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Polypyrrole nanostructures formed by electrochemical method on graphite impregnated with paraffin
Synthetic Metals 156 (2006) 514–518
Polypyrrole nanostructures formed by electrochemical method 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 2005 Available online 18 April 2006
Abstract Polypyrrole nanostructures were prepared electrochemically by template-free method on graphite impregnated with paraffin. The experimental parameters, such as testing temperature, polymerization potential, doped ions and the status of the electrode surface have significant effect on the morphology of the formed polypyrrole. Under various experimental conditions, fibrillar, taper and cauliflower polypyrrole can be obtained. By controlling the active sites, the polypyrrole nuclei will grow in one-dimensional pattern, and thus polypyrrole nanowires were obtained. The method should be useful for preparation other materials nanowires. © 2005 Published by Elsevier B.V. Keywords: Conductive polymers; Polypyrrole; Nanowires; Mechanism
1. Introduction Material systems of reduced size or dimensionality may, often do, exhibit properties different from those found in the bulk and in molecular state. Nanomaterials involve a wide-range of potential applications. Such phenomena are of considerable scientific and technological interests, particularly in the area of miniaturized, highly compact electronic devices. Nanowires and nanotubes are often prepared by template-route [1–8], selfassembly technique [9–11] and anisotropic growth of the nuclear [12,13]. The template might be nanopores of membrane or molecular sieves, macromolecules or cavity of macromolecular, channel of microbes, nanotubes and nano-particles. The self-assembly system relates to the micelle formation from surfactant, block polymers and supermolecular structure of some certain molecules. Inherently electronically conductive polymers, such as polypyrrole, polyaniline, polythiophene and poly(pvinylbenzene) have been intensively studied due to their potential applications in sensors and actuators as well as in
∗
Corresponding author. Tel.: +86 22 2740 4533; fax: +86 22 2740 4757. E-mail address: [email protected] (J. Wang).
0379-6779/$ – see front matter © 2005 Published by Elsevier B.V. doi:10.1016/j.synthmet.2005.10.011
electronic, electroluminescence, electrochromic and photoelectrochemical devices. Properties of these polymers depend on their microstructures and morphologies determined by the synthesis method. As an inherently conductive polymer, polypyrrole have been electropolymerzied on various homogeneous substrates for wide-range of purposes. The morphology of the polypyrrole formed on electrodes is normally in cauliflower form. Polypyrrole dendrite, wrinkle and other forms might be obtained under certain conditions. Polypyrrole nanotubes/nanofibers were first prepared in the pores of polycarbonate membranes by Penner and Martin [14]. After that, template method is a common route to fabricate polypyrrole nanotubes/nanofibers. Porous substrates [15,16], lipid tubule [17] and molecular cavity of cyclodextrin [18] are also used as template for synthesis polypyrrole nanowires. We reported the formation of polypyrrole nanowires on composite electrode under the induction of the polyanions [19–21]. But, our recent experimental results indicate that the induction role of the polyanions is not the unique factor that controls the morphology. The polypyrrole nanowires can be simply synthesized on graphite electrodes impregnated with paraffin. The control of electroactivity of the formed polypyrrole and the heterogeneous property of the electrode material might have much effect on the morphology.
J. Wang et al. / Synthetic Metals 156 (2006) 514–518
2. Experimental approach The electrode used in the experiments was a graphite rod with diameter of 8 mm. The graphite rod was pretreated with a boiling acidic solution containing hydrochloric acid (30%) and nitric acid (30%) for 2 h, and then rinsed thoroughly with de-ionized water. The treated rod was immersed in paraffin (m.p. 52 ◦ C) at 150 ◦ C until no bubbles appeared. Then the rod was polished by 1200# emery paper. The experiments were conducted at room temperature in a one-compartment glass cell in an environment of pure nitrogen. A saturated calomel electrode (SCE) and a platinum network were used as reference electrode and counter electrode, respectively. A buffer solution made from mixing sodium carbonate (0.20 M) and sodium dicarbonate (0.20 M) solution containing lithium perchlorate and pyrrole was used as electrolytic solution. The volume ratio of sodium carbonate (0.20 M) to sodium dicarbonate (0.20 M) solution may be changed from 1:5 to 5:1. In order to compare the effect of the electrolyte, phosphate buffer solution (0.20 M) at pH 6.86 and oxalate buffer solution (0.20 M) at pH 4.22 were also used. The electrochemical experiment was performed on PAR 273 Electrochemical System controlled by a computer. The morphology of PPy prepared by this method was examined under a scanning electron microscope (SEM) (Philip XL30). 3. Results Fig. 1A–C shows the polypyrrole nanowires of different length and diameter formed on the graphite surface by pass-
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ing varied charge during the polymerization. The figure clearly shows that polypyrrole nanowires can be electrogenerated on graphite impregnated with paraffin electrode from the carbonate buffer system with applied potential in the range of 0.75–0.85 V. Varying the ratio of sodium carbonate (0.20 M) to sodium dicarbonate (0.20 M) solution, except the diameter of the PPy nanowires gets smaller, PPy morphology has no obviously change. When polymerized at 0.80 V versus SCE, the diameters of the formed PPy nanowires may change from about 120 nm (formed in solution NaHCO3 :Na2 CO3 = 5:1) to 50 nm (formed in solution NaHCO3 :Na2 CO3 = 1:5). When polymerized at 0.95 V versus SCE, loose polypyrrole wires with diameter of several hundreds nanometers are obtained (Fig. 1D). At 1.05 V, the taper form (Fig. 1E) polypyrrole is formed. When the applied potential is higher than 1.05 V versus SCE, polypyrrole nanowires cannot be obtained. The morphology of polypyrrole polymerized at potential higher than 1.05 V versus SCE is shown in Fig. 1F. The fibrillar morphology might attribute to the low doping degree resulting in low current carrier (electron, hole, soliton, polaron or bipolaron) density of the formed polypyrrole under the low applied potentials. The low electroactivity of the formed polypyrrole in carbonate solutions further increases the resistant forces to the current carriers. The low electroactivity might be caused by the structure defects in the molecular chain [22]. Just as reported, the FTIR of the formed polypyrrole nanowires shows that C O (adsorbed wavelength at about 1698) is contained at the pyrrole ring (as shown in Fig. 2). The low current carrier density and the high resistant made the energy cannot be transferred from the electrode to the formed
Fig. 1. SEM pictures of polypyrrole electrogenerated on graphite composite electrode in carbonate solution. Potential-step polymerization at (A) 0.75 V vs. SCE with 100 s, (B) 0.80 V vs. SCE with 100 s, (C) 0.85 V vs. SCE with 100 s, (D) 0.95 V vs. SCE with 40 s, (E) 1.05 V vs. SCE with 15 s and (F) 1.15 V vs. SCE with 10 s.
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Fig. 2. FTIR of PPy nanowires prepared under different conditions: (A) in carbonate solution, (B) in phosphate solution and (C) in oxalate solution.
polypyrrole, and thus cannot generate enough electroactive sites on the formed polypyrrole to keep electropolymerization going on. When the applied potential lower than 0.95 V versus SCE, active sites cannot be produced on the formed polypyrrole except the end rooted on the electrode. Thus, under certain condition, pyrrole will be polymerized at the nanowire end attached on the electrode and the former formed polypyrrole will be pushed away from the electrode by the later one. The growth pattern will be a one-dimensional process. At 0.95 V, some active sites are electrogenerated, but less than those at 1.05 V and polypyrrole wires with diameter about several hundred nanometers is obtained. When the applied potential is higher than 1.05 V versus SCE, it will oxidize the formed polypyrrole and increase the doping degree. The high potential can generate enough current carriers to transfer energy to the formed polypyrrole and produce active sites at the formed polypyrrole and those make the polymerization keep on. When the polymerization potential is higher than 1.05 V, the growth pattern will be three-dimensional type and cauliflower morphology obtained. Last paragraph, the results show that the formation of the nanowire might result from the two reasons. The one is the low doping degree of the PPy, and the other is defected form at the polymeric chains. Fig. 3 is SEM pictures of polypyrrole
formed at 0.80 V versus SCE 100 s in 0.20 M phosphate buffer solution (pH 6.8), 0.20 M oxalate buffer solution (pH 4.22) and 0.20 M K4 [(CN)6 ]/K3 [(CN)6 ] solution, respectively. The SEM clearly shows that, as we early reported, in phosphate buffer solution PPy nanowires are formed, and in oxalate buffer solution and in 0.20 M K4 [(CN)6 ]/K3 [(CN)6 ] solution no PPy nanowire founded. The results indicate that the formation of PPy nanowire needs its low conductivity of the formed PPy and the low electroactivity of the dopants. No PPy nanowire formed in oxalate buffer solution and in 0.20 M K4 [(CN)6 ]/K3 [(CN)6 ] solution that is because its dopants might be reduced or oxidized by the formed 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 same time, FTIR spectrum clearly demonstrates that PPy formed in phosphate solution and oxalate solution have no adsorption at about 1698 cm−1 and the molecule has long conjugate length [23–25]. The long conjugate length means that the electrons might be delocalized and the polymer might has the high electrical conductivity. The formation mechanism of PPy nanowires might be attributed to its low conductivity (low density charge carriers) coming from the low doping degree. So, we can conclude that the low density of charge carriers is the main reason for formation of PPy nanowires. Nucleation-loop or trace-crossing usually happens in the first cycle when the materials deposit onto a foreign substrate [26]. Fig. 4 clearly shows that the trace-crossings appear from the first to the last-cycle in our experiments. In accordance with mentioned in last paragraph, the curves indicate that the polymerization of pyrrole occurs mainly at the graphite surface. Under a certain condition in this paper, the formed polypyrrole doped with carbonate ion loses its elelctroactivity, but the end of the polypyrrole contacts with the electrode. Reaction can be happened mainly at the interface of the graphite and polypyrrole. Thus, the later formed polypyrrole will push the former away from the electrode. Fig. 4 also shows that the maximum polymerization currents are getting smaller with the polymerization going on. This phenomenon is attributed to diffusion resistance of the formed polypyrrole which made less pyrrole monomer arrive at the interface of the graphite and polypyrrole. The moving to a higher potential of the crossing point means that the
Fig. 3. PPy formed in different electrolyte solutions. (A) Polymerization in phosphate buffer solution by potential-step method at 0.80 V vs. SCE with 100 s (B) Polymerization in oxalate buffer solution by potential-step method at 0.80 V vs. SCE with 100 s. (C) Polymerization in K4 [(CN)6 ]/K3 [(CN)6 ] solution by potential-step method at 0.80 V vs. SCE with 100 s.
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Fig. 4. Cyclic-voltammetric polymerization curves of pyrrole on graphite anode with scanning rate at 25 mV/s in carbonate solution.
Fig. 5. Relationships between ln[I(t)] vs. ln[Q(t)].
ratio of pyrrole polymerized at the graphite is getting smaller with the reaction keeping on. Temperature and doped ions are the two other factors that affect the density of current carriers. The band-gap of the long conjugate (conductive) polymers is much less than that of the normal compounds. The current carrier density of conductive polymer is very sensitive to the experimental temperature, and the relationship between conductivity and temperature of the polymers is usually the same as semiconductor [27]. The higher the temperature is, the denser the current carriers, the higher the conductivity. The oxidation of conductive polymer introduces positive charges into the polymer chain. The positive charge at the molecular chain is balanced by the doped ions. The size and the number of charges of the doped ions have significant effect on the structure of conductive polymer molecular chains and the conjugate length. The double charged doped ions have much more effect on the molecular structure than the single charged doped ions. The high charge might distort the polymeric chains and disturb its conjugate. The more ordered the molecular chain accumulation has, the longer the molecular conjugate length is, the higher the conductivity. When the experimental tem-
perature was elevated up to 30 ◦ C, the loose polypyrrole wires with diameter about several hundreds nanometers are obtained. Further elevating the temperature, cauliflower polypyrrole will be obtained. Substituting carbonate ions to perchlorate ions will improve the electroactivity of the formed polypyrrole, and cauliflower structure is produced. The heterogeneous structure of the electrode material plays an important role for the formation of polypyrrole nanowires. Nucleation and growth of polypyrrole might occur at any place of the homogeneous electrode. With the polymerization going on, the nuclei of polypyrrole will grow and connect with each other to form a polypyrrole film. Under a certain condition, the circular insulator, such as paraffin of conductor material will separate the electroactive sites from each other, and prevent the nuclei connect. Lowering the electroactivity, the formed polypyrrole nuclei cannot grow at other site other than the end connected with 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 consumed during potential-step electropolymerization. The slope with values of 0, 1/2 and 2/3 implies one-, two- and three-
Fig. 6. SEM pictures of polypyrrole electrogenerated on graphite composite electrode in carbonate solution. Potential-step polymerization at (A) 0.80 V vs. SCE with 30 s, 43.20 mC and (B) 0.80 V vs. SCE with 150 s, 209.41 mC.
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dimensional growth pattern, respectively [28]. The near zero slopes of the curves for all potentials under the given conditions indicate that polymerization occurs at one end of the polymer filaments. The increase of slopes with the applied potentials is attributed to the generation of electroactivity sites not only at the end of the nanowires, but also at the other place of the wires. The higher the potential applied, the denser the electroactivity sites generated, the larger the slope is. It could be concluded from the experimental results that polypyrrole nanowires are generated from graphite surface in one-dimensional growth pattern, and then they will be pushed away from the graphite electrode successively with the polymerization going on. Polypyrrole nanostructures electrogenerated at the same electrode with different polymerization charge at 0.80 V versus SCE are shown in Fig. 6. The growth pattern of the polypyrrole nuclei determines its morphology. Cauliflower polypyrrole will be obtained when the nuclei grow in a three-dimensional way. When the nuclei grow in a two-dimensional way, the morphology of the polypyrrole might be lamelliform or fibriform which are determined by the shape of the nuclei. Fibriform polypyrrole will be produced when the nuclei grow in one-dimensional pattern. The nearly same diameter of the formed polypyrrole nanowires presented in Fig. 4 indicates the polypyrrole nuclei grow in one-dimensional pattern, and this agree with the conclusion obtained from the relationship between ln[I(t)] versus ln[Q(t)]. In summary, by controlling the current carrier density in the formed polypyrrole and the heterogeneous property of the applied electrode, polypyrrole nanowires with different length and diameters can be simply obtained. Under a certain condition, the polypyrrole nuclei will grow in one-dimensional pattern. The method should be useful for preparation other materials nanowires. References [1] L. Jun, Z. Lu, Z. Jing, Adv. Mater. 15 (7–8) (2003) 579–581. [2] S.Z. Chu, K. Wada, S. Inoue, S. Todoroki, Electrochim. Acta 48 (20–22) (2003) 3147–3153.
[3] N.R. de Tacconi, K. Rajeswar, Electrochim. Acta 47 (16) (2002) 2603–2613. [4] M. Martin-Gonzalez, G.J. Snyder, A.L. Prieto, R. Gronsky, T. Sands, A.M. Stacy, Nano Lett. 3 (7) (2003) 973–977. [5] C.H. Liu, J.A. Zapien, Y. Yao, X.M. Meng, C.S. Lee, S.S. Fan, Y. Lifshitz, S.T. Lee, Adv. Mater. 15 (10) (2003) 838–841. [6] S. Valizadeh, J.M. George, P. Leisner, L. Hultman, Electrochim. Acta 47 (6) (2001) 865–874. [7] Y.G. Wang, X.G. Zhang, Electrochim. Acta 49 (12) (2004) 1957–1962. [8] Z.H. Wang, M.D. Watson, J.S. Wu, K. Mullen, Chem. Commu. 3 (2004) 336–337. [9] M. Bal, A. Ursache, M.T. Touminen, J.T. Goldbach, T.P. Russell, Appl. Phys. Lett. 81 (18) (2002) 3479–3481. [10] R. Djalali, S.Y. Li, M. Schmidt, Macromolecules 35 (11) (2002) 4282–4288. [11] K. Shin, K.A. Leach, J.T. Goldbach, D.H. Kim, J.Y. Jho, M. Tuominen, C.J. Hawker, T.P. Russell, Nano Lett. 2 (9) (2002) 933–936. [12] N.M. Hwang, W.S. Cheong, D.Y. Yoon, D.Y. Kim, J. Cryst. Growth 218 (1) (2000) 33–39. [13] B. Gates, B. Mayers, B. Cattle, Y.N. Xia, Adv. Funct. Mater. 12 (3) (2002) 219–227. [14] R.M. Penner, C.R. Martin, J. Electrochem. Soc. 133 (1986) 2206–2207. [15] D.J. Maia, A.J.G. Zarbin, O.L. Alves, M.A. De Paoli, Adv. Mater. 7 (9) (1995) 792–794. [16] J. Joo, K.T. Park, B.H. Kim, M.S. Kim, S.Y. Lee, C.K. Jeong, J.K. Lee, D.H. Park, W.K. Yi, S.H. Lee, K.S. Ryu, Synth. Met. 135 (1–3) (2003) 7–9. [17] M. Goren, Z. Qi, R.B. Lennox, Chem. Mater. 12 (5) (2000) 1222– 1228. [18] J.Y. Lee, S.M. Park, J. Electrochem. Soc. 147 (11) (2000) 4189–4195. [19] D.T. Ge, J.X. Wang, S.C. Wang, J. Mater. Sci. Lett. 22 (11) (2003) 839–840. [20] J.X. Wang, D.T. Ge, S.C. Wang, Chem. Lett. 32 (6) (2003) 528–529. [21] D.T. Ge, J.X. Wang, Z. Wang, S.C. Wang, Synth. Met. 132 (1) (2002) 93–95. [22] T. Osaka, T. Momma, H. Kanagawa, Chem. Lett. 4 (1993) 649–652. [23] M. Granstrom, O. Inganas, Polymer 36 (15) (1995) 2867–2872. [24] V.P. Menon, J. Lei, C.R. Martin, Chem. Mater. 8 (9) (1996) 2382–2390. [25] M.K. Song, Y.T. Kim, B.S. Kim, J. Kim, K. Char, H.W. Rhee, Synth. Met. 141 (3) (2004) 315–319. [26] R.E. Noftle, D. Pletcher, J. Electroanal. Chem. 227 (1–2) (1987) 229–235. [27] S. Roth, H. Bleier, W. Pukacki, Faraday Discuss. Chem. Soc. 88 (1989) 223–233. [28] M.L. Myrick, J.D. Noll, M.A. Nicholson, J. Electrochem. Soc. 145 (1) (1998) 179–185.
Polypyrrole nanostructures formed by electrochemical method 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 2005 Available online 18 April 2006
Abstract Polypyrrole nanostructures were prepared electrochemically by template-free method on graphite impregnated with paraffin. The experimental parameters, such as testing temperature, polymerization potential, doped ions and the status of the electrode surface have significant effect on the morphology of the formed polypyrrole. Under various experimental conditions, fibrillar, taper and cauliflower polypyrrole can be obtained. By controlling the active sites, the polypyrrole nuclei will grow in one-dimensional pattern, and thus polypyrrole nanowires were obtained. The method should be useful for preparation other materials nanowires. © 2005 Published by Elsevier B.V. Keywords: Conductive polymers; Polypyrrole; Nanowires; Mechanism
1. Introduction Material systems of reduced size or dimensionality may, often do, exhibit properties different from those found in the bulk and in molecular state. Nanomaterials involve a wide-range of potential applications. Such phenomena are of considerable scientific and technological interests, particularly in the area of miniaturized, highly compact electronic devices. Nanowires and nanotubes are often prepared by template-route [1–8], selfassembly technique [9–11] and anisotropic growth of the nuclear [12,13]. The template might be nanopores of membrane or molecular sieves, macromolecules or cavity of macromolecular, channel of microbes, nanotubes and nano-particles. The self-assembly system relates to the micelle formation from surfactant, block polymers and supermolecular structure of some certain molecules. Inherently electronically conductive polymers, such as polypyrrole, polyaniline, polythiophene and poly(pvinylbenzene) have been intensively studied due to their potential applications in sensors and actuators as well as in
∗
Corresponding author. Tel.: +86 22 2740 4533; fax: +86 22 2740 4757. E-mail address: [email protected] (J. Wang).
0379-6779/$ – see front matter © 2005 Published by Elsevier B.V. doi:10.1016/j.synthmet.2005.10.011
electronic, electroluminescence, electrochromic and photoelectrochemical devices. Properties of these polymers depend on their microstructures and morphologies determined by the synthesis method. As an inherently conductive polymer, polypyrrole have been electropolymerzied on various homogeneous substrates for wide-range of purposes. The morphology of the polypyrrole formed on electrodes is normally in cauliflower form. Polypyrrole dendrite, wrinkle and other forms might be obtained under certain conditions. Polypyrrole nanotubes/nanofibers were first prepared in the pores of polycarbonate membranes by Penner and Martin [14]. After that, template method is a common route to fabricate polypyrrole nanotubes/nanofibers. Porous substrates [15,16], lipid tubule [17] and molecular cavity of cyclodextrin [18] are also used as template for synthesis polypyrrole nanowires. We reported the formation of polypyrrole nanowires on composite electrode under the induction of the polyanions [19–21]. But, our recent experimental results indicate that the induction role of the polyanions is not the unique factor that controls the morphology. The polypyrrole nanowires can be simply synthesized on graphite electrodes impregnated with paraffin. The control of electroactivity of the formed polypyrrole and the heterogeneous property of the electrode material might have much effect on the morphology.
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2. Experimental approach The electrode used in the experiments was a graphite rod with diameter of 8 mm. The graphite rod was pretreated with a boiling acidic solution containing hydrochloric acid (30%) and nitric acid (30%) for 2 h, and then rinsed thoroughly with de-ionized water. The treated rod was immersed in paraffin (m.p. 52 ◦ C) at 150 ◦ C until no bubbles appeared. Then the rod was polished by 1200# emery paper. The experiments were conducted at room temperature in a one-compartment glass cell in an environment of pure nitrogen. A saturated calomel electrode (SCE) and a platinum network were used as reference electrode and counter electrode, respectively. A buffer solution made from mixing sodium carbonate (0.20 M) and sodium dicarbonate (0.20 M) solution containing lithium perchlorate and pyrrole was used as electrolytic solution. The volume ratio of sodium carbonate (0.20 M) to sodium dicarbonate (0.20 M) solution may be changed from 1:5 to 5:1. In order to compare the effect of the electrolyte, phosphate buffer solution (0.20 M) at pH 6.86 and oxalate buffer solution (0.20 M) at pH 4.22 were also used. The electrochemical experiment was performed on PAR 273 Electrochemical System controlled by a computer. The morphology of PPy prepared by this method was examined under a scanning electron microscope (SEM) (Philip XL30). 3. Results Fig. 1A–C shows the polypyrrole nanowires of different length and diameter formed on the graphite surface by pass-
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ing varied charge during the polymerization. The figure clearly shows that polypyrrole nanowires can be electrogenerated on graphite impregnated with paraffin electrode from the carbonate buffer system with applied potential in the range of 0.75–0.85 V. Varying the ratio of sodium carbonate (0.20 M) to sodium dicarbonate (0.20 M) solution, except the diameter of the PPy nanowires gets smaller, PPy morphology has no obviously change. When polymerized at 0.80 V versus SCE, the diameters of the formed PPy nanowires may change from about 120 nm (formed in solution NaHCO3 :Na2 CO3 = 5:1) to 50 nm (formed in solution NaHCO3 :Na2 CO3 = 1:5). When polymerized at 0.95 V versus SCE, loose polypyrrole wires with diameter of several hundreds nanometers are obtained (Fig. 1D). At 1.05 V, the taper form (Fig. 1E) polypyrrole is formed. When the applied potential is higher than 1.05 V versus SCE, polypyrrole nanowires cannot be obtained. The morphology of polypyrrole polymerized at potential higher than 1.05 V versus SCE is shown in Fig. 1F. The fibrillar morphology might attribute to the low doping degree resulting in low current carrier (electron, hole, soliton, polaron or bipolaron) density of the formed polypyrrole under the low applied potentials. The low electroactivity of the formed polypyrrole in carbonate solutions further increases the resistant forces to the current carriers. The low electroactivity might be caused by the structure defects in the molecular chain [22]. Just as reported, the FTIR of the formed polypyrrole nanowires shows that C O (adsorbed wavelength at about 1698) is contained at the pyrrole ring (as shown in Fig. 2). The low current carrier density and the high resistant made the energy cannot be transferred from the electrode to the formed
Fig. 1. SEM pictures of polypyrrole electrogenerated on graphite composite electrode in carbonate solution. Potential-step polymerization at (A) 0.75 V vs. SCE with 100 s, (B) 0.80 V vs. SCE with 100 s, (C) 0.85 V vs. SCE with 100 s, (D) 0.95 V vs. SCE with 40 s, (E) 1.05 V vs. SCE with 15 s and (F) 1.15 V vs. SCE with 10 s.
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Fig. 2. FTIR of PPy nanowires prepared under different conditions: (A) in carbonate solution, (B) in phosphate solution and (C) in oxalate solution.
polypyrrole, and thus cannot generate enough electroactive sites on the formed polypyrrole to keep electropolymerization going on. When the applied potential lower than 0.95 V versus SCE, active sites cannot be produced on the formed polypyrrole except the end rooted on the electrode. Thus, under certain condition, pyrrole will be polymerized at the nanowire end attached on the electrode and the former formed polypyrrole will be pushed away from the electrode by the later one. The growth pattern will be a one-dimensional process. At 0.95 V, some active sites are electrogenerated, but less than those at 1.05 V and polypyrrole wires with diameter about several hundred nanometers is obtained. When the applied potential is higher than 1.05 V versus SCE, it will oxidize the formed polypyrrole and increase the doping degree. The high potential can generate enough current carriers to transfer energy to the formed polypyrrole and produce active sites at the formed polypyrrole and those make the polymerization keep on. When the polymerization potential is higher than 1.05 V, the growth pattern will be three-dimensional type and cauliflower morphology obtained. Last paragraph, the results show that the formation of the nanowire might result from the two reasons. The one is the low doping degree of the PPy, and the other is defected form at the polymeric chains. Fig. 3 is SEM pictures of polypyrrole
formed at 0.80 V versus SCE 100 s in 0.20 M phosphate buffer solution (pH 6.8), 0.20 M oxalate buffer solution (pH 4.22) and 0.20 M K4 [(CN)6 ]/K3 [(CN)6 ] solution, respectively. The SEM clearly shows that, as we early reported, in phosphate buffer solution PPy nanowires are formed, and in oxalate buffer solution and in 0.20 M K4 [(CN)6 ]/K3 [(CN)6 ] solution no PPy nanowire founded. The results indicate that the formation of PPy nanowire needs its low conductivity of the formed PPy and the low electroactivity of the dopants. No PPy nanowire formed in oxalate buffer solution and in 0.20 M K4 [(CN)6 ]/K3 [(CN)6 ] solution that is because its dopants might be reduced or oxidized by the formed 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 same time, FTIR spectrum clearly demonstrates that PPy formed in phosphate solution and oxalate solution have no adsorption at about 1698 cm−1 and the molecule has long conjugate length [23–25]. The long conjugate length means that the electrons might be delocalized and the polymer might has the high electrical conductivity. The formation mechanism of PPy nanowires might be attributed to its low conductivity (low density charge carriers) coming from the low doping degree. So, we can conclude that the low density of charge carriers is the main reason for formation of PPy nanowires. Nucleation-loop or trace-crossing usually happens in the first cycle when the materials deposit onto a foreign substrate [26]. Fig. 4 clearly shows that the trace-crossings appear from the first to the last-cycle in our experiments. In accordance with mentioned in last paragraph, the curves indicate that the polymerization of pyrrole occurs mainly at the graphite surface. Under a certain condition in this paper, the formed polypyrrole doped with carbonate ion loses its elelctroactivity, but the end of the polypyrrole contacts with the electrode. Reaction can be happened mainly at the interface of the graphite and polypyrrole. Thus, the later formed polypyrrole will push the former away from the electrode. Fig. 4 also shows that the maximum polymerization currents are getting smaller with the polymerization going on. This phenomenon is attributed to diffusion resistance of the formed polypyrrole which made less pyrrole monomer arrive at the interface of the graphite and polypyrrole. The moving to a higher potential of the crossing point means that the
Fig. 3. PPy formed in different electrolyte solutions. (A) Polymerization in phosphate buffer solution by potential-step method at 0.80 V vs. SCE with 100 s (B) Polymerization in oxalate buffer solution by potential-step method at 0.80 V vs. SCE with 100 s. (C) Polymerization in K4 [(CN)6 ]/K3 [(CN)6 ] solution by potential-step method at 0.80 V vs. SCE with 100 s.
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Fig. 4. Cyclic-voltammetric polymerization curves of pyrrole on graphite anode with scanning rate at 25 mV/s in carbonate solution.
Fig. 5. Relationships between ln[I(t)] vs. ln[Q(t)].
ratio of pyrrole polymerized at the graphite is getting smaller with the reaction keeping on. Temperature and doped ions are the two other factors that affect the density of current carriers. The band-gap of the long conjugate (conductive) polymers is much less than that of the normal compounds. The current carrier density of conductive polymer is very sensitive to the experimental temperature, and the relationship between conductivity and temperature of the polymers is usually the same as semiconductor [27]. The higher the temperature is, the denser the current carriers, the higher the conductivity. The oxidation of conductive polymer introduces positive charges into the polymer chain. The positive charge at the molecular chain is balanced by the doped ions. The size and the number of charges of the doped ions have significant effect on the structure of conductive polymer molecular chains and the conjugate length. The double charged doped ions have much more effect on the molecular structure than the single charged doped ions. The high charge might distort the polymeric chains and disturb its conjugate. The more ordered the molecular chain accumulation has, the longer the molecular conjugate length is, the higher the conductivity. When the experimental tem-
perature was elevated up to 30 ◦ C, the loose polypyrrole wires with diameter about several hundreds nanometers are obtained. Further elevating the temperature, cauliflower polypyrrole will be obtained. Substituting carbonate ions to perchlorate ions will improve the electroactivity of the formed polypyrrole, and cauliflower structure is produced. The heterogeneous structure of the electrode material plays an important role for the formation of polypyrrole nanowires. Nucleation and growth of polypyrrole might occur at any place of the homogeneous electrode. With the polymerization going on, the nuclei of polypyrrole will grow and connect with each other to form a polypyrrole film. Under a certain condition, the circular insulator, such as paraffin of conductor material will separate the electroactive sites from each other, and prevent the nuclei connect. Lowering the electroactivity, the formed polypyrrole nuclei cannot grow at other site other than the end connected with 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 consumed during potential-step electropolymerization. The slope with values of 0, 1/2 and 2/3 implies one-, two- and three-
Fig. 6. SEM pictures of polypyrrole electrogenerated on graphite composite electrode in carbonate solution. Potential-step polymerization at (A) 0.80 V vs. SCE with 30 s, 43.20 mC and (B) 0.80 V vs. SCE with 150 s, 209.41 mC.
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dimensional growth pattern, respectively [28]. The near zero slopes of the curves for all potentials under the given conditions indicate that polymerization occurs at one end of the polymer filaments. The increase of slopes with the applied potentials is attributed to the generation of electroactivity sites not only at the end of the nanowires, but also at the other place of the wires. The higher the potential applied, the denser the electroactivity sites generated, the larger the slope is. It could be concluded from the experimental results that polypyrrole nanowires are generated from graphite surface in one-dimensional growth pattern, and then they will be pushed away from the graphite electrode successively with the polymerization going on. Polypyrrole nanostructures electrogenerated at the same electrode with different polymerization charge at 0.80 V versus SCE are shown in Fig. 6. The growth pattern of the polypyrrole nuclei determines its morphology. Cauliflower polypyrrole will be obtained when the nuclei grow in a three-dimensional way. When the nuclei grow in a two-dimensional way, the morphology of the polypyrrole might be lamelliform or fibriform which are determined by the shape of the nuclei. Fibriform polypyrrole will be produced when the nuclei grow in one-dimensional pattern. The nearly same diameter of the formed polypyrrole nanowires presented in Fig. 4 indicates the polypyrrole nuclei grow in one-dimensional pattern, and this agree with the conclusion obtained from the relationship between ln[I(t)] versus ln[Q(t)]. In summary, by controlling the current carrier density in the formed polypyrrole and the heterogeneous property of the applied electrode, polypyrrole nanowires with different length and diameters can be simply obtained. Under a certain condition, the polypyrrole nuclei will grow in one-dimensional pattern. The method should be useful for preparation other materials nanowires. References [1] L. Jun, Z. Lu, Z. Jing, Adv. Mater. 15 (7–8) (2003) 579–581. [2] S.Z. Chu, K. Wada, S. Inoue, S. Todoroki, Electrochim. Acta 48 (20–22) (2003) 3147–3153.
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