High Surface Potential Research Articles

In situ surface potential evolution along Au/Gd:CeO2 electrode interfaces

We present an investigation of electroactive Au/gadolinium doped ceria electrode interfaces under CO2/CO co-electrolysis environments using a combination of in situ high temperature scanning surface potential microscopy (HT-SSPM) and modified Poisson-Cahn (PC) models. Here charged surface adsorbate-oxygen vacancy interactions manifested in HT-SSPM potential profiles as small perturbations of opposite sign in reference to the applied biases. The positive deviation of surface potential on Au from applied cathodic biases is attributed to the work function difference between gold (ϕAu ∼ 5.31 eV) and graphitic carbon deposits (ϕC ∼ 5.0 eV) formed through CO disproportionation. The negative potential deviation from the applied anodic bias is attributed to negatively charged carboxylates. Results of the PC model confirmed the affinity of oxygen vacancies for the surface, thus supporting in situ experimental evidence of surface vacancy accumulation/depletion processes induced by cathodic/anodic biases.

Partially Hydrolyzed Poly(n-propyl-2-oxazoline): Synthesis, Aqueous Solution Properties, and Preparation of Gene Delivery Systems.

Random copolymers of n-propyl-2-oxazoline and ethylenimine (PPrOx-PEI) were prepared by partial acidic hydrolysis of poly(n-propyl-2-oxazoline) (PPrOx). Dynamic and electrophoretic light scattering and diffusion-ordered NMR spectroscopy were utilized to investigate aqueous solution properties of the copolymers. Above a specific cloud point temperature, well-defined nanoparticles were formed. The latter consisted of a core composed predominantly of PPrOx and a thin positively charged shell from PEI moieties that mediated formation of polyplexes with DNA. The polyplexes were prepared at 65 °C at varying N/P (amine-to-phosphate groups) ratios. They underwent structural changes upon temperature variations 65-25-37 °C depending on N/P. At N/P < 2, the polyplex particles underwent minor changes because of formation of a surface layer of DNA that acted as a barrier and prevented swelling and disintegration of the initial particles. Dramatic rearrangements at N/P ≥ 2 resulting in large swollen microgel particles were overcome by coating of the polyplex particles with a cross-linked polymeric shell. The shell retained the colloidal stability and preserved the physicochemical parameters of the initial polyplex particles while it reduced the high surface potential values. Progressive loss of cytotoxicity upon complexation with DNA and coating of polyplex particles was displayed.

Pressure driven transport of neutral macro-solute in microchannel with porous wall at high surface potential

Mass transport of macro-solutes under streaming potential in a microchannel with porous wall is presented in this study. The streaming field under high wall potential and overlapping electric double layer with coupled velocity and concentration of neutral solutes is computed numerically in this work. Streaming potential increases by 7 folds as κH decreases from 0.7 to 0.1 at ζ=-50mV, where, κ-1 is Debye length, H is channel half height and ζ is wall zeta potential. Mass transport in terms of Sherwood number is calculated including osmotic effect of the solution and selective retention of macrosolutes. Electroviscous effects resulting from streaming potential and overlapping electric double layer at higher wall zeta potential have significant effect on velocity field and the mass transport of macrosolutes. Apparent viscosity due to this effect increases upto 3.2 times the solution viscosity at κH=0.2 for ζ=-50mV. This effect is intensified upto 4 times at higher wall potential, ζ=-200mV at κH=0.5. At these points, the velocity profile suffers the most adversely affecting the mass transfer reducing Sherwood number to 4 (about 5 times reduction compared to higher κH) at ζ=-200mV and κH=0.5. Sherwood number reduces by three to four folds at the critical κH values, representing the minimum. The induced electrical field due to the streaming potential is higher compared to axial pressure drop for |ζ|>50mV at κH<0.3. The electric double layer overlaps for the values of κH upto 0.7 corresponding to ζ=-50mV. In case of high ζ (−200mV), the centerline potential can be as high as 75% of the wall potential for κH=1.5. The minimum wall Peclet number (Pew‾) occurs at κH=0.5 for ζ=-200mV, which is five times less than the situation without electrokinetic effects.

Communication between nitric oxide synthase and positively-charged surface and bone formation promotion

Despite the effects on physiology of bone marrow mesenchymal stem cells (BMSCs) and bone tissue, biological signal communication between bone implants and them is seldom employed as a guidance to create an osteo-inductive interface. Herein, the positively-charged surface is constructed on bone implant from the perspective of mediation of nitric oxide synthase (NOS) expression to signal BMSCs osteo-differentiation. In vitro and in vivo results indicate that the proper surface potential on the positively-charged surface affects NOS to express a high level of inducible nitric oxide synthase (iNOS) in three NOS isoforms of the contacted BMSCs, upregulates their osteogenetic expression, and ultimately foster new bone growth. However, an excessively high surface potential produces substantial immunomodulatory effects thereby offsetting the aforementioned advantages. This study demonstrates that fine-tuning of the positively-charged surface and proper utilization of the communication between NOS and bone implants promote bone formation.

Precipitantless Crystallization of Protein Molecules Induced by High Surface Potential

In the context of protein crystallization, surface decorated with heterogeneous topographical features decreases the energy barrier for nucleation, thereby facilitating crystallization; a precipitant is, nevertheless, required to be used. Here we eliminate the need of such precipitant by using a combined effect of nanoscopic surface undulations and charges on a substrate. Using surface instabilities as a tool for generating such features on polymeric materials, we show that intrinsic curvature of nanofeatures (<10 nm) coupled with surface charges lead to spatial gradient in potential as high as 140 V·μm–1, where curvature gets maximum. These surfaces show remarkable ability to induce nucleation not achieved by any other conventional process. They induce precipitantless nucleation of proteins, directed crystallization of a specific protein from a mixture of two or more species, and even simultaneous crystallization from a mixture of proteins. These results signify large scale molecular ordering at the bulk...

Critical adsorption of polyelectrolytes onto planar and convex highly charged surfaces: the nonlinear Poisson–Boltzmann approach

We study the adsorption–desorption transition of polyelectrolyte chains onto planar, cylindrical and spherical surfaces with arbitrarily high surface charge densities by massive Monte Carlo computer simulations. We examine in detail how the well known scaling relations for the threshold transition—demarcating the adsorbed and desorbed domains of a polyelectrolyte near weakly charged surfaces—are altered for highly charged interfaces. In virtue of high surface potentials and large surface charge densities, the Debye–Hückel approximation is often not feasible and the nonlinear Poisson–Boltzmann approach should be implemented. At low salt conditions, for instance, the electrostatic potential from the nonlinear Poisson–Boltzmann equation is smaller than the Debye–Hückel result, such that the required critical surface charge density for polyelectrolyte adsorption increases. The nonlinear relation between the surface charge density and electrostatic potential leads to a sharply increasing critical surface charge density with growing ionic strength, imposing an additional limit to the critical salt concentration above which no polyelectrolyte adsorption occurs at all. We contrast our simulations findings with the known scaling results for weak critical polyelectrolyte adsorption onto oppositely charged surfaces for the three standard geometries. Finally, we discuss some applications of our results for some physical–chemical and biophysical systems.

Controlling the Relative Areas of Photocathodic and Photoanodic Terraces on the SrTiO3(111) Surface

SrTiO3(111) surfaces have been heated in air at 1250 °C, either alone or with reservoirs of TiO2 and SrTiO3 powders. The surface structure and properties were measured by atomic force microscopy, scanning Kelvin probe microscopy, X-ray photoelectron spectroscopy (XPS), and photochemical marker reactions that leave solid products on the surface at the site of the photochemical reaction. All of the surfaces are made up of atomically flat terraces of two distinguishable types. One terrace has a relatively higher surface potential and promotes the photochemical reduction of Ag+. The other terrace has a relatively lower positive potential and promotes the photochemical oxidation of Pb2+. XPS measurements show that the concentration of titanium at the surface increases with the fraction of TiO2 in the powder reservoir during annealing, and marker reactions show the fraction of terraces that promote oxidation increases with the titanium content. The fractional area of terraces that promote oxidation was controll...

High voltage surface potential measurements in ambient conditions: Application to organic thin-film transistor injection and transport characterization

A scanning surface potential measurement technique suited for thin-film devices operating under high voltages is reported. A commercial atomic force microscope has been customized to enable a feedback-controlled and secure surface potential measurement based on phase-shift detection under ambient conditions. Measurements of the local potential profile along the channel of bottom-gate organic thin-film transistors (TFTs) are shown to be useful to disentangle the contributions from the channel and contacts to the device performance. Intrinsic contact current-voltage characteristics have been measured on bottom-gate, top-contact (staggered) TFTs based on the small-molecule semiconductor dinaphtho[2,3-b:2′,3-f]thieno[3,2-b]thiophene (DNTT) and on bottom-gate, bottom-contact (coplanar) TFTs based on the semiconducting polymer polytriarylamine (PTAA). Injection has been found to be linear in the staggered DNTT TFTs and nonlinear in the coplanar PTAA TFTs. In both types of TFT, the injection efficiency has been found to improve with increasing gate bias in the accumulation regime. Contact resistances as low as 130 Ω cm have been measured in the DNTT TFTs. A method that eliminates the influence of bias-stress-induced threshold-voltage shifts when measuring the local charge-carrier mobility in the channel is also introduced, and intrinsic channel mobilities of 1.5 cm2 V−1 s−1 and 1.1 × 10−3 cm2 V−1 s−1 have been determined for DNTT and PTAA. In both semiconductors, the mobility has been found to be constant with respect to the gate bias. Despite its simplicity, the Kelvin probe force microscopy method reported here provides robust and accurate surface potential measurements on thin-film devices under operation and thus paves the way towards more extensive studies of particular interest in emerging fields of solid-state electronics.

Significantly enhanced surface electrical conduction of poly(butylene terephthalate) insulators by direct fluorination

Commercialization of cyclic butylene terephthalate oligomers has opened the door to bulky poly(butylene terephthalate) (PBT) matrix composite parts. In view of the potential applications of PBT in high voltage insulation or other sectors, where surface charge can play an unwanted role, PBT sheets were surface fluorinated in a laboratory vessel using a F2/N2 mixture with 12.5% F2 by volume at the different temperatures of 25, 55, and 85 ° C for the same time of 30 min, to enhance surface electrical conduction and to investigate the effect of fluorination temperature. Conductivity measurements reveal that the fluorinated samples have a much higher surface conductivity than the unfluorinated sample, and surface conductivity significantly increases with fluorination temperature. In accordance with the conductivity measurements, surface potential measurements after corona charging show that the fluorinated samples have a lower or much lower initial surface potential or even no surface potential, strongly depending on fluorination temperature and ambient humidity, compared with a high surface potential of the unfluorinated sample. ATR-IR analyses indicate that fluorination led to substantial changes in surface composition and structure, depending on fluorination temperature. SEM cross section and surface images show an increase of the fluorinated layer thickness with fluorination temperature and a modification of the surface morphology. Contact angle measurements of water show that fluorination also led to a significant increase in surface hydrophilicity, especially at higher fluorination temperatures. The intrinsic increase in surface electrical conduction is considered to be a result of competition between surface compositional changes and surface structural changes or the chemical traps and the physical traps.

A computational study of multiple surface-directed phase separation in polymer blends under a temperature gradient

The surface-directed phase separation (SDPS) phenomena of a model binary polymer blend quenched into the unstable region of its binary symmetric upper critical solution temperature phase diagram is numerically investigated using a mathematical model composed of the nonlinear Cahn–Hilliard (CH) theory for phase separation along with the Flory–Huggins–de Gennes (FHdG) free energy functional. The SDPS occurs in a square domain with a linear temperature gradient along the horizontal direction and with all sides having short range surface potential h1. The effects of different quench depth, diffusion coefficient, surface potential, and temperature gradient were studied numerically. The numerical results indicate that there is a simultaneous competition between the four surfaces in attracting the preferred polymer. The side with a higher surface potential would win the competition against the side with a lower surface attraction in the case of a uniform quench. The numerical results also indicated a later transition time for higher values of h1. As surface potential increased, the transition time from complete wetting to partial wetting occurred at a later time on the surface. The impact of different temperature gradient ΔT*/Δx* values on the surface enrichment rate with fixed temperature at one surface and higher temperature at the opposite surface was studied for the first time within a multiple surface potential set up. The results showed that higher values of ΔT*/Δx* increased the growth rate of the preferred polymer on the surface adding to the thickness of the wetting layer. The transition time from complete wetting to partial wetting occurred slightly later at the lower temperature side.

A modified Poisson-Boltzmann theory: Effects of co-solvent polarizability

In this paper within a field-theoretical approach taking into account explicitly a co-solvent with a nonzero dipole and a polarizability tensor, we derive a modified Poisson-Boltzmann equation. Applying the modified Poisson-Boltzmann equation, we formulate a generalized Gouy-Chapman theory for the case when an electrolyte solution is mixed with a polar co-solvent having a large polarizability. We show that an increase of the co-solvent concentration as well as the co-solvent polarizability lead to a significant increase of differential capacitance at sufficiently high surface potentials of the electrode, whereas the profile of the electrostatic potential becomes considerably more long-ranged. On the contrary, an increase in the permanent dipole of the co-solvent only weakly affects the differential capacitance.

The coupling effects of electric field and clay mineralogy on clay aggregate stability

This paper aims to investigate the effect of clay mineralogy on aggregate stability under the consideration of soil particle surface electric field in electrolyte solution. Aggregates of montmorillonite, kaolinite, and their compounds in different mass ratio were used as experimental materials. The degree to which the aggregates broke down under different electrolyte concentrations was evaluated both qualitatively and quantitatively. The aggregate breakdown process was observed with a video microscope. Furthermore, the percentage of particles (micro-aggregates and single particles) in diameter of <10, <5, and <2 μm to the total sample mass were measured based on Stokes equation. Different electrolyte concentrations correspond to different surface potential values around the particle surface. (1) The amount of released small particles for aggregate of clay compounds was several times higher than that for montmorillonite aggregate after breakdown. (2) Under high surface potential conditions, aggregate breakdown in a way of explosion was observed, and the critical surface potential values for the explosion increased with the increase of kaolinite content in aggregates. (3) The content of released particles for clay compound aggregates with high kaolinite content was much higher than that for aggregates with low kaolinite content under high surface potential values. The coupling effects of electric field and clay mineralogy determined the aggregate stability of montmorillonite and clay compounds. This indicated that soil aggregates with relatively high silt content would be less stable than those with relatively high clay content under heavy rainfall; the most erosive soil might not be clay soils, but the silt soils, especially those under sudden storm after long-time drought.

Differential capacitance of the electric double layer: the interplay between ion finite size and dielectric decrement.

We study the electric double layer by combining the effects of ion finite size and dielectric decrement. At high surface potential, both mechanisms can cause saturation of the counter-ion concentration near a charged surface. The modified Grahame equation and differential capacitance are derived analytically for a general expression of a permittivity ε(n) that depends on the local ion concentration, n, and under the assumption that the co-ions are fully depleted from the surface. The concentration at counter-ion saturation is found for any ε(n), and a criterion predicting which of the two mechanisms (steric vs. dielectric decrement) is the dominant one is obtained. At low salinity, the differential capacitance as function of surface potential has two peaks (so-called camel-shape). Each of these two peaks is connected to a saturation of counter-ion concentration caused either by dielectric decrement or by their finite size. Because these effects depend mainly on the counter-ion concentration at the surface proximity, for opposite surface-potential polarity either the cations or anions play the role of counter-ions, resulting in an asymmetric camel-shape. At high salinity, we obtain and analyze the crossover in the differential capacitance from a double-peak shape to a uni-modal one. Finally, several nonlinear models of the permittivity decrement are considered, and we predict that the concentration at dielectrophoretic saturation shifts to higher concentration than those obtained by the linear decrement model.

Charging of poly(methyl methacrylate) (PMMA) colloids in cyclohexyl bromide: locking, size dependence, and particle mixtures.

We studied suspensions of sterically stabilized poly(methyl methacrylate) (PMMA) particles in the solvent cyclohexyl bromide (CHB; εr = 7.92). We performed microelectrophoresis measurements on suspensions containing a single particle species and on binary mixtures, using confocal microscopy to measure the velocity profiles of the particles. We measured the charge of so-called locked PMMA particles, for which the steric stabilizer, a comb-graft stabilizer of poly(12-hydroxystearic acid) (PHSA) grafted on a backbone of PMMA, was covalently bonded to the particle, and for unlocked particles, for which the stabilizer was adsorbed to the surface of the particle. We observed that locked particles had a significantly higher charge than unlocked particles. We found that the charge increase upon locking was due to chemical coupling of 2-(dimethylamino)ethanol to the PMMA particles, which was used as a catalyst for the locking reaction. For particles of different size we obtained the surface potential and charge from the electrophoretic mobility of the particles. For locked particles we found that the relatively high surface potential (∼ +5.1 kBT/e or +130 mV) was roughly constant for all particle diameters we investigated (1.2 μm < σ < 4.4 μm), and that the particle charge was proportional to the square of the diameter.

Modeling water flux and salt rejection of mesoporous γ-alumina and microporous organosilica membranes

The water and ion transport through a mesoporous γ-alumina membrane and a microporous organosilica membrane was simulated using the extended Nernst–Planck equation combined with models for Donnan, steric and dielectric interfacial exclusion mechanisms. Due to the surface charge within the pore, the electroviscous effect was introduced in the model. The modified model fits well the rejection and permeability data for both membranes. The organosilica membrane shows a higher selectivity compared to the γ-alumina membrane, but the permeate flux is lower. At low ionic strength the electroviscous effect lowers the water flux through the γ-alumina membrane. The electroviscous effect is negligible for the organosilica membrane because its absolute surface potential (~20mV) is low compared to the γ-alumina membrane (~60mV). The simulation shows that the electroviscous effect should be included for the membranes with high surface potential (>20mV) and a pore size below 2–5 times of the electroviscous double layer thickness.

Charging and characterization of non-patterned organic micro electret arrays

This paper presents the charging and characterization of an organic micro electret array for vibration energy harvesting. Micro sized electrets with stable and high surface potential are essential for the long-term effectiveness of micro electrostatic power generators which harvest low-level vibration energy. In this study, a localized corona charging method to obtain micro sized electret areas on non-patterned macro sized dielectric thin film has been proposed to solve the problem of low charging efficiency and fast charge decay of micro sized electrets. This method involves using a shadow mask to transfer charge patterns and a triode corona charging configuration to facilitate charge penetration. Charging efficiency of 93.6% is achieved on an electret area of 100 µm × 100 µm, and 87% of its initial surface potential remains on the shrunken area of 50 µm × 50 µm after 240 days of storage. A technique combining SEM surface topography and non-contact measurement of the average surface potential has been developed to map the charge distribution on locally charged dielectric thin film and measure the surface potential on the micro sized charged area by incorporating the layout characteristic of a micro electret array.

Nanocrystalline zinc titanate coatings for corrosion protection

Nanocrystalline zinc titanate (ZnTiO3) coatings were deposited by atomic layer deposition (ALD) on AISI 52100 steel to study ZnTiO3 corrosion protection. The main aim of this study was to determine how the coating can inhibit pitting formation of 52100 steel in both saline solution and simulated body fluid (SBF) media. Potentiodynamic polarization tests revealed increasing corrosion potentials for the ALD ZnTiO3-coated steel specimens with protective efficiencies of 83 and 73% in saline solution and SBF, respectively. Atomic force microscopy analysis of both the ZnTiO3-coated and uncoated steel revealed a high surface potential value for the ZnTiO3-coated sample as compared to the uncoated sample with a relatively low surface potential. The corrosion rates for the coated steels in both media were calculated and observed to drop by an order of magnitude. Due to the conformality, homogeneity and density of the ALD ZnTiO3 coatings, migration of Cl− and OH− ions from the electrolytes were inhibited resulting in the improved corrosion resistance.

22am2-F7 荷電されたマイクロピラー上における液体表面の挙動

Electret that can be used in the liquid environment has been realized for the first time toward better performance of electret-based generators and actuators. By using super-lyophobic overhanging pillar surface, stable Cassie-Baxter (CB) state is sustained even with low-surface-tension hexadecane liquid and with high surface potential. Pillar surface with a SiO_2 electret layer has been successfully charged with soft X-ray photoionization previously developed. Surface potential as high as -150 V has been sustained after applying hexadecane droplet on the pillar surface. The interface behavior of CB droplets is systematically examined by changing the surface potential of the pillar.

Regimes of streaming potential in cylindrical nano-pores in presence of finite sized ions and charge induced thickening: An analytical approach

We obtain approximate analytical expressions for the streaming potential and the effective viscosity in a pure pressure-driven flow through a cylindrical pore with electrokinetic interactions, duly accounting for the finite size effects of the ionic species (steric effects) and charge-induced thickening. Our analytical results show a remarkable agreement with the numerical solution even for high surface potentials and small channel radii. We demonstrate a consistent increment in the predicted value of the streaming potential and effective viscosity when finite size effects of the ionic species are accounted for. In addition to this, we account for the radial variation of in the viscosity of the fluid due to charge-induced thickening. We show that this so-called viscoelectric effect leads to a decrease in the induced streaming potential especially at high steric factors and high surface potentials. However, the viscoelectric effect, which is prominent at high zeta potential and narrow channels, does not cause significant changes in the electrokinetic conversion efficiency. These results shed light on the interesting confluence of the steric factor, the channel radius, the electrical double layer screening length, and the surface charge density in conjunction with the charge induced thickening, and thus provide ion-size dependent analytical framework for accurate system design and better interpretation of electrokinetic data.

Hydrogen evolution reaction measurements of dealloyed porous NiCu

Porous metals are of interest for their high surface area and potential for enhanced catalytic behavior. Electrodeposited NiCu thin films with a range of compositions were electrochemically dealloyed to selectively remove the Cu component. The film structure, composition, and reactivity of these samples were characterized both before and after the dealloying step using scanning electron microscopy, energy-dispersive spectroscopy, and electrochemical measurements. The catalytic behavior of the dealloyed porous Ni samples towards the hydrogen evolution reaction was measured and compared to that of the as-deposited samples. The dealloyed samples were generally more reactive than their as-deposited counterparts at low overpotentials, making the dealloying procedure a promising area of exploration for improved hydrogen evolution catalysts.