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Stability of MoCl5GICs in various solutions
S¥11TH|TIIC Ilfl|TRILS ELSEVIER
Synthetic Metals 94 (1998) 235-238
Stability of MoC15-GICs in various solutions M. Inagaki *, G. Watanabe Graduate School of Engineering, Hokkaido University, Kita-ku, Sapporo 060, Japan Received 22 January 1998; accepted 26 January 1998
Abstract Stability of graphite intercalation compounds of MoCI5 (MoC15-GICs) with stage-2 and -4 structure was studied in various solutions. The change in the X-ray powder pattern of these GICs could be hardly observed, so they were proved to be stable not only in water at room temperature but also in boiling water, acetone, KCl-saturated aqueous solution and CC14, all of which can dissolve MoCI5 itself. In boiling water, however, the stability of GICs was found to depend strongly on the flake size of the host graphite of the starting GICs: decomposition was observed on those with flake size of 10 ixm, but those of size 400 Ixm were perfectly stable. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Graphite intercalation compounds; Molybdenum; Chemical stability
1. Introduction Much attention has been focused on the structure and properties of graphite intercalation compounds (GICs), but not on their stability, although they were indicated to have potential in their science and application [ 1,2]. It has been pointed out that metal chloride-GICs have many valuable features, such as electrical conductivity, magnetic property, etc. Though they were said to be relatively stable in an atmospheric environment, their stability has not been examined in detail, and reports on it are often contradictory. The present authors have studied the synthesis of stable GICs of metal chlorides combined with either other metal chlorides, such as PbC12 and KCI, or organic molecules, such as chloroform [3-8]. MoC15-GICs with stage-4 structure were synthesized by using highly crystallized graphite films prepared from polyimides and their electronic properties were studied [9]. On the same MoC15-GIC films, detailed scanning transmission microscopy (STM) and atomic force microscopy (AFM) observations were carried out [ 10], showing a superstructure due to the commensurate position of chlorides with graphite layers and also a boundary structure between intercalated and non-intercalated domains. The electromagnetic properties of the MoCIs-GICs were also measured by other authors [9,11]. Recently, MoCIs-G1C with stage-1 structure was * Corresponding author. Tel.: +81 11 706 6575; fax: +81 11 706 6575; e-mail: [email protected] 0379-6779/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PHS0379-6779( 9 8 ) 0 0 0 2 0 - 4
successfully synthesized with the coexistence of MoOCI3 and MoC15 [ 12,13 ]. These works suggested high stability of these MoC15-GICs, but there has been no detailed examination of their stability yet. Knowledge on stability of these GICs is important information needed to develop their practical applications. In the present work, the stability was studied of MoC15GICs prepared from flaky natural graphite powders of different sizes in various liquid media.
2. Experimental Sample MoC15-GICs were prepared from natural graphite powders with different average flake sizes of 10, 85 and 400 txm by mixing with a reagent grade anhydrous MoCI5 and heating at 300°C for 7 days in a glass tube after evacuation. For the synthesis of stage-2 GIC, the mixing ratio of MoC15 to natural graphite host was selected to be 1/8 in molar ratio and 1/40 for stage 4. The formed GICs were separated from unreacted MoCI5 by washing with acetone until no trace of unreacted MoC15 was detected in the rinsed acetone. The stage number of GICs thus obtained was determined by Xray powder diffraction using Ni-filtered Cu Ke~ radiation. The spacings for the diffraction peaks were calibrated by referring to the inner standard of crystalline Cr203 powders. The stability of MoC15---GICs in distilled water either at room temperature or at the boiling point was evaluated by stirring GICs (about 0.05 g) in 500 dm 3 of distilled water for
M. lnagaki, G. Watanabe / Synthetic Metals 94 (1998) 235-238
236
different periods from 7 to 30 days. The Mo and Cl ions dissolved in the water were detected by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) and by precipitation with Ag ions, respectively. The structure of GICs after being kept in the solutions was also examined by measuring the X-ray diffraction (XRD) pattern. Identity periods along the c-axis I, and the full widths at half-maximum (PAVHM) of either the 004 or 006 diffraction line were determined by referring to the inner standard of crystalline Cr203 and averaging from fine measurements. The stability of the sample GICs was also examined in pure acetone, KCl-saturated aqueous solution and CC14.Distilled water and acetone were selected in the present work because they are known to be good solvents for MoC15. MoC15 is also dissolved in KClsaturated aqueous solution and CC14.
: o°
o~ o
as-prepared
22
24 20 / deg.
26 (Cu KU)
28
solution
I.
FWHM of
/ yam
004 line / de~.
as-prepared water at r.t. water at b.p. acetone KCI aq. CC14
1.262 1.262 1.264 1.262 1.264 1.262
0.12 0.13 0.15 0.15 0.13 0.14
~ o
o
stage-2 I~ =I. 2 5 8 n m
i~
o
o
M o
a g e9-248 n m Ics t=1.
o: ~: : 1
o o
o
o
.., . . . . . . . 40
+ 0.01 ± 0.01 ± 0.03 + 0.03 -+ 0.02 ± 0.01
FWHM of the GICs. This 20 range is selected to detect the 002 line of host graphite and two OOl lines of the starting GICs. After being kept in various liquids for 15 days, no change is observed in the starting stage structure and I, value, but there is a tendency of broadening of the diffraction lines to increase slightly. No peaks corresponding to higher stage structure are detected. A trace of the 002 line for graphite is detected only after boiling in distilled water for 15 days. The evaluation of Mo ions by ICP-AES was impossible due to the concentration of Mo ions being lower than that of the detection limit. On the other hand, after the addition of 0.1 M of AgNO3 aqueous solution into water kept for 15 days in the presence of the GICs, the water became opaque, being more marked in the case of water boiled with GICs. This was due to formation of AgCI, but not so much as to be collected as precipitate. Therefore, it was confirmed that C1 ions existed in the water, which were reasonably supposed to originate from the GICs. The present experimental results show that the present GIC is very stable in different solutions into which MoCi5 itself
In Fig. 2, XRD patterns in an angle range between 20 and 29 ° in 20 are compared for GICs with stage-2 structure prepared from an average flake size of 85 Ixm and kept in respective solutions for 15 days, together with the table of I~ and
30
aq.
Fig. 2. Changes in XRD patterns of MoCIs-GICshaving stage-2 structure and flake size 85 ~m, and in identity period I, and FWHMof the 004 line of GICs in various solutions.
3.2. Stability of GICs in various solutions
20
at b.p.
CC14
20
XRD patterns of starting GIC samples are shown in Fig. I. They show only 001 diffraction lines of GICs and no peak for graphite is detected. All diffraction lines are indexed, as shown in Fig. 1, on the basis of stage number n and identity period Ic by assuming an interlayer spacing between carbon layers as 0.335 nm and that intercalated with MoCI5 as 0.586 nm [ 14]. Two GICs thus prepared are concluded to be single phase with either stage-2 or -4 structure, in which I~. values of 1.258 and 1.928 nm, respectively, are in agreement with data already reported [ 14,15 ]. These MoC15-GICs obtained are very stable in ambient air, and their own I, values never change in storage for at least 1 year, except Ibr the sample whose average size is 10 ~m.
i0
water
KCI
3.1. Characterization of starting MoCI~-GIC samples'
o
at r.t.
acetone
3. Results and discussion
. o~
water
, ~. 1_1 50
60
i0
20
30
40
50
60
20 / d e g . (Cu Kct) 20 / d e g . (Cu Ka) Fig. I. XRD patterns of MoC15-GICs synthesized from natural graphite with average flake size 85 ~m.
M. lnagaki, G. Watanabe / Synthetic Metals 94 (1998) 235-238
dissolves very quickly, showing only slight decomposition even in boiling water. The GICs with stage-2 structure were put into distilled water both at room temperature and at the boiling point before washing out the unreacted excess MoCi5 with acetone. It is known that acetone is not only a good solvent for MoCl5 but also immediately reacts with MoCI~ to foNn MoOCI:~. Even alter being kept for 30 days, no decomposition of GICs was detected. This result shows that the procedure for the preparation of the present starting sample GICs, particularly washing with acetone, is not the main reason for their high stability in various solutions.
237
~: _-/k~
as-prepared
_-/k~
In order to understand the effect of the stage structure of MoCIs-GIC on their stability, exactly the same experiment was carried out on the GIC with stage-4 structure. In Fig. 3, XRD patterns taken after the sample had been kept for 15 days in various solutions are shown lor the GIC with stage4 structure prepared ti'om natural graphite with 85 p,m size. As pointed out for stage-2 GICs, no changes in I, value and F W H M on keeping the samples in the solutions and also no additional diffraction lines corresponding to higher stage structure are observed. Only a trace of graphite 002 line is detected alter being kept in boiling water for 15 days. It is concluded that MoCIs-GICs are very stable in various solutions which can dissolve MoC15 itself, irrespective of their stage structure.
water
at r.t.
water
at b.p.
acetone KC1
aq.
CC14 22
24 20 / deg. solution
3.3. Effect ()['stage structure
o °
as-prepared water at r.t. water at b.p. acetone KCI aq. CC14
26 (Cu Ks)
28
x.
FWHM of
/ nm
006 line / deg.
1.928 1.933 1.934 1.929 1.934 1.928
0.13 0.13 0.14 0.14 0.14 0.13
± ± ± ± ± ±
0.01 0.01 0.01 0.01 0.01 0.01
Fig. 3. Changes in XRD patterns of MoCIs-GICshaving stage-4 structure and flake size 85 ~m, and in identity period L and FWHM of the 006 line of G1Cs m various solutions. (a) o
as-prepared w a t e r at r.t. 15 days 30 days w a t e r at b.p. 15 days
3.4. Effect o.fflake size of host graphite
30 days 20
22
24
26
28
o
o
(b)
XRD patterns of the GICs with stage-2 structure which were prepared trom flake sizes 400 and 10 ~m of host graphite are shown in Fig. 4 (a) and (b), respectively, alter being kept in distilled water at room temperature and at the boiling point for 7 to 30 days. In the case of GICs with average size of 400 Ixm (a), no indication of decomposition of GICs is seen in the XRD pattern, and no trace of the graphite line even after 30 days in boiling water. No appreciable changes were detected in the I, value and FWHM. The MoC15-GICs prepared from 400 txm natural graphite are very stable even in boiling water, more than those from 85 ixm shown in Fig. 2. In the case of GICs with the size of about 10 Ixm, on the other hand, certain deterioration of crystallinity in stage-2 structure cart be confirmed even after 7 days at room temperature (Fig. 4(b) ). After being kept at the boiling point, the XRD pattern shows the existence of graphite, more clearly alter 15 than 7 days, and even the 004 line for stage-2 structure is difficult to detect. By boiling the GICs with 10 ~m size in distilled water, their stage-2 structure decomposed to mixed stage structures, which could not be identified, and graphite. Even though the GIC was kept at room temperature, certain degradation of crystallinity is observed. From the present experimental results, stability of MoCIsG1Cs depends strongly on their flake size, 10 Ixm GIC flakes
:
o
as-prepared w a t e r at r.t. 7 days
\
15 days w a t e r at b.p. 7 days 15 days
20
22
24 20 / deg.
26 (Cu Ka)
28
Fig. 4. Changes in XRD patterns of MoCIs-GICswith stage-2 and flakesize 400 txm (a) and I0 p~m(b) in water at room temperature and at the boiling point. being decomposed but 400 ~m being perfectly stable in boiling water.
4. Concluding remarks In the present work, the stability of binary MoCIs-GICs prepared from natural graphite with different flake sizes was experimentally confirmed in various liquid media: distilled
238
M. lnagaki, G. Watanabe / Synthetic Metals 94 (1998) 235-238
water at room temperature and at the boiling point, acetone, KCl-saturated aqueous solution and CC14, all of which can easily dissolve MoC15 itself. Their stability did not seem to depend on their stage structure, but on their flake size. GICs with 400 I~m size were found to be almost perfectly stable but those with 10 ~zm showed certain decomposition, which was more marked in boiling water. It should be pointed out that this high stability of binary MoC15-GICs is rather exceptional, in comparison with other GICs, even with other metal chloride--GICs which have been said to be stable. For this stabilization of MoC15-GICs, a different mechanism can be assumed: local decomposition at the edges of the GIC flakes, formation of stable metal oxide and/or oxychloride at the edge surfaces, etc. However, direct evidence for these mechanisms is difficult to obtain in an experiment. The present authors are carrying out more observations on these GICs by means of microprobe analyses, focusing on the edge part of the GIC flakes, but have not yet obtained satisfactory results.
References [ 1] [2] [3] [4] [5] [6] [7] [8] [9] [ 10] [ 11 ] [ 12] [ 13] [ 14] [ 15]
M.S. Dresselhaus, G. Dresselhaus, Adv. Phys. 30 ( 1981 ) 139. M. Inagaki, J. Mater. Res. 4 (1989) 1560. M. Inagaki, Z.D. Wang, Synth. Met. 20 (1987) 1. M. Inagaki, T. Mitsuhashi, Y. Soneda, J. Chim. Phys. 84 (1987) 1439. Y. Soneda, M. Inagaki, Z. Anorg. Allg. Chem. 610 (1992) 157. Y. Soneda, M. Inagaki, Solid State Ionics 63-65 (1993) 523. M. Inagaki, M. Ohira, Mol. Cryst. Liq. Cryst. 244 (1994) 101. Z.D. Wang, M. Inagaki, Synth. Met. 44 ( 1991 ) 165. Y. Kaburagi, Y. Hishiyama, Y. Soneda, M. Inagaki, Synth. Met. 73 (1995) 49. V. Vignal, H. Konno, S. Frandrois, M. Inagaki, J. Mater. Res. to be published. M. Suzuki, A. Furukawa, H. Ikeda, H. Nagano, J. Phys. C: Solid State Phys. 16 (1983) LI211. J. Mittal, M. Inagaki, Synth. Met. in press. J. Mittal, M. Inagaki, Solid State Ionics to be published. M. Suzuki, L.J. Santodonato, I.S. Suzuki, B.E. White Jr., E.J. Cotts, Phys. Rev. B 43 (1991) 5805. A.W.S. Johnson, Acta Crystallogr. 23 (1967) 770.
Synthetic Metals 94 (1998) 235-238
Stability of MoC15-GICs in various solutions M. Inagaki *, G. Watanabe Graduate School of Engineering, Hokkaido University, Kita-ku, Sapporo 060, Japan Received 22 January 1998; accepted 26 January 1998
Abstract Stability of graphite intercalation compounds of MoCI5 (MoC15-GICs) with stage-2 and -4 structure was studied in various solutions. The change in the X-ray powder pattern of these GICs could be hardly observed, so they were proved to be stable not only in water at room temperature but also in boiling water, acetone, KCl-saturated aqueous solution and CC14, all of which can dissolve MoCI5 itself. In boiling water, however, the stability of GICs was found to depend strongly on the flake size of the host graphite of the starting GICs: decomposition was observed on those with flake size of 10 ixm, but those of size 400 Ixm were perfectly stable. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Graphite intercalation compounds; Molybdenum; Chemical stability
1. Introduction Much attention has been focused on the structure and properties of graphite intercalation compounds (GICs), but not on their stability, although they were indicated to have potential in their science and application [ 1,2]. It has been pointed out that metal chloride-GICs have many valuable features, such as electrical conductivity, magnetic property, etc. Though they were said to be relatively stable in an atmospheric environment, their stability has not been examined in detail, and reports on it are often contradictory. The present authors have studied the synthesis of stable GICs of metal chlorides combined with either other metal chlorides, such as PbC12 and KCI, or organic molecules, such as chloroform [3-8]. MoC15-GICs with stage-4 structure were synthesized by using highly crystallized graphite films prepared from polyimides and their electronic properties were studied [9]. On the same MoC15-GIC films, detailed scanning transmission microscopy (STM) and atomic force microscopy (AFM) observations were carried out [ 10], showing a superstructure due to the commensurate position of chlorides with graphite layers and also a boundary structure between intercalated and non-intercalated domains. The electromagnetic properties of the MoCIs-GICs were also measured by other authors [9,11]. Recently, MoCIs-G1C with stage-1 structure was * Corresponding author. Tel.: +81 11 706 6575; fax: +81 11 706 6575; e-mail: [email protected] 0379-6779/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PHS0379-6779( 9 8 ) 0 0 0 2 0 - 4
successfully synthesized with the coexistence of MoOCI3 and MoC15 [ 12,13 ]. These works suggested high stability of these MoC15-GICs, but there has been no detailed examination of their stability yet. Knowledge on stability of these GICs is important information needed to develop their practical applications. In the present work, the stability was studied of MoC15GICs prepared from flaky natural graphite powders of different sizes in various liquid media.
2. Experimental Sample MoC15-GICs were prepared from natural graphite powders with different average flake sizes of 10, 85 and 400 txm by mixing with a reagent grade anhydrous MoCI5 and heating at 300°C for 7 days in a glass tube after evacuation. For the synthesis of stage-2 GIC, the mixing ratio of MoC15 to natural graphite host was selected to be 1/8 in molar ratio and 1/40 for stage 4. The formed GICs were separated from unreacted MoCI5 by washing with acetone until no trace of unreacted MoC15 was detected in the rinsed acetone. The stage number of GICs thus obtained was determined by Xray powder diffraction using Ni-filtered Cu Ke~ radiation. The spacings for the diffraction peaks were calibrated by referring to the inner standard of crystalline Cr203 powders. The stability of MoC15---GICs in distilled water either at room temperature or at the boiling point was evaluated by stirring GICs (about 0.05 g) in 500 dm 3 of distilled water for
M. lnagaki, G. Watanabe / Synthetic Metals 94 (1998) 235-238
236
different periods from 7 to 30 days. The Mo and Cl ions dissolved in the water were detected by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) and by precipitation with Ag ions, respectively. The structure of GICs after being kept in the solutions was also examined by measuring the X-ray diffraction (XRD) pattern. Identity periods along the c-axis I, and the full widths at half-maximum (PAVHM) of either the 004 or 006 diffraction line were determined by referring to the inner standard of crystalline Cr203 and averaging from fine measurements. The stability of the sample GICs was also examined in pure acetone, KCl-saturated aqueous solution and CC14.Distilled water and acetone were selected in the present work because they are known to be good solvents for MoC15. MoC15 is also dissolved in KClsaturated aqueous solution and CC14.
: o°
o~ o
as-prepared
22
24 20 / deg.
26 (Cu KU)
28
solution
I.
FWHM of
/ yam
004 line / de~.
as-prepared water at r.t. water at b.p. acetone KCI aq. CC14
1.262 1.262 1.264 1.262 1.264 1.262
0.12 0.13 0.15 0.15 0.13 0.14
~ o
o
stage-2 I~ =I. 2 5 8 n m
i~
o
o
M o
a g e9-248 n m Ics t=1.
o: ~: : 1
o o
o
o
.., . . . . . . . 40
+ 0.01 ± 0.01 ± 0.03 + 0.03 -+ 0.02 ± 0.01
FWHM of the GICs. This 20 range is selected to detect the 002 line of host graphite and two OOl lines of the starting GICs. After being kept in various liquids for 15 days, no change is observed in the starting stage structure and I, value, but there is a tendency of broadening of the diffraction lines to increase slightly. No peaks corresponding to higher stage structure are detected. A trace of the 002 line for graphite is detected only after boiling in distilled water for 15 days. The evaluation of Mo ions by ICP-AES was impossible due to the concentration of Mo ions being lower than that of the detection limit. On the other hand, after the addition of 0.1 M of AgNO3 aqueous solution into water kept for 15 days in the presence of the GICs, the water became opaque, being more marked in the case of water boiled with GICs. This was due to formation of AgCI, but not so much as to be collected as precipitate. Therefore, it was confirmed that C1 ions existed in the water, which were reasonably supposed to originate from the GICs. The present experimental results show that the present GIC is very stable in different solutions into which MoCi5 itself
In Fig. 2, XRD patterns in an angle range between 20 and 29 ° in 20 are compared for GICs with stage-2 structure prepared from an average flake size of 85 Ixm and kept in respective solutions for 15 days, together with the table of I~ and
30
aq.
Fig. 2. Changes in XRD patterns of MoCIs-GICshaving stage-2 structure and flake size 85 ~m, and in identity period I, and FWHMof the 004 line of GICs in various solutions.
3.2. Stability of GICs in various solutions
20
at b.p.
CC14
20
XRD patterns of starting GIC samples are shown in Fig. I. They show only 001 diffraction lines of GICs and no peak for graphite is detected. All diffraction lines are indexed, as shown in Fig. 1, on the basis of stage number n and identity period Ic by assuming an interlayer spacing between carbon layers as 0.335 nm and that intercalated with MoCI5 as 0.586 nm [ 14]. Two GICs thus prepared are concluded to be single phase with either stage-2 or -4 structure, in which I~. values of 1.258 and 1.928 nm, respectively, are in agreement with data already reported [ 14,15 ]. These MoC15-GICs obtained are very stable in ambient air, and their own I, values never change in storage for at least 1 year, except Ibr the sample whose average size is 10 ~m.
i0
water
KCI
3.1. Characterization of starting MoCI~-GIC samples'
o
at r.t.
acetone
3. Results and discussion
. o~
water
, ~. 1_1 50
60
i0
20
30
40
50
60
20 / d e g . (Cu Kct) 20 / d e g . (Cu Ka) Fig. I. XRD patterns of MoC15-GICs synthesized from natural graphite with average flake size 85 ~m.
M. lnagaki, G. Watanabe / Synthetic Metals 94 (1998) 235-238
dissolves very quickly, showing only slight decomposition even in boiling water. The GICs with stage-2 structure were put into distilled water both at room temperature and at the boiling point before washing out the unreacted excess MoCi5 with acetone. It is known that acetone is not only a good solvent for MoCl5 but also immediately reacts with MoCI~ to foNn MoOCI:~. Even alter being kept for 30 days, no decomposition of GICs was detected. This result shows that the procedure for the preparation of the present starting sample GICs, particularly washing with acetone, is not the main reason for their high stability in various solutions.
237
~: _-/k~
as-prepared
_-/k~
In order to understand the effect of the stage structure of MoCIs-GIC on their stability, exactly the same experiment was carried out on the GIC with stage-4 structure. In Fig. 3, XRD patterns taken after the sample had been kept for 15 days in various solutions are shown lor the GIC with stage4 structure prepared ti'om natural graphite with 85 p,m size. As pointed out for stage-2 GICs, no changes in I, value and F W H M on keeping the samples in the solutions and also no additional diffraction lines corresponding to higher stage structure are observed. Only a trace of graphite 002 line is detected alter being kept in boiling water for 15 days. It is concluded that MoCIs-GICs are very stable in various solutions which can dissolve MoC15 itself, irrespective of their stage structure.
water
at r.t.
water
at b.p.
acetone KC1
aq.
CC14 22
24 20 / deg. solution
3.3. Effect ()['stage structure
o °
as-prepared water at r.t. water at b.p. acetone KCI aq. CC14
26 (Cu Ks)
28
x.
FWHM of
/ nm
006 line / deg.
1.928 1.933 1.934 1.929 1.934 1.928
0.13 0.13 0.14 0.14 0.14 0.13
± ± ± ± ± ±
0.01 0.01 0.01 0.01 0.01 0.01
Fig. 3. Changes in XRD patterns of MoCIs-GICshaving stage-4 structure and flake size 85 ~m, and in identity period L and FWHM of the 006 line of G1Cs m various solutions. (a) o
as-prepared w a t e r at r.t. 15 days 30 days w a t e r at b.p. 15 days
3.4. Effect o.fflake size of host graphite
30 days 20
22
24
26
28
o
o
(b)
XRD patterns of the GICs with stage-2 structure which were prepared trom flake sizes 400 and 10 ~m of host graphite are shown in Fig. 4 (a) and (b), respectively, alter being kept in distilled water at room temperature and at the boiling point for 7 to 30 days. In the case of GICs with average size of 400 Ixm (a), no indication of decomposition of GICs is seen in the XRD pattern, and no trace of the graphite line even after 30 days in boiling water. No appreciable changes were detected in the I, value and FWHM. The MoC15-GICs prepared from 400 txm natural graphite are very stable even in boiling water, more than those from 85 ixm shown in Fig. 2. In the case of GICs with the size of about 10 Ixm, on the other hand, certain deterioration of crystallinity in stage-2 structure cart be confirmed even after 7 days at room temperature (Fig. 4(b) ). After being kept at the boiling point, the XRD pattern shows the existence of graphite, more clearly alter 15 than 7 days, and even the 004 line for stage-2 structure is difficult to detect. By boiling the GICs with 10 ~m size in distilled water, their stage-2 structure decomposed to mixed stage structures, which could not be identified, and graphite. Even though the GIC was kept at room temperature, certain degradation of crystallinity is observed. From the present experimental results, stability of MoCIsG1Cs depends strongly on their flake size, 10 Ixm GIC flakes
:
o
as-prepared w a t e r at r.t. 7 days
\
15 days w a t e r at b.p. 7 days 15 days
20
22
24 20 / deg.
26 (Cu Ka)
28
Fig. 4. Changes in XRD patterns of MoCIs-GICswith stage-2 and flakesize 400 txm (a) and I0 p~m(b) in water at room temperature and at the boiling point. being decomposed but 400 ~m being perfectly stable in boiling water.
4. Concluding remarks In the present work, the stability of binary MoCIs-GICs prepared from natural graphite with different flake sizes was experimentally confirmed in various liquid media: distilled
238
M. lnagaki, G. Watanabe / Synthetic Metals 94 (1998) 235-238
water at room temperature and at the boiling point, acetone, KCl-saturated aqueous solution and CC14, all of which can easily dissolve MoC15 itself. Their stability did not seem to depend on their stage structure, but on their flake size. GICs with 400 I~m size were found to be almost perfectly stable but those with 10 ~zm showed certain decomposition, which was more marked in boiling water. It should be pointed out that this high stability of binary MoC15-GICs is rather exceptional, in comparison with other GICs, even with other metal chloride--GICs which have been said to be stable. For this stabilization of MoC15-GICs, a different mechanism can be assumed: local decomposition at the edges of the GIC flakes, formation of stable metal oxide and/or oxychloride at the edge surfaces, etc. However, direct evidence for these mechanisms is difficult to obtain in an experiment. The present authors are carrying out more observations on these GICs by means of microprobe analyses, focusing on the edge part of the GIC flakes, but have not yet obtained satisfactory results.
References [ 1] [2] [3] [4] [5] [6] [7] [8] [9] [ 10] [ 11 ] [ 12] [ 13] [ 14] [ 15]
M.S. Dresselhaus, G. Dresselhaus, Adv. Phys. 30 ( 1981 ) 139. M. Inagaki, J. Mater. Res. 4 (1989) 1560. M. Inagaki, Z.D. Wang, Synth. Met. 20 (1987) 1. M. Inagaki, T. Mitsuhashi, Y. Soneda, J. Chim. Phys. 84 (1987) 1439. Y. Soneda, M. Inagaki, Z. Anorg. Allg. Chem. 610 (1992) 157. Y. Soneda, M. Inagaki, Solid State Ionics 63-65 (1993) 523. M. Inagaki, M. Ohira, Mol. Cryst. Liq. Cryst. 244 (1994) 101. Z.D. Wang, M. Inagaki, Synth. Met. 44 ( 1991 ) 165. Y. Kaburagi, Y. Hishiyama, Y. Soneda, M. Inagaki, Synth. Met. 73 (1995) 49. V. Vignal, H. Konno, S. Frandrois, M. Inagaki, J. Mater. Res. to be published. M. Suzuki, A. Furukawa, H. Ikeda, H. Nagano, J. Phys. C: Solid State Phys. 16 (1983) LI211. J. Mittal, M. Inagaki, Synth. Met. in press. J. Mittal, M. Inagaki, Solid State Ionics to be published. M. Suzuki, L.J. Santodonato, I.S. Suzuki, B.E. White Jr., E.J. Cotts, Phys. Rev. B 43 (1991) 5805. A.W.S. Johnson, Acta Crystallogr. 23 (1967) 770.