Preparation, Microstructure And Properties Of Nanomaterials With Core-shell And Hollow Nanostructures

The rational design and controllable synthesis of nanomaterials with special morphologies is the driving force for the continuous development of current material science and human society. The capabilities of nanomaterials with core-shell structure are different from or better than that of the core or shell single components. The preparation of core-shell materials can realize the design and optimization of the structure and properties of materials, and it is the attractive topic in material area. Hollow materials obtained by removal of the core have widely potential applications in high-technology fields because they have low density, large specific surface areas and well permeability.The nanoscale functional properties of core-shell and hollow nanostructures are closely related to shape, size, and surface properties. A key challenge to the ultimate exploitation of this class of novel nanomaterials is the development of abilities to control the size, morphology and structure of the core-shell and hollow structures. Due to this, the aim of our work is to synthesize hierarchical nanostructures with unique morphology and combined special characteristics, and to investigate the main factors affecting their microwave absorption properties and electrochemical properties. The primary coverage is summarized as follows:1. Double-shelled yolk-shell microspheres with spinel Fe3O4 cores and tetragonal rutile SnO2 double shells were successfully synthesized by combining Stober process and hydrothermal reaction. By controlling the synthetic parameters, various yolk-shell microspheres with different core sizes, interstitial void volume and shell thicknesses could be readily synthesized. The size, morphology, microstructure, crystallographic structure, and magnetic properties were characterized in detail.Furthermore, the microwave absorption properties of these microspheres were investigated in terms of complex permittivity and permeability. The electromagnetic data demonstrated that the as-synthesized double-shelled yolk-shell microspheres exhibited significantly enhanced microwave absorption properties compared with pure Fe3O4 and core-shell Fe₃O4@SiO2 microspheres. Importantly, these double-shelled yolk-shell microspheres with different core sizes, interstitial void volume and shell thicknesses showed obvious differences in absorption properties. The maximum reflection loss value of these double-shelled yolk-shell microspheres could reach-36.5 dB at 7 GHz with a thickness of 2 mm, and the absorption bandwidths with the reflection loss lower than-20 dB was up to 8.5 GHz. Our results suggested that these unique double-shelled yolk-shell microspheres could be effective in microwave absorption enhancement, which therefore offered an avenue for achieving novel high-efficient absorbers with light weight, wide absorption frequency, and strong absorption characteristics for microwave absorption applications.2. Hierarchical hollow Li4Ti5O12 urchin-like microspheres with ultra-high specific surface area of over 140 m2 g-1 and diameter more than 500 nm have been successfully synthesized by combining the versatile sol-gel process and hydrothermal reaction. Moreover, time-dependent reactions reveal that both etching process and in-situ transformation process occur during the hydrothermal reaction.The as-synthesized hollow urchin-like microspheres exhibit excellent electrochemical performance with a high specific capacity of 120 mA h g-1 at 20 C and more long cycling stability than that of Li4TisO12 particles commercially bought. Ex situ electron energy loss spectroscopy （EELS） analysis for Li4Ti5O12 microspheres at different charge-discharge stages indicates that only partial titanium are reduced to Ti3+ while the others still stay in Ti4+ states and a phase transformation occurs that spinel phase Li4Ti5O12 transferring into the rock-salt phase Li7Ti5O12. All the results suggest that these Li4Ti5O12 microspheres may be attractive candidate anode materials for lithium ion batteries.