Kzx

Figure 3. Ligand elimination mechanism in hydride- and alkyl-modified Mg-Al tert-butoxides.

metastable precursor is responsible, in the latter case, for the formation of a composite material. The thermolysis of alkyl silicon alkoxides (Rx-Si(OR)4_x) to produce SiC/SiO2 composite or that of LaAl3(OPr')12 to form LaAlO3/Al2O3 belongs to the first category, whereas the disproportionation of a Cu(I) precursor to give Cu(0) and Cu(II) species from a single source exemplifies the second category. Since these composites originate from a single chemically homogeneous source, the different phases are homogeneously interdis-persed, and it is possible to control their three-dimensional structures and the grain sizes of the individual phases.

Phase I Phase II (5)

For instance, the Mg-Al isopropoxide, [Mg2Al3(OPr')13] (Fig. 4), which can be prepared in good yield by reacting Mg and Al(OPr ')3 in refluxing isopropyl alcohol [317], serves as a unimolecular source to obtain a biphasic oxide-oxide composite of composition MgAl2O4/MgO [321]. The X-ray powder diffractogram of the oxide-oxide nanocomposite

2Mg2Al3(OPr' )13 MgAl2O4 + MgO

Organic by-products Phase I Phase II (6)

obtained from the above precursor shows that both phases are crystalline (Fig. 5). The TEM image revealed a homogeneous biphasic material in which MgO grains are evenly dispersed in a nanocrystalline matrix of spinel particles.

The current interest in the design of nanomaterials at the molecular scale is largely driven by the ability of chemical methods to permit the control of size, shape, and purity, which ultimately influence the final properties of the nano-materials. In this context, the use of SSPs is highly favored to achieve an atomic-scale mixing of the elements in target nanomaterial. The concept of using a single molecular source or a molecular building block is derived in principle from biomolecular, supramolecular, or polymeric structures where a repetitive arrangement of one or more fundamental structural units can lead to novel architectures and properties [33]. However, for inorganic materials, the molecular

Figure 4. Molecular structure of [Mg2Al3(OPr')13]. Reprinted with permission from [317], J. A. Meese-Marktscheffel et al., Chem. Mater. 5, 755 (1993). © 1993, American Chemical Society.

Figure 3. Ligand elimination mechanism in hydride- and alkyl-modified Mg-Al tert-butoxides.

Figure 4. Molecular structure of [Mg2Al3(OPr')13]. Reprinted with permission from [317], J. A. Meese-Marktscheffel et al., Chem. Mater. 5, 755 (1993). © 1993, American Chemical Society.

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