Competitive doping in polypyrrole

Synthetic Metals, 41-43 (1991) 295-299

295

COMPETITIVE DOPING IN POLYPYRROLE

D.J. WALTON, D.M. HADINGHAM, C.E. HALL, I.V.F. VINEY and A. CHYLA* Department of Applied Physical Sciences, Coventry Polytechnic, Priory Street, Coventry CVl 5FB, U.K.

ABSTRACT The electrooxidation

of pyrrole in electrolytes containing both perchlorate

and tetrasulphonato copper (II) phthalocyanine anions produced polypyrrole films from aqueous methanol that were TSCuPc-rich. methods gave similar results.

Potentiostatic and galvanostatic

However, a similar procedure in acetonitrile pro.

duced films that were perchlorate-rich.

Electrical conductivity of the films

mirrored composition, agreeing with the trend that perchlorate-rich films have higher conductivities than phthalocyanine-rich ones.

INTRODUCTION Polypyrrole is particularly tolerant of dopant anion [I], and electrooxidation of pyrrole in various electrolyte medium can produce free-standing polymer film containing not only simple anions such as perchlorate (CI04), tetrafluoroborate

(TFB) or p-toluene sulphonate (PTS), but also containing

relatively esoteric anionic species such as poly (styrene sulphonate) tetrasulphonato copper (II) phthalocyanine

[2] or

(TSCuPc) [3,4].

We now report the results of a study into the competitive doping of polypyrrole whereby the polymer is prepared from an electrolyte solution containing more than one dopant anion.

The anionic species chosen were perchlorate and

TSCuPc and two solvent systems: acetonitrile and aqueous methanol were employed.

EXPERIMENTAL Pyrrole (Aldrich) was freshly distilled prior to use.

Potassium perchlorate,

*Permanent address: Institute of Physical and Organic Chemistry, Technical Univer sity of Wroclaw, Wroclaw, Poland

0379-6779/91/$3.50

© Elsevier Sequoia/Printed in The Netherlands

296 tetra n-butyl ammonium perchlorate, phthalocyanine

3,4',4",4"'tetrasulphonato

tetrasodium salt and p-toluene

reagents or the best available

equivalent.

(II)

The tetra n-butyl ammonium salt of

TSCuPc was prepared by double decomposition hydrogen sulphate and the tetrasodium

copper

sulphonic acid were analar grade

between tetra n-butyl ammonium

salt of TSCuPc by mixture of the salt in

water followed by extraction with dichloromethane.

The deep blue dichloromethane

solution was dried over anhydrous magnesium sulphate and evaporated

to dryness

giving fine blue crystals. Polypyrrole

films containing C104 and TSCuPc anions were produced both galvano-

statically and potentiostatlcally

in an undivided cell using indium tin oxide

(ITO) coated glass anodes and stainless

steel or platinum cathodes.

monomer was present at 0.1M; and electrolyte

concentrations

0.02M for perchlorate

and from OM to 0.005M for TSCuPc

thereby corresponding

to a similar number of charges).

Galvanostatic

Pyrrole

varied from OM to

(which is a tetraanion,

control was maintained by a Thurlby PL 320 galvanostat and a _2 was passed for 1 hour. Electrolyte solutions at

current of density of imA cm perchlorate/sulphonate employed.

ionic ratios of 0:100, 25:75, 50:50,

Experiments were performed

perchlorate respective

and the tetrasodium

in 3:1 methanol/water

75:25 and 100:0 were using potassium

salt of TSCuPc; or in acetonitrole

using the

tetra n-butyl ammonium salts.

Potentiostatic

control was maintained by an EG&G MODEL 273 Potentiostat against

a saturated calomel reference electrode 3:1 aqueous methanol.

(SCE).

Experiments were all performed

in

A series of films was prepared at the same electrolyte

ratios as above, all at +1250 mV (vs. SCE); then a second series of films was prepared at a constant

sulphonate

to perchlorate

tials varying between +1000 and +1500 mY.

All

ionic ratio of i:i, at potenelectrolyses

were maintained

until the passage of 10 Coulombs of charge. Free-standing

films were examined

for compostion by elemental microanalysis

and energy dispersive analysis of X-rays

(EDAX).

The temperature

film conductivity was also measured over the temperature four probe

dependence

of

range 293- 77 K, by the

technique using J2E signal generator at i000 Hz [5,6].

RESULTS AND DISCUSSION Film composition

from aqueous methanol

Table i gives the anion content of polymer films obtained both galvanostatically and potentiostatically acetonitrile medium.

from aqueous methanol,

as a function of CI04/TSCuPc

Data is obtained from EDAX measurements

Table 2 gives supportive microanalytical statically-prepared analytical

films.

techniques

and galvanostatically

ratio in the preparative

from

electrolyte

of C1 and S.

data for representative

galvano-

Some discrepancy between the bulk and surface

is evident but the trend is clear.

297 TABLE 1 The atomic percentage

ratio of sulphur and chlorine determined

analysis and room temperature

conductivity

of galvanostatically

statically prepared PPy films from different Solvent, method of deposition

Film a) number

from EDAX

Film thickness, ~m

and potentio-

solvents. EDAX atomic percentage S CI

Room temp. conductivity, S/cm

Aqueous methanol, galvanostatically

i 2 3 4 5

30 30 20 52 58

12.45 35.44 63.73 70.83 69.87

83.90 45.64 6.45 3.42 -

0.72 0.56 0.35 0.07 0.05

Acetonitrile, galvanostatically

1 2 3 4 5

18 33 30 25 93

1.60 5.43 10.32 22.81 79.37

96.70 92.13 87.23 73.19 7.32

24.15 11.65 7.09 5.71 0.01

Aqueous methanol, potentiostatically at 1250 mV

i 2 3 4 5

20 27 20 28 18

4.68 61.55 66.85 67.91 79.38

92.02 18.26 16.13 13.16 3.87

0.90 0.08 0.07 0.06 0.03

Aqueous b" methanol ) potentio" ~ statically cj

1 2 3 4

15 15 20 35

65.11 64.50 66.85 68.01

17.80 17.17 16.13 13.23

0.06 0.06 0.07 0.05

a) Respective anion CI0~/SO S ratio: 1 = i00:i, 2 = 75:25, 3 = 50:50, 4 = 25:75 and 5 = i:i00. b) Constant electrolyte ionic CI04/SO S = 50:50 ratio. c) Respective deposition potentials: 1 = 1000 mV, 2 = ii00 mV, 3 = 1250 mV, 4 = 1400 mV and 5 = 1500 mV. The films obtained as expected,

from 100% solutions

but the striking

containing

feature of this compositional

of the film from the i:i CI0~/SO S anion mixture. of anions, which corresponds perchlorate

either CI04 or TSCuPc are study is the behaviour

Here, despite the equivalence

to a 4:1 molar excess of the more mobile monovalent

ion, the ensuing film is almost completely doped with TSCuPc.

This result implies that it is possible

to produce polypyrrole

films contain-

ing unusual or esoteric dopants by employing only a small amount of this species. Moreover

from EDAX data in Table i, it can be seen that only a slight trend

towards increased

sulphur occurs at higher potentials,

fluence of electrode potential upon film composition fluence of electrolyte Film composition

suggesting

that the in-

is much less than the in-

anion ratio.

from acetonitrole

The compositional

data from mixed-anion

electrolytes

is also given in Table

I, and here it can be seen that the equal CI0~/SO S system now produces that are perchlorate-rich. therefore observed.

films

An inversion of anion selectivity with solvent is

This may be connected with solubility and solvation

factors,

298 TABLE 2 The atomic percentage galvanostatically Electrolyte composition of films

ratios of the elements present in three representative

produced films.

Percentage of elements by elemental microanalysis, found (calculated) mass % C H N C1 S Cu 0

100% Pc in 49.84 3.62 13.87 aqueous (49.79)(4.6) (14.52) methanol 100% KCI0 in aqueous me thano 1

5.55 2.20 23.01 (5.54)(2.74)(22.80)

42.42 3.10 12.16 10.36 (41.33)(4.33)(12.05)(10.17)

i:I mixture of anions a)

1.91 -

50.83

3.65

13.90

Empirical ratios a)

0.19

0.11 -

-

31.86 (32.12)

4.54

2.55

24.34

The empirical formulae of these films was determined, pyrrole rings present to counterions was determined.

formulae

16Py:IPc:21H20

3Py: ICI04:3H20

so the number of poly-

since the tetrabutyl ammonium salt of TSCuPc is not greatly soluble in acetonitrile, and during electrolysis the anode is observed.

some agglomeration

However,

of strongly coloured TSCuPc at

this material washes readily from the poly-

pyrrole film and is not incorporated

as dopant.

Film conductivities Presented solvent

in Table 1 conductivities

(exemplified by 100% CI04) are higher than 3:1 methanol/water;

general Cl04-rich

films have higher conductivities

diminution of conductivity anions

of the films obtained from acetonitrile

has been previously

than TSCuPc-rich

and in

ones.

This

observed for other poly-sulphonate

[7]. (It should be noted here that passage of too-high currents

through

Cl04-rich films can produce explosive decomposition.) It is also seen that electrode potential produces

little effect upon

conductivity within the deposition potential range of +I000 to +1500 mV. However,

the anomalous conductivity

galvanostatically

from water/methanol

having almost exclusively

for the equal-ionic-ratio

film prepared

is closer to that of Cl04-rich

film despite

TSCuPc as the dopant.

CONCLUSIONS Potentiostatic

and galvanostatic

containing perchlorate tetrasulphonato polypyrrole anion,

preparations

and tetrasulphonato

phthalocyanine

anion.

copper

of polypyrrole

from mixtures

(Ii) phthalocyanine

anions in

This procedure allows the production of

containing an esoteric anion by use of only small amounts of this

the electrolyte

readily available salt.

properties

of the solution being bolstered by a more

299

In acetonitrile are obtained

the reverse behaviour

from mixed electrolyte

controlled by choice of preparation Preliminary suggests

behaviour,

distinct

dependence

of conductivity also

doping procedure offers a means to influence

and the conductivity

films

showing that selectivity may be

conditions.

studies on the temperature

that the competitive

composition

is observed and perchlorate-rich

solutions,

characteristics

of polypyrroles

the

to produce new

from the effects of anion identity normally imposed during

polymerisation.

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Handbook of Conducting

2

D.J. Walton et al, J. Chem. Soc. Chem. Commun.,

Polymers,

T.A. Skotheim,

3

R.A. Bull, F.R. Fan and A.J. Bard, J. Electrochem.

4

D.J. Walton,

C.J. Hall and A. Chyla,

5

A.R. Blythe,

Polymer Testing,

6

G. Wegner and J. Ruhe, Faraday Discuss.

7

A. Diaz, Chemica Scripta,

New York 1986.

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