Standard Electrochemical Behavior of High-Quality, Boron-Doped Polycrystalline Diamond Thin-Film Electrodes

Abstract

Standard electrochemical data for high-quality, boron-doped diamond thin-film electrodes are presented. Films from two different sources were compared (NRL and USU) and both were highly conductive, hydrogen-terminated, and polycrystalline. The films are acid washed and hydrogen plasma treated prior to use to remove nondiamond carbon impurity phases and to hydrogen terminate the surface. The boron-doping level of the NRL film was estimated to be in the mid 10¹⁹ B/cm³ range, and the boron-doping level of the USU films was ∼5 × 10²⁰ B/cm^-3 based on boron nuclear reaction analysis. The electrochemical response was evaluated using Fe(CN)₆^3-/4-, Ru(NH₃)₆^3+/2+, IrCl₆^2-/3-, methyl viologen, dopamine, ascorbic acid, Fe^3+/2+, and chlorpromazine. Comparisons are made between the apparent heterogeneous electron-transfer rate constants, k^o_app, observed at these high-quality diamond films and the rate constants reported in the literature for freshly activated glassy carbon. Ru(NH₃)₆^3+/2+, IrCl₆^2-/3-, methyl viologen, and chlorpromazine all involve electron transfer that is insensitive to the diamond surface microstructure and chemistry with k^o_app in the 10^-2−10^-1 cm/s range. The rate constants are mainly influenced by the electronic properites of the films. Fe(CN)₆^3-/4- undergoes electron transfer that is extremely sensitive to the surface chemistry with k^o_app in the range of 10^-2−10^-1 cm/s at the hydrogen-terminated surface. An oxygen surface termination severely inhibits the rate of electron transfer. Fe^3+/2+ undergoes slow electron transfer at the hydrogen-terminated surface with k^o_app near 10^-5 cm/s. The rate of electron transfer at sp² carbon electrodes is known to be mediated by surface carbonyl functionalities; however, this inner-sphere, catalytic pathway is absent on diamond due to the hydrogen termination. Dopamine, like other catechol and catecholamines, undergoes sluggish electron transfer with k^o_app between 10^-4 and 10^-5 cm/s. Converting the surface to an oxygen termination has little effect on k^o_app. The slow kinetics may be related to weak adsorption of these analytes on the diamond surface. Ascorbic acid oxidation is very sensitive to the surface termination with the most negative E_p^ox observed at the hydrogen-terminated surface. An oxygen surface termination shifts E_p^ox positive by some 250 mV or more. An interfacial energy diagram is proposed to explain the electron transfer whereby the midgap density of states results primarily from the boron doping level and the lattice hydrogen. The films were additionally characterized by scanning electron microscopy and micro-Raman imaging spectroscopy. The cyclic voltammetric and kinetic data presented can serve as a benchmark for research groups evaluating the electrochemical properties of semimetallic (i.e., conductive), hydrogen-terminated, polycrystalline diamond.