Structure and Oxidation Behavior of Nickel NanoparticlesSupported by YSZ(111)

摘要

Nickel nanoparticles supported by the yttria-stabilized zirconia (111) surface show several preferential epitaxial relationships, as revealed by in situ X-ray diffraction. The two main nanoparticle orientations are found to have their [111] direction parallel to the substrate surface normal and ∼41.3 degrees tilted from this direction. The former orientation is described by a cube-on-cube stacking at the oxide–metal interface and the latter by a so-called coherent tilt strain-relieving mechanism, which is hitherto unreported for nanoparticles in literature. A modified Wulff construction used for the 111-oriented particles results in a value of the adhesion energy ranging from 1.4 to 2.2 Jm², whereby the lower end corresponds to more rounded particles and the upper to relatively flat geometries. Upon oxidation at 10^–3 Pa of molecular oxygen and 673 K, a NiO shell forms epitaxially on the [111]-oriented particles. Only a monolayer of metallic nickel of the top (111) facets oxidizes, whereas the side facets seem to react more severely. An apparent sizeincrease of the remaining metallic Ni core is discussed in relationto a size-dependent oxidation mechanism, whereby smaller nanoparticlesreact at a faster rate. We argue that such a preferential oxidationmechanism, which inactivates the smallest and most reactive metalnanoparticles, might play a role for the long-term degradation ofsolid oxide fuel cells.