Figure shows the SEM images of resulting products prepared under different reaction temperatures of 40, 60, and 80°C, in which Co ion concentration was 0.01 M and the magnetic field of 0.4 T. It is apparent that the mean diameter of the wires prepared at higher temperature is much less than at lower temperature, whereas the aspect ratio of the wires increased with the temperature firstly and reached its highest value of about 450 at 60°C and then decreased with increased temperature. The diameter is about 4 μm at 40°C (see Figure ) and 800 nm at 60 and 80°C (see Figure ), respectively. The wires fabricated at 60°C are much smoother, longer, and more uniform. This may be derived from the thermodynamic influence on the nucleation velocity and the subsequent fast growth of the crystals, which results in smaller size of the Co particles [16
]. When the reaction temperature is sufficiently high up to 80°C, the average length of the corresponding Co nanowires appears to decrease from 350 to 200 μm as a result of the influence of thermal kinetics interaction. Consequently, we choose temperature of 60°C as the optimal reaction temperature for the preparation of Co wires.
SEM images with the same magnification times of Co nanostructures prepared at different reaction temperatures: a 40°C; b 60°C; c 80°C.
The influence of Co ion concentration on morphology of Co wires has also been investigated. Figure shows the SEM images of products with different Co ion concentration under the same external magnetic field of 0.4 T. It can be seen that the Co ion concentration has a strong influence on the morphology of products. The average diameter of the wires increases with increased Co ions concentration. It is about 800 nm for concentration of 0.01 M (see Figure ) and about 1.5 μm for 0.1 M (see Figure ), whereas the average length decreases from 350 to 50 μm. When the concentration reached 0.5 M, the corresponding products tend to further agglomerate and grow thicker, which results in the potato-like structure with the average diameter of 2 μm (see Figure ). The possible reason is that the primary Co crystallites tend to aggregate together to form spherical particles to decrease the surface energy when the concentration increased.
SEM images of products prepared in different Co ion concentrations under a 0.40 T magnetic field: a 0.01 mol/l; b 0.1 mol/l; c 0.5 mol/l.
In order to investigate the effect of the external magnetic field on the morphology of the resulting products, magnetic fields with different intensity were applied, where the Co ion concentration and the reaction temperature were 0.01 M and 60°C, respectively. As shown in Figure , only some bulky particles were observed in the absence of the external magnetic field. Under a low magnetic field of 0.15 T, some short, unsmooth, and thick wires were obtained. The average length and diameter are respectively about 200 and 1.5 μm (see Figure ). The surface of the wires was irregular, at which lots of Co particles are not aligned along its magnetic anisotropy direction in order to reduce the surface energy. When the intensity of the external magnetic field was increased to 0.40 T, the corresponding Co nanowires appeared to be significantly elongated. The orientation of the Co nanoparticles is further promoted, which results in some parallel self-assembled arrays of Co nanowires. The wires have aspect ratio of about 450 with average diameter of 800 nm and length up to 350 μm (see Figure ). It can be concluded that the magnetic field had played a very important role in forming 1D nanostructure, and therefore the length of the wires can be easily controlled by adjusting the intensity of the external magnetic field.
SEM images of samples prepared under different magnetic fields: a 0 T; b 0.15 T; c 0.40 T.
The XRD pattern of the Co nanowires prepared at 0.01 M concentration under an external magnetic field of 0.40 T is shown in Figure . All the diffraction peaks can be well indexed to hexagonal-phase cobalt, with lattice constants of a = 2.492 Å and c = 4.025 Å, which is well consistent with the standard card (JCPDS 89-4308, P63/mmc, a = 2.505 Å, c = 4.089 Å). Compared with the neighboring peak (101), the relative intensity of peak (002) in patterns increases significantly, which indicates the oriented growth of cobalt crystallites. There is no other impurity observed, suggesting the prepared Co nanowires have high purity.
XRD pattern of Co nanowires prepared at 60°C under a 0.40 T magnetic field.
Figure displays the hysteresis loops of the samples prepared without an external magnetic field (Zero-field), with a low external magnetic field of 0.15 T (0.15 T-field) and with an external magnetic field of 0.40 T (0.40 T-field), respectively. The saturation magnetization (MS
) of the samples is respective 127, 138, and 160 emu/g, which are all smaller than that of bulk Co materials (168 emu/g). The reduced magnetizations result from the surface effect of Co nanostructures, in which lots of Co atoms of the surface are not aligned along its magnetic anisotropy direction in order to reduce the surface energy. Therefore, the magnetic moments of these Co atoms cannot be aligned along the magnetic field due to the strong exchange interaction. On the other hand, the Zero-field sample had coercivity (HC
) value of 103 Oe, whereas the 0.15 T-field sample was 84 Oe and 65 Oe for 0.40 T-field one. These values are much lower than that of bulk cobalt material of 1500 Oe. These differences may be attributed to the different magnetic anisotropy manner [18
]. The magnetocrystalline anisotropy and shape anisotropy are two main anisotropy energies existed in magnetic materials, which can induce different coercivity. These two anisotropies cause the nanowires to exhibit a vortex-like magnetization distribution so that the moment directions can easily turn parallel to the external magnetic field, which leads to the reduction in the coercivity of the products.
Hysteresis loops of Co nanostructures measured at different intensity of applied field: Zero-field, 0.15 T-field, and 0.40 T-field.
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.