Ab Initio Molecular Dynamics Simulation of the Dissociation of Ethanol

Article pubs.acs.org/JPCC

Ab Initio Molecular Dynamics Simulation of the Dissociation of Ethanol on a Nickel Cluster: Understanding the Initial Stage of MetalCatalyzed Growth of Carbon Nanotubes Tomoya Oguri,† Kohei Shimamura,‡ Yasushi Shibuta,*,† Fuyuki Shimojo,‡ and Shu Yamaguchi† †

Department of Materials Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan Department of Physics, Kumamoto University, 2-39-1 Kurokami, Chuo-ku, Kumamoto 860-8555, Japan

‡

ABSTRACT: The dissociation of ethanol molecules on a nickel cluster was investigated by ab initio molecular dynamics and nudged-elastic-band (NEB) simulations to discuss the initial stage of metal-catalyzed growth of carbon nanotubes via alcohol catalytic chemical vapor deposition. Both C− C and C−O bonds in ethanol molecules are dissociated on the nickel cluster, which is followed by the formation of various reaction products such as hydrogen atoms and molecules, carbon monoxide, oxygen atoms, water, ethylene, methane, and their fragments. Moreover, the NEB analysis revealed that the activation energy for C−H bond dissociation in the fragment molecules on the nickel cluster is just approximately one-eighth of the bond-dissociation energy of the corresponding C−H bond without the influence of the nickel cluster. This confirms that the nickel cluster acts as the activator of the dissociation process of carbon-source molecules by reducing the activation energy.

1. INTRODUCTION The process of formation of carbon nanotubes (CNTs)1 has been widely investigated since its discovery. It is well-known that a catalytic metal is needed for the synthesis of single-walled carbon nanotubes (SWNTs), although multiwalled carbon nanotubes (MWNTs)2 can be formed without a catalytic metal.3 Therefore, the role of the catalytic metal in the growth of SWNTs has been widely studied.4−7 Since catalytic chemical vapor deposition (CCVD) has become a major technique for the synthesis of SWNTs,8−10 the mechanism of formation of SWNTs on catalytic metal nanoparticles has been a particular focus. In 2003, on the basis of classical molecular dynamics (MD) simulations, Shibuta and Maruyama11 proposed a metalcatalyzed growth model for the initial cap structure of the SWNT in which graphite networks precipitate after isolated carbon atoms dissolve into a nickel cluster and then a cap structure is formed on the surface of the cluster. The publication of the metal-catalyzed growth model was followed by many computational studies: Ding and co-workers12,13 (classical MD), the Barbuena group14−16 (classical MD), Shibuta and co-workers17−21 (classical MD), Neyts and coworkers22−24 [hybrid MD/Monte Carlo (MC) with the ReaxFF potential], Amara and co-workers25−28 (tight-binding grand-canonical MC), the group of Irle and Morokuma29−31 [density-functional tight-binding (DFTB) method], and Raty et al.32 (Car−Parrinello MD). The metal-catalyzed cap growth process was then confirmed by in situ environmental transmission electron microscopy (TEM) observations.33,34 Therefore, the metal-catalyzed cap growth model is now widely accepted. © XXXX American Chemical Society

On the other hand, the nature of the process whereby molecules of the carbon source dissociate on the metal catalyst is not yet well-understood, although it is empirically known that the yield and quality of the SWNT products strongly depend on the choice of the carbon source and additives. For example, the use of ethanol as the carbon source in the CCVD process [called alcohol CCVD (ACCVD)] yields large amounts of SWNTs without amorphous carbons in comparison with the CCVD process using hydrocarbons.5 Moreover, the addition of a small amount of water enhances the activity and lifetime of the catalytic metal drastically, and nanotube forests with heights of up to 2.5 mm can repeatedly be synthesized (this is called the supergrowth CVD technique).35 However, it is not straightforward to treat the dissociation and subsequent cap formation processes simultaneously in computational simulations because of the discrepancy between the time scales for these processes. In addition, a low-impact interatomic potential describing the dissociation of the carbon-source molecules appropriately has not been established, making it difficult to investigate the initial dissociation process using classical MD simulations. Meanwhile, Mueller et al.36 used classical MD simulations with the ReaxFF potential to investigate the dissociation of hydrocarbons on a nickel cluster as the initial phase of CNT growth. Although the adsorption and decomposition of hydrocarbons were closely analyzed in their study, their results did not reach the subsequent cap formation Received: March 27, 2013 Revised: April 19, 2013

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dx.doi.org/10.1021/jp403006m | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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process even with the classical MD simulation. Therefore, almost all of the computational works on the SWNT formation mechanism have not taken dissociation of carbon-source molecules into account but instead have been limited to the cap formation process starting from isolated carbon atoms. Under these circumstances, we investigated the dissociation of ethylene molecules on a nickel cluster as the initial stage of the SWNT formation process37,38 as well as the dissociation of methane molecules on a Ni(111) surface as the initial stage of graphene formation34 using ab initio MD simulations performed on a high-performance parallel computer. The results of these studies can be summarized as follows: hydrogen atoms dissociate from ethylene molecules and drift on the nickel surface. Once the nickel surface is saturated with dissociated hydrogen atoms, hydrogen molecules are formed and separate from the nickel cluster. On the other hand, C−C bonds in ethylene were never broken during the simulation. Therefore, it can be concluded that pairs of carbon atoms, rather than isolated carbon atoms, are the smallest components available for the synthesis of CNTs in the case of ethylene as the carbon source. Thus, ab initio MD simulations can provide much information on the initial stage of SWNT formation that is not yet straightforward to observe directly by experiment. In the present study, the dissociation of ethanol molecules on the nickel cluster was examined to investigate the initial stage of metal-catalyzed growth of CNTs by ACCVD. After the simulation methodology, which follows the approach used in our previous studies,37−39 is summarized in section 2, the perspective of whole reactions in the system is investigated in section 3.1, and section 3.2 focuses on the ethanol C−C and C−O bond dissociation processes. Next, the subsequent process of water formation from fragments is analyzed in section 3.3, and the role of the nickel cluster in the initial dissociation process is discussed on the basis of nudged-elasticband (NEB) analysis in section 3.4. Section 4 provides a concluding discussion.

Figure 1. Initial configuration of the calculation cell. A 32-atom nickel cluster was placed at the center of a 15 Å × 15 Å × 15 Å cubic cell, and 30 ethanol molecules were allocated around the nickel cluster at random. Blue, sky-blue, white, and red spheres represent Ni, C, H, and O atoms, respectively, and hereinafter the same applies.

this study, the MD simulations were carried out for 10 164 fs (42 000 steps) at 1500 K. We note that recent syntheses of CNTs typically have been carried out at temperatures somewhat lower than 1500 K and on clusters somewhat bigger than Ni32, and therefore, the parameters in this study were chosen in consideration of the computational cost. A population analysis48,49 generalized to the PAW method50 was then examined to clarify the change in the bonding properties of atoms associated with the dissociation reaction of ethanol molecules. By expansion of the electronic wave functions in an atomic-orbital basis set,51,52 the bond-overlap populations, Oij(t), were obtained for each atomic pair between ith and jth atoms, and the gross populations, Zi(t), were obtained for each atom. Oij(t) and Zi(t) give a semiquantitative estimate of the strength of the covalent bond between atoms i and j and the charge on atom i, respectively. Moreover, NEB analysis53 was employed to estimate the activation energies of the dissociation reactions found in the MD simulations. In the NEB method, the initial (reactant) and final (product) configurations are connected by a chain consisting of M − 1 replicas with elastic springs, which is then optimized toward the minimum-energy path. The value of M for intermediate replicas was chosen to be 10 in this study. Optimized structures of atomic configurations just before and after the dissociation process in the MD simulation were employed as the initial and final configurations, and intermediate replica configurations were prepared by linear interpolation. A homemade code developed by one of the authors (F.S.) was used for all of the calculations in this study.

2. SIMULATION METHODOLOGY Ab initio MD simulations were employed to study the dissociation of ethanol molecules on the nickel cluster. The electronic states were calculated using the projector augmented-wave (PAW) method,40,41 which is an all-electron electronic structure calculation method within the frozen-core approximation. Projector functions were generated for the 3d, 4s, and 4p states of nickel atoms; the 2s and 2p states of carbon atoms; the 2s and 2p states of oxygen atoms; and the 1s state of hydrogen atoms. In the framework of density functional theory, the generalized gradient approximation42 was used for the exchange−correlation energy. The Γ point was used for Brillouin zone sampling. The plane-wave cutoff energies were 30 and 300 Ry for the electronic pseudowave functions and the pseudocharge density, respectively. The energy functional was minimized iteratively using a preconditioned conjugate-gradient method.43,44 Figure 1 shows the initial configuration of the calculation system: a nickel cluster with 32 atoms, which was annealed in advance at 1500 K, was placed in the center of a cubic cell of dimensions 15 Å × 15 Å × 15 Å, and 30 ethanol molecules were randomly allocated around the nickel cluster. Periodic boundary conditions were employed. The equations of motion were integrated numerically using an explicit reversible integrator45 with a time step of 0.242 fs, and the Nosé− Hoover thermostat46,47 was used for temperature control. In

3. RESULTS AND DISCUSSION 3.1. A Variety of Reaction Products Are Formed on the Nickel Cluster. Figure 2 shows a snapshot of the atomic configuration at 8989 fs for the nickel cluster and various products formed by the dissociation reactions on the nickel cluster. Undissociated ethanol molecules have been omitted from the snapshot for clarity. Various products such as hydrogen atoms (H), a hydrogen molecule (H2), carbon monoxide (CO), an oxygen atom (O), water (H2O), ethylene (C2H4), methane (CH4), and various fragment molecules B



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Figure 2. Snapshot of the atomic configuration at 8989 fs for the nickel cluster and the reaction products formed by the dissociation reactions on the nickel cluster. Undissociated ethanol molecules have been omitted for clarity.

formed via dissociation of ethanol molecules and subsequent production reactions were observed on the surface. Interestingly, dissociation of both the C−C and C−O bonds in ethanol molecules as well as hydrogen dissociation (i.e., dissociation of C−H and O−H bonds) was observed in this simulation, whereas only hydrogen dissociation was observed and no C−C bonds were broken in our previous simulation of ethylene dissociation on the nickel cluster.37,38 This is reasonable since the bond-dissociation energy (defined as the standard enthalpy change for homolytic bond cleavage54) of the C−C double bond in ethylene (ca. 7.5 eV)54 is much larger than that of the C−C single bond in the hydrocarbon (ca. 4.0 eV).54 In total, one C−C bond dissociation and four C−O bond dissociations were observed during the calculation over 10 000 fs. Methane and carbon monoxide were formed from the C−C bond dissociation, and ethylene, water, the oxygen atom, and various fragment molecules were formed from the four C−O bond dissociation processes. These dissociation processes are closely analyzed in section 3.2. Figure 3 shows the numbers of reaction

Figure 4. Time series of the number of (top to bottom) dissociated hydrogen atoms (both isolated atoms and atoms forming hydrogen molecules), hydrogen molecules, oxygen atoms, OH fragments, and water during the calculation. The dissociated hydrogen atoms in the top graph are classified into three groups as illustrated in the inset.

forming hydrogen molecules), hydrogen molecules, oxygen atoms, OH fragments, and water during the calculation. The hydrogen atoms in the ethanol molecule can be categorized into three groups, as illustrated in the Figure 4 inset: the H atom bonded to the O atom [group (I)], the two H atoms bonded to the methylene carbon [group (II)], and the three H atoms bonded to the methyl carbon [group (III)]. Therefore, the number of dissociated hydrogen atoms is broken down into three groups in the top panel of Figure 4. Here, the hydrogen atom was regarded as dissociated from the ethanol molecule when the distance between it and the counterpart (the C or O atom) was over 1.5 Å. Hydrogen molecules were regarded as generated when the distance between the two hydrogen atoms was within 1.1 Å. An OH fragment was then regarded as generated when the the hydrogen atom was within 1.5 Å of an oxygen atom that was more than 2.0 Å from any carbon atom. The fluctuations within 100 steps (24.2 fs) have been smoothed in the figures for clarity in all of the graphs except in the top panel, where time series of the numbers of dissociated hydrogen atoms is shown as it is. On the whole, the total number of dissociated H atoms increased rapidly until 2 ps, converged at 15−18 atoms from 2 to 6 ps, and then increased again after 6 ps. It seems that hydrogen atoms are dissociated from the ethanol molecule with ease on the bare surface of the nickel cluster in the first phase until 2 ps. As dissociated hydrogen atoms cover the surface of the nickel cluster with hopping, they inhibit the additional adsorption and dissociation of ethanol molecules in the second phase. In the third phase, dissociated hydrogen atoms begin to form hydrogen molecules,

Figure 3. Numbers of reaction products in the system at 5082 and 10 164 fs (21 000 and 42 000 steps).

products formed at 5082 and 10 164 fs (21 000 and 42 000 steps). The number of hydrogen atoms formed is much larger than those of the other products, which means that hydrogen dissociation happens more frequently than other dissociations such as C−C and C−O bond dissociations. Moreover, the number of hydrogen molecules increased between 5082 and 10 164 fs, whereas the numbers of most of other products did not increase during this period. Figure 4 shows the time series of the numbers of all dissociated hydrogen atoms (both isolated atoms and atoms C



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Figure 5. (a) Snapshots of the atomic configuration and (b) time series of bond-overlap populations Oij(t) and gross populations Zi(t) during the C−C bond dissociation at ca. 600 fs. For clarity, the other ethanol and fragment molecules are not shown in the snapshots.

water. These water formation processes are closely analyzed in section 3.3. In addition, water dissociation reactions were also observed in the simulation. For example, the following water dissociation process was confirmed at ca. 8 ps:

which detach from the surface of the nickel cluster. Therefore, hydrogen atoms in the ethanol and fragment molecules can be dissociated again on the nickel cluster, which results in an increase in the number of dissociated hydrogen atoms again. The formation and detachment of hydrogen molecules was also observed in our previous simulation of the dissociation of ethylene molecules on the nickel cluster at a similar time scale.37,38 In contrast, such formation of hydrogen molecules was not observed in the previous simulation of methane dissociation on a flat Ni(111) surface.39 Therefore, the curvature of the nickel surface may enhance the formation of hydrogen molecules from hydrogen atoms on the surface of the nickel cluster. We note that formation of hydrogen molecules from hydrocarbon species on the Ni(111) surface was reported in a classical MD simulation55 in which 200 CHx molecules were impacted consecutively and covered the Ni(111) surface with a much higher density than in our previous simulation.39 In regard to the aforementioned groups of hydrogen atoms, there was no significant difference in the number of dissociated hydrogen atoms among the three groups, which means that the dissociation probability is not very different among three groups and therefore that there is no fixed order of hydrogen dissociation among the three groups. This is also reasonable since the bond-dissociation energy of the hydroxyl group in alcohols (ca. 4.5 eV)54 is close to that of C−H bonds (ca. 4.2 eV).54 In comparison with the case of ethylene molecules, the number of dissociated hydrogen atoms on the nickel cluster in the ethanol system is larger. Moreover, it was observed that ethylene molecules tend to absorb to the nickel cluster frequently, whereas such absorption was not observed in the case of the ethanol molecules. In regard to the formation and dissociation of other species, the number of water molecules increased as the number of OH fragments decreased, which means that the OH fragments bond to hydrogen atoms to form

O + H 2O → 2OH

(1)

That is, two OH fragments were generated at the same time as one oxygen atom and one water molecule disappeared (Figure 4). 3.2. C−C and C−O Bond Dissociation on the Nickel Cluster. Next, the C−C and C−O bond dissociation processes were closely investigated on the basis of bond-overlap population analysis and the atomic configuration. As described above, one C−C bond dissociation and four C−O bond dissociations were observed during the calculation over 10 000 fs. Figure 5a shows snapshots of the atomic configuration during the C−C bond dissociation. The time series of bondoverlap populations Oij(t) and gross populations Zi(t) associated with the atoms labeled in the snapshots are shown in Figure 5b. In the atomic configuration at 500 fs, a CH3CHOH fragment, which is formed by dissociation of a methylene C−H bond of an ethanol molecule, is adsorbed on the nickel cluster. At 520 fs, the hydrogen atom labeled “H1” bonds to the nickel atom labeled “Ni2” and dissociates from the oxygen atom labeled “O1”. Accordingly, the bond-overlap population for the carbon atom labeled “C1” and the O1 atom increases remarkably, which means that the C−O bond becomes stronger after the O−H bond breaks. Subsequently, the hydrogen atom labeled “H2” bonds to the nickel atom labeled “Ni1” and dissociates from C1 at ca. 610 fs. During the above processes, the carbon atom labeled “C2” forms an antibonding state with Ni1 (i.e., the bond-overlap population takes a negative value), whereas the C1−Ni1 bond-overlap population indicates a bonding state. Finally, C2 dissociates D



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forms an antibonding state with the Ni3 atom, as shown in the bond-overlap population. Subsequently, O2 dissociates from C3 at 201 fs, forming an ethylene and an OH fragment. The formed OH fragment changes counterparts from Ni3 to Ni5 and keeps moving on the surface of the nickel cluster with hopping, while the ethylene molecule stays in the same position for a certain time. There is no significant change in the value of the bond-overlap population for the C3−C4 bond during the process, which means that the C3−C4 bond remains in a single-bond state rather than changing to a double-bond state, even though ethylene is formed during the process. This occurs because the C3 and C4 atoms are bonded to the Ni3 and Ni4 atoms, respectively, which agrees with the previous simulation of ethylene dissociation on the nickel cluster.37,38 The gross population of C3 becomes more negative after the C−O bond breaks, whereas a significant change in the gross population was not observed for the surrounding atoms. Encouragingly, the formation of ethylene molecules by ethanol dissociation was experimentally confirmed by in situ mass spectroscopic analysis during CNT synthesis by the ACCVD technique.56 It is significant in this study to confirm that the dissociation process observed in this computational study agrees with actual experimental observations. 3.3. Formation of Water on the Nickel Cluster. In addition to the various dissociation processes described above, several OH fragments generated from the C−O bond dissociations bonded to hydrogen atoms to form water molecules during the calculation over 10 000 fs. In particular, two types of water formation processes were found: in one, the OH fragment bonds to a dissociated hydrogen atom floating on the nickel cluster, and in the other, the OH fragment bonds to a hydrogen atom in another fragment molecule. Figure 7a shows snapshots of the atomic configuration during the first of these water formation processes. The time series of Oij(t) and Zi(t) associated with atoms labeled in the snapshots are shown in the Figure 7b. In this process, the hydrogen atom labeled “H5” approaches the OH fragment adsorbed to the nickel cluster and bonds to the oxygen atom labeled “O3” at 822 fs. After H5 dissociates from the nickel atom labeled “Ni7” at 826 fs, O3 dissociates from the nickel atom labeled “Ni6”, forming the water molecule. In terms of the gross populations, ZH1(t) shifts from a negative to a positive value after formation of the O3− H5 bond at 822 fs. This demonstrates that isolated hydrogen atoms on the nickel cluster can have a negative charge. Snapshots of the atomic configuration during the second water formation process are shown in Figure 8. After a CH3COH fragment approaches an OH fragment on the Ni cluster, the hydrogen atom labeled “H6” in the CH3COH fragment bonds to the oxygen atom labeled “O5” in the OH fragment at 8516 fs. O5 then dissociates from the nickel atom labeled “Ni9” at 8639 fs. At almost the same time, H6 dissociates from the oxygen atom labeled “O4”, generating the water molecule. Thus, it was confirmed that water molecules can be generated via various processes in the presence of OH fragments. Therefore, one can say that the unique feature of the ACCVD process compared with the other CCVD process using hydrocarbons is the presence of water in the reaction product, which may act as an activator of the CNT growth, as in the supergrowth CVD technique.35 Moreover, we note that the hydrogen atoms constituting the water molecules often switched with other hydrogen atoms in the neighboring fragments or ones floating on the nickel cluster during the subsequent calculation, which is consistent with the hydrogen

from C1 at ca. 678 fs, forming a CH3 fragment and carbon monoxide. The CH3 fragment afterward bonds to a hydrogen atom floating on the nickel cluster, forming a methane molecule. In terms of the gross population, ZH1(t) drops from positive to zero or slightly negative after the O1−H1 bond is broken at 520 fs, and at the same time, ZC1(t) shifts from a negative to a positive value. On the other hand, ZO1(t) remains negative, although its magnitude slightly decreases. In regard to the C−O dissociations, the first of the four different observed processes is discussed here. We note we did not find any strict order of the multistep dissociations among the four C−O dissociation processes. Figure 6 shows snapshots

Figure 6. (a) Snapshots of the atomic configuration and (b) time series of bond-overlap populations Oij(t) and gross populations Zi(t) during the C−O bond dissociation at ca. 200 fs. For clarity, the other ethanol and fragment molecules are not shown in the snapshots.

of the atomic configuration and the time series of bond-overlap and gross populations associated with atoms labeled in the snapshots during the C−O bond dissociation observed at ca. 200 fs. In the atomic configuration at 165 fs, a CH2CH2OH fragment, which is formed by dissociation of a methyl C−H bond of an ethanol molecule, adsorbs on the Ni cluster. Concretely, the carbon atom labeled “C4” bonds to the nickel atom labeled “Ni3”. Next, the oxygen atom labeled “O2” bonds to Ni3 at 177 fs. Accordingly, the carbon atom labeled “C3” E



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bond-switching events observed in the previous ab initio MD study of hydrogen production from water on an aluminum cluster.57 3.4. Activation Energy for C−H Bond Dissociation by NEB Analysis. Next, the activation energy of the dissociation reaction on the nickel cluster was estimated by the NEB method in order to discuss the role of the nickel cluster in the dissociation process from the quantitative point of view. The bond-dissociation energy of the corresponding dissociation reaction without the nickel cluster was also calculated for comparison. Here, the C−H bond dissociation from the CH3CHO fragment observed at ca. 600 fs (Figure 5a) was examined. The initial and final configurations [structures (d) and (f) in Figure 9, respectively] were obtained by extracting

Figure 7. (a) Snapshots of the atomic configuration and (b) time series of bond-overlap populations Oij(t) and gross populations Zi(t) during the formation of water from an OH fragment and a dissociated hydrogen atom floating on the nickel cluster at ca. 800 fs. For clarity, the other ethanol and fragment molecules are not shown in the snapshots. Figure 9. Energy profile along the reaction path of the C−H bond dissociation in the CH3CHO fragment on the nickel cluster (red circles) and along the homolytic bond cleavage of the corresponding C−H bond in the free-standing CH3CHO fragment (blue circles). Insets (a−c) and (d−f) show the initial, intermediate/barrier, and final configurations without and with the nickel cluster, respectively.

the atomic configurations of the fragment and the nickel cluster just before and after the dissociation and then optimizing the structures. Structure (e) in Figure 9 shows the transition state obtained by the NEB calculation. The red circles in the plot in Figure 9 represent the energy profile along the reaction path of the C−H bond dissociation in the CH3CHO fragment on the nickel cluster. The activation energy for the reaction was estimated to be 0.61 eV. For comparison, the bond dissociation energy of the corresponding C−H bond in the free-standing CH3CHO fragment (that is, without the influence of the nickel cluster) was calculated as follows. The initial configuration [(a) in Figure 9] was extracted from the atomic configuration of the CH3CHO fragment in Figure 9d, and the final one [(c) in Figure 9] was prepared by homolytic bond cleavage of the

Figure 8. Snapshots of the atomic configuration during the other type of water formation process, at ca. 8500 fs, in which the OH fragment bonds to a hydrogen atom in another fragment molecule. For clarity, the other ethanol and fragment molecules are not shown in the snapshots.

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Notes

corresponding C−H bond to a distance of 6 Å. Both the initial and final configurations were optimized, and the energy profile along the homolytic bond cleavage (blue circles in the plot in Figure 9) was obtained by the NEB calculation. The bonddissociation energy was estimated to be ca. 5 eV, which almost agrees with the previously reported bond-dissociation energy of C−H bonds in hydrocarbons.54 This means that the C−H bond dissociation is unfavorable without the presence of the nickel cluster because of the high bond dissociation energy. On the other hand, the activation energy of the C−H bond dissociation for the same fragment molecule on the nickel cluster is just approximately one-eighth of the bonddissociation energy without the nickel cluster. In addition, compared with the case without the influence of the nickel cluster, the final configuration after the C−H bond dissociation on the nickel cluster is energetically stable as a result of the interactions between the dissociated fragments and the nickel atoms. These facts confirm that the nickel cluster enhances the dissociation process of the carbon-source molecules.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Part of this research was supported by a Grant-in-Aid for Young Scientists (A) (24686026) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.

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4. CONCLUSION By means of ab initio MD simulations, the dissociation of ethanol molecules on a nickel cluster was closely investigated. Both the C−C and C−O bonds in the ethanol molecules dissociated during the calculation, whereas in contrast, the C− C bond dissociation was not observed in our previous study of the dissociation of ethylene molecules on the nickel cluster. Methane and carbon monoxide molecules were formed after the C−C bond dissociation, and ethylene and water molecules were formed after the C−O bond dissociation and subsequent O−H bond formation. As a consequence, many reaction products were observed on the nickel cluster after the calculation over 10 000 fs. As the C−H bond dissociation proceeds, hydrogen atoms cover the nickel cluster with hopping on the nickel surface, inhibiting further hydrogen dissociation. However, the formation and subsequent detachment of hydrogen molecules enable further C−H bond dissociation. Moreover, the activation energy of the C−H bond dissociation on the nickel cluster was estimated by the NEB calculation and found to be much smaller than the bond-dissociation energy of the corresponding C−H bond without the influence of the nickel cluster. In summary, it can be concluded that one of the important roles of the nickel cluster in the initial growth process of CNTs is to enhance the dissociation of carbon-source molecules by reducing the activation energy. We note, however, that there must be other roles for the catalytic metal during CNT growth in addition to the role discussed in this study. For example, the epitaxy between the graphite network and the crystalline metal surface is considered to act as a template during CNT and graphene growth.5,19,20 Finally, we emphasize the novel aspect of this study, namely, that the initial dissociation process of carbon-source molecules during the CNT growth was examined on the basis of the ab initio MD simulation combined with the quantitative estimation of the activation energy by the NEB analysis. In contrast, most of the other MD simulations on this matter involve the cap formation process starting from isolated carbon atoms.

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Ab Initio Molecular Dynamics Simulation of the Dissociation of Ethanol

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