Metal 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... 

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... 

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 ... 

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 ... 

Well ordered mesoporous silicate films were prepared in supercritical carbon dioxide.[218] In the synthesis in aqueous or alcoholic solution, film morphology of preorganized surfactants on substrate cannot be fully prescribed before silica-framework formation, because structure evolution is coincident with precursor condensation. The rapid and efficient preparation of mesostructured metal oxides by the in situ condensation of metal oxides within preformed nonionic surfactants can be done in supercritical CCU- The synthesis procedure is as follows. A copolymer template is prepared by spin-coating from a solution containing a suitable acid catalyst. Upon drying and annealing to induce microphase separation and enhance order, the acid partitions into the hydrophilic domain of the template. The template is then exposed to a solution of metal alkoxide in humidified supercritical C02. The precursor diffuses into the template and condenses selectively within the acidic hydrophilic domain of the copolymer to form the incipient metal oxide network. The templates did not go into the C02 phase because their solubility is very low. The alcohol by-product of alkoxide condensation is extracted rapidly from the film into the C02 phase, which promotes rapid and extensive network condensation. Because the template and the metal oxide network form in discrete steps, it is possible to pattern the template via lithography or to orient the copolymer domains before the formation of the metal oxide network. 

Figure 14 A hybrid organic-transition metal oxide network.

A lot of research has also been devoted to the infiltration of macroporous templates by reactive components. Porous polymeric material was synthesized by either infiltration of a monomer-initiator mixture with subsequent polymerization [29], or by infiltration of a prepolymer solution, which can be UV-cured afterwards [27]. A quite common route to fabricate metal oxide networks is to infiltrate the precursor structure with its corresponding sol-gel solution, which eventually hydrolyzes and solidifies in the desired porous shape. This technique has been shown for a great variety of materials (compare Table 2 in [10], Table 1 in [37], and Table 1 in [38]), such as silica [52], titania [30,50], zirconia [30] or aliunina [30], just to mention a few. Another pathway to metal oxide structures was introduced by Park et al., who precipitated acetate salt solutions of the desired material in the free voids. After addition of oxalic acid the porous metal oxide was formed during the combustion of the latex template [73]. 

Fig. 5.1 Templated electrodeposition of metal oxides. (1) The electrochemical refiUing process starts at the FTO surface and progresses trough the voided DG channels. The deposition process is restricted to areas that are not covered by SU-8, thereby creating a visible design pattern in the electroplated V2O5. (2) Removal of the styrenic template yields the free-standing, mesoporous DG-structured metal oxide network...

Molecular precursor -F Organic solvent—No water, Metal oxide network... 

The sol-gel method is generally employed for the synthesis of metal oxide NPs as well as oxide nanocomposites. The sol-gel process involves the hydrolysis and condensation of metal precnrsors [43,44]. Further condensation and polymerization will lead to three-dimensional metal oxide networks forming the gel. The sol-gel process can be either in aqueous or non-aqueous medium. In the aqueous sol-gel process, oxygen for the formation of the oxide is supplied by water molecules. In the... 

The reaction takes place at the water/ oil interface. An aqueous solution containing Fe2+/Fe3+ salts is added to an organic solvent containing the stabilizer. The sol-gel reaction is performed at room temperature and is based on the hydroxylation and condensation of molecular precursors in solution and leads to a three-dimensional metal oxide network, the wet gel. Heat treatments are further needed to acquire the final crystalline state (Liu et al., 1997 Kojima et al., 1997). The properties of the gel are dependent on the structure created during the sol stage of the sol-gel process. 

The sol-gel process starts with a solution of metal oxides (usually metal-alkoxy compounds such as tetraorthoethylsilicate (TEOS) are used) that undergo hydrolysis and poly-condensation to form a rigid matrix of cross linked metal-oxide network followed by thermal evaporation to form a matrix with interconnecting pores. The sensing agents are added during the process of condensation and are encapsulated into the gel structure formed around them. 

The point we would like to discuss in this chapter is that, importantly, sol-gel chemistry has allowed the concepts of molecular organic and inorganic chemistry together with those of supramolecular and macromolecular chemistry to enter the field of materials science. Therefore, it has allowed the design of new synthetic routes, taking advantage of the kinetic reactivity of the molecular precursors and of the intermolecular interactions that can develop during the formation of the three-dimensional polymeric metal oxide network. 

Hierarchically porous metal oxide networks can be formed via a spontaneous self-formation phenomenon from metal alkoxides in aqueous solution [113]. Two chemical processes, hydrolysis and condensation, are involved in this spontaneous self-formation procedure to target hierarchically porous structures [114,115]. In fact, the hydrolysis and condensation rates are generally comparable for metal alkoxides [116]. The condensation rate is directly proportional to the rapid hydrolysis rate of reactive metal alkoxides [117,118]. It is well known that the rapid reaction rate of metal alkoxides plays the key role in the formation of hierarchically porous metal oxides [119,120]. The self-formation procedure to form hierarchically porous materials can be achieved by dropping liquid metal alkoxide precursors into an aqueous solution. In this section, the features of self-formation procediu-e and the resulting hierarchically porous materials are summarized. 

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