The Scientific World Journal

Volume 2013 (2013), Article ID 410594, 5 pages

http://dx.doi.org/10.1155/2013/410594

Research Article

Wet Chemical Controllable Synthesis of Hematite Ellipsoids with Structurally Enhanced Visible Light Property

Chengliang Han,1,2 Jie Han,¹ Qiankun Li,¹ and Jingsong Xie¹

¹Department of Chemical and Material Engineering, Hefei University, Hefei 230601, China

²Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, China

Received 4 August 2013; Accepted 5 September 2013

Academic Editors: K. Prokeš and D. Zhang

Copyright © 2013 Chengliang Han et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

A facile and economic route has been presented for mass production of micro/nanostructured hematite microcrystals based on the wet chemical controllable method. The as-prepared samples were characterized using X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and UV-Vis absorption spectroscopy. The results showed that the product was mesoporous α-Fe₂O₃ and nearly elliptical in shape. Each hematite ellipsoid was packed by many α-Fe₂O₃ nanoparticles. The values of vapor pressure in reaction systems played vital roles in the formation of porous hematite ellipsoids. Optical tests demonstrated that the micro/nanostructured elliptical hematite exhibited enhanced visible light property at room temperature. The formation of these porous hematite ellipsoids could be attributed to the vapor pressure induced oriented assembling of lots of α-Fe₂O₃ nanoparticles.

1. Introduction

Hematite (α-Fe₂O₃) is the oldest known iron oxide mineral and is widely applied in catalysts, gas sensors, pigments, and promising photoanodes for solar cells [1–4]. Over the past ten years, various hematite nanostructures with a well-defined shape such as nanorods [5], nanowires [6], and nanobelts [7] have been obtained successfully by directional growth techniques [8–10], template guiding [11, 12], and decomposition of shape-regular iron precursors [13, 14]. However, the previous reported shape-controllable synthetic methods have some limitations. For example, the used shape-controlling reagents such as some templates and surfactants are usually more expensive and hard to wash out. Additionally, solid reaction processes for α-Fe₂O₃ may introduce some other iron oxide phases and release harmful gases for possible environmental pollution. Consequently, developing simple and economic methods for preparation of hematite nanomaterials as well as the modification of their sizes, morphology, and porosity has been intensively pursued not only for their fundamental scientific interest but also for many technological applications. This work presents a new method to produce large-scale α-Fe₂O₃ micro/nanostructured porous ellipsoids. The size and shape of as-prepared α-Fe₂O₃ can be well controlled only by simply regulating the values of vapor pressure in reaction systems. The results reported in this paper mainly encompass the formation mechanism of porous α-Fe₂O₃ with different morphologies and their corresponding visible light properties at room temperature.

2. Materials and Methods

2.1. Preparation of the Samples

The micro/nanostructured hematites were prepared as follows. Firstly, the Fe(OH)₃ precursors were precipitated from FeCl₃·6H₂O solution by adding proper ammonia (NH₃·H₂O). And the above Fe(OH)₃ with the solution was transferred into a 100 mL Teflon autoclave with a pressure gage for detecting internal vapor pressure. Then, the above sealed Teflon autoclave was heated to 453 K, and the internal vapor pressure was kept under 1.45 × 10⁵ Pa for 2 h. After cooling to ambient temperature, the bright red powder was ultrasonically rinsed for several times in deionized water and ethanol, respectively. Finally, the sample was collected by a centrifuge and dried in a vacuum oven at 353 K for 6 h.

2.2. Characterization of the Samples

X-ray powder diffraction (XRD) patterns were recorded on a Philips X’pert diffractometer using CuKα radiation . Scanning electron microscopy (SEM) was recorded on a Sirion 200 FEI field emission scanning electron microscope. The transmission electron microscopic (TEM) micrographs were taken with a JEOL-2010 field emission transmission electron microscope with an accelerating voltage of 200 kV. The UV-Vis absorption spectra were taken on a spectrophotometer (Cary 5E UV-Vis-NIR) from 200 to 1200 nm.

3. Results and Discussion

3.1. XRD and Morphology of As-Prepared Samples

After Fe(OH)₃ precursors in solution were heated at 453 K for 2 h in Teflon autoclave with the internal vapor 1.45 × 10⁵ Pa or so, the red powders were collected by a centrifuge. The corresponding X-ray diffraction (XRD) was illustrated in Figure 1. It can be found that all the diffraction peaks can be indexed to pure α-Fe₂O₃ (JCPDS NO. 89-2810). The intense peaks of the XRD pattern indicate that the as-prepared powders were well-crystallized α-Fe₂O₃.

Figure 1: XRD patterns of the as-synthesized sample and standard α-Fe₂O₃ powder (the line spectrum).

FE-SEM observations have shown that each as-prepared α-Fe₂O₃ particle is of collective-like morphology with nearly elliptical in shape and nanoscaled surface roughness, as illustrated in Figure 2(a) and its inset. Each hematite ellipsoid is 500 nm and 120 nm in major and short axis, respectively (seen from Figure 2(a)). Further microstructural examination conducted for such α-Fe₂O₃ ellipsoids was shown in Figure 2(b). It was confirmed that many α-Fe₂O₃ nanoparticles (~20 nm) reconstructed the porous hematite ellipsoid. The surface of every ellipsoid is also thus rough.

Figure 2: Microstructural examination of α-Fe₂O₃ ellipsoids. (a) Low-magnification SEM image; (b) low-magnification TEM image (insets: corresponding high-magnification).

In general, the values of pressure in reaction systems will determine the chemical reaction start at the same reaction temperature. So different composition products can be gained under different values of pressures. Our deep and systematic studies have revealed that pure goethite (α-FeOOH) nanorods (seen from Figures 3(a) and 4(a)) will be obtained when the value of reactive pressure is less than 1.02 × 10⁵ Pa. And the mixture of α-FeOOH nanorods and α-Fe₂O₃ ellipsoids will be acquired when the value of pressure is between 1.15 × 10⁵ Pa and 1.45 × 10⁵ Pa, as shown in Figures 3(b) and 4(b), respectively.

Figure 3: XRD patterns of samples under different pressures ((a) 1.02 × 10⁵ Pa (b) 1.15 × 10⁵ Pa (c) 1.62 × 10⁵ Pa).

Figure 4: Morphology of samples at different pressures ((a) 1.02 × 10⁵ Pa, (b) 1.15 × 10⁵ Pa, and (c) 1.62 × 10⁵ Pa).

3.2. Effect of Vapor Pressure

When the value of pressure is next to 1.62 × 10⁵ Pa or higher, pure α-Fe₂O₃ nanocrystals will be achieved (seen from Figure 3(c)). Hence, a lower value of reactive pressure was not beneficial to the formation of α-Fe₂O₃. However, higher value of pressure will lead to lots of homogeneous α-Fe₂O₃ solid quasicubic particles (seen from Figure 4(c)) instead of the porous α-Fe₂O₃ ellipsoids, as illustrated in Figure 2. Therefore, it can be concluded that the values of vapor pressure in reaction systems will influence not only the composition of the products but also the morphologies.

3.3. Formation of Porous α-Fe₂O₃ Ellipsoids

The formation process of α-Fe₂O₃ porous ellipsoids could be described in the following three stages. First of all, the Fe(OH)₃ precursors would be formed by the precipitation between Fe³⁺ and OH⁻ at room temperature (1). With subsequent heating at 453 K, the Fe(OH)₃ will begin to decompose into α-FeOOH (2), which is finally dehydrated to α-Fe₂O₃ molecules (3). Secondly, when the concentration of α-Fe₂O₃ molecules was supersaturated, ultrafine α-Fe₂O₃ particles would be formed in the solution by nucleation and growth:

Finally, the as-formed hematite nanoparticles would be further coarsened by an oriented-assembling mode in order to reduce the surface energy of α-Fe₂O₃ particles which were polar crystals and tended to spontaneously assemble [15, 16]. The proper value of vapor pressure will help α-Fe₂O₃ nanocrystals migrate and readjust more easily in a reactor. As a result, with the assemble going on, the porous hematite ellipsoid packed with many α-Fe₂O₃ nanoparticles would be formed. The above three stages have been illustrated in Figure 5. We acknowledge that the true formation mechanism of porous α-Fe₂O₃ elliptical structures is still unclear. However, it is obvious that the values of vapor pressure in a reactor are of great importance in the growth of porous α-Fe₂O₃ ellipsoids.

Figure 5: Schematic illustration for formation of porous α-Fe₂O₃ ellipsoids.

3.4. Ultraviolet Visible Optical Properties

The optical absorption measurement of as-prepared α-Fe₂O₃ products was conducted at room temperature, which may help the understanding of the electronic structure and size effect.

Figure 6(a) shows the optical absorption spectrum of the quasicubic α-Fe₂O₃ nanostructures, and the optical absorption feature was observed at wavelength around 390 nm (3.18 eV) and had an inflection and shift to the short wavelength compared to that reported by Li et al. [17]. Figure 6(b) shows the optical absorption spectrum of the ellipsoid-like micro/nanostructured α-Fe₂O₃, and the absorption peak is about 590 nm (2.10 eV). The different optical adsorption properties of two different shaped α-Fe₂O₃ can be attributed to their different structures. In a word, the as-obtained porous α-Fe₂O₃ ellipsoids were extended for potential application in photodegradating pollutants under visible light. The relative studies are our next work.

Figure 6: UV-Vis spectrums of α-Fe₂O₃ from various pressures: (a) 1.62 × 10⁵ Pa; (b) 1.45 × 10⁵ Pa.

4. Conclusions

In conclusion, we have demonstrated mass production of micro/nanostructured elliptical hematite by a wet chemical pressure-controlled method. Our investigations have showed that the values of vapor pressure in reaction systems have been considered to play vital roles in the formation of porous α-Fe₂O₃ ellipsoids. The vapor pressure induced oriented assembling mechanism of polar hematite nanocrystals has been inferred. Importantly, the micro/nanostructured porous α-Fe₂O₃ ellipsoids with excellent visible light property can be used as a novel potential photocatalyst for removal of some toxic chemicals.

Acknowledgments

The authors wish to acknowledge with thanks the support of this research by the National College Students’ Innovation and Entrepreneurship Training and Practice of Foundation of Hefei University (Grant no. 201211059008), Natural Science Foundation of Education Department of Anhui Province (Grant no. KJ2012B148), Natural Science Foundation of Hefei University (Grant no. 13RC09), and Anhui Provincial Natural Science Foundation (Grant no. 1308085QB35).

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