Fabrication of shape controlled Fe3O4 nanostructure

M A TE RI A L S CH A RACT ER IZ A TI O N 61 ( 20 1 0 ) 4 8 9 –4 9 2

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Short communication

Fabrication of shape controlled Fe3O4 nanostructure Y.Y. Zheng, X.B. Wang, L. Shang, C.R. Li⁎, C. Cui, W.J. Dong⁎, W.H. Tang, B.Y. Chen Key Laboratory of Advanced Textile Materials and Manufacturing Technology, Ministry of Education, Department of Physics, Center for Optoelectronics Materials and Devices, College of Science, and Nanometer Measurement Lab, Zhejiang Sci-Tech University, Hangzhou 310018, People's Republic of China

AR TIC LE D ATA

ABSTR ACT

Article history:

Shape-controlled Fe3O4 nanostructure has been successfully prepared using polyethylene

Received 15 September 2009

glycol as template in a water system at room temperature. Different morphologies of Fe3O4

Received in revised form

nanostructures, including spherical, cubic, rod-like, and dendritic nanostructure, were

16 January 2010

obtained by carefully controlling the concentration of the Fe3+, Fe2+, and the molecular weight

Accepted 18 January 2010

of the polyethylene glycol. Transmission Electron Microscope images, X-ray powder diffraction patterns and magnetic properties were used to characterize the final product.

Keywords:

This easy procedure for Fe3O4 nanostructure fabrication offers the possibility of a generalized

Fe3O4

approach to the production of single and complex nanocrystalline oxide with tunable

Nanostructure

morphology.

Morphology

1.

Introduction

Due to their unique electronic, optical, magnetic and physical/ chemical properties, application of nanomaterials with different morphologies comprises a significant aspect of today's endeavors in nanotechnology [1]. To exploit and optimize the advantages of nanostructures, a number of researchers have focused their attention on controlling the morphology of nanostructures through specific synthesis techniques [2,3]. Although several techniques have been developed for the synthesis of nanoscale compounds, morphology control is quite difficult because little is quantitatively known about crystallization [4]. Now, much effort is devoted to exploring synthesis methods and controlling morphology [2,3,5]. Magnetic nanostructure, in particular, has shown interesting application potentials in semiconducting, information storage, color imaging, bio-nanomedicine, magnetic refrigeration, gas sensors, and ferrofluids [6–10]. Among the variety of magnetic materials, shape-controlled Fe3O4 nanostructures

© 2010 Elsevier Inc. All rights reserved.

have been widely used due to their strong size and shape dependent properties [11]. Many methods for the synthesis of Fe3O4 nanostructure have been developed [12]. For instance, Si et al. reported a coprecipitation method to synthesize iron oxide nanoparticles [13]. Fried and Sun developed a hydrothermal reaction of Fe3+ in the presence of a weak reducing agent and sonochemical decomposition of hydrolyzed Fe2+ salt respectively, each forming Fe3O4 [14,15]. Wang and Xin presented a γ-irradiation-induced chemical change from β-FeOOH to Fe3O4 [16]. Further, organic solution phase decomposition routes and sol– gel methods are widely used in the synthesis of iron oxide nanoparticles and magnetic nanoparticles [17]. In all of these methods, relatively high temperatures, special conditions, long reaction time or tedious procedures are required. Furthermore, the obtained products are often comprised of a single morphology. In this paper, we demonstrate a simple route for the preparation of shape-controlled Fe3O4 nanostructures with a variety of morphologies, including spherical,

⁎ Corresponding authors. Tel.: +86 571 8684575; fax: +86 571 86843575. E-mail addresses: [email protected] (C.R. Li), [email protected] (W.J. Dong). 1044-5803/$ – see front matter © 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2010.01.008

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cubic, rod-like, and dendritic nanostructure, by carefully controlling the reaction concentrations with polyethylene glycol (PEG) as a template at room temperature. This method requires no complex apparatus or techniques, and the synthesis time is very short.

2.

Experimental

2.1.

Materials

The starting chemical reagents used in this work were: iron(II) nitrate (Fe(NO3)2·4H2O), iron(III) nitrate (Fe(NO3)3·9H2O), ammonium nitrate (NH4NO3), ammonium hydroxide solution (28.0–30.0%, NH3·H2O) and polyethylene glycol, all of which were analytical grade and without further purification.

2.2.

Experimental Procedure

In a typical experiment, appropriate amounts of analytically pure Fe(NO3)2·4H2O, Fe(NO3)3·9H2O, 4.04 g NH4NO3 and PEG were dissolved into 100 mL distilled water. To prevent some oxidative reactions, the system was kept under nitrogen atmosphere. After the solution had been bubbled with nitrogen for 30 min, 0.1 mol/L NH3·H2O was introduced slowly into the system to adjust the pH above 11. Then the system was continuously bubbled with nitrogen for 20 min to remove oxygen. Then the black precipitates were formed. The black precipitates were collected from the solution by centrifugalization, washed with distilled water several times, and dried in air at room temperature.

2.3.

Characterization

The morphology and size of the Fe3O4 nanostructures were observed on a Hitachi-8100IV Transmission Electron Microscope (TEM), which allows the direct imaging of nanostructures and provides information on the quality of individual particles. Xray powder diffraction (XRD) patterns employing a scanning rate of 0.02°/s in 2θ range from 20° to 70° were performed on a Siemens D5005 X-ray diffractometer with CuKα (λ = 1.5418 Å) at

room temperature. The magnetic properties were analyzed with a Quantum Design MPMS-XL SQIUD magnetometer.

3.

Results and Discussion

Under proper condition, the magnetite can be obtained by reaction of the Fe3+ and Fe2+ salts. In this experiment, the molar ratio of iron to PEG 20,000 and the concentration of iron are very important factors affecting the morphology of the Fe3O4 nanostructure. The concentration of PEG 20,000 was fixed to 1.0 × 10− 3 mol/L. When the concentration of Fe3+ is 0.2 mol/L and Fe2+ is 0.1 mol/L, 30-nm Fe3O4 nanocubes were obtained as shown in Fig. 1a. When the concentration of Fe3+ is 2.0 × 10− 2 mol/L and Fe2+ is 1.0 × 10− 2 mol/L, Fe3O4 nanorods with a diameter of about 10 nm and a length ranging from 100 to 200 nm can be obtained (Fig. 1b). Fig. 1c shows the TEM image of the product obtained with the Fe 3+ concentration 2.0 × 10− 3 mol/L and Fe2+ concentration 1.0 × 10− 3 mol/L. This image shows clusters of Fe3O4 dendrites. With careful observation, it can be found that the branch was composed of small nanorods. As mentioned above, the concentrations of iron led to a dramatic change in the morphology of the nanostructure. At high concentrations of iron, a high nucleation rate competes favorably with a lower growth rate of the already formed particles, allowing a large amount of nucleation to take place before suitable environmental conditions allow the nuclei to grow, forming cubic nanostructures. When the concentration of the iron is low, rod-like Fe3O4 nanostructures form. With further decreased iron concentration, dendritic Fe3O4 nanostructures are obtained. The PEG also plays an important role in the morphology of the Fe3O4 nanostructures. To investigate the effects of PEG on the morphology of Fe3O4 nanostructures, different molecular weights of PEG were studied when the Fe3+ concentration was 2.0 × 10− 2 and the Fe2+ concentration was 1.0 × 10− 2 mol/L. Fig. 2a shows the Fe3O4 nanostructure produced with these Fe2+ and Fe3+ concentrations and 1.0 × 10− 3 mol/L PEG 400. Nanoparticles with a 15 nm diameter were obtained, but these particles were often aggregated into large clusters. Fig. 2b

Fig. 1 – TEM images of Fe3O4 nanostructures at different concentrations of Fe3+ and Fe2+ solutions:(a) Fe3+: 0.2, Fe2+: 0.1 mol/L, (b) Fe3+: 2.0 × 10− 2, Fe+ 2:1.0 × 10− 2 mol/L, (c) Fe3+: 2.0 × 10− 3, Fe2+:1.0 × 10− 3 mol/L.

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Fig. 2 – TEM images of Fe3O4 nanostructures for different molecular weights of PEG: (a) 400, (b) 10,000, and (c)100,000.

shows the Fe3O4 nanorods derived from 1.0 × 10− 3 mol/L PEG 10,000 solution. Nanorods with 20 nm diameter and 150 nm average lengths were obtained. When 1.0 × 10− 3 mol/L PEG 100,000 was used in this system, nanowires of several hundred nanometers in length can be obtained from the solution (as shown in Fig. 2c). So PEG plays an important role in the formation of Fe3O4 nanostructures. It has been proposed that oxygen in the PEG can interact with metal ions, and the nanostructure can grow along the chain [18]. This means the PEG produces a self-assembly of ordered nanostructures in the solution. It was confirmed that different shaped nanostructures formed when the molecular weight of PEG increased. When the molecular weight of PEG increased from 400 to 10,000 and then to 100,000, the morphology of the nanostructure will change from nanoparticles to nanorods and then to nanowires (the size and shape of Fe3O4 under different experimental conditions are listed in Table 1).

XRD patterns of the Fe3O4 nanostructures demonstrate cubic structure by the characteristic peaks [2θ = 30.2° (220), 35.5°(311), 43.2° (400), 54.5° (422), 57.1° (511), 62.7° (440), (JCPDS card No.: 880315)]. Fig. 3a shows the XRD pattern of the 30-nm cubic Fe3O4 nanostructure. The XRD patterns confirmed the nanorods and nanowires are also pure Fe3O4 phase as shown in Fig. 3b and c. NH3·H2O also plays a key role in the formation of Fe3O4 nanostructure. In this system, the NH3·H2O acts as a pH buffer and reacts with water to provide the system with a slow and constant supply of OH− anions [19]. When the NH3·H2O was replaced by NaOH during the synthesis, only irregularly shaped Fe3O4 nanoparticles are found. The slow supply of a low content of OH− anions could be a main driving force for the formation of the Fe3O4 nanostructures. The formation of Fe3O4 is therefore believed to proceed via the following steps: [20] − NH3 H2 O→NHþ 4 þ OH

ð1Þ

Fe2þ þ 2OH− →FeðOHÞ2

ð2Þ

Fe3þ þ 3OH− →FeðOHÞ3

ð3Þ

Table 1 – The size, shape or phase of Fe3O4 under different experimental conditions. Reagent concentrations (mol/L)

PEG molecular weight and concentration (mol/L)

Morphology

−2 C3+ Fe = 2.0 × 10 −2 C2+ Fe = 1.0 × 10

CPEG 100000 =1.0×10− 3 Nanofiber

C3+ Fe = 0.2 mol/L C2+ Fe = 0.1 mol/L −2 C3+ Fe = 2.0 × 10 −2 = 1.0 × 10 C2+ Fe

CPEG 20000 =1.0×10− 3

Nanocube

CPEG 20000 = 1.0 × 10− 3 Nanorod

−3 C3+ Fe = 2.0 × 10 −3 C2+ = 1.0 × 10 Fe −2 C3+ Fe = 2.0 × 10 2+ CFe = 1.0 × 10− 2

CPEG 20000 = 1.0 × 10− 3 Branchlike fractals CPEG 10000 = 1.0 × 10− 3 Nanorod

−2 C3+ Fe = 2.0 × 10 −2 = 1.0 × 10 C2+ Fe

CPEG 400 = 1.0 × 10− 3

Nanoparticle

Size

Several hundred nm in lenghth 20–30 nm 10 nm in diameter 100–200 nm in length – 20 nm in diameter 150 nm in length 15 nm in diameter

Fig. 3 – X-ray powder diffraction patterns of Fe3O4 nanostructures: (a) as-prepared nanocubes, (b) nanorods, and (c) nanowires.

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the support of the Scientific Research Foundation of Zhejiang Province Office of Education.

REFERENCES

Fig. 4 – Hysteresis loops of the 30-nm Fe3O4 nanostructures measured at room temperature.

FeðOHÞ2 þ 2FeðOHÞ3 →Fe3 O4 þ 4H2 O

ð4Þ

The reaction will not proceed unless NH3·H2O is present, and this parameter provided an opportunity to control the reaction products. It can provide a uniformity of the solution to give a good environment for the growth of high quality Fe3O4 nanostructures. Furthermore, it causes a competition between the precipitation and shape-controlled nanostructures reactions, in which the nucleation, and thus the growth of Fe3O4 could be adjusted. The use of NH3·H2O not only can adjust the pH but also can provide a relative “clean” reaction environment, which will increase the formation opportunity for different shape Fe3O4 nanostructures with PEG present. The magnetic properties of Fe3O4 nanostructures were investigated on a Quantum Design MPMS-XL SQIUD magnetometer. The hysteresis loop of 30-nm Fe3O4 nanostructures was measured at 300 K, as shown in Fig. 4, and no pronounced hysteresis loop was observed. The zero remanence and coercivity indicated the superparamagnetic behavior of the Fe3O4 nanostructures, which was consistent with the result reported in the literature [21,22].

4.

Conclusion

In summary, Fe3O4 nanostructures have been prepared via a simple technique at room temperature using PEG as a template. It was found that the concentrations of Fe2+ and Fe3+, PEG molecular weight, and NH3·H2O play a significant role in the formation and growth of the Fe3O4 nanostructures. These unusual nanostructures may have important applications in excellent magnetic materials. This method may be extended to prepare novel nanostructures of other systems.

Acknowledgements This work was supported by the National Natural Science Foundation of China (Nos. 20701033, 10874153, 60806045); SRF for ROCS, SEM; China Postdoctoral Science Foundation funded project (No. 20090450174); and the Science Foundation of Zhejiang Sci-Tech University (No. 0713675-Y). L.S is grateful to

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