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J. Phys. Chem. C 2007, 111, 9136-9141
Large-Scale Synthesis of Fe3O4 Nanosheets at Low Temperature Kok Chung Chin,†,‡ Ghee Lee Chong,† Chee Kok Poh,‡,§ Li Hui Van,† Chorng Haur Sow,†,‡ Jianyi Lin,‡,§ and Andrew Thye Shen Wee*,†,‡ NUS Nanoscience and Nanotechnology InitiatiVe, Block S13, 2 Science DriVe 3, Singapore 117542, Department of Physics, National UniVersity of Singapore, 2 Science DriVe 3, Singapore 117542, and Institute of Chemical and Engineering Sciences, 1 Pesek Road, Jurong Island, Singapore 627833 ReceiVed: February 1, 2007; In Final Form: April 17, 2007
We introduce a simple approach to synthesize high-quality Fe3O4 nanosheets at low temperature. By oxidizing pure Fe substrates in acidic solution on a hot plate maintained at 70 °C, we have grown the nanosheets in large-scale. The samples were characterized by SEM, XRD, TEM, micro-Raman, and VSM. Flowing O2 gas into the solution during oxidation enhances the growth rate of the nanosheets and thereby shortens the growth time from 24 h to 15 min. The magnetic property of the sample was also investigated. This technique can potentially be extended for large-scale synthesis of other metal-oxide nanosheets such as ZnO.
1. Introduction Metal-based nanostructures have attracted much attention due to their potential applications in different areas, including the electronics,1,2 optical,3 and life science4 industries. Low dimensional nanomaterials, such as nanorods, nanosheets, nanobelts and nanowires, display unique properties that are different from their bulk phases due to their large surface area and small size. In particular, intensive fundamental research has been performed on nanoscale magnetic materials, as it is believed these materials could lead to the fabrication of ultra-high-density magnetic recording devices or magnetic sensors. Ross proposed that selfassembled arrays of magnetic nanostuctures are needed for the production of future data storage devices with densities up to 150 Gbit/cm2.5 Recently, Martorana et al. fabricated micrometer scale ferromagnetic iron (Fe) spheres for magnetic encoder applications.6 Besides data storage applications, magnetic nanoparticles have also been extensively studied in the biomedical field. Dickson et al. introduced a method to synthesize a biocompatible ferrofluid known as magnetoferritin.7 Magnetoferritin is a colloidal solution containing iron oxide magnetic nanoparticles that can be used to detect membrane modifications associated with malaria or Alzheimer’s disease.8 Magnetite, Fe3O4, is a ferromagnetic material with Curie temperature of about 580 °C. It is known to be one of the most magnetic mineral with spin polarization close to 100% as per band-structure calculations. Thus, Fe3O4 is a promising material for many magnetic related applications. The discovery of its gas sensing ability and applications in lithium-ion batteries have highlighted the need of a more effective fabrication approach.9-11 Over the past decades, researchers have proposed several fabrication and synthesis approaches encompassing both physical and chemical methods such as sol-gel methods, thermal oxidation, hydrothermal synthesis, and chemical vapor deposition. However, most of these techniques involve unfavorable high growth temperatures (300-600 °C) and long growth * Corresponding author. Phone: (65) 65166362. Fax: (65) 67790350. E-mail:
[email protected]. † NUS Nanoscience and Nanotechnology Initiative. ‡ National University of Singapore. § Institute of Chemical and Engineering Sciences.
times.12-16 With the widespread applications of magnetic nanomaterials, the challenge is to obtain new practical ways to synthesize low-dimensional magnetic nanostructures at low formation temperature and reasonable process time. In this paper, we introduce a novel bottom-up technique to prepare good quality and homogeneous Fe3O4 nanosheets on Fe substrate in large-scale. As in our previous work on synthesizing FeOOH nanowalls by the electrochemical approach, this method also demonstrates a low-temperature process with rapid growth rate.17 The synthesis has yielded uniform nanosheet-like structures, with typical dimension of about a micrometer in length and 20-40 nm in thickness. Moreover, these structures were found to anchor securely on the substrate. The chemical oxidation process is similar to other Fe3O4 nanostructures growth by the conventional hydrothermal technique.18,19 In most conventional hydrothermal processes, the end products are usually in powder form. This limits the utilization of the products in many applications such as gas sensing, electron field emission, magnetic data storage and supercapacitors which require the nanocomposites to adhere securely onto the substrate surface. In our work, the Fe3O4 nanostructures were grown directly on the Fe substrate. Hence, it is convenient to handle the product for subsequent experiments. 2. Experimental Methods We synthesized Fe3O4 nanosheets by oxidizing Fe substrates in acidic solution on a hotplate. Pure Fe foils (99.5% purity, Goodfellow) with a thickness of 0.5 mm and a dimension of 2 × 2 cm2 were polished with sandpaper (360 grits), rinsed with distilled water and then dried. The acidic solution used in the growth process consisted of hydrochloric acid (HCl) and 0.1 M potassium chloride (KCl). The solution was prepared by adding 0.04 mL of 25% HCl into 900 mL of 0.1 M KCl. The pH of the solution was adjusted to be around 3 as indicated by a pH paper (Universalindikator, Merck). To prepare Fe3O4 nanosheets on Fe substrates, 10 mL of the solution was loaded into a glass beaker and then heated to around 70 °C by a hotplate. An iron foil sample was then immersed into the solution. The Fe foil thus provided the raw Fe material needed for Fe3O4 nanosheet formation. A magnetic
10.1021/jp070873g CCC: $37.00 © 2007 American Chemical Society Published on Web 06/14/2007
Large-Scale Synthesis of Fe3O4 Nanosheets
J. Phys. Chem. C, Vol. 111, No. 26, 2007 9137
Figure 2. X-ray diffraction patterns of the Fe substrate and Fe3O4 nanosheets. The shaded box represents the diffraction peaks for Fe3O4 phases.
Figure 1. Optical photographs taken at two slightly different orientations perpendicular to Fe substrate surface (a) before and (b) after treated for 24 h in acidic solution. (c and d) Corresponding SEM images of the Fe substrate surfaces. Large area (2 × 2 cm2) uniform synthesis of Fe3O4 nanosheets is observed.
bar rotated at a rate of 120 rpm provided continuous stirring to the solution during the growth processes. After 24 h, the beaker was removed from the hotplate and cooled in ambient. The asgrown sample with dark green surface color (presumably due to the hydrated magnetite or green rust) was then immersed into 50 mL of distilled water for about 1 h before dried in N2 environment. The morphologies of the synthesized Fe3O4 nanosheets were characterized with a JEOL JSM-6400F field emission scanning electron microscope (FE-SEM), operating at 5 kV in high vacuum. X-ray diffraction (XRD) and micro Raman spectroscopy were utilized to analyze the chemical composition and crystallographic structure of the nanocomposites. The XRD patterns were recorded on a BRUKER D8 Advance X-ray Diffractometer using Cu KR radiation (λ ) 1.541 87 Å), scanning in the 2θ range of 20-90° for about 3 h while Raman spectra were measured by a computer controlled Renishaw system at 532 nm wavelength. Crystalline structures of the nanosheets were also studied with a JEOL JEM-3010F Transmission Electron Microscope (TEM) operating at 300 kV in high vacuum. Magnetization of the sample was characterized in a Lake Shore 7400 series vibrating sample magnetometer (VSM) at room temperature. 3. Results and Discussion 3.1. As-Grown Fe3O4 Nanosheets. Figure 1a,b shows the optical photographs of the substrate before and after growth. The shiny Fe surface turned to dark/black color after treatment for 24 h, suggesting the formation of magnetite (Fe3O4) on the Fe surface, rather than other phase of iron oxide such as Fe2O3. The corresponding SEM images of the Fe substrate and Fe3O4 nanosheets grown in 70 °C acidic solution are shown in panels c and d, respectively, of Figure 1. The SEM observations reveal the formation of dense nanosheet-like structures on the Fe substrate. The nanosheets are anchored securely on the substrate surface. The edges of the nanosheets are irregular and the lengths
Figure 3. Micro Raman spectrum of as-grown nanosheets. Two Fe3O4 peaks were observed at 670 and 540 cm-1.
of the nanosheets are in the region of a micrometer, with thicknesses of a few tenths of nanometers. To analyze the composition and crystal structures of the nanosheets, the sample was subjected to XRD measurements. Figure 2 shows typical XRD peaks of both Fe substrate (gray line) and as-synthesized sample (dark line). Magnetite is believed to be the major crystalline phase for the synthesized nanosheets as identified by the new diffraction peaks at 30.2°, 35.6°, 43.3°, 57.3°, and 62.9°. These peaks correspond to five indexed planes (220), (311), (400), (511), and (440), respectively, of magnetite. The positions and the relative intensity ratios of the diffraction peaks match those of published results for magnetite.14,20,21 To further investigate the composition and the nature of the as-grown iron oxide nanosheets, micro Raman spectroscopy was carried out in range of 450-750 cm-1, and the spectrum is shown in Figure 3. The strongest peak is at 670 cm-1, which is attributed to A1g mode of magnetite, according to Phase et al.22 We also observed a weaker T2g mode at near 540 cm-1. These peaks are two main characteristic peaks for magnetite.23,24 A possible mechanism for the formation of magnetite nanosheets is proposed. When the Fe substrate is immersed in the acidic solution, iron will pass into solution, usually as ferrous ions (Fe2+). Under an oxygen-deficient environment, ferrous hydroxide or Fe(OH)2 forms.25 Intermediate oxidation products of ferrous hydroxide, such as dark green hydrated magnetite which is a mixture of Fe3+ and Fe2+ ions, also form and selfassemble on the substrate surface. Although black magnetite
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normally forms at the pH range from 7 to 12, some reports demonstrate magnetite formation at pH
