Regular stepped surfaces

Figure 3. The other common imperfection site on a (111) surface is a kink. This 7(111) X (310) surface shown in perspective is a regularly stepped and kinked surface with a step sequence beginning every seventh row and with a kink site at every third atom in the row. The atoms in kink sites have a coordination number of six...

Figure 3.44 Schematic representation of i(f) transients of condensed 2D Meads phase formation at monatomic steps, (a) substrate surface with regular step spacing (b) substrate surface with irregular step spacing (arbitrary step distribution).

As already indicated, a weU-defmed step-wise (Type VI) isotherm is obtained when a noble gas or lower hydrocarbon is adsorbed on a basal graphitic surface at an appropriate temperature [7, 11]. The regular steps can extend up to four or five molecular layers, but become less sharp with increased distance from the adsorbent surface. An increase in temperature also produces a progressive blurring of the layer-by-layer adsorption [7]. The appearance of such regular multilayer steps in isotherms on uniform surfaces supports the view that (a) the influence of the surface structure can extend well beyond the first adsorbed layer and (b) the multilayer steps are associated with a form of localized physisorption. 

Since the catalytic activity takes place not only on the three-component system Cu/ZnO/AljOj, but already on pure ZnO, the first step in a thorough analysis of the reaction mechanism is to investigate how the reactants, intermediates and products adsorb on ZnO. However, since ZnO catalysts are mostly prepared in wet chemical processes, it is difficult to find out whether the active sites are Zn or 0 atoms on regular low-index surface planes, edges or corners of ZnO micro crystals, defects such as O vacancies, or even more complicated species. Of course, the simplest possibility is to study processes at regular ZnO surface planes, as is always done in the surface science approach to heterogeneous catalysis. 

Stepped Surfaces. Steps on vicinal surfaces are interesting because they represent a set of one-dimensional (ID) nanostructures. A regular step array is most often observed [155-158] though at low temperatures the energetic minimum can be a faceted surface [159]. The origin of this order is the step-step... 

Wulfhekel, W. et al., Regular step formation on concave-shaped surfaces on 6H-SiC(0001). Surface Science, 2004. 550(1-3) 8-14. 

Although all real surfaces have steps, they are not usually labelled as vicinal unless they are purposely misoriented in order to create a regular array of steps. Vicinal surfaces have unique properties, which make them useful for many types of experiments. For example, steps are often more chemically reactive than terraces, so that vicinal surfaces provide a means for investigating reactions at step edges. Also, it is possible to grow nanowires by deposition of a metal onto a surface of another metal in such a way that the deposited metal diflfiises to and attaches at the step edges [3]. 

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