Encapsulation of Rh Nanoparticles Supported on TiO2(110)-(1 × 1) Surface: XPS and STM Studies

The encapsulation (decoration) of Rh nanoparticles supported on a TiO 2 (110) surface was studied by X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). In the case of a stoichiometric and well-ordered TiO 2 (110)-(1 × 1) surface covered by 3 monolayers (ML) of rhodium, there is no indication for thermally induced decoration of the metal particles by the suboxide phase of titania in the temperature range 300−800 K in ultrahigh vacuum (UHV). The encapsulation can be induced, however, by a few minutes annealing in a H 2 atmosphere of 10 -4 mbar at 750 K. The islands of TiO x phase on the top of the rhodium particles were clearly detected by STM. In the case of specially grown giant Rh crystallites (diameter of 10−15 nm) annealing in H 2 at 750 K results in dramatic morphological changes (corrosion or disruption) of the original nanoparticles accompanying the encapsulation process. Pretreatment of the stoichiometric TiO 2 (110)-(1 × 1) surface by Ar + bombardment (creation of surface and subsurface Ti 3+ states) and the postdeposition of Rh on it results in encapsulation of Rh particles after annealing in UHV (in the absence of hydrogen).