Oxidation of Al doped Au clusters: A first principles studyChinagandham Rajesh and Chiranjib Majumder Citation: The Journal of Chemical Physics 130, 234309 (2009); doi: 10.1063/1.3149849 View online: http://dx.doi.org/10.1063/1.3149849 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/130/23?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Real space pseudopotential calculations for size trends in Ga- and Al-doped zinc oxide nanocrystals with wurtziteand zincblende structures J. Chem. Phys. 141, 094309 (2014); 10.1063/1.4893478 Influence of the cluster dimensionality on the binding behavior of CO and O2 on Au13 J. Chem. Phys. 136, 024312 (2012); 10.1063/1.3676247 Catalytic activity of Pd ensembles over Au(111) surface for CO oxidation: A first-principles study J. Chem. Phys. 134, 054704 (2011); 10.1063/1.3551617 Theoretical investigation of the interaction of CH 4 with Al 2 and Al 3 neutral and charged clusters J. Chem. Phys. 132, 154701 (2010); 10.1063/1.3376174 Formation of titanium-solute clusters in alumina: A first-principles study Appl. Phys. Lett. 84, 4795 (2004); 10.1063/1.1760598

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Oxidation of Al doped Au clusters: A first principles studyChinagandham Rajesh1 and Chiranjib Majumder2,a�

1RMC, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India2Chemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India

�Received 2 February 2009; accepted 30 April 2009; published online 18 June 2009�

Using first principles method we report the oxidation of Al doped Au clusters. This work is dividedinto two parts: �i� the equilibrium structures and stability of Al doped Aun−1 clusters �n=2–7,21� and �ii� the interaction of O2 with stable clusters. The calculations are performed usingthe plane wave pseudopotential approach under the density functional theory and generalizedgradient approximation for the exchange and correlation functional. The optimized geometries ofAun−1Al clusters indicate that the substitution of Au by Al results an early onset ofthree-dimensional structures from tetramer onwards. This is different from the results of transitionmetal doped Au clusters, where the planar conformation of Au clusters retains up to heptamer. Thestability of Aun−1Al clusters has been analyzed based on the binding energy, second difference inenergy, and the energy gaps between the highest occupied molecular orbital and lowest unoccupiedmolecular orbital energy levels. Based on the energetics, the Au3Al and Au5Al clusters are found tohave extraordinary stability. The oxidation mechanism of Al doped Au clusters have been studied bythe interaction of O2 with Al, Au, AuAl, Au3Al, and Au20Al clusters. It is found that the oxidationof Aun−1Al clusters undergoes via dissociative mechanism, albeit significant charge transfer from Alto Au. Moreover, the O2 molecule prefers to attach at the Al site rather than at the Au site. © 2009American Institute of Physics. �DOI: 10.1063/1.3149849�

I. INTRODUCTION

Gold clusters have been receiving considerable attentionsince some time because of their special physical and chemi-cal properties. Gold, which is inert in the bulk, shows sig-nificant catalytic activity in its nanoform. A large number ofreactions have been demonstrated1–6 to verify the potential ofgold nanoparticles as an active catalyst. Further applicationsof nanoscale gold particles as building blocks for nanostruc-tured materials,7 electronic devices,8–12 and nanocatalysis13,14

is on a surge. Considering the sensitivity of nanoparticles onsize and shape, it has become important to study the effect ofdoping, which modifies the electronic and geometric struc-ture of these clusters. Thus a thoughtful selection of the im-purity atoms for tuning their chemical reactivity might leadto enhanced performance of these clusters.

Extensive experimental and theoretical studies have beencarried out to elucidate the geometric and electronic struc-tures of gold clusters.15–34 Recent studies have shown thatthe ground state structures of small gold clusters are planarup to n=11.19 Further studies have shown that the groundstate structures of small gold clusters differ from other coin-age metal clusters.33–40 For example, using experimental andtheoretical techniques, Weis et al.33 have shown that Agn

+

clusters adopt three-dimensional �3D� structures from pen-tamer onwards. The reason for the preference of planar struc-tures by gold clusters up to large cluster sizes was attributedto the relativistic effects that cause a shrinking of the size ofthe s orbitals and thus enhances the s-d hybridization.

It is well known from a large number of previous studies

that impurity atoms can strongly influence geometric, elec-tronic, and bonding properties of doped clusters. In this di-rection many studies have been done on doped gold clusters�mostly transition metal atoms as impurity� to enhance theirstability and reactivity. Recent experimental and theoreticalwork demonstrated that the introduction of a dopant atom ina metal cluster can change its structure and magnetic prop-erties significantly.41–62 Neukermans et al.44 investigated thestability of cationic clusters AunM+, with M varying from Scto Ni, by means of photofragmentation experiments. The re-sults have shown higher intensity for specific sizes of theclusters, which corresponds to the highly stable AunM+ clus-ters. A qualitative explanation was given in terms of jelliummodel, �electronic shell closing with 2 ,8 ,¯ electrons� inorder to understand the enhanced stability. In this series,Au5X+ �X=V,Cr,Mn,Fe,Co,Zn� clusters showed extra sta-bility, which was explained based on the structural planarityand the delocalized electrons.46,48 The enhanced stability ofAu5Zn+ cluster has been explained based on the stabilizationinduced via �-aromaticity with six delocalized s electrons.50

The effect of transition metal atom �Mo, W, Zr, and Hf�doping in Au12 and Au14 clusters was found to increase theenergy gap between the highest occupied molecular orbital�HOMO� and lowest unoccupied molecular orbital �LUMO�energy levels.51,52 The enhanced stability of the metal-dopedAu12 cluster was attributed to the aurophilic attractions, rela-tivistic effects, and closed-shell electron configuration. Wanget al.53 reported the first observation and characterization ofAu16

− and Au17− doped with a Cu atom �Cu@Au16

− andCu@Au17

− � by both photoelectron spectroscopy and DFT cal-culations. In another work the existence of tetrahedral MAu4

�M =Ti, Zr, Hf, U and Th� has been predicted.54 The chargea�Electronic mail: chimaju@barc.gov.in.

THE JOURNAL OF CHEMICAL PHYSICS 130, 234309 �2009�

0021-9606/2009/130�23�/234309/9/$25.00 © 2009 American Institute of Physics130, 234309-1

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distribution analysis of these clusters showed that the goldcarries a formal negative charge and acts as a halogen. Onthe other hand using the photoelectron spectroscopy Kiranand co-workers55 reported the hydrogen like behavior of Auin SiAu4 with an analogy of SiH4. In a recent study on elec-tronic structure and magnetic properties of transition-metal-doped Au clusters, MAu6 �M =Ti,V,Cr�, it is found that allthe MAu6

− and MAu6 clusters not only form planar struc-tures, but also the M atoms possess atomlike moments.57 ForTi doped Aun clusters, it is found that up to n=7, the equi-librium geometry prefers planar configuration and for n=12–16, Ti gets encapsulated inside a spherical cage formedby Au atoms.58

Although most of the studies focused on the interactionof Au clusters with transition metal atoms, few are availableto show the interactions of s- and p-block elements with Auclusters.59–62 A study on the atomic and electronic structuresof Au5M clusters, where M atom represents the second pe-riod elements, revealed that, with the exception of S, impu-rities with p electrons �Al, Si, P� adopt non-planar geom-etries while those with s electrons �Na, Mg� prefer planargeometries.61,62 In this work, we presented the atomic andelectronic structures of Aun−1Al clusters �n=2–7,21� andelucidated the results with a view to understand how chemi-cal bonding influences the stability and structure of smallsize atomic clusters. It should be mentioned here that al-though Al �3s2 ,3p1� and Au�5d106s1� are metals in their bulkand have similar crystal packing arrangements �fcc�, theatomic states have large difference in their electronegativityvalues. According to Pauling’s scale, the electronegativity ofAu and Al are 2.54 and 1.61, respectively.63 Therefore, it isexpected that the interaction between Al and Au would in-volve significant charge transfer or, in other words, thechemical bonding will have more ionic than covalent char-acter. Based on these motivations, in this work, we investi-gated the equilibrium structures of small Aun−1Al �n=2–7,21� clusters followed by their oxidation reactions. Theobjective of this study can be divided into two parts: �i� toinvestigate the effect of electropositive Al atom in disturbingthe stable planar conformations of small Au clusters and �ii�the oxidation mechanism of the Al doped Au clusters.

II. COMPUTATIONAL DETAILS

All calculations were performed using the density func-tional theory with projector augmented wave pseudopoten-tials and plane wave basis set as implemented in the VASP

code. The spin polarized Perdew–Wang generalized gradientapproximation has been used to calculate the exchange-correlation energy.64,65 A simple cubic supercell of side 20 Åhas been used and the Brillouin zone integrations were car-ried out using only the gamma point. The cutoff energy forall calculations was 500 eV, which ensured a good conver-gence of the energy. The self-consistent equations weresolved with an iterative matrix diagonalization scheme. Ge-ometry optimizations were performed with the conjugategradient algorithm and the geometries were considered to beconverged when the force on each atom became 0.01 eV/Åor less. The total-energy convergence was tested with respect

to the plane-wave basis set size and simulation cell size andthe total energy was found to be accurate to within 1 meV.

Test calculations were done for Au bulk and the dimersof Au and Al in order to verify the accuracy of our compu-tational methodology. In Table I we summarized the com-puted results obtained along with the experimental values.From our calculations the lattice parameters of the Au bulkwas found to be 4.13 Å, which is quite close ��1% error� tothe experimental value of 4.09 Å. The bulk cohesive energywas estimated to be 3.61 eV/atom, which is close to theexperimental value �3.81 eV/atom�.66 Further, for the Au2

dimer, the corresponding values of bond length and bindingenergy �BE� are estimated to be 2.53 Å and 1.17 eV/atom,which are in good agreement with experimental values of2.47 Å and 1.16 eV/atom, respectively.63 Similar good agree-ment was obtained for the Al2 dimer. The bond length andBEs were estimated to be 2.51 Å and 0.70 eV/atom, whichagree quite well with the experimental values of 2.46 Å and0.69 eV/atom.63

Initial calculations were carried out to obtain the groundstate geometries of Aun �n=2–7,20� clusters. The lowestenergy geometries of Aun �n=2–7� clusters form planar “W”structures and Au20 forms a Td structure, respectively. Theseresults were found to be consistent with previously reportedresults using different techniques.15–34 To obtain the equilib-rium geometries of Aun−1M clusters, the initial geometrieswere prepared by adding the M atom on each possible site ofa few low lying two-dimensional �2D� and 3D isomers ofAun−1 host cluster as well as by substituting one Au by an Matom from the Aun cluster.

III. RESULTS AND DISCUSSION

A. Aun−1Al clusters

1. Geometry

The optimized bond lengths of Au2, Au–Al, and Al2 are2.53, 2.32, and 2.51 Å, respectively, and the bond strengthfollows the trend as Al–Al�Au–Au�Au–Al �Table I�.This has further been corroborated by the higher BE andshorter bond length of the Au-–Al dimer, which are 1.83eV/atom and 2.32 Å, respectively. The interaction of Al withthe Au2 dimer forms obtuse angle isosceles triangle with�Au–Al–Au of 132° and the Al–Au distance of 2.34 Å.This is similar to that of Au3 cluster, which also forms an

TABLE I. Computed and experimental HEs and bond lengths of the dimersand bulk.

System

Bond length��

BE/atom�eV/atom�

Comp. Exp. Comp. Exp.

Au2 2.53 2.47 1.17 1.16��0.005�a

Au-Al 2.32 ¯ 1.83 1.68��0.06�a

Al2 2.51 2.46 0.70 0.69��0.06�a

Au-bulk 4.13b 4.09b 3.6c 3.8c,d

aReference 63.bLattice length of the bulk.cCohesive energy of the bulk.dReference 66.

234309-2 C. Rajesh and C. Majumder J. Chem. Phys. 130, 234309 �2009�

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open triangle with a �Au–Au–Au angle of 137°. The aver-age BE of the Au2Al cluster is calculated to be 1.96 eV/atom. The geometry of the tetramer cluster is important, as itis the smallest size for the onset of 3D configurations. Pre-vious studies on pure and transition metal impurity dopedgold clusters suggested that, at least up to hexamer, theyfavor planar configurations. For Au3Al, we found that a pla-nar D3h symmetric structure �Fig. 1� forms the most stableisomer, which is quite similar to that of AlCl3. The Au–Albond lengths and the angle between Au–Al–Au are found tobe 2.36 Å and 120°, respectively. Another isomer of the

Au3Al cluster, which forms 3D capped triangle, is 0.5 eVhigher in energy. The average BE of the Au3Al is estimatedto be 2.25 eV/atom.

For Au5, the ground state geometry forms a W shapedplanar structure. When Al replaces one Au atom, the moststable isomer of the Au4Al cluster adopts 3D tetrahedralshapes as shown in Fig. 1. A close look at this structurereveals that this structure consists of two parts: Au2Al andAu2 joining together. The interaction energy �IE� betweenAu2 and Au2Al is estimated to be 2.49 eV. The shortestAu–Au and Au–Al bond length is found to be 2.68 and 2.38Å, respectively. The Au–Al bond length is slightly elongatedto 2.45 Å when Al interacts with the Au2 fragment. The BEis estimated to be 2.13 eV/atom. The planar isomers ofAu4Al formed by capping the Au3Al cluster in plane werefound to be at least 0.15 eV higher in energy. The hexamer ofAu prefers to form a planar triangle structure as the lowestenergy isomer. When Al replaces one Au, the most stablegeometry of the Au5Al cluster forms a capped rhombuswhere Al atom is capping the rhombus formed by Au atomsand the additional Au atom connects at the top site of the Alatom. Other isomers with tetrahedron and octahedron motifswere found to be 0.36 and 0.66 eV higher in energy, respec-tively. The differences in the total energy for other low-lyingisomers are indicated in the Fig. 1. The BE and the shortestAu–Al bond length are found to be 2.25 eV/atom and 2.38Å, respectively.

For the heptamer, the Au cluster continues to favor pla-nar configuration over 3D geometry and forms edge cappedtriangle structure. For the Au6Al cluster, 3D structural motifscontinue to grow. The formation of the Au6Al can be viewedfrom three different perspectives: the Al atom can interactwith Au6, the Al atom can substitute one Au atom from Au7

cluster, and one Au atom is added with Au5Al cluster. Theresults show that the most stable geometry of the Au6Alcluster follows the trend of Au5Al cluster, where the addi-tional Au atom is connected at the edge site of the rhombusformed by four Au atoms. The average BE is estimated to be2.18 eV/atom and the smallest Au–Al bond length is found tobe 2.32 Å. Interestingly we note that the planar isomer ofAu6Al cluster, similar to Au7 was found to form one of thelow-lying isomers with an excess energy of 0.43 eV.

Li et al.11 have shown that the Au20 cluster prefers Td

geometry as the lowest energy isomer with nine atoms of Auon each face of the tetrahedron. The Au atoms on the tetra-hedral face have similar atomic arrangement to that of Au�111� surface with reduced coordination. Several possiblesites including the top, bridge, edge capping, triangular cap-ping, substitutional, and interstitial positions were consideredfor the generation of the initial isomers for the Au20Al clus-ter. A comparison of total energy among all these isomerssuggests that the Al atom prefers to cap the threefold hollowsite of the Au20 cluster from outside as shown in Fig. 1.Another isomer with Al occupying the interstitial space ofAu20 was also found to be stable on the potential energysurface but 0.80 eV higher in energy compared to the lowestenergy isomer. The shortest Au–Al distance is found to be2.50 Å, which is longer compared to the Au–Al distances in

Sreerama

AuAl Au2Al Au3Al

Au4Al-a

∆E = 0.0

Au4Al-b

∆E = 0.15

Au4Al-c

∆E = 0.27

Au6Al-a

∆E = 0.0

Au6Al-b

∆E = 0.23

Au6Al-c

∆E = 0.27

Au6Al-d

∆E = 0.43

Au5Al-a

∆E = 0.0

Au5Al-b

∆E = 0.36

Au5Al-c

∆E = 0.66

Au20Al-a

∆E=0.0

Au20Al-b

∆E=0.23

Au20Al-c

∆E=0.80

FIG. 1. Lowest energy isomers of Aun−1Al �n=2–7,21� clusters along withfew other low lying isomers �∆E in eV�.

234309-3 Oxidation of Al doped Au clusters J. Chem. Phys. 130, 234309 �2009�

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the smaller clusters, which might be the result of the in-creased coordination. The BE of Au20Al is found to be 2.41eV/atom.

2. Energetics

The average BE of Aun and AunAl clusters is calculatedas

BE�Aun� = − �E�Aun� − n�E�Au��/�n� ,

BE�Aun−1Al� = − �E�Aun−1Al� − �n − 1��E�Au�

− E�Al��/�n� .

Figure 2 shows the BE of Aun and Aun−1Al clusters as afunction of the total number of atoms. Apart from an oddeven alteration in the stability pattern, two important pointshave been noted from this BE plot: �i� the incorporation of Alatom improves the overall stability of the host Au clustersand �ii� Au3Al is the most stable cluster in this series. Thehigher stability of the Aun−1Al clusters is attributed to thehigher bond strength of Au–Al than that of Au–Au �3.66 and2.34 eV, respectively�. The trend in the relative stability ofthe Aun−1Al and Aun clusters can also be understood by cal-culating the second order difference in energy as shown inFig. 3. The effect of the odd-even alteration in the relativestability order is evident from the oscillatory pattern. For theseries of Aun−1Al clusters, the results suggest that Au3Al andAu5Al are more stable as compared to their neighbors. Thehigher stability of these clusters has been corroborated by the

large energy gaps between HOMO and LUMO levels, whichare estimated to be 2.55 and 1.92 eV for Au3Al and Au5Al,respectively. In order to verify the electronic stability ofthese clusters, we calculated the adiabatic ionization poten-tials, which results 6.8 and 7.38 eV for Au5Al and Au3Al,respectively. In addition to the even number of electrons,these two clusters have more reasons to be extra stable. TheAu5Al cluster has a total of eight number valence electrons,which corresponds to the closed configuration of electronshell model. For the Au3Al cluster, it contains six numbervalence electrons and is planar geometrically. Both these cri-teria could be contributing to achieve an extra stability of theAu3Al cluster through resonance stabilization known as aro-matic stability.

3. Electronic structure

To understand the electronic stability of Aun clusters inpresence of Al atom, the energy gaps between the HOMOand LUMO energy levels of Aun and Aun−1Al clusters arelisted in Table II. The even electron systems were found tohave higher gaps compared to the odd electron systems. Thehigher energy gaps are observed for AuAl, Au3Al, andAu5Al, which are estimated to be 2.4, 2.5, and 1.92 eV, re-spectively. It has been noticed that the energy gap of Aun

�n=2 and 4� increased by Al substitution. However for Au6,the Al substitution leads to reduce the energy gap. This hasbeen attributed to the additional stability of pure Au6 clusterdue to planar aromatic nature of the cluster.

To further analyze the nature of bonding and the effectof the Al doping on the Aun clusters, we analyzed the elec-tronic density of state �EDOS� spectrum. For the sake ofsimplicity we only plotted the EDOS of even numbered clus-ters �Au2, Au4, Au6, AuAl, Au3Al, and Au5Al� only asshown in Fig. 4. In general, it is seen that the presence of Alin gold clusters results in a red shift of the eigenvalue spec-trum. This has been attributed to the electronic charge trans-fer from Al to the host Au clusters. However, an exceptionwas observed for the Au4 cluster. The HOMO of the Au4

cluster is destabilized because of the antiaromatic nature ofthe four electrons. However after the substitution of one ofthe Au atoms by Al, the HOMO becomes stabilized signifi-cantly. To further illustrate the charge transfer of Al to Auhost, we analyzed the charge density difference of the AuAland Au3Al clusters as shown in Fig. 5, which clearly shows

FIG. 2. �Color online� The average BE of Aun �circles� and Aun−1Al�squares� clusters �n=2–7�.

TABLE II. Summary of the average binding energy �BE/atom�, interaction energy �IE�, energy gap between theHOMO and LUMO energy levels �HLG=HOMO-LUMO�, the shortest Au–Al bond distances of Aun−1Al�n=2–7,21� clusters, and the charge on Al atom in the Aun−1Al clusters.

SystemBE/atom

�eV/atom�Bond length

��IE

�eV�HOMO-LUMO gap

�eV� Charge

AuAl 1.83 2.32 3.66 2.4 +1.21Au2Al 1.96 2.34 3.49 0.75 +2.04Au3Al 2.25 2.36 5.36 2.55 +2.86Au4Al 2.13 2.38; 2.45 4.41 0.52 +2.92Au5Al 2.25 2.38; 2.45 4.99 1.92 +2.98Au6Al 2.18 2.40; 2.47 3.68 0.2 +2.98Au20Al 2.41 2.68 3.30 0.28 +2.15

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significant charge depletion �blue� from the Al atom andcharge accumulation �red color� on the Au atoms.

B. Interaction of Aun−1Al clusters with O2

In order to understand the fundamentally important oxi-dation of metal nanoparticles, we performed the interactionof oxygen with stable Al doped Au clusters. Before attempt-ing to calculate the oxidation reactions, we compared theexperimental bond length and binding strength of Al–O andAu–O with our calculated values. From the optimized geom-etry we estimated the BE of Au–O and Al–O as 1.15 and2.53 eV/atom, respectively, which are in good agreementwith the experimental values of 1.15 and 2.59 eV/atom.63

The bond lengths are found to be 2.15 and 1.61 Å, which arealso consistent with the experimental values of 2.3 and 1.61Å, respectively.

In order to understand the interaction of O2 with atomicAu and Al, we performed the geometry optimization ofAl–O2 and Au–O2 complexes by keeping the molecularidentity of oxygen as well as its atomic form. The compari-son of total energy values suggests that when oxygen is at-

tached to Al in the atomic form �O–Al–O�, the structure is1.17 eV lower in energy in comparison with the isomerwhere oxygen interacts with Al in the molecular way�Al–O2�. Unlike Al, the most stable conformation of Au–O2

complex is obtained when the O2 molecule interacts with Auin the molecular form and the ground state configurationform an angled �Au–O–O� structure. The initial and finalconfigurations of the Au–O2 and Al–O2 are given in Fig. 6.The IE of O–Al–O and Au–O–O are found to be 4.3 and 0.48eV, respectively. The IE of the oxygen molecule withAun−1Al is calculated by

IE = − �E�Aun−1Al – O2� − E�Aun−1Al� − E�O2��

From our previous discussion we have seen that Al–Auforms partial ionic bond where Al transfers electronic chargeto Au. Thus it is of interest when O2 interacts with the Au–Aldimer, whether it approaches from the Au or Al side and howthe IE differs in comparison to their atomic states �Au or Al�.

FIG. 3. �Color online� Plot of second-order difference in total energy ��2E�of Aun �circles� and Aun−1Al �square� clusters �n=2–7� as a function ofsize.

FIG. 4. �Color online� The EDOS for the lowest energy isomer of theAun−1Al �n=2,4 ,6� clusters �dotted line� in comparison with the Aun�n=2,4 ,6� �line�.

FIG. 5. �Color� 2D projection of the charge density for AuAl���AuAl�−��Au�−��Al�� and Au3Al���Au3Al�−��Au3�−��Al�� as obtained from thedifference of the constituent species. The respective molecular structure ofAuAl and Au3Al are shown below the plot.

FIG. 6. �Color online� The initial and final configurations of the interactionof the O2 with Au and Al atoms.

234309-5 Oxidation of Al doped Au clusters J. Chem. Phys. 130, 234309 �2009�

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We optimized few initial configurations keeping O2 in differ-ent orientations with respect to the Al–Au. The result showsthat when O2 approaches Au, the IE is very small and itremains in the molecular form �Fig. 7�. However, when O2

approaches the Al side, the O–O bond dissociates. In thiscase the IE is estimated to be 1.85 eV. Similar atomic con-figuration was found to be stable when two predissociated Oatoms were allowed to react with the AuAl cluster. The Al–Obond length was found to be 1.73 Å and the O–Al–O anglewas found to be 84.2°. It should be noted that the IE of Alwith O2 in the AuAl cluster reduced in comparison to theinteraction of the naked Al atom. This is attributed to thepositive charge induced on the Al atom, which results instabilizing the Al energy levels and reduced overlap �or re-activity� of the frontier orbitals of Al with the O2 molecule.This is further corroborated by the blueshift of the projecteddensity of states �PDOS� of the Al atom in the AuAl–O2

cluster as compared to AuAl dimer �Fig. 8�, indicating anincreased positive charge on the Al atom or, in other words,a charge transfer from the Al atom to the oxygen moleculeleading to dissociation.

It is known that spin conservation rule plays an impor-tant role to control the reactions of oxygen in gas phase.67

Recent works by Burgert et al.68 and Chrétien and Metiu69

demonstrated this fact by studying the reactions of the oxy-gen molecule with the Al cluster anion and Au/TiO2 �110�surface, respectively. In order to illustrate the importance ofspin conservation in studying reactions involving oxygen, weused AuAl dimer as a model cluster. The total energy andgeometry optimizations were carried out by modifying theinteratomic separation of the O–O bond for their respectivespin states as well as by fixing the total spin state of theproduct as shown below.

AuAl + O2�singlet� → AuAl – O2�singlet�;

�H = − 1.73 eV,

AuAl + O2�triplet� → AuAl – O2�triplet�;

�H = − 1.85 eV.

The results reveal that when singlet O2 interacts withAuAl, the reaction does not favor bond dissociation. How-ever, when triplet O2 interacts with AuAl, the reaction under-goes O–O bond dissociation. Moreover, from the total energyconsideration, the stability of the triplet product is 0.51 eVmore stable than the singlet.

The Au3Al cluster is found to be the most stable clusterwith a high BE, large HOMO-LUMO gap �2.55 eV�, andhigh IE. From the comparison of EDOS of Au3Al andAu4�Fig. 4� clusters we note that the frontier orbitals ofAu3Al are more stabilized compared to Au4, even after in-corporation of Al. So, in order to understand the chemicalreactivity of Au3Al with O2, molecular O2 is allowed to in-teract with the cluster approaching from different orienta-tions in space and a few low-lying isomers are shown in Fig.

FIG. 7. �Color online� The initial and final configurations of the interactionof O2 with the AuAl cluster.

FIG. 8. Comparison of the PDOS plot for Al atom in the �a� gas phase, �b�AuAl dimer, and �c� AuAl–O2 complex.

234309-6 C. Rajesh and C. Majumder J. Chem. Phys. 130, 234309 �2009�

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9. It was observed that when molecular oxygen was allowedto interact with the Au3Al cluster through Au atoms, it doesnot interact with them. So, in order to understand the prefer-ential mode of interaction between the molecular and atomicforms of O2 with the Au atoms of Au3Al, calculations werecarried out with the O–O bond elongated to the dissociationlimit and oriented in the same molecular plane of the Au3Alcluster. It was observed after the geometry optimization thatmolecular oxygen separates out of the cluster, which clearlyreflects a reduced reactivity of the Au atoms. On the otherhand when molecular oxygen approaches the Au3Al clusterin a perpendicular direction to the basal plane of the Au3Al,O2 dissociates and the disassociative adsorption is predictedto induce large relaxations in the gold part of the complex�Fig. 9�. Au3Al–O2 gets highly stabilized because of thelarge structural relaxation involved. The IE is found to be2.56 eV, which is higher compared to AuAl cluster IE withoxygen molecule.

Several orientations were tried to scan the possiblemodes of approach of the oxygen molecule to interact withAu20Al cluster. Basically, molecular O2 and atomic oxygenspecies were allowed to interact with Au20Al and the finalconfigurations are given in Fig. 10. It was observed that

when oxygen adsorbs on the Al atom as a molecule, it dis-sociates into its atomic form. The interoxygen distances wereestimated to increase to 1.50 Å. The O–Al–O angle wasfound to be 47.5°. Further it was observed that when the O2

molecule approaches linearly to the Al atom, the Au20Al–O2

also relaxed into the same configuration. It was also foundthat when predissociated O species were allowed to reactwith the cluster, with O–Al–O angle �65°, the cluster re-laxed into the same old configuration that was observed inthe first two cases. However, for the O–Al–O angle �65°�configuration 4, of Fig. 10�, the optimized geometry wastotally different, where the O–Al–O angle was estimated tobe 143° and O is connected to both Al and Au. This configu-ration was found to be the lowest energy configuration withadditional stabilization energy of 0.6 eV.

IV. CONCLUSIONS

In this work we presented several geometrical isomers ofAl doped Au clusters �Aun−1Al clusters, n=2–7 and 21� andthe oxidation reaction of few stable clusters. The stability of

FIG. 9. �Color online� The initial and final configurations of the interactionof O2 with the Au3Al cluster.

FIG. 10. �Color online� The initial and final configurations of the interactionof O2 with the Au20Al cluster.

234309-7 Oxidation of Al doped Au clusters J. Chem. Phys. 130, 234309 �2009�

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these clusters was analyzed based on their BE, second orderdifference in energy, and energy gap between the HOMO andLUMO energy levels. For small clusters, it is found thatAu3Al and Au5Al clusters are more stable than their neigh-bors. The reason for such higher stability has been attributedto their specific electronic structures related to the planarresonance stabilization and electron shell filling effect forAu3Al and Au5Al clusters, respectively. The electron chargedensity analysis of these clusters suggests that Al atom trans-fers electronic charges toward Au, resulting in higher elec-tronic charge on the Au atoms. This is corroborated by theshift of energy levels to less negative values or in otherwords imparts activity in its electronic level diagram.

In order to understand the fundamentally important oxi-dation of metal nanoparticles we also performed the interac-tion of oxygen with stable Al doped Au clusters. The resultssuggest that oxygen prefers to bind at the Al site to Au andthe adsorption of oxygen still favors disassociative chemi-sorptions mechanism. However, the IE of O2 with Aun−1Alclusters is smaller in comparison to the interaction of thenaked Al atom. This can be attributed to the reduced valenceelectrons on the Al in Aun−1Al clusters.

ACKNOWLEDGMENTS

The authors are thankful to the members of the Com-puter Division, BARC, for their continuous support and theuse of the supercomputing facility.

1 Metal Clusters, edited by W. Ekardt �Wiley, Chichester, 1999�.2 U. Heiz, S. Abbet, A. Sanchez, W.-D. Schneider, H. Hakkinen, and U.Landman, Proceedings of the 117th Nobel Symposium on Phys. Chem.Clusters, 2001 �unpublished�, pp. 87–98.

3 M. Haruta, Catal. Today 36, 153 �1997�.4 M. Valden, X. Lai, and D. W. Goodman, Science 281, 1647 �1998�.5 G. C. Bond and D. T. Thompson, Catal. Rev.- Sci. Eng. 41, 319 �1999�.6 A. Sanchez, S. Abbet, U. Heiz, W.-D. Schneider, H. Hakkinen, R. N.Barnett, and U. Landman, J. Phys. Chem. A 103, 9573 �1999�.

7 H.-G. Boyen, G. Kästle, F. Weigl, B. Koslowski, C. Dietrich, P. Ziemann,J. P. Spatz, S. Riethmüller, C. Hartmann, M. Möller, G. Schmid, M. G.Garnier, P. Oelhafen, Science 297, 1533 �2002�.

8 U. Landman, W. D. Luedtke, N. A. Burnham, and R. J. Colton, Science248, 454 �1990�.

9 H. Hakkinen, R. N. Barnett, A. G. Scherbakov, and U. Landman, J. Phys.Chem. B 104, 9063 �2000�, and references therein.

10 J. G. Hou, B. Wang, J. Yang, X. R. Wang, H. Q. Wang, Q. Zhu, and X.Xiao, Phys. Rev. Lett. 86, 5321 �2001�; C. J. Kiely, J. Fink, M. Brust, D.Bethell, and D. J. Schiffrin, Nature �London� 396, 444 �1998�; B. Wang,X. Xiao, X. Huang, P. Sheng, and J. G. Hou, Appl. Phys. Lett. 77, 1179�2000�.

11 J. Li, X. Li, H. J. Zhai, and L.-S. Wang, Science 299, 864 �2003�.12 H. Hakkinen, R. N. Barnett, and U. Landman, Phys. Rev. Lett. 82, 3264

�1999�.13 R. Meyer, C. Lemire, S. K. Shaikhutdinov, and H. J. Freund, Gold Bull.

27, 72 �2004�; G. J. Hutchings, and M. Haruta, Appl. Catal., A 291, 1�2005�; D. T. Thompson, Top. Catal. 38, 231 �2006�.

14 H. Häkkinen and U. Landman, J. Am. Chem. Soc. 123, 9704 �2001�; M.Walter and H. Häkkinen, Phys. Chem. Chem. Phys. 8, 5407 �2006�.

15 C. L. Cleveland, U. Landman, T. G. Schaaff, M. N. Shafigullin, P. W.Stephens, and R. L. Whetten, Phys. Rev. Lett. 79, 1873 �1997�.

16 H. Hakkinen and U. Landman, Phys. Rev. B 62, R2287 �2000�.17 F. Furche, R. Ahlrich, P. Weis, C. Jacob, S. Gilb, T. Bienweiler, and M.

Kappes, J. Chem. Phys. 117, 6982 �2002�.18 K. Koga, H. Takeo, T. Ikeda, and K. I. Ohshima, Phys. Rev. B 57, 4053

�1998�.19 H. Hakkinen, M. Moseler, and U. Landman, Phys. Rev. Lett. 89, 033401

�2002�.20 B. Palpant, B. Prevel, J. Lerme, E. Cottancin, M. Pellarin, M. Treilleux,

A. Perez, J. L. Vialle, and M. Broyer, Phys. Rev. B 57, 1963 �1998�.21 J. J. Zhao, X. S. Chen, and G. H. Wang, Phys. Lett. A 189, 223 �1994�.22 H. Handschuh, G. Gantefor, P. S. Bechthold, and W. Eberhardt, J. Chem.

Phys. 100, 7093 �1994�.23 I. L. Garzon and A. Posada-Amarillas, Phys. Rev. B 54, 11796 �1996�.24 J. M. Soler, M. R. Beltran, K. Michaelian, I. L. Garzon, P. Ordejon, D.

Sanchez-Portal, and E. Artacho, Phys. Rev. B 61, 5771 �2000�.25 O. D. Haberlen, S. C. Chung, M. Stener, and N. Rosch, J. Chem. Phys.

106, 5189 �1997�.26 I. L. Garzon, K. Michaelian, M. R. Beltran, A. Posada-Amarillas, P.

Ordejon, E. Artacho, D. Sanchez-Portal, and J. M. Soler, Phys. Rev. Lett.81, 1600 �1998�.

27 R. N. Barnett, C. L. Cleveland, H. Hakkinen, W. D. Luedtke, C. Yan-nouleas, and U. Landman, Eur. Phys. J. D 9, 95 �1999�.

28 J. L. BelBruno, Heteroat. Chem. 9, 651 �1998�.29 N. T. Wilson and R. L. Johnston, Eur. Phys. J. D 12, 161 �2000�.30 T. Li, S. Yin, Y. Ji, G. Wang, and J. Zhao, Phys. Lett. A 267, 403 �2000�.31 T. G. Schaaff, W. G. Cullen, P. N. First, I. Vezmar, R. L. Whetten, W. G.

Cullen, P. N. First, C. Gutierrez-Wing, J. Ascensio, and M. J. Jose-Yacaman, J. Phys. Chem. 101, 7885 �1997�.

32 K. J. Taylor, C. L. Pettiette-Hall, O. Cheshnovsky, and R. E. Smalley, J.Chem. Phys. 96, 3319 �1992�.

33 P. Weis, T. Bierweiler, S. Gilb, and M. M. Kappes, Chem. Phys. Lett.355, 355 �2002�.

34 G. Dietrich, S. Kruckeberg, K. Lutzenkirchen, L. Schweikhard, and C.Walther, J. Chem. Phys. 112, 752 �2000�.

35 S. Gilb, P. Weis, F. Furche, R. Ahlrichs, and M. M. Kappes, J. Chem.Phys. 116, 4094 �2002�.

36 H. Grönbech and W. Andreoni, Chem. Phys. 262, 1 �2000�.37 H. Hakkinen, B. Yoon, U. Landman, X. Li, H.-J. Zhai, and L.-S. Wang, J.

Phys. Chem. A 107, 6168 �2003�.38 R. Rousseau and D. Marx, J. Chem. Phys. 112, 761 �2000�.39 G. Gantefor, H. Handschuh, H. Moeller, C.-Y. Cha, P. Bechthold, and W.

Eberhardt, Surf. Rev. Lett. 3, 399 �1996�.40 V. B. Bonacic-Koutecky, J. Burda, R. Mitri, M. Ge, G. Zampella, and P.

Fantucci, J. Chem. Phys. 117, 3120 �2002�; J. Zhao, J. Yang, and J. G.Hou, Phys. Rev. B 67, 085404 �2003�; J. Wang, G. Wang, and J. Zhao,ibid. 66, 035418 �2002�; M. Lee, M. Ge, B. R. Sahu, P. Tarakeshwar, andK. S. Kim, J. Phys. Chem. B 107, 9994 �2003�.

41 K. Koszinowski, D. Schroder, and H. Schwarz, ChemPhysChem 4, 1233�2003�.

42 H. Hakkinen, S. Abbet, A. Sanchez, U. Heiz, and U. Landman, Angew.Chem. Int. Ed. 42, 1297 �2003�.

43 B. R. Sahu, G. Maofa, and L. Kleinman, Phys. Rev. B 67, 115420�2003�.

44 S. Neukermans, E. Janssens, H. Tanaka, R. E. Silverans, and P. Lievens,Phys. Rev. Lett. 90, 033401 �2003�.

45 D. W. Yuan, Y. Wang, and Z. Zeng, J. Chem. Phys. 122, 114310 �2005�.46 E. Janssens, H. Tanaka, S. Neukermans, R. E. Silverans, and P. Lievens,

Phys. Rev. B 69, 085402 �2004�.47 W. Bouwen, F. Vanhoutte, F. Despa, S. Bouckaert, S. Neukermans, L. T.

Kuhn, H. Weidele, P. Lievens, and R. E. Silverans, Chem. Phys. Lett.314, 227 �1999�.

48 E. Janssens, H. Tanaka, S. Neukermans, R. E. Silverans, and P. Lievens,N. J. Phys. 5, 46 �2003�.

49 H. Tanaka, S. Neukermans, E. Janssens, R. E. Silverans, and P. Lievens,J. Am. Chem. Soc. 125, 2862 �2003�.

50 H. Tanaka, S. Neukermans, E. Janssens, R. E. Silverans, and P. Lievens,J. Chem. Phys. 119, 7115 �2003�.

51 P. Pyykko and N. Runeberg, Angew. Chem. Int. Ed. 41, 2174 �2002�; B.Kiran, X. Li, H.-J. Zhai, L.-F. Cui, and L.-S. Wang, ibid. 43, 2125�2004�.

52 Y. Gao, S. Bulusu, and X. C. Zeng, J. Am. Chem. Soc. 127, 15680�2005�.

53 L.-M. Wang, S. Bulusu, H.-J. Zhai, X.-C. Zeng, and L.-S. Wang, Angew.Chem. Int. Ed. 46, 2915 �2007�.

54 L. Gagliardi, J. Am. Chem. Soc. 125, 7504 �2003�.55 X. Li, B. Kiran, J. Li, H. J. Zhai, and L.-S. Wang, Angew. Chem. Int. Ed.

41, 4786 �2002�.56 C. E. Klots, J. Chem. Phys. 92, 5864 �1988�.57 X. Li, B. Kiran, L.-F. Cui, and L.-S. Wang, Phys. Rev. Lett. 95, 253401

�2005�.58 M.-X. Chen and X. H. Yan, J. Chem. Phys. 128, 174305 �2008�.59 M. S. Rong, F. M. Liu, X. Y. Li, Y. F. Zhao, and X. G. Jing, Chem. Pap.

234309-8 C. Rajesh and C. Majumder J. Chem. Phys. 130, 234309 �2009�

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

129.93.61.27 On: Sat, 22 Aug 2015 02:01:08

61, 308 �2007�.60 M. Heinebrodt, N. Malinowski, F. Tast, W. Branz, I. M. L. Billas, and T.

P. Martin, J. Chem. Phys. 110, 9915 �1999�.61 C. Majumder, A. K. Kandalam, and P. Jena, Phys. Rev. B 74, 205437

�2006�.62 C. Majumder and S. K. Kulshreshtha, Phys. Rev. B 73, 155427 �2006�.63 CRC Handbook of Chemistry and Physics, 49th ed., edited by R. C.

Weast �CRC, Cleveland, 1969�.64 G. Kresse and J. Hafner, Phys. Rev. B 47, 558 �1993�; G. Kresse and J.

Furthmuller, Phys Rev. B 54, 11169 �1996�; Comput. Mater. Sci. 6, 15

�1996�; G. Kresse and J. Hafner, Inst. Chem. Eng. Symp. Ser. 6, 8245�1994�; Phys. Rev. B 49, 14251 �1994�.

65 P. Perdew, in Electronic Structure of Solids ’91, edited by P. Ziesche andH. Eschrig �Akademie, Berlin, 1991�.

66 C. Kittel, Introduction to Solid State Physics, 7th ed. �Wiley, New York,1996�.

67 K. E. Shuler, J. Chem. Phys. 21, 624 �1953�.68 R. Burgert, H. Schnöckel, A. Grubisic, X. Li, S. T. Stokes, K. H. Bowen,

G. F. Ganteför, B. Kiran, and P. Jena, Science 319, 438 �2008�.69 S. Chrétien and H. Metiu, J. Chem. Phys. 129, 074705 �2008�.

234309-9 Oxidation of Al doped Au clusters J. Chem. Phys. 130, 234309 �2009�

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