Solid State Communications 150 (2010) 198–200

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Solid State Communications

journal homepage: www.elsevier.com/locate/ssc

Collision synthesis of unique carbon nanomaterials inspired by theAllende meteoriteS. Ohara a,∗, Z. Tan a, J. Noma a,b, T. Hanaichi a,c, K. Sato a, H. Abe aa Joining and Welding Research Institute, Osaka University, 11-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japanb Technology Development Headquarters, Kurimoto Ltd., 2-8-45 Shibatani, Suminoe-ku, Osaka 559-0021, Japanc Ultrastructure Research Institute, Hanaichi Co. Ltd., 36 Teramae, Ida-cho, Okazaki, Aichi 444-0076, Japan

a r t i c l e i n f o

Article history:Received 26 August 2009Accepted 26 October 2009by C.H.R. ThomsenAvailable online 30 October 2009

PACS:61.46.+w61.82.Rx95.10.-a07.79.Lh

Keywords:A. NanostructuresB. Chemical synthesisB. NanofabricationsC. Crystal structure and symmetry

a b s t r a c t

A simple shock event approach inspired by the Allende meteorite to produce sophisticated carbon nano-materials is reported. It is demonstrated that unique carbon nanostructures, including carbon nanotubes,carbon onions, and new carbon nanorings are synthesized by high-speed ball-milling of steel balls. Thecarbon nanorings have diameter of several tens of nanometers. It is considered that the carbon nanoma-terials are formed from around the surface of steel balls under local high temperatures induced by thecollision energy in ball-milling process.

© 2009 Elsevier Ltd. All rights reserved.

Elementary nanostructures have potential in the design ofmacroscopic material structures with finely tuned properties.Thus, the fabrication of tailor-made carbon nanomaterials whilecontrolling the shape andmorphology is a current topic of interest.Carbon nanotubes (CNTs) are on the cusp of commercial exploita-tion as multifunctional components in next generation compositematerials; for example, CNTs are being considered as unique rein-forcement fibers in polymer matrices [1]. Moreover, other carbonnanomaterials may offer additional choices in designing materi-als with finely tuned properties. For example, carbon onions arepotential lubricants [2], while carbon nanohooks can be used tomake nanovelco [3]. The unique mechanical properties associatedwith nanohorns [4], nanojunctions [5] and nanotori [6] may alsorealize materials synthesized with specialized mechanical proper-ties. However, despite their importance, controlling the shape andmorphology of carbonnanomaterials is a difficult task, and the syn-thesis of carbon nanorings remains a challenge. Herein we report asimple shock event approach inspired by the Allende meteorite toproduce sophisticated carbon nanomaterials. We demonstrate the

∗ Corresponding author. Tel.: +81 6 6879 4370; fax: +81 6 6879 4370.E-mail address: ohara@jwri.osaka-u.ac.jp (S. Ohara).

0038-1098/$ – see front matter© 2009 Elsevier Ltd. All rights reserved.doi:10.1016/j.ssc.2009.10.035

formation of CNTs, carbon onions, and carbon nanorings by a novelcollision process.In amicrostructural study, Smith andBuseck have observed car-

bon nanostructures, including graphitic carbon and carbon onionsin the Allende carbonaceous chondrite meteorite [7]. Some heav-ily shocked chondrite meteorites in the early solar system havemelted veins, which contain quenched high-pressureminerals andnumerous metallic iron compounds. The Allende meteorite hasrecorded shock eventswith pressures less than 4 to 5GPa [8]. Thesefindings have inspired us to choose a collision process using high-energy ball-milling as a shock event to synthesize carbon nano-materials. In the planetary ball-milling system, centrifugal force isgenerated by combining the rotation and revolution of the pots sothat compressible and shearing forces occur between the balls.In this experiment, a very powerful centrifugal force of 150

gravities (G) was achieved by a high-speed ball-milling apparatus(High-G, Kurimoto Ltd., Japan). Commercial stainless steel balls,such as SUS440C, which is a solid solution of iron (Fe, 83 wt.%),chromium (Cr, 16 wt.%), and carbon (C, 1 wt.%), were selected asshocked raw materials because the balls contain a similar amountof C as carbonaceous chondrites. The steel balls, which had a 3mmdiameter, were treated for 3 h in an air atmosphere by the high-speed ball-milling apparatus. The samples on the surface of thesteel balls were dispersed in ethanol by an ultrasonic treatment,

S. Ohara et al. / Solid State Communications 150 (2010) 198–200 199

Fig. 1. Typical HRTEMmicrographs of carbon nanomaterials synthesized by high-speed ball-milling of stainless steel balls: (a) graphitic carbons, (b) carbon onions, and (c)curved and looped SWCNTs. Inset in (a) is the electron diffraction of the carbon nanostructures shown in (a).

Fig. 2. (a) HRTEMmicrograph and (b) – (d) AFM images of carbon nanorings. AFM images are measured using a non-contact method. (b) Lowmagnification, (c) and (d) highmagnification.

andwere subsequently characterized by high-resolution transmis-sion electron microscopy (HRTEM, JEM-2100F, JEOL, Japan) andatomic forcemicroscopy (AFM, NanoNavi S-image SPM system, SII,Japan).Fig. 1 shows HRTEM micrographs of the samples obtained

by a high-speed ball-milling treatment. Nanostructures similarto those found in the Allende meteorite were clearly observed(Fig. 1(a)), and the lattice fringe spacing of the area surroundedby the circle (upper part in Fig. 1(a)) was 0.34 nm. The materialis an ordered graphite because the lattice fringes correspond tothe (002) basal planes of the graphite structure. A fast Fouriertransform (inset in Fig. 1(a)) of the entire HRTEM image revealed a0.342 nm (002) lattice fringe spacing. Carbon onions, which havehollow spherical or polyhedral graphitic nanostructures, were alsoobserved (Fig. 1(b)). Moreover, CNTs were synthesized by theball-milling of steel balls (Fig. 1(c)). Due to their very small

diameters, the tubes were flexible, and were often observed ascurves and loops rather than straight strings.Through a detailed HRTEM observation of the obtained car-

bon nanostructures, we found ring-like carbon nanostructures, ap-proximately 10 to 20 nm in diameter (Fig. 2(a)). However, TEMobservations alone cannot confirm that these nanostructures arecarbon nanorings or hollow carbon nanocapsules. Therefore, wecharacterized the carbon nanostructures by AFM. Consequently,a ring-shape morphology was clearly observed, and each carbonnanostructure was dispersed separately on a fresh mica substrate(Fig. 2(b)–(c)). The height of ring-shape carbon nanostructureswasapproximately 1.5 nm (Fig. 2(d)). Thus, the carbon nanostructure isa carbon nanoring. Although the diameter of the carbon nanoringsobserved by AFM is slightly larger than that observed by HRTEM,this difference may be due to the radius of the tip curvature of theAFM probe.

200 S. Ohara et al. / Solid State Communications 150 (2010) 198–200

Fig. 3. Plausible scheme for the formation of carbon nanomaterials by a novel collision process inspired by the Allende meteorite.

To date, accidental occurrences of ring structures during CNTsyntheses have been reported [9,10]. Moreover, ring formation hasbeen reported by physical coiling [11] and a chemical reaction [12].However, all these rings have diameters of about a few hundrednanometers because one ring consists of numerous SWCNTs. Incontrast, the carbon nanorings obtained in this study are very smallcompared to rings fabricated from CNTs, so that the nanoringsherein may consist of one and/or several CNT components. To elu-cidate whether the resulting nanorings are complete and seamlesscarbon nanostructures, we are accurately characterizing the car-bon nanorings.Fig. 3 schematically depicts the formation of carbon nanomate-

rials via high-speed ball-milling of stainless steel balls. In additionto carbon nanomaterials, spherical steel nanoparticles coated withamorphous carbons were synthesized (TEMmicrographs in Fig. 3).Typically, the nanosized spherical shape of the resultant steel wasformed by quenching themetal vapor. Dravid et al. have reported asimilar morphology for spherical metal nanoparticles coated withgraphite [13]. According to them such spherical nanocapsules canbe synthesized using a molten metal pool in a graphite crucibleunder arc plasma conditions. Hence, the temperature of local re-gions around the surface of steel balls probably increases up to sev-eral thousands degree Celsius by the collision of 150 G to generatemetal vapor and carbon on the surface of the steel balls. Conse-quently, various graphitic nanostructures are formed during cool-ing of the C vapor as it is drastically moved by the milling action.In addition, the initial nanoparticles of the steel formation from thevapor likely act as catalysts for CNT formation. Although the reasoncarbonnanorings are formed remains unclear, it is possible that theshearing stress in the reaction area plays a key role in the ring for-mation because highly curved carbon nanostructures produced byball-milling of graphite particles have been reported [14,15].Recently, the ball-milling process has become commonplace

not only as grinding machines, but also as reactors in which var-ious functional materials can be created bymechanochemical syn-thesis. A simple milling process will reduce both CO2 generationand energy consumption during materials production. Dachilleet al. have reported that mechanical treatment of simple labo-ratory grinders, mortars, and similar devices can generate highpressures above 1 GPa [16], and such high pressure agrees withthe shock events in the Allende meteorite [8]. In a conventionalmechanochemical synthesis like mechanical alloying, a solid statereaction occurs under high-pressure conditions by mechanical ac-tion. On the other hand, the gas-phase reaction occurs underlocal high temperatures induced by the collision energy in our ball-milling process, which results in phase separated unique carbonnanomaterials.

In summary, our study offers a simple collision approach basedon the shock events of the Allende meteorite to produce so-phisticated carbon nanomaterials. Unique carbon nanostructures,including CNTs, carbon onions, and carbon nanorings, can be syn-thesized by high-speed ball-milling of steel balls. Future studieson the collision process should realize the fabrication of seam-less carbon nanorings and carbon nanotori, e.g., nanochain andnanomaile [6]. In addition to materials science, carbon nanoma-terials with unique shapes and morphologies synthesized by acollision process are attracting much attention in astronomy be-cause carbon nanomaterials may be the interstellar carbon mate-rials with a 220 nm hump in the interstellar extinction curve [17]or the gas carrier Q in meteorites [18]. Hence, investigations of theabsorption spectra of the obtained carbon nanomaterials are cur-rently under way.

Acknowledgments

The authors express their gratitude to Dr. C. Koike and Dr.H. Chihara for their valuable discussions about meteorites andinterstellar materials. This work was partially supported by aScientific Research Grant from the Ministry of Education, Culture,Sports, Science and Technology of Japan (S.O.).

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