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Vol. 8. Issue 3.
Pages 2481-3388 (May - June 2019)
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Vol. 8. Issue 3.
Pages 2481-3388 (May - June 2019)
Original Article
DOI: 10.1016/j.jmrt.2017.07.007
Open Access
Electrical and magnetic properties of NiTiO3 nanoparticles synthesized by the sol–gel synthesis method and microwave sintering
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Pavithra C.a,
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madhuriw12@hotmail.com

Corresponding author.
, Madhuri W.a,b
a Center for Crystal Growth, VIT University, Vellore, Tamil Nadu 632 014, India
b Leibniz Institute for Solid State and Materials Research, Dresden 01069, Germany
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Table 1. Conductance parameters from Cole–Cole plot.
Abstract

In this paper, we focused on microwave sintered NiTiO3 nanoparticles synthesized via sol–gel method. The crystal structure was determined by the X-ray diffraction. Vibrational bands related to Ni–O and Ti–O bands were confirmed using the Fourier transform infrared spectrum. These NiTiO3 ceramics obeyed semiconductor behavior of Arrhenius type. The activation energy was found to be 0.04μeV. The M–H curve exhibited superparamagnetic behavior at room temperature.

Keywords:
NiTiO3
X-ray diffraction
HRSEM
Impedance analysis
M–H curve
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1Introduction

Inorganic materials especially in lead titanate, barium titanate, strontium titanate, cobalt titanate and nickel titanate have wide applications in electronic industry. These are piezoelectric in nature. A few of them can exhibit an anti-ferromagnetism [1]. Metal oxide nanoparticles are important to develop advanced catalyst and sensory materials [2]. Titanium based perovskite ATiO3 (A=Pb, Ba, Sr, Zn and Ni) are widely used in photoemission and photocatalysis. NiTiO3 is used as an photocatalyst in the removal of organic pollutants from gas sensitivity [3]. NiTiO3 is an n-type semiconductor with wide-spread usage [4]. At room temperature, it exhibits antiferromagnetism due to order–disorder transition between Ni and Ti which is also responsible for high curie transition temperature, Tc=1570 [5].

Different methods are successfully synthesized the NiTiO3 such as, solid state, hydrothermal, molten salt, polymeric precursor, electrospinning and a sol–gel method. Electrospinning method yielded good results in X-ray diffraction at 600°C calcination temperature though there is no addition TiO2 was present. Of all synthesis methods, sol–gel technique is easy to process, having homogenate and controlled reaction [6–12]. The work objective is to synthesize NiTiO3 nano-particles by sol–gel and then process by sintering, this is the first time to report the microwave sintered NiTiO3 ceramics. Compared to conventional sintering, microwave sintering consumes less energy and time. The ceramics sintered in microwaves have exhibited high density, uniform grain growth and homogeneous [13–15]. Therefore, in this paper, we present how NiTiO3 nanoparticles are obtained by the sol–gel synthesis, microwave processing and its structural, microstructural, electrical and magnetic characterization.

2Experimental details2.1Sol–gel synthesis of NiTiO3 nanoparticles

Initially, analytical grade nickel nitrate [Sigma-Aldrich, 97.0%, Ni(NO3)2·6H2O] and titanium(IV) butoxide [Sigma-Aldrich, 97.0%, Ti(OC4H9)4] are taken in a stoichiometric ratio. Then nickel nitrate is dissolved in 16.88ml of glacial acetic acid under continuous stirring. This mixed solution is dehydrated at 100°C for 20min and is cooled to room temperature. Then, titanium (IV) butoxide is added very slowly under constant stirring for 45min at room temperature. Next, the mixture of ethanol and water are added drop by drop to prevent fast gelation. Then the gel is heated at 100°C in an oven to obtain nanoparticles. Further, the prepared powder is calcined at 730°C for 45min at the rate of 30°C per min in a microwave furnace. The green sample is grinded using agate mortar for 6h. Finally, the powder is pressed into pellet and densification is done at 1000°C for 45min at the rate of 30°C per min in a microwave furnace.

2.2Characterizations

X-ray diffraction is carried out using powder X-ray instrument (Bruker D8) with Cu Kα radiation (1.5406Å). FT-IR is recorded using Siemens Instrument. The surface morphological and compositional analyses were studied using high resolution scanning electron microscope (FEI Quanta FEG 200) and energy dispersive X-ray spectrometer (EDX). Conductivity and impedance spectroscopy of nickel titanate are carried out using the LCR Hi TESTER (HIOKI-3532-50). The sample is placed between the electrodes of the computer interfaced oven. The magnetic properties of nickel titanate are studied by VSM instrument (LAKESHORE-7407).

3Results and discussion3.1Structural and microstructural characterization

Fig. 1 shows the X-ray diffraction pattern of NiTiO3. All the peaks of X-ray diffraction pattern are matching with JCPDS file no 83-0198 and [5]. The appearance of the small additional TiO2 rutile phase peak is represented by * which is also confirmed in the JCPDS file no 87-1165. Approximately 15% of rutile phase is estimated by Reitvelds analysis. The rutile phase is more stable than anatase phase at a high temperature [16]. These peaks represent rhombohedral crystal structure and R3 space group. The lattice parameters are evaluated using the powder X software and are found to be a=b=5.03Å and c=13.79Å. The volume of the unit cell is found to be 3483, these values match with those in Ref. [5]. Density (according to Archimedes principle) and porosity are estimated using relation (1) and (2). The values are to be 5.046g/cm3 and 0.143. The theoretical density (from X-ray studies) is found using the relation (3) the value is 5.89g/cm3.

where M is the molecular weight of the corresponding composition, N is the Avogadro's number (6.026×1023atomsmol−1) and a3 is the volume of the unit cell.

Fig. 1.

X-ray diffraction of NiTiO3.

(0.07MB).

The average crystal size is 46nm calculated using the relation (4):

where k=0.9, λ is the wavelength of X-rays Cu Kα radiation (∼1.54Å), β is the full width half maximum and θ is the angle of the diffraction.

Fig. 2 shows the differential scanning calorimetry (DSC) and thermo-gravimetric analysis (TGA) of NiTiO3. DSC is used to analyze the endothermic and/or exothermic process of the material, and it measures the phase change, purity, evaporation, melting point, crystallization and heat capacity etc. The DSC curve shows the initial endothermic peak at 76.02°C, the dehydration from 53°C to 130°C. An exothermic peak at 309.24°C is a decomposition of residual organic compounds. A small peak at 400°C is due to the formation of NiO. A small shoulder around 420°C indicated the initiation of ordered solid state transitions leading to the formation of NiTiO3. TGA curve shows the decomposition of the materials at three points indicating the weight loss of the material. The first weight loss is due to dehydration of water molecules, and the observed weight loss is 15.13%. The second weight loss has been observed from 100°C to 320°C is 34.94%. It has occurred due to decomposition of nickel, titanate and residual organic compounds. The third weight loss 320°C to 600°C due to degradation of butoxide. Finally, crystallization of NiTiO3 has occurred [6,8,10,17] after 600°C with no weight loss.

Fig. 2.

TGA and DSC curve of NiTiO3.

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The bond formation in the sample is studied using FT-IR analysis, in the range of 4000–400cm−1. The characteristic vibration of metal–oxygen bond is supposed to be in the range 700–400cm−1. Fig. 3 shows the FT-IR spectrum of NiTiO3. The wavelengths of 673.15cm−1 and 526.56cm−1 are stretching vibrations of Ni–O and Ti–O bonds. The sharp absorption peak at 459.05cm−1 confirms the metal titanate bond Ti–O–Ni [5]. Fig. 4(a) and (b) shows the morphology and elemental analysis of NiTiO3. The high-resolution scanning electron microscope (HRSEM) image shows (Fig. 4(a)) uniform grain growth and size distribution of NiTiO3. The particle size is between 56.5nm and 71.9nm. The energy dispersive spectrum in Fig. 4(b) shows no impurities in the synthesized NiTiO3.

Fig. 3.

FT-IR spectra of NT.

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Fig. 4.

(a) HRSEM and (b) EDAX pattern of NiTiO3.

(0.43MB).
3.2Electrical analysis3.2.1Impedance analysis

The complex impedance spectroscopy technique gives an insight into various contributions of the electrical characteristics of the material. The Cole–Cole plot is plotted between the real and imaginary parts of the impedance. The shape of these plots is usually like two semicircles where the semicircle at a lower frequency is electrical contribution due to grain boundaries while the second arises due to grains and occurs at higher frequencies [18]. Fig. 5 shows the Cole–Cole plot of NiTiO3 in 400–500°C temperature range. Center of the semicircles lies below the X-axis called as non-Debye relaxation. In the plot, it can be noted a single semi-circle is present at higher frequency region. This indicates the intra-grain conduction and no effect of grain boundaries. The relaxation frequency and time at all temperatures can be estimated using the relation:

where ω=2πνmax, νmax is applied frequency corresponding to the arc maximum. The conduction of the material is calculated using the relation:
where the intercept of the semicircle with X-axis gives the bulk resistance Rb (ohm), L is the thickness and A is the cross-sectional area of the pelletized NiTiO3 sample (in cm), and σ is the conductivity (Scm−1). The inset of Fig. 5 represents the equivalent RC circuit of the bulk conductance of NiTiO3. The value of bulk capacitance is calculated using the relation 2πνmaxRbCb=1 (or) ωRbCb=1. The value of relaxation time can be evaluated using the Rb and Cb values. The calculated values are tabulated in Table 1. It is observed that the relaxation time is decreased with an increase in temperature. It is indicating an increase in the electrical conductivity with increase in temperature. Similar semiconducting behavior is noticed and reported [19,20] in polycrystalline complex titanium ceramics. Fig. 6 shows the variation of σdc with the inverse of temperature (1000/T). The linearity of the plot is associated with thermally activated behavior. The activation energy is found to be 0.041μeV. Frequency dependence of conductivity is plotted at various temperatures ranging from 490°C to 560°C in Fig. 7. At low frequencies, the conductance is steady and low which may be accounted for dc conduction. A linear increase is noticed from 0.1MHz to 5MHz, which indicated the dominant role of grains in conduction mechanism than grain boundaries. A small shoulder (kink) noticed at 4MHz must be due to instrumental error as 5MHz is the limit of the instrument.

Fig. 5.

Impedance spectra of NT at various temperatures.

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Table 1.

Conductance parameters from Cole–Cole plot.

Temperature(°C)  Rb(×107ohm)  CbpF  τ(s)  σdc (Scm−1)Cole–Cole plot 
400  5.0192  6.345  3.184×10−4  5.454×10−8 
420  2.9509  7.708  2.274×10−4  9.282×10−8 
440  1.0485  5.062  5.307×10−5  2.612×10−7 
460  0.2815  5.655  1.592×10−5  9.728×10−7 
480  0.1438  5.533  7.961×10−6  1.903×10−6 
500  0.1060  5.002  5.307×10−6  2.581×10−6 
520  0.0539  5.903  3.184×10−6  5.078×10−6 
540  0.0484  3.283  1.592×10−6  5.649×10−6 
560  0.0449  3.540  1.592×10−6  6.091×10−6 
Fig. 6.

Arrhenius plot of NiTiO3.

(0.09MB).
Fig. 7.

Conductance spectra of NT at different temperature.

(0.15MB).
3.3Magnetic studies

The field dependence of magnetic moment in an applied field of ±15kV at room temperature is shown in Fig. 8. The narrow peak shows the existence of ferromagnetic materials in the sample. At low-temperatures NiTiO3 exhibits antiferromagnetic behavior while ferromagnetic behavior at high temperatures, as reported elsewhere [5]. In the entire range of applied field, NiTiO3 is not saturated. This may be due to high antiferromagnetic interactions at room temperature. The spin interaction between the Ni2+–Ni2+ ions is more, so high magnetic field is required to align the spins in the field direction. The coercive field, retentivity and magnetization are 195.72G, 220.91×10−6emu and 15.994×10−3emu respectively. These low values indicate that NiTiO3 is super paramagnetic in nature.

Fig. 8.

M–H loop of NiTiO3.

(0.08MB).
4Summary and conclusion

In summary, we have obtained with success NiTiO3 nanoparticles synthesized by the sol–gel method and processed in microwaves at 1000°C for 45min. X-ray diffractions patterns confirm the rhombohedral phase and dense NiTiO3 formation. Thermal analysis of DSC and TGA is confirms the crystallization stating point at 600°C. FTIR characterization of NiTiO3 confirms the Ni–O, Ti–O and Ni–O–Ti bond formations and their corresponding vibrational frequencies. HRSEM analysis revealed uniform grain growth with a particle size distribution between 56nm and 71nm. EDX spectrum confirms the purity of synthesized NiTiO3. Conductivity studies confirm the semiconducting nature of NiTiO3 and activation energy is estimated to be 0.04μeV. Magnetometry measurements have revealed superparamagnetic behavior at room temperature.

Conflicts of interest

The authors declare no conflicts of interest..

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Journal of Materials Research and Technology

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