M. D. Krotova

Electrochemical behavior of amorphous carbon films : kinetic and impedance-spectroscopy studies

Abstract Impedance spectra and potentiodynamic curves of oxidation and reduction reactions in the quinone/hydroquinone and Ce3+/4+ systems were measured in a 0.5M H2SO4 solution on amorphous carbon thin-film electrodes grown by magnetron sputtering or ion source techniques. The electrode equivalent circuit contains a constant phase element. Only narrow-bandgap (“graphitelike”) amorphous carbon is electrochemically active; however, the wider bandgap (“diamondlike”) material also acquires the activity on “doping” it with platinum (ca 10%) in the course of film growth. The admixture of platinum does not effect film conductivity; its action probably is of catalytic character. In its electrochemical activity, the platinum-containing amorphous diamondlike carbon films resemble boron-doped polycrystalline diamond.

Synthetic semiconductor diamond electrodes: elucidation of the equivalent circuit for the case of frequency-dependent impedance

Abstract Analysis of the frequency dependence of the impedance of boron-doped diamond thin film electrodes resulted in elucidation of their equivalent circuit. The latter generally comprises a frequency-independent capacitance (or a constant phase element) and a resistance, connected in parallel, with a series-connected “bulk” resistance. For electrodes whose impedance contains a constant phase element, a frequency-dependent Mott-Schottky plot enabled us to determine the flat-band potential. The constant phase element was shown to describe properties of the space charge region in diamond, rather than those of surface states. The behaviour of diamond electrodes is often affected by a series (“Helmholtz”) capacitance, which may be compared with the space charge capacitance of a semiconductor.

Synthetic semiconductor diamond electrodes: The comparative study of the electrochemical behaviour of polycrystalline and single crystal boron-doped films

Capacitance and potentiodynamic measurements were conducted on single crystal (homoepitaxial) and polycrystalline boron-doped diamond thin-film electrodes. The impedance characteristics and kinetic data in the Ce3+/4+ redox system, even if having a great deal of variability, appeared to be similar for the two kinds of diamond electrodes, whereas the kinetics of redox reactions on crystalline diamond and amorphous diamond-like carbon films differ significantly. These data in aggregate lead to a tentative conclusion that the electrochemical behaviour of polycrystalline diamond is determined by the diamond crystallites proper, rather than by the disordered carbon of the intercrystallite boundaries. The concentration of uncompensated acceptors in diamond was estimated from the linear and non-linear impedance data. The ways of presentation of Mott–Schottky plots for semiconductor electrodes are discussed for the case of frequency-dependent differential capacitance.

Effect of Crystal Structure on the Behavior of Diamond Electrodes Electrochemical Characteristics of Individual Crystal Faces

Effects of crystal structure on the electrochemistry of boron-doped high temperature-high pressure (HTHP) diamond single crystals grown from a Ni-Fe-C melt are studied. On the {111}, {100}, and {311} faces, the linear and nonlinear electrochemical impedance measurements were performed and the rate of electron transfer for Fe(CN) 3-/4- 6 was evaluated. Like polycrystalline chemical vapor deposited films the HTHP electrodes equivalent circuit includes a constant phase element. The uncompensated acceptor concentration in the semiconductor diamond was determined from Mott-Schottky plots and amplitude-demodulation measurements and was found to vary in the range of 10 18 to 10 21 cm -3 . The difference in the electrochemical behavior of individual crystal faces is primarily attributed to different boron concentrations in the growth sectors associated with the faces.

Synthetic semiconductor diamond electrodes : determination of acceptor concentration by linear and non-linear impedance measurements

The non-compensated boron concentration in thin films of boron-doped polycrystalline diamond, chemically vapour deposited onto tungsten or silicon substrates, was determined by the methods of differential capacitance and amplitude demodulation. In the case of frequency-independent capacitance, both methods give practically the same concentration value. For samples with frequency-dependent capacitance, an upper estimate of the concentration was obtained (using both methods) of the same order of magnitude. In this case, the electrode impedance includes a constant-phase element.

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’.

Photosplitting of water in a photoelectrolyser with solid polymer electrolyte

Abstract A photoelectrolyser with a semiconductor (SrTiO 3 , TiO 2 ) photoanode for splitting water at the expense of solar energy was designed, with a membrane of sulphonated polytetrafluorethylene used as an electrolyte. The flat-band potential of the semiconductor was determined from the photocurrent data, and the characteristics of the semiconductor electrodes in contact with the solid polymer electrolyte and with aqueous H 2 SO 4 solution were found to be similar. The quantum efficiency of the anodic photocurrent is as high as 0.8.

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.