Ali

The first study of Sn deposition on Pt( 111) was reported by Paffet and Windham in 1989 [42] and a subsequent one on the same system was published by Campbell in 1990 [1]. In both studies, two LEED patterns were observed after annealing a 2x2 and a ( /3x v ) R30°. Both superstructures were interpreted in terms of incorporation of the tin layer in the first platinum layer, but only a qualitative examination of the LEED pattern was performed. Subsequently the results of low energy alkali ion scattering spectroscopy ALISS [43, 21] could be quantitatively interpreted as due to ordered, single atomic layer surface alloys. The ion scattering results have been confirmed and expanded by a quantitative LEED study [34]. The atomic structure of both phases corresponds exactly to that of the topmost layer of the phases with the same periodicity observed on the on PtsSnflll). The LEED and ALISS results for the Sn/Pt(lll) system were confirmed by a recent STM study reported by Batzill et al. [44]. Even though atomic resolution was not attained in this study (only the surface unit mesh could be observed), the results are closely comparable to the atomically resolved ones obtained on the PtaSn(l 11) surface [35]. 

The alloyed c(2x2)-Sn structure on Pt(lOO) was found to be unstable and to quickly transform into the (3 /2 x /2) R45°phase which was found to be stable up to annealing temperatures of 1000 K. It was not possible to propose a complete model for this phase, however the ALISS results remained very similar to those for the c(2x2) phase. It was therefore suggested that the the local structure of the (3 /2 x y/2) R45°is the same as that of the c(2x2). Indeed the c(2x2) periodicity can also be written in an equivalent manner as ( /2 x /2) R45°. The extra 2>y/2 periodicity observed for the Sn-Pt(lOO) surface can be due to a specific step arrangement or periodic domains of pure Sn atoms every three lattice spacing along the [100] azimuth.lt appears that the formation of the (3 /2 X /2) R45°is accompanied by the disappearance of tin atoms from the subsurface region. 

Abstract Thermally stable, ordered surface alloys of Sn and Pt that isolate threefold Pt, twofold Pt, and single-Pt atom sites can be produced by controlled deposition of Sn onto Pt single crystals and annealing. The strnctnre was established by characterization with several techniques, including ALISS, XPD, LEED, and STM. Chemisorption and catalysis studies of these well-defined, bimetallic surfaces also define the overall chemical reactivity of Pt-Sn alloys, clarify the role of a second-metal component in altering chemistry and catalysis on Pt alloys, and develop general principles that describe the reactivity and selectivity of bimetallic alloy catalysts. 

Figure 7 (a) ALISS polar scan for an fee Pt(l 11) crystal along the [—211] azimuth. The peak... 

Figure 17 Two possible structures for the c(2 x 2) Sn-Pt(lOO) surface, (a) Overlayer model with the Sn atoms located above the Pt(l 11) surface plane in threefold hollow sites, (b) Surface alloy model with the Sn atoms replacing every second Pt atom in the surface plane, (c) ALISS is ideally suited to distinguish between these two structures with high accuracy, as indicated by the shift in the critical angle for Sn-scattering upon alloying between 720-760 K. (From Ref. 73.)...

Recently ALISS experiments and simple classical theory have been used to directly get the surface structure of TiO2(110) [41]. Figure 20 shows an unrelaxed stoichiometric Ti02 surface with bridging oxygen rows. 

Figure 21 shows the Li ALISS polar scans obtained along the [001] and [ 110] azimuths of the Ti02(l 10)-p(l x 1) surface and the corresponding theoretical simulations. Peaks I through VI have been reproduced in the theoretical simulations, and the difference with bulk structure is illustrated in Figure 21b. The structure determined from the critical angles shows that the bridging oxygens are located at 1.2 0.1 A above the sixfold titanium atoms at the surface. A large relaxation of about —18 + 4% (—0.6 + 0.1 A) was observed between the first and second titanium layers.

Figure 21 ALISS polar scans of a TiO2(110)-p(l x 1) surface taken along the (a) [001] direction and (b) [—110] direction using 1-keV Li" " ions backscattered at 160°. The solid circles are experimental data points and the solid curve is the result of theoretical calculations. Dotted lines demonstrate simulations for an error in critical angle by +1.0°. The dashed line shows the simulations for the bulk structure shown in Figure 20. (From Ref. 41.)...

---