Titanate nanofibers of various sizes and layered structure were prepared from inorganic titanium compounds by hydrothermal reactions. These fibers are different from "refractory" mineral substances because of their dimension, morphology, and significant large ratio of surface to volume, and, surprisingly, they are highly reactive. We found, for the first time, that phase transitions from the titanate nanostructures to TiO(2) polymorphs take place readily in simple wet-chemical processes at temperatures close to ambient temperature. In acidic aqueous dispersions, the fibers transform to anatase and rutile nanoparticles, respectively, but via different mechanisms. The titanate fibers prepared at lower hydrothermal temperatures transform to TiO(2) polymorphs at correspondingly lower temperatures because they are thinner, possess a larger surface area and more defects, and possess a less rigid crystal structure, resulting in lower stability. The transformations are reversible: in this case, the obtained TiO(2) nanocrystals reacted with concentrate NaOH solution, yielding hollow titanate nanotubes. Consequently, there are reversible transformation pathways for transitions between the titanates and the titanium dioxide polymorphs, via wet-chemical reactions at moderate temperatures. The significance of these findings arises because such transitions can be engineered to produce numerous delicate nanostructures under moderate conditions. To demonstrate the commercial application potential of these processes, we also report titanate and TiO(2) nanostructures synthesized directly from rutile minerals and industrial-grade rutiles by a new scheme of hydrometallurgical reactions.

Titanium oxides with one-dimensional nanostructure are of significant interest for the electrochemical lithium insertion and extraction because of their large specific surface area and numerous surface defects. Nanotubes with 10-15 nm outer diameters and 200 ∼ 400 nm long were prepared by a reaction between rutile and caustic soda under hydrothermal conditions. These nanotubes are protonated titanate and can be converted into the anatase nanotubes after calcination at 500°C in argon atmosphere. The anatase nanotubes exhibited a high reversible discharge capacity, excellent high-rate discharge capability, and good cycle stability under the large current density.

Various sized hollow nanotubes and solid nanorods are synthesized from rutile powder (particle size ≈ 120–280 nm) using a relatively simple chemical approach in alkaline solution. The nanotubes and nanorods occur as hydrated phases: TiO2·1.25H2O and TiO2·1.0H2O, respectively. The rutile particles react in concentrated NaOH solution under hydrothermal conditions, yielding layered sodium titanate in the form of either polycrystalline nanotubes or single‐crystal nanorods. The form of the product depends on the temperature and time of hydrothermal reaction: Therefore, this is a report of the template‐free control of the degree of crystallinity, crystal structure, and morphology of these types of nanoscale sodium titanate products. By treating the nanotubes and nanorods with dilute HCl, the sodium ions within them could be exchanged for protons, and the morphology of the nanotubes and nanorods is retained, resulting in hydrogen titanate nanotubes and nanorods. The electrochemical performance of dehydrated hydrogen titanate nanotubes and nanorods is explored in terms of their potential performance as anode materials for lithium‐ion batteries. The discharge capacity is higher for thin anatase nanorods converted from hydrogen titanate nanotubes when compared to the calcined (at 500 °C and 700 °C) products of hydrogen titanate nanorods. The significance of these findings is the possibility of fabricating delicate, nanostructured materials directly from industrial raw materials, because the natural mineral of titanium dioxide and most of the raw industrial TiO2 products exist in the rutile phase.

The titanium oxides with one-dimensional (ID) nanostructure are of significance in electrochemical lithium insertion owing to their high specific surface area and pore volume. In this study, nanorods with diameters of ca. 3-5 nm and lengths of 40-60 nm were prepared through the hydrothermal treatment of a hydrolysate obtained from TiCl4 with caustic soda as demonstrated by HRTEM. These nanorods are protonated titanate and can be converted into the anatase (TiO2) nanorods by a calcination at 400 degreesC. The anatase nanorods have a large specific surface area of 314 m(2)/g and a high pore volume of 1.514 cm(3)/g, respectively. The anatase TiO2 nanorods exhibit a large initial electrochemical lithium insertion capacity of 206 mAh/g and good reversibility. The splitting and multi peaks in cyclic voltammograms associated with differing site occupations are ascribed to the formation of the imperfection of the TiO2 nanorod lattice, which facilitates the transport of lithium in surface defects and bulk materials.

Hydrogen titanate nanofibers synthesized by a hydrothermal reaction, are chemically reactive, readily reacting with dilute acid. This reaction is a topochemical process in which in situ phase transition from H-titanate to anatase takes place and the product retains the fibril morphology. The extent of this reaction can be precisely controlled, allowing us to achieve a delicate composite structure at nanoscale: long titanate fibers of 40-100 nm thick and up to 30 mum long covered with anatase nanocrystals of 10-30 nm. The structure has desired photocatalytic properties and can be separated readily after use. This study demonstrates new opportunities to create delicate inorganic nanostructures with advanced functions by wet chemical reactions.

The electrochemical properties of an electrode of carbon nanotubes decorated with metallic nickel particles were investigated. A high initial discharge capacity of 297 mAh g(-1) of the decorated carbon nanotubes was obtained under a discharge current density of 1000 mA/g, after eliminating the capacity of Ni-P alloy, due to the improvement of the electrocatalytic, activity and the hydrogen adsorption of the surface Ni-P alloy. However, the electrode of decorated carbon nanotubes was found to show a poor charge/discharge cycle life owing to the dramatic increase of surface reaction resistance measured by means of electrochemical impedance spectra, which arose probably from destruction of the structure at the interface of the carbon nanotubes and the coating layer and the passivation of nanoscale-size metallic nickel particles during cycling. (C) 2001 The Electrochemical Society.

In this letter, carbon nanotubes were synthesized by chemical vapor deposition of methane using a hydrogen storage alloy of LaNi5 with a CaCu5 type of hexagonal structure as a catalyst. It was found that surface treatment of LaNi5, alloy particle in KOH solution was effective for providing catalytic sites for carbon nanotube growth. The reaction gas of methane could easily penetrate and reach the catalytic sites on metallic nickel through the decomposed loose surface layer of La2O3. The metallic nickel on the surface layer of LaNi5 alloy particle was dominant for growth of carbon nanotubes. The composite of carbon nanotubes with hydrogen storage alloy will fulfil a potential application in hydrogen energy field.

Carbon nanotubes were synthesized by catalytic decomposition of methane using a hydrogen storage alloy of LaNi5 with a CaCu5 type of hexagonal structure as a catalyst. It was found that surface treatment of LaNi5 alloy particle in KOH solution was effective for providing catalytic sites for carbon nanotube growth. The reaction gas of methane could easily penetrate and reach the catalytic sites on metallic nickel through the decomposed loose surface layer of La2O3. The metallic nickel on the surface layer of LaNi5 alloy particle was dominant for growth of carbon nanotubes. The composite of carbon nanotubes with hydrogen storage alloy would be expected to have a potential application in the field of hydrogen energy.