I

/SNS

^a GBJT

Scheme 6.

Furthermore, the SSPs can facilitate the formation of those ceramic materials (e.g., nitrides) whose syntheses are severely constrained by the reaction conditions. Transition metal nitrides possess free energies of formation that are extremely less favorable than corresponding oxides, principally because of the tremendous strength of the N=N bond (227 kcal mol-1) relative to O2 (119 kcal mol-1). Consequently, the preparation of [MNJ from the respective elements is entropically disfavored at the high temperatures necessary to ensure sufficient diffusion rates. In this case, the metal amides, given the covalent nature of metal-nitrogen bonds, act as convenient SSPs to obtain metal nitride phases at low temperatures.

Given the importance of molecular chemistry routes in the synthesis of nanomaterials, the challenge is to develop synthetic rationales for a customized assembly of molecular building blocks from the fundamental components (metals and ligands) that would facilitate a prefabrication of suitable precursor to any desired nanomaterial. However, it would be overly simplistic to assume that a single molecular source is the only structure from which target material can be obtained. Given the versatility and development of synthetic chemistry, the range of potential molecular structures that can be used to obtain new materials and nano-materials appears to be unlimited. A major bottleneck in this regard is the fact that molecular control of materials is not a predictive science, and it is difficult to anticipate the phase structure of the resulting solid from the knowledge of the precursor design. Most of the work performed to demonstrate the strength of molecular chemistry methods in achieving better control over the phase purity and composition of the materials is based on an explanation with hindsight, achieved after the experiment was concluded. In this context, there is an urgent need to develop an understanding of the precursor-material relationship based on the mechanism of precursor decomposition and the structural and chemical factors that govern (or disfavor) molecular control over the process of material formation. The ability to answer the question, Can the molecular structure of the precursor species influence the phase of the final solid product? is the driving force for the current quest for SSPs to nanoscale materials.

Despite the potential advantages of metal-organic precursors as molecular sources for the low-temperature synthesis of nanomaterials, their application to most of the materials is limited because of the unavailability of compounds containing all of the phase-forming elements in a single source. In most of the cases, the selection of a suitable precursor is dictated by the requirements of the material and the synthesis methods. Subject to the processing conditions (solid, solution, or gas phase), the inherent properties of the precursor can impose limitations on the building-block approach. For example, the pyrophoric nature, poor solubility, and extremely low vapor pressure of a compound can limit its application in solid-state, liquid, and gas-phase synthesis, respectively. The other related and equally important parameters are the stability of the molecular framework and stoichiometry of the cations. The fact that the construction of metal-organic species (e.g., alkoxides, amides, metal-metal bonded clusters, etc.) is based on the principles of charge neutrality and electron count limits the choice when precursors to nonstoichiometric compounds or doped materials are sought.

To understand what precursor chemistry implies, the isolation and characterization of preceramic aggregates are necessary, because similar molecular building blocks can arrange themselves in different ways to produce compounds with different metal stoichiometries. For example, hetero-metal alkoxides are invariably formed in a mixture of constituent alkoxides; however, a simple mixing of two alkoxides, M(OR)x and M'(OR)y, is no guarantee of the formation of a single precursor species MM'(OR)J+y mainly due to the structure potential (I, II, and III, Scheme 7) of alkoxide lig-ands. As a result, in addition to the desired precursor, several other compounds may be present in an uncharacterized precursor mixture, depending upon the chemical requirements of the individual components and reaction conditions. For example, Ba and Zr(Ti) alkoxides can combine in different ways to produce mixed-metal Ba-Zr(Ti) clusters with different structures and metal ratios [331]. Of course, the

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