Oxide network

The percolation argument is based on the idea that with an increasing Cr content an insoluble interlinked cliromium oxide network can fonn which is also protective by embedding the otherwise soluble iron oxide species. As the tlireshold composition for a high stability of the oxide film is strongly influenced by solution chemistry and is different for different dissolution reactions [73], a comprehensive model, however, cannot be based solely on geometrical considerations but has in addition to consider the dissolution chemistry in a concrete way. 

Figure 17 summarizes the avadable sol—gel processes (56). The process on the right of the figure involves the hydrolysis of metal alkoxides in a water—alcohol solution. The hydrolyzed alkoxides are polymerized to form a chemical gel, which is dried and heat treated to form a rigid oxide network held together by chemical bonds. This process is difficult to carry out, because the hydrolysis and polymerization must be carefully controlled. If the hydrolysis reaction proceeds too far, precipitation of hydrous metal oxides from the solution starts to occur, causing agglomerations of particulates in the sol. 

Film formation proceeds through condensation of hydroxyls to create a three-dimensional oxide network ... 

Our approach, to achieve a high dispersion of the metal compound while the oxide network is formed, is to employ metal complexes of the type LnM[X(CH2)3Si(OR)3]y in the sol-gel process [2]. The metal ions then cannot aggregate because of complexation, and the metal complexes cannot leach because they are linked to the oxidic support. These complexes are formed in situ on reaction of silanes of the type X(CH2)nSi(OR)3 with suitable metal salts. 

Figure 9.1. Scheme of micelle templating. The circular objects correspond to micelles and the gray color to the condensed inorganic oxide network. 

Scheme 11.9 Oxidation network of glycerol and yields (Y) of product at different pH and with discrete catalysts. (After [74, 76]).

In general, metal nanoparticles are obtained via reduction of metal complexes, such as metal chlorides, by chemical agents (chemical reduction), or by electrons (electrodeposition). Hybrids of metal oxides are obtained by oxidation, network formation or precipitation of precursors such as metal nitrates and acetates [144]. 

Note 2 In sol-gel processing the term applies to the heating of a polymer network containing metal compounds to convert it into an oxide network. 

Shchukin DC, Schattka JH, Antonietti M, Curasu RA (2003) Photocatalytic properties of porous metal oxide networks formed by nanoparticles infiltration in a polymer gel template. J Phys Chem B 107 952-957... 

Condensation or precipitation The species that are so formed may be unstable and in a second step may condense upon elimination of molecules such as ROH or H20, thus leading to the formation of a hydroxide or oxide network. This step corresponds to an inorganic polymerization such as (9) ... 

Xerogels Ag, Au Two steps (1) gamma radiation initiation of metal-particle growth in a solution containing metal ions and multifunctional silanes (2) xerogels with metal clusters grafted on an oxide network prepared via hydrolysis and condensation 76... 

Figure 10.7 (a) Two-dimensional schematic of pure, oxide network glass of composition... 

IR spectra measurements as well as variation of the film thickness, shrinkage, and refractive index demonstrated substantial differences in the mechanisms of thermal decomposition of films prepared from the exclusively metal alkoxide precursor and from the metal alkoxides modified by 2-ethylhexanoic acid. These differences affect the evolution of film microstructure and thus determine the different dielectric properties of the obtained films. The dielectric permittivity of the films prepared from metal alkoxide solutions was relatively low (about 100) and showed weak dependence ofthe bias field. This fact may be explained by the early formation of metal-oxide network (mostly in the... 

The Ti and Hf compounds are monomers, whereas the Zr complex was dimeric. As found by Hrncir and Skiles, the series of M[OSi(OBu-t)3]4 compounds are all moisture-sensitive. TGA studies indicate that the Ti complex decomposes cleanly at temperatures >240 °C and gives a ceramic yield of 25 wt% whereas the theoretical ceramic yield for TiSiC>4 should be 29.07 wt%. The primary gaseous thermolysis products were identified by mass spectroscopy to be isobutene and water. A likely pathway for decomposition appears to involve /1-hydrogen elimination followed by condensation of the resulting Si—OH groups to generate isobutene, water and an oxide network as shown in equations 66 and 67. 

Sol-gel as a synthetic method was discussed in Section 4.3. Here we revisit the topic, with the emphasis shifted to particle size. Sol-gel techniques have been reviewed extensively (Wright and Gommerdijk, 2001). The basic principle underlying the sol-gel method involves the condensation of metal alkoxides to form metal oxide networks with entrapped alcohol and water ... 

Fig. 8. (a) Transmission electron micrograph of a Cu/ZnO = 30/70 binary catalyst (40) 60 A copper spheres are placed on crystalline zinc oxide network, (b) Dark field image of the copper crystallites in the area shown in the bright field image (a) obtained using the [111] reflection of copper. [Adapted with permission from J. Catal. 57, 339 (1979). Copyright (1979) Academic Press, New York.]... 

Silicon and aluminum, of course, are not unique in their ability to form tetrahedrally coordinated oxide networks. The element phosphorus, at the right of silicon in the periodic table, frequently assumes tetrahedral coordination with oxygen. With phosphorus in the +5 oxidation state as phosphate, aluminum phosphate possesses many structural similarities to silica 1) A1P0 is isoelectronic with S120 . 2) The average of the ionic radii of... 

A wide variety of organic species can be dissolved with metal alkoxides in a common solvent. Hydrolysis and condensation reactions lead to the formation of an oxide network in which organic molecules remain trapped. However, the synthetic routes often have to be adapted to the properties of the immobilized species. 

Organic species can also be chemically bonded to the oxide network via Si-C bonds. Many organoalkoxysilane precursors, R4 Si(OR), are commercially available. They contain nonhydrolyzable Si-C bonds, so that organic moieties are not removed upon hydrolysis. Depending on the nature and amount of organic and inorganic components, it becomes possible to tailor the microstructure and even obtain multifunctional materials. 

Sol-gel chemistry has been extensively studied during the past decade. The basic reactions, hydrolysis, condensation, and complexation, involved in the formation of oxide networks from molecular precursors are now quite well described. The sol-gel process brings new opportunities in the field of materials science. It allows a powderless processing of glasses and ceramics. Coatings can be easily deposited directly from the solution onto a wide variety of substrates. Many patents have been taken and several products are now on the market. Other industrial applications will undoubtedly be developed during the next few years. 

The smface chemistry of the as-synthesized NCs has been exploited in order to develop methods for anchoring the semicondnctor NCs directly to the sol-gel network. The approaches investigated are analogous to methods discussed in Section 2 with respect to organically modified gels, with one major exception - the colloidal NC, and not jnst the starting metal salt, is directly tethered to the metal-oxide network ... 

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