Microstructure of Six-Pointed Starlike Anatase Aggregates

Abstract

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.

Size, Composition and Structural Changes of TiO2 Rutile Nanoparticles by Pulsed Laser Ablation in Water

“…The two minimum band gaps are significantly narrower than that of polycrystalline rutile thin film (E g = 3 34 eV) and bulk anatase (E g = 3 39 eV). 27 …”

Morphology of anatase crystals and their aggregates synthesized hydrothermally from aqueous mixtures of titanium alkoxide and different alkylammonium hydroxides

“…Previously, we 4) and other groups 5), 6) reported the synthesis of sixpointed starlike anatase aggregates having a six-fold symmetry ( Fig. 1), although anatase has a tetragonal lattice.…”

“…4) At fist, the preparation of transparent aqueous sols of titanate colloids by using N(C2H5)4OH, N(C3H7)4OH and N(C4H9)4OH instead of N(CH3)4OH, and the crystal structure of the titanate colloids were examined. The aqueous mixtures were prepared at R ≥ 0.5.…”

“…4) In order to examine the formation process of the starlike anatase aggregates, anatase was synthesized from aqueous mixtures of TIP and alkylammonium hydroxides having different alkyl chains. The morphology of the synthesized anatase crystals and the starlike anatase aggregates were dependent of the type of NR4OH, as well as the formation rate of anatase was.…”

“…2. 4) If this formation process of the acicular crystals is right, the use of NR4OH with long alkyl chains instead of N(CH3)4OH can influence the morphology of the synthesized crystals and their aggregates, because the intercalation of NR4OH with long alkyl chains into the interlayer of lepidocrocite-type titanates leads to the exfoliation of the titanate layers. 10),11) In this study, anatase crystals and their aggregates were hydrothermally synthesized from …”