Non-Metallic Adsorbates

The available data are inadequate to permit a detailed analysis of the various factors that control the ordering of metal monolayers on metal surfaces. It is likely that both the electronic interaction between the two metals and the relative atomic sizes should be important in determining the nature of ordering in the monolayer. 

The frequent occurrence of ordered fractional-coverage adsorption indicates that adsorbate-adsorbate interactions at close range (S 5 A) are often repulsive. Island formation can occur simultaneously, showing that at larger separations these interactions can become attractive. 

In the last few years LEED studies of high Miller index or stepped surfaces have become more frequent. Almost all of these studies have been on fee metals, where the atomic structure of these surfaces consists of periodic arrays of terraces and steps. A nomenclature which is more descriptive of the actual surface configuration has been developed for these surfaces, as described in Section III. In Table 5.5 the stepped surface nomenclature for several high Miller index surfaces of fee crystals has been tabulated. In Fig. 5.1 the location of these high Miller index surfaces are shown on the 

Substrate Adsorbed Structure Nearest Heat of Deposition Substrate Technique of Surface structures observed References 

Substrate Adsorbed Structure Nearest Heat of Deposition Substrate 

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In this section, the surface chemistry of non-metals adsorbed as thin layers, films or SAMs on gold surfaces is discussed. Although attachment by a sulfur atom is by far the most predominant binding motif, many other elements may be used to bind to gold. Particular focus is given here to surface binding through atoms other than those already extensively covered in the literature. 

In comparison, too, direct calorimetric determination of heats of adsorption can be less accurate, although less tedious, than heat-of-immersion determinations. Lack of accuracy can occur with poorly conducting, non-metallic adsorbents, where long equilibrium times are required for vapor phase adsorption or where surface sites do not fill in strict accordance with the site energy distribution of the solid surface. Atoms or molecules can be expected to stick to the first part of the surface they strike when strong chemisorption occurs and then molecules are likely not to move freely over... 

Conventionally RAIRS has been used for both qualitative and quantitative characterization of adsorbed molecules or films on mirror-like (metallic) substrates [4.265]. In the last decade the applicability of RAIRS to the quantitative analysis of adsorbates on non-metallic surfaces (e.g. semiconductors, glasses [4.267], and water [4.273]) has also been proven. The classical three-phase model for a thin isotropic adsorbate layer on a metallic surface was developed by Greenler [4.265, 4.272]. Calculations for the model have been extended to include description of anisotropic layers on dielectric substrates [4.274-4.276]. 

For films on non-metallic substrates (semiconductors, dielectrics) the situation is much more complex. In contrast with metallic surfaces both parallel and perpendicular vibrational components of the adsorbate can be detected. The sign and intensity of RAIRS-bands depend heavily on the angle of incidence, on the polarization of the radiation, and on the orientation of vibrational transition moments [4.267]. 

This review is structured as follows. In the next section we present the theory for adsorbates that remain in quasi-equilibrium throughout the desorption process, in which case a few macroscopic variables, namely the partial coverages 0, and their rate equations are needed. We introduce the lattice gas model and discuss results ranging from non-interacting adsorbates to systems with multiple interactions, treated essentially exactly with the transfer matrix method, in Sec. II. Examples of the accuracy possible in the modehng of experimental data using this theory, from our own work, are presented for such diverse systems as multilayers of alkali metals on metals, competitive desorption of tellurium from tungsten, and dissociative... 

The relative importance of the two mechanisms - the non-local electromagnetic (EM) theory and the local charge transfer (CT) theory - remains a source of considerable discussion. It is generally considered that large-scale rough surfaces, e.g. gratings, islands, metallic spheres etc., favour the EM theory. In contrast, the CT mechanism requires chemisorption of the adsorbate at special atomic scale (e.g. adatom) sites on the metal surface, resulting in a metal/adsorbate CT complex. In addition, considerably enhanced Raman spectra have been obtained from surfaces prepared in such a way as to deliberately exclude one or the other mechanism. 

As the electrostatic interaction between the solvated ions and the metal is indirect, it is virtually independent of the chemical nature of the ions these latter are said to be non-specifically adsorbed. 

One of the basic assumptions of the d band model is that E0 is independent of the metal. This is not a rigorous approximation. It will for instance fail when metal particles get small enough that the sp levels do not form a continuous (on the scale of the metal-adsorbate coupling strength) spectrum. It will also fail for metals where the d-states do not contribute to the bonding at all. The other basic assumption is that we can estimate the d contribution as the non-self-consistent one-electron energy change as derived above ... 

Figure 7.4. Schematic model of the Electrical Double Layer (EDL) at the metal oxide-aqueous solution interface showing elements of the Gouy-Chapman-Stern-Grahame model, including specifically adsorbed cations and non-specifically adsorbed solvated anions. The zero-plane is defined by the location of surface sites, which may be protonated or deprotonated. The inner Helmholtz plane, or [i-planc, is defined by the centers of specifically adsorbed anions and cations. The outer Helmholtz plane, d-plane, or Stern plane corresponds to the beginning of the diffuse layer of counter-ions and co-ions. Cation size has been exaggerated. Estimates of the dielectric constant of water, e, are indicated for the first and second water layers nearest the interface and for bulk water (modified after [6]).

Most of the contemporary research areas that utilize surface chemistry techniques employ thin organic films that have been physically or chemically adsorbed onto a solid (usually metallic) substrate. The use of conducting metal surfaces is due not only to their relevance to many different fields, but also to the fact that many surface spectroscopic techniques (Table I) need such a surface in order to produce high-quality spectra. This criterion effectively eliminated many interesting non-metallic surfaces from study using modem surface-sensitive spectroscopic methods. 

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