Stepwise Melting Model for the Formation Mechanism of Ni Catalyst Nanoparticles for Carbon Nanotube Growth

Different thicknesses of Ni films were deposited on Si substrates and heat treated at 973 and 1073 K. The results showed that Ni nanoparticles with different diameters and shapes were formed from different thicknesses. To find the mechanisms of the Ni nanoparticle formation from Ni nanofilms, our previous model on the heterogeneous transformation of Ni nanofilms to nanopartides and geometrical and kinetic calculations were used and compared with the experimental results. It was found that the mechanism for formation of particles is a stepwise melting of layers of specific thickness, dependent upon the working temperature, e.g., 0.5 nm thick layers at 1073 K. Nanofilms transform to nanoparticles which do not exceed 150 nm in diameter and 40 degrees in wetting angle. Films with a thickness of more than 12.9 nm show a different behavior. The predictions are in excellent agreement with the experimental results.