Hydrothermal tuning of anatase–brookite TiO₂ nanostructures: influence of synthesis parameters on surface charge, crystallite size, and photocatalytic activity

Titanium dioxide (TiO₂) nanoparticles were synthesized via a sol–gel route followed by hydrothermal treatment, and the combined effects of reaction time and temperature on structure–property–activity relationships were systematically investigated using a central composite rotatable design (CCRD). Eleven samples were prepared under controlled hydrothermal conditions and characterized by complementary structural, optical, surface, and colloidal techniques, including XRD with Rietveld refinement, Raman spectroscopy, BET surface area analysis, zeta potential measurements, and photoacoustic spectroscopy (PAS). All materials consisted predominantly of anatase with brookite as a secondary phase (65.7–83.9%). Crystallite size (4.5–7.0 nm) was the only structural parameter significantly affected by synthesis conditions (p < 0.05), increasing mainly with hydrothermal temperature, whereas phase composition and optical band gap remained essentially unchanged. Textural properties, including BET surface area, exhibited statistically significant dependence on hydrothermal parameters, while band gap energy and surface-related optical features remained remarkably stable, indicating electronic robustness of the anatase–brookite system. All samples showed high photocatalytic activity toward methylene blue degradation under halogen irradiation, achieving near-complete removal within 90 min and apparent first-order rate constants of 0.050–0.133 min⁻¹, comparable to commercial TiO₂ P25. Additional experiments using phenol, a more recalcitrant pollutant, demonstrated complete degradation under predominantly visible irradiation without pH adjustment. Ecotoxicological assays using Lemna minor revealed low residual ecotoxicity for treated effluents and nanoparticle leachates. Response surface modeling confirmed hydrothermal temperature as the dominant factor controlling crystallite growth and surface-related properties, while photocatalytic efficiency remained statistically unaffected within the investigated design space. Overall, the results highlight the intrinsic robustness of anatase–brookite TiO₂ and its suitability for environmentally relevant photocatalytic applications.