Solid surface methods

G.A. Somorjal, Chemistry in Two Dimensions Surfaces, Cornell University Press (1981). (Composition, structure and reactions of solid surfaces, methods and results, many of them tabulated.)... 

The interface between a solid and its vapor (or an inert gas) is discussed in this chapter from an essentially phenomenological point of view. We are interested in surface energies and free energies and in how they may be measured or estimated theoretically. The study of solid surfaces at the molecular level, through the methods of spectroscopy and diffraction, is taken up in Chapter VIII. 

A number of methods that provide information about the structure of a solid surface, its composition, and the oxidation states present have come into use. The recent explosion of activity in scanning probe microscopy has resulted in investigation of a wide variety of surface structures under a range of conditions. In addition, spectroscopic interrogation of the solid-high-vacuum interface elucidates structure and other atomic processes. 

Swanson L W and Davis P R 1985 Work function measurements Solid State Physics Surfaces(Methods of Experimental Physics 22) cd R L Park and M G Lagally (New York Academic) chi... 

Our intention is to give a brief survey of advanced theoretical methods used to detennine the electronic and geometric stmcture of solids and surfaces. The electronic stmcture encompasses the energies and wavefunctions (and other properties derived from them) of the electronic states in solids, while the geometric stmcture refers to the equilibrium atomic positions. Quantities that can be derived from the electronic stmcture calculations include the electronic (electron energies, charge densities), vibrational (phonon spectra), stmctiiral (lattice constants, equilibrium stmctiires), mechanical (bulk moduli, elastic constants) and optical (absorption, transmission) properties of crystals. We will also report on teclmiques used to study solid surfaces, with particular examples drawn from chemisorption on transition metal surfaces. 

The diversity of approaches based on HF (section B3.2.3.4) is small at present compared to the diversity found for DFT. For solids, HF appears to yield results inferior to DFT due to the neglect of electron correlation, but being a genuine many-particle theory it offers the possibility for consistent corrections, in contrast to DFT. Finally, the QMC teclmiqiies (section B3.2.3.41 hold promise for genuine many-particle calculations, yet they are still far from able to offer the same quantities for the same range of materials and geometries as the theories mentioned before. With this wide range of methods now introduced, we will look at their application to chemisorption on solid surfaces. 

Computational solid-state physics and chemistry are vibrant areas of research. The all-electron methods for high-accuracy electronic stnicture calculations mentioned in section B3.2.3.2 are in active development, and with PAW, an efficient new all-electron method has recently been introduced. Ever more powerfiil computers enable more detailed predictions on systems of increasing size. At the same time, new, more complex materials require methods that are able to describe their large unit cells and diverse atomic make-up. Here, the new orbital-free DFT method may lead the way. More powerful teclmiques are also necessary for the accurate treatment of surfaces and their interaction with atoms and, possibly complex, molecules. Combined with recent progress in embedding theory, these developments make possible increasingly sophisticated predictions of the quantum structural properties of solids and solid surfaces. 

A vast amount of research has been undertaken on adsorption phenomena and the nature of solid surfaces over the fifteen years since the first edition was published, but for the most part this work has resulted in the refinement of existing theoretical principles and experimental procedures rather than in the formulation of entirely new concepts. In spite of the acknowledged weakness of its theoretical foundations, the Brunauer-Emmett-Teller (BET) method still remains the most widely used procedure for the determination of surface area similarly, methods based on the Kelvin equation are still generally applied for the computation of mesopore size distribution from gas adsorption data. However, the more recent studies, especially those carried out on well defined surfaces, have led to a clearer understanding of the scope and limitations of these methods furthermore, the growing awareness of the importance of molecular sieve carbons and zeolites has generated considerable interest in the properties of microporous solids and the mechanism of micropore filling. 

Analytical separations may be classified in three ways by the physical state of the mobile phase and stationary phase by the method of contact between the mobile phase and stationary phase or by the chemical or physical mechanism responsible for separating the sample s constituents. The mobile phase is usually a liquid or a gas, and the stationary phase, when present, is a solid or a liquid film coated on a solid surface. Chromatographic techniques are often named by listing the type of mobile phase, followed by the type of stationary phase. Thus, in gas-liquid chromatography the mobile phase is a gas and the stationary phase is a liquid. If only one phase is indicated, as in gas chromatography, it is assumed to be the mobile phase. 

Liquids, directly solids, following dissolution solids, surfaces, and thin films with special methods (e.g., laser ablation)... 

This article discusses why one would choose nonresonant multiphoton ionization for mass spectrometry of solid surfaces. Examples are given for depth profiling by this method along with thermal desorption studies. 

The atom flux sputtered from a solid surface under energetic ion bombardment provides a representative sampling of the solid. Sputtered neutral mass spectrometry has been developed as method to quantitatively measure the composition of this atom flux and thus the composition of the sputtered material. The measurement of ionized sputtered neutrals has been a significant improvement over the use of sputtered ions as a measure of flux composition (the process called SIMS), since sputtered ion yields are seriously affected by matrix composition. Neutral panicles are ionized by a separate process after sputter atomization, and SNMS quantitation is thus independent of the matrix. Also, since the sputtering and ionization processes are separate, an ionization process can be selected that provides relatively uniform yields for essentially all elements. 

Before equations such as Eqs. 6, 7 and 8 can be used, values for the surface energies have to be obtained. While surface energies of liquids may be measured relatively easily by methods such as the du Nouy ring and Wilhelmy plate, those of solids present more problems. Three approaches will be briefly described. Two involve probing the solid surface with a liquid or a gas, the third relies on very sensitive measurement of the force required to separate two surfaces of defined geometry. All involve applying judicious assumptions to the experimental results. 

Many of the most widely used methods are based on measuring the contact angles of a series of test liquids on the solid surface, and evaluating the surface energies via Young s equation, Eq. 4 above. 

The adsorption of gas onto a solid surface can also be used to estimate surface energy. Both inverse gas chromatography (IGC) and isotherm measurement using the BET method [19] have been used. Further discussion and detailed references are given by Lucic et al. [20] who compare the application of IGC, BET and contact angle methods for characterising the surface energies of stearate-coated calcium carbonate fillers. 

Sec. Ill is concerned with the description of models with directional associative forces, introduced by Wertheim. Singlet and pair theories for these models are presented. However, the main part of this section describes the density functional methodology and shows its application in the studies of adsorption of associating fluids on partially permeable walls. In addition, the application of the density functional method in investigations of wettability of associating fluids on solid surfaces and of capillary condensation in slit-like pores is presented. 

Another method of simulating chemical reactions is to separate the reaction and particle displacement steps. This kind of algorithm has been considered in Refs. 90, 153-156. In particular. Smith and Triska [153] have initiated a new route to simulate chemical equilibria in bulk systems. Their method, being in fact a generalization of the Gibbs ensemble Monte Carlo technique [157], has also been used to study chemical reactions at solid surfaces [90]. However, due to space limitations of the chapter, we have decided not to present these results. 

A very simple, though indirect, method of surface analysis is the measurement of the angle of contact that a liquid makes with the solid surface being analyzed. This method has been widely used to study changes introduced in a polymer surface by various treatments. 

A more complex but more versatile separation method is chromatography, a technique widely used in teaching, research, and industrial laboratories to separate all kinds of mixtures. This method takes advantage of differences in solubility and/or extent of adsorption on a solid surface. In gas-liquid chromatography, a mixture of volatile liquids and gases is introduced into one end of a heated glass tube. As little as one microliter (10-6 L) of sample may be used. The tube is packed with an inert solid whose surface is coated with a viscous... 

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