CoWO4 composite and its application in sonocatalytic degradation of tetracyclines

Accepted Manuscript Fabrication of a novel Z-scheme SrTiO3/Ag2S/CoWO4 composite and its application in sonocatalytic degradation of tetracyclines Jing Qiao, Hongbo Zhang, Guanshu Li, Siyi Li, Zhihui Qu, Meng Zhang, Jun Wang, Youtao Song PII: DOI: Reference:

S1383-5866(18)32539-5 https://doi.org/10.1016/j.seppur.2018.10.058 SEPPUR 15045

To appear in:

Separation and Purification Technology

Received Date: Revised Date: Accepted Date:

24 July 2018 25 October 2018 25 October 2018

Please cite this article as: J. Qiao, H. Zhang, G. Li, S. Li, Z. Qu, M. Zhang, J. Wang, Y. Song, Fabrication of a novel Z-scheme SrTiO3/Ag2S/CoWO4 composite and its application in sonocatalytic degradation of tetracyclines, Separation and Purification Technology (2018), doi: https://doi.org/10.1016/j.seppur.2018.10.058

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Separation and Purification Technology 00 (2018) 000-000

Fabrication of a novel Z-scheme SrTiO3/Ag2S/CoWO4 composite and its application in sonocatalytic degradation of tetracyclines Jing Qiao a, Hongbo Zhang a, Guanshu Li b, Siyi Li a, Zhihui Qu b, Meng Zhang a, Jun Wang a, *, Youtao Song b,* a b

College of Chemistry, Liaoning University, Shenyang 110036, P. R. China

College of Environment, Liaoning University, Shenyang 110036, P. R. China

Received 00 July 2018; received in revised form 00 July 2018; accepted 00 July 2018; Available online 00 July 2018

ABS TRACT A ternary SrTiO3/Ag2S/CoWO4 composite as an effective Z-scheme sonocatalyst was prepared. X-ray diffractometer (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), UV-vis diffuse reflectance spectroscopy (DRS) and photoluminescence (PL) spectroscopy were applied for characterization of prepared samples. Tetracycline (TTC) was used as a target pollutant for investigating performance of Z-scheme SrTiO3/Ag2S/CoWO4 sonocatalyst. Some affecting factors, ultrasonic irradiation time, ultrasonic irradiation power, solution pH, recycling times, sonocatalyst dosage and initial TTC concentration, on sonocatalytic degradation were explored and trapping experiments of free radicals (•OH and h+) were also carried out. The sonocatalytic degradation route of TTC was confirmed based on the intermediates. The experimental results revealed that, because of existence of Ag2S as trapezoid electronic channel and co-catalyst, Z-scheme SrTiO3/Ag2S/CoWO4 composite displayed outstanding sonocatalytic performance. Therefore, it can be predicted that SrTiO3/Ag2S/CoWO4 as Z-scheme sonocatalyst has potential application in environmental purification. © 2018 Elsevier B.V. All rights reserved. Keywords: Z-scheme SrTiO3/Ag2S/CoWO4 system, Trapezoid electronic channel, Sonocatalytic degradation, Tetracyclines.

* Corresponding author. Tel.: +86 024 62207861; Fax: +86 024 62202053. E-mail addresses: [email protected] (J. Wang); [email protected] (J. Wang); [email protected] (Y. Song). http://dx.doi.org/10.1000/j.seppur.2018.00.000 0000-0000/© 2018 Elsevier Ltd. All rights reserved. 1

Separation and Purification Technology 00 (2018) 000-000

Fabrication of a novel Z-scheme SrTiO3/Ag2S/CoWO4 composite and its application in sonocatalytic degradation of tetracyclines Jing Qiao a, Hongbo Zhang a, Guanshu Li b, Siyi Li a, Zhihui Qu b, Meng Zhang a, Jun Wang a, *, Youtao Song b,* a

College of Chemistry, Liaoning University, Shenyang 110036, P. R. China

b

College of Environment, Liaoning University, Shenyang 110036, P. R. China

ARTI C LE

I N FO

Article history: Received 00 July 2018 Revised 00 July 2018 Accepted 00 July 2018 Available online 00 July 2018

Keywords: Z-scheme SrTiO3/Ag2S/CoWO4 system Trapezoid electronic channel Sonocatalytic degradation Tetracyclines

ABS TRACT A ternary SrTiO3/Ag2S/CoWO4 composite as an effective Z-scheme sonocatalyst was prepared. X-ray diffractometer (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), UV-vis diffuse reflectance spectroscopy (DRS) and photoluminescence (PL) spectroscopy were applied for characterization of prepared

* Corresponding author. Tel.: +86 024 62207861; Fax: +86 024 62202053. E-mail addresses: [email protected] (J. Wang); [email protected] (J. Wang); [email protected] (Y. Song). http://dx.doi.org/10.1000/j.seppur.2018.00.000 0000-0000/© 2018 Elsevier Ltd. All rights reserved. 2

samples. Tetracycline (TTC) was used as a target pollutant for investigating performance of Z-scheme SrTiO3/Ag2S/CoWO4 sonocatalyst. Some affecting factors, ultrasonic irradiation time, ultrasonic irradiation power, solution pH, recycling times, sonocatalyst dosage and initial TTC concentration, on sonocatalytic degradation were explored and trapping experiments of free radicals (•OH and h+) were also carried out. The sonocatalytic degradation route of TTC was confirmed based on the intermediates. The experimental results revealed that, because of existence of Ag2S as trapezoid electronic channel and co-catalyst, Z-scheme SrTiO3/Ag2S/CoWO4 composite displayed outstanding sonocatalytic performance. Therefore, it can be predicted that SrTiO3/Ag2S/CoWO4 as Z-scheme sonocatalyst has potential application in environmental purification. © 2018 Elsevier B.V. All rights reserved.

1. Introduction Antibiotics have been broadly used in pharmaceuticals because of strong inhibitive ability and destroyed effect in ecological environment [1,2]. As one of the widely used antibiotics, tetracyclines are used as antimicrobial agents in medical treatments and feed supplements in animal husbandry [3-5]. However, researchers have detected tetracyclines in eggs, milk, vegetables, meat and other food [6]. Because of benzene skeleton structure and high hydrophility, tetracyclines are difficult to be destroyed and easy to concentrate in aquatic environment [2]. So far, aquatic environment pollution caused by the overuse of tetracyclines has aroused wide concern. Consequently, the removal of tetracyclines from environment has been a important work [7,8]. Currently, some ways have been proposed to remove stubborn organic pollution from wastewater, such as chemical flocculation, adsorption, ion exchange, activated sludge, etc. [9,10]. Nevertheless, these traditional treatments of wastewater generally have many disadvantages, such as long processing cycle, inexhaustive degradation, easily cause the secondary pollution and so on. Moreover, the antibiotics are some special organic molecular compounds, which have relatively stable chemical

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structures and compositions [11]. Therefore, it is an exigent mission to search an effective and environmentally friendly way for removing stubborn organic pollutant [12]. Among AOPs, recently developed sonocatalytic degradation technology has aroused increasing attention. The ultrasonic wave can be used instead of the light wave to excite the sonocatalyst, executing degradation experiment. Specifically, ultrasound has advantages of powerful penetrability and easy operation in comparison with light [11]. Ultrasonic degradation of organic pollutants is related to the cavitation effect resulted from ultrasonic irradiation. When the ultrasound is used to irradiate liquid, the cavitation bubbles occur, enlarge, condense and collapse [13,14]. The whole process takes place for a short period of time and causes the emergence of sonoluminescence and “hot-spots”. The “hot-spots” can produce high temperatures and high pressures that exceed 5000 K and 1000 atm [15-17], respectively. Particularly, when the sonocatalyst exists, a synergistic effect of ultrasonic and solid catalyst particles can be found in degradation of organic pollutants [18]. In recent years, Z-scheme sonocatalyst has been widely concerned because it is effective for elimination of stubborn pollutants [19,20]. Being different from traditional charge-transfer type, Z-scheme system is proved to be effective in boosting the photo-induced electron-hole (e--h+) pairs separation and obtaining more positive valence band (VB) and more negative conduction band (CB) [21,22]. Because of these obvious advantages, the Z-scheme heterostructure sonocatalyst has been considered as the ideal sonocatalytic system for degradation organic pollutants [23]. In order to constitute an ideal Z-scheme sonocatalyst, selecting semiconductors with appropriate energy band is crucial. Among a large amount of semiconductors, Strontium titanate (SrTiO3) with perovskite structure has a wide band-gap (3.40 eV) [24]. Because of excellent catalytic performance and heat endurance, SrTiO3 should be one of the most promising sonocatalyst [25,26]. Cobalt tungstate 4

(CoWO4) as one of tungstate semiconductor possesses a relatively narrow band-gap (2.70 eV) [27]. The combination of SrTiO3 and CoWO4 not only can effectually broaden the response range of light, but also get more oxidative VB of CoWO4 and more reductive CB of SrTiO3 [28]. In the past years, it has been proposed that the use of Cu, Ag and Au as electronic channel to boost charge-transfer is an effective way [29-31]. However, the used noble metals have narrow and fixed Fermi energy-levels. When the potential difference between VB of the first semiconductor and the CB of the second semiconductor is relatively wide, it may not be ideal to use above noble metals as electronic channel. Because Fermi energy-levels of noble metal are not only away from VB of SrTiO3, but also away from CB of CoWO4. With this in mind, an extreme narrow band-gap semiconductor, like a “conductive ladder”, can be used as conductive channel. It is very conducive to e- from CB of CoWO4 flow to the VB of SrTiO3 by combining with a narrow band-gap semiconductor. As electronic channel, Silver sulfide (Ag2S) can boost the electron transfer. Like a ladder, Ag2S (ECB = 0.00 eV, EVB = +1.00 eV and ΔEbg = 1.00 eV) forms relatively denser electronic energy-levels. The VB of Ag2S is close to the VB of SrTiO3, and CB of Ag2S is close to the CB of CoWO4. It should be very conducive to e- transfer from the CB of CoWO4 to the VB of SrTiO3 through Ag2S as “conductive ladder”. Thus, such combination of e- from the CB of CoWO4 and h+ from the VB of SrTiO3 can efficiently restrain the recombination of electron-hole (e--h+) pairs of CoWO4 and SrTiO3. In addition, the Ag2S can also serve as co-catalyst. It is also conducive to the e- transfer from the CB of SrTiO3 to Ag2S surface and can be used for reduction O2, forming •O2-. The h+ on the VB of CoWO4 can be used for oxidation organic pollutants into CO2 and H2O. In this work, SrTiO3, CoWO4 and Ag2S are used to fabricate a novel Z-scheme SrTiO3/Ag2S/CoWO4 sonocatalyst, executing degradation experiment of tetracyclines under ultrasonic irradiation. 5

In the present work, a novel Z-scheme sonocatalyst composite, SrTiO3/Ag2S/CoWO4, is chosen to carry out sonocatalytic degradation of tetracyclines under ultrasonic irradiation. And then, a series of degradation experiments of tetracyclines are performed under some changeable conditions such as ultrasonic irradiation power, solution pH, catalyst dosage and TTC initial concentration. Besides, the effects of ultrasonic irradiation time, recycling times and free radical trapping agents on sonocatalytic performance of Z-scheme SrTiO3/Ag2S/CoWO4 are also investigated. LC-MS analysis is employed to identify the formed intermediates during the sonocatalytic degradation. Finally, a probable sonocatalytic degradation mechanism caused by Z-scheme SrTiO3/Ag2S/CoWO4 is also presented. The results displayed that the SrTiO3/Ag2S/CoWO4 as a Z-scheme sonocatalyst could effectually degrade tetracyclines under ultrasonic irradiation.

2. Experimental 2.1. Materials and equipments Ti(OBu)4, Sr(NO3)2 and NaOH were used to prepare strontium titanate (SrTiO3). Na2WO4·2H2O and Co(NO3)2·6H2O were used to prepare cobalt tungstate (CoWO4). Na2S·9H2O and AgNO3 were used to prepare silver sulfide (Ag2S). Tetracycline (TTC), Oxytetracycline (OTC), Chlotetracycline (CTC) and Deoxytetracycline (DTC) were used to carry out sonocatalytic degradation. Their structures are given as displayed in Scheme 1. Dimethylsulfoxide (DMSO) and ethylene diamine tetraacetic acid (EDTA) were used as trapping agents of •OH and h+. All materials with analytical grade in this study are purchased through Sinopharm Chemical Regent Co, Ltd of China.

Scheme 1.

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Muffle furnace (SX-4-10, Tianjin Experiment Apparatus Company, China) and oven (101-1AB, Tianjin Experiment

Apparatus Company,

China)

were used to prepare the Z-scheme

SrTiO3/Ag2S/CoWO4 composite. X-ray powder diffractometer (XRD, D-8, Bruker-axs, Germany), scanning electron microscope (SEM, JEOL JSM-5610LV, Hitachi Corporation, Japan) and transmission electron microscope (TEM, JEOL JEM2100, Hitachi Corporation, Japan) were used to determine the crystalline phase and surface morphology of prepared samples. Energy dispersive X-ray spectroscopy (EDX, JEOL JSM-5610LV, Hitachi Corporation, Japan) and X-ray photoelectron spectroscopy (XPS, XSAM800, Shimadzu-Kratos, Japan) were used to determine the element type and composition content of Z-scheme SrTiO3/Ag2S/CoWO4 composite. UV-vis diffuse reflectance spectroscopy (DRS, UV-3600, SHIMADZU, Japan) was used to measure the optical diffuse absorption spectra of prepared samples. Photoluminescence spectroscopy (PL, F4500, Hitachi Corporation, Japan) was used to determine the recombination rate of the photo-generated electron-hole pairs in semiconductor. UV-vis spectrometer (Cary 50, Varian Company, USA) was used to inspect the sonocatalytic degradation processes of antibiotics in aqueous solution. LC-MS machine (Agilent 1290 Infinity/6460 UHPLC/MS, America) was used to detect the degradation intermediates of tetracycline (TTC). Controllable Serial-Ultrasonics apparatus (KQ-300, six transducer, Kunshan Company, China) was adopted to irradiate the antibiotic in aqueous solution, operating at ultrasonic frequency of 40 kHz and output power of 300 W.

2.2. Fabrication of sonocatalysts 2.2.1. SrTiO3 SrTiO3 was prepared by solvothermal procedure [32]. 3.40 g Ti(OBu)4 was put into a vessel with

7

40 mL absolute ethanol. The solution was blended with 2.11 g Sr(NO3)2, and then 10 mL of 5.00 mol/L NaOH solution was slowly added. The turbid liquid was put into a 100 mL capacity Teflon-lined stainless steel antoclave and maintained at 180 ºC for 24 h. After cooling to the room temperature naturally and washing several times by using ethanol and distilled water, and dried at 70 ºC for 12 h. After calcination at 550 ºC for 2.0 h, the SrTiO3 was obtained.

2.2.2. CoWO4 CoWO4 was prepared via hydrothermal approach [33]. 2.90 g Na2WO4·2H2O and 3.29 g Co(NO3)2·6H2O were joined in 40 mL distilled water. The turbid liquid was continuously stirred for 1.0 h and then put into a 100 mL capacity Teflon-lined stainless steel antoclave and maintained at 180 ºC. After 24 h, the autoclave was cooled to room temperature, the precipitates were gained with centrifugation, rinsed with distilled water several times. The final product was dried at 60 ºC for 12 h. Finally, the solid was heated at 450 °C for 2.0 h and then the CoWO4 was obtained.

2.2.3. Ag2S Ag2S was prepared via ion-exchange way [34]. 3.38 g AgNO3 was blended with 30 mL distilled water. 1.68 g Na2S·9H2O in aqueous solution was slowly added. The mixture was acutely stirred for 6.0 h. Subsequently, the black powder was filtrated and washed with deionized water three times. The black precipitates was dried overnight at 60 °C in the dark, and then heated at 300 °C for 1.0 h. In the end, the sintered powder was cooled to room temperature. The Ag2S was obtained.

2.2.4. SrTiO3/CoWO4 SrTiO3/CoWO4 composite was prepared by isoelectric point method and high temperature calcination method [19]. The isoelectric points of SrTiO3 and CoWO4 are 3.50 and 4.60 [25,27],

8

respectively. 0.91 g SrTiO3 powder was added into 40 mL distilled water and then followed by 20 min of drastic stirring. Subsequently, 1.53 g CoWO4 powder was added into above solution and then adequately dispersed for 20 min. The solution pH was adjusted to 4.0, the surface charges of CoWO4 particles and SrTiO3 particles became positive and negative, respectively. The electrostatic attraction forces can be generated between CoWO4 particles and SrTiO3 particles. Thus, one SrTiO3 particle can combine with only one CoWO4 particle. Afterwards, the turbid liquid was mild stirred for 0.50 h, heated to 70 °C and kept at invariable temperature for 0.50 h. Subsequently, the dried mixture was rinsed sequentially with distilled water. Then the final product was dried at 60 ºC for 10 h and calcination at 300 °C for 1.0 h. Finally, the SrTiO3/CoWO4 composite were obtained.

2.2.5. SrTiO3/Ag2S SrTiO3/Ag2S composite was prepared by isoelectric point method and high temperature calcination method. The isoelectric points of SrTiO3 and Ag2S are 3.50 and 10.20 [34], respectively. 1.83 g SrTiO3 was joined into 20 mL deionized water and then adequately dispersed for 20 min. Subsequently, 24 mg Ag2S was joined into above solution with stirred fiercely. The solution pH was adjusted to 6.80, the surface charges of Ag2S particles and SrTiO3 particles became positive and negative, respectively. The electrostatic attraction forces can be generated between Ag2S particles and SrTiO3 particles. Thus, one SrTiO3 particle can synchronously combine with one Ag2S particle. The homogeneous suspension was heated to 70 °C and kept at invariable temperature for 0.50 h with constantly magnetic stirring. The turbid liquid was mild stirred for 0.50 h. After centrifugation and lavation, the resulting solid was dried at 60 °C for 12 h. Then, the final product was heated in muffle furnace by calcination at 300 °C for 1.0 h. Finally, the SrTiO3/Ag2S composite were obtained.

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2.2.6. SrTiO3/Ag2S/CoWO4 The SrTiO3/Ag2S/CoWO4 composite was prepared by isoelectric point method and high temperature calcination method. The isoelectric points of SrTiO3, CoWO4 and Ag2S are 3.50, 4.60 and 10.20, respectively. Firstly, 1.85 g SrTiO3/Ag2S and 3.06 g CoWO4 were joined into 20 mL distilled water and then adequately dispersed for 20 min. The solution pH was adjusted to 7.10, the surface charges of SrTiO3 particles and CoWO4 particles became negative, while surface charges of Ag2S particles were became positive. Hence, there are electrostatic repulsion forces between SrTiO3 particles and CoWO4 particles, and there are electrostatic attraction forces between CoWO4 particles and Ag2S particles. Therefore, the CoWO4 particles and Ag2S particles attract each other and the CoWO4 particles and SrTiO3 particles repel each other. Whereafter, the mixture was heated to 70 °C and maintained for 0.50 h. The turbid liquid was mild stirred for 0.50 h. After centrifugation and lavation, the separated deposit was put into a crucible and heated in a muffle furnace by calcination at 300 °C for 1.0 h. At last, the SrTiO3/Ag2S/CoWO4 composite were obtained.

2.3. Measurements of sonocatalytic performance of SrTiO3/Ag2S/CoWO4 Sonocatalytic degradation experiments of antibiotic were implemented in a 150 mL erlenmeyer flask placed in an ultrasonic irradiation apparatus (300 (length) × 180 mm (width) × 120 mm (height), KQ-300, 40 kHz, 300 W, six transducers, Kunshan ultrasonic apparatus Company, China) under the air atmosphere. To reach the adsorption-desorption equilibrium, the mixture which composed of TTC (10.00 mg/L) and sonocatalyst (1.00 g/L) was stirred in the dark for half an hour. Subsequently, the sonocatalytic degradation of TTC was implemented under ultrasonic irradiation for 300 min. A certain of solution volume was taken out every 50 min and detected by UV-vis spectrometer. The degradation

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percentages were measured base on absorbance change of TTC solution employing following formula:

Reduction ratio (%)  (1 - At /A 0 )  100 % Where A0 and At represent absorbance of TTC before ultrasonic irradiation and after ultrasonic irradiation in a given time, respectively. Meanwhile, some effecting factors, for instance, trapping agents, ultrasonic irradiation time and recycling experiments on sonocatalytic performance of Z-scheme SrTiO3/Ag2S/CoWO4 were also researched by sonocatalytic degradation of TTC solution. For verify universality of sonocatalytic performance of Z-scheme SrTiO3/Ag2S/CoWO4, the degradations of oxytetracycline (OTC), chlorotetracycline (CTC) and deoxytetracycline (DTC) were also implemented.

3. Results and discussion 3.1. Characterization of the synthesized samples Fig. 1 gives XRD patterns of SrTiO3, CoWO4, Ag2S, SrTiO3/CoWO4 and SrTiO3/Ag2S/CoWO4. As demonstrated in Fig. 1(SrTiO3), the primary diffraction peaks of synthesized SrTiO3 locate at 32.43°, 39.97°, 46.44° and 57.74°, respectively, corresponding with (110), (111), (200) and (211) crystal planes. It is discovered that these diffraction peaks are consistent with SrTiO3 standard card (JCPDS: 35-0734) data [32], which proves that the SrTiO3 has been successfully synthesized. As displayed in Fig. 1(CoWO4), the primary diffraction peaks are locate at 19.56°, 31.18°, 36.91° and 54.60°, respectively, corresponding to the (001), (-111), (200) and (-202) crystal planes. Apparently, the XRD pattern of synthesized CoWO4 is consistent with standard data of monoclinic CoWO4 (JCPDS: 15-0867) [35], proving the formation of CoWO4 particles. In Fig. 1(Ag2S), the diffraction peaks clearly appear at 29.19°, 31.69°, 34.57°, 37.90° and 43.55°, respectively, which can be perfectly 11

indexed in the (111), (-112), (-121), (-103) and (200) of Ag2S (JCPDS: 14-0072) [34]. It proves that the Ag2S has been successfully synthesized. Fig. 1(SrTiO3/CoWO4) displays characteristic diffraction peaks of SrTiO3 and CoWO4 and have slight changes compared with pure SrTiO3 and pure CoWO4. It indicates that the SrTiO3/CoWO4 composite has also been obtained. In Fig. 1(SrTiO3/Ag2S/CoWO4), besides normative peaks of SrTiO3 and CoWO4, the peaks corresponding Ag2S can be obviously observed. Meanwhile, some diffraction peaks (at 32.43° (110) of SrTiO3, 37.90° (-103) of Ag2S and 54.60° (-202) of CoWO4) slightly broaden compared with those of their monomers, which may be caused by the interface interaction between SrTiO3 and Ag2S and between Ag2S and CoWO4. The above results demonstrate that the Z-scheme SrTiO3/Ag2S/CoWO4 composite may be prepared.

Fig. 1. The shape and size of SrTiO3, CoWO4, Ag2S and SrTiO3/Ag2S/CoWO4 were observed by SEM and corresponding images were given in Fig. 2. In Fig. 2(SrTiO3), there are numerous irregular clumps crystal nanoparticles with size from 100 nm to 200 nm, which should attribute to SrTiO3 nanoparticles [36]. From Fig. 2(CoWO4), it is clearly seen that prepared powder is composed of nanoparticles with homogeneous and nearly spherical morphology with size of approximate 50 nm, which are identified as CoWO4 nanoparticles [37]. It can be discovered in Fig. 2(Ag2S) that spherical nanoparticles with diameter of nearly 20 nm, which should be Ag2S nanoparticles [38]. As seen in Fig. 2(SrTiO3/Ag2S/CoWO4), there are numerous irregular clumps crystal nanoparticles with size range of 100-200 nm and nearly spherical nanoparticles with average size of 50 nm, which should attribute to SrTiO3 and CoWO4 nanoparticles, respectively. In addition, it can be discovered that there are spherical nanoparticles with diameters about 20 nm, which should be attributed to Ag2S nanoparticles.

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Moreover, the Ag2S nanoparticles as co-catalyst are loaded on surface of SrTiO3 and as “conducting ladder” inserted between SrTiO3 and CoWO4 particles. These discoveries confirm that predicted Z-scheme SrTiO3/Ag2S/CoWO4 has been successfully obtained.

Fig. 2. Fig. 3 gives TEM image of Z-scheme SrTiO3/Ag2S/CoWO4 composite. In Fig. 3(a), it can be observed that average sizes of SrTiO3 and CoWO4 are about 100 nm and 50 nm, respectively. According to the preparation process, the much small particles with size of 20 nm between SrTiO3 and CoWO4 should be Ag2S. In addition, it can be discovered that SrTiO3 and CoWO4 particles are connected by small grain diameter of Ag2S particles. Further, many clear lattice fringes can be discovered in Fig. 3(b). Through the calculation, the crystal faces of composite particles can be obtained. It is found that lattice space of SrTiO3 is 0.398 nm, which can be ascribed to (200) plane of SrTiO3. Moreover, the calculated lattice space of CoWO4 is 0.291 nm, ascribing to (-111) plane of CoWO4. Meanwhile, the lattice fringe spacing of 0.258 nm is corresponding to (-121) crystal plane of Ag2S. The calculated lattice spaces approach with those in previously reported literatures [35,39,40].

Fig. 3. Fig. 4 shows EDX spectrum of prepared Z-scheme SrTiO3/Ag2S/CoWO4 composite. In Fig. 4, it can be discovered that seven elements are detected, including O, W, Sr, Co, Ti, Ag and S. and that, it can indicate existence of SrTiO3, Ag2S and CoWO4 sample. Basically, the atom proportion obtained is close to our expected reasonable ratio of Z-scheme SrTiO3/Ag2S/CoWO4 composite. It confirms that predicted Z-scheme sonocatalyst has been obtained. This result is consistent with above characterizations. Specifically, in EDX can discover peaks of Ag and S elements. It indicated that 13

Ag2S as both electronic channel and co-catalyst exist in Z-scheme system, forming Z-scheme SrTiO3/Ag2S/CoWO4 composite.

Fig. 4. The chemical components and surface chemical states of Z-scheme SrTiO3/Ag2S/CoWO4 composite were analyzed by XPS and the results were shown in Fig. 5. From Fig. 5 (SrTiO3/Ag2S/CoWO4), it can be found that seven strong peaks appear at 36.08 eV, 134.08 eV, 178.05 eV, 359.01 eV, 459.05 eV, 530.08 eV and 781.06 eV, respectively, which should belong to W (4f), Sr (3d), S (2p), Ag (3d), Ti (2p), O (1s) and Co (2p). From Fig. 5(Sr), it can be seen that the Sr (3d) contains two peaks at 132.96 eV and 134.70 eV, which are assigned to Sr (3d 5/2) and Sr (3d 3/2), respectively. In Fig. 5(Ti), the binding energies at 458.46 eV and 464.15 eV belong to Ti (2p 3/2) and Ti (2p 1/2), respectively, confirming the Ti4+ presence [32]. In Fig. 5(Co) two peaks are observed at 780.38 eV and 796.28 eV, which are ascribed to the binding energies of Co (2p 3/2) and Co (2p 1/2), respectively. In Fig. 5(W), two peaks are observed at 35.17 eV and 37.30 eV, which are ascribed to the binding energies of W (4f 7/2) and W (4f 5/2) [33], respectively. From Fig. 5(Ag), the positions of Ag (3d 5/2) and Ag (3d 3/2) are locate at 367.78 eV and 373.95 eV, which are the characteristic peaks of Ag+ ion. In addition, In Fig. 5(S), the binding energies of S (2p) present at 161.54 eV and 162.72 eV, which belong to S (2p 5/2) and S (2p 3/2) [41], respectively, which are the characteristic peaks of S2ion [34]. At the same time, the XPS spectrum of O (1s) is resolved to three peaks with the energies of 529.96 eV, 531.21 eV and 532.5 eV, which could be assigned to the lattice oxygen O2-, adsorbed oxygen O- and adsorbed oxygen O2- [32], respectively. From whole Fig. 5, it can be concluded that the Z-scheme SrTiO3/Ag2S/CoWO4 composite has successfully been prepared.

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Fig. 5. In Fig. 6((SrTiO3) and (CoWO4)), it can be discovered that absorption edges of SrTiO3 and CoWO4 appear at about 400 nm and 480 nm, respectively, And that, in Fig. 6(Ag2S), it can be found that the absorption range of Ag2S is relatively wide, covering entire visible light range. It manifests that Ag2S is a narrow band-gap semiconductor sulfide and suitable as preconceived conductive passageway in Z-scheme system. Besides, it can be discovered that the absorption edges of SrTiO3/CoWO4 and SrTiO3/Ag2S/CoWO4 appear at about 480 nm and 490 nm, respectively. Comparing with the pure SrTiO3 and CoWO4, the absorption edges of the SrTiO3/CoWO4 and SrTiO3/Ag2S/CoWO4 composites show clear redshift. This result illustrates that SrTiO3/CoWO4 and SrTiO3/Ag2S/CoWO4 can improve utilization efficiency for light from sonoluminescence. Meanwhile, the optical band-gaps of prepared samples can be estimated from the following formula:

h  A(h - E bg )1/2 (α: absorptioncoefficient, h: Plankconstant, ν : light frequency, A: a constantand Ebg : band gap.) As displayed in Fig. 6((SrTiO3) and (CoWO4)), the calculated band-gaps of prepared SrTiO3 and CoWO4 are 3.30 eV and 2.70 eV, respectively, which are close to literature references values [32,33]. Fig. 6(Ag2S) demonstrates that the calculated band-gap of Ag2S is 1.50 eV. Due to the small particle size (about 20 nm), the size effect may be generated, leading to a relatively wider band-gap than the values of literature references [34]. In addition, the band-gaps of SrTiO3/CoWO4 and SrTiO3/Ag2S/CoWO4 are found to be 2.70 and 2.60 eV, respectively. It can be found that band-gaps of SrTiO3/CoWO4 and SrTiO3/Ag2S/CoWO4 are narrower than pure SrTiO3 and CoWO4, which is beneficial to improving utilization efficiency for light from sonoluminescence.

15

Fig. 6. Photoluminescence (PL) spectrum is a significant method to confirm the recombination of the photo-generated electron-hole pairs in semiconductor. Generally, lower intensity signal in PL spectrum indicates lower recombination rate of electron-hole pairs. Conversely, higher intensity signal shows higher recombination rate of electron-hole pairs [41]. Fig. 7 shows the PL spectra of SrTiO3 and CoWO4 semiconductor as well as SrTiO3/Ag2S/CoWO4 composite under 350 nm and 380 nm wavelength excitation lights, respectively. It can be seen that PL intensities of pure SrTiO3 at 620 nm and CoWO4 at 500 nm are both very high, which means that excited electrons of SrTiO3 and CoWO4 return to VB and recombine with h+ highly effectively. However, when SrTiO3/Ag2S/CoWO 4 composite was excited under 350 nm and 380 nm wavelength excitation lights, respectively, the PL intensities both significantly diminished in comparison with those of pure SrTiO3 and pure CoWO4. This result shows that the recombination of photo-induced e--h+ pairs can be distinctly retarded in SrTiO3/Ag2S/CoWO4 composite owing to rapid interfacial e- transfer [42]. In addition, the introduction of Ag2S nanoparticles as trapezoid electronic channel and co-catalyst can further enhance charge separation efficiency, and then increase number of charge carriers participating in sonocatalytic reaction. Therefore, SrTiO3/Ag2S/CoWO4 composite as a novel Z-scheme sonocatalyst probably reveals outstanding sonocatalytic performance in degradation of tetracyclines.

Fig. 7. 3.2. UV-vis spectra of TTC solutions with ultrasonic irradiation time and comparison of degradation percentages The

UV-vis

spectra

of

tetracycline

TTC 16

solution

in

the

presence

of

Z-scheme

SrTiO3/Ag2S/CoWO4 sonocatalyst within 300 min ultrasonic irradiation and the corresponding sonocatalytic degradation percentages calculated based on two absorption peaks at 277 nm and 360 nm were shown in Fig. 8. Since the TTC has double bond and conjugated double bonds, the - * electron transition can occur under the excitation of matching wavelength light. In addition, there are some heteroatoms containing solitary electrons in double bond, so the n-* electron transitions will also occur [4,5]. Two obvious absorption peaks were observed in Fig. 8(a). So it can be inferred that the peaks at 277 nm and 360 nm are the characteristic absorption peaks of TTC [6]. The change of these absorption peaks can be used to evaluate the degradation process of TTC and measure the degradation percentages. It can be found that the absorbance peak intensities of TTC at 277 nm and 360 nm show obvious decline trends with increasing ultrasonic irradiation time in the presence of Z-scheme SrTiO3/Ag2S/CoWO4 sonocatalyst, which testifies that TTC molecules were gradually destroyed. When ultrasonic irradiation time reaches 300 min, the calculated degradation percentages based on two absorption peaks at 277 nm and 360 nm, were 85.64 % and 87.30 %, respectively. This indicates that most of TTC molecules can be degraded by using Z-scheme SrTiO3/Ag2S/CoWO4 sonocatalyst under ultrasonic irradiation.

As given in Fig. 8(b), the obtained degradation percentages based on the absorption peak at 277 nm gradually increase with increasing ultrasonic irradiation time and they were 38.18 %, 59.10 %, 68.62 %, 72.58 %, 79.60 % and 85.64 %, respectively, at 50 min, 100 min, 150 min, 200 min, 250 min and 300 min under given experimental conditions. And that, based on the absorption peak at 360 nm they were 40.80 %, 60.30 %, 70.50 %, 75.60 %, 80.70 % and 87.30 %, respectively, at 50 min, 100 min, 150 min, 200 min, 250 min and 300 min. The results showed that most of the TTC can be degraded by Z-scheme SrTiO3/Ag2S/CoWO4 sonocatalyst under ultrasonic irradiation for 300 min. It 17

indicates that the use of Z-scheme SrTiO3/Ag2S/CoWO4 sonocatalyst for the sonocatalytic degradation of TTC in aqueous solutions is a viable method.

Fig. 8. 3.3. Influence of ultrasonic irradiation time and reaction kinetics The

degradation

effects

of

TTC

were

researched

for

five

systems,

including

SrTiO3/Ag2S/CoWO4/US, SrTiO3/CoWO4/US, SrTiO3/US, CoWO4/US and US alone. The corresponding sonocatalytic degradation percentages of TTC in water solution within 300 min at every 25 min were detected by using UV-vis spectrometer and calculated results were exhibited in Fig. 9(a). As expected, degradation percentages of TTC for five systems all increase gradually along with the extension of ultrasonic irradiation time. Thereinto, it can be seen that, for ultrasonic irradiation alone, the degradation percentage is only 34.22 % after ultrasonic irradiation of 300 min. It displays that ultrasonic irradiation alone is low efficiency for TTC degradation. For SrTiO3/US and CoWO4/US, the degradation percentages are 60.00 % and 52.40 %, respectively. It shows that existence of sonocatalyst can significantly improve degradation percentage of TTC under ultrasonic irradiation. Moreover, for SrTiO3/CoWO4/US degradation percentage is 70.30 %, which indicates that Z-scheme SrTiO3/CoWO4 system obtained through combination of two semiconductors can further enhance sonocatalytic degradation effectiveness of TTC. Particularly, the highest degradation percentage (86.47 %) in five systems can be obtained when improved Z-scheme system with Ag2S, US/SrTiO3/Ag2S/CoWO4, was used. It manifests that Ag2S as both electronic channel and co-catalyst can further enhance sonocatalytic performance of SrTiO3/CoWO4 for degradation of TTC under ultrasonic irradiation. For quantitatively contrasting degradation effects of TTC for above five systems, their first-order

18

kinetics models were investigated and obtained results were displayed in Fig. 9(b). It shows that all -ln(At/A0) (At and A0 represent instant absorbance of TTC and initial absorbance of TTC data for reaction and irradiation time can be deemed to about linear relationships. Hence, for these five systems, the courses of sonocatalytic degradation can be verified to be first-order kinetics reactions. In this experiment, the corresponding values of kinetic equations were listed in Table 1. Rate constants of five systems are 0.0070 min−1, 0.0041 min−1, 0.0030 min−1, 0.0023 min−1 and 0.0015 min−1, respectively. Comparatively, the sequence of rate constants are US/SrTiO3/Ag2S/CoWO4 > US/SrTiO3/CoWO4 > US/SrTiO3 > US/CoWO4 > US. It can be verified again that US/SrTiO3/Ag2S/CoWO4 provides a highest degradation rate of TTC under ultrasonic irradiation. Thus, Z-scheme SrTiO3/Ag2S/CoWO4 is a more effective sonocatalyst in TTC degradation.

Fig. 9. Table 1. 3.4. Influences of Z-scheme SrTiO3/Ag2S/CoWO4 dosage and TTC initial concentration The impact of sonocatalyst dosage on the degradation of TTC was studied by using different SrTiO3/Ag2S/CoWO4 dosages from 0.50 g/L to 1.50 g/L. As shown in Fig. 10(a), the sequence of sonocatalytic degradation percentages from high to low are 1.00 g/L > 1.50 g/L > 0.50 g/L at any ultrasonic irradiation time. In addition, rate constants for 0.50 g/L, 1.00 g/L and 1.50 g/L sonocatalyst dosages are 0.0039 min−1, 0.0070 min−1 and 0.0046 min−1, respectively. Comparatively, the sequence of rate constants is also 1.00 g/L > 1.50 g/L > 0.50 g/L. It can be found that too much and too little of sonocatalyst dosage can lead to decline of the degradation percentage. The too little sonocatalysts provide less reactive sites, which leads to generation of less reactive oxygen species. By increasing the 19

sonocatalyst particles, more cavitation bubbles are produced, which is beneficial for the cavitation effect to provide more energy. However, too much sonocatalysts can impact ultrasound transmission, resulting in lower sonocatalytic activity of sonocatalyst [43]. Therefore, the following experiments were performed using 1.00 g/L of SrTiO3/Ag2S/CoWO4 as optimum quantity. The effect of TTC initial concentration was investigated by changing its concentration from 5.0 mg/L to 15 mg/L and the results were given in Fig. 10(b). It could be observed that the sequence of sonocatalytic degradation percentages from high to low are 10 mg/L > 15 mg/L > 5.0 mg/L at any ultrasonic irradiation time, and the highest degradation percentage can be obtained when initial concentration of TTC is 10 mg/L. In addition, the rate constants for 5.0 mg/L, 10 mg/L and 15 mg/L TTC concentrations are 0.0036 min−1, 0.0070 min−1 and 0.0045 min−1, respectively. Comparatively, the sequence of rate constants is also 10 mg/L > 15 mg/L > 5.0 mg/L. It indicates that too low and too high contaminant concentrations can both lead to decline of the degradation percentage. When the contaminant concentration is 5.0 mg/L, it becomes more difficult that the generated free radicals capture contaminant molecules. And that, when the initial concentration of TTC is 15 mg/L, the degradation percentage is also reduced. The reason may be that the sonocatalyst surface is covered with overfull TTC molecules, resulting in less energy absorption [12]. It is worth noting that, for 15 mg/L concentration of TTC solution, the degradation percentage is still close to 70 %. It can be made a prediction that the sonocatalytic degradation of organic pollutants is a very feasible method, even for relatively high concentration of organic pollutants.

Fig. 10. 3.5. Influences of ultrasonic irradiation power, solution pH, scavengers, recycling

20

experiments and tetracyclines kind Fig. 11(a) demonstrates the influence of ultrasonic irradiation power (100-500 W) on the sonocatalytic degradation of TTC. It can be found that the degradation percentages of TTC gradually increases with increasing ultrasonic irradiation power. Under ultrasonic irradiation for 300 min, along with the increase of ultrasonic generator power from 100 W to 300 W, the degradation percentage is increased from 55.66 % to 86.76 %. Apparently, increasing ultrasonic irradiation power is helpful to enhance the TTC degradation efficiency. The high ultrasonic irradiation power leads to the formation of more collapsing bubbles, which causes the generation of more hydroxyl radicals. In addition, the high ultrasonic irradiation power intensifies the turbulence of the solution, which consequently results in enhancing of the mass transfer rate of reactive radical species, contaminant molecules and produced intermediates between the surface of sonocatalyst and solution. The improvement of mass transfer under high power of ultrasonic irradiation enhanced the degradation efficiency [16]. However, along with the further increase of ultrasonic irradiation power from 300 W to 500 W, the degradation percentage is increased only slightly from 86.76 % to 88.69 %. It indicates that the positive impact increasing ultrasonic irradiation power is limited. Therefore, in order to economically treat TTC by using sonocatalytic degradation method, it is necessary to optimize the ultrasonic irradiation power. Fig. 11(b) shows the influences of solution pH on sonocatalytic degradation of TTC by using Z-scheme SrTiO3/Ag2S/CoWO4 sonocatalyst. At pH = 3.0, 6.0 and 9.0, the degradation percentages of TTC were 55.61 %, 85.48 % and 64.85 %, respectively. It can be found that degradation efficiency at pH = 6.0 was higher than ones at pH = 3.0 and 9.0. Apparently, the degradation efficiency is related to interaction between the TTC molecules and surface of sonocatalyst particles. Because the points of zero charge (pzc) of SrTiO3 and CoWO4 are 3.50 and 4.60 [44], respectively. The TTC has three pKa 21

(pKa1 = 3.30, pKa2 = 7.68 and pKa3 = 9.70) [5]. Therefore, when the pH value of the solution is 3.0, the TTC exists in cationic form and the surface charge of the sonocatalyst become positive. Therefore, the electrostatic repulsion forces can be generated between sonocatalyst particles and TTC molecules, which leads to decline of the degradation percentage. In addition, when the pH value of the solution is 9.0, TTC was anionic form and the surface charge of the sonocatalyst become negative. Hence, there also are electrostatic repulsion forces between TTC molecules and sonocatalyst particles, which also leads to decline of the degradation percentage. Nevertheless, when the pH value of the solution is 6.0, the TTC molecules are neutral and the surface of sonocatalyst particles are charged negatively. The electrostatic repulsion forces disappear between TTC molecules and sonocatalyst particles, which leads to a higher degradation percentage at pH = 6.0 than that of pH = 3.0 or pH = 9.0. The experimental results revealed that the highest degradation percentage of TTC was taken place at pH = 6.0, which was considered as an optimum value for subsequent experiments. It is feasible to understand degradation mechanism of TTC by verifying the production of •OH and h+ in the sonocatalytic system. It was reported that DMSO and EDTA were deemed to be •OH and h+ trapping agents [45], respectively. Therefore, the EDTA and DMSO were added to the reaction system to assess effects of •OH and h+. In Fig. 11(c), the results display that sonocatalytic degradation percentage of TTC without trapping agents was up to 86 %. Nevertheless, after adding trapping agents, the degradation percentages both became lower distinctly than that of without trapping agents and were 51.73 % and 33.79 %, respectively. The results reveal that EDTA and DMSO can suppress sonocatalytic degradation of TTC. Particularly, the degradation percentage adding into •OH trapping agent (DMSO) distinctly reduced, which displays that contribution of •OH is slightly larger than that h+ in sonocatalytic degradation of TTC. The generated •OH comes from two parts: (1) the e- on CB of 22

SrTiO3 react with adsorbed O2 and generate •O2−, and then •O2− can become •OH. (2) The h+ on VB of CoWO4 oxidize water molecules, generating •OH. Therefore, sonocatalytic degradation of TTC is conjunct oxidations of h+ and •OH. Moreover, the generated •OH plays a critical role. It is worth pointing out that the reusability of any sonocatalyst is a crucial factor for its practical applications. To confirm the reusability of Z-scheme SrTiO3/Ag2S/CoWO4 sonocatalyst, four times recycle experiments were carried out under invariable experimental conditions, containing initial concentration of 10.00 mg/L TTC, usage amount of 1.00 g/L Z-scheme SrTiO3/Ag2S/CoWO4 and ultrasonic irradiation time of 300 min and acquired results were presented in Fig. 11(d). It can be discovered that there is no obvious decrease about degradation percentage of TTC under ultrasonic irradiation within four cycles. The sonocatalytic degradation percentage of TTC still reached 65 % even after four cycles. It explains that the Z-scheme SrTiO3/Ag2S/CoWO4 composite is an excellent sonocatalyst with quite stable and high sonocatalytic performance. Therefore, it shows that the sonocatalytic activity of Z-scheme SrTiO3/Ag2S/CoWO4 can be maintained for a long time to effective degradation of TTC under ultrasonic irradiation. As everyone knows, tetracyclines have various structures and compositions due to different substituent groups. The effluents of pharmaceutical factory contain multiple tetracyclines compounds. Thus, it is necessary to investigate degradation efficiencies of all tetracyclines caused by Z-scheme SrTiO3/Ag2S/CoWO4 sonocatalyst. In this study, several representative tetracyclines, TTC, OTC, CTC and DTC, were utilized to perform the sonocatalytic degradation. As can be seen in Fig. 11(e), all of these tetracyclines could be distinctly degraded in existence of Z-scheme SrTiO3/Ag2S/CoWO4 under ultrasonic irradiation for 300 min and degradation percentages exceed 80 %. It manifests that, for these tetracyclines with different compositions and structures, Z-scheme SrTiO3/Ag2S/CoWO4 reveals 23

relatively outstanding sonocatalytic degradation performance. Therefore, this experiment testifies that Z-scheme SrTiO3/Ag2S/CoWO4 composite is feasible sonocatalyst to treat organic pollutants.

Fig. 11. 3.6. HPLC-MS spectrum The intermediates of TTC in sonocatalytic degradation were analyzed by using LC-MS and corresponding mass spectrum was given in Fig. 12(a). It can be found that, under ultrasonic irradiation after 200 min, some new peaks appear in mass spectrum, which indicates nine main intermediates have been generated during sonocatalytic process. The mass peak at m/z 444.9 corresponds to TTC, and others at m/z 416.9, 300.0, 279.1, 242.2, 175.1, 163.9, 144.0, 128.9 and 102.2 relate to the nine intermediates [1,46-49]. Hence, this result indicated that TTC could be decomposed to some smaller molecules in degradation process. According to m/z values and possible molecular structures of intermediates, it can be inferred that TTC molecules were attacked by the generated ROS. That is, the TTC molecules were degraded to low-molecular weight organic compounds through dehydroxylation, deamination, demethylation and ring opening reaction. Finally, these intermediates would also be mineralized into CO2, H2O and some inorganic ions. The Total Organic Carbon (TOC) analysis showed that the sonocatalytic degradation percentage of TTC under ultrasonic irradiation after 300 min was 61.73 %. Based on the sonocatalytic degradation intermediates of TTC and previous works, the possible decomposition pathway was presented in Fig. 12(b). These intermediates are some nontoxic, low toxic, low activity and unstable organic pollutants. Even if they exist in a short period of time or is not completely mineralized, it is also safe for human and environment.

Fig. 12. 24

3.7. Mechanism and process of sonocatalytic degradation of organic pollutants caused by Z-scheme SrTiO3/Ag2S/CoWO4 Based on experimental results, a possible process of sonocatalytic degradation of tetracyclines caused by Z-scheme SrTiO3/Ag2S/CoWO4 sonocatalyst under ultrasonic irradiation was presented in Fig. 13. It is well known that sonolysis has been extensively used to degrade the organic pollutants. The chemical effect of ultrasonic irradiation comes from acoustic cavitation. The collapse of bubbles leads to productions of light with quite wide wavelength range and hot-spot with local high temperature and local high pressure [50,51]. Under such extreme conditions, the highly reactive radical species such as •OH can be produced, which can oxidize organic pollutants in water [52,53]. Apparently, most of photocatalysts can also be excited by using ultrasound to perform photocatalytic reaction. Therefore, the sonocatalytic degradation process is similar with photocatalytic reaction. The different semiconductors could be excited by appropriate wavelength of lights to produce e- on CB and h+ on VB. Besides, the ultrasonic cavitation effect can produce “hot-spot” with 5000 K high temperature in water, which makes H2O splitting and then producing •OH. As strong oxidizing agents, the •OH can effectively break organic pollutants [18]. However, because the produced •OH number is not much, ultrasonic irradiation alone is low efficacy for degradation of organic pollutants. Since the ‘‘hot-spot” can also excite photocatalyst [11,54], some sonocatalysts can be joined to enhance the degradation effect of organic pollutants. At present, due to the prominent separation efficiency of photo-induced e--h+ pairs, the Z-scheme sonocatalyst can effective realize degradation of organic pollutants [11,55]. In Z-scheme sonocatalytic system, the photo-induced e- migrate from CB of first semiconductor to VB of second semiconductor, recombining with the h+ [56]. As a consequence, in Z-scheme 25

sonocatalytic system the efficient charge separation can be achieved, obtaining the e- with powerful reduction capacity on CB of second semiconductor and the h+ with powerful oxidation capacity on VB of first semiconductor [57], respectively. In addition, the short-wavelength lights can be utilized for stimulating wide band-gap sonocatalyst, and the long-wavelength lights can be utilized for stimulating narrow band-gap sonocatalyst. Thus, the light respond scope can be broadened through the combination of wide and narrow band-gap semiconductors, which enhances the utilization ratio of lights from sonoluminescence [29]. Particularly, after introducing a more narrow band-gap semiconductor as a “conductive ladder”, the recombination of photo-induced e--h+ pairs may well be inhibited, enhancing the sonocatalytic performance of Z-scheme sonocatalyst. In this work, the combination of a wide band-gap semiconductor SrTiO3 (ECB = -1.26 eV, EVB = +2.14 eV and ΔEbg = +3.40 eV) [24] and a narrow band-gap semiconductor CoWO4 (ECB = -0.11 eV, EVB = +2.59 eV and ΔEbg = +2.70 eV) [27] can form an ideal Z-scheme sonocatalytic system. Under ultrasonic irradiation, SrTiO3 and CoWO4 can both produce photo-induced e--h+ pairs. And that, the eon CB of CoWO4 can transfer to VB of SrTiO3 and combine with the h+. However, because potential difference of VB of SrTiO3 and CB of CoWO4 is relatively large, the photo-induced e- on CB of CoWO4 is difficult to enter VB of SrTiO3. Nevertheless, this problem could be overcome through introducing the more narrow band-gap Ag2S (ECB = 0.00 eV, EVB = +1.00 eV and ΔEbg = 1.00 eV) as a trapezoid electronic channel [34]. Particularly, CB position of Ag2S is close to that of CoWO4, and VB position of Ag2S is close to that of SrTiO3. Because adding of Ag2S forms some relatively denser electronic energy-levels, the photo-induced e- on CB of CoWO4 could easily transfer to VB of SrTiO3 through conductive ladder (Ag2S). Therefore, the combination of SrTiO3, CoWO4 and Ag2S can not only promote recombination of photo-induced e- on CB of CoWO4 with h+ on VB of SrTiO3, but also 26

possess more positive valence band (VB) and more negative conduction band (CB) at the same time, which obviously improves the sonocatalytic performance of Z-scheme SrTiO3/Ag2S/CoWO4 composite sonocatalyst. Besides, on surface of SrTiO3, Ag2S as co-catalyst could provide more active sites and make e- on CB of SrTiO3 react with O2, generating •O2-. Further, the •O2- becomes •OH through several chemical reactions. In general, according to migration pathway of photo-induced eand combination model of e- and h+, a CB with much strong reduction capacity and a VB with much strong oxidation capacity could be obtained, thus enhancing sonocatalytic degradation ability [9]. Finally, •OH and h+ oxidize the contaminants and mineralize them into CO2, H2O and inorganic ions [47]. The reaction mechanism of Z-scheme SrTiO3/Ag2S/CoWO4 in tetracyclines degradation under ultrasonic irradiation is illustrated in Fig. 13 and detailed process may be as following:

Ultrasonicirradiation (cavitation effect) light (sonoluminescence)  heat (hot spot) SrTiO3/Ag2S/CoWO4  light (short wavelength) [SrTiO3] * /Ag2S/CoWO4

[SrTiO3] * /Ag2S/CoWO4  h /VB/SrTiO3  e - /CB/SrTiO3 SrTiO3/Ag2S/CoWO4  light (long wavelength)  SrTiO3/Ag2S/[CoWO4] *

SrTiO3/Ag2S/[CoWO4]*  h /VB/CoWO4  e - /CB/CoWO4 h /VB/SrTiO 3  e - /CB/CoWO4  light  heat e - /CB/SrTiO3  O2   O2 ( on Ag2S) 

 O2

 H  HO2

HO2  H2O  H2O2   OH

H2O  h   OH  H 

H2O2  heat (hot spot) 2  OH H2O  heat (hot spot)  OH   H 27



 H  O2   O2

 H

Tetracyclines (TCs)   OH  D   D    CO2  H2O  inorganicions Tetracyclines (TCs)  h  /VB/ CoWO4  D   D    CO2  H2O  inorganicions

Fig. 13. 4. Conclusions A ternary Z-scheme sonocatalyst, SrTiO3/Ag2S/CoWO4, was successfully fabricated. Its sonocatalytic performance was assessed by degradation of TTC in aqueous solution under ultrasonic irradiation. SrTiO3/Ag2S/CoWO4 composite displays an outstanding sonocatalytic performance for degradation of TTC. The excellent performance is attributed to its special composition and construction. Ag2S particles in SrTiO3/Ag2S/CoWO4 act as trapezoid electronic channel and co-catalyst, respectively, which boosts transfer efficiency of photo-induced e- and restrains recombination of e--h+ pairs. The recycling experiments verify high stability of Z-scheme SrTiO3/Ag2S/CoWO4 composite. A high sonocatalytic degradation percentage (86.47 %) of TTC could be obtained when 300 min ultrasonic irradiation time, 1.00 g/L SrTiO3/Ag2S/CoWO4 adding quantity and 10.00 mg/L TTC initial concentration were employed. Sonocatalytic degradation ability of Z-scheme SrTiO3/Ag2S/CoWO4 could further be verified by degrading other several tetracyclines, including oxytetracycline (OTC), chlorotetracycline (CTC) and deoxytetracycline (DTC). The degradation percentages for all these tetracyclines could exceed 80 %. LC-MS spectrum reveals that TTC molecules can be decomposed into a suite of non-toxic intermediates. Therefore, it can be forecasted that Z-scheme SrTiO3/Ag2S/CoWO4 is a usable sonocatalyst to dispose organic pollutants.

28

Acknowledgements The authors greatly acknowledge the National Science Foundation of China (21371084 and 31570154) for financial support. The authors also thank our colleagues and other students for their participating in this work.

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Content of Scheme, Figures and Table: Scheme 1. Molecular structure of some tetracyclines. Fig. 1. XRD patterns of SrTiO3, CoWO4, Ag2S, SrTiO3/CoWO4 and SrTiO3/Ag2S/CoWO4. Fig. 2. SEM images of prepared SrTiO3, CoWO4, Ag2S and SrTiO3/Ag2S/CoWO4. Fig. 3. TEM images of prepared Z-scheme SrTiO3/Ag2S/CoWO4 composite with different magnifications (a: 20 nm and b: 5.0 nm). Fig. 4. EDX spectrum of prepared Z-scheme SrTiO3/Ag2S/CoWO4 composite. Fig. 5. XPS of SrTiO3/Ag2S/CoWO4 (whole), Sr (3d), Ti (2p), Co (2p), W (4f), Ag (3d), S (2p) and O (1s). Fig. 6. UV-vis diffuse reflectance spectra (DRS) of prepared SrTiO3, CoWO4, Ag2S, SrTiO3/CoWO4 and SrTiO3/Ag2S/CoWO4. Fig. 7. Photoluminescence (PL) spectra of prepared SrTiO3, CoWO4 and SrTiO3/Ag2S/CoWO4. Fig. 8. UV-vis spectra of TTC solutions with ultrasonic irradiation time (a) and comparison of degradation percentages calculated based absorption peaks at 277 nm and 360 nm (b). Fig. 9. Influence of ultrasonic irradiation time (a) and corresponding reaction kinetics (b) on the sonocatalytic degradation of TTC. Fig. 10. Influences of Z-scheme SrTiO3/Ag2S/CoWO4 dosage (a) and TTC initial concentration (b) on the sonocatalytic degradation. Fig. 11. Influences of ultrasonic irradiation power (a), pH (b), scavengers (c), recycling experiments (d) and tetracyclines kind (e) on sonocatalytic activity of Z-scheme SrTiO3/Ag2S/CoWO4. Fig. 12. LC-MS analysis (a) and intermediate products (b) of TTC in sonocatalytic degradation. Fig. 13. Sonocatalystic degradation mechanism of TTC in presence of Z-scheme SrTiO3/Ag2S/CoWO4 composite under ultrasonic irradiation. Table 1. Kinetic data the TTC sonocatalystic degradation process using various sonocatalysts.

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Scheme:

Scheme 1. Molecular structure of some tetracyclines.

36

Figures:

Fig. 1. XRD patterns of SrTiO3, CoWO4, Ag2S, SrTiO3/CoWO4 and SrTiO3/Ag2S/CoWO4.

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Fig. 2. SEM images of prepared SrTiO3, CoWO4, Ag2S and SrTiO3/Ag2S/CoWO4.

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Fig. 3. TEM images of prepared Z-scheme SrTiO3/Ag2S/CoWO4 composite with different magnifications (a: 20 nm and b: 5.0 nm).

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Fig. 4. EDX spectra of prepared Z-scheme SrTiO3/Ag2S/CoWO4 composite.

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Fig. 5. XPS of SrTiO3/Ag2S/CoWO4 (whole), Sr (3d), Ti (2p), Co (2p), W (4f), Ag (3d), S (2p) and O (1s).

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Fig. 6. UV-vis diffuse reflectance spectra (DRS) of prepared SrTiO3, CoWO4, Ag2S, SrTiO3/CoWO4 and SrTiO3/Ag2S/CoWO4.

42

Fig. 7. Photoluminescence (PL) spectra of prepared SrTiO3, CoWO4 and SrTiO3/Ag2S/CoWO4.

43

Fig. 8. UV-vis spectra of TTC solutions with ultrasonic irradiation time (a) and comparison of degradation percentages calculated based absorption peaks at 277 nm and 360 nm (b).

44

Fig. 9. Influence of ultrasonic irradiation time (a) and corresponding reaction kinetics (b) on the sonocatalytic degradation of TTC.

45

Fig. 10. Influences of Z-scheme SrTiO3/Ag2S/CoWO4 dosage (a) and TTC initial concentration (b) on the sonocatalytic degradation.

46

Fig. 11. Influences of ultrasonic irradiation power (a), pH (b), scavengers (c), recycling experiments (d) and tetracyclines kind (e) on sonocatalytic activity of Z-scheme SrTiO3/Ag2S/CoWO4.

47

Fig. 12. LC-MS analysis (a) and intermediate products (b) of TTC in sonocatalytic degradation.

48

Fig. 13. Sonocatalystic degradation mechanism of TTC in presence of Z-scheme SrTiO3/Ag2S/CoWO4 composite under ultrasonic irradiation.

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Table 1. Kinetic data the TTC sonocatalystic degradation process using various sonocatalysts. k (min-1)  10-3

R2

Sample

Kinetic expression

US/SrTiO3/Ag2S/CoWO4

y = 0.0070t + 0.0506

7.0

0.9896

US/SrTiO3/CoWO4

y = 0.0041t + 0.0324

4.1

0.9917

US/SrTiO3

y = 0.0030t + 0.0562

3.0

0.9851

US/CoWO4

y = 0.0023t + 0.5550

2.3

0.9930

US

y = 0.0015t + 0.0140

1.5

0.9831

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Research Highlights The Research Highlights of this paper are listed below: 

A new Z-scheme sonocatalyst SrTiO3/Ag2S/CoWO4 is designed to degrade tetracyclines.



Ladder electronic channel Ag2S speeds electron transfer from CB/SrTiO3 to VB/CoWO4.



Co-catalyst Ag2S on SrTiO3 surface forms active sites to promote charge separation.



SrTiO3/Ag2S/CoWO4 can quickly degrade tetracyclines under ultrasonic irradiation.

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Graphical abstract Separation and Purification Technology 000 (2018) 000-000

Article Fabrication of a novel Z-scheme SrTiO3/Ag2S/CoWO4 composite and its application in sonocatalytic degradation of tetracyclines Jing Qiao a, Hongbo Zhang a, Guanshu Li b, Siyi Li a, Zhihui Qu b, Meng Zhang a, Jun Wang a,

*, Youtao Song b,*

a

College of Chemistry, Liaoning University, Shenyang 110036, P. R. China

b

College of Environment, Liaoning University, Shenyang 110036, P. R. China

52