Bi2WO6 heterojunctions: Hydrothermal fabrication and sonophotocatalytic degradation of organic pollutants

Bi2WO6 heterojunctions: Hydrothermal fabrication and sonophotocatalytic degradation of organic pollutants

Journal Pre-proof 3+ N/Ti co-doping biphasic TiO2/Bi2WO6 heterojunctions: Hydrothermal fabrication and sonophotocatalytic degradation of organic pollu...

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Journal Pre-proof 3+ N/Ti co-doping biphasic TiO2/Bi2WO6 heterojunctions: Hydrothermal fabrication and sonophotocatalytic degradation of organic pollutants Mingxuan Sun, Yuan Yao, Wen Ding, Sambandam Anandan PII:

S0925-8388(19)34418-4

DOI:

https://doi.org/10.1016/j.jallcom.2019.153172

Reference:

JALCOM 153172

To appear in:

Journal of Alloys and Compounds

Received Date: 4 September 2019 Revised Date:

21 November 2019

Accepted Date: 23 November 2019

3+ Please cite this article as: M. Sun, Y. Yao, W. Ding, S. Anandan, N/Ti co-doping biphasic TiO2/ Bi2WO6 heterojunctions: Hydrothermal fabrication and sonophotocatalytic degradation of organic pollutants, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/j.jallcom.2019.153172. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

Graphical Abstract

N/Ti3+ codoping multiphasic TiO2/Bi2WO6 heterojunctions were prepared and showed excellent sonophotocatalytic degradation of pollutants.

N/Ti3+

co-doping

biphasic

TiO2/Bi2WO6

heterojunctions:

Hydrothermal fabrication and sonophotocatalytic degradation of organic pollutants

Mingxuan Suna,b*, Yuan Yaoa, Wen Dinga, Sambandam Anandanc

a

School of Materials Engineering, Shanghai University of Engineering Science,

Shanghai 201620, China b

State Key Laboratory of Photocatalysis on Energy and Environment, Fuzhou

University, Fuzhou, 350002, P. R. China c

Nanomaterials and Solar Energy Conversion Lab, Department of Chemistry,

National Institute of Technology, Tiruchirappalli 620 015, India

*Corresponding author: Prof. Mingxuan Sun School of Materials Engineering, Shanghai University of Engineering Science, Shanghai 201620 (China) E-mail: [email protected]; [email protected], Tel.: +86 21 67791208 Fax: +86 21 67791207

1

Abstract This study describes the one-step in situ hydrothermal construction of N/Ti3+ co-doping biphasic TiO2/Bi2WO6 heterojunctions (NT-TBWx) and investigates their sonophotocatalytic degradation of organic contaminants in water. The obtained samples were characterized in detail including X-ray photoelectron spectroscopy, X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and Ultraviolet diffuse reflectance spectroscopy, etc. Their photocatalytic, sonocatalytic, and sonophotocatalytic performance were compared by the degradation of organic pollutants and found the degradation rate according to the order: photocatalysis < sonocatalysis < sonophotocatalysis. In addition, NT-TBWx samples present better sonophotocatalytic activity for the removal of methylene blue (MB), p-Nitrophenol, Rhodamine B, and levofloxacin compared with that of NT-TiO2 and pure TiO2. Especially, the highest sonophotocatalytic degradation rate is demonstrated for NT-TBW1. Additionally, the excellent stability of NT-TBWx samples was demonstrated by four cycles of sonophotocatalytic measurements with a tiny decline. Superoxide radical (·O2-) was found to dominate the sonophotocatalytic degradation process of MB. The synergic effect among the doping level, heterophase junction, and heterojunctions as well as between sonocatalysis and photocatalysis contribute to superior sonophotocatalytic activity. This work provides a new alternative architecture of TiO2-based nanomaterials and promotes their application in environmental issues.

Key words: N/Ti3+ codoping; TiO2/Bi2WO6 heterojunctions; hydrothermal construction; sonophotocatalytic activity; environmental remediation 2

1. Introduction Over the past few years, continuous research interest has been devoted to develop advanced oxidation processes (AOP’s) for the removal of organic pollutants in water such as ozonization, hydrogen peroxide, sonocatalysis, Fenton-like reaction, and photocatalysis, etc [1-6]. A hybrid of two or more different AOPs techniques has been usually confirmed to be more efficient for wastewater treatment than individual process [7]. As we know, photocatalysis has been demonstrated to be a green and effective route for the degradation of pollutants [8]. Additionally, a variety of reports have demonstrated that sonocatalysis under ultrasound irradiation can promote various synthesis reaction [9] and contaminants decomposition [10]. In sonocatalytic process, ultrasound can provide energies in different forms for the fabrication of materials and the degradation of organic pollution, such as ultrasonic cavitation, mechanical vibration, and thermal effect, etc. Recent reports revealed that the combination of sonocatalysis and photocatalysis named sonophotocatalysis could markedly enhance the decomposition efficiency of organic contaminants and reduce costs compared to sonocatalysis or photocatalysis alone [11,12]. Both of sonocatalytic and photocatalytic processes can generate active radicals but through different reaction mechanisms. The extreme reaction conditions of ultrasound cavitation induce the generation of H● and ●

OH radicals via the sonolysis of H2O, whereas the photoinduced electrons and holes-

on the surface of photocatalyst can react with the absorbed O2 and H2O to generate O● and ●OH radicals. Thus, the combination of sonocatalysis and photocatalysis is expected to provide a mass of radicals in a short time, which can boost the oxidation process for the degradation of organic compounds into CO2 and H2O [13,14]. After Fujishima reported the application of Titania (TiO2) in photocatalytic 3

water splitting in 1972, TiO2 has been one of the most widely studied photocatalysts because of its merits of high stability, non-toxicity, low cost, and appropriate band structure for redox reactions, etc [15,16]. However, the intrinsic drawbacks of TiO2 involving wide band-gap energy (3.0~3.2 eV) with only UV light response and rapid recombination of photoinduced electrons and holes tremendously reduce its solar energy utilization efficiency [17]. To overcome these shortcomings, numerous attempts on modification of TiO2 have been devoted until now such as doping, coupling, and sensitization, etc [18-23]. Recently, multiple modification methods have attracted much attention for the synergistic promotion of the photocatalytic performance of TiO2. For example, codoping with N and Ti3+ ions of TiO2 has been confirmed to effectively vary its band structure and electronic transition modes [24,25]. Generally, the N 2p states doping creates energy-band above the valence band (VB) and Ti3+ causes continuous vacancy band of electronic states below the conduction band (CB) [26-28]. Thereby, Ti3+ and N co-doping equivalently reduce the bandgap of TiO2, which account for the visible light absorption and result in high photocatalytic properties [29,30]. Furthermore, due to the difference in band-edge positions and the built-in electric field of the heterophase junction, the interfacial charge transportation among different crystal phases of TiO2 ensured the effective separation of photogenerated electrons and holes [31-33]. Thus, the co-decoration of TiO2 by N/Ti3+ codoping and heterophasic junction can further improve the light utilization ability and enhance the photocatalytic performance. Bismuth tungstate (Bi2WO6) with band-gap energy around 2.8 eV has been considered to be an attractive modification for TiO2 due to its matchable energy band structure with TiO2, visible light response, lamellar structure, and high photochemical stability and redox ability [34,35]. Several studies have demonstrated that 4

TiO2/Bi2WO6 heterojunctions exhibited highly superior photocatalytic activity [36,37]. For example, Li et al [38] investigated the hydrothermal fabrication of TiO2/Bi2WO6 heterojunction, which exhibited excellent photocatalytic activity and stability. Song et al

[39]

reported

the

higher

photocatalytic

performance

of

P25/Bi2WO6

nanocomposites than P25 and Bi2WO6 alone for the visible light-driven degradation of ethylene. At the contact interface between P25 and Bi2WO6 heterojunctions, the grain is refined and lattice defects are produced, which can expose more active sites to improve the photocatalytic degradation process. Until now, several reports have emerged on sonophotocatalysis over TiO2-based nanomaterials for the degradation of organic pollutants [40]. In our previous research work, we developed N/Ti3+ codoping triphasic TiO2 and demonstrated the superior photocatalytic degradation of methylene blue and resorcinol dyes [41]. We also constructed heterojunctions of N/Ti3+ codoping multiphasic TiO2/BiOBr for enhanced sonocatalytic degradation of p-Nitrophenol, Rhodamine B, and methylene blue [10]. In this work, we developed a series of N/Ti3+ codoping biphasic TiO2/Bi2WO6 heterojunctions with different quality of Bi2WO6 by an in-situ hydrothermal process. Their photocatalytic, sonocatalytic, and sonophotocatalytic activity on the removal of organic contaminants including Methylene blue, p-Nitrophenol, Rhodamine B, and levofloxacin were systematically investigated and compared. Moreover, recycle experiments and active species capturing experiments were also performed. The further modification of N/Ti3+ codoping multiphasic TiO2 with Bi2WO6 can deliver better photocatalytic performance due to their synergistic effect. To our knowledge, this work may be the first report about the simultaneously multiple modifications of TiO2 by N/Ti3+ codoping, biphasic junctions, and Bi2WO6 heterojunctions as well as their sonophotocatalytic performance on removing organic pollutants. 5

2. Experimental methods 2.1 Synthesis. The N/Ti3+ codoping biphasic TiO2/Bi2WO6 heterojunctions (NT-TBWx) were synthesized using a hydrothermal method, as schematically illustrated in Scheme 1. Briefly, 0.01 mol sodium tungstate (Na2WO4, AR, 99.0%) and 0.02 mol bismuth nitrate (Bi(NO3)3·5H2O, AR, 99.0%) were dissolved in 70 mL of deionized water. Thereafter, 0.01 mol titanium nitride (TiN, 40 nm, 99.9%) was dispersed in the above-mixed solution under 30 min magnetic stirring. The resulted suspension was autoclaved in a Teflon-sealed reactor (100 mL) at 200oC for 36 h. The dark blue precipitates thus obtained by centrifuging and consecutively washing with deionized water and ethanol for three times, followed by drying in an oven at 70oC for 12 h. In addition, the calculated initial amount of Bi(NO3)3·5H2O and Na2WO4 in the synthesis system was adjusted to prepare a series of NT-TBWx samples. Amongst, x equals to 0.1, 1, and 2, which represent the molar ratio of Bi2WO6 to N/Ti3+ codoping biphasic TiO2. For comparison, the same hydrothermal condition with TiN only was applied for fabrication of the N/Ti3+ codoping triphasic TiO2 nanoparticles named as NT-TiO2. Pure TiO2 was also prepared via the sol-gel process of titanium butoxide (TBOT, CP, 98.0%) [42]. 2.2 Characterizations X-ray diffractometer (Panalytical X' Pert, Holland) was employed for X-ray diffraction (XRD) patterns with Cu-Kα radiation (40 kV, 40 mA). The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were severally investigated on JEOL JSM-7000F (Japan) and FEI Tecnai F20 (USA) microscopes. The optical properties were analyzed by UV-vis diffuse reflectance 6

spectra (UV-vis DRS) on Shimadzu UV 2600 spectrophotometer (Japan). The X-ray photoelectron spectroscopy (XPS, RBD upgraded PHI-5000 C ESCA system, PerkinElmer) was applied for the analysis of elemental state with radiation source Al/Mg Kα and the binding energy of 284.6 eV for C 1s was used as an internal reference. The specific surface area was investigated using N2 adsorption-desorption analysis operated on Micrometrics ASAP 2020 V4.01at 77 K (USA). 2.3 Sonophotocatalytic performance The sonophotocatalytic degradation of organic pollutions was carried out in a glass reactor over the as-prepared samples under both ultrasonic irradiation and UV-vis light illumination. Thereinto, ultrasound was provided by the ultrasonic cleaner (35 kHz, 180 W, SK3300LHC, KUDOS) and UV-vis light illuminates from a 500 W Xenon lamp (100 mW/cm2, CHF-XM-500 W). The inner liquid surface of glass reactor is somewhat lower than that of outer water in ultrasonic cleaner. The addition of ice packs in ultrasonic cleaner was used to control the temperature of the water bath around the glass reactor at around 25oC. Methylene blue (MB, 5 mg/L), Rhodamine B (20 mg/L), p-Nitrophenol (25 mg/L), and levofloxacin (20 mg/L) aqueous solution were selected as organic contaminant models. The concentration of remaining organic pollutants was examined by UV-vis spectrophotometer (UV1901, YOKE). For comparison, sonocatalysis was performed under ultrasound irradiation without light illumination, and photocatalysis was conducted with stirring under UV-vis light illumination. In each experiment, 7.5 mg catalysts were dispersed into 15 mL organic pollutant taken in a beaker and kept stirring in the dark for 2 h to achieve the equilibrium of dye adsorption/desorption. Then, for MB solution, the suspension was sampled, centrifuged, and measured at 10 min intervals for 50 min to record the 7

degradation of MB under irradiation. For other organic pollutants, the degradation process proceeded for 100 min under continuous irradiation. Recycle sonophotocatalytic experiments of the NT-TBWx samples were also performed by using NT-TBW1 sample for the degradation of MB. After each cycle, the catalysts were collected by centrifuging and subsequently washed for three times by deionized water and ethanol subsequently. After drying at 80oC for 6 h, the collected catalysts was reused in the next sonophotocatalytic cycle for the degradation of MB. Totally, the recycle experiments were carried out for four cycles to investigate the stability of NT-TBWx samples. Active species capture testing experiments were conducted to monitor the main active species in the sonophotocatalytic degradation of MB. Generally, the addition of 0.2 mM p-benzoquinone (BQ), ammonium oxalate (AO), and tert-butanol (TBA) was used as the quencher of ●O2-, h+, and ●OH, respectively. 2.4 Photocurrent and EIS tests The transient photocurrent and electrochemical impedance spectra (EIS) were measured on Shanghai Chenhua electrochemical workstation (CHI660E, China) and PARSTAT 4000 electrochemical workstation under UV-vis light illumination, respectively. Generally, a 500 W Xenon lamp (CHF-XM-500 W) was used as the UV-vis light source. The electrochemical cell with a plane quartz window was composed of three-electrode and 0.5 M Na2SO4 electrolyte, where the obtained samples, saturated calomel electrode (SCE), and platinum wire were applied as working electrode, reference electrode, and counter electrode, respectively. Furthermore, the EIS tests were recorded in a frequency range from 0.01 Hz to 100 kHz.

8

3. Results and discussion Fig. 1 depicts the XRD plots of pristine TiO2, NT-TiO2, and NT-TBWx samples. The pure TiO2 (curve a in Fig. 1) shows a collection of characteristic diffraction peaks for anatase phase of (101), (004), (200), (105), and (211) crystal planes at 2θ values of 25.27°, 37.92°, 48.06°, 53.85° and 55.21°, respectively (JCPDS. 89-4921). For NT-TiO2 sample (curve b in Fig. 1), it can be clearly seen the co-existence of three phases including anatase, rutile, and brookite. Thereinto, the peaks at 25.36°, 37.87°, 48.06°, 53.98°, and 55.10° are assigned to (101), (004), (200), (105), and (211) facets of anatase phases (JCPDS. 65-5714), while the peaks at 27.55° and 36.18° correspond to the (110) and (101) facets of rutile phases (JCPDS. 89-4920). Additionally, the tiny peak at 30.85° is indexed to (121) facet of brookite phase (JCPDS No.76-1936). Interestingly, only peaks for anatase and rutile are observed for NT-TBWx samples (curves c-e in Fig. 1), and diffraction peaks corresponding to brookite disappear. This phenomenon may be due to the varied synthetic environment caused by raw materials Bi(NO3)3·5H2O or Na2WO4, or the suppression of formation of brookite crystal phase by the product Bi2WO6. The diffraction peaks at 2θ values of 28.55°, 32.90°, 36.16°, 47.21°, 56.04°, 58.75°, 76.02°, and 78.26° are attributed to (103), (200), (202), (220), (303), (107), (109), and (307) crystal planes of tetragonal Bi2WO6 (JCPDS No. 26-1044). The strong Bi2WO6 signals indicate high crystallinity in the composites, which make TiO2 diffraction peaks relatively weaker. Furthermore, with the increase of the molar ratio of Bi2WO6 from 0.1 to 2, the intensity of diffraction peaks for Bi2WO6 gradually get stronger whereas the intensity of TiO2 diffraction peaks gets decreased or even almost invisible for samples of NT-TBW1 and NT-TBW2. Anyhow, the XRD analysis suggests anatase TiO2, rutile TiO2, and Bi2WO6 exist together in the obtained NT-TBWx samples. 9

Fig. 2 presents FE-SEM (Fig. 2A and 2B), TEM (Fig. 2C), and HR-TEM (Fig. 2D) of the as-prepared samples. It is easily found from Fig. 2A that the prepared NT-TiO2 samples comprise of abundant nanorods, some nanoparticles, and a very small amount of nanosheets with good dispersion. For NT-TBW1 sample in Fig. 2B, NT-TiO2 nanomaterials anchor on the large lamellar structure of Bi2WO6 sheets. The statistics results for random selection of 20 points for thickness of lamellar structure are shown in the inset of Fig. 2B, suggesting thickness distribution of lamellar structure is mainly around 10-15 nm. During the synthesis process, NT-TiO2 nanomaterials occupy on the active sites instead of Bi2WO6 whereas the growth of nearly perfect Bi2WO6 single crystals is achieved from the residual Bi and W reactants in the solution. As a result, N/Ti3+ codoping multiphasic TiO2 nanoparticles are intimately anchored on the surface of Bi2WO6. In addition, the BET surface areas are confirmed to be about 61, 79, and 82 m2 g-1 for pristine TiO2, NT-TiO2, and NT-TBW1 by N2 adsorption-desorption analysis, respectively. The slightly larger surface areas for NT- TBW1 imply more active sites expose to the organic pollution, which contribute to the improvement of photocatalytic activity. To observe detail microstructure, NT-TBW1 sample was chosen for TEM and HR-TEM investigation. Fig. 2C provides the TEM images of NT-TBW1 sample, further confirming that N/Ti3+ codoping multiphasic TiO2 nanoparticles closely couple on the surface of flake Bi2WO6. The clear observation of intimate contact between NT-TiO2 and Bi2WO6 confirms the heterojunctions are formed between them. Fig. 2D displays the HR-TEM image of NT-TBW1 sample. Clear lattice fringes can be observed, which indicate the high crystallinity of the obtained samples. As shown, the lattice fringes of 0.234 and 0.351 nm are indexed to (112) and (101) planes of anatase phase while the spacing of 0.324 nm corresponds to (110) plane of rutile phase. The 10

lattice fringes of 0.271 and 0.311 nm are severally attributed to (200) and (103) planes of Bi2WO6. The HR-TEM results are in accordance with XRD analysis. Additionally, the lattice fringes for anatase, rutile, and Bi2WO6 are in close contact to one another, indicating that heterojunctions are formed not only at the contact interface of NT-TiO2/Bi2WO6 but also at the contact interface of anatase/rutile phases. Fig. 3 presents the XPS spectra of NT-TBW1 composites. As displayed in Fig. 3A, the characteristic peaks for Ti, O, Bi, W, N, and C elements were observed in the XPS survey spectrum. There is no carbon source in the synthesis process, thus, the appearance of carbon peak may come from CO2 absorbed on the surface of the samples. For further XPS analysis, the high-resolution XPS spectra are displayed from Fig. 3B to Fig. 3F, respectively. In Fig. 3B, the N1s XPS spectrum shows a characteristic peak at 399.5 eV corresponding to TiO-N bonds [43], which implies N element exists in the form of interstitial nitrogen in NT-TBW1 sample [44]. In Fig. 3C, Ti 2p peaks at 458.4, 464.1, 458.9, and 465.0 eV for Ti3+ 2p3/2, Ti3+ 2p1/2, Ti4+ 2p3/2, and Ti4+ 2p3/2 are observed from the fitting results [23], whereas the peak at binding energy of 466.5 eV is ascribed to Bi 4d [45]. In Fig. 3D, the fitting results of O 1s spectra exhibits peaks for different oxygen states at 529.9, 530.7, and 532.4 eV. Amongst, the peak at 529.9 eV is assigned to Ov defects in TiO2 matrix or lattice oxygen in crystalline Bi2WO6 [46,47]. Additionally, the peaks at 530.7 and 532.4 eV are ascribed to the oxygen in Ti-O bond and surface hydroxyl groups, respectively [48]. In Fig. 3E, the two strong peaks of Bi 4f located at 159.1 and 164.4 eV are severally attributed to Bi 4f5/2 and Bi 4f7/2 for Bi2WO6, corresponding to the characteristic peaks of Bi3+ [49]. Furthermore, the difference of binding energy between Bi 4f7/2 and Bi 4f5/2 is found to be 5.3 eV, which corresponds to the characteristic of normal Bi 4f state [50]. In Fig. 3F (W 4f), the peaks at 35.4 eV for W 11

4f7/2 and 37.5 eV for W 4f5/2 confirm the existence of W6+ in NT-TBW1 sample [49]. Combined with the XRD and HR-TEM results as well as XPS analysis, the successful synthesis N/Ti3+ co-doping biphasic TiO2/Bi2WO6 heterojunctions are achieved. Fig. 4 depicts the optical absorption properties of NT-TBWx, NT-TiO2, and TiO2 samples. As shown in Fig. 4A, all the NT-TBWx (x=0.1, 1, 2) samples show longer absorption edge and stronger absorption intensity in the visible region than pristine TiO2. Moreover, although the strongest absorption intensity of visible light was observed for sample NT-TiO2, which could be attributed to the darkest color, all the NT-TBWx samples present a red shift in the optical absorption edge compared with that of NT-TiO2. The phenomenon indicates the introduction of Bi2WO6 can improve the optical absorption properties of NT-TiO2. For further investigation, the band gap of the obtained samples can be calculated from the [αhv]1/2 vs photon energy plots [51] with results shown in Fig. 4B. According to the intercept of the tangent to the X axis, the band gaps for pristine TiO2, NT-TiO2, NT-TBW0.1, NT-TBW1, and NT-TBW2 are roughly estimated to be 3.10 eV, 2.47 eV, 2.36 eV, 2.05 eV, and 2.20 eV, respectively. Evidently, the sample NT-TBW1 possesses the narrowest band gap values among all of the samples. It is noted that both Bi2WO6 coupling and N/Ti3+ co-doping lead to the enhanced visible light absorption intensity and narrowed band gap of TiO2, which contributes in promoting the efficient utilization of solar energy. To detect the sonophotocatalytic activity of the obtained NT-TBWx samples, sonophotocatalytic degradation of organic pollutants were measured and analyzed (Fig. 5). Fig. 5A displays the sonophotocatalytic degradation of MB over different samples. As presented in Fig. 5A, a significant decrease of MB concentration is observed in the sonophotocatalytic process over NT-TBWx samples. All NT-TBWx samples exhibit quite high sonophotocatalytic decomposition rates of MB and almost 12

all the MB dyes are degraded after 50 minutes. The highest sonophotocatalytic degradation efficiency of MB is up to 98% for NT-TBW1, and the values are 63%, 65%, 82%, 91%, 96%, and 58% over pure TiO2, pristine Bi2WO6, NT-TiO2, NT-TBW0.1, NT-TBW2, and without

catalyst, respectively. Obviously,

the

sonophotocatalytic performance of NT-TBWx samples is much better than that of NT-TiO2 and pure TiO2, which is due to the improved light response and electron-hole separation ability. In addition, NT-TBW2 shows a decline in sonophotocatalytic activity for MB degradation compared to that of NT-TBW1. This is because an excess amount of Bi2WO6 coupling plays a negative effect on sonophotocatalytic performance. The phenomenon may be explained as follows: an excessive combination of Bi2WO6 in NT-TBWx samples can occupy active sites of TiO2 or act as a recombination center of photogenerated electron-hole pairs [52]. In a word, there is an optimal coupling content of Bi2WO6 for NT-TBWx samples, which can achieve the highest sonophotocatalytic activity. Fig. 5B depicts the evaluation of photocatalytic, sonocatalytic, and sonophotocatalytic activity of NT-TBW1 catalysts for the degradation of MB. As shown, the MB sonophotodegradation (under both ultrasound and light irradiation) efficiency is 98%, which are 1.09 and 1.52 folds larger than that of sonodegradation (under ultrasound irradiation without light illumination, 90%) and photodegradation (under light illumination with stirring, 79%) alone, respectively. Obviously, the degradation efficiencies of MB follow the order: sonophotodegradation > sonodegradation > photodegradation. In addition, photocatalytic, sonocatalytic, and sonophotocatalytic degradation of other contaminants (Rhodamine B, p-Nitrophenol, and levofloxacin) over NT-TBW1 sample for 100 min was also investigated and the relevant results are displayed in Fig. 5C. As shown, the degradation efficiencies of 13

levofloxacin, p-Nitrophenol, and Rhodamine B are 51%, 67%, and 97% for sonophotocatalysis, respectively. The degradation efficiencies are 92%, 55%, and 42% under ultrasound irradiation alone, while the values are 76%, 39%, and 19% under light

irradiation

alone.

Evidently,

it

is

further demonstrated

that

sonophotocatalytic activity of NT-TBW1 sample is better than that of either sonodegradation or photodegradation alone, and it is usability in the degradation of other contaminants. Usually, the sonodegradation is related to the sonoluminescence and ●OH concentration from sonolysis of water because of the cavitation. And, the photodegradation has a relation with the photoinduced electron-hole pairs. For sonophotodegradation, sonocatalysis and photocatalysis play a synergistic effect on the degradation, which results in the best degradation of pollutants. The stability and reusability of NT-TBW1 catalysts were also examined by four recycle sonophotodegradation of MB (Fig. 5D). The sonophotocatalytic degradation efficiencies of MB are severally calculated to be 99%, 97%, 95%, and 90% over NT-TBW1 for once, twice, thrice, and four times. It can be found that the sonophotocatalytic activity of NT-TBW1 shows a fairly small decrease after four successive degradation cycles with a 91% retention of the initial degradation efficiency. Additionally, the XRD patterns and SEM images of NT-TBW1 after four times recycle are severally displayed in Fig. 5E and 5F. By comparison, it is noted that the XRD patterns for NT-TBW1 sample are almost the same before and after recycle. Also, there is no visible change of the morphology for NT-TBW1 sample after recycle in Fig. 5F compared with that of NT-TBW1 sample before recycle in Fig. 2B. The results suggest that the prepared NT-TBW1 sample is relatively stable in the sonophotocatalytic process of this study. Transient photocurrent test was carried out to confirm the favorably affect of 14

Bi2WO6 modification on NT-TiO2 for light harvest and transport of photo-induced electron-hole pairs under UV-vis light illumination. As depicted in Fig. 6A, the prompt and reproducible response of TiO2, NT-TiO2, and NT-TBW1 photoelectrodes to UV-vis light is clearly observed. In addition, NT-TBW1 photoelectrode presents higher photocurrent value than that of TiO2 and NT-TiO2 photoelectrodes. The enhanced photocurrent value confirms that the introduction of Bi2WO6 into NT-TiO2 can facilitate the generation and separation of more electrons and holes. The improved charge transfer of NT-TBW1 photoelectrode can be demonstrated by EIS results, as displayed in Fig. 6B. Evidently, the modification of Bi2WO6 on NT-TiO2 leads to a decreased diameter of the semicircle as compared to that of TiO2 and NT-TiO2, which suggest smaller resistance of NT-TBW1 for charge transfer, implying a faster charge transfer rate in the NT-TBW1 photoelectrodes. A proposed mechanism for the sonophotodegradation of organic contaminants in the presence of NT-TBWx catalysts is illustrated in Scheme 2. Synergistic effect between

sonocatalysis

and

photocatalysis

contributes

to

the

enhanced

sonophotocatalytic performance of NT-TBWx samples. The sonocatalytic degradation of organic pollutants induced by ultrasound is mainly attributed to the acoustic cavitation process. Acoustic cavitation can provide an extra ●OH from the pyrolysis of water, more available surface area by increasing the uniformity of the dispersion, and more active sites by removing intermediates from the surface. In addition, it can also accelerate mass transfer between the interface of liquid/solid and the adsorption activity of reactant on the surface. Particularly, acoustic cavitation can result in the phenomenon called sonoluminescence, which can provide flashlight with a longwavelength and short-wavelength for a photocatalytic process. The photocatalytic degradation of organic pollutants promoted by flash light from acoustic cavitation and 15

UV-vis light from Xe lamp is due to the formation of effectiveness photoinduced electrons and holes of NT-TBWx samples. Generally, the semiconductor materials are activated to generate electrons and holes when the energy of irradiated light is equal or greater than its energy of bandgap. In NT-TBWx samples, both N/Ti3+ codoping TiO2 and Bi2WO6 can absorb UV and visible light due to their satisfying band structures. Thereinto, although TiO2 is a UV light response photocatalyst, nitrogen atoms and Ti3+ can locate impurity level above the VB and continuous local states below CB of TiO2, respectively. Both of them can result in the visible light response of TiO2. Bi2WO6 itself can be excited by UV-vis light due to its bandgap of ~2.8 eV. Furthermore, the heterophasic junction of TiO2 and TiO2/Bi2WO6 heterojunctions can promote the separation of photoinduced electrons and holes due to the different CB and VB positions and the formative built-in electric field among them [36,37,41]. The electrons and holes effectively separated and transported to the surface of NT-TBWx samples can react with the surface absorbed OH- and O2 to generate ●OH and ●O2-, respectively. In sonophotocatalytic degradation process, the formed ●O2-, ●OH, and h+ can oxidize the organic pollutants to small molecular such as CO2 and H2O. The influence of ultrasonic power and frequency on the sonophotocatalytic degradation of MB over sample NT-TBW1 was investigated with results presented in Fig. 7A. It can be clearly seen that the degradation ratio of MB is improved as the increase of ultrasonic power and frequency, which may be attributed to the enhanced ultrasonic cavitation for the degradation process. Moreover, in order to examine the active species in the degradation process, the sonophotodegradation of MB was carried out by the addition of radical scavengers over NT-TBW1 sample [53]. As shown in Fig. 7B, the presence of tert-butanol, p-benzoquinone, and ammonium oxalate as the quencher of ●OH, ●O2-, and h+ in the sonophotocatalytic system all 16

significantly reduce the degradation efficiency of MB, suggesting that all three of oxidative species contributes to the sonophotocatalytic degradation of MB. However, the order for the suppression effect of sonophotocatalysis is ●O2- > ●OH > h+, indicating that the main degradation of MB is through ●O2- radical-mediated process.

4. Conclusions In this work, the use of sonophotocatalytic system is demonstrated to be a facile approach for the degradation of organic pollution in environmental governance. A series of N/Ti3+ codoping biphasic TiO2/Bi2WO6 heterojunctions are successfully synthesized through one-step in-situ hydrothermal processes at 200oC for 36 h and the resulting heterojunctions are investigated systematically by a collection of characterizations. The sonophotocatalytic activity of NT-TBWx samples in the degradation of methyl blue, p-Nitrophenol, levofloxacin, and Rhodamine B is found to be much higher than that of photocatalysis and sonocatalysis. Moreover, NT-TBWx samples exhibit higher sonophotocatalytic activity compared to pure TiO2 and NT-TiO2. Amongst, the sample NT-TBW1 shows the highest sonophotocatalytic degradation, which is confirmed to be the optimal coupling amount. Also, NT-TBWx samples were detected by four cycles of sonophotocatalytic measurements with a 91% retention of the initial degradation efficiency of MB. Superoxide radical (●O2-) is found to play be the main role in the sonophotocatalytic degradation of MB. The enhanced sonophotocatalytic performance of NT-TBWx samples is attributed to the synergistic effect between sonocatalysis and photocatalysis as well as among N/Ti3+ codoping, heterophase junctions, and heterojunctions. This work constructs a new selectable

structure

of

TiO2-based

nanomaterials

and

demonstrates

sonophotocatalytic performance in addressing environmental-related issues. 17

their

Acknowledgments The financial support from Training Program for Young Teachers in Shanghai Colleges and Universities (ZZgcd14010), Innovation Program of Shanghai Municipal Education Commission (15ZZ092), the Talent Program of Shanghai University of Engineering Science (2018RC082017), Startup Foundation of Shanghai University of Engineering Science (No Xiaoqi 2014-22), and the Open Project Program of the State Key Laboratory of Photocatalysis on Energy and Environment (Grant No. SKLPEE-KF201710), Fuzhou University is appreciated. Additionally, we appreciate the referee’s very valuable comments, which help us a lot to improve the quality of the manuscript.

18

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Ti3+

self-doped

Figure captions Fig. 1 XRD patterns of TiO2 (a), NT-TiO2 (b), NT-TBW0.1 (c), NT-TBW1 (d), and NT-TBW2 (e) samples Fig. 2 FE-SEM images of NT- TiO2 (A) and NT-TBW1 samples (B, inset is the statistics results for thickness of lamellar structure); TEM (C) and HR-TEM (D) images of NT-TBW1 sample Fig. 3 XPS spectra of NT-TBW1 sample: the survey spectrum (A), N 1s (B), Ti 2p (C), O 1s (D), Bi 4f (E), and W4f (F). Fig. 4 UV-vis absorption spectra (A) and the corresponding [αhv]1/2 vs hv plots (B) of TiO2, NT-TiO2, and NT-TBWx samples Fig. 5 The sonophotocatalytic degradation of MB over different samples (A); The comparison of photocatalytic, sonocatalytic, and sonophotocatalytic degradation of MB (B) and other organic contaminants (C) in the presence of NT-TBW1; The recycle sonophotodegradation of MB over NT-TBW1 (D); The XRD patterns of NT-TBW1 before (a) and after (b) four times recycle (E); FE-SEM image of NT-TBW1 after four times recycle (F) Fig. 6 Transient photocurrent curves (A) and EIS Nyquist plots (B) of pristine TiO2, NT-TiO2, and NT-TBW1 under UV-vis light illumination Fig. 7 (A) The effect of ultrasonic power and frequency on the sonophotocatalytic degradation of MB over sample NT-TBW1; (B) Investigation of active species in the sonophotocatalytic process of the degradation of MB over sample NT-TBW1 Scheme 1 Illustration for the preparation of N/Ti3+codoped biphasic TiO2/Bi2WO6 heterojunctions by one-step in situ hydrothermal method Scheme 2 Possible mechanism for the sonophotocatalytic degradation of organic contaminants over NT-TBWx heterojunctions 26

Figure 1

−Rutile −Bi2WO6

Intensity (a.u.)

−Anatase −Brookite

(e)

(d)

(c)

(b) (a)

20

30

40

50

60

70

80

2Theta(degree) Fig. 1 XRD patterns of TiO2 (a), NT-TiO2 (b), NT-TBW0.1 (c), NT-TBW1 (d), and NT-TBW2 (e) samples

27

Figure 2

28

Fig. 2 FE-SEM images of NT- TiO2 (A) and NT-TBW1 samples (B, inset is the statistics results for thickness of lamellar structure); TEM (C) and HR-TEM (D) images of NT-TBW1 sample 29

NT-TBW1

0

Bi 4p1/2

Bi 4p3/2

N 1s

C 1s

Bi 4d Ti 2p

O 1s

Bi 4f

(A)

Bi 5d W 4f

Intensity (counts·s-1)

Figure 3

100 200 300 400 500 600 700 800 900

Binding energy (eV)

(B)

Intensity(count·s-1)

N 1s 399.5eV

394

396

398

400

402

Binding energy(eV)

30

404

Ti 2p

Intensity(count·s-1)

(C)

466.5eV

465eV 458.9eV 464.1eV 458.4eV

456

460

464

468

472

476

Binding energy(eV)

O 1s

Intensity(count·s-1)

(D) 529.9eV

530.7eV 532.4eV

527

528

529

530

531

532

533

Binding energy(eV)

31

534

535

(E)

Bi 4f

Intensity(count·s-1)

159.1eV 164.4eV

156

158

160

162

164

166

168

Binding energy(eV)

(F)

W 4f

Intensity(count·s-1)

35.4eV 37.5eV

33

34

35

36

37

38

39

40

Binding energy(eV) Fig. 3 XPS spectra of NT-TBW1 sample: the survey spectrum (A), N 1s (B), Ti 2p (C),

O 1s (D), Bi 4f (E), and W4f (F). 32

Figure 4

Fig. 4 UV-vis absorption spectra (A) and the corresponding [αhv]1/2 vs hv plots (B) of

TiO2, NT-TiO2, and NT-TBWx samples 33

Figure 5

34

35

Fig. 5 The sonophotocatalytic degradation of MB over different samples (A); The

comparison of photocatalytic, sonocatalytic, and sonophotocatalytic degradation of MB (B) and other organic contaminants (C) in the presence of NT-TBW1; The recycle sonophotodegradation of MB over NT-TBW1 (D); The XRD patterns of NT-TBW1 before (a) and after (b) four times recycle (E); FE-SEM image of NT-TBW1 after four times recycle (F) 36

Figure 6

Fig. 6 Transient photocurrent curves (A) and EIS Nyquist plots (B) of pristine TiO2

(a), NT-TiO2 (b), and NT-TBW1 (c) under UV-vis light illumination

37

Figure 7

Fig. 7 (A) The effect of ultrasonic power and frequency on the sonophotocatalytic

degradation of MB over sample NT-TBW1; (B) Investigation of active species in the sonophotocatalytic process of the degradation of MB over sample NT-TBW1

38

Scheme 1

Scheme 1 Illustration for the preparation of N/Ti3+codoped biphasic TiO2/Bi2WO6

heterojunctions by one-step in situ hydrothermal method

39

Scheme 2

Scheme 2 Possible mechanism for the sonophotocatalytic degradation of organic

contaminants over NT-TBWx heterojunctions

40

Highlights: ►One-step hydrothermal synthesis of N/Ti3+ co-doping multiphasic TiO2/ Bi2WO6 heterojunctions ►The removal of methyl blue, p-Nitrophenol, Rhodamine B, and levofloxacin was investigated ►The order of catalytic degradation rate was photocatalysis < sonocatalysis < sonophotocatalysis ►The synergic effect of the structure, sonocatalysis, and photocatalysis contribute to the enhancement

Author contribution section Mingxuan Sun designed and supervised this study for all the characterizations and measurements. Yuan Yao synthesized the samples and carried out the photocatalytic, sonocatalytic, and sonophotocatalytic experiments. Wen Ding performed the photocurrent and EIS measurement. Sambandam Anandan mainly improved the English writing of the manuscript. Furthermore, all authors contributed to analyzing the results and writing the manuscript

Declaration of interests 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.