys. Rev. B 47 (1993) 10706. [9] X.c. Lo´pez Lozano, C. Mottet, H.-C. Weissker, Effect of alloying on the optical properties of Ag–Au nanoparticles, J. Phys. Chem. C 117 (2013) 3062–3068.
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