ELECTROCHEMICAL CHARACTERIZATION OF DOPED AND UNDOPED CVD DIAMOND DEPOSITED BY MICROWAVE PLASMA

Undoped and doped polycrystalline diamond films have been deposited ov er molybdenum substrate by microwave plasma CVD process using a methan e and hydrogen gas mixture at a pressure of 35 +/- 0.5 Torr. A.C. impe dance of undoped diamond films in 0.5 M NaCl solution has been determi ned as a function of time and compared to the results obtained with mo lybdenum substrate. Double-layer capacitance, solution resistance, fil m resistance, film capacitance, and polarization resistance have been determined using the data plotted in Nyquist and Bode formats. Existen ce of two time constants due to electrical double-layer and film capac itance at the diamond/solution interface is clear from the impedance d ata. D.C, polarization techniques such as linear and Tafel polarizatio n have been used to evaluate the undoped diamond-coated molybdenum, do ped-diamond-coated molybdenum? and molybdenum for corrosion resistance characteristics in terms of charge-transfer coefficients and corrosio n rate. The interfacial resistance of the undoped diamond electrode/so lution interface is in the range of 10(8) Ohm . cm(2) according to imp edance spectroscopy studied over a wide frequency range and Tafel pola rization. This indicates the faradaic reaction rates are insignificant at the interface. Corrosion rate of diamond (doped and undoped) is th ree orders of magnitude lower than bare molybdenum substrate. The meas ured double-layer capacitance of diamond/solution interface is substan tially lower than the molybdenum. Cyclic voltammetry has been used to evaluate the molybdenum, platinum, and diamond (doped and undoped)-coa ted molybdenum materials in 0.5 M NaCl solution. Diamond is stable wit h 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 a nd oxygen during cathodic and anodic polarization, respectively, accor ding to cyclic voltammetry. Redox kinetics of ferri/ferro cyanide in 0 .5 M NaCl solution at the doped diamond and platinum electrodes has be en investigated.