Synthetic Metals 156 (2006) 514–518

Polypyrrole nanostructures formed by electrochemicalmethod on graphite impregnated with paraffin

Jixiao Wang ∗, Xiaoping Mo, Dongtao Ge,Yun Tian, Zhi Wang, Shichang Wang

State Key Laboratory of Chemical Engineering, Chemical Engineering Research Center,School of Chemical, Engineering and Technology, Tianjin University, Tianjin 300072, China

Received 26 April 2005; received in revised form 28 September 2005; accepted 5 October 2005Available online 18 April 2006

Abstract

Polypyrrole nanostructures were prepared electrochemically by template-free method on graphite impregnated with paraffin. The experimentalparameters, such as testing temperature, polymerization potential, doped ions and the status of the electrode surface have significant effect ontBm©

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he morphology of the formed polypyrrole. Under various experimental conditions, fibrillar, taper and cauliflower polypyrrole can be obtained.y controlling the active sites, the polypyrrole nuclei will grow in one-dimensional pattern, and thus polypyrrole nanowires were obtained. Theethod should be useful for preparation other materials nanowires.2005 Published by Elsevier B.V.

eywords: Conductive polymers; Polypyrrole; Nanowires; Mechanism

. Introduction

Material systems of reduced size or dimensionality may,ften do, exhibit properties different from those found in theulk and in molecular state. Nanomaterials involve a wide-rangef potential applications. Such phenomena are of considerablecientific and technological interests, particularly in the areaf miniaturized, highly compact electronic devices. Nanowiresnd nanotubes are often prepared by template-route [1–8], self-ssembly technique [9–11] and anisotropic growth of the nuclear12,13]. The template might be nanopores of membrane orolecular sieves, macromolecules or cavity of macromolec-

lar, channel of microbes, nanotubes and nano-particles. Theelf-assembly system relates to the micelle formation from sur-actant, block polymers and supermolecular structure of someertain molecules.

Inherently electronically conductive polymers, suchs polypyrrole, polyaniline, polythiophene and poly(p-inylbenzene) have been intensively studied due to theirotential applications in sensors and actuators as well as in

electronic, electroluminescence, electrochromic and photo-electrochemical devices. Properties of these polymers dependon their microstructures and morphologies determined bythe synthesis method. As an inherently conductive poly-mer, polypyrrole have been electropolymerzied on varioushomogeneous substrates for wide-range of purposes. Themorphology of the polypyrrole formed on electrodes isnormally in cauliflower form. Polypyrrole dendrite, wrinkleand other forms might be obtained under certain conditions.Polypyrrole nanotubes/nanofibers were first prepared in thepores of polycarbonate membranes by Penner and Martin [14].After that, template method is a common route to fabricatepolypyrrole nanotubes/nanofibers. Porous substrates [15,16],lipid tubule [17] and molecular cavity of cyclodextrin [18] arealso used as template for synthesis polypyrrole nanowires. Wereported the formation of polypyrrole nanowires on compositeelectrode under the induction of the polyanions [19–21]. But,our recent experimental results indicate that the inductionrole of the polyanions is not the unique factor that controlsthe morphology. The polypyrrole nanowires can be simplysynthesized on graphite electrodes impregnated with paraffin.The control of electroactivity of the formed polypyrrole and the

∗ Corresponding author. Tel.: +86 22 2740 4533; fax: +86 22 2740 4757.E-mail address: jxwang@tju.edu (J. Wang).

heterogeneous property of the electrode material might havemuch effect on the morphology.

379-6779/$ – see front matter © 2005 Published by Elsevier B.V.

oi:10.1016/j.synthmet.2005.10.011

J. Wang et al. / Synthetic Metals 156 (2006) 514–518 515

2. Experimental approach

The electrode used in the experiments was a graphite rod withdiameter of 8 mm. The graphite rod was pretreated with a boil-ing acidic solution containing hydrochloric acid (30%) and nitricacid (30%) for 2 h, and then rinsed thoroughly with de-ionizedwater. The treated rod was immersed in paraffin (m.p. 52 ◦C) at150 ◦C until no bubbles appeared. Then the rod was polished by1200# emery paper. The experiments were conducted at roomtemperature in a one-compartment glass cell in an environmentof pure nitrogen. A saturated calomel electrode (SCE) and aplatinum network were used as reference electrode and counterelectrode, respectively. A buffer solution made from mixingsodium carbonate (0.20 M) and sodium dicarbonate (0.20 M)solution containing lithium perchlorate and pyrrole was usedas electrolytic solution. The volume ratio of sodium carbon-ate (0.20 M) to sodium dicarbonate (0.20 M) solution may bechanged from 1:5 to 5:1. In order to compare the effect of theelectrolyte, phosphate buffer solution (0.20 M) at pH 6.86 andoxalate buffer solution (0.20 M) at pH 4.22 were also used. Theelectrochemical experiment was performed on PAR 273 Elec-trochemical System controlled by a computer. The morphologyof PPy prepared by this method was examined under a scanningelectron microscope (SEM) (Philip XL30).

3. Results

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ing varied charge during the polymerization. The figure clearlyshows that polypyrrole nanowires can be electrogenerated ongraphite impregnated with paraffin electrode from the car-bonate buffer system with applied potential in the range of0.75–0.85 V. Varying the ratio of sodium carbonate (0.20 M)to sodium dicarbonate (0.20 M) solution, except the diameterof the PPy nanowires gets smaller, PPy morphology has noobviously change. When polymerized at 0.80 V versus SCE, thediameters of the formed PPy nanowires may change from about120 nm (formed in solution NaHCO3:Na2CO3 = 5:1) to 50 nm(formed in solution NaHCO3:Na2CO3 = 1:5). When polymer-ized at 0.95 V versus SCE, loose polypyrrole wires with diam-eter of several hundreds nanometers are obtained (Fig. 1D). At1.05 V, the taper form (Fig. 1E) polypyrrole is formed. When theapplied potential is higher than 1.05 V versus SCE, polypyrrolenanowires cannot be obtained. The morphology of polypyrrolepolymerized at potential higher than 1.05 V versus SCE is shownin Fig. 1F. The fibrillar morphology might attribute to the lowdoping degree resulting in low current carrier (electron, hole,soliton, polaron or bipolaron) density of the formed polypyr-role under the low applied potentials. The low electroactivity ofthe formed polypyrrole in carbonate solutions further increasesthe resistant forces to the current carriers. The low electroac-tivity might be caused by the structure defects in the molecularchain [22]. Just as reported, the FTIR of the formed polypyr-role nanowires shows that C O (adsorbed wavelength at about1Te

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Fig. 1A–C shows the polypyrrole nanowires of differentength and diameter formed on the graphite surface by pass-

ig. 1. SEM pictures of polypyrrole electrogenerated on graphite composite electroith 100 s, (B) 0.80 V vs. SCE with 100 s, (C) 0.85 V vs. SCE with 100 s, (D) 0.95 V0 s.

698) is contained at the pyrrole ring (as shown in Fig. 2).he low current carrier density and the high resistant made thenergy cannot be transferred from the electrode to the formed

de in carbonate solution. Potential-step polymerization at (A) 0.75 V vs. SCEvs. SCE with 40 s, (E) 1.05 V vs. SCE with 15 s and (F) 1.15 V vs. SCE with

516 J. Wang et al. / Synthetic Metals 156 (2006) 514–518

Fig. 2. FTIR of PPy nanowires prepared under different conditions: (A) in car-bonate solution, (B) in phosphate solution and (C) in oxalate solution.

polypyrrole, and thus cannot generate enough electroactive siteson the formed polypyrrole to keep electropolymerization goingon. When the applied potential lower than 0.95 V versus SCE,active sites cannot be produced on the formed polypyrrole exceptthe end rooted on the electrode. Thus, under certain condi-tion, pyrrole will be polymerized at the nanowire end attachedon the electrode and the former formed polypyrrole will bepushed away from the electrode by the later one. The growthpattern will be a one-dimensional process. At 0.95 V, someactive sites are electrogenerated, but less than those at 1.05 Vand polypyrrole wires with diameter about several hundrednanometers is obtained. When the applied potential is higherthan 1.05 V versus SCE, it will oxidize the formed polypyrroleand increase the doping degree. The high potential can gen-erate enough current carriers to transfer energy to the formedpolypyrrole and produce active sites at the formed polypyr-role and those make the polymerization keep on. When thepolymerization potential is higher than 1.05 V, the growth pat-tern will be three-dimensional type and cauliflower morphologyobtained.

Last paragraph, the results show that the formation of thenanowire might result from the two reasons. The one is thelow doping degree of the PPy, and the other is defected format the polymeric chains. Fig. 3 is SEM pictures of polypyrrole

formed at 0.80 V versus SCE 100 s in 0.20 M phosphate buffersolution (pH 6.8), 0.20 M oxalate buffer solution (pH 4.22) and0.20 M K4[(CN)6]/K3[(CN)6] solution, respectively. The SEMclearly shows that, as we early reported, in phosphate buffer solu-tion PPy nanowires are formed, and in oxalate buffer solutionand in 0.20 M K4[(CN)6]/K3[(CN)6] solution no PPy nanowirefounded. The results indicate that the formation of PPy nanowireneeds its low conductivity of the formed PPy and the low elec-troactivity of the dopants. No PPy nanowire formed in oxalatebuffer solution and in 0.20 M K4[(CN)6]/K3[(CN)6] solutionthat is because its dopants might be reduced or oxidized by theformed PPy. The FTIR indicates the structure defects (C O at the� position of pyrrole ring, FTIR adsorption at about 1698 cm−1)exist in PPy synthesized from carbonate solution. At the sametime, FTIR spectrum clearly demonstrates that PPy formed inphosphate solution and oxalate solution have no adsorption atabout 1698 cm−1 and the molecule has long conjugate length[23–25]. The long conjugate length means that the electronsmight be delocalized and the polymer might has the high elec-trical conductivity. The formation mechanism of PPy nanowiresmight be attributed to its low conductivity (low density chargecarriers) coming from the low doping degree. So, we can con-clude that the low density of charge carriers is the main reasonfor formation of PPy nanowires.

Nucleation-loop or trace-crossing usually happens in the firstcycle when the materials deposit onto a foreign substrate [26].FfimmUdohTfmgoai

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ig. 3. PPy formed in different electrolyte solutions. (A) Polymerization in pholymerization in oxalate buffer solution by potential-step method at 0.80 V vs. Sethod at 0.80 V vs. SCE with 100 s.

ig. 4 clearly shows that the trace-crossings appear from therst to the last-cycle in our experiments. In accordance withentioned in last paragraph, the curves indicate that the poly-erization of pyrrole occurs mainly at the graphite surface.nder a certain condition in this paper, the formed polypyrroleoped with carbonate ion loses its elelctroactivity, but the endf the polypyrrole contacts with the electrode. Reaction can beappened mainly at the interface of the graphite and polypyrrole.hus, the later formed polypyrrole will push the former away

rom the electrode. Fig. 4 also shows that the maximum poly-erization currents are getting smaller with the polymerization

oing on. This phenomenon is attributed to diffusion resistancef the formed polypyrrole which made less pyrrole monomerrrive at the interface of the graphite and polypyrrole. The mov-ng to a higher potential of the crossing point means that the

ate buffer solution by potential-step method at 0.80 V vs. SCE with 100 s (B)ith 100 s. (C) Polymerization in K4[(CN)6]/K3[(CN)6] solution by potential-step

J. Wang et al. / Synthetic Metals 156 (2006) 514–518 517

Fig. 4. Cyclic-voltammetric polymerization curves of pyrrole on graphite anodewith scanning rate at 25 mV/s in carbonate solution.

ratio of pyrrole polymerized at the graphite is getting smallerwith the reaction keeping on.

Temperature and doped ions are the two other factors thataffect the density of current carriers. The band-gap of the longconjugate (conductive) polymers is much less than that of thenormal compounds. The current carrier density of conductivepolymer is very sensitive to the experimental temperature, andthe relationship between conductivity and temperature of thepolymers is usually the same as semiconductor [27]. The higherthe temperature is, the denser the current carriers, the higher theconductivity. The oxidation of conductive polymer introducespositive charges into the polymer chain. The positive charge atthe molecular chain is balanced by the doped ions. The size andthe number of charges of the doped ions have significant effecton the structure of conductive polymer molecular chains and theconjugate length. The double charged doped ions have muchmore effect on the molecular structure than the single chargeddoped ions. The high charge might distort the polymeric chainsand disturb its conjugate. The more ordered the molecular chainaccumulation has, the longer the molecular conjugate lengthis, the higher the conductivity. When the experimental tem-

Fig. 5. Relationships between ln[I(t)] vs. ln[Q(t)].

perature was elevated up to 30 ◦C, the loose polypyrrole wireswith diameter about several hundreds nanometers are obtained.Further elevating the temperature, cauliflower polypyrrole willbe obtained. Substituting carbonate ions to perchlorate ionswill improve the electroactivity of the formed polypyrrole, andcauliflower structure is produced.

The heterogeneous structure of the electrode material playsan important role for the formation of polypyrrole nanowires.Nucleation and growth of polypyrrole might occur at any place ofthe homogeneous electrode. With the polymerization going on,the nuclei of polypyrrole will grow and connect with each otherto form a polypyrrole film. Under a certain condition, the circularinsulator, such as paraffin of conductor material will separatethe electroactive sites from each other, and prevent the nucleiconnect. Lowering the electroactivity, the formed polypyrrolenuclei cannot grow at other site other than the end connectedwith electrode.

The ln[I(t)] versus ln[Q(t)] is given in Fig. 5, in which the I(t)is referred to polymerization current and the Q(t) to charges con-sumed during potential-step electropolymerization. The slopewith values of 0, 1/2 and 2/3 implies one-, two- and three-

F lectrow

ig. 6. SEM pictures of polypyrrole electrogenerated on graphite composite eith 30 s, 43.20 mC and (B) 0.80 V vs. SCE with 150 s, 209.41 mC.

de in carbonate solution. Potential-step polymerization at (A) 0.80 V vs. SCE

518 J. Wang et al. / Synthetic Metals 156 (2006) 514–518

dimensional growth pattern, respectively [28]. The near zeroslopes of the curves for all potentials under the given conditionsindicate that polymerization occurs at one end of the polymerfilaments. The increase of slopes with the applied potentials isattributed to the generation of electroactivity sites not only at theend of the nanowires, but also at the other place of the wires. Thehigher the potential applied, the denser the electroactivity sitesgenerated, the larger the slope is. It could be concluded fromthe experimental results that polypyrrole nanowires are gener-ated from graphite surface in one-dimensional growth pattern,and then they will be pushed away from the graphite electrodesuccessively with the polymerization going on.

Polypyrrole nanostructures electrogenerated at the same elec-trode with different polymerization charge at 0.80 V versus SCEare shown in Fig. 6. The growth pattern of the polypyrrolenuclei determines its morphology. Cauliflower polypyrrole willbe obtained when the nuclei grow in a three-dimensional way.When the nuclei grow in a two-dimensional way, the morphol-ogy of the polypyrrole might be lamelliform or fibriform whichare determined by the shape of the nuclei. Fibriform polypyr-role will be produced when the nuclei grow in one-dimensionalpattern. The nearly same diameter of the formed polypyrrolenanowires presented in Fig. 4 indicates the polypyrrole nucleigrow in one-dimensional pattern, and this agree with the con-clusion obtained from the relationship between ln[I(t)] versusln[Q(t)].

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In summary, by controlling the current carrier density inhe formed polypyrrole and the heterogeneous property of thepplied electrode, polypyrrole nanowires with different lengthnd diameters can be simply obtained. Under a certain condition,he polypyrrole nuclei will grow in one-dimensional pattern.he method should be useful for preparation other materialsanowires.

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