Phenomenal Properties of Gold Nanoclusters with No Chemical

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Phenomenal Properties of Gold Nanoclusters with No Chemical Interaction with O: Field Effect and Electron Pumping 2

Ali Moghaddasi, and Mansour Zahedi J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp401605j • Publication Date (Web): 03 Sep 2013 Downloaded from http://pubs.acs.org on September 4, 2013

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The Journal of Physical Chemistry

Phenomenal Properties of Gold Nano Clusters with No Chemical Interaction with O2: Field Effect and Electron Pumping

Ali Moghaddasi, Mansour Zahedi* Department of Chemistry, Faculty of Sciences, Shahid Beheshti University G.C., Evin, Tehran, Iran 19839, P.O. box 19839-63113 Tel: 98-21-29902889, Fax: 98-21-22431661 m-zahedi@ sbu.ac.ir [email protected]

*

Corresponding Author

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ABSTRACT: Molecular clusters of gold built from Au(111) layers, namely, Au27, and Au28 have been designed and optimized using the density functional theory (DFT) approach (B3LYP) with the LANL2DZ basis. The Au27 species are perfect three layer cluster models, while Au28 include an extra Au dopant atom. A brief survey of the experimental literature, as well as theoretical studies of O2 adsorbed on Au clusters, results in a peculiar puzzle. On the one hand, O2 adsorbed on Au nano clusters is greatly activated and its electronic states are shifted. On the other hand, it is reported that the O2 molecule does not have any chemical bonded type interaction with Au clusters. Our calculations for O2 adsorption on Au27 and Au28 nano clusters suggest two basic mechanisms for nonchemical bonding interactions and the simultaneous activation of an O2 molecule in a chemical process. Gold clusters can act as a destabilizing perturbing potential field for the electronic state of the adsorbing species. Thus the well known gold metal’s nobility can be accounted by the aforementioned field effect. Concurrently, Au clusters can pump electron density to the already destabilized states. Thus we believe that these complementary effects of gold nano clusters namely, the negative electric field, as well as electron pumping, are the main causes of the O-O bond weakening.

Keywords: Field effect; Electron pumping; Gold nano cluster; Oxidative site; Charge transfer; Gold surface; DFT study


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I. Introduction: Gold is commonly regarded as the most inert element [1]. However, the discovery of the exceptional catalytic properties of gold nanoparticles for low temperature CO oxidation [2] initiated great interest due to its promising applications and spawned a large number of studies devoted to the understanding of the reaction mechanism [3-6]. Considerable effort to characterize the chemical interactions between gold clusters and a variety of molecules has been made [7-16]. However, Liu et al. calculated that the O2 dissociation barrier is larger than 2 eV on nonsupported Au clusters. In addition, at the Au/TiO2 interface the dissociation barrier is 0.52 eV [17]. Above theoretical results bring about an initial impression that O2 has a significant interaction with Au. Also, some more theoretical studies have been available with their basic results summarized above [18-30]. Contradictory to aforementioned report, results on the electronic state of gold clusters in the presence of O2 using X-ray photoelectron spectroscopy (XPS) suggest a weak and nonchemical bonded interaction of O2 adsorptions on gold clusters [31]. In addition, Gao et al. showed that the binding energy of O2 on gold is smaller than that of Ar [32]. Thus, based on aforementioned reports, spontaneous dissociation of molecular oxygen on the Au surface is not energetically favorable. Contrary to the above XPS data, Vishnu Kamath et al. reported the destabilization of the electronic states of O2 adsorbed on gold clusters [33]. Their reported UV Photoelectron Spectroscopy (UPS) spectra clearly show three peaks of the dioxygen species around 6.4, 9.2, and 12.5 eV, due to πg*, πu and 2pσg molecular levels, respectively. In order to compare the orbital energies of the adsorbed O2 with those of free oxygen molecule in the aforementioned



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states, they reported energy values of 10.5, 14.9, and 17.6 eV for πg*, πu and 2pσg, respectively [34]. Finally, Fischer [35] has assigned a band at 850 cm-1 in the high-resolution electron energy loss spectrum of oxygen adsorbed on a thin layer of Au(111)-covering a Ni surface to the OO stretching frequency of the molecularly adsorbed species. Interestingly, when the gold layer becomes thicker, above feature completely vanishes from the spectrum. This stretching frequency is considerably lower than that of free molecular oxygen (1555 cm-1). This brief survey of the literature of O2 adsorbed on Au clusters results in a peculiar puzzle. On the one hand, O2 adsorbed on Au nano clusters is greatly activated and its electronic states are shifted. On the other hand, it is reported that the O2 interaction with Au is even slightly weaker than that of an Ar atom, while no chemical interaction between O2 and gold clusters has been observed. To reconcile these contrasting observations and to come up with a solution for this puzzle, the adsorption of O2 on some gold nano clusters have been investigated theoretically. The complementary effects of the Au nano clusters, namely a field effect as well as an electronic charge pumping effect, have been shown to account for the observed experimental behavior.

II.

Theoretical and Computational Models

A. Cluster Model The essential features of the cluster embedding technique used in this study are illustrated in Figures 1a and 1b. The basic active cluster that is treated by DFT calculations is the Au27(Au(111)) cluster, with a size around 0.6 nm. While, the remainder of cluster includes 216 atoms up to its border. B. Computational Details


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All geometry optimizations and energy minimization were performed using the Gaussian-03 program (G03) [36].

The lower energy structures were obtained using the two-layer

ONIOM2 [37]. The universal force field approach (UFF), with no charges assigned to atoms, was used for the low-level calculations (Figures 1a and 1b). High-level calculations were performed using the density functional theory (DFT) approach (B3LYP) with the LANL2DZ basis set. The B3LYP functional along with the relativistic pseudo-potential LANL2DZ have been used in the literature [38] to study different Aun clusters, (n= 27 and 28). The Total number of gold atoms is 243 and 244 in the Au27 and Au28 model nano clusters respectively. Also all calculations have been carried out with the extended basis set 6-311+G*/LANL2DZ and the corresponding data are reported adjacent to the data of LANL2DZ basis set wherever appropriate. The Au27 moiety includes three Au(111) layers with nine atoms in each layer, while the Au28 species comprises just one extra dopant gold atom than the former. We allowed for full relaxation of the Au27/Au(111) clusters. Two layers were then kept fixed and a dopant gold atom was added and the cluster was then allowed to once more relax. For O2 adsorption on Au28 cluster molecules, three layers of Au28 were kept fixed and the dopant gold atom and the O2 molecule were allowed to relax. In order to study the O2 interaction with the gold cluster, a systematic scan of the distance between the two has been carried out. The O atom farther from the surface has been kept fixed at each instance (reported distances are those of the closer O atom to the surface), while the O-O bond length and its orientation have been allowed to relax. The gold clusters employed for above scan are the obtained full optimized geometries in which all three layers have been fixed. It can be questioned that the density functional do not correctly describe weak interactions and in general, the Au-Au distances calculated using DFT and pseudo-potentials or effective core potentials (ECP) are overestimated. To obtain reasonable values of distances and angles in gold compounds, it is necessary to perform the calculation at the MP2 level with atomic f 5 ACS Paragon Plus Environment


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functions on Au atoms [39,40]. However, a DFT calculation at the B3LYP level shows no significant change in the calculated Au-Au distance compared with the corresponding method at the MP2 level [39].

III.

Result and discussions

A. Analyzing the nature of adsorption energy of O2 on Au27 and Au28 clusters: natural bond orbital (NBO) analysis. In order to arrive at an explanation of the oxidative nature of gold clusters in oxidation reaction such as that of CO molecule, it will be informative to analyze the various possible chemical interactions of O2 adsorption on gold nano clusters (orbital-orbital interaction). To reach such goal, interactions between atomic orbitals, bond orbitals, and lone pair orbitals can be obtained by using NBO analysis. Such analysis was also employed to obtain second order interaction energies (∆Eij2) and intermolecular donor–acceptor interactions, arising from the noncovalent terms [41]. ∆Eij2 represents noncovalent delocalization effects which are associated with interactions between filled (donor) and unfilled (acceptor) orbitals. Such interactions are naturally described as being of ‘‘donor–acceptor”, ‘‘charge transfer”, or generalized ‘‘Lewis base–Lewis acid” type. Latter interaction is used to present hydrogen bonding and other strong forms of van der Waals interactions [41]. Tables 1 and 2 summarize some lone pair hybridization and antibonding interactions of O2 molecule and aforementioned Au27 and Au28 clusters. Results involved for a few donor– acceptor interactions of O2 adsorption on Au27 and Au28 clusters are also summarized in Tables 1 and 2, respectively. Some NBO second order interaction energies (∆Eij2) found greater than 1 kcal mol-1 for these donor–acceptor interactions are mentioned in these tables too. By taking a closer look at data of these tables, it is obvious that oxygen molecule does


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not interact to Au27 and Au28 clusters covalently since second order interaction energies range from 1-20 kcal mol-1. Interactions lying in the aforementioned range can be classified as van der Waals, hydrogen bonding and electrostatic type interactions. Calculated NBO results with the extended basis set (6-311+G*/LANL2DZ) combination for O2/Au27 and for O2/Au28 systems show the same trend as those summarized in Tables 1 and 2 and have been reported in supplementary information as Table 1s and Table 2s respectively. These weak interactions don’t satisfy the Vishnu Kamath et al. reports of destabilization of the electronic states of O2 adsorbed on gold clusters [33] as mentioned previously. So, alternative explanation on the nature of O2 adsorption on gold clusters will be required. B. Charge transfer between O2 adsorbed on an Au27 cluster (field effect) As stated above, for more explanations position of the O2 molecule was scanned from 10 Å to a distance that it assumes at the full optimization geometry. At each scan step, the O-O bond length and its orientation towards the Au27 cluster (Au(111) perfect surface with no additional dopant gold atom) (Figure 2) were allowed to relax. Then the O2 charge, O2 vibrational frequencies and molecule dipole moment were calculated. The results of these calculations are shown in Table 3. As can be seen from the results, by decreasing the O2 distance, the induction of a dipole moment is followed by charge losses on the O2 molecule (for B3LYP/LANL2DZ data). It is noteworthy that the induced dipole (dind) direction is away from the cluster. As further data in the same table reveals, the O-O bond length shortens as the distance between the O2 and the Au cluster decreases. Surprisingly the O2 bond length and its stretching frequency at a large distance of the O2 from the perfect gold cluster (10Å), slowly converges to that of the free O2 molecule. By combining the observed trends in Table 3 and taking into consideration the destabilization of electronic states reported by Vishnu Kamath et al. [33 ] (Figure 3), it can be concluded that there is a perturbation effect that simultaneously destabilizes the electronic state of the oxygen molecule and induces a dipole moment. As is further discussed in the following sections, the above perturbation can exhibit 7 ACS Paragon Plus Environment


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the effects of a negative field. Moreover, such field effect can account for the nobility of gold metal and its known resistance against oxidation.

C. Competition between the O2 charge loss and Au28 cluster electron pumping As for Au27, scanning the O2 molecule-Au28 distance (Figure 4) was followed by the calculation of charges, stretching frequency and its dipole moment at each step, with the results summarized in Table 4. Unlike the perfect cluster, due to a decrease in the adsorption distance of the O2 molecule at the dopant Au atom site (defect site), the charge on the O2 molecule increases, its bond length increases (Figure 5), while the O2 vibrational frequency decreases. This result is in agreement with our previous work on the site dependence of charge transfer on Au nano clusters [38]. An Au cluster has a tendency to pump electrons to defect sites. Also as O2 is moved further away from the Au28 cluster, the orientation of the induced dipole moment of the oxygen molecule changes direction and resembles that of O2 on an Au27 cluster (for LANL2DZ data), as explained previously. Thus, one can conclude that at a close O2-Au28 distance, the electron pumping phenomenon is the dominant effect, whereas, at longer distances (for LANL2DZ data), a long range field effect, as that observed for Au27 perfect cluster, can be found to account for the observed trends (Figure 5). In order to investigate the effect of larger number of dopant atoms as well as cluster shape on the O2 interaction with the cluster species, some calculations have been performed which their results are as follows.

Figure 6 presents full optimized geometry and relevant

parameters for Au29 cluster interaction with O2. Aforementioned cluster moiety was obtained by adding another gold dopant atom to already optimized Au28 species. As it is revealed from this figure, further weakening of O-O bond at the presence of two defect sites has been occurred. O2 molecule adsorbed on cluster with two defect sites (Au29) receives higher electron density due to amplification of electron pumping effect and subsequently experiences larger bond length. Also O2 vibrational frequency on Au29 cluster has been 8 ACS Paragon Plus Environment

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significantly lowered by 52cm-1 relative to the corresponding structure of Au28. Analogous effects have been observed for O2 while interacting with stepped shape cluster models in some preliminary calculations which support above observations. Also Moghaddasi et al. have reported that electron pumping can be considered as a function of atoms location on the cluster surface. Since by decreasing the cluster size the number of corners per unit area of cluster is expected to rise, electron pumping is increased [38]. It is necessary to remember that the interaction of O2 with Gold cluster cannot be classified as chemical interaction categories due to experimental and theoretical studies.

D. Molecular Orbital (MO) analysis of free and adsorbed O2 molecule on Au27 and Au28 nano clusters As stated earlier, it was concluded that at the surfaces of gold clusters such as Au27, a perturbation effect, that simultaneously destabilizes the electronic state of the oxygen molecule and induces a dipole moment, exists. As was further discussed in the former sections, the above perturbation can exhibit the effects of a negative field. To study the nature of such field, molecular orbital (MO) analysis has been carried out to shed more light on latter effect. The MOs of O2 molecule both adsorbed on gold nano clusters (O2/Au27, O2/Au28) as well as isolated O2 molecule have been calculated. After visualization of each MO of the former, the perturbation effect of the cluster on the O2 energy states have been investigated via their comparison with those of the isolated O2. Figures 7a and 7b present the MOs of isolated O2 on the left which are correlated to the corresponding MOs of O2/Au27 on the right, in each case. Taking a closer look at the MOs of latter, a clear splitting of O2 MOs at the presence of Au27 nanocluster and thereby removal of degeneracy of original free O2 spin orbitals is evident. This splitting is a reminder of the Stark or Zeeman field effects on splitting of degenerate states of molecules. More explanation regarding the origin of aforementioned 9 ACS Paragon Plus Environment


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effect will follow next when spin multiplicities of the involved species are taken into account. Another point to mention from this figure is the absence of any overlap between MOs of O2 and Au27 system except for HOMO states which show a very weak interaction.

This

observation is in agreement with the earlier NBO analysis that suggested no covalent interaction between them. Figures 8a and 8b illustrate analogous results for O2 and O2/Au28 systems. As it is indicated by Figure 8a, O2/Au28 MOs are destabilized relative to those of free O2 in this case and no splitting has occurred.

It should be emphasized that each

perturbed O2/Au28 energy state is twofold degenerate due to electron spin (Figure 8a). More explanation regarding the difference between Figures 8a and 8b will follow next when spin multiplicities of the involved species are taken into consideration. Moreover, O2 MOs show more interactions with those of Au28 relative to that of O2/Au27 mentioned above. Such difference can be attributed to the fact that Au28 nano cluster has defect compared to Au27. In order to take the O2 spin states into account, we have run calculations for O2/Au27 and O2/Au28 systems with various spin states. O2 in its low lying electronic state enjoys two spin multiplicities, namely a triplet ground state (

and two singlet excited states

and

being 7918 and 13195 cm-1 respectively above the ground state. The O2/Au27 system was optimized with the total spin multiplicities of doublet and quartet while, the O2/Au28 system with the singlet and triplet states. To observe the influence of the spin state on the resulting optimized geometries, one needs to refer to their molecular orbitals, especially the HOMO and LUMO ones. Looking at several of the latter more important MOs in the O2/Au27 and O2/Au28 systems, it has become evident that the oxygen molecule has either no contribution or assumes a very week share of the aforementioned MOs. In other words, the HOMO and LUMO orbitals can be mainly attributed to pure gold atoms. Thus, it is expected


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that gold atoms impart different spin field effects on to the embedded O2 molecule due to various spin states involved. Figures 7a and 7b illustrate the MO’s for the optimized O2/Au27 moiety with the doublet and quartet spin states, respectively. As can be seen, the two figures are almost identical and the only difference is that the O2’s MO energy states are slightly more stable for quartet spin state than those of doublet state as expected Figures 8a and 8b depict the MO’s for the optimized O2/Au28 species with the singlet and triplet spin multiplicities. An interesting observation of the latter figure is the fact that not only the O2/Au28 MO’s are almost all destabilized for the triplet spin state similar to that of the singlet case, but all of them are split into two new states as well. Thus due to the presence of unpaired electron spins in all above cases except for the singlet spin state, the splitting of the MO’s is observed as expected. So far, our calculated results in harmony with reported experimental as well theoretical results suggest that no covalent electronic orbital interaction present between O2 and two Au27 and Au28 nano clusters. While at the same time the aforementioned shifting as well as splitting of perturbed O2 states is nothing but an indication of presence of a strong perturbing field. In order to justify such argument, simulation calculations have been carried out to explore the effect of both electric and magnetic fields on the O2 energy states. Induction of an electric field up to 0.015 au caused no splitting or destabilization of O2 states and just introduced a very weak dipole moment. However, application of a spin magnetic field of 0.005 au resulted in considerable splitting of O2 MOs analogous to O2/Au27 system while use of an applied spin magnetic fields of 0.005 au and a negative electric field destabilized most of the O2 MOs similar to what was observed in singlet O2/Au28 system earlier. Therefore it is clear that gold clusters can induce a spin field to the energy states of O2 molecule. It can be



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proposed that O2/Au27 system with doublet and quartet spin states and O2/Au28 species with triplet spin multiplicity can introduce a pure spin field due to their unpaired electrons. One expects that for large clusters with odd number of atoms or electrons, due to the delocalization of pure spin in the whole system, same effect of inducing spin field as the even number ones will be observed. Therefore aforementioned spin field effect and the observed O2 MOs destabilization nicely corroborates with that reported by Vishnu Kamath et al. of the destabilization of the electronic states of O2 adsorbed on gold clusters [33]. E. Mechanism for nobility of gold clusters and charge transferring: Mulliken electronegativity (chemical potential (µ)) of O2, Au27 and Au28 molecules Sections B and C are indicative of the odd observation that, contrary to the electronegativity values, this is O2 which has lost electron compared to Au clusters. To come up with a proper explanation, mechanism for charge transfer from O2 to gold clusters has been presented in this section. For more clarity, the Mulliken electronegativity (µ) of molecular O2, Au27 and Au28 have been calculated. The chemical potential (µ) of a molecule [42] is defined as: µ=δEtot⁄δN (the first derivative of the total energy (Etot) with respect to the total number of electrons (N)). Because of the discontinuous nature of µ, it can be clearly shown that Potential) when

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Phenomenal Properties of Gold Nanoclusters with No Chemical

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