Preparation Of Carbon Nanomaterials From Metal-organic Frameworks

In the view of configuration,carbon nanomaterials can be divided into 0-dimensional particles,1-dimensional wire and 2-dimensional film structures,such as the carbon nano-onions?CNOs?,carbon nnaotubes?CNTs?and carbon nano-sheets?CNSs?,respectively.Because of their unique molecular structures,carbon nanomaterials show excellent chemical and physical properties,and have been widely used in many fields.In this paper,the metal-organic frameworks?MOFs?were synthesized by using nickel chloride as the metal source and terephthalic acid as the organic ligand.The CNOs,CNTs and carbon nano-sheet/NiS?CNS/NiS?were prepared by pyrolysis of the Ni?bdc??ted?0.5 at different conditions.The influences of the process conditions such as pressure,pyrolysis temperature,maintaining time and heating rate on the morphology and structure of the products were investigated,and the growth mechanism and electrochemical properties were also investigated.With the increases of pyrolysis temperature,maintaining time and heating rate,the average diameter of the prepared CNOs increases and the hollow structure also becomes distinct.In addition,the pyrolysis pressure also has a significant effect on the morphology of the product;therefore,we can obtain the optimum preparation conditions.In the open tubular furnace,under atmospheric conditions,the CNOs are well dispersion with outer thickness and onion diameters of about 4 nm and 13 nm,respectively,which were obtained in the carbonization temperature of 500 ? for 5 h at a rate of 5 ?/min.In the sealed high-pressure furnace,the CNOs are well dispersion with outer thickness and onion diameters of about 16.7 nm and 130 nm,respectively,which were obtained in the carbonization temperature of 400 ? for 5 h at a rate of 5 ?/min.According to the analysis of XRD and Raman data,the CNOs were obtained in two different environments with good purity and crystallinity.The electrochemical tests show that as-synthesized CNOs in the optimum conditions of the open tubular furnace after acid washing exhibit high reversible capacity,high-rate performance and excellent cycling stability.The specific capacity of CNOs is 655.3 mA h g^-1 after 100 cycles at 50 mA g^-1,and 196.7 mA h g^-1 after 500 cycle at 3A g^-1.We also use Ni?bdc??ted?_0.5 as precursor to prepare the CNTs and their nitrogen-doped products?N-CNTs?.In the open tubular furnace under normal pressure,the higher the temperature the larger heating rate is propitious to the preparation of CNTs.In the sealed high-pressure furnace under high pressure conditions,the lower the temperature the slower heating rate is propitious to the preparation of CNTs.Therefore,the optimum preparation conditions can be obtained.In the open tubular furnace,the CNTs are well dispersion with outer wall diameter,internal diameters and length of about 5 nm,9 nm and 1 um respectively,which were obtained in the carbonization temperature of 900 ? for 5 h at a rate of direct warming?to reach the carbonization temperature directly after the sample?;in the sealed high-pressure furnace,the CNTs are well dispersion with outer wall diameter,internal diameters and length of about 16 nm,5 nm and 500 um respectively,which were obtained in the carbonization temperature of 500 ? for 20 h at a rate of 5 ?/min.And N-CNTs were successfully prepared in the sealed high-pressure furnace,using the process that triethylenediamine escapes from Ni?bdc??ted?0.5 during pyrolysis and self-deposits back on CNTs under sealed atmosphere which leads to the formation of N-CNTs.Based on data of EDS and XPS,nitrogen content is 2.5%in the as-prepared N-CNTs.The as-synthesized N-CNTs exhibits good electrochemical performance.The specific capacity of N-CNTs is 607.4 mA h g^-1 after at 50 mA g^-1,and 83 mA h g^-1 at 3 A g^-1.The nano-carbon sheet/NiS composites with different morphologies were prepared by adding elemental sulfur.The mixture of precursor Ni?bdc??ted?0.5 and sulfur with a certain ratio is pyrolyzed under high pressure conditions.It is found that the morphology and structure of the sample changed with the ratio of the Ni?bdc??ted?0.5 and sublimed sulfur.The electrochemical test results show that the carbon nano-sheet/NiS composite prepared with a mass ratio of 1:2?MOFs:S?exhibits high reversible capacity,good high-rate performance and excellent cyclic stability:798.3 mA h g^-1 after 50 cycles at 50 mA g^-1,and 53 mA h g^-1 at 3A g^-1,respectively.