High-Yield Growth of Carbon Nanotubes on Composite Fe/Si/O Nanoparticle Catalysts: ACar-Parrinello Molecular Dynamics and Experimental Study

Chad J. Unrau, Richard L. Axelbaum, and Cynthia S. Lo*Department of Energy, EnVironmental, and Chemical Engineering, Washington UniVersity in St. Louis, St.Louis, Missouri 63130

ReceiVed: September 25, 2009; ReVised Manuscript ReceiVed: May 4, 2010

Single-walled carbon nanotubes (SWCNTs) have recently been synthesized at high catalyst yield (∼90%)using a composite iron/silicon-oxide nanoparticle catalyst in a gas-phase diffusion flame environment. Sincecatalyst yields without silicon are less than 10%, the role of silicon in improving catalyst yield must bestudied to understand the molecular-scale factors that govern carbon nanotube nucleation and growth. In thiswork, Car-Parrinello molecular dynamics simulations are employed to investigate the structure of Fe/Si andFe/Si/O nanoparticle catalysts at synthesis temperatures (1300 K). The simulations show that silicon is uniformlydispersed on the iron surface when oxygen is not present, but covers only one hemisphere of the particlesurface when oxygen is present to form a silica “cap”. These results are consistent with the results of substratesynthesis and the phase diagram of this Fe/Si/O system. The structure of the catalyst particle when oxygenand silicon are present thus facilitates the preferential decomposition of a carbon precursor on the Fe-richside of the particle. On the basis of this finding, SWCNTs will nucleate preferentially on Fe/Si/O with segregatedphases compared to catalyst particles with a uniform surface composition that typically become encapsulatedin carbon before nucleation can occur. High catalyst yields are also demonstrated on Fe/Al/O catalysts, whichindicate that high yields are not specific to the presence of silicon in the particle. The results of this studysupport the hypothesis that the addition of silicon or aluminum, in the presence of oxygen, to iron oxide-based catalysts results in a nonuniform surface composition that facilitates SWCNT nucleation.

1. Introduction

The unique properties and applications1 of single-walledcarbon nanotubes (SWCNTs) have generated significant aca-demic and industrial interest in designing transition metal-basedcatalysts2 (e.g., iron, nickel, cobalt), and more recently transitionmetal-oxide catalysts,3 for nanotube nucleation and growth.4

These catalysts decompose a gas-phase carbon source, such asacetylene or carbon monoxide,5 which provides carbon atomsthat may assemble in ringlike configurations on the catalystsurface to nucleate a carbon nanotube.6

Numerous methods have been developed for SWCNT syn-thesis that can generally be classified according to the catalystenvironment: affixed to a substrate or freely floating in the gasphase. Substrate methods (e.g., those achieved via chemicalvapor deposition (CVD))7 have received the most attention, sincethey provide good control over nanotube length, diameter, andpurity. On the other hand, gas-phase methods have the advantageof being volumetric, continuous processes, which are desirablefor applications such as composite materials that require largequantities of nanotubes and industrial-scale synthesis methodsto make them economically feasible.8

Although gas-phase synthesis methods have several distinctadvantages, they suffer from several disadvantages, such as (1)low catalyst yields, where only a small percentage of catalystsform nanotubes, (2) short catalyst lifetimes, and (3) low catalystnumber density, which is necessary to keep the particle sizesmall. Recently, a high catalyst yield (>90%) was achieved inthe gas phase by using composite iron/silicon/oxygen catalysts

synthesized in an enriched-oxygen inverse diffusion flame.9

Without silicon in the system, however, the catalyst yield wasless than 10%. Thus, further investigation is required tounderstand the growth mechanism of SWCNTs on Fe/Si/Ocatalysts.

Single-walled carbon nanotube formation is thought to beginwith the formation of an initial hemispherical carbon cap onthe surface of the catalyst particle.10-12 This has been observedboth experimentally and computationally by using moleculardynamics (MD) simulations. MD simulations have proven tobe particularly valuable for modeling the growth mechanismof SWCNTs, since the small size of catalysts (on the order of1 nm) makes experimental observation of SWCNT nucleationdifficult. Classical,13,14 density functional theory-based tightbinding,15 and ab initio16,17 molecular dynamics have beenutilized to study SWCNT formation. The latter approachemploys a quantum mechanical treatment of the nuclear andelectronic motion that provides a balance between accuracy insimulation of bond breakage and formation and computationaltime. Thus, ab initio methods are preferred for modeling theinitial steps of SWCNT formation, while classical methods arepreferred for the simulation of continued nanotube growth overlonger simulation times.

In this work, we employ Car-Parrinello molecular dynamics(CPMD)18 to investigate the structure of Fe/Si/O catalysts andthe function of silicon in improving catalyst yield. In the caseof an Fe/Si catalyst without oxygen, the most stable configu-ration consists of silicon atoms distributed uniformly on thesurface of a spherical iron cluster. On the other hand, whenoxygen is present, silicon remains on one hemisphere of thecluster surface, with the majority coordinated with oxygen to

* To whom correspondence should be addressed. Phone: (314) 935-8055.Fax: (314) 935-7211. E-mail: clo@wustl.edu.

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10.1021/jp909255r 2010 American Chemical SocietyPublished on Web 05/18/2010

form a silica-like phase. We propose that this behavior resultsin a faster deposition rate of carbon on the iron-rich side of thecluster than on the silicon-rich side. Preferential carbon deposi-tion may increase catalyst yield by allowing sufficient time forSWCNT cap formation before the particle becomes encapsulatedin carbon. This hypothesis is supported by experiments that showhigh catalyst yield may also be achieved by adding elementssuch as aluminum to the iron oxide catalyst.

2. Computational Model

The CPMD code v3.13.119 was utilized to calculate theelectronic structure, properties, and reactivity of the catalystsystem, using density functional theory (DFT) calculations andCar-Parrinello molecular dynamics (CPMD) simulations. Theelectron exchange and correlation energies were calculated byusing the generalized gradient approximation of Perdew, Burke,and Ernzerhof,20 and core electrons were treated with use ofVanderbilt ultrasoft pseudopotentials.21 The local spin densityapproximation was employed to include spin polarization in thecalculations, and an initial multiplicity was estimated based onthe number of iron atoms in the cluster. Geometry optimizationswere performed with an orbital convergence of 10-5 Ha, anenergy convergence of 5 × 10-4 Ha, and a plane-wave cutoffof 25 Ryd.

CPMD simulations were performed at 1300 K, which issimilar to the temperature at which SWCNTs form in thediffusion flames described in the next section. This temperaturewas maintained by velocity rescaling of the ions, using theBerendsen thermostat.22 This thermostat was also used on theelectrons to minimize the transfer of energy between ions andelectrons. All calculations were performed with use of periodicboundary conditions by placing the catalyst system in a 16 Åcubic box. This box was sufficiently large that the cluster wascalculated as if it were an isolated system, rather than part of aperiodic structure.

All clusters were constructed based on a 0.8 nm iron spherewith atoms in FCC positions. This crystal structure is charac-teristic of bulk Fe above 1189 K and has been observedpreviously in iron nanoparticles.23 The cluster size was chosenso that it would be large enough to represent catalyst particlesthat form nanotubes under typical experimental conditions whileoptimizing for computational accuracy and cost. The numberof atoms in the cluster was kept constant for all simulations,while varying the ratio of iron, silicon, and oxygen atoms. TheFe:Si and Fe:O atom ratios were set to be 2:1 and 1:1,respectively, which were obtained from experimental charac-terization of the atomic compositions under optimal conditionsfor SWCNT formation in diffusion flames.

3. Experimental Methods

A detailed description of the experimental setup has beenpreviously published.9 Briefly, a triaxial inverse diffusion flamewas established on a burner with a 12.8-mm-diameter centraljet and a 6.35-cm-diameter coannular tube. A second coannulartube was used to introduce nitrogen, which acted as a sheathflow. Pure oxygen was introduced through the jet at 3.7 mg/swhile ethylene and nitrogen were introduced through the firstcoannular tube at 15.6 and 145.3 mg/s, respectively. Ferroceneand aluminum acetylacetonate were introduced with the fuelstream as precursors for iron and aluminum. The flow rate offerrocene and aluminum acetylacetonate was 0.5 mg/min and0.4 mg/s, respectively, to yield a catalyst Fe:Al molar ratio ofapproximately 2:1.

The catalyst yield was estimated from the particle sizedistribution, which was measured with a fast-quench dilutionprobe (0.5 mm inlet) coupled to a scanning mobility particlesizer (SMPS). The SMPS consisted of a Kr85 bipolar ion source,a TSI 3081 differential mobility analyzer (DMA), a TSI 3776condensation particle counter (CPC), and a PC to run thesoftware. The samples for analysis by scanning electronmicroscopy (SEM) were collected on a filter downstream ofthe flame, and subsequently mounted on copper mesh grids witha lacey carbon coating.

4. Results and Discussion

4.1. Influence of Silicon on the Structure of Iron Catalysts.Several studies have shown that the catalyst yield for gas-phasesynthesis of single-walled carbon nanotubes on iron or iron oxidecatalysts is extremely low (∼10%), even under optimal condi-tions such as uniform catalyst size and composition.24-26 Instead,the majority of the catalyst particles become encapsulated in adisordered carbon matrix. On the other hand, a high catalystyield has been observed in gas-phase diffusion flame synthesiswhen silicon is added to iron oxide catalysts.9 Similarly, wheniron or iron oxide catalysts are placed on a silicon or silicasubstrate, the yield for carbon nanotube formation is muchgreater (∼90%).27,28 Thus, the presence of silicon likely altersthe mechanism controlling catalyst particle encapsulation versusnanotube formation in gas-phase synthesis and possibly evenin substrate synthesis.

The role of silicon in the nanotube nucleation process maybe elucidated by first considering the likely mechanisms for theassembly of carbon atoms on a pure iron or iron oxide particleversus a particle containing some fraction of silicon atoms onthe surface. A pure iron or iron oxide catalyst particle on theorder of 1 nm in size that is suspended in the gas phase willhave a uniform surface composition29 and is of an appropriatesize to produce a SWCNT. As described in the Introduction,the nanotube formation process begins with the decompositionof carbon-containing molecules on the catalyst particle surface.Since the pure iron or iron oxide catalyst has a uniform surfacecomposition, decomposition will occur evenly over the surfaceof the particle, which results in uniform coverage of carbonatoms. These carbon atoms may either bond together toencapsulate the catalyst or diffuse to one side of the catalyst toform a hemispherical cap. The former scenario is more likelysince the characteristic time for the encapsulation process isless than the time needed for diffusion of carbon atoms.

It thus appears likely that the addition of silicon to an ironor iron oxide catalyst may result in a different catalyst surfacecomposition, leading to nonuniform behavior of carbon atomson the catalyst surface. To investigate this possibility, Fe/Siclusters without oxygen were constructed and modeled with useof DFT calculations and CPMD simulations. Although oxygenis known to be present in the catalysts used in oxygen-enricheddiffusion flames, it was not considered in this part of the studyin order to isolate the effect of silicon on the catalyst surfacestructure and potential for SWCNT nucleation.

In Fe/Si catalysts, several configurations for the silicon phasewere considered: (1) Si concentrated in the core of the particle,(2) Si distributed uniformly on the surface, and (3) Si concen-trated to one hemisphere of the catalyst surface. The totalenergies of the different configurations, as computed with DFT,were compared to gain predictive insight on the preferredlocation of silicon in these catalysts. For these calculations, threecatalysts were constructed from a 0.8 nm (43 atom) cluster ofiron with atoms in fcc positions. In each case, 13 iron atoms

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were replaced with silicon to yield an Fe:Si ratio of ap-proximately 2:1, which was experimentally determined to bethe optimum ratio for catalyst yield in oxygen-enriched diffusionflames.27 Three clusters are shown in Figure 1, each with adifferent silicon configuration. Iron and silicon are representedby the blue and yellow spheres, respectively.

Geometry optimizations were performed on each of theseclusters at 0 K, using DFT to determine which configuration isthe most energetically favorable. The cluster with siliconconcentrated in the core (Figure 1a) was much less stable, being6 eV higher in total energy than the clusters where silicon waslocated on the surface (Figure 1b,c). These calculations thusindicate that the lowest energy configuration for Si is similarto that shown in Figure 1b, where Si is distributed uniformlyover the catalyst surface.

Although the cluster in Figure 1b represents the preferredconfiguration for Fe/Si clusters at 0 K, the most energeticallyfavorable configuration may be different at the growth temper-atures (∼1300 K) present in diffusion flames. To investigatethis possibility, a CPMD simulation was conducted at 1300 Kfor up to 5 ps on the cluster with an initial Si cap, as shown inFigure 1c. This cluster was chosen since the silicon atoms inFigure 1c should distribute uniformly over the surface givensufficiently long simulation times if the preferred configurationof silicon is actually similar to that shown in Figure 1b. Thestructure of the cluster after 5 ps is shown in Figure 2.

Figure 2 shows that over the course of the simulation, siliconredistributes on the surface of the cluster from a cap configu-ration (Figure 1c) to a more disperse configuration similar tothat in Figure 1b. Thus, the most favorable structure of these

Fe/Si clusters appears to be one where silicon is distributeduniformly over the surface of the cluster. This result hasinteresting implications for both substrate and gas-phase syn-thesis. Several studies have shown that when iron catalysts areplaced on silicon substrates, the catalyst yield is high, butcatalyst lifetimes are relatively short; this leads to a maximumnanotube length of roughly 1-10 µm.23,30 Short catalyst lifetimeshave been attributed to the formation of an iron silicide, whichis thought to be inactive for nanotube growth. The results shownin Figure 2 support this hypothesis, since the preferred structurefor Fe/Si catalysts at high temperature appears to be a uniformdistribution of Si atoms over the Fe surface. This is in contrastto the configuration showing segregation of iron and silicon(Figure 1c), which corresponds to minimal interaction in asupported catalyst configuration between the iron particles andthe silicon substrates.

With respect to gas-phase synthesis, Fe/Si catalysts wouldlikely form in the configuration shown in Figure 2. The structureof this Fe/Si catalyst is similar in structure to iron and iron oxidecatalysts in that all possess a uniform surface composition.29

Thus, any dissociation of adsorbed carbon-containing molecules(to provide carbon atoms for nanotube formation) would beexpected to occur uniformly over the catalyst surface. Thisuniform distribution of carbon may not allow for preferentialdiffusion of carbon to form an SWCNT cap on one side of thecatalyst: The result would be that the carbon would encapsulatethe gas-phase catalyst particle. This conclusion is supported bothby experiment24,25,31 and ab initio molecular dynamics simula-tions.16

4.2. Influence of Silicon on the Structure of Iron OxideCatalysts. In contrast, experiments have shown that whenoxygen is present during substrate synthesis, catalyst lifetimesare dramatically improved, and nanotubes of over 100 µm inlength can be obtained.23,32 The investigation of the catalyststructure in these studies revealed that the iron and siliconremained segregated over the course of nanotube growth. Inthe present study, the addition of oxygen to the Fe/Si clustershown in Figure 1c would likely result in silicon remaining onone side of the catalyst particle instead of dispersing over thesurface.

To investigate this possibility, a surface oxide layer was addedon the silicon side of the cluster in Figure 1c. Oxygen atomswere added to the system to give an Si:O ratio of 1:2, which isconsistent with experiment (Fe:O ratio of 1:1).33 The oxygenwas added to the silicon side as opposed to the iron side sincethe Ellingham diagram indicates that iron will be reduced inthe presence of silicon. The resulting Fe/Si/O cluster is shownin Figure 3a. Oxygen atoms are represented by the smaller redspheres.

A 5 ps CPMD simulation was performed on the cluster shownin Figure 3a at 1300 K, and the resulting cluster is shown inFigure 3b. Figure 3c shows the total energy of the cluster as afunction of time, which indicates that the system has equilibratedafter ∼1 ps. Silicon tends to remain segregated from the ironwhen oxygen is present (Figure 3b), which is analogous to theexperimental observations in the Fe/Si/O catalyst/substratesystem. Entropically, one might expect the silicon and oxygento diffuse around the particle surface but this does not occurdue to the large negative enthalpy of formation of SiO2. Sinceiron is significantly more active toward acetylene dissociation,34

carbon precursor dissociation (acetylene in the case of ourdiffusion flames) will occur more rapidly on the iron-rich sideof the catalyst than on the silicon-rich side. Consequently, anSWCNT cap may form and lift off of the iron-rich surface of

Figure 1. Illustrations of 0.8 nm clusters with (a) silicon (yellow)concentrated in the core, (b) silicon concentrated on the catalyst surfaceuniformly, and (c) on one-half of the surface. The lowest energyconfiguration is shown in panel b, with the configuration in panel cbeing 1 eV higher in total energy, and the configuration in panel abeing 6 eV higher in total energy.

Figure 2. An illustration of the structure of the cluster shown in Figure1c after a 5 ps CPMD simulation at 1300 K.

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the catalyst with subsequent carbon atoms adding to the capedge to form the nanotube wall. This concept is supported bythe results of Raty et al.,16 who performed ab initio moleculardynamics simulations on 1 nm iron particles. Their resultsshowed that a pure iron particle becomes encapsulated in carbon.On the other hand, when hydrogen is affixed to one side of theparticle to represent a substrate, carbon atoms assembled onthe other side of the particle to form a carbon cap that is theprecursor for subsequent SWCNT growth.

4.3. Feasibility of Carbon Nanotube Nucleation on Com-posite Fe/Si/O Catalysts. To confirm that the Fe/Si/O clustershown in Figure 3b could also result in SWCNT formation,carbon atoms were added to this cluster in two differentconfigurations as shown in Figure 4. Carbon atoms arerepresented by the small gray spheres.

Figure 4a shows six carbon atoms separated from each otheron the iron surface of the cluster while Figure 4b shows thesecarbon atoms arranged in a hexagonal ring. Geometry optimiza-tions were performed on both of these clusters at 0 K todetermine which configuration was energetically favorable. Thecalculations indicated that the ring configuration was favoredby 2 eV compared to the isolated carbon configuration. Theseresults indicate that as carbon atoms are supplied to the surfaceof the catalyst, they will preferentially bond with each otherrather than diffusing on the surface of the catalyst particle, which

would eventually cause encapsulation. Moreover, since carbonwill be supplied more rapidly to the iron-rich side versus thesilicon-rich side of the particle, a SWCNT may nucleatepreferentially on the iron-rich side as discussed above.

4.4. Experimental Verification of the Role of Silicon andAluminum in Carbon Nanotube Nucleation. The role ofsilicon in gas-phase synthesis of SWCNTs thus appears to besimilar to that of a substrate in chemical vapor deposition. InCVD, one role of the substrate is to prevent carbon fromdiffusing on the catalyst particle and encapsulating it before anSWCNT can nucleate. The presence of silicon in gas-phasesynthesis of SWCNTs on Fe/Si/O catalysts gives the catalyst asimilar structure to that of the substrate system (Figure 3b).Consequently, carbon source dissociation will occur morerapidly on the iron side of the catalyst, allowing an SWCNT tonucleate before the catalyst becomes encapsulated.

On the basis of the results presented above, silicon essentiallyplays a steric or geometric role in improving catalyst yield forgas-phase nanotube synthesis. If this is the case, however, thena high catalyst yield should also be possible if elements otherthan silicon are added to the catalyst as long as the catalyststructure is similar to that depicted in Figure 3b. Iron catalystson an alumina substrate have been shown to behave in a similarmanner to that of the Fe/Si/O system, in that the iron andaluminum remain segregated in the particle.35 To determine if

Figure 3. An illustration of (a) an Fe/Si cluster with an oxide layer over the silicon, (b) the structure of the catalyst in panel a after a 5 pssimulation at 1300 K, and (c) a plot of the cluster energy versus time showing that the cluster has equilibrated after just 1 ps. Color key: oxygen,red; iron, blue; silicon, yellow.

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an Fe/Al/O catalyst could also result in the high catalyst yieldachieved with the Fe/Si/O catalysts, aluminum acetylacetonateand ferrocene were added to the diffusion flame described inthe Experimental Methods section. These precursors providealuminum and iron atoms for particle formation through theirdecomposition near the flame surface. The flow rates of theseprecursors were such that the Fe:Al ratio of the catalysts wasapproximately 2:1, which matches the optimum compositionof the Fe/Si/O catalysts. Figure 5a shows the size distributionof particles emerging from the flame when Fe/Al/O catalystsare present. A scanning electron micrograph of SWCNTsproduced from these catalysts is shown in Figure 5b.

In a previous study, we determined that if bare catalysts andSWCNTs are present in the flame, the size distribution will bebimodal, so the number of particles associated with each modecan be used to estimate the catalyst yield.36 The size distribution

in Figure 5a, with the mode toward the right side of thedistribution corresponding to SWCNTs, indicates that thecatalyst yield is high (∼90%). Figure 5b shows an SEM ofSWCNTs produced from Fe/Al/O catalysts. The measurednanotube diameters were 1-2 nm and the lengths were up toseveral micrometers. The results shown in Figure 5 are quitesimilar to those obtained with Fe/Si/O catalysts in terms ofcatalyst yield and SWCNT size.9 On the basis of these resultsand the structure of iron catalysts during SWCNT synthesis onsilica or alumina substrates, we expect that Fe/Si/O and Fe/Al/O catalysts have a similar structure with silicon or aluminumseparated from iron. The ternary phase diagrams for theFe-Si-O and Fe-Al-O systems support this catalyst structure,as both phase diagrams show a region of two immiscible liquids(iron and silica or alumina melts) at high temperature for themolar concentrations of elements employed in this study.37

Figure 4. An illustration of six carbon atoms (gray spheres) added to the cluster of Figure 3b in (a) a separated arrangement and (b) a hexagonalring arrangement.

Figure 5. The particle size distribution of the flame when Fe/Al/O catalysts are present is shown in panel a, while panel b shows many SWCNTsproduced from Fe/Al/O catalysts. The large size of the right-most mode in the size distribution relative to the other mode indicates that the catalystyield is high.

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Conditions higher than the melting point of iron are consideredwhen evaluating the implications of the phase diagrams sincecatalysts of approximately 1 nm in size are expected to be liquidor at least exhibit a liquid-like behavior for the temperaturespresent in our diffusion flames. The phase diagrams also showthat when oxygen is not present, a single melt of either Fe/Sior Fe/Al exists.37 This is consistent with the results presentedin Figure 2 and the results of substrate synthesis in the absenceof oxygen.23

The combined results of (1) CPMD simulations on Fe/Si/Oclusters, (2) experimental characterization of both Fe/Si/O andFe/Al/O clusters, and (3) the phase diagrams for both systemsare consistent with the hypothesis that a catalyst is formed witha nonuniform surface composition. Part of the surface is iron-rich while the other part is iron-deficient. This would be expectedto lead to a higher rate of dissociation of the carbon precursoron the iron-rich part of the catalyst, since iron is morecatalytically active, and thus a higher concentration of carbonon that part of the particle. Consequently, an SWCNT cap mayform before the particle becomes encapsulated in carbon. Theprobability of cap formation is likely determined by the Fe:Si(or Fe:Al) ratio as shown in ref 3 and reproduced here in Figure6.

For high Fe:Si ratios, only a small percentage of the catalystsurface will be covered with silicon. Thus, the catalyst surfacewould be similar to that of a pure iron or iron oxide particle,which is known to result in low catalyst yields. For low Fe:Siratios, most of the catalyst would be covered with silicon (Figure1b), which would likely render it inactive toward decompositionof the carbon source and prevent the subsequent formation ofa nanotube. The optimum Fe:Si ratio of 2:1 found fromexperimental studies most likely represents the optimum cover-age of one hemisphere of the catalyst surface for achieving bothcarbon source decomposition and SWCNT cap formation.

5. Conclusions

The results of this study suggest that silicon may improveiron oxide catalyst yield by reducing carbon precursor decom-position on silicon-rich areas of the catalyst surface. If oxygenis present, the silica phase is segregated from the iron oxidephase, as indicated by Car-Parrinello molecular dynamicssimulations on model systems and the phase diagram for thissystem. Carbon precursor decomposition is expected to occur

preferentially on the iron-rich hemisphere of the particle, whichmay allow for a SWCNT cap to form. The probability of capformation appears to depend on the percentage of the catalystsurface covered with silica. Fe/Al/O catalysts were also testedand demonstrated to give high catalyst yields, as seen withsimilar results achieved with Fe/Si/O catalysts. The addition ofother elements to the reaction may thus also result in highcatalyst yields, so long as the carbon precursor decompositionoccurs preferentially on one side of the catalyst.

Acknowledgment. This research was funded by the NASAMissouri Space Grant.

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JP909255R

Figure 6. The catalyst yield as a function of the Fe:Si ratio.

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