Characterization and preparation of nano-sized a-Fe₂O₃ particles by solid phase method

Du Yanjun, Du Baoan, Zhao Lei, Lv Jiantong
(College of Chemistry and Environmental Science, Hebei University, Baoding 071002, China)

Abstract Nano-sized a-Fe₂O₃ particles have been attracting great attentions due to its excellent physical and chemical properties in recent years. In this paper, nano-sized a-Fe₂O₃ particles were prepared by solid phase method using Fe(NO₃)₃·9H₂O and NaOH as raw materials. A series of experimental researches were carried out in order to obtain the nano-sized iron oxide powders with mono-phase and good dispersion. X-ray diffraction (XRD) and transmission electron microscopy (TEM) were used to investigate the products. The best preparation parameters were also obtained through these series of contrastive experiments. The results from XRD showed that the products were a-Fe₂O₃. It could be seen from TEM that their shapes were peanut-like, and the sizes of the particles were around 50 nm.
Keywords Nano-sized a-Fe₂O₃; preparation; characterization; solid phase method

1. INTRODUCTION
Nano-sized a-Fe₂O₃ particles are widely used as functional materials because of its excellent physical and chemical properties. There are comprehensive applications in magnetic materials, gas-sensing materials, catalyst materials and so on ^[1]. Generally, nanoparticles of a-Fe₂O₃ can be prepared by sol-gel method ^[2,3], microemulsion method ^[4], hydrothermal method ^[1,5], microwave method ^{[6, 7,8]}, solid phase method^[9], and so on. In the present paper, nanoparticles of a-Fe₂O₃ were prepared by solid-phase method using Fe(NO₃)₃·9H₂O and NaOH as raw materials. Different experimental conditions were carried out in order to obtain the nano-sized iron oxide powders with mono-phase and good dispersion. Compared with other methods, the solid phase method applied for preparation of nano-sized a-Fe₂O₃ particles has advantages of low cost of production, simple process and short period of production, which are suitable for industrial production. We hope that the method described in the paper is useful for production of nano-sized a-Fe₂O₃ on industrial scale.

2. EXPERIMENTAL
The nano-sized a-Fe₂O₃ particles were prepared as follows: 10 g Fe(NO₃)₃·9H₂O and 3 g NaOH were put into agate mortar, then were mixed and grinded thoroughly. With grinded, the mixture became glue gradually, and then solidified. The solidified materials were grinded to powders, and then washed three times with water and ethanol, respectively. The prepared materials were decompressing filtered, air dehydration naturally at ambient temperature, and then the precursors were obtained. Then the precursor was grinded, and calcined for different period at different temperature. 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 XRD analysis of the nanoparticles of a-Fe₂O₃
The crystal structure of the five samples were analysed by X-ray diffraction using CuKa radiation (CuKa, l_Cu = 0.154178 nm). The conditions of calcinations were showed in Table 1. The XRD patterns of samples were illustrated in Fig. 1.

Table 1. The conditions for calcinations of the precursors

According to Fig.1, the sample #1 was α-Fe₂O₃ and belongs to hexagonally, the shapes of peaks were in accordance with the other references ^[1,10]. However, the 3.66 characteristic peak was absent in the XRD pattern of sample #1, which indicated some impurity of g-Fe₂O₃ was contained in sample #1. Thus, pure a-Fe₂O₃ nano-sized particles could not be obtained by calcining the precursor for 2h at 673K. On the other hand, the reaction reagents could not be changed into a-Fe₂O₃ completely due to the lower temperature and shorter period, some impurities and the residual precursor still remained in sample #1 ^[6]. The sample #2 was better than the sample #1, although some impurities were still contained in the sample #2. According to Fig.1, the sample #3, #4 and #5 were pure a-Fe₂O₃.
    Seen from Fig. 1, with the temperature of calcinations ascending and the period extending, the X-ray diffraction peaks became higher and narrower. The average sizes of particles were calculated by the Scherrer formula:
D = k·l/(b·COS q)
    where D (nm) is the primary diameter of the product, k equals 1.075, l_Cu (nm) is 0.154178, b(arc) is integral peak width, q (degree) is angle of diffraction. X-ray diffraction peaks with low angular degree (generally 2q ≤50°) were selected for calculation, and the average value was obtained. These results were showed in Table 2. From Table 2, it could be seen that the primary diameter of particles was in the range of 20-30 nm.

Table 2. Average primary diameter of samples by calculation according to XRD data

3.2 TEM analysis of nano-sized particles of a-Fe₂O₃
The TEM photograph of sample #3 was shown in Fig. 2. According to Fig. 2, the sizes of the particles were about 50 nm and their shapes were peanut-like. The TEM photographs of the other samples were similar to Fig. 2, so these photographs were not shown in the present paper. Commonly, the calculated primary diameter based on XRD was less than the primary diameter based on TEM, our result was accordant with this phenomenon in this study.

4. CONCLUSIONS
In this study, nano-sized a-Fe₂O₃ particles were prepared by solid phase method using Fe(NO₃)₃·9H₂O and NaOH as raw materials. The prepared production was characterized by X-ray diffraction and transmission electron microscopy. The experiment showed that the temperature and period of calcinations were important factors influencing preparation of nano-sized a-Fe₂O₃ particles. The best preparation parameters were also obtained through contrastive experiments. The results from XRD showed that the products were a-Fe₂O₃. It could be seen from TEM that their shapes were peanut-like, and the sizes of the particles were around 50 nm. The technology for preparation of nano-sized a-Fe₂O₃ particles by solid phase method had advantages of low cost of production, simple process and short period of production, which were suitable for industrial production.

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纳米α-Fe₂O₃粒子的固相法制备和表征
杜艳君，杜宝安，赵磊，吕建通
(河北大学化学与环境科学学院，保定，071002)
摘要 由于纳米a-Fe₂O₃粒子具有良好的物理化学性质，因此一直受到相关领域科学家的广泛关注。在本文中，以Fe(NO₃)₃·9H₂O和NaOH固体为原料，采用固相法制备了a-Fe₂O₃纳米粒子。为了制备单相、分散性好的产品，进行了一系列的实验条件摸索。利用X射线衍射(XRD)和透射电子显微镜(TEM)对产品进行了表征。通过实验比较，优化了制备条件。经XRD检测，表明产品为a-Fe₂O₃，TEM显示a-Fe₂O₃粒子外形呈花生状，粒径约为50 nm。
关键词 纳米a-Fe₂O₃；制备；表征；固相法