The formation of boron nitride (BN) nanotubes via a solid-state process is demonstrated. The nanotubes are produced by first ball-milling hexagonal BN powder to generate highly disordered or amorphous nanostructures, followed by annealing at temperatures up to 1300 °C. The annealing leads to the nucleation and growth of hexagonal BN nanotubes of both cylindrical and bamboo-like morphology. Unlike previous mechanisms for nanotube formation, the reordering and solid-state growth process of our nanotubes does not involve deposition from the vapor phase nor chemical reaction

Chadderton, Lewis

Chen, Ying

Fitz Gerald, John

Williams, James S

Applied Physics Letters

The Australian National University

APPLIED PHYSICS LETTERS VOLUME 74, NUMBER 20 17 MAY 1999DownA solid-state process for formation of boron nitride nanotubesYing Chena)Joint Department of Engineering, Department of Electronic Materials and Engineering,Research School of Physical Sciences and Engineering, Australian National University,Canberra, ACT 0200, AustraliaLewis T. ChaddertonAtomic and Molecular Physics Laboratories, Research School of Physical Sciences and Engineering,Australian National University, Canberra, ACT 0200, AustraliaJohn Fitz GeraldResearch School of Earth Sciences, Australian National University, Canberra, ACT 0200, AustraliaJames S. WilliamsDepartment of Electronic Materials and Engineering, Research School of Physical Sciences andEngineering, Australian National University, Canberra, ACT 0200, Australia~Received 27 January 1999; accepted for publication 26 March 1999!The formation of boron nitride~BN! nanotubes via a solid-state process is demonstrated. Thenanotubes are produced by first ball-milling hexagonal BN powder to generate highly disordered oramorphous nanostructures, followed by annealing at temperatures up to 1300 °C. The annealingleads to the nucleation and growth of hexagonal BN nanotubes of both cylindrical and bamboo-likemorphology. Unlike previous mechanisms for nanotube formation, the reordering and solid-stategrowth process of our nanotubes does not involve deposition from the vapor phase nor chemicalreactions. ©1999 American Institute of Physics.@S0003-6951~99!04320-X#nderoederialNathnioicmorrongdhToitwduc-ac-nedis-as-ath,portlerNinofre11i-ind atby280ubeeseretronuc-Hollow nanostructures, in particular, nanotubes ananocages, which can be formed in carbon1 a d other com-pound materials such as boron nitride,2 are currently gener-ating much interest. Such structures are normally formehigh temperature by deposition2 or chemical reaction3 from avapor phase. We recently reported that boron nitride~BN!nanotubes can be prepared by a reactive ball-milling procat room temperature,4 during which elemental boron powdewas first ball milled in an atmosphere of ammonia gas, flowed by annealing in nitrogen up to 1400 °C. Nanotubwith diameters varying from 20 to 150 nm were obtaineThis formation process for nanotubes occurred at a tempture far below that of the familiar arc-discharge2 orlaser-ablation5 methods. In particular, the starting materfor ball milling is elemental boron powder rather than a Bcompound and the nanotube formation process duringnealing may have been initiated by chemical reaction. Inpresent letter we demonstrate that ball milling of hexagoBN powder, followed by annealing, leads to the nucleatand growth of BN nanotubes via a solid-state process whdoes not involve deposition from the vapor phase nor checal reactions.Hexagonal boron nitride powder of a purity of 98%better was used as the starting material. The ball-milling pcess was carried out in a rolling laboratory ball mill usihardened steel balls with a diameter of 25.4 mm anstainless-steel cell.6 The containment cell was loaded witseveral grams of BN powder together with four balls.prevent contamination with oxygen, the cell was purged wnitrogen gas several times and a pressure of 300 kPaestablished prior to milling. Following milling, as-millea!Electronic mail: ying.chen@anu.edu.au2960003-6951/99/74(20)/2960/3/$15.00loaded 24 May 2011 to 130.56.105.85. Redistribution subject to AIP licensdatssl-s.a-n-ealnhi--ahaspowders were heated under N2 gas flow ~1 ml min21! atdifferent temperatures up to 1300 °C for about 10 h. Strtures of samples were investigated by means of x-ray diffrtion ~XRD! analysis using Co radiation~l50.1789 nm! atroom temperature. Chemical compositions were examiusing x-ray energy-dispersive spectroscopy~XEDS! in aJEOL ~JSM6400! scanning electron microscope. Transmsion electron microscopy~TEM! was carried out using bothPhilips EM430~300 kV! and a JEOL 2000 scanning tranmission electron microscopy~200 kV! instruments. Powdersamples were dispersed in acetone using an ultrasonic band a drop of the suspension was placed on a copper supgrid covered with holey carbon film. Differential thermaanalyses~DTA! were made in a Shimadzu analyzer undnitrogen flow.Figure 1~a! shows a TEM micrograph of hexagonal Bpowder first ball milled for 140 h at room temperaturenitrogen gas, followed by annealing at a temperature1200 °C for 10 h in a nitrogen flow. BN nanotubes aclearly observed which have an outer diameter of aboutnm and an inner diameter of 3 nm. Microdiffraction on indvidual tubes reveals that walls are multilayers of BN~002!planes, with a lattice spacing of 3.3860.04 Å .Different microstructures of tubes can be observedannealed samples. For example, a sample anneale1300 °C for 10 h@Fig. 1~b!# shows three ‘‘bamboo-like’’tubes, each of which are divided into several segmentstransverse internal walls. The biggest has a diameter ofnm and the smallest a diameter of about 120 nm. Each tcontains a metal particle at its tip. XEDS reveals that thparticles are mainly Fe with a little Ni and Cr, which acontaminants from the stainless-steel container. The elecdiffraction inset shows that the wall has a hexagonal str0 © 1999 American Institute of Physicse or copyright; see http://apl.aip.org/about/rights_and_permissionsvosesicilloler ieai-nte,urreeboasexBinck-aingl isgevi--hedur-onal-ngbeorgh2961Appl. Phys. Lett., Vol. 74, No. 20, 17 May 1999 Chen et al.Downture. The outer diameter seems to be determined by the oall size of the metal particles and the inner diameter is clto the projected width of the edge of the Fe particle. Thtubes are analogous to carbon ‘‘bamboo’’ tubes.7Nanotubes are not observed in milled samples whhave not been heated, nor are they observed when unmBN powder is heated. This shows that formation of nanstructures can only take place during annealing of ball-milpowders. The nanostructure of the ball-milled BN powdeessential to the ultimate growth of nanotubes during anning. Figure 2 is a typical TEM micrograph showing the typcal morphology of a sample after milling. Aggregates cosisting of particles below 50 nm are observed. The selecarea diffraction pattern~inset! shows three diffuse ringswhich suggests a disordered nanocrystalline BN structassociated with~002!, ~100!, and ~110! planes with latticespacings of 3.61, 2.14, and 1.24 Å , respectively. The large~002! interlayer distance and the absence of a thrdimensional structure are features characteristic of a turtratic structure.8,9 TEM also reveals that iron particles withsize below 1mm are dispersed in milled BN powders ainclusions.XRD analysis also supports the conclusion that the hagonal BN phase is transformed to a highly disorderedphase after ball milling. The turbostratic BN structure isdicated by two weak and broadened peaks~referred to asa1and a2! in the XRD pattern shown in Fig. 3~a!. The meanFIG. 1. TEM micrographs taken from the 140 h milled sample followitwo different heat treatments:~a! 1200 °C for 10 h, the electron diffractionpattern inset is taken from the two filaments; and~b! 1300 °C for 10 h, theelectron diffraction pattern inset is taken from the wall of the longest tuloaded 24 May 2011 to 130.56.105.85. Redistribution subject to AIP licenser-eehed-dsl--de,-s--N-lattice spacing for thea1 peak is about 3.56 Å~though thisvalue has a high level of uncertainty because of the baground correction!. The XRD patterns including XEDS datalso show the presence of iron contaminant from the millballs and the cell itself. The measured overall Fe leveabout 3.8 wt %. After heating at 1200 °C for 10 h in flowinnitrogen, the hexagonal BN structure was recovered, asdenced by the XRD pattern shown in Fig. 3~b!. Pronounceddiffraction peaks of the Fe and Fe~C! phases are also observed@see Fig. 3~b!#. A sample milled for 140 h was alsoanalyzed using DTA up to 1250 °C in nitrogen gas. Tsomewhat featureless curve obtained~not shown! stronglysuggests that no activated chemical reactions take placeing heating. Also, no evidence for a sharp crystallizatievent is observed from DTA and the behavior during anne.FIG. 2. TEM micrograph taken from the BN sample after ball milling f140 h without heat treatment.FIG. 3. X-ray diffraction patterns taken from the BN sample after millinfor 140 h~a!, and from the milled sample after heating at 1200 °C for 10~b!. a1, a2: turbostratic BN,1: hexagonal BN,* : a-Fe, ands: Fe~C!austenite alloy~stainless steel!.e or copyright; see http://apl.aip.org/about/rights_and_permissionsanosioinrooiceatioeliBNobainabtealnynathngtmenceareavero-o--a aodshF.G.ett.H.2962 Appl. Phys. Lett., Vol. 74, No. 20, 17 May 1999 Chen et al.Downing is more consistent with gradual reordering, defectsnealing, and grain growth.The above results clearly show formation of BN nantubes during annealing of disordered BN nanostructure1200 °C and above. It is important to note that precipitatfrom vapor or liquid BN phases is impossible: the annealtemperatures are simply too low. Nanotubes appear to gdirectly from adjacent disordered, nanosized BN grains, bwith and without the aide of Fe~contaminant! particles. Fur-thermore, since the starting phase is BN and no chemreaction has been detected by DTA during the heat trment, there is no reason to suggest that chemical reacare involved in the formation process of the nanotubTherefore, the current results clearly demonstrate a sostate nucleation and growth process for the formation ofnanotubes, which is distinctly different from either a vapphase~arc-discharge and laser-ablation!2,5 or chemical reac-tion ~catalysis!3 method identified previously.The metastable disordered BN nanostructure aftermilling and its structural rearrangement during annealhold the key to understanding the process. The metastnanostructure contains a large surface area. Fine Fe~Cr andNi! particles with sizes in the range from several nanometo 1 mm are also dispersed through the sample. Duringnealing, the~002! lowest free-energy10 layers for hexagonaBN grow from the disordered BN powder. Indeed, matwo-dimensional~002! sheets are observed in samples anealed at low temperatures~less than 1100 °C! and these maybe the precursors of nanotubes. The~002! layers appear togrow to a particular thickness depending on temperaturethen extend into various tubular morphologies, with or wiloaded 24 May 2011 to 130.56.105.85. Redistribution subject to AIP licens--atngwthalt-nss.d-rllglersn--nd-out the catalytic effect of metal particles. At higher annealitemperatures~around 1300 °C!, the growth is faster and mostubes are both thicker and longer. It is interesting that socylindrical tubes appear to have grown without the preseof a contaminant particle whereas the bamboo-like tubesmost often capped with an Fe particle which appears to hcatalyzed the growth.In conclusion, boron nitride nanotubes have been pduced by the annealing of ball-milled BN powders in nitrgen. Ball milling is crucial in forming disordered BN nanostructures from which the nanotubes appear to grow visolid-state process, quite distinct from other known methfor nanotube synthesis.The authors thank P. Willis for providing them witsome hexagonal BN powders, and also S. Stowe andBrink for assistance with the TEM.1S. Iijima, Nature~London! 354, 56 ~1991!.2N. G. Chopra, R. J. Luyken, K. Cherrey, V. H. Crespi, M. L. Cohen, S.Louie, and A. Zettle, Science269, 2045~1995!.3P. Gleize, M. C. Schouler, P. Gadelle, and M. Caillet, J. Mater. Sci.29,1575 ~1994!.4Y. Chen, J. Fitz Gerald, J. S. Williams, and S. Bulcock, Chem. Phys. L299, 260 ~1999!.5D. Golberg, Y. Bando, M. Eremets, K. Takemura, K. Kurashima, andYusa, Appl. Phys. Lett.69, 2045~1996!.6Y. Chen, T. Halstead, and J. S. Williams, Mater. Sci. Eng., A206, 24~1996!.7Y. Saito and T. Yoshikawa, J. Cryst. Growth134, 154 ~1993!.8J. Thomas, N. E. Weston, and T. E. O’Connor, J. Am. Chem. Soc.84,4619 ~1963!.9S. I. Hirano, T. Yogo, S. Asada, and S. Naka, J. Am. Ceram. Soc.72, 66~1989!.10J. Abrahamson, Carbon19, 217 ~1973!.e or copyright; see http://apl.aip.org/about/rights_and_permissions

A solid-state process for formation of boron nitride nanotubes

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