A polytriphenylamine derivative exhibiting a four-electron redox center as a high free radical density organic cathode

Abstract

A high free radical density polytriphenylamine derivative, poly(1-N,1-N,4-N-triphenyl-4-N-[4-(N-[4-(N-phenylanilino)phenyl]anilino)phenyl]benzene-1,4-diamine) (PFTP) has been synthesized and investigated as the cathode material of organic free radical batteries for the first time. The molecular structure, thermal-stability, morphology, spectral characteristics and electrochemical properties of the prepared polymer were characterized by Raman spectroscopy (RS), electron spin resonance (ESR), pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS), MALDI-TOF mass spectrometry, thermogravimetric analysis (TGA), ultraviolet visible spectroscopy (UV-vis), scanning electron microscopy (SEM), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Also, the charge–discharge properties of the prepared polymers were studied by galvanostatic charge–discharge testing. Compared to PTPA, it was found that PFTP not only had improved thermal stability and morphology, but also exhibited obvious four-electron redox characteristics. At a discharge current of 20 mA g⁻¹, the PFTP electrode presented a multi-stage discharge platform and an improved discharge capacity of 74.2 mA h g⁻¹ compared to PTPA, which could be attributed to the high free radical density and the improved morphology of the obtained novel polymer. The above exploratory work made it promising to design the multi-electron redox centers of the polytriphenylamine-based polymer to obtain improved electrochemical performance.

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Introduction

There is no end to researchers pursuing high-performance and sustainable energy storage devices, especially considering the desire today for longer working times of portable electronic devices, as well as the extended range and lower cost of electric vehicles.^1–3 Among all storage devices, lithium ion batteries represent promising power sources for the future because of their advantages in energy density and cyclability. As one of the main components of the battery, the cathode materials of lithium ion batteries play an important role in the technological development of the cell, which is presently undergoing a development bottleneck due to limited theoretical capacities, limited mineral resources and the waste treatment process for the traditional inorganic intercalation cathode materials.^4–6 As an alternative, organic electrode materials for rechargeable batteries have received considerable attention in recent years, and have shown more and more advantages for the development of sustainable and versatile energy storage devices beyond lithium ion batteries.^7,8 So far, various organic electroactive materials have been explored as positive electrode materials, particularly conducting polymers,^9,10 organosulfur compounds,¹¹ carbonyl-based compounds^12–15 and stable radical polymers,^16,17 etc.

Among all endeavors, stable radical polymers and their derivatives have been explored as cathode-active charge-storage materials for lithium ion batteries due to their good cycling stability and stable voltage platform.¹⁸ Polytriphenylamine (PTPA) and its derivatives containing triphenylamine radical units and a highly conductive polyparaphenylene (PPP) back-bone combined with the high energy density of the electroactive polyaniline unit, belong to a family of radical polymers, which have been widely applied as organic electroluminescence (EL) materials, photo-conduction materials and organic solar cell materials.¹⁹ More recently, PTPA-based functional polymers have been studied as the electrode-active material for supercapacitors and lithium ion batteries,^20–23 and they display good electrochemical performances and relatively smooth charging and discharging voltage platforms (∼3.6 V), resulting from the π-conjugated electron-structure and reversible redox radical nature of the triphenylamine substructure in the polymers. In order to obtain applicable organic cathodes with higher energy density, it is necessary to increase the specific capacity of the organic cathode material by the means of novel molecular structure design. According to the formula for the calculation of the theoretical specific capacity of an organic electrode material (C (mA h g⁻¹) = [n × F (C mol⁻¹)]/M_w (g mol⁻¹), in which C, n, F and M_w respectively mean the theoretical specific capacity, the transferred electron number in each structural unit, the Faraday constant and the molecular weight of the structural unit), there are two ways to improve the theoretical capacity of the material: one is adopting multi-electron reactions and another is to reduce the molecular weight of the structural unit. In our prior work, we have prepared a PTPA derivative (poly [N,N,N,N-tetraphenylphenylenediamine] (PDDP)) with a two electron redox process in every structural unit of the polymer, which exhibited double electron electrochemical characteristics and an improved cell performance as the cathode material of lithium ion batteries.²³ Inspired by this previous work, we will attempt to make a further exploration of the novel PTPA-based free radical polymer with multiple redox centers, in order to obtain further understanding of the relationship between the molecular structure of an electro-active polymer and its electrochemical properties.

In this paper, a polytriphenylamine derivative, poly(1-N,1-N,4-N-triphenyl-4-N-[4-(N-[4-(N-phenylanilino)phenyl]anilino) phenyl]benzene-1,4-diamine) (PFTP), with four redox centers in the structural unit has been designed and prepared. The material’s structure and electrochemical characteristics as well as the cell performance for the novel triphenylamine free radical-based cathode material were also systematically investigated. Also, the charge–discharge mechanism of the prepared polymer during the charge–discharge process was explored, correspondingly.

Experimental

Materials

(4-Bromo-phenyl)-diphenyl-amine (98%) and triphenylamine (98%) were purchased from Aladdin-Reagent Co. Tri-tert-butylphosphine (PtBu₃, 1.0 M), potassium tert-butoxide (KO^tBu, 98%) and palladium acetate (Pd(OAc)₂, AR) were purchased from Energy Chemical Reagent Co. N,N′-diphenyl-benzene-1,4-diamine (>98%) was purchased from TCI (Shanghai) Development Co. All other reagents were received as analytical grade and used without further purification.

Material synthesis

Synthesis of the FTP monomer. The synthesis process of the monomer (FTP) is shown in Scheme 1.

Scheme 1 The synthesis route to FTP.

1-N,1-N,4-N-Triphenyl-4-N-[4-(N-[4-(N-phenylanilino)phenyl]anilino)phenyl]benzene-1,4-diamine (FTP) was synthesized through an Ullmann reaction: 4.8454 g (15 mmol) of (4-bromo-phenyl)-diphenyl-amine, 1.3016 g (5 mmol) of N,N′-diphenyl-benzene-1,4-diamine, 1.6872 g (15 mmol) of KO^tBu, 0.05 g of Pd(OAc)₂ and 5 mL of PtBu₃ were dissolved in 40 mL of toluene in a N₂ atmosphere. Then the mixture was stirred at 110 °C for 12 h. After reaction, the solution was filtered and then washed with toluene to remove the unconverted reactants. The residue was dried under vacuum at 80 °C for 12 h. The product was acquired as a cement colored solid power in an 87.90% yield. ¹H NMR (500 MHz, CDCl₃) δ 7.25 (t, J = 7.8 Hz, 12H), 7.11 (t, J = 6.7 Hz, 12H), 7.04–6.94 (m, 18H). MS (EI): calculated for C₅₄H₄₂N₄ m/z: 746.34, found m/z: 746.14.

Chemical polymerization of PTPA and PFTP. The polymers of poly(triphenylamine) (PTPA) and poly(1-N,1-N,4-N-triphenyl-4-N-[4-(N-[4-(N-phenylanilino)phenyl]anilino)phenyl]benzene-1,4-diamine) (PFTP) were prepared by a chemical oxidative polymerization method in chloroform (30 mL) with iron (III) chloride hexa-hydrate as the oxidant. In a N₂ atmosphere, the solution containing 1 mmol of monomer (TPA or FTP) and 3.5 mmol of iron (III) chloride hexa-hydrate was stirred at 40 °C for 24 h. The final mixture was poured into methanol to deposit the polymer product, and then filtered and washed with deionized water and ethanol. The polymers were dried under vacuum at 60 °C. The colors of PTPA and PFTP turned out to be yellow and green, respectively.

Material characterization

Raman spectra were recorded on a Lab RAM HR UV800 (JOBIN YVON, France). The electron spin resonance (ESR) spectra were recorded on a BRUKER A300 spectrometer (Switzerland). Pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS) was carried out on a vertical microfurnace pyrolyzer (PY2020iD, Frontier Lab Ltd, Fukushima, Japan), which was directly attached to a gas chromatograph (CP-3800, Varian, USA) equipped with a flame ionization detector (FID). MALDI-TOF mass spectrometry (MALDI-TOF-MS) was carried out on an UltrafleXtreme spectrometer (Bruker Daltonics Co.). UV-vis spectra were recorded on a Varian Cary 100 UV-vis spectrophotometer (Varian, USA). Thermogravimetric analysis (TGA) was recorded on a Q5000IR (Ta, USA), and the measurements were carried out from room temperature to 800 °C at a heating rate of 10 °C min⁻¹ in a nitrogen atmosphere. Scanning electron microscopy (SEM) measurements were performed using a Hitachi S-4800 scanning electron microscope (Hitachi, Japan).

Electrochemical measurements

The cathodes were prepared by coating a mixture, containing the as-prepared polymers, acetylene black and PVDF binder in a weight ratio of 50 : 40 : 10 with N-methyl-2-pyrrolidone (NMP) as the solvent, on circular Al current collector foils, followed by drying at 60 °C for 10 h. The electrochemical performances were evaluated with CR2032-type coin cells assembled in an argon-filled glove box, with the prepared cathode, a metal lithium foil as the anode, polypropylene film (Celgard 2300) as the separator and a solution of 1 M LiPF₆ dissolved in ethylene carbonate (EC) and dimethyl carbonate (DMC) (EC/DMC = 1 : 1, v/v) as the electrolyte. All of the organic solvents and the electrolyte were of battery grade. The charge–discharge measurements were carried out on a LAND CT2001 A in the voltage range of 2.5–4.2 V vs. Li/Li⁺, using a constant current density at room temperature. The cyclic voltammetry (CV) tests were performed with a CHI 660E electrochemical working station in 0.1 M LiCO₄/CH₃CN vs. Ag/AgCl at a scan rate of 100 mV s⁻¹. Spectroscopy (EIS) experiments were also carried out using a CHI 660E electrochemical working station, with the measured lithium ion half-cells at a charge stage, the applied frequency ranging from 0.1 Hz to 1 MHz and an applied amplitude of 5 mV.

Results and discussion

Material characterization

Fig. 1 shows the Resonance Raman spectra of the as-prepared polymers. As can be seen, the main spectral characteristics of the triphenylamine moieties are exhibited in PTPA and PFTP samples, which are located as follows: 1165 cm⁻¹ (C–H in-plane stretching), 1285 cm⁻¹ (C–C inter-ring stretching), 1490 cm⁻¹ (C=N stretching), and 1608 cm⁻¹ (C–C ring stretching).²⁴ Also, a new peak for the symmetric stretching of N–Ar–N is observed clearly at 1351 cm⁻¹ in the spectrum of PFTP, but, comparatively, it is not found in PTPA, indicating that the novel polymer possesses the p-phenylene structure and PFTP has been synthesized successfully. What’s more, the peak intensity for C–N–C stretching at 1490 cm⁻¹ in the spectrum of PFTP is stronger than that of PTPA, which is in accordance with the increased density of the N-phenylene structure in the obtained PFTP polymer. To confirm the existence of free radicals in the polymers, Electron Spin Resonance (ESR) for both PTPA and PFTP was also explored, accordingly. As shown in Fig. 2, in the inset box, a small and broad peak (g value equal to 2.00654) shown in the spectrum demonstrates the existence of free radicals in PTPA. For PFTP, the free radical characteristic peak, i.e., g value located at 2.00340 with the obvious broad peak shape and the enhanced peak intensity, can be attributed to the specific molecular structure of PFTP with an improved free radical density.

Fig. 1 Resonance Raman spectra of the PTPA and PFTP samples measured at 632.81 nm.

Fig. 2 ESR of PTPA and PFTP measured in the powder state.

Pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS) and MALDI-TOF mass spectrometry (MALDI-TOF-MS) are two effective identification techniques to analyze the molecular structure of polymer materials, as well as the molecular weight and the degree of polymerization of the polymers, respectively.^25,26 As shown in Fig. S1 (seen in the ESI†), the Py-GC-MS chromatogram of PFTP shows three distinctive peaks, which are located at 31.25, 48.02 and 53.14 minutes, corresponding to the compounds with a molecular ion at 245.08, 336.08 and 412.11 m/z in the mass spectrum, respectively. Those three characteristic peaks correspond to the fragments of triphenylamine, N,N,N′-triphenyl-benzene-1,4-diamine and N,N,N,N-tetraphenylphenylenedi-amine, respectively, indicating that those fragments are present in the obtained PFTP polymer. From the MALDI-TOF-MS spectra of the as-prepared polymers (as shown in Fig. S2-1 and S2-2 of the ESI†), some information can be acquired about the molecular weight and the degree of polymerization of the prepared polymers (as shown in Tables S1 and S2 of the ESI†). For PTPA, the lowest and the highest molecular weights of the polymers combined with the ion are 731 and 3890 Da, corresponding to the polymerization degree changing from three to sixteen in the polymer. As to PFTP, the lowest and the highest molecular weights of the polymers combined with the ion are 745 and 2978 Da, in accordance with the polymerization degree changing from one to four in PFTP. The above results indicate that the prepared polytriphenylamine-based derivatives are actually oligomers and/or macromolecules, which limits the number of triphenylamine moieties in both PTPA and PFTP (the FTP monomer can be defined as the integration of four triphenylamine moieties) not to exceed sixteen.

UV-vis spectroscopy (normalized absorbance) is a powerful tool to explore the electronic structure characteristics of molecules, and was applied to investigate the electronic characteristics of the PTPA and PFTP samples in N,N-dimethylformamide (DMF). As can be seen in Fig. 3, PTPA exhibits one absorption peak at 353.5 nm, which is attributed to the π–π* electron transition of the triphenylamine units. For PFTP, the corresponding absorption peak for the π–π* electron transition clearly red-shifts from the 353.5 nm seen in PTPA to 412.5 nm, which is due to the prepared novel PFTP’s extended triphenylamine structure which compacts the free radical density more closely in the triphenylamine-based unit of the polymer. As a result, the charge carrier transportation in the PFTP polymer becomes more smooth and favorable, leading to the red-shift phenomenon for the absorption peak of the π–π* electron transition in the triphenylamine units.^27,28

Fig. 3 UV-vis spectra of (a) PTPA and (b) PFTP.

The thermal stabilities of the PTPA and PFTP samples were also measured by thermogravimetric analysis (TGA), and the measurements were carried out from room temperature to 800 °C with a heating rate of 10 °C min⁻¹ in a N₂ atmosphere. As shown in Fig. 4, the small weight loss before about 200 °C for PTPA and PFTP is mainly attributed to the loss of the residual water and solvent molecules remaining in the polymers. With further temperature programming, a multi-step mass loss for PTPA started at about 310 and continued until 760 °C. This occurred during the temperature-rise period, and is possibly caused by the polymerization degree ranging from three to sixteen (as proved by MALDI-TOF-MS spectra). In contrast, the thermogram of PFTP shows only one obvious narrow thermal degradation process, starting at about 400 °C and ending at 490 °C, which is attributed to the narrow molecular change range of PFTP from one to four. Also, it indicates the improved thermal stability of PFTP compared to PTPA. Generally, the improved thermal stability of PFTP will be beneficial to its practical application as a safe cathode of lithium ion batteries in the future.

Fig. 4 TGA of PTPA and PFTP, measuring from room temperature to 800 °C at a heating rate of 10 °C min⁻¹ in a N₂ atmosphere.

Fig. 5 displays the morphology of the as-obtained polymers. As can be seen in Fig. 5a and c, PTPA presents a dense packing sheet-like morphology with significant aggregation, which may be caused by the steric torsion inducing nature of the big triphenylamine units in the polymer. In contrast, as shown in Fig. 5b and d, PFTP exhibits a loose stacking structure assembled by many smaller particles. We speculate that this variant morphology for PFTP is a result of the unique structure of the FTP monomer, which has an obvious effect on the obtained polymer molecular structure and aggregation behavior as well as the morphology of the resultant polymer. Also, this loose stacking morphology of PFTP is favourable for the electrolyte to diffuse and migrate into the composite electrode, leading to the close contact of the electrode-active material and the electrolyte ions during the charge–discharge process, which is very crucial for obtaining the desired electrochemical performance for the electrode materials of lithium ion batteries.

Fig. 5 SEM images of samples (a) PTPA and (b) PFTP; (c) and (d) partially enlarged SEM images of (a) and (b), respectively.

Electrochemical performance

Fig. 6 shows the cyclic voltammetry (CV) profiles of both PTPA and PFTP, collected in 0.1 M LiClO₄/acetonitrile solution vs. Ag/AgCl at a scanning rate of 100 mV s⁻¹. As shown in Fig. 6a, a couple of very well defined redox peaks can be seen in the CV profile of PTPA. The redox peaks of PTPA at about 1.18 V and 0.52 V with a redox potential separation of about 0.66 V can be attributed to the redox transformation of the triphenylamine-based free radical/triphenylamine during the charge–discharge process, which was accompanied by the insertion/extraction of both Li⁺ and PF₆⁻ into and out of the electrode. The approximately symmetrical peak shape suggests good insertion/extraction reversibility of the produced PTPA. In contrast, there are some obvious differences for the CV profile of the PFTP electrode, in which four pairs of redox peaks, located at 0.41/0.46, 0.62/0.65, 0.72/0.76 and 0.98/1.04 V/V respectively (reduction/oxidation), are presented in the curve (as shown in Fig. 6b). The multiple peaks characteristic of PFTP indicates the four free radical center structure in the FTP units of PFTP, which undergoes a four electron gain/loss reaction during the charge–discharge process. Moreover, the potential separations for the four redox pairs of PFTP are 0.05, 0.03, 0.04 and 0.06 V, respectively, which are obviously smaller than that of PTPA, implying that the electrode polarization is small during the electrochemical oxidation/reduction process and the electrode reactivity is improved by the multi-electron reaction. It is speculated that the compact triphenylamine structure in PFTP results in a higher free radical density, which benefits the migration of the free radical electron located at the central nitrogen atom of the triphenylamine moieties, leading to the smaller potential separation. This result has further proven the obtained UV-vis spectra’s results, in which the facile electron migration in the polymer is in favor of the clear absorption peak red-shifts of PFTP.

Fig. 6 Cyclic voltammograms (CV) of PFTP and PTPA in 0.1 M LiClO₄/CH₃CN vs. Ag/AgCl at a scan rate of 100 mV s⁻¹: (a) PFTP; (b) PTPA; (c) PFTP and PTPA together.

Charge–discharge performance

The charge–discharge behaviors of the obtained polymers as the cathode of lithium ion batteries were further investigated by the lithium ion half-cell method. The initial charge–discharge profiles of the polymers at 20 mA g⁻¹ between 2.5 and 4.2 V are shown in Fig. 7A. As can be seen, PTPA exhibits a discharge capacity of 62.3 mA h g⁻¹ in the initial cycle, with a gradually declining voltage plateau in the voltage range of 3.5–4.1 V. In contrast, under the same conditions, the PFTP electrode shows an initial discharge capacity of 74.2 mA h g⁻¹ with multi-stage voltage platforms, which is in accordance with the redox couples observed in the CV analysis (Fig. 6). Therein, the high voltage platform in the voltage range of 3.7–4.2 V is attributed to the highest redox couple, which provides a discharge capacity of about 23.4 mA h g⁻¹, while the other lower voltage platforms in the voltage range of 2.5–3.7 V are corresponding to the reduction reaction of the other three overlapped redox couples at low voltage, which in total contribute about 50.8 mA h g⁻¹ to the discharge capacity. It can be assumed that the multi-stage discharge curve characteristics of PFTP can be attributed to a one-by-one multi-stage redox reaction process of four electrons in the FTP units; while the improved discharge capacity for the PFTP electrode can be ascribed to a higher free radical density, as well as the resulting high theoretical capacity of PFTP. In addition, the tiny particle morphology of PFTP compared to that of PTPA further facilitates the diffusion of the electrolyte solution to the active-polymer center and will be conducive to the improvement of the PFTP performance to some degree. The low capacity obtained in our experiment, compared to reported results,²⁰ is mainly due to the different cell testing system adopted; a three electrode system is used in the previous report,²⁰ while a lithium ion button cell is applied in our experiment. Furthermore, the un-optimized electrode preparation may be a reason for the low capacity for both PTPA and PFTP. However, as the same electrode preparation technology and testing conditions have been applied in our experiment, the obtained results can be compared in general.

Fig. 7 (A) Initial charge–discharge profiles of (a) PFTP and (b) PTPA electrode material at a constant current of 20 mA g⁻¹ between 2.5 and 4.2 V in LiPF₆ EC/DMC (v/v, 1 : 1) electrolyte versus Li/Li⁺; (B) cycling stability of PTPA and PFTP electrodes at a constant current of 20 mA g⁻¹ between 2.5 and 4.2 V in LiPF₆ EC/DMC (v/v, 1 : 1) electrolyte versus Li/Li⁺.

The cycling stability of the electroactive polymers as the cathode materials of lithium ion batteries was also investigated between 2.5–4.2 V at a constant current of 20 mA g⁻¹. As shown in Fig. 7B, the PTPA electrode exhibits a comparatively stable cycling performance, and after 50 cycles, its discharge capacity changes from 62.3 mA h g⁻¹ to 58.4 mA h g⁻¹ with a 6.3% loss of the initial capacity. PFTP exhibits unstable and fluctuating cycling performances, but has no obvious capacity decay, even after 50 cycles. Also, it can be considered that this degeneration of the cycle performance is possibly caused by significant re-aggregation of PFTP morphology during the charge–discharge cycles, due to the pristine looser stacking micro-structure of PFTP than that of PTPA. Furthermore, it can be observed that although there is unstable fluctuation of the discharge capacity of PFTP, the discharge specific capacity of PFTP after 50 cycles is still higher than that of PTPA, demonstrating that PFTP electrodes present excellent electrochemical stability.

Additional electrochemical measurements for PTPA and PFTP were conducted by means of electrochemical impedance spectroscopy (EIS). A Nyquist plot of the impedance spectra taken under open circuit conditions for cells with the considered cathodes is shown in Fig. 8. In these impedance plots, the initial intercept of the spectrum at the Z_re axis at high frequencies corresponds to the resistance of the electrolyte (R_e). The semicircle in the middle impedance frequencies represents the charge transfer reaction resistance (R_ct), while the straight lines with a slope of about 45° at low frequencies is connected to the lithium ion diffusion in the cathode material and is described by the Warburg impedance (Z_w), which shows the diffusion-controlled process.^29,30 As is shown in Fig. 8, the R_e is almost the same for the two cells with different cathode materials, indicating that there is no significant change in ionic conductivity of the electrolyte or mobility of ions with the different cathode-based cells during the cycling process. However, the charge transfer resistance (R_ct) varies with the different cathode materials; the R_ct for the PTPA and PFTP electrodes are 644 Ω and 175 Ω, respectively. The reduced charge transfer resistance of the PFTP electrode may be attributed to its unique structure and high free radical density, which results in the charge migration occurring smoothly and quickly along the polymer backbone. Furthermore, the smaller particles and the loose stacking structure of PFTP are beneficial to obtain a higher specific surface area, and make the electrolyte penetration easier during the charge–discharge process, which also enhances the decrease of the charge transfer reaction resistance (R_ct).

Fig. 8 EIS of PTPA and PFTP samples in the Li/electrolyte/sample configuration.

Conclusions

A polytriphenylamine derivative (PFTP) with a multiple radical electron structure was successfully synthesized through a chemical oxidation reaction, and the corresponding molecular structure and material characteristics of the polymer were studied in detail. For the first time, PFTP was applied as the cathode material of lithium ion batteries and its electrochemical properties were explored in comparison to PTPA. The results indicated that PFTP as the cathode material exhibited a four-electron redox process and a multi-stage discharge platform. After discharge testing at 20 mA g⁻¹ between 2.5 and 4.2 V, PFTP presented an improved discharge capacity of 74.2 mA h g⁻¹, which was obviously higher than that of PTPA. What’s more, the discharge capacity of PFTP exhibited an unstable cycling performance with a little fluctuation during the cycling testing, but its discharge capacity even after 50 cycles was still higher than that of PTPA, indicative of its excellent capacity retention performance. UV-vis spectra and EIS tests demonstrated the smoother and quicker charge migration in the PFTP polymer bulk than that in PTPA, which was ascribed to the intensive free radical density structure and the improved morphology of PFTP. The improved electrochemical performance of PFTP compared to PTPA indicated that it is a promising strategy to design and prepare polytriphenylamine derivatives with high free radical density, to obtain an advanced organic cathode for lithium ion batteries.

Acknowledgements

The Project was supported by the National Science Foundation of China (Grant No. 51573099), the Natural Science Foundation of Liaoning Province, China (Grant No. 2015020441) and the National Science Foundation for Post-doctoral Scientists of China (Grant No. 2015M570524). This work also was supported by the analysis and testing foundation of Zhejiang University of Technology.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra03248f

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