A comparison of Pt overlayers on α-Al2O3(0001), ZnO(0001)Zn, and ZnO(0001)O.

The growth of Pt films on ZnO(0001)Zn and ZnO(0001)O and their adsorption properties for CO have been studied and compared to results obtained for Pt on α-Al2O3(0001). Samples were prepared by vapor deposition of Pt onto flat substrates in ultra high vacuum, and metal coverages were measured using a quartz-crystal, film thickness monitor. On both ZnO surfaces, Auger electron spectroscopy (AES) and transmission electron microscopy (TEM) indicate that Pt grows in as a 2D film at 300 K, in contrast to results for Pt on α-Al2O3(0001) where 3D particles are formed under these conditions. Furthermore, transmission electron diffraction (TED) showed that the Pt grew as an ordered overlayer on the ZnO crystals. Heating the Pt/ZnO surfaces resulted in the formation of Pt particles starting at ∼650 K, but the Pt particles were still oriented with the ZnO. On both ZnO surfaces, temperature programmed desorption (TPD) curves for CO from high Pt coverages (>one monolayer) and from Pt particles formed by heating thinner films were identical to TPD curves for CO from Pt on α-Al2O3(0001). The main desorption features were observed at 400 and 510 K and are interpreted as being due to different site geometries on the Pt by comparison to results from Pt single crystals. However, for Pt at submonolayer coverages prior to particle formation, the TPD curves for CO from Pt on ZnO(0001)Zn were shifted down in temperature by ∼60 K compared to results for Pt on ZnO(0001)O. These results are interpreted as indicating a chemical interaction between Pt and the Zn+2 ions which are in direct contact. [ABSTRACT FROM AUTHOR]