Structural and chemical properties of ac(2×2)−Ti/Pt(100)second-layer alloy: A probe of strong ligand effects on surface Pt atoms

Abstract

We investigated the structure and chemisorption properties of a Ti/Pt(100) surface alloy using Auger electron spectroscopy (AES), low energy electron diffraction (LEED), scanning tunneling microscopy (STM), x-ray photoelectron spectroscopy (XPS), x-ray photoelectron diffraction (XPD), low-energy alkali ion scattering (ALISS), and temperature programed desorption (TPD). Samples were prepared by evaporating Ti onto a clean hex-Pt(100) reconstructed surface at 300 K. After annealing the sample to 800 K, a $c(2\ifmmode\times\else\texttimes\fi{}2)$ LEED pattern was observed that sharpened as the temperature was increased to 920 K. Further annealing to 1000 K caused this $(2\ifmmode\times\else\texttimes\fi{}2)$ LEED pattern to become diffuse because of the onset of disorder in the surface layers resulting from Ti diffusion into the bulk. Using XPD and ALISS, we determined that this LEED pattern is due to an ordered alloy structure that has Ti atoms present in the second layer of a $c(2\ifmmode\times\else\texttimes\fi{}2)\ensuremath{-}\mathrm{T}\mathrm{i}/\mathrm{P}\mathrm{t}(100)$ surface alloy, and not in the topmost layer. Thus, the surface layer of this alloy is pure Pt. XPS showed that the Ti $2p$ peak from the surface alloy is shifted by 1.4 eV to higher binding energy than that of a thick Ti film, and the Pt $4f$ peak is shifted by 0.1 eV higher from that of the clean hex-Pt(100) reconstructed surface, consistent with the formation of strong intermetallic bonds upon alloying. The chemisorption properties of the surface alloy were probed using CO and ${\mathrm{H}}_{2}$ adsorption. CO adsorbed reversibly on the alloy, desorbing in TPD in a broad peak with a maximum at 376 K. This is lower by 132 K from the CO desorption peak from a clean Pt(100) surface. Thermal desorption of ${\mathrm{H}}_{2}$ showed a similar peak shift to lower temperature, and much less hydrogen (20%) adsorbed on the $c(2\ifmmode\times\else\texttimes\fi{}2)\ensuremath{-}\mathrm{T}\mathrm{i}/\mathrm{P}\mathrm{t}(100)$ alloy than on Pt(100). These results show that second layer Ti atoms exert a strong ``ligand'' or electronic effect on Pt atoms at the surface. Thus, the $c(2\ifmmode\times\else\texttimes\fi{}2)\ensuremath{-}\mathrm{T}\mathrm{i}/\mathrm{P}\mathrm{t}(100)$ surface alloy represents an interesting model system for studying ligand effects on the chemistry of a bimetallic alloy surface.