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Synthesis of PS−CoFe2O4 Composite Nanomaterial with Improved Magnetic Properties by a One-Step Solvothermal Method Ming Zhong, Peng Fei, Xiaorui Fu, Ziqiang Lei, and Bitao Su* Key Laboratory of Eco-Environment-Related Polymer Materials, Ministry of Education of China, Key Laboratory of Polymer Materials of Gansu Province, College of Chemistry and Chemical Engineering, Northwest Normal University, No. 967 Anning East Road, Lanzhou 730070, People’s Republic of China ABSTRACT: A novel one-step solvothermal method was developed and polystyrene (PS)−cobalt ferrite (CoFe2O4) composite nanomaterial (PS−CoFe2O4) was successfully synthesized by this method using ferric chloride hexahydrate (FeCl3·6H2O), cobalt chloride hexahydrate (CoCl2·6H2O), and the monomer styrene as the precursors. Fourier transform infrared (FT-IR) spectroscopy, X-ray diffraction (XRD), and transmission electron microscopy (TEM) demonstrated the successful preparation of the PS−CoFe2O4 composite material, with the two phases of polystyrene and CoFe2O4 being hybridized at the nanoscale and some strong interaction existing between them. The results of thermogravimetric analysis (TGA) indicated that the inorganic component could improve the thermal stability of the polymer PS. Importantly, the magnetic properties of the as-prepared PS− CoFe2O4 samples were found to be superior to those of pure CoFe2O4, prepared under the same conditions (180 °C for 12 h). The improved magnetic properties can mainly be attributed to the hybrid structure, the high degree of crystallinity, and the size of CoFe2O4 in the composite material.

1. INTRODUCTION In recent years, considerable efforts have been focused on the design and controllable preparation of organic−inorganic composite nanomaterials,1−7 because such nanomaterils combine the properties of both the inorganic and the organic components, such as thermal stability, mechanical strength, or optical properties, and simultaneously present some new properties when they composite on the nanoscale.8 Organic− inorganic composite materials have been used in versatile fields such as chemical sensing, catalysis, optics, microelectronics, coatings, and medicines,9,10 because of their widely applicable multifunctionality. Polymers have been used as the organic component and prepared by many methods. Among the polymers used in these materials, polystyrene (PS)6,8 has received a great deal of attention because of its good properties such as partially positively charged CH2 groups,11 easy synthesis, low cost, chemical resistance, water resistance, and easy molding. Spinel ferrite magnetic nanomaterials are known to exhibit unique properties such as enhanced magnetic moments, exchange-coupled dynamics, quantization of spin waves, and giant magnetoresistance, resulting in new potential applications in electromagnetic interference (EMI) shielding,12 supercapacitance,13 biomedicine,14,15 and so on. As a magnetic material, CoFe2O4, one of the most important spinel ferrites, exhibits negative surface charge without any surfactant11 and shows a relatively large magnetic hysteresis that is different from that of other spinel ferrites such as superparamagnetism ferrites. It has very broad prospects in various applications such as high-density recording media and magnetic fluids because of its unique physical and chemical properties, large magnetocrystalline anisotropy, high coercivity, moderate saturation magnetization, large magnetostrictive coefficient, chemical stability, and mechanical hardness.16−21 However, the question © XXXX American Chemical Society

of how to combine a magnetic component with a polymer remains the focus of intensive investigations. Several reported methods of synthesizing polymer−magnetic composite nanomaterials involve relatively long procedures. Primarily, magnetic nanomaterials have been synthesized through coprecipitation,22 sol−gel processing,23 microemulsion techniques,24 and hydrothermal processing,25 and then, the monomer molecules are polymerized on the surface of the magnetic component or inorganic oxide seeded on the polymer to produce polymer−magnetic composite nanomaterials. Sometimes, the aid of a coupling/modifying agent has been necessary during the preparation of inorganic−organic material. For instance, Huang and Tang26 used presynthesized PS latex and FeCl2/FeCl3 as the precursor to synthesize polystyrene coated by Fe3O4 nanoparticles through a homogeneous seeded growth. Prasanna et al.27 used a sol−gel citrate−nitrate method to synthesize CoFe2O4 and then prepared PANI/CoFe2O4 nanocomposites by an in situ polymerization. Zhang et al.28 reported a method called “swelling−diffusion−interfacialpolymerization”. Stearic acid-modified ferroferric oxide (Fe3O4−SA) was first prepared by the coprecipitation method. Subsequently, the surface of Fe3O4−SA was modified with sodium dodecylbenzene sulfonate (SDBS), and then styrene was added with ammonium peroxodisulfate (APS) as an initiator to form core−shell Fe3O4@polystyrene (Fe3O4@PS) nanoparticles. In general, the as-prepared composite materials had core−shell structures. However, the relatively complex schedules, long reaction times, and high reaction temperatures involved in their preparation are common problems. Therefore, Received: January 28, 2013 Revised: May 11, 2013 Accepted: May 28, 2013

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these synthetic procedures are difficult to use for preparing inorganic−organic hybrid nanomaterials and to apply in largescale and economical production. According to the procedures and conditions of preparing inorganic oxides and some polymers, especially those with πelectron structures or positively charged polymer chains,11 we recently designed a novel method, called a one-step solvothermal method, and successfully synthesized PS− CoFe2O4 composite nanomaterial using this method, in which two phases are hybridized in a nearly monodisperse manner on the nanoscale. This method solves the problems described in the previous paragraph. The reactions involved are as follows

Scheme 1. Schematic Representation of the One-Step Formation of PS−CoFe2O4 Nanocomposite by a Solvothermal Process

Corp., Tokyo, Japan). Cu Kα radiation (λ = 0.154187 nm) was used with a generator voltage of 40 kV and a current of 60 mA. The average particle sizes of the samples were calculated by the Scherrer equation D = 0.89λ/(cos θ), where D is the average particle size, λ is the X-ray wavelength, β is the full width at half-maximum (fwhm), and θ is the diffraction angle. The morphologies of the samples were characterized by transmission electron microscopy (TEM) using a JEM-1200 EX/S microscope (JEOL, Tokyo, Japan). The samples were dispersed in ethanol and vibrated for 3 h, after which they were deposited on a copper grid covered with a perforated carbon film. An FTS3000FX infrared spectrometer (Digilab Inc., Canton, MA) was employed for Fourier transform infrared (FT-IR) spectroscopy analysis in the range of 400−4000 cm−1. The KBr pellet technique was used to prepare samples for recording IR spectra. Thermogravimetric analysis (TGA) was performed on a TA Instruments (New Castle, DE) 2050 thermogravimetric analyzer at a heating rate of 10 °C/min from 10 to 800 °C in air atmosphere. The magnetic properties were detected by vibrating sample magnetometer (Lakeshore 7304, Lake Shore Cryotronics Inc., Westerville, OH).

Transmission electron microscopy (TEM) and thermogravimetric analysis (TGA) suggest that some strong interaction exists between polystyrene and CoFe2O4 in PS−CoFe2O4. The structure, morphology, and magnetic properties of the obtained samples were also investigated in detail by X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, and vibrating sample magnetometry (VSM).

2. EXPERIMENTAL PROCEDURE 2.1. Materials. The monomer styrene (S) was obtained from Tianjin Guangfu Chemical Industry Research Institute. Ferric chloride hexahydrate (FeCl3·6H2O) and cobalt chloride hexahydrate (CoCl2·6H2O) were purchased from Yantai Shuangshuang Chemical Co., Ltd., and Shanghai Zhongqin Chemical Reagent Co., Ltd., respectively. Ethyl alcohol was supplied by Anhui Ante Biological Chemical Co., Ltd. Potassium persulfate (KPS) and ammonia solution were both received from Tianjin Zhiyuan Chemical Reagent Co., Ltd. All of these reagents were analytically pure. Distilled water was used throughout. 2.2. Synthesis of the Samples. A one-step solvothermal method was carried out to prepare PS−CoFe2O4 composite nanomaterials without any surface modification of the CoFe2O4 magnetic particles. FeCl3·6H2O (2.1674 g), CoCl2·6H2O (0.9518 g) (2:1 molar ratio of Fe3+ to Co2+) and 2 mL of monomer S were added to 40 mL of an ethanol/deionized water mixture (3:1, v/v) and placed in a 100 mL autoclave, and the pH of the system was adjusted to 10 by the dropwise addition of ammonia solution. Then, N2 was allowed into the autoclave to exclude air for 30 min, and a catalytic amount of KPS was added. Finally, the autoclave was left in an oven at constant temperature (140−180 °C) for 12 h to investigate the effects of temperature on the preparation of PS−CoFe2O4 composite nanomaterial with optimal magnetic properties. The product was centrifuged and washed several times with deionized water and ethanol. Then, the final sample, in which the amount of PS was about 34%, was obtained after being dried in an oven at 100 °C for 12 h. Pure CoFe2O4 was also prepared using the same method without the addition of the monomer. The possible formation mechanism of the PS− CoFe2O4 composite nanoparticles is shown in Scheme 1. 2.3. Characterization. X-ray diffraction (XRD) patterns were recorded in the range of 2θ = 1−90° by step scanning with a Rigaku D/Max-2400 X-ray diffraction meter (Rigaku

3. RESULTS AND DISCUSSION 3.1. X-ray Diffraction Analysis. X-ray diffraction patterns of pure CoFe2O4 obtained at 180 °C and of PS−CoFe2O4 obtained at 140, 160, and 180 °C are presented in Figure 1. From the pattern of pure CoFe2O4, it can be found that, in the

Figure 1. X-ray diffraction patterns of (a) pure CoFe2O4 obtained at 180 °C and (b−d) PS−CoFe2O4 obtained at temperatures of (b) 140, (c) 160, and (d) 180 °C. B

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respectively, in agreement with the results from the XRD patterns. Nanocrystallites of cobalt ferrite present aggregation, whereas PS−CoFe2O4 shows a relatively good monodispersion, which indicates that PS can effectively prevent the aggregation of magnetic CoFe2O4 nanoparticles. Figure 2c, a highmagnification TEM image of pure CoFe2O4, further displays the aggregation phenomenon and weak crystallization. In contrast, PS−CoFe2O4 (Figure 2d) shows a good distribution and crystallinity, and the lattice-fringe spacing (marked in red in Figure 2d) of 0.299 nm is assigned to the (220) planes of CoFe2O4. 3.3. FT-IR Spectroscopic Characterization. FT-IR spectra of PS, CoFe2O4, and PS−CoFe2O4 are shown in Figure 3. In the spectrum of pure PS, the peaks at 695 and 757

2θ range from 25° to 80°, all peaks match well with facecentered cubic (fcc) cobalt ferrite (JCPDS card 22-1086), corresponding to Miller indices (220), (311), (400), (422), (511), (440), and (533). The average size of the CoFe2O4 particles was 13 nm. In the patterns of the PS−CoFe2O4 samples, the peaks corresponding to fcc cobalt ferrite also appeared, indicating that PS does not affect the phase structure of CoFe2O4. However, their intensities increased and their halfwidths decreased with increasing solvothermal temperature, which indicates that the degree of crystallinity and the size of the samples also increased. The average sizes of CoFe2O4 particles in the PS−CoFe2O4 composites obtained at 140, 160, and 180 °C were 11, 13, and 14 nm, respectively. The broad band observed in the range of 15−25° is possibly due to the packed PS chains and, thus, extensive π−π orbital overlap in the side chains of PS.29 These results confirm the presence of PS and CoFe2O4 in the material prepared by the proposed onestep method. A comparison of the patterns of pure CoFe2O4 and PS− CoFe2O4 prepared at the same temperature 180 °C (Figure1a,d) shows that the diffraction intensity of the latter is stronger than that of the former, indicating that PS is advantageous for the regular growth of CoFe2O4 to form more perfect crystals. In addition, in the spectra of PS− CoFe2O4, the diffraction peaks of CoFe2O4, especially corresponding to Miller indices (311), shifted to higher 2θ values (see the inset of Figure 1). This phenomenon suggests that PS has an effect on the lattice structure of CoFe2O4 and that some strong interaction exists between CoFe2O4 and PS. The size (11 nm) of the pure CoFe2O4 particles was smaller than that (14 nm) of the CoFe2O4 particles in the PS− CoFe2O4 composite. 3.2. TEM Analysis. Figure 2 presents TEM images of pure CoFe2O4 and PS−CoFe2O4 prepared at the same temperature of 180 °C at different magnifications. From the lowmagnification images of pure CoFe2O4 and PS−CoFe2O4 (Figure 2a,b), it can be clearly observed that the sizes of the pure CoFe2O4 and the PS−CoFe2O4 composite particles are all nanoscale, and their average sizes are 12 and 15 nm,

Figure 3. FT-IR spectra of (a) PS, (b) CoFe2O4, and (c) PS− CoFe2O4 nanoparticles.

cm−1 are attributed to the C−H out-of-plane vibrations of the benzene ring and are also a characteristic of a singly substituted benzene ring. The absorbance peaks at 1447 and 1492 cm−1 correspond to CC stretching of the benzene ring,8 and those at 2846 and 3024 cm−1 correspond to −C−H and C−H stretching vibrations of PS. In the IR spectrum of pure CoFe2O4, the band at about 596 cm−1 is the typical absorbance of the MO inherent stretching vibration, and those at about 1600 and 3500 cm−1 are O−H stretching absorbances from the surface O−H of CoFe2O4 and adsorbed H2O. Compared to the spectra of PS (Figure 3a) and CoFe2O4 (Figure 3b), in the spectrum of PS−CoFe2O4 (Figure 3c), all of the characteristic peaks of PS and CoFe2O4 appear, but their intensities decrease in the composite material. These results confirm the successful preparation of a PS− CoFe2O4 hybrid material at the nanoscale. 3.4. Thermogravimetric Analysis. Figure 4 shows TGA curves for pure PS and PS−CoFe2O4. As revealed in Figure 4, the TGA curve of PS−CoFe2O4 shows an obvious weight loss at about 314 °C, whereas the beginning of the loss for pure PS occurs at about 228 °C, attributed to the degradation of PS in both cases. The two samples reached constant-weight status at 422 and 408 °C, respectively. The results reveal that the thermal stability of the PS in the composite material is higher than that of pure PS. The good thermal stability of the PS in the composite material can be understood by the fact that some strong interaction between PS and CoFe2O4, also deduced from the XRD results for PS−CoFe2O4, restricts the thermal

Figure 2. TEM images of (a,c) CoFe2O4 and (b,d) PS−CoFe2O4 nanoparticles. C

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Table 1. Magnetic Parameters of Pure CoFe2O4 and PS− CoFe2O4 Prepared at Different Temperatures temperature (°C) sample CoFe2O4

PS−CoFe2O4

magnetic parameters

140

160

180

Ms (emu/g) Mr (emu/g) Hc (Oe) Ms (emu/g) Mr (emu/g) Hc (Oe)

43.2 0.72 22.82 17.5 0.50 42.01

45.7 1.92 57.00 42.6 1.84 58.52

44.5 3.12 103.64 46.6 5.58 223.49

pure CoFe2O4 (also seen in Figure 5). The coercivity value (223.49 Oe) of PS−CoFe2O4 composite material is more than twice that (103.64 Oe) of pure CoFe2O4. Figure 6 shows the typical hysteresis loops of bare CoFe2O4 and PS−CoFe2O4 prepared at the optimal temperature of 180

Figure 4. TGA curves of (a) pure PS and (b) PS−CoFe2O4.

motion of PS in the composite and therefore improves its thermal stability. The weight loss of PS−CoFe2O4 was about 34%, meaning that the content of PS was about 34% in the prepared PS−CoFe2O4 composite material. 3.5. Magnetic Property Measurements. The magnetic properties of the CoFe2O4 and PS−CoFe2O4 nanomaterials were investigated by VSM. The magnetization curves, in an applied magnetic field sweeping from −12 to +12 kOe at room temperature, of pure CoFe2O4 (Figure 5A) and PS−CoFe2O4 composite (Figure 5B) obtained at different temperatures of 140, 160, and 180 °C are shown in Figure 5. The magnetic parameters of all samples, namely, the saturation magnetization (Ms), remanent magnetization (Mr), and coercivity (Hc), are also listed in Table 1. As shown in Figure 5 and Table 1, the magnetic parameters of pure CoFe2O4 change with increasing reaction temperature, where the value of Hc increases markedly from 22.82 to 103.64 Oe but the change in the saturation magnetization (Ms) is not notable. Compared to those of pure CoFe2O4, the magnetic properties of PS−CoFe2O4 composite nanomaterials are remarkably improved when the temperature changes from 140 to 180 °C. For example, the Ms value increases from 17.5 to 46.6 emu/g, the Mr value from 0.50 to 5.58 emu/g, and the Hc value from 42.01 to 223.49 Oe. It is important that the magnetic properties, especially the coercivity Hc, of the PS−CoFe2O4 composite material prepared at a proper temperature of 180 °C are markedly superior to those of

Figure 6. Magnetic hysteresis (M−H) curves of (a) pure CoFe2O4 and (b) PS−CoFe2O4 nanoparticles at 180 °C.

°C. As-prepared CoFe2O4 and PS−CoFe2O4 nanomaterials obtained at 180 °C exhibit an obvious hysteretic behavior. In Figure 6 and the inset, it is further clearly observed that the saturation magnetization (Ms) and coercivity (Hc) of PS− CoFe2O4 composite are 46.60 emu/g and 223.49 Oe,

Figure 5. Magnetization curves of pure (A) CoFe2O4 and (B) PS−CoFe2O4 prepared at temperatures of (a) 140, (b) 160, and (c) 180 °C. D

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(2) Malika, M. A.; Wani, M. Y.; Hashim, M. A. Microemulsion method: A novel route to synthesize organic and inorganic nanomaterials. Arabian J. Chem. 2012, 5, 397. (3) Gao, Y.; Song, Y. H.; Zheng, Q. Miniemulsion polymerized titania/polystyrene core−shell nanocomposite particles based on nanotitania powder: Morphology, composition and suspension rheology. Colloids Surf. A 2012, 411, 40. (4) Li, Y. X; Yin, D. P; Wang, Z. Q; Li, B.; Xue, G. Controlling the heterocoagulation process for fabricating PS−CoFe2O4 nanocomposite particles. Colloids Surf. A 2009, 339, 100. (5) Zhang, M.; Gao, G.; Zhao, D. C.; Li, Z. Y.; Liu, F. Q. Crystallization and Photovoltaic Properties of Titania-Coated Polystyrene Hybrid Microspheres and Their Photocatalytic Activity. J. Phys. Chem. B 2005, 109, 9411. (6) Xu, H.; Cui, L. L.; Tong, N. H.; Gu, H. C. Development of High Magnetization Fe3O4/Polystyrene/Silica Nanospheres via Combined Miniemulsion/Emulsion Polymerization. J. Am. Chem. Soc. 2006, 128, 15582. (7) Bourgeat-Lami, E. J. Organic−Inorganic Nanostructured Colloids. Nanosci. Nanotechnol. 2002, 2, 1. (8) Zhang, M.; Gao, G.; Li, Z. Y.; Liu, F. Q. Titania-Coated Polystyrene Hybrid Microballs Prepared with Miniemulsion Polymerization. Langmuir 2004, 20, 1420. (9) Bronstein, L. M.; Joo, C.; Karlinsey, R.; Ryder, A.; Zwanziger, J. W. Nanostructured Inorganic−Organic Composites as a Basis for Solid Polymer Electrolytes with Enhanced Properties. Chem. Mater. 2001, 13, 3678. (10) Zhang, X. J.; Bagwe, R. P.; Tan, W. H. Development of OrganicDye-Doped Silica Nanoparticles in a Reverse Microemulsion. Adv. Mater. 2004, 16, 173. (11) Martins, P.; Caparros, C.; Gonçalves, R.; Martins, P. M.; Benelmekki, M.; Botelho, G.; Lanceros-Mendez, S. Role of Nanoparticle Surface Charge on the Nucleation of the Electroactive βPoly(vinylidene fluoride) Nanocomposites for Sensor and Actuator Applications. J. Phys. Chem. C. 2012, 116, 15790. (12) Kamchi, N. E.; Belaabed, B.; Wojkiewicz, J. L.; Lamouri, S.; Lasri, T. Hybrid Polyaniline/Nanomagnetic Particles Composites: High Performance Materials for EMI Shielding. J. Appl. Polym. Sci. 2013, 127, 4426. (13) Kumbhar, V. S.; Jagadale, A. D.; Shinde, N. M.; Lokhande, C. D. Chemical synthesis of spinel cobalt ferrite (CoFe2O4) nano-flakes for supercapacitor application. Appl. Surf. Sci. 2012, 259, 39. (14) Noppakun, S.; Christopher, C. B.; Cuie, W.; James, W. Transition metal-substituted cobalt ferrite nanoparticles for biomedical applications. Acta Biomater. 2013, 9, 5830. (15) Koon, G. N.; Lihan, T.; En-Tang, K. Magnetic Core−Polymer Shell Nanoparticles: Synthesis and Biomedical Applications. Nanotechnol. Life Sci. 2011, 4, 347. (16) Bao, N. Z.; Shen, L. M.; An, W.; Padhan, P.; Turner, C. H.; Gupta, A. Formation Mechanism and Shape Control of Monodisperse Magnetic CoFe2O4 Nanocrystals. Chem. Mater. 2009, 21, 3458. (17) Leng, C.; Wei, J.; Liu, Z.; Shi, J. Influence of imidazolium-based ionic liquids on the performance of polyaniline−CoFe2O4 nanocomposites. J. Alloys Compd. 2011, 509, 3052. (18) Zi, Z.; Sun, Y.; Zhu, X.; Yang, Z.; Dai, J.; Song, W. Synthesis and magnetic properties of CoFe2O4 ferrite nanoparticles. J. Magn. Magn. Mater. 2009, 321, 1251. (19) Turtelli, S.; Giap, R.; Duong, V.; Nunes, W.; Grossinger, R.; Knobel, M. Magnetic properties of nanocrystalline CoFe 2 O 4 synthesized by modified citrate-gel method. J. Magn. Magn. Mater. 2008, 320, 339. (20) Eshraghi, M.; Kameli, P. Magnetic properties of CoFe2O4 nanoparticles prepared by thermal treatment of ball-milled precursors. Curr. Appl. Phys. 2011, 11, 476. (21) Elsayed, A. H.; Mohy Eldin, M. S.; Elsyed, A. M.; Abo Elazm, A. H.; Younes, E. M.; Motawe, H. A. Synthesis and Properties of Polyaniline/Ferrites Nanocomposites. Int. J. Electrochem. Sci. 2011, 6, 206.

respectively, which are higher than the values of 44.50 emu/g and 103.64 Oe, respectively, for pure CoFe2O4 prepared under the same conditions. In theory, adding nonmagnetic PS to CoFe2O4 should weaken the magnetic properties of the composite material. However, we found improved magnetic properties of CoFe2O4 due to the addition of PS. We believe that the hybrid nanostructure of the composite material makes a greater contribution to the improvement of the magnetic properties. First, the hybrid nanostructure offers an opportunity for PS to fully enwrap the CoFe2O4 particles, resulting in an increase in the anisotropy of the CoFe2O4 particles. In addition, the integration of the partially positively charged CH2 groups of the PS chain and the negatively charged CoFe2O4 component without any surfactant generates an electronic interaction,11,30 resulting in the change of the charge density of the CoFe2O4 surface, which affects the spinning mechanism of electrons in the system.31 We also found that the amount of S affects the magnetic properties of CoFe2O4 as well, but the specific details are still being investigated at the moment and will be reported shortly. According to crystal principles, the surface anisotropy becomes greater when the degree of crystallinity of a material is higher. In the PS−CoFe2O4 composite material, the CoFe2O4 crystals become more perfect because of the presence of PS (see Figure 2d). In addition, the size of a magnetic material is also a factor, and the magnetic properties, especially the saturation magnetization (Ms), increase with increasing size. This relationship between size and magnetic properties was observed for the materials prepared in this study.

4. CONCLUSIONS In this study, PS−CoFe2O4 composite nanomaterial was successfully prepared using a novel one-step solvothermal method with FeCl3, CoCl2, and the monomer styrene (S) as the precursors. Compared to those of pure CoFe2O4, the magnetic properties of the as-prepared PS−CoFe2O4 samples are improved, mainly because of the hybrid structure, the high degree of crystallinity, and the size of the composite material.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] or [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was performed with the financial support of the National Natural Science Foundation of China (Project 21174114), the Program for Changjiang Scholars and Innovative Research Team in University of Ministry of Education of China (Project IRT1177), the Scientific and Technical Plan Project of Gansu Province (No. 1204GKCA006), the Natural Science Foundation of Gansu Province (Project 1010RJZA024), and the Scientific and Technical Innovation Project of Northwest Normal University (nwnu-kjcxgc-03-63).

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