Crystal surface developement

B. Growth Mechanism and Rate 1. Crystal Surface Development... 

The visible crystals that develop during a crystallization procedure are built up as a result of growth either on nuclei of the material itself or surfaces of foreign material serving the same purpose. Neglecting for the moment the matter of impurities, nucleation theory provides an explanation for certain qualitative observations in the case of solutions. 

A cmcial development for zinc phosphate coatings came in 1943 when it was found that more uniform and finer crystals would develop if the surface was first treated with a titanium-containing solution of disodium phosphate (6). This method of crystal modification is a prime reason for the excellent paint (qv) adhesion seen on painted metal articles. 

Morphology. A crystal is highly organized, and constituent units, which can be atoms, molecules, or ions, are positioned in a three-dimensional periodic pattern called a space lattice. A characteristic crystal shape results from the regular internal stmcture of the soHd with crystal surfaces forming parallel to planes formed by the constituent units. The surfaces (faces) of a crystal may exhibit varying degrees of development, with a concomitant variation in macroscopic appearance. 

LEED is the most powerfiil, most widely used, and most developed technique for the investigation of periodic surface structures. It is a standard tool in the surface analysis of single-crystal surfaces. It is used very commonly as a method to check surface order. The evolution of the technique is toward greater use to investigate surface disorder. Progress in atomic-structure determination is focused on improving calculations for complex molecular surface structures. 

The most arresting development is the use of an STM tip, manipulated to move both laterally and vertically, to shepherd individual atoms across a crystal surface to generate features of predeterminate shapes an atom can be contacted, lifted, transported and redeposited under visual control. This was first demonstrated at... 

U02(N03)2 6 H20 showed unusual behaviour [62] in that there was no induction period to dehydration, the generation of specialized nuclei was apparently unnecessary since water evolution occurred by desorption at existing crystal surfaces and no migratory interface was developed. 

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. 

Thus the study of surfaces has emerged as an important focus in the chemical sciences, and the relationship between surfaces of small systems and their performance has emerged as a major technological issue. Flow in microfluidic systems—for example, in micromechanical systems with potential problems of stiction (sticking and adhesion) and for chemistry on gene chips—depends on the properties of system surfaces. Complex heterogeneous phases with high surface areas—suspensions of colloids and liquid crystals—have developed substantial... 

A remarkable progress has been made in the last several years in electrocatalysis on single crystal surfaces. This parallels the progress in surface science and it has been partly stimulated by developments in that field, mostly regarding the preparation and characterization of surfaces. New advances in preparation of surfaces outside of high vacuum, achieved in electrocatalytic studies, also helped this trend. 

Model studies on single crystal surfaces are also helpful in developing an understanding of the effects of surface additives on catalyst performance. Electronegative, electroneutral (i.e. metals) and electropositive additives can all be studied. The influence of additives on the bond strengths and structure of... 

The key to understanding such processes lies in our ability to dissect the catalytic event into its separate components. Numerous ingenious experiments have been performed by workers in the field of catalysis for many years, and it is not the intent of this article to review these contributions. It is important to note that such studies have advanced the field of catalysis to a refined science and that a number of general observations have been developed which serve as guidelines for the development and improvement of catalytic materials. Insofar as surface science and the study of reactions on macroscopic single crystal surfaces is related to catalysis, its purpose should therefore be to contribute a more exact and, thereby, a more general understanding of the basic phenomena involved. 

Steps, In a real crystal where dislocations are present, there are two types of steps the step that begins and ends on the boundary of the surface (Fig. 3.13a), and the step that starts on the surface and terminates on a boundary (Fig. 3.4). If a step starts on a surface, this is a place where a screw dislocation meets the surface. At 0 K, steps tend to be straight, but as the temperature is raised (F > 0 K), step roughness develops and the structure of the step includes a number of kinks, adsorbed atoms (adatoms or adions), and vacancies (Fig. 3.16). Steps can be of monatomic height or, as in the case of a real crystal surface, polyatomic height. 

We shall proceed from a concept which in a certain sense is contrary to that of the two-dimensional gas. We shall treat the chemisorbed particles as impurities of the crystal surface, in other words, as structural defects disturbing the strictly periodic structure of the surface. In such an approach, which we first developed in 1948 (I), the chemisorbed particles and the lattice of the adsorbent are treated as a single quantum-mechanical system, and the chemisorbed particles are automatically included in the electronic system of the lattice. We observe that this by no means denotes that the adsorbed particles are rigidly localized they retain to a greater or lesser degree the ability to move ( creep ) over the surface. 

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