Copper Nanoparticles with High Index Facets on Basal and Vicinal ZnO Surfaces

We investigated the orientation and morphology of Cu nanoparticles grown under ultrahigh-vacuum conditions on ZnO(0001), ZnO(0001̅), and ZnO(101̅4) single crystal surfaces by scanning tunneling microscopy, high-energy grazing incidence X-ray diffraction, low-energy electron diffraction, and scanning electron microscopy. The (111) oriented Cu NPs on basal ZnO showed only small area fractions of high indexed Cu(225) and Cu(331) facets. Cu NPs grown on ZnO(101̅4) show alignment of Cu [111] with the ZnO [0001] direction, which is at an angle of 24.8° to the ZnO(101̅4) surface normal. Because of this tilt, the NPs exhibit a shape with a larger fraction of high indexed facets such as (335), (221), (113), and (551̅). In addition, the direct interaction of subsequent Cu(111) planes to the underlying substrate results in unequal amounts of ABCA and ACBA stacked NPs. Small NPs are found to interact strongly with the vicinal surface, giving rise to a surface corrugation with a multiple of the surface step distance. The high density of low-coordinated Cu surface atoms potentially increases the overall catalytic activity for methanol synthesis and CO2 hydrogenation reactions.