CO Adsorption on Ag Nanoclusters Supported on Carbon Nanotube: A

Apr 3, 2014 - Department of Physical Chemistry, Razi University, 67149-67346 Kermanshah, Iran. ABSTRACT: Molecular dynamics simulations are used to ...
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CO Adsorption on Ag Nanoclusters Supported on Carbon Nanotube: A Molecular Dynamics Study Hamed Akbarzadeh,*,† Hamzeh Yaghoubi,† Amir Nasser Shamkhali,‡ and Farid Taherkhani§ †

Department of Chemistry, Faculty of Basic Sciences, Hakim Sabzevari University, 96179- 76487Sabzevar, Iran Department of Chemistry, Faculty of Basic Sciences, University of Mohaghegh Ardabili, 56199-11367Ardabil, Iran § Department of Physical Chemistry, Razi University, 67149-67346 Kermanshah, Iran ‡

ABSTRACT: Molecular dynamics simulations are used to study CO adsorption on Ag nanoclusters ranging from 38 to 500 Ag atoms, supported on carbon nanotube. Each nanocluster was simulated under various pressures of CO gas at different temperatures. The absolute value of enthalpy of adsorption was calculated for all of the nanoclusters in constant coverage which is increased sharply by decreasing cluster size. This increasing trend with coverage reaches a maximum around 0.75 ML for Ag108. Also, the structural changes are irreversible in such a way that by gradually decreasing the pressure to zero, the nanocluster geometry is not reversed to its initial structure in vacuum conditions. It was found that structural irreversibility increases with the size. Also, the difference between diffusivity of Ag nanoclusters in vacuum and CO atmosphere increases with the size.

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INTRODUCTION Ag nanoclusters are one of the important metallic clusters which have several applications such as catalysis,1 electrocatalysis,2 nano-optics,3,4 solar cells,5 molecular sieve membranes,6 and antibacterial activities.7−11 The size and morphology of Ag nanoparticles are affected by different parameters.12,13 Meanwhile, the special electrical, chemical, and mechanical characters14 of carbon nanotubes lead them to be widely used in the construction of chemical sensors and biosensors, especially in the field of supporting materials.15 The large surface area of carbon nanotubes provides the possibility to deposit metallic nanoparticles on their surface in order to enhance their properties.13,15 Many experimental studies exist in which syntheses of carbon nanotubes with various diameters are reported. Lijima and Ichihashi synthesized single-walled carbon nanotubes (SWCNs) with a diameter of 1 nm.16 Cheung et al. synthesized carbon nanotubes with average diameters of 3, 7, and 12 nm using Fe nanoclusters as catalyst by the chemical vapor deposition (CVD) method.17 Beside these investigations, Ag nanoparticles deposited on carbon nanotubes have gained more industrial applications such as heterogeneous catalysis, sensors, and microelectronics.18−21 One of the important factors that can affect the shape of metallic nanoclusters is the influence of gas atmosphere around the cluster, which usually leads to the adsorption of gases on their surface.22 CO adsorption on metal electrodes can be considered as a model system in electrocatalysis and interfacial electrochemistry, in such a way that it can be used as a probe molecule to obtain some information about the surface structure and its morphology using various spectroscopic methods such as infrared spectroscopy.23 Also, the other © 2014 American Chemical Society

importance of CO adsorbates is the fact that CO is a ubiquitous catalytic poison. The adsorption of CO on the Ag surfaces has been a subject of numerous experimental and theoretical studies concerning their structural, dynamical, electronic, and thermodynamic properties. McElhiney et al. studied Xe and CO adsorption on Ag(111) surface by low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), and electron energy loss spectroscopic (EELS) measurements.24 They reported a value of −6.46 ± 0.36 kcal/mol for enthalpy of CO adsorption on Ag(111) surface. Gajdoš et al. studied CO adsorption on Co, Ni, Cu, Ru, Rh, Pd, Ag, Ir, Pt, and Au surfaces using density functional theory (DFT) methods.25 Their calculations propose adsorption energy of −3.69 kcal/ mol for CO adsorption on Ag(111) surface, and they compared it by the experimental data of McElhiney et al. (−6.46 kcal/ mol).Yim et al.26 identified two competing substrate relaxation mechanisms of the CO/Ag(110) system. First are the shortrange interactions which prefer tilt angle for CO and alleviation of Fermi surface nesting. Second is the long-range interactions which lead to weak interaction of CO with Ag(110) surface while the electrons are localized in C-p and O-p orbitals.26 It is noticeable that CO adsorption on metal clusters may have different characteristics in comparison with macroscopic surfaces due to the large surface/volume ratio in clusters. Also, thermodynamic properties of clusters usually are size dependent. Beside these difficulties, several successful experiments have been reported in which small sizes of Ag nanoclusters are prepared and characterized. Bromann et al. deposited Ag7 and Received: December 17, 2013 Revised: March 17, 2014 Published: April 3, 2014 9187

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Ag19 clusters on the Pt(111) surface.27 Shimizu et al. deposited Ag nanoclusters in the sizes between 0.8 and 4.2 nm on the θAl2O3 support in order to prepare a catalyst for hydrogenation of nitroaromatics.28 Xu and Suslick synthesized Ag nanoclusters with diameters below 2 nm using sonochemical techniques.29 Moreover, many theoretical studies have been performed in order to determine energy of adsorption of gas molecules on the metal nanoclusters. Wang et al. calculated adsorption energy of CO and O2 molecules on the Ag55, Au25Ag30, and Au55 nanoclusters using the DFT method.30 They reported −10.15 kcal/mol for adsorption energy of CO on Ag55 nanocluster. Fleischer et al. investigated composition-dependent selectivity in the coadsorption of H2O and CO on pure and binary silver−gold clusters.31 They determined that the coadsorption effect occurs at a crossover in the molecular binding energies of CO and water with these clusters. Tang et al. investigated the CO adsorption on the Au55, Ag55, and Cu55 clusters at the all-electron relativistic BVP86/DNP levels.32 Their computational results indicated that CO adsorption energy on the Ag55 cluster varies from −10.38 to −11.30 kcal/ mol, depending on the type of adsorption site.32 Arafune et al. determined the adsorption site and vibrational energies of CO on a clean Ag(001) surface using scanning tunneling microscopy, inelastic electron tunneling spectroscopy with a scanning tunneling microscope, and high-resolution electron energy loss spectroscopy.33 They found that the CO molecules are chemisorbed very weakly on the Ag(001) surface. Ortigoza et al. presented a first-principles study of the nature of the binding of a c(2 × 2)-CO overlayer on Ag(001) and the origin of CO−CO interactions during the adsorption process.34 They proposed that the binding of CO aggregately enhances the intermolecular (CO−CO) force constants. Their results indicate that CO is chemisorbed on Ag(001) surface and follows Blyholder model of donation and back-donation of electrons between CO and metal orbitals.34 Bloch et al. investigated adsorption of CO and CO2 on the large pore sized Ag/SiO2 nanocomposite using the microcalorimetry method.35 Their results indicate that the Ag/SiO2 nanocomposite can be considered as an interesting candidate for the adsorption of trace amounts of CO in the presence of CO2. Gas phase effects on substrate-supported nanoclusters have been studied in more theoretical and experimental methods.36−39 Penza et al. deposited Au nanoclusters on the singlewalled carbon nanotubes with diameters in the range of 10−40 nm in order to create a gas sensor for NH3, CO, N2O, H2S, and SO2 molecules.40 Hrapovic et al. deposited Cu, Au, and Pt nanoclusters on the carbon nanotubes with diameters in the range of 10−20 nm in order to electrochemically determination of explosive nitroaromatic compounds.41 We use molecular dynamics (MD) simulations to study CO adsorption on Ag nanoclusters ranging from 38 to 500 Ag atoms with average diameters from 0.85 to 1.90 nm, supported on carbon nanotube with 15.34 nm diameter. The mentioned size range of Ag nanoclusters in this work is selected in such a way that to be in the range of Shimizu et al.28 and Xu and Suslick29 and also are in the range of particle sizes of interest for catalytic and electrocatalytic processes.42−45 Also, the selected diameter of carbon nanotube is in the range of that used in experimental studies of Penza et al.40 and Hrapovic et al.41 Please note that the focus of this work is on the effects of cluster size on the adsorption characteristics of CO gas for which a specific diameter of carbon nanotube is needed. The motivation of this research is to understand the behavior of gas

molecules near the surface of Ag nanoclusters supported on carbon nanotube and the variation of thermodynamic, structural, and dynamic properties of adsorbates with nanocluster size and environmental conditions.

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SIMULATION DETAILS MD simulations at constant temperature and volume (NVT) were carried out by the DLPOLY 2.20 program.46,47 The Berendsen thermostat48 was applied to all of the species in the simulation box in order to maintain their temperature with a relaxation time of 0.01 ps. The equations of motion were integrated using the Verlet leapfrog algorithm49 with time step of 1 fs. The simulations were carried out for 1 ns of equilibration followed by production time of 2 ns for calculated properties. Five fcc-like AgN nanoclusters with N = 38, 108, 256, 405, and 500 Ag atoms were placed on a single-walled carbon nanotube with 15.34 Å diameter and 40.31 Å length and 800 carbon atoms. In order to investigate the effect of CO gas on nanoclusters, the CO gas molecules were randomly distributed in a cubic box, the dimensions of which were 42.34 × 42.34 × 100.34 Å3, and periodic boundary conditions added in all directions. Therefore, the initial simulation box was composed of the Ag nanocluster on carbon nanotube, together with 400 CO molecules at random positions. Then, the pressure is reduced smoothly by removing some CO gas molecules. By using this method, pressure is reduced near the zero at several steps, and after each step, the final configuration was used as initial configuration for the next step. The values of pressure were calculated using the van der Waals equation of state (VWEOS). It is noticeable that pressures vary with temperature. Since various temperatures are considered for each nanocluster in order to investigate the temperature dependency of adsorption, the pressure of 400 CO molecules on the surface is temperature dependent. For instance, the calculated pressures caused by 400 CO molecules using VWEOS are 86.57, 157.47, 228.37, and 299.28 atm for temperatures equal to 300, 500, 700, and 900 K, respectively. Therefore, the highest pressure is applied with 299.28 atm at 900 K created by including 400 gas molecules around the system. Paying attention to this fact, we report pressure on the surface by the number of CO gas molecules, in order to simplify future discussions. Also, it is important to be emphasized that simulations are carried out in NVT ensemble, in which the total pressure of the system is fluctuated. However, by consideration of CO gas molecules, we fixed gas pressure on the surface, not the total pressure of the system. In other words, the system is assumed to have two phases: surface (Ag nanocluster + carbon nanotube) and gas (CO molecules). Simulations were performed using many-body quantum Sutton−Chen (QSC) potential50,51 in order to describe the Ag−Ag interactions. The QSC potential parameters for Ag atom are ε = 0.033 147 eV, a = 4.05 Å, n = 7, m = 6, and c = 16.399. The rest interactions (gas−gas, gas−Ag, and gas− nanotube) were modeled by the Lennard-Jones (LJ) 12−6 potential.52,53 The LJ parameters for dissimilar atoms were obtained by utilizing the geometric mean for ε and the arithmetic mean for σ.54 The CO gas was considered as a twosite model with a fixed interatomic distance of 1.163 Å.55 Furthermore, the charge of C and O atoms in the CO molecule are +0.107 and −0.107 au,55 respectively. In order to reduce the computational time, the positions of carbon atoms of the nanotube were fixed, which means that we had a static substrate. It is noticeable that on the basis of previous studies 9188

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fixing of nanotube atoms has negligible effect on the results of simulation.56,57The accuracy of the results of mentioned simulations was examined by comparison of MD results with those obtained previously by DFT calculations.58−60 This goal was achieved by performing a separate simulation in order to calculate the adsorption energy (Eads) of CO on the Ag55 nanocluster, which was calculated previously with the DFT method by Wang et al:30 Eads = ECO−cluster − ECO − Ecluster

use Nads/Ntotal instead of Nads/Nsurf. However, the results of adsorption isotherms for Nads/Ntotal are more complicated and have some crossover points between different isotherms which lead to difficulties in discussion about them. Since these nanoclusters have layer structures, Nsurf is easily calculated by counting the Ag atoms located at outermost layer. In order to take into account the number of adsorbed gas molecules, the Ag−C radial distribution function (RDF) was used. The position of the first peak in RDF was accepted as a criterion that gas molecules were adsorbed in that distance. It can be observed that as the size of the nanocluster becomes smaller, it reaches a saturation state faster. The number of the nanocluster surface atoms decreases with decreasing size; thus, the gas atoms can cover the surface of the smaller nanocluster at lower pressures. As is obvious from Figure 1, at constant temperature and pressure conditions, the smaller nanoclusters have more coverage than larger ones due to the lower number of surface atoms and adsorption sites in smaller nanoclusters. Also, it is well-known that when the temperature increases, the coverage difference between nanoclusters decreases, and the effect of nanocluster size on coverage gradually disappears because at higher temperatures desorption of gas molecules is increased. The adsorption isotherms of CO gas on Ag256 nanocluster were calculated at 300, 500, 700, and 900 K, as shown in Figure 2. This figure illustrates that the saturation pressure is in direct

(1)

The calculated Eads for Ag55 nanocluster by MD simulation using the same force field (QSC potential for Ag−Ag interactions and LJ(12,6) for the rest of them) is −9.52 kcal/ mol. This result is in good agreement with −10.15 kcal/mol calculated by Wang et al.30 using the DFT method.

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RESULTS AND DISCUSSION Thermodynamic Properties of Adsorption. Figure 1 shows the adsorption isotherms of AgN nanoclusters (with N =

Figure 2. Adsorption isotherms of Ag256 at four temperatures.

relation with the temperature in such a way that lower saturation pressures occur at lower temperatures. Also, by comparing the coverage values at different temperatures in unsaturated regions of adsorption isotherms, it is observed that the decline of the isotherm is larger at lower temperatures and then at temperatures near melting point becomes smaller. As the results show, increasing the temperature decreases the coverage. This is fundamentally in consistent with the fact that at higher temperatures gas molecules have more kinetic energy, which leads to less chance of being adsorbed. When the gas molecules are physisorbed on the nanocluster surface, increasing the temperature makes the adsorbed system unstable and therefore decreases the coverage. Figure 3 exhibits the temperature dependence of coverage at the pressure caused by 400 CO molecules around the Ag nanoclusters of mentioned sizes. This figure shows a decrease of the coverage when cluster size increases. As cluster size decreases, the larger fraction of surface atoms is obtained.

Figure 1. Adsorption isotherms of Ag nanoclusters of various sizes studied in this work at 300 (a) and 500 K (b).

38, 108, 256, 405, and 500) deposited on a carbon nanotube (with 15.34 Å diameter and 40.31 Å length) at 300 and 500 K. Nads/Nsurf (coverage) is the ratio of the number of monolayeradsorbed gas molecules and the total number of Ag surface atoms (which is donated by ML). It is noticeable that one can 9189

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Figure 3. Temperature dependence of coverage at the pressure caused by 400 CO molecules around the Ag nanoclusters of various sizes studied in this work.

Figure 4. Deformation parameter R(t) fluctuation of the Ag256 nanocluster with time at temperature range of 100−900 K.

temperatures between 600 and 700 K, owing to surface melting and broadening of the nanocluster on the substrate surface. Above the melting point of the Ag256 nanocluster, the fluctuations of R(t) become larger, showing the liquid phase with greater atomic motions. In order to determine the melting point of the carbon nanotube-supported Ag nanoclusters, the specific heat capacities were calculated in constant volume for them. For all the five Ag nanoclusters, the maximum of specific heat capacity corresponds to their melting point, i.e., 540, 680, 820, 940, and 1010 K for nanoclusters with 38, 108, 256, 405, and 500 atoms, respectively. The melting points calculated by R(t) are in good agreement with those calculated by the specific heat capacities. Since the melting point of various sizes of nanoclusters is different, the mentioned four temperatures in which the temperature dependence of coverage is studied are different for other clusters, as it is obvious from Figure 3. Figure 5 shows the coverage values of Ag nanoclusters versus nanocluster size at the pressure caused by 400 CO molecules around them at 300 K. It is obvious that coverage increases when the nanocluster size decreases, and this trend is more considerable when nanocluster size tends to smaller ones. Paying attention to this, seven Ag38 clusters were used instead

Because surface atoms have less binding energy, compared to the inner atoms, therefore by decreasing the size of the cluster and increase of the surface/volume ratio, cohesive energy is reduced and a more unstable structure is attained. This instability of smaller clusters can be described by Wulff construction in which cohesive energy of the N-atom cluster is related to that of the bulk as follows:61 1/2 EC(N ) ⎛ Ba ⎞ =⎜ ⎟ E b0 ⎝ Bt ⎠

(2)

where EC(N) and Eb0 are cohesive energies of N-atom cluster and complete crystalline bulk, respectively. Ba is the rest bond number or actual bond number of the cluster surface, and Bt is the bond number in the perfect crystalline bulk. As it is obvious from this equation, the most stable structure of a nanocluster is obtained when the maximum Ba/Bt value is reached. Therefore, these surface atoms may have a greater tendency to adsorb gas molecules in order to increase their cohesive energy. Consequently, a decrease in the coverage with increase of nanocluster size should be expected which is in agreement with Figure 3. It is also noticeable that the temperature dependence of coverage is greater for the smaller nanoclusters. Figure 4 shows the variation of the deformation parameter R(t) within time at time periods of 200 ps at temperature range of 100−900 K for the Ag256 nanocluster. R(t) was calculated using the equation N

R (t ) =

1 ∑ [(Xi(t ) − Xcm(t ))2 + (Yi(t ) − Ycm(t ))2 N i=1 1/2

+ (Zi(t ) − Zcm(t ))2 ]

(3)

Here N is the total number of Ag atoms in the nanocluster, Xi, Yi, and Zi are the coordinates of the ith atom in the x, y, and z directions at time t, and the cm denotes the X, Y, and Z coordinates of the cluster center of mass at the same time. This figure illustrates that the nanocluster has solid state at low temperatures, characterized by small values and few fluctuation in R(t). As the temperature increases, the R(t) fluctuation increases because of the incremental atomic motions with temperature. A sudden increment in the R(t) value occurs at

Figure 5. Coverage versus nanocluster size at the pressure caused by 400 CO molecules at 300 K. 9190

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absolute value of enthalpy of adsorption increases sharply with decreasing cluster size, from 2.9 to 3.9 kcal/mol. This trend can be described by lower cohesive energy of smaller nanoclusters. As mentioned in the case of Figure 3, the larger surface/volume ratio of in small Ag nanoclusters leads to reduce of cohesive energy, and surface atoms have more tendencies to adsorb CO molecules in order to increase their cohesive energy, as implied from eq 1. Therefore, the absolute value of enthalpy of adsorption increases by decreasing of the cluster size. Also, Figure 6b illustrates the enthalpy of adsorption versus coverage for Ag108 nanocluster. At coverage of 0.75 ML, the absolute value of enthalpy apparently reaches the maximum, 3.45 kcal/ mol, then decreases rapidly in the range of 0.75−0.35 ML, and reaches a nearly constant trend at coverage values below 0.35 ML. These results are in good agreement with experimental results.61 Structural Properties of Adsorption. Figure 7 (top) shows the snapshots of Ag256 nanocluster at 300 K in three conditions: vacuum, at the pressure caused by 400 CO molecules, and in vacuum after removing the mentioned CO molecules (the gas molecules have been deleted for clarity). Gas phase causes the nanocluster to roll up around the nanotube, and the interface structure between the nanocluster and the nanotube is expanded. After the pressure gradually was decreased to zero, the nanocluster structure is not reversed to its initial one in vacuum. Other nanoclusters (except Ag38 due to its small size) show similar irreversibility, too. Figure 7 (bottom) shows the nanocluster layer in contact with the nanotube surface at the same conditions mentioned above. The expansion of the interface structure is observed in both the presence of gas phase and zero pressure. In this layer, the Ag atoms were placed in the middle of the carbon nanotube aromatic rings (note that due to the curvature of the nanotube, this case can be observed by looking in the normal direction to the nanotube surface), because for this state the Ag−C interactions are maximum. Also, it can be observed that the Ag atoms arrange in forms of rectangular lattices. Maybe, the irreversibility of structural changes occurs due to this coherent interface structure. The calculated Ag−Ag bond lengths are in the range of 2.64 ± 0.25 Å. This bond length is in good agreement with theoretical64 and experimental65,66 results. It can be concluded that the presence of CO gas 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, the interaction between the Ag nanocluster surface atoms and the gas tends to stabilize the surface atoms on the Ag nanocluster. Figure 8 shows the radial distribution function (RDF) of Ag−C for Ag nanoclusters in the same conditions mentioned in Figure 7. The evolution of these curves and increment of peaks are corresponding to structural changes which were illustrated in Figure 7. Three points can be understood from Figure 8: First, when the nanocluster size increases, the height of the peak of that layer in contact with the substrate (the first peak) decreases. Second, there is no difference between the RDF of Ag38 in different conditions. This means that after the pressure was gradually decreased to zero the nanocluster structure did not differ with its initial structure in vacuum. Third, as the size of the nanoclusters increased, the difference in height of the peaks was greater. It means that structural irreversibility increases with size.

of the Ag256 nanocluster. These seven nanoclusters were located on the carbon nanotube, separately. Then they were simulated at the pressure caused by 400 CO molecules, and the coverage was calculated. Although seven Ag38 clusters together can adsorb almost 200 CO molecules according to the adsorption isotherms, the mentioned system adsorbs almost 170 CO molecules. However, the Ag256 nanocluster only adsorbed 100 CO molecules. Consequently, an adsorbent containing smaller granules can adsorb significantly greater amounts of adsorbates. Also, the natural logarithm of P/P0 versus the inverse of temperature was calculated for adsorption isotherms of CO gas, in such a way that at each isotherm, the pressure was defined and plotted.62,63 The Clausius−Clapeyron equation describes the relation between natural logarithm of P/P0 and 1/T as ⎡P⎤ Δ H Δ S ln⎢ 0 ⎥ = ads − ads ⎣ P ⎦coverage RT R

(4)

where P is the gas pressure, P0 is a reference pressure, T is temperature, R is the gas constant. Also, H, and S are enthalpy and entropy, respectively, and the subscript “ads” refers to the adsorption phenomenon. Then, the enthalpy of adsorption was calculated by slope of the curves. Figure 6a exhibits the enthalpy of adsorption of CO gas for all nanoclusters in 0.25 ML of coverage. It seems that higher absolute values for enthalpy of adsorption are achieved at the lower cluster size and

Figure 6. Enthalpy of adsorption versus nanocluster size in coverage 0.25 (a) and the enthalpy of adsorption versus coverage for Ag108 (b). 9191

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Figure 7. Snapshots of Ag256 nanocluster in three conditions (top) and their interface structures (bottom) at 300 K (in bottom the Ag atoms are shown in blue for clarity).

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Dynamic Properties of Adsorption. In order to investigate the nanocluster diffusion, the mean-square displacement (MSD) is applied using the equation MSD(t ) =

1 N

CONCLUSION In this research CO adsorption on Ag nanoclusters ranging from 38 to 500 Ag atoms, supported on a carbon nanotube, was investigated by molecular dynamics simulations. The manybody QSC potential was used to describe the Ag−Ag interactions; also, the LJ (12−6) potential was applied for the rest of interactions (such as Ag−CO, Ag−nanotube, CO−CO). Since, as mentioned in the Introduction, one of the most important applications of nanocluster−nanotube systems is the preparation of new generations of gas sensors, one serious question should be answered: what size of Ag nanocluster on the carbon nanotube is appropriate for construction of a typical gas sensor? Answering this question needs a brief reconsideration of the results. By consideration of the mentioned results in previous sections, the following important points can be concluded: 1. The position of the first peak in RDF is considered as a criterion that gas molecules are adsorbed in that distance. The decline of the coverage relative to the pressure is smaller at higher temperatures and then at temperatures near melting point becomes smaller. Temperature dependency of coverage for the smaller nanoclusters is more significant. If one wants to prepare a chemical or physical sensor, the temperature dependency of its functionality should be small in order to be applicable in various physical conditions. Therefore, from this point of view, very small nanoclusters are not suitable, unless the measurement of temperature is considered. 2. The enthalpy of adsorption was calculated for all of the nanoclusters in coverage of 0.25 ML. Results show that the absolute value of enthalpy of adsorption increases sharply from

N

∑ (|ri(t + Δt ) − ri(t )|)2 i=1

(5)

Here, ri(t) is the position vector of the atom i at time t, and N is the total number of nanocluster atoms. Figure 9 illustrates MSD of nanoclusters in vacuum (a) and in vacuum after reducing the CO pressure to zero (b) at 300 K. By comparing Figures 9a and 9b, it can be concluded that the diffusivity of nanoclusters after exposing to CO has decreased. This decrease in the diffusivity is greater for larger nanoclusters because the larger nanoclusters create larger interface with the nanotube in the presence of gas pressure, which leads to stronger interaction between cluster and nanotube. As this correlation between cluster and nanotube is increased, the diffusivity is more reduced. Generally, diffusivity increases with size. Two types of different forces cause these movements. The first type of the forces arises from the nanocluster− carbon nanotube interactions and the second type from the Ag−Ag interactions in the nanocluster itself. The surface atoms of Ag nanocluster in the presence of CO gas were redistributed to create a nanocluster with lower energy. This redistribution changes the nanocluster−carbon nanotube interfacial contact area and so the nanocluster−carbon nanotube interactions. These interaction changes lead to the structural variation on the cluster surface and so the Ag−Ag interactions which lead to the change of MSD value to some extent. 9192

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Figure 9. MSD of nanoclusters in vacuum (a) and in vacuum after reducing the CO pressure to zero (b) at 300 K.

adsorption is a measure of interactions between adsorbate and adsorbent. Thus, higher interactions are not desirable due to the fact that strong interactions may make larger structural changes and the possibility of reversible changes is decreased. Also, it is noticeable that the other important property of a sensor is its sensitivity. It means that an appropriate sensor should have reversibility in low coverage values, and the coverage dependency of enthalpy of adsorption is not suitable in these circumstances. Therefore, the very small nanoclusters are not suitable for a mentioned typical gas sensor. 3. Also, the structure of Ag nanoclusters in vacuum and in zero pressure after exposing to CO atmosphere has been investigated. It was found that after the pressure was gradually decreased to zero, the nanocluster structure is not reversed to its initial structure in vacuum. It was shown that structural irreversibility increases with size. The MSD of nanoclusters in vacuum and in zero pressure after exposing to CO atmosphere were studied. The results show that the diffusivity of nanoclusters after exposing to CO has decreased. Moreover, the diffusivity of Ag nanoclusters increases with size. As discussed earlier, the reversibility of structural changes in a sensor is very important. In addition to that, structural stability of sensor is also important. It is obvious that diffusion of nanocluster on the surface leads to more structural instability.

Figure 8. Radial distribution function (RDF) of Ag-nanotube in vacuum, at the pressure caused by 400 CO molecules and in vacuum after reducing the CO pressure to zero at 300 K. (a) Ag38, (b) Ag256, and (c) Ag500.

2.9 to 3.9 kcal/mol, with decreasing cluster size. Also, the enthalpy of adsorption versus coverage was plotted for Ag108. The enthalpy is almost constant in coverage values below 0.35 ML. Above 0.35 ML, the absolute value of enthalpy of adsorption increases rapidly with the coverage and approaches a maximum value at 0.75 ML. One of the most important properties of a sensor is its reversibility. The enthalpy of 9193

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Therefore, large sizes of nanoclusters may not be suitable for preparation of a sensor at this point of view. These results imply that the selection of an appropriate size of the nanocluster to use it in a sensor is not a straightforward method. It needs many investigations for all of the related parameters and conditions in order to achieve an optimized size of nanocluster.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Tel +98 915 3008670; Fax +98 571 4003323 (H.A.). Notes

The authors declare no competing financial interest.

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