Facile synthesis of inverse spinel NiFe2O4 nanocrystals and their superparamagnetic properties

Facile synthesis of inverse spinel NiFe₂O₄ nanocrystals and their superparamagnetic properties

Jie Tan^*; Wei Zhang; Ai-Lin Xia

Anhui Key Laboratory of Metal Materials and Processing, School of Materials Science and Engineering, Anhui University of Technology, Anhui, Maanshan 243002, China

ABSTRACT

Spinel NiFe₂O₄ nanocrystals have been obtained by means of a novel composite-hydroxide-salt-mediated approach, which is based on a reaction between metallic salt and metallic oxide in the solution of composite-hydroxide-salt eutectic at ~225 ºC and normal atmosphere without any organic dispersant or capping agent. The obtained products are characterized by an X-ray diffraction (XRD), a transmission electron microscopy (TEM) and an alternating gradient magnetometer (AGM). The formation process of NiFe₂O₄ nanosheet is proposed to begin with a ‘‘dissolution-recrystallization’’ which is followed by an ‘‘Ostwald ripening’’ mechanism. The NiFe₂O₄ nano-octahedrons can be obtained through adjusting the reaction water content in the hydroxide melts at constant temperature. At 300 K, magnetic hysteresis loops at an applied field of 15 kOe show zero coercivity, indicating the superparamagnetic behavior of the as-prepared NiFe₂O₄ nanocrystals.

Keywords: NiFe₂O₄ nanocrystals, morphology, mechanism, magnetic properties

1. Introduction

Nanocrystalline spinel ferrites with the general formula MFe₂O₄ (M = divalent metal ions, e.g. Ni, Co, Cu, Zn, Mg, Mn, Cd, etc.) are attractive for their interesting magnetic, magnetoresistive, and magneto-optical properties. As a kind of important spinel ferrite, NiFe₂O₄ has attracted much interest because of its fascinating magnetic and electromagnetic properties¹. NiFe₂O₄ powders have been studied to use in many fields, such as ferrofluids^2,3, catalysts⁴, gas sensors^5-8, biomedicine⁹ and so on^10-12. NiFe₂O₄ is a cubic ferromagnetic oxide with a typical inverse spinel structure where Ni²⁺ ions occupy octahedric B-sites and Fe³⁺ ions equally distributed between tetrahedric A-sites and octahedric B-sites¹³. Nanoscale NiFe₂O₄ ferrites have been successfully synthesized by various methods including sol-gel^11,14-16, solid-state reaction^6,17-18, co-precipitation^7,12,19,20, mechanochemical²¹, rheological phase reaction method²², pulsed wire discharge²³, arc plasma assisted gas phase synthesis method²⁴, combustion²⁵, surfactant-assisted refluxing method²⁶, micromulsion²⁷, electrospinning²⁸, thermolysis of mixed metal-oleate complexes²⁹, sonochemical³⁰ and hydrothermal^31-35.

In this paper, we report a novel one-step, simple method to directly synthesize uniform, mass-production nanostructured NiFe₂O₄ by the composite-hydroxide-salt-mediated (CHSM) method. The method is based on the reaction between one metallic oxide and the other metallic oxide in a solution of molten mixed potassium nitrate and potassium hydroxide eutectic at ~225 ºC and normal atmosphere. Furthermore, because of no addition of organic dispersant or capping agent in the reaction system, the final product can be easily purified. The main advantage of this method is the easy recycle of by-products, due to application of the salt nitrate and hydroxide of the same metal. The morphology of NiFe₂O₄ can be controlled by adjusting the content of H₂O and reaction duration. The reaction mechanism has also been discussed. In addition, the dependence of magnetic properties on morphologies and grain size of final products has been clearly observed.

2. Experimental

All chemicals used, such as NiCl₂·6H₂O, Fe₂O_3, were analytical grade reagents and without any further purification. Synthesis of NiFe₂O₄ nanomaterials was carried out by the composite-hydroxide-salt-mediated (CHSM) method without using any capping agent. The synthesis steps are as follows: (1) a total of 20 g of KOH and KNO₃ was mixed at a ratio of 63.8:36.2 and placed in a 25ml Teflon vessel. (2) A mixture of 0.5 mmol NiCl₂·6H₂O and 0.5 mmol Fe₂O₃ each was used as the raw material for reaction, 0-2 mL H₂O were also put into the vessel, and was placed on the top of the hydroxide and salt nitrate in the vessel. (3) Then, the Teflon vessel was put into a furnace preheated to 235 ºC. (4) After the hydroxide and salt nitrate were totally molten (about 30 minutes later), the molten reactants were mixed uniformly by shaking the covered vessel. (5) After the setting reaction time (12, 24, 48 hours), the vessel was taken out and cooled to room temperature naturally. The product was collected by centrifugation and through washing with deionized water and ethanol. (6) The products (NiFe₂O₄) synthesized at 235 ºC with 0 mL H₂O for 12 hours, 24 hours, 48 hours and with 2 mL H₂O for 48 hours were designated as sample S1, S2, S3 and S4, respectively.

Phase analysis of the products was performed by X-ray diffraction measurement (XRD, D8-Advance, Germany) with the use of Cu Ka radiation (l = 1.5418 Å) in the 2q range from 20º to 70º . The morphology and the size of the synthesized samples were characterized by a transmission electron microscopy (TEM, JEM-100CXII, Japan) with the emission voltage of 120 kV. The magnetic hysteresis loops was obtained at room temperature on a alternating gradient magnetometer (AGM, Micromag Model 2900) with a maximum external field H_m ≈ 1194 kA.m^–1 (15 kOe).

3. Results and Discussion

The crystal structure and phase purity of the synthesized products was characterized by XRD. Figure 1 shows the XRD patterns of samples S1-S4. It can be seen in Figure 1a that S1 was poorly crystallized. However, the crystallization quality of NiFe₂O₄ was improved with the increase of reaction time (Figure 1b, c). The labeled diffraction peaks at (311), (222), (400) and (440) reveals the information of typical inverse spinel structure according to the standard value for bulk NiFe₂O₄ phase (JCPDS file No.22-1086). In the XRD pattern of sample S4 that 2 mL water was used during reaction (Figure 1d), the characteristic diffraction peak (220) of spinel NiFe₂O₄ begins to emerge. The results of XRD indicate that the crystallinity of samples is dependent of reaction time and water content.

Figure 2 gives the TEM images of samples S1-S4. In Figure 2a, a large number of amorphous products (the dark in the figure) as well as a small number of NiFe₂O₄ nanocrystals formed in S1, which agrees with the weak peaks in XRD spectrum (Figure 1a). However, only a few amorphous product is found in Figure 2b, and it almost disappears in Figure 2c. Irregular morphology of the NiFe₂O₄ nanosheets with peculiar shape was formed in S2 and S3. The possible growth mechanism has been presented as "oriented aggregation" of primary nanoparticles, which involve self-assembly of adjacent particles in a common crystallographic orientation and joined at a planar interface. This involves the spontaneous decrease of overall energy in the whole system^12,36. However, when 2 mL is used, NiFe₂O₄ nano-octahedrons instead of nanosheets were obtained in S4 (Figure 2d). Our experiments have revealed that the morphology can be controlled through adjusting the reaction water content in the hydroxide melts with constant temperature.

Based on the time-dependent morphology evolution described above, the formation process of NiFe₂O₄ nano-octahedrons could be proposed to be the ‘‘dissolution-recrystallization’’ firstly and then be the ‘‘Ostwald ripening’’ mechanism. At the initial stage, a large amount of nano-crystallites nucleate and grow into nanosheets to minimize the overall energy of the system. A small amount of water in the hydroxide melts could alter the viscidity and acidity in the melts, which may be the key factor to affect phase crystallization³⁷. Owing to water addition, quick nucleation and growth rate may directly result in the formation of NiFe₂O₄ nano-octahedrons. As is known, the KOH/KNO₃ composite melts possess large viscidity but H₂O reduces the viscidity in the melts and increases the diffuse ability of the melt atoms. As reported by Wang³⁸, the ratio of growth rate along the <100> and <111> directions (R) determines the geometrical shape of a crystal. The octahedron consisting of eight highly stable {111} planes resulted from a much higher growth rate along the <100> direction than the <111> direction due to the lowest energy of the {111} surfaces. Generally, low reaction rate is favorable to fully exhibit the crystalline habit. The theoretical growth habit of the MFe₂O₄ oxometallates crystal is the octahedron because the {111} surfaces have the lowest energy³⁹. Here, the inverse spinel NiFe₂O₄ nanocrystalline has an octahedron shape.

From the above experimental results, a possible reaction mechanism for the synthesis of NiFe₂O₄ in hydroxide and salt solution is suggested as follows. Although the melting points (T_m) of both pure potassium hydroxide and potassium nitrate are over 300 ºC, T_m = 404 ºC for KOH and T_m = 337 ºC for KNO₃, the eutectic point at KOH/KNO₃ = is only about 225 ºC, the eutectic point at KOH/KNO₃ = 63.8:36.2 is only about 225 ºC. During the reaction process, hydroxides work not only as the solvent but also as the reactant for reducing the reaction temperature. In the molten hydroxide, Fe₂O₃ reacts with KOH to form a hydroxide-soluble K₂Fe₂O₄.

At the same time, NiCl₂ reacts with hydroxide to form Ni(OH)₂, which is dissolved in the hydroxide solutions:

The K₂Fe₂O₄ produced in (1) reacts with the Ni(OH)₂ produced in process (2) and to form an indissoluble solid NiFe₂O₄:

Because the viscosity of hydroxide is large, the formation of NiFe₂O₄ is slow and it is not easy for the nanostructures to agglomerate, which maybe the key for receiving dispersive single-crystalline nanostructures during the reaction without using an organic surface-capping material. The hydroxides mediate the reaction, but they are not part of the final nanostructures.

The magnetic properties of this nanostructured ferrite system were studied with the help of AGM. Figure 3 shows the hysteresis loops of sample S1, S3, S4. Zero coercivity features the three NiFe₂O₄ nanocrystals, which indicates the presence of superparamagnetic behavior. The saturation magnetization (M_s) are about 22.9, 41.4 and 48.5 emu.g^–1 for S1, S3 and S4, respectively, as shown in Figure 3. The reported value of the saturation magnetization, calculated using Neel’s sublattice theory for cubic inverse spinel NiFe₂O₄ is 50 emu.g^–1[40] and reported value of the saturation magnetization experimentally observed for bulk NiFe₂O₄ is 56 emu.g^–1[41]. Chkoundali et al.⁴² pointed out that the large values of saturation magnetizations usually correlated with the highest crystallinity of the nanoparticles. Therefore, compared with bulk NiFe₂O₄ ferrite, due to the different crystallinity as shown in Figures 1 and 2, the smaller M_s and the increasing trend from S1 to S4 in our case is rational.

4. Conclusions

In summary, NiFe₂O₄ nanocrystals have been synthesized via the CHSM approach, which is simple, low-cost, high-yield and easy-recycle for by-products. The crystallization and morphology of NiFe₂O₄ nanocrystals can be controlled through adjusting the reaction water content in the hydroxide melts with constant temperature. The magnetic hysteresis loops with zero coercivity at room temperature suggest the superparamagnetic behavior of NiFe₂O₄ nanocrystals.

Acknowledgements

The work is financially supported by the National Natural Science Foundation of China (Grant No. 11204003).

Received: April 12, 2012

Revised: September 17, 2012

• 1. Zhang DE, Zhang XJ, Ni XM, Zheng HG and Yang DD. Synthesis and characterization of NiFe₂O₄ magnetic nanorods via a PEG-assisted route. Journal of Magnetism and Magnetic Materials 2005; 292:79-82. http://dx.doi.org/10.1016/j.jmmm.2004.10.097
• 2. Sousa MH, Hasmonay E, Depeyrot J, Tourinho FA, Bacri JC, Dubois E et al. NiFe₂O₄ nanoparticles in ferrofluids: evidence of spin disorder in the surface layer. Journal of Magnetism and Magnetic Materials. 2002; 572:242-245.
• 3. Sousa MH, Tourinho FA, Depeyrot J, Da Silva GJ and Lara MCFL. New electric double-layered magnetic fluids based on copper, nickel, and zinc ferrite nanostructures. Journal of Physical Chemistry B 2001; 105:1168-1175. http://dx.doi.org/10.1021/jp0039161
• 4. Sreekumar K and Sugunan S. Ferrospinels based on Co and Ni prepared via a low temperature route as efficient catalysts for the selective synthesis of o-cresol and 2,6-xylenol from phenol and methanol. Journal of Molecular Catalysis A: Chemical. 2002; 185:259-268. http://dx.doi.org/10.1016/S1381-1169(02)00074-2
• 5. Luo LQ, Li QX, Xu YH, Ding YP, Wang X, Deng DM et al. Amperometricglucose biosensor based on NiFe₂O₄ nanoparticles and chitosan. Sensors and Actuators, B 2010; 145:293-298. http://dx.doi.org/10.1016/j.snb.2009.12.018
• 6. Liu YL, Wang H, Yang Y, Liu ZM, Yang HF, Shen GL et al. Hydrogen sulfade sensing properties of NiFe₂O₄ nanopowder doped with noble metals. Sensors and Actuators, B 2004; 102:148-154. http://dx.doi.org/10.1016/j.snb.2004.04.014
• 7. Yang LF, Xie YA, Zhao HY, Wu XH and Wang YD. Preparation and gas-sensing properties of NiFe₂O₄ semiconductor materials. Solid-State Electronics 2005; 49:1029-1033. http://dx.doi.org/10.1016/j.sse.2005.03.022
• 8. Chu XF, Jiang DL and Zheng CM. The preparation and gas-sensing properties of NiFe₂O₄ nanocubes and nanorods. Sensors and Actuators, B. 2007; 123:793-797. http://dx.doi.org/10.1016/j.snb.2006.10.020
• 9. Fan HM, Yi JB, Yang Y, Kho KW, Tan HR, Shen ZX et al. Single-crystalline MFe₂O₄ nanotubes/nanorings synthesized by thermal transformation process for biological applications. ACS Nano 2009; 3(9):2798-2808. http://dx.doi.org/10.1021/nn9006797
• 10. Vestal CR and Zhang ZJ. Effects of surface coordination chemistry on the magnetic properties of MnFe₂O₄ spinel ferrite nanoparticles. Journal of the American Chemical Society 2003; 125:9828-9833. http://dx.doi.org/10.1021/ja035474n
• 11. Duque JGS, Souza EA, Meneses CT and Kubota L. Magnetic properties of NiFe₂O₄ nanoparticles produced by a new chemical method. Physica. B 2007; 398:287-290. http://dx.doi.org/10.1016/j.physb.2007.04.030
• 12. Sivakumar P, Ramesh R, Ramanand A, Ponnusamy S and Muthamizhchelvan C. Preparation of sheet like polycrystalline NiFe₂O₄ nanostructure with PVA matrices and their properties. Materials Letters 2011; 65:1438-1440. http://dx.doi.org/10.1016/j.matlet.2011.02.026
• 13. Chinnasamy CN, Narayanasamy A, Ponpandian N, Chattopadhyay K, Shinoda K, Jeyadevan B et al. Mixed spinel structure in nanocrystalline NiFe₂O₄ Physical Review B 2001; 63:184108-184116. http://dx.doi.org/10.1103/PhysRevB.63.184108
• 14. Ahmed MA, El-Dek SI, El-Kashef IM and Helmy N. Structural and magnetic properties of nano-crystalline Ag⁺ doped NiFe₂O₄ Solid State Science 2011; 13:1176-1179. http://dx.doi.org/10.1016/j.solidstatesciences.2010.11.002
• 15. Srivastava M, Ojha AK, Chaubey S and Materny A. Synthesis and optical characterization of nanocrystalline NiFe₂O₄ structures. Journal of Alloys and Compounds 2009; 481:515-519. http://dx.doi.org/10.1016/j.jallcom.2009.03.027
• 16. Li LP, Li GS, Smith RL and Inomata H. Microstructure evolution and magnetic properties of NiFe₂O₄ nanocrystals dispersed in amorphous silica. Chemistry of Materials 2000; 12:3705-3714. http://dx.doi.org/10.1021/cm000481l
• 17. Ceylan A, Ozcan S, Ni C and Shah SI. Solid state reaction synthesis of NiFe₂O₄ nanoparticles. Journal of Magnetism and Magnetic Materials 2008; 320:857-863. http://dx.doi.org/10.1016/j.jmmm.2007.09.003
• 18. Kamala Bharathi K, Markandeyulu G and Ramana CV. Structural, magnetic, electrical, and magnetoelectric properties of Sm- and Ho-substituted nickel ferrites. Journal of Physical Chemistry C. 2011; 115:554-560. http://dx.doi.org/10.1021/jp1060864
• 19. Niasari MS, Davar F and Mahmoudi T. A simple route to synthesize nanocrystalline nickel ferrite (NiFe₂O₄) in the presence of octanoic acid as a surfactant. Polyhedron. 2009; 28:1455-1458. http://dx.doi.org/10.1016/j.poly.2009.03.020
• 20. Sivakmar P, Ramesh R, Ramanand A, Ponnusamy S and Muthamizhchelvan C. Synthesis and characterization of NiFe₂O₄ nanosheet via polymer assisted co-precipitation method. Materials Letters 2011; 65:483-485. http://dx.doi.org/10.1016/j.matlet.2010.10.056
• 21. Yang HM, Zhang XC, Ao WQ and Qiu GZ. Formation of NiFe₂O₄ nanoparticles by mechanochemical reaction. Materials Research Bulletin 2004; 39:833-837. http://dx.doi.org/10.1016/j.materresbull.2004.02.001
• 22. Jiang J and Yang YM. Facile synthesis of nanocrystalline spinel NiFe₂O₄ via a novel soft chemistry route. Materials Letters 2007; 61:4276-4279. http://dx.doi.org/10.1016/j.matlet.2007.01.111
• 23. Kinemuchi Y, Ishizaka K, Suematsu H, Jiang WH and Yatsui K. Magnetic properties of nanosize NiFe₂O₄ particles synthesized by pulsed wire discharge. Thin Solid Films. 2002; 407:109-113. http://dx.doi.org/10.1016/S0040-6090(02)00021-4
• 24. Nawale AB, Kanhe NS, Patil KR, Bhoraskar SV, Mathe VL and Das AK. Magnetic properties of thermal plasma synthesized nanocrystalline nickel ferrite (NiFe₂O₄). Journal of Alloys and Compounds 2011; 509:4404-4413. http://dx.doi.org/10.1016/j.jallcom.2011.01.057
• 25. Alarifi A, Deraz NM and Shaban S. Structural, morphological and magnetic properties of NiFe₂O₄ nano-particles. Journal of Alloys and Compounds 2009; 486:501-506. http://dx.doi.org/10.1016/j.jallcom.2009.06.192
• 26. Jiang J, Yang YM and Li LC. Surfactant-assisted synthesis of nanostructured NiFe₂O₄ via a refluxing route. Materials Letters 2008; 62:1973-1975. http://dx.doi.org/10.1016/j.matlet.2007.10.063
• 27. Zhang DE, Tong ZW, Xu GY, Li SZ and Ma JJ. Templated fabrication of NiFe₂O₄ nanorods: Characterization, magnetic and electrochemical properties. Solid State Science 2009; 11:113-117. http://dx.doi.org/10.1016/j.solidstatesciences.2008.05.001
• 28. Wang ZL, Liu XJ, Lv MF, Chai P, Liu Y and Meng J. Preparation of ferrite MFe₂O₄ (M=Co, Ni) ribbons with nanoporous structure and their magnetic propertie. Journal of Physical Chemistry B 2008; 112:11292-11297. http://dx.doi.org/10.1021/jp804178w
• 29. Bao NZ, Shen LM, Wang YH, Padhan P and Gupta A. A Facile thermolysis route to monodisperse ferrite nanocrystals. Journal of the American Chemical Society 2007; 129:12374-12375. http://dx.doi.org/10.1021/ja074458d
• 30. Shafi KVPM, Koltypin Y, Gedanken A, Prozorov R, Balogh J and Felner I. Sonochemical preparation of nanosized amorphous NiFe₂O₄ Particles. Journal of Physical Chemistry B 1997; 101:6409-6414. http://dx.doi.org/10.1021/jp970893q
• 31. Li H, Wu HZ and Xiao GX. Effects of synthetic conditions on particle size and magnetic properties of NiFe₂O₄ Powder Technology 2010; 198:157-166. http://dx.doi.org/10.1016/j.powtec.2009.11.005
• 32. Satyanarayana L, Madhusudan Reddy K and Manorama SV. Nanosized spinel NiFe₂O₄: A novel material for the detection of liquefied petroleum gas in air. Materials Chemistry and Physics 2003; 82:21-26. http://dx.doi.org/10.1016/S0254-0584(03)00170-6
• 33. Chen LY, Dai H, Shen YM and Bai JF. Size-controlled synthesis and magnetic properties of NiFe₂O₄ hollow nanospheres via a gel-assistant hydrothermal route. Journal of Alloys and Compounds 2010; 491:L33-L38. http://dx.doi.org/10.1016/j.jallcom.2009.11.031
• 34. Wang JY, Ren FL, Jia BP and Liu XH. Solvothermal synthesis and characterization of NiFe₂O₄ nanospheres with adjustable sizes. Solid State Communications 2010; 150:1141-1144. http://dx.doi.org/10.1016/j.ssc.2010.03.021
• 35. Sato T, Sue K, Suzuki W, Suzuki M, Matsui K, Hakuta Y et al. Rapid and continuous production of ferrite nanoparticles by hydrothermal synthesis at 673 K and 30 MPa. Industrial and Engineering Chemistry Research 2008; 47:1855-1860. http://dx.doi.org/10.1021/ie071168x
• 36. Wang L and Gao L. Morphology transformation of hematite nanoparticles through oriented aggregation. Journal of the American Ceramic Society. 2008; 91:3391-3395. http://dx.doi.org/10.1111/j.1551-2916.2008.02537.x
• 37. Hu CH, Yan W, Wan BY, Zhang KY, Zhang Y and Tian YS. Water-induced structure phase transition of CdSe nanocrystals in composite hydroxide melts. Physica E 2010; 42:1790-1794. http://dx.doi.org/10.1016/j.physe.2010.01.050
• 38. Wang ZL. Transmission electron microscopy of shape-controlled nanocrystals and their assemblies. Journal of Physical Chemistry B 2000; 104:1153-1175. http://dx.doi.org/10.1021/jp993593c
• 39. Müller-Buschbaum H. The crystal chemistry of AM₂O₄ oxometallates. Journal of Alloys and Compounds 2003; 349:49-104. http://dx.doi.org/10.1016/S0925-8388(02)00925-8
• 40. George M, John AM, Nair SS, Joy PA and Anantharaman MR. Finite size effects on the structural and magnetic properties of sol-gel synthesized NiFe₂O₄ powders. Journal of Magnetism and Magnetic Materials 2006; 302:190-195. http://dx.doi.org/10.1016/j.jmmm.2005.08.029
• 41. Sepelak V, Tkacova K, Boldyrev VV, Wibmann S and Becker KD. Mechanically induced cation redistribution in ZnFe₂O₄ and its thermal stability. Physica B: Condensed Matter. 1997; 234-236:617-9. http://dx.doi.org/10.1016/S0921-4526(96)01061-7
• 42. Chkoundali S, Ammar S, Jouini N, Fievet F, Molinie P and Danot M et al. Nickel ferrite nanoparticles: elaboration in polyol medium via hydrolysis, and magnetic properties. Journal of Physics: Condensed Matter. 2004; 16: 4357-4372. http://dx.doi.org/10.1088/0953-8984/16/24/017

Publication Dates

• Publication in this collection
  22 Nov 2012
• Date of issue
  Feb 2013

History

• Received
  12 Apr 2012
• Reviewed
  17 Sept 2012