Formation of graphite oxide

Synthetic Metals, 34 (1989) 157-162

257

FORMATION OF GRAPHITE OXIDE

M. MERMOUX Science des Surfaces et Mat~riaux Carbon,s, INP Grenoble,

BP 75, F-38402,

associ~ au CNRS (UA 413)

Saint Martin d ' H ~ r e s

(France)

Y. CHABRE Laboratoire de Spectrom~trie Physique, Universit6 Joseph ~ourier-Grenoble Saint Martin d ' H ~ r e s

associ~ au CNRS (UA 08)

I, BP 87, F-38402,

(France)

ABSTRACT Preparation of graphite oxide in nitric acid media has been performed and analyzed step by step. Evidence is given that graphite oxide is obtained by hydrolysis of an intermediary compound which is the result of an overoxidation of the second stage nitric acic GIC. A preliminary characterization

of

this intermediary compound is presented.

INTRODUCTION Interest

in graphite

oxide has

positive electrode material

increased

after

in lithium batteries

it has been proposed [I]. Electrochemical

as

tests

have clearly shown that it compares well with graphite fluoride in terms of electrical capacity,

as well as for energy and power density

[2].

From 13C NMR studies we have shown recently that graphite oxide contains covalently carbons

linked epoxy

[2,3].

An

(etheral) groups,

average

formula

close

hydroxy groups and sp 2 coordinated to C400H

has

been

agreement with the earlier hypothesis of Hofmann and coworkers It is generally accepted that graphite oxide

(hereafter

proposed,

in

[4].

G.O.)

can only be

obtained by oxidation of graphite acid salts, such as nitrate or bisulfate, with strong oxidizing agents such as KCIO 3, KMnO 4, CIO 2, sis

is enforced

electrochemical

when reminding way:

that G.O.

for example

can also be obtained

electrochemical

sulfuric acid leads to the formation of the first which givesG.O,

by overoxidation

etc...This hypothe-

oxidation

through

an

of graphite

in

stage graphite bisulfate

[5]. But very scarce information

has been

given on the mechanism of this overoxidation.

0379-6779/89/$3.50

© Elsevier Sequoia/Printed in The Netherlands

158 We focus our attention on these questions, mine how w e r e

particularly in order to deter-

formed the epoxy and hydroxy groups.

doing step by step synthesis with X-ray diffaction,

This has been obtained

in nitric acid, as in the Brodie process

FTIR spectroscopy,

[6],

13C NMR and DTA studies at differ-

ent steps to characterize the intermediary compounds. SAMPLES PREPARATION In our previous

studies

modified Brodie method

G.O.

preparation

[7]. In this study,

was carried out according

to a

in order to examine the different

steps of the synthesis we stopped the reaction before adding KClO 3 and before the washing

of the product.

For this,

from Madagascar,

10~m grain size)

is put in concentrated nitric acid,

anhydrous

KCIO 3 is added gradually.

then

and finally dried at room tempera-

Great care is taken to prevent atmosphere exposition dur-

ing all this process. atmosphere,

(natural,

Then it is washed several times with

nitric acid and filtrated,

ture under vacuum.

powder

The solution is heated at 70°C-80°C un-

til the powder turns to a green color. concentrated

the graphite

When treated with water,

or within hours at ambient

this green compound thus obtained turns to a yellow one which is

G.O.. Samples have also been prepared in this way but with heavy water

(D20)

in the latter step, for comparative studies by NMR and FTIR. RESULTS AND DISCUSSION X-ray diffraction In fig.l are shown the diffraction various

steps

of the G.O.

profiles

preparation.

It

of the products

is known

that

obtained

graphite

at

reacts

spontaneously with nitric acid to give a second stage graphite intercalation compound la),

(GIC) [8]. This GIC is observed with

in agreement

with previous

results.

an 11.15 A Ic parameter

The green compound,

potassium chlorate has been added to the solution, close to 8 A

(Fig

obtained when

has an Ic parameter value

(Fig. ib), whereas the yellow compound obtained after action of

the water or exposure to ambient ed in fig Ic, characteristic

atmosphere gives the X-ray profile present-

of G.O.,

with an Ic parameter value depending

on the water content of the sample [7]. A value close to 6 A is observed for the the well dried G.O. samples. FTIR Spectroscopy The FTIR spectra of the of the G.O. pellets

preparation

pressed

with

compounds obtained at the above mentioned

are presented

KBr,

in Fig 2. They

under an argon atmosphere.

were The

steps

recorded, spectrum

from

of the

159

50

40

30 20

20

10

i 50

4O

3O 20

I!

2O

10

!

~

I

,

i

,

~

50

J

,

i

,

I

I

4O

Fig.

I.

second

to

stage

nitric

O-H

groups.

,

J

I

Proof

of

and

hydrogen

chlorate

one obtains

ance of a new band at 1070 cm ported by Hadzi and Novak its

mode

the

I

,

i

in fig.2a:

the strong band

at 1375

the band at 3400 cm

can be assigned

bonding

different

between

the

at 2940 cm-i[8].

the spectrum -i

oxide.

presented

of the

the appear-

of C-O bonds,

as reon this

O-H

may be characteristic

After action

in fig.2b:

HNO 3

[9]. The band at 1375 cm -I is still present

intensity

of

,

c) graphite

is reported

of NO 3 ions

is given by the band centered

but

,

I0

green compound,

potassium

spectrum

i

of: a) second stage nitric acid GIC

molecules

stretching

,

20

patterns

acid GIC

zs characteristic the

,

20

X-ray diffraction b) intermediary

cm

,

3O •

c

has

increased,

groups.

The

as

compared

water-washed

to

that

compound

of

the

presents

the

FTIR spectrum 2c: it is that of G.O., characterized by another band at 1370 -i cm , due to the appearance of acidic tertiary C-OH groups. An increase of the

relative

amount

of the O-H groups

firmed by measurements

on D20 washed

bonds with a stretching that Hadzi

and Novak

tons in G.O. that

part

washing

of

samples,

in which

This

has been

case one observes

mode at 2560 cm -I. This must be compared

failed

can be replaced the

is also observed.

protons

to exchange

proton

with deuterium

by strong basic reagents of

step of the preparation,

the

G.O.

structure

only.

comes

conO-D

to the fact

in G.O.:

pro-

One can conclude

during

the

which rmlst be seen as a hydrolysis.

water-

160

~000

~0

3~80

a§ZO

Z~60

Z~O0

I~0

1~eo " I|20

#60

k~AVENUMBER

Fig. 2.

FTIR spectra of: a) second stage nitric acid GIC; b) intermediary green compound;

c) graphite oxide; d) deuterated graphite oxide.

13C high resolution NMR We have done preliminary 13C CPMAS measurements of the compound obtained by action of the potassium chlorate on the second stage nitric acid GIC. Spectra of this overoxidized second stage GIC are similar to that of G.O.. In both cases they are characterized by three lines, ppm from TMS, respectively sp

2

hybridized carbons

shifted by 60.2, 71.2 and 132

attributed to epoxy groups , hydroxy groups and

[3]. But

the

intensity

of

the

line due to the hy-

droxy groups is smaller in the intermediary compound than in G.O.. In agreement with the results of the FTIR,

this confirms that part of the protons

comes from the final water-washing step.

161 DTA analvsis

The thermal behaviors of the various samples are presented in fig.3. Concerning the second stage nitric acid GIC

(fig.3a) the presence of an endo-

thermic peak which begins at 50°C shows that the compound turns to a higher stage one on heating. The DTA of G.O. 100°C,

assigned

to its dehydration,

(fig.3c) shows an endothermic peak at and a strong exothermic

corresponding to its exfoliation. A similar behaviour in the case of the intermediary compound,

one at 240°C

(Fig.3b)

is observed

but one can notice that there is

no endothermic peak which could be attributed to dehydration or to nitric acid

deintercalation.

present

in this

It

seems

that

intermediary green

all

the

chemical

groups

compound are covalently

which

are

linked to the

carbon skeleton. Exfoliation f

so

Ex follation

~

IOO

200

~sfl

*

('C)

so

i I

,-4

50

a

Fig.

3.

~00

200

;SO

1fct

~

b

DTA o f : a) second s t a g e n i t r i c compound,

i~o

/~o~

75o T(c}

e

a c i d GIC, b) i n t e r m e d i a r y g r e e n

c) graphite oxide

CONCLUSIONS

The preliminary results reported in this paper show that action of the potassium chlorate on the second stage nitric acid GIC leads to an intermediary compound which is transformed in graphite oxide by an hydrolysis. new green compound contains covalent C-O bonds, have sp 3 coordinated carbons.

The observation we have done to date is

sufficient enough to have a clear picture of this compound. have not yet clearly Ic parameter

This

so its carbon skeleton must not

In particular we

identified the chemical groups which are present. The

(close to 8 A)

is consistent with a first stage compound in

which nitrate groups would be covalently bounded to a puckered carbon skeleton.

The

(i00) diffraction line leads to a 1.52 A value for the C-C dis-

162

tance, G.O.

as in G.O.. whereas

For the first time we were able to prepare deuterated

previous

reports

have clearly

shown that exchange

of proton

with deuteron does not occur when G.O. is simply put into heavy water. This confirms that part of the O-H groups present in G.O. comes from the hydrolysis final step in the formation process. So,

as proposed by Fischer and coworkers

[I0], it seems that the charge

transfer compounds which can be formed by reacting graphite with electron acceptors can be classified in two distinct groups:

the ones obtained from

weakly oxidizing species which give GICs in which the guest species is ionic and in which the transferredcharge is delocalized, leading to a metallic behavior;

the others obtained from overoxidation of the first ones which are

covalent insulator compounds in which the transferred c h ~ e s

are localized on

the C-O bonds. In this latter case the carbon skeleton is no more planar but puckered. According

to these

results,

chemical

overoxidation

graphite bisulfate GIC by potassium permanganate,

of the

second

stage

as done in the Staudenmai-

er or Hummers and Offeman synthesis method to prepare G.O.,

also would give

a covalent compound similar to the one that we described in this paper. Concerning this new green compound, phite nitrate,

that we think to be a first stage gra-

further studies are currently in progress in order to deter-

mine its stoichiometry and to get a detailed scheme of its structure.

REFERENCES

1

Ph. Touzain, R. Yazami and J. Maire, French patent 83.8266

2.

M. Mermoux,

3.

M. Mermoux and Y. Chabre , ~oceedings of the International

(1983) Ph.D. Thesis,

INP, Grenoble,

Colloauium on Lavered Comoounds.

France,1988

Pont ~ Mousson .France. 8-10

march 1988. D. Gu~rard and P. Lagrange eds., Universit~ de Nancy, France, 1988, pp 261. 4.

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Z. Anora. u.

Allaem. Chem.. 291 (1957) 205. 5.

J.O. Besenhard and H.P. Fritz, Anaew. Chem. Int. Ed.Enal..22

6.

B.C. Brodie, Ann. Chim. Phvs,, ~

7.

R. Yazami, Ph. Touzain, Y. Chabre, D. Berger and M. Coulon,

(1983) 950. (1855) 351.

Rev. Chim. Min.. 22 (1985) 398. 8.

Ph. Touzain,

9.

D. Hadzi, and A. Novak, Trans. Faraday Soc., 51 (1955) 1614.

i0

J.E. Fischer, Brucker,

Synthetic Metals. A. Metrot,

1 (1979/1980) 3

P.J. Flanders,

Phys. Rev. B23 (1981) 5576.

W.R. Salanek and C.F.