# **Heterogeneous Photocatalysis A Solution for a Greener Earth**

Edited by Stéphanie Lambert and Julien Mahy Printed Edition of the Special Issue Published in *Catalysts*

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## **Heterogeneous Photocatalysis: A Solution for a Greener Earth**

## **Heterogeneous Photocatalysis: A Solution for a Greener Earth**

Editors **St´ephanie Lambert Julien Mahy**

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*Editors* Stephanie Lambert ´ Department of Chemical Engineering University of Liege ` Liege ` Belgium

Julien Mahy Department of Chemical Engineering University of Liege ` Liege ` Belgium

*Editorial Office* MDPI St. Alban-Anlage 66 4052 Basel, Switzerland

This is a reprint of articles from the Special Issue published online in the open access journal *Catalysts* (ISSN 2073-4344) (available at: www.mdpi.com/journal/catalysts/special issues/ photocatal green).

For citation purposes, cite each article independently as indicated on the article page online and as indicated below:

LastName, A.A.; LastName, B.B.; LastName, C.C. Article Title. *Journal Name* **Year**, *Volume Number*, Page Range.

**ISBN 978-3-0365-6296-4 (Hbk) ISBN 978-3-0365-6295-7 (PDF)**

© 2023 by the authors. Articles in this book are Open Access and distributed under the Creative Commons Attribution (CC BY) license, which allows users to download, copy and build upon published articles, as long as the author and publisher are properly credited, which ensures maximum dissemination and a wider impact of our publications.

The book as a whole is distributed by MDPI under the terms and conditions of the Creative Commons license CC BY-NC-ND.

 She achieved her Ph.D. in Applied Sciences at the University of Liege in 2003, with her study entitled "Development ` of Pd, Ag and Cu based mono- and bimetallic cogelled catalysts and their use in hydrodechlorination and oxidation reactions". After working in the position of Engineer in a Belgian chemical company (2004–2005), and two postdoctoral stays at the DCE of the University of Illinois at Chicago in 2006 and at the Institute Charles Gerhardt in Montpellier in 2007, she joined DCE as FRS-FNRS Associate Researcher, in which she develops heterogeneous catalysts for sustainable chemistry. Since October 2022, she has been Director of the Research Unit of Chemical Engineering. Prof. Stephanie Lambert ´ has published over 115 publications, 12 book chapters, holds 1 patent, and has an h-index of 27. She has also delivered 25 invited lectures and she is the Guest Editor of two Special Issues in the journal *Catalysts* and one Special Issue in the journal *Processes*.

Key Topics: materials engineering; heterogeneous (photo)catalysis; sol–gel process; environmental engineering; inorganic biomaterials

ORCID : 0000-0001-9564-1270

Institutional website: https://www.chemeng.uliege.be/cms/c 3482790/fr/chemeng-repertoire? uid=u182186

Research Gate: https://www.researchgate.net/profile/Stephanie-Lambert-8 URL Google Scholar: https://scholar.google.be/citations?user=MhgEhjMAAAAJ&hl=fr

#### **Julien Mahy**

Julien G. Mahy is a chemical engineer at the University of Liege (ULi ` ege, Belgium). His Ph.D. ` thesis, conducted under the supervision of Prof. Stephanie Lambert and Prof. Beno ´ ˆıt Heinrichs, focused on the development of a TiO2 aqueous sol–gel process in order to produce, at large scale, photocatalysts with hydrophilic property and high activity, both under visible and UV/visible light, for water and air remediation. In 2018, after a short period working in the industry in the CRM group as a project leader, he accepted a postdoctoral position in collaboration between the NCE (ULiege) ` and the "Institut fur Energie- und Umwelttechnik e.V. (IUTA)", Duisburg (Germany) to work on the ¨ development of a quaternary treatment for water in order to degrade residual organic micropollutants using oxidation processes. From July 2019 to June 2021, he worked as a postdoctoral researcher at the Institute of Condensed Matter and Nanosciences (IMCN) at the Universite catholique de Louvain ´ under the supervision of Pr. Sophie Hermans. Their work focused on the development of low-cost sensors for the detection of air pollution by producing onion-like composite nanoparticles.

Since October 2021, Julien Mahy has been an FNRS postdoctoral researcher at the Universite´ de Liege under the supervision of Prof. St ` ephanie Lambert and Prof. Nathalie Job and at ´ the INRS, Quebec, under the supervision of Prof. Patrick Drogui. His work focuses on the ´ development of inorganic materials and processes in environmental applications such as water and air decontamination process by photocatalysis, adsorption and electrochemistry.

Key Topics: TiO2; photocatalysts; water treatment; sol–gel process; adsorption

Institutional website: https://www.chemeng.uliege.be/cms/c 3482790/fr/chemeng-repertoire? uid=u217056

Research Gate: https://www.researchgate.net/profile/Julien-Mahy?ev=hdr xprf URL Google Scholar: https://scholar.google.com/citations?hl=fr&user=ohH-zUgAAAAJ

### *Editorial* **Heterogeneous Photocatalysis: A Solution for a Greener Earth**

**Julien G. Mahy 1,2,\* and Stéphanie D. Lambert 1**

Since the beginning of the industrial era, various human activities have steadily increased, leading to rapid technological developments and high population growth. In consequence, the expanding industry has heavily polluted the atmosphere, soil, and water, with negative consequences for humans and the environment.

To decrease pollution emissions, various chemical, physical, and biological treatment methods have been developed. The major technics for treating wastewater are based on wastewater treatment plants using dry cleaning, decantation, and biological treatments. Occasionally, pollutant molecules are not eliminated by these processes; therefore, other technics can be used as secondary treatments to remove these small residual fractions of pollution. Among these methods, photocatalysis is a well-developed technic in the past few years. Through a photocatalyst and using light's energy, photocatalysis allows the production of highly reactive species that can react and decompose organic molecules, yielding, in the best case, the final decomposition products CO2 and H2O. The most commonly used photocatalysts are titanium dioxide (TiO2), zinc oxide (ZnO), and tin oxide (SnO2).

The papers contributing to this Special Issue address innovative photocatalytic processes for environmental applications. In what follows, we provide a synopsis of the obtained results in the 17 papers published in this Special Issue.

1. TiO2-based photocatalysts

In this collection of articles, TiO2 was used as the photocatalyst for depollution applications, and different doping and shaping were used to produce photoefficient materials.

Received: 16 June 2022 Accepted: 21 June 2022 Published: 23 June 2022 **Publisher's Note:** MDPI stays neutral

**Citation:** Mahy, J.G.; Lambert, S.D. Heterogeneous Photocatalysis: A Solution for a Greener Earth. *Catalysts* **2022**, *12*, 686. https:// doi.org/10.3390/catal12070686

with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

In a contribution by Almeida et al. [1], the efficiency of photodegradation, the volatilization profile of bidentates, and the role of reactive oxidizing species (ROS) were explored for nanocrystalline TiO2 modified with bidentate ligands (acetylacetone). In this study, TiO2- ACAC CTC calcined at 300 ◦C (TiO2-A300) was applied for the photocatalytic degradation of chlorophenol (4-CP) and tetracycline (TC) under low power visible light (26 W). Furthermore, the ROS scavengers isopropanol and benzoquinone were added for studying the photocatalytic role of •OH and •O2 − radicals. The photocatalytic abatement of tetracycline (68.6%), performed via TiO2-A300, was two times higher than that observed for chlorophenol (31.3%) after 6 h, indicating the distinct participation of ROS in the degradation of these pollutants. The addition of the ROS scavenger revealed •O2 − radicals as primarily responsible for the high efficiency of TiO2-ACAC CTC under reduced visible light. On the other hand, the •OH radicals were not efficiently generated in the CTC. Therefore, the development of heterostructures based on TiO2-ACAC CTC can increase the generation of ROS through coupling with semiconductors capable of generating •OH under visible light.

In the study of Pascariu et al. [2], Ag–TiO2 nanostructures were prepared via electrospinning, followed by calcination at 400 ◦C. Morphological characterization revealed the presence of one-dimensional uniform Ag–TiO2 nanostructured nanofibers, with a diameter from 65 to 100 nm, depending on the Ag loading, composed of small crystals interconnected

with each other. Structural characterization indicated that Ag was successfully integrated as small nanocrystals without affecting much of the TiO2 crystal lattice. Moreover, the presence of nano-Ag was found to contribute to reducing the band gap energy, which enables the activation by the absorption of visible light while, at the same time, delaying the electron–hole recombination. Tests of their photocatalytic activity in methylene blue, amaranth, Congo red, and orange II degradation revealed an increase by more than 20% in color removal efficiency at an almost double rate for the case of 0.1% Ag–TiO2 nanofibers compared with pure TiO2.

In another study [3], Mancuso et al. successfully prepared different tri-doped TiO2 photocatalysts (Fe-N-P/TiO2, Fe-N-S/TiO2, Fe-Pr-N/TiO2, Pr-N-S/TiO2, and P-N-S/TiO2) and tested them in the photocatalytic removal of thiacloprid (THI) under UV-A, visible, and direct solar light irradiation. The physical–chemical properties of the prepared catalysts revealed that dopants were effectively incorporated into the anatase TiO2 lattice, resulting in a decrease in the energy band gap. The reduction in photoluminescence intensity indicated a lower combination rate and longer lifespan of photogenerated carriers of all doped samples in comparison with the undoped TiO2. The doped photocatalysts promoted photodegradation under UV-A light irradiation and also extended the optical response of TiO2 to the visible light region. Fe-N-P tri-doped TiO2 sample exhibited the highest THI photodegradation degree (64% under UV-A light, 29% under visible light, and 73% under solar light).

In the contribution of Benkhennouche-Bouchene et al. [4], TiO2 that was prepared using a green aqueous sol–gel peptization process was co-doped with nitrogen and zirconium to improve and extend its photoactivity to the visible region. For all doped and co-doped samples, TiO2 nanoparticles with sizes ranging from 4 to 8 nm were formed of anatase–brookite phases. X-ray photoelectron (XPS) measurements showed that nitrogen was incorporated into the TiO2 materials through Ti–O–N bonds, allowing light absorption in the visible region. The XPS spectra of the Zr-(co)doped powders showed the presence of TiO2–ZrO2 mixed oxide materials. Under visible light, the best co-doped sample yielded a degradation of p-nitrophenol (PNP) equal to 70%, instead of 25% with pure TiO2 and 10% with P25 under the same conditions. Similarly, the photocatalytic activity improved under UV–Vis, reaching 95% with the best sample, compared with 50% with pure TiO2.

In the study of Eun et al. [5], sol–gel-synthesized N-doped and carbon–nitrogen– sulfur (CNS)-doped TiO2 solutions were deposited on upconversion phosphor using a dip-coating method. Scanning electron microscopy (SEM) imaging showed that there was a change in the morphology of TiO2 coated on NaYF4:Yb,Er from spherical to nanorods caused by additional urea and thiourea doping reagents. Fourier transform infrared (FTIR) spectroscopy further verified the existence of nitrate–hyponitrite, carboxylate, and SO4 2− because of the doping effect. NaYF4:Yb,Er composites coated with N- and CNSdoped TiO2 exhibited a slight shift in UV–Vis spectra toward the visible light region. The photocatalytic reactivity with CNS-doped TiO2/NaYF4:Yb,Er surpassed that of the undoped TiO2/NaYF4:Yb,Er for the MB solution and toluene. The photocatalytic activity was increased by CNS doping of TiO2, which improved light sensitization as a result of band gap narrowing due to impurity sites.

In the presented study by Mahy et al. [6], the objective was to improve the efficiency of TiO2 photocatalysts via activation treatments and through modification with palladium nanoparticles and doping with SiO2. X-ray diffraction provided evidence that the crystallographic structure of TiO2 was anatase and that Pd was present, either in its oxidized form after calcination or in its reduced form after reduction. The results on methylene blue degradation showed that the photocatalytic activity of the catalysts was inversely proportional to the content of silica present in the matrix. A small amount of silica improved the photocatalytic activity, compared with the pure TiO2 sample. By contrast, a high amount of silica delayed the crystallization of TiO2 in its anatase form. The introduction of Pd species increased the photocatalytic activity of the samples because it allowed for a decrease in the rate of electron–hole recombination in TiO2. The reduction treatment improved the activity

of photocatalysts, regardless of the palladium content, owing to the reduction of Ti4+ to Ti3+, and the formation of defects in the crystallographic structure of anatase.

A review of the green synthesis of TiO2 is also presented in this issue [7]. Indeed, in this study, the authors reported an eco-friendly process for producing TiO2 via colloidal aqueous sol–gel synthesis, resulting in crystalline materials without a calcination step. Three types of colloidal aqueous TiO2 were reviewed: the as-synthesized type obtained directly after synthesis, without any specific treatment; the calcined, obtained after a subsequent calcination step; and the hydrothermal, obtained after a specific autoclave treatment. This eco-friendly process is based on the hydrolysis of a Ti precursor in excess of water, followed by the peptization of the precipitated TiO2. Depending on the synthesis parameters, the three crystalline phases of TiO2 (anatase, brookite, and rutile) can be obtained. The morphology of the nanoparticles can also be tailored by the synthesis parameters. The most important parameter is the peptizing agent. Indeed, depending on its acidic or basic character and also on its amount, it can modulate the crystallinity and morphology of TiO2. Colloidal aqueous TiO2 photocatalysts are mainly being used in various photocatalytic reactions for organic pollutant degradation. The as-synthesized materials seem to have equivalent photocatalytic efficiency to the photocatalysts post-treated with thermal treatments and the commercial Evonik Aeroxide P25, which is produced via a high-temperature process.

TiO2 in form of film is also presented in the contribution of De Ceglie et al. [8]. In this study, the photocatalytic efficiency of an innovative UV-light catalyst consisting of a mesoporous TiO2 coating on glass fibers was investigated. Photocatalytic activity of the synthesized material was tested, for the first time, on a secondary wastewater effluent spiked with nine pharmaceuticals (PhACs), and the results were compared with the photolysis used as a benchmark treatment. Interestingly, the novel photocatalyst led to an increase in the degradation of carbamazepine and trimethoprim (about 2.2 times faster than the photolysis). Several transformation products (TPs) resulting from both the spiked PhACs and the compounds naturally occurring in the secondary wastewater effluent were identified through UPLC–QTOF/MS/MS. Some of them, produced mainly from carbamazepine and trimethoprim, were still present at the end of the photolytic treatment, while they were completely or partially removed by the photocatalytic treatment.

#### 2. Composite materials as photocatalysts

In the papers under this theme, different composite materials were produced to be used as photocatalysts.

In one study [9], Ghazzy et al. reported a visible-light-induced, trimetallic catalyst (Cu0.5Zn0.5Fe2O4) prepared through green synthesis using Tilia plant extract. The spinel crystalline material was ~34 nm. In benign reaction conditions, the prepared photocatalyst oxidized various benzylic alcohols with excellent yield and selectivity toward aldehyde with 99% and 98%, respectively. Aromatic and aliphatic alcohols (such as furfuryl alcohol and 1-octanol) were photocatalytically oxidized using Cu0.5Zn0.5Fe2O4, LED light, and H2O2 as oxidant, 2 h reaction time, and ambient temperature. The advantages of the catalyst were found in terms of reduced catalyst loading, activating catalyst using visible light in mild conditions, high conversion of the starting material, and recyclability of up to 5 times without loss of selectivity.

In the contribution of Mahy et al. [10], raw clays, from Cameroon, were modified with semiconductors (TiO2 and ZnO) to improve their depollution properties with the addition of photocatalytic properties. Cu2+ ions were also added to the clay via an ionic exchange to increase the specific surface area. The presence of TiO2 and ZnO was confirmed by the detection of anatase and wurtzite, respectively. The composite clays showed increased specific surface areas. The adsorption property of the raw clays was evaluated on two pollutants—namely, fluorescein (FL) and p-nitrophenol (PNP). The experiments showed that the raw clays can adsorb FL but were not efficient for PNP. To demonstrate the photocatalytic property resulting from the added semiconductors, photocatalytic experiments were performed under UV-A light on PNP. These experiments showed degradation up to

90% after 8 h of exposure with the best ZnO-modified clay. The proposed treatment of raw clays seems promising to treat pollutants, especially in developing countries.

In another contribution [11], Dao et al. prepared co-doped NiTiO3/g-C3N4 composite photocatalysts using a modified Pechini method to improve their photocatalytic activity toward methylene blue photodegradation under visible light irradiation. The combination of Co-doped NiTiO3 and g-C3N4 and Co-doping into the NiTiO3 lattice synergistically enhanced the photocatalytic performance of the composite photocatalysts. X-ray photoelectron spectroscopy results for the Co-doped NiTiO3/g-C3N4 composite confirmed Ti–N linkages between the Co-doped NiTiO3 and g-C3N4. In addition, characteristic X-ray diffraction peaks for the NiTiO3 lattice structure clearly indicated the substitution of Co into the N–TiO3 lattice structure. The composite structure and Co-doping of the C–x composite photocatalysts (x wt % Co-doped NiTiO3/g-C3N4) decreased the emission intensity of the photoluminescence spectra but also the semicircle radius of the Nyquist plot in electrochemical impedance spectroscopy, giving the highest Kapp value (7.15 × 10−3 min−1 ) for the Co-1 composite photocatalyst.

In the study of Bazta et al. [12], pure and Ce-modified ZnO nanosheet-like polycrystalline samples were successfully synthesized via a microwave-based process. The XRD results showed that the obtained photocatalysts were composed of hexagonal, wurtzitetype crystallites in the 34–44 nm size range. The microscopy showed nanosheet-shaped crystallites, with a composition close to Ce0.68Zn0.32Ox. Importantly, the STEM–XEDS characterization of the photocatalyst samples revealed that Ce did not incorporate into the ZnO crystal lattice as a dopant but that a heterojunction formed between the ZnO nanosheets and the Ce–Zn mixed oxide phase nanoparticles. The optical analysis revealed that in the ZnO:Ce samples optical band gap was found to decrease to 3.17 eV in the samples with the highest Ce content. It was also found that the ZnO:Ce (2 at.%) sample exhibited the highest photocatalytic activity for the degradation of methylene blue (MB) when compared with both the pure ZnO and commercial TiO2–P25 under simulated sunlight irradiation.

3. Pilot and comparative process studies, and other photocatalytic processes

In this last collection of papers, pilot reactors were tested for larger-scale photocatalytic processes, and other advanced oxidative processes were explored.

In one study [13], Faccani et al. offered easily scalable solutions for adapting TiO2 based photocatalysts, which were deposited on different kinds of fabrics and implemented in a 6 L semipilot plant, using the photodegradation of rhodamine B (RhB) as a model of water pollution. They took advantage of a multivariable optimization approach to identify the best design options in terms of photodegradation efficiency and turnover frequency (TOF). Surprisingly, in the condition of use, the irradiation with a light-emitting diode (LED) visible lamp appeared as a valid alternative to the use of UV LED. The identification of the best design options in the semipilot plant allowed scaling up the technology in a 100 L pilot plant suitable for the treatment of industrial wastewater.

In the presented paper by Gonzalez-Burciaga et al. [14], several advanced oxidative processes were used to degrade 6-mercaptopurine (6-MP), a commonly used cytostatic agent. To degrade 6-MP, three processes were applied: photolysis (UV-C), photocatalysis (UV-C/TiO2), and their combination with H2O2. Each process was performed with variable initial pH (3.5, 7.0, and 9.5). Pilot-scale reactors were used, using UV-C lamps as a radiation source. Kinetic calculations for the first 20 min of reaction proved the significance of the addition of H2O2: In UV-C experiments, the highest k was reached under pH 3.5, k = 0.0094 min−1 , while under UV-C/H2O2, k = 0.1071 min−1 was reached under the same initial pH; similar behavior was observed for photocatalysis since k values of 0.0335 and 0.1387 min−1 were calculated for UV-C/TiO2 and UV-C/TiO2/H2O2 processes, respectively, also under acidic conditions. Degradation percentages here reported for UV-C/H2O2 and UV-C/TiO2/H2O2 processes were above 90% for all tested pH values.

In another contribution [15], Gonçalves et al. investigated the impact of different oxidation processes on the maprotiline degradation pathways via liquid chromatography–highresolution mass spectrometry (LC/HRMS) experiments. Semiconductors photocatalysts—

namely, Fe–ZnO, Ce–ZnO, and TiO2—proved to be more efficient than heterogeneous photo-Fenton processes in the presence of hydrogen peroxide and persulfate. No significant differences were observed in the degradation pathways in the presence of photocatalysis, while the SO4 −-mediated process promoted the formation of different transformation products (TPs). Species resulting from ring openings were observed with higher persistence in the presence of SO4 −. In silico tests on mutagenicity, developmental/reproductive toxicity, Fathead minnow LC50, D. Magna LC50, and fish acute LC50 were carried out to estimate the toxicity of the identified transformation products. Low toxicant properties were estimated for TPs resulting from hydroxylation onto the bridge rather than aromatic rings, as well as those resulting from ring openings.

In the contribution of Tejera et al. [16], an integral treatment process for landfill leachate reverse osmosis concentrate (LLROC) was designed and assessed, aiming to reduce organic matter content and conductivity, as well as to increase its biodegradability. The process consisted of three steps. First, a coagulation/flocculation treatment (removal of the 76% chemical oxygen demand (COD), 57% specific ultraviolet absorption (SUVA), and 92% color) was utilized. In the second step, a photo-Fenton process resulted in enhanced biodegradability (i.e., the ratio between the biochemical oxygen demand (BOD5) and the COD increased from 0.06 to 0.4), and an extra 43% of the COD was removed at the best-trialed reaction conditions. A UV-A light-emitting diode lamp was tested and compared with conventional high-pressure mercury vapor lamps, achieving a 16% power consumption reduction. Finally, an optimized 30 g L−1 lime treatment was implemented, which reduced conductivity by 43%, and the contents of sulfate, total nitrogen, chloride, and metals by 90%. Overall, the integral treatment of LLROC achieved the removal of 99.9% color, 90% COD, 90% sulfate, 90% nitrogen, 86% Al, 77% Zn, 84% Mn, 99% Mg, and 98% Si, in addition to significantly increasing biodegradability up to BOD5/COD = 0.4.

In the presented paper by Selvanathan et al. [17], the green synthesis of nickel oxide nanoparticles using phytochemicals from three different sources was employed to synthesize nickel oxide nanoparticles (NiOx NPs) as an efficient nanomaterial-based electrocatalyst for water splitting. Nickel (II) acetate tetrahydrate was reacted in presence of aloe vera leaves extract, papaya peel extract, and dragon fruit peel extract, respectively, and the physicochemical properties of the biosynthesized NPs were compared with sodium hydroxide (NaOH)-mediated NiOx. Based on the average particle size calculation from Scherrer's equation, using X-ray diffractograms and field-emission scanning electron microscope analysis revealed that all three biosynthesized NiOx NPs had smaller particle sizes than that synthesized using the base. Aloe-vera-mediated NiOx NPs exhibited the best electrocatalytic performance, with an overpotential of 413 mV at 10 mA cm−2 and a Tafel slope of 95 mV dec−1 . Electrochemical surface area (ECSA) measurement and electrochemical impedance spectroscopic analysis verified that the high surface area, efficient charge-transfer kinetics, and higher conductivity of aloe-vera-mediated NiOx NPs contribute to its low overpotential values.

In conclusion, this Special Issue entitled "Heterogeneous Photocatalysis: A Solution for a Greener Earth" gives an overview of the latest advances in the development of innovative photocatalytic processes. These studies pave the path for the development of innovative processes for a greener Earth.

Finally, we are grateful to all authors for their valuable contributions and to the editorial team of *Catalysts* for their kind support, especially to Pamela Li for her constant help and availability during this Special Issue submission and publication process.

**Funding:** This research received no external funding.

**Acknowledgments:** J.G.M. and S.D.L. are grateful to F.R.S.-F.N.R.S. for, respectively, his postdoctoral position and her Senior Research Associate position.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

## *Article* **TiO2-Acetylacetone as an Efficient Source of Superoxide Radicals under Reduced Power Visible Light: Photocatalytic Degradation of Chlorophenol and Tetracycline**

**Lucas A. Almeida , Anja Dosen, Juliana Viol and Bojan A. Marinkovic \***

Department of Chemical and Materials Engineering, Pontifical Catholic University of Rio de Janeiro (PUC-Rio), Rio de Janeiro 22453-900, Brazil<EMAIL_ADDRESS>(L.A.A<EMAIL_ADDRESS>(A.D<EMAIL_ADDRESS>(J.V.)

**\*** Correspondence<EMAIL_ADDRESS>
**Abstract:** Visible light-sensitive TiO2 -based nanomaterials are widely investigated for photocatalytic applications under high power (≥300 W) UV and visible light. The formation of charge transfer complexes (CTCs) between bidentate ligands and nanocrystalline TiO2 promotes visible light absorption and constitutes a promising alternative for environmental remediation under reduced visible light power. However, the efficiency of photodegradation, the volatilization profile of bidentates, and the role of reactive oxidizing species (ROS) are not fully understood. In this study, thermogravimetric analyses coupled with mass spectroscopy (TGA-MS) were performed on TiO2 -Acetylacetone (ACAC) CTC. TiO2 -ACAC CTC calcined at 300 ◦C (TiO2 -A300) was applied for the photocatalytic degradation of chlorophenol (4-CP) and tetracycline (TC) under low power visible light (26 W). Furthermore, the ROS scavengers isopropanol and benzoquinone were added for studying the photocatalytic role of •OH and •O2 − radicals. The TGA-MS showed the release of ACAC fragments, such as ethyl ions and acetone, in the range between 150 ◦C and 265 ◦C, while between 300 ◦C and 450 ◦C only CO2 and H2O were released during oxidation of ACAC. The photocatalytic abatement of tetracycline (68.6%), performed by TiO2 -A300, was ~two times higher than that observed for chlorophenol (31.3%) after 6 h, indicating a distinct participation of ROS in the degradation of these pollutants. The addition of the ROS scavenger revealed •O2 − radicals as primarily responsible for the high efficiency of TiO2 -ACAC CTC under reduced visible light. On the other hand, the •OH radicals are not efficiently generated in the CTC. Therefore, the development of heterostructures based on TiO2 -ACAC CTC can increase the generation of ROS through coupling with semiconductors capable of generating •OH under visible light.

**Keywords:** ligand-to-metal charge transfer; anatase; oxygen-based bidentate diketone; sol-gel; TGA-MS; remediation of aqueous pollutants

#### **1. Introduction**

The intensive use of drugs for the treatment of human health and industrial wastewater disposal are potential sources of contamination of aqueous effluents [1–4]. Currently, water treatment plants are not designed for the remediation of pharmaceutical and personal care products (PPCPs) [1,2]. An example of PPCPs is the antibiotic tetracycline, used to prevent bacterial infections, found in sewage from the pharmaceutical industry, hospitals, and livestock [5,6]. The remediation of organic compounds, such as chlorophenol, is a priority due to its high toxicity, low biodegradability, and carcinogenic and endocrine disruptive properties [4,7].

In this context, photocatalysis (an advanced oxidative process) is an alternative and sustainable technology for the remediation of these pollutants. The development of new photocatalysts mainly seeks to increase the capacity of the absorption of sunlight, i.e., to expand absorption from the ultra-violet (UV) region to the visible spectrum, since the

**Citation:** Almeida, L.A.; Dosen, A.; Viol, J.; Marinkovic, B.A. TiO2-Acetylacetone as an Efficient Source of Superoxide Radicals under Reduced Power Visible Light: Photocatalytic Degradation of Chlorophenol and Tetracycline. *Catalysts* **2022**, *12*, 116. https:// doi.org/10.3390/catal12020116

Academic Editors: Stéphanie Lambert and Julien Mahy

Received: 17 December 2021 Accepted: 7 January 2022 Published: 19 January 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

visible region comprises ~45% of the solar spectrum [6,8–12]. In addition, the maximization of the photogeneration of reactive oxygen species (ROS), such as •O2 − and •OH, plays a key role in photocatalytic activity [13,14]. This maximization occurs through the increase in the formation of e−/h+ pairs and, consequently, their interaction with the O2 and H2O molecules to generate •O2 − and •OH, respectively.

Thus, the visible light sensitization mechanism denominated as the ligand-to-metal charge transfer (LMCT) has gained recognition in the development of TiO2-based nanomaterials [15], although the effect of point defects should also be considered [16–18]. The LMCT mechanism promotes, in theory, the efficient photogeneration of superoxide radicals under visible light [16] and consists of the direct injection of e− from the highest occupied molecular orbital (HOMO) of a chelating ligand into the conduction band (CB) of a semiconductor, such as TiO2 [19]. The LMCT mechanism occurs via bonds formed between the chelating ligands (small organic molecules) and the TiO2 surface [15,16,19].

The LMCT complex based on TiO2 and acetic acid (monodentate ligand) showed photodegradation of phenol higher than 90% after 1.5 h exposure to a blue light-emitting diode (LED) lamp (20 W) [20]. In addition, another LMCT complex, made of TiO2 and salicylic acid (bidentate ligand), revealed the potential for the selective aerobic oxidation of amines to imines under blue LED irradiation (3 W), having a selective yield of 92% of the desired product, N-benzylidenebenzylamine [21]. Other LMCT complexes of TiO2 and bidentate ligands, such as glucose [22] and alizarin [23], showed ~100% photocatalytic reduction of Cr (VI) during 1 h under visible light irradiation (Xenon lamp 300 W). Furthermore, the nanocrystalline charge transfer complex (CTC) between TiO2 and acetylacetone (bidentate) calcinated at 300 ◦C in air revealed ~100% NOx gas photodegradation for 2 h under visible light irradiation (24 W) [16]. Another study on the amorphous TiO2-Acetylacetone complex revealed ~90% degradation of 2,4-dichlorophenol after 24 h in the absence of irradiation in the presence of high catalyst concentration (1 g·L −1 ) [24]. However, as the authors are aware, a CTC such as TiO2-bidentate has not been investigated thus far for photocatalytic applications in drug degradation.

Moreover, the understanding of the role of reactive radicals on the photocatalytic activity of these complexes is not fully understood. Experimental analyses performed by electron paramagnetic resonance (EPR) [16,25] and theoretical studies based on density functional theory (DFT) reported for the TiO2-Acetylacetone [25] and TiO2-Thiosalicylic acid [26] complexes demonstrate the capacity of the LMCT mechanism to produce superoxide (•O2 −). On the other hand, Fourier transform infrared (FTIR) analysis reveals the presence of adsorbed hydroxyls on the surface of these LMCT complexes [16,20–22,25] that may participate in the formation of •OH if electronic holes exist in the valence band (VB) of TiO2. However, to the best of our knowledge, the individual potential of ROS in the LMCT systems has not been evaluated for photocatalytic applications. The ROS scavengers have been, however, already employed to other photocatalysts in photocatalytic measurements in the liquid medium to assess the individual potential of each ROS [4,6,27–30].

The measurements of photocatalytic degradation of chlorophenol and tetracycline by CTC, combined with the use of •O2 − and •OH scavengers, would clarify the actual potential of each ROS in the photocatalytic abatement.

Photocatalytic degradation in a liquid medium under visible light irradiation is mostly carried out with a light power superior to 300 W [30–33]. The high energy consumption of the lamps follows the opposite path to the sustainable nature of photocatalysis. Therefore, a lamp with reduced power such as residential lamps of 26 W will be applied in this research for the degradation of aqueous pollutants.

This study has as the main goals (1) the evaluation of the potential of TiO2-Acetylacetone CTC calcinated at 300 ◦C in air for the photodegradation of chlorophenol and tetracycline under reduced power visible light (26 W) and (2) an understanding of ROS (•O2 − and •OH) roles in photocatalysis performed by TiO2-Acetylacetone using scavengers, such as benzoquinone and isopropanol. Furthermore, the volatilization profile of TiO2-ACAC CTC

was studied up to 550 ◦C by thermogravimetric analysis coupled with mass spectroscopy (TGA-MS) to confirm the presence of acetylacetone at temperatures above 150 ◦C. and TiO2-A300 and their first derivatives (DTG) (Figure S1, see Supplementary Materials) were in accordance with the results previously reported by Almeida et al. [16].

scavengers, such as benzoquinone and isopropanol. Furthermore, the volatilization profile of TiO2-ACAC CTC was studied up to 550 °C by thermogravimetric analysis coupled with mass spectroscopy (TGA-MS) to confirm the presence of acetylacetone at tempera-

TiO2-A-RT, TiO2-ACAC, and TiO2-A300 were analyzed by TGA-MS to confirm the presence of ACAC above 150 °C through the release of ACAC fragments or the release of

The TGA curve of the TiO2-A-RT sample (Figure 1a) revealed an expressive mass loss of 29.5 wt.% at temperatures below 150 °C due to dehydration of the material. Furthermore, the mass loss between 150 °C and 450 °C is associated with the release of acetylacetone, as documented by TGA-MS results (Figure 1b). The mass loss of organic species between 150 °C and 450 °C was ~15 wt.% and approximately equal to the mass loss of the TiO2-ACAC xerogel for the same temperature region [16]. The TGA curves of TiO2-ACAC

#### **2. Results** Figure 1b illustrates the evolution profiles of released gases as a function of temperature for the TiO2-A-RT sample. In the first temperature stage of volatilization between 30

tures above 150 °C.

the entire ACAC molecule on heating.

**2. Results**

#### *2.1. Evaluation of TiO2-ACAC CTC Volatilization Profile by TGA-MS* °C and 150 °C, species with *m*/*z* of 17, 18, 19, and 20, corresponding to water were detected

*2.1. Evaluation of TiO2-ACAC CTC Volatilization Profile by TGA-MS*

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TiO2-A-RT, TiO2-ACAC, and TiO2-A300 were analyzed by TGA-MS to confirm the presence of ACAC above 150 ◦C through the release of ACAC fragments or the release of the entire ACAC molecule on heating. [34]. In the second temperature stage between 150 °C and ~265 °C, the *m*/*z* of 43, 44, and 58 were identified, owing to the release of acetyl ions (CH3C≡O+), CO2, and acetone, respectively [35]. Bowie et al. [35] showed that *m*/*z* of 43 and 58 are associated with the frag-

The TGA curve of the TiO2-A-RT sample (Figure 1a) revealed an expressive mass loss of 29.5 wt.% at temperatures below 150 ◦C due to dehydration of the material. Furthermore, the mass loss between 150 ◦C and 450 ◦C is associated with the release of acetylacetone, as documented by TGA-MS results (Figure 1b). The mass loss of organic species between 150 ◦C and 450 ◦C was ~15 wt.% and approximately equal to the mass loss of the TiO2- ACAC xerogel for the same temperature region [16]. The TGA curves of TiO2-ACAC and TiO2-A300 and their first derivatives (DTG) (Figure S1, see Supplementary Materials) were in accordance with the results previously reported by Almeida et al. [16]. mentation of the ACAC molecule, where the most intense peak of the ACAC volatilization in the mass spectrum is *m*/*z* 43, belonging to acetyl ions. Acetone and CO2 release in this temperature range are in accordance with Acik et al. [36] who observed the release of both species from the amorphous TiO2-ACAC xerogel. In the third temperature region of mass loss between 300 °C and 450 °C, an increase of CO2 release was identified (*m*/*z* = 44 and 45) together with water release, which is in accordance with the observation of Acik et al. [36] and Madarász et al. [37] for amorphous TiO2-ACAC xerogel and crystalline titanium oxobis(acetylacetonate), respectively.

**Figure 1.** (**a**) TGA and DTG curves of TiO2-A-RT sample; (**b**) Evolution profiles of gaseous species as monitored by TGA-MS for TiO2-A-RT sample. **Figure 1.** (**a**) TGA and DTG curves of TiO2 -A-RT sample; (**b**) Evolution profiles of gaseous species as monitored by TGA-MS for TiO2 -A-RT sample.

The profiles of gaseous species released from nanocrystalline TiO2-ACAC and TiO2- A300 samples are shown in Figure 2. The volatilization stages of TiO2-ACAC (Figure 2a) follow a similar path as TiO2-A-RT (Figure 1). TiO2-ACAC water release occurs in the Figure 1b illustrates the evolution profiles of released gases as a function of temperature for the TiO2-A-RT sample. In the first temperature stage of volatilization between 30 ◦C and 150 ◦C, species with *m*/*z* of 17, 18, 19, and 20, corresponding to water were detected [34]. In the second temperature stage between 150 ◦C and ~265 ◦C, the *m*/*z* of 43, 44, and 58 were identified, owing to the release of acetyl ions (CH3C≡O+ ), CO2, and acetone, respectively [35]. Bowie et al. [35] showed that *m*/*z* of 43 and 58 are associated with the fragmentation of the ACAC molecule, where the most intense peak of the ACAC volatilization in the mass spectrum is *m*/*z* 43, belonging to acetyl ions. Acetone and CO2 release in this temperature range are in accordance with Acik et al. [36] who observed the release of both species from the amorphous TiO2-ACAC xerogel. In the third temperature region of mass loss between 300 ◦C and 450 ◦C, an increase of CO2 release was identified (*m*/*z* = 44 and 45) together with water release, which is in accordance with the observation of Acik et al. [36] and Madarász et al. [37] for amorphous TiO2-ACAC xerogel and crystalline titanium oxobis(acetylacetonate), respectively.

The profiles of gaseous species released from nanocrystalline TiO2-ACAC and TiO2-A300 samples are shown in Figure 2. The volatilization stages of TiO2-ACAC (Figure 2a) follow a similar path as TiO2-A-RT (Figure 1). TiO2-ACAC water release occurs in the temperature range between 30 ◦C and 150 ◦C, followed by a two-stage release of molecules due to ACAC oxidation above 150 ◦C. In the interval from 150 ◦C to ~265 ◦C, only acetyl ions

(*m*/*z* = 43) and CO2 (*m*/*z* = 44) were released. In addition, in the third temperature stage (300–450 ◦C), CO2 (*m*/*z* = 44 and 45) and water release were reported. The absence of acetone release in the TiO2-ACAC xerogel probably occurred due to overnight drying at 100 ◦C, reducing the excess ACAC content in the sample. stage between 300 °C and 450 °C was due to CO2 and water release. The volatilization step of the ACAC oxidation products above 300 °C, for all studied TiO2-ACAC CTC samples, indirectly contributes to proving that the ACAC is responsible for the sensitization of TiO2 in visible light in the samples calcined at 300 °C, as previously reported [16]. due to ACAC oxidation above 150 °C. In the interval from 150 °C to ~265 °C, only acetyl ions (*m*/*z* = 43) and CO2 (*m*/*z* = 44) were released. In addition, in the third temperature stage (300 °C –450 °C), CO2 (*m*/*z* = 44 and 45) and water release were reported. The absence of acetone release in the TiO2-ACAC xerogel probably occurred due to overnight drying

temperature range between 30 °C and 150 °C, followed by a two-stage release of molecules

On the other hand, the TGA-MS data of the TiO2-A300 sample (Figure 2b) revealed only two distinct temperature stages of gases volatilization. The first stage between 30 °C and 150 °C was related to a release of water (*m*/*z* = 17, 18 e 19, and 20), while the second

temperature range between 30 °C and 150 °C, followed by a two-stage release of molecules due to ACAC oxidation above 150 °C. In the interval from 150 °C to ~265 °C, only acetyl ions (*m*/*z* = 43) and CO2 (*m*/*z* = 44) were released. In addition, in the third temperature stage (300 °C –450 °C), CO2 (*m*/*z* = 44 and 45) and water release were reported. The absence of acetone release in the TiO2-ACAC xerogel probably occurred due to overnight drying

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at 100 °C, reducing the excess ACAC content in the sample.

at 100 °C, reducing the excess ACAC content in the sample.

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**Figure 2.** Evolution profiles of gaseous species as monitored by TGA-MS for (**a**) TiO2-ACAC and (**b**) TiO2-A300 samples. **Figure 2.** Evolution profiles of gaseous species as monitored by TGA-MS for (**a**) TiO2 -ACAC and (**b**) TiO2 -A300 samples.

*2.2. Chlorophenol Photodegradation and ROS Efficiency under Reduced Power Visible Light*  Figure 3 shows the photocatalytic degradation of chlorophenol using TiO2-A300 CTC during the period of 6 h. Photolysis did not promote pollutant degradation (green curve). On the other hand, the TGA-MS data of the TiO2-A300 sample (Figure 2b) revealed only two distinct temperature stages of gases volatilization. The first stage between 30 ◦C and 150 ◦C was related to a release of water (*m*/*z* = 17, 18 e 19, and 20), while the second stage between 300 ◦C and 450 ◦C was due to CO2 and water release. The volatilization step of the ACAC oxidation products above 300 ◦C, for all studied TiO2-ACAC CTC samples, indirectly contributes to proving that the ACAC is responsible for the sensitization of TiO2 in visible light in the samples calcined at 300 ◦C, as previously reported [16]. (**a**) (**b**) **Figure 2.** Evolution profiles of gaseous species as monitored by TGA-MS for (**a**) TiO2-ACAC and (**b**) TiO2-A300 samples.

#### *2.2. Chlorophenol Photodegradation and ROS Efficiency under Reduced Power Visible Light 2.2. Chlorophenol Photodegradation and ROS Efficiency under Reduced Power Visible Light*

Figure 3 shows the photocatalytic degradation of chlorophenol using TiO2-A300 CTC during the period of 6 h. Photolysis did not promote pollutant degradation (green curve). Figure 3 shows the photocatalytic degradation of chlorophenol using TiO2-A300 CTC during the period of 6 h. Photolysis did not promote pollutant degradation (green curve).

**Figure 3.** Chlorophenol photodegradation by TiO2-A300 photocatalyst with and without the addition of scavengers. The green curve stands for photolysis. **Figure 3.** Chlorophenol photodegradation by TiO2 -A300 photocatalyst with and without the addition of scavengers. The green curve stands for photolysis.

The TiO2-A300 CTC revealed photocatalytic abatement of 31.3% ± 3.1% for 4-CP after 6 h (Figure 3). Li et al. [32] quantified visible light abatement of 4-CP as 29.9% after 2 h using graphene grafted titania/titanate nanosheets under UV-Vis light of 500 W for the same concentrations of pollutants and photocatalyst used in our study. Furthermore, 4-CP degradation using TiO2-A300 was nearly half of the degradation reported by Li et al. [32] after 2 h, however, with a light source with 20 times lower power. The TiO2-A300 CTC revealed photocatalytic abatement of 31.3% ± 3.1% for 4-CP after 6 h (Figure 3). Li et al. [32] quantified visible light abatement of 4-CP as 29.9% after 2 h using graphene grafted titania/titanate nanosheets under UV-Vis light of 500 W for the same concentrations of pollutants and photocatalyst used in our study. Furthermore, 4- CP degradation using TiO2-A300 was nearly half of the degradation reported by Li et al. [32] after 2 h, however, with a light source with 20 times lower power.

The addition of the •OH radical scavenger (IPA) did not significantly impact the degradation efficiency of TiO2-A300 (Figure 3). The observed photodegradation of 4-CP was 32.8% ± 3.1% after 6 h and was within the error of the degradation measured for TiO2-A300 without the addition of the IPA scavenger. On the other hand, Moraes et al. [4] revealed that with the addition of the IPA, the degradation of 4-CP (10 mg·L −1 ) using nanometric anatase (0.2 g·L −1 ) was about two times lower in comparison to the degradation without the addition of the IPA, however, under UV light and after 3 h. Therefore, these data indicate that hydroxyl radicals played an important role in the degradation of 4-CP for TiO2 under UV light, since it promoted the formation of e−/h+ pairs and, consequently, •OH radials. The results reported by Moraes et al. [4] are in accordance with studies that show that •OH radicals have a dominant role in photodegradation of 4-CP [32,38]. However, for TiO2-A300 CTC under visible light, it was not possible to detect the role of •OH radicals in the photodegradation of 4-CP. Therefore, our results strongly indicate that TiO2-A300 CTC does not efficiently generate •OH radicals under visible light. The addition of the •OH radical scavenger (IPA) did not significantly impact the degradation efficiency of TiO2-A300 (Figure 3). The observed photodegradation of 4-CP was 32.8% ± 3.1% after 6 h and was within the error of the degradation measured for TiO2- A300 without the addition of the IPA scavenger. On the other hand, Moraes et al. [4] revealed that with the addition of the IPA, the degradation of 4-CP (10 mg·L−1) using nanometric anatase (0.2 g·L−1) was about two times lower in comparison to the degradation without the addition of the IPA, however, under UV light and after 3 h. Therefore, these data indicate that hydroxyl radicals played an important role in the degradation of 4-CP for TiO2 under UV light, since it promoted the formation of e–/h+ pairs and, consequently, •OH radials. The results reported by Moraes et al. [4] are in accordance with studies that show that •OH radicals have a dominant role in photodegradation of 4-CP [32,38]. However, for TiO2-A300 CTC under visible light, it was not possible to detect the role of •OH radicals in the photodegradation of 4-CP. Therefore, our results strongly indicate that TiO2-A300 CTC does not efficiently generate •OH radicals under visible light.

On the other hand, the addition of the •O2 − scavenger (BQ) caused a strong reduction of the photodegradation potential of TiO2-A300 (Figure 3). The 4-CP abatement after the addition of BQ was only 10.9% ± 2.1% after 6 h interval. Therefore, the •O2 − plays an important role in the photocatalytic activity of TiO2-A300 during the degradation of 4-CP. According to Su et al. [39] and Fónagy et al. [40], benzoquinone is predominantly reduced by •O2 − radicals and e− and, consequently, degraded. Therefore, •O2 − generated by TiO2-A300 CTC preferentially degrades BQ molecules instead of degrading 4-CP, as illustrated in Figure 4. The expressive 76.7% ± 2.1% BQ degradation and low 4-CP degradation after 6 h demonstrate the efficient generation of •O2 − radicals under visible light from TiO2-A300. On the other hand, the addition of the •O2− scavenger (BQ) caused a strong reduction of the photodegradation potential of TiO2-A300 (Figure 3). The 4-CP abatement after the addition of BQ was only 10.9% ± 2.1% after 6 h interval. Therefore, the •O2− plays an important role in the photocatalytic activity of TiO2-A300 during the degradation of 4-CP. According to Su et al. [39] and Fónagy et al. [40], benzoquinone is predominantly reduced by •O2− radicals and e– and, consequently, degraded. Therefore, •O2− generated by TiO2- A300 CTC preferentially degrades BQ molecules instead of degrading 4-CP, as illustrated in Figure 4. The expressive 76.7% ± 2.1% BQ degradation and low 4-CP degradation after 6 h demonstrate the efficient generation of •O2− radicals under visible light from TiO2- A300.

**Figure 4. Figure 4.** The absorbance of the BQ band situated at 246 nm and the 4-CP band at 224 nm, over time. The absorbance of the BQ band situated at 246 nm and the 4-CP band at 224 nm, over time.

#### *2.3. Tetracycline Photodegradation and ROS Efficiency under Reduced Power Visible Light* TiO2 core-shell heterostructure decorated by Pt nanoparticles (0.08 g·L−1) under UV-Vis light at 300 W after 2 h. However, the performance of TiO2-A300 was achieved with 11.5

*2.3. Tetracycline Photodegradation and ROS Efficiency under Reduced Power Visible Light*

photolysis (green curve) did not show pollutant degradation during the test time.

Figure 5 shows the photocatalytic degradation of tetracycline using TiO2-A300. The

The TiO2-A300 CTC showed a high photocatalytic degradation of 68.6% ± 3.4% of TC after 6 h (Figure 5). In addition, the maximum photodegradation of TC up to the abatement stabilization was 83% after 11 h (Figure S2). This maximum abatement of TC from TiO2-A300 was similar to the 86.7% degradation reported by Wang et al. [6] for the WO3-

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Figure 5 shows the photocatalytic degradation of tetracycline using TiO2-A300. The photolysis (green curve) did not show pollutant degradation during the test time. times lower light power with the use of visible instead of UV light and without the use of precious metals.

**Figure 5.** Tetracycline photodegradation by TiO2-A300 with and without the addition of scavengers. The green curve stands for photolysis. **Figure 5.** Tetracycline photodegradation by TiO2 -A300 with and without the addition of scavengers. The green curve stands for photolysis.

The use of isopropanol as the •OH scavenger revealed an abatement of 74.6% ± 8.7% after 6 h, within the error bars of the degradation without the addition of the IPA scavenger. In contrast, TC degradation under UV-Vis light by heterostructures, such as ZnO/GO/Ag3PO4 [41], Ag3PO4/AgBr/g-C3N4 [42], and Bi2W2O9/g-C3N4 [43] revealed a decrease in TC abatement due to the addition of IPA. These results reported for the heterostructures indicate that •OH radicals contribute to the degradation of TC when generated [5]. Therefore, the result obtained for TiO2-A300 suggests that TiO2-A300 is not capable of producing •OH under visible light. The TiO2-A300 CTC showed a high photocatalytic degradation of 68.6% ± 3.4% of TC after 6 h (Figure 5). In addition, the maximum photodegradation of TC up to the abatement stabilization was 83% after 11 h (Figure S2). This maximum abatement of TC from TiO2-A300 was similar to the 86.7% degradation reported by Wang et al. [6] for the WO3-TiO2 core-shell heterostructure decorated by Pt nanoparticles (0.08 g·L −1 ) under UV-Vis light at 300 W after 2 h. However, the performance of TiO2-A300 was achieved with 11.5 times lower light power with the use of visible instead of UV light and without the use of precious metals.

The TC photodegradation with the addition of BQ was significantly reduced to 21.8% ± 7.6% using TiO2-A300 after 6 h (Figure 5). The reduction observed in our study agrees with the literature, where different ceramic heterojunctions showed low TC degradation efficiency with the addition of BQ under high power UV or UV-Vis light sources (>300 W) [6,42,44]. He et al. [5] reported that •O2− and H2O2 radicals are capable of attacking the benzene ring of tetracycline, oxidizing the N-dimethyl group and the -C(O)NH2 group, suggesting that •O2− plays an important role in the photodegradation pathway of TC. Therefore, the TC degradation behavior observed in Figure 5 is mainly due to the efficient The use of isopropanol as the •OH scavenger revealed an abatement of 74.6% ± 8.7% after 6 h, within the error bars of the degradation without the addition of the IPA scavenger. In contrast, TC degradation under UV-Vis light by heterostructures, such as ZnO/GO/Ag3PO4 [41], Ag3PO4/AgBr/g-C3N4 [42], and Bi2W2O9/g-C3N4 [43] revealed a decrease in TC abatement due to the addition of IPA. These results reported for the heterostructures indicate that •OH radicals contribute to the degradation of TC when generated [5]. Therefore, the result obtained for TiO2-A300 suggests that TiO2-A300 is not capable of producing •OH under visible light.

generation of •O2− radicals by TiO2-A300 CTC. The TC photodegradation with the addition of BQ was significantly reduced to 21.8% ± 7.6% using TiO2-A300 after 6 h (Figure 5). The reduction observed in our study agrees with the literature, where different ceramic heterojunctions showed low TC degradation efficiency with the addition of BQ under high power UV or UV-Vis light sources (>300 W) [6,42,44]. He et al. [5] reported that •O2 − and H2O2 radicals are capable of attacking the benzene ring of tetracycline, oxidizing the N-dimethyl group and the -C(O)NH2 group, suggesting that •O2 − plays an important role in the photodegradation pathway of TC. Therefore, the TC degradation behavior observed in Figure 5 is mainly due to the efficient generation of •O2 − radicals by TiO2-A300 CTC.

#### **3. Discussion**

The TGA-MS results of TiO2-A-RT, TiO2-ACAC, and TiO2-A300 samples evidenced the fragments of ACAC oxidation release, such as acetyl ions, acetone, CO2, and water between 150 ◦C and ~265 ◦C, while only CO2 and water release were detected at higher temperatures between 300 ◦C and 450 ◦C (Figures 1b and 2). Two distinct steps of ACAC fragments release for TiO2-A-RT and TiO2-ACAC, between 150 ◦C and 450 ◦C, are in accordance with the TGA-MS and FTIR-MS analysis reported by Acik et al. [36] for amorphous TiO2-ACAC xerogel. The volatilization profile of ACAC from TiO2-A300 (Figure 2b) converges well with the mass loss reported by Almeida et al. [16] for the same material since CO2 and water were released as ACAC oxidation products above 300 ◦C. Therefore, it is worth noting for the understanding of the CTC mechanism that nanocrystalline TiO2-ACAC samples preserved a part of ACAC when calcined at temperatures higher than 150 ◦C, such as 300 ◦C, as applied for TiO2-A300.

The results on the photocatalytic activity of TiO2-A300 support the hypothesis that a TiO2-ACAC CTC would be able to degrade aqueous pollutants under reduced power visible light. The TiO2-A300 showed photocatalytic potential for degradation of both aqueous pollutants 4-CP and TC. In addition, TiO2-A300 did not show adsorption of pollutants in the dark 1 h before turning on visible light (Figure S3). Therefore, the abatement of all the pollutants was the result of the photocatalytic activity of TiO2-ACAC CTC.

In our previous study [16], TiO2-A300 revealed a high efficiency (~100%) for photodegradation of NOx gas (100 ppm) during 2 h under reduced power visible light (24 W). Therefore, the results presented in the current research additionally increase the photocatalytic applicability of TiO2-ACAC CTC.

Zhu et al. [41] reported 96.3% degradation of TC (30 mg·L −1 ) after 75 min, with ~50% of the reported degradation values associated with the adsorption phenomenon in the dark. The authors used for this purpose ZnO/GO/Ag3PO4 photocatalyst (1 g·L −1 ) under low power visible light (65 W). Therefore, photodegradation of TC reported in our study after 6 h (68.6%) was higher than documented by Zhu et al. (46.3%) [41] with 2.5 lower visible light power and using four times less photocatalyst. Additionally, the capacity of degradation of TC is ~two times higher than that observed for 4-CP using TiO2-A300, after 6 h, indicating a distinct participation of ROS in the degradation of these two pollutants.

#### *ROS Generation Efficiency in TiO2-A300 CTC*

One of the main goals of our study was understanding the roles of ROS (•O2 − and •OH) in photocatalysis performed by TiO2-A300 CTC, using BQ and IPA scavengers. The photodegradation of 4-CP and TC under visible light with the addition of •O2 − scavenger confirms the efficient generation of •O2 − radicals by TiO2-ACAC CTC (Figures 3 and 5), in accordance with the EPR characterization and DFT studies carried out on LMCT complexes [16,25,26]. On the other hand, the •OH radicals were not efficiently generated by TiO2-A300 CTC under visible light, resulting in the lower photodegradation of 4-CP than the photodegradation of TC, since •OH radicals are the dominant ROS in the 4-CP photodegradation [4,7,32], while •O2 − radicals play a key role in TC degradation [5].

Figure 6 represents an update of the photocatalytic reactions conducted by TiO2-A300 CTC for the ROS generation and consequent degradation of 4-CP and TC. Additionally, the photocatalytic reactions for hydrogen production are also presented in Figure 6.

**Figure 6.** Scheme of electronic bands in TiO2-A300 CTC and photocatalytic reactions for ROS generation. The position of BV and BC of anatase at redox potential scale, in accordance with normal hydrogen electrode (NHE), was adopted from literature [8,9], as well as the positions of the ROS species. The position of HOMO of acetylacetone was previously established in [16]. The position of LUMO of acetylacetone was arbitrarily added, since it does not have a role in photocatalytic and water splitting events and is higher than 3.2 eV. **Figure 6.** Scheme of electronic bands in TiO2 -A300 CTC and photocatalytic reactions for ROS generation. The position of BV and BC of anatase at redox potential scale, in accordance with normal hydrogen electrode (NHE), was adopted from literature [8,9], as well as the positions of the ROS species. The position of HOMO of acetylacetone was previously established in [16]. The position of LUMO of acetylacetone was arbitrarily added, since it does not have a role in photocatalytic and water splitting events and is higher than 3.2 eV.

Figure 6 shows the electron transfer from ACAC HOMO to TiO2 (anatase) CB and, consequently, the formation of •O2− radicals due to the reaction between O2 adsorbed on the surface of anatase and free e– from CB (Equation (1)). Figure 6 shows the electron transfer from ACAC HOMO to TiO2 (anatase) CB and, consequently, the formation of •O2 − radicals due to the reaction between O2 adsorbed on the surface of anatase and free e− from CB (Equation (1)).

$$\rm O\_2 + e^- \rightarrow \rm {^\bullet O\_2}^-,\tag{1}$$

tion medium [9]. A possible additional explanation for the O2 presence in an aqueous solution is the water splitting mechanism (Equation (2)) [8,9]. The h+ generated in the HOMO of the ACAC may promote water splitting, i.e., decomposing H2O into O2 and H+ through an oxidation half reaction as shown in Figure 6. 2 H2O + 4 h+ → O2 + 4 H+, (2) The efficient •O2 − generation is also a consequence of a high O2 presence in the reaction medium [9]. A possible additional explanation for the O2 presence in an aqueous solution is the water splitting mechanism (Equation (2)) [8,9]. The h+ generated in the HOMO of the ACAC may promote water splitting, i.e., decomposing H2O into O2 and H+ through an oxidation half reaction as shown in Figure 6.

$$2\text{ H}\_2\text{O} + 4\text{ h}^+ \rightarrow \text{O}\_2 + 4\text{ H}^+,\tag{2}$$

which is another ROS capable of degradation of aqueous pollutants due to its ability to attack benzene rings and the N-dimethyl group [5]. The energy barrier of the formation of H2O2 from •O2− radicals (Equation (3)) is 0.3 eV below the normal hydrogen electrode potential (NHE) [13]. Therefore, a part of •O2− generated by TiO2-A300 can be transformed into H2O2 and can continue to degrade 4-CP and TC. •O2− + e− + 2 H+ → O2 + H2O2, (3) Additionally, the efficient production of •O2 − radicals can also form H2O2 [9,45], which is another ROS capable of degradation of aqueous pollutants due to its ability to attack benzene rings and the N-dimethyl group [5]. The energy barrier of the formation of H2O2 from •O2 − radicals (Equation (3)) is 0.3 eV below the normal hydrogen electrode potential (NHE) [13]. Therefore, a part of •O2 − generated by TiO2-A300 can be transformed into H2O2 and can continue to degrade 4-CP and TC.

$$\text{\textbullet O}\_2\text{\textbullet }^\text{-} + \text{e}^- + 2\text{ H}^+ \rightarrow \text{O}\_2 + \text{H}\_2\text{O}\_2\text{\textbullet }^\text{-} \tag{3}$$

2 H+ + 2 e− → H2, (4)

tron holes from HOMO of ACAC are capable of oxidizing water, forming H+ (Equation (2)), and the e– in the CB of TiO2 may reduce H+ to H2 (Equation (4)). As seen in Figure 6, the arrangement of CB of TiO2 and HOMO of ACAC in the TiO2- ACAC CTC may provide photocatalytic production of H2 from water splitting. The electron

holes from HOMO of ACAC are capable of oxidizing water, forming H+ (Equation (2)), and the e– in the CB of TiO2 may reduce H+ to H2 (Equation (4)).

$$2\ \text{H}^+ + 2\ \text{e}^- \to \text{H}\_2\tag{4}$$

Finally, some previous studies demonstrated the photocatalytic potential of Z-scheme heterostructures based on WO3 [6,46], SnO2 [47], and BiVO4 [48] for TC and NOx degradation and hydrogen production under UV-Vis light. These low-bandgap semiconductors can absorb visible light and generate •OH radicals via VB and CB with positive redox potential. The Z-scheme heterojunction consists of the recombination of the CB electrons of these low-bandgap semiconductors with the VB of another semiconductor with a lower positive redox potential, providing a reduction in the recombination rate of e−/h+ pairs in the low-bandgap semiconductors, such as WO3, SnO2, and BiVO4 and, consequently, an increase in the formation of •OH radicals. Therefore, coupling TiO2-ACAC CTC with semiconductors capable of generating •OH radicals under visible light may maximize the generation of ROS and increase of photocatalytic efficiency of these new heterojunctions.

#### **4. Materials and Methods**

#### *4.1. Materials*

All reagents were purchased from Sigma-Aldrich and used as obtained. Titanium isopropoxide (Ti(OiPr)4, 97%), acetylacetone (ACAC, ≥99%), ethanol (≥99.8%), and nitric acid (65%) were used for the synthesis of TiO2-ACAC CTC nanoparticles. The aqueous pollutants were chlorophenol (4-CP, ≥99%) and tetracycline (TC, ≥98%). Benzoquinone (BQ, ≥98%) and isopropanol (IPA, ≥99.5%) were used as ROS scavengers.

#### *4.2. Synthesis of TiO2-Acetylacetone Charge Transfer Complex*

The synthesis of TiO2 anatase nanoparticles coupled with acetylacetone (CTC) was carried out by the sol-gel route as reported by Scolan and Sanchez [49]. In addition, as previously reported [12,16], the molar ratios for hydrolysis (H = [H2O]/[Ti]), acidity (H+ = [H+ ]/[Ti]), and complexing (A = [ACAC]/[Ti]) were kept at 100, 0.027 and 2, respectively, to produce TiO2-ACAC xerogel with mean crystal size ~2.5 nm.

In the adopted procedure, 30 mL of Ti(OiPr)4 was added dropwise into a solution of 20 mL of ACAC with 100 mL of ethanol (1:5 *v*/*v*). The obtained yellow solution remained under magnetic stirring for 40 min at room temperature. Afterward, 180 mL of HNO3 solution (0.015 M) was dropped slowly into the yellowish solution under continuous stirring. The dark orange solution obtained was heated to 60 ◦C and kept under magnetic stirring for 8 h. Next, the sol of TiO2 was dried overnight in Petri dishes at room temperature, and a gel was formed (TiO2-A-RT). Finally, the gel was dried at 100 ◦C overnight to obtain a red-yellowish xerogel of TiO2-Acetylacetone (TiO2-ACAC). The TiO2-ACAC xerogel, grounded in an agate mortar, was calcined in air at 300 ◦C for 2 h in a Tubular Maitec-INTI FET 1600/H furnace. The as-prepared powder was denoted TiO2-A300.

#### *4.3. Thermogravimetric Analysis Coupled with Mass Spectroscopy (TGA-MS)*

Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were performed on a Perkin-Elmer Simultaneous Thermal Analyzer STA-6000 under synthetic air flow (130 mL·min−1 ) with a heating rate of 10 ◦C·min−1 and within the temperature range between 30 ◦C and 550 ◦C. A mass of ~25 mg was used.

To identify the volatilized species from TiO2-ACAC and confirm the presence of ACAC in the TiO2-A300 sample, a mass spectrometer OmniStar/ThermoStar-GSD 320 O3 (Pfeiffer Vacuum) was coupled to the STA-6000. The *m*/*z* range until 105 was scanned with the measuring time of 0.5 s·amu−1 .

#### *4.4. Measurement of the Photodegradation of Chlorophenol and Tetracycline and the Efficiency of ROS*

The photocatalytic potential of TiO2-A300 for degradation of 5 mg·L −1 of 4-CP and TC under visible light irradiation was evaluated. In accordance with [16], the inorganic part of TiO2-A300 CTC is composed of nanocrystalline anatase. Two other materials, TiO2- A-RT and TiO2-ACAC (where the inorganic part also consists of nanocrystalline anatase), were not evaluated since they previously showed inferior photocatalytic efficiency [16]. In addition, a measurement of the TC photodegradation until stabilization of TC abatement was performed.

All measurements were performed with 0.2 g·L −1 of the photocatalyst. The light source used was a DULUX D/E 26 W residential fluorescent lamp with an irradiance of 0.23 W·cm−2 and an emission of light in the wavelength range from 400 nm to 700 nm. The photocatalytic system is shown in Figure S4. The lamp is housed in a cylindrical quartz bulb (length 11 cm, internal diameter 3.2 cm, and thickness 0.15 cm) unable to absorb visible light. The photoreactor was immersed in a bath with cold water circulation to maintain the system at room temperature. The photocatalytic tests were carried out under vigorous magnetic stirring in two stages. In the first stage, the pollutant and the photocatalyst were kept in the dark for 1 h to attain the absorption–desorption equilibrium. After this period, the lamp was turned on, and 5 mL aliquots of supernatant were acquired at intervals of 1 h for 6 h. The supernatant was filtered through a Merck Millipore filter (0.45 µm) and analyzed by an Agilent UV-Vis spectrophotometer (model 8453). The absorption at the wavelengths of 224 nm and 358 nm was accompanied for photodegradation of 4-CP and TC during the test time, respectively. The observed reduction in these wavelengths for each pollutant was used as input to determine the percentage of photodegradation performed by TiO2-A300 CTC, as seen in Figure S5 and Table S1. The degradation data obtained were presented through the mean value together with the respective standard deviations (Table S1) since all photocatalytic tests were performed in triplicate to ensure repeatability of results.

The ROS efficiency in TiO2-ACAC CTC was evaluated by adding the scavenger molecule BQ (5.4 mg·L −1 ; 0.05 mmol·L −1 ) to inhibit the participation of the •O2 − radical in the degradation of 4-CP and TC. On the other hand, the addition of the IPA scavenger (6.0 mg·L −1 ; 0.1 mmol·L −1 ) was performed to inhibit the participation of the •OH radical. The scavengers were added to the 5.0 mg·L −1 solutions of 4-CP or TC before starting the photocatalytic tests to ensure a mass ratio close to 1:1 between the pollutant and the scavenger.

#### **5. Conclusions**

The TGA-MS indirectly proved the presence of ACAC on TiO2-A-RT, TiO2-ACAC xerogel, and TiO2-A300 at temperatures higher than 150 ◦C. The release of acetyl ions and acetone between 150 ◦C and ~265 ◦C was detected, while at temperatures above 300 ◦C, and not higher than 450 ◦C, the release of CO2 and water was detected as the products of ACAC oxidation.

The TiO2-A300 photodegrades 4-CP and TC under reduced power visible light. The photocatalytic abatement of tetracycline (68.6%) was ~two times higher than that observed for chlorophenol (31.3%) after 6 h, indicating a distinct potential of ROS in the degradation of these pollutants.

The addition of BQ scavenger on the photocatalytic degradation of 4-CP and TC proved TiO2-A300 CTC to be a powerful source of superoxide (•O2 −) radicals under visible light, promoting a high photodegradation of TC (68.6%) after 6 h. However, the •OH scavenger does not promote the reduction in photodegradation of 4-CP and TC, proving TiO2-ACAC CTC is not an efficient generator of •OH radicals under visible light.

The photocatalytic potential reported for TiO2-A300 may stimulate new studies on hydrogen production process from water splitting under visible light due to the redox potential arrangement of CB of TiO2 and HOMO of ACAC. Furthermore, the development of heterostructures based on TiO2-A300 CTC, and similar CTC, can increase the generation of ROS through coupling with semiconductors capable of generating •OH under visible light and, therefore, capable of formation of more powerful photocatalysts.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/article/10 .3390/catal12020116/s1, Figure S1: TGA and DTG curves of (a) TiO2-ACAC xerogel and (b) TiO2- A300 sample, Figure S2: Tetracycline photodegradation by TiO2-A300, Figure S3: Curves of adsorption (1 h in dark) and photocatalytic activity of 4-CP and TC using TiO2-A300 of (a) 4-CP and (b) TC by TiO2-A300 without scavengers addition during 6 h, Figure S4: Photocatalytic system for aqueous pollutants degradation, Figure S5: Absorbance of (a) 4-CP band, situated at 224 nm and (b) TC band at 358 nm, over the time, Table S1: 4-CP and TC degradation data obtained in the first repetition of the photocatalytic test using TiO2 -A300.

**Author Contributions:** Conceptualization, L.A.A. and B.A.M.; synthesis and overall characterizations, L.A.A.; funding acquisition, B.A.M.; data interpretation, L.A.A. and B.A.M.; TGA-MS analysis, A.D. and J.V.; TGA-MS interpretation, A.D. and J.V.; writing—original draft preparation, L.A.A.; writing—review and editing, L.A.A., A.D. and B.A.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** The authors are grateful to FAPERJ for financial support through the projects E-26/210.585/2019 and E-26/210.046/2021.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data underlying this article will be shared on reasonable request from the corresponding author.

**Acknowledgments:** This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior-Brasil (CAPES)–Finance Code 001. B.A.M. is grateful to CNPq (National Council for Scientific and Technological Development) for a Research Productivity Grant. L.A. is grateful to ANP (Agência Nacional do Petróleo, Gás Natural e Biocombustíveis) and Finep (Funding Authority for Studies and Projects) for a doctoral degree scholarship.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

## *Article* **Innovative Ag–TiO2 Nanofibers with Excellent Photocatalytic and Antibacterial Actions**

**Petronela Pascariu 1,\*, Corneliu Cojocaru 1 , Anton Airinei 1 , Niculae Olaru 1 , Irina Rosca 1 , Emmanouel Koudoumas 2 and Mirela Petruta Suchea 2,3,\***

**Abstract:** Ag–TiO2 nanostructures were prepared by electrospinning, followed by calcination at 400 ◦C, and their photocatalytic and antibacterial actions were studied. Morphological characterization revealed the presence of one-dimensional uniform Ag–TiO2 nanostructured nanofibers, with a diameter from 65 to 100 nm, depending on the Ag loading, composed of small crystals interconnected with each other. Structural characterization indicated that Ag was successfully integrated as small nanocrystals without affecting much of the TiO2 crystal lattice. Moreover, the presence of nano Ag was found to contribute to reducing the band gap energy, which enables the activation by the absorption of visible light, while, at the same time, it delays the electron–hole recombination. Tests of their photocatalytic activity in methylene blue, amaranth, Congo red and orange II degradation revealed an increase by more than 20% in color removal efficiency at an almost double rate for the case of 0.1% Ag–TiO2 nanofibers with respect to pure TiO2 . Moreover, the minimum inhibitory concentration was found as low as 2.5 mg/mL for *E. coli* and 5 mg/mL against *S. aureus* for the 5% Ag–TiO2 nanofibers. In general, the Ag–TiO2 nanostructured nanofibers were found to exhibit excellent structure and physical properties and to be suitable for efficient photocatalytic and antibacterial uses. Therefore, these can be suitable for further integration in various important applications.

**Keywords:** nano-Ag–TiO2 nanostructured nanofibers; electrospinning; UV-visible light assisted photocatalytic activity; photodegradation kinetics; antimicrobial activity

#### **1. Introduction**

Titanium dioxide (TiO2) is considered a suitable compound for decomposing wastes and antimicrobial action due to its photocatalytic nature and because it is a chemically stable, non-toxic, inexpensive, and quite safe substance. In particular, various types of photocatalysts have been developed using TiO2 for wastewater treatment employing various methods including immobilized catalyst systems, membrane separation, and gravitational separation systems. As the TiO2 semiconductor band gap energy (3.2 eV) limits its activity when used in visible light, in many of these studies, an important issue to be considered is the activation of the photocatalysts with solar radiation, as this can improve the energy efficiency and consequently the economic viability of the process. In that respect, TiO2 is doped with various elements to enhance visible light activity. As a result, numerous photocatalysts have been developed, such as composite, co-doped, and co-catalysts compounds, their photocatalytic performance depending strongly on their morphological, structural, and textural properties [1–8].

There are many studies regarding the antifungal and antibacterial properties of TiO2 nanoparticles in various forms against a broad range of both Gram-positive and Gram-

**Citation:** Pascariu, P.; Cojocaru, C.; Airinei, A.; Olaru, N.; Rosca, I.; Koudoumas, E.; Suchea, M.P. Innovative Ag–TiO2 Nanofibers with Excellent Photocatalytic and Antibacterial Actions. *Catalysts* **2021**, *11*, 1234. https://doi.org/10.3390/ catal11101234

Academic Editors: Stéphanie Lambert, Julien Mahy and Detlef W. Bahnemann

Received: 10 August 2021 Accepted: 9 October 2021 Published: 13 October 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

negative bacteria, properties needed in sectors such as food, textiles, medicine, water disinfection, and food packaging [9,10]. In general, the antimicrobial activity of nanostructured TiO2 is greatly dependent on the photocatalytic performance of TiO2, which depends strongly on its morphological, structural, and textural properties, as was mentioned before [1–4].

Studies have shown that the crystalline structure and morphology of TiO2 nanoparticles, are influenced by growth process parameters such as temperatures, starting concentration of precursors, pH, etc. The potential health impact and toxicity to the environment of nanomaterials is also currently an important matter to be addressed. As an example, metal oxide nanoparticles (NPs) conventionally synthesized using chemical methods, have shown different levels of toxicity to test organisms [11–13]. Their toxicity seems to be mainly related to the small size that permits easy penetration through cellular membranes and its light dependent properties. One way to avoid nanoparticulate free circulation is to immobilize them onto substrates or larger structures [14].

Regarding the improvement of the electron–hole pair generation and the enlargement of the spectral absorption domain of TiO2, many studies have been performed related to the inclusion of metal/non-metal ions in the structure of TiO2, the dye functionalization on the TiO2 surface in dye-sensitized solar cells (DSSC), and the growth of noble metals onto the TiO2 surface [15]. Silver (Ag) is among the most interesting metals used as a dopant to modify the structure of TiO2 because it has the particular property to prevent the recombination of electron–hole pairs. Moreover, Ag can generate surface plasmon resonance with TiO2 under visible light. These changes provided by Ag doping lead to a significant improvement of the photocatalytic activity, a fact confirmed by other authors [16–18]. On the other hand, Ag nanoparticles possess a broad spectrum of antibacterial, antifungal, and antiviral properties. Ag nanoparticles have the ability to penetrate bacterial cell walls, changing the structure of cell membranes and even resulting in cell death. Their efficacy is due not only to their nanoscale size but also to their large ratio of surface area to volume. They can increase the permeability of cell membranes, produce reactive oxygen species, and interrupt the replication of deoxyribonucleic acid by releasing silver ions.

There are studies showing that the inclusion of Ag in the structure of TiO2 leads to enhance photocatalytic efficiency, as well as antimicrobial properties. Over the years, a large volume of reported research was focused on obtaining 0D systems (nanoparticles) based on Ag doped TiO2, which were tested in terms of photocatalytic and antimicrobial performances [13,19–22]. As an example, studies showed that TiO2-NPs had efficient antimicrobial activity against *E. coli*, *S. aureus*, methicillin-resistant *S. aureus*, *K. pneumoniae* [23,24]. However, little attention was paid to the development of Ag doped TiO2 nanofibers by the electrospinning-calcination technique, as well as studies of their performances in photocatalytic dye degradation and antimicrobial action. As an example, Zhang et al. [15] prepared hierarchical structures composed of TiO2 fibers on which Ag nanoparticles were grown to improve the photocatalytic efficiency for Rhodamine B (RhB) dye degradation. Moreover, nano-Ag-decorated TiO2-nanofibres proved that the inclusion of Ag exhibited an increased antimicrobial effect on *S. aureus* and *E. coli* [25]. Recently, Roongraung et al. [18] reported the photocatalytic performance of Ag doped TiO2 nanofibers for photocatalytic glucose conversion.

Although the research on TiO2 has a very long history and its applications are almost countless as the respective publications are too, this semiconductor has the potential to offer even today very interesting results worth being further investigated. This paper reports the development and optimization of pure TiO2 and Ag–TiO2 photocatalytic nanostructured nanofibers, fabricated by electrospinning followed by calcination at high temperature. Nanofibers can be well stacked together in larger 3D structures, thus, they can better be immobilized in membranes or other kinds of support, offering at the same time a much larger active surface than that for the case of nano-powders, while they can easily be recovered from the active medium used in applications. At the same time, the presence of Ag can lead to both an effective photocatalytic activity and antimicrobial action and

a sunlight-driven activation, as compared to pure TiO2. Therefore, the integration of such materials into food packaging, medical textiles, and other healthcare related items can be a quite innovative approach for real-life applications. In the present study, we report the fabrication of materials with remarkable photocatalytic efficiency of 99% at a constant rate of k = 1.29 × 10−2 min−1 that was found for the 0.1% Ag–TiO2 sample for the case of Methylene Blue dye degradation. Additionally, these materials proved able to degrade up 99% of other organic dyes with constant rates in the range of 4.57 × 10−3 to 7.28 × 10−2 min−1 depending on the dye nature, all degradation tests being performed under UV-visible light irradiation from a halogen lamp, without intervening to acidify the solution or add H2O2. These materials proved to be also reusable.

#### **2. Results**

#### *2.1. X-rays Diffraction (XRD) Analysis*

Typical XRD diffractograms of the reference pure TiO2 and the Ag–TiO2 nanostructured nanofibers with different Ag content, analyzed in the range of 2θ = 5–80◦ are shown in Figure 1.

**Figure 1.** X-ray diffraction patterns of pure TiO2 and Ag–TiO2 nanostructured nanofibers materials with different Ag content.

The grazing incidence X-ray diffraction patterns present multiple diffraction peaks, assigned unambiguously as anatase phase with lattice parameters: a = b = 0.38 nm; c = 0.94 nm according to ICDD (International Center for Diffraction Data) database card No. 001-0562. The diffraction peaks located at 25.30◦ , 37.96◦ , 47.94◦ , 54.41◦ , 54.93◦ , 62.77◦ , 69.24◦ , and 75.17◦ , correspond to the (101), (103), (200), (105), (211), (213), (116) and (107) Miller indices. An investigation of the position of the diffraction peaks shows that the lattice constant of anatase is preserved for different Ag content (the respective values for the (101) diffraction peak are shown in Table 1). In order to identify the diffraction peaks, in Figure 1 the positions of Ag diffraction peaks were added according to card no. 001-1164. It is clear that numerous very small diffraction peaks remain unassigned. It

exists a possibility to be an unknown organic compound contaminant, but usually, the organic compounds give diffraction peaks below 30◦ . Additionally, it exists the possibility that some traces of unknown titanates compounds to appear (brookite was unsuccessfully checked). The second possibility is more plausible, as the Energy Dispersive X-ray Analysis (EDX) investigations did not show the presence of other elements. However, the very small peaks cannot be assigned to any expected material.

**Table 1.** Structural parameters of pure TiO2 , and Ag–TiO2 nanostructured nanofibers.

Spacing distance between crystallographic planes (dhkl), the lattice parameters a and c, the crystallite size (D), microstrain (ε) and band gap energy (*E*g).

> As the lattice constant remains unmodified, these findings suggest that interstitial doping of Ag did not take place. This observation is strengthened by the observation that the size of the crystalline domains remains practically unchanged at different Ag content. The mean crystallite size was evaluated with two independent methods, Scherrer's equation and Rietveld refinement [26,27]. The respective values are presented in Table 1. In addition, Rietveld refinement allowed the evaluation of the lattice strain, which was increasing from 0.13% (pure TiO2 sample) to 0.52% (TAg1), reaching 0.53% at the highest Ag content (Table 1). Even though Ag was detected by EDX (as will be shown later), there is no diffraction peak of Ag observed in the XRD patterns, a fact that can be attributed to its existence as small crystals having a size below the resolution limit of the technique, observation that agrees with previous literature data [28,29]. In any case, the XRD findings indicate that the Ag content variation did not affect the unit cell of TiO2 or the crystal quality.

#### *2.2. Morphological Characterization*

The morphological characterization of the nanostructured nanofibers was carried out by Scanning Electron Microscopy (SEM) analysis. Representative samples of Ag–TiO2 nanostructured nanofibers, after calcination at 400 ◦C, were investigated and their images are presented in Figure 2.

As can observe in Figure 2, one-dimensional uniform Ag–TiO2 nanostructured nanofibers were obtained. The average value of the fiber diameters was evaluated from the measurement of 100 diameters for each sample using Image-J software. Histogram of fiber diameter distribution plotted from SEM images analysis of pure TiO2 (T400) and Ag doped TiO2 (TAg1 and TAg5) are given in Figure S1 of the Supplementary material. The average width of the fibers was found to vary between 65 and 100 nm, depending on the Ag loading, the mean fiber diameter decreasing as the Ag concentration increases. It was also observed that Ag doping leads to a slight variation of the dimensional distribution of nanofiber widths. As the Ag concentration increases, less thinner fibers are present. Moreover, it is interesting to indicate the particular nanostructuring of the fibers, that is, each fiber is composed of small crystals interconnected with each other. This observation was confirmed by Transmission Electron Microscopy (TEM). The elemental composition of the obtained nanofibers was studied using the energy dispersive X-ray (EDX) technique. Typical EDX spectra of Ag–TiO2 nanostructured nanofibers (TAg1, TAg3 and TAg5) are presented in Figure 3.

**Figure 2.** Small and larger magnification SEM images of Ag–TiO2 nanostructured nanofibers: pure TiO2 × 10,000 (**a**), 0.1% Ag–TiO2 × 50,000 (**b**), 5.0% Ag–TiO2 × 50,000 (**c**), TiO2 × 50,000 (**d**), 0.1% Ag–TiO2 × 150,000 (**e**), and 5.0% Ag–TiO2 × 150,000 (**f**).

**Figure 3.** EDX spectra of 0.1% Ag–TiO2 (**a**), 1.0% Ag–TiO2 (**b**) and 5.0% Ag–TiO2 (**c**).

The EDX characterization confirmed the presence of Ti, O, Ag, and Pt elements. No other impurity elements were identified. The appearance of Pt in the EDS spectra is due to the sample's preparation by Pt metallization for SEM/EDS measurements. The atomic percentages of Ag in the analyzed samples were found to be 0.05%, 0.21%, 0.40%, 0.51% and 0.6% respectively, values depending on the amount of precursor solutions. Some examples of EDX spectra are presented in Figure 3.

TEM characterization of pure and Ag doped TiO2 nanofibers confirmed their polycrystalline nature and nanostructuring. It can be noticed that the crystallites forming the doped materials are slightly smaller than in the case of pure TiO2 and that their order of magnitude is <10 nm as observed by XRD characterization too. Some examples are presented in the TEM images shown in Figure 4.

**Figure 4.** Smaller (scale 100 nm) and larger magnification (scale 50 nm) TEM images of Ag–TiO2 nanostructured nanofibers: pure TiO2 (**a**), 0.1% Ag–TiO2 (**b**), 5.0% Ag–TiO2 (**c**), TiO2 (**d**), 0.1% Ag–TiO2 (**e**), and 5.0% Ag–TiO2 (**f**).

> The specific surface area characterized by BET as a function of Ag concentration is illustrated in Table 2, while the weight change/relative humidity graphs are included in Supplementary material (Figure S2). It was found that as the Ag concentration increased, the specific surface area of the TiO2 nanofibers slightly increased for low Ag concentration but it dropped for the 5% Ag TiO2. This might be due to the lower diameters size of the fibers and the higher degree of crystallites agglomeration.

**Table 2.** BET data of pure TiO2 and Ag doped TiO2 nanostructured materials.

#### *2.3. Raman Spectroscopy Measurements*

Several authors [22,30–38] have shown in their research that the anatase phase has a tetragonal structure consisting of six Raman active modes, including A1g located at 516 cm−1 , 2B1g corresponding to 397 and 516 cm−1 and 3Eg from 144, 198 and 638 cm−1 , where the A1g mode overlapped with the B1g mode at 516 cm−1 [22,32,39].

In Figure 5, which displays the Raman spectra of Ag–TiO2 nanostructured nanofibers with different Ag content, one can be seen that the Raman bands observed correspond to the anatase phase of pure TiO2, even for those samples containing Ag. In particular, the Eg modes at 144 and 638 cm−1 correspond to the symmetric stretching vibration of O–Ti–O bond, whereas the B1g and A1g Raman modes at 397 and 516 cm−1 can be assigned to the symmetric and anti-symmetric bending vibration of O–Ti–O [40,41]. In any case, the peaks for the Ag containing samples were found to present small shifts (about 3 nm) to longer wavelengths. Additional peaks corresponding to Ag were not detected in any of the Ag containing samples.

**Figure 5.** Raman spectra of pure TiO2 and Ag–TiO2 nanostructured nanofibers materials.

It can be noted that the results of Raman characterization are in good agreement with the XRD observations. All Ag-containing materials have a similar structure of the TiO2 anatase phase, suggesting that the Ag was successfully integrated as small nanocrystals without affecting the TiO2 crystal lattice as intended. The slight red-shift of the peaks in the Raman spectra may be due to the slight distortion caused by the incorporation of metal, particles of various sizes, nature of defects, and so on [17–19,32].

#### *2.4. Optical Properties and Band Gap Energy*

It is known that the band gap energy is the most important optical parameter that significantly influences photocatalytic activity. Diffuse reflectance spectra recorded for pure TiO2 and Ag–TiO2 nanostructured nanofibers are illustrated in Figure 6, where a slight shift of about 10 nm to blue can be noticed for the Ag containing samples. The reflectance data were processed according to the method indicated in reference [39] for indirect bandgap semiconductors and the corresponding values are given in Table 1. The Eg values for the Ag/TiO2 nanostructures are much lower than those corresponding to pure TiO2 due to the Ag doping process. As can be observed, the presence of nano-Ag leads to decreased values of around 2.70 eV for the optical band gap, as compared to the 3.01 eV gap of pure TiO2. This means that photons with lower energy can generate electron–hole pairs and the photocatalytic activity of such materials can be activated even under visible light irradiation. Many studies [13,40] have shown that this decrease of the band gap may be due to the occurrence of new energy levels in the band gap range of the composite materials.

**Figure 6.** Optical properties: (**a**) reflectance spectra and (**b**) Tauc plots of Ag–TiO2 nanostructured nanofibers materials.

#### *2.5. Photoluminescence Analysis*

In the context of studies of a photocatalytic material, it is of great importance to gather information on the active surface sites of the catalyst and on how they affect the dynamics of adsorption and photoactivated transformations of the targeted species. In this regard, studies of photoluminescence (PL) properties of the material are very well suited and useful. PL phenomena in semiconductors are driven by diffusion and recombination of photogenerated charges, which typically occurs in a thin region beneath the semiconductor surface (typical widths of few tenths of nm if the excitation is provided at photon energy larger than the bandgap), making it very sensitive to small local variations. To observe how the Ag doping affects the carrier recombination and diffusion phenomena in TiO2, PL characterization using different excitation wavelengths was performed to see the excitation states involved in the emission and to observe the occurrence of sub-bandgaps. Figure 7 shows the PL spectra for the studied materials, excited at different wavelengths (λex = 280, 300, 320 and 340 nm).

TiO2 has an indirect band-edge configuration and hence its PL emission occurs at wavelengths longer than the bandgap wavelength: that is, the PL of TiO2 is not caused by band-to-band transitions but involves localized states. [42] The fluorescence spectra of TiO2 nanostructures normally display three bands, assigned to self-trapped excitons, oxygen vacancies and surface defects [18,24,33,35–37]. In particular, these emission bands are located in the violet, the blue (460 nm) and the blue-green (485 nm) regions respectively, which can be attributed to self-trapped excitons localized on TiO6 octahedral (422 nm) [36,37], and to oxygen related defect sites or surface defects (460 and 485 nm) [38]. Moreover, the band edge emission around 364 nm corresponds to free exciton recombination in TiO2 materials [35,36]. As can be seen, all materials present the same emission bands, but with slightly different intensities. In particular, the PL intensity of the Ag–TiO2 nanostructured nanofibers was found lower as compared to that of pure TiO2. As is known, the emission

intensity is related to the recombination of electron–hole pairs in the structure of TiO2 [13]. In addition, the low intensity in the fluorescence spectra suggests that the photoexcited electron–hole pairs can be achieved at a longer time, which is beneficial in the processes of photocatalytic degradation. Thus, these findings suggest that the presence of nano Ag has a distinct effect on limiting the electron–hole recombination, as the photoexcited electron may be captured by the Ag nanoparticles that behave as an electron storage source on the TiO2 surface [13]. Nano Ag presence also contributed significantly to reducing the band gap energy and facilitating the activation by the absorption of light in the visible region, along with delaying the electron–hole recombination. Therefore, the presence of nano-Ag offers several advantages in the functionality of the Ag–TiO2 nanostructured nanofibers. Additionally, it is expected that the best photocatalytic activity under the visible irradiation would be performed for an optimal nano Ag concentration level in TiO2.

**Figure 7.** Emission spectra of pure TiO2 and Ag–TiO2 nanostructured nanofibers at different excitation wavelengths λex = 280 nm (**a**), 300 nm (**b**), 320 nm (**c**) and 340 nm (**d**).

#### *2.6. Photocatalytic Properties*

2.6.1. Methylene Blue Dye Degradation

Methylene blue (MB) (C0 = 10 mg/L) was used to evaluate the photocatalytic activity of the grown materials. The dye degradation was performed under a halogen lamp light irradiation (400 W) and the amount of photocatalyst was maintained at 0.4 g/L for all samples. Typical UV-VIS absorption spectra recorded for MB dye solution degradation up 300 min under halogen lamp light irradiation in presence of pristine TiO2 and 0.1% Ag–TiO2 nanostructured nanofibers are shown in Figure 8. It can be observed that the intensity of the absorption band corresponding to a wavelength at 665 nm decreases with the increase of the irradiation time. In addition, all Ag–TiO2 nanostructured nanofibers

1

show a faster decreasing tendency of colorant concentration as compared to pure TiO2. Regarding the color removal efficiency, this is shown in Figure 8c. The maximum degradation efficiency was found for the TAg1 sample, having a value of 97.05%. The kinetics of the photodegradation process under visible light irradiation was also evaluated.

**Figure 8.** UV-VIS absorption spectra for the degradation of MB dye (10 mg/L) at various irradiation times in the presence of pure TiO2 (**a**), 0.1% Ag–TiO2 nanostructured nanofibers (**b**), and (**c**) color removal efficiency obtained for all materials after the end of the photodegradation.

2.6.2. Kinetics of the Photodegradation Process

Kinetics plots of the photodegradation of MB in aqueous solutions under the halogen lamp irradiation in the presence of Ag–TiO2 nanostructured nanofibers are presented in Figure 9. The data were interpolated to the pseudo-first-order (PFO) kinetic model by using the nonlinear regression technique. The goodness-of-fit was estimated by chi-square statistic test (χ 2 -value). Thus, the decay of MB dye concentration versus time was fitted to PFO equation, which can be expressed as:

$$\mathbb{C}\_{t} = \mathbb{C}\_{0} \cdot e^{-kt} \tag{1}$$

where *C*0 is the initial MB dye concentration (~10 mg/L), *k* is the pseudo-first-order reaction rate constant (min−1 ), and *t* is the irradiation time (min). The calculated parameters of the PFO model are listed in Table 3.

**Figure 9.** Kinetics plots of MB dye decay against irradiation time during the photodegradation process under halogen lamp in the presence of Ag–TiO2 nanostructured nanofibers catalysts. Solid and dash lines represent predictions given by PFO kinetic model. Experimental conditions: catalyst dosage = 0.4 g/L; *T* = 23 ± 2 ◦C; pH 7.0 ± 0.2.

**Table 3.** Kinetic parameters for MB dye photodegradation under visible light in the presence of Ag–TiO2 nanostructured nanofibers catalysts.

As observed from Table 3, the presence of nano Ag in TiO2 nanofibers increases the rate constant (*k*). This effect was found more important for a 0.1% content of Ag in the electrospun solution. Thus, the optimal formulation of the developed materials when acting as photocatalyst seems to be the 0.1% Ag–TiO2 (TAg1 sample), as this presents the maximum value of both the constant rate (k = 1.29 × 10−2 min−1 ) and the degradation efficiency of MB dye (97.05%). Furthermore, the kinetics for the degradation of methylene blue (MB), Congo red (CR), amaranth and orange II dyes under TAg1 photocatalyst are presented below, in which the dye concentration was maintained at 10 mg/L (Figure 10). The photolysis test (without catalysts) for all four dyes was performed under fluorescent bulb light irradiation for 300 min of irradiation, and the corresponding spectra are presented in Figure S3, Supplementary material.

**Figure 10.** Comparative study showing the kinetics curves for degradation of various dyes under visible light in the presence of sample TAg1. Solid and dash lines represent predictions given by PFO kinetic model. The experimental conditions are: catalyst dosage = 0.4 g/L, *T* = 23 ± 2 ◦C, pH 7.0 ± 0.2.

The UV-visible absorption spectra (Figure S4, Supplementary material) have been recorded for the degradation of MB, CR, amaranth, and orange II dye solutions in the following experimental conditions: initial dye concentration=10 mg/L for all dyes, catalyst dosage = 0.4 g/L, irradiation time = 300 min, *T* = 23 ± 2 ◦C and pH 7.0 ± 0.2. From Figure S3, it can be noted that color removal efficiency varies between 75% and 98%, depending on the type of dye. The highest degradation efficiency (99%) was found for Congo red dye, the efficiency being achieved in a shorter time (30 min) as compared to other dyes.

A comparative study is reported in Figure 10 showing the photodegradation kinetics of different dyes under visible light using TAg1 as a catalyst. Experimental data were interpolated to PFO-kinetic model and the calculated parameters are summarized in Table 4.

**Table 4.** Kinetic parameters for photodegradation of different dyes under the fluorescent bulb light in the presence of 0.1% Ag–TiO2 nanostructured nanofibers photocatalyst (TAg1).

According to Table 4, the highest rate constant (7.28 × 10−2 min−1 ) was observed for Congo red dye photodegradation, and the lowest one (4.57 × 10−3 min−1 ) for the orange-II dye, respectively. Comparing the present results with others reported on appropriate photocatalysts (Ag doped TiO2 nanostructures) [16–22,43], one can observe the excellent performance of our samples, as these are capable to degrade up 99% of dyes depending on the dye nature, with constant rates between 4.57 × 10−3 and 7.28 × 10−2 min−1 . In addi-

tion, all the degradation tests on the fabricated samples were done using a moderate amount of catalyst 0.4 g/L, fluorescent bulb light irradiation (400 W), and temperature (23 ± 2 ◦C), pH (7.0 ± 0.2) and without intervening to acidify the solution or add H2O2. For comparative purposes, Harikishore et al. reported a rate constant value of 8.16 × 10−3 min−1 for methylene blue (C0 = 5 ppm) dye degradation in the presence of 1.5g/L amount of catalyst under UV lamp irradiation [43]. Recently, Ali et al. [22] obtained Ag-doped TiO2 nanoparticles with improved photocatalytic and antibacterial properties. They found a degradation efficiency of 96% for MB degradation (C0 = 10 mg/L) in the presence of a 1g/L catalyst under visible light irradiation (500 W).

#### 2.6.3. Photocatalyst Reuse

The degradation efficiency after several photodegradation cycles is an important parameter to evaluate the stability of the photocatalyst. Figure 11c) shows a degradation efficiency registered after 5 cycles of reuse under visible light irradiation for one hour. It was noticed that the photocatalytic efficiency was maintained at about 98% even after five cycles for Congo red (C0 = 10 mg/L) dye degradation. In addition, the stability of the reused photocatalyst was confirmed by XRD and SEM characterization. XRD pattern and an SEM image for recycled photocatalyst are presented in Figure 11a,b. It can be observed that both, crystallinity and morphology are preserved after its use.

**Figure 11.** SEM image and XRD pattern of TAg1 reused material after five cycles (**a**,**b**); color removal efficiency after 60 min in the presence of both materials (TAg1 reused and pristine) (**c**).

#### *2.7. Antimicrobial Activity*

All samples were also tested for their antibacterial activity against *S. aureus* and *E. coli*. Samples T400 and TAg1 did not show any antibacterial action. TAg3 and TAg5 proved to be quite effective against the Gram-positive bacterial strain represented by *S. aureus*, while TAg2 and TAg5 against the Gram-negative bacterial strain represented by *E. coli*. The highest efficiency observed, regardless of the bacterial strain, was found in sample TAg5 that is at the highest Ag concentration (5%). The nanostructures were in general more efficient against *E. coli* (19 mm of inhibition zone) than against *S. aureus* (16 mm of inhibition zone). An image of a culture plate is presented in Figure 12.

From this image, the diameters of the inhibition zones of each tested sample can be calculated and are presented in Table 5.

**Table 5.** Antimicrobial activity of the tested compounds against the reference strains.

**Figure 12.** Image of a culture plate for evaluation of antimicrobial activity against *S. aureus* and *E. coli*.

The MIC of the samples against *E. coli* was as low as 2.5 mg/mL for sample TAg5, and 5 mg/mL for the same sample against *S. aureus* (see Table 6).

**Table 6.** Antimicrobial activities of the samples against *S. aureus* and *E. coli*. MIC: minimum inhibitory concentration.

The final trial was focused on the determination of the minimum bactericidal concentration (MBC), which is recognized as the lowest concentration that destroyed all bacterial cells. This was investigated by spreading 10 µL samples from the wells on MH agar plates. However, MBC against both bacterial strains could not be estimated, as a higher concentration of Ag within the samples was probably required. The samples had better antibacterial activity against the Gram-negative strain represented by *E. coli* probably due to its cell wall composition, distinctive rod shape and extracellular matrix [44]. Literature states that when *E. coli* is exposed to light activated Ag–TiO2 the oxidative damage to the bacterial cell envelope may occur which plays a significant role in biocidal activity [45]. In this particular case, the antibacterial activity is directly correlated with the silver concentration within the samples [46].

#### **3. Materials and Methods**

#### *3.1. Materials*

Titanium (IV) isopropoxide (TTIP), silver nitrate (AgNO3), glacial acetic acid (CH3COOH), ethanol, polyvinylpyrrolidone (PVP) (Mw = 1.300.000) of analytical grade purchased from Sigma-Aldrich (Sigma-Aldrich/Merck KGaA, Darmstadt, Germany) were used for the preparation of the Ag-doped TiO2 nanostructures. Methylene blue, Congo red, orange II, and amaranth dyes were procured from Sigma-Aldrich (Sigma-Aldrich/Merck KGaA, Darmstadt, Germany) and utilized without further purification.

#### *3.2. Preparation of Ag–TiO2 Nanostructured Nanofibers*

Pure TiO2 and Ag–TiO2 nanostructured nanofibers were developed using electrospun solutions, which were obtained by mixing two initial solutions, solution 1 containing 0.75 mL of TTIP in 1.5 mL acetic acid and solution 2 consisting of 0.25 g PVP in 2.5 mL ethanol. This solution was used to prepare the pure TiO2 blank sample (named T400), while in the case of Ag–TiO2 nanostructures, these were prepared by adding to solution 2 different amounts of AgNO3 as follows: 0.5 mg corresponding to a 0.1% mass percentage (sample TAg1), 2.5 mg for 0.5% (sample TAg2), 5 mg for 1.0% (sample TAg3), 15 mg for 3.0% (sample TAg4) and 25 mg for 5.0% (sample TAg5). The electrospinning was performed using the set-up already described in our previous publications [47,48]. The electrospinning parameters used to obtain the fibrous materials were: 25 kV high voltage, 15 cm tip to collector distance, and 0.75 mL/h feed flow rate. The Ag–TiO2 nanostructured nanofibers were obtained after removal of PVP matrix using calcination in air at 400 ◦C with a heating rate of 15 ◦C/min for 4 h.

#### *3.3. Characterization*

The crystallographic structure of all materials was studied using X-ray diffraction (XRD) technique performed with a Rigaku SmartLab-9kW diffractometer (Rigaku Corporation, Tokyo, Japan). The experimental spectra were analyzed using PDXL software developed by Rigaku Corp., Tokyo, Japan. The respective morphological characteristics were examined using a Verios G4 UC Scanning Electron Microscope (Thermo Fisher Scientific, Waltham, MA, USA) equipped with an energy dispersive spectrometer (EDX) (AMETEK, Tokyo, Japan), EDAX Octane Elite. The morphology of the fibers based on pure and doped TiO2 was studied using a Hitachi High-Tech HT7700 Transmission Electron Microscope (TEM) (Hitachi, Tokyo, Japan), operated in high contrast mode at 120 kV accelerating voltage. BET analysis was performed using a fully automated gravimetric analyzer IGASorp supplied by Hidden Analytical, Warrington (UK), with an ultrasensitive microbalance, which was used to measure dynamic water vapor sorption capacity of the samples by the weight change with variation of humidity at a constant temperature. Each sample was dried in flowing nitrogen (250 mL/min) until the weight of the sample was in equilibrium at RH < 1%. Experiments were carried out at 25 ◦C in the relative humidity (RH) range 0–90%, with 10% humidity steps, each having a pre-established equilibrium time between 10 and 20 min (minimum time and time out, respectively).

A Horiba Scientific LabRam HR Evolution spectrometer (HORIBA Scientific, Palaiseau, France) was used for Raman spectroscopy measurements, which were done in the 25–1650 cm−1 frequency range, employing excitation at 532 nm, acquisition time 100 s, 3 accumulations and a grating of 600 gr/mm. The optical properties were evaluated by UV-vis diffuse reflectance spectra, recorded on an Analytik Jena (Jena, Germany) SPECORD UV/Vis 210+ spectrophotometer with an integrating sphere. Emission spectra were recorded employing a luminescence spectrometer Perkin Elmer LS55 (PerkinElmer, Inc., Waltham, MA, USA).

#### *3.4. Photocatalytic Activity*

The photocatalytic activity of Ag–TiO2 nanostructured nanofibers was evaluated by studying the photodegradation of methylene blue (MB), amaranth, Congo red (CR) and orange II dye solutions under UV-visible light irradiation supplied by a 400 W halogen lamp (Model TG-2503.0l, Elbi Electric and Lighting, Bragadiru, Romania) with the emission spectrum presented in Figure S5 in supporting material file. The distance between the light source (lamp) and the reactor was 30 cm. Deionized water was used to prepare the dye solution. In all photocatalytic measurements, the following experimental conditions were employed: initial dye concentration=10 mg/L for all dyes, catalyst dosage = 0.4 g/L (the catalyst loading that proved previously optimal for reference sample T400), maximum irradiation time = 300 min, T = 23 ± 2 ◦C and pH 7.0 ± 0.2. The pH was recorded for the initial and final solutions and was found to be 7 in both cases. Initially, the dye solution and the catalyst were continuously magnetically stirred (500 rotation/min) in the absence

of light for 30 min (time needed to establish an adsorption equilibrium). After this, the solution was irradiated with visible light for 5 hours, and during photodegradation, a quantity of 3 ml of the solution was extracted at the given time intervals to assess the change in the concentration. All absorption spectra for collected samples were measured after 12 h of settling when the catalyst was completely found as sediment. Absorption spectra were recorded using UV–Vis spectrophotometer SPECORD 210+ (Analytik Jena gmbh, Jena, Germany).

The photocatalytic efficiency (%) for degradation of the dyes was evaluated using the following relationship [13]:

$$\text{Photocatalytic efficiency} \left( \% \right) = \frac{\text{C}\_0 - \text{C}\_e}{\text{C}\_0} \times 100 \tag{2}$$

where C0 = initial dye concentration (mg/L); Ce = the dye concentration after irradiation time (mg/L).

#### *3.5. Antimicrobial Activity*

The antimicrobial activity of the materials was determined by disk diffusion assay against Staphylococcus aureus ATCC25923 and Escherichia coli ATCC25922. Both microorganisms were stored at −80 ◦C in 20–40% glycerol. The bacterial strains were refreshed in tryptic soy agar (TSA) at 36 ± 1 ◦C for 24 h. Microbial suspensions were prepared with these cultures in sterile solutions to obtain turbidity optically comparable to that of 0.5 McFarland standards.

Volumes of 0.3 ml from each inoculum were spread on the Petri dishes. The sterilized paper disks were placed on plates and an aliquot (20 µL) of the samples with a concentration of 10 mg/ml was added. To evaluate the antimicrobial properties, the growth inhibition was measured under standard conditions after 24 h of incubation at 36 ± 1 ◦C under solar light illumination. All tests were carried out in triplicate to verify the results. After incubation, the diameters of inhibition zones were measured by using Image J version 1.53e software (Madison, WI, USA), and were expressed as the mean ± standard deviation (SD) performed with XLSTAT software Ecology version 2019.4.1 software (New York, NY, USA) [49,50].

#### *3.6. Determination of Minimum Inhibitory Concentration (MIC)*

The concentration of a sample that prevented the growth of bacteria is in general recognized as minimum inhibitory concentration (MIC). This was investigated by a broth dilution method, which was performed in 96-well microtiter plates using the resazurin reduction assay concept [49]. Bacterial culture grown to log phase was adjusted to 1 × 108 cells/mL in Muller–Hinton (MH) broth. Inoculants of 50 µL were mixed with 50 µL of serial dilutions of samples and were subsequently incubated at 36 ± 1 ◦C for 24 h. Resazurin was prepared at 0.015% by dissolving 0.015 g, vortexed and filter sterilized (0.22 µm filter) and stored at 4 ◦C for a maximum of 2 weeks after preparation. After incubation for 24 h at 36 ± 1 ◦C, resazurin (0.015%) was added to all wells (20 µL per well), and these were further incubated for 2–4 h more. MIC was determined by reading the fluorescence at λex = 575 nm and λem = 590 nm with FLUOstar Omega Microplate Reader (BMG LABTECH, Ortenberg, Germany). To determine accurately the MIC values, the experiments were performed in triplicate.

#### **4. Conclusions**

Ag–TiO2 nanostructured nanofibers with different Ag content were produced using the electrospinning technique followed by calcination at 400 ◦C for 4 h and were tested regarding their photocatalytic and antimicrobial action. XRD characterization confirmed the formation of anatase TiO2 crystalline structure, without identifying additional peaks corresponding to other phases (brookite, rutile) or other secondary peaks corresponding to the Ag presence, which indicates that Ag was successfully integrated as small nanocrystals without affecting much of the TiO2 crystal lattice. Regarding the lattice strain, this was found to increase from 0.13% (pure TiO2 sample) to 0.52% (TAg1), reaching 0.53% at the highest Ag content. These observations were also confirmed by the Raman spectroscopy analysis. SEM characterization proved the formation of one-dimensional uniform Ag–TiO2 nanostructured nanofibers of excellent quality, with a diameter in the range of 65 to 100 nm. The optical band gap energy decreased to 2.4 eV for the Ag–TiO2 nanostructured nanofibers as compared to 3.01 eV of pure TiO2, which leads to an improvement of the photocatalytic activity under visible light irradiation. A remarkable photocatalytic efficiency of 99% at a constant rate of *k* = 1.29 × 10−2 min−1 was found for the 0.1% Ag–TiO2 sample for the case of methylene blue dye degradation. Additionally, these materials proved able to degrade up 99% of other organic dyes with constant rates in the range of 4.57 × 10−3 to 7.28 × 10−2 min−1 depending on the dye nature, all degradation tests being performed under visible light irradiation, without intervening to acidify the solution or add H2O2. The present study focused on materials optimization for photocatalytic applications. Further studies regarding the structure and nano-properties of these materials as well as solving the photocatalytic mechanism and by-products are ongoing. The Ag–TiO2 nanostructured nanofibers materials showed promising antimicrobial responses against Gram-positive (*S. aureus*) and Gram-negative (*E. coli*) strains. The sample with a higher concentration of Ag (5%) was found to present a MIC value of 2.5 mg/mL against *E. coli* and 5 mg/mL against *S. aureus*. Based on these results, one can conclude that Ag–TiO2 nanostructured nanofibers were developed with excellent structure and suitable physical properties for photocatalytic and antibacterial action. Further studies regarding the possibility of integration of these materials into food packaging, medical textiles, and other healthcare related items will be performed.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/article/ 10.3390/catal11101234/s1, Figure S1. Histogram of fiber diameter distribution plotted from SEM images analysis of pure TiO2 (T400) and Ag doped TiO2 (TAg1 and TAg5). Figure S2. Dynamic vapor sorption isotherms for pure TiO2 and Ag doped TiO2 materials. Figure S3. UV-VIS absorption spectra of the photolysis test for all 4 dyes performed under visible light irradiation for 300 minutes. Figure S4. UV-VIS absorption spectra recorded for the degradation of (a) MB, (b) CR, (c) Amaranth and (d) Orange II dyes. Figure S5. Lamp spectrum.

**Author Contributions:** Data Curation, P.P., C.C., N.O., E.K. and M.P.S.; Formal Analysis, P.P., A.A., I.R., E.K. and M.P.S.; Funding Acquisition, P.P.; Investigation, P.P. and I.R.; Methodology, P.P. and I.R.; Project Administration, P.P.; Resources, P.P., C.C., A.A. and N.O.; Supervision, P.P.; Validation, P.P., E.K. and M.P.S.; Visualization, P.P. and M.P.S.; Writing—Original Draft, P.P., C.C., E.K. and M.P.S.; Writing—Review and Editing, P.P., E.K. and M.P.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was partially supported by a grant of the Romanian Ministry ofResearch, Innovation and Digitization, CNCS/CCCDIUEFISCDI, project number PN-III-P1-1.1-TE-2019-0594, within PNCDI III.

**Data Availability Statement:** The raw and processed data required to reproduce these findings cannot be shared at this time due to technical or time limitations. The raw and processed data will be provided upon reasonable request to anyone interested anytime until technical problems are being solved.

**Acknowledgments:** Special thanks to Cosmin Romanitan for helping with the XRD characterization. M. P. Suchea contribution was partially financed by the Romanian Ministry of Research, Innovation and Digitization through "MICRO-NANO-SIS PLUS" core Programme.

**Conflicts of Interest:** The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

#### **References**

## *Article* **Photocatalytic Degradation of Thiacloprid Using Tri-Doped TiO2 Photocatalysts: A Preliminary Comparative Study**

**Antonietta Mancuso 1,\*, Wanda Navarra 2 , Olga Sacco 2,\* , Stefania Pragliola 2 , Vincenzo Vaiano 1 and Vincenzo Venditto 2**

**Abstract:** Different tri-doped TiO2 photocatalysts (Fe-N-P/TiO2 , Fe-N-S/TiO2 , Fe-Pr-N/TiO2 , Pr-N-S/TiO2 , and P-N-S/TiO2 ) were successfully prepared and tested in the photocatalytic removal of thiacloprid (THI) under UV-A, visible, and direct solar light irradiation. The physical-chemical properties of the prepared catalysts were analyzed by different characterization techniques, revealing that dopants are effectively incorporated into the anatase TiO2 lattice, resulting in a decrease of the energy band gap. The reduction of photoluminescence intensity indicates a lower combination rate and longer lifespan of photogenerated carriers of all doped samples in comparison with the un-doped TiO2 . The doped photocatalysts not only significantly promote the photodegradation under UV-A light irradiation but also extend the optical response of TiO2 to visible light region, and consequently improve the visible light degradation of THI. Fe-N-P tri-doped TiO2 sample exhibits the highest THI photodegradation degree (64% under UV-A light, 29% under visible light and 73% under solar light).

**Keywords:** tri-doped TiO2 ; thiacloprid; photocatalysis; water treatment

#### **1. Introduction**

Today, the most important commercial insecticides available on the market are neonicotinoids, thanks to their high insecticidal activity, adequate water solubility, and high stability [1]. Usage has spread beyond agriculture to home garden [2]. Unsurprisingly, widespread use of neonicotinoids has led to an almost ubiquitous environmental presence of these pollutants, including in surface water and groundwater [3]. The neonicotinoid insecticides, both at high and low concentrations (e.g., LC50 of 5 ng/bee) [4], bind to nervous systems causing receptor blockage, paralysis, and death. Thiacloprid (THI) is an insecticide, belonging to the second-generation neonicotinoid pesticides, introduced by Bayer Crop Science with the name Calipsol [5]. Like all neonicotinoid insecticides, THI selectively acts on the insect nervous system as an agonist of the nicotinic acetylcholine receptors. This unique mode of action makes THI highly applicable for controlling the biological effect on insects which developed resistance to conventional chlorinated hydrocarbons, organophosphate, carbamate, and pyrethroid insecticides [6]. Due to its high water solubility and recalcitrance to biodegradation, THI results still persistent in water after the conventional wastewater treatment [7]. Thus, various technologies have been tested to degrade THI from water. Unfortunately, all the proposed techniques suffer to relative low removal efficiency and require continued use of electricity and chemicals [8,9]. Therefore, it is of great importance to develop a reliable and effective method for removing THI from water.

Heterogeneous photocatalysis is an advanced oxidation technology, which offers a promising alternative to remove a great variety of organic pollutants with high efficient

**Citation:** Mancuso, A.; Navarra, W.; Sacco, O.; Pragliola, S.; Vaiano, V.; Venditto, V. Photocatalytic Degradation of Thiacloprid Using Tri-Doped TiO2 Photocatalysts: A Preliminary Comparative Study. *Catalysts* **2021**, *11*, 927. https:// doi.org/10.3390/catal11080927

Academic Editors: Stéphanie Lambert and Julien Mahy

Received: 30 June 2021 Accepted: 29 July 2021 Published: 30 July 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

degradation rate. TiO2 is the most widely studied photocatalyst because of its excellent properties, such as non-toxic nature, water insolubility, low cost, favorable band edges position, photochemical stability, and high light conversion efficiency [10,11].