Crystallization Mechanism of Gel-Derived SiO₂–TiO₂ Amorphous Nanobeads Elucidated by High-Temperature In Situ Experiments

SiO₂–TiO₂ amorphous nanobeads with a TiO₂ content up to 50 mol % were synthesized by sol–gel spray drying. Their crystallization during heat treatments was then characterized by in situ high-temperature techniques including X-ray diffraction, Raman spectroscopy, and particularly high-resolution transmission electron microscopy. Intrinsic nanoscale chemical modulations could be identified already in the as-prepared nanobeads and were shown to play a major role for the nonisochemical precipitation of TiO₂ nanocrystals during heating experiments: the size and compositional contrast of such fluctuations progressively evolved and increased until the emergence of long-range ordering. The formation of TiO₂ polymorphs occurred according to Ostwald’s rule of stages, with the metastable TiO₂(B) phase acting as a precursor to stabilize anatase and rutile. The temporary appearance of TiO₂(B) nanocrystals at early annealing stages was interpreted as the first direct experimental observation of subcritical crystalline nuclei.