| Boron-doped, polycrystalline and single-crystal diamond electrodes were fabricated by hot-filament chemical vapor deposition and characterized for electrochemical applications. Near-single-crystal diamond films were grown homoepitaxially on natural ⟨100⟩ diamond windows and ⟨111⟩ diamond macles. Diamond electrodes were also fluorinated using CF4.; For 24 hour growth runs, 10 mum thick, well-faceted, polycrystalline films were obtained; homoepitaxial single-crystal films exceeded 4 mum in thickness and gave the macles or windows a deep blue color. Secondary ion mass spectroscopy indicated a uniform doping level of approximately 10 21 boron atoms/cm3, ca., 6000 ppm. Nuclear reaction analysis confirmed boron concentrations between 5000 and 8200 ppm. Electrode resistivities ranged from 0.05 to 0.001 Ohm-cm.; Characteristic voltammetry features of high-quality polycrystalline diamond electrodes in 0.5M H2SO4 were a wide potential range for water stability (-1.25 to +2.3 V vs. standard hydrogen electrode) and low baseline current. On low-quality electrodes, a narrower potential range and higher baseline current approached the behavior of highly-oriented pyrolytic graphite or glassy carbon electrodes. A redox couple at E½ = +1.83 V corresponded to a combination of a reversible oxidation at the grain boundaries, e.g., by a change in oxygen functionality, and an irreversible oxidation, i.e., etching, process.; Single-crystal diamond electrodes displayed the wide potential range of water stability and low baseline current, but the redox couple at +1.83 V was absent, verifying that it corresponds to grain boundary reactivity. Fluorination of polycrystalline electrodes also eliminated the redox couple, suggesting that the fluorine passivates the grain boundary reactivity.; Polarization of as-grown diamond electrodes caused an irreversible addition of oxygen to the surface. The nature of the oxygen termination was investigated by attenuated total reflection FTIR through a 50 mum diamond-coated silicon wafer, while the diamond-coated surface was polarized in H2SO 4. The spectra suggested that hydroxyl groups were among the oxygen functionalities added to the surface during anodic polarization.; The wide potential window of water stability permits electrochemical couples with large positive or negative standard potentials to be investigated without interference from water electrolysis. For example, oxidation of commonly used copper plating additives, polyalkylene glycol and disodium (bis(3-sulfopropyl)) disulfide, were easily measured in solutions on polycrystalline diamond. |
