Range Of Sweep Rates Research Articles

Transient Hole Trapping in Individual GeSi Quantum Dot Grown on Si(001) Studied by Conductive Atomic Force Microscopy

Electrical properties of individual self-assembled GeSi quantum dots grown on Si substrates are investigated by using conductive atomic force microscopy at room temperature. By controlling the bias voltage sweep in a certain fast sweep rate range, a novel current peak is observed in the current–voltage characteristics of the quantum dots. The current peaks are detectable only during the backward voltage sweep immediately after a forward sweep. The current peak position and intensity are found to depend strongly on the voltage sweep conditions. This kind of current–voltage characteristic under fast sweep is very different from the ordinary steady state current behaviour of quantum dots measured previously. The origin of this phenomenon can be attributed to the transient hole trapping in the potential well formed by the quantum dot sandwiched between the native oxide layer and the bottom Si substrate.

Kinetics of redox mediator tris(4-bromophenyl)amine in acetonitrile and ionic liquid [BMIm][PF6]: Oxidation of benzyl and cyclohexyl alcohols

A reversible one-electron oxidation of tris(4-bromophenyl)amine (TBPA) in acetonitrile or 1-butyl-3-methylimidazoliumhexafluorophosphate [BMIm][PF6] media unambiguously generates the cation-radical (TBPA + ), and its reduced form is catalytically regenerated in a follow-up chemical reaction with benzyl alcohol in the presence of 2,6-lutidine. The electron-transfer kinetics for these transformations has been investigated by a detailed comparison of the results obtained in both media. The apparent standard heterogeneous rate constant k app 0 for an interfacial single-electron transfer of the mediator in acetonitrile containing 0.1 M Et 4NClO 4 was estimated using cyclic voltammetry (CV) profiles within a wide range of sweep rates (20–1000 mV s −1). The slightly higher k app 0 value for the TBPA/TBPA + couple at a Pt electrode surface was found to be (2.1 ± 0.8) 10 −2 cm s −1 compared to (1.4 ± 0.6) 10 −2 cm s −1 at a glassy carbon (GC) surface. k app 0 values estimated in [BMIm][PF6] are comparatively two orders of magnitude lower (6.6 ± 0.5 × 10 −4 cm s −1 and 4.9 ± 0.4 × 10 −4 cm s −1 at Pt and GC, respectively). It has been found that the second-order homogeneous rate constant for the oxidation of benzyl alcohol by TBPA + in acetonitrile (12.10 M −1 s −1) was estimated to be greater by one order of magnitude than that in ionic liquid [BMIm][PF6] (4.20 M −1 s −1). Similar experiments carried out for the oxidation of 2-chlorobenzyl alcohol, 4- tert-butylcyclo hexanol and 4-methylcyclohexanol by the TBPA + showed no noticeable catalytic effect due to large potential difference of the mediator ( E p/2 = 0.74 V vs. Fc/Fc +), compared with those of the substrates.

Electro-oxidation of d-xylose on platinum and gold electrodes in alkaline medium

The electrocatalytic oxidation of d-xylose was performed in alkaline medium at both platinum and gold electrodes. The higher catalytic activity of gold was clearly demonstrated. Cyclic voltammetric studies were performed in a wide range of sweep rates (0.010–1.00 V s −1), various concentrations of d-xylose and of hydroxide. The influence of temperature was also studied. Kinetic data was obtained. The chromatographic analysis has identified xylonic acid (0.5 to 5×10 −3 mol dm −3) as the main oxidation product produced during the electrolysis of d-xylose, on Pt and Au electrodes, respectively, at potentials corresponding to the main oxidation peaks. The d-xylose conversion yield was found to be higher on Au than on Pt (98% against 26%). Other carboxylic acids, in concentrations lower than 10 −3 mol dm −3, were identified by high-performance liquid chromatography (HPLC), as secondary products, namely glyceric; glycolic and formic acids, during the electrolysis on both electrode materials. On the other hand, tartaric (4.6%) and glyoxilic acids (14.0%) have been identified during the electrolysis on Au and Pt electrodes, respectively. Traces of tartronic and of oxalic acids have also been identified. HPLC studies have demonstrated the higher selectivity of Au electrodes for the production of xylonic acid.

Thermal runaway prediction for impregnated activated carbons from isothermal DSC measurements

Isothermal DSC measurements are used to extract the kinetics of the reaction between activated carbon, which has been impregnated with K 2CO 3, and air at elevated temperature. For the carbon studied here, the reaction takes place in two apparently independent processes. In the first process, the surface functional groups react with the carbon and with air, and in the second process the remaining carbon reacts with air. The impregnate causes the dramatic acceleration of the reaction compared to the reactivity of the pure base carbon in air. The reaction of the surface functional groups with air obeys the following reaction for the fractional degree of conversion, d α 1/d t= γ 1 exp(− E a1/ k B T) α 1 m (1− α 1) n , with m and n approximately equal to −0.95 and 0.5, respectively. The reaction of the remaining carbon with air follows zero-order reaction kinetics. The obtained reaction kinetics can fit the results of isothermal DSC measurements over a wide range of temperature and scanning DSC measurements over a wide range of sweep rates, without any adjustment of parameters. The reaction kinetics, among other experimentally-derived parameters, were then used to calculate the temperature–time profiles for samples of carbon held in stainless steel mesh cylinders of various diameters within a heated constant-temperature oven. These calculations are shown to agree well with experiments at numerous temperatures for two cylinder radii using the same set of kinetic parameters used to fit the DSC results. Some of the temperatures and radii were selected such that thermal runaway of the carbon sample would occur, and the simulations modeled the time and temperature of this runaway accurately. Therefore, it should now be possible to simulate the temperature–time response of this particular impregnated carbon to conditions of high-temperature storage, such as those of the IATA oven exposure test or to storage within a shipping container exposed to a particular thermal history.

Thickness-dependent dynamic hysteresis scaling behavior in epitaxial Fe/GaAs(001) and Fe/InAs(001) ultrathin films

The dynamic hysteresis scaling behavior in epitaxial Fe/GaAs(001) and Fe/InAs(001) thin films (thickness range 7.3–150 Å) has been investigated as a function of Fe film thickness in the field sweep rate range 0.005–1000 kOe/s using the magneto-optic Kerr effect. The hysteresis loop area A follows the scaling relation A∝(dH/dt)α. We find two distinct dynamic regimes: the low dynamic regime in the sweep rate range 0.005–250 kOe/s, and the high dynamic regime beyond 250 kOe/s. There is a marked increase in α between the low and high dynamic regimes which we attribute to the dominant reversal mechanism changing from domain wall motion to nucleation. In the low dynamic regime α is a decreasing function of Fe film thickness, and this behavior is attributed to the effect of interface-induced pinning.

Relation of Structural Changes to Electron-Transfer Parameters in Fulvalenediyl Dirhodium Complexes Demonstrating Quasi-Reversible Two-Electron Voltammetry

The oxidations of two fulvalenediyl (Fv) dirhodium complexes [(Fv)Rh2(CO)4, 1, and (Fv)Rh2(CO)2(μ-dppm), 2] have been studied by voltammetry, coulometry, IR spectroelectrochemistry, UV−VIS spectroscopy, and X-ray crystallography. Both complexes undergo net two-electron oxidations to dications having a Rh−Rh bond. The M−M bond distances are 2.837(1) Å in 22+ and 4.511(1) Å in 2. Quasi-reversible cyclic voltammetric responses were exploited over a range of sweep rates to give CV curves successfully fit to an EE mechanism by digital simulations: for 1, E°‘1 = −0.10 V (vs Fc), ks1 = 0.013 cm s-1, (1 − α1) = 0.66, E°‘2 = −0.245 V, ks2 = 0.044 cm s-1, α2 = 0.5; for 2, E°‘1 = −0.76 V, ks1 = 0.035 V, (1 − α1) = 0.75, E°‘2 = −0.776 V, ks2 > 0.2 cm s-1, α2 = 0.5. Complementary results were obtained for the reduction of the dications 12+ or 22+. The effect of the bridging fulvalenediyl ligand is to retard the oxidation process by an amount consistent with the calculated barrier to rotation around the C−C bond of the η5,η5‘-C10H8 ligand. In both complexes the 0/1+ redox process is slower than the 1+/2+ process. The electron transfer rates are consistent with a progressive increase in the Rh−Rh bond order in the 0/1+/2+ charged complexes rather than full M−M bond formation and cleavage in a single step.

Identification of the adsorbed intermediate of the electrooxidation of chloride by the cv technique

The adsorbed intermediate of the chloride electrochemical oxidation process on a RuO 2 TiO 2 mixed oxide electrode was studied by cv technique in a large range of sweep rate. The existence of a reverse adsorption peak was directly proved by the preliminary results summarised here. The electrocatalytic activity of different electrodes used as working electrode for chloride oxidation can be qualified by the cv method.

A potentiodynamic study of the electrolytic formation of AgO

Cyclic voltammetry has been used to study the electrolytic formation of AgO at a silver electrode in aqueous KOH solutions. A range of sweep rates and limiting positive potentials have been used. The conditions under which an anodic peak appears on the negative-going scan have been determined. The results are interpretated in terms of two processes in the formation of AgO: nucleation and crystal growth. One of these is more energy-dependent while the other is more time-dependent. The electrode reaction mechanism is the same in 1.0 as in 6.5 M KOH.

Conductivity in AgI films grown by silver corrosion in a solid-state cell

The conductivity of AgI thin films grown in a Ag/β-AgI/2I 2Py cell by solid-state reactions was measured between 10 and 60°C employing a pulse method together with coulometric titration. A triangular voltage pulse method is discussed assuming a simple model of the system impedance which consists of two RC series circuits and an ohmic resistance with a sweep rate range from 1 to 1000 V s −1. The different circuit elements are separated and agreement with experiment is observed in Z versus pulse duration as well as in the system response curve for a single pulse. The calculated values for each component as well as its modifications with the charge state of the cell are discussed.

Mechanism of the Electrochemical Reduction of Vitamin B 12r

The previous studies of the electrochemical reduction of vitamin B 12r by polarography and cyclic voltammetry (CV) have been restricted to a few pH values. In CV, the sweep rates used were limited to very low values owing to adsorption phenomena, hampering a detailed analysis of the reduction mechanism. The aim of the present communication is to report and discuss the results obtained by CV at several pH values ranging from 0 to 8. The adsorption phenomena were minimized by addition of a solubilizing salt at pH above 3 enabling a large range of sweep rates to the explored. It is shown, onder these conditions, that the base-off/base-on tautomerism of vitamin B 12r, i.e. the cleavage and binding of the 5,8-dimethyl-benzimidazole of the nucleotide side-chain, plays a major role in the oxido-reduction mechanism of the B 12 r-B 12s couple. The analysis of the CV results leads to the description of the mechanism and to the evaluation of various kinetic and thermodynamic characteristic factors of the B 12r-B 125 couple.

Search for surface structural effects in electrocatalysis. II. Platinized platinum electrodes

The adsorptive properties of platinized electrodes, as indicated by cyclic voltammetry, depended on the potential of platinum electrodeposition. Electrodes deposited above 0.05 V v. reversible hydrogen electrode were similar to smooth platinum, while those deposited at negative potentials showed anomalous behaviour, particularly with regard to hydrogen sorption. Measurements with such electrodes over a range of sweep rates showed slow sorption of about two hydrogen atoms in excess of the usual fast adsorption of one hydrogen atom per surface platinum atom. Freshly electrodeposited platinum had a specific activity for methanol oxidation of one-quarter to one-half that of smooth platinum. The lower activity was attributed to the effects of disordered surface structure and/or small particle size. Anodic-cathodic cycling produced surface recrystallization and activities close to that for smooth platinum.

The Electrochemical Oxidation of Ethylene: Comparison of Results by the Potential Sweep and Steady‐State Methods

The electrochemical oxidation of ethylene on platinum electrodes in 1N sulfuric acid at 80 °C was studied by the potential sweep method over a wide range of sweep rates . The results were compared with those obtained previously under steady‐state conditions. Over three decades of sweep rate an apparent Tafel slope of approximately was observed. This was shown to be consistent with the behavior for a totally irreversible diffusion‐controlled reaction. In the same region the peak potential was found to shift 65 mv/decade increase in sweep rate and the peak current was proportional to the square root of the sweep rate with a proportionality constant in agreement with the calculated value.It is shown that the application of the potential sweep may change the mechanism of the process taking place (in the present case the change is from activation control in steady state to diffusion control during potential sweep). During potential sweep a totally irreversible, diffusion‐controlled reaction will give rise to a linear Tafel plot with a slope about 35% higher than the mechanistically significant slope of 2.3 RT/αF. Further, an apparent passivation region can be observed, due to depletion of the solution near the electrolyte from reactant molecules and quite independent of the nature of the electrode surface. During potential sweep measurements the partial coverages θ by various species on the surface cannot reach their steady‐state value. The kinetic parameters derived from such measurement, therefore, cannot be used to represent the results under steady‐state conditions, unless the sweep rate is made sufficiently slow to correspond in practice to steady state.