Novel synthesis of TiO2 nanoparticles by sol-gel and solvothermal approaches assisted by glycerol: Application to the photocatalytic degradation of synthetic dyes under solar irradiation

• Glycerol-assisted synthesis of TiO 2 nanoparticles via sol–gel and solvothermal approaches. • Photocatalytic activity was tested under natural sunlight on a binary dye mixture (malachite green/methyl red). • Global degradation (GD %) depends strongly on mixture concentration, catalyst dose, dyes ratio, and pH. Titanium dioxide (TiO 2 ) nanoparticles were successfully synthesized via two environmentally friendly liquid-phase routes, namely sol-gel (TiO 2 -SG) and solvothermal (TiO 2 -SV) using glycerol as principal reaction medium and stabilizing agent. This approach enables a direct comparison of the influence of synthesis routes under identical green chemical conditions. Structural and morphological analyses revealed that TiO 2 -SG exhibits a highly crystalline anatase phase with larger crystallite size and lower specific surface area (76.21 m 2 /g), whereas TiO 2 -SV presents smaller crystallites, a significant amorphous fraction, and a higher surface area (254.6 m 2 /g). Optical studies showed a band gap narrowing from 2.91 eV (TiO 2 -SG) to 2.70 eV (TiO 2 -SV) which can be attributed to defect-induced electronic states. The photocatalytic performance was evaluated under natural solar irradiation using a binary dye mixture of malachite green (MG) and methyl red (MR), thus providing a more realistic model of wastewater. The degradation efficiency was strongly influenced by operational parameters, with optimal performance achieved at pH 9 and catalyst doses of 1.2 mg/mL (TiO 2 -SG) and 0.8 mg/mL (TiO 2 -SV). Despite its superior adsorption capacity, TiO 2 -SV exhibited lower photocatalytic efficiency compared to TiO 2 -SG. Kinetic analysis revealed predominant pseudo-zero-order behavior for both catalysts, indicating surface saturation conditions, while competitive interactions between MG and MR reduced degradation rates in the binary mixture.