Synthesis And Application Of Two-dimensional Nanosheets And Their Hierarchical Architectures

Two dimensional nanosheets possess large exposed surface, rich active sites and unique photoelectric properties which make them useful in areas such as electrochemical energy storage, photoelectricity and catalysis. The performance of nanosheets strongly depends on the morphologies and structures, thus the design, controllable preparation and functional application of hierarchical architectures assembled by nanosheets become the development tendency of nanoscience. Hierarchical architecture assembled by nanosheets is a novel hierarchical architecture with multi-level and multi-component, in which nanosheet units arrange or assemble in accordance with certain rules. This novel structure not only inherits the advantages of nanosheet units but also possesses the coupling effect and synergistic effect of hierarchical architecture, which expands the application of nanosheets in electrochemical energy storage, catalysis, biological detection and so on. This thesis focuses on the design and controllable preparation of hierarchical architectures assembled by nanosheets, carrying out researches on material structures and properties to develop their application in lithium ion battery (LIBs) and surface enhanced Raman spectroscopy (SERS). The main research contents are as follows:A modified hydrothermal approach without strong base and acid circumstances was developed to synthesize TiO2(B) sphere hierarchical architecture and bunchy hierarchical architecture. These TiO2(B) hierarchical architectures are constructed by the ultrathin TiO2(B) nanosheets and possess large surface area and stable structures, contributing to the fast diffusion and reversible storage of lithium ions. The electrochemical lithium storages of the as-prepared TiO2(B) hierarchical architectures were investigated. Benefiting from the unique structural advantages, the as-prepared TiO2(B) bunchy hierarchical architecture exhibits high specific capacity of 186.8 mA h g-1 after 1000 cycles at current density of 5 C, the corresponding capacity fading is less than 8%. The improved lithium storage preformance of TiO2(B) hierarchical architecture has been proved, which promotes the development of TiO2 anodes for LIBs.A hierarchical nanocomposite of T1O2 nanowires decorated with molybdenum disulfide nanosheets (TiO2@MoS2) was synthesized by a facile and low-cost glucose-assisted hydrothermal approach. The growth mechanisms of TiO2@MoS2 have been discussed. The electrochemical lithium storages of TiO2@MoS2 hierarchical architectures have been investigated for their application in LIBs. A synergistic effect of lithium storage is demonstrated on TiO2@MoS2, leading to improved electrochemical profermance compared with either individual component. After 100 cycles at 100 mA g-1, the TiO2@MoS2 anode shows a high reversible capacity of 544 mA h g-1 with the Coulombic efficiency of over 99%. The TiO2@MoS2 nanocomposite also displays an excellent rate capability with a specific capacity of 414 mA h g-1 at 1000 mA g-1.Low-crystallinity molybdenum sulfide (LCMS) and CNTs@LCMS were synthesized by a facile and low temperature solvothermal method. The results from materials characterization shows that the as-prepared LCMS is composited of M0S2 layers mixed with amorphous MoS3, in which the ratio of S and Mo is 2.75. The lithium storages performance of LCMS and CNTs@LCMS have been investigated by electrochemical measurements. They exhibit an unusual electrochemical process for lithium storage compared to typical MoS2 anode. Especially, CNTs@LCMS exhibits an persistent ascending trend in capacity. The unusual ascending capacity is ascribed to the increasing lithium storage caused by new-formed MoS3, combined with the reduced volume change derived from well-dispersed LCMS on CNTs. Furthermore, the ascending performance is proved to be able to effectively extend the circulation life (up to 200%) for lithium-ion batteries by mathematical modeling and calculation. These results provide reference for the diversity study of transition metal sulfide nanosheets.Smart hybrids of Zn2GeO4 nanoparticles and ultrathin g-C3N4 layers (Zn2GeO4/g-C3N4 hybrids) were realized by a facile solution approach, where g-C3N4 layers acted as an effective substrate for the nucleation and subsequent in situ growth of Zn2GeO4 nanoparticles. As a anode materials for LIBs, the Zn2GeO4/g-C3N4 sandwich-like hierarchical architecture exhibits highly reversible capacity of 1370 mA h g-1 at 200 mA g-1 after 140 cycles and excellent rate capability of 950 mA h g-1 at 2000 mA g-1. A synergistic effect is demonstrated on the two building blocks of Zn2GeO4/g-C3N4 hybrids for lithium storage, which leads to the excellent electrochemical performance. These results cast new light on the development of high-energy and high-power anode materials.Ag core-shell hierarchical microstructures, with nanosheet-assembled microspheres as the core and dendrites coated on the surface, were synthesized by electrodeposition. The growth mechanisms of these Ag microstructures were systematically investigated through time-dependent morphological evolution. A transformation stage from the microspheres to the dendrites is found in the morphological evolution of Ag core-shell hierarchical microstructures, which is affected by the deposition time and voltage. This lays the foundation for morphology control of products. As effective SERS substrates, the Ag core-shell hierarchical microstructures exhibit excellent surface-enhanced Raman scattering (SERS) ability, showing great potential.