Showing papers on "Cyclic voltammetry published in 1995"

A New Charge Storage Mechanism for Electrochemical Capacitors

Abstract: The hydrous form of ruthenium oxide (RuO[sub 2][center dot]xH[sub 2]O) has been demonstrated to be an excellent electrode material for electrochemical capacitors. This material, as prepared by a sol-gel process at low temperatures, is amorphous and electrically conductive. The specific capacitance is over 720 F/g. This value is at least two times higher than the highest value ever reported for such materials. The charge storage mechanism is believed to involve bulk electrochemical protonation of the oxide. This discovery opens a new avenue of research in the field of high energy density electrochemical capacitors.

Fast-scan cyclic voltammetry of 5-hydroxytryptamine

Abstract: Fast-scan cyclic voltammetry, a demonstrated analytical method for the in vivo detection of dopamine, is extended to the detection of in vitro and in vivo 5-hydroxytryptamine (5-HT) with the use of a specific potential wave form applied at 1000 V/s. The wave form, 0.2 to 1.0 to -0.1 to 0.2 V, is employed to accelerate electrode response times which are significantly slower with other wave forms due to the adsorption of 5-HT. The scan rate of 1000 V/s enables follow-up reactions which lead to the formation of strongly adsorptive products to be outrun. The peak current at a carbon fiber disk microelectrode exposed to 1 microM 5-HT in flow injection experiments is 1 nA, with a half-rise time of less than 200 ms. The peak current of Nafion-coated electrodes exposed to the same concentration of 5-HT is 5 nA, with a half-rise time on the order of 400 ms. The rate of adsorption of 5-HT was determined to be 4.22 +/- 0.33 s-1. Several compounds present in brain tissue as well as the pharmacological agents used to elicit 5-HT release in the caudate of the rat were evaluated. Those which gave a response could be differentiated from 5-HT on the basis of respective oxidative and reductive peak potentials. Nafion-coated electrodes were used to monitor transient increases in both dopamine and exogenous 5-HT in the caudate of the anesthetized rat in response to electrical stimulation. The rate of cellular uptake of 5-HT was shown to be 3-fold slower than dopamine uptake. NS-15841

Cyclic voltammetric studies of charge transfer reactions at highly boron-doped polycrystalline diamond thin-film electrodes

Abstract: Cyclic voltammetry and ac impedance analysis were used to measure the background current response and capacitance of interfaces formed at as grown (untreated) boron-doped polycrystalline diamond thin-film electrodes in contact with aqueous electrolytes. The diamond films (∼1 cm 2 , 15 μm thick ; carrier concentration, ∼10 17 cm -3 ) were grown on conducting Si substrates by plasma-enhanced chemical vapor deposition. Cyclic voltammetry was also used to determine the charge transfer reactions of several redox analytes at as grown and chemically wet etched diamond thin-film electrodes and to study the effect of surface pretreatment, including Fe(CN) 6 3-/4- , IrCl 6 2-/3- , Ru(NH 3 ) 6 3+/2+ , dopamine, 4-methylcatechol, MV 2+/+/0 , and ferrocene. The electrochemical response exhibited by the films is explained using two models : (i) traditional electron transfer at a p-type semiconductor-electrolyte interface and (ii) electron transfer at a composite electrode composed of nondiamond carbon impurities contained within a diamond matrix such that k° nondiamond >> k° diamond.

Electrical and optical properties of porous nanocrystalline TiO2 films

Abstract: The optical and electrochemical properties of nanocrystalline TiO{sub 2} electrodes in acidic aqueous solution are reported. Cyclic voltammetry, UV/vis/NIR attenuance, EPR, photocurrent spectroscopy, and electrochemical impedance measurements are presented. At negative applied potentials the TiO{sub 2} films turn black, and an EPR spectrum appears which we attribute to Ti(III) species. The results are described in terms of a model in which the reduction of TiO{sub 2} leads to band edge unpinning. 36 refs., 9 figs.

Potential-Dependent Electron Injection in Nanoporous Colloidal ZnO Films

Abstract: We have quantitatively investigated the charging of colloidal ZnO films by cyclic voltammetry and potential step measurements. Up to six electrons can be stored on each colloidal particle of the film (mean particle diameter 5.9 nm) before zinc reduction occurs. Fluorescence and absorbance of the films are already strongly influenced in potential regions where less than one electron is stored on each particle. Potentiostatically controlled and time-resolved photocurrent measurements show the influence of trap filling as well as the role of the electrolyte on the charge transport through the films.

Interactions between Organized, Surface-Confined Monolayers and Liquid-Phase Probe Molecules. 4. Synthesis and Characterization of Nanoporous Molecular Assemblies: Mechanism of Probe Penetration

Abstract: We report a detailed study of the physical and chemical properties of nanoporous, organomercaptan, self-assembled monolayers (SAMs). The present work expands upon our previous finding that two-component SAMs can serve as primitive models for studying molecular recognition phenomena. The monolayers self-assemble from solution-phase mixtures of two organomercaptans. One of these, which w call the framework component, is an n-alkanethiol (Au/HS(CH 2 ) 15 CH 3 ) about 25 A thick. The other is a much shorter organomercaptan (Au/HS(C 6 H 4 )OH), which we call the template, that induces defects within the much longer, kelp-forest-like framework. The number density of the templates on the metal surface is controlled by the template concentration in the ethanol solution from which it self-assembles and the total time allotted for self-assembly. Under the conditions used in this study the defects induced by the templates are of molecular dimensions and present at a number density of about 10 9 /μm 2 . The synthesis of these defect structures is reproducible and the nanoporous films themselves are stable under a wide variety of conditions. We characterize the monolayers using electrochemical methods. For example, we expose the probes to single- and multicomponent mixtures of redox probe molecules (including: Ru-(NH 3 ) 6 3+ , Fe(CN) 6 4- , Mo(CN) 8 4- , Fe(CN) 4 (bpy) 2- , Fe(bpy) 2 (CN) 2 , and cytochrome c) that have different hydrated radii, ionic charges, and heterogeneous electron-transfer rate constants. Cyclic voltammetry reveals that the probes are selectively passed through the nanoporous SAMs depending upon their properties. For probes of similar dimensions, selectivity is most closely correlated to ionic charge, although other factors are also important. We also find that the extent of probe penetration, and therefore the pore structure, is determined by the type and concentration of the electrolyte

Studies on the Chemical Syntheses and on the Characteristics of Polyaniline Derivatives

Abstract: SYNOPSIS Syntheses of parent polyaniline and methyl, methoxy, and ethoxy ortho-substituted polyanilines were performed using the conventional chemical methodology and monitored using the new open-circuit-potential ( V,) profile technique. The intermediate pernigraniline oxidation state was identified and isolated at the V,,, maximum (A) during the conventional chemical synthesis of poly(o-methoxyaniline) in the emeraldine oxidation state. The introduction of the substituent on the aniline ring leads to longer polymerization times and lower V,, values. Syntheses in the presence of two different monomers in solution were also investigated and showed preferential polymerization of the monomer with the lowest V, potential. All polymers produced were characterized by elemental analysis, gel permeation chromatography, UV-VIS spectroscopy, and cyclic voltammetry. The influence of the substituent on the V,, profile and on the polymer characteristics are rationalized in terms of steric and electronic effects. 0 1995 John Wiley & Sons, Inc.

Electrochemistry of Mono‐ through Hexakis‐adducts of C60

Abstract: The first systematic electrochemical study by cyclic voltammetry (CV) and rotating-disk electrode (RDE) of the changes in redox properties of covalent fullerene derivatives (2–11) as a function of increasing number of addends is reported. Dialkynylmethanofullerenes 2–4 undergo multiple, fullerene-centered reduction steps at slightly more negative potentials than C60 (1; see Table and Fig. 1). The two C-spheres in the dumbbell-shaped dimeric fullerene derivative 4 show independent, identical redox characteristics. This highlights the insulating character of the sp3-C-atoms in methanofullerenes which prevent through-bond communication of substituent effects from the methano bridge to the fullerene sphere. In the series of mono- through hexakis-adducts 5–11, formed by tether-directed remote functionalization, reductions become increasingly difficult and more irreversible with increasing number of addends (see Table and Fig. 2). Whereas, in 0.1M Bu4NPF6/CH2Cl2, the first reduction of mono-adduct 5 occurs reversibly at −1.06 V vs. the ferrocene/ferricinium couple (Fc/Fc+), hexakis-adduct 11 is reduced irreversibly only at − 1.87 V. Hence, with incremental functionalization of the fullerene, the LUMO of the remaining conjugated framework is raised in energy. Reduction potentials are also dependent on the relative spatial disposition of the addends on the surface of the fullerene sphere. Observed UV/VIS spectral changes and changes in the chemical reactivity along the series 5–11 are in accord with the results of electrochemical measurements. Further, with increasing number of addends, the oxidation of derivatives 5–11 becomes more reversible. Whereas oxidations are increasingly facilitated upon going from mono-adduct 5 (+1.22 V) to tris-adduct 7 (+0.90 V), they occur at nearly the same potential (+0.95 to +0.99 V) in the higher adducts 8–11. This indicates that the oxidations occur in these compounds at a common sub-structural element, for which a ‘cubic’ cyclophane is proposed (see Fig. 3). This sub-structure is fully developed in hexakis-adduct 11.