Core/Shell Morphology in Ti Clusters Prepared by Plasma Gas Condensation and Post Annealing

Abstract

Ti clusters have been prepared by a plasma-gas-condensation cluster deposition system (PGCCD) with controlling an Ar gas flow rate, R_Ar and a He gas flow rate, R_He, and observed by transmission electron microscopy (TEM) at their as-prepared and annealed states. In Ti clusters whose sizes are about 18 nm, an hcp phase is predominant in the as-prepared state, while TiO_X crystallites are formed on the hcp phase cores after annealed at 623 K, revealing a core-shell morphology. In small Ti clusters whose sizes are about 10 nm, an fcc phase coexists with the TiO_X phase in the as-prepared state, while the TiO_X phase is predominant after annealed at 623 K, revealing no core-shell morphology. The electrical resistivity, ρ, of these cluster assemblies increases with decreasing temperature. After annealed at 623 K, the absolute ρ value of hcp Ti cluster assemblies increases remarkably while that of fcc cluster assemblies does not change so much. These results demonstrate the difference in the oxidation behaviors and morphologies between large and small Ti clusters.