J. CHEM. SOC. FARADAY TRANS., 1994, 90(9), 1241-1244 Influence of Polymer Structure on the Electrochemistry of Phenothiazine Dyes incorporated into Nafion Films Swamidoss A. John and Ramasamy Ramaraj" School of Chemistry, Madurai Kamaraj University, Madurai-625 021, India Electrochemical properties of phenothiazine dyes, thionine (TH+) and methylene blue (MBf), incorporated into Nafion films have been studied at different acid concentrations. The TH+ and MB+ dyes adsorbed by Nafion films gave an unusual redox wave at more positive potentials in addition to the usual redox wave at less positive potentials in 0.05 mol dm-3 H,SO,. Upon continuous cycling for 45 min the unusual redox wave disappeared. The two redox waves observed at 0.05 mol dm-3 H,SO, were accounted for by the distribution of electroactive species in different domains of the Nafion polymer film, i.e.the lower void volume of the interfacial region and the higher void volume of the ionic cluster region. The same dyes incorporated into Nafion films in 0.5 mol dm-3 H2S04 gave only the usual redox wave, which was accounted for by the molecules residing only in the ionic cluster region of the swelled Nafion film. The same electrochemical behaviour was also observed in Na+- and H+-Nafion film. The effects of different concentrations of dyes in the Nafion films were also studied in order to understand the influence of the polymer film structure on the electrochemical properties of phenothiazine dyes. Electrodes modified with the perfluorosulfonate membrane, with low charge (e.g.ferrocene) gave an additional redox Nafion, have received much attention, owing to its outstand- wave. However, highly charged molecules (e.g. [Ru(bpy): '3)ing chemical and thermal stability.' An electrode surface did not give any additional oxidation or reduction waves. '' modified with Nafion film containing electroactive species There are only a few reports in the literature on the elec- exhibits interesting electr~catalytic~.~ and photo~hemical~,~ trochemical properties of dye molecules incorporated into properties. In the last decade, much effort has been devoted to elucidate the mechanisms of diffusional charge transport in Nafion polymer films at electrode surfaces.'*' Nafion membrane provides a unique solid matrix contain- ing a fluorocarbon backbone with pendant chains terminated with sulfonate head groups which are responsible for its ion- exchange and swelling proper tie^.^,^ The cluster network model of Gierke and Hsu' described the ionic regions of Nafion film as spherical with interconnecting channels. Later Yeager and Steck" proposed a three-phase structure, i.e., a hydrophobic fluorocarbon phase, a -SO; ion cluster region and an interfacial region between these two (Fig.1). The effects of the water uptake characteristics of the Nafion film,'' curing humidity," and variations in the properties of Nafion due to different preparation condition^'^ have also been subjects of recent interest. The first direct evidence for the existence of multiple-phase structure in a Nafion film was reported by R~binstein'~ using cyclic voltammetric experi- ments, This was later supported by further evidence from Vining and Meyer" and Harth et a/.'' that uncharged mole- cules gave an additional oxidation wave, whereas molecules Fig.1 Schematic structure of Nafion polymer film: H, hydrophobic fluorocarbon phase; C, hydrophilic -SO; ionic cluster region; I, interfacial region; g,SO; group of the polymer Nafion-film-coated electrodes."-19 Studies were mainly carried out to investigate the charge-transport properties of the dyes and also to elucidate the dimerization kinetics of the dye molecules in Nafion film^.'^.'^ Even though electro-chemical behaviour such as the transport properties and absorption spectral characteristics of thionine (TH +) and methylene blue (MB') at Nafion-coated electrodes has been investigated, the influence of Nafion-film structure on these dyes has not yet been studied.These dye-modified electrodes found applications in solar energy conversion, and electro- chromic and photochromic devices.20-22 In the present paper, we report the observation of an unusual redox wave for thionine (TH') and methylene blue (MB') dyes incorpor- ated into Nafion-film-coated electrodes in 0.05 mol dm-3 H2S0,. Studies were also carried out using Na+- and H+- Nafion films and different dye concentrations in the Nafion film. Experimental Materials A 5% solution of Nafion 117 (Aldrich, 1100 EW) was diluted to 1-2% with methanol before use.Thionine and methylene blue (Aldrich) were purified by chromatographic methods.23 All other chemicals were of analytical grade and used as received. Apparatus and Methods A three-electrode cell was used with 1 cm2 platinum plates as working and counter electrodes and a saturated calomel elec- trode (SCE) as the reference. Cyclic voltammograms were run using an (EG&G) PAR 273A potentiostatlgalvanostat equipped with an RE0151 recorder. The surface coverage (r)was determined using a coulometric method by measuring the charge required in a potential-step experiment to reduce the dye molecules q~antitatively.~~ The diffusion coefficients were determined by chronoamperometry.25 Nafion films were prepared by casting a known volume of 1% or 2% Nafion solution onto a 1 cm2 platinum plate and the solvent was evaporated from the surface of the electrode at room temperature for 10 min.Then, the Nafion-coated electrode was washed with distilled water and stored in dis- tilled water for 30 min. The thickness of the Nafion film was calculated from the amount of Nafion present in the solution and the density of Nafion (2 g ~rn-~).’~,” It has been reported” that the Nafion film thickness did not change sig- nificantly when the film was immersed in 0.2 mol dm-3 Na,SO, for 2 h. The Na+-Nafion film was prepared by soaking the Nafion-coated electrode in dilute NaOH solution and the H+-Nafion film by soaking in dilute H2S04.26*27 The dye-incorporated Nafion-film-coated electrodes were prepared by dipping the Nafion-coated electrode in a solu- tion containing a known concentration of dye.The solutions were purged with nitrogen for 30 min before each experiment to exclude dissolved oxygen. Results and Discussion Recently, the influence of Nafion-film structure with hydro- philic and hydrophobic environments on molecules of low charge was reported.16 Such an effect was not noticed in the case of phenothiazine dyes under the experimental conditions employed in previous studies.’ 7-19 Earlier studies of TH’ and MB+ incorporated into Nafion films in 0.5 mol dmd3 H2S04 have shown a well defined reversible redox wave in the potential region of +0.4-0.0 V us. SCE.17-” Cyclic voltammograms recorded for the Nafion-coated electrode dipped into a mixture containing TH+ and 0.05 mol dm-3 H2S04 are shown in Fig.2(a). In the first cycle, TH’ in the Nafion film gave two well defined redox waves at 0.16 and 0.39 V us. SCE. The latter wave is unusual for TH’ in Nafion films.” Upon continuous cycling, the wave at 0.39 V us. SCE decayed and that at 0.16 V us. SCE increased [Fig. 2(a)]. When the Nafion-coated electrode was dipped into a mixture containing TH’ and 0.5 mol dm-3 H2S04, the redox wave at 0.39 V us. SCE disappeared during the first cycle and a wave at 0.18 V us. SCE appeared, as shown in Fig. 2(c). Very similar behaviour was also observed for MB+ using Nafion-coated electrodes [Fig. 2(b) and (43.The cyclic voltammogram recorded for a Nafion-coated electrode dipped into a mixture containing MB+ and 0.05 mol dm-3 H2S04 showed two redox waves at 0.14 and 0.40 V us.SCE [Fig. 2(b)]. When 0.5 mol dm-3 H2S04 was used as support- ing electrolyte, only the usual redox wave was observed at 0.18 V us. SCE [Fig. 2(4]. I I J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 In the case of ferrocene, the cyclic voltammograms showed an unusual wave, in addition to the usual redox wave.l4.l6 It was suggested that ferrocene (Fc) and ferrocenium (Fc’) (a neutral molecule and an ion of low charge, respectively) occupy different domains, such as the sulfonate ionic cluster region and an interfacial region existing between the hydro- phobic fluorocarbon domain and the ionic cluster region.However, in the present study the TH’ and MB’ molecules possess one positive charge and upon two-electron reduction these dye molecules form leuco-dyes with one positive charge [eqn. (1)and (2)].’79’9>28 TH+ + 2H’ + 2e--,TH; (1) MB+ + 2H+ + 2e-+MBH; (2) These dye molecules are hydrophobic in nature and upon adsorption by Nafion film in the presence of 0.05 mol dmd3 H2S04, might occupy both the ionic cluster and interfacial regions, as in the case of ferrocene molecules,’6 to give two redox waves, as inferred in Fig. 2(a) and (b). Furthermore, upon continuous cycling, the dye molecules present in the interfacial region might be transported to the ionic cluster region, owing to the involvement of H+ ions in the redox reaction [eqn.(1) and (2)J Hence the unusual redox wave decreases and the usual redox wave increases. The unusual redox waves obtained for TH+ and MB+ are assigned to molecules adsorbed by the interfacial region of the Nafion film. Upon increasing the acid concentration, the dye mol- ecules are adsorbed by the well solvated ionic cluster region of the Nafion film and the unusual redox waves are no longer observed. The cyclic voltammograms obtained for MB’ incorpor-ated into Na+- and H+-Nafion-coated electrodes in 0.05 mol dmd3H2S04 exhibit two redox waves as shown in Fig. 3(a) and 3(b). The corresponding electrochemical data are given in Table 1. The amounts of electroactive species for the two redox reactions were obtained coulometrically in the first cycle by stepping the potential from +0.7 to +0.3 V and from +0.3 to -0.1 V us.SCE. The amount of electroactive species in 0.5 mol dm-3 H2S0, is obtained by stepping the potential from +0.7 to -0.1 V us. SCE. The corresponding electrochemical data are given in Table 2. The amount of electroactive TH’ and MB’ species in the H+-Nafion film is higher than that in the Na+-Nafion film. Note that it has already been reported that lower amounts of electroactive h (b, d :d I I I I I I~ I 7 T -T 1-1 0.7 0.5 0.3 0.1 017 0.5 0.3 0.1 E/V vs. SCE Fig. 2 Continuous cyclic voltammograms recorded at Nafion-coated electrodes immersed in 0.05 mol dm-3 H2S04 with (a) TH+, (6) MB +. Cyclic voltammograms recorded at Nafion-coated elec- trodes immersed in 0.5 rnol dmd3 H2S04 with (c) TH+, (d) MB+.Thickness of the Nafion film is 2.5 pm. 0.7 V 150 PA 0.7 v 100 mV EfV vs. SCE Fig. 3 Cyclic voltammograms of MB+ incorporated into (a) Na+-Nafion film and (b) H+-Nafion film in 0.05 rnol dm-3 H,SO,; (c) Na+-Nafion film and (d) H+-Nafion film in 0.5 rnol dm-3 H,SO,. Thickness of the Nafion film is 2.5 pm. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 1 Electrochemical data for TH' and MB' dyes incorporated into Nafion-film-coated electrodes in 0.05 rnol dm-3 H2S04 (thickness of Nafion film, 2.5 pm) electrode system 1.2 3 9 T(E2)/109rnol cm-2 DaPp(E2)/1O8cm2 s-' E~JVus. SCE T(E1)/108rnol cm-' E:,/V us. SCE D,pp(E1)/lOBcm2 s-l 4.15 6.32 0.36 1.14 0.18 1.13 6.22 9.82 0.36 0.83 0.19 3.43 1.30 5.63 0.35 1.86 0.13 1.12 5.18 8.72 0.38 0.83 0.14 2.25 I, Pt/H+-Nf/TH+;5 Pt/Na+-Nf/TH+;3, Pt/H+-Nf/MB+;& Pt/Na+-Nf/MB+.El, usual redox wave; E2, unusual redox wave. species are adsorbed by Na+-Nafion than by H+-Nafi~n;~' this is reflected in Table 2. The diffusion coefficients (D,,,), or charge transport, of TH+ and MB+ incorporated into the H+-Nafion and Na+- Nafion films were measured in 0.05 and 0.5 mol dm-3 H2S04 at different dye concentrations in the film, and the data are given in Tables 1 and 2, respectively. To determine the diffusion coefficients for dyes at Nafion films by chrono- amperometric experiments, the potential was stepped from +0.7 to +0.3 V and from +0.3 to -0.1 V us.SCE in 0.05 mol dm-3 H,S04, and for 0.5 mol dm-3 H2S04 the poten- tial was stepped from +0.7 to -0.1 V us. SCE. The diffusion coefficients for the dyes in the Na+-Nafion film are much higher than those for the dyes in the H+-Nafion film. This is because the concentration of the electroactive species in the Na+-Nafion film is lower than that in the H+-Nafion film. It has already been reported" that the diffusion coefficients decrease with increasing concentration of dyes in the film. The total amount of electroactive species determined for the two redox reactions in 0.05 mol dm-3 H2S04 is almost equal to the amount of electroactive dye molecules measured in 0.5 mol dm-3 H2S04.This observation shows that the electro- active dye molecules adsorbed by the interfacial region are transported immediately to the ionic cluster region at higher acid concentrations.The concentration of MB+ adsorbed by the Nafion film was varied from 1.30 x lo-' mol cm-2 to 1.14 x lo-* mol cm-2 and cyclic voltammograms in 0.05 mol dm-3 H2S04 were recorded and are shown in Fig. 4(a) and (b).When the amount of MB+ adsorbed in the Nafion film is low (1.30 x lo-' mol crn-'), a less intense unusual redox wave appeared. At higher concentrations of MB+ (1.14 x lo-* mol cm-2), the unusual redox wave was more intense. These observations show that at lower concentrations of MB', the dye molecules are mostly adsorbed by the sulfonate ionic cluster regions. At higher concentrations of MB +,the dye molecules are adsorbed by the interfacial region as well as the ionic cluster region of the swollen Nafion film.The swollen Nafion film consists of solvated -SO, head groups and spherical counter-ion-solvent clusters of 40 A diameter, resembling a reverse micellar structure with a hydrophobic fluorocarbon chain.".' 1*30*31 Since the inter- facial region contains higher concentrations of fluorocarbon chain, the interphase is more hydrophobic and the interfacial Table 2 Electrochemical data for TH' and MB' dyes incorporated into Nafion-film-coated electrodes in 0.05 rnol dmP3 H2S04 (thickness of Nafion film, 2.5 pm) electrode system' r/108 mol cm-2 Ep,/V us. SCE DaPd1O8cm2 s-l 1. 2 1.25 0.98 0.25 0.24 1.35 3.84 3 1.89 0.19 1.98 4 1.04 0.19 3.80 a As for Table 1.A .o LI 0.7 E/V vs. SCE Fig. 4 Cyclic voltammograms of MB+ incorporated into a Nafion film of thickness 2.5 pm in 0.05 rnol dm-3 H2S04: (a) r = 1.30 x mol cm-2 and (b)r = 1.14 x rnol cm-' region has a lower void volume than the cluster region.30 In the present study, in 0.05 rnol dm-3 H,SO, the dye mol- ecules enter into the film and preferentially reside in the higher void volume of the cluster region to give the usual redox wave. At increased concentrations, the dye molecules also enter into the lower void volume of the interfacial region to give the unusual redox wave. The ionic cluster region is well solvated at higher acid concentrations.Under these conditions the dye molecules reside only in the well solvated ionic cluster region to give the usual redox wave. 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