Morphology microcrystals

Fig. 5. Silver haUde grain morphologies for (a) cubic, precipitated in an environment having a silver ion concentration, [Ag" ], of ca 2.5 x 10 mol/L (b) octahedral, ca 6.0 x 10 mol/L and (c) tabular microcrystals, ca 1.0 x 10 ° mol/L. A cross section of a tabular grain revealing double parallel twin planes...

Comparing the above-mentioned results with the data of morphological survey of islet films of gold on a ZnO surface [116- 168] leads to an inference that the maximum of curve 1 is associated with changes in the geometric surface of the Au islet film as it grows, while the maximum at curve 2 is connected with changes in the mean size of microcrystals in the islet film. 

We reported the preparation of sophisticated bipolar three-dye photonic antenna materials for light harvesting and transport [22]. The principle is illustrated in Figure 1.12. Zeolite L microcrystals of cylinder morphology are used as host for organizing several thousand dyes as monomers into well-defined zones. 

Batis-Landoulis, H. Vergnon, P. (1983) Magnetic moment of y-Pe203 microcrystals, morphological and size effects. J. Mat. Sd. 18 3399-3403... 

In this review, the relationships between structure, morphology, and surface reactivity of microcrystals of oxides and halides are assessed. The investigated systems we discuss include alkali halides, alkaline earth oxides, NiO, CoO, NiO-MgO, CoO-MgO solid solutions, ZnO, spinels, cuprous oxide, chromia, ferric oxide, alumina, lanthana, perovskites, anatase, rutile, and chromia/silica. A combination of high-resolution transmission electron microscopy with vibrational spectroscopy of adsorbed probes and of reaction intermediates and calorimetric methods was used to characterize the surface properties. A few examples of reactions catalyzed by oxides are also reported. 2001... 

Which types of faces do the microcrystals expose (or, alternatively, what are their morphologies) ... 

How flat and regular are the faces defining the morphologies of the microcrystals ... 

The morphological information contained in the electron micrographs of polycrystalline materials (constituted by a multitude of variably shaped polyhedra) is difficult to extract and does not give straightforward and unambiguous information about the shapes of the microcrystals. In the images obtained with HRTEM only the two-dimensional (2D) projections of the single 3D polyhedra in various orientations relative to the electron beam are obtained. 

Second, at low coverages, the vibrational perturbation induced by adsorption on cationic sites located on different faces of the same microcrystal is primarily determined by the coordinative unsaturation of the cation (which in turn is a complex function of the structure of the face). This statement implies that the vibrational spectra of diatomic molecules adsorbed on low-surface-area materials (in which the crystallites exhibit only a few dominant faces) are usually characterized by the presence of a small number of narrow peaks—one for each exposed face. Therefore, vibrational spectra of adsorbed species provide morphological information that can be compared with information derived from HRTEM and SEM studies of the same microcrystals. 

C. Predicting Microcrystal Morphology Introduction to Geometric, Force Field, and Ab Initio Modeling Approaches... 

The morphology of a microcrystal depends in a complex way on the thermodynamics and kinetics that determine the stabilities of the faces and their growth. Currently, an exhaustive theory of crystal growth in different atmospheres is not available nevertheless, a reasonable prediction of surface morphology based on the bulk crystalline structure of the solid is possible in many cases. 

The relationship between the growth rate and 4 1 can be explained easily if it is considered that the faces with highest dhki also show the highest concentration of chemical and/or electrostatic bonds within the plane of the face and the minimal density of bonds in the direction perpendicular to the plane of the face. When oxides and halides are considered, the previous rule can also be formulated in the following way Faces exposing ions with the lowest coordinative unsaturation are the most stable and least reactive and hence determine the final morphology of the microcrystals under thermodynamic equilibrium conditions. 

Fig. 6 shows clearly that the anionic species formed at threefold coordinated sites are preferentially affected by the morphological modifications induced by the sintering and resulting from a dramatic decrease in the abundance of surface O T anions as the dimension and perfection of the microcrystals gradually increase (Fig. 5). Of course, on the nearly perfect microcrystals of MgO smoke, anionic species were not observed.

NiO is a cubic oxide characterized by ionicity and lattice parameters very similar to those of MgO. Furthermore, the preparation procedures of the two oxides may be similar (i) Stoichiometric high-surface-area NiO is prepared (as is MgO) from the hydroxide precursor by decomposition under vacuum and (ii) low-surface-area materials are obtained by progressive sintering at high temperatures. The evolution of the microcrystal morphology on passing from high- to low-surface-area (sintered) NiO is also similar to that for MgO, as demonstrated by Escalona Platero et al. (73,265,266) the final habit of the microcrystals is represented by nearly perfect cubes predominantly defined by atomically flat (001) faces and terraces. 

Because of the high purity, the excellent morphological definition, and the good optical properties of the microcrystals, the surface and catalytic properties of ZnO powders prepared by Zn combustion have been investigated extensively by spectroscopic techniques. The results of these investigations are well suited to comparison with results obtained with single crystals and less well-defined samples. 

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