The Single-Crystal Method of Studying Surface Reactions

The Single-Crystal Method of Studying Surface Reactions 

If the crystal is prepared in the form of a sphere, all possible faces will be exposed a number of times on the surface. In the case of the face-centered cubic metals, for example, the (100) face will appear 6 times, the (111) face 8, the (110) face 12, and so on. Therefore, all faces appear on the surface a number of times, and from the symmetry of the patterns which form during reaction, as described later, the different crystal faces may be 

Several different types of reaction patterns may form on the sphere as the reaction proceeds. In cases where the reaction products actually build up on the surface of the metal, as in oxidation, the oxide film will form more rapidly on one face than on another. When the films are in the range of 200 to 2,500 A. and the sphere is examined by placing a tube of white paper over the crystal, the different thicknesses of oxide on the different faces appear as a regular pattern of interference colors of great beauty. The symmetry of one of these highly colored patterns is shown in Fig. 1. In electrodeposition on a crystal sphere at a low current density the metal will deposit more rapidly on one face than another, and the sphere is converted into a polyhedron, or small facets are formed on the different faces which may be seen under the microscope. 

In the case of catalytic reactions where the reaction products do not build up on the surface but pass on, as in the case of the reaction of hydrogen and 

When larger areas of a particular face are desired, flat surfaces may be cut parallel to a particular crystal plane, and methods have been developed as described later, whereby measurements may be carried out on a single face and the effect of edges and other faces may be eliminated. A particularly convenient specimen is a spherical crystal with plane surfaces cut on it parallel to the particular faces which it is desired to study. The actual measurements can be made on the flat faces, and the pattern on the spherical surface can be used as a sensitive test for possible contaminants and improper control of experimental conditions. 

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III. The Single-Crystal Method of Studying Surface Reactions. 63... 

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