Coupling effect of piezomaterial and DSA catalyst for degradation of metronidazole: Finding of induction electrocatalysis from remnant piezoelectric filed

Journal of Catalysis 381 (2020) 530–539

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Coupling effect of piezomaterial and DSA catalyst for degradation of metronidazole: Finding of induction electrocatalysis from remnant piezoelectric filed Jinxi Feng a, Jingxiang Sun a, Xiaosheng Liu a, Jinzhu Zhu a, Shuanghong Tian a, Rong Wu b,⇑, Ya Xiong a,* a School of Environmental Science and Engineering, Guangdong Provincial Key Laboratory of Environmental Pollution Control and Remediation Technology, Sun Yat-Sen University, no. 135, Xingang Xi Road, Guangzhou 510275, PR China b School of Geography and Planning, Sun Yat-Sen University, no. 135, Xingang Xi Road, Guangzhou 510275, PR China

a r t i c l e

i n f o

Article history: Received 26 August 2019 Revised 25 October 2019 Accepted 25 November 2019

Keywords: Piezo-material Piezocatalysis SnO2 Metronidazole

a b s t r a c t Piezocatalysis is an emerging green advanced-oxidation technology for the degradation of toxic pollutants; however, piezomaterials are not generally excellent electrocatalysts. Herein, two complex piezocatalysts (SnO2/t-BaTiO3 and SnO2-Sb/t-BaTiO3) consisting of an electrocatalyst and piezomaterial, respectively, are designed to investigate the coupled effect of piezocatalytic and electrocatalytic processes on the degradation of pollutants. SnO2, especially SnO2-Sb, can considerably enhance the piezocatalytic degradation of the antibiotic metronidazole (MNZ). The piezocatalytic rate constants of SnO2/tBaTiO3 and SnO2-Sb/t-BaTiO3 are 2.3 and 3.0 times that of t-BaTiO3, respectively. In the degradation process, SnO2 and SnO2-Sb are confirmed to act as dimensionally stable anode-like (DSA-like) catalysts because they can increase the response of the piezoelectrochemical anodic current and the generation of OH radicals as well as reduce the formation of 1O2. Significantly, the piezoelectric field can drive not only the free charges of t-BaTiO3 and its remnant piezoelectric field but also those of the loaded electrocatalyst SnO2 to participate in the degradation of MNZ. These results indicate that the coupling of an excellent electrocatalyst and piezomaterial is an effective method to promote piezocatalytic efficiency. Ó 2019 Elsevier Inc. All rights reserved.

1. Introduction Since the piezoelectric effect of ZnO nanobelts was discovered in 2003 [1], the applications of the nano-piezoelectric effect have attracted significant attention [2–5]. Among them, the piezoelectric degradation of pollutants is one of the most concerned applications. In 2010, Hong et al. found that the piezoelectric effect of fibrous nano-ZnO and dendritic nano-BaTiO3 could directly drive the degradation of acid orange 7 in situ. Subsequently, the authors recognized that the piezoelectric potential could directly enhance or suppress the chemical processes occurring at the piezoelectric material-solution interface, which exceeded the range of electron migration in the usual piezoelectric effect; therefore, they termed this phenomenon piezoelectric catalysis or piezocatalysis [6]. Recently, an increasing number of investigations about the piezocatalytic degradation of pollutants have been reported. More than a dozen piezoelectric materials have been investigated, such

⇑ Corresponding authors. E-mail addresses: [email protected] (R. Wu), [email protected] (Y. Xiong). https://doi.org/10.1016/j.jcat.2019.11.037 0021-9517/Ó 2019 Elsevier Inc. All rights reserved.

as, BaTiO3, ZnO, MoS2, PbTiO3, BiFeO3, PbZrxTi1
xO3, WS2, BiOIO3, Bi2O2(OH)(NO3), BiOCl, Bi4Ti3O12, etc. [7–12]. Some of these materials have been reported as new piezocatalysts, but most are used to prove the effectiveness of modification methods [13–16]. For example, Ma et al. found that defective MoS2 exhibited a higher piezoelectric catalytic activity than that of intact MoS2. The material could degrade 99% of the ciprofloxacin in 30 s. This finding denoted that regulating the structural defects of piezoelectric catalysts was an effective way to improve the activity [16]. Huang et al. reported that the partial substitution of I5+ with V5+ in BiOIO3  could increase the concentrations of O
2 and OH radicals in piezoelectric catalysis by 1.7 and 3.3 times, respectively, due to the enhancement in macroscopic polarization. Accordingly, the authors considered that macroscopic polarization was also an effective method to promote the catalytic activity of piezoelectric catalysts [9]. In addition, Hao et al. found that the generation rate of OH in the piezoelectric catalytic process of Bi2O2(OH)(NO3) nanosheets with exposed {0 0 1} crystal planes reached as high as 7.13 lmol L
1 h
1. This was the first investigation to correlate the piezoelectric catalytic activity with the crystal surface exposure of a piezoelectric material [10]. Of note is that these

J. Feng et al. / Journal of Catalysis 381 (2020) 530–539

improvements have been basically achieved by tuning the chemical components and structure of the piezoelectric materials themselves and not by introducing other catalysts. Starr et al. have once predicted that piezocatalytic processes were very similar to those of electrolysis, and their only difference laid in the power supply. The power of the former was from the built-in piezoelectric field of the strained piezomaterial while that of the latter was from the external DC (direct current) potential [17]. Moreover, Huang and Hao et al. directly labeled cathodes and anodes as the two opposite surfaces of nano-piezoelectric materials with positive and negative polarization charges, respectively. In other words, the deformed piezoelectric nanoparticle was very similar to a special electrochemical nano-reactor with self-generating power [9,10]. As we all know, the efficiency of an electrochemical reactor is very dependent on the nature of the electrocatalyst. However, piezoelectric materials themselves are generally not excellent electrocatalysts, therefore, it is possible to improve the piezocatalytic efficiency by modifying piezomaterials with excellent electrocatalysts because the modification of electrodes with electrocatalysts is an effective method to promote the electrochemical efficiency [18–22]. To date, no related investigation has been reported. Presently, an abundance of electrocatalysts, such as borondoped diamond, Pt, IrO2, RuO2, MnO2, PbO2 and SnO2 etc., have been developed to remove toxic organic pollutants. Among them, SnO2 and doped SnO2, as dimensionally stable anode (DSA) electrocatalysts, have continuously attracted considerable attention because they are relatively low-cost and possess a high oxygen evolution potential (OEP) and inert surface for the weak physical adsorption for OH. Hence, these compounds facilitate the direct extraction of electrons from organic molecules [23], and are considered to be some of the most promising electrocatalysts for the oxidation of recalcitrant organic contaminants [24,25]. Therefore, in this work, SnO2 and Sb-doped SnO2 (SnO2-Sb) are used as representatives of excellent DSA electrocatalysts to modify piezoelectric nano-materials. This work mainly focuses on the investigations of their catalytic performance in a periodic piezoelectric field, including their enhancement effect on the piezodegradation of pollutants, their ability to tune the piezogeneration of reactive oxygen radicals (ROS), etc. The main aims of these investigations are to demonstrate the hypothesis that the coupling of piezoelectric nano-materials with typical DSA electrocatalysts can improve the piezocatalytic degradation efficiency of pollutants and to reveal the mechanism of these improvements, especially the role of strain-induced piezoelectric charges. To these ends, tetragonal BaTiO3 (t-BaTiO3) is designated as the tested piezomaterial because t-BaTiO3 is the representative of green piezomaterials, and metronidazole (MNZ, 2-methyl-5-nitroimidazole-1-ethanol) is selected as the targeted pollutant because it is an extensively used, refractory and potentially carcinogenetic drug, which has recently become a significant concern as an emerging toxic contaminant [26–28].

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2.2. Preparation of piezocatalytic materials 2.2.1. Preparation of t-BaTiO3, SnO2/t-BaTiO3 and SnO2-Sb/t-BaTiO3 Nano-t-BaTiO3 is synthesized by the reported hydrothermal method [29]. SnO2/t-BaTiO3 is prepared by a two-step process [30]. First, Sn(OH)4/t-BaTiO3 is obtained by the reported hydrolysis–oxidation process. That is, as-prepared t-BaTiO3 powder (0.5 g) is dispersed in a 100 mL ethanol solution with sonication. Then, a SnCl2-containing ethanol solution is dripped into the above slurry and stirred for 2 h. After adding 10 mL of 1 mol L
1 NH3H2O aqueous solution, the slurry is continuously stirred for 24 h in a 30 °C water bath, and then, the as-prepared products are filtered, washed with distilled water and dried in an oven at 150 °C for 6 h. Second, Sn(OH)4/BaTiO3 is sintered for 2 h at 400 °C (heating rate = 20 °C min
1) to form SnO2/t-BaTiO3. SnO2-Sb/t-BaTiO3 is prepared by the same experimental process as above except that the SnCl2 solution is replaced by a SnCl2-SbCl3 mixed solution. 2.2.2. Preparation of piezo film electrodes The t-BaTiO3 piezo film electrode is prepared by a two-step procedure consisting of dip-coating and a thermal treatment. Briefly, 0.05 g piezomaterial is added to 10 mL of a 0.005% Nafion aqueous solution, and the mixture is sonicated for 30 min to form a homogeneous suspension solution. The material suspension is loaded onto an ITO (Indium tin oxide) glass plate (2 cm  2 cm) by the dip-coating method, dried for 15 min in an oven at 100 °C and then sintered in a muffle furnace at 400 °C (heating rate = 20 °C min
1) for 1 h. The loading, drying and sintering are repeated three times, and the resulting film has a thickness of ca. 2–3 lm, as estimated from an SEM image. The SnO2/t-BaTiO3 and SnO2-Sb/t-BaTiO3 piezo film electrodes are obtained by the same preparation procedure as above except for replacing t-BaTiO3 with SnO2/t-BaTiO3 and SnO2-Sb/t-BaTiO3, respectively. 2.2.3. Preparation of t-BaTiO3@SiO2 and SnO2/(t-BaTiO3@SiO2) t-BaTiO3@SiO2 is synthesized using the method reported by Stöber et al. First, 0.2 g t-BaTiO3 is added to a mixture solution containing 200 mL ethanol, 75 mL distilled water and 2.5 mL NH3H2O (28%) and vigorously stirred for 30 min. Second, 7.5 mL TEOS is dripped into the suspension mixture in a 30 °C water bath. Third, the above mixture is stirred for 4 h in a 30 °C water bath. The product is obtained by centrifugation, washing with distilled water and alcohol several times, and drying in an oven at 60 °C for 12 h. Finally, the dried product is sintered for 2 h at 1000 °C (heating rate = 20 °C min
1) to form t-BaTiO3@SiO2. SnO2/(t-BaTiO3@SiO2) is prepared by loading SnO2 onto the obtained t-BaTiO3@SiO2 using the same process as that for the preparation of SnO2/t-BaTiO3. 2.3. Characterization and analysis

2. Experimental 2.1. Reagents and chemicals Cubic BaTiO3 (c-BaTiO3), MNZ, Ti(OC2H5)4, dimethyl sulfoxide (DMSO), 2,2,6,6-tetramethylpiperidine (TEMP), 5,5-dimethyl-1pyrroline N-oxide (DMPO), methanol (HPLC), sodium hydroxide (NaOH), ammonia solution (NH3H2O, 28%), acetic acid, ethyl alcohol absolute and tetraethyl orthosilicate (TEOS) are obtained from Aladdin Chem. Co., Ltd. (China), Ba(OH)28H2O, SnCl2 and SbCl3 are all purchased from Sinopharm Chemical Reagent Co., Ltd. All reagents and chemicals are of analytical grade unless otherwise stated.

The surface topography and piezoelectric property of the piezomaterials are determined by atomic force microscopy (AFM, Bruker, ICON) and piezoresponse force microscopy (PFM, Bruker, ICON), respectively. The elemental mapping image and morphology of the powders are obtained by scanning electron microscopy (SEM, JEOL, JSM-6330F) with an energy dispersive X-ray spectrometer (EDS, Inca300 Oxford). X-ray diffraction (XRD) spectra of the samples are recorded with a D-MAX 2200 VPC diffractometer (Rigaku Corporation, Japan) with Cu Ka radiation at 40 kV and 30 mA. High-resolution transmission electron microscopy (HRTEM) images are obtained on a JEOL JEM-2010HR. X-ray photoelectron spectroscopy (XPS) measurements are carried out on an ESCALAB 250, Thermo-VG Scientific (UK) system. The BET surface

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areas of the samples are obtained by the physical adsorption of N2 at 77 K on an auto-adsorption system (Autosorb-6, Quanta chrome) and calculated by the Brunauer-Emmet-Teller (BET) equation. The amounts of the loaded Sn and Sb are determined using ICP-OES (ICPS 7500, Shimadzu, Japan) after the sample is dissolved in a strong acid by microwave digestion. The reactive oxygen species (ROS, O–2, OH and 1O2) are detected via the electron spin resonance (ESR) technique at ambient temperature (Bruker A300-1012 spectrometer, Germany) after they are trapped by DMPO (100 mmol L
1) in DMSO, DMPO (100 mmol L
1) and TEMP (200 mmol L
1) in water. The concentration of MNZ is quantified by high-performance liquid chromatography (HPLC, Shimadzu LC-15C) with a UV detector set at 318 nm. The mobile phase is a mixture of water/methanol 30/70 (V/V) eluted at flow-rate 0.8 mL min
1. The injection volume is 20 lL, and the oven temperature is set at 30 °C. The degradation intermediates are analyzed by liquid chromatography-mass spectrometry (LC–MS/MS, Thermo Fisher TSQ Quantum Ultra). The piezoelectrochemical current (current-time curves) and Nyquist plots (EIS, electrochemical impedance spectroscopy) of the piezo film electrodes are determined in a self-designed piezoelectrochemical reactor by an electrochemical workstation (Reference 600+, Gamry) (The piezoelectrochemical currents in a 0.1 mol L
1 Na2SO4 solution are performed at an open-circuit voltage; The Nyquist plots in a 0.1 mol L
1 Na2SO4 solution are performed at an open-circuit voltage in a frequency range from 100 kHz to 1 mHz with an amplitude of 10 mV). The reactor consists of three electrodes: a standard calomel electrode as the reference electrode, a Pt electrode as the counter electrode and a piezo film electrode as the work electrode (as shown in S1). 2.4. Piezocatalytic experiments The piezocatalytic performance of MNZ is carried out using ultrasonic vibration (CPX2800H-C, Branson) at a low frequency (40 kHz, 110 W) to trigger the piezomaterials. The piezomaterial (0.05 g) is added to a 200 mL reactor containing 50 mL MNZ aqueous solution (10 mg L
1), whose initial pH value is approximately 6.5. The reaction solution is kept at approximately 25 °C in the dark. After a given irradiation time, the samples are extracted at set time intervals. After being filtered with 0.22 lm PFTE syringe filters, the concentration of residual MNZ is determined by HPLC. 3. Results and discussion 3.1. Composition and morphology of the loaded t-BaTiO3 The hydrothermally prepared t-BaTiO3 is a kind of white powder with an average size of ca. 90 nm and a sphere-like micromorphology (Fig. S2). After t-BaTiO3 is mixed with the SnCl2 or SnCl2-SbCl3 ethanol solution and then oxidized by air, the elements of Sn and Sb are evenly dispersed on the surface of t-BaTiO3, as shown in the EDS-mappings (Fig. S3). At the same time, some small particles with a size of 3–5 nm on the surface of t-BaTiO3 can be formed (Fig. 1a-c). Their HRTEM images (Fig. 1d-f) show many well-defined lattice fringes with inter planar spacings of 0.241 nm and 0.245 nm. The former value matches the distance between the SnO2{2 0 0} orientation planes, while the latter value is slightly greater than the former. The increase in the distance is consistent with the doping effect of larger Sb ions observed from the X-ray diffraction pattern by Yadav et al. [31]. The results preliminarily indicate that SnO2 and SnO2-Sb are anchored on the surface of t-BaTiO3. This indication is further confirmed by XPS spectra. As shown in Fig. 2c, Sn(II) is continuously decreased, as Sn(IV) is incessantly formed with prolonged reaction time. After

Fig. 1. TEM (left) and HRTEM (right) of t-BaTiO3 (a, d), SnO2/t-BaTiO3 (b, e) and SnO2-Sb/t-BaTiO3 (c, f).

42 h, the XPS peaks of Sn(II) are basically nonexistent, showing that Sn(II) is completely oxidized to Sn(IV) by air. However, the Sb element in SnO2-Sb remains trivalent and is not oxidized by air. Although SnO2 and SnO2-Sb are demonstrated as being loaded on the surface of t-BaTiO3, the color, XRD pattern (Fig. 2a) and SEM image (Fig. S2) of the loaded t-BaTiO3 present no clear alteration. In addition, the BET surface areas of the samples are only slightly different, as shown in Table 1. These similarities between the un- and loaded t-BaTiO3 may be because the loading amount is so little that these changes cannot be apparently observed.

3.2. Piezoelectric and electrochemical characteristic of the loaded t-BaTiO3 Fig. 3 presents the AFM images and PFM mapping of SnO2-Sb/tBaTiO3. As seen from the figure, although the surface of t-BaTiO3 is loaded with SnO2-Sb, the piezoeffect can still be clearly observed, and the PFM map is very similar to the AFM topography image. However, the loading of SnO2, especially SnO2-Sb, can considerably decrease the impedance of the t-BaTiO3 film electrode, (i.e., improve its conductivity), as indicated in the Nyquist plots (Fig. 4a). The improvement in conductivity is favorable for the mobility of piezocharge carriers, possibly leading to an enhancement in the piezocatalytic activity. The piezoelectrochemical current response of the piezoelectric nano-particle films in solution (0.1 mol L
1 Na2SO4) under sonicated waves is firstly observed. As shown in Fig. 4b, the current

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Fig. 2. XRD patterns (a) and XPS spectra (b) of t-BaTiO3, SnO2/t-BaTiO3 and SnO2-Sb/t-BaTiO3. XPS spectra (c) of Sn(II) and Sn(IV) for various oxidation times and XPS spectra (d) of Sb(III).

Table 1 Featured parameters of the loaded BaTiO3. Piezomaterial

Surface area (m2 g
1)

Piezo electrochemical current (lA)

Piezo degradation efficiency (%)

Rate constant (k, min
1)

Average intensity of ESR peak (103) 

1



11.5 26.1 30.5

524.0 222.6 219.5

56.7 65.7 118.6

OH

t-BaTiO3 SnO2/t-BaTiO3 SnO2-Sb/t-BaTiO3

25.5 29.04 31.79

0.50 0.63 1.14

36.7 62.8 74.2

0.0079 0.0180 0.0235

O2

O–2

*In the piezomaterials, the amount of SnO2 loading is 0.4 at%, and the amount of doped Sb in SnO2 is 2.5 at%.

is rapidly generated when the sonication is turned on. The piezoelectrochemical current responses of the SnO2/t-BaTiO3, and especially, SnO2-Sb/t-BaTiO3 films are apparently stronger than that of the pure t-BaTiO3 film. The main peak current of the SnO2-Sb/tBaTiO3 film is 1.38 mA, which is 1.5-fold that of the pure t-BaTiO3 film. Moreover, the shape of the piezoelectrochemical current response curve of SnO2-Sb/t-BaTiO3 is different from that of tBaTiO3. It is noteworthy that there are several t-BaTiO3 nanoparticles in these films, and each nano-particle t-BaTiO3 can theoretically drive the piezocatalytic redox in water. Their main reactions are as follows [32]:

t-BaTiO3 + mechanical energy ! t-BaTiO3 (e, h)

ð1Þ

h + H2 O ! OH + Hþ + e

ð2Þ

e + O2 ! O2 —

ð3Þ

As shown in reactions (2) and (3), the film will output current when the rate of water oxidation is faster than that of O2 reduction because more residual electrons will be present at the t-BaTiO3 piezo film electrode (i.e., the working electrode) and they will flow towards the Pt electrode (i.e., the counter electrode). The current is generated by the electrocatalytic reactions via a piezopotential driving force, therefore, the output is referenced as the piezoelectrochemical current, which is different from the usual piezocurrent from the pure physical process of t-BaTiO3 deformation. It can be

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Fig. 3. Topography AFM images (a and c) and PFM response maps (b and d) of t-BaTiO3 and SnO2-Sb/t-BaTiO3.

inferred from the observed considerable increase in current that the loading of SnO2, and especially SnO2-Sb, can promote the piezoelectrochemical oxidation of water to generate OH on t-BaTiO3. 3.3. Enhancement effect of SnO2 and SnO2-Sb

Fig. 4. (a) Nyquist plots of the three kinds of piezoelectric film electrodes (t-BaTiO3, SnO2/t-BaTiO3 and SnO2-Sb/t-BaTiO3) in a 0.1 mol L
1 Na2SO4 solution. (b) The piezoelectrochemical currents of t-BaTiO3, SnO2/t-BaTiO3 and SnO2-Sb/t-BaTiO3 (insert: the response of the piezoelectrochemical current of SnO2-Sb/t-BaTiO3 to a pulsed ultrasound with a 20 s on–off frequency) (electrolyte: 0.1 mol L
1 Na2SO4 solution).

The piezodegradation efficiency of MNZ is used as an active indicator for the piezocatalyst. Fig. 5a presents the change in the piezodegradation efficiency of MNZ with the amount of loaded SnO2. The piezodegradation efficiency firstly increases rapidly to 62.8% and then basically remains constant after the loading amount exceeds 0.4 at% (at%: atomic percent). The maximum degradation efficiency is approximately 2 twice that of the pure t-BaTiO3. The apparent increase indicates that a small amount of SnO2 can considerably improve the piezocatalytic activity of tBaTiO3. Fig. 5b presents the dependence of the piezocatalytic degradation of MNZ efficiency on the oxidation time of loaded Sn(II) by air. The degradation efficiency linearly increases from 36.7% to 62.8% with the time of Sn(II) oxidation for the first 24 h; however, after 24 h, the degradation efficiency remains basically unchanged, staying at approximately 63%. Comparing Figs. 2c and 5b, it is found that the change tendency is consistent with amount of SnO2 formed in 24 h; however, after 24 h, the degradation efficiency is reduced, although the amount of SnO2 is continuously increasing due to the oxidation of Sn(II). The maximum degradation efficiency does not correspond to the complete oxidation of Sn(II). The facts indicate that the loading of SnO2 can improve the piezocatalytic activity of t-BaTiO3, but SnO2 containing a small amount of Sn(II) possesses a more evident enhancement in the piezocatalytic activity. According to Wang et al.’s investigation on SnO2 as a traditional electrocatalyst, the residual Sn(II) ions were readily doped into the SnO2 structure, possessing the selfdoping effect. The self-doping effect could enhance the electrocatalytic activity by improving the conductivity of SnO2 [33]. The above piezocatalytic enhancement is also possibly dependent on the doping effect, a possibility that is further confirmed by the doping of Sb(III) into SnO2.

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Fig. 5. Dependence of piezodegradation efficiency on (a) SnO2 loading amount (SnO2/t-BaTiO3), (b) Sn(II) oxidation time (SnO2/t-BaTiO3) and (c) Sb(III)-doping amount in SnO2 (SnO2-Sb/t-BaTiO3).

As shown in Fig. 5c, the degradation efficiency of MNZ is linearly increased to 74.2% with the amount of the doped Sb(III) in SnO2 and is then reduced after the doping amount exceeds to 2.5 at%, emerging as a volcano-like plot. Soheilnia and Xie et al. reported that the electrochemical activity of an SnO2 electrode could be significantly improved by doping with Sb(III) due to the increase in electrode conductivity [34,35]. Therefore, these results are also consistent with Starr et al.’s prediction that the piezoelectric catalytic process of nano-piezoelectric materials is similar to that of traditional electrolytic cells [17,36]. Fig. 6 presents the piezodegradation kinetic curves of MNZ for various processes. The kinetic curves can be well fitted by the first-order kinetics equation, ln(C0/Ct) = kt, where k, C0 and Ct are the rate constant (min
1) and the concentrations (mol L
1) of MNZ at reaction times of 0 min and t min. The as-fitted k values for different various processes are listed in Table 1. The k values of SnO2-Sb/t-BaTiO3 and SnO2/t-BaTiO3 are 0.0235 min
1 and 0.0180 min
1, respectively, and are 3.0- and 2.3-times that of tBaTiO3 (0.0079 min
1). Such great kinetic enhancement effects further demonstrate that the modification of piezomaterials with electrocatalysts is an effective way to improve the piezocatalytic activity.

3.4. Enhancement mechanism of SnO2 and SnO2-Sb for piezocatalysis In the enhanced degradation of MNZ, sonic waves are used as the vibrational energy. Theoretically, the enhanced effect is also possibly dependent on sonic catalysis. However, it has been demonstrated for low-frequency sonic waves that the sonic catalysis of pure t-BaTiO3 only slightly contributed to the degradation of organic pollutants [37], whereas the contribution of ultrasound catalysis for the t-BaTiO3 loaded with SnO2 or SnO2-Sb cannot be ruled out. To evaluate their contributions, SnO2/c-BaTiO3 and SnO2-Sb/c-BaTiO3 are used as probe materials for sonocatalysts, because the features of c-BaTiO3 are very similar to those of tBaTiO3 except for a lack of piezoactivity. As shown in Fig. 6, the k values of SnO2/c-BaTiO3 and SnO2-Sb/c-BaTiO3 are both only approximately 0.004 min
1, which are similar to the k value of pure sonication. The above results clearly show that enhancement role of SnO2 and SnO2-Sb on the t-BaTiO3-mediated degradation of MNZ stems from an improvement in the piezocatalytic activity, not sonic catalysis. To further understand the enhancement mechanism of SnO2 and SnO2-Sb, the ESR spectra of the main ROS, O–2, 1O2 and OH, in the piezoprocesses of t-BaTiO3, SnO2/t-BaTiO3 and SnO2-Sb/tBaTiO3 are all determined because of the strong oxidative abilities

Fig. 6. Piezocatalytic kinetics of MNZ for various processes.

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of these ROS towards organic pollutants. It can be observed from these ESR spectra (Fig. 7 and Table 1) that the processes present various changes in the ESR peak intensity for different ROS. For  OH, the order of the ESR intensity is SnO2-Sb/t-BaTiO3 > SnO2/tBaTiO3 > t-BaTiO3, while the order of the ESR intensity for 1O2 is SnO2-Sb/t-BaTiO3 < SnO2/t-BaTiO3 < t-BaTiO3. For O–2, the order of the ESR intensity is SnO2-Sb/t-BaTiO3 > SnO2/t-BaTiO3 > t-BaTiO3. In brief, the loading of SnO2, especially SnO2-Sb, results in the increase in OH and O–2 and the decrease in 1O2. According to the traditional electrochemical principle, the formation of the initial O–2 radical is by the reduction of O2 at the cathode (reaction 4) while that of the initial OH and 1O2 are both from the oxidation of H2O at the anode (reactions 5–6), in which generations of OH and 1O2 are competing. The DSA catalysts SnO2 and SnO2-Sb, with a high OEP, are favorable to the generation of OH (reaction 5) [23–25].

Cathode: O2 + e ! O2 —

ð4Þ

Anode: H2 O ! OH + Hþ + e

ð5Þ

2H2 O !1 O2 + 4Hþ + 4e

ð6Þ

Combining the electrochemical principle with the changes in the above ESR spectra, the increase in OH and the decrease in 1 O2 are consistent with the characteristics of SnO2 and SnO2-Sb as typical DSA catalysts in piezoreactions. At the same time, the increase in ESR intensity of O–2 for SnO2-Sb is consistent with the cathodic reduction reaction (1). The consistency shows that SnO2-Sb can also improve the piezocatalytic reduction efficiency of the piezomaterial. This result seems slightly different from the electro-catalytic feature in a traditional electrochemical reactor. This difference may be due to the special electrochemical phenomenon of cyclically changing the piezopolarization field [38]. Although the special enhancement mechanism of SnO2-Sb for the generation of O–2 remains quite unclear, it is possibly related to two factors. First, the good conductivity of SnO2-Sb, as shown in the Nyquist plots (Fig. 4a), is beneficial to the transfer of charge carriers to the active site of O2 reduction. Second, the extra electrons from the catalytic oxidation of SnO2-Sb may promote the reduction of O2 (reaction 4); however, SnO2-Sb is possibly not the active site of the reduction. The speculation is confirmed by

the piezocatalytic-reduction of Ag(I) ions, the EDS element mappings of Sn and Ag (Fig. 8a-c) and the XPS spectra of Ag(0) (Fig. S7) that show the reduced metal Ag(0) is not almost located at the Sn in SnO2. In other words, the active site of the piezoelectric catalytic reduction reaction is located on the bare surface of tBaTiO3 for SnO2/t-BaTiO3. Recently, several groups have demonstrated that the charges of piezocatalytic reactions were from intrinsic free carriers, rather than the strain-induced piezoelectric charges in the piezomaterials because the piezoelectric charges were bound polarization charges and could not be freely moved. Piezocatalytic reactions only drove the intrinsic free charge carriers to the surface of the material [7,39–42]. However, in general, the intrinsic free charge carriers in piezomaterials are few and they cannot completely balance piezoelectric charges [9,10], therefore, there is a residual piezoelectric potential in the strained piezomaterial. The present issue is whether the residual piezoelectric potential can drive free charges in the loaded electrocatalysts? In other words, can the piezoelectric potential drive the free charges in the loaded electrocatalysts to take part in the redox reactions, in addition to the intrinsic free charges of the piezomaterials? To answer this question, t-BaTiO3 is coated with vitrified SiO2 to prevent the transfer of the intrinsic free charges of the piezoelectric materials to the external environment. It can be seen from Fig. 8d that the efficiency of piezocatalytic degradation for t-BaTiO3@SiO2 is only 15.4%. The efficiency is apparently less than that of pure t-BaTiO3, being similar to only that of pure sonication. This result indicates that the shell of vitrified SiO2 can indeed prevent the intrinsic free charges of piezoelectric materials to participate in the degradation reaction of MNZ. However, the piezocatalytic degradation efficiency for SnO2/(tBaTiO3@SiO2) reaches as high as 58.6%. The free charges participating in the degradation reaction should come from the external SnO2 because the free charges of t-BaTiO3 are enclosed in the shell of the vitrified SiO2. Moreover, according to the principle of the piezoelectric nano-generator, in which the piezopotential can drive the flow of electrons in an external conductor to generate current [3], it can be deduced that the free charges in the external SnO2, participating in the degradation reaction, should be driven by the piezopotential. Therefore, it may be concluded that the piezopotential can drive not only the intrinsic free charges of t-BaTiO3 but also the free charges of the loaded SnO2 to participate in the degradation reaction. In other words, the enhancement effect of the loaded SnO2 can be partly attributed to the participation of free

Fig. 7. ESR spectra of reactive oxygen species (ROS) in the various piezosystems.

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Fig. 8. EDS element mappings of (a) Sn (blue points), (b) Ag (yellow points), and (c) Sn + Ag (Ag obtained by the piezocatalytic reduction of SnO2/t-BaTiO3). (d) The piezocatalytic activities of t-BaTiO3@SiO2 and SnO2/(t-BaTiO3@SiO2).

charges from the loaded SnO2 in the degradation reactions of MNZ under the driving force of the piezopotential. 3.5. Possible pathway of MNZ piezocatalytic degradation The intermediates from the piezocatalytic degradation of MNZ are detected by LC-MS. The eight MS peaks with m/z = 187, 159, 157, 142, 118, 100, 90 and 60 are observed in their +ESI scan spectra (Fig. S8). Referring to the attribution of the MS peaks of the intermediate products from MNZ degradation in other processes [43–45], the data can be attributed to the following eight compounds: (2-methyl-5-nitroimidazol-1-yl) acetic acid (I), (5-hydroxy-2-

methylimidazol-1-yl) acetic acid (II), 2,5-dihydroxyimidazol-1ylacetic acid (III), 1-hydroxyethy-2-methyl 5-hydroxyimidazol (IV), 2,5 dihydroxy imidazole (V), 2-acetamidoacetic acid (VI), formamidoformic acid (VII) and acetic acid (VIII). According to the features of these compounds, it can be inferred that the degradation of MNZ starts with its hydroxyl and nitro groups. The hydroxyl is oxidized to produce the intermediate (I), while the nitro group is attacked to form the intermediate (II). The two primary intermediates are further oxidized to result in a series of secondary intermediates with an imidazole ring (III)–(V) and unidentified rearrangement products, finally degrade into small molecular organic and inorganic compounds. The main degradation intermediates and pathway are

Fig. 9. Possible mechanisms of MNZ piezocatalytic degradation with SnO2-Sb/t-BaTiO3 as the piezocatalyst.

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similar to the reported those of MNZ degradation by OH radicals, in which the OH radicals are generated with a typical boron-doped diamond (BDD) anode [45,46]. This similarity possibly denotes that the piezocatalytic degradation of MNZ may also be by the oxidation of piezogenerated OH radicals, not other ROS. The possibility is further demonstrated by the experiment of OH scavenging with tertbutyl alcohol (TBA) because the scavenging of OH leads to the decrease in the degradation of MNZ to 19% for a 60- min piezocatalytic process of SnO2-Sb/t-BaTiO3. The efficiency is only similar to that of the pure sonication. Therefore, combining the analysis of LC-MS with the above investigation about the enhancement effect of SnO2-Sb and the scavenging of OH, the main pathway of the enhanced piezocatalytic degradation of MNZ can be described in Fig. 9. This work confirms Starr et al.’s prediction that the piezoelectric catalytic process of nano-piezoelectric materials is similar to that of traditional electrolytic cells [17,47], and the combination of excellent electrocatalysts with piezoelectric nanomaterials is a new and efficient way to improve the piezocatalytic efficiency of pollutant removal. 4. Conclutions In this work, the piezocatalytic perfomance of three kinds of nano-materials, t-BaTiO3, SnO2/t-BaTiO3 and SnO2-Sb/t-BaTiO3, are invesigated. It is found that SnO2, specially SnO2-Sb, can considerably enhance the piezo-electrochemical current of H2O piezo-oxidation and the piezocatalytic activity of t-BaTiO3 for MNZ degradation. The enhancement originates from catalytic generation of OH. In these processes, SnO2 and SnO2-Sb can also act as DSA-like catalysts in the cyclic piezo-field. These results confirm Starr et al.’s prediction that the piezoelectric catalytic process of nano-piezoelectric materials is similar to that of traditional electrolytic cells. Significantly, they indicate that the combination of excellent electrocatalysts with piezoelectric nanomaterial is a new and efficient way to improve the piezo-catalytic efficiency of pollutants.

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

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