A hydrothermal method for preparation of α-Fe2O3 nanotubes and their catalytic performance for thermal decomposition of ammonium perchlorate

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Abstract

A hydrothermal method for α-Fe2O3 nanotube preparation is described which requires no surfactants or templates. The crystalline structure and morphology of the as-prepared powder have been characterized by using X-ray powder diffraction, Fourier transform infrared spectroscopy, and transmission electron microscopy. The results showed that the average length, diameter, and wall thickness of the highly crystalline α-Fe2O3 nanotubes were about 200–1000 nm, 100–150 nm, and 25–30 nm, respectively. The α-Fe2O3 nanotubes were used as a catalyst for thermal decomposition of ammonium perchlorate at low temperature. A detailed reaction mechanism for ammonium perchlorate decomposition over α-Fe2O3 nanotubes is proposed.

Introduction

The environmentally friendly α-Fe2O3 (hematite), an n-type semiconductor (Eg = 2.1 eV), is the most stable iron oxide under ambient conditions. It is widely used in catalysts, gas sensors, photoelectrodes for solar energy conversion, clinical therapy and diagnosis, etc. [1], [2], [3], [4], [5]. Most of these functions depend strongly on the structure and morphology of the semiconductor materials, and one-dimensional (1D) α-Fe2O3 nanostructured material exhibits better electrochemical performance than that of bulk samples. Therefore, tremendous efforts have been directed in recent years to the synthesis and investigation of properties of low-dimensional α-Fe2O3 nanostructures. However, the synthesis of α-Fe2O3 nanotubes is still a challenge. So far, many methods have been developed effectively, including using a simple hydrothermal route based on a FeCl3 solution in the presence of NH4H2PO4 [6], [7], [8], rolling up layered precursors [9], treating thermally silica coating of β-FeOOH nanoparticles [10], preparing of α-Fe2O3 nanotubes on Fe Foil [11], and so on [12], [13], [14], [15]. For most methods, the preparation process requires templates. The template can increase the production cost and introduce the impurities. In addition, removal of the template has also increased the complexity of the process. In this article, we report a simple and efficient synthesis method for preparing α-Fe2O3 nanotubes in large quantities without additional surfactants or templates. In our study, the raw materials are K4[Fe(CN)6] and H2O2. Therefore, [Fe(CN)6]4− ions are first oxidized to [Fe(CN)6]3− ions by H2O2, and then the [Fe(CN)6]3− ions dissociate slowly into Fe3+ ions (Kf = 1.0 × 1035), which can help control the morphology of iron oxide. The as-prepared α-Fe2O3 nanotubes exhibited a high catalytic activity toward thermal decomposition of ammonium perchlorate (AP) at low temperature when tested as catalysts.

Section snippets

Preparation of α-Fe2O3 nanotubes

All reagents were of 99.9% purity from Tianjin Chemical Reagents Company and were used without further purification. In a typical procedure, under vigorous stirring, 1 mmol of K4[Fe(CN)6]·3H2O was dissolved in 20 mL of distilled water at room temperature. Then 5 ml 30% H2O2 was added to form a heterogeneous solution, which was transferred into 30 mL Teflon-lined autoclave. The autoclave was then directly heated to 160 °C for 100 min, followed by cooling to room temperature under ambient conditions.

XRD characterization of α-Fe2O3 nanotubes

The crystallinity of the as-synthesized α-Fe2O3 nanotubes and nanorods was examined by XRD. Fig. 1 shows the powder diffraction patterns of the obtained samples. The XRD patterns shown in Fig. 1a (nanotubes) and b (nanorods) indicate that peaks could be perfectly indexed to a pure rhombohedral crystalline phase of α-Fe2O3 according to JCPDS card 81-2810. No peaks of any other phases or impurities were detected from XRD patterns, indicating the high purity of the product.

Morphological characterization of α-Fe2O3 nanotubes

The morphology and size

Conclusions

In summary, we developed a novel synthesis method for selective preparation of nanotubes and nanorods of α-Fe2O3. The approach presented here can overcome the disadvantages of using assisted surfactant or template methods for the synthesis of these nanotubes and nanorods. The experimental results reveal that α-Fe2O3 nanotubes exhibited enhanced catalytic activities for the thermal decomposition of AP. This study provides a simple option for preparation of good burning rate catalysts for

Acknowledgment

This work was supported by Development Program of Science and Technology of Tianjin (06TXTJJC14400).

References (17)

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    It is evidenced that reducing the particle size of Fe2O3 to nanometer scale contributes to decreasing the decomposition temperature and apparent activation energy of AP. A variety of nanostructured Fe2O3 have been explored, including nanoparticles [33,34], nanotube [35], core–shell structure [36], mesoporous structure [37], and nanoflakes [38]. To avoid agglomeration, nano-sized Fe2O3 is usually anchored onto carbon supports, such as graphene and its derivatives [39–41].

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