Growth of Copper on Single Crystalline ZnO: Surface Study of a Model Catalyst

Abstract

Copper, vapor-deposited on the polar, Zn-terminated ZnO(0001) surface is investigated in view of its suitability as model system for the technologically important Cu/ZnO catalyst. The structure and electronic properties of Cu clusters on ZnO(0001)-Zn have been studied with scanning tunneling microscopy (STM), low energy electron diffraction (LEED), ultraviolet photoelectron spectroscopy (UPS), and low-energy He + ion scattering (LEIS). At room temperature copper grows as two-dimensional (2D) clusters only at very low coverages of 0.001-0.05 equivalent monolayers (ML). At coverages greater than 0.01 ML, 3D clusters start to develop. This is contrasted to Cu growth on the oxygen-terminated ZnO(0001bar) surface, where a strong adhesion between Cu and the ZnO substrate results in an initial wetting of the surface by Cu. On ZnO(0001)-Zn, surface roughness and sputter damage change the growth mode to more 2D-like. Annealing in UHV results in well-separated, hexagonal clusters rotationally aligned with the substrate. Annealing of 2-5 ML Cu deposits on the ZnO(0001)-Zn surface in 10 )6 mbar O 2 results in the formation of a (Ö3 · Ö3)R30°superstructure with respect to the ZnO lattice. This superstructure likely contains Cu + sites. The suitability of the different surface morphologies to probe specific sites that are thought to be active for catalytic processes is discussed.

Properties of oxide thin films and their adsorption behavior studied by scanning tunneling microscopy and conductance spectroscopy

“…Nucleation and growth of Cu has been explored on the polar ZnO (0001) surface, revealing the formation of 3D particles on the Zn-terminated, but 2D-wetting growth on the O-terminated surface due to the larger Cu-O interaction in the latter case. 361,362 On the non-polar ZnO(10-10), mainly the adsorption of hydrogen 35,363 and water 286,311,364 has been studied. Interestingly, binding of a single H-atom per surface unit cell gives rise to a metallization of the initially semiconducting oxide, as demonstrated by STM conductance spectroscopy and HREELS (Fig.…”

“…Unfortunately, the number of such experiments performed with the STM is surprisingly small. Most of them deal with polar ZnO(0001) surfaces, 361,363 or explore the hydroxylation of thin oxide films with polar character, for instance of Cr 2 O 3 /Cr(110) 402 or more recently of FeO/Pt(111). 303 For the latter system, also experiments on other atomic and molecular adsorbates have been performed with low-temperature STM, which shall be presented in the following.…”

Small Cu-clusters on ZnO(0 0 0 1)–Zn: Nucleation and annealing behavior

“…Dulub et al [11] found 3D-cluster growth already for very low coverages >0.01 ML. The Cu-clusters are aligned with respect to the ZnO-substrate and the LEED pattern shows (1 1 1)-orientation with the lattice constant of bulk Cu(1 1 1) (21% smaller relative to the ZnO lattice) [4].…”

“…In catalysis this system is very important because it plays a significant role in the technologically important methanol synthesis from CO/CO 2 /H 2 reactants and hydrogen production from methanol by the reverse water gas-shift reaction [4][5][6][7]. The oxide (ZnO) is an essential part of the catalyst and although many groups work on catalyst characterisation, the mechanisms that make this combination of material such a successful catalyst are still unidentified.…”

CuO/ZnO catalysts for methanol steam reforming: The role of the support polarity ratio and surface area

“…Typical ZnO materials exhibit a würtzite structure with the polar planes corresponding to the basal planes of the hexagonal würtzite unit cell [36][37][38]. The würtzite configuration has preferential exposure of non-polar facets (lower polarity ratio).…”