The Controllable Synthesis And Growth Mechanism Of One-Dimension Carbon Nanomaterials

Carbon nanotubes and carbon nanofibers have many unique properties,such as high electric conductivity,high surface area,and high mechanical strength.These unique properties result in many potential applications,such as catalyst supports, hydrogen storage,composite materials,nano-electronic and nano-mechanical devices, and field emission devices.The synthesis parameters,such as types and particle sizes of catalysts,reaction temperatures,type of support materials,and type of carbon sources may greatly affect the growth process of carbon nanotubes and carbon nanofibers.Among these growth parameters,particle size of catalysts and reaction temperature undoubtedly play key roles in the structure properties of carbon nanotubes and carbon nanofibers.In this thesis,one-dimensional carbon nanomaterials,carbon nanofibers and carbon nanotubes,were synthesized by the pyrolysis of acetylene or ethylene using different metal nanoparticle catalysts or catalyst precursors.Vertically aligned carbon nanofibers were prepared successfully by the decomposition of acetylene with copper catalyst coated on an iron substrate.The products were characterized by SEM,TEM, AFM,XRD,DTA/TG,and so on.The effects of catalyst particle size and reaction temperature on the morphology of carbon nanofibers and carbon nanotubes were investigated.The determining factors in fiber alignment using copper catalyst were studied.The growth mechanism of the obtained carbon nanomaterials was also discussed.Helical carbon nanofibers were synthesized at low temperature by the catalytic decomposition of acetylene with metal nanoparticles prepare by a physical method. Good yields and reproducibility were obtained.Small size effect for synthesis of helical carbon nanofibers was found.Carbon nanofibers were synthesized by catalytic decomposition of acetylene with copper nanoparticles prepared by an evaporation method and Al₂Cu nanoparticles using a hydrogen-arc plasma method.Different morphologies(helical and straight) of carbon nanofibers were observed.The catalyst particle size had a considerable effect on the morphology of carbon nanofibers. Helical carbon nanofibers were grown on copper nanoparticles with a grain size less than 50 nm.The following characteristics of the helical carbon nanofibers were discovered.Firstly,there are always only two helical nanofibers symmetrically grown over a single catalyst nanoparticle,which is below 50 nm in size.Secondly,the two helical nanofibers have absolutely opposite helical senses,namely,one is left-handed coiled,and the other is right-handed coiled.Thirdly,the two helical nanofibers are identical in coil diameter,coil length,fiber diameter,cycle number,coil pitch and cross section.Finally,the fiber diameter of the helical nanofibers is approximately equal to the grain size of the nanocatalyst particle responsible for their growth,but the coil diameter almost twice the size of the catalyst nanoparticle.Therefore,based on the information mentioned above,we conclude that the two helical fibers follow a mirror-symmetric growth mode.When the catalyst particle size was larger than 50 nm, straight carbon nanofibers were obtained dominantly.This symmetric growth mode was induced by the shape changes in catalyst nanoparticles during catalyzing the chemical vapor deposition of acetylene.The shape changes were caused by the changes in surface energy resulting from the acetylene-adsorption on the nanoparticles.The catalytic activity anisotropy of the particle surfaces was the essential condition that fiber can be grown in a helical morphology.Helical and straight carbon nanofibers were synthesized by the catalytic decomposition of acetylene at a low temperature using Cu(NO₃)₂, CuCO₃·Cu(OH)₂·xH₂O and copper tartrate as catalyst precursors.Helical carbon nanofibers with a symmetric growth mode were grown on catalyst nanoparticles with a grain size less than 50 nm.If the catalyst particle size was in the range of 50-300 nm,straight carbon nanofibers were obtained dominantly.Vertically aligned carbon nanofibers were produced by the decomposition of acetylene with copper catalyst coated on an iron substrate.Interestingly,we find that catalyst particles are located at the middle of the formed carbon nanofibers.TEM images reveal that there are only two straight nanofibers grown over a single catalyst nanoparticle.The fiber diameter is approximately equal to the grain size,of the catalyst particle.The angles between the two straight nanofibers are 180.The catalyst particles undergo shape changes during the growth of vertically aligned carbon nanofibers.In contrast,the carbon nanofibers grow in a random orientation, when using copper nanoparticle catalysts coated on a glass plate by a sol-gel method or an evaporation method.The determining factors in fiber alignment using copper catalyst are as follows:(1) Dense copper catalyst particles on a substrate.(2) Catalyst particles are large enough.Two straight nanofibers vertically grown over a single catalyst nanoparticle on a substrate.Because of the dense copper catalyst particles, each fiber is supported by surrounding fibers in the growth process.Attributed to sparse distribution,the carbon nanofibers without supporting by surrounding fibers grow in random orientations on a glass substrate base coated by copper films using a sol-gel method.Because helical carbon nanofibers with a "V"-type mirror-symmetric growth mode were grown on copper nanoparticles with a small grain size,the carbon nanofibers grown in random orientations on glass substrates base coated by copper nanoparticles using an evaporation methodTwo types of carbon nanomaterials,including carbon nanofibers and carbon nanotubes,were synthesized at different temperatures by the catalytic pyrolysis of acetylene or ethylene with iron,cobalt,and nickel nanoparticles prepared using a hydrogen-arc plasma method.The structures of the products were closely related to the reaction temperature,and could be changed from fibers to tubes by simply increasing the reaction temperature.At low temperature,such as the growth with iron nanoparticles under 650℃,the growth with nickel nanoparticles under 550℃,and growth with cobalt nanoparticles under 500℃,carbon nanofibers were obtained. While at higher temperature,such as the growth with iron nanoparticles between 710℃-800℃,the growth with nickel nanoparticles between 650℃-850℃,and the growth with cobalt nanoparticles between 550℃-850℃,carbon nanotubes were the dominant products.The reaction temperature had a significant effect on the activities of the catalysts and carbon sources.At high temperature,the carbon source gas decomposed to produce carbon atoms,which dissolved at catalyst surface exposed to gas and diffused into metal nanoparticles and deposited on the rear side.As for the growth of carbon nanofibers at lower temperatures,we infer that,in this case,maybe there is no dissolution and diffusion of carbon atoms into the catalyst particles,and a surface catalytic polymerization reaction occurs.Namely,carbon atoms decomposed from the carbon source gas deposited on the surface of catalyst to form carbon nanofibers.Bamboo-like carbon nanotubes were synthesized by an arc method using copper as catalyst.The growth mechanism for bamboo-like carbon nanotubes was discussed.