Core-shell Structure Model Research Articles

Iron Oxide Nanoparticles: Tunable Size Synthesis and Analysis in Terms of the Core–Shell Structure and Mixed Coercive Model

Iron oxide nanoparticles (NPs) are currently a very active research field. To date, a comprehensive study of iron oxide NPs is still lacking not only on the size dependence of structural phases but also in the use of an appropriate model. Herein, we report on a systematic study of the structural and magnetic properties of iron oxide NPs prepared by a co-precipitation method followed by hydrothermal treatment. X-ray diffraction and transmission electron microscopy reveal that the NPs have an inverse spinel structure of iron oxide phase (Fe3O4) with average crystallite sizes (DXRD) of 6–19 nm, while grain sizes (DTEM) are of 7–23 nm. In addition, the larger the particle size, the closer the experimental lattice constant value is to that of the magnetite structure. Magnetic field-dependent magnetization data and analysis show that the effective anisotropy constants of the Fe3O4 NPs are about five times larger than that of their bulk counterpart. Particle size (D) dependence of the magnetization and the non-saturating behavior observed in applied fields up to 50 kOe are discussed using the core–shell structure model. We find that with decreasing D, while the calculated thickness of the shell of disordered spins (t ∼ 0.3 nm) remains almost unchanged, the specific surface areas Sa increases significantly, thus reducing the magnetization of the NPs. We also probe the coercivity of the NPs by using the mixed coercive Kneller and Luborsky model. The calculated results indicate that the coercivity rises monotonously with the particle size, and are well matched with the experimental ones.

Design of superhalogens using a core-shell structure model.

Superhalogens, which have larger electron affinity than any halogen, play an important role in physical chemistry and materials design because of their applications in hydrogen storage and lithium-ion batteries. Inspired by the unique geometries and electronic properties of II-VI/III-V cage clusters, particularly the experimentally synthesized B12N12, we propose a core-shell structure model to design new superhalogens. The idea is assessed by conducting ab initio calculations on endohedral cage clusters X@B12N12 (X = F, Cl, Br) and other similar systems. With an exceptionally large electron affinity of 5.36 eV, the stable F@B12N12 cluster behaves as a novel superhalogen that can serve as a building block for Li salts and hyperhalogens. The findings highlight a new route for the discovery of superhalogens and provide useful building blocks for the bottom-up design of materials.

The Abnormal Optical Property and Room‐Temperature Exchange Bias Behavior in Na‐ and Ru‐Codoped BiFeO3 Nanoparticles

Bi0.98Na0.02Fe1−xRuxO3 (x = 0, 0.01, 0.02) samples were prepared via a sol–gel method. The formation of the desired materials was confirmed using X‐ray diffraction. The tetravalent Ru doping could effectively reduce the number of oxygen vacancy, and subsequently reduce the leakage current. Interestingly, the optical band gap was narrowed gradually in our samples, which seems contradictory with the variation in oxygen vacancy. To better understand this abnormal optical property, the effect of bandwidth was analyzed based on the change in Fe–O bond length and Fe–O–Fe bond angle, and the corresponding phenomenological qualitative model was proposed. The magnetization was increased with Ru substitution. In addition, an exchange bias (EB) phenomenon without field cooled process was observed at room temperature for all the samples, which was explained by introducing a core–shell structure model. Moreover, the EB behavior becomes more pronounced at 5 K for 1% Ru‐doped sample.

Effect of different oxide thickness on the bending Young's modulus of SiO2@SiC nanowires.

The surface or sheath effect on core-shell nanowires plays an important role in the nanomechanical test. In the past few years, SiC nanowires have been synthesized using various methods with an uneven and uncontrollable amorphous silicon dioxide sheath. The bending Young’s modulus of the SiC nanowires has scarcely been measured, and the effect of the oxide sheath has not been taken into account. In this paper, SiO2-coated SiC (SiO2@SiC) nanowires were synthesized using the chemical vapor deposition method, followed by thermal reduction. Scanning electron microscopy and transmission electron microscopy show that the SiO2@SiC nanowires in this paper have diameters ranging from 130 ~ 150 nm, with the average thickness of SiO2 layer approximately 14 nm. After different processing times with 1 mol/L NaOH, approximately 5 nm, 9 nm, 14 nm silicon dioxide layers were obtained. The results of the three-point-bending test show that the modulus of SiO2@SiC nanowires is found to clearly decrease with the increase in oxide thickness and the influence of the oxide sheath should not be ignored when the layer thickness is above 5 nm. Young’s modulus of the SiO2@SiC nanowires calculated in this study by the core-shell structure model is in good agreement with the theoretical value.

Phase composition and surface properties of nylon-6 nanofibers prepared by nanospider technology at various electrode distances

Phase composition, morphology and surface properties of nylon-6 nanofibers prepared by Nanospider technology have been studied for dependence on spinning distance using a combination of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), electrokinetic analysis, and scanning electron and transmission electron microscopy (SEM, TEM). The effect of the electric field strength on the nanofiber phase composition was investigated via the variable distance of the electrodes. Quantitative XRD phase analysis revealed the dependence of the phase composition on the electrode distance, which in the case of roller electrospinning, differs from that by melt spinning. A combination of XRD, XPS, and TEM suggested a core-shell structure model of the nanofibers. The XPS and electrokinetic analysis revealed the difference in surface chemistry and zeta potential at the face and reverse side of the nanofiber textile adjacent to a polypropylene (PP) antistatic spunbond, which may be important in subsequent chemical modification of nanofiber textiles and in its use for tissue engineering.

Study of electrocaloric effect in barium titanate nanoparticle with core–shellmodel

A quantitative core–shell structure model on the grain size effects is applied to investigate the effect of grain size on the electrocaloric effect of BaTiO3 nanoparticles. In this study, considering the grain surface effects, the nanoparticle is composed of inner tetragonal core and surface cubic layer. Theoretical analysis indicates that the adiabatic temperature change peak is depressed and diffuse in BaTiO3 nanoparticles. With decreasing grain size, adiabatic temperature change peak moves to lower temperature. Moreover, it is worth noting that the maximum of adiabatic temperature change increases with the increase of the applied electric field gradient and decreases with the increase of the initialfield.

The Superstable 25 kDa Monolayer Protected Silver Nanoparticle: Measurements and Interpretation as an Icosahedral Ag152(SCH2CH2Ph)60 Cluster

A cluster obtained in high yield from the reduction of a silver-thiolate precursor, Ag-SCH(2)CH(2)Ph, exhibited a single sharp peak near 25 kDa in the matrix-assisted laser desorption mass spectrum (MALDI MS) and a well-defined metal core of ~2 nm measured with transmission electron microscopy (TEM). The cluster yields a single fraction in high-performance liquid chromatography (HPLC). Increased laser fluence fragments the cluster until a new peak near 19 kDa predominates, suggesting that the parent cluster-Ag(152)(SCH(2)CH(2)Ph)(60)-evolves into a stable inorganic core-Ag(152)S(60). Exploiting combined insights from investigations of clusters and surface science, a core-shell structure model was developed, with a 92-atom silver core having icosahedral-dodecahedral symmetry and an encapsulating protective shell containing 60 Ag atoms and 60 thiolates arranged in a network of six-membered rings resembling the geometry found in self-assembled monolayers on Ag(111). The structure is in agreement with small-angle X-ray scattering (SAXS) data. The protective layer encapsulating this silver cluster may be the smallest known three-dimensional self-assembled monolayer. First-principles electronic structure calculations show, for the geometry-optimized structure, the development of a ~0.4 eV energy gap between the highest-occupied and lowest-unoccupied states, originating from a superatom 90-electron shell-closure and conferring stability to the cluster. The optical absorption spectrum of the cluster resembles that of plasmonic silver nanoparticles with a broad single feature peaking at 460 nm, but the luminescence spectrum shows two maxima with one attributed to the ligated shell and the other to the core.

Core-shell structure and size effect in barium titanate nanoparticle

A quantitative core-shell structure model on the grain size effect of nano-BaTiO 3 ceramics has been proposed on the basis of the Ginsburg–Landau–Devonshire thermodynamic theory. In this study, we have considered the existence of surface effects, which leads to the core-shell structure in BaTiO 3 nanoparticles consisting of inner tetragonal core, gradient lattice strain layer, and surface cubic layer. The size effects on the spontaneous polarization, Curie temperature, c/ a ratio, and electrocaloric and dielectric properties of BaTiO 3 ferroelectric nanoparticle are successfully explained. Theoretical and experimental results for BaTiO 3 nanoparticle are compared and discussed.

Mechanism study of nitrate reduction by nano zero valent iron

This study investigates the fate of nitrogen species during nitrate reduction by nano-scale zero valent iron (NZVI) and related reaction mechanisms. The NZVI used for the experiments was prepared by chemical reduction without a stabilizing agent. NZVI has great ability to reduce nitrate. However, the question of what end-product results from nitrate reduction by NZVI has sparked controversy. Establishing nitrogen mass balance by quantitative analysis of aqueous phase and gas-phase nitrogen species, this study clearly determines that nitrate was converted to ammonium ion followed by ammonia stripping under a strong alkaline condition, which leads to a decrease in the total aqueous nitrogen amount. Moreover, some of the major reactions, which consisted of nitrate reduction, ammonia production, and ammonia stripping were modelled by pseudo first-order kinetics. According to the model estimation results, additional reaction mechanisms would exist in an early stage of reaction. This might be due to the adsorption and desorption reaction which could be explained by the core–shell structure model.

Contact-resonance atomic force microscopy for nanoscale elastic property measurements: Spectroscopy and imaging

Quantitative measurements of the elastic modulus of nanosize systems and nanostructured materials are provided with great accuracy and precision by contact-resonance atomic force microscopy (CR-AFM). As an example of measuring the elastic modulus of nanosize entities, we used the CR-AFM technique to measure the out-of-plane indentation modulus of tellurium nanowires. A size-dependence of the indentation modulus was observed for the investigated tellurium nanowires with diameters in the range 20–150 nm. Over this diameter range, the elastic modulus of the outer layers of the tellurium nanowires experienced significant enhancement due to a pronounced surface stiffening effect. Quantitative estimations for the elastic moduli of the outer and inner parts of tellurium nanowires of reduced diameter are made with a core–shell structure model. Besides localized elastic modulus measurements, we have also developed a unique CR-AFM imaging capability to map the elastic modulus over a micrometer-scale area. We used this CR-AFM capability to construct indentation modulus maps at the junction between two adjacent facets of a tellurium microcrystal. The clear contrast observed in the elastic moduli of the two facets indicates the different surface crystallography of these facets.

Theoretical study of the optical absorption properties of Au–Ag bimetallic nanospheres

The dependence of nonlinear optical response properties of bimetallic alloying and core–shell structure spherical nanoparticles was investigated as a function of relative composition. Numerical calculations based on Drude model and quasi-static theory were developed to describe these absorption properties. With the increasing gold content, the absorption peaks of both alloying Au–Ag nanospheres and core–shell structure nanospheres red-shift from 402 to 526 nm in a nonlinear way. Furthermore, the linearity of alloying nanospheres is better than that of core–shell structure model. In core–shell structure nanospheres, the shell composition characters play a more important role to influence the optical properties of the bimetallic nanoparticles.

A control on the photoluminescence properties in P-passivated nanocrystalline ZnO films

P-passivated nanocrystalline ZnO films on InP substrates were prepared by thermal oxidation of Zn films. By a simple thermal annealing cycle process, uniform photoluminescence (PL) samples with controllable visible emission and intense UV emission were obtained at different annealing conditions. Through a detailed study of the photoluminescence spectra of P-passivated nano-ZnO films vs the annealing-temperature and annealing-time, it is shown that applying different annealing time or temperatures provides a very practical technique to control the deep-level defect emission. A core-shell structure model of the surface passivation was helpfully used to discuss the P-passivation mechanism.

Structure and photoluminescence of Mn-passivated nanocrystalline ZnO thin films

We have studied the structure and the photoluminescence of Mn-passivated nanocrystalline ZnO thin films. The ZnO thin films were prepared by thermally oxidizing ZnS:Mn films grown by low-pressure metalorganic chemical vapor deposition. The structural properties of the ZnO films were examined by X-ray diffraction and X-ray photoelectron spectroscopy. It was demonstrated that the Mn passivation could dramatically change the emission characteristics of nanocrystalline ZnO thin films. The photoluminescence spectra of the films with an optimized Mn-doped concentration showed only ultraviolet emission, while the green emission was fully quenched due to the Mn passivation of the ZnO nanocrystallite surface. A core-shell structure model of the surface passivation is presented. The note of the oxygen vacancy (Vo**) as the dominant recombination center for green emission is discussed.

ANALYSIS OF EDDY-CURRENT-LOSS IN Fe-BASED AMORPHOUS WIRES

On the basis of core-shell structure model of amorphous wires, for Fe-based amorphous wire we consider that there are an axial anisotropy Kc in the core and a radial anisotropy Ks in the shell, both of Kc and Ks are functions of λs and σ0,the constants of stress energy density. We deduce the ac circular permeability μφ(r) and calculate the anomaly factor of low-frequency eddy-current-loss for this model,η=2.2, from Maxwell equation and Ohm's law. Comparison of the theoretical value with the experimental results indicates that the magnetic structure of Fe-based amorphous wires can be well described by this improved core-shell structure model.