Morphological changes and surface intermediates during C2H2 exposure on nickel nanocrystals: surface carbide as an early stage of carbon nanotubes nucleation

Abstract

We present an imaging study of the reshaping of Ni 3D single crystals when exposed to C2H2 at 873K. This has been achieved using Field Emission/Ion Microscopy techniques which provide high resolution micrographs of the apex of sharp metallic tips. The latter realistically model a single catalytic grain and reactive gases can be introduced in the analysis chamber, providing direct in situ imaging of surface reactions. When adding C2H2(g) to a H2(g) atmosphere, at 873K, the tip morphology switches from a strongly facetted form to a new equilibrium shape where step edges-containing crystal planes, like {113}, extend. We propose, in agreement with other authors’ hypothesis, that carbon preferentially adsorbs on step edges and therefore induce the extension of related crystal planes. These regions could then act as preferential sites for graphene sheets nucleation. We have then used an atom-probe apparatus, which consists in a FIM coupled with a ToF mass spectrometer, to analyze the adsorbed species and the composition of the (sub)surface during the ongoing experiment. We have associated this reshaping phenomenon with the formation of a surface carbide as NiC and NiC2 compounds were detected. We propose that the formation of this surface carbide, along with the induced morphological changes, is a prerequisite for graphene - and thus CNTs - nucleation on nickel catalysts.info:eu-repo/semantics/nonPublishe