bulk TiO2 powder is treated with NaOH solution in a Teflon-lined autoclave at 100–1508C for several hours, followed by an acid treatment in HCl.[14,47,48] The general formation of the tube geometry is based on exfoliation of TiO2 crystal planes in the alkali environment and stabilization as TiONa+. This step is followed by a rolling of the nanolayer sheets into tubes during cooling or in the HCl treatment. Some debate exists in literature on the exact cause for the rolling-up, and several experimental factors are crucial to indeed obtain a tubular material. Such tubular structures consist of multishells, with an inner shell diameter of about 5 nm, a shell spacing of less than 1 nm, and an average tube diameter of about 10 nm. The amount of synthesized nanotubes and also their length (typically several tens to hundred nanometers) and size distribution depend on the specific reaction conditions.[48]