Effect of a Gold Cover Layer on the Encapsulation of Rhodium by Titanium Oxides on Titanium Dioxide(110)

Abstract

The present study focuses on the thermal behavior of continuous rhodium thin films (∼10 nm) deposited on a TiO 2 (110) substrate at 300 K and covered by a continuous ultrathin gold film (∼1 nm), applying scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and low-energy ion scattering (LEIS) techniques. This arrangement facilitated a rather clear-cut investigation into the effect of the Au cover layer on the wellknown encapsulation/decoration of Rh by TiO x , since the contribution of the titania substrate to the XPS and LEIS spectra could be avoided. Upon annealing a monometallic Rh film, the diffusion of Ti and O from the TiO 2 (110) substrate on top of the Rh film is essentially complete up to 850 K, leading characteristically to the formation of a "pinwheel" encapsulation layer with a TiO 1.2 stoichiometry. The dewetting of the Rh film does not take place up to 950 K. Upon annealing of the Rh film in the presence of a continuous Au cover layer, the diffusion/accumulation of O was completely blocked up to 850 K, but metallic Ti was well detectable by XPS. Since LEIS spectra were dominated by the Au peak, and almost no Ti signal was observable up to this temperature, the metallic Ti is mainly stabilized at subsurface positions, probably either at the Rh−Au interface or within the Au film, in any case in contact with Au. The complete separation of O and Ti diffusion in this temperature range can be attributed to the weak interaction between oxygen and gold. Raising the sample temperature up to 930 K induces substantial changes. The diffusion of O is no longer kinetically hindered, and it reacts with a part of the alloyed Ti to form wellseparated TiO 2 nanoclusters on top of the Au shell. However, no oxygen is stabilized at subsurface sites in the bimetallic film. The rest of Ti alloyed in the subsurface is detectable by STM as ringlike features. The "pinwheel" TiO 1.2 encapsulation layer appearing typically on a pure Rh film did not form on the bimetallic Au−Rh film at any of the temperatures investigated.

Variation of SMSI with the Au:Pd Ratio of Bimetallic Nanoparticles on TiO2(110)

“…The absence of encapsulation, and Au(111) termination of the nanoparticles at this higher Au concentration is in line with our expectations. It is also consistent with earlier work where it was reported that a thick layer of Au covering Rh nanoparticles on TiO 2 (110) hinders the diffusion of Ti and O species onto the top facet up to ~ 900 K [ 32 ].…”

“…Moreover, a Au overlayer on a VIII. B metal can strongly hinder the SMSI process [ 31 , 32 ], while alloy formation at higher temperatures can also occur [ 41 , 42 ].…”

Monitoring in situ the colloidal synthesis of AuRh/TiO2 selective-hydrogenation nanocatalysts

“…Extensive work on this system was performed by Kiss,Óvári and coworkers. [4][5][6][7] Using either chemical or physical preparation techniques, they showed that Au tends to cover the Rh surface. Using scanning transmission electron microscopy (STEM), Chantry et al 8,9 highlighted the tendency of Rh either to deposit in an uneven way on Au nanorods and/or to form mixed alloy layers at the topmost surface depending on kinetic issues during the formation of such nanostructures.…”

Understanding and controlling the structure and segregation behaviour of AuRh nanocatalysts

“…Although several groups have succeeded in synthesizing solid-solution nanoalloys of bulk-immiscible elements using colloidal methods 11 13 14 , these structures are generally metastable and cannot resist the thermal treatment needed for removing the stabilizing agents. As the thermodynamic properties of bulk alloys can influence the structure of their nanosized counterparts, some of us have recently compared the well-known bulk-miscible Au-Pd system 7 12 16 to the rarely studied bulk-immiscible Au-Rh system 11 13 17 in terms of mixing behaviour and reactivity at the nanoscale 9 15 18 . As seen for 3 nm-sized nanoparticles (NPs) anchored on well-defined rutile titania nanorods, Au and Pd atoms form a solid-solution alloy, whilst Au and Rh atoms segregate into single-phase domains within the NPs 15 .…”