Homogeneous film formation

Homogeneous film formation is likely due to the film-forming characteristics of PEDOTiPSS at room temperature. 

Let us assume that the total surface of an electrode is in an active state, which supports dissolution, prior to anodization. The application of a constant anodic current density may now lead to formation of a passive film at certain spots of the surface. This increases the local current density across the remaining unpassivated regions. If a certain value of current density or bias exists at which dissolution occurs continuously without passivation the passivated regions will grow until this value is reached at the unpassivated spots. These remaining spots now become pore tips. This is a hypothetical scenario that illustrates how the initial, homogeneously unpassivated electrode develops pore nucleation sites. Passive film formation is crucial for pore nucleation and pore growth in metal electrodes like aluminum [Wi3, He7], but it is not relevant for the formation of PS. 

Finally, even if these criteria are satisfied, there remains the question of whether the product will adhere to form a film or just precipitate homogeneously in the solution. This is the most difficult criterion to answer a priori. The hydroxide and/or oxy groups present on many substrates in aqueous solutions are likely to be quite different in a nonaqueous solvent (depending on whether hydroxide groups are present or not). Another factor that could conceivably explain the general lack of film formation in many organic solvents is the lower Hamaker constant of water compared with many other liquids this means that the interaction between a particle in the solvent and a solid surface will be somewhat more in water than in most other liquids (see Chapter 1, van der Waals forces). From the author s own experience, although slow precipitation can be readily accomplished from nonaqueous solutions, film formation appears to be the exception rather than the rule. The few examples described in the literature are confined to carboxylic acid solvents (see later). 

The deposition occurs in parallel with homogeneous precipitation, suggesting that film formation is due to adhesion of crystals from the solution. This is supported by SEM pictures that show scattered crystal formation, with gradual den-... 

Table 7.2 summarizes a range of homogeneous precipitation reactions. Details of all these reactions can be found in Ref. 2 (this book, in spite of its age, is required reading for anyone wishing to pursue this line more recent books may exist, but will probably not reduce its value). It should fu-st be stressed that the material in this final section relates to precipitates rather than to films. However, with some effort (in some cases only a little or none, as seen from the common film formation occurring in urea precipitations), it is reasonable to expect extension to form films of the same materials in at least some cases. 

As noted earlier, our principal interest will be in an open, flowing CVD system. In order to correctly interpret the phenomena occurring in such systems, it will be necessary to study chemically reacting gas flows with nonuniform flow and temperature fields. And, of course, we will have to understand the surface reactions that lead to the solid film formation. Within the reacting gas, we will consider homogeneous gas phase reactions. At the surface, we have... 

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