Synthesis And Characterization Of Core-Shell Structure Nanocomposites

The interest in core-shell structure nanocomposites stems from the fact that their properties (optical, electrical, mechanical, chemical, tribological, catalytic etc.) are a function of their size, composition and structure. Therefore, effective strategies to build tailored nanocomposites are required in order to meet the ever-increasing demands placed on materials synthesis and performance by nanotechnology. The creation of core-shell particles is also of interest from a fundamental and academic viewpoint. Here, we synthesized novel CFs/Fe₃O₄, Ni/ZnO, Ni/SnO₂, MSS/SnO₂, FG/SnO₂ and HGWS/SnO₂ composites by various methods. Their magnetic, optical, microwave absorptive properties have been evaluated, and some important results have been obtained.Carbon fibers coated by nanocrystalline Fe₃O₄ has been successfully fabricated via sol-gel method combined with annealing under vacuum. The results show that a complete and uniform Fe₃O₄ coating on carbon fibers can be obtained in the temperature range of 300-550°C, with coating thickness ca. 800nm composed by Fe₃O₄ nanoparticles of mean sizes ca. 30nm. The method used can be easily extended to synthesize other metal oxide coating on CFs. The microwave absorptive properties are evaluated for the CFs-phenol formaldehyde resin and CFs/Fe₃O₄-phenol formaldehyde resin composite. The results show the shielding effectiveness of 8wt.% CFs/Fe₃O₄-phenol formaldehyde resin composite is superior to that of 8wt.% CFs-phenol formaldehyde resin composite. Greater shielding effectiveness of CFs/Fe₃O₄ composite obtained at a low weight fraction (16wt.%) is abstractive. However, the microwave absorption decreases distinctly when the weight fraction increases to 30wt.% and 50wt.%. Thus, we can conclude that 16wt.% CFs/Fe₃O₄ composite is proper weight fraction for CFs/Fe₃O₄ as absorbers in providing phenol formaldehyde resin as insulating matrix for electromagnetic performance. After coating Fe₃O₄ nanoparticles, CFs begin to oxide at 550°C, about 100°C higer than that of pristine CFs. CFs /Fe₃O₄ composites lead to CFs/γ- Fe2O3 at 300°C in air, and convert to CFs/α-Fe2O3 at 500°C. After CFs/Fe₃O₄ composites are exposed to air at 700°C, all CFs were consumed, and the residual composites are tubularα-Fe2O3. Someone has reported to synthesize iron oxide using sol-gel method with ferric nitrate and ethylene glycol as regents. Unfortunately, pure magnetite nanoparticles cannot be obtained. Here, magnetite (Fe₃O₄) nanoparticles have been successfully synthesized by sol-gel method combined with annealing under vacuum with ferric nitrate and ethylene glycol as regents. The phase structures, morphologies, particle sizes, chemical composition, and magnetic properties of Fe₃O₄ nanoparticles have been characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectrometer (EDS) and vibrating sample magnetometer (VSM). The results indicate that the size, the corresponding saturation magnetization value and coercivity value of Fe₃O₄ nanoparticles increase with the increase of synthesized temperature. The results inspire us to tune the size, magnetization value and coercivity value only by varying the annealing temperatures. Other metal oxide can be prepared by the method above.Core-shell structure Ni/ZnO, MSS/SnO₂ and Ni/SnO₂ nanocomposites are formed by a facile and low-temperature precipitaion method. The nanocomposites have been characterized by XRD, TEM, PL and VSM.The The results indicate that Ni or MSS have been coated by ZnO or SnO₂, and Ni/ZnO, MSS/SnO₂ and Ni/SnO₂ nanocomposites can be obtained without further annealing treat. Comparing with the saturation magnetizations of Ni or MSS, those of nanocomposites decrease obviously. The results inspire us to tune the magnetic properties only by changing the coating. The phase transformation of Ni/ZnO nanocomposites treated at various temperature has been studied, no change take place at 200oC, part of Ni/ZnO nanocomposites convet to Ni0.9Zn0.1O at 400oC, most of Ni/ZnO nanocomposites convet to Ni0.9Zn0.1O at 800oC. The photoluminescence spectra of Ni/ZnO exhibit two emission peaks centered around 380-390 nm and 510 nm, respectively. The strong ultraviolet (UV) emission corresponds to the near band edge (NBE). Obvious red-shift of the UV peaks with the increasing treating temperatures can be observed, explained by quantum confinement effect of increased ZnO size with the increasing treating temperatures. Compared with UV emission intensities of Ni/ZnO nanocomposites treated at various temperatures, UV emission increases at 200oC, attributed to the improving crystalline of ZnO. However, the UV emission decreases at 400oC, the reason leading to the result is the decrease of ZnO. More ZnO take place phase trasformation at 800oC, as a result the UV emission decreases more obviously. The prominent deep-band green emission around 510 nm is usually believed to relate to oxygen deficiency. Compared the green emission intensity of Ni/ZnO nanocomposites, the emission decreases sharply at 200oC because increasing crystalline of the ZnO. However, the emission increases at 400oC, because Ni nanoparticles"rob"inevitably oxygen from the"shell"of ZnO, as a result more oxygen defects appear. No green emission can be observed at 800oC because most of ZnO transform. The R.T. PL measurements of MSS/SnO₂ and Ni/SnO₂ nanocomposites show three emission peaks near 390 nm, 440 nm and 480 nm. The emission near 390 nm might be due to oxygen vacancies. The emission at 440 nm Ni or MSS have been coated by ZnO or SnO₂, and Ni/ZnO, MSS/SnO₂ and Ni/SnO₂ nanocomposites can be obtained without further annealing treat.Comparing with the saturation magnetizations of Ni or MSS, those of nanocomposites decrease obviously. The results inspire us to tune the magnetic properties only by changing the coating. The phase transformation of Ni/ZnO nanocomposites treated at various temperature has been studied, no change take place at 200oC, part of Ni/ZnO nanocomposites convet to Ni0.9Zn0.1O at 400oC, most of Ni/ZnO nanocomposites convet to Ni0.9Zn0.1O at 800oC. The photoluminescence spectra of Ni/ZnO exhibit two emission peaks centered around 380-390 nm and 510 nm, respectively. The strong ultraviolet (UV) emission corresponds to the near band edge (NBE). Obvious red-shift of the UV peaks with the increasing treating temperatures can be observed, explained by quantum confinement effect of increased ZnO size with the increasing treating temperatures. Compared with UV emission intensities of Ni/ZnO nanocomposites treated at various temperatures, UV emission increases at 200oC, attributed to the improving crystalline of ZnO. However, the UV emission decreases at 400oC, the reason leading to the result is the decrease of ZnO. More ZnO take place phase trasformation at 800oC, as a result the UV emission decreases more obviously. The prominent deep-band green emission around 510 nm is usually believed to relate to oxygen deficiency. Compared the green emission intensity of Ni/ZnO nanocomposites, the emission decreases sharply at 200oC because increasing crystalline of the ZnO. However, the emission increases at 400oC, because Ni nanoparticles"rob"inevitably oxygen from the"shell"of ZnO, as a result more oxygen defects appear. No green emission can be observed at 800oC because most of ZnO transform. The R.T. PL measurements of MSS/SnO₂ and Ni/SnO₂ nanocomposites show three emission peaks near 390 nm, 440 nm and 480 nm. The emission near 390 nm might be due to oxygen vacancies. The emission at 440 nm...