Standard electrochemical behavior of high-quality, boron-doped polycrystalline diamond thin-film electrodes

Standard electrochemical data for high-quality, boron-doped diamond thin fi lm electrodes are presented. Films from two different sources were compared (NRL and USU) and both were highly conductive, hydrogen-terminated, and po lycrystalline, The films are acid washed and hydrogen plasma treated prior to use to remove nondiamond carbon impurity phases and to hydrogen terminat e the surface. The boron-doping level of the NRL film was estimated to be i n the mid 10(19) B/cm(3) range, and the boron-doping level of the USU films was similar to 5 x 10(20) B/cm(-3) based on boron nuclear reaction analysi s. The electrochemical response was evaluated using Fe(CN)(6)(3-/4-), Ru(NH 3)(6)(3+/2+), IrCl62-/3-, methyl viologen, dopamine, ascorbic acid, Fe3+/2, and chlorpromazine, Comparisons are made between the apparent heterogeneo us electron-transfer rate constants, k(app)(o), observed at these high-qual ity diamond films and the rate constants reported in the literature for fre shly activated glassy carbon. Ru(NH3)(6)(3+/2+), IrCl62-/3-, methyl viologe n, and chlorpromazine all involve electron transfer that is insensitive to the diamond surface microstructure and chemistry with k(app)(o) in the 10(- 2)-10(-1) cm/s range. The rate constants are mainly influenced by the elect ronic properties of the films. Fe(CN)(6)(3-/4-) undergoes electron transfer that is extremely sensitive to the surface chemistry with k(app)(o) 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. Fe3+/2 + undergoes slow electron transfer at the hydrogen-terminated surface with k(app)(o) near 10(-5) cm/s, The rate of electron transfer at sp(2) carbon e lectrodes is known to be mediated by surface carbonyl functionalities; howe ver, this inner-sphere, catalytic pathway is absent on diamond due to the h ydrogen termination. Dopamine, like other catechol and catecholamines, unde rgoes sluggish electron transfer with k(app)(o) between 10(-4) and 10(-5) c m/s, Converting the surface to an oxygen termination has little effect on k (app)(o). The slow kinetics may be related to weak adsorption of these anal ytes on the diamond surface. Ascorbic acid oxidation is very sensitive to t he surface termination with the most negative E-p(ox) observed at the hydro gen-terminated surface. An oxygen surface termination shifts E-p(ox) positi ve by some 250 mV or more. An interfacial energy diagram is proposed to exp lain the electron transfer whereby the midgap density of states results pri marily 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 c an serve as a benchmark for research groups evaluating the electrochemical properties of semimetallic (i.e., conductive), hydrogen-terminated, polycry stalline diamond.