Helical carbon nanofibers with a symmetric growth mode Original Research Article

Helical carbon nanofibers with a symmetric growth mode were synthesized by the decomposition of acetylene with a copper catalyst. There were always only two helical fibers symmetrically grown over a single copper nanocrystal. The two helical fibers had opposite helical senses, but had identical cycle number, coil diameter, coil length, coil pitch, cross section, and fiber diameter. The irregular tips and helical reversals of the two helical fibers further revealed the symmetric growth mode. This mirror-symmetric growth mode was induced by the shape changes in copper nanocrystals during catalyzing the decomposition of acetylene. Upon contacting the initial copper nanocrystals with irregular shapes, acetylene began to decompose to form two straight fibers (the irregular tips). At the same time, shape changes in copper nanocrystals began. Once they changed from an irregular to a regular faceted shape, the two straight fibers ceased to grow and two regular helical nanofibers with opposite helical senses began to grow. If the regular faceted nanocrystals continue to change shapes during fiber growth, the two helical fibers possibly changed helical senses at the same time, resulting in helical reversals. The shape changes were caused by the changes in surface energy resulting from the acetylene-adsorption on the copper nanocrystals.