polypyrrole polystyrenesuiphonate films

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P o l j w r r In ternat iona l 31

( 1993)45-50

Electrochemical,

FTIR

and

Morpho logical Study

of

Polypyrrole-Polystyrenesu phonate

Conducting F i l m s

M. J. Gonzalez-Tejera,

M .

A. de la Plaza,b

E.

Sanchez de la Blanca

I .

Hernandez-Fuentes

Departamento dc Quimica Fisica

I ,

Facultad de Ciencias Quimicas, Universidad Complutense, 28040 Madrid, Spain

Departamento de Quimica Fisica, Universidad Nacional de Educacion a Distancia, 28040 Madrid, Spain

(Received 18 December 1991; revised version received 21 April 1992; accepted 28 May 1992)

Abstract: The electrochemical bchaviour, FTIR spectrum and

t he

morphology of

polypyrroleepolystyrenesulphonate (PPy-PSS) films obtained po tentiostatically

a t 0 . 6V in a

0.05~

y

+

1.7 x

~ O - M

NaPSS medium have been analysed. Different

electrochemical parameters have been modified in order to establish the first

polaron formation, the reversibility of the redox process and

the

doping

nicch a n ism.

The F TIR spectrum confirms the existence

of

C=O groups in the film structure

and two possible explanations are suggested. SEM reveals a higher homogeneity in

these films than

in

PPy films doped with smaller counterions.

K q .

i iwh: conducting polymer, polypyrrole, electrochemical and FTIR study.

INTRODUCTION

Polypyrrole (PPy) is t h e mo s t co mmo n co n d u c t in g

polymer because of i ts application to battery electrode

material a nd electrochroniic devices. Th e characterist ics

of

the result ing polymeric material depend on the

electrochemical conditions of synthesis. In this sense PPy

has been obtained i n aqueou s and non-aqueous media by

several authors, ' - 3 and the influence of the counterion

(e.g. size, geometry, charge, organic o r no t o rganic nature,

etc.) has been studied in some cases.4

Compo site systems can be prepared by the electro-

chemical polymerization

of

PPy in the presence of a

soluble anionic polyelectrolyte such as sodium polysty-

renesulphonate (NaPSS). l 3 Using this polyelectrolyte

it is possible to have

a

charge controllable membrane in

which the fixed charges are controlled electrochemically.6

The membrane electroneutrali ty is preserved by the

penetra tion of the e lec tro ly te ca t ion (N a+ ) in to the

PPy-PSS matrix producing a pseudo-c athodic doping.69 7

Th e s tabi l ity of the membrane genera ted depends on

the degree of entanglement of the polymeric counterions

and the po lypyr ro le s t ruc ture which is enhanced as the

oxidation state increases.

In o r d e r to establish better knowledge of the electro-

chemical mechanism

of

PPy-PSS m em bran e generation,

the a im of th is paper has been focused on producing

different internal oxidation sta tes by cyclic voltamme try.

This technique a lso g ives in forma t ion abo ut the revers i -

bility of the redox process and the porosi ty of t h e

composi te f i lm genera ted . I t s morphology has been

analysed using scanning electron microscopy and i ts

conduct iv ity by th e four -prob e method .

Spectroscopic meth ods have been used to elucidate the

s t r u c tu r e of PPy f i lms obtained with different counterions

such as BF,,14 C10, 5* 6 a n d K N O 3 . l 7The re a re several

differences, but the C=O groups presence in the f i lm

structure i s the most s ign i fican t . The

FTIR

spect rum of

PPy-PSS has been studied

to

clarify the presence of the

C=O

groups.

45

P ol j wrr .

InternarionalO959-8103/93/ 06.00

993

SCI.

rinted

in

Great Britain

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46

M . J . Gnnzulez-Tcyera M . A .

de

la Pluzu, E. Sanchez de la Blunca I . Hernundez-Fucntes

E X P E R l M

E N T A L

PPy

films

were obtained potentiostatically at

0.6

V using

times from 600 s to 4 h. Film thickness was determined by

weighing using the PPy density given by Dia z Hall.18

The anodic deposition and the electrochemical charac-

terization of PPy films were carried o ut in two different

cells. with

compartment and three electrodes in each

one.

A

saturated calomel electrode SCE) was employed

as a reference electrode. The electrical contact between

working and refercnce electrodes was through a Luggin

capillary. A Pt-mesh counter-electrode of large area was

employed and a platinum sheet

10

x 20m m2 was used as

the working electrode. Before each electrodeposition the

working electrode was sonicated in acetone 5 min) after

being treated with hot and concentrated sulphuric acid

a n d

rinsed copiously

with

ultrapure Milli-Q water.

P j ri-ole (Py j (Aldrich Chemical Co .) was previously

distillod under vacuum and stored in the dark under

reduced pressure at low temperature. Th e electrolyte used

was sodium polystyrenesulphonate (Na PPS ) (Aldrich

<.liemicnl Co.). The solution concentration was 0 . 0 5 ~

Py + 1.7

x

10-

'M

NaPSS. Th e solution w as slightly acidic

i n

order to improve the quality of the deposits5 and was

dcosygenated w i t h nitrogen. The experiments were run at

room tcmper;iture

i n

a nitrogen atmosphere.

Cj clic voltammot ric nieasurenieiits were perform ed

\ v i t h ;I Wenking 6XFR 0.5 potentiostat in conjunction

u i t h

ii

triangular wave generator Wenking model

VS

G72

and a Sefram X Y type Y t

25

recorder.

FTlR spectra of the films

in

the oxidized solid state

werc dctorniined froni it pressed disc of the sample mixed

with K B r powder, using a Nicolet 60 SX.

A n S E M IS1 DS-130 with an R-X spectrometer (ED X )

Kwex

XOOO

I I and

a

Si/Li detector were used to analyse

the morphology of the polymeric films formed.

The conducti\.ity o f the films was measure d by the fou r-

probe method.

RESULTS A N D

DISCUSSION

Figure shows cyclic voltam mo gram s of a PPy film

potcntiosta~icallygrown

i n

the conditions mentioned

in

the Experimental section and recorded in the same

medium. The voltammo grams were obtained by potential

sweeping between

-0.8

and + 0 . 6 V at a scan rate, 1 1 , of

30mV

s -

I . Four peak potentials are observed in Fig. 1.

I n the anod ic sweep two different oxidation states were

detected in the PPy matrix (pea ks A and B). Peak A

(E,(A) = -0.75 V) (where E, = peak potential) represents

a weak oxidation sta te (polaron state ) generated in the

PPy matrix, i n which electroneutrality was conserved by

penetration of PSS anion s from the electrolytic solutio n.

The valuc of this first oxidation potential is much lower

than that of polypyrrole films prepared using low

molecular weight electrolyte' an d we can attr ibu te this

effect to the macromolecular nature of the counterion. A

4 5 1 P P y l N a P S S

1.7.10-

M

I I

4 5 -

P P y l N a P S S 1 . 7 . l O - M

v = 30

mVs-1

30

-

N

E 15-

u

E

n

7 0-

-15

-

-30

-

- 0.5 0 0.5

E / V S C E )

I

I I

- 0.5 0 0.5

E / V S C E )

Fig 1 Cyclic charactcriration vo l t ammograms of PPy

in

1.7

x

1 o - ' M

NaPSS.

redox couple at peak A is not formed because thc

oxidation state generated is very weak and unstable. At

highcr positive potential peak B appears E,, B) 0.12 V),

which corresponds to a more intense oxidation state of

PPy and consequently to a higher incorporation of PSS-

anions, producing an intricate entanglement of PPy and

PSS- backbones s imilar to a me m br ar ~e .~ .~ . ').20 Further

studies by electron microscopy must be done in order to

analyse the PSS- distribution

in

PPy films.

n thccathodiccycle twopeaksarcalso present. Peak

C

(E ,(C j= -0 .7V ) is related t o the oxidized species

generated in peak B (AE,(B -

C

=

0.58

V). According to

Gen ies Sy ed ?' there is a clear irreversibility in the

formation of oxidized and reduced species, but i,,c/ip,13

remains close to unity, suggesting that the reaction is

almost reversible (where

i,,c

and i, ,13are peak C an d peak

B curren t density respectively).' Finally, peak D (E,(D) =

f0 .1 V) can be attr ibuted to the penetration of the Na

cation into the PPy- PSS membrane (pseudo-cathodic

dop ing) when the m embrane is organized.

PSS- incorpora ted durin g either electrochemical

polymerization or oxidation processes does not expe-

rience any redox reaction.

With cycling, the current densities of peaks

B

and C

decrease, because of the loss ofelectroactivity of thc PPy-

PSS membrane in the highest oxidation state, while the

current densities of peaks A and D increase.

The influence of the value of the positive and negative

end potentials at

\ ' =

30 m V s- ' on the electrochemical

behaviour of PPy films has been studied. F igure

2

shows

cyclic voltammograms obtained using a constant initial

potential El= -0.8V ) an d a variable positive end

potential, from 0.08 to

0.85V.

Q JQ ratios have been

calculated in all cases and the corresponding values were

very close to unity, indicating that all the PPy oxidized

P O L Y M E R

INTERNATIONALVOL. 31, NO. 1,1993

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Polypyrrole-polystyrenesulphonate

conducting

Jilms

47

E.

a

1 125 mA c n i

PPy I N o P S S

30 mV 5-1

I I

O 5

EIV ISCEI

0 0

Fig.

2. Cyclic voltammograms

of

a 1.13pm standard PPy layer

in 1.7 x 10 2~ NaPSS

with

different Ef

was reduced (where Q , and

Q,

are cathodic and anodic

charge respectively).

The negative end potential has also been modified, see

Fig. 3,

in

the range -0.1 to

-1.OV.

The end positive

potential was maintained constant in all cases

E ,

=

0.55

V).

From Fig.

3

it

can be seen that peak

A

clearly appears only when the sweep begins at Ei

0.7V

I

I

- 0 5 0 0 5

-50

-10

I V ISCEI

Fig. 3.

Cyclic voltammogram s

of a

1.13

p m

standard PPy layer

in

1.7 x

1 0 - 2 ~aPSS with different E .

I I

FPy11710 2M N aPSS

I

E/V SCEI

Fig.

4. Effect of potential scan rate on the cyclic voltammo-

30; ...... 50.

70. 90

grams shape:

- 9 , ,

, ,

_ _ _ - , 110

m V s - ' .

and this is the reason why in previous studies of PPy-

PSS- f ilms this peak has not been d e~ cr ib ed .~ . ' he

evolution of ip,c/ip.8ratios over the different voltamm-

ogram s shows a slow increase of resistivity in the polym er

film, which means a higher degree of oxidation as the

cath odic potential reaches a mo re negative value. Finally,

the evolution of hydrogen when the sweep begins at

Ei= .OV can be observed.

Th e variation of the potential scan rate can give further

insight into the mechanism of charge transport and

electrode processes. Potential scan rate was varied in the

range 10-130mV s - l and the an odic peak potential shifts

positively, as show n in F ig.

4.

Th e peak's cu rren t increases

as the scan rate rises. A plot of the anodic (B)and cathodic

peak (C)current as a function of v ' ' ~ s represented in Fig.

5.

A

linear relationship with

a

slope very close to

0 5

is

found, which is an indication that the electron transfer

process is controlled by a semi-infinite diffusion

condi t ion.2.22-24

I

I I

loo0

,,*

10000

3

Fig.

5. Plot of i as a funct ion of v * : --------

, p B; -

ips?

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48

M . J . Gonzalez-Tejera

M .

A . de

la Plaza, E.

Sanchez de la Blanca I . Hernandez-Fuentes

500r l

0 50

100

150

v / rnv s-1

Fig. 6. Capacitive current versus scan rate plot for a

0 8

pm PPy

film to a constant potential

0.35

V).

In agreement with K O er a/ ’ we have observed in our

cyclic voltammograms recorded at different scan rates a

decay in the faradaic currents after oxidation peak

potential. All the oxidation processes finished in the

region of 0.35 V and the current measured should arise

from a capacitive component of the current density.

Currents measured

in

this potential region (0.35 V) show

linear relationships with scan rates (Fig. 6). Fro m the

slope of this line the PPy film c apac itance w as calculated,

(b )

Fig.

7.

S E M s ofPPy

films

potentiostatically ob tained in NaPSS

medium. Time deposit: (a ) 30min;

(b)

240min.

C =

0.27 m F cm -2 . The comparison between the value of

the capacitance determined by us in PPy-PSS films and

the corresponding magnitude for PPy films with similar

thickness doped with ClO, anio ns2 ndicates tha t the PPy

films electropolymerized in NaPSS medium are less

porous. With regard

to

the assumption that th e PPy films

are composed of many essentially identical non-

interconnected channelsz5our lower capacitance value is

an indication that the polymeric counteranion closes

some

of

the channels, improving the film homogeneity.

Figure 7 shows SEM pictures of the PPy-PSS deposits

obtained during 30 and 240min with thicknesses of 1.13

and 13.4

pm

espectively. Th e homogeneity of bo th films

is higher than that obtained for PPy films in other

electrolytes with smaller

counter ion^^^-^'

and is in

agreement with the film porosity discussed above.

The measured electronic conductivity

of

the film of

13.4pm thickness was 10.8 S c m -

’.

N o change

in

the film

conductivity after 3 mon ths in contact with air was found.

FTIR SPECTRUM

Figure 8 shows the FTIR spectrum of the oxidized

PPy-PSS films

E ,

= 0 5 5

V).

Th e spectrum was measured

from 4000 to 400cm -

’,

In the high frequency region NH

an d C H stretching bands are observed. The region below

1800cm - was specially analysed because the most

significant bands of the film studied appear there. The

main characteristic ba nds observed are included

in

Table

1 , together with t he literature FT IR results for PPy films

doped with ClO, counterions.I6

Analysis of the FTIR PPy-PSS spectrum reveals the

following characteristics. Vibrations due to CH out-of-

plane bending are observed about 916,951 and 762cm -’.

A band appea rs at arou nd 1025 cm - ’ ,which is attributed

to the NH in-plane bending vibration. In the region

121( r l

1

82 cm can be found two CH in-plane bendings,

one abou t 1 204 cm -’ assigned to the CH bending of the

95.6

78.2 I I I I I I I I

1860

1700 1560 1380

1220 1060 9M

7LO r

WAVENUMBERS

Fig.

8. FTIR spcctrum of a PPy film generated

in 1.7

x I O - ’ M

NaPSS

medium.

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Polypyrrok-polystyrenesulphonate

conduct ing films

49

TABLE 1.

FTIR data of PPy sys tem

Mode PPy-PSS PPy-cIo;

(from Ref. 16)

(C-0)

stretching

(C- C)

stretching pyrrole ring

Ring stretching

(C-H)

bending

(N-H)

bending deformation

(C-H)

out-of-plane

C--C

stretching of

C,H,-C-C

group

R-SO; characteristic band

1700cm-'

1559

1466

1380

1319-1 300

1204-1 182

1025

951

91 6

762

1636

1

370-1 300.

1200-1 180

1 71 7 cm-'

1567

1482

1360

1302

1191-1 178

1040-1 035

965-964

91 0-900

781 776

polymer neutral s tate and the other about 118 2cm -'

assigned to the oxidized polymer state.

PPy ring stretching characteristic ban ds can be found at

1466, 1380 and 1319-1300cn1-' an d the most characte r-

ist ic one, a very strong band, at abo ut 155 9cm -'.

The band assignments mentioned are in agreement

with

the literature results for PPy films doped with ClO,

(see Table I ) .

Th e presence of R-SO; grou ps ca n be justified by the

strong band that appears at 120 4cm -'; there is also a

weak band at around 163 6cm -' that could be attr ibuted

to a C-C stretching band belonging to the C,H,C=C

group. This is a confirmation of the presence of PSS

groups in the polymer.

The weak band that appears at approximately

170 0cm -' is attributed to the existence of C=O groups

in

the structure . This is

in

accord with the results obtained

using others

counter ion^,'^^'^.^^

and was also confirmed

in the U V region in the case of polymerization in aqu eou s

solution. The intensity of this band can be understood

as a consequence of the very low percentage of C=O

groups in the PPy structure.

The presence of C=O groups and the electroactivity

lost could mean either a nucleophilic attack by water

molecules and/or hydroxylic groups belonging to the

medium that break the n-conjugated structure of the

polymer, o r pos sib le PPy ring r ~ p t u r e . ~

CONCLUSIONS

Two oxidation states have been detected by cyclic

voltammetry in PPy-PSS- films. T he first, appearing at

very low values of the oxidation potential, corresponds t o

a weak oxidation state; the second corresponds to more

intense oxidation. PSS- has become entangled with the

PPy matrix producing a polymeric membrane which is

very homogeneous a t the highest oxidation po tential and

with a conductivity value above that of semiconductor

materials.

The FTIR spectrum confirms the existence of a

characteristic band belonging to C=O stretching vibra-

tions. This means that C=O groups are produced in the

PPy-PSS f i lm structure , as well as in PPy films generated

in

ClO, and NO, aqueous media, and two possible

explanations are suggested.

ACKNOWLEDGEMENTS

The authors wish

to

thank DGICYT (Spain) (Project

PB89-0089) for financial suppo rt. Than ks are due to D rs

D. Rueda and

C.

Arribas (In stituto de la Materia, CSIC)

for the conductivity measurements and to Dr J. Palacios

and Mrs Bajon (Instituto de Catalisis, CSIC) for the

SEMs.

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