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

• Published in: Crystal growth & design Vol. 23; no. 6; pp. 4545 - 4555
• Main Authors: Zandonà, Alessio, Véron, Emmanuel, Helsch, Gundula, Canizarès, Aurélien, Deubener, Joachim, Allix, Mathieu, Genevois, Cécile
• Format: Journal Article
• Language: English
• Published: American Chemical Society 07-06-2023
• ISSN: 1528-7483; 1528-7505
• DOI: 10.1021/acs.cgd.3c00300

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.