Vertically Aligned Large-Diameter Double-Walled Carbon Nanotube Arrays Having Ultralow Density

We have selectively synthesized ultralow density (0.015 ± 0.002 g/cm 3 ) double-walled carbon nanotube (DWNT) arrays. The low mass density is due to the large diameter and low-site density of DWNTs in the arrays. The diameter of the as-grown DWNTs varies from 4.5 to 14.8 nm with a mean value of 7.9 nm. The role of the thickness of Fe catalyst layer is evaluated for the selective growth of large-diameter DWNTs. Our DWNT arrays have promising applications in areas such as solar absorbers and super-hydrophobic surfaces.