Thin Solid Films 458(2004) 47–51

0040-6090/04/$ - see front matter� 2003 Elsevier B.V. All rights reserved.doi:10.1016/j.tsf.2003.11.273

A method for cathodic polymerization of aniline by in situelectrogenerated intermediate at gold surface

Yong Shao, Yongdong Jin, Xuping Sun, Shaojun Dong*

State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences,Ren Min St. 5625, Changchun 130022, Jilin, PR China

Received 6 May 2003; received in revised form 22 October 2003; accepted 21 November 2003

Abstract

Polyaniline(PANI) was cathodically synthesized at an evaporated gold electrode using an in situ electrogenerated intermediateas oxidant during reduction of the dissolved oxygen. The obtained PANI layer showed an electrochemical response similar to thatsynthesized by the conventionally anodic polymerization, and the average rate for the growth of PANI layer at polycrystallinegold electrode was 1.59 nm h , while that at the Au(111) electrode was 4.93 nm h . Based on these results, the thickness ofy1 y1

the resulted layer can be easily controlled at molecular level for potential nanodevice applications. The obtained PANI layershowed morphology from an island-like nanostructure to an ultrathin film, depending on the crystal orientation of the electrodeused.� 2003 Elsevier B.V. All rights reserved.

Keywords: Conducting polymers; Surface plasmon resonance; Ultrathin film; Electrochemistry; Atomic force microscopy

1. Introduction

The unique properties of polyaniline(PANI) havecontinuously attracted more interest due to its potentialapplications in various fields, for example, electronic–photonic transductionw1–3x, and electro-driven microd-evices w4x. However, prompt switching of conductingpolymers between the conducting and non-conductingforms can be effectively exploited only for very thinfilms of high integrity w5x, and the ultrathin film ofconducting polymers has properties in improving theresponse time for its gas sensorw6x. Also, the qualityand performance of the polymers depend mainly on theadopted synthesis methodw7x. The control of the struc-ture and morphology of the films is thus essential whenenvisioning practical applications. Therefore, pursuit forthe development of new methods, besides classicallychemical and electrochemical synthesis, is necessary toobtain the needed special features of PANI. However, itis difficult to obtain a very thin and even film of PANIwith thickness at the controllable molecular level by the

*Corresponding author. Tel.:q86-43-1526-2101; fax:q86-43-1568-9711.

E-mail address: dongsj@ns.ciac.jl.cn(S. Dong).

conventionally anodic polymerization. Although thetechnique of molecular self-assembly has been devel-oped for the fabrication of ultrathin films of conductingpolymers w8–10x, the preliminary dissolution of con-ducting polymers in an appropriate solvent is still a keyproblem. Recently, Heinzew11x reported the formationof patterns of conducting polymers, in which the mon-omer was oxidized by anodically electrogenerated oxi-dant produced at the probe tip of the scanningelectrochemical microscope. However, this strategy can-not avoid direct oxidation of the monomer at the anodicprobe. At cathode, employing in situ electrogeneratedintermediate as an oxidant may solve the problem by acontrollable way in which the intermediate can undergoa further kinetically slow electrochemical reaction at theelectrode surface but react promptly with aniline mon-omers present in solution. Herein, we tried to do thiswith the cathodically electrogenerated intermediate asoxidant during reduction of the dissolved oxygen(O ).2

2. Experimental

Surface plasmon resonance(SPR)measurements weredone by the SPR apparatus(SPR 2000, Electronic

48 Y. Shao et al. / Thin Solid Films 458 (2004) 47–51

Fig. 1. Cyclic voltammgrams(CVs) of the dissolved O in 0.2 M2

H SO free of aniline at polycrystalline gold electrode. Inset showed2 4

CVs of the resulted PANI layer in 0.2 M H SO free of aniline in the2 4

potential range fromy0.3 to 0.6 V(a) and fromy0.3 to 1.0 V(b).The layer was obtained by electrolysis 1 h aty0.2 V in 0.2 MH SO containing 0.1 M aniline.2 4

Fig. 2. SPR responses to the growth process of PANI layer producedby electrolysis 1 h aty0.2 V in 0.2 M H SO containing 0.1 M2 4

aniline. Curves(a)–(f) were recorded per 10 min. The inset showedthe shift of the SPR angle during the growth process at polycrystallinegold electrode.

Institute of Chinese Academy of Sciences, China) withangle resolution less than 10 degree. The glass slidey3

covered with 50 nm gold film for SPR was pressed ontothe base of a half-cylindrical lens(ns1.61) via anindex-matching oil. Linearly p-polarized light having awavelength of 670 nm from a diode laser was directedthrough the prism onto the gold film in the Kretshmannconfiguration. The intensity of the reflected light wasmeasured as a function of the angle of incidenceu,using a photodiode with a chopperylock-in amplifiertechnique. For SPR in situ combined with electrochem-ical investigation(CHI 800, Shanghai Chenhua Instru-ments Co., China), the as-prepared goldyglass substrateswere mounted against the Teflon cuvette with 1-mlvolume using a Kalrez O-ring, which provided a liquid-tight seal. The Teflon cuvette allowed for the simulta-neous recording of the SPR data and the application ofa voltage biased to the gold film. The gold film on theglass slide used for the excitation of surface plasmonmodes also served as the working electrode. A platinumwire served as the counter electrode, and Agy(AgCl,saturated KCl) as the reference electrode. All potentialwere recorded with respect to this reference electrode.The measurements of atomic force microscopy

(AFM) were imaged by a SPA400 equipped with a SPI-3800 controller(Seiko Instruments Industry Co., Tokyo,Japan) at room temperature. The tip type was SN-AF01(Seiko Instruments Co.) and the cantilever used wasfabricated from Si N with a spring constant of 0.02 Ny3 4

m. All images were recorded with a scan rate of 2.0 Hzand repeated several times. X-Ray diffraction(XRD)analysis was carried out on DyMax 2500 VyPC X-raydiffractometer using Cu(40 kV, 200 mA) radiation.

3. Results and discussion

An intermediate during reduction of the dissolvedO was first served as an in situ electrogenerated oxidant2

for the cathodic polymerization of aniline. Fig. 1 gavethe cyclic voltammgram of dissolved O in 0.2 M2

H SO showing a reduction wave between 0.10 andy2 4

0.30 V (vs. AgyAgCl). Thus the strategy for anilinepolymerization by the intermediate of O reduction was2

carried out aty0.20 V. SPR was used to follow thegrowth of PANI at evaporated gold film electrode withfilm thickness 50 nm. Fig. 2 illustrated the SPRresponses to the process. As the reaction proceeded withcontinuous electrolysis, the SPR angle gradually shiftedto higher values, but the reflectance at SPR angle slightlyincreased, which was consistent with the anodic growthof PANI at gold electrode previously also convinced bySPR w12x. Additionally, the electrochemical activity ofthe obtained PANI, a prime quality for PANI, wasobserved as in the inset of Fig. 1. However, as in theinset of Fig. 2 showing the time-dependent shift of theSPR angle, the growth with a constant rate was obviousonly after 15 min electrolysis, which indicated an induc-tion period but longer than that needed for the conven-tional polymerization convinced by literaturesw13,14x.All these observations demonstrated that PANI layerwas formed by our proposed method.

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Fig. 3. The kinetics of shifts of SPR angle in the case of no appliedpotential(a), electrolysis aty0.2 V (b), further continuous electrol-ysis after b(c), electrolysis aty0.2 V with saturated O in solution2

(d) at the polycrystalline electrode, and electrolysis aty0.2 V at theAu (111) electrode(e). In all cases, the electrolyte was 0.2 MH SO containing 0.1 M aniline and was saturated by air except the2 4

condition d. Inset showed the XRD patterns for the correspondingpolycrystalline gold electrode(a) and Au(111) electrode(b).

We also checked the kinetics during the growth ofPANI by SPR. As shown in Fig. 3, when the electrodepotentialy0.2 V was applied the shift of the SPR anglewas much more obvious than that obtained when thecircuit was open(curve a). However, when the electrodepotentialy0.2 V was applied but the dissolved O was2

purged by nitrogen the SPR angle changed very slightlywith time and very same result was obtained as thecircuit was open(data not shown). Furthermore, whenan aniline solution saturated by air was used, PANIgrew with a constant rate during 2 h because the slopesof curve b and c were similar. However, when anilinesolution saturated by pure O was used, a faster rate for2

the growth of PANI was observed(curve d). Obviously,in our case the aniline polymerization was very relatedto the reduction of the dissolved O at gold electrode.2

However, the crystal orientation of the gold electrodeused had a profound effect on the growth rate of PANI.The inset in Fig. 3 showed the XRD patterns of thegold electrode used. The Au(111) electrode possessedenhancement upon the growth of PANI compared tothat at polycrystalline gold electrode(curve e). Thethickness of the obtained PANI layer was extracted byfitting to the four-phase Fresnel equationsw15x using theknown refractive index of PANI at 670 nmw1x. Thusthe obtained results indicated that a shift of 0.2358 inthe resonance minimum angle was corresponding to aPANI layer thickness of 4.0 nm. Therefore, the averagerate for the growth of the PANI layer at the polycrystal-

line gold electrode was 1.59 nm h , while that at they1

Au (111) electrode was 4.93 nm h . These rates arey1

obviously much slower than that obtained by the con-ventionally potentialstatic bulk electrolysisw1x. In theusually used cyclic voltammetry for the polymerizationof aniline, the layer thickness was more than 1.0 nmeven by the first scanw12x. However, in our case, thePANI layer in thickness less than 1.0 nm can be easilycontrolled. This fact suggests that the fabrication ofultrathin films of conducting polymers with molecular-level control over thickness can be developed as thatconstructed by self-assembly of conducting polymersw16x. The resulted ultrathin film may be found applica-tions in microfabrications for nanodevices. Due to theultrathin film having very weak responses to otherspectroscopic characteristics such as infrared or ultravi-oletyvisible spectroscopy, no attempt was made to char-acterize and verify its quality.The morphology of PANI layer obtained also had a

strong dependence on the crystal orientation of theelectrode used. As shown by AFM in Fig. 4, PANIseemed to grow preferentially at certain sites of thepolycrystalline gold electrode surface, and the PANIstructures formed were well separated with each other.Therefore, some randomly scattered nanoislands of PANIwere constructed. However, the growth at single crystalAu (111) electrode was observed with relatively densestructure and thus to some extent an ultrathin film ofPANI was obtained. Since the surface roughness of asingle crystal electrode was more obvious than that ofthe polycrystalline electrode, the obtained ultrathin filmappeared to have some variation in thickness over theareas scanned. But the morphological contrast wasobvious (Fig. 4b,d). Based on these observations andthe results of Fig. 3, we believe that the morphologyand the average growth rate of PANI film can becontrolled via using the electrode with different crystalorientation.From these observations, it is very obvious that PANI

can be formed at gold electrode with the electrogener-ated intermediate as oxidant during reduction of thedissolved O . In fact, Strbacw17x demonstrated that O2 2

reduction occurred as a 2e process to hydrogen peroxide(H O ) on most of the gold facets and some crystal2 2

facets showed a pronounced sensitivity to O reduction.2

But the applied electrode potential also affected thisprocess. It is well known that a normal polycrystallinegold surface reduces O to H O by a two-electron2 2 2

mechanism at more positive potential than 0.30 V butbelow this potential H O is further reduced and a four-2 2

electron current is approachedw18,19x. However, the Au(111) facet has been found inactive for H O reduction2 2

in the whole potential region up to hydrogen evolutionw20,21x. Therefore, H O , which has been used an2 2

oxidant for aniline polymerization in solution catalyzedby Fe w22x, should be served as an oxidant in our2q

50 Y. Shao et al. / Thin Solid Films 458 (2004) 47–51

Fig. 4. AFM of the resulted PANI layer after electrolysis 1 h aty0.2 V in 0.2 M H SO containing 0.1 M aniline. The PANI grown at the2 4

polycrystalline gold electrode showed some island-like nanostructures(b) whereas that grown at the Au(111) electrode showed an ultrathin film(d). The control experiments were also imaged for comparison at the polycrystalline gold electrode(a) and at the Au(111) electrode(c). Imagesizes3=3 mm .2

case and thus Au(111) facet could catalyze anilinepolymerization by the electrogenerated H O . Thus the2 2

formation of PANI nanoislands should be also ascribedto the contribution from the included Au(111) compo-nents at polycrystalline gold electrode. The fact that thenanoislands had an appearance of random nucleation(Fig. 4b) should reflect the arbitrary distribution of Au(111) facet within the polycrystalline electrode. Thusthe patterns of PANI layer should be obtained if thepatterns of Au(111) were constructed at polycrystallinegold electrode. Although the deposition of PANI madethe reduction of the dissolved O difficult(see the inset2

of Fig. 1), the average growth rate of PANI seemed tobe unaffected upon increasing the PANI thickness. Thefact indicated that the generating rate of H O at the2 2

electrode surface by electrochemistry was much fasterthan its reaction rate with aniline or oligomer over theelectrode surface. One may doubt the reaction efficiencyof the electrochemically produced H O with aniline2 2

monomers due to diffusion of H O from the electrode2 2

surface to solution. In fact, the concentration of aniline,

0.1 M in our case, was much higher than that ofdissolved O in solution and that of the further produced2

H O at the electrode surface. Thus H O could prefer-2 2 2 2

entially and kinetically react with aniline before itsdiffusion towards solution. Although the applied reduc-tion potential was disadvantageous for PANI polymeri-zation, the produced H O always dynamically2 2

maintained the Nernst equilibrium and made the reactioncontinue.In summary, PANI was cathodically synthesized by

in situ electrogenerated H O as an oxidant. Au(111)2 2

facet had a pronounced activity towards aniline polym-erization. The PANI obtained showed morphology fromrandomly scattered island-like nanostructures to an ultra-thin film, depending on the crystal orientation of theelectrode used. Due to the low concentration of theelectrogenerated oxidant over the electrode surface,which is proportional to that of dissolved O , the2

polymerization of the monomer can be carried out witha controllable rate to form ultrathin film. Thus themorphology of the resulted film should be, to some

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extent, improved in comparison with conventionallyelectrochemical methods. Based on these, smaller andhigh qualitative microdevicesw4x may be envisaged.

Acknowledgments

This work was supported by special funds for majorstate basic research of China(2002CB713803) and theNational Natural Science Foundation of China(No.20275036 and No. 20211130506).

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