Enhanced Photocatalytic Degradation Performance by Fluid-Induced

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Enhanced Photocatalytic Degradation Performance by Fluid-induced Piezoelectric Field Lili Ling, Yawei Feng, Sa Yan, Donglai Pan, Hao Ge, Hao Li, Hexing Li, and Zhenfeng Bian Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b00946 • Publication Date (Web): 21 Jun 2018 Downloaded from http://pubs.acs.org on June 23, 2018

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Environmental Science & Technology

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Enhanced Photocatalytic Degradation Performance

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by Fluid-induced Piezoelectric Field

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Yawei Feng,† Hao Li, † Lili Ling, Sa Yan, Donglai Pan, Hao Ge, Hexing Li,* and Zhenfeng

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Bian*

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Education Ministry Key and International Joint Lab of Resource Chemistry and Shanghai Key

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Lab of Rare Earth Functional Materials, Shanghai Normal University, Shanghai 200234, PR

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China

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ABSTRACT: The introduction of piezoelectric field has been proven a promising method to

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enhance photocatalytic activity by preventing photoelectron-hole recombination. However, the

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formation of piezoelectric field requires additional mechanical force or high-frequency ultrasonic

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baths, which limits its potential application in industrial scale. Therefore, it is of great practical

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significance to design the catalyst which can harvest the discrete energy such as the fluid

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mechanical energy to form the electric field. Herein, PZT/TiO2 catalyst with a core-shell

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configuration was prepared by a simple coating method. By collecting the mechanical energy of

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water, an internal piezoelectric field was induced. Under 800 rpm stirring, transient photocurrent

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measured on PZT/TiO2 electrode is about 1.7 times higher than that of 400 rpm.

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Correspondingly, the photocatalytic degradation rate and mineralization efficiency of RhB, BPA,

ACS Paragon Plus Environment

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phenol, p-chlorophenol much improved, showing the promoting effect of piezoelectric field

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generated directly from harvesting the discrete fluid mechanical energy.

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INTRODUCTION

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Photocatalysis has been demonstrated as a promising technology for pollutants degradation

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and hydrogen generation by solar energy.1-6 However, it is still challenging to improve the low

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photocatalytic efficiency as the strong tendency of recombination between photo-generated

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electrons and holes.7-11 Generally, an external field is proposed to suppress the recombination of

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carriers and promote the photocatalytic performance.12-19 Recently, the built-in electric fields

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from piezoelectric materials, including wurtzite and perovskite, have been applied to spatially

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separate the photo-generated charges and thus enhance the performance of photocatalytic

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activity.20-27 Several previous reports have demonstrated that the piezoelectric field is an

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effective way to promote the charge separation and reduce the recombination rate. For example,

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Chang et al. reported that rhombohedral ZnSnO3 nanowires had the synergistic piezo-

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photocatalytic property under an external stress and ultrasonic waves.28,29 Xue et al. proposed a

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design to enhance the organic dye photocatalytic degradation on carbon-fiber-loaded ZnO

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nanowires by a periodically applied force.22 Chang et al. demonstrated that an improvement of

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1.4 times for rhodamine B (RhB) degradation on Ag-modified ZnO nanowires by bending-

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induced piezoelectric property.30 Furthermore, the built-in electric field originated from BaTiO3

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nanocrystal by the alternately variation of ultrasonic generator not only can enhance the

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photocatalytic activity of Ag2O, but can depress the self-corrosion of Ag2O nanoparticles by the

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enhanced separation of photo-generated charges.31 However, the formation of piezoelectric field

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requires additional mechanical force or high-frequency ultrasonic baths, which limits its potential


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application in industrial scale. Therefore, it is of great practical significance to design the catalyst

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that can harvest the discrete micro- or nano- energy to form the electric field, such as the fluid

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mechanical energy.

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Lead zirconate titanate (PZT) is a typical piezoelectric material used as energy harvesters,

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ultrasonic transducers and actuators.32,33 The piezoelectric coefficient of PZT (d33: 500-600

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pC/N) is much exceeding to wurtzite ZnO and perovskite BaTiO3,34-36 making PZT competent

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for discrete energy harvesting. In this work, we designed the PZT/TiO2 catalyst with a core-shell

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configuration by coating titania (TiO2) nanoparticles on the surface of piezoelectric PZT

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microsphere. The results indicated that the photocatalytic degradation performance was related to

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the induced piezoelectric field, as evidenced by the Rhodamine B (RhB), Bisphenol A (BPA),

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phenol and p-chlorophenol degradation rate improved with the stirring speed ranges 200 to 800

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rpm. Transient photocurrent output on PZT/TiO2 photoelectrode also confirmed this piezo-

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enhanced phenomenon, indicated by the notably enlarged current output. In addition, the lead

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element was not released in the solution, indicating the stability of the PZT/TiO2 catalyst. This

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study shows the advantage and potential of PZT/TiO2 for enhancing activity in photocatalytic

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degradation of organic pollutants by harvesting the discrete fluid mechanical energy to generate

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piezoelectric field.

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MATERIALS AND METHODS

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Materials. Lead oxide (Pb3O4), zirconium dioxide (ZrO2), titania (TiO2), bismuth oxide

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(Bi2O3), ferric oxide (Fe2O3) were purchased from Sinopharm Chemical Reagent Co., Ltd.,

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Shanghai. Tetrabutyl titanate (TBOT), concentrated ammonia solution (28 wt%), and SiO2 (Φ: 2

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µm) were purchased from Aladdin Bio-Chem Technology Co., Shanghai. Ethanol was purchased


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from Richjoint Chemical Reagent Co., Ltd. All the chemicals were analytical pure (AR) and

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used without any further purification. Deionized water was used throughout the experiment.

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PZT powder. PZT powder was prepared by a conventional solid-state reaction method

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according to the previous work.37,38 Raw powders, including Pb3O4, ZrO2, TiO2, Bi2O3, Fe2O3,

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were mixed by ball milling for 6 h with the presence of ethanol. The powders were mixed at a

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molar ratio of Pb: Zr: Ti: Bi: Fe = 100: 97: 3: 10: 10. After filtration and drying, the mixtures

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were heated to 850 °C and kept for 2 h in a muffle furnace. Followed by a second milling, the

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powders were pressed into pellets and calcined at 1200 °C for 2 h. After cooling down to room

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temperature, the sample was crushed into fine powder.

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PZT/TiO2 photocatalyst. Firstly, 0.4 g PZT fine powder was dispersed in ethanol (135 mL),

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and mixed with concentrated ammonia solution (0.2 mL) under ultrasound for 10 min.

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Afterwards, 0.75 mL of TBOT was added dropwise, and the reaction was performed in a sealed

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flask for 12 h at 50 °C under continuous magnetic stirring to form the seed layer.39 The resultant

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powders were separated, and followed by ethanol washing. Secondly, the obtained powders were

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re-dispersed in 135 mL of ethanol with 2 ml of concentrated ammonia. Later, another dosage of

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TBOT was added dropwise. The mixture was transferred into a 200 mL Teflon-lined autoclave,

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and placed in oil bath (24 h, 110 °C) under continuous magnetic stirring. Washed with ethanol,

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the product was dried at 80 °C overnight. The resulting sample was calcined at 500 °C in air for

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2 h to remove any organic species and improve the crystallinity. Finally, the sample was

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grounded into fine powders and reserved for the subsequent characterization and catalytic

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activity evaluation. The thickness of TiO2 nanoparticle layer coated on the surface of PZT

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microsphere was rather dependent on the TBOT dosage in the second step, so an optimized

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TBOT dosage (2 ml) was added in this second step.


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SiO2/TiO2 sample was obtained by the same method as the preparation of PZT/TiO2. And the

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pure TiO2 sample was obtained according to the second step without any dispersed PZT or SiO2

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powder.

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Characterization. The crystal structures of samples were characterized by using X-ray

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diffraction (XRD, Rigaku D/MAX-2000, Cu Kα source). The morphology of samples was

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investigated by a field emission scanning electron microscope (FESEM, HITACHI S4800) and

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transmission electron microscope (TEM, JEOL JEM 2100 and 2100F). The steady-state optical

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absorption was measured by UV-visible spectrophotometer (Shimadzu, UV 2600) with an

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integrating sphere attachment and used BaSO4 as reflectance standard at room temperature.

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Surface electronic states was determined by X-ray photoelectron spectroscopy (XPS, PHI 5000

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Versaprobe II). The shift of the binding energy due to relative surface charging was corrected

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using the C1S level at 284.8 eV as an internal standard. The specific surface area was measured

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by nitrogen sorption on an auto-adsorption system (Micromeritics TriSrar II 3020, at 77 K).

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Photocatalytic activity test. For a typical piezo-photocatalytic run, photocatalyst dispersion

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(5.0 g/L PZT/TiO2 or 3.6 g/L SiO2/TiO2) containing Rhodamine B (RhB, 10 mg/L), Bisphenol A

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(BPA, 10 mg/L), phenol (10 mg/L) or p-chlorophenol (10 mg/L) was equilibrated for 30 minutes

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at 25 °C under dark condition. Then, the piezo-photocatalytic process was initiated by a LED

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(Aulight CEL-LED100) with a light power density of 15 mW/cm2 at a certain stirring speed

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controlled by a magnetic stirrer. Finally, the solution was centrifuged at 9500 rpm to remove the

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photocatalyst particles. The concentration of RhB, BPA or p-chlorophenol remained in solution

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was analyzed by a UV-vis spectrophotometer (Xinmao UV 7504/PC) at the characteristic

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wavelength. The concentration of phenol was analyzed by gas chromatograph mass spectrometer

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(Shimadzu, GCMS-QP2010 SE) equipped with an SH-Rxi-5Sil MS column in SIM model. The


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total organic carbon (TOC) content was determined by a TOC analyzer (Elementar vario TOC

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select). Graphite tube atomic absorption spectrometry (Varian AA240Z) was used to

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quantitatively analyze the residual lead element in the solution.

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Photoelectric measurement. Photoelectrochemical analysis was measured in a conventional

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three-electrode electrochemical station (CHI 660E). PZT/TiO2 photoelectrode was prepared by

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coating PZT/TiO2 sample on a piece of ITO glass (20mm*20mm) and served as the working

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electrode. The counter electrode and reference electrode consisted of a platinum sheet

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(10mm*20mm) and saturated calomel electrode (SCE), respectively. A monochromator (365 nm,

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4 W) was used as the UV light source and positioned 8 cm away from the photoelectrochemical

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cell. The transient photocurrent was measured in Na2SO4 solution (0.5 mol/L) with zero bias

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voltage. The photoelectric measurement of SiO2/TiO2 electrode was also performed in the same

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process.

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RESULTS AND DISCUSSION

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PZT sample shows microspheres with average diameter around 1.5 - 2.5 µm, as the SEM images

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displayed in Figure 1a and Figure S1. The XRD patterns in Figure 2a demonstrate the obtained

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PZT crystal structure is orthorhombic (JCPDS, No.89-8012), which is ferroelectric at room

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temperature. Besides, the selected-area electron diffraction patterns of the slice sections (Figure

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S2c, S2f) indicate the as-made PZT is monocrystalline for an individual particle. It is an

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incontrovertible fact that the prepared PZT can be piezoelectric for the uniform orientation of

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spontaneous polarization despite no extra high voltage polarization applied for its single crystal

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structure. Uniform TiO2 nanoparticles were deposited on the surface of PZT as shown in the

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high-resolution SEM image (Figure 1c). The TEM image in Figure 1e also indicates TiO2


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photocatalyst is successfully coated on the PZT surface, as the lattice fringes in the HRTEM

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image (Figure 1f) indicates the (101) facet of anatase TiO2. After coating, the composite

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PZT/TiO2 shows the characteristic diffraction peaks of anatase TiO2 (JCPDS, No.21-1272) (101)

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and (200) facets positioned at 25.4° and 48.1° (Figure 2a and Figure S4). Steady-state UV-vis

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absorption was employed to explore the optical properties of samples. The PZT/TiO2 sample

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exhibits the characteristic absorption of TiO2 and PZT (Figure S5). Besides, the absorption

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intensity of PZT/TiO2 sample is a little higher than the equivalent physical mixture of PZT and

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TiO2. The valence state of the Ti in PZT/TiO2 was determined by XPS (Figure 2b). The weak

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peaks at 465.6 and 458.4 eV are assigned to the binding energies of Ti 2p1/2 and Ti 2p3/2 from

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Ti4+ in PZT, while the peaks at 463.9 and 458.3 eV represent the binding energies of Ti 2p1/2 and

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Ti 2p3/2 from Ti4+ in TiO2, which shifted 0.6 eV to lower values compared to the pure TiO2

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(Figure S6b). This binding energy shift may result from the strained contact of TiO2

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nanoparticles and PZT microsphere in the drying process.40 N2 adsorption-desorption isotherms

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of the samples (Figure S7) showed that the BET surface area of PZT/TiO2 (37.5 m2/g) is larger

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than that of the physical mixture (28.4 m2/g), which may result from the uniform distribution of

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TiO2 nanoparticles in the sample.

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A SiO2/TiO2 core-shell configuration (Figure S8-S11) was also prepared as the reference sample

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by the same method as PZT/TiO2 preparation. SEM, TEM images (Figure S8) and XRD patterns

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(Figure S9) indicate that similar anatase TiO2 nanoparticles successfully deposited on the surface

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of SiO2 microparticles as the shell layer. In addition, the UV absorption intensity of SiO2/TiO2

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sample (Figure S10) is rather higher than the physical mixture and the BET specific surface area

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of SiO2/TiO2 (66.8 m2/g, Figure S11) is much larger than that of PZT/TiO2 (37.5 m2/g) due to the

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supporting effect of SiO2 microparticles and enhanced pore volume.


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The comparison of photocatalytic degradation of RhB was carried out to evaluate the activity of

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piezo-photocatalyst composite. A high-pressure mercury lamp (Figure S12a) with 400 nm cut-off

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filter was used as the UV source. Due to the piezo-catalytic effect of PZT rather than the physical

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adsorption by particles according to the previous reports,37,41 the concentration of RhB decreased

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slightly (0.6 % within 5 min, Figure S13) in the dark condition over the physical mixture of PZT

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and TiO2. Under UV irradiation, PZT/TiO2 sample displayed a higher RhB degradation rate

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(0.041 mg·L-1·min-1 at the pseudo-zero-order), almost 3.4 times of the physical mixture sample

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(0.012 mg·L-1·min-1) (Figure S13). In addition that the increasing of specific surface area

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(,/ ⁄, = 1.3, Figure S7) coupled with the heterojunction effect has

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the contribution to this enhanced performance, the piezoelectric field induced from PZT core by

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RhB solution turbulence may also contribute to the promotion of the photocatalytic performance.

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On the basis that piezoelectric field originated from PZT particle can contribute to the charge

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separation and force the free electrons and holes migration to the surface, the photocatalytic

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performance can be modulated by the ambient stress. Thus, transient photocurrent measurement

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was carried out to investigate the character of piezopotential-depended spatially separation and

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migration of free charge carriers on PZT/TiO2 electrode. As shown in Figure 3a, the PZT/TiO2

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electrode showed no distinguishing current output in the dark condition, even suffering from the

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different amplitude turbulence applied by magnetic stirring. Under UV irradiation, 0.79 µA

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photocurrent output was measured, indicating the PZT/TiO2 sample with UV-response.

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However, it showed that the photocurrent output lightly depressed (0.74 µA) at 400 rpm stirring.

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This phenomenon might be cause by the interference of the UV absorption from stirring switch

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since 10 % decreased in intensity on the pure TiO2 photoelectrode under the same condition

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(Figure S14) and almost 33 % decreased on SiO2/TiO2 electrode (Figure 3b). By improving the


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rotation speed to 600 rpm, the photocurrent output increased to 1.12 µA. The current output

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reached to 1.98 µA when the rotation speed increased to 800 rpm, almost 1.7 times higher than

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that at 400 rpm. As a contrast, the SiO2/TiO2 electrode has no change in current output at

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different stirring speeds. Since SiO2 has no piezoelectric property, the behavior of charge carriers

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was not affected by the increased strain stress applied by stirring electrolyte solution. The above

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photocurrent output results make clear that the stress-boosted piezoelectric field can effectively

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suppress the recombination and enhance the spatial transportation of photo-generated carriers.

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The performance of RhB degradation on PZT/TiO2 sample was investigated to clear the role of

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the induced piezoelectric field in piezo-photocatalysis. A magnetic stirrer was used to tune the

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rotation condition of RhB solution to achieve different piezoelectric field in intensity by applying

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various magnitudes of compressive stress on the surface of PZT/TiO2 microsphere. The results

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showed that the remained RhB concentration in aqueous solution was relied on the stirring. As

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shown in Figure 4a and Figure S15, RhB solution has been totally decolorized (100% removed

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of the dye molecules) within 80 min at 800 rpm. Whereas, there was still 25 % (2.5 mg/L) RhB

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molecules remained at 200 rpm, 10% (1 mg/L) remained at 600 rpm. As for the controlling

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group, there is only about 25% (2.5 mg/L) of RhB removed in the dark condition and about 32%

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(3.2 mg/L) removed under UV irradiation without any stirring (Figure S16). Experimental results

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on decolorization of RhB over SiO2/TiO2 (Figure 4b) and bare TiO2 (Figure S17a) confirm the

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photocatalytic performance is not affected by the solution turbulence ranges from 200 rpm to 800

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rpm. Though the promotion effect for photocatalytic dye degradation is not as notable as the

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evaluation of photocurrent output from the electrode (2.7 times), which might be due to the

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difference between the direct current signal measured by the photocurrent test and photocatalytic

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performance measured is generated by free radicals which requires a number of reaction steps


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from the photogenerated charges. Therefore, the above results proved the enhancement of

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photocatalytic decolorization caused by piezoelectric-field induced from discrete water

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mechanical energy. However, if the stirring velocity is more than 1200 rpm, as shown in Figure

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S15f and Figure S17b, the scattering of light is dominated by the solution, which reduces the

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light absorption of photocatalyst, thus affecting the enhancement of photocatalytic performance.

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This phenomenon was also appeared on pure TiO2 and SiO2/TiO2 composite (Figure S17a, c).

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It’s worthy to note that, though SiO2/TiO2 sample possess the large specific surface area than

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PZT/TiO2 ( , / ⁄,/ = 1.8 ), it need almost 100 min to fully realize the

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decolorization process, a bit longer than that over PZT/TiO2 sample (equivalent TiO2) at 800 rpm

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(80 min). The TOC removal ratio in RhB solution over PZT/TiO2 within 80 min was also

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investigated. The decreasing TOC content (Figure S18) from 200 to 800 rpm implies the

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improved mineralization, also confirming the enhanced photocatalytic performance with the

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intensive piezoelectric field. As we expected, the removed TOC content within 80 min at 800

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rpm over PZT/TiO2 (65% in percentage, Figure 4c and Figure S18) is exceeding to the amount

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that over SiO2/TiO2 at 800 rpm within 100 min (51% in percentage, Figure 4c and Figure S19),

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showing the advantage of higher efficiency in TOC removal on piezo-photocatalyst by

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harvesting the discrete fluid mechanical energy.

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In addition, examples about photocatalytic degradation of other organic compounds, such as

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BPA (Figure 4d and Figure S20-22), phenol (Figure 4e) and p-chlorophenol (Figure 4f), were

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displayed to visualize the above certified piezoelectric field improved photocatalytic

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performance, which excluded the possibility of self-degradation of color dye under UV

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irradiation and indicated the universality of this strategy for photocatalytic performance

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enhancement originated form piezoelectric field induced by discrete fluid mechanical energy.


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According to the above results in Figure 4 and Figure S15-S22, it’s clear to conclude that the

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photocatalytic performance could be enhanced by the boosted piezoelectric field from discrete

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fluid mechanical energy. Furthermore, we detected the main species by using the trapping agents

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to inhibit active species during the piezo-photocatalytic process. As shown in Figure S23a, it

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clearly indicated that the holes (h+) were the main active oxidative species. Tert-butanol as

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hydroxyl radicals (·OH) scavenger and N2 bubbling to suppress the formation of superoxide

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anions (·O2-) also had a certain inhibitory effect on the photocatalytic activity which suggested

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these radicals were also the active oxidative species in the degradation process. The trapping

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tests in SiO2/TiO2 photocatalytic degradation process (Figure S23b) suggested that it is

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essentially the same as piezo-photocatalysis by these same active oxidative species. Recycling

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tests (Figure S24) showed that PZT/TiO2 composite could be reused for eight times without any

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significant deactivation. Besides, there were no obvious exfoliation of TiO2 nanoparticles in the

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shell layer (Figure S25) and no residual Pb element detected (Table S1) in the RhB-totally-

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removed solution, indicating the composite was physicochemically stable and nontoxic in

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application.

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Based on the above results, a schematic diagram on the general photocatalytic reaction enhanced

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by piezoelectric filed induced by discrete fluid mechanical energy is illustrated in Figure 5a.

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When PZT/TiO2 is suffered different stress by water stirring, piezoelectric field in PZT

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monocrystalline core with different magnitude will be originated. To better understand the

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carrier transportation in TiO2 shell under the influence of piezoelectric field, only simplified

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band structure diagrams of TiO2 at the interface of solution are given in Figure 5b. Under UV

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irradiation, free electron-hole charge pairs are excited in TiO2 shell nanoparticles, then carriers

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cross the energy barrier between TiO2 catalyst and solution to achieve the redox reaction. As


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mentioned above, the recombination of free electrons and holes restrains the photocatalytic

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efficiency. When compressive strain is applied to the PZT particle, piezoelectric field generated.

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Due to the polarization, the effective separation of free charges in TiO2 particles is realized. The

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stronger piezoelectric field induced by the higher stirring rate will further improve the separation

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and transportation of free charges. Specifically speaking, the positive piezoelectric polar charges

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at the PZT surface will attract the negative free electrons on the conduct band of TiO2 to the

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photocatalyst-piezoelectric material interface and push the positive holes on the valence band of

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TiO2 to the photocatalyst-solution interface, thus resulting in a favorable situation for

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photocatalytic degradation and the better performance. On the opposite direction, the negative

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piezoelectric polar charges at the PZT surface will attract the positive holes in TiO2 to the

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photocatalyst-piezoelectric material interface and push the negative free electrons in TiO2 to the

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photocatalyst-solution interface. Owing to the existence of nanopores (Fig. S7b) which provided

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the mass transfer channel, the holes attracted to the direction of PZT core can still contribute to

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the compounds degradation, thus also leading to the better performance.

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PZT/TiO2 piezo-photocatalytic configuration was prepared by coating technology. Piezoelectric

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electric fields were produced by stirring at different speeds. The rate of photocatalytic

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degradation increased with the increase of stirring speed ranges from 200 to 800 rpm, showing

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the promoting effect of the piezoelectric field by facilitating charge separation and transportation

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to inhibit photoelectron-hole recombination. This work shows the possibility of harvesting the

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discrete fluid mechanical energy and converting it into the piezoelectric field to improve the

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photocatalytic performance. More importantly, this strategy supplies a general way to design

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piezo-photocatalytic system for degradation of organic pollutants in the practical application.

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ASSOCIATED CONTENT


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Supporting Information: The Supporting Information is available free of charge on the ACS

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Publications website at http://pubs.acs.org. SEM, TEM images and SAED patterns of PZT; SEM

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images, XRD patterns, absorption spectra, XPS spectra, N2 adsorption−desorption isotherms of

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PZT/TiO2 samples; SEM images, TEM images, XRD patterns, absorption spectra, N2

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adsorption−desorption isotherms of SiO2/TiO2 samples; Emission spectra of high pressure

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mercury lamp and UV LED; RhB degradation performance on PZT/TiO2 and the physical

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mixture; photocurrent output from TiO2 electrode; absorption spectra and TOC content of RhB

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solution, BPA solution; recycling tests.

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AUTHOR INFORMATION

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Corresponding Author

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* [email protected], [email protected]

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Author Contributions

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†These authors contributed equally to this work.

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Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENT

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This work is supported by National Natural Science Foundation of China (21761142011,

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21407106, 21522703), Ministry of Education of China (PCSIRT_IRT_16R49) and International

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Joint Laboratory on Resource Chemistry (IJLRC). Research is also supported by The Program

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for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher

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Learning.


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Figure 1. SEM and TEM images of PZT and PZT/TiO2 samples. SEM image of (a) PZT

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Figure 2. (a) The XRD patterns of PZT, TiO2 and PZT/TiO2 samples. (b) High resolution XPS

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Figure 3. Photocurrent output on (a) PZT/TiO2 electrode and on (b) SiO2/TiO2 electrode under

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UV irradiation at different stirring speed.

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Figure 4. (a) Photocatalytic performance of RhB degradation on PZT/TiO2 and (b) SiO2/TiO2

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under different conditions. (c) The remained TOC content in RhB solution using PZT/TiO2 as

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(black column). (d)-(f) The degradation performance of BPA, phenol, p-chlorophenol on

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PZT/TiO2 under different stirring speed.

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Figure 5. (a) A diagram illustration on the proposed piezoelectric field enhanced photocatalytic

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reaction and (b) the simplified band structure diagrams of TiO2 at the photocatalyst-solution

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interface with different PZT polar direction.

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