A novel approach to fabrication of superparamagnetite hollow silica/magnetic composite spheres

We described a method for synthesizing hollow silica/magnetic composite spheres using sulfonic acid functionalized hollow silica spheres (SAFHSS) as templates. The Fe₃O₄ nanoparticles were deposited on or imbedded in the hollow silica shell by a precipitation reaction. The morphologies, composition and properties of the hollow composite spheres were characterized by transmission electron microscopy, Fourier transform infrared analysis, X-ray diffraction measurement and vibrating-sample magnetometry measurement. The results indicated crystal sizes and amount of the Fe₃O₄ nanoparticles on the SAFHSS. The magnetic properties of the hollow composite spheres were controlled by adjusting the proportion between Fe²⁺ and Fe³⁺ and iron ion total concentration. When appropriate loading species were added into the system, superparamagnetite hollow composite spheres were obtained. The method also could be applicable to prepare other superparamagnetite hollow silica/ferrite composite spheres.     

Magnetic behavior of core–shell particles

We have prepared composite magnetic core–shell particles using the process of soap-free emulsion polymerization and the co-precipitation method. The shell of the synthesized composite sphere is cobalt ferrite (CoFe₂O₄) nanoparticles and the core consists of poly(styrene-co-methacrylic acid) polymer. The mean crystallite sizes of the coated CoFe₂O₄ nanoparticles were controlled in the range of 2.4–6.7 nm by the concentration of [NH₄⁺] and heated temperature. The magnetic properties of the core–shell spherical particles can go from superparamagnetic to ferromagnetic behavior depending on the crystalline sizes of CoFe₂O₄.     

Electromagnetic and microwave absorption properties of Fe3O4 magnetic films plated on hollow glass spheres

Magnetic hollow spheres of low density were prepared by plating Fe₃O₄ magnetic films on hollow glass spheres using ferrite plating. The complex permeability and permittivity of spheres–wax composites were measured in the range of 2–18 GHz. The complex permeability and permittivity increased, and the dielectric and magnetic losses were improved as the volume fraction of the magnetic spheres in the composites increased from 60% to 80%, which also resulted in a great improvement of microwave absorption properties. For composites with volume fraction 80%, its magnetic resonance frequency was at about 13 GHz and it appeared three loss peaks in the calculated reflection loss curves; the bandwidth less than −10 dB was almost 4 GHz which was just in the Ku-band frequencies (12–18 GHz) and a minimum reflection loss of −20 dB was obtained when the thickness was 2.6 mm; the microwave absorbing properties were mainly due to the magnetic loss. The results showed that the magnetic spheres composites were good and light microwave absorbers in the Ku-band frequencies.     

Synthesis and characterization of hard magnetic composites—Hollow microsphere/titania/barium ferrite

Hard magnetic composites—hollow microsphere (core)/titania (intermediate layer)/barium ferrite (magnetic shell) (M/T/B) were prepared by wet-chemical method. Barium ferrite nanoparticles were directly coated on the rutile titania-coated hollow microsphere forming light hard magnetic composites using sol-gel technique. The prepared composites were characterized with FESEM, EDS, XRD and vibrating sample magnetometry. The composites are composed of barium ferrite, hematite, titania and mullite. For the samples with 40 wt.% barium ferrite, its specific saturation magnetization with titania is increased to 17.88 emu/g in comparison with 9.6 emu/g without titania. The function of titania in the composites is also discussed.     

Preparation and photocatalytic property of α-Fe2O3 hollow core/shell hierarchical nanostructures

We report the preparation of a novel kind of α-Fe₂O₃ hollow core/shell hierarchical nanostructures self-assembled by nanosheets. A green precursor powder is first prepared using nontoxic and inexpensive FeCl₃ and urea in ethylene glycol by a surfactant-free solvothermal method at 160 °C for 15 h. The α-Fe₂O₃ hollow core/shell hierarchical nanostructures are obtained by the thermal treatment of the green precursor powder. The as-prepared α-Fe₂O₃ hollow core/shell hierarchical nanostructures are porous, and exhibit a good photocatalytic activity for the degradation of phenol. The samples are characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM).     

Preparation and microwave absorption properties of electroless Co-P-coated nickel hollow spheres

Co-P-coated nickel hollow spheres (NHSs) were prepared by electroless plating technology. The morphology and component content of Co-P coating varies with the change of sodium citrate concentration in elctroless plating solution. And as phosphorus content increases in coatings, resulting in smaller grain, coercivity of microspheres decreases. The microwave absorption properties of spheres-wax composite were investigated in the range of 2-18 GHz. Both permittivity and permeability increase with an increase of cobalt content in coatings. For composite layer, a minimal reflection loss (RL, −36.9 dB) of was predicted at 8.1 GHz with a thickness of 3 mm.     

核壳结构磁性复合纳米材料的可控合成与性能

具有核/壳结构的磁性复合纳米材料是十分重要的功能材料,其综合物性受材料微结构的影响,而这很大程度上又取决于复合体系的可控合成.本文综述了近二十年来有关核/壳磁性复合纳米材料的制备、表征及性能研究方面的进展,讨论的体系主要有：铁氧体基永磁/软磁（反铁磁）复合纳米材料、非磁性体包覆磁性核而成的复合纳米材料、用磁性颗粒催化合成的碳基复合纳米材料、基于交换偏置效应而设计的复合纳米材料、核-壳同轴结构的一维复合纳米材料和核/壳/壳三元结构的磁性复合纳米材料等.构建复合体系的组分包括M型永磁铁氧体、3d过渡金属（及其合金、氧化物、碳化物）、多铁化合物、非磁性体（比如绝缘体、半导体、有机分子）和碳材料等,着重分析了复合纳米材料的热稳定性、光致发光性能、光电催化能力、电化学特性、微波吸收性能、磁电阻效应、永磁体性能、高频软磁特性、交换偏置效应及其相关现象.最后,对核/壳结构磁性复合纳米材料的未来发展趋势进行了展望,并在基础研究和改性应用方面提出了一些建议.     

Study on the surface erosion route to the fabrication of TiO2 hollow spheres

Three-dimensional (3D) architecture of TiO₂ hollow sphere has many excellent and interesting performances that attract significant attention nowadays. In this paper, a simple surface erosion approach to the fabrication of TiO₂ hollow spheres via the hydrothermal process has been developed. The morphologies and the phase were characterized by scanning electron microscopy (SEM) and X-ray diffractometer (XRD). The results indicate that the anatase-type TiO₂ hollow spheres with a diameter of ∼1 μm are successfully synthesized. The shell thickness of TiO₂ hollow spheres is ∼150 nm and the size of hollow cavity is ∼600 nm. By the control experiments, the influence of ammonium fluoride and hydrogen peroxide on the hollow spherical structures was studied. Hydrogen peroxide acts as both the oxidant and the bubble generator, ammonium fluoride is crucial for the erosion and dissolution of titanium, the detailed dissolution-crystallization mechanism for the formation of TiO₂ hollow spheres was also proposed.     

Ferromagnetic resonance of cobalt nanoparticles in the polymer shell

The magnetic properties of cobalt spherical nanoparticles (～ 5–9 nm in size) in a polymer shell are investigated using ferromagnetic resonance (FMR) spectroscopy. The metal-polymer complex is prepared through the frontal polymerization of the cobalt acrylamide (CoAAm) complex, followed by the thermolysis at a temperature of 643 K. Analysis of the ferromagnetic resonance spectra demonstrates that the material has a high blocking temperature of ～700 K. The anisotropy constant equal to 0.5 erg/cm³ is somewhat larger than the anisotropy constants characteristic of cobalt macrostructures. This difference is associated with the predominance of the surface anisotropy of nanoparticles. The surface anisotropy constant is calculated to be 0.17 erg/cm², and the anisotropy field is determined to be ～350 Oe. It is revealed that the polymer shell affects the magnetic properties of nanoparticles.     

Fabrication of porous BaSnO3 hollow architectures using BaCO3@SnO2 core-shell nanorods as precursors

We present a strategy to synthesize porous BaSnO₃ hollow architectures with that were 150-300 nm in diameter and 1.5-5 μm in length using precursor of BaCO₃@SnO₂ nanorods prepared by hydrothermal treatment. BaCO₃@SnO₂ nanorods, consisting of a BaCO₃ core and a SnO₂ shell, could be used effectively for the solid-state synthesis of polycrystalline BaSnO₃ powder at 800 °C (lower than convention for BaCO₃ and SnO₂ mixtures). The core/shell structure of the precursor could play a role as a structural directing template for preparing BaSnO₃ hollow architectures during the calcination process. The X-ray diffractometer (XRD), scanning electron microscope (SEM), and transmission electron microscope (TEM) are employed to characterize the structures and morphologies. When applied to DSSC, the porous BaSnO₃ hollow architectures exhibit distinct photovoltaic effect.     

Low-density core-shell composite hollow microspheres with tunable magnetic properties

Low-density (about 0.9 g/cm³) composite core-shell hollow microspheres with tunable magnetic properties were fabricated by Ni-Fe-P deposition on hollow glass microspheres (HGM) with modified electroless plating process. The effects of mole ratio of Fe²⁺/Ni²⁺, concentration of the reducer and pH value of the solution on the magnetic properties of the products were investigated. In conclusion, the increase in the mole ratio of Fe²⁺/Ni²⁺ and pH value of the solution could improve the soft magnetic properties of composite microspheres remarkably, while the increase in the concentration of NaH₂PO₂ had the opposite effect. The as-obtained metallic shells were amorphous and the crystallization got better with increased annealing temperature after plating. In addition, the saturation intensity of the composite microspheres was enhanced monotonically by increasing the annealing temperature. This work provided a facile and effective strategy to fabricate core-shell composite hollow microspheres with tailored magnetic properties.     

CoFe2O4/空心微球复合体的制备与吸波性能

采用化学共沉淀法,在空心微球上包覆一层CoFe2O4,得到一种低密度的空心磁性微球．磁测量结果表明,磁场下退火制备的CoFe2O4样品反位缺陷减少,从而导致饱和磁化强度随退火磁场的增强而增大．吸波性能测试结果表明,包覆结构的CoFe2O4/空心球样品是一种轻质的微波吸收材料.     

CoFe_2O_4/空心微球复合体的制备与吸波性能

采用化学共沉淀法,在空心微球上包覆一层CoFe2O4,得到一种低密度的空心磁性微球.磁测量结果表明,磁场下退火制备的CoFe2O4样品反位缺陷减少,从而导致饱和磁化强度随退火磁场的增强而增大.吸波性能测试结果表明,包覆结构的CoFe2O4/空心球样品是一种轻质的微波吸收材料.     

Preparation and microwave characterization of submicrometer-sized hollow nickel spheres

The hollow structural submicrometer-sized nickel spheres were successfully fabricated by the autocatalytic reduction method. Because of the metallic and ferromagnetic behaviour of the nickel spheres, the low-density microspheres could obtain high dielectric constant and magnetic loss in microwave frequencies. The abrupt variation of the real part and the sharp peaks of the imaginary part of permittivity and permeability were observed for the micrometer-sized and nanometer-sized nickel hollow spheres. Reflection loss less than −25 dB were predicted over 11 GHz with a thickness of 1.5–2.0 mm.     

Synthesis, characterization and photodegradation study of mixed-phase titania hollow submicrospheres with rough surface

Titania hollow submicrospheres with mixed phase (anatase-brookite or anatase-rutile) were synthesized via the combination of hydrothermal treatment and calcination of submicrospheres consisting of a polystyrene core and an amorphous TiO₂ shell. After hydrothermal treatment, amorphous titania shell could be transformed to anatase-brookite shell consisting of loose packed titania nanocrystals, which could be further converted to anatase-brookite (below 700 °C) or anatase-rutile titania (700-800 °C) hollow spheres with rough surface via calcination. The loose packing of titania nanocrystals not only inhibited the transformation temperature from anatase to rutile, but also provided titania hollow submicrospheres with high photodegradation activity of Rhodamine B. The photocatalytic activity of titania hollow spheres increased firstly then decreased when the calcination temperature was varied in the range of 450-800 °C, while hollow spheres obtained via calcinating at 700 °C exhibited the highest photocatalytic activity, which was five times higher than that of counterpart without hydrothermal treatment.     

Synthesis of magnetic and lightweight hollow microspheres/polyaniline/Fe3O4 composite in one-step method

After hollow microspheres (HM) were surface modified, a layer of electromagnetic polyaniline/Fe₃O₄ composite (PAN/Fe₃O₄) was successfully grafted onto the surface of the self-assembled monolayer coated HM, resulting in HM/PAN/Fe₃O₄ composites. In this approach, γ-aminopropyltriethoxy silane was adopted to form a well-coating monolayer with amino groups for the graft polymerization of aniline, which played an important role in fabricating the core-shell structure. FeCl₃ was used as the oxidant not only for aniline to form PAN, but also for FeCl₂ to prepare the magnets. The structure, morphologies, and magnetic properties of the as-prepared samples were characterized by scanning electron microscopy, Fourier transform infrared spectroscopy, X-ray diffraction and vibrating sample magnetometer. The results indicated that the HM/PAN/Fe₃O₄ composites possess low density (ρ < 1.0 g/cm³), controllable morphology, and good magnetic properties at room temperature (saturation magnetization Ms = 8.32 emu g⁻¹ and coercive force Hc ≈ 0).     

Polishing behavior of PS/CeO2 hybrid microspheres with controlled shell thickness on silicon dioxide CMP

Organic-inorganic composite microspheres with PS as a core and CeO₂ nanoparticles as a shell were synthesized by in situ decomposition reaction of Ce(NO₃)₃ on the surfaces of PS microspheres prepared through soap-free emulsion polymerization. The shell thickness of the composite microspheres could be turned by varying the concentration of Ce(NO₃)₃ in the reaction solution. The whole process required neither surface treatment for PS microspheres nor additional surfactant or stabilizer. The as-synthesized PS/CeO₂ composite microsphere samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and thermogravimetric analysis (TGA). Oxide chemical mechanical polishing (CMP) performance of the PS/CeO₂ composite abrasives with different shell thickness was characterized by atomic force microscopy (AFM). The results indicated that the as-prepared core-shell structured composite microspheres (220-260 nm in diameter) possessed thin shell (10-30 nm) composed of CeO₂ nanoparticles (particle diameter of 5-10 nm), and the final CeO₂ contents of the composite microspheres ranged from 10 to 50 wt%. A possible mechanism for the formation of PS/CeO₂ composite microspheres was discussed also. The CMP test results confirmed that the novel core-shell structured composite abrasives are useful to improve oxide CMP performance. In addition, there is an obvious effect of shell thickness of the composite abrasives on oxide CMP performance.     

Fabrication and optical properties of 3D composite photonic crystals of core-shell structures

Three-dimensional (3D) composite colloidal photonic crystals with SiO₂ core and ZnO shell were fabricated on borosilicate glass (BSG) substrate by a two-stage deposition method. Scanning electron microscopy (SEM) measurements show that both the pre-deposited SiO₂ and SiO₂/ZnO core-shell structures are oriented with their (1 1 1) axes parallel to the substrates. Optical measurement reveals that the periodic arrays exhibit a photonic band gap in the (1 1 1) direction. The optical properties of SiO₂/ZnO core-shell structures strongly depend on the size dispersions of colloidal spheres and the intrinsic defects in the sample.     

Biomass-Templated Preparation of Hierarchically Multi-Functional Ni/NiO@C Hollow-Fibers

Two series of hierarchically Ni/NiO@C hollow-fibers with magnetic, adsorptive, and electrocatalytic oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) properties are successfully prepared via a facile and green cotton fiber (CF)-templated dipping/adsorbing-calcining strategy. The effect of calcination temperature and CF amount on morphologies, components, and properties of the as-prepared materials is investigated. The results demonstrate that a proper CF template can endow the as-obtained materials with hierarchically hollow-fiber morphology and multi-components (Ni⁰, NiO, and C) at a certain calcination temperature, therefore providing excellent magnetic, adsorptive, and electrocatalytic OER and HER properties. Specifically, the unique hierarchical morphology and synergistic effect between Ni⁰ and NiO remarkably enhance OER and HER properties of as-prepared samples. This strategy is low-cost, simple, eco-friendly, and could be extended to prepare other composites with multi-components and functionalities.     

Preparation and characterization of upconversion luminescent LaF3:Yb,Er/LaF3 core/shell nanocrystals

LaF₃:Yb³⁺,Er³⁺/LaF₃ core/shell nanocrystals were successfully synthesized using solvothermal method. The crystal structure, morphology and photoluminescence properties of as-prepared nanocrystals were investigated in detail. XRD patterns show that the obtained LaF₃:Yb³⁺,Er³⁺ core and LaF₃:Yb³⁺,Er³⁺/LaF₃ core/shell nanocrystals exhibit hexagonal structure. The average particle size is about 9.3 nm and 11.4 nm for core and core/shell nanocrystals, respectively. Compared with LaF₃:Yb³⁺,Er³⁺ nanocrystals, both the upconversion emission intensity and the lifetime increase in LaF₃:Yb³⁺,Er³⁺/LaF₃ core/shell nanocrystals. The enhancement can be attributed to the LaF₃ shell which can eliminate the nonradiative centers on the surface of LaF₃:Yb³⁺,Er³⁺ nanocrystals.