Simultaneously obtaining fluorescent carbon dots and porous active carbon for supercapacitors from biomass

Abstract

We present a facile and green approach to simultaneously synthesize fluorescent carbon dots and porous active carbon for supercapacitors via a two-step carbonization process from a widely available protein-rich biomass precursor – soybeans. Fluorescent soybean carbon dots (SCDs) with inherent nitrogen-doping were obtained in the first low-temperature carbonization step, while hierarchical porous carbon with an interconnected microstructure was obtained by follow-up high-temperature carbonization with the insoluble residues. The effect of the activating agent (KOH) on the microstructure, conductivity, and nitrogen-doping degree of the porous carbon were explored. Significantly, we demonstrate that much less corrosive KOH (weight ratio: KOH/residue = 0.5/1) is needed to achieve a high specific surface area (1663.1 m² g⁻¹) and remarkable capacitive performance with high specific capacitance (337.3 F g⁻¹ at 1 A g⁻¹). This work provides a completely distinctive way for full utilization of biomass.

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Introduction

Carbon-based materials, such as porous carbons,¹ carbon nanotubes,² graphene³ and carbon dots⁴ are attracting much attention in various fields due to their special physical and chemical properties. For example, porous carbon materials with regular pores, large specific surface area, good thermal stability and chemical stability have found application in CO₂ capture,^5,6 catalysis,⁷ gas adsorption/separation,⁸ energy storage^9,10 etc. Carbon dots, owing to their unique fluorescence properties, stability, biocompatibility and low toxicity, have exhibited potential applications in bio-imaging,¹¹ drug delivery,¹² sensing⁴ and light emitting diode materials.¹³

Up to now, various precursors including fossil materials,^14–16 polymers^6,17 and biomass materials^18–20 have been successfully used to prepare carboneous materials, among which the development of value-added carbon materials from renewable biomass resources is of special meaning in terms of environment protection and commercializing carbon materials. A wide variety of biomass resources have been explored to prepare porous carbon materials for supercapacitors. For example, carbon electrodes obtained from cocoon,²¹ watermelon,²² peanut shell,²³ cow dung,²⁴ rice husk,²⁵ fungi,²⁶ eggplant,²⁷ loofah sponge²⁸ etc. have presented improved electrochemical performances. Renewable bio-based precursors, including orange juice,²⁹ soy milk,³⁰ peach gum polysaccharide,³¹ beer³² and chitosan³³ are also showing promise in preparing fluorescent carbon dots by hydrothermal treatment. So far, most studies on biomass carboneous materials only focus on obtaining single products, and the conversion efficiency and yield of some carbon materials, such as carbon dots, are very low.

Additionally, recent studies also show that doped heteroatoms (such as N, O and S) in the carbon materials play important roles in improving the electrochemical and optical performances of carbon materials.^34,35 Especially, nitrogen as the heteroatom of most attention has been found able to enhance the electrical conductivity, wettability, capacitance, and photoluminescence.^36,37 The incorporation of nitrogen was usually achieved by treating carbon materials with ammonia gas.³⁸ In our previous effort, nitrogen-doped biomass carbon dots with significantly improved photoluminescence was obtained by a mild hydrothermal treatment to hemicellulose dissolved in ammonia water.³⁹ Carbonization nitrogen-containing carbon precursors such as melamine, gelation, or chitosan is another promise way of obtaining efficient N-doping.^18,40,41

Herein, we report a novel two-step carbonization method through which N-doped fluorescent carbon dots (SCDs) and porous carbon (SC) can be simultaneously obtained from natural nitrogen-rich biomass soybean (as shown in Fig. 1). A first low temperature carbonization (200 °C) resulted in fluorescent carbon dots, while a follow-up high temperature carbonization (750 °C) gave porous carbon with high specific surface area (1663.1 m² g⁻¹) and unique interconnected micropores and mesopores network. When tested as supercapacitor electrodes, the SC demonstrate excellent capacitive performance with high specific capacitance of 337.3 F g⁻¹ at a current density of 1 A g⁻¹ in 6 M KOH. More importantly, this study demonstrate a green and efficient way for obtaining porous carbon with desired microstructures by using much less erosive KOH activation (about 1/4 to 1/8 of the previous reported values).^42–44 This method provides a new way for fully utilization of biomass resources while minimizing the production of waste, and it can be extended to other nitrogen-containing biomass residue for achieving sustainable electrode materials.

Fig. 1 A schematic diagram of the preparation route of the SCDs and porous carbon.

Experimental section

Materials

Soybean was purchased from local supermarket, and grounded into powder before use. Polytetrafluoroethylene (PTFE) solution (65 wt%) was purchased from Aladdin Chemistry Co., Ltd. Acetylene black and nickel foam were used as purchased. Potassium hydroxide and hydrochloric acid were of analytical-grade and purchased from Aladdin Chemistry Co., Ltd. Deionized water obtained by Merck Milipore Elix Reference 5 was used throughout the experiments.

Preparation of SCDs

The soybean was grinded into powder, and then placed into a pipe furnace for carbonization at 200 °C for 3 h under an argon atmosphere. The carbonized products were then dispersed in deionized water followed by magnetic stirring for a few hours. The mixture was refrigerator centrifuged at high speed (20 000 rpm) twice to separate the supernatant and dark brown solid products. The supernatant was further purified by filtration with 0.22 μm millipore filters, the obtained transparent and yellowish solution exhibited strong blue fluorescence under a 365 nm UV lamp.

Preparation of SC

The dark brown residue obtained above was dried under vacuum at 50 °C to a constant weight. In the following, the dark brown solid product was immersed in KOH solution for 10 h (weight_KOH/weight_residue = 0, 0.5, 1, 2, 3). The activated dark brown solid product with KOH was dried at 80 °C and further pyrolyzed in a ceramic crucible at 750 °C for 1.5 h under an argon atmosphere with a heating rate of 3 °C min⁻¹. The obtained product was washed with 1.0 M HCl to remove inorganic impurity and then with deionized water till the filtrate became neutral. The products were finally dried overnight at 80 °C in a vacuum. The samples were denoted as SC_x, where SC is the soybean carbon, x is the KOH/residue weight ratio.

Electrochemical performance of SC_x in 6 M KOH

To prepare the testing electrode, the mixed slurry containing 80 wt% SC material, 10 wt% carbon black, and 10 wt% PTFE was mixed and pressed onto a nickel foam current collector. The as-formed electrodes (with a mass of SC approximately 60–80 mg and an area of 1.0 cm²) were then dried at 100 °C in a vacuum oven. Electrochemical characterizations were performed in a three-electrode system, which included the above loaded nickel foam as working electrodes, a Pt wire and a Hg/HgO as counter electrodes and reference electrodes, respectively.

Characterization

Optical absorption spectra (UV-vis) of the SCDs were measured on a UV-3600 (Shimadzu, Japan) spectrometer. A FL-7000 fuorescence spectrometer (Hitachi, Japan) was used to follow the fuorescence spectra of the SCDs. Atomic-force microscopy (AFM) was employed to observe SCDs using a Nanoscope III (Veeco Co. Ltd., USA). Transmission electron microscopy (TEM) was performed using a JEM-2100 operated at an accelerating voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) results were detected on an AMICUS (Shimadzu, Japan) spectrometer with X-ray source Mg Kα, using monochromated Mg Kα radiation (1253.6 eV). X-ray diffraction (XRD) was determined by a D/max-IIIA X-ray diffractometer, in which the high-intensity monochromatic nickel-filtered Cu Kα radiation (λ = 0.15418 nm) was generated at 40 kV and 40 mA. Samples were scanned at a speed of 1° min⁻¹, range from 2θ = 5–80° with step size of 0.02° at room temperature. The Raman spectra were obtained on LabRAM Aramis Smart Raman Spectrometer (HORIBA Jobin Yvon, France). Nitrogen sorption analysis was carried out using an ASAP 2020 accelerated surface area and porosimetry instrument (Micromeritics), equipped with an automated surface area, at 77 K using Brunauer–Emmett–Teller (BET) calculations for the surface area. The pore size distribution plots were recorded from the desorption branch of the isotherms based on the Barrett–Joyner–Halenda (BJH) model. All electrochemical characterizations were carried out on a CHI660e electrochemical workstation (Shanghai Chenhua Instruments Co.) at room temperature.

Results and discussion

Preparation of soybean CDs by low-temperature pre-carbonization

Fig. 1 illustrates the schematic diagram for the preparation of SCDs and SCs. The carbonization process includes two steps treatments: the first step is a low temperature (200 °C) pre-carbonization, through which the fluorescent carbon dots is obtained, and the microstructure and the heteroatoms (e.g. N, O, S) of the raw material soybean are well kept; the second step is a high temperature carbonization (750 °C) with KOH activation, through which the residue of pre-carbonization is partially graphitized to obtain highly porous active carbon.

The optical properties of SCDs were characterized by UV-vis and photoluminescence spectra. In the UV-vis spectrum (Fig. 2a), the SCDs showed two shoulder peaks located at around 275 and 320 nm, similar to previous reports.¹⁸ These shoulder peaks are corresponding to p–p* transition of C=C bonds and n–p* transition of C=O bonds, respectively.⁴⁵ The SCDs had a maximum emission at around 405 nm under 330 nm excitation and the emissions of SCDs were dependent upon excitation wavelength (Fig. 2b), which is typical for carbon dots.³⁹ When the excitation was beyond 330 nm, the emission red shifted and gradually decreased with the increment of excitation, indicating different surface defects on the surface of SCDs.⁴⁶ The quantum yield of SCDs was calculated as 3.17% by using quinine sulfate as standard reference (known 54% at 360 nm excitation). Since soybean contains abundant heteroatoms due to the high protein content, the soybean based carbon dots have inherent N-doping that was considered effective in modifying surface state, and thus achieving brighter fluorescence emissions.⁴⁷ AFM and TEM were conducted to investigate the morphology of SCDs (Fig. 2c and d). Morphology characterizations illustrate that the SCDs were spherical and well-dispersed. The particle diameter of SCDs was in the range of 2–5 nm, while the topographic heights determined by AFM were mostly between 1–2 nm, which is in consistent with previous reports.³⁹ XPS analysis was used to further investigate the chemical structure of the SCDs. As shown in Table 1, the SCDs mainly consisted of C, O and N. A few amount of S was also detected, which should be attributed to the natural protein component of soybean. The C/O ratio of original soybean was 4.89, but this was slightly increased to 5.10 in the resultant SCDs, which indicated that a partly dehydration of soybean occurred in the process of low-temperature carbonization treatment. The C 1s signals (Fig. 3b) could be fitted by four peaks, including C–C/C–H at 284.7 eV, C–N at 285.6 eV, C–O at 286.5 eV and C=N/C=O at 288.2 eV, indicating a graphitic core and N-doped carboxylate-rich surface in SCDs.⁴⁸ The chemical state of N element was clarified by N 1s peaks (Fig. 3c). N element mainly existed in N–H bond (amine or amide)⁴⁹ and others were involved in C–N and N–O moieties. The results indicate fluorescent carbon dots with inherent surface functionalization by O- or N-containing groups and typical excitation dependent photoluminescence emission were successfully obtained in the pre-carbonization step. And the obtained fluorescent carbon dots has excellent stability compared with those obtained by hydrothermal method. It's water solution has almost no precipitation after one year.

Fig. 2 Optical and morphological characterization of SCDs: (a) absorption spectra of the SCDs (inset: photograph under day light (left) and UV radiation (right)); (b) fluorescence spectra with different excitations ranging from 300 nm to 360 nm (inset: curves of Em changes with Ex); (c) AFM image with a height of section trace between two particles (inset); (d) TEM image of SCDs.

Table 1 Elemental composition of soybean and SCDs

Sample	C	O	N	S
Soybean	81.43%	16.66%	1.67%	0.24%
SCDs	81.28%	15.92%	2.49%	0.32%

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Fig. 3 Chemical characterization of SCDs: (a) XPS spectrum of SCDs; (b) C 1s peaks analysis; (c) N 1s peaks analysis.

Preparation of porous SCs by high temperature carbonization

After the fluorescent SCDs were separated, the pre-treated chars were activated with KOH to generate nanoscale pores. As an efficient and traditional method to generate abundant pores, KOH activation has been widely used on various carbon materials to increase surface area and improve electrochemical performance.⁵⁰ Since KOH is erosive and the afterwards neutralization of excess KOH would need a lot of hydrochloric acid, green method requiring less KOH is highly desired.

Here in this work, the effect of KOH on the structure and properties of porous carbon was investigated, wherein soybean chars were firstly impregnated with different amount of KOH aqueous solution at mild condition before being carbonized at a medium high temperature (750 °C). The mass ratio of KOH to carbon residue was varied to control the pore microstructures as well as elemental composition of the SCs. As a control, the sample without KOH activation was also characterized and compared. As shown in the scanning electron microscopy (SEM) image, porous carbon prepared at 750 °C without KOH activation has no obvious cavities (Fig. 4a). For the samples with KOH activation, the resultant porous carbons demonstrate the similar porous structures as well as smooth surface (Fig. 4b–e). It can be seen that the porous carbon activated with more KOH forms larger cavities than those prepared with less KOH, showing that more carbon structures were burned off during the activation process with more activating agent. These results were in consistent with previous reports.²⁴

Fig. 4 Micro-structures of SCs: (a), (b), (c), (d) and (e) are SEM images of SC₀, SC_0.5, SC₁, SC₂ and SC₃; (f) is TEM images of SC_0.5.

The transmission electron microscopy (TEM) in Fig. 4f reveals that there are some unique interconnected micropores and mesopores network within the SC_0.5, and meso/micropores channels can also be observed clearly. These mesopores and interconnections of the carbon materials provided a short pathway for transportation and penetration of electrolyte ions, which were important for fast ion transfer and minimized inner-pore resistance.^51–53

X-ray diffraction (XRD) patterns of the porous carbons with KOH activation are shown in Fig. 5a. Two broad characteristic peaks are located at around 24° and 43°. A well-developed graphitic stacking peak at 24° corresponds to the (002) plane of graphite, while a weak peak at 43° corresponds to the (100) plane of graphite.⁵⁴ These characteristic peaks indirectly ensure the conductivity required for electrochemical application.⁵⁵ With the increment of the proportion of activating agent from 0.5 to 2, the intensity of the (002) diffraction peak at 24° of porous carbon significantly reduces but dramatically broadens and the weak peak at 43° almost disappeared. The change in X-ray pattern indicated that the pure graphitic crystalline structures of the soybean could be destroyed by high content KOH in the chemical activation process. With careful observation, a little increase in the intensity of the (002) diffraction peak from SC₂ to SC₃ presented, probably due to the intrinsic complex hierarchical structure of the soybean.

Fig. 5 (a) XRD pattern of SC_0.5, SC₁, SC₂ and SC₃; (b) Raman spectra of SC_0.5, SC₁, SC₂ and SC₃.

The Raman spectroscopy results confirmed above mentioned discussions (Fig. 5b). In the Raman spectra of the carbon materials, the G band (∼1580 cm⁻¹) is the vibration of sp²-bonded carbon atoms in a 2D hexagonal lattice, while the D band (∼1320 cm⁻¹) is ascribed to edges, other defects, and disordered carbon.⁵⁶ The I_D/I_G ratio of band intensities is a measure of the degree of structural order with respect to a perfect graphitic structure.^57,58 Here, the I_D/I_G ratio of SC_0.5, SC₁, SC₂, and SC₃ were determined to be 1.05, 1.42, 1.76, and 1.31, respectively. The I_D/I_G increases with increasing ratio of KOH/residue from 0.5 to 2, as a result of the exposure of more edges during the pore evolution process. These results indicated that higher ratio of KOH/residue leads to less structural alignment. And a little decrease from SC₂ to SC₃ is possibly owing to the intrinsic complex hierarchical structure of the soybean. The square resistance of SC_0.5, SC₁, SC₂, and SC₃ are determined to be 9.2, 16.0, 29.0 and 40.0 Ω sq⁻¹, respectively. The sample treated with the least KOH resulted in the highest conductivity. This result further confirms that a high degree of intralayer condensation of carbon materials is good to improve their electrical conductivity.

The N₂ adsorption–desorption analysis and pore size distribution (PSD) curves of the synthesized activated carbons are shown in Fig. 6. All the profiles of activated carbon in Fig. 6a reveal similar type I/IV with an increasing slope at higher relative pressures. A type-H4 hysteresis loop extending from P/P₀ = 0.45 to 1 was observed for the samples activated by KOH, indicating the coexistence of both micropore and mesopore structures in these materials.^59,60 The pore size distribution in Fig. 5b shows higher amount of pores in both microporous and mesoporous region for SC and the PSD shift from micropore to mesopore with the increase of the KOH ratio. The pore size of all samples mainly distributes in the range of t 1–7 nm. Table 2 displays the porosity data obtained from N₂ sorption isotherm analysis. It can be seen that the porosity of the resultant carbon materials was significantly influenced by the mass ratio of activating agent. The sample without KOH activation only possesses a very low surface area of 6.92 m² g⁻¹. For the samples with KOH activation, the specific surface area is greatly enlarged to 742.5–1663.1 m² g⁻¹. It can be observed that SC_0.5 has the highest BET surface area as well as the highest micropore volume, whereas SC₁ has slightly lower BET surface area with less micropores. Moreover, increasing the proportion of activating agent from 1 to 2 and 3, the BET surface area and total pore volume of porous carbon are significantly decreased and the average pore diameter is gradually widened from 1.79 to 1.93 and 2.29 nm, probably due to the collapse of pores during the carbonization process.⁶¹ At high KOH/char ratio of 3, excessive burn-off of carbon surface takes place, which leads to breakage in carbon structure, and thereby both surface area and pore volume decrease substantially.²⁴ Both micropores (<2 nm) and mesopores (>2 nm) are needed for supercapacitors, because micropores contribute to high specific surface area for ion adsorption, while larger pores facilitate the efficient diffusion and transport of electrolyte ions.

Fig. 6 (a) Nitrogen adsorption–desorption isotherms; (b) pore size distribution (PSD) profiles of SC samples.

Table 2 Pore characteristics of porous carbon materials

Samples	SBET (m^2 g^−1)	SLangmuir (m^2 g^−1)	Vpore (cm^2 g^−1)	Daver (nm)
SC0	6.0	6.92	0.0113	7.53
SC0.5	1663.1	1788.0	0.7379	1.77
SC1	1577.7	1671.9	0.7069	1.79
SC2	1094.8	1153.2	0.5278	1.93
SC3	742.5	784.8	0.4257	2.29

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Table 3 showed the elemental analysis of SCs from EDX and XPS. The chemical compositions of these SC materials were found mainly consist of C, N and O. It can be seen that there is a decrease in the element of C content and an increase in O content with increasing ratio of KOH. The reduced N-doping level is mainly associated with a high ratio of KOH, which accelerates the decomposition of N-containing frameworks by breaking the C–N bonds to release bonded N.^61,62 It also implies that insufficient defects were created in the carbon network because doped N atoms create structural defects in the carbon material and give rise to more active sites, which is beneficial to rate and cycling performances of supercapacitors. Moreover, the accessible N-containing species would provide chemically active sites and facilitate the power density of supercapacitors since N atoms in heterocyclic rings may influence the spin density and charge distribution of neighboring C atoms and then activate them.^51,63,64 There is a trade-off between the specific surface area, the N-doping level and the number of micropores. Anyway, active hierarchically porous carbons with high specific surface area of up to 1663.1 m² g⁻¹, large pore volume and heteroatoms doping can be obtained by varying the KOH/residue ratio.

Table 3 Chemical composition of SC_x materials determined by energy-dispersive X-ray spectroscopy (EDX) measurements and X-ray photoelectron spectroscopy (XPS)

Samples	EDX (wt%)			XPS (atom%)
	C	O	N	C	O	N
SC0	78.8	16.11	5.09	75.84	23.19	0.97
SC0.5	87.20	12.57	2.06	84.4	14.98	0.82
SC1	82.8	12.57	1.69	83.39	15.78	0.62
SC2	78.24	16.61	3.42	77.35	22.04	0.61
SC3	69.19	23.82	4.61	77.32	22.05	0.64

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Electrochemical capacitive performance of the SCs

The aforementioned discussion demonstrates that the porous carbon derived from soybean is promising candidate materials for energy storage. To confirm this, the electrochemical performances of SC_0.5 and SC₁ as electrode materials for supercapacitor were further investigated in an KOH electrolyte using a three-electrode system. Fig. 7a shows a Nyquist plot of SC_x carbon electrode materials in 6 M KOH in a frequency range from 10 kHz to 10 mHz. The equivalent series resistances were 0.53 Ω and 0.69 Ω for SC_0.5 and SC₁, respectively, showing good ionic conductivity in aqueous electrolytes. An almost vertical line of SC_0.5 was obtained that represents the dominance of double-layer charge-storage at low frequencies, indicating the electrolyte can easily access the pores.^53,65 The Nyquist plots in Fig. 7a show an increment in the relative size of semicircles from the cell with SC_0.5 to SC₁ as expected.

Fig. 7 Electrochemical performance characteristics measured in a three-electrode system in the 6 M KOH electrolyte. (a) Electrochemical impedance spectra of SC_0.5 and SC₁ under the influence of an ac voltage of 5 mV. (b) Cyclic voltammograms of SC_0.5 at different scan rates. (c) Charge–discharge curves of SC_0.5 at different current densities. (d) Specific capacitances of SC_0.5 and SC₁ at different current densities.

Fig. 7b shows cyclic voltammograms (CVs) of SC_0.5 at different scan rate in 6 M KOH. It shows that SC_0.5 exhibited a rectangular shaped voltammetry characteristics even at a high potential scan rate of 100 mV s⁻¹. The rectangular CVs is typical for an ideal electrical double-layer capacitance behavior based on ionic adsorption and exchange.⁶⁶ The CVs of the SC_0.5, revealed that it have excellent charge propagation.

The capacitive performance of the porous SC_0.5 was further tested with galvanostatic charge–discharge experiments at various current densities. The galvanostatic charge–discharge curves are exhibited in Fig. 7c. It can be seen that all curves are almost symmetrical and linear at increased current densities ranging from 1 to 20 A g⁻¹, which is a typical characteristic of an ideal capacitor with good electrochemical reversibility. The discharging profiles of all electrodes show slight deviation from linearity, showing the coupled EDL- and pseudocapacitance.^67,68

Fig. 7d compares the capacitance retention of SC_0.5 and SC₁ at a range of current density from 1 to 20 A g⁻¹. At the current density of 1 A g⁻¹, the specific capacitance of 337.3 and 315.4 F g⁻¹ were obtained for SC_0.5 and SC₁, respectively. At higher current density, the specific capacitance decreased due to the steric limitations of materials over which ions can only partially penetrate into the micropores.^69,70 The highest capacitance is demonstrated by SC_0.5 in good agreement with its higher specific surface area and more favorable amount of micropore and mesopore as compared to the other materials. In addition, it has less breakdown of aligned structural domains in the carbon matrix and thus a better conductivity with the less KOH. The reason is that KOH activation tends to attack the graphitic structure domains in a carbon matrix, resulting in a highly disordered porous structure. While SC₁ has slightly lower specific capacitance, this may be caused by a higher burn off of carbon networks leading to breakage into smaller particles with larger amount of KOH, resulting in decrease in amount of active surface for charge storage and increase in electrical resistivity. The long term performance of SC_0.5 maintains at about 90% of the initial specific capacitance over 3000 cycles, which shows that this SC_0.5 electrode displays excellent stability.

Conclusions

In summary, we proposed a two-step carbonization method for simultaneously soybean carbon dots and porous carbon. The soybean carbon dots obtained in the first-step low-temperature carbonization exhibited bright blue fluorescence under UV radiation. The porous carbon with high surface area obtained in the second-step high-temperature carbonization was successfully used to construct high performance supercapacitors, which exhibited high specific capacitance of 337.3 F g⁻¹ in 6 M KOH at a current density of 1 A g⁻¹ and good stability. Our strategy provided a novel and efficient way for fully utilization of biomass resources by conversing them to value-added materials.

Acknowledgements

This work was supported by the Independent Study Projects of the State Key Laboratory of Pulp and Paper Engineering (2016TS01, 2015C08, 2015ZD03), National Science Foundation of China (51673072), the Science and Technology Program of Guangzhou, China (201504010033), the New Century Excellent Talents in University (NCET-13-0215), the Fundamental Research Funds for the Central Universities, SCUT (201522036) and the Opening Project of the Key Laboratory of Polymer Processing Engineering, Ministry of Education, China (KFKT-201401).

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