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.
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.
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