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The bonding of organic molecules to transition metal oxide surfaces such as TiO2 is a central concern in the construction and optimization of molecule-based devices. Organic–inorganic interfaces are receiving increasing attention due to both fundamental and application interests. Additional interest derives from the role of TiO2 as a support for metal catalysts such as Au nanoclusters, which, e.g., act as an excellent catalyst for the low temperature CO oxidation reaction. The cluster size has to remain within an optimum range for the catalyst to preserve its activity. However, exposure to high pressure CO gas leads to the coalescence of the Au clusters. Thus it is important to hinder this growth process. The idea developed here is to use cysteine as a space between the clusters, since it strongly binds to the TiO2 surface via its carboxylic group, as shown here. It is well-known that the thiol group of cysteine interacts with gold, which then might establish the missing link between the gold clusters and the spacer molecules.
We evaporated L-cysteine onto a rutile (110) TiO2 surface and investigated the resulting adsorbate system using x-ray photoelectron spectroscopy (XPS). O 1s XPS shows that the molecules bind to the surface through their carboxylic groups. From the C 1s, S 2p, and N 1s spectra we were able to distinguish between monolayer and multilayer structures. During these measurements we observed very rapid beam damage due to irradiation, which meant that the experiment had to be performed with extreme care. We studied the effect of sample temperature to the rate and amount of beam damage. Different effects of the radiation on both the mono and multilayer are reported. We also discuss the co-deposition of Au and L-cysteine on the TiO2 surface.
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