Extrinsic Point Defects in TiO₂–Acetylacetone Charge-Transfer Complex and Their Effects on Optical and Photochemical Properties

Abstract

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.

Tetracycline Removal through the Synergy of Catalysis and Photocatalysis by Novel NaYF4:Yb,Tm@TiO2-Acetylacetone Hybrid Core-Shell Structures

“…As a result, an increased number of reactive oxygen species are generated on the TiO 2 -Acac shell. As shown in some previous studies through the correlation between EPR-spin trapping and tetracycline photodegradation using scavengers [18,19], tetracycline degradation over TiO 2 -Acac occurs via interactions with electronic holes and, principally, superoxide radicals. On the other hand, catalytic activity of the NaYF 4 :Yb,Tmbased structures was corroborated through shifting of the tetracycline absorption band, originally situated at 358 nm.…”

“…The visible spectrum tail recorded for NaYF 4 :Yb,Tm@TiO 2 -Acac (Figure 5) may be fully ascribed to the ligand-to-metal charge transfer phenomenon between TiO 2 -Acac, where the electrons from the highest occupied molecular orbital (HOMO) of acetylacetone are directly injected into the conduction band (CB) of TiO 2 [17]. However, even more intense absorption within the visible spectrum of NaYF 4 :Yb,Tm@TiO 2 -Acac 300 can be additionally attributed to the extrinsic oxygen vacancies generated in the anatase shell layer through oxidation decomposition of Acac [18,19]. The same feature was recently reported for another charge transfer complex, namely, TiO 2glutaric acid, after calcination at 270 • C [20].…”

“…Bearing the above in mind, the current work presents, for the first time as the authors are aware, catalytic and photocatalytic activity, towards tetracycline, of the novel core-shell structures, in which TiO 2 -based shell is sensitive to the Vis light. We already showed that the formation of LMCT in TiO 2 -Acac nanoparticles effectively shifts TiO 2 absorption to Vis range, boosting its photocatalytic efficiency toward different pollutants [17][18][19]. In this work, an additional light-harvesting feature was achieved by the NIR-to-Vis conversion capability of β-NaYF 4 :Yb,Tm phase.…”

“…It Is relevant to note that tetracycline is one of the most widely used antibiotics in humans and in domestic animals, so developing effective sunlight-driven photocatalysts for its removal from wastewater represents a great challenge. To address this issue, besides TiO 2 -Acac and TiO 2 -glutaric acid charge transfer complexes [18][19][20], Co-TiO 2 mesoporous structures [21], CdS/TiO 2 , Cu 2 O/TiO 2 , and MoS 2 /TiO 2 composites [22][23][24], magnetic graphene oxide loaded TiO 2 :Ce [25], and TiO 2 /Fe 2 O 3 heterojunction [26] were tested up to now, but there is no report about photocatalytic performance of the UC@TiO 2 -based core-shell structures.…”

Extrinsic Oxygen Vacancies Formation during Crystallization of Al2W3O12 by Calcination in Air

“…Point defects play an essential role in the physical properties of materials, influencing a wide range of applications such as thermal and chemical sensors, optical devices, and transparent infrared windows. ,− In semiconductors and insulators, these defects can affect electrical, thermal, optical, and photochemical properties by modifying the band gap energy and, hence, its radiation absorption, by forming intermediate energy states within the band gap. ,, TiO 2 nanofibers showed a formation of oxygen vacancies when heat-treated in 100% Ar atmosphere, significantly lowering the optical band gap from 3.33 to 2.18 eV. MgFe 2 O 4 photocatalysts calcined in air displayed, on the other hand, an increase of the band gap as the calcination temperature and oxygen vacancies concentration increased, going from 1.99 eV at 650 °C to 2.17 eV at 750 °C.…”