Morphology of multiwall WS2 nanotubes

Recent progress in the synthesis of bulk quantities of long WS2 nanotubes has allowed them to be considered as potential candidates in various applications, specifically in nanotechnology. In order to evaluate their efficacy in such applications and to obtain a better insight into their growth mechanism, morphological studies were performed both by scanning electron microscopy and transmission electron microscopy. Three types of WS2 nanotubes, a few microns in length, were found. The nanotubes denoted "thin" have a diameter up to 30 nm and are circular; those denoted "thick" have a larger diameter, up to 150 nm, and are often polygonal. A third kind of nanotube, bundled or fused, could be also discerned. Differences in the morphology of the nanotube caps were also observed: The majority of "thin" nanotubes exhibit a head close to hemispherical, while the "thick" ones have a peculiar 90 degrees apex. It is argued that the two discernible morphologies are obtained through two distinct growth mechanisms, as discussed in the text.