Composition of a surface layer

CPAA may be employed to determine trace element concentrations in bulk solid material, but its importance in our present context is that it permits the characterization of a thin surface layer, i.e. the mass of the analyte element per surface unit, with a good detection limit and outstanding accuracy. For example the composition of a surface layer (or foil) of known thickness can be determined, or, conversely, the thickness of a surface layer of known concentration. Depth profiling or scanning is not possible, and a disadvantage of the method is that heating occurs during irradiation. It is also not possible to discriminate between different oxidation states of the analyte element or between different compounds. 

Ion implantation also has promise in other tields involv ino surface technology for example, new metallurgical phases w ith prior unknown properties can be I untied. In some eases. Mich as heav y implantations of tantalum irt copper of phosphorus in iron, amorphous or glassy phases can be formed. Or. if the implanted atoms ore mobile, inclusions and precipitates can he formed as. for example, implanted argon and helium atoms are insoluble in metals and may form bobbles. The composition of a surface layer can be changed by differential sputtering caused by the implanted ions. 

In certain areas of science and engineering, the elemental composition of a surface layer of solid that is a few angstroms to a few tens of angstroms thick is of much greater interest than the bulk elemental composition. Fields where surface composition is extremely important include heterogeneous catalysis, corrosion and adhesion studies, embrittlement properties, and studies of the behavior and functions of biological membranes. 

Generally, the chemical composition of the surface of a solid differs, often significantly, from the interior or bulk of the solid. I hus far in this lext. we have focused on analytical methods that provide information about bulk composition of solids only. In certain areas of science and engineering, however, the chemical composition of a surface layer of a solid is much more important than is the bulk composition of the material. 

Spectroscopic surface methods provide both qualitative and quantitative chemical information about the composition of a surface layer of a solid that is a few tenths of nanometers (a few angstroms) to a few nanometers (lens of angstroms) thick. In this section we describe some of the most widely used of these spectroscopic techniques. ... 

The chemical potentials of components within surface layer p depend on the composition of the layer and its surface tension y. The dependence of p on the composition of a surface layer is given by the known relation [48]... 

More rigorous thermodynamic relations valid for adsorption layers which undergo a phase transition could be derived based on the requirement that the chemical potentials in either phase should be equal to each other. The phases are represented by the surfactant bulk solution, the non-condensed (surface solution) and the condensed part of the surface layer. The dependence of p- on the composition of a surface layer is given by the Butler equation (2.7). The chemical potential of the i component in the condensed phase comprised of the given component only (f x = 1) can be derived from Eq. (2.7) as... 

This inference can also be made by comparing the curves of distribution of pores in their size (Figure 33.7). One of the few properties common to samples 34 and 36 is hydrophobicity of their surface layers. Thus, by varying the composition of a surface layer of xerogels it is possible to exert an effect on selectivity of their sorption. 

From the above-stated it follows that synthesis of poly-organosiloxane xerogels with a bifunctional surface layer is possible when a use is made of functional groups of various nature. In this case the composition of a surface layer can be varied over a wide range (in our experiments the SH/NH2 ratio was equal to 4 0, 3 1, 2 2, or 1 3). 

The partial mass thickness(es) determined can be used to calculate (1) the elemental composition of a surface layer or (2) the total mass thickness knowing the stoichiometry of the surface layer. Moreover, the thickness in nm or pm can be calculated knowing the density of the surface layer material. 

Surface oxides may also be examined directly by removing them from the base metal. Iodine in dry methanol will dissolve most metals, leaving the oxide unaffected. Electron diffraction, especially if combined with elemental analysis, provides a powerful method of studying the structure and composition of a surface layer on a metal. 

This approach comes to neglecting the variation of composition as a function of depth in the explored zone and to determine the composition of a surface layer... 

An RBS spectrum contains information about the mass of the scattering atoms, the composition of the surface layer, the depth of scattering atoms, and the thickness of a surface layer. 

The purpose of diffusion coatings is not to produce a coating of another metal on the substrate, but to change the composition of the surface layers... 

A general problem existing with all multicomponent catalysts is the fact that their catalytic activity depends not on the component ratio in the bulk of the electrode but on that in the surface layer, which owing to the preferential dissolution of certain components, may vary in time or as a result of certain electrode pretreatments. The same holds for the phase composition of the surface layer, which may well be different from that in the bulk alloy. It is for this reason that numerous attempts at correlating the catalytic activities of alloys and other binary systems with their bulk properties proved futile. 

Significant depletion of In, enrichment in iodine and simultaneous reduction of selenium to Se° was observed by XPS for n-CuInSe2 photoelectrode surfaces in an I2—1 —Cu+—HI electrolyte [129]. Over a distance of 100 A the composition of the surface layer gradually changes to that of the substrate. With the help of UPS analysis the surface film, assumed to be CuISe3, was characterized as p-type semiconductor [130],... 

A thermodynamically acceptable explanation for the solubility behavior of solid solutions at x = 0.868 is needed. First, we shall assume that OHA-FA solid solutions are ideal. If the composition of the surface layer of the solid particles is given by Equation (49), then the following equations can be derived (2) ... 

If more than one component is present in the droplet, the evaporation rate depends on the miscibility of the components. For totally miscible components, distillation of the more volatile component(s) occurs, and the droplet composition changes as evaporation proceeds. If the components are partially miscible or insoluble, evaporation may be greatly retarded by the formation of a surface layer. In the case of evaporation of a surfactant... 

Kinetic parameters of zinc electrode processes in water-organic mixtures depend strongly on the composition of the surface layer, which is modified by the adsorption of organic solvent on the electrode and also on reactant solvation [46], which is changing with a solvent composition. 

All the above studies indicated clearly that reduction of solvent, salt, and additives (e.g., H20) by Li contribute together to the buildup of the surface films on lithium in solutions. It should be emphasized that XRD, XPS, and AES studies of Li electrodes, as well as the indirect identification of surface species from studies of reactions of lithiated graphite or Li/Hg amalgam with electrolyte solutions, could not provide specific enough information on the chemical composition of the surface films. Moreover, application of XPS for Li electrodes may induce secondary surface reactions. Visible changes appear on Li surfaces during XPS measurements. More specific information on the composition of the surface layers formed on Li could be obtained by surface-sensitive FTIR spectroscopy that was introduced into this field in the middle of 1985 by Yeager et al. [84,85,178], and which is a nondestructive technique. 

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