Boron trichloride graphite intercalation compound studied byselected area electron diffraction and scanning tunneling

microscopy

J. Walter* , H. Shioyama

Osaka National Research Institute, AIST, MITI, 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan

Received 21 September 1998; accepted 7 December 1998

Abstract

Boron trichloride was intercalated from a 1.0 M solution of BCl3 in heptane at 658C. A mixture of a stage-2 and a stage-3graphite intercalation compound without unreacted graphite was estimated from an X-ray diffraction pattern. A scanningtunneling microscope study showed the occurrence of a Moire´ structure. This pattern can be interpreted as a commensuratesupperlattice to the underlying graphene lattice. This hexagonal in-plane structure formed by guest particles shows thefollowing relationship to the pristine graphite lattice as well as to the pristine BCl3 lattice: a(Moire) � 5 × a(graphite)� 2 ×a(BCl3) � 1225 pm. The structure is rotated by 298 ^ 0.38 in respect to the carbon lattice. This surface structure was confirmedby selected area electron diffraction data.q 1999 Elsevier Science Ltd. All rights reserved.

Keywords:A. Inorganic compounds; C. Electron diffraction; C. Scanning tunneling microscopy (STM); D. Superlattices

1. Introduction

Graphite intercalation compounds (GICs) were widelystudied with regard to their physical properties [1,2].Owing to the high anisotropy in the sample, suchcompounds can be described as low-dimensional materials.These unique physical properties of GICs are the reason forthe high consideration in academic fields. However, aserious problem against technical applications is the lowenvironmental stability of all intercalated carbons. Somegood achievements were done during the last few years toenhance the environmental stability of graphite intercalationcompounds [3–9]. One method to describe the environmen-tal stability of intercalated materials is to estimate the in-plane structure of an as-prepared compound and comparethis structure to the pattern determined on environmentalaged samples.

The in-plane structures of some GICs were studied bydiffraction methods, as electron or X-ray diffraction, XRD

(MoCl5-GIC [10], TaCl5-GIC [11], BiCl3-GIC [12]) and byscanning tunneling microscopy, STM (TaCl5-GIC [11],BiCl3-GIC [13], CoCl2-GIC [14], AlCl3-GIC [15] and alkalimetal-GICs [16]). In the current work we describe a super-lattice which could be observed by selected area electrondiffraction (SAED) and either by STM. This structure willbe interpreted with regard to the underlying graphite latticeas well as to the lattice of pristine boron trichloride. Borontrichloride can intercalated from the gas phase into graphite,the stage formation of the BCl3-GICs is strongly correlatedto the temperature by the synthesis [17].

2. Experimental

Highly orientated pyrolytic graphite (HOPG) was used ashost material. Graphite slices of 3× 3 mm sizes wereimmersed in a 1.0 M solution of boron trichloride inheptane, and the suspension heated to 658C for one day.

Scanning tunneling microscope images were obtainedwith a Nanoscope III from Digital Instruments Inc. Thesetpoint current varied from 1.5 to 2.75 nA. The bias rangedfrom 10 to 100 mV. The sample was cleaved along the basal

Journal of Physics and Chemistry of Solids 60 (1999) 737–741

0022-3697/99/$ - see front matterq 1999 Elsevier Science Ltd. All rights reserved.PII: S0022-3697(98)00345-X

* Corresponding author. Tel.:181-727-519-615; fax:181-727-519-622.

E-mail address:walter@onri.go.jp (J. Walter)

J. Walter, H. Shioyama / Journal of Physics and Chemistry of Solids 60 (1999) 737–741738

Fig

.1.

XR

Dpa

ttern

ofan

as-p

repa

red

BC

l3-

GIC

;Cu k

Ka

lra

diat

ion,

40kV

acce

lera

ting

volta

ge,1

50m

Aan

ode

curr

ent,

step

-siz

e0.

028.

Am

ixtu

reof

ast

age-

2B

Cl

3-G

IC(I

c�

1298

^40

pm)a

nda

stag

e-3

BC

l 3-G

IC(I

c�

1633

^60

pm)

was

obta

ined

,no

unre

acte

dgr

aphi

tew

asde

tect

ed.

J. Walter, H. Shioyama / Journal of Physics and Chemistry of Solids 60 (1999) 737–741 739

planes to obtain a fresh surface, which was not in contactwith the solution before. All measurements were performedon air at room temperature. All images are filtered and Four-ier transformed.

The handling of the sample in air must be done veryquickly; BCl3-GIC is air sensitive. However, the compoundis stable for a short time. The prismatic faces of the inter-calation compound were sealed with silver paste. This hastwo advantages for measuring the sample: (i) GICs arehighly anisotropic materials, so a better electrical bridgebetween the basal plane and the sample holder is estab-lished. (ii) The silver paste seals the prismatic faces of thehost lattice and can act as a barrier against hydrolysis. Inter-calation compounds decompose via a hydrolyzing gradientfrom the prismatic edges to the center of the compound [9].It is well known that the size of the host lattice can influencethe environmental stability of GICs, a large specimen shouldbe used [8]. These two effects enhance slightly the environ-mental stability of BCl3-GIC. So it was possible to exposethe sample for a short time to air to perform the measure-ments.

3. Results and discussion

3.1. X-ray diffraction measurement

An X-ray diffraction pattern of the as-prepared intercala-tion compound (Fig. 1) gives evidence that BCl3 is interca-lated into graphite. A mixture of a stage-2 BCl3-GIC (Ic �1298 ^ 40 pm) and a stage-3 BCl3-GIC (Ic � 1633 ^

60 pm) was obtained, no unreacted graphite could bedetected. The broad and unsymmetrical reflections indicatea disordered sample, which is typical for all kinds of layeredmaterials. The guest lattice is also a layered material. Owingto the intercalation process the disorder in the system isincreased. The intercalation process was performed manytimes the sample always showed the huge disorder. XRDpattern of disordered graphite intercalation compounds canshow the following unusual behavior [18,19]:

• Some reflections of one identity period are sharp andothers of the same identity period are broad.

• Some (00l) reflections can be shifted and other (00l)reflections do not shift, so for this reason it is possibleto calculate different identity periods for the same stage;their standard deviation is high.

• The intensity ratios of (00l) reflections from one stageare not constant, and it is possible that weaker reflections

are completely absent in XRD pattern of disorderedsamples.

Sometimes not all of the graphite reacted and a graphite(002) reflection could be observed together with (00l) reflec-tions of BCl3-GIC.

3.2. Scanning tunneling microscopy study

Fig. 2(a) shows a STM image of BCl3-GIC with a Moirepattern. Such a hexagonal in-plane pattern shows ana(Moire)-axis of 1225 pm (see bar inside the STM image). Moire´pattern occurs when two hexagonal lattices with differenta-axis build one common structure. Graphite has a wellknown hexagonal structure (space group: P63/mmc,a(graphite)� 245.4 pm,c(graphite)� 670 pm) [20]. Solid borontrichloride shows also a hexagonal lattice (spacegroup P63)with a(BCl3)� 614 pm andc(BCl3)� 660.6 pm [21]. From theknowledge of these two lattice parameters and both crystalsystems, the observed Moire´ pattern can easily be inter-preted.

Boron trichloride forms a hexagonal in-plane structure inaddition to the underlying hexagonal graphite lattice. Owingto the different lattice parameters a Moire´ pattern occurs.The observed Moire´ pattern shows close relationships to thepristine lattice parameter of graphite as well as to the latticeparameter of pristine BCl3.

J. Walter, H. Shioyama / Journal of Physics and Chemistry of Solids 60 (1999) 737–741740

Fig. 2. (a) STM images of BCl3-GIC. Clearly the carbon lattice and an additional Moire´ pattern can be observed. This superlattice shows ana-axis of 1225 pm (see indicated bar). Thea-axis of the superlattice shows a close relationship to thea-axis of pristine graphite and to pristineBCl3: a(Moire)� 5 × a(graphite)� 2 × a(BCl3)� 1225 pm. (b) Schematic drawing of the Moire´ pattern observed by STM. Hatched circles representedthe Moirepattern, the pattern formed by lines represented the host lattice. The unit cell of graphite as well as a half unit cell of the Moire´ patternis indicated, it is a commensurate structure. The Moire´ pattern is rotated by 298 ^ 0.358 in respect to the host lattice. Owing to this rotation asmall misfit to the underlying carbon lattice occurs,a(Moire) � 1225 pm, see indicated bar in Fig. 2(a).

Fig. 3. SAED images of BCl3-GIC, obtained with a magnificationof 200 000×, 300 kV accelerating voltage, camera length 2 m. Theaxis of the host lattice (a* gr) as well as the axis of the guest lattice(a*M) are indicated by large arrows. Thea*M-axis is rotated by 298^0.38 in respect toa*gr-axis. The reflections (are indicated by smallarrows) of the host lattice are bright and the reflections of the guestlattice are weak.

• Experimental• a(Moire) � 2 × a(BCl3) � 1225 pm• a(Moire) � 5 × a(graphite)� 1225 pm

• Theoretical• 2 × a(BCl3) � 1228 pm• 5 × a(graphite)� 1227 pm

Thea-axis of the Moirepattern is rotated by 298 ^ 0.358in respect of thea-axis of graphite. Fig. 2(b) shows a sche-matic drawing of the Moire´ pattern. The unit cell of graphiteis indicated as well as a half unit cell of the superlattice. Therotation by 298 in respect toagr is shown. The guest lattice isa commensurate lattice in regard to the host lattice. Owingto the rotation a small misfit results. The rotation of a guestlattice in respect to the host is certainly attributed to thedifficulty of incorporation of a layered material in anotherlayered structure.

3.3. Selected area electron diffraction study

The in-plane lattice of the as-prepared intercalationcompound (Fig. 3) was also investigated by SAED. Thispattern was obtained at a magnification of 200 000× andgives a further evidence for the successful intercalation ofboron trichloride in HOPG. Fig. 3 shows the in-planediffraction pattern with indicated axis of the host lattice(a*gr) and the guest lattice (a*M). Carbon and BCl3 crystal-lize in a hexagonal system, so the angle between botha*gr-axis or between botha*M-axis is 608. The guest lattice isrotated by 298 ^ 0.38 in respect to the host lattice. Somereflections of the host as well as of the guest lattice areindicated. Thed(100)gr could be used as internal standardand the camera constant could be so refined to 3.93×10212 m2.

• d(hkl)M experimentald(hkl)M theoretical• d(200)M� 538.4 pmd(200)M� 531.7 pm• d(400)M� 269.2 pmd(400)M� 265.9 pm• d(900)M� 117.7 pmd(900)M� 118.2 pm

The lattice parametera was determined to:aexp� 1236^12 pm (theor.:a � 1228 pm). The electron diffraction dataconfirm the superlattice with regard to their symmetry aswell as in regard to the lattice constant.

4. Conclusion

The intercalation of BCl3 from a heptane solution into

graphite is successful. The intercalation could be confirmedby X-ray diffraction, selected area electron diffraction andby scanning tunneling microscopy. Ina-direction the latticeparameter of pristine BCl3 is about half of the value deter-mined for intercalated BCl3. The superlattice shows also aclose relationship to pristine graphite. Ina-direction thelattice parameter of pristine graphite is about a fifth of thevalue determined for intercalated BCl3; it is a commensuratesuperlattice which is rotated by 298 ^ 0.38 in respect to thehost lattice.

Acknowledgements

Jurgen Walter is grateful to the Alexander von Humboldt-Foundation (AvH, Germany) and the Science and Technol-ogy Agency (STA, Japan) for his Japan fellowship.

References

[1] O. El-Shazly, J. Phys. Chem. Solids 58 (1997) 149.[2] Y.V. Zubavichus, Y.L. Slovokhotov, L.D. Kvacheva, Y.N.

Novikov, Physica B 208/209 (1995) 549.[3] J. Walter, H. P. Boehm, Carbon 33 (1995) 1121.[4] J. Walter, M. Maetz, Mikrochim. Acta 127 (1997) 183.[5] J. Walter, Solid State Ionics 101/103 (1997) 833.[6] P. Capkova, D. Rafaja, J. Walter, H.P. Boehm, K.F. van

Malssen, H. Schenk, Carbon 33 (1995) 1425.[7] J. Walter, H. Shioyama, Y. Sawada, Carbon 37 (1999) 41.[8] J. Walter, H. Shioyama, Synth. Met. 92 (1998) 91.[9] J. Walter, Synth. Met. 89 (1997) 39.

[10] A.W. Syme Johnson, Acta Crystallogr. 9 (1956) 421.[11] J. Walter, H. Shioyama, Y. Sawada, S. Hara, Carbon 36 (1998)

1277.[12] P. Behrens, V. Woebs, K. Jopp, W. Metz, Carbon 26 (1988)

641.[13] J. Walter, H. Shioyama, Y. Sawada, Carbon 36 (1998) 1811.[14] M. Tanaka, W. Mizutani, T. Nakahizu, N. Morita, S. Yama-

zaki, H. Bando, M. Ono, K. Kujimura, J. Microscopy (Oxford)152 (1988) 183.

[15] N. Ikemiya, E. Shimazu, S. Hara, H. Shioyama, Y. Sawada,Carbon 34 (1996) 277.

[16] D. Anselmetti, V. Geiser, D. Bradbeck, G. Overney, R.Wiesendanger, H.J. Gu¨ntherodt, Synth. Met. 38 (1990) 157.

[17] A.G. Freeman, J.H. Johnston, Carbon 9 (1971) 667.[18] D. Hohlwein, W. Metz, Z. Kristallogr. 139 (1974) 279.[19] W. Metz, E.J. Schulze, Z. Kristallogr. 142 (1975) 409.[20] R.C. Weast, M.J. Astle (Eds.), Handbook of Chemistry and

Physics, 63, CRC Press, Boca Raton, 1982, pp. 208.[21] M.A. Rollier, A. Riva, Gazz. Chim. Ital. 77 (1947) 361.

J. Walter, H. Shioyama / Journal of Physics and Chemistry of Solids 60 (1999) 737–741 741