Nanoarchitectonics for Mesoporous Materials

© 2012 The Chemical Society of Japan

Bull. Chem. Soc. Jpn. Vol. 85, No. 1, 1­32 (2012)

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Accounts

Nanoarchitectonics for Mesoporous Materials Katsuhiko Ariga,*1,2 Ajayan Vinu,*1,3 Yusuke Yamauchi,*1,4,5 Qingmin Ji,1 and Jonathan P. Hill1,2 World Premier International (WPI) Research Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044 1

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JST, CREST, 1-1 Namiki, Tsukuba, Ibaraki 305-0044

Australian Institute for Bioengineering and Nanotechnology (AIBN), Corner College and Cooper Rds (Bldg 75), The University of Queensland, Brisbane Qld 4072, Australia 3

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Faculty of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555

Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012 5

Received May 31, 2011; E-mail: [email protected], [email protected], [email protected]

Although mesoporous materials have well-defined pore structures, these fine materials can surprisingly be produced by employing a set of conventional and simple procedures such as mixing, heating, filtration, and washing, using low-cost materials. They can be regarded as easy-to-make bulk nanostructured materials. Mesoporous materials have great potential for use in both macroscopic applications and nanotechnology. In this account, we introduce examples of recent developments in mesoporous materials involving innovations in their components and structural designs and concentrating on our own recent progress. These examples include syntheses of mesoporous silica, metal oxides, semiconductive materials, metals, alloys, organic composites, biomaterial composites, carbon, carbon nitride, and boron nitride, as innovative components. As structural innovations for mesoporous materials, various film preparations, pore alignments, and hierarchic structures are described together with their related functions including sensing and controlled release of target molecules.

1. Why Mesoporous Materials? Bulk Quantities and Precise Structures There is little doubt that nanotechnology and related technologies are making significant contributions to current research and development which will eventually be felt in our day-to-day lives. Device miniaturization in such products as cellular phones and portable computers has irrevocably changed our lifestyles. Portable devices allow us to work and communicate wherever we are in the world in contrast to the previous situation where machine operation required a human presence. Greater freedom in our work and leisure dispositions should diminish over-concentration of populations in cities and may ultimately reduce energy consumption and waste production. So far, device innovations have been due to development of top-down approaches, especially sophisticated microfabrication techniques.1­7 However, those top-down techniques are not useful for materials innovation where chemical processes are used as the operating principle. Rather, so-called bottom-up approaches are expected to create novel functional materials having well-controlled structural features with nanometric

precision. Bottom-up approaches rely on self-assembly processes to form selected structures through spontaneous association of atoms, molecules, clusters, and particles.8­18 Furthermore, assisted assembly techniques using external influences are provided by Langmuir­Blodgett (LB) method19­28 and layer-by-layer (LbL) adsorption29­36 and provide significant contributions for materials’ fabrication. According to these concepts, molecular complex formation,37­41 molecular array control,42­52 and microscopic structural design53­61 have been widely investigated leading to generation of a large quantity of scientific knowledge. Ideally, materials should be produced in a series of easy processes to yield bulk quantity without loss of nanometric structural features and some categories of material already satisfy this condition. For example, formation of metal­organic frameworks (MOF) is by self-assembly through coordination between metal cations and appropriate organic ligands to produce large amounts of materials with well-defined pores and frameworks.62­74 Another successful example of bulk nanostructured materials are the so-called mesoporous materials. These materials are produced by using template syntheses of

Published on the web December 23, 2011; doi:10.1246/bcsj.20110162

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Figure 1. Classification of porous materials into microporous, mesoporous, and macroporous materials according to their pore diameters.

self-assembled micelle structures and low-cost material sources such as silica and carbon. The latter materials are available in a huge variety of pore sizes and geometries as well as in various compositions. In this account, we focus on mesoporous materials as potentially useful nanostructured substances. The examples introduced here can also be described as architectonic materials of nanoscale precision so that we can apply the general term “nanoarchitectonics”75­77 to describe these mesoporous materials. General features of mesoporous materials are briefly mentioned, then our recent innovations of mesoporous materials regarding their components and structures are introduced, together with examples from other groups. 2. Mesoporous Materials, Past and Present Porous materials have attracted attention in many practical fields including chemical, environmental/energy, optics, electronics, medical, and biotechnological applications. This is due to their regular pore structures which are useful for materials adsorption, selection, sensing, removal, storage, and release as well as properties based on enhanced surface areas.78­83 According to an IUPAC definition, these porous materials are classified into three major categories depending on their pore sizes: microporous materials with pore sizes below 2 nm, mesoporous materials with pore sizes between 2 and 50 nm, and macroporous materials with pore sizes exceeding 50 nm (Figure 1). Zeolites are representative microporous materials that have found many uses in practical applications including catalysis and as adsorbents. However, these applications are often limited by their small pore size. In other advanced applications, the 2nd category of porous materials, i.e., mesoporous materials, make a larger contribution because the range of their pore sizes offers accommodation of complex chemicals, molecular complexes, and even biomolecular materials. The functionalities of large complex molecules, which can depend on their flexible conformational changes, motion, or diffusion, are more likely to be preserved within mesopores. Accordingly, ordered mesoporous materials prepared through the self-assembly of surfactants have attracted growing interest due to their special properties which include uniform mesopores and high specific surface areas.

Figure 2. Illustration of the synthetic procedure for typical mesoporous silica.

The first step in the history of mesoporous materials was made in Japan with the initial report on mesoporous material (KSW-1) published by Kuroda et al. of Waseda University in 1990. In that case, mesoporous silica was formed by using the intercalated complex of a layered polysilicate kanemite with alkyltrimethylammonium cationic surfactants (Cn-TMA) whose mesoporous structures were confirmed by N2 adsorption.84­87 The pore diameters could be easily tuned simply by changing the alkyl number of Cn-TMA which was removed during processing for mesopore formation. About two years later, a Mobil group famously reported preparation of mesoporous silica using alkyltrimethylammonium bromide (Cn-TAB) or chloride (Cn-TAC) as removable templates and sodium silicate as silica source under alkaline conditions (Figure 2).88,89 The materials have a two-dimensional hexagonal (Mobil Composition of Matter-41, MCM-41), bicontinuous cubic (MCM-48), and lamellar (MCM-50) mesostructures.90,91 They proved the regular porous structure by TEM observation. These impressive findings initiated much research on mesoporous materials accompanied by a dramatic increase in the number of publications in corresponding areas. Currently, a few thousand papers per year are published by the mesoporous materials community. Over the past 20 years, significant advances in the synthesis of new mesoporous materials toward practical applications have been made. Selection of starting materials and conditions has created a great variety of mesoporous materials. Some representative examples are briefly shown here. Tanev and Pinnavaia prepared silica using neutral amine as template to give HMS (Hexagonal Mesoporous Silica) whose mesostructures generally possess slightly disordered hexagonal structures and thicker walls.92 Bagshaw et al. reported the synthesis of a disordered mesoporous material, MSU-1 (Michigan State University) by using poly(ethylene oxide) as a structure directing agent.93 Stucky and co-workers reported use of amphiphilic triblock copolymer of poly(ethylene oxide) and poly(propylene oxide) as the structure directing reagent for preparation of highly

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ordered large pore mesoporous silica designated as SBA-15 (Santa Barbara Amorphous) with thicker pore walls and twodimensional hexagonal structure.94,95 That group also reported the preparation of MCF (Meso Cellular Form) type materials where triblock copolymers stabilized oil in water microemulsions were used as templates.96 Recently, several efforts have been made for rapid large-scale synthesis of mesoporous silica materials. Endo et al. have developed an evaporation-todryness technique (known as the vacuum solvent evaporation method) for rapid syntheses of some kinds of mesoporous silica materials under acidic conditions.97,98 Matsukata and co-workers have recently developed rapid crystallization of MCM-41 by using a new evaporation-to-dryness technique using aqueous ammonia solution in a wide range of molar ratios of tetraethyl orthosilicate (TEOS), water, and surfactants.99,100 There was no difference in the d100 values between as-prepared and calcined MCM-41, implying the formation of a highly dense silicate framework. The MCM-41 obtained after calcination shows a high degree of thermal stability and water resistance. To date, mesoporous silica materials with different macroscopic morphologies, for example, spheres, helicoids, tops, low-dimensional objects such as fibers and thin films, have also been reported.101­104 The morphology of the mesoporous silica materials can be finely tuned by controlling the synthesis conditions, namely, the temperature, solution pH, surfactant-to-silica ratio, nature of the surfactants, and nature of the silica sources. Incorporation of heteroatoms such as Cu, Zn, Al, B, Ga, Fe, Cr, Ti, V, Sn, and U into mesoporous silica framework has been widely investigated.105­119 Methodology to prepare mesoporous silica via the template synthesis can be extended to preparation of various mesoporous metal oxides such as TiO2 and Al2O3 as well as synthesis of mesoporous aluminophosphate93,120,121 and metal phosphates.122­124 Conversion of materials from silica to other materials is also possible. A typical example involves preparation of mesoporous carbon. Ryoo et al. first realized synthesis of ordered mesoporous carbon CMK-1 using MCM48 silica as template and sucrose as the carbon source.125 The first well-ordered mesoporous carbon (CMK-3) that was a faithful replica of the template was synthesized using SBA-15 as a template.126 Other progress in nonsilaceous mesoporous materials are described later in this paper. Modifications in preparation methods of mesoporous materials have been regularly proposed.81,127­136 For example, Kuroda and co-workers reported use of simple dialysis for preparation of colloidal mesoporous silica nanoparticles less than 20 nm in diameter.137 Surfactant structure-directing reagents were removed from colloidal mesostructured silica nanoparticles by dialysis while retaining particle dispersion. The dispersity of precursor nanoparticles could be preserved after the removal and this method overcomes possible disadvantage of the conventional centrifugation and redispersion process for the preparation of dispersed mesoporous silica nanoparticles. Okubo and co-workers recently developed mesoporous nanoparticles with excellent stability and dispersibility for antirefrection coating.138 A variety of materials have been selected as templates. In a unique proposal, Kapoor et al. investigated use of food grade emulsifier polyglycerol esters of fatty acids (PGEFA) as a soft-

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template with n-decane as a swelling agent.139 The interconnected pore channels extending from the exteriors of the microspheres’ shell to their interiors are very useful for accommodation and controlled release of functional substances including medicines. Enhanced encapsulation of vitamin B3 precursor and cumulative in vitro release of vitamin B3 with a considerable pharmacokinetic rate using a simple pH trigger mechanism was demonstrated. Tatsumi, Che, and co-workers used unusual anionic surfactants for synthesis of mesoporous silica, which led successfully to production of mesoporous silica of chiral morphology.16,140­142 Okubo and co-workers proposed use of novel tri(quaternary ammonium) surfactants with benzene-core for synthesis of ordered pore silica.143 A well-ordered two-dimensional hexagonal silica structure with relatively small pores was generated due to the formation of a stable mesophase where surfactants were stacked to form cylindrical assemblies. Introduction of organic functional groups inside of mesoporous materials is important for development of their functional applications. Such interior modification with organic moieties can be obtained through cocondensation, grafting, and related techniques.144­148 Yamauchi et al. reported stoichiometric modification of mesoporous silica with thiol groups by a one-step spray dry technique.149 With this method, the amount of embedded thiol groups in the mesopore channels can be stoichiometrically controlled by changing the compositions of the initial precursor solutions. Increase of adsorption capacity of mercury(II) ions was observed in proportion to the amount of the thiol groups embedded in the frameworks. Vinu and coworkers demonstrated the preparation of a triflic acid (TFA)functionalized cage-type mesoporous silica with tunable pore diameters by the wet impregnation method.150 Catalytic activity of the TFA-functionalized mesoporous silica was much higher than that of zeolites and metal-substituted mesoporous catalytic materials in the synthesis of coumarin derivatives. The stability of the catalyst is extremely good and it can be reused several times without much loss of catalytic activity. 3. Component Innovation One of the important progressing subjects in mesoporous material preparation is invention of various nonsilaceous mesoporous materials. In this section, we comprehensively summarize research on nonsilaceous mesoporous materials. 3.1 Mesoporous Metal Oxides, Semiconductive Materials, and Others. 3.1.1 Metal Oxides: Titania (TiO2) is a fascinating semiconducting material which has been much investigated in recent years due to its strong oxidizing and reducing ability under UV light irradiation, resulting in various scientific and industrial possibilities including removal of hazardous organic substances and dye-sensitized solar cells.151­161 In particular, mesoporous titania with ordered pore structures is anticipated to be useful as a photocatalyst and support for wet-type solar cells. Sakai and co-workers succeeded in synthesizing crystalline mesoporous titania.162 Combination of a TiOSO4 precursor with a cetyltrimethylammonium bromide (CTAB) template controlled the formation rate of anatase-type crystal nuclei and crystalline anatase particles with the hexagonal pore structure were obtained. Sanchez and co-workers produced highly organized and

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Figure 3. Preparation of multilayered titania within mesoporous silica channels.

oriented mesoporous titania thin films using a poly(ethylene oxide)-based structure-directing reagent.163,164 Dai and coworkers proposed a unique method to immobilize titania layers at the inner pore walls of mesoporous silica through a layer-bylayer modification of the nonhydrolytic surface sol­gel process.165 Multilayered titania was prepared on mesoporous silica SBA-15 channels through the direct condensation between titanium precursors (Figure 3). Zhang and Pinnavaia elucidated the stepwise reaction process leading to the formation of mesostructured forms of £-Al2O3 from aluminium salts in the presence of both triblock and diblock surfactants as the structure-directing reagents.166 Kim and co-workers proposed so-called nanopropping using a tailored gemini surfactant containing a siloxane moiety for the preparation of mesoporous zirconia.167 Mesoporous structures with zirconia tend to collapse or transform to a poorly ordered structure due to thermal damage. The siloxane moiety introduced into the gemini surfactants is necessary to stabilize the mesostructure even after the calcination of the materials at high temperature. Domen and co-workers synthesized mesoporous niobium oxide with three-dimensional structure and excellent textural parameters.168 The initial wormhole-like mesoporous structure was dramatically changed to highly ordered three-dimensional arrays by the addition of cations under precisely controlled conditions. Zhu et al. successfully prepared various mesoporous metal oxides using organic functionalized mesoporous silica as template that involves the adsorption of inorganic precursors onto the amine-functionalized mesoporous silica materials and the thermal decomposition of the precursor to assemble the individual metal hydroxide nanoparticles in the mesochannels followed by subsequent removal of the amine-functionalized silica by hydrofluoric acid.169 Various mesoporous metal oxides such as Cr2O3, V2O5, WO3, and Fe2O3 with ordered structure were synthesized using this strategy by choosing appropriate metal precursor solutions. Mesoporous mixed oxides have been also developed. For example, mixed titanium­vanadium oxides were fabricated using codeposition from aqueous solutions as reported by

Shyue and De Guire.170 Nazar and co-workers reported the synthesis of mesoporous transparent conducting oxides composed of an electronically conductive and stable indium­tin oxide (ITO) framework.171 Sanchez and co-workers reported the preparation of ordered mesoporous crystalline networks composed of multicationic metal oxides having perovskite, tetragonal, or ilmenite structures through evaporationinduced self-assembly (EISA).172 This method was applied to synthesize nanocrystalline mesoporous films of dielectric SrTiO3, photoactive MgTa2O6, or ferromagnetic semiconducting CoxTi1¹xO2¹x. Yang et al. also demonstrated a general method for the synthesis of mesoporous metal oxides including TiO2, ZrO2, Al2O3, Nb2O5, Ta2O5, WO3, HfO2, SnO2, and mixed oxides such as SiAlO3.5, SiTiO4, ZrTiO4, Al2TiO5, and ZrW2O8, with semicrystalline frameworks.173 Recently, hierarchically macro-mesoporous metal oxides have been attracting great attention because of their great potentials as catalyst supports or catalysts.174­176 Zhou et al. reported very simple preparation of hierarchically macro-mesoporous titania and alumina by simple dropwise addition of titanium and aluminum alkoxides to an ammonia solution in the presence or absence of surfactant molecules, CTAB.177 The addition of CTAB does not direct the formation of macropores, but significantly affects the mesopore size, the surface area, and the pore volume. Various types of mesoporous metal oxides have been developed.178­180 3.1.2 Semiconductive Materials and Others: Mesoporous structures of other inorganic substances have been reported. Bhaumik and Inagaki reported mesoporous titanium phosphates that have many potential uses such as ion exchangers, acid­base catalysts, photocatalysts, and liquid-phase oxidation catalysts.181 Zhao and co-workers proposed an acid­base pair concept for synthesis of various mesoscopically ordered metal phosphates with large-pores.182 The larger acidity or alkalinity difference between the metal and/or nonmetal sources leads to an appropriate pH of the desired sol­gel reactions during the entire preparation without using extra acidic or basic reagents. Mesoporous structures of semiconductive materials could be tremendously important in wide areas of technological developments because they are required for the construction of modern materials. Mesoporous metal sulfides were prepared by the group of Stupp through a soft-templating approach using surfactants.183­185 Mesostructured CdS and ZnS were prepared by combining the corresponding metal ions and H2S in the presence of the surfactant molecules. Mesostructured CdTe films have also been prepared through a direct liquid crystal templating method using nonionic polymeric surfactants.186­189 Gao et al. used the nanocasting method to introduce mesoporosity in CdS semiconductors.190 Similarly, several mesoporous metal sulfides such as ZnS, In2S3, and CuS have been prepared through the nanocasting technique.191 3.2 Mesoporous Metals and Alloys. 3.2.1 Metals: Owing to their metallic frameworks, mesoporous metals with high electroconductivity and high surface areas hold promise for a wide range of potential applications, such as electronic devices, magnetic recording media, and metal catalysts.192 Attard et al. made a historic attempt to prepare various mesostructures through the soft-templating approach by using organic structure-directing agents such as surfactants in very high concentrations.193 In their method, higher concentration

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of the surfactant is employed to produce a lyotropic liquid crystalline phase for so-called lyotropic liquid crystal templating (LLCT). The LLCT method was previously used by Tanori et al. for preparation of metal nanoparticles in surfactant solutions or lamellar lyotropic liquid crystals (LLCs).194 Attard et al. introduced this LLCT concept to the chemical reduction processes, resulting in mesoporous platinum.195 The same group also successfully demonstrated the preparation of mesoporous platinum films using an electrodeposition method,196 which involves the electrodeposition of platinum metals from appropriate salts dissolved in the aqueous domains of the LLC phases of nonionic surfactants onto highly polished gold electrodes. Mesoporous platinum metals with lamellar, twodimensional (p6mm), and three-dimensional cubic structures
(Ia3d) have also been prepared through modification of synthetic conditions.197,198 Mesoporous platinum films can be used as amperometric sensors for the detection of hydrogen peroxide in aqueous solutions over a wide range of concentrations as reported by Evans et al.199 Park et al. reported that mesoporous platinum gives a stronger response to the glucose molecule than other interfering species such as L-ascorbic acid and 4-acetamidophenol.200 The electrochemical deposition technique has also been extended to the fabrication of mesoporous palladium films. Bartlett et al. also demonstrated that mesoporous palladium films can be converted to the ¢-hydride phase and back to palladium without a change in mesoporosity.201 High-surface-area mesoporous rhodium films were successfully synthesized using the LLCT method through electrochemical deposition. Moreover, the LLCT method was also applied for the fabrication of various other metals such as Co, Sn, and Ni using chemical or electrochemical reduction of the corresponding metal salts.202­205 Sun et al. also prepared hexagonally ordered mesostructured germanium by linking polymeric forms of the anion K2Ge9 with the cationic surfactants through electrostatic interactions followed by the oxidation with ferrocenium cations.206 Armatas and Kanatzidis reported the synthesis of cubic mesostructured germanium with gyroidal channels separated by amorphous walls.207,208 Yamauchi and Kuroda proposed a novel convenient pathway called evaporation-mediated direct templating (EDIT) via solvent evaporation for fabrication of mesoporous metals at the micrometer-scale.209 The EDIT process is composed of two basic steps: (1) LLC formed by solvent evaporation and (2) the reduction of a metal species in the presence of LLC (Figure 4).

Figure 4. Evaporation-mediated direct templating (EDIT) process for synthesis of mesoporous metal films.

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The creation of 2D hexagonally-ordered mesoporous Pt films inside a micrometer scale channels was reported by this method.210,211 A precursor solution as an LLC former is prepared by mixing distilled water, surfactant, Pt species, and ethanol as a volatile solvent. Because the viscosity of the precursor solution is very low, the precursor solution can be efficiently introduced into the channels by capillary force. After the volatile solvent preferentially evaporates, the LLC with Pt complexes is formed entirely inside the channel. By applying electrodeposition, mesostructured Pt can be deposited only inside the channels. Furthermore, well-ordered Pt thin films with a macro­meso bimodal pore system were prepared by combining the EDIT method with colloidal crystal templating.212 Before LLC coating, polystyrene spheres were assembled onto a gold-coated Si substrate by dip-coating. Then, a precursor solution as an LLC former was introduced into the confined space among the spheres by capillary force. After the formation of LLC, the Pt deposition is carried out in the presence of LLC which is filled in the space between the spheres. After deposition, the resulting film is placed in ethanol, toluene, and water to remove the surfactants, undeposited Pt species, and polystyrene spheres. The resulting Pt film possesses macropores, interconnected windows, and mesopores. The structure of the well-ordered metallic film with a hierarchical pore system was fully proven by X-ray diffraction (XRD) patterns, scanning electron microscopic (SEM) images, and optical appearance. The EDIT process is fundamentally different from the conventional evaporation-induced self-assembly (EISA) process which has been utilized for the formation of mesoporous films. In the EDIT system, a homogeneous precursor solution (as a LLC former) is initially prepared, and then preferential evaporation of ethanol accelerates to form LLC mesophases. During LLC formation, no reactions among metal species are carried out. The dissolved metal species are stabilized in the aqueous LLC domain. In contrast, in the EISA system, the polymerization of inorganic species occurs simultaneously during the solvent evaporation. Several types of phase transformations of mesoporous structures have often been observed in the films, because various interactions between the inorganic species and surfactants are inevitable. The experimental conditions such as humidity and temperature are very sensitive. In the EDIT system, the final LLC mesophases after preferential evaporation of ethanol can be directly predicted by a certain phase diagram of ternary compositions (surfactant + water + metal species) independent of the amount of ethanol solvent. In direct templating from LLC, the LLC mesophases act as true templates. Therefore, it is possible to predict the final structures from the original LLC mesostructures. This point is quite different from the conventional EISA system. This convenient EDIT pathway is important for the rapid production of functional nanoscale devices utilizing mesoporous metals.213 Furthermore, giant mesopores can also be prepared by using lyotropic liquid crystals made of large block copolymers.214,215 The entire synthetic procedure is simple and can be extended to various mesoporous alloys by using precursor solutions including mixed metal ions.216­219 Yamauchi and co-workers have made several distinct advances in the preparation of mesoporous metals. They

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Figure 5. Periodic mesoporous organosilicates (PMO) with various organic components.

demonstrated the control of pore size and surface structure of cage-type mesoporous platinum using single- and dualtemplates consisting of a silica nanoparticle assembly and a block copolymer.220 Block-copolymer-assisted metal deposition in a confined space of hard-templates should be effective for further control of crystal surfaces by using proper capping agents. This progress is important because varied crystallinity and pore surfaces are essential to the development of catalytic activity and optical properties. This group reported synthesis of unique nanoporous metallic objects composed of Au@Pd@Pt triple-layered core­shell colloids with a Au core, Pd inner layer, and nanoporous Pt outer shell.221 The synthesis of this object can be done in a one-step procedure known as autoprogrammed synthesis. The proposed autoprogrammed synthesis rationally utilizes the temporal separation of the depositions of Au, Pd, and Pt affording spontaneous step-by-step formation of triple-layered core­shell colloids. 3.2.2 Alloys: The synthesis methodology used for the preparation of mesoporous metals can be extended to mesoporous alloys. The LLCT strategy has actually been used for the synthesis of mesoporous alloys such as Pt­Pd, Te­Cd, Ni­Co, Pt­Ru, and Pt­Ni through chemical or electrochemical coreduction of two-metal species.222­225 The metallic composition of mesoporous alloys can be tuned simply by varying the concentration of the metals. In the case of Pt­Ni alloys, their composition can be varied even though the standard potentials of Pt and Ni are completely different. Interestingly, mesoporous Pt­Ru alloys exhibited a better performance in the electrochemical oxidation of methanol or CO than that of the corresponding alloys made from nanoparticles.223 This improvement would be attributed to altered electronic and surface textures. Saramat et al. reported the preparation of mesoporous Pt­Al2O3 nanocomposite catalysts.226 Its CO oxidation catalytic performance was less sensitive to CO poisoning than the nonporous metal­metal oxide nanocomposites. Yamauchi, Kuroda, and co-workers successfully synthesized Pt­Ru fibers via the EDIT method using diluted precursor solutions that were embedded within porous anodic alumina membranes.227

The Pt and Ru were uniformly dispersed in the fibers, were rarely oxidized, and were present in the intermetallic alloy state. The mesoporous fibers obtained had unique stacked donut-like mesochannels due to the confined effect of the channels of porous anodic alumina membranes. Their mesopore walls consisted of connected nanoparticles and were in a crystalline alloy state. They also demonstrated the synthesis of highly ordered multicomponent (Ni, Co, and Fe) mesostructured magnetic alloys with controllable compositions.228 The saturation magnetization could be controlled through adjusting the metal compositional ratios. The proposed electroless deposition of Ni, Co, and Fe species from LLCs of a nonionic surfactant can lead to highly ordered mesostructured alloys in spite of difficulties in homogeneous deposition of multicomponent alloys because of the potential existence of complicated phases. 3.3 Mesoporous Structures with Organic and Biological Components. 3.3.1 Periodic Mesoporous Organosilica: Although most of the scientific methodologies of mesoporous materials are oriented to inorganic substances, the introduction of organic components to mesoporous structures can enhance their potential capabilities in other fields especially organic chemistry, supramolecular chemistry, and biological chemistry. For introduction of organic moieties in their frameworks, one unique approach lies in the synthesis of periodic mesoporous organosilicates (PMO) using organic molecules with multiple alkoxysilane groups as silica source (Figure 5).229­239 PMO materials are prepared by two methods: either cocondensation of an organotrialkoxysilane precursor appended with one alkoxide group R­Si(OR)3 and tetraalkoxysilane Si(OR)4 or by use of a bridged-organosilane (RO)3Si­R­ Si(OR)3.240 Changes in the organic groups (R) permit synthesis of a wide range of materials with differing surface properties. Inagaki et al. reported on the preparation of two-dimensional and three-dimensional hexagonal mesoporous organosilicas by using 1,2-bis(trimethoxysilyl)ethane (BTME).241 Subsequently, various compositional controls of organosilica frameworks have also been achieved by many other researchers.242­249 Crystalline frameworks of mesoporous organosilica films were

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realized by hydrothermal preparation under the basic condition, as reported by the Ha group.250 The organosilica framework was formed by hydrolysis and condensation of an organosilane precursor, 1,4-bis(triethoxysilyl)benzene. Generally, basic conditions for the preparation of phenylene-bridged mesoporous organosilica tend to yield a crystalline framework, in contrast to those obtained under acidic conditions.251 TEM images revealed that the repeating units of SiO1.5­C6H4­SiO1.5 exist parallel to the mesochannels. Yamauchi et al. used a spraydrying system for large-scale production in the preparation of microspheres of PMO with ethane and benzene fragments using 1,2-bis(triethoxysilyl)ethane or 1,4-bis(triethoxysilyl)benzene with TEOS.252 Organic components of the frameworks could be tuned by changing the molar ratios of initial precursor solutions. Mesoporous organosilica films have also been prepared by hydrolysis and condensation of organosilane precursors such as (C2H5O)3Si­R­Si(OC2H5)3 (where “R” is a bridging organic group) based on the EISA method. The frameworks have various functionalities with both organic and inorganic properties. Mesoporous organosilica films are of great use as lowdielectric-constant (low-k) materials, which are much in demand in the semiconductor chemistry due to the increasing device and interconnect density of microelectronics. Lu et al. prepared mesoporous organosilica film by spin-coating or dipcoating a mixture of TEOS and 1,4-bis(triethoxysilyl)ethene (as an organosilica source).253 TEM images showed a highly ordered mesostructure over the entire coated area. Ozin et al. also prepared mesoporous organosilica films with highly ordered mesostructures for low-k films by using various bridged organosilicone precursors.254,255 The films obtained exhibited good thermal stability and hydrophobicity, which showed potential utility as low-k films in microelectronics. The dielectric constants were found to decrease with increasing organic content. The value of methylene-bridged mesoporous organosilica film had the lowest value (1.8). Self-standing mesoporous organosilica films were also prepared by casting a solution on a flat Teflon vessel.256 After the vessel had been cured, the solidified film could be peeled off from the Teflon vessel to obtain the self-standing films. 3.3.2 Polymers: Mesoporous structures of organic polymers have been also investigated.257­260 For example, Ozin and co-workers prepared polymeric fibers through the nanocasting technique by polymerizing formaldehyde and phenol inside the channels of mesoporous silica MCM-41 followed by etching of the mesoporous silica template.261 Zhao et al. successfully prepared highly ordered mesoporous polymeric resins through the EISA process.262­264 Amphiphilic triblock copolymers were added to low-molecular-weight polymers of phenol and formaldehyde, followed by a thermopolymerization process to produce highly ordered mesoporous polymers. A large number of hydroxy groups in the precursor can interact with the nonionic surfactants through hydrogen bonding, which is advantageous for formation of well-ordered mesoporous polymers. They reported synthesis of a whole family of mesoporous polymers with different structures, including mesostructures with lamellar, two-dimensional hexagonal (p6mm), body-centered cubic, and bicontinuous three
dimensional cubic symmetries (Ia3d) upon varying the

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polymeric precursor-to-surfactant ratios. Ikkala and co-workers prepared mesoporous phenolic resins by crosslinking phenolic resin precursors with a block copolymer (polystyrene-blockpoly(4-vinylpyrimidine) (PS-block-P4VP)), thermopolymerization, and removal of the surfactant by pyrolysis.265 Mallouk and co-workers reported a strategy for the synthesis of ordered mesoporous polymers using colloidal crystals as template.266 The mesoporous polymers were prepared by replication of colloidal crystals made from silica spheres. 3.3.3 Biomaterial Composites: Immobilization of biomaterials into various nanostructures such as supermolecular lipid bilayers,267­274 self-assembled monolayers,275­279 Langmuir­Blodgett films,280­288 and layer-by-layer assemblies,289­297 have been widely investigated. However, most of these structures are mechanically weak and sometimes obstruct material diffusion, and these features are not advantageous for practical uses of biomaterials such as enzymes. Solid porous materials are robust enough to guarantee mechanical stability. However, microporous materials such as zeolites do not possess pore geometries suitable for accommodation of relatively large biomolecules. In contrast, mesoporous materials possess structural stability and appropriately sized voids for accommodation of biomaterials.298­306 Therefore, research on immobilization of biomaterials in mesoporous materials has become an attractive target. Vinu and co-workers have systematically investigated adsorption of proteins to mesoporous materials.307­315 Adsorption of lysozyme onto three different mesoporous silica materials, C12-MCM-41, C16-MCM-41, and SBA-15, whose pore diameters are 3.54, 4.10, and 10.98 nm, respectively, showed maximal adsorption of 13.4, 28.1, and 35.3 ¯mol g¹1 for C12-MCM-41, C16-MCM-41, and SBA-15, respectively. Structural stability of the adsorbed proteins was confirmed using diffuse reflectance Fourier transform infrared spectroscopy for the lysozyme molecules. Similar observations were made using other proteins such as cytochrome c and myoglobin. The data obtained were compared with simplified porefilling models as illustrated in Figure 6,316,317 where mesopore channels are represented as a cylinder of diameter D while a sphere of diameter d is assigned to a protein molecule. Four models, (a) separated single-molecular adsorption, (b) separated double-molecular adsorption, (c) separated triple-molecular adsorption, and (d) interdigitated triple-molecular adsorption with interdigitation by 1/4 of the protein diameter through changing relative orientation, assigned volume occupations as 2/3 © (d/D)2, 4/3 © (d/D)2, 2 © (d/D)2, and 8/3 © (d/D)2, respectively. Assuming that d = 4 nm and D = 10 nm, which roughly corresponds to adsorption of the used proteins in mesopore of SBA-15 materials, the pore occupation values by protein molecules are calculated as 11, 21, 32, and 43% for the cases of (a), (b), (c), and (d), respectively. Comparison of the experimentally obtained values indicates that protein molecules adsorbed in mesopores are in well-packed state as suggested by models (c) and (d). Physical conditions are also important factors in protein adsorption to mesoporous silica as exemplified in Figure 7. Protein adsorption generally displays maximum values at around the isoelectric point of the protein. Suppression of lateral repulsion between the protein molecules may more

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Figure 6. Pore-filling models for protein adsorption within mesopore channels.

Figure 7. Physical factors to influence adsorption of protein molecules to mesoporous silica.

strongly influence the adsorption process than the electrostatic interaction between the silica surface and the protein surface while incorporation of heteroatoms in mesoporous silica framework modifies protein adsorption profiles. For example, adsorption capacities of lysozyme onto aluminium-substituted mesoporous materials are always larger than those of the corresponding nonsubstituted materials. Acidic sites at aluminium atoms introduce strong electrostatic interactions with protonated basic amino acid residues (lysine, arginine, histidine, N-terminal, and so on) of the protein.

Blin et al. demonstrated that hexagonally arranged pore structures could be prepared while maintaining the glucose oxidase at an appropriate concentration.318 Galarneau and co-workers proposed use of neutral lecithin and cationic dodecylamine as structure-directing reagents in the presence of lactose for direct protein immobilization.319 Ying and coworkers reported a pressure-driven method for entrapping lipase in siliceous mesocellular foam where a high pressure (3000­ 5000 psi) was used to achieve high enzyme loading within the support (up to 275 mg per gram of silica).320 Wang and Caruso demonstrated preparation of nanoporous protein spheres.321,322 In the initial step, proteins such as lysozymes were adsorbed into mesopores of silica spheres. Several washing cycles removed loosely adsorbed protein and then the protein-loaded mesoporous silica spheres were dispersed in an aqueous solution of poly(acrylic acid) at pH 4.5. At that pH condition, poly(acrylic acid) has the same charge as the silica substrate but is oppositely charged to the proteins. Therefore, poly(acrylic acid) most likely associates with the proteins through electrostatic interactions. Crosslinking using 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride enhanced the stability of the protein­polyelectrolyte complex. Finally, the silica component was removed by exposure of the composite to a HF/NH4F buffer, resulting in nanoporous protein spheres. The materials obtained are expected to be applied in biosensing, catalysis, separations, and controlled drug delivery. Apart from proteins, superior structural characteristics of mesoporous materials, such as high surface area and large pore volume, are also advantageous for hybridization of other biopolymers. Deming and co-workers reported use of polypeptides as a structure-directing reagent of nanostructured silica.323 Bellomo and Deming recently reported formation of monoliths composed of silica­polypeptide hexagonal platelets utilizing a nonionic, ¡-helical polypeptide in concentrated solutions.324 Naik and co-workers found that poly-L-lysine promoted the precipitation of silica from a silicic acid solution within minutes and that the polypeptide secondary structure transition occurred during the silicification reaction.325 The formation of the hexagonal silica platelets is attributed to the poly-L-lysine helical chains that are formed in the presence of monosilicic acid and phosphate. Ariga and co-workers have pioneered the preparation of mesoporous silica films filled with an amphiphilic polypeptide that was composed of a hydrophilic poly(ethylene glycol) chain (MW, ca. 3000) and a hydrophobic peptide segment with defined length (leucine 16mer).326 Structural regularity of the prepared transparent films was verified by XRD and TEM measurements. Preservation of ¡-helix structure of the poly(leucine) was confirmed by CD and FT-IR spectroscopy. Immobilization of DNA and RNA in well-defined nanostructures is an attractive research subject. Fujiwara et al. reported that duplex DNA in protonated phosphoric acid form can adsorb onto mesoporous silicates even in low salt aqueous solution.327 DNA adsorption behavior depended much on pore size of mesoporous silica. Solberg and Landry accomplished DNA adsorption onto acid-prepared mesoporous silica.328 While mesoporous silica alone adsorbed a negligible amount of DNA, exchanging divalent cations such as Mg2+ and Ca2+ into mesoporous silica significantly enhanced DNA adsorption.

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Figure 8. Lizard template method for mesoporous silica that enables both dense modification of the pore interior with biofunctions and good accessibility by external guests.

Advantages of modification of mesoporous silica with amino groups for DNA adsorption was also suggested in a report by Zhang et al.329 Mann and co-workers proposed preparation of nanohybrids between mesoporous silica and gold nanoparticles mediated through DNA hybridization.330 Mesoporous silica was first modified with a 3¤-amino-terminated 12-base single strand oligonucleotide (5¤-TCT-CAA-CTC-GTACH(OH)-CH2CH(OH)-NH2) where gold nanoparticles of 11 nm diameter were separately capped with a 5¤-thiol-terminated 12-base oligonucleotide (HS-propyl-CGC-ATT-CAG-GAT-3¤). Recently, Ariga, Aida, and co-workers developed a new synthetic method for mesoporous silica that enables both dense modification of the pore interior with biofunctions and good accessibility by external guests (Figure 8).331 The organosilane including alanine residue was covalently attached to the silica framework upon sol­gel reaction with tetraethoxysilane and cleavage and removal of the alkyl tail by selective hydrolysis of the ester at the C-terminal leaves open pores with a surface covalently functionalized by the alanine residue. As one can easily imagine, the template behaves like a “lizard,” whose head bites the silica wall and whose tail can be cleaved off. Therefore, the “lizard template” term was applied to describe this method. Temperature-programmed desorption (TPD) analysis with NH3 as a basic guest confirmed exposure of the alanine C-terminal within the silicate channel. The amount of adsorbed ammonia in the hydrolyzed porous silica was comparable to the amount of immobilized alanine C-terminal. Immobilization of functional groups within defined spaces is a good strategy for design of reactors such as artificial enzymes. Aida and co-workers used the hybrid structure for reactor applications and illustrated the catalytic capability of unhydrolyzed materials on acetalization of a ketone, such as cyclohexanone, in ethanol under mild conditions (Figure 9).332 Cyclohexanone and ethanol are incorporated into the silica channels at the hydrophobic inner domain and hydrophilic outer shell, respectively, which can be activated through hydrogen-bonding interactions with the amide NH and carbonyl groups of the peptide functionalities. The

Figure 9. Hybrid structure of mesoporous silica and peptide segments for reactor applications.

acetalization should involve transient carbocationic intermediates that can be generated by a highly polar environment containing concentrated ammonium salt functionalities. 3.4 Mesoporous Carbon and Related Materials. 3.4.1 Carbon: Porous carbon materials with regular-sized nanopores have received attention due to their versatility and shape selectivity.333­337 These characteristics are potentially useful for

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Figure 10. Illustration of conversion of mesoporous silica to CMK-type mesoporous carbon.

chromatographic separation systems, catalysts, nanoreactors, battery electrodes, capacitors, energy-storage devices, and biomedical devices. Synthesis of highly regular microporous carbon materials were reported by Kyotani and co-workers.338,339 Later, Ryoo et al. invented carbon materials with mesoporous structures (the CMK family) using sucrose as the carbon source and mesoporous silicates such as MCM-48, SBA-1, and SBA-15 as hard templates (Figure 10).125,126 Independently and somewhat later, Hyeon et al. used a similar approach and reported the synthesis of well-ordered mesoporous carbons designated the SNU family.340­342 For the synthesis of mesoporous carbon materials, the replica (or nanocasting) route is most often applied using structurally regular mesoporous silica. It involves impregnation of appropriate carbon sources into the template structures containing three-dimensional networks of well-ordered mesoporous channels, carbonization, and template removal. Modification of synthetic conditions creates variations of mesoporous carbon structures. For example, Hartmann and Vinu reported the method for systematic variation of the SBA-15 structure simply by controlling the synthesis temperature.343,344 Mesoporous carbon CMK-3 with pore diameters ranging from 3 to 6.5 nm were successfully prepared using these pore size-tuned SBA-15 template. Ryoo and co-workers prepared mesoporous carbon CMK-5 composed of carbon nanopipes using SBA-15 as the template.345,346 The partial filling of the pores with the template is necessary for successful synthesis of tubular-type mesoporous carbon. Schüth and co-workers used mesoporous aluminosilicate as template for preparation of mesoporous carbons similar to CMK-5.347 Yamauchi, Sugimoto, and co-workers demonstrated CMK-5type mesoporous carbons with different amounts of fullerene cage by using a fullerenol-based precursor solution.348 The

fullerene cages embedded in the frameworks are electrochemically active, showing high potential as an electrode material for an electric double-layer capacitor. Che et al. prepared largepore three-dimensional bicontinuous mesoporous carbons with
symmetry using mesoporous monoliths with cubic cubic Ia3d structure as the template.349 Kleitz et al. reported the synthesis
structure using mesoporous of mesoporous carbon with Ia3d silica materials with three-dimensional structures, prepared by different hydrothermal treatment using butanol as a structure modifier.350 Choi and Ryoo reported the synthesis of mesoporous structures of polymer­carbon composite. The obtained materials have polymer-like chemical properties with a strong improvement of the stability of the mesopores against mechanical compression, thermal, and chemical treatments.351 Mokaya and co-workers reported the use of chemical vapor deposition to supply the carbon source in a replicatype synthesis for the preparation of graphitic mesoporous carbons.352 Vinu et al. used mesoporous aluminosilicates with variable pore diameters for the synthesis of mesoporous carbon materials.353­355 This mesoporous aluminosilicate material acts as both an acid catalyst for the polymerization of the carbon source and as the template for mesoporous carbon synthesis. The specific surface area, the specific pore volume, and the pore diameter of the obtained materials are significantly higher than those of CMK-3. Lee et al. employed mesoporous silica templates with different wall thicknesses as the templates to control the pore diameter of the resulting mesoporous carbon materials.356 Vinu et al. proposed a simple method for pore structure controls of mesoporous carbons through a controlled pore-filling technique.357 This technique involves a simple variation of the concentration of sucrose molecules during their infiltration into the mesoporous matrix of the silica template. The unit cell constant of the resulting mesoporous carbon materials increases from 9.80 to 10.62 nm with increasing the sucrose-to-silica weight ratio from 1.25 to 5 probably due to changes in the void spaces inside the channels upon filling with different amounts of sucrose molecule. The specific surface area increases from 724 to 1570 m2 g¹1 during decreasing the sucrose-to-template ratio from 5 to 0.8. The thickness of the pore wall of the mesoporous carbon materials increases from 5.62 to 6.74 nm with increasing the sucrose-to-silica weight ratio from 1.25 to 3.3 and then decreases to 6.42 nm with further increase of the amount of sucrose. Lee et al. synthesized mesoporous carbon materials with pore diameters in excess of 10 nm using mesocellular foam as the template.358 Large-pore diameters can only be achieved through a partial filling of the pores of the mesocellular foams with phenol that is later polymerized inside the channels with formaldehyde. Oda et al. reported a mesocellular foam with a main cell size of 24 nm and a window size of 18 nm with closed hollow spherical pores using double impregnation of sucrose into the channels of mesoporous cellular silica foam materials.359 Kim et al. used aromatic carbon precursors instead of sucrose molecules to prepare graphitic mesoporous carbons.360 Unfortunately, mesoporous carbon materials with disordered graphitic sheets, which are aligned perpendicular to the template walls, were obtained. Pinnavaia and co-workers used aromatic carbon precursors such as naphthalene, anthracene,

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and pyrene to synthesize graphitic mesoporous carbons.361 The obtained materials had much higher electrical conductivity than those of amorphous mesoporous carbons. Fuertes et al. demonstrated preparation of graphitic mesoporous carbons with an electrical conductivity of 0.3 S cm¹1 using poly(vinyl chloride) as the carbon precursor.362,363 They also synthesized graphitic carbons using polypyrrole as the carbon precursor and FeCl3 as the catalyst, which promoted the formation of a graphitic structure. The material obtained showed a superior performance for electrochemical double-layer capacitors over nongraphitic carbons. Gierszal et al. reported the synthesis of mesoporous carbon using KIT-6 as the template that is mesoporous silica consisting of an interpenetrating bicontinuous network of channels.364 There has also been much related research progress in recent years.365­376 Tang and Xie reported a novel template route for mesoporous carbon, by using ZnO nanoparticles as mesoporogens.377 The ZnO nanoparticles (as mesoporogen) and polyacrylamide (as carbon source) were mixed under mechanical stirring. A carbon network was successfully formed around the ZnO nanoparticles by the carbonization of polyacrylamide in non-oxygen atmosphere. Following the removal ZnO, mesoporous carbon materials could be obtained. Yamauchi et al. prepared nanoporous carbon fibers through carbonization of an Al-based porous coordination polymer with furfuryl alcohol at 1000 °C under an inert gas atmosphere.378 During the carbonization process, the Al species aggregated to form £-alumina nanoparticles. By chemical treatment with HF, the £-alumina nanoparticles could be easily removed yielding pure nanoporous carbon. Interestingly, the fibrous morphology of the original material was retained after the carbonization. From the N2 adsorption­desorption isotherms, an increase in the BET surface area upon increasing the loading amount of furfuryl alcohol was observed. The maximum surface area and pore volume of the obtained carbon materials reached 513 m2 g¹1 and 0.844 cm3 g¹1, respectively. 3.4.2 Carbon Nanocage: Vinu and co-workers extended the concept of the replica route for innovation of a novel type of nanocarbon, namely, carbon nanocage (Figure 11).379­381 In the synthesis of carbon nanocage materials, three-dimensional large cage-type face-centered cubic mesoporous silica materials (KIT-5) are used as inorganic templates. The standard procedure for the preparation of the mesoporous carbon CMK-3, namely using mesoporous silica SBA-15 as inorganic template, is not suitable in this case due to the higher bulk density and lower pore volume of KIT-5 compared to the other mesoporous silicates such as MCM-48 and SBA-15. Modified synthesis procedure and other parameters should be considered where the appropriate weight ratio (ca. 2.5) of water-to-silica template (KIT-5) is a crucial factor for a successful synthesis. Carbon nanocage materials with different pore diameters were prepared using different KIT-5 template, which were synthesized at different temperatures, with sucrose as the carbon source. Carbon nanocage prepared under optimized condition possesses pores with pore and cage diameters of 5.2 and 15.0 nm, specific surface areas of 1600 m2 g¹1, and specific pore volumes of 2.10 cm3 g¹1. Because of the large-pore volume of the carbon nanocage materials, carbon nanocages are expected to possess a superior capability for the adsorption

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Figure 11. Synthesis of carbon nanocage.

Figure 12. Comparison of adsorption capability for organic dye between activated carbon, carbon nanocage, and mesoporous carbon through simple filtration.

of biomolecules. Adsorption isotherms indicate a higher capacity of the carbon nanocages for lysozyme adsorption than that observed for CMK-3. The superior adsorption capability of the carbon nanocage materials for small molecules was also demonstrated, as shown in Figure 12, by a simple adsorption experiment of Alizarin Yellow dye.382,383 An aqueous solution (2 g L¹1, ca. 5 mL) was passed through a bed of the respective carbon material (2.5 mg) deposited on top of a cotton plug in a pipette and with application of a slight pressure. When compared with the control test without carbon, the carbon nanocage materials completely removed the dye, while activated carbon powder

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and mesoporous carbon CMK-3 were not effective for removal of the dye under these conditions. Subsequent quantitative experiments on adsorption of the bioactive tea components, caffeine, catechin, and tannic acid were carried out using the adsorbents, carbon nanocage, activated carbon, CMK-3, and mesoporous silica SBA-15, which were dispersed in an aqueous solution of the tea components. All three carbon adsorbents had similar adsorption capacity for caffeine, although caffeine was hardly adsorbed by hydrophilic SBA-15. Superior capacity of carbon adsorbents relative to SBA-15 was also evident for adsorption of catechin. Interestingly, carbon nanocage materials exhibited larger adsorption capacity for tannic acid with a two-step adsorption, while CMK-3 showed lower capacity in single-step mode. Competitive adsorption (guest selection) of catechin and tannic acid on the carbon nanocage adsorbent using solutions containing equal weights of the guests unexpectedly gave adsorption behaviors. Catechin adsorption was suppressed drastically by the presence of tannic acid, especially at lower concentrations. Diminished catechin adsorption is caused by preferential adsorption of tannic acid on carbon nanocage. A highly selective separation of catechin and tannic acid can be demonstrated by using appropriate conditions. Surprisingly, use of carbon nanocage as adsorbent provided a highly selective adsorption of tannic acid (ca. 95%) in a simple onepot process. Adsorption behavior of nucleosides (adenosine, guanosine, and thymidine) onto carbon nanocages were also investigated.384 When compared with mesoporous silica, porous carbons exhibit superior adsorptive performance. We serendipitously observed a pronounced selectivity between purine-base and pyrimidine-base nucleosides by carbon nanocage. This finding is useful for design of materials for applications in adsorption-based separations and as column stationary phases for separation of costly and important gene-related materials. The binding behaviors of two kinds of intercalators, methyl violet and 3,6-diaminoacridine hydrochloride, to various mesoporous materials, carbon nanocage, mesoporous carbon CMK-3, activated carbon, and mesoporous silica SBA-15 were also compared.385 Carbon nanocage, due to its unique cage type structure, shows large adsorption capacity compared to other adsorbents like mesoporous carbon CMK-3, activated carbon, and mesoporous silica SBA-15. It was also demonstrated that carbon nanocage can inhibit intercalation of methyl violet to DNA. The latter finding indicates that carbon nanocage is an excellent material for sequestration of DNA intercalators and could be used for entrapment of many deleterious aromatic molecules, which are increasingly common environmental contaminants. Very recently, Inagaki et al. demonstrated that carbon nanocage prepared as a carbon replica of KIT-5 mesoporous silica showed much higher electric double-layer capacitance and Li-accumulation capacity than CMK-3 and conventional activated carbon, due to the high specific surface area and mesopore volume as well as the sparse carbon network.386 3.4.3 Carbon Nitride: Carbon nitride (CN) is a wellknown and fascinating material that has attracted attention because the incorporation of nitrogen atoms in the carbon nanostructure can enhance the mechanical, conducting, field

Figure 13. Synthesis of mesoporous carbon nitride.

emission, and energy storage properties. Vinu et al. have combined the chemical synthesis and nanotemplating routes for the preparation of carbon nitride with porous structure using mesoporous silica as template (Figure 13).387­390 A highly ordered mesoporous CN material, designated MCN-1, with a uniform pore size distribution, high specific surface area, and a high specific pore volume were obtained. In its synthesis, mesoporous silica SBA-15 as the template was added to a mixture of ethylenediamine and carbon tetrachloride followed by heat-treatment in a nitrogen flow to carbonize the polymer and dissolution of the silica framework in HF, resulting in regular mesoporous carbon nitride. The XRD pattern of the MCN-1 material confirmed a twodimensional hexagonal lattice (p6mm). The corresponding pore size distribution of MCN-1 is centered at 4.0 nm and its specific surface area and the specific pore volume were calculated to be 505 m2 g¹1 and 0.55 cm3 g¹1, respectively. X-ray photoelectron spectroscopy (XPS) data of mesoporous carbon nitride yield more detailed information. The XPS C1s spectrum could be deconvoluted into four peaks with binding energies of 289.3, 287.5, 285.7, and 284.1 eV. The lowest energy contribution fitted for C1s in MCN-1 (284.1) is assigned to pure graphitic sites in the amorphous CN matrix and the peak at 285.7 eV is attributed to the sp2-hybridized carbon bound to nitrogen inside the aromatic structure. The highest energy contribution, 289.3 eV, is assigned to sp2-hybridized carbon, while the contribution at 287.5 eV is assigned to the sp2-hybridized carbon in the aromatic ring attached to NH2 groups. The XPS nitrogen corelevel 1s spectrum shows two peaks centered at 397.8 and 400.2 eV. The peak at the highest binding energy (400.2 eV) corresponds to nitrogen atoms trigonally bound to all sp2 carbons or to two sp2 carbon atoms and one sp3 carbon atom in an amorphous CN network, while the peak at 397.8 eV is attributed to sp2-hybridized nitrogen bound to carbon. Structures of mesoporous carbon nitride can be tuned as reported by them. MCN-type materials with different pore diameters were prepared using different SBA-15 templates that were originally synthesized at different temperature. The change in the pore diameter of the MCN-type materials can be realized upon increasing the pore diameter of the template. They also reported that the degree of polymerization in the pores of the silica template can easily be controlled by varying the weight ratio of ethylenediamine to carbon tetrachloride in the pore channels. It was also confirmed that the optimum weight

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Figure 14. Encapsulation of gold nanoparticles within nanochannels of mesoporous carbon nitride.

ratio of ethylenediamine to carbon tetrachloride is around 0.45 for the synthesis of well-ordered MCN-type materials with appreciable textural parameters and high nitrogen content. Vinu and co-workers reported particle-type mesoporous carbon nitride.391 Using mesoporous ultrasmall silica nanoparticles as template, well-ordered mesoporous carbon nitride nanoparticles with a size smaller than 150 nm and a high nitrogen content (i.e., C4N2) were synthesized. The prepared materials showed basic catalytic properties, and are the first highly ordered, mesoporous, metal-free basic catalysts for the transesterification of ¢-keto esters with excellent conversion and 100% product selectivity. These materials are anticipated to be used as drug delivery systems and adsorbents for capturing CO2. The basic sites of mesoporous carbon nitrides can also be used for the selective removal of slightly acidic phenols.392 The mesoporous carbon nitride adsorbent is highly stable and can be reused for the removal of phenol from contaminated water. Mesoporous carbon nitride can be used as a support for nanomaterial preparation. Metal nanoparticles are important in the fields of catalysis, separation, magnetism, optoelectronics, and microelectronics owing to their unique physical and chemical properties. However, the overall performance of these metal nanoparticles is dependent on the size, shape, crystal structure, and the textural parameters. Although the size of the nanoparticle can be controlled by nanoporous support strategy, stabilization and reduction of the particles on the porous surface after their formation is quite challenging. Vinu and co-workers demonstrated that gold nanoparticles can be encapsulated inside the nanochannels without any stabilizing or size-controlling chemical agents (Figure 14).393 Mesoporous carbon nitride materials have three different functions namely stabilizer, size controller, and reducing agent. Ultrasmall gold nanoparticles inside the confined nanoporous matrix were found to be highly active, selective, and recyclable for the synthesis of fine chemicals such as propargylamines through the coupling reaction of benzaldehyde, piperidine, and phenyl acetylene. Propargylamines are intermediates for the construction of nitrogencontaining biologically active molecules and for the synthesis of polyfunctional amino derivatives. Several researchers have reported ordered mesoporous carbon nitrides.394­400 Antonietti and co-workers have reported preparation of mesoporous carbon nitrides of high nitrogen content by condensation of cyanamide, dicyandiamide, or melamine under ammonia elimination.401,402 The morpho-

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logical and structural properties of carbon nitrides can be controlled to a certain extent by different silica templates. The surface areas can be controlled in the range between 80 and 450 m2 g¹1 by changing original mesoporous silica templates and nitrogen sources. Besides hard-templates, several types of nanoporous carbon nitride solids were prepared by using softtemplates such as ionic liquid, nonionic surfactants, and block polymers.403 Xia and Mokaya synthesized nitrogen-doped mesoporous carbon materials with graphitic pore walls through the chemical vapor deposition of acetonitrile on mesoporous silica template.404 Graphitization of the materials could be controlled by adjusting the carbonization temperature. Vinu et al. also reported synthesis of hexagonally-ordered N-doped mesoporous carbon using aniline as the source for both carbon and nitrogen and using SBA-15 as the template.405 They also applied this method to the preparation of N-doped mesoporous carbon materials with three-dimensional large pore structure using mesoporous silica KIT-6 as the template.406 The material obtained possesses a well-ordered pore system with threedimensional cubic structure containing an enantiomeric pair of independent interpenetrating three-dimensional continuous networks of mesoporous channels that are mutually intertwined and separated by carbon walls. 3.4.4 Boron Carbonitride and Boron Nitride: Gas-phase reactions are frequently used for the postsynthetic modification of mesoporous materials. For example, Ozin and co-workers prepared periodic mesoporous aminosilicates containing functional amine groups in the framework of a mesoporous network material through thermal ammonolysis of periodic mesoporous organosilica.407 Wan et al. synthesized mesoporous silicon oxynitrides by heating fresh SBA-15 precursors in ammonia in a flow-through quartz tube reactor.408 Unlike these previous approaches, Vinu and co-workers used gas-phase reaction of framework compositions of mesoporous structures. They realized the preparation of mesoporous boron nitride (MBN) and mesoporous boron carbonitride (=boron carbide nitride) (MBCN) from mesoporous carbon through gas reaction for substitution of carbon by nitrogen and boron.409­411 This strategy can be regarded as the thirdgeneration approach for the synthesis of mesoporous materials after soft-template and hard-template (replica or nanocasting) routes (Figure 15). This strategy is referred to as the elemental substitution method. The MBN and MBCN with very high specific surface areas and specific pore volumes were prepared through substitution reactions at high temperatures, using a well-ordered hexagonal mesoporous carbon (CMK-3) as the template and boron trioxide as boron source under nitrogen gas flow as nitrogen source. Their preparation can be tuned by synthetic temperature. The MBNs were basically synthesized at higher temperature such as 1750 °C, while the MBCN can be obtained through the substitution reaction at considerably lower temperatures (1450­1550 °C) with short reaction times. Pore structure of the MBN and the MBCN are not perfectly homogeneous with some irregularity in the shapes of the mesopores. Surface area of typical MBCN was reported as being 740 m2 g¹1 with specific pore volumes of 0.69 cm3 g¹1, while the specific surface area and the specific pore volume of the MBN remained relatively low at 565 m2 g¹1 and 0.53 cm3 g¹1, respectively. Evaluation of MBN materials by

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Figure 15. Syntheses of mesoporous BN and mesoporous BCN by elemental substitution method.

electron energy-loss spectroscopy (EELS) showed that the boron-to-nitrogen ratio was confirmed to 1.0. The XPS survey spectrum of the MBN showed sharp signals for boron and nitrogen without indication of any peaks for other elements. On the other hand, the EELS analyses on MBCN showed decrease of the carbon content as the synthesis temperature increased. The MBN and MBCN materials obtained are chemically inert and quite resistant to oxidation. Therefore, they are useful as catalyst supports operating at high temperature even in an oxidative atmosphere. In addition, they could also find a potential use in the preparation of batteries and fuel cells. 4. Structural Innovation Major distinct characteristics of mesoporous materials are their controlled pore dimensions, high surface areas, and large pore volumes. These structural features create many advantages in several applications such as catalysis, material removal, and drug delivery. However, these factors are not common to all mesoporous materials. More advanced controls for mesoporous structures can be performed with other parameters including morphology, pore alignment, and orientation. In the following sections, we highlight these aspects as structural innovation of mesoporous materials, where film preparation, pore alignment control, and construction of hierarchic structures are described. 4.1 Films of Mesoporous Structures. 4.1.1 Mesoporous Silica Films: Transparent films of mesoporous materials prepared on a flat substrate through self-assembly of surfactants have attracted attention because of their potential applications in optical and electronic devices. Preparation of mesoporous films is an active research area.412 Mesoporous silica films can be prepared by two different methods. First, a hydrothermal deposition process based on heterogeneous nucleation and growth of mesostructured silica seeds can be used.413,414 This highly reproducibly method provides mesoporous silica films with improved crosslinking. The reactions are well controlled in a closed vessel and thus are not influenced by ambient factors such as humidity.415­418 However, the film formation

Figure 16. Immobilization of one-dimensional columnar charge-transfer (CT) assemblies within mesoporous silica channel.

proceeds slowly under the harsh low pH conditions employed, and the kind of substrates that afford continuous films is very limited. The other method is by solvent evaporation and was first proposed by Ogawa for the first time.419,420 By spin-coating precursor solutions including ethanol, inorganic species, and surfactants, mesoporous films with various mesostructures such as lamellar and two-dimensional hexagonal mesostructures can be prepared. After the first report, Brinker and co-workers demonstrated a sol­gel-based dipcoating method, evaporation-induced self-assembly (EISA).421 These methods are usually employed to rapidly produce continuous mesoporous film with controlled composition and thickness on the entire area of a substrate.422­425 Films prepared from thin mesoporous silica structures are often transparent so that it becomes possible in control photonic and related properties in confined nanospaces. Aida and coworkers reported the first example of immobilization of one-dimensional columnar charge-transfer (CT) assemblies in mesoporous silica film through sol­gel reaction with CT complexes of an amphiphilic triphenylene donor and various acceptors (Figure 16).426 The individual CT columns exhibit red-shifted absorption bands possibly due to a long-range structural ordering within mesopore channels, which displayed neither solvatochromism nor guest exchange behaviors. Ariga and co-workers demonstrated the use of oligopeptide-functionalized surfactants for synthesis of mesoporous silica hybrids with peptide segments confined in regular mesopores (Figure 17).427 A spin-coating method effectively provided transparent mesoporous films. The mesostructured silica­peptide composites obtained provide an optically asymmetric environment for dopant molecules where photoisomerization behaviors of spiropyran were investigated using alanylalanine

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Figure 17. Mesoporous silica hybrids with peptide segments confined in regular mesopores for asymmetric photoisomerization behaviors of spiropyran.

surfactants as the host peptides.428 Isomerization between the spiropyran form and the merocyanine form was repeated upon alternate irradiation of the films with visible light (420 nm) and UV light (280 nm), respectively. The film with the spiropyran form showed clear circular dichroism (CD) activity in the region from 250 to 400 nm for the L-peptide host and a complete mirror image of the CD spectrum was obtained with D-peptide host. In contrast, only negligible CD signals originating from the guest could be observed for the film containing the merocyanine form. This system could be applied for development of nondestructive memory devices. Nishiyama and co-workers developed vapor-phase synthesis for mesoporous silica films. Nonionic poly(ethylene oxide)­ poly(propylene oxide)­poly(ethylene oxide) amphiphilic triblock copolymer (EO106PO70EO106, Pluronic F127) was used as a templating agent.429­431 The surfactant film was prepared on a silicon wafer by spin-coating using ethanol-based solution including Pluronic F127. The surfactant film was then placed in a closed vessel together with a small amount of TEOS and HCl solution. The surfactant film was exposed to saturated TEOS vapor under autogenous pressure. Under vapor infiltration process, a nanophase transition from a lamellar structure to a two-dimensional cage structure of a silica­surfactant nanocomposite was confirmed. After removal of surfactants by calcination, the silica thin films with two-dimensionally connected cage-like mesopores were obtained. The film thicknesses are very sensitive to the triblock copolymer concentration in the initial surfactant solutions. Ultrathin silica films with a monolayer of uniform nanopores can be fabricated on a silicon substrate432 and the number of layers can be tuned

easily. Furthermore, this process is applicable to various surfactants, such as CTAB and Brij 30 (C12H25(EO)4OH), to change the mesostructures in the films.433­435 Films synthesized by vapor infiltration show a lower concentration of residual Si­OH groups compared to the films prepared by a conventional EISA method. Also, the films show high thermal stability up to 900 °C and high hydrothermal stability. 4.1.2 Mesoporous Non-Siliceous Films: Because the framework compositions in the mesoporous films are crucial factors in their properties, compositional controls are important for practical applications.436­441 For example, metal oxides are anticipated to show unique electronic properties. Generally, mesoporous metal oxides have been prepared by solvent evaporation using various metal alkoxides or metal chlorides. Because the reactivities of metal alkoxides are much higher than those of silicon alkoxide, controlling hydrolysis and condensation is crucial. Slow condensation reaction (i.e., slow polymerization reaction of metal alkoxide) is necessary to produce highly ordered mesostructures. Several strategies have been reported for controlling the reactions including optimization of pH or sol compositions in precursor solutions. For example, under strongly acidic conditions, the condensation reaction is effectively hindered by protonation of the M­OH nucleophilic species, in which small oligomers can remain during the selfassembly process of surfactants (i.e., solvent evaporation process).442 The use of additives such as acetic acid is also effective for controls of the hydrolysis and condensation.443 Grosso et al. reported the preparation of optically uniform mesoporous TiO2 thin films of high regularity.444 Nonionic surfactants, F127 and Brij 58, and tetrachlorotitanium were

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used as structure-directing agents and inorganic precursor, respectively. The films were prepared by dip-coating. The mesochannels were organized into a two-dimensional hexagonal mesostructure with the c axis aligned parallel to the substrate. Calcination under low temperature (350 °C) permitted successful crystallization with the retention of ordered mesostructure. Alberius et al. demonstrated a general synthetic strategy to prepare mesoporous silica and titania films with cubic, two-dimensional hexagonal, and lamellar mesostructures. The structural controls were achieved by varying the surfactant concentration.445 At high surfactant concentrations, the lamellar mesostructure was formed while at low surfactant
mesostructure was formed. concentrations the cubic Im3m Strategies based on the EISA method can be applied to preparation of mesoporous non-siliceous metal oxide films with different framework compositions. Sanchez and other groups have prepared several mesoporous non-siliceous films with various metal oxides.446­449 Miyata et al. proposed a novel strategy for the mesoporous non-siliceous metal oxide films. Well-ordered mesostructured tin oxide thin films were prepared by combining dip-coating and a subsequent treatment with water vapor. Although the film after dip-coating is opaque and poorly ordered, a short post-treatment with water vapor improved the transparency and structural ordering.450 Moreover, the water-vapor treatment can induce the crystallization of the framework at low temperature, which may be applied in industrial processes. Hillhouse et al. successfully removed the surfactants from mesostructured tin oxide films prepared through the water-vapor treatment.451 By careful thermal treatment, consisting of heating at temperatures ranging from 50 to 250 °C with a 30 °C step and a 12 h pause at each temperature, a highly ordered mesoporous tin oxide film with a 3D face-centered orthorhombic nanostructure was obtained. The crystallization of frameworks upon calcination collapses the mesoporous films. Several groups have successfully retained ordered mesostructures upon calcination by using surfactants with high thermal stability. Smarsly’s group prepared mesoporous non-siliceous metal oxide films with various framework compositions by using the novel surfactant, poly(ethylene-co-butylene)-block-poly(ethylene oxide) (KLE).452 KLE has a high thermal stability caused by strong hydrophobic­hydrophilic contrast in its chemical structure. Niederberger et al. reported a new synthetic route to mesoporous tin oxide structures by self-assembly of nanosized tin oxide crystal nanoparticles and block copolymer.453 After calcination, the mesoporous structure was fully retained without shrinkage because of the use of crystal nanoparticles. This elegant method is powerful tool for preparing well-defined mesoporous tin oxide with fully crystalline framework. Kimura et al. proposed the crystallinity-controlled synthesis of TiO2 for processing of surfactant-assisted mesoporous TiO2.454 This approach is based on the poor reactivity of phosphites with transition-metal species for preventing the continuous condensation of metal species in solution. Yamauchi and co-workers reported the extraordinary antibacterial properties of mesoporous titania films.455 This antibacterial effect was controlled by several significant factors such as surface area, anatase crystallinity, and surface morphology. These parameters could be controlled by varying the applied

calcination temperature. To further enhance the antibacterial properties, Yamauchi and co-workers focused on modification of metal nanoparticles in mesoporous anatase films.456 They succeeded in drastically enhancing antibacterial activity by uniformly depositing Ag nanoparticles on mesoporous anatase films. Surprisingly, mesoporous titania films with Ag nanoparticles can inactivate around 90% E. coli bacteria within only 2 min under UV irradiation. Preparation of mesoporous film structures can be extended to metallic materials. Yamauchi, Kuroda, and co-workers reported the synthesis of mesostructured Pt films with extralarge periodicity from lyotropic liquid crystals (LLCs) consisting of block copolymers (polystyrene-b-poly(ethylene oxide), PS-b-PEO) on Au substrates by electrochemical deposition.457 Films with three different geometries (two-dimensional hexagonal, lamellar, and cage) can be obtained by controlling the compositional ratio between block copolymers and Pt species in precursor solutions. This synthetic strategy should be widely applicable not only in single metal systems but also in binary and ternary alloy systems. 4.2 Pore Alignment. 4.2.1 Advantageous Points of Pore Alignment: The control of mesopore arrangement in thin films is important for practical applications including molecular-scale devices. The guest species introduced into the onedimensional mesopores can be oriented in a particular direction, showing macroscopically anisotropic properties in the films. Some examples have been reported. Yang et al. demonstrated incorporation of laser dye rhodamine 6G inside uniaxially aligned mesochannels prepared by applying an electronic field in confined space.458 The rhodamine 6G molecules were highly concentrated inside the mesopores, thereby producing the spontaneous emission caused by the aligned laser dye molecules. Miyata, Kuroda, and co-workers achieved highly uniaxial alignments of one-dimensional mesochannels on rubbing-treated polyimide film. By using such mesoporous silica films with a uniaxially oriented mesochannels, they demonstrated excellent alignment control of cyanine dyes.459 The cyanine dyes existed in a monomeric state, as was confirmed by absorption spectrum. Interestingly, mesoporous films containing the cyanine dyes showed anisotropic absorption behavior in which the maximum absorption was obtained when polarization was parallel to the mesochannels (cyanine dyes). For comparison, in mesoporous silica films with randomly oriented mesochannels, isotropic absorption was confirmed. Tolbert et al. also succeeded in alignment control of semiconducting polymers inside uniaxially aligned mesochannels.460,461 The semiconducting polymer was aligned along the uniaxial mesospace, showing strong polarized absorption and fluorescence. Moreover, the confinement leads to low-threshold amplified spontaneous emission from the aligned polymer chains. In another example, alignment of each nanowire was totally controlled over the entire substrate by electrochemical replication using the mesoporous silica film with uniaxially aligned mesochannels.462 A conductive substrate coated with the mesoporous silica film was used as a working electrode. After metal deposition followed by removal of silica template, a distinct macroscopic optical anisotropy was observed. The totally aligned Pt nanowires showed anisotropic surface

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Figure 18. Incorporation of a leuco dye into a mesoporous TiO2 electrode with vertical pores for high-resolution electrochromic display.

plasmon resonance. Also, the transmittance and reflectance of visible light were largely dependent on the polarization with respect to the alignment direction of the Pt nanowires. Such metal nanowires are expected to be applied in many optical applications. Vertical alignment of mesopores can lead to improved function. For example, Yamauchi et al. demonstrated a high-speed and high-resolution electrochromic passive-matrix display using a leuco dye with a mesoporous TiO2 electrode with vertical pores (Figure 18).463 In this case, the vertical pores of the electrode can support effective diffusion of leuco dyes perpendicular to the electrode and can prevent the diffusion of the dye around the electrode. Such application of leuco dyes to electrochromic display (ECD) devices has high potential to realize a full-color reflective display with low production costs. 4.2.2 Control by External Fields: The use of external fields is an effective tool for materials chemistry because application of anisotropic external fields can induce alignment effects in many substances. Hillhouse et al. demonstrated alignment control of mesochannels in mesoporous silica films by using a flow field.464 Films were prepared on a borosilicate substrate by hydrothermal deposition whereby the precursor solution was pumped through a glass capillary and continuously flowed over the substrate in the required direction. Aligned tubular domains could be clearly observed by SEM observation indicating that continuous flow can be used to induce the alignment of mesochannels. A superconducting magnet with a high magnetic field (>10 T) has been developed for various applications.465 One of the most convenient and versatile methods for the orientation of materials is the application of a high magnetic field; this is effective even for paramagnetic or diamagnetic materials with extremely small

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magnetic susceptibility.466­471 It has been shown that, when a high magnetic field is applied, rod-like macromolecules formed through the self-assembly of surfactants are uniaxially oriented due to the magnetic anisotropies of the constituent surfactant molecules.472­474 The alignment control of mesochannels in mesoporous silica monolith was accomplished through application of strong magnetic fields (11.7 T) as reported by Tolbert et al.475 Unpolymerized hexagonal silicate­surfactant LLCs were prepared and subsequently oriented in a strong magnetic field by heating the samples above their isotropic-to-anisotropic phase transition temperatures followed by slow cooling in the magnetic field. Tubular micelles of the silicate­surfactant LLCs were aligned parallel to the magnetic field direction, and the resulting structure was well characterized by XRD measurements. The alignment obtained was successfully preserved after polymerization of the silicate species by acid treatment. Yamauchi, Kuroda, and co-workers applied a strong magnetic field for preparation of mesoporous films.476­478 Two types of surfactants, Pluronic P123 (poly(ethylene oxide)­ poly(propylene oxide)­poly(ethylene oxide) triblock copolymers (EO20­PO70­EO20)) and CTAB, were used as structuredirecting agents. The ethanol-based precursor solution was cast or spin-coated onto a glass substrate, and subsequently dried under an applied magnetic field (12.0 T). The applied magnetic directions were varied along the parallel and perpendicular to the substrate, respectively. Cross-sectional TEM image and XRD measurements revealed that the mesochannels had been aligned with the magnetic field. From in-plane XRD measurements, it was shown that the alignment degree of P123-based mesoporous film was much higher than that of CTAB-based mesoporous film. Thus, magnetic field techniques can be used to control orientation within mesoporous films so that it becomes possible to prepare mesoporous films with designed alignments of mesochannels. In another approach, Walcarius et al. reported preparation of mesoporous silica film with mesochannels aligned perpendicular to the substrate under potentiostatic conditions on various conductive substrates.479 The cationic surfactant, CTAB, was used as a structuredirecting agent. The cross-sectional and top-view TEM images showed perfectly aligned mesochannel formations perpendicular to the substrates. 4.2.3 Use of Confined Spaces: Confinement can influence the alignment of mesochannels. When mesoporous materials with one-dimensional tubular mesochannels are formed in confined spaces, the mesochannels are induced to take up the most thermodynamically stable alignment. Several researchers have reported that confined space such as porous anodic alumina membranes can orient mesochannels in a particular direction. In their synthetic approach, surfactant-based precursor solutions penetrate into the matrices and are dried until complete evaporation of the solvents has occurred. When CTAB was used as a structure-directing agent, the long axes of the mesochannels are aligned with the long axes of the porous anodic alumina (PAA) channels (i.e., columnar orientation) (Figure 19).480,481 However, when nonionic surfactants, such as block copolymers P123 and Brij 56, are used as larger structure-directing agents, in most cases, the mesochannels are

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Figure 19. Alignment of the long axes of the mesopores along with the long axes of the porous anodic alumina channels.

oriented perpendicular to the PAA channels and circularly packed like stacked donuts.482­484 Other researchers have also demonstrated that the orientation directions of the mesochannels strongly depend on the synthesis conditions (e.g., the kind of surfactant, surfactant concentration, reaction temperature, and humidity).485,486 Confined spaces created by electron-beam lithography are also very effective for orientation controls of mesochannels. Wu et al. proposed a new alignment method in which a silica precursor solution including a triblock copolymer P123 was deposited on linear resist molds.487 The effect of the width size of the mold on the final orientation of the mesochannels was studied by X-ray diffraction (XRD) and high-resolution scanning electron microscopy (HRSEM). Highly aligned mesochannels with their long axes running parallel to both the substrate and the long axis of the mold were obtained when the precursor was deposited on a mold with 0.5 ¯m width. Interestingly, the mesochannels could be partially induced to perpendicularity in the 0.1 ¯m microchannels. As another example, Trau et al. reported the formation of mesostructured silica patterns with aligned mesochannels by combining the use of microcapillaries and applied electronic fields.488 The reactant solution was introduced to the microcapillaries, and an electronic field applied parallel to their long axes so that the mesochannels were aligned along the long axes of the microcapillaries. After removal of the molds, the patterned bundles of orientated nanotubules remained on the substrates. The combination of top-down lithographical technologies and bottom-up self-assembly chemistry will be important for creating hierarchically ordered porous structures. 4.2.4 Use of Smart Substrates: Miyata et al. reported the preferential control of mesochannels in mesoporous silica films prepared on a substrate coated with rubbing-treated polyimide film. The uniaxially-aligned mesochannels were brought about by the hydrophobic interactions between the alkyl chains of surfactant and elongated polyimide chains.489­491 The mesoporous films were prepared either by hydrothermal deposition or by EISA. In-plane rocking curves clearly indicate that the mesochannels are aligned perpendicular to the rubbing direction with a very narrow alignment distribution. The full width at half-maximum of the hydrothermally-prepared films is larger

Figure 20. Rubbing as a facile process for uniaxial orientation control of mesochannels in silica films.

than that of the EISA film. This difference results from the harsh conditions of the hydrothermal deposition method. Yamauchi and co-workers also demonstrated rubbing methods in a facile process for uniaxial orientation control of mesochannels in silica films using two different kinds of surfactant (P123 and Brij 56) in their lyotropic liquid crystalline states at high concentrations (Figure 20).492 Strict control of uniaxial orientation of the mesochannels was achieved without disorder or damage to the hexagonal arrangement while permitting formation of multilayered mesoporous silica thin films have different orientations of mesochannels. Photo-orientation techniques are also useful for alignment control of mesochannels. Seki et al. reported the alignment control of mesochannels in mesoporous silica films by using photo-orientation.493 Mesochannels were aligned due to the interactions between surfactant and aligned polymer on substrate. Unlike the rubbing method, however, this method can be applied to local patterning techniques using a photomask. From optical microscopic images, it was proven that the direction of cracks runs perpendicular to the polarization direction of the UV light source. This result indicates the potential for alignment control of mesochannels in various directions by using photo-orientation techniques. Control of substrate surface can lead to the formation of mesoporous silica film with perpendicularly aligned mesochannels. Koganti et al. reported preparation of mesoporous silica films with perpendicularly aligned mesochannels using P123 as a structure-directing agent.494 Substrates modified using poly(ethylene oxide)­poly(propylene oxide) random copolymer or P123 were used as a chemically neutral interface, and resulted in the formation of perpendicularly aligned mesochannels. Tolbert et al. demonstrated nanometer-scale epitaxy, in which honeycomb arrangements of mesopores on the top surface in cubic mesoporous films were utilized as the patterns.495 Although this paper improved the degree of perpendicular

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orientation, the size of the obtained mesopores ranged only from 10 to 15 nm. Therefore, oriented mesochannels of smaller dimensions (