Numerical analysis of Debye screening effect in electrode surface potential mapping by scanning electrochemical potential microscopy

We investigate the effects of the probe apex geometry, overlap of the electric double layers (EDLs) and Debye screening on surface potential mapping with scanning electrochemical potential microscopy (SECPM). The simulation consists of scanning a tip parallel to the electrode surface over a charged hemispherical nano-particle adsorbed on the electrode surface. As expected, a clear dependence of the apparent size of the imaged particle on the probe apex geometry has been noticed. The Debye screening has a significant effect on the probe sensitivity, while the electrolyte concentration affects the observed size of the imaged particles.     

Activation energy of electrochemical reaction at a constant value of electrode potential

Equations were derived interrelating ideal activation energy W (determined at the constant Galvani potential value), real activation energy A (determined at a constant overpotential), and the so called formal activation energy Ω (calculated at a constant electrode potential vs. an arbitrarily chosen reference electrode). The obtained equations include only the parameters of the studied reaction and no characteristics of the reaction occurring on the reference electrode. The type of the Ω dependence on the electrode potential is established. Conditions are found, under which the real and formal activation energies of the electrochemical reactions can simultaneously remain constant in the given temperature range and it becomes possible to use them in the integral forms of the Arrhenius equation.     

Size-dependence enhancement in electrocatalytic activity of NiHCF-gold nanocomposite: potential application in electrochemical sensing

A nanocomposite of nickel hexacyanoferrate (NiHCF) was made with gold nanoparticles (AuNPs) of two different sizes (20 and 80 nm as AuNP(red) and AuNP(blue) respectively), synthesized via 3-glycidoxypropyltrimethoxysilane mediated reduction of 3-aminopropyltrimethoxysilane treated gold chloride and characterized by scanning electron microscopy and UV-VIS spectroscopy. The size of AuNPs was found to influence the two pairs of reversible voltammetric peaks of cation rich and cation deficient NiHCF. Such influence was identified from cyclic voltammetry of nanocomposite modified electrodes and applications during electrochemical sensing of two different analytes hydrazine and glutathione (GSH). Electrochemical sensing of hydrazine was based on cyclic voltammetry and differential pulse voltammetry (DPV) found as a function of sodium deficient NiHCF and was greatly amplified with increasing AuNPs nanogeometry. NiHCF alone is not an efficient electrode material for GSH analysis at the level required, however, the presence of AuNPs introduces size dependent sensitive and selective detection of GSH. GSH sensing based on linear sweep voltammetry (LSV) was found to be mediated by the potassium rich form of NiHCF redox couple in the presence of AuNPs. The results justified electrochemical detection of these analytes based on a mediated mechanism and support the role of AuNPs for facilitated electrochemical activity of NiHCF based systems as a function of nanogeometry.     

Switching in organic devices caused by nanoscale Schottky barrier patches

We have identified a possible electronic origin of metal filaments, invoked to explain the switching behavior of organic devices. Interfaces of two representative organics polyparaphenylene (PPP) and poly(2-methoxy-5-2-ethyl-hexyloxy-1,4-phenylenevinylene) with Ag are investigated using ballistic emission microscopy. Nanometer scale spatial nonuniformity of carrier injection is observed in ballistic electron emission microscopy images of both interfaces. The measured Schottky barrier (SB) appears to be consistent with metal states tailing into the gap of the PPP. We find that the SB values exhibit a distribution, even for the diodes with low ideality factors. The implications of this distribution on the measured physical properties of the diode are discussed, in light of work on devices of similar geometry, published in the literature. We also demonstrate that patches of low SB are likely to nucleate current filaments which can cause local ionization and are reported to be responsible for the switching behavior observed in metal-organic, metal-CuS and Ag-AgSe structures.     

Kinetic study of the degradation of a potential local anesthetic drug in serum using the DNA-based electrochemical biosensor

DNA-drug association interaction at the DNA modified screen-printed electrode for 1-methyl-2-piperidinoethylester of 2-hexoxyphenylcarbamic acid was found leading to an accumulation of the drug within the DNA layer. A procedure for the determination of drug in blood serum matrix using the protein precipitation and voltammetric measurement of the electroactive drug with the DNA biosensor was obtained and an effort was done to apply it for an assay of the drug enzymatic degradation in human and rabbit sera at 37 degrees C.     

Mass transfer enhancement for mesh electrode in a tubular electrochemical reactor using experimental and numerical simulation method

The effect of electrode configuration on mass transfer rate in a tubular electrochemical reactor was investigated with the limiting-current method. The expression of mass transfer enhancement factor was derived from the dimensionless equations for different electrode configurations. Furthermore, the local velocity distribution in the tubular electrochemical reactor was simulated by computational fluid dynamics technology. These simulation results explained the reason of mass transfer enhancement for mesh electrode: the rotation of the solution is induced by high rates of shear, and small eddies are formed. The local velocity distributions in bulk for X-dimension get flatter because of the lateral momentum transfer. Due to the impact and obstruction of electrode holes, the turbulence of the electrolyte gets drastic and the mass transfer performance in the tubular electrochemical reactor is improved.     

General expression of the linear potential sweep voltammogram in the case of diffusionless electrochemical systems

The equation of the linear potential sweep voltammogram is derived for any degree of reversibility of the electrochemical reaction for the following methods: surface voltammetry when both the oxidized and the reduced forms are strongly adsorbed, and a Langmuir isotherm is obeyed, thin layer voltammetry, and linear potential sweep coulometry. The results are expressed in one mathematical form valid for the three cases. The transfer coefficient and the rate constant of the electrochemical reaction can be deduced from an experimental study of the variations of the peak potentials as a function of the sweep rate.     

Improvement in reflective-emissive dual-mode properties of electrochemical displays by electrode modification

We studied the electrochromic (EC) and electrochemiluminescent (ECL) properties of a novel dual-mode display (DMD) cell that was enabled for reflective and emissive modes of representation by introducing both EC and ECL materials into an electrochemical cell. We fabricated EC, ECL, and DMD cells based on a simple-mixture solution or modified electrodes and compared their properties to clarify the advantage of a DMD system based on modified electrodes. Both the solution- and modified electrode-based DMDs showed EC properties in the reflective mode under dc bias application and ECL properties in the emissive mode under ac bias application. Although the solution-based DMD cell featured a very simple structure, some improvements related to side reaction and quenching reaction were required. The modified electrode-based DMD cell was fabricated to improve these aspects. The advantage of the DMD model based on the modified electrodes was certainly suggested by comparisons of the results with those of EC, ECL, and DMD cells based on a simple-mixture solution.     

Preparation of a mercury film electrode modified by tri-n-octylphosphine oxide and the electrochemical properties of this electrode

Mercury film electrodes have been prepared on silver metal and silver-coated glassy-carbon supports and have been modified by a film of tri-n-octylphosphine oxide (TOPO) in a poly(vinyl chloride) matrix. The electrodes have been characterized in detail and the effects of the modifying film parameters on their electrochemical properties have been studied. It has been shown that these electrodes permit selective and sensitive determinations of many metals. The most important parameters are the thickness of the modifying film, the modifier-to-matrix ratio in the modifying film and the base electrolyte composition. Data concerning the reactions of a number of metal ions on the modified electrode are given.     

Pulsed electrochemical detection of orotic acid by an activated potential waveform at a gold working electrode following anion-exchange chromatography

The application of activated pulsed amperometric detection (APAD) for the determination of orotic acid (OrA) in real samples at a gold working electrode in alkaline solutions, in combination with anion-exchange chromatography, is reported. Such an activated potential waveform was designed with an initial step that involves the formation of redox active species (e.g., adsorbed AuOH/AuO), which in turn is halted upon lowering the applied potential at the detection value while the adsorbed gold hydroxide/oxide species are still catalytically active. A direct comparison between the activated potential waveform and the more commonly used pulsed amperometric detection showed roughly a 20-fold increase in sensitivity. The chromatographic separation of OrA was accomplished by using a microbore anion-exchange column eluted with an isocratic mobile phase composed of 100 mM NaOH+40 mM NaNO(3). Orotic acid was determined at the concentration ranges of 0.2-30 microM (r=0.9997) with an absolute detection limit of 80 pg (10 microL injected). The levels of OrA in cows' milk samples evaluated by standard additions, using 5-aminoorotic acid as an internal standard, ranged from 56 to 126 mg/L. Lower levels were found in raw sheep's milk (<20 mg/L). The assay is shown to be very useful in clinical investigations where relatively high levels of OrA in human urine are correlated to metabolic diseases.     

Quantitative model of electrochemical Ostwald ripening and its application to the time-dependent electrode potential of nanocrystalline metals

The contact of a metastable nanocrystalline metal ensemble with a metal ion electrolyte leads to an electrochemical Ostwald ripening. The kinetics is modeled on the level of irreversible thermodynamics for the case that the rate is controlled by the electrode/electrolyte transfer resistance. In particular, the kinetic behavior of medium-sized particles and the time dependence of the electromotive force is investigated. Even though it is expressed in electrochemical terms (mixed potential), the modeling is also applicable to chemical Ostwald ripening as long as it is interfacially controlled. Under these conditions, the kinetics exhibits, even though not self-accelerating, strong similarities to selection dynamics, with the competition stemming from the cannibalistic nature of the process.     

Kinetic analysis in sub-monolayer systems by gas chromatography

A simple experimental arrangement is proposed for the kinetic analysis of heterogeneous catalytic processes by gas chromatography. The technique is illustrated by sub-monolayer micropulse activity data of i-PrOH dehydration over -Al₂O₃. Some peculiarities of the kinetic analysis of sub-monolayer activity data are also discussed.

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. -Al₂O₃. .

     

纳米电极界面的构建和茶碱的直接电氧化测定

以镍铬合金为基体制备嵌入式超薄碳糊前驱膜,然后在薄膜表面构筑单壁碳纳米管/聚L-酪氨酸复合电极.SEM结果表明膜表面的碳纳米管复合物呈立体枝状渗透性结构,电化学实验证明表面膜上的复合纳米结构改变了基体的电化学性质,使之可用作研究电极.以茶碱为目标物考察其在该电极上的伏安行为,在0.1 mol/L的HClO4底液中,茶碱的2.5次微分峰电流与其浓度在1.0×10-6～8.0×10-4 mol/L 范围内呈良好的线性关系,相关系数为0.9971,方法可用于茶碱的定量测定.     

Mass-transfer problems in the electrochemical systems

The problems concerning quantitative analysis of mass-transfer processes in the electrochemical systems (ECS) are briefly reviewed. The interrelation between the mass-transfer problems in the electrochemical and heat systems is considered. Various approaches to the numerical determination of distributions of concentrations of ions of all types and electrochemical potential in the ECS are considered. The methods of allowance for the migration transfer and choosing the boundary conditions for the transfer equations and other problems are considered. Some actual lines of development of the theory of mass-transfer in the ECS are pointed out.     

Kinetic patterns of the interaction between ammonia and nanoscale films of bismuth

Transformations in nanoscale films of bismuth at T = 293 K depending on the thickness of film (1–56 nm) and duration (1 min-5400 h) of contact with gaseous ammonia are studied by means of optical spectroscopy, microscopy, and gravimetry. Depending on the initial thickness of the bismuth films, the kinetic curves of degree of conversion are satisfactorily described by linear, inverse logarithmic, parabolic, and logarithmic laws. The contact potential for Bi and BiN films is measured, along with the photovoltage for Bi-BiN systems. An energy band diagram is constructed for Bi-BiN systems. A model for the transformation of Bi films is been proposed that includes stages of ammonia adsorption, redistribution of the charge carriers in the Bi-BiN contact field, and the formation of bismuth nitride.     

Kinetic prediction of functional group distributions in thermosensitive microgels

A kinetic model accounting for the copolymerization of up to four comonomers is applied to predict both chain and radial functional group distributions in carboxylic-acid-functionalized poly(N-isopropylacrylamide) (NIPAM)-based microgels. The model can accurately predict the experimentally observed radial distributions of functional monomers in microgels prepared using a variety of different carboxylic-acid-functionalized monomers with significantly different hydrophobicities, copolymerization kinetics, and reactivities, without requiring the use of adjustable parameters. Multimodal distributions can both be predicted and experimentally generated by copolymerizing two -COOH-containing monomers with widely different reactivities. Chain distributions and monomer block formation can also be probed using the kinetic model, allowing for qualitative predictions of the potentiometric titration behavior of the microgels. The kinetic model reported herein therefore provides the first available analytical method for semiquantitatively predicting and controlling functional group distributions in bulk-polymerized microgel systems.     

A simplified representation of the potential produced by Gaussian charge distributions

A procedure is presented which allows a more economical representation of the potential produced by orbital charge distributions in which the orbitals are expanded in terms of a finite set of polynomial Gaussian functions. The basic idea is that the products of pairs of Gaussian basis functions, on which the charge distributions are expanded, are expressed in terms of a new basis set of optimally chosen single Gaussian functions. Such a procedure has been tested in a particular case and a few possible applications have been suggested.     

Scanning electrochemical mapping of spatially localized electrochemical reactions induced by surface potential gradients

The influence of a surface potential gradient on the location and extent of electrochemical reactions was examined using a scanning electrochemical microscope. A linear potential gradient was imposed on the surface of a platinum-coated indium tin oxide electrode by applying two different potential values at the edges of the electrode. The applied potentials were used to control the location and extent of several electrochemical reactions, including the oxidation of Ru(NH3)6(2+), the oxidation of H2, and the oxidation of H2 in the presence of adsorbed CO. Scanning electrochemical mapping of these reactions was achieved by probing the feedback current associated with the oxidation products. The oxidation of Ru(NH3)6(2+) occurred at locations where the applied potential was positive of the formal potential of the Ru(NH3)6(2+/3+) redox couple. The position of this reaction on the surface could be spatially translated by manipulating the terminal potentials. The rate of hydrogen oxidation on the platinum-coated electrode varied spatially in the presence of a potential gradient and correlated with the nature of the electrode surface. High oxidation rates occurred at low potentials, with decreasing rates observed as the potential increased to values where platinum oxides formed. The extent of oxide formation versus position was confirmed with in-situ ellipsometry mapping. In the presence of adsorbed carbon monoxide, a potential gradient created a localized region of high activity for hydrogen oxidation at potentials between where carbon monoxide was adsorbed and platinum oxides formed. The position of this localized region of activity could be readily translated along the surface by changing the terminal potential values. The ability to manipulate electrochemical reactions spatially on a surface has potential application in microscale analytical devices as well as in the discovery and analysis of electrocatalytic systems.