ELECTROCHEMISTRY AT BORON-DOPED DIAMOND FILMS GROWN ON GRAPHITE SUBSTRATES - REDOX, ADSORPTION AND DEPOSITION PROCESSES

Highly boron-doped (atomic concentration similar to 10(21) cm(-3)) con ducting diamond films were grown on graphite substrates by microwave a ssisted vapor deposition from a gaseous feed of hydrogen and methane a nd solid boron. These diamond films of ca. 5 to 10 mu m thickness comp osed of crystals of up to 10 mu m size were characterized by both surf ace analytical techniques such as Raman spectroscopy, atomic force mic roscopy (AFM), and scanning electron microscopy (SEM), and electrochem ical techniques. The reduction of Ru(NH3)(6)(3+), the oxidation of chl orpromazine, and the reduction of PB(2+)in aqueous media were studied in order to investigate the processes involving oxidation and reductio n as well as adsorption and deposition. The one-electron oxidation of chlorpromazine in aqueous 0.1 M KCI and the one electron reduction of Ru(NH3)(6)(3+) in aqueous 0.1 M KCI gave well defined cyclic voltammet ric responses at diamond electrodes with peak currents proportional to the square root of the scan rate consistent with diffusion control at a macroscopically uniformly active electrode. For the oxidation of ch lorpromazine at concentrations less than 4 mM adsorption of the neutra l chlorpromazine at the diamond electrode was additionally detected. A t very low scan rates a transition to sigmoidally shaped responses occ urred which may be attributed partly to the presence of areas of low o r no conductivity on a microscopic level. The reduction of Pb2+ in aqu eous 0.1 M HClO4, allowed the deposition of metallic lead on the diamo nd surface. Studies by ex-situ SEM and in-situ AFM show that Pb nuclei are formed and distributed inhomogeneously over the polycrystalline d iamond film in the active surface areas, irrespective of grain boundar ies. However, little or no Pb deposition occurred in some less active areas with dimensions of 2 to 100 mu m. This effect may be attributed to regions of poor conductivity between the graphite substrate and the diamond film coupled to low lateral conductivity of the film itself. The size of the regions of lower activity decreases with higher applie d over-potential. A characteristic delay in the re-dissolution process of the Pb deposit was observed and attributed to both poor adhesion a nd a resistive contact between deposit and electrode surface. (C) 1998 Elsevier Science S.A.