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.
