V. Ya. Mishuk

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.

Application of nonlinear A.C. methods in the investigation of the electrical double layer properties

Abstract This paper presents the results of an investigation of the electrical double layer properties for the systems Hg/NaF, Cd/NaF, Hg/Na2SO4 carried out by use of nonlinear a.c. methods, based on the quadratic approximation of the current–voltage characteristic of the cell, the two-frequency method, and the amplitude demodulation method. By use of these methods it is possible to measure the potentials of zero charge of metals at higher electrolyte concentrations than is possible with the cell impedance measurement method.

Synthetic Semiconductor Diamond Electrodes: Electrochemical Characteristics of Individual Faces of High-Temperature-High-Pressure Single Crystals

Effects of crystal structure on the electrochemistry of boron-doped high-temperature-high-pressure diamond single crystals grown from an Ni–Fe–C–B melt are studied. On the {111}, {100}, and {311} faces, the linear and nonlinear electrochemical impedance spectra and the electrochemical kinetics in the Fe(CN)63_/4_ redox system are measured. The acceptor concentration in the diamond interior adjacent to these faces was determined from the Mott–Schottky plots and the amplitude-demodulation measurements. It varies in the 1018 to 1021 cm–3 range. The difference in the electrochemical behavior of individual crystal faces is primarily attributed to different boron acceptor concentrations in the growth sectors associated with the faces.

Polarization Diagram of the Electrochemical Impedance of Second Order: A Theory that Accounts for the Diffusion of Reactants

With the aim of improving diagnostic possibilities of second-order impedance spectroscopy, a theory is put forward for a polarization diagram of a second-order impedance for reactions with charge transfer. The theory takes into account the diffusion of discharging particles in the absence of their specific adsorption.

Second-order impedance spectroscopy of an electroreduction reaction with allowance for the Frumkin correction

A theory of second-order impedance spectroscopy is developed. The theory is good for analyzing an electroreduction reaction in surface-inactive supporting electrolytes with allowance made for the structure of the electrical double layer (the Frumkin correction). According to the theory, a measured second-order impedance contains, as a factor, an effective charge transfer coefficient. The latters dependence on potential has a minimum connected with the diffuseness of the electrical double layer. The theoretical notions are experimentally confirmed on the basis of a real system.

Second-order Impedance in Electrode Kinetics

The kinetics of electrode reactions with a rather severe influence of the EDL structure is studied by nonlinear second-order impedance spectroscopy. Polarization impedance spectra and potential dependences of a nonlinear impedance are obtained for the reaction of electroreduction of the ferricyanide anion on the cadmium and bismuth electrodes in surface-inactive supporting NaF and Na2SO4 electrolytes. The results of measurements for the reaction Eu3+ + e → Eu2+ on the bismuth and mercury electrodes are presented. It is shown that such important parameters of EDL as the potential of zero charge and the second derivative of potential with respect to the charge of the electrode surface can be determined directly from experimental curves even under conditions of occurrence of a faradaic process.

Nonlinear Impedance of Liquid In–Ga Electrode in Aqueous Solutions of a Symmetrical Surface-Inactive Electrolyte

Experimental data related to the potential dependence of nonlinear characteristics of the electrical double layer at a liquid In–Ga electrode in aqueous solutions of a symmetrical surface-inactive electrolyte are obtained for the first time. It is shown that, as opposed to polycrystalline Cd and Pb electrodes, on a liquid (atomically smooth) In–Ga electrode, as on Hg, there is a clear intersection of the potential dependences of a nonlinear signal for different concentrations of a 1–1-valence surface-inactive electrolyte at one point. The intersection point exactly corresponds to the potential of zero charge of an electrode undistorted by specific adsorption of ions. It is established that, when estimating hydrophilicity of metals by a nonlinear impedance method, most information is provided by the region of average negative charges, rather than by the region near zero charge. It is shown that, as opposed to a linear impedance method, the nonlinear impedance method makes it possible to determine, directly from experiment, quantities that directly characterize the metal–solvent chemisorption interaction in a pure form; at the same time, these quantities are criteria of lyophilic nature of metals. Quantities that characterize the metal–solvent chemisorption interaction, obtained by the linear and nonlinear impedance methods are in good agreement, which confirms the validity of the approach we proposed earlier for separating the difference between reciprocal capacitances of the inner part of the electrical double layer on Hg and metal M, ΔHgMC-1i= 1/CHgi– 1/CMiinto physical (ΔHgMC–1)physand chemical constituents. This coincidence also confirms correctness of numerical values obtained earlier for quantities (ΔHgMC–1)phys.