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Available online at www.sciencedirect.com

Synthetic Metals 157 (2007) 1085–1090

Short communication

On the porosity of polypyrrole films

Allan Hallik, Ants Alumaa, Heisi Kurig, Alar Janes, Enn Lust, Juri Tamm ∗Institute of Physical Chemistry, University of Tartu, Jakobi 2, 51014 Tartu, Estonia

Received 27 June 2007; received in revised form 20 September 2007; accepted 31 October 2007Available online 21 December 2007

bstract

Polypyrrole (PPy) films doped with anions of various size, charge and chemical nature (inorganic, surfactant, with aromatic ring) werelectrochemically synthesized and investigated by low-temperature N2 sorption experiments at −196 ◦C. The specific surface area, total poreolume, average pore radius, pore size distribution and other parameters for oxidized PPy films using dodecylsulfate, 2-naphthalene sulfonate,,5-naphthalene disulfonate, poly(4-styrenesulfonate), tosylate, perchlorate, nitrate and chloride as dopant ions, were calculated. The obtained

ata show that although the average pore radius of investigated mesoporous PPy films (17–19 A) is practically independent of the dopant anionsed, however the latter determines the total pore volume and specific surface area values in different PPy materials investigated. As the total poreolumes for PPy films doped with large amphiphilic anions show the smallest values, the corresponding values for PPy/small inorganic anions, arep to 2 times higher.

2007 Elsevier B.V. All rights reserved.

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eywords: Polypyrrole; Counter-ion; Porosity; Surface area; BET method; Nit

. Introduction

Conducting polymers, among these polypyrrole and its com-osites have attracted a great attention due to their possiblepplications in many fields of technology, among others asnergy storage materials (batteries and supercapacitors) [1–9]r active layers for ion/molecule sensitive electrodes [1,10–14].t is well known that in both fields one of the most impor-ant factors is the macroscopic and microscopic porosity of the

aterials used. In the case of energy storage materials upon theast peculiarity depends the accessibility of ions of electrolyteolution to sorption/reaction sites of solid matrix and thereforehe total “working” internal surface area and total double-layerapacitance or pseudocapacitance of the material. Likewise theonic conductivity, related to the mobility of ions inside theorous system and thus also the rate of electrochemical pro-esses is dependent on the porosity of material used [4,5,15]. Inrder to improve the electrolyte access to reaction centers and

ecrease the mass transfer resistance in conducting polymers,improving consequently their electrical capacitance proper-ies), the porosity of these polymers have been enlarged by their

∗ Corresponding author. Fax: +372 7 375 160.E-mail address: juri.tamm@ut.ee (J. Tamm).

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379-6779/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.synthmet.2007.10.017

adsorption

imultaneous deposition with carbon nanotubes [8,9,13,14]. Inuch a way as well is enhanced the surface area of PPy andbtained its higher sensor sensitivity to detectable NH3 vapors14].

It is well known that many physical and (electro)chemicalroperties of synthesized PPy films are dependent on the dopant-on used. A lot of investigations have proved the influence of theature and size of the dopant on PPy mechanical properties (theensile strength, Young’s modulus, viscoelasticity, etc.), electri-al properties (conductivity), hydrophobicity/hydrophilicity andlso on properties, more related to PPy film porosity (sphere ofnterest of current paper) as pore size, film density, the distanceetween PPy chains, degree of structure order, degree of elec-rostatic cross-linking, isotropy/anisotropy, and permeability toases/ions [1,7,16–24].

Suematsu et al. have reported that the increase of the numberf sulfonate groups in the dopant anion (naphthalene sulfonates)esults in higher degree of electrostatic cross-linking and muchore microporous structure with enhanced diffusion of ions in

he PPy films [7]. In Refs. [16,18] it is concluded that the poros-ty of polymer layers decreases with increasing size of dopant

nion. Likewise, the lower porosity is achieved by decreasinghe doping level [18]. Besides, due to their higher degree ofrder, the structure of PPy films doped with large anions isrreversibly changed when these films are electroreduced in elec-

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rolyte solution. It was also found that PPy layers doped withmphiphilic dopant anions like dodecylsulfate and dodecylben-enesulfonate gave layers with short range order, whereas smallF4

− or ClO4− or (trifloro)methylsulfonate anions produced

morphous porous layers, with rough surface morphology. Thiss in accordance with stated in Ref. [17] results that PPy filmsoped with ClO4

−, BF4− and PF6

− were quite sparse, with morehan 70% void volume. On the other hand PPy films doped alsoith the counter anions of small or medium size as Tos−, SO4

2−nd NO3

− showed compact structure (less than 20% void vol-me) with apparent densities close to their flotation densities17]. This kind of quite essential structure/porosity differenceetween PPy films, doped with similar ions or with dopantsf not very different size was also observed in Refs. [24–26].o, PPy/Tos film was found more smooth and compact in com-arison with the PPy/Cl film. Besides, proceeding from inverseas chromatography method, a possibility of certain microp-rosity of PPy/Cl films is concluded, which was not found toe characteristic for PPy/Tos films [25]. The PPy films, withopants even more similar than in above given case, such asPy/Cl and PPy/Br showed a significant difference in surfacetructures in microscale and particularly in nanoscale range.he PPy/Br film was found to have nanostructure, whereas thePy/Cl film surface was smooth in nanoscale [26]. Hakanssont al. [24] have concluded from their studies of the influence ofhe dopants of different size on PPy properties, that dopant sizeeflects in different unit cell volumes of the polymer. When smallopants like Cl− ions or benzenesulfonate ions have little effectn the molecular matrix, the larger dopants result in larger unitell volumes. Differently from some above given literary data,Py/small dopant films as PPy/I and PPy/Br showed high den-ity and compact structure [24], but PPy doped with bulky anionss dodecylbenzenesulfonate or anthraquinone-2-sulfonate, gaveparse structure.

It must be mentioned that despite the existence of certainmount of qualitative estimative literature data and still feweruantitative data on the PPy porosity, the corresponding arean literature is poorly represented, due to which many conclu-ions and explanations are uncertain. To obtain some quantitativeorosity data on PPy, doped with the anions of various size,lectric charge and chemical nature, the present study was under-aken.

. Experimental

Polypyrrole films (with the estimated thickness from 50 to00 �m) having dry mass of about 35–70 mg were synthesizedalvanostatically on the Pt-foil (5 cm2) electrodes at currentensity i = 2 mA cm−2 from an aqueous solutions of 0.1 M pyr-ole in 0.1 M supporting electrolyte. Some small monovalentnions as Cl−, ClO4

− and NO3− were chosen as dopants.

he large monovalent dopants were represented by tosylateTos−), 2-naphthalene sulfonate (NS−), dodecylsulfate (DDS−)

nd the large polymeric dopant by poly(4-styrenesulfonate)PSS−) and divalent dopants by SO4

2− and 1,5-naphthaleneisulfonate (NDS2−). For electrodeposition in conventional one-ompartment cell the Pt-wire was used as the counter electrode

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ls 157 (2007) 1085–1090

nd an aqueous saturated KCl | Ag | AgCl (Ag/AgCl) electrodeas the reference electrode.Pyrrole (Aldrich) was distilled under vacuum and kept refrig-

rated in the dark. The reagents of analytical and reagent gradeere used as received. NaClO4 was additionally purified by

ecrystallisation from H2O. The solutions were prepared usingilli Q+ water.After electrodeposition the synthesized films were thor-

ughly washed with Milli Q+ water (leaching the films 5 + 5 minn water and exchanging the water amount once) to removehe excessive electrolyte, present in the film pores; dried andeparated from substrate Pt-foil, using razor blade and plasticheet.

The low-temperature nitrogen adsorption experiments werearried out at the boiling temperature of liquid nitrogen−196 ◦C) by using Quantachrome Instrument Nova 1200e. Thisemperature is deeply below polypyrrole glass transition temper-ture, which, depending on dopant, ranges from 95 to 215 ◦C28,29] and therefore the polymer is in shrunk form. Beforeach experiment the PPy film was kept during 4 h at 50 ◦C underacuum (p = 10 mTorr) to remove the traces of water from thelm pores. It is known that PPy films are thermally stable up

o 150 ◦C and below this temperature only the desorption ofdsorbed water occurs [25].

For comparison, the porosity of one polypyrrole filmPPy/SO4) was additionally characterized by applying ben-ene sorption method. The adsorption of benzene at PPy filmas studied at room temperature by using computer controlledeighing system of PPy samples in benzene vapor at normal

tmospheric pressure. The initial weight of the PPy test sam-le was 28 mg and the changes of the weight in time due toenzene adsorption were registered every 10 s till the sampleeight stabilized, indicating the reaching of adsorption equilib-

ium of benzene on porous PPy. The pore volume inside the film,s was calculated according to the equation:

s = m2 − m1

m1 × dbenzene,

here m1 and m2 are the initial and final weights of the PPyample and dbenzene = 0.879 g/cm3 is the density of benzene.

. Results and discussion

.1. Adsorption isotherms

The low-temperature N2 adsorption isotherms for PPy filmsoped with different anions are presented in Fig. 1. It must beoted that the desorption curves practically coincide with thedsorption ones. As seen, the linear relationship between relativeressure and adsorbed gas volume with different slopes for dif-erent PPy/dopant films is in effect to the full extent of measuredressures. The validity of such a kind extensive linear adsorptionsotherm as the analogue of Henry’s law is, although expectable

t small enough pressure values, however somewhat problematict quite high (p/p0) values. According to the theoretical consid-rations, after monolayer adsorption at very low pressures, theultilayer adsorption goes on up to the point where follow-

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ig. 1. Low-temperature N2 adsorption isotherms for PPy films doped withnions, noted in figure.

ng new adsorptive molecules already interact with (instead ofrevious flat surface) curved surface and beginning from thisoint, due to capillary condensation, the deviation of the curverom linearity should start. The relative pressure (p/p0) value athich the capillary condensation at mesoporous materials starts,epends on the pore size distribution function [30–32]. As con-luded in Ref. [33], in the case of N2 or Ar adsorption in linearesopores, the individual local geometries as the pore mouth, a

lind end, or a single constriction do not affect the shape of sorp-ion isotherms, and therefore the pore space should be regardeds a statistical ensemble of pore segments with a lot of quenchedisorder.

In our case, the lack of the isotherm of type, characteristic toommon micro- or mesoporous adsorbent [32], may be causedy nonwetting phenomenology, which may be related with poordhesion of a liquid to a solid. It is known that in the case ofonwetting liquid its vapors condensate in capillaries at higher

ressure than on the occasion of condensation at flat surface. Inny case the reducing of the adsorption isotherms to the linearnes, seems to point to the lack or inessentiality of microporosityor PPy at low-temperature, also to the non-formation of the

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able 1he porosity parameters for PPy films doped with different anions

arameter Dopant-ion of PPy film

ClO4− Cl− NO3

− Tos−

BET (m2 g−1) 429.8 370.7 350.3 259.7

tot (cm3 g−1) 0.37 0.35 0.32 0.23

AV (A) 17.3 18.8 18.3 17.7

BJH (m2 g−1) 240.5 224.2 201.4 148.4

BJH (m3 g−1) 0.36 0.34 0.31 0.22

NLDFT (m2 g−1) 135.4 128.8 116.6 83.8

NLDFT (m3 g−1) 0.34 0.32 0.29 0.21vv (BJH) 54.1 50.6 46.0 33.6vv* (BJH) 51.9 47.1vv (Vtot) 55.8 52.3 48.0 34.4

BET, BET surface area; Vtot, total pore volume; rAV, average pore radius; % vv, peercent of void volume in PPy film, using different dPPy values for different PPy/dopaPPy/Tos), d = 1.83 g/cm3 (PPy/PSS) presented by Holzhauser and Bouzek [46]. The

ls 157 (2007) 1085–1090 1087

oncave surfaces in PPy mesopores, which would call forth theapillary condensation process. It is worth to note here that theodel of compact polymer layer, given by Vorotyntsev et al. [34]

nd the model of homogeneous porous membrane, developed byhrenbeck et al. [35], can both be used for PPy characterization.

.2. PPy surface and volume parameters

The various surface and volume parameters, characteristico PPy/different dopant films, calculated from obtained adsorp-ion isotherms according to BET [32,36] and density functionalheory (DFT) [32,37] theories, are given in Table 1. The calcula-ion of specific surface area SBET was performed up to a nitrogenelative pressure (p/p0) = 0.2 and the total volume of pores Vtotas obtained at the conditions near to the saturation pressure/p0 = 0.99. It should be noted that the specific surface areasnd pore volumes were determined using the classical macro-copic thermodynamic Barrett–Joyner–Halenda (BJH) theoryuitable for mesoporous materials with pore width from 20 to00 A (SBJH, VBJH) as well as by the non-local density functionalheory (NLDFT), using the so-called slit-shaped pores modelor micropores (SNLDFT, VNLDFT) [32,37,38]. BJH-method isased on the assumption that equilibrium between the gas phasend the adsorbed phase during desorption is determined by twoechanisms: (1) physical adsorption on the pore walls (whichould occur to the same extent whether the area involved con-

tituted walls of pores or a flat surface impenetrable to nitrogen)nd (2) capillary condensation in the “inner capillary volume.”o define the relationship between volume of capillary conden-ate and relative pressure, the classical Kelvin equation, relatingapor pressure depression to capillary radius, is used [38]. Bysing NLDFT as one of the particular cases of DFT, also theore size distribution in PPy films was determined. It is wortho mention that this (DFT) theory, based on statistical mechan-cs, connects macroscopic properties to the molecular behavior,

nd allows (by minimizing a free-energy functional) to calculatehe equilibration density profile for all locations in the pore. Theesults in Table 1 shows that BET surface areas for different PPylms vary from 188 to 430 m2/g. Thus, the SBET for bare PPy

NS− NDS2− SO42− DDS− PSS−

187.6 246.2 391.2 233.7 277.90.18 0.22 0.36 0.21 0.2518.8 17.9 18.5 17.9 18.0114.2 140.2 227.9 131.3 158.70.17 0.21 0.34 0.20 0.2464.5 80.6 133.5 75.7 92.20.16 0.20 0.33 0.19 0.2325.7 31.8 51.7 29.8 35.8

58.6 43.726.4 33.1 54.3 31.3 37.6

rcent of void volume in PPy film, calculated at dPPy = 1.50 g/cm3 [1]; % vv*,nt films as: d = 1.57 g/cm3 (PPy/Cl), d = 1.70 g/cm3 (PPy/SO4), d = 2.10 g/cm3

explanation of abbreviations is here and in the text.

1088 A. Hallik et al. / Synthetic Metals 157 (2007) 1085–1090

Fig. 2. (a) Surface area vs. pore radius plots, using NLDFT method (surfacehvd

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istograms) for PPy films doped with anions, noted in figure. (b) Pore volumes. pore radius plots, using NLDFT method (volume histograms) for PPy filmsoped with anions, noted in figure.

n its oxidized form is approximately 5–7 times less than thepecific surface area of activated carbon electrodes, operatinglectrostatically in organic solutions of electrolyte (S from 1000o 3000 m2/g). The SBET for PPy exceeds 2–4 times the corre-ponding values of another supercapacitor material—transitionetal oxides in aqueous solutions of electrolyte, in which oper-

tion is based on the electrochemical faradaic surface reactionsSBET around 100 m2/g) [15,39].

In comparison with some PPy—inorganic oxide nanocom-osites [6], our bare PPy films showed the SBET values of 4–5imes higher, and possess practically the same SBET values as theesoporous pure carbon nanotubes discussed by Frackowiak

nd Beguin in Ref. [15] (ca. 100–410 m2/g).The SBJH and particularly SNLDFT values are essentially lower

n comparison with the classical SBET values (Table 1). This kindf effect has also been observed elsewhere [36,37,40], but inur case the differences are larger than those for carbonaceousaterials studied.The surface area or pore volume versus pore radius (r) plots

or differently doped PPy films, obtained using NLDFT method,re presented in Fig. 2. According to these histograms and tohe differential pore size distribution dV(d)–r plots (Fig. 3),

he only porosity region observed is the mesopore one, withhe maxima near 16 A, practically independent of the dopantsed. The obtained plots (see Fig. 2) point to the lack of theicropore region for PPy synthesized galvanostatically. From

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ig. 3. Differential pore size distribution dV(d) vs. pore radius plots (NLDFTethod) for PPy films doped with anions, noted in figure.

he obtained data results that the PPy films in their oxidizedorm under investigated conditions, are in shrunk state causedy very low-temperature and lack of the swelling effect of sol-ent. However it is possible that the formation of such state ofuite closely adhered PPy chains has extinguished some pre-eding microporosity. A certain apparent similarity concerningelaxation/sticking together effect of PPy chains is also knownrom cyclic voltammetry measurements on the occasion of long-erm holding of PPy electrodes at externally applied potential ort open circuit potential condition in electrolyte solutions or sol-ents. After this kind of electrode “treatment”, the followingelectro)chemical processes may be essentially aggravated [41]ue to decreased porosity of the electrode material.

.3. PPy pore size distribution and its influence on redoxrocess

According to the data in Fig. 2 quota of the smallest poresith radius (r = 10–20 A) to the total pore volume of PPy/SO4

lectrode is ca. 15%, the corresponding value for the pores with= 10–50 A is ca. 65% and for pores with r between 10 and00 A is ca. 92%. So the degree of larger mesopores (r > 100 A)emains below 10% and the histogram curve reaches approx-mate zero volume relatively quickly at around 150 A for r.hese proportions/tendencies are similar also in the case ofther PPy films investigated. From the experimental results itecomes evident that the obtained sizes of the (meso)pores andheir distribution found for measured PPy films, enable not onlyhe motion of “dry” but also of hydrated ions in these mesopores.o the comparison of average pore radius rav = 17–19 A (BETethod) of this research with the comparable hydrated (rhydr) or

ffective radii (reff) of common cations and anions, discussed initeratures [42,43], shows that rav exceeds the latter ones approx-mately 4–5 times or even 7–9 times as compared with reff inef. [43]. Such multiple difference is valid as well in the case of

trongly hydrated bivalent ions and bulky tetraalkylammoniumations (for example tetrapropylammonium or tetrabutylam-

onium cations). It is worth to mention that for rigid carbonaterials it is concluded that in principle, the pores larger thanA could be accessible electrochemically for electrolyte ions

n aqueous solutions [15]. The proceeding of redox process

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n PPy is much more complicated. First our previous studiesf PPy/DDS and PPySO4 films [20,27,44] have shown thaton penetration ability into PPy film can depend both on theirdry” Pauling radius r or on rhydr values, being dependent onlm “history”. Even more, from experimental results it becamevident that the bulky triethylbenzylammonium and tetrabuty-ammonium ions practically cannot penetrate the PPy/DDS filmnd even tetraethylammonium ions permeate the film with bigifficulties or, in the case of PPySO4 films, do not permeatehe film at all. Similar effect was also found in the case ofPy/Tos films [45]. It is evident that the existence of quite bigesopores in PPy matrix alone cannot guarantee the good acces-

ibility of many (redox)active sites to moving ions, and the othermportant properties of PPy matrix and penetrating ions, asheir hydrophilic/hydrophobic nature, deformability, etc. muste taken into account. Probably the ions, moving in wetted byolution PPy matrix, themselves have to create a certain kind oficroporosity to access the redoxactive sites.In our previous paper it was established that the “elevated

orosity” can be obtained by use of other solvents instead ofater [27]. Thus, it was found that even bulky poorly mobile

n aqueous solutions dopant DDS− ions, can be easily removedrom PPy matrix, swollen in alcohols or acetonitrile by elec-rochemical reduction or by soaking polymer electrodes inlectrolyte solutions of these solvents. Therefore the conduct-ng polymers, having swelling capacity, differ from rigid carbon

aterials, where in many aprotic solvents due to increased sizef solvated ions, their mobility inside the pores decreases and theumber of non-accessible pores increases, causing the decreasef the electrical capacitance of the material with narrow micro-ores [15,47–49].

The comparison of the porosity of dry PPy films of the presenttudy with the porous structure of the other well known repre-entatives of conducting polymers as polyaniline (PANI) andolyparaphenylene (PPP) swollen in electrolyte solution [3]hows the closeness of the corresponding porosity values. Soccording to the data in Table 1 the percent of void volume inPy dry films was found mainly to be in the range of 25–55%,epending mainly on the dopant, but also dependent somewhatn the calculation method and PPy film density used. The inter-al porosity values in the range of some tens of percent werelso found for PANI and PPP films [3]. The specific surfacereas S, from 188 to 430 m2/g for PPy (Table 1) or 250–600 m2/gor PANI and PPP [3] seem to be quite typical for electricallyonductive polymers. Quite interesting is the absence of essen-ial microporosity for all above mentioned conducting polymerstudied at different conditions. The medium pore radii exceededn both cases the value of 10 A. The essential difference in upperimit of pore radii for PPy (100–150 A) and for PANI and PPP103 to 104 A) are presumably essentially achieved by swellingffect.

.4. The influence of dopant nature on PPy porosity

The comparison of the porosity parameters of studied PPylms doped with the counter ions of different size, charge andature allow to conclude that although the mentioned parame-

ab

ls 157 (2007) 1085–1090 1089

ers practically have no effect on the average pore radius, theytill have a considerable influence on the total pore volume (V)nd specific surface area (S) values. According to the data fromable 1, the following row of increasing percent of void volumef PPy films doped with different anions can be brought forth:

NS− < DDS− < NDS2− < Tos− < PSS− < NO3− < Cl−

< SO42− < ClO4

−.

he obtained arrangement shows that from among the studiedlms, more compact are the ones containing large oligomericurfactants such as naphthalene (di)sulfonates and dodecylsul-ate. In addition to existing electrostatic PPy+ and negativelyharged dopant anion interaction this effect may at least par-ially be caused by the �-electronic donor–acceptor interactionetween the aromatic ring and the oxidized form of PPy, bothesulting in suitable and more compact packing of PPy chainsith large dopant anions [41]. Additionally to the “better pack-

ng” effect the larger anions fill more spaciously, than smallnions, the void space of PPy matrix. The possible advantage ofDS− ions to form more compact films can be explained by itsexible “tail” and mono-charged “head” structure. Somewhatparser PPy film structure is generated by smaller alkyl ben-ene sulfonate (Tos−) and polymeric surfactant anions (PSS−).n the case of (PSS−) anions the very big chain and repeatingver chain unit sulfonate groups may cause some hindrances toense packing. As evidenced by the data in Table 1 the smallydrophilic inorganic anions produce the films with the spar-ity approximately from 1.5 to 2 times more than by using ofnionic surfactants as dopants. The belonging of double-chargedO4

2− ion to the group of dopants, producing the PPy layers withigher porosity seems somewhat unexpected as compared withhe results presented in Ref. [17] or our previous results [20],ccording to which as evidenced by SEM microphotographs,O4

2− ions give smooth and very different, as compared withPy/ClO4 film, surface. Evidently the surface morphology aloneannot sufficiently characterize the whole polymer layer struc-ure.

Additionally, the porosity of one film (PPy/SO4) was charac-erized by applying benzene sorption method. The porosity value

S ≈ 0.02 cm3/g, obtained using benzene adsorption method isssentially lower than the corresponding pore volume valuesalculated from nitrogen adsorption measurements. The reasonor ineffective adsorption of benzene onto PPy is not clear, butt is obviously related with benzene (chemical nature) peculiar-ty, among this (at least as one of the reasons) with the recentlystablished low surface activity and weak adsorption of non-olar benzene at the PPy/solution boundary [41], also with theifferent experimental conditions—essentially higher tempera-ure (room temperature) for benzene adsorption as comparedith the temperature for N2 adsorption (−196 ◦C).

. Conclusions

The porous structure of PPy films doped with variousnions of different size, charge and nature were investigatedy low-temperature N2 sorption measurement method. Differ-

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nt calculation methods as BET, BJH and NLDFT were used toharacterize quantitatively the specific surface area, total poreolume, average pore radius and pore size distribution of dopedPy films. It was established that these PPy films have signifi-ant mesoporosity with the pore radii (r) basically in the rangerom 10 to 100 A. It should be noted that nearly 66% of the poresave radius from 10 to 50 A. The performed experiments did nothow considerable microporosity of studied PPy films, whichan at least partially explain the essential mobility aggravationsf larger ions in PPy layers soaked in the electrolyte solutions.he comparison of the porosity parameters of studied PPy filmsoped with the counterions of different type allowed to concludehat although the parameters of dopant ions practically had noffect on the average pore radius of the PPy film (rav = 17–19 A),owever they had a considerable influence on the total pore vol-me (V) and specific surface area (S) values. The more compacttructure of the films containing large oligomeric surfactants asodecylsulfate and naphthalene (di)sulfonate anions, can prob-bly be explained by the better film package and more effectiveccupation of the void space in PPy by big dopants. The smallydrophilic inorganic anions produced the PPy films approxi-ately from 1.5 to 2 times more sparse than the large anionic

urfactants.

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