Electrochemical characterization of doped and undoped CVD diamond deposited by microwave plasma

Abstract

Abstract Undoped and doped polycrystalline diamond films have been deposited over molybdenum substrate by microwave plasma CVD process using a methane and hydrogen gas mixture at a pressure of 35±0.5 Torr. A.C. impedance of undoped diamond films in 0.5 M NaCl solution has been determined as a function of time and compared to the results obtained with molybdenum substrate. Double-layer capacitance, solution resistance, film resistance, film capacitance, and polarization resistance have been determined using the data plotted in Nyquist and Bode formats. Existence of two time constants due to electrical double-layer and film capacitance at the diamond/solution interface is clear from the impedance data. D.C. polarization techniques such as linear and Tafel polarization have been used to evaluate the undoped diamond-coated molybdenum, doped-diamond-coated molybdenum, and molybdenum for corrosion resistance characteristics in terms of charge-transfer coefficients and corrosion rate. The interfacial resistance of the undoped diamond electrode/solution interface is in the range of 108 Ω · cm2 according to impedance spectroscopy studied over a wide frequency range and Tafel polarization. This indicates the faradaic reaction rates are insignificant at the interface. Corrosion rate of diamond (doped and undoped) is three orders of magnitude lower than bare molybdenum substrate. The measured double-layer capacitance of diamond/solution interface is substantially lower than the molybdenum. Cyclic voltammetry has been used to evaluate the molybdenum, platinum, and diamond (doped and undoped)-coated molybdenum materials in 0.5 M NaCl solution. Diamond is stable with respect to molybdenum and platinum over a wide potential range (+ 1.5 to −1.5 V) for the decomposition of H2O to occur to evolve hydrogen and oxygen during cathodic and anodic polarization, respectively, according to cyclic voltammetry. Redox kinetics of ferri/ferro cyanide in 0.5 M NaCl solution at the doped diamond and platinum electrodes has been investigated.