Ordered pores

Figure 4.18 TEM images showing the mesostructures of Ti02 species dispersed in Si02 transparent thin films with (a) a hexagonal ordered pore arrangement and (b, c) cubic symmet7. Reproduced from [247] (2003) with permission from Elsevier.

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. 

F. Li, L. Zhang, and R. M. Metzger, On the growth of highly ordered pores in anodized aluminum oxide, Chem. Mater. 10, 2470 (1998). 

Using a different Zintl anion as building block, we and others prepared mesoporous germanium with well-ordered pore structure and high internal surface area [43,44],... 

Transition metal complexes encapsulated in the channel of zeolites have received a lot of attention, due to their high catalytic activity, selectivity and stability in field of oxidation reactions. Generally, transition metal complex have only been immobilized in the classical large porous zeolites, such as X, Y[l-4], But the restricted sizes of the pores and cavities of the zeolites not only limit the maximum size of the complex which can be accommodated, but also impose resistance on the diffusion of substrates and products. Mesoporous molecular sieves, due to their high surface area and ordered pore structure, offer the potentiality as a good host for immobilizing transition complexes[5-7]. The previous reports are mainly about molecular sieves encapsulated mononuclear metal complex, whereas the reports about immobilization of heteronuclear metal complex in the host material are few. Here, we try to prepare MCM-41 loaded with binuclear Co(II)-La(III) complex with bis-salicylaldehyde ethylenediamine schiff base. 

The DTG curves for uncalcined HR-B2 and B3 still exhibited some similarity to those of good-quality samples, such as HR-A1 - A3, but resemble more closely the DTG curve for an uncalcined lamellar phase [22], XRD revealed the presence of hexagonally ordered domains in the calcined HR-B2 and B3 samples, whereas adsorption data showed the presence of ordered pore domains of the size in accord with the XRD interplanar spacing (see Table 2). 

Many applications of AFM to pillared clays or zeolites have not specifically addressed the porosity characteristics, but rather the occurrence of adsorbed surface Al species in, e.g., pillared montmorillonite [41], or the crystal growth processes, adsorption on porous surfaces and the surface structure of natural zeolites [42]. Sugiyama et al. [43] succeeded to reveal the ordered pore structure of the (001) surface of mordenite after removal of impurities that clogged the pores. The authors indicated that resolution in AFM imaging of zeolites is significantly affected by the magnitude of the periodical corrugation on the crystal surface, so that if the surface contains deep pores only the pore structure, but not the atomic structure, can be resolved. 

For cement-like materials an overestimation of the pore size is expected, since the contact angle of < 130° is much lower than the standard value of 140°. However, the availability of cement-based materials with an ordered pore structure in the appropriate pore size range is limited and as a consequence these results cannot be discussed here. 

Even today, adsorbents which are employed in separation techniques do not fulfil the following criteria a high capacity combined with fast mass transfer kinetics. The solution to this problem is provided by nature itself. There, aU adsorption processes are performed in a very effective manner using the principle of hierarchy. It is the ultimate goal to mimic this property and to manufacture a man-made adsorbent featuring a high degree of hierarchically ordered pore structure. 

Figure 1 Concept for the synthesis of highly ordered pore structures.

Each of the three characterisation techniques explored in the present study can be criticised in terms of experimental procedure or mathematical interpretation of the raw data. Nevertheless, in the present study, a remarkable agreement is obtained between the results obtained with the three techniques. It is interesting to note that this is the case with such large pore samples of heterogeneous pore size distribution. These results are in good agreement with comparison studies carried out on other more ordered pore systems [6]. 

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