Effects of annealing Ni catalyst in nitrogen-containing gases on the surface morphology and field-emission properties of thermal chemical vapor deposited carbon nanotubes

Ni catalyst was annealed in N2O/N2/NH3 in order to study the effects of the annealing on the surface morphology and field-emission characteristics of synthesized carbon nanotubes (CNTs). The morphology and composition of the CNTs show that the grain size and composition of the Ni catalyst can be modified by N2O, N2, and NH3 pretreatments. Scanning electron microscopy images of evaporated Ni after N2O pretreatment show that the Ni particles are more uniformly distributed, and their sizes are relatively small. It is found that the CNTs grown on Ni pretreated with N2O have the highest surface density and the highest emission current. This improvement is accomplished through the more uniformly distributed and smaller Ni particles obtained by N2O pretreatment. The growth of amorphous carbon is also suppressed. The reasons for this improvement are that nitrogen and oxygen atoms dissociated from N2O in the pretreatment can oxidize the catalyst particles and inhibit their growth so that the resulting Ni metal nuclei become smaller and more uniformly distributed, which is beneficial to CNT growth.