Combined in situ small- and wide-angle X-ray scattering studies of TiO₂ nanoparticle annealing to 1023 K

Combined in situ small- and wide-angle X-ray scattering (SAXS/WAXS) studies were performed in a recently developed laboratory setup to investigate the dynamical properties of dry oleic acid-capped titanium dioxide nanorods during annealing in an inert gas stream in a temperature interval of 298-1023 K. Aggregates formed by the titanium dioxide particles exhibit a continuous growth as a function of temperature. The particle size determined with SAXS and the crystallite size refined from WAXS show a correlated growth at temperatures above 673 K, where the decomposition of the surfactant is expected. At temperatures above 823 K, the particle and crystallite sizes increase rapidly. An increasing discrepancy between particle and crystallite size indicates growth of a shell structure on the single-crystalline core of the particles. This was confirmed by high-resolution transmission electron microscopy studies of the sample. Transmission electron microscopy shows a transformation from a rod to a spherical particle shape; the WAXS data indicate that the shape change occurs in a temperature interval of 773-923 K. The highly crystalline titanium dioxide particles remain in the metastable anatase phase during the entire annealing process. The transition to the thermodynamically stable rutile phase was not observed at any temperature, in agreement with existing experimental observations.