Fabrication and Characterization of Ni-Carbon nanofibers by Electrospinning Technique

Carbon nanofibers with the distribution of magnetic Ni metal (Ni-CNFs) have been synthesized using Nickel (II) acetate tetrahydrate (Ni (OCHCH3)2.4H2O), polyacrylonitrile (PAN) as precursors by electrospinning technique. The as-prepared nanofibers were dried in a vacuum oven, stabilized, and followed by carbonization at 850 °C in N2 atmosphere. Their structural phase composition, microstructure, and magnetic properties have been investigated. The phase formation of pure Ni-CNF without any impurity has been confirmed by X-ray diffraction (XRD) pattern. The average diameter of the nanofibers and the Ni nanoparticles has been calculated to be 159±25.72 nm and 67±11.24 nm, respectively. Energy-dispersive X-ray spectroscopy (EDX) measurement has been carried out to confirm the presence of Ni and C elements with their weight%. Raman spectrum attributes the presence of two characteristic absorption peaks of carbon materials. Room temperature (300 K) magnetic measurement shows the ferromagnetic behavior of Ni-CNF. These nanofibers have the potential to be used in a wide variety of applications including These materials have wide variety of applications as in different magnetic devices (magnetic sensors, magnetic recording devices, and soft magnetic materials.), microwave absorption materials, supercapacitors and batteries etc.


Introduction
During the last decades, one-dimensional (1D) carbon nanofibrous materials with magnetic transition metal (Fe, Co, Ni) or metal oxide-based have taken the central part of research in different fields of science and technology because of their superior electrical and magnetic properties [1,2].Therefore, many efforts have been made to prepare carbon-based metal (Fe, Co, Ni) nanofibers and study their various applications [1,4].These materials can be used in many applications like microwave absorption, supercapacitor, magnetic recording media, magnetic refrigeration system, and magnetic devices, etc [3][4][5][6].Basically, from application point of view, nanofibers are best in comparison to other nanoshapes because, the ratio of surface area to volume is high and also the longest axial ratio characteristics [2].D Li. et al. study the microwave absorption properties of Ni/C porous nanofibers by electrospinning technique and also calculate the saturation magnetization of 14.1 emu/g and coercivity of 113.5 Oe [7].Gong et al. prepared ultrafine Ni fibers and investigate their magnetic as well as electromagnetic characteristics [8].There are many techniques developed to synthesize nanofibers such as electrospinning, template-assisted, self-assembly, interfacial polymerization, electrochemical deposition and seeding approach [1].Among all, electrospinning is one of the simplest, low-cost, high-yield nanofiber production techniques.In this paper, we explain the preparation of Ni-Carbon nanofibers by electrospinning technique and characterize their structural, morphological, elemental, and magnetic properties.

Materials and methods
CNF loaded with magnetic Ni was prepared by electrospinning and thermal carbonization.In a typical procedure, 1 g of polyacrylonitrile (PAN) was mixed in 10 ml of N, N dimethyl formamide (DMF) and magnetic stirred the solution over 10 hours to ensure the dissolution of PAN into the solvent.Secondly, 1 gm of nickel (II) acetate tetrahydrate (Ni (OCHCH 3 ) 2 .4H 2 O) was mixed in 10 ml of DMF.After dissolving both the solution completely, mix these two solutions with 1:1 ratio to obtain the final electrospun solution.Then the solution was loaded into 5 ml plastic syringe and connected with a syringe pump, placed at a distance of 10 cm from the collector.To prepare the electrospun nanofibers a voltage of 12 kV, solution flow rate of 0.8 ml/h and drum rotation speed of 400 rpm was applied to the instrument.The electrospinning process was performed at.The as-prepared nanofibers were dried at 60 C in a vacuum oven and stabilized at 230 C in air atmosphere followed by carbonization at 850 C in N 2 atmosphere for 2 h with a heating rate of 5 C/min.
The crystal structure of the prepared Ni-CNF was investigated using X-ray diffraction (XRD).The fiber and nanoparticles diameter with surface morphology were examined using a field emission scanning electron microscope (FESEM).The elementals present in the synthesized nanofibers were determined by energy-dispersive X-ray analysis (EDX).Raman spectroscopy measurement was taken in the range of 750 -2000 cm -1 using a Renishaw micro-Raman spectrometer.The magnetization hysteresis loop was determined by vibrating sample magnetometer (VSM) measurement.

Structural properties
The phase composition of prepared Ni-carbon nanofiber was analyzed by XRD pattern, shown in Figure 1

Raman Measurement
Raman spectra was further carried out to get structural information and identify the presence of amorphous carbon in the Ni-CNF.As presented in Figure 3, Ni-CNF has two characteristic Raman peaks at about 1350 cm-1 and 1590 cm-1 can be referred to as D band and G band respectively.Basically, the D band corresponds to (sp3 vibration) graphitic structural disorder whereas the G band corresponds to (sp2 vibration) C=C stretching vibration [9].This result is a good agreement with the XRD result.

Magnetic Properties
The investigation of the magnetic properties of the Ni-CNF was characterized by the vibrating sample magnetometer (VSM) at room temperature (300 K) and the hysteresis curve is shown in Figure 4.The ferromagnetism of Ni-CNF is clearly observed from different magnetic parameters like magnetization (M at 2T), coercivity (Hc), and remanent magnetization (Mr).Ni-CNF exhibits the value of magnetization (at 2T), coercivity, and remnant magnetization of 43.5emu/g, 46.6 Oe and 1.8 emu/g respectively, revealed a good ferromagnetic characteristic.The room temperature magnetization value of Ni-CNF is lower than the bulk Ni metal (58.57emu/g) due to the presence of the diamagnetic CNF.
Here CNF acts as a magnetically dead layer on the surface of the Ni metal [5,10].As shown in Figure 4 (b) the synthesized nanofibers exhibit higher coercivity (46.6 Oe) than the bulk Ni (0.7 Oe).Additionally, because of the high value of magnetization and coercivity, this Ni-CNF could be a promising material to be used as flexible magnets.