Langmuir 1994,10, 1657-1659
1657
Surface Modification of Carbon and Platinum Electrodes Using Tic14 Stanton Ching,' Raymond C. Dudek, and Elie A. Tabet Department of Chemistry, Connecticut College, New London, Connecticut 06320
William S. Willis and Steven L. Suib Department of Chemistry and Institute of Materials Science, University of Connecticut, Storrs, Connecticut 06268 Received April 7, 1994"
Chemically modified electrodes with supported titanium complexes have been generated for the first time by the reaction of Tic& with hydroxylated surfaces of glassy carbon or platinum electrodes. The resulting monolayers of titanium oxychloride species are not electroactive, but they can be evaluated with scanning Auger microprobe analysis and with a subsequent derivatization that uses (hydroxymethyllferrocene as a redox probe. The modified electrodes are stable under inert atmosphere but readily degrade upon exposure to air. Treatment of electrodes with other Ti complexes such as Ti(OEt14,Cp2TiCl2,and CpTiClS gives either no reaction or electrochemically unstable surface moieties. Introduction Early transition metal complexes with hydrolytically unstable metal-ligand bonds are effective reagents for derivatizing solid oxide surfaces, eq 1. Supported species from these reactions have been studied as promoters in heterogeneous catalysis and as precursors for thin metal oxide fiis.'a Electrode surfaces can similarly be modified using well-established reactions with hydrolytically unstable silanes.= These procedures have been used to generate electrode interfaces that are tailored to specific tasks. We have been exploring the possibility of using early transition metal complexes as precursors for supported Lewis acids in chemically modified electrodes. Our goal is determine if these types of systems are able to combine chemical activation and electron transfer phenomena on electrode surfaces. In particular, it would be interesting to learn if supported early transition metal species can promote different electrode processes compared to silanes due to their ability to expand their coordination sphere. Despite considerable research on covalently bonded silanes with electrodes, analogous chemistry using hydrolytically unstable early transition metal complexes has not been investigated? Here we report the successful preparation and characterization of a new type of modified electrode that features covalently bonded titanium metal centers. This system is generated by reactions of carbon and Pt surfaces with Tick. *Abstract published in Advance ACS Abstracts, May 15, 1994. (1)(a) Iwasawa, Y. In Tailored Metal Catalysts; Iwasawa, Y., Ed.; Reidel: Dordrecht, 1986; Chapter 1. (b) Hartly, F. R. Supported Metal Complexes; Reidel: Dordrecht, 1985; Chapter 3. (c) Yermakov, Yu. I.; Kuznetsov, B. N.; Zakharov, V. A. Studies in Surface Science and Catalysis; Elsevier: Amsterdam, 1981; Vol. 8, Chapter 2. (d) Candlin, J. P.; Thomas, H.Adu. Chem. Ser. 1974, No. 132, 212. (2) (a) Srinivasan, S.; Datye, A. K.; Hampden-Smith, M.; Wachs, I. E.; Deo, G.; Jehng, J. M.; Turek, A. M.; Peden, C. H.F. J. Catal. 1991,131, 260. (b) McDaniel, M. P.; Welch, M. B.; Dreiliig, M. J. J. Catal. 1983,
82, 118. (3) Abruna, H. D. Coord. Chem. Rev. 1988,86,135. (4) Murray, R. W. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 19&1; Vol. 13, p 191. (5) Murray, R. W. Acc. Chem. Res. 1980,13,136. (6)To our knowledge, only one such syetem haa been reported (a) Malpas, R. E.; Mayers, F. R.; Osborne, A. G.J.Electroanal. Chem. 1983, 153,97. (b) Malpas, R. E. J. Electroanal. Chem. 1981,117,347.
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Experimental Section Methods and Materials. All manipulationswere carried out under a prepurified Nz atmosphere using standard Schlenk line and syringe techniques' or in a Braun MB-150-M glovebox. Reagent grade solvents were distilled from appropriate drying agentss and stored under Nz prior to use. Tic4 (Aldrich) was used without further purification and stored under NZas a 10% solution in hexane. (Hydroxymethy1)ferrone(HMFc),CpTiCh, CpzTiClz, and Ti(0Et)r (Strem) were used as received. The [BhN][PFe] supportingelectrolytewas prepared from [BaNIBr and HPF6 according to a literature procedure.9 Instruments and Equipment. Cyclic voltammograms were recorded with a Bioanalytical Systems CV-1B potentiostat coupled with a Yokogawa 3025 X-Y recorder. Glassy carbon (GC) and Pt electrodes were fabricated by sealing the walls of 2.0 mm diameter GC rods (Atomergic)or 1.5 mm diameter Pt (JohnsonMatthey)wires withTeflon shrouds. Electrochemistry was typicdy carried using 0.1 M [BaNl[PF6] in CH&N with a Pt wire auxiliary electrode and either a Ag wire quasi-reference or AglAgNOS reference electrode. Auger electron spectroscopy was performed with a Perkin-Elmer PHI 610 scanning Auger microprobe. In most cases, sampleswere handled with a PerkinElmer Model 04-llOA vacuum transfer vessel. However, some samples required brief (
