Metal Oxide And Metal Nanotubes: Synthesis, Characterization And Application

One-dimensional tubular-like nanostructures have attracted great interest since the discovery of Carbon Nanotubes (CNTs). The nanotubes can be applied in optoelectronic devices, sensors, energy storage and conversion, and so on, due to their special tubular-like nanostructure, large surface area and unique physical, chemical and biological properties. Therefore, the simple, large-quantity synthesis and applications of nanotubes are of great significance.In the dissertation, series of CNT-metal, CNT-metal oxide, metal oxide nanotubes and core-shell nanotubes have been achieved using CNTs as template. The formation mechanism of nanotubes and core-shell nanotubes has been discussed. Moreover, the as-synthesized metal oxide nanotubes and core-shell nanotubes have been applied in gas sensors and Li-ion batteries. The main innovative results are displayed as follows:(1) A layer-by-layer method has been developed to synthesize a serial of CNT-metal and CNT-metal oxide core-shell nanotubes, including CNT-Au, CNT-Pd, CNT-Pt, CNT-Sn, CNT-SnO₂, CNT-NiO, CNT-In₂O₃, CNT-CeO₂ core-shell nanotubes. The effects of the polyelectrolyte on the surface of CNTs, reductant and complexing agent during the reaction, the molar ratio of CNTs and metal salts on the morphology and composition have been systematically studied. It is indicated that the key to obtain the high-quality core-shell nanotubes is to control the reductive speed and the strong electrostatic attraction between CNTs and metal ions. We can also control the shell thickness by adjusting the molar ratio of CNTs and metal salts.(2) Uniform metal oxide nanotubes have been obtained by the calcinations of CNT-metal oxide core-shell nanotubes. To our best knowledge, it is the first time to report about the synthesis of a serial of metal oxide nanotubes on CNTs template. The as-synthesized metal oxide nanotubes have been applied in gas sensors and Li-ion batteries. The dense and porous In₂O₃ nanotubes have been synthesized using the above-mentioned method and applied in the ammonia gas sensor. Compared to In₂O₃ nanoparticles and nanowires, In₂O₃ nanotubes exhibit larger sensitivity, short response time and recovery time. Similarly, SnO₂ and NiO nanotubes have also been synthesized using CNTs as template and applied as anode materials of Li-ion batteries. These nanotubes exhibit large capacity and good cycling performance. It is believed that their special tubular-like nanostructure and large surface-to-volume ratio are responsible for the good performance in gas sensor and Li-ion batteries.(3) CNT-based composite core-shell nanotubes, such as CNT-Au-SnO₂, CNT-CeO₂-SnO₂ and CNT-Ag-NiO, have been synthesized by a modified layer-by-layer method using CNTs as template. As an example, CNTs-Au-SnO₂ core-shell nanotubes have been applied in gas sensor for detecting CO, which show large sensitivity, short response time and short recovery time, and good selectivity. After annealing of the above-mentioned core-shell nanotubes, CeO₂-SnO₂ and Ag-NiO composite nanotubes have been obtained. CeO₂-SnO₂ nanotubes have been applied in ethanol gas sensor, which show larger sensitivity and better selectivity compared to CeO₂-SnO₂ composite nanoparticles and pure SnO₂ nanotubes, respectively. Ag-NiO nanotubes have been applied in Li-ion batteries, which show large capacity and good cycling performance compared with pure NiO nanotubes. It is believed that their special tubular-like nanostructure, large surface area and the doping are responsible for the improveed performance in gas sensors and Li-ion batteries.(4) Metal oxide nanotubes and CNT-metal oxide core-shell nanotubes have been derived from metal carbonyl source using CNTs as template. Porous C03O4 nanotubes have been achieved by sonication and further annealing of cobalt carbonyl and CNTs solution in hexane. The as-synthesized Co₃O₄ nanotubes have been applied as anode materials of Li-ion batteries, which exhibit large capacity (1200mAh/g) and good cycling performance. To our best konwledge, it is the best performance when using Co₃O₄ as anode materials of Li-ion batteries by far. The special tubular-like nanostructure and large surface area are responsible for the good performance in Li-ion batteries.(5) Large-scale SnO₂ nanotubes have been synthesized at room-temperature for the first time using positive charged polyelectrolyte PDDA as soft template and Na_BH₄ as reductant. At the primarily stage, Sn nanorods have been obtained by reduction of SnCl₄ with assistance of PDDA as soft template. Subsequently, Sn nanorods have been oxidized and turn into SnO₂ nanotubes directed by Kirkendall effect along with the reaction time. The effects of the molar ratio of SnCl₄ and PDDA and the charged nature of polyelectrolyte on morphology of SnO₂ nanotubes have been systematically investigated.