Post-Synthesis Heat Treatment Induces Polymorphic Homogenization and Band-Tail Narrowing in Sol–Gel TiO2 Nanostructures

Post-synthesis heat treatment provides a route for tuning the structural and optical response of TiO2 without necessarily altering its particle architecture. In this study, TiO2 nanostructures prepared by the sol–gel method were evaluated before and after a stepwise heat treatment from 250 to 400 °C using XRD/Rietveld, Raman spectroscopy, FE-SEM/EDS, statistical morphometry, and UV–Vis diffuse reflectance spectroscopy, with XPS performed on TiO2-400. TiO2-250 consisted of 96.76 ± 1.22 wt.% anatase and 3.24 ± 0.47 wt.% brookite, whereas anatase was the only crystalline phase detected in TiO2-400 within the XRD detection limit. Concurrently, the coherent crystalline domain size increased from 44.3 ± 2.6 to 53.6 ± 1.9 nm, without a statistically significant increase in the equivalent circular diameter of the particles. The apparent optical band gap increased from 3.14 ± 0.03 to 3.25 ± 0.03 eV, while the Urbach energy decreased by approximately 48%, from 189.4 ± 1.1 to 98.0 ± 0.4 meV. These results show that moderate heat treatment is accompanied by polymorphic homogenization and predominantly intraparticle crystalline/electronic reorganization, together with narrowing of the band-tail distribution without appreciable external particle growth. Overall, the study establishes a multiscale correlation between phase evolution, coherent-domain growth, morphological stability, and optical-edge narrowing in sol–gel TiO2 nanostructures.