Crystallographic faces

Recently, the In situ Raman scattering from Fe-TsPc adsorbed onto the low Index crystallographic faces of Ag was examined and the results obtained are shown In Fig. 5 (15). On the basis of the similarities of these spectra with those obtained for the macrocycle In solution phase, as well as the polarization behavior characteristics, It has been concluded that the most likely configuration Is that with the macrocycle edge-on with respect to the surface. This Is In agreement with conclusions reached from the UV-vlslble reflectance spectra. The preferred configuration, however, may depend on the particular macrocycle, as well as on the nature of the adsorption site. 

Liu and Zeng [88] showed that [001]-oriented petal-like Ti02 mesocrystals could be grown onto MWCNTs in aqueous solution (Fig. 16.6(c)). These examples show how in the presence of CNTs unusual morphologies for Ti02 nanocrystals could be prepared, with clear relevance for their photocatalytic and/or photoanode activity due to both different crystallographic faces exposed and different type of interface with nanocarbons on which they are supported. 

To do this, we must first investigate the origin of the variations in morphology exhibited by the same single crystal. For example, a single crystal might exhibit a polyhedral form bounded by flat crystallographic faces, a hopper or skeletal form bounded by faces with a step-wise depressed center at a face, or a dendritic form (see Fig. 2.1). 

The crystallographic face of silicon that is most important industrially is the Si(100) surface, followed by the Si(lll) surface. Both the (100) and (111) surfaces undergo extensive reconstructions, i.e., their surface atomic geometry differs significantly from that of the bulk. Moreover, the two surfaces have markedly different... 

Pt has repeatedly been found to be on the top of activity volcano curves (see Chapter 4) for ORR in acid electrolytes. After a number of inconclusive early studies, Markovic and coworkers demonstrated a clear structural dependence of the ORR activity on the crystallographic faces of Pt single-crystals [83-86]. The activity... 

Having obtained a suitable derivative, the crystallographer faces data collection again. Because derivatives must be isomorphous with native crystals, the strategy is the same as that for collecting native data. You can see that the phase problem effectively multiplies the magnitude of the crystallographic project by the number of derivative data sets needed. As I will show, at least two, and often more, derivatives are required. 

The foregoing electrostatic calculations hold, moreover, only for positions in the middle of a cubic face of a crystal of the NaCl type. Any deviation from this situation may result in a stronger electrostatic bond. Corners and edges of crystals, other crystallographic faces, lattice disturbances, etc., may all form active spots where the electrostatic adsorption of ions is relatively strong. We shall return to the problem of active spots in Sec. V,12. 

The forces between ions and metal surfaces, discussed in Sec. V,3 are far less influenced by active spots. Those spots that are active for nonpolar van der Waals forces are not active here. According to the simplified picture described in Sec. V,3, all crystallographic faces should give the same attraction if the equilibrium distance r0 were the same. This distance, however, will not be the same and for this reason as well as because of other minor differences, we may expect the actual surfaces also to be heterogeneous with respect to this contribution of adsorption forces though quantitatively far less outspoken than for the nonpolar van der Waals forces. 

The effect of active spots on the polarization of adsorbed molecules by a dielectric absorbent (Sec. V,6) is very great. The nature of the active spots is the same as of those which affect the attraction of ions or dipoles. Edges or corners of crystals, other crystallographic faces, and especially those places where the growth of individual crystal faces stopped, as well as lattice disturbances in the surface, will be active. 

These rather complicated effects may probably result in a rather heterogeneous distribution of heats of those chemisorptions on the surfaces of oxides and of salts where the surrounding ions suffer changes of character and charge simultaneously. Other crystallographic faces will also give variations in the strength of these chemisorptions (see Sec. VIII). 

Rhodin 221) has succeeded in preparing three different well-defined crystallographic faces of copper and in measuring the adsorption of nitrogen on these faces at various (low) temperatures. As may be seen from... 

Pig. 24. Heats of adsorption of nitrogen on copper as a function of the degree of coverage, 6. The solid lines are for the crystallographic faces, which are indicated the dashed line is for polycrystalline copper (221). 

An a priori heterogeneity may be, and is mostly, superimposed on the surface potential effect, This heterogeneity may be caused by the presence of different crystallographic faces or of impurities, by contaminations of the surface, or by dissolved impurities in the lattice or between the grains. 

Clearly, under conditions of diffusion control the rate of dissolution expressed in terms of the concentration of dissolving elements in the melt does not depend upon the atomic packing density of the crystallographic faces of any substance under investigation. Therefore, dissolution of single crystals of different orientation (line 2 in Fig. 5.7) is characterised by the... 

Pradier et al. (81) do not confirm the results obtained in the hydrogenation of butadiene on Pt(l 10) when they consider the (100) face. The activity decrease is no longer proportional to the sulfur coverage. They consider that the sulfur action changes with the crystallographic face the sulfur would adsorb on the diene sites over the (100) face, but on the hydrogen activation sites, over the (110) face. Moreover, the complexity of the sulfur action is not only seen by changing the substrate or the catalyst. It is... 

It was only after making measurements with solid electrodes that the concept of the energy associated with the electrode s electronic distribution in the interfacial region was introduced. This distribution depends on the electrode material as well as on its crystalline structure and exposed crystallographic face. However, it is interesting to see the historical evolution of the models, given that successively more factors that reflect the structure have been introduced. 

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