Mesoporous formation

The major design concept of polymer monoliths for separation media is the realization of the hierarchical porous structure of mesopores (2-50 nm in diameter) and macropores (larger than 50 nm in diameter). The mesopores provide retentive sites and macropores flow-through channels for effective mobile-phase transport and solute transfer between the mobile phase and the stationary phase. Preparation methods of such monolithic polymers with bimodal pore sizes were disclosed in a US patent (Frechet and Svec, 1994). The two modes of pore-size distribution were characterized with the smaller sized pores ranging less than 200 nm and the larger sized pores greater than 600 nm. In the case of silica monoliths, the concept of hierarchy of pore structures is more clearly realized in the preparation by sol-gel processes followed by mesopore formation (Minakuchi et al., 1996). 

As a result of the crystal growth process Si wafers usually show striations, a variation in the bulk Si resistivity in a concentric ring pattern with a spacing in the order of millimeters. This variation of the bulk Si resistivity modulates the current density and thereby the porosity, which results in an interface roughness [Lel6]. Mesopore formation due to breakdown at the pore tips is very sensitive to striations and can be used for their delineation. 

PS formed chemically without an applied potential in a mixture of HF and an oxidizing agent, e.g. HN03, is called a stain film. The similar nature of electrochemi-cally formed micro PS and stain films was pointed out in 1960 [Arl]. Stain films are usually microporous. They are of predominantly monocrystalline character and show visible PL [Scl6, Jil], If metal films are present on the silicon surface, mesopore formation may also be observed [LilO]. 

Mesopore Formation and Spiking in Low-Doped n-Type Silicon... 

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. 

Highly dispersed phases of CdS and ZnS are prepared in a NaX matrix by ion-exchange and gas phase sulfidation. The growth of sulfide particles exceeding supercage size is accompanied by mesopore formation. The rate of photocorrosion increases with increasing dispersion of the semiconductor phase. 

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]. 

Alkaline treatment of Fe-Z15, its Si/Al ratio of 16 being outside the optimal range for mesopore formation as shown in Fig. 1, leads to a significantly lower Si extraction of 290 mg F as compared to the iron-free system (570 mg f ), while a similar (limited) mesoporosity (50 m g ) has been measured (Table 1). The slight increase of 10 m g" in mesopore surface area must be attributed to the relatively high framework aluminium concentration in Z15, which stabilizes the zeolite framework against desilication and suppresses silicon extraction [18]. Similarly to s-FeS-at, the presence of non-framework iron does not facilitate sihcon dissolution. 

The degree of mesoporosity development in the Fe-Z25 series however appears to be dependent on the concentration of non-framework iron in the zeolite. As the iron loading increases from 0 to 1.1 wt.%, the mesopore surface area upon alkaline treatment progressively decreases from 195 to 115 m g (Fig. 2). Although the mesopore surface areas obtained decrease maximally a factor of 2, remarkably the silicon dissolution is 3 to 7 times lower than in the absence of iron. Similar to alkaline treatment of s-FeS, non-framework iron appears to interact with the extracted silicate species thereby avoiding its dissolution. In the presence of substantial non-framework iron species, further extraction of silicon and additional mesopore formation seems to be hindered. 

Alkaline treatment of Fe-MFI zeolites prepared via ion exchange or isomorphous substitution leads to combined micro- and mesoporous Fe-MFI structures. The preparation method highly determines the impact of the alkaline treatment. Iron in framework positions (iron sihcahte) directs the silicon extraction towards controlled mesopore formation similarly to framework aluminium in ZSM-5. Iron in non-framework positions inhibits leaching of silicon to the filtrate. Higher concentrations of non-framework iron, as obtained in ion-exchanged Fe-ZSM-5 catalysts, lead to a lower mesoporosity development. TTie iron nanoparticles appear to remain unchanged upon Eilkaline treatment and speculatively isolated and oligonuclear iron species are held responsible for the restricted silicon dissolution and mesoporosity development. 

In previous sections, it was explained how different precursors can be used to produce activated carbons and the type of porosity developed depending on the type of activation method applied. Thus, thermal activation imrmally yields adsorbents with a medium to liigh adsorption capacity, a medium micropore size distribution and no mesopore formation (except in the case of high bum-off ratios, where micropores may be of a large size). Phosphoric acid activation yields a carbon with a higher adsorption capacity than thermal activation and a wider micropore size distribution (even in the low mesopore range), whereas KOH yields extremely narrow microporous carbons. 

Figure 5 is a simplified diagram of the original synthesis route (i.e., Si-TUD-1). The first step - the formation step - involves a monomeric silica source (here, TEOS), triethanolamine ( TEA ), and optionally TEAOH. The TEA serves as a template for the mesopore formation. The TEAOH serves as both a source of quaternary cation (to generate some micropores if necessary) and a basic environment to accelerate TEOS hydrolysis. The reaction rate increases with pH (i.e., the [OH-]/ [Si02] ratio), which can also be achieved in part or wholly by increased temperature. The second step involves an aging/drying phase to establish the primary pore structure. The last step -... 

Ouyang H, Christopherson M, Fauchet PM (2005) Enhanced control of porous silicon morphology fi"om macropore to mesopore formation. Phys Stat Solidi (a) 202(8) 1396-1401 Pacholski C (2013) Photonic crystal sensors based on porous silicon. Sensors 13 4694-4713 Roura P, Costa J (2002) Radiative thermal emission from silicon nanoparticles a reversed story from quantum to classical theory. Eur J Phys 23 191-203 Scherer WG, Smith DM, Stein D (1995) Deformation of silica aerogels during characterisation. JNon Cryst Solids 186 309-315... 

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