Morphology octahedra

Figure 6 TEM characterization of the structure and morphology of Pd nanoparticles supported on MgO(l 0 0). (a) Electron diffraction pattern (b) top-view micrograph (c) profile view micrograph of an individual particle (d) drawing of the truncated octahedron shape of a Pd particle (e) shape of a large coalesced particle (0 truncated pyramid shape of a small (<7 nm) Pd particle.

FIGURE 15.5 Variation in crystal morphology for identical unit cells, (a) Crystals in the rhombohedral system that only have faces present in the unit cell (001), (010), and (100). (b) Cubic system the leftmost picture is a cube, the rightmost one a regular octahedron in between are intermediate shapes, (c) Examples of the various shapes that an ot-lactose monohydrate crystal (monoclinic) can assume in practice. In (a) and (b) the shapes are shown in perspective. In (c) we have projections (all in the same direction with respect to the axes of the unit cell). 

Nanoparticles are generally assumed to be spherical. However, an interesting interplay exists between the morphology and the packing arrangement, specially in small nanocrystals. If one were to assume that the nanocrystals strictly follow the bulk crystalline order, the most stable structure is arrived at by simply constraining the number of surface atoms. It is reasonable to assume that the overall polyhedral shape has some of the symmetry elements of the constituent lattice. Polyhedra such as the tetrahedron, the octahedron. 

The regular facets of a crystal are planes of the type described above. Here, the lattice architecture of the crystal is visible macroscopically at the surface. Figure 1.3-4 shows some surfaces of a cubic crystal. If the crystal had the shape or morphology of a cube, this would be described by e set of facets (100), (010), (001), (100), (OlO), (OOl). An octahedron would be described by (111), (111), (111), (111), (Hi), (111), (111), (ill). The morphology of a crystalline material may be of technological interest (in relation to the bulk density, flow properties, etc.) and can be influenced in various ways, for exanqtle by additives during the crystallization process. 

In pardcular, studies by HAADF-STEM electron tomography have clearly revealed a definite increase in the extent of (111) type facets exposed at the surface. From a situadon in which a balanced population of both rounded and faceted crystallites are observed in a fresh or SR-SO sample, the morphology of most of the SR-MO crystallites features shapes close to octahedrons or other more irregular shapes, in which 111 facets are still dominant. Fig. 2.6. ... 

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