Methylene Blue Photodegradation onto TiO2 Thin Films Sensitized with Curcumin: DFT and Experimental Study

Titanium dioxide (TiO2) thin films sensitized with curcumin were fabricated to investigate the influence of sensitization on their spectroscopic, optical, and photocatalytic properties. TiO2 films were prepared using different curcumin concentrations and characterized by FTIR, UV–Vis, and diffuse reflectance spectroscopy (DRS). The adsorption kinetics of curcumin on TiO2 were analyzed, and the photocatalytic performance was evaluated through methylene blue (MB) photodegradation under visible-light irradiation. FTIR spectra confirmed the successful anchoring of curcumin onto the TiO2 surface, while optical characterization revealed a significant enhancement in visible-light absorption. The band gap decreased from 3.2 eV (pure TiO2) to 1.8 eV (curcumin-sensitized TiO2). Furthermore, the curcumin adsorption onto semiconductor data fitted the pseudo-second-order kinetic model, yielding a maximum adsorption capacity of 12.0 mg·g−1. Density Functional Theory (DFT) calculations indicated that ligand-to-metal charge transfer (LMCT) transitions are responsible for the improved visible-light response. Photocatalytic tests demonstrated that all curcumin-sensitized TiO2 films were active under visible irradiation, confirming curcumin as an effective natural sensitizer for enhancing TiO2-based photocatalytic coatings.