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Preparation of sheet like polycrystalline NiFe2O4 nanostructure with PVA matrices and their properties
Materials Letters 65 (2011) 1438–1440
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Preparation of sheet like polycrystalline NiFe2O4 nanostructure with PVA matrices and their properties P. Sivakumar a,⁎, R. Ramesh b, A. Ramanand c, S. Ponnusamy b, C. Muthamizhchelvan b a b c
T. S. Srinivasan Centre for Polytechnic College and Advanced Training (CPAT-TVS), Vanagaram, Chennai, 600 095, Tamilnadu, India Center for Materials Science and Nano Devices, Department of Physics, SRM University, Kattankulathur, Kancheepuram, 603 203, Tamilnadu, India Department of Physics, Loyola College, Nungambakkam, Chennai, 600 034, Tamilnadu, India
a r t i c l e
i n f o
Article history: Received 27 November 2010 Accepted 4 February 2011 Available online 18 February 2011 Keywords: Nickel ferrite Nanosheet Oriented aggregation FTIR Spectra Magnetic properties
a b s t r a c t Sheet-like nickel ferrite (NiFe2O4) has been synthesised with PVA matrices using co-precipitation technique. The sheet is formed from the oriented aggregation of single crystalline NiFe2O4 nanoparticles with PVA as the structure directing template. The synthesised materials are characterised based on X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), highresolution scanning electron microscopy (HRSEM), Fourier transform infrared spectroscopy (FTIR) and vibrating sample magnetometry (VSM). The XRD results show that the nanocrystal contains single phase spinel structure of Fd3m space group. The existence of PVA with nanoparticles has been confirmed by FTIR spectra. The room temperature ferromagnetic property is exhibited by the as synthesised sample with high saturation magnetisation. © 2011 Elsevier B.V. All rights reserved.
1. Introduction In recent times, nanosized spinel ferrite particles have attracted considerable attention due to the enhancement of physical and chemical properties. As an important member of the ferrite family, nickel ferrite (NiFe2O4) has attracted significant research interest because of its fascinating magnetic and electromagnetic properties. NiFe2O4, an inverse spinel structure, shows ferromagnetism. NiFe2O4 is a soft ferrite with low coercivity, high saturation magnetization, chemical stability and electrical resistivity, which can make it an excellent material for magnetic resonance imaging (MRI) enhancement, magnetic recording media and electronic devices [1–4]. Recently, many attempts have been made to synthesise various NiFe2O4 nanostructures in order to explore and enhance its properties and design the technological applications. 1D ferrite nanostructure can be synthesised by sol–gel method, co-precipitation technique, microemulsion method, anodic aluminium oxide (AAO) template method, polymeric co-precipitation method and precursor method [5–9]. Amongst these many various synthesis methods, it is still critical to find simple and cost-effective routes to synthesise nanocrystalline NiFe2O4 by the utilisation of cheap, nontoxic and environmentally benign precursors. Co-precipitation of NiFe2O4 with
⁎ Corresponding author at: Department of Basic Engineering, T. S. Srinivasan Centre for Polytechnic College and Advanced Training (CPAT-TVS), No:1, TVS School Road, Vanagaram, Chennai, 600 095, Tamilnadu, India. Tel.: + 91 9790898204; fax: + 91 44 24760808. E-mail address: [email protected] (P. Sivakumar). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.02.026
PVA matrices is a good technique to synthesise the 1D nanostructure since it fulfils the above conditions. The self-assembly of the NiFe2O4 within the PVA polymer matrices is achieved by introducing Fe (NO3)3·9H2O and Ni(NO3)2·6H2O precursors into the polymer and by subsequent processes such as co-precipitation. Single crystalline NiFe2O4 nanoparticles are formed within the PVA matrices, which serve as the templating medium to form the polycrystalline NiFe2O4 nanosheet. The development of such mixed-NiFe2O4 PVA-based nanocomposites is targeting the functionalisation into device technologies. 2. Experimental details 2.1. Synthesis of NiFe2O4 nanosheet with PVA matrices Iron nitrate [(Fe (NO3)3·9H2O)], nickel nitrate [(Ni (NO3)2·6H2O)] and polyvinyl alcohol (PVA) are purchased from Aldrich and is used without any further purification. In a typical synthesis, 0.2 M (20 mL) solution of Fe (NO3)3·9H2O and 0.1 M (20 mL) solution of Ni (NO3)2·6H2O are prepared and vigorously mixed under stirring at 80 °C for 1 h. PVA is added into the solution as a capping agent. Here 0.2 g of PVA capped NiFe2O4 is labelled as sample-1 and 0.4 g PVA capped NiFe2O4 is labelled as sample-2. Subsequently, an appropriate amount of hydrazine hydrate is added drop by drop into the solutions and black colour precipitates are formed. Finally the precipitates are separated by centrifugation and dried in hot air oven at 100 °C for 4 h. The acquired substance was then ground into a fine powder and then annealed at 300 °C for 10 h.
P. Sivakumar et al. / Materials Letters 65 (2011) 1438–1440
1439
3. Results and discussion Fig. 1(a, and b) shows the powder XRD pattern of sample 1 and sample 2. The peaks are indexed with inverse spinel cubic phase with the space group of Fd3m, which is very well consistent with the JCPDS file (card no: 10-0325). The reflectance peak (311) at 35.7° is relatively weaker than the other peaks. “This indicates that crystallisation of particles has taken place along the easy direction (311) of NiFe2O4”. The crystallite size of NiFe2O4 is calculated using Scherrer's relation. D = 0:9λ = βcosθ
Fig. 1. XRD patterns of (a) sample-1 and (b) sample-2 of NiFe2O4 nanostructures.
2.2. Characterisation methods X-ray diffraction patterns of the samples are recorded using PANalytical X' pert pro X-ray diffractometer with CuKα radiation (1.5406 Å) source. The intensity data is collected over the range of 20°–80° using a step scan mode (0.06°/s). The HRTEM micrographs are obtained on a JEM 3010 (JEOL) transmission electron microscope with an accelerating voltage of 200 kV. The particle morphological features are imaged by high-resolution scanning electron microscopy (FEI Quanta FEG200) with an accelerating voltage of 25 kV. Fourier transform infrared (FTIR) spectra are recorded on Perkin Elmer 2000 FTIR spectrometer in KBr pellets. The magnetic measurements are carried out in a vibrating sample magnetometer (VSM, JDM-13) at room temperature.
where β is the broadening of the diffraction line measured at half maximum intensity (radians) and λ = 1.5406 Å, the wavelength of CuKα. The average crystallite size of sample-1 and sample-2 are 23 nm and 22 nm respectively. All the photographs presented in Fig. 2 clearly show that NiFe2O4 nanosheets can be formed from NiFe2O4 nanoparticles. Fig. 2 (a, b, and c) show small aggregation of primary NiFe2O4 nanoparticles which causes the formation of sheet like morphology. Although most of the nanosheets appear to be separate sheets, the surfaces are clearly not smooth, but contain some nanoparticles. It clearly indicates the initial stages for the growth of nanosheet. The aggregation of the nanoparticles increase to 0.4 g of the PVA capped NiFe2O4 nanoparticles. Finally irregular morphology of the nanosheet with peculiar shape has been formed (Fig. 2d, e, and f). The HRTEM results clearly show that NiFe2O4 nanosheets can be effectively formed from the oriented attachment of single crystalline NiFe2O4 nanoparticles spreading PVA in all directions. “The clear atomic lattice fringes can be observed, and the measured spacing of the crystallographic planes are about 0.25 and 0.28 nm, which are close to the (311) and (211) lattice planes of NiFe2O4 crystal respectively”. The possible growth mechanism has been presented as “oriented aggregation” of primary spherical nanoparticles, which involve self
Fig. 2. (a) HRSEM, (b) TEM and (c) HRTEM images of the sample-1 of NiFe2O4 nanoparticles. (d) HRSEM, (e) TEM and (f) HRTEM images of the sample-2 of NiFe2O4 nanosheet.
1440
P. Sivakumar et al. / Materials Letters 65 (2011) 1438–1440
60
(b)
(a)
1383 2922
(a)
2852
1622
3412 602
Magnetization (emu/g)
Transmittance (a.u)
40
(b)
20
0
-20
-40 3436 4000
3500
1639 2923 2852 3000
2500
2000
1500
1379 1000
500
Wavenumber (cm-1)
-60 -8000
-6000
-4000
-2000
0
2000
4000
6000
8000
Applied Field (Oe) Fig. 3. (a) FTIR of pure polyvinyl alcohol (PVA) and (b) FTIR of as synthesised NiFe2O4 nanosheet.
Table 1 Magnetic properties of synthesised NiFe2O4-PVA samples. Sample
Ms (emug−1)
Mr (emug−1)
Hc (Oe)
1 2
44.05 41.98
10.93 10.09
204.96 204.20
assembly of adjacent particles in a common crystallographic orientation and joined at a planar interface. This involves spontaneously reducing the overall energy of the whole system [10]. This present work, NiFe2O4 nanosheet is being formed by a similar mechanism. The long chain of PVA molecules attached on the crystallographic face of nanoparticles promotes the growth of oriented aggregates, which continue to grow into a sheet like morphology mainly driven by crystal packing force and stacking interactions among adjacent atoms. Finally, polycrystalline NiFe2O4 nanosheet has been formed (Fig. 2d, e, and f). Fig. 3 (a and b) show the FTIR of pure polyvinyl alcohol (PVA) and FTIR of as synthesised NiFe2O4 nanosheet. FTIR analysis of NiFe2O4PVA nanocomposite shows (Fig. 3) typical absorption bands at 3414 cm−1 that correspond to the stretching mode of the OH group. The Fe–O vibration mode of NiFe2O4 is found to be present near 603 cm−1 [11]. The band at 1383 cm−1 reveals the presence of ―CH2 asymmetric bending. No significant change in the C–H stretching bands at 2853 and 2926 cm− 1 is seen. However a remarkable change in the insignificant peak of ―CH bending band at 1383 cm− 1 is observed with respect to pure PVA. In addition, the ―OH bending band at 1639 cm− 1 of pure PVA has been blue-shifted to 1622 cm− 1. Magnetic properties are measured by a vibrating sample magnetometer. Fig. 4(a, and b) shows the hysteresis loops of the NiFe2O4 nanosheet at room temperature. The saturation magnetization (Ms), remanent magnetization (Mr), and coercivity (Hc) are summarised in Table 1. The magnetic properties of the materials have been believed to be dependent on the sample shape, magnetization direction, crystallinity etc. In the present work, the nanosheet is formed through the agglomeration of magnetic nanoparticles, but their sizes are close to bulk values; therefore, their magnetic properties should be between those of the nanoparticles and the bulk material [4]. The saturation magnetization values of sample-1 and sample-2 are 44.05 and 41.98 emu/g, respectively, which are smaller than the bulk values of 55 emu/g [12] but higher than those of the nanoparticles [13]. “The
Fig. 4. Hysteresis loop for (a) sample-1 and (b) sample-2 of NiFe2O4 nanosheet at room temperature.
reduction of Ms relative to bulk is mainly due to the spin canting existing in whole volume of the nanoparticles [14] and disordered the surface layer of the nanoparticles. In addition, the hysteresis loops showed that Ms is not saturated, indicating the existence of surface spin disorder of nanoparticles”. From Table 1, it is seen that saturation magnetization of sample-1 is higher than that of sample-2 whereas the coercivity and remanent magnetization for the two samples are almost very close to each other. This is due to the increase of concentration of paramagnetic PVA in sample-2 compared to sample-1. 4. Conclusion Polycrystalline NiFe2O4 nanosheet is synthesised by co-precipitation method using two concentrations of PVA as structural directing agent. Morphology of NiFe2O4 nanocrystals can easily be tuned with the varying concentration of PVA. The polycrystalline nanosheet is formed through orientation aggregation of each single crystalline nanoparticles. The average sizes of each crystallite are 23 nm and 22 nm for low concentration and high concentration of PVA respectively. TEM and HRSEM results show the clear sheet-like morphology of NiFe2O4. The synthesised product exhibits the room temperature ferromagnetic behaviour. References [1] Shi X, Wang SH, Swanson SD, Ge S, Cao Z, Van Antwerp ME, et al. Adv Mater 2008;20:1671–8. [2] Jun Hong, Dongmei Xu, Jiahui Yu, Peijun Gong, Hongjuan Ma, Side Yao. Nanotechnology 2007;18:135608–13. [3] Ziolo Ronald F, Giannelis Emmanuel P, Weinstein Bernard A, O'Horo Michael P, Ganguly Bishwanath N, Vivek Mehrotra, et al. Science 1992;257:219–23. [4] Wang Z, Liu X, Lv M, Chai P, Liu Y, Meng J. J Phys Chem B 2008;112:11292–7. [5] Chatterjee A, Das D, Pradhan SK, Chakravorty D. J Magn Magn Mater 1993;127: 214–8. [6] Doh SG, Kim EB, Lee BH, Oh JH. J Magn Magn Mater 2004;2238:272–6. [7] Jiang J, Yang YM, Li LC. Mater Lett 2008;62:1973–5. [8] Ji G, Tang S, Xu B, Gu B, Du Y. Chem Phys Lett 2003;379:484–9. [9] Li XD, Yang WS, Li F, Evans DG, Duan X. J Phys Chem Solids 2006;67:1286–90. [10] Wang L, Gao L. J Am Ceram Soc 2008;91:3391–5. [11] Salavati-Niasari M, Davar F, Mahmoudi T. Polyhedron 2009;28:1455–8. [12] Jiang J, Yang YM. Mater Lett 2007;61:4276–9. [13] Qi Liu, Hexiang Huang, Lifang Lai, Jianhua Sun, Tongming Shang, Quanfa Zhou, et al. J Mater Sci 2009;44:1187–91. [14] Bao N, Shen L, Wang Y, Padhan P, Gupta A. J Am Chem Soc 2007;129:12374–5.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Preparation of sheet like polycrystalline NiFe2O4 nanostructure with PVA matrices and their properties P. Sivakumar a,⁎, R. Ramesh b, A. Ramanand c, S. Ponnusamy b, C. Muthamizhchelvan b a b c
T. S. Srinivasan Centre for Polytechnic College and Advanced Training (CPAT-TVS), Vanagaram, Chennai, 600 095, Tamilnadu, India Center for Materials Science and Nano Devices, Department of Physics, SRM University, Kattankulathur, Kancheepuram, 603 203, Tamilnadu, India Department of Physics, Loyola College, Nungambakkam, Chennai, 600 034, Tamilnadu, India
a r t i c l e
i n f o
Article history: Received 27 November 2010 Accepted 4 February 2011 Available online 18 February 2011 Keywords: Nickel ferrite Nanosheet Oriented aggregation FTIR Spectra Magnetic properties
a b s t r a c t Sheet-like nickel ferrite (NiFe2O4) has been synthesised with PVA matrices using co-precipitation technique. The sheet is formed from the oriented aggregation of single crystalline NiFe2O4 nanoparticles with PVA as the structure directing template. The synthesised materials are characterised based on X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), highresolution scanning electron microscopy (HRSEM), Fourier transform infrared spectroscopy (FTIR) and vibrating sample magnetometry (VSM). The XRD results show that the nanocrystal contains single phase spinel structure of Fd3m space group. The existence of PVA with nanoparticles has been confirmed by FTIR spectra. The room temperature ferromagnetic property is exhibited by the as synthesised sample with high saturation magnetisation. © 2011 Elsevier B.V. All rights reserved.
1. Introduction In recent times, nanosized spinel ferrite particles have attracted considerable attention due to the enhancement of physical and chemical properties. As an important member of the ferrite family, nickel ferrite (NiFe2O4) has attracted significant research interest because of its fascinating magnetic and electromagnetic properties. NiFe2O4, an inverse spinel structure, shows ferromagnetism. NiFe2O4 is a soft ferrite with low coercivity, high saturation magnetization, chemical stability and electrical resistivity, which can make it an excellent material for magnetic resonance imaging (MRI) enhancement, magnetic recording media and electronic devices [1–4]. Recently, many attempts have been made to synthesise various NiFe2O4 nanostructures in order to explore and enhance its properties and design the technological applications. 1D ferrite nanostructure can be synthesised by sol–gel method, co-precipitation technique, microemulsion method, anodic aluminium oxide (AAO) template method, polymeric co-precipitation method and precursor method [5–9]. Amongst these many various synthesis methods, it is still critical to find simple and cost-effective routes to synthesise nanocrystalline NiFe2O4 by the utilisation of cheap, nontoxic and environmentally benign precursors. Co-precipitation of NiFe2O4 with
⁎ Corresponding author at: Department of Basic Engineering, T. S. Srinivasan Centre for Polytechnic College and Advanced Training (CPAT-TVS), No:1, TVS School Road, Vanagaram, Chennai, 600 095, Tamilnadu, India. Tel.: + 91 9790898204; fax: + 91 44 24760808. E-mail address: [email protected] (P. Sivakumar). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.02.026
PVA matrices is a good technique to synthesise the 1D nanostructure since it fulfils the above conditions. The self-assembly of the NiFe2O4 within the PVA polymer matrices is achieved by introducing Fe (NO3)3·9H2O and Ni(NO3)2·6H2O precursors into the polymer and by subsequent processes such as co-precipitation. Single crystalline NiFe2O4 nanoparticles are formed within the PVA matrices, which serve as the templating medium to form the polycrystalline NiFe2O4 nanosheet. The development of such mixed-NiFe2O4 PVA-based nanocomposites is targeting the functionalisation into device technologies. 2. Experimental details 2.1. Synthesis of NiFe2O4 nanosheet with PVA matrices Iron nitrate [(Fe (NO3)3·9H2O)], nickel nitrate [(Ni (NO3)2·6H2O)] and polyvinyl alcohol (PVA) are purchased from Aldrich and is used without any further purification. In a typical synthesis, 0.2 M (20 mL) solution of Fe (NO3)3·9H2O and 0.1 M (20 mL) solution of Ni (NO3)2·6H2O are prepared and vigorously mixed under stirring at 80 °C for 1 h. PVA is added into the solution as a capping agent. Here 0.2 g of PVA capped NiFe2O4 is labelled as sample-1 and 0.4 g PVA capped NiFe2O4 is labelled as sample-2. Subsequently, an appropriate amount of hydrazine hydrate is added drop by drop into the solutions and black colour precipitates are formed. Finally the precipitates are separated by centrifugation and dried in hot air oven at 100 °C for 4 h. The acquired substance was then ground into a fine powder and then annealed at 300 °C for 10 h.
P. Sivakumar et al. / Materials Letters 65 (2011) 1438–1440
1439
3. Results and discussion Fig. 1(a, and b) shows the powder XRD pattern of sample 1 and sample 2. The peaks are indexed with inverse spinel cubic phase with the space group of Fd3m, which is very well consistent with the JCPDS file (card no: 10-0325). The reflectance peak (311) at 35.7° is relatively weaker than the other peaks. “This indicates that crystallisation of particles has taken place along the easy direction (311) of NiFe2O4”. The crystallite size of NiFe2O4 is calculated using Scherrer's relation. D = 0:9λ = βcosθ
Fig. 1. XRD patterns of (a) sample-1 and (b) sample-2 of NiFe2O4 nanostructures.
2.2. Characterisation methods X-ray diffraction patterns of the samples are recorded using PANalytical X' pert pro X-ray diffractometer with CuKα radiation (1.5406 Å) source. The intensity data is collected over the range of 20°–80° using a step scan mode (0.06°/s). The HRTEM micrographs are obtained on a JEM 3010 (JEOL) transmission electron microscope with an accelerating voltage of 200 kV. The particle morphological features are imaged by high-resolution scanning electron microscopy (FEI Quanta FEG200) with an accelerating voltage of 25 kV. Fourier transform infrared (FTIR) spectra are recorded on Perkin Elmer 2000 FTIR spectrometer in KBr pellets. The magnetic measurements are carried out in a vibrating sample magnetometer (VSM, JDM-13) at room temperature.
where β is the broadening of the diffraction line measured at half maximum intensity (radians) and λ = 1.5406 Å, the wavelength of CuKα. The average crystallite size of sample-1 and sample-2 are 23 nm and 22 nm respectively. All the photographs presented in Fig. 2 clearly show that NiFe2O4 nanosheets can be formed from NiFe2O4 nanoparticles. Fig. 2 (a, b, and c) show small aggregation of primary NiFe2O4 nanoparticles which causes the formation of sheet like morphology. Although most of the nanosheets appear to be separate sheets, the surfaces are clearly not smooth, but contain some nanoparticles. It clearly indicates the initial stages for the growth of nanosheet. The aggregation of the nanoparticles increase to 0.4 g of the PVA capped NiFe2O4 nanoparticles. Finally irregular morphology of the nanosheet with peculiar shape has been formed (Fig. 2d, e, and f). The HRTEM results clearly show that NiFe2O4 nanosheets can be effectively formed from the oriented attachment of single crystalline NiFe2O4 nanoparticles spreading PVA in all directions. “The clear atomic lattice fringes can be observed, and the measured spacing of the crystallographic planes are about 0.25 and 0.28 nm, which are close to the (311) and (211) lattice planes of NiFe2O4 crystal respectively”. The possible growth mechanism has been presented as “oriented aggregation” of primary spherical nanoparticles, which involve self
Fig. 2. (a) HRSEM, (b) TEM and (c) HRTEM images of the sample-1 of NiFe2O4 nanoparticles. (d) HRSEM, (e) TEM and (f) HRTEM images of the sample-2 of NiFe2O4 nanosheet.
1440
P. Sivakumar et al. / Materials Letters 65 (2011) 1438–1440
60
(b)
(a)
1383 2922
(a)
2852
1622
3412 602
Magnetization (emu/g)
Transmittance (a.u)
40
(b)
20
0
-20
-40 3436 4000
3500
1639 2923 2852 3000
2500
2000
1500
1379 1000
500
Wavenumber (cm-1)
-60 -8000
-6000
-4000
-2000
0
2000
4000
6000
8000
Applied Field (Oe) Fig. 3. (a) FTIR of pure polyvinyl alcohol (PVA) and (b) FTIR of as synthesised NiFe2O4 nanosheet.
Table 1 Magnetic properties of synthesised NiFe2O4-PVA samples. Sample
Ms (emug−1)
Mr (emug−1)
Hc (Oe)
1 2
44.05 41.98
10.93 10.09
204.96 204.20
assembly of adjacent particles in a common crystallographic orientation and joined at a planar interface. This involves spontaneously reducing the overall energy of the whole system [10]. This present work, NiFe2O4 nanosheet is being formed by a similar mechanism. The long chain of PVA molecules attached on the crystallographic face of nanoparticles promotes the growth of oriented aggregates, which continue to grow into a sheet like morphology mainly driven by crystal packing force and stacking interactions among adjacent atoms. Finally, polycrystalline NiFe2O4 nanosheet has been formed (Fig. 2d, e, and f). Fig. 3 (a and b) show the FTIR of pure polyvinyl alcohol (PVA) and FTIR of as synthesised NiFe2O4 nanosheet. FTIR analysis of NiFe2O4PVA nanocomposite shows (Fig. 3) typical absorption bands at 3414 cm−1 that correspond to the stretching mode of the OH group. The Fe–O vibration mode of NiFe2O4 is found to be present near 603 cm−1 [11]. The band at 1383 cm−1 reveals the presence of ―CH2 asymmetric bending. No significant change in the C–H stretching bands at 2853 and 2926 cm− 1 is seen. However a remarkable change in the insignificant peak of ―CH bending band at 1383 cm− 1 is observed with respect to pure PVA. In addition, the ―OH bending band at 1639 cm− 1 of pure PVA has been blue-shifted to 1622 cm− 1. Magnetic properties are measured by a vibrating sample magnetometer. Fig. 4(a, and b) shows the hysteresis loops of the NiFe2O4 nanosheet at room temperature. The saturation magnetization (Ms), remanent magnetization (Mr), and coercivity (Hc) are summarised in Table 1. The magnetic properties of the materials have been believed to be dependent on the sample shape, magnetization direction, crystallinity etc. In the present work, the nanosheet is formed through the agglomeration of magnetic nanoparticles, but their sizes are close to bulk values; therefore, their magnetic properties should be between those of the nanoparticles and the bulk material [4]. The saturation magnetization values of sample-1 and sample-2 are 44.05 and 41.98 emu/g, respectively, which are smaller than the bulk values of 55 emu/g [12] but higher than those of the nanoparticles [13]. “The
Fig. 4. Hysteresis loop for (a) sample-1 and (b) sample-2 of NiFe2O4 nanosheet at room temperature.
reduction of Ms relative to bulk is mainly due to the spin canting existing in whole volume of the nanoparticles [14] and disordered the surface layer of the nanoparticles. In addition, the hysteresis loops showed that Ms is not saturated, indicating the existence of surface spin disorder of nanoparticles”. From Table 1, it is seen that saturation magnetization of sample-1 is higher than that of sample-2 whereas the coercivity and remanent magnetization for the two samples are almost very close to each other. This is due to the increase of concentration of paramagnetic PVA in sample-2 compared to sample-1. 4. Conclusion Polycrystalline NiFe2O4 nanosheet is synthesised by co-precipitation method using two concentrations of PVA as structural directing agent. Morphology of NiFe2O4 nanocrystals can easily be tuned with the varying concentration of PVA. The polycrystalline nanosheet is formed through orientation aggregation of each single crystalline nanoparticles. The average sizes of each crystallite are 23 nm and 22 nm for low concentration and high concentration of PVA respectively. TEM and HRSEM results show the clear sheet-like morphology of NiFe2O4. The synthesised product exhibits the room temperature ferromagnetic behaviour. References [1] Shi X, Wang SH, Swanson SD, Ge S, Cao Z, Van Antwerp ME, et al. Adv Mater 2008;20:1671–8. [2] Jun Hong, Dongmei Xu, Jiahui Yu, Peijun Gong, Hongjuan Ma, Side Yao. Nanotechnology 2007;18:135608–13. [3] Ziolo Ronald F, Giannelis Emmanuel P, Weinstein Bernard A, O'Horo Michael P, Ganguly Bishwanath N, Vivek Mehrotra, et al. Science 1992;257:219–23. [4] Wang Z, Liu X, Lv M, Chai P, Liu Y, Meng J. J Phys Chem B 2008;112:11292–7. [5] Chatterjee A, Das D, Pradhan SK, Chakravorty D. J Magn Magn Mater 1993;127: 214–8. [6] Doh SG, Kim EB, Lee BH, Oh JH. J Magn Magn Mater 2004;2238:272–6. [7] Jiang J, Yang YM, Li LC. Mater Lett 2008;62:1973–5. [8] Ji G, Tang S, Xu B, Gu B, Du Y. Chem Phys Lett 2003;379:484–9. [9] Li XD, Yang WS, Li F, Evans DG, Duan X. J Phys Chem Solids 2006;67:1286–90. [10] Wang L, Gao L. J Am Ceram Soc 2008;91:3391–5. [11] Salavati-Niasari M, Davar F, Mahmoudi T. Polyhedron 2009;28:1455–8. [12] Jiang J, Yang YM. Mater Lett 2007;61:4276–9. [13] Qi Liu, Hexiang Huang, Lifang Lai, Jianhua Sun, Tongming Shang, Quanfa Zhou, et al. J Mater Sci 2009;44:1187–91. [14] Bao N, Shen L, Wang Y, Padhan P, Gupta A. J Am Chem Soc 2007;129:12374–5.