Silicene: A Promising Surface to Achieve Morphological

Article pubs.acs.org/JPCC

Silicene: A Promising Surface to Achieve Morphological Transformation in Gold Clusters Krishnakanta Mondal,† C. Kamal,‡ Arup Banerjee,†,§ Aparna Chakrabarti,†,‡ and Tapan K. Ghanty*,∥ †

Homi Bhabha National Institute, ‡Indus Synchrotrons Utilization Division, and §BARC Training School at RRCAT, Raja Ramanna Centre for Advanced Technology, Indore 452013, India ∥ Theoretical Chemistry Section, Chemistry Group, Bhabha Atomic Research Centre, Mumbai 400085, India ABSTRACT: In recent years, gold clusters supported on solid materials have attracted considerable attention because of their various potential applications. With the objective of stabilizing the energetically less stable but catalytically more active planar form of finite sized gold clusters on some suitable solid support, we have theoretically investigated the morphological transformation of gold dimer, trimer, and Au20 clusters on pristine silicene/ Ag(111) surface. In contrast to previous schemes which have been suggested for the stabilization of planar structures by appropriate surfaces, the present proposal does not require any external influence such as doping or application of external electric field. In fact, we have exploited the special characteristics of the silicene surface, namely, buckled nature and weak Si−Si π bonding, which enable this surface to make strong Si−Au covalent bonds resulting in the two-dimensional planar Au20 isomer being more stable than its three-dimensional tetrahedral structure. In contrast to this, as already reported in the literature, the planar isomer of the Au20 cluster is not energetically favored when it is adsorbed on a pure graphene surface. Moreover, Bader charge density analysis indicates that the amount of charge transfer from the silicene surface to the planar Au20 cluster is considerably high, thereby increasing the possibility of enabling this composite system to act as a better catalyst. It would be interesting to investigate the adsorption of gold clusters on the silicene surface experimentally for the verification of our theoretical prediction. as a support;21,22,25 (ii) by using aluminum and nitrogen atom doped MgO and graphene surfaces, respectively, as supports;23,24 and (iii) by growing the system of cluster and MgO support in the presence of an electric field of strength 1 V/nm.26 The morphological transformation from 3D to 2D isomer of Au20 on these modified surfaces is attributed to the charge transfer from the substrate to the cluster. Moreover, it has been shown that the excess charge on the supported 2D isomer of Au20 cluster (Au20-P) makes it the better catalytic agent for carrying out the environmentally important CO oxidation reaction. However, it is experimentally observed that the growth of continuous ultrathin film (1−3 layers) of MgO is not possible.22 Moreover, doping has its own intrinsic complexity, and the use of external electric field to stabilize the planar form of Au20 gold cluster (Au20-P) is also not practicable for actual applications. It is then natural to search for a defect-free undoped surface that can stabilize the Au20-P cluster even in the absence of any external field. Here, we note that although graphene doped with nitrogen is capable of transforming the morphology of Au20 cluster, its pure form fails to transform the morphology.24 Recently, silicon-based 2D structures analogous to graphene has attracted many researchers because of their novel properties.27−32 Unlike graphene, this 2D structure of silicon, which is termed silicene, has a buckled honeycomb arrangement of Si atoms (as shown

1. INTRODUCTION Gold is one of the most unique elements in the periodic table, with the largest relativistic effect among all elements with Z < 100.1,2 The structure and properties of finite sized gold clusters are significantly altered because of the strong relativistic effect as compared to the corresponding copper and silver clusters, which are less affected.3−5 For instance, in gold clusters (Aun) 2-dimensional (2D) to 3-dimensional (3D) structural transition occurs for larger sizes (n > 11) as compared to the other two coinage metal clusters because of relativistically enhanced strong sd hybridization and d−d interaction.6,7 Moreover, larger gold clusters like Au20, Au32, Au55, etc. adopt 3D highly symmetrical tetrahedral or icosahedral structures.8−11 Among all the gold clusters investigated so far, Au20 is one of the most unique clusters and has been studied extensively because of its high symmetry and very high thermodynamic and chemical stabilities.8,9,12−17 In the ground state, Au20 possesses pyramidal (Au20-T) structure with a very high HOMO−LUMO gap of 1.77 eV.8 As a result, this cluster is less reactive and hence not very useful for catalytic reactions.14,18,19 On the other hand, if Au20 cluster had a 2D planar structure (Au20-P), it would exhibit catalytic activity better than its 3D counterpart.20−22 However, in gas phase, Au20-T is found to be energetically more stable (1.5−2.0 eV)8,9,12,23 than Au20-P; hence, a few studies have explored the possibility of tuning or modifying the 3D geometry of Au20 cluster by anchoring it to different kinds of surfaces.21−26 It has been shown that the stabilization of Au20-P can be achieved mainly in three possible ways: (i) by using an ultrathin layer of MgO on underlying metal surface (Mo(100)) © XXXX American Chemical Society

Received: December 1, 2014 Revised: January 15, 2015

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DOI: 10.1021/jp5119579 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

out using 250 eV of energy cutoff for plane waves and a k-mesh of 3 × 3 × 1.33 To study the adsorption of the Au20 cluster, we consider a 2 × 2 super cell of the previously constructed silicene/Ag(111) surface so that the interaction between the two neighboring images of the Au20 cluster is negligible. Because of the large size of this system, calculations corresponding to Au20/silicene/Ag(111) systems have been carried out using a single k-point. The size of the unit cells for Au20-P and Au20-T adsorbed on silicene/Ag(111) are taken to be 23.35 × 23.35 × 30 Å3 and 23.35 × 23.35 × 37 Å3, respectively. For all the calculations, we apply dipole corrections as implemented in MedeA-VASP and the tolerances for energy and force are chosen to be 10−5 eV and 0.02 eV/Å, respectively. As the dispersion forces are likely to play an important role in determining the properties of the cluster−cluster and cluster− surface systems,44−50 we use Grimme’s dispersion correction51 along with DFT (PBE + D2 method as implemented in VASP 5.3.5)39,52 to take into account the effect of van der Waals (vdW) interaction between the cluster and the surface. Note that for gold the values of C6 coefficient and R0 (vdW radius) of Au atom are not included in the program suit. For the present calculations we use the values of C6 and R0 for Au as 40.62 J nm6 mol−1 and 1.772 Å, respectively.50

in Figure 1). This buckled silicene has also been grown on various kinds of surfaces like Ag(111), ZrB2(0001), Ir(111),

Figure 1. (a) Top view and (b) side view of the free-standing, hexagonal, buckled 3 × 3 unit cell of silicene surface.

and Au(110).33−36 The buckling in silicene arises because of the mixing of sp2 and sp3 hybridizations rather than the purely sp2 hybridization present in graphene. Moreover, the strength of π bonds between two 3pz orbitals in silicene is weaker than the corresponding bond in graphene formed between two 2pz orbitals. Therefore, it is expected that any adsorbate on a silicene surface can easily break the π bonds and can form covalent-like bonds with silicon atoms. In this context, recently it has been shown that when a single Au atom is placed on a silicene surface, it binds very strongly to the surface.37 Subsequently, it has been revealed that the silicene supported single Au atom is also catalytically very active for the CO oxidation reaction.38 In the light of the above discussion, it is quite natural to explore whether a pure silicene surface can be employed for the stabilization of catalytically active planar structures of gold clusters. Moreover, it remains to be investigated whether the buckling present in silicene has any role in stabilizing the 2D isomer of gold clusters. Therefore, in the present paper our objective is to investigate the morphology and energetics of small (Aun, n = 1−3) as well as large (Au20) gold clusters adsorbed on silicene surface by using density functional calculations. We consider silicene surface supported on Ag(111) for all of our calculations in accordance with a recent experimental report.33 The rest of the paper is arranged in the following manner. In the next section we provide a description of the computational methodology employed for the calculations presented here. We discuss our main results in section 3, and the paper is summarized in section 4.

3. RESULTS AND DISCUSSION To check the reliability of our theoretical approach we first calculate the adsorption energy of a single Au atom adsorbed on a free-standing silicene. To this end we compare our results with the theoretical data already available in the literature.37,38 The optimized geometry of a 3 × 3 unit cell of silicene, which is employed for studying adsorption of a single Au atom, is shown in Figure 1. In this figure, D and U represent the two different Si atoms lying on two planes in silicene, and H indicates the center of the hexagon. To study the adsorption of a single Au atom, we place it on these three above-mentioned locations. The optimization of Au adsorbed on the 3 × 3 unit cell of silicene has been carried out with energy cutoff of 400 eV and k-mesh of 7× 7 × 1. In Table 1 we present the results of our Table 1. Calculated Results for the Adsorption of a Single Au Atom on the Free-Standing Silicene along with the Corresponding Reported Values adsorption energy (Eads)a

2. COMPUTATIONAL DETAILS We employ the density functional theory (DFT) in combination with projector augmented wave method as implemented in MedeA-VASP for all of our calculations.39−42 The exchange-correlation (XC) functional is approximated within the generalized gradient approximation (GGA) as proposed by Perdew, Burke, and Ernzerhof (PBE).43 Energy cutoff of 500 eV is used to obtain the converged results in the case of all the free gold clusters. To construct the silicene/ Ag(111) surface, we consider a 3 × 3 unit cell of silicene deposited on five layers of 4 × 4 unit cell of Ag(111).37 For optimization of this structure we allow all the atoms (Si and Ag) to relax except for the Ag atoms which reside within the bottom three layers of Ag(111). We find that the geometry of the silicene on Ag(111) does not change significantly when we freeze one, two, or three bottom layers of Ag(111). The smaller gold clusters (Aun, n = 1−3) are placed on the optimized silicene/Ag(111) surface. These calculations have been carried

adsorption sites

present results (eV)

reported (eV)b

H U D

2.39 1.99 2.21

2.316 1.977 2.159

a Eads = −{E(Au/surface) − E(surface) − E(Au)}, where E(Au/ surface), E(surface), and E(Au) are the energies of the Au atom deposited on surface (silicene), the bare surface, and the free single Au atom, respectively. bValues are taken from ref 37.

calculations along with the reported data. We observe that our results for the adsorption of a single Au atom on free-standing silicene match well with the previously reported data.37 Furthermore, in accordance with our aim of considering experimentally grown silicene/Ag(111) surface for studying the adsorption of gold clusters, we optimize the structure of the silicene/Ag(111) surface. We find that the silicene surface is buckled by 0.77 Å, which agrees well with both experimental B


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The Journal of Physical Chemistry C (0.75 Å)33 and theoretical (0.77 Å)33,37 results reported in the literature. We now proceed with the discussion of our results for the adsorption of various isomers of Aun (n = 1−3, and 20) clusters on the silicene/Ag(111) surface. To quantify the strength of interaction between the cluster and the surface (silicene/ Ag(111)), we calculate the interaction energy given by E i = −{E(cluster/surface) − E(cluster) − E(surface)} (1)

where E(cluster/surface), E(cluster), and E(surface) are the energies of the gold cluster adsorbed on silicene/Ag(111), bare gold cluster, and bare surface (silicene/Ag(111)), respectively, with the geometries the same as those in the composite system. To study the adsorption of the gold clusters, we consider different possible adsorption sites on the silicene/Ag(111) surface. For a single Au atom we find that the center of the hexagon of the silicene surface is the most favorable adsorption site. The interaction energy of a single Au atom on silicene/ Ag(111) is found to be 3.13 eV. To explain this strong bonding between silicene and Au ,we calculate the difference charge density by using ρdifference = ρAu/cluster+surface − ρsurface − ρAu/cluster (2) where ρAu/cluster+surface, ρsurface, and ρAu/cluster, are the charge densities of the gold cluster/Au-atom adsorbed on silicene/ Ag(111), bare surface (silicene/Ag(111)), and the bare gold cluster/Au-atom with the geometries the same as those in the composite system, respectively. We display the ρdifference for a single Au atom adsorbed on silicene/Ag(111) in Figure 2.

Figure 3. (a) Horizontal and (b) vertical configurations for Au2 and (c) horizontal and (d) vertical configurations for Au3. All are placed on silicene/Ag(111) surface. The values at the bottom indicate the relative energies of the respective configurations. The relative energy (Er, in eV) values are calculated with respect to the horizontal configuration of the respective cluster. The values outside the square brackets are obtained with the PBE XC-functional, and the numbers within the square bracket are calculated using the PBE+D2 method. Gray, blue, and yellow balls represent Ag, Si, and Au atoms, respectively.

these configurations are 1.05 and 0.44 eV lower for Au2 and Au3, respectively, as compared to their corresponding vertical counterparts. To take into account the effect of van der Waals interaction between the cluster and the surface, we use the PBE +D2 method and calculate the energies of the Aun (n = 2, 3) clusters deposited on the silicene/Ag(111) surface. We find that the horizontal configurations of the Aun clusters become more stable because of the incorporation of the van der Waals interaction, over its vertical counterparts (Figure 3). This may be attributed to the fact that more Au atoms are attached to the silicene/Ag(111) surface for the horizontal configuration than for its vertical counterpart. To get more insight into the effect of silicene/Ag(111) on the stability and geometry of the adsorbed Au2 and Au3 clusters, we calculate the interaction energies with and without Grimme’s dispersion correction and various bond lengths (Au−Au and Au−Si) and compare them with the binding energies and bond lengths of the free Au2 and Au3 clusters. The results of these calculations are compiled in Table 2. From this table we observe that the bond lengths of both Au2 and Au3 clusters increase as compared to their gas-phase values when they are placed on the silicene/Ag(111) surface. For Au2, the increments in bond lengths for horizontal and vertical configurations are 0.259 and 0.047 Å, respectively. Interestingly, the equilateral geometry (gas phase) of Au3 transforms into isosceles when it is adsorbed on the silicene/Ag(111) surface in

Figure 2. Difference charge density (ρdifference) with isosurface value of 0.003 e/Å3: (a) top view and (b) side view for Au/silicene/Ag(111) system. Red and green indicate the accumulation and depletion of charge, respectively. Gray, blue, and yellow balls represent Ag, Si, and Au atoms, respectively.

From this figure we observe that the accumulation of the excess charge occurs at the interface between Au and the silicene surface, but closer to the Au atom. This indicates that there is a strong covalent-like bonding between Au and Si atoms on the silicene/Ag(111) surface. Having discussed the adsorption of a single gold atom on silicene/Ag(111) surface, we now focus our attention on the adsorption of Au2 and Au3 clusters. For this purpose we choose two initial configurations of the clusters, namely, horizontal (plane of the cluster is parallel to the surface) and vertical (plane of cluster is perpendicular to the surface) to study their adsorptions. We find that various horizontal and vertical configurations are possible for both Au2 and Au3 clusters. The two most stable horizontal and vertical configurations for both Au2 and Au3 clusters are shown in Figure 3. It is observed that for both Au2 and Au3 their horizontal configurations adsorbed on silicene/Ag(111) are the most stable ones. Energetically, C


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Table 2. Interaction Energies with and without Grimme’s Dispersion Correction and Average Au−Au and Au−Si Bond Distances (dAu−Au and dAu−Si, Respectively) for the Deposited Aun (n = 2, 3) Clusters for two Different Adsorption Configurations (Horizontal and Vertical) Along with the Average Au−Au Bond Distances (dgAu−Au) and Binding Energies (Eb) of the Gas-Phase Aun (n = 2, 3) Clusters horizontala deposited cluster

dAu−Au (Å)

dAu−Si (Å)

Au2 Au3

2.776 2.852

2.419 2.436

Ei

PBE

verticala (eV)

3.39 5.46

Ei

PBE+D2

(eV)

4.45 6.59

dAu−Au (Å)

dAu−Si (Å)

EiPBE (eV)

EiPBE+D2 (eV)

dgAu−Au (Å)

Eb (eV)b

2.564 2.666, 2.876

2.462 2.312

2.39 4.60

2.94 5.32

2.517 2.673

2.26 3.53

a

EiPBE+D2 and EiPBE indicate the interaction energies obtained using PBE XC-functional with and without Grimme’s dispersion correction, respectively. bEb = −{E(cluster) − nE(Au)}, where E(cluster) and E(Au) are the energies of the free cluster and the Au atom, respectively, and n represents the total number of Au atoms in the cluster.

Ag(111) surface. To explore this possibility for larger clusters, we consider two isomers of Au20 cluster (Au20-P and Au20-T) with different dimensionalities (2D and 3D) and study their adsorptions on the silicene/Ag(111) surface. The optimized geometries of the Au20-P and Au20-T clusters adsorbed on silicene/Ag(111) surface are given in Figure 4. We note here

the vertical configuration. This is because the increase in the three Au−Au bonds in the Au3 cluster is not isometric. The Au−Au bond parallel to the surface increases by 0.203 Å, whereas the other two Au−Au bonds remain almost unchanged for Au3 adsorbed on silicene/Ag(111). On the other hand, in the case of the horizontal configuration of the adsorption of Au3 on silicene/Ag(111), the equilateral triangular nature of Au3 remains intact. Here, the increase in the Au−Au bond is 0.179 Å. Furthermore, to investigate the effect of van der Waals forces on the geometries of the adsorbed Aun clusters, we carry out geometry optimization calculations using the PBE+D2 method. We find that the geometries of the deposited clusters obtained using the PBE+D2 method are very close to the respective structures obtained with only the PBE functional. However, we note that the Au−Au bond length with PBE+D2 increases by around 0.01 Å over its PBE value, whereas the corresponding Au−Si bond length decreases by around 0.02 Å. Having discussed the structures of the adsorbed clusters on the silicene/Ag(111) surface, we now focus our attention on the interaction energy between the Aun clusters and the silicene/ Ag(111) surface. For this purpose we employ eq 1 to calculate the interaction energies. The results for interaction energies obtained using the PBE XC-functional both with and without Grimme’s dispersion correction are compiled in Table 2. From this table it is very clear that the incorporation of dispersion correction increases the interaction energy by around 1 eV (0.6 eV) for horizontal (vertical) configurations of Au2 and Au3 clusters. This result indicates that the van der Waals interaction is important for gold/silicene/Ag(111) systems and it leads to the higher stability of the horizontal configurations as compared to the vertical configuration of gold/silicene/Ag(111) systems. Moreover, we find that the interaction energies of the Au2 and Au3 clusters are higher than their binding energies (gas phase). This indicates that the Aun (n = 2, 3) clusters are more stable when they are placed on the silicene/Ag(111) surface. Furthermore, it is observed that for both Au2 and Au3 the interaction energy for the horizontal configuration is around 1 eV higher than that of their vertical counterparts. Higher stability of the horizontal configurations may be attributed to the strong covalent-like bonding between the Au and Si atoms.53,54 Here, it is important to note that the binding energy of AuSi dimer is 3.34 eV (experimental value is 3.26 eV),55 which is significantly higher than that of the Au2 dimer (2.26 eV). This larger binding energy of the AuSi dimer favors all the Au atoms in a cluster to lie on the silicene surface to form covalent-like bonds with the Si atoms, resulting in highly stable horizontal configurations. From the above discussion we expect that the planar isomer of the gold clusters may be stabilized over their threedimensional counterparts by depositing them on the silicene/

Figure 4. Optimized geometries of (a) Au20-P and (b) Au20-T. Both are placed on silicene/Ag(111) surface. The values at the bottom indicate the relative energies of the respective configurations. The relative energy (Er, in eV) values are calculated with respect to Au20-P. The values outside the square brackets are obtained with PBE XCfunctional, and the numbers within the square brackets are calculated using the PBE+D2 method. Gray, blue, and yellow balls represent Ag, Si, and Au atoms, respectively.

that the free Au20-T cluster is more stable by 1.61 eV as compared to the Au20P isomer (note that the reported values in the literature are in the range of 1.5−2.0 eV).8,9,12,23 Interestingly, when these isomers of the Au20 cluster are adsorbed on the silicene/Ag(111) surface, the abovementioned energy ordering gets reversed. We observe that Au20-P becomes 2.43 eV more stable than Au20-T when they are adsorbed on the silicene/Ag(111) surface. Note that in the PBE+D2 method the relative stability of Au20-P with respect to Au20-T increases to 3.99 eV. This indicates that the van der Waals interaction provides additional stability to the planar isomer of Au20 compared to its tetrahedral counterpart. To quantify the interaction between the cluster and surface, we calculate the interaction energy as defined in eq 1. The interaction energy between Au20-P and silicene/Ag(111) is found to be 18.73 eV, which is around 4 eV higher than that between Au20-T and the same surface (14.64 eV). Note that in the PBE+D2 method the interaction energies between the two isomers of the Au20 cluster and silicene/Ag(111) surface are found to be 26.22 and 19.57 eV for Au20-P and Au20-T, respectively. The high stability of the Au20-P cluster on silicene/ Ag(111) can be attributed to the fact that all 20 Au atoms make D


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To quantify the excess amount of charge acquired by the gold atoms, we perform Bader charge analysis. From this analysis we find that the amount of charge transferred from the surface to the Au20-P and Au20-T clusters are 0.26e and 0.18e, respectively, per Au atom. It is reported in the literature that the Au atoms with excess charge will serve as active sites for the adsorption of CO and O2 molecules.14,56,57 As Au20-P provides more active sites associated with higher amount of negative charge than Au20-T, it is then expected that the 2D planar isomer will exhibit higher catalytic activity for oxidation reactions as compared to the 3D counterpart. Here, we note that the amount of charge accumulation observed in the present study is much higher than values reported in the literature.21−26 Hence, we expect that Au20-P supported on silicene/Ag(111) will act as more efficient catalysts than clusters on previously reported supports.21−26

bonds with the surface, whereas for Au20-T, because of tetrahedral geometry, only 50% of the 20 Au atoms which lie on the base of the tetrahedron can form Au−Si bonds with Si atoms on the silicene surface. Thus, we find that a pristine silicene surface deposited on Ag(111) can be used to stabilize the 2D Au20 cluster over its 3D counterpart. Moreover, we note that the strong Au−Si bond may lead to wetting of the silicene surface by the gold atoms. We emphasize here that unlike silicene, the stabilization of 2D gold clusters over the 3D counterparts on the pristine graphene surface is found to be energetically not favorable.24 The origin of this distinct difference between the nature of interaction of gold clusters with graphene and silicene can be attributed to the weak π bonds in the buckled silicene surface,31,32 which can easily be broken to form strong Au−Si covalent-like bonds. Consequently, the buckled silicene surface leads to the stabilization of the catalytically active 2D planar Au20 cluster over its highly symmetric tetrahedral 3D counterpart when adsorbed on the silicene/Ag(111) surface. On the other hand, the pristine graphene surface is unable to stabilize the 2D isomer as compared to the corresponding 3D structure because of the presence of strong C−C π bonds and absence of any buckling in it. Furthermore, to gain more insight into the nature of bonding between the surface and the Au20 cluster, we calculate the difference charge density (ρdifference) for each isomers adsorbed on the silicene/Ag(111) surface by using eq 2. The results of these calculations are displayed in Figure 5. From this figure we

4. CONCLUSION Our density functional investigation demonstrates that the strong covalent-like bonding between the Au and Si atoms tend to keep all the Au atoms of the clusters directly attached to the pure silicene/Ag(111) surface. This leads to the stabilization of the catalytically active planar Au20 cluster over its highly symmetric tetrahedral counterpart when adsorbed on the silicene/Ag(111) surface. Note that this stabilization of planar structure over the tetrahedral counterpart is further enhanced when van der Waals interaction between the cluster and the surface is taken into account. In contrast to the pure graphene surface, the buckling nature of the pristine silicene surface associated with weak π bonds is largely responsible for the lowering of the energy of Au20-P isomer as compared to that of the Au20-T structure. It would indeed be interesting to investigate various chemical reactions on silicene supported nanoclusters of gold. The results reported in the present work clearly indicate that the recently discovered silicene surface has the potential to change the present landscape of gold-based catalysts and stimulate further experimental studies.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: 0091-22-25505151. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS K.M. gratefully acknowledges HBNI RRCAT for financial support. K.M. also acknowledges Dr. H. S. Rawat and Mr. P. K. Thander for their encouragements and kind assistance. K.M. acknowledges Mr. A. K. Das and Dr. S. Pal for useful discussions. Authors acknowledge Dr. P. K. Gupta, Dr. B. N. Jagatap, and Dr. G. S. Lodha for their encouragement and support. Computer Centre, RRCAT, and BARC computer division are gratefully acknowledged for providing computation facilities.

Figure 5. Difference charge density (ρdifference) with isosurface value of 0.002 e/Å3: (a) top view and (b) side view for Au20-P and (c) top view and (d) side view for Au20-T, both adsorbed on silicene/Ag(111) surface. Red and green indicate the accumulation and depletion of charges, respectively. Gray, blue, and yellow balls represent Ag, Si, and Au atoms, respectively.

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observe that the accumulation of the charge takes place at the interface region between the cluster and the surface, but closer to the Au atoms. This clearly indicates the presence of strong covalent-like bonding between Au and Si atoms, as was also observed for the smaller gold cluster. Moreover, we observe from Figure 5 that the accumulation of charges occurs around all 20 atoms in Au20-P, whereas only 10 atoms of Au20-T located nearest to the surface acquire the excess charges.

REFERENCES

(1) Pyykkö, P.; Desclaux, J. P. Relativity and the Periodic System of Elements. Acc. Chem. Res. 1979, 12, 276−281. (2) Pyykkö, P. Theoretical Chemistry of Gold. Angew. Chem., Int. Ed. 2004, 43, 4412−4456. (3) Häkkinen, H.; Moseler, M.; Landman, U. Bonding in Cu, Ag, and Au Clusters: Relativistic Effects, Trends, and Surprises. Phys. Rev. Lett. 2002, 89, 033401-1−033401-4.

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Supported MgO Films. Phys. Rev. Lett. 2008, 100, 056102-1−0561024. (27) Cahangirov, S.; Topsakal, M.; Akturk, E.; Sahin, H.; Ciraci, S. Two- and One-Dimensional Honeycomb Structures of Silicon and Germanium. Phys. Rev. Lett. 2009, 102, 236804-1−236804-4. (28) Sahin, H.; Cahangirov, S.; Topsakal, M.; Bekaroglu, E.; Akturk, E.; Senger, R. T.; Ciraci, S. Monolayer Honeycomb Structures of Group-IV Elements and III-V Binary Compounds: First-Principles Calculations. Phys. Rev. B: Condens. Matter Mater. Phys. 2009, 80, 155453-1−155453-12. (29) Ezawa, M. Valley-Polarized Metals and Quantum Anomalous Hall Effect in Silicene. Phys. Rev. Lett. 2012, 109, 055502-1−055502-5. (30) Houssa, M.; Pourtois, G.; Afanasev, V. V.; Stesmans, A. Can Silicon Behave Like Graphene? A First-Principles Study. Appl. Phys. Lett. 2011, 97, 112106-1−112106-3. (31) Kamal, C.; Chakrabarti, A.; Banerjee, A.; Deb, S. K. Silicene beyond Mono-LayersDifferent Stacking Configurations and Their Properties. J. Phys.: Condens. Matter 2013, 25, 085508-1−085508-10. (32) Jose, D.; Datta, A. Structures and Chemical Properties of Silicene: Unlike Graphene. Acc. Chem. Res. 2014, 47, 593−602. (33) Vogt, P.; Padova, P. D.; Quaresima, C.; Avila, J.; Frantzeskakis, E.; Asensio, M. C.; Resta, A.; Ealet, B.; Lay, G. L. Silicene: Compelling Experimental Evidence for Graphenelike Two-Dimensional Silicon. Phys. Rev. Lett. 2012, 108, 155501-1−155501-5. (34) Fleurence, A.; Friedlein, R.; Ozaki, T.; Kawai, H.; Wang, Y. Experimental Evidence for Epitaxial Silicene on Diboride Thin Films. Phys. Rev. Lett. 2012, 108, 245501-1−245501-5. (35) Enriquez, H.; Mayne, A.; Kara, A.; Vizzini, S.; Roth, S.; Lalmi, B.; Seitsonen, A. P.; Aufray, B.; Greber, T.; Belkhou, R.; Dujardin, G.; Oughaddou, H. Adsorption of Silicon on Au(110): An Ordered Two Dimensional Surface Alloy. Appl. Phys. Lett. 2012, 101, 021605-1− 021605-5. (36) Meng, L.; Wang, Y.; Zhang, L.; Du, S.; Wu, R.; Li, L.; Zhang, Y.; Li, G.; Zhou, H.; Hofer, W. A.; Gao, H. J. Buckled Silicene Formation on Ir(111). Nano Lett. 2013, 13, 685−690. (37) Lin, X.; Ni, J. Much Stronger Binding of Metal Adatoms to Silicene than to Graphene: A First-Principles Study. Phys. Rev. B: Condens. Matter Mater. Phys. 2012, 86, 075440-1−075440-9. (38) Li, C.; Yang, S.; Li, S. S.; Xia, J. B.; Li, J. Au-Decorated Silicene: Design of a High-Activity Catalyst toward CO Oxidation. J. Phys. Chem. C 2013, 117, 483−488. (39) Kresse, G.; Furthmuller, J. Efficient Iterative Schemes for ab Initio Total-energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 11169−11186. (40) VASP 5.2 programme package used here is fully integrated in the MedeA platform (Materials Design, Inc.) with a graphical user interface enabling the computation of the properties. (41) Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method. Phys. Rev. B: Condens. Matter Mater. Phys. 1999, 59, 1758−1775. (42) Blöchl, P. E. Projector Augmented-Wave Method. Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 50, 17953−17979. (43) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (44) Banerjee, A.; Chakrabarti, A.; Ghanty, T. K. Time-dependent Density Functional Theory Calculation of van der Waals Coefficient of Sodium Clusters. J. Chem. Phys. 2007, 127, 134103-1−134103-7. (45) Banerjee, A.; Autschbach, J.; Chakrabarti, A. Time-dependent Density-Functional-Theory Calculation of the van der Waals Coefficient C6 of Alkali-Metal Atoms Li, Na, K; Alkali-Metal Dimers Li2, Na2, K2; Sodium Clusters Nan; and Fullerene C60. Phys. Rev. A: At., Mol., Opt. Phys. 2008, 78, 032704-1−032704-9. (46) Kamal, C.; Ghanty, T. K.; Banerjee, A.; Chakrabarti, A. The van der Waals Coefficients between Carbon Nanostructures and Small Molecules: A Time-Dependent Density Functional Theory Study. J. Chem. Phys. 2009, 131, 164708-1−164708-11. (47) Tao, J.; Perdew, J. P. Communication: Non-additivity of van der Waals Interactions Between Nanostructures. J. Chem. Phys. 2014, 141, 141101-1−141101-4.

(4) De, H. S.; Krishnamurty, S.; Pal, S. Density Functional Investigation of Relativistic Effects on the Structure and Reactivity of Tetrahedral Gold Clusters. J. Phys. Chem. C 2009, 113, 7101−7106. (5) Thayer, J. S. Relativistic Effects and the Chemistry of the Heaviest Main-Group Elements. J. Chem. Educ. 2005, 82, 1721. (6) Xing, X.; Yoon, B.; Landman, U.; Parks, J. H. Structural Evolution of Au Nanoclusters: From Planar to Cage to Tubular Motifs. Phys. Rev. B: Condens. Matter Mater. Phys. 2006, 74, 165423-1−165423-6. (7) Bonačić-Koutecký, V.; Burda, J.; Mitrić, R.; Ge, M.; Zampella, G.; Fantucci, P. Density Functional Study of Structural and Electronic Properties of Bimetallic Silver−Gold Clusters: Comparison with Pure Gold and Silver Clusters. J. Chem. Phys. 2002, 117, 3120−3131. (8) Li, J.; Li, X.; Zhai, H.; Wang, L. S. Au20: A Tetrahedral Cluster. Science 2003, 299, 864−867. (9) Gruene, P.; Rayner, D. M.; van der Meer, A. F. G.; Lyon, J. T.; Meijer, G.; Fielicke, A. Structures of Neutral Au7, Au19, and Au20 Clusters in the Gas Phase. Science 2008, 321, 674−676. (10) Johansson, M. P.; Sundholm, D.; Vaara, J. Au32: A 24-Carat Golden Fullerene. Angew. Chem., Int. Ed. 2004, 43, 2678−2681. (11) Schmid, G.; Meyer-Zaika, W.; Pugin, R.; Sawitowski, T.; Majoral, J. P.; Caminade, A. M.; Turrin, C. O. Naked Au55 Clusters: Dramatic Effect of a Thiol-Terminated Dendrimer. Chem.Eur. J. 2000, 6, 1693−1697. (12) Kryachko, E. S.; Remacle, F. The Magic Gold Cluster Au20. Int. J. Quantum Chem. 2007, 107, 2922−2934. (13) Aikens, C. M.; Scatz, G. C. TDDFT Studies of Absorption and SERS Spectra of Pyridine Interacting with Au20. J. Phys. Chem. A 2006, 110, 13317−13324. (14) Molina, L. M.; Hammer, B. The Activity of the Tetrahedral Au20 Cluster: Charging and Impurity Effects. J. Catal. 2005, 233, 399−404. (15) Ghanty, T. K.; Banerjee, A.; Chakrabarti, A. Structures and the Electronic Properties of Au19X Clusters (X = Li, Na, K, Rb, Cs, Cu, Ag). J. Phys. Chem. C 2010, 114, 20−27. (16) Banerjee, A.; Ghanty, T. K.; Chakrabarti, A.; Kamal, C. Nonlinear Optical Properties of Au19M (M = Li, Na, K, Rb, Cs, Cu, Ag) Clusters. J. Phys. Chem. C 2012, 116, 193−200. (17) Wang, J.; Wang, G.; Zhao, J. Structures and Electronic Properties of Cu20, Ag20, and Au20 Clusters with Density Functional Method. Chem. Phys. Lett. 2003, 380, 716−720. (18) Gao, Y.; Shao, N.; Pei, Y.; Chen, Z.; Zeng, X. C. Catalytic Activities of Subnanometer Gold Clusters (Au16−Au18, Au20, and Au27−Au35) for CO Oxidation. ACS Nano 2011, 5, 7818−7829. (19) Mondal, K.; Banerjee, A.; Ghanty, T. K. Structural and Chemical Properties of Subnanometer-Sized Bimetallic Au19Pt Cluster. J. Phys. Chem. C 2014, 118, 11935−11945. (20) Martínez, A. Size Matters, but Is Being Planar of Any Relevance? Electron Donor-Acceptor Properties of Neutral Gold Clusters up to 20 Atoms. J. Phys. Chem. C 2010, 114, 21240−21246. (21) Zhang, C.; Yoon, B.; Landman, U. Predicted Oxidation of CO Catalyzed by Au Nanoclusters on a Thin Defect-Free MgO Film Supported on a Mo(100) Surface. J. Am. Chem. Soc. 2007, 129, 2228− 2229. (22) Harding, C.; Habibpour, V.; Kunz, S.; Antonietti, J.-M.; Farnbacher, A. N.-S.; Heiz, U.; Yoon, B.; Landman, U. Control and Manipulation of Gold Nanocatalysis: Effects of Metal Oxide Support Thickness and Composition. J. Am. Chem. Soc. 2009, 131, 538−548. (23) Mammen, N.; Narasimhan, S.; Gironcoli, S. Tuning the Morphology of Gold Clusters by Substrate Doping. J. Am. Chem. Soc. 2011, 133, 2801−2803. (24) Yamijala, S. SRKC; Bandyopadhyay, A.; Pati, S. K. NitrogenDoped Graphene Quantum Dots as Possible Substrates to Stabilize Planar Conformer of Au20 over Its Tetrahedral Conformer: A Systematic DFT Study. J. Phys. Chem. C 2014, 118, 17890−17894. (25) Ricci, D.; Bongiorno, A.; Pacchioni, G.; Landman, U. Bonding Trends and Dimensionality Crossover of Gold Nanoclusters on MetalSupported MgO Thin Films. Phys. Rev. Lett. 2006, 97, 036106-1− 036106-4. (26) Yoon, B.; Landman, U. Electric Field Control of Structure, Dimensionality, and Reactivity of Gold Nanoclusters on MetalF


Article

The Journal of Physical Chemistry C (48) Hatz, R.; Hänninen, V.; Halonen, L. Dispersion Interactions in Small Zinc, Cadmium, and Mercury Clusters. J. Phys. Chem. A 2014, 118, 5734−5740. (49) Girifalco, L. A.; Hodak, M. van der Waals Binding Energies in Graphitic Structures. Phys. Rev. B: Condens. Matter Mater. Phys. 2002, 65, 125404-1−125404-5. (50) Amft, M.; Lebègue, S.; Eriksson, O.; Skorodumova, N. V. Adsorption of Cu, Ag, and Au Atoms on Graphene Including van der Waals Interactions. J. Phys.: Condens. Matter. 2011, 23, 395001-1− 395001-10. (51) Grimme, S. Semiempirical GGA-type Density Functional Constructed with a Long-range Dispersion Correction. J. Comput. Chem. 2006, 27, 1787−1799. (52) Bučko, T.; Hafner, J.; Lebègue, S.; À ngyán, J. G. Improved Description of the Structure of Molecular and Layered Crystals: Ab Initio DFT Calculations with van der Waals Corrections. J. Phys. Chem. A 2010, 114, 11814−11824. (53) Barysz, M.; Pyykkö, P. Strong Chemical Bonds to Gold. High Level Correlated Relativistic Results for Diatomic AuBe+, AuC+, AuMg+, and AuSi+. Chem. Phys. Lett. 1998, 285, 398−403. (54) Kiran, B.; Li, X.; Zhai, H. J.; Cui, L. F.; Wang, L. S. [SiAu4]: Aurosilane. Angew. Chem., Int. Ed. 2004, 43, 2125−2129. (55) Huber, K. P.; Herzberg, G. Molecular Spectra and Molecular Structure. IV. Constants of Diatomic Molecules; Van Nostrand: New York, 1974. (56) Yoon, B.; Häkkinen, H.; Landman, U.; Wörz, A. S.; Antonietti, J. M.; Abbet, S.; Judai, K.; Heiz, U. Charging Effects on Bonding and Catalyzed Oxidation of CO on Au8 Clusters on MgO. Science 2005, 307, 403−407. (57) Sanchez, A.; Abbet, S.; Heiz, U.; Schneider, W. D.; Häkkinen, H.; Barnett, R. N.; Landman, U. When Gold Is Not Noble: Nanoscale Gold Catalysts. J. Phys. Chem. A 1999, 103, 9573−9578.

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