PW12O403-. 2. Physical, Spectroscopic

2. Physical, Spectroscopic and Electrochemical Characterization. T. F. Otero* .... and a blue cloud is formed around the electrode, flowing to the bot...
0 downloads 0 Views 217KB Size
Download PDF
10528

J. Phys. Chem. B 2000, 104, 10528-10533

Hybrid Materials Polypyrrole/PW12O403-. 2. Physical, Spectroscopic and Electrochemical Characterization T. F. Otero,* S. A. Cheng,† D. Alonso, and F. Huerta Laboratorio de Electroquı´mica, Facultad de Quı´micas, UniVersidad del Paı´s Vasco, P.O.Box 1072, E-20080 San Sebastia´ n, Spain ReceiVed: February 11, 2000; In Final Form: August 11, 2000

Physical, spectroscopic and electrochemical information on the potentiostatically synthesized hybrid material polypyrrole/PW12O403- was obtained by means of electron microscopy, UV-vis spectroscopy, in situ UVvis absorption-reflection spectroelectrochemistry, FTIR spectroscopy, EDX and cyclic voltammetry. The hybrid shows a very flat morphology and a two-layer structure after its generation. By reduction, the external layer, 76% weight of the material, is electrodissolved. This fraction contains PW12O403- anions and pyrrole oligomers. Continuous potential cycling of the insoluble hybrid adhered to the electrode gives rise to an increasing concentration of polyoxometalate in the background solution, thus proving that phosphotungstate anions were gradually rejected from the film. Films before and after electrodissolution of the soluble fraction show similar flat morphology. The insoluble fraction undergoes a strong morphological change during potential cycling, keeping almost constant both weight and stored charge. The oxidized state of the electrosoluble layer is partially soluble in DMSO and in mixtures of PC and EC, thus opening new ways for processing those materials. The two-layer structure of polypyrrole-based hybrid materials was also observed for other polyoxometalates, synthesized in different electrolytic media.

1. Introduction

2. Experimental Section

In the previous paper1 it was stated that any film of the hybrid material polypyrrole/PW12O403- electrogenerated at a constant potential undergoes significant weight loss (up to 80%) during the electrochemical reduction in 0.1 M LiClO4 acetonitrile solutions (free of polyoxometalate). So, it seems necessary to study and to characterize the nature of this process. First of all, it should be considered that some polyoxometalate anions could interchange during the oxidation-reduction process in order to keep the film electroneutrality. On this basis, the weight loss could be attributed to an irreversible release of polyoxometalate, as reported by Shimidzu.2 On the other hand, the weight loss could be the manifestation of an electrodissolution process where counterions as well as polymeric or oligomeric chains are involved. This second hypothesis seems supported by the presence of a specific cathodic peak at -0.48 V linked to the electrodissolution.1 The first hypothesis drives to a polypyrrole film, free of polyoxometalate ions, as the insoluble fraction. The second hypothesis drives to the presence of two layers of different hybrid materials: one of them electrosoluble and the second one insoluble. The different nature of those two materials would give rise to diverse applications, opening the soluble fraction a new way for the electrochemical processing of hybrid materials, similar to that studied and patented by our group related to multiheterocyclic conducting polymers.3 So, the aim of this paper is to apply different electrochemical, spectroelectrochemical, spectroscopic and solubilization techniques, altogether microscopic observations to characterize both parts, insoluble and soluble, of the material.

The hybrid material polypyrrole/polyoxometalate was synthesized in freshly prepared acetonitrile (Panreac, p.a) + 2% (v/v) ultrapure water solutions containing 0.1 M Pyrrole and 5 mM H3PW12O40. The polyoxometalate provides enough ionic conductivity to avoid the use of additional electrolytes. The working electrode was a platinum sheet with an area of 1 cm2 and the counter electrode was a 3 cm2 stainless steel plate. Potentials were measured against the Ag/AgCl reference electrode and are presented in this scale. The hybrid material was electrochemically tested by means of cyclic voltammetry (CV). The CV’s were carried out in 0.1 M LiClO4 acetonitrile solutions free of polyoxometalate. Films before and after potential cycling were also characterized by Fourier transform infrared spectroscopy (FTIRS), scanning electron microscopy (SEM), ex situ UV-visible spectroscopy and energy-dispersive X-ray (EDX). In situ absorption-reflection UV-visible spectroscopy was used to examine the electrode surface during the electrochemical experiments. The FTIR spectra were acquired with a Nicolet Magna 560 spectrometer. The ex situ UV-vis absorption spectra were collected with a Philips PU8700 spectrophotometer. The micrographs were obtained with a Hitachi S-270 scanning electron microscope. EDX data were obtained with a Philips DX4 spectrometer. The in situ absorption-reflection spectra were collected with an Oriel spectrometer equipped with a Xe-Ne 66006 Oriel lamp. The light is conducted to the electrochemical cell by an optic fibber. There goes through a quartz window, the solution and the polymer film, arriving to the mirror polished platinum electrode, where it is reflected. After crossing the polymer film, the solution and the window is collected in a second optic fibber and conducted to an Oriel 77173 photodiode array.

* Corresponding author. E-mail: [email protected]. † Permanent address: Department of Chemistry, Zhejiang University, Hangzhou 310027, China.

10.1021/jp000554o CCC: $19.00 © 2000 American Chemical Society Published on Web 10/21/2000

Hybrid Materials Polypyrrole/PW12O403-

Figure 1. Twenty consecutive cyclic voltammograms recorded for the hybrid film at 20 mV s-1 in 0.1 M LiClO4 acetonitrile solutions after the first cathodic polarization (linear curve). The arrows indicate the evolution of the voltammetric profile. The dotted curves are several consecutive votalmmograms of basic polypyrrole/ClO4- in the same background solution at 20 mV s-1. Basic polypyrrole/ClO4- film was electrogenerated at 1.2 V for 30 s in 0.1 M LiClO4 + 0.1 M Py acetonitrile solution.

3. Results and Discussion To characterize both the insoluble and the electrosoluble parts of the hybrid material, synthesis conditions similar to those employed in the first part of the work have been selected.1 So, the synthesis potential was kept constant at 1.2 V for 60 s in solutions containing 0.1 M pyrrole and 5 mM polyoxometalate. Films of about 1.4 µm thickness were thus obtained. When those films were transferred into 0.1 M LiClO4 acetonitrile solution for several minutes (up to 30) then rinsed, dried and weighed, the weight of the hybrid material keeps constant: it does not exist a dissolution process. In this context, we will study the electrochemical behavior of the films during the initial electrochemical reduction. The spectroscopic and microscopic characterization will be performed later. 3.1. Electrochemical Characterization. Once electrogenerated and weighed, any coated electrode was immersed in 0.1 M LiClO4 acetonitrile solution at 0.4 V and then submitted to a cathodic potential sweep down to -0.8 V to achieve its whole reduction. The film weight decreased from 0.2122 mg before cycling to 0.0509 mg after the two initial consecutive potential cycles, as was described in a previous paper.1 These results evidenced a potential-induced loss of mass of the hybrid during the first cathodic polarization. After the second potential cycle, the insoluble part of the material does not attain stable electrochemical behavior and the voltammograms undergo a continuous evolution (Figure 1). Anodic processes A and B and cathodic processes C and D in the first sweep decrease continuously upon cycling, meanwhile new increasing processes, E (anodic) and F (cathodic), appear. To compare these processes with those occurring in basic polypyrrole, several consecutive voltammograms performed in the same background solution using a thick layer of polypyrrole-perchlorate are shown (dotted curve in Figure 1, maxima G and H). Cathodic peaks D and F show lower overlapping than the anodic peak E and its associated shoulder A. We will attempt to quantify the experimental evolution of the consecutive voltammograms by integration of the charges involved in D and F, taking -0.41 V as the potential of separation between them. In Figure 2 we present the evolution of both the overall charge involved in the reduction process ([) and the charges measured for the decreasing peak D (b)

J. Phys. Chem. B, Vol. 104, No. 45, 2000 10529

Figure 2. Evolution of the charges involved in the cathodic processes from +0.4 to -0.8 V: (O) peaks C and D; (b) peak F; ([) overall charge. Data extracted from Figure 1.

and the increasing peak F (O). The charge involved in D diminishes continuously from the first cycle to the 15th and then remains almost constant. The behavior of the charge measured for process F is opposite up to the 17th cycle, just where the same stabilization takes place. It is interesting to note that the rate of the charge variation for both peaks is nearly the same, 0.71 mC per cycle for peak D and 0.73 mC per cycle for peak F, indicating the interdependency between both processes. As the overall charge involved in the reduction process keeps nearly constant, we deduce that the weight of the material should remain also constant. This constancy was proved by an ex situ gravimetric determination. These results and the evolution of the voltammetric profile point to changes either in the nature of the species involved in the redox process or in the structure of the hybrid material. We will try to clarify those facts. 3.2. Spectroscopic Experiments. In an attempt to monitor the process undergone by the hybrid during the first polarization at cathodic potentials, an in situ absorption-reflection UVvisible experiment was performed in experimental conditions parallel to those employed during the electrochemical dissolution.1 In this way, a mirror polished platinum electrode coated with the hybrid film was used as the working electrode in a spectroelectrochemical cell having a flat quartz window.4 The baseline was acquired previously for the uncoated electrode immersed in the background 0.1 M LiClO4 solution. Once the coated electrode is immersed in the solution, the potential was scanned in the cathodic direction from +0.4 to -0.8 V at 10 mV s-1 and, simultaneously, the absorption-reflection UVvis spectra were collected (one every second). Figure 3a shows the evolution of the recorded spectra as a function of the electrode potential. Those spectra contain information about the reduction processes of the polypyrrole fraction and also about those related to the weight loss and the eventual electroreduction of the polyoxometalate present in the film. The complete elucidation of those processes is out the scope of this paper and will be clarified in a future work, but now we will attempt to clarify whether the polymeric part of the material participates on the weight loss during the electrochemical reduction. In this way, we will consider one of the bands characteristic of the oxidized polypyrrole, the bipolaronic band4 appearing between 600 and 850 nm in the initial spectrum. We will follow the evolution of this band during the cyclic scanning of the potential. Both cyclic voltammogram and absorbances at the concomitant potentials have been depicted in Figure 3b. The density of bipolaronic states decreases very fast along the main reduction maximum (near -0.4 V) and along the second maximum (near -0.55 V). Parallel to the development of this second maximum,

10530 J. Phys. Chem. B, Vol. 104, No. 45, 2000

Otero et al.

Figure 4. Transmission FTIR spectrum collected for the background 0.1 M LiClCO4 solution after the hybrid material was submitted to one potential cycle from +0.4 to -0.8 V. Resolution 2 cm-1. 10 scans.

Figure 3. (a) In situ absorption-reflection UV-vis spectra for a hybrid film polypyrrole/polyoxometalate in a background 0.1 M LiClCO4 solution free of polyoxometalate. First potential cycle from +0.4 to -0.8 V. Scan rate 10 mV s-1. See text for further details. (b) First cathodic scan for a for a hybrid film polypyrrole/polyoxometalate performed in the spectroelectrochemical cell (s) and simultaneous absorbance changes related to the bipolaronic band at 820 nm (‚‚‚). (c) Absorbance spectra for 0.01 mM solution of polyoxometalate in 0.1 M LiClO4 acetonitrile solution (s) and for 0.1 M LiClO4 acetonitrile solution where a film of the hybrid material was submitted to an electrochemical reduction (‚‚‚).

the dissolution process takes place and a blue cloud is formed around the electrode, flowing to the bottom of the cell. During the subsequent anodic potential sweep, the absorbance increases along the oxidation peak attaining a constant value after the maximum, only a half of that observed before the electrochemical reduction. This means that the density of bipolaronic states at the end of the cycle is only a half of that shown by the original material. As the same electric potential is attained, this result means that a half of the polymeric chains present in the initial

film are lost. This points clearly to the presence of polypyrrole chains, probably as oligomers, in the soluble material lost from the electrode. Once we have an indication about the presence of oligomers in the soluble fraction, we will also investigate the possible presence of polyoxometalate ions. Figure 3c shows two absorption spectra obtained from a 0.01 mM solution of polyoxometalate in 0.1 M LiClO4 acetonitrile solution (s), and from a 0.1 M LiClO4 acetonitrile solution where a film of the hybrid material was submitted to an electrochemical reduction with formation of the subsequent dissolution cloud during the process (‚‚‚). The maximum appearing at 265 nm, characteristic of the polyoxometalate, is present in both cases. Those UV-vis experiments point to the existence of an electrodissolution process related to a fraction of the electrogenerated material: an oxidized state of macroions and oxidized oligomers of pyrrole. This material should be insoluble in their oxidized state and should become soluble under electrochemical reduction. From the in situ spectroscopic experiments, it has been suggested the presence of pyrrole oligomers in the hybrid film that can dissolve during the reduction process. This is an indirect method and the experimental results could be also interpreted in terms of structural changes occurring inside the film allowing a partial oxidation of the polymer. To check the presence of polypyrrole in the dissolved fraction, a transmission FT-IR experiment was performed. A just synthesized hybrid material was submitted to a single potential scan from 0.4 V down to -0.8 V, in dry 0.1 M LiClO4 acetonitrile solution to dissolve its electrosoluble fraction. Then, a sample of the background solution was examined by transmission FT-IR spectroscopy in a KBr pellet. The spectrum presented in Figure 4 shows several bands in the frequency range from 700 to 1500 cm-1. The bands coming from perchlorate appear at 941 cm-1 and at around 1100

Hybrid Materials Polypyrrole/PW12O403cm-1.5 The features at 791 and 972 cm-1 can be assigned clearly to distinct W-O vibrations, while the band at 1080 cm-1 corresponds to a P-O stretching vibration, all of them from polyoxometalate anions.6 Undoubtedly, the other bands that appear in the spectrum correspond to pyrrole vibrations. In particular, the band at 1405 cm-1 seems related to C-C stretching vibrations and that at 923 cm-1 appears in the frequency region for C-H out-of-plane deformations.5,7 This result confirms that some pyrrole oligomers simultaneously dissolve with polyoxometalate molecules upon film reduction. The electrochemical and gravimetric observations allow us to conclude that the potentiostatic generation of the hybrid material gives rise to a bilayer structure formed by two different materials (one of them electrosoluble and the other insoluble). This fact points to the concurrence of two different reaction mechanisms during the polymerization of pyrrole. The general electrochemical mechanism that has been reported by several authors8,9 would be responsible for the production of the insoluble part of the hybrid material. However, this mechanism seem not be followed by the electrosoluble part, otherwise this fraction would be insoluble as well. In a recent paper reported by Bielanski et al.10 it was shown that sorption of n-butylamine on the surface of a similar Keggintype polyoxometalate (SiW12O404-) occurs through the protonated nitrogen atom. The sorption process is subsequently accompanied by the addition of further amine molecules to the first layer of butylamine monomers. The occurrence of similar process for some pyrrole monomers and phosphotungstate anions in the solution of polymerization could yield adducts as those proposed by Bidan et al.11 Those adducts could be the electroactive species that undergo the electropolymerization process. The results of cyclic voltammograms (Figure 2 in ref 1) show that the insoluble part (24% of the weight) stores 75% of the whole charge, meanwhile the soluble fraction (76% of the weight) only stores 25% of the whole charge. Probably, this means that there exists a different polyoxometalate/polypyrrole ratio in both materials. Low oligomer content or very low electroactivity of such oligomers in the soluble fraction should also explain the little charge involved in their reduction. The existence of a proton’s gradient between the outer layer and the solution could induce a chemical polymerization to produce protonated (low electroactive) polypyrrole, as it has been observed before.12 This chemically polymerized material, located between the electrochemically generated insoluble film and the solution, would be the electrosoluble part that leaves the film upon reduction. The electrochemically induced solubilization explains, at least in part, the presence of the polyoxometalate in solution after voltammetric control of films electrogenerated under a constant potential.2 Also explains the stability of the films electrogenerated by potential cycling: the soluble fraction electrogenerated along the anodic sweep is lost during the cathodic one. Once stated that a fraction of the synthesized hybrid undergoes an electrochemically induced dissolution process in the first cathodic potential scan, it would be interesting to clarify what occurs during the continuous potential cycling (Figure 1) after the electrosoluble material has left the solid phase. To avoid any interference coming from the soluble material, a new experiment was designed. After submitting the just synthesized film to a single scan down to -0.8 V, the background solution was substituted by a clean one. The insoluble material was then cycled several times and samples of the new solution were collected after different number of cycles. Ex situ UV-visible spectroscopy was employed to estimate the amount of polyoxometalate released from the solid phase as a function of the number of potential cycles done between -0.8 and +0.4 V from

J. Phys. Chem. B, Vol. 104, No. 45, 2000 10531

Figure 5. Evolution of the amount of dissolved polyoxometalate as a function of the number of potential cycles done.

the second cycle. The amounts of PW12O403- were estimated from absorbance measurements (at λ ) 265 nm) and subsequent interpolation within a calibration line obtained experimentally. The weight of polyoxometalate in solution is depicted against the corresponding cycle in the plot of Figure 5. Results show clearly that the releasing of phosphotungstate takes place continuously during the potential cycling. The reversible redox reaction undergone by polypyrrole is always pPy0 T (pPy)n+ + ne-, so the referred releasing of polyoxometalate anions should affect the counterions involved in the reaction. The just synthesized hybrid film is formed by positive-charged polypyrrole chains and PW12O403- anions entrapped inside. In this way, the electroneutrality is guaranteed in the oxidized state. The annihilation of the positive charges in pPy+ during film reduction should be accompanied by Li+ insertion from the background solution to compensate the negative-charged polyoxometalates, because this background solution is free of polyoxometalate. Since the polyoxometalate is slowly released from the film during the potential cycling, the positive charges generated in the polypyrrole chains upon oxidation could not be fully compensated by the remaining polyoxometalate and some ClO4- electrolyte anions should penetrate the film to achieve this. The smaller the quantity of polyoxometalate retained, the larger the amount of perchlorate anions needed. Thus, from cycle to cycle, the main counterion involved in the redox process could change progressively from Li+ to ClO4-. Such a change could be at the origin of the evolution of the voltammetric profile observed in Figure 2. However, as it will be shown in the following section, this is not the only possible explanation to the voltammetric transformation observed. 3.3. Morphology and Composition Changes. The progressive alteration in the voltammetric profile (Figure 1) recorded when the material was submitted to consecutive potential cycles could also reveal alterations in the film morphology. To check this, the surfaces of freshly synthesized and cycled materials were observed by SEM. The micrographs obtained in these conditions are presented in Figure 6. After synthesis (Figure 6a) the film reveals a flat morphology, covering most of the substrate irregularities. Small white grains can also be detected at the polymer surface. Similar morphology is observed for the film submitted to a single reduction cycle (Figure 6b) but the irregularities present on the platinum substrate can be clearly detected after the electrodissoution of most of the material. From this observation we can conclude that the film thickness has decreased, but without any significant morphologic alteration. Finally, Figure 6c shows that, after 35 potential cycles, the

10532 J. Phys. Chem. B, Vol. 104, No. 45, 2000

Otero et al.

Figure 7. EDX spectra for the freshly synthesized film (a) and for the film submitted to 35 potential cycles (b).

Figure 6. SEM micrographs obtained for the freshly synthesized hybrid (a), for the film after a single reduction scan down to -0.8 V (b) and for the film submitted to 35 potential cycles (c).

morphology of the hybrid material appears highly altered spite that the stored charge remains almost constant along cycles. It is difficult to discern if the alteration in the voltammetric profile of Figure 2 is due to a change in the counterion responsible for the charge compensation (from Li+ to ClO4-) or to a morphologic/structural change. Probably, both phenomena contribute to this alteration. In this context, modifications in the bulk material composition supporting the first hypothesis can be observed by means of EDX experiments. The EDX spectra shown in Figure 7 were obtained after the transfer of different films to the vacuum chamber in their oxidized states. The spectrum for the just synthesized film (Figure 7a) shows the presence of a large amount of polyoxometalate, as derived from the intense tungsten peak near 1.8 keV and the shoulder at around 1.4 keV. Phosphorus is also detected near 2 keV. After 35 potential cycles, the spectrum of the hybrid material is dominated by the signal coming from the platinum substrate (similar signals were obtained after the second cycle). In agreement with SEM results, this observation can also be taken as an evidence of the low film thickness obtained after the electrodissolution of the soluble part. The tungsten peak is very weak in this spectrum and the ratio carbon-oxygen has increased considerably, when compared with Figure 7a. Finally, the peak appearing near 2.6 keV in Figure 7b is due to the presence of chlorine inside the film.

Although the EDX technique does not provide any quantitative information, these results clearly support the voltammetric and spectroscopic information reported above. Thus, it can be observed that in the freshly synthesized film the amount of PW12O403- is very high. The initial intensity ratio of the bands coming from tungsten at 1.8 keV and carbon at 0.3 keV changes greatly in Figure 6b and becomes reverse. The same occurs for the signal ratio C/O. These observations follow the significant loss of polyoxometalate undergone during the first cathodic scan and during the subsequent potential cycles. The detection of Cl after the potential cycles (Figure 7b) is clearly ascribable to the presence of perchlorate anions, thus showing that some of the polypyrrole positive charges are compensated by this species. As discussed above, it would be very interesting to check whether lithium cations are also present inside the film in its reduced state but, unfortunately, Li cannot be detected by EDX. 3.4. Behavior of the Material in Solvents. The experimental results presented above support the presence of two layers of different hybrid materials at the end of the electropolymerization process performed at constant potential. The first layer would be formed by a hybrid material polyoxometalate/cross-linked polypyrrole attached to the platinum electrode. A salt of the polyoxometalate and pyrrole oligomers, which can be electrochemically dissolved, would constitute the second layer between the inner polymer and the solution. This second layer is in the oxidized state when formed and is insoluble in the background solution. Its electrochemical reduction produces the electrodissolution, thus showing that only the reduced state is soluble in acetonitrile medium. This behavior offers a new possibility for processing hybrid materials. To check the possibility to obtain solutions of the oxidized state of the hybrid material films, freshly synthesized materials were weighed and immersed in different solvents (water, 3 M H2SO4, acetonitrile, dimethyl sulfoxide (DMSO), tetrahydrofuran, benzene, propylene ca-

Hybrid Materials Polypyrrole/PW12O403bonate (PC), and mixtures ethylene carbonate + propylene carbonate (EC + PC)) for several hours. After this time, the coated electrodes were extracted from the solvent, dried and then weighed again. Only two solvents are able to dissolve the material in its oxidized state: DMSO, for which up to a 60% of the weight dissolves and the mixture EC + PC, for which only a 25% of the weight dissolves. 3.5. Study of the Synthesis and Electrodissolution of Different Hybrids. After a careful revision of the literature related to hybrid materials, it was found that the significant electrodissolution process has never been described. Taking into account that most of the electrochemical syntheses described in the bibliography were performed in distinct electrolytic media with different hybrid materials, we repeated the experiments using several Keggin-type salts in different solvents (water, acetonitrile and mixtures of both) with and without a second electrolyte (LiClO4 or H2SO4). Thus, the behavior of the acid, the sodium and the tetrabutylammonium salts of PW12O403-, SiW12O404- and PMo12O403- were checked. From these experiments we concluded that all the films synthesized at a constant potential and then reduced in the background solution present a significant fraction (between 60% and 80%) of electrosoluble material. On the other hand, all the films formed by cyclic scanning of the potential present only the insoluble fraction. In this latter case the soluble fraction is liberated into the solution of synthesis during every cathodic excursion of the potential. Then, we have to conclude that the electrochemical synthesis of hybrid materials polypyrrole/Keggin polyoxometalates always yields a two-layered film. The film contains an inner, crosslinked, insoluble, electrochromic, and electroactive layer and an outer and electrosoluble layer, which is formed by a salt of pyrrole oligomers and the concomitant polyoxometalate. 4. Conclusions The present paper constitutes the first characterization of hybrid materials formed by polypyrrole and PW12O403- anions. It has been observed that the electrochemical polymerization of pyrrole in polyoxometalate solutions yields two distinct hybrid components. One of them is electrosoluble and, probably, it is formed through a chemical reaction parallel to the electrochemical process. This material leaves the solid fraction during a cathodic polarization down to -0.8 V, giving rise to specific voltammetric and spectroscopic features. The soluble part was analyzed by means of FTIR spectroscopy and it was concluded that both pyrrole oligomers and polyoxometalate ions were its main constituents. The insoluble part remains attached to the platinum surface and undergoes a slow releasing of PW12O403during the continuous potential cycling in acetonitrile/LiClO4 solutions free of polyoxometalate. In addition, this insoluble material also suffers significant morphological changes during the potential cycling. The material is thought to interchange mainly Li+ cations during the early charge/discharge processes but, as the polyoxometalate is released, the interchange of lithium seems substituted by that of ClO4-. Both contributions (structural and chemical) could be at the origin of the potential shift to more negative values observed for the characteristic redox peaks. The results presented here and those reported in the previous paper1 allow us to propose a model for the film growing. The hybrid material consists of two distinct components, whose mass ratio is nearly constant in all conditions (76% electrosoluble and 24% insoluble). This means that the rate of production of both materials is balanced. However, it seems unlikely that both

J. Phys. Chem. B, Vol. 104, No. 45, 2000 10533

Figure 8. Proposed model for the film growing in the synthesis solution. Arrows indicate the growing direction.

rates were affected in equivalent ways when the synthesis conditions were changed. In addition, from the comparison of the film surface before and after dissolution of the soluble part (Figure 6a,b), we conclude that the morphology of both materials is very similar. The model presented in Figure 8 seems compatible with these observations. In this scheme, the insoluble component is always attached to the platinum electrode and covers it completely, while the electrosoluble one grows on the surface of the former. The insoluble material grows in the inner layer from the soluble component located in the outer one. Thus, during the potential cycling in the solution containing neither pyrrole nor polyoxometalate, the soluble part leaves the hybrid material and the insoluble one is exposed to the electrolytic solution. This insoluble part undergoes morphological and chemical changes during further potential cycling. The oxidized state of the electrosoluble material is partially soluble only in DMSO and in mixtures of PC and EC. Potentiostatic electropolymerization of hybrid materials of polypyrrole in the presence of different Keggin-type salts, in different solvents and using different electrolytes showed that the two-layer structure of the electrogenerated film is a general fact. Acknowledgment. The government of the Basque Country and CEGASA have supported this work. We thank Dr. L. Irusta for her assistance during FTIR data acquisition. References and Notes (1) Otero, T. F.; Cheng, S. A.; Huerta, C. F. J. Phys. Chem. B 2000, 104, 10522. (2) Shimidzu, T.; Ohtani, A.; Aiba, M.; Honda, K. J. Chem. Soc., Faraday Trans. 1988, 84, 3941. (3) Otero, T. F.; Villanueva, S.; Brillas, E.; Carrasco, J. Acta Polym. 1998, 49, 433. (4) Otero, T. F.; Bengoechea, M. Langmuir 1999, 15, 1323. (5) Socrates, G. Infrared Characteristic Group Frecuencies; Wiley: New York, 1994. (6) Lee, K. Y.; Mizuno, N.; Okahara, T.; Misono, M. Bull. Chem. Soc. Jpn. 1989, 62, 1731. (7) Kofranek, M.; Kovar, T.; Karpfen, A.; Lischka, H. J. Chem. Phys. 1992, 96, 4464. (8) Genies, E. M.; Bidan, G.; Dı´az, A. J. Electrochem. Soc. 1983, 149, 101. (9) Handbook of Conducting Polymers; Skotheim, T. A., Ed.; Marcel Dekker: New York, 1986; Vol. 1. (10) Bielanski, A.; Micek-Ilnicka, A.; Gil, B.; Szneler, E.; Bielanska, E. An. Quim. 1998, 94, 268. (11) Bidan, G.; Genies, E. M.; Lapkowski, M. J. Electroanal. Chem. 1988, 251, 297. (12) Otero, T. F.; Rodrı´guez, J. J. Electroanal. Chem. 1994, 379, 513.