Faceted atomic structure

Fig. 4.15 (a) A 15 K He field ion image of a low temperature field evaporated silicon surface. (b) A computer simulation image of Si with (lxl) surfaces, (c) a 60 K Ne field ion image of a 720°C annealed silicon surface where well ordered atomic structures are developed at a few facets of the Si emitter surface, (d) When the Si tip is annealed at 800°C, almost all the facets are well developed. The atomic structures of all these facets are completely reconstructed. 

Several studies have been performed on alternative extended carbon support structures Pd/graphite rods [82], Pd/nanotubes [83], and Pd/nanofibers [84]. However, it is difficult to distinguish between catalyst size and atomic structure differences. Zhang et al. attributed the performance enhancements to an increase in the relative abundance of the more active Pd(lll) facet [82]. 

With a clear understanding of the differences between atoms with regards to their overlap binding abilities, let us now turn our attention to another facet of atomic structure and seek to relate the VOIE s of the first and second... 

Fig. 11 Formation of crystalline 3D super-lattices of tin nanoparticles a TEM view of a facetted super-crystal b SEM image showing particles included into a super-crystal as well as the organic surrounding c High resolution micrograph showing the alignment of the tin atomic planes inside the super-structure...

Figure 9.10 Like Pt, the ordered PtsCo phase has the fee structure and is represented by the thermodynamically most stable (111) facet. These diagrams show high symmetry adsorption sites identified for O and O2 on (a) monolayer Pt skin on PtsCoflll) and (b, c) PtsCoflll). Large circles represent Pt atoms, medium gray circles represent Co atoms, and small open circles represent O atoms. The surface unit cells are dehneated. (Reproduced with permission from Xu et al. [2004].)...

A key feature of this study was the structural information available on the model palladium nanoparticle catalyst. The mean particle size is 5.5 nm, containing on average 3000 atoms the majority of the particles are well formed with a (111) orientation and terminated by (111) facets with only a small fraction of (100) facets exposed. 

The rather low coordination in the (100) and (110) surfaces will clearly lead to some instability and it is perhaps not surprising that the ideal surface structures shown in Figure 1.2 are frequently found in a rather modified form in which the structure changes to increase the coordination number. Thus, the (100) surfaces of Ir, Pt and Au all show a topmost layer that is close-packed and buckled, as shown in Figure 1.3, and the (110) surfaces of these metals show a remarkable reconstruction in which one or more alternate rows in the <001 > direction are removed and the atoms used to build up small facets of the more stable (111) surface, as shown in Figure 1.4, These reconstructions have primarily been characterised on bare surfaces under high-vacuum conditions and it is of considerable interest and importance to note that chemisorption on such reconstructed surfaces can cause them to snap back to the unreconstructed form even at room temperature. Recently, it has also been shown that reconstructions at the liquid-solid interface also... 

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