Structural study on graphene-based particles prepared from old coconut shell by acid–assisted mechanical exfoliation

This work is aimed to identify the structure of the graphene-based particles (GPs) from old coconut shell as the raw material after the mechanical exfoliation processes. The burnt coconut shell was at first heated at 400 °C for 5 h in ambient air. The sample was then stirred in an acid solution (HCl) and then continued by ultrasonication and centrifugation. The exfoliated GPs were characterized by x-ray diffraction (XRD), particle size analyzer (PSA), Fourier-transform infrared spectroscopy (FTIR), scanning and transmission electron microscopy (SEM and TEM, respectively), atomic force microscopy (AFM), Raman spectroscopy, and synchrotron wide and small angle x-ray scattering (WAXS and SAXS, respectively). The XRD and WAXS analyses show Bragg peaks corresponding to a pure phase of reduced graphene oxide (rGO). PSA, SEM/TEM, AFM, and Raman analyses show that the use of HCl-assist in the solution during the exfoliation process has successfully reduced particle size of the obtained GPs. SAXS pattern of the exfoliated GPs using the assist of HCl, confirmed by TEM and AFM images, results in the specific particle sizes of between 1.42 and 4.99 nm. The present mechanical exfoliation technique has successfully been applied to obtain several nanometers of GPs and provides an alternative of simple synthesis of biomass – based graphene products.     

Catalysis under shell: Improved CO oxidation reaction confined in Pt@h-BN core–shell nanoreactors

Core-shell nanostructures consisting of active metal cores and protective shells often exhibit enhanced catalytic performance,in which reactants can access a small part of the core surfaces through the pores in the shells.In this study,we show that Pt nanoparticles (NPs) can be embedded into few-layer hexagonal boron nitride (h-BN) overlayers,forming Pt@h-BN core-shell nanocatalysts.The h-BN shells not only protect the Pt NPs under harsh conditions but also allow gaseous molecules such as CO and O2 to access a large part of the Pt surfaces through a facile intercalation process.As a result,the Pt@h-BN nanostructures act as nanoreactors,and CO oxidation reactions with improved activity,selectivity,and stability occur at the core-shell interfaces.The confinement effect exerted by the h-BN shells promotes the Pt-catalyzed reactions.Our work suggests that two-dimensional shells can function as robust but flexible covers on nanocatalyst surfaces and tune the surface reactivity.     

Templated,carbothermal reduction synthesis of mesoporous silicon carbide from carbon nanotube–mesoporous silica core–shell composite

Mesoporous materials are the subject of extensive interest due to their large surface area and multiscale structural order. These properties are especially relevant for applications such as catalyst supports in both chemical and electrochemical systems. The first part of this study details the synthesis of carbon nanotube–mesoporous silica core–shell composites starting with single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs) through micellar self-assembly. The formation of such a composite structure was verified using scanning electron microscopy and further analysis was carried out through X-ray diffraction (XRD). The subsequent refinement of the diffraction pattern revealed the silica shell to be of the continuous cubic (Ia3d) MCM48 structure. The mesoporous silica–carbon nanotube core–shell composite was later subjected to high-temperature carbothermal reduction. Subsequent XRD analysis showed that the reduction product was mesoporous silicon carbide (SiC). Thus, this study details a novel synthesis method for mesoporous SiC, which is an attractive material for possible diverse applications such as catalyst supports, intercalation electrodes and other emerging high technology areas.     

Toughening of epoxy using core–shell particles

An epoxy resin, cured using an anhydride hardener, has been modified by the addition of preformed core–shell rubber (CSR) particles which were approximately 100 or 300 nm in diameter. The glass transition temperature, T _g, of the cured epoxy polymer was 145 °C. Microscopy showed that the CSR particles were well dispersed through the epoxy matrix. The Young’s modulus and tensile strength were reduced, and the glass transition temperature of the epoxy was unchanged by the addition of the CSR particles. The fracture energy increased from 77 J/m² for the unmodified epoxy to 840 J/m² for the epoxy with 15 wt% of 100-nm diameter CSR particles. The measured fracture energies were compared to those using a similar amount of carboxyl-terminated butadiene-acrylonitrile (CTBN) rubber. The CTBN particles provided a larger toughening effect when compared to CSR particles, but reduced the glass transition temperature of the epoxy. For the CSR-modified epoxies, the toughening mechanisms were identified using scanning electron microscopy of the fracture surfaces. Debonding of the cores of the CSR particles from the shells was observed, accompanied by plastic void growth of the epoxy and shell. The observed mechanisms of shear band yielding and plastic void growth were modelled using the Hsieh et al. approach (J Mater Sci 45:1193–1210). Excellent agreement between the experimental and the predicted fracture energies was found. This analysis showed that the major toughening mechanism, responsible for 80–90% of the increase in fracture energy, was the plastic void growth.     

III-nitride core–shell nanowire arrayed solar cells

A solar cell based on a hybrid nanowire–film architecture consisting of a vertically aligned array of InGaN/GaN multi-quantum well core–shell nanowires which are electrically connected by a coalesced p-InGaN canopy layer is demonstrated. This unique hybrid structure allows for standard planar device processing, solving a key challenge with nanowire device integration, while enabling various advantages by the nanowire absorbing region such as higher indium composition InGaN layers by elastic strain relief, more efficient carrier collection in thinner layers, and enhanced light trapping from nano-scale optical index changes. This hybrid structure is fabricated into working solar cells exhibiting photoresponse out to 2.1 eV and short-circuit current densities of ~1 mA cm(-2) under 1 sun AM1.5G. This proof-of-concept nanowire-based device demonstrates a route forward for high-efficiency III-nitride solar cells.     

Fast synthesis,formation mechanism,and control of shell thickness of CuS–polystyrene core–shell microspheres

The silanol-modified polystyrene microspheres were prepared through dispersion polymerization. Then copper sulfide particles were grown on silanol-modified polystyrene through sonochemical deposition in an aqueous bath containing copper acetate and sulfide, released through the hydrolysis of thioacetamide. The resulting particles were continuous and uniform as characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Fourier transform infrared, thermogravimetric analysis and UV–vis absorption spectroscopy were used to characterize the structure and properties of core–shell particles. The results showed the coating thickness of CuS shell can be controlled by the amount of silanol and the UV–vis absorption intensity of PSt/CuS composite also changed with the coating thickness of CuS.     

Development of electrosprayed artesunate-loaded core–shell nanoparticles

Objective: Artesunate (ART) is proven to have potential anti-proliferative activities, but its instability and poor aqueous solubility limit its application as an anti-cancer drug. The present study was undertaken to develop coaxial electrospraying as a novel technique for fabricating nanoscale drug delivery systems of ART as the core–shell nanostructures.

Methods: The core–shell nanoparticles (NPs) were fabricated with coaxial electrospraying and the formation mechanisms of NPs were examined. The physical solid state and drug–polymer interactions of NPs were characterized by X-ray powder diffraction (XRPD) and Fourier transform infrared (FTIR) spectroscopy. The effects of materials and electrospraying process on the particle size and surface morphology of NPs were investigated by scanning electron microscopy (SEM). The drug release from NPs was determined in vitro by a dialysis method.

Results: The ART/poly(lactic-co-glycolic) acid (PLGA) chitosan (CS) NPs exhibited the mean particle size of 303?±?93?nm and relatively high entrapment efficiency (80.5%). The release pattern showed an initial rapid release within two hours followed by very slow extended release. The release pattern approached the Korsmeyer–Peppas model.

Conclusions: The present results suggest that the core–shell NPs containing PLGA and CS have a potential as carriers in the anticancer drug therapy of ART.     

The effective thermoelectric properties of core–shell composites

Thermoelectric materials are capable of converting heat directly into electricity and vice versa, and they have been explored for both waste heat recovery and thermal management. In this work, we analyze axially symmetric thermoelectric problems, motivated by energy harvesting using waste heat from an automobile exhaust pipe. Thermoelectric field distributions in both homogeneous shell and core–shell composites are solved, and the effective thermoelectric properties of the core–shell composites are analyzed. Numerical results show that higher thermoelectric conversion efficiency can be achieved in core–shell composites, and the mechanism responsible for the enhanced conversion efficiency is also identified. The analysis thus points to a new direction in developing high-performance thermoelectric materials.     

Polymer composites filled with core–shell structured nanofillers: effects of shell thickness on dielectric and thermal properties of composites

Journal of Materials Science: Materials in Electronics - In this study, we explore poly(vinylidene fluoride) (PVDF) filled with the core–shell nanofillers of silicon dioxide-coated...     

Temperature-dependent resonance energy transfer from CdSe–ZnS core–shell quantum dots to monolayer MoS<Subscript>2</Subscript>

We investigated the temperature-dependent resonance energy transfer (ET) from CdSe–ZnS core–shell quantum dots (QDs) to monolayer MoS₂. QDs/MoS₂ structures were fabricated using a spin-coating method. Photoluminescence (PL) spectra and decay curves of the QDs/MoS₂ structures were measured in the temperature range of 80?400 K. The results indicate that the PL intensity of the QDs decreased approximately 81% with increasing temperature, whereas that of the MoS₂ increased up to a maximum of 78% at 300 K because of the combined effect of thermal quenching and the ET in the QDs/MoS₂ structures. The ET efficiency and ET rate also exhibited similar variation trends, both increased with increasing temperature from 80 to 260 K and then decreased until 400 K, resulting in a maximum ET efficiency of 22% and an ET rate of 1.17 ns^–1 at ~260 K. These results are attributed to the varied distribution of the localized excitons and free excitons in the QDs/MoS₂ structures with increasing temperature.

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Investigation of optical properties of core–shell silicon nanowires

The ability to control the size, orientation, composition and morphology of silicon nanowires (SiNWs) presents an ideal platform for exploring a wide range of potential technological applications. In this work, we demonstrated the detail study of optical properties of highly disordered core–shell SiNWs that were grown by atmospheric pressure chemical vapor deposition. The microstructure of SiNWs was characterized by field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). The TEM study shows that the SiNWs consists of crystalline core silicon surrounded by thick amorphous silicon oxide. The total diameter including the outer SiO₂ sheath was 60–80 nm. The reflection and absorption of a-SiO₂/c-SiNWs were affected by process parameter like silane flow rate and hydrogen dilution. The optical reflection of SiNWs decreased with increasing photon energy across the visible and near the ultraviolet range, approaching moth's eye antireflection. Specifically, a minimum reflection of 2–3% was observed at 400 nm. The band gap is estimated at ∼1.32 eV by quasi-direct band Tauc's plot. The sum of localized states at the band edge is ∼0.53 eV. Straight SiNWs have lower reflection than those of nanoparticles mixed SiNWs and coil mixed SiNWs. The reflection and absorption of SiO₂/SiNWs were confirmed to respond strongly to infrared with increasing H₂ flow rate.     

Controllable synthesis and characterization of Ag@AgBr core–shell nanowires

Ag@AgBr core–shell nanowires have been synthesized in large quantities via a redox reaction between Ag nanowires and FeBr₃ in solution at room temperature. The effect of the molar ratio of Fe:Ag on the formation and optical absorption of the Ag@AgBr core–shell nanowires was systematically studied. The results showed that Ag nanowires were converted into Ag@AgBr core–shell nanowires and finally into AgBr nanorods with the increase of the molar ratio of Fe:Ag. At the same time, the optical absorption of Ag nanowires decreased gradually and disappeared finally. In addition, the growth mechanism of the Ag@AgBr core–shell nanowires was also discussed in detail.     

Current advances in precious metal core–shell catalyst design

Precious metal nanoparticles are commonly used as the main active components of various catalysts. Given their high cost, limited quantity, and easy loss of catalytic activity under severe conditions, precious metals should be used in catalysts at low volumes and be protected from damaging environments. Accordingly, reducing the amount of precious metals without compromising their catalytic performance is difficult, particularly under challenging conditions. As multifunctional materials, core–shell nanoparticles are highly important owing to their wide range of applications in chemistry, physics, biology, and environmental areas. Compared with their single-component counterparts and other composites, core–shell nanoparticles offer a new active interface and a potential synergistic effect between the core and shell, making these materials highly attractive in catalytic application. On one hand, when a precious metal is used as the shell material, the catalytic activity can be greatly improved because of the increased surface area and the closed interfacial interaction between the core and the shell. On the other hand, when a precious metal is applied as the core material, the catalytic stability can be remarkably improved because of the protection conferred by the shell material. Therefore, a reasonable design of the core–shell catalyst for target applications must be developed. We summarize the latest advances in the fabrications, properties, and applications of core–shell nanoparticles in this paper. The current research trends of these core–shell catalysts are also highlighted.     

CeNi nanoparticles with shell structure for hydrogen storage

A new kind of Ce-Ni nanoparticle was prepared by hydrogen arc plasma method. The nanoparticles consist of a large Ni core and a thin outer shell of CeNi alloy and CeO₂. A large quantity of hydrogen was stored in the particles, which was released at about 400 °C. The particles possess a lot of defects, dislocations and twin faults, which increase the number of surface active centers of the particles. The mechanism of shell structure formation of the nanoparticles is discussed in terms of the low solubility of Ce in Ni, and surface segregation under non-equilibrium cooling conditions.     

Four-node mixed Hu–Washizu shell element with drilling rotation

In this paper, enhanced four-node shell elements with six DOFs/node based on the Hu–Washizu (HW) functional are developed for Green strain. The drilling rotation is included through the drilling rotation constraint equation. The key features of the approach are as follows.

1. The shell HW functional is derived from the shell potential energy functional, which is an alternative to the derivation from the three-dimensional HW functional. This method is more versatile as it enables the derivation of the so-called partial HW functionals, with different treatment of the bending/twisting part and the transverse shear part of strain energy.
2. For the membrane part of HW shell elements, a seven-parameter stress, a nine-parameter strain and a two-parameter enhanced assumed displacement gradient enhancement are selected as optimal. The assumed representations of stress and strain are defined in skew coordinates in the natural basis at the element's center. This improves accuracy and has positive theoretical consequences.
3. The drilling rotation constraint equation is treated by the perturbed Lagrange method. The faulty term resulting from the equal-order approximations of displacements and the drilling rotation is eliminated, and one spurious mode is stabilized using the gamma method. The proposed formulation is insensitive to the element's distortions and yields a large radius of convergence in the examples involving in-plane bending.

The performance of 4 four-node shell HW elements, having different bending/twisting and transverse shear parts, is analyzed on several numerical examples. Such aspects are considered as: accuracy, radius of convergence, required number of iterations of the Newton method or the arc-length method and time of computations. The element with 29 parameters (HW29) is selected as the best performer. Copyright © 2011 John Wiley & Sons, Ltd.     

Correction to: Polymer composites filled with core–shell structured nanofillers: effects of shell thickness on dielectric and thermal properties of composites

Journal of Materials Science: Materials in Electronics -     

Effective electron emitters by molybdenum oxide-coated carbon nanotubes core–shell nanostructures

In this article, we showed that simple metal oxide coatings such as MoO₃ can be an effective enhancer for carbon nanotubes (CNTs) in field emission (FE) performance. For comparison, the FE properties of the pristine vertically aligned multi-walled CNTs with the metal oxide-coated CNTs were investigated. The metal oxide coating of the pristine CNTs was carried out by metal–organic chemical vapor deposition (MOCVD) method at 400 °C using Mo(CO)₆ as the precursor. The core–shell structure of the nanocomposite was studied by transmission electron microscopy (TEM). X-ray photoelectron spectroscopy (XPS) results showed that the surface of the coating material was mainly MoO₃. FE test indicated that the MoO₃-coated CNTs film exhibited an enhanced performance than the pristine CNTs with a turn-on field of 1.33 V μm⁻¹ and a field enhancement factor β estimated to be ~7000. Ultraviolet photoelectron spectroscopy (UPS) results confirmed a lower electron emission barrier height for MoO₃-coated CNTs than for the pristine CNTs. The mechanism of the enhanced FE performance is discussed based on Schottky barrier effect.     

Bi-layered polymer–magnetite core/shell particles: synthesis and characterization

Polymer magnetic core particles receive growing attention due to these materials owing magnetic properties which are widely used in different applications. The prepared composite particles are characterized with different properties namely: a magnetic core, a hydrophobic first shell, and finally an external second hydrophilic shell. The present study describes a method for the preparation of bi-layered polymer magnetic core particles (diameter range is 50–150 nm). This method comprises several steps including the precipitation of the magnetic iron oxide, coating the magnetite with oleic acid, attaching the first polymer shell by miniemulsion polymerization and finally introducing hydrophilic surface properties by condensation polymerization. The first step is the formation of magnetite nanoparticles within a co-precipitation process using oleic acid as the stabilizing agent for magnetite. The second step is the encapsulation of magnetite into polyvinylbenzyl chloride particles by miniemulsion polymerization to form a magnetic core with a hydrophobic polymer shell. The hydrophobic shell is desired to protect magnetite nanoparticles against chemical attack. The third step is the coating of magnetic core hydrophobic polymer shell composites with a hydrophilic layer of polyethylene glycol by condensation polymerization. Regarding the miniemulsion polymerization the influence of the amount of water, the mixing intensity and the surfactant concentration were studied with respect to the formation of particles which can be further used in chemical engineering applications. The resulting magnetic polymer nanoparticles were characterized by particle size measurement, chemical stability, iron content, TEM, SEM, and IR.     

The specially orthotropic GRP multi-layered cylindrical shell—The radial patch load

A theoretical approach for the multi-layered, cross-ply GRP cylindrical shell with through thickness symmetry subject to a general loading system is presented in Ref. 1. The method is here used to examine the behaviour of the shell when subjected to a uniformly distributed radial load, and to explore the capability of the design procedure proposed by the recently revised BS 4994, to predict adequately stress values that can be used in the design of multi-layered systems.