Positive Charge Density Research Articles

Enhanced activation of periodate by iodine-doped granular activated carbon for organic contaminant degradation

In this study, iodine-doped granular activated carbon (I-GAC) was prepared and subsequently applied to activate periodate (IO4−) to degrade organic contaminants at ambient temperature. The physicochemical properties of GAC and I-GAC were examined using scanning electron microscopy, N2 adsorption/desorption, Raman spectroscopy, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. No significant difference was observed between the two except for the existence of triiodide (I3−) and pentaiodide (I5−) on I-GAC. The catalytic activity of I-GAC towards IO4− was evaluated by the degradation of acid orange 7 (AO7), and superior catalytic performance was achieved compared with GAC. The effects of some influential parameters (preparation conditions, initial solution pH, and coexisting anions) on the catalytic ability were also investigated. Based on radical scavenging experiments, it appeared that IO3 was the predominant reactive species in the I-GAC/IO4− system. The mechanism underlying the enhanced catalytic performance of I-GAC could be explained by the introduction of negatively charged I3− and I5− into I-GAC, which induced positive charge density on the surface of I-GAC. This accelerated the interaction between I-GAC and IO4−, and subsequently mediated the increasing generation of iodyl radicals (IO3). Furthermore, a possible degradation pathway of AO7 was proposed according to the intermediate products identified by gas chromatography-mass spectrometry.

Surface effects of liquid metal amoeba

Liquid metals (LM) such as eutectic gallium-indium and gallium-indium-tin are important functional liquid-state metal materials with many unique properties, which have attracted wide attentions especially from soft robot area. Recently the amoeba-like transformations of LM on the graphite surface are discovered, which present a promising future for the design and assemble of self-fueled actuators with dendritically deformable body. It appears that the surface tension of the LM can be significantly reduced when it contacts graphite surface in alkaline solution. Clearly, the specific surface should play a vital role in inducing these intriguing behaviors, which is valuable and inspiring in soft robot design. However, the information regarding varied materials functions underlying these behaviors remains unknown. To explore the generalized effects of surface materials in those intriguing behavior, several materials including glass, graphite, nickel and copper oxides (CuO) were comparatively investigated as substrate surfaces. Important results were obtained that only LM amoeba transformations were observed on graphite and CuO surfaces. In order to identify the proper surface condition for LM transformation, the intrinsic properties of substrate surfaces, such as the surface charge and roughness, as well as the specific interaction with LM like wetting behavior and mutual locomotion etc., were characterized. The integrated results revealed that LM droplet appears more likely to deform on surfaces with higher positive surface charge density, higher roughness and less bubble generation on them. In addition, another surface material, CuOx, is identified to own similar ability to graphite, which is valuable in achieving amoeba-like transformation. Moreover, this study offers a fundamental understanding of the surface properties in realizing LM amoeba transformations, which would shed light on packing and structure design of liquid metal-based soft device within multi-material system.

Dynamics of the ions in liquid argon detectors and electron signal quenching

A study of the dynamics of the positive charges in liquid argon has been carried out in the context of the future massive time projection chambers proposed for dark matter and neutrino physics. Given their small mobility coefficient in liquid argon, the ions spend a considerably longer time in the active volume with respect to the electrons. The positive charge density can be additionally increased by the injection, in the liquid volume, of the ions produced by the electron multiplying devices located in gas argon. The impact of the ion current on the uniformity of the field has been evaluated as well as the probability of the charge signal quenching due to the electron–ion recombination along the drift. The study results show some potential concerns for massive detectors with drift of many meters operated on surface.

Electrical double layer ion transport with cell voltage-pulse potential coupling circuit for separating dilute lead ions from wastewater

Continuous separation of dilute Pb2+ from wastewater was demonstrated by using a membrane containing highly conductive multiwalled carbon nanotubes (MWCNTs) in an electrochemically switched ion permselective (ESIP) process, with a cell voltage–pulse potential coupling circuit. The membrane with high selectivity for Pb2+ separation was fabricated by pressure filtering MWCNT solution through a polytetrafluoroethylene (PTFE) film. In the separation system, the directional uptake and release of Pb2+ were realized by modulating the positive and negative charge densities on the electrical double layer (EDL) of the membrane, together with the application of an external electric field. The effects of the operating parameters on the flux of Pb2+ and membrane permselectivity were investigated. The highest Pb2+ flux across the membrane occurred with considerable permselectivity at a cell voltage of 0.8V, a pulse width of 40s, and a membrane MWCNT content of 30mg. The system reduced the Pb2+ concentration in the solution from 30 to 0.36ppm with a current efficiency of 46.1%. Such a novel EDL membrane-based ESIP system could be used for wastewater treatment.

Pectin adsorption on amorphous Fe/Al hydroxides and its effect on surface charge properties and Cu(II) adsorption

The purpose of this study was to elucidate the mechanisms for pectin-enhanced adsorption of heavy metal cations on variable charge minerals. Batch experiments were conducted to investigate the adsorption of pectin and copper(II) by amorphous Fe/Al hydroxides. The morphology, mineralogy, and functional groups of pectin–Fe/Al hydroxides were examined using X-ray diffraction, scanning electron microscopy, and Fourier transform infrared spectroscopy analysis. The amount of pectin adsorbed by amorphous Al(OH)3 was much greater than that by amorphous Fe(OH)3 at pH values between 3.5 and 6.5 due to the higher positive charge density on Al(OH)3 and greater electrostatic attraction between the hydroxide and pectin compared with Fe(OH)3. The addition of pectin decreased the positive surface charge on amorphous Fe and Al hydroxides. The presence of pectin enhanced the adsorption of Cu(II) by the Fe and Al hydroxides. The increase in Cu(II) adsorption on amorphous Fe hydroxide was more obvious at low pH values than at higher pH values, while an opposite changing trend was observed for amorphous Al hydroxide. At pH 3.9, 4.3, and 4.9, pectin increased Cu(II) adsorption by Fe hydroxide from 24.4, 76.6, and 177.0 mmol/kg to 61.6, 98.8, and 192.0 mmol/kg, i.e., Cu(II) adsorption was increased by 37.2, 22.2, and 15.0 mmol/kg, respectively. At pH 4.3 and 4.9, pectin increased Cu(II) adsorption by Al hydroxide from 3.7 and 27.0 mmol/kg to 17.3 and 69.4 mmol/kg, i.e., Cu(II) adsorption was increased by 13.6 and 42.4 mmol/kg, respectively. The greater adsorption of pectin by Al hydroxide was mainly responsible for the larger enhancement of pectin on Cu(II) adsorption on Al hydroxide at higher pH values compared with Fe hydroxide. The adsorption of pectin on Fe and Al hydroxides decreased the positive charge on the hydroxides and thus enhanced the adsorption of Cu(II) by the hydroxides.

Magnetic Porous Carbonaceous Material Produced from Tea Waste for Efficient Removal of As(V), Cr(VI), Humic Acid, and Dyes

Magnetic porous carbonaceous (MPC) materials derived from tea waste were synthesized by an integrated biosorption–pyrolysis process and were applied as adsorbents for wastewater cleanup. On the basis of various characterizations, we demonstrated that the formation mechanism of γ-Fe2O3 anchored on the porous carbonaceous material surface consisted of the adsorption of iron ions and then the γ-Fe2O3 nucleation and growth through pyrolysis at alternative peak temperatures (300–500 °C). The sample pyrolyzed at 300 °C (MPC-300) showed good capacities for As(V) (38.03 mg g–1) and Cr(VI) (21.23 mg g–1) adsorption, outperforming that of commercial bulk Fe2O3 and many other materials. Moreover, the large available positive charge density can facilitate the effective adsorption of anionic dye (MO) and humic acid (HA) on the γ-Fe2O3 surface while the adsorption performance is sluggish for cationic dyes (MB and RhB). Relatively, the adsorption isotherms could significantly conform to the Langmuir model, and the pseud...

Impact of front-side point contact/passivation geometry on thin-film solar cell performance

In this work, we perform an extensive campaign of three-dimensional numerical simulations of CIGS solar cell structures to investigate the effect of a surface-passivated CIGS with point contacts openings on the cell performance parameters (Jsc, Voc, FF and η). Detailed analysis of the combination of passivation thickness, point contact size and pitch is performed under the hypothesis of highly defective CIGS front surface and ideal chemical passivation: efficiencies close to the case of ideal (i.e., defect-free) CdS/CIGS interface can be achieved by optimized nanometer-scale point contact arrays. To account for field-effect passivation due to positive residual charge density, Qf, within the passivation layer, we vary Qf in the range 1010–1013cm−2 under the two extreme scenarios of ideal or ineffective chemical passivation. Several examples of CIGS cells with different buffer layers (CdS, ZnO, ZnMgO, In2S3, Zn(O, S)) are also analyzed. We find that a positive Qf in the interval 1012– 5·1012cm−2 can help completely recover the ideal cell efficiency, irrespective of the chemical passivation effect and even in the presence of unfavorable conduction band alignment at the buffer/CIGS heterojunction. This may help devising solutions with buffer materials alternative to CdS, boosting the performance of otherwise surface-limited cells. The effect of grain boundary defect density and position with respect to point contacts is also addressed, with a grain dimension of 750nm.

Self-Assembled Structures of Benzoic Acid on Au(111) Surface

Electrochemical scanning tunneling microscopy combined with cyclic voltammetry were employed to explore the self-assembly of benzoic acid (BA) on a Au(111) substrate surface in a 0.1-M HClO4 solution. At the negatively charged surface, BA molecules form two highly ordered physisorbed adlayers with their phenyl rings parallel to the substrate surface. High-resolution scanning tunneling microscopy images reveal the packing arrangement and internal molecular structures. The striped pattern and zigzag structure of the BA adlayers are composed of parallel rows of dimers, in which two BA molecules are bound through a pair of O–H···O hydrogen bonds. Increasing the electrode potential further to positive charge densities of Au(111) leads to the desorption of the physisorbed hydrogen-bonded networks and the formation of a chemisorbed adlayer. BA molecules change their orientation from planar to upright fashion, which is accompanied by the deprotonation of the carboxyl group. Furthermore, potential-induced formation and dissolution of BA adlayers were also investigated. Structural transitions between the various types of ordered adlayers occur according to a nucleation and growth mechanism.

Dark neutron stars

There is good evidence that electron-positron pair formation is not present in that section of the pulsar open magnetosphere which is the source of coherent radio emission, but the possibility of two-photon pair creation in an outer gap remains. Calculation of transition rates for this process based on measured whole-surface temperatures, combined with a survey of gamma-ray, X-ray and optical luminosities, expressed per primary beam lepton, shows that few Fermi LAT pulsars have significant outer-gap pair creation. For radio-loud pulsars with positive polar-cap corotational charge density and an ion-proton plasma there must be an outward flow of electrons from some other part of the magnetosphere to maintain a constant net charge on the star. In the absence of pair creation, it is likely that this current is the source of GeV gamma-emission observed by the Fermi LAT and its origin is in the region of the outer gap. With negative polar-cap corotational charge density, the compensating current in the absence of pair creation can consist only of ion or protons. These neutron stars are likely to be radio-quiet, have no observable gamma-emission, and hence can be described as dark neutron stars.

Efficiency of Iron-Based Oxy-Hydroxides in Removing Antimony from Groundwater to Levels below the Drinking Water Regulation Limits

This study evaluates the efficiency of iron-based oxy-hydroxides to remove antimony from groundwater to meet the requirements of drinking water regulations. Results obtained by batch adsorption experiments indicated that the qualified iron oxy-hydroxide (FeOOH), synthesized at pH 4 for maintaining a high positive charge density (2.5 mmol OH−/g) achieved a residual concentration of Sb(III) below the EU drinking water regulation limit of 5 μg/L by providing an adsorption capacity of 3.1 mg/g. This is more than twice greater compared either to similar commercial FeOOHs (GFH, Bayoxide) or to tetravalent manganese feroxyhyte (Fe-MnOOH) adsorbents. In contrast, all tested adsorbents failed to achieve a residual concentration below 5 μg/L for Sb(V). The higher efficiency of the qualified FeOOH was confirmed by rapid small-scale column tests, since an adsorption capacity of 3 mg Sb(III)/g was determined at a breakthrough concentration of 5 μg/L. However, it completely failed to achieve Sb(V) concentrations below 5 μg/L even at the beginning of the column experiments. The results of leaching tests classified the spent qualified FeOOH to inert wastes. Considering the rapid kinetics of this process (i.e., 85% of total removal was performed within 10 min), the developed qualified adsorbent may be promoted as a prospective material for point-of-use Sb(III) removal from water in vulnerable communities, since the adsorbent’s cost was estimated to be close to 30 ± 3.4 €/103 m3 for every 10 μg Sb(III)/L removed.

Modulation of interfacial and electrical properties of HfGdO/GaAs gate stacks by ammonium sulfide passivation and rapid thermal annealing

Abstract In this work, the interface chemistry and the reduction of GaAs surface species by using (NH 4 ) 2 S solution prior to Gd-doped HfO 2 (HGO) thin film deposition and the removal of Ga Oxides and elemental As by rapid thermal annealing (RTA) have been investigated by X-ray photoelectron spectroscopy (XPS). Additionally, the effect of the surface passivation and rapid thermal annealing on the electrical properties of MOS capacitors based on sputtering-derived HGO as gate dielectric on GaAs substrate has been detected by means of capacitance-voltage ( C-V ) and leakage current density-voltage ( J-V ) measurements. Based on electrical analysis, it can be noted that the constantly improvement of electrical properties, such as the decreases of flat band voltage (V fb ), hysteresis (ΔV fb ), oxide charge density (O ox ), border trapped oxide charge density (N bt ) and leakage current density, have been observed. Especially, the dielectric constant of 16.72, flat band voltage V fb of 1.19 V, hysteresis ΔV fb of 0.04 V, leakage current density of 1.54 × 10 −5 A/cm 2 at bias voltage of 1 V, total positive charge density and border trap charge density of 6.09 × 10 12 cm −2 and 2.54 × 10 11 cm −2 , respectively render 600 °C-annealed HGO thin films, potential high-k gate dielectrics in future CMOS devices.

Synthesis of antibiotic loaded polylactic acid nanoparticles and their antibacterial activity against Escherichia coli O157:H7 and methicillin-resistant Staphylococcus aureus

Polymeric nanoparticles are promising nanotechnology tools to fight pathogenic bacteria resistant to conventional antibiotics. To synthesize polylactic acid nanoparticles loaded with ofloxacin and vancomycin, and to determine their antibacterial activity against Escherichia coli O157:H7 and methicillin-resistant Staphylococcus aureus (MRSA). We synthesized ofloxacin or vancomycin loaded polylactic acid nanoparticles by the emulsification-solvent evaporation method, and characterized them by dynamic light scattering, laser Doppler electrophoresis and scanning electron microscopy. We evaluated in vitro antibacterial activity of ofloxacin- and vancomycin-loaded polylactic acid nanoparticles against E. coli O157:H7 and MRSA using the broth microdilution method. Ofloxacin- and vancomycin-loaded polylactic acid nanoparticles registered a positive surface charge density of 21 mV and an average size lower than 379 nm. In vitro minimum inhibitory concentration (MIC50) of ofloxacin-polylactic acid nanoparticles was 0,001 μg/ml against E. coli O157:H7, i.e., 40 times lower than the free ofloxacin (MIC50: 0.04 μg/ml), indicating enhanced antibacterial activity while the in vitro MIC50 of vancomycin-polylactic acid nanoparticles was 0,005 μg/ml against MRSA, i.e., 100 times lower than that of free vancomycin (MIC50: 0.5 μg/ml). Polylactic acid nanoparticles loaded with ofloxacin and vancomycin showed a higher antibacterial activity. Polymeric nanoparticles are a possible alternative for drug design against pathogenic bacterial strains of public health interest.

Single positively charged particle trapping in nanofluidic systems

High-throughput and contact-free trapping of single nano-objects in an aqueous solution is of substantial interest for fundamental and applied research. One of the several trapping methods is geometry-induced electrostatic (GIE) trapping that allows for passive spatial confinement of single nanoparticles in nanofluidic devices. In aqueous environments (pH>2) glass and silicon dioxide surfaces acquire a net negative surface charge density due to the self-dissociation of terminal silanol groups. Thus, with native glass/silicon-based GIE-trapping devices only negatively charged nano-objects can be trapped, limiting the applications of this method. In this work, we have performed surface modifications of glass-based GIE-trapping nanofluidic devices to enable the trapping of positively charged nanoparticles. For surface functionalization of the devices, a layer transfer of poly-(ethyleneimine) electrolytes were used which provides a net positive surface charge density. We demonstrate the successful confinement of positively charged 60nm gold nanoparticles inside the functionalized devices, and present a comparison study between trapping of negatively- and positively charged particles in native and functionalized devices, respectively.

Highly effective electronic passivation of silicon surfaces by atomic layer deposited hafnium oxide

This paper investigates the application of hafnium oxide (HfO2) thin films to crystalline silicon (c-Si) solar cells. Excellent passivation of both n- and p-type crystalline silicon surfaces has been achieved by the application of thin HfO2 films prepared by atomic layer deposition. Effective surface recombination velocities as low as 3.3 and 9.9 cm s−1 have been recorded with 15 nm thick films on n- and p-type 1 Ω cm c-Si, respectively. The surface passivation by HfO2 is activated at 350 °C by a forming gas anneal. Capacitance voltage measurement shows an interface state density of 3.6 × 1010 cm−2 eV−1 and a positive charge density of 5 × 1011 cm−2 on annealed p-type 1 Ω cm c-Si. X-ray diffraction unveils a positive correlation between surface recombination and crystallinity of the HfO2 and a dependence of the crystallinity on both annealing temperature and film thickness. In summary, HfO2 is demonstrated to be an excellent candidate for surface passivation of crystalline silicon solar cells.

Comparative Assessment of Nonlocal Continuum Solvent Models Exhibiting Overscreening

AbstractNonlocal continua have been proposed to offer a more realistic model for the electrostatic response of solutions such as the electrolyte solvents prominent in biology and electrochemistry. In this work, we review three nonlocal models based on the Landau-Ginzburg framework which have been proposed but not directly compared previously, due to different expressions of the nonlocal constitutive relationship. To understand the relationships between these models and the underlying physical insights from which they are derive, we situate these models into a single, unified Landau-Ginzburg framework. One of the models offers the capacity to interpret how temperature changes affect dielectric response, and we note that the variations with temperature are qualitatively reasonable even though predictions at ambient temperatures are not quantitatively in agreement with experiment. Two of these models correctly reproduce overscreening (oscillations between positive and negative polarization charge densities), and we observe small differences between them when we simulate the potential between parallel plates held at constant potential. These computations require reformulating the two models as coupled systems of local partial differential equations (PDEs), and we use spectral methods to discretize both problems. We propose further assessments to discriminate between the models, particularly in regards to establishing boundary conditions and comparing to explicit-solvent molecular dynamics simulations.

Structure-activity relationships and action mechanisms of collagen-like antimicrobial peptides.

An antimicrobial triple-helical peptide, R3, was previously obtained from a collagen-like combinatorial peptide library. In this research, based on structure-activity relationship studies of R3, a more potent peptide, RR4, with increased positive net charge and charge density relative to R3, was developed. RR4 exhibited antimicrobial activity against both Gram-negative and Gram-positive bacterial strains, including multidrug-resistant strains. Its action could be attributed to entry into cells and interactions with intercellular molecules such as DNA/RNA that inhibited cell division rather than increasing bacterial membrane permeability. Furthermore, RR4 exhibited remarkable stability in serum and low cytotoxicity.

Hybrid fluid‐particle simulation of whistler‐mode waves in a compressed dipole magnetic field: Implications for dayside high‐latitude chorus

AbstractIn this work we present a methodology for simulating whistler‐mode waves self‐consistently generated by electron temperature anisotropy in the inner magnetosphere. We present simulation results using a hybrid fluid/particle‐in‐cell code that treats the hot, anisotropic (i.e., ring current) electron population as particles and the background (i.e., the cold and inertialess) electrons as fluid. Since the hot electrons are only a small fraction of the total population, warm (and isotropic) particle electrons are added to the simulation to increase the fraction of particles with mass, providing a more accurate characterization of the wave dispersion relation. Ions are treated as a fixed background of positive charge density. The plasma transport equations are coupled to Maxwell's equations and solved in a meridional plane (a 2‐D simulation with 3‐D fields). We use a curvilinear coordinate system that follows the topological curvature of Earth's geomagnetic field lines, based on an analytic expression for a compressed dipole magnetic field. Hence, we are able to simulate whistler wave generation at dawn (pure dipole field lines) and dayside (compressed dipole) by simply adjusting one scalar quantity. We demonstrate how, on the dayside, whistler‐mode waves can be locally generated at a range of high latitudes, within pockets of minimum magnetic field, and propagate equatorward. The obtained dayside waves (in a compressed dipole field) have similar amplitude and frequency content to their dawn sector counterparts (in a pure dipole field) but tend to propagate more field aligned.

Oxygen-Molecule Adsorption and Dissociation on BCN Graphene: A First-Principles Study.

Boron and nitrogen co-doped (BCN) graphene is an attractive material for use as a metal-free oxygen reduction reaction electrocatalyst and as other catalysts due to its unique structure and electronic properties. Reported here is the structure, determined by using density functional theory, of the active O2 -dissociation site of BCN graphene containing different types of BN cluster. The results show that the edge termination and shape of substitutional BN clusters are two important factors that determine the catalytic activity of BCN graphene for the dissociation of molecular oxygen. N-Terminated triangular BN (t-BN) cluster doping can reduce the energy barrier more effectively compared to a t-BN with a B edge or quadrangular BN cluster. Interestingly, the B atom neighboring the N edge, only in the case of N-terminated t-BN doping, is determined to be the most active site for O2 dissociation due to the barrier being as low as 0.08 eV. The electronic structure calculations reveal that in addition to the large positive charge densities, the catalytic activity of graphene enhanced by B,N doping is also attributed to the increased density of states of the π* states of the active site around the Fermi level.

High-k dielectric materials for the gate oxide of a MIS capacitor: effect of interface states on the C–V characteristics

The capacitance-voltage (C---V) characteristics of metal-insulator-semiconductor (MIS) capacitors are investigated by solving in 1D the self-consistent equations using the Silvaco ATLAS device simulation package. In this paper, ambient and high temperature C---V characteristics for both positive and negative interface charge densities are studied. The results obtained from the simulation show that the C---V characteristics at ambient temperature change from high frequency to a low frequency for $$D_\mathrm{it} > \hbox {1e12}\, \hbox {cm}^{-2}\,\hbox {eV}^{-1}$$Dit>1e12cm-2eV-1. We find that the shift in threshold voltage has a strong dependence on the positive and negative interface charge densities. Also, a less significant shift in threshold voltage for the highest temperature operation is found which is in the opposite direction for negative and positive interface charge densities. Electrons and holes that occupy interface traps become charged and contribute to the threshold voltage shift and a hump in the C---V characteristics.

Development of shell cross-linked nanoparticles based on boronic acid-related reactions for self-regulated insulin delivery

Shell cross-linked nanoparticles were fabricated by the complexation of poly(3-methacrylamido phenylboronic acid) (PMAPBA) and thiolated chitosan (chitosan-SH) via boronic acid-related reactions. The formation of PMAPBA/chitosan-SH nanoparticles was confirmed by transmission electron microscopy, dynamic light scattering, and UV spectroscopy. The nanoparticles had a narrow size distribution with a relatively high positive charge density, and the size and zeta potential of the nanoparticles correlated with the chitosan-SH concentration. Furthermore, owing to the cross-linking of the nanoparticle shell, insulin was encapsulated in the nanoparticles with a loading capacity of up to 18%. The release of insulin from the nanoparticles slowed down because of the presence of disulfide bonds and increased with increasing glucose level in the medium. The structure of the released insulin was not distorted. More importantly, the nanoparticles had good cytocompatibility, as demonstrated by in vitro experiments. The simplicity of this strategy along with a high loading capacity, glucose sensitivity, and cytocompatibility of the produced nanoparticles should significantly boost their application in self-regulated insulin delivery.