Observing core-shell electrons

The removal of a core electron requires energy that is characteristic primarily of the atom concerned, so X-ray photoelectron spectroscopy (XPS) offers us the ability to identify the constituent atoms of any sample. By far the largest proportion of XPS studies are carried out to determine properties of surfaces, and so do not fall within the remit of this book. More information on this technique is given in the on-line supplement for Chapter 9. 

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It is noteworthy that the HRTEM cannot distinguish core and shell even by combining X-ray or electron diffraction techniques for some small nanoparticles. If the shell epitaxially grows on the core in the case of two kinds of metals with same crystal type and little difference of lattice constant, the precise structure of the bimetallic nanoparticles cannot be well characterized by the present technique. Hodak et al. [153] investigated Au-core/Ag-shell or Ag-core/Au-shell bimetallic nanoparticles. They confirmed that Au shell forms on Ag core by the epitaxial growth. In the TEM observations, the core/shell structures of Ag/Au nanoparticles are not clear even in the HRTEM images in this case (Figure 7). 

Recently five monometallic (Au, Pd, Pt, Ru, Rh) nanoparticles were investigated as electron mediators together with four core/shell bimetallic (Au/Pd, Au/Pt, Au/Rh, Pt/ Ru) nanoparticles [53,194-196]. The linear relationship was observed between the electron transfer rate coefficients and the hydrogen generation rate coefficient as shown in Figure 15. 

The electron density i/ (0)p at the nucleus primarily originates from the ability of s-electrons to penetrate the nucleus. The core-shell Is and 2s electrons make by far the major contributions. Valence orbitals of p-, d-, or/-character, in contrast, have nodes at r = 0 and cannot contribute to iA(0)p except for minor relativistic contributions of p-electrons. Nevertheless, the isomer shift is found to depend on various chemical parameters, of which the oxidation state as given by the number of valence electrons in p-, or d-, or /-orbitals of the Mossbauer atom is most important. In general, the effect is explained by the contraction of inner 5-orbitals due to shielding of the nuclear potential by the electron charge in the valence shell. In addition to this indirect effect, a direct contribution to the isomer shift arises from valence 5-orbitals due to their participation in the formation of molecular orbitals (MOs). It will be shown in Chap. 5 that the latter issue plays a decisive role. In the following section, an overview of experimental observations will be presented. 

The more recently developed cryo-TEM technique has started to be used with increasing frequency for block copolymer micelle characterization in aqueous solution, as illustrated by the reports of Esselink and coworkers [49], Lam et al. [50], and Talmon et al. [51]. It has the advantage that it allows for direct observation of micelles in a glassy water phase and accordingly determines the characteristic dimensions of both the core and swollen corona provided that a sufficient electronic contrast is observed between these two domains. Very recent studies on core-shell structure in block copolymer micelles as visualized by the cryo-TEM technique have been reported by Talmon et al. [52] and Forster and coworkers [53]. In a very recent investigation, cryo-TEM was used to characterize aqueous micelles from metallosupramolecular copolymers (see Sect. 7.5 for further details) containing PS and PEO blocks. The results were compared to the covalent PS-PEO counterpart [54]. Figure 5 shows a typical cryo-TEM picture of both types of micelles. 

A similar increase in the values for the hyperfine constants and parameters of the P,T-odd interactions when the correlations with the core shells (primarily, 5s, bp) are taken into account is also observed for the BaF molecule [93], as one can see in Table 3. Of course, the corrections from the 4/-electron excitations are not required for this molecule. The enhancement factor for the P,T-odd effects in BaF is three times smaller than in YbF mainly because of the smaller nuclear charge of Ba. 

Finally, if the role of Au is essentially to act as a structural promoter for Pd, it is expected that the support influences this effect. The observation of Hutchings et al. [ 105 ] that Pd-Au/carboncatalysts showthe presenceofahomogeneous alloy (both Au and Pd on the surface) and improved reactivity with respect to Pd-Au/oxide (Ti02, A1203 and Fe203), where a core-shell structure is observed, is thus not surprising. This result evidences that Au has a structural effect more than an electronic effect [106]. 

Gohy et al. studied the solution properties of micelles formed by two polystyrene-frZock-poly(2-vinylpyridine)-block-poly(ethylene oxide) (PS-b-P2VP- -PEO) copolymers in water by dynamic light scattering and transmission electron microscopy [92]. Spherical micelles were observed that consist of a PS core, a P2VP shell and a PEO corona. The characteristic sizes of core, shell and corona were found to depend on the copolymer composition. The micellar size increased at pH<5 due to P2VP block protonation (Fig. 19). 

From observations in transmission electron micrography (TEM), carbon soot particles are found to have an idealized onion-like shelled structure, as shown in Fig. 14.1.8. Carbon onions varying from 3 to 1000 nm in diameter have been observed experimentally. In an idealized model of a carbon onion, the first shell is a C6o core of /h symmetry, the second shell is C240 (comprising 22 x 60... 

The core-shell structure of PU-functionalized MWNTs was found via transmission electron microscopy (TEM) by Chen et al. (47). Ultra-high resolution TEM indicated that a layer of an amorphous structure with a thickness in the range of 2 to 5 nm was formed on the surface of the MWNT-PU. Functionalized MWNT samples could be dispersed very well in highly polar solvents and formed a homogenous solution, which was stable even after 10 days. Neither sedimentation nor aggregation of the MWNT bundles was observed. 

In first approximation, one might expect to observe a single electron line from the photoexcitation of a core s level and a spin-orbit doublet from levels of higher l. Line widths, in this approximation, would be a function of the core vacancy lifetimes set by the various filling mechanisms. As mentioned earlier, the core shell... 

When homoconjugation leads to electron or bond delocalization, and thereby to a change in the properties of a molecule, homoconjugation becomes chemically relevant. In fact, electrons are always delocalized over the space of a molecule. However, it has turned out that it is extremely useful to consider bonding, lone-pair and inner-shell electrons to be essentially localized in the bond, lone-pair or core region, respectively. This assumption is the basis of the concept of electron or bond localization and reflects the fact that many properties of a molecule can be reproduced in terms of bond or atom contributions. Of course, neither bond localization nor electron localization refers to any observable molecular property. They simply suggest that most molecules behave as if their bonds were localized and that their properties can be reproduced with the help of bond increments. With the concept of bond localization a large body of experimental data on molecular properties can be rationalized, i.e. bond or electron localization is a heuristic concept ... 

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