Facile preparation of CoNi₂S₄@NiSe nano arrays on compressed nickel foam for high performance flexible supercapacitors

Abstract

CoNi₂S₄@NiSe nano arrays with hierarchical structure are in situ prepared on compressed nickel foam for a high performance flexible supercapacitor by a three-step solution-based method which involves a one-step solvothermal process and a two-step hydrothermal process. During the preparation process, the nickel foam serves not only as a support but also as the nickel source for NiSe nanorods, which is beneficial for improving the electronic conductivity and reducing the contact resistant of the active materials/current collector. The CoNi₂S₄@NiSe nano arrays with ample active sites and porous structure contribute to increasing the electrochemical performance. The flexible supercapacitor based on CoNi₂S₄@NiSe nano arrays was prepared, which exhibits a high areal capacitance of 312.95 mF cm⁻² at a scan rate of 5 mV s⁻¹ and good cycling stability with 97.59% capacitance retention after 1000 cycles. Meanwhile, it also exhibits excellent electrochemical stability which can maintain great capacitive performance even in the bending state. Therefore, the CoNi₂S₄@NiSe nano arrays with hierarchical structure on the compressed nickel foam could be a promising electrode material for high performance flexible supercapacitors.

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1. Introduction

In recent years, with rapidly increasing demands for high performance and flexible electronic devices, flexible energy storage technology has attracted great interest.^1–4 Supercapacitors, also known as ultra-capacitors, have received considerable attention due to their high power density, low maintenance costs, fast charge and discharge processes, and so forth.^5–11 According to the charge storage mechanism, the supercapacitors are divided into two different types: electrical double-layer supercapacitors (electrostatic energy storage mechanism) and pseudocapacitors (redox energy storage mechanism).¹² Due to the limited specific capacitance and energy density of electrical double-layer supercapacitors, pseudocapacitors have been attracted lots of interests.^8,13–16

Ni-Based materials, such as NiO,¹⁷ Ni(OH)₂ ^18,19 and NiS,^20–23 have been widely investigated as the electrode materials for supercapacitors owning to their low cost and nontoxic than RuO₂.^24,25 Attributed to the valence electronic configuration of Ni (3d⁸4s²) and the small difference in electronegativity between Ni (χ = 1.9) and Se (χ = 2.4), Ni and Se can form a variety of complexes with varied stoichiometries²⁶ with multiple oxide states, which indicate that they're beneficial for being the electrode materials for pseudocapacitor.²⁷ Compared with the conductive oxide nanostructures such as TiO₂,²⁸ SnO₂ ²⁹ and ZnO³⁰ nanorod arrays as the core materials, the NiSe nano arrays as the cores not only can provide effective electron transport path but also can contribute to capacitance.²⁷ In addition, the NiSe nanorod is believed as good electrode material for pseudocapacitor, it not only has multiple oxide states, but also can in situ grow on nickel foam with the nickel foam as nickel source, and the electrode materials exhibit excellent kinetics of charges and ions with short diffusion paths.²⁷ However, as the single-metal-component (Ni-based) materials, NiSe nanorod has relative low conductive activity and single morphology compared to the binary-metal-component.^31–36 CoNi₂S₄, as one of binary-metal-component materials with easy synthesis, high conductivity, ample active sites and abundant redox reactions,^37–39 can in situ grow on NiSe nanorods with specific morphology. It is believed that decorating NiSe nanorods with CoNi₂S₄ will benefit for preparing the high performance electrode materials for pseudocapacitors.

In this paper, with the aim of designing and fabricating high performance electrode materials for flexible supercapacitors, CoNi₂S₄@NiSe nano arrays in situ grown on the compressed nickel foam with hierarchical structure were synthesized. During the synthesis process, the nickel foam serves not only as a support but also as the nickel source for NiSe nanorods, CoNi₂S₄ nanosheets were further in situ grown on the NiSe nanorods. CoNi₂S₄@NiSe nano arrays with hierarchical structure have multiple oxide states and ample redox reactions to realize high performance energy storage. The electrode of CoNi₂S₄@NiSe nano arrays with abundant active sites and porous structure provide abundant ion transmission paths. In addition, the nickel foam maintains its flexibility after growing active materials, owing to the press process before active materials was grown on it. The electrochemical properties of CoNi₂S₄@NiSe nano arrays on compressed nickel foam were further studied as electrode materials for flexible supercapacitors.

2. Materials and experimental section

2.1 Materials

Selenium powder, ethylene glycol, ethanediamine, Ni(NO₃)₂·6H₂O, Co(NO₃)₂·6H₂O, Na₂S·9H₂O, nickel foam and hexamethylenetetramine (HMT) were purchased from Sigma-Aldrich. All chemicals were analytical grade, used as received without any further purification, and their aqueous solutions were prepared with deionized water.

2.2 Experimental

2.2.1 Preparation of flexible substrate. Flexible substrate is of great significant in fabricating the flexible device. In order to get the flexible nickel foam, we have disposed and saved nickel foam as follows: firstly, a piece of nickel foam (2 cm × 2 cm, 146.5 mg) was immersed in 5% NaOH and 5% Na₂CO₃ aqueous (remove the possible surface oil layer), 3 M HCl (neutralize the alkaline), acetone, ethanol, 6 M HCl (remove the possible surface oxide layer) and deionized water for 10 min in ultrasonic apparatus, respectively; secondly, the washed nickel foam was pressed as the strips current collector in the pressure of 3 MPa for ten seconds; last, the compressed nickel foam was saved in the ethylene glycol to prevent it from further oxidation (Fig. 1a). And the compressed nickel foam demonstrates remarkable flexibility, which benefits for preparing flexible electrode (Fig. 1b).

Fig. 1 Compressed nickel foam (a) the store state and (b) the bending state.

2.2.2 Preparation of NiSe nanorods. The growth of NiSe nanorods on compressed nickel foam has been realized through a typical one-step solvothermal process. 16 mL ethylene glycol solvent was added into 50 mL Teflon-lined stainless steel autoclave, and then 80 mg of the selenium powder was added into it with continuous stirring. About 16 mL of the ethanediamine solvent was then dropped into the selenium mixture with continuous stirring at room temperature for 30 min. After removing the magnetic stir bar, a piece of prepared nickel foam was immersed the reaction solution and maintained 140 °C for 24 h. After the constant temperature oven cooled to room temperature naturally, the NiSe nanorods on nickel foam were prepared. NiSe@nickel foam was washed with ethanol and deionized water alternatively several times, and dried at room temperature naturally. The mass loading of NiSe nanorods on the nickel foam is about 0.287 mg by subtracting the amount of nickel foam from the total of CoNi₂S₄@NiSe@nickel foam.

2.2.3 Preparation of CoNi₂S₄@NiSe nano arrays. The in situ growth of CoNi₂S₄ nanosheets on the NiSe nanorods was achieved by a facile two-step hydrothermal method. The 54.4 mg Ni(NO₃)₂·6H₂O, 109.36 mg Co(NO₃)₂·6H₂O and 157.44 mg HMT were dissolved with 75 mL deionized water in the beaker, and were stirred at room temperature for 30 min to get the light pink solution. And then the solution was transferred into a clean Teflon-lined stainless steel autoclave with the volume as 100 mL. NiSe@nickel foam was immersed in the reaction solution and maintained 95 °C for 8 h. After the Teflon-lined stainless steel autoclave cooled to room temperature naturally, the Ni–Co precursor on the NiSe@nickel foam was prepared. After washing with ethanol and deionized water alternatively several times, the sample was dried at room temperature naturally.

In the next step, 45.34 mg Na₂S·9H₂O was dissolved with 75 mL deionized water in the beaker, with continuous stirring at room temperature for 30 min, then the solution was transferred into a clean Teflon-lined stainless steel autoclave with the volume as 100 mL. NiSe@nickel foam with Ni–Co precursor was immersed in the reaction solution and maintained 120 °C for 8 h. After the Teflon-lined stainless steel autoclave cooled to room temperature, the novel 3D hierarchical structure of CoNi₂S₄@NiSe nano arrays electrode materials were prepared. After washing with ethanol and deionized water alternatively several times, the samples were dried at room temperature naturally. The mass loading of CoNi₂S₄@NiSe nano arrays is about 0.866 mg by subtracting the amount of nickel foam from the total of CoNi₂S₄@NiSe@nickel foam.

2.2.4 Flexible supercapacitor device fabrication. The CoNi₂S₄@NiSe@nickel foam (1 cm × 1 cm) electrodes were fabricated as the aqueous symmetric supercapacitor with sandwich structure, and 6 M KOH solutions was used as the electrolyte. After that, the device was encapsulated by polycarbonate film to avoid leakage of the electrolyte. And the similar device based on NiSe@nickel foam (1 cm × 1 cm) electrodes was prepared for comparison.

2.3 Materials characterization

The morphologies and structures of these samples were characterized by scanning electron microcopy (SEM, JEOL JSM-7600F), high resolution transmission electron microscopy (HRTEM, JEOL 2100F) and X-ray diffraction (XRD, SHIMADZU XRD-7000) by using Cu Kα (λ = 0.15406 nm) radiation. All electrochemical tests were carried out on a electrochemical workstation (CHI660E, Chenhua, Shanghai), including cyclic voltammetry (CV), galvanostatic charge–discharge (GCD) and electrochemical impedance spectroscopy (EIS). Cycling performance of present supercapacitor based on GCD were tested in Land battery testing system (CT2001A). All the cells areas were approximate to 1 cm × 1 cm.

Areal specific capacitance is concluded from the CV curves based on eqn (1), the gravimetric specific capacitance is calculated from the CV curves based on eqn (2), the gravimetric specific capacitance is calculated from the GCD curves based on eqn (3). E_D and P_D are the energy and power density of the supercapacitor, which have been calculated as shown in the eqn (4) and (5), respectively.

In these equations, C_A is the areal specific capacitance (F cm⁻²), I is the constant discharging current (A), and the value of is equal to the area of the CV curves, A is the total area of the electrode material (cm²), Δt is the discharge time (s), ν is the scan rate (V s⁻¹), ΔV is the potential window during the charge and discharge process (V), C_m is the gravimetric specific capacitance (F g⁻¹), m is the mass of active materials (g), V_max is the operation potential (V).

3. Results and discussion

Fig. 2 shows the procedure for preparation of CoNi₂S₄@NiSe@nickel foam electrode materials. Solvothermal and hydrothermal techniques were utilized to fabricate the composite electrode. First, the NiSe nanorods were grown on the compressed nickel foam, and the nickel foam serves not only as a support but also as the nickel source for NiSe nanorods. Then, the CoNi₂S₄ nanosheets were in situ grown on the NiSe nanorods. In order to maintain the flexible of nickel foam and improve the electrochemical performance of the electrode, nickel foam was pressed before the active materials was grown on it.

Fig. 2 Schematic diagram for in situ growing CoNi₂S₄@NiSe nano arrays on compressed nickel foam.

Fig. 3 shows the difference between the pressed nickel foam and unpressed one. Compared with the unpressed nickel foam,^40–45 electrode based on the pressed one is efficient in keeping its flexibility and improving the electrochemical performance, we can get the reasons as follow: in terms of flexibility, the pressed nickel foam become more compact, and more reactions happened on the surface of nickel foam, which reduce the inner reaction of nickel foam to maintain its flexibility (Fig. 3a and b); in terms of electrochemical performance, due to the electrochemical performance of supercapacitor is mainly decided by the active materials on the surface of substrate, and the pressed nickel foam make more reactions occurred on the substrate surface than the unpressed one, so that its performance was improved (Fig. 3c and d). Thus, the electrode based on the pressed nickel foam is efficient in keeping its flexibility and improving the electrochemical performance.

Fig. 3 Structures of the supercapacitors with (a) pressed nickel foam and (b) unpressed nickel foam as substrate, the CVs of supercapacitors based on (c) CoNi₂S₄@NiSe@nickel foam and (d) NiSe@nickel foam with pressed and unpressed nickel foam as substrate.

3.1 Morphology and structure characterization

The well-defined electrode materials of CoNi₂S₄@NiSe@nickel foam with hierarchical structure was verified by XRD (Fig. 4), SEM and HRTEM (Fig. 6). As shown in Fig. 4a, the crystal structure of NiSe was verified, which is made up of hexagonal NiSe phase (JCPDS 02-0892, 2θ = 29.8°, 32.8°, 44.4°, 49.9°, 59.7°, 61.2°, 69.1° attributed to the (100), (101), (102), (110), (103), (201) and (202) planes) (Fig. 4a). Then, the crystal structure of CoNi₂S₄ was verified (JCPDS 24-0334, 2θ = 31.4°, 38.4°, 47.2°, 50.4°, 68.8° belongs to the (311), (400), (422), (511) and (444) planes) (Fig. 4b), and the strong peaks marked with asterisks (Fig. 4a and b) were originated from nickel foam. The XPS spectra are shown in Fig. 5. Ni 2p_3/2 and Ni 2p_1/2 peaks appeared at 855.4 and 873.2 eV, respectively (Fig. 5a). The peak at 852.9 eV was corresponding to metallic Ni 2p, which came from unreacted nickel substrate. These peaks have two shake-up satellites at 861.7 and 880.2 eV, which indicated that Ni was in the Ni²⁺ oxidation state. In Fig. 5b, one pair of binding energies at 780.0 and 796.2 eV correspond to Co 2p_3/2 and Co 2p_1/2, respectively. Another pair appeared in the higher energy region around 781.4 and 801.2 eV. Two kinds of cobalt oxidation state: Co²⁺ and Co³⁺ can be confirmed from the two pairs of doublet which are characteristic of Co 2p and Co 3p.⁴⁶ As shown in Fig. 5c, two main peaks and one shake-up satellite are presented in the S 2p spectrum. The binding energy at 162.5 eV is typical of metal–sulfur bonds in the ternary metal sulfides, while the binding energy at 169.9 eV can be attributed to the sulfur ions with higher oxide state of S₄O₆²⁻ at the surface owing to partly oxidation of CoNi₂S₄.⁴⁷ The Se 3d features of NiSe shown in Fig. 5d, and the peak at 68.7 eV can be assigned to SeS_x.²⁷ These results were in good agreement with the XRD pattern. Through the SEM, we can see that NiSe was grown on nickel foam uniformly (Fig. 6a), and the NiSe nanorods (70–100 nm in diameter and several micrometers in length) were grown around the skeleton of nickel foam (Fig. 6b). Meanwhile, these nanorods were made up of many smaller nanorods (Fig. 6c), such as a channel was composed of many paths, which benefited for ion and charge transmission. Not only that, and CoNi₂S₄ nanosheets (several nanometers in thickness) were in situ growth on the NiSe nanorods (Fig. 6d–f), which will further add the active sites to improve the kinetics of charges and ions in the electrode. Fig. 6g presents the HRTEM images of an individual NiSe nanorod, it reveals that the NiSe has a lattice fringe with interplane spacing of 0.232 nm, corresponding to plane of NiSe. The HRTEM image of CoNi₂S₄@NiSe@nickel foam reveals that the CoNi₂S₄ nanosheets were grown around the NiSe nanorod (Fig. 6h). These results demonstrate that the CoNi₂S₄ nanosheets were successfully in situ growth on the NiSe nanorods, which is advantage of charges and ions transporting.

Fig. 4 XRD patterns of (a) NiSe@nickel foam and (b) CoNi₂S₄@NiSe@nickel foam electrode materials.

Fig. 5 XPS characterization of the CoNi₂S₄@NiSe nano arrays.

Fig. 6 SEM images of (a–c) NiSe@nickel foam and (d–f) CoNi₂S₄@NiSe@nickel foam, HRTEM images of (g) NiSe@nickel foam and (h) CoNi₂S₄@NiSe@nickel foam electrode materials.

3.2 Electrochemical measurements

To demonstrate the electrochemical performance benefits of CoNi₂S₄@NiSe@nickel foam, the symmetry flexible supercapacitor based on CoNi₂S₄@NiSe@nickel foam was tested by CV tests, GCD tests and EIS tests (Fig. 7a, d and f), and the same tests have been done on the flexible supercapacitor based on NiSe@nickel foam for comparison (Fig. 7b, e and g).

Fig. 7 CV curves of flexible supercapacitors based on (a) CoNi₂S₄@NiSe@nickel foam and (b) NiSe@nickel foam at different scan rates, (c) CV comparison image of flexible supercapacitors based on CoNi₂S₄@NiSe@nickel foam and NiSe@nickel foam at the scan rate of 5 mV s⁻¹; GCD curve of flexible supercapacitors based on (d) CoNi₂S₄@NiSe@nickel foam and (e) NiSe@nickel foam at different current density; (f) impedance comparison spectra of flexible supercapacitors based on CoNi₂S₄@NiSe@nickel foam and NiSe@nickel foam, the inset image shows the high frequency region of spectrum.

Fig. 7a and b show the typical CV curves of flexible supercapacitors with the potential window from 0 to 0.8 V at various scan rates ranging from 5 to 50 mV s⁻¹. The flexible supercapacitor based on CoNi₂S₄@NiSe@nickel foam show a higher specific capacitance than that of flexible supercapacitor based on NiSe@nickel foam, which is attributed to the CoNi₂S₄ nanosheets layer (Fig. 7c). CoNi₂S₄ nanosheets not only increase the active sites of the electrode materials, but also increase the kinds of redox reactions, thus resulting in improved capacitive performance. The possible reversible faradaic redox reactions during the cathodic and anodic sweeps between the NiSe@nickel foam and electrolyte, as well as between the CoNi₂S₄@NiSe@nickel foam and electrolyte were presented. Since the sulfur is in the same family with oxygen, the redox reaction of mechanism of the CoNi₂S₄ is similar to the CoNi₂O₄ in the alkaline electrolyte,^48–50 which can be described as the following equations:

Accordingly the redox reaction of nickel sulfide, cobalt sulfide and nickel selenide with OH⁻ can be illustrated as follows:^51–53

It was clear that these well-defined redox peaks can be explained as related with reversible faradaic reactions of MX/MXOH and MXOH/MXO (M = Ni and Co, X = S and Se). With increasing the sweep rates, the anodic peak current density increases and the cathodic peak current density decreases, which suggests that a relatively low resistance of the electrode and fast redox reactions at the interface of the electrode and electrolyte.³⁴

GCD tests within a potential window from 0 to 0.8 V were carried out for further evaluating the capacitive performance at the current densities of 3, 5, 8, 10 and 15 mA cm⁻² (Fig. 7d and e). The specific capacitance of the supercapacitor was calculated via the eqn (3). These results exhibit that the calculated specific capacitance decreases with the current density increasing, and capacitance values of flexible supercapacitor based on CoNi₂S₄@NiSe@nickel foam can get as high as 311.25, 216.88, 180, 156.25 and 136 mF cm⁻² at the applied current densities of 3, 5, 8, 10 and 15 mA cm⁻². These values are in agreement with capacitance values of CV curves and higher than that of flexible supercapacitor based on NiSe@nickel foam. The enhanced electrochemical performance could be attributed to the multiple oxide states and abundant redox reactions in the binary-metal composite as well as the ample active sites in the hierarchical structure.

To further evaluate the electrochemical behaviors of the flexible supercapacitors, the EIS measurements were carried out at a frequency ranging from 0.01 to 100 kHz. And the corresponding Nyquist plots of two supercapacitors are shown in Fig. 7f. In the low-frequency region, both of the flexible supercapacitors exhibit a more vertical line leaning to imaginary axis more than 45°, suggesting the more facile electrolyte ions diffusion to the active material and more ideal capacitor behavior, which further confirms that the electrode can remain good capacitive performances^54–56 In the high-frequency region, the internal resistance (R_s), including electrolyte resistance, active materials ohmic resistance and active materials/current collector' contact resistant, can be read from the intersection of the high-frequency part at the real axis of the pattern.⁵⁷ The R_s of flexible supercapacitors based CoNi₂S₄@NiSe@nickel foam is 0.5764 Ω, and that of flexible supercapacitors based on NiSe@nickel foam is 0.3743 Ω (Fig. 7f inset one). The favorable low value of R_s may be attributed to the conductive substrate and the in situ growth of the active materials on the compressed nickel foam. In the intermediate frequency region, there is a negligible semicircle, indicating a particularly low charge transfer resistance. According to the comparison image of the EIS (Fig. 7f), in the high-frequency region, no distinct semicircle shape is observed, demonstrating the fast charge transfer behaviour inside the electrode during its charge–discharge process, which benefit from the hierarchical structure. In the low-frequency region, the straight line of flexible supercapacitor based on CoNi₂S₄@NiSe@nickel foam lean more toward the imaginary axis, indicating the lower diffusion resistance. The reduced Faraday resistance rendered by the multiple oxide states and abundant redox reactions in the intriguing material combination and the ample active sites in the hierarchical structure lead to enhanced electrochemical reaction.

In addition, the cycling life and specific capacitance retention of the flexible supercapacitors are shown in Fig. 8a and b. Flexible supercapacitor based on CoNi₂S₄@NiSe@nickel foam (Fig. 8a) can maintain 97.59% and flexible supercapacitor based on NiSe@nickel foam (Fig. 8b) can keep 89.73% after 1000 cycles of CV at the scan rate of 50 mV s⁻¹. Areal specific capacitance vs. scan rates is plotted (Fig. 8c), the areal specific capacitance of flexible supercapacitors based on CoNi₂S₄@NiSe@nickel foam is 312.95 mF cm⁻², the gravimetric specific capacitance is 1686.028 F g⁻¹ and the energy density is 30.97 μW h cm⁻² at the scan rate of 5 mV s⁻¹. These values not only are much higher than those of the single component electrode of NiSe but also conventional supercapacitors.³⁷ Accordingly, the areal specific capacitance of flexible supercapacitors based on NiSe@nickel foam is 176.75 mF cm⁻², the gravimetric specific capacitance is 851.912 F g⁻¹ and the energy density is 9 μW h cm⁻² at the scan rate of 5 mV s⁻¹. The cycling performance of supercapacitor based on CoNi₂S₄@NiSe@nickel foam by the test of galvanostatic charge–discharge has been presented in Fig. S1.† Specific energy and specific power are the two key factors for evaluating capacitance of the flexible supercapacitors. Fig. 8d is the Ragone plot (power density vs. energy density) of flexible supercapacitor based on CoNi₂S₄@NiSe@nickel foam, which shows relatively good performance. Through the above results, we can know the flexible supercapacitor based on CoNi₂S₄@NiSe nano arrays with ample active sites and porous structure, which provides abundant ion transmission paths to enhanced the electrochemical performance. In addition, multiple oxide states and abundant redox reactions in the electrode of CoNi₂S₄@NiSe with hierarchical structure also contribute to increasing the electrochemical performance. The electrochemical performance for CoNi₂S₄@NiSe@nickel foam in this study and some reported Ni-based compounds electrodes has been presented in Table S1† for comparison.

Fig. 8 Cycling performance of flexible supercapacitors based on (a) CoNi₂S₄@NiSe@nickel foam and (b) NiSe@nickel foam at the scan rate of 50 mV s⁻¹ for 1000 cycles; (c) areal capacitance dependence on scan rates of flexible supercapacitors based on NiSe@nickel foam and CoNi₂S₄@NiSe@nickel foam; (d) Ragone plot of the flexible supercapacitor based on CoNi₂S₄@NiSe@nickel foam.

In order to evaluate the flexibility of the device, we have test its capacitance retention in bending state. Fig. 9 is the CV curves of flexible supercapacitor based on CoNi₂S₄@NiSe@nickel foam in bending state. Compared to the device without bending, the capacitance performance of the device in the bending state is slightly improved, which can be explained as follows: in the bending state, the distance of electrode will be shorter than the without bending, which is more beneficial for the ion moving in the electrolyte to enhance the specific capacitance. This result confirms the highly flexible nature of the CoNi₂S₄@NiSe@nickel foam electrode.

Fig. 9 Capacitance retention of flexible supercapacitor based on CoNi₂S₄@NiSe@nickel foam with and without bending.

4. Conclusions

In summary, we combined solvothermal and hydrothermal methods to fabricate hierarchical structure of CoNi₂S₄@NiSe@nickel foam nano arrays for high performance flexible supercapacitor. The NiSe nanorods and CoNi₂S₄ nanosheets provide abundant active sites, which are beneficial for the kinetics of charges and ions in the electrode. The capacitive performance of flexible supercapacitor based on CoNi₂S₄@NiSe@nickel foam was superior to that of the flexible supercapacitor based on NiSe@nickel foam at all scan rates and discharge current densities. The newly synthesized electrode material CoNi₂S₄@NiSe@nickel foam with hierarchical structure also exhibits a high areal capacitance, a high cycling stability and excellent electrochemical stability. The flexible electrode based on nickel foam can be got by pressing nickel foam before the active material was grown, which provide a method to fabricate flexible supercapacitors.

Acknowledgements

We thank the National Natural Science Foundation of China (Grant No. 21572030, 21272033, 21402023) and Technology Innovative Research Team of Sichuan Province of China (No. 2015TD0005) for financial support.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra20871a

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