# Preparation of Magnetically Driven Nickel Phosphide Nanowires and Their Electrochemical Properties
## 3 Results and Discussion
### Effect of pH and Reducing Agent on the Composition of Nickel Phosphide  
Nickel phosphide is typically synthesized according to the following chemical reactions [27; 28].  
\[\left(\mathrm{NiR_{n}}\right)^{2+}+\mathrm{H_{2}PO_{2}}^{-}+3\mathrm{OH^{-}} \rightarrow\mathrm{Ni}+\mathrm{HPO_{3}}^{2-}+2\mathrm{H_{2}O}+\mathrm{nR} \tag{3}\]  
\[\mathrm{H_{2}PO_{2}}^{-}+2\mathrm{H^{+}}+\mathrm{e^{-}}\rightarrow\mathrm{P}+ 2\mathrm{H_{2}O} \tag{4}\]  
Here, R denotes a salt (generally succinate or pyrophosphate). A complex of a salt and nickel (NiR)\({}^{2+}\) is reduced by \(\mathrm{H_{2}PO_{2}}^{-}\) derived from a reducing agent (e.g., sodium hypophosphite). The concentrations of the Ni source and reducing agent and the pH directly affect the phosphorus and nickel contents in the product after chemical reduction.  
Figure 2: EDS spectra together with chemical composition ratios of the nickel phosphide nanowires prepared at different values of magnetic field strength: (**a**) sample 0L, (**b**) sample 2L, and (**c**) sample 18L from Table 1; and (**d**) their XRD patterns.  
As shown in reaction Equations (3) and (4), the phosphorus and nickel contents in nickel phosphide are directly affected by the pH and reducing agent concentration during chemical reduction. When the pH is increased, reaction (3) acts to increase the nickel content and decrease the phosphorus content, and reaction (4) is suppressed. On the other hand, the phosphorus content increases when the reducing agent concentration increases because the reducing agent is a phosphorus precursor [27]. In this study, we tried to obtain nickel phosphides with three different nickel:phosphorus composition ratios using a combination of pH levels and reducing agent concentrations (Samples 18L, 18M, and 18H in Table 1. Sample 18L is the same as sample 18L in Section 3.1. Here, M denotes a medium phosphorus content, and H denotes a high phosphorus content). The magnetic field strength was fixed at 18 mT, the value at which the nanowires were best formed in Section 3.1.  
The shapes of the synthesized nickel phosphide nanowires are shown in Figure 1c (sample 18L) and Figure 3a,b (samples 18M and 18H, respectively), and the contents of nickel and phosphorus are summarized in Figure 3c. With regard to the composition, the lower the pH and higher the reducing agent concentration, the higher the proportion of phosphorus in the nickel phosphide, as discussed above. In addition, with an increase in the proportion of phosphorus, the nickel phosphide particle sizes became non-uniform, particle aggregation around the nanowire became apparent, and the aspect ratio of the nanowires decreased. It is quite likely that the changes in the nanowire shape according to the phosphorus content are related to the magnetization of nickel phosphides. The magnetization of nickel phosphides in a fixed magnetic field decreased as the phosphorus content increased or nickel content decreased, resulting in non-uniform nanowires and their small aspect ratio. Based on the structure analysis (Figure 3d), no clear crystallinity was observed in all specimens.  
Figure 3: Scanning electron micrographs of the nickel phosphide nanowires prepared at different combinations of pH and reducing agent content: (**a**) sample 18M and (**b**) sample 18H in Table 1. Inset figures are the magnified images. (**c**) Phosphorus and nickel contents in the samples obtained from the EDS analysis and (**d**) their XRD patterns.