The Formation and Electrochemical Activity of Microporous Diamond Thin Film Electrodes in Concentrated KOH

Abstract

The corrosion resistance and electrochemical activity of boron‐doped, polycrystalline diamond thin films were evaluated before and after potentiodynamic cycling between −0.7 and 0.7 V vs. Ag/AgCl in 15 weight percent KOH. The maximum anodic current densities at 0.7 V ranged from 0.3 to 6.0 mA/cm2. Two types of corrosion processes were observed. In one case, low‐quality films grown using a 2.9% C/H ratio developed circular pits (100 to 300 nm diam) located almost exclusively at the intercrystalline grain boundaries, creating a microporous film. For the most part, the diamond microcrystallites were morphologically unaffected by the polarization, but in a few areas, extreme pitting and smoothing of the microcrystallites occurred. In a second case, low‐quality films grown using a 1% C/H ratio experienced corrosion that advanced laterally across the diamond microcrystallite surfaces commencing at facet corners and step edges. High‐quality films grown using a 1.4% C/H ratio exhibited no signs of corrosion or morphological damage. A corrosion mechanism is proposed whereby the nondiamond carbon impurities exposed at the surface are preferentially etched away by anodic polarization. The electrochemical activity of the films, before and after cycling, was probed using cyclic and differential pulse voltammetry with . The morphological and surface chemical changes produced during the microporous film formation led to an increase in the oxidation and reduction peak currents by up to a factor of eight. The heterogeneous electron‐transfer rate constant, , decreased slightly by a factor of four after formation of the microporous film. It is supposed that the electron‐transfer reactions are not occurring exclusively through the nondiamond carbon impurity sites but rather that these impurities serve primarily an electronic role by supplying charge carriers to the film, thereby reducing resistivity.