Yu. V. Pleskov

Electrochemistry of Diamond: A Review

Because of its extraordinary chemical stability, diamond is a perspective electrode material to be used in electrochemistry and electrochemical engineering. In this review-article, the results of basic studies in the synthetic-diamond electrochemistry are summarized: the electrochemical kinetics, photoelectrochemistry, electrochemical impedance spectroscopy. Relations between the semiconductor nature and crystal structure of diamond and its electrochemical behavior are revealed. Prospects for using diamond electrodes in the electroanalysis, electrosynthesis, and environmentally-oriented industry are outlined.

Regularities of photoemission from metals into electrolyte solutions

Summary The regularities of photoemission from metal into electrolyte solution upon electrode illumination with ultraviolet light (365 nm) have been investigated. The experiments were carried out on mercury and thallium and indium amalgams in KCl, KF and Na 2 SO 4 solutions containing H 3 O + and N 2 O as electron acceptors. In concentrated electrolyte solutions, the photoemission current in a wide range of potentials, φ, is described by the equation, I=A (φ 0 −φ) 5/2 . The threshold potential φ 0 , determined by extrapolation of the dependence, I 0.4 −φ to I =o, does not depend on the metal nature and is equal to −0.31±0.03 V (SCE). The electronic work function of the system, mercury-solution, at the zero-charge potential has been found to be 3.26 eV. In dilute electrolyte solutions, the effect of the ψ 1 -potential on photocurrent has been established. The effect of the ψ 1 -potential on photoemission proper can be quantitatively taken into account by substituting in the formula for I , φ−ψ 1 for the electrode potential, φ. A method of determining the zero-charge potential of metal based on the photocurrent measurements in dilute solutions has been proposed. With increasing supporting electrolyte concentration from 10 −3 to 1 M , the “corrected” photocurrent decreases. This effect has been shown to be associated with the effect of electrolyte concentration on the secondary processes occurring in solution and involving solvated electrons.

Photoemission of electrons into electrolytes from solid metals with poor hydrogen adsorption properties

Summary Investigation of photoelectron emission from lead, cadmium, indium and bismuth into aqueous solutions has supported the basic relationships obtained earlier for the mercury electrode, namely the five-halves law and the independence of the photoemission threshold on the nature of the metal. A linear relationship is established between the mean hydration range of electrons and their energy. The photoemission method is used to study the double-layer structure and kinetics of some electrochemical reactions on solid metals. A new method is suggested for determination of zero charge potential from the intersection of photocurrent potential curves obtained in solutions with variable electrolyte concentration. Effects of ψ ′-potential and specific adsorption on the total photoemission process in dilute solutions are analysed. Investigation of atomic hydrogen reactions on a bismuth electrode has led to the discovery of the activationless process of electrochemical desorption. This reaction mostly involves participation of water molecules and not of hydrogen ions. Kinetic characteristics of the NO 3 2− ion-radical reactions were measured.

Synthetic Diamond Electrodes: Photoelectrochemical Investigation of Undoped and Boron‐Doped Polycrystalline Thin Films

The photoelectrochemical behavior of polycrystalline diamond films, grown on tungsten substrates by chemical vapor deposition (CVD), has been studied in 0.5M H 2 SO 4 solution. Different types of films have been investigated, with or without intentional doping with boron. Experimental results point to a p-type behavior of boron-doped samples, while the presence of an inversion of the photocurrent sign reveals an insulating behavior of undoped samples. Photocurrent spectra display different threshold energies, which have been related with possible defect levels inside both types of diamond films

Electron photoemission as a new method for studying the electric double layer structure and the kinetics of electrochemical reactions

Summary The effect of various factors on the electron photoemission proper and on further transformations in solution of hydrated electrons and the products of their interaction with acceptors has been studied. Specific adsorption does not change the photoemission threshold if the adsorption layer thickness remains less than the de-Broglie wavelength of emitted electrons. Otherwise adsorption has a strong effect on photoemission. At low acceptor concentrations, when a large portion of emitted electrons is not captured in solution but returns to the electrode, the dependence of photocurrent on electrode potential is no longer described by the “5/2-law”. The mean distance from the electrode at which emitted electrons undergo hydration has been estimated. The electron work function for the mercury/aqueous solutions system at the potential of zero charge is 3.0 eV. The photoemission method has been applied to the investigation of the kinetics of electrochemical transformation of atomic hydrogen on the mercury electrode. It has been shown that atomic hydrogen enters into both conjugate reactions—ionization and cathodic reduction—being in the same initial state (adsorbed on the electrode or dissolved in electrolyte). One of these reactions (apparently, electrochemical desorption) is activationless. The electrochemical desorption reaction occurs with the participation both of hydrogen ions and water. The rate constant of bulk disintegration of unstable ion-radical NO32− has been measured as well as its oxidation transfer coefficient on mercury.

Threshold effect of admixtures of platinum on the electrochemical activity of amorphous diamond-like carbon thin films

The kinetics of electrode reactions in a Fe(CN) redox system on thin-film electrodes of amorphous diamond-like carbon 3yy4y 6 with admixtures of platinum (0–15 at.%) have been studied. The wide-gap diamond-like carbon is not in itself electrochemically active; however, it acquires electrochemical activity upon introducing platinum into the DLC bulk during the film deposition. The effect of platinum is shown to be of a threshold nature: the electrochemical current appears at approximately 3 at.% and saturates at approximately 10 at.% of Pt. By contrast, the differential capacitance increases continuously with increasing Pt content; and the DLC resistivity practically does not change. The observed effects are explained in terms of a model assuming a non-uniform character of both electric conductance in the DLC bulk and the catalytic effect of platinum at the electrode ysolution interface. 2002 Elsevier Science B.V. All rights reserved.

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 diamond, a new electrode material for electroanalysis

Depending on the doping level, diamond exhibits properties of either a semiconductor or a semimetal. Heavily doped “metallic” diamond was found to be a corrosion-resistant electrode, suitable for electrochemical syntheses and analyses. The advantages of synthetic diamond in electroanalytical chemistry are its corrosion resistance, good reproducibility of electrochemical properties, low background currents, and selectivity to a number of reactions used to develop electroanalytical methods.

Lithium intercalation into amorphous-silicon thin films: An electrochemical-impedance study

Lithium intercalation into 0.25-μm-thick films of amorphous silicon is studied using the electrochemical-impedance technique. An equivalent circuit, proposed for such electrodes, comprises the electrolyte resistance and three units connected in series, each unit being a parallel combination of a resistance and a constant-phase element. The units relate to the charge transfer processes at the silicon/electrolyte interface, charge transfer though the passive film on the silicon, and the lithium diffusion into the silicon bulk. During potential cycling, changes occur largely in the unit related to the passive film. The lithium diffusion coefficient in the amorphous silicon is estimated as ∼ 10−13 cm2 s−1.

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.