Improved photocatalytic and gas sensing properties of α-Fe2O3 nanoparticles derived from β-FeOOH nanospindles

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Abstract

α-Fe2O3 nanoparticles were synthesized through calcining the as-synthesized spindle-like β-FeOOH precursors at 600 °C. X-ray powder diffraction (XRD) and transmission electron microscopy (TEM) results showed that β-FeOOH nanospindles with a diameter of ~50 nm and lengths up to 100–150 nm were readily changed to α-Fe2O3 nanoparticles with a size of 30–150 nm after heat-treatment for 2 h. The photocatalytic performances of the as-prepared samples were evaluated by photocatalytic decolorization of methylene blue (MB) in the presence of H2O2 at ambient temperature. The results indicated that the α-Fe2O3 nanoparticles exhibited the highest photocatalytic activity compared with the β-FeOOH nanospindles and the commercial α-Fe2O3 powders. The gas-sensing measurement results demonstrated that the products showed an excellent gas response to ethanol and acetone. The results showed that these α-Fe2O3 nanoparticles may have potential applications in gas sensor and photocatalysts.

Introduction

Hematite (α-Fe2O3) is the most thermodynamically stable iron oxide with n-type semiconducting properties (Eg=2.1 eV) under ambient conditions. It has been extensively used in many fields, such as magnetics [1], [2], catalysts [3], [4], gas-sensing [5], [6], pigments [7] and water treatment [8], [9], as well as in other biological and medical fields due to its low cost, environmental friendliness, and fascinating physicochemical properties. Nanostructured materials are expected to have improved physicochemical properties compared with bulk materials due to their size effects, large surface area to volume ratios and possible quantum confinement effects [10].

Many efforts have been directed to fabricate α-Fe2O3 nanostructures with specific size and morphology [11], [12], [13] because of their unique electrical, optical and magnetic properties for the potential applications in a lot of fields. Stimulated by these intriguing properties and extensive applications, a variety of methods have been reported for the synthesis of α-Fe2O3 nanostructures, including the vapor–solid growth method, sol–gel approach, hydrothermal technique and the chemical precipitation process. Zhonglin Wang groups have synthesized uniform α-Fe2O3 nanowire arrays by a vapor–solid method, and systematically studied the growth machanism of the α-Fe2O3 aligned arrays [14]. Wael Hamd et al. fabricated α-Fe2O3 mesoporous films by a template-directed sol–gel method combined with a dip-coating approach and followed by annealing at various temperatures in air [15]. Guohong Qiu et al. have synthesized nanosized α-Fe2O3 powder by a microwave-assisted hydrothermal reaction of Fe(NO3)3 in the presence of urea at 120 °C [16]. Although there are a lot of successes in the synthesis of α-Fe2O3 nanostructures, a facile method is still required.

Herein, α-Fe2O3 nanoparticles are prepared by a combined method of hydrolysis and calcination. The hydrolysis of FeCl3 solution gives uniformly sized β-FeOOH nanospindles that serve as the precursor for α-Fe2O3 nanoparticles. Despite the size and shape changes, the calcination-derived α-Fe2O3 nanoparticles show phase pure hematite and high crystallinity. Moreover, the well-defined β-FeOOH precursor may provide a natural defense from the heavy agglomeration of α-Fe2O3 nanoparticles, resulting in network-like nanoparticle aggregates. These networks should have an effective contact surface area and accessible defuse pathway for the molecule adsorption and desorption. It is thus expected that the as-synthesized α-Fe2O3 nanoparticles show high photo-catalytic activity and gas-sensing performance. The visible light photo-degradation of MB on the synthesized α-Fe2O3 nanoparticles shows their obviously higher ability than that of the β-FeOOH precursor. Furthermore, the gas-sensing measurements of the synthesized α-Fe2O3 nanoparticles exhibit an excellent ethanol- and acetone-sensing performance.

Section snippets

Sample preparation

All the reagents used in the experiment were analytical grade without further purification. The precursor β-FeOOH was obtained from hydrolysis of diluted aqueous iron chloride (FeCl3·6H2O) solution. In a typical process, 0.02 mol L−1 aqueous solution of FeCl3 was heated at 80 °C for 24 h under magnetic stirring. After cooling to room temperature, the product was washed with deionized water and ethanol for several times, and then dried at 40 °C in air for 8 h. After that, the as-obtained β-FeOOH was

Structure characterization

Fig. 1 shows the XRD patterns of the precursor and the as-synthesized sample. Fig. 1a exhibits the precursor fabricated by the hydrolysis of iron chloride solution. It is obvious that almost all the reflections can be readily indexed to a tetragonal β-FeOOH phase (JCPDS no. 34-1266). A complete conversion from β-FeOOH to α-Fe2O3 is achieved by the calcination of the spindle-shaped β-FeOOH, which can be proved by the XRD pattern in Fig. 1b. The product shows a hexagonal structure of α-Fe2O3

Conclusions

In conclusion, α-Fe2O3 nanoparticles were prepared through annealing the β-FeOOH precursor obtained from hydrolysis of FeCl3·6H2O. Moreover, the gas sensing property and the photocatalytic activity of the synthesized products were evaluated. The results showed that the α-Fe2O3 nanoparticles exhibited excellent photocatalytic properties towards MB and high gas response to ethanol and acetone, suggesting that these products may have potential applications in the field of photocatalysis and gas

Acknowledgments

We appreciate the financial supports of NSFC (21266031), Scientific and Technological Assistance Projects in Xinjiang (201191102) and International Cooperation Projects in Xinjiang (20116011).

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