Predictable Shrinkage during the Precise Design of Porous Materials

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Predictable shrinkage during the precise design of porous materials and nanomaterials Saikat Dutta, Kevin C.-W. Wu, and Tatsuo Kimura Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b02435 • Publication Date (Web): 25 Sep 2015 Downloaded from http://pubs.acs.org on October 4, 2015

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Chemistry of Materials

Predictable shrinkage during the precise design of porous materials and nanomaterials Saikat Dutta1, Kevin C.-W. Wu1* and Tatsuo Kimura2* 1

Department of Chemical Engineering, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan. 2 Inorganic Functional Materials Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Shimoshidami, Moriyama-ku, Nagoya 463-8560, Japan. ABSTRACT: The advent of ordered porous materials and nanomaterials with controlled porosity has opened new frontiers in the field of materials chemistry. Materials with ordered micro-, meso-, and macropores, including hierarchical pores, provide exceptional functions that make them suitable for uses in a wide variety of advanced device materials, media for electrochemical phenomena, and components for sophisticated functional products. In the fabrication of ordered porous structures that would enhance the function of materials, the shrinkage of frameworks plays a vital role in the formation of unique structures of porous materials and nanomaterials. To reveal the role of isotropic and anisotropic shrinkages in the process of pore construction, this review provides an overview of previous experiments that have focused on such framework shrinkages of highly porous materials prepared using organic templates and further assesses how shrinkages (isotropic for particles and anisotropic for films) are fascinating for porous materials design. This type of study will stimulates new strategies on the emerging challenges and opportunities to utilize high-grade and predictable design for obtaining nanomaterials including unique porous materials of relevance in multiple areas such as energy. 1. INTRODUCTION Porous materials are generally classified according to their pore diameter (D) from microporous (D < 2 nm) to mesoporous (2 nm < D < 50 nm) and macroporous (D > 50 nm) regions, and they have been synthesized through different routes by using organic structure-directing agents. Using small organic amine and ammonium molecules leads to the formation of zeolites and related microporous materials,1,2 while periodic meso1

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and macroporous materials can be obtained using amphiphilic organic molecules (e.g., surfactant, block copolymer) and polymer beads, respectively.1-12 In the case of zeolitic materials, the periodicity of porous structures is accurately identified by their individual crystal structures. Although more than 200 crystal structures of synthetic zeolites have been reported thus far, the compositional design of frameworks is almost limited to the isomorphous substitution of metal species into their frameworks. Thus, the acidic and catalytic properties of metal-substituted zeolites depend on the state of doped metal species.13 A wide variety of synthetic processes with nanostructural, compositional, and morphological designs have been utilized for the preparation of meso- and macroporous materials, because inorganic frameworks are mainly built through the sol-gel reactions of metal alkoxides.14 Regarding synthetic pathways based on the sol-gel chemistry, oxide frameworks of meso- and macroporous materials are always amorphous and must be condensed after the calcination-assisted removal of commonly found surfactant assemblies and polymer (e.g., polystyrene, PS) beads. Moreover, thermal treatment at high temperatures leads to partial and/or full crystallization of oxide frameworks with the deformation or complete collapse of ordered nanostructures.15,16 In addition to such a framework crystallization, condensation can be used in the precise design of porous materials, particularly for obtaining unique nanostructures that are difficult to be formed through the conventional surfactant chemistry. From the viewpoint of the materials design, a new approach for constructing nanostructures with highly reliable and diverse properties is scientifically crucial and challenging. In this review, we summarize condensation-assisted strategies for designing porous materials precisely in which the meso- and macroporous materials are respectively synthesized using supramolecular assemblies and uniformly aggregated colloids. Compared with the common soft- and hard-templating methods, isotropic and anisotropic shrinkages provide the precise and reliable designs for constructing new nanostructured materials, and this is beneficial for various emerging applications. In this review, we discuss major roles of these shrinkages for the high-level design of porous materials and derivative nanomaterials. Moreover, the ordering of porous structures depending on the soft- and hard-templating approaches has been described to confirm this concept for demanded materials involving ordered porous structures. The role of anisotropic shrinkage in developing nanostructures of films containing periodic porous structures is discussed by referring to relevant case studies. The extent to which the shrinkage of pores in isotropic particles affects the application profiles for periodic porous structures is addressed. In addition, we evaluate the factors responsible for the 2


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shrinkage restriction of periodic pores. From this perspective, we suggest highlighting the roles of the framework shrinkages in developing isotropic and anisotropic porous materials and provide certain future scopes. 2. PREDICTION OF FINAL POROUS STRUCTURES AFTER SHRINKAGES OF ORDERED POROUS MATERIALS Ordered mesoporous films have been synthesized using the evaporation-induced self-assembly (EISA) process.17 Such films have been developed for the size-dependent separation of bulky proteins,18 humidity control based on water sorption,19 and trace-level and selective detection of vaporized molecules.20,21 This synthetic process has been applied to recover powders when the films are scratched off from substrates and crashed. After a coating with precursor solutions, the concentration of surfactant molecules increases as the solvent evaporates, and the self-organization of the surfactant molecules starts above their critical micelle concentration.17,22 A combination of the EISA process with the sol-gel chemistry concerning oxide species directs the formation of ordered nanostructures.17, 22-24 Final porous structures after the thermal combustion of organic templates such as supramolecular assemblies and polymer beads are strictly decided by the nanostructure of the organic templates accompanied with framework densification processes that are briefly illustrated in Figure 1. Frameworks are densified by dehydration, decomposition, solvent removal, etc. followed by further condensation and microporosity elimination by viscous sintering as well as the synerisis process.25-28 The whole structure shrinkage occurs on the basis of Laplace Force at the curved surface of the pores. In this review, we introduce structural variations after the EISA process and survey the uniqueness of the structural deviation at meso- and macroscales for the precise design of porous materials and nanomaterials, including the structural evolution through crystallization in the case of transition metal oxides.

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Figure 1. Schematic illustration of typical framework densification processes. 2.1. Ordered porous thin films Films generally shrink anisotropically; that is, the direction perpendicular to that of the substrate, during calcination or other thermal treatments. This shrinkage is caused by the condensation of frameworks because either end of the film is anchored to a substrate. This type of macroscopic variation is not limited to porous films; anisotropic shrinkage has also been observed for conventional dense films.17 For obtaining high-quality (i.e., crack-free) ordered porous films, as-made films should be calcined under appropriate conditions for removing organic structure-directing agents. Depending on composition, porosity, and thickness of the films, adhesion force of the films to the substrate interface, and the conditions during thermal treatment, impossibility of the films to relax its lateral tensile stresses during condensation leads to the formation of cracks in macroscale. In many papers, as-made films have frequently been calcined at temperatures appropriate for each composition with a slow heating rate (e.g., 1 °C min-1), and then crack-free films with ordered nanoporous structure can be obtained. Although the direction of the framework condensation is not limited, the films are shrunk anisotropically. Ordered structures are thus converted to their distorted ones. In addition, extraordinary structural variations are observed when frameworks are not only condensed but also crystallized during calcination. In this section, we introduce some compelling cases of the construction of meso- and macroporous films by framework condensation, removal of 4


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templates, and film anisotropic shrinkage. 2.1.1 Mesoporous thin films Anisotropic structural variations in ordered mesostructures can be explained by schematic illustrations (Figure 2). For example, a 2-D hexagonal mesoporous structure (space group; p6mm) is generally fabricated in a direction parallel to that of the substrate, and its anisotropic shrinkage leads to the formation of ellipsoidal cylinders.29-32 Such 2-D hexagonal mesoporous materials, such as SBA-15, have micropores inside silicate frameworks,33-37 in addition to the ordered mesopores. In this case, the micropores parallel to the substrate disappear after complete anisotropic shrinkage (top of Figure 2). When the cylindrical 1-D mesopores are vertically aligned to the substrate, the pore shape is almost retained after the anisotropic shrinkage, although the length of the 1-D mesopores, which is similar to the film thickness, is shortened after the condensation of frameworks.38 Thus, the microporosity in the frameworks is considered lost by the substantial shrinkage of the film. In the case of micropores inside frameworks, interconnected pore windows existing between cage-type spherical pores (space groups; Im3തm, Fm3തm, and P63/mmc) are compressed with anisotropic shrinkage of films and finally vanish (middle and bottom of Figure 2; micropores are not shown for clarity). Vertical connectivity of the spherical pores after compression depends on the initial mesostructural orderings and orientations in the film forms. The shrinkage-driven conversion involves conversion from spherical to ellipsoidal pores with reduction of the mean pore size and partial disappearance of pore windows. Thus, the resultant porous structures are predicted by considering the initial periodicity of cage-type mesopores.

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Figure 2. Structural design by the anisotropic shrinkage of ordered porous films. In the case of inorganic oxide films with cylindrical mesopores aligned in a direction parallel to that of a substrate, 2-D hexagonal orderings are deformed and collapsed by the complete crystallization of frameworks.39,40 When frameworks do not crystallize by heating even at considerably high temperatures, large deviations from the original mesostructures can be observed. For example, mesoporous carbon films with regular slit-like pores are formed after the anisotropic shrinkage of a 2-D hexagonal mesoporous structure.41 Vertical connectivity through pore windows inside a cubic (Im3തm) mesoporous structure is almost maintained according to the mesostructural orientation even after the shrinkage of carbon-based frameworks, thus leading to a marked reduction in the film thickness. Transition metal oxide frameworks are preferred to be crystallized by heating, and ordered mesoporous structures are then often deformed by the partial crystallization of the frameworks. However, these structures are occasionally transformed into regular structures such as nanopillar arrays with high porosity.42-46 This behavior has been observed for cage-type mesoporous materials and is promoted by crystallization of oxide frameworks (Figure 3).47-51 The deformation of mesoporous structures is occasionally avoided by expansion of macrospaces embedded by polymer beads and emulsions.52,53 However, the described structural variation

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induced by crystallization of the titania frameworks can be applied for designing a nanostructure with improved crystallinity. (Im3തm)

(P63/mmc)

Figure 3. Sequential formation of nanopillar arrays from ordered mesoporous materials with cage-type pores that are closely packed as Im3ത m and P63/mmc symmetries. Reprinted with permission from refs 45 and 46. Copyrights 2010 and 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. *This illustration is to be mounted using the double column.

We further explain the nanostructural transformation from cage-type mesoporous materials to regularly arranged nanopillars that involves using the exact experimental data (Figure 4).43 A vertical mesoporosity of the mesoporous titania film features a nanostructural transformation from P63/mmc (i.e., ABABAB stacking of spherical mesopores), in which another close-packed mesostructure, Fm3ത m (i.e., ABCABC stacking), was mixed in the original film. Because both P63/mmc and Fm3തm are highly stable close-packed mesostructures, selectively obtaining their single phases is considerably difficult. The transformation is supposed to be induced by the large contraction of the film (approximately 40%) in a direction perpendicular to that of the substrate with the crystallization of frameworks during the thermal process. As viewed from the [100] zone axis, the 3-D hexagonal structure shows an alternatively organized ABABAB stacking of the porous layers (spherical white and gray) surrounded by amorphous titania (black area). A significant contraction in a direction perpendicular to that of the substrate occurred when the as-synthesized film was calcined at 200 °C. This large contraction caused the merging of pores along the c axis of the initial 3-D hexagonal structure, leading to the formation of an inverse mesospace in the final structure. As viewed from the [001] zone axis, the areas marked as “Ti” are the regions 7



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of titania existing throughout the film thickness. On calcination, Ti is the center of the shrinkage along the c axis of the 3-D hexagonal structure and crystallizes into the anatase phase. Thus, the 3-D hexagonal structure and nature of titania are considered crucial for structural transformation. Recently, the formation of a slit-like nanostructure was also achieved through anisotropic shrinkage of a 2-D hexagonal mesoporous titania prepared using Pluronic P123.54

Figure 4. A plausible mechanism of the structural transformation of a 3-D hexagonal (P63/mmc) mesostructured titania film; schematic of the film before and after calcination at 200 °C and 400 °C, which are viewed from the cross section and top of the surface, with corresponding high-resolution SEM images after calcination at 400 °C. Reprinted with permission from ref 43. Copyright 2006 American Chemical Society. 2.1.2. Macroporous thin films 3DOM materials are obtained as a film by applying careful procedures based on the colloid crystal templating approach.11,12 A wide range of compositions can be fabricated as frameworks in 3DOM powders and films by using clear precursor solutions.55-60 In addition, a colloidal crystal templating by using silica nanoparticles has been employed for obtaining carbon films with a symmetric 3-D ordered mesopore topology (ca. 10~50 nm).61 Uniform-sized colloids, such as spherical beads of PS and poly(methyl methacrylate) (PMMA), are regularly stacked, similar to opal structures, and precursor solutions containing inorganic species are penetrated into the interparticle spaces and solidified without the collapse of the opal-like structures. In this case, the orientation of the stacked structures is very limited to a few structures because spherical hard colloids are preferred to be packed hexagonally as a suitable structure. The polymer beads are finally eliminated by calcination. As illustrated in Figure 5, the anisotropic shrinkage of 8


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films should be observed with the condensation of frameworks during calcination. Although the spherical shape of the hard colloids is not changed after heating, the anisotropic shrinkage of the films is actually feasible after the removal of the stacked polymer beads.62,63 Thus, the spherical macropores are compressed into oval-like pores by anisotropic shrinkage. The 3DOM material is finally converted to nanoparticle arrays when spherical polymer beads are stacked extremely close to each other, as reported in the case of corresponding powder samples64-67 When polymer beads are stacked more loosely, such an isolation is avoided even on calcination at high temperatures because the continuous networks of oxide frameworks in the loosely packed opal structure are thicker than those in the closely packed opal structure.68

Figure 5. Structural design by the anisotropic shrinkage with controlled condensation of inorganic frameworks. Considering the formation of continuous oxide networks between assemblies and aggregates as supramolecular templates, further design is possible for obtaining porous materials69-71 and those found in amorphous photonic crystals with short- and long-range orders.72,73 This type of porous material has never been found among highly porous materials prepared artificially by using organic templates. Particularly, the use of flexible soft colloids for porous materials design is distinct from the aforementioned colloid crystal templating approach.69-71 Such flexible colloids can be prepared using polystyrene-block-poly(oxyethylene) (PS-b-PEO) diblock copolymers with a strong hydrophobic-hydrophilic contrast. Such block copolymers have thus far been utilized as surfactants for fabricating oxide films with uniform and spherical large mesopores and 9



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macropores.74-77 However, they can be designed as flexible spherical colloids by the addition of an appropriate amount of water to their concentrated solutions.69-71,78,79 Then, continuous oxide networks are formed in a resultant film after complete removal of the organic template when flexible colloids are packed loosely (not regularly), and considerable condensation of the frameworks leads to the formation of reticular type oxide networks with sufficient spaces (Figure 6). According to this concept, reticular type porous titania films can be fabricated successfully. The reticular type porous films have an extremely high surface area, which is quite helpful for the accommodation of bulky proteins and diffusion of target molecules.69,71 This is a new and high-grade synthetic concept of obtaining ordered porous materials by using organic templates combined with the condensation of frameworks.

Figure 6. Structural design by the anisotropic shrinkage with controlled condensation of inorganic frameworks. Reprinted with permission from refs 69 and 71. Copyrights 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim and 2014 AIP Publishing LLC. 2.2. Ordered porous particles In general, the conditions for calcination are not limited to powders (particles) 10


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because powders, as well as monoliths, are shrunk isotropically with multidirectional condensation of frameworks. However, for ordered porous particles (e.g., powders and monoliths), unique structural variations have also been reported when frameworks are condensed markedly with the isotropic shrinkage of particles, particularly during the subsequent condensation of ordered macroporous powders. 2.2.1. Mesoporous particles In contrast to films, mesoporous structures in powder form are shrunk isotropically whole the entire particles. Accordingly, the shapes of the uniform mesopores become small with the rational reduction in their pore diameters. In the case of SBA-15 type silica,80,81 cylindrical mesopores are partly connected by micropores existing inside the silicate frameworks,33-37 and 2-D hexagonal structures are then replicated to carbon and other components completely.82-84 Although the number of such micropores can be decreased by substantial shrinkage of the silicate frameworks during high-temperature heating,85-87 the cylindrical mesopores still open even after the micropores disappear. The microporosity inside the frameworks is also controllable; for example, the number of micropores increases by the dehydration of ethylene oxide blocks. According to the synthesis temperature, micropores connecting the primary cylindrical mesochannels are expanded to mesopores, and this is reflected in the adsorption features of SBA-15 type silica.35 In addition, similar micropores are present inside the frameworks of cage-type mesoporous silica. However, a more compelling behavior is the thermal shrinkage and subsequent isolation of spherical mesopores (Figure 7).87,88 After calcination at 450 °C, the N2 adsorption-desorption isotherm for the cage-type mesoporous materials with pore windows is typical. Although the adsorbed amount of N2 molecules drastically decreases with an increase in the calcination temperature above 550 °C, an ordered structure assignable to the original fcc type (Fm3ത m) mesostructure is observed at 640 °C.88 The results indicate that the cage-type mesopores are isolated through the substantial shrinkage of pore windows by the condensation of silicate frameworks. This structural variation is confirmed for SBA-16 type silica with a bcc type (Im3ത m) mesostructure.88 In this case, silica powders having large mesopores were investigated by considering the most significant condition for thermally induced structural transition between open and closed spherical pores. The condition was that the pore window between spherical mesopores is significantly smaller than the diameter of the cage-type mesopores.

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Figure 7. N2 adsorption-desorption isotherms and small-angle X-ray scattering patterns of large-pore FDU-12 (Fm3തm) with open mesopores after calcination at 450 °C and closed mesopores after calcination at 550 °C and 640 °C. Reprinted with permission from ref 88. Copyright 2008 American Chemical Society. 2.2.2. Macroporous particles Macrostructural variation in the 3DOM particles prepared by dual templating with polymer beads and surfactants is shown in Figure 8, which is totally induced by the condensation of frameworks. The main 3DOM structures are fabricated through the replication of the void spaces in the fcc colloidal crystals of spherical polymers such as PMMA.11,12 The isotropic shrinkage of the particles leads to a simple reduction in the spherical macropores with the decrease in the particle size. Further condensation of the frameworks leads to a structural modification of a skeleton type and/or polycrystalline frameworks.89,90 Finally, two types of nanoparticles, which are defined by interparticle spaces surrounded by four or six beads, are formed through considerable shrinkage of frameworks;64-66 the shape of the nanoparticles gradually becomes close to that of a cube and/or sphere to minimize the surface energy. The initially formed inverse opal structures are disassembled into nanocubes (six spheres) and smaller spheroids (four spheres) during calcination. After the assimilation of small spheroids by nanocubes, the resultant nanocubes are consequently self-organized as simple cubic arrays.

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Figure 8. Structural design by the isotropic shrinkage of 3DOM particles including their conversion to an ordered array of uniform nanocrystals. Reprinted with permission from ref 65. Copyright 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Silica- and carbon-based nanocube arrays have been designed thus far, as well as those obtained for a binary (TiO2-P2O5) system.64-67 Along the [001] direction, uniformly oriented nanocubes on every two adjacent fcc layers occupy complementary positions. Adjacent layers of the originally structured nanocubes may interpenetrate and collapse to restack regularly, forming a simple cubic lattice. This may become a general route for creating a periodic structure with cubic and/or other symmetries, providing a new avenue for the precise design of porous materials and nanomaterials. This unusual transformation from fcc nanostructures to simple cubic arrays relies on the special positioning and orientation of template-confined nanoparticles and may occur in parallel with the disassembly process. The confinement of a precursor solution (e.g., phenolic resin and Pluronic F127) in a PMMA colloidal crystal permits the control of external morphology. This method offers either monoliths with hierarchical (macroporous and mesoporous) porosity or uniform nanoparticles with ordered/disordered mesopores.67 A generalized synthesis of mesoporous single crystals and mesocrystals of metal oxides (e.g., TiO2, ZrO2, SnO2, CeO2) has been developed using a colloidal crystal templating with SiO2 spheres as a nanoreactor.91 3. RESTRICTION TO SHRINKAGE In this review, we introduce precise but facile structural designs at various scales. However, considerable attention has been paid to several factors that restrict shrinkage of porous materials. Such restriction effects have been realized through either the strong 13



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interaction of frameworks with surfaces of hard templates or doping of heteroatoms into frameworks. For fabricating hierarchical porous materials on a porous anodic aluminum oxide (AAO) membrane,92 a restriction effect results in an anomalous increase in the mesopore size and pore windows by the shrinkage of polymer frameworks during carbonization on the AAO membrane surface. It is also possible to expand the whole mesoporous structure not only due to the increase of the carbon framework density but also a great loss of framework volume with decomposition of large organic ligands. The pore diameter of the mesoporous resol carbonized rods prepared on the AAO membrane having 90-nm cylinders by using Pluronic F127 increased from 11 nm (350 °C) to 13.5 nm (500 °C) and 15 nm (700 °C). As illustrated in Figure 9, a strong interaction between the polymeric resol molecules and hydroxyl (-OH) groups at the AAO surfaces results in the expansion of mesopores with the shrinkage of frameworks. In contrast, a unique behavior has been introduced;52,53 soft macrospaces embedded in mesoporous titania films are expanded and then prevented from deforming the ordered mesoporous structures by relaxing tensile stress of the films.

Figure 9. Restriction effect exerted through strong interactions with hard templates; anomalous expansion of uniform mesopores during carbonization of FDU-16 prepared using Pluronic F127, which are directly observed using TEM (e.g., inside AAO at 350 °C and 700 °C, inside SiO2 opal at 700 °C) and also confirmed by N2 adsorption– desorption measurements. Reprinted with permission from refs 92 and 93. Copyrights 2009 The Royal Society of Chemistry and 2013 American Chemical Society.

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A SiO2 opal hard template exerts a similar effect on the shrinkage restriction of carbon frameworks during thermosetting and carbonization (Figure 9).93 Regardless of pore symmetries (e.g., Ia3തd FDU-14, p6mm FDU-15, and Im3തm FDU-16), mesopores are expanded by the restriction effect between the template and polymeric resol precursors. For example, the pore diameter of resol carbonized materials prepared using Pluronic F127 followed by carbonization at 350, 500, 700, and 900 °C is 12.5, 14.9, 18.1, and 18.1 nm, respectively, and these values are retained after the chemical etching of the SiO2 opal template. This technique that entails using rigid constituents, such as silica, is an effective strategy for preventing the shrinkage of carbon-based frameworks and affords large mesopores.94-96 Being different from this system, flexible spherical copolymer aggregates can be then utilized as the new organic templates for obtaining reticular type porous structures in films after substantial shrinkage of oxide frameworks even in the presence of spherical templates,69-71 we have explained before (see Figure 6). In the soft-templating approach to prepare ordered mesoporous carbon (OMC) powders, we have often observed remarkable framework shrinkages with the significant decrease of pore sizes.97-99 Being related to such a shrinkage behavior, extraordinary shrinkage of the film is observed and then slit-like pores (anisotropic rectangular, cmm) are for example formed from cylindrical mesopores (2-D hexagonal, p6mm) aligned parallel to the substrate.41 Accordingly, expansion of initial original mesopore size is important for considering resultant pore sizes after substantial shrinkage. A continuous pore expansion (22.9~37.4 nm) of ordered (Fm3തm) mesoporous carbon films prepared using PS230-b-PEO125, was observed by adding low-molecular-weight homopolystyrene (e.g., h-PS49) without the deformation of the mesostructure, although the pore walls become extremely thin (3.6 nm) because the mesostructural shrinkage is restrained by adding h-PS49.100 As the chain length of h-PS49 is considerably shorter than of the PS segment of PS230-b-PEO125, the small amount of h-PS49 can solubilize continuously into the core of PS microdomains and increases the hydrophobic PS core volume, which is caused by the reduction in interaction enthalpy. This method is applied for accessing films having enough porous spaces. A similar consideration was also confirmed; large pores (~15 nm) were consequently formed even after calcination at a higher temperature (e.g., 1000 °C) with crystallization to rutile phase of titania.101 By using a mesostructured titania prepared using spherical colloids constructed by silica-like cores and hydrophilic PEO tethers. The spherical nanoparticles can act as a hard-template. On the other hand, hypercross-linked robust organic frameworks are designed by a Friedel-Crafts alkylation of phenol resin with 1,4-bis(chloromethyl)benzene).102 In this 15



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case, a serious framework shrinkage during carbonization of phenol resin frameworks is successfully avoided to maximize well-preserved organic functions. Robustness of main frameworks is very important for considering not only resultant porous structure but also designed functions of the frameworks. Even in a binary system, the robustness of frameworks is also applied for designing resultant framework and porous structures. A silica-carbon composite precursor was prepared by using a tri-constitutent co-assembly approach; chemical (HF) etching of the silicate frameworks creates small pores (>2.5 nm) in the residual carbon-based frameworks.96 A similar approach provides ordered mesoporous films of carbon and silica, which can be obtained from corresponding carbon-silica composite.103 According to the synthetic conditions, the density of surface Si-OH groups is varied; thus, the interaction at the surfaces of the rigid components can be managed depending on the polymerization and cross-linking degree of the silicate species. In addition, the incorporation of metal, metal oxide, and heteroatoms into the carbon frameworks suppresses the framework shrinkage.104,105 The presence of unshrinkable components such as highly condensed silica in binary systems is quite advantageous for a 3-D structural design in macroscale. For example, two types of core-shell particles constructed by ordered mesoporous carbon cores and mesoporous or non-porous silica shells are prepared through the soft-templating using surfactants.106,107 Although the mesoporous carbon cores shrink in both cases, silica shells change in different ways depending on the porosity. Shrinkage of the mesoporous silica shells coincides with substantial shrinkage of mesoporous carbon cores, leading to the formation of rattle-like nanospheres having dual ordered mesopores (3.1 nm in carbon, 5.8 nm in silica).106 When non-porous silica layers are completely covered with spherical mesoporous carbon particles, shrinkage of silica hardly occur by heating while the mesoporous carbon cores shrink significantly.107 As in the case of OMC films, such silica shells are considered as the hard substrate and then hemispheres of OMC particles are formed after the anisotropic shrinkage induced by the silica-carbon interface tension inside the spherical silica shells. 4. CONCLUSIONS AND OUTLOOK This review provides the first coverage of “framework shrinkage” in various highly porous materials prepared using organic templates such as supramolecular assemblies and colloidal particles. By thoroughly investigating the synthetic processes that occur at micro-, meso-, and macroscales, we can understand the formation processes of fairly complicated porous materials, such as colored materials with some kinds of ordering in nature,72,73 based on biomimetism and bioinspiration.108,109 According to the overall 16


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insights provided in this review, resultant micro-, meso-, and macrostructures after indispensable but controlled shrinkage of frameworks can be predicted by considering (1) ordering of as-synthesized materials occupied by organic templates, (2) initial network of main frameworks, (3) interaction at the interface of composite materials (e.g., substrate, colloid, confined vessel), (4) flexibility of organic templates, (5) incorporation and mixture of other components in main frameworks, and (6) initial condensation degree of main frameworks (e.g., density of frameworks, OH density, and porosity). This careful consideration is quite essential for the precise design of porous materials and nanomaterials and the planning of suitable processes through manufacturing of the materials to construct advanced devices, for example, through step-by-step fabrication of materials (films) and molding/filling of materials inside other substances (particles). Although we introduced this consideration concerning “framework shrinkage” for the high-level design of porous materials and derivative nanomaterials, further investigation will be required or should be considered for understanding the demanded functions of such designed materials toward practical applications. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] Notes The authors declare no competing financial interest. Biographies Dr. Kevin C.-W. Wu studied at the Department of Materials Science and Engineering, The University of Tokyo, Japan and received his Ph.D. in 2005. After that, he worked with Prof. Kazuyuki Kuroda of Waseda University, Japan as a post-doc from April 2005 to September 2006. Since then he has been working on the orientation control of 2-D hexagonal mesoporous thin films. From October 2006 to July 2008, he joined Prof. Victor S.-Y. Lin’s group at Iowa State University, USA as a post-doc and worked on the intracellular controlled drug delivery using oriented mesoporous silica nanoparticles. He was appointed an assistant professor at the Department of Chemical Engineering, National Taiwan University, in August 2008 and was prompted as an associate professor in 2012. His current interest is the synthesis of mesoporous nanoparticles and metal-organic frameworks with desired structural orientation and functionalities for biomedical applications and cellulosic conversion. Dr. Tatsuo Kimura studied at Waseda University, Japan, and received his Ph.D. degree 17



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in 1999. After working as a research associate at Waseda University in 1998, he joined National Industrial Research Institute of Nagoya (NIRIN) and Toyota Central R&D Labs., Inc. as a post-doctoral fellow. He was appointed as a researcher in NIRIN in 2000 which has been integrated as the AIST Chubu of National Institute of Advanced Industrial Science and Technology (AIST) since 2001. He has continuously researched a wide variety of surfactant assisted mesoporous materials for the precise designs of composition and nanostructure in frameworks with mesoscale and macroscale controls. At the beginning of his research career, cutting-edge materials (e.g., first mesoporous AlPO in 1996, first crystalline mesoporous silica from a layered polysilicate in 1999, first non-silica-based hybrid mesoporous materials in 2003) were reported. Besides, he has a great interest in new porous materials design for practical applications and recently focuses on beautiful structural variations of morphologically controlled mesoporous and macroporous materials (oxides and hybrids). ACKNOWLEDGEMENTS T.K. acknowledges the major support of “the Environmentally Friendly Sensor Project” granted by the Ministry of Economy, Trade and Industry (METI), Japan with a partial support by JSPS KAKENHI Grant Number 26288110. REFERENCES 1. Corma, A. From microporous to mesoporous molecular sieve materials and their use in catalysis. Chem. Rev. 1997, 97, 2373. 2. Davis, M. E. Ordered porous materials for emerging Applications. Nature 2002, 417, 813. 3. Ciesla, U.; Schüth, F. Ordered mesoporous materials. Microporous Mesoporous Mater. 1999, 27, 131. 4. Kresge, C. T.; Roth, W. J. The discovery of mesoporous molecular sieves from the twenty year perspective. Chem. Soc. Rev. 2013, 42, 3663. 5. Wan, Y.; Zhao, D. On the controllable soft-templating approach to mesoporous silicates. Chem. Rev. 2007, 107, 2821. 6. Wan, Y.; Shi, Y. F.; Zhao, D. Y. Designed synthesis of mesoporous solids via nonionic-surfactant-templating approach. Chem. Commun. 2007, 897. 7. Hsueh, H.-Y.; Yao, C.-T.; Ho, R.-M. Well-ordered nanohybrids and nanoporous materials from gyroid block copolymer templates. Chem. Soc. Rev. 2015, 44, 1974. 8. Bastakoti , B. P.; Li, Y.; Kimura, T.; Yamauchi, Y. Asymmetric block copolymers for supramolecular templating of inorganic nanospace materials. Small 2015, 11, 1992. 18


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109. Wegst, U. G. K.; Bai, H.; Saiz, E.; Tomsia, A. P.; Ritchie, R. O. Bioinspired structural materials. Nature Mater. 2014, 14, 23. Graphic TOC

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Predictable Shrinkage during the Precise Design of Porous Materials

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