Electrochemical and Spectroelectrochemical Studies of Phenothiazine

Phenothiazine Dyes Immobilized in Nafion Film. S. Abraham John and R. Ramaraj*. School of Chemistry, Madurai Kamaraj University, Madurai-625 021, Indi...
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Langmuir 1996, 12, 5689-5695

5689

Electrochemical and Spectroelectrochemical Studies of Phenothiazine Dyes Immobilized in Nafion Film S. Abraham John and R. Ramaraj* School of Chemistry, Madurai Kamaraj University, Madurai-625 021, India Received November 22, 1995. In Final Form: July 24, 1996X The absorption of phenothiazine dyes, thionine (TH+), azure-A+ (Az-A+), and methylene blue (MB+) into Nafion film was studied, and the standard free energy of hydrophobic interaction (∆G°h) was calculated for the dyes. Electrochemical and spectroelectrochemical studies of these dyes in Nafion film were also carried out to understand the electrochemical behavior and their interaction with the Nafion film. TH+ and Az-A+ showed different electrochemical behavior in Nafion film when compared to aqueous solution. The hydrophobicity of reduced dye molecules was found to influence the electrochemical properties of the dye molecules in Nafion film. The spectroelectrochemical studies of dyes in Nafion film showed complete reduction of monomer, dimer, and protonated dye molecules. Upon the oxidation of reduced dye molecules in Nafion film, dimer and protonated dyes were formed.

Introduction Nafion, a perfluorosulfonate ion-exchange polymer, has been used extensively in preparing chemically modified electrodes and ion-exchange membranes.1 The unique properties of Nafion membrane are (i) the outstanding chemical and thermal stabilities, (ii) preconcentrating the electroactive and photoactive cations even from dilute solution, and (iii) the multiphase structure consisting of a fluorocarbon hydrophobic phase, a hydrophilic ionic cluster, and interfacial regions (Figure 1). The cationic molecules are exchanged into the Nafion film by both electrostatic and hydrophobic interactions due to the presence of sulfonate head groups (-SO3-) and a fluorocarbon chain.2-4 A wide variety of applications of Nafion have been reported involving immobilization of electroactive and photoactive molecules into the Nafion film.5-8 It is also used in chlor-alkali cells,9 fuel cells,10 and batteries.11 The strong acidic environment provided by the protonated form of Nafion membrane is used to carry out organic rearrangements and isomerizations.12,13 Numerous investigations were also carried out to understand the rates and * To whom correspondence should be addressed. Fax: +91 452 859139. E-mail: ramaraj%[email protected]. Telephone: +91 452 859146. X Abstract published in Advance ACS Abstracts, October 15, 1996. (1) (a) Yeager, H. L.; Steck, A. J. Electrochem. Soc. 1981, 28, 1880. (b) Eisenberg, A., Yeager, H. L., Eds. Perfluorinated Ionomer Membranes; ACS Symposium Series 180; American Chemical Society: Washington, DC, 1982. (c) Rubinstein, I. In Applied Polymer Analysis and Characterization; Mitchell, J., Jr., Ed.; Hauser: Munnich, 1991; Vol. II. (d) Martin, C. R.; Rubinstein, I.; Bard, A. J. J. Am. Chem. Soc. 1982, 104, 4817. (e) Buttry, D. A.; Saveant, J. M.; Anson, F. C. J. Phys. Chem. 1984, 88, 3086. (f) Rubinstein, I. J. Electroanal. Chem. 1985, 188, 227. (g) Vining, W. J.; Meyer, T. J. J. Electroanal. Chem. 1987, 237, 191. (h) Whiteley, L. D.; Martin, C. R. J. Phys. Chem. 1989, 93, 4650. (2) Safranji, A.; Gershuni, S.; Rabani, J. Langmuir 1993, 9, 3676. (3) Martin, C. R.; Dollard, K. J. J. Electroanal. Chem. 1983, 159, 127. (4) Szentirmay, N. M.; Martin, C. R. Anal. Chem. 1984, 56, 1898. (5) Harth, R.; Mar, U.; Ozer, D.; Bettelheim, A. J. Electrochem. Soc. 1989, 136, 3863. (6) Gobi, K. V.; Ramaraj, R. J. Electroanal. Chem. 1994, 368, 77. (7) Anson, F. C.; Tsou, Y.; Saveant, J. M. J. Electroanal. Chem. 1984, 178, 1134. (8) Fan, F.-R.; Liu, H.-Y.; Bard, A. J. J. Phys. Chem. 1985, 89, 4418. (9) Grot, W. Chem. Ing. Tech. 1978, 50, 299. (10) Laconti, A. B.; Fragal, A. R.; Boyack, J. R. Proc. Electrochem. Soc. 1977, 77, 354. (11) Yeo, R. S.; McBreen, J.; Kissel, G.; Kulesa, F.; Srinivasan, S. J. Appl. Electrochem. 1980, 10, 741. (12) Prakash, G. K. S.; Olah, G. A. In Acid-Base Catalysis; Tanabe, K., Hattori, H., Yamaguchi, T., Taaka, T., Eds.; Kodausha: Tokyo, 1989. (13) Childs, R. F.; Mika-Gibala, A. J. Org. Chem. 1982, 47, 4204.

S0743-7463(95)01066-3 CCC: $12.00

Figure 1. Schematic structure of Nafion film: H ) hydrophobic fluorocarbon region; C ) hydrophilic ionic cluster region; I ) interfacial region; circled minus sign ) -SO3- group of the polymer.

mechanisms of charge transport in the Nafion films.3,14-16 Both physical diffusion and electron transfer diffusion were involved in the charge transfer diffusional process within the Nafion film. The diffusional mode of the cation [(trimethylammonio)methyl]ferrocene (CpFeTMA+) was almost exclusively physical14 whereas tris(2,2′-bipyridine)ruthenium(II) ([Ru(bpy)3]2+) mostly involves electron transfer.15 Efforts were also taken to improve the quality and mechanical strength of the Nafion film.17 The procedure involves the replacement of an ethanol-water mixture from the Nafion solution with a high-boiling solvent followed by removal of solvent at an elevated temperature. Recently, the influence of water content on the photophysical and electrochemical properties of cationic molecules incorporated into the Nafion film has been reported.18-21 (14) White, H. S.; Leddy, J.; Bard, A. J. J. Am. Chem. Soc. 1982, 104, 4811. (15) Buttry, D. A.; Anson, F. C. J. Am. Chem. Soc. 1983, 105, 685. (16) Sharp, M.; Lindholm, B.; Eva-Lotta, L. J. Electroanal. Chem. 1989, 274, 35. (17) Moore, R. B.; Martin, C. R. Macromolecules 1988, 21, 1334. (18) Pourcelly, G.; Oikonomou, A.; Gavach, C.; Hurwitz, K. A. J. Electroanal. Chem. 1990, 287, 43. (19) Striebel, K. A.; Scherer, G. G.; Haas, O. J. Electroanal. Chem. 1991, 304, 289. (20) Lin, R. J.; Onikubo, T.; Nagai, K.; Kaneko, M. J. Electroanal. Chem. 1993, 348, 189 and references cited therein. (21) Porat, Z.; Rubinstein, I.; Zinger, B. J. Electrochem. Soc. 1993, 140, 2501.

© 1996 American Chemical Society

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Studies on the photophysical, electrochemical, and photoelectrochemical properties of phenazine and phenothiazine dyes incorporated into Nafion film were reported.22-27 These dyes find applications in solar energy conversion systems,28-31 electrochromism,32,33 and oxidation of NADH.34,35 The earlier studies on phenothiazine dyes in Nafion film were mainly focused on the dimerization kinetics and diffusional properties.22,23,25 In the present investigation, the absorption, electrochemical, and spectroelectrochemical studies of phenothiazine dyes in Nafion film were carried out. It has been found that the interaction of reduced dye molecules with the hydrophobic fluorocarbon chain material of the Nafion film influences the electrochemistry of the phenothiazine dyes. Experimental Section A 5% Nafion solution (Aldrich, EW 1100, dissolved in a mixture of lower aliphatic alcohol and water) was diluted to 1% with ethanol. The dyes, thionine (TH+), azure-A (Az-A+), and methylene blue (MB+) were purified by chromatographic methods.36,37 All other chemicals were of analytical grade and used without further purification. The solution-processed Nafion solution was prepared by adding dimethyl sulfoxide to the 1% Nafion solution and heated at elevated temperature to remove ethanol and water.17 The recast Nafion film (RC-Nf) was prepared by using Nafion in ethanol. Solution-processed Nafion solution was used to prepare solution-processed Nafion film (SPNf). For absorption spectral and adsorption studies, RC-Nf or SPNf was prepared by casting 6 µL of 1% Nafion on a quartz glass plate and dried in air at room temperature for 30 min. The film thickness was calculated as 0.30 µm using a density of 2 g/cm3 for dry Nafion film.1d The Nafion film prepared on a quartz glass plate was dipped in a dye solution and then washed after dye absorption. The H+-Nf and Na+-Nf films were prepared by soaking the Nafion film in 0.1 M H2SO4 and 0.1 M NaOH for 30 min, respectively.24 For electrochemical experiments, RC-Nf or SP-Nf was prepared by casting 9 µL of 1% Nafion solution onto a 1 cm2 platinum plate (Pt) and dried at room temperature for 5 min. Then the Nafioncoated Pt plate was washed and kept in distilled water for 30 min. The thickness of the Nafion film was calculated as 0.56 µm using a density of 1.58 g/cm3 for the wet film.3 The electrode was dipped in a known concentration of dye solution for 3-5 min. Then the electrode was washed and dipped in distilled water and used for electrochemical experiments. The dry Nafion film was prepared by evaporating the solvent from the cast solution for 30 min and was not soaked in water. A three-electrode cell was used with a dye-incorporated Nafioncoated 1 cm2 Pt plate (represented as Pt/Nf/dye) as working electrode, a 1 cm2 Pt plate as counter electrode, and a saturated calomel electrode (SCE) as reference. Cyclic voltammograms (22) Kuwabata, S.; Nakamura, J.; Yoneyama, H. J. Electroanal. Chem. 1989, 261, 363. (23) Guadalupe, A. R.; Liu, K. E.; Abruna, H. D. Electrochim. Acta 1991, 36, 881. (24) Gopidas, K. R.; Kamat, P. V. J. Phys. Chem. 1990, 94, 4723. (25) Mika, A. M.; Lorenz, K.; Szczurek, A. J. Membr. Sci. 1989, 41, 163. (26) John, S. A.; Ramaraj, R. J. Chem. Soc., Faraday Trans. 1994, 90, 1241. (27) John, S. A.; Gobi, K. V.; Ramasubbu, A.; Ramaraj, R. Res. Chem. Intermed. 1992, 18, 203. (28) Rabinowitch, E.; Epstein, L. F. J. Chem. Phys. 1940, 8, 551. (29) Clark, W. D. K.; Eckert, J. Sol. Energy 1975, 17, 147. (30) Albery, W. J.; Foulds, A. W.; Hall, J.; Hillmann, A. R.; Egdell, R. G.; Orchard, A. F. Nature 1979, 282, 793. (31) Tamilarasan, R.; Natarajan, P. Nature 1981, 292, 1002. (32) Kuwabata, S.; Mitsui, K.; Yoneyama, H. J. Electroanal. Chem. 1990, 281, 97. (33) Lenza, R. O.; Junato, S.; Zagal, J. H. J. Electroanal. Chem. 1995, 389, 197. (34) Ohsaka, T.; Tanaka, K.; Tokuda, K. J. Chem. Soc., Chem. Commun. 1993, 222. (35) Tanaka, K.; Tokuda, K.; Ohsaka, T. J. Chem. Soc., Chem. Commun. 1993, 1770. (36) Bergmann, K.; Okonski, C. T. J. Phys. Chem. 1963, 67, 2169. (37) Kamat, P. V.; Lichtin, N. N. J. Phys. Chem. 1981, 85, 814.

John and Ramaraj were recorded on a EG & G PAR 273A potentiostat/galvanostat equipped with an RE0151 recorder. The surface coverage (Γ) was determined by a coulometry method.38 The apparent diffusion coefficient, Dapp, was determined by chronoamperometry plots of i vs t-1/2.1d The absorption spectral measurements were made on a JASCO Model 7800 UV-visible spectrophotometer. For spectroelectrochemical studies, the 1% Nafion solution was coated on a 1 cm2 ITO (indium tin oxide) plate and dried at room temperature and then dipped in distilled water for 30 min. The Nafion-coated ITO electrode was dipped in the dye solution for 3-5 min, and after absorption of the dye, the electrode was washed with distilled water before use (represented as ITO/Nf/ dye). The absorption spectra were recorded in situ during the potential sweep using an Otsuka Electronics IMUC-7000 diode array multichannel detection system, as described elsewhere.20 The electrochemical cell used for spectroelectrochemical studies was a quartz cell equipped with an ITO/Nf/dye electrode as working electrode, Pt wire as counter electrode, and a SCE as reference electrode. The solutions for electrochemical and spectroelectrochemical studies were purged with nitrogen for 30 min before each experiment.

Results and Discussion Kinetics of Absorption of Dyes into RC-Nf and SPNf. The kinetics of absorption of dyes into RC-Nf and SP-Nf were studied in 1-3 min (eq 1).39

Ft )

4(Dt)1/2 π1/2l

(1)

where Ft is the fractional attainment (Qt/Q∞), Qt is the concentration of dye at time t, Q∞ is the maximum saturation concentration, D is the diffusion coefficient, and l is the thickness of the Nafion film. When the Nafion film was dipped in an aqueous solution of dye, the absorbance of the dye in aqueous solution decreased with time, confirming the incorporation of dye into the Nafion film. Plots of Ft vs t1/2 obtained for the absorption of dyes into Nafion films in 1-3 minutes showed straight lines, confirming the absorption of dyes into RC-Nf and SP-Nf is a diffusion-controlled process. The diffusion coefficient (D) was determined from the slope of the plot of Ft vs t1/2. The D values calculated for TH+, Az-A+, and MB+ dyes in RC-H+-Nf are 1.3 × 10-11, 2.0 × 10-11, and 4.5 × 10-11 cm2/s, and those in SP-H+-Nf are 1.0 × 10-11, 1.3 × 10-11, and 2.6 × 10-11 cm2/s. The D values of TH+, Az-A+, and MB+ in RC-Na+-Nf are 2.8 × 10-11, 4.3 × 10-11, and 9.5 × 10-11 cm2/s, and those in SP-Na+-Nf are 2.0 × 10-11, 4.0 × 10-11, and 8.3 × 10-11 cm2/s. The D values observed for the phenothiazine dyes in Nafion film are comparable to those reported for rhodamine 6G39 and 7-amino-4-methylcoumarine (C-120) laser dye40 in Nafion film. The kinetics of absorption of phenothiazine dyes by Nafion film revealed that the dye uptake was slow and diffusion controlled. Exchange of Phenothiazine Dyes in Nafion Film with Monovalent Metal Ion. The exchange of monovalent metal ion (K+) in the presence of phenothiazine dyes was studied (eqs 2 and 3). K

D+b + M+Nf y\z D+Nf + M+b

(2)

K ) ([D+]Nf[M+]b)/([D+]b[M+]Nf)

(3)

The specificity constant K was determined by the following method.2 The concentration of D+ in Nafion film ([D+]Nf) (38) Oyama, N.; Anson, F. C. J. Electrochem. Soc. 1980, 127, 640. (39) Mohan, H.; Iyer, R. M. J. Chem. Soc., Faraday Trans. 1992, 88, 41. (40) Nandan, D.; Mohan, H.; Iyer, R. M. New Developments in Ion Exchange; Kodansha Ltd.: Tokyo, 1991; p 491.

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was calculated from the differences between the initial (([D+]b)ini) and equilibrium concentrations ([D+]eq) (eq 4).

[D+]Nf ) V/v([PS+]b)ini - ([PS+])eq

(4)

where V and v are the volumes of solution and Nafion film, respectively. [D+]b was measured spectrophotometrically before and after equilibration with Nafion film (eq 5).

[D+]b ) ([D+]b)ini - (v/V)[D+]Nf

(5)

[M+]b was calculated by eq 6.

[M+]b ) [M+]ini - (v/V)[M+]Nf

(6)

Figure 2. Structure of phenothiazine dyes: (A) thionine; (B) azure-A; (C) methylene blue.

[M+]Nf was calculated from the difference between the saturation concentration of D+ in Nafion film and [D+]Nf (eq 7).

[M+]Nf ) [D+]Nfsat - [D+]Nf

(7)

The saturation concentration of dyes in RC-Nf was measured by equilibrating the Nafion film with dye solution in the absence of monovalent metal ions. The saturation concentrations of TH+, Az-A+, and MB+ are 1.20 ( 0.05, 1.40 ( 0.05, and 1.62 ( 0.05 M, respectively. We will assume that the free energy of interaction of dyes with Nafion film can be separated into electrostatic and hydrophobic components. Furthermore, it is assumed that inorganic ions have only electrostatic interaction and that the electrostatic interaction remains constant when exchanging monovalent inorganic ions for monovalent organic cations.2 Thus the standard free energy of the hydrophobic interaction of dyes with Nafion will, with the above assumptions, be given by eq 8.

∆G°h(D+) ) -RT ln K

(8)

The calculated ∆G°h values for TH+, Az-A+, and MB+ in RC-Nf based on K+ ion are -5.02 ( 0.01, -5.10 ( 0.01, and -5.23 ( 0.01 kcal/mol. The ∆G°h values obtained for dyes in Nafion film show that the dyes are having not only electrostatic interaction but also hydrophobic interaction with the Nafion film. Similar studies were also carried out using SP-Nf, and the calculated ∆G°h values for dyes based on K+ are -5.04 ( 0.01, -5.10 ( 0.01, and -5.22 ( 0.01 kcal/mol for TH+, Az-A+, and MB+, respectively. The similar ∆G°h values obtained for dyes in SPNf as in the case of RC-Nf show the electrostatic and hydrophobic interactions of dyes with SP-Nf are almost the same as in the case of RC-Nf. The ∆G°h values obtained for TH+, Az-A+, and MB+ show that MB+ is more hydrophobic in nature and interacts comparatively more strongly with Nafion film than the other two dyes. Absorption Spectral Studies of SP-Nf/Dyes. The structures of the phenothiazine dyes TH+, Az-A+, and MB+ used in this investigation are shown in Figure 2. The very dilute aqueous solutions of the dyes (1 × 10-5 M), a blue-shifted shoulder band was observed due to dimer dyes. In strong acidic solution, a new red-shifted band was observed for protonated dyes. The absorption spectra recorded for MB+ absorbed into wet SP-H+-Nf and at different drying times at room (41) Rabinowitch, E.; Epstein, L. F. J. Am. Chem. Soc. 1941, 63, 69. (42) Spencer, W.; Sutter, J. R. J. Phys. Chem. 1979, 83, 1573.

Figure 3. Absorption spectra recorded for wet H+-SP-Nf/MB+ at different drying times: (a) 0 min; (b) 5 min; (c) 10 min.

temperature are shown in Figure 3. Under set conditions, MB+ absorbed into SP-H+-Nf showed an absorption band at 650 nm for monomer MB+ along with a band at 745 nm for protonated MB+ and a shoulder band at 590 nm for dimer MB+. When the wet SP-H+-Nf/MB+ is slowly dried, the absorption band due to monomer MB+ at 650 nm gradually decreased with a simultaneous increase in the absorbance at 745 nm due to protonated MB+. It has been already reported that dry H+-Nf provides a strong acidic environment to the incorporated molecules.43 Due to the strong acidity of H+-Nf under the dry conditions, the dye molecules undergo protonation. Similar absorption spectral changes were also observed for dyes in RCH+-Nf. The absorption spectral changes recorded for the wet SP-Na+-Nf/TH+ with different drying times at room temperature are shown in Figure 4. Under the wet conditions, the absorption spectrum showed two bands at 595 and 554 nm due to monomer and dimer TH+, respectively. When the wet SP-Na+-Nf/TH+ film is slowly dried, the absorption band due to dimer TH+ increased with a simultaneous decrease in the absorbance due to monomer TH+. Similar spectral changes were also observed for Az-A+ and MB+ dyes in wet SP-Na+-Nf. The (43) Weir, D.; Scaiano, J. C. Tetrahedron 1987, 43, 1617.

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Figure 6. Plots of anodic and cathodic peak currents against the square root of the scan rate: (A) RC-Nf/TH+; (B) RC-Nf/ Az-A+; (C) RC-Nf/MB+. Table 1. Electrochemical Data of Phenothiazine Dyes Incorporated into Wet RC-Nf Films

Figure 4. Absorption spectra recorded for wet Na+-SP-Nf/ TH+ at different drying times: (a) 0 min; (b) 5 min; (c) 10 min.

dye TH+ Az-A+ MB+ a

Figure 5. Cyclic voltammograms of (A) TH+, (B) Az-A+, and (C) MB+ at a plain Pt electrode and of (D) Pt/RC-Nf/TH+, (E) Pt/RC-Nf/Az-A+, and (F) Pt/RC-Nf/MB+ electrodes in 0.5 M H2SO4. Current range: (A-C) 50 µA; (D-F) 100 µA. Scan rate ) 50 mV/s.

dyes absorbed into RC-Na+-Nf also showed very similar absorption spectral changes. Under the wet conditions, the presence of water inside the Nafion film increases the volumes of the ionic cluster and the interfacial regions of the Nafion film,20 which favors the formation of monomer dye. In dried SP-Na+-Nf the water content is less and the volumes of the ionic cluster and interfacial regions become narrow. This leads to a higher local concentration of dye in the shrank interfacial and ionic cluster regions and pushes the monomer-dimer equilibrium toward the dimer dye. Electrochemistry of Phenothiazine Dyes in RCNf. The cyclic voltammograms recorded for TH+, Az-A+, and MB+ in 0.5 M H2SO4 using a plain Pt electrode are shown in Figure 5A-C. All the dyes undergo a twoelectron redox process.44,45 The linear plots of peak currents against the square root of the scan rate showed

ipa/ipca

Dappox (109 cm2/s)

Dappred (109 cm2/s)

Dappox/Dappred

1.42 1.03 0.68

1.7 2.0 3.8

2.4 2.3 3.2

0.70 0.86 1.18

Scan rate ) 50 mV/s.

that the electron transfer process was diffusion controlled. For these dyes, the anodic peak current was lower than the cathodic peak current at all scan rates. This has been attributed to the dimerization of dyes in solution during oxidation.44 The cyclic voltammograms recorded for TH+, Az-A+, and MB+ incorporated into RC-Nf dipped in 0.5 M H2SO4 are shown in Figure 5D-F, and the electrochemical data are given in Table 1. TH+ and Az-A+ showed different electrochemical behavior in Nafion film than in solution. TH+ showed a higher anodic peak current than cathodic peak current, and the ipa/ipc ratio is higher than 1 in Nafion film (Table 1). Az-A+ showed almost the same anodic and cathodic peak currents, and the ipa/ipc ratio is nearly 1. However, MB+ in Nafion film showed a very similar behavior as observed in solution; i.e., the ipa/ipc value is less than 1. This observation for Nf/MB+ has already been reported by Kuwabata et al.,22 and it has been discussed that the dimerization of MB+ in Nafion film during oxidation was the reason for the observed difference in anodic to cathodic peak current, as in the case of aqueous solution.44 If the dimerization was the only reason for the observed change in anodic and cathodic peak currents, it should also be reflected in Nf/TH+ and Nf/Az-A+. It has been recognized from the spectroelectrochemical studies that during oxidation of dyes in Nafion film, dimerization and also protonation occurred not only in Nf/MB+ but also in Nf/TH+ and Nf/Az-A+ (vide infra). The plots of peak currents against the square root of the scan rate for TH+, Az-A+, and MB+ in Nafion film are shown in Figure 6. Even though all the dyes showed linearity, the slopes of anodic to cathodic peak current differed very much. The ipa/ipc ratios and the apparent diffusion coefficients of oxidized dyes (Dappox) and reduced dyes (Dappred) calculated (44) Vetter, K. J.; Bardeleben, J. Z. Electrochem. 1957, 61, 135. (45) Murthy, A. S. N.; Reddy, K. S. J. Chem. Soc., Faraday Trans. 1 1984, 84, 1745.

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Figure 7. Cyclic voltammograms of (A) RC-Nf/TH+ and (B) RC-Nf/MB+ in 0.5 M H2SO4 at every 1 min interval: (a) 0 min; (b) 5 min. Scan rate ) 50 mV/s.

from the completely reduced dye and completely oxidized dye, respectively, in RC-Nf are given in Table 1. The ratio Dappox/Dappred differed for the dyes in RC-Nf (Table 1). The ratio Dappox/Dappred for TH+ is less than that for MB+. The lower value of Dappred than Dappox for MB+ was attributed to the interaction of the reduced form of MB+ with the hydrophobic interfacial region of the Nafion film.23 The diffusion coefficients calculated for the more highly charged (oxidized) form of the hydrophobic redox couples in Nafion film were always higher than the diffusion coefficient of the lower charge (reduced) molecules.3 This has been understood by the hydrophobic interaction of the molecule with the Nafion polymer chain material. The hydrophobicities of the reduced dyes observed from the peak currents and diffusion coefficients are in the order MB+ > Az-A+ > TH+. The hydrophilic and hydrophobic domains in the Nafion film are in close proximity,1a-c and hence both hydrophilic and hydrophobic interactions play an important role in determining the relative stability of the electroactive ions in Nafion film.46 It has been found that the interaction of the incorporated molecules with the hydrophobic region of the Nafion film influenced the formal potentials of metal complexes.47 In the case of phenothiazine dyes, the hydrophobicity of the reduced dye molecules played a major role in determining the electrochemical properties of these molecules in Nafion film. The reduced form of MB+ is more hydrophobic than Az-A+ and TH+, and it strongly interacts with the organic chain material of the Nafion film. This led to a larger cathodic peak current than the anodic peak current for MB+ in Nafion film. The reduced form of TH+ was less hydrophobic than the reduced MB+, and hence it comparatively weakly interacts with the hydrophobic organic chain material of the Nafion film. The interaction of dyes with the hydrophobic chain material is also understood by studying the stability of the reduced dyes in Nafion film. The cyclic voltammograms recorded by oxidative scan for TH+ and MB+ in RC-Nf dipped in 0.5 M H2SO4 at different time intervals are shown in Figure 7. During the oxidative scan, the initial potential was held at 0.0 V vs SCE. The anodic and cathodic peak currents observed for TH+ in Nafion film decreased on cycling from 0.0 to 0.5 V vs SCE at regular intervals of time, and leaching of reduced TH+ (46) Porat, Z.; Tricot, Y. M.; Rubinstein, I. J. Electroanal. Chem. 1991, 315, 217. (47) Tsou, Y. M.; Anson, F. C. J. Electrochem. Soc. 1984, 131, 595.

Figure 8. Cyclic voltammograms of wet (A) RC-Nf/Az-A+ and (B) SP-Nf/Az-A+ in 0.5 M H2SO4; scan rate ) 50 mV/s. Table 2. Electrochemical Data of Phenothiazine Dyes Incorporated into Wet RC-Nf and SP-Nfa nature of film

dye

RC-Nf

TH+

SP-Nf

Az-A+ MB+ TH+ Az-A+ MB+

ipcb (µA)

Dappred (109 cm2/s)

450 550 310 280 340 180

0.53 1.08 1.33 0.90 1.30 1.70

a Both RC-Nf/dye and SP-Nf/dye electrodes prepared under similar experimental conditions. b Scan rate ) 50 mV/s.

from the Nafion film was observed during the scan (Figure 7A). The leaching of TH+ into solution was confirmed by recording the absorption spectrum of the supporting electrolyte solution. However, in the case of RC-Nf/MB+, cycling at regular intervals of time from an applied potential of 0.0 V vs SCE for several minutes did not bring about leaching of reduced MB+ from Nafion film into solution (Figure 7B). It clearly showed that the interaction of the reduced MB+ with the hydrophobic chain material of the Nafion film stabilizes the reduced MB+ in the Nafion film. In the case of Az-A+, the leaching of dye was very much lower than that for TH+ during the cycling. These observations showed that the hydrophobicity of the reduced dye molecules influenced the electrochemistry of phenothiazine dyes in Nafion film. Effect of Film Preparation on the Electrochemical Behavior of Dyes in Nafion Film. The cyclic voltammograms recorded for Az-A+ in wet RC-Nf and SP-Nf dipped in 0.5 M H2SO4 are shown in Figure 8. The cathodic peak current and Dappred determined for TH+, Az-A+, and MB+ in wet RC-Nf and SP-Nf are given in Table 2. Wet RC-Nf and SP-Nf were dipped in the same concentration of Az-A+ under similar experimental conditions, and it was found that the absorption of Az-A+ in wet RC-Nf was much higher than that in wet SP-Nf. This observation was supported by the fact that the peak current observed for Az-A+ in wet RC-Nf was higher than that in wet SP-Nf (Figure 8 and Table 2). Moore and Martin17 studied the chemical and morphological properties of SP-Nf and RCNf and found that RC-Nf contains higher amounts of water than SP-Nf. Due to the presence of higher water content in dry RC-Nf than in dry SP-Nf, the volumes of the ionic

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John and Ramaraj

Figure 9. Plots of cathodic peak currents against the square root of the scan rate: (a) wet RC-Nf/Az-A+; (b) dry RC-Nf/AzA+; (c) wet SP-Nf/Az-A+; (d) dry SP-Nf/Az-A+.

cluster and interfacial regions are larger than those for SP-Nf and they absorbed a higher amount of Az-A+. Very similar electrochemical behavior was also observed for TH+ and MB+ in wet and dry RC-Nf and SP-Nf films. The plots of peak current against square root of scan rate for Az-A+ in wet and dry RC-Nf and SP-Nf are shown in Figure 9. The slopes of the plots of peak currents vs ν1/2 observed for Az-A+ in wet and dry RC-Nf are higher than those for wet and dry SP-Nf. This result showed that the volume of ionic cluster and interfacial regions in dry RC-Nf was still higher than that for wet SP-Nf and accommodates a higher amount of dye molecules inside RC-Nf. Also the solvent treatment minimizes reswelling of SP-Nf. The Dappred values calculated for the dyes in SP-Nf are higher than that of RC-Nf (Table 2), and this is due to the lower concentration23,48 of dyes in SP-Nf than in RC-Nf. Spectroelectrochemistry of Phenothiazine Dyes in Nafion Film. The spectroelectrochemical studies were carried out for Nf/dyes both by scanning and at potentiostatic condition in 0.5 M H2SO4. The absorption spectra recorded for a higher concentration of Az-A+ (0.022 M) in Nafion film during the scan from 0.7 to -0.3 V vs SCE at 1 mV/s are shown in Figure 10A. The absorption spectrum recorded at 0.7 V vs SCE showed three absorption bands at 595, 635, and 730 nm due to dimer, monomer, and protonated Az-A+, respectively (Figure 10A). During the potential scan from 0.5 to -0.3 V vs SCE, the absorbances due to monomer, dimer, and protonated Az-A+ decreased (Figure 10A). At -0.3 V vs SCE all the dye molecules underwent reduction and the absorption spectrum completely disappeared. During the reverse scan from -0.3 to 0.7 V vs SCE, the original absorption spectrum was fully recovered (Figure 10B). The spectroelectrochemical study was also carried out under potentiostatic condition using both higher and lower concentrations of Az-A+ in Nafion film. When a potential of -0.1 V vs SCE was applied to the ITO/Nf/Az-A+ (0.012 M) electrode, the absorption spectrum completely disappeared due to the complete reduction of Az-A+ in Nafion film. The absorption spectral changes observed for the same ITO/Nf/Az-A+ electrode at an applied potential of 0.6 V vs SCE are shown in Figure 11. After 150 s, the absorption bands observed at 630 and 730 nm due to the monomer and protonated Az-A+ and the shoulder band at 595 nm due to dimer were completely developed. These observations showed that the dimerization and protonation of Az-A+ occurred in Nafion film during oxidation (Figures 10 and 11). Similar spectroelectrochemical studies were also carried out for TH+ and MB+. The absorption spectral changes (48) Buttry, D. A.; Saveant, J. M.; Anson, F. C. J. Phys. Chem. 1984, 88, 3086.

Figure 10. Spectroelectrochemical changes of ITO/Nf/Az-A+ during the scan (A) from 0.7 to -0.3 V and (B) from -0.3 V to 0.7 V vs SCE at a scan rate of 1 mV/s. [H2SO4] ) 0.5 M.

Figure 11. Spectroelectrochemical changes of ITO/Nf/Az-A+ at an applied potential of 0.6 V vs SCE: [H2SO4] ) 0.5 M.

recorded for TH+ (0.016 M) in Nafion film at an applied potential of 0.0 V vs SCE in 0.5 M H2SO4 are shown in Figure 12. The absorption spectra initially showed three absorption bands at 560, 590, and 670 nm due to dimer, monomer, and protonated TH+, respectively, and at an applied potential of 0.0 V vs SCE the absorption spectrum almost disappeared due to reduction of TH+. When a potential of 0.5 V vs SCE was applied to the Nf/TH+ electrode, the original spectrum reappeared with a small decrease in the absorbance due to leaching of TH+. The spectroelectrochemical studies were carried out for MB+ (0.017 M) in Nafion film during the scan from 0.5 V to -0.3 V vs SCE at 1 mV/s (Figure 13). During the scan from 0.5 V to -0.3 V vs SCE, MB+ in Nafion film underwent reduction and the absorption spectrum disappeared at the end of the scan. The absorption spectrum was fully

Phenothiazine Dyes Immobilized in Nafion Film

Langmuir, Vol. 12, No. 23, 1996 5695

dimer, and protonated dye molecules in the Nafion film during the reductive scan are given in eqs 9-11

D+ + 2e- + 2H+ f DH2+

(9)

(D+)2 + 2e- + 2H+ f DH2+ + D+

(10)

DH2+ + 2e- + H+ f DH2+

(11)

where D+, (D+)2, and DH2+ are monomer, dimer, and protonated dye molecules and DH2+ is the leuco dye molecule. The overall reaction of eqs 9 and 10 is given in eq 12

(D+)2 + 4e- + 4H+ f 2DH2+ Figure 12. Spectroelectrochemical changes of ITO/Nf/TH+ at an applied potential of 0.0 V vs SCE: [H2SO4] ) 0.5 M.

(12)

The dimerization and protonation of dye molecules during the oxidative scan are given in eqs 13-15

DH2+ f D+ + 2e- + 2H+

(13)

D+ + D+ f (D+)2

(14)

D+ + H+ f DH2+

(15)

Conclusions

Figure 13. Spectroelectrochemical changes of ITO/Nf/MB+ during the potential scan from 0.5 to -0.3 V vs SCE at a scan rate of 1 mV/s: [H2SO4] ) 0.5 M.

recovered for MB+ during the oxidative scan from -0.3 V to 0.5 V vs SCE. This observation showed that the reduced MB+ in Nafion film was not leached. At an applied potential of 0.5 V vs SCE, all the dyes, TH+, Az-A+, and MB+ showed three absorption bands due to monomer, dimer, and protonated forms. When the potential was switched to -0.3 V vs SCE, all three bands due to monomer, dimer, and protonated forms disappeared due to complete reduction of dye molecules in Nafion film. It was observed from the spectroelectrochemical studies that the oxidation of dyes in Nafion film brings about dimerization and protonation. The reductions of monomer,

The calculated standard free energy of hydrophobic interaction of dyes with Nafion film (∆G°h) reveals that MB+ shows relatively more hydrophobic interaction than TH+ in Nafion film. The electrochemical data obtained for MB+ in Nafion film show that reduced MB+ strongly interacts with the hydrophobic region of the Nafion film and does not leach from the Nafion film, unlike TH+ and Az-A+, which do. The spectroelectrochemical changes observed for Nf/dye show that the monomer, dimer, and protonated dyes in Nafion film undergo reduction during the reductive scan. Upon oxidation, dimerization and protonation of dye molecules in the Nafion film were observed. Among the reduced dye molecules, MB+ strongly interacts with the hydrophobic region of the Nafion film and is stabilized. Acknowledgment. The financial support from the Department of Science and Technology and the Department of Atomic Energy is gratefully acknowledged. S.A.J. thanks the Council of Scientific and Industrial Research for a Senior Research Fellowship. We thank Professor Masao Kaneko, Ibaraki University, Japan, for the spectroelectrochemical studies. LA951066O