Pd clusters (4-5 nm) epitaxially grown on clean surfaces of MgO microc
ubes were annealed at 450-degrees-C in pure O2 or H-2 at pressures of
10(-7) and 10(-1) Torr. The samples were investigated by high-resoluti
on transmission electron microscopy in order to image both the profile
view and the interface Pd/MgO. Particles annealed in oxygen at 10(-7)
Torr develop a slightly rounded shape. Their lattice parameter is exp
anded by 2%, as in the case of the same clusters annealed in vacuum. A
nnealing under a higher pressure of oxygen induces a full rounded shap
e and produces PdO on the surface and edges of the Pd particles. Parti
cles annealed in H-2 retain the shape of a half-octahedron truncated a
t the top by (100) faces and their lattice parameter is dilated by 3-4
%. (C) 1994 Academic Press, Inc.