Acidic Hydrothermal Conversion of Degussa P25 into Rutile Nanocrystals and Particle Morphology Control

Hydrothermal conversion of Degussa P25 into pure rutile has been investigated at 180°C under acidic conditions. Detailed characterizations of the hydrothermal products were achieved via XRD, TEM, BET, Raman scattering, and UV-vis absorption spectroscopy. The results show that the conversion process is a zero order reaction with a constant anatase conversion rate of ∼1.469 wt%/h. The existent ∼17.9 wt% of rutile in the starting P25 powder does not seem to appreciably seed the phase transition or lead to an epitaxial growth of the rutile crystals. Quasi-equiaxed rutile nanocrystals (∼41 nm, ∼31.5 m2/g), a morphological form most difficult to synthesize via soft-chemical processing, have been obtained at a HNO3/TiO2 molar ratio of 12. Higher acidity or the presence of Cl- tends to yield rutile nanorods, and the effects of Cl- are more significant. These phenomena have been deciphered by considering the solution chemistry of the titanium ionic species and the construction mechanisms of the rutile lattice. Both the equiaxed and the rodlike rutile nanocrystallites were found to be oxygen stoichiometric and possess an indirect interband transition energy of ∼2.93 eV. The rutile nanocrystals obtained via this strategy may find applications as nano-whiteners in cosmetics and pigments.