Synthesis and Magnetic Properties of Maghemite (γ-Fe2O3) Short-Nanotubes
© The Author(s) 2010
Received: 26 March 2010
Accepted: 3 June 2010
Published: 17 June 2010
We report a rational synthesis of maghemite (γ-Fe2O3) short-nanotubes (SNTs) by a convenient hydrothermal method and subsequent annealing process. The structure, shape, and magnetic properties of the SNTs were investigated. Room-temperature and low-temperature magnetic measurements show that the as-fabricated γ-Fe2O3 SNTs are ferromagnetic, and its coercivity is nonzero when the temperature above blocking temperature (T B). The hysteresis loop was operated to show that the magnetic properties of γ-Fe2O3 SNTs are strongly influenced by the morphology of the crystal. The unique magnetic behaviors were interpreted by the competition of the demagnetization energy of quasi-one-dimensional nanostructures and the magnetocrystalline anisotropy energy of particles in SNTs.
KeywordsShort-nanotubes γ-Fe2O3 Magnetic properties
In recent years, the assembled nanostructures of magnetic iron oxide materials have attracted widespread interest because of their diverse applications, such as magnetic fluids, data storage, catalyst, and bionanotechnology [1–3]. One-dimensional (1D) nanostructures are very appealing, owing to many unique physical and chemical properties based on their high intrinsic anisotropy and surface activity [4, 5]. Especially, understanding the correlation between the magnetic properties and the morphology of nanostructures is a prerequisite for widespread applications of nanomagnetism in data storage and bioseparation areas . However, it is crucial to choose the materials for the construction of nanostructure materials and devices with adjustable physical and chemical properties. Among the various magnetic materials, the cubic spinel structured maghemite (γ-Fe2O3) represents an important class of magnetic transition metal oxide materials in which oxygen atoms form a fcc close-packed structure . Moreover, γ-Fe2O3 is an ideal candidate for fabrication of luminescent and magnetic dual functional nano-composites due to its excellent transparent properties [8–10].
The search for new geometries is an important aspect for magnetic iron oxide nanomaterials, and past research mainly has lead to structures such as nanoparticles, hollow nanoparticles [1, 11–13]. Generally, the lowest energy state of a magnetic particle depends on its size, shape, strength and character of its anisotropy, especially the shape of nanomaterials can influence its magnetic properties in different ways. Magnetic quantities such as anisotropy and coercivity are important for many present and future applications in permanent magnetism, magnetic recording, and spin electronics . More recently, the magnetic properties of nanoparticles, nanocages, nanowires, and nanochains have been reported [13, 15–18]. However, reports on the template-free synthesis and magnetic properties of γ-Fe2O3 SNTs are very scarce so far [8, 19, 20]. In the present work, we demonstrated an efficient and facile approach for large-scale synthesis of γ-Fe2O3 SNTs by hydrothermal and subsequent annealing process. The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) results showed that the obtained products were short-tubular structures. The room-temperature and low-temperature magnetic properties of these SNTs were investigated. The study of pure γ-Fe2O3 SNTs and their magnetic properties is a key issue, not only for practical applications but also for fundamental understanding.
At first, the starting materials were prepared by a hydrothermal treatment of iron (III) chloride with sulfate and phosphate additives. In a typical experimental procedure, 0.27 g FeCl3·6H2O, 7 mg NaH2PO4·2H2O, and 19.5 mg Na2SO4 aqueous solutions were mixed together and then double-distilled water was added to the mixture to keep the final volume at 25 mL. After ultrasonic dispersion, the mixture was transferred into a Teflon-lined stainless steel autoclave with a capacity of 30 mL for hydrothermal treatment at 220°C for 12 h. After the autoclave was allowed to cool to room temperature, the precipitate was separated by centrifugation, washed with double-distilled water, and dried under vacuum at 120°C. Then, as-obtained dried α-Fe2O3 powders were annealed in a tubular furnace at 300°C under a continuous hydrogen flow for 5 h. The furnace was allowed to cool to room temperature while still under a continuous hydrogen gas flow. Finally, the above sample was annealed at 400°C for 2 h in oxygen atmosphere with the heating rate of 5°C/min.
The morphologies and microstructures of as-synthesized samples were characterized by scanning electron microscopy (FEI Nova 400 NanoSEM), transmission electron microscopy (JEOL JEM-2010(HT)), and high-resolution transmission electron microscopy (JEOL JEM-2010 FET (UHR)). The operating voltages of the SEM and TEM were 25 and 200 kV, respectively. The crystal structure of the samples was determined by X-ray diffraction (XRD) (Cu Kα radiation, λ = 0.1542 nm). The Brunauer-Emmett-Teller (BET) surface area of the annealing samples was analyzed by nitrogen adsorption in a Micromeritics ASAP 2020 nitrogen adsorption apparatus. The composition of as-synthesized samples was measured by attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy (Nicolet iS10). Magnetic measurements were performed on a Quantum Design physical property measurement system (PPMS). The powder sample was filled in a diamagnetic plastic tube, and then the packed sample was put in a diamagnetic plastic straw and impacted into a minimal volume for magnetic measurements. Background magnetic measurements were checked for the packing material.
Results and Discussion
where the first term results from the contribution of shape anisotropy energy of SNTs and the second term is due to the contribution of magnetic crystalline anisotropy energy of small particles. In the Eq. 1, here the q is the geometric factor (for a prolate spheroid, q varies between the limits of 2.0816 for a sphere and 1.8412 for an infinite cylinder, and for an oblate sphere, q gradually increases from 2.0816 for a sphere to 2.115 for an infinite plate ), D is the average diameter of the SNTs, d is the small particle diameter, K 1 is the first-order magnetic anisotropy constant (4.6 kJ/m3 for γ-Fe2O3), A is the exchange stiffness constant (A = 10−11 J/m), p C is a coefficient of dimensionless quantity related to the crystal structure (P C ~ 0.5), and l ex is the exchange length According to Eq. 1, the coercivity was estimated and the values was about 82 Oe. This result indicates that the coercivity of γ-Fe2O3 SNTs was mainly originated from the small nanocrystallines. Moreover, taking account into that T B is defined as T B = K A V/25k B, where K A is the magnetic anisotropy constant, V is the magnetic core volume, and k B is the Boltzmann constant . The total magnetic core volume will decrease with the increase in applied field. Because the saturation magnetic flux density is small, such materials are easily magnetically saturated, thereby making it impossible to reduce their volumes. In other words, magnetic core volume is the most significant factor determining the inductance value, and the size and thickness reductions are difficult to be attained unless the magnetic properties of magnetic materials are improved .
The approach used in this study provides a simple and inexpensive method for the preparation of stable and magnetic γ-Fe2O3 SNTs. The as-synthesized SNTs are ferromagnetic at room temperature, which may have potential applications in biotechnology, biomedicine, and fundamental science. The results reveals that the self-assembly strategy is an efficient way to create novel nanostructured systems. Further detailed studies on the formation mechanism of the magnetic SNTs are currently under investigation.
The author thanks the National Basic Research Program of China (973 Program, No. 2009CB939704), the National Nature Science Foundation of China (No. 10775109, 10905043), the Specialized Research Fund for the Doctoral Program of Higher Education (No. 20070486069), Young Chenguang Project of Wuhan City (No. 200850731371, 201050231055), the Specialized Research Fund for the Young Teacher of Wuhan University(No. 1082010) and the PhD candidates self-research (including 1 + 4) program of Wuhan University in 2008 (No. 20082020201000008) for financial support.
This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.
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