Structure, characterization, and magnetic/electrochemical properties of Ni-doped BiFeO3 nanoparticles

The BiFe1−xNixO3 (x  =  0, 0.05, 0.1, 0.2, and 0.3) nanoparticles were prepared by a simple solution method. Their nanostructures were characterized by x-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), x-ray absorption spectroscopy (XAS) and gas absorption techniques. The magnetic properties of the nanoparticles were studied by using a vibrating sample magnetometer (VSM). The increasing of Ni content with decreasing of crystallize size can improve magnetization. Moreover, the samples were fabricated as electrodes to study the electrochemical properties by cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS). The high specific capacitances of the electrodes are in the range of 193–514 F g−1. Although the increasing of the Ni content leads to decreasing of the specific capacitances, the 5% Ni-doped BiFeO3 can improve the capacity retention (82%) after 500 cycles at 10 A g−1.


Introduction
Perovskite BiFeO 3 (BFO) is a popular multiferroic material due to the co-existence of ferroelectricity and ferromagnetism [1,2], its potential applications in data storage, sensors, and spintronic devices, as well as the interesting physics involved in understanding their properties [1,3]. The multiferroic material exhibits spontaneous polarization and antiferromagnetism ordering with a high ferroelectric Curie temperature T c of ~1103 K and an antiferromagnetic Neel T N temperature of ~643 K [1,3]. Several studies have been devoted to the improving of the multiferroic properties of BiFeO 3 with reduction of size of nanoscale on improvement of the magnetic properties of BiFeO 3 through cation substitution realized by B-site (Fe-site) doping, such as nonmagnetic metal ion of Cu [4] or magnetic ions of Co [5] and Ni [6][7][8] enhanced magnetization. The magnetization suppression in bulk BiFeO 3 occurs due to its spin spiral structure. Size-dependent magnetic properties of BiFeO 3 are strongly correlated with decreasing nanoparticle size below cycloidal spin wavelength of ~62 nm and uncompensated spin at the surface [9]. (Nd, Ni) co-doped BiFeO 3 can improve magnetization due to the suppression of spin cycloid structure of the particle size and the decrease in crystallite size with increasing of Ni content [6]. Thus, The Ni atom can be considered as a candidate for the enhancement of magnetic properties due to the fact that its radius ion is similar for substitution which may be attributed to the size effect of nanostructures and their magnetic properties.
Moreover, BiFeO 3 -based nanomaterials have been used as potential active electrode materials for electrochemical supercapacitors which have attracted great attention over the past few decades due to their having higher power density and longer life cycle than batteries and higher energy density than conventional dielectric capacitors, which suggest potential applications in electric vehicles, power sources, portable electronics, and other devices [10]. Generally, electrochemical capacitors are classified into two types on the basis of the charge storage mechanism used such as (1) electric doublelayer capacitors (EDLCs), which depend on the non-Faradic charge separation at the interface between an electrode (such as carbon materials with very high surface area) and an electrolyte [11,12]; and (2) pseudocapacitors, which depend on electron transfer that occurs near the electrode/electrolyte interface through a fast reversible redox reaction (such as oxide materials) [11,13]. Nowadays, transition metal oxides such as TiO 2 , MnO 2 , NiO, Co 2 O 3 , MoO 3 , V 2 O 5 , and Fe 2 O 3 are studied for supercapacitor applications due to their high pseudocapacitance, which have a higher capacitance performance than normal electric double-layer capacitors, low cost, and environmental friendliness [14,15]. In particular, the high specific capacitance and high reliability of hydrous RuO 2 has been found to be very high. However, the restrictive price and toxicity of RuO 2 have limited practical use [15]. Many research studies have tried to improve the electrochemical performance by fabrication of various forms of BiFeO 3 as electrode materials, such as perovskite BiFeO 3 thin-film electrodes [16], BFeO 3 nanorods on porous anodized alumina (AAO) templates [17] as well as Cu-doped BiFeO 3 nanoparticles electrodes [4]. The BiFeO 3 nanoparticles showed higher specific capacitance (513.5 F g −1 ) than BiFeO 3 thinfilm (81 F g −1 ) and BFeO 3 nanorods (450 F g −1 ). Moreover, BiFe 0.95 Cu 0.05 O 3 can improve the specific capacitance (568.13 F g −1 ) and the capacity retention (77.13%) after 500 cycles due to pore size distribution.
These factors explain the motivation for this work. For the above reasons, we chose Ni metal ion as the substituent to study and to clearly clarify the mechanisms underlying the effects of Ni addition to the magnetic properties of BiFeO 3 nanoparticles. Additionally, the Ni-doped BiFeO 3 nanoparticles were used as candidates for electrode materials for electrochemical supercapacitors and the effects on the electrochemical performances of Ni-doped BiFeO 3 were studied.  3 and deionized water seven to ten times and dried in an oven at 100 °C.

Particle characterization
The XRD patterns of the Ni-doped BiFeO 3 nanoparticles were investigated by using XRD (D2 Advance Bruker) analysis with Cu Kα at λ = 0.15406 nm. The Rietveld refinement technique with TOPAS software was used to investigate the crystal structure. For the space groups, the space groups of R3c (JCPDS No. 86-1518) for the rhombohedral phase, Pbam (JCPDS No. 72-1832) for the orthorhombic phase and Fd3m (ICSD No. 40040) for the cubic phase were used. The crystallite size of the nanocrystalline samples was measured from the line broadening analysis based on the Debye-Scherer equation [18] where D is the crystallite size (nm), λ is the x-ray wavelength, θ is the diffraction angle, and β is the full width at half maximum (FWHM) intensity. Moreover, the particle sizes were calculated from the surface area of BiFe 1−x Ni x O 3 nanoparticles using the following equation [19][20][21] where D BET is the average particle size (nm), ρ is the crystallographic density (g cm −3 ) and A is the specific surface area according to the BET isotherm (m 2 g −1 ). The particle morphology was examined by scanning electron microscopy (SEM; JSM-7800F) and transmission electron microscopy (TEM; FEI TECNAI G 2 20). In order to determine the valence state of Ni and Fe, x-ray absorption near edge spectra (XANES) of Ni and Fe K-edge spectra were recorded in the fluorescence and transmission modes, respectively at the SUT-NANOTEC-SLRI XAS Beamline (BL 5.2) (electron energy, 1.2 GeV; bending magnet; beam current, 80-150 mA; (1.1-1.7) × 10 11 photon s −1 ) at the Synchrotron Light Research Institute (SLRI), Nakhon Ratchasima, Thailand. The normalized XANES data were processed and analyzed using ATHENA software which included an IFEFFIT package [22,23]. The surface area (S p ) and pore characterizations were obtained from N 2 adsorption technique by BEL SORP-miniII after degassing at 80 °C for 18 h. The total specific area (S BET ) and total pore volume (V pore ) and pore size distribution were investigated by the Brunauer-Emmett-Teller (BET) and Barret-Joyner-Halenda (BJH) methods, respectively, with the same equipment.

Measurements of magnetic properties
The magnetic properties of the nanoparticles were measured using the vibrating sample magnetometer (VSM) option in the Quantum Design Versalab instrument. The hysteresis loops were collected in magnetic fields from 30 kOe to −30 kOe at various temperatures from 350 K to 50 K. Zero-field cooled (ZFC) and field-cooled (FC) temperature-dependent magnetization curves were measured with applied magnetic fields of 500 Oe from 350 to 50 K.

Preparation of electrodes and electrochemical measurements
The working electrodes were prepared by mixing the Ni-doped nanoparticles, acetylene black and a polyvinylidene difluoride (PVDF) binder (weight ratio of 80:10:10) using n-methyl-2 pyrrolidinone (NMP) as a solvent to form a slurry on a nickle foam current collector. Then, the electrode was dried at 70 °C for 12 h and pressed at 20 MPa, respectively. Each working electrode contained about 3 mg of electroactive material and the area of coating was about 1 cm 2 . The electrochemical measurement was employed to explore the electrodes for electrochemical supercapacitor application with 6 M KOH aqueous electrolyte in a three-electrode system on Metrohm Autolab PGSTAT 302N, which consists of the active materials, a platinum wire and Ag/AgCl electrodes as working, counter, and reference electrodes, respectively. The electrochemical impedance spectroscopy (EIS) test was collected with a frequency range of 0.1 Hz-100 kHz. Cyclic voltammetry (CV) was performed at a potential window in the range of −1.2 V to 0.3 V and different scan rates of 5, 10, 20, 40, 60, 80, and 100 mV s −1 were applied. The enclosed area of the CV curve can be used to estimate the electrochemical specific capacitance (C CV ) using the following equation [24] where I is the response current density discharge current (A cm −2 ), υ is the potential scan rate (mV s −1 ), m is the mass of the electroactive materials in the electrodes (g cm −2 ), and ΔV is the potential window (V). For the galvanostatic charge-discharge (GCD) measurements, the GCD curves at different current densities of 1, 2, 5, 10, 15, and 20 A g −1 were tested to investigate the electrochemical performances of the Ni-doped BiFeO 3 electrodes. The specific capacitance (C GCD ), was calculated using the following equation [ where i is the discharged current (A), ΔV is the potential window (V), and Δt is the discharge time (s).

Structural analysis
The XRD patterns of the BiFe 1−x Ni x O 3 (x = 0, 0.05, 0.1, 0.    figure 4(a). Clearly, the oxidation state of Ni is not 3+, but could be 2+. Figure 4(b) shows the XANES  Parameters   figure 5. Generally, the pores of the materials are classified into three groups according to pore size distributions namely, micropores (pore size <2 nm), mesopores (2-50 nm), and macropores (>50 nm). The presence of micropores, mesopores and macropores in particles is shown by the BJH curve (inset of figure 5).
This may be related to capacitance and capacity retention, which will be discussed in section 3.3. Table 2 shows the specific surface area (S BET ), the mean pore diameter (D MP ), the total pore volume (V TP ), and the particle size (D BET ) of    In general, the decrease in the size of BiFeO 3 nanoparticles is related to an increase in surface area [9]. In this research, the decreases in the crystallite size led to a sharp increase in the surface area from 3.64 m 2 g −1 of the BiFeO 3 sample to 21.6 m 2 g −1 in the BiFe 0.7 Ni 0.3 O 3 sample. The particle size decreases from 197.8 nm of un-doped samples to 32.9 nm in 30 % Ni-doped samples were calculated by using the gas absorption technique.
A comparison of the average crystallite size calculated by XRD and the average particle size estimated by BET showed that the average particle size calculated by BET is larger than the crystallite size calculated by XRD in all samples except for the BiFe 0.95 Ni 0.05 O 3 sample as shown in figure 6. The difference in the results occurs from aggregates and/or agglomerates of crystals, which indicates that the particles include several crystallites [21]. The presence of the Ni ion in the Fe 3+ site acts as an inhibitor and results in a decrease of crystallite size. The inhibition is mainly because of the surface energy of BiFeO 3 with the addition of dopant [6,25]. In this research, we confirm that all the samples with higher concentrations of Ni dopant showed a decrease in particle size which shows a tendency to increase their specific surface area, and total pore volume as calculated and cited in table 2. The increase in the average pore diameter occurs from the agglomeration of the particles that causes sintering of the pores into small ones with decreasing homogeneity of the dimensions and number of pores [21,26].  3) samples in this study at room temperature were 0.51, 6.43, 12.20, and 19.12 emu g −1 and at 50 K of temperature were 2.87, 7.45, 14.12, and 22.12 emu g −1 , respectively. These results were found to be higher than those reported in the literature, which were 5 % and 25% Ni-doped BiFeO 3 at 50 K of temperature (1.29 and 8.04 emu g −1 ) [7], 10 % Ni-doped BiFeO 3 at room temperature (~3.04 emu g −1 ) [8], and 5% Ni-doped BiFeO 3 at room temperature (~1.4 emu g −1 ) [30].
The observed increases in the magnetization may arise for two reasons: (1) the magnetization is mainly dependent on the Ni content which provides strong evidence of the effects of the sizes of the BiFeO 3 nanoparticles. It is known that particles on the nanoscale exhibit significantly different properties from bulk BFeO 3 [31]. Improved magnetization may be due to suppression of the spin cycloid structure of the particle size when it is less than 62 nm which causes the intrinsic spiral spin structure to be incompletely suppressed and the decreases in crystallite size with increases of Ni content results in an increase in surface-volume ratio and the contribution of uncompensated spin at the surface to the total magnetic moment of the particle increases. (2) The high M s of NiFe 2 O 4 nanoparticles are between 32.1 and 49.1 emu g −1 measured at 300 to 80 K of temperature, respectively [32]. So, the increases of the secondary phase of the NiFe 2 O 4 nanoparticles in the BiFe 1−x Ni x O 3 (x = 0.05 to 0.3) samples may cause an increase in saturation magnetization with a decrease in grain size [32][33][34].
All the samples show the hysteresis loops are field dependent on magnetization measurements indicating weak  Specific surface area (S BET ), mean pore diameter (D MP ), total pore volume (D TP ), particle size (D BET ) of BiFe 1−x Ni x O 3 (x = 0, 0.05, 0.1, 0.2, and 0.3) nanoparticles.
where a and b are constants, and D is the particle size. Thus, the coercivity may decrease with an increase in particle size above a critical size. The H c of BiFeO 3 and BiFe 0.95 Ni 0.05 O 3 samples decrease due to a decrease in the crystallite size of BiFeO 3 and an increase in temperature. This conforms to the crystallite-size and the temperature-dependent behavior of BiFeO 3 [9].

Electrochemical measurements
The cyclic voltammetry (CV), galvanostatic chargedischarge (GVD), and electrochemical impedance spectr oscopy (EIS) analyses were used to evaluate the electrochemical performance of the BiFe 1−x Ni x O 3 (x = 0, 0.05, 0.1, 0.2, and 0.3) electrodes. All these electrochemical measurements were conducted in 6 M KOH solution using a threeelectrode system. Figures 9(a)-(e) show the CV curves of the BiFe 1−x Ni x O 3 nanoparticles. The CV measurements were performed between −1.2 V and 0.3 V at different potential scan rates of 5-100 mV s −1 in 6 M KOH solution. The samples exhibited a pseudocapacitive behavior. Redox peaks were observed for all the samples, indicating the redox transitions of the nanoparticles between different valence states. The current response of all electrodes was enhanced when the scan rates were increased. The height of the peak current varied and a progressive shift in the peaks to higher potentials was observed with increasing scan rates from 5 to 100 mV s −1 . The calculated specific capacitances versus scan rates are plotted in figure 9(f). The specific capacitances of all the samples decrease with increasing scan rates. This is attributed to the presence of inner active sites, which completely inhibit the redox transitions at higher scan rates of CV, probably owing to the diffusion effect of protons within the electrodes [38]. All the electrodes exhibited the highest specific capacitance at a scan rate of 5 mV s −1 . The maximum specific capacitance of 397.3 F g −1 at a scan rate of 5 mV s −1 was obtained for the pure BiFeO 3 sample. The specific capacitance of the nanoparticles depends linearly on Ni doping concentrations with continuously decreases.
The galvanostatic charge-discharge behavior of the electrodes at current densities from 1 to 20 A g −1 are shows in figures 10(a)-(e). The nonlinear curves confirm the pseudacapacitive behavior of the material. The discharge curve of the electrodes consists of two parts: a steep voltage (IR) drop due to internal resistance and a capacitive component (curved portion) related to the voltage change due to changes in energy within the capacitor [39]. This (IR) drop is a common phenom enon occurring in transition metal oxides [40,41]. The galvanostatic charge-discharge curves measured in all samples show that current density increases with decreases of the discharge time. The maximum specific capacitance of 513.5 F g −1 at 1 A g −1 current density was obtained from the undoped sample. The specific capacitance at all current densities also continuously decreased from x = 0.05 to x = 0.3 as shown in figure 10(f). This decrease in the capacitance is due to the fact that the surface of the electrode is inaccessible at high charge-discharge rates [16], increasing in ionic resistivity and decreasing in charge diffusion deeper into the inner active sites [40,42]. Therefore, the specific capacitance of the electrodes at a low current density should be suitable for practical applications. At a current density of 1 A g −1 , all the electrodes exhibited the highest specific capacitance.
In general, increase in the specific surface area in electrochemical capacitors is a likely reason for the increase in the specific capacitance, especially in carbon materials. On the contrary, the specific capacitance of these BiFe 1− samples (at 1-20 A g −1 for GCD measurement) with increases in the specific surface area. However, specific capacitance does not only depend on surface area, but also on other factors, such as the pore size distribution and pore volume [4,43,44]. All the samples have distributions of different sizes of pores, namely, micropores, mesopores and macropores, as shown in figure 5, indicating that they have a porous structure, which is specific to supercapacitor materials [43,44]. The decreases in the specific capacitance of the BiFe 1−x Ni x O 3 samples with increases in Ni doping can possibly be attributed to the following: (i) all samples enriched with mesopores show a mean pore diameter of BiFeO 3 smaller than the Ni-doped samples; (ii) with regard to mesopore distribution, the BiFeO 3 samples showed small mesopore sizes (~3.3 nm) which were smaller than the 10, 20 and 30% Ni doping samples (~24.5 nm), this provides more active sites for chemical reactions [45] and (iii) with regard to macropore distribution, the BiFeO 3 sample showed the largest pore in the diameter range of macropores, which provide relatively greater accessibility to the electrolyte for surface adsorption and intercalation, and rapid electrolyte transport and diffusion into the inner region of the electrodes [27,[46][47][48].
The cycling performance of the BiFe 1−x Ni x O 3 (x = 0.05, 0.1, 0.2 and 0.3) electrodes at 10 A g −1 current density are shown in figure 11. The life cycle (stability) of the electrodes is important for practical applications. The capacity retentions of the Ni-doped BiFeO 3 samples with x = 0, 0.1, 0.20, and 0.30 were 58, 42, 38 and 35%, respectively, after 500 cycles. Capacity retention can be improved by Ni content. BiFe 0.95 Ni 0.05 O 3 showed higher capacity retention than the BiFeO 3 electrodes. The capacity retention of the BiFe 0.95 Ni 0.05 O 3 electrode (82%) in this work was higher than that of the BiFe 0.95 Cu 0.05 O 3 electrode (77.13%) [4]. It increased to 102 % after 80 cycles, and then slightly decreased to 82% after 500 cycles. The capacity retention of over 100% in this electrode was due to the additional cycles needed to fully activate the sample [49,50]. Improved capacity retention of the BiFe 0.95 Ni 0.05 O 3 sample may be due to the small mesopore size of about 2.4 nm. This provides more active sites for chemical reactions [45]. This may lead to improvements in the capacity retention in the BiFe 0.95 Ni 0.05 O 3 electrode. The decreases in the capacity retention with increases of Ni content with x = 0.1 to x = 0.3 may be due to the macropore size distribution, which tends to decrease. This can lead to the suppression of electrolyte diffusion into the inner region of the electrode and active sites for chemical reactions [15,45]. Moreover, increases in the NiFe 2 O 4 phase composition may influence the specific capacitance due to the fact that the specific capacitance of NiFe 2 O 4 nanoparticles (42.8 F g −1 ) [51] is lower than that of BiFeO 3 nanoparticles (397.3 F g −1 ) at the same scan rate of 5 mV s −1 in 6 M KOH solution [4]. This indicates that the increases in the phase composition of