One-dimensional (1D) nanostructures of spinel ferrite have been a subject of intense research for their interesting chemical and physical properties which different from those of bulk materials  . Spinel ferrite with a general formulae MFe2O4 (where M = Co, Ni, Fe, Mg, Mn, Zn, and Cu) are widely used for many kinds of industrial applications such as optical, catalytic, sustainable hydrogen production application and electronic and magnetic devices    . Among, NiFe2O4 is a one of most investigated spinel ferrite because of their remarkable properties such as high electrical resistivity, high mechanical hardness, large permeability at high frequency and chemical stability. The structural and magnetic properties of NiFe2O4 are particularly affected at cation distribution and the type of substitution   . Among many ion doping, Cu substitution NiFe2O4 have been the subject of extensive investigation because of the high frequency application as magnetic materials  . NiFe2O4 is a completely inverse spinel (Fe)[NiFe]O4), Ni2+ have a strong preference for octahedral site (B-site), while CuFe2O4 is a partial inverse spinel (CuxFe1-x)[Cu1-xFe1+x]O4, Cu2+ have a preference for tetrahedral site (A-site) and B-site. The substitution of Cu in NiFe2O4 brings about a structural phase transition, makes them a suitable material for various technological applications due to the interesting magnetic and electrical properties   .
In earlier work, with increasing copper content the saturation magnetization of Ni1-xCuxFe2O4 microparticles prepared by double-sintering method decreases linearly, whereas coercivity decreases up to x = 0.6 and then increases  . The effect of Cu substitution on chemical states of surface ions and surface composition in Ni1-xCuxFe2O4 spherical nanoparticles prepared by sol-gel combustion method  , and the effect of Cu2+ substitution on electromagnetic properties of Ni1-xCuxFe2O4 nanoparticles is well studied  . Similar structure and magnetic properties are obtained for Ni1-xCuxFe2O4 nanostructures prepared by citrate-gel auto combustion technique  , microwave-induced combustion  , co-precipitation method   , and ceramic method   . Compared to commercial mechanical process, electrospinning represents a simple, effective and convenient method for generating 1D nanofibers  . One of the most important advantages of electrospinning is the ability to control the component of composites, morphology and diameter of nanofibers. Electrospun nanofibers have been applied in a broad range of applications owing to their large specific surface area, high aspect ration, and good dimensional stability  .
In this paper, a series of Ni1-xCuxFe2O4 (where, x = 0.0, 0.3, 0.5, 0.7, 1.0) nanofibers have been prepared by electrospinning method. The effect of Cu substitution on structural, morphology and magnetic properties of NiFe2O4 nanofibers will be studied.
2.1. Preparation of Ni1-xCuxFe2O4 Nanofibers
In this study, the raw materials including Ni(NO3)2・6HO2, Cu(NO3)2・3HO2, Fe(NO3)3・9HO2 and PVP (polyvinylpyrrolidone, Mw ≈ 1,300,000) were of analytical grade and purchased from Tianjin Guangfu. DMF (N, N-Dimethylfor- mamide, 99.7% purity, Tianjin Guangfu, China) and ethanol (100% purity) were used as solvents. In the solution preparation, 0.1 g of PVP was dissolved in mixture of ethanol and DMF with a weight ratio of 1:1, followed by magnetic stirring for 2 h to ensure the dissolution of PVP. Then Ni(NO3)2・6HO2, Cu(NO3)2・3HO2, Fe(NO3)3・9HO2 with a molar ratio of (1 − x):x:2 were added into the mixture solution. After having been stirred for 2 h, the homogeneous viscous solution was transferred into a plastic syringe in which a needle made of stainless steel was connected to a high-voltage equipment. The applied voltage was kept at +15 kV during the electrospinning process. The nanofibers were collected on a piece of aluminum foil about 15 cm below the tip of needle. All electrospinning processes were carried out at room temperature. The collected Ni1-xCuxFe2O4/PVP precursor nanofibers were dried at 80˚C for 3 h, and calcined at 650˚C for 3 h in ambient atmosphere with a heating rate of 1˚C/min.
The calcined nanofibers were characterized by X-ray diffraction (XRD) pattern using CuKα radiation with λ = 0.15418 nm (PANalytical diffractometer). The scanning electron microscope (SEM, Hitachi S-4800) and transmission electron microscope (TEM, TecnaiTM G2 F30, FEI) were employed to analyze morphology and microstructure of samples. Infrared spectra were obtained using Fourier transform infrared spectroscopy (FT-IR, Nicolet 6700) in the 400 - 4000 cm−1 range. The magnetic properties of nanofibers were measured at room temperature using vibrating sample magnetometer (VSM. Lakeshore 7403, USA) with a maximum applied field of 12 kOe.
3. Results and Discussion
3.1. Structural Studies
The effect of Cu substitution on structural and morphology of NiFe2O4 nanofibers was been studied by XRD, SEM and TEM. The XRD patterns of synthesized Ni1-xCuxFe2O4 nanofibers calcined at 650˚C for 3 h are shown in Figure 1. All of main diffraction peaks are indexed as the cubic spinel structure, no second phase can be detected. The position of peaks is slightly shifted to lower angle with increasing Cu2+ content. The diffraction peaks of samples for x ≤ 0.5 correspond to
Figure 1. X-ray diffraction patterns of Ni1-xCuxFe2O4 (0.0 ≤ x ≤ 1.0) nanofibers calcined at 650˚C for 3 h.
NiFe2O4, while the diffraction peaks of samples for x ≥ 0.7 correspond to CuFe2O4. The lattice constant a is calculated by using equation:
where d is the interplanar distance and h, k, l is the Miller indices of plane  . The average crystallite size D is calculated using Debye-Scherrer’s formula with respect to peak plane (311). The values of a and D are extracted and listed in Table 1. From Table 1 it can been seen that lattice constant increases with increasing Cu2+ content. The increased a may be explained on the bigger ionic radii of Cu2+ ions (0.72 Å) than Ni2+ ions (0.69 Å), indicating Cu2+ ions can be effectively built into NiFe2O4 lattice. The average crystallite size increases initially with Cu2+ contents, the maximum D occurs at x = 0.5 (D = 24.1 nm), and then decreases. The variation of a and D with Cu2+ content mainly attributed to the Cu2+ ions insert into the cubic spinel structure, the similar trends were observed in Ni1-xCuxFe2O4 nanoparticles prepared by sol-gel combustion method  .
3.2. Morphological Studies
The morphology of Ni1-xCuxFe2O4 nanofibers were investigated by SEM and TEM. Figure 2 shows the SEM images of Ni1-xCuxFe2O4 nanofibers calcined at 650˚C. It can be seen that all samples remained as continuous and randomly oriented morphology, the diameter of nanofibers is about 50 - 60 nm. The surface of nanofibers is smooth when x less than 0.3, rough surface were observed after x increasing to 0.5 and 0.7, the surface of CuFe2O4 nanofibers(x = 1.0) consists of small open porosity. A similar result was also observed in
Ni0.5-xCuxZn0.5Fe2O4 nanofibers with x = 0.0 - 0.5 prepared by electrospinning  . The Cu2+ content has some influences on morphology of Ni1-xCuxFe2O4 nanofibers. Figure 3 shows the typical TEM (a-b) and HRTEM (c-d) images of Ni0.5Cu0.5Fe2O4 nanofibers, respectively. From Figure 3(a) and Figure 3(b), it can be seen that these nanofibers exhibited a fibrous, continuous and good dispersity morphology, and a nanofiber is composed of randomly aligned nanoparticles. This is well consistent with that observed from SEM (Figure 2). In HRTEM image of Ni0.5Cu0.5Fe2O4 nanofibers (Figure 3(c)), the crystalline phase has well-resolved lattice fringes. The value of distance between the adjacent
Table 1. Parameters extracted from XRD and VSM for Ni1-xCuxFe2O4 nanofibers system: lattice contant (a), average crystallite size (D), saturation magnetization (Ms), remanent magnetization (Mr) and coercivity (Hc).
Figure 2. SEM images of Ni1-xCuxFe2O4 nanofibers with different Cu2+ content: (A) x = 0.0; (B) x = 0.3; (C) x = 0.5; (D) x = 0.7 and (E) x = 1.0.
Figure 3. (a) (b) TEM images; (c) HRTEM image; and (d) SAED pattern of Ni0.5Cu0.5Fe2O4 nanofibers.
lattice is 2.95 Å, which is in agreement with the XRD patterns. As shown in Figure 3(d), selected area electron diffraction (SEAD) of Ni0.5Cu0.5Fe2O4 nanofibers consists of multiple intense rings, indicating that the sample has a polycrystalline nature.
3.3. FT-IR Studies
The ideal spinel structure consists of two sub-lattices, namely tetrahedral sites (A) and octahedral sites (B). Different charge combinations of metal cations are distributed in A and B sites. Therefore, the magnetic properties of spinel ferrite are to a large extent determined by the class of metal ions and cation distribution among the A and B sites. In Ni1-xCuxFe2O4 nanofibers, the replacement of Ni2+ ions with Cu2+ ions at B sites will influence the magnetic properties of samples. FT-IR spectra is usually assigned to the vibration of ions in crystal lattice, which can been used to confirmed the positions of Ni2+, Cu2+, and Fe3+ ions in spinel structure. The vibrating sample magnetometer is used to measure the magnetic properties of samples.
Figure 4 shows the typical FT-IR spectra of Ni1-xCuxFe2O4 nanofibers recorded
Figure 4. FT-IR spectra of Ni1-xCuxFe2O4 (0.0 ≤ x ≤ 1.0) nanofibers.
between 4000 and 400 cm−1. In the range of 1000 - 400 cm−1, two main absorption bands of ferrite are appear. The absorption band υ1 = 580 cm−1 is assigned to the stretching vibration of tetrahedral complexes (Fe3+-O2−), and the absorption band υ2 = 400 cm−1 is attributed to the octahedral complexes (Fe3+-O2−). The peak intensity of υ1 decreases with increasing Cu2+ contents, while the position band is shifted to lower frequencies. Synchronously, the intensity and position of υ2 changed slightly with x. Similar results are observed in Ni1-xCuxFe2O4 nanoparticles prepared by sol-gel combustion method   . The difference in band position of υ1 and υ2 can be related to the difference in Fe3+-O2 bond lengths at A sites and B sites. It was found that the Fe-O distance at A sites (1.89 Å) is smaller than that of the B sites (2.03 Å)   . When Ni2+ ions is replaced by Cu2+ ions, due to charge imbalance some Fe3+ ions shift from A sites to B sites, making the Fe3+-O2 stretching vibration in greater. So the decrease in peak intensity of υ1 with increasing Cu2+ content is mainly attributed to the change in Fe3+-O2 bands.
3.4. Magnetic Studies
The magnetic structure of spinel ferrite is ferrimagnetic, the magnetic moments of A and B sites are coupled antiparallel to each other. There are twice as many B sites filled, so there is a net magnetic moment equal to the difference between the two sites. The magnetization behavior of spinel ferrite can be understood in Neel’s model. In Ni1-xCuxFe2O4 nanofibers, the composition and cation distribution among the A and B sites will influence the magnetic properties of samples. Figure 5 shows magnetic hysteresis loops for Ni1-xCuxFe2O4 nanofibers measured at room temperature, the values of saturation magnetization (Ms), remanent magnetization (Mr) and coercivity (Hc) are calculated from loops and given in Table 1. From the figure it is noticed that the value of Ms decreases linearly with increasing Cu2+ content, while Mr gradually decreases up to x = 0.7, then increases small for x = 1.0. The Hc decreases up to x = 0.5 with x, after it sharply
Figure 5. Magnetic hysteresis loops for Ni1-xCuxFe2O4 (0.0 ≤ x ≤ 1.0) nanofibers at room temperature.
increase to 723.9 Oe for x = 1.0.
According to Neel’s model, the magnetic moment per formula is expressed as:
where MB and MA are magnetic moments of B and A sites in μB. It is well know NiFe2O4 is an inverse spinel structure, all Ni2+ ion and a Fe3+ ion occupy B sites, other Fe3+ ion occupy A sites. CuFe2O4 is a partial inverse spinel structure with 85% Cu2+ at B sites, other 15% at A sies  . The magnetic moment of Cu2+ ions (1.0 μB) is smaller than Ni2+ ions (2.3 μB)  . The Cu2+ ions are substituted instead of Ni2+ ions result in a decrease in net magnetic moment of samples. A part of Cu2+ ions occupy A sites leading to a migration of Fe3+ ions from A sites to B sites. With increasing Cu2+ content, the super-exchange interaction between A and B sites decreases. Therefore, the decrease trend in saturation magnetization and remanent magnetization is agreed with that of an expected decrease in Ni1-xCuxFe2O4 nanofibers.
The variation of Hc with Cu2+ contents can be understood on basis of domain structure, anisotropy and critical diameter  . The initial decrease trend of Hc (x ≤ 0.5) is due to the increase in crystallite size, which is observed in XRD results. This may be attributed to the magnetization mechanism which is a domain rotation process. The Hc value of 723.9 Oe obtained for CuFe2O4 nanofibers in present work is higher than the value of 93.7 Oe and 151.0 Oe of CuFe2O4 nanoparticles prepared by double-sintering method and coprecipitation method, respectively   . This values is also higher than Hc = 625.0 Oe for CuFe2O4 nanofibers prepared by electrospinning method  . The high value of Hc in this paper may be attributed to the magnetocrystalline and shape anisotropy. The magnetocrystalline anisotropy of CuFe2O4 nanofibers is about 0.6 × 105 erg cm−3, while shape anisotropy is calculated to be ks = 1.7 × 105 erg cm−3 using the measured Ms (31.8 emu g−1)  , which is higher than magnetocrystalline anisotropy. Therefore, the high Hc of CuFe2O4 nanofibers mainly come from shape anisotropy if we neglect the dipolar interactions between nanofibers. In Refer. 15, the breakdown of fibers morphology result to lower Hc than this work, while in this paper CuFe2O4 sample retain favorable nanofibers morphology.
The class of metal ions and cation distribution among A and B sites will affect the magnetic properties of spinel ferrite. Nanofibers morphology produced a difference characteristic compare with nanoparticles ones. In this paper, Ni1-xCuxFe2O4 nanofibers (0.0 ≤ x ≤ 1.0) were prepared by electrospinning method, the effect of copper substitution on structure, morphology and magnetic properties of NiFe2O4 nanofibers is studied. Increasing the Cu2+ ion causes an increase in lattice constant due to the larger ionic radii of Cu ion. All samples remain as continuous fibers morphology, while surface of nanofibers with x ≤ 0.3 is smooth, it becomes rough and porous for x = 0.5 - 0.7 and x = 1.0. FT-IR spectra is used to confirmed the positions of Ni2+, Cu2+, and Fe3+ ions in spinel structure. Magnetic properties of Ni1-xCuxFe2O4 nanofibers are studied using vibrating sample magnetometer at room temperature. With increasing Cu2+ content, the saturation magnetization (Ms) was observed to decrease, while the coercivity (Hc) decreased up to x = 0.5 and then sharply increased to 723.9 Oe for x = 1.0. The high coercivity of CuFe2O4 nanofibers compare with nanoparticles samples mainly comes from shape anisotropy of nanofibers. These observations provided by this work gave a fundamental understanding of nanofibers morphology prepared by electrospinning method.
This paper was supported by the Science and Technology Fund of Guizhou (J2143), the Scientific Research Fund of Guizhou Normal College (13BS014) and the Grant of Guizhou Normal College (107003001455).