Du Yanjun, Zheng Yanju, Du Baoan, Zhang Jingfeng
(College of Chemistry and Environmental Science, Hebei University, Baoding 071002, China)

Abstract Nanosized ZnFe₂O₄ powders were prepared by co-precipitation method using ZnSO₄·7H₂O and FeCl₃·6H₂O as raw materials. The composition of crystal phase and the sizes of particles were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The effect of pH value, concentration of reagents, and ultrasonic on preparation were studied. Finally, and the optimum conditions were obtained by comparison experiment.
Keywords co-precipitation method; preparation; nanosized ZnFe₂O₄

1. INTRODUCTION
Due to its paramagnetic property, ZnFe₂O₄ possessing normal spinel structure is widely used for industrial materials including magnetic materials, gas sensor, absorbent material for hot-gas desulphurization, and semiconductor photo catalyst.^[1-4] Traditionally, nanosized ZnFe₂O₄ is prepared by wet-milling process^[1], Hydrothermal synthesis^[2], solvent thermal process^[3], shock wave treatment^[5], sol-gel method^[6], solid state reaction^[7], combustion synthesis^[8]. However, all these methods are very complicated for obtaining nanosized ZnFe₂O₄.
    A new technique for preparation of nanosized ZnFe₂O₄ powders by co-precipitation method using ZnSO₄·7H₂O and FeCl₃·6H₂O as raw materials is reported in this study. The most important advantage of this technique is its feasibility for large-scale production as well as its simplicity and low cost.

2. EXPERIMENTAL
A series of FeCl₃ solutions were obtained by dilution: 0.2, 0.4, 0.6, 0.8, 1.0 mol× L^-1. And the ZnSO₄ solutions were also achieved: 0.1, 0.2, 0.3, 0.4, 0.5 mol× L^-1. The FeCl₃ solution and the ZnSO₄ solution were mixed in a beaker with a molar ratio of 1(Zn):2(Fe). 1 mol× L^-1 NaOH was used for adjusting the pH value of the mixture ranged from 8 to 13. Then, the mixture was stired for 30 min, decompression filtered, washed by distillated water 3 times, and then washed by ethanol 2 times. The filter cake was exposed to the air for 24 hrs of dehydration naturally. Then the precursor was obtained. The precursor was calcined at 973 K for 6 h. Finally, the products were obtained. The phase compositions of the products were examined by XRD. The sizes and shapes of the products were testified by TEM.

3. RESULTS AND DISSCUSSION
3.1 Optimization of the preparation
The pH of the reaction system plays an important role in the research. When the pH was less than 8, Zn(OH)₂ was not precipitated completely, then the Zn:Fe was less 1:2; when the pH was larger than 9, it was difficult to wash and leach the precipitate, the products was hard for aggradation. Finally, the results showed that pH value ranged from 8 to 9 is the best choice.
    The effect of concentrations of solution for preparation was also very important. The capacity for preparation become larger when the concentration becomes lower, and then the production cost become also much higher. The higher of the concentrations, the impurities in product were much more. Finally, the optimum concentration of the raw materials were FeCl₃ 0.6 mol× L^-1 and ZnSO₄ 0.3 mol× L^-1.
3.2 XRD analysis of the ZnFe₂O₄ nanoparticles

Fig. 1 XRD patterns of the products
(a) dispersed in ethanol and treated by ultrasonic for 30min and
(b) not treated by ultrasonic. FeCl₃: 0.6 mol× L^-1; ZnSO₄: 0.3 mol× L^-1; pH = 9.

Fig. 2 TEM photograph of the sample
(×50000 times; FeCl₃: 0.6 mol× L^-1; ZnSO₄: 0.3 mol× L^-1; pH = 9.)

3.4 Mechanism of the preparation of nanosized ZnFe₂O₄ by co-precipitation method
The mechanism of prepared nanosized ZnFe₂O₄ powders can be divided into the follow two steps. Firstly, the highly dispersed and easily decomposed precursor was prepared; secondly, the precursor was thermal decomposed.
    Co-precipitation process includes single-phase　co-precipitation process and multiple-phase co-precipitation process. When the precipitation is simplex compound or single-phase　solid solution, the process is called single-phase　co-precipitation process, the ions precipitate at the same time. When the precipitation is mixture, the process is called multiple-phase co-precipitation process, the order of precipitating of ions had no effect on the form of the precipitation. In the study, the color of the precursor prepared by hydrolyzed　FeCl₃·6H₂O and the color of the precursor prepared by FeCl₃·6H₂O without hydrolyzation was different. In other words, the precipitation of ferric ion in advance affects the form of the precipitation. So it could be concluded that the process of the technology is single-phase　co-precipitation process.

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