Controlled Synthesis, Characterization, and Crystallization of Ni−P Nanospheres

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Abstract
The size- and composition-controlled synthesis of Ni−P nanospheres from nickel chloride and sodium hypophosphite has been systematically investigated by changing the conditions, such as the ratio of the starting materials, pH value, and reduction temperature. It was found that when the starting ratio of H2PO2-/Ni2+ was changed the size and chemical composition of the nanoparticles changed simultaneously. Within a suitable pH range, the phosphorus content was altered without affecting the particle size. Increasing the reduction temperature resulted in smaller Ni−P nanospheres but invariable phosphorus content. The Ni−P nanospheres were amorphous when the phosphorus content was higher than 10.0 mol %, while lower phosphorus content led to a composite of amorphous Ni−P and face-centered cubic (fcc) Ni. During postsynthesis calcinations, amorphous Ni−P nanospheres with a low phosphorus content directly crystallized to Ni3P and fcc Ni. However, the specimens with high phosphorus content crystallized via some intermediate phases such as Ni5P2 and Ni12P5. In the latter, an amorphous P-rich shell was developed simultaneously. A preliminary catalytic test of growth of carbon nanofibers on the Ni−P nanospheres has been carried out.