Hierarchical microstructure in mesoporous SnO2 beadsHierarchical microstructure in mesoporous SnO2 beadsK Suresh et al.

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.