In situ electron spin resonance and cyclic voltammetric studies of polyaniline

Synthetic Metals, 29 (1989) E227-E234

E227

IN SITU ELECTRON SPIN RESONANCE AND CYCLIC VOLTAMMETRIC STUDIES OF POLYANILINE

S. M. Yang~ and T. S. Lin Department of Chemical Engineering, National Central University, Chung-Li (Taiwan)

ABSTRACT The conductivities of polyaniline can degree of oxidation and conductions

the

were suggested

degree

be

controlled by

adjusting the

of protonation. Both polaron and bipolaron

for polyaniline.

Epr is the direct method to

distinguish between these two conducting mechanisms.

In-situ electron spin

resonance studies of polyaniline at various applied potentials in media of pH 0.49, 2.09, 3.96, and 9 are reported. The g factor of the epr signal is 2.0036 and does not vary with the applied

potential.

decreases to a minimum in a potential

The peak to peak linewidth

range which depends on the pH of the

media. On the other hand, the spin density increases to a maximum in the same potential range. The pH-potential range where the highly conductive form exists matches very well the range of narrow linewidth and high spin density of the epr signal. Cyclic voltammograms were also studied in media of various pH. The potential ranges between the first and the second redox processes in media of various pH also match with the potential ranges where the highly conductive form exist. We propose that the conductive form of polyaniline consists of highly mobile radical cations or polarons existing between the first and the second redox processes in cyclic voltammograms.

INTRODUCTION Recently considerable attention has been focused on polyaniline with respect to its potential as a conducting polymer [i-4]. Among the electro-conducting organic polymers studied, polyaniline is the only one whose conductivity can be controlled by adjusting two independent variables namely the degree of oxidation and the degree of protonation

0379-6779/89/$3.50

[5,6]. Depending on the oxidation state,

the

© Elsevier Sequoia/Printed in The Netherlands

E228 structure of polyaniline , and reduced amounts

of oxidized

conductive

and reduced

[4]. Polyaniline

electrodes devices

consists in various

units, -NH- B - N H - B

After acid-doping,

for e l e c t r o c h r o m i c

the conducting

Both p o l a r o n

[I0] and b i p o l a r o n

direct method

to distinguish

voltammograms

of polyaniline

corresponding

units.

display

mechanism

[ii] c o n d u c t i o n s

useful as thin film

[ 8 ], and e l e c t r o n i c

media of different conductivity

various

Epr is the

mechanisms.

two redox processes,

The cyclic

the redox reactions We report here

applied potentials and cyclic voltammograms

pH. Comparing

studies,

is not clear.

were suggested.

to these two redox processes are not clear [12,13].

in-situ epr studies at

of equal

this form is highly

of polyaniline

between the two conducting indicate

units, -N=Q=N-B

form consists

is stable in air and potentially

[7], e l e c t r o d e

[9]. However,

ratios of oxidized

[4]. The e m e r a l d i n e

optical

the results

spectra and c y c l i c

conducting mechanism of polyaniline

in

of epr studies with those of the voltammetric

studies,

the

was further

purified

by

liquid

was

can be clarified.

Experimental Chemicals Aniline,

obtained

distilling obtained. without

several

Hydrochloric further

from M e r c k (reagent

times o v e r acid

zinc

grade),

powder

, obtained

until

a colorless

from Merck was reagent grade and used

purification.

In-situ epr measurement Polyaniline

was formed under a constant applied

SCE on a 0.3 mm p l a t i n u m wire in I aniline. HCI

HCI aqueous

The platinum wire with polyaniline

solution

performed

of the a p p r o p r i a t e

in an e l e c t r o l y t i c

spectrometer. current

M

on

On a p p l y i n g a potential,

reached a steady value.

solution

hours.

applied

and the measurements

was then decreased

0.1M

The e x p e r i m e n t

the epr signal was recorded

The applied potential was until

potential

0.7 V versus

with was

with a Bruker ER 2000D 10/12 epr

the epr signal was recorded repeatedly

spin density was calculated

of

containing

it was then equilibrated

pH for s e v e r a l

epr flat cell,

potential

after the

then increased

a maximum applied

potential.

were repeated.

and The The

from the product of peak intensity and the square of

the peak to peak linewidth and DPPH was used as the reference. Cyclic voltammetry Polyaniline solution

was

formed

of 0.i M aniline

out under constant controlled was

then

washed

voltammograms with a REO091

with

on a 1.6 mm diameter

in 1 M

hydrochloric

potential

hydrochloric

Pt working

electrode

The syntheses

of

appropriate

pH and

in a

were carried

at 0.7 V versus SCE. Polyaniline

acid

were obtained with a PAR model XY recorder.

acid.

the

362 scanning potentiostat

formed cyclic

equipped

E229

RESULTS AND DISCUSSION In-situ epr study As reported signal

previously

without

fine

is shown in figure

constant

The g factor

potential.

The variation

when the a p p l i e d

the epr spectrum of polyaniline

structure.

vary with the a p p l i e d free electron.

[14],

is

around

2.0036 and does not

This v a l u e a p p r o a c h e s

the g factor of the

of peak to peak linewidth with applied

1. In a m e d i u m of pH 0.49, the l i n e w i d t h

potential

shows a single

is b e l o w 0.05 V, decreases

when the applied potential

remains at 3 gauss

to 2 gauss and remains

is above 0.I V. Similar trends are observed

in media of pH 2.09 and 3.96. In a m e d i u m of pH 2.09, the l i n e w i d t h around 3 gauss below 0 . 1 V and

linewidth

drops d r a m a t i c a l l y

remains

remains

and then decreases gradually

potential

at 1.3 gauss

at 1.3 gauss between

gauss at 0.45 V.

above

0.3 V. In medium

0.15 V and 0.3 V and increases of the l i n e w i d t h

narrowing of the epr line profile.

at

of

pH 3.96,

the

gradually

to 3.5

may be due to the m o t i o n a l

If the hopping rate for spins is faster than

the inverse of the epr time scale (relaxation time),

u1 c3 cr~

remains

with increasing applied

from 4.75 gauss at 0.05 V to 2 gauss at 0.I V and

The d e c r e a s i n g

averaged magnetic environment.

potential

the spin will experience an

This results in a narrower epr line profile.

4

T3 o

C3

'"

Z

2

1 00 0.1 0.2 0.3 0.4 0.5 -0.1 APPLI ED POT E NTIAL(V vs SCE ) i

~

i

,

,

,

i

i

i

Fig. 1. The variation of peak to peak linewidth of the epr signal p o t e n t i a l in media of pH 0.49,o ; 2.O9,e ; 3.96, v .

with applied

E230 The variation of the spin density with the applied potentials in media of pH 0.49,

2.09,

and 3.96 are shown

in figure

2. As shown

in figure

2a, when

polyaniline in the reduced state is oxidized, the spin density remains low until the

applied

dramatically

potential

reaches

0.2

V;

the

spin

density

then

increases

to a maximum v a l u e at 0.22 V then decreases to a constant v a l u e

above 0.3 V. When polyaniline is reduced the spin density remains at a constant v a l u e until

the potential

drops to 0.2 V, the spin density increases

to a

m a x i m u m at 0.18 V and then decreases dramatically. The maxima at 0.22 V in the oxidation

cycle

and 0.18 V in the reduction

c y c l e are due to the p o l a r o n

formation and the subsequent decreasing of spin density to a constant

value

corresponds to the formation of the polaron lattice as suggested by Epstein and coworkers [15]. Similarly, in figure 2b, the spin density increases above 0.16 V and reaches a maximum at 0.28 V; it then decreases gradually up to 0.5 V in the oxidation cycle. In the reduction cycle the spin density remains at a constant v a l u e down to 0.18 V and passes a m a x i m u m at 0.i V and then a p p r o a c h e s zero b e l o w -0.13 V. In figure 2c, the spin density increases a b o v e 0.i V, after passing a m a x i m u m at 0.19 V and a p p r o a c h e s zero a b o v e 0.4 V in the oxidation cycle. The spin density remains around zero in the reduction cycle and increases below 0.13 V. After passing a maximum at -0.04 V, the spin density drops to near zero below -0.I V. The applied potentials in these experiments cannot exceed 0.5 V, since the degradation of the polymer occurs above this potential. Hysteresis is observed in the epr signal upon reduction. The same by M a c D i a r m i d

[16],

Genies

c l e a r l y understood [16].

[17],

and G l a r u m

phenomenon was observed

[18 ]. This p h e n o m e n o n is not

Comparing figure 1 with figure 2, the

decreases when the signal intensity increases.

linewidth

The narrow linewidth appears in

the potential range where the spin density is significant. The results indicate the spin density increases in the same potential range where the mobility of the electron spin increases. medium of pH 9,

We also performed the

in-situ epr experiment with a

no epr signal was observed in the potential range studied.

Cyclic Voltammetry Figure

3 shows the effect of the pH of the m e d i u m on El/2 of the redox

processes. The result is similar to those reported p r e v i o u s l y

[13]. The peak

potential of the first anodic wave (and the corresponding cathodic wave) is pH independent in the pH range of 1 to 5 and shifted to higher v a l u e at lower pH (not shown in the figure). The r e s u l t s indicate that

in the pH range of 1 to 5

the oxidation process does not involve proton addition or elimination. At pH i, the reduced form is not protonated (Kb=10-14 for diphenyl amine) so the first redox process in cyclic voltammogram corresponds to the following equation:

E231

÷ (I)

The formation of the radical cations was supported by the in-situ epr studies. In the potential range between the first and the second redox waves, significant spin densities were observed. When the pH of the medium is lower than i, the amine form is protonated. The oxidation process forming the radical cations involves the loss of proton, and the E]/2 is pH dependent.

80 k--

>-

60

4O

< 20 l--

m ci.-

i

i

i

i

i

I

I

i

i

I

I

1

I

i

< 20 >'-15

I---

z W

10 5

Z i

u~ 40

A

Of

LL]

(c)

30

w 20 _J

W I

-O2 -0.1

0.0

0.1 0.2

03

0.4

0.5

APPLIED POTENTIAL(V vs SCE) Fig. 2. The variation of spin density with applied potential in media of pH 0.49, (a); 2.09, (b); 3.96, (c).

E232

The peak potential of the second anodic wave (and the corresponding cathodic wave) is pH dependent. The slope of the plot is 0.109 V per pH unit. The results indicate that the

reaction involves the loss of two protons per electron. The

following reaction corresponding to the second redox process was proposed:

( - ~ N H~ ~

-ne-

-)n-2nl-+> I (-N=~=N~ ) n

+

(2)

This result was also supported by the in-situ epr study. Beyond the potential of the second anodic wave, the epr signal vanished.

0,7 W

~

Q5 ¸

~I/3 0.4 I=_

> >

0.3

0.1 o

0

i

2

i

[] ,

3 pH

i

i

4

i

5

Fig. 3. Dependence of El/2 of the first redox process, m, and the second redox process,+ , on pH of themedia.

Conducting mechanism The conductivity of polyaniline depends on the

degree of protonation and

the degree of oxidation. In figure 3 the pH of the medium where the conductivity

g233

are

greater

potential

than

1 o h m - l c m -I

are

plotted

(data taken from reference

form show a b s o r p t i o n potential optical

potential

voltammograms

densities

6c). The optical

recorded

The

in media

oxidation

are significant

significant

of

various

potential

and

and electron

form consists

voltammogram.

the d e l o c a l i z e d

of o x i d a t i o n

pH

mobilities

of highly

mobile

in figure

redox w a v e s

pH is p l o t t e d

the

matches

form as d e t e r m i n e d

plotted

range

cations

radical

that the conductive (polarons)

between the two redox waves in the cyclic

I

the

epr

The results

cations

4 for spin in the

indicate

as shown by the The conductive

range between the first and the second redox waves

We propose

radical

in the c y c l i c

are large are plotted

one another.

by

4. The

in figure

where

spin density and narrow linewidth of the epr signal.

form exists in the potential in cyclic

6c) is a l s o

the first and the second

same figure. The ranges approximately the conductive

range

spectra of the conductive

to the c o n d u c t i v e

(data from r e f e r e n c e

ranges between

comparison.

the

m a x i m a at 800 nm and 420 nm. The range of the o x i d a t i o n

and the pH c o r r e s p o n d i n g

spectrum

against

I

form of polyaniline

which is the s t r u c t u r e

is

existing

voltammogram.

I

•

I

i

!

~-"

I

,7 JJ

- J

t p ~'1

7- 5

_

9

-

.,"~

,,7o

,,9

-0.6

I

I

I

- 0.2

0.2

0.6

E/V

vs

10

Ag/AgC[

Fig. 4. A p H - p o t e n t i a l d i a g r a m that shows the range of p r o t o n a t i o n states and oxidation states for the conductive form of polyaniline to exist. --- represent the range where c o n d u c t i v i t y > 1 S , o represent the range where the optical spectrum show m a x i m a at 420 and 800 nm, v represent the range between the two redox waves of cyclic voltammograms,a represent the range determined using an epr signal.

CONCLUSIONS The results different

of in-situ epr studies at various applied

pH indicate the highly mobile

radicals

potentials

in media of

exist in a pH-potential

range.

E234 Comparing the results obtained from conductivity studies, and cyclic voltammetric

studies,

optical spectra

studies, the pH-potential

range where the

radical exists matches well with the range where the conductive form exists.

ACKNOWLEDGEMENTS The financial support from NSC, Republic of China, under the grant NSC760208-M008-15 is greatly acknowledged. The authors thank professor C. P. Cheng for helpful discussion and kind assistance of the experiment.

REFERENCES i. A. G. MacDiarmid,

J. C. Chiang, M. Halpern, W. S. Huang, S. L. Mu, N. L. D.

Somasiri, W. Wu and S. I. Yaniger, Mol. Cryst. Liq. Cryst., 121 (1985) 173. 2. W. R. Salaneck and I. LundstrSm, Syn. Met., 13 (1986) 291. 3. W.R. Salaneck, B. Liedberg, O. Ingan~s, R. Erlaudsson and I. Lundstrom, Mol. Cryst. Liq. Cryst,

121 (1985)191.

4. J. C. Chiang and A. G. MacDiarmid, Syn. Met., 13 (1986) 193. 5. E. W. Paul, A. J. Ricco and M. S. Wrighton, J. Phys. Chem., 89, (1985) 1441. 6. (a) P. M. McManus,

S. C. Yang,

and R. J. Cushman,

J. Chem.

Soc., Chem.

Commun. (1985) 1556. (b) P. M. McManus,

R. J. Cushman and S. C. Yang, J. Phy. Chem., 91 (1987)

744. (c) R. J. Cushman, P. M. McManus,

and S. C. Yang, J. Electroanal

Chem., 291

(1986) 335. 7. A. F. Diaz and J. A. Logan, J. Electroanal. Chem., Iii (1980) 111. 8. T. Kobayashi, H. Yoneyama, and H. Tamurq, J. Electroanal. Chem., 161 (1984) 419. 9. E. M. Geni~s and M. Lapkowski, Syn. Met.~ 21 (1987) 117. i0. G. E. Wnek, Syn. Met., 15 (1986) 213. ii. M. Kaya, A. Kitani and K. Sasaki, Chem. Lett. (1986) 147. 12. W. S. Huang, B. D. Humphrey and A. G. MacDiarmid,

J. Chem. Soc. Faraday

Trans. 1, 82 (1986)2385. 13. W. W. Focke, G. E. Wnek, and Y. Wei, J. Phys. Chem., 91 (1987) 5813. 14. S. M. Yang and S. H. Chien,

J. Chin. Chem.

Soc.,

33 (1986) 285.

15. A. J. Epstein, J. M. Ginder, F. Zuo, R. W. Bigelow, H. S. Woo, D. B. Tanner, A. F. Richter, W. S. Huang, and A. G. MacDiarmid, Syn. Met., 18 (1987) 303. 16. A. G. MacDiarmid, J. C. Chiang, A. F. Richter, and A. J. Epstein, Syn. Met., 18 (1987) 285. 17. E. M. Geni~s and M. Lapkowski, Syn. Met., 21 (1987) 117. 18. S. H. Glarum and J. H. Marshall,

J. Phys. Chem., 90, (1986) 6076.