Spatially confined Fe2O3 in hierarchical SiO2@TiO2 hollow sphere exhibiting superior photocatalytic efficiency for degrading antibiotics

Spatially confined Fe2O3 in hierarchical SiO2@TiO2 hollow sphere exhibiting superior photocatalytic efficiency for degrading antibiotics

Chemical Engineering Journal 380 (2020) 122583 Contents lists available at ScienceDirect Chemical Engineering Journal journal homepage: www.elsevier...

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Chemical Engineering Journal 380 (2020) 122583

Contents lists available at ScienceDirect

Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej

Spatially confined Fe2O3 in hierarchical SiO2@TiO2 hollow sphere exhibiting superior photocatalytic efficiency for degrading antibiotics

T

Sai Zhanga, Jianjian Yib, Juanrong Chenb,c, , Zhengliang Yina, Tao Tanga, Wenxian Weid, ⁎ Shunsheng Caoa, Hui Xub, ⁎

a

Research School of Polymer Materials, School of Materials Science and Engineering, Jiangsu University, Zhenjiang 212013, China School of the Environment and Safety Engineering, Jiangsu University, Zhenjiang 212013, China c Surface Engineering Precision Institute, School of Aerospace, Transport and Manufacturing, Cranfield University, Bedfordshire MK43 0AL, United Kingdom d Testing Center, Yangzhou University, Yangzhou, Jiangsu 225009, China b

HIGHLIGHTS

GRAPHICAL ABSTRACT

confined Fe O in hier• Spatially archical SiO @TiO (SFT) photo2

2

3

2

catalyst was prepared.

We introduce confinement effect to overcome disadvantages of directly immobilizing Fe2O3 on the surface of TiO2 by encapsulating Fe2O3 in photocatalyst, which exhibits a higher photocatalytic activity.

degradation of antibiotics • Awascomplete achieved under natural sunlight irradiation.

intermediates and • Transformation pathways of antibiotics were presented.

work provided a new insight for • This constructing other metal oxides confined photocatalysts.

ARTICLE INFO

ABSTRACT

Keywords: SiO2-Fe2O3@TiO2 Hierarchical structure Confinement effect Antibiotics Nature solar light

Although TiO2-based photocatalysts have achieved great successes for the degradation of organic pollutants, the complete removal of antibiotics is hard to be realized because of its unique macromolecular ring structure under solarlight irradiation. Herein, this work demonstrates the rational design of the hierarchical hollow SiO2-Fe2O3@TiO2 (SFT) photocatalyst by introducing spatially confined Fe2O3 as a modifier of TiO2, in which inner SiO2 serves as a carrier to support and disperse Fe2O3 in order to obtain small size of Fe2O3 (2–6 nm), while outer TiO2 acts as a bounding wall to protect Fe2O3 from aggregation and abscission. The as-synthesized SFT photocatalyst not only can overcome easy corrosion, dissolution and deactivation of Fe2O3 during the photoreaction process, but also can substantially enhance the adsorption of antibiotics because of its hierarchical hollow structure, facilitating the separation of electron-hole pairs and prolonging the trapping of incident light. Therefore, the SFT photocatalyst manifests the complete removal of antibiotics under simulated solar light irradiation. The intermediates of antibiotics were analyzed by liquid chromatography-mass spectrometry (LC/MS) and the possible degradation pathway was proposed accordingly. Besides, SFT photocatalyst exhibits an excellent recyclability due to confinement effect. Especially, the assynthesized SFT also achieves the 100% degradation rate of antibiotics under natural sunlight irradiation, efficiently overcoming the incomplete removal of antibiotics for many previous TiO2-based photocatalysts.



Corresponding authors at: School of the Environment and Safety Engineering, Jiangsu University, Zhenjiang 212013, China (J. Chen). E-mail addresses: [email protected] (J. Chen), [email protected] (H. Xu).

https://doi.org/10.1016/j.cej.2019.122583 Received 10 March 2019; Received in revised form 19 August 2019; Accepted 22 August 2019 Available online 22 August 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.

Chemical Engineering Journal 380 (2020) 122583

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1. Introduction

J&K Chemical Company Ltd. Methacryloxyethyltrimethyl ammonium chloride (DMC, 80% in water) was obtained from Sigma Aldrich Company Ltd. All other chemicals were of analytical grade and purchased from Sinopharm Chemical Reagent Co, Ltd. (China) and were used as received without further treatment except styrene, which was purified using NaOH solution.

The abuse of antibiotics produces high levels of antibiotic residues, leading to a serious threat to ecosystems [1–3]. Heterogeneous photocatalysis is a highly promising approach for the removal of antibiotics due to low cost, easy recovery and high-efficient energy conversion [4,5]. TiO2-based materials have been considered as efficient photocatalysts due to their fascinating properties [6,7], however, the wide bandgap (~3.2 eV) of pure TiO2 limits its light absorption to UV light, making it inefficient for the conversion of solar energy [8]. Numerous strategies including ion doping, metal decoration, clay modification and heterojunction have been introduced to enhance the visible-light activity of TiO2 [9–13]. For example, Bhatia et al prepared Bi-Ni codoped TiO2 and found that it degraded 86% of ofloxacin after 6 h under solar-light irradiation [14]. Chen et al synthesized Au-CuS-TiO2 nanobelts with high degradation (96%) of oxytetracycline under simulated sunlight irradiation [15]. However, the complete removal of antibiotics under solar light irradiation still remains a challenge [15–18]. Inspiringly, metal oxide decorating offers a new solution to enhance the visible-light efficiency of TiO2 [19]. Hematite (α-Fe2O3) is an appropriate candidate because of its favorable bandgap, abundance, and environmental compatibility [20]. Unfortunately, the heterojunction of Fe2O3 and TiO2 belongs to Type I band alignment, in which the photoexcited charge carriers tend to accumulate in Fe2O3, suggesting that the charge separation is hard to be improved [21]. To suppress such accumulation, a small amount of ultra-small Fe2O3 (2–4 nm) is required to shorten the migration length of charge carriers from bulk to surface of catalyst, indicating that photoinduced charge can rapidly participate in photoreaction process [19], on the contrary, a higher amount decreases the distance between trapping sites and Fe2O3 nanoparticles would become recombination centers of photoinduced e−/h+ pairs through quantum tunneling [22]. Besides, the geometry and position of Fe2O3 decorating has a significant effect on the photocatalytic activity [23]. Confinement effect has proved that metal nanoparticles (MNPs) encapsulated in photocatalysts could exhibit higher photocatalytic activity than MNPs loaded the surface of photocatalysts [24], which has been further confirmed by encapsulating Fe2O3 into mesopores SiO2 manifesting an enhanced photocatalytic activity compared to commercial α-Fe2O3 [25]. However, most of previous investigations paid more attention to the preparation of Fe2O3/ TiO2 composites by anchoring Fe2O3 onto the surface of TiO2 [26–28], which resulted in many potential disadvantages such as obstructing light absorption [29], and undergoing exfoliation, corrosion and dissolution during photo-degradation process. More importantly, the Fe2O3 loaded on the surface of TiO2 inhibits the production of radicals because the holes trapped by Fe2O3 present weaker oxidizing power than the holes generated in TiO2 [28]. Therefore, the rational design of highly efficient Fe2O3/TiO2 photocatalytic systems that can overcome these limitations is very urgent. Here, we design a novel photocatalyst by encapsulating Fe2O3 nanoparticles into hierarchical SiO2@TiO2 hollow sphere (SFT: SiO2Fe2O3@TiO2), in which inner SiO2 serves as a carrier to support and disperse Fe2O3 in order to obtain small size of Fe2O3 nanoparticles because SiO2 can decrease agglomeration rate of Fe2O3 [30], while outer TiO2 acts as a bounding wall to encapsulate and protect Fe2O3. Thanks to this rational design, SFT not only achieves good energy-level match between TiO2 and Fe2O3, but also efficiently overcomes the disadvantages of directly immobilizing Fe2O3 on the surface of TiO2, leading to superior photocatalytic efficiency and stability for degrading antibiotics. Therefore, this work offers a new insight in constructing other multi-semiconductor systems with high photocatalytic efficiency.

2.2. Preparation of SiO2-Fe2O3@TiO2 hollow spheres The synthesized CPS (cationic polystyrene sphere)@SiO2 particles according our previous work [31,32] were dispersed into 30 mL absolute ethanol, followed with the injection of 20 mL Fe(NO3)3 aqueous solution (0.2 wt%). After that, the mixed solution was stirred for 6 h and then dried at 50 °C, obtaining CPS@SiO2-Fe3+. Subsequently, CPS@ SiO2-Fe3+ was further served as the template for in situ polymerization with styrene, DVB and DMC at 70 °C, forming CPS@SiO2-Fe3+@CPS. Finally, outer TiO2 was further coated on CPS@SiO2-Fe3+@CPS via sol–gel process [33,34], preparing SiO2-Fe2O3@TiO2 photocatalyst by pyrolysis of CPS (450 °C). For comparison, SiO2@TiO2 (ST) and SiO2@TiO2-Fe2O3 (STF) hollow spheres were prepared through the same process, as shown in Supplementary Materials. 2.3. Photocatalytic activity evaluation The photocatalytic activity was investigated under light irradiation (Chang Zhou SIYU XQ 350W Xeon lamp) including visible light (λ > 420 nm), simulated solar-light (with an AM 1.5 G filter) and natural sunlight (sunny or cloudy day at 11:00 am, coordinate: 32.21396, 119.458012) at 25 °C. Typically, 10 mg catalyst was dispersed into 50 mL TC (10 mg L−1) or ENR (5 mg L−1) aqueous solution with a glass vessel, and then the mixed solution was stirred for 30 min in dark to achieve adsorption–desorption equilibrium (Fig. S1). After that, the catalytic system was exposed to light irradiation. Periodically, 3 mL of reaction solution was withdrawn, separated by 0.22 μm millipore filter and then analyzed by recording variations in the absorption in UV–vis spectra using Shimadzu UV-2600 spectrophotometer (TC: λ = 357 nm [35]; ENR: λ = 271 nm [36]), calculating the removal rate according to the calibration curves (Fig. S2). Incidentally, the reported photocatalytic values are the average of three measurements. 2.4. The degradation pathway Intermediates of antibiotics were measured using liquid chromatography-mass spectrometry (LC/MS) (Thermo LXQ) equipped with an electrospray ionization (ESI) source. After photoreaction, the filter liquor was analyzed by HPLC with a VWD detector (wavelength 355 nm (TC), 276 nm (ENR)) and a chromatographic column (Elite; particle size 5.0 μm, 4.6 × 250 mm). Water 0.1% formic acid-acetonitrile (50/50, v/ v) was used as the mobile phase at a flow rate of 0.3 mL min−1 at 35 °C, injection volumes were 5.0 μL. MS spectra was scanned with a positive ion mode at a mass range of 100–500 m/z. 3. Results and discussion 3.1. The morphology and structure of SFT hollow sphere Fig. 1 briefly demonstrates our design thought and the fabrication process of the SiO2-Fe2O3@TiO2 (SFT) hollow spheres. The CPS@SiO2 was synthesized according our previous work [31], and then was used as template to adsorb Fe3+ due to the electrostatic interaction. To obtain the ultra-small Fe2O3 nanoparticles, an ultrathin cross-linked CPS was coated on the surface of CPS@SiO2-Fe3+, inhibiting the aggregation of Fe2O3. After that, TiO2 layer was coated on the surface of CPS@SiO2Fe3+@CPS. Finally, SFT hollow spheres can be obtained by pyrolysis of CPS at 450 °C. For comparison, SiO2@TiO2 (ST) and SiO2@TiO2-Fe2O3 (STF) hollow spheres were prepared via the same process.

2. Experimental 2.1. Materials Titanium tetrabutoxide (TBT, > 99%), azobisisobutyronitrile (AIBN, > 99%), and divinylbenzene (DVB, > 80%) were obtained from 2

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The typical SEM image shows that the as-prepared SFT has highly uniform and intact sphere (Fig. 2a), and the hierarchical hollow structure can be clearly observed from the several broken spheres (Fig. 2a inset), which is further confirmed by TEM image (Fig. 2b, c). The cavity can be clearly seen from the sharp contrast of the gray scale between the shell and hollow interior. The small gap between SiO2 and TiO2 indicated by arrows can be found from the higher magnification of TEM image in Fig. 2c, confirming the hierarchical structure of SFT. In addition, HRTEM image (Fig. 2d) shows that the SFT consists of anatase TiO2 with lattice spacing of 0.35 nm ((1 0 1) plane) and α-Fe2O3 with lattice spacing of 0.25 nm ((1 1 0) plane), confirming the existence of Fe2O3 with small size of 2–6 nm (indicated by ellipses). The EDS mapping images ascertain that the Si, Fe, Ti, O and C signals are uniformly distributed in SFT samples, as shown in Fig. 2e-j. All these results confirm the successful preparation of the hierarchical SFT hollow spheres. N2 adsorption/desorption isotherm shows that the SFT photocatalyst possesses a high specific surface area (~180 m2 g−1) because of its hierarchical hollow structure, beyond the comparative values of SiO2 (~121 m2 g−1) and TiO2 (~55 m2 g−1) hollow spheres, as demonstrated in Fig. 3a. Clearly, higher surface area and the mesopore (~3.48 nm) can allow pollutant molecules to rapidly access photocatalyst and provide more active sites for photocatalytic reaction. The phase composition and structure of the SFT, STF and ST hollow spheres were identified by X-ray powder diffraction (XRD). Fig. 3b shows that the crystal peaks of SFT hollow spheres at 2θ = 25.4°, 37.9°, 48.0°, 53.8°, 54.9° and 62.8° are indexed to the standard XRD patterns of anatase TiO2 (JCPDS: No. 21-1272), indicating the formation of anatase TiO2. It should be pointed out that the peak intensity of STF is evidently weaker than SFT and ST, suggesting that the Fe2O3 nanoparticles deposited on the surface of TiO2 decrease the crystallinity of TiO2 [37]. This result may decrease the separation efficiency of photogenerated carriers because the density of crystal defects is increased with the decrease in crystallinity [38]. Incidentally, the signal of αFe2O3 is not detected due to low loading amount and ultra-small crystallinity, as reported in previous publication [22]. On the other hand, the sizes of TiO2 in SFT, STF, and ST have been calculated by using Debye-Scherer equation, and the parameters of the unit cell ((a) and (c)) of the three materials have also been determined by jade software and given in Supplementary Materials.

3.2. The UV–vis absorbance spectra and surface chemical status of SFT hollow spheres Fig. 4a, b show the UV–vis absorption spectra and corresponding Kubelka–Munk plots of SFT and STF photocatalysts. STF photocatalyst exhibits stronger visible-light absorption than SFT photocatalyst because the exposed Fe2O3 (~2.2 eV) usually can boost visible-light absorption of TiO2 [20]. However, the as-synthesized SFT manifests a stronger UV light absorption than STF because Fe2O3 on the surface of TiO2 blocks TiO2 to absorb UV light [39]. Incidentally, Fe2O3 decorating does not affect the bandgap of TiO2 (Fig. 4b). The XPS full spectrum (Fig. S3) of SFT photocatalyst shows that the Ti, O, C, and Si peaks can be observed with the binding energies of Ti2p, O1s, C1s and Si2p, suggesting that SFT sample consists of SiO2, C and TiO2. Similarly, the signal of Fe is hard to be detected because of its extreme low amount (~0.45 wt%) [40]. Fig. 5a shows that the peaks of Ti2p located at ~464.13 eV (Ti2p1/2) and 458.54 eV (Ti2p3/2) both manifest positive shifts than corresponding peaks of pure TiO2 (~463.96 eV and 458.24 eV). The higher binding energies can be attributed to the interaction between TiO2 and Fe2O3 as well as C species. The positive shift of O1s binding energy (Fig. 5b) is also clearly seen for SFT, indicating that the formation of oxygen vacancies [41]. The XPS spectrum of C1s can be divided into three peaks at 284.79, 286.65 and 288.64 eV, indicating the existence of three different chemical environments. The 286.65 and 288.64 eV are related to oxygen bound species CeO and C]O respectively, indicating the formation of carbonate species [42]. While the main peak at 284.79 eV is dominant by elemental carbon, which can serve as an adsorbent for antibiotics [42,43]. No peak at ~281 eV (Ti-C bond) is observed, suggesting that the C element is not doped into TiO2 lattice [43]. In addition, Si2p spectrum of SFT has no similar shift in comparison to pure SiO2, indicating that SiO2 does not affect the binding energy of TiO2. 3.3. Photocatalytic activity of SFT hollow spheres 3.3.1. Visible-light photocatalytic activity Under visible-light irradiation, the degradation of TC is almost negligible, indicating that TC is very difficult to be photo-mineralized in the absence of catalyst. As shown in Fig. 6a, SiO2@TiO2 (ST) may remove ~72.1% TC after 80 min, while almost the same degradation rate of TC is obtained for SiO2@TiO2-Fe2O3 (STF) photocatalyst, suggesting that a

Fig. 1. Schematic illustration of design and fabrication process of SFT hollow spheres. 3

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Fig. 2. SEM (a), TEM (b, c), HRTEM (d), and EDS elemental mapping (e–j) images of SFT hollow spheres.

small amount of Fe2O3 loaded on the surface of TiO2 does not evidently improve the photocatalytic activity. By contrast, the degradation rate of TC over the as-synthesized SFT increases to ~80% under the same conditions. In addition, the degradation of ENR under visible light irradiation was also investigated (Fig. 6b). After 40 min, STF and ST obtain higher removal rates than SFT due to the higher adsorption amount of ENR in dark. Interestingly, the removal rate of ENR over STF and ST photocatalysts do not increase with the prolongation of irradiation time, while SFT achieves the highest degradation rate (~70.5%) after 160 min, indicating an enhanced photocatalytic activity.

synthesized SFT photocatalyst exhibits 100% TC degradation, indicating the complete removal of TC (Fig. 7a). While STF and ST photocatalysts only achieve ~90% TC degradation. In addition, the effect of pH on photocatalytic degradation of TC over SFT photocatalyst was carried out under simulated solar-light irradiation. The result shows that SFT almost remains the same degradation efficiency in pH range 3–7 (Fig. S4), while the degradation rate of TC evidently decreases when the pH is adjusted to 9 and 11 because the photogenerated holes favor the production of hydroxyl radicals rather than the directly oxidation of TC and the hydroxyl ions are able to scavenge hydroxyl free radicals under basic pH values [44]. Such an enhanced photocatalytic activity of SFT becomes more obvious towards the degradation of enrofloxacin (ENR) under simulated solar-light irradiation (Fig. 7b). After 80 min, the degradation rate of ENR for SFT photocatalyst reaches 100%, far beyond the values of ENR for ST (~68%) and STF (~65%). Therefore, the higher photocatalytic activity of SFT is ascribed to confinement effect because of the same

3.3.2. Simulated solar-light photocatalytic activity In comparison with the visible-light activity of TC, the self-degradation of TC is improved to 14.8% after 140 min under simulated solarlight irradiation (Fig. 7). This result shows that even solar light irradiation does not resulted in high removal of TC. By contrast, the as-

Fig. 3. BET isotherms and BJH pore size distribution (a) of SFT, SiO2 and TiO2 hollow spheres, XRD patterns (b) of SFT, STF and ST hollow spheres. 4

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Fig. 4. UV–vis absorption spectra (a) and Band gap energy (b) of SFT and STF hollow spheres.

loading amount of Fe2O3 and similar adsorption amount of TC and ENR.

natural sunlight irradiation in different weather conditions, as recorded in Fig. 8. The self-degradation of TC still is very weak even under strong sunlight irradiation (sunny weather), while TC is completely decomposed (100%) within 80 min in the presence of SFT photocatalyst (Fig. 8c). Excitedly, even in cloudy weather, the almost complete

3.3.3. Natural sunlight photocatalytic activity To evaluate the practical potential of the as-synthesized SFT photocatalyst, the photocatalytic activity was further investigated under

Fig. 5. XPS spectra of the full survey spectrum: Ti 2p (a), O 1s (b), C 1s (c), and Si 2p (d). 5

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Fig. 6. Photocatalytic activities of SFT, STF and ST photocatalysts towards degradation of TC (a) and ENR (b) under visible-light irradiation (λ > 420 nm, Cat.: 0.2 mg mL−1, TC: 10 mg L−1, ENR: 5 mg L−1).

Fig. 7. Photocatalytic activities of SFT, STF and ST photocatalysts towards degradation of TC (a) and ENR (b) under simulated solar-light irradiation (AM 1.5G irradiation, Cat.: 0.2 mg mL−1, TC: 10 mg L−1, ENR: 5 mg L−1).

degradation of TC can also be achieved after 140 min (Fig. 8d). Moreover, Fig. 8e shows that ENR is also completely degraded after 80 min (sunny weather), while the removal rate of ENR also approaches to ~98% in the cloudy day (Fig. 8f), indicating the good potential of practical application.

photoreaction process. This result may be attributed to the pollutants or intermediates adsorbed and/or accumulated on the surface of STF photocatalyst that occupy the photocatalytic active sites and block the supply of oxygen molecules to the active sites [47], impairing the photocatalytic activity of STF. Although the superoxide radicals are produced almost at the same rate for two photocatalysts, which is demonstrated by electron spin resonance (ESR) (Fig. 9c), transition metal may establish coordination bonds with organic molecules, and thus block the sorption sites [48,49]. As a result, the photoelectrons cannot rapidly react with oxygen, the accumulated photoelectrons will rapidly recombine with photogenerated holes. On the other hand, molecules adsorbed on the surface of STF photocatalyst are harder to be degraded because the holes in Fe2O3 have weaker oxidation power than those of TiO2 because the valence band (VB) position of TiO2 is more positive than Fe2O3 [50]. While SFT photocatalyst can efficiently decompose the refractory intermediates to avoid poisoning and deactivation than STF photocatalyst because of outer surface without Fe2O3 covering. Such a phenomenon seems more evident in the case of degrading ENR (Fig. 7b) because the STF exhibits a high adsorption capacity (~56%) in dark that insignificantly blocks the sorption sites, poisoning the STF photocatalyst. Besides, SFT photocatalyst exhibits a higher photocurrent and smaller arc radius than STF photocatalyst (Fig. 10), suggesting a more

3.4. Photocatalytic degradation mechanism The active species during the degradation process of TC are very significant to understand the difference of the photocatalytic activity between SFT and STF photocatalysts. Various scavenger agents, such as t-butanol (t-BuOH), triethanolamine (TEOA) and N2, were employed to capture hydroxyl radicals (%OH), photogenerated holes (h+) and superoxide radicals (%O2−), respectively [45,46]. As shown in Fig. 9, the addition of t-BuOH has little effect on the degradation of TC, indicating that hydroxyl radical is not the main active specie. By contrast, after adding TEOA, the degradation of TC is significantly inhibited, suggesting that holes with strong oxidation power play a critical role in opening the robust macromolecular ring of TC. Interestingly, when N2 is bubbled into the photoreaction system, the degradation of TC over SFT photocatalyst is greatly restrained, while the value of TC over STF photocatalyst almost remains unchanged before and after N2 bubble, suggesting that superoxide radicals are not nearly involved in the STF 6

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Fig. 8. Photocatalytic activities of SFT photocatalyst towards degradation of TC (c, d) and ENR (e, f) under natural sunlight in different weather (Solar irradiation, Cat.: 0.2 mg mL−1, TC: 10 mg L−1, ENR: 5 mg L−1).

effective separation of charge carriers [51]. Therefore, SFT photocatalyst has more holes on outer surface of TiO2 to decompose antibiotic molecules directly [52].To further understand the interfacial charge transfer in the heterojunction, the Mott-Schottky plots and UV–vis absorption spectra were used to analyze the charge migration pathway (Fig. 11a, b). According to the intercept of the axis with potential values, the flat band potential (Efb) of TiO2 and Fe2O3 are determined to about −0.96 V and −0.25 V vs Ag/AgCl, corresponding with −0.36 V and 0.35 V vs normal hydrogen electrode (NHE) (pH = 0), respectively [53]. The Mott-Schottky plots of Fe2O3 and TiO2 both hold the positive slopes (a, b), indicating that they are n-type

semiconductors [53]. As the Efb is about 0.1 V below the CBM (CB minimum) potential for n-type semiconductor [54], the CBM of TiO2 and Fe2O3 are −0.46 V and 0.25 V vs NHE, respectively. Given the band gap of TiO2 (3.2 eV) and Fe2O3 (2.1 eV) (Fig. S5), the VBMs (VB maximum) of TiO2 and Fe2O3 are determined to be 2.74 V and 1.85 V, respectively. Accordingly, the charge migration pathway is proposed (Fig. 11c). Due to the more positive CBM of Fe2O3 than TiO2, photogenerated electrons in TiO2 can easily transfer to CB of Fe2O3, which is thermodynamically more favorable, and then further migrate to the surface to reduce oxygen. Holes remaining in VB of TiO2 can easily transfer to the surface for oxidation reactions. In this sense, Fe2O3 acts 7

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Fig. 9. Degradation of TC over the SFT (a) and STF (b) samples with addition of different sacrificial agents (t-BuOH, TEOA and N2 under simulated solar-light irradiation (AM 1.5G irradiation, Cat.: 0.2 mg mL−1, TC: 10 mg L−1), ESR spectra of DMPO-%O2− (c) and DMPO-%OH (d) adducts in SFT and STF photocatalysts aqueous dispersion system.

While the outside Fe2O3 in STF sample can partially block TiO2 from absorbing incident light due to shading effect, leading to low quantum efficiency [29], as illustrated in Fig. 12.

as a co-catalyst in SFT system to enhance the oxidation power of TiO2. Moreover, Fe2O3 is well protected by outer TiO2, ensuring that the outer surface of TiO2 presents highly efficient oxidation reactions.

Fig. 10. Transient photocurrent response of SFT and STF photocatalysts (a), EIS Nyquist plots of the electrodes (b). 8

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Fig. 11. Mott-Schottky plots for TiO2 (a) and Fe2O3 (b), Schematic of charge migration pathway (c).

3.5. Transformation intermediates and possible pathways

and amine group of TC are easily attacked by radicals because they are functional groups with relatively high electron density [55,56]. Pathway I is the hydroxylation process. Different positions of TC molecule are attacked by hydroxyl radical and the hydroxylated TC is transformed to possible structures of TC1 or TC2, and then TC3 [55,57].

To obtain the degradation pathway of antibiotics, the intermediates were monitored by MS and two possible degradation pathways of TC were proposed (Fig. 13 and Table S1). The double bond, phenolic group

Fig. 12. Schematic of illustrating the photocatalytic mechanism for the enhanced photodegradation of SFT compared with STF photocatalyst. 9

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Fig. 13. The proposed structures of intermediates and the suggest transformation pathways of photocatalytic tetracycline degradation.

The degradation pathway reveals that hydroxyl radical is not the mainly active specie because TC molecule structure is not damaged or broken in hydroxylation process, suggesting hydroxyl radical quenching, which fairly agrees with the results of active species. Pathway II is the attack path of holes and superoxide radicals. As the lower energy of N-C bond, the attack of holes leads to the formation of multiple N-demethylation process, producing intermediate TC4 and then TC5 [56]. After the further attack of holes and superoxide radicals,

the TC5 is decomposed to TC6 via ring opening reaction. The intermediates of TC7 and TC8 are attributed to decarboxylation and oxidation reaction. Owing to the open-ring and the oxidation reaction, photogenerated intermediates are further transformed to carboxylic acids (TC9) or alcohols and ultimately decomposed to CO2 and H2O. Moreover, the possible degradation pathway of ENR was also proposed, as shown in Fig. 14. The ENR is firstly transferred to ENR1 through ring opening reaction [58]. ENR1 can be further transferred to

Fig. 14. The proposed structures of intermediates and the suggest transformation pathways of photocatalytic enrofloxacin degradation. 10

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light irradiation. Especially, SFT photocatalyst manifests an excellent recyclability and chemical stability because of confinement effect compared with STF, offering a new insight in constructing other metal oxides confined photocatalysts. Acknowledgements Financial supports of this research from the National Natural Science Foundation of China (Grants 21707054), China Postdoctoral Science Foundation (No. 2016M601744), and the China Scholarship Council (CSC), and Postgraduate Research & Practice Innovation Program of Jiangsu Province (KYCX19_1620). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cej.2019.122583. References Fig. 15. The recycle of SFT and STF for TC photodegradation under simulated solar-light irradiation (AM 1.5G irradiation, Cat.: 0.2 mg mL−1, TC: 10 mg L−1).

[1] B. Wang, X.L. Lv, D.W. Feng, L.H. Xie, J. Zhang, M. Li, Y.B. Xie, J.R. Li, H.C. Zhou, Highly stable Zr(IV)-based metal-organic frameworks for the detection and removal of antibiotics and organic explosives in water, J. Am. Chem. Soc. 138 (2016) 6204–6216. [2] Z. Lu, F. Chen, M. He, M. Song, Z. Ma, W. Shi, Y. Yan, J. Lan, F. Li, P. Xiao, Microwave synthesis of a novel magnetic imprinted TiO2 photocatalyst with excellent transparency for selective photodegradation of enrofloxacin hydrochloride residues solution, Chem. Eng. J. 249 (2014) 15–26. [3] T. Wang, W. Quan, D. Jiang, L. Chen, D. Li, S. Meng, M. Chen, Synthesis of redoxmediator-free direct Z-scheme AgI/WO3 nanocomposite photocatalysts for the degradation of tetracycline with enhanced photocatalytic activity, Chem. Eng. J. 300 (2016) 280–290. [4] S. Zhao, J. Chen, Y. Liu, Y. Jiang, C. Jiang, Z. Yin, Y. Xiao, S. Cao, Silver nanoparticles confined in shell-in-shell hollow TiO2 manifesting efficiently photocatalytic activity and stability, Chem. Eng. J. 367 (2019) 249–259. [5] L. Jing, Y. Xu, M. Xie, J. Liu, J. Deng, L. Huang, H. Xu, H. Li, Three dimensional polyaniline/MgIn2S4 nanoflower photocatalysts accelerated interfacial charge transfer for the photoreduction of Cr(VI), photodegradation of organic pollution and photocatalytic H2 production, Chem. Eng. J. 360 (2019) 1601–1612. [6] Y. Choi, H.-I. Kim, G.-H. Moon, S. Jo, W. Choi, Boosting up the low catalytic activity of silver for h2 production on Ag/TiO2 photocatalyst: thiocyanate as a selective modifier, ACS Catal. 6 (2016) 821–828. [7] X. Liu, P. Lv, G. Yao, C. Ma, P. Huo, Y. Yan, Microwave-assisted synthesis of selective degradation photocatalyst by surface molecular imprinting method for the degradation of tetracycline onto Cl-TiO2, Chem. Eng. J. 217 (2013) 398–406. [8] X. Feng, P. Wang, J. Hou, J. Qian, Y. Ao, C. Wang, Significantly enhanced visible light photocatalytic efficiency of phosphorus doped TiO2 with surface oxygen vacancies for ciprofloxacin degradation: synergistic effect and intermediates analysis, J. Hazard. Mater. 351 (2018) 196–205. [9] H. Choi, D. Shin, B.C. Yeo, T. Song, S.S. Han, N. Park, S. Kim, Simultaneously controllable doping sites and the activity of a W-N codoped TiO2 photocatalyst, ACS Catal. 6 (2016) 2745–2753. [10] L.Q. Liu, S.X. Ouyang, J.H. Ye, Gold-Nanorod-photosensitized titanium dioxide with wide-range visible-light harvesting based on localized surface plasmon resonance, Angew. Chem. Int. Ed. Engl. 52 (2013) 6689–6693. [11] D. Papoulis, Halloysite based nanocomposites and photocatalysis: a review, Appl. Clay Sci. 168 (2019) 164–174. [12] A.P. Singh, N. Kodan, B.R. Mehta, A. Held, L. Mayrhofer, M. Moseler, Band edge engineering in BiVO4/TiO2 heterostructure: enhanced photoelectrochemical performance through improved charge transfer, ACS Catal. 6 (2016) 5311–5318. [13] Y. Xiao, X. Sun, J. Li, S. Chen, C. Zhao, L. Jiang, L. Yang, S Cao Cheng, Simultaneous formation of a C/N-TiO2 hollow photocatalyst with efficient photocatalytic performance and recyclability, Chin. J. Catal. 40 (2019) 765–775. [14] V. Bhatia, A.K. Ray, A. Dhir, Enhanced photocatalytic degradation of ofloxacin by co-doped titanium dioxide under solar irradiation, Sep. Purif. Technol. 161 (2016) 1–7. [15] Q. Chen, S. Wu, Y. Xin, Synthesis of Au-CuS-TiO2 nanobelts photocatalyst for efficient photocatalytic degradation of antibiotic oxytetracycline, Chem. Eng. J. 302 (2016) 377–387. [16] S. Yu, Y. Wang, F. Sun, R. Wang, Y. Zhou, Novel mpg-C3N4/TiO2 nanocomposite photocatalytic membrane reactor for sulfamethoxazole photodegradation, Chem. Eng. J. 337 (2018) 183–192. [17] A. Kaur, A. Umar, S.K. Kansal, Sunlight-driven photocatalytic degradation of nonsteroidal anti-inflammatory drug based on TiO2 quantum dots, J. Colloid. Interfaces Sci. 459 (2015) 257–263. [18] L. Rimoldi, E. Pargoletti, D. Meroni, E. Falletta, G. Cerrato, F. Turco, G. Cappelletti, Concurrent role of metal (Sn, Zn) and N species in enhancing the photocatalytic activity of TiO2 under solar light, Catal. Today 313 (2018) 40–46. [19] L. Liu, Z. Ji, W. Zou, X. Gu, Y. Deng, F. Gao, C. Tang, L. Dong, In situ loading transition metal oxide clusters on TiO2 nanosheets As Co-catalysts for exceptional

ENR2 or ENR3 after loss of an aldehyde group. The ENR4 is produced by the loss of an aldehyde group, and then is transferred into ENR5 via dealkylation. After that, ENR5 undergoes oxidation, producing the ENR6. The ENR6 changes to RNR7 through the loss of an aldehyde group, which further is transferred to ENR8 by defluorination. ENR8 is subjected to dealkylation and is transferred to ENR9. These intermediates may further be oxidized through ring opening reactions and generate inorganic acid ions and organic acids [58], which are finally mineralized to NO3−, CO2 and H2O. 3.6. Stability of SFT photocatalyst Stability of photocatalyst is an important factor in evaluating its practical potential [59–61], and thus a series of recycling experiments were performed using SFT and STF as photocatalysts towards the degradation of TC and ENR under simulated sunlight irradiation. After each photodegradation experiment, the sample was separated by centrifugation, washed with distilled water, and ultimately dried at 50 °C for the next test. As shown in Fig. 15, after five consecutive runs, the removal rate of TC over STF photocatalyst decreases from ~90% to ~75%. By contrast, the removal rate of TC over the as-synthesized SFT photocatalyst almost remains unchanged even after five recycle tests. On one hand, the decrease in the reusability of the STF may be attributed to the photo-corrosion and dissolution of Fe2O3 during photoreaction process. On the other hand, it may be caused by the chemisorption of the TC molecule and intermediates poison the photocatalyst because there was no significant loss of degradation rate of ENR over STF compared with that of TC (Fig. S6). In addition, the hierarchical structure and crystalline morphology of SFT are still very well after five recycle tests, indicating the good chemical stability (Figs. S7 and 8). 4. Conclusion In summary, we designed and synthesized the hierarchical SiO2Fe2O3@TiO2 hollow spheres. By confining Fe2O3 inside photocatalyst, the SFT photocatalyst not only can efficiently overcome deactivation, shading effect, easy corrosion and dissolution arising from directly immobilizing Fe2O3 on the surface of TiO2, but also substantially enhance the adsorption of antibiotic molecules because of its hierarchical hollow structure, facilitating the separation efficiency of e−/h+ pairs due to the ultra-small size Fe2O3. Therefore, the SFT photocatalyst presents the complete removal of TC and ENR under simulated solar11

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S. Zhang, et al. high photoactivity, ACS Catal. 3 (2013) 2052–2061. [20] X. She, J. Wu, H. Xu, J. Zhong, Y. Wang, Y. Song, K. Nie, Y. Liu, Y. Yang, M.T.F. Rodrigues, R. Vajtai, J. Lou, D. Du, H. Li, P.M. Ajayan, High efficiency photocatalytic water splitting using 2D alpha-Fe2O3/g-C3N4 Z-scheme catalysts, Adv. Energy Mater. 7 (2017). [21] Z. Lin, P. Liu, J. Yan, G. Yang, Matching energy levels between TiO2 and α-Fe2O3 in a core–shell nanoparticle for visible-light photocatalysis, J. Mater. Chem. A 3 (2015) 14853–14863. [22] A. Banisharif, A.A. Khodadadi, Y. Mortazavi, A. Anaraki Firooz, J. Beheshtian, S. Agah, S. Menbari, Highly active Fe2O3-doped TiO2 photocatalyst for degradation of trichloroethylene in air under UV and visible light irradiation: experimental and computational studies, Appl. Catal. B: Environ. 165 (2015) 209–221. [23] L. Qin, X. Pan, L. Wang, X. Sun, G. Zhang, X. Guo, Facile preparation of mesoporous TiO2(B) nanowires with well-dispersed Fe2O3 nanoparticles and their photochemical catalytic behavior, Appl. Catal. B: Environ. 150–151 (2014) 544–553. [24] D. Wang, G. Yang, Q. Ma, M. Wu, Y. Tan, Y. Yoneyama, N. Tsubaki, Confinement effect of carbon nanotubes: copper nanoparticles filled carbon nanotubes for hydrogenation of methyl acetate, ACS Catal. 2 (2012) 1958–1966. [25] L.H. Wee, M. Meledina, S. Turner, K. Custers, S. Kerkhofs, G. van Tendeloo, J.A. Martens, Hematite iron oxide nanorod patterning inside COK-12 mesochannels as an efficient visible light photocatalyst, J. Mater. Chem. A 3 (2015) 19884–19891. [26] J. Mahajan, P. Jeevanandam, Synthesis of TiO2@α-Fe2O3 core–shell heteronanostructures by thermal decomposition approach and their application towards sunlight-driven photodegradation of rhodamine B, New J. Chem. 42 (2018) 2616–2626. [27] B. Sun, W. Zhou, H. Li, L. Ren, P. Qiao, F. Xiao, L. Wang, B. Jiang, H. Fu, Magnetic Fe2O3/mesoporous black TiO2 hollow sphere heterojunctions with wide-spectrum response and magnetic separation, Appl. Catal. B: Environ. 221 (2018) 235–242. [28] J.A. Libera, J.W. Elam, N.F. Sather, T. Rajh, N.M. Dimitrijevic, Iron(III)-oxo centers on TiO2 for visible-light photocatalysis, Chem. Mater. 22 (2010) 409–413. [29] J. Xie, R. Jin, A. Li, Y. Bi, Q. Ruan, Y. Deng, Y. Zhang, S. Yao, G. Sankar, D. Ma, J. Tang, Highly selective oxidation of methane to methanol at ambient conditions by titanium dioxide-supported iron species, Nat. Catal. 1 (2018) 889–896. [30] A. Nadar, A.M. Banerjee, M.R. Pai, S.S. Meena, R.V. Pai, R. Tewari, S.M. Yusuf, A.K. Tripathi, S.R. Bharadwaj, Nanostructured Fe2O3 dispersed on SiO2 as catalyst for high temperature sulfuric acid decomposition—Structural and morphological modifications on catalytic use and relevance of Fe2O3-SiO2 interactions, Appl. Catal. B: Environ. 217 (2017) 154–168. [31] S. Cao, J. Chang, L. Fang, L. Wu, Metal nanoparticles confined in the nanospace of double-shelled hollow silica spheres for highly efficient and selective catalysis, Chem. Mater. 28 (2016) 5596–5600. [32] Y. Zhang, Y. Zhao, S. Cao, Z. Yin, L. Cheng, L. Wu, Design and synthesis of hierarchical SiO2@C/TiO2 hollow spheres for high-performance supercapacitors, ACS Appl. Mater. Interfaces 9 (2017) 29982–29991. [33] J. Shao, W. Sheng, M. Wang, S. Li, J. Chen, Y. Zhang, S. Cao, In situ synthesis of carbon-doped TiO2 single-crystal nanorods with a remarkably photocatalytic efficiency, Appl. Catal. B: Environ. 209 (2017) 311–319. [34] Y. Zhang, J. Chen, H. Tang, Y. Xiao, S. Qiu, S. Li, S. Cao, Hierarchically-structured SiO2-Ag@TiO2 hollow spheres with excellent photocatalytic activity and recyclability, J. Hazard. Mater. 354 (2018) 17–26. [35] R. Daghrir, P. Drogui, Tetracycline antibiotics in the environment: a review, Environ. Chem. Lett. 11 (2013) 209–227. [36] T.S. Anirudhan, F. Shainy, J. Christa, Synthesis and characterization of polyacrylic acid- grafted-carboxylic graphene/titanium nanotube composite for the effective removal of enrofloxacin from aqueous solutions: adsorption and photocatalytic degradation studies, J. Hazard. Mater. 324 (2017) 117–130. [37] B. Zhao, G. Mele, I. Pio, J. Li, L. Palmisano, G. Vasapollo, Degradation of 4-nitrophenol (4-NP) using Fe–TiO2 as a heterogeneous photo-Fenton catalyst, J. Hazard. Mater. 176 (2010) 569–574. [38] R. Marschall, Semiconductor composites: strategies for enhancing charge carrier separation to improve photocatalytic activity, Adv. Funct. Mater. 24 (2014) 2421–2440. [39] Q. Sun, W. Leng, Z. Li, Y. Xu, Effect of surface Fe2O3 clusters on the photocatalytic activity of TiO2 for phenol degradation in water, J. Hazard. Mater. 229–230 (2012) 224–232. [40] N. Wang, Q. Sun, R. Bai, X. Li, G. Guo, J. Yu, In Situ confinement of ultrasmall Pd clusters within nanosized silicalite-1 zeolite for highly efficient catalysis of hydrogen generation, J. Am. Chem. Soc. 138 (2016) 7484–7487. [41] Y. Zhang, J. Chen, L. Hua, S. Li, X. Zhang, W. Sheng, S. Cao, High photocatalytic activity of hierarchical SiO2@C-doped TiO2 hollow spheres in UV and visible light

towards degradation of rhodamine B, J. Hazard. Mater. 340 (2017) 309–318. [42] Z. Jiang, W. Wei, D. Mao, C. Chen, Y. Shi, X. Lv, J. Xie, Silver-loaded nitrogen-doped yolk–shell mesoporous TiO2 hollow microspheres with enhanced visible light photocatalytic activity, Nanoscale 7 (2015) 784–797. [43] L. Zhao, X. Chen, X. Wang, Y. Zhang, W. Wei, Y. Sun, M. Antonietti, M.-M. Titirici, One-step solvothermal synthesis of a carbon@TiO2 dyade structure effectively promoting visible-light photocatalysis, Adv. Mater. 22 (2010) 3317–3321. [44] L. Yue, S. Wang, G. Shan, W. Wu, L. Qiang, L. Zhu, Novel MWNTs–Bi2WO6 composites with enhanced simulated solar photoactivity toward adsorbed and free tetracycline in water, Appl. Catal. B: Environ. 176–177 (2015) 11–19. [45] H. Zhang, W. Wang, H. Zhao, L. Zhao, L.-Y. Gan, L.-H. Guo, Facet-dependent interfacial charge transfer in Fe(III)-grafted TiO2 nanostructures activated by visible light, ACS Catal. 8 (2018) 9399–9407. [46] X. Lv, T. Wang, W. Jiang, Preparation of Ag@AgCl/g-C3N4/TiO2 porous ceramic films with enhanced photocatalysis performance and self-cleaning effect, Ceram. Int. 44 (2018) 9326–9337. [47] T.T. Xiao, Z. Tang, Y. Yang, L.Q. Tang, Y. Zhou, Z.G. Zou, In situ construction of hierarchical WO3/g-C3N4 composite hollow microspheres as a Z-scheme photocatalyst for the degradation of antibiotics, Appl. Catal. B: Environ. 220 (2018) 417–428. [48] H.U. Farouk, A.A.A. Raman, W.M.A.W. Daud, TiO2 catalyst deactivation in textile wastewater treatment: current challenges and future advances, J. Ind. Eng. Chem. 33 (2016) 11–21. [49] G. Zhang, G. Kim, W. Choi, Visible light driven photocatalysis mediated via ligandto-metal charge transfer (LMCT): an alternative approach to solar activation of titania, Energy Environ. Sci. 7 (2014) 954–966. [50] S. Kment, F. Riboni, S. Pausova, L. Wang, L. Wang, H. Han, Z. Hubicka, J. Krysa, P. Schmuki, R. Zboril, Photoanodes based on TiO2 and alpha-Fe2O3 for solar water splitting - superior role of 1D nanoarchitectures and of combined heterostructures, Chem. Soc. Rev. 46 (2017) 3716–3769. [51] C. Gao, T. Wei, Y. Zhang, X. Song, Y. Huan, H. Liu, M. Zhao, J. Yu, X. Chen, A Photoresponsive Rutile TiO2 Heterojunction with Enhanced Electron-Hole Separation for High-Performance Hydrogen Evolution, Adv. Mater. (2019) 1806596. [52] X. Liu, Z. Xing, Y. Zhang, Z. Li, X. Wu, S. Tan, X. Yu, Q. Zhu, W. Zhou, Fabrication of 3D flower-like black N-TiO2-x@MoS2 for unprecedented-high visible-light-driven photocatalytic performance, Appl. Catal. B: Environ. 201 (2017) 119–127. [53] W. Yin, L. Bai, Y. Zhu, S. Zhong, L. Zhao, Z. Li, S. Bai, Embedding metal in the interface of a p-n heterojunction with a stack design for superior z-scheme photocatalytic hydrogen evolution, ACS Appl. Mater. Interfaces 8 (2016) 23133–23142. [54] C. Zeng, Y. Hu, H. Huang, BiOBr 0.75I0.25/BiOIO3 as a novel heterojunctional photocatalyst with superior visible-light-driven photocatalytic activity in removing diverse industrial pollutants, ACS Sustain. Chem. Eng. 5 (2017) 3897–3905. [55] J. Wang, D. Zhi, H. Zhou, X. He, D. Zhang, Evaluating tetracycline degradation pathway and intermediate toxicity during the electrochemical oxidation over a Ti/ Ti4O7 anode, Water Res. 137 (2018) 324–334. [56] L. Ren, W. Zhou, B. Sun, H. Li, P. Qiao, Y. Xu, J. Wu, K. Lin, H. Fu, Defects-engineering of magnetic γ-Fe2O3 ultrathin nanosheets/mesoporous black TiO2 hollow sphere heterojunctions for efficient charge separation and the solar-driven photocatalytic mechanism of tetracycline degradation, Appl. Catal. B: Environ. 240 (2019) 319–328. [57] Y. Yang, Z. Zeng, C. Zhang, D. Huang, G. Zeng, R. Xiao, C. Lai, C. Zhou, H. Guo, W. Xue, M. Cheng, W. Wang, J. Wang, Construction of iodine vacancy-rich BiOI/ Ag@AgI Z-scheme heterojunction photocatalysts for visible-light-driven tetracycline degradation: transformation pathways and mechanism insight, J. Chem. Eng. 349 (2018) 808–821. [58] H. Guo, N. Jiang, H. Wang, N. Lu, K. Shang, J. Li, Y. Wu, Pulsed discharge plasma assisted with graphene-WO3 nanocomposites for synergistic degradation of antibiotic enrofloxacin in water, Chem. Eng. J. 372 (2019) 226–240. [59] J. Yi, X. She, Y. Song, M. Mao, K. Xia, Y. Xu, Z. Mo, J. Wu, H. Xu, H. Li, Solvothermal synthesis of metallic 1T-WS2: a supporting co-catalyst on carbon nitride nanosheets toward photocatalytic hydrogen evolution, Chem. Eng. J. 335 (2018) 282–289. [60] Y. Liu, J. Kong, J. Yuan, W. Zhao, X. Zhu, C. Sun, J. Xie, Enhanced photocatalytic activity over flower-like sphere Ag/Ag2CO3/BiVO4 plasmonic heterojunction photocatalyst for tetracycline degradation, Chem. Eng. J. 331 (2018) 242–254. [61] X. Feng, P. Wang, J. Hou, J. Qian, C. Wang, Y. Ao, Oxygen vacancies and phosphorus codoped black titania coated carbon nanotube composite photocatalyst with efficient photocatalytic performance for the degradation of acetaminophen under visible light irradiation, Chem. Eng. J. 352 (2018) 947–956.

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