BaTiO3 heterostructures for high-efficiency catalytic oxidation

Journal Pre-proof Piezophototronic effect in enhancing charge carrier separation and transfer in ZnO/ BaTiO3 heterostructures for high-efficiency catalytic oxidation Xiaofeng Zhou, Shuanghao Wu, Chunbo Li, Fei Yan, Hairui Bai, Bo Shen, Huarong Zeng, Jiwei Zhai PII:

S2211-2855(19)30834-1

DOI:

https://doi.org/10.1016/j.nanoen.2019.104127

Reference:

NANOEN 104127

To appear in:

Nano Energy

Received Date: 2 August 2019 Revised Date:

20 September 2019

Accepted Date: 21 September 2019

Please cite this article as: X. Zhou, S. Wu, C. Li, F. Yan, H. Bai, B. Shen, H. Zeng, J. Zhai, Piezophototronic effect in enhancing charge carrier separation and transfer in ZnO/BaTiO3 heterostructures for high-efficiency catalytic oxidation, Nano Energy (2019), doi: https://doi.org/10.1016/ j.nanoen.2019.104127. 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 Ltd.

Graphical abstract

Specific ultrasound excitation of ZnO/BaTiO3 heterostructures resulted in a piezoelectric potential difference of 414.40 mV, which is much higher than pure BaTiO3 and ZnO. Consequently, the photoexcited electron–hole pairs can be easily separated and transfer in ZnO/BaTiO3, thus creating more free radicals to accelerate piezophotocatalytic oxidation processes in aqueous solution.

Piezophototronic effect in enhancing charge carrier separation and transfer in ZnO/BaTiO3 heterostructures for high-efficiency catalytic oxidation Xiaofeng Zhoua, Shuanghao Wua, Chunbo Lib, Fei Yana, Hairui Baia, Bo Shena, Huarong Zengb, Jiwei Zhaia,* a

Key Laboratory of Advanced Civil Engineering Materials of Ministry of Education,

Functional Materials Research Laboratory, School of Materials Science and Engineering, Tongji University, Shanghai 201804, China b

Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050,

China *Corresponding author. E-mail address: [email protected] (J. Zhai).

Abstract Rapid charge transfer in various piezoelectric semiconductors is usually limited, which has thus hardly achieved a higher mechanochemical potential elicited by macroscopic polarization for advanced oxidation processes. Here we propose a piezoelectric ZnO/BaTiO3 heterostructure with an elevated macroscopic polarization under ultrasonic activation and, therefore, should be amenable to facilitate photoexcited electron-hole pairs with more efficient separation and transfer; that is, ZnO/BaTiO3 heterostructures have an enhanced piezophototronic effect. Under excitations of concurrent ultrasound at ultrasonic power of 120 W and simulated sunlight at light intensity of 100 mW cm–2 irradiation, the catalytic oxidation capability to dyeing wastewater degradation with ZnO/BaTiO3 is dramatically 1

increased, and its oxidation reaction rate constant can be up to 1.20×10–1 min–1, which is 1.50 and 2.00 times that of BaTiO3 and ZnO, respectively. In contrast, ultrasonic agitation or simulated sunlight irradiation of ZnO/BaTiO3 results in an oxidation reaction rate constant of 3.46×10–2 or 5.60×10–2 min–1, correspondingly, which are both lower than the piezophototronic effect triggered. Additionally, the finite element simulation shows that the as-obtained ZnO/BaTiO3 creates a piezoelectric potential difference of 414.40 mV, which is higher than that of as-obtained BaTiO3 (409.50 mV) and ZnO (33.00 mV). This work will provide references for understanding the correlation between macroscopic polarization and catalytic oxidation processes, and also contribute to the development and applications of multi-energy harvesting piezoelectric materials.

Keywords: Piezophototronic effect; Heterostructures; Polarization; Piezoelectric potential; Catalytic oxidation

1. Introduction Photocatalytic advanced oxidation procedures have long been used as an appealing treatment option for energy conversion and environmental remediation [1-4]. However, the practical use of traditional photocatalysis is usually restricted by its quantum efficiency below 10% [5]. Efforts to overcome the relatively poor photocatalytic activities, such as bandgap engineering [6, 7], nanostructured morphology maneuvering [8-11], heterojunctions constructing [12-20], metallic or non-metallic doping [21, 22], single atoms loading [23, 24] and lattice defects introducing [25-27] have been extensively studied over the past decades. Although 2

some of these strategies promoted photocatalytic oxidation processes to a certain extent, there is still a huge potential for efficient charge transfer and quantum yields in photocatalytic oxidation processes. For example, when the external magnetic field was applied to the photocatalytic reaction system with α-Fe2O3/reduced graphene oxide hybrid photocatalysts, it could be seen that the enhanced photocatalytic performance is due to the fact that more charge carrier transfer participates in the photocatalytic oxidation reactions accompanied by the action of the magnetic field [28]. Similarly, the supplementary thermal stress [29], microwave [30, 31] and external electric fields [32] also contributed to the charge carrier separation and transfer in photocatalysts, and enabled the design of multi-energy harvesting photocatalysts more possible. However, these methods typically rely on precise experimental conditions and require artificial driven force, where photocatalytic costs and operational burdens increase, and are therefore crucial to developing new strategies for improving photocatalytic oxidation processes. It is well known that piezoelectric semiconductors having non-centrosymmetric structures possess a piezoelectric potential for spontaneous electrical polarization, which facilitates the generation of a built-in electric field that is initiated by periodic mechanical energy or ultrasonic agitation [33-37]. Therefore, the saturated electrostatic field can be easily interrupted by the alternating built-in electric field generated in the piezoelectric semiconductors, and then an electric field driven force is produced incessantly to accelerate the separation and transfer of the photogenerated charge carrier. Since Wang and colleagues discovered this phenomenon [38, 39], it 3

has been widely used in the design and improvement of light-emitting diodes [40, 41], photodetectors [42-45], photocells [46], solar cells [33, 47-49], and other optoelectronic devices [50-52]. Meanwhile, the coupling among piezoelectricity, semiconductor charge transport and optical processes in piezoelectric materials directly provides a unique method to manipulate redox reactions induced by the charge carrier at the interface of piezoelectric semiconductors, such as water splitting [53-55], chemical synthesis [56], and wastewater treatment [57-60]. In addition, mechanical energy, such as wind, tide and sound energy, is an ubiquitous energy source in nature, hence it is significant to investigate the structural relationship between piezoelectric semiconductors and piezophototronic effect at a micro level. Recently, the study on piezoelectrically-assisted photoexcited charge carrier separation and transfer in various photocatalytic applications has been mainly manifested in three aspects. One prevalent strategy is to form junctions by coupling semiconductor photocatalysts and piezoelectric materials to realize the judicious integration of semiconductor’s optoelectronic properties and piezoelectric effect. For example, Wang and Liu et al. [60] proposed an Ag2O/BaTiO3 hybrid photocatalyst, which exhibited enhanced photocatalytic activity and cyclic performance under periodic ultrasonic excitation for the piezoelectric BaTiO3 created a polar charge induced the built-in electric field that increased the separation of photoinduced carriers. Simultaneously, Wang and Lin et al. [61] found that piezoelectric BaTiO3 can tune the interface of p-type Ag2O and n-type TiO2 to decrease the recombination of photogenerated charge carriers and significantly improve the photoelectrocatalytic 4

performance. Based on the same principle, Zhai and Liu et al. [59] made use of Ag2S/ZnO to assess its catalytic activity and cyclic stability for organic dyes degradation under simultaneous ultrasound and light irradiation. Due to the strain-initiated piezopotentials from the bent ZnO nanowires, the Ag2S/ZnO composites showed a large improvement in photocatalytic and recycling performance for organic dyes degradation in aqueous solution compared to pure ZnO nanowires. Soon after, CuS/ZnO [62], TiO2/ZnO [63], TiO2/BiFeO3 [64], TiO2/Pb(Zr,Ti)O3 [65] and MoS2/KNbO3 [66] to name a few, have fallen within the realms of piezophotocatalysis. Compared to the isolated photocatalysis or piezocatalysis processes, these hybrid structures showed a higher catalytic oxidation capability to wastewater decomposition and/or hydrogen evolution under the piezophototronic effect due to the piezoelectric potential induced by-mechanical energy that can act as a driven force to efficiently separate photoexcited charge carrier in time. Second, well-dispersed noble or non-noble metal nanocrystals on piezoelectric semiconductors were carried out as a major focus area in photocatalytic oxidations. For example, Wang et al. [57] obtained an excellent photocatalytic activity using Au/BaTiO3 plasmonic photocatalysts by synergism of BaTiO3 mediated macroscopic polarization and Au nanoparticles mediated localized surface plasmon resonance. Similar effects were obtained by Au/MoS2 [67] and Al/BaTiO3 [68] plasmonic photocatalysts. Third, due to its simple preparation method and pure composition, single piezoelectric materials such as ZnO nanowires or nanorods [69, 70], ZnSnO3 nanowires [71], CdS nanosheets [72], and organolead halide perovskite CH3NH3PbI3 [55] have become 5

key players for piezophotocatalysis. Although a large volume of publications have been devoted to development of photocatalysts with improved oxidation capabilities, few examples have shown that homotype piezoelectric heterostructures mediated highly-efficient catalytic oxidation reactions based on piezophototronic effects. In addition, the mechanisms of catalytic oxidations initiated by piezophototronic effect are still unclear, so it is urgent to study its intrinsic relationship connection with piezoelectric materials. BaTiO3 as a “star” piezoelectric material, has a higher piezoelectric coefficient and dielectric constant, but its electric conductivity is very poor. In contrast, ZnO has greater electrons mobility, whereas its piezoelectric properties are not very obvious. In this research, we report a novel piezoelectric ZnO/BaTiO3 heterostructure by combining BaTiO3 and ZnO with a solvothermal reaction and impregnation sintering. The catalytic oxidation processes of ZnO/BaTiO3 were comparably investigated under piezophototronic effect. The results showed that ultrasonic agitation or simulated sunlight irradiation of ZnO/BaTiO3 resulted in a greater catalytic oxidation capability for degradation of wastewater in contrast to pure BaTiO3 and ZnO. In particular, the contaminant degradation ratio in the presence of ZnO/BaTiO3 can be up to 97% in 30 min under the excitations of both ultrasonic agitation and simulated sunlight irradiation, which is higher than that of isolated BaTiO3 (88%) and ZnO (84%). Furthermore, the piezoelectric potential of ZnO/BaTiO3 was analyzed using the finite element simulation, and a reasonable mechanism was proposed. Through this mechanism, the piezophototronic effect enhanced catalytic oxidation processes with 6

ZnO/BaTiO3 heterostructures. 2. Materials and methods 2.1 Preparation of BaTiO3 crystallites BaTiO3, a mixed-structure of nanoparticles/nanorods-powder, was prepared by a solvothermal method at 200℃ for 12 h. Typically, 1.00 g of the surfactant (Polyethylene glycol 6000) was first dissolved in 25 ml of ethanol in a 100 ml flask. After vigorous agitation at 30℃, the surfactant had completely dissolved, resulting in a semitransparent and viscous micellar solution. Then, 1.00 ml of tetrabutyl titanate (C16H36O4Ti) solution was gently dropped into the micellar solution at the same temperature. Subsequently, the addition of 10.00 ml of an aqueous KOH solution (2.00 mol L–1) and the continuous stirring led to a white stable emulsion. Next, some 0.32 g of Ba(OH)2·8H2O was added to the mixture, which was actively stirred for a several minutes. The resulting solution was transferred to a 100 ml Teflon-lined stainless-steel autoclave and heated in a conventional oven at 200℃ for 12 h. The as-formed products were collected by centrifugation and washed by formic acid (0.1 mol L–1) and deionized water for three times, respectively. Afterward, it was dried in an oven at 105℃ under and calcined in the atmosphere at 800℃ for 2 h. 2.2 Preparation of ZnO/BaTiO3 heterostructures A highly reproducible impregnation method was used to prepare ZnO/BaTiO3 heterostructures. In a typical experiment, a certain amount of Zn(NO3)2℃6H2O was dissolved in a 5.00 ml of volumetric flask with deionized water, then added dropwise to 0.20 g of as-prepared BaTiO3 with vigorous stirring. After all the water had 7

evaporated, the mixture was immediately transferred to a high-quality alumina crucible equipped with a lid, then heated at a ramping rate of 3℃ min–1 and finally held at 400℃ for 1 h in a muffle furnace. After the product was cooled to room temperature, the samples were then washed thrice with deionized water and stored for characterization and catalytic activity tests. Fig. S1 stepwisely illustrated the preparation processes of the ZnO/BaTiO3 heterostructures. The as-prepared samples are represented by ZBT-x, where x is the concentration values of the aqueous solution of Zn(NO3)2·6H2O (i.e., x = c/[c]). ZnO was prepared by the same procedure just only without adding BaTiO3. 2.3 Materials characterization Scanning electron microscopy (SEM) measurements were performed on a Nova NanoSEM 450 with a scanning voltage of 10 kV. Microstructure and selected area electron diffraction (SAED) were observed by high resolution transmission electron microscope (TEM JEM-2100F). The piezoelectric response was characterized using an atomic force microscope (Seiko SPA400) with the function of a piezoresponse force microscope (PFM) for the local polarization experiments. X-ray diffraction (XRD) measurements were taken using a D/MAX 2550VB diffractometer with Cu-kα (wavelength 0.1542 nm). XPS was performed on a Thermo Scientific XPS K-alpha machine using monochromatic Al-Kα radiation. UV–Vis absorption spectra were obtained on a U-4100 spectrophotometer fitted with an integrating sphere. The powder samples were subjected to reflectance measurements using a standard barium sulphate powder as a reference. The reflection measurements were converted to 8

absorption spectra using Kubelka–Mulk transformation. Photoluminescence (PL) spectra were measured using a photomultiplier, and a cutoff filter (Hitachi, F-7000). The illumination was carried out by the 325 nm band from a 500 W high-pressure mercury lamp and a wavelength range of 450 to 650 nm. A finite element calculation was performed using COMSOL to help elucidate the relationship between experimental data and theoretical calculations. 2.4 Electrochemical measurements The ZnO/BaTiO3 electrodes were coated on a conductive fluoride-doped tin oxide (FTO) glass substrates (2.2 mm thickness, 15 Ω sq–1, 84% transmittance). After a standard cleaning procedure (sonication in acetone, ethanol, and deionized water), 50 µL of sample dispersions (5.0 mg ml–1) were deposited onto the substrates by drop casting to form ZnO/BaTiO3 electrodes. The electrodes were then dried at 60℃ for 12 h, and then the electrodes were cut into a square with an area of 1.00 cm2. Electrochemical impedance spectroscopy (EIS) measurements were taken in the dark from 105 to 1 Hz using an electrochemical workstation (CHI760E) with ZnO/BaTiO3, Pt wire and Ag/AgCl as working, counter and reference electrodes, respectively. The photocurrents of ZnO/BaTiO3 electrodes were detected under the Xe lamp irradiation. Mott–Schottky plots of ZnO/BaTiO3 electrodes were taken at a frequency of 500, 1000 and 1500 Hz, respectively. In all cases, the aqueous electrolyte contained 0.50 mol L–1 Na2SO4. 2.5 Catalytic activity tests The catalytic oxidations of the ZnO/BaTiO3 heterostructures have been evaluated 9

for piezoelectric, photo and piezophoto-degradation of dyeing wastewater containing rhodamine B (RhB). The catalytic oxidation reactions were carried out in 200 ml beaker containing 100 ml of wastewater (10.00 mg L–1) in the presence of 1.00 g L–1 of catalysts. The macroscopic polarization of catalysts was initiated by an ultrasound provided by a digital ultrasonic generator at different ultrasonic powers (i.e., 80, 120, and 200 W) with fixed frequency of 40 kHz. The simulated sunlight source came from a 500 W Xe lamp, with an irradiation of different light intensities (i.e., 10, 30 and 100 mW cm–2) and its wavelength ranges from 300 to 900 nm (i.e., UV-Vis light). For comparison, the ultraviolet (UV) and visible light photocatalytic activities were also evaluated with a 500 W Xe lamp equipped with a 300 ~ 350 nm cutoff filter or a 420 nm cutoff filter as the UV or visible light source, respectively. The intensity of the simulated sunlight was monitored with an all-weather light radiation self-recording instrument (QTS-4). The suspension includes catalysts and the contaminants were stirred in the dark for 30 min to establish adsorption-desorption equilibrium prior to testing. Aliquots (~ 5.0 ml) of the reaction mixture were centrifuged during the reaction processes and analyzed using a UV–Vis absorption spectroscopy. After the wastewater purification was completed, the catalysts were recovered by filtration, and then dried. These catalysts were reused in the next cycle under the same conditions. A detailed schematic diagram of the test apparatus is shown in Fig. S2. 3. Results and discussion Representative SEM images of BaTiO3, ZnO and ZnO/BaTiO3 (ZBT-0.10) are shown in Fig. S3a-c, respectively. As presented in Fig. S3a, the as-prepared BaTiO3 10

nanocrystalline consists of nanoparticles/nanorods hybrid structure. It is important to note that the surface of BaTiO3 is not smooth or continues, thus providing more active sites to interact with contaminants. In previous research, Bao et al. [73] discovered that ultrasonic agitation of (Ba,Sr)TiO3 nanowires/nanoparticles resulted in higher piezocatalytic activity compared to pure (Ba,Sr)TiO3 nanoparticles, but lower than (Ba,Sr)TiO3 nanowires. Although this observation revealed that piezoelectric (Ba,Sr)TiO3 nanowires have a large macroscopic polarization under ultrasound activation at ambient temperature, they failed to clarify the direct correlation between the (Ba,Sr)TiO3 hybrid structure and the catalytic oxidations. As motivated by this finding, piezoelectric ZnO/BaTiO3 heterostructures was prepared directly from the as-obtained BaTiO3, as schematically depicted in Fig. S1. The SEM image of pure ZnO (Fig. S3b) exhibits the irregular particles with an average diameter of approximately 60 µm, which is a typical characteristic of solid phase synthesis. For ZnO/BaTiO3 (ZBT-0.10) heterostructures (Fig. S3c), it can be seen that nanoparticles/nanorods and irregular nanoparticles are cross-linked with each other. Interestingly, when reducing the loading of ZnO particles in the BaTiO3, there is a corresponding decrease in the size of ZnO. The representative TEM image of ZnO/BaTiO3 (ZBT-0.10) (Fig. 1a) shows that ZnO nanoparticles randomly are distributed on the surface of BaTiO3 (red circle), and the aspect ratios of BaTiO3 indicate that it has axial dense structures, which contributes to the establishment of the polarization electric field and is consistent with the corresponding SEM images (Fig. S3a, c). The high resolution TEM image (Fig. 1b) clearly reveals the crystal lattices of 11

ZnO and BaTiO3, respectively, indicating that the formation of ZnO/BaTiO3 heterostructures. The corresponding SAED pattern is displayed as the inset of Fig. 1b, confirming the monocrystalline nature of the ZnO/BaTiO3 heterostructures. To investigate the piezoelectric response of ZnO/BaTiO3 heterostructures, PFM tests were conducted as shown in Fig. 1c, d. The topographic image shows the surface structure of ZnO/BaTiO3, which is consistent with SEM images analysis. However, the piezoelectric image is different from the topographic image, indicating that ZnO/BaTiO3 has excellent piezoelectric properties. Furthermore, the piezoelectric image clearly shows a darker and brighter flat region, which is attributable to the corresponding distribution of weakly piezoelectric ZnO and strong piezoelectric BaTiO3. In addition, the ZnO/BaTiO3 heterostructures has size ranging from 10 to 20 nm (Fig. S3d), which is determined by the line profile of the piezoelectric image (Fig. 1d). This further supports the good formation of ZnO/BaTiO3 heterostructures with seamless connections.

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Fig. 1. (a) TEM and (b) High resolution TEM images of ZnO/BaTiO3 (ZBT-0.10), and the inset is the corresponding SAED pattern from (b). PFM images of (c) topography, (d) piezoelectric image for ZnO/BaTiO3 (ZBT-0.10).

The phase structures of BaTiO3, ZnO and ZnO/BaTiO3 were studied by XRD, as shown in Fig. 2a. It can be observed that the as-obtained ZnO exhibits characteristic diffraction peaks at 31.8°, 34.4°, 36.3°, 47.5°, 56.6°, 62.9°, 66.4°, 67.9°, 69.1° and 76.9°, corresponding to the (100), (002), (101), (102), (110), (103), (200), (112), (201) and (202) planes, respectively, showing a hexagonal phase with high crystallinity (JCPDS: 36-1451, space group P63mc). The as-prepared BaTiO3 exhibits characteristic diffraction peaks at 21.9°, 31.4°, 38.6°, 44.9°, 50.6°, 55.9°, 65.4°, 69.9°, 74.4°, 78.6° and 82.9°, corresponding to the (100), (110), (111), (200), (210), (211), (220), (300), (310), (311) and (222) planes, respectively, which can be indexed as a tetragonal phase BaTiO3 (JCPDS: 31-0174, space group P4mm). For XRD patterns of ZnO/BaTiO3, it can be seen that the characteristic diffraction peaks of ZnO gradually appear with increasing ZnO content of in the mixture, and the crystallinity of BaTiO3

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is well maintained. Furthermore, the hybrid phases of ZnO/BaTiO3 are clean without any impurity phase. These observations indicate that ZnO/BaTiO3 heterostructures have been successfully synthesized. The surface compositions and chemical states of the elements in ZnO/BaTiO3 were investigated by XPS measurements. As shown in the survey scanning XPS spectrum (Fig. 2b), the photoelectron lines observed at binding energies of about 285, 460, 530, 780, and 1044 eV correspond to C 1s, Ti 2p, O 1s, Ba 3d, and Zn 2p, respectively. The high-resolution Ba 3d spectrum data (Fig. 2c) show that the peaks of Ba 3d5/2 and Ba 3d3/2 are located at 778.5 and 793.7 eV, corresponding to Ba atoms in the perovskite structure; while the peaks of Ba 3d5/2 and Ba 3d3/2 are located at 779.9 and 795.1 eV, corresponding to Ba atoms in the non-perovskite structure. In the spectrum of O 1s (Fig. 2d), the non-symmetric peaks can be attributed to the existence of an –OH group on the surface of ZnO/BaTiO3 [74]. For the Ti 2p XPS spectrum (Fig. 2e), the main peak of Ti 2p3/2 splits off from the other peaks, and the main peaks of the 2p1/2 have asymmetric profiles. At the same time, the satellite peak in the dotted frame can be attributed to the inelastic scattering process on the spectral distribution of the Ti 2p spectrum for perovskite structure BaTiO3 [74]. In Fig. 2f, two doublets of 1021.1 and 1044.3 eV are assigned to the signals of Zn 2p3/2 and Zn 2p1/2, respectively. These observations suggest that Ba2+, O2–, Ti4+, and Zn2+ are present in the ZnO/BaTiO3, respectively.

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Fig. 2. (a) XRD patterns of the as-prepared ZnO, BaTiO3 and ZnO/BaTiO3. (b) XPS survey spectrum and high resolution (c) Ba 3d, (d) O 1s, (e) Ti 2p, and (f) Zn 2p spectra of ZnO/BaTiO3.

The UV–Vis absorption spectra of BaTiO3, ZnO and ZnO/BaTiO3 are shown in Fig. 3a. The intrinsic absorption edges of ZnO and BaTiO3 are 405 and 416 nm, respectively, while the ZnO/BaTiO3 lies between them. It can be seen from Fig. 3b that the bandgap of BaTiO3 and ZnO are 3.06 and 2.98 eV, respectively, indicating that BaTiO3 and ZnO have an absorbance to light with wavelength below 420 nm due 15

to their bandgap being less than 3.10 eV. This observation promoted us to study their catalytic oxidations of contaminants by piezophototronic effect. Also, the influence of the band edge positions of BaTiO3 and ZnO on the oxidation reactions will be discussed later. To explore the ability of charge carrier separation and transfer in ZnO/BaTiO3, a PL test was conducted as shown in Fig. 3c. It can be seen that ZBT-0.10 shows the lowest PL intensity in comparison with ZBT-0.01 and ZBT-1.00, suggesting that ZBT-0.10 has a prolonged charge carrier lifetime under the simulated sunlight irradiation, thus resulting in enhanced photocatalytic activity [75]. The photocurrent–time response curves of BaTiO3, ZnO and ZnO/BaTiO3 (Fig. 3d) further reveal that the ZBT-0.10 has a strong photoelectric conversion capability. It is worth noting that the photocurrent of all electrodes decreases sharply at the moment the light turns on (elliptic frame) and then tends to be stable due to the partial recombination of the photogenerated electron–hole pairs on the electrode surface under low bias. These results are consistent with the PL spectrum (Fig. 3c) and the catalytic oxidation performances described below.

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Fig. 3. (a) UV–Vis absorption spectrum of BaTiO3, ZnO, and ZnO/BaTiO3. (b) Plots of (F(R)E)1/2 versus Eg of BaTiO3, ZnO, and ZnO/BaTiO3. (c) PL spectrum of ZnO/BaTiO3. (d) Photocurrent–time response curves of BaTiO3, ZnO, and ZnO/BaTiO3 electrodes.

The piezocatalytic oxidation capabilities of BaTiO3, ZnO and ZnO/BaTiO3 heterostructures were studied under ultrasonication at different ultrasonic powers (i.e., 80, 120, and 200 W) to catalytically degrade dyeing wastewater (Fig. 4a-f). Clearly, the contaminant degradation is inappreciable in the absence of piezocatalysts with the prolonging of reaction time, indicating that sonolysis and adsorption of the contaminants were negligible. Encouragingly, over time it can be seen that the degradation of contaminants is related to the ultrasonic agitation of BaTiO3, ZnO and ZnO/BaTiO3. In addition, as the ultrasonic power increases from 80 to 200 W, the contaminant degradation ratios of ZBT-0.10 heterostructures continuously increase, with the maximum degradation efficiency of 97% in 75 min at 200 W, while the degradation efficiencies are 39% at 80 W and 92% at 120 W. This observation suggests that the elevated ultrasonic power is beneficial to the improvement of piezocatalytic

performances

of

ZnO/BaTiO3

heterostructures.

Besides,

the

contaminant degradation ratio of ZBT-0.10 heterostructures is significantly higher than that of ZBT-0.01, ZBT-1.00, pure BaTiO3 and ZnO under ultrasound excitation at

17

the same ultrasonic power. For example, the contaminant degradation ratio of ZBT-0.10 (93%) was about 3.10 and 1.84 times that of BaTiO3 (30%) and ZnO (50%) in 90 min under ultrasound activation at ultrasonic power of 120 W. Although the contaminant degradation ratios of ZBT-0.01 and ZBT-0.10 are about 47% and 73%, respectively, under the same reaction conditions. Based on these results, it can be reasoned that the piezocatalytic activities of BaTiO3, ZnO and ZnO/BaTiO3 heterostructures rely on the ultrasonic power, and the optimal piezocatalytic activity of ZnO/BaTiO3 heterostructures occurred at the ultrasonic power of 200 W. This is a natural progression for larger ultrasonic power results in greater deformation of ZnO/BaTiO3 heterostructures, which can initiate higher piezoelectric potential to inhibit the recombination of polar charges, thus improving the piezocatalytic activity of ZnO/BaTiO3 heterostructures. The first-order linear relationship is revealed by the plots of ln(C0/C) vs reaction time, which is usually written in the form of an exponential decay equation －In(C/C0) = kt

(1)

where k is the pseudo-first-order reaction rate constant (min−1) [76]. As expected, the kinetics of ultrasound-mediated contaminant degradation reaction catalyzed by the ZnO/BaTiO3 heterostructures follows first-order reactions (Fig. 4b, d, f). It is worth noting that the reaction rate constants of ZBT-0.10 are up to 6.56×10–3, 1.53×10–2, and 3.95×10–2 min–1 by capitalizing on ultrasonic power of 80, 120, and 200 W, respectively. Obviously, the reaction rate constant of ZnO/BaTiO3 heterostructures increases with the ultrasonic power increases. Under ultrasonication at 120 W, the 18

contaminant degradation reaction rate constant of ZBT-0.10 heterostructures is 6.78 times that of BaTiO3 (5.10×10–3 min–1) and 4.21 times that of ZnO (8.20×10–3 min–1), respectively. These two observations further support a truth that higher ultrasonic power in beneficial to higher piezocatalytic activity of ZnO/BaTiO3 heterostructures.

Fig. 4. Piezocatalytic degradation efficiencies of contaminants as a function of time and their corresponding kinetic curves for BaTiO3, ZnO, and ZnO/BaTiO3 heterostructures under ultrasonication at ultrasonic powers of (a, b) 80 W, (c, d) 120 W, and (d, e) 200 W. The ultrasonic 19

frequency is fixed at 40 kHz.

The photocatalytic oxidation to contaminants with BaTiO3, ZnO and ZnO/BaTiO3 heterostructures were evaluated by degradation of dyeing wastewater under simulated sunlight irradiation at different light intensities (i.e., 10, 30, and 100 mW cm–2), as shown in Fig. 5a-f. When no photocatalysts are used in the photoreaction system, only negligible contaminants are eliminated upon light irradiation, indicating that the self-photosensitized bleaching of contaminants can be substantially ignored. Interestingly, the ZnO/BaTiO3 heterostructures show much enhanced photocatalytic activity compared with that of pure BaTiO3 and ZnO under simulated sunlight irradiation at the fixed light intensity, which is agreement with the photocurrent-time response curves (Fig. 3d). This observation suggests that the synergistic effect of ZnO and BaTiO3 plays an important role in the photocatalytic oxidation processes. In addition, it can be observed that the contaminant photodegradation ratio by using ZBT-0.10 heterostructures reaches 82% after 45 min under simulated sunlight irradiation at 100 mW cm–2, but only less than 33% of contaminants are decomposed with ZBT-0.10 heterostructures under simulated sunlight irradiation at 10 mW cm–2 in the same period. When the light intensity turns from 10 to 30 mW cm–2, the photocatalytic activity of ZBT-0.10 heterostructures is significantly enhanced, and the corresponding contaminant degradation ratio is 77% within 45 min. Although the contaminant almost completely decomposed in the presence of ZBT-0.10 heterostructures in 75 min under the simulated sunlight irradiation at light intensity of 30 or 100 mW cm–2, there are more residual

20

contaminants in the presence of BaTiO3, ZnO, ZBT-0.01 and ZBT-1.00 photocatalysts under the simulated sunlight irradiation at 30 mW cm–2 than that at 100 mW cm–2. This result can be ascribed to the higher light intensity provides more photons to the photocatalysts at per unit time, thus creating more free charge carriers on the photocatalysts surface, which is contribute to advancing photocatalytic oxidation processes. In addition, the degradation reaction rate constants of BaTiO3, ZnO and ZnO/BaTiO3 are all less than 18% after 90 min under the room light irradiation at light intensity around 1 mW cm–2 (Fig. S4), indicating that the light intensity has a positive effect on photocatalytic activity in this photoreaction system. The kinetic curves (Fig. 5b, d, f) show that the reaction rate constant of ZnO/BaTiO3 heterostructures gradually increases with the light intensity increases, and the reaction rate constant of ZBT-0.10 heterostructures (5.64×10–2 min–1) under the simulated sun irradiation at 100 mW cm–2 is more than 5-fold that of at 10 mW cm–2. This means that 100 mW cm–2 represents the optimal light intensity for photocatalytic contaminant degradation.

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Fig. 5. Photocatalytic degradation efficiencies of contaminants as a function of time and their corresponding kinetic curves for BaTiO3, ZnO, and ZnO/BaTiO3 heterostructures under the simulated solar irradiation at light intensities of (a, b) 10 mW cm–2, (c, d) 30 mW cm–2, and (e, f) 100 mW cm–2.

For comparison, the piezophotocatalytic activities of BaTiO3, ZnO and ZnO/BaTiO3 heterostructures were performed under concurrent ultrasonic at different ultrasonic powers with fixed frequency of 40 kHz and simulated sunlight irradiation at light intensity of 100 mW cm–2, as shown in Fig. 6a-f. After piezophotocatalysis for 30 min, the contaminant degradation ratios in the presence of ZBT-0.10 heterostructures are 46%, 97%, and 86% by capitalizing on ultrasonic power of 80, 120, and 200 W, respectively. Furthermore, the degradation rate constant of ZBT-0.10 heterostructures reaches 1.18×10–1 min–1, which is 7.70 and 2.11 times of the value under only ultrasound activation at 120 W (1.53×10–2 min–1) and only simulated

22

sunlight irradiation at 100 mW cm−2 (5.60×10–2 min−1). This observation indicates that the contaminant degradation rate via piezophotocatalysis is much higher than that via piezoelectric or photo-catalysis. Additionally, the piezophotocatalytic activity of ZnO/BaTiO3 is first increases and then decreases with the ultrasonic power ranging from 80 to 200 W when the light intensity is fixed at 100 mW cm–2. Two aspects can be attributed to this phenomenon: one is the piezoelectric effect in ZnO/BaTiO3, which failed to reflect effectively when the ultrasonic power is low to 80 W, and the other one is higher ultrasonic power, which causes an excessive deformation of ZnO/BaTiO3, thus limiting the utilization of free charge carriers in aqueous solutions.

23

Fig. 6. Piezophotocatalytic degradation efficiencies of contaminants as a function of time and their corresponding kinetic curves for BaTiO3, ZnO, and ZnO/BaTiO3 heterostructures under the simulated sun irradiation together with ultrasonication at different ultrasonic powers, that is (a, b) 80 W, (c, d) 120 W, and (e, f) 200 W.

Fig. S5 shows the comparison of the contaminant degradation ratios for BaTiO3, ZnO and ZnO/BaTiO3 within 30 min for the piezoelectric, photo, and piezophoto-catalysis when the ultrasonic power is fixed at 120 W and the light intensity is fixed at 100 mW cm–2. For the blank control group, it can be seen that the contaminants are slightly degraded under ultrasonic agitation, which may originate from the ultrasound-induced extrusion, collision, and vibration of solution molecules on the oxidation of contaminant molecules [77]. However, the contaminant degradation ratio of ZBT-0.10 is as high as 76%, which is higher than BaTiO3 (20%) and ZnO (26%). This increased piezocatalytic catalytic oxidation capability results from the macroscopic polarization enhanced piezoelectric effect of ZnO/BaTiO3 heterostructures, which implies that ZnO/BaTiO3 heterostructures can promote the polarization-induced surface charges on the surface of BaTiO3 and ZnO, leading to more efficient separation and transfer. When the reaction system was exposed to light irradiation, it was apparent to observe that the photocatalytic oxidation capabilities of all samples were superior to piezocatalytic oxidation, except for ZBT-0.10. This 24

observation in part due to the mobility of the photogenerated charge carrier is faster than the mobility of charges caused by induced polarization charges, in part because light irradiation is more likely to cause surface charge recombination of ZBT-0.10. Compared with the isolated ultrasonic activation or light irradiation, the contaminant removal ratio by BaTiO3, ZnO and ZnO/BaTiO3 improves when the light and ultrasound are applied together to the reaction system. In particular, the degradation ratio of BaTiO3 was increased to 78% by the mediated piezophototronic effect, which is 1.86 and 3.90 times that of the light induced (42%) and ultrasound-induced oxidations (20%), respectively. In addition, the contaminant degradation ratio of ZnO can be up to 84%, which is 2.55 times that of photocatalytic oxidations (33%) and 3.11 times that of piezocatalytic oxidations (27%). Moreover, the contaminant degradation ratio of ZnO/BaTiO3 initiated by piezophototronic effect is much higher than that induced by light or ultrasound. These observations clearly indicate that the macroscopic polarization contributes to the separation and transport of charge carriers in ZnO/BaTiO3, thereby resulting in the production of large amounts of reactive oxygen species in the wastewater. In addition, the comparisons of the oxidation reaction rate constants for BaTiO3, ZnO and ZnO/BaTiO3 highlight that the ultrasonic agitation of ZnO/BaTiO3 initiate the piezoelectric effect, which is beneficial to improve the photocatalytic oxidation processes. Based on these observations, we propose a plausible mechanism by which ultrasound induced photocatalytic advanced oxidations with ZnO/BaTiO3 heterostructures. That is when the piezoelectric ZnO/BaTiO3 heterostructures encountered in ultrasonic activation, a polar 25

charge-stabilized electric field is formed, and thereby generates a piezoelectric potential in response to the deformation energy. Due to the inherent chemical potential of ZnO/BaTiO3 heterostructures, the photoexcited electrons and holes can be separated more efficiently driven by that polarization-induced piezoelectric potential. Moreover, the newly formed surface charges initiated by polar charges can then take part in the catalytic oxidation reactions. As a consequence, more charge carriers are utilized to produce reactive oxygen species and the photocatalytic activity of ZnO/BaTiO3 heterostructures is largely improved. To understand the photocatalytic pathways for the contaminant degradation on ZnO/BaTiO3 heterostructures, the photocatalytic performances of BaTiO3, ZnO and ZnO/BaTiO3 heterostructures were investigated under both UV and visible light irradiation. As shown in Fig. 7a-d, the photocatalytic activity of ZBT-0.10 heterostructures under both UV and visible light irradiation is much enhanced compared with that of pure BaTiO3 and ZnO. Due to the ZnO/BaTiO3 heterostructures have a weak absorbance to visible light (Fig. 3a), it can be reasoned that the enhancement

of

both

semiconductor

photoexcitation

and

indirect

dye

photosensitization in the ZnO/BaTiO3 heterostructures. This phenomenon has been verified by previous literatures [78, 79]. Surprisingly, the photocatalytic activity of ZnO/BaTiO3 heterostructures driven by visible or UV light is significantly improved through ultrasound activation at ultrasonic power of 80 W (Fig. 7e-h). This result further indicates that ultrasound-mediated piezophotocatalysis is an effective way to promote advanced oxidation processes. 26

27

Fig. 7. Photocatalytic degradation efficiencies of contaminants as a function of time and their corresponding kinetic curves for BaTiO3, ZnO, and ZnO/BaTiO3 heterostructures under (a, b) visible, and (c, d) ultraviolet light irradiation. Piezophotocatalytic degradation efficiencies of contaminants as a function of time and their corresponding kinetic curves for BaTiO3, ZnO, and ZnO/BaTiO3 heterostructures under (e, f) concurrent ultrasonic and visible light, and (g, h) concurrent ultrasonic and ultraviolet light irradiation. The ultrasonic frequency was fixed at 40 k.

In order to gain a deeper understanding of the role of free radicals in the catalytic oxidation process, several comparative experiments were performed and several additives were added to the reaction system under ultrasonic activation with a power of 120 W and simulated sunlight irradiation with intensity of 100 mW cm–2, as shown in Fig. 8a-c. When phenylhydrazine (BQ, as ̇O2– scavenger) was added to the reaction system, it could be seen that the contaminant degradation rate of ZnO/BaTiO3 decreaseds by 67% in 90 min under concurrent ultrasonic and light irradiation. When ethylenediamine tetraacetate dehydrate (EDTA, as h+ scavengers) and tert-butyl alcohol (TBA, as ˙OH scavengers) were added to the reaction system, the corresponding contaminant degradation ratio was observed to have decreased by 38% and 23% in the first 30 min, but after 90 min of reaction, there is a little residual contaminant. These observations indicate that ultrasound agitation together with light irradiation of ZnO/BaTiO3 produces abundant ˙O2– radicals and slight ˙OH radicals and h+. It is worth noting that these scavengers are self-degraded under ultrasonic agitation, especially for TBA molecules. The linearity of In(C0/C) versus reaction time shows that the reactions follow a pseudo-first-order reaction kinetics (Fig. 8b). In addition, to confirm the stability of the high piezophotocatalytic activity of the ZnO/BaTiO3 heterostructures, the ZBT-0.10 sample was selected to conduct recycling experiments for the contaminant degradation under the concurrent ultrasonic at 28

ultrasonic power of 120 W and simulated sunlight irradiation at light intensity of 100 mW cm–2, as shown in Fig. 8d. Between each run, the catalysts were dried at 105℃ to evaporate all absorbed reactants and products. Clearly, the catalysts present a similar contaminant removal ratio and degradation reaction rate across six runs, indicating that the as-synthesized ZnO/BaTiO3 heterostructures are stable under concurrent ultrasonic and simulated sunlight irradiation, which is important for its practical application.

Fig. 8. (a) Piezophotocatalytic oxidation contaminant degradation with ZnO/BaTiO3 (ZBT-0.10) in the presence of h+ (EDTA), ˙OH (TBA), and ˙O2– (BQ) scavengers. (b) The fitting plot of In(C/C0) versus reaction time. (c) Comparisons of oxidation reaction-rate constants with different radicals’ scavengers. (d) The consecutive cycling catalytic oxidation performances of ZnO/BaTiO3 (ZBT-0.10) toward the bleaching of contaminants.

To study the charge carrier transport properties of BaTiO3, ZnO and ZnO/BaTiO3, the EIS tests were conducted, as shown in Fig. 9a. Usually, a smaller radius implies a 29

faster rate of electron–hole pairs separation and transfer while a larger radius generally indicates a larger charge transfer resistance, thus a lower separation efficiency of the photogenerated electron–hole pairs [80]. Obviously, the pure ZnO showed the largest radius compared to all of the other samples, followed by pure BaTiO3. Interestingly, when ZnO is combined with BaTiO3, the spectrum radius becomes smaller, while the ZBT-0.10 sample shows the lowest radius in all of these samples. When the ZnO content further increases beyond its optimum value, the charge transfer resistance increases again, probably because the excess ZnO can serve as a recombination center rather than providing a charge separation pathway, which accelerates the recombination of electron–hole pairs in ZnO/BaTiO3. This observation suggests that ZBT-0.10 has a superior charge transfer capability to other samples. Fig. 9b-f show the Mott–Schottky plots of BaTiO3, ZnO and ZnO/BaTiO3 at different frequencies (i.e. 500, 1000, and 1500 Hz, respectively). It can be observed that the Mott–Schottky plots of all samples exhibit positive slopes at various frequencies, implying that all of the samples are n-type semiconductors [81]. The flat band potential, Vfb, is determined from a plot of C–2 versus potential as the x intercept and the carrier concentration can be calculated using the slope of the linier region according to the Mott–Schottky equation for n-type semiconductor

(2) where C and A are the interfacial capacitance and area, respectively, ε is the dielectric constant of the material, ε0 is the permittivity of free space, ND is the carrier

30

concentration, V is the applied potential, kB is Boltzmann’s constant, T the absolute temperature, and e is the electronic charge [81]. The term kBT/e is small at room temperature and therefore neglected. From Fig. 9b-f, it can be reasoned that the ZnO/BaTiO3 (ZBT-0.10) has a higher electron donor density in contrast to pure BaTiO3 and ZnO because of the slope of the Mott–Schottky plot is inversely proportional to the ND, which is beneficial to catalytic oxidation processes. Moreover, the flat band potential derive from the x-intercept in the Mott–Schottky plot of ZnO/BaTiO3 (ZBT-0.10) is －0.71 V, which is more negative than the BaTiO3 (－ 0.51) and ZnO (－0.68 V), indicating that the conduction band (CB) potential of ZnO/BaTiO3 is more positive than BaTiO3 and ZnO, which means that the ZnO/BaTiO3 are more likely to accept the hot electrons jumping from valence band (VB) to CB, thus resulting in the improvement of utilization of electrons and holes. Notably, the flat band potentials of ZBT-0.01 and ZBT-1.00 down shift 0.13 and 0.95 V in contrast to ZBT-0.10, respectively, suggesting that the appropriate energy level potential is an important role for catalytic oxidation processes. In addition, the narrower band gap of ZnO/BaTiO3 heterostructures is in agreement with the red shift of the intrinsic absorption edges in the UV–Vis spectrum (Fig. 3a, b).

31

Fig. 9. (a) EIS Nyquist plots of the BaTiO3, ZnO and ZnO/BaTiO3. (b–f) Mott–Schottky plots of BaTiO3, ZnO and ZnO/BaTiO3 at different frequencies.

To understand the direct correlation between piezoelectric potential and catalytic oxidation reactions, a computational simulation was performed using the finite element method (COMSOL Multiphysics 5.4). It is known that ultrasonic agitation of water results in numerous tiny bubbles, which subsequently vibrate, grow, accumulate acoustic energy, and finally collapse when they reach the critical and unstable size. Thus, a shock wave with a peak pressure of ~108 Pa is released and transit to the surface of the adjacent piezoelectric nanocrystals, thus resulting in the deformation of nanocrystals and macroscopic polarization (i.e., piezoelectric potential) [57, 73, 82]. Fig. 10 shows the volumetric strain and gradient piezoelectric potential distributions along the z-axis on the surface of BaTiO3, ZnO and ZnO/BaTiO3 in response to authentic simulation models. According to the as-obtained BaTiO3 hybrid structure, 32

three kinds of simulation models are built for BaTiO3, namely, BaTiO3 nanorod, BaTiO3 nanoparticle and BaTiO3 nanoparticles/nanorod. Fig. 10b, d show that external pressure is applied to the BaTiO3 nanorod (Fig. 10a) and BaTiO3 nanoparticle (Fig. 10c) give rise to a piezoelectric potential differences of 316.00 and 31.39 mV, respectively, while the piezoelectric potential difference increases to 409.50 mV for BaTiO3

nanoparticles/nanorod

(Fig.

10e,

f),

suggesting

that

BaTiO3

nanoparticles/nanorod hybrid structure can be easier to polarization by mechanical energy stimuli. In light of the as-prepared ZnO particles have an irregular shape, two kinds of simulation models are separately selected, i.e., ZnO cuboid and ZnO nanoparticle. Analogously, the ZnO cuboid (Fig. 10e) shows a piezoelectric potential difference of 4330.00 mV (Fig. 10h), and its nanoparticle is 33.00 mV (Fig. 10j). This observation suggests that the size of ZnO has a great influence on piezoelectric potential, and further explains that the piezocatalytic oxidation ability of ZnO is superior to BaTiO3 (Fig. 10a). However, the photocatalytic oxidation ability of ZnO is lower than BaTiO3 can be ascribed to its lower photocurrent (Fig. 3d) and larger charge transfer resistance (Fig. 9a). Notably, the piezoelectric potential difference of ZnO/BaTiO3 reaches 414.40 mV (Fig. 10l), which is much higher than BaTiO3 and ZnO in the same simulation conditions, suggesting that ZnO/BaTiO3 heterostructures create an enhancement macroscopic polarization, and thus resulting in an mechanochemical potential sufficiently large to promote catalytic oxidations.

33

34

Fig. 10. COMSOL simulations performed on BaTiO3, ZnO and ZnO/BaTiO3 under a sonication pressure at 108 Pa showing the volumetric strain and the corresponding surface piezoelectric 35

potential distribution generated on (a, b) BaTiO3 nanorod, (c, d) BaTiO3 nanoparticle, (e, f) BaTiO3 nanoparticles/nanorod hybrid structure, (g, h) ZnO cuboid, (i, j) ZnO nanoparticle, and (k, l) ZnO/BaTiO3.

To understand how the ZnO/BaTiO3 heterostructures induced the catalytic oxidations to contaminant degradation by piezophototronic effect, the plausible mechanisms of charge transfer are proposed. The band edge positions of BaTiO3 and ZnO are calculated by the empirical equation EVB＝χ－Ee－0.5Eg ECB＝EVB－Eg

(3) (4)

where EVB and ECB are the valence band and conduction band edge potential, respectively, Ee is the energy of free electrons on the hydrogen scale (~ 4.5 eV), Eg is the band gap, and the χ values for the BaTiO3 and ZnO are about 5.13 and 5.79 eV, respectively [83]. The ECB values of the BaTiO3 and ZnO are calculated at about −0.90 and −0.20 eV, respectively. Correspondingly, the EVB are estimated to be about 2.16 and 2.78 eV, respectively. The results show that the band edge positions of BaTiO3 and ZnO match well with the conditions of n-n type heterostructures. Moreover, both the EVB of BaTiO3 and ZnO are higher than that the O2/H2O potential of 1.23 eV, so that the OH– or H2O in aqueous solution can be converted into ˙OH radicals. In addition, the CB electrons are able to reduce the O2 in water to generate ˙O2– radicals because of the CB potential of BaTiO3 and ZnO are lower than O2/O2– potential of －0.13 eV (Fig. 11). These free radicals provide an ideal precondition for the catalytic oxidation reactions to the contaminant degradation. As shown in Fig. 11 (left), under light irradiation, the photogenerated electrons in BaTiO3 can be

36

transferred to the conduction band of ZnO, on the other hand, the holes in the valence band of ZnO can be transported to the valence band of BaTiO3, thus efficiently prevent the photoinduced electrons and holes recombination in BaTiO3 and ZnO, which results in the enhancement of photocatalytic oxidation property under light irradiation. As mentioned in the early literature [84], the ZnO presents a higher electron mobility in comparison with BaTiO3, thus resulting in more ˙O2– radicals generation on the ZnO surface (Fig. 8a-c). When the ZnO/BaTiO3 is in an elevated pressure field provides by ultrasound (the right panel of Fig. 11), the piezoelectric polarization (Ppz) of the ZnO/BaTiO3 lead to the formation of the bound surface charges, followed induce a built-in electric field inside the ZnO/BaTiO3. The built-in electric fields drive the free electrons and holes to transfer toward opposite directions (solid arrows) and react with O2 and/or OH–, thus resulting in more free radicals generate to promote oxidation abilities. Besides, some free electrons and holes will transfer along the dotted arrows direction when the intensity of the polarization electric fields decrease, so that interact with surrounding O2 and/or OH– to form the ˙O2– and ˙OH radicals, respectively, further contributing to the catalytic oxidation processes to contaminant degradation.

37

Fig. 11. Schematic illustration of the electron–hole pairs separation and transfer in ZnO/BaTiO3 mediate by light irradiation and/or ultrasonic activation.

4. Conclusions In conclusion, we describe here the first example of a piezophototronic effect initiates high-efficiency catalytic oxidation to dyeing wastewater with piezoelectric ZnO/BaTiO3 heterostructures. The high durability ZnO/BaTiO3 heterostructures with single crystals are prepared by a facile method combining a solvothermal reaction and impregnation sintering. When the ZnO/BaTiO3 is excited by ultrasonic agitation or light irradiation, specific free radicals such as ˙O2– and ˙OH are generated on its surface and thus resulting in catalytic oxidation to contaminants degradation, but the entire piezoelectric or photo-catalytic oxidation processes are extremely slow because of there are still contaminant residues after oxidations of 90 min. However, under the optimal excitations of ultrasound together with light irradiation, the contaminant degradation ratio of ZnO/BaTiO3 is as high as 97% in 30 min, which is larger than pure BaTiO3 (88%) and ZnO (84%) in the same period. More encouragingly, the oxidation reaction-rate constant of ZnO/BaTiO3 by piezophototronic effect mediated is much higher than that mediated by ultrasound or light irradiation. The finite

38

element simulation further confirms that ZnO/BaTiO3 heterostructure has a superior piezoelectric potential difference in contrast to BaTiO3 and ZnO. These observations suggest that charge carrier elicited by piezophototronic effect can be more easily transferred in ZnO/BaTiO3 heterostructures and thus resulting in abundant free radical generation

to

strengthen

catalytic

oxidation

processes

for

water-soluble

contaminations. Despite this study provides a unique approach to complement the advanced catalytic oxidation processes, but striving for the utilization of environmental vibrations and/or natural light to efficiently driven the charge carrier separation and transfer in piezoelectric semiconductors will be the focus of future work.

Acknowledgements This work was financially supported by the National Natural Science Foundation of China (No. 51772211) and Instrument Developing Project of Chinese Academy of Sciences (No. ZDKYYQ20180004).

References [1] T. Hisatomi, K. Domen, Reaction systems for solar hydrogen production via water splitting with particulate semiconductor photocatalysts, Nat. Catal. 2 (2019) 387-399. [2] I. Azzouz, Y.G. Habba, M. Capochichi-Gnambodoe, F. Marty, J. Vial, Y. Leprince-Wang, T. Bourouina, Zinc oxide nano-enabled microfluidic reactor for water purification and its applicability to volatile organic compounds, Microsyst. 39

Nanoeng. 4 (2018) 17093-17099. [3] Z. Zou, J. Ye, K. Sayama, H. Arakawa, Direct splitting of water under visible light irradiation with an oxide semiconductor photocatalyst, Nature 414 (2001) 625-627. [4] B.C. Hodges, E.L. Cates, J.-H. Kim, Challenges and prospects of advanced oxidation water treatment processes using catalytic nanomaterials, Nat. Nanotech. 13 (2018) 642-650. [5] M. Setvin, X. Shi, J. Hulva, T. Simschitz, G.S. Parkinson, M. Schmid, C. Di Valentin, A. Selloni, U. Diebold, Methanol on Anatase TiO2 (101): Mechanistic Insights into Photocatalysis, ACS Catal. 7 (2017) 7081-7091. [6] K.-W. Park, A.M. Kolpak, Optimal methodology for explicit solvation prediction of band edges of transition metal oxide photocatalysts, Commun. Chem. 2 (2019) 79-88. [7] A. Aziz, A.R. Ruiz-Salvador, N.C. Hernández, S. Calero, S. Hamad, R. Grau-Crespo, Porphyrin-based metal-organic frameworks for solar fuel synthesis photocatalysis: band gap tuning via iron substitutions, J. Mater. Chem. A 5 (2017) 11894-11904. [8] W. Jiao, L. Wang, G. Liu, G.Q. Lu, H.-M. Cheng, Hollow Anatase TiO2 Single Crystals and Mesocrystals with Dominant {101} Facets for Improved Photocatalysis Activity and Tuned Reaction Preference, ACS Catal. 2 (2012) 1854-1859. [9] A. Dhakshinamoorthy, Z. Li, H. Garcia, Catalysis and photocatalysis by metal organic frameworks, Chem. Soc. Rev. 47 (2018) 8134-8172. [10] A. Li, W. Zhu, C. Li, T. Wang, J. Gong, Rational design of yolk–shell nanostructures for photocatalysis, Chem. Soc. Rev. 48 (2019) 1874-1907. [11] R.S. Datta, J.Z. Ou, M. Mohiuddin, B.J. Carey, B.Y. Zhang, H. Khan, N. Syed, A. Zavabeti, F. Haque, T. Daeneke, K. Kalantar-zadeh, Two dimensional PbMoO4: A photocatalytic material derived from a naturally non-layered crystal, Nano Energy 49 (2018) 237-246. [12] S. Linic, P. Christopher, D.B. Ingram, Plasmonic-metal nanostructures for efficient conversion of solar to chemical energy, Nat. Mater. 10 (2011) 911-921. [13] M.J. Kale, T. Avanesian, P. Christopher, Direct Photocatalysis by Plasmonic Nanostructures, ACS Catal. 4 (2014) 116-128. [14] J. Chen, C.S. Bailey, Y. Hong, L. Wang, Z. Cai, L. Shen, B. Hou, Y. Wang, H. Shi, J. Sambur, W. Ren, E. Pop, S.B. Cronin, Plasmon-Resonant Enhancement of Photocatalysis on Monolayer WSe2, ACS Photonics 6 (2019) 787-792. [15] D.F. Swearer, H. Robatjazi, J.M.P. Martirez, M. Zhang, L. Zhou, E.A. Carter, P. Nordlander, N.J. Halas, Plasmonic Photocatalysis of Nitrous Oxide into N2 and O2 Using Aluminum–Iridium Antenna–Reactor Nanoparticles, ACS Nano (2019) 8076-8086. [16] W. Bi, X. Li, L. Zhang, T. Jin, L. Zhang, Q. Zhang, Y. Luo, C. Wu, Y. Xie, Molecular co-catalyst accelerating hole transfer for enhanced photocatalytic H2 evolution, Nat. Commun. 6 (2015) 8647-8653. [17] S. Yu, X.-B. Fan, X. Wang, J. Li, Q. Zhang, A. Xia, S. Wei, L.-Z. Wu, Y. Zhou, 40

G.R. Patzke, Efficient photocatalytic hydrogen evolution with ligand engineered all-inorganic InP and InP/ZnS colloidal quantum dots, Nat. Commun. 9 (2018) 4009. [18] Z. Wang, Y. Inoue, T. Hisatomi, R. Ishikawa, Q. Wang, T. Takata, S. Chen, N. Shibata, Y. Ikuhara, K. Domen, Overall water splitting by Ta3N5 nanorod single crystals grown on the edges of KTaO3 particles, Nat. Catal. 1 (2018) 756-763. [19] F. Parrino, M. Bellardita, E.I. García-López, G. Marcì, V. Loddo, L. Palmisano, Heterogeneous Photocatalysis for Selective Formation of High-Value-Added Molecules: Some Chemical and Engineering Aspects, ACS Catal. 8 (2018) 11191-11225. [20] C. Zhou, C. Lai, C. Zhang, G. Zeng, D. Huang, M. Cheng, L. Hu, W. Xiong, M. Chen, J. Wang, Y. Yang, L. Jiang, Semiconductor/boron nitride composites: Synthesis, properties, and photocatalysis applications, Appl. Catal. B Environ. 238 (2018) 6-18. [21] I. Shown, S. Samireddi, Y.-C. Chang, R. Putikam, P.-H. Chang, A. Sabbah, F.-Y. Fu, W.-F. Chen, C.-I. Wu, T.-Y. Yu, P.-W. Chung, M.C. Lin, L.-C. Chen, K.-H. Chen, Carbon-doped SnS2 nanostructure as a high-efficiency solar fuel catalyst under visible light, Nat. Commun. 9 (2018) 169-178. [22] P. Kanhere, P. Shenai, S. Chakraborty, R. Ahuja, J. Zheng, Z. Chen, Mono- and co-doped NaTaO3 for visible light photocatalysis, Phys. Chem. Chem. Phys. 16 (2014) 16085-16094. [23] B.-H. Lee, S. Park, M. Kim, A.K. Sinha, S.C. Lee, E. Jung, W.J. Chang, K.-S. Lee, J.H. Kim, S.-P. Cho, H. Kim, K.T. Nam, T. Hyeon, Reversible and cooperative photoactivation of single-atom Cu/TiO2 photocatalysts, Nat. Mater. 18 (2019) 620-626. [24] J. Di, C. Chen, S.-Z. Yang, S. Chen, M. Duan, J. Xiong, C. Zhu, R. Long, W. Hao, Z. Chi, H. Chen, Y.-X. Weng, J. Xia, L. Song, S. Li, H. Li, Z. Liu, Isolated single atom cobalt in Bi3O4Br atomic layers to trigger efficient CO2 photoreduction, Nat. Commun. 10 (2019) 2840-2846. [25] G. Ou, Y. Xu, B. Wen, R. Lin, B. Ge, Y. Tang, Y. Liang, C. Yang, K. Huang, D. Zu, R. Yu, W. Chen, J. Li, H. Wu, L.-M. Liu, Y. Li, Tuning defects in oxides at room temperature by lithium reduction, Nat. Commun. 9 (2018) 1302-1310. [26] A. Naldoni, M. Altomare, G. Zoppellaro, N. Liu, Š. Kment, R. Zbořil, P. Schmuki, Photocatalysis with Reduced TiO2: From Black TiO2 to Cocatalyst-Free Hydrogen Production, ACS Catal. 9 (2019) 345-364. [27] S. Bai, N. Zhang, C. Gao, Y. Xiong, Defect engineering in photocatalytic materials, Nano Energy 53 (2018) 296-336. [28] J. Li, Q. Pei, R. Wang, Y. Zhou, Z. Zhang, Q. Cao, D. Wang, W. Mi, Y. Du, Enhanced Photocatalytic Performance through Magnetic Field Boosting Carrier Transport, ACS Nano 12 (2018) 3351-3359. [29] L. Wang, S. Liu, Z. Wang, Y. Zhou, Y. Qin, Z.L. Wang, Piezotronic Effect Enhanced Photocatalysis in Strained Anisotropic ZnO/TiO2 Nanoplatelets via Thermal Stress, ACS Nano 10 (2016) 2636-2643. [30] S. Horikoshi, H. Hidaka, N. Serpone, Environmental Remediation by an 41

Integrated Microwave/UV-Illumination Method. 1. Microwave-Assisted Degradation of Rhodamine-B Dye in Aqueous TiO2 Dispersions, Environ. Sci. Technol. 36 (2002) 1357-1366. [31] S. Horikoshi, F. Hojo, H. Hidaka, N. Serpone, Environmental Remediation by an Integrated Microwave/UV Illumination Technique. 8. Fate of Carboxylic Acids, Aldehydes, Alkoxycarbonyl and Phenolic Substrates in a Microwave Radiation Field in the Presence of TiO2 Particles under UV Irradiation, Environ. Sci. Technol. 38 (2004) 2198-2208. [32] P.C. Sevinc, B. Dhital, V. Govind Rao, Y. Wang, H.P. Lu, Probing Electric Field Effect on Covalent Interactions at a Molecule–Semiconductor Interface, J. Am. Chem. Soc. 138 (2016) 1536-1542. [33] L. Zhu, Z.L. Wang, Recent Progress in Piezo-Phototronic Effect Enhanced Solar Cells, Adv. Funct. Mater. (2018) 1808214-1808231. [34] M. Wang, B. Wang, F. Huang, Z. Lin, Enabling PIEZOpotential in PIEZOelectric Semiconductors for Enhanced Catalytic Activities, Angew. Chem. Int. Ed. 131 (2019) 7606-7616. [35] Piezotronics and Piezo-Phototronics, Wiley Encyclopedia of Electrical and Electronics Engineering, (2012) 1-18. [36] Y. Zhang, W. Jie, P. Chen, W. Liu, J. Hao, Ferroelectric and Piezoelectric Effects on the Optical Process in Adv. Mater. and Devices, Adv. Mater. 30 (2018) 1707007-1707041. [37] Z.L. Wang, J. Song, Piezoelectric Nanogenerators Based on Zinc Oxide Nanowire Arrays, Science 312 (2006) 242-246. [38] Y. Hu, Y. Chang, P. Fei, R.L. Snyder, Z.L. Wang, Designing the Electric Transport Characteristics of ZnO Micro/Nanowire Devices by Coupling Piezoelectric and Photoexcitation Effects, ACS Nano 4 (2010) 1234-1240. [39] Z.L. Wang, Piezotronic and Piezophototronic Effects, J. Phys. Chem. Lett. 1 (2010) 1388-1393. [40] K. Kim, Y. Jeon, K. Cho, S. Kim, Enhancement of Trap-Assisted Green Electroluminescence Efficiency in ZnO/SiO2/Si Nanowire Light-Emitting Diodes on Bendable Substrates by Piezophototronic Effect, ACS Appl. Energy Mater. 8 (2016) 2764-2773. [41] R. Bao, C. Wang, L. Dong, R. Yu, K. Zhao, Z.L. Wang, C. Pan, Flexible and Controllable Piezo-Phototronic Pressure Mapping Sensor Matrix by ZnO NW/p-Polymer LED Array, Adv. Funct. Mater. 25 (2015) 2884-2891. [42] F. Zhang, S. Niu, W. Guo, G. Zhu, Y. Liu, X. Zhang, Z.L. Wang, Piezo-phototronic Effect Enhanced Visible/UV Photodetector of a Carbon-Fiber/ZnO-CdS Double-Shell Microwire, ACS Nano 7 (2013) 4537-4544. [43] W. Wu, L. Wang, R. Yu, Y. Liu, S.-H. Wei, J. Hone, Z.L. Wang, Piezophototronic Effect in Single-Atomic-Layer MoS2 for Strain-Gated Flexible Optoelectronics, Adv. Mater. 28 (2016) 8463-8468. [44] F. Xue, L. Chen, J. Chen, J. Liu, L. Wang, M. Chen, Y. Pang, X. Yang, G. Gao, J. Zhai, Z.L. Wang, p-Type MoS2 and n-Type ZnO Diode and Its Performance Enhancement by the Piezophototronic Effect, Adv. Mater. 28 (2016) 3391-3398. 42

[45] C. Liu, M. Peng, A. Yu, J. Liu, M. Song, Y. Zhang, J. Zhai, Interface engineering on p-CuI/n-ZnO heterojunction for enhancing piezoelectric and piezo-phototronic performance, Nano Energy 26 (2016) 417-424. [46] Y. Hu, Y. Zhang, Y. Chang, R.L. Snyder, Z.L. Wang, Optimizing the Power Output of a ZnO Photocell by Piezopotential, ACS Nano 4 (2010) 4220-4224. [47] K. Gu, D. Zheng, L. Li, Y. Zhang, High-efficiency and stable piezo-phototronic organic perovskite solar cell, RSC Adv. 8 (2018) 8694-8698. [48] D.Q. Zheng, Z. Zhao, R. Huang, J. Nie, L. Li, Y. Zhang, High-performance piezo-phototronic solar cell based on two-dimensional materials, Nano Energy 32 (2017) 448-453. [49] G. Hu, W. Guo, R. Yu, X. Yang, R. Zhou, C. Pan, Z.L. Wang, Enhanced performances of flexible ZnO/perovskite solar cells by piezo-phototronic effect, Nano Energy 23 (2016) 27-33. [50] R. Yu, X. Wang, W. Wu, C. Pan, Y. Bando, N. Fukata, Y. Hu, W. Peng, Y. Ding, Z.L. Wang, Temperature Dependence of the Piezophototronic Effect in CdS Nanowires, Adv. Funct. Mater. 25 (2015) 5277-5284. [51] M. Kumar, H.-S. Kim, G.-N. Lee, D. Lim, J. Kim, Piezophototronic Effect Modulated Multilevel Current Amplification from Highly Transparent and Flexible Device Based on Zinc Oxide Thin Film, Small 14 (2018) 1804016-1804021. [52] L. Zhang, Y. Fu, L. Xing, B. Liu, Y. Zhang, X. Xue, A self-powered flexible vision electronic-skin for image recognition based on a pixel-addressable matrix of piezophototronic ZnO nanowire arrays, J. Mater. Chem. C 5 (2017) 6005-6013. [53] M.B. Starr, J. Shi, X. Wang, Piezopotential-Driven Redox Reactions at the Surface of Piezoelectric Materials, Angew. Chem. Int. Ed. 51 (2012) 5962-5966. [54] H. Huang, S. Tu, C. Zeng, T. Zhang, A.H. Reshak, Y. Zhang, Macroscopic Polarization Enhancement Promoting Photo- and Piezoelectric-Induced Charge Separation and Molecular Oxygen Activation, Angew. Chem. Int. Ed. 56 (2017) 11860-11864. [55] M. Wang, Y. Zuo, J. Wang, Y. Wang, X. Shen, B. Qiu, L. Cai, F. Zhou, S.P. Lau, Y. Chai, Remarkably Enhanced Hydrogen Generation of Organolead Halide Perovskites via Piezocatalysis and Photocatalysis, Adv. Energy Mater. (2019) 1901801-1901807. [56] M.B. Starr, X. Wang, Coupling of piezoelectric effect with electrochemical processes, Nano Energy 14 (2015) 296-311. [57] S. Xu, L. Guo, Q. Sun, Z.L. Wang, Piezotronic Effect Enhanced Plasmonic Photocatalysis by AuNPs/BaTiO3 Heterostructures, Adv. Funct. Mater. 29 (2019) 1808737-1808744. [58] W. Tong, Y. Zhang, H. Huang, K. Xiao, S. Yu, Y. Zhou, L. Liu, H. Li, L. Liu, T. Huang, M. Li, Q. Zhang, R. Du, Q. An, A highly sensitive hybridized soft piezophotocatalyst driven by gentle mechanical disturbances in water, Nano Energy 53 (2018) 513-523. [59] Y. Zhang, C. Liu, G. Zhu, X. Huang, W. Liu, W. Hu, M. Song, W. He, J. Liu, J. Zhai, Piezotronic-effect-enhanced Ag2S/ZnO photocatalyst for organic dye 43

degradation, RSC Adv. 7 (2017) 48176-48183. [60] H. Li, Y. Sang, S. Chang, X. Huang, Y. Zhang, R. Yang, H. Jiang, H. Liu, Z.L. Wang, Enhanced Ferroelectric-Nanocrystal-Based Hybrid Photocatalysis by Ultrasonic-Wave-Generated Piezophototronic Effect, Nano Lett. 15 (2015) 2372-2379. [61] Z. Liu, L. Wang, X. Yu, J. Zhang, R. Yang, X. Zhang, Y. Ji, M. Wu, L. Deng, L. Li, Z.L. Wang, Piezoelectric-Effect-Enhanced Full-Spectrum Photoelectrocatalysis in p–n Heterojunction, Adv. Funct. Mater. (2019) 1807279-1807286. [62] D. Hong, W. Zang, X. Guo, Y. Fu, H. He, J. Sun, L. Xing, B. Liu, X. Xue, High Piezo-photocatalytic Efficiency of CuS/ZnO Nanowires Using Both Solar and Mechanical Energy for Degrading Organic Dye, ACS Appl. Energy Mater. 8 (2016) 21302-21314. [63] Z. Wang, T. Hu, H. He, Y. Fu, X. Zhang, J. Sun, L. Xing, B. Liu, Y. Zhang, X. Xue, Enhanced H2 Production of TiO2/ZnO Nanowires Co-Using Solar and Mechanical Energy through Piezo-Photocatalytic Effect, ACS Sustainable Chem. Eng. 6 (2018) 10162-10172. [64] Y.-L. Liu, J.M. Wu, Synergistically catalytic activities of BiFeO3/TiO2 core-shell nanocomposites for degradation of organic dye molecule through piezophototronic effect, Nano Energy 56 (2019) 74-81. [65] Y. Feng, H. Li, L. Ling, S. Yan, D. Pan, H. Ge, H. Li, Z. Bian, Enhanced Photocatalytic Degradation Performance by Fluid-Induced Piezoelectric Field, Environ. Sci. Technol. 52 (2018) 7842-7848. [66] S. Jia, Y. Su, B. Zhang, Z. Zhao, S. Li, Y. Zhang, P. Li, M. Xu, R. Ren, Few-layer MoS2 nanosheet-coated KNbO3 nanowire heterostructures: piezo-photocatalytic effect enhanced hydrogen production and organic pollutant degradation, Nanoscale 11 (2019) 7690-7700. [67] T.-M. Chou, S.-W. Chan, Y.-J. Lin, P.-K. Yang, C.-C. Liu, Y.-J. Lin, J.-M. Wu, J.-T. Lee, Z.-H. Lin, A highly efficient Au-MoS2 nanocatalyst for tunable piezocatalytic and photocatalytic water disinfection, Nano Energy 57 (2019) 14-21. [68] L. Guo, C. Zhong, J. Cao, Y. Hao, M. Lei, K. Bi, Q. Sun, Z.L. Wang, Enhanced photocatalytic H2 evolution by plasmonic and piezotronic effects based on periodic Al/BaTiO3 heterostructures, Nano Energy 62 (2019) 513-520. [69] X. Xue, W. Zang, P. Deng, Q. Wang, L. Xing, Y. Zhang, Z.L. Wang, Piezo-potential enhanced photocatalytic degradation of organic dye using ZnO nanowires, Nano Energy 13 (2015) 414-422. [70] K. Wang, Z. Fang, X. Huang, W. Feng, Y. Wang, B. Wang, P. Liu, Enhanced selectivity of methane production for photocatalytic reduction by the piezoelectric effect, Chem. Commun. 53 (2017) 9765-9768. [71] M.-K. Lo, S.-Y. Lee, K.-S. Chang, Study of ZnSnO3-Nanowire Piezophotocatalyst Using Two-Step Hydrothermal Synthesis, J. Phys. Chem. C 119 (2015) 5218-5224. [72] Y. Zhao, X. Huang, F. Gao, L. Zhang, Q. Tian, Z.-B. Fang, P. Liu, Study on water 44

splitting characteristics of CdS nanosheets driven by the coupling effect between photocatalysis and piezoelectricity, Nanoscale 11 (2019) 9085-9090. [73] B. Yuan, J. Wu, N. Qin, E. Lin, D. Bao, Enhanced Piezocatalytic Performance of (Ba,Sr)TiO3 Nanowires to Degrade Organic Pollutants, ACS Appl. Nano Mater. 1 (2018) 5119-5127. [74] X. Xu, L. Xiao, Y. Jia, Z. Wu, F. Wang, Y. Wang, N.O. Haugen, H. Huang, Pyro-catalytic hydrogen evolution by Ba0.7Sr0.3TiO3 nanoparticles: harvesting cold–hot alternation energy near room-temperature, Energy Environ. Sci. 11 (2018) 2198-2207. [75] J. Xie, R. Jin, A. Li, Y. Bi, Q. Ruan, Y. Deng, Y. Zhang, S. Yao, G. Sankar, D. Ma, J. Tang, Highly selective oxidation of methane to methanol at ambient conditions by titanium dioxide-supported iron species, Nat. Catal. 1 (2018) 889-896. [76] C. Jin, D. Liu, J. Hu, Y. Wang, Q. Zhang, L. Lv, F. Zhuge, The role of microstructure in piezocatalytic degradation of organic dye pollutants in wastewater, Nano Energy 59 (2019) 372-379. [77] J. Wu, N. Qin, D. Bao, Effective enhancement of piezocatalytic activity of BaTiO3 nanowires under ultrasonic vibration, Nano Energy 45 (2018) 44-51. [78] J. Zhao, T. Wu, K. Wu, K. Oikawa, H. Hidaka, N. Serpone, Photoassisted Degradation of Dye Pollutants. 3. Degradation of the Cationic Dye Rhodamine B in Aqueous Anionic Surfactant/TiO2 Dispersions Under Visible Light Irradiation: Evidence for the Need of Substrate Adsorption on TiO2 Particles, Environ. Sci. Technol. 32 (1998) 2394-2400. [79] M. Guan, C. Xiao, J. Zhang, S. Fan, R. An, Q. Cheng, J. Xie, M. Zhou, B. Ye, Y. Xie, Vacancy Associates Promoting Solar-Driven Photocatalytic Activity of Ultrathin Bismuth Oxychloride Nanosheets, J. Am. Chem. Soc. 135 (2013) 10411-10417. [80] K.H. Reddy, K. Parida, P.K. Satapathy, CuO/PbTiO3: A new-fangled p–n junction designed for the efficient absorption of visible light with augmented interfacial charge transfer, photoelectrochemical and photocatalytic activities, J. Mater. Chem. A 5 (2017) 20359-20373. [81] A.J.E. Rettie, H.C. Lee, L.G. Marshall, J.-F. Lin, C. Capan, J. Lindemuth, J.S. McCloy, J. Zhou, A.J. Bard, C.B. Mullins, Combined Charge Carrier Transport and Photoelectrochemical Characterization of BiVO4 Single Crystals: Intrinsic Behavior of a Complex Metal Oxide, J. Am. Chem. Soc. 135 (2013) 11389-11396. [82] M. Pan, C. Zhang, J. Wang, J.W. Chew, G. Gao, B. Pan, Multifunctional Piezoelectric Heterostructure of BaTiO3@Graphene: Decomplexation of Cu-EDTA and Recovery of Cu, Environ. Sci. Technol. 53 (2019) 8342-8351. [83] Y.-L. Wang, Y. Tian, Z.-L. Lang, W. Guan, L.-K. Yan, A highly efficient Z-scheme B-doped g-C3N4/SnS2 photocatalyst for CO2 reduction reaction: a computational study, J. Mater. Chem. A 6 (2018) 21056-21063. [84] N. Kirkwood, B. Singh, P. Mulvaney, Enhancing Quantum Dot LED Efficiency by Tuning Electron Mobility in the ZnO Electron Transport Layer, Adv. Mater. Interfaces 3 (2016) 1600868-1600874. 45

Highlights • ZnO/BaTiO3 was prepared by combining solvothermal and impregnation sintering. • ZnO/BaTiO3 has a higher piezoelectric potential difference than BaTiO3 and ZnO. • The flat band potential of ZnO/BaTiO3 is more negative than BaTiO3 and ZnO. • ZnO/BaTiO3 has superior catalytic oxidations initiated by piezophototronic effect. • ZnO/BaTiO3 can efficiently utilize ultrasound and light irradiation simultaneously.