Synthesis And Growth Mechanism Of Helical Carbon Nanofibers

Helical carbon nanomaterials have been attracting tremendous attention owing totheir outstanding physical and chemical properties. Because of their novel structure,helical carbon nanomaterials are expected to be applied as nanoelectronic devices,hydrogen storage materials, electromagnetic wave absorption materials, etc. In thispaper, helical carbon nanofibers （CNFs） were synthesized by chemical vapordeposition （CVD） of acetylene using supported copper nanocrystals as catalyst at thelow temperature of 250℃. The products were characterized by SEM, TEM, XRD, IR,DSC/TG and so on. The main contents of our work are as follows:1. Synthesis of supported catalyst by impregnation methodCu/MgO catalyst was prepared by impregnation method using Cu（NO₃）₂·3H₂Oas catalyst source, MgO powder as the support. The average grain size of Cu/MgO isabout 30-40 nm. The effect of types of support （SiO₂, TiO₂, Al₂O₃, MgO） as well asthe weight ratios of Cu/support （1:1, 1:5, 1:10） on the morphology of carbon fiberswere studied. The experiment results indicate that MgO support is advantage for thepreparation of helical CNFs with high purity. As for the same type of support, as thedecrease of the ratios of Cu/support, the content of helical CNFs in the productincreases. And the fiber diameter becomes uniform. These results indicate that smallerratio of Cu/support is effective for the preparation of uniform helical CNFs.2. Preparation of helical CNFs by chemical vapor deposition （CVD）CNFs were synthesized by the chemical vapor deposition of acetylene withCu/MgO as catalyst at the low temperature of 250℃. The investigation indicates thatmost of the CNFs are helical with fiber diameter of about 150 nm and fiber length ofseveral micrometers to ten micrometers. The relationship between the morphology of CNFs and reaction temperatures was discussed. The content of helical CNFsdecreases by increasing the reaction temperature from 250℃to 400℃keeping otherreaction conditions unchanged. The fiber diameter increases from 100 nm to 400 nm.The coil diameter also decreases from 200 nm to about 100 nm. The morphologies ofCNFs become diverse with the increase of curved or straight CNFs in the product.3. Growth mode of helical CNFs grown on copper nanocrystalsTEM and SEM analysis shows that these helical CNFs exhibit a symmetricgrowth mode. There are always only two helical nanofibers symmetrically grown overa single copper nanocrystal. The two fibers have absolutely opposite helical senses,but identical cycle number, coil diameter, coil length, coil pitch and fiber diameter.According to TEM projection, the copper nanocrystals have regular faceted shapesafter fiber growth. The copper nanocrystals underwent the shape changes fromirregular to regular faceted shapes during the chemical vapor deposition of acetylene.We consider that the shape changes are caused by the changes in surface energyresulting from the acetylene-adsorption on the copper nanocrystals.4. Growth mechanism of helical CNFs with symmetric growth modeThe shape changes of copper nanoparticles from irregular to regular duringchemical vapor deposition of acetylene as well as the transfer of catalytic active siteson the crystal surfaces of catalyst are considered to be the essential conditions thatfiber can be grown symmetrically in helical morphology. Acetylene begins to depositto form the initial ’irregular tips’ on contacting the initial copper nanocrystals withirregular shapes. At the same time, because of the changes in surface energy resultingfrom the acetylene-adsorption, the shape changes of copper nanocrystals begin whilethe catalytic active sites begin to transfer on surface of copper nanocrystals. Once theshapes of copper nanocrystals become regular, and if the distribution of active sites onconvexs of crystal surface is different, the CNFs will be grown in helical morphology;and if the different distribution of active sites on convexs of crystal surface is alsosymmetrical, the helical CNFs with symmetrical growth mode will be obtained.