MoS2 nanocomposites using a novel exfoliated-MoS2–MgCl Bi-supported Ziegler-Natta catalyst via in-situ polymerization

Accepted Manuscript Fabrication of Polyethylene/MoS2 nanocomposites using a novel exfoliated-MoS2– MgCl Bi-supported Ziegler-Natta catalyst via in-situ polymerization He-Xin Zhang, Eun-Bin Ko, Jae-Hyeong Park, Young-Kwon Moon, Xue-Quan Zhang, Keun-Byoung Yoon PII:

S0266-3538(16)31174-5

DOI:

10.1016/j.compscitech.2016.10.019

Reference:

CSTE 6550

To appear in:

Composites Science and Technology

Received Date: 8 September 2016 Revised Date:

14 October 2016

Accepted Date: 22 October 2016

Please cite this article as: Zhang H-X, Ko E-B, Park J-H, Moon Y-K, Zhang X-Q, Yoon K-B, Fabrication of Polyethylene/MoS2 nanocomposites using a novel exfoliated-MoS2–MgCl Bi-supported ZieglerNatta catalyst via in-situ polymerization, Composites Science and Technology (2016), doi: 10.1016/ j.compscitech.2016.10.019. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT Fabrication of Polyethylene/MoS2 Nanocomposites Using a Novel ExfoliatedMoS2–MgCl Bi-supported Ziegler-Natta Catalyst via in-situ Polymerization

He-Xin Zhang,a,b Eun-Bin Ko,a Jae-Hyeong Park,a Young-Kwon Moon,a Xue-Quan

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Zhang*b and Keun-Byoung Yoon*a

University, Daegu, 702-701 Korea

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a. Department of Polymer Science and Engineering, Kyungpook National

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b. Key Lab. of Synthetic Rubber, Changchun Institute of Applied Chemistry, Chinese Academy of Science, Changchun, 130022 China.

ABSTRACT A novel exfoliated-MoS2 (EMoS2)–MgCl bi-supported Ziegler-Natta catalyst was synthesized by the reaction of Grignard reagent and EMoS2, followed by

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anchoring with TiCl4. The tendency of the individual EMoS2 layers to aggregate was prevented by covering them with solid-state Ziegler-Natta catalyst. The resultant

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catalyst was applied to ethylene polymerization, forming polyethylene (PE)/EMoS2 organic-inorganic hybrid nanocomposites. The resultant products had a flake-shape

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morphology, with EMoS2 well dispersed in the PE matrix. In addition, the thermal stability and mechanical properties of PE were significantly enhanced by the introduction of EMoS2.

KEYWORDS Ziegler-Natta catalyst, MoS2, Polyethylene, Nanocomposite, In-situ polymerization

Introduction

ACCEPTED MANUSCRIPT In recent decades, polymers have become increasingly prominent in the field of organic–inorganic hybrids, which aim to combine the properties of polymers with those of inorganic fillers.[1-4] The organic polymer includes ease of processing, low weight, flexibility, extremely high versatility in materials

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design with regard to structural and architectural characteristics, and consequently, material properties. Inorganic materials exhibit superior thermal and mechanical behavior as well as unique optical, electrical, catalytic, and

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magnetic properties, especially when characterized by their nanoscale dimensions.

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Molybdenum disulfide (MoS2) is a two-dimensional (2D) layered inorganic material. The basic unit of MoS2 is composed of a molybdenum atom coordinated to six sulfur atoms, two layers of sulfur atoms forming a sandwich structure with a layer of molybdenum atoms in the middle. Each sulfur atom is

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coordinated to three molybdenum atoms within a single 2D MoS2 layer.[5-7] The MoS2 monolayer has been reported to have an extraordinarily high breaking strength (~23 GPa) and Young’s modulus (~300 GPa), which are

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higher than those of chemically reduced graphene.[8,9] Thus, these fascinating

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properties make exfoliated MoS2 (EMoS2) an attractive substitute in the fabrication

of

high

performance

organic–inorganic

polymer

nanocomposites.[10-13] Typically, EMoS2 are formed by charge transfer to the MoS2 layers after the insertion of lithium atoms between the MoS2 layers followed by hydration of the Li-intercalated MoS2.[14] The resultant EMoS2 is used as a filler for solution blending with polymer. Hu et al. reported a solution blending method to prepare EMoS2-based nanocomposites with poly(methyl methacrylate) (PMMA), polystyrene (PS), and poly(vinyl alcohol) (PVA).[15-

ACCEPTED MANUSCRIPT 17] The resultant polymer/EMoS2 nanocomposites exhibited enhanced mechanical properties, thermal stabilities, and fire resistance. Wang et al. suggested an in-situ polymerization approach to produce polyamide-6/EMoS2 nanocomposites.[18] The modified MoS2 significantly enhanced the thermal

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stability and mechanical properties of polyamide-6. Although the preparation of polar polymer-based EMoS2 nanocomposites has been successfully achieved, the dispersion of EMoS2 in non-polar polymers, such as two widely used

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plastics, polyethylene (PE) and polypropylene (PP), has rarely been reported.[19]

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According to the literature, MoS2 reacts easily with n-butyllithium, during which Mo4+ cations are reduced to Mo3+, and Li+ cations become intercalated between the MoS2 layers, in a process developed over 40 years ago.[14] Grignard reagents, RMgX (X=halide), are organometallic reagents that exhibit

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similar properties to n-butyllithium in some chemical reactions. Therefore, a Grignard reagent could easily react with MoS2 in the same manner as nbutyllithium. With this reaction in mind, we designed a novel EMoS2–MgCl bi-

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supported Ti-based Ziegler-Natta catalyst, in which possible aggregation

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between the individual EMoS2 layers will be prevented by the covered catalyst. Thus, during polymerization, the layered EMoS2 fillers will be well dispersed within the polymer matrix.

Experimental Materials Molybdenum disulfide (MoS2, ~6 µm, Sigma-Aldrich), n-butyllithium (2.5 M in hexanes, Sigma-Aldrich), n-butylmagnesium chloride (BuMgCl, 2.0M in

ACCEPTED MANUSCRIPT THF, Sigma-Aldrich), triethylaluminum (TEA, 1.0 M in hexane, SigmaAldrich), and titanium tetrachloride (TiCl4, >99%, Sigma-Aldrich) were used as received. Polymerization-grade ethylene was provided by Korea Petrochemical Ind. Co., Ltd., Korea. n-Hexane was distilled from sodium/benzophenone under

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N2 prior to use.

Exfoliation of MoS2

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Exfoliation of MoS2 was carried out according to the literature.[14] Typically, MoS2 (1 g) was placed in the autoclave and n-butyllithium (10 mL, 2.5 M in n-

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hexane) was added. The autoclave was heated at 90 °C for 12 h under an argon atmosphere, cooled to room temperature, and the product was filtered and washed several times with anhydrous n-hexane. The resultant MoS2 was vacuum-dried and immersed in distilled water (1 L) under ultrasonication at

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ambient temperature for 4 h to produce a colloidal suspension of EMoS2. The suspension was neutralized with dilute acid (1 M HCl), and the product was then washed with distilled water (3 × 1 L) and vacuum freeze-dried to obtain

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EMoS2 powder.

Preparation of EMoS2–MgCl-supported Ziegler-Natta catalysts The reactor was charged with EMoS2 (1 g) and THF (100 mL), and BuMgCl (20 mL) was added under stirring. The suspension was refluxed at 80 °C for 48 h under an argon atmosphere. The reactor was then cooled to room temperature and the product was filtered, washed with THF (3 × 100 mL) and anhydrous nhexane (5 × 100 mL). The resulting EMoS2–MgCl bi-support was suspended in n-hexane (200 mL) under ultrasonication for 60 min. Then, TiCl4 (10 mL) was

ACCEPTED MANUSCRIPT added dropwise to the EMoS2–MgCl suspension at room temperature, after which the temperature was increased to 80 °C and the suspension was refluxed for 4 h. The mixture was filtered to remove unreacted TiCl4 and washed several times with hot n-hexane. The obtained powdery black catalyst was dried under

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vacuum at 60 °C for 3 h. The Mg and Ti contents in the resultant catalyst were determined by inductively coupled plasma-atomic emission spectroscopy (ICPAES) analysis (MoS2: 21.8 wt%, Mg: 9.7 wt%, and Ti: 9.5 wt%). For

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comparison, the catalyst was also synthesized in the absence of MoS2. Typically, BuMgCl (10 mL) and n-hexane (200 mL) were placed in a 500 mL

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reactor, then TiCl4 (10 mL) was added dropwise under an argon atmosphere at room temperature, after which the temperature was increased to 80 °C and the suspension was stirred for 4 h. The mixture was filtered to remove unreacted TiCl4 and washed several times with hot n-hexane. The obtained powdery

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yellow catalyst was dried under vacuum at 60 °C for 3 h. The Mg and Ti contents in the resultant catalyst were determined by ICP-AES analysis (Mg:

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2.2 wt% and Ti: 13.9 wt%).

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Ethylene Polymerization

Polymerization was performed in a 300-mL glass reactor equipped with a magnetic stirring bar. The reactor was back-filled three times with N2 and charged with the required amount of n-hexane. At the stipulated temperature, the reaction solution was stirred under ethylene (1 atm) for the desired period, before TEA cocatalyst was added to the reactor. After cocatalyst addition, the catalyst was injected into the reactor, and polymerization was initiated under a continuous feed of ethylene. Ethylene pressure was kept constant throughout

ACCEPTED MANUSCRIPT the polymerization using a bubbler. After 30 min, the polymerization was terminated by adding a 10% HCl-methanol solution. The mixture was poured into methanol (500 mL) to precipitate the polymer, which was collected and

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dried under vacuum at 60 °C until a constant weight was achieved.

Characterization

The Mg and Ti contents in the catalyst were determined by ICP-AES

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(PerkinElmer, Optima 7300DV). Scanning electron microscopy (SEM) images were taken on a JEOL JSM-6380LV microscope. The morphologies of the

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support and catalyst were studied using an optical microscope (ANA-006, Leitz, Germany) and recorded using a charge-coupled device (CCD) camera. X-ray diffraction patterns were obtained on a Philips X-Pert PRO MRD diffractometer equipped with Cu-Kα radiation.

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The melting temperature (Tm) of the obtained polymer was determined using differential scanning calorimetry (DSC; DSC131evo, Setaram) at a heating rate of 10 °C/min. The sample was heated to 200 °C and held in the molten state for

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3 min to eliminate any influence of thermal history. The polymer melt was then

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cooled to 30 °C at a rate of 10 °C/min. The melting point was determined in the second scan. Decomposition temperature analysis was conducted under N2 atmosphere by thermogravimetric analysis (TGA; Setaram Labsys evo) with a programmed heating rate of 10 °C/min from 30 to 800 °C. The tensile mechanical properties of the polymers were measured using a universal testing machine (Instron M4465). The sample sizes for the tensile drawing experiment were 5.0 × 75.0 × 1.0 mm3. The sample gauge length was 40.0 mm, and the crosshead speed was 50.0 mm/min.

ACCEPTED MANUSCRIPT Results and discussion

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EMoS2–MgCl-supported Ziegler-Natta Catalyst

Scheme 1. Preparation of EMoS2–MgCl bi-supported Ziegler-Natta catalyst and

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PE/EMoS2 nanocomposites.

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The preparation of the EMoS2–MgCl bi-supported Ziegler-Natta catalyst and PE/EMoS2 nanocomposites with well-dispersed inorganic MoS2 fillers is illustrated in Scheme 1. The EMoS2–MgCl bi-support was prepared by the reaction of a Grignard reagent (BuMgCl) with EMoS2, followed by treatment

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with excess TiCl4, to generate Mg/Ti catalyst species on the EMoS2 surface. Possible aggregation between the individual EMoS2 layers was prevented by the covered solid-state catalyst. The resultant EMoS2–MgCl bi-supported

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Ziegler-Natta catalyst was then applied to ethylene polymerization for the

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preparation of PE/EMoS2 nanocomposites.

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Figure 1. SEM images of (a) commercial MoS2, (b) EMoS2, (c) EMoS2–MgCl

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bi-supported Ziegler-Natta catalyst, and (d) marked region in (c).

The morphologies of commercial MoS2, EMoS2, and the EMoS2–MgCl bi-

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supported Ziegler-Natta catalyst were studied by SEM analysis. As shown in Figure 1a, a sheet structure and diameter of several micrometers was observed

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for commercial MoS2. Additionally, the commercial MoS2 fillers clearly contain a large number of tightly stacked MoS2 layers. In comparison with commercial MoS2, EMoS2 is composed of crumpled sheets, with some layers separated from the rest (Figure 1b). This demonstrated the successful exfoliation of commercial MoS2 into graphene-like nanosheets. For EMoS2– MgCl bi-supported Ziegler-Natta catalyst, a layered morphology was clearly observed (Figure 1c), in which the EMoS2 sheets were wrapped with solid-state

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catalyst without EMoS2 showed irregular particles (Figure S3).

Figure 2. XRD patterns of commercial MoS2, EMoS2, and EMoS2–MgCl/TiCl4

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catalyst.

To confirm successful exfoliation, XRD analysis was conducted on commercial

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MoS2, EMoS2, and the EMoS2–MgCl bi-supported Ziegler-Natta catalyst. As shown in Figure 2, an intense reflection at 2θ = 14.3° (corresponding to an interlayer distance of 0.62 nm) was observed for commercial MoS2, which was attributed to the (002) diffraction peak of MoS2. After exfoliation, the (002) diffraction peak at 14.3° was drastically reduced, and a new peak at 7.8° (corresponding to an interlayer distance of 1.13 nm) was observed. This indicated the successful exfoliation of MoS2. Additionally, the fact that the peak

ACCEPTED MANUSCRIPT at 14.3° had not completely disappeared demonstrated that a few stacked MoS2 layers remained after exfoliation. The remaining stacked MoS2 sheets could be ascribed to the reassembly of single MoS2 layers during the solvent removing process before analysis. Interestingly, for the EMoS2–MgCl bi-supported

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Ziegler-Natta catalyst, no obvious sharp diffraction peaks were observed, despite the intensity being magnified 1000 times. This result confirmed our hypothesis that possible aggregation of the EMoS2 layers was prevented by the

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surface covered solid-state catalyst.

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Preparation and Properties of PE/EMoS2 Nanocomposites

Table 1. Results of Ethylene Polymerization by BuMgCl–TiCl4 Catalyst and EMoS2– MgCl Bi-supported Ziegler-Natta Catalyst. Cat.

Cat. (mg)

[Al]/[Ti]

Activity (kg/mol-Ti•h)

EMoS2 (wt%)

Tm (°C)

Tc (°C)

Xc (%)

1

BuMgCl/TiCl4

100

50

17.3

-

133.2

116.2

52.5

50

100

50.5

0.44

135.8

116.8

52.7

100

50

32.3

0.68

135.6

117.6

54.4

200

25

20.2

1.09

135.5

117.8

55.1

300

17

13.5

1.64

135.1

118.2

54.9

500

10

9.1

2.42

134.8

119.4

55.2

3 4 5 6

EMoS2/MgCl/TiCl4

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2

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Entry

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Polymerization conditions: n-hexane (100 mL), TEA cocatalyst, 30 min, 1 atm, 40 °C.

The ethylene polymerization behavior of the EMoS2–MgCl bi-supported ZieglerNatta catalyst was evaluated after activation with the TEA cocatalyst. For comparison, the ethylene polymerization of the BuMgCl–TiCl4 catalyst was also studied. As shown in Table 1, the catalytic activity of BuMgCl–TiCl4 was much lower than that of the EMoS2–MgCl bi-supported Ziegler-Natta catalyst when the same catalyst weight was added (Entries 1 and 3). This could be due to the introduction of EMoS2 leading

ACCEPTED MANUSCRIPT to an increase in catalyst surface area, resulting in a more active sites being expose to the ethylene monomer dissolved in the polymerization medium. By controlling the catalyst feed weight and [Al]/[Ti] ratio, PE/EMoS2 nanocomposites with EMoS2

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contents of 0.44~2.42 wt% were obtained.

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Figure 3. Optical images of PE and PE/EMoS2 nanocomposites.

Due to the morphology replication characteristics of supported Ziegler-Natta

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catalysts, the polymer morphology could be controlled by modification of the

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catalyst morphology.[20-22] Therefore, the morphologies of the resultant PE and PE/EMoS2 nanocomposites directly mirror the catalyst morphologies. As shown in Figure 3, PE obtained using the BuMgCl–TiCl4 catalyst was comprised of irregularly shaped white particles, while the PE/EMoS2 nanocomposites had a flaked shape (1–3 mm) and homogeneous gray color. Additionally, no high-contrast black EMoS2 sheets were observed. Thus, we believe that the EMoS2 fillers were completely wrapped in PE and homogeneously dispersed in the PE/EMoS2 nanocomposites. 11

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Figure 4. Optical micrographs of (a) PE, and PE/EMoS2 nanocomposites with EMoS2 contents of (b) 0.44 wt%, (c) 0.68 wt%, (d) 1.09 wt%, (e) 1.64 wt%, and (f) 2.42 wt%.

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In order to characterize the dispersion of EMoS2 in the obtained PE/EMoS2 nanocomposites, both PE and PE/EMoS2 nanocomposites were hot-pressed into

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films. The polymer films were analyzed using an optical microscope in transparent mode, and the micrographs are summarized in Figure 4. The EMoS2

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fillers were found to be highly compatible with the PE matrix in the micrographs. Additionally, it was clear that the EMoS2 fillers were homogeneously dispersed in the PE/EMoS2 nanocomposites. By increasing the EMoS2 feed ratio, an increased EMoS2 density was observed. Good dispersion of the EMoS2 in the PE matrix was found for all PE/EMoS2 nanocomposites.

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Figure 5. XRD spectra of MoS2 and PE/EMoS2 nanocomposites with different

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amounts of EMoS2 fillers.

To confirm EMoS2 was present without aggregation in the PE/EMoS2

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nanocomposites, the resultant nanocomposites were characterized by XRD analysis; the XRD spectra are shown in Figure 5. The PE and PE/EMoS2 nanocomposites were

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characterized by diffraction peaks at 2θ = 21.4° and 23.8°, associated with the (110) and (200) reflections of PE, respectively. No conspicuous diffraction peaks, except for the crystalline diffraction peaks of the PE matrix, were observed, indicating that no aggregation of the EMoS2 had occurred in the PE/EMoS2 nanocomposites. Thus, we expected the PE/EMoS2 nanocomposites to exhibit excellent thermal and mechanical properties.

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ACCEPTED MANUSCRIPT The effect of EMoS2 fillers on the crystallization of PE/EMoS2 nanocomposites was characterized by DSC, and typical DSC curves are summarized in Figure S4. As shown in Table 1, PE produced using the BuMgCl/TiCl4 catalyst had a non-isothermal crystallization peak temperature (Tc) of 116.2 °C and a Tm of 133.2 °C. The Tc and

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Tm values of PE were shifted to higher temperatures after incorporating the EMoS2 fillers. The changes in Tm could be ascribed to interactions between the EMoS2 fillers and PE matrix, which restricted the motion of the PE chain.[23] The increased Tc

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resulting from the introduction of EMoS2 demonstrated that EMoS2 fillers could act as nucleating agents to induce PE crystallization. Naffakh et al. [24] reported a similar

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phenomenon for isotactic-PP/MoS2 nanocomposites produced by a melt mixing method, finding that MoS2 affected the crystallization of nanocomposites remarkably, which can be attributed to the nucleating effect of MoS2 on the monoclinic α-crystal

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form of PP.

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Table 2. Effect of EMoS2 Content on the Thermal Stability of PE/EMoS2 Nanocomposites. Td5% (°C)

Tdmax (°C)

Char yield (wt%)

-

399.8

469.2

0.9

0.44

424.6

482.7

1.6

0.68

424.4

483.3

3.4

1.09

427.9

482.6

6.0

1.64

433.7

485.2

7.1

2.42

451.9

488.7

8.7

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EMoS2 content (wt%)

Thermal stability is a crucial property of polymeric materials, as it is often the limiting factor for both polymer processing and end-use applications. Therefore, the thermal degradation of PE and PE/EMoS2 nanocomposites 14

ACCEPTED MANUSCRIPT containing different amounts of EMoS2 fillers were investigated by TGA under a N2 atmosphere. The results are given in Table 2, while TGA and derivative thermogravimetry (DTG) curves are shown in Figure S5. The addition of EMoS2 fillers was found to have a notable influence on the thermal behavior of

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PE. As shown in Figure S5, all TGA curves exhibited a single degradation process. In comparison with PE, the thermal degradation temperatures gradually shifted to a higher-temperature field with increasing EMoS2 content,

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inferring a significant improvement in the thermal oxidation stability of PE. Table 2 shows that degradation temperature at 5 wt% loss (Td5%) and

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decomposition temperature at the maximum weight-loss rate (Tdmax) of all PE/EMoS2 nanocomposites are higher than pure PE, and increase with increasing EMoS2 content. By incorporating EMoS2, the Td5% of the PE/EMoS2 nanocomposites increased to ~452 °C, which was ~50 °C higher than that of

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pure PE. The significant enhancements in PE thermal stability after the introduction of EMoS2 fillers could correspond with the good dispersion of EMoS2 in the PE matrix, which might act as an insulator between the heat

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source and the polymer surface where combustion occurs. In addition, the

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resultant PE/EMoS2 nanocomposites all produced higher char yields than that of PE (0.9 wt% at 600 °C). As shown in Table 3, the char yields of PE/EMoS2 nanocomposites with 0.44, 1.09, and 2.42 wt% MoS2 were 1.6, 6.0, and 8.7 wt% respectively. These results were not surprising, because Mo, as a transition metal element, can catalyze the char formation of polymers, and sulfur can improve the flame retardancy of polymers.[25,26] Additionally, EMoS2 has a layered structure, like montmorillonite (MMT) and graphene, which can

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ACCEPTED MANUSCRIPT improve the thermal stability of the polymer due to a physical barrier effect that retards the diffusion of degradation products, gases, and heat.

PE/EMoS2 Nanocomposites

Tensile Strength (MPa) 25.6±1

0.44 0.68 1.09 1.64 2.42

33.9±1 38.4±1 36.7±1 40.1±2 45.4±2

480±20

Elongation at break (%) 1000±80

510±20 625±20 610±30 680±20 760±30

1110±80 1220±80 1150±70 990±70 930±70

Modulus (MPa)

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Neat PE

EMoS2 content (wt%) -

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Various EMoS2 Contents.

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Table 3. Mechanical Properties of PE and PE/EMoS2 Nanocomposites with

Since the optical images and XRD results reveal that EMoS2 are fully

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exfoliated and well dispersed in the PE matrix, a considerable enhancement in mechanical performance of PE matrix will be expected by incorporating the EMoS2 sheets with large aspect ratio. Clearly, as shown in Table 3, the tensile

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strength, modulus, and elongation at break values of the resultant PE nanocomposites were significantly enhanced by even very low EMoS2

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nanofiller loadings. The maximum increase in tensile strength and modulus is 77% and 58%, respectively. Simultaneously, the elongation at break of the nanocomposites reduces with increasing EMoS2 loading. However, the maximum decrease in elongation at break values is less than 7%, while the elongation at break value is enhanced for the relatively lower EMoS2 loading samples. Hu et al. [27] reported that the largest increase in tensile strength, modulus and elongation at break values was 16.8%, 37.5% and 9.6% for PE/MoS2 nanocomposites that prepared from solution mixing method, which is 16

ACCEPTED MANUSCRIPT lower than our results. For the similar amount of EMoS2 filler (0.68 wt%) added, the increase in tensile strength, modulus and elongation at break values was 50.0%, 30.2% and 22.0%, respectively. These results indicated that the PE/EMoS2 nanocomposites obtained by in-situ polymerization had a

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remarkable stiffness-toughness balance.

Conclusions novel

EMoS2–MgCl-supported

Ti-based

Ziegler-Natta

catalyst

was

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successful synthesized through the reaction of a Grignard reagent and EMoS2,

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followed by anchoring with TiCl4. After in-situ polymerization of ethylene, we successfully fabricated PE/EMoS2 nanocomposites with well-dispersed EMoS2 nanofillers. The resultant PE/EMoS2 nanocomposites displayed enhanced thermal stability and mechanical properties compared with PE obtained using a

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MoS2-free catalyst system. With incorporation of EMoS2 filler, the Td5% increased by ~52 °C and the tensile strength, modulus, and elongation at break values were increased by 77, 58 and 22%, respectively. Thus, this work

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provides a facile approach to the production of high-performance PE with good

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thermal stability and an excellent stiffness-toughness balance.

ACKNOWLEDGMENT This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (NRF-2015R1D1A1A0161012). The authors would also like to acknowledge the financial support from National Natural Science Foundation of China (No. U1462124).

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