The thermally induced interaction of Cu and Au with ZnO single crystal surfaces

Abstract

The reaction of Cu‐clusters with a polar and a mixed terminated single crystalline ZnO‐substrate upon thermal treatment in UHV is studied in comparison with Au‐films. Scanning tunneling microcopy and spectroscopy in combination with photoemission experiments reveal the geometrical and chemical changes in the Cu‐cluster system on ZnO(0001)‐Zn and ZnO(10$\overline {1} $0) upon annealing up to 770 K. On ZnO($10\overline {1} 0$) the Cu‐clusters show a roof like outline with Cu(111) side facets. The data points to a Cu(110) interface with the ZnO‐substrate. The interface of the Cu‐clusters and the ZnO‐substrate was investigated by the controlled removal of clusters using STM‐tip manipulation exposing the “footprints” of the clusters. On both investigated ZnO surfaces an entrenching of the Cu‐clusters during annealing was found which partly explains the observed decrease of the amount of Cu visible above the ZnO‐substrate level upon annealing. Even at higher annealing temperature the main body of the cluster surface is still pure copper. No large scale oxidation or brass formation was found. Scanning tunneling spectroscopy shows an increased density of occupied states at the cluster perimeter which is possibly relevant as an active site in catalysis.

Treatment of delocalized electron transfer in periodic and embedded cluster DFT calculations: The case of Cu on ZnO (100)

“…Tang et al modeled methanol synthesis over a Cu “strip”, a few ML high, adsorbed on ZnO(10 0) using periodic DFT+U calculations, and concluded that hydrogen spillover to/from atoms at the Cu/ZnO interface contributes to the catalytic activity. Empirical force‐fields have also been used to model larger Cu particles on ZnO(10 0), and the Cu particles were found to preferentially adopt 3‐D shapes, in agreement with available experimental data …”

Enhanced wetting of Cu on ZnO by migration of subsurface oxygen vacancies

“…The growth of Cu on the polar ZnO(0001) surface 24 25 26 , as well as thermally induced effects 27 28 29 30 31 32 33 have been the subject of extensive debate and have also been intensively investigated theoretically 34 35 36 . Previous investigations on Cu/ZnO(0001) (refs 31 , 33 ) left several important questions about the thermally induced behaviour of this system, such as disappearance of 3D-like Cu NPs 31 , Cu-induced changes of the ZnO surface structure 31 and Cu–Zn alloying 37 38 upon annealing, unanswered. Part of this debate originates from the incomplete understanding of the structure of the ZnO(0001) surface itself.…”

Multi-phase-field model for surface and phase-boundary diffusion

“…As a forecast on future applications of the proposed model, the annealing dynamics of nano-clusters on initially flat surfaces is investigated. The reported craterlike structure with an elevated perimeter [10] is found to be a transient non-equilibrium state during nano-cluster annealing, thus shedding light onto this complex process from a dynamic perspective. The proposed multi-phasefield method is generic and can be used to study any type of surface and phase-boundary phenomena which contains conserved fields.…”

“…The equilibrium configurations shown in Fig. 7 do not resemble the geometry of the nano-clusters observed in [10] where elevated perimeters have been observed which surround the 'crater-like' entrenchments of the nano-clusters. This perimeter can be explained as intermediate state of the entrenching process, in a sense that the entire system is not in equilibrium.…”