Metal oxide nanocrystals synthesis

Based on the above analysis, the development of metal oxides of nanometric dimensions can result in devices and materials with superior performance. However, these developments are directly related to the development of synthetic methods that allow for controlled particle size, particle morphology, and deposition. Once again, the bottom-up methods of wet chemical nanocrystal synthesis are apparently the most viable approach to achieve such control. Compared with the control attained in the synthesis of metal and 11-lV semiconductor nanocrystals, the control of metal oxide nanocrystals is still incipient, particularly insofar as the synthesis of complex metal oxide nanocrystals (oxides formed of more than one cation) is concerned. 

The synthesis of metal oxide nanocrystals by wet chemical processes can be divided basically into two major groups (a) chemical synthesis method based on the hydrolysis of metal alkoxides or metal halides (b) chemical synthesis based on the nonhydrolytic method. Examples of these methods are described below. 

The chemical synthesis of metal oxide nanocrystals based on hydrolysis falls into two major groups hydrolysis of metal alkoxides hydrolysis of metal halides, and other inorganic salts. Metal alkoxide compounds are defined as compounds that have metal-oxygen-carbon bonds. Si(OC2H5)4 (tetraethyl orthosUicate or TEOS), for instance, is an alkoxide compound. This class of compound is highly reactive with water. Because the hydroxyl ion (OH ) becomes bonded to the metal of the organic precursor, this reaction is called hydrolysis. Equation (50) shows a typical hydrolytic reaction of an alkoxide compound [100] ... 

Besides the common features of most metal oxides as chemically stable and mechanically robust materials, this class of compounds can also provide unique functions for ceramics, high refractive index and magnetic materials, catalysis, and biocompatibility. A wide range of porous metal oxide materials have been synthesized over the last decade. In general three distinct pathways have been undertaken a sol-gel synthesis of the metal alkoxide infiltration with an appropriate metal salt and subsequent conversion or infiltration or codeposition with preformed metal oxide nanocrystals. 

Heat-up is a simple but effective method of synthesizing highly uniform nanocrystals, which yields a degree of size uniformity comparable to that of the best result from the hot injection method. This method is adopted mainly for the synthesis of metal oxide nanocrystals. In this section, we describe the synthetic procedure of iron oxide nanocrystals via the heat-up method as a representative example (23-25). 

The precursors used for the iron oxide nanocrystal synthesis via the heat-up method are various iron carboxylatc complexes, including the most widely used iron-oleate complex. Generally, when heated, metal carboxylatc complexes thermally decompose at temperatures near 300°C or higher to produce metal oxide nanocrystals along with some byproducts, such as CO, CO2, H2, water, ketones, esters, and various hydrocarbons. It is thought that the decomposition reaction proceeds via the formation of thermal See radicals liom metal carboxylatc (26, 27) ... 

Nanocrystals are zero-dimensional particles and can be prepared by several chemical methods, typical examples being reduction of salts, solvothermal synthesis and the decomposition of molecular precursors, among which the first method is the most commonly used in the case of metal nanocrystals. Metal oxide nanocrystals are generally prepared by the decomposition of precursor compounds such as metal acetates, acetylacetonates and cupferronates in appropriate solvents, often under solvothermal... 

Synthesis forms a vital aspect of the science of nanomaterials. In this context, chemical methods have proved to be more effective and versatile than physical methods and have therefore, been employed widely to synthesize a variety of nanomaterials, including zero-dimensional nanocrystals, one-dimensional nanowircs and nanotubes as well as two-dimensional nanofilms and nanowalls. Chemical synthesis of inorganic nanomaterials has been pursued vigorously in the last few years and in this article we provide a perspective on the present status of the subject. The article includes a discussion of nanocrystals and nanowires of metals, oxides, chalcogenides and pnictides. In addition, inorganic nanotubes and nanowalls have been reviewed. Some aspects of core-shell particles, oriented attachment and the use of liquid-liquid interfaces are also presented. 

The inverse-micelle approach may also offer a generalized scheme for the preparation of monodisperse metal-oxide nanoparticles. The reported materials are ferroelectric oxides and, thus, stray from our emphasis on optically active semiconductor NQDs. Nevertheless, the method demonstrates an intriguing and useful approach the combination of sol-gel techniques with inverse-micelle nanoparticle synthesis (with OTO erafe-temperature nucleation and growth). Monodisperse barium titanate, BaTiOs, nanocrystals, with diameters controlled in the range from 6-12nm, were prepared. In addition, proof-of-principle preparations were successfully conducted for Ti02 and PbTiOs. Single-source alkoxide precmsors are used to ensure proper stoichiometry in the preparation of complex oxides (e.g. bimetallic oxides) and are commercially available for a variety of systems. The... 

Recent examples include the batch synthesis of C0AI2O4 nanocrystals and the continuous synthesis of nano-hydroxyapatite. In the first example, the researchers wanted to prepare metal oxide particles that could be easily... 

N. L. Wu, S. Y. Wang, and I. A. Rusakova. Inhibition of crystallite growth in the sol-gel synthesis of nanocrystalline metal oxides. Science, 285 1375-1377, 1999 A. Vioux. Nonhydrolytic sol-gel routes to oxides. Chem. Mater., 9 2292-2299, 1997 J. Rockenberger, E. C. Scher, and A. P. Alivisatos. A new nonhydrolytic single-precursor approach to surfactant-capped nanocrystals of transition metal oxides. J. Am. Chem. Soc., 121 11595-11596, 1999... 

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