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Insights into the Role of Ferroelectric Polarization in Piezocatalysis of Nanocrystalline BaTiO 3

Jiang Wu, Qi Xu, Enzhu Lin, Baowei Yuan, Ni Qin, Santhosh Kumar Thatikonda, and Dinghua Bao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b01991 • Publication Date (Web): 04 May 2018 Downloaded from http://pubs.acs.org on May 5, 2018

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ACS Applied Materials & Interfaces

Insights into the Role of Ferroelectric Polarization in Piezocatalysis of Nanocrystalline BaTiO3 Jiang Wu, Qi Xu, Enzhu Lin, Baowei Yuan, Ni Qin*, Santhosh Kumar Thatikonda, and Dinghua Bao* State Key Laboratory of Optoelectronic Materials and Technologies, School of Materials Science and Engineering, Sun Yat-Sen University, Guangzhou 510275, China

ABSTRACT Piezoelectric effect, commonly known as a change in electric polarization in piezoelectric/ferroelectric materials under mechanical stress, is extensively employed as a driving force for catalytic degradation of organic pollutants. However, the relationship between electric polarization and piezocatalytic activity is still unclear. In this work, we investigated the role of ferroelectric polarization in the piezocatalytic activity of BaTiO3 nanoparticles through annealing BaTiO3 at different temperatures or poling BaTiO3 at different electric fields. The BaTiO3 nanoparticles annealed at 800 oC exhibit effectively enhanced piezocatalytic activity compared with those annealed at other temperatures. The polycrystalline particles annealed at higher temperatures exhibit a greatly reduced catalytic activity. After poling, the piezocatalytic activity of the polycrystalline BaTiO3 particles was obviously improved. In addition, we identified the free radical species and the intermediate products of catalytic reaction. We also well explained the dependence of electric polarization in BaTiO3 piezocatalyst on annealing temperature and ultrasonic vibration theoretically. Our study indicates that increasing ferroelectric polarization (but not crystallite size) can effectively enhance piezocatalytic activity. We believe that the

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present work provides a clear understanding of role of ferroelectric polarization in piezocatalysis. KEYWORDS: piezocatalysis, ferroelectric polarization, barium titanate, annealing, poling.

1. INTRODUCTION The demand for environmental protection continues to increase as a result of worldwide population growth and rapid industrial development. It is especially difficult to deal with soluble organic dye pollutants in water due to their high stability, which can lead to serious health and environmental problems. To address this severe problem, development of environment friendly, convenient and inexpensive technologies for degrading organic pollutants is urgently required.1, 2 Very recently, modulation of polarization in ferroelectric nanomaterials by ultrasonic vibration was proposed as an effective method to enhance the degradation efficiency of photocatalysis.3-7 The beneficial effect of ferroelectric materials in photocatalysis was introduced by the presence of built-in electric field which can promote the separation of photo-generated charge carriers.4 The polarization induced by mechanical stress, known as piezoelectricity, has been extensively employed as the driving force for catalytic degradation of organic pollutants in the dark.8-18 For instance, Hong et al. demonstrated that the aqueous solution of acid orange 7 (AO7) can be degraded by ultrasonic vibration in the presence of the piezoelectric BaTiO3 micro-dendrites.9 A similar degradation behavior was observed when adopting Pb(Zr0.52Ti0.48)O3 (PZT) fibers as the piezocatalyst.11 Lv et al. investigated “piezo-Fenton process” using BaTiO3 in the presence of metal ions, and found H2O2 was generated in situ, which is facilely turned to


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hydroxyl radical with strong oxidative power, leading to degradation of organic pollutants.13 The MoS2 and MoSe2 nanoflowers exhibited an ultrahigh piezocatalytic activity under continuous ultrasonic vibration.15, 19 In the very recent report by Feng et al., the authors claimed that the piezocatalytic degradation of Rhodamine B (RhB) can be achieved by low frequency magnetic stirring when the PZT catalyst as prepared by a conventional solid-state reaction method were employed.17 Compared with the existing catalytic technologies such as photocatalysis and electrocatalysis, piezocatalysis is especially attractive for the ability to utilize the prevalent mechanical vibration and to reduce the dependence on other conditions, such as light and electricity.20-23 The study of piezocatalysis was primarily performed on the classic ferroelectric materials. The correlation between the crystal/microcrystal structures and the functional properties is of essential importance in material design. As is well known, the BaTiO3 perovskite undergoes a structural transition at the Curie temperature (120 oC), and the low-temperature-stable structure of a polar crystal class (P4mm) is responsible for the spontaneous polarization.24 The piezoelectric property of a ferroelectric crystal has a strong dependence on its spontaneous polarization. Since the crystalline characteristics as well as the ferroelectric properties of fine BaTiO3 powders can be adjusted by thermal annealing process,25 we are eager to reveal the role of ferroelectric polarization in the piezocatalytic property of BaTiO3 through thermal annealing process. In this work, the BaTiO3 nanoparticles were annealed in a wide temperature range from room temperature to 1200 oC. The piezocatalytic activity of BaTiO3 reaches the maximum in 800 oC-annealed sample, while the catalytic properties of the samples annealed at higher temperatures are greatly reduced. The decrease in piezocatalytic



activity of high-temperature-annealed samples is due probably to the sintering of nanocrystals. It is known that the random distribution of spontaneous polarization in a polycrystalline grain generally results in zero microscopic polarization. Hence, the effect of poling treatment on the piezocatalytic activity of the thermally annealed BaTiO3 samples was further investigated. After being poled in a strong electric field, the piezocatalytic activity of the polycrystalline BaTiO3 particles was obviously improved. We demonstrated that increasing ferroelectric polarization (but not crystallite size) can effectively enhance piezocatalytic activity.

2. EXPERIMENTAL SECTION The original BaTiO3 nanoparticles used in this work were purchased from Aladdin Co., Shanghai, China. The nanoparticles were annealed at various temperatures from 200 o

C to 1200 oC (hereafter abbreviated as BTO-200, BTO-400, BTO-600, BTO-800,

BTO-1000 and BTO-1200, respectively). The poling treatment of the samples was performed in a self-made insulating Teflon mold by applying a static high-voltage electric field to the powder compact for 30 min at room temperature. To avoid electric breakdown, the powders were dried at 120 oC for 12 h. Then, the powders were pressed into a compact in the mold with two copper blocks below and above the compact. The copper blocks, which were fixed in the mold with screws to keep a tight contact with the powder compact, also acted as the top and bottom electrodes for poling. During the electric poling process, the applied voltage up to 4 kV mm-1 can be steadily maintained for more than 30 min, which enables sufficient poling of BaTiO3 particles. X-ray diffraction (XRD) with monochromatic Cu Kα radiation (Rigaku D/MAX 2200 VPC) was used to determine crystallographic information. Room temperature


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Raman spectra were measured with a laser micro-Raman spectrometer (Renishaw inVia). The morphology and corresponding selected-area electron diffraction (SAED) patterns of BaTiO3 samples were examined by high resolution transmission electron microscopy (TEM, JEM-2100, JEOL), operating at 200 kV. The nitrogen (N2) adsorption/desorption isotherms were measured using a Quantachrome Autosorb-IQ instrument at about 77 K. Prior to the test, the particles were degassed under vacuum at 350 oC for 5 h, and then, the specific surface area was calculated by Brunauer-Emmett-Teller (BET) method. In a typical piezocatalytic measurement, 0.1 g of BaTiO3 samples were dispersed in 100 mL of 5 mg L-1 MO aqueous solution. Before the application of ultrasonic vibration, the aqueous dispersion was stirred in the dark for 30 min to establish an adsorption-desorption equilibrium between catalysts and MO molecules. To avoid the effect of photocatalysis, the whole piezocatalytic experiment was carried out in the dark environment. At time intervals, 3 mL of the aqueous dispersion was periodically collected and centrifuged to obtain a clear solution. The clear solution was analyzed by Ultraviolet-visible (UV-vis) absorption spectroscopy using a Shimadzu UV-3600 spectrophotometer. The reactive oxidation species generated in the piezocatalysis system were investigated using the electron spin resonance (ESR) technique at ambient temperature

(Bruker

A300-10-12

spectrometer).

High

performance

liquid

chromatography (HPLC) analysis was carried out by an Agilent 1200 equipped with a Thermo C18 column (250 mm×4.6 mm, 5 µm). The aqueous dispersion after piezocatalytic degradation was centrifuged and filtered through a 0.5 µm cellulose membrane filter. Acetonitrile/Ammonium acetate (10 mM) was used as a mobile phase with 23/77 (V/V) and 0.8 ml min−1 as a flow rate.



3. RESULTS AND DISCUSSION The XRD patterns of these samples confirm the formation of perovskite structure (JCPDS 31-0174). As shown in Figure 1a for the original BaTiO3 sample, the degenerated (200) peak usually refers to the cubic-like perovskite structure. No obvious change in the crystal structure is observed as the annealing temperature varies from 200 o

C to 600 oC. When the annealing temperature was increased to higher temperatures, the

(200) peak splits gradually into two distinct diffraction peaks, which implies a coherent tetragonal distortion of the perovskite structure.26 As shown in Figure 1b, the c/a ratio calculated from the XRD spectra increased suddenly at 800 oC, while the full width at half maximum (FWHM) of the strongest (110) peak decreases significantly with the increasing annealing temperature, indicating an activated recrystallization upon annealing at 800 oC.27 Since the information provided by XRD is limited to the global structure of the crystal, Raman spectroscopic analysis was performed to investigate the local distortions of the lattice.28 As shown in Figure 1c, all the investigated samples contain the characteristic peaks of tetragonal structure centering near 249 cm-1, 306 cm-1, 515 cm-1 and 715 cm-1.29 It has been well established that the ideal cubic BaTiO3 (Oh symmetry) is Raman inactive because of the isotropic distribution of the electrostatic forces around the Ti4+ ions.29 Therefore, the distinct bands observed in the Raman scattering profile of all samples should be assigned to the non-centrosymmetric structure. The peak at 306 cm-1 is usually associated with the asymmetric vibration of [TiO6] octahedra, while the peak at 715 cm-1 is related to the highest frequency longitudinal optical mode (LO) with A1 symmetry.30 The results indicate a tetragonal distortion of [TiO6] octahedra in all the


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investigated BaTiO3 samples, which can give rise to piezocatalytic effect although XRD pattern indicates a global cubic-like symmetry. It is known that the spontaneous polarization Ps of perovskite-type structure is mainly originated from the atom displacement relative to the center of the octahedron along the ferroelectric axis and the increased tetragonality (c/a) is conducive to enhancing ferroelectric properties.24 The spontaneous polarization Ps of the BaTiO3 samples can be deduced by the following formula:31

Ps =

e V

∑ Z ′δ′ (i)

(1)

i

i

, where V is the unit cell volume, δ(i) represents the atomic displacements along the ferroelectric axis, and Z i′′ ( ZTi′′ = 5.7, Z O′′ = −3.1) is the apparent charge,31 which allow for the contribution from the ionic polarizability. In general, the spontaneous polarization of polycrystalline particles can be greatly reduced due to the random orientation of electric dipoles. Therefore, the calculation is valid only for single crystalline samples. Here, we used XRD Rietveld refinements to obtain the V and δ(i) values of the annealed BaTiO3 nanoparticles with single crystalline structure (BTO-RT to BTO-800). The calculated spontaneous polarization values of BaTiO3 samples as well as the unit cell volumes and the atomic displacements are summarized in Table 1. The results indicate that the spontaneous polarization of BaTiO3 nanoparticles increases with the increasing annealing temperature. Table 1. The unit cell volume, atoms displacement, and the calculated spontaneous polarization of BaTiO3 nanoparticles. Temperature (oC) Cell volume (Å3)

RT

200

400

600

800

64.72

64.573

64.362

64.345

64.294



Atom displacement

δ(Ti)

(Å)

δ(OI)

0.00229

0.00242

0.00198

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0.00299

0.00392

−0.00770 −0.00772 −0.01465 −0.01757 −0.02291

δ(OII) −0.01210 −0.01124 −0.01045 −0.00957 −0.01010 2

Ps (µC/cm )

18.4

17.9

22.1

25.1

36.4

The detailed morphology and microstructure of the BaTiO3 samples are shown in Figure 2. The samples annealed below 800 oC (original BTO, BTO-600) are mostly spherical particles with a uniform diameter of about 150 nm (see Figure 2a and 2d). The lattice fringes in the high resolution TEM (HRTEM) images (Figure 2b and 2e) extend throughout the whole nanoparticle as an indication of the single crystalline structure, which is also demonstrated by the sharp and bright dots in the SAED patterns (Figure 2c and 2f). For BTO-800, the average diameter was about 200 nm (Figure 2g), and the single crystalline nature was still maintained (Figure 2h and 2i). By increasing the annealing temperature from 1000 oC to 1200 oC, the crystalline size increases to micrometer levels (Figure 2j and 2m). The appearance of different lattice fringes orientations (Figure 2k and 2n) and bright circular rings in the typical SAED pattern (Figure 2l and 2o) indicates that the particles are of polycrystalline structure. The piezocatalytic property of the BaTiO3 nanoparticles was investigated by applying continuous ultrasonic vibration to the dye aqueous dispersion at a frequency of 40 kHz and a power of 80 W (see Figure 3a). When ultrasound propagates through the fluid, the acoustic pressure will fluctuate in waveform. As shown schematically in Figure 3b, water molecules are extruded in the positive phase and torn apart in the negative phase of pressure change, thus leads to the formation and growth of cavitation bubbles.32


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When the cavitation bubbles collapse at a critical size (usually several tens µm), the impact force on the adjacent BaTiO3 nanoparticles can be as high as 108-109 Pa.32, 33 The response of an individual BaTiO3 nanocrystal to the fluctuation of hydraulic pressure was simulated by the finite element method (FEM) with the aid of Comsol Multiphysics.34 The details of FEM simulation are given in the Supporting Information. The simulated deformation and electric potential from the selected phases (point 1 to 5 in Figure 3b) are plotted in Figure 3c and 3d, respectively. Initially, the nanocube is in a free state (point 1) with no potential difference. The compressive deformation of the BaTiO3 nanocube upon increasing pressure (point 2) is most significant along the polar axis. The induced polarization orients in the opposite direction of polar axis, resulting in negative potential on the top and positive potential on the bottom. The maximum potential difference (eg. 98 mV in a 100 nm-scaled BaTiO3 nanocube) is achieved at point 3. While the pressure drops off at a point (point 4) midway to 0 (point 5), the piezoelectric potential is drawn back together with the polarization. The distribution of current density in the BaTiO3 and water domains is shown in Figure 3e. The arrows represent the current direction only. In a dielectric medium such as barium titanate, the conduction electric current is ignorable due to the highly insulative property of the dielectrics. For this reason, only the displacement current was taken into account in this model for ideal dielectrics. The r displacement current density ( J D ) is defined as the derivative of electric displacement

r D to time r r r r r r dD d ε 0 E + P dE dP = = ε0 + JD = dt dt dt dt

(

)

(2)



r where ε 0 is the permittivity of vacuum, P is the electric polarization in the dielectric material. It is clear that the displacement current density is determined by the transient variations of polarization vector and electric field vector with time. Therefore, the displacement current density is not necessarily in the same direction of electric field. In special cases, displacement current density possibly has opposite direction with electric field.

r Moreover, since there is no external electric field, E in the first term on the right side of the eq. 2 comes from the depolarization field generated by the surface polarization r charge. The second term ( dP dt ) is called polarization current, which comes from the

separation of positive and negative charges in the dielectric materials. Apparently, in the r r interior of BaTiO3 nanoparticle, J D is dominated by dP dt . Whereas in the water r surrounding the BaTiO3 nanoparticle, J D is dominated by the polarization of water

induced by depolarization field. That is, r r dE J D = ε 0ε r dt

(3)

In the ascending stage of ultrasonic pressure (the point 2), the displacement current in the interior of BaTiO3 nanoparticle is directed from the top to the bottom, in r accordance with the positive dP . Thus, the current direction is opposite to that of electric

field. Meanwhile, in the water the displacement current is directed from bottom to the the r top, in parallel with positive dE . On the contrary, when the ultrasonic pressure decreases

from the maximum value to 0 at the point 4, the current direction is inversed in both the r r interior of BaTiO3 nanoparticle and water due to the negative dP and dE . In this case,

the current direction in BaTiO3 is same with that of electric field, while opposite in water.


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The piezocatalytic performance of the prepared BaTiO3 samples was examined by the degradation of MO in aqueous dispersion under continuous ultrasonic vibration. The relative concentration ratios C/C0 of MO over different piezocatalysts are plotted as a function of degradation time (Figure S1 in the Supporting Information). Given that the piezocatalytic degradation process follows the first order kinetics reaction, the kinetics of the degradation process can be represented as a linear dependence of ln(C0/C) on time (t), as shown in Figure 4a. The blank experiments in absence of either ultrasonic vibration or BaTiO3 catalyst show negligible degradation of MO. The result confirms that the piezoelectric effect is necessarily required by the piezocatalysis. When the annealing temperature is below 800 oC, the annealing temperature is too low to cause a notable change in crystalline property. Therefore, no obvious change in piezocatalytic efficiency was observed. With increasing annealing temperature, the piezocatalytic efficiency of BaTiO3 firstly increases and then decreases, giving the optimum value at 800 oC. The rate constants k, which are obtained from the linear fit of ln(C0/C) - t plots, are compared in Figure 4b. It can be seen that the BTO-800 sample shows the highest k value (0.019 min-1), which is nearly 4 times than that of BTO-1200 (0.005 min-1). The regularity was confirmed by repeating the degradation experiments under same conditions. According to the results of structural analysis, the samples annealed below 600 oC have a single crystalline structure. Hence, the piezoelectric catalytic efficiency is almost unchanged due to the little change in morphology and crystallinity. As shown in Figure 4d, the increase in piezocatalytic efficiency at 800 oC can be associated with the enhanced ferroelectric polarization (P2 > P1), resulting from the stronger tetragonal distortion, which is clearly indicated by the splitting of (200) peak. Although the BTO-1000 and



BTO-1200 are also crystallized with a tetragonal structure, the piezocatalytic efficiencies of these two samples are much lower than that of BTO-800. The reason for reduction of piezocatalytic activity at higher temperatures is relatively complicated. Two major effects have been caused by the high temperature annealing process: crystalline size increase and poly-crystallization. In our previous work, the grain morphology of BaTiO3 nanowires has essential influence on the piezocatalytic efficiency.34 We demonstrated by FEM that the piezoelectric potential difference between the two terminals of the polar axis of a BaTiO3 crystal increased in proportion with the grain size. Therefore, an increase in crystalline size is expected to show beneficial effect on the piezocatalytic performance. On the other hand, the sintering of nanocrystals results in recrystallization and large polycrystalline particles. Due to the random distribution of electric domains in different grains, the total polarization within a polycrystalline particle will be greatly reduced (P3 < P2), leading probably to the decrease of the piezocatalytic degradation efficiency.

To verify this possibility, the 1200 oC-annealed BaTiO3 samples were poled under different static high-voltage electric fields. Figure S2 in the Supporting Information shows the degradation efficiency of MO with the BTO-1200 samples poled at 0 kV mm-1, 2 kV mm-1, and 4 kV mm-1 for 30 min at room temperature, respectively. The rate constants of poled BTO-1200 as derived from the linear fit in Figure 4c are compared in Figure 4d. It is clear that the piezocatalytic activity of the poled samples has been obviously enhanced. The rate constant for BTO-1200 poled under 4 kV mm-1 (0.009 min-1) is almost 2 times of that of the unpoled sample (0.005 min-1). Since no other experimental factors have been changed in the poling process, the improved piezocatalytic property can only be attributed to the effect of poling treatment. We believe


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that the poling makes polarization directions in an individual polycrystalline particle align along electric field direction, which will result in increased polarization. It is worth noting that only the low-temperature (≤ 800 oC) annealed samples can be influenced by the poling process. This is reasonable because the polarization of ferroelectric single crystals with single electric domain is independent on the external electric field. The difference in polarization between single crystalline particle, poled and unpoled polycrystalline particles is shown schematically in the insets of Figure 4d. The experiment confirms that the ferroelectric polarization of the individual ferroelectric particle plays the key role in influencing the piezocatalytic property. The piezocatalytic activity can be enhanced by improving either the spontaneous polarization of the single crystals or the total polarization of polycrystalline particles. The dependence of degradation efficiency on the amount of piezocatalyst is shown in Figure 5a-b. The relative concentration C/C0 of MO over different addition of piezocatalysts are plotted in Figure S3 in the Supporting Information. The optimal concentration of BaTiO3 is around 1 g L-1. Further increase in the amount of catalysts has led to the reduced piezocatalytic efficiency. The result is consistent with a previous report, in which the decreased piezocatalytic efficiency is attributed to higher probability of the collisions between particles and particles, which may lead to the neutralization of surface charges.35 In addition, a higher solid loading possibly leads to a reduced absorption density of the MO dye at the particle surfaces in a BaTiO3 dispersion. This in turn results in a reduced surface-charge density at the BaTiO3 particles in an aqueous dispersion and, possibly, enhances degree of agglomeration. Besides the MO dye molecules, other organic dyes such as methylene blue (MB)



and Rhodamine B (RhB) can also be degraded by piezocatalytic process (Figure 5c-d). The relative concentration C/C0 of MO, RhB and MB over BTO-800 are plotted in Figure S4 in the Supporting Information. The rate constant of MO degradation is much higher than that of MB and RhB degradations. The phenomenon can be explained by the different charge types of dye molecules, which usually show a great influence on their adsorption ability on catalyst surface. It is reported the BaTiO3 nanoparticles are positively charged in near-neutral pH environment.36 Thus, BaTiO3 is easily capable of adsorbing anionic MO ions but difficult to adsorb cationic MB and cationic RhB, resulting in a higher piezocatalytic activity toward MO. For practical applications, the reuse of a piezocatalyst should also be considered. To evaluate the stability and recyclability of BaTiO3, a recycling utilization of the BTO-800 nanoparticles for the degradation of MO was performed. As shown in Figure S5 in the Supporting Information, there is no significant decrease in piezocatalytic activity after being recycled for eight times. The results indicate the excellent stability and recyclability of BaTiO3 nanoparticles, which is beneficial for its long-term use in the degradation of organic pollutants. To reveal the mechanism in piezocatalytic process, a series of controlled experiments were performed by introducing tert-butyl alcohol (TBA), benzoquinone (BQ), and ethylene diamine tetra-acetate dehydrate (EDTA-2Na) additives as scavengers for ⋅OH , ⋅O −2 radicals and holes ( h + ) into the piezocatalytic reaction system.37 The relative concentration ratios C/C0 of MO over BTO-800 with different scavengers are plotted in Figure S6 in the Supporting Information. As shown in Figure 5e-f, the degradation efficiency of MO was remarkably suppressed by the addition of these


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scavengers. The inhibition performance for the degradation of MO follows the order TBA > BQ > EDTA-2Na. The results suggest that the piezocatalyst allows in-situ generation of ⋅OH , ⋅O −2 radicals and holes, which further reacted with dye molecules to stimulate the degradation process. Among these effective radicals, ⋅OH is the dominant reactive species, ⋅O −2 has some positive effect, and h+ is the minor reactive species in the piezocatalytic process. Spin trapping is a valuable fool for the study of free radicals. The ESR spin trapping with 5,5-dimethyl-1-pyrroline N-oxide (DMPO) was used to identify the reactive oxidation species. As shown in Figure 6, the ESR signals obtained with DMPO in aqueous and methanol dispersions of BaTiO3 correspond to hydroxyl and superoxide radicals, respectively. Evidently, no sign of radicals was detected before the ultrasonic treatment. After applying ultrasonic vibration for 2 min, the characteristic peaks of both DMPO- ⋅OH and DMPO- ⋅O −2 can be detected. The intensities of the ESR signals increase with the prolonged ultrasonic vibration. The results confirm that ⋅OH and ⋅O −2 radicals are the main oxidative species in the piezocatalytic system. Nanoparticles usually have strong adsorption capacity. It is necessary to know whether adsorption affects degradation result. For this purpose, the nitrogen (N2) adsorption/desorption isotherms were measured. All the isotherms show a single and very narrow hysteresis loop in the relative pressure range of 0.8-1.0 (Figure S7 in the Supporting Information). The obtained BET specific surface areas of BTO-25, BTO-400, BTO-800 and BTO-1200 is 13.5 m2 g-1, 21.1 m2 g-1, 4.7 m2 g-1 and 0.23 m2 g-1, respectively. The relatively high adsorption ability was obtained in BTO-25 and BTO-400, which however possess lower catalytic activity compared with BTO-800. Based on these



results, we believe that the adsorption of dye molecules is not the main reason for the degradation. A direct evidence to verify the dye degradation is provided by HPLC. Figure 7 exhibits the HPLC profile of the reaction products collected at 0 min, 80 min and 160 min. In the starting solution, only one peak (A) exists at retention time tR = 18.5 min corresponding to MO molecule.38 After 80 min of ultrasonic vibration, several additional peaks appear at shorter retention times (eg. peak B and C at 7.8 min and 9.4 min, respectively). Meanwhile, the intensity of the characteristic MO peak is greatly reduced. According to the literature,38 peak B and C belong to the species of m/z 290 (one methyl group left) and the parent molecule m/z 304, respectively. After 160 min of ultrasonic vibration, the MO signal completely disappears, leaving only weak peaks of the intermediate products. The above results indicate that the MO molecule has been decomposed to smaller molecules. On the basis of the experimental results, the mechanism of the piezocatalytic degradation can be described as follows. As schematically shown in Figure 8, the ultrasonic oscillation produces cavitation bubbles in the aqueous dispersion. When the bubble explodes, it produces a high pressure of up to 108 Pa,39 which acts on the adjacent BaTiO3 nanoparticles and leads to the piezoelectric effect.34 While a piezoelectric polarization (Ppz) is introduced by the compression stress, an internal piezoelectric field can be built. The small amount of ultrasound activated electron-hole pairs will be separated and attracted in opposite directions towards the catalyst's surfaces. Holes can directly react with the adsorbed organic molecules or trapped by OH − to generate ⋅OH radicals, and then arose the oxidative decomposition. On the other hand, the electrons


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will be trapped by dissolved oxygen, where ⋅O −2 radicals are likely to generate and trigger the reductive decomposition.

4. CONCLUSIONS In summary, a fundamental relationship between ferroelectric polarization and the piezocatalytic performance was demonstrated through structure modification by thermal annealing or enhancing the polarization of polycrystalline powders. The BaTiO3 nanoparticles were annealed at different temperatures, and the 800 oC annealed samples have the best catalytic performance. This is due to the formation of stronger tetragonal distortion in the sample and the maintenance of the single crystal morphology. Annealing at higher temperatures resulted in degraded piezocatalytic activity. The reason is attributed to the formation of polycrystalline structure by sintering. It was demonstrated that the ferroelectric polarization can significantly enhance the piezocatalytic activity through poling the piezocatalyst under electric field. Furthermore, a detailed analysis of piezocatalysis process has been performed through FEM stimulation. We believe that this study provides a clear understanding of role of ferroelectric polarization in piezocatalysis.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: The relative concentration ratios C/C0 of MO over different piezocatalysts, different poling electric fields, different addition of catalyst, different dye solutions, different radicals’ scavengers, nitrogen adsorption/desorption isotherm of the BaTiO3 samples, cycling measurement of piezocatalytic degradation and detailed description of the FEM



simulation are presented in the Supporting Information.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (N.Q.). *E-mail: [email protected] (D.B.). Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 51372281 and 51202298) and Natural Science Foundation of Guangdong Province, China (No. 2015A030311019).

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Figures

Figure 1. (a) XRD patterns. (b) FWHM and c/a of the BaTiO3 samples with respect to the annealing temperature. (c) Room-temperature Raman spectra of the original and the annealed BaTiO3 samples.



Figure 2. TEM, HRTEM images and SAED patterns of BaTiO3 crystallites: (a-c) unannealed BTO, (d-f) BTO-600, (g-i) BTO-800, (j-l) BTO-1000, (m-o) BTO-1200.


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Figure 3. Piezoelectric effect induced by ultrasonic vibration in fluid. (a) Schematic diagram of the piezocatalysis experiments. (b) Ultrasonic cavitation and the fluctuation of pressure due to the implosive collapse of a cavitation bubble. Distribution of (c) displacement, (d) electric potential and (e) current density in BaTiO3 nanocrystals from the selected phases (point 1 to 5) of impact force by COMSOL simulation.



Figure 4. Effect of BaTiO3 piezocatalysts on the degradation of MO. (a) Plots of ln(C0/C) versus the ultrasonic vibration time for the degradation of MO under different conditions. (b) The kinetic rate constant as a function of the annealing temperature. (c) Piezocatalytic degradation of MO using poled BTO-1200 as catalysts. (d) Comparison of rate constants between different catalysts. The insets are the schematic diagrams of ferroelectric domain structures in the corresponding samples.


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Figure 5. Plots of ln(C0/C) versus the ultrasonic vibration time and the kinetic rate constant under different experiment conditions: (a, b) different content of piezocatalyst; (c, d) different kind of dye solutions; (e, f) different free radical scavengers.



Figure 6. ESR spectra of radical adducts trapped by DMPO over BTO-800: (a) aqueous dispersion and (b) methanol dispersion under ultrasonic vibration.


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Figure 7. High performance liquid chromatograms monitored in full scan corresponding to MO solution after being degraded with different ultrasonic vibration time.



Figure 8. The proposed mechanism of piezocatalysis, which contains immigrations of electron-hole pairs and redox reaction.


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