Mesoporous structures surfactants self-assembled

The ability of using mixed [Geg.nSin]" clusters as starting building blocks, which are soluble in ethylenediamine, allowed us to prepare mesoporous Ge/Si alloy semiconductors. These structures were synthesized as described above by the oxidative self-polymerization of mixed [Geg.nSin]" clusters with the assistance of self-assembled cationic surfactants (3). 

Synthesis of solid state materials using surfactant molecules as template has been extensively used in this decade. Among the advantages of the use of amphiphilic molecules, the self-assembling property of the surfactants can provide an effective method for synthesising ceramic and composite materials with interesting characteristics, such as nanoscale control of morphology, and nano or mesopore structure with narrow and controllable size distribution [1-5]. 

Figure 3.5 shows the organized mesoporous structure of one of these materials. The wide variety of organic molecules with self-assembling properties allows for the synthesis of a myriad of solids with controlled porosity. Cationic surfactants, like the hexadecyltrimethylammonium used to synthesize MCM-41 [26], or three-block copolymers (hydrophilic-hydrophobic-hydrophilic) are two good examples. 

There are several successful examples of the self-assembly of preformed zeolite primary and secondary structural building units (nanoclusters) with surfactant micelles. This self-assembly will form ordered mesoporous aluminosilicates with strong acidity and high hydrothermal stability. 

The synthesis of mesoporous silica films typically begins with the preparation of precursor solutions. These solutions contain a silica source (typically an alkoxide, although chloride and colloidal precursors can be used), a surfactant molecule used to template the mesostructure, an acid or base catalyst, and solvents. The nanoscale structure is then formed by a cooperative self-assembly of monomeric or partially... 

The chemistry involved in the formation of mesoporous silica thin films is qualitatively well understood. However, specific reaction mechanisms of the individual steps are still debated. In addition, owing to the complexity of the sol-gel reaction pathways and cooperative self-assembly, full kinetic models have not been developed. From the time of mixing, hydrolysis reactions, condensation reactions, protonation and deprotonation, dynamic exchange with solution nucleophiles, complexation with solution ions and surfactants, and self-assembly, all occur in parallel and are discussed here. Although the sol-gel reactions involved may be acid or base catalyzed, mesoporous silica film formation is carried out under acidic conditions, as silica species are metastable and the relative rates of hydrolysis and condensation reactions lead to interconnected structures as opposed to the stable sols produced at higher pH. Silicon alkoxides are the primary silica source (tetramethyl orthosilicate, tetraethyl orthosilicate, tetrapropyl orthosilicate, etc.) and are abbreviated TMOS, TEOS, and TPOS, respectively. Starting from the alkoxide, Si(OR)4, in ROH and H2O solution, some of the general reactions are ... 

---