Intercalation of pyrographite by NO2+ and NO+ salts

Mat. R e s . B u l l . , Vol. 15, p p .

1627-1634,

0025-5408/80/111627-08502.00/0

1980.

Printed in the USA.

Copyright (c) Pergamon Press Ltd.

INTERCALATION OF PYROGRAPHITE BY NO2+ AND NO+ SALTS XX

XXX

X

D. Billaud x, A. Pron , F. Lincoln Vogel and A. H6rold (Laboratoire de Chimie du Solide Mineral, Associ~ au C.N.R.S. N° 158, Service de Chimie Min~rale Appliqu~e, UniversitQ de Nancy I C.O. N° 140 - 54037 Nancy CQdex - France) xx (Department of Chemistry - University of Pennsylvania Philadelphia PA 19104 - Etats-Unis) xxx (Department of Electrical Engineering and Science University of Pennsylvania - Philadelphia PA 19104 - Etats-Unis) (Received September 4, 1980; Communicated by P. Hagenmuller) ABSTRACT BF4", PF6" and SbF6- ions have been intercalated into pyrographite HOPGby chemical oxidation. The graphite is oxidized by NO2+ (or NO+ in certain experiments) coming from NO2BF4, NO2PF6 and NO2SbF6 (or NOSbF6) salts dissolved in dry nitromethane. X-ray diffraction allows us to determine the identity period along the axis leading to the stage n and the interpianar distance dI. Relative weight change leads us to believe that the MFx" anions are solvated by the solvent. Chemical analyses confirm this hypothesis allowing us to give to these compounds the ideal formula C~3RMFx (CH3NO2)y.

INTRODUCTION A large group of compounds called nitryl(N02 +) or nitrosyl (NO+) salts was used for almost 20 years in organic synthesis as convenient n i t r i t i n g agents (1). In a previous paper (2) we found that the above n i t r y l salts of BF4", PF6- and SbF6" can be used for oxidizing 11systems Qf graphite with concomittant intercalation of the fluoride anions. We established the experimental conditions for ~ynthesis and measured~ axis repeat distances and room temperature basal plane resistivity as a function of stage. In this work we focus our interest on the interaction of nitrosyl (NO+) salts of the above mentioned anions, with graphite. In addition, the nature of intercalant was studied for selected intercalation compounds by using N.M.R. and elemental analysis techniques. EXPERIMENTAL Pyrographite (HOPG) of Union Carbide Corporation was cut and cleaved to standard squares 0.5 cm x 0.5 cm x 0.25 mm thick in order to perform in 1627

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situ X-ray characterization and resistivity measurements. NO2BF4, NO2SbF6, NOPF6 and NO2SbF6 were purchased from Alpha Chemicals, Inc., NO2PF6 from Ozark Mahonino Co., CH3N02 from Fischer Scientific Co. The apparatus described elsewhere (2) is made of Pyrex glass. The main body contains the salt prealably heated under vacuum at 110°C to remeve volatile products of hydrolysis and the dried solvent brought on the salt by d i s t i l l a t i o n to form a solution of given concentration. The pyrographite f i r s t l y outgassed is holded in a rectangular Pyrex tube connected to the main body of the apparatus. Finally, the solvent containing the dissolved salt is discharged into the graphite and the intercalation reaction, generally performed at room temperature, occurs. With saturated NO2+ salts solution, the reaction starts quickly when a gas (probably NO2) is evolved. The solutions were diluted to obtain better control over the rate of the reaction, and i t is possible to have a gradual intercalation which needs several weeks to pass from the higher stages (n : 10) to the richer. Using NO+ salts leads to nothing with BF4- and to a d i f f i c u l t intercalation with PF6" and SbF6". The compounds formed in these conditions are always of higher stages than those obtained with NO2+. For example as NO2SbF6 leads to a stage I compound, NOSbF6 leads only to a stage II compound. The richest compounds synthesized from NOPF6 belong to stage IV when these are stage II compounds when using NO2PF6. In all cases, the rates of the reaction, performed with NO+ salts, even with saturated solutions and at highest temperatures, are very slow. X-ray studies and resistivity measurements are made in situ but weight uptake and thickness were measured only after a transferal in an argon glove bag. .

RESULTS

a) Radiocristal lographi c studies The study of the OOZ X-ray diffraction reflections enable us to determine the identity period along the ~) axis I c and subsequently to calculate the interplanar distance dl and the stage n according to the equation : I c = dI + (n-l) 3.35. The study of the hkO reflections gives data i f the in plane organization of the intercalated species is sufficient. Figure 1 shows examples of 00£ reflections diagrams for the three systems studied, We have isolated stage I I , I l l , IV and V tetrafluoroborate compounds, stage II to VIII hexafluorophosphate compounds and stage I to VIII hexafluoroantimonate compounds. The correlation of stage by X-ray and the macroscopic relative increasing of the thickness AZ/£ is shown in the table I. Finally, the study At room temperature of the hkO reflections diagrams made on graphite-hexafluoroantimonate compounds of stage I shows only reflections attributable to graphite (100 and 110 peaks). We can conclude that, at least at room temperature, no in plane organization of the intercalated species exist. b) Chemical anal~sisand weight uptake The increasing of weight of the richest compounds is shown in table I. The elemental analysis is d i f f i c u l t to perform because of the variety of the species that can be intercalated : fluoride anions, salt nmlecules, solvent. Nevertheless some chemical analyses performed by (a) Centre de Microanalyse du C.N.R.S. (France) and (b) Schwarzkopf Microanalytical Laboratory Woodside N.Y.

Vol. 15, No. 11

PYROGRAPHITE INTERCALATION

1629

~T

AL~ " ~

2t

~

'

~

-

-

-

30

i

20

---:~" ';'

• ,

~ . . . . . . .

t"

IG

S

o

8

"

a~

m

ores ~

om~ 3O

110

FIG. 1 (A) OOZ reflections of stage II and I l l graphite-tetrafluoroborate compounds (MoKe radiation)

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Vol. 15, No, i i

~6

-

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FIG. I (B) 00£ reflections of stage II and I I I graphite-hexafluorophosphate compounds (MoK~ radiation)

Vol. 15, No. 11

PYROGRAPHITE INTERCALATION

. z." . . . . . .J ~.

2e

~

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.....

r

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~to

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l~

FIG. 1 (C) OOZ reflections of stage I and II graphite-hexafluoroantimonate compounds (MoK~ radiation)

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(on samples of ca. 20 mg and 150 mg respectively) are listed in table II. TABLE I Correlation of Stage, c Axis Repeat Distance, Relative Thickness Increasing and Weight Uptake

~Ul

o

Fluorides Stage

I c (A) J

Amlm

From thickness

X- ray

.70

.68

i

11.25

.434 .366

BF4" 14.58

PF6" m

SbF6

.45

11.10 14.44

.79

8.05

1.45

11.38

.72

.656

.533

• 436 1.40 .698

1.2 0.72 - 0.735(N0~+) 0.722 (NO+)

TABLE I I Analytical Data of Stage I Intercalation Compound Formed by the Reaction of Nitrylnexafluoroantimonate with Graphite in Nitromethane %Sb

%C Found (a)

49+1

%F

19+2

18+2

not

m

.i

Found ~'Gi Calc. for C23SbF61.7CH3NO2

%N

%H not

o (c) Total )

measured _as.ured{

47.55

19.72

19.43

3.0

)0.99

# 9.31

100.00

48.18

19.77

18.51

3.87

0.84

tI 8.83

100.00l

!

;hemical analyses performed by (a) Centre de Microanalyse du CNRS (France) (b) Schwarzkopf Microanalyti cal Lab.Woodside (c) Oxygen determined by difference c) Basal plane resist!vit¥ Pa In situ resistivity measurements along ~ axis were_ performed using the contactless method of Zeller et al. (3). The curves representing the variation of R.T. Pa of compounds synthesized from NO2+ salts as a function of stage have been-published elsewhere (2). We report in table I I I the comparison of the resistivities Pa measured on compounds synthesized from NO2+ and NO+ salts

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PYROGRAPHITE

INTERCALATION

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TABLE I l l Room Temperature Basal Plane Resistivity as a Function of Stage for Intercalation Compounds synthesized from NO2+ and NO+ salts Pa (u~.cm) Fluoride

PF6

Stage NO2+

NO+ )2.3to2.6 2.5 2.3to2.6

l i

4 5

2.2 to 2.5

6

2.1 to 2.5

!

2

4.0to4.3 3.4 3

!

3

SbF6

4

i

4.1 I3.3to3.5 ~ 3 I

i

This table I l l shows that in both cases, the Pa values are similar. d) 19F and 31p NMR studies In order to establi)h the n~))ure of the species intercalated in the graphite we have performed ~F and o~P NMR studies on the second stage PF6graphite (4). Besides the main (A) reaction (A) NO2PF6 + nC ~ NO2 + C+PF6" a side (B) reaction may occur without change in stoechiometry : (B) C+PF6" ÷ CnF + PF5 The narrow absorption lines of ca. 240 Hz FWHM(~o = 56.4 ~z) imply rapid translational motion and rapid isotropic rotation of the intercalant. The^indire~)~ spin-spin coupling for the PF6- ion has been observed both in the ~F and o~p spectra ; the coupling constant JPF of 708 + 20 Hz is identical to that observed for the PF6- ion in aqueous-solution.'This coupling constant, the fluorine doublet and the phosphorus septet confirm that the PF6- ion is th~ only P-F species intercalated in a stage 2 PF6- - graphite. Thus, reaction (A) only occurs. This conclusion, however, cannot be drawn for the SbF6- - graphite compounds since antimony fluorides are, in general, better fluorinating agents than the phosphorus fluorides. Under these conditions, reaction (B) may occur partially which might explain the higher resistivities observed in the SbF6" graphite system than in the PF6" - graphite one. DISCUSSION .The intercalation . . . of BF4", . PF . " . and . SbF6- anions, in the graphite, implies Its s~multaneous oxldlzatlon. ~hls oxldlzatlon may be reallzed by chemical way, by reducting in NO2 the very oxidizing NO2+ ion or by electrochemical way (5). In this last case,the anodic oxidization ofHOPGhasbeen made in a KPF6 molar solution in propylene carbonate milieu. The very close structural similitude of the compounds obtained by these different techniques in various solvents (nitromethane and propylene carbonate respectively) enable us to confirm that these compounds are graphite fluorophosphate. In particular, the voltammograms show an anodic peak corresponding approximatively to the oxidation of the graphite in the state C48+, for a stage II graohite hexafluorophosphate.

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Taking in account the chemical analyses, the weight uptakes and the NMR studies we can give to these graphite-fluorides compounds the ideal formula : C:3X" (CH3NO2)y (X : BF4", PF6", SbF6") with Y, depending on the stage, being comprised between 1.7 and 2.5. These values are about the sametnatthose admitted for the graphite - hydrogenosulfate compounds (6,8).No evidence of neutral NO2Xmolecules intercalated has been found. The interplanar distance allows us to calculate the thickness of the intercalated layer which seems to be imposed by the fluoride ions, the size of which being larger than that of CH3NO2. One can notice the low values of the basal plane resistivity measured in the PF6" - graphite intercalation compounds, whatever the salt (NOX or NO2X) uti I i zed. The electrochemical intercalation of solvated BF4", AsF6-, SbF6" is in progress and may be correlated with other works (7). ACKNOWLEDGMENTS The authors wish to thank Dr. Moore of Union Carbide for providing the HOPGused in these experiments.

I. 2. 3. 4.

5. 6.

REFERENCES See for example : G.A. Olah, S.J. Kuhn and S.H. Flood, J. Amer. Chem. Soc., 83, 4581 (1961). D. Billaud, A. Pron and F.L. Vogel, To be published in Synthetic Metals. C. Zeller, A. Denenstein and G.M.T. Foley, Rev. Sci. Inst., 50,71 (1979). G.R. Miller, H.A. Resing, P. Brant, r,1.J. Moran, F.L. Vogel, A. Pron and D.BILLAUD~roceedings of the 2nd International Conference on Intercalation Compounds of Graphite -Provincetown - Massachussets, 27 (1980). H.A. Resing, J.P. Reandon, D.C. Weber, P. Brand, F.L. Vogel, T.C. Wu, D. Billaud and A. Pron, Proceedings NATOSummerSchool on Magnetic Resonance in ColloTd and Interface Science, Menton, France (1980). D. Billaud, A. Metrot, P. Willmann and A. HQrold, To be published. D.E. Nixon, G.S. Parry and A.R. Ubbelhode, Proc. Roy. Soc., A_291, 324

(1966). 7. J.O. Besenhard and H.P. F r i t z , Electroanal. Chem., 53, 329 (1974). 8. A. M~trot, J. Fischer, Proceedings of the 2nd International Conference on Intercalation Compounds of Graphite - Provincetown - Massachussets, (1980).