Six-pointed starlike anatase aggregates were synthesized hydrothermally from aqueous sols of tetramethylammonium titanate colloids having a lepidocrocite-type layered structure. Their microstructure was examined by transmission electron microscopy (TEM). The star-shaped aggregates had a 6-fold symmetry similar to hexagonal crystals, although anatase has a tetragonal lattice. Acicular anatase nanocrystals extending along the c-axis direction were aligned parallel to each other to form one-half of the aggregate arm and were connected with the acicular crystals in the other half on the {112} plane by twinning. The fact that the angle between the ( 112) and (112 j ) planes is about 60°contributes to the 6-fold symmetry of the starlike aggregates. In a higher resolution TEM image, fringe contrast was observed normal to the c-axis of the anatase crystals. The interval was close to the basal spacing of the original layered tetramethylammonium titanate, although the X-ray diffraction (XRD) pattern did not show any peaks of the titanate. It was inferred that the anisotropic self-assembly of the layered-titanate colloids, their topotactic structural conversion to anatase, and the twin formation of anatase crystals play important roles in the formation of the acicular anatase crystals and the star-shaped aggregates.

Anatase was hydrothermally synthesized from transparent aqueous sols including layered titanate colloids. The reaction sols were prepared from titanium isopropoxide and different alkylammonium hydroxides (NR4OH). The influence of the type of NR4OH on the morphology of the anatase crystals and their aggregates was examined. We previously reported that although anatase has a tetragonal lattice, six-pointed starlike anatase aggregates were synthesized from the sols prepared using N(CH3)4OH. With increasing the alkyl chain length of NR4OH, the number of the starlike aggregates decreased, and their arms became thicker and shorter. When N(C4H9)4OH was used, needlelike anatase crystals elongated along the c-axis were formed rather than starlike aggregates. The use of NR4OH with long alkyl chains were expected to retard stacking of the layered titanate during hydrothermal synthesis. The stacking and exfoliation of the layered titanate colloids may play an important role in the morphology of the resulting anatase crystals and their aggregates.

We hydrothermally synthesized anatase films on Ti substrates from transparent aqueous solutions of layered titanate colloids. The morphology and wetting properties of the films were examined. The films consisted of columnar aggregates grown on the Ti substrates, which resulted in high orientation. Randomly oriented films were obtained when alumina was used as a substrate, whereas the sol-gel precoating of a titanium oxide (TiO 2 ) layer on alumina substrates resulted in similar orientation as that of the films on Ti substrates. Thus, the presence of TiO 2 on the substrate surface plays an important role in the orientation of columnar aggregates. The columnar aggregates were similar in microstructure to the arms of a type of six-pointed star-like anatase aggregate synthesized hydrothermally from the same reaction solutions. They were composed of twins of tapered anatase nanocrystals, which provided a rough film surface. The film surface was highly hydrophilic and oleophilic due to the nanoscale roughness even when the film was not irradiated with UV light.

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