Alkoxides based sol-gel processing

Takahashi Y., Ohsugi A., Arafuka T., Ohya T., Ban T., Ohya Y. Development of new modifiers for titanium alkoxide-based sol-gel process. J. Sol-Gel Sci Technol. 2000 17 227-238 Taylor D.J., Fabes B.D. Laser processing of sol-gel coatings. J. Non-Cryst. Solids 1992 147/148 ... 

Ohya, T., Ban, T., and Ohya, Y. (2000) Development of new modifiers for titanium alkoxide-based sol-gel process. 

Most of 3DOM metal oxides are fabricated by an alkoxide-based sol-gel process. The typical procedures include (i) preparation of a colloidal crystal template by ordering monodispersive microspheres (e.g., PS, PMMA or silica) into a face-centered close-packed array (ii) infiltration of a metal alkoxide solution into voids in the colloidal crystals, and in-situ solidification of the precimsor via a sol-gel process and (iii) template removal via a calcination or extraction pathway [99,217-219], Using the alkoxide-based sol-gel method, 3DOM oxides of Si, Ti, Zr, Al, Sb, W, Fe, and 3DOM mixed oxides of some of them can synthesized due to the moderate reactivity of their alkoxide preciuTsors [99], However, most of the other metal alkoxides exhibit high reactivity and their reactions are too quick to be... 

Hybrid, chelate, or molecularly modified precursor routes also utilize alkoxide compounds for the B-site species. Unlike true sol-gel processes, these routes rely on molecular modification of alkoxide compounds through reactions with other reagents, namely chelating ligands, such as acetic acid, acetylacetone, or amine compounds. Despite this difference, chelate processes stiU share several common attributes with methoxyethanol-based sol-gel processes, most importantly, the formation of oligomeric species during solution synthesis. 

This type of reaction leads to the formation of an inorganic polymer or a three-dimensional network formed of metal oxianions. The above-described process is called metal alkoxide-based sol-gel. The literature contains excellent reports providing in-depth analyses of this method [100,223]. The sol-gel process allows for very good chemical homogeneity and offers the possibility of obtaining metastable phases, including the amorphous phase. This process normally promotes the formation of amorphous metal oxides, which require thermal or hydrothermal treatment to promote crystallization. Several factors affect the sol-gel process, including the kind... 

Matsuzaki K., Aral D., Tanabe N., Mukaiyama T., Ikemura M. Continuous silica glass fiber produced by sol-gel process. J. Non-Cryst. Solids 1989 112 437-441 Meyer Jr. R., Shrout T., Yoshikawa S. Lead zirconium titanate fine fibers derived from alkoxide-based sol-gel technology. J. Am. Ceram. Soc. 1998 81 861-868 Mizuno T., Phalippou J., Zarzyeki J. Evolution of the viscosity of solutions containing metal alkox-ides. Glass Technol. 1985 26 39-45... 

As in the case of silica-based sol-gel processes, M—OH groups can also be created by hydrolysis of M—OR groups, that is, by addition of water to metal alkoxides. As outlined above, metal alkoxides are stronger Lewis acids than silicon alkoxides, and the formation of higher coordinated species is easier. Nucleophilic attack at the metal is thus facilitated, and the hydrolysis rates are strongly increased. For example, the hydrolysis rate of Ti(OR)4 is about 10 times faster than that of Si(OR)4 with the same alkoxide substituents. The reactivity of some tetravalent isopropoxides in hydrolysis reactions increases in the following order [4] ... 

The sol-gel process can be performed with a variety of metal alkoxides. The most common metal alkoxides used in the sol-gel process are silicon-based alkoxides such as tetraethoxysilane (TEOS) and tetramethoxysilane (TMOS). Silicon-based sol-gel process results in the formation of an inorganic polymer of silica, Si02, with tetrahedral imit cell geometry. A dried gel xerogel after firing at 700 °C will transform completely to fused sflica. Additional frequently used metal alkoxides for the sol-gel process are transition metal alkoxides such as Ti,... 

The most common sol-gel process employs metal alkoxides of network forming elements (M(0R) where M is Si, B, Ti, Al, etc. and R is often an alkyl group) as monomeric precursors. In alcohol/water solutions the alkoxide groups are removed stepwise by hydrolysis reactions, generally employing acid or base catalysts, and are... 

Silica-based materials obtained by the sol-gel process are perhaps the most promising class of functional materials capable to meet such a grand objective. In the sol-gel process liquid precursors such as silicon alkoxides are mixed and transformed into silica via hydrolytic polycondensation at room temperature. Called soft chemitry or chimie douce, this approach to the synthesis of glasses at room temperature and pressure and in biocompatible conditions (water, neutral pH) has been pioneered by Livage and Rouxel in the 1970s, and further developed by Sanchez, Avnir, Brinker and Ozin. 

Biological species such as enzymes, whole cells, antibodies and even bacteria can all be successfully entrapped in silica sol-gel matrices, often with enhancement of activity with respect to the free biologicals. In these cases, the process is adapted to eliminate toxic alcohols which are typically released in conventional sol-gel processes based on the hydrolysis of silicon alkoxides. Two such methods are the use of silicon alkoxide... 

The Stober method is also known as a sol-gel method [44, 45], It was named after Stober who first reported the sol-gel synthesis of colloid silica particles in 1968 [45]. In a typical Stober method, silicon alkoxide precursors such as tetramethylorthosili-cate (TMOS) and tetraethylorthosihcate (TEOS), are hydrolyzed in a mixture of water and ethanol. This hydrolysis can be catalyzed by either an acid or a base. In sol-gel processes, an acidic catalyst is preferred to prepare gel structure and a basic catalyst is widely used to synthesize discrete silica nanoparticles. Usually ammonium hydroxide is used as the catalyst in a Stober synthesis. With vigorous stirring, condensation of hydrolyzed monomers is carried out for a certain reaction time period. The resultant silica particles have a nanometer to micrometer size range. 

This chapter focuses on silica synthesis via the microemulsion-mediated alkoxide sol-gel process. The discussion begins with a brief introduction to the general principles underlying microemulsion-mediated silica synthesis. This is followed by a consideration of the main microemulsion characteristics believed to control particle formation. Included here is the influence of reactants and reaction products on the stability of the single-phase water-in-oil microemulsion region. This is an important issue since microemulsion-mediated synthesis relies on the availability of surfactant/ oil/water formulations that give stable microemulsions. Next is presented a survey of the available experimental results, with emphasis on synthesis protocols and particle characteristics. The kinetics of alkoxide hydrolysis in the microemulsion environment is then examined and its relationship to silica-particle formation mechanisms is discussed. Finally, some brief comments are offered concerning future directions of the microemulsion-based alkoxide sol-gel process for silica. 

These reactions are responsive to both acid and base catalysis, and can be manipulated to give a variety of silica products, e.g., discrete particles, monolithic gels, films, and fibers. This technique of materials synthesis via alkoxide hydrolysis has become known as sol-gel processing (17). It should be noted, however, that under certain conditions, gelation may be confined only to the interior of discrete particles (base-catalyzed systems), while the sol may consist of polymeric networks rather than individual particles (acid-catalyzed systems). 

In 1968, Stober et al. (18) reported that, under basic conditions, the hydrolytic reaction of tetraethoxysilane (TEOS) in alcoholic solutions can be controlled to produce monodisperse spherical particles of amorphous silica. Details of this silicon alkoxide sol-gel process, based on homogeneous alcoholic solutions, are presented in Chapter 2.1. The first attempt to extend the alkoxide sol-gel process to microemul-sion systems was reported by Yanagi et al. in 1986 (19). Since then, additional contributions have appeared (20-53), as summarized in Table 2.2.1. In the microe-mulsion-mediated sol-gel process, the microheterogeneous nature (i.e., the polar-nonpolar character) of the microemulsion fluid phase permits the simultaneous solubilization of the relatively hydrophobic alkoxide precursor and the reactant water molecules. The alkoxide molecules encounter water molecules in the polar domains of the microemulsions, and, as illustrated schematically in Figure 2.2.1, the resulting hydrolysis and condensation reactions can lead to the formation of nanosize silica particles. 

The electrochemical technique can be used also for direct synthesis of bimetallic alkoxides. For instance, the anodic dissolution of rhenium in the methanol-based electrolyte that already contained MoO(OMe)4, permitted to prepare with a good yield (60%) a bimetallic complex RevMov,02(OMe)7, with a single Re-Mo bond [904], Application of the same procedure permitted the preparation of complex alkoxide solutions with controlled composition for sol-gel processing of ferroelectric films [1777]. 

Routes to monomeric , mononuclear , monolanthanide alkoxides, enolates, siloxides and aryloxides - an expanded title which will put the scope of the article in a more concrete form. The synthesis of mononuclear alkoxides, in particularly homoleptic derivatives [1], was decisively stimulated by the discovery of high temperature superconducting ceramics based on YBa2Cu307<, where yttrium represents the lanthanide elements [2]. The support of volatile and highly soluble molecular precursors is a prerequisite for synthesizing thin films of these materials by means of MOCVD [3] and sol gel processes [4], respectively. More recently, lanthanide alkoxide reagents became established in... 

The silica gel is amorphous. Using high-resolution electron microscopy, it is known that its amorphous framework is made up of small globular (primary) particles having sizes of 10 to 20 A (Rouquerol, Rouquerol and Sing, 1999). An alternative route involves reactions of silicon alkoxides with water, and a wide variety of materials can be made this way (Jones 1989 Brinker and Sherer, 1990). The processes based on this route are referred to as sol-gel processing, and they offer many promising possibilities. For silica gel, the reaction is... 

On the other hand, the progress of wet-processes as preparative techniques of metal oxide films has been remarkable. The so-called soft solution process that provides oxide layers by means of electrochemical oxidation of a metal surface is expanding as a synthetic method of various mixed metal oxides with controlled thickness [2], The two-dimensional (2D) sol-gel process based on the hydrolysis of metal alkoxides at the air/water interface has been reported as a preparative technique of ultrathin oxide films (Fig. 6.1a) [3]. It is also known that LB films of metal complexes of long-chain alkyl carboxylic acid can be converted to metal oxide films after removal of organic component by oxygen plasma [4] and UV-ozone treatments (Fig. 6.1b) [5]. Preparation of metal oxide... 

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