Template-free Synthesis of One-dimensional Cobalt Nanostructures by Hydrazine Reduction Route
© Zhang et al. 2010
Received: 8 September 2010
Accepted: 14 September 2010
Published: 1 October 2010
One-dimensional cobalt nanostructures with large aspect ratio up to 450 have been prepared via a template-free hydrazine reduction route under external magnetic field assistance. The morphology and properties of cobalt nanostructures were characterized by scanning electron microscopy, X-ray diffractometer, and vibrating sample magnetometer. The roles of the reaction conditions such as temperature, concentration, and pH value on morphology and magnetic properties of fabricated Co nanostructures were investigated. This work presents a simple, low-cost, environment-friendly, and large-scale production approach to fabricate one-dimensional magnetic Co materials. The resulting materials may have potential applications in nanodevice, catalytic agent, and magnetic recording.
KeywordsCobalt nanowires Magnetic field assistance Hydration hydrazine reduction
In recent years, nanostructure materials have been actively studied due to their novel properties and potential applications. Among them, much attention has been focused on one-dimensional (1D) magnetic materials such as Fe, Co, and Ni due to their potential applications in nanodevice, biosensor, and magnetic recording [1–3]. 1D cobalt nanostructures with uniform shape and high purity have become increasingly required for specific uses in many areas, such as in high-density information storage, magnetic sensors, commercial batteries, and catalysts.
Various approaches have been developed to synthesize cobalt nanostructures [4–6]. For example, Puntes et al. prepared Co nanodisks by rapid decomposition of carbonyl cobalt in the presence of trioctylphosphane (TOP) and oleic acid (OA) . Legrand et al. applied a physical method to synthesize 3D supra-organization of Co nanocrystals . The most common method to fabricate 1D Co nanostructures is based on porous anodic aluminum oxide (AAO) templates. Li et al. prepared Co nanowire arrays in alumina arrays by using a chemical electrodeposition method . Other templates such as polyaniline or polycarbonate membranes, diblock copolymer, and mesoporous silica have also been applied to fabricate magnetic Co nanowires [10–12]. In view of the complexity of multi-step template preparation and low production, it is imperative to develop simpler template-free methods for the fabrication of magnetic Co nanowires.
In recent years, our research group has reported a chemical solution reduction approach for fabricating 1D Ni nanostructures assisted by magnetic fields [13–15]. This may be a more promising method to prepare cobalt nanostructures in terms of its low cost and potential for large-scale production. In this study, we fabricated Co nanowires with large aspect ratio under normal pressure in absence of any templates or surfactants. The influence of reaction temperature, Co ion concentration and magnetic field intensity on the formation and morphology of Co nanowires were investigated. The magnetic properties of Co nanostructures with different morphology were also evaluated.
After the beaker cooled down to room temperature, the gray solid product floating on the beaker was collected by using a magnet. Then it was washed with distilled water and absolute ethanol several times and finally dried in a vacuum oven at 60°C for 24 h.
The size and morphology analyses were performed using a field emission scanning electron microscope (SEM, Ultra55, Zeiss). The crystal structure was characterized by an X-ray polycrystalline diffractometer (XRD, D8 Advance, Bruker) using Cu Ka radiation (λ = 1.54056 nm) with graphite monochromator. The hysteresis properties were measured on a vibration sample magnetometer (VSM, Lake Shore 7400).
Result and Discussion
The possible mechanism for the formation of Co nanowires under applied magnetic field may be expressed as following: At first, Co ions were reduced by strong reduction agent of hydrazine hydrate and turned to tiny spherical particles. Then the magnetic Co particles aligned along the magnetic field direction to form one-dimensional nanostructures under the magnetic driving force. The cobalt nanowires retained their linear structure after kept in ultrasonic bath for 10 min, which proved that the nanowires displayed a good mechanical strength.
A simple, low-cost, environment-friendly approach of preparation magnetic Co nanowires was developed. In this method, the nanowires were fabricated in an ethylene glycol solution at normal pressure by assistant of magnetic field without any templates or surfactants. The prompt wires have average length of up to 350 μm and aspect ratio of up to 450. It was found that the nanowires are elongated with the increasing intensity of magnetic field, and there is no wires formed in absence of magnetic field. The reaction temperature and Co ion concentration have also strong influences on formation and morphology of nanowires. This method provides a new approach to fabricate magnetic nanowires under normal pressure and may be the most promising candidate to produce large-scale magnetic nanowires, which broads their practical applications.
This research was supported by the Hi-Tech Research and Development Program of China No. 2007AA03Z300, Shanghai-Applied Materials Research and Development fund No. 07SA10, National Natural Science Foundation of China (No. 50730008, 50902092), Shanghai Science and Technology Grant (No. 0752 nm015, 1052 nm06800), National Basic Research Program of China No. 2006CB300406, and the fund of Defence Key Laboratory of Nano/Micro Fabrication Technology.
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