V.G. Ralchenko

Effect of crystal structure on the electrochemical behaviour of synthetic semiconductor diamond: Comparison of growth and a nucleation surfaces of a coarse-grained polycrystalline film

Comparative studies of the electrochemical behaviour of the growth and nucleation surfaces of a free-standing boron-doped polycrystalline diamond film grown in a microwave plasma CVD reactor are performed. The uncompensated acceptor concentration in diamond is determined from the electrochemical impedance (Mott–Schottky plots), uncompensated boron acceptor concentration from infrared absorption measurements, and the total boron concentration, by the SIMS method. In the diamond bulk adjacent to the nucleation surface, constituted from submicrometre-sized crystallites, both the boron concentration and the total acceptor concentration are found to be significantly higher than near the growth surface, where the film crystallinity is more perfect. This difference is tentatively attributed to the increased concentration of crystal lattice defects near the nucleation surface. These defects, in addition to boron atoms, play the role of acceptors in diamond.

Electrochemical properties of undoped CVD diamond films vacuum- annealed at 1775-1915 K

Abstract Electrochemical behavior of vacuum-annealed undoped polycrystalline diamond films is studied. The 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 –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 of the electrode properties can be attributed to gradual change of the thickness and/or properties (first and foremost, conductivity) of the non-diamond carbon phase formed along the intercrystallite boundaries upon the annealing and outcropping at the film surface as ‘active sites’.

Semiconductor properties of nanocrystalline diamond electrodes

The semiconductor properties of nitrogenated nanocrystalline diamond electrodes and their corrosion transformations caused by electrochemical experiment in indifferent electrolyte (1 M K2SO4) were studied by the electrochemical impedance spectroscopy method. It was shown that after electrochemical measurements a narrow diamond peak at 1335.7 cm−1 appears in the Raman spectrum; formerly the peak was hidden at a background of the intense signal inherent to graphite-like carbon. It was suggested that the corrosion damage caused by the exposure to electrochemical experiment resulted in a decrease of relative amount of nondiamond (graphite-like) carbon in the subsurface layer in the nanocrystalline diamond. By using Mott-Schottky plots, the nanocrystalline diamond was shown having n-type conductance. Within the bounds of the “effective medium” approach, the nanocrystalline diamond’s flat-band potential in aqueous solution and the noncompensated donor apparent concentration were estimated.

Benzene Oxidation at Diamond Electrodes: Comparison of Microcrystalline and Nanocrystalline Diamonds

A comparative study of benzene oxidation at boron-doped diamond (BDD) and nitrogenated nanocrystalline diamond (NCD) anodes in 0.5 M K(2)SO(4) aqueous solution is conducted by using cyclic voltammetry and electrochemical impedance spectroscopy. It is shown by measurements of differential capacitance and anodic current that during the benzene oxidation at the BDD electrode, adsorption of a reaction intermediate occurs, which partially blocks the electrode surface and lowers the anodic current. At the NCD electrode, benzene is oxidized concurrently with oxygen evolution, a (quinoid) intermediate being adsorbed at the electrode. The adsorption and the electrode surface blocking are reflected in the impedance-frequency and impedance-potential complex-plane plots.

Electrochemical behavior of nitrogenated nanocrystalline diamond electrodes

Nitrogenated nanocrystalline diamond films with controlled conductivity are deposited from microwave plasma in CH4-Ar-H2-N2 gas mixtures. They are characterized using atomic force microscopy, Raman spectroscopy, and electrophysical measurements. Their electrochemical properties are studied by cyclic voltammetry and electrochemical impedance spectroscopy. Kinetic parameters of reactions in [Fe(CN)6]3-/4- redox system are determined. The character of electrode behavior is controlled by the degree on nitrogenation. With the increasing of the nitrogen content in the reaction gas mixture (from 0 to 25%), the potential window somewhat narrows, the background current increases, the reversibility of reactions in the [Fe(CN)6]3-/4- redox system increases. By and large, the transition occurs from the electrochemical behavior of a “poor conductor” to that of a metal-like electrode.

Vacuum-annealed undoped polycrystalline diamond: a new electrode material

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.

Electrodes of strongly nitrogenated nanocrystalline diamond

Electrochemical behavior of nanocrystalline diamond films grown in microwave plasma initiated in Ar-CH4-H2-N2 mixtures containing 30 to 90 vol % N2 is studied. Thin-film nanocrystalline-diamond electrodes grown at this high (30 to 90 vol %) N2 content in reactor behave as nearly metal-like: reaction in the [Fe(CN)6]3−/4− redox couple proceeds in a reversible manner. Generally, with the increasing of the N2 content in the reactor the electrochemical behavior of a “poor conductor” gives place to the metal-like one; in a sense, the material’s electrochemical activity saturates and does not change beyond some critical value of the N2 content (∼30 vol %). This conclusion is substantiated by the study of Raman spectra of the nanocrystalline diamond films: at this N2 content the diamond-graphite structure of the material is stabilized.

Nanocrystalline nitrogenated diamond: An N-type electrode material

Nitrogenated nanocrystalline diamond thin films are grown from arc-plasma in CH4/Ar/H2/N2-gas mixtures and characterized by AFM-and Raman spectroscopy, X-ray diffraction and resistivity measurements. It is shown by Mott-Schottky plots taken in indifferent electrolyte (0.5 M H2SO4) that the behavior of the nitrogenated nanocrystalline diamond thin-film electrodes resembles that of n-type semiconductor; the donor effective concentration therein is estimated.

Synthetic Semiconductor Diamond Electrodes: Comparison of Electrochemical Impedance at the Growth and Nucleation Surfaces of a Coarse-Grained Polycrystalline Film

A comparative study of the impedance of the growth and nucleation surfaces of a free-standing film of polycrystalline diamond deposited from RF-plasma and moderately doped with boron is performed. Using Mott–Schottky plots, the acceptor concentration is determined. It is shown that in the diamond bulk adjacent to the film growth side, which has more perfect crystal structure, this concentration is much lower than that near the nucleation side, where the film consists of submicron-sized grains.