Excess Surface Charge Research Articles

Temperature Jump Methods for Measuring the Pme of RuO2

The potential of maximum entropy (PME) is used to understand the structure and charging of the electrochemical double layer. Closely related to the potential of zero charge (PZC), the PME occurs when disorder of the solvent molecules at the surface reaches a maximum. Using a laser induced temperature jump (LITJ) to measure the PME has been shown to be a convenient method for metal electrodes. We present experiments using LITJ to investigate a metal oxide thin film and film thickness. The PME of these thin films appears to change with thickness, correlating with surface strain where a thinner film shifts the PME closer to OER potentials in turn reducing the overpotentials required. Our experiments are shown to be reproducible and independent of sweep rate or the vertex potentials selected. Interestingly we observe a hysteresis with scan direction, of 100-250 mV which we nominally attribute to surface adsorbates. Our results highlight the link between excess surface charge and catalytic activity and how this may be tuned via surface strain. Figure 1

Atomistic origin of high grain boundary resistance in solid electrolyte lanthanum lithium titanate

Lanthanum lithium titanate is one of the promising electrolytes for solid-state lithium-ion batteries due to its high bulk ionic conductivity up to ∼10−3 S/cm. However, the practical application of this material has been bottlenecked by high grain boundary (GB) resistance, while the underlying mechanism is still under debate. Here we report a comprehensive study with direct evidence to reveal the origin of high GB resistance in La2/3–xLi3xTiO3 (LLTO). Atomic-scale observations via advanced scanning transmission electron microscopy show that the GBs are uniformly subject to subsurface segregation of La atoms to compensate for the excess surface charges. The La segregation leads to an abrupt decrease of charge carrier concentration neighboring GBs and hence is supposed to have deleterious effect on GB conductivity. The findings suggest a novel mechanism of space-charge-induced cation segregation, which shed lights on the intrinsic origin of low GB ionic conductivity in LLTO.

Controlling corrosion protection of mild steel in acidic environment via environmentally benign organic inhibitor

The current research employed electrochemical techniques to evaluate a green corrosion inhibitor Subphthalocyanine chloride (SubPc) to control the corrosion mitigation of mild steel in acidic environments. The corrosion rates and hydrogen evolution rates were measured by weight loss. The inhibitor was found to be highly effective with a corrosion-inhibiting efficiency up to 97.6% at a concentration of 150 ppm in 1 M HCl solution loaded with synthesized (SubPc) through variable concentrations. According to the obtained information from the polarization technique, the corrosion current density, icorr, decreases with increasing the concentration levels of (SubPcs) inhibitors in 1 M HCl medium. The inhibition mechanism of mild steel has been suggested as well as its excess surface charge was determined employing electrochemical impedance spectroscopy (EIS). The results suggest that the inhibitory functions by forming a protective barrier on the steel surface, hindering the corrosion process. The present study provides a new insight into developing eco-friendly and cost-effective corrosion inhibitor to protect mild steel from acidic corrosion.

Electrocatalytic Mechanisms for an Oxygen Evolution Reaction at a Rhombohedral Boron Monosulfide Electrode/Alkaline Medium Interface.

Rhombohedral boron monosulfide (r-BS) with a layer stacking structure is a promising electrocatalyst for an oxygen evolution reaction (OER) within an alkaline solution. We investigated the catalytic mechanisms at the r-BS electrode/alkaline medium interface for an OER using hybrid solvation theory based on the first-principles method combined with classical solution theory. In this study, we elucidate the activities of the OER at the outermost r-BS sheet with and without various surface defects. The Gibbs free energies along the OER path indicate that the boron vacancies at the first and second layers of the r-BS surface (VB1 and VB2) can promote the OER. However, we found that the VB1 is easily occupied by the oxygen atom during the OER, degrading its electrocatalytic performance. In contrast, VB2 is suitable for the active site of the OER due to its structure stability. Next, we applied a bias voltage with the OER potential to the r-BS electrode. The bias voltage incorporates the positive excess surface charge into pristine r-BS and VB2, which can be understood by the relationship between the OER potential and potentials of zero charge at the r-BS electrode. Because the OH- ions are the starting point of the OER, the positively charged surface is kinetically favorable for the electrocatalyst owing to the attractive interaction with the OH- ions. Finally, we qualitatively discuss the flat-band potential at a semiconductor/alkaline solution interface. It suggests that p-type carrier doping could promote the catalytic performance of r-BS. These results explain the previous measurement of the OER performance with the r-BS-based electrode and provide valuable insights into developing a semiconductor electrode/water interface.

Tunable Fluorescence from 2D Assemblies of LnW10 and P2W18 Polyoxometalates Clusters with Quaternary Ammonium‐type AIEgens

AbstractTwo‐dimensional (2D) AIE‐inorganic nanocomposites exhibit high stability and high fluorescence activity. However, limited by the types and properties of 2D inorganic materials, it's challenging to realize complex structures and functions. We designed a quaternary ammonium‐type AIE ligand, TPECholine. We chose TPECholine and POM clusters as building blocks and synthesized a series of single‐layer nanosheets (SLNSs) by assembling LnW10 or P2W18 POM clusters with TPECholine. By regulating the types of POM clusters and ligands, the morphology and fluorescence intensity of the SLNSs can be finely tuned. Due to the restriction of the intramolecular motions of AIEgens by the SLNSs, nanosheets can exhibit promoted quantum yield (up to 76 %). In addition, thanks to the sub‐nanometric sizes and excess surface charges, SLNSs exhibit excellent solvent compatibility, including water, chloroform, ethanol, etc. And the nanosheets showed high fluorescence intensity in these solvents.

Tunable Fluorescence from 2D Assemblies of LnW10 and P2W18 Polyoxometalates Clusters with Quaternary Ammonium-type AIEgens.

Two-dimensional (2D) AIE-inorganic nanocomposites exhibit high stability and high fluorescence activity. However, limited by the types and properties of 2D inorganic materials, it's challenging to realize complex structures and functions. We designed a quaternary ammonium-type AIE ligand, TPECholine. We chose TPECholine and POM clusters as building blocks and synthesized a series of single-layer nanosheets (SLNSs) by assembling LnW10 or P2W18 POM clusters with TPECholine. By regulating the types of POM clusters and ligands, the morphology and fluorescence intensity of the SLNSs can be finely tuned. Due to the restriction of the intramolecular motions of AIEgens by the SLNSs, nanosheets can exhibit promoted quantum yield (up to 76%). In addition, thanks to the sub-nanometric sizes and excess surface charges, SLNSs exhibit excellent solvent compatibility, including water, chloroform, ethanol, etc. And the nanosheets showed high fluorescence intensity in these solvents.

A comprehensive study on the adsorption, corrosion inhibition efficiency and stability of acriflavine on mild steel in 1 M HCl solution

A comprehensive study was conducted on the adsorption, corrosion inhibition efficiency and stability of acriflavine (ACF) on mild steel (MS) in 1 M HCl solution. Electrochemical studies were conducted to determine the corrosion protection ability of the inhibitor after short and long immersion times. The MS surface after exposing to the uninhibited and inhibited solutions were characterized by SEM, EDX, AFM and contact angle measurements. The excess surface charge of the metal and some thermodynamic parameters were derived after 1 h and 6 h exposure times and a possible adsorption and inhibition mechanism was discussed. The electrochemical stability of the inhibitor film formed on the metal surface was studied using potentiodynamic and potentiostatic techniques. It was found that ACF adsorbs on the MS surface via dominantly chemical and weakly physical interactions and forms a protective surface film. The film was adherent, uniform, protective and electrochemically stable. The inhibitor film provides maximum inhibition efficiencies of 92.8% and 97.1% at 1 mM after 1 h and 6 h immersion times. ACF mitigates the rates of both anodic metal dissolution and cathodic hydrogen evolution reactions. The excess surface charge of the steel in the inhibited solution was negative, which becomes more evident after longer exposure time.

The use of methanol extract of Rheum Ribes (Işgın) flower as a natural and promising corrosion inhibitor for mild steel protection in 1 M HCl solution

There is a tendency towards natural products with the increase in sensitivity to the environment, people and other living things around the world. In recent years, there has been a similar trend regarding inhibitors used in the metals protection against corrosion. In this study, the adsorption and protection ability of Rheum ribes (RR) flower extract on mild steel (MS) corrosion in 1 M HCl solution were investigated with the help of many electrochemical techniques. The MS surface after exposing to the test solutions was examined by energy dispersive X-ray spectroscopy (EDX), scanning electron microscopy (SEM) and contact angle measurements. Some thermodynamic parameters corresponding to the adsorption of extract molecules on the MS surface and excess surface charge of the steel in the extract containing solution were calculated and a possible mechanism of the corrosion inhibition process was discussed. The durability of the extract film formed on the MS surface was examined under potentiodynamic and potentiostatic conditions. It was found that an adherent and uniformly distributed extract film formed on the MS surface, which is responsible for the inhibition of corrosion. The flower extract provided 94.7 % and 98.4 % protection efficiencies at 1000 ppm concentration after 1 h and 6 h exposure. RR flower extract mitigated both anodic and cathodic reactions, predominantly cathodic process. The extract molecules interact and bond to the MS surface via physical and chemical interactions. The extract could be suggested as a promising natural corrosion inhibitor for MS protection in 1 M HCl solution.

Oxygen Adsorption on Polar and Non-Polar Zn:ZnO Heterostructures from First Principles.

Zn:ZnO nanostructures have been studied extensively due to their potential use in many applications, such as oxygen scavengers for food packaging applications. Under atmospheric conditions, ZnO grows on the surface of Zn via an oxidation process. The mechanisms governing Zn oxidation are still not fully understood, with classical oxidation models, such as the Cabrera Mott, underestimating the oxide thickness of Zn:ZnO core-shell structures. In this work, Ab initio DFT calculations were performed to assess the adsorption properties of oxygen molecules on Zn:ZnO heterostructures to help elucidate the mechanisms involved in the growth of a ZnO film on a Zn substrate. Results suggest that the charge transfer mechanism from the Zn:ZnO heterostructures to the adsorbed oxygen layer can be promoted by two different processes: the electronic doping of ZnO due to the formation of the Zn:ZnO interface and the excess surface charge due to the presence of dangling bonds on the as cleaved ZnO.

Multivalent Ion-Modulated Electron Transfer Processes in Carbon Nanopipettes.

Conductive nanopipettes with both an electroactive interface and a pipet geometry have been recognized as powerful multifunctional probes in various electrochemical sensing and imaging applications. As confined inside the nanopipette, the excess surface charges at the solid/solution interface would then play a dominant role in the resulting charge transport processes. Herein, the effects of a multivalent ion on the resulting electron transfer (ET) processes in the carbon nanopipettes are investigated with both experimental and simulation methods. The multivalent cations (i.e., Ca2+, Mg2+, Co2+, and Ni2+) are shown to strongly adsorb at the negatively charged carbon surface and attract more Fe(CN)64- ions inside the cavity, as indicated by the increasing ET current responses. In addition to elucidating the fundamental charge transport processes in conductive nanopipettes to afford better usage as electrochemical probes, these results could also help in the development of new sensing methods for measuring the non-electroactive ions in biological or environmental systems.

The Cation Role in Ion Transport Selectivity of Gold-Plated Nanotubes

Ion permselectivity is an important ionic transport property exhibited in nanopores and nanotubes. Permselectivity develops due to the strong interactions between ions in solution and the nanopore/nanotube surfaces. Here, we study the ion permselectivity of gold-plated nanotube membranes. Gold-plated membranes were obtained by using an electroless gold plating method to deposit gold nanotubes into the pores and on the faces of track-etched polycarbonate membranes. These membranes have the potential to display cation permselectivity due to the excess surface charge density present on the membrane from chemisorbed Cl- anions. The cation permselectivity of membranes with 9.9 + 0.6 nm diameter nanotubes was studied potentiometrically by using a concentration cell. A new Debye-sphere based model was presented here to describe the extent of permselectivity. However, some ion specific effects with the gold-plated membrane are observed that cannot be interpreted by Debye theory. These ion-specific surface interactions lead to a reduction in the negative surface charge, causing no permselectivity for some cations as demonstrated by their lower than ideal cation transference numbers. Quantification of the surface charge density by XPS indicated the gold-plated membranes are highly charged due to Cl- chemisorption with a surface charge density of ~ 5.3 uC per cm2 corresponding to 3.3 x 1013 Cl--ions per cm2.

Ionic Transport Triggered by Asymmetric Illumination on 2D Nano-Membrane.

Ionic transport and ion sieving are important in the field of separation science and engineering. Based on the rapid development of nanomaterials and nano-devices, more and more phenomena occur on the nanoscale devices in the field of thermology, optics, mechanics, etc. Recently, we experimentally observed a novel ion transport phenomenon in nanostructured graphene oxide membrane (GOM) under asymmetric illumination. We first build a light-induced carriers’ diffusion model based on our previous experimental results. This model can reveal the light-induced ion transport mechanism and predict the carriers’ diffusion behavior under different operational situations and material characters. The voltage difference increases with the rise of illuminate asymmetry, photoresponsivity, recombination coefficient, and carriers’ diffusion coefficient ratio. Finally, we discuss the ion transport behavior with different surface charge densities using MD simulation. Moderate surface charge decreases the ion transport with the same type of charge due to the electrostatic repulsion; however, excess surface charge blocks both cation and anion because a thicker electrical double layer decreases effective channel height. Research here provides referenced operational and material conditions to obtain a greater voltage difference between the membrane sides. Also, the mechanism of ion transport and ion sieving can guide us to modify membrane material according to different aims.

Sample-dependent Dirac-point gap inMnBi2Te4and its response to applied surface charge: A combined photoemission andab initiostudy

Recently discovered intrinsic antiferromagnetic topological insulator MnBi$_2$Te$_4$ presents an exciting platform for realization of the quantum anomalous Hall effect and a number of related phenomena at elevated temperatures. An important characteristic making this material attractive for applications is its predicted large magnetic gap at the Dirac point (DP). However, while the early experimental measurements reported on large DP gaps, a number of recent studies claimed to observe a gapless dispersion of the MnBi$_2$Te$_4$ Dirac cone. Here, using micro($\mu$)-laser angle-resolved photoemission spectroscopy, we study the electronic structure of 15 different MnBi$_2$Te$_4$ samples, grown by two different chemists groups. Based on the careful energy distribution curves analysis, the DP gaps between 15 and 65 meV are observed, as measured below the N\'eel temperature at about 10-16 K. At that, roughly half of the studied samples show the DP gap of about 30 meV, while for a quarter of the samples the gaps are in the 50 to 60 meV range. Summarizing the results of both our and other groups, in the currently available MnBi$_2$Te$_4$ samples the DP gap can acquire an arbitrary value between a few and several tens of meV. Further, based on the density functional theory, we discuss a possible factor that might contribute to the reduction of the DP gap size, which is the excess surface charge that can appear due to various defects in surface region. We demonstrate that the DP gap is influenced by the applied surface charge and even can be closed, which can be taken advantage of to tune the MnBi$_2$Te$_4$ DP gap size.

The Surface-Vacancy Model—A General Theory of the Dissolution of Minerals and Salts

The kinetics of the dissolution of salts and minerals remains a field of active research because these reactions are important to many fields, such as geochemistry, extractive metallurgy, corrosion, biomaterials, dentistry, and dietary uptake. A novel model, referred to as the surface-vacancy model, has been proposed by the author as a general mechanism for the primary events in dissolution. This paper expands on the underlying physical model while serving as an update on current progress with the application of the model. This underlying physical model envisages that cations and anions depart separately from the surface leaving a surface vacancy of charge opposite to that of the departing ion on the surface. This results in an excess surface charge, which in turn affects the rate of departing ions. Thus, a feedback mechanism is established in which the departing of ions creates excess surface charge, and this net surface charge, in turn, affects the rate of departure. This model accounts for the orders of reaction, the equilibrium conditions, the acceleration or deceleration of rate in the initial phase and the surface charge. The surface-vacancy model can also account for the effect of impurities in the solution, while it predicts phenomena, such as ‘partial equilibrium’, that are not contemplated by other models. The underlying physical model can be independently verified, for example, by measurements of the surface charge. This underlying physical model has implications for fields beyond dissolution studies.

Polar zinc oxide surface in electrolyte solutions: an atomic view of reconstruction, hydration and surface states.

The stabilization mechanism of the Zn-terminated (Zn-) ZnO(0001) surface in electrolyte solutions has been investigated by using atomic-resolution liquid-environment atomic force microscopy (AFM) and an electrochemical method. The electrochemically measured pH dependence of the flat band potential of the Zn-ZnO(0001) surface indicated the adsorption of OH groups onto the (0001) surface in the wide pH range of 1-13. Atomic-scale AFM images of the Zn-ZnO(0001) surface showed a well-ordered hydroxide superstructure in an alkaline solution but a disordered structure in an acidic solution, which is probably attributed to the rapid diffusion of the adsorbed OH groups. Furthermore, the density of the O-terminated step edge on the Zn-ZnO(0001) surface in an acidic solution was higher than that in an alkaline solution. From these findings, we concluded that the excess positive charges of the Zn-ZnO(0001) surface are compensated by the adsorbed OH groups and the O-terminated step edges. In acidic solutions, a higher density of the O-terminated step edge is required for charge compensation. In addition, it was found that potential-dependent reversible surface reconstruction occurs in the local transition area with disordered step orientation by electrochemical AFM. We concluded that the reconstruction compensates the excess surface charges of the local transition area which are induced and varied by potential-dependent local surface states.

Structural and electronic properties of monodomain ultrathin PbTiO3/SrTiO3/PbTiO3/SrRuO3 heterostructures: A first-principles approach

First-principles calculations within the local density approximation were carried out to explain the ground state and electronic properties of a vacuum/PbTiO3/SrTiO3/PbTiO3/SrRuO3 multilayer in a monodomain phase. Open-circuit boundary conditions were assumed, considering the electric displacement field, D, as the fundamental electrical variable. The direction and the magnitude of D can be monitored by proper treatment of the PbO surface layer, introducing external fractional charges Q in the surface atomic layers by means of virtual crystal approximation. Different excess or deficit surface charges (from Q=±0.05 to Q=±0.15) were considered, corresponding to small values of the polarization (up to ±0.16C/m2) in both directions. The layer-by-layer electric polarization, tetragonality, and the profile of the electrostatic potential were computed, as well as the projected density of states, as a function of electric displacement field. The magnitude of D is preserved across the dielectric layers, which translates into a polarization of the SrTiO3 spacer layer. The tetragonality of the two PbTiO3 layers is different, in good agreement with experimental x-ray diffraction techniques, with the layer closer to the free surface exhibiting a smaller value. This is attributed to the interplay with surface effects that tend to contract the material in order to make the remaining bonds stronger. Our calculations show how the final structure in this complex oxide heterostructure comes from a delicate balance between electrical, mechanical, and chemical boundary conditions.

Stochastic Particle Approach Electrochemistry (SPAE): Estimating Size, Drift Velocity, and Electric Force of Insulating Particles.

Stochastic particle impact electrochemistry (SPIE) is considered one of the most important electro-analytical methods to understand the physicochemical properties of single entities. SPIE of individual insulating particles (IPs) has been particularly crucial for analyses of bioparticles. In this article, we introduce stochastic particle approach electrochemistry (SPAE) for electrochemical analyses of IPs, which is the advanced version of SPIE; SPAE is analogous to SPIE but focuses on deciphering a sudden current drop (SCD) by an IP-approach toward the edge of an ultramicroelectrode (UME). Polystyrene particles (PSPs) with and without different surface functionalities (-COOH and - NH3) as well as fixed human platelets (F-HPs) were used as model IPs. From theory based on finite element analysis, a sudden current drop (SCD) induced by an IP during electro-oxidation (or reduction) of a redox mediator on a UME can represent the rapid approach of an IP toward an edge of a UME, where a strong electric field is generated. It is also found that the amount of current drop, idrop, of an SCD depends strongly on both the size of an IP and the concentration of redox electrolyte. From simulations based on the SPAE model that fit the experimentally obtained SCDs of three types of PSPs or F-HP dispersed in solutions with two redox electrolytes, their size distribution histograms are estimated, from which their average radii determined by SPAE are compared to those from scanning electron microscopic images. In addition, the drift velocity and corresponding electric force of the PSPs and F-HPs during their approach toward an edge of a Pt UME are estimated, which cannot be addressed currently with SPIE. We further learned that the estimated drift velocity and the corresponding electric force could provide a relative order of the number of excess surface charges on the IPs.

Light scattering by a charged infinite cylinder in a transparent medium

Although charged particles exist widely in a plasma environment or an electrolyte, the problem of electromagnetic scattering by a charged cylinder has remained unsolved. In this paper, using the tangential boundary conditions with a surface current induced by excess surface charges, we calculate the extinction, scattering and absorption efficiencies for a charged infinite cylinder upon which a plane wave is normally incident. For two polarizations of the incident light, we show the different effects of the excess surface charges on the optical efficiencies. For p- polarization, the extinction efficiencies own a platform structure where the extinction is in proportion to the density of surface charges, which corresponds to the light absorption by the excess surface charges. For s- polarization, the excess surface charges will induce a new resonance at the dielectric function of the cylinder larger than − 1 and a shift on the anomalous resonance which can only exist for the dielectric function near and smaller than − 1. The increasing surface charge density can shift these resonances to a larger size parameter and broaden the widths of the anomalous resonances. Furthermore, considering a specific case of a charged MgO cylinder, we identify a blue shift of the resonances near the longitudinal optical phonon frequency.

The enhancement effect of surface charges on forward and backward scattering of submicron polydisperse particles

The existence of excess surface charges can change the complex refractive index and scattering properties of the submicron single particle. When charged submicron particles constitute a polydisperse particle system, the overall optical behavior will be affected by the number of particles, size distribution, surface charge density, and uneven complex refractive index. If the classical Mie theory based on electrically neutral particles is used to analyze such a particle system, it will inevitably lead to erroneous results. In this paper, the calculation model for scattering properties of charged spherical polydisperse particle systems is established, based on Mie theory. Considering the difference in the complex refractive index of the different size particles in the particle system, we analyzed the change characteristics of the optical properties of the submicron particle system before and after charged. Through calculation, it is found that the non-uniform particle size and complex refractive index in the polydisperse particle system have a certain buffer effect on the change of each optical coefficient, compared with the monodisperse particle system. Taking different Rosin-Ramler distributions as examples, we discussed the enhancement effect of surface charges on forward and backward scattering of submicron particle systems. It is found that the enhancement of forward scattering is more effective for particle systems with smaller median diameters. This means that the enhancement of charge helps to achieve visible light measurement of submicron or even nanometer particle systems.

Electrically controlled magnetism in iron thin film

The coupling of ferroelectric and ferromagnetic order parameters in low dimension materials has been of great interest due to the potential spintronic applications. In this report, by employing first-principles density-function-theory calculation without including spin-orbital coupling, it is demonstrated that the magnetization of iron thin film increases in proportion to the external electric field and it prefers to be perpendicular to the film plane. This intriguing property stems from the spin-dependent screening of the electric field which leads to a spin imbalance of the excess surface charge. The result is in agreement with previous publications. We suggest that the electrically controlled magnetism might be used in advanced magnetic and electronic devices.