International Journal of Electrochemistry

Guest Editors: Fethi Bedioui, Tebello Nyokong, and José H. Zagal

Surface Electrochemistry: Structured Electrode, Synthesis, and Characterization

Surface Electrochemistry: Structured Electrode,Synthesis, and Characterization

International Journal of Electrochemistry

Surface Electrochemistry: Structured Electrode,Synthesis, and Characterization

Guest Editors: Fethi Bedioui, Tebello Nyokong,and José H. Zagal

Copyright © 2012 Hindawi Publishing Corporation. All rights reserved.

This is a special issue published in “International Journal of Electrochemistry.” All articles are open access articles distributed under theCreative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided theoriginal work is properly cited.

Editorial Board

Maria Carmen Arévalo, SpainShen-Ming Chen, TaiwanAbel César Chialvo, ArgentinaJean-Paul Chopart, FranceAdalgisa R. de Andrade, BrazilSergio Ferro, ItalyGerd-Uwe Flechsig, GermanyRubin Gulaboski, GermanyShengshui Hu, ChinaMehran Javanbakht, IranJiye Jin, Japan

Emilia Kirowa-Eisner, IsraelBoniface Kokoh, FranceEmmanuel Maisonhaute, FranceGrzegorz Milczarek, PolandValentin Mirceski, MacedoniaMohamed Mohamedi, CanadaAngela Molina, SpainDavood Nematollahi, IranKenneth I. Ozoemena, South AfricaMarı́a Isabel Pividori, SpainMiloslav Pravda, Ireland

Manfred Rudolph, GermanyBenjamı́n R. Scharifker, VenezuelaAuro Atsushi Tanaka, BrazilGermano Tremiliosi-Filho, BrazilHamilton Varela, BrazilJay D. Wadhawan, UKJose H. Zagal, ChileSheng S. Zhang, USAJiujun Zhang, CanadaXueji Zhang, USA

Contents

Surface Electrochemistry: Structured Electrode, Synthesis, and Characterization, Fethi Bedioui,Tebello Nyokong, and José H. ZagalVolume 2012, Article ID 405825, 2 pages

Size and Shape Control of Gold Nanodeposits in an Array of Silica Nanowells on a Gold Electrode,Amy E. Rue and Maryanne M. CollinsonVolume 2012, Article ID 971736, 9 pages

Zinc-Nickel Codeposition in Sulfate Solution Combined Effect of Cadmium and Boric Acid,Y. Addi and A. KhouiderVolume 2011, Article ID 742191, 7 pages

Cyclic Voltammetry and Impedance Spectroscopy Behavior Studies of Polyterthiophene ModifiedElectrode, Naima Maouche and Belkacem NessarkVolume 2011, Article ID 670513, 5 pages

Surface Potential of Polycrystalline Hematite in Aqueous Medium, Tajana Preočanin, Filip Stipić,Atid̄a Selmani, and Nikola KallayVolume 2011, Article ID 412731, 6 pages

Surface of Alumina Films after Prolonged Breakdowns in Galvanostatic Anodization,Christian Girginov and Stephan KozhukharovVolume 2011, Article ID 126726, 5 pages

Hindawi Publishing CorporationInternational Journal of ElectrochemistryVolume 2012, Article ID 405825, 2 pagesdoi:10.1155/2012/405825

Editorial

Surface Electrochemistry: Structured Electrode, Synthesis,and Characterization

Fethi Bedioui,1 Tebello Nyokong,2 and José H. Zagal3

1 Unité de Pharmacologie Chimique et Génétique et Imagerie, UMR CNRS 8151-U INSERM 1022,Chimie ParisTech and Université Paris Descartes, 11 rue Pierre et Marie Curie, 75231 Paris, France

2 Department of Chemistry, Rhodes University, Grahamstown 6140, South Africa3 Departamento de Quı́mica de los Materiales, Facultad de Quı́mica y Bioloǵıa, Universidad de Santiago de Chile, Casilla 40,Sucursal Matucana, Santiago 9170022, Chile

Correspondence should be addressed to Fethi Bedioui, fethi-bedioui@chimie-paristech.fr

Received 1 December 2011; Accepted 1 December 2011

Copyright © 2012 Fethi Bedioui et al. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Electrochemistry has become a multidisciplinary science,and a growing emphasis in the last decades has been focusedon the chemical control of the structure of the electrode/solution interface in order to achieve desired reactivity forelectron transfer processes involving target molecules. Thus,modified electrode surfaces have become a wide topic thatcovers several issues of interfacial electrochemistry, nano-technology, material science, sensors for biology and envi-ronmental sciences, and so forth.

The aim of this special issue is to show, through recentupdated significant examples, how the electrochemical tech-niques allow the unique characterization of specific proper-ties of micro- and nanostructured materials that offer variedpossibilities of uses and the preparation of specific typesof ordered materials that take advantage of electrochemicalsynthetic methods such as structuring nanosized wires anddots, to cite only two examples.

Five original research articles are presented in this issueand illustrate some of the efforts in developing electro-chemical strategies related to structured surfaces. Threethemes are concerned: electrochemical analysis of electrodesurfaces upon specific chemical treatments, electrochemicaldeposition of structured metal deposits and electrochemicalcharacterization of polymer-coated electrodes. In the paperentitled “Surface of alumina films after prolonged break-downs in galvanostatic anodization” by C. Girginov and S.Kozhukharov, the authors investigated the breakdown phe-nomena at continuous isothermal (20◦C) and galvanostatic(0.2–5 mA cm−2) anodizing of aluminum in ammoniumsalicylate in dimethylformamide (1 M AS/DMF) electrolyte.

Data on topography and surface roughness parameters of theelectrode after electric breakdowns are obtained as a functionof anodization time. The results are discussed on the basisof perceptions of avalanche mechanism of the breakdownphenomena, due to the injection of electrons and their multi-plication in the volume of the film.

In the paper entitled “Surface potential of polycrystalinehematite in aqueous medium” by T. Preočanin et al., the sur-face potential of polycrystalline hematite in aqueous sodiumperchlorate environment as a function of pH was examined.Surface potential of hematite was obtained from measuredelectrode potential of a nonporous polycrystalline hematiteelectrode, and interpretation of the equilibrium data wasperformed by applying the surface complexation model.

In the paper entitled “Zinc-nickel codeposition in sulfatesolution combined effect of cadmium and boric acid” by Y.Addia and A. Khouider, the combined effect of cadmiumand boric acid on the electrodeposition of zinc-nickel froma sulfate has been investigated. It is shown that the pres-ence of cadmium ion decreases zinc in the deposit. Lowconcentration of CdSO4 reduces the anomalous nature ofZn-Ni deposit while boric acid decreases current density andshifts potential discharge of nickel and hydrogen to morenegative potential.

In the paper entitled “Size and shape control of gold nan-odeposits in an array of silica nanowells on a gold electrode”by A. E. Rue and M. M. Collinson, ordered arrays of hemi-spherical nanowells were formed in a sol-gel derived silicafilm on a gold electrode using 500 nm diameter polystyrenelatex spheres as templates. The conductive domain located

2 International Journal of Electrochemistry

at the bottom of each nanowell upon template removalwas enlarged via electroless deposition from a gold platingsolution. The structured electrodes thus formed were char-acterized using scanning electron microscopy and atomicforce microscopy. Electroless deposition in the nanowellsproduced (near) sphere-like nanostructures of gold, thesize of which depended on the incubation time in theplating solution and the size of the conductive domain.Longer exposure times yielded nanostructures that filledthe nanowell whereas smaller exposure time yielded muchsmaller structures. Significantly larger, rougher deposits wereformed in nanowells with large conductive domains. Theelectrochemical response observed at these electrodes wasstrongly dependent on the extent of long-range packing, thepresence of defect sites in the film and their relative spacing,and the redox species in solution.

Finally, in the paper entitled “Cyclic voltammetry andimpedance spectroscopy behavior studies of polyterthio-phene modified electrode” by N. Maouche and B. Nessark,a study of the electrochemical behaviour of terthiophene andits corresponding polymer, which is obtained electrochemi-cally as a film by cyclic voltammetry (CV) on platinum elec-trode, is presented. The analysis focuses essentially on theeffect of two organic solvents acetonitrile and dichloro-methane on the electrochemical behaviour of the obtainedpolymer. The electrochemical behavior of this materialwas investigated by cyclic voltammetry and electrochemicalimpedance spectroscopy (EIS). The impedance plots showthe semicircle which is characteristic of charge transfer resis-tance at the electrode/polymer interface at high frequencyand the diffusion process at low frequency.

Fethi BediouiTebello Nyokong

José H. Zagal

Hindawi Publishing CorporationInternational Journal of ElectrochemistryVolume 2012, Article ID 971736, 9 pagesdoi:10.1155/2012/971736

Research Article

Size and Shape Control of Gold Nanodeposits in an Array ofSilica Nanowells on a Gold Electrode

Amy E. Rue and Maryanne M. Collinson

Department of Chemistry, Virginia Commonwealth University, Richmond, VA 23284, USA

Correspondence should be addressed to Maryanne M. Collinson, mmcollinson@vcu.edu

Received 7 May 2011; Revised 26 October 2011; Accepted 27 October 2011

Academic Editor: Tebello Nyokong

Copyright © 2012 A. E. Rue and M. M. Collinson. This is an open access article distributed under the Creative CommonsAttribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work isproperly cited.

Ordered arrays of hemispherical nanowells were formed in a sol-gel-derived silica film on a gold electrode using 500 nm diameterpolystyrene latex spheres as templates. The conductive domain located at the bottom of each nanowell upon template removalwas enlarged via electroless deposition from a gold plating solution. The structured electrodes thus formed were characterizedusing scanning electron microscopy and atomic force microscopy. Depending on the method used to make the films, the extent ofthe long-range packing and the size of the conductive domain changed. Electroless deposition in the nanowells produced (near)sphere-like nanostructures of gold, the size of which depended on the incubation time in the plating solution and the size of theconductive domain. Longer exposure times yielded nanostructures that filled the nanowell, whereas smaller exposure time yieldedmuch smaller structures. Significantly larger, rougher deposits were formed in nanowells with large conductive domains. Theelectrochemical response observed at these electrodes was strongly dependent on the extent of long-range packing, the presence ofdefect sites in the film and their relative spacing, and the redox species in solution.

1. Introduction

Patterned or structured electrodes have been shown to haveconsiderable importance in many areas of science and tech-nology, particularly electroanalytical applications [1–3]. Thedevelopment of strategies for the fabrication of such mate-rials is needed as the desire to make smaller, more sensitive,and more selective devices becomes increasingly important.One means to create structured electrodes involves strategi-cally combining sol-gel chemistry and templating together[4, 5]. In such a process, for example, a suitable templateis doped into a silica sol and cast on a surface such as anelectrode. Upon subsequent removal of the template, voidsremain in the microporous oxide network. Examples of vari-ous types of templates that have been used include molecules,surfactants, and latex spheres [3–7]. In prior work, we haveshown how an array of silica nanowells can be formed onan electrode surface and how these structured electrodescan be used to electrochemically grow copper nanoparticlesand conducting polymers from the bottom-up [8]. Thesuccess of this method depends on the extent at which, upon

removal of the latex sphere, the underlying electrode surfaceis exposed, enabling it to be used as a microelectrode [9].Providing there are no defects (pin holes) in the insulatingsilica surrounding the nanowells, the electrochemistry onlytakes place in the nanowell, thus restricting growth to thisarea [8]. The advantages of growing arrays of nanoparticleswithin a porous inorganic network using this approachare the following: (a) the interparticle spacing betweennanoparticles can be controlled by the size of the templateused to form the nanowell, (b) the size and shape of theresultant nanoparticles/nanostructures can be controlled bythe size and shape of the nanowell, and (c) the silica supporthelps prevent aggregation of the nanoparticles as they beginto grow larger [8, 10, 11]. From an electroanalytical pointof view, enlarging the conductive domains located at thebottom of the nanowell also provides a means to potentiallyincrease the sensitivity of the microelectrode array [12].The importance of nanoparticle arrays in electroanalyticalchemistry has been well documented in the literature [12–15].

2 International Journal of Electrochemistry

Method 1

Sol

Method 2

Au

Au

Au

Silica Au

PS

AuAu3+

NH2OH·HCl

1 min

15 min

3 hrs

Sol/SDS/PS

CHCl3

Figure 1: Simplified cartoon that shows the fabrication of silica nanowells on the gold electrode and the subsequent enlargement of theconductive domains exposed after template removal.

In this work, we describe an alternative approach forexpanding the conductive domain at the bottom of theporous nanowells that is less sensitive to defect sites (pinholes) in the silica framework and more amenable to differ-ent metals. The procedure involves the formation of the silicananowells on a gold electrode using polystyrene latex spheresas the sacrificial template followed by the electroless growthof the gold microelectrode that is exposed when the templateis removed. The height of the gold nanoparticles can be easilyadjusted by changing the incubation time in the electrolessplating solution. In related work recently published, thesilica nanowells were formed from the vapor depositionof dimethyldichlorosilane around an array of latex spheresformed on quartz. To form the gold nanoparticle array, theexposed surface (hydroxides) was immersed in a solution oftin ions, followed by silver ions, and then gold ions [11]. Inanother paper, shallow nanowells were formed on a siliconwafer via the vapor deposition of chlorosilanes around thelatex sphere [10, 16, 17]. The hydroxylated surface locatedat the bottom of the nanowells was then functionalized withmercaptopropyltrimethoxysilane and subsequently used tobind gold nanoparticles [10]. In contrast to these papers,the current method is a one-step method that does notrequire multiple reaction steps or chemical modification.It utilizes the sol-gel process [18], which means variousreagents can be entrapped into the microporous networkto create a chemical sensor [19–23], and it is directlyformed on an electrode surface and thus making it astructured electrode ideally suited for electroanalytical appli-cations.

2. Experimental

2.1. Templated Silica Film Formation

2.1.1. Spin Cast Method for Colloidal Crystal Formation. Goldelectrodes (EMF, Ithaca NY) were cut to ∼1.5 cm × 1.5 cmand then cleaned by successive sonications in ethanol, and

water twice for 15 minutes each followed by plasma cleaningat 10 W (Southbay PE-2000, DC bias of −100 V) to makethe slides hydrophilic. The silica sol was prepared by stirring0.25 mL tetramethoxysilane (TMOS, 99%, Acros), 1.20 mLmethanol (MeOH), 1.15 mL water, and 0.015 mL 0.1 MHCl for 30 minutes and then aged for two days. Sodiumdodecylsulfate (SDS) was added (5 mM) to the sol before itwas mixed with 0.4 μm polystyrene latex spheres (PS, fromInvitrogen, formally Interfacial Dynamics Company (IDC),sulfate stabilized, 8%) at a 1 : 1 (v/v%) ratio. The SDS wasadded as a wetting agent to improve the quality of the spherepacking [7]. The solution was viciously mixed, then pipettedon to the surface of a clean gold electrode, and spin coated at3000 rpm for 30 seconds. After the films were dried overnighton the bench top, they were submerged in chloroform forthree hours to remove the PS spheres.

2.1.2. Evaporative Method for Colloidal Crystal Formation.Gold (Au) electrodes were cut into ∼1.5 cm × 1.5 cmsquares and cleaned by successive sonications in 2-propanoland water followed by boiling piranha solution (7 : 3)for 5 minutes. (Caution: piranha solutions are extremelydangerous and react violently with organic materials). Theelectrodes were rinsed well with water and ethanol andsoaked in a 5 mM cysteamine in ethanol (EtOH) for 24hours. To form the colloidal crystal, a modified version ofthe vertical deposition method was used [24]. In this work,the modified electrode was vertically placed in a glass vial,and a mixture of ethanol and 0.5 μm spheres in a 33 : 1 V/Vratio was added. The solution was slowly evaporated at50◦C by placing the vial in a thermostated oven or a waterbath. A closed water bath was preferred because it tended tominimize fluctuations in temperature and ambient humidity.Evaporation took approximately 24 hours and resulted in awell-packed, single layer (or near single layer) of spheres onthe gold electrode.

To form the silica nanowells on the surface of the goldelectrode, a silica sol (0.2 mL TMOS, 0.06 mL MTMOS

International Journal of Electrochemistry 3

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Figure 2: Tapping mode AFM images (5 μm × 5 μm) of (a) a bare gold electrode, (b) hexagonally packed PS spheres in a silica film-coatedgold electrode, and (c) the nanowells formed after chemical removal of the PS spheres.

(methyltrimethoxysilane, 97%), 0.1 mL MeOH, 0.3 mLwater, and 0.06 mL 0.1 M HCl, stirred for 1 hour) was dilutedwith ethanol 1 : 100, and the diluted sol was then spin coatedover the packed spheres at 8000 or 6000 rpm for 60 seconds.To ensure the sol effectively penetrated the voids in thecolloidal crystal, the substrates were first plasma cleaned for2 minutes at 10 W (DC bias of −100 V), and the diluted solalso was allowed to rest on the substrate for one minute priorto electrode rotation. The films were dried overnight, and thePS spheres were removed via soaking in chloroform to formthe silica nanowells on the gold electrode. In some cases, tofurther hydrolyze the sol, 10 μL of 0.16 M KOH was added tothe sol and stirred for an additional 30 minutes prior to spincoating.

2.2. Electroless Gold Deposition. To insure that all PS wasremoved from the film, the substrates were rinsed withfresh chloroform and plasma cleaned for 2 minutes at15 W (DC bias of −150 V) prior to submerging in a 1 : 100(v/v%) solution of hydrogen tetrachloroaurate (1 mM) andNH2OH·HCl (0.45 mM) for varying time lengths. Duringdeposition, the solution was placed on a shaker at 180 rpm.The solution was replaced with a fresh solution at 15 minuteintervals. The films were rinsed with water then dried on thebench before further characterization.

2.3. Electrochemistry. Electrochemistry was performed in athree-electrode cell using a saturated Ag/AgCl electrode as areference and a Pt wire as the auxiliary on a CH instrumentspotentiostat. The electrode area was defined by a quarter inchdiameter hole punched into a piece of Teflon tape and coppertape was attached to the edge of the gold slide as a lead.Ferrocene methanol (1 mM) and potassium ferricyanide(1 mM) in 0.1 M KCl were used as the redox probes.

3. Results and Discussion

3.1. Overview. Previous work in our lab has shown thatsilica nanowells can be formed on glassy carbon electrodesby templating with latex spheres [8, 9]. Because thesenanowells are open at the bottom, thus exposing theelectrode underneath, they can be used as nanosized reactionvessels to do electrochemistry [8]. In essence, they are thenanosized equivalent of a round-bottom flask containingan immobilized microelectrode as depicted in the cartoonshown in Figure 1. In the present work, two methods wereused to form the silica nanowells. Both methods utilize thesol-gel process [18], which is a versatile method used toprepare inorganic and organic-inorganic hybrid materials[19–23]. The first method (Figure 1, method 1) is the moretraditional way to make sol-gel-derived films, which firstinvolves doping the silica sol with latex spheres and spincoating the composite sol on the electrode surface. In thesecond method (Figure 1, method 2), a near monolayer ofspheres is formed first, followed by spin coating an undopedsilica sol on this surface. These structured electrodes arethen used as reaction vessels to electrolessly grow goldnanoparticles of varying height and width, defined in partby the size of the nanowell and the electroless growthconditions. It is known in the literature that exposure ofgold nanoparticles to HAuCl4 in the presence of the reducingagent NH2OH, either in solution or immobilized on asubstrate, results in particle growth [25]. It was hypothesizedthat the exposed gold electrode located at the bottom ofthe silica nanowell can be used as a reduction site for golddeposition, and nanoparticles were not necessary to achievethis affect. This same method could be applied to any goldsurface of limited dimensions such as the small area ofgold exposed at the bottom of the nanowell upon templateremoval. The size/microstructure of the gold deposits canbe controlled by the length of exposure time to the growth

4 International Journal of Electrochemistry

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Figure 3: 5 μm × 5 μm AFM images of gold deposited on (a) a bare gold electrode (electroless deposition for 6 minutes) and in silicananowells grown by electroless deposition for (b) 1 minute, (c) 15 minutes, and (d) 3-hour exposure times. (e) Cross-section plots overlaidfor clarity (blue = 1 min, red = 15 min, and green = 3 hours).

solution (Figure 1, bottom) and the area exposed to solutionupon removal of the PS latex sphere. No prior modificationof the surface is necessary.

3.2. Gold Nanoparticles Formed in Silica Nanowell ElectrodesPrepared via Method 1. In this method, silica sols aredoped with PS spheres and then spin cast on the electrodesurface. The packing of the spheres on the surface of theelectrode depends on many factors including surface charge(electrostatic repulsion), surface hydrophilicity, and surfacecleanliness [6, 7]. In this work, gold electrodes and sulfate-stabilized PS spheres are used. Figure 2(a) shows a tappingmode atomic force microscope (AFM, Veeco MultimodeSPM) image of the surface of a bare gold electrode. As can beseen, the bare electrode consists of a flat surface with small,random roughness. After spin coating, the silica sol/latexsphere mixture on the gold electrode, localized regions ofthe surface shows a hexagonally packed monolayer of the PSspheres, Figure 2(b). After template removal, silica nanowells

are formed on the surface as shown in Figure 2(c). The depthof the nanowell largely depends on the speed at which thesphere-doped sol is cast, the viscosity of the sol, and size ofthe latex sphere [8]. At a spin speed of 3000 rpm and a spheresize of 400 μm, the nanowells have an average dimensionof 500 nm in diameter and a well depth of 300 nm. Thesenanowells are open at the top (evident in the AFM images)and the bottom (evident from AFM section scans and goldgrowth (described below)), thus exposing the underlyingnanostructured surface.

The nanowells, such as those shown in Figure 2(c)can be used as reaction vessels to electrolessly grow goldnanoparticles. In prior work, we utilized a similar electrolessdeposition procedure to grow nanostructured gold wires inmicron size channels formed from bacteria [26]. Figure 3(a)shows an area of the substrate not covered by packed spheres,which, due to the generally poor interaction between goldand silica, results in almost no film formation [27]. Exposureof the bare gold electrode surface to the gold growth solution

International Journal of Electrochemistry 5

x50 500 μm 0000 06/May/1120 kV

(a) (b)

(c)

Figure 4: SEM images of typical sphere packing on a gold electrode using (a) the spin-coating method and (b) the vertical depositionmethod. (c) SEM of the nanowells formed in a film prepared via the vertical deposition method. (Scale bar: (a) 500 μm, (b) 50 μm, and (c)10 μm).

resulted in increased roughness of the surface, with theoccasional large asymmetrical gold deposit. The electrolessdeposition of gold within the nanowells, however, was foundto be markedly different compared to that observed at abare gold electrode, as in Figures 3(b) and 3(c). Electrolessdeposition in the nanowells produced (near) sphere-likedeposits of consistent size throughout the film. The growthof single particles in the nanowells is likely due to there beingonly a few exposed points of roughness from the underlyingsubstrate. A single small growth is formed at those points,and then the particle increases in the same manner as ananchored, pregenerated nanoparticle formed in solution,free from competition and eventual merger with adjacentdeposition sites as seen in the bare substrate [28]. Thisgrowth progression is seen in the time-dependent growthstudy. A one-minute exposure results in the formationof a deposit with an observable height of about ∼20 nm(Figure 3(b)), which is similar to gold nanoparticles typicallygenerated in solution [29]. As the exposure time continues,the particle increases in size while retaining its basic shape(Figure 3(c)). There appeared to be a limit to this growth,however, as it was observed that growth would not continuepast a height of ∼150 nm (Figure 3(d)), even when left inthe growth solution overnight (data not shown). Figure 3(e)shows an overlay of the cross-sections of the nanowells

before and after electroless deposition, ranging from particleformation to the growth limit. As can be seen, the conductivedomain located at the bottom of the nanowell progressivelyincreases as the incubation time in the growth solutionincreases.

3.3. Gold Nanoparticles Formed in Silica Nanowell ElectrodesPrepared via Method 2. The spin-coating method, whilesimple and easy to do, does not produce nanowell arrayswith significant long-range order on gold electrodes thatwould likely be required for electroanalytical applications.Part of the problem likely results from the surface of goldnot being easily wetted by the silica sol, as well as the lackof available hydroxyl groups for the silica to bind with [27,30]. To address this shortcoming, the method used to packthe colloidal crystal on the electrode surface was separatedfrom silica film formation. There have been many methodsdescribed in the literature for the formation of colloidal crys-tals on gold with long-range order [2, 6, 7, 31]. Many of thesepublished methods are ideally suited for forming multilayerfilms, which would be more challenging in this study. Inthis work, the method with the greatest success for forminga monolayer or near-monolayer of PS spheres with long-range order is a variant of the vertical deposition/solvent

6 International Journal of Electrochemistry

150 nm

(a)

250 nm

(b)

Figure 5: SEM images of (a) a narrow and (b) a wide conductive domain located at the bottom of the silica nanowells on a gold electrode.(Scale bar: (a) 150 nm, (b) 250 nm).

evaporation method [2, 24]. In this method, the substrate isvertically positioned in a vial containing a dilute suspensionof spheres in ethanol [2, 24]. As the solvent slowly evaporates,a compact layer of spheres remain on the surface [24]. Inthis work, the concentration of latex spheres in ethanolwas kept relatively low (1 : 33 V/V PS spheres : EtOH), andevaporation took place at an elevated temperature (50◦C). Toinsure a steady evaporation rate, the evaporation setup washeated in a closed water bath to provide consistent heatingand humidity. In contrast to a room-temperature-baseddeposition, a packed monolayer (or near monolayer) of PSspheres can be formed in 24 hours. This method produceda vastly superior colloidal crystal with long-range packingof spheres, mostly in a single layer, compared to the spin-coating method. Figure 4 shows representative 157× 110 μmscanning electron microscope images (SEM, Hitachi FE-SEMSU-70, 5 kV, platinum/gold sputter to minimize chargingeffects) of a single (or near single) layer of PS spheres on agold electrode formed by spin coating the sphere-doped solon a gold electrode (Figure 4(a)) or via the vertical depo-sition method (Figure 4(b)). While the spin-coated packedelectrode shows good packing in localized areas, the packingis weblike over the entire surface, leaving a large amountof the substrate exposed. The vertical deposition method,however, exhibits good packing on both the microscopicand macroscopic length scales (Figure 4(b)). In Figure 4(c),an SEM of the nanowells formed after template removal isshown.

Once the colloidal crystal was formed on the goldelectrode, a silica sol could be spin cast directly over thislayer of spheres to ultimately form the silica nanowells onthe gold electrode (Figure 4(c)) [26]. The silica nanowellsproduced by this method have many similarities to that ofthe doping method (described above) in that dimensionsof the nanowells are similar. One major difference is in thevariation of the diameter of the underlying exposed electrodesurface (the conductive domain), which has consequencesfor the electroless deposition of gold. The variables thatwill likely influence the diameter of the bottom of the

nanowell include the degree at which the sol is hydrolyzedand condensed (its age), the viscosity of the sol (dilutionfactor), the time the sol is placed on the surface before theelectrode is rotated, and the speed at which the sol is cast overthe packed spheres. Fresh silane precursors and faster spinspeeds (8000 rpm) tend to produce wider bottoms, whileaged silane (one day) and slower speeds (6000 rpm) producenarrower bottoms. Because speed influences film thickness,the original sol was diluted more when the spin speed wasdecreased to retain nearly the same nanowell depth. Figure 5shows representative examples of the sizes of the conductivedomain formed at the bottom of the nanowells on a goldelectrode. The “wide bottom” nanowells expose a substratearea of about 70–80% that of the mouth of the nanowell,whereas the “narrow bottom” nanowells expose roughly 30–40%. The sides of the nanowell shown in Figure 5(a) exhibita more particle-like surface relative to that observed in thenanowell shown in Figure 5(b). These observed differencesin the microstructure of the silica network are attributedto differences in the aging of the silane precursors priorto making the sol. In Figure 5(b), the silane precursorswere freshly removed from a nitrogen environment andimmediately used to prepare the sol, whereas in Figure 5(a),the silanes were partially hydrolyzed via exposure to air priorto use.

Films that exhibited large conductive domains at thebottom of the nanowells produced gold deposits withasymmetric shapes and sizes (Figure 6(b) inset). Films withsmaller conductive domains produced deposits consistentwith those observed with the doped films (Figure 3), witha single sphere-like particle (Figure 6(a) inset). The growthin the wide bottom appears to be a hybrid of the growthpatterns on the bare gold surface and in the narrow bottomwells. The shape of the deposit reflects the natural roughnessof the gold surface under the film but is able to grow pastthe point commonly seen on the bare surface, similar to thenanowells with smaller conductive domains. It is believedthat this is the result of the wider conductive domains havingseveral initial growth sites within the nanowell that merge

International Journal of Electrochemistry 7

(a)

(b)

Figure 6: SEM images of (a) narrow bottom and (b) widebottom films. Insets display representative gold deposits withinsilica nanowells. (Scale bars: 20 μm, inset: 250 nm).

together at longer growth times. By combining the verticaldeposition method of film formation with the gold growthmethod utilized in the spin-coated slides, a large, well-packedarray of nanowells containing gold particles of controllablesize were formed (Figure 6).

3.4. Electrochemistry. The voltammetry of ferrocene meth-anol (FcCH2OH) and potassium ferricyanide (Fe(CN)6

3−)were examined at electrodes prepared via method 2. InFigure 7(a), cyclic voltammograms (CVs) for FcCH2OHat a bare gold electrode and a silica film/sphere-coatedelectrode can be observed. As expected, at a bare goldelectrode, the voltammogram has a diffusion-controlledshape characteristic of a redox probe in solution transferringelectrons at the surface of a large planar electrode. At thesphere-coated electrode, different electrochemical responseswere observed depending on the quality of the packing ofthe latex spheres (and the redox probe, described below).When the electrode contained closely spaced defects, suchas those formed at surface dislocations and areas not coatedwith silica, and/or at pinholes between spheres, the CVof FcCH2OH looks almost identical to that obtained ata bare electrode. In this case, the diffusion layers at theindividual defect sites overlap and the surface appears asone big electrode [32, 33]. In contrast, when the electrode

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(b)

Figure 7: CVs of ferrocene methanol (1 mM) in 0.1 M KCl at0.1 V/s. (a) Red: bare gold, blue: silica film/sphere-coated electrodewith many defects in the colloidal crystal, and black: silicafilm/sphere-coated electrode with fewer defects in the colloidalcrystal. (b) Red: bare gold, blue: after sphere removal via soakingin chloroform for 3 hrs, and black: after particle growth for 45minutes.

is covered with a densely packed layer of spheres withfew defects and better long-range packing, the oxidation offerrocene methanol is hindered, and the voltammetric curveis suppressed relative to that observed at a bare electrode.Upon removal of the spheres leaving behind a closely spacedarray of nanowells, the voltammetry once again looks likethat observed at a bare electrode because of the overlapof diffusion layers at the closely spaced nanowells, as inFigure 7(b) [8]. Once the spheres are removed, the qualityof the packing of the colloidal crystal (i.e., the presence ofdefects/pin holes) cannot be assessed from the voltammetrybecause the electrochemistry that takes place at a defect sitecannot be distinguished from the electrochemistry that takesplace in the nanowell [8]. Further growth of the exposedgold electrode via electroless deposition for 45 min does not

8 International Journal of Electrochemistry

60

30

0

0.5 0.4 0.3 0.2 0.1 0

Potential (V)

Cu

rren

t (μ

A)

−30

−60

Figure 8: CVs of potassium ferricyanide (1 mM) in 0.1 M KCl at0.1 V/s. Red: bare gold, blue: silica film/sphere-coated electrode,black: silica film after sphere removal via soaking in chloroform for3 hrs, and green: silica film after gold particle growth for 45 minutes.

produce distinct changes: the voltammetry of FcCH2OHlooks identical to that observed at nanowell electrode.

Very different results were observed using Fe(CN)63−

as the redox probe. Fe(CN)63− is very similar in size to

FcCH2OH, but is negatively charged. Thus, electrostaticinteractions will be more important compared to the casefor FcCH2OH, which is initially neutral. Under near-neutral pH conditions, the silica matrix will be negativelycharged. Figure 8 shows the CVs at a bare electrode, a silicafilm/sphere-coated electrode, the nanowell electrode, andthe nanowell electrode with the conducting Au domainsenlarged. In contrast to that observed using FcCH2OH, asignificantly diminished response is observed for Fe(CN)6

3−

at the silica/sphere-coated electrode. The voltammetric curveexhibits a sigmoidal-shaped response indicative of the pres-ence of pinholes/defects that are relatively far apart. Afterremoval of the spheres, the voltammetric shape remainssigmoidal-like but increases in size. The negatively chargedsilica matrix obviously hinders access of Fe(CN)6

3− tothe underlying gold electrode relative to that observed forFcCH2OH. By increasing the size of the Au conductingdomain via electroless deposition, electrostatic repulsionbetween the redox probe and the silica framework is lessened,and the CV of Fe(CN)6

3− more closely resembles thatobserved at the bare electrode.

4. Conclusion

Nanostructured gold electrodes were created through theselective electroless deposition of gold into nanowells formedin a silica film on a gold electrode. The size and shapeof the deposited particles were controlled via exposuretime and changes in film characteristics. Method 1, whichinvolved spin coating a doped sol on a gold electrode, wassimple and easy and yielded nanowells that all had similarsize conductive domains. When used as a chemical reactorto electrolessly expand the conductive domain, uniform

gold deposits were obtained. The nanowells, however, werenot uniformly distributed/packed across the entire surfaceof the electrode, potentially limiting their application inelectroanalytical chemistry. Method 2, while a more time-consuming 2-step method, yielded a much more uniformlypacked array of nanowells. The conductive domains locatedat the bottom of the nanowells, however, varied in size. Bothnanowells with large conductive domains and nanowellswith small conductive domains were obtained by judiciouslychanging the sol-gel processing conditions. The size of theconductive domain significantly influenced the size andstructure of the gold deposit electrolessly formed at thebottom of the well. Initial electrochemistry experimentsusing two different redox probes, ferrocene methanol andpotassium ferricyanide, showed interesting results. Mostsignificantly, the shape of the cyclic voltammetric curveand the magnitude of the faradaic current were stronglydependent on the long-range packing and the presenceof defects in the film and the redox probe in solution.For potassium ferricyanide, enlargement in the conductivedomains lessens electrostatic repulsion effects observed fromthe silica network and results in a more traditional-shapedvoltammetric similar to that obtained a bare electrode.

Acknowledgments

The authors gratefully acknowledge support of this workfrom the National Science Foundation (CHE-0618220, CHE-0847613). A. E. Rue thanks Altria for a fellowship. They alsoacknowledge the support of the VCU Nanomaterials CoreCharacterization (NCC) facility and Dr. Dmitry Pestov andMs. Bo Zhao for their help with the SEM.

References

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[8] M. Kanungo, P. N. Deepa, and M. M. Collinson, “Template-directed formation of hemispherical cavities of varying depthand diameter in a silicate matrix prepared by the sol-gel

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[10] J. R. Li, K. L. Lusker, J. J. Yu, and J. C. Garno, “Engineeringthe spatial selectivity of surfaces at the nanoscale using particlelithography combined with vapor deposition of organosi-lanes,” ACS Nano, vol. 3, no. 7, pp. 2023–2035, 2009.

[11] W. Ahn and D. K. Roper, “Periodic nanotemplating byselective deposition of electroless gold island films on particle-lithographed dimethyldichlorosilane layers,” ACS Nano, vol. 4,no. 7, pp. 4181–4189, 2010.

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[14] X. Luo, A. Morrin, A. J. Killard, and M. R. Smyth, “Applicationof nanoparticles in electrochemical sensors and biosensors,”Electroanalysis, vol. 18, no. 4, pp. 319–326, 2006.

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[31] P. N. Bartlett, J. J. Baumberg, P. R. Birkin, M. A. Ghanem,and M. C. Netti, “Highly ordered macroporous gold andplatinum films formed by electrochemical deposition throughtemplates assembled from submicron diameter monodispersepolystyrene spheres,” Chemistry of Materials, vol. 14, no. 5, pp.2199–2208, 2002.

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SAGE-Hindawi Access to ResearchInternational Journal of ElectrochemistryVolume 2011, Article ID 742191, 7 pagesdoi:10.4061/2011/742191

Research Article

Zinc-Nickel Codeposition in Sulfate Solution Combined Effectof Cadmium and Boric Acid

Y. Addi and A. Khouider

Faculté de Chimie, USTHB, BP 09, Bab Ezzouar, Alger 16111, Algeria

Correspondence should be addressed to Y. Addi, addi yassine@yahoo.fr

Received 18 February 2011; Revised 26 June 2011; Accepted 12 July 2011

Academic Editor: José H. Zagal

Copyright © 2011 Y. Addi and A. Khouider. This is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properlycited.

The combined effect of cadmium and boric acid on the electrodeposition of zinc-nickel from a sulfate has been investigated. Thepresence of cadmium ion decreases zinc in the deposit. In solution, cadmium inhibits the zinc ion deposition and suppresses itwhen deposition potential value is more negative than −1.2 V. Low concentration of CdSO4 reduces the anomalous nature of Zn-Ni deposit. Boric acid decreases current density and shifts potential discharge of nickel and hydrogen to more negative potential.The combination of boric acid and cadmium increases the percentage of nickel in the deposit. Boric acid and cadmium.

1. Introduction

Great interest has been shown in the possibilities offered bythe electrodeposition of alloys, particularly in the automotiveindustry. It is well known that alloys improved mechanical[1–3] and chemical properties of metals. Zn-Ni alloy depositson iron increases strongly the corrosion resistance [2, 3].

The electrodeposition of Zn-Ni alloys is classified byBrenner [4] as an anomalous codeposition, where zinc,which is the less noble metal, is preferentially deposited. Al-though this phenomenon has been known since 1907, thecodeposition mechanisms of zinc and nickel are not wellunderstood.

Several studies were carried out to explain this behaviour[5] in contradiction with the thermodynamic.

The nature of the anomalous codeposition has beenextensively investigated, and one of the explanations is theformation of the hydroxide precipitate of the less noblemetal at the cathode. The hydroxide is caused by a localincrease of pH. This precipitate may suppress deposition ofthe noble metal [6]. It was noticed that anomalous code-position occurred even at low current densities [7], wherehydrogen formation is unable to cause large alkanizationeffects. Another explanation is based on the underpotential(UDP) of zinc on nickel-rich zinc alloys or on nickel

nuclei [8]. Matlosz [9] uses a two-step reaction mechanisminvolving adsorbed monovalent intermediate ions for bothelectrodeposition of iron and nickel, as a single metal, andcombines the two to develop a model for codeposition.Anomalous effects are assumed to be caused by preferentialsurface coverage due to differences in tafel rate constants forelectrodeposition.

Many studies were carried out to attempt minimizing theinhibition phenomenon. Ashassi et al. [10] used a pulsedcurrent in order to decrease zinc hydroxide formation atthe cathodic surface. They consider that anomalous co-deposition is due to the slow kinetic of nickel on steel. Huand Bai [11] showed the possibility to obtain a compositionof the deposit equal to that of the solution by the use ofcyclic voltammetry. From Zhou and JO’Keefe work [12], theanomalous codeposition can be reduced by adding additivesas tin.

It has been reported that the deposition of nickel needslow overpotential to create the initial nucleus [13] and the de-posit grows at low potentials.

It has been noticed that normal codeposition takes placeat low current density. Boric acid and some other reactivehave the ability to decrease the current density. According toKarwas and Hepel [14] boric acid inhibits zinc depositionshifting the nickel content in alloys toward Ni-rich phases.

2 International Journal of Electrochemistry

From Šupicová and all work [15], boric acid shifts thereduction peak to more negative potentials and increases thepeak height. It improves morphology and brightness, as wellas the adhesion of the deposited Ni. In addition it acts as acatalyst lowering the overvoltage which allows the depositof nickel instead of H2. For Hoare [16], boric acid plays amore important role than that of a buffer. Boric acid forms acomplex from which nickel can be discharged more easily.

The present work focuses on the study of the Zn-Ni alloyelectrodeposition in sodium sulfate baths in presence of boricacid and cadmium. In order to identify the effect of boric acidon current density, polarizations curves were plotted withvarious boric acid concentrations. The nucleation potentialwas studied in the presence of boric acid. The behaviour ofcadmium on the zinc-nickel deposition was investigated.

2. Experimental

All experiments were carried out in a three-electrode cellwith a capacity of 250 mL. A stainless-steel electrode witha surface of 2 cm2 as a working electrode, a titanium gridcoated with ruthenium oxide (Ti/RuO2) as a counter elec-trode, and an electrode of calomel saturated (ECS) as areference electrode (Hg2Cl2/Hg). The reference electrode wasimmersed in the upper section of a luggin capillary, and itsend was placed as close as possible to the working electrodeto minimize errors due to ohmic drop in the electrolytes.The counter electrode was placed in parallel with the workingelectrode at a distance of 2 cm. Potentiodynamic polarizationcurves were measured at a scan of 5 mV/s starting from opencircuit with a potentiostat (Radiometer analytical PGP201)assisted by a VoltaMster 4 software.

All solutions were freshly prepared with analytical gradechemicals (Merck) and distilled water. The electrolyte for thealloy deposition was a solution of 0.5 M sodium sulfate. TheZn-Ni alloys deposits were obtained at room temperaturefrom electrolyte containing 0.02 M of ZnSO4·7H2O and0.02 M of NiSO4·6H2O.

The working electrode was polished with abrasive paper(#320, #1200 and #2000 SiC) and finished with polishingslurry of 0.5 μm Al2O3 powder followed by washing with dis-tilled water then with acetone and finally dried under warmairflow.

Before each experiment, the solution was purified withpure nitrogen for 10 mn to release oxygen dissolved andmaintained under nitrogen atmosphere during the run ofexperiments.

The temperature was kept constant at 25◦C by using awater-jacketed cell. The solution was agitated with a mechan-ical stirrer in order to have uniform bulk pH. The rotationalspeed of the stirrer was kept at 250 rpm for all experiments.

After deposition, the electrode is removed from the so-lution rinsed with distilled water and then dried.

In order to determine the alloy composition, the workingelectrode was immersed in concentrated nitric acid to re-move the deposits obtained.

Boric acid 0 MBoric acid 0.3 MBoric acid 0.8 M

−100

−80

−60

−40

−20

0

20

40

60

80

−1.5 −1 −0.5 0 0.5

I(m

A)

E (V)

Figure 1: Polarization curves of 0.5 M sodium sulfate solution fordifferent concentrations of boric acid.

The surface morphology of the deposit was analysedusing scanning electron microscopy (JSM-5500LV, SEM,Japan).

The composition of the deposits was determined usingatomic absorption spectrophotometry (PERKIN-ELMER,AA 5000). Deposition current efficiency was calculated foreach deposit using electrochemical equivalents of Ni (II) andZn (II).

3. Results and Discussions

3.1. Effect of Boric Acid on the Polarization and PotentialCurves. Figure 1 shows the effect of boric acid on the po-larization curves of a solution containing 0.5 M of sodiumsulfat. The measured current density decreases with increas-ing boric acid concentration suggesting that boric acidinhibits protons reduction that is extended to more negativepotentials. This effect is attributed to adsorption of boric acidspecies on the substrate.

Figure 2 shows the influence of boric acid on the curvespotential—time during the deposition of zinc-nickel alloy onsteel at constant current density. The nucleation potentialand the nucleation time are where a sudden change in theslope occurs. This sudden change in the slope is an indicationof diffusion controlled reduction system. In the presence ofboric acid the nucleation potential decreases and nucleationtime increases. Therefore, in the presence of boric acidthe nucleation time increases and the nucleation potentialshifts to positive values. Thus boric acid by reducing nu-cleation potential leads to reduce anomalous deposition andhydrogen evolution reaction.

3.2. Effect of Cadmium on Zinc and Nickel Deposition.Figure 3 shows the effect of cadmium on the deposition ofzinc. With a concentration ten-times smaller than that ofzinc, the presence of cadmium decreases the deposition of

International Journal of Electrochemistry 3

−1.7

−1.6

−1.5

−1.4

−1.3

−1.2

−1.1

0 50 100

E(V

)

Time (s)

Boric acid 0 MBoric acid 0.8 M

Figure 2: Chronopotentiometric curves at 5 mA/cm2 on steel elec-trode.

−3.5−3

−2.5−2

−1.5−1

−0.50

0.5

1

1.5

2

−1.5 −1 −0.5 0

I(m

A/c

m2)

E (V)

|Zn2+| = 0.02 M|Zn2+| = 0.02 M |Cd2+| = 0.002 M|Zn2+| = 0.02 M |Cd2+| = 0.002 M

Figure 3: Cyclic voltammetry at a scan of 5 mV/s of a metallic so-lution containing |Na2SO4|= 0.5 M at pH= 3.5.

zinc. Moreover in the presence of cadmium, the depositionof zinc is suppressed when the cathodic potential sweepis reversed at −1.2 V represented by dotted lines in thevoltammogram.

In Figure 4, cyclic voltammograms behavior of steel inthe bath solutions were represented. As for the previousfigure, the peak observed at −0.95 V/SCE disappears in thepresence of cadmium indicating that it concerns the zinc.Many authors attribute this peak to the formation of δ-phase

−0.02

−0.015

−0.01

−0.005

0

0.005

0.01

0.015

0.02

−1.5 −1 −0.5 0

I(A

/cm

2)

E (V)

|Zn2+| = 0.02 M|Zn2+| = |Ni2+| = 0.02 M|Zn2+| = |Ni2+| = 0.02 M |Cd2+| = 0.001 M

Figure 4: Cyclic voltammograms at a scan rate 5 mV/s of a metallicsolution containing |Na2SO4|= 0.5 M at pH= 3.5.

−1.5−1

−0.50

0.5

1

1.5

2

2.5

−1.2 −1 −0.8 −0.6 −0.4 −0.2 0I(m

A/c

m2)

E (V)

|Zn2+| = |Ni2+| = 0.02 M |Cd2+| = 0.001 M|Zn2+| = |Ni2+| = 0.02 M |Cd2+| = 0.001 Mboric acid = 0.8 M

Figure 5: Cyclic voltammetry at a scan of 5 mV/s of a metallic so-lution containing |Na2SO4|= 0.5 M at pH= 3.5.

(Ni3Zn22). This observation confirms that zinc is inhibitedby cadmium regardless of the form in which it appears.

We can notice, on the other hand, that cadmium has noeffect on the nickel deposit.

3.3. Combined Effect of Boric Acid and Cadmium on Zinc-Nickel Deposition. Figure 5 shows the combined effect ofboric acid and cadmium on the deposition of zinc and nickel.As mentioned above, cadmium has a strong effect on the zincdeposit. It reduces zinc deposition. The presence of boric aciddecreases the zinc deposit while it shifts the reduction peakof nickel to more negative potentials and increases the peakheight.

Boric acid has an effect on the cadmium deposition sincethe intensity of the anodic stripping decreases.

The effect of the presence of Cd2+ on the electrode-position of nickel can be seen from Figure 6. The currentremained unchanged when the concentration of Cd2+ is

4 International Journal of Electrochemistry

−1.4−0.9−0.4

0.1

0.6

1.1

1.6

2.1

2.6

−1.5 −1 −0.5 0I

(mA

)

E (V)|Ni2+| = 0.02 M|Ni2+| = 0.02 M |Cd2+| = 0.001 M|Ni2+| = 0.02 M |Cd2+| = 0.01 M|Ni2+| = 0.02 M |Cd2+| = 0.02 M

Figure 6: Anodic stripping voltammetry at a scan of 5 mV/s of a metallic solution containing |Na2SO4|= 0.5 M |H3BO3|= 0.8 M at pH= 3.5.

55

60

65

70

75

80

0 2 4

Zn

wei

ght

(%)

Concentration of CdSO4 (g/L)

(a)

0 2 4

Concentration of CdSO4 (g/L)

18

19

20

21

22

23

24

Niw

eigh

t(%

)

(b)

Figure 7: Effect of cadmium concentration on (a) Zn and (b) Ni at 4 mA/cm2.

lower than 0.02 M. At 0.02 M the current of redissolution ofnickel decreases and the decrease may be attributed to theadsorption of cadmium [17], which blocks the active sitesof the cathode surface and inhibits electrocrystallisation ofnickel.

Figure 7 illustrates the influence of cadmium on nickeland zinc in the deposit.

At 4 mA/cm2 the increases of cadmium in the bathdecreases the zinc content and not nickel content. The zinccontent decreased from 77% to 57% while the cadmium wasincreased from 0.5 to 3 g/L. The figure also shows that thecomposition can be easily controlled when, lower amount ofcadmium (0.5 and 1 g/L) is introduced in the bath.

Figure 8 show, the effect of amount of cadmium sulfaton the morphology of final deposits at a current densityof 4 mA/cm2. When cadmium sulfat concentration is below2 g/L, a smooth and uniform deposition can be obtained.Above 2 g/L of sulfat cadmium leads to a non-uniform sur-face with higher cadmium content on the surface.

A comparison of the optical micrographs in Figure 9shows a remarkably refined Zn-Ni coating structure in thepresence of boric acid. Coating in the presence of boric acidlooked compact, bright, and smooth.

Figure 10 shows the effect of boric acid on the surfacemorphology of Zn-Ni alloy deposits in the presence ofcadmium and cadmium boric acid. In the presence ofcadmium boric acid smaller particles in the range of 1-2 μmare observed. The decrease of particles in presence of boricis due to the preferred homogeneous nickel deposit thatoccurs at lower current densities. Boric acid acts on loweringcurrent density.

The current efficiency is plotted with the molar ratio|Zn2+|/|Ni2+| in the Figure 11.

The increase of the current efficiency, for the ratiobetween 0 < |Zn2+|/|Ni2+| < 1, results in the strong re-duction of nickel. The increase is more important for bathscontaining boric acid and boric acid-cadmium than for thosewithout boric acid. This observation confirms that boricacid increases the percentage of nickel in the deposit and

International Journal of Electrochemistry 5

Mag

10000x

Spot

3.5

HV30.0 kV

Scan142.01 s

DetETD

WD7.3 mm

8/20/200412:45:04 PM

5.0μm

(a)

Mag

10000x

Spot

3.5

HV30.0 kV

DetETD

WD7.3 mm

5.0μm8/20/200412:08:50 PM

Scan94.8 s

(b)

Figure 8: SEM photographs of electrodeposition of Zn-Ni on steel at 4 mA/cm2. (a) |CdSO4|= 1.5 g/L, (b) |CdSO4|= 2.5 g/L.

HV15.0 kV

Mag

2000x

Sig

SE

HFW0.14 mm

WD8.5 mm

Pressure0.75 Torr

Spot

5.050.0μm

SE

(a)

HV5.0 kV

Mag

2000x

Sig

SE

HFW0.14 mm

WD6.8 mm

Pressure0.85 Torr

Spot

5.0

50.0μm

SE

(b)

Figure 9: SEM Zn-Ni electrodeposited on steel at 4 mA/cm2 from a bath containing 0.02 M ZnSO4, 0.02 M NiSO4, and 0.5 M Na2SO4. (a)Without boric acid, (b) |H3BO3|= 0.8 M.

Mag

10000xSpot

3.5

HV30.0 kV

Scan94.67 s

DetETD

WD10.0 mm

8/19/200110:48:16 AM

5.0μm

(a)

Mag

10000x

Spot

5.0

HV30.0 kV

Scan94.67 s

DetETD

WD9.7 mm

8/19/200110:39:04 AM

5.0μm

(b)

Figure 10: SEM Zn-Ni electrodeposited on steel at 4 mA/cm2 from a bath containing 0.02 M ZnSO4, 0.02 M NiSO4, and 0.5 M Na2SO4. (a)|Cd2+|= 0.001 M, (b) |Cd2+|= 0.001 M boric acid= 0.8 M.

6 International Journal of Electrochemistry

0

20

40

60

80

100

120

0 1 2 3 4 5

Cu

rren

teffi

cien

cy(%

)

Zn2+/Ni2+

Boric acid 0 M |Cd2+| = 0 MBoric acid 0.8 M |Cd2+| = 0 MBoric acid 0.8 M |Cd2+| = 0.001 M

Figure 11: Current efficiency on the molar ratio plotted at 5 mA/cm2 in a solution containing |Na2SO4|= 0.5 M.

inhibits the deposition of zinc and protons discharge. Boricacid would be expected to influence reactions that take placeat the electrode surface as hydrogen evolution, zinc, andnickel deposition. According to Karwas and Hepel [14], boricacid suppresses the secondary nucleation while it increasesthe primary nucleation density. However, cadmium has noeffect on the current efficiency. For |Zn2+|/|Ni2+| ratio valuesgreater than 1 the current efficiency is slightly dependenton the changes in the |Zn2+|/|Ni2+| ratio. For |Zn2+|/|Ni2+|ratio values higher than 2, the percentage of nickel in thedeposit slightly decreases. This diminution is attributed tothe weak rate of hydrogen reaction on the cathodic surfacerecovered with zinc compared to the one observed on baresteel for example [18].

Conclusion

The electrodeposition of Zn-Ni alloys is anomalous when thepercentage of nickel in the deposit is lower than that in thesolution. Boric acid extends the proton discharge potentialto more cathodic values. It shifts the reduction peak to morenegative potentials, and increases the peak height. Boric acidenhances the Ni nucleation process, and improved deposi-tion of Ni consequently decreases anomalous deposition.

Cadmium decreases strongly the deposition of zinc buthas no effect on nickel. As shown in the morphology ofZn-Ni-Cd with varying CdSO4 concentration well-definedparticles are seen up to 2 g/L of CdSO4 and nonuniformsurface was obtained beyond 2 g/L.

For a ratio |Zn2+|/|Ni2+| = 0.5, and in the presence ofcadmium-boric acid at a concentration of 0.8 M the co-deposition tends toward normal deposition since the pointon the curve corresponding to the ratio 0.5 is almostcoincident with that on the curve CRL. Boric acid enhancesthe current efficiency while cadmium has no effect on currentefficiency.

References

[1] Y. P. Lin and J. R. Selman, “Electrodeposition of corrosion-resistant Ni-Zn alloy. I.Cyclic voltammetric study,” Journal ofthe Electrochemical Society, vol. 140, no. 5, pp. 1299–1303,1993.

[2] Y. Miyoshi, “State of the art in precoated steel sheet for au-tomotive body materials in Japan,” ISIJ International, vol. 31,no. 1, pp. 1–10, 1991.

[3] S. Wakano, A. Shibuya, Y. Hobo, and M. Nishihara, “Corro-sion Performance of Zn alloy Precoated Steels for AutomotiveBody,” ISIJ International, vol. 18, p. 967, 1983.

[4] A. Brenner, Electrodeposition of Alloys: Principles and Practice,vol. 1-2, Academic Press, New York, NY, USA, 1963.

[5] B. C. Baker and A. C. West, “Electrochemical impedance spec-troscopy study of nickel-iron deposition II. Theoretical inter-pretation,” Journal of the Electrochemical Society, vol. 144, no.1, pp. 169–175, 1997.

[6] H. Fukushima, T. Akiyama, and K. Higashi, “Electrodeposi-tion behavior of Zn-Ni alloys from sulfate baths over a widerange of current density,” Metallurgy, vol. 42, no. 3, pp. 242–247, 1988.

[7] J. Horkans, “Effect of plating parameters on electrodepositedNiFe,” Journal of the Electrochemical Society, vol. 128, no. 1, pp.45–49, 1981.

[8] M. J. Nicol and H. I. Philip, “Underpotential deposition andits relation to the anomalous deposition of metals in alloys,”Journal of Electroanalytical Chemistry, vol. 70, no. 2, pp. 233–237, 1976.

[9] M. Matlosz, “Competitive adsorption effects in the electrode-position in iron-nickel alloys,” Journal of the ElectrochemicalSociety, vol. 140, no. 8, pp. 2272–2279, 1993.

[10] H. Ashassi-Sorkhabi, A. Hagrah, N. Parvini-Ahmadi, and J.Manzoori, “Zinc-nickel alloy coatings electrodeposited froma chloride bath using direct and pulse current,” Surface andCoatings Technology, vol. 140, no. 3, pp. 278–283, 2001.

[11] C. C. Hu and A. Bai, “The inhibition of anomalous codepo-sition of iron-group alloys using cyclic voltammetry,” Journalof the Electrochemical Society, vol. 149, no. 11, pp. C615–C622,2002.

International Journal of Electrochemistry 7

[12] Z. Zhou and T. J. O’Keefe, “Modification of anomalousdeposition of Zn-Ni alloy by using tin additions,” Surface andCoatings Technology, vol. 96, no. 169, p. 191, 1997.

[13] C. Müller, M. Sarret, and M. Benballa, “Some peculiaritiesin the codeposition of zinc-nickel alloys,” Electrochimica Acta,vol. 46, no. 18, pp. 2811–2817, 2001.

[14] C. Karwas and T. Hepel, “Morphology and composition ofelectrodeposited cobalt-zinc alloys and the influence of boricacid,” Journal of the Electrochemical Society, vol. 136, no. 6, pp.1672–1678, 1989.

[15] M. Šupicová, R. Rozik, L. Trnková, R. Oriňáková, and M.Gálová, “Influence of boric acid on the electrochemical dep-osition of Ni,” Journal of Solid State Electrochemistry, vol. 10,no. 2, pp. 61–68, 2006.

[16] J. P. Hoare, “Boric acid as a catalyst in nickel plating solutions,”Journal of the Electrochemical Society, vol. 134, no. 12, pp.3102–3103, 1987.

[17] K. Kazuo, S. Takahiro, and S. Kunioa, “Morphology evolutionof zinc-nickel binary alloys electrodeposited with cadmiumadditive,” Journal of the Electrochemical Society, vol. 142, no.10, pp. L193–L195, 1995.

[18] Z. F. Lodhi, J. M. C. Mol, A. Hovestad, H. Terryn, and J. H.W. de Wit, “Electrodeposition of Zn-Co and Zn-Co-Fe alloysfrom acidic chloride electrolytes,” Surface and Coatings Tech-nology, vol. 202, no. 1, pp. 84–90, 2007.

SAGE-Hindawi Access to ResearchInternational Journal of ElectrochemistryVolume 2011, Article ID 670513, 5 pagesdoi:10.4061/2011/670513

Research Article

Cyclic Voltammetry and Impedance Spectroscopy BehaviorStudies of Polyterthiophene Modified Electrode

Naima Maouche and Belkacem Nessark

Laboratoire d’Electrochimie et Matériaux, Département de Génie des Procédés, Faculté de Technologie, Université Ferhat Abbas,19000 Sétif, Algeria

Correspondence should be addressed to Naima Maouche, m naima2001@yahoo.fr

Received 10 May 2011; Accepted 20 June 2011

Academic Editor: José H. Zagal

Copyright © 2011 N. Maouche and B. Nessark. This is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properlycited.

We present in this work a study of the electrochemical behaviour of terthiophene and its corresponding polymer, which isobtained electrochemically as a film by cyclic voltammetry (CV) on platinum electrode. The analysis focuses essentially onthe effect of two solvents acetonitrile and dichloromethane on the electrochemical behaviour of the obtained polymer. Theelectrochemical behavior of this material was investigated by cyclic voltammetry and electrochemical impedance spectroscopy(EIS). The voltammograms show that the film of polyterthiophene can oxide and reduce in two solutions; in acetonitrile, theoxidation current intensity is more important than in dichloromethane. The impedance plots show the semicircle which ischaracteristic of charge-transfer resistance at the electrode/polymer interface at high frequency and the diffusion process at lowfrequency.

1. Introduction

As other conjugated conducting polymers, polythiopheneand its oligomers can be polymerized from their monomersin solutions by electrochemical methods. The electrochemi-cal synthesis is advantageous method: polymers are formedin the doped state; films generally possess interesting elec-trochemical and good semiconductor properties [1] andrelatively good stability in air for both the neutral andoxidised states [2–7].

The mechanism of the electropolymerisation of con-ducting polymers and polyheterocycles occurred by thecoupling via α-α bonding of monomer radical cation afterits oxidation at the electrode, and the protons are removedfrom dihydrodication leading to neutral species [8–10]. Asthe dimer is more easily oxidized than the monomer, itis immediately oxidized. The chain elongation occurs bythe addition of new monomer radical cation leading topolymerization and forms the insoluble polymeric species,which subsequently deposits onto the electrode [11].

Conducting polymer-modified electrodes have beenwidely investigated because of their potential application

in areas such as electrocatalysis [12, 13], sensors [14, 15],corrosion [16–18], batteries [19, 20], electronic displays, anddevices [21–24].

In our previous work [25], we have studied the roleof P3T in corrosion of stainless steel; the results wereimportant and show effectively that the film of P3T willdecrease the corrosion rate. In this paper, we are interestedin performing an electrochemical characterisation of P3Tfilms electrochemically synthesized, in two organic solvents:acetonitrile (CH3CN) and dichloromethane (CH2Cl2), atplatinum substrates. We want to show how the mediumof the analysis is depending upon the electrochemical andelectronic behaviour of the formed films.

2. Experimental

The solvents acetonitrile and dichloromethane, Aldrich pureproducts for analysis, were used without further purification.Terthiophene (3T) was purchased from Aldrich (98%),and it was used as received; the supporting electrolyte salttetrabutylammonium perchlorate (TBAP) was purchased

2 International Journal of Electrochemistry

from Fluka; this salt was first dried at a temperature of 80◦Cfor 4 h before its use for the preparation of solution.

Electrochemical measurements were carried out onpotentiostat/galvanostat Voltalab 40 interfaced with a PCunder Voltamaster 4 software, in a three-electrode cellconsisting of: platinum working electrode (diameter 2 mm),which was served as a substrate for the deposition of poly-terthiophene (P3T), wire platinum as auxiliary electrode,and a saturated calomel electrode (SCE) as reference.

Electrochemical impedance spectroscopy measurementswere performed using an impedancemeter Z-computerTACUSSEL controlled by microcomputer HEWLETT-PACKARD. The assembly is coupled to a plotter anda printer. The impedance spectra were recorded in thefrequency range 105 Hz–10−3 Hz with an amplitude of10 mV, in three-electrode electrochemical cell. All experi-ments were carried out at room temperature.

Before the electrochemical deposition of polyterthio-phene for each experiment, the working electrode waspolished by 0.03 μm alumina slurry and rinsed with distilledwater and acetone for the removal of excess 3T monomer.

3. Results and Discussion

3.1. Electropolymerisation of Terthiophene. Figure 1 showsthe cyclic voltammograms (12th cycles) 10−2 M of 3Tdissolved in CH3CN solution containing 0.1 M of TBAPsupporting electrolyte at Pt electrode, in potential scansbetween 0 and 1.2 V versus SCE with scan rate of 50 mV/s.The first scan shows that the terthiophene is oxidized to itsradical cation at potential of +1.07 V [26], and immediatelythe polymer was formed. In the reverse cathodic scan, apolymer reduction peak was observed at a potential of 0.70 V.The current of the oxidative peak progressively increaseswith the number of cycles indicating the formation and thegrowth of conducting polymer film and suggests that thereis systematic increase in the electrode area as a result of theactual deposition of P3T [27, 28]. The films formed are stableon the electrode and can easily be transferred to an electrolytesolution in absence of monomer.

3.2. Cyclic Voltammetry Characterisation. After polymeriza-tion, the working electrode was extracted from the cell,rinsed with acetone, and dried with a gentle nitrogen flux,then analysed by cyclic voltammetry and impedance spec-troscopy in the monomer-free solution of CH3CN/TBAP andCH2Cl2/TBAP, respectively.

Figure 2 shows the cyclic voltammograms of P3T filmin CH3CN/0.1 M of TBAP solution, recorded at differentvoltage scan rates 10 (a), 20 (b), 50 (c), and 100 mV/s.

The voltammogram recorded at scan rate of 10 mV/s(inset of Figure 2) shows a single oxidation peak located at1.06 V and two reduction processes, one at around 0.96 Vand the other less intense at 0.74 V. However, we observethat, except for their oxidation potential, which depends onthe chain length of the oligomer, the shape of voltammetriccurves is similar, and the same behavior was also observedwith different thiophene oligomers [29–33].

−0.0002

−0.0003

−0.0001

0

0.0001

0.0002

0.0003

0.0004

0.0005

0.0006

I(m

A)

E (V/SCE)

0 0.2 0.4 0.6 0.8 1 1.2

Figure 1: Electrochemical polymerization of 3T (a) 1rst cycle,repetitive cycling (b) in a solution of TBAP 0.1 M in acetonitrile,scan rate 50 mV/s.

0 0.2 0.4 0.6 0.8 1 1.2

−20−10

0

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30

40

50

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−100

−50

0

50

100

150

200

(d)

(a)I(μ

A)

E (V/SCE)

Figure 2: Cyclic voltammograms corresponding to P3T film in asolution of CH3CN/TBAP 0.1 M, recording for different scan rates:(a) 10, (b) 20, (c) 50, and (d) 100 mV/s.

The film formed on the platinum electrode is electro-chemically active and shows a reversible change of colour,a red in the oxidation state and blue in the reductionone. This phenomenon of P3T based on reversibly colouredelectrochromic materials has become the subject of manyinterest applications. The electrochemical stability of thefilm was shown by repetitive voltammograms which remainessentially stable with cycling, and no evolution of theoxidation or the reduction current is observed. The stabilityof the oligothiophenes in air is already discussed in the

International Journal of Electrochemistry 3

0 0.2 0.4 0.6 0.8 1 1.2

−60

−40

−20

0

20

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80

100(d)

(a)

I(μ

A)

E (V/SCE)

0 0.2 0.4 0.6 0.8 1 1.2

−40−20

0

20

40

60

I(μ

A)

E (V/SCE)

Figure 3: Cyclic voltammograms corresponding to P3T film in a0.1 M solution of TBAP in dichloromethane recording for differentscan rates; (a) 10, (b) 20, (c) 50, and (d) 100 mV/s.

literature [3, 34], and it is one of the most importantproperties of polyoligothiophenes.

The film of P3T was furthermore studied in CH2Cl2/TBAP in absence of monomer. The corresponding voltam-perograms are presented in Figure 3. Also it can be shownthat the film can oxide and reduce, and practically thesame allure of voltammograms is obtained; however, theelectroactivity of the film in CH3CN was more defined andhas a high oxidation current.

The voltammogram obtained here do not show clearlythe anodic peaks except in the return scan; we observe apeak at 0.51 V attributable to the reduction of the filmdeposited on the electrode. So, contrary to the analysisrealized in acetonitrile medium, in dichloromethane, theoxidation peak is poorly defined and is concealed, probablybecause of strong participation of capacitive current. Thecurrent intensity of the anodic and cathodic peak increaseswith scan rate. Also, the waves and the oxidation peaks donot appear when curves are recorded with high scan rates(v > 20 mV/s), this is the result of the participation of alarge capacitive current which becomes important as the scanrate increases. The difference (ΔEp) between the oxidationpotential and the reduction one increases with the scan rates.The proportionality of the peak intensity to the scan ratessuggests that the oxidation of electroactive polymer obtainedon the electrode surface is limited by a diffusional process[30].

These results suggest that the thickness of the film issmaller than the diffusion layer thickness of counteranionson the cyclic voltammetric time scale used here, which mustdiffuse in and out during the doping and dedoping processes.The oxidation peaks shifted to more positive potential at scanrates increase; this suggests that as explained in reference [33](i) the electron transfer from the electrode to the polymerfilm may be slow; (ii) the rate of electron transfer may becontrolled by the diffusion of counter ions at a scan rate faster

Table 1: Electrical parameters corresponding to P3T in CH3CN/TBAP and in CH2Cl2/TBAP.

CH3CN/TBAP CH2Cl2/TBAP

Re (Ω) 100 500

Rct (Ω) 500 2600

C (μF) 0.019 0.036

than 10 mV/s; and/or (iii) the ohmic potential drop acrossthe film may be significant.

3.3. Electrochemical Impedance Spectroscopy Characterization.Figure 4 gives the Nyquist plots of the electrodes coatedby P3T/Pt, obtained in solution of CH3CN/TBAP 0.1 M(Figure 4(a)) and in CH2Cl2/TBAP 0.1 M (Figure 4(b)).It can be seen that the two impedance curves have asemicircle in the high-frequency region for each medium,and the semicircle in acetonitrile is smaller than that indichloromethane. The high-frequency part gives electrolyteresistance (Re) by the distance from the original value, andthe width of the semicircle gives the estimated impedance ofthe studied film, that is, ohmic resistance or ionic charge-transfer resistance (Rct) of the polymer-solution interface bythe intercept of the semicircle with the real axis [35, 36].This behaviour is typical of impedance diagram of polymerfilm-coated metals in the asymmetric metal/film/electrolyteconfiguration [37].

Towards low or middle frequency region, the impedancediagrams show a straight line with slope of 45◦ which indi-cates a diffusion-controlled Warburg behavior, attributableto the semiinfinite diffusion of protons at the polymer-electrolyte interface [38].

From the Nyquist plots, the kinetic parameters can beeasily deduced. The value of transfer resistance allows us todetermine the corresponding capacity C (capacitance of theinterface electrode/polymer), using the relationship: 2πfRC =1 for the frequency where the imaginary part has a maximumvalue on the half semicircle. The obtained parameters fromthe impedance plots of P3T are presented in Table 1.

As shown from Table 1, the values of Re, Rct, and Care less important in acetonitrile than in dichloromethanemedium, indicating that the charge-transfer process of P3Ton the Pt electrode is more easier and fast in acetonitrilethan in dichloromethane, and the film in CH2Cl2 has ahigh resistance. These results confirmed what we obtainedin cyclic voltametry.

Generally, the electrochemically results obtained fromthe impedance diagrams of conducting organic polymersare modelled by an equivalent electrical circuit. Many ofthem have been proposed, in the literature, and in general,for the most part of cases, the equivalent circuit can beassimilated to a circuit of the Randles model, more or lessmodified according to the experimental conditions. The elec-trochemical parameters of P3T film are modelled by Randlescircuit (Figure 5), which gives the uncompensated electrolyteresistance and serially connected to the capacitance of double

4 International Journal of Electrochemistry

0

2

4

6

8

10

12

×104

0 2 4 6 8 10 12×104

−ZIm

(·cm

−2)

ZRe ( ·cm−2)

0 5 10 15 200

−ZIm

(·cm

−2)

ZRe ( ·cm−2)10

5H

z

2.15·1

04H

z

1.67·1

03H

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×102

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20×102

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04H

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(a)

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−2)

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0 1 2 3 4 5 6 7 8 9 10

×103

−ZIm

(·cm

−2)

ZRe ( ·cm−2)

105

Hz

6·1

03H

z

3.6·1

02H

z

1.67·1

0H

z

0 0.5 1 1.5 2 2.5 3 3.5 4

0123456789

10×103

(b)

Figure 4: Nyquist plots of Pt modified electrode with P3T, recorded in: (a) CH3CN/TBAP, (b) CH2Cl2/TBAP. Plots were recorded in thefrequency range from 105 Hz to 10−3 Hz at free potential.

Cdl

Re

RtZw

Figure 5: Electrical equivalent circuit corresponding to P3T/electrode interface (Randles circuit).

layer which is in parallel with the resistance of the chargetransfer and Warburg impedance.

4. Conclusion

Electrochemical characterisation of P3T film was carried outusing CV and EIS techniques. CV measurements showedthat the electroactivity of polyterthiophene films intensityincreases with increase of scan rates and it is more importantin CH3CN than in CH2Cl2. However, the reduction peak cur-rents are broader and displaced to more negative potential.EIS revealed that, at high frequencies, charge-transfer processdominates with a semicircle, and at low frequencies, diffusionprocess dominates with a slope line of 45◦. The electricalparameters were found to be more significant in CH3CN.

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[7] L. B. Groenendaal, F. Jonas, D. Freitag, H. Pielartzik, andJ. R. Reynolds, “Poly(3,4-ethylenedioxythiophene) and itsderivatives: past, present, and future,” Advanced Materials, vol.12, no. 7, pp. 481–494, 2000.

[8] E. M. Genies, G. Bidan, and A. F. Diaz, “Spectroelectrochem-ical study of polypyrrole films,” Journal of ElectroanalyticalChemistry, vol. 149, no. 1-2, pp. 101–113, 1983.

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[14] M. Boopathi, M.-S. Won, and Y.-B. Shim, “A sensor foracetaminophen in a blood medium using a Cu(II)-conductingpolymer complex modified electrode,” Analytica Chimica Acta,vol. 512, no. 2, pp. 191–197, 2004.

[15] K. B. Crawford, M. B. Goldfinger, and T. M. Swager, “Na+ spe-cific emission changes in an ionophoric conjugated polymer,”Journal of the American Chemical Society, vol. 120, no. 21, pp.5187–5192, 1998.

[16] D. W. DeBerry, “Modification of the electrochemical andcorrosion behavior of stainless steels with an electroactivecoating,” Journal of the Electrochemical Society, vol. 132, no. 5,pp. 1022–1026, 1985.

[17] H. Hammache, L. Makhloufi, and B. Saidani, “Corrosionprotection of iron by polypyrrole modified by copper usingthe cementation process,” Corrosion Science, vol. 45, no. 9, pp.2031–2042, 2003.

[18] R. Hasanov and S. Bilgiç, “Monolayer and bilayer conductingpolymer coatings for corrosion protection of steel in 1 MH2SO4 solution,” Progress in Organic Coatings, vol. 64, no. 4,pp. 435–445, 2009.

[19] C. Y. Wang, G. Tsekouras, P. Wagner et al., “Functionalisedpolyterthiophenes as anode materials in polymer/polymerbatteries,” Synthetic Metals, vol. 160, no. 1-2, pp. 76–82, 2010.

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