Vacuum-annealed undoped polycrystalline CVD diamond: a new electrode material

Vacuum-annealing imparts conductivity to initially insulating undoped polycrystalline chemical–vapor-deposited diamond, thus turning it to a possible electrode material. The diamond film annealed at 1775 K appeared to be practically not conducting. With further increase in the annealing temperature above 1825 K, the film effective resistivity decreased from initial value of 10 11 to 10 12 Ω cm down to less than 0.1 Ω cm; the differential capacitance increased from ∼10 −3 to ∼50 μF per 1 cm 2 of geometrical surface; the transfer coefficients for electrochemical reactions in the [Fe(CN) 6] 3−/4− redox solution increased from ∼0.2 to 0.5; and the degree of reversibility of the electrochemical reaction increased. The observed changes in the electrode properties are attributed to gradual change in the thickness and/or properties (first and foremost, conductivity) of the nondiamond carbon phase formed along the intercrystallite boundaries upon the annealing; the conducting phase is outcropping at the film surface as an array of microelectrodes (“active sites”).