Visible-Light Dye-Sensitized Activation of TiO2 for Phenol Degradation Under Mild Aqueous Conditions

A visible-light photocatalytic approach for TiO2 activation is presented using a phenothiazine–quinolinium cationic dye as a molecular photosensitizer. To minimize self-sensitization artifacts and improve mechanistic interpretation, phenol, a colorless and non-photosensitizing pollutant, was selected as the target contaminant. The pre-synthesized dye was adsorbed onto commercially available TiO2 without further modification of the TiO2 crystal lattice or metal doping. Under the employed white-LED irradiation and mild aqueous conditions, the system achieved approximately 70% phenol removal within 60 min. The experimental data were reasonably described by an apparent pseudo-first-order kinetic model (k ≈ 0.01478 min−1). Radical-scavenging experiments provided indirect evidence for the involvement of hydroxyl- and superoxide-related reactive species. Control experiments showed that dark adsorption, direct phenol photolysis, and the activity of bare TiO2 contributed only minimally under the investigated conditions, supporting a sensitization-assisted photocatalytic process. Supernatant analysis and four-cycle reuse experiments indicated predominantly heterogeneous behavior and short-term operational stability, with no substantial sensitizer release detectable by the applied UV–Vis method. The overall observations are consistent with a proposed sensitization-assisted pathway involving dye excitation, possible interfacial electron transfer to TiO2, oxygen reduction, and reactive oxygen species formation. The mechanistic interpretation is presented in accordance with the experimental evidence available in the present study. The results demonstrate the potential of molecular dye sensitization as an approach for extending the photocatalytic response of TiO2 under white-LED irradiation toward the removal of colorless organic contaminants from water.