Kinetics study of mixed-phase TiO2 photocatalyst: Degradation of Rhodamine 6G and Methylene blue under UV-Vis irradiation

Abstract

This study aimed to investigate the photocatalytic degradation and reaction kinetics of Rhodamine 6G and Methylene Blue using sol gel synthesized mixed-phase Titanium dioxide (TiO 2 ). X-ray Diffraction (XRD) confirmed a 48.8%:51.2% rutile:anatase composition based on the (110) rutile and (101) anatase diffraction planes. Brunauer-Emmett-Teller (BET) surface analysis revealed a pore volume of 0.0216 cm 3 /g, confirming the mesoporous nature of the photocatalyst. UV-Vis spectroscopy showed absorption peaks at 430 nm and 323 nm, corresponding to band gap energies of 2.57 eV (rutile) and 3.14 eV (anatase), verifying the coexistence of both crystalline phases. Photocatalytic experiments demonstrated that R6G and MB reached 74% and 85% degradation under UV light after 120 minutes, while under visible light R6G and MB achieved 65% and 35% degradation, respectively. Kinetic modelling revealed that all systems follow first-order kinetics: MB exhibited rate constants of 0.01841 min -1 under UV (t 1 / 2 = 37.63 min) and 0.00440 min -1 under visible light (t 1 / 2 = 157.50 min). R6G showed first-order rate constants of 0.01051 min -1 under UV (t 1 / 2 = 65.96 min) and 0.00870 min -1 under visible light (t 1 / 2 = 79.66 min). Scavenger experiments identified superoxide (O 2 • - ) as the dominant reactive species under visible light irradiation. UV excitation favored hydroxyl (·OH) or singlet oxygen ( 1 O 2 ), depending on the dye. These findings demonstrate that phase-engineered TiO 2 with controlled rutile–anatase interfaces enhances charge separation and maximizes UV driven photocatalysis. Visible-light activity, however, is largely dye-dependent. Furthermore, this study provides mechanistic insight into mixed-phase TiO 2 for dye degradation and supports its potential use in solar-assisted water purification. • TiO 2 48.8% rutile and 51.2% anatase was synthesized using a sol-gel method. • XRD, UV-Vis, and TEM analyses confirm the anatase-rutile mixed-phase of the TiO 2 . • High UV photocatalytic activity: MB (85%) and R6G (74%) after 120 minutes. • Degradation of both dyes under visible light is primarily governed by superoxides.