Adsorption of a Methylthio Radical on Silver Nanoparticles: Size

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Adsorption of a Methylthio Radical on Silver Nanoparticles: Size Dependence David Becerril, and Cecilia Noguez J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 08 Nov 2014 Downloaded from http://pubs.acs.org on November 9, 2014

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The Journal of Physical Chemistry

Adsorption of a Methylthio Radical on Silver Nanoparticles: Size Dependence David Becerril and Cecilia Noguez∗ Instituto de Física, Universidad Nacional Autónoma de México, Apartado Postal 20-364, México DF 01000, México E-mail: [email protected]

∗

To whom correspondence should be addressed

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ACS Paragon Plus Environment


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Abstract A systematic study of the the adsorption of a methylthio on different sites of silver nanoparticles of 13, 55, 147 and 309 atoms with icosahedral symmetry as well as on the (111) surface is performed using density functional theory. Ab initio molecular dynamics were used to obtain the adsorption energies, atomic positions, and electronic properties of the lowest-energy configurations. Different adsorption sites and orientations of the molecule were tested. The electronic density of states also show a size dependence, where a transition from discrete to more like-band structure is found. Adsorption of a second methylthio is also studied. It was observed that final structures, adsorption energies and electronic density of states strongly depended on the nanoparticle size, and thus, on the atomic coordination number, where bridge configurations had the lowest total energy and the highest adsorption energy for all sizes except Ag13 .

KEYWORDS: silver nanoclusters, adsorption energy, thiol adsorption, structure property relationships

Introduction Nanosystems containing metal nanoparticles (NPs) have attracted much attention because of their novel and promising properties, such as electronic, optical and catalytic. 1–3 Many of the physical and chemical properties of NPs depend on size and shape and may therefore be tailored. 4 Due to this, one of the main areas in nanoscience research is the ability to functionalize nanoparticles with high degree of control to build complex nanosystems with emergent or improved functions. 5–8 Hybrid nanosystems containing metal nanoparticles and organic molecules have attracted attention because their novel and promising properties contrasting with those of their constituents. 9 Size, shape, assembly, electronic, optical, magnetic, and chemical properties can be manipulated for specific applications in, for example, charge and energy transfer, electronic transport, photonics, catalysis, photovoltaics, sensing, among others. 10–15 2


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The capacity to design hybrid nanosystems with specific functions is critical to our understanding and anticipating the properties and behavior under different conditions. 16 However, the production of well-defined hybrid nanosystems with reproducible properties remains challenging since it requires the control of the shape and size distributions of the metal nanoparticles; and of the sites and number of adsorbed ligands of a certain kind. Additionally, metallic NPs generally coalesce when assembled as bare entities and one needs ways to protect the individual clusters and stop their agglomeration. One of the successful approaches to stabilize the new materials is to attach ligands with high affinity to metal atoms, such as organothiols. 17 Passivation using thiols can lead to atomically precise NPs; 5–8,17–28 small Aun and Agn nanoclusters with n ranging from tens to few hundreds. Recently, the synthesis and single-crystal X-ray structure characterization of ultrastable Ag thiolate clusters have been reported, 27,28 with a silver thiolate protecting layer consisting of Ag2 S5 capping units. This capping structure was theoretically and experimentally confirmed at the same time in thiol-stabilized Ag44 and Au12 Ag32 nanoparticles. 27 Thiol-stabilized metallic nanoparticles have attracted significant research interest due to their importance in both fundamental science and technological applications. Understanding how the reactivity of such nanoparticles varies with size, shape and adsorption site is crucial for the rational design of, for example, new catalysts. 29–33 In this direction it has been recognized that reactivity depends on the size and shape of the metal nanoparticles and that the synthesis of well-controlled particles could be critical for their applications. For instance, it was found that adsorption properties of CO on Pd clusters with different number of atoms show two adsorption mechanisms. 33 For large particles, the adsorption values slowly decrease with cluster size from the asymptotic value for an (ideal) infinite surface. But for small clusters, the adsorption energies drop quite steeply with increasing cluster size. These opposite trends yielded the lowest adsorption energies. Many studies of adsorption of thiols on noble metal surfaces have been carried out, most of these have focused on methanethiolate on the (111) surfaces of the noble metals. 34–38 It

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is generally accepted that thiol adsorption is favored on bridge or hollow sites. 39 Studies of adsorption of thiols on the surface of metallic NPs, mostly on gold, have been carried out. 40–44 Comparative studies of adsorption of thiols on Au, Ag and Cu surfaces have shown differences in the geometrical characteristics and electronic structure. 39,43 For example, binding energies for adsorption of methylthio on bridge sites and fcc hollow sites is largest for Cu and Ag than on Au. 38,39 In clusters, it has been reported a pronounced charge density rearrangement found for Cu, charge is depleted from the metal site and polarized toward the thiolate. A similar pattern is obtained for Au and charge rearrangement for Ag is less pronounced. 43 The differences in bond lengths, tilt angle, binding energy and other characteristics are attributed to the different chemical properties of each metal such as the participating orbitals in each bond. In this work we present a first-principle study of the electronic and structural properties as well as the adsorption process of one SCH3 on Ag icosahedral NPs of different sizes and atomic coordination numbers. We tested different adsorption sites, methyl molecule’s orientations to find the lowest energy cases. Our results shows two different adsorption paths that are related to the atomic coordination number, as well as to the electronic properties that evolve from molecular like states to band like states. The atomic arrangements characteristics for the most representative adsorption site cases are also discussed, as well as the adsorption of a second SCH3 . We believe that these results can provide insights in the properties of thioled-protected silver NPs.

Methodology Icosahedral Nanoparticles Bulk Ag is a face centered-cubic (fcc) crystal, where each atom has twelve nearest neighbors. Equilibrium Ag structures formed by tens, hundreds or thousands of atoms are no longer pieces of cubic lattices, and nanoparticles with morphologies like icosahedra, decahedra, 4


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octahedra, etc., are observed. 45–50 At small sizes and room temperatures, it has been found that icosahedral shapes dominate. 51–53 Therefore, we consider small NPs with icosahedral symmetry where a single methylthio (SCH3 ) is adsorbed. Since the NP has high symmetry, the number of unique adsorption sites reduces. The icosahedral NP can be thought of as being constructed by twenty tetrahedral shaped pieces combined together, where each tetrahedron shows an internal fcc structure with a (111) surface plane, which is known to be the most stable. 53 The planes of symmetry are present at all NP sizes, therefore a structural description of the atoms must be given to correctly define the adsorption sites, as we discuss in the supporting information file. The smallest of the icosahedral NP is composed with one layer (n = 1, see supportng information), i.e., of an atom in the geometrical center and 12 atoms in a first or external shell, and an atomic nuclearity of 13, Ag13 . In this particular case, the twelve surface atoms make up the 12 vertices and are (6) coordinated, while the interior atom is (12) coordinated. The next cluster size is formed with the previous one, Ag13 with 13 interior atoms and 42 atoms on the exterior shell (n = 2) for a total of 55 atoms or Ag55 . Now, each of the 20 faces has six constituent atoms, being three of them vertex atoms, i.e. (6) coordinated, and three are edge atoms, i.e. (8) coordinated. Now, the 13 interior atoms are (12) coordinated. The next size has the Ag55 interior atoms and 92 surface atoms with n = 3 composing the Ag147 . Here, faces have 10 atoms each, with three vertex(6) atoms, six edge(8) atoms and one atom at its center or face atom that is (9) coordinated. Finally, the Ag309 NP with n = 4 has Ag147 in its interior and 162 surface atoms, with faces made of 15 atoms of which three are vertex(6), nine are edge(8) and three are face(9) atoms. The NPs described above and the coordination number of their surface atoms are shown in Figure 1.

Adsorption Sites When adsorbed by the surface the methanethiol molecule SHCH3 undergoes a S−H bond rupture, where the H atom passivating the S atom is removed. The SCH3 radical is adsorbed 5



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Figure 1: Left: Coordination number of atoms on one face of Ag13 , Ag55 , Ag147 , and Ag309 . Right: Corresponding atomic models, where atoms on one face are shown in yellow color. through the S atom at the silver surface. 54–58 Since surface atoms are identified by their coordination number and because adsorption energy is expected to depend on it, we have defined adsorption sites according to three parameters: the number of atoms that constitute the site, their coordination number, and the orientation of the methyl molecule. According to the number of atoms that constitute the adsorption site, we have top (one atom), bridge (two atoms), and hollow (three atoms) sites. The Ag13 has all its surface atoms with coordination number (6) and therefore has one type of top site, Ag55 two, while Ag147 and Ag309 both have three types of top sites. There is one type of bridge site present on the Ag13 surface, two on Ag55 , four on Ag147 , and five on the Ag309 . Following the same reasoning, hollow adsorption sites are found. A listing of the different sites can be seen in Table 1. Notice that two different types of bridge sites with the same coordination number can be found in Ag147 6


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and Ag309 , one composed with atoms located at the edge and denoted by a superscript (e), like (8)-(8)(e) ; and those with atoms at the face denoted by a superscript (f), like (8)-(8)(f) . Table 1: Adsorption sites on NPs according to size and atomic coordination number. Size Ag13 Ag55 Ag147 Ag309

Top (6) (6),(8) (6),(8),(9)

Bridge (6)-(6) (6)-(8), (8)-(8) (6)-(8), (8)-(8)(e) (8)-(8)(f) , (8)-(9) (6),(8), (9) (6)-(8), (8)-(8)(e) (8)-(8)(f) , (8)-(9) (9)-(9)

Hollow (6)-(6)-(6) (6)-(8)-(8) (6)-(8)-(8) (8)-(8)-(9) (6)-(8)-(8) (8)-(8)-(9) (9)-(9)-(9)

The unique orientations of the methyl group will depend on the location of the adsorption site with respect to the icosahedron’s symmetry planes. For example, all top-vertex or top (6) sites have unique configurations when the S−C bond is oriented between the edge and its nearest plane of the NP face, spanning 36◦ . For all Bridge Face sites made up by two edge atoms, the S−C bond can be uniquely oriented when spanning the 180◦ between pointing towards the vertex and perpendicular to the edge.

Computational Details First-principle calculations were done within the density functional theory (DFT) as implemented in the siesta code, 59 using general-gradient approximation (GGA) and PerdewBurke-Ernzerhof (PBE) exchange-correlation functional, with scalar-relativistic norm-conserving pseudopotentials and a double-ζ polarized basis set of numerical atomic orbitals. Lowest total energy configurations were obtained using unconstrained relaxations. A force tolerance of 0.01 eV/Å was used as a convergence criteria in the molecular dynamic calculations. To confirm that relaxed configurations were correctly converged smaller force tolerances up to 0.001 eV/Å were tested in some cases, but 0.01 eV/Å proved to be the optimum convergence criteria. These DFT parameters have been used in previous studies with organic molecules in

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ligand-protected gold clusters 60 and silver NPs. 61 The success of this kind of DFT methods in the structure prediction of Aum (SR)n clusters has been recently discussed elsewhere. 62 Once a low-energy structure was obtained, we calculated the energy given up by the system due to the adsorption of the methylthio. The adsorption energy was calculated using the expression Eads =

EAgn + ESCH4

1 − E H2 2

− E[Agn −SCH3 ] ,

(1)

where E[Agn −SCH3 ] , EAgn , ESCH4 and EH2 are the total energies of the relaxed structures correspond to the methylthio adsorbed on the silver cluster of n atoms (Agn −SCH3 ), the bare cluster Agn , the methanethiol molecule SCH4 and the hydrogen molecule H2 . The term in parenthesis is the total energy of the bare cluster and methanethiol without interaction. The second term is the total energy of the system after adsorption and Eq. (1) measures the reduction of total energy due to adsorption. Previous studies of adsorption of SCH3 on metallic surfaces used an alternative expression to calculate adsorption energy: 39

Eads = EAgn + ESCH3 − E[Agn −SCH3 ] ,

(2)

using the total energy of the SCH3 radical instead of the term (ESCH4 − 21 EH2 ). However, we choose Eq. 1 since it better describes the observed physical process. 63,64 The difference due to the use of Eq. 1 can be adjusted by a constant term given by [ESCH4 − 12 EH2 ] − ESCH3 . Therefore, Eq. 1 gives lower adsorption energy than Eq. 2. Notice that the highest adsorption energy situation corresponds to the lowest-energy configurations. As defined in Eq. (2), when adsorption is favorable the energy, Eads , is a positive quantity. Differences in energy tend to cancel systematic errors in the DFT calculations, therefore our calculations of Eads are expected to be confident up to meV.

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Adsorption Energies and Geometrical Configurations For all sizes, it was found that top sites had lowest adsorption energy among all the sites for a given Agn . These sites also exhibit the smallest dependence with the orientation of methyl. Except for Ag13 , bridge sites have largest adsorption energies for a given Agn , with a large dependence on the molecule orientation, varying the energy up to ∼ 40%. When methylthio was initially placed on hollow sites in Ag55 , Ag147 , and Ag309 , upon molecular dynamics the ligand is displaced to a bridge site. On the other hand, Ag13 proved to be the most reactive of the NP’s studied here, the hollow site having largest adsorption energy of all the sites and NP’s tested. These different results for Ag13 , and the rest mean that it is possible find at least two different adsorption paths that depend on the NP’s size, as we will discuss along this section.

The Ag13 nanoparticle. In this NP only three different adsorption sites are present: top (6), bridge (6)-(6) and hollow (6)-(6)-(6), as we have depicted in Fig. 2. Additionally, we have tested many molecule’s orientations. Table 2 shows the adsorption energy and relaxed geometrical parameters for the configurations of highest adsorption energy or lowest energy configurations for each site. It can be observed that the adsorption energy rises as the S atom bonds with more Ag atoms, therefore the hollow site is preferred. Additionally, Ag−S bond lengths increase as S bonds with more Ag atoms, with an increment of ∼ 6% of bond lengths between top and hollow sites. Of the different orientations tested for a given site, configurations with highest adsorption energy were the most symmetric with respect to the Ag−S−C bonding angles. The bridge site showed the highest dependence on the orientation of the methyl, varying Eads up to 0.760 eV (38%), where the lowest-energy case is when the molecule is highly anti-symmetric with respect to the Ag−Ag bonding angles. The most stable site for this NP size is the hollow, but here the Ag−S−C angles is totally different from the other cases, as

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observed in Table 2. On the other hand, the top site had the smallest dependence of energy with respect to the orientation of methyl varying 0.001 eV (less than 1%). Nevertheless, top sites are the less stable configurations.

Figure 2: Atomic configurations of highest adsorption energy for each site of Ag13 . S atom is shown red and Ag atoms making up the site are shown in blue, otherwise Ag atoms are in grey. (a) hollow, (b) bridge and (c) top sites. Table 2: Adsorption energies and relaxed geometrical parameters of configurations with highest adsorption energies of Ag13 . Hollow(6)-(6)-(6) Bridge(6)-(6) Top(6)

Eads (eV) Ag−S (Å) Ag−S−C (◦ ) 2.349 2.51 127.1 2.51 130.1 2.52 129.2 1.978 2.49 105.6 2.49 102.8 1.226 2.35 104.2

The Ag55 nanoparticle. This NP has two differently coordinated surface atoms, (6) for vertex and (8) for edge atoms. Table 3 shows results of the cases with highest adsorption energies, including the relaxed geometrical parameters for each site. Previous results of the adsorption of SCH3 on icosahedral Ag55 have been reported. 61 From Table 3 we can see that bridge are the preferred adsorption sites for Ag55 . In all cases, configurations with highest adsorption energy were most symmetric with respect to the bond angles, with small difference of only 0.2◦ for the bridge face site where Ag atoms involved in the adsorption have the same 10


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coordination number; and 5.2◦ for the bridge edge site where Ag atoms in the adsorption have different coordination number. It can be seen that the atom with larger coordination number has a smaller bond angle. Both bridge cases are the most favorable adsorption sites. Particularly, highest adsorption energies for bridge sites are obtained when the methyl is oriented perpendicular to the Ag−Ag bond or bridge, as observed in Figure 3. Hollow sites achieve highest adsorption energy, so that SCH3 changes position toward the vicinity of a bridge site. This accounts for identical adsorption energy between bridge and hollow sites that transform to bridge sites. It is also noticed that at higher adsorption energy their is a smaller S−C bond. Table 3: Adsorption energies and relaxed geometrical parameters with highest adsorption energies configurations of Ag55 . Site Bridge Face(8)-(8) Hollow Face(8)-(8)-(8) (transforming to a Bridge Face)

Hollow Vertex(6)-(8)-(8) (transforming to a Bridge Face)

Bridge Edge(6)-(8) Top Edge(8) Top Vertex(6)

Eads (eV) Ag−S (Å) 1.277 2.52 2.52 1.276 3.51(8) 2.52(8) 2.52(8) 1.275 3.13(6) 2.51(8) 2.51(8) 1.147 2.50(6) 2.50(8) 0.579 2.39 0.313 2.38

Ag−S−C (◦ ) 109.5 109.7 109.4 109.8 108.3 108.9 106.4 101.2 104.2 104.2

Figure 3: Final configurations of Ag55 bridge sites with highest adsorption energy. (a) and (b) Bridge Face with different orientations and (c) Bridge Edge. S atom is shown red and Ag atoms making up the adsorption site are shown in blue, otherwise Ag atoms are in grey, translucent Ag atoms are from the subjacent shell. 11



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Bridge sites continue to be the most sensitive to the methyl orientation, for example, the bridge edge site varies up to 0.570 eV. 61 This variation occurs when the S−C bond is initially aligned with the icosahedral edge, but relaxation moves the methyl towards the vicinity of the top edge site with an adsorption energy almost identical to this latter. The Bridge Face site with highest adsorption energy aligns the S−C bond against the Ag−Ag bridge towards the edge, as shown in Fig. 3(a). While the configuration that aligns the S−C against the Ag−Ag bridge towards a vertex shown in Fig. 3(b), displays a lower adsorption energy with a variation of 0.112 eV, due only to a change of the molecule orientation. This latter orientation locates the methyl group over a hollow site that have a Ag atom from the underlying shell directly beneath it, known as a hcp site. However, the preferred orientation of SCH3 at the Bridge Face (8)-(8) locates the methyl group over an fcc site instead of an hcp site. The difference is that the hcp site has a second-layer Ag atom just below the site, whereas the fcc site does not, showing an influence of the underlying shells. We find that this feature does not remain the same for all Agn but depends on the NP size. On a previous study of adsorption of SCH3 on Ag(111) surfaces, 39 the preferred orientation locates the methyl over a hcp site and not over the fcc site, like in Ag55 . The main difference between the inifinite (111) surface and Ag55 is the atomic coordination number, where the surface only have (9) coordinated atoms. In the surface study, a variation in energy of 0.09 eV on the bridge site of (9) coordinated atoms in the hcp and fcc sites was found, 39 which is close to the variation found here in Ag55 at the bridge face site of 0.112 eV between the fcc and hcp configurations.

The Ag147 Nanoparticle. This NP has atoms of coordination (6) at the vertex and (8) edge atoms, and it is the first to give rise to a surface atom of coordination number (9), located in the center of each face, as shown in Fig. 1. It should be noticed that as the icosahedral NPs grow, their surface will be made predominately with atomic coordination (9), accordingly with the infinite (111) 12


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surface. Adsorption energies and relaxed geometrical parameters for the adsorption on the Ag147 NP are reported in Table 4. A general reduction of adsorption energy is observed, the highest adsorption energy of the Ag147 cases reducing 0.084 eV (∼6%) with respect to the highest adsorption energy of Ag55 cases, and these reducing about 1.072 eV (∼45%) with respect to Ag13 . Table 4: Adsorption energies and relaxed geometrical parameters of configurations with highest adsorption energies of bridge cases of Ag147 . Site Bridge Edge(8)-(8) Bridge Edge(6)-(8) Bridge Face

(8)-(9)

Bridge Face

(8)-(8)

Eads (eV) Ag−S (Å) 1.193 2.50 2.50 1.192 2.49 2.54 1.012 2.52(8) 2.56(9) 0.991 2.55(8) 2.56(8)

Ag−S−C (◦ ) 104.1 104.1 105.5 102.2 113.7 110.1 106.9 107.4

Bridge sites continue to have the highest adsorption energy particularly when the S−C bond is oriented perpendicularly to the Ag−Ag bond. However, Bridge Edge sites are now preferred over Bridge Face sites. For the Bridge Face (8) − (8) case the S−C bond can be perpendicular to the Ag−Ag bond with two different orientations, pointing to the vertex atom (hcp site) or the center atom (fcc site). Again the energetically favored orientation for this site is when the S−C bond points to the center atom or fcc site. The Bridge Edge (8)−(8) site is pointing always to a hcp site and has the highest adsorption energy. The symmetric coordination number of the site allows bonding lengths and angles to be nearly equal, opposed to Bridge Edge (6) − (8) site that displays a smaller Ag−S−C bond angle by 3.2◦ and larger bond length 0.05 Å (1.96%) for the atom with larger coordination. This asymmetry is also observed for the Bridge Face (8) − (9) case where the atomic coordination differ only by one and have a difference in bond angle and length of 3.6◦ and 0.04 Å, respectively. Bridge Edge adsorption sites have angles between 102.2◦ − 105.5◦ while Bridge Face sites have Ag−S−C angles between 106.9◦ − 113.7◦ , which shows a notable difference in bonding angles between 13



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Edge and Face sites. It is remarkable that Bridge Edge (8) − (8) and Bridge Edge (6) − (8) are degenerated in energy. The relaxation of the Hollow sites continued to move SCH3 to the vicinity of the closest bridge site of highest adsorption energy. The top sites show a significant difference of adsorption energy with respect to bridge and hollow sites, up to 0.580 eV. The difference in energy between bridge sites and top sites decreases as the NP increases in size from 0.752 eV for Ag13 , 0.689 eV for Ag55 and 0.581 eV for Ag147 . However, this energy is still large enough to make top sites the less suitable ones.

The Ag309 Nanoparticle. This NP requires a significant computational effort to perform molecular dynamics. Previous studies of adsorption of SCH3 on silver surfaces showed that bonding is favored in the vicinity of the bridge site. 38,39 Also, our DFT calculations found that bridge sites have considerable higher adsorption energy compared with other sites. Therefore, for the Ag309 NP mostly bridge sites are studied. The adsorption energies and relaxed geometrical parameters for the adsorption on the Ag309 NP are shown in Table 5. There is an overall raise in the highest adsorption energy of 0.118 eV (9.89%) with respect to Ag147 . Considering only the bridge face sites, there is a raise in energy of 0.218 eV (21.5%). All bridge face sites show a rise in adsorption energies with respect to Ag147 , while the bridge edge (6)-(8) diminishes. Edge sites continue to exhibit slightly smaller Ag−S−C angles than face sites. Figure 4 shows the preferred orientations of the bridge face sites; where bridge face sites (9)-(9) and (8)-(9) prefer to orient the methyl over an fcc hollow, while bridge face site (8)-(8) prefers the hcp hollow orientation. This latter marks a change in the preference of orientations displayed by the Ag55 and Ag147 NPs towards the infinite surface. The hollow site integrated by (9) coordinated atoms was also studied. This site is highly symmetric resulting in equal S−Ag bonding lengths and bonding angles for all Ag atoms. The S−C bond was oriented perpendicular to the face of the NP similarly to the hollow site of the Ag13 NP. Although this site does not have the highest adsorption energy of the Ag309 bridge sites, it is close 14


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Figure 4: Bridge Face configurations of energetically preferred orientations of Ag309 : (a) (9)-(9), (b) (8)-(9) and (c) (8)-(8). enough in energy to the bridge sites to be a likely site for molecule adsorption. Table 5: Adsorption energies and relaxed geometrical parameters of configurations with highest adsorption energies for the Ag309 . Site Bridge Edge(8)-(8) Bridge Face(9)-(9) Bridge Face(8)-(9) Bridge Edge(6)-(8) Hollow Face

(9)-(9)-(9)

Bridge Face

(8)-(8)

Eads (eV ) Ag−S (Å) 1.311 2.50 2.50 1.230 2.53 2.54 1.190 2.53(8) 2.54(9) 1.101 2.50(6) 2.51(8) 1.025 2.55 2.55 2.55 1.003 2.54 2.55

Ag−S−C (◦ ) 104.4 104.2 105.7 107.7 109.8 108.7 105.2 103.1 131.3 131.3 131.5 108.8 108.0

The Ag(111) surface It is expected that at a limiting case of adsorption on icosahedral NP surface will have similar attributes to the adsorption on infinite Ag(111) surfaces. It has been shown that adsorption of SCH3 on silver surfaces is favored in the vicinity of the bridge site 39 and because the bridge site is favored for most NP sizes, we restricted our study to the bridge sites of (111). Here, the surface was modeled using a slab of 36 Ag atoms distributed into 9 layers with four atoms per layer, where the three middle layers held fixed during relaxation in order to simulate bulk silver. A vacuum thickness equivalent to 8 inter planer distances 15



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(25 Å) was introduced between the slabs to avoid interactions between neighboring cells before and upon molecular adsorption. Then, periodic boundary conditions are applied in the three directions. Two SCH3 molecules, one on each side of the slab, were adsorbed onto the surface. In previous studies 38,39 large clusters rather than periodic cells were used to simulate low coverage adsorption on the surface, our results are found to be in good agreement. We first relaxed the bare surface, little surface relaxation and a contraction of the first inter-layer distances were found. The adsorption of SCH3 causes a slight contraction of the outer inter-plane distances, however it has an opposite effect on the interior planes, causing an increase in the inner inter-plane distances. Geometrical features are very similar to the Bridge Face (9)-(9) site in Ag309 , for instance bond angles continue to differ by 2 degrees, however Ag−S bond lengths are slightly larger than since now all surface atoms are 9 coordinated. In this case, the calculated adsorption energy of 0.9 eV was the lowest of all bridge sites tested. Being the limiting case, this result defines an overall tendency of decreasing adsorption energy as the size increases.

Atomic Deformation Before molecule adsorption, edge, vertex and center atoms of each shell are clearly grouped ¯ to the NP geometric center in the icosahedral symmetry. by well defined average distance (R) Inter-layer distances between the shells layers of the NP were also computed and compared with the inter-plane distance of the bulk and the semi-infinite surface. As we have mentioned, each face of the icosahedral NP has a fcc (111) facet, as the NP grows the limiting case for each face is the (111) surface. The (111) surface showed little structural relaxation, where the first interlayer distance between the surface layer and the first internal layer contracted from the calculated bulk interlayer distance to 2.39 Å, in approximately 0.75%. This is in good agreement with experimentally measured contractions of less than 2%. 65 We consider the inter-layer distances of the NPs, which is found by measuring the distances between planes of the NP with a particular orientation of the (111) facet, it is expected for these 16


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distances to approach the inter-layer distances of the fcc (111) surface, these distances are shown in Table 6. Table 6: Average inter-shell distances. ∆core−1 indicates distance from the geometric center to the first shell of atoms and successively. Distances are given in Angstroms (Å). The interplane distance in the limiting case of the surface was of 2.39 Å. Ag13 Ag55 Ag147 Ag309

∆core−1 2.20 2.22 2.21 2.19

∆1−2

∆2−3

∆3−4

2.28 2.27 2.28

2.29 2.29

2.38

NPs inter-layer distances were found to be smaller than bulk inter-plane distances, but they increase with the NP’s size. It is found that distances of inner or core atoms are closer to the distances corresponding to the (111) surface and bulk, than distances among atoms belong to the outer layer. It is clear that as the NP grows, the inter-layer distances increase. Also, for a given NP size, the inner planes are distributed more closely than the inter-layer distance between the surface plane and the first interior plane. The interplane distances of the NP clearly approach the inter-plane distances that we calculated for the surface and bulk, as shown in Figure 5.

Figure 5: Ag-Ag bond lengths of inner (in green) and outer (in red) atoms of bare NPs with respect to the size. Dashed lines are the corresponding bond lengths values in the (111) surface (red) and bulk (green). Now, to measure the effects of adsorption on the geometry of the cluster, we calculate the ¯ and their respective standard deviations, σ. It was found that Ag change in distance of R, 17



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NPs undergo little surface reconstruction as opposed, for example to Au NP that show larger deformation of the cluster after adsorption. 44,62,66 While core atoms have small deviations from the average distance, the surface atoms making up the adsorption site can undergo larger displacements. The atoms that are displaced most are those with which the S atom bonds to, while the rest of the Ag atoms are moved relatively little. It is found that distortion due to adsorption depends on the adsorption site and NP size. We now investigate the atomic distance distribution of each of the NPs. Ag13 has one shell of 12 atoms, all of which are located at an average distance of 2.77 Å. Ag55 has two shells, the outer shell posses edge and vertex atoms that are located at an average distance of 4.82 Å, for edge atoms and 5.53 Å, for vertex atoms. Therefore, Ag55 will display 3 characteristic atomic distribution distances, one for the first shell and two for the second. In a similar way Ag147 has 6 and Ag309 has 10 characteristic atomic distribution distances. We notice that for smaller clusters larger distortions, which are confirmed by the standard deviation that tends to decrease with respect to size of the NP. For instance, the largest standard deviation for Ag13 is about 0.06 Å, while for Ag309 is only 0.02 Å. Figure 6 shows the atomic distance distribution of the surface atoms of cases with highest adsorption energy of each Agn size. Here, we can distinguish between atoms making up the adsorption site in blue and nearest neighbors in green. Previously, the comparison of SCH3 adsorption between Ag55 and Au55 was discussed. 66 It was found that molecule adsorption is favored in both metals when the molecule is bond to two metal atoms forming a bridge. Low-energy metal-thiolate NPs show different atomic structure depending on the atomic specie. For instance, Ag NPs are minimally reshaped while Au NPs are more susceptible to be distorted due to the sulfur interaction. By comparing energies in both types of NPs, it is found that adsorption of the thiol group is favored up to 30% on Au rather than on Ag.

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Figure 6: Atomic distance distribution of atoms in the outer layer with respect to the center of mass of NPs sites with highest adsorption energy: Ag13 Hollow (6)-(6)-(6), Ag55 Bridge Face (8)-(8), Ag147 Bridge Edge (8)-(8) and Ag309 Bridge Edge (8)-(8). Atoms making up the adsorption site are shown in blue, nearest neighbors in green. Dotted lines correspond to the average values of the corresponding bare NPs.

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Electronic Properties To obtain more information about the adsorption we look at the electronic density of states (DOS) of the different systems. The energies of the occupied electronic states of the SCH3 molecule are shown in Figure 7. The HOMO state of SCH3 at the Fermi level 67 is doubly degenerated made up primarily of S p orbitals oriented perpendicular to the C−S bond, as previously found. 68 Also in agreement with previous calculations, the HOMO-1 state is found to be at −3.316 eV below the Fermi energy and the HOMO-2, which is also doubly degenerate, at −5.086 eV. It can be seen in Figure 7 that the S atom strongly contributes to the HOMO and HOMO-1 molecular orbitals.

Figure 7: DOS of occupied states of SCH3 in blue. Contribution from the S atom only is shown in orange. Fermi level has been set at zero. In Figure 8 we present the density of states (DOS) of the outer layers of bare NPs and the Ag(111) surface. Here, we present contributions from 4d and 5s orbitals, where the Fermi level is set at zero eV. We see that at energies close to the Fermi level there is a predominance of 5s over 4d orbitals for all NP sizes. The high symmetry of the bare NPs before adsorption leads to highly degenerated eigenstates, which can be seen most clearly for the smaller clusters Ag13 and Ag55 . Here, the well-defined peak structures of DOS are associated to degenerated molecular-like electronic states, because of the NPs five-fold symmetry. As the NP grows, there is an increase of occupied states leading to a nearly continuum of 4d orbitals for Ag147 and Ag309 below −2.5 eV. However, above −2.5 eV the DOS is still discrete, as can be seen in Figure 8. Finally, the DOS of the Ag(111) surface shows a continuum of occupied 20


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Figure 8: DOS of bare NPs projected onto the 5s (purple) and 4d (light blue) atomic orbitals of the outer layer atoms. The Fermi level is set at zero eV. The dark blue colour results from the overlapping orbitals.

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states close to the Fermi level, furthermore, there is a larger presence of 4d orbitals located at lower energies than in the NPs cases. This behaviour of the surface can be associated to the formation of two-dimensional bands projected along the direction perpendicular to the surface plane. To give a measure of the evolution and position of the occupied states as a function of the NPs size, we calculate the centroids of 4d and 5s orbitals of occupied states for the differently coordinated atoms of bare NPs, as R g(E)EdE centroid = R , g(E)dE

(3)

where g(E) is the density of states as a function of energy E, where the integral is done up to the Fermi Level. In Figure 9 we show the centroid of the 4d and 5s orbitals of occupied states for the differently coordinated atoms at the outer layer, i.e. vertex (6), edge (8), and face (9), as well as core (12) atoms. For comparison, we also include centroid values of Ag(111) for atoms with coordination (9) in the outer layer and (12) in the bulk. It is seen that the position of the centroid heavily depends on the coordination of the atoms with a lower centroid for larger coordinated atoms. Additionally, their is a general tendency of increasing energy with respect to the number of atoms of the NP. This tendency is very clear for core atoms with coordination (12), where it is observed that already for NP sizes of 309 atoms the energy difference of the 5s centroid is of less than 0.2 eV with respect to the corresponding centroid of the (111) surface, and even smaller for the 4d centroid. On the other hand, centroid values for the outer layer atoms are still lower than those of the surface, but it is expected that as the NP size increases the outer layer will contain more (9) coordinated atoms resembling Ag(111). It is interesting to notice that there are two different behaviors for NPs with and without face or center atoms at the outer layer, i.e. atoms with coordination number (9). For instance, a maximum value of the centroid is found in all the out layer atoms of Ag55 , i.e. for (6) and (8) coordinated atoms, with respect to the other NPs

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sizes. Then, these centroids slightly decrease in Ag147 and increase again for Ag309 , being slightly larger the values of Ag309 than those of Ag55 , showing a tendency to the surface values, as expected. For comparison, centroids of bulk 4d and 5s were calculated to be at −4.26 and −4.78 eV, respectively, which are slightly smaller than those of Ag(111).

Figure 9: Centroid of the 4d (bottom) and 5s (top) occupied states of the outer layer and core atoms of bare NPs. Contributions from vertex (H), edge (N), face (), and core (•) atoms with different coordination numbers are shown. Dashed lines indicates the centroid of the 5s occupied states of the Ag(111) surface for core atoms at −4.96 eV and at outer layer atoms at −2.66 eV, and of the 4d occupied states for core and outer layer atoms at −4.34 eV and −3.31 eV, respectively. The pink solid line corresponds to the energy of the HOMO-1 molecule. Perturbation theory suggests a chemical interaction to be characterized roughly by the expression: 69 ∆E =

|Vij | , Ei − Ej

(4)

where Vij is the interaction between states |i > and |j > and Ei − Ej is their energy gap. Therefore, as smaller the gap, larger chemical reaction. Since the methylthio is adsorbed via the S atom, which has largest presence in the HOMO and HOMO-1 states, it is expected that NP’s with centroids closer to such energies be more reactive. In the case of Ag13 the centroid of 5s occupied orbitals of vertex(6) atoms is about −3.49 eV, which is close to the

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molecule HOMO-1 state found at −3.32 eV, while 4d occupied orbitals centroid of vertex(6) atoms is at −3.88 eV. Therefore, the symmetry of the 5s atoms and the one of the NP can be responsible of the fact that the largest adsorption configuration is the hollow site, only for this particular size, Ag13 . For the next size, Ag55 , the closest centroid to the HOMO-1 is the one of the 5s edge(8) atoms at −3.88 eV, while the one of vertex(6) atoms is far at −2.92 eV. Now, the 4d centroid is slightly closer, but not as much as the 5s centroid. Therefore, the preferred adsorption site for Ag55 is with edge(8) atoms, the Bridge Face(8)-(8) case. Notice that for this particular NP it is not possible to have a symmetric environment with edge(8) atoms, like in Ag13 , because of the presence the second-layer Ag atoms just below the site. For the next size, Ag147 , now there are face(9) atoms. Here the situation is more complex since 5s centroids of vertex(6), edge(8), and face(9) atoms are very close at −3.06, −3.66, and −3.84 eV, respectively. Again the centroid of the edge(8) atoms is closer to HOMO-1 and the largest adsorption energy is found for the Bridge Edge(8)-(8), followed by the Bridge Edge(6)-(8). Now in Ag309 the 5s centroids of edge(8) and face(9) atoms are at −3.51 and −3.71, respectively, and the one of the vertex(6) is at −2.93 eV. Therefore, the Bridge Edge(8)-(8) followed by the Bridge Face(9)-(9) are the largest adsorption energy sites for the molecule in Ag309 . Finally, adsorption on the the Ag(111) surface proved to be the least reactive in agreement with the fact that the 5s centroid of face(9) atoms are at −2.66 eV, farther to HOMO-1 than the NPs values. In this clear tendency, it is conclude that the 4d states do not play a significant role in the adsorption process. Upon molecule adsorption both the NP and S states lose their high degeneracy causing a large distortion in the DOS and a spreading of both the S and silver states to larger energy regions. 61,66 For all cases, S states are shifted toward lower energies, with its centroid changing from −5.29 eV to values between −6.02 and −7.32 eV after adsorption, where the largest change is found for the smallest NP and the smallest change for Ag309 , as listed in Table 7. The centroid value for the (111) surface upon adsorption is −5.95 eV. A decrease in the value of the S centroid can be associated to a passivation of the p orbital and a lowering

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Table 7: Centroids of S occupied orbitals upon adsorption, and 4s and 5d Ag occupied orbitals before and after adsorption for NPs with highest adsorption energy. Core(12) S

Center (9)

Edge(8)

Vertex (6)

Ag13 Bare Hollow (6)-(6)-(6)

5s 4d -7.20 -5.09 -7.32 -6.89 -4.91

Ag55 Bare B.Face (8)-(8)

-5.55 -4.44 -6.18 -5.58 -4.49

Ag147 Bare B.Edge (8)-(8)

-5.24 -4.39 -3.85 -4.05 -3.66 -4.00 -3.06 -3.81 -6.08 -5.22 -4.46 -3.87 -4.08 -3.69 -4.03 -3.04 -3.85

Ag309 Bare B.Edge (8)-(8)

-4.94 -4.37 -3.71 -4.08 -3.51 -3.98 -2.93 -3.80 -6.07 -5.11 -4.40 -3.81 -4.02 -3.53 -3.93 -2.87 -3.73

5s

4d

5s

4d

5s 4d -3.49 -3.88 -3.71 -4.05

-3.60 -3.77 -2.92 -3.65 -3.50 -3.86 -2.99 -3.72

of its energy. A clear difference is seen in the properties and tendencies of the two smaller NPs, Ag13 and Ag55 . For these sizes their is a larger interaction between the adsorbate and the core 4d and 5s Ag states, as observed from Table 7 that shows the centroid values of 5s and 4d occupied orbitals before and after adsorption for the sites with larger adsorption energies, i.e. the hollow site of Ag13 , and the different bridge sites of Ag55 , Ag147 , Ag309 , and Ag(111). Core states for Ag13 and Ag55 are shifted towards higher energies while their vertex and edge states are shifted towards lower energies. While a decrease in the value of the centroid can be associated to a passivation of the orbital, an increase can be seen as a donation of electrons from that orbital and a raising in energy of its states. For these small sizes the adsorption energy can be correlated to the position of the S centroid, as can be seen in Figure 10, lower S centroid leads to larger adsorption energy. It is seen that for these NP sizes, lowering the energy of the S states is a crucial parameter determining the adsorption energy. The larger NPs, Ag147 and Ag309 , show less interaction with core atoms. The variation of the centroids in adsorption energy with respect to the different bridge sites 25



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is in general negative for Ag147 and Ag309 , and not positive like in smaller NPs. The values of the S centroid all fall within −6.08 and −5.99 eV, this is a variation of only 0.09 eV with respect to the adsorption sites, we then see that the value of the S centroid is less definitive in determining the value of the adsorption energy. The adsorption energy may therefore be determined more by the local rearrangement of the few atoms involved in the adsorption process, as suggested in the atomic distortion discussed above.

Figure 10: Value of the S centroid plotted against the adsorption energy for Ag13 and Ag55 NPs.

Adsorption of two SCH3 molecules Calculation were carried out to investigate the change in adsorption energy when more than one SCH3 molecule are adsorbed. Two SCH3 molecules were adsorbed onto the Ag55 NP at different bridge sites that show the highest adsorption energy. Of the many possible ways of adsorbing two molecules we found that the adsorption energy per molecule diminishes between 6 − 9%, with respect to the adsorption of only one molecule, when the SCH3 molecules do not bond with a common Ag atom. When the SCH3 molecules do bond with a common Ag atom, the adsorption energy variations are considerably larger, with a decrease of up to 20% with respect to the adsorption of one molecule. This variation depends on the adsorption site and the coordination number of the Ag atom which is being shared by the SCH3 26


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molecules.

Figure 11: Configurations of two SCH3 molecules adsorbed on Ag55 . Configurations (a)-(c) are of two Bridge Face (8)-(8) sites and (d)-(f) of two Bridge Edge (6)-(8) sites. Adsorption energy per molecule is shown in each case. The different configurations that were tested can be seen in Figure 11. Recall that the Bridge Face (8)-(8) site has the largest adsorption energy of all sites for the Ag55 NP and one molecule (1.277 eV). We can see that for case (a) the two SCH3 molecules are located at opposing faces and the adsorption energy diminishes by only 8.9%. In case (b) the two SCH3 molecules are adsorbed onto adjacent faces, the adsorption energy for this case diminishes by 9.7%. The difference in adsorption energy between case (a) and (b) is less than 1% even though the two SCH3 molecules bond to Ag atoms that are largely separated in case (a) and nearest neighbors in case (b). For case (c) the two SCH3 molecules bond to a common Ag atom, this reduces the adsorption energy per molecule by 20.9% with respect to the adsorption of one molecule, a considerably larger reduction compared to cases (a) and (b). For the Bridge Edge (6)-(8) a similar behavior was found. Therefore, the highest adsorption energy per molecule was found when SCH3 molecules were placed at opposing faces, case (d). In case (e) the Ag atoms which bond to the SCH3 molecules are nearest neighbors and the adsorption energy reduces by 5.6%. The largest change in adsorption energy was found when the two SCH3 molecules bonded to a common Ag atom. The greater difference occurred when the Ag atom being shared was (8) coordinated, as seen in case (f), with a 27



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decrease of adsorption energy of 22%. For the cases where the two SCH3 molecules do not bond with a common Ag atom the structural parameters vary little from the case of the adsorption of one molecule. At the Bridge Face (8)-(8) site, when a single SCH3 molecule is adsorbed, the two Ag−S−C angles are equal. It can be seen that for case (a) the difference between these angles is of 0.2 degrees and for (b) the difference is of 1 degree, which shows only small differences from the single molecule case. For case (c) we can see that the symmetry is broken in the bond lengths and bond angles. For this case the two SCH3 molecules bonds more strongly with the Ag atom that is not shared by the molecules, as can be seen by the smaller bond distance. The Ag−S−C angles differ more from the single molecule adsorption with a larger bond angle created between the shared edge Ag atom, the S atom and the C atom. Similar results are found for the bridge edge configurations. The largest difference from the adsorption of one SCH3 molecule is found in the case where a (8) coordinated Ag atom is shared. We can see that for case (f) the SCH3 bonds more strongly with the vertex atoms, and therefore leads to a larger bond length difference between (6) and (8) coordinated atoms as well as larger Ag−S−C bond angle difference. The structural parameters of the cases studied are shown in the supporting information file. The previous results suggest that adsorption energy per molecule will decrease with an increasing amount of adsorbed molecules. However, the results found in the adsorption of one SCH3 such as geometrical parameters, preferred adsorption site and adsorption energy, will not radically differ so long as the molecules do not bond to a common Ag atom. This type of shared Ag atom configuration is energetically unfavorable and would be an unlikely configuration for adsorption.

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Conclusions From the systematic study of the adsorption process of one SCH3 molecule on Ag icosahedral NPs of different sizes by using first-principle methods we conclude that bridge sites are most energetically favorable. Bridge symmetric configurations are favored, such as sites with the same coordination number, i.e. (8) − (8) or (9) − (9) sites. We found that smaller particles are more reactive, with larger adsorption energies, with a tendency of decreasing reactivity with a limit value found at the infinite (111) surface. We found that the HOMO-1 and HOMO molecule electronic states are most involve in adsorption with the 5s orbitals from Ag. Silver nanoparticles show little distortion after molecular adsorption, for all NP sizes. We observed an increasing of the Ag−Ag bond length with increasing NP size. Atoms with larger atomic coordination have energy centroids at lower energies. These centroids increase and are expected to tend to the those of the infinite surface at large NP size. The electronic density of states of small clusters show highly degenerated eigenstates, with well-defined peak structures. As the NP grows, there is an increase of occupied states leading to a nearly continuum of 4d orbitals for Ag147 and Ag309 below −2.5 eV. Finally, adsorption is less favored when two sulfur atoms share a single Ag atom. We believe that these results can provide insights into the properties of thioled-protected silver NPs.

Acknowledgement We acknowledge useful discussions with Dr. Francisco Hidalgo. This project was partially supported by DGAPA-UNAM PAPIIT IN104212-3 and CONACyT 179454 grants.

Supporting Information Available Supporting information can be found on-line: This material is available free of charge via the Internet at http://pubs.acs.org/.

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References (1) Huang, X.; El-Sayed, I. H.; Qian, W.; El-Sayed, M. A. Cancer Cell Imaging and Photothermal Therapy in the Near-Infrared Region by Using Gold Nanorods. J. Am Chem. Soc. 2006, 128, 2115–2120. (2) Haruta, M.; Daté, M. Advances in the Catalysis of Au Nanoparticles. Appl Catal A: Gen 2001, 222, 427 – 437. (3) Kamat, P. Photophysical, Photochemical and Photocatalytic Aspects of Metal Nanoparticles. J. Phys. Chem. B 2002, 106, 7729–7744. (4) Noguez, C. Surface Plasmons on Metal Nanoparticles: The Influence of Shape and Physical Environment. J. Phys. Chem. C 2007, 111, 3806–3819. (5) Jin, R.; Qian, H.; Wu, Z.; Zhu, Y.; Zhu, M.; Mohanty, A.; Garg, N. Size Focusing: A Methodology for Synthesizing Atomically Precise Gold Nanoclusters. J. Phys. Chem. Lett. 2010, 1, 2903–2910. (6) Jin, R. Quantum Sized, Thiolate-Protected Gold Nanoclusters. Nanoscale 2010, 2, 343–362. (7) Shichibu, Y.; Negishi, Y.; Tsukuda, T.; Teranishi, T. Large-Scale Synthesis of Thiolated Au25 Clusters via Ligand Exchange Reactions of Phosphine-Stabilized Au11 Clusters. J. Am. Chem. Soc. 2005, 127, 13464–13465. (8) Zhu, M.; Qian, H.; Jin, R. Thiolate-Protected Au24 (SC2 H4 Ph)20 Nanoclusters: Superatoms or Not? J. Phys. Chem. Lett. 2010, 1, 1003–1007. (9) Buck, M. R.; Bondi, J. F.; Schaak, R. E. A Total-Synthesis Framework for the Construction of High-Order Colloidal Hybrid Nanoparticles. Nat. Chem. 2012, 4, 37–44.

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Page 31 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(10) Yan, J.-Y.; Zhang, W.; Duan, S.; Zhao, X.-G.; Govorov, A. O. Optical Properties of Coupled Metal-Semiconductor and Metal-Molecule Nanocrystal Complexes: Role of Multipole Effects. Phys. Rev. B 2008, 77, 165301. (11) Feng, Y.; He, J.; Wang, H.; Tay, Y. Y.; Sun, H.; Zhu, L.; Chen, H. An Unconventional Role of Ligand in Continuously Tuning of Metal-Metal Interfacial Strain. J. Am. Chem. Soc. 2012, 134, 2004–2007. (12) Cheng, K.; Kothapalli, S.-R.; Liu, H.; Koh, A. L.; Jokerst, J. V.; Jiang, H.; Yang, M.; Li, J.; Levi, J.; Wu, J. C.; Gambhir, S. S.; Cheng, Z. Construction and Validation of Nano Gold Tripods for Molecular Imaging of Living Subjects. J. Am. Chem. Soc. 2014, 136, 3560–3571. (13) Galvan-Moya, J. E.; Altantzis, T.; Nelissen, K.; Peeters, F. M.; Grzelczak, M.; LizMarzan, L. M.; Bals, S.; Van Tendeloo, G. Self-Organization of Highly Symmetric Nanoassemblies: A Matter of Competition. ACS Nano 2014, 8, 3869–3875. (14) Zhao, Q.; Ji, M.; Qian, H.; Dai, B.; Weng, L.; Gui, J.; Zhang, J.; Ouyang, M.; Zhu, H. Controlling Structural Symmetry of a Hybrid Nanostructure and its Effect on Efficient Photocatalytic Hydrogen Evolution. Adv. Mater. 2014, 26, 1387–1392. (15) Wu, Z.; Wu, Y.; Pei, T.; Wang, H.; Geng, B. ZnO Nanorods/ZnS Center Dot(1,6hexanediamine)(0.5) Hybrid Nanoplates Hierarchical Heteroarchitecture With Improved Electrochemical Catalytic Properties for Hydrazine. Nanoscale 2014, 6, 2738– 2745. (16) Ruan, L.; Ramezani-Dakhel, H.; Lee, C.; Li, Y.; Duan, X.; Heinz, H.; Huang, Y. A Rational Biomimetic Approach to Structure Defect Generation in Colloidal Nanocrystals. ACS Nano 2014, 8, 6934–6944. (17) Kumar, S.; Bolan, M. D.; Bigioni, T. P. Glutathione-Stabilized Magic-Number Silver Cluster Compounds. J. Am Chem. Soc. 2010, 132, 13141–13143. 31



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(18) Pedicini, A. F.; Reber, A. C.; Khanna, S. N. The Effect of Sulfur Covalent Bonding on the Electronic Shells of Silver Clusters. J. Chem. Phys. 2013, 139, 164317. (19) Jadzinsky, P. D.; Calero, G.; Ackerson, C. J.; Bushnell, D. A.; Kornberg, R. D. Structure of a Thiol Monolayer-Protected Gold Nanoparticle at 1.1 Å Resolution. Science 2007, 318, 430–433. (20) Maity, P.; Xie, S.; Yamauchi, M.; Tsukuda, T. Stabilized Gold Clusters: From Isolation Toward Controlled Synthesis. Nanoscale 2012, 4, 4027–4037. (21) Tsukuda, T.; Tsunoyama, H.; Negishi, Y. Metal Nanoclusters in Catalysis and Materials Science; Elsevier: Amsterdam, 2008; pp 373–382. (22) Shafai, G.; Hong, S.; Bertino, M.; Rahman, T. S. Effect of Ligands on the Geometric and Electronic Structure of Au13 Clusters. J. Phys. Chem. C 2009, 113, 12072–12078. (23) Harkness, K. M.; Tang, Y.; Dass, A.; Pan, J.; Kothalawala, N.; Reddy, V. J.; Cliffel, D. E.; Demeler, B.; Stellacci, F.; Bakr, O. M.; McLean, J. A. Ag44(SR)304-: A Silver-Thiolate Superatom Complex. Nanoscale 2012, 4, 4269–4274. (24) Qian, H.; Zhu, Y.; Jin, R. Size-Focusing Synthesis, Optical and Electrochemical Properties of Monodisperse Au38(SC2H4Ph)24 Nanoclusters. ACS Nano 2009, 3, 3795–3803. (25) Qian, H.; Zhu, Y.; Jin, R. Isolation of Ubiquitous Au40(SR)24 Clusters from the 8 kDa Gold Clusters. J. Am. Chem. Soc. 2010, 132, 4583–4585. (26) Negishi, Y.; Takasugi, Y.; Sato, S.; Yao, H.; Kimura, K.; Tsukuda, T. Magic-Numbered Aun Clusters Protected by Glutathione Monolayers (n = 18, 21, 25, 28, 32, 39): Isolation and Spectroscopic Characterization. J. Am Chem. Soc. 2004, 126, 6518–6519. (27) Yang, H.; Wang, Y.; Huang, H.; Gell, L.; Lehtovaara, L.; Malola, S.; Hakkinen, H.; Zheng, N. All-Thiol-Stabilized Ag-44 and Au12Ag32 Nanoparticles with Single-Crystal Structures. Nat. Commun. 2013, 4, 2422. 32


Page 32 of 38

Page 33 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(28) Desireddy, A.; Conn, B. E.; Guo, J.; Yoon, B.; Barnett, R. N.; Monahan, B. M.; Kirschbaum, K.; Griffith, W. P.; Whetten, R. L.; Landman, U.; Bigioni, T. P. Ultrastable Silver Nanoparticles. Nature 2013, 501, 399–402. (29) Bell, A. T. The Impact of Nanoscience on Heterogeneous Catalysis. Science 2003, 299, 1688–1691. (30) Pei, Y.; Shao, N.; Gao, Y.; Zeng, X. C. Investigating Active Site of Gold Nanoparticle Au55(PPh3)12Cl6 in Selective Oxidation. ACS Nano 2010, 4, 2009–2020. (31) Lopez-Acevedo, O.; Kacprzak, K. A.; Akola, J.; Hakkinen, H. Quantum Size Effects in Ambient CO Oxidation Catalysed by Ligand-Protected Gold Clusters. Nat. Chem. 2010, 2, 329–334. (32) Yudanov, I. V.; Genest, A.; Schauermann, S.; Freund, H.-J.; Rösch, N. Size Dependence of the Adsorption Energy of CO on Metal Nanoparticles: A DFT Search for the Minimum Value. Nano Lett. 2012, 12, 2134–2139. (33) Yudanov, I. V.; Metzner, M.; Genest, A.; Rösch, N. Size-Dependence of Adsorption Properties of Metal Nanoparticles: A Density Functional Study on Palladium Nanoclusters. J. Phys. Chem. C 2008, 112, 20269–20275. (34) Hayashi, T.; Morikawa, Y.; Nozoye, H. Adsorption State of Dimethyl Disulfide on Au(111): Evidence for Adsorption as Thiolate at the Bridge Site. J. Chem. Phys. 2001, 114, 7615–7621. (35) Gottschalck, J.; Hammer, B. A Density Functional Theory Study of the Adsorption of Sulfur, Mercapto, and Methylthiolate on Au(111). J. Chem. Phys. 2002, 116, 784–790. (36) Ghiringhelli, L. M.; Caputo, R.; Site, L. D. Alkanethiol Headgroup on Metal (111)Surfaces: General Features of the Adsorption onto Group 10 and 11 Transition Metals. J. Phys: Condens. Mat. 2007, 19, 176004. 33



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(37) Yourdshahyan, Y.; Rappe, A. M. Structure and Energetics of Alkanethiol Adsorption on the Au(111) Surface. J. Chem. Phys. 2002, 117, 825–833. (38) Akinaga, Y.; Nakajima, T.; Hirao, K. A Density Functional Study on the Adsorption of Methanethiolate on the (111) Surfaces of Noble Metals. J. Chem. Phys. 2001, 114, 8555–8564. (39) Cometto, F. P.; Paredes-Olivera, P.; Macagno, V. A.; Patrito, E. M. Density Functional Theory Study of the Adsorption of Alkanethiols on Cu(111), Ag(111), and Au(111) in the Low and High Coverage Regimes. J. Phys. Chem. B 2005, 109, 21737–21748. (40) Barngrover, B. M.; Aikens, C. M. Incremental Binding Energies of Gold(I) and Silver(I) Thiolate Clusters. J. Phys. Chem. A 2011, 115, 11818–11823. (41) Kleis, J.; Greeley, J.; Romero, N.; Morozov, V.; Falsig, H.; Larsen, A.; Lu, J.; Mortensen, J.; DulĆak, M.; Thygesen, K.; Norskov, J.; Jacobsen, K. Finite Size Effects in Chemical Bonding: From Small Clusters to Solids. Catal. Lett. 2011, 141, 1067–1071. (42) Yudanov, I. V.; Sahnoun, R.; Neyman, K. M.; Rösch, N.; Hoffmann, J.; Schauermann, S.; Johánek, V.; Unterhalt, H.; Rupprechter, G.; Libuda, J.; Freund, H.-J. CO Adsorption on Pd Nanoparticles: Density Functional and Vibrational Spectroscopy Studies. J. Phys. Chem. B 2003, 107, 255–264. (43) Kacprzak, K. A.; Lopez-Acevedo, O.; Häkkinen, H.; Grönbeck, H. Theoretical Characterization of Cyclic Thiolated Copper, Silver, and Gold Clusters. J. Phys. Chem. C 2010, 114, 13571–13576. (44) Häkkinen, H. The Gold-Sulfur Interface At the Nanoscale. Nat. Chem. 2012, 4, 443– 455.

34


Page 34 of 38

Page 35 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(45) Tao, A.; Sinsermsuksakul, P.; Yang, P. Polyhedral Silver Nanocrystals with Distinct Scattering Signatures. Angew. Chem. Int. Ed. 2006, 45, 4597–4601. (46) Xia, Y.; Xiong, Y.; Lim, B.; Skrabalak, S. Shape-Controlled Synthesis of Metal Nanocrystals: Simple Chemistry Meets Complex Physics? Angew. Chem. Int. Ed. 2009, 48, 60–103. (47) Gracia-Pinilla, M.; Ferrer, D.; Mejia-Rosales, S.; Perez-Tijerina, E. Size-Selected Ag Nanoparticles with Five-Fold Symmetry. Nano. Res. Lett. 2009, 4, 896–902. (48) Tsuji, M.; Ogino, M.; Matsuo, R.; Kumagae, H.; Hikino, S.; Kim, T.; Yoon, S.-H. Stepwise Growth of Decahedral and Icosahedral Silver Nanocrystals in DMF. Cryst. Growth Des. 2010, 10, 296–301. (49) Henglein, A.; Giersig, M. Formation of Colloidal Silver Nanoparticles: Capping Action of Citrate. J. Phys. Chem. B 1999, 103, 9533–9539. (50) Morones, J. R.; Elechiguerra, J. L.; Camacho, A.; Holt, K.; Kouri, J. B.; Ramirez, J. T.; Yacaman, M. J. The Bactericidal Effect of Silver Nanoparticles. Nanotechnol. 2005, 16, 2346–2353. (51) Baletto, F.; Ferrando, R. Structural Properties of Nanoclusters: Energetic, Thermodynamic, and Kinetic Effects. Rev. Mod. Phys. 2005, 77, 371–423. (52) Angulo, A. M.; Noguez, C. Atomic Structure of Small and Intermediate-Size Silver Nanoclusters. J. Phys. Chem. A 2008, 112, 5834–5838. (53) González, A. L.; Noguez, C.; Beránek, J.; Barnard, A. S. Size, Shape, Stability, and Color of Plasmonic Silver Nanoparticles. J. Phys. Chem. C 2014, 118, 9128–9136. (54) Nuzzo, R. G.; Zegarski, B. R.; Dubois, L. H. Fundamental Studies of the Chemisorption of Organosulfur Compounds on Au( 111). Implications for Molecular Self-Assembly on Gold Surfaces. J. Am. Chem. Soc. 1987, 109, 733–740. 35



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(55) Sellers, H.; Ulman, A.; Shnidman, Y.; Eilerss, J. E. Structure and Binding of Alkanethiolates on Gold and Silver Surfaces: Implications for Self-Assembled Monolayer. J. Am. Chem. Soc. 1993, 115, 9389–9401. (56) Lee, K.-H.; Park, S.-H.; Kim, H. The Adsorption of Methanethiol and Benzenethiol on Silver Surface. Bull. Korean Chem. Soc. 1995, 16, 89–95. (57) Harris, A. L.; Rothberg, L.; Dubois, L. H.; Levinos, N. J.; Dhar, L. Molecular Vibrational Energy Relaxation at a Metal Surface: Methyl Thiolate on Ag(111). Phys. Rev. Lett. 1990, 64, 2086–2089. (58) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. SelfAssembled Monolayers of Thiolates on Metals as a Form of Nanotechnology. Chem. Rev. 2005, 105, 1103–1169. (59) Soler, J. M.; Artacho, E.; Gale, J. D.; Garcia, A.; Junquera, J.; Ordejon, P.; SanchezPortal, D. The SIESTA Method for Ab Initio Order-N Materials Simulation. J. Phys.: Condens. Matter 2002, 14, 2745–2779. (60) Sánchez-Castillo, A.; Noguez, C.; Garzón, I. L. On the Origin of the Optical Activity Displayed by Chiral-Ligand-Protected Metallic Nanoclusters. J. Am. Chem. Soc. 2010, 132, 1504–1505. (61) Hidalgo, F.; Cecilia Noguez, Optical Activity of Achiral Ligand SCH3 Adsorbed on Achiral Ag55 Cluseters: Relationship Between Adsorption Site and Circular Dichroism. ACSNano 2013, 7, 513–521. (62) Jiang, D.-e. The Expanding Universe of Thiolated Gold Nanoclusters and Beyond. Nanoscale 2013, 5, 7149–7160. (63) Lee, J.-G.; Lee, J.; Yates, J. T. Comparison of Methanethiol Adsorption on Ag(110)

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and Cu(110) Chemical Issues Related to Self-Assembled Monolayers. J. Phys. Chem. B 2004, 108, 1686–1693. (64) Heinzt, R.; Rabe, J. P. Scanning Tunneling Microscopy Investigation of Sulfide and Alkanethiolate Adlayers on Ag(111). Langmuir 1995, 11, 506–511. (65) R.J Culbertson, P. S., L. C. Feldman; oehm, H. B. A Computer Modeling Study of Perfect and Defective Silver (111) Surfaces. J. Phys. Chem. B 2003, 107, 3528–3534. (66) Hidalgo, F.; Noguez, C.; Olvera de la Cruz, M. Metallic Influence on the Atomic Structure and Optical Activity of Ligand-Protected Nanoparticles: A Comparison Between Ag and Au. Nanoscale 2014, 6, 3325–3334. (67) We used the same notation as reported in Ref. [68] for comparison, where they considered the Fermi level at the HOMO and it was set to zero. In our calculations the HOMO was found at −5.27 eV and the LUMO at 0.82 eV, therefore the Fermi level is at −2.22 eV. (68) Karhánek, D.; Bucko, T.; Hafner, J. A density-functional study of the adsorption of methane-thiol on the (111) surfaces of the Ni-group metals: II. Vibrational spectroscopy. J. Phys: Condens. Matter 2010, 22, 265005. (69) Hoffmann, R. A Chemical and Theoretical Way to Look at Bonding on Surfaces. Rev. Mod. Phys. 1988, 60, 601–628.

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Adsorption of a Methylthio Radical on Silver Nanoparticles: Size

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