Fabrication of a flexible free-standing film electrode composed of polypyrrole coated cellulose nanofibers/multi-walled carbon nanotubes composite for supercapacitors

Abstract

Today, flexible energy storage systems have become attractive alternatives for applications in portable electronic devices. This account reports a simple and low-cost “dipping and polymerization” method for preparing flexible and free-standing supercapacitor film electrodes. The process consisted of depositing polypyrrole (PPy) coating on the surface and inside the network of an entangled cellulose nanofibers (CNFs)/multi-walled carbon nanotubes (MWCNTs) film. The electrochemical performance of the resulting CNFs/MWCNTs/PPy hybrid electrode was evaluated and compared with that of CNFs/MWCNTs film electrodes. The CNFs/MWCNTs/PPy nanocomposite electrode exhibited a particular entangled 3D network structure with an elevated specific capacitance of 288 F g⁻¹, obtained at a scan rate of 5 mV s⁻¹. This value is higher than that of the CNFs/MWCNTs electrode, calculated to be 32.2 F g⁻¹. Furthermore, the CNFs/MWCNTs/PPy electrode exhibited excellent redox reversibility and cycle stability. This novel procedure could provide an effective method for achieving flexible, free-standing, high-performance, low-cost, and environmentally friendly materials for use in supercapacitor electrodes.

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1. Introduction

Nowadays, efficient renewable energy conversion technologies and high-performance energy storage devices have become an essential alternative to overcome the limited gasoline resources and the excessive use of fossil fuels.¹ In this respect, tremendous efforts have been devoted to the development of supercapacitors due to their numerous advantages, including high energy-density, large capacitance, rapid charge/discharge capability, high electrical conductivity, long life-cycle stability, and low environmental impact.^2,3

A number of materials have been investigated as supercapacitor electrodes and they can be divided into three categories: (i) conducting polymers, (ii) metal oxides, and (iii) activated carbons.⁴ Conductive polymers received great attention due to their several benefits, such as low cost if compared to noble metal oxides and rapid reversible doping–dedoping ability during the charge–discharge processes. If compared to high surface carbon, conductive polymers also exhibit high charge densities.⁵ Among polymers, polypyrrole (PPy) is a typical conductive polymer with ease of synthesis, interesting electroactivity, great storage ability, and excellent environmental stability.⁶ PPy has been widely researched for various applications such as supercapacitors,^7,8 thermoelectric^9,10 and Li-batteries.¹¹ However, its poor mechanical properties and the long-term cycle life instability, induced by the repeated volumetric swelling/shrinking issued from charge–discharge processes have limited its application as individual electrode material.¹² These limitations could be overcome through forming composites between the PPy and other materials such as carbon nanostructures.¹³

Carbon nanotubes (CNTs) are considered as promising materials for supercapacitor electrodes because of their excellent conductivity, substantial surface area, robust mechanical strength, and porous entangled networks that could promote fast electron transfer, thus, improving the utilization of active materials.¹⁴ Because of their numerous advantages, considerable literatures are available on CNTs/PPy composite electrodes for various applications.^15–17

To meet the fast-growing market demands for wearable and portable electronics, flexible and lightweight power sources are desirable, but their applications in “smart textiles” and hybrid electric vehicles remain challenging.¹⁸ As a result, flexible supercapacitors, a type of flexible energy devices, have been widely investigated. Several substrates have recently been tested for flexible supercapacitors electrodes, including plastics, textiles, graphene papers, and cellulose nanofibers.¹⁹ Among these, cellulose is the most abundant and sustainable biopolymer found on the planet. The derived cellulose nanofibers (CNFs) have several interesting characteristics like high aspect ratios, excellent mechanical properties, and great flexibility.²⁰ CNFs can create multi-channels and mesoporous structure ideal for absorption and transport of water, as well as the essential ions through the outer and inner surface of the films.²¹ Furthermore, one-dimensional CNFs possess an appropriate hydrophilic characteristics, which can be used as aqueous electrolyte nano-reservoirs. This property can effectively reduce the transmission distance of the electrolyte ions, and CNFs also can enhance the wettability of the mesopores with aqueous electrolytes to improve the utilization efficiency of mesopores.²²

This paper reports a simple “dipping and polymerization” method for the fabrication of PPy-coated CNFs/MWCNTs film, which is self-stand, bind-free, and can be used as a flexible electrode for supercapacitors. The schematic representation of the synthesis technique of the CNFs/MWCNTs/PPy composite film is illustrated in Fig. 1. The PPy coating was found all over the surface and inside the network of the entangled porous CNFs/MWCNTs structure, which was strongly bond to guarantee the integrity of the electrode. To the best of our knowledge, only a handful of studies have been reported on CNFs/MWCNTs/PPy hybrid films as flexible electrodes. The flexible CNFs/MWCNTs/PPy hybrid films obtained in this study demonstrated outstanding performance in the all-solid-state supercapacitors.

Fig. 1 Schematic diagram of the CNFs/MWCNTs/PPy nanocomposite film synthesis process.

2. Experimental

2.1 Materials

Bamboo powders (moso bamboo) were purchased from Zhejiang Lishui, China. The powders were first passed through a 60 mesh, then air dried and stored at room temperature. The MWCNTs powders (97% purity, 10–30 nm diameters) were received from Shenzhen Nanotech Port Co., Ltd. Pyrrole monomer, ferric chloride (FeCl₃), and sodium dodecyl benzene sulfonate (SDBS) were purchased from Shanghai Ling Feng Chemical Reagent Co., Ltd. Benzene, potassium hydroxide, sodium chlorite, acetic acid, hydrochloric acid, ethanol, and other chemicals were of analytical grade and used without further purification.

2.2 Methods

The chemical purification of bamboo cellulose tissues was carried out according to the published literature.²³ First, solvent extraction was performed in a Soxhlet apparatus with a 2 : 1 mixture of toluene/ethanol at 90 °C for 6 h. The samples were then delignified using a solution of acidified sodium chlorite at 75 °C for 1 hour, and the process was repeated five times. To remove the hemicelluloses, residual starch, and pectin, the samples were treated with 2 wt% potassium hydroxide at 90 °C for 2 h. Afterward, the samples were further treated with an acidified sodium chlorite solution at 75 °C for 1 hour, followed by treatment with 5 wt% potassium hydroxide at 90 °C for 2 h. In the end, highly purified cellulose fibers were prepared by treatment with 1 wt% hydrochloric acid solution at 80 °C for 2 h. During the process, the samples were rinsed with deionized water until their pH becomes neutral then kept in a water-wollen state. Following the chemical treatment, the CNFs were obtained by milling the purified cellulose using a grinder (MKCA6-2, Masuko Sangyo Co., Ltd., Japan) at 1500 rpm.²⁴

MWCNTs powders (25 mg) were dispersed in deionized water with a 100 mg SDBS dispersant using an ultrasonic wave cell pulverizer (XO-1200, Nanjing Xianou Biological Technology Co., Ltd., China) for 30 min at an output power of 960 W. An amount of CNFs (2.5 g, 1 wt%) were dispersed in deionized water and sonicated for 30 min at 960 W. Subsequently, the MWCNTs suspension and CNFs suspension were sufficiently mixed by sonication for 30 min at 960 W. The final mixture was drained to obtain a CNFs/MWCNTs filter cake by vacuum filtration, which was then immersed repeatedly in alcohol until all the water was replaced, then followed by a freeze-drying process to yield the CNFs/MWCNTs electrode.

The CNFs/MWCNTs/PPy electrode was prepared through the process of “dipping and polymerization”. In a typical procedure, the non-freeze-dried CNFs/MWCNTs film was first dipped into a 50 mL aqueous solution containing 10 mmol pyrrole monomer for 1 h. A solution of FeCl₃ (50 mL, 0.1 M) was then added dropwise to the solution and the polymerization process was maintained at 0 °C for 2 h under continuous mechanical stirring. Afterward, the electrode was washed with 0.3 M HCl followed by deionized water and ethanol. Finally, the CNFs/MWCNTs/PPy electrode was obtained after a freeze-drying process of the film for 24 h.

2.3 Characterization methods

The morphology of the samples was determined by a scanning electron microscope (FE-SEM, Hitachi S-4800, Japan) operating at 5.0 kV. The X-ray diffraction (XRD) patterns were carried out on an Ultima IV multipurpose X-ray diffraction system (Ultima IV, Rigaku, Japan) using Cu Kα radiation (40 kV and 30 mA) at a scan rate of 5° min⁻¹ and 2θ angles of 10° to 50°. Raman spectra were recorded on a DXR Raman spectrophotometer (Thermo Scientific, USA) using a 532 nm laser source. Fourier transform infrared spectroscopy (FTIR) measurements were recorded with a spectrometer (Nicolet iS10, Thermo Electron Corp., USA) operating at the attenuated total reflectance (ATR) device in the frequency range of 500–4000 cm⁻¹ and a resolution of 4 cm⁻¹. Thermogravimetric analysis (TGA) was carried out using Pyris 1 thermogravimetric analyzer (Perkin-Elmer Cetus Instruments, Norwalk, CT). The samples were heated from room temperature to 600 °C under a nitrogen gas atmosphere at a heating rate of 20 °C min.

The electrochemical studies were performed on a CHI600E electrochemical workstation (Chenhua, Shanghai) connected to a three electrode cell containing 1 M H₂SO₄ aqueous solution. A platinum sheet and a standard calomel electrode (SCE) were used as the counter and reference electrodes, respectively. The cyclic voltammetry (CV) scans were recorded from −0.2 to 0.8 V at different scan rates of 5, 10, 30, 50, and 100 mV s⁻¹. The galvanostatic charge–discharge (GCD) tests were performed at different current densities of 0.2, 0.3, 0.5, 1 and 2 A g⁻¹. The electrochemical impedance spectroscopy (EIS) measurements were carried out at the open circuit potential in the frequency range of 0.01 Hz to 100 kHz using an alternating current and amplitude of 5 mV.

3. Results and discussion

3.1 SEM analyses

Fig. 2a shows a typical SEM image of the cellulose nanofibers, where extremely long length nanofibers with three-dimensional porous network could be observed. On the other hand, the CNFs/MWCNTs hybrid film exhibited an entangled network structure with randomly oriented 1D fiber-like structure with pores of varying dimension (Fig. 2b). The structures paved the way for the fabrication of PPy coated MWCNTs and CNFs. As shown in Fig. 2c, pure PPy particles exhibited agglomerated globular structure with diameters ranging from 150 to 200 nm. The average size of the agglomerates was estimated to 300–400 nm. The formation of uniformly distributed PPy coating on the CNFs/MWCNTs film is depicted in the SEM images of the composites (Fig. 2d). It will be noted that the entangled network was preserved after the PPy deposition. The interpenetrating CNFs/MWCNTs/PPy films presented a porous structure, which effectively enabled a better transport of the electrolytes to access the active-sites.

Fig. 2 SEM images of (a) CNFs, (b) CNFs/MWCNTs, (c) PPy, and (d) CNFs/MWCNTs/PPy.

3.2 XRD analyses

Fig. 3 depicts the XRD patterns of the pristine CNFs film, MWCNTs powders, PPy powders, and CNFs/MWCNTs/PPy composite electrode. Two characteristic peaks of CNFs were observed at around 2θ of 15.6° and 22.5°, corresponding respectively to the crystal planes (110) and (200).²⁵ The XRD pattern of the CNTs exhibited a sharp and high-intensity peak at 25.9° (002) and a lower intensity peak at 42.7° (100), attributed to diffractions from the interlayer spacing and in-plane regularity, respectively.²⁶ On the other hand, the PPy presented a broad diffraction peak at 2θ of 25.6° related to the intermolecular spacing of pyrrole. The broadness of the peak is characteristic of the amorphous PPy due to the scattering from PPy chains at the interplanar spacing.²⁷ It was observed that the CNFs/MWCNTs/PPy hybrid electrode had a typical crystallite reflections (200, 002) associated with both the CNFs and MWCNTs, indicating the presence of CNFs and MWCNTs in the hybrid electrode. Also, it can be noted that the crystalline structure of CNFs and MWCNTs were hindered by the amorphous coating of PPy. This induced weaker and broader diffraction peaks of CNFs and MWCNTs, where some of the peaks even vanished in the hybrid electrode. Therefore, the results from XRD confirmed the presence of interactions among CNFs, MWCNTs, and PPy.

Fig. 3 XRD patterns of (a) CNFs film, (b) MWCNTs powders, (c) PPy powders, and (d) CNFs/MWCNTs/PPy film.

3.3 Raman analyses

The characteristic Raman bands of CNFs (Fig. 4a) appeared at about 2891, 1388, 1092, and 897 cm⁻¹, attributed to the symmetric and asymmetric stretching vibrations of CH₂, CH₂ deformation vibrations, C–O–C glycosidic link asymmetric stretching, and the in-plane symmetric stretching of the C–O–C.²⁸ The Raman spectra of MWCNTs depicted a typical peak at 1569 cm⁻¹ (G band), assigned to the Raman active E_2g mode analogous to that of graphite (Fig. 4b). The disorder-induced D band observed at 1337 cm⁻¹ was caused by the defects in the structure, which showed a second-order harmonic (G′ band) at 2670 cm⁻¹.²⁹ The intensity ratio of the D band to the G band (I_D/I_G) was estimated to be 0.86 (<1), suggesting lower defects in the MWCNTs. The Raman spectrum of PPy powders shown in Fig. 4c, illustrated a peak located at 1350 cm⁻¹, which was assigned to the ring stretching vibrations of the C–C bonds. The peak at 1572 cm⁻¹ depends on the C=C band ratios and its shift is related to the doping level of the PPy. Therefore, shifts in the peak position are a useful tool for evaluating the conductivity of the PPy.³⁰ For the CNFs/MWCNTs/PPy hybrid electrode, two strong peaks related to MWCNTs overlapped with the C=C backbone stretching vibrations and C–C ring stretching vibrations of PPy (Fig. 4d).³¹ The intensity of MWCNTs peaks obviously increased, rendering the CNFs and PPy bands invisible.

Fig. 4 Raman spectra of (a) CNFs film, (b) MWCNTs powders, (c) pure PPy powders, and (d) PPy/MWCNTs/CNFs film.

3.4 FT-IR spectra

The FT-IR spectroscopy is a useful qualitative technique for the evaluation of structural characterization of composites and their constituent materials. In the case of CNFs film (Fig. 5a), a broad band at 3330 cm⁻¹ was attributed to the stretching of hydroxyl groups. The band at 2894 cm⁻¹ was assigned to the aliphatic C–H stretching vibrations. The low-intensity peak at 1640 cm⁻¹ resulted from the H–O–H stretching vibrations of the absorbed water molecules in carbohydrate structure. The bands at 1430 cm⁻¹ were characteristics of the bending modes of –CH₂ groups. The sharp and steep band observed at 1026 cm⁻¹ was due to the presence of the C–O–C pyranose ring skeletal vibrations. The peak at 896 cm⁻¹ was attributed to the C₁–H deformation vibrations.²³ Due to the symmetrical structure of dried pure MWCNTs powders, significant peaks are rarely observed in the FTIR spectrum (Fig. 5b). The characteristic bands at 1516 cm⁻¹ were attributed to a mixed C=C and inter-ring C–C vibrations of the PPy units. The band at 1438 cm⁻¹ was related to the C–N stretching vibrations in the Py ring. The characteristic bands at 1272, 1127, 1011, and 960 cm⁻¹ were respectively assigned to the breathing vibrations of the pyrrole ring, N–H deformation vibrations, C–H in-plane deformation mode, and C–H out-of-plane ring deformation (Fig. 5c).³²

Fig. 5 FTIR spectra of CNFs film (a), MWCNTs powders (b), PPy powders (c) and the CNFs/MWCNTs/PPy film electrode (d).

The spectrum of the CNFs/MWCNTs/PPy hybrid electrode shown in Fig. 5d presented typical features of PPy. The absorbed peaks of cellulose have almost vanished, suggesting that the CNFs/MWCNTs film was completely wrapped by PPy.³³ Consequently, the FT-IR analysis further confirmed the strong intermolecular interactions between CNFs, MWCNTs and PPy.

3.5 Thermal stability

TGA was used to assess the thermal stability of the CNFs, MWCNTs, PPy, and CNFs/MWCNTs/PPy. Fig. 6 revealed that most of the samples have some absorbed moisture in addition to MWCNTs, which was released at the water boiling point of 100 °C. The thermal decomposition of CNFs was basically divided into three stages, a weight loss up to 4% was observed at 50–150 °C, attributed to the evaporation of water and absorbed molecules. During the second stage, CNFs showed a significant weight loss at 250–400 °C estimated to 28%. The latter was induced by the removal of small molecular fragments such as O–H and CH₂–OH groups. The third weight loss observed at 400–600 °C was linked to the decomposition of cellulose backbone.³⁴ In comparison, the MWCNTs showed a very good thermal stability, where 97.2% of the mass was preserved at 600 °C. On the other hand, the PPy exhibited an insignificant mass loss at 50–125 °C, and a steady decrease in mass loss at 125–600 °C, which reached 72% loss at 600 °C due to polymer chain degradation.³⁵ For CNFs/MWCNTs/PPy hybrid electrode, the onset of weight loss shifted from 125 °C observed for PPy to 250 °C for the hybrid electrode. Also, it was noticed that the hybrid electrode displayed higher amount of residue (73%) than pure CNFs (18%) at 600 °C.

Fig. 6 TGA curves of CNFs film, MWCNTs powders, PPy powders, and the CNFs/MWCNTs/PPy film electrode.

These findings demonstrated that the PPy layer was the most external and more stable component that acted as a protective barrier of the CNFs and MWCNTs layers against thermal degradation. The introduction of MWCNTs improved the thermal stability of the hybrid electrode.

3.6 Electrochemical characterization

The electrochemical behaviors of the CNFs/MWCNTs and CNFs/MWCNTs/PPy electrodes were carried out using cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), and electrochemical impedance spectroscopy (EIS) measurements. Fig. 7a exhibited the CV curves of the CNFs/MWCNTs/PPy film electrode in the potential range between −0.2 and 0.8 V at different scan rates. It was observed that all of the CV curves displayed a typical hybrid electrode performances. The hybrid electrode combines the electrical double-layer capacitance from MWCNTs and the pesudocapacitance property from PPy.³⁶ For a proper comparison, Fig. 7c showed the CV curves of the CNFs/MWCNTs and CNFs/MWCNTs/PPy electrodes at the same scan rate of 5 mV s⁻¹. It could be seen that the CNFs/MWCNTs/PPy electrode exhibited a superior energy storage capacitance. The specific capacitance of the electrode can be calculated from the CV curve and the equation as follows:
where C_Scv is the specific capacitance (F g⁻¹), is the integrated area of the CV curve, m is the mass of the active material present on the electrodes (g), ν is the potential scan rate (mV s⁻¹), and ΔV is the scanned potential window (V). Applying of the equation to the experimental results showed that the CNFs/MWCNTs/PPy electrode delivered greater specific capacitance if compared to that of the CNFs/MWCNTs electrode (288 F g⁻¹ versus 32.2 F g⁻¹, respectively). The latter was mainly attributed to the pseudocapacitances of PPy. And we calculated the C_Scv of the two electrodes at different scan rates from 5 to 100 mV s⁻¹ and compared them in Fig. 7e.

Fig. 7 The electrochemical performances of the two samples: CV curves of the CNFs/MWCNTs/PPy electrode at different scan rate (a), and at a scan rate of 5 mV s⁻¹ in comparison with the CNFs/MWCNTs electrode (c). Galvanostatic charge/discharge curves of the CNFs/MWCNTs/PPy electrode at different current densities (b), and at 0.2 A g⁻¹ in comparison with the CNFs/MWCNTs electrode (d). Specific capacitance of the two electrodes as a function of scan rate (e). Specific capacitance of the two electrodes as a function of current density (f). Cycle stability of the CNFs/MWCNTs/PPy electrode at 2 A g⁻¹. The inset showed the first 10 cycles of the charge–discharge curves (g). EIS measurements at open circuit potential (h).

To further evaluate the electrochemical performances of the film electrodes, galvanostatic charging/discharging experiments were performed in the potential range from 0 to 0.8 V. As shown in Fig. 7b, all the charge and discharge curves of the CNFs/MWCNTs/PPy electrode showed nearly triangular symmetrical shape at different current densities, indicative of the excellent reversibility of the hybrid material. Furthermore, the CNFs/MWCNTs/PPy electrode showed a much larger specific capacitance than that of the CNFs/MWCNTs electrode at a current density of 0.2 A g⁻¹ (Fig. 7d). This agrees well with results obtained from the CV measurements. The pseudocapacitance of PPy was considered to be the primary source contributing to the relevant charge storage. The specific capacitance (C_s) was calculated from the following equation:

C_Scd = (IΔt)/(mΔV)

where C_Scd is the specific capacitance of the electrodes (F g⁻¹), I is the charge–discharge current (A), Δt is the time of discharge (s) corresponding to the voltage difference ΔV between the upper and lower potential limits (V), and m is the mass of the active material (g). The C_Scd value obtained for the CNFs/MWCNTs/PPy electrode at a current density of 0.2 A g⁻¹ was 180.9 F g⁻¹, notably higher than the obtained value for the CNFs/MWCNTs electrodes (17.1 F g⁻¹). The C_Scd of the two electrodes at different current density from 0.2 A g⁻¹ to 2 A g⁻¹ were calculated (Fig. 7f).

The cycling stability of electrodes at constant current densities is critical for the performance of supercapacitors in practical applications. As shown in Fig. 7g, the capacitance of the CNFs/MWCNTs/PPy electrode retained 83% of its original value after 1000 of continuous charge/discharge cycles at a current density of 2 A g⁻¹. This meant that the electrode had a good cycling stability and a high degree of reversibility during the repetitive charge–discharge cycling processes. This was attributed to the three-dimensional network structure of the composite electrode, beneficial for an efficient propagation of the electric charges. The inset of Fig. 7g showed typical charge–discharge curves during the first 10 cycles of continuous operation.

The EIS measurements were performed to further characterize the electronic and transport properties in CNFs/MWCNTs/PPy and CNFs/MWCNTs film electrodes. The impedance plot presented in Fig. 7h could be divided into high-frequency and low-frequency components. For the CNFs/MWCNTs/PPy electrode, the semicircle was nearly absent in the low-frequency region, suggesting that electron transfer resistance was significantly low or negligible. However, a semicircle with larger diameter was observed for the CNFs/MWCNTs electrode, indicative of a low charge transfer rate due to the elevated electron transfer resistance.³⁷ In the high-frequency region, an ideal polarizable capacitance caused a straight line along the imaginary axis. As a result, the impedance curve of the CNFs/MWCNTs/PPy electrode looked more parallel to the imaginary axis than that of the CNFs/MWCNTs electrode, proposing that the CNFs/MWCNTs/PPy electrode had a better capacitive performance.

In terms of flexibility, Fig. 8a clearly depicted the free-standing black CNFs/MWCNTs/PPy electrode with outstanding flexibility and no cracks formed upon bending. This was attributed to the mechanical reinforcement of the cellulose nanofibers, as well as the inherent flexibility of both the PPy and the MWCNTs. The high conductivity of the CNFs/MWCNTs/PPy film can be used as a flexible conductor to connect components in an electric circuit. A small blue LED was lighted when the CNFs/MWCNTs/PPy film was used as a connecting lead under a 3.0 V voltage (Fig. 8c).

Fig. 8 Digital photographs of the flexible CNFs/MWCNTs/PPy film (a and b). Optical images of an LED illuminated when nanostructured CNFs/WMCNTs/PPy nanocomposite film was used as the connecting wire in the bending states (c).

4. Conclusions

The novelty of the reported fabrication technique consisted of using both dipping and polymerization processes in the synthesis of CNFs/MWCNTs/PPy film electrodes to yield high performance, flexible, free-standing supercapacitor electrodes. The nanostructure and composition of the composites were found to play a crucial role for the fulfilment of the three-dimensional and reticular hybrid composite electrodes. PPy was coated on both the surface and inside network of the entangled MWCNTs/CNFs film to yield improved absorption and electrolytes transport phenomena throughout the film. The maximum specific capacitance of the CNFs/MWCNTs/PPy electrode was estimated by cyclic voltammetry measurement to be 288 F g⁻¹ at 5 mV s⁻¹, which was much higher than that of the CNFs/MWCNTs film electrode estimated to only 32.3 F g⁻¹. Furthermore, the CNFs/MWCNTs/PPy electrode retained 83% of its initial capacitance at 2 A g⁻¹ after 1000 charge/discharge cycles. The mechanical flexibility of the supercapacitor electrodes based on CNFs/MWCNTs/PPy may open novel avenues for facile development of biomass-derived renewable and lightweight electrode materials for supercapacitors.

Acknowledgements

This work is financially supported by National Natural Science Foundation of China (31170514 and 31370557); Jiangsu Province Natural Science Foundation of China (BK20150875); Graduate Cultivation Innovative Project of Jiangsu Province; Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD); the Doctoral Program of Higher Education (20113204110011) and the Analysis & Test Center of Nanjing Forestry University.

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