Fe3O4–Au and Fe2O3–Au Hybrid Nanorods: Layer-by-Layer Assembly Synthesis and Their Magnetic and Optical Properties
© The Author(s) 2010
Received: 26 May 2010
Accepted: 15 July 2010
Published: 1 August 2010
A layer-by-layer technique has been developed to synthesize FeOOH–Au hybrid nanorods that can be transformed into Fe2O3–Au and Fe3O4–Au hybrid nanorods via controllable annealing process. The homogenous deposition of Au nanoparticles onto the surface of FeOOH nanorods can be attributed to the strong electrostatic attraction between metal ions and polyelectrolyte-modified FeOOH nanorods. The annealing atmosphere controls the phase transformation from FeOOH–Au to Fe3O4–Au and α-Fe2O3–Au. Moreover, the magnetic and optical properties of as-synthesized Fe2O3–Au and Fe3O4–Au hybrid nanorods have been investigated.
KeywordsLayer-by-layer Hybrid nanomaterials Iron oxide Magnetic properties
Hybrid nanomaterials consisting of two or more different nanoscale functionalities have attracted much attention due to their novel combined properties and technological applications [1, 2]. Among them, iron oxide–Au (Fe3O4–Au, α/γ-Fe2O3–Au) nanocomposites are of great importance for their combined optical and magnetic properties and potential applications in the fields of biotechnologies and catalysts [3–8]. Up to now, many methods have been developed to synthesize various Fe3O4–Au and α/γ-Fe2O3–Au nanocomposites [9–19]. For example, Yu et al.  reported the synthesis of dumbbell-like Fe3O4–Au nanoparticles using decomposition of Fe(CO)5 on the surface of the Au nanoparticles followed by oxidation in 1-octadecene. Fe3O4–Au core–shell nanoparticles could be prepared with room-temperature coating of Au on the surface of Fe3O4 nanoparticles by reducing HAuCl4 in a chloroform solution of oleylamine . Wu et al.  prepared magnetic Fe3O4–Au nanoparticles by the controlling a combination of chemically tunable chelating layer modifications for magnetic core and further deposition of Au on the amine-functionalized Fe3O4 surface. Bao et al.  reported the synthesis of γ-Fe2O3–Au nanoparticles with different Au shell thickness by reducing HAuCl4 on the surface of γ-Fe2O3 nanoparticles. Moreover, the synthesis and transformation of 1D nanostructures and their hybrids are of particular interest due to their immense applications [20–22]. However, to the best of our knowledge, there is no report for the controllable synthesis of Fe2O3–Au and Fe3O4–Au hybrid 1D nanostructures.
Layer-by-layer technique is based on the electrostatic attraction between charge species, and it has been widely used to synthesize nanocomposites [23–28]. More recently, this technique has been realized to prepare hybrid 1D nanostructures [29–36]. Herein, we use layer-by-layer technique to synthesize uniform FeOOH–Au hybrid nanorods that can be controllably transformed into Fe2O3–Au and Fe3O4–Au hybrid nanorods. The magnetic and optical properties of as-synthesized Fe2O3–Au and Fe3O4–Au hybrid nanorods have been investigated.
Poly (sodium 4-styrenesulfonate) (PSS) and Poly (allylamine hydrochloride) (PAH) were purchased from Alfa Aesar Co. Ltd. All the chemicals were of analytical grade without further purification. First, FeOOH nanorods were prepared by a hydrothermal route described elsewhere . Second, the pristine FeOOH nanorods were modified by polyelectrolyte (PAH/PSS/PAH) in sequence via layer-by-layer assembly. Briefly, 10 mg FeOOH nanorods was sonicated for 1 h in 50 ml 1 M NaCl solution, and 80 mg PAH was added and stirred for 0.5 h. Subsequently, the excess PAH was removed by six repeated centrifugation/wash cycles. Similarly, the PSS and PAH layers were then coated on the surface of the PAH-modified FeOOH nanorods to obtain the PAH/PSS/PAH-modified FeOOH nanorods. Third, FeOOH–Au nanorods were fabricated by chemical reaction using HAuCl4, trisodium citrate, and NaBH4 as reactants on PAH/PSS/PAH modified FeOOH nanorod templates. The resulting solid products were centrifuged, washed with distilled water and ethanol to remove the ions possibly remaining in the final products, and finally dried at 80°C in air.
For the synthesis of α-Fe2O3–Au nanorods, the as-prepared FeOOH–Au nanorods were heated to 500°C for 3 h in air. While for the synthesis of Fe3O4–Au nanorods, the as-prepared FeOOH–Au nanorods were heated to 400°C for 3 h under H2/Ar (10% H2) atmosphere.
The obtained samples were characterized by X-ray powder diffraction (XRD) using a Rigaku D/max-ga X-ray diffractometer with graphite monochromatized Cu Kα radiation (γ = 1.54178 Å). The morphology and structure of the samples were examined by transmission electron microscopy (TEM, JEM-200 CX, 160 kV), field emission scanning electron microscopy (FESEM, Hitachi S-4800) and high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2010). The infrared (IR) spectra were measured with a Nicolet Nexus FTIR 670 spectrophotometer. Magnetization measurements were carried out using a physical property measurement system (PPMS-9, Quantum Design). The optical absorption of the products was examined by a Perkin–Elmer Lambda 20 UV/vis Spectrometer. BET surface area and pore volume were tested using Beckman coulter omnisorp 100cx.
Results and Discussion
FeOOH–Au hybrid nanorods have been synthesized via layer-by-layer assembly, which can be transformed into α-Fe2O3–Au and Fe3O4–Au hybrid nanorods by controllable annealing process. The strong electrostatic attraction between AuCl4− and polyelectrolyte-modified FeOOH nanorods plays the most important role in the uniform deposition of Au nanoparticles. The annealing atmosphere determines the phase transformation from FeOOH–Au to α-Fe2O3–Au and Fe3O4–Au. The as-synthesized Fe3O4–Au hybrid nanorods show the high saturation magnetizations, and α-Fe2O3–Au hybrid nanorods show the low saturation magnetizations, respectively. The UV–vis analysis indicates that both Fe3O4–Au and α-Fe2O3–Au hybrid nanorods show a broad peak located at about 525 nm. It is believed that the as-synthesized Fe3O4–Au and α-Fe2O3–Au hybrid nanorods can be applied in biotechnologies and catalysts, respectively.
The authors thank the Doctoral Science Foundation of Zhejiang Sci-Tech University (no. 0803611-Y) and National Natural Science Foundation of China (no. 50976106) for financial support.
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