Investigation On Controllable Growth And Mechanism Of Carbon Nanostructures At Atomic Resolution

Carbon nanomaterials have been widely used in many fields,such as new energy,information,environment,biology and aerospace,owing to their excellent optical,electrical,mechanical and chemical stability.To deeply reveal and understand the nucleation and growth mechanism of carbon nanomaterials is the cornerstone for the controllable synthesis and functionalization of carbon nanomaterials.Transmission electron microscopy,Raman spectroscopy,etc.,combined with preparation methods and calculation are usually used to speculate its growth mechanism.But these processes are carried out after the completion of growth,which cannot reflect the nucleation and growth process of carbon nanomaterials in real time.Environmental transmission electron microscopy(ETEM)can be used to study the atomic migration and evolution at atomic scale in situ,which provides a basis for the nucleation and growth evolution and characterization of carbon nanomaterials with high spatial and temporal resolution.However,the controlled growth and mechanism of autocatalytic carbon nanomaterials remain to be further studied.In this thesis,metal phthalocyanine was used as catalyst precursor and solid carbon source,the effect of catalyst and temperature on the evolution process of nucleation and growth of carbon nanomaterials was studied in situ by ETEM at the atomic scale,to reveal the nucleation and growth mechanism of carbon nanomaterials under different interface interactions,which would lay a foundation for the further realization of accurate and controllable preparation of carbon nanostructured materials.The specific research results are as follows:(1)The carbon nanostructures growth at temperature dependent interface was studied via in-situ pyrolysis of cobalt phthalocyanine(CoPc)by ETEM,and the effect mechanism of interfacial interaction on the growth of carbon nanostructures was revealed.The results indicates that the growth modes of carbon nanostructures,i.e.,base-growth or tip-growth,are dependent on the interface interaction between catalyst and substrate controlled by temperature.High temperature favors tipgrowth of bamboo CNTs with larger catalyst nanoparticles(NPs)(～20 nm),while low temperature favors the base-growth of GSs with smaller NPs(～10 nm).The average growth rate of 131.3 nm/s at 850℃ was about 60 times compares with that at 750℃.As a result,higher temperature is beneficial to reduce the interaction force between catalyst nanoparticles and substrate.When the temperature is higher than the critical temperature for tip growth,the force between Co3C nanoparticles and carbon nanoshell can overcome the interaction force between Co3C nanoparticles and substrate,and promote the rapid tip growth of graphite shell to form bamboo carbon nanotubes.In other words,higher temperature is conducive to the tip growth of carbon nanotubes.(2)Based on the in-situ pyrolysis of CoPc at atomic scale,the nucleation mechanism of graphite shell was studied,and the rearrangement of graphite shell was found.The nucleation,growth and rearrangement mechanism of graphite shell formed by CoPc pyrolysis was also revealed by ETEM.The results show that under the high vacuum condition of ETEM and heated to 850℃,the CoPc provides Co to form Co-Co3C as active catalyst and carbon source for the growth of graphite shell.Further study found that the carbon adsorbed in the catalyst migrated to the outermost layer of graphite through the distortion of the graphite shell,thus reconstituting the new graphite layer.The structural dynamics of the catalyst shows that the carbon concentration in the catalyst changes dynamically during the process of carbon migration,and the adsorbed carbon continues to grow through the deformation of the graphite layer so that the carbon atoms will penetrate and rearrange to form a new shell.In this work,the solid carbon source is used instead of gas carbon source,which not only improves the growth efficiency of carbon nanotubes,but also solves the problem that the atmosphere is not conducive to highresolution imaging in ESTM.And this work also provides a new way to study the controllable growth and mechanism of other graphite materials.(3)Based on the autocatalytic growth of graphite shell via pyrolysis of different central metals phthalocyanine,the behavior of carbon atoms in different catalysts and the structure-activity relationship between catalyst surface and interface were studied at atomic scale.And the dynamic carbon atom evolution growth process of graphite shell catalyzed by transition metals Ni,Co,Cu and Zn was also investigated.The mechanism of graphite shell growth catalyzed by different central metals was revealed.The results show that,through the in situ study of NiPc and CuPc cracking and growing graphite shell by heating at 850℃ at atomic scale,it is found that for the Ni catalyst,the carbon atom firstly diffuses into the metal phase to form the carbide of Ni,and then diffuses to the surface to form the graphite shell after reaching carbon saturation.It is inferred that the carbon atom is extruded from the Ni(-1-1-1)surface,because the carbon graphite shell is distorted to some extent to fit the lattice.For Cu catalyst,the formation of graphite shell can be explained by the surface growth model,that is,carbon aggregates and migrates on the surface of Cu catalyst to form graphite shell.In addition,the pyrolysis growth of graphite shell by CoPc and ZnPc heated at 850℃ was further investigated.The result indicates that the growth mechanism of graphite shell by Co and Ni,Zn and Cu is similar.Therefore,different activities of catalysts lead to different binding energies between catalysts and carbon,and different interactions between carbon atoms and catalysts,resulting in different growth mechanism of graphite materials.This work provides guidance for the design of catalysts for the controllable growth of carbon nanostructures.