Electrochemistry of boron -doped diamond thin-film electrodes: Electronic and morphologic structure effects

Chemically vapor deposited, conductive diamond films are emerging as an attractive electrode alternative to more conventional sp 2 bonded carbon and metal electrodes. Diamond possesses many technologically useful properties that have generated numerous possibilities in electroanalysis and electrosynthesis. However, the combination of its electrically insulating origin, nearly metallic resistive characteristics when heavily doped, surface inertness, and defective nature makes the understanding of electron transfer at boron-doped diamond (BDD) thin-films complex.;To better understand which physical, chemical, and electronic factors influence the electrochemistry of this material, films grown on Si and Pt in four different laboratories, using slightly different conditions were studied. The electrodes were continuous polycrystalline films with varying amounts of adventitious non-diamond carbon impurity phases. The polycrystalline BDD electrodes, once immersed in an aqueous electrolyte, have differential capacitances that range from 1 to 12 μF/cm2 between −0.5 and +1.0 V vs SCE and an extremely small background current envelope between the electrolysis potentials of the electrolyte. The overpotential required for electrolyte electrolysis is much larger at BDD than at more conventional electrodes due to its inert, hydrogen passivated surface.;Heavily doped BDD electrodes (>1018 B/cm3) are metallic with regard to electron transfer. Aqueous redox couples that undergo simple, one-electron, outer sphere heterogeneous electron transfers are kinetically reversible over a wide range of potentials at BDD. In contrast, redox species requiring stabilization through interaction with the electrode surface during electrolysis demonstrate more irreversible kinetics. This behavior is attributed to the low affinity for adsorption of the BDD surface.;The chemical composition of BDD may be altered by anodic polarization. This pretreatment results in no morphological damage; however, the surface atomic O/C ratio increases from 0.02 to approximately 0.20. Concomitant with the increase in surface oxygen, electron transfer rates for Fe(CN)6 −3/−4 become irreversible, indicative of a site-specific, inner-sphere electron transfer of this couple at BDD electrodes. This situation can be reversed by acid washing and rehydrogenating the film in a hydrogen plasma. These results unequivocally rule out the influence of adventitious non-diamond carbon phases as the sole sites for electron transfer.