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Structural and paramagnetic behavior of spinel NiCr2O4 nanoparticles synthesized by thermal treatment method: Effect of calcination temperature
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Structural and paramagnetic behavior of spinel NiCr2O4 nanoparticles synthesized by thermal treatment method: Effect of calcination temperature Syuhada Abu Bakar a, Nayereh Soltani a,n, W. Mahmood Mat Yunus a, Elias Saion a, Afarin Bahrami b a b
Department of Physics,Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia Islamic Azad University, EslamShahr Branch, Iran
art ic l e i nf o
a b s t r a c t
Article history: Received 16 February 2014 Received in revised form 2 May 2014 Accepted 3 May 2014 by E.V. Sampathkumaran
Spinel nickel chromite nanoparticles were synthesized using a simple thermal treatment method. The effect of calcination temperatures on the final properties of obtained materials was carefully examined using various characterization techniques.The infrared spectra of nickel chromite (NiCr2O4) revealed the characteristic bonds of metal–oxygen for Ni–O and Cr–O bands around 600 and 470 cm 1, respectively. The powder X-ray diffraction patterns exhibited the formation of normal spinel phase of NiCr2O4 in the calcination process at temperature between 550 and 850 1C. From transmission electron micrographs, nanosized particles with average size of 7–64 nm were observed at calcination temperatures of 550– 850 1C, respectively. The calcined samples at 750 and 850 1C exhibited paramagnetic behavior with g-factor values of 1.92 and 2.15, peak-to-peak line width of 25.59 and 117.02 Oe and resonance magnetic field of 342.04 and 306.49 Oe, respectively. Variation in the value of g-factor, peak-to-peak line width and resonance magnetic field can be attributed to the dipole–dipole and super exchange interactions. & 2014 Published by Elsevier Ltd.
Keywords: A. Nanostructures C. Electron microscopy C. Infrared spectroscopy C. X-ray diffraction D. Magnetic properties
1. Introduction Spinels are a class of binary transition metal oxides with general composition formula of AB2O4, where A is a divalent cation and B is a trivalent cation [1]. Spinels have a cubic structure with the crystal group of Fd3m, having three different cation distribution patterns between the sites which are known as normal, random and inverse. In the ideal normal spinel, A and B ions occupy the tetrahedral and octahedral sites, respectively; in the inverse spinel, half of the B ions enter the fourfold coordination and all the A ions migrate to the octahedral sites [2–4].The physical and catalytic properties of spinels are influenced by the nature and the oxidative state of transition metal ions and also by their distribution in the spinel structure [5]. Chromites are considered as a significant normal type spinel structure with potential applications in the fields ranging from applied physics and material sciences to geophysics [3]. One of the particular chromite, nickel chromite (NiCr2O4) is identified as a
n
Corresponding author. E-mail address: [email protected] (N. Soltani).
promising catalytic material for a number of industrial processes [6–8]. Since achievement of high surface area is greatly desired for this important application [9], the synthesis of nanosized nickel chromite particles have attracted considerable attention and research efforts. Among different methods in preparing spinel type material, thermal treatment offers an approach because of its ability to control the movement of the metal ions and oxygen atoms during heating process to occupy either tetrahedral or octahedral sites [5,10]. Recently, a simple thermal treatment method to synthesize metal oxides has been introduced in our laboratory [11,12]which offers advantages such as high flexibility, reproducibility with constant quality, environmentally friendly process, low cost production and easy to handle. It has also significant capability to amend the chemical structure of material resulting in desirable properties such as size controlling of nanoparticles. In the present study, nanosized nickel chromite was synthesized from an aqueous solution containing nickel nitrate, chromium nitrate, polyvinyl pyrrolidone, and deionized water followed by drying, grinding and calcination at various temperatures. The characteristics of the final products were studied with various techniques to verify the influence of calcination temperature on
http://dx.doi.org/10.1016/j.ssc.2014.05.002 0038-1098/& 2014 Published by Elsevier Ltd.
Please cite this article as: S.A. Bakar, et al., Solid State Commun (2014), http://dx.doi.org/10.1016/j.ssc.2014.05.002i
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the crystallization, morphology, particle size and paramagnetic properties of synthesized nickel chromite nanoparticles.
2. Experimental Nickel nitrate (Acros Organic) and chromium nitrate (RiedeldeHa&n) were used as metal precursors, polyvinyl pyrrolidone (PVP10000, Sigma Aldrich) as capping agent, and deionized water as solvent. All materials were analytical grade products and used without further purification. In a typical procedure, an aqueous solution of PVP (3 wt%) was prepared under stirring at 80 1C. Subsequently, 2 mM of chromium nitrate and 1 mM of nickel nitrate were added to the polymer solution under stirring. Thereafter, the obtained colorless solution was poured into a glass Petri dish and dried in the air at 110 1C for 24 h. The dried orange solid chromites were crushed and ground for 2 h to form powder. The calcinations of the powder were conducted at 550, 650, 750 and 850 1C for 4 h with heating rate of 2 1C/min (the optimum time and heating rate) to decompose the organic compounds and crystallize the nanoparticles. The products were characterized by X-ray diffraction (XRD) at the scanning rate of 51/min in the 2θ range 20–701 using a Philips X-ray diffractometer (7602 EA Almelo) with Cu Kα radiation (λ ¼0.1542 nm). Infrared spectra (280–4000 cm 1) were recorded using an FT-IR spectrometer (Perkin Elmer model 1650), with the samples pressed onto diamond-coated CsI pellets. Both the XRD and the FT-IR results were used to establish the formation of nickel chromite nanocrystals at different calcination temperatures. The morphology, particle size and size distribution of samples were determined using the transmission electron microscopy (TEM) micrographs (HTACHI H-7100 TEM) operating at an accelerating voltage of 100 kV. Magnetic characterization of the nickel chromite nanoparticles was analyzed using the electron spin resonance (ESR) spectrometer (JOEL JES-FA200 EPR) at room temperature by placing the small amount of powder samples in a quartz tube.
3. Results and discussions 3.1. Interaction between PVP and transition metal ions Possible interactions between the PVP capping agent and transition metal ions in the synthesis of nickel chromite nanoparticles are schematically shown in Fig. 1.
Fig. 1. A proposed mechanism of interaction between PVP and transition metal ions precursors in synthesis of nickel chromite nanoparticles.
Nickel (II) and chromium (III) ions are bound by the strong ionic bonds between the metallic ions and the amide group in a polymeric chain. The role of PVP as a stabilizer for dissolved metallic salts is through steric and electrostatic stabilization of the amide groups of the pyrrolidone rings and the methylene groups. In the drying step, the PVP stabilizer may decompose to a limited extent, thereby producing shorter polymer chains that are capped on the surface of metallic ions. The uniform immobilization of metallic ions in the cavities of the polymer chains favors the formation of uniformly-distributed solid solution of the metallic oxides in the calcination process. The influence of PVP is not restricted only to the solution and drying steps; it also affects the nucleation and growth of the nickel chromite nanoparticles in the calcination step. In this stage, although PVP will be decomposed, the carbon residual that bonded on the surface of nanoparticles can protect them from uncontrolled growth and agglomeration [12–14]. 3.2. XRD analysis The formation of nickel chromite compound and its spinel structure were confirmed by XRD studies. Fig. 2 shows the XRD patterns of dried (110 1C) and calcined samples in various temperatures. The broad peak in the diffractogram of dried sample shows the amorphous structure. Transformation to crystalline phase started at 550 1C with appearance the low intensity diffraction peaks which indicate the presence of single phase nickel chromite spinel structure. The calcination process caused the moving of atoms to their own lattice completely where Ni2 þ ions occupy the tetrahedral (A) site while Cr3 þ ions have preference for octahedral (B) site. With increasing of calcination temperature up to 850 1C, the sharper and narrower peaks with substantial intensity became manifest which led to formation of particles with higher degree of crystallinity and bigger size. For all calcined samples, the XRD peaks observed at 2θ values closely match the (111), (220), (311), (400), (511) and (440) crystalline planes of the face-centered cubic (FCC) spinel structure of NiCr2O4 (ICDD PDF 23-1271) with lattice parameter of 8.3160 Å and unit cell volume of 575.10 Å3. From the width of the XRD peaks, the mean crystalline size can be calculated using the Debye–Scherrer's equation [15] as follows: D¼
0:9λ β cos θ
ð1Þ
where D is the average crystallite size, λ is X-ray wavelength (0.1542 nm), β is angular line width of half-maximum intensity (FWHM) of the diffraction peak, and θ is Bragg's angle in degrees.
Fig. 2. (Color online) XRD patterns of as-prepared and calcined NiCr2O4 nanoparticles at different temperatures.
Please cite this article as: S.A. Bakar, et al., Solid State Commun (2014), http://dx.doi.org/10.1016/j.ssc.2014.05.002i
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The average crystallite sizes of calcined samples estimated by Debye–Scherrer's equation for the preferred plane of (311) are listed in Table 1. From the table, it can be seen that the average crystallite size slightly rises with the calcination temperature from 550 to 750 1C (lies in the range of 9–16 nm) and considerably increases at calcination temperature of 850 1C (63 nm). This is also evidence for the intensification in crystallinity that originates from the increment of crystalline volume ratio due to enlargement of particle size [16].
important peaks are at 3393, 1626, 886, 779, 593 and 472 cm 1, corresponding to the stretching and bending vibrations of O–H, CQO, C–C, C–NQO, Ni–O and Cr–O, respectively. The broadband absorption peak at 3393 cm 1 is due to the presence of hydroxyl group and physically adsorbed water molecule. Other peaks are associated to the PVP matrix and NiCr2O4 nanostructure. At 650 1C, there is still a trace of a broadband absorption peak of hydroxyl group at 3146 cm 1 but at higher temperature, the FT-IR results only exemplify absorption bands of spinel NiCr2O4 nanoparticles (Table 1) and carbon residue caused by PVP decomposition.
3.3. FTIR analysis
3.4. TEM results
The infrared spectroscopy provided the important information about the vibration frequency of functional groups and network structures of the samples. The IR spectra of NiCr2O4 nanoparticles in the range of 280–4000 cm 1 for different calcination temperatures are shown in Fig. 3. Six main regions can be recognized in the spectra of NiCr2O4 at lowest temperature of 550 1C. In this case, the most
The average sizes and size distribution of nanoparticles were evaluated from TEM images using Image tool software considering at least 100 nanoparticles for each sample. Fig. 4 shows the TEM images and corresponding particle size distribution histograms of NiCr2O4 nanoparticles calcined at different temperatures. Morphological evaluation of the TEM images shows that NiCr2O4 nanoparticles are generally uniform in shape and have relatively narrow size distributions. These results indicate the critical role of using the capping agent (PVP). During the heating process, the thermal decomposition of PVP causes the formation of carbonaceous residual which are adsorbed as a thin layer on the surface of nanoparticles and prevents uncontrolled growth of nanoparticles, although the capping characteristics differ from the original material [17–19]. The average particle size increased with increasing calcination temperature from 550 to 850 1C. This suggests that several neighboring particles fuse together to increase particle sizes by melting their surfaces. The smallest average particle size of 7 nm was obtained at 550 1C while it reaches up to 64 nm at the highest calcination temperature of 850 1C. These results are in fair concurrence with those determined from XRD patterns.
Table 1 Obtained data of NiCr2O4 nanoparticles at different calcination temperatures. Samples Calcination temperature (1C)
1 2 3 4
550 650 750 850
Average size (nm)
FT-IR bands (cm 1)
XRD TEM Ni–O
Cr–O
9.2 7.0 593 12.8 12.7 602 16.1 24.4 597 63.8 63.9 594
472 477 473 463
gΔHpp factor (Oe)
Hr (Oe)
– – 1.92 2.15
– – 342.04 306.49
– – 25.59 117.02
3.5. ESR results Fig. 5 shows the broad and symmetrical resonance signals of NiCr2O4 nanoparticles calcined at 750 and 850 1C which indicate the existence of Ni2 þ and Cr3 þ ions. With increasing the calcinations temperature from 750 to 850 1C, the resonance signal of NiCr2O4 become broader and the value of peak-to-peak line width (ΔHpp) ascend from 2.59 Oe to 117.02 Oe but the resonant magnetic field (Hr) decreases from 342.04 Oe to 306.49 Oe. This suggests that the addition of Cr3 þ ions at higher temperature in B site causes an increase in the super exchange interaction, contributing to the increase of the internal field and decrease of the resonance field [20]. The value of g-factor can be determined using g ¼ hν=μB H r
Fig. 3. (Color online) FT-IR spectra of NiCr2O4 nanoparticles calcined at different temperatures.
ð2Þ
where h is Planck's constant and v is the microwave frequency. The evaluated value of g-factor rises from 1.93 to 2.15 when the calcination temperature increases from 750 to 850 1C. These results imply that microscopic magnetic interaction increases with increasing the particle size. It is to be noted that for calcination temperature of 550 and 650 1C, no resonance signal was observed possibly due to the dipole–dipole and super exchange interaction produce in the nanoparticles. The strong dipole–dipole interaction affect the line width and g-factor which given a larger values while the strong super exchange interaction may produce a small value of line width and g-factor [21]that cannot be detected by ESR spectroscopy. Furthermore, the ‘merging effect’ causes the summation of symmetrical resonance lines which cannot be imitated of any shape when the line shifts towards the central position or the line become broad or cramped[22].
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Fig. 4. (Color online) TEM images and corresponding size distribution histograms of NiCr2O4 nanoparticles calcined at different temperatures.
those determined using TEM images ranging from 7.0 nm to 63.9 nm. NiCr2O4nanoparticles prepared at calcination temperature of 750 and 850 1C showed paramagnetic behavior with the g-factor values of 1.92 and 2.15,the resonance magnetic field of 342.04 and 306.49 Oe and peak-to-peak line width of 25.59 and 117.02 Oe respectively. The dipole–dipole interaction and super exchange interaction contribute to the increasing and decreasing values of g-factor, resonance magnetic field and peak-to-peak line width.
Acknowledgments
Fig. 5. (Color online) ESR spectra of NiCr2O4 nanoparticles calcined at 750 and 850 1C.
This work was supported by the Ministry of Higher Education of Malaysia under the FRGS and RUGS grants. The authors would also like to thank the staff of the Faculty of Science and Bioscience Institute of the University Putra Malaysia, who contributed to this work.
4. Conclusions References Nickel chromite nanoparticles have been successfully synthesized using the thermal-treatment method followed by calcination step. This method is a cost-effective and easy route for preparing fine crystalline oxides at low temperatures and in a short reaction time. The XRD patterns of NiCr2O4 confirmed the fine crystalline single phase FCC spinel structure. With increasing the calcination temperature, nanoparticles with higher crystallinity and larger size were obtained. The crystallite size estimated from XRD was found ranging from 9.2 to 63.8 nm which are comparable with
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Contents lists available at ScienceDirect
Solid State Communications journal homepage: www.elsevier.com/locate/ssc
Structural and paramagnetic behavior of spinel NiCr2O4 nanoparticles synthesized by thermal treatment method: Effect of calcination temperature Syuhada Abu Bakar a, Nayereh Soltani a,n, W. Mahmood Mat Yunus a, Elias Saion a, Afarin Bahrami b a b
Department of Physics,Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia Islamic Azad University, EslamShahr Branch, Iran
art ic l e i nf o
a b s t r a c t
Article history: Received 16 February 2014 Received in revised form 2 May 2014 Accepted 3 May 2014 by E.V. Sampathkumaran
Spinel nickel chromite nanoparticles were synthesized using a simple thermal treatment method. The effect of calcination temperatures on the final properties of obtained materials was carefully examined using various characterization techniques.The infrared spectra of nickel chromite (NiCr2O4) revealed the characteristic bonds of metal–oxygen for Ni–O and Cr–O bands around 600 and 470 cm 1, respectively. The powder X-ray diffraction patterns exhibited the formation of normal spinel phase of NiCr2O4 in the calcination process at temperature between 550 and 850 1C. From transmission electron micrographs, nanosized particles with average size of 7–64 nm were observed at calcination temperatures of 550– 850 1C, respectively. The calcined samples at 750 and 850 1C exhibited paramagnetic behavior with g-factor values of 1.92 and 2.15, peak-to-peak line width of 25.59 and 117.02 Oe and resonance magnetic field of 342.04 and 306.49 Oe, respectively. Variation in the value of g-factor, peak-to-peak line width and resonance magnetic field can be attributed to the dipole–dipole and super exchange interactions. & 2014 Published by Elsevier Ltd.
Keywords: A. Nanostructures C. Electron microscopy C. Infrared spectroscopy C. X-ray diffraction D. Magnetic properties
1. Introduction Spinels are a class of binary transition metal oxides with general composition formula of AB2O4, where A is a divalent cation and B is a trivalent cation [1]. Spinels have a cubic structure with the crystal group of Fd3m, having three different cation distribution patterns between the sites which are known as normal, random and inverse. In the ideal normal spinel, A and B ions occupy the tetrahedral and octahedral sites, respectively; in the inverse spinel, half of the B ions enter the fourfold coordination and all the A ions migrate to the octahedral sites [2–4].The physical and catalytic properties of spinels are influenced by the nature and the oxidative state of transition metal ions and also by their distribution in the spinel structure [5]. Chromites are considered as a significant normal type spinel structure with potential applications in the fields ranging from applied physics and material sciences to geophysics [3]. One of the particular chromite, nickel chromite (NiCr2O4) is identified as a
n
Corresponding author. E-mail address: [email protected] (N. Soltani).
promising catalytic material for a number of industrial processes [6–8]. Since achievement of high surface area is greatly desired for this important application [9], the synthesis of nanosized nickel chromite particles have attracted considerable attention and research efforts. Among different methods in preparing spinel type material, thermal treatment offers an approach because of its ability to control the movement of the metal ions and oxygen atoms during heating process to occupy either tetrahedral or octahedral sites [5,10]. Recently, a simple thermal treatment method to synthesize metal oxides has been introduced in our laboratory [11,12]which offers advantages such as high flexibility, reproducibility with constant quality, environmentally friendly process, low cost production and easy to handle. It has also significant capability to amend the chemical structure of material resulting in desirable properties such as size controlling of nanoparticles. In the present study, nanosized nickel chromite was synthesized from an aqueous solution containing nickel nitrate, chromium nitrate, polyvinyl pyrrolidone, and deionized water followed by drying, grinding and calcination at various temperatures. The characteristics of the final products were studied with various techniques to verify the influence of calcination temperature on
http://dx.doi.org/10.1016/j.ssc.2014.05.002 0038-1098/& 2014 Published by Elsevier Ltd.
Please cite this article as: S.A. Bakar, et al., Solid State Commun (2014), http://dx.doi.org/10.1016/j.ssc.2014.05.002i
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the crystallization, morphology, particle size and paramagnetic properties of synthesized nickel chromite nanoparticles.
2. Experimental Nickel nitrate (Acros Organic) and chromium nitrate (RiedeldeHa&n) were used as metal precursors, polyvinyl pyrrolidone (PVP10000, Sigma Aldrich) as capping agent, and deionized water as solvent. All materials were analytical grade products and used without further purification. In a typical procedure, an aqueous solution of PVP (3 wt%) was prepared under stirring at 80 1C. Subsequently, 2 mM of chromium nitrate and 1 mM of nickel nitrate were added to the polymer solution under stirring. Thereafter, the obtained colorless solution was poured into a glass Petri dish and dried in the air at 110 1C for 24 h. The dried orange solid chromites were crushed and ground for 2 h to form powder. The calcinations of the powder were conducted at 550, 650, 750 and 850 1C for 4 h with heating rate of 2 1C/min (the optimum time and heating rate) to decompose the organic compounds and crystallize the nanoparticles. The products were characterized by X-ray diffraction (XRD) at the scanning rate of 51/min in the 2θ range 20–701 using a Philips X-ray diffractometer (7602 EA Almelo) with Cu Kα radiation (λ ¼0.1542 nm). Infrared spectra (280–4000 cm 1) were recorded using an FT-IR spectrometer (Perkin Elmer model 1650), with the samples pressed onto diamond-coated CsI pellets. Both the XRD and the FT-IR results were used to establish the formation of nickel chromite nanocrystals at different calcination temperatures. The morphology, particle size and size distribution of samples were determined using the transmission electron microscopy (TEM) micrographs (HTACHI H-7100 TEM) operating at an accelerating voltage of 100 kV. Magnetic characterization of the nickel chromite nanoparticles was analyzed using the electron spin resonance (ESR) spectrometer (JOEL JES-FA200 EPR) at room temperature by placing the small amount of powder samples in a quartz tube.
3. Results and discussions 3.1. Interaction between PVP and transition metal ions Possible interactions between the PVP capping agent and transition metal ions in the synthesis of nickel chromite nanoparticles are schematically shown in Fig. 1.
Fig. 1. A proposed mechanism of interaction between PVP and transition metal ions precursors in synthesis of nickel chromite nanoparticles.
Nickel (II) and chromium (III) ions are bound by the strong ionic bonds between the metallic ions and the amide group in a polymeric chain. The role of PVP as a stabilizer for dissolved metallic salts is through steric and electrostatic stabilization of the amide groups of the pyrrolidone rings and the methylene groups. In the drying step, the PVP stabilizer may decompose to a limited extent, thereby producing shorter polymer chains that are capped on the surface of metallic ions. The uniform immobilization of metallic ions in the cavities of the polymer chains favors the formation of uniformly-distributed solid solution of the metallic oxides in the calcination process. The influence of PVP is not restricted only to the solution and drying steps; it also affects the nucleation and growth of the nickel chromite nanoparticles in the calcination step. In this stage, although PVP will be decomposed, the carbon residual that bonded on the surface of nanoparticles can protect them from uncontrolled growth and agglomeration [12–14]. 3.2. XRD analysis The formation of nickel chromite compound and its spinel structure were confirmed by XRD studies. Fig. 2 shows the XRD patterns of dried (110 1C) and calcined samples in various temperatures. The broad peak in the diffractogram of dried sample shows the amorphous structure. Transformation to crystalline phase started at 550 1C with appearance the low intensity diffraction peaks which indicate the presence of single phase nickel chromite spinel structure. The calcination process caused the moving of atoms to their own lattice completely where Ni2 þ ions occupy the tetrahedral (A) site while Cr3 þ ions have preference for octahedral (B) site. With increasing of calcination temperature up to 850 1C, the sharper and narrower peaks with substantial intensity became manifest which led to formation of particles with higher degree of crystallinity and bigger size. For all calcined samples, the XRD peaks observed at 2θ values closely match the (111), (220), (311), (400), (511) and (440) crystalline planes of the face-centered cubic (FCC) spinel structure of NiCr2O4 (ICDD PDF 23-1271) with lattice parameter of 8.3160 Å and unit cell volume of 575.10 Å3. From the width of the XRD peaks, the mean crystalline size can be calculated using the Debye–Scherrer's equation [15] as follows: D¼
0:9λ β cos θ
ð1Þ
where D is the average crystallite size, λ is X-ray wavelength (0.1542 nm), β is angular line width of half-maximum intensity (FWHM) of the diffraction peak, and θ is Bragg's angle in degrees.
Fig. 2. (Color online) XRD patterns of as-prepared and calcined NiCr2O4 nanoparticles at different temperatures.
Please cite this article as: S.A. Bakar, et al., Solid State Commun (2014), http://dx.doi.org/10.1016/j.ssc.2014.05.002i
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The average crystallite sizes of calcined samples estimated by Debye–Scherrer's equation for the preferred plane of (311) are listed in Table 1. From the table, it can be seen that the average crystallite size slightly rises with the calcination temperature from 550 to 750 1C (lies in the range of 9–16 nm) and considerably increases at calcination temperature of 850 1C (63 nm). This is also evidence for the intensification in crystallinity that originates from the increment of crystalline volume ratio due to enlargement of particle size [16].
important peaks are at 3393, 1626, 886, 779, 593 and 472 cm 1, corresponding to the stretching and bending vibrations of O–H, CQO, C–C, C–NQO, Ni–O and Cr–O, respectively. The broadband absorption peak at 3393 cm 1 is due to the presence of hydroxyl group and physically adsorbed water molecule. Other peaks are associated to the PVP matrix and NiCr2O4 nanostructure. At 650 1C, there is still a trace of a broadband absorption peak of hydroxyl group at 3146 cm 1 but at higher temperature, the FT-IR results only exemplify absorption bands of spinel NiCr2O4 nanoparticles (Table 1) and carbon residue caused by PVP decomposition.
3.3. FTIR analysis
3.4. TEM results
The infrared spectroscopy provided the important information about the vibration frequency of functional groups and network structures of the samples. The IR spectra of NiCr2O4 nanoparticles in the range of 280–4000 cm 1 for different calcination temperatures are shown in Fig. 3. Six main regions can be recognized in the spectra of NiCr2O4 at lowest temperature of 550 1C. In this case, the most
The average sizes and size distribution of nanoparticles were evaluated from TEM images using Image tool software considering at least 100 nanoparticles for each sample. Fig. 4 shows the TEM images and corresponding particle size distribution histograms of NiCr2O4 nanoparticles calcined at different temperatures. Morphological evaluation of the TEM images shows that NiCr2O4 nanoparticles are generally uniform in shape and have relatively narrow size distributions. These results indicate the critical role of using the capping agent (PVP). During the heating process, the thermal decomposition of PVP causes the formation of carbonaceous residual which are adsorbed as a thin layer on the surface of nanoparticles and prevents uncontrolled growth of nanoparticles, although the capping characteristics differ from the original material [17–19]. The average particle size increased with increasing calcination temperature from 550 to 850 1C. This suggests that several neighboring particles fuse together to increase particle sizes by melting their surfaces. The smallest average particle size of 7 nm was obtained at 550 1C while it reaches up to 64 nm at the highest calcination temperature of 850 1C. These results are in fair concurrence with those determined from XRD patterns.
Table 1 Obtained data of NiCr2O4 nanoparticles at different calcination temperatures. Samples Calcination temperature (1C)
1 2 3 4
550 650 750 850
Average size (nm)
FT-IR bands (cm 1)
XRD TEM Ni–O
Cr–O
9.2 7.0 593 12.8 12.7 602 16.1 24.4 597 63.8 63.9 594
472 477 473 463
gΔHpp factor (Oe)
Hr (Oe)
– – 1.92 2.15
– – 342.04 306.49
– – 25.59 117.02
3.5. ESR results Fig. 5 shows the broad and symmetrical resonance signals of NiCr2O4 nanoparticles calcined at 750 and 850 1C which indicate the existence of Ni2 þ and Cr3 þ ions. With increasing the calcinations temperature from 750 to 850 1C, the resonance signal of NiCr2O4 become broader and the value of peak-to-peak line width (ΔHpp) ascend from 2.59 Oe to 117.02 Oe but the resonant magnetic field (Hr) decreases from 342.04 Oe to 306.49 Oe. This suggests that the addition of Cr3 þ ions at higher temperature in B site causes an increase in the super exchange interaction, contributing to the increase of the internal field and decrease of the resonance field [20]. The value of g-factor can be determined using g ¼ hν=μB H r
Fig. 3. (Color online) FT-IR spectra of NiCr2O4 nanoparticles calcined at different temperatures.
ð2Þ
where h is Planck's constant and v is the microwave frequency. The evaluated value of g-factor rises from 1.93 to 2.15 when the calcination temperature increases from 750 to 850 1C. These results imply that microscopic magnetic interaction increases with increasing the particle size. It is to be noted that for calcination temperature of 550 and 650 1C, no resonance signal was observed possibly due to the dipole–dipole and super exchange interaction produce in the nanoparticles. The strong dipole–dipole interaction affect the line width and g-factor which given a larger values while the strong super exchange interaction may produce a small value of line width and g-factor [21]that cannot be detected by ESR spectroscopy. Furthermore, the ‘merging effect’ causes the summation of symmetrical resonance lines which cannot be imitated of any shape when the line shifts towards the central position or the line become broad or cramped[22].
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Fig. 4. (Color online) TEM images and corresponding size distribution histograms of NiCr2O4 nanoparticles calcined at different temperatures.
those determined using TEM images ranging from 7.0 nm to 63.9 nm. NiCr2O4nanoparticles prepared at calcination temperature of 750 and 850 1C showed paramagnetic behavior with the g-factor values of 1.92 and 2.15,the resonance magnetic field of 342.04 and 306.49 Oe and peak-to-peak line width of 25.59 and 117.02 Oe respectively. The dipole–dipole interaction and super exchange interaction contribute to the increasing and decreasing values of g-factor, resonance magnetic field and peak-to-peak line width.
Acknowledgments
Fig. 5. (Color online) ESR spectra of NiCr2O4 nanoparticles calcined at 750 and 850 1C.
This work was supported by the Ministry of Higher Education of Malaysia under the FRGS and RUGS grants. The authors would also like to thank the staff of the Faculty of Science and Bioscience Institute of the University Putra Malaysia, who contributed to this work.
4. Conclusions References Nickel chromite nanoparticles have been successfully synthesized using the thermal-treatment method followed by calcination step. This method is a cost-effective and easy route for preparing fine crystalline oxides at low temperatures and in a short reaction time. The XRD patterns of NiCr2O4 confirmed the fine crystalline single phase FCC spinel structure. With increasing the calcination temperature, nanoparticles with higher crystallinity and larger size were obtained. The crystallite size estimated from XRD was found ranging from 9.2 to 63.8 nm which are comparable with
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