Crystal facets-predominated oxygen evolution reaction activity of earth abundant CoMoO4 electrocatalyst

Journal of Alloys and Compounds 781 (2019) 460e466

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Crystal facets-predominated oxygen evolution reaction activity of earth abundant CoMoO4 electrocatalyst Zailun Liu a, b, c, d, 1, Chen Yuan a, b, c, d, 1, Fei Teng a, b, c, d, * a Jiangsu Engineering and Technology Research Centre of Environmental Cleaning Materials (ECM), Nanjing University of Information Science & Technology, 219 Ningliu Road, Nanjing 210044, China b Collaborative Innovation Centre of Atmospheric Environment and Equipment Technology (CICAEET), Nanjing University of Information Science & Technology, 219 Ningliu Road, Nanjing 210044, China c Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution Control (AEMPC), Nanjing University of Information Science & Technology, 219 Ningliu Road, Nanjing 210044, China d School of Environmental Science and Engineering, Nanjing University of Information Science & Technology, 219 Ningliu Road, Nanjing 210044, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 4 September 2018 Received in revised form 17 November 2018 Accepted 3 December 2018 Available online 8 December 2018

The surface property of an electrocatalyst is crucial for oxygen evolution reaction (OER). In this work, we have successfully prepared CoMoO4 nanorods (NR) with {100} facets exposed mainly and CoMoO4 nanosheets (NS) with {010} exposed mainly via a facile hydrothermal route. It is found that although the CoMoO4 NS has a 7 times higher surface area (49.3 m2 g
1) than the CoMoO4 NR (7.0 m2 g
1), the CoMoO4 NR has a 5.7 times higher OER current density (8.93 mA cm
2 cat) than the CoMoO4 NS (1.56 mA cm
2 cat) at 550 mV. Its higher intrinsic OER activity is mainly attributed to the exposed {100} facets of CoMoO4 NR. Compared with {010} facets (0.41 Jm-2), the {100} facets with a higher surface energy (0.54 Jm-2) are more reactive, and the more Co atoms on (100) surface can provide more OER active sites. This work demonstrates that the control of active facets exposed is a promising way for the development of efficient OER catalysts. © 2018 Elsevier B.V. All rights reserved.

Keywords: Intrinsic activity Facets CoMoO4 Oxygen evolution reaction (OER)

1. Introduction In recent years, the increasing demand for power and energy has impelled the development of various clean, sustainable energy technologies [1e4]. In particular, hydrogen and oxygen production of water splitting is considered as a promising energy conversion technology to replace fossil fuels [5,6]. However, oxygen evolution reaction (OER) undergoes a complex four-electron oxidation process, thus resulting in sluggish kinetic process and requiring a high overpotential [7e9]. Precious metals such as ruthenium and iridium oxides are good OER catalysts, but their scarcity and high cost have seriously limited their industrial applications [10e12]. Hence, it is necessary to develop inexpensive, earth-abundant OER catalysts [9]. Up to now, earth-abundant 3d transition metals compounds (Mn, Fe, Co and Ni) are considered as promising catalysts for the replacement of precious metals due to their environmental

* Corresponding author. Jiangsu Engineering and Technology Research Centre of Environmental Cleaning Materials (ECM), Nanjing University of Information Science & Technology, 219 Ningliu Road, Nanjing 210044, China. E-mail address: [email protected] (F. Teng). 1 Z. Liu and C. Yuan contribute this work equally. https://doi.org/10.1016/j.jallcom.2018.12.026 0925-8388/© 2018 Elsevier B.V. All rights reserved.

friendliness and low cost. Amongst, cobalt-based OER catalysts have attracted tremendous interest for water oxidation, for example, cobalt phosphate [13], perovskite oxides [14,15], Co3O4 [16], Co3S4 [17], NiCo2O4 [18], CoMn2O4 [19], CoMoO4 [20e23], and so on. However, their electrocatalytic activities are still obviously lower than precious metal oxides. Thus, it is still a big challenge to develop highly active earth abundant OER electrocatalysts. It has been demonstrated that catalyst morphology has a significant influence on catalytic activity [24]. Over the past two decades, controllable synthesis of nanostructures has attracted an increasing interest [25e27]. Up to date, various CoMoO4 nanostructures, including nanorods [20], nanowires [21], nanospheres [28], nanosheets [23], have been synthesized and their electrochemical performances were investigated. However, the effect of crystal facets on the OER performance of CoMoO4 has not been reported so far. Thus, it is highly desirable to develop an efficient electrocatalyst from the viewpoint of crystallographic facets. In this work, we have successfully developed CoMoO4 nanorods (NR) with {100} facets exposed mainly and CoMoO4 nanosheets (NS) with {010} exposed mainly via a facile hydrothermal route. It is found that at 550 mV, the OER current density is 8.93 mA cm
2 cat for CoMoO4 NR, which is 5.7 times higher than that (1.56 mA cm
2 cat) for CoMoO4

Z. Liu et al. / Journal of Alloys and Compounds 781 (2019) 460e466

NS, although the CoMoO4 NS has 7 times higher surface area (49.3 m2 g
1) than the CoMoO4 NR (7.0 m2 g
1). Theoretical calculations show that the surface energy of {100} facets (0.54 J/m2) is higher than that of {010} facets (0.41 J/m2). As a result, their different electrocatalytic efficiencies have been mainly attributed to their different facets exposed, but not conventional BET area that decides the OER performances of CoMoO4, which has been reported before. 2. Experimental 2.1. Materials preparation 2.1.1. CoMoO4 nanorods (NR) The CoMoO4 nanorods were synthesized by a simple hydrothermal method. In detail, 1 mmol Co(NO3)2$6H2O was dissolved in 30 mL distilled water, then the solution was stirred at room temperature for 10 min by a magnetic stirrer. After 1 mmol of Na2MoO4$2H2O was added to the solution above, the mixture was stirred for 30 min to form a solution. Then, the solution was transferred to a 50-mL Teflon-lined autoclave and was heated at 150  C for 5 h. After the reaction completed, the sample was collected and washed with distilled water for several times and dried at 60  C for 24 h. Finally, the dried products were placed in a quartz tube and calcined at 500  C for 3 h in a tube furnace.

461

by a thin catalyst film was used as the working electrode. Typically, the working electrode was prepared as follows, 4 mg of measured catalyst materials and 30 ml Nafion solution (5 wt-%, Du Pont) were dispersed in 1 ml water-ethanol mixture solution with the volume rate of 3:1, sonicated for 1 h to form a homogeneous ink. Then 2 ml of the homogeneous ink was pipetted onto the glass carbon electrode and dried under room temperature. And final the catalyst loading on the surface of the glass carbon electrode is 0.1 mg/cm2. The oxygen evolution activities of different catalyst materials were recorded in 1 M KOH (PH 13.6) electrolyte at room temperature with a scan rate of 5 mV s
1. All of the electrochemical measurements were iRcompensated. The potentials of LSV curves and Tafel plots are reported vs the reversible hydrogen electrode (RHE), based on the Nernst equation. According to the following calculation: E (vs RHE) ¼ E (vs Ag/AgCl) þ EºAg/AgCl þ 0.0592 pH. The electrochemical active surface area of different catalyst materials was estimated by measuring the double-layer capacitances based on the electrochemical method reported on literature [29]. The cyclic voltammorgrams (CVs) were obtained with different scan rates from 20 to 120 mV s
1 in the potential range of 0.1e0.3 V (vs. Ag/AgCl). The stability of catalysts was measured at a scan rate of 100 mV s
1 for 2000 cycles. The electrochemical impedance spectroscopy (EIS) data were measured with frequencies range from 100 kHz to 0.1 Hz, and the impedance data were fitted to a simplified equivalent circuit, obtained the series and charge-transfer resistances.

2.1.2. CoMoO4 nanosheets (NS) CoMoO4 nanosheets can be easily prepared through a simple hydrothermal route. Typically, 1 mmol Co(NO3)2$6H2O was dissolved in 30 ml distilled water, then the solution was stirred at room temperature for 10 min by a magnetic stirrer. After 1 mmol of Na2MoO4$2H2O and 1 mmol of urea were added to the above solution, the mixture was stirred for 30 min to form a solution. Then, the solution was transferred to a 50 ml Teflon-lined autoclave and was heated at 120  C for 24 h. After the reaction was complete, the sample was collected and washed with distilled water several times and dried at 60  C for 24 h. Finally, the dried products were placed in a quartz tube and calcined at 500  C for 3 h in a tube furnace.

All calculations were carried out using the density of functional theory (DFT) with the exchange-correlation function described by GGA-PBE [30]. A (1  1) super cell of CoMoO4 was used and enabled by the CASTEP package in Materials Studio, in which the projector augmented wave (PAW) method represented the electron-ion interaction with a kinetic energy cutoff of 300 eV after convergence calculation [31,32]. During optimizations, the energy and force converged to 2  10
6 eV/atom and 10
3 eV/Å, respectively. The vacuum in all the models was kept at 12 Å.

2.2. Materials characterization

3. Results and discussion

The fine surface structures of the samples were characterized by high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2100F), equipped an electron diffraction (ED) attachment with an acceleration voltage of 200 kV and an energy dispersive X-ray spectrometer (EDS). The crystal phases of the samples were characterized by X-ray diffraction (XRD, Rigaku D/max-2550VB) with graphite monochromatized Cu Ka radiation (l ¼ 0.154 nm), operating at 40 kV and 50 mA at a scan rate of 7 min
1. X-ray photoelectron spectroscopy (XPS) measurements were carried out on a VG ESCALAB MKII XPS system used Mg KaX-rays as the excitation source with a voltage of 12.5 kV and power of 250 W, the C1s peak at 284.8 eV of the surface adventitious carbon used as the reference standard for all the binding energies. Nitrogen sorption isotherms were performed at 77 K and <10
4 bar on a Micromeritics ASAP2012 gas adsorption analyzer, each sample was degassed at 160  C for 6 h before measurements, and the surface area was calculated by the Brunauer-Emmett-Teller (BET) method.

CoMoO4 NR or NS could be easily prepared through a facile hydrothermal route combined with the subsequent thermal treatment, and the experimental details are placed in electronic supporting information (ESI). As shown in Fig. 1a, all the strong diffraction peaks of the samples are in good agreement with the standard cards (JPCDS: 21e0868), demonstrating the formation of phase-pure CoMoO4. Moreover, we have calculated the intensity ratios of (400)/(003) peaks for CoMoO4 NR and (040)/(400) peaks for CoMoO4 NS. The intensity ratios of (400)/(003) peaks for CoMoO4 NR is 0.87, which is lower than 1.00 of bulk CoMoO4. This suggests that the crystal of CoMoO4 NR is dominated by {100} planes. For CoMoO4 NS, the intensity ratios of (040)/(400) peaks is 0.83, which is lower than 2.00 of bulk CoMoO4. This result suggests that the crystal of CoMoO4 NS is dominated by {010} planes. It is well known that CoMoO4 has a monoclinic structure, in which Co and Mo atoms occupy octahedral and tetrahedral sites [22], respectively (Fig. 1b). Interestingly, the edge-sharing CoO6 octahedra are interconnected by MoO4 tetrahedra to constitute a 3D structure, which is beneficial for ion diffusion [33]. Besides, the X-ray photoelectron spectroscopy (XPS) was employed to reveal the elemental composition and chemical states. The survey XPS spectra of CoMoO4 NR and CoMoO4 NS reveal the existence of Co, Mo, C, and O elements (Fig. S1, seeing ESI). The Co 2p XPS spectra (Fig. 1c) of CoMoO4 NR and NS show two major peaks at a low combining energy of 780.8 eV and a high combining energy of

2.3. Electrochemical measurements Electrochemical measurements were carried out on a standard three-electrode electrochemical cell using CHI 660D electrochemical working station at room temperature. Pt wire was used as counter electrode and Ag/AgCl (saturated KCl-filled) as the reference electrode. A commercial glass carbon with a diameter of 3 mm covered

2.4. Theory calculations

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Fig. 1. (a) XRD patterns of CoMoO4 nanorods (NR) and nanosheets (NS); (b) crystal structure of CoMoO4; (cee) XPS spectra of CoMoO4 NR and NS: (c) Co 2p; (d) Mo 3d; (e) O 1s.

797 eV, corresponding to Co 2p3/2 and Co 2p1/2, respectively; which is in good agreement with the reported results [34,35]. The peak fitting analysis of Co 2p shows that the chemical species of Co can be identified as Co2þ (780.6 and 782.0 eV) [36]. In the Mo 3d XPS spectra (Fig. 1d), we notice two peaks at 232.4 eV and 235.6 eV, corresponding to Mo 3d5/2 and Mo 3d3/2, respectively. The Mo 3d XPS spectra reveal the existence of Mo6þ [37,38] The O 1s peaks at 530.4 eV (Fig. 1e) indicate the existence of O2
species [39]. Furthermore, the CoMoO4 NR and CoMoO4 NS have been characterized by scanning electron microscopy (SEM) and highresolution transmission microscopy (HRTEM). Fig. 2a shows that CoMoO4 NR have the length of ca. 5 mm and the diameters of 200e500 nm, with the aspect ratios of ca. 10e25. Fig. 2b,c shows the HRTEM images of CoMoO4 NR. The interplanar spacings are determined to be 0.271 nm, corresponding to the (022) planes of CoMoO4 (Fig. 2c). This result indicates that CoMoO4 NR preferentially grow along the [011] direction. The selected area electron diffraction (SAED) pattern of CoMoO4 NR reveals its single-crystalline nature, and the (003) and (022) planes of CoMoO4 are determined (the inset of Fig. 2c). The cross-product of [001] and [011] is [100] direction (Fig. S2a, seeing ESI), meaning that the SAED image is obtained along the [100] direction of CoMoO4 crystal. On base of the above analysis, it can be concluded that the {100} facets are exposed for CoMoO4 NR. Besides, Fig. 2d shows that CoMoO4 NS are about 100 nm long and 10 nm thick; the HRTEM images (Fig. 2e and f) reveal that for CoMoO4 NS, the spacings of 0.348 nm can be assigned to (002) planes of CoMoO4, suggesting that CoMoO4 NS preferentially grow along the [001] direction. The SAED pattern of CoMoO4 NS (Fig. 2f insert) also reveals the single-crystalline nature, and the (002) and (400) planes of CoMoO4 are determined (the inset of Fig. 2f). The cross-product of [001] and [100] is [010] direction (Fig. S2b, seeing ESI), indicating that the SAED image is performed along the [010] direction of CoMoO4 crystal. On the basis of the above analysis, it can be concluded that the {010} facets are exposed for CoMoO4 NR.

Fig. S3 shows the pore size distribution and the nitrogen adsorptiondesorption isotherms of CoMoO4 NR and CoMoO4 NS. The BET surface areas are determined to be 7 m2 g
1 and 49.3 m2 g
1 for CoMoO4 NR and CoMoO4 NS, respectively. The pore volume is calculated to be 0.04 cm3 g
1 and 0.44 cm3 g
1 for CoMoO4 NR and CoMoO4 NS, respectively. And the pore size is around 4 nm and 215 nm for CoMoO4 NR and CoMoO4 NS, respectively. The OER activity of CoMoO4 NR and CoMoO4 NS for water oxidation were measured using a typical three-electrode system in 1 M KOH solution, in which the glassy carbon electrode (GCE) carried with the samples was used as the working electrode (seeing ESI). Catalysts were uniformly drop-casted on a GCE with a loading of 0.1 mg/cm2. Before test, the electrode underwent 60 cyclic voltammetric scans so as to reach a stable state, then OER activity was probed by linear sweep voltammetry (LSV) at a scan rate of 5 mV s
1. The polarization curves of the samples have all been normalized by BET area, as shown in Fig. 3a. In comparison with the CoMoO4 NS, the LSV curves of CoMoO4 NR exhibits higher current and onset current under a certain applied voltage, directly confirming that the CoMoO4 NR owns a higher intrinsic electrocatalytic activity for water oxidation. Fig. 3b shows the OER current densities at different overpotentials. The OER current densities over CoMoO4 NR are 0.38, 3.29 and 8.93 mA cm
2 cat at 350, 450 and 550 mV, respectively, which is much higher than those (0.12, 0.74, 1.56 mA cm
2 cat) of CoMoO4 NS. The LSV curves and OER current densities demonstrate that CoMoO4 NR is more efficient for water oxidation than the CoMoO4 NS. In addition, the catalytic activity of CoMoO4 NR surpasses those of Co related electrocatalysts (Table S1, ESI) [40e45]. To further insight into the OER activity, Tafel plots are shown in Fig. 3c, and the slopes are obtained by the Tafel equation. The Tafel slope is 72 mV/dec for CoMoO4 NR, which is obviously smaller than that (98 mV/dec) for CoMoO4 NS. The smaller Tafel slope illustrates that the OER process can rapidly occur over CoMoO4 NR electrocatalyst. The smaller Tafel slope can lead to a

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463

Fig. 2. SEM and HRTEM images of the samples: (aec) CoMoO4 NR; (dee) CoMoO4 NS.

Fig. 3. The electrocatalytic properties of CoMoO4 NR and NS: (a) Polarization curves.

remarkable increment rate of OER with overpotentials, which is beneficial for practical applications. To further assess the intrinsic activities of CoMoO4 NR and NS, their turnover frequencies (TOFs) at different overpotentials were calculated and plotted as a function of overpotential. The TOF value is calculated from the equation [46]:

TOF ¼

JA 4F m

J is the current density at an overpotential. A is the surface area of catalysts (based on BET). F is the faraday constant (96485 C/mol). m is the moles of active material deposited onto GCE. Herein, we calculated the TOFs on base of the assumption as follows: we assumed that all Co ions, as the active sites, can take part in the OER reaction. It is worth noting that in fact, not all Co ion in CoMoO4 is involved in the electrocatalytic reaction [9,46]. Hence, the as-

calculated TOFs represent the lowest limit of the active sites. As shown in Fig. 3d, CoMoO4 exhibits a linear increase in TOF with overpotential, and the CoMoO4 NS shows much higher TOF, in comparison to the CoMoO4 NR. For instance, at an overpotential of 0.55 V, the TOF (0.43 s
1) of CoMoO4 NS is much higher than that (0.35 s
1) of CoMoO4 NR. The higher TOF of CoMoO4 NS mainly attributed to its higher BET area. In general, the active site number of an OER catalyst is directly proportional to the electrochemically active surface area (ECSA) [17,36,47]. The ECSAs of the catalysts were compared using their electrochemical double-layer capacitance (Cdl), and the Cdl values of CoMoO4 catalysts are estimated by using a simple cyclic voltammetry method [17,47]. Fig. S4 (a,b) (seeing ESI) shows the cyclic voltammorgrams (CVs) at different scanning rates (20e120 mV s
1) in the potential range of 0.1e0.3 V (vs. Ag/AgCl). The differences in current density (DJ ¼ Ja-Jc) at a potential of 0.2 V (vs. Ag/AgCl)

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Z. Liu et al. / Journal of Alloys and Compounds 781 (2019) 460e466

Fig. 4. (a) Nyquist plots (the inset of equivalent circuit); (b) Polarization curves over CoMoO4 NS before and after the 1000 CV sweeps between 100 mV and 200 mV (vs. Ag/AgCl).

against scanning rates are plotted to get the slope that is equivalent to twice of Cdl. As shown in Fig. S4c (seeing ESI), CoMoO4 NR has a much higher Cdl of 0.9 mF/cm2cat, compared with CoMoO4 NS (0.37 mF/cm2cat). The Cdl is relative to ECSA, the current density divided by Cdl can reflect the intrinsic activity [48]. Usually, the electrocatalytic reaction kinetics can be revealed by electrochemical impedance spectroscopy (EIS) [49]. Fig. 4a shows the Nyquist plots of CoMoO4 NR and NS, and the inset of Fig. 4a presents the simulated equivalent electric circuit (EEC). The simulated EEC consists of an electrolyte solution resistance (Rs), a constant phase element (CPE) accounting for the electrical doublelayer capacitance, a charge transfer resistance (Rct) and a Warburg impedance (W) accounting for the diffusion resistance. In the Nyquist plots, the semicircle in the low-frequency range represents Rct, while Rs appears in the high-frequency range [49]. On one hand, the Rs values were measured to be 3.15 U and 2.42 U for CoMoO4 NR and NS, respectively. The low Rs values indicate that the electrolyte solution resistance is not susceptible to the surface condition of electrode. On the other hand, the Rct values were calculated to be

22.65 U and 13.48 U for CoMoO4 NR and CoMoO4 NS, respectively. The lower Rct value of CoMoO4 NS indicates that CoMoO4 NS has a much higher electron transfer ability than CoMoO4 NR. It is obvious that the conductivity order of CoMoO4 NR and CoMoO4 NS is not consistent with the their OER activity order, suggesting that the conductivity is not the crucial impact factor for the activity. Besides the OER activity, the durability is another critical parameter for a good electrocatalyst. Fig. 4b shows the OER polarization curves for 1000 sweeps. After 1000 sweeps, the final polarization curve of CoMoO4 NR nearly does not change, demonstrating that the CoMoO4 NR is a highly stable OER electrocatalyst. On base of the results above, only charge-transfer resistance and turnover frequency can be related to the BET area. However, the BET area can not be applicable to explain the difference of intrinsic activity between CoMoO4 NS and CoMoO4 NR. In order to clarify the underlying mechanism for the higher electrocatalytic efficiency of CoMoO4 NR, we further studied the surface structures of CoMoO4 through density functional theory (DFT) calculations. As shown in Fig. 5, both {100} and {010} facets were constructed with the same

Fig. 5. (a) Crystal cell, (b) (100) facet and (c) (010) facet of CoMoO4.

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Table 1 Calculated surface energies of different CoMoO4 facets. Facet

Thickness

Relaxed energy (eV)

Surface area (Ǻ2)

Surface energy (J/m2)

(100) (010) Bulk

2 2 /


37808.1202
37807.7625
37811.5606

50.83 73.80 /

0.54 0.41 /

Appendix A. Supplementary data

Table 2 The different atom number of CoMoO4. Facet

atom number

atom ratio

Co

Mo

O

Co/Mo/O

(100) surface (010) surface Bulk

2 2 8

4 2 8

2 4 32

4/8/4 2/2/4 1/1/4

thickness. Furthermore, each surface is a nonpolar surface and each side of the slab keeps atomic stoichiometry. Geometry relaxations were performed with the midlayers fixed. The surface energy (g) was calculated using the formula as follows [50].

g¼

Eslab
nEbulk 2A

where Eslab is the total energy of the slab, Ebulk is the total energy of bulk unit cell, n is the number of bulk unit cells contained in the slab, and A is the surface area of each side of the slab. The calculation results (Table 1) showed that the surface energy of {100} facets (0.54 J/m2) is higher than that of {010} facets (0.41 J/m2), indicating that the {100} facets should be more reactive than {010} facets. Furthermore, we have investigated the surface atom composition of (100) and (010) facets (Table 2). It was found that the Co atom number on (100) surface is more than that on the (010) surface, suggesting that the (100) surface have more Co active sites for OER evolution. On the other hand, oxygen atom of (100) surface is deficient. Therefore, we could cautiously speculate that the deficiency of oxygen atom on (100) surface may facilitate the adsorption of OH
on these sites. Thereby, the higher intrinsic activity of CoMoO4 NR can be mainly attributed to the two main factors as follows: 1) higher-energy {100} facets exposed; 2) more Co active sites at (100) facets. 4. Conclusions In conclusion, we have successfully prepared CoMoO4 NR with {100} facets exposed mainly and NS with {010} facets exposed mainly by a facile hydrothermal method. The obviously higher intrinsic OER activity of the CoMoO4 NR than NS was mainly attributed to the higher-surface-energy {100} facets exposed for NR. Compared with {010} facets, the higher-surface-energy {100} facets are more reactive and the more Co atoms on (100) surface can provide more OER active sites. Acknowledgements This work is financially supported by National Science Foundation of China (21377060), the Project of Science and Technology Infrastructure of Jiangsu (BM201380277), Scientific Research Foundation for the Returned Overseas Chinese Scholars of State Education Ministry (2013S002), the Key Project of Environmental Protection Program of Jiangsu (2013005), Six Talent Climax Foundation of Jiangsu (20100292),“333” Outstanding Youth Scientist Foundation of Jiangsu (20112015), and Jiangsu Science Foundation of China (BK2012862).

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