The formation and stability of Rh nanostructures on TiO2(110) surface and TiOx encapsulation layers

Rh overlayers formed on slightly oxygen deficient TiO2(1 1 0) single crystals, as well as on ultrathin encapsulation titania layers prepared on Rh multilayers were characterized by AES, LEIS, XPS, TPD and work function (WF) measurements. Rh deposition on TiO2(1 1 0) below 0.1 monolayer (ML) Rh coverage led to electron transfer from the metal toward the TiO2(1 1 0) surface. Annealing of Rh multilayers up to 950 K in UHV resulted in the surface diffusion of titanium and oxygen ions into a TiOx encapsulation layer of definite stoichiometry and a thickness of a few atomic layers. The accompanying 0.3-0.6 eV WF enhancement at circle dot(Rh) = 2-6 ML can be attributed to the smoothing of the Rh overlayer and the formation of a continuous TiOx dipole layer consisting of positively charged titanium ions at the metal-oxide interface and negative oxygen ions in the topmost layer. De-encapsulation of Rh particles was observed on a TiO2 sample less reduced in its bulk, revealing the roles of bulk and surface substrate stoichiometry on the decoration process. Increasing the thickness of Rh multilayers supported on the TiO2(1 1 0) single crystal hampered ion diffusion and consequently, it led to an increase in the temperature characteristic of the completion of the encapsulation. Deposition of additional Rh on the TiOx encapsulation layers covering Rh multilayers resulted in the growth of Rh particles having a similar height up to 1 ML. LEIS data indicated that the decoration of the second metal layer by titania was hindered. It occurred at a temperature more than 100 K higher than that characteristic of the TiO2(1 1 0) surface at the same Rh coverage. Aspects of the produced structures in relation to the formation of protective oxide layers, modification of surface work function, catalysis and metal-insulator-metal (MIM) devices are considered. (C) 2013 Elsevier B.V. All rights reserved.