Differences in the electrorheological response of a particle suspension under direct current and alternating current electric fields

We have observed an unusual reduction of shear stress with increasing shear rate under direct current electric fields, for an electrorheological fluid composed of sulfonated poly(styrene-co-divinylbenzene) particles dispersed in silicone oil. At all shear rates, the shear stress under the electric field is larger than that in the absence of the field, indicating that there is still some field-induced agglomeration of the particles. In contrast, the behavior under alternating current electric fields is the Bingham-fluid-type response commonly observed with electrorheological fluids. It is suggested that the conventional dipole–dipole interaction approach based on simplified microstructural models would be unable to explain these phenomena. Received: 27 November 2000 Accepted: 22 May 2001     

Activation of glassy carbon electrodes by alternating current electrochemical treatment

Glassy carbon electrodes subjected to “severe” electrochemical treatment is saturated chloride solutions exhibit enhanced electrochemical reversibility compared to untreated electrodes. Apparently faster rates of electron-transfer result in lowering of the overpotential by 110–300 mV for eight electroactive species tested. Systematic alterations in the response in terms of peak potential, sensitivity, background current, and stability are reported. Scanning electron micrographs show the introduction of pits of random size distribution. From the practical point of view, the most significant improvement is observed in differential pulse measurements, that are highly sensitive to small changes in the rate of electron transfer. In amperometric detection for liquid chromatography, the pretreated electrodes are shown to facilitate the quantitation of different eluting species. It appears that the surface and bulk properties of the glassy carbon have profound effects on the observed behavior.     

Faradaic alternating current response of the adsorbed redox couple

A simple theoretical model to explain the a.c. voltammetry of the adsorbed reactant and product is developed. The theory indicates that maximum a.c. responses of the reversible redox reactions appear out of phase, whilst for the totally irreversible reaction, response appears in phase with the excitation signal. Theoretical predictions are compared with a.c. voltammograms of berberine.     

Thermodynamic stabilities of semiconductor electrodes

The application of the Pourbaix diagram to the prediction and understanding of the thermodynamic stability of semiconductor materials is discussed. With the aid of flat band potential information, the thermodynamic stability of semiconductor materials under various operating conditions, types of electrodes (n- or p-) to be used, conversion efficiencies, types of redox couples to be chosen for a given semiconductor electrode material, and the proper operating pH region can be determined. It is shown from the available information that all the semiconductor materials examined, i.e., CdS, CdSe, CdTe, and GaP may be unstable with respect to anodic dissolution and that p-type electrodes may be successfully used as photocathodes for the photoassisted electrolysis of water to hydrogen. GaAs is shown to be unstable with respect to both anodic and cathodic dissolution.     

On the mechanism of electrochemical dispersion of platinum under the action of alternating current

The behavior of platinum electrodes in alkali solutions under the action of pulse alternating current was studied. A mechanism of electrochemical dispersion of platinum was proposed. An important step of the mechanism is the discharge and cathodic insertion of alkali metal cations into the crystalline lattice of platinum followed by the decomposition of intermetallic compound Pt _m Na upon the interaction with water. Dispersion is facilitated by the presence of an oxide film on the platinum surface formed in the anodic period of pulses, a periodical alternation of electrode polarity resulting in the electrochemical injection of vacancies from the metal bulk to the surface, and hydrogen and oxygen formation in the corresponding periods.     

Macroscopic theory of electrochemical transducer of alternating current,based on two-dimensional model with mesh cathodes

Basing on approximate equations describing real processes, a possibility of the presenting in general closed form of the theoretical frequency dependence of diffusion current density in the entire frequency range is considered.     

Study of the underlying electrochemistry of polycrystalline gold electrodes in aqueous solution and electrocatalysis by large amplitude fourier transformed alternating current voltammetry

Polycrystalline gold electrodes of the kind that are routinely used in analysis and catalysis in aqueous media are often regarded as exhibiting relatively simple double-layer charging/discharging and monolayer oxide formation/removal in the positive potential region. Application of the large amplitude Fourier transformed alternating current (FT-ac) voltammetric technique that allows the faradaic current contribution of fast electron-transfer processes to be emphasized in the higher harmonic components has revealed the presence of well-defined faradaic (premonolayer oxidation) processes at positive potentials in the double-layer region in acidic and basic media which are enhanced by electrochemical activation. These underlying quasi-reversible interfacial electron-transfer processes may mediate the course of electrocatalytic oxidation reactions of hydrazine, ethylene glycol, and glucose on gold electrodes in aqueous media. The observed responses support key assumptions associated with the incipient hydrous oxide adatom mediator (IHOAM) model of electrocatalysis.     

Nonlinear alternating current responses of electrorheological solids

When a composite containing nonlinear dielectric particles suspended in a host medium is subjected to a sinusoidal alternating current (ac) electric field, the dielectric response of the composite will generally consist of ac fields at frequencies of higher order harmonics. For an electrorheological (ER) solid under structure transformations due to external fields, we apply the Ewald-Kornfeld formulation to derive the local electric fields and induced dipole moments explicitly, and then we perform the perturbation expansion method to extract their fundamental and third-order harmonics analytically. It is shown that the degree of anisotropy of the ER solid can affect these harmonics significantly. Our results are well understood in the spectral representation theory. Thus, it seems possible to perform a real-time monitoring of the structure transformation by measuring the nonlinear ac responses of ER solids.     

A study of the mechanism of response of liquid ion exchanger calcium selective electrodes: Part III. Membrane behaviour under non-zero current conditions

The I-E response of the liquid membrane of the calcium selective electrode is studied under constant or linearly varying current and voltage. An increase in the membrane resistance, recorded when an electrical current crosses the membrane, is due to the outflow of Cl^? ions initially present in the membrane. When calcium ions are replaced by alkaline ions inside the membrane at constant current, the decrease of the membrane resistance due to an ion exchange is in agreement with the conductivity measurements (Part II). When the applied voltage is imposed besides the ion exchange one must take into account the interfacial overpotential to explain the important rectification effect observed. The interfacial transfer constant rate of alkaline ions seems greater than that of Ca²⁺ ion.     

Reversible redox processes of poly(anilines) in layered semiconductor niobate films under alternate UV-vis light illumination

The synthesis of polyaniline (PANI) with semiconducting layered niobate (NbO) to form PANI/NbO hybrid materials and their reversible color change under a unique redox process under the influence of UV and/or visible light have been investigated. The in-situ polymerization of anilinium chlorides (ANI) packed in a regular orientation in a bilayer structure within the NbO interlayers led to PANI/NbO hybrid powders by heat treatment using (NH(4))(2)S(2)O(8) as the catalyst. The resulting PANI of these hybrids showed the characteristics of a fully oxidized quinoid form, i.e., pernigranine (PG). The PANI/NbO suspension in H(2)O was cast on a glass substrate to form a PANI/NbO film after evaporation of the water with a good parallel orientation of the NbO layers against the glass substrate. Upon UV light irradiation in the presence of a reductant such as MeOH, the violet-colored PANI (PG) polymers within the NbO interlayers were reduced by the NbO-induced photocatalytic reactions and led to a colorless PANI, i.e., leucoemeraldine (LE). Moreover, the resulting colorless PANI/NbO films reverted back to a blue-colored PANI, i.e., emeraldine (EM), due to oxidation by the surrounding O(2) gas. The PANI/NbO hybrid films were able to retain repetitive and reversible photoinduced patterning for over 50 cycles under such alternate UV and visible light irradiation.     

Dynamic polarizabilities of polyaromatic hydrocarbons using coupled-cluster linear response theory

Coupled-cluster theory with single and double excitations is applied to the calculation of optical properties of large polyaromatic hydrocarbons. Dipole polarizabilities are reported for benzene, pyrene, and the oligoacenes sequence n=2-6. Dynamic polarizabilities were calculated on polyacences as large as pentacene for a single frequency and for benzene and pyrene at many frequencies. The basis set effect was studied for benzene using a variety of basis sets in the Pople [Theor. Chim. Acta 28, 213 (1973)] and Dunning [J. Chem. Phys. 90, 1007 (1989)] families up to aug-cc-pVQZ and the Sadlej pVTZ basis [Collect. Czech. Chem. Commun. 53, 1995 (1998)], which was used exclusively for the largest molecules. Geometries were optimized using HF, B3LYP, PBE0, and MP2 and compared to experiment to measure method dependence and the possible role of bond-length alternation. Finally, the polarizability results were compared to four common density functionals (B3LYP, BLYP, PBE0, PBE).     

Mathematical treatment of concentration profiles and anodic current of amperometric enzyme electrodes with chemically-amplified response

Formulae are presented for explicit solutions of partial differential equations describing the transient and stationary behaviour of concentration profiles and the anodic current in amperometric chemically-amplified enzyme electrodes. The mathematical treatment is based on reaction/diffusion models with irreversible first-order catalytic reactions. Numerical results for an L-lactate-sensing electrode are used to demonstrate the application of the derived formulae.     

The determination of ethanol by alternating current tensammetry

A method for the determination of ethanol in vodka by phase-sensitive a.c. tensammetry is described; the only prior treatment of the sample is dilution with water and addition of KCl The positive wave of the tensammetric response was used. Ethanol concentrations in the range 2–10% can be determined with a relative imprecision of 0.5%. Other species having surfactant properties, such as the fusel oils found in most distilled liquors, interfere.     

Green functions in the theory of semiconductor surfaces

Electronic properties of a few technologically important semiconductor surfaces, explored in surface Green function calculations, are presented and briefly discussed in comparison with experimental data from high-resolution surface spectroscopy. The emphasis is on results of first-principles calculations employing the local density approximation or the generalized gradient approximation of density functional theory. The systems addressed comprise of the prototype surfaces of the elemental semiconductors diamond and Si, as well as the group IV compound semiconductor SiC. The examples show that surface Green function calculations, as performed by Maria St licka and Sydney Davison in their early work on the surfaces of model systems, such as linear monoatomic chains or the Kronig–Penney model, can nowadays be applied to efficiently evaluate electronic properties of real surfaces. The results of such ab initio Green function calculations are found to be in very good agreement with experimental data.     

Mechanisms of water photooxidation at n-TiO2 rutile single crystal oriented electrodes under UV illumination in competition with photocorrosion

Photoetching is known to compete with water photooxidation at n-TiO₂ rutile electrodes in contact with aqueous H₂SO₄ solutions under UV illumination and anodic bias. A mechanism based on the generation of bridging hydroxyl species from the adsorption of water molecules at photoinduced bridging oxygen vacancies is proposed in order to explain the competition between both photoreactions. This mechanism, designated as Redox Photooxidation (RP) Mechanism, correlates the atomic arrangement of the TiO₂ surface with its photocatalytic activity, considering that the first step for water photooxidation is the photogeneration of bridging oxygen/hydroxyl radicals associated with intrinsic bandgap surface states, via inelastic transfer of free valence band holes to bridging oxygen/hydroxyl groups, depending on the electrolyte pH. The critical distance between adjacent bridging oxygen/hydroxyl radicals allows their covalent bonding with generation of surface-bound peroxide species, which are further photooxidized leading to oxygen evolution. The RP mechanism allows to explain literature experimental results concerning surface modifications of n-TiO₂ rutile during photoetching in competition with water photooxidation, as well as their dependence on crystal orientation. The photogeneration of chemisorbed peroxo species, intermediates of the oxygen evolution reaction, detected by MIRIR spectroscopy, as well as experimental results obtained from PL and DEMS experiments are also interpreted in the light of the RP mechanism. A comparative analysis with the nucleophilic attack (NA) Mechanism, an alternative model proposed recently to explain photoelectrochemical water oxidation at n-TiO₂ rutile, is presented.     

Modelling the electric potential distribution in the dark in nanoporous semiconductor electrodes

This study concerns the electric potential distribution in the dark in nanocrystalline porous semiconductor electrodes, in full depletion conditions. Since band bending in a single colloidal particle is small, the idea is to develop a model that accounts for the total potential drop resulting from the equilibration between the Fermi level and the redox potential in the solution. As preliminary steps, the band bending and potential distribution in a planar electrode and also in a colloidal semiconductor particle are reviewed. In order to overcome the limitations of results based on these geometries, a model based on a columnar shape is developed. The Poisson equation is solved in the columnar electrode, with careful consideration of the boundary conditions. A large potential drop is shown to take place at the back contact. To complete the study, the effect of the depletion zone in the transparent conducting oxide is analysed. Simple expressions are derived that permit evaluation of how the total potential drop is distributed between the electrode and the substrate. From this, the strength and spatial range of the electric field in the electrode can be estimated. Received: 25 June 1998 / Accepted: 15 December 1998