Hydrothermal splitting of titanate fibers to single-crystalline TiO ₂ nanostructures with controllable crystalline phase, morphology, microstructure, and photocatalytic activity

A simple and efficient inorganic acid-assisted hydrothermal route has been developed for the synthesis of single-crystalline TiO₂nanostructures with use of protonic tetratitanate hydrate fibers as a precursor. A variety of TiO₂ nanostructures, including aligned nanorods, nanoporous nanostructures, nanocubes, and diamondshaped nanocrystals, have been prepared with this method. The morphology, crystalline phase, composition, average grain size, band gap, and microstructure of the nanostructures have been determined as a function of the nature and concentration of the inorganic acid (HCl, HNO ₃, or H₂SO₄) used for the synthesis. The rutile phase is obtained by using HCl for a concentration range of 0.1-0.4 M and also for 6 M concentration, HNO₃ for a concentration range of 0.1-0.2 M and >7 M, and H₂SO₄ for a narrow concentration range of 0.1-0.9 M. Likewise, the anatase phase is obtained by using both HCl and HNO₃ for concentrations of around 0.9 M, and a broader concentration range of 2-7 M with H₂SO₄. An increase in the band gap of the TiO₂ nanostructures from 2.97 to 3.35 eV is observed as the crystalline phase changes from rutile, to a mixture of rutile and anatase, and subsequently to pure anatase. Correspondingly, the photocatalytic activity of the synthesized nanostructured materials is found to be dependent on the crystalline phase, composition, and surface area.