Studies on structural and magnetic properties of pristine and nickel-filled carbon nanotubes synthesized using LaNi5 alloy particles as a catalyst

• Pristine CNTs and Ni-filled CNTs were synthesized over LaNi5 alloy catalyst by CVD.
• The structural and magnetic properties of pristine CNTs and Ni-filled CNTs compared.
• Catalyst oxidation step plays a key role in the formation of Ni-filled CNTs.
• The surface segregation mechanism was suggested for the growth of Ni-filled CNTs.
• The Ni-filled CNTs exhibit superior ferromagnetic behavior.

Pristine carbon nanotubes (CNTs) and nickel-filled CNTs (Ni-filled CNTs) were synthesized by chemical vapor deposition using lanthanum nickel alloy (LaNi5) particles as a both catalyst and source for the Ni-filling. Pristine CNTs and Ni-filled CNTs were formed without and with catalyst oxidation of LaNi5 alloy, respectively. Transmission electron microscopy analysis clearly illustrated that the formation of Ni-filling during the growth of CNTs was due to the catalyst oxidation. X-ray diffraction and selected area electron diffraction analysis demonstrated that the encapsulated Ni exists as a single crystal face centered cubic structure. Compared with the pristine CNTs, the first order and second order Raman spectra of Ni-filled CNTs showed a disorder induced D band, an increased ID/IG   ratio, and a decreased IG′/IG ratio. The magnetic properties of the Ni-filled CNTs were studied by vibrating sample magnetometer at room temperature. Coercivity value of Ni-filled CNTs reached the maximum at 445.87 Oe, which is significantly larger than that of the bulk Ni at room temperature. The Ni-filling process through the catalyst oxidation is explained using the “surface segregation” mechanism. In addition, we have suggested the possible growth model involved in their formation.