Vacuum-Annealed Undoped Polycrystalline Diamond: a New Electrode Material

Abstract

Electrochemical behavior of undoped polycrystalline diamond films annealed in a vacuum at 1775 to 1915 K is studied. The annealing at temperatures over 1825 K imparts conductance to initially insulating films, which permits using them as electrode material. With further increase in the annealing temperature to above 1915 K, the effective resistivity decreases from initial value of 1011 to 1012 Ω cm down to less than 0.1 Ω cm, the differential capacitance increases from ∼10–3 to ∼50 μF per cm2 of geometrical surface, the transfer coefficients for electrochemical reactions in the Fe(CN)63–/4– redox solution increase from ∼0.2 to 0.5, and the degree of reversibility of the electrochemical reaction increases. The observed changes in the electrode properties are caused by the formation, upon the annealing, of a nondiamond phase at the intercrystallite boundaries and defect areas in the crystallites; the outcroppings of the conducting phase play the role of active sites at the electrode surface. With increasing annealing temperature, both the amount of this phase and its conductivity increase.