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Materials Research

Print version ISSN 1516-1439

Mat. Res. vol.16 no.1 São Carlos Jan./Feb. 2013  Epub Nov 22, 2012 

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




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 ferrofluids2,3, catalysts4, gas sensors5-8, biomedicine9 and so on10-12. 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-gel11,14-16, solid-state reaction6,17-18, co-precipitation7,12,19,20, 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 hydrothermal31-35.

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, Fe2O3, 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 Hm ≈ 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 system12,36. 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 (Tm) of both pure potassium hydroxide and potassium nitrate are over 300 ºC, Tm = 404 ºC for KOH and Tm = 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 (Ms) 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 Ms 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.



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



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Received: April 12, 2012
Revised: September 17, 2012




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