Indian Journal of Chemistry Voi.39A, May 2000, pp. 501-506

Influence of doping on the structure and electrical properties of poly( aniline)

Al-Nakib Chowdhury*, Ruhul Amin Khan & Md. Motasser Hossain

Department of Chemistry, Bangladesh University of Engineering & Technology, Dhaka- I 000, Bangladesh

Received 5 Seprember /999; revised 5 Jan11ary 2000

Poly(aniline) has been synthesized electrochemically from an aqueous solution of aniline and perchloric acid at different constant current densities of 0.30, 0.60, 0.90, 1.20 and 1.50 mA cm·1 in order to prepare the polymers at different doped states. The doped polymers thus synthesized are found to exhibit excellent crystallinity and higher conductivity than those of dedoped one. Analysis of the studied samples by UV -vi s spectroscopy also indicates differences in the structure of doped and dedoped polymers. Effects of ambient atmosphere and heat ing on the doped polymers have also been examined based on the measurements of X-ray diffraction pattern , d.c. conductivity together with the optical studies. An irreversible deactivation of the structure and electrical properties has been observed upon heating and ageing the polymers in air.

Poly(aniline) (PA) has been known for many years 12

in various ill-defined forms such as aniline black , emaraldine, nigraniline, etc. synthesized by the chemical or electrochemical oxidation of aniline. A significant progress in the studies of better controlled material s has been achieved only in the last decadeo.x. This revival of

the interest in PA is mainly associated with the facility

of its preparation, its good stability and high potential for

technological application~. Among the organic conduc­

tors , PA is the only conducting polymer whose proper­

ties not only depend on its oxidation states but also on its

protonated state which, in turn , is associated with the pH

of the medium where it is equilibrated 111-12

• The polymer

is electroactive in solution of pH<4 and its conductivity

can be varied over I 0 order of magnitude depending on

the protonation level 13• Several oxidation states of PA

also exist, corresponding to different amine (-NH-) to

imine (=N-), benzenoid to quinoid rings ratios. These

characteristics have been pointed out to play an impor­tant role on the properties of doped PA n-l'i. Although significant advances in the understanding of PA have been achieved, still many problems in the study of struc­ture, environmental stability etc are yet to be resolved to meet the stringent requirements necessary for techn o­logical application. In this paper, we report the structural changes of PA on doping, trying to relate these changes to the electrical and optical properties of the polymer. Effects of ambient atmosphere and heating on the struc­ture, electrical and optical properties of PA have also

been discussed .

Materials and Methods Aniline purchased from E. Merck, Germany, was di s­

tilled twice under nitrogen atmosphere. Perchlori c acid (60%) and dimethylformamide (DMF) also from E.Merck, Germany, were used as received. Doubly distilled water was used as solvent to prepare all the electrolytic solu­tions.

Electropolymerization of aniline was performed with a potentiostat (Hokuto Denko, HA-151 , Japan) on a Pt electrode (6.00 x 2.75 cm·2 ) in an aqueous solution (pH - 0.6) containing 0.5 M aniline and 0.8 M perchloric acid. PA samples were synthesized at different cun·ent densi­ties of0.30, 0 .60, 0.90, 1.20 and 1.50 mA cm·2 . Neutrali­zation (dedoping) of the PA was carried out by treating the thus synthesized polymers at O.OV in an aqueous so­lution containing 0.8 M perchloric acid . Thus neutralized polymers were then doped in the same monomer free electrolyte solution at a current density at which the sample

was synthesized initially. Polymer samples thus prepared were washed with the same perchloric acid solution and then dried and stored in a des iccator under vacuum. Cy­clic voltametric studies of the PA film and the Pt e lec­trode were carried out with the same potentiostat cou­pled with a X-Y recorder (F-5C, Riken Denshi Co, Ltd , Japan) . A saturated calomel electrode (SCE) and a Pt plate were used as reference and counter electrodes, respectively, throughout the study. All the electrochemical measurements were performed at room temperature .

A conventional two probe method was employed to measure the d.c. conductivity of the PA pellets (I mm thick). Electrical contacts were made by silver paste .

502 INDIAN J CHEM, SEC. A, MAY 2000

..-.. (b) = (a) ell .._..

£ .., = Ql ...

..s

...0.2 0.0 0.2 0.4 0.6 0.8 1.0 04 14 24 34 44 04 14

Electrode PotenUal (V vs SCE) 29 (degree)

Fig. I- CV of a PA film on Pt electrode in an aqueous soluti on containing 0.8 M perchloric acid. Dotted curve shows the CV of the Pt electrode in the same electrolytic solu­tion . Scan speed was 100 mVs·'.

t;J~ (d)

,......

~ = Ul(b) . ... '-'

t-o -~

~) j..__ "' c -

ru~ 04 14 24 34 44 04 14 24 "34 44

28 (degree)

Fig. 2- X-ray diffraction patterns as a function of the Bragg angle.

28 for PA samples. (a) : dedoped and (b-0 : doped at

current densities of 0.3, 0.6, 0.9. 1.2 and 1.5 mAcm·2,

respectively.

The observed resistance for each sample was read di­rectly from an autoranging microvolt (Keithley I 97 A, USA) . IR spectra of the powdered PA were frequently obtained by mixing and grinding the samples with dry and pure KBr. T he spectra were recorded on a IR spectrophotometer (IR-470, Sh imadzu, Japan) in the region of 4000-400 cm 1

• UY- vis optical analysis of

Fig. 3- X- ray di ffraction patterns of the doped PA samples after (a) exposing to ambient atmosphere for 24 hours and (b) heat­ing in air for 12 hours.

10

-0

....... .... I

6 u

VJ 1(}-1 -t;>

~ u :I

"0 c 0 1 (}-2 u

Current Density (rnA cm-2)

Fig. 4-- Change in conductivity of PA samples with current density that employed for doping.

the PA solution in DMF was performed in a spectropho­tometer (model no. 200-l 0, Hitachi , .T apan) coupled with a synchronized recorder (model no. 200, Hitachi , Japan). The sample solutions were diluted to a vi sual extent in such a way that the optical density did not exceeded 2. The X-ray diffraction (XRD) pattern of the powdered PA samples were recorded on an automatic X-ray diffractometer (JDX-8P, JEOL Ltd, Japan) using Cu(Ni) radiation of wavelength l .54A operated at 30 KV and 20 rnA and the scan speed was maintained at 2° min ·1 •

CHOWDHURY eta/. : EFFECT OF DOPING ON POLY (ANILINE) 503

Results and Discussion Figure I shows the cyclic voltammogram (CV) of a

PA film and a bare Pt electrode in a monomer free aque­ous solution containing 0.8 M perchloric acid . The result shows that PA can be switched between its oxidised and neutral states. Colour of PA changed from deep green to yellowish-green during electrochemical switching from oxidised to neutral state, respectively. These observa­tions seem to be consistent with those of poly(pyrrole) and poly( thiophene) suggesting that anodic oxidation and neutralization of the present PA lead it to be doped and dedoped, respectively. Further more, it can be seen from Fig .! .that the CV of the present system is composed of mainly two redox couples. This behaviour indicates the presence of variable electroactive region in the polymer film . These observations are consistent with the previ­ous studies 16

-1x, suggesting that during anodic treatment

of the PA film both protonation and anion doping are indeed taking place. IR studies of the oxidised PA showed

absorption bands in the range 1170-1050 em·' and 1680-

1640 em·' indicating the presence of CI04

- ions (may

prevail as aniline perchlorates) and -N= sites which might occur by the anodic oxidation of the polymer.

Structural analysis of the powdered samples of both the doped and dedoped PA was carried out using wide angle X-ray diffraction. The X-ray diffraction patterns

as a function of the Bragg angle, 28 at A= I .54 A of the above samples are presented in Fig. 2. Measurements were done immediately after taking the samples out of the evacuated desiccator. For all the doped and dedoped samples the results show that the scattering pattern con­sists of crystalline as well as amorphous peaks. Compar­ing the results depicted in Fig.2, it can be seen that the number of crystalline peaks in the pattern of the doped PA (curve b-f) are much higher than that of the dedoped one (curve a) . However, from the result it can be consid­ered that the dedoped PA structure is mostly amorphous in nature, in contrast, its doped state exhibits excellent crystallinity. The unique crystallinity of the anodically oxidised PA samples may arise from the current density dependent morphological changes. In fact, morphologi­cal changes upon doping have already been reported by previous workers 1 ~ 20 • Excellent crystallinity of the PA samples may arise also from the existence of -N= sites in the polymer chain which might occur by the high de­gree of oxidation of the polymer. The molecular chain with this -N= group probably can move more easily to achieve regular crystalline arrangement.

The effects of the ambient atmosphere and heating on the structure of the doped PA samples are depicted in Fig.3(a) and 3(b), respectively. Prior to X-ray analysis vacuum dried powdered PA sample synthesized at the

current density of 0.6 rnA cm·2 was kept in ambient at­mosphere for 24 hours while the dried sample prepared at the current density of 0.9 rnA cm·2 was heated in air at 50"C for 12 hours. The results show that although the pattern consists of both amorphous and crystalline peaks, their structures seem to be amorphous rich. Again, com­paring the result with the findings depicted in Fig.2(c) and 2( d), it can be seen that the number and the intensity of the peaks are much higher than those appeared in Fig.3. These observations indicate that on heating or age­ing in air, the unique crystallinity of the PA decays to an amorphous rich structure. On exposure to air the poly­mer probably absorbed water to cause degradation of the film. In fact, degradation ofPA in aqueous media has already been reported to occur via the imine form of PA which subsequently hydrolyses to form quinone­hydroquinone structures2 1

•

The change in conductivity of the PA samples with current densities employed for doping the polymers is depicted in Fig.4. Conductivity at the null current density corresponds to the dedoped state of PA. The studied samples were stored soon after synthesis in an evacu­ated desiccator till the conductivity measurements com­mence. The conductivity increases by more than three order of magnitudes to reach 4.5 S em·' when a PA film was oxidised anodically. The curve in Fig.4 is character­istic of conventional doping of a conducting polymer, i.e. the conductivity rises rapidly at the beginning of the dop­ing process and then shows a little change even though the PA becomes more highly doped . This result demon­strates that the conductive doped form of PA can be obtained by anodic treatment of the polymer. The ob­served switching from insulator (dedoped) to conductive (doped) state ofPA may arise from the presence of CI0

4-

ion and -N= sites that occurred by the anodic oxidation of the PA . This argument is in agreement with the pre­vious studies'\ where imine form of PA has been re­ported as its conductive state. It is worthwhile noting that crystallinity of the samples also increases upon dop­ing of PA as can be seen in Fig.2.

Figure 5 represents the influence of ambient atmos­phere and heating on the conductivity of the PA. For this purpose, the procedure for the synthesis of PA samples and their ageing and heat treatment in air was followed exactly similar to those samples used for X-ray meas­urements described in Fig.3 except for the PA pellet that was employed in the conductivity measurements. The result shows that the conductivity of the polymer decays gradually with time for both the effects. The change in conductivity of the PA film seems to be rather faster in the initial hours . Comparing the results depicted in Fig.S ,

504 INDIAN J CHEM , SEC. A, MAY 2000

'\:_) J- ~

2 -

(a)

1 1- (b) ..() o-o-o-o o-o-o

I I I I I

5 10 15 20 25 0

Time (h)

Fig. 5- Conductivity changes ofPA samples when (a) heating in air

at 50°0 C, (b) ageing in ambient atmosphere and (C) evaCU­

ated after ageing it 24 hours in ambient atmosphere.

., c 0

(n)

100 JOO 600 900 100 300 600 900

Wnvc Length (um)

Fig. 6-- UV- vis spectra for the PA samples , (a): dedoped and (b-1) :doped at the current densiti es of0.3 , 0.6. 0.9. 1.2 and 1.5

mAcm·2, respectively.

~· ·;; c 0

(a) (b)

100 300 600 900 100 300 600 900

Wave Length (nm)

Fig. 7- UV -vis spectra of the doped PA samples after (a) exposing

to ambi ent atmosphere for 24 hours and (b) heating in air

for 12 hours.

Conduction Band

> > > .. .. .. ..... "'

0 ..... 00 ..... ..; .... .--;

Bipolaron anti-bonding

state

Bipoaron bonding state

Fig. 8- Experimental band model o f PA based on the optical

measurement.

it can be seen that on heating (curve a) the decay in conductivity is much higher than that of the polymer on ageing in air (curve b). Evidently, this suggests instability of the PA samples although a mechan ism for the deacti­vation has not been investigated fu rther in the present work. Chowdhury et a/. 22 also reported a decrease in

the work function and conductivity of the anion-doped

poly(3-methyl thiophene) film when exposed to air. Al­though deactivation of PA takes place to some extent, the result shows that PA remains fairly conductive even after 12 hours heat ing or standing 24 hours in air. How­ever, the observed deactivation of PA upon exposure to air was found to be irreversible. The result is also shown in Fig.5 (curve c). This was done by measuring the con­ductivity at every 2 hours interval prior to evacuation of the deactivated PA and the evacuation with occasional

CHOWDHURY eta/.: EFFECT OF DOPING ON POLY (AN ILINE) 505

conductance measurements were continued for 12 hours. The decayed conductivity was found not to be recov­ered under thi s circumstance indicating the possibility of a structural destruction. In fact, XRD pattern depicted in Fig.3 also suggests structural disorderliness to have oc­curred upon heating or ageing the PA in air.

UV-vis spectra of dedoped (curve a) and doped (curve b-f) PA are presented in Fig.6. DMF solutions of PA prepared immediately after taking the samples out of the evacuated desiccator were employed. The possible electronic processes that took place under the electro­magnetic UV -vi s irradiation on the samples have been reflected on the result. In the dedoped state of PA, two peaks at 335 and 450 nm and a weak shoulder around 660 nm have been observed. Although the peaks at :n5 nm seems to be unchanged, the peak at 450 nm and the shoulder observed at around 660 nm becomes prominent for the doped PA samples synthes ized at different cur­rent den sities. These observations are consistent with the previous study23 suggesting that the peaks observed in the spectra around 335 nm both for doped and dedoped

samples may correspond to the inter band n-n* (val­

ance band to conduction band) transition while the other two transitions at around 450 and 660 nm may be re­sponsible for the PA conductivity by forming bipolaron as mid-gap state. It is to be noted that the peak at 450 nm and a weak shoulder at around 660 nm observed in the spectrum of the dedoped PA seem to be unlikely. How­ever, this may arise from a little amount of dopant that might remain in the polymer even after dedoping it under the experimental conditions employed here. This argu­ment seems to support the observed conductance of the

dedoped PA which has already been reported 10" to be in

the order of I 0·7 - I 0·9 S cm·1• Observed yellowish-green

colour of the present dedoped PA, in contrast to yellow colour for the sim ilar dedoped sample reported by early workers 1, may also support incomplete dedoping of the present sample .

Figure 7 represents the optical spectra of doped PA samples after exposing the polymer 24 hours in an ambi­ent atmosphere (curve a) and after heating the polymer for 12 hours in air (curve b). The PA samples were pre­pared at constant current densit ies of 0 .6 and 0.9 mA cm·2 for the measurements depicted in Fig.7(a) and 7(b), respectively. Although the resu lt shows peaks at 335 and 450 nm, in contrast to the results presented in Fig .6(c) and 6(d), in the present case the shoulder at 660 nm seems to disappear. Since, this transition is considered to be as­sociated with the conductivity of polymer, the results in­dicate that on heating or ageing in air, a decrease in the

conductivity of PA would occur. In fact, the measured conductivity depicted in Fig.5(a) and 5(b) seems to be in agreement with the observed optical study. On heat ing or ageing in air, structural degradation of PA may take place and that may account for the observed decrease in conductivity of the samples. XRD studies with these sam­ples, indeed, showed a decrease in crystallinity upon heat­ing or ageing the polymer in air, as can be seen in Fig.3 .

Spectroscopic data also provide elucidation of band structure of PA as depicted in Fig.8. The possible elec­tronic transition that occurred upon irradiation of PA with UV-vis light have also been shown in Fig. 8. The band gap assigned for this system has been found to be 3.7 e V which is in good agreement with the previous finding 2~ .

On the basis of these studies we found phenomenological effect, i.e. the effect of doping, that controls the struc­tural and electrical properties of PA. Both crystallinity and conductivity of PA increase during switching from dedoped to doped states, respectively. An irrevers ible deactivation of the structure and electrical properties occurs upon heating or ageing PA in air. Therefore, in view of possible applications for such conductive poly­mers , it would be worth protecting them with an insulat­ing and non- oxygen-affected layer.

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