Ionic conduction and electrode polarization in a doped nonpolar liquid

Electrical current versus potential relationships were measured for solutions of dodecane containing the charge control agent poly(isobutylene succinimide) (PIBS) at various concentrations. Both one-dimensional (parallel planar electrodes) and two-dimensional (strip electrodes) fields were studied. The initial current was proportional to the applied voltage for both electrode configurations. Using the initial decay rate of the current (t < 0.5 s) in the planar electrode cell and the Gouy-Chapman model for electrode polarization, we determined the diffusion coefficient of the charge carriers (micelles) in the solution, from which we calculated their effective radius to be 10 nm. The constancy of the carrier radius over a 7-fold change in PIBS concentration, along with the proportionality between conductivity and concentration, supports the hypothesis that the charged species result from the interactions between two micelles. The experimentally determined geometric factor (cell constant) relating current to applied potential at time zero for the strip electrode cell agrees with the value predicted from the solution of Laplace's equation for the electrical potential in this system. The intermediate-time (0.5-3.0 s) decay rate of current was faster than predicted from the classical Gouy-Chapman theory of the double layer, possibly because of volume fraction effects in the double layer. The very long-time (minutes to hours) residual current that we observed is not explained, but we suspect that some charge transfer across the electrode must have occurred because there was insufficient ion capacity (i.e., amount of PIBS) in the solution to account for the total charge transferred through the cell.     

Photoelectrochemical investigation of ultrathin film iron oxide solar cells prepared by atomic layer deposition

Atomic layer deposition was used to grow conformal thin films of hematite with controlled thickness on transparent conductive oxide substrates. The hematite films were incorporated as photoelectrodes in regenerative photoelectrochemical cells employing an aqueous [Fe(CN)(6)](3-/4-) electrolyte. Steady state current density versus applied potential measurements under monochromatic and simulated solar illumination were used to probe the photoelectrochemical properties of the hematite electrodes as a function of film thickness. Combining the photoelectrochemical results with careful optical measurements allowed us to determine an optimal thickness for a hematite electrode of ～20 nm. Mott-Schottky analysis of differential capacitance measurements indicated a depletion region of ～17 nm. Thus, only charge carriers generated in the depletion region were found to contribute to the photocurrent.     

Electrostatically stabilized metal oxide particle dispersions in carbon dioxide

We report electrostatic stabilization of micrometer-sized TiO(2) particles at long range (several micrometers) in liquid and supercritical CO(2) despite the ultralow dielectric constant, as low as 1.5. The counterions were solubilized in dry reverse micelles, formed with a low-molecular weight cationic perfluoropolyether trimethylammonium acetate surfactant, to prevent ion pairing with the particle surface. Dynamic light scattering and settling velocities indicate a particle diameter of 620-740 nm. The electrophoretic mobility of -2.3 x 10(-8) m(2)/V s indicated a particle charge on the order of -1.7 x 10(-17) C, or 105 elementary negative charges per particle. The balance of particle compression by an electric field versus electrostatic repulsion generated an amorphous arrangement of particles with 5-9 mum spacing, indicating Debye lengths greater than 1 mum. Scattering patterns also indicate that chains of particles may be achieved in CO(2) by dielectrophoresis with alternating fields. The electrostatic stabilization has been achieved by solubilizing a small concentration of counterions in only a small fraction of the reverse micelles in the double layer. Whereas many low-molecular weight surfactants have been shown to form reverse micelles in CO(2), very few polymers are able to stabilize micrometer-sized colloids sterically. Thus, electrostatic stabilization has the potential to expand markedly the domain of colloid science in apolar supercritical fluids.     

Changes in zeta potential caused by a dc electric current for thin double layers

An asymptotic solution was obtained to describe one-dimensional, steady-state transport of a symmetric binary electrolyte normal to two large parallel electrodes, in the limit in which the Debye length is infinitesimal compared to the distance separating the two electrodes. Despite the nonzero ion flux, Boltzmann's equation continues to describe the relationship between either ion concentration and the electrostatic potential inside the diffuse part of the double layer, while local electroneutrality applies outside, even for current densities approaching the limiting value. In the absence of ion adsorption or dissociation reactions at the electrodes, the magnitude of any charge or zeta potential arising on the electrodes at zero current is determined by the equilibrium constant for the redox reactions which would exchange ionic charge carriers for electric charge carriers at the electrode surface. Nonzero current causes the ionic strength of the bulk to vary with position. This perturbs the Debye length of the diffuse cloud on either electrode: it is the local ionic strength just outside the cloud which determines the Debye length for that cloud. Nonzero current also changes the zeta potential. The dimensionless rate of change dζ/dJ was as large as 30.     

Layer-by-Layer Self-Assembled Multilayer Films Composed of Graphene/Polyaniline Bilayers: High-Energy Electrode Materials for Supercapacitors

Multilayer assemblies of uniform ultrathin film electrodes with good electrical conductivity and very large surface areas were prepared for use as electrochemical capacitors. A layer-by-layer self-assembly approach was employed in an effort to improve the processability of highly conducting polyaniline (PANi) and chemically modified graphene. The electrochemical properties of the multilayer film (MF-) electrodes, including the sheet resistance, volumetric capacitance, and charge/discharge ratio, were determined by the morphological modification and the method used to reduce the graphene oxide (GO) to reduced graphene oxide (RGO) in the multilayer films. The PANi and GO concentrations could be modulated to control the morphology of the GO monolayer film in the multilayer assemblies. Optical ellipsometry was used to determine the thickness of the GO film in a single layer (1.32 nm), which agreed well with the literature value (～1.3 nm). Hydroiodic acid (HI), hydrazine, or pyrolysis were tested for the reduction of GO to RGO. HI was found to be the most efficient technique for reducing the GO to RGO in the multilayer assemblies while minimizing damage to the virgin state of the acid-doped PANi. Ultimately, the MF-electrode, which could be optimized by fine-tuning the nanostructure and selecting a suitable reduction method, exhibited an excellent volumetric capacitance, good cycling stability, and a rapid charge/discharge rate, which are required for supercapacitors. A MF-electrode composed of 15 PANi/RGO bilayers yielded a volumetric capacitance of 584 F/cm(3) at a current density of 3.0 A/cm(3). Although this value decreased exponentially as the current density increased, approaching a value of 170 F/cm(3) at 100 A/cm(3), this volumetric capacitance is one of the best yet reported for the other carbon-based materials. The intriguing features of the MF-electrodes composed of PANi/RGO multilayer films offer a new microdimensional design for high energy storage devices for use in small portable electronic devices.     

电极材料对NPB/Alq3有机电致发光器件性能的影响

采用不同材料作为有机电致发光器件(OELDs)的电极, 制备了基本结构为[阳极/NPB(40 nm)]/Alq3(50 nm)/阴极]的异质结双层器件, 并通过改变OELDs器件的阴极或阳极来研究电极材料对器件光电性能的影响. 研究结果表明, 各器件电流-电压(I-V)关系的基本特征与陷阱电荷限制电流(TCLC)机制的拟合情况相符. 由于有机材料本身能级的无序性以及载流子迁移率对温度和电场的依赖性, 不同电极的载流子注入能力与其功函数并无直接关系. 双层器件中由于空穴传输层的引入, 使得载流子复合区域位于有机层异质结界面处, 降低了金属阴极对激子的猝灭作用, 从而大大提高了器件性能. 此外, 金属电极OLEDs器件结构具有的微腔效应会导致发射光谱的位移和谱峰宽度变窄, 这表明通过对金属电极的表面改性和优化可使器件性能超过常规结构的器件.     

Electrochemical impedance study of the hematite/water interface

Reactions taking place on hematite (α-Fe(2)O(3)) surfaces in contact with aqueous solutions are of paramount importance to environmental and technological processes. The electrochemical properties of the hematite/water interface are central to these processes and can be probed by open circuit potentials and cyclic voltammetric measurements of semiconducting electrodes. In this study, electrochemical impedance spectroscopy (EIS) was used to extract resistive and capacitive attributes of this interface on millimeter-sized single-body hematite electrodes. This was carried out by developing equivalent circuit models for impedance data collected on a semiconducting hematite specimen equilibrated in solutions of 0.1 M NaCl and NH(4)Cl at various pH values. These efforts produced distinct sets of capacitance values for the diffuse and compact layers of the interface. Diffuse layer capacitances shift in the pH 3-11 range from 2.32 to 2.50 μF·cm(-2) in NaCl and from 1.43 to 1.99 μF·cm(-2) in NH(4)Cl. Furthermore, these values reach a minimum capacitance at pH 9, near a probable point of zero charge for an undefined hematite surface exposing a variety of (hydr)oxo functional groups. Compact layer capacitances pertain to the transfer of ions (charge carriers) from the diffuse layer to surface hydroxyls and are independent of pH in NaCl, with values of 32.57 ± 0.49 μF·cm(-2)·s(-φ). However, they decrease with pH in NH(4)Cl from 33.77 at pH 3.5 to 21.02 μF·cm(-2)·s(-φ) at pH 10.6 because of the interactions of ammonium species with surface (hydr)oxo groups. Values of φ (0.71-0.73 in NaCl and 0.56-0.67 in NH(4)Cl) denote the nonideal behavior of this capacitor, which is treated here as a constant phase element. Because electrode-based techniques are generally not applicable to the commonly insulating metal (oxyhydr)oxides found in the environment, this study presents opportunities for exploring mineral/water interface chemistry by EIS studies of single-body hematite specimens.     

Thermally Stimulated Processes in a Liquid Crystal Polymer

Measurement of voltage-induced thermal depolarization current and calculation of the rate of depolarization as well as the parameters of drift mobility and conductivity of charge carriers for melt-extruded neat unreinforced grade A950 VECTRA® resin - a wholly aromatic copolyester - strongly suggest that an irreversible minor transition centered around 65°C is the primary thermal process related to molecular realignment. Changes in capacitance values with temperature also show this to be the most active temperature region. A major depolarization peak at 100°C having the characteristics of a T_g cannot be justified as due to glass transition but likely to result from molecular motions involving long range intermolecular order. The interpretation for both transitions can be supported by the mechanical response of this polymer. An important outcome of this work is the assertion that contrary to current thinking, it is the number of charge carriers and not viscosity alone that will have to be considered in future development of fast response liquid crystal displays along with the development of newer liquid crystal polymer structures.     

Zeta potential of dodecyltrimethylammonium hydroxide micelles in water

 The electrophoretical mobility of dodecyltrimethyl-ammonium hydroxide micelles has been measured at two different concentrations giving values similar to that determined in other surfac-tants. There is a good agreement between micelle ionization degrees computed from zeta potential measurements and those from ion-selective electrodes experiments. This demonstrates that electrophoresis experiments may be replaced by the simpler ion-selective electrode measurements to determine micelle surface potential. It has also been concluded that ion-selective electrodes detect only the non-micellised ions, that only free ions contribute to the intermicellar solution ionic strength, and micelles do not affect the result, and that the dependence of the electrophoretic mobility on the soap concentration is due to the reduction of the micelle net charge when the ionic strength of the intermicellar solution arises. Received: 2 December 1996 Accepted: 24 February 1997     

Inverse micelles as charge carriers in nonpolar liquids: Characterization with current measurements

Despite the increasing importance of charges in nonpolar liquids for practical applications and fundamental research, their origin, nature and behavior are not yet completely understood. The most widely (but not generally) accepted view is that in mixtures of a nonpolar liquid with surfactant, inverse micelles act as charge carriers. A lot of research is still needed to support this view, and to gain a fundamental understanding of the electrical properties of inverse micellar solutions. In this article, we discuss transient current measurements as a valuable technique for the characterization of charged inverse micelles in nonpolar liquids, and we illustrate how they can be used to study a large number of properties, such as the concentration of both neutral and charged inverse micelles, their mobility, size, aggregation number and valency, and their behavior and generation in the bulk and at surfaces.     

From small charged molecules to oligomers: a semiempirical approach to the modeling of actual mobility in free solution

According to Stokes' treatment, the ionic mobility of particles, which are small with respect to Debye length, is usually considered to be proportional to the nominal charge and inversely proportional to the hydrodynamic radius. Experimentally, it is well known, however, that the ionic mobility of a small multicharged molecule does not depend linearly on its nominal charge in a wide range. This behavior can be accounted for by a condensation of the charge or a modification of the friction coefficient with the charge. This paper presents a semiempirical modeling of the actual mobility based on the assumption of additivity of frictional contributions pertaining to the uncharged molecular backbone and to each charged or uncharged moiety. Condensation of the charge was not considered. The model first appeared to be suitable for multicharged analytes having a characteristic dimension smaller than the Debye length, such as benzene polycarboxylic acids and polysulfated disaccharides. This approach was then adapted to account for the actual mobilities of singly and evenly charged oligomers (N-mers) having a dimension smaller than or similar to the Debye length. Rather good experimental agreement was obtained for polyalanines and polyglycines (N < or = 6), fatty acid homologs, fully sulfonated polystyrene oligomers (N < or = 13), and polycytidines (N < or = 10). Especially the influence of the polymerization degree on the mobility of oligomers having identical charge densities was clarified. It is also shown that the electrophoretic contribution to the overall friction coefficient increases linearly with the nominal charge but hardly depends on the chemical nature of the charged moieties. This model should be of interest to evaluate the role of various physicochemical phenomena (hydrodynamic and electrophoretic frictions, hydrodynamic coupling, charge condensation) involved in the migration of charged oligomers.     

Charge stabilization in nonpolar solvents

While the important role of electrostatic interactions in aqueous colloidal suspensions is widely known and reasonably well-understood, their relevance to nonpolar suspensions remains mysterious. We measure the interaction potentials of colloidal particles in a nonpolar solvent with reverse micelles. We find surprisingly strong electrostatic interactions characterized by surface potentials, |ezeta|, from 2.0 to 4.4 k(B)T and screening lengths, kappa(-1), from 0.2 to 1.4 microm. Interactions depend on the concentration of reverse micelles and the degree of confinement. Furthermore, when the particles are weakly confined, the values of |ezeta| and kappa extracted from interaction measurements are consistent with bulk measurements of conductivity and electrophoretic mobility. A simple thermodynamic model, relating the structure of the micelles to the equilibrium ionic strength, is in good agreement with both conductivity and interaction measurements. Since dissociated ions are solubilized by reverse micelles, the entropic incentive to charge a particle surface is qualitatively changed from aqueous systems, and surface entropy plays an important role.     

Methotrexate-loaded biodegradable polymeric micelles: preparation, physicochemical properties and in vitro drug release

Polymeric micelles based on amphiphilic diblock copolymers methoxy poly(ethylene glycol)-polylactide with various hydrophobic lengths were designed as carriers of poorly water-soluble anticancer drug methotrexate (MTX). Relationship between physicochemical characteristics of micelles and release behavior was explored. The critical micelle concentration was determined by fluorescence spectroscopy using 9-chloromethyl anthracene as fluorescence probe. Core-shell type polymeric micelles were prepared by free-surfactant dialysis technique. The mean size of micelles loaded with MTX was 50-200 nm with narrow polydispersity. Physicochemical properties of drug-loaded micelles were evaluated. In vitro release behavior of MTX was also investigated. MTX was continuously released from micelles and less than 50% MTX was released in 5 days. Release rate was dependent on chemical structures of micelles and enhanced by decreasing polylactide lengths.     

纳米纤维聚苯胺在电化学电容器中的应用

采用脉冲电流方法（PGM）合成了具有纳米纤维结构的导电聚苯胺（PANI）.扫描电子显微镜对膜层观察表明, PANI膜是由直径约为100 nm的掺杂态聚苯胺纤维交织而成.以纳米纤维状聚苯胺组成电化学电容器,研究了其电化学电容性能,并与恒电流方法（GM） 制备的颗粒状PANI电容器性能进行了比较.结果表明,在相同的沉积电量下,PGM制备的纳米纤维状PANI电化学电容器比颗粒状PANI电化学电容器具有更大的电容容量,其电化学电容器的比电容可高达699 F•g－1,能量密度为54.6 Wh•kg－1.并且该电化学电容器具有良好的充放电性能和循环寿命.     

Synthesis and electrochemical characterization of carbon nanotubes decorated with nickel nanoparticles for use as an electrochemical capacitor

Attachment of nickel nanoparticles on multiwalled carbon nanotubes (MWCNTs) was conducted to explore the influence of Ni loading on the electrochemical capacitance of MWCNT electrodes. A chemical impregnation leaded to homogeneously disperse Ni particles onto the surface of MWCNTs, and the Ni particles were found to be an average size of 30–50 nm. The capacitive behavior of the MWCNT electrodes was investigated in 6 M KOH, by using cyclic voltammetry (CV), charge–discharge cycling, and ac electrochemical impedance spectroscopy. CV measurements showed that the Faradaic current was found to increase with the Ni coverage, indicating that the presence of Ni would enhance the pseudocapacitance through the redox process. Equivalent circuit analysis indicated that both of electrical connection and charge transfer resistances accounted for the major proportion of the overall resistance and were found to decrease with the amount of nickel. A linearity relationship between the total capacitance and the Ni population reflected that each Ni particle exhibits an identical electrochemical activity in enhancing the electrochemical capacitance. The overall electrochemical capacitance (including double layer capacitance and pseudocapacitance) of Ni-MWCNT electrode can reach a maximum of 210 F/g over 500 cycles.     

Charge transport in self-organized pi-stacks of p-phenylene vinylene oligomers

We have studied the mobility of charge carriers along self-organizing pi-stacks of hydrogen-bonded phenylene vinylene oligomers in solution, by time-resolved microwave conductivity measurements. The value deduced for the mobility along the stacks is 3 x 10(-3) and 9 x 10(-3) cm2/(V s) for holes and electrons, respectively. Additionally, we have calculated the mobility along the pi-stacks using a hopping model based on parameters from density functional theory. The mobility values obtained from these calculations are in good agreement with the experimental values if it is assumed that there are relatively large twist angles between neighboring molecules in the stack. It is shown that a significantly higher mobility can be attained if the twist angle between neighboring oligomers is reduced.     

Electrical conduction of poly(vinyl alcohol) films

PVA films (30–45 μm thickness) were prepared by casting. dc electrical measurements were done over the temperature range 293–353 K. Two different conduction mechanisms are suggested. The first one is ohmic and extends to 303 K. Above this temperature a space-charge-limited conduction mechanism, with protons as charge carriers, predominates. Dependences of the voltage-reversion current temperature and field were investigated and were attributed to a clean-up effect of charge carriers. The variance in the activation energy, calculated from the conductivity curves and from the mobility curves, led to a discussion of the origin of the charge carriers. No change in conductivity was observed for the PVA films irradiated with UV (of wavelength 365 nm) at 303 K, i.e., PVA is a photostable material under these conditions.     

Electrochemical behavior of nitroxy radical monolayers prepared on tin oxide electrodes by the Langmuir—Blodgett method: An unusual effect of concentration of supporting electrolytes on cyclic voltammograms

The photoemission current (j) and the differential capacitance of the double layer (C) on 20-200 nm thick bismuth films have been measured. The current j as a function of potential (E) follows the 5/2 law for films of thickness exceeding 80 nm and for bulk bismuth. The current j as a function of E for thin films exhibits steps whose width increases as the thickness decreases. The absolute values of j and C also decrease for thinner films. These effects are manifestations of the quantum size effect in the energy spectrum of charge carriers in bismuth. Estimations of the degree of metallization of the bismuth surface are presented.     

Thin-film solid-state electroluminescent devices based on tris(2,2'-bipyridine)ruthenium(II) complexes

The behavior of light-emitting electrochemical cells (LEC) based on solid films ( approximately 100 nm) of tris(2,2'-bipyridine)ruthenium(II) between an ITO anode and a Ga-In cathode was investigated. The response times were strongly influenced by the nature of the counterion: small anions (BF(4)(-) and ClO(4)(-)) led to relatively fast transients, while large anions (PF(6)(-), AsF(6)(-)) produced a slow time-response. From comparative experiments of cells prepared and tested in a glovebox to those in ambient, mobility of the anions in these films appears to be related to the presence of traces of water from atmospheric moisture. An electrochemical model is proposed to describe the behavior of these LECs. The simulation results agreed well with experimental transients of current and light emission as a function of time and show that the charge injection is asymmetric at the two electrodes. At a small bias, electrons are the major carriers, while for a larger bias the conduction becomes bipolar.     

Atomistic simulation on charge mobility of amorphous tris(8-hydroxyquinoline) aluminum (Alq3): origin of Poole-Frenkel-type behavior

The atomistic simulation of charge transfer process for an amorphous Alq(3) system is reported. By employing electrostatic potential charges, we calculate site energies and find that the standard deviation of site energy distribution is about twice as large as predicted in previous research. The charge mobility is calculated via the Miller-Abrahams formalism and the master equation approach. We find that the wide site energy distribution governs Poole-Frenkel-type behavior of charge mobility against electric field, while the spatially correlated site energy is not a dominant mechanism of Poole-Frenkel behavior in the range from 2x10(5) to 1.4x10(6) V/cm. Also we reveal that randomly meshed connectivities are, in principle, required to account for the Poole-Frenkel mechanism. Charge carriers find a zigzag pathway at low electric field, while they find a straight pathway along electric field when a high electric field is applied. In the space-charge-limited current scheme, the charge-carrier density increases with electric field strength so that the nonlinear behavior of charge mobility is enhanced through the strong charge-carrier density dependence of charge mobility.