Boron-Doped Diamond Films for Electrochemical Applications

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Department, Case

Western Reserve

ABSTRACT Diamond electrodes possess unique chemical stability, a very wide potential window of water stability, and low background currents. These properties give rise to numerous possible applications, for example, electrosynthesis and electrodestruction reactions at extreme potentials and environmental conditions and as a sensor electrode in aggressive environments. Furthermore, the study of semiconducting diamond electrodes promises to lead to greater understanding of the surface chemistry of diamond and of electronic levels and surface states in doped diamond. In this paper the reactivity of diamond electrodes and their use in a molten salt environment, as a sensor element, and for characterizing diamond are discussed. INTRODUCTION Diamond electrodes may find uses in electrochemistry in two principal areas: (i) synthesis (or destruction), where an applied potential is used to bring about a desired electrochemical reaction, and (ii) analysis, where the current/potential response of an electrode is used to determine the type and concentration of a species. These applications require stable, chemically robust, and economical electrodes. Diamond electrodes, fabricated by chemical vapor deposition, meet these requirements for a wide range of applications. The first reports of

electrochemical studies on diamond were in the mid 1980s." 2 The field has attracted increasing attention and two reviews of fundamental studies and emerging applications of diamond electrochemistry have recently appeared. 3' 4 The majority of studies of diamond electrodes have focused on their unusual chemical stability, their differences in electrochemical behavior from conventional carbon electrodes, and their photoelectrochemical response. Although thermodynamically unstable with respect to graphite, diamond's atomically dense structure makes it unreactive in aggressive environments that attack highly oriented pyrolytic graphite, e.g., oxidizing acids. The hydrogen terminated asgrown surface of diamond is alkane-like and is not a favorable surface for adsorption. This is believed to give rise to highly irreversible kinetics (high overpotentials) for electrode processes that involve adsorbed intermediates. For example, this may be the reason for the very large overpotentials observed for both hydrogen and oxygen evolution from water. The resulting large potential "window" for water stability makes diamond electrodes attractive for some sensor applications. The wide potential window also permits the electrodes to be used at very cathodic potentials; Tenne et a• reduced nitrate and nitrite ions to ammonia on diamond. Simple oneelectron, outer sphere reactions are far more reversible on diamond.6 The hydrogen termination is stable in air at room temperature, and electrodes remain hydrophobic for months in contact with air. The electrochemical behavior of polycrystalline diamond electrodes may be influenced by non-diamond, sp 2 , carbon at the grain boundaries and differences in levels of boron 217 Mat. Res. S