EISA processing

The number of publications concerning utilization of the EISA process for fabrication of different structured materials is counted in the hundreds, which is far beyond the possibilities of this chapter to review in depth. Rather, we intend to provide a brief introduction into EISA and its application to the fabrication of functional thin films for electronic applications (e.g., electro-chromic layers and solar cells), with a special focus on fabrication of crystalline mesoporous films of metal oxides. Attention will also be given to techniques used to evaluate the pore structure of the thin films. For the other aspects of the EISA process, for example its mechanism,4 strategies for preparation of crystalline porous metal oxides,5 mesoporous nanohybrid materials,6 periodic organic silica materials,7,8 or postgrafting functionalization of mesoporous framework,9 we kindly recommend the reader to refer to the referenced comprehensive reviews. 

Formation of organized mesoporous structures in the EISA process is generally governed by three major parameters (1) the nature and composition of the precursor solution, which predefines the chemical composition and, to a great extent, the mesostructure of the final film (2) the deposition process,... 

Self-assembly of the precursor components in the EISA process starts after evaporation of part of the volatile solvent, which progressively enriches the solution with respect to surfactant, precursor, and water. When their concentration in the deposited him achieves a certain critical level, mesostructure formation takes place. The latter is in equilibrium with the processing atmosphere. So the relative humidity, as well as the temperature during him deposition, represent some of the most important parameters influencing the mesostructure formation. 

Brinker et al. developed a simple evaporation-induced self-assembly (EISA) process that allows the rapid production of patterned porous or nanocomposite materials in the form of films, fibers, or spheres. Starting from a homogeneous solution of soluble silica and surfactant below the critical micellar concentration, evaporation of ethanol increased surfactant concentration, driving self-assembly of silica-surfactant micelles and their further organization. By... 

Evaporation-induced Self-assembly (EISA) Process... 

Figure 8.40 Approximate trajectory taken in ethanol-water-CTAB phase space during the EISA process. Point A corresponds to the initial composition of entrained solution, Point B is near the drying line, and Point C corresponds to the dried product. Reproduced with permission from [180]. Copyright (1999) Wiley-VCH...

This mechanism has been utilized in casting, spin-coating and dip-coating processes, and a variety of films differing in pore size and pore orientation has been synthesizedJ The principle has been investigated extensively for silica-based systems. Recently, it was shown that mesostructured transition metal oxide based systems can also be synthesized by using the EISA process. 

Andersson et al. [55] recently demonstrated the synthesis of spherical, mesoporous silica particles using an approach that combines emulsion-based precipitation methods [56,57] with the EISA process. This synthetic route, termed the emulsion and solvent evaporation (ESE) method, produced well-ordered two-dimensional (2D) hexagonal mesoporous silica microspheres. Because the emulsions were prepared in bulk using vigorous stirring, the droplets, and therefore the resulting particles, were produced with a broad particle size distribution. 

MFFDs provide a robust approach to form monodisperse emulsion drops [8]. It has been demonstrated that microfluidic-generated drops can function as both morphological tanplates and chemical reactors for the synthesis of monodisperse polymer [44,45,63] and biomolecular microparticles [48]. Below, we summarize our recent results on a novel procedure for fabrication of weU-defined monodisperse, mesoporous silica particles. It is based on MFFD ulsification of an aqueous-based sol [64] with subsequent EISA processing utilizing the aforementioned ESE method [55]. 

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