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Apr 2, 2010 - Polygonal ring formation, and hence SWNT growth itself, was driven by the continual, simultaneous extension of these polyyne chains and ...
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Polyyne Chain Growth and Ring Collapse Drives Ni-Catalyzed SWNT Growth: A QM/MD Investigation Alister J. Page,† Stephan Irle,*,‡ and Keiji Morokuma*,†,§ Fukui Institute for Fundamental Chemistry, Kyoto UniVersity, Kyoto 606-8103, Japan, Institute for AdVanced Research and Department of Chemistry, Nagoya UniVersity, Nagoya 464-8602, Japan, and Cherry L. Emerson Center for Scientific Computation and Department of Chemistry, Emory UniVersity, Atlanta, Georgia 30322 ReceiVed: January 27, 2010; ReVised Manuscript ReceiVed: March 9, 2010

A mechanism describing Ni38-catalyzed single-walled carbon nanotube (SWNT) growth has been elucidated using quantum mechanical molecular dynamics (QM/MD) methods. This mechanism is dominated by the existence of extended polyyne structures bound to the base of the initial SWNT cap-fragment. Polygonal ring formation, and hence SWNT growth itself, was driven by the continual, simultaneous extension of these polyyne chains and subsequent “ring collapse” (i.e., self-isomerization/interaction of these polyyne chains). The rate of the former exceeded that of the latter, and so this mechanism was self-perpetuating. Consequently, the observed kinetics of Ni38-catalyzed SWNT growth were increased substantially compared to those observed using other transition metal catalysts of comparable size. 1. Introduction 1,2

Since their initial discovery in 1993, single-walled carbon nanotubes (SWNTs) have become the cornerstone of a new domain of scientific and industrial research at unprecedentedly small scales. The synthesis of SWNTs, via techniques such as arc discharge and chemical vapor decomposition (CVD) in conjunction with transition metal catalysts, is now performed routinely at the commercial scale. Nevertheless, the SWNT community has yet to grasp completely the details of the underpinning mechanisms of SWNT nucleation and growth at the atomic level. Consequently, there are many aspects of SWNT growth mechanisms that are the subject of current debate, and others which are yet to be comprehended at all. Current experimental techniques, furnishing spatial and temporal resolutions on the order of ca. 0.1 nm and 1 ms, respectively, are unable to provide atomically resolved mechanistic details of the SWNT growth process. Molecular dynamics (MD) methods, on the other hand, are uniquely positioned to provide time evolution data that may contribute to our understanding of such mechanisms. There have been a number of MD investigations of Nicatalyzed SWNT growth, and related phenomena, reported in the literature.3-8 The first such MD investigation was reported by Shibuta and Maruyama in 20023 and elaborated upon in 2003.4 These investigations established the roles of both Ni vapor and condensed Ni nanoparticles during the initial stages of the SWNT nucleation process. In remarkable agreement with contemporary CVD results,9-11 these authors successfully nucleated an sp2-hybridized SWNT cap-precursor on a variety of Ni nanoparticles. It was in these pioneering investigations that one of the critical roles now commonly ascribed to the metal catalyst particle in CVD synthesis (viz. the prevention of the closure of the assembling fullerene cages) was observed for the first time * To whom correspondence should be addressed. E-mail: sirle@ iar.nagoya-u.ac.jp (S.I.); [email protected] (K.M.). † Kyoto University. ‡ Nagoya University. § Emory University.

in theoretical simulations. In the intervening years, Shibuta and co-workers have extended these initial investigations of Nicatalyzed SWNT growth on a number of occasions.6-8 Most recently, Shibuta and Elliot7 reported on the interactions between graphene sheets and Fe/Ni bulk/nanoparticles. Despite the ingenuity and scope of these MD/MC investigations, these studies relied on various classical force fields to describe the atomic interactions throughout the respective simulations. This made the simulation of SWNT growth phenomena over nanosecond time scales possible. However, quantum mechanical (QM) effects that are important in the context of SWNT nucleation/growth, such as π-conjugation in sp2-hybrized carbon systems, involvement of transition metal open-shell d orbitals, and changes in the degree of charge transfer, were ignored entirely in such approaches. A notorious consequence of omitting these particular QM effects is the spurious sp3-hybridized/sp2-hybridized carbon ratio12 predicted by classical force fields (compared to DFT data13) in the simulations of gas-phase carbon systems. Such systems are typical during fullerene formation.14 In addition, recent tightbinding Monte Carlo (TBMC) simulations of Amara et al.5 concerning the SWNT nucleation process on Ni(100) are not in the position to elaborate on the time evolution, and hence dynamics, of kinetically favored species such as polyyne chains capable of large-amplitude motions across large areas of real space. To our knowledge there have been no investigations reported in the literature concerning Ni-catalyzed SWNT growth using a QM/MD approach to date. A reliable, atomistically resolved mechanism of Ni-catalyzed SWNT growth therefore remains unknown. This shortcoming is the primary focus of the present work. In this work, we wish to report QM/MD simulations of Nicatalyzed SWNT growth. In particular, we will detail the growth mechanism of a nascent SWNT cap-fragment catalyzed by a Ni38 particle. The simulations presented here complement those reported recently by our group,15,16 which dealt with SWNT growth catalyzed by a variety of Fe catalyst particles. Thus, we will compare and contrast both the mechanism and kinetics of

10.1021/jp100790e  2010 American Chemical Society Published on Web 04/02/2010

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Figure 1. C40Ni38 model system employed during SCC-DFTB/MD simulation of SWNT growth. Cyan and gray spheres represent C and Ni atoms, respectively.

SWNT growth as a function of the catalyst particle composition and size. Our theoretical approach is based on the self-consistentcharge density functional tight-binding (SCC-DFTB/MD) method,17 and so includes QM effects as described by the density functional theory (DFT), at a computational cost closer to that of classical force-field and first principles MD simulations. 2. Computational Details Single-walled carbon nanotube growth has been investigated in this work using nonequilibrium QM/MD. The equations of motion for all MD simulations were integrated using the Velocity-Verlet scheme with a 1 fs time step. The nuclear temperature was maintained using a Nose´-Hoover chain thermostat18 connected to the degrees of freedom of the system. The quantum chemical potential was evaluated “on the fly” at each iteration of the MD simulation using the SCC-DFTB method,17 in conjunction with a finite electronic temperature19,20 (Te ) 10 000 K) and the DFTB parameters developed previously by our group.21 The occupancy of each molecular orbital was therefore described by a Fermi-Dirac distribution function of its energy (and therefore varied continuously over [0, 2] near the Fermi level). The use of a finite electronic-temperature approach accounts for the open-shell nature of the model system employed here (the latter of which possesses unterminated carbon bonds and numerous near-degenerate Ni d orbitals). In particular, the convergence of the SCC-DFTB wave function is improved dramatically using a finite electronic temperature. It has also been established previously during investigations of similar SWNT-catalyst particle systems that using Te ) 0 K leads to a less reactive catalytic surface.15,16,22,23 The model system employed consisted of a single icosohedralC60 hemisphere possessing a (5,5) edge (C40), bound to a Ni38 cluster (Figure 1). The cap-fragment, although constructed manually, resembles closely the cap structures that we have “grown” in previous simulations of Fe38-catalyzed cap fragment nucleation.24 The Ni cluster itself is a segment from the fcc lattice, and it was chosen so that direct comparisons between Ni38- and Fe38-catalyzed16 SWNT growth rates/mechanisms could be made. Following the geometry optimization of the model system and an annealing period of 10 ps at 1500 K,

Figure 2. Structures of Trajectories 1-10 following 50 ps of SCCDFTB/MD simulation. Color conventions as in Figure 1; pink spheres represent newly added carbon atoms.

growth of the C40 cap-fragment was induced by manually supplying gas-phase carbon atoms to the C40-Ni38 boundary region at 0.5 ps intervals, following the protocols described in refs 22 and 25. Each supplied carbon atom was ascribed a velocity of 0.129 eV (equivalent to the MD simulation temperature of 1500 K) directed at a randomly selected “target carbon”. A target carbon is defined here as those carbons that exhibited either an sp-hybridized C-C bond or a C-Ni bond (i.e., all target carbon atoms resided at the C40-Ni38 boundary). A total of 100 carbon atoms were supplied in this manner over a period of 50 ps. Ten independent trajectories were calculated using randomized initial conditions; these trajectories are denoted using roman numerals 1-10. 3. Results and Discussion The final structures of Trajectories 1-10, following 50 ps of SWNT growth simulation using SCC-DFTB/MD, are depicted in Figure 2. It is immediate from this figure that the extension of the original C40 cap-fragment along the axis of growth (i.e., vertically, with respect to Figure 2) occurred in the majority of these trajectories. Trajectories 3, 4, 5, 6, 9, and 10 best illustrate the growth process in this case (a more detailed discussion of Trajectory 6 will follow below). These “successful” trajectories clearly show that relatively clean catalyst surfaces remained. That is, following 50 ps of SWNT growth simulation, the catalyst surface observed in these trajectories remained largely exposed and amenable to the further deposition of carbon species

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Page et al.

Figure 3. SWNT growth Trajectory 6 following (a) 10 ps, (b) 20 ps, (c) 30 ps, (d) 40 ps, and (e) 50 ps SCC-DFTB/MD simulation. Color conventions as in Figure 2.

(in this case carbon atoms). This behavior of the Ni38 catalyst nanoparticles is correlated with the dominance of longer polyyne species attached to the catalyst surface. On the contrary, Trajectory 8 has a greater number of carbon atoms and smaller polyyne species (for instance, C2) on the Ni38 surface. Concomitantly, a less productive extension of the C40 cap-fragment occurred in this trajectory, in that the extension of the sp2hybridized carbon network was against the axis of growth, tending instead toward the encapsulation, and hence deactivation, of the Ni38 catalyst. In Trajectory 8 after 50 ps of SWNT growth simulation, several Ni atoms migrated toward the capfragment. More notably, a few Ni atoms were even incorporated into the lowest layers of the cap-fragment. This incorporation is a specific symptom of a more general phenomenon, viz. the dispersion of surface Ni atoms by C and C2 species bound to the Ni38 surface. We conclude therefore that the “health” of the catalyst particle (i.e. the extent to which its constitution is maintained) correlates directly with the successful extension of the nascent SWNT cap-fragment along the growth axis (i.e., SWNT growth). We have recently arrived at a similar conclusion upon comparison of Fe55- and Ni55-catalyzed SWNT growth.15 Comparison between Ni38- and Ni55-catalyzed SWNT growth and Fe38- and Ni38-catalyzed SWNT growth will be presented in the discussion that follows. Presently, we turn to the detailed discussion of the explicit Ni38-catalyzed SWNT growth mechanism observed in our SCC-DFTB/MD simulations. Now we will discuss Trajectory 6, as it best illustrates many of the typical features associated with successful extension of the sp2-hybridized carbon network observed in Trajectories 1-10. The time evolution of Trajectory 6 is featured in Figure 3. The instantaneous cap height and corresponding ring addition statistics during the course of the SCC-DFTB/MD simulation are shown in Figure 4. It is evident from comparison of these two figures that the extension of the sp2-hybridized carbon network coincided with an increase in the numbers of constituent 5-, 6-, and 7-membered rings. For instance, following 50 ps of SWNT growth simulation, a total of nine 5-membered, eight 6-membered, and nine 7-membered rings were added to the original C40 cap-fragment. These ring-addition statistics are typical of all Ni38-catalyzed SWNT growth simulations performed in this work (see Table 1). The addition of 3- and 4-membered rings was generally uncommon, due to the inherent instability of these structures. These smaller ring structures were therefore deemed insignificant with respect to the SWNT growth mechanism as a whole. This cap growth-ring addition relationship is now well established following a number of prior investigations,15,16,22-26 and so it was also anticipated in this work. One consequence of this relationship is the production of local regions in the sp2-hybridized carbon network exhibiting positive curvature. Such structural characteristics were also anticipated in this work and can be observed, for example, in

Figure 4. (a) Height of C40 cap and (b) corresponding changes in the number of 3-, 4-, 5-, 6- and 7-membered rings as a function of time of SWNT growth Trajectory 6.

TABLE 1: Comparison of Average Addition of 5-, 6-, and 7-Membered Ring Addition and Growth Statistics of Fe38and Ni38-Catalyzed SWNT Growth Simulationsa 5-membered ring 6-membered ring 7-membered ring cap growth (Å) (growth rate (×10-2 Å ps-1))

Fe38

Ni38

Fe38/Ni38

6.2 9.2 4.3 1.77 ( 0.79 (3.54 ( 1.59)

9.2 10.2 6.5 3.65 ( 0.82 (7.30 ( 1.64)

1:1.48 1:1.11 1:1.51 1:2.06

a Data averaged over 10 SCC-DFTB/MD trajectories following 50 ps simulation (error bars denote 95% confidence intervals). Fe data from ref 16.

Figures 2 and 4. This “local curvature” is the result of the phasealternating addition of 6-membered rings and 5-/7-membered (i.e., defect) rings into the growing cap structure. Nevertheless, careful consideration of Figure 3 suggests a different mechanism underlying the extension of the nanotube sidewall in the case of Ni38 growth, compared to, for example, Fe-catalyzed growth.

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Figure 5. Typical polygonal ring addition mechanism observed during SCC-DFTB/MD simulation of Ni38-catalyzed SWNT growth (Trajectory 6). In this case, the alternating extension and collapse (via self-isomerization) of a single polyyne chain results in the formation of a 6-5-7-6 ring system. Color conventions as in Figure 1; yellow spheres represent carbon atoms in newly formed polygonal rings. (a), (c), (e), and (g) represent polyyne extension; (b), (d), (f), and (h) represent polyyne collapse (and ring formation). Times are given with respect to the beginning of the simulation.

In the latter case, we have recently observed an unmistakable correlation between sharp increases in SWNT growth rates and additions of 6-membered rings. That is, Fe-catalyzed SWNT growth is driven almost entirely by the addition of 6-membered rings at the SWNT base. From Figure 3, however, it is evident that this correlation between growth rate and 6-membered ring addition is far more tempered in the present Ni38-catalyzed growth. For example, between ca. 18 and 25 ps, the most noticeable structural change in the sp2-hybridized carbon network is the addition of seven 6-membered rings (Figure 3b). In addition, very few 5- and 7-membered rings were added to the growing cap-fragment. Based on prior knowledge, we are led to anticipate a noticeable increase in SWNT growth rate. Nevertheless, this is not observed in Figure 3a. On the contrary, the growth rate during the first ca. 30 ps is essentially constant. Similarly, between 25 and 40 ps, the SWNT fragment increased in length by ca. 3.4 Å (Figure 3a), despite the number of 6-membered rings actually decreasing during this period (Figure 3b). These disparities between Fe-catalyzed SWNT growth trends and those observed here for Ni38-catalyzed SWNT growth suggest that the mechanism of SWNT growth in the latter case is of a significantly different origin. We now turn to an explicit discussion of this SWNT growth mechanism. Although this discussion focuses on the SWNT growth mechanism observed in Trajectory 6 alone, this mechanism is a common feature of all the trajectories that exhibited successful SWNT growth (see above). The Ni38-catalyzed SWNT growth mechanism is driVen by the simultaneous polyyne extension and subsequent ring collapse of polyyne chains bound to the base of the SWNT cap-fragment. The ring collapse of these polyyne chains, akin to the one deemed to be the major driving force during fullerene self-assembly,14,27 is brought about by either autoisomerization or interaction with adjacent polyyne structures on the Ni38 surface. In either case, the collapse and subsequent shortening of these polyyne chains results in the formation of new sp2-hybridized carbon ring structures at the base of the

SWNT, and hence the growth of the SWNT itself. The accumulation of these polyyne chains is driven by the continual migration of deposited carbon across the Ni38 surface toward the base of the SWNT. This accumulation begins almost immediately after carbon atoms are supplied to the complex, and it is a key element to the ensuing growth mechanism. Initially, these polyyne chains consist primarily of either carbon atoms or C2 units bound to the lowest layer of 6-membered rings in the C40 cap. The subsequent continued deposition of carbon atoms, however, serves to elongate these nascent polyyne moieties throughout the growth process. Critically, the rate of this extension is greater than the rate at which the polyyne chains collapse and become part of the SWNT itself. One example of this process, which is typical of the present mechanism, is illustrated in Figure 5 and the accompanying movie provided in the Supporting Information. This figure begins explicitly at 14.72 ps after the beginning of the growth simulation. The initial step in this mechanism (Figure 5a,b) features the interaction of the C4 unit and the adjacent C2 unit, also bound to the SWNT rim, both of which can be seen in Figure 5a. This interaction produced a new 6-membered ring at 17.06 ps (Figure 5b). Concomitantly, this C4 collapsed, becoming a C3 unit as a result of this ring formation step. In the 1.8 ps that follow, this shortened polyyne chain underwent significant extension, ultimately becoming a C7 polyyne chain (Figure 5c). At t ) 18.86 ps (Figure 5d), the two carbon atoms in this C7 unit closest to the SWNT rim interacted with a neighboring 7-membered ring. This interaction resulted in the formation of a new 5-membered ring. Once again, the collapse of the C7 polyyne chain was observed in this step. The next phase of polyyne extension (between Figure 5d and e) was notably slower than that observed between Figure 5b and c, taking ca. 15 ps. The resulting polyyne chain featured two distinct arms and consisted of 7 carbon atoms in total. Interestingly, this Y-shaped polyyne chain was formed by the dissociative recombination of the terminal C2 moiety. Y-shaped

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polyynes, similar to the structure observed in Figure 5e, have been observed in recent QM/MD simulations of Fe38-catalyzed SWNT nucleation.24 The occurrence of such a structure in this instance was therefore not unexpected. After 33.38 ps of SWNT growth simulation, a third isomerization of the polyyne chain was observed and resulted in the formation of a new 7-membered ring (Figure 5f). Consequently, the polyyne chain was reduced to a C3 unit. It is interesting to note here that this polyyne chain, during the subsequent period of extension (which lasted ca. 9 ps, Figure 5f,g), formed short-lived 5-membered rings on two occasions via both self-isomerization and interaction with a neighboring C3 chain. The latter case is visible in Figure 5g. On both occasions, these 5-membered rings were unstable and dissociated in favor of two independent Cn chains. By 42.82 ps, the polyyne chain had once again been extended significantly and consisted of 6 carbon atoms. A further 0.12 ps of simulation resulted in the formation of a new 5-membered ring, seen in Figure 5h. In this case, the interaction of two adjacent polyyne chains resulted in ring formation. The structure shown in Figure 5h persisted until the end of this SWNT growth trajectory (50 ps in total). Based on these simulation results, we therefore propose that this ring formation mechanism (and hence SWNT growth), driVen by the continual, alternating polyyne growth and ring collapse, is self-perpetuating, proVided that carbon feedstock (in this case carbon atoms) remains aVailable on the catalyst nanoparticle surface. An apparent limitation of this proposed mechanism is that if the rate of carbon addition into the SWNT structure is very small, then ring formation may proceed faster than polyyne extension. That is, the polyyne-extension/ringcollapse mechanism may depend sensitively on the rate of carbon addition. However, subsequent SCC-DFTB/MD simulations of Ni38-catalyzed growth using a slower carbon supply rate (1 C/10 ps, a factor of 20 slower than that employed in this work) have demonstrated that extended polyyne chains still accumulate at the base of the SWNT structure in the manner observed in this work, despite using a slower supply rate. In effect, although a slower carbon supply rate yielded a slower polyyne chain growth rate, it also yielded less frequent ring formation at the SWNT base. The net effect of these two phenomena is that the dependence of the polyyne-extension/ ring-collapse mechanism on carbon supply rate is less sensitive than what might be expected. An analogous trend has been observed previously with respect to Fe38-catalyzed SWNT growth.16 In addition, the self-perpetuity of this mechanism depends entirely on the relative rates of polyyne extension and collapse. These relative rates, in turn, depend on the extent to which the catalyst surface allows new C-C bonds to persist. In the context of SWNT nucleation and growth, C-C bonds act as a “thermodynamic sink”, in that C-C bond formation is thermodynamically much more favorable than C-C bond dissociation. We have shown previously24 that this irreversibility drives the initial nucleation of a SWNT on transition metal catalysts itself. It follows therefore that a weaker catalyst-carbon interaction promotes C-C bond formation, and therefore polyyne extension. A weak catalyst-carbon interaction therefore promotes SWNT nucleation and growth, compared to a stronger catalyst-carbon interaction. Of course, if we consider a hypothetical cap-catalyst interaction that is zero (or at most negligible), our argument fails; the catalyst would no longer be able to hold the growing sp2-hybridized carbon network open, and the subsequent behavior of the sp2-hybridized carbon structure would resemble more closely that typical of fullerene formation.14

Page et al. A comparison of growth rates and ring addition statistics in terms of cap-catalyst energies shows that this proposed relationship between catalyst-carbon interaction strength and SWNT growth kinetics is indeed the case. This comparison is made in Table 1 with respect to Fe38- and Ni38-catalyzed SWNT growth simulations. It is evident from this table that the addition rates of 5-, 6-, and 7-membered rings for Ni38-catalyzed SWNT growth are dramatically larger than those for Fe38-catalzyed SWNT growth. Concomitantly, the rate of growth of the SWNT itself using a Ni38 catalyst particle is higher compared to that obtained using an Fe38 particle. Interestingly, there is not a linear correlation between the ratios of 5-/6-/7-membered ring additions observed in Fe38- and Ni38-catalyzed SWNT growth, and the respective growth rates. For example, it is evident from Table 1 that 5-, 6-, and 7-membered rings are added 48%, 11%, and 51% faster on average in Ni38-catalyzed SWNT growth, compared to Fe38-catalyzed SWNT growth. However, we have determined the Ni38-catalyzed SWNT growth rate to be more than twice that of Fe38-catalyzed SWNT growth. These contrasting ring addition/growth rates are attributed directly to the rate at which carbon is incorporated into the growing sp2-hybridized carbon network. In the case of Ni-catalyzed SWNT growth, we observe here and in our recent simulations of Ni55-catalyzed SWNT growth15 that polyyne extension proceeds more quickly compared to polyyne collapse. According to the mechanism discussed above, this facilitates a faster extension of the nanotube sidewall than is observed for Fe38-catalyzed SWNT growth. From comparison of the number of 6-membered rings formed during Fe38- and Ni38-catalyzed SWNT growth, the polyyne extension-collapse mechanism described here does not provide any obvious preference toward 6-membered ring formation. This was anticipated, since 5-, 6-, and 7-membered rings were formed indiscriminately as the mechanism proceeded, as shown in Figure 5. It is likely, however, that further annealing of the SWNT structure would lead to isomerization of 5- and 7-membered rings, thereby increasing the population of 6-membered rings in the sp2-hybridized carbon network. We have recently observed similar, albeit more tempered, trends with respect to ring addition/growth rates in simulations of Fe55- and Ni55-catalyzed SWNT growth.15 In the latter case, the ratios of 5-, 6-, and 7-membered ring addition (after 50 ps of growth simulation) using Fe55 and Ni55 catalysts were 1:1.45, 1:1.38, and 1:1.34, respectively. Similarly, the ratio between these respective growth rates was 1:1.69. This smaller growth rate ratio for Fe55/Ni55 than for Fe38/Ni38 (1:2.06) is ascribed to the larger surface area and volume of Fe55/Ni55 catalyst nanoparticles, both of which suppress the efficiency of the polyyne extension-collapse mechanism by providing a greater domain that the chemical decomposed carbon moieties may explore before being incorporated into the polyyne chains bound to the SWNT base. In turn, the Ni55-catalyzed SWNT growth process is decelerated. Explicit differences separating the Ni38- and Ni55-catalyzed SWNT growth mechanisms were not observed, despite the difference in catalyst particle diameter. That is, Ni55-catalyzed SWNT growth is also dominated by the polyyne extensioncollapse mechanism outlined above. Similarly, the mechanisms of Fe38- and Fe55-catalyzed SWNT growth are essentially of the same nature.15,16 However, we do observe a substantial difference in the kinetics of SWNT growth using these two different Ni catalyst nanoparticles. Explicitly, the Ni55-catalyzed SWNT growth rate at 50 ps of growth simulation was 4.87 × 10-2 Å ps-1. Ni38-catalyzed SWNT growth is therefore 50% faster compared to Ni55-catalyzed SWNT growth. Once again,

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this slow-down of SWNT growth using Ni55 is attributed directly to the larger surface area and volume of the Ni55 catalyst nanoparticle.

at Kyoto University as well as at the Research Center for Computational Science (RCCS) at the Institute for Molecular Science (IMS).

4. Conclusions

Supporting Information Available: Computational details, xyz coordinates of Trajectories 1-10 following 50 ps SCCDFTB/MD simulation, and QuickTime movie depicting Trajectory 6. This material is available free of charge via the Internet at http://pubs.acs.org.

In summary, we have presented in this work the first atomistic description of the Ni-catalyzed SWNT growth mechanism based on a QM/MD method. This mechanism was dominated by a polyyne extension-collapse process, which drove the formation of new sp2-hybridized carbon ring structures at the base of the SWNT. In addition, this mechanism was found to depend critically on the relative rates of polyyne extension and collapse. In the case of Ni38-catalyzed growth, the former exceeded the latter, and so this mechanism, and hence SWNT growth itself, was observed to be self-perpetuating provided that carbon feedstock was readily available. Conversely, we have observed previously15,16 that for both Fe38- and Fe55-catalyzed SWNT growth the opposite was generally the case. We therefore elucidated that the rate at which these polyyne chains are extended depended on the stability of C-C bonds located on the surface of the catalyst nanoparticle. In turn, this stability depended on the strength of the catalyst-carbon interaction; the C40-Fe55 and C40-Ni55 binding energies per carbon were calculated to be 1.78 and 1.06 eV, respectively.15 Equivalent trends have recently been determined by Larsson and co-workers,28,29 who employed DFT to calculate the relative adhesion energies between model SWNTs and Fe, Ni, Co, Pd, Cu, and Au clusters. Significant differences in the dynamics of SWNT growth using Fe and Ni catalysts were consequently established. Most notably in this respect was the fact that Ni38-catalyzed SWNT growth is faster than Fe38catalyzed SWNT growth by a factor of 2.06 based on our simulations. This agrees qualitatively with observations from CVD, laser evaporation, and carbon arc experiments indicating that Ni is a better catalyst than Fe with respect to the rate of SWNT growth.30 Nevertheless, upon consideration of the respective rates of 5- and 7-membered ring addition for Ni- and Fe-catalyzed SWNTs, it is reasonable to presume that the use of Ni catalysts may impede “chirality-controlled” SWNT growth (compared to Fe catalysts), due to the higher population of defect structures in the growing nanotube sidewall. Acknowledgment. This work was in part supported by a CREST (Core Research for Evolutional Science and Technology) grant in the Area of High Performance Computing for Multiscale and Multiphysics Phenomena from the Japanese Science and Technology Agency (JST). One of the authors (S.I.) also acknowledges support by the Program for Improvement of Research Environment for Young Researchers from Special Coordination Funds for Promoting Science and Technology (SCF) commissioned by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan. Simulations were performed using the computer resources at the at the Academic Center for Computing and Media Studies (ACCMS)

References and Notes (1) Iijima, S.; Ichihashi, T. Nature 1993, 363, 603. (2) Bethune, D. S.; Klang, C. H.; de Vries, M. S.; Gorman, G.; Savoy, R.; Vazquez, J.; Beyers, R. Nature 1993, 363, 605. (3) Shibuta, Y.; Maruyama, S. Phys. B 2002, 323, 187. (4) Shibuta, Y.; Maruyama, S. Chem. Phys. Lett. 2003, 382, 381. (5) Amara, H.; Bichara, C.; Ducastelle, F. Surf. Sci. 2008, 602, 77. (6) Shibuta, Y.; Elliott, J. A. Chem. Phys. Lett. 2006, 427, 365. (7) Shibuta, Y.; Elliott, J. A. Chem. Phys. Lett. 2009, 472, 200. (8) Shibuta, Y.; Maruyama, S. Chem. Phys. Lett. 2007, 437, 218. (9) Cassell, A. M.; Raymakers, J. A.; Kong, J.; Dai, H. J. Phys. Chem. B 1999, 103, 6484. (10) Dai, H.; Rinzler, A. G.; Nikolaev, P.; Thess, A.; Colbert, D. T.; Smalley, R. E. Chem. Phys. Lett. 1996, 260, 471. (11) Maruyama, S.; Kojima, R.; Miyauchi, Y.; Chiashi, S.; Kohno, M. Chem. Phys. Lett. 2002, 360, 229. (12) Zheng, G. S.; Irle, S.; Elstner, M.; Morokuma, K. J. Phys. Chem. A 2004, 108, 3182. (13) Marks, N. A.; Cooper, N. C.; McKenzie, D. R.; McCulloch, D. G.; Bath, P.; Russo, S. P. Phys. ReV. B 2002, 65, 075411. (14) Irle, S.; Zheng, G.; Wang, Z.; Morokuma, K. J. Phys. Chem. B 2006, 110, 14531. (15) Page, A. J.; Minami, S.; Ohta, Y.; Irle, S.; Morokuma, K. Carbon, submitted for publication. (16) Page, A. J.; Ohta, Y.; Okamoto, Y.; Irle, S.; Morokuma, K. J. Phys. Chem. C 2009, 113, 20198. (17) Elstner, M.; Porezag, D.; Jungnickel, G.; Elsner, J.; Haugk, M.; Frauenheim, T.; Suhai, S.; Seifert, G. Phys. ReV. B 1998, 58, 7260. (18) Martyna, G. J.; Klein, M. L.; Tuckerman, M. J. Chem. Phys. 1992, 97, 2635. (19) Weinert, M.; Davenport, J. W. Phys. ReV. B 1992, 45, 13709. (20) Wentzcovitch, R. M.; Martins, J. L.; Allen, P. B. Phys. ReV. B 1992, 45, 11372. (21) Zheng, G.; Witek, H. A.; Bobadova-Parvanova, P.; Irle, S.; Musaev, D. G.; Prabhakar, R.; Morokuma, K.; Lundberg, M.; Elstner, M.; Kohler, C.; Frauenheim, T. J. Chem. Theory Comput. 2007, 3, 1349. (22) Ohta, Y.; Okamoto, Y.; Irle, S.; Morokuma, K. J. Phys. Chem. C 2009, 113, 159. (23) Ohta, Y.; Okamoto, Y.; Irle, S.; Morokuma, K. Carbon 2009, 47, 1270. (24) Ohta, Y.; Okamoto, Y.; Page, A. J.; Irle, S.; Morokuma, K. ACS Nano 2009, 3, 3413. (25) Ohta, Y.; Okamoto, Y.; Irle, S.; Morokuma, K. ACS Nano 2008, 2, 1437. (26) Ohta, Y.; Okamoto, Y.; Irle, S.; Morokuma, K. Phys. ReV. B 2009, 79, 195415. (27) Hunter, J. M.; Fye, J. L.; Roskamp, E. J.; Jarrold, M. F. J. Phys. Chem. 1994, 98, 1810. (28) Ding, F.; Larsson, P.; Larsson, J. A.; Ahuja, R.; Duan, H.; Rosen, A.; Bolton, K. Nano Lett. 2008, 8, 463. (29) Larsson, P.; Larsson, J. A.; Ahuja, R.; Ding, F.; Yakobson, B. I.; Duan, H.; Rose`n, A.; Bolton, K. Phys. ReV. B 2007, 75, 115419. (30) Huang, Z. P.; Wang, D. Z.; Wen, J. G.; Sennett, M.; Gibson, H.; Ren, Z. F. Appl. Phys. A: Mater. Sci. Process. 2002, 74, 387.

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