Metal Atom Catalyzed Enlargement of Fullerenes

A metal catalyzed enlargement of fullerenes has been demonstrated by in situ high-resolution transmission electron microscopy. It was found that carbon atoms and clusters can be continuously incorporated into a closed fullerene cage at a high temperature, leading to an increase in the diameter and consequently the formation of giant fullerene with the assist of adsorbed metal atoms. Density functional theoretical simulations indeed suggest that the activation energy for the carbon incorporation and the associated Stone-Wales transformation can be substantially reduced due to the presence of metal atoms, which should be of key importance for the fullerene growth.

Figure 1

(a) A schematic image for the experimental setup: a single MWNT was bridged between a W tip and a Pt wire. (b) A five-wall MWNT containing two fullerenes in the hollow core was chosen. A few W clusters were observed (indicated with white arrowheads) in the MWNT. The white rectangular marked the regions of interest for (c) and (d). (c) Under a small voltage (ca. 0.2 V and $4\mu \mathrm{A}$), no obvious structural change was found. (d) When the applied voltage reached 1.6 V (a current of $\sim 45\mu \mathrm{A}$), a few kinks (indicated with unfilled white arrowheads) were found to form and migrate along the walls. Scale bar is 2 nm.

Figure 2

Serial HR-TEM images showing the W metal assisted enlargement of fullerene: (a) The W cluster jumped onto larger fullerene’s cage (white arrowheads). (b)–(e) The fullerene started to grow and obviously inflated. The W cluster was found to be highly mobile on the fullerene cage. (f) After the W cluster detached, the fullerene growth stopped. During the whole process, the applied voltage and current were kept unchanged as about 1.7 V & $50\mu \mathrm{A}$. Scale bar is 2 nm.

Figure 3

Schemes for two different pathways proposed for the incorporation of a ${\mathrm{C}}_2$ into the carbon network (a) without and (b) with the anticipation of heptagon. (c),(d) Two initial configurations considered for the W atom to assist insertion of ${\mathrm{C}}_2$ into the fullerene cage. The models (c) and (d) correspond to the pathways (a) and (b), respectively.

Figure 4

Transition energy barrier for a SW defect into four hexagons by a simple 90° rotation of a C-C dimer (in purple) with the adsorption of a W atom (in blue).