Structural study on Co-Ni bimetallic nanoparticles by X-ray spectroscopy

The Co_100-xNi_x (x ) 0, 30, 50, and 70) bimetallic nanoparticles (NPs) synthesized by a solvothermal method show interesting shape changes from aggregated clusters to ordered nanorings with increasing Ni concentration. A comprehensive structural study of these Co _100-xNi_x NPs is performed with X-ray diffraction and X-ray absorption fine structure spectroscopy. It is shown that the pure Co NPs consist of a majority of the fcc structure and a minority of the hcp phase. When 30% Ni is added, alloyed Co-Ni NPs with an inhomogenous atomic distribution are formed. The further addition of Ni to ≥50% could considerably improve the crystallinity and homogeneity of the alloyed NPs, as suggested by the fact that the disorder degree (σ²) is gradually lowered and the coordination number ratios N_Co-Co/N_Co-Ni and N _Ni-Ni/N_Ni-Co gradually approach the stoichiometry of the NPs. The ordered and homogeneous fcc structure helps to strengthen the magnetocrystalline anisotropic interactions, which may facilitate the head-to-tail assembly of the Co-Ni NPs.