charge transfer processes at poly-o-phenylenediamine and poly-o-aminophenol films

Electrochimica Acta 50 (2005) 1573–1585

Charge transfer processes at poly-o-phenylenediamineand poly-o-aminophenol films

O. Levina, V. Kondratieva, V. Maleva,b,∗,1

a Department of Chemistry, St. Petersburg University, Petrodvoretz 198504, Russiab Institute of Cytology, Russian Academy of Sciences, St. Petersburg 194064, Russia

Abstract

A comparative study of charge transfer at electrodes modified with the title polymer films was performed mainly in 1 M perchlorate solutionsat variable pH by using different electrochemical methods. Experiments with Nafion®-coated electrodes were carried out to elucidate the roleof the supporting electrolyte anions, also exploiting the results of EQCM measurements. In both films, the charge transport is complicatedby background conductance that most likely results from bulk residual charge. Both polymers are doped by hydrogen ions and the effects ofsupporting electrolyte anions are negligible. The results obtained from low-amplitude measurements showed that the charge transport throughb ncy range,m concerningt©

K

1

bepttpmppdiTsi

andona-

re tod im-ithinro-ainslms,apa-ionsrds,e traps

deas-lmsre-

der-th de-spec-

0d

oth polymer films is accompanied by irreversible injection processes at the film interfaces. A capacity dispersion in the low-frequeost likely related to the existence of different states of hydrogen ions in the film, was observed for both polyamines. The problems

he treatment of the impedance data obtained for slow injection processes are also discussed.2004 Elsevier Ltd. All rights reserved.

eywords:Electroactive polymers; Cyclic voltammetry; Impedance spectroscopy; Chronoamperometry; Injection processes; Porosity

. Introduction

Electrochemically prepared conducting polymers formedy compounds with conjugated� bonds are of a great inter-st to modern electrochemistry, due to a wide spectrum ofossible applications. This is the reason why a lot of elec-

rochemical studies are devoted to elucidation of the chargeransfer mechanisms at the electrodes modified with sucholymer films. In the wide variety of organic conducting poly-ers, films synthesised from aromatic amines take a speciallace. Such polymers as poly-o-phenylenediamine (PPDA),oly-o-aminophenol (PAP), and derivatives are doped by hy-rogen ions[1–3], rather than by supporting electrolyte an-

ons, as observed for polythiophenes, polypyrroles, etc.[4,5].he most general situation takes place with polyaniline films,ince these can be doped by both supporting electrolyte an-

ons and hydrogen ions[6].

∗ Corresponding author. Tel.: +7 812 428 6900; fax: 7 812 428 6939.E-mail address:[email protected] (V. Malev).

1 ISE member.

It is usually assumed that redox reactions of PPDA-PAP-modified electrodes result from protonation/deprottion of polymers’ nitrogen atoms, as shown inScheme 1a andb [1,2,7,8].

Although the reactions represented in the scheme asome extent questionable, the existence of mobile anmobile forms of hydrogen ions seems to be probable wthe bulk of the film. For example, one can think that hydgen ions constrained with nitrogen atoms of polymer chdo not contribute to the electrical conductance of the fiand only a small part of such constrained groups is cble to dissociate producing the mobile form of hydrogenand, thus, providing the film for conductance. In other wothere are sound reasons to suppose the presence of somfor hydrogen ions within the bulk of the film, which provibinding of the ions with polymer film fragments, as it wassumed by Vorotyntsev et al. in the case of polypyrrole fi[9]. The binding phenomenon was partially analysed incent papers[9,10]. The main consequence of these consiations was the appearance of the capacity dispersion wicreasing ac frequency. Therefore, the use of impedance

013-4686/$ – see front matter © 2004 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2004.10.028

1574 O. Levin et al. / Electrochimica Acta 50 (2005) 1573–1585

Scheme 1. Species involved in the protonation/deprotonetion reaction ofPAP (a) and PPDA (b).

troscopy seems to be an adequate approach to verify this pre-diction with electrodes modified by the above films. Unfor-tunately, the treatment of impedance results is often compli-cated by accompanying experimental or/and theoretical dif-ficulties. These difficulties can be removed, at least partially,if impedance measurements are complemented by cyclicvoltammetry (CV) and low-amplitude chronoamperometry(LAC). Though the latter technique gives, in principle, thesame information as that from impedance spectroscopy, theopportunity of a comparison between the results of both theabove methods decreases a possible uncertainty in the conclusions drawn out. The use of an electrochemical quartz crystalmicrobalance (EQCM) technique also plays an important rolein the elucidation of the complex events occurring under suchcases.

PPDA- and PAP-modified electrodes were the objects ofpreceding electrochemical research (see, for example, Refs.[1–3,7,11–26]. As a result of these studies it was establishedthat, first, hydrogen ions actually play the role of doping ions,though supporting electrolyte anions are also present in thebulk film. This was a result of radiometric measurements onthese films[17]. The impedance spectra recorded on bothof them showed different frequency-limited regions, includ-ing a semi-circle portion, a 45-slope line, corresponding tothe Warburg region, and a nearly vertical line resulting froma w-f -t cou-p oly-m s ofc s ob-t sultso ed

that supporting electrolyte anions in parallel with hydrogenions participate in charge transfer within PPDA- and PAP-modified electrodes. Having performed a complicated analy-sis of the corresponding transport scheme[23], they showed asatisfactory agreement between the used representations andthe observed peculiarities of charge transfer through the poly-mers. During the subsequent studies[24,25], these authorshave modified their scheme by taking into account a porousnature of the electrodes studied. To treat the impedance resultsobtained with PAP-modified electrodes, Nieto and Tucceri[3]used a transport model, which accounts for a chemical reac-tion between hydrogen ions and oxidised sites of the polymer.In scope of the model, the concentration of hydrogen ionswithin a film was assumed constant, but obviously influencingthe dynamics of charge transport. These authors also observeda good correlation between the predicted and experimentalresults. Here, a principle difference of the impedance resultsreported in[24,25]and those of studies[1,2,7,21]should beemphasised. The data of the first works manifest the capac-ity dispersion in the region of low-ac frequencies, whereasdirect indications on the dispersion are absent in the secondstudies.

As it is probably clear from what has been said above, au-thors of all the above studies did not use explanations relatedto the binding phenomenon that might be proposed for thefi arget com-m acityd mo-b t toe sult-i thep n. Its olec-ub co-e lingw lter-n able.A stedu n thisr

fi hichm ationo , asa h ac ly-m ca-pt elec-t s.O isper-s tiono PAPfi im

finite film thickness, within the high-, middle-, and lorequency ranges, respectively[2,8]. These findings were inerpreted in terms of the well-known representations ofled transport of electrons and hydrogen ions through per films[1,2,7,21]. At the same time, alternative scheme

harge transport were also applied to explain the resultained with the films in question. Based upon the above ref radiometric studies[17], Inzelt and co-workers propos

-

lms under consideration. Although such aspect of chransfer was not discussed in the cited papers, one canent the reasons of doing this. In the absence of the capispersion, the assumption about equilibrium betweenile and immobile forms of hydrogen ions is sufficienxplain the observed results. A fast proton transport re

ng from proton transfer between nitrogen atoms alongolymer chain might also be an alternative explanatiohould be evidenced that the assumption about intermlar proton transfer was for the first time made in Ref.[1],ut resulted from the estimates of the effective diffusionfficient within PPDA films. As concerns the authors deaith the capacity dispersion effects, they preferred the aative explanations that seemed them to be more probdditional remarks related to the above studies will be linder subsequent discussion of the results obtained iesearch.

Our recent study of PPDA-modified electrodes[22] con-rmed the existence of two electron transfer reactions, wight be assigned to a consecutive protonation/deprotonf the both nitrogen groups of a polymer structural unitssumed inScheme 1b. Some evidences that allowed suconclusion will be given in this paper. Unlike PPDA, poer chains of PAP contain only one type of groupsable of protonation/deprotonation (seeScheme 1a). We,

herefore, hoped to observe some differences in therochemical behaviour of PPDA- and PAP-modifying filmur preceding research also revealed the capacity d

ion of PPDA film electrodes and, therefore, observaf the same effect might be expectable in the case oflms. A comparison of the behaviour was the initial a

O. Levin et al. / Electrochimica Acta 50 (2005) 1573–1585 1575

of this study, though its results posed unexpected questionsconnected with both experimental features of the objectsstudied and theoretical difficulties of treating the obtaineddata.

For the sake of shortness, we only cite in this sectionthat porosity of the films compared seems to be responsi-ble for some peculiarities in the relevant properties. More-over, the obtained data demonstrate a direct reduction of hy-drogen ions, most likely at the electrode interfaces. At thesame time, a limiting character of charge carrier injectionprocesses into a film is observed especially in the case ofPAP-modified electrodes. As it was shown by Vorotyntsevet al. [27], treatment of the results obtained in the condi-tions of slowness of the injection processes is complicatedby accounting for the effects of the interfacial charging. “Be-sides the parameters characterizing the transport processesof the charge species in the bulk film”, the impedance equa-tions derived for this case “contain four characteristics ofeach interface”, namely the charge transfer resistance, theinterfacial capacity, and the so-called “double-layer numberand asymmetry factor”. In other words, there arises a multi-parameter problem of fitting the obtained impedance results.Even if such a problem can be solved, the reliability of theparameters found would be questionable. To remove the rel-evant doubts, it is necessary one to make sure that the ob-t tionsa gingt olytec odelr m in-t singp thispw flu-e harget cor-r ly-m thatV tem,n lec-t port-i theg e re-s eemsu orre-s eoreti whyw aperw willb ses,t othi o ther y toa entalr r thef

2. Experimental

PPDA and PAP films were synthesised according to a po-tentiodynamic procedure on a polished glassy carbon (GC)disc electrode, 0.6 cm diameter (i.e. 0.28 cm2 area). A Pt wireelectrode (1 cm long and 0.07 cm diameter) was sometimesused as a conducting substrate. To synthesise the films, weprepared solutions containing 0.05 M monomer and 0.05 MHClO4, and cycled the electrode potential from 0.200 to1.200 V, according to the procedure described in Ref.[28].Twice-distilled water and reagent grade reactants were used.All measurements were performed in a hermetically sealedthree-electrode cell at 25◦C. To remove dissolved oxygen,extra purity grade argon was bubbled through the solutionsbefore the measurements. The applied potentials are referredto an Ag/AgCl electrode in a saturated KCl solution. Electro-chemical measurements [cyclic voltammetry, low-amplitudechronoamperometry, and electrochemical impedance spec-troscopy (EIS)] were performed in LiClO4 and HClO4 so-lutions at different pH and constant ionic strength (1 M). Inseveral experiments with PPDA films, solutions of H2SO4and HBF4 were also used. Special experiments were per-formed with PPDA- and PAP-modified electrodes coveredwith Nafion® films in order to elucidate the role of support-ing electrolyte anions in charge transfer within the films stud-i tedq tor,S

tat–g nter-f ARM ostat( ndM ncem from1 ade.W -t noisec ltagea tudeo d upt kepta . Toc msw ents.O , theo

3

elec-t lp sion,r de-t med

ained parameters are in accord with theoretical predicnd this accordance takes place in a wide region of chan

he experimental variables (electrode potential, electroncentration). In our opinion, this means that some mepresentations about the injection processes at the filerfaces must inevitably be introduced to solve the ariroblem. The Butler–Volmer equation is often used forurpose (see, for example Refs.[1,3,23]). Meanwhile, it isell-known that the equation does not account for the innce of double layers on the rate of heterogeneous c

ransfer reactions (it neglects the so-called Frumkin’section) and, therefore, its applicability to conducting poers seems to be doubtful. Here, it should be indicated

orotyntsev[28] proposed to use a more complicated sysamely the same modified electrode bathed with an “e

roactive solution, containing a redox couple and a supng electrolyte”, in order to solve the problem of findingreat number of parameters from fitting the impedancults. Although such a complementation of the results sseful, it does not completely solve the above problem; cpondence between the determined parameters and th

cal predictions will remain unclear. That is the reasone shall continue the relevant consideration in the next phere, unlike the Vorotyntsev’s approach, a model onee used to explicitly define the rates of injection proces

he “double-layer numbers”, and “asymmetry factors” of bnterfaces. The necessity of such a consideration is alseason why, in this paper, we will restrict ourselves onlsemi-quantitative discussion of the obtained experim

esults and postpone their more detailed treatment fouture.

-

ed. EQCM measurements with 5 MHz AT-cut gold-coauartz crystal (QCM100 controller and QCM25 oscillaRS, USA) pursued the same aim.The used equipment included a PI-50.1 potentios

alvanostat with a PR-8 programmer and a Grafit-2 fast iace, Autolab PGSTAT30 (Eco Chemie, Netherlands), P

273 impedance system, M273 potentiostat–galvanEG&G, PARC), M5315 two-channel preamplifier, a5301 computer-controlled lock-in amplifier. Impedaeasurements were performed within the frequency00 kHz to 0.1 Hz at five discrete frequencies per dece also tried to use low frequenciesf< 0.1 Hz, but the ob

ained impedance values were, as a rule, doubtful due toontributions in that frequency range. The applied ac vomplitude was 5 mV; in the case of LAC we choose amplif 20 mV, though a linear current response was observe

o amplitudes of 50–60 mV. The studied electrodes weret every selected potential value for not less than 5 minheck the electroactivity of the films, cyclic voltammograere recorded before and after the relevant measuremnly if the recorded currents differed no more than 10%btained results were then processed.

. Results and discussion

Results of our preceding research on PPDA-modifiedrodes were published in Ref.[20]. In this paper, we wilartly complement these results and renew their discuseferring the readers to the indicated paper for moreails. As to PAP-modified electrodes, we have perfor


Fig. 1. Typical CVs of polymer-modified glassy carbon electrodes (1) andbare electrodes (2) in 1 M HClO4 solutions, represented as functions ofcurrentI divided on scan speedv vs. electrode potentialE. Scan rates: (–)50 mV/s; (—) 100 mV/s; (. . .) 200 mV/s: (a) PPDA film of 223 nm, (b) PAPfilm of 12 nm.

analogous studies and their results will be discussed belowextensively.

3.1. Cyclic voltammetry

Typical CV curves of PPDA- and PAP film-modified elec-trodes at three different scan rates,v, are represented inFig. 1aand b, respectively (curve 1). The current is normalised withrespect tov. Comparison with the CV curves obtained withthe same bare GC electrode is also performed in these figuresIt is evident that, in both cases, the CV traces for modifiedelectrodes outside the regions, within which the films areelectroactive, exhibit very similar shape as those of unmod-ified electrodes. Moreover, similarly to the bare electrode,the electrical chargesQc andQa consumed during cathodicand anodic cycles, respectively, differ from each other, andthe former amount of charge turns out to be higher than thelatter one. In other words, there appears some dc componentof current flow for both films under cycling the potential over

the indicated ranges, i.e. the phenomenon of the so-calledFaradaic rectification is observed. The indicated differencebecomes the smaller, the smaller the cathodic value of elec-trode potentials reached on cycling. The difference is also thesmaller, the smaller the concentration of hydrogen ions in thebathing solution at a given range of cycling potential. Thisobviously means that charge transfer processes at the stud-ied electrodes include a process, which is not directly relatedwith the films’ electroactivity. In our opinion, such a processis the electroreduction of hydrogen ions.

One can also notice that the currents recorded outside theregion of the film electroactivity are even higher than those ofunmodified electrodes; the difference can amount to severaltimes (see below), depending on the pre-treatment of the elec-trode surface and on the electrodeposition conditions. Such adifference could hardly be expected if the hydrogen ions werereduced at the GC surface; reduction at the film/solution in-terface and/or the film volume seems more probable. Thisaccompanying reduction is more pronounced in the case of aPt substrate, which forced us to use such an electrode only ina limited number of experiments. At a first sight, the observedinfluence of the substrate nature on electroreduction of hy-drogen ions is in favour of the assumption that the reductiontakes place at the substrate surface. However, this influencemost likely results from different Fermi levels of electronsw , fromd op.I ACm hichh cessw at ofP

theire layerc er It isr madei ore,s esultf to ab un-m doxa s ofs back-g -f rrentv sureda e 2 inF es oft clingpT sameu s wasuo ges,q odic

.

ithin the compared substrates and, as a consequenceifferent values of the film/solution interface potential dr

n any case, it follows from these data that EIS and Leasurements should be carried out at potentials at wydrogen electroreduction does not occur. The last proas more expressed in the case of PPDA films than thAP ones.

As it was established for unmodified GC electrodes,lectrical capacity exceeds by 10–20 times the double-apacity (20–40�F/cm2). This might be ascribed to somoughness or/and to porosity of the electrode surface.easonable to think that the same assumption can ben the case of coatings with polymer films. We, therefupposed that CV curves of both modified electrodes rrom two different processes. A former one correspondsackground film conduction, similar to that observed forodified GC, while a latter one is determined only by rectivity of the films. In order to separate the contributionuch processes to the total CV trace, subtraction of theround current,If (E), from the overall one,I(E), was per

ormed. According to this procedure, the background cualues were estimated to be proportional to those meat the same potentials on bare GC electrodes (see curvig. 1a and b). Besides, we assumed that the current valu

he modified electrodes recorded near anodic limits of cyotentials are practically equal to background currentsIf (E).he ratio of such currents to the currents observed on thenmodified electrodes at the same electrode potentialsed to calculate background currentsIf (E) within the regionsf the film electroactivity. As a result, the cathodic charc =Qc −Qf , corresponding to the area between the cath


Fig. 2. Dependences of calculated cathodicqc (�), anodicqa (©) and back-groundQf (�) charges (a) PPDA film of 138 nm, (b) PAP film of 13 nm.

branch of the CV curves and the relevant background trace,are practically coincided with the relevant anodic charges,qa =Qa−Qf , calculated in a similar way (Qf is the chargeresulted from the assumed background conduction).Fig. 2aand b report the dependence of these charges on the poten-tial scan rate,v. The same figures also report the analogousdependence ofQf , estimated for both modified electrodes ac-cording to the above assumption. No significant pH-effect onthe calculated charges,qa andqc, in the pH range 0–2, couldbe evidenced.

As it is clear fromFig. 2, chargesqc andqa are somewhatdifferent at low scan rates, which is most likely related topossible errors in the above described background correctionin this region ofv-values due to increasing contributions ofthe hydrogen electroreduction to the experimentally obtained

Fig. 3. Background capacity of PPDA (�) and PAP (©) film electrodes asa function of the film thickness.

chargesQc andQa. Since, anodic chargesqa for both filmsslow down their increase with decreasing the scan rate, thevalues determined atv = 50 mV/s were used for the furtherfilm thickness estimation. The film thickness was determinedaccounting for the polymer doping levels that were reportedpreviously[2,18]. Changing the time of synthesis, we couldacquire PPDA-modifying films with a thickness up to 1�m,while in the case of PAP it did not exceed 50 nm, due to theself-limiting effects, caused by low polymer film conductiv-ity [8,29]. Changes of theQf -values at varying the time ofsynthesis were also observed. To account for such changes,we used a background capacity,Cf . Such a capacity was eval-uated as the ratio of background currents to the scan rate, atv = 50 mV/s, in the region of high-anodic potentials (+0.4 to+0.6 and +0.5 to +0.7 V for PPDA and PAP, respectively).Results of these calculations given inFig. 3 show that, forthe both cases, capacityCf slows down its increase with in-creasing the film thickness, and probably tends to a limitingvalue of about 1 mF/cm2. This permits one to suppose thatthe porosity (or roughness) status of a film/solution transientlayer is stabilised at increasing the film thickness.

The observed background conductance should correspondsome charge transfer through a film to charge the metal/filmand film/solution interfaces. This requires the presence of acompensating charge or a special mechanism of its genera-t rgef ucha e thefi ob ctiv-i allt weenc

ion within the bulk film. In our opinion, some residual chaormed during the electrodeposition of a film can play s

role if the corresponding charged species do not leavlm under its cycling. In any case, theQf -values result te comparable with those corresponding to the redox a

ty of the studied films, at least under their relatively smhickness, as it can be deduced from a comparison bethargesqa,c andQf given byFig. 2.


Fig. 4. CVs of polymer-modified GCE in 0.01 M HClO4 solutions.ν = 20 mV/s: (a) PPDA film of 180 nm, (b) PAP film of 5 nm.

It is noteworthy that cathodic and anodic peak currentsi(p) = I(p)–I(p)

f , as computed by the background subtraction,did not result to be proportional tov1/2 for both films stud-ied; the slopes of the linear plots of lni(p) versus lnv wereequal to 0.75 and 0.9 for PPDA and PAP films, respectively.This probably means that injection of one or two types ofcharge carriers within the films is a limiting step of the to-tal charge transfer process under the conditions of our CVmeasurements.

Continuing the discussion of the above data, it is nec-essary to evidence a complicated form of the CV curvesobtained for PPDA films. In parallel to the well-definedpeaks, a couple of shoulders in CV curves of PPDA films areobserved (seeFig. 1a). The appearance of such shoulders,i.e. the presence of at least two redox processes, becomesmore evident for 0.01 and 0.1 M HClO4 solutions, where abroad anodic CV peak is splitted into two ones at low scanrates (seeFig. 4a). This splitting might serve the evidenceof protonation/deprotonation processes involving two differ-ent nitrogen-containing groups of the polymer chains (seeScheme 1a). A confirmation of such a conclusion will begiven below, when discussing the LCA results obtained withPPDA films.

As it was expected (see Section1), a similar splittingis undetectable at any pH and potential scan rates in thecase of PAP films, which is also in favour of the conclu-sion gained. At the same time, the peaks observed in CVcurves of PAP-modified GC electrodes are quite broad (seeFig. 4b). One cannot, therefore, exclude that these peaks cor-respond to reduction/oxidation processes with participationof nitrogen groups at different energetic states, resulting frominhomogeneity of the studied films. An alternative explana-tion, namely the existence of pronounced repulsive interac-tions between nitrogen-containing groups of polymer chains,is also possible.

It should be complemented here that CV curves recordedon Pt electrode modified by the same PPDA film resultedslightly different in 1 M HClO4, H2SO4, and HBF4 solutions,but the peak potentials in such curves differ no more than30 mV from each other. This observation supports a possibleinclusion of anions into the film[17,22]. In that connection,the question about the role of supporting electrolyte anionsin redox reactions of the studied polymers becomes actual.To get the relevant answer, we have performed additionalCV measurements with PPDA- and PAP-modified electrodescovered by Nafion® thin films. The latter is known to hinderanionic permeability due to its own negative charge[30,31],so the films covered with this polymer should significantlyc ncov-e ’ par-t nte hes et mpli-c nderc e oft f theea acids Oa db

car-r Them -t t re-s pre-l lec-t ieldst rodes,� w),i esis( ngt po-t rus-sA ifiede mainr d

hange their electrical properties, as compared to the ured ones, in the case of supporting electrolyte anions

icipation in the redox activity of the films. No significaffects of Nafion® were observed for the both kinds of ttudied films, as it is seen fromFig. 5a and b. This allows ono conclude that perchlorate anions, at least, are not iated in the redox reactions of the modified electrodes uonsideration. As to the above-indicated small influenche supporting electrolyte anion nature on CV curves olectrodes not covered by Nafion®, it probably results fromdifference between the activity coefficients of the usedolutions. At least for the cases of 1 M solutions of HCL4nd H2SO4, this difference is essential[32] and is reflectey an incomplete dissociation of the sulphuric acid.

To confirm the conclusion drawn out, we have alsoied out EQCM measurements with the films studied.icrobalance frequency change,�fmc, of the modified elec

rodes was measured simultaneously with their currenponse under the electrode potential cycling. During theiminary measurements with Prussian blue-modified erodes we have convinced that the used installation yhe regular results on the mass response of the electm=�fmc/k (wherek, the constant of the Sauerbrey la

n both the regime of the Prussian blue film synth�m/�Q(E) = 692, whereQ(E), the charge consumed durihe oxidation/reduction process) and that for cycling theential of the prepared electrodes within the range of the Pian blue/Prussian green transition (�m/�Q(E) = 37) [33].nalogous measurements with PPDA- and PAP-modlectrodes gave the mass numbers about 3–5 in theanges of the cycled potentials (from−100 to 200 mV an


Fig. 5. CVs of polymer-modified gold electrode in 1 M HClO4 solutions:(a) PPDA film, curve 1—without Nafion® covering, curve 2—Nafion® cov-ered PPDA film, (b) PAP film, curve 1—without Nafion® covering, curve2—Nafion® covered PAP film, curve 3—Nafion® thickness is increased.

from 0 to 300 mV for PPDA- and PAP-films, respectively).These numbers were somewhat increased in the regions ofmore positive potentials, similarly to that was indicated byInzelt et al.[26], but their maximum values did not exceed10–12 if the anodic potentials were not too high (<300 and<400 mV for PPDA- and PAP-films, respectively). It shouldbe emphasised that the conductance parameterRm character-izing the viscoelastic contribution[34] was sufficiently lowunder the determinations performed (Rm < 600 Ohm), so theresults obtained can be considered as reliable ones. One cancertainly explain the observed decrease of the mass num-bers as compared to the molecular weight of ClO4

− anionsas the consequence of some compensating effects. Howeverthe simplest way is to think about an insignificant insertion ofthe anions into the studied films. Thus, these data, as well asthe previously observed small influence of the anion’s natureand the Nafion® covering on CV curves of the electrodes,most likely show that perchlorate anions do not participate inthe redox reactions of PPDA- and PAP-modified electrodes.The same sum of the results also indicates that possible poresof the studied films are of either molecular sizes or they aresettled only within the surface region of the films.

3.2. Low-amplitude methods: electrochemicalimpedance spectroscopy and chronoamperometry

In order to facilitate further discussion, we will give be-low well-known theoretical results[35] applied usually inthe context of a quantitative treatment of experimental data.The developed theory should be considered as a simplifiedone, since it does not take into account the impedance of thesubstrate/film and film/solution interfaces. As it was men-tioned in Section1, the proper theory of the homogeneouspolymer film impedance was developed in[27]. The consid-eration that also accounts for electron exchange reactions atthe film/solution interface was recently correctly made byVorotyntsev[28].

According to the simplified theory of polymer filmimpedance[35], a linear dependence of real and imaginaryparts of the Warburg impedance on the square root of the acfrequency, namely the equation

ReZW − ReZW(∞) = −ImZW = RT (t2e + t2x)

2F2ACeff√

(πDefff )(1)

can be used to estimate the effective diffusion coefficient,Deff, of charge carriers inside redox polymer films. In Eq.(1), ReZW(∞) is the value of the film impedance extrap-o erso -c -c -d qualt in-tc a-t nsh

a

−w lm[ theo cf tb freef d re-g yt

C

wthe

fi l tot , thed from

,

lated at f=∞; te and tx are the transference numbf electrons and counter-ions, respectively;cx, the conentration of counter-ions in the bulk;C, the total conentration of redox sites;θR =CR/C, the fraction of reuced sites, so that the fraction of oxidised sites is e

o (1− θR); α0, the parameter characterizing short-rangeeractions;A, the electrode area;Ceff ={[(1 +α0θR(1− θR))x +CR(1− θR)]/cxCR(1− θR)}−1, the effective concentrion of charge carriers within the bulk film; other notatioave their usual meaning.

The same theory gives a low-frequency limit of−ImZ(f)ccording to the equation:

ImZ = 12πfClf atf > 0, (2)

hereClf is the low-frequency capacity of a polymer fi35]. Thus, theClf -values can be found by analysingbtained dependence of−ImZ(f) in the region of low-a

requencies. However, finding of theClf -values might noe possible if the equilibrium between binding and

orms of counter-ions is not achieved inside the studieion of ac frequencies[10]. The Clf quantity is defined b

he equation

lf = F2LCeff

RT, (3)

hereCeff satisfies the relation indicated above.The above theory does not account for charging

lm/substrate and film/solution double layers in parallehe processes of injection of charge carriers. Howeveresired result is usually obtained due to the transition


Scheme 2. Equivalent circuits representing a series of electrical models of polymer films with two charge carriers: (a) Mathias & Haas’ model[35]; (b) Musiani’smodel[36]; (c) Vorotyntsev’s model[28]. Rx is the resistance of the film bulk,Rx(z) andCx(z) are the resistance and the capacitance ofz interface of the film,wherez= 0 for metal/film interface andz=L for film/solution interface. Subscript index “x” means the type of charge carrier;x= i for immobile charge carriers(electrons) andx= m for mobile ones (counter-ions).Clf is the low-frequency capacitance of the film;Rs is the resistance of the bathing electrolyte.

Scheme 2a corresponding to the Mathias & Haas’ consid-eration toScheme 2b that takes the charging processes intoaccount. As it is known, such a transition can yield the ap-pearance of semi-circles in the Nyquist plots[36]. Here wewould like to indicate that a stricter version of the impedancetheory should correspond toScheme 2c, according to whichthe contributions of counter-ions and electrons to capacitivecurrents within the film/substrate and film/solution doublelayers, respectively, are taken into account. The correspond-ing analysis for the films with two kinds of charge carrierswas performed in the most general form by Vorotyntsev[28].The principle point of his consideration is the fact that theelectrical flow densities of electrons and counter-ions at thefilm/substrate and film/solution interfaces coincide, respec-tively, with the measured current density only if the chargingof double layers is negligible. Meanwhile, the requirementof such a coincidence is always used in theoretical works,dealing with charge transport through conducting polymers,and plays the role of boundary conditions to the correspond-ing boundary value problem (see, for example[35,36]). Inother words, there are forcible reasons to think that, in thecase of irreversible injection processes at the film interfaces,the proper corrections on charging the double layers should,first of all, be accounted for under treating the obtained ex-perimental results. Since, our preceding CV data allow toa s, we

are planning the use of the approach indicated in our subse-quent papers. Additional explanations to its application willbe given in the next paper[37].

Returning to the simplified theory of charge transport, weindicate that, under equilibrium conditions of the charge car-riers injection processes, a Cottrellian current trend is knownto be observed over the most part of the time interval of therelaxation process[38]:

I(t) =(

Deff

πt

)1/2AF2�ECeff

RT (t2e + t2x), (4)

where�E is the amplitude of the potential perturbation. It isseen from Eq.(4) that the slope tgα = dI/d(t−1/2) is propor-tional to (Deff)1/2. Hence, such a slope bears the same infor-mation as that coming from the Warburg impedance data. Thismeans, in particular, that the product tgα·tgβ = dI/d(t−1/2) ·d[ReZW]/d(f−0.5) should satisfy the relation:

dI

d(t −1/2)

d[ReZW]

d(f− 0.5)= �E

2π, (5)

as it is seen from a comparison of Eqs.(1) and (4). Thiscircumstance is most suitable to use for verification of self-consistency of the results obtained with either method[39].According to Eq.(4), the necessary condition of such ver-i inear
ssume the irreversible character of injection processe fication is that the observed current responses are l


Fig. 6. Impedance spectra of 780 nm PPDA film. 1 M HClO4 solution: (a)E= 0 mV, (b)E=−100 mV, (c)E=−200 mV.

for the potential perturbation applied. As it was mentionedabove (see Section2), preliminary measurements showedthat this requirement was satisfied up to|�E| not exceed-ing 50–60 mV.

Concluding this short review of the theory, we would liketo once more emphasise its simplified character. Besides theremarks indicated above, the principle proposition of the the-ory about a structural homogeneity of the films modelledprovokes some doubts as to its applicability to real cases.Since, the polymer films studied here manifest their inhomo-geneity, one cannot expect a complete coincidence of theirelectrochemical properties with those predicted by the the-ory even if it contains the desired corrections on chargingthe double layers. However, a qualitative agreement mightbe expectable, since the observed porosity effects are not toohigh, at least in the case of sufficiently thick films.

Typical impedance spectra of PPDA films are given inFig. 6a–c as the corresponding Nyquist’s plots. The inserts inthe figures show segments of the spectra in the high-frequencyregion. As it is evidenced, the slopes of the plots exceed unity,but are near to one either at decreasing frequency or at elec-trode potentials approaching the formal potential value. Onecan consider these segments as a result of overlapping a semi-circular part and the Warburg one of the film impedance, asit was in essence proposed by Komura et al.[1,2] in the caseo lter-n ancec f fre-q filmi -c at-it thei al po-t rburgr du s, thet to bee ortr uality

seems questionable, but the value of the effective diffusioncoefficient increases only four times at the transition to thecase of eitherte = 1, tx = 0 or te = 0, tx = 1, as seen from Eq.(1). A more significant increase of the parameter should obvi-ously take place if one assumes that the fraction of hydrogenions capable to transfer inside the film bulk is small as com-pared to the maximum doping level of the polymers. Therealso exists another origin for obtaining the increased valuesof Deff, as it will be explained below.

It was mentioned previously that the calculatedDeff-valuesdepend on pH, but such dependence was less pronounced thanthat given in Ref.[2]. Besides, the obtained data are one totwo orders of magnitude smaller than those reported for thesame films by Komura et al.[2]. The reason for such a highdifference is the fact that theDeff estimations of Ref.[2] wereperformed with the use of the low-frequency capacityClf de-termined in the cited work. Since,Clf is proportional to the

F of thes se

f PPDA-modified GC electrodes. However, only an aative representation of the real and imaginary impedomponents as a function of the reciprocal square root ouency revealed the Warburg equivalent of the PPDA

mpedance, as seen inFig. 7. The effective diffusion coeffiients calculated for PPDA films (for 2 electrons participng to the redox reaction) varied in the range from 1× 10−10

o 2× 10−10 cm2/s, depending on the pH value. Since,mpedance spectra were mainly recorded near the formential, due to the observed disappearance of the Waegions at different potentials, the value ofθR = 0.5 was usender the above estimations. To perform the estimation

ransference numbers of charge carriers were supposedqual to each other, i.e.te = tx = 0.5, and the absence of shange interactions was assumed. Certainly, the above eq

ig. 7. Real and imaginary parts of PPDA film impedance as functionsquare rooted frequency. (�) Real part; (�) imaginary part, film thicknesquals 780 nm.


Fig. 8. Impedance spectra of 23 nm PAP film. 1 M HClO4 solution: (a)E=−50 mV, (b)E= 50 mV, (c)E= 150 mV.

effective concentrationCeff, the latter can be replaced byClfin Eq. (1). One can also see that theCeff-values and, hence,theClf ones might strongly decrease with increasing the at-traction constantα0 if the latter exceeds unity (see the abovedefinition ofCeff). This means that theDeff-values will be themore, the higher the positive values ofα0. In other words,our neglecting of short-range interactions could be responsi-ble for obtaining decreased values ofDeff. Unfortunately, wecannot use the indicated way ofDeff-calculations, since, un-like the Komura’s measurements, the capacity dispersion wasobserved in our experiments, as it was mentioned in Section1 (see also below).

Similar impedance spectra were obtained for PAP films,with the only difference that impedance values were one or-der of magnitude higher than those recorded for PPDA films(seeFig. 8). At the same time, for PAP films the estimationsof Deff carried out above were hardly possible, because ofthe absence of the well-defined Warburg region in the corre-sponding spectra. This absence of the Warburg dependence inPAP spectra and its presence in PPDA ones are in accord withthe above-indicated features of CV curves of GC electrodesmodified with these films; the slopes of the lnip versus lnvplots were equal to 0.75 and close to 1 in the cases of PPDAand PAP films, respectively. Taking these values into account,one can really expect the appearance of diffusion limitations( sely,t

thei dif-f nsisto thefi incet ts aren dencew milarri mea-st . Wec city

Fig. 9. Low-frequency part of 780 nm PPDA film impedance, represented asa dependence of the imaginary part vs. inverse ac frequency. (�) E= 0 mV;(©) E=−100 mV; (�) E=−200 mV; all curves recorded in 1 M HClO4solution.

values of the studied films and restricted ourselves only totheir estimations, as it is indicated below. In that connection,we also note that the difference between the slopes of the indi-cated lines diminished and practically vanished in the limit ofsmall film thickness. Slopes of these lines could in principle

Fig. 10. Low-frequency part of 15 nm PAP film impedance, represented asa dependence of the imaginary part vs. inverse frequency. (�) E=−50 mV;(©)E= 50 mV; (�)E= 100 mV; all curves recorded in 1 M HClO4 solution.

i.e. the Warburg impedance) for PPDA films and, converheir absence for PAP ones.

Fig. 9 illustrates the low-frequency dependence ofmaginary component of PPDA impedance spectra aterent potentials. As it is evident, the reported curves cof two lines and an intermediate region in between. Onlyrst lines satisfy approximately the impedance theory, sheir intercepts are near to zero. The second intercepon-zero. It should be emphasised that such a depenas observed at all the studied electrode potentials. Si

esults were also obtained for PAP films (seeFig. 10). Here,t is necessary to once more indicate that the impedanceurements occurred impossible for frequenciesf< 0.1 Hz dueo the noise fluctuations observed in this frequency regionould not, therefore, find out the real low-frequency capa


be used to find capacitiesClf of such films. However, the ob-tained capacities were practically of the same value as thosedetermined in result of the subtraction procedure describedin Section3.1. That was not surprising, since, under the con-ditions indicated, the redox activity of the films is small ascompared to their background conductance.

d(−ImZ)/d(f−1) slope values were used to calculate theapparent capacitiesClf

(ap) of the films, according to the rela-tion

Clf(ap) = [d(−ImZ)/d(f−1)] −1

2π, (6)

and then to compare these values with the low-frequency ca-pacity,Clf , as calculated by Eq.(3). Even if maximum valuesof Clf

(ap) corresponding to the second lines ofFigs. 8and 9were used in such a comparison, theClf -values turned outapproximately three times higher than apparent capacitiesClf

(ap). It should be added that the calculated quantityClf(ap)

increased with increasing the film thickness and highest val-ues were near the formal potentials, i.e. the above quantityshowed a behaviour in qualitative agreement with the predic-tions of the simplified theory.

Hence, the above data demonstrate the capacity dispersionin the low-frequency region. One can, therefore, assume thatthe first line of the dependence of ImZ versus 1/f, where ana anti-t riumb ptedt cies,ts sedo ce oft llyh of theI en-c eent bvi-o dif-f ction1 f thed orousei thato ervedp ders andt tionso t al.[ re-s olytea g theo r theq tionsd rousm lyi for-

tunately, the corresponding theoretical problems seem to bevery complicated.

Fig. 11a and b represents the obtained transient currentsfor both modified electrodes as a function of the reciprocalof the square root of time,t−1/2. One can see from the fig-ure that a two-phase relaxation is observed for PPDA filmsin 0.01 and 0.1 M HClO4 solutions. However, the currentrelaxation became similar to a one-phase process for 1 MHClO4 solution. These results are in accord with the previ-ous CV data and, thus, confirm the primary conclusion aboutthe two-step character of the charge transfer in these films. Itis also seen that the Cottrell equation is approximately validfor the I(t) versust−1/2 dependence of PPDA system. Aver-aged dI/d(t−1/2) slopes show the well-defined maximum nearthe formal potentials, at least in the case of diluted HClO4 so-lutions[22]. Moreover, the verification of Eq.(4)showed thatit is really fulfilled for the studied PPDA films; the product ofthe above slopes was about a theoretical value (3–4 mV for|�E| = 20 mV). This means, in particular, that theDeff-valuescalculated above are to consider as reliable ones, within thevalidity of the assumptions made for their estimation.

As to the current relaxation observed for PAP films, itdoes not satisfy the Cottrell equation. This is clearly seenfrom Fig. 10b, which illustrates a practical absence of anydependence of the recorded currents on the electrode poten-to rrentr (datan flu-e faceso ioni ,a usly,t AP-m War-b nd isa evantC le fort films( r pa-r theo ce atP PDAfi

udem ds oft es att ree-m -d ncyr aticaa rans-p laint indingt ups

pproximate proportionality between the compared quies is observed, corresponds to the absence of equilibetween free and bound forms of hydrogen ions. If acce

his means that, in the corresponding range of ac frequenhe effective concentration of charge carriersCeff should beignificantly smaller than that follows from estimations ban the doping levels of the polymers. As a consequen

he assumption, the relevantDeff-values must be essentiaigher than those estimated above. The remaining part

mZ versus 1/f curves corresponding to more small frequies reflects a gradual transition to the equilibrium betwhe different forms of counter-ions. This explanation ously corresponds to the initial expectations of finding

erent states of counter-ions inside the film bulk (see Se), but one can also propose a different interpretation oispersion phenomenon observed. From the theory of plectrodes[40] and relevant electrochemical studies[41], it

s known that their impedance behaviour differs fromf electrodes with a smooth surface. Therefore, the obseculiarities of low-frequency impedance of the films untudy can be ascribed to their porosity. This explanationhe assumption about anions’ participation in redox reacf the polymers were used in recent papers by Inzelt e

23–25]. To be brief, we only repeat here that, first, ourults do not show an essential effect of supporting electrnions and, secondarily, our above remarks concerninrdinary used boundary conditions seem to be valid fouantitative analysis of the cited papers. These objeco not mean our negative attitude to “the advanced poodels” of conducting polymer films; we would like to on

ndicate the necessity of their correct consideration. Un

ial, in contrast with the prediction of Eq.(4). A more thor-ugh analysis shows that the main part of recorded cuesponses well fits a two-exponential time dependenceot represented), which most likely indicates a limiting innce of charge carrier injection processes at both interf PAP films. A slow component of the current relaxat

s also recorded sometimes att→ ∞, but its observation iss a rule, complicated by the noise fluctuations. Obvio

he observed deviation from the Cottrell relaxation of Podified electrodes corresponds to the absence of the

urg region in impedance spectra of these electrodes also in agreement with the discussed features of the relV curves. As concerns the physical reason responsib

his absence, the small thickness of the synthesised PAPL≤ 50 nm) and, hence, the small values of the Fourieameter,L2/Deff, seems to be the main reason for this. Onther hand, higher values of the charge transfer resistanAP film interfaces, as compared to the resistance of Plms, can also be an additional reason.

Thus, our results obtained with the help of low-amplitethods show that the charge transfer through both kin

he films is complicated by irreversible injection processhe film interfaces. This conclusion is in a qualitative agent with the results reported in[1,2], but, unlike the inicated works, the capacity dispersion in the low-frequeange is observed for the films formed from both arommines. The same dispersion effect was established in[3,23]nd discussed in terms of different schemes of charge tort within the polymers. We are rather disposed to exp

he observed capacity dispersion as a consequence of bhe hydrogen ions with polymer nitrogen-containing gro


Fig. 11. Typical chronoamperogramms in the Cottrell coordinates. (a) 854 nm PPDA film in 0.01 M HClO4 + 0.99 LiClO4 solution, (�) E=−200 mV, (©)E=−100 mV, (b) 24 nm PAP film in 0.01 M HClO4 + 0.99 LiClO4 solution, (©) E= 0 mV, (�) E= 100 mV.

than to assign it to the porosity of the films and simultane-ous participation of supporting electrolyte anions in chargetransfer through the polymer films[24,25]. In that connec-tion, the scheme of[3] seems us to be more realistic, sinceit, in essence, accounts for binding the hydrogen ions. In anycase, the established slowness of injection processes at thefilm interfaces needs more thorough procedures of fitting theimpedance data than those applied in the cited works. As itwas explained above, the proper fitting could be based on theresults obtained in[27], but needs a preliminary introductionof some model representations.

4. Conclusions

The performed comparative study of PPDA- and PAP-modified electrodes shows an intricate character of chargetransfer within these systems. In both cases, the charge trans-port is complicated by the background conductance of the

films that most likely results from some residual charge intheir bulk and their porosity. Besides, the hydrogen ion elec-troreduction takes place within the ranges of sufficiently high-cathodic potentials. The both polymers are doped by hydro-gen ions and the effects of supporting electrolyte anions arenegligible, at least in the case of perchlorate anions. Slow-ness of the charge carrier injection processes is also estab-lished for both polymers, but it is more expressed for PAP-modified electrodes. The observed capacity dispersion in thelow-frequency range is most likely related to the existence ofdifferent forms (mobile and bound) of hydrogen ions in thebulk film.

Acknowledgements

The authors are thankful to the Eco Chemie Company andDr. Marcin Palys for supplying electrochemical complex Au-tolab PGSTAT30 with an impedance adapter. Financial sup-


port from the Russian Foundation for Basic Research (Grants## 01-03-32-326, 04-03-33018) is also gratefully acknowl-edged.

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charge transfer processes at poly-o-phenylenediamine and poly-o-aminophenol films

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