Porous alumina membranes

Template-based synthesis involves the fabrication of the desired material within the pores or channels of a nanoporous template. A template may be defined as a central structure within which a network forms in such a way that removal of the template creates a filled cavity with morphological and/or stereochemical features related to those of the template. Track-etch membranes, porous alumina, and other nanoporous structures have been characterized as templates. Electrochemical and electroless depositions, chemical polymerization, sol-gel deposition, and chemical vapor deposition have been presented as major template synthetic strategies. Template-based synthesis can be used to prepare nanostructures of conductive polymers, metals, metal oxides, semiconductors, carbons, and other solid matter... 

Another method of template-assisted synthesis, mainly used for the growth of metal nanowires, involves the deposition of metal into the cyUndrical pores or channels of an inert, non-conductive nanoporous electrode material. Track etch membranes, porous alumina, nanoporous conductive rubber polymers, metals, semiconductors, carbons and other solid materials have been used as templates to prepare nanometer-sized particles, fibrils, rods and tubules. The experimental set... 

Yamaguchi, A., Uejo, F., Yoda, T., Uchida, T., Tanamura, Y., Yamashita, T. and Teramae, N. (2004) Self-assembly of a silica-surfactant nanocomposite in a porous alumina membrane. Nature Materials, 3, 337-341. 

McCarley, K.C. and J.D. Way, Development of a model surface flow membrane by modification of porous /-alumina with octadecyltrichlorosilane, Sep. Purif. Techno., 25,195-210, 2001. 

Suzuki, F., K. Onozato and Y. Kurokawa. 1987. Gas permeability of a porous alumina membrane prepared by the sol-gel process. J. Non-Cryst. Solids 94 160-62. 

Figure 1.8 SEM image of porous alumina membrane, used in templating. (Reproduced by permission of The Royal Society of Chemistry from [36].)...

In a related approach, these same porous alumina membranes serve as a mask through which O2 plasmas are used to etch underlying carbon films. This etching process produces honeycomb carbon structures that are positive replicas of the alumina-membrane mask. This process has successfully produced honeycomb structures of both diamond and graphitic carbon, with pore sizes in the carbon replica around 70 nm. 

Imai H, Takei Y, Shimizu K, Matsuda M, Hirashima H (1999) Direct preparation of anatase Ti02 nanotubes in porous alumina membranes. J Mater Chem 9 2971-2972 Michailowski A, A1 Mawlawi D, Cheng GS, Moskovits M (2001) Highly regular anatase nanotubule arrays fabricated in porous anodic templates. Chem Phys Lett 349 1-5 Jung JH, Kobayashi H, van Bommel KJC, Shinkai S, Shimizu T (2002) Creation of novel helical ribbon and double-layered nanotube Ti02 structures using an... 

Imai H, Take Y, Shimizu K, Matsuda M, Hirashima H (1999) Direct preparation of anatase Ti02 nanotubes in porous alumina membranes. J Mater Chem 9 2971-2975... 

It should be mentioned that alternative possibilities to prepare similar membranes include the use of a porous alumina membrane as matrix, with the titania nanotubes grown in the channels. Nanoporous alumina membranes are commercial products, also synthesized by anodic oxidation. The commercial Whatman Corporation anodic membrane has holes of about 20-nm diameter at the top of the membrane and about 200-nm diameter at the bottom of membrane. Within these pores Ti02 nanotubes fabricated by template synthesis and water vapour hydrolysis could be grown, but non-uniform membrane characteristics are obtained due to the non-uniform pores of the commercial alumina... 

Several commercial flat membrane supports have been used including polyethylene [235], polyamide [230, 231], cellulose [236, 237], PVDF [229, 238], PTFE [222, 239], polyurethane [235] and porous alumina [240]. Fibres and hollow fibres of glass [241], polypropylene [233, 234] or PVDF [242] were also employed as support. 

Figure 9. Left experimental spectrum obtained in the center of a hole in porous alumina. Right three LELS relativistic simulations. (Reprinted from Surface Science 532-535 Zabala N, Rivacoba, Garcia de Abajo F.J. and Pattantyus A., Cherenkov radiation effects in EELS for nanoporous alumina membranes, 461-467 Copyright (2003) with permission from Elsevier)...

In this context, a novel and interesting method for the preparation of such systems has recently been reported. The pyrolysis of a nickel naphthalocyanine (Nc), a homolog of Pc with a larger tt system, previously organized in the nanoscale channels of a porous alumina membrane that acts as a template, leads to the formation of... 

Different ways have been proposed to prepare zeolite membranes. A layer of a zeolite structure can be synthesized on a porous alumina or Vycor glass support [27, 28]. Another way is to allow zeolite crystals to grow on a support and then to plug the intercrystalline pores with a dense matrix [29], However, these two ways often lead to defects which strongly decrease the performance of the resulting membrane. A different approach consists in the direct synthesis of a thin (but fragile) unsupported monolithic zeolite membrane [30]. Recent papers have reported on the preparation of zeolite composite membranes by hydrothermal synthesis of a zeolite structure in (or on) a porous substrate [31-34]. These membranes can act as molecular sieve separators (Fig. 2), suggesting that dcfcct-frcc materials can be prepared in this way. The control of the thickness of the separative layer seems to be the key for the future of zeolite membranes. 

A new class of artificial membranes, based on lipid bilayers supported on porous alumina, were recently studied by Hennesthal and Steinem using AFM [58], In this case, tip-sample convolution effects affecting the detection of pores (see Section 2.3) were clearly demonstrated by the authors on comparing the average pore size of the alumina support as obtained by SEM (60 nm) and by AFM (50 nm). 

Romer W, Steinem C. Impedance analysis and single-channel recordings on nano-black hpid membranes based on porous alumina. Biophys. J. 2004 86 955-965. 

Furthermore, vacuum evaporation has been used to deposit a dense membrane layer of Pd or Pd-Pt alloys on a porous alumina support [Saito, 1988]. 

This method has been applied to ceramic membranes (e.g., gamma-alumina membranes) and compared to other methods such as nitrogen adsorption/desorption and thermoporometry (to be discussed next) in Figure 4.13. It can be seen that the mean pore diameter measured by the three methods agrees quite well. The pore size distribution by permporometry, however, appears to be narrower than those by the other two techniques. Similar conclusions have been drawn regarding the comparison between permporometry and nitrogen adsorption/desorption methods applied to porous alumina membranes [Cao et al., 1993]. The broader pore size distribution obtained from nitrogen adsorption/desorption is attributable to the notion that the method includes the contribution of passive pores as well as active pores. Permporometry only accounts for active pores. 

The year 1980 marked the entry of a new type of commercial ceramic membrane into the separation market. SPEC in France introduced a zirconia membrane on a porous carbon support called Carbosep. This was followed in 1984 by the introduction of alumina membranes on alumina supports, Membralox by Ceraver in France and Ceraflo by Norton in the U.S. With the advent of commercialization of these ceramic membranes in the eighties, the general interest level in inorganic membranes has been aroused to a historical high. Several companies involved in the gas diffusion processes were responsible for this upsurge of interest and applications. 

There are a variety of porous inorganic membranes in the market today. Highlighted in Table 5.1 are selected major commercial inorganic membranes according to their material type. So far the most widely used inorganic membranes are alumina membranes, followed by zirconia membranes. Porous glass and metal (such as stainless steel and silver) membranes have also begun to attract attention. 

Figure 5.8 Flux decline of a porous alumina membrane system with Lime due to fouling [Gumming and Turner, 1989]...

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