Deposition dynamics and chemical properties of size-selected Ir clusters on TiO2

We report a study of Ir-n/TiO2 samples prepared by size and energy-selected deposition of Ir-n(divided by) (n = 1, 2, 5, 10, 15) on rutile TiO2(110) at room temperatures. The Ir clusters are found to be formally in the zero oxidation state, and there are no significant shifts in Ir 4f binding energy with cluster size. Over a wide range of impact energies, both Ir XPS intensity and peak position are constant, indicating constant sticking coefficient, and no impact-driven redox chemistry. Low energy ion scattering spectroscopy (ISS) suggests that the deposited Ir clusters remain largely intact, neither fragmenting nor agglomerating, and retaining 3-D structures for the larger sizes. For impact energies above 10 eV/ atom, comparison of ISS and XPS data show that the Ir clusters are penetrating into the TiO2 surface, with the extent of penetration increasing with both per atom energy and cluster size. Temperature programmed desorption (TPD) of CO is used to further characterize the deposited Ir-n. This system shows pronounced substrate-mediated adsorption (SMA) in low CO exposures, with strong dependence on cluster size. ISS and sputtering experiments indicate that CO adsorbed via SMA is bound differently than CO adsorbed in high dose experiments. In experiments with sequential (CO)-O-16 and (CO)-O-18 doses, facile (CO)-O-16-->(CO)-O-18 exchange is observed for Ir-5 and larger clusters, but not for Ir-2. The peak CO desorption temperature is found to decrease with cluster size. The cycle of CO adsorption and heating comprising a TPD experiment have a dramatic effect on the sample morphology, leading to encapsulation of Ir by a thin TiOx layer. (C) 2003 Elsevier B.V. All rights reserved.