THE FORMATION AND ELECTROCHEMICAL ACTIVITY OF MICROPOROUS DIAMOND THIN-FILM ELECTRODES IN CONCENTRATED KOH

The corrosion resistance and electrochemical activity of boron-doped, polycrystalline diamond thin films were evaluated before and after pot entiodynamic cycling between -0.7 and 0.7 V vs. Ag/AgCl in 15 weight p ercent KOH. The maximum anodic current densities at 0.7 V ranged from 0.3 to 6.0 mA/cm(2). Two types of corrosion processes were observed. I n one case, low-quality films grown using a 2.9% C/H ratio developed c ircular pits (100 to 300 nm diam) located almost exclusively at the in tercrystalline grain boundaries, creating a microporous film. For the most part, the diamond microcrystallites were morphologically unaffect ed by the polarization, but in a few areas, extreme pitting and smooth ing of the microcrystallites occurred. In a second case, low-quality f ilms grown using a 1% C/H ratio experienced corrosion that advanced la terally: across the diamond microcrystallite surfaces commencing at fa cet corners and step edges. High-quality films grown using a 1.4% C/H ratio exhibited no signs of corrosion or morphological damage. A corro sion mechanism is proposed whereby the nondiamond carbon impurities ex posed at the surface are preferentially etched away by anodic polariza tion. The electrochemical activity of the films, before and after cycl ing, was probed using cyclic and differential pulse voltammetry with I rCl6-2/-3. The morphological and surface chemical changes produced dur ing the microporous film formation led to an increase in the oxidation and reduction peak currents by up to a factor of eight. The heterogen eous electron-transfer rate constant, K-app(0), decreased slightly by a factor of four after formation of the microporous film. It is suppos ed that the electron-transfer reactions are not occurring exclusively through the nondiamond carbon impurity sites but rather that these imp urities serve primarily an electronic role by supplying charge carrier s to the film, thereby reducing resistivity.