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Nucleation and growth of carbon nanotubes and nanofibers: Mechanism and catalytic geometry control
Carbon 114 (2017) 411e417
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Perspectives
Nucleation and growth of carbon nanotubes and nanofibers: Mechanism and catalytic geometry control Luis Sousa Lobo Universidade Nova de Lisboa, Requimte, Chemistry Department, FCT, 2829-516 Caparica, Portugal
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 August 2016 Received in revised form 1 December 2016 Accepted 2 December 2016 Available online 2 December 2016
Carbon nanotubes and nanofibers with certain shape, size and structure are desired. The study of the kinetics of nucleation and growth of carbon nanotubes and nanofibers is an important key to understand and control the growth process. That knowledge will greatly improve our capacity to optimize structural shapes and increase growth rates. This Perspectives article draws from the literature on carbon nanotube growth and analyzes them to reveal some aspects underlying reaction mechanisms. In each catalyst nanoparticle a constant carbon bulk diffusion flux operates between two distinct catalytic areas with different roles: 1) Surface catalysis producing carbon atoms in some areas; 2) Graphene nucleation and growth, in other areas. Preliminar solid-state changes e obeying 2nd Fick’s law e may take place. Subsequent kinetic linearity is the sign that a steady-state 1st Fick’s law controlled growth process has been established. Data from the literature on diverse crystal orientations activity are discussed. Catalyst duality may be based on different crystal faces or on solid-state phases prevailing during steady-state growth. Growth of carbon nanotubes from Ni nanoparticles usually described as “octopus” carbon offers evidence of the role of geometry, pentagon formation “catalysis” and catalyst duality operating at low temperatures. © 2016 Published by Elsevier Ltd.
1. Introduction There is pressure nowadays to produce nanotubes efficiently and with a given geometry due to its expanding multiple uses [1e9]. But the mechanism is not well understood: “The central problem of nanotube science is still the mechanism” [3]. The early proposals for the mechanism of catalytic carbon formation involving bulk diffusion of carbon atoms through the catalyst were based on detailed thermogravimetric kinetic studies of carbon formation from hydrocarbons on metals: on Ni in the range 200e350 C [10], at 1,000 C [11] and on Ni, Co and Fe and other transition metals in the range 350 C and 700 C [12]. Based on the kinetics observed all those authors concluded that C diffusion through Ni was a step in the carbon growth process and calculated the respective activation energy, based in operating conditions where that step was assumed to be rate controlling (Table 1). Most of the studies on catalytic carbon formation in the period 1970e1990 aimed at minimizing the problem of catalyst poisoning in several processes, particularly steam reforming. Since the 1990’s
E-mail address: [email protected]. http://dx.doi.org/10.1016/j.carbon.2016.12.005 0008-6223/© 2016 Published by Elsevier Ltd.
the scientific interest in understanding carbon formation aims at optimizing the shape, the rate and the density of carbon nanotubes and nanofibers, initially grown mostly at high temperatures - up to 2,000 C. Several alternative routes of carbon formation are known. The mechanisms of 3 alternative routes in carbon formation have been discussed in a recent paper [14]. Table 2 lists and briefly describes the 3 routes. The routes of carbon nanotubes (CNTs) formation may be initiated pyrolytically or catalytically. We call hybrid route the formation of carbon particles pyrolytically in the gas phase, impinging on the surface of a catalyst, dissolving carbon atoms and nucleating and growing graphite elsewhere on the surface of the catalyst. In the nucleation and growth of CNTs by a catalytic route, three factors must be accounted for [14]: 1) The dual catalyst: different and separate areas, some active in gas decomposition (or carbon black solubilization, in pyrolysis) and others active in carbon nucleation and growth; 2) The meaning of kinetic linearity under steady-state, both in catalytic carbon formation and catalytic carbon gasification; 3) The need to form 6 pentagons to get perpendicular CNTs growth, after initial graphene nucleation.
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Table 1 Early thermogravimetric studies of C formation on Ni (assuming carbon bulk diffusion). Year
Authors
Ref.
Gases
T C
Ea
1967 1968 1971
Baggioni, Eyraud Lafiteau, Jacque Lobo, Trimm
[10] [11] [12,13]
C2H2, C2H4 CH4 C2H2, 4 olefins
200e350 C 1,000 C 350e800 C
22,5 37 33
Ea: exponential dependence on temperature of C diffusion in Ni, Kcal/mole.
In a spheroid single crystal nano-particle various low index flat surfaces will appear and high index areas may predominate. The discussion about the catalytic particles being solid or liquid does not apply in surface catalysis: liquids do not have the solid-state crystal structure that is the essence of surface catalysis. We will try to explain how nanotubes can nest on flat solid surfaces of appropriate metal catalysts using Euler’s law and local geometry. High selectivity in nucleation is the key to get various
Table 2 Three alternative kinetic routes in carbon formation [12]. 3 Routes
Process description
Remarks
Dual catalyst route
Carbon formation by surface catalysis decomposition, carbon bulkdiffusion, nucleation and growth e in different surface areas of the catalyst Pyrolytic decomposition, particle impingement, dissolution in catalyst, selective graphite nucleation/growth. Pyrolytic decomposition, forming carbon black particles, usually in layers - sometimes spheres (C60) or tubes.
Initial solid-state changes (2nd Fick’s law) followed by steady-state linear growth (1st Fick’s law).
Hybrid route Pyrolysis route
Carbon formation on foils or catalyst layers is also being used to produce graphene layers. The geometry is a key factor in this type of reactions, both in nucleation and in the growth process. This will be discussed below. The duality required to get a sustained growth may be based on different crystal orientations or on distinct solidstate phases (one in equilibrium with graphite and a different one in equilibrium with the reacting gas). The case of octopus carbon nucleation and growth help illuminate the geometry requirements for CNTs growth [15e19]. The recent paper by Saavedra et al. is of particular interest in view of the detailed observations and statistics presented [18]. Spheroid metal particles are nowadays deposited on substrates to produce CNTs by CVD (chemical vapor deposition): understanding the initial changes in shape, the crystal nano-faces prevailing and the geometry of carbon nucleation and growth is essential to optimize the process. The metals active in the dual catalyst route are Ni, Co and Fe. For the hybrid route there is no need for surface catalysis. Pt, Pd, Cu and many other transition metals are active. It is important to understand the differences in the growth mechanism of carbon tubes (perfectly cylindrical, with one or several graphene walls) and carbon fibers (graphene structure arranged apparently as stacked cones). This will be discussed below, at point 4. 2. CNT growth initiation: 6-pentagon rule and pentagon forming catalysis Iijima et al. [20e22] observed nanotube caps of different shapes and based their analysis on Euler’s theorem concerning polyhedrons and their number of faces (F), vertices (V) and edges (E): F þ V ¼ E þ 2. A consequence is the fact that a hexagonal lattice of any size or shape can only form a closed structure by the inclusion of 12 pentagons. They showed how 6 pentagons could explain the structure of hemispherical caps at the end of nanotubes. The context of Ijima’s work was an apparently non-catalytic arc discharge process, but the CNTs grew at the negative end of the electrode. The carbon nucleation stage must be selective. If nucleation took place over the entire surface, the catalyst would be poisoned and growth would stop. Best catalysts should exhibit selective duality, with certain areas enabling nucleation and other areas (most of the surface) catalyzing carbon formation (or receiving pyrolytic carbon black deposition - hybrid route).
Initial solid-state changes followed by steady-state growth; risk of encapsulation. Various particle shapes: laminar, spherical, tubular.
particular growth processes. The formation of a graphene layer may be seen as a series of hexagon rounds expanding from an initial hexagon, in successive rounds of 6, 12, 18, 24, etc. hexagons (Fig. 1b). The “next round” has always 6 more hexagons. When two pentagons are formed, the next round will have only 4 hexagons more (two less than in a planar expansion). The growth will in fact include from then on only 4 hexagons more in each successive round, instead of 6 more each time. But when a total of six pentagons are formed, the next round (and all subsequent ones) will have the same stable number of hexagons: a hollow graphene tube with a stable number of hexagons is then growing. So the graphene growth orientation is turned 90 in relation to the initial flat growth. A drawing describing CNT nucleation and growth is shown in Fig. 1-a,c. The bending of the initial layer takes place at the edges of that nano surface, as the neighboring carbon atoms coming from the bulk of the metal are at a lower level (Fig. 1-c). The “pentagon formation catalysis” of Ni(111) nano faces can be explained in the following way: Assuming a nanoparticle with 100 nm, the 8 Ni(111) hexagonal facets may be 5e10 nm across and hexagonally shaped. When the growing graphene reaches the border of the facet the next carbon atoms coming from the bulk will be at lower level. Along the zigzag graphene borders the extension will be a range of hexagons slightly bended downwards. But at each corner of the Ni(111) hexagonal facet there is one armchair hexagon. A rehybridization of sp2 s and p p-orbitals, as discussed by Haddon [23] will be induced by geometry/topology. The best fit is then a pentagon not a hexagon with an adequate pyramidalization angle. The 6 corners of the Ni hexagon facet will then “catalyze” the formation of the 6 pentagons needed for the growth to proceed as a nanotube exactly in a perpendicular direction. Discussions of the role of pentagons in other carbon structures, including C60, can be found in the books by Peter Harris [3] and by Kroto and Walton [24]. The work of Ijima mentioned above stimulated the study of these structures. The main differences to the catalytic route discussed here are: 1) Non-catalytic nucleation and growth in the gas phase require much higher temperatures (up to 2000e3000 C; 2) The two catalytic carbon formation processes (dual catalyst route and hybrid route) involve carbon bulk diffusion through the catalyst, graphene nucleation and sustained growth, but the initiation process is substantially different as discussed elsewhere. “Pentagon catalysis” on Ni (111) nanofacets does in fact enable growing nanotubes at temperatures up from 350 C. In the
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Fig. 1. “Pentagon forming catalysis”: carbon nanotubes nucleation and growth a) Graphene nucleation; b) Pentagon rule for 90 graphene surface turn; c) Turn of carbon atoms source (step) at the end of the (111) plateau; d) Fitting of graphene on Ni (111) surface (just 1% adjustment needed: 0.246 nme0.249 nm). (A colour version of this figure can be viewed online.)
work of Iijima the positive (pentagons) and negative (heptagons) declinations are discussed but in relation to narrow nanotubes formed pyrolytically at very high temperatures (2000e3000 C). The catalytic formation on specific surfaces of nanoparticles enables a much better control at much lower temperatures (300 Ce550 C). This nanogeometry is at a scale just above atomic level geometry.
change substantially with the mode of preparation and with environment. This has been the object of various studies and will be briefly discussed below. Octopus carbon growth is a good example that provides evidence of the effectiveness of catalyst duality, as discussed next and shown in Fig. 2. Let us consider a nano-catalyst particle
3. Surface duality at nano level: the case of octopus carbon Of the 3 different routes for carbon formation the dual catalyst route prevails at low temperatures. In this route the catalyst shows a dual role: surface gas reaction in certain areas, a flux of carbon inside the catalyst and carbon growth on other areas [14]. Nano catalysts are very effective for catalytic carbon formation because the bulk diffusion step is short e and so, faster. However, surface duality is essential for the catalyst to be effective: surface catalysis being effective to decompose the reactant gas in certain areas and carbon nucleation and growth being effective in other areas, as remarked above. It is known that carbon nucleation varies substantially in different grains, evidencing selectivity in nucleation in different crystal faces or surface structures [14]. That selectivity is found at all levels, from 102 cm scale (observed on a foil) to 10 nm nanoparticles: the solid-state structure is just the same. However, the shape and prevailing crystal surfaces of the catalyst particles
Fig. 2. Understanding the geometry of octopus carbon growth by assuming the location of the facets in a face centered cube crystal spheroid particle: a) Spheroid Ni particle and location of Ni (111) eight nanofaces; b) Solid state transitions before nucleation and steady-state octopus oriented CNTs growth. (A colour version of this figure can be viewed online.)
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approximately spherical e whose shape depends on its history e but which is in fact a single crystal. A Ni crystal cube would show 6 (100) faces (Fig. 2-a). Rounding up the eight corners would give 8 (111) faces; cutting the edges it would show 12 (110) faces. This helps to visualize the fact that a spheroid Ni particle shows 8 minute hexagonal (111) facets as well as 12 (110) facets, and 6 (100) facets. The fitting of graphene to a (111) Ni face is excellent (ca. 1% difference, check Fig. 1-d). So, when experimental conditions are such that nucleation of graphene is happening only on (111) faces, a tendency to grow nanotubes in about 8 zones with octahedral symmetry is explained. The equilibrium shape of crystals is usually discussed based on the equilibrium crystal shape, as discussed below. We choose here to relate the spheroid shape with reference to an imaginary cube to help understanding the number of its facets and geometry. With this geometry in mind, when nucleation and growth take place on a particular set of facets the observed behavior can be better understood. Is there a preferential growth in 6, 8 or 12 legs? This will be a key to confirm the favored crystal orientation prevailing for nucleation. In Fig. 2-b) the successive solid-state changes that precede carbon growth on a Ni crystallite are shown schematically: carbide formation, carbon nucleation, and carbide recession. Savedra et al. using C2H2 and Ni sputtered on top of a Cu film studied “carbon legs that grew from a faceted catalyst” at ca. 500 C [18]. They concluded that in the range of particle diameters 50e200 nm the number of legs was mostly between 7 and 10. The growth areas are almost restricted to 8 facets with octahedral orientation, most likely coinciding with octahedral Ni (111) planes. Some Ni (110) and Ni (100) nanofacets could also be engaged in nucleation. In other studies the number of legs is much higher and so we may suspect that nucleation is not so selective in those systems and working conditions. The radial growth of thinner and more numerous legs could be controlled by the size of the catalyst particle. The same tendency to octahedral symmetry in the nucleation surface areas and growth of carbon legs is visually obvious in the work of Tavares et al. [15] and Louis et al. [17]. The role of alloying should then be seen from various perspectives: 1) Activity for surface catalysis, producing carbon atoms; 2) Stability of small particle sizes; 3) Reasonable carbon bulk diffusivity; 4) Selectivity in nucleation and appropriate geometry for “pentagon catalysis”, as discussed next. How does a graphite layer growing on a Ni(111) surface turn into a perpendicular graphene tube? We will see that the 90 turn has a consistent explanation. Tavares et al. discussed the possible mechanism of octopus carbon growth in Ni-Cu alloys in relation to the possible changes of the crystal facets with alloy composition [15]. The concepts of catalyst duality and geometry used above are a safe and correct way to explain the phenomenon in general terms. An external carbide (when stable and catalytically active) is not the cause of the driving force for carbon growth but a consequence of the steady-state dynamic equilibrium established. In fact, when an external carbide is the phase in equilibrium with the gas phase, its stability may be sustained by the gradient (driving force) of carbon through the catalyst when this step is rate determining (large C activity gradient). When the diffusion step is not rate-controlling Ni3C is unstable.
interface contact of a particle with a graphite layer. Shape adaptation by solid-solid contact is related with the easiness of the surface atoms to diffuse laterally facilitating the contact. A surface energy gain promotes the change. Let us assume that selective carbon nucleation is a critical aspect of successful carbon growth. If carbon nucleation does not occur, carbon growth will not start. On the other hand, if carbon nucleation is very easy and takes place everywhere, poisoning of the entire catalyst surface will occur and the reaction stops. Selective nucleation is the key. This selectivity is helped by the fact that initiation of the growth process leads to a reduction of the carbon concentration inside the catalyst. A fascinating proof of this behavior is the fact that previous nucleation of carbon on one side of a Ni crystal foil does in fact inhibit nucleation on the opposite side of the foil [25]. Alternative modes of selective nucleation are observed: a) On flat basal contact. When the preferential (selective) nucleation takes place at the flat contact surface between catalyst nanoparticle and substrate (Fig. 3-a), graphene growth will extend laterally to the perimeter of that contact surface and subsequently will bend upward when grasping more carbon atoms from the neighboring interstitial carbon atoms available inside the catalyst solid structure. The particle will be lifted but growth goes on. If new flat layers of graphene nucleate on the bottom surface of the nanoparticle, a CNT with several layers will grow.
4. Alternative nucleation and growth geometries Many reviews have classified and discussed the various growth processes observed. We will discuss those alternatives again from the point of view of the catalyst duality concept. The catalyst substrate and the catalyst carbon good contact require a reaction temperature above the Tammann temperature of the catalyst particle so that its shape may adjust effectively (TTa > 0.50 mp in Kelvin). There is some similarity between a sintering process and an
Fig. 3. Alternative nucleation modes and subsequence growth mechanisms. The several modes of carbon growth depend mostly on the prevailing nucleation area. a,b) Pentagon formation is the key to bend the initial graphene layer to a CNT. Euler’s rule evidences the need to have 6 pentagons formed for a 90 bend, turning the flat graphene into a perpendicular tube; c,d) Observed TEM dynamics of CNFs growth evidence the spiral conic structure of these fibers which grow turning around; e) Growth by successive graphene layers, each pushing up previously formed ones. (A colour version of this figure can be viewed online.)
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b) On external crystalline face. Let us consider the case where easier nucleation takes place on a particular crystal surface not in contact with the substrate (Fig. 3-b). In this case growth evolves in a similar manner, but the particle remains in contact with the substrate. When this type of nucleation is not successful in bending to form a CNT but successive graphene layers nucleate underneath, growth takes place as schematized in Fig. 3-e, superimposing successive graphene layers. c) Inside embedded conical contact. A very interesting case is carbon nanofibers (CNFs) growth. Nucleation starts at a particular point of a conical contact catalyst/substrate (Fig. 3-c) when the nanoparticle is embedded in the substrate. With a selective nucleation and graphene growth inhibiting alternative nucleation, the “just born” graphene sheet grows turning around the conical contact surface in opposite directions. When the two growth parts meet at the opposite side, one goes “up”, remains in contact with the catalyst and will continue to grow. The other part loses contact with the catalyst and stops growing - it remains as the limit of the conic spiral CNF that goes on growing at the other end. When observed under in-situ electron microscopy the successive layers are seen to appear very regularly but alternating at each side, as described by Helveg et al.: “… interfacial step edge formation is observed at either side of the Ni nanocrystal at different instants” [26]. This should be regarded as proof of the spiral cone growth (check Fig. 3-d). Under TEM a sheet is only seen when observed from a top view. It is almost transparent and invisible when observed perpendicularly to its surface [27].
5. Catalytic behavior of nanoparticles, crystal phases, crystal shape, crystal faces Recently the present author published review papers aiming at clarifying the reverse reaction: catalytic carbon gasification. Geometry and graphite/catalyst contact are essential elements of the role of the catalyst particles, described as carbonworms [28,29]. In carbon formation geometry and crystal structure are also an essential element in the role of the catalyst particles in the formation of carbon structures, as described above. We can analyze that in more detail. Table 3 summarizes studies on the activity of different crystal faces observed in catalytic carbon formation. It is well established that decomposition of CO or hydrocarbons on Ni takes place on faces (100) and (210) but not on (111). It is also well established by Geus and co-workers that in the case of Ni turns to carbide initially, facilitating carbon nucleation on a (111) face, but once graphite is formed recession of the carbide is immediate, as discussed below [1]. Ni3Ce is the phase in equilibrium with the gas phase but Ni is the phase in equilibrium with carbon. Duality is the
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basis for the carbon atoms flux. In this case catalyst duality is based in different solid-state phases - not just different crystal faces. In the case of Co there is a change from hexagon to bcc structure above 425 C and turning back to hexagonal below 375 C [43]. Above 425 C the catalytic behavior of the crystal faces is similar to Ni, with almost the same cell size. There is evidence that Co may stay with the fcc crystal structure below 400 C when in small particles [44]. Co carbide should play the same role as in Ni in the catalysis of carbon formation. Fe has a different crystal structure and excretion of carbon takes place at the (100) faces. Kherer and Leidherser recorded many details of carbon formation on different crystal orientations of Ni, Co and Fe [31]. Co (fcc) and Ni showed a similar behavior. With Fe the analysis is more complex the due to presence of various carbides. They used CO as carbon source. The shape and size of the crystal particles depend on various factors, including the solid-state layer crystallinity where they were formed. Early observations showed the effect of previous annealing (accelerating growth) or previous H2 reduction (delaying carbon growth) [12e14]. Tesner et al. studied carbon fibers formation using C2H2 and nichrome wire [45] and Oberlin et al. studied filamentous carbon formation from benzene [46]. Tibbetts and co-workers studied carbon growth from CH4 on Fe and steel [47,48]. The nature and dimensions of the catalyst particles were observed and discussed. Various recent studies on the preparation of supported nanoparticles have identified ways to prepare metal particles with alternative sizes and shapes. A very recent international study on Co defines conditions to prepare Co particles with 3e10 nm diameter supported on commercial silica using NaBH4 as precipitating agent [49]. It would be interesting to identify the crystal orientation of the predominant facets. After the studies by Iijima in the early 90s [21,22] the interest in producing low cost carbon fibers with particular properties increased tremendously. The production of single-wall CNTs had important contributions from Bethune and co-workers [50,51], Smalley and co-workers [52e54] and Terrones and co-workers [55,56]. Terrones et al. discussed the carbon helices frequently observed [57,58]. Several recent reviews include important remarks on geometry [4e6]. The formation of single vs. multiple walls depends on kinetic and thermodynamic conditions favoring or inhibiting successive nucleation after the first one. Iijima et al. and Bethune et al. announced in the same issue of Nature in 1993 the successful production single wall CNTs [59,60]. Pre-treatments are now being used to accelerate and optimize CNTs growth by many authors. Merkulov et al. [61] also found ways to get a sharpening of the CNTs. The control of both the supporting film (usually SiO2) and the catalyst film by varying the preparation mode has recently been explored in various ways to make the CNTs grow faster and with the desired structure. It has been a trial and error approach to optimize carbon growth with a certain structure -
Table 3 Different catalytic activity of different crystal faces (Ni, Co, Fe). Study
1st author (ref.)
Year
Gas
Metal
T C
Remarks (reac./nucleat.)
C formed Nucleation Face changes Nucleation Nucleation Nucleation Adsorption Reac./Growth Role of faces Role of faces Surface catal. Nucl./Growth
Leidherser [30] Kherer [31] Cunningham [32] Lobo [16] Bernardo, Lobo [15] Eizenberg [33] Eizenberg [34] Schouton [35,36] Audier [37e39] Vogt [40] Labohm, Gijze [41]. Lahiri [42]
1948 1954 1957 1971 1975 1979 1979 1979 1980 1987 1982 2011
CO CO/H2 C2H4/H2 Olefins C2H2 CH4/H2 CH4/H2 CH4 CO/CO2 CH4/CO CO/CH4 C2H4,105
Ni Fe, Co, Ni Ni Ni foils Ni crystal Ni Ni Ni Fe, Co Ni, Fe Ni Ni
550 420e600 50e200 400e600 445 700e800 700e800
C on (111), not (100), (110) Faces with carbon pictured Reaction on crystal faces C growth: grain sensitive Growth inhibits nucleation C on Ni(111) faces At Ni (100) þ (210) faces Reac. (100), Growth (111) (111) faces free of carbon Excretion: Ni(111),Fe (100) Reaction: (100)þ(110) Growth on Ni(111)
600 150e550 >150 460
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fast if possible. Understanding the mechanism allows a more efficient approach to optimize growth conditions. As remarked elsewhere, cause-effect misunderstandings seem to occur frequently in explaining this reaction’s alternative mechanisms. To understand a mechanism kinetics is more important than thermodynamics. The detailed study of Bromley and Strickland constable on carbon formation on Ni below 500 C was an important contribution to that understanding [14,62]. The solid-state phases of the nanoparticles prevailing during reaction, their size, shape and crystallographic orientation determine the catalytic activity. So, we may say that when the shape, crystal faces, solid-state phases open a reaction route, the catalyst will be active and selective for a certain type of growth. The best way to determine the kinetic rate-determining step is to draw an Arrhenius plot including an indication of the reaction orders. The external film diffusion step (flat line), or the catalytic surface reaction (variable reaction order) or the carbon bulk diffusion step (zero reaction order) will be rate controlling. The correlation between the Tammann Temperature and the onset of catalytic activity in carbon gasification is well known [29]. The effective solid-solid contact between catalyst and carbon is facilitated, allowing easy carbon solubility [14,29]. Speaking of melted “liquid surfaces” is misleading: a state is tri-dimensional. The Tammann effect is related with ease of surface self-diffusion and so of solid-solid contact adjustment, as in sintering of powder particles. 6. Concluding remarks Comprehensive kinetics is a useful basis to understand the critical aspects to focus on in each particular system. When a steady state carbon formation rate is established linearity of the weight vs. time register is observed. To successfully control CNTs and CNFs growth the following aspects must be known and adjusted when possible: a) Size, shape, solid-state phase(s), crystal structure, and orientation of the nanoparticles; b) Gas chemistry, surface catalysis; b) Carbon nucleation and initial shape formation; c) Steadystate carbon growth kinetics. Acknowledgements I thank Mana for revising and Tiago Monteiro for the drawings. References [1] K. De Jong, J.W. Geus, Carbon nanofibers: catalytic synthesis and applications, Catal. Rev. Sci. Eng. 42 (4) (2000) 481e510. [2] E. Lamouroux, P. Serp, Catalytic routes towards single wall carbon nanotubes, Catal. Rev. Sci. Eng. 49 (3) (2007) 341e405. [3] P.J.F. Harris, Carbon Nanotube Science, Cambridge U. Press, Cambridge, 2009. [4] J. Zhang, M. Terrones, C.R. Park, R. Mukherjee, M. Monthioux, N. Koratkar, Y.S. Kim, R. Hurt, E. Frackowiack, T. Enoki, Y. Chen, Y. Chen, A. Bianco, Carbon science in 2016: status, challenges and perspectives, Carbon 98 (2016) 718e732. [5] J.-P. Tessonier, S.D. Su, Recent progress on the growth mechanism of carbon nanotubes, ChemSusChem 4 (2011) 824e847. [6] V. Jourdain, C. Bichara, Current understanding of the growth of carbon nanotubes in chemical vapor deposition, Carbon 58 (2013) 2e39. [7] G.D. Nessim, M. Seita, D.L. Plata, K.P. O’Brien, A.J. Hart, E.R. Meshot, C.M. Reddy, P.M. Gschwend, C.V. Thompson, Precursor gas chemistry determines the crystallinity of carbon nanotubes synthesized at low temperature, Carbon 49 (2011) 804e810. [8] L.L. Patera, C. Africh, R.S. Weatherup, R. Blume, S. Bhardwaj, C. CastellarinCudia, A. Knop-Ghrike, R. Schloggl, G. Comelli, S. Hofmann, C. Cepek, In situ observations of the atomistic mechanism of Ni catalyzed low temperature graphene growth, ACS Nano 7 (9) (2013) 7901e7912. [9] N. Halonen, J. Maklin, Low temperature growth of MWCNTs by CVD, Phys. Status Solid 248 (2011) 11. [10] M. Escoubes-Baggioni, J. Quinson, C. Eyraud, Reaction du methane, de thane, de l’ethyle ne et de l’acetyle ne sur le nickel divise , Bull. Soc. Chim. Fr. l’e (1967) 2435.
tique des reactions de carburation et de [11] H. Lafitau, L. Jacque, Etude cine haute temperature, Bull. Soc. Chim. Fr. (1968) decarburization du nickel a 4779. [12] L.S. Lobo, D.L. Trimm, Complex temperature dependencies of rate of carbon deposition on nickel, Nat. Phys. Sci. 234 (44) (1971) 15e16. [13] L.S. Lobo, J.L. Figueiredo, C.A. Bernardo, Carbon formation and gasification on metals. Bulk diffusion mechanism: a reassessment, Catal. Today 178 (2011) 110e116. [14] L.S. Lobo, Catalytic carbon formation: clarifying the alternative kinetic routes and defining a kinetic linearity for sustained growth concept, React. Kinet. Mech. Catal. 118 (2016) pp.393e414. [15] T. Tavares, C.A. Bernardo, I. Alstrup, J.R. Rostrup-Nielsen, Reactivity of of carbon deposited on nickel-copper alloy catalysts, J. Catal. 100 (1986) 545e548. [16] C. Pham-Huu, R. Vieira, B. Louis, A. Carvalho, J. Amadou, About the octopuslike growth mechanism of carbon nanofibers over graphite supported nickel catalyst, J. Catal. 240 (2006) 194e202. [17] B. Louis, R. Vieira, A. Carvalho, J. Amadou, M.J. Ledoux, C. Pham-Huu, Carbon nanofibers grown over graphite supported Ni catalyst: relationship between octopus-like growth mechanism and macro-shaping, Top. Catal. 45 (1e4) (2007) 75e80. [18] M.S. Saavedra, G.D. Sims, L.N. McCartney, V. Stolojan, J.V. Anguita, Y.Y. Tan, Catalyzing the production of multiple arm carbon octopi nanostructures, Carbon 50 (2012) 2141e2146. [19] K. Dasgupta, J. Jieshtharaj, P. Bhashkar, D. Sen, S. Banerjee, Growth of carbon octopus-like structures from carbon black in fluidized bed, Mater. Express 3 (1) (2013) 51e66. [20] S. Iijima, Helical microtubules of graphitic carbon, Nature 35 (1991) 56e58. [21] S. Iijima, T. Ichiachi, Y. Ando, Pentagons, hexagons and negative curvature in graphite microtubules growth, Nature 356 (1992) 776e778. [22] S. Iijima, P.M. Ajayan, T. Ichiashi, Growth model for carbon nanotubes, Phys. Rev. Lett. 69 (21) (1992) 3100e3103. [23] R.C. Haddon, The Fullerenes: Powerful Carbon-based Electron Acceptors. Ref. (23), pp. 53e62. [24] H.W. Kroto, D.R.M. Walton (Eds.), The Fullerenes, Cambridge U. Press, 1993. [25] C.A. Bernardo, L.S. Lobo, Carbon 14 (1976) 287e288. [26] S. Helveg, J. Sehested, J.R. Rostrup-Nielsen, Whisker growth in perspective, Catal. Today 178 (2011) 42e46. [27] A.A. Chen, M.A. Vannice, J. Philips, Morphology of graphite supported ironmanganese catalyst particles: formation of hollow spheres during oxidation, J. Catal. 116 (1989) 568e585. [28] L. Lobo, Catalytic carbon gasification: review of observed kinetics and proposed mechanisms or models - highlighting carbon bulk diffusion, Catal. Rev. Sci. Eng. 55 (2013) 210e254. [29] L.S. Lobo, S.A.C. Carbineiro, Kinetics and mechanism of catalytic carbon gasification, Fuel 183 (2016) 457e469. [30] H. Leidhaiser, A.T. Gwathmey, The selective deposition of carbon on the (111) face of a nickel crystal in the catalytic decomposition of carbon monoxide, J. Am. Chem. Soc. 70 (3) (1954) 1206. [31] V.J. Kherer, H. Leidheiser, The catalytic decomposition of carbon monoxide on large metallic single crystals, J. Phys. Chem. 58 (7) (1954) 550e555. [32] R.E. Cunningham, A.T. Gwathmey, The reaction of hydrogen and ethylene on several faces of a single crystal of nickel, Adv. Catal. 9 (1957) 25e36. [33] M. Eisenberg, J.M. Blakely, Carbon monolayer phase condensation on Ni (111), Surf. Sci. 82 (1) (1979) 228e236. [34] M. Eisenberg, J.M. Blakely, Carbon interaction with nickel surfaces e monolayer formation and structure stability, J. Chem. Phys. 71 (8) (1979) 3467e3477. [35] F.C. Schouten, O.L.B. Gijzeman, G.A. Bootsna, Interaction of methane with Ni(111) and Ni(100) e diffusion of carbon into nickel through the (100) surface e AEG-LEED- study, Surf. Sci. 87 (1) (1979) 1e12. [36] F.C. Schouten, O.L.B. Gijzeman, G.A. Bootsna, Reaction of methane with nickel single-crystal surfaces and the stability of surface nickel carbides, Bull. Soc. Chim. Belg. 88 (7e8) (1979) 541e557. [37] M. Audier, M. Renaud, L. Bonnetain, Disproportionation of CO on Ni and Co, J. Chim. Phys. Physichochim. Biol. 74 (5) (1977) 533e538. [38] M. Audier, M. Coulon, Relative crystallographic orientations of carbon and metal in a filamentous catalytic carbon, Carbon 18 (1) (1980) 73e76. [39] M. Audier, M. Coulon, L. Bonnetain, Disproportionation of CO on iron cobalt alloys: I. Thermodynamic study; II. Kinetic study of Fe-Co alloys of different compositions; III. Kinetic Laws of the Carbon Growth and Catalyst Fragmentation, Carbon 21 (1983) 93e111. [40] E.T.C. Vogt, A.J. van Dillen, J.W. Geus, in: B. Delmont, G.F. Froment (Eds.), Prevention of Growth of Filamentary Carbon in Supported Iron and Nickel Catalysts. Catalyst Deactivation, Elsevier, 1987, pp. 221e233. [41] (a) F. Labhom, C.W.R. Engelen, O.L.J. Gijzeman, J.W. Geus, G.A.J. Bootsma, Chem. Soc. Faraday Trans. I 78 (1982) 2435; (b) O.L.J. Gijeman, M.M.J. van Zandvoort, F. Labohm, Carbon monoxide on Ni(111), J. Chem. Soc. Faraday Trans. 2 80 (1984) 771e781. [42] J. Lahiri, T. Miller, L. Adamska, I. Oleynik, M. Batzill, Graphene growth on Ni (111) by transformation of a surface carbide, Nano Lett. 11 (2011) 518e522. [43] J.E. Bidaux, R. Schaller, W. Benoit, Study of the hcp-fcc phase transition on cobalt by internal friction and elastic modulus measurements in the kHz frequency range, Phys. Colloq. C8 48 (12) (1987). Decembre. [44] E.A. Owen, D.M. Jones, Effect of grain size on the crystal structure of Co, Proc.
L.S. Lobo / Carbon 114 (2017) 411e417 Phys. Soc. Sec. B 67 (6) (1954). [45] P.A. Tesner, E.Y. Robinovich, I.S. Rafalkes, E.F. Arefieva, Formation of carbon fibers from acetylene, Carbon 8 (4) (1970) 435. [46] A. Oberlin, M. Endo, T. Koyama, Filamentous growth of carbon through benzene decomposition, J. Cryst. Growth 32 (3) (1976) 335e349. [47] G.G. Tibbetts, Lengths of carbon fibers grown from iron catalyst particles in natural gas, J. Cryst. Growth 73 (1985) 431e438. [48] G.G. Tibbetts, Improved yield of carbon fibers by pyrolysis of natural gas in stainless steel tubes, Carbon (1985) 423e430. [49] G. Di Carlo, M. Lualdi, A.M. Venezia, M. Boutonnet, M. Sanchez-Dominguez, Design of cobalt nanoparticles with tailored structural and morphological properties via O/W and W/O microemulsions and their deposition onto silica, Catalysts 5 (2015) 442e459. [50] C.-H. Kiang, W.A. Goddard, R. Beyers, D.S. Bethune, Carbon nanotubes with single-layer walls, Carbon 33 (1995) 903e914, 995. [51] C.-H. Kiang, W.A. Goddard, R. Beyers, J.R. Salem, D.S. Bethune, Catalytic effects of heavy metals on the growth of carbon nanotubes and nanoparticles, J. Phys. Chem. Solids 57 (1996) 35e39. [52] T. Guo, P. Nickolaev, A. Thess, D.T. Colbert, R.E. Smalley, Catalytic growth of single-walled nanotubes by laser vaporization, Chem. Phys. Lett. 243 (1995) 49e54. [53] H. Dai, G. Andrew, G. Rinzler, P. Nickolaev, A. Thess, D.T. Colbert, R.E. Smalley, Single-wall nanotubes produced by metal-catalyzed disproportionation of carbon monoxide, Chem. Phys. Lett. 260 (1996) 471e475. [54] P. Nicolaev, A. Thess, G. Rinzler, D.T. Colbert, R.E. Smalley, Diameter doubling
417
of single-wall nanotubes, Chem. Phys. Lett. 266 (1997) 422e426. [55] M. Terrones, N. Grobert, J. Olivares, J.P. Zhang, H. Terrones, K. Kordatos, W.K. Hsu, J.P. Hare, P.D. Twnsend, K. Prassides, A.K. Cheetham, H.W. Kroto, D.R.M. Walton, Controlled production of aligned-nanotube bundles, Nature 388 (1997) 52e55. [56] M. Terrones, N. Grobert, J.P. Zhang, H. Terrones, J. Olivares, W.K. Hsu, J.P. Hare, A.K. Cheetham, H.W. Kroto, D.R.M. Walton, Preparation of aligned carbon nanotubes catalyzed by laser-etched cobalt thin films, Chem. Phys. Lett. 285 (1998) 299e305. [57] H. Terrones, A.L. MacKay, Y. Shibuta, J.A. Elliot, From C60 to negatively curved graphite, Prog. Cryst. Growth Charact. 34 (1997) 25e36. [58] M. Terrones, G. Terrones, H. Terrones, Structure, chirality, and formation of giant icosahedral fullerenes and spherical graphitic onions, Struct. Chem. 13 (2002) 373e384. [59] D.S. Bethune, C.H. Kiang, M.S. Devries, Cobalt-catalyzed growth of carbon nanotubes with single-atomic-layerwalls, Nature 363 (1993) 605e607. [60] S. Iijima, T. Ichiashi, Single-shell carbon nanotubes of 1-nm diameter, Nature 363 (1993) 603e605. [61] I.A. Merkulov, M. Yoon, D.B. Geohegan, How the shape of catalyst nanoparticles determines their crystallographic orientation during carbon nanofiber growth, Carbon 60 (2013) 41e45. [62] J. Bromley, R.F. Strickland-Constable, Role of nucleation process in the reactions between carbon monoxide and nickel, Trans. Faraday Soc. 10 (1960) 1492e1500.
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Perspectives
Nucleation and growth of carbon nanotubes and nanofibers: Mechanism and catalytic geometry control Luis Sousa Lobo Universidade Nova de Lisboa, Requimte, Chemistry Department, FCT, 2829-516 Caparica, Portugal
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 August 2016 Received in revised form 1 December 2016 Accepted 2 December 2016 Available online 2 December 2016
Carbon nanotubes and nanofibers with certain shape, size and structure are desired. The study of the kinetics of nucleation and growth of carbon nanotubes and nanofibers is an important key to understand and control the growth process. That knowledge will greatly improve our capacity to optimize structural shapes and increase growth rates. This Perspectives article draws from the literature on carbon nanotube growth and analyzes them to reveal some aspects underlying reaction mechanisms. In each catalyst nanoparticle a constant carbon bulk diffusion flux operates between two distinct catalytic areas with different roles: 1) Surface catalysis producing carbon atoms in some areas; 2) Graphene nucleation and growth, in other areas. Preliminar solid-state changes e obeying 2nd Fick’s law e may take place. Subsequent kinetic linearity is the sign that a steady-state 1st Fick’s law controlled growth process has been established. Data from the literature on diverse crystal orientations activity are discussed. Catalyst duality may be based on different crystal faces or on solid-state phases prevailing during steady-state growth. Growth of carbon nanotubes from Ni nanoparticles usually described as “octopus” carbon offers evidence of the role of geometry, pentagon formation “catalysis” and catalyst duality operating at low temperatures. © 2016 Published by Elsevier Ltd.
1. Introduction There is pressure nowadays to produce nanotubes efficiently and with a given geometry due to its expanding multiple uses [1e9]. But the mechanism is not well understood: “The central problem of nanotube science is still the mechanism” [3]. The early proposals for the mechanism of catalytic carbon formation involving bulk diffusion of carbon atoms through the catalyst were based on detailed thermogravimetric kinetic studies of carbon formation from hydrocarbons on metals: on Ni in the range 200e350 C [10], at 1,000 C [11] and on Ni, Co and Fe and other transition metals in the range 350 C and 700 C [12]. Based on the kinetics observed all those authors concluded that C diffusion through Ni was a step in the carbon growth process and calculated the respective activation energy, based in operating conditions where that step was assumed to be rate controlling (Table 1). Most of the studies on catalytic carbon formation in the period 1970e1990 aimed at minimizing the problem of catalyst poisoning in several processes, particularly steam reforming. Since the 1990’s
E-mail address: [email protected]. http://dx.doi.org/10.1016/j.carbon.2016.12.005 0008-6223/© 2016 Published by Elsevier Ltd.
the scientific interest in understanding carbon formation aims at optimizing the shape, the rate and the density of carbon nanotubes and nanofibers, initially grown mostly at high temperatures - up to 2,000 C. Several alternative routes of carbon formation are known. The mechanisms of 3 alternative routes in carbon formation have been discussed in a recent paper [14]. Table 2 lists and briefly describes the 3 routes. The routes of carbon nanotubes (CNTs) formation may be initiated pyrolytically or catalytically. We call hybrid route the formation of carbon particles pyrolytically in the gas phase, impinging on the surface of a catalyst, dissolving carbon atoms and nucleating and growing graphite elsewhere on the surface of the catalyst. In the nucleation and growth of CNTs by a catalytic route, three factors must be accounted for [14]: 1) The dual catalyst: different and separate areas, some active in gas decomposition (or carbon black solubilization, in pyrolysis) and others active in carbon nucleation and growth; 2) The meaning of kinetic linearity under steady-state, both in catalytic carbon formation and catalytic carbon gasification; 3) The need to form 6 pentagons to get perpendicular CNTs growth, after initial graphene nucleation.
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Table 1 Early thermogravimetric studies of C formation on Ni (assuming carbon bulk diffusion). Year
Authors
Ref.
Gases
T C
Ea
1967 1968 1971
Baggioni, Eyraud Lafiteau, Jacque Lobo, Trimm
[10] [11] [12,13]
C2H2, C2H4 CH4 C2H2, 4 olefins
200e350 C 1,000 C 350e800 C
22,5 37 33
Ea: exponential dependence on temperature of C diffusion in Ni, Kcal/mole.
In a spheroid single crystal nano-particle various low index flat surfaces will appear and high index areas may predominate. The discussion about the catalytic particles being solid or liquid does not apply in surface catalysis: liquids do not have the solid-state crystal structure that is the essence of surface catalysis. We will try to explain how nanotubes can nest on flat solid surfaces of appropriate metal catalysts using Euler’s law and local geometry. High selectivity in nucleation is the key to get various
Table 2 Three alternative kinetic routes in carbon formation [12]. 3 Routes
Process description
Remarks
Dual catalyst route
Carbon formation by surface catalysis decomposition, carbon bulkdiffusion, nucleation and growth e in different surface areas of the catalyst Pyrolytic decomposition, particle impingement, dissolution in catalyst, selective graphite nucleation/growth. Pyrolytic decomposition, forming carbon black particles, usually in layers - sometimes spheres (C60) or tubes.
Initial solid-state changes (2nd Fick’s law) followed by steady-state linear growth (1st Fick’s law).
Hybrid route Pyrolysis route
Carbon formation on foils or catalyst layers is also being used to produce graphene layers. The geometry is a key factor in this type of reactions, both in nucleation and in the growth process. This will be discussed below. The duality required to get a sustained growth may be based on different crystal orientations or on distinct solidstate phases (one in equilibrium with graphite and a different one in equilibrium with the reacting gas). The case of octopus carbon nucleation and growth help illuminate the geometry requirements for CNTs growth [15e19]. The recent paper by Saavedra et al. is of particular interest in view of the detailed observations and statistics presented [18]. Spheroid metal particles are nowadays deposited on substrates to produce CNTs by CVD (chemical vapor deposition): understanding the initial changes in shape, the crystal nano-faces prevailing and the geometry of carbon nucleation and growth is essential to optimize the process. The metals active in the dual catalyst route are Ni, Co and Fe. For the hybrid route there is no need for surface catalysis. Pt, Pd, Cu and many other transition metals are active. It is important to understand the differences in the growth mechanism of carbon tubes (perfectly cylindrical, with one or several graphene walls) and carbon fibers (graphene structure arranged apparently as stacked cones). This will be discussed below, at point 4. 2. CNT growth initiation: 6-pentagon rule and pentagon forming catalysis Iijima et al. [20e22] observed nanotube caps of different shapes and based their analysis on Euler’s theorem concerning polyhedrons and their number of faces (F), vertices (V) and edges (E): F þ V ¼ E þ 2. A consequence is the fact that a hexagonal lattice of any size or shape can only form a closed structure by the inclusion of 12 pentagons. They showed how 6 pentagons could explain the structure of hemispherical caps at the end of nanotubes. The context of Ijima’s work was an apparently non-catalytic arc discharge process, but the CNTs grew at the negative end of the electrode. The carbon nucleation stage must be selective. If nucleation took place over the entire surface, the catalyst would be poisoned and growth would stop. Best catalysts should exhibit selective duality, with certain areas enabling nucleation and other areas (most of the surface) catalyzing carbon formation (or receiving pyrolytic carbon black deposition - hybrid route).
Initial solid-state changes followed by steady-state growth; risk of encapsulation. Various particle shapes: laminar, spherical, tubular.
particular growth processes. The formation of a graphene layer may be seen as a series of hexagon rounds expanding from an initial hexagon, in successive rounds of 6, 12, 18, 24, etc. hexagons (Fig. 1b). The “next round” has always 6 more hexagons. When two pentagons are formed, the next round will have only 4 hexagons more (two less than in a planar expansion). The growth will in fact include from then on only 4 hexagons more in each successive round, instead of 6 more each time. But when a total of six pentagons are formed, the next round (and all subsequent ones) will have the same stable number of hexagons: a hollow graphene tube with a stable number of hexagons is then growing. So the graphene growth orientation is turned 90 in relation to the initial flat growth. A drawing describing CNT nucleation and growth is shown in Fig. 1-a,c. The bending of the initial layer takes place at the edges of that nano surface, as the neighboring carbon atoms coming from the bulk of the metal are at a lower level (Fig. 1-c). The “pentagon formation catalysis” of Ni(111) nano faces can be explained in the following way: Assuming a nanoparticle with 100 nm, the 8 Ni(111) hexagonal facets may be 5e10 nm across and hexagonally shaped. When the growing graphene reaches the border of the facet the next carbon atoms coming from the bulk will be at lower level. Along the zigzag graphene borders the extension will be a range of hexagons slightly bended downwards. But at each corner of the Ni(111) hexagonal facet there is one armchair hexagon. A rehybridization of sp2 s and p p-orbitals, as discussed by Haddon [23] will be induced by geometry/topology. The best fit is then a pentagon not a hexagon with an adequate pyramidalization angle. The 6 corners of the Ni hexagon facet will then “catalyze” the formation of the 6 pentagons needed for the growth to proceed as a nanotube exactly in a perpendicular direction. Discussions of the role of pentagons in other carbon structures, including C60, can be found in the books by Peter Harris [3] and by Kroto and Walton [24]. The work of Ijima mentioned above stimulated the study of these structures. The main differences to the catalytic route discussed here are: 1) Non-catalytic nucleation and growth in the gas phase require much higher temperatures (up to 2000e3000 C; 2) The two catalytic carbon formation processes (dual catalyst route and hybrid route) involve carbon bulk diffusion through the catalyst, graphene nucleation and sustained growth, but the initiation process is substantially different as discussed elsewhere. “Pentagon catalysis” on Ni (111) nanofacets does in fact enable growing nanotubes at temperatures up from 350 C. In the
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Fig. 1. “Pentagon forming catalysis”: carbon nanotubes nucleation and growth a) Graphene nucleation; b) Pentagon rule for 90 graphene surface turn; c) Turn of carbon atoms source (step) at the end of the (111) plateau; d) Fitting of graphene on Ni (111) surface (just 1% adjustment needed: 0.246 nme0.249 nm). (A colour version of this figure can be viewed online.)
work of Iijima the positive (pentagons) and negative (heptagons) declinations are discussed but in relation to narrow nanotubes formed pyrolytically at very high temperatures (2000e3000 C). The catalytic formation on specific surfaces of nanoparticles enables a much better control at much lower temperatures (300 Ce550 C). This nanogeometry is at a scale just above atomic level geometry.
change substantially with the mode of preparation and with environment. This has been the object of various studies and will be briefly discussed below. Octopus carbon growth is a good example that provides evidence of the effectiveness of catalyst duality, as discussed next and shown in Fig. 2. Let us consider a nano-catalyst particle
3. Surface duality at nano level: the case of octopus carbon Of the 3 different routes for carbon formation the dual catalyst route prevails at low temperatures. In this route the catalyst shows a dual role: surface gas reaction in certain areas, a flux of carbon inside the catalyst and carbon growth on other areas [14]. Nano catalysts are very effective for catalytic carbon formation because the bulk diffusion step is short e and so, faster. However, surface duality is essential for the catalyst to be effective: surface catalysis being effective to decompose the reactant gas in certain areas and carbon nucleation and growth being effective in other areas, as remarked above. It is known that carbon nucleation varies substantially in different grains, evidencing selectivity in nucleation in different crystal faces or surface structures [14]. That selectivity is found at all levels, from 102 cm scale (observed on a foil) to 10 nm nanoparticles: the solid-state structure is just the same. However, the shape and prevailing crystal surfaces of the catalyst particles
Fig. 2. Understanding the geometry of octopus carbon growth by assuming the location of the facets in a face centered cube crystal spheroid particle: a) Spheroid Ni particle and location of Ni (111) eight nanofaces; b) Solid state transitions before nucleation and steady-state octopus oriented CNTs growth. (A colour version of this figure can be viewed online.)
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approximately spherical e whose shape depends on its history e but which is in fact a single crystal. A Ni crystal cube would show 6 (100) faces (Fig. 2-a). Rounding up the eight corners would give 8 (111) faces; cutting the edges it would show 12 (110) faces. This helps to visualize the fact that a spheroid Ni particle shows 8 minute hexagonal (111) facets as well as 12 (110) facets, and 6 (100) facets. The fitting of graphene to a (111) Ni face is excellent (ca. 1% difference, check Fig. 1-d). So, when experimental conditions are such that nucleation of graphene is happening only on (111) faces, a tendency to grow nanotubes in about 8 zones with octahedral symmetry is explained. The equilibrium shape of crystals is usually discussed based on the equilibrium crystal shape, as discussed below. We choose here to relate the spheroid shape with reference to an imaginary cube to help understanding the number of its facets and geometry. With this geometry in mind, when nucleation and growth take place on a particular set of facets the observed behavior can be better understood. Is there a preferential growth in 6, 8 or 12 legs? This will be a key to confirm the favored crystal orientation prevailing for nucleation. In Fig. 2-b) the successive solid-state changes that precede carbon growth on a Ni crystallite are shown schematically: carbide formation, carbon nucleation, and carbide recession. Savedra et al. using C2H2 and Ni sputtered on top of a Cu film studied “carbon legs that grew from a faceted catalyst” at ca. 500 C [18]. They concluded that in the range of particle diameters 50e200 nm the number of legs was mostly between 7 and 10. The growth areas are almost restricted to 8 facets with octahedral orientation, most likely coinciding with octahedral Ni (111) planes. Some Ni (110) and Ni (100) nanofacets could also be engaged in nucleation. In other studies the number of legs is much higher and so we may suspect that nucleation is not so selective in those systems and working conditions. The radial growth of thinner and more numerous legs could be controlled by the size of the catalyst particle. The same tendency to octahedral symmetry in the nucleation surface areas and growth of carbon legs is visually obvious in the work of Tavares et al. [15] and Louis et al. [17]. The role of alloying should then be seen from various perspectives: 1) Activity for surface catalysis, producing carbon atoms; 2) Stability of small particle sizes; 3) Reasonable carbon bulk diffusivity; 4) Selectivity in nucleation and appropriate geometry for “pentagon catalysis”, as discussed next. How does a graphite layer growing on a Ni(111) surface turn into a perpendicular graphene tube? We will see that the 90 turn has a consistent explanation. Tavares et al. discussed the possible mechanism of octopus carbon growth in Ni-Cu alloys in relation to the possible changes of the crystal facets with alloy composition [15]. The concepts of catalyst duality and geometry used above are a safe and correct way to explain the phenomenon in general terms. An external carbide (when stable and catalytically active) is not the cause of the driving force for carbon growth but a consequence of the steady-state dynamic equilibrium established. In fact, when an external carbide is the phase in equilibrium with the gas phase, its stability may be sustained by the gradient (driving force) of carbon through the catalyst when this step is rate determining (large C activity gradient). When the diffusion step is not rate-controlling Ni3C is unstable.
interface contact of a particle with a graphite layer. Shape adaptation by solid-solid contact is related with the easiness of the surface atoms to diffuse laterally facilitating the contact. A surface energy gain promotes the change. Let us assume that selective carbon nucleation is a critical aspect of successful carbon growth. If carbon nucleation does not occur, carbon growth will not start. On the other hand, if carbon nucleation is very easy and takes place everywhere, poisoning of the entire catalyst surface will occur and the reaction stops. Selective nucleation is the key. This selectivity is helped by the fact that initiation of the growth process leads to a reduction of the carbon concentration inside the catalyst. A fascinating proof of this behavior is the fact that previous nucleation of carbon on one side of a Ni crystal foil does in fact inhibit nucleation on the opposite side of the foil [25]. Alternative modes of selective nucleation are observed: a) On flat basal contact. When the preferential (selective) nucleation takes place at the flat contact surface between catalyst nanoparticle and substrate (Fig. 3-a), graphene growth will extend laterally to the perimeter of that contact surface and subsequently will bend upward when grasping more carbon atoms from the neighboring interstitial carbon atoms available inside the catalyst solid structure. The particle will be lifted but growth goes on. If new flat layers of graphene nucleate on the bottom surface of the nanoparticle, a CNT with several layers will grow.
4. Alternative nucleation and growth geometries Many reviews have classified and discussed the various growth processes observed. We will discuss those alternatives again from the point of view of the catalyst duality concept. The catalyst substrate and the catalyst carbon good contact require a reaction temperature above the Tammann temperature of the catalyst particle so that its shape may adjust effectively (TTa > 0.50 mp in Kelvin). There is some similarity between a sintering process and an
Fig. 3. Alternative nucleation modes and subsequence growth mechanisms. The several modes of carbon growth depend mostly on the prevailing nucleation area. a,b) Pentagon formation is the key to bend the initial graphene layer to a CNT. Euler’s rule evidences the need to have 6 pentagons formed for a 90 bend, turning the flat graphene into a perpendicular tube; c,d) Observed TEM dynamics of CNFs growth evidence the spiral conic structure of these fibers which grow turning around; e) Growth by successive graphene layers, each pushing up previously formed ones. (A colour version of this figure can be viewed online.)
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b) On external crystalline face. Let us consider the case where easier nucleation takes place on a particular crystal surface not in contact with the substrate (Fig. 3-b). In this case growth evolves in a similar manner, but the particle remains in contact with the substrate. When this type of nucleation is not successful in bending to form a CNT but successive graphene layers nucleate underneath, growth takes place as schematized in Fig. 3-e, superimposing successive graphene layers. c) Inside embedded conical contact. A very interesting case is carbon nanofibers (CNFs) growth. Nucleation starts at a particular point of a conical contact catalyst/substrate (Fig. 3-c) when the nanoparticle is embedded in the substrate. With a selective nucleation and graphene growth inhibiting alternative nucleation, the “just born” graphene sheet grows turning around the conical contact surface in opposite directions. When the two growth parts meet at the opposite side, one goes “up”, remains in contact with the catalyst and will continue to grow. The other part loses contact with the catalyst and stops growing - it remains as the limit of the conic spiral CNF that goes on growing at the other end. When observed under in-situ electron microscopy the successive layers are seen to appear very regularly but alternating at each side, as described by Helveg et al.: “… interfacial step edge formation is observed at either side of the Ni nanocrystal at different instants” [26]. This should be regarded as proof of the spiral cone growth (check Fig. 3-d). Under TEM a sheet is only seen when observed from a top view. It is almost transparent and invisible when observed perpendicularly to its surface [27].
5. Catalytic behavior of nanoparticles, crystal phases, crystal shape, crystal faces Recently the present author published review papers aiming at clarifying the reverse reaction: catalytic carbon gasification. Geometry and graphite/catalyst contact are essential elements of the role of the catalyst particles, described as carbonworms [28,29]. In carbon formation geometry and crystal structure are also an essential element in the role of the catalyst particles in the formation of carbon structures, as described above. We can analyze that in more detail. Table 3 summarizes studies on the activity of different crystal faces observed in catalytic carbon formation. It is well established that decomposition of CO or hydrocarbons on Ni takes place on faces (100) and (210) but not on (111). It is also well established by Geus and co-workers that in the case of Ni turns to carbide initially, facilitating carbon nucleation on a (111) face, but once graphite is formed recession of the carbide is immediate, as discussed below [1]. Ni3Ce is the phase in equilibrium with the gas phase but Ni is the phase in equilibrium with carbon. Duality is the
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basis for the carbon atoms flux. In this case catalyst duality is based in different solid-state phases - not just different crystal faces. In the case of Co there is a change from hexagon to bcc structure above 425 C and turning back to hexagonal below 375 C [43]. Above 425 C the catalytic behavior of the crystal faces is similar to Ni, with almost the same cell size. There is evidence that Co may stay with the fcc crystal structure below 400 C when in small particles [44]. Co carbide should play the same role as in Ni in the catalysis of carbon formation. Fe has a different crystal structure and excretion of carbon takes place at the (100) faces. Kherer and Leidherser recorded many details of carbon formation on different crystal orientations of Ni, Co and Fe [31]. Co (fcc) and Ni showed a similar behavior. With Fe the analysis is more complex the due to presence of various carbides. They used CO as carbon source. The shape and size of the crystal particles depend on various factors, including the solid-state layer crystallinity where they were formed. Early observations showed the effect of previous annealing (accelerating growth) or previous H2 reduction (delaying carbon growth) [12e14]. Tesner et al. studied carbon fibers formation using C2H2 and nichrome wire [45] and Oberlin et al. studied filamentous carbon formation from benzene [46]. Tibbetts and co-workers studied carbon growth from CH4 on Fe and steel [47,48]. The nature and dimensions of the catalyst particles were observed and discussed. Various recent studies on the preparation of supported nanoparticles have identified ways to prepare metal particles with alternative sizes and shapes. A very recent international study on Co defines conditions to prepare Co particles with 3e10 nm diameter supported on commercial silica using NaBH4 as precipitating agent [49]. It would be interesting to identify the crystal orientation of the predominant facets. After the studies by Iijima in the early 90s [21,22] the interest in producing low cost carbon fibers with particular properties increased tremendously. The production of single-wall CNTs had important contributions from Bethune and co-workers [50,51], Smalley and co-workers [52e54] and Terrones and co-workers [55,56]. Terrones et al. discussed the carbon helices frequently observed [57,58]. Several recent reviews include important remarks on geometry [4e6]. The formation of single vs. multiple walls depends on kinetic and thermodynamic conditions favoring or inhibiting successive nucleation after the first one. Iijima et al. and Bethune et al. announced in the same issue of Nature in 1993 the successful production single wall CNTs [59,60]. Pre-treatments are now being used to accelerate and optimize CNTs growth by many authors. Merkulov et al. [61] also found ways to get a sharpening of the CNTs. The control of both the supporting film (usually SiO2) and the catalyst film by varying the preparation mode has recently been explored in various ways to make the CNTs grow faster and with the desired structure. It has been a trial and error approach to optimize carbon growth with a certain structure -
Table 3 Different catalytic activity of different crystal faces (Ni, Co, Fe). Study
1st author (ref.)
Year
Gas
Metal
T C
Remarks (reac./nucleat.)
C formed Nucleation Face changes Nucleation Nucleation Nucleation Adsorption Reac./Growth Role of faces Role of faces Surface catal. Nucl./Growth
Leidherser [30] Kherer [31] Cunningham [32] Lobo [16] Bernardo, Lobo [15] Eizenberg [33] Eizenberg [34] Schouton [35,36] Audier [37e39] Vogt [40] Labohm, Gijze [41]. Lahiri [42]
1948 1954 1957 1971 1975 1979 1979 1979 1980 1987 1982 2011
CO CO/H2 C2H4/H2 Olefins C2H2 CH4/H2 CH4/H2 CH4 CO/CO2 CH4/CO CO/CH4 C2H4,105
Ni Fe, Co, Ni Ni Ni foils Ni crystal Ni Ni Ni Fe, Co Ni, Fe Ni Ni
550 420e600 50e200 400e600 445 700e800 700e800
C on (111), not (100), (110) Faces with carbon pictured Reaction on crystal faces C growth: grain sensitive Growth inhibits nucleation C on Ni(111) faces At Ni (100) þ (210) faces Reac. (100), Growth (111) (111) faces free of carbon Excretion: Ni(111),Fe (100) Reaction: (100)þ(110) Growth on Ni(111)
600 150e550 >150 460
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fast if possible. Understanding the mechanism allows a more efficient approach to optimize growth conditions. As remarked elsewhere, cause-effect misunderstandings seem to occur frequently in explaining this reaction’s alternative mechanisms. To understand a mechanism kinetics is more important than thermodynamics. The detailed study of Bromley and Strickland constable on carbon formation on Ni below 500 C was an important contribution to that understanding [14,62]. The solid-state phases of the nanoparticles prevailing during reaction, their size, shape and crystallographic orientation determine the catalytic activity. So, we may say that when the shape, crystal faces, solid-state phases open a reaction route, the catalyst will be active and selective for a certain type of growth. The best way to determine the kinetic rate-determining step is to draw an Arrhenius plot including an indication of the reaction orders. The external film diffusion step (flat line), or the catalytic surface reaction (variable reaction order) or the carbon bulk diffusion step (zero reaction order) will be rate controlling. The correlation between the Tammann Temperature and the onset of catalytic activity in carbon gasification is well known [29]. The effective solid-solid contact between catalyst and carbon is facilitated, allowing easy carbon solubility [14,29]. Speaking of melted “liquid surfaces” is misleading: a state is tri-dimensional. The Tammann effect is related with ease of surface self-diffusion and so of solid-solid contact adjustment, as in sintering of powder particles. 6. Concluding remarks Comprehensive kinetics is a useful basis to understand the critical aspects to focus on in each particular system. When a steady state carbon formation rate is established linearity of the weight vs. time register is observed. To successfully control CNTs and CNFs growth the following aspects must be known and adjusted when possible: a) Size, shape, solid-state phase(s), crystal structure, and orientation of the nanoparticles; b) Gas chemistry, surface catalysis; b) Carbon nucleation and initial shape formation; c) Steadystate carbon growth kinetics. Acknowledgements I thank Mana for revising and Tiago Monteiro for the drawings. References [1] K. De Jong, J.W. Geus, Carbon nanofibers: catalytic synthesis and applications, Catal. Rev. Sci. Eng. 42 (4) (2000) 481e510. [2] E. Lamouroux, P. Serp, Catalytic routes towards single wall carbon nanotubes, Catal. Rev. Sci. Eng. 49 (3) (2007) 341e405. [3] P.J.F. Harris, Carbon Nanotube Science, Cambridge U. Press, Cambridge, 2009. [4] J. Zhang, M. Terrones, C.R. Park, R. Mukherjee, M. Monthioux, N. Koratkar, Y.S. Kim, R. Hurt, E. Frackowiack, T. Enoki, Y. Chen, Y. Chen, A. Bianco, Carbon science in 2016: status, challenges and perspectives, Carbon 98 (2016) 718e732. [5] J.-P. Tessonier, S.D. Su, Recent progress on the growth mechanism of carbon nanotubes, ChemSusChem 4 (2011) 824e847. [6] V. Jourdain, C. Bichara, Current understanding of the growth of carbon nanotubes in chemical vapor deposition, Carbon 58 (2013) 2e39. [7] G.D. Nessim, M. Seita, D.L. Plata, K.P. O’Brien, A.J. Hart, E.R. Meshot, C.M. Reddy, P.M. Gschwend, C.V. Thompson, Precursor gas chemistry determines the crystallinity of carbon nanotubes synthesized at low temperature, Carbon 49 (2011) 804e810. [8] L.L. Patera, C. Africh, R.S. Weatherup, R. Blume, S. Bhardwaj, C. CastellarinCudia, A. Knop-Ghrike, R. Schloggl, G. Comelli, S. Hofmann, C. Cepek, In situ observations of the atomistic mechanism of Ni catalyzed low temperature graphene growth, ACS Nano 7 (9) (2013) 7901e7912. [9] N. Halonen, J. Maklin, Low temperature growth of MWCNTs by CVD, Phys. Status Solid 248 (2011) 11. [10] M. Escoubes-Baggioni, J. Quinson, C. Eyraud, Reaction du methane, de thane, de l’ethyle ne et de l’acetyle ne sur le nickel divise , Bull. Soc. Chim. Fr. l’e (1967) 2435.
tique des reactions de carburation et de [11] H. Lafitau, L. Jacque, Etude cine haute temperature, Bull. Soc. Chim. Fr. (1968) decarburization du nickel a 4779. [12] L.S. Lobo, D.L. Trimm, Complex temperature dependencies of rate of carbon deposition on nickel, Nat. Phys. Sci. 234 (44) (1971) 15e16. [13] L.S. Lobo, J.L. Figueiredo, C.A. Bernardo, Carbon formation and gasification on metals. Bulk diffusion mechanism: a reassessment, Catal. Today 178 (2011) 110e116. [14] L.S. Lobo, Catalytic carbon formation: clarifying the alternative kinetic routes and defining a kinetic linearity for sustained growth concept, React. Kinet. Mech. Catal. 118 (2016) pp.393e414. [15] T. Tavares, C.A. Bernardo, I. Alstrup, J.R. Rostrup-Nielsen, Reactivity of of carbon deposited on nickel-copper alloy catalysts, J. Catal. 100 (1986) 545e548. [16] C. Pham-Huu, R. Vieira, B. Louis, A. Carvalho, J. Amadou, About the octopuslike growth mechanism of carbon nanofibers over graphite supported nickel catalyst, J. Catal. 240 (2006) 194e202. [17] B. Louis, R. Vieira, A. Carvalho, J. Amadou, M.J. Ledoux, C. Pham-Huu, Carbon nanofibers grown over graphite supported Ni catalyst: relationship between octopus-like growth mechanism and macro-shaping, Top. Catal. 45 (1e4) (2007) 75e80. [18] M.S. Saavedra, G.D. Sims, L.N. McCartney, V. Stolojan, J.V. Anguita, Y.Y. Tan, Catalyzing the production of multiple arm carbon octopi nanostructures, Carbon 50 (2012) 2141e2146. [19] K. Dasgupta, J. Jieshtharaj, P. Bhashkar, D. Sen, S. Banerjee, Growth of carbon octopus-like structures from carbon black in fluidized bed, Mater. Express 3 (1) (2013) 51e66. [20] S. Iijima, Helical microtubules of graphitic carbon, Nature 35 (1991) 56e58. [21] S. Iijima, T. Ichiachi, Y. Ando, Pentagons, hexagons and negative curvature in graphite microtubules growth, Nature 356 (1992) 776e778. [22] S. Iijima, P.M. Ajayan, T. Ichiashi, Growth model for carbon nanotubes, Phys. Rev. Lett. 69 (21) (1992) 3100e3103. [23] R.C. Haddon, The Fullerenes: Powerful Carbon-based Electron Acceptors. Ref. (23), pp. 53e62. [24] H.W. Kroto, D.R.M. Walton (Eds.), The Fullerenes, Cambridge U. Press, 1993. [25] C.A. Bernardo, L.S. Lobo, Carbon 14 (1976) 287e288. [26] S. Helveg, J. Sehested, J.R. Rostrup-Nielsen, Whisker growth in perspective, Catal. Today 178 (2011) 42e46. [27] A.A. Chen, M.A. Vannice, J. Philips, Morphology of graphite supported ironmanganese catalyst particles: formation of hollow spheres during oxidation, J. Catal. 116 (1989) 568e585. [28] L. Lobo, Catalytic carbon gasification: review of observed kinetics and proposed mechanisms or models - highlighting carbon bulk diffusion, Catal. Rev. Sci. Eng. 55 (2013) 210e254. [29] L.S. Lobo, S.A.C. Carbineiro, Kinetics and mechanism of catalytic carbon gasification, Fuel 183 (2016) 457e469. [30] H. Leidhaiser, A.T. Gwathmey, The selective deposition of carbon on the (111) face of a nickel crystal in the catalytic decomposition of carbon monoxide, J. Am. Chem. Soc. 70 (3) (1954) 1206. [31] V.J. Kherer, H. Leidheiser, The catalytic decomposition of carbon monoxide on large metallic single crystals, J. Phys. Chem. 58 (7) (1954) 550e555. [32] R.E. Cunningham, A.T. Gwathmey, The reaction of hydrogen and ethylene on several faces of a single crystal of nickel, Adv. Catal. 9 (1957) 25e36. [33] M. Eisenberg, J.M. Blakely, Carbon monolayer phase condensation on Ni (111), Surf. Sci. 82 (1) (1979) 228e236. [34] M. Eisenberg, J.M. Blakely, Carbon interaction with nickel surfaces e monolayer formation and structure stability, J. Chem. Phys. 71 (8) (1979) 3467e3477. [35] F.C. Schouten, O.L.B. Gijzeman, G.A. Bootsna, Interaction of methane with Ni(111) and Ni(100) e diffusion of carbon into nickel through the (100) surface e AEG-LEED- study, Surf. Sci. 87 (1) (1979) 1e12. [36] F.C. Schouten, O.L.B. Gijzeman, G.A. Bootsna, Reaction of methane with nickel single-crystal surfaces and the stability of surface nickel carbides, Bull. Soc. Chim. Belg. 88 (7e8) (1979) 541e557. [37] M. Audier, M. Renaud, L. Bonnetain, Disproportionation of CO on Ni and Co, J. Chim. Phys. Physichochim. Biol. 74 (5) (1977) 533e538. [38] M. Audier, M. Coulon, Relative crystallographic orientations of carbon and metal in a filamentous catalytic carbon, Carbon 18 (1) (1980) 73e76. [39] M. Audier, M. Coulon, L. Bonnetain, Disproportionation of CO on iron cobalt alloys: I. Thermodynamic study; II. Kinetic study of Fe-Co alloys of different compositions; III. Kinetic Laws of the Carbon Growth and Catalyst Fragmentation, Carbon 21 (1983) 93e111. [40] E.T.C. Vogt, A.J. van Dillen, J.W. Geus, in: B. Delmont, G.F. Froment (Eds.), Prevention of Growth of Filamentary Carbon in Supported Iron and Nickel Catalysts. Catalyst Deactivation, Elsevier, 1987, pp. 221e233. [41] (a) F. Labhom, C.W.R. Engelen, O.L.J. Gijzeman, J.W. Geus, G.A.J. Bootsma, Chem. Soc. Faraday Trans. I 78 (1982) 2435; (b) O.L.J. Gijeman, M.M.J. van Zandvoort, F. Labohm, Carbon monoxide on Ni(111), J. Chem. Soc. Faraday Trans. 2 80 (1984) 771e781. [42] J. Lahiri, T. Miller, L. Adamska, I. Oleynik, M. Batzill, Graphene growth on Ni (111) by transformation of a surface carbide, Nano Lett. 11 (2011) 518e522. [43] J.E. Bidaux, R. Schaller, W. Benoit, Study of the hcp-fcc phase transition on cobalt by internal friction and elastic modulus measurements in the kHz frequency range, Phys. Colloq. C8 48 (12) (1987). Decembre. [44] E.A. Owen, D.M. Jones, Effect of grain size on the crystal structure of Co, Proc.
L.S. Lobo / Carbon 114 (2017) 411e417 Phys. Soc. Sec. B 67 (6) (1954). [45] P.A. Tesner, E.Y. Robinovich, I.S. Rafalkes, E.F. Arefieva, Formation of carbon fibers from acetylene, Carbon 8 (4) (1970) 435. [46] A. Oberlin, M. Endo, T. Koyama, Filamentous growth of carbon through benzene decomposition, J. Cryst. Growth 32 (3) (1976) 335e349. [47] G.G. Tibbetts, Lengths of carbon fibers grown from iron catalyst particles in natural gas, J. Cryst. Growth 73 (1985) 431e438. [48] G.G. Tibbetts, Improved yield of carbon fibers by pyrolysis of natural gas in stainless steel tubes, Carbon (1985) 423e430. [49] G. Di Carlo, M. Lualdi, A.M. Venezia, M. Boutonnet, M. Sanchez-Dominguez, Design of cobalt nanoparticles with tailored structural and morphological properties via O/W and W/O microemulsions and their deposition onto silica, Catalysts 5 (2015) 442e459. [50] C.-H. Kiang, W.A. Goddard, R. Beyers, D.S. Bethune, Carbon nanotubes with single-layer walls, Carbon 33 (1995) 903e914, 995. [51] C.-H. Kiang, W.A. Goddard, R. Beyers, J.R. Salem, D.S. Bethune, Catalytic effects of heavy metals on the growth of carbon nanotubes and nanoparticles, J. Phys. Chem. Solids 57 (1996) 35e39. [52] T. Guo, P. Nickolaev, A. Thess, D.T. Colbert, R.E. Smalley, Catalytic growth of single-walled nanotubes by laser vaporization, Chem. Phys. Lett. 243 (1995) 49e54. [53] H. Dai, G. Andrew, G. Rinzler, P. Nickolaev, A. Thess, D.T. Colbert, R.E. Smalley, Single-wall nanotubes produced by metal-catalyzed disproportionation of carbon monoxide, Chem. Phys. Lett. 260 (1996) 471e475. [54] P. Nicolaev, A. Thess, G. Rinzler, D.T. Colbert, R.E. Smalley, Diameter doubling
417
of single-wall nanotubes, Chem. Phys. Lett. 266 (1997) 422e426. [55] M. Terrones, N. Grobert, J. Olivares, J.P. Zhang, H. Terrones, K. Kordatos, W.K. Hsu, J.P. Hare, P.D. Twnsend, K. Prassides, A.K. Cheetham, H.W. Kroto, D.R.M. Walton, Controlled production of aligned-nanotube bundles, Nature 388 (1997) 52e55. [56] M. Terrones, N. Grobert, J.P. Zhang, H. Terrones, J. Olivares, W.K. Hsu, J.P. Hare, A.K. Cheetham, H.W. Kroto, D.R.M. Walton, Preparation of aligned carbon nanotubes catalyzed by laser-etched cobalt thin films, Chem. Phys. Lett. 285 (1998) 299e305. [57] H. Terrones, A.L. MacKay, Y. Shibuta, J.A. Elliot, From C60 to negatively curved graphite, Prog. Cryst. Growth Charact. 34 (1997) 25e36. [58] M. Terrones, G. Terrones, H. Terrones, Structure, chirality, and formation of giant icosahedral fullerenes and spherical graphitic onions, Struct. Chem. 13 (2002) 373e384. [59] D.S. Bethune, C.H. Kiang, M.S. Devries, Cobalt-catalyzed growth of carbon nanotubes with single-atomic-layerwalls, Nature 363 (1993) 605e607. [60] S. Iijima, T. Ichiashi, Single-shell carbon nanotubes of 1-nm diameter, Nature 363 (1993) 603e605. [61] I.A. Merkulov, M. Yoon, D.B. Geohegan, How the shape of catalyst nanoparticles determines their crystallographic orientation during carbon nanofiber growth, Carbon 60 (2013) 41e45. [62] J. Bromley, R.F. Strickland-Constable, Role of nucleation process in the reactions between carbon monoxide and nickel, Trans. Faraday Soc. 10 (1960) 1492e1500.