4914
Langmuir 1996, 12, 4914-4920
Electrochemical Transfer at Anionic Clay Modified Electrodes. Case of 2,2′-Azinobis(3-ethylbenzothiazoline-6-sulfonate) S. Therias,† C. Mousty,*,† C. Forano,‡ and J. P. Besse‡ Laboratoire d’Electrochimie Organique, URA 434, Universite´ Blaise Pascal, 63177 Aubie` re Cedex, France, and Laboratoire de Physico-Chimie des Mate´ riaux, URA 444, Universite´ Blaise Pascal, 63177 Aubie` re Cedex, France Received February 2, 1996. In Final Form: April 17, 1996X The electrochemical behavior of the 2,2′-azinobis(3-ethylbenzothiazoline-6-sulfonate) (ABTS), a reversible redox probe, has been examined after it has been intercalated into the interlayer spacing of anionic clays (also called layered double hydroxides, LDH). The cyclic voltammetry response of [Zn-Cr-ABTS] and [Zn-Al-ABTS] LDHs modified electrodes was compared to that obtained with ABTS adsorbed species on [Zn-Cr-TA] and [Zn-Al-TA] LDHs where TA ) terephthalate anion. The stable radical ABTS•+ intercalated in the LDHs structure was studied by UV-visible spectroelectrochemistry.
I. Introduction In the last 20 years, a new approach to electrocatalysis emerged using molecular materials attached to electrode surfaces,1 since the kinetics of an electrochemical reaction and sometimes even the electrode reaction product may depend on the composition of the electrode. Research arose focusing on the preparation and the characterization of electrodes modified with inorganic structured materials such as zeolites and clays. The key features of inorganic lattices are their shape and size selectivity in chemical reactions. Zeolites have well-defined pores and channels, clays exhibit layerlike structures, in both cases a threedimensional architecture is built on the electrode surface allowing study of molecular recognition effects in the design of electrocatalysts for specific substrates and of analytical devices. Clays exhibit a flexible sheet or layerlike structure with relatively low interlayer bonding which gives rise to swelling properties. So they appear as a good host structure for electroactive guest molecules. Most of the studies on clay-modified electrodes have been done with the swelling cationic clays known as smectite clays.2 Since 1987, several papers have dealt with the application of synthetic anionic clays to electrode preparation.3-10 These materials, also called layered double hydroxides (LDH), present special interest because the hydrotalcite type structure has an anion exchange property and can act as a passive discriminator allowing preconcentration of an * Author to whom correspondence should be addressed. † Laboratoire d’Electrochimie Organique, URA 434. ‡ Laboratoire de Physico-Chimie des Mate ´ riaux, URA 444. X Abstract published in Advance ACS Abstracts, August 1, 1996. (1) Murray, R. W. Molecular Design of Electrodes Surface. In Techniques of Chemistry; Murray, R. W., Ed.; Wiley: New York, 1992; Vol. 22, p 1. (2) Bard, A. J.; Mallouk, T. Molecular Design of Electrodes Surface. In Techniques of Chemistry; Murray, R. W., Ed.; Wiley: New York, 1992; Vol. 22, p 271. (3) Itaya, K.; Chang, H. C.; Uchida, I. Inorg. Chem. 1987, 26, 624. (4) Shaw, B.; Deng, Y.; Strillacci, F.; Carrado, K.; Fessehaie, M. J. Electrochem. Soc. 1990, 137, 3136. (5) Keita, B.; Belhouari, A.; Nadjo, L. J. Electroanal. Chem. 1991, 314, 345. (6) Keita, B.; Belhouari, A.; Nadjo, L. J. Electroanal. Chem. 1993, 355, 235. (7) Shaw, B.; Creasy, K. E. J. Electroanal. Chem. 1988, 243, 209. (8) Gaillon, L.; Battioni, P.; Bedioui, F.; Devynck, J. J. Electroanal. Chem. 1993, 347, 435. (9) Mousty, C.; Therias, S.; Forano, C.; Besse, J.-P. J. Electroanal. Chem. 1994, 271, 69. (10) Qiu, J.; Villemure, G. J. Electroanal. Chem. 1995, 395, 159.
S0743-7463(96)00101-1 CCC: $12.00
analyte or reactant. In particular, inorganic electroactive anionic species such as [Fe(CN)6]3-, [Mo(CN)8]4-, [IrCl6]2-,3,4,10 and oxometalates5,6 have been studied at anionic clay modified electrodes. Electrochemical behavior of organic molecules, simply adsorbed at the clay surface or previously exchanged in the interlayer lattice, has also been investigated.4,7-9 Precursor materials commonly used to prepare electrodes are Mg6Al2(OH)16X‚4H2O denoted [Mg-Al-X],3,4,8 Zn2Al(OH)6X‚nH2O denoted [Zn-Al-X],4-7 and Zn2Cr(OH)6X‚2H2O denoted [Zn-Cr-X].9 More recently, Villemure et al.10 used [Ni-Al-X]- and [Ni-Fe-X]-modified electrodes. The exchangeable anions (X) are chloride, nitrate, or carbonate anions. In our study, the starting material was previously exchanged by organic electroactive molecules bearing a sulfonate group before preparation of the modified electrodes.9 Anionic clay films were prepared on an electrode as reported for smectite clays.2 They were cast by slow evaporation of a measured volume of a colloidal suspension (typically 5-50 µL of a solution containing 2-20 g L-1 of clay), placed on the conductive electrode surface. The thickness of the films was estimated to be 70-100 nm.3,9 Anionic clay modified electrodes were also prepared by methods similar to those reported previously for zeolitemodified electrodes. They are composite electrodes containing a clay/carbon mixture pressed into electrode grids,8 clay/carbon/polymer composites,7 and clay/polymer composites.4 The electrochemical response of redox species accumulated in the clay film was studied by cyclic voltammetry and showed stable diffusion-controlled waves. The effective diffusion coefficients of incorporated anions in clay films generally are in the range 10-12-10-11 cm2 s-1.3,9 During an accumulation step, the peak current intensities increased as a function of the soaking time, showing that an exchange reaction occurred within the clay film. However, as reported for smectite clay films, only a fraction (2-20%) of the species incorporated into the clay film was electroactive.9,10 [Fe(CN)6]3- incorporated into [Zn-Al-Cl],4 [Zn-Cr-Cl],11 and [Ni-Al-Cl]10 films showed a particular behavior which was ascribed to the formation of a Prussian Blue like layer on the clay particles, whereas no film formed on the [Mg-Al-Cl] surface. Moreover, results obtained with catechol,7 phenol,4, metatungstate,5,6 and (11) Therias, S.; Mousty, C. Unpublished results.
© 1996 American Chemical Society
Electrochemical Transfer at Clay-Modified Electrodes
Langmuir, Vol. 12, No. 20, 1996 4915
Table 1. Elemental Analysis and Basal Spacings of [Zn-Cr-X] and [Zn-Al-X] clay
preparation method
MII/MIII
LAB1′
[Zn-Cr-ABTS]
CAB2 EAB2 CTA2 TAB2
[Zn-Al-Cl] [Zn-Al-ABTS] [Zn-Al-ABTS] [Zn-Al-TA] [Zn-Al-TA] + ABTS ads
coprecipitation coprecipitation exchange (reflux) exchange (room temp) exchange (T ) 50 °C, pH ) 5) reverse exchange (room temp) coulometry (Eappl ) 0.8 V) coprecipitation coprecipitation exchange (reflux) coprecipitation exchange (room temp)
1.9 1.8 1.9 1.8 1.8
LAB1
[Zn-Cr-Cl] [Zn-Cr-ABTS] [Zn-Cr-ABTS] [Zn-Cr-TA] [Zn-Cr-TA] + ABTS ads. [Zn-Cr-ABTS]
sample CAB1 EAB1 ETA1 TAB1
a
exchange (%)
1.9
100 90 99a 0 (20 % ads) 100
1.9
100
2.7 1.8 2.6 4 4
100 60 100a 0 (10% ads)
remaining Cl (%)
d (Å)
100
