Controllable Synthesis And Lithium Storage Properties Of Mof-derived Carbon And Carbon/Metal Composite Nanostructures

Metal-organic frameworks （MOFs） are a kind of complexes with a periodically net structure formed by metal ions or metal-oxo units as nodes and organic molecules as struts. The ordered porous structures of MOFs can be constructed by selecting ligands and central atoms properly and adjusting the configuration of the structures. In this dissertation, the pyrolysis mechanism of MOFs was investigated systematically, based on which the pyrolysis process of MOFs can be controlled to design and synthesize the required nanostructures. The MOF-precursor was used as template to prepare porous carbon materials with high porosity and fractal structures, by adjusting the process parameters of the precursor pyrolysis.Besides, by varying the ligands and metal ions one can change the pyrolysis nature of the MOF-precursors. The design and synthesis of metal oxide nanostructures, carbon nanomaterials and their composites were achieved using the interaction between metal and carbon atoms by controlling the pyrolysis process of precursors. The lithium storage performance of the resulting carbon-based nanomaterials were investigated using as anode materials, and the mechanism of lihtium storage in closed pores and super-micropores in porous carbon was also investigated.The pyrolysis process of MOF-precursors was studied systematically which would be instructive and informative for the design and preparation of MOF-derived nanostructures. A systematic investigation was carried out to study the pyrolysis process of a Zn-BDC （H2BDC =terephthalic acid） MOF, which revealed the structural and compositional evolution by in-situ DRIFT, TG-DSC and X-ray methods. It was found that the pyrolysis of Zn-BDC mainly experiences three stages, ie. the removal of guest molecules, the enhancement of structure regularity and the occurrence of pyrolysis and carbonization reactions. The analysis shows that, the carbonization start before the occurrence of pyrolysis reactions of secondary building unit （SBU） in the third stage. In addition,the effect of ligands and metal elements on the thermostability and pyrolysis charateristics of MOF-precursors were also investigated,exhibiting the great effects of the components of MOF-precursors on the morphologies and structures of the pyrolysis products.Hierarchical porous carbon materials with fractal structures were prepared using Zn-BDC as precursor. The roles of pyrolysis temperature,the vacuum degree of pyrolysis atmosphere and the evaporation of reduced Zn on pyrolysis process were investigated to adjust the pyrolysis and carbonization reactions and the pore forming process, to realize the design and controllable synthesis of the resulting structures. N2 sorption and small angle X-ray scattering （SAXS） were used to investigate the pore structures and the fractal structures of the porous carbon products.The results show the marked impact of pyrolysis temperature on the pore structures. All the products exhibit the highly developed porous and the multifractal structures. The specific surface area shows a burst from 1435 m² g^-1 to 2565 m² g^-1 when the temperature rised from 900 ℃ to 1000 ℃.This can be attributed to the evaporation of the reduced Zn. The analysis results of SAXS show that the specific surface area of all the products are higher than those from N2 sorption method, indicating the pore constrictions hinder the diffusion of N2 molecules, in some parts of closed pores and blind pores, or the open micropores that are too narrow to accommodate two layers of N2 molecules, and the surface area of this kind of pores even can be as large as 538 m² g^-1. Besides, according to the results of SAXS, all the products show both volume fractal and pore fractal structures. The well-developed hierarchical pore structure and multifractal structures can provide more lithium storage active sites and favorable conditions for the infiltration of electrolyte and the diffusion of lithium ions. All the products exhibit both high lithium storage capacity and excellent cyclability. The sample pyrolyzed at 1000 ℃ under vacuum shows an ultrahigh reversible specific capacity of 2016 mA g^-1 at 0.2 C after 60 cycles, and a high-rate performance of as high as 600 mA g^-1 at 3 C. The investigation of lithium storage mechanism by SAXS indicates that there exists a dramatic positive correlation between surface area of closed pores and lithium storage capacity of porous carbons, and the SAXS structure parameters, such as fractal parameters, correlation distance, carbon wall thickness and Porod radii, are also related to the lithium storage capacity.By choosing the appropriate ligands, we have obtained ZnO nanosheets by controlling the evolution of intermediate structure during the pyrolysis of precursor. Zn-TED-BDC （TED = triethylenediamine） MOF was used as precursor, of which the TED molecules will escape from the the crystal structure during the pyrolysis. The leave of TED struts will lead to the formation of a stacked intermediate structure, which can provide a favorable path for the migratation of ZnO to form nanosheets at the surface of the carbon matrix. It was found that the pyrolysis temperature has a significant impact on the morphology of ZnO on the surface of carbon matrix. At 500 ℃, the nanosheets was formed; however, with the increase of temperature, only ZnO nanoparticles can be observed.According to the previous experiences, both the stacked structure of carbon matrix and the ZnO nanosheets can provide valuable lithium storage performance. The ZnO nanosheets/squeezebox-like porous carbon composite shows a high reversible capacity of 950 mAh g^-1 even after 100 cycles at 50 mA g^-1 when was used as the anode material for Lithium-ion batteries.The metal central atoms also play an important role on the stability of MOF-precursor; therefore, one can utilize the poor stablity of the precursor to prepare carbon nanostructures by controlling the pyrolysis process. A Al-BDC MOF was used as precursor to prepare porous carbon nanobelts. The direct pyrolysis leads to the fragmentation of the precursor crystalline, which finally forms the alumina-loaded porous carbon nanobelts. The products show an empty pod structure after acid washing.The increasing pyrolysis temperature will cause the morphology change from belts to particles. All the products show a large surface area, and the mesopores have made a major contribution to the surface area and pore volume. In order to promote the lithium storage performance of the carbon nanobelts, a N-doped process was introduced to give a reversible capacity as high as 1400 mAh g^-1.