Three Component Systems ABEx 531 Heterometal Oxides

The preparation of three-component systems poses a challenge for the molecular design of the precursor because of the compatibility of the intrinsic physicochemical properties of the individual building units. Moreover, it demands unambiguously characterized precursors with element ratios corresponding to that of the nanomaterial of choice. A variety of interesting molecular precursors have been synthesized and/or modified for a size- and phase-selective synthesis of nanomaterials (Table 2), which provide the proof of the concept for the molecular-level synthesis of materials.

Stoichiometric MgAl2O4 films have been successfully prepared from magnesium-aluminum isopropoxide containing the appropriate Mg:Al ratio (1:2) [259-261]. However, for the preparation of transition metal-aluminum, -gallium, or -iron spinels, the fusion of component oxides [543, 544] at high temperatures (1000-1600 °C), ball milling [545], self-propogating combustion synthesis [546], coprecipitation reactions [547], polymer route [548], and hydrothermal synthesis [549] have been used. Wickham et al. have used mixed acetates, M3Fe6(OOCCH3)17(O)3(OH>12pyridine (M = Ni, Co, Mn), as crystalline precursors to stoichiometric ferrites. These complexes contain a large number of pyridine molecules of crystallization, and rapid removal of the coordinated ligands can lead to the segregation of ions. Moreover, they require high temperatures (800-1000 °C) to transform into the respective ferrites. Apblett et al. synthesized a water-soluble Ni-Fe complex, [Ni(H2O)6][FeCl(EDTA)H]2• 4H2O [266], as a single precursor to nickel ferrite; however, the pyrolysis of this compound leads to phase separation, and the sample heat-treated at 600 °C shows Fe2O3 and NiFe2O4 as the crystalline phases. Further heating at 900 °C is essential for obtaining stoichiometric nickel ferrite.

Mathur et al. [244, 550] have used heterobimetallic precursors of the type [MM2(OR)8] (M = Co, Zn; M' = Al, Ga, Fe) for low-temperature synthesis of high-purity spinel particles by the microemulsion method. For a comparative evaluation of the single- and multicomponent approaches, the CoAl2O4 spinel was synthesized from the single source [CoAl2(OBu')8] (Fig. 37) and by the use of a stoichiometric mixture of [Al(OPr')3]4 and Co(OR)2 (R = -C(C6H5)3). The XRD patterns (Fig. 38) of the two CoAl2O4 samples reveal that the single-source synthesis yields, under similar experimental conditions, higher phase purity and crystallinity compared with the spinel obtained from a stoichiometric mixture of Al and Co sources. The expected differences in the hydrolysis susceptibility of the isopropoxide and phenox-ide derivatives [551] cause different hydrolysis kinetics that adversely affect the homogeneity (with respect to the cation distribution) of the precursor solution. As a result, minor phase separation occurs in the dual-source system (Fig. 38).

Figure 37. Molecular structure of [CoAl2(OBu')8].

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