"Characterization and Chemical Activity of Co and Co-Au Clusters on TiO2(110)"
Friday, August 9, 2013, 10:30 am
Berkner Hall, Room A
Hosted by: Evgeny Nazaretski
The nucleation and growth of pure Co clusters and Co-Au bimetallic clusters on vacuum-annealed (reduced) TiO2(110) have been studied by scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), low energy ion scattering (LEIS), temperature programmed desorption (TPD), and density functional theory calculations (DFT). On vacuum-annealed TiO2(110), the Co clusters grow as three-dimensional islands with average height from 3 to 5 Ã… at coverages between 0.02 and 0.25 ML. In addition, the high nucleation density of the Co clusters and lack of preferential nucleation at the step edges demonstrate that diffusion is slow for Co atoms on the TiO2 surface. Co clusters become partially encapsulated by titania upon heating to 800 K. XPS experiments show that there is little reduction of the titania surface by Co. A comparison of the metal-titania binding energies calculated by DFT for Co, Au, Ni and Pt indicates that stronger metal-titania interactions correspond to lower diffusion rates on the surface, as observed by STM. Co-Au bimetallic clusters are formed by first depositing Co atoms, followed by the addition of the more mobile Au atoms. For clusters with a total coverage of 0.25 ML, the cluster density increases and average cluster height decreases as the fraction of Co is increased. Annealing to 800 K results in cluster sintering and selective encapsulation of Co, for clusters of all compositions. LEIS experiments indicate that the surfaces of the bimetallic clusters are Au-rich. The desorption of CO from the bimetallic clusters demonstrates that the presence of the CO adsorbate induces diffusion of Co to the cluster surface. DFT calculations confirm that for a 50%Co-50% Au structure, the surface is predominantly Au in the absence of CO, and that CO-induced-diffusion of Co to the cluster surface is thermodynamically favorable.