BaFe12O19-chitosan Schiff-base Ag (I) complexes embedded in carbon nanotube networks for high-performance electromagnetic materials

The multiwalled carbon nanotubes/BaFe12O19-chitosan (MCNTs/BF-CS) Schiff base Ag (I) complex composites were synthesized successfully by a chemical bonding method. The morphology and structures of the composites were characterized with electron microscopy, Fourier transform infrared spectroscopy and X-ray diffraction techniques. Their conductive properties were measured using a four-probe conductivity tester at room temperature, and their magnetic properties were tested by a vibrating sample magnetometer. The results show that the BF-CS Schiff base Ag (I) complexes are embedded into MCNT networks. When the mass ratio of MCNTs and BF-CS Schiff base is 0.95:1, the conductivity, Ms (saturation magnetization), Mr (residual magnetization), and Hc (coercivity) of the BF-CS Schiff base composites reach 1.908 S cm−1, 28.20 emu g−1, 16.66 emu g−1 and 3604.79 Oe, respectively. Finally, a possible magnetic mechanism of the composites has also been proposed.

It is also known that chitosan (CS) Schiff-base is a type of important organic-magnetic material due to its low-cost possessing, environmental friendliness and inherent chirality [28][29][30][31] . Its excellent electric and magnetic properties result from charge transfer along the -C=N-group and the magnetic moment formed by the electronic rotation of -C=N-, respectively [32][33][34] . Hence, CS Schiff-base has been chosen to adjust the overall magnetic properties of the final composites. Therefore, we attempted to graft the CS Schiff-base onto the surface of BF to obtain enhanced magnetic and conductive properties. However, the conductivity of BF-CS Schiff-base composites is still poorer than common semiconductor materials. Presently, carbon nanotubes (CNTs) are well known for their high aspect ratio, high Young's modulus, chemical stability, nanometric dimensions and good electrical properties 35 . CNTs as dopant materials to improve the conductivity of electromagnetic composites have been reported. Wu's group prepared CNTs/carbonyl iron complex absorbers and found their conductivity to be 13.54 S cm −1 when the ratio of CNTs is 6.6% 36 . Poly((α -methylstyrene)-co-butylmethacrylate) grafted multi-walled CNTs (MCNTs) with conductivity of 1.695 S cm −1 were synthesized by Liu et al. 37 . Wang's group reported that the hybrid super-aligned carbon nanotube/carbon black conductive networks deliver excellent electrical conductivity and capacity in lithium ion batteries 38 . Therefore, we used MCNTs to build the conductive networks and absorb onto the surface of the BF-CS Schiff-base composites to prepare MCNTs/BF-CS Schiff base composite materials. In addition, Ag (I) metal ions coordinated with N atoms of the -CH=N-group can change the permeability and permittivity of Schiff base to gain improved electromagnetic properties 39 , so we further fabricate the MCNTs/BF-CS Schiff base Ag (I) complexes with a conductive network structure under mild conditions.
The as-prepared MCNTs/BF-CS Schiff base Ag (I) complexes exhibit excellent conductivity and relatively high M s compared to the pure BF-CS Schiff base. The reason for the enhancement was studied, and the magnetic mechanism is proposed according to the effects of each component in the composites. Our work reveals the important role of conductive networks in the optimization of electromagnetic materials.
Purification of MCNTs. MCNTs were added into concentrated nitric acid and refluxed for 5 h at 90 o C, and the precipitate was filtered and washed with 0.1 mol L −1 HCl and deionized water three times. Finally, the product was dried under vacuum at 50 o C for 24 h.
Synthesis of BF-CS Schiff base. 2.0 g chitosan was dissolved into 50 mL diluted acetic acid (pH = 1). Then 2.0 g BF was added into the above solution with ultrasonic treatment for 0.5 h. After that, a 10 mol/L NaOH solution was slowly dropwise added into the above solution until the pH value of the system equaled 13. The mixture was heated to 60 o C. Then, two drops of the 25% glutaraldehyde were added into the mixture stirring for 2 h. Finally, the precipitate was filtrated and washed with deionized water, ethanol, and acetone, respectively. The product was dried under vacuum at 50 o C for 12 h. After that, 1.0 mL glyoxal was added into 80 mL absolute ethanol, stirring for 10 min. Then, 2.0 g of the above product was added into the above solution refluxing for 12 h at 75 o C. After that, the precipitate was filtered and washed with ethanol 3 times. Finally, the precipitate was dried under vacuum at 30 o C for 10 h. Characterization. Fourier transform infrared (FTIR) spectra were obtained using a Nicolet 5700 FTIR with the KBr method. X-ray diffraction (XRD) patterns of the samples were conducted by using a Philps-pw3040/60 diffractometer with Cu Kα radiation (λ = 0.15418 nm). The morphologies and microstructure of the samples were observed by a scanning electron microscope (SEM, Nova NanoSEM450) and a transmission electron microscope (TEM, JEOL JEM2010), respectively. The electrical conductivities were measured with a four-probe resistivity instrument (SDY-4) at room temperature using pressed pellets of sample powder with a thickness of about 1 mm and a diameter of 1 cm. A Lakeshore 7404 vibrating sample magnetometer was used to measure the magnetization in applied magnetic fields within the range of − 10 to + 10 kOe at room temperature.

Results and Discussion
Synthesis route of MCNTs/BF-CS Schiff base Ag (I) complexes. Figure 1 illustrates the synthesis route of MCNTs/BF-CS Schiff base Ag (I) complexes. The MCNTs/BF-CS Schiff base Ag (I) complexes are prepared through a chemically bonded method using purified MCNTs with a large number of -COOsurface groups. MCNTs are adsorbed onto the surface of BF-CS Schiff base Ag (I) complexes by hydrogen bonding interactions between MCNTs and BF-CS Schiff base, so BF-CS Schiff base Ag (I) complexes are successfully embedded into the MCNT network. The network can certainly provide numerous electronic transfer paths to enhance the conductive behaviors of the composite. Moreover, the combination of inorganic magnet BF with organic magnet CS Schiff-base may yield an outstanding magnet with a balance of permittivity and permeability. Meanwhile, Ag (I) metal ions coordinated with N atoms of the -CH=N-groups can adjust the permeability and permittivity of the Schiff base to gain optimal electromagnetic properties 39  Chemical bonding between components. Figure 3 gives FTIR spectra of (a) BF-CS Schiff base, Microstructures of the composites. SEM and TEM have been employed to observe the microstructure of composites. As shown in Fig. 4a, the SEM image directly shows that the BF-CS Schiff-base composite (without MCNTs) is assembled by aggregation of BF and CS particles. It seems that the BF particles are almost covered by the CS Schiff-base. In Fig. 4b Fig. 5a indicates that the BF-CS Schiff base composites have been produced. The black components are BF, and CS Schiff base locates on the surface of BF. As shown in Fig. 5b-d, the BF-CS Schiff base Ag (I) complexes, as an inorganic-organic hybrid magnetic material, are embedded uniformly into the MCNT network. The network provides paths for electron transfer, which is helpful for improving the conductivity of the MCNTs/BF-CS Schiff base Ag (I) complexes. Therefore, we can conclude that the MCNTs/BF-CS Schiff base Ag (I) complexes with network structures have been prepared.
The elemental composition of MCNTs/BF-CS Schiff base Ag (I) complexes has been analyzed by EDS. Figure 6 gives  Conductivity properties. Figure 7 shows the electrical conductivity measurements of (a) BF-CS  Table 1. The conductivity of the BF-CS Schiff base is only 0.002 S cm −1 , because the BF is an insulator and only CS Schiff base contributes to the total conductivity. From Fig. 7, we can see that the conductivity of MCNTs/BF-CS Schiff base Ag (I) complexes increases with the contents of MCNTs. When the mass ratio of MCNTs and BF-CS Schiff base is 0.15:1, the conductivity of MCNTs/BF-CS Schiff base Ag (I) complexes reaches 0.037 S cm −1 . When the ratio is 0.95:1, their conductivity goes up to 1.908 S cm −1 . Three possible reasons account for the conductivity changes of MCNTs/BF-CS Schiff base Ag (I) complexes. First, the CS Schiff base contributes to the total conductivity. Second, BF-CS Schiff base Ag (I) complexes enhance the conductivity of composites due to doping effects of Ag (I) ions. Third, the MCNT network in the MCNTs/ BF-CS Schiff base Ag (I) complexes (Fig. 8IV) drastically improves their conductivity. The more the MCNTs are there, the bigger the improvement is.      composites reach 28.20 emu g −1 , 16.66 emu g −1 and 3604.79 Oe, respectively. According to previous reports 21 , it is understood that a non-magnetic CS coating layer and soft magnetic MCNTs impede the total magnetization and cause a decrease in the saturation magnetization. In addition, the H c of MCNTs/ BF-CS Schiff base Ag (I) complexes initially drops and then goes up owing to the influence of MCNTs. The detailed analysis of the magnetic mechanism of MCNTs/BF-CS Schiff base Ag (I) complexes will be discussed as follows.
The proposed magnetic mechanism of the MCNTs/BF-CS Schiff base Ag (I) complexes is shown in Fig. 8(I-III). There are at least three factors related to the overall magnetic performance. First, BF as an inorganic magnetic component in the composites plays an important role for the total magnetic properties. Figure 8I shows the BF hysteretic behaviors with M s , M r , and H c of 46.04 emu g −1 , 28.62 emu g −1 and 3610.15 Oe, respectively. In addition, MCNTs with M s about 14.02 emu g −1 possess soft magnetic behaviors (Fig. 8II) due to the quantum size and microscopic aggregates. Moreover, Schiff-base metal complexes also contribute to the magnetic properties. In Fig. 8III, when the Ag (I) complexes are excited, the electron of N atoms jumps into the 5s orbit of Ag to generate magnetic moments. When a magnetic field is applied, the Schiff base Ag (I) complexes perform obviously magnetic behaviors. This is due to the   fact that electrons are the primary stakeholder for magnetic materials, and the total magnetic moments of atoms are the sum of electronic orbital and spin magnetic moments. Therefore, the magnetic mechanism tends to be complicated and diversified for the MCNTs/BF-CS Schiff base Ag (I) complexes. In summary, the magnetic properties of the MCNTs/BF-CS Schiff base Ag (I) complexes are a result of the relationships between BF, the CS Schiff-base Ag(I) complex and MCNTs.

Conclusions
The This study also provides a feasible way to readily optimize electromagnetic materials through inorganic-organic hybrid structures for magnetic applications.