Ind. Eng. Chem. Res. 2010, 49, 5323–5338
5323
An Updated Review of Synthesis Parameters and Growth Mechanisms for Carbon Nanotubes in Fluidized Beds Kieran J. MacKenzie,* Oscar M. Dunens, and Andrew T. Harris Laboratory for Sustainable Technology, School of Chemical and Biomolecular Engineering, UniVersity of Sydney, NSW 2006, Australia
Research published since 2006 on the synthesis of carbon nanotubes (CNTs) using chemical vapor deposition (CVD) in a fluidized bed reactor is reviewed. A complete account of experimental procedures, including upstream treatments (catalyst preparation, calcination, and reduction), synthesis conditions, and downstream processes (purification) is presented in an attempt to determine the effect of these variables on carbon nanotube morphology, diameter, yield, and quality. The formation and growth mechanisms of carbon nanotubes by CVD is reviewed in detail in an attempt to account for discrepancies in the properties of CNTs produced from experiments at superficially similar conditions. This reveals that the underlying variables that appear to control growth are not directly manipulated in the CVD process; rather they are determined by complex interactions between variables. Thus, the current “change-one-factor-at-a-time” experimental paradigm, which assumes orthogonal variables, is the most likely source of the conflicting experimental results reported in the literature, and does not give insight into CNT growth or permit “global” process optimization. 1. Introduction Fluidized bed chemical vapor deposition (FBCVD) has been identified as one of the most promising large-scale carbon nanotube (CNT) synthesis techniques. In early 2007, we published a review of developments in this field;1 however, the pace of development in the nanotechnology literature necessitates continued evaluation and discussion of new issues. Hence, the purpose of this review is to update and extend our previous work to reflect the current state of knowledge. Many areas will not be reiterated: an overview of CNTs, their properties and applications, has previously been discussed, and analysis of pre2006 FBCVD experimental results has been undertaken by See and Harris,1 Philippe et al.,2 and Son et al.3 In our previous work, variables affecting CNT growth and design and operational issues specific to fluidized beds were discussed. The conflicting results presented in the literature prior to 2007 were indicative of a poorly understood and controlled process. Therefore, we begin this updated review with an analysis of new fluidized bed CVD experimental data (section 1) to examine progress in understanding the factors influencing CNT growth. In section 2, we identify issues related to fluidized bed design based on apparatus implemented in the literature, and in section 3, we discuss the effect of temperature on CNT growth. The experimental discrepancies, and difficulty in determining trends in the literature data, leads to a detailed analysis on the CNT growth mechanism, as it relates to FBCVD in section 4, in order to explain the difficulties in reconciling results published at superficially similar experimental conditions. A complete list of experimental work published from 2006 to date is reported in Table 1. The inclusion criteria for this review required experiments be conducted in a fluidized bed using a CVD system or, in a number of instances, pyrolysis. All available upstream (catalyst preparation, calcination, reduction), synthesis (gas type, flow rates, temperature, etc.), and downstream (purification) steps have been summarized. Product characterization has been reported based on a number of metrics * To whom correspondence should be addressed. E-mail:
[email protected]. Phone: +61-2-90366244. Fax: +612-93512854.
including yield, nanotube diameter, and quality, although it must be noted that bias does exist in some values due to inconsistent purification conditions among published studies. This detail in variable reporting is far more comprehensive than previous reviews in an attempt to more clearly elucidate the effect of parameters on CNT growth. 2. Fluidized Bed Design Previously identified problems with fluidized bed reactor design and dimensionality, e.g. the use of inappropriately sized reactors, have still not been addressed,1 and incomplete and/or deficient result sets continue to be presented in a large number of FBCVD studies. As shown in Table 1, many studies employ 900 °C) using an Fe-Mo/MgO catalyst with ethanol as the carbon source. CNT quality also shows a positive temperature relationship with the Raman ID/IG ratio falling with increasing temperature to the maximum investigated of 950 °C. Lolli et al.6 monitored the effect of temperature in a CO fed CoMoCAT process (on silica supports) and found that above 850 °C the silica started sintering, resulting in low yields. They reported increasing temperature increased the SWCNT diameter, shifting from a dominant (6,5) at 700-750 °C to (7,6) and (8,7) at 850 °C. While nanotube diameter was found to increase with temperature due to changes in metal catalyst particle size, the chiral angle of the CNTs stayed close to the armchair line. Plotting the reported effects of temperature on yield and quality (degree of graphitization inferred from Raman spectroscopy) from all suitable data reported in Table 1 is shown in Figures 1 and 2. The general trend present in Figure 1 supports the intuitive positive relationship between temperature and yield (due to accelerating carbon source decomposition); however, this is largely attributable to the majority of data coming from Philippe and co-workers.2,26 The remaining data on yield are widely scattered, despite attempts to account for differences in carbon source and catalyst type. Insufficient data is available to determine the optimal synthesis temperature for a given carbon source over a specific catalyst. Experimental data showing the effect of temperature on nanotube quality in Figure 2 again appear to display an intuitive positive relationship: the higher the temperature, the more energy is available to permit carbon atoms to form the desired crystalline structure. However, the available data is difficult to reconcile with this explanation due to the product materially influencing the IG/ID ratio (SWCNTs have a significantly higher ratio) and the variation in results using identical catalysts and carbon sources. For example, at 900 °C IG/ID ratios from 2.84 to 1011 are reported with both using Fe-Mo catalyst and a CH4 carbon source. This difference should not be significant if this simple intuitive explanation is correct, and the variables accounted for are the only contributing factors; obviously, they are not. It is also interesting to note the trade-off between yield and quality. Studies achieving high quality generally do not report the yield (e.g., the works of Liu et al.,12 Ouyang et al.,4 and Yu et al.11) but, given the competing nature of these goals, the yields are likely low. For example, Liu et al.9 (IG/ID of ∼9) report a yield of 0.005 g of CNTs per gram of metal catalyst. In contrast,
Ind. Eng. Chem. Res., Vol. 49, No. 11, 2010
Figure 3. Effect of temperature on CNT diameter. Symbols represent carbon source and colors indicate catalyst type.
reported high yields (>20 g CNT/g catalyst metal) universally result in lower quality (G/D ∼1), suggesting that these two production objectives may not be simultaneously maximized. 3.2. Effect of Temperature on Nanotube Diameter. Despite much research, the effect of temperature on CNT diameter remains unclear. Thiele et al.17 reported a positive diametertemperature relationship between 600 and 800 °C with C2H2 over Fe, as do Ouyang et al.4 between 700 and 850 °C with CH4 on Fe-Mo/MgO. Lolli et al.6 concur, finding higher temperatures increase diameter, but the CNTs remain close to the armchair chiral line, shifting from a dominant (6,5) at 700-750 °C to (7,6) and (8,7) at 850 °C with CO using the CoMoCAT process. Experiments by Niu and Fang7 also indicated a positive diameter-temperature relationship but the effect is only significant with metallic and not semiconducting SWCNTs using CH4 with Mo-Co/MgO catalysts. The positive diameter-temperature relationship is not universally reported. Son and co-workers3,32 reported an inverse relationship, with decreasing mean CNT diameter and standard deviation (13.6-6.3 nm and standard deviation 2.2-1.2 nm) as temperature increased between 800 and 950 °C using CH4. While Liu et al.12 and Philippe et al.2 reported no relationship at all, with consistent SWCNT diameters from 750-950 °C using ethanol on a Fe-Mo/MgO catalyst and MWCNT diameters from 500-750 °C using C2H4 on a Fe/Al2O3 catalyst, respectively. The lack of consensus is borne out by the scattered temperature-diameter data shown in Figure 3, where no discernible trend can be determined even accounting for catalyst and carbon source variations. Note that where diameter ranges are reported, the average value has been used. Figure 3 shows that a large range of CNT diameters can be effectively synthesized over a range of temperatures using a
5327
variety of carbon sources and catalyst types, but there is no obvious relationship with temperature. The enlarged figure, of the region up to 15 nm, shows evidence of two diameter regionssthose below 2 nm and those greater than 8 nm. This likely relates to a SWCNT to MWCNT transition, but it is important to note that even the rarer small diameter CNTs can be synthesized using single (Fe) and binary metal catalysts (Co-Mo, Fe-Mo, Co-W) and a range of carbon sources with both high and low thermal stability (CO, CH4, C2H5OH, and C2H2). This data is strongly suggestive that CNT diameter is affected by but is not a direct function of temperature, catalyst type, and/or carbon source, with other factors playing significant roles. Comparing this discussion to our previous review and the recent review of Danafar et al.,42 it is clear that there has not been significant progress in the understanding of key variables on CNT growth. It should be noted that detailed experimental data required for this analysis may be held by commercial operators (Bayer, Arkema) but is not publically available in the published literature. Thus, further analysis along these lines with publically available data on other variables (e.g., carbon source, catalyst metal and loading, duration, etc.) or a statistically valid meta-analysis is ultimately futile. A cursory examination of the larger pool of data from fixed bed CNT studies results in a similar conclusion. The discussion of See et al.22 offers an explanation for this: variables are not orthogonal in the system and significant variables extend further than temperature, carbon source, and catalyst. It is these additional variables, and the ignored interaction effects, that offer an explanation into the difficultly in determining trends from current experimental data. At this point, it is worth investigating the CNT growth mechanism in more detail to elucidate these variable interactions and how they can help explain the differing results in superficially similar experimental studies. 4. Insights into the Carbon Nanotube Growth Mechanism Although the structure of a CNT is generally visualized as rolling a graphene sheet into a cylinder, the actual growth of CNTs does not proceed along this route due to the formation of an energetically unfavorable seam. Thus while the macroscopic view of CNT growth is relatively simple and largely derived from the models developed to explain carbon fiber growth, there is still considerable uncertainty regarding the details of CNT growth, with no published models adequately explaining all experimental observations. Furthermore, it remains unclear if all CNT growth is mechanistically consistent as suggested by Dai43 or multiple mechanisms exist depending on synthesis technique, specific conditions, and product.44 The mechanisms for CNT growth via CVD generally stem from the vapor-liquid-solid (VLS) mechanism proposed in 1964 by Wagner and Ellis45 and developed for carbon filaments by Baker and co-workers.46-49 In this model the carbon source is decomposed on the surface of a metallic catalyst, with hydrogen being abstracted back into the gas phase and the carbon diffusing over, and potentially throughout, the catalyst particle. Carbon saturation in the metal occurs either by reaching the carbon solubility limit in the metal at a given temperature or by lowering the solubility limit by a relative temperature decrease. Once carbon levels reach super saturation, solid carbon species are precipitated, with morphology dependent on the precise system conditions. Depending on the strength of the metal-substrate interaction, precipitation growth is often de-
5328
Ind. Eng. Chem. Res., Vol. 49, No. 11, 2010
Figure 4. CNT growth by base growth and tip growth, adapted from the work of Sinnott et al.50
scribed as tip growth (weak interaction) or base growth (strong interaction) as shown in Figure 4. Little51 has expanded this generalized carbon filament process into a detailed CNT specific 12-step formation mechanism consisting of (1) carbon atomization; (2) carbon expansion and motion; (3) carbon fixation and rehybridization; (4) carbon confinement; (5) carbon cooling; (6) media distortion; (7) carbon cluster nucleation; (8) carbon cluster growth; (9) CNT nucleation; (10) CNT growth; (11) metal particle formation and reconstruction; and (12) CNT growth termination. In reality, the steps do not necessarily proceed strictly in order; however, the first six steps are faster, involving energetic transformations, transitions, and transport, while the latter six are slower, involving mass transport and transformations. The most salient steps common to both the traditional VLS mechanism and that proposed by Little are discussed below. 4.1. Carbon Supply and Diffusion. During CVD, the actual carbon source is thought to derive primarily from the catalytic cracking of the introduced carbon source on the catalyst, which subsequently diffuses to the site of CNT growth. While the most ubiquitous rationalization, there is experimental evidence to suggest that this is not the only pathway, with hydrocarbon decomposition and transport occurring via the support material, the CNT itself, or attaching directly from the gas phase. Louchev et al.52 studied CNT forest growth using CVD and found that during forest growth, the carbon source is unable to reach the catalyst; instead they chemisorb and diffuse over the CNTs before migrating to the growth site. Schneider et al.53 reported similar transport phenomena, synthesizing MWCNTs in the pores of porous aluminum oxide without the presence of a catalyst. Given that the MWCNTs almost completely filled the alumina pores, it is thought that the diffusion required for growth was due to migration along the CNT or through the alumina template material. Bell et al.54 report that during plasma enhanced CVD (PECVD), the catalyst function is simply that of stabilization and that the carbon is added directly from the gas phase by C2H2 addition. These results are suggestive of multiple growth and transportation mechanisms; however, the subsequent discussion will be focused on the processes more likely to occur in the FBCVD system. It has been proposed that the rate limiting step in CNT formation is the growth of CNTs themselves and not the nucleation step.55 In the case of Fe catalysts, activation energy for SWCNT formation of 37-40 kJ/mol has been reported,56,57 significantly lower than the activation energy of carbon diffusion in bulk Fe (70-80 kJ/mol),55 signifying that dimensionality and temperature factors affecting diffusion are nontrivial. The mode
Figure 5. Illustration of the temperature gradient that can arise across a catalytic metal particle during CNT growth. The carbon flow (arrowed black dots) in the catalyst particle is driven by the temperature gradient (arrowed lines). (Reprinted with permission from ref 60. Copyright 2004 Elsevier.)
(surface or bulk), actual carbon species involved, and the driving force for carbon diffusion remain unclear. Surface versus bulk diffusion is discussed in section 4.1.6. Atomic carbon is the most commonly assumed diffusing species, but due to the polymerization of carbon at the site of the CNT it is difficult to determine. Other researchers have proposed that the actual precursors to growth are C2 or C3 molecules.58 However it is unclear if these are formed at the site of the growing CNT or are the actual diffusing species derived from either incomplete decomposition of the carbon source (e.g., hydrogen abstraction only) or the rapid polymerization of atomic carbon in the metal. Due to the relative size differences in diffusing species, this may have a significant impact on the rate of diffusion and consequently CNT growth. Reilly and Whitten59 presented a compelling argument suggesting that free radical chemistry is the key factor in understanding CNT growth by CVD and any mechanisms relying on simplistic atomic carbon species are naive. Baker and co-workers originally proposed a temperature gradient driving force for carbon fiber growth due to the exothermic decomposition of hydrocarbons at the exposed catalyst surface and endothermic precipitation at the rear catalyst particle faces (Figure 5). While this satisfactorily explains large fiber growth with exothermic hydrocarbon decomposition, Baker and co-workers noted at the time that this mechanism could not account for the formation of carbonaceous products from endothermic decompositions such as carbon monoxide, which generates heat flow in the opposite direction. Furthermore the presence of a temperature gradient on very small metallic particles used in CNT growth (6000 atoms have been assembled into a coherent tubular structure, the complete reorientation back to the thermodynamically stable graphite requires more energy than is available in CVD systems, thus accounting for the persistence of synthesized CNTs. However this curving of the graphene layers introduces an extra elastic term into the free energy equation of nucleation and growth, leading to a lower limit (∼10 nm) to the diameter of carbon fibres that can form from curved graphite layers.81 This explains the hollow core of carbon filaments; however, most CNTs possess dimensions far below 10 nm. Charlier and Iijima44 invoked quantum considerations to explain the smaller
5332
Ind. Eng. Chem. Res., Vol. 49, No. 11, 2010
Figure 13. Possible carbon precipitation morphologies for 60 atom carbon nuclei and their corresponding energies showing the energetic preference of the nucleation of capped structures: (a) an isolated flat graphite flake; (a) a flat graphite flake perpendicular to a Ni surface; (c) an open-ended 5,5 tube on metal; (d) a hemispherical cap on metal; (e) a capped 5,5 tube on metal; (f) an isolated C60 fullerene. Adapted from the work of Fan et al.83 Table 3. Total Energy of Different Structures from Figure 13 flake flake open tube large cap capped tube configurations (a) (b) (c) (d) (e) ngraphite ncylinder nsphere ndanglingbond Ncarbon-metal Etotal (eV)a a
80
80 80
20 0 45.9
C60 (f)
14 6 32.1
10 10 31.3
83
45 40
90
14 16.8
10 17.3
24.0
Etotal ) ngraphEgraph + ndangEdang + nc-mEc-m + ncylEcyl + nsphEsph.
CNT formation. They report that it is no longer the “fluid nature” of the metal cluster, but the chemical interaction between the 3d electrons and the carbon π electrons that stabilize and permit the small diameter CNTs. 4.3. Carbon Nanotube Nucleation. The steady state growth of tubular carbon products is explained by energetic considerations and carbon diffusion; however, these factors do not adequately explain CNT nucleation. As carbon is absorbed on the metal catalyst, it is thought to attract, rehybridize, and be incorporated into growing carbon clusters, preferentially at specific facets.51 The CNT nucleation is the forced distortion of graphene nanosheets due to the instability of unsaturated dangling carbon bonds on the nanosheet as a result of limited or no passivating metal atoms.51 Nucleation typically occurs on an “active” spot on the catalyst56 caused by nonhomogeneous nanoscale surface features. The nucleation over a surface feature potentially explains why CNT growth has been reported in the absence of a metal catalyst:53 surface topography and roughness is sufficient to induce the required graphene curvature to effectively catalyze the cap formation. Dai et al.82 were the first to propose this cap (or “yarmulke”) mechanism. Because the nanoscale metal particle contains a high percentage of surface atoms, it possesses an unusually high surface energy per atom. Since carbon has low surface energy, the formation of a carbon cap with its edges chemisorbed to the metal will reduce the overall surface energy. Energy calculations using density functional theory (DFT) by Fan et al.83 with Ni show that the cap forms as it is more energetically favorable to incorporate some pentagons as they have fewer dangling bonds per atom and they can deform into a dome to better saturate the dangling bonds at their edges. The energetic cost of curvature resulting from the pentagon induced doming is orders of magnitude lower than retaining the dangling bonds (0.067 vs 2 eV); hence, its formation is favored. Figure 13 depicts possible carbon precipitation and their total energies
illustrating the preference for capped structures (when the metal surface is Ni). Table 3 demonstrates that “open” CNTs are energetically unfavorable, yet their experimental observation suggests that additional stabilizing mechanisms occur, such as the presence of mobile “passivating” bonds.44 Note the lower energy configuration of the tubular structures compared to the flat graphene layers, supporting the curvature explanation of Tibbetts,80 and the energy of fullerenes compared to graphite in the absence of catalyst metal, explaining their stability in the absence of supports and prevalence in the arc discharge and laser ablation techniques. 4.3.1. Nucleation Stability. The stability of the CNT nuclei is crucial to permit successful CNT growth. The simulations of Fan et al.83 on Ni indicate energetic stability for the cap; however, stability varies with the catalyst metal depending on the metal-carbon adhesion strength. To understand the critical role of the adhesion strengths and why there might be a limiting value for SWCNT growth, Ding et al.84 considered the energies associated with eq 3. metal-SWCNT f metal + open-ended SWCNT f metal + capped SWCNT (3) The free energy of this reaction must be positive for growth to be stable under equilibrium conditions; otherwise, the chemisorbed open end of the nanotube would spontaneously form a cap.84 That is, the free energy of an individual capped CNT and isolated metal nanoparticle must exceed an open-ended stabilized CNT bound to the metal; otherwise, the system will spontaneously decompose into the two products and prevent further CNT growth as illustrated in Figure 14 for Fe and Au. Using this hypothesis, Ding et al.84 examined the carbon-metal adhesion strengths for a number of metals and tube (n,m) indicies and found that Fe, Co, and Ni have sufficiently large adhesion strength to stabilize the open CNT end during growth (∼1.5-3 eV) while the binding energy of Cu, Pd, and Au ( 0). Weakly interacting metals like Au do not have sufficient binding strength (∆E < 0) to prevent cap formation and separation of the SWCNT from the catalyst particle. (Reprinted from ref 84. Copyright 2008 American Chemical Society.)
Figure 15. MD simulations of SWCNT growth with different carbon-metal interaction strengths. The open end of a cap on a Fe50 catalyst particle (a) closes after the addition of carbon atoms when the carbon-metal interaction was weak (a f b) and remains open to support SWCNT elongation (a f c) when the carbon-metal interaction is strong. (Reprinted from ref 84. Copyright 2008 American Chemical Society.)
4.3.2. Nucleation Size. According to the calculations of Fan et al.,83 it is favorable for the cap to increase its diameter up to 10 nm before increasing tube length. Given the experimental evidence of far smaller diameters, it is proposed that the diameter is determined in practice by particle size and kinetic factors. Once the cap has matched the particle diameter or if the carbon supply rate exceeds the rate at which bonds in the cap can rearrange, the radius of the cap will no longer increase and carbon atoms will be added to the base of the cap, extruding the CNT. This mechanism may explain conflicting reports on whether the diameter of a CNT is directly proportional to the catalyst particle diameter: if the carbon supply kinetics are sufficiently rapid, the CNT will be smaller than the catalyst particle; if the supply kinetics are slower, then the cap will continue to expand until it has reached the most energetically favorable dimensions or the edge of the nanoparticle, resulting in a direct CNT-particle size correlation. 4.4. Chirality. Carbon atoms can readily rearrange while still in contact with the catalyst surface, with a migration energy of
