Structural changes of Ag crystals during reaction with nitric oxide

Field ion microscopy (FIM) was used to study the reaction of nitric oxide (NO) with silver (Ag) field emitter tips. The clean and almost hemispherical shape of the Ag tip apex was found to undergo restructuring during field-free interaction with 150 L NO (1 L=1.3×10−4 Pa·s) at 400 K. Imaging at 57 K revealed an enlarged central (111) pole and a smaller number of layers leading to peripheral planes. Thus, crystal coarsening occurred during reaction with NO gas. Moreover, planes lying along the 〈100〉 zone lines appeared dark and could only be recovered by field evaporation of a large number of surface layers. Areas of the former {012} planes appeared enlarged. These structural changes are interpreted as being caused by oxygen atoms released during NO decomposition. Cleaning the oxygen-covered tip by field evaporation and heating it to 400 K reproduced this reconstruction form. This finding is explained by the formation of subsurface oxygen and its resegregation to the surface. However, if the sample, either clean or oxygen-covered, was sputter-cleaned and subsequently heated to 400 K, only slight changes of the surface structure were found. These changes involved increased local disorder and somewhat enlarged low-index planes as compared to the field-evaporated clean Ag specimen.