Ag7Au6 Cluster as a Potential Gas Sensor for CO, HCN, and NO

Article pubs.acs.org/JPCC

Ag7Au6 Cluster as a Potential Gas Sensor for CO, HCN, and NO Detection Yongliang Yong,*,†,‡ Chao Li,† Xiaohong Li,† Tongwei Li,† Hongling Cui,† and Shijie Lv† †

College of Physics and Engineering, Henan University of Science and Technology, Luoyang 471003, People’s Republic of China Department of Physics, Zhejiang University, Hangzhou 310027, People’s Republic of China

‡

S Supporting Information *

ABSTRACT: Ag−Au bimetallic clusters have demonstrated extreme sensitivity, which can be theoretically explained by the conductivity change of the clusters induced by the absorption process, to molecules such as CO, H2S, and so forth. Recently, a 13-atom alloy quantum cluster (Ag7Au6) has been experimentally synthesized and characterized. Here, the adsorption of CO, HCN, and NO on the Ag7Au6 cluster was investigated using density functional theory calculations in terms of geometric, energetic, and electronic properties to exploit its potential applications as gas sensors. It is found that the CO, HCN, and NO molecules can be chemisorbed on the Ag7Au6 cluster with exothermic adsorption energy (−0.474 ∼ −1.039 eV) and can lead to finite charge transfer. The electronic properties of the Ag7Au6 cluster present dramatic changes after the adsorption of the CO, HCN, and NO molecules, especially its electric conductivity. Thus, the Ag7Au6 cluster is expected to be a promising gas sensor for CO, HCN, and NO detection.

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INTRODUCTION The environmental gas monitoring and controlling is now recognized as an important issue for our safety and health. Much research has been focused on the development of suitable gas-sensitive materials for continuous monitoring and setting off alarms for hazardous chemical vapors present beyond specified levels.1−4 Chemical gases such as CO, HCN, and NO are highly toxic to human beings and animals as they inhibit the consumption of oxygen by body tissues. They are colorless, odorless, and tasteless, and thus human beings do not have timely alertness to their presence. For example, exposure levels of 100 ppm of HCN which would result in death are about 1 h or less in some cases, while exposure levels of 500 ppm of HCN are within 15 min.5 Higher concentration levels will result in faster onset of symptoms or death. Therefore, effective methods for sensing these three toxic gases are highly desired. Bimetallic nanoclusters (also called “nanoalloys”6) are of great interest from both the fundamental and the technological points of view not only because of their new degrees of freedom for understanding their electronic and geometric properties of clusters but also because of their potential applications in catalysis,7−11 optics,12−14 nanoelectronics,15 and sensing.16 In particular, silver−gold (Ag−Au) clusters have been investigated extensively from a computational point of view17−27 as well as experimentally.28−37 Their unique physicochemical properties depend on the shape and structure of the clusters, the surface segregation of the clusters, and the alloying extent or atomic distribution in nanoclusters. Recently, Ag−Au clusters are investigated as potential catalysts or sensors © XXXX American Chemical Society

for removal and detection of toxic molecules. In this regard, adsorption of toxic molecules on Ag−Au clusters has attracted several theoretical and experimental studies.38−45 Experimental investigations on CO adsorption on AunAgm (n = 10−45, m = 0, 1, 2) clusters at 140 K indicate that the CO molecule adsorption on closed electronic shell systems is the most reactive.38 Neumaier et al. have studied low-pressure CO adsorption on AunAgm (n + m < 7) clusters both theoretically and experimentally.39,40 They found that the CO molecule always binds to a gold atom in clusters, and the gold−CO bond strength is decreased with increasing cluster size. It is also observed that the maximum number of adsorbed CO molecules was found to strongly depend on cluster charge and composition as well.41 These studies indicate that AunAgm clusters with definite size, geometry, and composition may be used as CO sensors. However, to the best of our knowledge, no systematic theoretical or experimental work has been reported for NO and HCN molecule adsorption on Ag−Au clusters. Therefore, it would be interesting and important to find out whether the CO, HCN, and NO molecules can be absorbed on Ag−Au clusters and whether there would be sufficient charge transfer between the Ag−Au clusters and the molecules to make the Ag−Au clusters excellent sensors for detecting CO, HCN, and NO gases. The 13-atom alloy quantum cluster (Ag7Au6) is of particular interest not only because of a number of closed-shell highReceived: March 5, 2015 Revised: March 13, 2015

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DOI: 10.1021/acs.jpcc.5b02151 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C symmetry “magic” structures which are precursors of larger, stable high-symmetry structures17,26,46 but also because the Ag7Au6 cluster has been synthesized by a galvanic exchange reaction.34 In this present work, we employ first-principles calculations to accurately describe adsorption of the toxic CO, HCN, and NO molecules on the Ag7Au6 cluster, in order to reveal some clues for chemical sensor design. In the following section, we present the essentials of the computational methods, which are then followed by the results and a discussion.

2. COMPUTATIONAL DETAILS In this work, all calculations are performed using the spinpolarized density functional theory (DFT) implemented in the DMOL3 program (Accelyrs Inc.).47,48 The generalized gradient approximation formulated by Wang and Perdew (PW91)49 is employed to describe the exchange-correlation energy functional. Density-functional semicore pseudopotentials (DSPPs)50 fitted to all-electron relativistic DFT results and double numeric basis sets supplemented with d polarization functions (i.e., the DNP set) are selected. Self-consistent field procedures are performed with a convergence criterion of 10−6 au on the energy and electron density. The geometries are fully optimized without any symmetry constraints. We use a convergence criterion of 10−3 au on the gradient and displacement and 10−5 au on the total energy in geometrical optimization. To make sure the obtained lowest-energy structures are real local minima, normal-mode vibrational analysis was applied. All of the energy minima obtained for the lowest-energy clusters had no imaginary frequencies. The charge transfer between the Ag7Au6 cluster and absorbed molecules is analyzed based on Hirshfeld analysis, which is based directly on the electron density as a function of space. The accuracy of the GGA-PW91 functional as implemented in the DMOL software package for investigating the structures and properties of Ag−Au clusters has been confirmed by Kuang et al.51 They considered AuAg, Au2, and Ag2 clusters for which experimental or theoretical data are available for comparison. They found that the results are in good agreement with the experimental results. To further check the reliability of our calculations in this study, we calculated the binding energy of CO on the Au3Ag3+ cluster as a test using our methods. We find that the calculated binding energy of 0.91 eV is in good agreement with the previous study.39 Therefore, we are confident that the adopted computational method is reliable and accurate enough for this study.

Figure 1. Structures of the lowest-lying isomers of Ag7Au6 clusters. Values in parentheses are symmetries, relative energies in eV, and Boltzmann distribution percentages (298.15 K). Here and in the following figures, Ag atoms are light blue, and Au atoms are yellow.

shown in the Supporting Information. It is found that the stable configuration of the Ag7Au6 cluster is Au surface-segregated hollow structures with symmetry of C3v, which is in agreement with the trends of the family of 13-atom Au−Ag clusters.25 Chen et al.25 have observed a family of Au surface-segregated structures for core−shell AgnAu13−n (n = 1, 2, 3, 5, 7, 8, 9, 12) and hollow AgnAu13−n (n = 4, 6, 10, 11) clusters, whose stability is enhanced by directional charge transfer. However, the most stable structure we obtained is different from that of Chen et al.,25 which is a hollow structure with symmetry of C1. We also calculated the structure that Chen et al. obtained and found that its binding energy is higher by 0.506 eV than the structure we obtained, which indicates that the structure we obtained is much more stable. The trend of hollow structures we found is very different from the general trends of 34-atom and 38-atom Ag−Au clusters. The most stable structures of 34-atom24 and 38-atom19,24 Ag−Au clusters are Ag surface-segregated core− shell structure, as predicted from many-body Gupta empirical potentials. The energy difference between the most stable isomer and the second stable isomer is 0.206 eV. However, the energy difference between the second stable isomer and other stable isomer is about 0.041−0.125 eV. Furthermore, the conformational populations for the Ag7Au6 clusters were determined using the Boltzmann distribution equation, using the relative energies of each isomer. The Boltzmann distributions of the Ag7Au6 clusters at 298.15 K are displayed in Figure 1. It is found that the C3v isomer as shown in Figure 1A is the thermodynamically most stable structure, making up 35.2% of the population according to the Boltzmann distribution at 298.15 K. These results indicate that, from the energetic point of view, the most stable one would be particularly prone to synthesis. The most stable structure exhibits a gap of about 0.165 eV between the highest occupied (HOMO) and the lowest unoccupied (LUMO) molecular orbitals, and the other stable isomers have HOMO−LUMO gap values around 1.85 eV. Barron et al. have found that the Kohn−Sham HOMO−LUMO gap value of about 0.2 eV is a general feature of the AgnAu13−n (n = 0−13) clusters.17 However, it can be found that there are small variations with a value of about 0.05 eV in general (see the Figure 10 in their paper). In other words, the Kohn−Sham HOMO−LUMO gap values for the 13-atom Ag−Au clusters have been concentrated in the range of 0.15−0.25 eV. Therefore, our results further

3. RESULTS AND DISCUSSION 3.1. Pristine Ag7Au6 Cluster. First, we determined the most stable structural isomers of the neutral Ag7Au6 cluster, which were obtained during an exhaustive search for the lowestenergy geometries. In order to obtain the most stable structure, we consider as many initial chemical configurations as possible, which are set up (i) by random selections of atomic positions in three-dimensional space and (ii) replacing the atoms in 13atom metallic clusters within the fact that the 13-atom metallic clusters have been extensively studied.52−56 We also consider all the possible structures of the 13-atom icosahedron, decahedron, cuboctahedron, and buckled biplanar (BBP) as Ag7Au6 initial configurations, which is mentioned in ref 17. The six most stable isomers along with their symmetry and relative energy are presented in Figure 1, and their Cartesian coordinates are B

DOI: 10.1021/acs.jpcc.5b02151 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C indicate that the Kohn−Sham gap value of about 0.2 eV is a general feature of the 13-atom Ag−Au clusters.17 The most stable structure has a total spin of 1 μB, which is determined by Mulliken population analysis. 3.2. Molecular Adsorption. In this section, the adsorption of the CO, HCN, and NO molecules on the most stable structure of Ag7Au6 cluster is investigated. In order to obtain the best forms for the three molecules adsorbed on the Ag7Au6 cluster, a molecule was initially placed on all possible locals of the Ag7Au6 cluster surface. The optimized most stable configurations of molecule−cluster complexes are displayed in Figures 2−4, and the obtained results (such as adsorption

Figure 4. Two stable configurations of the NO molecule adsorbed on the Ag7Au6 cluster. Distances are in angstroms. The O atom is red, and the N atom is blue.

Table 1. Calculated Adsorption Energy (Eads), Distance between the Molecule and Ag7Au6 Cluster (D, Defined as the Distance between the Binding Site and the Close Atom of the Molecule), HOMO−LUMO Gap (Eg), Charge Transfer from the Ag7Au6 Cluster to Molecule (ET), and Total Magnetic Moment (μT) for the (CO, HCN, and NO) Molecule Adsorption on the Ag7Au6 Cluster Figure 2. Two stable configurations of the CO molecule adsorbed on the Ag7Au6 cluster. Distances are in angstroms. The C atom is gray, and the O atom is red.

systema

Eads (eV)

D (Å)

Eg (eV)

ET (e)

μT (μB)

CO-a CO-b HCN-a HCN-b NO-a NO-b

−0.660 −0.607 −0.474 −0.419 −1.039 −0.818

2.099 2.002 2.335 2.373 2.155 2.152

0.161 0.204 0.188 0.194 0.526 0.357

0.188 0.178 0.123 0.121 −0.108 −0.120

1.0 1.0 1.0 1.0 0 0

a

Notations: e.g., CO-a represents the optimized stable structure of the CO molecule adsorbed on the Ag7Au6 cluster as shown in the corresponding figure 2a.

configuration the adsorption energy is −0.660 eV, and the molecule−cluster distance is 2.099 Å, indicating that CO is chemisorbed on the Ag7Au6 cluster. This result is different from the previous reports,39,40 which found that the CO molecule always binds to a gold atom in AunAgm (n + m < 7) cationic clusters. To understand the difference, we have calculated the reaction of the Ag7Au6+ cluster with the CO molecule. We have considered various possible initial adsorption geometries, and it was found that the most stable configuration for CO adsorbed on the Ag 7 Au 6 + cluster is a CO molecule adsorbed preferentially on a Au atom with the adsorption energy of −0.775 eV. Further, we also obtained the most stable configuration of CO binding to a Ag atom in the Ag7Au6+ cluster, and the value of its adsorption energy is −0.461 eV, which is much higher by 0.314 eV than that of the most stable configuration of CO adsorbed on the Au atom. These results indicate that the difference between our results and previous work39,40 is due to the different charge state. For the most stable configuration of the CO molecule binding to a Ag atom in a cluster, it is found that there is a net charge of 0.057 e transferred from the Ag7Au6 cluster to the CO molecule, which is calculated using the Hirshfeld method. It is known that nearly all other charge schemes give significantly larger atomic charges than the Hirshfeld scheme, and there appears to be a consensus that Hirshfeld charges are too small.57 However, the Hirshfeld scheme is certainly more reliable than Mulliken, Bader, and

Figure 3. Two stable configurations of the HCN molecule adsorbed on the Ag7Au6 cluster. Distances are in angstroms. The C atom is gray, the H atom white, and the N atom blue.

energy, HOMO−LUMO gap, and charge transfer) are listed in Table 1. The adsorption energy (Eads) of a molecule on the Ag7Au6 cluster is defined as Eads = E(cluster−molecule) − E(cluster) − E(molecule)

where E(cluster−molecule), E(cluster), and E(molecule) stand for the energies of the molecule adsorbed on the Ag7Au6 cluster, the pristine Ag7Au6 cluster, and the corresponding molecule, respectively. By this definition, Eads < 0 corresponds to exothermic adsorption leading to local minima stable toward dissociation into a cluster and gas molecules. Then, we consider the CO molecule adsorption on the Ag7Au6 cluster. After full relaxation, the CO molecule adopts the orientation of the C atom pointing to the cluster, and the configuration of the CO molecule located on top of one Ag atom is the most stable, as shown in Figure 2a. At this C

DOI: 10.1021/acs.jpcc.5b02151 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C Weinhold schemes.58 Furthermore, there is a general trend for CO adsorption on AunAgm cationic clusters: a decrease in the experimentally determined binding energy with increasing silver content of the cluster.39 Our results show that the binding energy of CO adsorption on Ag7Au6 cationic clusters is 0.775 eV, which is much smaller than that of CO adsorption on Ag3Au3 cationic clusters (0.91 eV). Our results further indicate the trend would remain in larger cationic clusters. We also found the most stable configuration of a CO molecule binding to a Au atom in a cluster, as shown in Figure 2b. The interaction between the C atom and Au atom leads to exothermic adsorption energy of −0.607 eV, which is only 0.053 eV higher than the most stable one, and the formation of the C−Au bond (bond distance is 2.002 Å). This may indicate that CO molecules are chemisorbed on the Ag and Au sites in clusters simultaneously, while CO molecules are plentiful around the Ag7Au6 cluster. Meanwhile, for the most stable configuration of CO molecule binding to a Au atom in a cluster, there is a charge transfer (0.016 e) from the Ag7Au6 cluster to the CO molecule. The two CO−cluster complexes are found to have no imaginary frequencies using the method of normalmode vibrational analysis, indicating they are stable. Comparing the HOMO−LUMO gap of CO−cluster complexes with the pristine cluster clarifies significant influences of CO adsorption on the cluster electronic properties (see Table 1). In order to obtain the most stable configuration of single HCN adsorbed on the Ag7Au6 cluster, various possible initial adsorption geometries including single (hydrogen, carbon, or nitrogen), double (H−C or C−N), and triple H−C−N bonded atoms to Ag and Au atoms on different adsorption sites are considered. Results of our calculations are summarized in Table 1, and the final optimized configurations are shown in Figure 3. We find that the configuration of the N atom in the HCN molecule binding to a Ag atom in the Ag7Au6 cluster is the most stable. The adsorption energy of the most stable configuration is −0.474 eV and about 0.055 eV lower than that of the second most stable configuration. It can be seen from Figure 3 that the two most stable configurations have slight structural differences, which mainly reflected on the HCN adsorption sites. For the two most stable configurations, there is an unambiguous charge transfer of about 0.12 e from the Ag7Au6 cluster to the HCN molecule, and the distance between the molecule and cluster is less than 2.4 Å, which is similar to the case of HCN adsorbed on the Ag8 cluster.59 Although we have considered as many initial structures as possible, we find that the HCN molecule cannot be chemically adsorbed on a Au atom in the Ag7Au6 cluster, which indicates that the configuration of the HCN molecule adsorbed on a Au atom is much more energetically unfavored. The HOMO−LUMO gap for the two most stable structures of the HCN molecule adsorbed on the Ag7Au6 cluster is 0.188 and 0.194 eV, respectively, which is much larger than that of the pristine Ag7Au6 cluster (0.165 eV). This illustrates that HCN molecule adsorption obviously influences the electronic properties of the Ag7Au6 cluster. Because of the configurational similarity between CO and NO molecules, the initial structures of the NO molecule adsorbed on the Ag7Au6 cluster are analogous to the cases of the CO molecule adsorbed on the Ag7Au6 cluster. However, after full relaxation, the NO molecule adopts the orientation of the N atom pointing to the cluster, and the configuration of NO located on the top of an Au atom in the Ag7Au6 cluster is the most stable, as shown in Figure 4. This result is in contrast

with the cases of CO or HCN molecules adsorbed on the Ag7Au6 cluster. At the most stable configuration, the adsorption energy is −1.039 eV, and the distance between the cluster and molecule is 2.155 Å. The second most stable configuration has an adsorption energy of −0.818 eV, with the N atom of the NO molecule binding to a Au atom in a cluster. It is obvious that the adsorption energies of the NO molecule on the Ag7Au6 cluster are much larger than the cases of CO and HCN molecules on the Ag7Au6 cluster. The highly exothermic adsorption of NO on the Ag7Au6 cluster may be attributed to the structural deformation of the Ag7Au6 cluster. As mentioned above, the Ag7Au6 cluster has a C3v symmetry. However, after the adsorption of the NO molecule, the symmetry of the cluster changes into C2v. Since the NO molecule is an electron donor, some charge (about 0.122−0.136 e) is transferred from the NO molecule to the Ag7Au6 cluster for the two most stable configurations. Surprisingly, we find that the HOMO−LUMO gaps for the two most stable configurations of the NO molecule on the cluster are 0.526 and 0.357 eV, respectively, which are much larger than that of the pristine Ag7Au6 cluster, and their total magnetic moments are zero, indicating the magnetism of the Ag7Au6 cluster disappears after NO adsorption. Comparing the HOMO−LUMO gap and magnetic properties of the molecule−cluster complexes with the pristine cluster clarifies significant influences of NO adsorption on the cluster electronic properties, while CO and HCN exhibit similar effects. Now, we discuss the potential application of the Ag7Au6 cluster for CO, HCN, and NO detection based on our results. For CO, HCN, and NO detection, the three molecules should be adsorbed on the Ag7Au6 cluster and have sufficient charge transfer with the cluster to influence the electrical conductivity of the Ag7Au6 cluster. We find that the Ag atoms are reactive to CO and HCN molecules, while the Au atoms are reactive to CO and NO molecules. Our results suggest that all three molecules can be chemically adsorbed on the cluster with proper adsorption energies (≥0.47 eV), which are large enough to prevent spontaneous desorption at room temperature. Meanwhile, our results further indicate that there are apparent charge transfers (0.057−0.122 e) between the cluster and molecule. For example, the CO molecule is adsorbed on the Ag7Au6 cluster with a charge transfer of 0.057 e from the cluster to the CO molecule, while the NO molecule is adsorbed on the Ag7Au6 cluster with a charge transfer of 0.122 e from the NO molecule to the Ag7Au6 cluster. These two factors make the Ag7Au6 cluster suitable for CO, HCN, and NO detection. We further consider the influences of the three-molecule adsorption on electronic properties of the Ag7Au6 cluster. The HOMO− LUMO gaps for the configurations of CO, HCN, and NO adsorbed on the Ag7Au6 cluster have been summarized in Table 1. The change of the HOMO−LUMO gaps (Eg) in the configurations of CO, HCN, and NO adsorbed on the Ag7Au6 cluster is in the range of 0.188−0.526 eV (without regard for the case of CO adsorption with the C atom binding to the Ag atom), indicating high sensitivity of the electronic properties of the Ag7Au6 cluster toward the three-molecule adsorption, especially the NO adsorption. Although the most stable configuration of CO adsorption on the Ag7Au6 cluster is CO adsorption on the Ag sites of the cluster, its HOMO−LUMO gap is only 0.161 eV, which is slightly less than that of the pristine cluster. It should be noted that, as mentioned above, the stable configuration of CO adsorbed preferentially on the Au atom is just 0.053 eV higher in energy than that of the stable D

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The Journal of Physical Chemistry C

larger than those for N2 or CO2. However, it is found that the O2 molecule is chemisorbed on the Ag7Au6 cluster, and its adsorption energy is −0.638 eV. There is a charge transfer of 0.312 e from the molecule to the cluster. Further, the HOMO− LUMO gap for the most stable configuration of the O2 molecule on the cluster is 0.606, which is much larger than that of the pristine Ag7Au6 cluster. These results indicate that the conductivity change of O2 adsorbed on the cluster is much larger than the cases of CO, HCH, and NO adsorbed on the Ag7Au6 cluster, which can be used to distinguish the adsorbed molecules. This case is similar to silicon carbide nanotubes. Wu et al. have demonstrated that SiC nanotubes can be potential efficient gas sensors for CO and HCN detection,61 although the adsorption of O2 on SiC nanotubes results in strong Si−O bondings and charge transfers from the O2 molecule toward the SiC nanotube.62 By all accounts, the Ag7Au6 cluster would be an excellent gas sensor for CO, HCN, and NO detection. The theoretical results should be confirmed experimentally.

configuration of CO adsorbed on the Ag atom. Because of the minor energy difference, the two configurations can be viewed as the ground-state configuration. It is known that the change in Eg value of the configurations leads to the electric conductivity change of the Ag7Au6 cluster according to the following equation60

⎛ −Eg ⎞ σ ∝ exp⎜ ⎟ ⎝ 2kT ⎠ where σ is the electric conductivity of the configurations; k is the Boltzmann’s constant; and T is the thermodynamic temperature. Therefore, the CO, HCN, and NO molecules can be detected by calculating the conductivity change of the Ag7Au6 cluster before and after the adsorption process. In combination with the reasonable adsorption energies and apparent charge transfer, the Ag7Au6 cluster can be expected to be an excellent gas sensor for CO, HCN, and NO detection. Furthermore, we have considered the adsorption of O2, CO2, and N2 molecules on the Ag7Au6 cluster. The most stable configurations of O2, CO2, and N2 molecules adsorbed on the Ag7Au6 cluster are shown in Figure 5, and the relevant results

4. CONCLUSIONS The interaction between the Ag7Au6 cluster and CO, HCN, and NO molecules was studied using density functional calculations in order to find a novel sensor for CO, HCN, and NO detection. Our results showed that the three molecules are chemisorbed on the Ag7Au6 cluster with reasonable adsorption energies and apparent charge transfer. The electronic properties of the Ag7Au6 cluster present dramatic changes after the adsorption of the CO, HCN, and NO molecule, especially its electric conductivity. Therefore, it is suggested that the Ag7Au6 cluster is a promising candidate for CO, HCN, and NO detection.

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Figure 5. Most stable configuration of (a) O2, (b) CO2, and (c) N2 molecules adsorbed on the Ag7Au6 cluster. The C atom is gray, the O atom red, and the N atom blue.

S Supporting Information *

Cartesian coordinates of all the cluster structures as shown in this work. This material is available free of charge via the Internet at http://pubs.acs.org.

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Table 2. Calculated Adsorption Energy (Eads), Distance between Molecule and Ag7Au6 Cluster (D, Defined as the Distance between the Binding Site and the Close Atom of the Molecule), HOMO−LUMO Gap (Eg), and Charge Transfer from the Ag7Au6 Cluster to the Molecule (ET), for the (O2, CO2, and N2) Molecule Adsorption on the Ag7Au6 Cluster system

Eads (eV)

D (Å)

Eg (eV)

ET (e)

O2 CO2 N2

−0.638 −0.052 −0.098

2.382 3.482 3.022

0.606 0.164 0.165

−0.312 0.020 0.013

ASSOCIATED CONTENT

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 11304080 and No. 51302065).

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are summarized in Table 2. It is found that CO2 and N2 are physisorbed on the cluster with their adsorption energies of −0.052 and −0.098 eV, respectively. The charge transfer from the Ag7Au6 cluster to CO2 (or N2) is only 0.02 e (or 0.013 e), which is much smaller than the cases of CO, HCH, and NO adsorbed on the Ag7Au6 cluster. More importantly, the HOMO−LUMO gap of the Ag7Au6 cluster is hardly changed by the adsorption of the CO2 (or N2) molecule, indicating that there is little change for the conductivity of the Ag7Au6 cluster before and after the adsorption process of CO2 and N2 molecules. These results indicate that the magnitudes of conductivity changes for the CO, HCH, and NO molecules are

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DOI: 10.1021/acs.jpcc.5b02151 J. Phys. Chem. C XXXX, XXX, XXX−XXX