J. Phys. Chem. 1996, 100, 7143-7147
7143
Electrodes Modified with Synthetic Clay Minerals: Electron Transfer between Adsorbed Tris(2,2′-bipyridyl) Metal Cations and Electroactive Cobalt Centers in Synthetic Smectites Yan Xiang and Gilles Villemure* Department of Chemistry, UniVersity of New Brunswick, Bag SerVice #45222, Fredericton New Brunswick E2B 6E2, Canada ReceiVed: September 27, 1995; In Final Form: December 23, 1995X
Clay-modified electrodes (CMEs) were prepared with two synthetic cobalt smectites. Large increases in the peak currents were observed in the cyclic voltammograms of tris(2,2′-bipyridyl) metal cations ([M(bpy)3]2+, M ) Os, Fe, or Ru) adsorbed in these CMEs when the potential was scanned high enough to oxidize redox active cobalt centers in the lattices of the synthetic clays. The cathodic charges obtained in the cobalt clay were 18.6, 25.7, and 12.3 times larger than those obtained in electrodes modified with a natural montmorillonite for the osmium, ruthenium, and iron bipyridyl cations, respectively. These increases were attributed to electron transfer between the intercalated cations and the oxidized cobalt centers. The redox active cobalt centers in the clay lattices functioned as relays, transferring electrons between adsorbed cations that otherwise would be electrochemically inactive and the conductive substrate. In some cases, nearly 100% of the cations present in the cobalt CMEs participated in the electrochemical reaction. The increases in the peak currents required longer oxidation times in CMEs prepared with the more ordered of the two synthetic cobalt smectites. This was consistent with the lower concentration of electroactive Co(II) in the well-crystallized clay containing fewer defect sites.
Introduction Clay-modified electrodes (CMEs) are prepared by depositing thin films of smectite clays on conductive substrates.1,2 The aim is to use the chemical and physical properties of the clay to control the sensitivity or selectivity of the electrode. Smectites have many desirable properties as electrode surface modifiers: high thermal and chemical stabilities, large surface area, flexible adsorption capability, and catalytic potential.3-5 However, natural smectites are not electronically conductive.6 Electron transport in CMEs depends on a combination of diffusion of adsorbed electroactive species through the films and electron hopping between adsorbed species. This causes two problems. The first is the low electroactive fractions. The low mobility of many cations in clay films means that only a small fraction of them can reach the conductive substrate to participate in the electrochemical reaction.7,8 Second, the electron transfer processes in CMEs do not actually occur in the clay interlayer spaces. To participate in the electrochemical reaction, the intercalated species must first diffuse out of the interlayer spaces.9 Thus, the opportunity to exploit the geometry of the gallery spaces is lost. The electroactive fractions in CMEs can be increased by using more mobile electroactive species.10 Clay films can also be made more porous by extensive swelling of the clay in dilute electrolytes prior to the measurements.11 However, to have net electron transfer occur deep within the interlayer spaces requires an alternative charge transport mechanism. One possibility would be to make use of electrochemically active transition metal centers in the clay lattice as acceptor/donor sites to relay electrons between ions in the interlayer spaces and the conductive substrate. Redox active iron sites in the lattice of smectites have long been implicated in the charge transport process in CMEs.6,12-16 The involvement of electroactive transition metal sites in the * Author to whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, March 15, 1996.
0022-3654/96/20100-7143$12.00/0
electrochemistry of electrodes modified with other nonconductive solids has also been reported. de Castro-Martin et al.17,18 described redox active Ti(IV) sites in zeolite-modified electrodes. Recently we observed electron transfer between [Fe(CN)6]4- or [Mo(CN)6]4- ions adsorbed in electrodes modified with a Ni-Al-Cl LDH and redox active nickel sites in the lattice of the LDH.19 Natural smectites do not normally contain large amounts of transition metals other than iron.4 We therefore undertook an investigation of electrode modifications with synthetic smectites. Procedures for the preparation of smectites containing most of the first-row transition metals are available in the literature.3 In a previous paper we described the preparation of two cobalt smectites and showed that some of the Co(II) centers in these clays were electroactive.20 We report here on the cyclic voltammograms of [M(bpy)3]2+ (M ) Os, Fe, and Ru) cations adsorbed in these synthetic cobalt smectites. Large enhancements in the peak currents were observed. They are attributed to electron transfer between the adsorbed ions and the oxidized cobalt centers in the clay lattices. Experimental Section Materials. [Fe(bpy)3]Cl2 and [Os(bpy)3]Cl2 were prepared by literature procedures.21,22 CoCl2‚6H2O was obtained from Fisher Scientific (Fair Lamn, NJ). [Ru(bpy)3]Cl2 and all other chemicals were obtained from Aldrich (Milwaukee, WI) and used without further purification. The natural montmorillonite clay STx-1 (Gonzales County, TX) was obtained from the Source Clay Minerals Repository (University of Missouri, Columbia, MO). Its sodium form was prepared and its
