# Preparation of Magnetically Driven Nickel Phosphide Nanowires and Their Electrochemical Properties
## 3 Results and Discussion  
### Effect of Magnetic Field Strength on the Composition and Structure of Nickel Phosphide  
The difference in the composition and structure of the nickel phosphides according to the magnetic field strength was analyzed (Samples 0L, 2L, 18L in Table 1). Figure 1a-c show the electron micrographs of nickel phosphides obtained at magnetic fields of 0, 2, and 18 mT, respectively (hereafter, each specimen is denoted as sample 0L, sample 2L, or sample 18L, where the numbers represent the magnetic field strength and the letter L indicates a low phosphorus content). In the absence of a magnetic field, the agglomeration between particles was very irregular, and the overall structure was severely bent (sample 0L, Figure 1a). However, when a magnetic field was applied, the irregular and curved growth pattern changed to a one-dimensional growth pattern, even with a relatively low magnetic field (sample 2L, Figure 1b). Nanowires with highly uniform diameters and high aspect ratios grew under a high magnetic field (sample 18L, Figure 1c). It is considered that the wire growth of nickel phosphides in a magnetic field occurs due to the ferromagnetic properties of nickel. Nickel phosphide nanoparticles containing a large amount of nickel are arranged in a line along the magnetic flux under a magnetic field and combine to grow into nanowires. Because of the natural convection effect of the solution during synthesis, the stronger the magnetic field, the better the definition of the nanowire formed; this is consistent with previous literatures on the synthesis of pure nickel nanowires in a magnetic field [21; 22].  
Figure 2a-c show the composition analysis results for samples 0L, 2L, and 18L, respectively. The phosphorus content in the three specimens was approximately 7-8% when the effect of oxygen resulting from surface oxidation caused by atmospheric exposure of the specimen after synthesis was eliminated. This implies that the magnetic field has virtually no effect on the composition. In addition, only broad peaks arising from the presence of Ni appear in the X-ray diffraction pattern, suggesting that the synthesized nickel phosphides are amorphous or low-crystalline materials. It is known that when the phosphorus content is relatively low, the crystallinity of nickel phosphides is sensitive to the phosphorus content. According to literature on the synthesis of nickel phosphides by electroless plating [25; 26], nickel phosphides are crystalline when the phosphorus content is very low, but they become increasingly amorphous as the phosphorus content is increased. When the phosphorus content is 10 wt% or more, they are completely amorphous. Thus,  
Figure 1: Scanning electron micrographs of the nickel phosphide nanowires prepared at different values of magnetic field strength: (**a**) sample 0L, (**b**) sample 2L, and (**c**) sample 18L in Table 1. Inset figures are the magnified images.  
the composition and structure analysis results for the nickel phosphides synthesized in this study are consistent with those in existing literature.