Alessio Zandonà (Orléans / FR), Cécile Genevois (Orléans / FR), Aurélien Canizarès (Orléans / FR), Emmanuel Véron (Orléans / FR), Joachim Deubener (Clausthal-Zellerfeld / DE), Mathieu Allix (Orléans / FR)
Abstract text (incl. figure legends and references)
SiO2-TiO2 glasses and gels have attracted a long-standing scientific and technological interest due to their very low thermal expansion [1] and to their use as precursors for the synthesis of photocatalytic glass-ceramic materials [2]. Despite their simple binary composition, the crystallization of SiO2-TiO2 glasses and gels can be rather complex and encompass several metastable stages marked by phase transformation between different TiO2 polymorphs, such as TiO2(B), anatase and rutile [2]. In this work, the structural ordering and compositional reorganization of spray-dried amorphous nanobeads of composition 50 SiO2 – 50 TiO2 and 83 SiO2 – 17 TiO2 is studied in situ at high temperature in a JEOL ARM200F cold FEG TEM, operating at 80 kV and equipped with a Protochips heating holder.
The nucleation and growth of TiO2 crystals in a single amorphous nanobead (diameter ~50 nm) were first observed in the range 30-600 ºC, continuously acquiring TEM micrographs while heating. Subsequently, the role and evolution of compositional heterogeneities were evaluated in STEM-ADF mode, performing EELS mappings at various stages of heat treatment. Combined to in situ Raman spectroscopy and XRD, the results provide an unprecedentedly complete view of the crystallization mechanism in these materials: as-prepared nanobeads already contain TiO2-enriched nanodomains, which evolve in size and composition with increasing temperature, eventually undergoing long-range ordering and yielding TiO2 crystals. More generally, our study highlights potential, challenges and limitations of high-temperature (S)TEM experiments for the elucidation of crystallization processes in oxide materials.
Figure 1. Emergence and growth of anatase crystals in a nanobead of composition 50 SiO2 – 50 TiO2, observed in situ during heating in the TEM.
References
[1] Schultz, P. C. Binary Titania-Silica Glasses Containing 10 to 20 Wt% TiO2. Journal of the American Ceramic Society 59, 214–219 (1976)
[2] Zandona, A. et al. Spray-Dried TiO2(B)-Containing Photocatalytic Glass-Ceramic Nanobeads. Advanced Functional Materials 31, 2007760 (2021).