Surface specificity

The site specificity of reaction can also be a state-dependent site specificity, that is, molecules incident in different quantum states react more readily at different sites. This has recently been demonstrated by Kroes and co-workers for the Fl2/Cu(100) system [66]. Additionally, we can find reactivity dominated by certain sites, while inelastic collisions leading to changes in the rotational or vibrational states of the scattering molecules occur primarily at other sites. This spatial separation of the active site according to the change of state occurring (dissociation, vibrational excitation etc) is a very surface specific phenomenon. 

Experimental investigations of the model system of dye molecules adsorbed onto surfaces of polystyrene spheres have finuly established the sensitivity and surface specificity of the SHG method even for particles of micrometre size [117]. The surface sensitivity of die SHG process has been exploited for probing molecular transport across the bilayer in liposomes [118], for measurement of electrostatic potentials at the surface of small particles [119] and for imaging... 

Tn general, the. solvent-accessible surface (SAS) represents a specific class of surfaces, including the Connolly surface. Specifically, the SAS stands for a quite discrete model of a surface, which is based on the work of Lee and Richards [182. They were interested in the interactions between protein and solvent molecules that determine the hydrophobicity and the folding of the proteins. In order to obtain the surface of the molecule, which the solvent can access, a probe sphere rolls over the van der Waals surface (equivalent to the Connolly surface). The trace of the center of the probe sphere determines the solvent-accessible surjace, often called the accessible swface or the Lee and Richards surface (Figure 2-120). Simultaneously, the trajectory generated between the probe and the van der Waals surface is defined as the molecular or Connolly surface. 

Auger electron spectroscopy is the most frequently used surface, thin-film, or interface compositional analysis technique. This is because of its very versatile combination of attributes. It has surface specificity—a sampling depth that varies... 

By a fortunate coincidence, the depth into the solid from which information is provided by the techniques described here matches the above definition of a surface almost exactly. These techniques are, therefore, surface-specific, in other words, the information they provide comes only from that very shallow depth of a few atom layers. Other techniques can be surface sensitive, in that they would normally be regarded as techniques for bulk analysis, but have sufficient sensitivity for certain elements that can be analyzed only if they are present on the surface only. 

Tab. 1.1. Surface-specific analytical techniques using particle or photon excitation. The acronyms printed in bold are those used for methods discussed in more details in this publication.

Tab. 1.2. Surface-specific analytical techniques using non-particle excitation.

Electron spectroscopic techniques require vacuums of the order of 10 Pa for their operation. This requirement arises from the extreme surface-specificity of these techniques, mentioned above. With sampling depths of only a few atomic layers, and elemental sensitivities down to 10 atom layers (i. e., one atom of a particular element in 10 other atoms in an atomic layer), the techniques are clearly very sensitive to surface contamination, most of which comes from the residual gases in the vacuum system. According to gas kinetic theory, to have enough time to make a surface-analytical measurement on a surface that has just been prepared or exposed, before contamination from the gas phase interferes, the base pressure should be 10 Pa or lower, that is, in the region of ultrahigh vacuum (UHV). 

After XPS, AES is the next most widely used surface-analytical technique. As an accepted surface technique AES actually predates XPS by two to three years, because the potential of XPS as a surface-specific technique was not recognized immediately by the surface-science community. Pioneering work was performed by Harris [2.125] and by Weber and Peria [2.126], but the technique as it is known today is basically the same as that established by Palmberg et al. [2.127]. 

The reasons AES is a surface-specific technique have been given in Sect. 2.1.1, with reference to Fig. 2.2. The normal range of kinetic energies recorded in an AES spectrum would typically be from 20 to 1000 eV, corresponding to inelastic mean free path values of 2 to 6 monolayers. 

Compared with XPS and AES, the higher surface specificity of SSIMS (1-2 mono-layers compared with 2-8 monolayers) can be useful for more precise determination of the chemistry of an outer surface. Although from details of the 01s spectrum, XPS could give the information that OH and oxide were present on a surface, and from the Cls spectrum that hydrocarbons and carbides were present, only SSIMS could be used to identify the particular hydroxide or hydrocarbons. In the growth of oxide films for different purposes (e.g. passivation or anodization), such information is valuable, because it provides a guide to the quality of the film and the nature of the growth process. 

Filter aids should have low specific surface, since hydraulic resistance results from frictional losses incurred as liquid flows past particle surfaces. Specific surface is inversely proportional to particle size. The rate of particle dispersity and the subsequent difference in specific surface determines the deviations in filter aid quality from one material to another. For example, most of the diatomite species have approximately the same porosity however, the coarser materials experience a smaller hydraulic resistance and have much less specific surface than the finer particle sizes. 

There are still other surface analysis techniques including ellipsometry, surface enhanced Raman scattering, light scattering, nano-hardness measurements etc. which are used for specific investigations. It is, however, already evident from this discussion that many new and powerful techniques now are available which offer the capability of investigating various aspects of polymer surfaces on a molecular level. Some of those techniques are surface specific while others can be used for the analysis of buried interfaces, too. 

Crystal surface specificity of the potential of zero charge, 152 Current-potential curves for bipolar membranes, 228 of iron dissolution in phosphoric acid,... 

Potential of zero charge cont.) contribution of the solvent, 158 Conway and Colledan, and the determination of, 34 on copper, and aqueous solution, 89 crystal phase and, 44 crystal face specificity of, 21 and the crystal surface specificity, 152 DeLevie, on the effect of the density of broken bonds on, 75 dependence upon crystal phase, 154 dependence upon time of measurement, 150,151... 

The alkali promotion of CO dissociation is substrate-specific, in the sense that it has been observed only for a restricted number of substrates where CO does not dissociate on the clean surface, specifically on Na, K, Cs/Ni( 100),38,47,48 Na/Rh49 and K, Na/Al(100).43 This implies that the reactivity of the clean metal surface for CO dissociation plays a dominant role. The alkali induced increase in the heat of CO adsorption (not higher than 60 kJ/mol)50 and the decrease in the activation energy for dissociation of the molecular state (on the order of 30 kJ/mol)51 are usually not sufficient to induce dissociative adsorption of CO on surfaces which strongly favor molecular adsorption (e. g. Pd or Pt). 

Microscopy methods based on nonlinear optical phenomena that provide chemical information are a recent development. Infrared snm-frequency microscopy has been demonstrated for LB films of arachidic acid, allowing for surface-specific imaging of the lateral distribution of a selected vibrational mode, the asymmetric methyl stretch [60]. The method is sensitive to the snrface distribntion of the functional gronp as well as to lateral variations in the gronp environmental and conformation. Second-harmonic generation (SHG) microscopy has also been demonstrated for both spread monolayers and LB films of dye molecules [61,62]. The method images the molecular density and orientation field with optical resolution, and local qnantitative information can be extracted. 

The second approach is to study real catalysts with in situ techniques such as infrared and Mossbauer spectroscopy, EXAFS and XRD, under reaction conditions, or, as is more often done, under a controlled environment after quenching of the reaction. The in situ techniques, however, are not sufficiently surface specific to yield the desired atom-by-atom characterization of the surface. At best they determine the composition of the particles. 

The surface actlve/surface inactive difference between p-polarlsed/ s-polarised radiation has enabled an alternative modulation technique, polarisation modulation, to be developed (15,16). In electrochemical applications, it allows surface specificity to be achieved whilst working at fixed potential and without electrochemical modulation of the interface. It can be implemented either on EMIRS or on SNIFTIRS spectrometers and can be very valuable in dealing with electrochemically irreversible systems however, the achievable sensitivity falls well short of that obtained with electrochemical modulation. It should also be noted that its "surface specificity" is not truly surface but extends out into the electrolyte with decreasing specificity to about half a wavelength. 

Second-order NLO processes, including SFG, are strictly forbidden in media with inversion symmetry under the electric dipole approximation and are allowed only at the interface between these media where the inversion symmetry is necessarily broken. In the IR-Visible SFG measurement, a visible laser beam (covis) and a tunable infrared laser beam (cOir) are overlapped at an interface and the SFG signal is measured by scanning cOir while keeping cOvis constant. The SFG intensity (Isfg) is enhanced when coir becomes equal to the vibration levels of the molecules at the interface. Thus, one can obtain surface-specific vibrational spectra at an interface... 

Hyper-Raman spectroscopy is not a surface-specific technique while SFG vibrational spectroscopy can selectively probe surfaces and interfaces, although both methods are based on the second-order nonlinear process. The vibrational SFG is a combination process of IR absorption and Raman scattering and, hence, only accessible to IR/Raman-active modes, which appear only in non-centrosymmetric molecules. Conversely, the hyper-Raman process does not require such broken centrosymmetry. Energy diagrams for IR, Raman, hyper-Raman, and vibrational SFG processes are summarized in Figure 5.17. 

In this chapter, we will focus on some of the recent developments in understanding the influence of solution and electrochemical conditions over model single-crystal surfaces. Specifically, we will review work applying electronic structure methods to probe electrocatalytic mechanisms occurring at this complex interface. 

The availability of thermodynamically reliable quantities at liquid interfaces is advantageous as a reference in examining data obtained by other surface specific techniques. The model-independent solid information about thermodynamics of adsorption can be used as a norm in microscopic interpretation and understanding of currently available surface specific experimental techniques and theoretical approaches such as molecular dynamics simulations. This chapter will focus on the adsorption at the polarized liquid-liquid interfaces, which enable us to externally control the phase-boundary potential, providing an additional degree of freedom in studying the adsorption of electrified interfaces. A main emphasis will be on some aspects that have not been fully dealt with in previous reviews and monographs [8-21]. 

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