A new recipe (Shen et al. DOI) for yet another physical manifestation of the carbon nanofiber. These nanofibers are usually produced from hydrocarbons with Chemical vapor deposition but the new recipe shows solid carbon sources are also useful.
Take activated carbon, oxidise in sulfuric acid/nitric acid. Add solution to a solution of iron(III) chloride in ethanol. Stir. Heat carbon/catalyst mix at 1500°C. Slide under an electron microscopy. Observe bead in for a haircut.
The central blob is described as eutectic C@Fe (ratio 1:1.63). From it the carbon nanofibers grow, diameter diminishing along the way and occasionally coalescing at the ends perhaps due to melting. The spheres can be removed in a HCl wash.
How does it form? The purpose of the initial oxidation is to create defects in the carbon lattice through oxygen-containing functional groups. The iron/carbon core is actually molten at the processing temperature of 1500°C. No nanofiber formation takes place below this temperature.
The nanofiber growth is a liquid/solid process. Solid carbon dissolves into the Fe/C sphere, as the concentration of carbon increases it precipitates at the surface. The tip of each nanofiber is high in metal particles that are pushed out.
The authors note that the crystallinity of the the fibers corresponds with the crystallinity of the carbon source material. This observation may not be in agreement with the formation model which relies on molten carbon.