Synthesis , Characterization , and Magnetic Properties of Pure and EDTA-Capped NiO Nanosized Particles

The effect of ethylenediaminetetraacetic acid (EDTA) as a capping agent on the structure, morphology, optical, and magnetic properties of nickel oxide (NiO) nanosized particles, synthesized by coprecipitation method, was investigated. Nickel chloride hexahydrate and sodium hydroxide (NaOH) were used as precursors. The resultant nanoparticles were characterized by X-ray diffraction (XRD), transmission electronmicroscopy (TEM), and scanning electronmicroscopy (SEM). XRD patterns showed that NiO have a face-centered cubic (FCC) structure. The crystallite size, estimated by Scherrer formula, has been found in the range of 28–33 nm. It is noticed that EDTA-cappedNiOnanoparticles have a smaller size than pure nanoparticles.Thus, the addition of 0.1M capping agent EDTA can form a nucleation point for nanoparticles growth. The optical and magnetic properties were investigated by Fourier transform infrared spectroscopy (FTIR) and UV-vis absorption spectroscopy (UV) as well as electron paramagnetic resonance (EPR) and magnetization measurements. FTIR spectra indicated the presence of absorption bands in the range of 402– 425 cm, which is a common feature of NiO. EPR for NiO nanosized particles was measured at room temperature. An EPR line with g factor ≈1.9–2 is detected for NiO nanoparticles, corresponding to Ni ions. The magnetic hysteresis of NiO nanoparticles showed that EDTA capping recovers the surface magnetization of the nanoparticles.


Introduction
Nanosized nickel oxide (NiO) is a significant transition metal oxide that has garnered attention as a strong candidate for many fields including superparamagnetic devices, photovoltaic devices, electrochemical supercapacitors, magnetic materials, catalysis, smart windows, fuel cell, and photovoltaic devices [1].These nanostructured particles are regarded as a p-type semiconductor having large exciton binding energy with stable wide bandgap (3.6-4.0 eV).Bulk NiO is an antiferromagnetic insulator with a Néel temperature of 523 K [2].They exhibit many unique magnetic, optical, electronic, and chemical properties that are significantly different than those of bulk-sized NiO particles due to their quantum size and surface effects [1].
Several methods were used to prepare NiO nanoparticles like sol-gel process [3], thermal decomposition [4], polymer-matrix assisted synthesis [5], and spray-pyrolysis [6].Some of these techniques suffer from the difficulty in size homogeneity and dispersion of NiO nanoparticles.Generally, most techniques aim to reduce the costs of chemical synthesis and to produce materials for technological applications.However, in solution-based chemical methods for nanoparticles synthesis, a capping agent is generally added to control the size as well as the shape of nanoparticles and to prevent agglomeration of the synthesized particles.Many kinds of organic compounds such as surfactants and polymers have been used to direct or confine anisotropic growth.These capping reagents can bind with the various crystallographic surfaces to alter their growth rates [7].
Several researches have synthesized capped NiO nanoparticles using various techniques with different capping agents.Nowsath Rifaya et al. [8] synthesized nanostructured NiO particles through a simple, novel, and cost-effective chemical capping method using nickel chloride, alanine, ethanol, and ammonia.Alanine has served as capping molecule and blocked the active sites at growing surfaces.It was found that the resultant nanoparticles will be biocompatible in nature and very useful for biosensor related applications.Moreover, Davar et al. [9] synthesized NiO nanoparticles using thermal decomposition of [bis(2-hydroxyacetophenato)nickel(II)] in oleylamine surfactant.The development of such surfactant was to control nanocrystal size, shape, and distribution size.Also, nickel oxide nanoparticles (25 nm) have been synthesized by Fereshteh et al. [10] via decomposition of a new precursor nickel octanoate Ni(octa) 2 in the presence of oleylamine (C 18 H 37 N) and triphenylphosphine (C 18 H 15 P), in mild conditions.To control the particle size and morphology, combination of C 18 H 37 N and C 18 H 15 P were applied as surfactants.They play an important role in preventing aggregation of NiO.Babu et al. [11] prepared cubic phase of NiO nanoparticles with an average size of 25 nm by microwave synthesis method using citric acid as a capping agent.The observed TEM images demonstrated that NiO-CA produces nanoparticles having average size of 25 nm.
Among various types of stabilizers used for the preparation of nanoparticles, the water soluble polymer such as ethylenediaminetetraacetic acid, abbreviated as EDTA, has significant importance.EDTA has proved its efficiency in controlling the size and morphology of synthesized nanoparticles [12,13].Reddy et al. [14] synthesized Zn 1− Cr  S ( = 0.00, 0.005, 0.01, 0.02, and 0.03) nanoparticles through chemical coprecipitation method using EDTA as a capping agent.It was found that the produced nanoparticles were sterically stabilized by EDTA.Also, Sadjadi and Khalilzadegan [15] reported the synthesis of EDTA-capped and ethylene glycol EG-capped CdS nanoparticles separately.The XRD analysis revealed formation of hexagonal form when we used EDTA as a surfactant agent, whereas a face-centered cubic structure was obtained by using EG as surfactant agent.It was found that EDTA-capped nanoparticles exhibit smaller crystal size than EG-capped particles.Devi and Singh [16] prepared nanoparticles of Eu 3+ doped YPO 4 by coprecipitation method.Trisodium citrate dihydrate and EDTA were used as a complexing agent.With the addition of citrate and EDTA, there is a slight shift towards the lower wavelength in emission peaks.A broad peak at 250 nm is observed due to the Eu-O charge transfer band in the excitation spectra.Emission intensity decreases with complexing agent because of decrease of particle size as well as decrease of number of Eu 3+ activators per unit volume.
This work aims to synthesize pure and EDTA-capped NiO nanosized particles by a simple low cost method, the coprecipitation method, at 500 ∘ C. Structural, compositional, morphological, optical, and magnetic studies of the prepared samples were carried out by X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared (FTIR), UV-visible spectra, EPR, and magnetization techniques to determine the optimum conditions for preparing well-nanosized NiO particles., 95%, Sigma Aldrich) into 126 ml distilled water, respectively.The obtained solution was magnetically stirred at room temperature and a certain volume of 4 M NaOH was added dropwise to the solution to adjust its pH to 12.The final pH of the mixture was fixed to ≈12, a highly basic condition, which is convenient for the direct preparation of NiO crystals.The reaction was stirred for 2 h at 60 ∘ C. Afterwards, the resultant green product was washed thoroughly with distilled water for removal of reaction residues until the pH is 7 and then dried at 90 ∘ C for 24 h in air.Then, the dried ingots were separated and heated at 500 ∘ C for 5 h.Due to this annelation the color of the resultant powder changes from green to black.

Experimental Techniques
The same procedure was applied to prepare pure NiO particles free from 3.72 g EDTA at the same annealing temperature.

Nanoparticles Characterization. X-ray diffraction (XRD)
patterns of pure and EDTA-capped NiO nanoparticles were obtained using the Bruker D8 advance powder diffractometer with Cu-K radiation ( = 1.54056Å) in the range of 10 ∘ ≤ 2 ≤ 80 ∘ .Transmission electron microscopy (TEM) was applied to study the morphology and crystalline size of NiO nanoparticles using Jeol transmission electron microscope JEM 100CX, operated at 80 kV.The samples were examined using scanning electron microscope (SEM) with the objective of determining the particle shape.SEM images for pure and EDTA-capped NiO nanoparticles were performed on ASC-2100 from Serene Technology, Korea, with 20 kV.The FTIR analysis was carried by FTIR 8400S Shimadzu.The optical studies were measured using the ultraviolet-visiblenear infrared (NIR) spectrophotometer V-670 that measures the absorption spectra at a wavelength of 4000 nm-350 nm at room temperature.A vibrating sample magnetometer (VSM), Lakeshore 7410, was used to investigate the magnetic properties of nickel oxide nanoparticles.The EPR spectra were performed at room temperature using a Bruker Elexsys 500 EPR spectrometer operating at the X-band frequency (≈9.491GHz) with a field modulation frequency of 100 kHz.
The magnetic field was scanned in the range of 500-6500 G and the used microwave power was 0.64 mW.A powder sample of 100 mg was taken in a quartz tube for EPR measurements.

Results and Discussion
where (ℎ, , ) are the miller indices.The value of lattice constant for (111) plane was found to be 0.417 nm for both pure and EDTA-capped NiO nanoparticles.This value is very close to that for bulk NiO taken from (JCPDS card number 04-0835), which confirms the well growth of NiO.It is noted that the addition of EDTA has no effect neither on the positions of main NiO peaks nor the lattice constant .The average crystalline size () of the NiO particles was estimated from the line broadening measurement [20].
where  = 1.54056 nm is the wavelength of the X-ray,  = FWHM (full width at half maximum),  is diffraction angle obtained from 2 values corresponding to maximum intensity peak in XRD pattern (200), and  is an empirical constant equal to 0.9.The average crystalline size with the lattice constant  of pure and EDTA-capped NiO nanosized particles were calculated and listed in Table 1.
It is clearly noted from Table 1 that EDTA-capped NiO nanoparticles have smaller crystalline size than pure NiO nanoparticles.Thus, the addition of 0.1 M capping agent EDTA prevents the agglomeration of nanoparticles and forms a nucleation point for their growth [21].

Transmission Electron Microscopy. Figures 2(a) and 2(b)
show the TEM images of pure and EDTA-capped NiO nanoparticles at 500 ∘ C. As seen, the uniform NiO nanoparticles with narrow size distribution and sphere shapes with weak agglomeration are obtained [22].Most of the particle sizes are less than 40 nm.The dispersed particles in the TEM image reveal that the coprecipitation method used in this work can be considered appropriate for preparation of nickel oxide nanosized particles.One of the key factors for this desired result is the use of the capping agent EDTA, resulting in significant size reduction of nanoparticles contributing to their stability.A mechanism involves formation of strong covalent bond between polymeric chains and surfaces of nanoparticles that is responsible for stearic hindrance by which nanoparticles remain stable for months [23,24].Crystalline sizes obtained from TEM images are presented in Table 1.The tabulated data of the particle sizes obtained from TEM is well matched with those measured from the XRD peak broadenings.This difference is probably due to the fact that the samples for TEM measurements were prepared from reaction solution and not from powder as in SEM.At this point, aging conditions play a decisive role in the particles sizes.Particle sizes in reaction solution are susceptible to aging effects that alter their morphological shapes.

Capping Mechanism.
Figure 4 shows the interaction of the EDTA reaction sites with Ni 2+ ion of the nickel chloride in the solution.In this mechanism, nickel chloride hexahydrate reacts with NaOH, and then it yields pure NiO nanoparticles.
Further, pure NiO nanoparticles react with EDTA [C 10 H 16 N 2 O 8 ] and it formed a Ni-EDTA complex ion.Such capping of NiO nanoparticles with ethylenediaminetetraacetic acid prevents their agglomeration and controls their shape and morphology resulting in lower size than pure NiO nanoparticles as depicted by XRD and TEM.Both spectra display various significant absorption peaks.In both spectra, broad absorption band centered at 3440-3459 cm −1 is attributed to the O-H stretching vibrations band and the weak band near 1630 cm −1 is assigned to H-O-H bending vibrations mode.The appearance of such bands is representative of adsorbed water molecules on the external surface of NiO samples during handling to record FTIR spectra.These observations provided the evidence of the effect of hydration NiO nanoparticles.The absorption bands in the region of 1000-1500 cm −1 are assigned to the Osymmetric and asymmetric stretching vibrations and the C-O stretching vibration [25].The broad peak at about 402-425 cm −1 is assigned to stretching mode of NiO, which is clear evidence about the presence of the crystalline NiO [24,26].Moreover, the broadness of this band suggests that the NiO powders are nanocrystals [24].The presence of a band in the region of 1120 cm −1 in EDTA-capped nanoparticles spectrum may be attributed to bending bonds which result to coordinate bonding between EDTA and Ni 2+ indicating capping on NiO nanoparticles.It is clearly observed that the addition of EDTA does not modify or alter the position of peaks in the IR spectra of NiO nanoparticles.

FTIR Spectra Analysis.
3.6.UV-Vis Spectra. Figure 6 shows the optical absorption spectra of pure and EDTA-capped NiO nanoparticles at 500 ∘ C obtained by ultrasonic dispersion in absolute ethanol.The absorption edge is observed in the range of 280-350 nm [17].The absorption peaks of NiO nanoparticles show a blue shift which may be attributed to the change in crystallite size of nanoparticles, morphology, quantum confinement of the particles, and surface effects [17].The energy bandgaps (  ) of the samples are calculated using the theoretical relation between the coefficient of absorption () and the energy of the photon (ℎ]) as follows: where  is a constant,   is the energy bandgap of the material, and exponent  depends on the type of the transition.For direct and allowed transition  = 1/2; for indirect transition  = 2 and for direct forbidden transition  = 3/2.Energy bandgap   was obtained using the intercept of the linear portion of the curve (ℎ]) 2 versus ℎ] to  = 0 as shown in Figure 7.The obtained energy gap was found to be 3.085 eV for pure NiO and 3.116 eV for EDTAcapped NiO nanoparticles.Such increase in the bandgap may be attributed to the quantum confinement effect created by EDTA surfactant [15,[27][28][29].The resultant values of   of NiO nanoparticles are found to be less than that obtained in literature [30]; this may be due to the preparation method used.Khalaji and Das [30] synthesized nickel oxides nanoparticles via thermal decomposition method of nickel Schiff base complexes which results in high values of   .distribution function and the EPR parameters such as value, peak to peak line width (Δ pp ), resonance field (  ), and spin-spin relaxation time constant ( 2 ) were calculated and listed in Table 1.The values of  factor, the most important parameter for the description of the spin system, were calculated using the following equation [31,32]:
The -value provides information concerning the molecular motion, the paramagnetic properties, and the symmetry of ions [34].It is observed that -value around 1.9-2.09could be attributed to the Ni 2+ ions [35].The peak to peak line width (Δ pp ) decreases with the addition of capping agent.Such decrease in Δ pp may be due to the decrease in magnetic dipolar interaction and the reduction in particle size [36].However, the slight decrease in relaxation time with the addition of EDTA is due to the decrease in electron motion and weakening of superexchange interaction in lattice [34,37].The magnetization value plays a dominant role in the evaluation of the resonance field and the line width [38].The decrease in the resonance field with the addition of capping agent EDTA may be attributed to the increase in the internal magnetic field.Hence, a low field is required for the resonance in EDTA-capped NiO nanoparticles.This result agrees with these obtained by VSM measurement.In addition, both loops pass through the origin [39] and show antiferromagnetic behavior of NiO nanoparticles.The saturation magnetization (  ), the remnant magnetization (  ), the coercivity (  ), and squareness ratio () ( =   /  ) are listed in Table 1.The tabulated data show that coercivity   decreases with the addition of capping agent EDTA, indicating that   is a function of the particle size that decreases with the addition of EDTA.This variation of   with particle size can be explained on the basis of domain structure, critical size, and surface as well as interface anisotropy of the crystal.A crystallite will spontaneously break up into a number of domains in order to reduce the large magnetization energy if it is a single domain [40].The ratio of the energy before and after division into domains varied as √  [41], where  is the particle size.So the energy reduces as  decreases, which suggests that the crystallite prefers to remain with single domain behavior for quite small .However, the magnetization,   and   , of EDTAcapped NiO nanoparticles is enhanced as compared to that of the pure NiO nanoparticles because of bonding to EDTA.Such capping recovers the surface magnetization of the nanoparticles [42].From Table 1, it is clear that  < 0.5 for all samples indicating that the particles interact by magnetostatic interaction and the anisotropy decreases in crystal lattice [37,43].
Comparing our results with those obtained by Babu et al. [11], it was found that that use of either EDTA or citric acid as capping agents controls the size of nanoparticles and prevents their agglomeration and enhances the saturation magnetization of NiO nanoparticles.However, using citric acid as a capping agent results in lower sized NiO nanoparticles (25 nm) with a ferromagnetic nature and an   value of 0.8 emu/g.It has been strongly concluded that the magnetic behavior of NiO nanoparticles depends upon the size of nanoparticles.

Conclusion
NiO nanoparticles were prepared by a low cost-effective method, the coprecipitation method.The effect of the addition of capping agent EDTA to NiO nanoparticles was studied.XRD patterns showed that NiO have a face-centered cubic (FCC) structure of crystalline size in the range of 28-33 nm.It is clearly noted that EDTA-capped NiO nanoparticles had a smaller size than uncapped nanoparticles.Hence, the addition of 0.1 M capping agent EDTA can form a nucleation point for nanoparticles growth.The Fourier transform infrared (FTIR) spectra indicate the presence of absorption bands in the range of 402-425 cm −1 , which is a common feature of NiO.An EPR line with  factor ≈1.9-2 is detected for NiO nanoparticles, corresponding to Ni 2+ ions.The magnetic hysteresis of the investigated NiO nanoparticles showed that EDTA capping recovers the surface magnetization of the nanoparticles.
Figure 1: X-ray diffraction patterns for NiO Nps in the absence and presence of EDTA at 500 ∘ C.

3. 3 .
SEM Micrographs.Figures 3(a) and 3(b) show high resolution SEM images of pure and EDTA-capped NiO nanoparticles at 500 ∘ C. NiO exhibits a three-dimensional network of randomly oriented sheet-like structures.As seen from the figure, the addition of EDTA, the capping agent, controls the size as well as the shape of nanoparticles and prevents their agglomeration.It was noted that particle shapes and sizes estimated from TEM images are rather smaller than those estimated from SEM images.TEM does not clarify the formation of NiO nanosheets.

Figure 5
reveals the FTIR spectra of pure and EDTA-capped NiO nanoparticles at 500 ∘ C. The spectra were recorded in the range of 4000 cm −1 -350 cm −1 .

Figure 5 :Figure 6 :
Figure 5: FTIR spectrum of pure and EDTA-capped NiO nanoparticles at room temperature.

Figure 9 :
Figure 9: M-H loops for pure (a) and EDTA-capped NiO nanoparticles (b) at room temperature.

3. 8 .
Room Temperature Magnetic Hysteresis.Magnetization measurements of pure and EDTA-capped NiO nanoparticles performed at room temperature are shown in Figures 9(a) and 9(b).Pure and EDTA-capped NiO nanoparticles depict almost linear response to the applied magnetic field.
[19]sharpness and the intensity of the peaks are quite an indication of the well crystalline nature of the prepared nanoparticles.These results confirm that at 500 ∘ C the starting materials were decomposed completely to NiO.Similar patterns were observed by Gondal et al. for NiO particles prepared by pulsed laser ablation technique[19].The lattice constant  of NiO can be calculated for prominent peak (111) using Bragg's equation: [18]X-RayDiffractionPatterns.Figure1displays the Xray diffraction patterns for pure and EDTA-capped NiO nanoparticles annealed at 500 ∘ C. Both XRD spectra revealed the formation of face-centered cubic (FCC) NiO (JCPDS card number 47-1049) with space group Fm3hm ([225])[18].The five dominant diffraction peaks at approximately 2 = 37.34 ∘ , 43.32 ∘ , 62.9 ∘ , 75.4 ∘ , and 79.39 ∘ can be perfectly related to (111), (200), (220), (311), and (222) crystal planes, respectively.There are no peaks obtained for capping agent EDTA or Ni indicating that the nanocrystalline NiO obtained via this method consists of ultrapure phase without impurities.

Table 1 :
Variation of crystalline size, optical, and magnetic properties of pure and EDTA-capped NiO nanosized particles.
Resonance.EPR spectroscopy is a useful technique for studying the materials with unpaired electrons and defects.In order to understand the effects of chelating agent EDTA on the magnetic properties of NiO nanoparticles, EPR measurement was carried out on the pure and EDTA-capped NiO nanoparticles annealed at 500 ∘ C at room temperature.EPR plots are depicted in Figure8.It is clearly observed that pure NiO nanoparticles exhibit a broad resonance peak.The single line broad EPR spectra for these prepared NiO particles were analyzed using Lorentzian