J. Phys. Chem. C 2007, 111, 2623-2630
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The Role of Dislocations at the Catalyst-Wall Interface in Carbon Nanotube Growth Ludovico M. Dell’Acqua-Bellavitis,*,†,§ Jake D. Ballard,‡,§ Robert Vajtai,‡,§ Pulickel M. Ajayan,‡,§ and Richard W. Siegel‡,§ Department of Engineering Science, Department of Materials Science & Engineering, and Rensselaer Nanotechnology Center, Rensselaer Polytechnic Institute, Troy, New York 12180-3590 ReceiVed: August 23, 2006; In Final Form: NoVember 3, 2006
Ex situ transmission electron microscopy (TEM) performed on catalytically grown multiwall carbon nanotubes identified two types of catalyst-nanotube wall interfaces. The interfaces consisted of crystalline domains with different orientations: twist and twin boundaries in correspondence with quasi-spherical particles closer to the nanotube base and tilt boundaries in correspondence with high aspect ratio, tapered particles further from the base. TEM suggests that the domain boundaries maintain a rather steady position coupled to the catalytic particles, whereas the carbon atoms precipitate along the nanotube axis away from the particles. It is concluded that the relative movement of the carbon atoms with respect to the dislocations comprising the nanotube domain boundary located at the catalyst-wall interface is a significant elementary process in nanotube crystal growth driven by surface diffusion. The results appear consistent with the concurrence of base and tip growth for the catalytic synthesis of carbon nanotubes.
Introduction The traditional view on carbon nanotube catalytic growth maintains two distinct possible scenarios on the direction of carbon precipitation in the catalytic particle, with respect to a set of coordinates centered on the substrate where nanotube nucleation and growth occurs. If the particle adherence to the surface of the substrate is strong, then carbon precipitates from the top surface of the particle and the tube continues to grow with the particle anchored to the substrate. This is called the base growth model. In cases where the catalyst particle attachment to the surface of the substrate is sufficiently weak that it can be separated from the substrate easily, then carbon precipitation occurs at the bottom surface of the particle and the tube lifts the particle as it grows. In this case, the top end of the tube is decorated with the catalyst particle, and this scenario is aptly called the tip growth model. A previous study on the catalytic synthesis of multiwall carbon nanotubes (MWCNTs) led to the formulation of a concurrent base and tip growth model for successive catalyst particles.1,2 According to this model, in the initial stage of nanotube synthesis, dominated by base growth, the tubes grow in random orientation with open tips from the particle, and the dangling bonds from the open tips are energetically unstable until a second catalytic particle from the catalyst-precursorcontaining carbonaceous atmosphere is formed at the opened tip of the tube, minimizing the system free energy. These particles can nucleate and grow at the CNT tip by catalyst precursor molecule chemisorption, pyrolysis, and assembly. At this occurrence, base growth from the initial catalyst particles continues and a new synthesis mechanism of tip growth is initiated. The two processes of base and tip growth continue * Corresponding author. Address: Rensselaer Nanotechnology Center, Rensselaer Polytechnic Institute, Troy, New York 12180-3590. Tel: 518/ 2763011. Fax: 518/2766540. E-mail:
[email protected]. † Department of Engineering Science. ‡ Department of Materials Science & Engineering. § Rensselaer Nanotechnology Center.
simultaneously over subsequent iterations as soon as additional catalyst particles are added to the nanotube chain, and each nanotube is formed by a succession of iterative catalytic steps that concurrently catalyze carbon in the upward (in the case of base growth) and in the downward (in the case of tip growth) directions (Figure 1). Although the efficacy of the growth process has been generally regarded to decrease for the first nanotube domains to be synthesized, which are adjacent to the substrate, recent experimental findings also confirm a sustained growth from lower nanotube portions.3 As the synthesis proceeds, the nanotubes orient perpendicular to the substrate because of the presence of van der Waals forces among adjacent tubes, which inhibit growth in the horizontal orientation (Figure 1). In a more recent study, we have analyzed the interface between catalytic particles and the graphitic layers at the nanotube wall using ex situ transmission electron microscopy (TEM), conducting a structural comparison of nanotubes cooled at different rates in an attempt to explore the validity of the model of concurrent base and tip growth within this critical interfacial region.4 This study determined that the presence of stresses is invariant to different cooling conditions and must therefore be created during nanotube synthesis itself. It is well known in crystallographic systems that any boundary between different orientation domains generally contains dislocations. Hence, in the present work, we take off from the reasonable assumption that the MWCNT domain boundaries are composed of dislocations and we address how iron nanoparticles, encapsulated in the interior part of a nanotube, with no direct access to the gas phase, can act as a catalyst for growth by virtue of strain-induced dislocations on the nanotube walls that act as the pathways for nanotube growth. A recent study by Helveg et al.5 presented time-resolved, high-resolution in situ TEM observations of the formation of nanoscale carbon fibers, a carbon allotrope characterized by a less-ordered lattice structure than CNTs. In the study by Helveg et al., the growth of the nanofiber appears to be promoted by
10.1021/jp0654617 CCC: $37.00 © 2007 American Chemical Society Published on Web 01/23/2007
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Figure 1. Representation of the model of concurrent base and tip growth modes for consecutive catalyst particles. All pictures represent the growth of a small-diameter nanotube consisting of a single graphitic sheet (left to right). Selected portions of the sequence are magnified on the upper portion of the figure, as indicated by the arrows. The nanotubes originate from a catalyst particle (p1) adhering to the substrate. Base growth (color-coded in red) is initiated from this particle and leads to the creation of dangling bonds on one end of the nanotube, which incorporate new catalyst particles (p2). As new catalyst particles are incorporated within the nanotube structure, the mismatch in lattice orientation and the relative difference in lattice spacing between the existing domain of a nanotube and the contacting catalytic particles create a twist and twin domain boundary at the catalyst-wall interface, while the shape of the catalyst particle and its lattice orientation also dynamically change under the influence of the compressive stress by the graphitic walls, leading to a moderate increase in aspect ratio found in type 1 catalyst particles. The presence of type 1 catalyst-wall interfaces is generally limited to the first 30 µm of the nanotubes. Subsequent catalyst particles (p3, p4) reaching the tip of the tube for lengths above 30 µm also feature dynamic shape transitions from low to high aspect ratio found in type 2 catalyst-wall interfaces, while a tilt domain boundary is created at the catalyst-wall interface. The progressive thickening of the nanotube wall with a reduction in I.D. for longer nanotube lengths leads to extensive shearing of the catalyst particle and large aspect ratio: base growth from the initial catalyst particles continues and a new synthesis mechanism of tip growth is initiated (color-coded in green) by the highly sheared particles. The two processes continue simultaneously for subsequent iterations, as soon as additional catalyst particles are added to the nanotube chain. The differences in the catalytic efficiency of base growth and tip growth are expressed by an increasing proportion of green sections over red ones for different nanotube domains. Tip growth becomes increasingly dominant for the domains of the nanotube farther away from the base. As the synthesis proceeds, the nanotubes orient perpendicular to the substrate because of the presence of van der Waals forces between adjacent tubes.
sudden and reiterative changes in the form of the catalyst particle itself, from quasi-spherical to tapered. In particular, the nucleation and growth of the graphitic walls of the fiber appear to be driven by single-atom local step-edge protrusions continuously developing and disappearing in the catalyst, acting as the growth sites for graphene layers and ensuring that the catalyst maintains facets exposed to the vapor, therefore avoiding carbon encapsulation, which would result in growth cessation. The present paper reports ex situ TEM evidence regarding the mechanisms for chemical vapor deposition (CVD) growth of MWCNTs and confirms the interaction between CNTs and two distinct types of shapes of catalytic particles: (i) quasispherical, low aspect ratio and (ii) tapered, high aspect ratio ones. Similar to the study by Helveg et al., the TEM evidence presented here leads one to consider the two types of shapes of catalytic particles as dynamic evolutions formed under the compressive forces applied by the nanotube walls on the catalyst particle, from an otherwise spherical geometry under stressfree conditions. In addition, the two types of catalyst particles appear at segregated loci on the nanotube length: the quasi-spherical, low aspect ratio particle type is found exclusively in close proximity to the substrate, at nanotube lengths 30 µm. The ex situ evidence gathered in this study demonstrates the appearance of the two distinct types of catalytic particles at different time onsets in the synthesis process, therefore leading one to exclude the possibility of reiterative, cyclical transitions alternating from the quasi-spherical to the tapered form within the same particle.
In order for the reader to better understand the analysis and interpretation of this evidence, brief summaries of the materials and method of procedure, followed by the results, are presented here. Further details are presented in ref 4. Materials and Method of Procedure Nanotube Synthesis. Arrays of aligned multiwall carbon nanotubes (MWCNT) were synthesized catalytically using ferrocene (C10H10Fe) as the catalyst precursor, and xylenes (C6H4(CH3)2) were used as the carbon source. Carbon nanotubes were synthesized on a silicon dioxide substrate. Argon was used to carry the reactants to a heated chamber. The quality and quantity of the nanotubes obtained during the synthesis process were optimized by the variation of four parameters in a multidimensional design of experiment. These parameters were: (i) argon flow; (ii) concentration of xylenes-ferrocene solution; (iii) feeding rate of xylenes-ferrocene solution inside the reactor; and (iv) furnace operating temperature.1 Each of these parameters was maintained constant within each synthesis experiment. Good carbon nanotube synthesis was obtained only when the feeding rate and the concentration of xylenesferrocene solution delivered inside the reactor were, respectively, in the ranges of 3-18 mL/hr and 2.5-15 µg/mL. Below those ranges, only sporadic carbon nanotubes could be synthesized because of the paucity of carbon source, whereas above the upper limits of those ranges only amorphous carbon was produced. Transmission Electron Microscopy (TEM). Portions of the CNT arrays were removed from the silicon dioxide substrate with a razor blade and dispersed in N,N-dimethyl-formamide
Dislocations at the Catalyst-Wall Interface
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Figure 2. Brightfield planview transmission electron micrographs of type 1 catalyst-wall interfaces for a multiwall CNT. Type 1 interfaces were found close to the nanotube base (on the lower side of each micrograph) and were characterized by quasi-spherical, low aspect ratio catalytic particles and by sharp discontinuities in the alignment of the crystalline planes comprising the nanotube walls. Such discontinuities were found to precede the position of the catalytic particle with respect to the direction of nanotube growth. (a) On one side the lattice mismatch between adjacent graphite crystal planes was found to be equal to 60° with a sharply discontinuous interface. (b) On the left side the lattice mismatch between adjacent graphite crystal planes was found to be equal to 120° with a sharply discontinuous interface, while on the diametrically opposite side the lattice mismatch featured a lower angle equal to 30° and a continuous interface. (a and b) The catalyst particle associated with the type 1 interface exhibited lattice fringes, which were oriented parallel to those of the nanotube graphitic walls emanating from the particle.
(C3H7NO) at a concentration equal to 0.15 g/mL. The solution was sonicated in a water bath for 2 h until the nanotubes were uniformly dispersed. Five drops of the resulting solution were positioned on a copper TEM grid coated with lacey carbon that was laid on filter paper. A Philips CM 12 transmission electron microscope was used to investigate individual nanotubes comprising the array. This instrument featured a resolution limit of 3.0 Å and a lanthanum hexaboride filament used in conjunction with conventional pole pieces. The operating voltage used for the microscopy was equal to 120 kV. Results Several samples were studied positioning the microscope stage across tens and even hundreds of different portions. The following description of the results is based on the experience thus gleaned on the shape and relative dimensions of both the carbon nanotubes and the catalytic particles. The observations reported here were confirmed in three different batches of vertically aligned nanotube samples, featuring the same length, that had been synthesized using identical conditions and cooled at three different cooling rates (which our earlier work4 demonstrated, yielded identical structures). The brightfield transmission electron micrographs reported in Figure 2 and Figure 3 were selected as the most representative examples of two well-defined and distinct types of catalystwall interfaces for individual multiwall CNTs. These three micrographs were obtained from samples cooled at 1600 °Ch-1. The type of catalyst-wall interface represented in Figure 2, referred to as type 1, was found close to the nanotube base and was characterized by low aspect ratio catalytic particles and by sharp discontinuities in the alignment of the crystalline planes comprising the CNT walls. With respect to the direction of nanotube growth, these discontinuities preceded the position of the catalytic particle and were found to contact the catalytic particle on a portion of its surface characterized by its greatest curvature. The discontinuities in the graphitic sheets comprising the nanotube walls were never found to follow the catalytic particle with respect to the direction of nanotube growth. The catalyst particle associated with the type 1 interface exhibited lattice planes that were oriented parallel to those of the nanotube graphitic walls emanating from the particle, as illustrated in Figure 2. However, the lattice planes of the catalyst particle
Figure 3. Brightfield planview transmission electron micrograph of a type 2 catalyst-wall interface for multiwall CNTs. On average, type 2 interfaces were found farther apart from the nanotube base than interfaces of type 1 and were characterized by tapered, high aspect ratio catalytic particles and by continuous crystalline planes comprising the nanotube walls. The nanotube walls exhibited striations in correspondence to the position of the catalytic particle and were found to be continuously straight in correspondence to the position of the catalytic particle. The catalyst particle exhibited two-dimensional lattice fringes, which were consistently oriented at 60° with respect to the nanotube graphitic walls.
were oriented at an angle with respect to the nanotube graphitic walls preceding the particle. For the majority of type 1 interfaces, the manner in which the nanotube walls emanating from the particle connected to the wall sections preceding the position of the catalytic particle differed on diametrically opposite sides of the CNT: on one side, the lattice mismatch between adjacent graphite crystal planes was found to be a multiple of 60° with a discontinuous
2626 J. Phys. Chem. C, Vol. 111, No. 6, 2007 interface, whereas on the diametrically opposite side, the lattice mismatch featured a lower angle multiple of 30° and a continuous interface. These observations are excluded to be an artifact from specimen preparation in light of their presence within nanotube samples prepared with alternative methods. The type of catalyst-wall interface represented in Figure 3, referred to as type 2, was found farther from the nanotube base than interfaces of type 1 and is characterized by high aspect ratio catalytic particles and by continuous crystalline planes comprising the nanotube walls. In no instance could a type 2 catalyst interface be found on or next to the nanotube base. The catalytic particles exhibited an aspect ratio that varied in the approximate range from 2 to 10. The nanotube walls exhibited image striations corresponding with the position of the catalytic particle. The angle between the nanotube walls and the corresponding striations was found to be relatively constant with an average value of either 30° or 60° across several different nanotube samples, as measured on planview TEM by tilting and rotating the stage across several projections. The angle between the inner graphene layer of a nanotube wall and the Fe crystal lattice was found to vary in multiples of 30° from one nanotube to the other. As indicated in our previous work,4 type 2 catalyst-wall interfaces illustrated in Figure 3 exhibited thicker walls (i.e., larger numbers of coaxial graphitic tubes) than type 1 interfaces (wall thicknesstype2 > wall thicknesstype1). Thickening of the walls resulted in a net reduction of the inner diameter (I.D.) from type 1 to type 2 catalyst-wall interfaces (I.D.type1 > I.D.type2) and was accompanied by an increase in the outer diameter (O.D.) from type 1 to type 2 interfaces (O.D.type2 > O.D.type1). All of the striations evident on the nanotube walls were selectively located on the graphitic layers at the interface between the catalyst particle and the nanotube walls; no dislocations were found in the nanotube walls between different catalyst particles. Twenty transmission electron micrographs were taken at low magnification (
