Large Piezoelectric Strain with Superior Thermal Stability and

Mar 7, 2018 - ... of Education & International Center for Dielectric Research, Xi'an Jiaotong University , Xi'an 710049 , China. ‡ Department of Che...
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Functional Inorganic Materials and Devices

Large Piezoelectric Strain with Superior Thermal Stability and Excellent Fatigue Resistance of Lead-free Potassium Sodium Niobate-Based Grain Orientation-Controlled Ceramics Yi Quan, Wei Ren, Gang Niu, Lingyan Wang, Jin Yan Zhao, Nan Zhang, Ming Liu, Zuo-Guang Ye, Liqiang Liu, and Tomoaki Karakit ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b01554 • Publication Date (Web): 07 Mar 2018 Downloaded from http://pubs.acs.org on March 7, 2018

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Large Piezoelectric Strain with Superior Thermal Stability and Excellent Fatigue Resistance of Lead-free Potassium Sodium Niobate-Based Grain Orientation-Controlled Ceramics Yi Quan1, Wei Ren*,1, Gang Niu*,1, Lingyan Wang*,1, Jinyan Zhao1, Nan Zhang1, Ming Liu1, Zuo-Guang Ye1, 2, Liqiang Liu3, Tomoaki Karakit3.

1

Electronic Materials Research Laboratory, Key Laboratory of the Ministry of

Education & International Center for Dielectric Research, Xi’an Jiaotong University, Xi’an 710049, China, 2

Department of Chemistry and 4D LABS, Simon Fraser University, Burnaby, British

Columbia, V5A1S6, Canada 3

Department of Intelligent Systems Design Engineering, Faculty of Engineering,

Toyama Prefectural University, 5180 Kurokawa, Imizu,Toyama 939-0398, Japan

KEYWORDS KNN-based ceramics, lead-free piezoelectric ceramics, large piezoelectric strain, thermal stability, RTGG. 1 ACS Paragon Plus Environment

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ABSTRACT

Environment-friendly lead-free piezoelectric materials with high piezoelectric response which is stable in a wide temperature range are urgently needed for various applications. In this work, grain orientation-controlled (with a 90% c-oriented texture) (K,Na)NbO3 (KNN)-based ceramics with large piezoelectric response (d33*) = 505 pm V-1 and a high Curie temperature (TC) of 247 oC have been developed. Such a high d33* value varies by less than 5% from 30 oC to 180 oC, showing a superior thermal stability. Furthermore, the high piezoelectricity exhibits an excellent fatigue resistance with the d33* value decreasing within only by 6% at a field of 20 kV cm-1 up to 107 cycles. These exceptional properties can be attributed to the vertical morphotropic phase boundary (MPB) and the highly c-oriented textured ceramic microstructure. These results open a pathway to promote lead-free piezoelectric ceramics as a viable alternative to lead-based piezoceramics for various practical applications, such as actuators, transducers, sensors and acoustic devices in a wide temperature range.

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Introduction Piezoelectric materials have recently attracted great attention due to their capability to transform a mechanical input into an electrical output or vice versa, which is therefore technologically relevant.1 They are used in a broad variety of devices, such as actuators, motors, sensors, accelerometers, transducers and acoustic devices, etc. 2, 3 In one of the widest applications of piezoelectric materials, actuators, which are technologically important in the fields of smart materials and microelectromechanical systems (MEMS), high strain from electromechanical coupling is required. Lead zirconate titanate Pb(Zr,Ti)O3 (PZT)-based piezoceramics are the widely used materials because of their excellent piezoelectric strain, high thermal stability and acceptable working life.4, 5, 6, 7, 8 Despite their great properties, the lead in PZT ceramics is a pollution source for our environment and health because of its toxicity. As a result, the worldwide legislations for minimizing the usage of the toxic materials, especially Pb and PbO in the end consumer products, have been implemented. 4 The research on lead-free piezoelectric materials to replace the Pb-based piezoelectric ceramics has been intensively carried out in recent years.6, 7, 8, 9, 10 Because of their high TC, large piezoelectric property, strong ferroelectricity and environment friendly chemical component, lead-free potassium sodium niobate (KNN)-based piezoelectric ceramics have become one of the most promising candidates to replace the PZT-based ceramics.11, 12 Saito et al. reported PZT-like large strain in grain orientation-controlled KNN-based lead-free ceramics using the reactive templated grain growth (RTGG) 3 ACS Paragon Plus Environment

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method. 13 Recently, superior piezoelectric coefficients of 570 and 490 pC N-1 have been reported by Wang et al. and by Wu et al.14, 15 respectively and high piezoelectric strain d33* = 470 pm V-1 has been shown by Zhang et al.16 700pC N-1 reported by Li et al.17 for KNN-based ceramics. However, some major problems in KNN-based ceramics still need to be addressed. Particularly, it is of great difficulty to improve their thermal stability while maintaining good piezoelectric properties. Furthermore, their fatigue resistance also requires further improvement. 9, 18

The excellent piezoelectric properties in the PZT system are due to its tetragonalrhombohedral morphotropic phase boundary (MPB). And their thermal stability of the piezoelectric parameters thanks to the MPB providing in a wide working temperature range, from room temperature to more than 200oC.19, 20 21 We named it vertical MPB.22 Therefore, a straightforward method to improve the piezoelectric properties and their thermal stability of KNN-based ceramics is to develop a vertical MPB in KNN-based systems. However, the MPB in the KNN-based ceramics is not similar to the classical MPB in PZT, the regular KNN MPB which shows a lower piezoelectric response than PZT between two monoclinic phases.23, 24, 25 Furthermore, During a heating process, the phase of pure KNN ceramics changes from an orthorhombic to a tetragonal phase at around 220 oC and shows a great piezoelectric response under the phase transition area, what we call it polymorphic phase transition (PPT).26 Then the phase transport

to a cubic phase at around 420 oC. Several KNN-based ceramics 4 ACS Paragon Plus Environment

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with enhanced properties have been developed and the main reason of their properties enhancement has been attributed to the shift of the tetragonal-orthorhombic PPT toward room temperature. 11, 27 Karaki et al. firstly pointed out that MPB between the tetragonal-rhombohedral of KNN-based ceramics can be found in the (K,Na,Li)NbO3-BaZrO3

(KNLN-BZ) binary system. More interestingly, this MPB

can be adjusted by adding a third component (Bi,Na)TiO3 (BNT) into the KNN-BZ system. By introducing BZ into KNLN, an inclined MPB can be found and thus the phase around the MPB changes from rhombohedral to tetragonal and then to cubic upon heating.22 By varying the third component BNT concentration, the MPB slope can be adjusted. In particular, the slope of MPB can be tuned into a vertical one in 0.915(K0.45Na0.5Li0.05)NbO3-0.075BaZrO3-0.01(Bi0.5Na0.5)TiO3.22, 28, 29

To further improve the piezoelectric properties of KNN-base materials, two approaches are usually adopted: the first one is the grain orientation-controlled process with the RTGG method, which is known as a “texturing process”. The texturing process can lead to the enhancement of macroscopic strains and polarizations due to domain switching.30 For example, Saito et al. c-oriented textured KNN-based ceramic and its d33*

developed the

is two times higher than

the non-textured ceramic.13, 31 The other approach consists in the reduction of the PPT temperature to be just above room temperature. 13, 32 However, this would cause a poor thermal stability for the properties. To solve this problem, a material with a 5 ACS Paragon Plus Environment

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vertical MPB is required. 22, 33 For the piezoelectric ceramics with the co-existing tetragonal-rhombohedral phases (i.e. MPB), with the switching of domains will be permited in the angle of 71o, 90o, 109o and 180o, it will be easy to pole and will show significant macroscopic strain. 30 An ideal method to further improve the piezoelectric properties of KNN-based ceramics would be the combination of these two approaches. In addition, from an application point of view, fatigue resistance of piezoelectric ceramics is of great importance. There are two major ways to improve the fatigue endurance of piezoelectricity. The first one is to keep the ceramic away from the MPB or PPT, which is not viable because it will result in a huge decrease in the piezoelectric response. 34, 35 Furthermore, some previous works reported that c-oriented rhombohedral ferroelectric materials show excellent fatigue resistance, while c-oriented rhombohedral ferroelectric materials demonstrate a much poorer fatigue resistance.36, 37, 38 Therefore, for the piezoelectric materials with co-existing tetragonal-rhombohedral phases, the alternative approach is to control the orientation of piezoelectric materials.

In this work, we report an effective solution to solve the above-mentioned issues by combining the texturing and vertical MPB approaches to improve the piezoelectric strain, thermal stability and fatigue resistance in the KNN-based lead-free piezoelectric ceramics. A large piezoelectric strain (d33*) of 505 pm V-1 and a high TC of 247 oC are obtained. Furthermore, the piezoelectric properties of the ceramic 6 ACS Paragon Plus Environment

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exhibits great thermal stability, with the value of d33* changing by is less than ± 5% in a temperature range from 30 oC to 180 oC. The ceramics also show great fatigue resistance with the d33* value remaining 469 pm V-1, and undergoing only little degradation within 6% after 107 cycles under a drive field of 20 kV cm-1. The lead-free piezoelectric ceramics developed in this work demonstrate a great potential for a wide spectrum of applications such as actuators and transducers that can operate in a wide temperature range.

Experimental Section The NaNbO3 template was synthesized by topochemical microcrystal method is two steps. Firstly, Bi2.5Na3.5Nb5O18 (BNN5) as synthesized from a mixture of Na2CO3 (99.8%), Nb2O5 (99.5%), and Bi2O3 (99%) under 1120-1180 oC for 2-6 hours with a temperature rising rate of 5 oC min-1 using molten NaCl (99.5%). Subsequently, NaNbO3 template particles were synthesized in molten NaCl from the BNN5 particles and Na2CO3 for 2-8 hours at 1020-1060 oC with a temperature rising rate of 5 oC min-1. (see Figure S3, S9, S10)

The textured ceramics of 0.865(K0.476Na0.471Li0.053)NbO3-0.119BaZrO3-0.016(Bi0.5Na0.5)TiO3 were synthesized by the RTGG method. The raw materials included K2CO3 (99%), Na2CO3 (99.8%), 7 ACS Paragon Plus Environment

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Li2CO3 (98%), Nb2O5 (99.5%), BaCO3 (99%), ZrO2 (99%), Bi2O3 (99%), and TiO2 (98%). After 15-hours ball milling in ethanol, the mixture was calcined at 800 oC for 2 hours with a temperature rising rate of 5 oC min-1. For the RTGG process, the matrix and templates were mixed in a 19:1 molar ratio with solvent, binder and plasticizer by ball milling for 3 hours to obtain tape-casting slurry, and laminated to fabricate green compacts. After being dried, the green compacts were punched and pressed at 400 MPa with diameter 8mm, thickness 1mm, and then were removed by heating the compacts at 600 oC for 6 hours with a temperature rising rate of 3 oC min-1 to remove the organics materials. Finally, the compacts were annealed at 1200 oC for 2 hours with a temperature rising rate of 3 oC min-1 in air to promote template growth. The electrode was painting in the size as well as the surface area of the ceramics and then heating at 600oC 1h with a temperature rising rate of 5 oC min-1.

The phase structure and the degree of texture were determined by a lab-based x-ray diffractometer (XRD, D/MAX-2400, Rigaku, Japan, Cu Kα radiation, 25oC). The temperature-dependent phases were determined by another x-ray diffractometer (XRD, D/MAX-2200, Rigaku, Japan, Cu Kα radiation). The surface and cross-sectional scanning electron microscopy (SEM) images were obtained by a field-emission SEM equipment (FEI Quanta, 250 FEG). The temperature dependences of the dielectric constant and dielectric loss were measured using a LCR meter (4980A, Aglient Technologies Inc, USA, 25oC). The piezoelectric strain, the P-E and S-E hysteresis 8 ACS Paragon Plus Environment

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loops (with a fixed frequency of 1 Hz at 30 kV cm-1, room temperature = 25oC) and the fatigue resistance (up to 107 cycles at 20 kV cm-1, 100 Hz, 25oC) were measured using a ferroelectric testing system (TF Analyzer 2000E, aixACCT, Germany) at various temperatures. The specimens were poled using a silicon oil bath at 60 oC and 30 kV cm-1 for 20 minutes. The piezoelectric coefficients were measured by a piezoelectric testing system (ZJ-1, CAS, China, 25oC). The piezo-response of the textured ceramics was characterized by a piezoresponse force microscopy (PFM, Dimension ICON, Brucker, USA, 20oC).

Results and discussion Figure 1a shows the unipolar piezoelectric strain of the textured KNLN-BZ-BNT ceramics measured at temperatures ranging from 30 oC to 180 oC under an external drive field up to 30 kV cm-1. It can be observed that in this temperature range the strain remains very stable, at ~1.6‰. The strain measurement has been repeated for 4 times (see Figure S2) and the results remain the same. The strain response is compared with that of the LF4T textured ceramics 13 and several well-known piezoelectric ceramics in Figure 1b, which shows the normalized d33* (d33*T/ d33*RT) as a function of measuring temperatures. Our ceramics (red) show a better thermal stability than that of LF4T (green) which changes by more than 5% once the temperature is above 80 oC. The variation of the d33* values of the KNLN-BZ-BNT 9 ACS Paragon Plus Environment

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textured ceramics in this work is less than ± 5% in the temperature range of 30 oC to 180 oC (within the gray shadow area in Figure 1b), which is better than several other typical piezoelectric ceramics, including the widely used PZT4 (light green), 0.92BNT-0.06BT-0.02KNN (navy blue), KNN-CZ (blue) and

KNN-BZ-BLT-Mn

(purple).13, 16, 26, 39

The textured KNLN-BZ-BNT ceramics also show a dramatically enhancement of the piezoelectric properties (see Table S1). The addition of the plate-like NaNbO3 (NN) template into the ceramics before sintering process induces highly c-oriented grains in the ceramics, as shown in the x-ray diffraction (XRD) pattern in Figure 2a. Compared to non-textured counterparts (see Figure S5), the textured ceramics show a relatively low (110)c intensity, but much higher intensities for the (001)c and (002)c reflections. This demonstrates that the specimen has a preferred crystallographic orientation along c. The degree of grain orientation defined as Lotgering’s factor (F00l), which is given by the following formula:40

F 00l =

P − P0 1 − P0

,

(1)

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where P is the Sum I (00l) /Sum I(hkl), and P0 is the Sum I0 (00l) / Sum I0 (hkl). Sum I is the summation of the peak intensities of the XRD pattern of the sample and Sum I0 is the summation of the peak intensities of equiaxed powder, respectively. The extracted F00l from Figure 2a is about 90%.

Figure 2b shows the contour map of the (200)c peak intensity of

the crushed

powder sample of the KNLN-BZ-BNT textured ceramics measured in the temperatures from 30 oC to 300 oC . The XRD patterns with the different temperatures can be found in Supporting Information (see Figure S7). In Figure 2b, it is evident that when the temperature is below the Curie temperature TC (~247 oC, marked by a dashed line), only one broad peak appears and there is no relative intensity changing throughout the temperature range of 30 oC to TC. According to the Bragg Law, a phase transition from MPB to rhombohedral will lead to only one sharp (200)c peak while a transition from MPB to tetragonal results in two separated sharp peaks. 22 Therefore, the observed results in Figure 2b show a vertical MPB from 30 oC to the Curie temperature. Thanks to this vertical MPB, the KNLN-BZ-BNT textured ceramics have the co-existing tetragonal and rhombohedral phases, thus avoiding the influence of PPT and possessing superior thermal stability.

The phase components

of the sample with increasing of temperature maintain stable with co-existing tetragonal and rhombohedral phases until the temperature reaches the Curie temperature. After that, the phase gradually changes to a cubic one. 11 ACS Paragon Plus Environment

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Figures 2c shows the phase image alone the c-direction of the KNLN-BZ-BNT textured ceramics measured by piezoresponse force microscopy (PFM). The light and dark areas in the image refer to the polarization towards up and down. Micro-sized domains organized into stripe-like patterns with clear domain walls are observed. And Figures 2d shows scanning electron microscopy (SEM) image of a thermally etched surface and a cross-sectional of KNLN-BZ-BNT textured ceramics, respectively. The extracted grain size is around 15-25 µm, which is much larger than that of the non-textured ceramics (1-2 µm, as shown in Figure S6).

Figure 3a exhibits the temperature dependences of the dielectric constant and dielectric loss of the KNLN-BZ-BNT textured ceramics under 1kHz frequency. In the measuring temperature range of 30 oC to 400 oC, the dielectric constant firstly increases from ~1600 to a maximum of ~4000 at the TC (~ 247 oC), and then decreases to ~1500 at 400 oC.

There is no evidence of additional phase transition on

both the dielectric constant and dielectric loss curves, suggesting a stable phase below the Curie temperature. Figure 3b shows the frequency dependences of the dielectric constant and dielectric loss at room temperature. It can be seen that the dielectric constant decreases from 1650 to 1360, while the loss tangent increases from 0.035 to

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0.053 as the frequency increases from 1kHz to 2MHz, showing a slight frequency dispersion of the dielectric properties in the measuring frequency range.

To characterize the thermal stability of the piezo-/ferroelectric properties, the temperature dependent polarization (P-E) and the bipolar strain (S-E) hysteresis loops were measured and the results are shown in Figure 3c-d, respectively. In Figure 3c, it can be seen that the remnant polarization (Pr) decreases from 12.5 µC cm2 -1 to 7.5µC cm2 -1 and maximum polarization (Pmax) decreases slightly from 20µC cm2 -1 to 17µC cm2 -1 with the temperature increasing from 30 oC to 180 oC. Similarly, Figure 3d shows that the bipolar strain around 1.6‰ of the S-E loops under 30 kV cm-1 shows little change between 30 oC and 180 oC, indicating an excellent thermal stability of piezoelectric strain in the KNLN-BZ-BNT textured ceramics.

From practical point of view, the majority of the targeted applications for piezoelectric ceramics, such as transducers and actuators, requires a long device life, namely, good fatigue resistance. Figure 4a shows the results of fatigue test of the piezoelectric strain of the KNLN-BZ-BNT textured ceramics up to 107 cycles under a unipolar drive of 20 kV cm-1. After 107 cycles, the strain level only drops from 1.6 ‰ to 1.5 ‰, representing a mere 6% decrease. Figure 4b-c, show the bipolar strain and polarization hysteresis loops displayed at various cycles. The asymmetries occurred in 13 ACS Paragon Plus Environment

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S-E loops during the fatigue cycles were caused by the accumulating of the charged in the boundaries of the domains. The charges form a local bias field which lead to the asymmetries in the hysteresis loops.41 Figure 4d exhibits that after 107 cycles, the d33* undergoes a small decrease of about 6% from the initial 501 pm V-1 to 469 pm V-1, and the Pr decreases from the initial 12.5 µC cm2 -1 to 10.1 µC cm2 -1. In contrast, under the same condition, the non-textured KNLN-BZ-BNT ceramics show a poorer fatigue resistance with d33* showing a larger decrease of about 9% from 250 pm V-1 to 228 pm V-1, and Pr decreases from 9.1µC cm2 -1 to 6.4 µC cm2 -1(see Figure S11). Similarly, the d33* of PZT PIC151 decreases more than 10% after 107 cycles under a drive field of 19.6 kV cm-1.42 The (Bi,Na)TiO3-BaTiO3 showed a much weaker fatigue resistance which decreased more than 50% after only 100 fatigue cycles.43 And the BiFeO3 showed a 40-50% decrease at switchable polarization after 1011 fatigue cycles.44 Therefore, the textured KNLN-BZ-BNT ceramics exhibit excellent fatigue resistance for their piezo- and ferro-electric properties.

Conclusion In conclusion, a large piezoelectric strain d33* = 505 pm V-1 with superior thermal stability was realized in the c-oriented textured KNLN-BZ-BNT ceramics. The achieved ceramics have a high Curie temperature of TC = 247 oC and small variation of d33* value (less than ± 5%) from 30 oC to 180 oC. Such excellent piezoelectric 14 ACS Paragon Plus Environment

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properties can be attributed to both the textured structure and vertical MPB. Furthermore, the textured lead-free KNLN-BZ-BNT ceramics also possess great fatigue resistance: after 107 cycles, the strain level only drops by 6%. These make them excellent candidates for piezoelectricity related applications such as actuators and transducers.

FIGURES

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Figure 1. a) Piezoelectric strain of the textured KNLN-BZ-BNT ceramics under a 3 kV mm-1 unipolar field measured at different temperatures 30 oC, 40 oC, 50 oC, 60 oC, 70 oC, 80 oC, 90 oC, 100 oC, 120 oC, 140 oC, 160 oC, and 180 oC. b) Comparison of the temperature dependence of the piezoelectric strain d33*T value of different piezoelectric ceramics, as normalized to its room temperature d33*RT value. The data for PZT4,13 LF4T,13 KNN-CZ,26 BNT-BT-KNN,39 KNN-BZ-BLT-Mn16 are taken from the respective references.

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Figure 2. a) XRD pattern of the textured KNLN-BZ-BNT ceramics. b) Contour plot of temperature-dependent (002)c diffraction for the textured KNLN-BZ-BNT ceramics after being crushed. The black dashed line marks the Curie temperature. (c) PFM image of the textured KNLN-BZ-BNT ceramics. (d) SEM image of a thermally etched a cross-sectional of the textured KNLN-BZ-BNT ceramics.

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Figure 3. Dielectric, ferroelectric and piezoelectric properties of the textured KNLN-BZ-BNT ceramics. a) Temperature-dependent dielectric constant (black curve) and loss tangent (red curve). b) Frequency-dependent dielectric constant and loss tangent. c) Ferroelectric P-E hysteresis loops, and d) piezoelectric strain S-E hysteresis loops displayed at 30 oC, 50 oC, 70 oC, 100 oC, 150 oC, and 180 oC, corresponding to the purple, blue, light green, green, orange and red curves, respectively.

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Figure4. a) Unipolar piezoelectric strain measured after 100, 102, 103, 104, 105, 106, and 107 cycles, respectively. b) Piezoelectric strain P-E hysteresis loops, and c) Ferroelectric S-E hysteresis loops, measured after the 100, 103, 106, and 107 cycles, corresponding to the of purple, light green, orange and red, respectively. d) d33* (black) and Pr (red) values measured after 100, 102, 103, 104, 105, 106, and 107 cycles of the textured KNLN-BZ-BNT ceramics.

ASSOCIATED CONTENT

Supporting Information. 19 ACS Paragon Plus Environment

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A supporting information includes additional experiment results is available.

AUTHOR INFORMATION Corresponding Author Wei Ren, Gang Niu, Lingyan Wang.

E-mail:

[email protected];

[email protected];

[email protected] Funding Sources National Natural Science Foundation of China (Grant No. 51332003, 51202184, 51602247 and 91323303) 111 Project” of China (B14040) NSF research Project of Shaanxi province of China (No. 2017JQ6003) Natural Sciences and Engineering Research Council of Canada (NSERC. Grant No. 2037730)

ACKNOWLEDGMENT This research was supported by the National Natural Science Foundation of China (Grant No. 51332003, 51202184, 51602247 and 91323303), the “111 Project” of China (B14040) and NSF research Project of Shaanxi province of China (No. 2017JQ6003). ZGY acknowledges the support from the Natural Sciences and Engineering Research Council of Canada (NSERC. Grant No. 2037730). 20 ACS Paragon Plus Environment

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