Controlled permeability of functionalized polypyrrole films by use of different electrolyte anion sizes in the electropolymerization step

71

J. Electroanal. Chem., 310 (1991) 71-87 Elsevier Sequoia S.A., Lausanne

Controlled permeability of functionalized polypyrrole films by use of different electrolyte anion sizes in the electropolymerization step S. Cosnier, A. Deronzier and J.-F. Roland Laboratoire d’Electrochimie Organique et de Photochimie R&dox (URA CNRS DIZIO), Universite Joseph Fourier Grenoble 1, BP 53X, 38041 Grenoble Cedex (France) (Received

14 November

1990; in revised form 22 January

1991)

Abstract A series of electrodes modified by polypyrrole films substituted by a tris-bipyridine ruthenium complex was prepared in CH,CN in the presence of a large variety of electrolytes having different anion sizes. A further exchange of the former incorporated anion with perchlorate associated with the electrochemical destruction of the conductivity of the polypyrrole backbone leads to modified electrodes exhibiting different behaviours toward the permeation of solutes like ferrocene and decamethylferrocene chosen as models. Utilization of toluenesulphonate or substituted toluenesulphonate with a long aliphatic chain as the supporting electrolyte in the polymerization media enhances markedly the permeability of the films with regard to modified electrodes prepared in a perchlorate electrolyte. In contrast, the incorporation of larger anions like anthraquinonesulphonate or naphthalenesulphonate increases the permeation of the films very little as a consequence of a lower ordered polymeric structure. The influence of other parameters such as the electropolymerization method and cross-linking degree of the polymer was also investigated.

INTRODUCTION

The modification of electrode surfaces by electropolymerization of N-substituted pyrroles by transition metal complexes or electroactive organic groups having catalytic activity has received considerable attention in the last few years [l]. However, although several significant results have already been obtained in the field of electrocatalysis, the moderate permeability of these materials [2-41 toward substrates, especially bulky molecules, remains a major problem. Polymeric films need fast mass transport rates to ensure a good efficiency of the electrocatalytic processes, especially for thick films. As an example, we showed that the permeability of polypyrrole films substituted by polypyridyl ruthenium complexes, in which the conductivity of the polypyrrole skeleton has been destroyed, toward a ferrocene 0022-0728/91/$03.50

0 1991 - Elsevier Sequoia

S.A.

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derivative decreases rapidly as their thickness (apparent surface concentration of the complex, I) and the bulkiness of the substrate increase [2]. In theory, the diffusional mobility of molecules through polymeric films can be optimized by the creation of large pores or cavities in the polymer during their electrochemical formation. In order to improve the permeation of functionalized polypyrrole films we have studied the influence of the bulkiness of the anions of the supporting electrolyte on the permeability of some polypyrrole films N-substituted by a tris-bipyridyl Run complex as I increases. The choice of this kind of modified electrode was motivated by its electrocatalytic properties after the incorporation of RuO, particles toward the oxidation of alcohols [5,6]. We found that the permeability of these functionalized polypyrrole films can be markedly improved by using a conveniently sized anion in the polymerization step followed by its exchange with a smaller one, e.g. the perchlorate anion. We showed that some other parameters, like the electropolymerization procedure and the cross-linking degree of the polymer, also have a significant effect on the permeability. EXPERIMENTAL

The electrochemical equipment has been described previously [2]. Acetonitrile (SDS chromasol) was used without further purification. All potentials are referred to the Ag/lO mM Ag+ in CH,CN reference electrode. Platinum disk electrodes (5 mm) were polished with 1 pm diamond paste. All experiments were run under an argon atmosphere in a dry box. Film preparations were made using a procedure similar to that reported in ref. 2. Details concerning the use of different electrolytes and the destruction of the polypyrrole conductivity are given further on in the text. The apparent surface coverage l? (in mol cmV2) was determined from the charge under the anodic Ru2+j3+ cyclic voltammetry peak when the electrode is transferred in clean electrolyte. The monomers [RUG]” and [Ru(bpy)(L2)2]2’ were prepared as described previously [2,5]. Ferrocene was recrystallized from methanol and decamethylferrocene (Strem) was, used as received. Preparation of electrolytes The structures and abbreviations of the electrolytes are given in Fig. 1. Tetraethylammonium perchlorate (TEAP) and tetrabutylammonium perchlorate (TBAP) were purified following procedures described previously [2]. Tetraethylammonium p-toluenesulphonate (TEATS) and tetrabutylammonium trifluoromethanesulphonate (TBATFMS) were obtained from Fluka. TEATS was dissolved in a minimum volume of CH,Cl, and precipitated by addition of diethyloxide. TEATS and TBATFMS were dried under vacuum at 80 o C for 72 h. Tetrabutylammonium 2naphthalenesulphonate (TBANS) was prepared in H,O by mixing the corresponding sodium salt (Fluka) and 50% excess of n-Bu,NHSO,. The mixture was extracted with CH,Cl,. The resulting solution was dried over Na,SO,, filtered and evaporated under vacuum. The resulting solid was dissolved in

73

w CF,SO, Trifluoromethanesulphonate TFMS -

~&,2

-0

so,

0

Dodecylbenzenesulphonate DDBS-

so;

ql$ so,1,5-Naphthalenedisulphonate NDSFig. 1. Structures and abbreviations

so;

Toluenesulphonate NS-

m

so,-

00

2-Naphthalenesulphonate NS-

@?pSOT 0 2Anthraquinonesulphonate ASof the electrolytes

used in the polymerization

step.

a minimum volume of CH,Cl, and dropped into stirred diethyloxide. The precipitate was collected and dried under vacuum at 80” C for 72 h. FAB-MS: m/z, positive mode, C- (TBA+),H+ 692, TBA+ 242; negative mode, 2C- TBA+ 656, C- 207. Tetrabutylammonium dodecylbenzenesulphonate (TBADDBS) was prepared in a similar way from its sodium salt (Sigma), purified by reprecipitation from heptane and dried under vacuum for 72 h. FAB-MS : m/z, positive mode, C- (TBA+)2 809, TBA+ 242; negative mode, C- 325. Tetrabutylammonium 2anthraquinonesulphonate (TBAAS) was prepared in a similar way from its sodium salt (Fluka) and dried under vacuum ( - 100 o C, 72 h). FAB-MS: m/z, positive mode, C- (TBA+),H+ 772, TBA+ 242; negative mode, 2C- TBA+ 816, C- 287. Tetrabutylammonium p-toluenesulphonate (TBATS) was prepared by mixing the corresponding sulphonic acid (Merck) with a stoichiometric amount of tetrabutylammonium hydroxide (40% in aqueous solution). The mixture was stirred overnight, diluted with ethanol and then evaporated almost to dryness under vacuum. The remaining slurry was diluted with CH,Cl,, filtered, dried over Na,SO,, filtered again and the solution was concentrated. After addition of cyclohexane, the product was obtained as a white powder by removing the solvent slowly under vacuum. The

14

electrolyte was dried under vacuum for 72 h. FAB-MS: m/z, positive mode, C(TBA+)2 655, TBA+ 242; negative mode, 2C TBA+ 584, C- 171. Tetrabutylammonium 1,5-naphthalenedisulphonate (TBANDS) was prepared in a similar way from its sulphonic acid (Aldrich) and dried under vacuum at 80 ’ C for 72 h. FAB-MS: m/z, positive mode, C2- (TBA+)3 1013, TBA+ 242; negative mode, C- 287. Scanning electron microscopy @EM) experiments Samples for SEM were prepared by controlled-potential oxidation of monomer solutions on a vitreous carbon disk (diameter 18 mm), previously polished with diamond paste, using an H-shaped electrochemical cell. The modified electrodes were carefully and copiously rinsed with CH,CN to avoid any free-electrolyte pollution. SEM experiments were performed by Laboratoire Consortium des Moyens Technologiques Communs (Institut National Polytechnique de Grenoble) using a JEOL 6400 microscope. RESULTS AND DISCUSSION

The permeability of our functionalized polypyrrole films versus their thickness (l?) was investigated by measuring the permeation of the electroactive substrates, ferrocene (Fc) and its bulky derivative decamethylferrocene (DmFc), through the polymers by cyclic voltammetry experiments in CH,CN. The permeation measurements were carried out with two kinds of polypyrrole-tris-bipyridine ruthenium(I1) complex films having different cross-linkage degrees. The films were obtained by the electropolymerization of the two monomer complexes [RUG]” and [Ru(bpy) (L2)2]2’ (bpy = 2,2’-bipyridine) on a platinum surface.

I-1

I-2

As previously demonstrated, the electropolymerization can be achieved by scanning the potential repeatedly over the redox system Ru2+13+ or by controlledpotential oxidation at the foot of this anodic peak [2,5,7]. However, the obtention of polymeric films exhibiting the regular polypyrrole electrochemical response requires careful control of the potential during their electropolymerization. Rapid destruction of the feature of polypyrrole conductivity can be accomplished by an overoxidation process by potentiostating the electrodes for 5 min at 1.5 V or by cycling

75

the potential continuously in the range O-l.5 V. The resulting new film remains remarkably adherent and retains the regular electroactivity of the ruthenium(I1) complex, especially the reversible Ru2+13+ peak system which lies around 0.9 V. Figure 2 shows the cyclic voltammograms of both films. The overoxidation of the polypyrrole backbone corresponds to an ill-defined chemical transformation of the polymer. It has been assumed that two-electron oxidation of the pyrrole units in the regular polypyrrole occurs at a high overvoltage in acetonitrile, leading to the dication. The latter reacted with residual water, leading to substitution products that can be further oxidized [8]. As a consequence, the conductivity of the polymer is lost. Then the regular quasi-reversible wave of the polypyrrole is replaced by a sharp prepeak due to the resistance of the film [9]. This prepeak vanished completely after several successive scans in the positive region (Fig. 2B). The permeation experiments were conducted with this type of film. It has been shown previously that Fc and DmFc are oxidized on these modified electrodes following two different processes [2]. As can be seen in Fig. 3, a first oxidation peak lies at the same potential as the bare electrode reaction indicating the direct oxidation on the metallic surface of the substrates which diffused through the film. The oxidation of Fc and DmFc is also mediated by the electrogenerated Ru3+ species contained in the film. This indirect process is characterized by sharp prepeaks on the edge of the Ru2+13+ oxidation peak. Since the Fc and polypyrrole electrochemical responses are close together, we used polymeric films without the polypyrrole feature to avoid mixing of the two signals. The permeation rate ( PE) is

Fig. 2. Cyclic voltammograms of a Pt/poly[Ru(L,),]2+ electrode (r = 3.6 X low9 mol cme2) in CH,CN + 0.1 M TEAP; u = 100 mV s-l. (A) Electrode obtained by controlled electrolysis at 0.7 V (B) ( -) Same electrode after potentiostating for 5 min at 1.5 V; (------) 15th scan between -0.3 and 1 V.

Fig. 3. &lic voltammograms of (A) 2 mM Fc and (B) 2 mM DmFc in CH,CN + 0.1 M TEAP; u = 100 mV s-l. Curves 1: at a bare Pt electrode; curves 2: at a Pt/polflRu(L1)J2+ electrode (r = 3.5 x low9 mol cme2).

expressed as the ratio of the peak currents for direct oxidation of Fc and DmFc at the modified electrodes and at a bare Pt electrode. Effect of the size of the electrolyte anion in the electropolymerization

step

Regular polypyrrole is obtained in an oxidized conducting state with approximately one in three pyrrole rings carrying a positive charge, containing counter-ions from the electrolyte. It has been shown that the nature of the anion has a large effect on the polymer formed [lo-161. As an example, a polypyrrole film electrochemically synthesized in the presence of Cl- showed permeability toward Cl- and anions of comparable diameter but prevented anions of larger diameter, such as C,H,SO;, penetrating the matrix [17]. Moreover, compared with perchlorate anions, large organic sulphonate anions gave a higher order to the polypyrrole [14]. A more complicated situation is encountered for our compounds taking account of the fact that monomers contain two or three pyrrole groups and of the bulkiness of the ruthenium complexes. Assumption of a linear chain structure as for regular polypyrrole [18] is inappropriate here. In our films, the polypyrrole chains are probably short and strongly branched or crosslinked, providing a quite rigid polymer [2,19]. It is worth noting that in the course of the electropolymerization process, besides the anionic counter-ions entrapped as dopants of the polypyrrole skeleton, two counter-ions per monomeric unit are also entrapped in the film since the ruthenium complexes carry a double positive charge. Taking account of these

77

features,

the electrochemical elaboration of poly[Ru(L,)J2+ and poly-Ru(bpy) modified electrodes in electrolytes containing large anions should produce @2)212’ films with expanded structures. Subsequent exchanges of the incorporated anions with smaller ones such as perchlorates should leave larger pores in the film which are able to ensure more efficient mass transport of permeants through the polymer. In order to confirm this hypothesis we prepared a series of Pt/poly[Ru(L,)J2+ modified electrodes having different I values by controlled-potential oxidation at 0.85 V of a 2 mM solution of [Ru(L,),] 2+ in CH,CN in the presence of different supporting electrolytes (0.1 M; see Experimental section and Fig. 1). The electrodes modified in this way were rinsed and cycled 15 times under the Ru2+i3+ peak system in CH,CN + 0.1 M TEAP to realize rapid exchange of the counter-ions (see next section) and to destroy the polypyrrole conductivity [7,20]. Although destruction of the conductivity probably implies a chemical modification of the polypyrrole structure associated with the ejection of the doping counter-ions [8], the polymeric film still contains two counter-ions per monomeric unit because of the presence of the ruthenium complex substituent. These anions are exchanged during the continuous scanning under the Ru2+j3+ peak system. The permeation measurements were made with a 2 mM solution of Fc and DmFc in CH,CN + 0.1 M TEAP. Whatever the incorporated anions and F, the cyclic voltammograms of the modified electrodes in the presence of these permeants have the same basic shape as that in Fig. 3. Obviously, the values of the ratio of the peak currents for direct oxidation of Fc and DmFc at the films and at a bare electrode ( PE) depend on I and on the nature of the anionic counter-ions. The electrochemical signals are reproducible and stable with time. No change of the current intensities was observed after the electrodes were left for several hours in the solution. This seems to prove that the variations of P, are not due to a kinetic effect arising from slow relaxation of the polymer. The plots of the permeation efficiency P, of Fc and DmFc vs. P with Pt/poly[Ru(L,),]‘+ modified electrodes prepared with various electrolytes are shown in Figs. 4A and 4B. For all the films the direct oxidation of the permeants decreased with an increase in film thickness. Obviously, the permeation is greater with Fc than with the bulky DmFc, showing molecular size discrimination of the polymeric films toward the solutes in contact with them. As expected, the highest permeability was reached with films prepared with large anions while those prepared in the presence of the smallest one (P-) were the least permeable. However, it is worth noting that the best results were obtained with TS-, which is not the largest anion (P, = 0.63 for Fc and 0.22 for DmFc against P, = 0.31 and 0.06 respectively with a film prepared in TEAP having an identical thickness, r = 4.5 X lo-’ mol cme2). As TS- and DDBS have a similar structure, the P, values of both the resulting polymers are similar with a slight difference for large I where DDBS appears as less efficient. Increase of the anion size induces a decrease of P,, as revealed with films using AS- in the electropolymerization step. This effect is dramatically pronounced with NS. Moreover, the use of NDS instead of NS- should reduce by half the number or size of the cavities in the polymers, and hence the permeability should be markedly less. In contrast, the

78

(B)

1.0

5 0.5

o.oL 0

12

3

108r/mo1 cms2

4

5

0.0’ 0

’

12

’

’

3

.

’

4

5

108r/mo1 cme2

Fig. 4. (A) Variations of Pa (ratio of the peak currents for direct oxidation on a Pt/poly[R~(Lt)~]~’ electrode and on a bare Pt electrode) with poly[Ru(L),] 2+ thickness (P), for Fc (2 mM) in CH,CN containing 0.1 M TRAP as the supporting electrolyte. The modified electrode was prepared by controlled-potential oxidation of the monomer in the presence of electrolyte anions of different sizes: (A) TS-; (+) DDBS-; (m) AS; (x) NS-; (0) TFMS-; (*) P-. (B) Variations of Pa with polflR~(L)s]~’ thickness (I’), for DmFc (2 mM) in CH,CN containing 0.1 M TEAP as the supporting electrolyte. The modified electrode was prepared by controlled-potential oxidation of the monomer in the presence of electrolyte anions of different sizes: (A) TS-; (+) DDBS-; (W)AS-; (X) NS-; (0) TFMS-; (t) P-.

permeation of Fc and DmFc remains unchanged, indicating that the size and number of the charge of the anions is no longer a deciding factor in that case. At least, as expected, the utilization of another small-sized anion such as TFMS- gives results close to those obtained with P-. With the view of elucidating the discrepancy observed with the largest anions, we tried to evaluate the efficiency of our technique relative to the replacement of the starting large anions trapped in the polymer by perchlorates. Since AS- is an electroactive species exhibiting a reversible redox couple (AS-+ e-* ASs2-; E,,, = - 1.18 V) out of the potential domain of the electroactivity of the modified electrode (the first reduction of the bpy ligand in the complex lies at - 1.70 V [2]), this electrochemical signal can serve as a probe to evaluate the concentration of AS- in the polymeric film. Leaving a modified electrode containing AS- as the counter-anion for 1 h in CH,CN + 0.1 M TEAP solution causes a 50% reduction in the intensity of the cathodic cyclic voltammetric peak of the AS-+ AS”- process toward the same signal of a modified electrode having an identical I (I = 1.0 x lo-’ mol cmp2) recorded immediately after its dipping in the TEAP electrolyte. This demonstrates clearly that the natural free exchange between the large anions entrapped in the film and perchlorate from the electrolyte is a slow process. In contrast, scanning the potential repeatedly over the range - 0.80 to - 1.3 V results in a continuous fast decrease in the size of the AS-/AS’2peaks, as shown in Fig. 5. The electro-induced forced expulsion of the AS- anion results from the formation of the radical anion ASa2- carrying two negative charges. As a consequence, the Ru2+ sites are now associated with half of the starting amount of

79

Fig. 5. Cyclic voltammogram of a Pt/polflRu(L,)J2+ -(AS-)2 electrode (r=l.OXlO-* mol cmp2) in CH,CN+O.l M TEAP. (a) Ru~+/~+ system; (b) evolution of the AS/AS”system during repetitive scans at 100 mV s-‘.

anthraquinone electrolyte. On the reverse scan, the released AS- species are not able to be associated back with Ru2+ sites since the perchlorates in solution are in large excess. The stabilization of the voltammogram after 20 cycles indicates that less than 10% of the initial amount of AS remains in the polymer. On the other hand, if an identical modified electrode is cycled continuously, first over the redox couple Ru2+13+ in a fresh CH,CN + 0.1 M TEAP solution, the signal of AS-+ ASe2- is almost totally lost after 15 scans (Fig. 6). Here the electrorelease of AS occurs during the Ru3+d Ru2+ electrochemical reduction step following a similar process described above. This experiment demonstrates that AS- can be exchanged efficiently with the smaller perchlorate anions by cycling the potential over the Ru 2+/3+ redox couple. It can be assumed reasonably that this electrochemically forced anion release is a general event for all the other modified electrodes containing large anions studied in this paper. Thus, the lowering of the permeability, taking place gradually from TS- up, with an increase in the size of the electrolyte anions (DDBS- and AS-) was probably not due to an incomplete anion exchange. A more appropriate explanation would be that high steric hindrance due to AS- exceeds the simple swelling properties of the polymer, inducing disordered growth of the latter. As a consequence, the creation of ill-defined paths in the film after the anion exchange step prevents the efficient permeation of Fc and DmFc. The low permeability of the films prepared with NS and NDS could be due to

80

Fig. 6. Cyclic voltammogram of a Pt/poly[Ru(Ll)s]2’ -(AS-), electrode (r=l.OXlO-s mol cmv2) in CH,CN+O.l M TEAP. (a) 15th cycle between 0 and 1.1 V on the Ru2+13+ system; (b) evolution of the system during repetitive scans at 100 mV s-’ after 15 oxidative cycles. AS-/AS2-

another phenomenon. Strong a-lr interactions between aromatic cycles can occur in the polymer in a similar way, as observed with molecules such as pyridine [21] and cyclophane [22] in solution. Furthermore, the forced proximity of NS- or NDS in the polymeric film facilitates the formation of such molecular associations. In such a situation, their exchange with the anions of the supporting electrolyte would be markedly more difficult. Effect of the electropolymerization

procedure

As shown previously, the preparation of a p~ly[Ru(L,)~]~+ modified electrode can be accomplished also by cycling the potential between 0 and 1.3 V [2]. In this case, electrochemical destruction of the conductivity of the polypyrrole matrix is not required since the resulting films do not exhibit this feature. For instance, we compared the permeation of Fc and DmFc through films of polflR~(L,),]~+ having similar I values prepared in an electrolyte containing TS- following the cyclic voltammetry and constant applied potential procedures. Data from Fig. 7 reveal that the polymers prepared by controlled-potential electrolysis exhibit a higher permeability than those prepared following the cyclic voltammetry procedure. The difference is 25 and 75% respectively for the permeation of Fc and DmFc with I = 4.5 X lo-* mol cm- 2. This difference has its origin in the difference in the ability of the ruthenium complex to entrap anions during the electropolymerization step. All along the controlled-potential oxidation process the oxidation state of

81

0.0

0

12 3 4 i08F/mo1 crnm2

5

Fig. 7. Variation of P, with polflRu(L,),] ‘+ thickness (r) for a film prepared in the presence of TSby repetitive scanning (0) or by controlled-potential electrolysis (+) and transferred into CH,CN+O.l ) 2 mM Fc as substrate; (- - - -) 2 mM DmFc as substrate. M TFAP. (-

ruthenium remains constant (+3). In contrast, during the cyclic voltammetry procedure the oxidation level of the complex oscillates between 2 + and 3 + , leading to a decrease in the amount of incorporated anions. Effect of the electrolyte bnion size of the electrolyte transfer solution We checked whether a subsequent anion exchange is needed to ensure an improvement in the permeability of the poly[R~(L,)~]~+ electroprepared films. As an example, we compared the permeability of films electropolymerized by the cyclic voltammetry procedure using TS- as the counter-anion in solutions containing TSand P- respectively as electrolyte (Fig. 8). The higher value of P, obtained with Pshows that the expansion of the polymeric structure is inefficient to ensure a clear improvement in the permeability. The second step of the procedure (replacement of the initial anion by a smaller one) is required. Effect of the electrolyte cation size in the electropolymerization

step

Although the most obvious deciding factor in improving the permeability of the film relates to the electrolyte anion size of the electropolymerization solution, the effect of the cation size must be considered since metallic complex polymeric films are known to retain some electrolyte in their structure. Data from Fig. 9 show that the steric effect depending on the bulkiness of the cation (TEA+ or TBA+) has an insignificant effect on P, when the steric hindrance of the associated anion is large (TS-, for instance). In contrast, if the associated anion is small (P-), the permeation efficiency is markedly enhanced with the largest electrolyte cation (TBA+) (see Fig. 10). This behaviour is in good agreement with the statement that a large amount of

82

0

12 3 4 108r/mo1cmw2

5

Fig. 8. Variations of P, with poly[R~(L~)~]~’ thickness (r) for Fc () and DmFc (- - - -) films prepared in the presence of Tsanions and transferred into CH,CN +O.l M TEAP (0) or in CH,CN + 0.1 M TBATS (A).

supporting electrolyte accumulates in the film during its preparation. In addition, the expansion of the polymer matrix seems to be controlled by the volume of the largest component (anionic or cationic) of the electrolyte. Effect of the polymeric

cross-linking degree

The improvement of the permeability by producing large cavities in the film implies that the rigidity of the expanded structure is strong enough to retain this structure when the electrode is transferred into a perchlorate electrolyte. As matter of fact, replacing the initial large anion by P- can lead to partial collapse of the cavities. Since the rigidity of the polymer depends on the cross-linking degree, we

0.0’

0

.

n .

1

’

’

2

3

.

’

4

.

5

iOBr/molcme2 Fig. 9. Variations of P, with poly[R~(L,),]~+ thickness (r) for Fc () and DmFc (----) for films prepared in the presence of TEATS (m) or TBATS (A) and transferred into CH,CN + 0.1 M TEAP.

83

IO* r/m0 1 cms2 Fig. 10. Variations of P, with poly[Ru(L,),]*+ thickness (r) for Fc () and DmFc (----) for films prepared in the presence of TEAP (m) or TBAP (A) and transferred into CH,CN + 0.1 M TEAP.

compared the permeability of the poly[R~(L,)~]*+ films with that of the poly[Ru(bpy)(L, )21 *+ films The corresponding monomer of the latter polymer containing only two pyrrole groups, poly[Ru(bpy)(L2)2]2’, is less cross-linked than [19]. This study was carried out in optimum conditions, i.e. ~ol~[Ru(L,),l*+ electropolymerization by controlled-potential electrolysis in TBATS electrolyte followed by transfer of the modified electrode into TEAP solution. Data from Fig. 11 show markedly larger P, values for polflRu(L,),]*+ than for poly-Ru[(bpy)

0.5

0

1

2

3

4

5

10Brlmo 1 cm-2 Fig. 11. Variations of P, with polymer thickness for Fc () and DmFc (- - - -) for films prepared in the presence of TEATS and transferred into CH,CN+O.l M TJZAP. (A) Pt/polY[Ru(L1)3]2+ electrodes; (0) Pt/poly-[Ru(bpy)(L2)2]2+ electrodes.

84

(L-&2’ demonstrating polymer.’

the strong influence

of the cross-linkage

degree of the

Morphokogv of the modified electrodes Since large differences in the permeability of our polymeric materials are observed, a large difference in their structure can be expected. Numerous studies have been devoted to the effect of the deposition conditions on the morphology of regular polypyrrole and polythiophene films in connection with their conductive properties. Physical analysis techniques such as scanning electron microscopy, transmission electron microscopy and X-ray diffraction have been used to demonstrate the influence of numerous parameters on the structure of these conducting polymers. For instance, it has been reported that the film thickness and the electrolyte anion size strongly influence the surface of poly(3-methylthiophene) [23]. In the same way, the degree of order observed in the polypyrrole is highly dependent on the

Fig. 12. Scanning electron micrograph of poly[Ru(L,),] 2+ film surfaces (r=2SXlO-’ mol cmw2). (a) 350) and (b) ( X 350): films obtained in CH,CN + 0.1 M TBAP; (c) (X 350) and (d) (X 3500): films obtained in CH,CN + 0.1 M TBATS.

(X

85

incorporated dopant anions and also on the deposition conditions [24]. In contrast, only a few papers have been concerned with the micrographic analysis of the numerous polymeric films containing the ruthenium(I1) tris-bipyridine pendant group [25]. In particular, no systematic study of the effect of the electrolyte anion size on the polymeric structure has been reported yet. In an effort to understand the difference in permeation properties that we observed, preliminary SEM experiments were carried out for the samples prepared by the controlled-potential procedure in TBAP and TBATS electrolytes, having identical I’. Figure 12 shows the SEM of both films. The top views indicate that whilst the poly[Ru(L,):+, 2 P-1 film shows a surface structure 1151, on the contrary the very rough, “cauliflower-like” 2+ 2 TS-] surface appears very homogeneous and smooth except in polfiRu(L, )3 7 some places where some excrescences are seen. This observation must reflect a high regularity of the polymer structure. Pores or pinholes on the top surface of the films are not observable at this magnification

Fig. 13. Scanning electron micrograph of polfiRu(L,),j 2+ film cross-sections- (r= 2.5X lo-’ cm-‘). (a) (X350@: film obtained in CH,CN+O.l A4 TBAP; (b) (X3500) and (c) (X140@: obtained in CH,CN + 0.1 M TBATS.

mol films

86

( X 3500). The cross-section views (Fig. 13) confirm these observations and allow us to estimate the thickness of the dry films. For an identical l? (I’ k 2.5 x lo-’ mol cm-2), the thickness of polfiRu(L&+, 2 TS-] is 2.3 times higher than that of 2+ 2 P-1 This result corroborates the hypothesis that insertion of TS~oIflRu(L,), > . instead of P- increases the expansion of the polymeric structure. CONCLUSION

The permeability of functionalized polypyrrole films by cationic redox pendant groups such as the ruthenium tris-bipyridine complex was largely improved by the use of a supporting electrolyte having large anions in the electropolymerization step, followed by anion exchange with perchlorate anions. However, this behaviour has some limitations since the utilization of larger anions than toluenesulphonate or anions with similar structures induces a decrease of the permeability. This is probably due to a lower ordered polymeric structure. On the other hand, modified electrodes prepared by the constant-potential technique exhibit a better permeability than those obtained by the continuous cyclic voltammetric scanning technique. It has also been shown that the efficiency of the solute permeation is increased with films containing a high cross-linking degree as a consequence of the better rigidity of the polymeric structure. The application of such electrode materials to the catalytic oxidation of large molecular size alcohols is currently under way. We are also examining the possibility of extending this technique to improve the permeability of some other cationic polymeric films having catalytic activity. ACKNOWLEDGEMENTS

We thank Professor G. Cauquis for his interest in this work and PIRSEM Electrodes Modifiees) for partial financial support.

(ARC

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