Ordered mesoporous carbon-silica frameworks confined magnetic mesoporous TiO2 as an efficient catalyst under acoustic cavitation energy

Journal of Materiomics 6 (2020) 45e53

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Ordered mesoporous carbon-silica frameworks confined magnetic mesoporous TiO2 as an efficient catalyst under acoustic cavitation energy Pengpeng Qiu a, 1, Tao Zhao a, 1, Jeehyeong Khim b, Wan Jiang a, Lianjun Wang a, Wei Luo a, * a State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Institute of Functional Materials, Donghua University, Shanghai, 201620, China b School of Civil, Environmental and Architectural Engineering, Korea University, Seoul, 136-701, Republic of Korea

a r t i c l e i n f o

a b s t r a c t

Article history: Received 18 September 2019 Received in revised form 26 October 2019 Accepted 5 November 2019 Available online 16 November 2019

Herein, we report a biphase stratification strategy that enables the encapsulation of magnetic mesoporous TiO2 inside an ordered mesoporous C/SiO2 framework. The obtained composites exhibit high surface areas (up to 600 m2 g
1), large perpendicular pore sizes (up to 9 nm) and a strong magnetic response (~10.0 emu g
1), presenting significantly enhanced degradation activities toward pentachlorophenol (PCP) and bisphenol-A (BPA) under acoustic cavitation energy. The remarkable performance is ascribed to the synergistic effect from the unique structural modulation: 1) The large ordered mesopores favors the mass transfer, 2) The mesoporous C/SiO2 frameworks promote the adsorption of organic pollutants and enrich them close to the TiO2 surface and 3) The special spatial arrangement of different components facilitates the generation of cavitation bubbles, leading to the increase in the overall hydroxyl-radical-production rate. Moreover, owing to the effective confinement, the as-prepared materials possess an excellent stability and durability. More importantly, the catalysts can easily be recovered by a magnet and show an excellent reusability. It is believed that these results could provide an important insight for the development of an efficient, stable and facile recoverable catalyst for the acoustic chemical process. © 2019 The Chinese Ceramic Society. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Keywords: Fe3O4@mTiO2@mC/SiO2 Acoustic cavitation Magnetic separation Mechanical stability

1. Introduction Advanced oxidation technologies (AOTs) have recently attracted tremendous attention for the elimination of organic pollutants through powerful reactive oxidation species (ROS) such as hydroxyl radicals [1e4]. Among many conventional AOTs such as ozonolysis [5], photolysis [6,7], the Fenton process [8,9], and photocatalytic oxidation [10e13], the sonochemical process are of great interest owing to their unique features such as safety, easy-operation, and environmentally-friendly [14
16]. This technique is principally based on the acoustic cavitation, which involves the nucleation, growth, and violent collapse of microbubbles, leading to the formation of so called ‘hot spot’ with extremely high temperature (~5000 K) and pressure (~1000 atm) [17e19]. Under such high

* Corresponding author. E-mail address: [email protected] (W. Luo). Peer review under responsibility of The Chinese Ceramic Society. 1 P. P. Qiu and T. Zhao contributed equally.

energy input, water molecules can be pyrolyzed to produce highly reactive oxidative species such as OH, H and OOH, which are capable of mineralizing organic pollutants. In addition to chemical effects, the collapse of cavitation bubble could also lead to the emission of light, known as sonoluminescence (SL), which exhibits a wide wavelength range (200e700 nm) and a relatively high intensity (0.098 W, corresponded to a wavelength of 400 nm) [20]. In this regard, plenty of TiO2-based materials have been applied as highly effective catalysts to enhance the sonochemical degradation performance by utilizing SL owing to their excellent photocatalytic properties, nontoxicity and easy-production [21e26]. For example, Wang et al. synthesized a gold nanoparticle anchored TiO2 sonocatalyst, exhibiting a superior activity for degrading azo dyes [21]. The introduction of Au nanoparticles could greatly enhance both the oxidative and reductive reaction due to the improved photo-generated electrons and holes. Parizot and coworkers reported a Co3O4/TiO2 sonocatalyst for an efficient degradation of ethylenediaminetetraacetic acid. The high performance was due to the decrease of the

https://doi.org/10.1016/j.jmat.2019.11.003 2352-8478/© 2019 The Chinese Ceramic Society. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/).

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recombination rate of electron-hole pairs of excited TiO2 by the scavenge effect of Co3þ [25]. Another mechanism that determines the sonocatalytic performance is the number and strength of cavitation bubbles, closely related to the initial heterogeneous nucleation rate which could be well tuned by the surface properties of the catalyst such as roughness, pore size, wettability, and so on [27e32]. Till now, substantial research achievements have been made for examining the effects of wettability and roughness on heterogeneous cavitation nucleation, whereas patterning of a sonocatalyst surface with a mesoporous structure and investigation on the effects of the pore size and surface area was rarely considered. Besides the limited catalytic efficiency, another challenge remained in the practical application of nanosized sonocatalysts is the separation of them from the complex heterogeneous system for recovery. The integration with some magnetic recyclable components such as Fe3O4, g-Fe2O3, etc provides a very facile approach for the separation of the catalysts by simply using an external magnetic field [33e36]. Moreover, the catalyst in the ultrasonic system inevitably suffers from the continuous pitting of the high power microjets [37e39]. The mechanical stability of sonocatalyst could always be an important issue. Therefore, the rational design and synthesize a sonocatalyst with simultaneous properties of high efficiency, facile separation and strong mechanical stability is greatly desired. Herein, a mesoporous C/SiO2 framework wrapped magnetic mesoporous TiO2 composites (Fe3O4@mTiO2@mC/SiO2) have been fabricated via a bisphase stratification approach, followed by a calcination process at 650  C under N2. The adjustment of added TEOS amount during the synthesis are evaluated for the production of the composites with different physical properties such as pore size, pore volume and BET surface area. Then, the magnetic composites are examined as a catalyst for the degradation of pentachlorophenol (PCP) and bisphenol-A (BPA) under acoustic cavitation. We have found that the catalyst with a large pore size and a high surface area shows the best performance, which is independent on the type of pollutant. Moreover, the catalysts can be easily recycled within 5 min by using an external magnetic field and show an excellent reusability and mechanical stability. 2. Experimental section 2.1. Materials FeCl3$6H2O (98%), trisodium citrate (98%), anhydrous sodium acetate (ACS reagent, 99.0%), concentrated ammonia solution (28 wt%), hexadecyl trimethyl ammonium bromide (99%, CTAB), benzoic acid (99.5%), potassium penxosisulfate (K2S2O8, 99.0%), tetraethyl orthosilicate (TEOS, 98%), titanium (IV) isopropoxide (TIPO, 97%), BPA (Supelco, 99%), and PCP (98%) were bought from Sigma-Aldrich (USA). Anhydrous tert-butyl alcohol (TBA, 99.5%) were obtained from Duksan (Republic of Korea). Ethylenediaminetetraacetic disodium salt dehydrate (EDTA-2Na, 98%) was provided by Daejung Chemical & Metal Co., Ltd (Republic of Korea). Anhydrous ethylene glycol and ethyl alcohol were provied from Samchun Pure Chemicals. All chemicals used without any further purification. Millipore water was used for all experiments. 2.2. Synthesis of Fe3O4 nanoparticles The Fe3O4 nanoparticles were fabricated through a solvothermal method reported previously [40]. In brief, FeCl3$6H2O (3.25 g), sodium acetate (NaAc, 6.0 g), and trisodium citrate (1.3 g) were completely dissolved in ethylene glycol (80 mL) with magnetic stirring and the resultant solution was sonicated for 1 h.

Afterwards, the obtained solution was transferred to a Teflon-lined stainless-steel autoclave (100 mL in capacity) and heated at 200  C for 10 h. After cooling to room temperature, the black products were washed with deionized water and ethanol for 3 times, respectively and then dispersed into 50 mL of ethanol for further use. 2.3. Synthesis of core-shell structured Fe3O4@mTiO2 nanospheres The core-shell structured Fe3O4@mTiO2 nanospheres were prepared through a kinetic-controlled coating strategy, followed by an ultrasound assisted post-hydrolysis process [41]. Typically, an ethanol dispersion of the Fe3O4 magnetite particles obtained above (3.0 mL, 0.05 g mL-1) was added to a three-neck round-bottom flask with ethanol (100 mL) and ammonia solution (0.4 mL, 28 wt %). Subsequently, 0.7 mL of TIPO was added dropwise in 5 min, and the reaction was allowed to proceed for 20 h at 45  C under continuous mechanical stirring. The resultant products (denoted as Fe3O4@TiO2) were collected with a magnet and dispersed into Millipore water (45 mL) in a falcon tube. Then, the solution was sonicated for 1 h using an ultrasonic bath (frequency ¼ 40 kHz; power ¼ 80 W). The bath temperature was controlled at 25  C using a water-cooling system. Finally, the product was washed with ethanol and water for 3 times, respectively and then dispersed into 5 mL of water for further use (defined as Fe3O4@mTiO2). 2.4. Synthesis of Fe3O4@mTiO2@mC/SiO2 nanostructure The magnetic mesoporous TiO2 was encapsulated in the ordered mesoporous C/SiO2 framework through a biphase stratification strategy reported previously with a slight modification [42,43]. The obtained Fe3O4@mTiO2 water dispersion were added in a mixed solution consisting of CTAB (0.5 g, 1.3 mmol), deionized water (75 mL), and concentrated ammonia solution (0.8 mL, 28 wt %) with an ultrasonic treatment for 15 min. Into the aqueous solution, 20 mL of cyclohexane was added to form a bi-liquid phase system. Subsequently, certain amount of TEOS was added dropwise into the solution, and all the syntheses were carried out under a gentle stirring rate of 150 rpm for maintaining an oil-water bi-liquid phase. After reacting for 12 h at 45  C, the product (defined as Fe3O4@mTiO2@CTAB/SiO2) was collected with a magnet and washed with ethanol and water. Finally, the sample was dried at 60  C for overnight and transferred to a tubular oven for crystallizing TiO2 and carbonizing carbon at 650  C under a flow of N2. The dose of added silica precursor was adjusted (0.30, 0.50, 1.0 mL) and their corresponded products were denoted as Fe3O4@mTiO2@mC/ SiO2-1, Fe3O4@mTiO2@mC/SiO2-2, and Fe3O4@mTiO2@mC/SiO2-3, respectively. As a comparison, the sample Fe3O4@TiO2 obtained without post-hydrolysis process was directly coated with a mesoporous C/SiO2 framework using the same synthetic strategy as above. The obtained sample was designated as Fe3O4@TiO2@mC/ SiO2. 2.5. Materials characterization Nitrogen adsorption and desorption isotherms were analysed at 77 K with a Micromeritics Tristar 3020 analyser (USA). Prior to measurements, the samples were degassed in a vacuum at 180  C for 6 h. The specific surface areas were calculated using BrunauerEmmett-Teller (BET) method based on the adsorption data in the relative pressure range of P/P0 ¼ 0.04e0.2. The pore size distributions were derived from the adsorption branches of the isotherms using the Barrett-Joyner-Halenda (BJH) model and the total pore volumes (V) were estimated from the adsorbed amount at the relative pressure P/P0 of 0.995. X-ray diffraction (XRD) patterns

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were recorded on a Bruker D8X-ray diffractometer with Ni-filtered Cu Ka radiation (40 kV, 40 mA). Raman spectra were recorded on a combined Raman FT-IR spectrometer (LabRam ARAMIS IR2) with the excitation of a 532 nm laser. Transmission electron microscopy (TEM) images were taken on a JEOL 2011 microscope (Japan) operated at 200 kV. The magnetization was measured using a Vibrating Sample Magnetometer (EV9 including automatic sample rotation, Microsense, Japan) under a magnetic field of 10 KOe and a temperature of 24  C. 2.6. Sonocatalytic degradation measurements Fig. S1 shows the schematic illustration for the experimental set-up for carrying out the sonocatalytic test. A double-layered cylindrical container with a capacity of 1.25 L (F 10.0  16.0 cm) equipped with a cupehorn-type ultrasonic transducer (Mirae Ultrasonic MEGA-100, Republic of Korea) at the bottom was used as the sonoreactor. The frequency and power were controlled as 300 kHz and 100 W, respectively. The 800-mL reactor (F 7.0  20.0 cm) was immersed into a container containing 500 mL of water for the reaction. The solution temperature was measured using a thermometer (Tecpel DTM-318) and was maintained with a water jacket. The distance between the bottom of the reactor and container was about 6.0 cm. The catalytic experiments were tested in aqueous suspensions (100 mL) of organic pollutants (10 mg L
1) and catalyst (0.5 g L
1). At given time intervals, 0.5 mL of the suspension was removed using a 2-mL syringe and filtered by a membrane with a pore size of ~0.45 mm. The BPA and PCP concentration in the resultant filtrate was analysed on a HighPerformance Liquid Chromatography (HPLC, Agilent 1260) with a ZORBAX Eclipse Plus C18 column (4.6  100 mm, 5 mm) and a diode array UV detector (G4212B 1260 DAD, l ¼ 222 nm). 2.7. Hydroxyl radical measurements Benzoic acid was selected as the probe compound to detect the sono-generated hydroxyl radicals. The sonocatalytic reaction was proceeded in the presence of an excess amount of benzoic acid (1.1 g L
1), which ensured that all ∙OH formed during reaction was completely scavenged. The concentration of 4-hyrogenbenzoic acid (4-HBA) generated after benzoic acid scavenging ∙OH was measured by using HPLC. Owing to the adsorption of 4-HBA on the catalyst, we have summed the mass of 4-HBA from both the bulk solution and that adsorbed on catalyst. For measuring the 4-HBA adsorbed on the catalyst, the experiment was carried out as follows: The catalyst was recycled by applying a magnetic field and washed four times repeatedly by using 250 mL of water (Until no 4HBA was detected in current batch of washing water). The washing water was mixed together and 2 mL of resultant solution was withdrawn for HPLC analysis. 3. Results and discussion 3.1. Materials characterization The synthetic process for the uniform magnetic Fe3O4@mTiO2@mC/SiO2 is illustrated in Fig. 1. First, a compact amorphous TiO2 layer was deposited on the surface of Fe3O4 nanoparticles via a €ber coating method, resulting in the core-shell structured Sto Fe3O4@TiO2 nanospheres. Second, the resultant products were immersed into water and then subjected to an additional ultrasound assisted post-hydrolysis process, which led to the formation of mesoporous Fe3O4@mTiO2. Then, the oil-water biphase stratification strategy was applied to coat a uniform mesoporous CTAB/ SiO2 composite layer on the Fe3O4@mTiO2. Finally, after a thermal

Fig. 1. Schematic illustration for wrapping the magnetic mesoporous TiO2 inside the ordered mesoporous C/SiO2 framework.

treatment in N2 atmosphere, uniform magnetic mesoporous Fe3O4@mTiO2@mC/SiO2 with a highly crystalline anatase inner shell and an ordered dendritic mesoporous C/SiO2 outer shell were obtained. In a control experiment, the Fe3O4@TiO2 nanosphere were directly subjected to an oil-water biphase stratification process without post-hydrolysis treatment, which resulted in Fe3O4@TiO2@mC/SiO2 with disordered worm-like mesoporous pores. The magnetite particles were fabricated by a versatile hydrothermal method based on a high temperature reduction of Fe (III) salts with ethylene glycol in the presence of trisodium citrate. The obtained Fe3O4 particles possess a uniform spherical shape with an average particle size of ~130 nm (Fig. S2A). The Fe3O4@TiO2 nano€ber coating spheres prepared through the kinetics-controlled Sto method show a smooth surface with a diameter of ~180 nm (Fig. S2B). TEM image reveals that a TiO2 shell with a thickness of ~40 nm is uniformly coated onto the magnetic core, resulting in a well-defined core-shell structure. After the post-hydrolysis process, uniform mesoporous Fe3O4@mTiO2 nanospheres are obtained (Figs. S2C and D). TEM images show that the previous smooth TiO2 shells is converted into a rough one. Our previous work has found €ber that the TiO2 shells derived from the kinetics-controlled Sto coating method consists of large amount of incompletely hydrolyzed frameworks because low-content ammonia which contains limited water was used as a catalyst source as well as a reactant [44]. Therefore, the hydrolysis and condensation of TBOT are directed towards the middle structures (Ti(OH)4-n(OR)n, n > 1) rather than the fully condensed species (Ti(OH)4). After the ultrasound assisted post-hydrolysis of Fe3O4@TiO2 sample in water, the TieOR moieties on the surface of TiO2 can be further condensed to form TieOH groups, thus leading to a rough surface. Owing to the presence of large number of eOH groups on TiO2 surface, the further application of biphase stratification process leads to the deposition of a uniform mesoporous CTAB/SiO2 composite shell on the Fe3O4@mTiO2 nanospheres (Fig. S3 A, B, E and F). The effect of TEOS amount during the synthesis on the physical properties of the resultant composites was evaluated. As the added TEOS amount rises, the thickness of the CTAB/SiO2 shell increases. This result is

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reasonable because more precursor concentration leads to a faster reaction rate. Notably, after the core-shell Fe3O4@mTiO2@CTAB/ SiO2 samples being calcined at 650  C in N2, uniform honeycomblike structure with large open mesopores of ~9 nm (Fig. 2A) can be obtained. In addition, the TEM results show that the core-shell structures are well retained and the mesopore channels become open and clear which is due to the transformation of CTAB to amorphous carbon during annealing process (Figs. S3C, D, G and H, Fig. 2 B and C). The HRTEM image (Fig. S4) clearly shows that the TiO2 nanoparticles are well crystallized with a particle size of 6.5 nm and a d-spacing of 0.35 nm, which can be assigned to the d101 of anatase. Energy-dispersive X-ray (EDX) taken on the marked dark TEM image of the Fe3O4@mTiO2@mC/SiO2-1.0 (Fig. 2D) displays the characteristic peaks of Fe, Ti, C, Si and O, suggesting the coexistence of Fe3O4, TiO2, SiO2 and C. Note that the direct application of biphase stratification method on the Fe3O4@TiO2 without a post-hydrolysis process is not able to get uniform mesoporous C/ SiO2 coating because of the less amount of hydroxyl groups on the surface of TiO2, which is not favorable for the absorption of CTAþ ions and silica oligomers (Fig. S5). N2 sorption isotherms of the Fe3O4@mTiO2@mC/SiO2 nanospheres show a characteristic IV curve with a hysteresis loops close to H1-type and a sharp increase in the adsorption branch at a

relative pressure of P/P0 ¼ 0.65:0.8, further suggesting that mesoporous C/SiO2 shells contain uniform and large cylindrical pores (Fig. 3A). The BET surface area and pore volume of the Fe3O4@mTiO2@mC/SiO2 composites were calculated to be in the range of 334e600 m2 g
1 and increased as the rise of added TEOS amount, which agrees well with the increased mesoporous C/SiO2 shell thickness. The pore size distribution curve (Fig. 3B) of the core-shell Fe3O4@mTiO2@mC/SiO2 composites derived from the adsorption branch of the isotherms by using the BJH model clearly revealed a uniform pore size in the range of 7.0e8.7 nm. When the TEOS increased to 0.5 mL, the pore size attained the maximum and further increase of TEOS amount, no change of pore size was observed. To prove the presence of carbon, the Raman spectra was recorded in the range of 500e3000 cm
1 (Fig. 3C). In the Raman spectra of the resultant composites, the appearance of two prominent peaks D (1368 cm
1) and G (1590 cm
1), which can be assigned to the disordered hybridized carbon atoms (A1g) and the stretching vibrations of sp2 bonds of perfect graphite crystals (E2g mode), respectively, suggesting the presence of carbon [45,46]. The relatively large ID/IG ratio reveals a high degree of amorphous structure of the carbon. In addition, the carbon contents in the composites through TGA analyses (Fig. S6) is estimated to be 16.9%. The XRD pattern (Fig. 3D) of the Fe3O4@mTiO2@mC/SiO2-3 nanospheres displays several broad characteristic diffraction peaks, which are typical for amorphous silica, Fe3O4 and anatase TiO2, respectively. Based on the Scherrer formula, the particle size of TiO2 is estimated to be 6.3 nm, which corresponds well with the result obtained from the HRTEM image. Anatase TiO2 is not a thermally stable phase and it’s supposed to be converted to rutile beyond 500  C. However, TiO2 in this study can still keep anatase when heated at 650  C, possibly owing to the stabilization effect by the outer C/SiO2 shell. The magnetization saturation values of pristine Fe3O4 particles and Fe3O4@mTiO2@mC/SiO2-3 are measured to be ~58.4 and 10.0 emu g
1, respectively (Fig. S7). As a result of the superparamagnetic property and high magnetization, the core-shell Fe3O4@mTiO2@mC/SiO2-3 nanospheres in their homogeneous dispersion show fast motion under the applied magnetic field and quick dispersibility upon a slight shake when the magnetic field is removed. 3.2. Sonocatalytic performance

Fig. 2. SEM (A) and TEM (B and C) images of Fe3O4@mTiO2@mC/SiO2-3 composites and EDX analysis of Fe3O4@mTiO2@mC/SiO2-3 composites (D).

The performances of the resultant core-shell materials on the sonocatalytic degradation of PCP and BPA were examined (Fig. 4). Prior to the irradiation of US, the suspension was mechanically (200 rpm) stirred in dark for 60 min to reach the adsorption/ desorption equilibrium between the catalyst and pollutants. As a control, the performances of TiO2 and magnetic mesoporous TiO2 were also examined. TiO2 exhibits a negligible adsorption capacity for both of PCP and BPA, which is possibly due to the small BET surface area (25.0 m2 g
1). An approximately 10% of removal efficiency was obtained for magnetic mesoporous TiO2, suggesting the priority of the presence of a mesoporous structure. When core-shell structured Fe3O4@mTiO2@mC/SiO2 catalysts were added, the adsorption rate proceeds very fast (close to saturation within 5 min and reaching equilibrium within ~15 min), ascribed to the presence of ordered large open mesoporous channels in the catalysts. With the increase in the amount of TEOS precursor, the adsorption capacities of the resultant composites for both two pollutants increase, consistent to the increase in the BET surface area and pore size. In addition, all the resultant composites show larger adsorption capacities for PCP than for BPA, possibly owing to the fact that the former one (Kow ¼ 5.12) is more hydrophobic than the latter one (Kow ¼ 1.95). This is because the presence of carbon inside the

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Fig. 3. N2 sorption isotherms (A), pore-size distributions (B), Raman spectra (C) and XRD pattern (D) of the resultant composites. Fe3O4@mTiO2@mC/SiO2-1 (Green), Fe3O4@mTiO2@mC/SiO2-2 (Blue) and Fe3O4@mTiO2@mC/SiO2-3 (Red).

Fig. 4. The sonocatalytic degradation performance (A and B) and the pseudo first-order reaction kinetics (C and D) for PCP (A and C) and BPA (B and D) in the presence of various sonocatalysts. US alone (a), P25 (b), Fe3O4@mTiO2 (c), Fe3O4@mTiO2@mC/SiO2-1 (d), Fe3O4@mTiO2@mC/SiO2-2 (e), and Fe3O4@mTiO2@mC/SiO2-3 (f).

mesoporous channel enables the catalyst with hydrophobic property, which tends to adsorb more hydrophobic pollutants.

With the irradiation of ultrasound, almost 80% of pollutants can be degraded within 60 min in the absence of a catalyst. After the

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Fig. 5. (A) Sonocatalytic performances of Fe3O4@mTiO2@mC/SiO2-3 composites for the degradation of BPA in the presence of various radical scavengers, (B) Recycle tests of the Fe3O4@mTiO2@mC/SiO2-3 sonocatalyst for the degradation of BPA, and (C) HBA generation amount measured within 60 min.

addition of P25 catalyst, the degradation efficiency (~90%) improved, which is due to the formation of additional hydroxyl radicals. Although the magnetic mesoporous TiO2 shows a larger adsorption capacity than that P25, a comparable removal efficiency was finally achieved for both two organic pollutants. The addition of core-shell structured Fe3O4@mTiO2@mC/SiO2 composites as the catalysts greatly enhance the degradation performances. 100% of organic pollutants can be decomposed within 50 min. In addition, the Fe3O4@mTiO2@mC/SiO2-3 with the largest surface area and pore size shows the best degradation efficiency. To better compare the sonocatalytic performance of the resultant composites, the reaction kinetics were evaluated by fitting the experimental data with Langmuir Hinshelwood model (Fig. 4 C and D). The pseudo first-order kinetic was assumed because the initial concentration of target pollutants was low. The degradation rate constants for PCP and BPA in the absence of sonocatalyst were calculated to be 0.034 and 0.023 min
1, respectively. When P25 presents in the ultrasonic system, the degradation rate increased to 0.043 and 0.037 min
1, respectively, which is due to the enhanced hydroxyl radical rate promoted by the excited electron-hole pairs on TiO2 and reduction of cavitation threshold by inspiring a heterogenous nucleation mechanism. The magnetic mesoporous TiO2 shows slightly larger reaction rates for both PCP (0.045 min
1) and BPA (0.042 min
1). When the Fe3O4@mTiO2@mC/SiO2-1 composite was added, the reaction rate constants increased to 0.0760 and 0.066 min
1, respectively. Interestingly, as the added TEOS amount during the synthesis rises, the reaction rate of the resultant composites for both of the PCP and BPA increases, resulted from the

increased BET surface area, pore size and pore volume. The maximum reaction rates were achieved in the presence of Fe3O4@mTiO2@mC/SiO2-3 catalyst, which is independent on the organic pollutants. A series of trapping experiments were carried out to examine the major reactive species, which is involved in the sonocatalytic degradation of BPA (Fig. 5A). The activity was greatly enhanced after the addition of K2S2O8 as an electron scavenger, indicating that SL excited electrons is not the major species [47]. The great enchantment was owing to the efficient separation of electron-hole pairs. However, EDTA-2Na (electron hole scavenger) and TBA (hydroxyl radical scavenger) significantly reduced the degradation performance, suggesting that SL-excited holes and OH are the main contributor [48e50]. Note that by the addition of EDTA-2Na, the degradation performance decreased due to inhibiting the reaction of hþ with OH
or H2O to produce hydroxyl radicals. However, a significant amount of hydroxyl radicals are still generated through the cavitation effect. Therefore, the addition of TBA leads to a much smaller degradation efficiency, suggesting that OH plays the most important role in the degradation process. In this regard, the amount of OH generated in the presence of various catalyst was measured and compared (Fig. 5B). The maximum hydroxylradial-generation amount was achieved with Fe3O4@mTiO2@mC/ SiO2-3 catalyst, corresponding well with the degradation results. This is due to the fact that the large surface area and pore size provides more actives sites for the generation of cavitation bubbles. The recycling test of the composite Fe3O4@mTiO2@mC/SiO2-3 for the degradation of BPA was conducted (Fig. 5C). After each cycle,

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from its well-designed textual properties, as illustrated in Fig. 7. First, the large pore size and ordered mesoporous channels favors the mass diffusion between aqueous and solid phases. Second, the presence of ordered mesoporous C/SiO2 framework greatly enhances the adsorption of pollutants, which enriches them close to the exposed TiO2, facilitating the surface hydroxyl-radical assisted oxidation process. Third, the nucleation of cavitation bubbles within the pore structure is faster than that of at a smooth surface because mesopore corners could provide energetically preferred binding sites at which the new phase can be easily formed because of the reduced Gibbs free energy barriers [51,52]. Therefore, the mesoporous C/SiO2 shell supplies a large number of reactive sites for the generation of microbubbles, thus producing superior cavitation effects. Therefore, the high sonocatalytic performance of the Fe3O4@mTiO2@mC/SiO2 was a result of fast mass diffusion, enhanced adsorption rate, and accelerated nucleation rate. 4. Conclusion

Fig. 6. TEM images of fresh (A) and recycled (B) Fe3O4@mTiO2@mC/SiO2-3 sonocatalyst, and (C) FTIR spectra of BPA molecules, fresh and recycled Fe3O4@mTiO2@mC/SiO23 catalyst.

Magnetic mesoporous TiO2 wrapped in a mesoporous C/SiO2 framework was fabricated (Fe3O4@mTiO2@mC/SiO2) via a biphase stratification approach, followed by a calcination process at 650  C under N2. The effect of added TEOS amount during the synthesis was optimized for the production of the catalysts with favorable physical properties for enhancing the sonochemical degradation of PCP and BPA, respectively. The catalyst with a large pore size and a high surface area exhibited the best degradation performance, which is due to the large adsorption rate and high OH production rate. Moreover, the catalysts can be easily recycled within 5 min by using an external magnetic field and an almost constant activity is retained after four recycles. This study provides an important insight into the design and synthesis of an efficient, stable and facile recoverable sonocatalyst for degrading organic pollutants, which is very important from both fundamental and practical application viewpoints. Declaration of competing 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. Acknowledgment

Fig. 7. Schematic illustration of the sonocatalytic mechanism for the degradation of organic pollutants in the presence of the Fe3O4@mTiO2@mC/SiO2 catalyst.

the catalyst was collected by using a hand-held magnetic bar and used directly for the next recycle test without any further treatment. After four recycles, a slight decrease in the degradation performance was observed, which is possibly resulted from the accumulation of BPA molecules in the mesoporous channels. A clear BPA peak observed in the Fourier Transform infrared spectroscopy (FTIR) result of the recycled Fe3O4@mTiO2@mC/SiO2-3 sample supports the claim above (Fig. 6C). The TEM images before and after four cycles’ reuse (Fig. 6A and B) clearly showed that the ordered mesoporous channels were well retained after four cycles, suggesting the excellent mechanical stability of the catalyst. Based on the results above, the excellent performances of the Fe3O4@mTiO2@mC/SiO2 composites for the sonocatalytic degradation of organic pollutants can be ascribed to the synergistic effects

This work was supported by the NSF of China (Grant nos. 51822202 and 51772050), Shanghai Rising-Star Program (18QA1400100), Youth Top-notch Talent Support Program of Shanghai, Shanghai Scientific and Technological Innovation Project (19JC1410400), the Shanghai Committee of Science and Technology, China (19520713200), DHU Distinguished Young Professor Program and Fundamental Research Funds for the Central Universities. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jmat.2019.11.003. References [1] Matafonova G, Batoev V. Recent advances in application of UV light-emitting diodes for degrading organic pollutants in water through advanced oxidation processes: a review. Water Res 2018;132:177e89. [2] He K, Chen G, Zeng G, Chen A, Huang Z, Shi J, Huang T, Peng M, Hu L. Threedimensional graphene supported catalysts for organic dyes degradation. Appl Catal B Environ 2018;228:19e28. [3] Zhou Y, Xiang Y, He Y, Yang Y, Zhang J, Luo L, Peng H, Dai C, Zhu F, Tang L.

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Dr. Pengpeng Qiu is now a lecturer in the State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering at Donghua University. He received his Ph.D. from Korea University, South Korea, in 2017 supervised by Professor Jeehyeong Khim. Then, he worked as a postdoctoral researcher at Fudan University under Professor Dongyuan Zhao. His research interests focus on the design and synthesis of mesoporous materials for environmental application.

P. Qiu et al. / Journal of Materiomics 6 (2020) 45e53 Tao Zhao is a PhD candidate in the State Key Laboratory for Modification of Chemical Fibersand Polymer Materials, College of Materials Science and Engineering at Donghua University. Currently, his research is focusing on the designed synthesis of mesoporous materials with controllable structures and functions for energy conversion and storage as well as sensors.

Dr. Jeehyeong Khim received his B.S. (1984) and M.S (1986) from Seoul National University. Then, he joined University of Texas at Austin for PhD study (1994). He is now a Professor in the Department of Civil, Environmental and Architectural Engineering at Korea University in South Korea. His research interests include the design and synthesis of environmental catalysts, the research and development of advanced oxidation technology for wastewater treatment, water pollution and solid waste treatment processes and reactors.

Dr. Wan Jiang once worked at Shanghai Institute of Ceramics, CAS from 1999 to 2009. Now he is a distinguished Professor in Donghua University. Also he is the deputy director of Institute of Functional Materials and the director

53 of the Engineering Research Center of AGMT Ministry of Education, Donghua University.

Dr. Lianjun Wang received his PhD (2002) in Chemical Engineering from Dalian University of Technology. He then joined in the group of Professor Wan Jiang (the Shanghai Institute of Ceramics, CAS) and professor Zhijian James Shen (Stockholm University) as a postdoctral fellow. He is currently a professor at Donghua University. His current research interests focus on developing a new versatile processing strategy of fabricating Optical Silica Glasses derived from mesoporous silica by Spark Plasma Sinteirng.

Dr. Wei Luo is now a professor in the State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering at Donghua University. He obtained the Bachelor (2006) and Master (2009) degree at Nanjing Tech University (China). He received his PhD. in chemistry in 2014 from Fudan University supervised by Professor Dongyuan Zhao. He received the second prize of Natural Science Award of Ministry of Education (the third awardee). His research interests mainly include the synthesis of functional mesoporous and nanomaterials for energy storage and conversion as well as sensors.