Mesoporous film formation

In this section the thickness of the formation of a lyogel film with reference to the development of its thickness will be discussed. In latter sections the development of its microstructure will be treated. 

The production processes used are (i) dip-coating for plates and tubes, (and ii) spin-coating, mainly for plates. 

In the spin-coating process the liquid is applied to a rotating plate and is distributed across the plate by centrifugal forces [1,8]. 

Two main groups of lyogel film formation mechanisms can be distinguished (i) film coating, and (ii) slip casting. In the film-coating process, capillary forces in pores do not play a role and this process can be used also on dense, non-porous substrates. The slip cast process is widespread in the production of bulk ceramics but has only been recently applied to the production of membranes [3,9]. In this process capillary forces play a dominant role. 

In the discussion so far it has been assumed that the only interaction between liquid and substrate is by adhesion forces. This means that with porous substrates capillary forces should be absent. This situation can be obtained by filling the substrate pores with another liquid having a surface tension comparable to that of the colloidal solution or by making the substrate hydrophobic (in case of an aqueous solution). 

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The classic sensitizer dye employed in DSC is a Ru(II) bipyridyl dye, cis-bis(isothiocyanato)-bis(2,2/-bipyridyl-4,4/-dicarboxylato)-Ru(II), often referred to as N3 , or in its partially deprotonated form (a di-tetrabutyl-ammonium salt) as N719. The structure of these dyes are shown in 2 and 26. The incorporation of carboxylate groups allows immobilization of sensitizer to the film surface via the formation of bidendate coordination and ester linkages, whilst the (- NCS) groups enhance the visible light absorption. Adsorption of the dye to the mesoporous film is achieved by simple immersion of the film in a solution of dye, which results in the adsorption of a dye monolayer to the film surface. The counter electrode is fabricated from FTO-coated glass, with the addition of a Pt catalyst to catalyze the reduc-... 

Figure 5. Schematic representation of the pore structure of mesoporous silica, depicting the cylindrical pores in a regular hexagonal array. The sequential process of multilayer film formation and capillary condensation in this structure are illustrated.

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 ... 

In the discussion of the mesopore shape, the contact angle, is assumed to be zero (uniform adsorbed film formation). The lower hysteresis loop of file same adsorbate encloses at a common relative pressure depending to the stability of the adsorbed layer regardless of the different adsorbents due to the so called tensile strength effect. This tensile strength effect is not sufficiently considered for analysis of mesopore structures. The Kelvin equation provides the relationship between the pore radius and the amount of adsorption at a relative pressure. Many researchers developed a method for the calculation of the pore size distribution on the basis of the Kelvin equation with a correction term for the thickness of the multilayer adsorbed film. 

Li YB, Tong XL, He YN, Wang XG. 2006c. Formation of ordered mesoporous films from in situ structure inversion of azo polymer colloidal arrays. J Am Chem Soc 128 2220 2221. 

Lu Y, Ganguli R, Drewien CA, Anderson MT, Brinker CJ, Gong WL, Guo YX, Soyez H, Dunn B, Huang MH, Zink JI. 1997. Continuous formation of supported cubic and hexagonal mesoporous films by sol gel dip coating. Nature 389 364 368. 

Chen et al. [74] studied the photocatalytic behavior of Ti02-P25 coatings with nonionic surfactants (Tween 20) and confirm that S, pore volume/porosity, film thickness, and photocatalytic activity increase with the surfactant content. The addition of P25 to mesoporous layers of Ti02-Tween 20 can lead to the formation of bimodal pore size distribution, producing greater photocatalytic activity than the mesoporous films. 

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