Electron Transfer through Monolayers of Mixed Long-chain Aliphatic Acids and Alcohols

Properties of monolayers of cetyl, stearyl, and eicosyl alcohols and stearic and behenic acids and their mixtures are studied, along with the kinetics of a redox reaction of hemin adsorbed on these monolayers. Three-dimensional computer models of corresponding monolayers are constructed and compared with their electrochemical behavior. Properties of monolayers of cetyl alcohol in acid and alkaline solutions and stearyl alcohol in acid solutions correspond to package of molecules in a stretched conformation oriented normally to the electrode surface; for the other one-component systems, the monolayer structure is less regular. The hemin redox reaction rate has no correlation whatsoever with the monolayer thickness and is defined by the length of the electron transfer path (over a chain of covalent bonds and through van der Waals contacts between molecules). The stronger the deviation of the conformation of molecules from the stretched one, the shorter the effective path of the electron transfer. In mixed films of stearic acid and cetyl alcohol (1 : 6), it is sometimes possible to keep molecules of stearic acid in a stretched conformation, thus inhibiting the electron transfer along the hydrocarbon chain of this molecule to a maximum extent. The other mixed systems, while making the conformation more stretched, fail to provide for a completely regular structure.     

Electrochemical intercalation of lithium into diamond-pyrocarbon nanocomposites

Physics of the Solid State - The electrochemical behavior of composite materials based on nanodiamond and carbal is investigated in the course of cathodic intercalation of lithium from an LiPF6...     

Electrodes of synthetic diamond: The effects of Ti substrate pretreatment on the electrode properties

The electrode properties of boron-doped diamond thin films grown on Ti substrates by a hot-filament chemical vapor deposition technique are evaluated. The Ti substrate surface modifying conditions are devised, involving the surface roughening, annealing, and etching, which effectively improve the diamond electrode properties. The preetching of the Ti substrate produces the titanium hydride layer that can affect the boron-doped diamond film growth significantly. The substrate roughened surface obviously improved the diamond film adhesion and reduced the inner stress. The electrodes reveal minimal background current and better stability. A wider potential window, up to 3 V, is observed for the boron-doped diamond on the etched/annealed samples. The electrochemical activity of the electrodes in the Fe(CN) ₆ ^3-/4- redox system somewhat increases with increasing surface roughness. The apparent increase in the reversibility of the reaction may be explained by the decrease in the true current density. Suitability of the Ti-based boron-doped diamond electrodes for electroanalytical applications is exemplified by sensing the trace metal ions, such as Hg²⁺ and Pb²⁺.__________From Elektrokhimiya, Vol. 41, No. 4, 2005, pp. 387–396.Original English Text Copyright © 2005 by Pleskov, Evstefeeva, Krotova, Lim, Chu, Ralchenko, Vlasov, Kononenko, Varnin, Teremetskaya, Shi.This article was submitted by the authors in English.     

Reduction of oxygen and hydrogen peroxide on electrodes with adsorbed monolayer of aliphatic compounds

Cathodic reduction of oxygen and hydrogen peroxide on amalgamated platinum electrodes, which are coated with monolayers of long-chain aliphatic compounds cetyl alcohol (CA) and stearic acid (SA), is retarded as compared with the same reactions on clean mercury (or amalgam) surface. The oxygen reduction kinetics differ from that on mercury. The difference is explained by that oxygen diffuses into the monolayer and is reduced in it at a certain distance from the metal surface and only at the limiting current the reaction is forced onto the monolayer surface. In contrast to the oxygen reduction, the hydrogen peroxide reduction kinetics on electrodes with SA and CA monolayers is much closer to that on mercury, but with some quantitative distinctions. All results favor the H₂O₂ reduction at the monolayer/solution interface. The difference in the behavior of O₂ and H₂O₂ is explained by different polarity of these molecules: it is significantly more difficult to penetrate the hydrocarbon monolayer for polar H₂O₂ molecule than for nonpolar O₂ molecule.     

Electrode Properties of Tetrahedral Amorphous Carbon

Impedance spectra in a 2.5 M H₂SO₄ solution and kinetic characteristics of reactions in the Fe(CN) ₆ ^3−/4− redox system are measured for thin-film electrodes of tetrahedral amorphous carbon (ta-C). After an anneal in a vacuum at 700 to 900°C or implantation of C⁺ ions (10¹⁵ to 10¹⁷ ion/cm²), ta-C films acquire electrochemical activity, which can be explained by an increased content of sp ² carbon.__________Translated from Elektrokhimiya, Vol. 41, No. 7, 2005, pp. 866–871.Original Russian Text Copyright © 2005 by Evstefeeva, Pleskov, Kutsay, Bello.     

Effect of the Length of the Hydrocarbon Chain of Adsorbed Aliphatic Acids and Alcohols on the Electron Transfer Probability

The kinetics of redox reactions of hemin adsorbed over a monolayer of long-chain aliphatic alcohols, such as cetyl alcohol (C₁₆) and stearyl alcohol (C₁₈), and stearic (C₁₈), behenic (C₂₂), and melissic (C₃₀) acids is studied. The reaction inhibition on an alcohol monolayer (0.34 orders of magnitude per methylene group) is close to that reported in the literature. The inhibition on an acid monolayer is substantially smaller (0.2 to 0.07 orders), which is attributed to irregularity of the structure of the monolayer of carboxylic acids caused by the dissociation of carboxyl groups, especially in alkaline solutions. Violation of a regular structure of an acid monolayer is confirmed by a considerable irreversible increase in the electrode capacitance in alkaline solutions and by a decrease in the ohmic resistance of a monolayer of the longest saturated acid, i.e. triacontanoic acid. For a regular monolayer structure, when the length of an extended hydrocarbon chain defines the barrier thickness (alcohols), experimental data accord with predictions of the superexchange theory, which holds that the probability of a long-distance electron transfer depends on the length of the chain of covalent bonds over which the superexchange occurs. If the film structure is irregular (film thickness decreases with increasing hydrocarbon chain length (acids)), the packing of these bonds becomes essential, which allows the electron not to pass over all the bonds but to hop onto a nonbonded chain link nearby. The obtained data indicate that the electron transfer probability is defined by the electron's path, rather than by the barrier's geometrical thickness.     

Hydrogen peroxide reduction on amalgamated platinum electrodes covered with a monolayer of stearic acid

The reduction of hydrogen peroxide and, for comparison, oxygen on an amalgamated platinum electrode covered with a monolayer of stearic acid is studied by methods of polarization curves and impedance spectroscopy. In contrast with the oxygen reduction, the reduction of dissolved hydrogen peroxide occurs predominantly on the monolayer surface, rather than inside it. This is explained by the difficulty of penetration of the polar molecule of hydrogen peroxide into hydrocarbon environment.     

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.     

Synthetic Semiconductor Diamond Electrodes: Comparison of Electrochemical Behavior of the Growth and Nucleation Surfaces of Thin Polycrystalline Films

A comparative study of electrochemical kinetics in ferro- and ferricyanide solutions is performed. Electrochemical impedance spectra for the growth and nucleation sides of relatively thin films of boron-doped polycrystalline diamond synthesized by hot-filament CVD technology are taken. Concentrations of noncompensated acceptor near the growth and nucleation surfaces are estimated. It is shown that the growth and nucleation surfaces differ little in their electrochemical behavior, which is attributed to the absence of significant difference in the concentration of electrochemically active structural defects or the boron-acceptor impurity between the two sides of thin diamond films.     

Amorphous Diamond-like Carbon Electrodes: The Catalytic Effect of Platinum Admixture

The kinetics of electrode reactions in the Fe(CN)^3-/4- ₆ redox system is studied on the platinum-doped (Pt content up to 15 at. %) amorphous diamond-like carbon thin-film electrodes. The wide-gap diamond-like carbon is electrochemically inactive per se; yet, it acquires the electrochemical activity on adding platinum to the film bulk during the film growth. The effect of platinum is of a threshold character; at the same time, with increasing platinum content, the electrode differential capacitance increases smoothly, and the resistivity of the material remains practically constant. The effect is explained with a model which assumes that both the conductance in the film bulk and the catalytic action of platinum on the charge transfer at the electrode/solution interface have a nonuniform character.