Self-assembling structures of steroidal derivatives in organic solvents and their sol-gel transcription into double-walled transition-metal oxide nanotubes

Cholesterol-based aggregate-forming derivatives 1-4 were synthesized, and their self-assembled morphologies were investigated by TEM, powder XRD and CD. The TEM image of the self-assembled 1 exhibited a tubular structure with a uniform external diameter of ca. 560 nm, and wall thicknesses of 50 nm. On the other hand, the self-assembled 3 and 4 formed a spherical structure with ca. 2 μm outer diameter and a fiber structure with 200-300 nm diameters in acetonitrile, respectively. These results strongly support the view that the balance between solvophilic and solvophobic groups is important for the formation of the tubular structure by self-assembly in organic solvents. Sol-gel polymerization of transition-metal alkoxides such as Ti(Oi-Pr)₄, Ta₂(OBu)₅ and Zr(OBu)₄ was carried out using 1 as a template. The careful examination of TEM images revealed that the tube walls of the TiO₂, Ta₂O₅ and ZrO₂ nanotubes feature double layered structures. These results indicate that the self-assembled 1 acted as a template as well as a catalyst for sol-gel polymerization of inorganic precursors to produce novel double-walled tubular structures of transition-metal oxides. Furthermore, this sol-gel transcription gave binary transition-metal oxide nanotubes of TiO₂/ZrO₂. The crystalline structure of the TiO₂ nanotubes was investigated by powder XRD and HR-TEM by heating. The TiO₂ nanotubes annealed at 600°C exhibited the anatase phase whereas the TiO₂ nanotubes annealed at 1000°C exhibited the rutile phase. These results suggest that the crystalline structure of the TiO₂ obtained from the sol-gel reaction was changed by heating.