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Chlorine-Induced In-situ Regulation to Synthesize Graphene Frameworks with Large Specific Area for Excellent Supercapacitor Performance Yanyan Zhu, Huijuan Cui, Xin Meng, Jianfeng Zheng, Pengju Yang, Li Li, Zhijian Wang, Suping Jia, and Zhenping Zhu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b12677 • Publication Date (Web): 23 Feb 2016 Downloaded from http://pubs.acs.org on February 27, 2016

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ACS Applied Materials & Interfaces

Chlorine-Induced In-situ Regulation to Synthesize Graphene Frameworks with Large Specific Area for Excellent Supercapacitor Performance Yanyan Zhu,†,‡ Huijuan Cui,†,‡, Xin Meng,† Jianfeng Zheng† Pengju Yang,†,‡ Li Li,† Zhijian Wang†, Suping Jia,*,†and Zhenping Zhu*,† †State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taoyuan South Road 27, Taiyuan, 030001 China

‡University of Chinese Academy of Sciences, Beijing, 100049 China

ABSTRACT: Three-dimensional (3D) graphene frameworks are usually limited by a complicated preparation process and a low specific surface area. This paper presents a facile suitable approach to effectively synthesize 3D graphene frameworks (GFs) with large specific surface area (up to 1018 m2 g−1) through quick thermal decomposition of sodium chloroacetate, which are considerably larger than that of sodium acetate reported in our recent study. The chlorine element in sodium chloroacetate may possess a strong capability to induce in-situ activation and regulate graphene formation during pyrolysis in one step. These GFs can be applied as excellent electrode materials for supercapacitor and can achieve an enhanced supercapacitor performance with a specific capacitance of 266 F g−1 at a current density of 0.5 A g−1.

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KEYWORDS: Graphene, Framework, Supercapacitor, In-situ, Chlorine. 1. INTRODUCTION Graphene has elicited much scientific research as an ideal representative of 2D crystals because of its extraordinary electronic properties, large theoretical surface area (2630 m2g−1), and excellent mechanical flexibility1-5. To exploit these unique properties, initial studies focused on approaches to realize high-quality 2D graphene monolayers. To date, numerous methods have been developed to inhibit graphene π–π stacking, including mechanical cleavage1, epitaxial growth6-8, and chemical vapor deposition (CVD)9-12. Owing to its outstanding properties, graphene shows a promising potential for application in numerous

fields,

such

storage/conversion15-20.

as

in

sensors13,

electronics14,

and

energy

However, the application of graphene remains more

limited than expected based on theoretical data mainly because graphene is easily subject to irreversible agglomeration or restacking as a result of strong van der Waals forces among inter-graphene sheets. As a result, the accessible surface area is significantly reduced21, 22. Moreover, the poor connection among isolated graphene sheets could be regarded as building blocks to break the electron/ion

continuous

transport

pathway

and

seriously

inhibit

its

conductivity22, 23. Various effective methods have been presented to tune the geometry and shape of graphene sheets to 3D graphene architecture, such as foams, aerogels and sponges

24-35

. These porous framework architectures can

overcome the aggregation/restacking behavior of graphene sheets and increase 2 ACS Paragon Plus Environment

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the specific surface area. Therefore, 3D graphene frameworks (GFs) have been applied to numerous energy storage and conversion devices 33,34,36-40, because of their continuously interconnected porous structures, high electrical conductivity, and satisfactorily large specific surface area. The GFs synthesis process typically involves a template. For example, Chen et al.24 synthesized graphene foam through CVD method with porous nickel foam as a template. Choi et al.41 utilized polystyrene colloidal particles as a sacrificial template to fabricate macroporous graphene. However, the template removal stage is characterized by high production costs, limited mass production, and graphene loss. In addition, the template-free process has been proven effective for the preparation of porous graphene. For instance, Ruoff et al.20 performed KOH activation of microwave-exfoliated graphite oxide (GO) to prepare porous graphene paper. Kuang et al.42 prepared holey graphene through nitric acid oxidation and ultrasonic vibration of GO, followed by thermal reduction of porous GO. However, the preparation process consisted of multiple steps and required strongly alkaline or acidic solutions. Thus, the process was inherently dangerous and complicated. Therefore, a simple method must be developed to construct 3D GFs that can increase the practical specific surface area and promote electrolyte ion transport when used as electrode materials. Our group recently synthesized GFs successfully through the rapid decomposition of solid sodium acetate (NaAc) particles. The process is novel, simple, time-saving, and inexpensive43. However, the specific surface area of the generated GFs is low 3 ACS Paragon Plus Environment


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(220 m2 g−1). Consequently, developing facile synthesis processes to build 3D GFs that have a large specific surface area and incur low production costs to meet large-scale practical application requirements remains challenging. In the present paper, we report a template-free, facile one-step process to synthesize GFs with a large specific surface area of up to 1018 m2 g−1 through the rapid decomposition of sodium chloroacetate (NaAcCl). The chlorine element in sodium chloroacetate may play an important role in inducing the in-situ activation of graphene and generating pores during the formation process. Thus, the specific surface area of GFs is regulated. The synthesis approach is illustrated in Scheme 1. Owing to their connective porous structure channels and highly accessible surface area, the resultant GFs perform excellently as electrode materials in supercapacitors. The electrode based on GFs derived from NaAcCl (defined as GFs–NaAcCl) achieves a specific capacitance of 266 F g−1 at 0.5 A g−1, which is significantly higher than that of GFs (210F g−1 at 0.5 A g−1) obtained from NaAc (GFs–NaAc). Moreover, the GFs–NaAcCl electrode can retain 92.1% of its initial specific capacitance from a current density of 0.5 A g−1 to 20 A g−1.


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Scheme 1. The synthesis approach of GFs obtained from sodium acetate and sodium chloroacetate particles, respectively. 2. EXPERIMENTAL SECTION 2.1 Materials and methods In a typical synthesis procedure, 1.5 g of sodium chloroacetate was placed in a small ceramic boat, which was placed in a tubular quartz reactor. After the reactor was pre-heated under argon atmosphere to the desired temperature (1000 °C), the ceramic boat (which was originally positioned in the cool zone of the reactor) was moved to the desired temperature zone to facilitate the decomposition of the starting material by shifting the reactor. After one minute, the resultant black solids were shifted out, gradually cooled to room temperature, and then collected for direct analysis or further purification treatment. Finally, pure GFs were collected by washing the resultant black solids completely with abundant deionized water and drying at 60 °C for 24 h under vacuum conditions. The synthesis conditions and pyrolysis temperature for other organic acid sodium salts, including sodium acetate (NaAc), sodium dichloroacetate (NaAcCl2), and sodium trichloroacetate (NaAcCl3), were similar to that of NaAcCl. All the reagents were of analytic grade and used directly without further purification. 2.2 Characterization The morphologies of the sample were visualized through field emission scanning electron microscopy (JSM-7001F, operated at 10 kV), transmission 5 ACS Paragon Plus Environment


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electron microscopy (TEM; JEM-2100F, operated at 200 kV), and atomic force microscopy (AFM; Nanoscope 4 instrument). Raman spectrum was measured and collected at an excitation wavelength of 514.5 nm by a LABRAM-HR Raman system. X-ray photoelectron spectroscopy (XPS) measurements were implemented with an AXIS ULTRA DLD photoelectron spectrometer with an Al Kα X-ray source. X-ray diffraction (XRD) was conducted with a Bruker D8 Advance X-ray powder diffractometer with Cu Kα (λ=1.5406 Å) radiation. The nitrogen adsorption–desorption isotherms of the GFs samples were collected at 77.4 K with an ASAP 2020 (USA) instrument. Mass spectrometry (MS) was conducted with a mass spectrometer (Pfeiffer Vacuum OmniStar) that was directly connected to a synthesis reactor for in-situ analyses of the gases released during the reaction. 2.3 Electrochemical measurements Working electrodes were prepared by mixing 95 wt.% of GFs with 5 wt.% of polytetrafluoroethylene binder. The mixture (2 mg) was cast onto nickel foam current collectors (1 cm × 1 cm) to produce the electrodes. After drying at 60 °C for 10 h in a vacuum, the prepared electrodes were soaked overnight in electrolytes before electrochemical testing. Electrochemical tests were performed with a three-electrode system in 6 M KOH electrolytes, with Hg/Hg2Cl2 as a reference electrode and platinum as a counter electrode. Electrochemical performance was characterized by cyclic voltammetry (CV), galvanostatic charge/discharge, and electrochemical impedance spectroscopy 6 ACS Paragon Plus Environment

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(EIS) measurements. The CV tests were conducted at an electrochemical workstation (CHI660D). Galvanostatic measurements were carried on a galvanostatic charge/discharge device (LAND CT2001A). EIS measurements were performed on an electrochemical workstation (IM6ex, Zahner) within a frequency range of 100 KHz to 100 mHz and a perturbation amplitude of 10 mV. 3. RESULTS AND DISCUSSION Figure 1 shows the characterization results of the as-prepared products after the rapid decomposition of NaAcCl. These products were analyzed through XRD (Figure 1a). The NaCl crystals (JCPDS: 05−0628) are detected, unlike when NaAc materials are used as raw materials, the Na2CO3 are generated43. NaCl can easily be removed by washing the as-prepared products completely with distilled water at room temperature, and the obtained products (defined as GFs–NaAcCl) were analyzed through XRD and SEM. The XRD result of GFs– NaAcCl (Figure S1) shows the removal of NaCl after washing with water. As seen from the SEM image (Figure 1b), GFs–NaAcCl has an extensively interconnected framework structure with abundant open pores that is similar to that of GFs–NaAc (Figure S2), while the former appears to have richer networks structure than the latter, according to the SEM images in Scheme 1, Figure 1b and Figure S2. Figure 1c displays a thin film structure and a high-resolution TEM image (inset of Figure 1c), which reveals that GFs– NaAcCl are composed of a few layers. The AFM image (Figure 1d) also shows 7 ACS Paragon Plus Environment


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the thickness of GFs–NaAcCl (approximately 2.5 nm) and further implies that the GFs–NaAcCl consist of a few layers of graphene.

Figure 1. (a) X-ray diffraction pattern of products obtained from the rapid decomposition of the sodium chloroacetate at 1000 °C (b) SEM image of GFs-NaAcCl purified after washing with water, (c) TEM image of the GFs-NaAcCl, inset is a HRTEM image, and (d) A representative AFM image of the GFs-NaAcCl. In terms of the Raman spectrum (Figure 2a), GFs–NaAcCl show a sp2-hybrid-characterized G band, a defect-derived D band, and a broad 2D band, thus indicating the formation of graphitic carbon. This occurrence is similar to that observed in the spectra of graphene derived from NaAc and thermally or chemically reduced graphene oxides20, 21, 43-45. As illustrated in Figure 2b, the XPS analysis indicates that GFs–NaAcCl is composed of carbon atoms (91.5 at %) and approximately 8.5 at % oxygen atoms. The C 1s spectrum (inset of 8 ACS Paragon Plus Environment

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Figure 2b) for GFs–NaAcCl indicates that the four peaks centered at 284.4, 285.9, 287.4, and 289.0 eV are associated with the C=C, C−O, C=O, and −COOH groups, respectively46. The strong C=C peak reflects graphene with good graphitization, which agrees with the clear G peak detected in the Raman spectrum. In addition, the weak peak centered at 497.4 eV is assigned to Na Auger peak.

Figure 2. (a) Raman spectrum of the GFs-NaAcCl, (b) XPS survey spectrum of GFs-NaAcCl, and inset is the high-resolution C 1s spectrum of GFs-NaAcCl. Notably, GFs–NaAcCl has a Brunauer–Emmett–Teller (BET) specific surface area of 1018 m2g−1, which is significantly larger than that of GFs–NaAc (220 m2 g−1), as calculated through N2 absorption/desorption tests (Figure 3a). The aforementioned analysis suggests that GFs with large specific surface area can be obtained through the facile rapid pyrolysis of NaAcCl. The presence of chlorine in the reaction system is predicted to play a crucial role in regulating graphene formation.



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Figure 3. (a) Nitrogen adsorption/desorption isotherms curves and (b) pore size distributions deduced by means of the density functional theory method for the GFs derived from NaAc, NaAcCl and NaAcCl2. To investigate the effect of the chlorine element on the reaction process, raw materials, including NaAcCl2 and NaAcCl3, were processed under the same synthesis conditions. After the fast pyrolysis process, black solid products are derived from the NaAcCl2 materials. The XRD analysis results (Figure 4a) indicate that NaCl is generated in the pyrolysis powders. Figure 4b clearly shows that highly continuous and plentiful 3D network pore structures are obtained after washing the NaAcCl2 pyrolysis products sufficiently, similar to the NaAcCl products. The BET specific surface area of the NaAcCl2-based GFs (defined as GFs–NaAcCl2) is 534 m2 g−1 (Figure 3a). As per a comparison of the BET analysis results, the specific surface area of GFs–NaAcCl (1018 m2 g−1) is markedly higher than that of GFs–NaAc, whereas this area decreases to 534 m2 g−1 for GFs–NaAcCl2. Moreover, no graphene products are produced through the direct pyrolysis of NaAcCl3. The pore size distributions of GFs– NaAc, GFs–NaAcCl, and GFs–NaAcCl2 were analyzed through the density 10 ACS Paragon Plus Environment

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functional theory method to further identify the function of the chlorine element. As shown in Figure 3b, GFs–NaAcCl is basically composed of plentiful micropores (< 2 nm) and a small fraction of mesopores (2–3 nm). The quantities of mesopores (3–50 nm) and macropores in GFs–NaAc and GFs– NaAcCl are almost similar, however, the quantities of micropores and mesopores (2–3 nm) in GFs–NaAcCl are significantly higher than those in GFs–NaAc. As a result, the specific surface area of GFs–NaAcCl increases to 1018 m2 g−1. When chlorine content increased, the quantity of micropores in GFs–NaAcCl2 is less than that in GFs–NaAcCl, while the quantity of mesopores (10–50 nm) and macropores (50–100 nm) increased. Therefore, the specific surface area of GFs–NaAcCl2 is reduced to 534 m2 g−1. The results of the aforementioned BET specific surface area and pore size distribution analysis may be attributed to chlorine element, which may function as an in-situ activator of the graphene generated during NaAc pyrolysis and thus regulate the specific surface area of GFs. This theory is consistent with that of the chemical or physical activation process, in which weakly bonded carbon atoms are oxidized by the activation agent47-54. Specifically, when the Cl/C atomic ratio is 1/2 (NaAcCl), chlorine can initiate activation during the progress of carbonization and promote the generation of micropores, thereby increasing the specific surface area. With increasing chlorine content and Cl/C atomic ratio of 1 (NaAcCl2), chlorine may enhance graphene activation and induce the partial transformation of micropores into mesopores or macropores. Consequently, the 11 ACS Paragon Plus Environment


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specific surface area may decrease. Moreover, no graphene is generated when the Cl/C atomic ratio increased to 3/2 (NaAcCl3), suggesting that a high chlorine content may excessively activate all of the carbon. The generation of Cl2 or HCl gas in this process may activate the produced carbon, thereby producing voids of various sizes in the progress of carbonization47, 48, 55. This result can be visualized through MS analysis (Figure S3).

Figure 4. (a) X-ray diffraction pattern of products obtained from the rapid decomposition of NaAcCl2 at 1000°C, (b) SEM image of GFs-NaAcCl2 obtained from purified the pyrolysis products. Furthermore, graphene yield is an important parameter that is reflected in the carbon yield (proportion of the carbon content in graphene to the total content of carbon in organic sodium). The carbon yield for NaAcCl is roughly 30%, which is significantly higher than that of NaAc (approximately 3%). This phenomenon certainly suggests that chlorine plays a crucial role in regulating the assembly process of carbon atoms to graphene. The remarkable increase in carbon yield may be attributed to the following two factors. First, according to the MS results, the generation of HCl from NaAcCl can reduce the portion of carbon consumed, such as CH4 (Figure S4). Second, the pyrolysis products for 12 ACS Paragon Plus Environment

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NaAcCl and NaAc are NaCl and Na2CO3, respectively. Na2CO3 can decompose at 1000 °C (Na2CO3 = CO2 + Na2O), and the produced CO2 reacts with carbon (CO2= C+2CO), thereby consuming the generated carbon products56. By contrast, NaCl does not decompose at 1000 °C. The carbon yield of NaAcCl2 is approximately 25%, whereas NaAcCl3 does not produce carbon. This finding implies that chlorine not only facilitates the assembly of carbon atoms into graphene but also activates the graphene generation process. Therefore, the presence of an appropriate amount of chlorine in NaAc can facilitate the production of graphene with a large BET specific surface area and high carbon yield. GFs–NaAcCl is a particularly notable supercapacitor electrode material given its large specific surface area, rich oxygen functional group, and interconnected network structure; these features can promote electron/ion transport, facilitate specific

surface

area

accessibility,

graphene-based materials33,

41, 57-59

and

enhance

the

wettability

of

. CV and galvanostatic charge/discharge

measurements were obtained to investigate the capacitance performance of GFs–NaAcCl in a three-electrode device, which was compared with the GFs– NaAc sample. As depicted in Figure 5a, the CV curves for the GFs–NaAc and GFs–NaAcCl electrodes are rectangular at the scan rate of 50 mV s−1, which indicates the excellent charge storage capacity of the GFs-based electrode. According to the galvanostatic charge/discharge curves (Figure 5b), the GFs– NaAcCl modified electrode achieves a specific capacitance of 266 F g −1 at a 13 ACS Paragon Plus Environment


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current density of 0.5 A g−1, which is approximately 27% higher than that of the GFs–NaAc electrode (210 F g−1). The increased specific capacitance of the GFs–NaAcCl based electrode may be ascribed to the enlarged BET specific surface area as well as to the highly continuous network of open channels that facilitates efficient electron/ion transport and adsorbs additional electrolyte ions for capacitance formation60-62. The CV curves (Figure 5c) of the GFs-NaAcCl electrode retain a typical rectangle in increasing the sweep rate to 200 mV s-1. Furthermore, the GFs-NaAcCl electrode retains a specific capacitance of 245 F g−1 at a high rate of 20 A g−1.The retention rate is 92.1% at the initial specific capacitance of 266 F g−1 (Figure 5d). The galvanostatic charge and discharge curves at various current densities of GFs-NaAcCl are shown in Figure S5. These results suggest that the GFs–NaAcCl electrode exhibits excellent specific capacitance and high rate capability for application to supercapacitor58, 60.


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Figure 5. (a) CV curves of GFs-NaAc and GFs-NaAcCl based electrodes at a scan rate of 50 mV s-1 in 6 M KOH, (b) Galvanostatic charge and discharge curves at a current density of 0.5 A g-1 based on GFs-NaAc and GFs-NaAcCl, respectively, (c) CV curves of GFs-NaAcCl electrode measured at different scan rates in 6 M KOH, and (d) Specific capacitance of the GFs-NaAcCl based electrode obtained at various current densities. We further investigated the ion transport properties of the GFs–NaAcCl electrode through EIS analysis, and the resultant Nyquist curves are shown in Figure 6b. The series resistance (Rs), charge transfer resistance (Rct) at the electrode electrolyte interface, and Nernst diffusion impedance (ZN) were obtained (Table 1) by fitting the Nyquist curves with an equivalent circuit model (inset of Figure 6b). In the high-frequency region (same inset), the GFs–NaAcCl electrode has a lower Rct (0.35 Ω) than the GFs–NaAc material (0.45 Ω), thereby indicating an improved interface contact nature between the electrolyte ions and the electrode material. Hence, charge transfer is accelerated (Figure 6a) probably because GFs–NaAcCl has a large specific surface area and a unique 3D framework structure20,

60

. Moreover, GFs–

NaAcCl shows a smaller ZN (0.82 Ω) than that of GFs–NaAc (1.07 Ω), and GFs–NaAcCl is denoted by a sharp line in the low-frequency region, thus suggesting the improved electrolyte diffusion efficiency of the GFs– NaAcCl-based electrode.60 Relaxation time constant (τo), which represents the diffusion kinetics and relies on the porosity of electrode materials41,

60

, is 15

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extracted by plotting the imaginary part of the capacitance (Figure 6c) at different characteristic frequencies (f0). GFs–NaAcCl and GFs–NaAc display the f0 values of 0.89 and 0.53 Hz, respectively. The corresponding τo (=1/f0) is 1.12 s for the GFs–NaAcCl electrode, which is lower than that of the GFs– NaAc-based electrode (1.87 s). The shortened relaxation time of the GFs– NaAcCl-based electrode further suggests that the ion diffusion rate within this electrode is enhanced. These results imply that the specific capacitance of the GFs–NaAcCl electrode is enhanced, which can be ascribed to the enlarged ion accessible surface area and the accelerated ion transport in the framework channels of GF–NaAcCl20, 41, 60. Furthermore, the cycling life test conducted on the GFs–NaAcCl electrode indicates roughly 93.75% capacitance retention after 10,000 cycles at a current density of 6 A g −1 (Figure 6d), thereby demonstrating the excellent cycling stability of GFs–NaAcCl electrode materials.


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Figure 6. (a) Schematic of GFs as a working electrode material in the supercapacitor. (b) Nyquist curves of the GFs-NaAc and GFs-NaAcCl based electrodes. The inset shows the equivalent circuit model and the enlarged curves in the high-frequency region. (c) Imaginary capacitance vs. frequency plots of the GFs-NaAc and GFs-NaAcCl based electrodes. (d) Cycling stability of GFs-NaAcCl based electrode at a current density of 6 A g-1.

Table 1 Electrochemical impedance parameters for GFs-NaAc and GFs-NaAcCl based electrodes. CE GFs-NaAc GFs-NaAcCl

Rs (Ω)

Rct (Ω)

ZN (Ω)

0.67

0.45

1.07

0.66

0.35

0.82

τ (s) 1.87 1.12



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4. CONCLUSIONS In summary, we have reported a facile, suitable approach to effectively synthesize GFs with large specific surface area (up to 1018 m2g-1) through the fast pyrolysis of NaAcCl. Chlorine element performs a crucial function in activating and regulating graphene formation during pyrolysis. Therefore, GFs with large specific surface area can be generated by integrating graphene formation and the in-situ activation process in one step. The resultant GFs can serve as efficient supercapacitor electrode materials and enhance supercapacitor performance with a specific capacitance of 266 F g−1 at a current density of 0.5 A g−1. Moreover, the obtained GFs with large specific surface area are promising candidates for practical application in a broad range of fields, such as in solar cells, sensors, catalysis, lithium batteries, and fuel cells.

ASSOCIATED CONTENT Supporting Information. XRD pattern of GFs that was after washing fast pyrolysis sample of NaAcCl at 1000 oC. SEM image of GFs–NaAc. Mass spectrometry cycles obtained over the duration of heating for gas products detection during fast pyrolysis NaAcCl or NaAc at 1000 oC. Galvanostatic charge and discharge curves of GFs-NaAcCl at different current densities. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION 18 ACS Paragon Plus Environment

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Corresponding Author

*Tel: +86-351-4048715. Fax: +86-351-4048433. Email: [email protected]

*Email:[email protected]. ACKNOWLEDGMENT

The authors gratefully acknowledge the financial support from the Natural Science Foundation of China (Nos. 51202268), Shanxi Natural Science Foundation (Nos. 2013011013-4), Shanxi Natural Science Foundation (Nos. 2015021038)

and

Shanxi

Key

Scientific

and

Technological

Project

(Nos.MC2014-01). REFERENCES

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