Viscoelastic and Thermoreversible study of Water-Based TiO₂ Sol–Gels for High-Efficiency Photocatalytic Air Purification

Water-based sol–gel synthesis of TiO₂ is increasingly relevant for sustainable photocatalytic materials, but its strong temperature-sensitive hydrolysis–condensation behaviour often leads to poor viscosity control, unpredictable gelation, and non-uniform coatings. These rheological instabilities limit catalyst loading and reduce photocatalytic efficiency, particularly in gas-phase VOC degradation. This work investigates the viscoelastic evolution, thermoreversibility, and structural transitions of fully aqueous TTIP–acetic acid TiO₂ sols to establish processing conditions that yield uniform, high-performance coatings.

TiO₂ sols were aged at ambient temperature (Ta), thermally incubated at 50 °C (Ti), and subsequently cold stored at 4 °C (Tc). Viscosity was monitored using vibro-viscometry, while microstructural viscoelasticity was evaluated through DLS microrheology with 40 nm and 100 nm Au probes. Coatings prepared by dip-coating were examined via FE-SEM, profilometry, XRD, and UV-Vis spectroscopy. Gas-phase formaldehyde (10 ppm) degradation under UV-A irradiation (300–700 mL/min) was used to assess photocatalytic performance.

Ta sols remained weakly condensed (48–60 nm particles; ~1.8 mPa·s viscosity), producing thin, non-uniform coatings with low catalyst loading (0.016 mg/cm²) and poor HCHO degradation (<30%). Thermal incubation induced controlled condensation, optimized viscosity (~8 mPa·s), and generated subdiffusive microrheological behaviour, enabling uniform coatings with 0.106 mg/cm² loading and crystallite size ~7 nm. These coatings achieved 100%, 97%, and 78% HCHO degradation under UV-A irradiation at 300, 500, and 700 mL/min, respectively. Cold storage partially reversed the incubation effects, reducing viscosity, thickness, and photocatalytic activity.

Thermally modulated aqueous sol–gel processing enables precise control of TiO₂ sol viscoelasticity and thermoreversibility, allowing the formation of stable, uniform, nanocrystalline coatings with high gas-phase photocatalytic efficiency. This study establishes a rheology-guided pathway for designing sustainable, high-performance water-based TiO₂ photocatalytic materials.