Anatase/rutile mixed-phase TiO2 nanoparticles were synthesized through a simple sol-gel route with further calcination using inexpensive titanium tetrachloride as a titanium source, which effectively reduces the production cost. The structural and optical properties of the prepared materials were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and UV-vis adsorption. The specific surface area was also analyzed by Brunauer–Emmett–Teller (BET) method. The anatase/rutile mixed-phase TiO2 nanocomposites containing of rod-like, cuboid, and some irregularly shaped anatase nanoparticles (exposed {101} facets) with sizes ranging from tens to more than 100 nanometers, and rod-like rutile nanoparticles (exposed {110} facets) with sizes ranging from tens to more than 100 nanometers. The photocatalytic activities of the obtained anatase/rutile mixed-phase TiO2 nanoparticles were investigated and compared by evaluating the degradation of hazardous dye methylene blue (MB) under ultraviolet light illumination. Compared to the commercial Degussa P25-TiO2, the mixed-phase TiO2 nanocomposites show better photocatalytic activity, which can be attributed to the optimal anatase to rutile ratio and the specific exposed crystal surface on the surface. The anatase/rutile TiO2 nanocomposites obtained at pH 1.0 (pH1.0-TiO2) show the best photocatalytic activity, which can be attributed to the optimal heterojunction structure, the smaller average particle size, and the presence of a specific exposed crystal surface. The enhanced photocatalytic activity makes the prepared anatase/rutile TiO2 photocatalysts a potential candidate in the removal of the organic dyes from colored wastewater.

The morphology and exposed facet of the anatase-type TiO2 are very important to improve the photocatalytic activity and photovoltaic performance in dye-sensitized solar cells. In this work, we report the synthesis and the photocatalytic and dye-sensitized solar cell performances of anatase-type TiO2 single nanocrystals with exposed {010}- and [111]-facets and with various morphologies by using exfoliated tetratitanate nanoribbons as precursors. The precursor nanoribbons were prepared from the exfoliation of the protonated and, subsequently, tetramethylammonium/H(+) ion-exchanged K2Ti4O9. The colloidal suspension containing the nanoribbons was hydrothermally heated with a microwave-assistance at temperatures from 120 to 190 °C after pH was adjusted to 0.5-14. The dependence of the crystalline phases on temperature and pH indicated that anatase single phase can be obtained at pH 3-13 whereas temperatures higher than 160 °C. The [111]-faceted nanorod-shaped anatase nanocrystals were formed preferentially at pH ≤ 3, whereas the {010}-faceted anatase nanocrystals with morphologies of rhombic, cuboid, and spindle were preferentially at pH ≥5. The morphology observation revealed that the nanoribbons were transformed to anatase nanocrystals mainly by the topotactic structural transformation reaction accompanied by an Ostwald ripening reaction, and pH of the reaction solution took a critical role in the crystal morphology change. At pH ≤1, the mixture of anatase, rutile, and brookite were obtained at higher temperature conditions. The photocatalytic activity and photovoltaic performance were enhanced in an order of surface without a specific facet < [111]-faceted surface < {010}-faceted surface.

{010}-Facet-exposed anatase TiO2 crystals exhibit the highest photoreactivity among the exposed facets. To obtain a higher exposure rate of this facet, the work investigated the transformation of the nanosheets with cavities within the layers derived from a rock-salt-structured Li2TiO3 precursor. All the lithium ions were extracted from the precursor by H+/Li+ ion exchange in HCl aqueous solutions, and after tetramethylammonium ions were intercalated, the precursor can delaminated into the nanosheets. The [TiO3]2- nanosheets were hydrothermally treated under different temperatures and pH values. The results showed that the anatase phase was formed in a wider range of pH and temperature, compared with using nanoribbons of [Ti4O9]2- and nanosheets of [Ti1.73O4]1.07-. At low pH, [111]-faceted nanorod-shaped anatase nanocrystals were formed preferentially, and the nanocrystals preferentially grow along the [001] direction with the increase of solution pH, leading to a large percentage of {010} facets on their surface. The photocatalytic activity increases with the increase of exposure rate of {010} facets.