The evolution of hydrogen and oxygen was studied on diamond electrodes containing approximately 1021 boron atom/cm3. Voltammetry showed a wide potential window [−1.25 to +2.3 V vs. standard hydrogen electrode (SHE)] without significant water decomposition. This window was much narrower for poor quality diamond films with appreciable sp2 content. A redox couple observed at +1.7 V indicates oxidation of the diamond surface prior to oxygen evolution. The extent of surface oxidation increased with sp2 content. Anodic polarization made the diamond surface hydrophilic; x‐ray photoelectron spectroscopy showed an increase in oxygen coverage and the presence of carbon‐oxygen bonds. The estimated capacitance of the interface ranged from 0.05 μF/cm2 for high quality diamond to 5 μF/cm2 for low quality diamond. Preliminary measurements of the exchange current densities for oxygen and hydrogen evolution indicated slow kinetics compared to metals or highly oriented pyrolytic graphite.

Boron‐doped polycrystalline and near‐single‐crystal quality diamond electrodes were studied by voltammetry. A redox couple with E1/2=+1.83V vs. standard hydrogen electrode was detected on the polycrystalline electrodes but was absent on the single‐crystal electrodes. The results strongly suggest that the couple is associated with reactivity at the grain boundaries. Plasma fluorination of polycrystalline diamond electrodes using CF4 in a radio frequency plasma eliminated the redox couple at +1.83 V but did not alter the potential range of water stability. Cathodic polarization of as‐grown, polycrystalline diamond electrodes caused an irreversible addition of oxygen to the surface. Subsequent anodic polarization added additional oxygen and made the surface hydrophilic. Single‐crystal electrodes also displayed an increase in oxygen coverage upon both cathodic and anodic polarization. Voltammetry studies of electrodes covered with a thin sp2 carbon surface layer indicate that the redox couple at +1.83 V corresponds to multiple processes including the etching of sp2 carbon in the grain boundaries. © 1999 The Electrochemical Society. All rights reserved.

Thick films of polycrystalline GaN were grown at low pressures by direct reaction of atomic nitrogen with liquid Ga without the presence of a substrate. The crystals were confirmed to be wurtzitic GaN by x-ray diffraction, transmission electron microscopy, Raman spectroscopy, and elemental analysis. Photoluminescence spectra showed near band edge peaks and broad yellow band emission at both 298 and 10 K. The results show that atomic nitrogen is an attractive alternative to high pressure N2 for the saturation of liquid gallium with nitrogen for the synthesis of bulk GaN.

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Diamond was deposited on synthetic graphite, highly oriented pyrolytic graphite and on substrates covered with graphite powder. Scanning electron microscopy and transmission electron microscopy were used to examine the samples. A strong preference for nucleation of diamond on the edges of the graphite sheets was observed. The graphite and the diamond have a preferential orientation relationship in which the diamond (111) plane is parallel to the graphite (0001) plane, and the diamond [11̄0] direction is parallel to the graphite [112̄0] direction. This orientation means that the puckered hexagons in the diamond (111) plane retain the same orientation as the flat hexagons in the original graphite sheet. We conclude that the diamond can nucleate with an epitaxial relationship to the graphite. Some of the edges of the graphite sheets may have been converted to diamond by the atomic hydrogen.