Phase Transitions in C:Ni Nanocomposite Templates during Diameter-Selective CVD Synthesis of SWCNTs

Phase transitions in carbon: nickel nanocomposite templates during diameter-selective CVD synthesis of SWCNTs were studied. While almost conserving their pre-defined diameter distribution, as-deposited Ni₃C nanoparticles transform into fcc-NiO during activation in low-pressure air atmosphere, and are reduced to a mixture of fcc-Ni and Ni₃C under nanotube growth conditions. The first phase transition leads to a substitutional replacement of the protective carbon matrix by a protective oxide layer. The second one reflects competing reduction processes of NiO. A mechanism for the complementary roles of carbon matrix and Ni species in the three-step CVD synthesis is proposed that includes nanoparticle immobilization, carbon delivery and catalysis of nanotube growth.