Effect of Reaction Temperature on Size and Optical Properties of NiTiO 3 Nanoparticles

Nickel titanate (NiTiO3) nanoparticles were successfully prepared by wet-chemical method, using nickel acetate and titanium(IV) isopropoxide as Ni, Ti sources and citric acid as complexing reagent. The gel was calcined at different temperatures from 500-700 °C. Results of thermogravimetric analysis (TGA) are given. Fourier transform infrared spectrometry (FTIR), X-ray diffraction (XRD), Scanning electron microscopy (SEM),transmission electron microscopy (TEM), ultraviolet (UV) spectroscopy, vibrating sample magnetometer (VSM) were used to characterize the crystallization process, particle size, morphology, optical and magnetic properties of the calcined nanoparticles. TEM result reveals that the NiTiO3 was homogeneous and hexagon morphology with the grain size of 30-70 nm. The band gap values of the NiTiO3 nanoparticles were calculated to be 3.43, 3.39 and 3.31 eV. The magnetic property was confirmed that the NiTiO3 nanoparticles of super paramagnetic behavior in nature. Our results suggested that the temperature plays an important role in the particle size effect of nanocrystalline NiTiO3.


Introduction
The metal titanates based oxides including metals, such as nickel, cobalt, ferrite, zinc, copper and lead are universally known as inorganic functional materials with wide applications.For example, they are applicable for industries, such as semiconductor rectifiers, electrodes of solid oxide fuel cell, metal-air barrier, hydro carbonate catalyzers, color mixtures of surface coating and gas sensing devices, etc. [1][2][3][4][5] .
NiTiO 3 belong to the ilmenite structure.Ni and Ti atoms prefer octahedral coordination with alternating cation layers occupied by Ni and Ti alone 6 .On conventional solid state reaction, the preparation of NiTiO 3 nanoparticles requires high-temperature treatment over 1000 o C for an extended period until intermediate phases disappear.The traditional preparation methods of NiTiO 3 can produce large NiTiO 3 nanoparticles with uncontrolled morphologies due to their inherent problems such as high reaction temperature, heterogeneous solid phase reaction etc.By contrast with the traditional methods, the wet-chemical synthesis techniques, including sol-gel, sol-precipitation, combustion synthesis, chemical coprecipitation and hydrothermal synthesis, offer many distinctive advantages over traditional methods in the production of powders.There are few reports available regarding chemical synthesis of NiTiO 3 nanoparticles by citrate, maleate and propionic acid methods [7][8][9][10][11][12] .Thus, it is a meaningful work some routes at a low temperature to prepare ultra-fine particles of NiTiO 3 with a controlled morphology, narrow size distribution and high purity.
As a matter of fact, metal alkoxides and metal acetates are both easily dissolved in certain solvents such as methanol, ethanol, iso-proponal, ethylene and 2-methoxy ethanol 10 .In this study, we chose one typical wet-chemical synthesis method, citric acid complex, to try to prepare pure NiTiO 3 nanoparticles from titanium(IV) isopropoxide and nickel acetate.Citric acid and methanol (CH 3 OH) are used as the chelating agent and a solvent.Moreover, this synthesis process is easily controlled and convenient in comparison with other methods.The formation of process and structural characterization of NiTiO 3 phase have been investigated by thermogravimetric analysis (TGA), X-ray diffraction (XRD), fourier transform infrared (FTIR), scanning electron microscope (SEM), transmission electron microscope (TEM), ultra-violet vis absorption (UV) and vibrating sample magnetometer (VSM).

Experimental
NiTiO 3 nanoparticles were prepared along a synthetic procedure as summarized in Figure 1.A stoichiometric amount of nickel acetate and titanium(IV) isopropoxide with the cationic ratio of Ni: Ti = 1:1 were separately dissolved in methanol.Citric acid was then put into the Ti 4+ solution, and it kept in transparency.While the Ni 2+ solution was also mixed with the above-mentioned glassy solution, the precipitates came out.They were further heated at 90 o C in an oven for 12 h to remove the excess solvent and the agglomerate precursors in light green color were formed.After the organic precursor was calcined at 500-700 o C for 3 h, the phase-pured yellow NiTiO 3 nanoparticles were finally obtained after furnace cooling to room temperature.

Figure 1. Flow chart of the synthesis of NiTiO 3 nanoparticles by the wet chemical method
The size and crystalline phase of NiTiO 3 was determined using X-ray diffractometer Schimadzu model: XRD 6000 with CuKα radiation in the range 20-70 o (λ=0.154nm).Thermogravimetric analysis (TGA) was carried out with a Shimadzu 50-TG-DTA at a heating rate of 20 o C/min.The FT-IR spectrum of the samples was taken using a FTIR model Bruker IFS 66W spectrometer.UV-Vis absorption spectra of NiTiO 3 were recorded using a Varian Cary 5E spectrophotometer at room temperature in the range of 200-1000 nm.The surface morphologies of as-prepared NiTiO 3 the products were studied using the Hitachi S-4500 Scanning Electron Microscope.Transmission Electron Microscope (TEM) studies was taken using a model JEOL-2010 microscope with an accelerating voltage of 100 kV.The magnetic measurement was carried out in a Vibrating Sample Magnetometer (VSM: BHV-55, Riken Japan) at room temperature.

Results and Discussion
Figure 2 shows the TG curve of the citrate complex dried at 90 o C for 12 h.The weight loss below 100 o C was assigned to desorption of water.The weight loss between ~200 and ~450 o C was due to desorption of organic components in the precursor.At temperature above 450 o C no further weight loss was observed, which indicates complete removal of organic residues and the formation of crystalline NiTiO 3 .Thus, the minimum firing temperature to obtain organic-free precursor can be determined above 450 o C. Since the decomposition seems to have several steps such as: Figure 2. TGA Curve of the citrate complex dried at 90 o C for 12 h Figure 3(a-c) shows the XRD patterns of the NiTiO 3 nanoparticles calcined at different temperatures (500, 600 and 700 o C).At 500 o C, the crystallization of rhombohedral nickel titanate phase began along with traces of rutile ( R marked peaks) and anatase phase ( A marked peaks; Figure 2a).Further, by increasing the calcination temperature to 600 o C, both anatase and rutile phase was decreased with an increase in the intensity of NiTiO 3 phase (Figure 2b).The NiTiO 3 nanoparticles were further calcined at 700 o C (Figure 2c).However, at this temperature, the nanoparticles displayed sharp and intense peaks indicating crystalline rhombohedral NiTiO 3 phase.All the peaks corresponding to rhombohedral phase were well matched with database in JCPDS (File No. 33-960).According to the Debye Scherrer's formula, D=kλ/βcosθ where D is the average size of the particles, k=0.9, λ is the X-ray wavelength (0.154 nm for CuKα), β is the full width at half maximum of the diffracted peak (FWHM) and θ is the Bragg diffraction angle 13 .The crystalline sizes of NiTiO 3 were calculated to be 30, 45 and 65 nm for the samples prepared at temperatures 500, 600 and 700 o C respectively.The crystalline sizes were reveals that clearly with decreasing calcination temperature, the crystalline size also decrease.5a).Whereas (Figure 5b) an ellipsoid-like morphology was obtained when the temperature increased to 700 o C, which is well accord with the XRD results.And also it is smaller than those prepared by co-precipitation method 15 (about 90 nm) and solid state reaction method (sintered at above 1000 0 C with comparatively larger particle size about 1µm) 16 .
Figure 5(a-b).SEM morphologies of the as-synthesized NiTiO 3 nanoparticles calcined at 500 and 700 o C The morphologies and particle sizes were further investigated by TEM. Figure 6a shows the TEM image of the as-prepared NiTiO 3 nanoparticles calcined at 500 o C. The obvious rings of selective area electron diffraction (SAED) in the inset of Figure 6a indicate that all the products are polycrystalline.Figure 6a shows NiTiO 3 nanoparticle with hexagon morphology of size about ~30-40 nm.Most of the nanoparticles were well separated although some of them partially aggregated.Figure 6b reveals TEM pattern of sample calcined at 700 o C. Comparing to that of 500 o C, crystalline size of the products were increased to 60-70 nm.Moreover, the particle size of the samples obtained from TEM patterns are quite similar to those calculated from Scherrer's equation.can be calculated to be 3.43, 3.39 and 3.31 eV and are larger than the bulk NiTiO 3 (3.2eV).
The increasing trends of the band gap energy upon the decreasing particle size are well presented for the quantum confinement effect 18 .Figure 8 shows the hysteresis loops of NiTiO 3 nanoparticles at 500 o C. From the result, it was found that the saturated magnetization (M s ) of the fabricated NiTiO 3 nanoparticles was 15 emu/g and its saturated magnetic strength was 2760 Oe.In addition, the experiment show that is residual magnetism and coercive force (Hc) was close to 31.85, confirming that the magnetic particle was super paramagnetic particle.In other words, when the particle is small enough, and the anisotropy energy is too small to be identical with the heat motion, the direction of magnetization would no longer be fixed and the direction of easy magnetization would change irregularly, causing the phenomenon of super para magnetism.

Conclusion
This study has demonstrated the feasibility of synthesis of pure NiTiO 3 nanoparticles using wet-chemical synthesis route, metal-citrate complex at different temperatures (500, 600 and 700 o C).The TGA analysis indicates that the crystallization temperature of NiTiO 3 was above 450 o C. The chemical structure information of the products was studied by Fourier Transform Infrared (FT-IR) spectroscopy.Well crystallized NiTiO 3 nanoparticles could be synthesized at 700 o C for 3 h.The microstructures of NiTiO 3 nanoparticles have been evaluated using SEM and TEM and the grain sizes are shown to vary between ~30-70 nm.The significant optical properties of this material may be very interesting for further application on photo catalyst.The temperature can affect the particle size; the decreasing trends of temperature upon the decreasing particle size are well presented for the assynthesized NiTiO 3 nanoparticles.The magnetic property was studied using a vibrating sample magnetometer.We hope that the procedure mentioned in the experimental section can be a suitable route for the high-yield synthesis of NiTiO 3 nanoparticles.
Temperature on Size and Optical Properties 285

Figure 3 (
Figure3(a-c).XRD patterns of the NiTiO 3 nanoparticles calcined at different temperatures 500, 600 and 700 o C Figure4shows the FT-IR spectra of as-synthesized NiTiO 3 nanoparticles calcined at 500 and 700 o C. It is clear from Figure4athat the weak bands at 3391 and 1615 cm -1 were assigned to the stretching vibrations of surface hydroxyl (-OH) groups and another band around 1280 cm -1 was due to the bending vibration of the H−O−H bonds.However, as the temperature increased (Figure4b), the stretching vibration of -OH and H-O-H bands were completely disappeared, which is consistent with the TGA results.The very strong bands appeared at 617 and 690 cm -1 are due to the stretching of Ti-O bond and bending of O-Ti-O bond meanwhile the absorption at 547 cm -1 is result from Ni-O bond, which indicating 14 the formation of NiTiO 3 .

Figure 6a .
Figure 6a.TEM image of the as-prepared NiTiO 3 nanoparticles calcined at 500 o C.

Figure 6b .
Figure 6b.TEM image of the as-prepared NiTiO 3 nanoparticles calcined at 700 o CThe room temperature UV-Visible spectrum (Figure7a) of the as-prepared NiTiO 3 nanoparticles calcined at 500 o C has the absorption edge is at 364 nm, which is smaller than the band edge observed at 370 and 376 nm for calcined at 600 and 700 o C as shown in Figure7b & c.In all the cases, blue shifts were observed.Considering the blue shifts of the absorption positions from the bulk NiTiO 3 , the absorption onsets of the present samples can be assigned to the direct transition of electron in the NiTiO 3 crystal17 .The corresponding bandgap energies

Figure 7 .
Figure 7. UV-Vis absorption spectra of the as-prepared NiTiO 3 nanoparticles calcined at 500 o C.Figure8shows the hysteresis loops of NiTiO 3 nanoparticles at 500 o C. From the result, it was found that the saturated magnetization (M s ) of the fabricated NiTiO 3 nanoparticles was 15 emu/g and its saturated magnetic strength was 2760 Oe.In addition, the experiment show that is residual magnetism and coercive force (Hc) was close to 31.85, confirming that the magnetic particle was super paramagnetic particle.In other words, when the particle is small enough, and the anisotropy energy is too small to be identical with the heat motion, the direction of magnetization would no longer be fixed and the direction of easy magnetization would change irregularly, causing the phenomenon of super para magnetism.

Figure 8 .
Figure 8. Magnetic property of the as-prepared NiTiO 3 nanoparticles calcined at 500 o C