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Effects of Gas Adsorption on the Graphite-Supported Ag Nanoclusters: A Molecular Dynamic Study Hamed Akbarzadeh, Hamzeh Yaghoubi, Amir Nasser Shamkhali, and Farid Taherkhani J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 08 Nov 2013 Downloaded from http://pubs.acs.org on November 16, 2013
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Effects of Gas Adsorption on the Graphite-Supported Ag Nanoclusters: A Molecular Dynamic Study a
Hamed Akbarzadeh* , Hamzeh Yaghoubia, Amir Nasser Shamkhali b, Farid Taherkhanic, a
Department of Chemistry, Faculty of Basic Sciences, Hakim Sabzevari University, 9617976487 Sabzevar, Iran
b
Department of Chemistry, Faculty of Basic Sciences, University of Mohaghegh Ardabili,
56199-11367 Ardabil, Iran c
Department of Physical Chemistry, Razi University, 67149-67346 Kermanshah, Iran
*Author to whom be correspondence addressed: Email:
[email protected] Tel.: +98 915 3008670 Fax: +98 571 4003323 1 ACS Paragon Plus Environment
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Abstract
Molecular dynamics simulations were used to investigate the adsorption effects of different gas phases on structural and dynamical properties of graphite-supported Ag nanoclusters at various pressures, temperatures and cluster sizes. Three Ag nanoclusters with N= 38, 108, and 256 atoms in vacuum and under pressure of five gasses (He, Ar, Xe, H2, and CO) were simulated. The effect of each gas on nanoclusters at four temperatures and at various number of gas atoms (various pressures) were investigated. The adsorption, structural changes and dynamic properties were monitored as a function of cluster size, pressure and temperature. The adsorption isotherms, density profiles, deformation parameter, mean square displacement and diffusivity were calculated to study structural changes and dynamical properties of nanoclusters. It was found that the adsorption isotherms comply from the Langmuir type I. Also, the gas phase changes the vacuum cluster structure irreversibly. Furthermore, the gas phase increases the diffusivity of nanoclusters on the substrate surface, and this diffusivity increases with gas pressure.
Keywords: Sutton-Chen potential, Gas adsorption, Irreversible changes, Size dependence.
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Introduction Metallic nanoclusters are the subject of many experimental and theoretical researches due to their different physical, chemical, and electronic properties in comparison with the bulk material.1-2 In most cases, these differences are related to their large fraction of surface atoms. Ag nanoclusters possess many valuable optical properties that have opened the door to new approaches in sensing and imaging applications, offering a wide range of detection modes such as colorimetric, scattering, surface enhanced Raman spectroscopy (SERS), and metalenhanced fluorescence (MEF) techniques at extremely low detection limits.3 In many cases of heterogeneous catalysis and sensors, the nanoclusters must be sited on a substrate, and gaseous reactants are adsorbed on those surface.4 There are many evidences which emphasis that the catalytic activity of nanoparticles are size- and shape dependent.5-7 Therefore, investigation on dependency of the shape and structural changes of substrate-supported nanoclusters to the temperature and pressure of gas phase could give much important knowledge for better understanding of their catalytic activity. It is well documented that the catalytic activity of a metal surface depends on its atomic structure.8 The actual shape of an active catalyst is often thought to be related to the shape of clean particles observed by transmission electron microscopy (TEM) on model catalysts prepared in ultra-high vacuum (UHV) conditions.9 However, recent experiments are able to determine the structural and dynamic properties of metallic nanoclusters under more realistic conditions.4,10 Shape variations of Pd particles due to oxygen adsorption were investigated by Graoui et al.11 They prepared Pd particles under UHV on MgO single crystals which were in situ annealed under various pressures of O2 at high temperature, then observed ex situ via high-resolution transmission electron microscopy (HRTEM) and weak beam dark field (WBDF) imaging. They found that the Pd particles (10–15 nm) reach the equilibrium shape after annealing in UHV at 550°C (far below the melting point of Pd particles), because, the atomic diffusion on the surfaces is fast enough for these particle sizes ( . The parameters of QSC potential for Ag are =
0.033147eV, = 4.05 , = 7, = 6 and = 16.399.
In these simulations, we considered two different force field models for the graphite substrate: [1] The carbon atoms of graphite were fixed in their positions (indeed we have a static surface). [2] The optimized Tersoff potential22 was applied for C- C interactions of graphite. The
parameters of optimized Tersoff potential for graphite can be found in study of Lindsay and Broido.22
We compared the results obtained from the two models. There was no significant difference in results between the models (even at high temperatures and pressures). The reason is that the carbon support is a “bulk” extended surface which will not be affected by the presence of the nanoclusters. We also ran several simulations considering larger sized graphite sheets; the results were similar to the first graphite system (because the graphite is a bulk surface). Therefore, in this paper, we have reported the results obtained from the first model. For the rest of interactions, we used the Lennard-Jones (L-J) 12-6 potential, for which the parameters listed in Table 1.23-27 CO and H2 gases were considered as two-site models with fixed interatomic distance of 0.746 Ǻ for H223 and 1.163 Ǻ for CO.24 Also, the charge of C and O atoms in the CO molecule are +0.107 and -0.107 a. u., respectively.24 The L-J parameters for dissimilar atoms were obtained by the Lorentz–Berthelot mixing rules. The columbic long range interactions were calculated using the Ewald's method with a precision of 10-6. All interatomic interactions between the atoms in the simulation box were calculated within the cutoff distance of 9 Å. We used other potentials for Ag- CO and Ag- H2 interactions and compared the results with the results obtained from L-J potential. The Born- Mayer potential (B-M) was applied for Ag – CO interactions as study of Spruit et al.28 and Morse potential for Ag- H2 interactions as study of Yu et al.29 We ran several simulations considering these potentials for Ag- CO and Ag- H2, obtaining results similar to those obtained from the L-J parameters. It represents physisorption in these systems. In this study, we have reported the results of the L-J potential for Ag- CO and Ag- H2 gases.
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In order to count the number of adsorbed gas atoms or molecules, we used the Ag-Gas radial distribution function (RDF). The position of the first peak in the RDF was chosen as a distance that gas atoms were adsorbed. The first peak in Ag-Gas RDF represents the first layer of adsorbed gases that were located around the Ag naonocluster. Therefore, the number of adsorbed gas atoms or molecules was calculated by Eq. 3.
4! "' & #$ %
(3)
Here n1(r) is the number of adsorbed gas atoms or molecules in the first layer, g(r) is the radial distribution function, r1 is the first minimum in the RDF (the position of the first peak in RDF) and ρ is the gas phase density.
Table 1: Lennard-Jones 12-6 parameters used in this study
()
0.344700
2.644
0.002630
3.369
He-He 26
0.000900
2.550
26
0.008000
3.540
Xe-Xe 26
0.019900
4.050
0.002383
2.640
0.002601
3.120
0.004553
3.430
interaction Ag-Ag
25
+,∗ -+, 25
Ar-Ar
H-H
27
O-O 24 C-C (for CO molecule)
24
* The subscript # denotes the graphite.
*
The Ag-Ag and +, -+, parameters used only to calculate the other interactions by Lorentz-Berthelot rules (see text).
Results and discussions
I) Adsorption isotherms Figure 2 illustrates the pressure dependence of the number of adsorbed H2 molecules on the Ag256 at four temperatures. This figure illustrates that the adsorption increases with pressure and gradually reaches to a constant value, due to the nanocluster surface saturated by a layer of gas atoms, in accordance with adsorption isotherm of Langmuir type I.4,30 Also, as the results show, increasing the temperature decreases the number of adsorbed molecules. For better comparison, Figure 3 displays the temperature dependence of the number of adsorbed H2 molecules on the Ag38 and Ag256, in a constant H2 load of 160 H2 molecules. The number
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of adsorbed molecules decreases almost exponentially when the temperature increases and reduces significantly at temperatures close to the Ag nanoclusters melting point. This is fundamentally consistent with the fact that higher temperatures give the adsorbates more kinetic energy and this, in turn, results in less chance of being adsorbed. Also, this figure shows an increase in the number of adsorbed H2 molecules when cluster size increases at a constant temperature. As cluster size increases, a more number of atoms are on the surface of the cluster and consequently, the surface of the nanocluster saturates by more gas molecules. The same trends were observed for Xe, Ar, CO, and He gases. 140 300K 500K 700K 900K
Number of adsorbed gas atoms
120
100
80
60
40
20
0 0
100
200
300
400
500
600
Pressure (atm)
Figure 2. Adsorption isotherms for H2 on the Ag256 nanocluster at four temperatures in various pressures.
100 Ag256
Number of adsorbed gas atoms
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Ag38
80
60
40
20
0 200
400
600
800
1000
Temperature (K)
Figure 3. Temperature dependence of the number of adsorbed H2 molecules on the Ag256 and Ag38 nanoclusters in the presence of 160 molecules.
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Paying attention to the values of L-J parameters (gas-Ag interaction energy) of mentioned gases, it seems that the number of adsorbates should decrease in this trend: Xe > Ar > CO > H2 > He. However, this trend is not observed in some cases. For example, at the temperature
of 300K, the number of adsorbed atoms on the Ag 01 nanocluster is 120 for H2, 94 for CO, 80
for Xe and Ar, and 60 for He. These exceptions can be related to the size and charge effects. At 700K the mentioned trend is held true for all gases except for H2, whereas at 900K it is
true for all of them. At lower temperatures (300K), more H2 molecules can be accommodated because of their smaller size (He also has a small size, but its interaction energy with Ag is very low); whereas at higher temperatures (900K), because of desorption effects, the behavior reverts to the expected trend.
II) Effect of Gas phase on the Ag nanocluster structure Figure 4 displays snapshots of graphite supported Ag256 in the vacuum and in the atmospheres of various gases constituted from 400 gas particles at 300K (the gas species have been deleted in picture for clarity). In contrast to cluster structure in the vacuum conditions, the cluster structure in various gases shows a contraction in z direction (perpendicular to the graphite surface) and the number of cluster layers decreases. H2 and He have less effect on the cluster structure, CO and Ar have more effect on it, and Xe has the most effect. This phenomenon is expected, due to the greater mass of Xe atoms which leads to larger momentum. Therefore exerts a stronger impact on the surface. It can be concluded that the presence of adsorbed gas atoms or molecules exerts a significant effect on the Ag nanocluster surface atoms and has an impact on the overall cluster shape due to the high ratio of surface to bulk atoms in nanoclusters. Therefore, for all of the gases, the interaction between the Ag nanocluster surface atoms and the gas tends to stabilize the surface atoms on the Ag nanocluster.
Also, in order to examine the structural changes of the nanocluster, density profile 2 was used in z direction (perpendicular to the graphite surface) that was obtained by: 2
6 2 3 *4,54, 4 ∆8 3 9:; 3 ? is the surface area of the graphite, and
