Exciton-mediated optical transitions in PVA composites containing xerogel, amorphous, and crystalline silica from rice husks

Abstract

Polyvinyl alcohol (PVA) is a polymer known for its fluorescence; however, its relatively low exciton formation restricts its potential in optoelectronic devices. This study aims to enhance the optical properties of PVA by incorporating silica fillers (xerogel, amorphous, or crystalline) derived from rice husks, thereby creating silica-dispersed PVA composites. The composites were fabricated by solution casting at a PVA-to-filler mass ratio of 9:1. Structural, absorbance, and fluorescence analyses were performed on the samples. Embedding the different silica polymorphs enhanced photon absorption; moreover, it also improved excitation for the π–π* transitions by approximately 7.86, 1.72, and 1.01 times for PVA filled by xerogel, amorphous, and crystalline silica, respectively, compared to unfilled PVA. The composites also displayed a broadened excitation shoulder, enabling a broader excitation wavelength range. This enhancement is caused by the increase in the stiffness and bond strength of the PVA molecules induced by compressive stress resulting from its physical interactions with the filler, as well as B2 defect and hydroxyl dioxasilyrane clusters, which participate in promoting exciton formation. As a result, π–π* excitation is boosted, especially in the ultraviolet region, and an increased excitation shoulder intensity is observed between 360 and 425 nm. This higher excitation correlates with a greater emission intensity in the same hierarchical order as the excitation trend. The emission peak position remains relatively constant, with a slight Stokes shift toward the violet region owing to exciton internal conversion. PVA/silica composites also exhibited more complex emission patterns and a lower average lifetime than unfilled PVA.