ICAMS / Interdisciplinary Centre for Advanced Materials Simulation

Interaction of Cu-nanoclusters with piezoelectric substrates

Metal/oxide systems, e.g., Cu/ZnO, are used as catalyst in heterogeneous catalysis, which is a process where the phase of the catalyst differs from that of the reactants. These kinds of processes are of paramount importance in many industrial sectors, such as the energy and chemical industry, e.g., Haber-Bosch process. The aim of this project is to simulate and analyze such a metal/oxide system (Cu/ZnO).

Some experimental research of the interaction of Cu- and Au-cluster with different crystal surfaces of ZnO had been done already (see [1]). The attachment of the clusters, on the ZnO-surface was carried out by means of two different methods, molecular beam epitaxy (MBE) and metal organic chemical vapor deposition (MOCVD). Subsequently, the thus prepared model-catalysts were heated in ultra high vacuum (UHV) to various temperatures up to 770K. The study of the metal/oxide system was done by scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS) and photoelectron spectroscopy (PES). In this way, the geometric and chemical changes could be analyzed.

In case of the Au/ZnO-system the formation of a closed Au layer on the ZnO substrate was observed, which broke up into large island for temperatures above 600K. In contrast to this behavior, the formation Cu-clusters on the ZnO substrate was observed even at room temperature. Additionally, a loss of copper above the substrate level was visible, when the system was annealed in the rage 400-700K. To study the interface between Cu and ZnO, some of these Cu-clusters were removed with the STM and it was observed, that there was some entrenching of the clusters into the ZnO-substrate and an elevated perimeter around these cavities (see Figure 1). It should be remarked, that the entrenching does not explain completely the loss of the visible copper above the substrate surface.

STM-image of an area where the MBE deposited Cu-clusters on ZnO (annealed at 670 K) are partly removed by STM-induced surface modification. The image is high pass filtered andenhanced in contrast to improve the visibility [1].

Nevertheless these cluster perimeters are a point of interest, because at this border between Cu, ZnO and the surrounding vacuum, a high density of occupied electronic states was measured. This may indicate that the perimeter could be an interesting area for the catalytic activity of the Cu/ZnO system.

A simulation based on a phase field model can help to understand the different observed behaviour of Cu and Au on ZnO. The phase field method can be used to model the phase transitions and the electromagnetic interaction of the materials, by using the polarization and magnetization as an order parameter. Moreover the calculation of the electromagnetic field can be extended to the surrounding vacuum. In this way the electromagnetic field at the cluster perimeter can also further investigated.

[1] Köhler, U., Kroll, M., Löber, T., Birkner, A., Schott, V. and Wöll, C. "The thermally induced interaction of Cu and Au with ZnO single crystal surfaces." physica status solidi (b) , 6 1, 2013: 1222-1234 .

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