Synthesis of nickel ferrite nanoparticles by sol-gel method

doi:10.1016/S0025-5408(01)00620-1

Materials Research Bulletin

Volume 36, Issues 7–8, May–June 2001, Pages 1369-1377

https://doi.org/10.1016/S0025-5408(01)00620-1 Get rights and content

Abstract

NiFe₂O₄ nanoparticles were prepared by the sol-gel method using polyacrylic acid (PAA) as a chelating agent. It was shown that pure spinel NiFe₂O₄ nanoparticles with a diameter of 5∼30 nm and a specific surface area of 20.0∼55.2 m²/g could be produced by calcining the gel precursors with various molar ratios of PAA to total metal ions (0.5∼2.0) in air at 300°C for 2h. As the molar ratio of PAA to total metal ions increased, particle sizes decreased but the crystallinity increased, revealing PAA not only worked as a chelating agent to disperse the cations homogeneously but also contributed its combustion heat to increase the crystallinity of NiFe₂O₄ nanoparticles. Moreover, the magnetic analysis showed the resultant NiFe₂O₄ nanoparticles might be superparamagnetic. Their saturation magnetization (0.19 emu/mol at 298 K) was slightly lower than that for bulk materials due to small size and increased slightly with decreasing the temperature below 360 K

Introduction

Nanostructured materials exhibit unusual physical and chemical properties, significantly different from those of conventional bulk materials, due to their extremely small size or large specific surface area [1], [2], [3]. So, their preparation and characterization have attracted increasing attention in the past decade.

Nickel ferrite (NiFe₂O₄) is a well-known spinel magnetic material. Its preparation by the classical solid-state reactions requires a high calcination temperature and hence induces the sintering and aggregation of particles [4]. To produce nanosized ferrite particles, some techniques such as chemical coprecipitation [5], hydrothermal synthesis [6], hydrolysis of metal carboxylate in organic solvent [7], and aerosolization [8] have been developed.

The sol-gel method is a useful and attractive technique for the preparation of nanosized particles because of its advantages: good stoichiometric control and the production of ultrafine particles with a narrow size distribution in a relatively short processing time at lower temperatures. Recently, the use of polyacrylic acid (PAA) as a chelating agent in the sol-gel syntheses of LiCoO₂ and LiMn₂O₄ nanoparticles has been reported [9], [10]. Compared with citric acid, which is the conventional chelating agent, PAA has more carboxylic acid groups to form chelates with mixed cations and results in a sol. It also greatly aids in the formation of a cross-linked gel which may provide more homogeneous mixing of the cations and less tendency for segregation during calcination.

In this work, NiFe₂O₄ nanoparticles were synthesized by the sol-gel method using PAA as a chelating agent. The effects of PAA quantity and calcination temperature were investigated. The thermal decomposition behavior of gel precursor was examined, and the structure, size, specific surface area, and magnetic properties of the resultant nanoparticles were characterized.

Section snippets

Experimental

For the typical experimental procedures to prepare spinel NiFe₂O₄ nanoparticles, an aqueous solution containing nickel nitrate (0.4M) and ferric nitrate (0.8M) was prepared first, and then completely mixed with an aqueous solution of PAA. Phase separation was observed. With constant stirring, an appropriate amount of nitric acid was slowly added to this solution until a transparent green solution (pH 1∼3) was obtained. The resulting solution was evaporated at about 50°C until a transparent sol

Results

A typical TGA and DTA curve for the gel with molar ratio PAA-total metal ions equal to 1.0 is shown in Fig. 1. The TGA curve exhibits three distinct weight loss steps and the DTA curve shows one endothermic peak and two exothermic peaks. The first weight loss step in the temperature range of 25∼150°C, which was accompanied by an endothermic peak around 100°C in the DTA curve, is due to the loss of residual water in the gel. The second weight loss step in the temperature range of 150∼230°C

Discussion

The XRD analysis revealed that the synthesis of pure spinel NiFe₂O₄ could be achieved at a calcination temperature of 300°C, lower than the conventional solid-state reaction [4], by this preparation method. It is interesting and notable that, with increasing the molar ratio of PAA to total metal ions, the increased heat generated from the combustion of PAA did not result in the increase in the size of NiFe₂O₄ particles or the reduction in their specific surface area via the sintering of

Conclusions

Spinel NiFe₂O₄ nanoparticles have been synthesized by the sol-gel method using PAA as a chelating agent. Compared with the conventional method, this process showed good stoichiometric control and allowed the production of spinel NiFe₂O₄ powders at a relatively low temperature. By varying the PAA quantity and calcination temperature, the size, specific surface area, and crystallinity of NiFe₂O₄ nanoparticles could be controlled. It was also shown that the resultant NiFe₂O₄ nanoparticles might be

Acknowledgements

This work was performed under the auspices of the National Science Council of the Republic of China, under contract number NSC 88-2214-E006-017, to which the authors wish to express their thanks.

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