Electrodes Modified with Synthetic Clay Minerals: Electrochemistry of Cobalt Smectites

-Hydrothermal treatment of a mixture of silicic acid, cobalt chloride, sodium dithionite and sodium hydroxide at 250 ~ under 500 psi of argon produced a pink solid. X-ray powder diffraction (XRD), transmission electron microscopy (TEM) and electron diffraction data showed the product to be a well-crystallized smectite. SEM/EDX analysis gave a unit cell formula of [(Si8.05)(Co558)O20(OH)4]Na066. Heating the same mixture at 150 ~ without argon gave a less well ordered smectite of composition [(Si7.93)(COs.92)O20(OH)4]Nao.42. Two peaks were observed in the cyclic voltammograms of electrodes modified with films of these two clays recorded for the blank electrolytes in the absence of any adsorbed electroactive species. The first peak was attributed to the oxidation of a small fraction of the Co 2+ sites within the clay lattices to Co 3+. The second peak was assigned to further oxidation of these Co 3+ to Co 4+. Key Words--Clay-modified Electrodes, Cobalt, Cyclic Voltammetry, Smectites, Synthetic Clay. I N T R O D U C T I O N Clay-modif ied electrodes (CME) are prepared by deposit ing thin films o f clay onto conduct ive substrates (Bard and Mal louk 1992; Fitch 1990). The aim is to use the chemical and physical properties o f the clay to control the sensitivity or selectivity of the electrode toward solution species. However , natural clay minerals are not electronical ly conduct ive (Wang et al. 1989). Electron transport for C M E s depends upon a combinat ion of diffusion of adsorbed electroact ive species through the films, and electron hopping between the adsorbed species. This causes two problems. The first is low electroact ive fractions, The low mobility of ions within clay films means that only a small fraction of them can reach the conduct ive substrate to participate during the e lectrochemical reaction (Villemute and Bard 1990; King et al. 1987). Second, the electron transfer processes in C M E s do not actually occur within the clay interlayer spaces. To participate in the e lect rochemical reaction the intercalated species must first diffuse out of the interlayer spaces (Shaw 1989). Thus, the opportunity to exploit the geometry of the gal lery spaces is lost. The electroact ive fraction can be increased by using smaller more mobi le electroact ive species. Kaviratna and Pinnavaia (1992) reported that up to 80% of the [Ru(NH3)6] 3§ adsorbed into C M E s could be reduced electrochemical ly. The eff iciency of the diffusion o f ions in clay films can also be improved by swell ing o f the clay into dilute electrolytes prior to the measurements to make the films more porous (Fitch and Lee 1993). However , to have net electron transfer occur deep within the clay interlayer spaces requires an alternative charge transport mechanism. One possibility would be to make use o f e lectrochemical ly active transition metal centers within the clay lattice as acceptor/donor sites to relay electrons be tween ions within the interlayer spaces and the conduct ive substrate. Iron sites within the smectite lattices have long been implicated in charge transport in CMEs. Oyama and Anson (1986) reported that structural Fe sites within montmoril lonite could mediate the reduction of H20> Electroactive Fe sites within clay films have been used as part of the design of a glucose sensor (Ohsaka et al. 1990). Electron transfer from Fe 2+ has been proposed to account for the anomalously larger first anodic wave of [Ru(bpy)3] 2+ (Rudzinski and Bard 1986) and [Fe(bpy)3] 2§ (Villemure and Bard 1990) adsorbed into CMEs. The initial anodic to cathodic current ratio found for [Fe(bpy)3] 2§ for CMEs increased when the number of F C + sites in smectites was increased by partial reduction of the clay's structural Fe 3§ (Xiang and Villeinure 1992). Recently we have shown that redox active iron sites within a synthetic smectite could relay electrons between [Ru(NH3)6] 3§ and [Fe(bpy)3] 2§ ions coadsorbed into CMEs (Xiang and Villemure 1995). The involvement of redox active transition metal sites in the e lectrochemistry o f electrodes modified with other nonconduct ive solids has also been reported. Castro-Martin et al. (1993, 1994) described redox active Ti 4§ sites in zeol i te-modif ied electrodes. Electron t ransfer be tween adsorbed [Fe(CN)6] 4or [Mo(CN)s] 4ions and redox active Ni sites in an electrode modif ied with a Ni-A1 layered double hydroxide resulted in 4 to 6 fold increase of the electroact ive fractions (Qiu and Vil lemure 1995). Natural smecti tes do not normal ly contain large amounts of transition metals other than Fe (Newman 1987). Therefore, we undertook an invest igat ion o f electrode modificat ion with synthetic smectites. Procedures for the preparation of smectites containing Copyright 9 1996, The Clay Minerals Society 515 516 Xiang and Villemure Clays and Clay Minerals most of the first row transition metals are available from the literature (Guven 1988). We report the preparation of two Co 2+ smectites. Peaks are observed in the cyclic voltammograms of CMEs prepared with these 2 clays. They are attributed to the electrochemical activity of Co 2+ centers within the clays.