Double-Walled Boron Nitride NanotubesGrown by Floating Catalyst ChemicalVapor DepositionMyung Jong Kim,†,‡,§ Shahana Chatterjee,‡ Seung Min Kim,| Eric A. Stach,|Mark G. Bradley,‡ Mark J. Pender,† Larry G. Sneddon,*,‡ and Benji Maruyama*,†

Materials and Manufacturing Directorate, Air Force Research Laboratory,2941 Hobson Way, Wright Patterson Air Force Base, Ohio 45433, Department ofChemistry, UniVersity of PennsylVania, 231 South 34th Street, Philadelphia,PennsylVania 19104, UniVersal Technology Corporation, 1270 North Fairfield Road,Dayton, Ohio 45432, and School of Materials Engineering and Birck NanotechnologyCenter, Purdue UniVersity, 1205 West State Street, West Lafayette, Indiana 47907

Received June 12, 2008; Revised Manuscript Received August 10, 2008

ABSTRACT

One-dimensional nanostructures exhibit quantum confinement which leads to unique electronic properties, making them attractive as theactive elements for nanoscale electronic devices. Boron nitride nanotubes are of particular interest since, unlike carbon nanotubes, all chiralitiesare semiconducting. Here, we report a synthesis based on the use of low pressures of the molecular precursor borazine in conjunction witha floating nickelocene catalyst that resulted in the formation of double-walled boron nitride nanotubes. As has been shown for carbon nanotubeproduction, the floating catalyst chemical vapor deposition method has the potential for creating high quality boron nitride nanostructureswith high production volumes.

Carbon nanotubes (CNTs) have been of intense researchinterest owing to their remarkable electronic, mechanical,and optical properties.1 Unfortunately, their use in manynanoelectronic applications has been limited by the fact thatsemiconducting nanotubes are intimately mixed with metallicnanotubes in as-grown samples. Boron nitride nanotubes(BNNTs) are structural analogues of carbon nanotubes, wherealternating boron and nitrogen atoms replace carbon atomsin a hexagonal lattice structure. This elemental change resultsin a number of potential advantages over carbon nanotubes.These include a uniform band gap (∼5 eV) that is insensitiveto either diameter or chirality,2,3 band gap tunability by thegiant Stark effect,4,5 and oxidation resistance up to 800 °C.These properties make BNNTs attractive for nanoscaleapplications in, for example, nanoelectronics, optoelectronics,and nanocomposites.

Carbon nanotubes can be synthesized with relative easein laboratory and commercial settings via simple catalyticchemical vapor deposition (CCVD) techniques, thus allowingwidespread access for research and applications. On the other

hand, BN nanotubes have proven more difficult to reliablysynthesize on a large scale, and accordingly much less isknown about their properties and utility in potential applica-tions. Previous methods for BNNT syntheses have employedmany of the routes that have already been developed for CNTsyntheses, including arc discharge,6 laser ablation,7 ballmilling,8 substitution reactions using carbon nanotubes,9

pyrolysis of polymeric precursors,10 and chemical vapordeposition.11,12 However, the floating catalyst CVD tech-niques that have been exploited to provide high quality, largescale syntheses of CNTs have not yet been achieved forBNNT synthesis.

Here, we report that the use of low pressures of themolecular precursor borazine in conjunction with a floatingnickelocene catalyst results in the predominant formation ofdouble-walled BNNT structures. This floating catalyst methodhas the potential to produce BNNT materials continuouslyin a manner similar to the HiPco13 or CoMoCat14 processesused in the large scale production of carbon nanotubes.

As described in more detail in the Supporting Information(S1), the experimental CCVD setup consisted of a tubefurnace equipped with an alumina pyrolysis-chamber, alongwith the appropriate mass flow controllers, bubblers, andpumping system that allow for the controlled delivery of the

* To whom correspondence should be addressed. (B.M.) E-mail:Benji.Maruyama@wpafb.af.mil. Tel: 937-255-0042. (L.G.S.) Tel: 215-898-8632. E-mail: lsneddon@sas.upenn.edu.

† Air Force Research Laboratory.‡ University of Pennsylvania.§ Universal Technology Corporation.| Purdue University.

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10.1021/nl8016835 CCC: $40.75 2008 American Chemical SocietyPublished on Web 09/13/2008

borazine, nickelocene, and ammonia vapors. Nickelocene wasobtained from Strem, while borazine was synthesized andpurified as previously reported.15 Under an ammonia flow,the supply bubbler containing the solid nickelocene (100 mg)was heated with a heating tape at 80 °C, while at the sametime a separate bubbler containing the liquid borazine was

cooled under a nitrogen flow to the desired temperature usingan acetone and dry ice cooling bath. The two gas streamswere allowed to combine as they flowed into the hot zoneof the furnace. The relative nickelocene to borazine ratiowas controlled by both the temperatures of the bubblers andflow rates of the ammonia and nitrogen gases.

Figure 1. (a) Low- and (b) high-resolution TEM images, (c) SEM image, and (d) EELS data taken from the boron nitride nanofibersproduced via floating CCVD at a higher vapor pressure of borazine.

Figure 2. (a) SEM image, (b) low- and (c, d) high-resolution TEM images taken from the double-walled boron nitride nanotubes producedvia floating CCVD at lower borazine pressures.

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In a typical run, the flow ratio of ammonia to nitrogenwas 100 to 3 sccm, and the growth temperature was 1200°C. After several hours of reaction under these conditions,grown materials were deposited on a collection grid or afilter located in the down stream of the pyrolysis tube. Theseas-grown samples were then ultrasonicated in ethanol, andthe resulting solution dropped on a holey carbon TEM gridto enable study by transmission electron microscopy (TEM)and electron energy loss spectroscopy (EELS). The TEMand EELS analyses at University of Pennsylvania employeda JEOL 2010F, while those at Purdue University used a FEITitan 80-300 S/TEM.

Because of its volatility, ideal 1:1 B/N ratio, and preformedboron-nitride-like ring structure, borazine was selected asthe chemical precursor for BN nanotube growth in ourstudies. Previously Lourie et al. had used a feed streamcontaining in situ generated borazine and hydrogen toproduce multiwalled BNNT growth on nickel boride catalystparticles,16 but with in situ generation it is difficult to controlthe reactant pressure and ratios. We used borazine that hadbeen synthesized and purified in advance, and by simplycooling the borazine at different temperatures, it was possibleto carefully control its concentration in the reaction zone.As discussed below, such control proved important in theformation of different types of BN structures.

As illustrated by the SEM and TEM images in Figure1a-c, a reaction where the vapor pressure of borazine wasmaintained at 166 torr (20 °C, bubbler temperature, asdetailed in the Supporting Information (S2)) produced amixture of nanofibers and overcoated nanoparticle structures.The high-resolution TEM image in Figure 1b revealed thatsome of the nanostructures were similar to multiwallednanotubes with a hollow core along the center, but had poorcrystallinity. These nanofiber type structures were 500 nmto 1 µm long and had 20-30 nm diameters. The EELS data(Figure 1d) indicated they were pure BN, showing only theboron and nitrogen K-edges.

The SEM images in Figure 2a revealed that manybundlelike structures were produced when the growth reac-tion was carried out with a borazine pressure of only 22 torr(-20 °C, bubbler temperature). The low resolution imagein Figure 2b appears to be of single-walled nanotubes, butfurther analyses by high-resolution (HR) TEM (images inFigure 2c, d) showed that the structures are crystallinedouble-walled nanotubes with ∼2 nm diameters and lengthsranging from 100 to 200 nm. The mass yield of as-grownmaterials was approximately 20 mg/h with, as estimated fromthe HRTEM images, over 70% of the observed BN nano-tubes being double-walled along with much smaller amountsof single- and few-walled nanotubes. Catalyst particles werefrequently observed at the end of BN nanotubes (SupportingInformation (S3)). As estimated from the low-resolutionTEM images, one out of 20-30 nickel nanoparticles ap-peared to be activated for BN nanotube growth. Analogouscontrol experiments carried out without nickelocene did notresult in nanotube growth and thus provide further evidencefor the catalytic activity.

The EELS data in Figure 3a collected from the as-growndouble-walled nanotubes again showed the sharp boron andnitrogen K-edges that are characteristic of the sp2 hybridizedatoms of boron nitride, but unlike in the nanofiber EELSanalyses, the spectra also showed small carbon K edges. Thiscarbon most likely came from decomposed nickelocenecatalyst. The amount of carbon impurity seen in the BNnanofibers was negligible, due to the higher pressure ofborazine that was used during their syntheses. On the basisof previous reports,17,18 the observed broadly distributedcarbon K edges (Figure 3c), corresponding to π* transitions,can be assigned to amorphous carbon. The high resolutionTEM image (Figure 4a) also supports this interpretation,showing that the coatings on the outer surface of the tubeappear amorphous. Similar carbon K-edges were alsoobserved by Arenal et al. for single-walled BN nanotubesgrown by laser vaporization and were interpreted as arisingfrom carbon contamination during the growth process.19

To further investigate whether the carbon detected in EELS(Figure 3a) was part of the amorphous coating or carbonincorporated into the BN-graphene lattice, a sample washeated in air to remove the carbon. Oxidation of the as-grownmaterials in air at 600 °C for 30 min in a tube furnace

Figure 3. EELS data from (a) as-grown and (b) oxidized boronnitride double-walled nanotubes. (c) Inset shows magnified carbonK-edges from Figure 3a. Data demonstrate the removal of carbonby oxidation.

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changed their color from black to gray (Supporting Informa-tion (S4)). Carbon-containing materials including carbonnanotubes20 would have been oxidized under these condi-tions. Therefore, if the as-grown tubes contained anystructural carbon (i.e., BxCyNz nanotubes), the tubes wouldhave exhibited structural damage following oxidation.21 TheTEM images taken from the gray materials (Figure 4b)showed that the structural integrity of double-walled nano-tubes was preserved after oxidation. In addition, the amor-phous contaminants on the outer tube surface were largelyremoved, and the carbon K-edges disappeared from the EELSdata (Figure 3b). These results prove that carbon atoms werenot incorporated into the BN network of the grown nano-tubes.

The predominant production of double-walled BN nano-tubes was previously observed by Cumings et al. using aplasma-arc method22 and by Golberg et al.23 in the metaloxide promoted reaction of B2O3 with carbon nanotubes ina nitrogen atmosphere. Initially, it was proposed that“lip-lip” interactions were important in stabilizing thegrowing open edges of carbon nanotubes by forming bridgebonds between adjacent layers.24 In contrast to a C-C bond,a B-N bond is highly polar. Thus, “lip-lip” interactionssuch as those described above should be even more importantin the formation of BN nanotubes, even during catalyzedgrowth, and should favor the formation of double-wallednanotubes.25 Kinetics could also play a role. The fact thatdouble-walled BN nanotubes were produced at low borazinepressures suggests that the growth rate is limited by thesupply of BN precursor to the catalyst particles, whereasmultiwalled nanotube growth is thought to be limited by thediffusion through the catalyst particles, analogous to thegrowth processes for single-walled and doubled-walledcarbon nanotube growth.26 The presence of catalyst particleswith diameters larger than the associated BN nanotubes couldalso be evidence that nanotube growth is supply limited.However, a clear mechanism for double-walled nanotubeformation has not been clearly elucidated in any nanotubesystem and remains an open question.

In conclusion, BN nanotubes were synthesized via catalyticCVD using the floating catalyst, nickelocene. Depending onthe borazine pressure, the products are either BN nanotubes(lower pressure) or BN nanofibers (higher pressure). TEMand EELS data indicate that the as-grown BN nanotubes aredouble-walled and are pure BN nanotubes with a highcrystalline quality. It is significant that BN nanotubes canbe produced with a well-defined BN precursor and a floatingcatalyst in a manner similar to carbon nanotube CCVDsynthesis, since the CCVD method has the potential to becarried out under the continuous conditions needed for thehigh volume production of BN nanotubes.

Acknowledgment. Authors gratefully acknowledge sup-port from the AFOSR, Air Force Office of ScientificResearch through the UTC contract (Collaborative Researchand Development (CR&D) F33615-03-D-5801). S.M.K andE.A.S. acknowledge additional support from the ArmyResearch Office.

Supporting Information Available: (S1) Schematicdiagram of the CCVD setup for BN nanotube growth; (S2)the vapor pressure of borazine; (S3) the catalytic effect ofnickel nanoparticles; and (S4) pictures taken from (a) as-grown and (b) oxidized materials. This material is availablefree of charge via the Internet at http://pubs.acs.org.

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