Carbon 37 (1999) 41–45

The generation of nanometer-size tantalum particles in agraphite host lattice

J. Walter *, H. Shioyama, Y. SawadaOsaka National Research Institute, AIST, MITI, 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan

Received 7 January 1998; accepted 13 May 1998

Abstract

Tantalum(V ) chloride was intercalated from the gas phase into highly oriented pyrolytic graphite and into naturalgraphite. Second stage intercalation compounds were produced. The TaCl5–graphite intercalation compounds wereheated in a hydrogen atmosphere for 1 week at 200°C. The flakes showed no exfoliation, but reduction was insufficientat this temperature. A reduction for 1 week at 1000°C produced extensively exfoliated samples. After 1 week at 1000°Cin a hydrogen atmosphere, it was still possible to detect chlorine in some regions by energy-dispersive X-ray analysis.However, transmission electron microscope images showed the occurrence of nanometer-size particles within thegraphite host lattice, located near the prismatic edges and also at the center of the particles. Different morphologiesof the particles were observed, namely two-dimensional platelets and numerous one-dimensional ‘chains’, with particlesizes between 10 and 300 nm. Selected-area electron diffraction patterns of the particles gave evidence as to theirchemical nature. Metallic tantalum was formed by the reaction, the patterns gave also evidence that a small amountof TaCl5 had still survived. © 1998 Elsevier Science Ltd. All rights reserved.

Keywords: A. Intercalation compounds; B. Heat treatment; C. Transmission electron microscopy (TEM); C. Selected area electrondiffraction (SAED); D. Particle size

1. Introduction a threshold temperature, the guest species escapes inpart and the host lattice expands, this process constitut-ing ‘exfoliation’. The expansion in the c-direction canThe insertion of metals between the sheets of thebe up to ca 1000% [6 ]. A result of this exfoliation is agraphite lattice is of fundamental scientific and also ofhigher surface area, which is useful for catalysis. Yet,commercial interest. In recent years, many metal–graph-the exfoliated graphites do not lose their guests com-ites were synthesized by reduction of the metal chloridepletely, some intercalate can still survive the heat treat-graphite intercalation compound (GIC) precursors. Thement in some areas [7,8]. Exfoliated graphites find otherreduction can be performed in liquid media (e.g. byapplications besides their use in catalysis. Graphite foils,NaBH4 [1], LiAlH4 [1] n-BuLi [1], radical anions [2])which are in use as gaskets for high temperature applica-or from the gaseous phase by hydrogen gas [3] ortions, are produced from exfoliated H2SO4–HNO3–potassium vapor [4].graphite [9]. Some investigations were performed to useThe obtained compounds were assumed to be metal–-exfoliated graphites as obscurants on battlefields [10].graphites, for example, iron–graphite (or ‘iron–graphi-

Tantalum(V ) chloride has a high dielectric constant.mets’). Palladium–graphite is used as a catalyst inIn combination with the high anisotropy of intercalatedcyclohexene hydrogenation [5]. If a GIC is heated abovegraphites, the TaCl5–GICs should be interesting forapplications in electromagnetic interference shielding. Aproblem for all applications of intercalation compounds* Corresponding author. Tel: 0081 727 51 9615;

Fax: 0081 727 51 9622; e-mail: walter@onri.go.jp is their low environmental stability. All GICs decompose

0025-3227/98/$ — see front matter © 1998 Elsevier Science Ltd. All rights reserved.PII: S0008-6223 ( 98 ) 00185-7

42 J. Walter et al. / Carbon 37 (1999) 41–45

more or less rapidly in contact with moisture.TaCl5–GICs are stable for a few hours in water [11] orfor a few months in air [12–15], but the decompositionbegins near the prismatic edges, via tantalum oxychlor-ide [12,13] as an intermediate phase, with tantalumoxide formed as the final product after exposure for along time [16 ].

Recently, a study [17] has been published, whichinvestigated the electromagnetic interference shielding(EMS) behavior at megahertz and gigahertz frequenciesof tantalum–carbide coated carbon microcoils. Theanisotropy of TaCl5–GICs is higher than that of tanta-lum carbide. The intercalation process increases theelectrical conductivity parallel to the graphene sheets. Fig. 1. EDXA spectrum of a TaCl5–GIC after 1 week inFor this reason, the GICs should show a better EMS hydrogen at 200°C. The reduction is highly insufficient. Signalsbehavior than the carbides. for chlorine and tantalum are detectable (the copper signal is

a result of the Cu grid used).

2. Experimental samples were strongly exfoliated (Fig. 2). The TEMimages show a lot of metal particles located near the

Tantalum(V ) chloride–GICs were prepared by a stan- prismatic edges and also at the center of the flakes.dard technique [10] from highly oriented pyrolytic Fig. 3(a and b) shows bright-field images near thegraphite (HOPG) from Advanced Ceramic Corp.,

prismatic edges. An accumulation of tantalum particlesCleveland, OH, U.S.A., and from natural graphite (S40

directly at the edge of the sample can be recognized inflakes) from Graphitwerke Kropfmuhl, Kropfmuhl

Fig. 3(a), the center is not occupied by tantalum par-(Bavaria, Germany) as host materials. The graphite wasticles. However, Fig. 3(b) shows another region nearmixed with anhydrous TaCl5 from Kanto Chemical Co.,the prismatic edge. In this area, there are a few tantalumTokyo, Japan in a sealed glass ampoule in the presenceparticles at the prismatic edge, but the bulk of theof chlorine gas. The sealed ampoules were heated toparticles are located at the center of the sample of this400°C for 4 days. Second stage compounds were pro-region. The tantalum particle distribution is inhomogen-duced in the reaction.eous and their sizes vary to a large extent.One part of the samples was stored at 200°C for

Two different morphologies of tantalum particles can1 week in a hydrogen atmosphere with a flow rate ofbe observed by TEM. A first type of particles is disc-150 ml hydrogen per minute. Another part of the sampleshaped with semi-hexagonal habitus [Fig. 4(a)], as oftenwas heated to 1000°C under hydrogen (flow rateformed with a lot of metal–graphites. [18–20]. A second150 ml min−1) for 1 week.type of particles has quasi one-dimensional shapesBright field images and selected area electron diffrac-[Fig. 4(b)], with lengths much larger than lateral dimen-tion (SAED) pattern were obtained with a Hitachisions, which gives the particles a shape similar to ‘chains’H-9000 transmission electron microscope (TEM ), equ-

ipped with a LaB6 filament at 300 kV acceleratingvoltage. The energy-dispersive analysis was performedwith an energy-dispersive X-ray analysing (EDXA)system from Horiba, Kyoto, Japan.

3. Results and Discussion

No exfoliates of the sample was observed after theattempted reduction at 200°C. Only a few particles nearthe prismatic edges could be detected by TEM. Thereduction temperature of 200°C is highly insufficient.EDXA analysis (Fig. 1) shows the occurrence of tanta-lum and chlorine signals (the copper signal is a resultof the copper grid used, light elements cannot bedetected). Fig. 2. Photograph of exfoliated Ta–graphite, with HOPG as

The second sample was reduced at 1000°C for 1 week. host material. The precursor material was a second-stageTaCl5–GIC, heated at 1000°C for 1 week. The scale is in cm.TaCl5–graphite exfoliates at ca 300°C. After 1 week, the

43J. Walter et al. / Carbon 37 (1999) 41–45

(b)

(a)

(b)

(a)

Fig. 4. (a) Quasi two-dimensional (disc shaped) particlesFig. 3. TEM microphotograph of exfoliated Ta–graphite. (a)Particles located at the prismatic edge; in this region the center observed. Scale bar 100 nm. (b) Quasi one-dimensional par-

ticles with shapes similar to ‘chains’. Scale bar 100 nm.of the sample is free of particles. The scale bar is 100 nm. (b)Another region of the same sample, with the center rich inparticles, and the prismatic edge meanly free of particles. Thescale bar is 1000 nm. As seen in Fig. 5(b), it would appear that some tiny

particles have started to coagulate. The melting andboiling points of tantalum are very high. However,of fused spherical particles. Morphologically these types

of particles can be considered as being quasi one- surface diffusion of tantalum atoms becomes significantnear 950 K [21]. The TEM image (Fig. 6) shows thatdimensional, whereas the disc-shaped particles are rather

more two-dimensional. Fig. 5(a and b) shows the occur- the quasi one-dimensional particles consist of differenttiny particles.rence of the quasi one- and quasi two-dimensional

particles together in the same region. The particle size The chemical nature of the observed particles can be,in principle, metallic tantalum, tantalum carbide, orand distribution in Fig. 5(b) is very large, ranging from

a large quantity of tiny particles (ca 10 nm) to a few tantalum oxide. The formation of tantalum carbide ispossible by temperature treatment at ca 1000–1500°Cparticles with sizes ca 300 nm. It is very difficult to

estimate the average size of the particles from the [17] of a mixture of TaCl5 with carbon in a hydrogenatmosphere. On the other hand, it is well known thatobserved TEM images. The particle sizes vary strongly

from region to region, as does the size distribution. For fine metallic particles are very sensitive towards oxida-tion. Iron-filled carbon nanotubes immediately form inthis reason, we will not give an average size from TEM

measurements. air tiny magnetite (Fe3O4) particles [22].

44 J. Walter et al. / Carbon 37 (1999) 41–45

Fig. 6. The quasi one-dimensional particles are probably a pro-duct of coagulation of some tiny spherical particles. Scale bar50 nm.

(b)

(a)

Fig. 7. EDXA spectrum of a sample reduced for 1 week in aFig. 5. The occurrence of one- and two-dimensional particles in hydrogen atmosphere at 1000°C. A small chlorine signal istwo different regions. (a) Very large two-dimensional particles detectable in some areas (the copper signal is a result of thetogether with some small one-dimensional particles. Scale bar used Cu grid).1000 nm. (b) A very large size distribution of particles inanother region. The largest particles are two-dimensional plate-

the sample was exposed to ambient air. The SAEDlets. It appears that smaller particles apparently start to coagu-pattern and also the EDXA spectra give evidence thatlate. Scale bar 200 nm.a small amount of tantalum chloride still survived inthe host lattice. By reduction from a liquid phase, forexample by NaBH4, it may be possible to reduceAn EDXA spectrum (Fig. 7) shows only tantalum

and a very small chlorine signal (the copper peak is due TaCl5 completely and without exfoliation of the hostlattice.to the copper grid used; light elements cannot, in prin-

ciple, be detected). To estimate the chemical nature ofthe observed particles, an SAED investigation was car-ried out. Fig. 8(a) shows the bright field image of the 4. Conclusionsparticles, and the corresponding SAED pattern is shownin Fig. 8(b). The diffraction rings can be indexed as The reduction of TaCl5–GICs with hydrogen is pos-

sible at high temperatures; the graphite host latticemetallic tantalum and non-reacted TaCl5 [see Fig. 8(b)].This gives evidence that metallic tantalum was formed exfoliates at this temperature. The particles obtained are

distributed inhomogeneously in the host lattice: someby the hydrogen treatment in a hydrogen atmosphere athigh temperature. The SAED pattern was obtained some areas show a high, others low concentrations of the

particles. After reduction for 1 week, small chlorineweeks after completing the reduction, during which time

45J. Walter et al. / Carbon 37 (1999) 41–45

Acknowledgements

Jurgen Walter is grateful to the Science andTechnology Agency (Japan) and the Alexander vonHumboldt-Foundation (Germany) for his research fel-lowship in Japan.

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