Oxygen Activation on Nanometer-Size Gold Nanoparticles - The

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Oxygen Activation on Nanometer-Size Gold Nanoparticles Aleksandar Staykov,*,† Tomonori Nishimi,‡ Kazunari Yoshizawa,†,‡ and Tatsumi Ishihara†,§ †

International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), ‡Institute for Materials Chemistry and Engineering, and §Department of Applied Chemistry, Faculty of Engineering, Kyushu University, Fukuoka, 819-0395 Japan S Supporting Information *

ABSTRACT: The structure, electronic properties, and catalytic activity toward oxygen activation of gold nanoclusters with size between 10 and 42 atoms were investigated with first principle methods. Nanoparticle symmetry, bond lengths, and surface charge distribution were analyzed and compared to those of macroscopic gold surfaces. Irregular charge distribution was found on the surfaces of nanoparticles consisting of fewer than 30 gold atoms. Nanoparticles with more than 30 atoms were characterized with core−shell charge separation, e.g, positively charged core and negatively charged surface. The charge distribution on those nanoparticles significantly differs from the charge distribution on macroscopic gold surface. The structure and electronic properties of the gold nanoparticles were related to their catalytic activity toward the aerobic oxidation of organic molecules, e.g., cyclohexane. It was found that oxygen is activated by partially negatively charged surface gold atoms. Nanoparticles with sizes between 10 and 30 gold atoms could only activate oxygen over the negatively charged surface active sites, whereas larger nanoparticles could activate oxygen over the whole surface. The results are in good agreement and provide detailed understanding of recently published experimental data of aerobic oxidation on subnanometer gold nanoparticles (ACS Catal. 2011, 1, 2−6).

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INTRODUCTION The application of nanoparticles in heterogeneous catalysis is investigated intensively in the last decades due to their enhanced catalytic properties compared to those of bulk materials and macroscopic surfaces.1,2 Their catalytic activity is often related to their extended surface area compared to the bulk catalyst. The larger surface area provides access for more reacting molecules and leads to higher turnover frequencies. Beside the extended area, often the enhanced catalytic activity of nanoparticles is related to their finite size and geometry. The nanoparticles possess larger number of irregular surface areas, i.e., edges, adatoms and tips, compared to the macroscopic materials. Those surface areas are characterized with different geometry, i.e., bond lengths, bond angles, low-coordinated atoms, etc., compared to the periodic surface. Differences in the geometry affect the electron distribution on the surface and, as a result, the nanoparticles’ edges and tips are characterized with different surface-charge distribution from the periodic surface. Nanoparticles can be deployed on supporting surfaces, which can further tune their catalytic properties by surface−nanoparticle electron transfer.3 An additional advantage of the nanoraticles compared to the bulk catalysts is that they can be used in colloidal solutions, which significantly improves the accessibility of the reacting molecules to the catalytic surface.4 The activity of nanocolloidal palladium and palladium−gold bimetallic catalysts was demonstrated by Ishihara and coauthors in the direct synthesis of hydrogen peroxide.4 Tsukuda and coauthors demonstrated recently a precise method for atomnumber controlled gold nanoparticle synthesis.5 They achieved © 2012 American Chemical Society

the synthesis of nanoparticles with number of gold atoms between 10 and 80.5 That synthesis was performed by thiolligand stabilization in colloidal solution and the sequential removal of the supporting ligands. Unprotected gold nanoparticles show strong affinity to merge and form larger clusters.2 One possible way to avoid this conglomeration was to deploy them on hydroxiapatite surfaces.5 The size of the surface deployed nanoparticles was reported to be in the sub-2 nm regime.5 STEM images of 2−5 nm gold nanoparticles after the removal of the supporting ligands were provided by Hutching and coauthors.6 Those nanoparticles show well-ordered, highly symmetrical crystalline structures with clearly defined Au(111) and Au(100) planes.6 Beside the larger catalytic surface area, experimental studies suggest that the nanoparticles might possess different physical properties from the bulk materials.5−9 Haruta and coauthors reported size-dependence of the melting temperature of gold nanoparticles.7−9 The melting temperature is a material-specific property, which depends on the crystal structure and bond strength. Tsukuda and coauthors reported size dependence of the catalytic activity in their study of aerobic oxidation of organic molecules on sub-2 nm gold nanoparticles in aqueous solutions.5 Macroscopic gold is known as a material, which does not activate molecular oxygen and does not build stable oxides or surface oxide layers.2,3 However, small gold nanoparticles Received: February 26, 2012 Revised: July 8, 2012 Published: July 11, 2012 15992

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and CO oxidation on small gold clusters.13,21 Gao et al. investigated oxidation of CO on gold clusters with sizes of 16− 18, 20, and 27−30 atoms.13 In the proposed reaction mechanism CO and O2 are coadsorbed on neighboring Au atoms. They investigated the site dependence of the adsorption energy on neutral and anionic gold clusters. In their conclusion the catalytic activity was related to the geometry of the nanoparticle, e.g., with definition of cone angles.13 Shang and Liu investigated gold nanoparticles with sizes between 2 and 4 nm and up to 923 atoms including the solvent effect.21 They concluded that the reaction occurs on edge sites where O2 is adsorbed in side-on orientation to the surface on two gold atoms. They have shown that the aqueous solution is crucial for the catalytic activity.21 CO oxidation was investigated on gold clusters with 10 atoms where the dependence on the two and three-dimensional cluster structure was studied.22 Important for the understanding of the catalytic activity is the study23 of interaction between Au− and various small molecules, namely, O2, H2O, and CH4, as well as the combined experimental and theoretical work of Häkkinen and coauthors on the catalytic oxidation of CO by Au2−.24 It is worth noting that most theoretical studies suggest that anion nanoparticles show better catalytic activity toward oxidation reactions compared to the neutral species. In this study we investigate the structure and electronic properties of Au nanoparticles with sizes between 10 and 42 atoms. The aim of the study is to provide an understanding for their enhanced catalytic activity in oxidation reactions. This work focuses mainly on the first step of the oxidation, namely the activation of oxygen, related to the nanoparticles structure. The detailed investigation of the surface structure and electronic properties as well as the possible orientations and activation of the oxygen molecule on the surface would allow the further design and modification of the nanoparticles and their application in various processes.

show high activity toward oxygen activation, which suggests that their surfaces possess different geometry or electronic properties from the large two-dimensional surfaces. Tsukuda and coauthors tracked this property for gold clusters with sizes between 10 and 80 atoms.5 Gold nanoparticles with sizes between 10 and 30 atoms showed moderate turnover frequency for aerobic oxidation per atom, nanoparticles with sizes between 30 and 50 atoms showed highest turnover frequency per atom, and nanoparticles with sizes over 80 atoms showed low turnover frequency per atom.5 It is challenging to provide a theoretical explanation of the observed phenomenon, which would allow for the design of nanoparticles with controlled catalytic activity. The gold nanoparticle catalysis was compared with the thin-film gold catalysis. Chen and Googman have demonstrated that mono- and bilayer gold films deposited on metal oxides show catalytic activity toward CO oxidation, which is similar to that of the nanoparticles.3 They have concluded that the interaction between the surface-layer gold atoms and the first subsurface layer is of major importance for the catalytic activity.3 The experimental studies of Haruta,7−9 Ishihara,4 Tsukuda,5 Goodman,3 and Hutchings6 showed the interesting catalytic and physical properties of gold nanoparticles that arise from their finite size. The gold nanoparticle based catalysts can find applications in different processes in the preparative and synthetic chemistry, electrochemistry, colloidal chemistry, air purification, green energy, energy storage, etc. Finite size gold nanoparticles with precise control of their atom number were applied in the aerobic oxidation of organic molecules, e.g., cyclohexane.5 The industrial oxidation reactions are characterized with organic wastes and high-energy consumption, which can be avoided with the gold-nanoparticle catalysts. Gold nanoparticles can be used in the direct synthesis of H2O2 from H2 and O2.4,10−12 They can be applied in air purification using their ability to oxidize hazardous gases, e.g., CO, in aerobic conditions.13 Various types of theoretical methods were used to investigate the structure and catalytic activity of gold nanoparticles varying form molecular dynamics, density functional theory (DFT), and DFT with numeric functionals implemented in the Siesta and DMol3 programs. The switch between planar and nonplanar gold nanoparticles was investigated by Olsen et al. for clusters with sizes of 6−8 atoms.14 The high-symmetry structure of gold cluster with 13 atoms and its interaction with thiol linkers was investigated by Larsson et al.15 Extensive theoretical study of gold clusters with different size was performed by Zeng group.13,16−20 The ground state geometry and electronic properties of gold nanoparticles with different stoichiometry were established. They have investigated the lowenergy ground state geometries of neutral particles with 15−19 atoms showing that these particles are characterized with surface atoms only.16 The structural transition between “cagetype” structure to “pyramid-type” structure was investigated for the anions of particles with 16−20 atoms and it was shown that the transition occurs for particles with 18 atoms.17 The structural transition between “pyramid-type” structures to “core-shell” type structures was shown for anions with 21−25 atoms.18 The important structural properties of the core−shell particles were investigated for the anion cluster with 34 atoms.19 It was shown that this particle is characterized with a “soft” easy to deform shell and a rigid core. For the particle with 42 atoms, a fullerene type of structure was estimated.20 Theoretical studies were performed for aerobic oxidation of organic molecules on gold nanoparticles in aqueous solutions

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COMPUTATIONAL METHODS The structure and the electronic properties of gold clusters with sizes between 10 and 42 atoms were investigated with density functional theory (DFT) implemented in the Gaussian 09 program.25 The B3LYP26 hybrid DFT functional was employed in the study, and the computational results were verified with the BP8627 pure DFT functional. No qualitative differences were obtained from the calculations with different functionals. The LANL2DZ basis set with effective core potentials was used for the gold atoms, and the results were verified with the triple ζ SDD functional. Oxygen atoms were described with the 631G(d) basis set and the results were verified with 6-311+G(d) basis set. No qualitative differences were observed for the calculations with double ζ and triple ζ basis sets. Geometry optimization of the nanoparticles was performed for open-shell electronic configurations. The reported results correspond to open-shell singlet calculations. The results were compared with those for triplet and higher spin states; however, it was found that the low-spin state was characterized with lower energy. The investigated nanoparticles with 10, 18, 20, 24, 38, and 42 atoms are shown in Figure 1. All nanoparticles possess high symmetry, and their surfaces, with the exception of Au24, are characterized with planes with 100 and 111 Miller indexes. Symmetry was used wherever it was possible to speed up the calculations. The oxygen adsorption was investigated on different sites of the nanoparticles. The activation of the 15993

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with lower symmetry C2v (isosceles triangle). The symmetry of the nanoparticle affects the charge distribution and the bond lengths. The atoms on the tips of the triangle are negatively charged, whereas the central atom is positively charged. The base of the isosceles triangle is also negatively charged. The bond lengths vary between 2.68 and 3.00 Å. Au20 has the shape of a tetrahedron and was previously investigated by Li et al.34 It is characterized with C3v symmetry. The tips of the tetrahedron are negatively charged, whereas all other atoms are positively charged. The bond lengths on the surface vary between 2.73 Å and 3.08 Å. Particle Au18 is derived from Au20 after the removal of two tips of the tetrahedron. As a result of the removed atoms the symmetry point group is lowered from C3v (tetrahedron) to C2v. Au18 has different charge distribution from Au20. In the work of Zeng and coauthors it was shown that particles with 18 atoms mark the transition between “closecage” structure and “pyramid-type” structure.17 The remaining tetrahedron tips are negatively charged. The remaining atoms at the edge from which the tips were removed are also negatively charged. The bond lengths vary between 2.67 and 3.11 Å. Particle Au24 was investigated by Liu et al.35 It consists of three stacked gold hexagons toped on each side by two gold triangles. The particle has nanorod shape and is characterized with high symmetry, D3d. Both ends of the nanoparticle are negatively charged, whereas the middle part is positively charged. The bond lengths vary between 2.77 and 2.94 Å. The bond lengths and the charges of nanoparticles Au10, Au18, Au20, and Au24 are shown in Figure 2. The irregular charge

Figure 1. Structures of the investigated gold nanoparticles with 10, 18, 20, 24, 38, and 42 atoms.

oxygen molecule was estimated by the oxygen−oxygen bond elongation and the charge of the oxygen atoms. An additional study was performed for periodic twodimensional Au(111) and Pd(111) surfaces with plane-wave DFT method. The calculations were performed with CASTEP 5.5 software using PBA-GGA functional with 340 eV energy cutoff and 3 × 3 × 1 k-points.28 The calculations were performed for 3 × 3 atoms unit cell. Six layers were considered for the Pd(111) surface, and six, four, and three layers were considered for the Au(111) surface. A vacuum slab of 20 Å was included for all investigated models. Throughout this study Mulliken population analysis was used to estimate the partial atomic charge.29 Natural bonding orbitals30 (NBO) analysis was also performed; however, it is based on localized orbitals, which cannot be applied well in the case of the delocalized metal−metal bond. In this work we used orbital analysis to understand the interaction between the oxygen molecule and the gold nanoparticles. This method was already used for small clusters with sizes of 8−11 atoms.31 The Mulliken population analysis is based on the orbital population and describes well the delocalized metal−metal bonds. Its main drawback is its basis set dependence especially for compounds build by different chemical elements. However, in the case of gold clusters this effect should be minimal because they consist of one element, e.g., Au. We have tested the Mulliken charge accuracy by calculations with double ζ and triple ζ basis sets that yield qualitatively similar results.

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Figure 2. Bond lengths and Mulliken charge distribution in nanoparticles with sizes of 10, 18, 20, and 24 atoms. (A) Bond lengths. All distances are in Å. (B) Mulliken charges. The negatively charged atoms are in green, and the positively charged atoms are in white.

RESULTS AND DISCUSSION Structure and Properties of Gold Nanoparticles. The investigated gold nanoparticles are shown in Figure 1. All nanoparticles are characterized with a long-side diameter close to 1 nm, which is in agreement with the experimental observations.5 Nanoparticles Au10, Au18, Au20, and Au24 possess surface atoms only. Nanoparticles Au38 and Au42 possess surface atoms and one layer of subsurface atoms. Our computational results show that the properties of the nanoparticles consisting only of surface atoms differ from those of nanoparticles that consist of surface and subsurface layers. Nanoparticles with 10,32,33 18,17 20,34 and 2435 atoms were investigated previously, and their geometries were reported in the literature. Geometry optimization of Au10 shows that the ground state is not characterized with the highest possible symmetry, e.g., D3h (equilateral triangle) but

distribution and different Au−Au bond lengths are a result of the finite particles’ size, which affects the particles’ wave functions. Those nanoparticles are characterized with large deviations between the surface bond lengths (∼0.3 Å) and surface charges on the gold atoms. The particles’ geometry and electronic properties suggest that their surface will have sites with different catalytic activity. Negatively charged sites would help the initial step of reactions that require electrons. Different surface bond lengths would have different adsorption energies for diatomic molecules. The nanoparticles with 38 and 42 gold atoms are characterized with a surface layer and one subsurface layer. 15994

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charge difference, it was reported by Zeng and coauthors that the core and shell atoms of the nanoparticles show different structural properties.19 Experimentally, it is difficult to measure charge separation in nanoparticles; however, attempts were done by Yasumatsu and coauthors.36,37 They investigated platinum nanoparticles deployed on a Si surface. Owing to the surface particle interaction, the clusters form one-dimensional disk-like single layer structures. Such disk-like structures can approximately represent slices of nanoparticles. The STM measurement has shown that the central part is characterized with a positive charge and the periphery with a negative charge. They have also shown that oxygen reduction occurs preferably on the periphery of the disk.

Their geometry and electronic properties differ from those of nanoparticles with 10, 18, 20, and 24 gold atoms. Singlet ground state with low-lying triplet excited state was estimated for both clusters. Particle Au38 has C1 symmetry with structure close to the D4h point group. The bond lengths between the neighboring gold atoms vary between 2.77 and 3.07 Å. The bond lengths on the surface vary between 2.84 and 3.03 Å. The surface of Au38 is negatively charged, whereas the inner-layer atoms are positively charged. The results reveal that Au38 is characterized with a core−shell structure, e.g., positively charged core and negatively charged shell. The unified bond lengths and charge distribution on the surface suggest that the whole surface would have similar catalytic activity. The negatively charged surface atoms would catalyze the initial step of reaction, which require electrons. Particle Au42 has D4h symmetry. Its surface consists mainly of 111 planes. The bond lengths between the neighboring gold atoms vary between 2.82 and 2.99 Å. Au42 is characterized with unified negative charge distribution on the surface. The particle has a positively charged core and a negatively charged surface. The surface bond lengths and charge distributions of particles Au38 and Au42 are shown in Figure 3. Those results reveal that the whole surface will

Figure 4. Charge distribution on the Au(111) and Pd(111) surfaces. (A) Six layer Au(111) surface. (B) Six layer Pd(111) surface. (C) Three layer Au(111) surface. (D) Four layer Au(111) surface. The charge of the atoms in each layer is shown next to each model.

The negative surface charge is not a phenomenon related only to the finite-size nanoparticles, but it is also observed for macroscopic surfaces. We performed periodic calculations for Pd(111) and Au(111) surfaces modeled by six layers and 20 Å vacuum slab. The top two layers were relaxed while the rest of the atoms were fixed in order to model the structure of the bulk material. The computational results show that the atoms in the top layer of the Au(111) surface are negatively charged with −0.09 electrons, whereas the atoms in the second layer are positively charged with 0.09 electrons. The atoms in the third layer are characterized with a neutral charge. The charge separation is a result of the irregular potential, which exhibit the atoms on the surface compared to the subsurface atoms, as well as, the lower coordination of the atoms on the surface. Each surface atom is bound to 7 neighbors while each subsurface atom is bound to 12 atoms. The smaller charge separation is a result from the planar structure and the large number of subsurface layers, which provide electron density that equalizes the charges in the two top surface layers. The three and four layer models result in a core−shell-like charge separation between the surface atoms and the subsurface layers. The outer layers are negatively charged while the inner layers are positively charged. From the obtained results it can be concluded that the largest charge separation is obtained for nanoparticles that consist of two layers of atoms. Similar results

Figure 3. Surface bond lengths and Mulliken charge distribution in nanoparticles with 38 and 42 atoms. (A) Bond distances on the surface. All distances are in Å. (B) Charge distribution on the surfaces. (C) Charge distribution through slices of the nanoparticles. Negatively charged atoms are in green, and positively charged atoms are in white.

show the same catalytic activity. Those nanoparticles are also characterized with an interesting core−shell structure. The reason for the charge difference between the inner and the outer shells is the different surroundings of the gold atoms. Each gold atom from the inner shell possesses 12 neighbors; that is, it contributes with electron density to 12 metal−metal bonds. The atoms in the inner layer also exhibit relatively unified spatial potential interactions with the neighboring nuclei. Each atom in the outer shell possesses between 4 and 7 neighbors; that is, it contributes with fewer electrons to the metal−metal bonds compared to the atoms from the inner shell. The atoms on surface exhibit irregular spatial potential interaction with the neighboring nuclei. Toward the center of the nanoparticle they exhibit the interactions with the nuclei in the inner shell, while in the opposite direction the nanoparticle is terminated with the void. Those structural phenomena lead to the significant charge separation between the atoms in the core and the atoms in the shell of the nanoparticles. Beside the 15995

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are obtained for thin two-dimensional films. Larger nanoparticles with many subsurface layers and thicker twodimensional films would be characterized with limited charge separation between the surface layer and the first subsurface layer. The obtained results for periodic calculations of the six layer Pd (111) surface show larger charge difference between the top layer and the subsurface layer compared those for to the Au(111) surface. This result is in agreement with the experimental results for the better catalytic activity of palladium compared to gold.4,11 Oxygen Activation on the Nanoparticles’ Surface. The oxygen activation is estimated by the elongation of the oxygen− oxygen bond and the charge transfer from the nanoparticle to the oxygen molecule. Two possible orientations of the oxygen molecule on the nanoparticle surface were investigated. Those are the end-on and the side-on and orientation. In the end-on orientation the oxygen molecule is asymmetrically adsorbed on the surface with one oxygen atom bond to the gold and the other oxygen atom pointing to the void. This asymmetrical adsorption leads to different charge distribution on each oxygen atom. In the study of Gao et al. the end-on orientation is adopted in the reaction mechanism of CO oxidation on gold nanoparticles.13 In the side-on orientation the oxygen molecule is adsorbed parallel to the surface. Depending on the adsorption site both oxygen atoms may have equivalent charge distribution. Different end-on and side-on starting geometries were investigated for oxygen adsorption on Au10. The optimized stable Au10−O2 structures are shown in Figure 5.

kcal/mol. In panels C and D of Figure 5 are shown the geometries and relative energies of the side-on oxygen adsorption sites. In Figure 5C is shown the oxygen adsorption characterized with the lowest energy and longest oxygen− oxygen bond. The bond distance between the oxygen atoms is 1.366 Å. The oxygen is adsorbed on two equivalently (negatively) charged gold atoms. That leads to equivalent charge distribution on each oxygen atom. In Figure 5D is shown the oxygen adsorption on two gold atoms with different charges. That results in a shorter oxygen−oxygen distance, 1.319 Å, and higher energy, 1.13 kcal/mol. From the results shown in Figure 5 it can be concluded that the oxygen adsorption on positively charged gold atoms is energetically unfavorable. The side-on adsorption is energetically more favorable than the end-on adsorption. The elongation of the oxygen−oxygen bond is related to the energy of the nanoparticle−oxygen complex. The structures, which are characterized with lower energies, are also characterized with longest oxygen−oxygen bonds, i.e., better oxygen activation. The longest oxygen−oxygen bond is estimated for O2 adsorbed in side-on orientation over two gold atoms. For Au10, side-on geometry could be optimized only for oxygen adsorbed on the negatively charged base of the isosceles triangle. The calculated oxygen−oxygen bond is 1.37 Å, and the charge on each oxygen atom is −0.24 electrons. The obtained results show that O2 is activated on negatively charged active site of the nanoparticle. The analysis of the highest occupied orbitals of the Au10−O2 complex shows significant orbital amplitudes on the antibonding π* orbitals of the oxygen molecule. This means that electron density from the nanoparticle is transferred to the π* orbitals of the oxygen molecule. The occupied π* orbitals have nodal plane between the two oxygen atoms, which is perpendicular to the oxygen−oxygen bong. This nodal plane leads to reduced electron density between the oxygen nuclei. The π* orbitals of oxygen overlap well with the d orbitals of gold, which leads to electron density transfer between the oxygen molecule and the nanoparticle (Scheme 1). This result Scheme 1

Figure 5. End-on and side-on adsorptions of O2 on Au10. (A) End-on adsorption on a positively charged Au atom. (B) End-on adsorption on a bridge site between two negatively charged Au atoms. (C) Sideon adsorption on two equivalently, negatively charged Au atoms. (D) Side-on adsorption on two nonequivalently, negatively charged Au atoms. All distances are in Å.

shows that the side-on adsorption of oxygen is a stable chemisorptions state. In Figure 6 are shown the distances between the oxygen atoms and the gold nanoparticle, the spatial distribution of the highest occupied molecular orbital and the computed SCF electron density. The oxygen is bound to the nanoparticle surface through covalent type bonds. The NBO analysis shows bond order of 0.5 between each oxygen atom and the gold atoms from the nanoparticle. The bond order between the both oxygen atoms is 1. Spin polarization was not calculated for the oxygen atoms. The reason for that is the covalent type of bonding between the nanoparticle and the oxygen molecule. The end-on adsorption of O2 at the negatively charged vertices of Au10 is not favorable due to

In panels A and B of Figure 5 are shown the geometries and relative energies of the end-on oxygen adsorption sites. In Figure 5A O2 is adsorbed over the positively charged gold atom located in the middle of the triangle. The oxygen−oxygen bond length is 1.217 Å, which is close to that of the gas phase oxygen. The distance between the surface and the oxygen is 2.971 Å. That structure is characterized with the highest relative energy, 29.04 kcal/mol. In Figure 5B is shown an end-on oxygen adsorption on the nanoparticle’s edge. The distance between the gold atoms and the oxygen is 2.273 Å. The oxygen−oxygen bond is 1.276 Å. The oxygen−oxygen bond is slightly elongated compared to the gas-phase molecule. The relative energy is 4.66 15996

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Figure 6. Oxygen adsorbed on Au10. (A) Distances between the oxygen atoms and the gold nanoparticle. All distances are in Å. Gold atoms are shown with yellow color, oxygen atoms are shown with red color. (B) Spatial distribution of the highest occupied molecular orbital. (C) Computed SCF electron density.

Figure 7. Oxygen adsorbed on gold nanoparticles with 18, 20, and 24 gold atoms. (A) O2 adsorbed on Au18. (B) O2 adsorbed on Au20. (C) O2 adsorbed on Au24. All distances are in Å. Gold atoms are shown in yellow, and oxygen atoms are shown in red.

calculated for the negatively charged ends of the nanoparticle. It is worth noting that side-on oxygen adsorption is calculated only when the two gold atoms are characterized with similar charge. If there is a significant difference in the charge of the gold atoms then end-on adsorption is calculated. On the sideon adsorption site the oxygen−oxygen bond is elongated to 1.33 Å and the charge on each oxygen atom is −0.19 electrons. The analysis of the highest occupied orbitals of the Au24−O2 complex shows significant orbital amplitudes on the antibonding π* orbitals of the oxygen molecule. In Figure 7 are shown the distances between the oxygen atoms and the gold nanoparticle adsorbed on Au18, Au20, and Au24. The obtained results for oxygen adsorption on nanoparticles Au10, Au18, Au20, and Au24 show that two possible adsorption geometries exist, e.g., end-on and side-on O2 orientation. For the end-on adsorption the oxygen molecule is adsorbed on the top of a gold atom or on the bridge between two gold atoms. For the end-on adsorption the oxygen−oxygen bond is slightly elongated and there is a minor charge transfer from the nanoparticle to the oxygen molecule. The side-on adsorption occurs on a top-bridge-top site over two gold atoms. The oxygen−oxygen bond is significantly elongated to over 1.3 Å. There is a significant charge transfer from the nanoparticle to the oxygen molecule. The adsorption sites for side-on adsorption on gold nanoparticles are always negatively charged gold atoms. The obtained results show that not the entire nanoparticle surface possesses catalytic activity but only the negatively charged active sites. Thus, the turnover frequency per gold atom for oxidation reaction catalyzed by gold clusters consisting of less than 30 atoms is moderate. Oxygen is activated by electron transfer from negatively charged gold atoms to the antibonding π* orbitals of the oxygen molecule. Those orbitals are characterized with a nodal plane between the oxygen nuclei, and when occupied, they lead to an elongated oxygen−oxygen

the unfavorable spatial distribution of the frontier orbitals. The d orbital at a single gold atom does not overlap well with the π* orbitals of oxygen. That overlap is important for the O2 chemisorption on the nanoparticle and the oxygen−oxygen bond elongation. Similar results were previously obtained by Wells et al.,31 who showed that the overlap between the frontier orbitals of O2 and the gold nanoparticle determine the adsorption geometry. They have also shown that the electron transfer between the nanoparticle and the π* orbitals of O2 leads to the oxygen−oxygen bond elongation. Side-on adsorption of O2 on Au20 was not obtained. This result was expected because on Au20 only the tips of the tetrahedron are negatively charged. For side-on adsorption of O2 are required two neighboring, negatively charged gold atoms. On Au20 O2 is adsorbed end-on to a tip or edge. However, in end-on orientation the oxygen−oxygen bond is only slightly elongated to ∼1.25 Å and the charge on each oxygen atom is ∼−0.07 electrons. This result shows that the oxygen activation is strongly dependent on the nanoparticle atom number and geometry. Among the highest occupied orbitals of the Au20−O2 complex were not found orbitals with significant amplitude on the antibonding π* orbitals of the oxygen molecule. This is the main reason for the weak activation of the end-on adsorbed oxygen. On Au18 O2 is adsorbed side-on on the negatively charged site of the nanoparticle, while it is adsorbed in end-on orientation on the remaining tips. On the side-on adsorption site the oxygen− oxygen bond is elongated to 1.31 Å and the charge on each oxygen atom is −0.17 electrons. The analysis of the highest occupied orbitals of the Au18−O2 complex shows significant orbital amplitudes on the antibonding π* orbitals of the oxygen molecule. The results for oxygen adsorption on Au18 and Au20 demonstrate that a small change in the number of atoms or geometry leads to significant change in the nanoparticle activity. On the surface of Au24 side-on adsorption of oxygen was 15997

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Figure 8. Oxygen adsorbed on gold nanoparticle Au42. Different adsorption sites are considered. All distances are in Å. Gold atoms are shown in yellow, and oxygen atoms are shown in red.

bond. As a result of the good overlap between the π* orbitals of the oxygen molecule and the d orbitals of the gold atoms, significant electron density can be found between the nanoparticle surface and the oxygen molecule. This result shows that the side-on adsorption of oxygen on small gold nanoparticles leads to the formation of a stable chemisorption state. Nanoparticles Au38 and Au42 are characterized with core− shell structure, i.e., a positively charged core and a negatively charged shell. Owing to the negatively charged shell, it is expected that the entire nanoparticle surface can take part in the oxygen activation in contrast to nanoparticles Au10, Au18, Au20, and Au24, for which only the active sites could activate oxygen. Different starting geometries of the oxygen molecule on the Au38 were investigated. On nanoparticle Au38, the estimated bond lengths of the oxygen−oxygen bond vary between 1.30 and 1.36 Å. The estimated charges on each oxygen atom vary between −0.17 electrons and −0.24 electrons. On nanoparticle Au42, the estimated bond lengths of the oxygen−oxygen bond vary between 1.34 Å and 1.44 Å. The oxygen adsorption on the Au(100) surface results in the longest oxygen−oxygen bond among all investigated adsorption sites on all nanoparticles: 1.44 Å. At that site O2 is adsorbed over four gold atoms. The charge on each oxygen atom is −0.26 electrons. The structure is characterized with high symmetry: C2v point group. All other adsorption sites are characterized with an oxygen− oxygen bond shorter than 1.4 Å. Similar to nanoparticle Au38, the entire surface of Au42 is catalytically active and leads to elongation of the oxygen−oxygen bond. In Figure 8 are shown the obtained geometries for different Au42−O2 complexes. The analysis of the highest occupied orbitals of oxygen adsorbed on Au42 shows significant amplitudes on the antibonding π* orbitals of the oxygen molecule. The antibonding π* orbitals of the oxygen molecule overlap with the d orbitals of two neighboring Au atoms. Only for oxygen adsorbed on the Au(100) surface, the O2 π* orbitals overlap with the d orbitals of four neighboring gold atoms. As a result the oxygen−oxygen bond is significantly elongated to 1.44 Å. The turnover frequency of a catalyst rises significantly with the number of molecules that can be activated simultaneously. On nanoparticles consisting of less than 30 atoms, the negatively charged active sites are few and the activation of more than one molecule is significantly hindered. However, the whole surface of the nanoparticles consisting of more than 30 atoms is negatively charged. Owing to that negative charge the nanoparticle could activate simultaneously a larger number of oxygen molecules. Calculations were performed for nanoparticle Au42 with two and four oxygen molecules simulta-

neously adsorbed on the surface. The optimized geometries and the oxygen−oxygen bond lengths are shown in Figure 9. All oxygen−oxygen bonds are characterized with bond distances over 1.3 Å.

Figure 9. Two and four oxygen molecules adsorbed on gold nanoparticle Au42. All distances are in Å. Gold atoms are shown in yellow, and oxygen atoms are shown in red.

The catalytic activity of all investigated nanoparticles toward oxygen activation is summarized in Figure 10. Au20 can adsorb O2 only in end-on orientation. That geometry leads to limited charge transfer from the nanoparticle to the oxygen molecule and small elongation of the oxygen−oxygen bond. Au10, Au18, and Au24 can adsorb O2 in end-on and side-on orientation. The side-on adsorption occurs only on the negatively charged active sites. That geometry leads to significant charge transfer from the nanoparticle to the oxygen molecule and significant oxygen−oxygen bond prolongation. Au38 and Au42 can adsorb O2 on the entire surface in side-on orientation. Those nanoparticles can activate multiple O2 molecules simultaneously.

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CONCLUSION In this study we have investigated with first principle methods the structure and electronic properties of finite-size gold nanoparticle consisting of 10 to 42 gold atoms. The nanoparticles have diameters close to 1 nm. It was found that the geometry and electronic properties of those gold clusters depend on their size and shape and differ significantly from the properties of the macroscopic gold surface. The different geometry and electronic properties are expected to lead to different catalytic activity. The investigated gold nanoparticles are characterized with irregular charge distribution. Smaller nanoparticles, consisting of less than 30 gold atoms, are characterized with positively and negatively charged active sites on the surface. Larger nanoparticles consisting of more than 30 atoms are characterized with a core−shell structure, i.e., a positively charged core and a negatively charged shell. The 15998

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Figure 10. Size and geometry dependence of the catalytic activity of gold nanoparticles with size between 10 and 42 atoms. With green color are shown negatively charged atoms and with white color are shown positively charged atoms.

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ACKNOWLEDGMENTS This work was supported by World Premier International Research Center Initiative (WPI), Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT), Japan. K.Y. is thankful for Grants-in-Aid for Scientific Research (No. 22245028) from the Japan Society for the Promotion of Science (JSPS) and the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT), the Kyushu University Global COE Project, the Nanotechnology Support Project, the MEXT Project of Integrated Research on Chemical Synthesis, and CREST of the Japan Science and Technology Cooperation.

electronic properties of the gold nanoparticles were related to their catalytic activity in aerobic oxidation reactions. The level of oxygen activation was estimated by the oxygen−oxygen bond elongation after the adsorption on the gold nanoparticle surface. The activation occurs as a result of electron density transfer from the nanoparticle to the antibonding π* orbitals of the oxygen molecule. The π* orbitals of the oxygen overlap well with the d orbitals of the surface gold atoms. That results in electron density between the oxygen molecule and the nanopartilce; that is, a stable chemisorptions state is formed. The oxygen activation occurs easily on negatively charged surface atoms. For gold nanoparticles consisting of less than 30 atoms only negatively charged active sites lead to oxygen activation. The entire surfaces of the nanoparticles consisting of more than 30 atoms are catalytically active owing to their unique core−shell structure. Several oxygen molecules can be activated simultaneously on larger nanoparticles owning to their partial negative charge on the entire surface. That result is consistent with the experimental observation of gold nanoparticles’ size-dependence of the aerobic oxidation turnover frequency per gold atom.

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ASSOCIATED CONTENT

S Supporting Information *

Atomic Cartesian coordinates for the optimized geometries of nanoparticles Au10, Au18, Au20, Au24, Au38, and Au42. This material is available free of charge via the Internet at http:// pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +81-92-8022527. Notes

The authors declare no competing financial interest. 15999

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