Hierarchical microstructure in mesoporous SnO₂ beads

Tin oxide (SnO₂) mesoporous beads are a significant category of nanostructured materials with extensive applications in sensors and energy storage. In this study, we report the microstructural evolution of the mesoporous SnO₂ beads, particularly focusing on the formation of primary SnO₂ particles. The beads were synthesized through a solvothermal method and subsequently underwent heat treatment (calcination). The beads maintained a spherical shape regardless of the solvothermal temperature, which varied between 140 and 180 °C, causing the average bead size to increase from 120 to 550 nm. Initially, the beads were amorphous but became crystalline post-calcination at temperatures of 400 °C and above. SAXS data analysis indicated that the as-prepared beads contain a heterogeneous structure consisting of a tin-rich organic complex network with particle sizes ranging from 1.2 to 1.4 nm. Upon calcination at 300 °C and above, the size of these primary crystalline SnO₂ particles increases. It was determined from combined SAXS and XRD studies that these primary crystalline particles originate from an amorphous precursor in the as-prepared beads. The mesoporous structure forms only after calcination of the beads at 400 °C or higher due to the dissociation and evaporation of polyvinyl pyrrolidone (PVP), leaving empty spaces between the primary crystalline SnO₂ particles.