Gel preparation and properties

Although it is true that good crystals can occasionally be grown in substances that are not normally classified as gels, the general observation is that gels and, in particular, silica gels, are the best and most versatile growth media. Their preparation, structure, and properties therefore deserve attention. At the same time, it is useful to note that no clean-cut demarcation lines between gels, sols, colloidal suspensions, and pastes have ever been established. Standard descriptions of these materials are certainly available but they are not nearly as crisp as one would wish, and many practical substances must be regarded as borderline cases. Thus, for instance, Lloyd (1926) wrote disarmingly that ‘a gel is easier to recognize than to define’, and even 23 years later the best available characterization referred to a gel as ‘a two-component system of a semi-solid nature, rich in liquid’ (Alexander and Johnson, 1949). No one is likely to entertain illusions about the rigor of such a definition.

The materials which are ordinarily called gels include not only silica gel (e.g. as usually grown from sodium metasilicate solution), but also agar (a carbohydrate polymer derived from seaweed), gelatin (a substance closely related to proteins), soft soaps (potassium salts of higher fatty acids), a variety of oleates and stearates, polyvinyl alcohol, various hydroxides in water, and even (water-insoluble) tetraethoxysilane in the presence of electrolytes and co-solvents (e.g. methanol) or surface-active agents (Caslavska and Gron, 1984).

General principles

The problem of nucleation is of crucial importance in practical operations, since the crystals which grow in any particular gel system compete with one another for solute. This competition limits their size and perfection, and it is obviously desirable to suppress nucleation until, ideally, only one crystal grows in a predetermined and convenient place. The available techniques do not, as yet, allow us to reach this level of success, though they can sometimes approach it, and sometimes achieve it by happy accident. Growth of a solitary crystal of calcium oxalate, evidently of high perfection, is illustrated in Fig. 4.1.1; see Arora (1981).

Since the application of dislocation theory to these problems (e.g. Frank, 1949, 1950, 1951a, b and Burton et al., 1951) there has been a great increase in our knowledge of the manner in which crystals continue to grow, once growth has started. In comparison, the amount of precise information on the nature of that start is still only small. Always experimentally difficult, the problem is evidently simplest in vapors and melts because only one substance is then involved. It is a priori more complex in the case of solutions because of solute-solvent surface interaction and the possibility of nuclei in the course of formation being solvent contaminated (Smakula, 1962).

Crystals growth in gels is evidently a variant of growth in solution, with additional complications arising from the presence of the gel. In this sense, gel systems do not lend themselves well to nucleation studies of the most fundamental kind. Detailed quantitative considerations, though superficially tempting, are therefore not (or, at any rate, not yet) likely to be profitable in the present context.

When Crystal Growth in Gels (The Pennsylvania State University Press, University Park, Pennsylvania, 1970) was published, it was very much a playful ‘first book’ on the subject. Since then, the field as a whole has flourished, both in terms of practical technique and in terms of understanding. Inevitably, the advent of the computer has also left its mark. The present volume was prepared in order to provide an updated summary of our experience. Inter alia, it is intended as a guide to the literature, but it should not be taken as an encyclopedic or exhaustive evaluation of previously published material.

I am indebted to the late Dr. V. Vand for introducing me to the subject of crystal growth in gels in the 1960s and to many friends for their collaboration, their advice and, above all, for their forebearance in the face of enthusiasm. On this occasion, I would like to express special thanks to colleagues who have so generously contributed new illustrative material to this edition: J. Adair, H. Arend, S. K. Arora, H. Behm, A. S. Bhalla, B. Březina, J. F. Charnell, J. M. García-Ruiz, M. T. George, E. S. Halberstadt, J. Hanoka, F. Lefaucheux, P. Ramasamy and J. Ross. I owe a special debt to J. M. Garcia-Ruiz for many stimulating discussions, to Bonny Farmer for the exercise of her editorial skills, and to my wife, Bridget, for soothing the furrowed brow in times of crisis.

Fundamentals

At the beginning of this century, scientists generally believed that all adsorption phenomena were of the sort we have called physisorption. Some (unspecified) long range attractive force drew gas phase matter toward a solid and the increased concentration of the gaseous substance near the surface was thought to be analogous to the retention of the Earth's atmosphere by the gravitational field. The adsorbed layer was viewed as a ‘compressed vapor’ with little or no interaction with the atoms of the substrate. However, compelling experimental evidence soon accumulated that pointed to another, distinctly different, form of adsorption.

Langmuir (1916) introduced and extensively investigated the idea that there can exist strong, short range forces between adsorbates and a substrate. He regarded the arrangement of atoms at the surface of a solid as a sort of Chinese checkerboard that defines a specific density of potential adsorption sites. Foreign gas atoms that strike the surface may either bounce back into the gas phase or bind to one of these sites through formation of a surface chemical bond. The latter process is termed chemisorption and, in this view, it is not unreasonable to regard the adsorbate/substrate complex as an enormously large molecule.

Adsorption, and chemisorption in particular, lowers the free energy of any closed system that contains only a free surface and atoms or molecules in the gas phase.

Introduction

The preceding chapters have focused entirely on the equilibrium free energy state of an isolated clean crystal surface. Unfortunately, many of the most interesting conceptual (and commercial) issues in surface physics intimately involve the interaction of a solid surface with foreign matter. If the interaction is strong, it is necessary to treat the surface and the foreign material as a single combined system. This is the subject of Part 2 of this book. However, if the interaction is weak, the surface merely responds to the external perturbation while retaining its individual identity. In fact, any real experiment designed to probe the properties of even an isolated surface invariably perturbs the system and invokes a characteristic response. This response is determined by the low-lying excited states of the system.

For example, consider an experiment designed to determine the binding energy and dispersion of an electronic surface state. In practice, one uses photoemission spectroscopy to measure the kinetic energy and propagation vector of an electron ejected from the sample into the vacuum. However, what one actually measures is the energy and relative momentum of an excited electronic state (with a finite lifetime) that consists of 1023 electrons in the presence of the surface-localized hole left behind by the photoelectric event. A priori, this could be a horribly complex state of the interacting many-body system.

Introduction

A complete characterization of a solid surface requires knowledge of not only what atoms are present but where they are. Just as in the bulk, it is not that the atomic coordinates as such are of much direct interest. Rather, our concerns generally will center on electronic and magnetic properties and it is the geometrical arrangement of the surface atoms that largely determines the near-surface charge and spin density. Put another way, the nature of the surface chemical bond depends in detail on surface bond lengths and bond angles. The corresponding bulk structural issues normally are resolved by x-ray diffraction. Unfortunately, the extremely large penetration depth and mean free path of x-rays severely limits their routine use for surface crystallography. Consequently, much effort has been devoted to the invention and application of alternative experimental approaches to surface-specific structural analysis. Although a number of common techniques will be discussed below, it is a sobering fact that no single surface structural tool has emerged that can be used as easily and reliably as x-rays are used for the bulk.

Appeal to theory does not offer much relief. In principle, a solid adopts the crystal structure that minimizes its total energy. We know how to write down an exact expression for this energy; it is a parametric function of the exact position of all the ions in the material.

Introduction

Our penultimate chapter is concerned with the chemical physics of surface reactions. More precisely, the consistent microscopic perspective adopted throughout this book demands that we ask the following question: can the experimental and theoretical methods of surface physics provide a useful account of real-life surface reaction processes? Let us emphasize the word ‘useful’. It is one thing to construct a post facto analysis which faithfully reproduces some set of observations. It is quite another to formulate general principles which provide qualitative insight and lead to quantitative predictive power. The selected examples below are intended to demonstrate that this discipline is just now passing from the former perspective to the latter.

Surface reactions are complex events which come in many guises for many purposes. Often (but not always!) one begins with some combination of species in the gas phase: the reactants. In heterogeneous catalysis, the purpose of the surface is to confine the reactants to a two-dimensional space in order to increase the probability for collision and reaction. The sought-after reaction products desorb for collection with no material change in the surface itself. Compare this to metalorganic chemical vapor deposition (MOCVD) growth of compound semiconductors (Dupuis, 1984). Therein, the surface stimulates a decomposition reaction. Unwanted species desorb and the desired species incorporate themselves into the solid.

Introduction

The physics of chemisorption hinges on the static and dynamic properties of the surface chemical bond. Bond formation, bond stability and bond dissolution all are crucial to our subject. In this chapter we begin our investigation motivated by the well-known intimate connection between bonding and structure (see, e.g., O'Keeffe & Navrotsky (1981)). The spatial distribution, strength and reactivity of the electronic bonds within a chemisorption complex depend sensitively on the relative position of the adsorbate and substrate nuclei. In the best case, all bond distances and bond angles will be known to any desired numerical accuracy. Minimally, we should know the local symmetry of the adsorption site, the gross orientation of the adsorbate with respect to the surface and the nature of any structure within the adsorption layer itself. Unfortunately, in the world of surface science, none of these quantities reveals itself in a straightforward, mechanical fashion.

What is the geometrical arrangement of atoms in the surface region of a clean crystal after adsorption by a foreign species? Consider first the case of a single adatom. As discussed in Chapter 8 for the case of Au/NaCl(100), the most probable binding sites occur at substrate positions where the total adatom/substrate potential energy of interaction has minima.