Surface particle, atomic

The ion-bombardment-induced emission processes include electrons and photons, besides the emission of surface particles (atoms or molecules) in a charged or uncharged, and possibly excited, state. All of these emission products are emitted with a certain angular distribution. 

When particle impacts with a solid surface, the atoms of the surface layer undergo crystal lattice deformation, and then form an atom pileup on the outlet of the impacted region. With the increase of the collision time, more craters present on the solid surface, and amorphous transition of silicon and a few crystal grains can be found in the subsurface. 

Figure 1.22 schematically summarizes the principle of the preferred transition states without sharing of a common metal atom. Whereas we have earher discussed surface sensitivity as a function of the relative ratio of particle surface edge sites and surface terrace atoms, the discussion given above provides a principle for particle size shape differences. 

The turnover frequency (TOP) based on surface-exposed atoms significantly increases with a decrease in the diameter of the gold particle from 5 nm [66]. This feature is unique to gold, because other noble metals usually show TOFs that decrease or remain the same with a decrease in the diameter [7]. The decrease in particle size gives rise to an increase in corner or edge and perimeter of NPs and change in electronic structure however, the origin of size effects on catalytic activity for CO oxidation is not clear. 

In 1989, we developed colloidal dispersions of Pt-core/ Pd-shell bimetallic nanoparticles by simultaneous reduction of Pd and Pt ions in the presence of poly(A-vinyl-2-pyrrolidone) (PVP) [15]. These bimetallic nanoparticles display much higher catalytic activity than the corresponding monometallic nanoparticles, especially at particular molecular ratios of both elements. In the series of the Pt/Pd bimetallic nanoparticles, the particle size was almost constant despite composition and all the bimetallic nanoparticles had a core/shell structure. In other words, all the Pd atoms were located on the surface of the nanoparticles. The high catalytic activity is achieved at the position of 80% Pd and 20% Pt. At this position, the Pd/Pt bimetallic nanoparticles have a complete core/shell structure. Thus, one atomic layer of the bimetallic nanoparticles is composed of only Pd atoms and the core is completely composed of Pt atoms. In this particular particle, all Pd atoms, located on the surface, can provide catalytic sites which are directly affected by Pt core in an electronic way. The catalytic activity can be normalized by the amount of substance, i.e., to the amount of metals (Pd + Pt). If it is normalized by the number of surface Pd atoms, then the catalytic activity is constant around 50-90% of Pd, as shown in Figure 13. 

Changes in electronic properties, such as Fermi level shifts and changes in the DOS, which still have to be confirmed experimentally. For example, Yano and co-workers used Pt NMR to probe possible changes in electronic properties induced by particle size [Yano et al., 2006]. They concluded that the surface Knight shifts as well as Ep-LDOS of surface Pt atoms showed no noticeable size dependence in the particle size range from 4.8 down to 1.6 nm hence, electronic properties are independent of size in this interval. 

Now, we consider H, atoms produced from hydrogen molecules adsorbed on zinc oxide under the influence of electron (ion) impact. We suppose that in this case the energy released in interaction of an electron (ion) with an adsorbed molecule is enough to break any bond between hydrogen atoms. As a consequence, Hj atoms bounce apart over the surface. Hydrogen atoms produced in this case are similar to H atoms adsorbed on the oxide surface from the gas phase at small surface coverages. In other words, they can be chemisorbed as charged particles and thus may influence electric conductivity of zinc oxide. This conclusion is consistent with the experimental results. 

Crystalline solids have a regular geometric shape bound by plane surfaces that intersect at characteristic angles. Their shape results from the arrangement of the particles (atoms, ions, or molecules) within the crystals, in an... 

When an impacting particle transfers energy to a near surface carbon atom in an amount sufficient to overcome the lattice bond energy or surface binding energy, some carbon atoms may be displaced and move in a direction defined by the angle... 

It is noteworthy that surface carbon did not come from those CO molecules responsible for the HT peak but from sites that are able to disproportionate CO and correspond to the LT peak. Because the latter sites are important only on quite small particles, it is tempting to associate them with low coordination number surface metal atoms, the relative concentration of which increases rapidly as the particle size decreases below 2 nm (8). Thus, these atoms may be the sites responsible for the relatively weakly adsorbed state of CO. Results similar to our work were found on other Group VIII metals. In the case of a Ru/Si02 sample, Yamasaki et al. (9) have shown by infrared spectroscopy that the deposition of carbon occurs rapidly by CO disproportionation on the sites for weakly held CO. The disproportionation also occurred on a Rh/Al20 sample with 66% metal exposed so that appreciable concentrations of low coordination atoms are expected (10). 

It is well established that commercially important supported noble metal catalysts contain small metal crystallites that are typically smaller than a few nanometers. The surface of these crystallites is populated by different types of metal atoms depending on their locations on the surface, such as comers, edges, or terraces. In structure sensitive reactions, different types of surface metal atoms possess quite different properties. For example, in the synthesis of ammonia from nitrogen and hydrogen, different surface crystallographic planes of Fe metal exhibit very different activities. Thus, one of the most challenging aspects in metal catalysis is to prepare samples containing metal particles of uniform shape and size. If the active phase is multicomponent, then it is also desirable to prepare particles of uniform composition. 

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