Selective Chemistry of Single-walled Nanotubes - Structure and Reactivity

Structure and Reactivity

Reactivity of fullerene molecules with respect to addition chemistries is strongly dependent on the curvature of the carbon framework. Their outer surface (exohedral) reactivity increases with increase in curvature. In comparison with fullerene molecules single-walled nanotubes (SWNTs) are moderately curved. Consequently nanotubes are expected to be less reactive than most fullerene molecules due to their smaller curvature, but more reactive than a graphene sheet due to pyramidalization and misalignment of pi-orbitals. The strain of a carbon framework is also reflected in the pyramidalization angle (Ө_p) of the carbon constituents. Trigonal carbon atoms (sp2 hybridized) prefer a planar orientation with Ө_p=0° (i.e. graphene) and fullerene molecules have Ө_p= 11.6°. The (5,5) SWNT has Ө_p~6° for the sidewall. Values for other (n,n) nanotubes show a trend of increasing Ө_p (sidewall) with decrease in n. Therefore generally the chemical reactivity of SWNT increases with decrease in diameter (or n, diameter increases with n). Apart from the curvature SWNT reactivity is also highly sensitive to chiral wrapping (n,m) which determine its electronic structure. Nanotubes with n - m = 3i (i is an integer) are all metals and rest are all semiconducting (SC).