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Electrodeposition of composite films consisting of polypyrrole and mesoporous silica
Synthetic Metals 128 (2002) 57–62
Electrodeposition of composite films consisting of polypyrrole and mesoporous silica M. Nakayama, J. Yano, K. Nakaoka, K. Ogura* Faculty of Engineering, Department of Applied Chemistry, Yamaguchi University, 2557 Tokiwadai, Ube 755-8611, Japan Received in revised form 10 July 2001; accepted 16 October 2001
Abstract Electrochemical formation of composite films consisting of polypyrrole (PPy) and MCM particles has been presented, in which pyrrole is electrochemically oxidized in an aqueous solution with suspension of purely siliceous or aluminum-containing MCM-41. The composite films were characterized by scanning electron microscopy, X-ray diffraction and infrared reflection spectroscopy, and the electrochemical response of the PPy/MCM-deposited electrode to Fe(CN)63 investigated. From IR spectra, it is indicated that the polymerization of pyrrole takes place on the internal wall of MCM particles where the cationic PPy is charge-balanced by the negatively charged MCM. A PPy/SiMCM composite electrode prepared in the presence of higher concentration of pyrrole (30.5 M) shows a reproducible electrochemical response for Fe(CN)63. The CV curve on this electrode is comparable to that on pure PPy-modified electrode, and it is suggested that the negative charge on the mesopore surface is almost completely neutralized by the positive charge of PPy. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Mesoporous silica; Polypyrrole; Composite; Electrodeposition
1. Introduction Mesoporous silica MCM-41 is a new molecular sieve material consisting of a hexagonal array of uniformly sized cylindrical pores [1]. Due to the large pore diameter (1.5– 10 nm), this material has attracted much attention as a new host for large molecules. The internal surface of MCM-41 occupies the majority (about 95%) of the overall surface area (1000 cm2), which has been widely used in a variety of reactions involving the polymerization of included monomers [2,3]. The surface of purely siliceous MCM-41 shows negative charge due to the existence of silanol groups, and works as ion-exchange sites [4]. Mesoporous silica has been recently used to modify electrode surface [5], in which silica particles are dispersed in the polystyrene film coated on the electrode, and the enhancement of the voltammetric response for Fe(bpy)32þ/3þ couple is observed. Walcarius et al. have prepared a carbon paste electrode containing MCM-41 which is useful as an indicator electrode for copper and mercury ions [6]. In the present study, the composite film consisting of polypyrrole (PPy) and MCM particles was electrochemically prepared on an electrode with the object of developing a new functional electrode. In this composite, the negatively * Corresponding author. Tel.: þ81-836-85-9221; fax: þ81-836-32-2886. E-mail address: [email protected] (K. Ogura).
charged MCM particles are incorporated to PPy as a dopant. The PPy/MCM composite was characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy, and the modified electrode was examined on the voltammetric response for Fe(CN)63.
2. Experimental Purely siliceous MCM-41 (SiMCM) and aluminum-containing MCM-41 (AlMCM) with different ratios of Al/Si were synthesized according to the literature [7]. In the latter case, sodium aluminate was used as a source of aluminum. The obtained precipitates were filtered, washed thoroughly with doubly distilled water, and then calcined for SiMCM in air at 500 8C and for AlMCM in O2 flow at 550 8C to remove template molecules. The Al/Si molar ratio of the MCM sample was determined with electron probe X-ray microanalyzer (Horiba, EMAX-7000). The sample was further treated for 4 h at 250 8C in a reduced pressure prior to electrochemical use. Commercial A-type (Al/Si ¼ 1:0, Wako Pure Chemical) and Y-type (Al/ Si ¼ 0:7, Aldrich) zeolites were also used, and treated in the same manner as described above. Pyrrole (Wako Pure Chemicals) was twice distilled, and used immediately after preparation. Other chemicals were of reagent grade and
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were used as received. All solutions were prepared with doubly distilled water. Electrochemical polymerization of pyrrole was performed at a constant potential of þ0.7 V on a platinum and an indium tin oxide (ITO) electrode in a pyrrole solution with 0.1 M NaClO4 or suspension of MCMs (3.0 g dm3), and the electric charge applied in this polymerization was always 49 mC cm2 unless otherwise noted. The modified electrodes prepared in this manner were used for the measurement of cyclic voltammograms of Fe(CN)63 in a 0.1 M NaClO4 solution. Prior to this measurement, the electrodes were cycled between 0.2 and þ0.6 V in a 0.1 M NaClO4 solution until a stable redox response was obtained. A onecompartment three-electrode cell was used for all electrochemical experiments, and the working electrode was a Pt (1.0 cm2) or ITO (0.7 cm2) plate. A platinum wire was used as the counter electrode, and an Ag/AgCl/sat. KCl electrode as the reference electrode. The equipments used were a potentio/galvanostat (Hokuto Denko HA-301) and a function generator (Hokuto Denko HB-104). The deposited films were further characterized by SEM, XRD and FTIR reflection-absorption spectroscopy. SEM data were obtained with a Hitachi S2300. XRD patterns were recorded with a Shimadzu XD-D1 diffractometer with Cu Ka radiation (30 kV, 30 mA). FTIR measurements were performed on a Shimadzu FTIR spectrometer (type 8100 M) equipped with an external-reflection unit and a HgCdTe (MCT) detector cooled with liquid nitrogen.
3. Results and discussion 3.1. Characterization of MCM samples
Fig. 1. XRD patterns of MCM-41 samples with different aluminum contents: (a) Pure silica; (b) Al/Si ¼ 0:031; (c) Al/Si ¼ 0:064.
was observed in a solution containing a single component of either pyrrole or SiMCM. These results indicate that the electrochemical oxidation of pyrrole to the cationic PPy requires the incorporation of an anionic species to keep a charge balance. As described above, the surface of SiMCM possesses negative charge (balanced with exchangeable cations) owing to the existence of silanol groups, and a suspension of MCM in water can be considered to be an electrolyte. In fact, the MCM suspension shows a low conductivity, but it is enough to allow the electrochemical deposition. Hence, the current observed in the presence of
XRD patterns of the MCM samples are presented in Fig. 1. SiMCM (Fig. 1a) shows four peaks, corresponding to (1 0 0), (1 1 0), (2 0 0) and (2 1 0) diffraction lines typical of a hexagonal structure of MCM-41 with a d-spacing of 3.8 nm [1]. The AlMCM (Fig. 1b) with an Al/Si ratio of 0.031 exhibits the same pattern as that of SiMCM. A slight shift of these peaks to larger values of 2y upon the addition of Al suggests the shrinkage of lattice due to the interposition of aluminum [7]. For the AlMCM with larger amount of Al (Al/Si ¼ 0:064) (Fig. 1c), the broad peak due to diffraction line (1 0 0) was only observed, signifying the collapse of mesoporous structure. (This sample is denoted as amorphous aluminosilicate below). 3.2. Electrodeposition of composite films In Fig. 2, the current versus time curves on a Pt electrode during the potentiostatic oxidation are shown in various solutions. Some current is seen to pass in the solution with both pyrrole and SiMCM although this current is considerably small in comparison with that observed in a NaClO4 solution containing pyrrole. However, no significant current
Fig. 2. Current–time curves on a Pt electrode at a constant potential of þ0.7 V in aqueous solutions containing pyrrole ð0:5 MÞ þ SiMCM (3.0 g dm3) (a), pyrrole (0.5 M) (b), SiMCM (3.0 g dm3) (c). Inset: current–time curve at þ0.7 V in a 0.1 M NaClO4 solution containing 0.1 M pyrrole.
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pyrrole and SiMCM can be attributed to the deposition of PPy with SiMCM working as a charge compensator, and the deposited film be regarded as a composite consisting of PPy and SiMCM. The composite film prepared in such a manner was visually uniform over the substrate. SEM images of the PPy/SiMCM composites deposited on an ITO electrode are shown in Fig. 3a and b. The morphology of these images was quite similar to that of SiMCM. Fig. 3a and b show that the electrode surface is gradually covered with deposits by applying larger electric charge. These deposits are dissimilar
Fig. 4. XRD patterns of SiMCM powder (a) and composite films, (b, c) formed on an ITO electrode. The conditions for the formation of composite film: potential, þ0.7 V; electrical charge, 49 mC cm2; electrolyte, 0.1 M; NaClO4 pyrrole, 0.5 M; SiMCM, 3 g dm3. The pattern for the composite film (c) was taken after the SiMCM-modified electrode was cycled between 0.2 and þ0.6 V in a 0.1 M NaClO4 solution.
Fig. 3. SEM images of the composite films (a, b) and pure PPy film (c). The composite films were prepared on an ITO electrode at þ0.7 V in a solution containing 0.5 M pyrrole and 3.0 g dm3 SiMCM by applying electrical charges of 22 mC cm2 (a) and 49 mC cm2 (b). The pure PPy film was formed on the same electrode at þ0.7 V in the presence of 0.1 M pyrrole and 0.1 M NaClO4 by applying an electrical charge of 49 mC cm2 (c).
to hemispherical morphology typical to PPy (Fig. 3c). In Fig. 3b and c, the electrochemical deposition was done from pyrrole solutions containing suspension of SiMCM and NaClO4, respectively, by applying a constant electric charge of 49 mC cm2. The time necessary for this electric charge was very different in the former (150 min) and latter (40 s), which may be related to the poor and strong abilities of SiMCM and ClO4, respectively, as a charge compensator. The XRD pattern of the PPy/SiMCM composite is shown in Fig. 4b. Although the pattern is less defined compared to that of SiMCM powder (Fig. 4a), the main two peaks ((1 0 0) and (1 1 0)) can be observed at the corresponding positions. These peaks were not affected by the potential cycling (at least until 50 cycles) in the potential region between 0.2 and þ0.6 V (Fig. 4c). Hence, the mesoporous structure of SiMCM is suggested to be maintained during the electrodeposition and subsequent electrochemical processes. Fig. 5a shows an FTIR reflection spectrum of the PPy/ SiMCM composite in the region from 2000 to 750 cm1. The absorption bands appearing at 1238, 1100, 967 and 793 cm1 are all attributed to SiO2 [8]. This feature is almost identical to the spectrum of dehydrated SiMCM powder obtained on the transmission mode (Fig. 5b). On the other hand, the peaks at 1594, 1494, 1360 and 939 cm1 are attributable to PPy [9], because these peaks are close to those observed for pure PPy (Fig. 5c). Hence, the composite is demonstrated to be composed of SiMCM and PPy. The peak observed at 1638 cm1 for the dehydrated SiMCM (Fig. 5b) is assignable to the deformation vibration (dH–O–H) of the structural water bound to the surface silanol group. As seen in Fig. 5a, however, this peak disappeared for the SiMCM incorporated in the composite film. In crystalline MCM, most silanol groups exist on the mesopore walls since
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Fig. 5. FTIR spectra: (a) Composite film; (b) Dehydrated SiMCM powder in KBr; (c) Pure PPy film. The composite film was prepared in a solution containing 0.5 M pyrrole and 3.0 g dm3 SiMCM. The pure PPy film was formed in a solution containing 0.1 M pyrrole and 0.1 M NaClO4. Both films were obtained at þ0.7 V on a Pt electrode by applying an electrical charge of 49 mC cm2.
the external surface is only about 5% of the total equivalent area [3]. It is therefore suggested that PPy is formed on the surface of mesopores of MCM particles, and the structural water is replaced by the PPy chain. On the other hand, the peak at 1594 cm1 (Fig. 5a) is due to the combination of intra-ring C=C and inter-ring C–C vibration [9], which is located at higher wavenumber than the corresponding wavenumber (1561 cm1) of PPy (Fig. 5c). In general, this peak tends to shift to lower wavenumber as the conjugated length of polymer is increased [9]. Therefore, the PPy chain in the composite is considered to be shortened than the length of pure PPy. This finding is consistent with the fact that the molecular weight of polyaniline (35,000) chemically synthesized in an MCM host is smaller than that (52,000) formed in bulk [10]. Such a limitation in the polymer growth may be related to the restriction for the diffusion of monomer within mesopores. Similar composite films were prepared with AlMCM (Al/ Si ¼ 0:031), amorphous aluminosilicate (Al/Si ¼ 0:064), and Y- and A-type zeolites, and their FTIR spectra are shown in Fig. 6. The spectral features of Fig. 6a and b similar to that of PPy (Fig. 5c) indicate that the deposition of PPy is possible in the presence of mesoporous and amorphous aluminosilicates. A comparison between these figures and Fig. 5a suggests that the spectral intensity of SiO2 (1238, 1100 cm1) is decreased with an increase in aluminum content of the silicates. This is because the positively
Fig. 6. FTIR spectra of the composite films prepared in 0.5 M pyrrole solutions containing 3.0 g dm3 of AlMCM (a), Amorphous aluminosilicate (b), Y-zeolite (c), and A-zeolite (d). All films were obtained under the same conditions as those in Fig. 4.
charged PPy can be charge-balanced with only smaller quantity of the silicate with the enhanced negative charge (i.e. the increase in Al content). The band at 1564 cm1 owing to PPy in the composite with the amorphous aluminosilicate (Fig. 6b) is close to the wavenumber (1561 cm1) for PPy (Fig. 5c). This means that the growth of PPy is not restrained by the amorphous aluminosilicate with relatively large negative charge. As shown in Fig. 6c, the PPy/Y-zeolite composite presents the absorption bands due to PPy (1572, 1491, 1333 cm1), zeolite (1153, 972, 789 cm1) and structural water (1628 cm1). The formation of PPy within the channel in Y-zeolite has been first reported by Bein et al. [11], in which the polymerization is achieved by the reaction of pyrrole monomer with the oxidizing species (Fe(III) or Cu(II)) existing in zeolite. The polymerization is assumed to be not carried out in sodalite cages (0.3 nm windows) but in supercages of Y-zeolite (0.8 nm windows), which leads to the view that lattice water may remain during electropolymerization and thus to the appearance of the optical peak due to dH–O–H at 1628 cm1. On the other hand, as seen from Fig. 6d, the formation of composite was not observed with A-zeolite (pore size 0.4 nm, smaller than pyrrole). This result is probably caused by the small pore size of A-zeolite, and the negative charge existing on the external surface of zeolite particle is not enough to balance the cationic charge of PPy. Fig. 7 shows FTIR spectra of the PPy/SiMCM composite films obtained by the repeated potential cycling in the potential regions from 0.8 to þ0.2 V (a) and from 0.2
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Fig. 7. FTIR spectra of PPy/SiMCM composite. The modified electrodes were previously cycled for 50 times between 0.8 and þ0.2 V (a) and 0.2 and þ0.6 V (b) in a 0.1 M NaClO4 solution.
to þ0.6 V (b). In less noble potential region (Fig. 7a), the peak associated with SiO2 is not observed but those with PPy are apparent, indicating that SiMCM particles are detached from the composite when the PPy is reduced to the neutral state. In the latter potential region (Fig. 7b), PPy is in a conducting state, and the optical intensity of the peak due to SiO2 is large, confirming that SiMCM particles are electrostatically bound to PPy.
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Fig. 8. Cyclic voltammograms of composite- (a–c) and pure PPy-modified electrodes (d) in a solution with 0.1 M NaClO4 plus 1 mM K3Fe(CN)6. Scan rate, 20 mV s1. The composite films were prepared in solutions with 3.0 g dm3 SiMCM and various concentrations of pyrrole: (a) 0.05; (b) 0.2; (c) 0.5 M. The pure PPy film was formed in a solution containing 0.1 M pyrrole and 0.1 M NaClO4.
CV curves of the composite films prepared with 0.5 M pyrrole and AlMCM, amorphous aluminosilicate or Y-zeolite were measured in an NaClO4 solution containing K3Fe(CN)6 (Fig. 9). The composites with AlMCM (Fig. 9a) and amorphous aluminosilicate (Fig. 9b) show distinct CV curves
3.3. Electrochemical behavior of composite films Cyclic voltammograms (CVs) of PPy/SiMCM films prepared under various conditions were measured in a 0.1 M NaClO4 solution with 1.0 mM K3Fe(CN)6, and the resulting CV curves are shown in Fig. 8. As seen from this figure, the CV curves are considerably dependent on the concentration of pyrrole monomer used for the preparation of PPy/SiMCM composites. The charging current of the composite film increases with the increasing pyrrole concentration. Also, the current peak due to Fe(CN)63/4 increases with the monomer concentration, and the peak separation becomes small. Ill-defined CV curves of the composite film obtained with lower concentration of pyrrole monomer (20.2 M) are caused by less interaction between Fe(CN)63 and electroactive sites on PPy owing to the existence of negatively charged SiMCM. On the other hand, the composite prepared with higher concentration of pyrrole monomer (30.5 M) showed the well-defined CV curve comparable to that of pure PPy (Fig. 8d), suggesting that the negative sites on the mesopore surface are almost completely neutralized with cationic PPy.
Fig. 9. Cyclic voltammograms of the composite films: (a) PPy/AlMCM; (b) PPy/amorphous aluminosilicate; (c) PPy/ Y-zeolite in a solution with 0.1 M NaClO4 plus 1 mM K3Fe(CN)6. Scan rate, 20 mV s1.
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comparable to that of pure PPy. The CV curve of the PPy/Yzeolite composite (Fig. 9c) is quite similar to that of the PPy/ SiMCM composite prepared with lower concentration of pyrrole (Fig. 8a). Therefore, the negative sites on the mesopore surface of AlMCM and amorphous aluminosilicate are enough neutralized but those on the surface of zeolite not.
Acknowledgements This research was supported by the Japan Society for the Promotion of Science (No. 127536). References
4. Conclusions Composite films consisting of PPy and MCM particles were electrochemically prepared. XRD and infrared spectra revealed that the polymerization of pyrrole occurs within the mesopores of SiMCM by taking the place of water molecules. The oxidation of pyrrole to the cationic PPy proceeds by incorporating negatively charged SiMCM to keep a charge balance. The CV curve of Fe(CN)63 on the PPy/MCM-modified electrode which was prepared in the presence of higher concentration of pyrrole (30.5 M) was comparable to that observed on the pure PPy-coated electrode, suggesting that the negative sites on the mesopore surface were almost completely neutralized with cationic PPy.
[1] J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T.-W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins, J.L. Schlenker, J. Am. Chem. Soc. 114 (1992) 10834. [2] S.M. Ng, S. Ogino, T. Aida, K. Koyano, T. Tatsumi, Macromol. Rapid Commun. 18 (1997) 991. [3] K. Kageyama, S. Ogino, T. Aida, Macromolecules 31 (1998) 4069. [4] Z. Luan, J. Xu, L. Kevan, Chem. Mater. 10 (1998) 3699. [5] G. Villemure, T.J. Pinnavaia, Chem. Mater. 11 (1999) 789. [6] A. Walcarius, N. Luthi, J.-L. Blin, B.-L. Su, L. Lamberts, Electrochim. Acta 44 (1999) 4601. [7] J.M. Kim, J.H. Kwak, S. Jun, R. Ryoo, J. Phys. Chem. 99 (1995) 16742. [8] H.A. Benesi, A.C. Jones, J. Phys. Chem. 63 (1959) 179. [9] Y. Furukawa, S. Tazawa, Y. Fujii, I. Harada, Synth. Metals 24 (1988) 329. [10] C.-G. Wu, T. Bein, Science 264 (1994) 1757. [11] T. Bein, P. Enzel, Angew. Chem. Int. Ed. Engl. 28 (1989) 1692.
Electrodeposition of composite films consisting of polypyrrole and mesoporous silica M. Nakayama, J. Yano, K. Nakaoka, K. Ogura* Faculty of Engineering, Department of Applied Chemistry, Yamaguchi University, 2557 Tokiwadai, Ube 755-8611, Japan Received in revised form 10 July 2001; accepted 16 October 2001
Abstract Electrochemical formation of composite films consisting of polypyrrole (PPy) and MCM particles has been presented, in which pyrrole is electrochemically oxidized in an aqueous solution with suspension of purely siliceous or aluminum-containing MCM-41. The composite films were characterized by scanning electron microscopy, X-ray diffraction and infrared reflection spectroscopy, and the electrochemical response of the PPy/MCM-deposited electrode to Fe(CN)63 investigated. From IR spectra, it is indicated that the polymerization of pyrrole takes place on the internal wall of MCM particles where the cationic PPy is charge-balanced by the negatively charged MCM. A PPy/SiMCM composite electrode prepared in the presence of higher concentration of pyrrole (30.5 M) shows a reproducible electrochemical response for Fe(CN)63. The CV curve on this electrode is comparable to that on pure PPy-modified electrode, and it is suggested that the negative charge on the mesopore surface is almost completely neutralized by the positive charge of PPy. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Mesoporous silica; Polypyrrole; Composite; Electrodeposition
1. Introduction Mesoporous silica MCM-41 is a new molecular sieve material consisting of a hexagonal array of uniformly sized cylindrical pores [1]. Due to the large pore diameter (1.5– 10 nm), this material has attracted much attention as a new host for large molecules. The internal surface of MCM-41 occupies the majority (about 95%) of the overall surface area (1000 cm2), which has been widely used in a variety of reactions involving the polymerization of included monomers [2,3]. The surface of purely siliceous MCM-41 shows negative charge due to the existence of silanol groups, and works as ion-exchange sites [4]. Mesoporous silica has been recently used to modify electrode surface [5], in which silica particles are dispersed in the polystyrene film coated on the electrode, and the enhancement of the voltammetric response for Fe(bpy)32þ/3þ couple is observed. Walcarius et al. have prepared a carbon paste electrode containing MCM-41 which is useful as an indicator electrode for copper and mercury ions [6]. In the present study, the composite film consisting of polypyrrole (PPy) and MCM particles was electrochemically prepared on an electrode with the object of developing a new functional electrode. In this composite, the negatively * Corresponding author. Tel.: þ81-836-85-9221; fax: þ81-836-32-2886. E-mail address: [email protected] (K. Ogura).
charged MCM particles are incorporated to PPy as a dopant. The PPy/MCM composite was characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy, and the modified electrode was examined on the voltammetric response for Fe(CN)63.
2. Experimental Purely siliceous MCM-41 (SiMCM) and aluminum-containing MCM-41 (AlMCM) with different ratios of Al/Si were synthesized according to the literature [7]. In the latter case, sodium aluminate was used as a source of aluminum. The obtained precipitates were filtered, washed thoroughly with doubly distilled water, and then calcined for SiMCM in air at 500 8C and for AlMCM in O2 flow at 550 8C to remove template molecules. The Al/Si molar ratio of the MCM sample was determined with electron probe X-ray microanalyzer (Horiba, EMAX-7000). The sample was further treated for 4 h at 250 8C in a reduced pressure prior to electrochemical use. Commercial A-type (Al/Si ¼ 1:0, Wako Pure Chemical) and Y-type (Al/ Si ¼ 0:7, Aldrich) zeolites were also used, and treated in the same manner as described above. Pyrrole (Wako Pure Chemicals) was twice distilled, and used immediately after preparation. Other chemicals were of reagent grade and
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were used as received. All solutions were prepared with doubly distilled water. Electrochemical polymerization of pyrrole was performed at a constant potential of þ0.7 V on a platinum and an indium tin oxide (ITO) electrode in a pyrrole solution with 0.1 M NaClO4 or suspension of MCMs (3.0 g dm3), and the electric charge applied in this polymerization was always 49 mC cm2 unless otherwise noted. The modified electrodes prepared in this manner were used for the measurement of cyclic voltammograms of Fe(CN)63 in a 0.1 M NaClO4 solution. Prior to this measurement, the electrodes were cycled between 0.2 and þ0.6 V in a 0.1 M NaClO4 solution until a stable redox response was obtained. A onecompartment three-electrode cell was used for all electrochemical experiments, and the working electrode was a Pt (1.0 cm2) or ITO (0.7 cm2) plate. A platinum wire was used as the counter electrode, and an Ag/AgCl/sat. KCl electrode as the reference electrode. The equipments used were a potentio/galvanostat (Hokuto Denko HA-301) and a function generator (Hokuto Denko HB-104). The deposited films were further characterized by SEM, XRD and FTIR reflection-absorption spectroscopy. SEM data were obtained with a Hitachi S2300. XRD patterns were recorded with a Shimadzu XD-D1 diffractometer with Cu Ka radiation (30 kV, 30 mA). FTIR measurements were performed on a Shimadzu FTIR spectrometer (type 8100 M) equipped with an external-reflection unit and a HgCdTe (MCT) detector cooled with liquid nitrogen.
3. Results and discussion 3.1. Characterization of MCM samples
Fig. 1. XRD patterns of MCM-41 samples with different aluminum contents: (a) Pure silica; (b) Al/Si ¼ 0:031; (c) Al/Si ¼ 0:064.
was observed in a solution containing a single component of either pyrrole or SiMCM. These results indicate that the electrochemical oxidation of pyrrole to the cationic PPy requires the incorporation of an anionic species to keep a charge balance. As described above, the surface of SiMCM possesses negative charge (balanced with exchangeable cations) owing to the existence of silanol groups, and a suspension of MCM in water can be considered to be an electrolyte. In fact, the MCM suspension shows a low conductivity, but it is enough to allow the electrochemical deposition. Hence, the current observed in the presence of
XRD patterns of the MCM samples are presented in Fig. 1. SiMCM (Fig. 1a) shows four peaks, corresponding to (1 0 0), (1 1 0), (2 0 0) and (2 1 0) diffraction lines typical of a hexagonal structure of MCM-41 with a d-spacing of 3.8 nm [1]. The AlMCM (Fig. 1b) with an Al/Si ratio of 0.031 exhibits the same pattern as that of SiMCM. A slight shift of these peaks to larger values of 2y upon the addition of Al suggests the shrinkage of lattice due to the interposition of aluminum [7]. For the AlMCM with larger amount of Al (Al/Si ¼ 0:064) (Fig. 1c), the broad peak due to diffraction line (1 0 0) was only observed, signifying the collapse of mesoporous structure. (This sample is denoted as amorphous aluminosilicate below). 3.2. Electrodeposition of composite films In Fig. 2, the current versus time curves on a Pt electrode during the potentiostatic oxidation are shown in various solutions. Some current is seen to pass in the solution with both pyrrole and SiMCM although this current is considerably small in comparison with that observed in a NaClO4 solution containing pyrrole. However, no significant current
Fig. 2. Current–time curves on a Pt electrode at a constant potential of þ0.7 V in aqueous solutions containing pyrrole ð0:5 MÞ þ SiMCM (3.0 g dm3) (a), pyrrole (0.5 M) (b), SiMCM (3.0 g dm3) (c). Inset: current–time curve at þ0.7 V in a 0.1 M NaClO4 solution containing 0.1 M pyrrole.
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pyrrole and SiMCM can be attributed to the deposition of PPy with SiMCM working as a charge compensator, and the deposited film be regarded as a composite consisting of PPy and SiMCM. The composite film prepared in such a manner was visually uniform over the substrate. SEM images of the PPy/SiMCM composites deposited on an ITO electrode are shown in Fig. 3a and b. The morphology of these images was quite similar to that of SiMCM. Fig. 3a and b show that the electrode surface is gradually covered with deposits by applying larger electric charge. These deposits are dissimilar
Fig. 4. XRD patterns of SiMCM powder (a) and composite films, (b, c) formed on an ITO electrode. The conditions for the formation of composite film: potential, þ0.7 V; electrical charge, 49 mC cm2; electrolyte, 0.1 M; NaClO4 pyrrole, 0.5 M; SiMCM, 3 g dm3. The pattern for the composite film (c) was taken after the SiMCM-modified electrode was cycled between 0.2 and þ0.6 V in a 0.1 M NaClO4 solution.
Fig. 3. SEM images of the composite films (a, b) and pure PPy film (c). The composite films were prepared on an ITO electrode at þ0.7 V in a solution containing 0.5 M pyrrole and 3.0 g dm3 SiMCM by applying electrical charges of 22 mC cm2 (a) and 49 mC cm2 (b). The pure PPy film was formed on the same electrode at þ0.7 V in the presence of 0.1 M pyrrole and 0.1 M NaClO4 by applying an electrical charge of 49 mC cm2 (c).
to hemispherical morphology typical to PPy (Fig. 3c). In Fig. 3b and c, the electrochemical deposition was done from pyrrole solutions containing suspension of SiMCM and NaClO4, respectively, by applying a constant electric charge of 49 mC cm2. The time necessary for this electric charge was very different in the former (150 min) and latter (40 s), which may be related to the poor and strong abilities of SiMCM and ClO4, respectively, as a charge compensator. The XRD pattern of the PPy/SiMCM composite is shown in Fig. 4b. Although the pattern is less defined compared to that of SiMCM powder (Fig. 4a), the main two peaks ((1 0 0) and (1 1 0)) can be observed at the corresponding positions. These peaks were not affected by the potential cycling (at least until 50 cycles) in the potential region between 0.2 and þ0.6 V (Fig. 4c). Hence, the mesoporous structure of SiMCM is suggested to be maintained during the electrodeposition and subsequent electrochemical processes. Fig. 5a shows an FTIR reflection spectrum of the PPy/ SiMCM composite in the region from 2000 to 750 cm1. The absorption bands appearing at 1238, 1100, 967 and 793 cm1 are all attributed to SiO2 [8]. This feature is almost identical to the spectrum of dehydrated SiMCM powder obtained on the transmission mode (Fig. 5b). On the other hand, the peaks at 1594, 1494, 1360 and 939 cm1 are attributable to PPy [9], because these peaks are close to those observed for pure PPy (Fig. 5c). Hence, the composite is demonstrated to be composed of SiMCM and PPy. The peak observed at 1638 cm1 for the dehydrated SiMCM (Fig. 5b) is assignable to the deformation vibration (dH–O–H) of the structural water bound to the surface silanol group. As seen in Fig. 5a, however, this peak disappeared for the SiMCM incorporated in the composite film. In crystalline MCM, most silanol groups exist on the mesopore walls since
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Fig. 5. FTIR spectra: (a) Composite film; (b) Dehydrated SiMCM powder in KBr; (c) Pure PPy film. The composite film was prepared in a solution containing 0.5 M pyrrole and 3.0 g dm3 SiMCM. The pure PPy film was formed in a solution containing 0.1 M pyrrole and 0.1 M NaClO4. Both films were obtained at þ0.7 V on a Pt electrode by applying an electrical charge of 49 mC cm2.
the external surface is only about 5% of the total equivalent area [3]. It is therefore suggested that PPy is formed on the surface of mesopores of MCM particles, and the structural water is replaced by the PPy chain. On the other hand, the peak at 1594 cm1 (Fig. 5a) is due to the combination of intra-ring C=C and inter-ring C–C vibration [9], which is located at higher wavenumber than the corresponding wavenumber (1561 cm1) of PPy (Fig. 5c). In general, this peak tends to shift to lower wavenumber as the conjugated length of polymer is increased [9]. Therefore, the PPy chain in the composite is considered to be shortened than the length of pure PPy. This finding is consistent with the fact that the molecular weight of polyaniline (35,000) chemically synthesized in an MCM host is smaller than that (52,000) formed in bulk [10]. Such a limitation in the polymer growth may be related to the restriction for the diffusion of monomer within mesopores. Similar composite films were prepared with AlMCM (Al/ Si ¼ 0:031), amorphous aluminosilicate (Al/Si ¼ 0:064), and Y- and A-type zeolites, and their FTIR spectra are shown in Fig. 6. The spectral features of Fig. 6a and b similar to that of PPy (Fig. 5c) indicate that the deposition of PPy is possible in the presence of mesoporous and amorphous aluminosilicates. A comparison between these figures and Fig. 5a suggests that the spectral intensity of SiO2 (1238, 1100 cm1) is decreased with an increase in aluminum content of the silicates. This is because the positively
Fig. 6. FTIR spectra of the composite films prepared in 0.5 M pyrrole solutions containing 3.0 g dm3 of AlMCM (a), Amorphous aluminosilicate (b), Y-zeolite (c), and A-zeolite (d). All films were obtained under the same conditions as those in Fig. 4.
charged PPy can be charge-balanced with only smaller quantity of the silicate with the enhanced negative charge (i.e. the increase in Al content). The band at 1564 cm1 owing to PPy in the composite with the amorphous aluminosilicate (Fig. 6b) is close to the wavenumber (1561 cm1) for PPy (Fig. 5c). This means that the growth of PPy is not restrained by the amorphous aluminosilicate with relatively large negative charge. As shown in Fig. 6c, the PPy/Y-zeolite composite presents the absorption bands due to PPy (1572, 1491, 1333 cm1), zeolite (1153, 972, 789 cm1) and structural water (1628 cm1). The formation of PPy within the channel in Y-zeolite has been first reported by Bein et al. [11], in which the polymerization is achieved by the reaction of pyrrole monomer with the oxidizing species (Fe(III) or Cu(II)) existing in zeolite. The polymerization is assumed to be not carried out in sodalite cages (0.3 nm windows) but in supercages of Y-zeolite (0.8 nm windows), which leads to the view that lattice water may remain during electropolymerization and thus to the appearance of the optical peak due to dH–O–H at 1628 cm1. On the other hand, as seen from Fig. 6d, the formation of composite was not observed with A-zeolite (pore size 0.4 nm, smaller than pyrrole). This result is probably caused by the small pore size of A-zeolite, and the negative charge existing on the external surface of zeolite particle is not enough to balance the cationic charge of PPy. Fig. 7 shows FTIR spectra of the PPy/SiMCM composite films obtained by the repeated potential cycling in the potential regions from 0.8 to þ0.2 V (a) and from 0.2
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Fig. 7. FTIR spectra of PPy/SiMCM composite. The modified electrodes were previously cycled for 50 times between 0.8 and þ0.2 V (a) and 0.2 and þ0.6 V (b) in a 0.1 M NaClO4 solution.
to þ0.6 V (b). In less noble potential region (Fig. 7a), the peak associated with SiO2 is not observed but those with PPy are apparent, indicating that SiMCM particles are detached from the composite when the PPy is reduced to the neutral state. In the latter potential region (Fig. 7b), PPy is in a conducting state, and the optical intensity of the peak due to SiO2 is large, confirming that SiMCM particles are electrostatically bound to PPy.
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Fig. 8. Cyclic voltammograms of composite- (a–c) and pure PPy-modified electrodes (d) in a solution with 0.1 M NaClO4 plus 1 mM K3Fe(CN)6. Scan rate, 20 mV s1. The composite films were prepared in solutions with 3.0 g dm3 SiMCM and various concentrations of pyrrole: (a) 0.05; (b) 0.2; (c) 0.5 M. The pure PPy film was formed in a solution containing 0.1 M pyrrole and 0.1 M NaClO4.
CV curves of the composite films prepared with 0.5 M pyrrole and AlMCM, amorphous aluminosilicate or Y-zeolite were measured in an NaClO4 solution containing K3Fe(CN)6 (Fig. 9). The composites with AlMCM (Fig. 9a) and amorphous aluminosilicate (Fig. 9b) show distinct CV curves
3.3. Electrochemical behavior of composite films Cyclic voltammograms (CVs) of PPy/SiMCM films prepared under various conditions were measured in a 0.1 M NaClO4 solution with 1.0 mM K3Fe(CN)6, and the resulting CV curves are shown in Fig. 8. As seen from this figure, the CV curves are considerably dependent on the concentration of pyrrole monomer used for the preparation of PPy/SiMCM composites. The charging current of the composite film increases with the increasing pyrrole concentration. Also, the current peak due to Fe(CN)63/4 increases with the monomer concentration, and the peak separation becomes small. Ill-defined CV curves of the composite film obtained with lower concentration of pyrrole monomer (20.2 M) are caused by less interaction between Fe(CN)63 and electroactive sites on PPy owing to the existence of negatively charged SiMCM. On the other hand, the composite prepared with higher concentration of pyrrole monomer (30.5 M) showed the well-defined CV curve comparable to that of pure PPy (Fig. 8d), suggesting that the negative sites on the mesopore surface are almost completely neutralized with cationic PPy.
Fig. 9. Cyclic voltammograms of the composite films: (a) PPy/AlMCM; (b) PPy/amorphous aluminosilicate; (c) PPy/ Y-zeolite in a solution with 0.1 M NaClO4 plus 1 mM K3Fe(CN)6. Scan rate, 20 mV s1.
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comparable to that of pure PPy. The CV curve of the PPy/Yzeolite composite (Fig. 9c) is quite similar to that of the PPy/ SiMCM composite prepared with lower concentration of pyrrole (Fig. 8a). Therefore, the negative sites on the mesopore surface of AlMCM and amorphous aluminosilicate are enough neutralized but those on the surface of zeolite not.
Acknowledgements This research was supported by the Japan Society for the Promotion of Science (No. 127536). References
4. Conclusions Composite films consisting of PPy and MCM particles were electrochemically prepared. XRD and infrared spectra revealed that the polymerization of pyrrole occurs within the mesopores of SiMCM by taking the place of water molecules. The oxidation of pyrrole to the cationic PPy proceeds by incorporating negatively charged SiMCM to keep a charge balance. The CV curve of Fe(CN)63 on the PPy/MCM-modified electrode which was prepared in the presence of higher concentration of pyrrole (30.5 M) was comparable to that observed on the pure PPy-coated electrode, suggesting that the negative sites on the mesopore surface were almost completely neutralized with cationic PPy.
[1] J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T.-W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins, J.L. Schlenker, J. Am. Chem. Soc. 114 (1992) 10834. [2] S.M. Ng, S. Ogino, T. Aida, K. Koyano, T. Tatsumi, Macromol. Rapid Commun. 18 (1997) 991. [3] K. Kageyama, S. Ogino, T. Aida, Macromolecules 31 (1998) 4069. [4] Z. Luan, J. Xu, L. Kevan, Chem. Mater. 10 (1998) 3699. [5] G. Villemure, T.J. Pinnavaia, Chem. Mater. 11 (1999) 789. [6] A. Walcarius, N. Luthi, J.-L. Blin, B.-L. Su, L. Lamberts, Electrochim. Acta 44 (1999) 4601. [7] J.M. Kim, J.H. Kwak, S. Jun, R. Ryoo, J. Phys. Chem. 99 (1995) 16742. [8] H.A. Benesi, A.C. Jones, J. Phys. Chem. 63 (1959) 179. [9] Y. Furukawa, S. Tazawa, Y. Fujii, I. Harada, Synth. Metals 24 (1988) 329. [10] C.-G. Wu, T. Bein, Science 264 (1994) 1757. [11] T. Bein, P. Enzel, Angew. Chem. Int. Ed. Engl. 28 (1989) 1692.