Iron-based microstructured or nanostructured materials, including Fe, γ-Fe2O3, and Fe3O4, are highly desirable for magnetic applications because of their high magnetization and a wide range of magnetic anisotropy. An important application of these materials is use as an electromagnetic wave absorber to absorb radar waves in the centimeter wave (2−18 GHz). Dendrite-like microstructures were achieved with the phase transformation from dendritic α-Fe2O3 to Fe3O4, Fe by partial and full reduction, and γ-Fe2O3 by a reduction−oxidation process, while still preserving the dendritic morphology. The investigation of the magnetic properties and microwave absorbability reveals that the three hierarchical microstructures are typical ferromagnets and exhibit excellent microwave absorbability. In addition, this also confirms that the microwave absorption properties are ascribed to the dielectric loss for Fe and the combination of dielectric loss and magnetic loss for Fe3O4 and γ-Fe2O3.

Hierarchical Dendrite-Like Magnetic Materials of Fe3O4, γ-Fe2O3, and Fe with High Performance of Microwave Absorption

Colloidal inorganic nanocrystals stand out as an important class of advanced nanomaterials owing to the flexibility with which their physical-chemical properties can be controlled through size, shape, and compositional engineering in the synthesis stage and the versatility with which they can be implemented into technological applications in fields as diverse as optoelectronics, energy conversion/production, catalysis, and biomedicine. The use of microwave irradiation as a non-classical energy source has become increasingly popular in the preparation of nanocrystals (which generally involves complex and time-consuming processing of molecular precursors in the presence of solvents, ligands and/or surfactants at elevated temperatures). Similar to its now widespread use in organic chemistry, the efficiency of "microwave flash heating" in dramatically reducing overall processing times is one of the main advantages associated with this technique. This Review illustrates microwave-assisted methods that have been developed to synthesize colloidal inorganic nanocrystals and critically evaluates the specific roles that microwave irradiation may play in the formation of these nanomaterials.

Microwave-assisted synthesis of colloidal inorganic nanocrystals.

A porous ZnO nanosheet with a near-rectangular morphology has been successfully prepared through a simple solvothermal-annealing method using Zn5(OH)6(CO3)2 as a pore-directing agent. Moreover, the features of ZnO can be easily tuned by changing the annealing temperature. The evolution of defects along with the increase of annealing temperature has been revealed as follows: the content of surface oxygen vacancy of the as-prepared samples first increases and then decreases, however, the content of impurities decreases gradually. A clear relationship between the defects and photoluminescence/photocatalytic characteristics of ZnO is observed. The defect-related emission mechanism of the visible photoluminescence (PL) for the as-prepared ZnO samples has been proposed. In addition, the samples also show good activities for photo-degradation of phenol under UV light irradiation. ZnO-500 °C (annealed at 500 °C) exhibits the best photocatalytic activity, which is superior to that of commercial ZnO. The photocatalytic activity can be greatly influenced by the relative concentration ratio of surface defects to bulk defects, crystallization performance and specific surface area.

Defect-related photoluminescence and photocatalytic properties of porous ZnO nanosheets

In this study, we presented the efficiently adsorptive removal of organic dye, methyl orange (MO), from aqueous solution with the hydrothermal synthesized Mg–Al layered double hydroxide (LDH). The as-obtained product was characterized by X-ray diffraction (XRD), Fourier transformation infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). The adsorption characteristics of MO onto the Mg–Al LDH were evaluated in a batch adsorption process and systematically investigated by different experimental parameters, such as initial dye concentration, contact time, and solution pH. The adsorption kinetics were analyzed by pseudofirst-order, pseudosecond-order, Elovich, intraparticle diffusion, and Boyd models, which were well-described by the pseudosecond-order model. The equilibrium adsorption data were interpreted using the Langmuir, Freundlich, and Temkin isotherm models, which fitted well to both the Langmuir and Freundlich models. The monolayer adsorption capacity of the Mg–Al LDH toward MO was f...

Adsorption of Methyl Orange from Aqueous Solution on Hydrothermal Synthesized Mg–Al Layered Double Hydroxide

Hexagonal close-packed Ni (h-Ni) nanocrystals and face-centered cubic Ni (c-Ni) nanoflowers with uniform size and high dispersion have been successfully assembled on graphene nanosheets (GN) via a facile one-step solution-phase strategy under different reaction conditions, where the reduction process of graphite oxide (GO) sheets into GN was accompanied by the generation of Ni nanocrystals. The reduction of GO by this method is effective, which was confirmed by X-ray diffraction (XRD), Fourier transform infrared (FTIR) and Raman spectroscopy and is comparable to conventional methods. The phase and morphology of nickel can be easily tuned by varying the reaction temperature and solvent. It was shown that the as-formed h-Ni nanocrystals with a diameter as small as 3 nm are grown densely and uniformly on the graphene sheets, and as a result the aggregation of the h-Ni nanocrystals was effectively prevented. In addition, c-Ni nanospheres assembled by c-Ni nanocrystals with a size of 15 nm were also uniformly deposited on the graphene sheets. The investigation of the microwave absorbability reveals that the three Ni/GN nanocomposites exhibit excellent microwave absorbability, which is stronger than the corresponding Ni nanostructures.

/pdf/hexagonal-and-cubic-ni-nanocrystals-grown-on-graphene-phase-1t63wqv14q.pdf

Hexagonal and cubic Ni nanocrystals grown on graphene: phase-controlled synthesis, characterization and their enhanced microwave absorption properties

Novel hierarchical cobalt phosphate (alpha- and beta-Co2P2O7) and cobalt hydrogen phosphate hydroxide [Co-11(HPO3)(8)(OH)(6)] three-dimensional (3D) architectures with different morphologies were synthesized under a solvothemal condition. The form and shape of cobalt phosphate can be readily tuned by adjusting experimental parameters of the reaction system. The novel hierarchical alpha-CO2P2O7 microcrystals with basketry-like microstructures were obtained with a molar ratio of Co2+/H3PO4/CH3NH2/HO(CH2)(2)0OH of 1:1.2:1:108, the reaction temperature of 180 degrees C, and the reaction time of 5 days; dumbbell-like beta-Co2P2O7 microspheres were formed with the same reaction temperature and time but the molar ratio of 1: 1.2:1:54, and Co-11(HPO3)(8)(OH)(6) with peony-like nanostructures was prepared with the same molar ratio and reaction time as beta-Co2P2O7 but the reaction temperature of 120 degrees C. The samples were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), Fourier transform infrared spectra (FT-IR) technologies, a Quantum Design superconducting quantum interference device (SQUID) magnetometer, and a network analyzer. The formation mechanism of cobalt phosphate microstructures was investigated in detail.

Hierarchical Three-Dimensional Cobalt Phosphate Microarchitectures : Large-Scale Solvothermal Synthesis, Characterization, and Magnetic and Microwave Absorption Properties

Mg-Al spinel (MgAl2O4) nanorods and nanoplatelets transformed from Mg-Al layered double hydroxide (Mg-Al-LDHs) were synthesized via a combined hydrothermal method and calcination route using Al(NO3).9H2O and Mg(NO3)2.6H2O as raw materials. The nanorods and nanoplatelets were characterized by means of physical techniques, including powder X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microcopy (HRTEM), selected-area electron diffraction (SAED), Fourier transform infrared spectra (FT-IR), thermogravimetric (TG), and nitrogen adsorption-desorption isotherms. XRD patterns reveal that the Mg-Al-LDHs nanostructures were obtained under a hydrothermal reaction temperature of 200 degrees C and Mg-Al spinel nanostructures were fabricated via calcination of the Mg-Al-LDHs nanostructures at 750 degrees C. It can be seen from TEM that the sizes of the Mg-Al-LDHs nanoplatelets were about 20-40 nm and the diameters of the MgAl2O4 nanorods were ca. 6 nm. The HRTEM images indicate that the crystal lattice spaces of the MgAl2O4 nanorods and nanoplatelets are 0.282 and 0.287 nm, respectively.

From layered double hydroxide to spinel nanostructures: facile synthesis and characterization of nanoplatelets and nanorods

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Hierarchical Three-Dimensional Cobalt Phosphate Microarchitectures : Large-Scale Solvothermal Synthesis, Characterization, and Magnetic and Microwave Absorption Properties

From layered double hydroxide to spinel nanostructures: facile synthesis and characterization of nanoplatelets and nanorods