Synthesis And Microwave Absorption Properties Of Actinomorphic Tubular ZnO/Ferrite Nanoparticles Composites

The preparation of nanocomposite particles is a great challenge in the fields of synthetic chemistry and materials science, because nanocomposite particles have unique structural, mechanical, electronic, magnetic and optical proterties. With the development of technology and science, the request and design of materials is becoming higher and higher. Single non-conductivity materials or magnetic materials can not meet the growth of application requirements. In the present work, we report for the first time the actinomorphic tubular ZnO/CoFe₂O₄ nanocomposites and actinomorphic tubular ZnO/Fe₃O₄ by a simple chemical solution method. Characterization was accomplished using various techniques, such as powder X-ray diffraction, scanning electron microscopy, X-ray energydispersive spectroscopy, and so on.ZnO nanotubes bundles were synthesized by a single solution method at a mild temperature of 90 oC. From the X-ray powder diffraction patterns of the as-prepared pure ZnO, there is no diffraction peaks from other crystalline forms are detected, which indicates a high purity and crystallinity of these ZnO samples. The FESEM and TEM images indicate the detailed morphology of pure ZnO, the wall thickness of the ZnO nanotubes is about 60 nm, inner diameters of the tubes are about 350 nm, and each tube outer wall has the obvious six edge angles, which because of ZnO have a hexagonal structure.CoFe₂O₄ nanoparticles have been prepared at 90oC through coprecipitated method. We synthesize ZnO/CoFe₂O₄ nanocomposites with the same method. The diffraction peaks corresponding to both ZnO and CoFe₂O₄ can be seen clearly. Besides, there are no other diffraction peaks except ZnO and CoFe₂O₄. Hence, it is concluded that the as-synthesized core/shell structured composites are composed of crystalline ZnO and CoFe₂O₄. In comparison with FESEM images of pure ZnO, the average length of the ZnO nanotubes has not obviously changed, while the six edge angles of the outer wall have disappeared, and some small particles can clearly be found on the surface of the ZnO nanotube. It indicates that ZnO nanotubes have been coated with CoFe₂O₄ nanoparticles, and the size of CoFe₂O₄ nanoparticles are below 40 nm. From FESEM images of ZnO/CoFe₂O₄ composites with different relative content of ZnO. It can be seen that with the decreasing of relative content of ZnO, the coating thickness increases significantly. However, in compari- son with Figure 2a, the tubular structures are not obviously changed. The EDX pattern of the ZnO nanotubes shows only the presence of O and Zn elements. Co and Fe elements are found to be present after the nanotube is coated with CoFe₂O₄ nanoparticles, which provides powerful evidence for the successful coating of CoFe₂O₄ on the surface of ZnO nanotubes. This is consistent with the broad peaks of CoFe₂O₄ in the XRD spectra. It can be seen that the coating of CoFe₂O₄ nanoparticles is continuous and uniform. From the TEM image of typical area of actinomorphic tubular ZnO/CoFe₂O₄ nanocomposites. It can be seen that CoFe₂O₄ is coating the surface of ZnO (average 100 nm thick) as a thin layer. It also illustrates that CoFe₂O₄ nanoparticles coat ZnO nanotubes compactly, and there is no obvious crack in the core-shell of the ZnO/CoFe₂O₄ structure. This is in agreement with the result which is concluded from SEM. The magnetic properties of the assynthesized CoFe₂O₄ and the representative actinomorphic tubular ZnO/CoFe₂O₄ nanocomposites were measured at room temperature. A similar behavior for the actinomorphic tubular ZnO/CoFe₂O₄ nanocomposites was observed. In contrast, Ms of the ZnO coated with CoFe₂O₄ decreased to 27 emu/g, mainly due to the volume of the nonmagnetic of the total sample volume. From the the RAM reflectivity far field RCS method spectrum of five microwave test plates. It can be seen that the reflection loss of pure ZnO nanotubes and CoFe₂O₄ nanoparticles are rather low for all frequencies between 2～18 GHz and the peak values are 8.3 and 10.5 dB, respectively. For actinomorphic tubular ZnO/CoFe₂O₄ nanocomposites, the microwave absorption is evidently improved (much better than that of both ZnO nanotubes and CoFe₂O₄ particles). When the conthet of ZnO is 60%, the maximum reflection loss is 28.3 dB. The maximum reflection loss increases from 10.5 dB to about 28.3 dB for the weight ratio of CoFe₂O₄ = 40%. When the weight ratio of ZnO is 60%, the composites have good compatible dielectric and magnetic properties, and hence the microwave absorbing properties show the maximum value. However, when the weight ratio of ZnO is 40%, the maximum reflection loss decreases to 17.2 dB, which may be due to deteriorating the dielectric property when the weight ratio of CoFe₂O₄ exceeds a critical value. Hence, the CoFe₂O₄ particle-functionalized ZnO nanotubes exhibit a better microwave absorption. The improvement of microwave absorption obviously originates from the combination of ZnO nanotubes and CoFe₂O₄ nanoparticles. Therefore, the difference onreflection loss maximum as a function of the samples is associated with the magnetocrystalline anisotropy and structure anisotropy of as-synthesized ZnO nanotubes/CoFe₂O₄ nanocomposites.The synthesis of actinomorphic tubular ZnO/Fe₃O₄ nanocomposites was by a simple chemical solution method. For Fe₃O₄ nanoparticles, the typical characteristics of superparamagnetic behavior were observed, namely almost immeasurable coercivity and remanence. Saturated magnetization (Ms) were estimated to be Ms = 71.05 emu/g. In contrast, Ms of the ZnO coated with Fe₃O₄ decreased to 30.37 emu/g, mainly attributing to the volume of the non-magnetic to the total sample volume. Furthermore, the coercivity and remanence for the ZnO/Fe₃O₄ nanocomposites were also observed to tend towards zero, which is consistent with superparamagnetic behavior at the nanoscale dimensions of inorganic magnetic particles. Compared with that of pure ZnO nanotubes and Fe₃O₄ nanoparticles, enhanced electromagnetic wave absorption of actinomorphic tubular ZnO/Fe₃O₄ nanocomposites at 2¹8 GHz was observed and the possible mechanism was discussed. First, the actinomorphic tubular ZnO cluster comprises many crystalline tubes which are outwardly extended. The three-dimensional conductive paths will be formed when the actinomorphic tubular ZnO clusters distribute in the matrix. As electromagnetic wave impenetrate the cellular coating, the energy is induced into dissipative current by the conductive networks. Second, there exist multiple interfaces between Fe₃O₄ nanoparticles and Fe₃O₄ nanoparticles as well as Fe₃O₄ nanoparticles and ZnO nanotubes. The electromagnetic wave radiating on absorbents is partially absorbed, while the surplus electromagnetic wave will present diffuse reflections due to the multiple interfaces. During the process of diffuse reflections, the electromagnetic wave enters another process which the energy is induced into dissipative current.