Transmission electron microscopic observation showed that TiO2 nanotubes synthesized via a simple hydrothermal chemical process formed a crystalline structure with open-ended and multiwall morphologies. Unlike multiwalled carbon nanotubes, the TiO2 nanotube walls were not seamless. During alkali treatment, crystalline TiO2 raw material underwent delamination in the alkali solution to produce single-layer TiO2 sheets. TiO2 nanotubes were formed by rolling up the single-layer TiO2 sheets with a rolling-up vector of [001] and attracting other sheets to surround the tubes.

We report the synthesis and characterization of ordered multiferroic BiFeO 3 ͑BFO͒ nanotube arrays. BFO nanotubes with diameters of about 250 nm and lengths of about 6 m were fabricated by means of a sol-gel method utilizing nanochannel alumina templates. After postannealing at 700°C, the BFO nanotubes exhibited a polycrystalline microstructure, and x-ray diffraction and transmission electron microscopy study revealed that they are of a perovskite crystal structure. Significant ferroelectric and piezoelectric characteristics of BFO nanotubes have been demonstrated by means of piezoresponse force microscopy measurement. © 2005 American Institute of Physics. ͓DOI: 10.1063/1.2076437͔The prospect of a new generation of random access memory technology to handle the growing demand for memory is pushing several research fronts in this area, 1-4 such as magnetic random access memory and ferroelectric random access memory. Magnetism is involved with the local spin of electrons, while ferroelectricity represents a cooperative phenomenon that relies on the interaction of neighboring permanent electric dipoles in a crystal lattice. These two phenomena can coexist in some unusual perovskite-type oxide materials including magnetic elements, such as BiFeO 3 , 5-7 BiMnO 3 , 8,9 and TbMnO 3 , 10,11 termed multiferroics. Multiferroics have attracted much attention because of their promise in the realization of a new type of memory by a combination of ferroelectric and ferromagnetic properties. As the development of nanoscale electronics approaches a practical stage, it is quite natural to ask how the crystal structure and state of polarization are influenced by the shape and size of multiferroic materials. The synthesis of multiferroic nanostructures with a controllable size and shape is critical not only in new device application, such as high-density magnetically recorded ferroelectric memory, but also from a fundamental point of view.BiFeO 3 ͑BFO͒ is known to have rhombohedrally distorted perovskite structure. The space group R3c of BFO allows for a ferroelectric atomic displacement of below 1083 K and a weak ferromagnetism of below a Néel temperature T N of 643 K simultaneously. The enhancement of polarization and related properties in heteroepitaxially constrained BFO films with a thickness of 200 nm has been reported. The films display a room-temperature spontaneous polarization almost of an order of magnitude higher than that of the bulk. 3,12 More recently, a giant ferroelectric polarization of beyond 150 C/cm 2 has been obtained in BFO thin film deposited on a Pt/ TiO 2 / SiO 2 / Si substrate at 90 K. 13 Ordered BFO nanostructure arrays are of considerable interest for future applications, such as a vertical magnetic recording with an ultrahigh recording density. In this letter, we report the synthesis of the BFO nanotube arrays by using a sol-gel template method. Significant ferroelectric and piezoelectric characteristics of the BFO nanotube arrays have been revealed.Nanochannel alumina ͑NCA͒ templates were prepared by ...