Evaluation of surface potential from single crystal electrode potential

The new method of evaluation of the point of zero potential for the metal oxide exhibiting a saddle-like surface potential function Ψ ₀(pH), as obtained by acid base potentiometric titration using Single Crystal Electrode, was proposed. The electrode potential of sapphire single crystal electrode (A crystal plane, $11\bar{2}0$ ) was measured, point of zero potential and surface potentials were evaluated, and the results were analyzed using the Surface Complexation Model. The electroneutrality point corresponding to the point of zero potential was found to be at pH_pzp=7.0. Thermodynamic protonation equilibrium constants for the first and the second step of protonation were obtained as $\lg K_{1}^{\circ} = 12.7$ ; $\lg K_{2}^{\circ} = 1.2$ .     

Effects of biosurfactants on surface properties of hematite

Application of microorganisms as surface modifiers has focused our attention in recent times. The adsorption of biosurfactants can be a way for the solid surface modification. In the present investigation rhamnolipids produced by Pseudomonas aerugiosa were used to make the hematite surface modification. Experiments were carried out with pure mineral hematite. In this paper, the influence of biosurfactant addition on both the stability and the flotability of hematite suspensions has been studied in detail. The stability experiments were conducted using Turbiscan LAb apparatus, at constant pH conditions and mineral amount. The flotation experiments were carried out using Hallimond tube. The adsorption isotherms of biosurfactant onto the hematite particles were also determined. The experiments were carried out with broth and pure rhamnolipid. The results of those experiments were compared and discussed.     

Effect of solvent surface tension on the radius of hematite nanoparticles

In this work, the hematite (Fe₂O₃) nanoparticles were synthesized by homogeneous precipitation in alcohol (tert-butanol)/water mixed solvents with varied surface tension. The surface tension of the solvent was decreased from 55.8 to 15.9 mN m^?1 by the increasing of the alcohol content from 20 to 80 vol %. The size of the particles was determined by BET, XRD and TEM techniques. Based on XRD results, the crystalline phase of Fe₂O₃ in all samples was attributed to the cubic hematite structure. The results show that the average particle size of the prepared hematite samples is decreased from 38 to 14 nm upon decreasing surface tension from 55.8 to 15.9 mN m^?1.     

Water oxidation at hematite photoelectrodes: the role of surface states

Hematite (α-Fe(2)O(3)) constitutes one of the most promising semiconductor materials for the conversion of sunlight into chemical fuels by water splitting. Its inherent drawbacks related to the long penetration depth of light and poor charge carrier conductivity are being progressively overcome by employing nanostructuring strategies and improved catalysts. However, the physical-chemical mechanisms responsible for the photoelectrochemical performance of this material (J(V) response) are still poorly understood. In the present study we prepared thin film hematite electrodes by atomic layer deposition to study the photoelectrochemical properties of this material under water-splitting conditions. We employed impedance spectroscopy to determine the main steps involved in photocurrent production at different conditions of voltage, light intensity, and electrolyte pH. A general physical model is proposed, which includes the existence of a surface state at the semiconductor/liquid interface where holes accumulate. The strong correlation between the charging of this state with the charge transfer resistance and the photocurrent onset provides new evidence of the accumulation of holes in surface states at the semiconductor/electrolyte interface, which are responsible for water oxidation. The charging of this surface state under illumination is also related to the shift of the measured flat-band potential. These findings demonstrate the utility of impedance spectroscopy in investigations of hematite electrodes to provide key parameters of photoelectrodes with a relatively simple measurement.     

A simple method to evaluate the Coulomb potential at the polar surface of an ionic crystal

In the evaluation of the Coulomb potential at a polar surface of an ionic crystal modelled by a slab, a satisfactory criterion to determine the effect of the second surface of the slab is lacking. A method is proposed to remove the effect of the second surface such that the potential at a polar surface of a semi-infinite crystal can be uniquely calculated.     

Measurement of surface potential at silver chloride aqueous interface with single-crystal AgCl electrode

The surface potential at the silver chloride aqueous interface was measured by means of a single-crystal silver chloride electrode (SCr-AgCl). The measurements were conducted by titration of the KCl solution with AgNO3, and vice versa. The SCr-AgCl electrode potentials were converted to surface potentials psi(0) by setting zero at the point of zero charge at pCl = 5.2. The psi(0)(pCl) function was linear, with a slope 12% lower with respect to the Nernst equation. It was demonstrated that the surface potential at the silver halide aqueous interface could be interpreted by means of the surface complexation model, originally developed for metal oxides.     

Measurement of twist angle and surface torsional anchoring strength in a nematic liquid crystal cell

In this paper a novel method is developed for measuring the twist angle and surface torsional anchoring strength in nematic liquid crystal (NLC) cells. This method is based on a technique developed from the Jones optical propagation matrix. From the measurement, the actual twist angle in NLC cell and the deviation of the LC director on the boundary of the cell from the rubbing direction of the substrate are obtained. A theoretical discussion shows that the surface torsional anchoring strength has a great influence on the value of the deviation, and hence so it can be calculated.     

Director movement and potential function of surface stabilized ferroelectric liquid crystal cells by electrooptical investigations

Electrooptical investigations on a ferroelectric liquid crystal (DOBAMBC) were carried out on a well aligned sample in a 2 μm cell. On the basis of these measurements the complete average director movement during a switching process caused by an electric field can be calculated and will be discussed. The thermodynamic potentials at the S_A*-S_C* phase transition were determined from the experimental results with a modified Landau theory. They serve as a basis for a discussion of the phase transition and the influence of the surface orientation. Some preliminary results were obtained concerning the electrooptical behaviour of the S_H* phase.     

Voltage-expandable liquid crystal surface

Based on dielectrophoretic effect, we report a novel approach which can extensively spread a liquid crystal (LC) interface. With interdigitated striped electrodes, the droplets can be stretched along the striped electrode direction; while with zigzag interdigitated electrodes, the droplets can be further stretched sidewise. In our demonstration, the occupied area of a 1.9-mm-aperture LC droplet doped with 1.2 wt% black dye could be expanded over ～3.5× at 78 V(rms). The spreading and recovering times were measured to be ～0.39 s and ～0.75 s, respectively. The slower response time confirms the extreme expanding of the LC surface. The contrast ratio is over ～120?:?1 in transmissive mode. Color light switch was also demonstrated by spreading colored-dye doped LC droplets. The mechanical stability of the device was also evaluated. Liquid devices based on this cell structure have the advantages of good stability, simple operation and low power consumption. This work opens a new gateway for voltage controllable, polarization-insensitive, and broadband liquid photonic devices which may find numerous applications in switchable windows, variable optical attenuators, and displays.     

Measurement and modeling of the surface potential evolution of hydrated cement pastes as a function of degradation

Hydrated cement pastes (HCP) have a high affinity with a lot of (radio)toxic products and can be used as waste confining materials. In cementitious media, elements are removed from solution via (co)precipitation reactions or via sorption/diffusion mechanisms as surface complexation equilibria. In this study, to improve the knowledge of the surface charge evolution vs the degradation of the HCP particles, two cements have been studied: CEM-I (ordinary Portland cement, OPC) and CEM-V (blast furnace slag and fly ash added to OPC). Zeta potential measurements showed that two isoelectric points exist vs HCP leaching, i.e., pH. Zeta potential increases from -17 to +20 mV for pH 13.3 to pH 12.65 (fresh HCP states) and decreases from 20 to -8 mV for pH 12.65 to 11 (degraded HCP states). The use of a simple surface complexation model of C-S-H, limited in comparison with the structural modeling of C-S-H in literature, allows a good prediction of the surface potential evolution of both HCP. Using this operational modeling, the surface charge is controlled by the deprotonation of surface sites (>SO(-)) and by the sorption of calcium (>SOCa(+)), which brings in addition a positive charge. The calcium concentration is controlled by portlandite or calcium silicate hydrate (C-S-H) solubilities.     

Surface potential of hematite in aqueous electrolyte solution: hysteresis and equilibration at the interface

Electrostatic potential at the inner plane of the hematite aqueous interface, i.e., surface potential, was measured by means of a single-crystal hematite electrode. Acidic solutions were titrated with base and then back-titrated with acid. Surface potentials were evaluated from electrode potentials by setting the zero value at the isoelectric point. In the case of fast titrations the equilibration time was approximately 10 min, and significant hysteresis was obtained, more pronounced at higher electrolyte concentrations. Hysteresis disappeared in slow titration runs when the equilibration time was extended up to 120 min, and also when ultrasound was applied. Hysteresis was observed in the pH region close to neutrality, where the concentrations of potential-determining H+ and OH- ions are low. Equilibration was fast in acidic and basic regions. These results are explained on the basis of the kinetics of surface reactions, supported by the following rate of single-crystal electrode equilibration. It is concluded that the equilibration rate at the interface is specific for a given system and is not a general phenomenon. As several systems may undergo fast equilibration, such data may be regarded as equilibrium data and interpreted by the surface complexation model. In other cases, one should perform kinetic tests and apply extended equilibration times.     

Identification of low-index crystal planes of polycrystalline gold on the basis of electrochemical oxide layer formation

The electrochemical oxidation of single-crystal gold surfaces has been well studied, and the exposed crystal planes can be reliably distinguished based on the peak potentials of oxide formation. However, the multiple oxidation peaks of polycrystalline gold have not yet been unambiguously related to crystal planes. In this work, we used cyclic voltammetric responses of activated polycrystalline gold electrodes recorded in sulfuric acid solutions to allow constructing relationships between crystal planes and oxide peaks. The studies of oxide formation were complemented by measuring double-layer non-faradaic currents, lead underpotential deposition (Pb-upd), the oxygen reduction reaction (ORR), and the hydrogen evolution reaction (HER).

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Graphical abstract The link between three gold oxide current peaks and exposed low index crystal planes, viz. Au(100), Au(110) and Au(111) on polycrystalline gold electrode

     

Liquid crystal compound anti-ice surface

In some cold areas/regions, ice accumulation is harmful to aircrafts, highways and power lines. To overcome this challenge, many researchers have focused on developing anti-icing surfaces. In this paper, liquid crystal compound Cholest-5-en-3-ol(3β)-4-(2-propenyloxy)benzoate was synthesised, and a liquid crystal surface (LC surface) is prepared by heating the liquid crystal compound to 250°C and then cooling it. We determined ice-phobic properties of LC surface using differential scanning calorimetry (DSC) and polarised optical microscope (POM). A phenomenon that the freezing of water droplet on the LC surface is delayed was found. Compared with the two measurement methods, we obtained similar result that water freezing temperature was delayed nearly by 8°C on average on the LC surface. A process of ice/frost formation is observed using POM. The results displayed that the glass wafer without LC was covered completely by ice/frost, whereas on the LC surface at the same temperature no ice/frost was formed. Characteristics of LC surface were observed by field emission scanning electron microscope (FESEM) and Fourier transform infrared (FT-IR) imaging system. We suggest that the reason behind anti-ice surface is related to surface molecules and this is an important factor which may have an effect on anti-icing property.     

Fluctuations during catalytic CO oxidation on different crystal planes of a Pt field emitter

Fluctuations during catalytic CO oxidation on the (100), (111) and (113) planes of the [100]-oriented Pt field emitter tip are studied by FEM (field electron microscope). Intensity profiles and local time series are obtained by digitizing the image brightness in a rectangular probing window oriented along the [110]-direction. From the local time series the probability density distributions as well as the auto- and cross-correlation functions are computed. The results demonstrate that the fluctuation behavior is quite different on the different facets of a field emitter tip. On the facets studied here, the fluctuations observed in the monostable and in the bistable region of the reaction exhibit substantial differences in the degree of time- and spatial correlation.     

The surface potential of solvent and the intraphase pre-existing potential

Using own and literature data, the differences of real solvation energy for ferricenium and ferrocene in six solvents are found. These quantities are confronted with the calculated difference of the dielectric response energies plus nonelectrostatic energies for the redox couple. Such a comparison allows determining the sum of the surface and intraphase potentials. The comparison of these sums with the experimental values of the surface potential differences obtained by the measuring of Volta potentials allowed determining the differences of pre-existing intraphase potentials formed by solvent molecules on the ferrocene molecule. Thus, the intraphase potentials are evaluated for the first time, using an approach not based on the molecular-dynamic modeling. Using some approximations, the surface potentials of the studied solvents are found.