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244 MRS BULLETIN/APRIL 2004

IntroductionFirst discovered in 1991,1 carbon nano-

tubes are considered to be a new form ofcarbon material with many unique electri-cal, mechanical, and chemical properties.They have attracted much attention due topotential applications such as additives forhigh-strength polymer composites, elec-trode materials for high-capacity batteries,efficient field-emitters as electron sources,and functional components for nanoscaleelectronic devices. Over the years, basic re-search and applications exploration havebeen greatly accelerated by the synthesisof high-quality nanotube materials. Thefirst growth of high-quality and milligram-quantity single-walled carbon nanotubes(SWNTs) by laser ablation2 represented animportant milestone that enabled the studyof the intrinsic properties of nanotubes. It is clear that future developments innanotube-based science and technology willrely on the highly controlled synthesis ofnanotube materials.

Currently, there are four main challengesin the field of nanotube synthesis: (1) thedevelopment of low-cost, large-scale proc-esses for the synthesis of high-quality nano-tubes, including SWNTs; (2) control overthe structure and electronic properties ofthe produced nanotubes; (3) control over thelocation and orientation of the producednanotubes on a flat substrate; and (4) thedevelopment of a thorough understandingof the growth mechanism of nanotubes.

There have been many review articles3–6

and special issues of journals7–9 in additionto several books10–13 focusing on carbonnanotubes published during the pastseveral years. Recently, activities haveincreased in theoretical modeling and simu-lation of carbon nanotubes in order to un-derstand the growth mechanisms fromthe theoretical perspective.14–18 In this ar-ticle, we will present an overview of thecurrent state of the art in the synthesis ofcarbon nanotubes.

The preparation of high-quality SWNTswith high yield has been the goal of manyresearch endeavors. So far, arc-discharge,19,20

laser ablation,2 and chemical vapor depo-sition (CVD) are the three main methods forSWNT production. Arc-discharge and laserablation were the first methods that al-lowed synthesis of SWNTs in relativelylarge (gram) amounts. Both methods in-volve the condensation of hot gaseous car-bon atoms generated from the evaporationof solid carbon. However, the equipmentrequirements and the large amount of en-ergy consumed by these methods makethem suitable only on the laboratory scale.The CVD method, which can be easilyscaled up to industrial production levels,has become the most important commer-cial method for SWNT production.

Chemical vapor deposition is the termused to describe heterogeneous reactions inwhich both solid and volatile products are

formed from a volatile precursor throughchemical reactions, and the solid productsare deposited on a substrate.21–23 It has be-come a common method for thin-filmgrowth on various solid substrates. CVDof carbon has been successful in making car-bon films,24 fibers,25,26 and carbon–carboncomposites27 on an industrial scale for morethan 20 years; multiwalled carbon nanotube(MWNT) materials28,29 have also been pro-duced using CVD. Only recently has thegrowth of SWNTs using CVD becomepossible.30–34

Compared with arc and laser methods,the main advantage of CVD is the morestraightforward way to scale up produc-tion to industrial levels. Indeed, CarbonNanotechnology Inc. (Houston, Texas) isalready producing SWNTs on a pound perday scale, using a process called HiPCO(high-pressure catalytic decomposition ofcarbon monoxide), which is a floating cat-alyst CVD method. Another advantage ofCVD methods is that they allow more con-trol over the morphology and structure ofthe produced nanotubes. With the arc andlaser methods, only powdered sampleswith nanotubes tangled into bundles canbe produced. With the CVD methods, wecan produce well-separated individualnanotubes either supported on flat sub-strates or suspended across trenches. Thesenanotubes can be directly used to fabricatenanoscale electronics. For such applications,these well-separated nanotubes present abig advantage over bulk samples, since noseparation or purification of the nanotubesis needed. It is known that the purificationand separation processes may create de-fects in the nanotubes that can alter theirelectronic properties. In addition, there arealso recent reports that the orientation ofthe nanotubes35–38 and their diameters39–42

can be controlled by controlling the experi-mental parameters. In the following sec-tions, we will discuss in more detail thevarious methods for the synthesis ofSWNTs. We have divided the methodsinto two categories: bulk synthesis and sur-face synthesis.

Bulk Synthesis of SWNTsOver the last few years, several methods

have been developed that have the potentialfor industrial-scale preparation of nano-tubes. All of them are based on CVD meth-ods. Among these methods, four differentapproaches have been shown to be themost promising: methane CVD, HiPCO,CO CVD, and alcohol CVD.

Methane CVDMethane CVD was developed by Dai’s

group at Stanford; they first reported thesynthesis of bulk amounts of SWNTs by

Recent Advances inMethods of FormingCarbon Nanotubes

Jie Liu, Shoushan Fan, and Hongjie Dai

AbstractSince their discovery, carbon nanotubes, both single-walled and multiwalled, have

been a focus in materials research. Fundamental research and application developmenthinge on high-quality nanotube materials and controlled routes to their organization andassembly.The aim of this article is to provide updated information on recent progress inthe synthesis of carbon nanotubes.

Keywords: bulk synthesis, carbon nanotubes, chemical vapor deposition (CVD),multiwalled, single-walled, surface synthesis.

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Recent Advances in Methods of Forming Carbon Nanotubes

MRS BULLETIN/APRIL 2004 245

CVD from methane at 900°C.43 A systematicstudy of the catalyst leads to the conclu-sion that the best catalyst for their processis an Fe/Mo bimetallic catalyst supportedon an Al2O3-SiO2 hybrid material. TheAl2O3-SiO2 hybrid exhibits both strongmetal-support interaction from Al2O3 andbetter structural stability than either Al2O3or SiO2 alone at temperatures as high as900°C. The catalyst has a surface area of al-most 200 m2/g and a mesopore volume of 0.8 mL/g. Weight-gain measurementsshowed that the yield of SWNTs (the weightof SWNTs divided by the weight of thecatalysts) is �35% with a 30 min growthtime. Su et al. significantly improved theweight gain of this method to above 100%(30 min growth time) using Al2O3 aerogelsimpregnated with Fe/Mo nanoparticles asa catalyst.44 The catalyst was prepared bya sol-gel method, followed by supercriticaldrying. Simply evaporating the solvent ofthe wet gel would cause a collapse of thepore walls inside the gel by strong forcesfrom surface tension at the liquid/gas inter-face within the pores. The shrinkage wouldsignificantly reduce the surface area andpore volume of the dried gel. In the reportedmethod, the solvent in the wet gel is broughtto a supercritical state at high temperatureand high pressure, where no liquid/gasinterface is formed, before being removedfrom the wet gel. Therefore, the porousstructure survives this procedure. Theaerogel-based catalyst showed a surfacearea of 500–600 m2/g. Figure 1a shows atransmission electron microscopy (TEM)image of the SWNTs in the raw product.

Several groups used other hydrocarbonsand catalysts to prepare SWNTs. For ex-ample, Hafner et al. prepared SWNTs usingan extremely small amount of C2H4 dilutedby Ar and an Fe/Mo bimetallic catalystwith an Al2O3 support.33 Both single- anddouble-walled nanotubes were observedfor reaction temperatures from 700°C to850°C. However, methane is still the mostcommon gas used to prepare SWNTs. It hasbeen shown that when different catalystsare used, the optimal reaction conditionsare different. Unfortunately, there is noavailable systematic theory that can explainall of the experimental observations. Theseexperimental observations tend to be iso-lated from each other, and the observedrules only apply to the specific system. Takemethane as an example: when 2.5 wt%Co/MgO catalysts were used, 1000°C isthe reaction temperature that producedthe best quality nanotubes, as reported byColomer et al.45 On the other hand, Li et al.prepared high-quality SWNTs by CVDfrom methane at 850°C on an Fe/Mo catalyst with an MgO support,46 andHarutyunyan et al. reported high-quality

SWNT growth at low temperature (680°C)and low methane flow rate (40 cm3/min).47

Clearly, more systematic studies are neededin this research field to provide a better un-derstanding of the general growth mecha-nism and to explain all of the experimentalobservations.

HiPCOHiPCO stands for high-pressure catalytic

decomposition of carbon monoxide. It is amethod used to prepare SWNTs, usinghigh-pressure CO as the carbon source. Thecatalysts used in a HiPCO process areformed in the gas phase from a volatileorganometallic catalyst precursor intro-duced into the reactor. The organometallicspecies decompose at high temperature,forming metal clusters on which SWNTsnucleate and grow.

The HiPCO process was originally de-veloped by Smalley’s research group atRice University.48,49 In this procedure, high-pressure (�30–100 atm) and high-temperature (1050°C) CO with Fe(CO)5 asa catalyst precursor produced high-qualitySWNTs at a rate of approximately 450 mg/h.The product consists of entangled SWNTbundles interspersed with Fe nanoparticles(Figure 1b). Bronikowski et al. investigatedthe change in SWNT yield with COpressure.49 The production rate was foundto increase with increasing pressure up to

50 atm, indicating this is a surface-reaction-limited process. Currently, the HiPCO proc-ess is the only process that can make SWNTson a pound per day scale.

The catalysts used in the HiPCO proc-ess are formed from volatile organometal-lic precursors. The CVD methods that usesuch catalysts are normally called floatingcatalyst CVD methods.50,51 Other feed-gasand catalyst precursors have also beenwidely reported. For example, Cheng et al.used ferrocene to grow SWNTs with ben-zene as a carbon source; 52 Satishkumar et al.used metallocenes in an admixture withC2H2 in their floating catalyst synthesis;53

Bladh et al. used a CO and CO/H2 mixtureas a carbon feedstock to prepare SWNTs,54

and Ci et al. prepared high-quality SWNTsby pyrolysis of acetylene at 750–1200°C ina float-iron catalyst system.55 Recently, ul-tralong SWNT strands were synthesizedby Zhu et al.56 using a floating catalystmethod. In the reported method, a hexanesolution of ferrocene and thiophene wasintroduced into the reactor with hydrogenas the carrier gas. The as-grown SWNTstrands can be as long as 10–20 cm, with aYoung’s modulus ranging from 49 GPa to77 GPa. The main difference between thisnew method and previous methods52 isthat the furnace used in the CVD system ischanged from a horizontal position to a ver-tical position.

Figure 1.Transmission electron microscopy images of single-walled carbon nanotubes(SWNTs) synthesized by (a) methane chemical vapor deposition (CVD), (b) the HiPCOprocess (high-pressure catalytic decomposition of carbon monoxide), (c) the CoMoCat CO CVD process, and (d) alcohol CVD.



CO CVDCO was actually the first feed gas used

for the growth of SWNTs. Dai et al. per-formed the first CVD synthesis of SWNTsby Mo-catalyzed disproportionation of COat 1200°C in 1996.31 It was reported thatmost of the resulting SWNTs had catalyticparticles attached to the ends, indicatingthat the growth of SWNTs was catalyzed bypreformed nanoparticles. However, dueto safety reasons related to the use of CO,reports on the growth of SWNTs using COare limited, as compared with other feedgases. However, the use of CO as a feed gasdoes offer certain advantages over hydro-carbons. For example, Zheng et al. reportedthe use of CO and an Al2O3 aerogel-supported catalyst to synthesize SWNTs at900°C.57 Compared with samples madeusing the same catalyst and methane, theamount of amorphous carbon is greatly re-duced. An important advance in the COCVD method that makes it potentiallycommercial is the development of the Co-MoCat process by Resasco’s group at theUniversity of Oklahoma, who used Co-Mobimetallic catalysts and a fluidized bedCVD reactor to produce a large quantity ofSWNTs (Figure 1c).58,59 Even more inter-esting, it was found that the SWNTs pro-duced by their process have very narrowdiameter and chirality distributions.60

The impact of adding hydrogen to COCVD synthesis was studied by Bladh et al.54

and Zheng et al.61 It was found that hy-drogen can greatly enhance SWNT syn-thesis by CO disproportionation. The effectof hydrogen can be explained in two pos-sible ways: (1) hydrogen directly reactswith CO, producing carbon and H2O; and (2) hydrogen interacts with catalystnanoparticles, so that the activity of thecatalyst toward CO disproportionation isenhanced.61

Alcohol CVDThe most recent addition to the family of

CVD methods for SWNT production is thealcohol CVD method. Maruyama et al. re-cently reported the synthesis of high-purity SWNTs62 using alcohols such asmethanol and ethanol as a carbon source.The synthesis temperature is 700–800°C.The catalyst used in the method is abimetallic (Fe-Co) catalyst supported on azeolite. TEM and SEM showed that theproducts are very clean SWNTs (Figure 1d)without any amorphous carbon coating. Itis hypothesized that the OH radical formedat high temperature from alcohols can re-move the amorphous carbon efficientlyduring nanotube growth, leaving only pureSWNTs as a product. If proven to be scal-able, alcohols may become a better carbonsource for industrial-scale synthesis of

SWNTs because of their wide availabilityand their ease in handling and storage, ascompared with methane and carbonmonoxide.

Surface Synthesis of SWNTsDirect growth of SWNTs on flat surfaces

has several advantages over deposition ofSWNTs for device fabrication.3 No sonica-tion or oxidative purification is involved for surface-grown SWNT samples, whichgreatly reduces the possibility of defect for-mation on SWNTs. Patterns can be intro-duced by various lithographic techniques;this is especially useful for field emission,sensors, and other applications. Also, thesample is usually very clean after surfacegrowth, containing only SWNTs and nano-particles. Recent transport measurementsby several research groups have shown thatdevices made of CVD nanotubes grown di-rectly on substrates tend to exhibit better

performance than those produced usingSWNTs prepared by other methods.63–67

Recent progress in the surface growth ofSWNTs involves control of their diametersand of their orientation/alignment.

Diameter ControlTheoretical calculations have shown that

the electronic structures of SWNTs dependstrongly on diameter and chirality. There-fore, controlling the diameters of SWNTsis very important for both research and forindustrial applications. The diameter ofSWNTs is believed to be determined bythe size of the catalyst nanoparticles. Formost catalysts used for SWNT synthesis, thesize of the metal/metal oxide nanoparticlescannot be well controlled. As a result, theSWNTs that are produced usually exhibita broad distribution of diameters. Onemethod for solving this problem is to usepreformed, monodispersed nanoparticles

Figure 2. (a) Structure of a metal-containing molecular nanocluster found to be a goodcatalyst for chemical vapor deposition growth of single-walled carbon nanotubes (SWNTs).The icosahedral capsule comprises 30 Fe atoms (yellow atoms) connected with the 12(Mo)Mo5 pentagons, and the reduced Keggin nucleus containing 12 Mo atoms is in thecenter. (b) Atomic force microscopy (AFM) image of SWNTs grown using the molecularnanocluster as a catalyst. (c) Diameter distribution of SWNTs grown on chemically attachedFe/Mo nanoclusters on silicon dioxide surfaces. (d) Raman spectrum of SWNTs thus grown,showing a size range in good agreement with AFM measurements.



as a catalyst. Dai and co-workers loadediron atoms into the cores of horse spleenapoferritin, an iron-storage protein.40 Ironoxide nanoparticles were obtained byheating the artificial ferritin to 800°C in air.The size of the resulting nanoparticles canbe controlled by the number of iron atomsin the protein. Dai’s group also developeda method for making uniform catalystnanoparticles using a dendrimer as a tem-plate.68 Recently, a type of metal-containingmolecular nanocluster was found to be a good catalyst for the CVD growth ofSWNTs. This nanocluster has a well-defined chemical composition, with 84Mo atoms and 30 Fe atoms. Therefore, thediameters of all of these nanoparticles areidentical—about 1.3 nm after being re-duced in H2 at 900°C. SWNTs could begrown by either methane CVD or CO CVDusing this catalyst. The resulting SWNTshave remarkably uniform diameters, as re-vealed by AFM measurement and Ramanspectroscopy (Figure 2).42

Another approach to controlling the di-ameter of nanotubes is the use of templates.Tang et al. synthesized very uniform SWNTsinside 0.73 nm channels of microporousaluminophosphate crystallites by pyrolysisof tripropylamine.41 The resulting SWNTshave a diameter of 0.4 nm, which wouldbe unstable without the support providedby the channels of the template. Addition-ally, it was discovered that SWNTs pro-duced using the CoMoCat process, asdiscussed earlier, had very narrow size andhelicity distributions.60 The origin of such anarrow size distribution is currently underintensive study and discussion.

Orientation ControlOrdered SWNT architectures are of spe-

cial interest in both fundamental researchand industrial applications. To date, theCVD method has proven to be the mostsuccessful method for the synthesis ofSWNTs with controlled orientation.

In order to achieve orientation control,external guidance must be provided for theSWNT growth. This external force can befrom an electrical, gravitational, or magneticfield, or from the interaction between thegrowing SWNT and the feed gas. The firstdirect growth of suspended SWNTs wasdemonstrated by Dai’s group at Stanford.69

In their reported process, lithographicallypatterned silicon pillars were first fabri-cated on a substrate. Then, contact print-ing was used to transfer catalyst precursormaterials onto the tops of pillars. Finally,methane CVD was used to produce sus-pended SWNTs, with their orientations di-rected by the pattern of the pillars. Thesame research group also applied a lateralelectric field (�1 V/�m) during CVD

growth, and the SWNTs exhibited align-ment guided by the field (Figure 3a).35 Thisfield alignment effect originates from thehigh polarizability of SWNTs. Aligningtorques and forces on the nanotubes aregenerated by induced dipole moments. Bygrowing the nanotubes along different di-rections under an electric field, crossed nano-tube structures can be prepared (Figure 3b).

Another promising way of aligningSWNTs was recently developed in Liu’sgroup at Duke University, using the flowof the feed gas to grow SWNTs along spe-cific directions.38 In this new approach, Fe-Mo bimetallic nanoparticles were used ascatalysts, and a CO/H2 mixture was usedas the feed gas. The Si wafer containingpatterned catalysts was heated from roomtemperature to the reaction temperature(900°C) in a few seconds by a quick trans-fer of the sample from the outside of aheated furnace to the center. Such a “fast-heating” CVD process produces ultralong,aligned SWNTs, as shown in Figure 4a.The lengths of the nanotubes can reach1.5 cm after 20 min growth. More impor-tant, since no electrodes are needed to guidethe growth direction of the nanotubes,multidimensional nanotube structures canbe easily prepared by repeated growth ofnanotubes on the sample wafer along dif-ferent directions (Figure 4b). The growthmechanism of these long, aligned nanotubesis not clear. However, it is believed that the

long nanotubes were grown with a “tip-growth” mechanism, rather than the “base-growth” mechanism that was believed tobe the mechanism for most SWNT growth.In addition, the fast-heating process wasbelieved to cause convection of the gas flowdue to a temperature difference betweenthe sample and feed gas. Such a convec-tion flow of the feed gas lifted the nanotubes(with the catalysts on the tip) upward andkept them floating in the gas until theywere caught by the laminar flow of the feedgas above the sample surface. The activelygrowing nanotubes then floated in thefeed gas and grew along the direction ofthe laminar flow. This hypothesis is sup-ported by the observation of catalyst par-ticles at the ends of nanotubes and the factthat these long nanotubes can grow overbarriers and across trenches. The extremelength achieved and the control of orienta-tion demonstrated may prove their valuefor future nanoelectronics fabrication, sincethese nanotubes can be grown into pre-designed structures directly on a Si waferand can be converted into millions of de-vices by the evaporation of metal electrodeson a previously designed pattern.

Other DevelopmentsMetal-Free Synthesis of SWNTs

There have also been other interesting de-velopments in the synthesis of SWNTs,most notably, the growth of SWNTs with-

Figure 3. (a) Scanning electron microscopy image of single-walled carbon nanotubes grownunder an electric field. (b) Atomic force microscopy image of nanotube crosses produced bytwo steps of aligned growth in perpendicular electric fields.



out the use of any metallic catalysts. Themetal-free growth of SWNTs is attractivebecause these SWNTs contain no metalresidue that needs to be removed from themixture, as is a requirement for many ap-plications. In 2000, Gogotsi et al.70 reportedthe metal-free formation of SWNTs by thecarbonization of phenolic resin. The quan-tity of SWNTs obtained by their methodwas small, an amount that would need tobe increased for both research and practi-cal applications. Additionally, it was ob-served that the decomposition of SiC at hightemperature provides an alternative routefor the formation of carbon nanotubes.71–73

Arecent report from IBM shows that SWNTsgrown at high temperature (�1500°C) fromSiC materials have a very narrow diame-ter distribution.74 It was also observed thatthe direction of the nanotubes formed onthe surface of SiC tend to follow the atomicstructure of the surface. More recently,NASA has announced in a technology-transfer opportunity75 that they have de-veloped a metal-free arc method forpreparing high-quality SWNTs at a rate of2 g/h. The purity of the material is as highas 70%, and it contains no metal catalyststhat need to be removed; SWNTs preparedin this way would be of great interest formany applications.

Aligned MWNTs and Nanotube YarnsThere have also been some exciting recent

developments regarding the synthesis ofmultiwalled carbon nanotubes (MWNTs).Two of the most notable developments are

the growth of MWNTs selectively in areascovered with SiO2 rather than Si76 ormetals such as Ag,77,78 and the spinning ofa continuous nanotube yarn from a well-aligned MWNT array with no polymeradditive (Figure 5). While the area-selectivegrowth provides a convenient approach toproducing patterned nanotube structures,the continuous yarns can be directly used

in high-strength composite structures. Theyarn usually forms very thin ribbons, whichare several hundred micrometers wideand composed of parallel aligned thinthreads with diameters of several hun-dred nanometers. The yarn can be woveninto three-dimensional macroscopic struc-tures and used as a backbone for high-strength composite materials.

Figure 4. (a) Four connected scanning electron microscopy images showing millimeter-long well-aligned single-walled carbon nanotubes grownby means of chemical vapor deposition (CVD). (b) Scanning electron microscopy image of a two-dimensional nanotube network grown using a“fast-heating” CVD process.

Figure 5. Carbon nanotube yarns. (a) A 24-cm-long section of carbon nanotube yarn beingcontinuously pulled out from a free-standing carbon nanotube array, shown enlarged in (b)(�28� magnification). (c) Scanning electron microscopy (SEM) images of an alignedcarbon nanotube array grown on a silicon substrate (scale bar in inset, 200 nm). (d) SEMimage of the nanotube yarn shown in (a); inset is a transmission electron microscopy imageof a single thread of the yarn (scale bar in inset: 100 nm).



Summary and OutlookOverall, great progress has been made

over the last few years in developing var-ious chemical vapor deposition methodsfor the synthesis of single-walled carbonnanotubes. The yield of the CVD growthof SWNTs has been greatly enhanced, andcommercial production of SWNTs hasbeen realized. Control of the diameters andorientations of SWNTs has been demon-strated. The availability of a large quantityof SWNTs has been a benefit to the studyand application of these materials. Manypreliminary application tests have been suc-cessful and have shown great promise.For instance, nanotube field-emitters havebeen fabricated.79–81 They exhibit an ultra-low threshold for field emission. SWNTshave also been grown directly onto atomicforce microscope tips82 to offer significantimprovements in lateral AFM resolution,as compared with commercial siliconAFM tips.

However, several challenging issues stillremain. To date, the resulting raw materialconsists of an entangled mat of SWNTs,and the structure of individual tubes variesrandomly from zigzag and armchair tochiral form. All of these chiralities coexistin the product. Currently, complete controlof selectively growing metallic or semi-conducting SWNTs has not yet beenachieved. Most recently, preferential growthof semiconducting SWNTs (�85%) hasbeen observed in a plasma-enhanced CVD(PECVD) process at 600°C.83 It is likelythat preferential growth combined withpostgrowth treatment and separation caneventually solve the problem of obtainingmetallic versus semiconducting nanotubes.These remaining challenges, as well as thequest for a better understanding of the nan-otube growth mechanism, are the focus ofcurrent research in this field.

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