Thin Solid Films 519 (2011) 3596–3602

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Electrosynthesis and analysis of the electrochemical properties of a compositematerial: Polyaniline+titanium oxide

Souhila Abaci a, Belkacem Nessark a,⁎, Rabah Boukherroub b, Kamal Lmimouni b

a Laboratoire d'Electrochimie, Département de Génie des Procédés, Faculté de Technologie, Université Ferhat-Abbas 19000 Sétif, Algeriab Université des Sciences et Technologies de Lille, IEMN-UMR CNRS 8520, BP 60069 avenue Poincaré, 59652 Villeneuve d'Ascq Cedex, France

⁎ Corresponding author at: Laboratoire d'Electrochimde Génie des Procédés, Faculté de Technologie, UniverAlgeria. Tel./fax: +213 36 92 51 33.

E-mail address: b.nessark@yahoo.fr (B. Nessark).

0040-6090/$ – see front matter © 2011 Elsevier B.V. Adoi:10.1016/j.tsf.2011.01.277

a b s t r a c t

a r t i c l e i n f o

Article history:Received 16 January 2010Received in revised form 18 January 2011Accepted 21 January 2011Available online 31 January 2011

Keywords:Conducting organic polymerPolyanilineTitanium oxideCompositeCyclic voltamperometryElectrochemical impedance spectroscopy

The analysis of the electrochemical and spectroscopic properties of a composite material obtained startingfrom the polyaniline and TiO2 in H2SO4 medium, using cyclic voltamperometry, shows three redox couplescharacteristic of the different oxidation and reduction states of produced polymer. The electroactivity of thecomposite in acid mediumwas better than that obtained in basic medium. The impedance spectroscopy studyshows that the resistance of the film increases with the aniline concentration, but is not significantly affectedby the amount of TiO2 incorporated in polymer. The increase of pH decreases the resistance of the films andconsequently increases its conductivity.

ie et Matériaux, Départementsité Ferhat-Abbas 19000 Sétif,

ll rights reserved.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Polyaniline, existing under different oxidation states, can besynthesized either by chemical or electrochemical oxidation ofaniline. Like other conducting organic polymers, the polyaniline(PAn) was largely studied because of its metal behavior, goodchemical and thermal stabilities, redox reversibility and its highelectric conductivity when it is doped in acid medium [1,2]. Inaddition, polyaniline aroused a great interest due to its large field ofapplications like batteries [3], protection of metals against corrosion[4], electrocatalysis [5,6], biosensors (analysis of ADN, proteins, andantipollution) [7,8], electrochromism (flat-faced screens and diodes)and use in electronic components [9–12]. However, PAn shows nearlyno electroactivity in media with pH N4. Thus, the redox potential ofspecies to be oxidized and reduced by PAn should be within thepotential range in which PAn itself is electroactive. This restricts itsapplication in bioelectrochemistry, which requires an environmentneutral pH [7]. The synthesis of polyaniline is known for a long time. Itis in 1862 that Lethby [13] obtained it for the first time, by anelectrochemical synthesis. He describes this product as a dark greenprecipitate, deposited on the electrode. In 1986 Mac Diarmid (NobelPrize of Chemistry 2000) described the PAn like an organic conductingpolymer [14]. Polyaniline as an intrinsic conducting polymer shows a

marked redox behavior. In its reduced state, it is considered as aninsulator whereas when it is partially oxidized, it is a conductor(σ=10 S cm−1). The form of conducting polymer can be synthesizedby electrochemical oxidation of the entirely protoned form of the PAn[16,17].

The conducting polymer can be synthesized from a base form ofemeraldine acidified with varying pH or by electrochemical oxidationof the entirely protonic form of PAnwhich is leucoemeraldine. Variousfactors such as the oxidation degree of polymer, the protonation andits percentage influence the conductivity of PAn. The latter is alsoaffected by the degree of moisture content, morphology and texture ofthe polymer, the method of the deposit, the temperature and thedegree of crystallinity [15].

The polyaniline can indeed be prepared in various states ofoxidation. The reversibility controls its electronic structure and itselectrical properties [18]. As for other conducting polymers [19–21],the doping of the polyaniline can be carried out according tooxydoreduction reactions which are accompanied by a modificationof the number of electrons of the π system. The stability of thepolyaniline depends on its oxidation state, the doping agent, thephysical state of the sample and its environment (inert or oxidizingatmosphere).

Organic conducting polymers are materials which have applica-tions in the current industrial techniques, but start to bore at thisbeginning of themillennium. In 1977 Heeger andMac Diarmid [14,22]worked on the doping of polycetylene with iodine, giving a polymerhaving metallic properties, and thus leading to an increase inconductivity of about ten. The same group [23] also tested the PAn

-140

-70

0

70

140

(a)

E(V/SCE)0.0 0.2 0.4 0.6 0.8-0.2

-4

0

4

8

(b)

E(V/SCE)0.0 0.4 0.8

I(μA

/cm

2 )I(

mA

/cm

2 )

Fig. 1. Cyclic voltammograms corresponding to a solution of aniline (An) 10−1 Mdissolved in H2O/(LiClO4 0.1 M+H2SO4 0.5 M), obtained with v=10 mV/s, between−0.2 and 0.9 V/SCE, a) First cycle, and b) cycling.

3597S. Abaci et al. / Thin Solid Films 519 (2011) 3596–3602

as active electrodes in rechargeable batteries. The use of conductingorganic polymers such as polyaniline, polypyrrole and polythiophenemade it possible to obtain composites having high conductivity, andgood electrochemical, mechanical and optical properties. The dopingof the polyaniline in electrochemical terminology is equivalent to theelectrochemical oxidation or reduction. Thus, inorganic/organiccomposites have been investigated extensively in recent years dueto a wide range of potential use of these materials [24–26]. Oneimportant class of such hybrid materials is that in which the organicfraction is composed of conducting polymers, such as polyaniline orpolypyrrole. It is hopeful to obtain composite materials withsynergetic or complementary behaviors between polymer andinorganic matrices. For this reason, a large number of studies havebeen devoted to the preparation of polyaniline-TiO2 nano-compositematerials and their morphological characterization, such as size andshape of the oxide particles, degree of dispersion, kind of interaction,and interface between organic and inorganic compounds, etc. [27–29]. Schnitzler et al. [30] have prepared the polyaniline-TiO2

composite materials and characterized them. Their works provedthat TiO2 could improve the thermal stability of polyaniline.

Our study concerns the synthesis, and electrochemical andspectroscopic characterization of a PAn+TiO2 composite materialobtained from the polyaniline, and titanium oxide semiconductor. Thepurpose of this work is the use of this composite material in certainapplications like: energy storage, electrode material, photovoltaic cellsand electrocatalysis. Thus, the analysis of the reactional mechanism,transport charge anddiffusion, occurring inside this compositematerial,and the doping effect on the electrochemical propertieswere studied bycyclic voltamperometry and impedance spectroscopymeasurements, inorder to have information on its electrochemical properties.

2. Experimental details

In this study, distilled water is used as solvent. The medium ismade acid adding one of these acids H2SO4, HCl, or HClO4. Thesupporting electrolyte used is lithium perchlorate (LiClO4) (Flukaproduct), which is a pure salt for analysis. This electrolyte is chosenbecause of its solubility in organic and aqueous solution, and of itselectrochemical stability on a large domain of potential. The reagents(Aldrich product) are: titanium (IV) oxide (TiO2) rutile −99.9,powder, as doping semiconductor, and aniline with 99.99% purity asmonomer.

Cyclic voltamperometry was carried out in a glass cell with doublewalls closed by a cover, containing the solution, in which threeelectrodes are immerged: the working electrode is a platinum disk(Ø=2mm), the reference electrode is a saturated calomel electrodewith KCl (SCE) and the auxiliary electrode is a platinum wire. Thematerial composite was obtained by cycling, from the colloid: H2O/(LiClO4 0.1 M+H2SO4 0.5 M+An 0.1 M+TiO2 0.1 M), at v=10mV/s,between −0.2 and 0.9 V/SCE. The electrolysis was carried out understirring, in order to keep the TiO2 particles in suspension and let theparticles to be continuously in contact with the electrode surfacewherethe composite film is being deposited.

Electrochemical impedance spectroscopy (EIS) measurementswere performed using an alternative current voltage of 10 mV, atopen circuit potential (Eocp), in the frequency range 10−3–105 Hz. Thecell and the electrodes used are identical to those used in cyclicvoltamperometry. The study was related to films of polyaniline dopedor not by TiO2. In this case, the working electrode is a platinum disk(Ø=2 mm) modified by this film, obtained electrochemically bycycling on a potential domain ranging between −0.2 and 0.9 V/SCE.The analysis of the filmwas carried out in a system solvent/supportingelectrolyte, in absence of the monomer. The recording of the curvesI=f(E) or − Im(Z)=f(Re(Z)) is ensured by a standard potentiostatVoltalab PG 301, piloted by a microcomputer equipped with avoltamaster 4 software for the data processing. The surface morphol-

ogy of the films was investigated by scanning electron microscopy(ZEISS ultra 55). The operating voltage used is 1 kV. The apparatus iscoupled with energy dispersive X-ray spectroscopy. The EDXoperating parameters are HV:10 kV and Puls:12.84 kcps, where cpsis the current gross count rate.

3. Results and discussion

3.1. Electropolymerization of aniline in H2SO4 medium

Fig. 1 shows a successive cyclic voltamperograms corresponding toaniline 10−1 M dissolved in H2O/(LiClO4 0.1 M+H2SO4 0.5 M)solution, recorded in a potential range between −0.2 and 0.9 V/SCE,with scan rate v=10 mV/s.

The voltamperograms (Fig.1a) show during the positive scanpotential three anodic peaks at Epa1=0.226, Epa2=0.472 andEpa3=0.682 V/SCE respectively. During cathodic scan three peaks atEpc1=0.088, Epc2=0.406 and Epc3=0.586 V/SCE are observed re-spectively. These peaks correspond to different oxidation andreduction states of polyaniline. The difference between anodic andcathodic peak ΔEp (with ΔEp=EPa−EPc) of the three redox couplesare ΔEp1=0.138, ΔEp2=0.066 and ΔEp3=0.096 V/SCE, respectively.The second redox couple is the most reversible. This is justified by thevalue of ΔEp which is very close to 60 mV [31], variation which isgenerally obtained for reversible systems. In addition, the ratio of the

anodic and cathodic peak currentsIpa2Ipc2

is about 1.

3598 S. Abaci et al. / Thin Solid Films 519 (2011) 3596–3602

During the successive cyclic voltamperograms (Fig.1b), thepotential of oxidation of the first redox couple shifts slightly tomore positive values and that of the third couple moves towardsmorenegative values. However, the potential of the second redox coupleremains unchanged during cycling. The displacement of the potentialpeaks is accompanied by an increase in the current intensity of theoxidation and reduction peaks of the three redox systems. This is doneat the same time as the accumulation of a deposit of polyaniline on theelectrode surface. The current intensity and the potential of thedifferent peaks are stabilized after several cycles, attesting by this factthat a thermodynamic and kinetic stability of film is obtained on theelectrode. The increase of the two electrochemical parameters(current and potential) is weak only in the case of the second redoxcouple. However, it is noted that we observe the same peaks ofoxidation and reduction in the three cases of acid (HCl, HClO4, andH2SO4), but the peaks are verywell definite onlywith the sulfuric acid.

3.2. Cyclic voltamperometry of polyaniline on Pt electrode (PAn/Pt)

Fig. 2 shows the cyclic voltamperograms corresponding topolyaniline deposit, analyzed in H2O/(LiClO4 0.1 M+H2SO4 0.5 M)free solution, recorded with v=10 mV/s, between −0.2 and 0.9 V/SCE. Polyaniline is obtained by successive scanning of potential(cycling) starting from a solution of aniline 10−1 M. As the cyclingcontinues, a steady-state voltamperogram is reached after severalscans. On washing the modified electrode and transferring it into afree electrolyte, the cyclic voltamperogram revealed the same peaks

-6

0

6

12

(a)

E(V/SCE)0.0 0.4 0.8

-6

0

6

12

(b)

E(V/SCE)0.0 0.4 0.8

I(m

A/c

m2 )

I(m

A/c

m2 )

Fig. 2. Cyclic voltammograms relating to polyaniline (PAn/Pt), in a H2O/(LiClO4 0.1 M+H2SO4 0.5 M) solution, obtained at v=10 mV/s and between −0.2 and 0.9 V/SCE.a) First cycle, and b) cycling.

showing that the PAn polymer formed has the same electrochemicalbehavior than that observed for the monomer.

As shown by the cyclic voltamperograms, we note three anodicpeaks during the positive potential scan and three cathodic peaksduring negative scan (Fig. 2a). However, contrary to what wasobserved for the monomer, the intensity of the oxidation andreduction peaks of the polymer film decreases slightly during thecycling, and stabilizes after several cycles. The potential of theoxidation and reduction peaks moves towards more positive values.As shown in Fig. 2b, the cathodic peaks shift to positive potentials andtherefore the difference between the anodic and the cathodic peakpotentials (ΔEp) decreases in comparison with that observed duringthe electropolymerization of the monomer, indicating better revers-ibility. However, the peak ratio of the first redox couple is muchhigher than unity, which indicates that the film formed undergoes afollow-up reaction on this film. Moreover, cycling PAn film betweenreduced and oxidized states at PAn covered electrode is accompaniedby the decrease of peak intensity essentially for the first and the thirdtwo couples assignable to PAn, i.e. those corresponding to leucoemer-aldine/emeraldine and emeraldine/pernigraniline transitions. Simul-taneously the development of a new redox system (Epa2=0.472 V,Epc2=0.406 V) may be seen. Note also a positive shift of the first PAncouple simultaneously with a negative shift of the third couple,leading finally to their merging with the peaks of the second couple.The same shifts were noticed for substituted PAns [5,32], and wereattributed to steric and electronic effects provided by bulky organicdoping ions.

Fig. 3 shows the cyclic voltamperograms obtained for differentconcentrations of aniline 10−4, 10−3, 10−2, and 10−1 M dissolved inH2O/(LiClO4 0.1 M+H2SO4 0.5 M) solution. The voltamperograms arerecorded, in a potential domain ranging between−0.2 and 0.9 V/SCE,with v=50 mV/s.

It is noticed that the shape (intensity, potential, and number ofredox couple) of the cyclic voltamperogram varies with theconcentration of aniline. Cyclic voltamperogram shows at the weakconcentrations only one wave of oxidation and another of reductionwhich is very slightly marked. However, for strong concentration theCV shows three reversible redox couples. The peaks of the first couplewere most definite and intense. The second couple was mostreversible but it is the less intense. The concentration for which theoxidation and reduction peaks are well defined is 10−1 M. For that wechoose this concentration in what follows.

We note here that we remarked that the open circuit potentialdecreases with the increasing of the concentration of aniline. Thesevalues are: 0.493, 0.385, 0.340 and 0.130 V/SCE, respectively forthe concentrations 10−4, 10−3, 10−2 and 10−1 M. The experimental

-12

-6

0

6

12 (d)

(c)

(b)(a)

E(V/SCE)0.0 0.4 0.8

I(m

A/c

m2 ) (a) [An] = 10-4 M (c) [An] = 10-2 M

(b) [An] = 10-3 M (d) [An] = 10-1 M

Fig. 3. Cyclic voltammograms obtained for different aniline concentration (indicated onthe figure) dissolved in H2O/(LiClO4 0.1 M+H2SO4 0.5 M) solution, recorded withv=50 mV/s, between −0.2 and 0.9 V/SCE.

0

250

500

(e)

(d)

(c) (b)(a)

E(V/SCE)

(a) pH = 1 (d) pH = 12

(b) pH = 4.5 (e) pH = 14

(c) pH = 7

0.0 0.4 0.8

-42

-28

-14

0

14

28 pH = 1

I(μA

/cm

2 )

E(V/SCE)0.0 0.4 0.8I(

μA/c

m2 )

Fig. 5. Cyclic voltammograms relating to a solution of aniline 10−1 M dissolved in H2O/(LiClO4 0.1 M+TiO2 10−3 M), obtained for different pH values, with v=10 mV/s,between −0.2 and 0.9 V/SCE.

3599S. Abaci et al. / Thin Solid Films 519 (2011) 3596–3602

potential value, corresponding to the concentration of 10−1 M wassituated between the first and the second oxidation peak. Thissuggests that the chemical structure and the thermodynamicproperties of the electrochemical double layer change with anilineconcentration.

3.3. Synthesis of composite material (PAn+TiO2)/Pt

Fig. 4 shows the cyclic voltamperograms of the H2O/(LiClO4

0.1 M+H2SO4 0.5 M) solution, containing An (0.1 M) and TiO2 atdifferent concentrations indicated on the curve obtained atv=10 mV/s, between −0.2 and 0.9 V/SCE. The recording of cyclicvoltamperograms (first cycles), show three redox couples as thoseusually observed in the absence of TiO2. However, we observe a shiftof the oxidation potential of the first peak toward more positivevalues and the third toward more negative values, whereas theoxidation potential of the second redox couple remains practicallyunchanged. Also, the oxidation and reduction peak current intensitiesof the first two peaks increase slightly with the concentration of thesemiconductor (TiO2). However, the third oxidation peak remainsinvariant. We note here that the previous studies [33] have shownthat TiO2 incorporation into a PPy matrix during its electrochemicalsynthesis on mild steel under magnetic stirring leads to the formationof PPy+TiO2 composite films containing a significant quantity byweight. These composite films show a slight improvement in the PPycorrosion protection of mild steel [27]. Further studies of TiO2

incorporation onto Pt during electrochemical synthesis of PAn usingstronger electrolyte convection with looping to increase TiO2

concentration into PAn are shown in this work. The Pt coated withthe PAn+TiO2 composite films were then submitted to electrochem-ical characterizations.

The time of formation of the film on the electrode and theconcentration of the titanium oxide incorporated in the matrix ofpolymer, increase with stirring. The electropolymerization of anilineleads to the formation of thicker and homogenous PAn+TiO2

composite films on the platinum. Thus, stirring keeps the TiO2

particles in suspension and causes the particles to be continuously incontact with the electrode surface where the composite film is beingdeposited. The results confirm that this incorporation is greatlyfacilitated by mechanical stirring. In accord with what was reported[34], a very small quantity of TiO2 was detected in the compositematerials synthesized in the H2O/(LiClO4+H2SO4) electrolyte with-out stirring even using pH values weaker than 4, compared to thoseobtained in the same medium with stirring.

-140

0

140

280

(d)(c)(b)(a)

E(V/SCE)

0.0 0.4 0.8

I(μA

/cm

2 ) (a) [TiO2] = 0 M

(b) [TiO2] = 10-3 M

(c) [TiO2] = 10-2 M

(d) [TiO2] = 10-1 M

Fig. 4. Cyclic voltammograms relating to a solution of An 10−1 M dissolved in H2O/(LiClO4 0.1 M+H2SO4 0.5 M), obtained for different contents of TiO2 shown in thefigure, recorded at v=10 mV/s, between −0.2 and 0.9 V/SCE, on a Pt electrode(ϕ=2 mm).

Fig. 5 shows the cyclic voltamperograms (first cycles), of aniline10−1 M dissolved in H2O/(LiClO4 0.1 M+H2SO4 0.5 M+TiO2 10−3 M)colloidal solution, recorded in a potential range of −0.2 to 0.9 V/SCE,obtained for different pH of the solution, with a scan rate v=10 mV/s.The variation of pH is obtained by adding NaOH 2.5 10−3 M to thecolloid.

In a strongly acid mediumwe observe the oxidation and reductionpeaks of the three redox couples. Whereas, the increase of the pHleads to the disappearance of oxidation and reduction peaks usuallyobserved in acid medium and reveals only one irreversible oxidationpeak which is approximately at potential of oxidation of the thirdredox couple. The later is very intense at pH=14 and it correspondsto emeraldine/pernigraniline transition. The analysis of films obtainedin basic medium in a system solvent/supporting electrolyte showsthat these films are less electroactive. As reported [23], on pH risingabove 4, the inner PAn layer becomes deprotonated and thus acts asinsulator, making it difficult for electrons to transfer between theelectrode support and the outer redox polymer. In contrast, thin filmsare almost completely converted into the redox polymer and thus byclose contact with the platinum electrode are able to exchangeelectrons in a wide range of pH.

Thus, to investigate the electroactivity of the PAn+TiO2 compositefilm, the CVs of this latter have been recorded for different pH-valuesof solution. The results indicate that (PAn+TiO2)/Pt shows nearly noredox activity in media with pH N5. While it can be seen from Fig. 4that the modified electrode shows three pairs of redox peaks in pHacid (1 and 4.5) the two sets of peaks at around 0.226/−0.088 V and0.682/0.586 V are attributed to the transformations of leucoemer-aldine/emeraldine and emeraldine/pernigraniline, respectively, andthe third pair of peaks in the middle (0.472/0.406 V) is attributed tothe defects in the linear structure of the polymer. This is in accordancewith that reported by Lei Zhang et al. [7] who remarked that themiddle pair of peaks almost disappears when solution pH value ismore than 5.

3.4. Characterization by electrochemical impedance spectroscopy (EIS)

3.4.1. Effect of the concentration of anilineThe impedance spectra of PAn/Pt films measured at open circuit

potential (0.230 V/SCE) is shown as Nyquist diagrams in Fig. 6. The filmwas analyzed in aqueous solution containing (H2SO4 0.5 M+LiClO4

0.1 M). The polyaniline film is obtained starting from a variableconcentration (10−4, 10−3, 10−2, and 10−1 M) of the monomer in

(d)(c)(b)(a)

0.03 0.06

0.01

0.02

0.03

0.04

-Im

(Z)[

kΩ.c

m2 ]

Re(Z)[kΩ.cm2]

(a) [An] = 10-4 M

(b) [An] = 10-3 M

(c) [An] = 10-2 M

(d) [An] = 10-1 M

Fig. 6.Nyquist diagrams relating to films of PAn/Pt in a solution exempt ofmonomerH2O/(LiClO4 0.1 M+H2SO4 0.5 M), recorded on a frequency range 105 Hz and 5.10−2 Hz, at theabandonment potential, at a disturbance of 10 mV. The film is obtained starting from thesolutions containing various aniline concentrations 10−4, 10−3, 10−2, and 10−1 M.

(b)

(a)(a) pH = 1

(b) pH = 4.5

0.0 0.8 1.6

1.8

2.40.0

0.6

1.2

(e) (d) (c)

(c) pH = 7

(d) pH = 12

(e) pH = 14

0.4 0.8

0.1

0.2

0.3

0.0

-Im

(Z)[

kΩ.c

m2 ]

-Im

(Z)[

kΩ.c

m2 ]

Re(Z)[kΩ.cm2]

Re(Z)[kΩ.cm2]

Fig. 7. Nyquist diagrams relative to films of (PAn+TiO2)/Pt, obtained for various valuesof pH of the solution and analyzed in a solution in the absence of themonomer and TiO2.

3600 S. Abaci et al. / Thin Solid Films 519 (2011) 3596–3602

solution. The Nyquist diagrams were plotted on a frequency bandranging between 105 Hz and 0.5 Hz, with an alternative current voltageof 10 mV. As can be seen, EIS spectra have a single semi-circle in thehigh frequency region and a straight line in the low-frequency region,which are characteristic respectively of a process of charge transfer andanother of diffusion. The diameter of half-circle increases with theaugmentation of the concentration of aniline suggesting by this fact anaugmentation in resistance and thus a decrease in the conductivity offilm. All the half circles start from the same resistance. This means thatthe resistance of the electrolyte remains unchanged. The straight linesof diffusion are all parallel with the same slopes. It comes out from thisthat the diffusion process of the electroactive species is independent ofthe aniline concentration. We note here that, we have in all timesanalyzed the film in the same experimental conditions to avoid anychange that comes from themodification of film thickness. So, the filmswere analyzed by impedance spectroscopy when the cyclic voltammo-gram becomes reproducible indicating no change in the potential andcurrent peaks. This indicates that the film thickness remains constant.This thickness is approximated to a value of few microns. For example,measurements of film thickness of the polyaniline obtained underexperimental conditions more or less similar than thosewe used; showthat the thickness is of the order of several microns [35]. The optimizedthickness of the film calculated by a weight difference method,employing a sensitive microbalance was 0.9 μm [36].

However, we note here that the presence of TiO2 in the polymerfilm, affects slightly the electrochemical behavior of PAn. The compositefilms (PAn+TiO2)/Pt, obtained from different concentrations of theTiO2 for fixed concentration of aniline, analyzed in aqueous solutioncontaining (H2SO4 0.5 M+LiClO4 0.1 M), give an arc of circlecharacteristic of the charge transfer process at high frequencies,followed by a straight line at low frequencies, corresponding to adiffusion process. The Nyquist diagrams obtained are the same and this,whatever is the content of TiO2. This suggests that the phenomenonwhich governs the kinetic process is not affected by the presence of thetitanium oxide. This is probably due to the non electroactivity of TiO2

and its non-solubility in the medium. However the increase of thecurrent intensities observed in cyclic voltamperometry for differentpeaks can be due to the capacitive effect resulting to specific surfacewhich becomes more important and more porous.

It is deduced from the semicircle diameters that the resistanceof charge transfer polarization (Rp) of the composite material

(PAn+TiO2/Pt), increases with aniline concentration. The filmbecomes less conductive and more capacitive for high concentrations.According to what is shown in further studies [37–40], these featuresare typical for a polymer film-coated metal in the asymmetric metal/film/electrolyte configuration. This result implies that the filmobtained from the weak concentration of aniline increases theelectronic conductivity of the PAn/Pt and it can guarantee fasterelectron exchange on the electrolyte/electrode interface.

3.4.2. Effect of the pH of the solutionEIS has been employed to investigate the impedance changes of the

electrode surface in the modified process. Fig. 7 represents the Nyquistdiagrams corresponding to a modified platinum electrode by a(PAn+TiO2) composite material film obtained for different pH valuesof the solution. The variation of pH is obtained by dosage of the H2O/(LiClO4 0.1 M+H2SO4 0.5 M+TiO2 10−3 M+An 0.1 M), by a sodasolution (NaOH 2.5.10−3 M). This film is analyzed in an aqueoussolution containing (H2SO4 0.5 M+LiClO4 0.1 M), without monomerand dopant. The curves were recorded on a frequency ranging between105 Hz and 0.5 Hz, at the open circuit potential (Eocp=0.230 V/SCE) atan alternative current voltage of 10 mV. We note here that we haveremarked that the Eocp decreases with the increase of the pH of thesolution. The open circuit potential (Eocp) values obtained for pH=1,4.5, 7, 12 and 14, for example, are 0.563, 0.408, 0.393, 0.390 and0.380 V/SCE, respectively. This variation which is more significant foracid pH values is stabilized for basic pHs.

3601S. Abaci et al. / Thin Solid Films 519 (2011) 3596–3602

As shown, The Nyquist diagrams show a semicircle at highfrequencies and a straight line at low frequencies for pH=1, 12,and 14. The semicircle part corresponds to the electron transferlimited process and the linear part to the diffusion process. Whereas,for values of pH close to 4.5 and 7, we observe, a change of the diagramform. In this case, we notice the appearance of two successive arcs ofcircles at high frequencies, suggesting the presence of two processesof charge transfer, followed by a straight line to the low frequenciescharacteristic of diffusion process. In acidic medium, the semicircle isvery capacitive. However, in the basic pH, the half-circle radiusdecreases with the increase of the pH and the slope of the diffusionstraight line remains practically similar in all observed cases. Asshown by impedance diagrams, the polarization resistance of the(PAn+TiO2)/Pt composite, changes with the modifying pH andaniline concentrations. These changes of the resistance of polarization(Rp) indicate that the structure, the adherence to the electrodesurface and the physical properties of composite material PAn+TiO2

are very dependent to the concentration of aniline and the pH value ofthe colloidal solution.

3.5. Morphology and elemental composition

The morphology and elemental composition of formed PAn andPAn+TiO2 films were examined by SEM analysis. From the image ofthe PAn film in Fig. 8a, it is clearly observed that the electrodepositedpolymer layer on Pt substrate is homogeneous and compact. ThePAn+TiO2 composite SEM micrograph (Fig. 8b) suggests that theinorganic semiconductor particles with a grain size of 0.5 μmapproximately, were incorporated in organic conducting polymer,which consequently modifies themorphology of the film significantly.The inorganic semiconductor particles TiO2 are relatively piled upwith PAn. Also, the micrographs confirm the underlying distribution

Fig. 8. SEM images of (a) PAn/Pt, and (b) (PAn+TiO2)/Pt composite films.

of TiO2 in the PAn film by the presence of the big particles dispersed inthe polymer matrix, which consequently contributes to the formationof the pores with an increase in the specific surface of the compositematerial electrodeposited on the electrode.

The incorporation of titanium oxide in the polymer is confirmed bythe EDX analysis, which shows the presence of the intense rays oftitanium located at 4.50 and 0.40 keV (Fig. 9). Also, the EDX spectrum ofthe electrochemically prepared composite material film shows a signalof carbon (C) at 0.25 keV and nitrogen (N) at 0.4 keV characteristic ofthe PAn polymer. The signals of chlorine (Cl) at 2.62 and 2.82 keV andoxygen (O) at 0.53 keV indicate that the PAn film is doped by theperchlorate (ClO4

−) ions. This anion results from LiClO4, whichwas usedas supporting electrolyte. The presence of the important peak of sulfur(S) located at 2.34 keV shows that the film is also doped with SO4

−

anions resulting from sulfuric acid medium. Because the platinum isused as working electrode we observe also a peak at 2.07 keV of theplatinum. The other elements (Mn, Fe, Na, V, In,..) are characteristic ofthe presence of traces of impurities in the titanium oxide. Thus, thecontent of TiO2 in the composite film is confirmed by SEM and EDXanalyses. Therefore, we consider that the TiO2 particles can beincorporated in the polymer during the electropolymerization ofaniline, which led to the pigmentation of the polymer film, and theobtaining of the composite material PAn+TiO2 on the electrode. Thus,interesting electrochemical properties are obtained, allowing the use ofthis composite as electrode material in the field of electrochemistry,electronic and electrocatalysis applications.

4. Conclusion

We carried out a study on the synthesis and the analysis of theelectrochemical properties of a composite material obtained frompolyaniline, and titanium oxide, in order to have electrochemicalinformation and to use it as composite material in applications likeenergy storage and electrocatalysis. Thus, the effect of the pH and theaniline concentration on the electrochemical properties of PAn+TiO2

composite material was studied by cyclic voltamperometry andimpedance spectroscopy measurements.

The cyclic voltamperometry study shows three redox couples. Theoxidation and reduction peak currents increase during cycling,attesting to the formation of polymer film. The analysis usingimpedance spectroscopy shows that the resistance of the polymerincreases with the aniline concentration. The Nyquist diagrams showa charge transfer process at high frequencies followed by another ofdiffusion at low frequency values. The increase of pH decreases thefilm resistance, and consequently increases its conductivity, whereasthe resistance of the electrolyte RΩ is slightly affected by the variationof pH. The electroactivity of composite materials obtained in acidsolution was better than those synthesized in basic medium. So, amuchmore difference is marked in the properties and the structure ofthe film during the variation of pH. The PAn+TiO2 composite film hasbeen synthesized on Pt via electrochemical oxidation of aniline in thepresence of TiO2. Cyclic voltamperometry, electrochemical impedance

Fig. 9. EDX patterns of: a) PAn/Pt, and b) (PAn+TiO2)/Pt.

3602 S. Abaci et al. / Thin Solid Films 519 (2011) 3596–3602

spectroscopy and SEM techniques have been used to characterize thecomposite material. The PAn exhibits better electrochemical behaviorin acidic media, whichmakes it suitable for use as compositematerialsin electrochemical and catalysis applications.

The resulting composite-modified electrodes have been exploitedto achieve electrochemical and electrical applications as electrocatalysts and energy conversion as electrodes for photovoltaic cell.The strategy presented here may be of general interest since itprovides a route to build a composite material for the modification ofelectrode surfaces and related applications.

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