Metal oxide nanocrystals

Pinna, N., Garnweitner, G., Antonietti, M. and Niederberger, M. (2005) A general nonaqueous route to binary metal oxide nanocrystals involving a C-C bond cleavage. Journal of the American Chemical Society, 127 (15), 5608-5612. 

Metal oxide nanocrystals arc mainly prepared by the solvothermal decomposition of organometallic precursors. Solvothermal conditions afford high autogenous pressures inside the scaled autoclave that enable low-boiling solvents to be heated to temperatures well above their boiling points. Thus, reactions can be carried out at elevated temperatures and the products obtained are generally crystalli ne compared to those from other solution-based reactions. 

The very first report of two-dimensional arrays was of Fe304 nanocrystals [128]. Bentzon et al. observed that the ferrofluid obtained by thermolysis of iron penta-carbonyl upon drying (over a period of several weeks) yielded well ordered two-dimensional arrays of Fe304 nanocrystals. Since then, easier methods have been devised to obtain arrays of Fc304 nanocrystals [129]. Two-dimensional arrays of amine-capped metal oxide nanocrystals such as C03O4 have been obtained by start-... 

Apart from solvothermal methods, thermolysis of precursors in higli boiling solvents, the sol-gel method, hydrolysis and use of micelles have been employed to synthesize the metal oxide nanocrystals. Thus, Park et al. have used metal-oleates as precursors for the preparation of monodisperse Fe,0, MnO and CoO nanocrystals. 1-Octadecene, octyl ether and trioctylamine have been used as solvents. Hexagonal and cubic CoO nanocrystals can be prepared by the decomposition of cobalt acetylaceto-... 

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] ... 

Alivisatos and collaborators [257] demonstrated that transition metal oxide nanocrystals (7-Fc203, Mn304, CU2O) could be prepared using a nonhydrolytic process based on the thermal decomposition of metal Cupferron complexes M Cup (M = metal ion. Cup = CeHsNjNOjO ) in a hot solvent with surfactant. Their results suggest that a good level of control can be achieved when this approach is used to process metal oxide nanoparticles. 

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) ... 

It has been reported that single or mixed metal oxide nanoparticles, such as zinc oxide, copper oxide, aluminum oxide, or titanium oxide, incorporated into a filtration medium containing a binder matrix, can destroy bacteria (57). The metal oxide nanocrystals are included in amounts ranging from approximately 0.1 % up to about 10% by weight, based on the entire filtration medium. In a series of studies, it has been shown... 

Richards R, Li W, Decker S, Davidson C, Koper O, Zaikovski V, Volodin A, Rieker T, Klabunde KJ (2000) Consolidation of metal oxide nanocrystals. Reactive pellets with controllable pore structure that represent a new family of porous, inorganic materials. J. Amer. Chem. Soc. 122 4921 925. 

Zhu et al. [109] proposed an environmentally innocuous method of preparation by using a single-step solvothermal route in ethanol solutions. The procedure leads to simultaneous rGO reduction and iron or cobalt oxide precipitation due to the fact that the GO/rGO layers act as heterogeneous nucleation seeds during the precipitation of the metal oxide nanocrystals. In a related approach, Han et al. [110] were able to obtain Li4Ti50i2 particles anchored to rGO by solvothermal treatment of H2O/ EtOH-based suspensions of graphite oxide and the oxide powder. The process involves reduction of GO and attachment of the mixed oxide nanoparticles within a single step. 

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... 

Baratto C, Comini E, Faglia G, Sberveglieri G, Zha M, Zappettini A (2005) Metal oxide nanocrystals for gas sensing. Sens Actuators B 109 2-6... 

Future work may focus on the design of more selective and sensitive nanostructured interface at an atomic level by combining computational and experiments. For example, metal oxides nanocrystals exposed with different crystal facets which can selectively adsorb a specific metal ion can be computational screened and synthesized. In addition, issues related to the reproducibility and stability in the more complex environments need great investigation prior to the application in real samples. Furthermore, the developments of highly integrated detection system which can even ensure online analysis are highly expected. 

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