In summary, unique conical carbon filaments supported on vapor-grown carbon microfibers were synthesized by the ironcatalyzed pyrolysis of methane in the presence of hydrogen. The conical filaments exhibit axial cylindrical channels with open tips. A base-growth mechanism with delayed nucleation of the graphene sheets at protrusions is proposed for the formation of this unique structure.
Experimental
The reactor used for the fiber growth was a horizontal hot-wall quartz tube with a length and an inner diameter of 100 cm and 3 cm, respectively. The gas feeds (purities: CH 4 , 99.995 %; H 2 , 99.99999 %; He, 99.9999 %) was regulated by mass-flow controllers. The reactor was heated in a mixture of CH 4 and H 2 up to 1150 C and kept for 90 min at this temperature for fiber growth before it was cooled down to 950 C. H 2 was subsequently introduced for 10 min. The reactor was then cooled down to room temperature under He. The CH 4 /H 2 ratio was varied from 3:7 for the growth of the parent VGCFs to 2:8 for the growth of the secondary conical filaments. The total flow rate was always kept at 150 mL min ±1 (at standard temperature and pressure).
The parent carbon fibers were grown on rolled pieces of graphite foil (Union Carbide, Grafoil), which were impregnated in advance by an aqueous solution of iron nitrate (Fe[NO 3 ] 3 ´9H 2 O, Merck) to achieve an iron loading of 0.0045 mol m 2 . The average length and diameter of the parent VGCFs were 25 mm and 5.8 lm, respectively. The fibers were harvested from the foil and soaked for 24 h in a 0.16 M iron nitrate solution, which was prepared by dissolving iron nitrate into 65 % nitric acid. After filtration and drying at 100 C for 3 h, 0.3 g iron-loaded VGCFs were positioned in a quartz boat in the furnace for secondary filament growth.
The fiber morphology was studied by scanning electron microscopy (LEO Gemini 1530) and transmission electron microscopy (Philips CM 200 FEG). In order to avoid possible damage of the conical filaments, the as-grown sample was fixed directly in copper grids for the TEM measurement. Elemental analysis of Fe was performed using a Pye Unicam 7000 ICP±OES Spectrometer.