Rice husk-based silica: A structural and optical study of xerogel, amorphous, and crystalline phases

This study reports the xerogel silica fabrication derived from rice husks via the sol-gel route, which was continued with conventional calcination to produce both amorphous and crystalline phases. The primary objective is to elucidate the relationship between their structural characteristics and optical properties, analyzed by adequate analytical procedures. The xerogel and amorphous phases exhibited many structural defects, characterized by an amorphous framework featuring n-membered rings (n = 3 or 4) and a non-solid morphology alongside an elevated specific surface area, with the xerogel notably enriched in silanol groups. In contrast, the crystalline phase exhibited a well-ordered structure predominantly composed of cristobalite and tridymite, characterized by solid morphology and the lowest structural defects. Structural defects and silanol groups are critical for forming emission centers, leading to significantly higher emission intensity in the xerogel compared to the amorphous and crystalline phases. The xerogel and amorphous phases exhibit maximum emission within the blue spectral region, whereas the crystalline phase shows its peak emission in the green spectral region. The observed shifts in emission peaks are likely influenced by the number of defects that act as emission centers within or on the surface of the samples. Additionally, variations in emission intensity correlate with significant differences in lifetime measurements, recorded at 392.37 ns, 4.08 ns, and 1.24 ns for the xerogel, amorphous, and crystalline phases, respectively.