Gengping Wan

High densities of magnetic nanoparticles supported on graphene fabricated by atomic layer deposition and their use as efficient synergistic microwave absorbers

An atomic layer deposition (ALD) method has been employed to synthesize Fe3O4/graphene and Ni/graphene composites. The structure and microwave absorbing properties of the as-prepared composites are investigated. The surfaces of graphene are densely covered by Fe3O4 or Ni nanoparticles with a narrow size distribution, and the magnetic nanoparticles are well distributed on each graphene sheet without significant conglomeration or large vacancies. The coated graphene materials exhibit remarkably improved electromagnetic (EM) absorption properties compared to the pristine graphene. The optimal reflection loss (RL) reaches −46.4 dB at 15.6 GHz with a thickness of only 1.4 mm for the Fe3O4/graphene composites obtained by applying 100 cycles of Fe2O3 deposition followed by a hydrogen reduction. The enhanced absorption ability arises from the effective impedance matching, multiple interfacial polarization and increased magnetic loss from the added magnetic constituents. Moreover, compared with other recently reported materials, the composites have a lower filling ratio and smaller coating thickness resulting in significantly increased EM absorption properties. This demonstrates that nanoscale surface modification of magnetic particles on graphene by ALD is a very promising way to design lightweight and high-efficiency microwave absorbers.

Enhanced microwave absorption of ZnO coated with Ni nanoparticles produced by atomic layer deposition

In this work, flower-like ZnO coated by Ni nanoparticles has been synthesized by the reduction of NiO-coated ZnO produced by an atomic layer deposition (ALD) method. The structure and electromagnetic (EM) absorption properties of the as-prepared samples are investigated. By applying 800 ALD cycles of NiO deposition followed by a reduction process, the surfaces of ZnO are densely covered by Ni nanoparticles with a narrow particle size distribution and an average size of 13.1 nm. ZnO@Ni exhibits remarkably improved EM absorption properties compared to ZnO. The optimal RL calculated from the measured complex permittivity and permeability is −48.0 dB at 10.4 GHz. Moreover, the EM wave absorption less than −10 dB is found to reach 5.3 GHz for an absorber thickness of 1.5 mm. The enhanced absorption ability arises from the effective combination of multiple dielectric–magnetic loss mechanisms.

Preparation and microwave absorption properties of uniform TiO2@C core–shell nanocrystals

In this work, carbon-coated TiO2 (TiO2@C) core–shell nanocrystals have been synthesized by a simple acetylene decomposition method and further explored for the microwave absorbing application. Results demonstrate that a well-graphitized carbon layer with a thickness of about 3.5 nm can be uniformly coated on the surface of TiO2. It is found that the microwave absorption properties of TiO2@C are remarkably enhanced compared to the bare TiO2. The optimal RL calculated from the measured complex permittivity and permeability is −58.2 dB at 7.6 GHz for TiO2@C with a loading of 40 wt%. Whereas for TiO2@C with a loading of 60 wt%, the effective bandwidth of less than −10 dB is found to reach 5.0 GHz with the coating thickness of 2.2 mm. The enhanced performance can be attributed to the increased dielectric properties and the multiple relaxation processes caused by the core–shell composite materials.

Fabrication of carbon-coated NiO supported on graphene for high performance supercapacitors

In this work, carbon-coated NiO nanoparticles supported on graphene (NiO@C/graphene) have been synthesized by integrating an atomic layer deposition (ALD) technique with a simple acetylene decomposition method. Transmission electron microscopy, X-ray diffraction analysis, X-ray photoelectron spectroscopy and Raman results demonstrated that uniform carbon films were coated onto the surfaces of NiO nanoparticles supported on graphene. The electrochemical properties of the NiO@C/graphene were then investigated. The results showed that the special design enabled synergistic effects from graphene and the carbon layer to improve the electrochemical capacitive properties of NiO. As a supercapacitor electrode, the 400-NiO@C/graphene exhibits an initial specific capacitance of 408 F g−1 (1838 F g−1 for NiO) at 1 A g−1 and 68% is retained at 50 A g−1. After 2000 charge–discharge cycles, the specific capacitance improves the initial value of ∼28% at a high current density of 10 A g−1, suggesting a great potential for high performance supercapacitors.

The construction of carbon-coated Fe3O4 yolk-shell nanocomposites based on volume shrinkage from the release of oxygen anions for wide-band electromagnetic wave absorption

The demand for microwave absorbing materials with strong absorption capability and wide absorption band is increasing due to serious electromagnetic interference issues and defense stealth technology needs. Here the carbon-coated Fe3O4 (Fe3O4@C) yolk-shell composites were successfully synthesized in a large scale for the application of microwave absorption through an in-situ reduction process from carbon-coated γ-Fe2O3 precursor. The results show that the Fe3O4 nanoparticles are uniformly coated with a thin carbon layer about 10nm in thickness and a clear void about 1 nm in width between Fe3O4 core and carbon shell are formed due to the volume shrinkage during the reduction treatment. The obtained yolk-shell composites exhibit excellent microwave absorption properties. The absorption bandwidth with RL values exceeding -10dB is up to 5.4GHz for the absorber with a thickness of 2.2mm. The optimal RL can reach up to -45.8dB at 10.6GHz for the composite with a thickness of 3.0mm. The outstanding microwave absorption properties may be attributed to the multiple interfacial polarization, good impedance match and multiple reflections and scattering owing to the unique yolk-shell structures.

Growth Process and Optical Properties of SrWO4 Microcrystal Prepared by a Microwave-Assisted Method

A convenient microwave-assisted process has been applied to prepare Scheelite-type SrWO4 microcrystals. The phase structures and morphologies of the products obtained at different time have been characterized by X-ray diffraction, transmission electron microscopy, and field-emission scanning electron microscopy. The results show that SrWO4 with various morphologies including nanoparticles, nanorod, dumbbell, and sphere shapes were synthesized. A possible mechanism to illustrate the formation of the SrWO4 structures was discussed. The examination of photoluminescence (PL) property revealed that their PL emission properties of the as-prepared SrWO4 microcrystals can be dependent on the morphologies, degree of crystal, and sizes of SrWO4.

SYNTHESIS OF CROSS Bi2 WO6 MICROWAFERS WITH ENHANCED PHOTOCATALYTIC ACTIVITY UNDER VISIBLE LIGHT IRRADIATION

Novel cross Bi2 WO6 microwafers have been fabricated by a facile acetone-assisted solvothermal method in high quantity. The structure characterizations of the microwafers were investigated in detail by means of X-ray powder diffraction, scanning electron microscopy, and high-resolution transmission electron microscopy. The results indicate that the orthorhombic phase of Bi2 WO6 with high crystallinity can be obtained and each microwafer is polycrystalline in nature and organized by the nanoflake subunits. UV-visible diffuse reflectance spectrum of the prepared Bi2 WO6 microwafers demonstrates that they have absorption in the visible light region. The photocatalytic activity of cross Bi2 WO6 microwafers toward (Rhodamine B) RhB degradation under visible light was investigated, and it was found to be significantly better than that of Bi2 WO6 sample prepared by solid-state reaction (SSR-Bi2 WO6).