Mesopore Formation Mechanisms

The bias needed for breakdown decreases with pore diameter and becomes as low as 1 V independent of doping density for pore tip diameters below 20 nm. 

For moderately doped substrates the crossover from tunneling to avalanche breakdown occurs at pore diameters of about 500 nm, corresponding to a bias in excess of 10 V. Above doping densities of 1017 cm-3 breakdown is always dominated by tunneling. Tunneling is therefore expected to dominate all pore formation in the mesoporous regime and extends well into the lower macropore regime, while avalanche breakdown is expected to produce structures of macropor-ous size. 

For n-type doping densities below 1017 cm-3 and an anodization bias above 10 V, avalanche breakdown becomes relevant. The interface morphology generated in this regime is very complex and shows large etch pits, macropores and mesopores. The formation of this structure is not understood in detail. A hypothetical model will be discussed in Section 8.5. 

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An interesting question is whether such well-ordered pore arrays can also be produced in other semiconductors than Si by the same electrochemical etching process. Conversion of the macropore formation process active for n-type silicon electrodes on other semiconductors is unlikely, because their minority carrier diffusion length is usually not large enough to enable holes to diffuse from the illuminated backside to the front. The macropore formation process active in p-type silicon or the mesopore formation mechanisms, however, involve no minority carrier diffusion and it therefore seems likely that these mechanisms also apply to other semiconductor electrodes. 

Patarin, J., Lebeau, B. and Zana, R. (2002) Recent advances in the formation mechanisms of organized mesoporous materials. Current Opinion in Colloid and Interface Science,... 

The average pore size of PS structures covers four orders of magnitude, from nanometers to tens of micrometers. The pore size, or more precisely the pore width d, is defined as the distance between two opposite walls of the pore. It so happens that the different size regimes of PS characterized by different pore morphologies and different formation mechanisms closely match the classification of porous media, as laid down in the IUPAC convention [Iu2]. Therefore the PS structures discussed in the next three chapters will be ordered according to the pore diameters as mostly microporous (d<2 nm), mostly mesoporous (2 nm50 rim). Note that the term nanoporous is sometimes used in the literature for the microporous size regime. 

If the pore density is plotted versus the doping density of the silicon electrode, it can be seen that the micropore density is independent of doping, while the macropore and mesopore densities increase linearly with doping density, as shown in Fig. 6.10. This is a consequence of the QC formation mechanism being independent of doping, while the SCR-related mechanisms are not, as discussed in Section 6.2. 

In contrast to p-type electrodes, an n-type electrode is under reverse conditions in the anodic regime. This has several consequences for pore formation. Significant currents in a reverse biased Schottky diode are expected under breakdown conditions or if injected or photogenerated minority carriers can be collected. Breakdown at the pore tip due to tunneling generates mainly mesopores, while avalanche breakdown forms larger etch pits. Both cases are discussed in Chapter 8. Macropore formation by collection of minority carriers is understood in detail and a quantitative description is possible [Le9], which is in contrast to the pore formation mechanisms discussed so far. 

Since the discovery of the M41S materials with regular mesopore structure by Mobils scientists [1], many researchers have reported on the synthetic method, characterization, and formation mechanism. Especially, the new concept of supramolecular templating of molecular aggregates of surfactants, proposed as a key step in the formation mechanism of these materials, has expanded the possibility of the formation of various mesoporous structures and gives us new synthetic tools to engineer porous materials [2],... 

Many studies that have focused on the formation mechanism of various types of templated mesoporous materials have been summarized in a number of recent reviews,46 9 and it is still an ongoing topic of research.50 51 Here, we want to recapitulate only very briefly the useful interaction classification scheme between the surfactants (S) and the inorganic species (I), which was suggested by Huo et al.52,53... 

Following similar principles of combining the aggregate-forming properties of bifunctional molecules with low cost and low toxicity, a mesoporous silica material with a three-dimensional worm-like pore system was reported. Triethanol amine (TEA) was used as the SDA and TEOS as the silica source in this mesoporous silica, TUD-1.[48] The formation mechanism is depicted in Figure 1.3(a). The properties of the material can be easily tuned by modifications in the synthesis procedure, for example, the pore size of the material was found to be proportional to... 

Vis tad et al. (2001) Silicon-aluminum phosphates, SAPO 34 Phase formation role of thermal treatment + + Solid formation mechanism, intermediate phases during synthesis of mesoporous solids... 

Pinnavaia etal. used neutral alkylamines as templates to form disordered mesoporous silica, named hexagonal molecular sieves (HMS). The S°I° formation mechanism was proposed between neutral amine micelles (S°) and neutral inorganic precursors (1°). The interactions between S° and 1° were assumed to be hydrogen bonding. The resulting HMS has a worm-like pore structure, with thicker framework walls and smaller X-ray scattering domain sizes compared to M41S. 

Figure 16.8 Schematic illustration of the formation mechanisms for mesoporous Ti02 hollow and solid spheres (from [73]).

Yamazaki S and Tsutsumi K. Synthesis of A-type zeolite membrane using a plate heater and its formation mechanism. Micropor Mesopor Mater 2000 37 67-80. 

V. Lehmann, A. Luigart, and V. Corbel, On the morphology and the electrochemical formation mechanism of mesoporous silicon, Electrochem. Soc. Proc. 97(7), 132 1997. 

Figure 1. Schematic pathway for preparing surfactant-templated mesoporous silicas, illustrating a formation mechanism based on preformed liquid crystal (LC) mesophase (route A) or a cooperative process (route B). Reprinted from [20], Copyright (2008) WILEY-VCH Verlag GmbH Co.

Although studies on the subject of ordered mesoporous materials were started about 15 years ago, the unique structure and the properties of these materials attracted many scientists in different fields of research. Their efforts resulted in fruitful results that have been reported in thousands of publications. The flexibility and complexity of their synthesis and structure, and the extensive application potentials of mesoporous materials, create a huge opportunity for researchers and developmental scientists. This chapter will summarize the research results on mesoporous materials from syntheses, structures, formation mechanisms, compositions, morphologies, pore-size control, modifications, applications, challenges, and so on. 

Synthesis Characteristics and Formation Mechanism of Ordered Mesoporous Materials... 

Establishing a mechanistic understanding is needed for better control of the synthesis process. A better understanding of the formation mechanism via combined characterization techniques and modeling may lead to a more rational approach for tuning the pore structure of mesoporous materials. 

Grosso, D. Balkenenda, A.R. Albouy, P.A. Ayral, A. Amenitsch, H. Babonneau, F. Two-dimensional hexagonal mesoporous silica thin films prepared from block copolymers detailed characterization and formation mechanism. Chem. Mater. 2001, B (5), 1848-1856. 

Zhang, J. and Goldfarb, D., In situ investigations of the formation mechanism of mesoporous materials via the dynamics and ordering of spin-probes — pH and Si/surfactant effect, Microporous Mesoporous Mater., 48, 143, 2001. 

MFI zeolite upon alkaline treatment (see also Fig. 1) [6]. Following those results, an optimal framework Si/Al ratio of 25-50 for mesopore formation has been established. The fitting of the data in the range Si/Al ratio 50-200 was somehow arbitrary, due to lack of zeolites with a suitable Si/Al ratio. The increased mesopore surface area of 120 m g obtained upon desilication of FeS, coupled to a framework Si/Fe molar ratio of 77, however perfectly correlates with the previously proposed fitting, despite the different nature of the trivalent framework cation (solid circle in Fig. la). Additionally, the newly created mesoporosity centered around 20 nm also agrees well with the mesopore size vs. Si/Al ratio dependency, as shown in Fig. lb. These results provide supplementary convincing evidence of the crucial role of the trivalent metal cation in framework positions on the mesopore formation process, which appears to be independent on the nature of the trivalent metal cation. In addition, this confirms the universality of the pore formation mechanism as previously proposed for the alkaline treatment of MFI zeolites [18]. 

Mesostructured thin silica films can be prepared as freestanding films or supported by a variety of different substrates. In the following, a short description of the different synthesis methods is given the synthesis and the formation mechanism of mesoporous silica films has been reviewed in detail elsewhere. " ... 

Series of Special Issues cover various aspects of microporous molecular sieves, metal-organic frameworks and ordered mesoporous materials. Synthesis principles, templating, formation mechanisms, characterization methods, functionalization strategies, and applications are discussed in excellent and comprehensive review articles and more specific research reports. 

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