Synthesis And Activation Of Carbon Nano-onions And Their Phase Transition Under High Pressure

Carbon nano-onions(CNOs)is a new type of nano-sized carbon materials.They have displayed good forground for application in energy conversion and storage,catalysis,lubrication because of their excellent physical and chemical properties such as high specific surface area,high electric conductivity,prominent structural and chemical stability.In addition,CNOs are also ideal carbon resource for synthesizing nano-polycrystalline diamond(NPD)and nano twin diamond(NTD).The hardness,fracture toughness and heat stability of NPD and NTD surpass that of naturnal diamond and have more excellent application prospect in cutting tools and abrasive.In order to propel their industrial application,the most important technological problem to address is to realize the large-scale and controllable synthesis of CNOs.As anode materials for lithium-ion batteries(LIBs),the structure,size and degree of graphitization are key factors that have important effects on the properties of LIBs.Using CNOs which are prepared with different methods as carbon resource,the pressure and temperature for synthesizing NPD/NTD are also different.In this thesis,innovative achievements have been achieved base on how to realize the large-scale and controllable synthesis of CNOs,how to improve the properties of LIBs using CNOs as anode material and the phase transition of small-sized CNOs under high pressure.The results are displayed as follows:1.CNOs with different structures are successfully synthesized under high pressure and high temperature using nanodiamonds as precursor.The optimized condition for synthesizing spherical CNOs is 1GPa-1100℃-15 min.The optimized conditions for synthesizing polyhedral CNOs are 1GPa-1400℃-15 min or 1GPa-1000℃-90 min.The temperature for CNOs prepared via high pressure and high temperature is reduced by 300-500 ℃ compared to CNOs synthesized via nanodiamonds annealing in vaccum.The reason could be ascribed to oxygen-containing gases in the closed high pressure and high temperature(HPHT)synthesis system and oxygen-containing surface functional groups absorbed on the surface of nanodiamonds that reduce the graphitization temperature of nanodiamonds.The oxygen content of spherical CNOs synthesized via HPHT ups to 6.15 at.%,which surpasses CNOs prepared by nanodiamonds annealing in Ar(0.3 at.%).The C-O is the main surface functional group.2.The oxidation behavior in air of CNOs encapsulated with Fe-Ni alloy and their byproducts carbon nanotubes(CNTs)was studied.The properties of LIBs using the sample oxided at 500 ℃ for 1h as anode material was also studied.When the oxidation temperature is lower than 500 ℃,the production rate is higher than 70 wt.%.However,the production rate reduced quickly when the oxidation temperature is higher than 500 ℃.The reason could be attributed to the defect of carbon shell induced by the enlargement of the core caused by oxidation.XPS tests indicate that the content of oxygen-containing surface funtional groups increase with increasing the oxidation temperature.When oxided at 550 ℃,the oxygen content in surface funtional groups up to 10.23 at.%.However,the oxygen content reduced a little at 600 ℃.The content of α-Fe2O3 in the sample oxided at 500 ℃(α-Fe2O3@CNOs/CNTs)is 20.6 wt.% which is obtained from TG and EDS analysis.Using α-Fe2O3@CNOs/CNTs as anode material in LIBs,the discharge capacity is 541.4 mAhg-1 for the 100 th cycle at a discharge current of 100 mAg-1.The composite materials also display excellent rate capacities and cycle stability.The properties of α-Fe2O3@CNOs/CNTs as anode material in LIBs are superior to commecial graphite and CNTs without funtionlization.The high capacity is attributed to α-Fe2O3 which possesses a higher theoretical capacity than that of graphite as well as more Li+ storage sites provided by defective carbon shell.The excellent rate capacities and cycle stability could be ascribed to the high degree of graphitization of CNOs prepared by CVD and the one-dimensional structure of CNTs that is helpful to the fast transport of electron.3.The in-situ high pressure Raman tests indicate that at room temprature,a phase transition occurs at 7.4 GPa for small CNOs(S-CNOs)prepared nanodiamonds annealing in Ar because of the phase transition of spherical graphite layers to polyhedral graphite layers.Under high pressure,the structural stability of S-CNOs is lower than that of large CNOs(L-CNOs)because of curvature effect.After phase transition,the stability of S-CNOs is superior to that of graphite due to the nested graphite layers of CNOs.Compared to L-CNOs,S-CNOs have lower structural stability,higher surface free energy and sp3-C in the center of CNOs.All these properties are benificial for synthesizing NPD and it would reduce the synthesis conditions for NPD using S-CNOs as the starting materials.As has been certified by other researchers.