Particle-substrate orientations

An examination of a dark-field micrograph of a field such as that of Fig. 1 confirms the presence of an appreciable proportion of multiply twinned particles, and also shows that most of the particles are orientated with a 111 parallel to the substrate (29, 30). 

In other cases (sometimes for other areas of the same sample preparation) the particles are heavily twinned and show little correlation with the substrate orientation. It remains to be seen whether these differences may be associated with particular conditions, structures or chemical composition of the substrate surface. 

The predominance of van der Waals interactions at solid metal/A1203 interfaces is also shown by the fact that whatever the orientation of monocrystalline AI2O3 surface, Cu particles are orientated with (111) faces parallel to the A1203 surface (Soper et al. 1996). This orientation maximises the number of metal atoms per unit area in nearest-neighbour interactions with A1203. A similar behaviour was found for non-reactive fee metals on carbon substrates i.e., for systems with predominant van der Waals interfacial interactions (Section 8.1). 

Moreover, high-resolution electron microscopy and electron diffraction on lipid A-diphosphate rods (length of the order of pm and diameter of several nanometers) revealed that the rods were held to the truncated polyhedral with a 5-fold symmetry (see also Figure 7 and legend). It was possible to show that most of the lipid A-diphosphate particles were orientated in the [001] direction with respect to the substrate for one of the five deformed tetrahedral subunits that is the 5-fold axis was parallel to the surface of the substrate. 

Therefore, in many fundamentally oriented studies the complex catalyst is replaced by a simplified model, which is better defined. Such models range from supported particles from which all promoters have been removed, via well-defined particles deposited on planar substrates, to single crystals (Fig. 4.1). With the latter we are in the domain of surface science, where a wealth of informative techniques is available that do not work on technical catalysts. 

Crosslinked acrylic microgels in aqueous and non-aqueous media were patented as paint constituents in 1979 to improve the orientation of aluminum flake pigments, restrict the flow of the liquid coating on the substrate and restrict sagging [324]. As the patent speaks of emulsions, insolubility of the microgels and particle sizes up to 200 nm, it is questionable whether these polymers consisted of microgels only. 

For small particles supported on thin films of amorphous or microcrystalline materials it is not easy to determine whether there is any consistent correlation between the particle orientation and the orientation of the adjacent locally ordered region of the substrate. For some samples of Pt and Pd on gamma-alumina, for example, nanodiffraction shows that the support films have regions of local ordering of extent 2 to 5 nm. Patterns from the metal particles often contain spots from the alumina which appear to be consistently related to the metal diffraction spots. 

In this way it has been shown that for Pd particles on single crystal alpha-alumina films and for Au particles on magnesium oxide smoke crystals there may be wide variations of behavior depending on factors which are not immediately apparent. In some cases, the particles are single crystals and show a strong correlation between their orientations and the orientation of the substrate. 

---