Extrinsic Point Defects in TiO 2 -Acetylacetone Charge-Transfer Complex and Their Effects on Optical and Photochemical Properties.

TiO 2 -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 TiO 2 and charge-transfer complexes (CTCs) have been formed between chelating ligands, such as acetylacetone, and nanocrystalline TiO 2 (TiO 2 -ACAC). However, the influence of extrinsic oxygen vacancies on the photocatalytic performance of TiO 2 -based CTCs is unknown. In this work, surface/bulk extrinsic oxygen vacancies were introduced into TiO 2 -ACAC through calcination at 270 °C under static air, argon, and hydrogen atmospheres. TiO 2 -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 TiO 2 -ACAC CTCs under low-power visible-light radiation. The increased extrinsic oxygen vacancies concentration was not beneficial for the photocatalytic performance of TiO 2 CTCs, since bulk extrinsic oxygen vacancies additionally act as recombination centers. In fact, the TiO 2 -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 TiO 2 -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 TiO 2 -based CTCs.