THE STRUCTURE AND OXIDATION OF HIGHLY CAVITATED METAL-SURFACES - A STUDY BASED ON TEM AND RAMAN-SPECTROSCOPY

Highly cavitated layers formed in metal surfaces by He ion implantatio n are of special interest in relation to potential catalytic applicati ons. Here we report on a TEM study of coarse bubble structures produce d in two representative metals with particular potential as catalysts, Pt (fee) and Ti (hcp). For bubble sizes around 4 nm, the bubble struc tures (described as cellular) are remarkably similar to those previous ly reported for V. This suggests that at appropriate temperatures such bubble structures may be a universal response to He implantation, ind ependent of crystal type. For V, it is known that continued implantati on 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 Pro ceedings, Nucl. Instr. and Meth. B 127/128 (1997) 734). Here we demons trate for the first time that large bubbles in high concentration can also be produced in Pt and Ti by suitable choice of implantation tempe rature 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 ver y 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 aff ect the stoichiometry of the oxide phase with the cellular surface enh ancing the formation of the higher oxide phases, in particular V2O5.