The Controllable Synthesis And Growth Mechanism Of Helical Carbon Nanofibers

Helical nanofibers are synthesized by the chemical vapor deposition of acetylene with copper nanocrystals as a catalyst at the low temperature of 250℃. The copper nanocatalyst used in this study is obtained from the thermal decomposition of a copper tartrate precursor. The reaction conditions are simple, and good yields and reproducibility are obtained. The products are characterized by SEM, TEM, XRD, 1R, EDX, DSC/TG and so on. The nanofibers are regularly coiled in the form of a single coil with a coil diameter of 100 nm and dense coil pitches. The reaction proceeds mainly by the polymerization of acetylene over copper nanocrystals. These nanofibers have a new molecular structure between polyacetylene and carbon fibers. After a heat treatment at 900 ℃. helical carbon nanofibers are finally obtained.The morphologies of the fibers are in close relation to the reaction temperature. Coil diameters can be varied from 100 nm to 10 um by increasing the reaction temperature from 250℃ to 400℃ with other reaction conditions unchanged. When the reaction temperature is reduced to 200℃, the molecular structure of the obtained fibers is near that of trans-polyacetylene.It could be known by TEM and SEM analysis that these helical nanofibers exhibit a novel "V"-type mirror-symmetric growth mode. This is a new type of growth mode. There are always only two helical twin-nanofibers symmetrically grown over a single copper nanocrystal. The two helical nanofibers have absolutely opposite helical senses, but have identical cycle number, coil diameter, coil length, coil pitch, fiber diameter, and fiber cross section. They both have irregular tips with identical length. The two helical nanofibers usually change helical senses (helical reversals) at the coil position of the same cycle number. According to TEM projection, the copper nanocrystals have a regular faceted shape after fiber growth. Most of them show a rhombic shape. Most of the angles between the two helical nanofibers are about 70° or 110°.This symmetric growth mode is induced by the shape changes in copper nanocrystals during catalyzing the chemical vapor deposition of acetylene. Upon contacting the initial copper nanocrystals with irregular shapes, acetylene begins to deposit to form two straight fibers (the irregular tips). At the same time, shape changes in copper nanocrystals begin. Once they change from an irregular to a regular faceted shape, the two straight fibers cease to grow and two regular helical nanofibers with opposite helical senses begin to grow. The length of the irregular tips depends on the rate of the shape changes. If the regular faceted copper nanocrystals continue to change shapes during fiber growth, the two helical nanofibers possibly change helical senses at the same time, resulting in helical reversals and straight parts in the fibers. The rate of shape changes determines the reversal rate of helical senses and finally the length of the straight parts. The shape changes are caused bythe changes in surface energy resulting from the acetylene-adsorption on the copper nanocrystals. The catalytic activity anisotropy of the particle surfaces is the essential condition that fiber can be grown in a helical morphology.Similarly, copper nanocrystals, obtained from an aqueous copper sulfate solution by sodium borohydride reduction and from the decomposition of copper butyrate, oxalate, and lactate precursors, can also catalyze the growth of helical nanofibers with a symmetric growth mode. Whereas, copper nanoparticles prepared by the hydrogen-arc plasma only reveal ribbon-like fibers under identical reaction conditions. Therefore, the helical structures and the symmetric growth mode of the coiled nanofibers synthesized in this study are not induced by carboxyl anions and the chirality of the precursor. HRTEM investigations show that the shape regularity of the faceted catalyst particles included in the fibers accounts for the different morphologies of the resulting products. The thermal decomposition of precursors and sodium borohydride reduction of cop...