Sintering, oxidation, and chemical properties of size-selected nickel clusters on TiO2(110)

We report a study of Ni-n/TiO2 samples prepared by size-selected deposition of Ni-n(+) (n=1,2,5,10,15) on rutile TiO2(110). The effects of deposition energy and support preparation conditions on the oxidation state of the clusters are examined by x-ray photoelectron spectroscopy (XPS). On the stoichiometric surface, Ni-n is stable, but oxidation can be driven by increased impact energy. For TiO2 surfaces with chemisorbed oxygen, deposited Ni-n are oxidized even at low impact energies. Low energy ion scattering spectroscopy was used to characterize the dispersion of Ni on the support, and provide some insight into binding morphology. Small clusters bind preferentially to oxygen sites. Large clusters bind in compact geometries and appear to retain some three dimensional character on the surface. The data suggest that the clusters neither fragment, nor agglomerate, in room temperature deposition. Temperature programmed desorption (TPD) of CO was used to characterize deposited clusters. For these small clusters, no strong desorption features are observed in the temperature range above 140 K, where CO desorbs from TiO2. The lack of CO binding is discussed in terms of strong Ni-TiO2 binding. The ion scattering data indicate that there is significant sintering, and possibly partial encapsulation, of the Ni clusters during the TPD experiments. XPS reveals little change in oxidation state. This is the first study where the oxidation state and morphology of size-selected deposited clusters has been studied, before and after TPD. (C) 2002 American Institute of Physics.