TiO2-based visible-light-sensitive nanomaterials are widely studied for photocatalytic applications under UV–Vis radiation. Among the mechanisms of visible-light sensitization, extrinsic oxygen vacancies have been introduced into TiO2 and charge-transfer complexes (CTCs) have been formed between chelating ligands, such as acetylacetone, and nanocrystalline TiO2 (TiO2-ACAC). However, the influence of extrinsic oxygen vacancies on the photocatalytic performance of TiO2-based CTCs is unknown. In this work, surface/bulk extrinsic oxygen vacancies were introduced into TiO2-ACAC through calcination at 270 °C under static air, argon, and hydrogen atmospheres. TiO2-ACAC CTCs were characterized by X-ray powder diffraction, thermogravimetric analysis, diffuse-reflectance spectroscopy, photoluminescence, electron paramagnetic resonance (EPR), and X-ray photoelectron spectroscopy techniques. The correlation between EPR–spin trapping and tetracycline (TC) photodegradation, using scavengers, highlighted the key role of the superoxide radical in TC degradation by TiO2-ACAC CTCs under low-power visible-light radiation. The increased extrinsic oxygen vacancies concentration was not beneficial for the photocatalytic performance of TiO2 CTCs, since bulk extrinsic oxygen vacancies additionally act as recombination centers. In fact, the TiO2-ACAC CTC with the lowest extrinsic oxygen vacancies concentration exhibited the highest photocatalytic performance for TC degradation due to an adequate distribution of extrinsic bulk oxygen vacancies, which led to the trapped electrons undergoing repeated hopping, reducing the recombination rates and improving the efficiency in superoxide radicals production. Our findings indicated that TiO2-ACAC CTCs are able to degrade pollutants via interactions with electronic holes and principally superoxide radicals and also, provided fundamental information about the influence of surface/bulk extrinsic oxygen vacancies on the photocatalytic performance, lattice parameters, and optical and photochemical properties of TiO2-based CTCs.

No abstract

Novel hybrid core-shell structures, in which up-converting (UC) NaYF4:Yb,Tm core converts near-infrared (NIR) to visible (Vis) light via multiphoton up-conversion processes, while anatase TiO2-acetylacetonate (TiO2-Acac) shell ensures absorption of the Vis light through direct injection of excited electrons from the highest-occupied-molecular-orbital (HOMO) of Acac into the TiO2 conduction band (CB), were successfully synthesized by a two-step wet chemical route. Synthesized NaYF4:Yb,Tm@TiO2-Acac powders were characterized by X-ray powder diffraction, thermogravimetric analysis, scanning and transmission electron microscopy, diffuse-reflectance spectroscopy, Fourier transform infrared spectroscopy, and photoluminescence emission measurement. Tetracycline, as a model drug, was used to investigate the photocatalytic efficiencies of the core-shell structures under irradiation of reduced power Vis and NIR spectra. It was shown that the removal of tetracycline is accompanied by the formation of intermediates, which formed immediately after bringing the drug into contact with the novel hybrid core-shell structures. As a result, ~80% of tetracycline is removed from the solution after 6 h.