Preparations, Structure Tuning And Functional Applications Of Core-shell Nanomaterials

Core-shell nanoparticles are emerging classes of nanocomposite materials which could integrate the properties of core and shell to have synergistic function. Due to their structural and chemical diversity and tunability, they begin to be seen in a wide range of energy technology applications, including electrochemical energy storage, catalysis, thermal insulation, etc. However, in order to make real-world impact on humanity’s energy and environment problems, performance, life-cycle cost, manufacturing scalability and environmental safety challenges must be overcome. Technically, it has been challenging to synthesize monodisperse core-shell nanoparticles with a uniform shell thickness, or to consolidate them into stable bulk form. Scientifically, how the nanoscale dimension influences ion/electron transport, electrochemical reaction, phase transformation, mechanical deformation and structural degradation remain far from clear. In this PhD thesis, we focus on the preparation, characterization and energy application aspects of core-shell nanostructures, as well as some rudimentary mechanistic analysis and design. We have systematically prepared(a) traditional core-shell,(b) yolk-shell with void interspace, as well as(c) hollow shell structures using the sacrificial template method, in-situ chemical reactions and surface modifications which are often scalable, cheap and environmentally friendly. Transition-metal oxides, metallic aluminum and lanthanum zirconate are selected as the core and shell material candidates, and their structural tuning as well as functional applications are systemically optimized.Short diffusion distances make core-shell nanostructures ideal for electrochemical energy storage. To explore the controllable synthesis and performance optimization of traditional core-shell nanomaterials, C@MnO2 core-shell structure was prepared through an in-situ redox reaction between KMnO4 and reducing carbon template. The supercapacitor performance improved, but was still quite less than the theoretical capacity of MnO2. Therefore, we design and synthesize monodisperse sub-10 nm MnO2@C core-shell nanoparticles. The specific capacity then reaches 1103 Farad/gram at a scan rate of 1mV/s between 0-0.9V, up to 81% of the theoretical capacity of bulk MnO2, but with a high-rate near-surface redox mechanism. The capacity retention is 96% after 5000 cycles, making it potentially attractive for industry.Because battery requires larger capacity and therefore larger volume change of the active core than supercapacitor, Al@TiO2 yolk-shell structure was prepared with void interspace to accommodate the large volume change of the core in electrochemical cycling. When it was utilized as the anode material of lithium-ion batteries, its initial reversible capacity reached 1237 mAh/g at 1 C and stabilized to 1170 mAh/g after 500 cycles. The average Coulombic efficiency was 99.2% and capacity degradation was less than 0.01% per cycle. It has an exceptional high-rate performance, with a reversible capacity of 690 mAh/g at 10 C.When the core is completely removed, we obtain the hollow shell structure. Co3O4 hollow spheres were prepared using organic carbon spheres as sacrificial template, which were obtained through a hydrothermal process utilizing cheap glucose as the carbon source. It was found that the shell thickness could be easily tuned by a simple treatment of the carbon template with acid and alkali solutions. Furthermore, BET measurements revealed that the Co3O4 hollow spheres possessed meso pores and macro pores simultaneously, and high surface area. Subsequently, excellent catalytic performance of CH4 combustion was observed. In order to demonstrate the universality of our method, we synthesized hollow TiO2 nanospheres using a similar approach and its photocatalysis performance was also characterized.Finally, to show that core-shell nanoparticles can be consolidated into bulk form for thermal insulation, we developed a two-step sintering process to prepare La2Zr2O7 hollow-grain ceramics and hierarchically porous Co3O4 honeycombed monolith at centimeters scale. The monolith have ultra-high compressive specific strength(251 MPa at density of 1.86g/cm3), excellent thermal stability(up to 1400°C), and a thermal conductivity(0.016 W/m-1K-1) that is even lower than that of air(0.026 W/m-1K-1) due to nanoscale confinement of air molecule’s mean-free path.