Visible-Light-Active Black TiO₂ Nanoparticles with Efficient Photocatalytic Performance for Degradation of Pharmaceuticals

Special attention has recently been paid to surface-defective titanium dioxide and black TiO₂ with advanced optical, electrical, and photocatalytic properties. Synthesis of these materials for photodegradation and mineralization of persistent organic pollutants in water, especially under visible radiation, presents interest from scientific and application points of view. Chemical reduction by heating a TiO₂ and NaBH₄ mixture at 350 °C successfully introduced Ti³⁺ defects and oxygen vacancies at the surface of TiO₂, with an increase in the photocatalytic degradation of amoxicillin—an antibiotic that is present in wastewater due to its intense use in human and animal medicine. Three TiO₂ samples were prepared at different annealing temperatures to control the ratio between anatase and rutile and were subjected to chemical reduction. Electron paramagnetic resonance investigations showed that the formation of surface Ti³⁺ defects in a high concentration occurred mainly in the anatase sample annealed at 400 °C, contributing to the bandgap reduction from 3.32 eV to 2.92 eV. The reduced band gap enhances visible light absorption and the efficiency of photocatalysis. The nanoparticles of ~90 m²/g specific surface area and 12 nm average size exhibit ~100% efficiency in the degradation of amoxicillin under simulated solar irradiation compared with pristine TiO₂. Mineralization of amoxicillin and by-products was over 75% after 48 h irradiation for the anatase sample, where the Ti³⁺ defects were present in a higher concentration at the catalyst’s surface.