On the nature of the conducting state of polyaniline

Synthetic Metals, 29 (1989) E2I 1 E218

E211

ON THE NATURE OF THE CONDUCTING STATE OF POLYANILINE

M. Nechtschein, F. Genoud*, C. Menardo, K. Mizoguchi**, J.P. Travers,

and

B. Villeret DRF/Service de Physique/Dynamique de Spin (ER CNRS 216), Centre d'Etudes Nucl~aires de Grenoble, 85X, 38041 Grenoble c~dex (France)

ABSTRACT The are

role of disorder in the polyaniline properties is emphasized. Evidences

given against

a true

metallic state: the presence of two types of acidic

functions is not compatible with a regular periodic lattice ; the spin dynamics behavior

reflects incoherent carrier motion. Pulse and CW ESR data are consis-

tent with high spin concentration clusters (conducting islands), cluded

that a

conducting island

and it is con-

just consists of a single conducting polymer

chain.

INTRODUCTION Polyaniline (PANI) has emerged as a particularly interesting and complex material.

Actually, the

most fascinating

property of this compound lies in the

Insulator-to-Conductor transition which occurs as a function of the protonation level. Conductivity increases by ten orders of magnitude upon treatment in acidic solution, while the number of electrons on the polymer chains remains constant

[1-4]. Furthermore, protonation -i.e. simply addition of protons- results

in large increase in spin susceptibility [3,5]. Part of this spin susceptibility

has been

density

conducting in

shown recently

of states

of Pauli

type, giving evidence for finite

behavior of PANI as a metallic state, as proposed by Epstein et al.

terms of

value

to be

at the Fermi level [5]. It is thus tempting to describe the

of the

a polaron

lattice model.

However, other

data, such as the poor

conductivity, its hopping characteristic temperature dependence,

*Member of Universit4 J. Fourier, BP 68, 38402 St Martin d'Meres c4dex, France **Permanent address : Dept. of Physics, Tokyo Metropolitan Univ., Setagaya-Ku, Tokyo, Japan

0379-6779/89/$3.50

© Elsevier Sequoia/Printed in The Netherland,~

E212 and the large relative number of Curie spins are evidence for localization, suggest

a Fermi

conductivity

glass [6].

with

the

instead of homoqeneous lic

islands)

idea

of

protonation

a

metallic

with the observation

to the protonation

cal model considering

regions

state it has been proposed that

(insulating)

(metal-

[5,7]. This picture

that the Pauli susceptibility

is almost pro-

rate, y. It is also in agreement with a theoreti-

commensurability

effects

[8].

In the present work we report recent experimental nature of the conducting

and

the hopping features of the

there exist fully protonated domains

embedded in unprotonated

is consistent portional

In order to reconcile

results and we discuss the

rate.

RESULTS AND DISCUSSION Spin susce~t!hilityin We

the conducting

first concentrate

susceptibility,

state

our attention

on the factors which controle the spin

x s, in the conducting state. The x s data are given in Fig.l as

a function of the sample potential versus SCE, in IH HCl aqueous solution. have

been obtained

Measurements thicknesses, (prepared

using in

have as

been

well

iH HCl

situ ESR experiments

performed

as

on

a

on

sample

aqueous solution

in an electrochemical

electrochemical obtained

with (NH4)2S208

films

of

Data cell.

different

by chemical polymerization as oxidant).

The general

I0

JD

~8 u~

w6 •

i

m e~ E

o~

'*

N

6

e

I

.

u e

X~

/ 0

0./~ 0.8 potential/ V vs SCE

Fig. i. In situ ESR susceptibility vs potential of electrochemical films of di~ ferent thicknesses : i000 A (*), 2000 A (o), 7000 A (+), and chemical powder sample (~). The dashed line at 0.4 V indicates the potential of as prepared chemical samples.

E213 variation

of X s

versus V

conducting polymers.

is similar

to the

This has been discussed

doping level

formation,

should be

high

in thermal equilibrium

improved to

take into

(Xp). However,

doping

region,

considering

it

seems

lattice model

formation

[13]. The

samples, the

[9-i!~. At

account band

the decay of x s with increasing V in

that the conclusion

that only bipolarons

remain at full charge should be still valid. This statement w!th the polaron lattice model. We note that theoretical polaron

in other

at least in the case of PANI, as evidenced by the presence of Pauli

susceptibility the

this model

also observed

in previous papers and interpreted

in terms of polaron and bipolaron populations high

behavior

for which

[12], while another one concludes x s data

in favor of hipolaron

usually reported in literature

a potential

x s data from different

is not in agreement studies support the

refer to chemical

of 0.35 to 0.4 V vs SCE is measured.

authors are generally

that x s can vary in a large extent

in good agreement,

Although

it appears

:

a) As a function of the potential. Among others for the chemical samples x s can be

lowered well below the as-prepared

tion

by increasing

the potential

the X s data usually considered

sample value if one completes

the oxida-

above 0.4 V. This result gives evidence

for oxidized PANI are not an intrinsic

that

value.

b) With the sample preparation conditions. Data obtained with electrochemical films

are also

cyclic

presented

potential

scanning

NH4F,2.35 HF. One sees low.

in

Fig.1. The

in a

solution of

in Fig.l

aniline in

the eutectic mixture

that for the thinnest film x s at 0.4V is very

Thus, at least in this case, it seems clear that bipolarons

charged other

species in groups

the oxidized

[14].

state.

It is noteworthy,

behaviors between thin electrochemical a

films were deposited on a Pt wire by

function of

between

dependence the

the film thickness.

chemical

Similar results have been reported by

as evidenced in Fig.l,

that intermediate

film and chemical powder are observed as

What could be the reason for the difference

electrochemical

samples,

and

for

the film thickness

? We believe that the leading parameter is some kind of disorder

material.

Disorder

electrochemical films

and

are the basic

in

is expected

to be stronger in chemical powder that in

films. Furthermore,

it has been shown that the growth of PANI

during electropolymerization

gives rise to fractal surface,

explain the thickness dependence of the mean disorder

which could

[15].

Acidic functions A

study of

technique defined

to

[16]. We,

corresponding

functions are

imine groups,

functions

here,

acidic function

evidenced acidic

the acidic

summarize

at pK z 3, to

the main results.

In reduced PANI a well

with concentration

per ring C = 0.25, is

amine groups.

present

as expected.

(pK z 3, C = 0.14) are

in PANI has been achieved using titration

In the

oxidized state two types of

: one type at pK around 6 (C = 0.32) corresponds But in addition acidic functions of amine type

still present.

An estimate shows that about 25% of the

E214 amine groups can have two protons state can be represented

: -NH~-. The PAMI structure in the conducting

as follows

H

H

:

H+

H+

H

H+

=N H The

doubly protonated

either

nitrogen sites, which appear for pH < 3, can be thought

as regularly

Fermi

level, or

cases

their

spaced in

a superlattice,

randomly distributed,

presence

is

not

and then opening a gap at the

and then introducing

consistent

with

disorder.

In both

the proposed structure

for a

polaron lattice.

Condugtivity

and magnetism x e _ r ~

The dc conductivity aqueous

p!ptqgati£n

(q), and x s of chemical PANI samples equilibrated

solutions of

different

l

1

i

I

+~o

+

--

c o

10-'~ /

/

I

,

,

'

t

o÷ •

~

c ~u

/

+/

÷

/

/

/

10 6

/

/

/

eo

/+/

,

~1020

/

/

I

b

E "7~>,iO.2

in HCl

pH are shown in Fig.2 a) and b), respectively.

o

/

10-0

E ,0,

.

10

.

9

.

8

7

.

.

.

6

5

.

z,

3

;

:

c 111la

,

-

/

pH Fig. 2. Conductivity

(a), and susceptibility

i

I

I

I

I

i

9

8

7

6

5

I,

I

3

'

2

i

1

pH

(b) of PANI vs (#) decreasing,

and

(+) increasing equilibration pH

Equilibration

times as

long as

hysteresis

effect,

increasing,

or decreasing values.

y

the

data

protonation

depending

40 hours

superpose, process

on

which

itself.

whether However,

indicates The

have been the

pH

used. We has

been

notice a clear obtained

from

if they are plotted as a function of that

protonation

the rate

hysteresis

concerns the

is,

the relevant

thus,

parameter. In

Fig.3, ~ and spln susceptibility

function linear

of y. scale has

Measurements been chosen

(Pauli : x9, Curie : X c) are given as a

have been performed with a SQUID susceptometer. for o,

instead of

the log

A

plot usually used.

E215 o

"i',80 o%

1.5x1020 E

o

o

3

'E

g

I

.4(

Oo

2 /

.,Q 4-,..

•

> ..--

)

aJ

o

--1 t~

s

°:/

lo/O

,i! ,

, .....

0.1

/N

ul, I !

0.2 0.3 0'.4 0'3

Fig. 3. Susceptibility

as measured by SQUID : Pauli

function of the protonation

rate.

The conductivity

(,), and Curie vs protonation

(o), as a rate is repre-

sented by a hachured area to take into account

the large spread in the data.

Taking

the

into

represented apparent

a

threshold volume

percolation

transition.

The

like

data

spread

of

This can

of

The advantage

behavior

which for

xp and

of

are

data,

the

~

variation is

of the linear plot is to make

conductivity,

corresponds

xp

Epstein et

change

conducting

large

y = 0.3,

reported by

sudden

to

the

hachured area.

around

ratio.

already any

account by a

with a percolation

to Pc ~ 0.6 for the conducting

consistent with a linear variation, as

al. [5]. The striking point is the absence of xc,

when

crossing the Insulator-to-Conductor

be explained by non homogeneous protonation, giving rise

islands

(clusters)

embedded

in

an

insulating

sea. Further

evidences for this picture are supplied by the spin dynamics data.

Spin dynamics versus protonation An

analysis of

another tion

the spin

contribution to

on the

dynamics in

the conducting state is presented in

this conference [17]. Here, we concentrate our atten-

evolution of

the spin

dynamics as a function of the protonation

level. Firstly,

we analyze

(T2~), and spin been

measured by

spin

motion

frequency,

the vacuum

sealed sample data of Fig.4. The spin spin

lattice (TI~) relaxation rates, are given versus y. They have pulse ESR.

spectrum,

f(~).

These two With

-i Tle

relaxation rates probe the electronic the

motion

is probed at the Larmor

w e, while T2e-I also includes contribution at zero frequency. At high

-i This is evidence for strong y a narrow line is observed, and one has T -i l e = T2e.

E216 l

I

l

1013 - 0

i

O,

o °-'6''.!o o /" •

8O

v~

i

4~ ~

/

,P

OO~o -oo

/

"0

,,

7 4C

1011

~ 1010

i

o I

~

oo

o o o o o

.0- 0

[]

13 ~

/

7,

0

t~

ii

ul

1012

c:~ 10 9

10-1

/

/

6

c~ 10 8

o

D~ D,,

-

Im o

20

107

e,,

A

°

106 ,o J o o 0.1'

0:2 o'.3 oi y (=C,,N)

0.I

: T!~

rate.

' ' 2 0.3 0.4

' 0 I" 0.5 6

y (=OIN)

PANI samples ; inverse spin-spin relaxation

the protonation

R.T. 10-4

Fig. 4. X band ESR linewidth for sealed under vacuum

lattice relaxatlon

10-3

PANI

/ o

e oz

, ,

o.',

10-z P

• o

(o) and opened to air (o)

time : T2~

(o) and inverse spin-

(~) for sealed under vacuum samples,

The relaxation

as a function of

times Tle and T2e have been measured by

pulse ESR. Fi~. 5. Parallel

(o), transverse

(~) spin diffusion rates,

: DI/D # (o) as a function of the protonation

anisotropy

rates D# and D I have been determined from measurements

and motion

rate.

The diffusion

of the proton relaxation

time T 1 as a function of the frequency.

motional

narrowing

and

for

f(~e ) = f(0).

For

y < 0.3

the linewidth (T2~)

increases with decreasing y, while Tle remains roughly the same, so that one has

- i >> - I T2e Tle

interactions decrease

of ~H

interactlons high

of

~c << We"

those both

low

at low

are still

low

of

weak inter-chain obtained from and low

The ~H

linewidth on

is

not

deuteriation),

originated nor

in hyperfine

in g anisotropy (no

fields). Thus we can state that at low y the spin spin responsible for

the linewidth,

which is evidence for

clusters. Furthermore one has f(O) >> f(~e ), which is

dimensional

Thus, for

in high

y.

change

spin concentration

typical

very

at

(no

behavior,

isolated clusters coupling. These the T 1

NMR data,

with

a

we deduce

very

low cutoff frequency :

low dimensional motion with

results are in complete agreement with which evidence one dimensional motions

y regions [17]. The motion is highly anisotropic in the

E217 low

y region

magnitude have

(D# /D± m 104), and the anisotropy

for y

to

face

interactions ?

This problem

cluster is

statement

is consistent

occurs

hroadning

upon

paramagnetic

of a

around air

data of

the opened

y = 0.3.

collisions.

we discuss

to air

can

is At

be

features.

number. We

with spin

dynamics,

~ = 80 S/cm.

This

value. Taking

hoppings,

D# should

obtained.

We are,

due low

only

the

value

the conductivity

D#

just

be replaced

of

mean free

a metallic

an

Line

of Tle by

clusters are

is observed.

state to the light of the for

the

diffusion

c = nce2D#c2/kT,

for the bipolarons,

order

of

magnitude

that the determined

[18]. Assuming a metallic

is inconsistent Spin

follows.

surface

that real conduction

path along the chains,

state.

as

rate

where n c is

which are not

dynamics

larger than the

is

spin dynamics does restate, from the density

and from D~ we calculate the

I*. The obtained value

with any coherent motion, reflects

One

implies inter-chain

by D±, and good agreement with experiment

thus, confident

cell distances,

low y the

is the same, and we take n c = 0.4/ring.

into account

also the charge motion

carrier

the

found

assume that

is

At

but a dramatic

to a drastic shortening y

states at the Fermi level : 1.5 state/eV/ring,

unit

samples.

the short Tle of the surface spins is rapidly

From

we estimate

charge carrier

of

for the existence of

explained

the nature of the conducting

(D/ ~ 2.1012 rad/s)

concerned

This

contamination

the

flect

arguments

to the bulk. Then, the effect on the linewidth

dynamics

measured

than the inter-chain

single magnetic chain. We note that this

with the thermodynamic

on the

oxygen

Finally,

obtains

can the inter-cluster

explained easily if we admit that a given

can be

For y above percolation

transmitted

spin

How

in lowering the anisotropy

are about the same as for the vacuum sealed samples,

increase

affected.

as shown in Fig.5. Then, we

question.

areas.

now comment

linewidths

crossover,

puzzling

just composed

fully protonated We

percolation

following

he more efficient

interactions

spin

near the the

drops by more than 2 orders of

: i* = 6.10 -3 characteristic

a motion which is intrinsically

incoherent.

CONCLUSIONS Our

data

islands. have

very

chains. isolated formed

are

special

conducting upon charge

effects.

disorder chains.

we

Polyaniline

bipolarons, end

consistent

However,

but the As

shape :

the

they

presents

model

are

nothing

this unique

chains.

In

injection. bipolaron

shown also

should be

with

were led to the conclusion

a

At

conducting

that these conducting

islands

but single conducting polymer

property to

content almost ideally

all protonated

chain, polarons are

high doping level they tend to combine into

formation

by the

taken into

given

of fully protonated

is hindered by disorder

incoherent

account in

nature

of the

the description

and/or chain

spin dynamics,

of the conducting

E218 ACKNOWLEDGEMENTS We thank K. HOLCZER and the BRUKER Company for the pulse ESR experiments and A. PRON for preparation of deuterated samples. REFERENCES 1

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