Formation of anodic TiO2 nanotube arrays with ultra-small pore size

TiO 2 nanotubes prepared by anodization of titanium metal substrates are widely used as functional materials in various fields. Small pore size TiO 2 nanotube arrays are demanded because of their excellent performance in applications. However, the minimum pore size of TiO 2 nanotube arrays has been limited to ~15 nm (nm) despite of long time extensive studies. In the present work, we have fabricated TiO 2 nanotube arrays with diameter down to an unprecedented low value of ~6 nm in ammonium fluoride-containing ethylene glycol electrolyte by a simple one-step anodic oxidation method under optimized condition. The influences of concentration of ammonium fluoride, viscosity of solution, amount of adding water and applied voltage on the anodic growth of small pore TiO 2 nanotubes are studied systematically. Pore initiation and widening determined by the competition between chemical dissolution and field driven growth of anodic oxides under relative low formation field strength are investigated, where local concentration of fluoride anions and acidity near pore bottom are believed to play significant roles in the formation of nanoporous or nanotubular TiO 2 with ultra-small pore diameter and cell size. By sealing the reaction cell to prevent the solution from absorbing ambient water, typical TiO 2 nanotube arrays with a pore diameter of ~10.8 nm can grow up to ~1.1 μm in length, i.e. an aspect ratio up to 100, which is remarkable for TiO 2 nanotubes with such small pore size.