Baixe Ordered mesoporous and macroporous inorganic films and membranes e outras Notas de estudo em PDF para Engenharia Elétrica, somente na Docsity! Journal of Membrane Science 235 (2004) 53–72 Ordered mesoporous and macroporous inorganic films and membranes V.V. Guliants∗, M.A. Carreon, Y.S. Lin1 Department of Chemical and Materials Engineering, University of Cincinnati, Cincinnati, OH 45221-0012, USA Received 21 April 2003; received in revised form 29 December 2003; accepted 12 January 2004 Abstract This article reviews synthesis, structures and emerging applications of ordered inorganic meso- and macroporous films and membranes. Thin films or membranes of these groups of porous materials can be prepared by various techniques commonly used for film formation from liquid solution, including solvent evaporation, in situ growth from solution and other alternative methods. Although these techniques allow control over the thickness and pore structures in thin films or membranes, they currently offer limited control over the pore alignment relative to the substrate surface. The ordered mesoporous and macroporous thin films and membranes are highly promising for a range of potential applications in separations, chemical sensing, heterogenous catalysis, microelectronics and photonics as insulating layers of low dielectric constant and photonic bandgap materials. Furthermore, functionalization of the internal pore surfaces in these films and deposition of functional nanoparticles within the pores offer numerous new possibilities for molecular engineering of catalytic, photonic, and magnetic materials for advanced nanotechnological applications based on quantum confinement effect. © 2004 Elsevier B.V. All rights reserved. Keywords: Mesoporous; Macroporous; Pore structures 1. Introduction According to IUPAC [1], mesoporous and macroporous films or membranes refer to materials with pore diameters in the 2–50 and >50 nm ranges, respectively. The term “film” is used to describe a material in a specific planar geometry, while the membrane is defined as a thin physical barrier al- lowing selective transport of mass species. Thin films can be formed on any support (substrate), whereas membranes must be prepared on a porous support (or substrate), which allows transport of mass species. Mesoporous and macrop- orous films and membranes have been studied extensively in the past decades and many of them have found commer- cial applications [2,3]. Examples of these applications in- clude a large number of inorganic membrane microfiltration and ultrafiltration processes being used in industry for bev- erage and drinking water purification and wastewater treat- ment. The majority of these mesoporous and macroporous ∗ Corresponding author. Tel.: +1-513-556-0203; fax: +1-513-556-3473. E-mail addresses: vguliant@alpha.che.uc.edu (V.V. Guliants), jlin@alpha.che.uc.edu (Y.S. Lin). 1 Co-corresponding author. films and membranes were made by compacting nano- or micron-sized dense crystallites (such as - or -alumina crystals). Their pore structure is defined by the crystallite size and the manner in which they are packed. The pores of these films or membranes are not arranged in an ordered fashion as opposed to those in ordered mesoporous and macroporous films or membranes. The first ordered mesoporous materials, belonging to alu- minosilicate molecular sieves called M41S, were reported by researchers at Mobil over a decade ago [4]. The meso- porous aluminosilicate and other metal oxide materials with well-defined pore sizes up to 33 nm break past the pore-size constraint (<1.5 nm) of microporous zeolites. The ex- tremely high surface areas (>1000 m2/g) and precise tuning of pore sizes are among the many desirable properties that have made such materials the focus of great attention. The M41S materials also ushered in a new approach in materials synthesis where, instead of the use of single molecules as templating agents (as in the case of zeolites), self-assembled molecular aggregates or supramolecular assemblies are em- ployed as the structure-directing agents. Since the initial discovery of the M41S materials, many other ordered pore structures corresponding to different surfactant assemblies have been synthesized. Various aspects of these ordered 0376-7388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2004.01.019 54 V.V. Guliants et al. / Journal of Membrane Science 235 (2004) 53–72 mesoporous materials have been reviewed by Brinker (re- cent advances in porous inorganic materials) [5], Vartuli et al. (synthesis of the M41S family) [6], Stucky et al. (biomimetic synthesis of mesoporous materials) [7], Ra- man et al. (porous silicates templated by surfactants and organo-silicate precursors) [8], Antonelli and Ying (meso- porous transition metal oxide (TMS) family) [9], Behrens (mesoporous transition metal oxides) [10], Zhao et al. (aluminosilicate MCM-41) [11], and Sayari (catalytic ap- plications of MCM-41 and HMS) [12]. The most common ordered mesoporous structure is the two-dimensional hexagonal phase with the P6mm symme- try, consisting of close-packed hexagonal arrays of cylindri- cal surfactant micelles. Lamellar silicate-surfactant phases are also obtained, which are not stable to the removal of the surfactant which leads to a collapse of the silica lay- ers. Various cubic phases have also been reported. The bi- continuous cubic gyroid phase with the Ia3d symmetry is found in alkaline-catalyzed syntheses. This phase with its network of interconnected pores is much more attractive than the two-dimensional hexagonal phase for applications requiring diffusion of species into and out of the pore net- work. The Pm3n phase composed of spherical micelles in a cubic close-packed arrangement is found in both acid- and alkaline-catalyzed preparations. The initial work of the Mobil researchers involved only alkaline-catalyzed preparations and materials re- ported in these papers are referred to as MCM-41 (P6mm two-dimensional hexagonal phase), MCM-48 (Ia3d cubic phase) and MCM-50 (lamellar phase). Two-dimensional hexagonal mesoporous materials prepared by Inagaki and co-workers [13] from surfactant-intercalated kanemite are referred to as FSM-16. Subsequent preparation of materials in acidic medium led to ordered mesoporous materials with similar symmetries, but distinct wall properties: SBA-1 (Pm3n, cubic phase), SBA-2 (P63/mmc, three-dimensional hexagonally packed spherical micelle phase) and SBA-3 (P6mm, two-dimensional hexagonally packed cylinders) [14]. Other surfactant-templated phases include disor- dered materials with a worm-like network of channels: KIT-1 [15], MSU-1 [16], a well-defined intergrown cu- bic hexagonal phase STAC-1 [17] and ordered non-ionic alkyl-EOx-templated materials, e.g. cubic SBA-11 (Pm-3m), three-dimensional hexagonal SBA-12 (P63/mmc) [18]. Us- ing the same methodology, triblock-copolymer-templated materials with much larger pores have also been synthesized in acidic systems, e.g. two-dimensional hexagonal SBA-15 (P6mm) [19] and SBA-16 cubic cage structure (Im-3m) [18]. The technological potential of mesoporous materials for chemical separations and heterogeneous catalysis was rec- ognized at the time of their discovery [20]. An important feature of mesoporous materials is the recently discovered ability to form thin films. While the early work focused on the synthesis of bulk materials, methods for preparation of mesoporous materials in a thin film configuration at a thickness range of tens of nanometers to micrometers were reported in the last few years. The motivation for synthesis of mesoporous thin films originates from the appreciation of their technological potential as membranes, sensors [20], heterogeneous catalysts [20], and insulating layers of low dielectric constant for microelectronics [21]. These applica- tions require the ordered material in the form of thin film. Mesoporous thin films or membranes are characterized by bulk properties, such as symmetry, pore diameter, surface area and stability, and film-related parameters, such as pore orientation, film thickness, continuity and surface roughness. Although a number of reviews have appeared on bulk or- dered mesoporous materials, only one review on ordered mesoporous films has emerged to date with the emphasis on self-standing, unsupported films prepared at the liquid and air interface [22] despite increasing number of published studies on thin film ordered mesoporous materials and their technological importance. Ordered macroporous materials templated by ordered arrays of colloidal spheres and other shapes have also recently received interest from scientific community [23]. The present review is focused on solid substrate-supported thin films of the ordered mesoporous and macroporous materials that are promising for applica- tions in chemical sensing, separations and catalysis. We will first discuss the major preparation routes and mechanisms of formation of mesoporous thin films and then review poten- tial applications of these materials in membrane separations, heterogeneous catalysis and nanotechnology. The subjects of mesoporous and macroporous films will be discussed in separate sections because of the fundamentally different for- mation mechanisms and resulting pore structures. 2. Mechanisms of self-assembly 2.1. Ordered mesoporous materials 2.1.1. Liquid crystal templating mechanism The original M41S family of mesoporous molecular sieves is synthesized by reacting a silica source (e.g. tetraethylorthosilicate (TEOS), Ludox, fumed silica, sodium silicate), an alkyltrimethylammonium halide surfactant (e.g. cetyltrimethylammonium bromide (CTAB)), a base (e.g. sodium hydroxide or tetramethylammonium hydroxide (TMAOH)), and water at ≥100 ◦C for 24–144 h. Alumi- nosilicate M41S is synthesized by addition of an aluminum source to the synthesis mixture. The as-synthesized product contains occluded organic surfactant, which is removed by calcination at ∼500 ◦C in air to yield an ordered mesoporous material. A “liquid crystal templating” (LCT) mechanism of forma- tion was proposed by the Mobil researchers (Fig. 1), based on the similarity between liquid crystalline surfactant as- semblies (i.e. lyotropic phases) and M41S [4]. The common traits were the mesostructure dependence on the hydrocar- bon chain length of the surfactant tail group [24,26], the ef- fect of surfactant concentration, and the influence of organic V.V. Guliants et al. / Journal of Membrane Science 235 (2004) 53–72 57 produced mesostructured precipitates. Control of the con- densation of the silicates within the mesostructure by acidic vapor treatment led to the observation of cubic, lamellar and hexagonal phases as intermediate transformation phases. The previous theories have regarded the formation of MCM-41 as a series of events that occur homogeneously throughout an aqueous solution. Recent work has shown that MCM-41 might be formed heterogeneously. Regev [35] found evidence for MCM-41 intermediate structures in the form of clusters of rod-like micelles “wrapped” by a coat- ing of silicate through low-temperature transmission elec- tron microscopy (TEM) and small-angle X-ray scattering. The clusters of elongated micelles were found before pre- cipitation occurred. According to Regev [35], as the reac- tion progressed, the silicate species diffused to and deposited onto the individual surfaces of the micelles within the clus- ter. The clusters of elongated micelles eventually became clusters of silicate-covered micelles. Thus, the clusters of micelles served as nucleation sites for MCM-41 formation. 2.2. Ordered macroporous materials Colloidal crystals consisting of three-dimensional ordered arrays of monodispersed spheres, represent novel templates for the preparation of highly ordered macroporous inor- ganic solids, exhibiting precisely controlled pore sizes and highly ordered three-dimensional porous structures. This macroscale templating approach typically consists of three steps as shown in Fig. 4. First, the interstitial voids of the monodisperse sphere arrays are filled with precursors of var- ious classes of materials, such as ceramics, semiconductors, metals, monomers, etc. In the second step, the precursors Fig. 4. Experimental procedure to generate three-dimensional macroporous inorganic materials by templating against crystalline arrays of colloidal spheres (modified from [124]). Fig. 5. SEM image of colloidal array of ∼400 nm polystyrene spheres [130]. condense and form a solid framework around the spheres. Finally, the spheres are removed by either calcination or sol- vent extraction. The success of forming macroporous ordered structures is mainly determined by van der Waals interactions, wetting of the template surface, filling of the voids between the spheres and volume shrinkage of the precursors during so- lidification process. The colloidal crystal templating method may be used in combination with sol–gel, salt solution, nanocrystalline and other precursors to produce the inor- ganic three-dimensional macrostructures [23]. The colloidal crystal templates used to prepare three-dimensional macro- porous materials include monodisperse polystyrene (PS), poly(methyl methacrylate) (PMMA) and silica spheres. 58 V.V. Guliants et al. / Journal of Membrane Science 235 (2004) 53–72 Fig. 6. SEM image of macroporous inorganic material. The walls are com- posed from vanadium–phosphorus oxide crystals (modified from [130]). A typical SEM image of a colloidal array of polystyrene spheres used as a template in the synthesis of macroporous inorganic materials is shown in Fig. 5. Prior to precursor infiltration, these monodisperse spheres are ordered into close-packed arrays by sedimentation, centrifugation, verti- cal deposition or electrophoresis [36]. The final inorganic macroporous structure after the removal of spheres contains ordered interconnected pore structure shown in Fig. 6. Table 1 Ordered mesostructured films prepared by dip-coating Thin film structure Precursors Support Reference Three-dimensional cubic silica TEOS, ethanol, HCl, CTAB Alumina [37] Hexagonal and three-dimensional cubic silica ‖ TEOS, ethanol, HCl, PEO-PPO-PEO Silicon wafers and glass slides [38] Two-dimensional hexagonal silica ‖ TEOS, PEO-PPO-PEO, ethanol, HCl, pluronic F-127 Si(1 0 0) wafers [39] Two-dimensional hexagonal and rectangular titania ‖ TiCl4, ethanol, pluronic F-127, Brij 58 Glass or silicon substrates [40] Disordered tungsten oxide WCl6, pluronic P-123, ethanol Tin-doped indium oxide (ITO)-coated glass [41] Hexagonal silica TEOS, 3-(2,4-dinitrophenylamino)propyltriethoxysilane, CTAB Glass slides [42] Two-dimensional hexagonal and two-dimensional rectangular ZrO2 ZrCl4, C16H33(CH2CH2O)20OH ethanol, H2O Silicon [43] Two-dimensional hexagonal aluminum oxide normal to the substrate AlCl3, C16H33(CH2CH2O)20OH ethanol, H2O, NH4OH Glass plates or silicon wafers [44] Hexagonal silica ‖ TEOS, CTAB, ethanol, H2O, HCl Glass slide [45] Hexagonal silica TMOS, NaOH, NH3, H2O, CTABr Silicon wafers, borosilicate glass [46] One-dimensional hexagonal silica TEOS, HCl, H2O, F-127, EtOH Glass and silicon [47] One-dimensional hexagonal and cubic silica ‖ TEOS, HCl, H2O, EtOH, Brij 56 Silicon with oxide overlayer (SiO2/Si) and Au [48] Three-dimensional hexagonal silica, c-axes ⊥ TEOS, HCl, H2O, CTABr, EtOH Silicon wafers [49] Hexagonal and cubic silica ‖ TEOS, HCl, H2O, CTABr Silicon [50] One-dimensional hexagonal, three-dimensional hexagonal, lamellar and cubic silica TEOS, CTAB, Brij 56, HCl, H2O Silicon [51] Two-dimensional hexagonal silica membranes ‖ TEOS, HCl, H2O, alkyl trimethyl ammonium bromides (C12–C16) Glass or silicon [52] Three-dimensional hexagonal and cubic silica ‖ TEOS, HCl, H2O, gemini surfactants Silicon wafers or glass [53] One- and two-dimensional hexagonal silica ‖ TEOS, EtOH, HCl, H2O, CTAB, F-127 Glass or silicon wafers [54] Cubic and three-dimensional hexagonal silica ‖ TEOS, HCl, EtOH, CTAB Silicon wafers [55] ‖: Parallel to interface; ⊥: normal to interface. 3. Preparation of films and membranes 3.1. Mesoporous films and membranes 3.1.1. Solvent evaporation techniques The solvent evaporation techniques involve formation of a liquid film containing the solvent, surfactant and silica pre- cursor followed by evaporation of the solvent. Several meth- ods can be used to form liquid films. These include dip-, spin-coating and film casting. Solvent evaporation has been suggested as the driving force for the organization of surfac- tant species into mesoscale aggregates (i.e. lamellar, hexag- onal, cubic) around which condensation of silicate species takes place. The various mesostructured inorganic films and membranes prepared by the solvent evaporation techniques are summarized in Tables 1–3 [37–83]. The mesostructured films prepared by a dip-coating method are shown in Table 1. In this method, the substrates are withdrawn from a homogeneous precursor solution and the dip-coated solution is allowed to drain to a particular thickness [84]. The thickness of the film is mainly deter- mined by the rate of evaporation of the solvent and the viscosity of the solution. One of the advantages of this method is the facile formation of films on non-planar sur- faces. Casting is another solvent evaporation method that has been used for the preparation of mesostructured films. V.V. Guliants et al. / Journal of Membrane Science 235 (2004) 53–72 59 Table 2 Ordered mesostructured films prepared by casting Thin film structure Precursors Support Reference Hexagonal silica ‖ TEOS, CTAC, HCl Functionalized Si(1 0 0) wafers [56] Hexagonal silica ‖ TEOS, C12EO10 C16EO10, C16TAC, HCl, H2O Silica glass substrate coated with polyimide film [57] Hexagonal silica ‖ TEOS, cetylpyridinium chloride, H2O HCl Glass slides [58] Hexagonal silica ‖ TEOS, HCl, H2O, gemini surfactants, alkyl ammonium bromides (C12–C18) Glass [59] Hexagonal silica ‖ TEOS, HCl, H2O, CTACl Silicon wafers [60] Hexagonal silica ‖ TEOS, HCl, H2O, CTACl Mica–gold layer [61] Hexagonal silica ‖ TEOS, HCl, H2O, CTABr, EtOH Unsupported [62] Lamellar, one-dimensional hexagonal and cubic silica TEOS, PrOH, HCl, CTACl, H2O Glass [63] Two-dimensional hexagonal silica TMOS, NaOH, NH3, CTAB, CTAHS Silicon or glass [64] ‖: Parallel to interface. In this method, the solution is dropped on to the substrate and allowed to solidify, resulting in much thicker films. Some examples of mesostructured films prepared by casting and modified casting techniques are shown in Table 2. Spin-coating has been widely used for the preparation of mesostructured films by solvent evaporation summarized in Table 3. Four stages of spin-coating can be distinguished: deposition of the surfactant/inorganic solution, spin-up, spin-off and evaporation [84]. Initially, an excess of liquid Table 3 Ordered mesostructured films prepared by spin-coating Thin film structure Precursors Support Reference Hexagonal silica ‖ TMOS, HCl, CnTAC Pyrex glass [65] Hexagonal silica ‖ TEOS, CTACl, HCl Glass [66] Three-dimensional hexagonal silica c-axis ⊥ to the film plane TEOS, HCl, CTAB, ethanol Glass plates [67] Three-dimensional hexagonal silica ⊥ TEOS, CTAB, HCl, ethanol Glass plates [68] Hexagonal titania Ti(OCH(CH3)2)4, HCl, propanol Glass slides [69] Hexagonal silica ‖ TEOS, CTAC, HCl, pluronic P-123 Microslide glass, titanium, silicon [70] Hexagonal disordered titania Ti(OC4H9)4, ethanol, HCl, CTAC, BTAC Soda lime silica glass plates [71] Disordered silica TEOS, pluronic P-123, ethanol, polypropylene glycol Si(1 0 0) single crystal wafers or at-cut piezoelectric crystalline quartz [72] Oriented lamellar silica TEOS, ethanol, HCl, Fe-TMA Glass [73] Hexagonal silica Sodium silicate solution, H2SO4, CTACl ITO-coated glass [74] Hexagonal and cubic SiO2 ‖ TEOS, P-123, F-127, ethanol, H2O, HCl N-type silicon with SiO2 and Si3N4 [75] Disordered tungsten oxide WCl6, ethanol, P-123 ITO-coated glass, Pyrex glass [76] Hexagonal silica TMOS, NaOH, NH3, H2O, CTABr Silicon wafers, borosilicate glass [46] One-dimensional hexagonal silica ‖ TEOS, HCl, H2O, P-123 Glass [77] Hexagonal silica ‖ TEOS, HCl, H2O, P-123 Glass [78] One-dimensional hexagonal and cubic silica ‖ TEOS, HCl, H2O, EtOH, Brij 56 Silicon with oxide overlayer (SiO2/Si) and Au [48] Hexagonal silica TEOS, HCl, H2O, CTABr, EtOH Surface aluminum hydroxide on glass [79] One-dimensional hexagonal silica ‖ TEOS, HCl, H2O, CTACl, PrOH Glass [80] Lamellar, one-dimensional hexagonal silica ‖ TEOS, HCl, H2O, CTACl, PrOH Glass [81] Hexagonal and cubic silica ‖ TEOS, HCl, H2O, CTABr Silicon [50] Disordered titania Ti(OC4H9)4, EtOH, HCl, CTACl Glass plates [82] Lamellar, hexagonal and cubic silica ‖ TMOS, CTACl, HCl, H2O Glass [83] ‖: Parallel to interface; ⊥: normal to interface. is deposited on the surface of the substrate during the first stage. In the spin-up stage, the liquid flows radially outward by centrifugal force. In the spin-off stage, the excess of liq- uid flows to the perimeter and leaves in a form of droplets. In the final stage, evaporation takes place leading to the formation of uniform thin films. One of the most important advantages of this method is that the film tends to become very uniform in thickness. The main disadvantage of this method is that it can only be used with flat substrates. 62 V.V. Guliants et al. / Journal of Membrane Science 235 (2004) 53–72 Table 8 Bulk macroporous materials prepared using colloidal sphere templates by sol–gel method Colloidal template Macroporous framework Pore structure References 0.2–1.0 m ps SiO2 150–1000 nm, hcp [124] ∼0.35 m oil microemulsion droplets TiO2, SiO2 ZrO2 >50 nm, hcp [125] ∼0.47 m ps TiO2, ZrO2, Al2O3 320–360 nm, hcp [126] 0.4–0.7 m ps SiO2, TiO2, ZrO2, Al2O3, Fe2O3, Sb4O6, WO3, YZrO2 250–500 nm, ffc/hcp [127] ∼0.526 m ps SiO2 Disordered hcp, ∼250 nm [128] ∼0.4 m ps Vanadium phosphorus oxides 300–400 nm, hcp [129,130] ∼1 m ps SiO2, TiO2, ZrO2, PbTiO3, Pb(ZrTi)O3 Disordered hcp, 800–1000 nm [131] ∼0.7–0.85 m ps Eu2O3, Nd2O3, Sm2O3, ∼300–350 nm, hcp [132] 0.64 m ps TiO2, SiO2 ZrO2 Disordered hcp, ∼400 nm [133] be used to deposit thin films onto non-planar substrates. However, it is limited to electrodeposition of metals under mild conditions (temperature, pH, reactant concentration) needed to provide stable liquid crystal phases. Pulsed laser deposition (PLD) is another alternative de- position technique for creating continuous mesoporous thin films on non-planar surfaces. The laser ablation process in- volves irradiating a bulk mesoporous target with an intense laser beam, which results in the ejection and deposition of nanoscale fragments of the mesoporous structure on a substrate surface. Balkus Jr. et al. [122] have developed a guest assisted laser ablation technique which consisted of exchanging the bis(pentamethylcyclopentadienyl) cobal- tocenium ion (Cp2*Co+) or ferrocene followed by PLD. The exchanged species absorbed the UV laser irradia- tion during ablation preventing the generation of defects in the bulk mesoporous structure and resulting in deposi- tion of continuous, well-adhered and ordered mesoporous films. A post-PLD hydrothermal treatment resulted in a two-dimensional hexagonal MCM-41 phase without the preferential alignment of the pore channels relative to the substrate surface and the transition metal oxide meso- Table 9 Bulk macroporous materials prepared using colloidal sphere templates by other synthesis methods Colloidal template Macroporous framework Pore structure Synthesis method References 0.36–2.92 m ps TiO2 240–2000 nm, hcp Liquid phase chemical reaction [134] ∼0.4–0.7 m ps NiO 250–500 nm, hcp Salt precipitation and chemical conversion [135] 0.6–0.8 m ps MgO, NiO, Cr2O3, Mn2O3, Fe2O3, Co3O4, 380–550 nm, hcp Salt precipitation and chemical conversion [136] 0.56 m ps SiO2, TiO2 320–525 nm, fcc Nanocrystal incorporation [137] 0.21–0.26 m silica spheres CdSe ∼200–250 nm, fcc Nanocrystal incorporation [138] 0.23–0.48 m ps Polyurethane, polyacrylate, epoxy > 50 nm, ccp Polymerization [139] >0.1 m PS or silica spheres Polyacrylate-methacrylate, copolymer PAMC, polyurethanes > 60 nm, ccp, fcc Infiltration/polymerization [140] 0.2–0.4 m silica spheres Polyurethane, polystyrene, poly(methyl acrylate) 200–400 nm, hcp Polymerization [141] 0.13–0.3 m ps Ni, SnCo, Au, Si, Pt 150–200 nm, hcp Ion spraying/laser spraying [142] 0.466 m ps and 1 m silica spheres CdS, CdSe 400–800 nm, hcp Electrodeposition [143,144] 0.5–0.75 m ps Polypyrrole, polyaniline, polybiothiophene 300–700 nm, hcp Electrodeposition/polymerization [145] 0.33 m ps/0.336 m silica spheres, 0.33 m PMMA/ 0.336 m silica spheres TiO2, ZrO2, Al2O3, polypyrrole, PPV, CdS, AgCl, Au, Ni ∼270–330 nm, hcp Inverse opal templating [146] porous molecular sieve Nb-TMS1 exhibiting a worm-like interconnected pore structure. Although the pulsed laser deposition method can produce good quality ordered meso- porous films, sophisticated equipment is required for film production. It is unlikely that this method will be able to compete with solvent evaporation techniques in speed and degree of control over resulting film structures and pore orientations. 3.2. Macroporous films and membranes 3.2.1. Synthesis of macroporous materials Several synthesis methods have been used in the past to prepare three-dimensional macroporous inorganic materials. These include sol–gel, salt precipitation, nanocrystal infil- tration and polymerization. All the synthesis methods rely on the use of polymeric or inorganic template, usually in a form of spheres packed in a periodic fashion. Bulk ordered macroporous materials prepared by the sol–gel [124–133] and other methods [134–146] are summarized in Tables 8 and 9, respectively. The most representative methods to pre- pare inorganic macrostructures are briefly discussed further. V.V. Guliants et al. / Journal of Membrane Science 235 (2004) 53–72 63 Macroporous inorganic frameworks have been success- fully prepared using the sol–gel method, in which metal alkoxides dissolved in alcohol are impregnated into the voids of the colloidal (polymer) sphere arrays. Hydrolysis and condensation take place at the sphere surface, leading to the formation of the inorganic framework. Subsequent heat treatment to remove the polymer spheres results in an ordered macroporous metal oxide. The final macropore di- mensions are ∼15–30% smaller than the original size of the spheres due to the shrinkage of the inorganic framework. This is caused by a large volume loss during sol–gel pro- cess as an alcohol is evaporated. Significant shrinkage of the inorganic framework during template removal by heat treatment results in severe cracking and loss of long-range order. Therefore, the sol–gel method in combination with heat treatment to remove polymer spheres has not resulted to date in highly ordered macroporous materials for pho- tonic bandgap applications for which long-range order is required. Several single, binary and tertiary oxides have been prepared using the sol–gel chemistry: SiO2 [124]; TiO2, ZrO2, SiO2 [125]; TiO2, ZrO2, Al2O3 [126]; SiO2, TiO2, ZrO2, Al2O3, Fe2O3, Sb4O6, WO3, YZrO2 [127]; SiO2 [128]; V-P-O [129,130]; SiO2, TiO2, ZrO2, PbTiO3, Pb(ZrTi)O3 [131]; Eu2O3, Nd2O3, Sm2O3 [132]; TiO2, TiO2/SiO2 [133]; TiO2 [134]. Precipitation of metal salts, such as acetates and ox- alates, and oxides within the colloidal polymer sphere arrays and subsequent chemical conversion of the inor- ganic precursors is an alternative method to prepare ordered macroporous structures. This procedure is less sensitive to atmospheric humidity and allows the formation of ordered three-dimensional macroporous structures for compositions difficult to prepare by the sol–gel chemistry [23]. Yan et al. [135] reported the synthesis of macroporous NiO with 250–500 nm voids using templated precipitation and subse- quent chemical conversion of the inorganic precursors. The metal salt solution (acetates or nitrates) penetrated the void spaces between the spheres, and subsequent calcination of the macrocomposite removed the spheres and produced the desirable metal oxide. A number of ordered macroporous inorganic oxides have been prepared using the salt pre- cipitation method: MgO, Cr2O3, Mn2O3, Fe2O3, Co3O4 [136]. Ordered macroporous materials may be prepared by fill- ing the void spaces of colloidal sphere arrays with nanopar- ticles. This method offers the great advantage of incorporat- ing specific nanoparticles of desirable crystalline phases into the wall structure of the macroporous framework. Another major advantage of this method is that it results in a minor shrinking and cracking of the three-dimensional framework developed during template removal. Typically, the pore shrinkage is limited to 5–10%. Subramania et al. [137] used monodisperse PS spheres to template colloidal dispersions of silica and titania which led to three-dimensional struc- tures with 320–525 nm macropores. Vlasov et al. [138] used CdSe nanocrystals templated against monodisperse silica spheres to synthesize macroporous CdSe semicon- ductor. Polymerization of organic precursors around colloidal silica sphere arrays is a common method to produce ordered macroporous polymeric materials. The colloidal arrays of spheres are filled with a liquid monomer, which is subse- quently polymerized by a heat treatment or UV irradiation. Macroporous polyurethane, poly(acrylate–methacrylate), PMMA, polystyrene, epoxy, poly(methyl acrylate) have been prepared using this methodology [139–141]. Other miscellaneous techniques for the preparation of inorganic macrostructures include spraying techniques [142,147], electrodeposition [143–145,148] and inverse opal templating [140,146]. Spraying techniques have been used mainly in the preparation of macroporous films. Macroporous TiO2 was prepared using spray pyrolysis by depositing titanyl acetylacetonate onto silica spheres [142]. Macroporous Au was synthesized by ion spraying [147]. Electrodeposition techniques offer excellent control over the degree of filling and wall thickness. Growth of the desirable macrostructure occur galvanostatically or poten- tiostatically. Macrostructures of CdS, CdSe [143,144], ZnO [148], polypyrrole, polyaniline and polybithiophene [145] have been successfully prepared by electrodeposition. In inverse opal templating method, three-dimensional macrop- orous structures prepared by templating with opal structures can be subsequently used to form another opal replica. A wide range of compositions, (TiO2, ZrO2, Al2O3, polypyr- role, PPV, CdS, AgCl, Au, Ni) [146] have been prepared by this method. The ability to control wall thickness, pore size, elemen- tal and phase compositions makes the colloidal sphere array templating a versatile, attractive and flexible route for the synthesis of highly ordered macroporous materials with fine-tuned pore and framework architectures. The wall thickness of macroporous structures can be controlled by the hydrolysis/condensation rates of the inorganic precur- sors [127], the PS spheres packing [130] and by forming core-shell structures at the sphere surface (i.e. deposition of polyelectrolyte multilayers at the sphere surface) [133]. The pore size can be easily manipulated in the range of the sphere sizes, which are typically 100 nm to 50 m in di- ameter. Even smaller spheres (20 nm) can be prepared and used to template small-pore materials [36]. Furthermore, it is possible to build macroporous structures containing a specific crystalline phase by incorporating nanoparticles of desired phases in the voids of sphere arrays [137,138]. De- pending on the choice of the inorganic sources and template removal method, various crystalline phases can be obtained [130]. This suggests that the most critical aspects in the preparation of these macroporous structures are the ability of the precursors to infiltrate and condense between the spaces of the colloidal spheres without swelling or destroy- ing the template as well as the ability to avoid excessive grain growth which leads to a decrease in macroporosity and structural order. 64 V.V. Guliants et al. / Journal of Membrane Science 235 (2004) 53–72 Fig. 7. Cross sectional view of a ∼2.5 m copper film with ∼325 nm diameter voids prepared by the template self-assembly approach [149]. 3.2.2. Macroporous thin films and membranes Several examples of macroporous thin films and mem- branes have been also reported in recent years. Jiang et al. [149] disclosed a general methodology for preparation of macroporous metal films from colloidal arrays of silica spheres. Macroporous Ni, Cu, Ag, Au and Pt films were prepared by introducing metal nanocrystals in multilayer colloidal arrays of silica spheres. After the metals were de- posited in the interstitial spaces, the silica template was re- moved by etching in dilute HF. The final macroporous sam- ples displayed highly ordered free-standing ∼2 m thick metal films with three-dimensional interconnected spherical voids (200–400 nm). The SEM image of a ∼2.5 m thick Cu film prepared by this technique is shown in Fig. 7. The removal of the template at room temperature is most likely responsible for the formation of highly ordered macrop- orous structures without significant framework shrinkage. Subramania et al. [150–152] described a novel ceramic fabrication technique for thin-film TiO2 photonic crystals at visible wavelengths. Ordering of the PS spheres and for- mation of titania network were performed simultaneously. The colloidal PS suspension was spread on glass or silicon substrates and upon slow drying produced a thin film of or- dered PS spheres embedded in TiO2 matrix, followed by the removal of the spheres by heat treatment. The reflectance spectra of these films shifted systematically with the pore size (Fig. 8), providing the evidence of photonic crystal effects. Synthesis of macroporous TiO2 films by a modified sol–gel technique using silica spheres as a template was also reported by Turner et al. [153]. A typical film thicknesses of ∼3 m were obtained. Since the pores (∼300 nm) were highly ordered, the films possessed intense optical diffrac- tion properties. Khramov et al. [154] prepared macroporous silicate films by condensing a silica sol synthesized via sol–gel method with 0.1–1 m polystyrene latex spheres. After the film formation, PS spheres were removed by extraction with chloroform. Akin et al. [155] reported the synthesis of macroporous TiO2 films displaying 0.5, 16 and ∼50 m diameter pores. The ∼1 mm thick films were grown by forming colloidal mixtures of submicron-sized PS latex spheres that were interspersed in 50 nm sized ultrafine TiO2 powder. Ye et al. [156] synthesized ordered three-dimensional macroporous silica films by colloidal crystal templating. Optical mea- surements indicated that these films exhibited the typical behavior of a photonic bandgap material, indicating a crys- talline order in the sample. A robust Si–O–Si network was formed in the silica gel throughout the voids of PS sphere template prior to heat treatment, resulting in large-area Fig. 8. Wavelength of the primary reflective peak as a function of the polystyrene sphere diameter indicating photonic crystal effect of macro- porous TiO2 (modified from [150]). V.V. Guliants et al. / Journal of Membrane Science 235 (2004) 53–72 67 Ordered mesoporous silica coatings displaying bioactive properties can be used in biomedical applications, e.g. as medical implants as suggested by Gomez-Vega et al. [70]. Dag et al. [180] reported the preparation of silicon clus- ters inside hexagonal mesoporous silica films displaying photoluminescence and nanosecond luminescence lifetimes. These composites are promising as light-emitting diodes, optical interconnections and chemical sensors. Several examples of mesoporous silica films displaying low dielectric constants have been reported [181–186]. Yu et al. [181,182] prepared silica mesoporous thin films with low dielectric constant via sol–gel method. Preliminary re- sults on these silica films show a very positive prospective for intermetal dielectric applications. MacDougall et al. [183] synthesized mesoporous silica films with block copolymers showing very low dielectric constant (<2.3) which makes them useful for current and future microelectronic applica- tions. Ordered mesoporous materials are suitable hosts for the incorporation of nanoparticles with unique size-dependent properties for potential applications in adsorption, sensing, and catalysis. In particular, the discovery of the SBA-15 mesoporous silica has generated considerable interest in the areas of catalysis, separation and membrane reactors. However, the presence of framework microporosity in early SBA-15 materials that led to a three-dimensional-connected pore channel system limited their application as a model host system to investigate size-dependent behavior of matter in confined space. Tang et al. [187] prepared SiGe quantum dots in the pores of ordered mesoporous silica films. According to photolu- minescence measurements, these confined dots showed im- proved light emission required for optoelectronic devices. Guliants and co-workers [25,188] have recently reported synthesis of SBA-15 silica with low fraction of microp- ores and its use as a host for the nucleation and growth of 3–15 nm CdS crystals. Boilot and co-workers [189] and Rathousky and co-workers [190] incorporated CdS nanopar- ticles inside mesoporous silicate materials for potential ap- plications in nanoscale optoelectronic devices. Hirai et al. [191] incorporated CdS nanoparticles in reverse micellar systems into thiol-modified MCM-41. The resulting com- posite showed photocatalytic activity for H2 generation from water. Other nanoparticles have been successfully incorpo- rated inside mesoporous silica hosts. Fischer and co-workers [192] reported the confinement of CdSe nanoparticles inside MCM-41 pores. Zhang et al. [193] incorporated CdS, CuS and ZnS nanoclusters inside an MCM-41 host for potential applications in optoelectronics. These composites represent promising materials for the fabrication of light absorbers, quantum-dot lasers and as optical probes in biological stain- ing and diagnostics. It should be noted that several disordered mesoporous inorganic membranes, such as -alumina membranes with ∼4 nm pores, are commercially available [3]. These mem- branes are very robust and used in many commercial processes for filtration and separation. Amorphous and crystalline microporous inorganic membranes exhibiting excellent gas separation properties are also available and some of these have been commercialized for gas and liquid separations [194]. It is difficult for the ordered mesoporous membranes to compete with the well-established, robust disordered mesoporous inorganic membranes in conven- tional separation and filtration applications. A representative disordered mesoporous membrane is the 4 nm pore ceramic membrane made of -alumina which has been studied ex- tensively for several decades. Furthermore, the disordered mesoporous -alumina membrane has been commercialized for about two decades and offers good chemical, thermal and mechanical stability for a large number of industrial applica- tions. Nevertheless, these ordered mesoporous membranes can find new applications, which take advantages of their ordered pore structures and unique surface chemistries. One such potential application is the production of high quality polyethylene fibers using ordered mesoporous membranes with two-dimensional hexagonal pores oriented normal to the substrate surface as a molecular extruder [195]. 4.2.2. Macroporous films and membranes Ordered macroporous structures with pore sizes of a few hundred nanometers are highly attractive for new appli- cations in chromatography, (bio)catalysis, (bio)separations and photonics. Macroporous TiO2 films diffract visible light (i.e. photonic crystal effect) and can also be used in pho- tocatalysis and photonics [150,152,153]. Khramov et al. [154] described macroporous silica films that can selec- tively discriminate different probe compounds based on their charge. Therefore, various guest species may be trapped in the macropores and used in chemical sensing applications. Furthermore, macroporous bioceramic TiO2 films on Ti sur- faces can find applications in bone tissue–Ti implant inter- faces [155]. Tessier et al. [196] described the use of ordered macroporous Au structures in analytical applications, such as surface-enhanced Raman spectroscopy (SERS). Macro- porous structures containing semiconductor quantum dots show unique size-dependent optical and electronic proper- ties, which are not observed in corresponding bulk materials [138]. 5. Concluding remarks We have reviewed the synthesis, structures and emerg- ing applications of ordered meso- and macroporous inor- ganic films and membranes. Thin films and membranes of these porous materials can be prepared by various tech- niques commonly used for film formation from liquid so- lution. Although these techniques allow control over the thickness and the pore structure of the thin films, they have a limited ability to control the pore alignment relative to the substrate surface. The ordered mesoporous thin films and membranes are promising for potential applications in 68 V.V. Guliants et al. / Journal of Membrane Science 235 (2004) 53–72 (bio)separations, chemical sensing, heterogeneous catalysis and optoelectronics. Furthermore, facile functionalization of the internal pore surfaces in these films and deposition of nanoparticles in the pores offer numerous possibilities for the design of novel optical, magnetic and electrically active ma- terials (i.e. these novel materials can be used as capacitors, chemical sensors based on dielectric response of function- alized mesoporous silica films and low dielectric constant materials). Macrostructured films displaying pore diameters of a few hundred nanometers similar to the wavelength of visible light are promising as photonic crystals exhibiting unique optical properties. The emission of light through a photonic crystal can be manipulated in the region of the pho- tonic bandgap, which is of great interest for optoelectronics and photocatalytic applications. 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