- Nano Express
- Open Access
Synthesis of High-Aspect-Ratio Nickel Nanowires by Dropping Method
© Zhang et al. 2016
- Received: 24 December 2015
- Accepted: 9 February 2016
- Published: 1 March 2016
A facile and high-yield route, dropping method, has been used to synthesize Ni nanowires (NWs) with a high aspect ratio. Compared to the conventional chemical reduction method, the diameter of Ni NWs prepared by the dropping method distinctively decreased and the surface roughness was improved. After optimizing the process parameters such as the Ni ion concentration and volume of the dropped NiCl2·6H2O solution, the diameter and aspect ratio of the NWs are 70 nm and ~600, respectively. The possible synthesized process of the dropping method was discussed. This work presents a preferred approach to fabricate high-quality one-dimensional magnetic materials which have potential applications in electrochemical devices, magnetic sensors, and catalytic agents.
- Ni nanowires
- Dropping method
- Diameter and surface roughness
- Nanocrystalline materials
- Magnetic materials
Over the past decades, controlled growth of nanostructure magnetic materials has been attracting considerable attention owing to their size- or shape-dependent properties [1, 2]. As a type of important magnetic material, nickel nanowires (NWs) possess unique magnetic and catalytic properties, and it has been synthesized and studied because of their potential applications in catalytic , microwave-absorbing materials , sensors , and memory devices . To improve the performance of the materials and achieve its full potential in these applications, the preparation of Ni NWs with well-controlled size and surface roughness is critical.
The general methods for preparing Ni NWs usually require surfactants or additional template assistance, which are efficient in controlling the crystal growth and size of NWs [7, 8]. Unfortunately, these methods possibly lead to complicated fabrication processes, low cost efficiency, and low yield, which further limit the potential application. In recent years, synthesis of NWs by chemical reduction route is the preferred approach as it is low-costing, environmentally friendly, safe, and feasible for large-scale production. Liu, et al.  synthesized nickel NWs by assembly of small nanoparticles. Tang et al.  prepared the Ni NWs with chain morphology using the microwave-assisted method. However, to control the size and morphology of NWs synthesized by this method is still a challenge; it therefore becomes an interesting research field to precisely control the size and shape of these materials.
In this work, we propose a modified chemical reduction process, dropping method, to synthesize Ni NWs. In this method, the diameter of Ni NWs was effectively controlled, and the aspect ratio of NWs has been improved under a very low magnetic field (~0.05 T). Then, a possible synthesized process has been described. This method may further shed light on the fabrication approach on magnetic NWs with high quality.
During the synthesis process of Ni NWs by dropping method, 1.2 g sodium hydroxide (NaOH) was dissolved in 35 ml ethylene glycol (EG) under continuous magnetic stirring for 1 h. And then, 10 ml hydrazine hydrate (N2H4·H2O, 80 wt%) solution as a reducing agent was added into the NaOH and EG mixed solution with constant stirring to obtain a homogeneous solution. The as-prepared solution was placed in a magnetic field and heated to 80 °C at atmospheric pressure. The strength of the magnetic field in the container was 0.05 T generated by a NdFeB magnet. Then, 5 ml EG solution of nickel chloride hexahydrate (NiCl2·6H2O, 0.1 M) was added dropwise into the abovementioned solution. After about 10 min, a black fluffy solid product was formed and adsorbed on the inner surface of the container. The products were collected using a magnetic field and washed repeatedly with distilled water and ethanol and then dried at 60 °C for 12 h. The NWs prepared by the dropping method are named as DM-NWs. For comparison, Ni NWs were prepared by a conventional chemical reduction method . In this method, a homogeneous blue solution was obtained by fully mixing all the abovementioned solutions alternately and the Ni NWs were synthesized with the same conditions of the dropping method. The NWs synthesized by the conventional method are named as CM-NWs. The size and surface morphology analyses were performed using a field emission scanning electron microscope (SEM, Hitachi SU8010, Japan). The phase structure and composition of NWs were characterized by an X-ray polycrystalline diffractometer (XRD, D8 Advance, Bruker) using Cu Kα radiation with a graphite monochromator. The 2θ was scanned over the wide-angle range of 20°–90° at a rate of 1°/min to identify the structure. The detailed microstructure of the samples was studied by a transmission electron microscopy (TEM, JEM-2100, Japan).
In the dropping method, because the density of the NiCl2·6H2O solution is larger than that of the reduction agent, the droplet was diffusing and falling in the reduction agent which is different from that of the conventional method. In Fig. 4c, the drop-shaped area bounded by the dotted line indicates the diffusion region of the NiCl2·6H2O solution in the reduction agent, and the diffusion region increased with the reaction time increasing. Therefore, Ni reduction reaction will follow the dropping process of droplet; the Ni-induced nanoparticles are a graded distribution. In addition, owing to the little Ni content in droplet, there are not enough nanoparticles to increase the diameter of NWs during the formation process, i.e., the diameter of DM-NWs is smaller, and the NWs have a smooth surface compared to the CM-NWs as shown in Fig. 2a, b. Meanwhile, inasmuch as the Ni nanoparticles were distributed in a small area, the magnetic induction line will concentrate upon this area, which will enlarge the length of NWs (shown in Fig. 4d).
In summary, a facile, low-cost, environment-friendly improved approach, dropping method, to synthesize Ni NWs with a high aspect ratio has been developed. By optimizing the preparation conditions, the diameter of NWs decreased to 70 nm using this method. Compared to the conventional method, it is indicated that the dropping method is an effective technique to tune the diameter of Ni NWs. Based on the investigation of the structures of NWs and the influence of synthesis parameters, the difference of the synthesized process between DM-NWs and CM-NWs has been discussed. This improved synthesis method shows great potential value to synthesized high-quality NWs which could be applied in the fields of catalytic, magnetic, and electric sensors.
This work was supported by the National Nature Science Foundation of Beijing (4142047), the Beijing Higher Education Young Elite Teacher Project (YETP0684), and the Excellent Young Teachers Program of China University of Petroleum (ZX20150108).
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