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Electrochimica Acta 52 (2006) 723–727

Effects of dopants on percolation behaviors and gas sensingcharacteristics of polyaniline film

Ke Xu, Lihua Zhu ∗, Jing Li, Heqing Tang ∗Department of Chemistry, Huazhong University of Science and Technology, Wuhan 430074, PR China

Received 4 April 2006; received in revised form 13 May 2006; accepted 25 May 2006Available online 12 July 2006

bstract

Effects of dopants on the correlation between the polyaniline (PAn) film resistance (R) and the reduction charge (Q) injected to the PAn film wasnvestigated in dry acetonitrile solutions during electrochemical reduction by using the double potential step method. The R–Q correlation behavess S-type curves, leading to the determination of the critical reduction charge (Qc). The latter represents the reduction charge required for theormation of a continuous partially reduced phase in the PAn film. It was observed that the PAn film doped with sodium dodecylbenzene sulphateSDBS) yielded a smaller Qc than that doped with perchlorate, when the PAn films were electrochemically reduced under given conditions. The

esistance of the pre-doped PAn film will increase significantly when the film is injected with reduction charge more than Qc. Hence, a smaller Qc

eans that the film can respond to very light reduction (or dedoping), being indicative of better sensing ability toward alkaline and reducing gases.his was confirmed by the increased sensitivity of the PAn/SDBS sensor toward 100 ppm NH3 vapor, compared with the PAn/ClO4

− sensor.2006 Elsevier Ltd. All rights reserved.

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eywords: Polyaniline; Dopant; Gas sensor; Ammonia; Percolation theory

. Introduction

The discovery of conducting polymers has opened up manyew possibilities for devices combining optical, electrochemicalnd conducting properties [1]. These potential wide applicationsriginate from the changes in the electrical and optical propertiesf conducting polymers when they are chemically treated withxidizing or reducing agents. Among the conducting polymers,olyaniline (PAn) is appealing because of environmental stabil-ty, high electrical conductivity and excellent redox properties2]. After chemical treatment with alkaline or reducing agent,An can change from an initial electrically conducting state to annsulating state. This transition can be used in such applicationss chemical sensors detecting alkaline and reducing vapors [3].

Direct detection of NH3 and hydrazine is possible by using

onventional PAn, and the sensitivity can be further increased byabricating PAn nanofiber [4,5], partially attributed to increasedurface area, which allows fast diffusion of gas molecules into

∗ Corresponding authors. Tel.: +86 27 87543432; fax: +86 27 87543632.E-mail addresses: lhzhu63@yahoo.com.cn (L. Zhu),

qtang62@yahoo.com.cn (H. Tang).

ottcoldi

013-4686/$ – see front matter © 2006 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2006.05.060

he structures. In the case of the sensing, the detection pro-ess involves the redox or doping/dedoping of PAn. Thus, theopants, playing an important role in the intrinsic redox charac-eristics [6,7], will influence the sensing characteristics of PAn.or example, a blend film doped with bis (2-ethyl hexyl) hydro-en phosphate yields higher sensitivity and better reversibilityoward NH3 compared with that doped with HCl [8]. Although its known that the selection of dopants is important for PAn-basedas sensors, there has not a simple general method to evaluatehe effects of dopants on the sensitivity of PAn toward the tar-et gas, being unfavorable to fabricating high-performance gasensors.

Percolation theory can be used to study electric propertiesf conductor–insulator composites [9,10], being introduced toxplain the slow relaxation process of PAn [11–13]. Our previ-us work demonstrated that the reduction time dependence ofhe resistance of bulk film at a given reduction potential obeyshe scaling law of percolation theory [14]. The percolation pro-ess in the electrochemical reduction of PAn is similar to that

ccurring in PAn film being treated with a reducing or alka-ine gas [15]. The microstructure and distribution of two phasesetermines the change of film resistance [14], which shouldnfluence thereby the sensing properties of a resistance-type

724 K. Xu et al. / Electrochimica A

Fig. 1. Equivalent circuit of PAn film electrode during reduction. Rs is solutionrPta

stocecIsdttdo

2

tar

r0APbaacp

wcpoiE

i

wto

tttcett

6ts

aftoibcsgtgA1pdCattcitsbc

3

3fi

dpmtt(crfi

esistor, Cct and Rct represent the capacitor and charge-transfer resistor at theAn/electrolyte interface, Cf and Rf are correspondingly the capacitor and resis-or of the resistive PAn film, and Cox is the capacitor of the capacitive PAn filmt a less reduced state.

ensor. This inspires us to declare the percolation characteris-ics of the PAn film functioning as a gas sensor to a reducingr alkaline gas. The above-mentioned analogue between thehemical and electrochemical reduction processes allows us tovaluate the sensing ability of PAn film to NH3 by using electro-hemical methods, such as double potential step method [14].n the present work, by using double potential step method totudy the reduction process of PAn film doped with differentopants, we have observed that the potential dependence andhe dopant dependence of the percolation threshold, which signshe “conductor–insulator” transitions in reduction. These depen-encies have been further found to affect on the sensing abilityf PAn-based sensors toward NH3 gas.

. Experimental

Aniline (A.R.) was distilled under vacuum before used. Ace-onitrile (HPLC grade), anhydrous lithium perchlorate (A.R.)nd sodium dodecylbenzene sulphate (C.P.) were used aseceived.

PAn films were grown on an electrochemically cleaned Ptod as working electrode (with a geometrical surface area of.16 cm2), along with a Pt coil as counter electrode, and ag/AgCl electrode in 3 mol dm−3 KCl as the reference. The

An film growth was achieved by 70 cycles of potential cyclingetween −200 and +850 mV (versus Ag/AgCl/3 mol dm−3 KCl)t 50 mV s−1 in a solution of 1 mol dm−3 HCl + 0.2 mol dm−3

niline with or without addition of 1 mmol dm−3 sodium dode-ylbenzene sulphate (SDBS). After the polymerization, the pre-ared films were cleaned and then vacuum dried at 50 ◦C.

The change of the resistance of the PAn film was measuredith the double potential step method during the reduction pro-

ess in a 0.2 mol dm−3 LiClO4 acetonitrile solution. The princi-le and operation steps of this measurement were described inur previous work [14]. Based on the equivalent circuit shownn Fig. 1 (cf. Fig. 3 in our previous work [14]), we can deriveq. (1),

s = A1 exp

(− t

)+ A2 exp

(− t

)+ A3 exp

(− t

)(1)

τ1 τ2 τ3

here A1, A2, A3, τ1, τ2 and τ3 are expressions includinghe interfacial parameters of Rs, Rct, Rf, Cct, Cf and Cox. Thebtained step current curves were fitted by using Eq. (1), and

tvdr

cta 52 (2006) 723–727

hen these interfacial parameters were calculated. At the sameime, the charge consumed during the reduction at a given poten-ial, being referred to as the reduction charge, was recorded byhronoamperometry. Similarly, a Pt coil was used as counterlectrode, and a Ag/Ag+ (0.01 mol dm−3) electrode was used ashe reference, which the electrode potentials were referred tohroughout the present work unless specified elsewhere.

All the electrochemical operations were conducted on a CHI60A electrochemical analyzer (CH Instruments). All the solu-ions were degassed with nitrogen gas for 15 min before mea-urement, and a nitrogen blanket was kept during the measuring.

To evaluate the influence of dopants on the sensing char-cteristics of the PAn film toward NH3, concept sensors wereabricated as follows. On an ITO-coated glass substrate, a pat-ern of two parallel strips of ITO was formed by chemical etchingf the conducting ITO layer. The ITO strips were about 4 mmn length and 3 mm in width in the working area, and the gapetween the two parallel strips was about 10 �m. After beingleaned, PAn film was deposited on the patterned ITO sub-trate by using potential cycling method for the electrochemicalrowth of PAn film on the Pt electrode as described above. Athe end of the electrochemical growth, we could observe that theap between the two parallel strips was fully covered with PAn.fter that, the PAn film was oxidized at 0.6 V for 120 s in a freshmol dm−3 HCl solution and vacuum dried at 50 ◦C for 2 h. Therepared concept sensor was set in a 250 ml vessel filled up withry N2, and the terminals of the two ITO strips were connected aHI 660A electrochemical analyzer, by which a dc voltage waspplied between the two strips (as a two-electrode system) andhe current was recorded at the same time. The measurement inhe dry N2 atmosphere was taken as a control. After then, theurrent was recorded after a given amount of 25 �l NH3 wasnjected from bottom of the vessel (The NH3 concentration inhe box was evaluated as 100 ppm). The applied dc voltage waset at 1.2 V in each measurement. The resistance of the PAn filmetween the two ITO strips was calculated from the recordedurrent and the applied dc potential.

. Results and discussion

.1. Influences of dopants on percolation threshold of PAnlm during reduction

For a PAn film being reduced at given potentials, the depen-ences of the film resistance on reduction time and reductionotential were obtained by using the double potential stepethod. Under the same reduction conditions as the film resis-

ance measurement, we recorded chronoamperometric curves inhe course of the reduction, from which the reduction chargesi.e., the charges consumed during the specified reduction pro-ess) were calculated by integration of the current over theeduction time interval. In this way, we obtained the values oflm resistance and reduction charge in pair under given reduc-

ion conditions. At each given reduction potential, a series of thealues of film resistance and reduction charge in pair could beetermined by varying the reduction time. Then, we plotted theesistance of PAn film against the reduction charge as shown in

K. Xu et al. / Electrochimica Acta 52 (2006) 723–727 725

F uctioS

Fisg[sr

fitgipa(cetBcrdosaia(f

tpgtrsrrcd

bctsp(ttpctoic

mmivwd(c

ρ

wtvct

l

w

ig. 2. Dependences of PAn film resistance on reduction charge at various redDBS as dopant. An iso-reduction-time line is given for guide.

ig. 2. It should be noted here that we did all the measurementsn 0.2 mol dm−3 LiClO4 acetonitrile solution (aprotic organicolvent), which is aiming at avoiding any effects of the hydro-en evolution at high cathodic overpotential in aqueous solution16] and in turn ensuring that all of the injected charge is con-umed by the reduction of PAn chains, leading to increase of theesistance of the PAn film.

It can be seen from Fig. 2 that the resistance of the bulklm increases with reduction time at a given reduction poten-

ial, and with cathodic shifting of the reduction potential for aiven period of reduction time, being similar to that observedn aqueous solution [14]. According to the model of reductionrocess [14], which is based on the percolation model [11–13]nd electrochemical simulated conformational relaxation modelESCR) [16], the injected electrons compensates the positiveharges in oxidized PAn chains at the very initial stage of thelectrochemical reduction, resulting in decreasing of conduc-ivity of PAn film and forming of partially reduced clusters.eyond a certain period of reduction time, the partially reducedlusters increase in volume to produce a continuous partiallyeduced phase, in which the retained conducting clusters areispersed. This description match with the optical micrographf PAn film reduced at the potential of −0.2 V in acid aqueousolution [15]. The continuous partially reduced phase hindersnd even blocks the reduction of conducting clusters distributedn the film. Therefore, a deeper reduction requires a more neg-tive potential of reduction and/or a longer time of reductionmore experimental support to the above descriptions can beound in our previous work [14]).

It is rational that the resistance of PAn film increases dras-ically when partially reduced domains grow into a continuoushase in the previous oxidized PAn film. Therefore, at each ofiven reduction potentials, the film resistance increases little ashe reduction charge is increased at the very early stage of theeduction because the generated partially reduced domains aretill scattered in a continuous conductive phase. At about the

eduction charge for the formation of the continuous partiallyeduced phase, the electro-connection between substrate andonducting clusters is cut off, the resistance of PAn film increasesrastically. At this point, the gradient in the Rf–Q curve tends to

acfd

n potentials in acetonitrile solutions in the presence of (a) perchlorate and (b)

e maximum (being indefinite in theory). This value of reductionharge at this point may be takes as the percolation threshold (inhe percolation theory), corresponding to turn-on/turn-off tran-itions of the conducting pathway. The reduction charge at theercolation point can be defined as the critical reduction chargeQc). When the reduction charge is increased further beyond Qc,he charge transfer inside the film is realized by internal chargeransfer reaction between the conducting cluster and insulatinghase, which make movement and coalescence of conductinglusters [15]. Moreover, due to the increased amount of par-ially reduced phase in volume fraction and increased extentf the reduction, the film resistance increases further, but thencreasing becomes gradually slower. This leads to S-type R–Qurves as shown in Fig. 2.

From the S-type R–Q curves shown in Fig. 2, we can deter-ine the values of Qc. However, the relative errors for theeasurements around Qc are possibly great due to the signif-

cant changes of R in the region near Qc. It is better to get thealues of Qc by mathematical fitting based on the measured dataith greater reduction charges beyond Qc, following the wayescribed below. By using the scaling law of percolation theoryEq. (2)) [9], the resistivity of PAn film (ρf) during reductionan be represented as:

f = ρi

(ϕ − ϕc

1 − ϕc

)s

(ϕ > ϕc) (2)

here ρi and ϕ are the complex resistivity and the volume frac-ion of the insulating component in the PAn film, ϕc is the criticalolume fraction or the percolation threshold. Assuming that theontent of the partially reduced phase is linearly dependent onhe consumed reduction charge, Eq. (2) can be transformed as:

n Rf = ln

[ρi

l

A

(1

Qtot − Qc

)s]+ s ln(Q − Qc) (3)

here l/A is the geometric factor of PAn film (l is the thickness

nd A is the sectional area), Qtot is an amount of all the redoxharge (the charge required when the fully pre-reduced PAn isully oxidized at 0.2 V versus Ag/Ag+), Q is the reduction chargeuring the reduction and Qc is the critical reduction charge. Con-

726 K. Xu et al. / Electrochimica A

Fr

s(ri

ssodcdtptdpst

dtw

Fa1

rphtatipfittitwthiptraSwl

3t

iiwisrt

ig. 3. Critical reduction charges of PAn film doped with two dopants at differenteduction potentials.

equently, fitting the experimental data in Fig. 2 by using Eq. (3)indicated by the solid curves in Fig. 2.), we obtained the criticaleduction charges at different reduction potential as illustratedn Fig. 3.

It is clearly seen from Fig. 3 that as reduction potentialhifts to more negative values, the value of Qc decreases, pos-ibly due to the heterogeneity of reduction arising from greaterver-potentials and faster reduction. For visualized the effect ofopant, we compare the Rf–Q curve of PAn film doped with per-hlorate and SDBS in Fig. 4. This comparison indicates that theopant influences the percolation threshold and the changes ofhe resistance of PAn film. In contrast to the PAn film doped witherchlorate, the PAn film doped with SDBS needs less chargeo approach the percolation threshold, and the bulky dopantepresses the change of resistance after being reduced for aeriod of time (∼20 s). This difference is possibly due to thetrong interaction between the SDBS and the amine or imine inhe PAn chains.

Generally, the lower mobility and lower solvation of the bulkyopant counterions lead to a higher static interaction betweenhe counterions and the charges present in the PAn chain [17],hich would limit the reduction and retard the increase of the

ig. 4. Plot of resistance of PAn film doped with two dopants vs. reduction charget the same reduction potential of −0.6 V. The values of Qc were obtained as.17 and 1.81 mC for PAn/SDBS and PAn/ClO4

−, respectively.

di

cifpdcltcOtvntfit

i

cta 52 (2006) 723–727

esistance of PAn film under same reduction conditions com-ared with that for small dopant anions. In the meantime, theydrogen bonding between the N–H group in PAn chains andhe O–S group in SDBS [18] will promote this effect. Schmidtnd co-workers [19] identified that the incorporation of SDBS inhe PAn framework would prevent complete deprotonation of themines by dedoping with aqueous ammonia solution. The incom-lete deprotonation resulted in remaining bipolaron states in PAnlm, being vanished after consecutive dedoping by LiOH. The

wo distinct amine–imine species, alternating every one or twoetramer units and possessing different SDBS binding affinitiesn PAn backbone, were identified as the primary reason. Oncehe hydrogen bonding is broken up at some sites, this regionill become poorer conductive domain, i.e., more resistive par-

ially reduced domain. It is certain that destroying of this kind ofydrogen bonding requires much less reduction charge, thus Qcn the case of SDBS as dopant is smaller than that in the case oferchlorate as dopant. Moreover, because the lower mobility ofhe bulky dopant counterions and the difficulty in the completeeduction of the partially reduced domains or phase, it is easilyccounted for that the resistance of the PAn film pre-doped withDBS was considerable smaller than that of the film pre-dopedith perchlorate when the film was subjected to reduction for a

ong period of time at a same reduction potential.

.2. Sensing characteristics of two PAn-based sensorsoward NH3

Polypyrrole (PPy) has been well used in sensors for detect-ng NH3 dissolved in aqueous solution [20]. It was reported thatn an electrochemical sensing system, DBSA doped PPy filmas reduced by ammonia in solution [20]. The bulky dopant

ncreased the change of film resistance than chloride or othermall anion dopants at a given amount of ammonia, whichesulted in higher current response at the subsequently oxida-ion process and further lower detection limit than the chlorideoped films. The merit of the DBSA doping in PPy film is sim-larly observed for PAn-based gas sensor in our work.

In a PAn sensor toward NH3 gas, a pre-oxidized PAn filman be dedoped or even reduced by contacting NH3 gas, lead-ng to increasing of the film resistance. This sets the foundationor a PAn-based sensor toward NH3. In order to prepare a high-erformance sensor, we need a strong NH3-concentration depen-ence of the PAn film resistance and a response of the film tooncentration of NH3 as lower as possible, which determines theower limit of detection of the sensor. Based on above discussing,he injection of electrons in the electrochemical reduction willause the formation of the continuous partially reduced phase.nce the continuous partially reduced phase is formed, the resis-

ance of the PAn film will increase drastically. The smaller thealue of Qc, the less the reduction charge required to induce sig-ificant increasing of the PAn film resistance. This means that inhe case of NH3 sensor, the smaller the value of Qc for the PAn

lm, the smaller the threshold detection concentration level of

he sensor.Because the dopant can influence the value of Qc, we can

mprove the sensing ability of the sensor toward NH3 at low

K. Xu et al. / Electrochimica A

Ft

csptdwNogstt

ts2ficw(Nsw(r

4

trm

ccobtduwpoaUPt

A

dc

R

[

[[[[[[[

ig. 5. Response curves of PAn/ClO4− and PAn/DBSA sensors upon exposure

o 100 ppm NH3 vapors.

oncentrations. The PAn film pre-doped with SDBS requires amaller Qc for the formation of continuous partially reducedhase, which represents a great response in the film resistanceo the less reduction charge. This suggests that a PAn film pre-oped with SDBS can respond to lower concentration of NH3,hich inspired us to fabricate PAn-based concept sensors forH3 as described in Section 2. As shown in Fig. 5, the resistancef the SDBS-doped PAn sensor is increased to considerablyreater values compared with the perchlorate-doped PAn sen-or upon their exposure to 100 ppm NH3 vapors. This confirmshat the former possesses sensing ability toward NH3 better thanhe latter does.

It should be noted that the response time of the sensors seemso be several minutes as shown in Fig. 5. However, Fig. 5 cannothow the real response time of the sensor. As described in Section, during the response measurement, the concept sensor wasxed in the rather large box filled with dry nitrogen gas. Theontrol of the gas atmosphere was rather simple in our presentork. At the starting of the measurement, the gas to be detected

NH3) was injected at a corner of the box, and then the injectedH3 will diffuse from the corner to the sensor, which may take

everal minutes until a steady state is achieved. This is the reasonhy the time response of the sensor that seems to be very long

several minutes). If a constant NH3 flow is used instead, theesponse time of the sensor will be much shorter.

. Conclusion

The correlation between the film resistance and the reduc-ion charge injected to the PAn film during electrochemicaleduction was investigated by using the double potential stepethod. As reported previously [14], when the reduction

[[

[

cta 52 (2006) 723–727 727

harge is injected to the PAn film, a resistive layer composed ofontinuous partially reduced phase is formed at the early stagef reduction, which blocks the further reduction of remainedetter conductive domains. The reduction charge required forhe formation of such continuous partially reduced phase wasetermined as critical reduction charge Qc. It was found thatnder the same reduction conditions, the PAn film pre-dopedith SDBS yielded a smaller Qc than that of the PAn filmre-doped with perchlorate. This suggests that the responsibilityf the PAn-based sensor toward ammonia (or other alkalinend reducing gases) can be improved by using a larger dopant.sing concept sensors, we confirmed that the SDBS-doped

An sensor is more sensitive to diluted ammonia vapors thanhe perchlorate-doped PAn sensor is.

cknowledgement

This project was supported by the Scientific Research Foun-ation for the Returned Overseas Chinese Scholars, State Edu-ation Ministry of China.

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