Structure and formation ofH2Ti3O7nanotubes in an alkali environment

The structure and growth of ${\mathrm{H}}_2{\mathrm{Ti}}_3{\mathrm{O}}_7$-type nanotubes have been studied by first-principle calculations. It is shown that the asymmetry in the distribution of hydrogen on the two sides of the surface layer of a ${\mathrm{H}}_2{\mathrm{Ti}}_3{\mathrm{O}}_7$ crystal plate provides a sufficient driving force for the formation of the ${\mathrm{H}}_2{\mathrm{Ti}}_3{\mathrm{O}}_7$ nanotube. Hydrogen deficiency on one side of the surface layer of a ${\mathrm{H}}_2{\mathrm{Ti}}_3{\mathrm{O}}_7$ plate results in a surface tension that increases with increasing hydrogen deficiency and may eventually overcome the coupling from the layers beneath driving the surface layer to peel off from the crystal plate and roll into a tubular structure. While the radius of the resultant nanotube is determined mainly by the layer coupling energy, the thickness of the tube wall is determined by the residual charges on the peeled surface layer. Both the radius and the wall thickness may in principle be controlled via modifying the layer coupling strength and the net charges on the surface.