Carbon Vol. 36, No. 9, pp. 1277–1284, 1998© 1998 Elsevier Science Ltd

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ELECTRON DIFFRACTION AND SCANNINGTUNNELING MICROSCOPE STUDIES OF TaCl5-GRAPHITE

INTERCALATION COMPOUNDS

J. W,a,* H. S,a Y. Sa and S. HbaOsaka National Research Institute, AIST, MITI, 1-8-31 Midorigaoka, Ikeda, Osaka 563, Japan

bDepartment of Materials Science and Processing, Osaka University, 2-1 Yamada-Oka, Suita 565, Japan

(Received 25 March 1997; accepted in revised form 22 October 1997)

Abstract—Tantalum(V) chloride (TaCl5) was intercalated into highly oriented pyrolytic graphite andinto natural graphite flakes. X-ray diffraction measurements show that second stages were obtained. Thefresh intercalation compounds were investigated by scanning tunneling microscopy in air and by electrondiffraction. The commensurate (E7×E7) superlattice of tantalum(V ) chloride shows a rotation of±19.1° against the graphite lattice. Additional superstuctures of longer dimensions were detected byscanning tunneling microscopy. A Moire pattern and some bands were observed. The graphiteintercalation compounds were immersed in water for two hours at room temperature and dried at 100°Cin air. X-ray diffraction showed that the original stages were preserved after exposure to water. Thescanning tunneling micrographs of samples exposed to water show a pattern similar to that of pristinegraphite, the superlattice had disappeared. Electron diffraction studies on such samples gave evidencethat the intercalate is rearranged and forms amorphous layers. In the case of water-immersed samples,no surface structures of the intercalate were observable with scanning tunneling microscopy. © 1998Elsevier Science Ltd. All rights reserved.

Key Words—A. Intercalation compounds, B. intercalation, C. scanning tunneling microscopy (STM),C. electron diffraction, D. superlattices.

1. INTRODUCTION mensurate superlattices in binary [11–14] and ternary[18,19] alkali metal GICs. However, the monomeric

The in-plane structure of graphite intercalation com-and dimeric, zero-dimensional metal chlorides, show

pounds (GICs) has been investigated for the last 40 more similarities in their superlattices to alkali metalyears. In this time, the structures of some donor and GICs as to GICs with one- or two-dimensionalacceptor GICs were investigated by X-ray diffraction intercalates.( XRD) (e.g. FeCl3-GIC [1], MoCl5-GIC [2], K-GIC Molybdenum(V ) chloride GIC exist in two or[3], PdCl2-GIC [4], ZnCl2-GIC [5] and SbCl5-GIC more phases [2]. One of them contains dimeric[6 ]) and by electron diffraction (ED) (e.g. Mo2Cl10 molecules, which exhibit a precise lateral,SbCl5-GIC [7,8] and also MoCl5-GIC [2]). In recent one-dimensional fit of the dimers to the graphite hostyears, some papers were published in which the lattice [2]. Syme Johnson [2] gave no relation betweenin-plane structure of GICs was deduced from 7Li the chlorine atoms and the adjacent graphene sheets.magic-angle spinning nuclear magnetic resonance The second structure is hexagonal with a disordered(MAS-NMR) [9], X-ray photoelectron spectroscopy cation distribution, the dimeric relationship is no( XPS) [10], scanning tunneling microscopy (STM) longer distinguishable [2].[11–20] and atomic force microscopy (AFM) [21– In the case of antimony(V ) chloride intercalation23]. compounds, two interpenetrating, commensurate

The intercalates can be classified from their superlattices (±16.1° E39×E39 and ±19.1°in-plane structure into zero-dimensional compounds E7×E7) were found with freshly prepared samples(e.g. alkali metals, or monomeric and dimeric metal [8]. After exposure to air, one of them disappearedchlorides), one-dimensional intercalates, which form and only the ±19.1° E7×E7 superlattice waschains between the layers of the host lattice (e.g. observable [8]. A commensurate–incommensuratePdCl2, CuCl2), and two-dimensional intercalated transition of the superlattices in SbCl5–graphite bylayers (e.g. CoCl2, FeCl3 or CrCl3). cooling has been described in refs. [7,8].

Chain- and layer-forming intercalates build incom- The structure of TaCl5–graphite has not yetmensurate superlattices with regard to the graphite been studied. Earlier work has shown thathost lattice [17,18,22,24]. This is likely a result of TaCl5–graphite from flakes of natural graphite asthe strong intralayer interactions of these guest host material is relatively stable in air. An XRDspecies. The small, very mobile and zero-dimensional pattern of samples exposed to air for 1.5 days atalkali metals form commensurate and also incom- room temperature shows a slight line broadening of

the 00l lines [24]. This pattern gives no sign of*Corresponding author. E-mail: walter@onri.go.jp products of hydrolysis. The line broadening of the

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1278 J. W et al.

(a)

(c)

(b)

Fig. 1. (a) Selected-area electron diffraction image of an as-synthesized 2nd stage TaCl5–graphite. The pattern shows additionalreflections to the hexagonal carbon lattice. The unit cell of the superlattice is rotated. (b) Schematic drawing with indexationof important reflections of the SAED pattern in (a). (c) Selected-area electron diffraction image of a 2nd stageTaCl5–graphite, after immersion in water for two hours at room temperature. The pattern is now much clearer, additional to

the graphite reflections, only a diffuse halo is observable.

00l reflections is indicative of an increasing staging first product is tantalum oxychloride (TaOCl3).Scanning electron microphotographs (SEM) give evi-disorder [25]. X-ray diffraction measurements show

that the stage-2 TaCl5-GIC gradually converted to dence that samples exposed for short times did notdeintercalate [26,27]. However, samples exposed forstage-3 after contact with water at 90°C for two

hours and drying overnight at 120°C in air [24,26 ]. a long time showed decreasing Cl/Ta ratios, andtantalum oxide was formed as the final product onIn a moisture environment the Cl/Ta ratio decreased

near the edges of the intercalated flakes. In the first the prismatic faces [27]. The environmental stabilityof TaCl5 intercalated pitch-based carbon fibersfew micrometers from the edges, the Cl/Ta ratio was

estimated to be approximately three by an electron [26,28] and vapor grown carbon fibers [28] isdifferent from TaCl5-GICs with graphite flakes asmicroprobe analysis (EMPA), whereas the Cl/Ta

ratio in the bulk is larger than five [26 ]. Samples host material and so we will not discuss intercalatedcarbon fibers further.exposed to water and air for a long time show a wide

variation of the Cl/Ta ratio [27]. A line scan with anelectron microprobe on such an aged sample showed

2. EXPERIMENTALthat the Cl/Ta ratio varied from #1 to >5 [27].When water molecules diffuse into the GIC and Highly oriented pyrolytic graphite (HOPG) from

Union Carbide (Type ZYA) and natural graphitehydrolyze the intercalated tantalum(V ) chloride, the

1279Electron diffraction and scanning tunneling microscope studies

(a)

(b)

Fig. 2. (a) X-ray diffraction pattern of an as-synthesized 2nd stage TaCl5–graphite, HOPG as host material. (b) X-ray diffractionpattern of the sample after immersion in water for two hours at room temperature.

flakes (sieve fraction 1–2 mm) of >99.8% purity measured. The samples were protected againsthumidity by a thin polyethylene foil.from Kropfmuhl (Bavaria) were used as the host

materials. The graphite samples were mixed with For STM measurements the samples were transfer-red to a Nanoscope III (Digital Instruments Inc.)anhydrous TaCl5 ( Kanto Chemical Co. Inc.) in an

ampoule in the presence of chlorine gas [29]. After and measured in air at 25°C. The STM imageswith an atomic resolution were obtained by the con-sealing of the ampoules, the reactions were performed

for four days at 400°C. After removing the samples stant height mode (tunneling current mapping).Commercially available Pt–Ir tips from Digitalfrom the ampoule, X-ray diffraction patterns were

1280 J. W et al.

Fig. 3. A large-area STM image of the basal face of TaCl5–graphite. A long-periodic structure is visible. The distance betweenthe bands is 0.90 nm (#E13×a0).

Instruments Inc. were used. The images were The intercalation compound from S40 flakes wasa mixture of a 2nd stage (Ic=1330±38 pm) and aobtained by a set-point current (It) of 1.0 to 2.3 nA

and with a bias voltage of ±10 to ±100 mV. All 3rd stage (Ic=1610±9 pm), with domination of the3rd stage. The second stage in this sample has a littlesamples were freshly cleaved along the basal planes

before beginning the scans. broader 00l lines than the HOPG sample. This is thereason for the slightly different identity periods ofAfter observation of the fresh samples, the com-

pounds were submersed in water for two hours at the two second stages.room temperature and then dried for 1.5 days at100°C in air. After this procedure, the X-ray diffrac- 3.2 The bulk structure of water-exposed TaCl

5–

tion pattern was observed again. The STM studies graphiteon these samples were also repeated under the same Figure 1(b) shows that some spots of the SAEDconditions as before. pattern have disappeared after the immersion of the

A freshly prepared 2nd stage TaCl5-graphite com- sample in water for two hours at room temperature.pound (HOPG as host material ) and a sample sub- The SAED image is now much clearer. A diffusemersed in water for two hours were investigated by halo is observed in addition to the hexagonal patternED. The measurements were performed with a of the carbon sheets. A periodic long-range structureHitachi HF-2000 FE-TEM, using an accelerating of the intercalated molecules is no longer detectable.voltage of 200 kV. A rearrangement of the guest molecules produces a

random orientation with regard to the host lattice.An XRD measurement of this sample gives evidence

3. RESULTS AND DISCUSSIONthat even after the immersion in water, the original

3.1 The bulk structure of as-synthesized stage (Ic=1268±17 pm) was not changed. A slightTaCl

5–graphite line broadening of the 00l reflections is evidence of

An XRD pattern of TaCl5-intercalated HOPG the beginning of increasing staging disorder (Fig. 2(a)showed a pure 2nd stage compound (Ic= and 2(b)).1253±25 pm). No reflections of unreacted graphitewere observed. A selected-area electron diffraction 3.3 The superlattice of as-synthesized

TaCl5–graphite analyzed by STM(SAED) image of this compound is shown in

Fig. 1(a). The pattern obtained is complex, but shows For the STM studies, flat regions on the surfaceof the samples were investigated. Figure 3 shows aclearly the occurrence of a superlattice with regard

to the hexagonal carbon structure. The lattice con- typical STM image of a large area of freshTaCl5–graphite. The mean distance between thestant of the superlattice was estimated from

this ED pattern as 0.65 nm, it is a commensurate bands in Fig. 3 is 0.90 nm (#E13×a0). The hori-zontal distance between the bright spots of the inter-(E7a

0×E7a

0) superlattice (graphite, a0=0.2454 nm

[30]). This structure is rotated by ±19.1° with respect calate in a band has distances of 0.56 nm (Fig. 3). Ahigher magnification shows that the bright spots ofto the host lattice.

1281Electron diffraction and scanning tunneling microscope studies

(a)

(b)

Fig. 4. (a) A typical surface of TaCl5–graphite at higher magnification. In addition to the graphite lattice, a commensurate±19.1° (E7×E7) superlattice (bright spots) is visible. The image shows an instrumental distortion between the x and y

directions. (b) A schematical drawing of (a). Top: the (E7×E7) superlattice; bottom: the unit cell of graphite.

the intercalate form a hexagonal pattern (Fig. 4(a)), sities are detected, similar to those in Fig. 4(a). Asection analysis of this Moire image shows that thewhich is slightly rotated against the hexagonal carbon

lattice (spacing 0.25 nm), see Fig. 4(b) for a sche- distance between these very intense spots is 1.1 nmmatic drawing. The spots of the intercalate are of (#E21×a0). After water exposure for two hoursdifferent intensities. The Fourier transformation of at room temperature, the STM image of thethe STM image of the intercalation compound sample is just similar to the images of pristine S40(Fig. 5) shows the hexagonal carbon lattice and one graphite.additional superlattice, which fits with the intensebright spots of Fig. 4(a). 3.4 A comparison between STM and electron

diffraction dataIn the case of S40 flakes as host material, a Moirepattern was observed on the fresh compound Tantalum(V ) chloride is isomorphous to

niobium(V ) chloride [31]. In the solid state, the(Fig. 6(a)). This pattern is shown in a higher magni-fication in Fig. 6(b). Bright spots with different inten- molecules are dimers, with two edge-sharing octahe-

1282 J. W et al.

Fig. 5. Fourier transformation of the image in Fig. 4(a). Additional to the host lattice is a visible structure that fits with theobserved bright spots in Fig. 4(a).

dra [31]. After exposure of intercalated graphite to the 00l reflections [24,26 ]. Such line broadenings, area result of staging disorder, described by Metz andmoisture, tantalum oxychloride was formed in a

small zone near the edges of the flakes [27]. For the Hohlwein [25]. An amorphous layer between thegraphene sheets, should give no explicit structure forpolymerization of the dimers, the intercalate must be

rotated against the tantalum–tantalum axes by the underlying intercalation compound by STM. Butelectron microprobe data (EMPA) [27] and particleapproximately 45°. A simple geometrical construction

using bond lengths of the dimeric TaCl5 [31] and induced X-ray (PIXE) measurements [32] give evi-dence that tantalum chloride is not hydrolyzed in thevan der Waals radii, shows that tantalum(V ) chloride

must be intercalated in this way. This orientation of center of the sample.the dimers brings five chlorine atoms close to theupper and the other five chlorine atoms close to the 4. CONCLUSIONlower adjacent graphene sheets. The tantalum atoms

With STM it is possible to observe the surfaceare inserted at mid-level between these chlorine layers.structure of GICs. The obtained images show theWith scanning tunneling microscopy it is not possiblesuperstructure of the intercalation compound andto image the chlorine atoms in real space. The STMsome additional long periodical structures. Theseimages show the electronic environment in the sur-long-period structures can not be verified by Fourierface, and not necessarily the position of chlorinetransformation of the STM images.atoms. The electron density in the graphene sheets is

It is useful to verify STM results with independentinfluenced by the underlaying intercalate. Our com-techniques. Our experiments have shown that freshparative investigations with STM and SAED showedintercalated TaCl5–graphite has a superlattice, whichthat the (E7×E7) superlattice is rotated by ±19.1°is the same as observed earlier in SbCl5–graphite [8].relative to the graphite host lattice. This finding is

in good agreement with published data for Acknowledgements—J. Walter is grateful to the AlexanderSbCl5–graphite [8]. von Humboldt foundation (AvH) and the Science and

Technology Agency (STA) for his Japan fellowship.A possible explanation for the existence of severalsuperlattices with large dimensions is attributed tocharge density waves (CDWs) at the surface [20]. REFERENCESWater-immersed samples show STM images similar

1. Cowley, J. M. and Ibers, J. A., Acta Crystallogr., 1956,to pristine graphite. No effect on the surface structure9, 421.

of the intercalate was detectable (Fig. 7). However, 2. Syme Johnson, A. W., Acta Crystallogr., 1967, 23, 770.XRD gives evidence that the original stage is retained 3. DiCenzo, S. B., Phys. Rev. B, 1982, 26, 5878.

4. Behrens, P., Ehrich, J., Metz, W. and Niemann, W.,after water exposure. Electron diffraction confirmsSynth. Met., 1989, 34, 199.these results. The in-plane structure disappears after

5. Behrens, P., Beuthien, H., Eickhoff, H. P., Metz, W.contact with water, and an amorphous layer formed and Niemann, W., Synth. Met., 1988, 23, 95.between the graphene sheets (Fig. 1(a)). Water- 6. Rayment, T., Schlogl, R. and Thomas, J. M., Phys.

Rev. B, 1984, 30, 1034.exposed compounds show small line broadenings of

1283Electron diffraction and scanning tunneling microscope studies

(a)

(b)

Fig. 6. (a) A long-range image of TaCl5-intercalated S40 flakes showing a Moire pattern. (b) The Moire pattern with a highermagnification, the distance between the bright spots in the Moire pattern is #E21×a0.

7. Clark, R., Elzinga, M., Gray, J. N., Homma, H., 14. Kelty, S. P. and Lieber, C. M., Phys. Rev. B, 1989,40, 5856.Morelli, H. D., Winokur, M. J. and Uher, C., Phys.

Rev. B, 1982, 26, 5250. 15. Biensan, P., Roux, J. C., Saadaoui, H. and Flandrois,S., Microscopy, Microanalysis, Microstructure, 1991, 2,8. Thomas, J. M., Schlogl, R., Jones, W. and Korgul, P.,

Carbon, 1983, 21, 409. 465.16. Olk, C. H., Heremans, J., Dresselhaus, M. S., Speck,9. Sato, K., Noguchi, M., Demachi, A., Oki, N. and Endo,

M., Science, 1994, 264, 556. J. S. and Nicholls, J. T., Phys. Rev. B, 1990, 42, 7524.17. Olk, C. H., Heremans, J., Dresselhaus, M. S., Speck,10. Mordkovich, V. Z., Synth. Met., 1996, 80, 243.

11. Anselmetti, D., Wiesendanger, R., Geiser, V., Hidber, J. S. and Nicholls, J. T., J. Vac. Sci. Technol. B, 1991,9, 1055.H. R. and Guntherodt, H. J., J. Microscopy, 1988,

182, 509. 18. Lang, H. P., Thommen-Geiser, V. and Wiesendanger,R., Ultramicroscopy, 42, 43, 1992, 44, 624.12. Anselmetti, D., Wiesendanger, R. and Guntherodt, H.

J., Phys. Rev. B, 1989, 39, 11135. 19. Lang, H. P., Wiesendanger, R., Thommen-Geiser, V.and Guntherodt, H. J., Phys. Rev. B, 1992, 45, 1829.13. Anselmetti, D., Geiser, V., Brodbeck, D., Overney, G.,

Wiesendanger, R. and Guntherodt, H. J., Synth. Met., 20. Kelty, S. P., Lu, Z. and Lieber, C. M., Phys. Rev. B,1991, 44, 4064.1990, 38, 157.

1284 J. W et al.

Fig. 7. An STM image of a TaCl5-GIC after immersion in water for two hours at room temperature. No surface structure ofthe intercalate is visible.

21. Ikemiya, N., Okazaki, Y., Hara, S. and Nakajima, T., 27. Walter, J., Synth. Met., 1997, 89, 39.28. Walter, J. and Shioyama, H., Synth. Met., 1998, 92, 91.Surf. Sci. Lett., 1993, 291, 759.

22. Ikemiya, N., Okazaki, Y., Hara, S. and Nakajima, T., 29. Scharff, P., Stumpp, E. and Moritz, H., Proceedings ofCarbon∞88, International Conference on Carbon andCarbon, 1994, 32, 1191.

23. Ikemiya, N., Shimazu, E., Hara, S., Shioyama, H. and Graphite. Newcastle upon Tyne, England, 1988, p. 206.30. Handbook of Chemistry and Physics, Vol. 63, ed. R. C.Sawada, Y., Carbon, 1996, 34, 277.

24. Walter, J., Untersuchungen zur Stabilitat von interca- Weast and M. J. Astle. CRC Press, Boca Raton, FL,1982, p. B-208.lierten Graphiten und Kohlenstoff osern gegen Luftund

Wasser. Dr. rer. nat. thesis, University of Munich, 31. Zalkin, A. and Sands, D. E., Acta Crystallogr., 1958,11, 615.Munich, Germany, 1993.

25. Metz, W. and Hohlwein, D., Carbon, 1975, 13, 87. 32. Walter, J. and Maetz, M., Mikrochim. Acta, 1997,127, 183.26. Walter, J. and Boehm, H. P., Carbon, 1995, 33, 1121.