Article pubs.acs.org/JPCC
Carbon Coating Precedes SWCNT Nucleation on Silicon Nanoparticles: Insights from QM/MD Simulations K. R. S. Chandrakumar,†,∥ Alister J. Page,†,¶ Stephan Irle,*,‡ and Keiji Morokuma*,†,
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†
Fukui Institute for Fundamental Chemistry, Kyoto University, Kyoto 606-8103, Japan Department of Chemistry, Graduate School of Science, Nagoya University, Nagoya 464-8602, Japan § Cherry L. Emerson Center for Scientific Computation and Department of Chemistry, Emory University, Atlanta, Georgia 30322, United States
‡
ABSTRACT: Nucleation of single-walled carbon nanotubes (SWCNTs) from C2 molecules adsorbed on silicon nanoparticles (SiNPs) has been investigated using quantum chemical molecular dynamics (QM/MD) simulations. The SWCNT nucleation mechanism was observed to be strikingly different from that of traditional, transition metal catalyzed SWCNT growth. Most notably, neither a bulk Si carbide phase nor the precipitation of carbon from the nanoparticle bulk was observed to precede SWCNT nucleation on SiNPs. Instead, the intermediate stage during SWCNT nucleation featured the formation of a “carbon-coated” Si surface, i.e., one covered with networks of sp2-hybridized carbon. In addition, the QM/MD simulations indicate that the growth of these sp2-carbon networks was dependent exclusively on the dynamic motion of the polyyne chains formed on the catalyst surface. Analysis of the SiNP phase during SWCNT nucleation also indicated that nucleation proceeded while the Si catalyst remained in the solid phase. Thus, it is concluded that the SWCNT nucleation mechanism presented here was consistent with a vapor−solid−solid mechanism, rather than a vapor−liquid−solid mechanism. This conclusion correlates with recent findings concerning SWCNT nucleation on SiO2 catalyst nanoparticles (Page et al. J. Am. Chem. Soc. 2011, 133, 621).
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yet to be attained.14 To date there is no experimental evidence indicating that the carbide phase necessarily precedes the nucleation and growth of SWCNTs, and indeed, environmental TEM images have also supported the existence of a pure metal phase during growth in the case of nickel.15 Anisimov et al. went in fact so far to say that cementite (Fe3C) particles are catalytically inactive.16 To the contrary, Homma et al.17 observed the nucleation and growth process of CNTs following C2H2 CVD on Fe catalysts, concluding that graphitic networks are formed on cementite (Fe3C) nanoparticles, with CNTs subsequently being extruded from them. In addition, these authors observed that carbon atoms can diffuse/migrate through the nanoparticle bulk during growth. Several other investigations18−21 support the formation of transition metal carbide nanoparticles, such as Ni3C and Fe3C. However, recent X-ray photoemission studies22 point to the possibility that carbide peaks observed in XPS spectra are in fact due to the existence of a surface carbide structure, as opposed to a bulk carbide structure. This conclusion is in line with the proposal of Haratyunyan et al.,14 and subsequently Amara et al.,23−26 that a surface, or subsurface carbide structure, as opposed to bulk carbide, precedes the growth of CNTs and graphene on
INTRODUCTION Carbon nanotubes (CNTs) and single-walled CNTs (SWCNTs) have attracted great interest in recent years due to their unique physical and chemical properties, and their range of potential applications.1,2 However, many of these applications require CNT structural properties (i.e., (n,m) chirality and diameter) to be precisely controlled.3,4 In recent years, the synthesis via chemical vapor deposition (CVD) has been improved substantially toward this goal by the optimization of CVD catalysts,5−8 substrates,9 carbon feedstock,10 etching agents,11 etc. Despite these advances, a method by which a SWCNT with arbitrary (n,m) chirality may be synthesized in situ remains elusive to date. A detailed understanding of the nucleation and growth mechanisms on various catalyst particles is therefore crucial. With respect to traditional, transition metal catalysts (such as Fe, Co, Ni, etc., and alloys thereof), the vapor−liquid−solid (VLS) mechanism12,13 is the prevailing mechanism used to describe SWCNT growth. The VLS mechanism entails (in order) the decomposition of the hydrocarbon CVD feedstock on the metal catalyst, the formation of a liquid or partially molten metal carbide phase, and the nucleation and growth of a solid phase CNT, via the diffusion/precipitation of carbon from the nanoparticle bulk to its surface. Several experimental and theoretical studies of the nature of this intermediate carbide stage have been carried out in the past. A clear consensus regarding the stability of the metal carbide phase, however, is © 2013 American Chemical Society
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transition metal catalysts. Theoretical simulations performed within our own group also point to this possibility.27 Most recently, CNT synthesis on “nontraditional” catalysts, such as SiO2, SiC, Si, Ge, Al2O3, ZrO2, and even nanodiamonds, has been reported.28−39 Since these catalyst species are nonmagnetic in nature, their use may potentially aid the synthesis of defect-free SWCNTs for use in electronic applications. The solid phase of these covalent nanoparticles also provides a potential route to the in situ control of SWCNT chirality and diameter during growth. Recent experimental and theoretical investigations of SWCNT nucleation from SiO2 and SiC33,35,36 catalysts have revealed that SWCNT nucleation in these cases differs fundamentally from nucleation using traditional, transition metal catalysts. These reports not only point to the possibility that the SWCNT nucleation mechanism on silicon-based catalysts is independent of the catalyst employed, they also show that the role of the carbide phase in these cases is fundamentally different from that for transition metal catalysts. Yet, at present, the precise atomistic mechanism explaining SWCNT nucleation on the Si-based catalysts, however, remains unexplored. This shortcoming is the focus of the present work. In this work, we will present a systematic study of the SWCNT nucleation mechanism on SiNPs using quantum mechanical molecular dynamics (QM/MD) simulations. SWCNT nucleation here is induced using a model CVD process, in which gas phase C2 moieties are adsorbed onto a model Si58 nanoparticle at 1250 K. In addition, we will demonstrate that neither the formation of a bulk carbide phase nor the precipitation/diffusion of carbon through the catalyst bulk/subsurface is observed prior to SWCNT nucleation. Instead, we will show that SWCNT nucleation initially necessitates the saturation of the SiNP surface with carbon, after which an sp2-hybridized carbon network is formed. Finally, these results will be discussed in the context of the recently proposed mechanisms of SWCNT nucleation and growth on SiO2 and SiC catalysts.35,36
of carbon, viz., 100 and 150 carbon atoms (denoted as low and high [C], respectively), have been employed. Once these concentrations were reached, the respective systems were annealed at constant temperature for a further 175 and 162.5 ps for low [C] and high [C], respectively. The total simulation time is thus 200 ps. The trajectories for low and high [C] simulations were replicated 10 times and will be referred to as trajectories 1l-10l and 1h-10h, respectively. Since the initial velocities of each trajectory were randomly generated, all trajectories were statistically independent. All MD simulations presented in this work employed an NVT ensemble (i.e., one with constant temperature and volume), with the system temperature being maintained by a Nosé−Hoover chain thermostat (chain length 3).42−44 The Newtonian equations of motion were integrated using the Velocity-Verlet scheme45 with a 1 fs time step. The quantum chemical potential and atomic gradients of the system were calculated “on the fly” at each MD iteration using the selfconsistent-charge density-functional tight-binding (SCCDFTB) method46 in conjunction with a finite electronic temperature (Te = 1500 K). The occupancy of each molecular orbital of the system was thus defined using a Fermi−Dirac distribution function of its energy and so varied continuously between 0 and 2. The use of a finite electronic temperature has previously been shown35,40 to be crucial in the convergence of the SCC-DFTB equations for systems containing many neardegenerate molecular orbitals and unsaturated, dangling carbon bonds, such as those of concern in this work.
COMPUTATIONAL METHODOLOGY The computational methodology employed in this work has been described in detail elsewhere.35,40 Briefly, CNT nucleation was simulated using model SiNPs and carbon dimer (C2) units. The model SiNP, a Si58 cluster, was generated by truncating the crystal structure of bulk silicon.41 The approximate dimension of the nanoparticle was 1.2 × 0.98 × 0.65 nm3. This model geometry, shown in Figure 1a, was optimized before being equlibrated at 1250 K for a period of 10 ps. All MD simulations employed 10.0 × 10.0 × 10.0 nm3 periodic boundaries. Initial velocities of all atoms were randomly drawn from a Boltzmann distribution at 1250 K. It is noted that, due to the large surface/ bulk ratio, the starting geometry of the ordered crystalline structure of the nanoparticle relaxed into an amorphous one during this structural optimization (see Figure 1b). After the 10 ps of thermal equilibration at 1250 K, SWCNT nucleation was induced by “shooting” gas-phase C2 units from randomly chosen, isotropically distributed points around the SiNP, at a rate of 1 C2/0.5 ps. The initial velocity of each incident carbon dimer coincided with the nuclear temperature of the entire system (1250 K). Our group has used a similar shooting approach in previous simulations of Fe-catalyzed SWCNT nucleation.40 In order to analyze the dependence of the SWCNT nucleation mechanism and kinetics on the SiNP surface carbon concentration, two different final concentrations
RESULTS AND DISCUSSION We begin our discussion by considering SWCNT nucleation induced by both low and high carbon concentration on the SiNP surface. The final structures of all trajectories at two different carbon concentrations, low and high, following 200 ps SWCNT nucleation simulation are shown in Figure 2. It is immediate from this figure that the catalyst was chemically stable upon the addition of C2 species (as opposed to SiO235), although the structure of the catalyst surface became slightly disrupted. Most of the adsorbed C2 units remained on the Si surface, with only a few carbon atoms penetrating inside the cluster. The incident C2 species supplied to the Si catalysts mostly formed linear polyyne chains consisting of ca. three to six carbon atoms. Typically, both ends of the polyyne chains were connected to the SiNP through SiC σ-bonding and were evidently very stable, existing for long periods of time. It is also noted that the flexibility, or mobility, of these chains was restrictedthat is, the surface Si atoms held the polyyne chains in place throughout the simulation. The resulting structures were thus typically characterized by the formation of amorphous SiC surface/subsurface regions featuring extended polyyne chains. The core of the nanoparticle, however, remained composed entirely of Si atoms. The evolution of trajectory 6l is depicted in Figures 3 and 4, respectively; corresponding polygonal carbon ring formation is
Figure 1. Structure of the Si58 nanoparticle after (a) geometry optimization and (b) after 10 ps equilibration at 1250 K.
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Figure 2. Final structures of trajectories (a) 1l-10l and (b) 1h-10h at 1200 K after 200 ps, where subscripts “l” and “h” indicate low and high carbon concentration simulations. Blue- and black-colored spheres represent silicon and carbon atoms, respectively. Pure carbon rings are highlighted by yellow where present.
Figure 4. Snapshots from trajectory 6l for the formation of successive carbon ring formation leading to the formation of SWCNT cap type of structure. The number indicates time in picoseconds. Color scheme as in Figure 2; yellow spheres represent the carbon atoms related to the pentagons shown at 117.2 ps.
Figure 3. Snapshots from trajectory 6l, depicting the formation of carbon polyyne chains and pentagons at the surface of the catalyst. The number indicates time in picoseconds. Color scheme as in Figure 2; yellow spheres represent the carbon atoms related to the pentagon shown at 46.2 ps.
When additional C2 were adsorbed and/or other polyyne chains were formed nearby in its vicinity, further carbon ring condensation was observed, with the original carbon ring acting as an “anchor” for the growing sp2-carbon network. This process of additional and successive ring formation (as shown in Figure 4) is found to be relatively slow in this case. For instance, as shown in Figures 3 and 5, the first ring was a pentagon that formed from a carbon chain Y-junction at ca. 46.2 ps, with its next covalently liked neighbor ring being formed only after further ca. 115 ps. The slowness of carbon ring formation is at odds with previous QM/MD simulations of Fe38-catalyzed SWCNT nucleation.40 In the latter case, the formation of the original polygonal ring, although formed in a similar manner to that observed here, was the rate-limiting step of the nucleation process. Subsequent ring condensation, and thus SWCNT cap nucleation, was then significantly more labile than in the case of Si58. The origin of this inhibition of
illustrated in Figure 5. In the current discussion, we focus only on trajectory 6l, as it is indicative of all computed trajectories. The mechanism of polygonal ring formation here consists of several stages. Initially, as the carbon units were adsorbed on the catalyst, they tended to react with each other, forming extended polyyne chains. It can be seen from Figure 3 that such a polyyne chain with 6 atoms was formed at 10.6 ps, with both ends being terminated by Si atoms. As the populations of surface polyyne chains increased, interaction between them was observed, resulting in bridging structures connecting the linear chains. Ultimately, these bridging structures led to the formation of polygonal carbon rings. In the present case, a pentagonal ring was formed at 46.2 ps through such an interaction of two linear chains between 27.4 and 31.7 ps. Once the ring is formed on the Si58 surface, it was evidently stable. 4240
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Figure 5. Evolution of the carbon-only rings formed during the nucleation process on silicon nanoparticles. (a) Low [C], trajectory 6l, and (b) high [C], trajectory 6h.
Figure 6. Change in the trajectory average of the coordination number of carbon atoms during the nucleation process on silicon nanoparticles at (a) low [C] and (b) high [C].
Si58C100 and Si58C150 complexes. Following ca. 50 ps of annealing, there was a slow decrease in sp-carbon atom population and the concomitant increase in the population of sp2-carbon. This clearly demonstrates that the polyyne condensation process (which resulted in a change in carbon hybridization from sp to sp2), and thus SWCNT nucleation, was extremely slow in a relative sense. Although this sp → sp2 conversion feature is similar for both low and high [C] simulations, there is a clear difference in the population of spand sp2-hybridized carbon. In the case of the high [C] simulations, the rate at which the population of sp2-carbon increased, particularly during the first 150 ps, was much higher than that for low [C] simulations. This indicates that the formation of an extended sp2-hybridized network, the prerequisite for SWCNT nucleation, is enhanced in the presence of a carbon-saturated catalyst surface. This latter observation is consistent with Figure 5, which shows that the number of polygonal rings is higher in the case of high [C] than in the case of low [C]. Notably, SWCNT cap nucleation is observed during these high [C] simulations. We now turn to a discussion of the structures observed prior to SWCNT nucleation. In particular, we will address the question of whether or not a Si-carbide phase precedes the SWCNT nucleation process. To this end we have monitored the radial distribution of carbon within the Si58 catalyst (with respect to the Si58 center of mass) throughout the simulation. It is noted here that the radius of the cluster is approximately 4.5
polygonal carbon ring formation is ascribed to the relative strengths of the Si−C and C−C interactions and their impact on the ability of Si−C bonds to be broken in favor of C−C bonds. For example, the bond strengths of the C−C, C−Si, and Si−Si bonds from experimental diatomic binding energies are 5.98, 5.23, and 4.12 eV, respectively. For comparison, the Fe− Fe and Fe−C bond strengths are 0.78 and 1.78 eV, respectively. The mechanism of sp2-hybridized carbon network formation on Si catalysts is therefore remarkably different from that observed for traditional transition metal catalysts, such as Fe, Ni, and Co cases.28,41,47 The improved kinetics of polyyne formation and SWCNT nucleation observed in the latter case were driven by the more volatile or weak nature of these metal−metal and metal−carbon interactions. This in turn promoted the diffusion/oscillation and consequent coalescence of polyyne chains over the metal surface. Hence, the Sicatalyzed SWCNT nucleation mechanism is fundamentally different from that observed using traditional, transition metal catalysts. It is, however, similar to the SWCNT nucleation mechanism observed in our simulations on SiO2 and SiC nanoparticles.35,36 This SWCNT nucleation process is also consistent with the observed dynamics of carbon hybridization. Figure 6 shows the populations of sp-, sp2-, and sp3-hybridized carbon atoms observed during both low and high [C] simulations. It is evident that the formation of sp- and sp2-carbon network was almost instantaneous with the final carbon densities for the 4241
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Figure 7. Solubility of carbon species inside the silicon cluster for (a) low [C] and (b) high [C]. Number of carbon atoms present inside a sphere of radius equivalent to the distance of each carbon atom from the center-of-mass (COM) of the cluster.
Å. These data are shown in Figure 7a. It is evident that the population of carbon atoms at the surface of the Si 58 nanoparticle is very different from that at the core. For example, only two carbon atoms were found at 4 Å from the Si58 center of mass at 20 ps, and this population was maintained until 200 ps. At a distance of 4.5 Å from the Si58 center of mass (i.e., the catalyst surface), this population increases to five carbon atoms, whereas a maximum of 14 carbon atoms were observed at 7 Å. At larger distances, the number of carbon atoms decreases, despite that the volume of the shell is proportional to the distance. A similar pattern is also found for the high [C] case (Figure 7b). For example, at distances of 4, 4.5, and 7.5 Å from the Si58 center of mass, 2, 3, and 21 carbon atoms, respectively, are found. Interestingly, no significant change in this general pattern is observed even after 200 ps, for both carbon concentrations. Approximately 97% of all carbon atoms were located on the SiNP surface. The remaining carbon atoms penetrate the surface and reside in the SiNP subsurface region. The possibility of the diffusion of carbon within the nanoparticle bulk is therefore very unlikely, and the catalyst surface is uniformly covered by sp- and sp2-hybridized carbon. The two prerequisites for VLS growth of CNTs are that carbon is soluble in the catalyst nanoparticles and that the growth temperature should be above the eutectic melting point of the catalyst itself. As shown above, the solubility of carbon inside the Si catalyst is essentially negligible; in order to discount the possibility of a VLS model of SWCNT nucleation in this case, the melting behavior of these SiNPs must be established. The accepted bulk melting point of Si is ca. 1683 K.47 The time evolution of the Lindemann index for the Si58 nanoparticle and the Si58C100 complex is shown in Figure 8. The generally accepted threshold value indicating the solid− liquid phase transition is in the range of 0.10−0.15.48−50 It is evident from Figure 8 that the Si58 nanoparticle at 1250 K ultimately exists as a liquid, according to this definition, with the Lindemann index clearly exceeding 0.15 after ca. 20 ps. Remarkably however, the Lindemann index of the Si58C100 complex equilibrates to ca. 0.068. This indicates that a phase transition, from liquid to solid, takes place upon the adsorption of carbon adatoms on the Si surface. In addition, all carbon adatoms are adsorbed on the Si surface, with very few diffusing into the subsurface or bulk regions. This phase transition is presumably driven by the strong interactions observed between surface silicon atoms and adsorbed carbon atoms.35,36,51
Figure 8. Time evolution of the Lindemann index for Si58 and Si58C100 particle at 1250 K.
Further, due to these relatively strong interactions, we anticipate that the inability of carbon to dissolve into the catalyst nanoparticle is not related to the adsorption rate of carbon employed here. These specific characteristics are contrary to those central to the VLS mechanism. Consequently, it is concluded that SWCNT nucleation on SiNPs cannot be described using the VLS mechanism. Instead, we propose that this process is governed by a vapor−solid−solid (VSS) mechanism, in which the catalyst remains in the solid phase. This conclusion is consistent with recent theoretical and experimental findings concerning SWCNT nucleation on SiO235,51 and SiC36 catalyst nanoparticles. For SiO2, SiC, and Si, it is therefore now apparent that SWCNT nucleation proceeds via a VSS mechanism featuring a solid catalyst nanoparticle predominantly composed of Si. These investigations have also shown that in each case, SWCNT nucleation first requires the saturation of the catalyst surface with carbon, thus confirming the prediction of Homma and co-workers.29 Thus, the mechanisms of SWCNT nucleation on these Si-based catalysts are essentially the same, featuring a common intermediate. In each case, the factors governing the VSS mechanism are the stability of the catalyst, the insoluble nature of the carbon adatoms inside the catalyst, and the inhibited dynamics of polyyne chains adsorbed on the catalyst surface. 4242
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Present Addresses
CONCLUSIONS The presented QM/MD simulations demonstrate that SWCNT nucleation on Si catalyst nanoparticles from chemisorbed C2 particles at an environmental temperature close to 1250 K proceeds with the silicon particle remaining in the solid state and in the absence of a silicon carbide phase. This is consistent with the much lower activation energy for the diffusion of carbon in iron (0.75 eV)52 compared to that of carbon in Si (2.93 eV)53 between 1200 and 1650 K. Furthermore, we found that (i) carbon atoms neither dissolve nor diffuse through SiNPs; hence, a Si-carbide phase is not observed; (ii) SWCNT nucleation is driven by the formation of a carbon network through linear polyyne carbon chains on the SiNP surface; and (iii) these polyyne chains are exceptionally stable, and their constrained rotational/translational motion impedes the formation of sp2-carbon networks (and thus SWCNT nucleation) on the Si surface. An important consequence of these facts is that the SWCNT yield on Si catalysts should be much lower than for iron group metal catalysts (as has been shown experimentally37). These observations also suggest that the VLS type of mechanism is not applicable in the case of SiNP catalysts. While the concept of a VSS mechanism governing SWCNT growth seemed somewhat foreign until recently, it is noted that it is consistent with the latest experimental reports of SWCNT growth on Si-based catalysts. In particular, Homma et al. have demonstrated single- and double-walled CNT growth from semiconductor nanoparticles (Si, Ge, SiO2, and SiC),29,37 concluding that a carbon-coated, solid nanoparticle precedes SWCNT growth. The SWCNT yields obtained with SiO2, SiC, and Si were generally lower than those obtained with the irongroup metals. The more recent experiment by Liu et al.33 concerning the growth mechanism of SWCNTs on Si/SiOx catalysts also revealed that the active catalyst is in the form of solid and amorphous state and that SWCNT growth proceeds via the VSS mechanism. It is also noted here that the growth of several other inorganic nanowires, such as GaAs, InP, and ZnO nanowires,54−56 are governed by a VSS mechanism. In summary, a mechanism of the SWCNT nucleation on SiNPs has been proposed on the basis of QM/MD simulations. These simulations have demonstrated that the intermediate stage for the nucleation of SWCNT features neither a carbide phase nor the precipitation of carbon through the catalyst via diffusion. Si-catalyzed SWCNT nucleation is instead preceded by the formation of an sp2-hybridized carbon network on the catalyst surface, driven by the coalescence of adsorbed polyyne chains. This fact, in addition to the observation that the SiNP exists in the solid phase at 1250 K, i.e., close to the experimental growth temperature,37 leads to the conclusion that the VSS growth mechanism is responsible for SWCNT nucleation in this instance, rather than the VLS mechanism. Consequently, SWCNT nucleations on Si, SiC, and SiO2 proceed via essentially indistinguishable pathways that are fundamentally different from the established mechanisms using traditional transition metal catalysts. It is expected that these types of catalyst can play a vital role in realizing chiralitycontrolled SWCNT growth via CVD synthesis.
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Theoretical Chemistry Section, Bhabha Atomic Research Center, Mumbai400094, India. ¶ Department of Chemistry, School of Environmental and Life Sciences, The University of Newcastle, Callaghan 2308, Australia. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS 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) at Kyoto University, and in part by a U.S. AFOSR grant (FA9550-10-1-0304) at Emory University. A.J.P. acknowledges the Kyoto University Fukui Fellowship. S.I. also acknowledges the support from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of Japan under the Strategic Programs for Innovative Research (SPIRE) and the Computational Materials Science Initiative (CMSI).
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (S.I.); keiji.morokuma@ emory.edu (K.M.). 4243
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