The structure and oxidation of highly cavitated metal surfaces: a study based on TEM and Raman spectroscopy

Abstract

Highly cavitated layers formed in metal surfaces by He ion implantation are of special interest in relation to potential catalytic applications. Here we report on a TEM study of coarse bubble structures produced in two representative metals with particular potential as catalysts, Pt (fcc) and Ti (hcp). For bubble sizes around 4 nm, the bubble structures (described as cellular) are remarkably similar to those previously reported for V. This suggests that at appropriate temperatures such bubble structures may be a universal response to He implantation, independent of crystal type. For V, it is known that continued implantation to high He doses, beyond the ordered and cellular stages of bubble development, can lead to structures involving large bubbles (diameters exceeding 10–12 nm) in high concentration (companion paper, these Proceedings, Nucl. Instr. and Meth. B 127/128 (1997) 734). Here we demonstrate for the first time that large bubbles in high concentration can also be produced in Pt and Ti by suitable choice of implantation temperature and He dose. The results encouraged us to examine the chemical activity of a helium implanted surface. The particular example chosen was the oxidation of cellular structures in V. It was found that Raman spectroscopy is a powerful complement to TEM in investigating the very thin oxide layers involved and that: (i) the oxides grow as oriented crystallites with an abrupt interface to the underlying V, regardless of the surface structures; and (ii) the nature of the surface can affect the stoichiometry of the oxide phase with the cellular surface enhancing the formation of the higher oxide phases, in particular V 2O 5.