Size controlled and morphology tuned fabrication of Fe3O4 nanocrystals and their magnetic properties

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Abstract

Size controlled and morphology tuned fabrication of Fe3O4 nanocrystals were achieved by a simple hydrothermal method using (NH4)2Fe(SO4)2·6H2O, hexamethylenetetramine, and sodium sulfate as original reaction materials. Fe3O4 particles with controlled size ranging from 160 nm to 2 μm were obtained by adding an appropriate amount of NH4Ac to the reaction system. Fe3O4 quadrangular nanoplates with side length of about 500 nm were fabricated by adding optimum amount of urea. Box-like Fe3O4 nanocrystals were synthesized by adjusting the mol ratio between (NH4)2Fe(SO4)2·6H2O and hexamethylenetetramine. The as-synthesized products were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), and selected area electron diffraction (SAED). Room-temperature magnetization curves that measured by vibrating sample magnetometer show morphology and size dependent magnetism.

Introduction

During the past few years, considerable researches have been done on the morphology and size controlled synthesis of micro- and nano-scale materials due to their unique chemical and physical properties that are relevant to their shape and size [1], [2], [3], [4]. The design and synthesis of various magnetic architectural structures are of great interest and have actively been pursued due to their widely application such as pigmentation [5], recording materials [6], catalysts [7], ferrofluids [8], chemical sensors [9], and electro photographic developers [10].

Among all magnetic materials, magnetite (Fe3O4) has attracted much attention. It has not only been generally used as a very important ferromagnetic material for pigment, recording materials, catalysis, magnetocaloric refrigeration, printing ink, and magnetic resonance imaging, but also widely been considered as an ideal candidate for biological applications such as a tag for sensing and imaging [11], [12], a drug-delivery carrier for antitumor therapy [13], [14], an immunosensor for the detection of carcinoembryonic antigen in clinical immunoassay [15], and an activity agent for medical diagnostics [16] due to its good hydrophilic and biocompatible properties. Therefore, we are interested in developing simple method for size controlled and morphology tuned fabrication of Fe3O4 nanocrystals. For a long period of time, morphology controlled fabrication of nanomaterials was usually achieved by two ways: (i) adding surfactant [17]; (ii) adjusting the pH values by acid or alkali [18], [19]. Since sulfates influenced fabrication of tungsten oxide nanocrystals with different morphologies was reported [20], soluble salts were testified to play important roles in the synthesis of nanomaterials in solution environment. Morphology controlled or well-dispersed nanomaterials were obtained by adding optimum amount of soluble salts [20], [21]. In this paper, (NH4)2Fe(SO4)2·6H2O and hexamethylenetetramine were used to synthesis of well-dispersed Fe3O4 nanocrystals in the presence of sodium sulfate. Ammonia acetate and urea were used to tune the size and morphologies of the products.

Section snippets

Experimental details

All the chemicals are analytical grade and purchased from Shanghai Chemical Reagents. In a typical procedure, 5 mmol (NH4)2Fe(SO4)2·6H2O, 2.5 mmol hexamethylenetetramine, and 0.5 g sodium sulfate were dissolved in 30 ml distilled water. For fabrication Fe3O4 particles with bigger size, different amount of NH4Ac was added into the solution. Fe3O4 nanoplates were obtained by adding 0.3 g urea to the solution. After stirring for 20 min, the homogeneous green solution was transferred into a 50 ml Teflon

Results and discussion

Typical XRD patterns of the products synthesized by 5 mmol (NH4)2Fe(SO4)2·6H2O, 2.5 mmol hexamethylenetetramine, and 0.5 g Na2SO4 in the presence of different amount of NH4Ac (a, 0 g; b, 0.1 g; c, 0.25 g; d, 0.5 g) and optimum amount of urea (e, 0.3 g) are shown in Fig. 1. The peaks of all samples can be indexed as face centered cubic Fe3O4 with lattice constant a = 8.391 Å, which are in good agreement with magnetite (JCPDS card No. 19-0629). No diffraction peaks other than those from Fe3O4 were obtained,

Conclusions

In conclusion, well-dispersed Fe3O4 nanoparticles with controlled diameter from 160 nm to 2 μm, box-like morphology, and quadrangle nanoplates were synthesized by a simple hydrothermal method in the presence of sodium sulfate. We propose solution environments that determined by ion species and concentration play crucial roles in the dispersability and growth of magnetite. This soluble ion tuned synthesis will probably be widely adopted for size and morphology controlled fabrication in future. The

Acknowledgements

We gratefully acknowledge the financial support from the Teaching and Research Award Program for Outstanding Young Teachers (MOE, China), Natural Science Foundation of China (NSFC, 50972075), key projects of Chinese Ministry of Education (D209083), and Education Office of Hubei Province (D20081304 and CXY2009A004). Moreover, the authors are grateful to Dr. Jianlin Li at Three Gorges University for his kind support to our research.

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    Present address: College of Mechanical and Material Engineering, Three Gorges University, 8 Daxue Road, Yichang 443002, PR China.

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