9Feature Synthesis of Hierarchically Porous Materials Based onGreen Easy-Leaching ConceptGe Tian, Li-Hua Chen, Xiao-Yu Yang, and Bao-Lian Su
9.1Introduction
Hierarchically porous materials, featuring high specific surface areas, multimodalor multiscale porosity, tunable pore sizes, 3D interconnectivity, and rich surfacechemistry [1–37], hold great promise, particularly in catalysis and separation pro-cesses, where optimization of diffusion and confinement regimes is required.While micro- and mesopores provide size and shape selectivity for the guestmolecules, enhancing the host–guest interactions, the presence of macropores canconsiderably favor the diffusion of the guest molecules and thus their accessibilityto the active sites [38–57]. In practice, there have been many successful proceduresto prepare this kind of materials by templating methods, that is, colloidal crystaltemplating [1], emulsion templating [2, 3], supramolecular aggregates [7], and bio-materials [8] as well as phase separation by using polymers [10] and posttreatmentprocedures [11].
In this century, it is apparent that the full potential of as-prepared hierarchicallyporous materials, even all nanostructured materials, will be realized only whenthese materials are not only synthesized in large quantities with reproducible size,shape, structure, crystallinity, and composition but also prepared and assembledusing green, environmentally responsible methodologies. Green chemistry can bebroadly defined as the conscious reduction and/or elimination of hazardous start-ing materials, reactions, reagents, solvents, reaction conditions, and associatedwastes in manufacturing processes. For synthesis of macroporous structures, forexample, the postsynthetic removal of the templates requires additional processingsteps that can be costly, wasteful, and of environmental concern. Clearly, theseproblems would be easily overcome if spatial patterning of the inorganic phasecould be achieved in the absence of organic templates or by using easy-removalor recycle template. Recently, some environmentally friendly procedures, using aneasy-leaching template (also called organic template-free), have also been developedand used to synthesize hierarchically porous materials [12–18], such as gas expul-sion, salt template, ice template, and selective leaching. These methods not only
270 9 Feature Synthesis of Hierarchically Porous Materials Based on Green Easy-Leaching Concept
produced hierarchically porous materials with high quality and good multistructurebut also resulted in the extension of the hierarchical porous structure.
An unusual feature of the title methods that is worthy of emphasis is thatthe templates (or so-called ‘‘pore formers’’) are easily removed by leaching. Forexample, gas can be easily released, ice and salt can be easily solvated in water,and one phase from a biphasic composite can be easily leached. This greentemplating easy-leaching method to obtain hierarchically porous materials isgenerally atom economical and benign to human health and environment inthe synthesis of hierarchically porous materials. There are some superiorities orfeatures as following:
1) use of cost-effective, nontoxic precursors (utilization of air and aqueous sol-vents);
2) minimization of reaction steps – reduce waste, reagent use, and power con-sumption;
3) room-temperature (or low-temperature) synthesis under ambient conditions,if at all possible; and
4) efficiency of scale-up.
9.2Hierarchically Structured Porous Materials Synthesized by Easy-Leaching AirTemplates
Macroskeletal mesoporous silicate foams with open-cell randomly shapedmacrovoids on the (sub)millimeter scale have been prepared from metastablePEO-surfactant air–liquid foams induced by strong stirring under neutral aqueousconditions [6]. It was claimed that the macroscale morphologies were tuned bychanging the turbulence of the reaction media, which is certainly a difficultparameter to control. Since then, a bubbling process has been proposed toproduce air–liquid foams [58–60], which may allow complete control over thecell size and shape of the bubbles and a more easily maintained liquid fractionof the foam. Silica macroporous scaffolds with vermicular-type mesoporositywere obtained by this air–liquid foaming sol–gel process, in which nitrogenwas bubbled through a mixture of a surfactant and sol–gel precursor [58], whilethe TiO2 macrocellular scaffolds obtained had poor mesoporosity arising fromthe void space induced by the random aggregation of nanoparticles within thefoam walls [59]. Macroscopic cell morphologies were tuned by changing theair/liquid: foam ratios and the size of the nitrogen bubbles, while wall topologieswere varied by changing the surfactant. Spraying aqueous siliceous solutionscontaining alkyltrimethylammonium surfactants under high pH conditionsresulted in mesoporous silica foams with hierarchical trimodal pore structures(macrovoids and two kinds of mesopores of around 3 and 40 nm) (Figure 9.1) [60].The macrocellular structure was produced with air bubbles, which was stabilizedthrough a rapid condensation reaction during drying. The bimodal mesopore
9.2 Hierarchically Structured Porous Materials Synthesized by Easy-Leaching Air Templates 271
Hexagonal array ofCTAC micelles
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Figure 9.1 Schematic model for the hierarchical trimodal pore structure of silica foamsby a bubbling process. Reproduced from Ref. [60] by permission of the Royal Society ofChemistry.
distribution was derived from the surfactant micelles and the interparticle spacesof silica nanoparticles (Figure 9.2) [60]. This hierarchical architecture of themesoporous silica foams, having extremely low bulk density (0.01 g cm−3), highmesopore volume (>2 cm3 g−1), and specific surface area (>1000 m2 g−1), isattributable to the self-assembly of air bubbles, surfactant-siliceous complexes, andmesostructured silicate particles during the spraying, condensation, and dryingprocesses, respectively. However, this method looks complex, and it would bedifficult to control all the parameters.
Preformed polymer foams are also good candidates for templating macrop-orous structures. Monolithic polystyrene foams, preformed by polymerization ofstyrene either in the continuous or the dispersed phase of highly concentratedwater/oil emulsions, have been used to synthesize meso/macroporous inorganicoxide monoliths by imbibition of a self-assembling block-copolymer/sol–gel mix-ture [61]. After calcination to remove the organic components, the resultingmeso–macroporous silica, titania, and zirconia materials retained their macro-scopic shapes and possessed independently adjustable meso- and macroporestructures. The meso–macroporous silica monoliths prepared from the W/O(water-in-oil) polystyrene foams and PEO–PPO–PEO triblock copolymer speciesconsisted of cellular macropores 0.3–2 μm in diameter, interconnected by win-dows approximately 0.2–0.5 μm in diameter with wall thicknesses of approximately100 nm, and highly ordered mesopores, 5.1 nm in size. Alternatively, preformed
272 9 Feature Synthesis of Hierarchically Porous Materials Based on Green Easy-Leaching Concept
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Figure 9.2 Typical SEM image of macrovoids and mesopore II of silica foams preparedunder standard conditions (0.52 M TEOS and 0.25 M CTAC). Inset shows TEM images ofthe ordered mesoporous structure. Reproduced from Ref. [60] by permission of the RoyalSociety of Chemistry.
mesoporous silica nanoparticles were used as building blocks to coat polyurethanefoam, leading to mineralization of the foam. Subsequent elimination of the or-ganic foam by calcination resulted in monolithic macrocellular silica foams with atrimodal pore system (small mesopores–large mesopores–macropores) [62]. Tex-tural large mesopores/macropores (in the 20–70 nm range) have their origin inthe interparticle voids, and the small intraparticle mesopore system (2–3 nm indiameter), owing to the supramolecular templating effect of the surfactant.
9.3Hierarchically Structured Porous Materials Synthesized by Easy-Leaching IceTemplate
Ice-templated materials have long been referred to as freeze-casting (also knownas freeze-drying or freeze-gelation) [63–65]. This simple technique has been used toproduce porous, complexly shaped polymeric or ceramic parts. In freeze-casting,ceramic slurry is poured into a mold and frozen. The frozen solvent acts temporarilyas a binder to hold the part together for demolding. Subsequently, the part issubjected to freeze-drying to sublimate the solvent under vacuum, thus avoidingthe drying stresses and shrinkage that may lead to cracks and warping duringnormal drying. After drying, the compacts are sintered in order to fabricate a porousmaterial with improved strength, stiffness, and desired porous microstructure thatis generated during freezing. By controlling the growth direction of the ice crystals,it is possible to impose a preferential orientation of the porosity in the final material.
Certain superiorities of the freeze-drying process are summed up as following:
• First, green template: aqueous solutions are often used to prepare porousmaterials by freeze-drying, wherein water is an environment-friendly solvent andthe use of ice crystals as templates (or so-called ‘‘porogens’’ and ‘‘pore former’’) isgreen and sustainable. This is particularly beneficial for biological applications.
• Second, easy purification of products: when removing the solvent, thefreeze-drying process does not bring impurities into the samples, and a furtherpurifying process is therefore not necessary.
• Third, variety of the porous structures: numerous pore morphologies and nanos-tructures (from 2D to 3D structure) can be obtained by changing variables duringfreezing.
• Fourth, general technology in various materials: to date, the freeze-drying processhas been applied to a variety of different materials, including ceramics, polymers,composites made of both, and hydrogels.
Macroporous ice-templated polymeric materials were first reported more than40 years ago [66, 67], and their properties, rather unusual for polymer gels, soonattracted attention. Since then, many different polymers (e.g., poly(l-lactic acid)and poly(dl-lactic-co-glycolic acid) [68, 69], gelatin [70], g-PGA/chitosan (CHI)[70–72], collagen and elastin [73], collagen-glycosaminoglycan [74], and albumin-cross-linked polyvinylpyrrolidone (PVP) hydrogels [75], among others) havebeen widely used in biomedicine (e.g., for tissue engineering and drug deliverypurposes) most likely because of the biocompatible character of the process.Macroporous ice-templated ceramic materials (in particular, bundles of alignedsilica fibers) [76] were first reported in 1980, their production being based onprevious studies of freeze-drying techniques for the synthesis of metal and metaloxide powders [77–79]. Since 1980, numerous works (albeit less than for polymers)have also reported ice-templating processes for the preparation of different porousceramic materials with a uniform microstructure [80, 81]. A wide variety ofpolymers have also been synthesized by this technique, which are different instructure from ceramics produced by similar method (Figure 9.3) [82, 83].
9.3.1Ceramics
Different types of ceramics have hence been freeze-casted, including alumina [84],silicon nitride [85], and NiO–YSZ (Y2O3-stabilized ZrO2) [86]. The first idea is totake advantage of the specific porosity template by ice, thereby processing materialsthat could be used for gas filtration, separation filters, catalyst supports, and soon. The requirements are very specific to each application, and the main interestof the technique in such cases is the control of the total porosity, orientation ofthe porosity, and the control of its characteristics (shape and size). The porousstructure is based on the interaction between the ice front and the incorporatedceramic particles. A second approach was developed, with interest not in the porousstructure but rather in the possibility of processing complex-shaped ceramic parts,by obtaining net-shaped green bodies [87]. In such cases, dense materials aredesired, so that the suspensions are highly loaded and no porous structures aredeveloped. With all ceramic materials, a densification stage at high temperature isnecessary after ice sublimation in order to consolidate the structure and bind theparticles together.
274 9 Feature Synthesis of Hierarchically Porous Materials Based on Green Easy-Leaching Concept
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Figure 9.3 SEM images of cross and longitudinal sections of well-patterned microchan-neled materials of different natures (ceramics and polymers) formed by ice-templatingprocesses. Reproduced from Refs. [82] and [83] by permission of the American ChemicalSociety and the Royal Society of Chemistry, respectively.
9.3.2Polymer
Polymer scientists have already taken advantage of the technique for a wide varietyof materials, including collagen [88], CHI [89], agarose [90], and alginate [91]. Severalstrategies have been identified to modify the pore structure, including modificationof the freezing regime, with faster freezing rates yielding smaller pores andthe use of additives, such as acetic acid or ethanol [88], increasing the amountof constitutional super-cooling due to solute rejection. Hence, drugs, proteins,or any other active substance can be efficiently incorporated into the scaffoldsduring the processing, without affecting their biological activity. Finally, extremelyhigh porosity (>90%) can be easily achieved. The process is somewhat simplerthan with ceramics, where a high-temperature densification stage is necessary toconsolidate the scaffolds. In the case of collagen, for example, water removal causescross-linking between the collagen aggregates, and the scaffolds are ready to useafter the ice is sublimated.
9.3.3Hydrogels (Silica)
It is also worth mentioning the application of freeze-casting to hydrogels, such assilica gel [92]. The phase separation occurring between water and the freshly gelled
hydrogels is used to tailor the pore structure. This results in a unique structure withmacro-, meso-, and micropores in advanced materials that might find numerousapplications in separation and reaction processes.
These ice spheres then play the role of a template in the formation of 3D intercon-nected macroporous metal oxides or polymers. When precursor hydrogels, whichare freshly gelled and which contain an adequate quantity of solid components,are unidirectionally frozen under conditions in which the pseudo steady-stategrowth of ice crystals can continue, an array of polygonal ice rods with fairlyuniform diameters grows in parallel to the freezing direction. After the removalof the ice template by elevating the temperature, large monolithic materials withordered macropores are obtained (Figure 9.4) [93]. The straight macropores havea polygonal cross section and are parallel to the freezing direction. Besides theirordered macroporosity, micro- and mesopores develop inside the honeycomb wallsthrough the freeze-drying of SMHs soaked in tert-butyl alcohol. It was found thatthe macropore size of the SMHs can be controlled by changing the rate of immer-sion into a cold bath and the freezing temperature, without changing the micro-and mesoporosity of their honeycomb walls. The thickness of the honeycomb wallswas affected by the SiO2 concentration and the macropore size. The porosity ofthe honeycomb walls could be controlled to be microporous as well as mesoporousby a hydrothermal treatment of the as-prepared SMHs in basic aqueous solutions.Moreover, SMHs with developed mesopores showed a higher stability against heattreatment. The method developed by Nishihara et al. [93] is thus quite versatilefor the preparation of hierarchically porous materials with a variety of chemicalcompositions.
9.3.4Composites
Following the examples of ceramics and polymers, composites made of both ma-terials have been processed, opening up a new class of functional properties. Suchcomposites include hydroxyapatite (HAP)/collagen [94], cerium oxide/poly(vinylalcohol) (PVA), or PVA/silica [95]. The addition of micro- or nanoparticles can bejustified for several reasons, such as improvement of mechanical properties oractivation of catalytic properties. The structure is logically similar to that of poly-mers and ceramics. del Monte et al. have recently prepared hierarchically organizedstructures by the application of the ice-templating process to a silica hydrogel alsocontaining proteins or liposomes (Figure 9.5) [96, 97]. The former comprised anesterase protein (pig liver esterase, PLE) dispersed in PVA/silica hybrid hydrogel,which resulted in a hierarchical biohybrid material exhibiting a very sophisticatedstructure with up to six levels of space organization. Furthermore, these hierar-chical biohybrid materials show an interesting dual character, which allows forsharing of tasks, some entities supporting the structure (e.g., colloidal silica andPVA nanodomains) and others providing functionality (e.g., in this case, PLE).
With a similar approach, the ice-templating process has also been applied to the3D structured composite of multiwall carbon nanotubes (MWCNT) and CHI [82],
276 9 Feature Synthesis of Hierarchically Porous Materials Based on Green Easy-Leaching Concept
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Figure 9.4 Morphology and structure of silica monoliths exhibiting a microhoneycombstructure. (a) An overall image. SEM images of (b) cross section, (c) microchannelstructure, and (d) longitudinal section. (e) Detail of a cross section. Nitrogen isotherms(f) are also represented. The inset shows the mesopore size distribution in the desorptionbranches. Reproduced from Ref. [93] by permission of the Royal Society of Chemistry.
which favors MWCNT dispersion and ensures homogeneity of the suspension [98].The resulting 3D architectures are highly porous (specific gravity about ∼10−2)and extremely conductive (up to 2.5 S cm−1, depending on the MWCNT content)because of MWCNT interconnections in the macrostructure (Figure 9.6) [82]. Theachievement of CNT-based 3D architectures is of special relevance since, except fora few recent cases [99], most of the arrays prepared to date with controlled areasand nanotube lengths were two-dimensional (2D) [100–104].
Figure 9.5 (a) SEM image and scheme rep-resenting PLE encapsulation within the hy-brid structure of PVA-silica. The bar is 5 μmlong. (b) Evolution of the enzymatic activ-ity of PLE during formation of silica (opentriangle) and PVA-silica (open circle) sam-ples: (A) gelation and (B) freeze-drying. (C)
Evolution of the enzymatic activity of PLE insolution (solid circle) and in the PVA-silicasample (open circle), with aging time. Linesrepresent a guide for the eye. Reproducedfrom Ref. [96] by permission of John Wiley &Sons, Inc.
9.3.5Development of Methodology
The freeze-drying processes are excellent candidates for ice templating. Theice-templating method consists of freezing, keeping the frozen state, and de-frosting of precursors (or colloid systems), forming a water solution (or suspensionor a hydrogel). Thus, the formation of crystalline ice (hexagonal ice, typically)causes every solute originally dispersed in the aqueous medium to be expelled
278 9 Feature Synthesis of Hierarchically Porous Materials Based on Green Easy-Leaching Concept
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Figure 9.6 (a) SEM image of a cross-sectioned monolithic Pt/MWCNT 3D architecture(the bar is 20 μm in length). The inset shows a TEM micrograph of the MWCNT surfacedecorated with Pt nanoparticles (here the bar is 50 nm). (b) Plot of current density (solidsymbols, left ordinate) and normalized current (open symbols, right ordinate) versus massof Pt/MWCNT 3D monolith (abscissa). The data presented were obtained after scanning for10 cycles (5 mV s−1 scan rate) to ensure a stable response. Reproduced from Ref. [82] bypermission of the American Chemical Society.
to the boundaries between adjacent ice crystals (Figure 9.7) [105]. Subsequentfreeze-drying gives rise to a macroporous matter during high-vacuum sublimationof ice. The freeze-drying process also allows for the achievement of monoliths thatpreserve the size and shape of the container submitted to freezing.
During the freezing step, solvent crystals grow and solute molecules are excludedfrom the frozen solvent until the sample is completely frozen. Different conditions
of freezing, including the freezing temperature, solute concentration, solvent type,and direction of freezing, can have a great impact on the resulting pore structureof the materials. By controlling the direction of freezing, the growth of ice crystalscan be orientated in one direction, a process called directional freezing (Figure 9.7)[63, 105, 106]. Drying is usually the most time-consuming step in the processdirectly related to the ice sublimation rate and is determined by factors such aslevel of vacuum, shelf temperature, sample volume and exposed surface area, andproduct resistance [107].
The main process control variables available to tailor the final morphology werefirst studied by Tamon and coworkers [93, 108] and more recently overviewedextensively by Tomsia and coworkers [109, 110]. One of the most interestingobservations made by the latter authors concerns the structural heterogeneity ofthe sample in the freezing direction (Figure 9.8) [110]. Three distinct zones canbe clearly distinguished in the samples, each characterized by a particular poreshape and dimension. In zone 1, the closest to the initial cold finger, no porosityat all is observed, and the material is dense. In the second zone, the material ischaracterized by a cellular morphology.In the upper zone (zone 3), the ceramicis lamellar (with interlamellar separation defined by λ, see Figure 9.8), with longparallel pores aligned in the direction of movement of the ice front.
There have been some further significant development in methodology offreezing approach, for example, organic solutions for biocompatible scaffolds;combination of emulsions and freezing for porous materials and nanoparticles,microwires/nanofibers by the controlled freezing of colloidal suspensions anddilute polymer solutions.
Organic solutions have been freeze-dried mainly for the preparation of porous hy-drophobic biodegradable polymers such as scaffolds for tissue engineering. Porouspoly(caprolactone) (PCL) was prepared by freezing a PCL–tetrahydrofuran solution
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Figure 9.7 Schematic representation of the directional freezing process. Ice crystals growin one direction, and the solutes (such as particles, polymeric molecules, or a mixture) areexcluded and solidified between the crystals. Reproduced from Ref. [105] by permission ofJohn Wiley & Sons, Inc.
280 9 Feature Synthesis of Hierarchically Porous Materials Based on Green Easy-Leaching Concept
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Figure 9.8 SEM images of the evolutionof the ice-front morphology and final mi-crostructure. Homogeneous layer thicknessis attained at about 200–250 mm above theimmersion level – the layer thickens progres-sively with the separation from the immer-sion level and then becomes constant. Tilt-ing of the lamellae in the first frozen zonecan be observed near the immersion level,
at the bottom of the vertical-section micro-graph (arrow). The horizontal cross-sectional(parallel to the ice front) micrographs revealthe corresponding evolution of the porousstructure and hence the interface morphol-ogy (depicted diagrammatically on the right).Reproduced from Ref. [110] by permission ofElsevier B.V.
and then freeze-drying at −80 ◦C. The effects of the concentrations of the polymer onscaffold properties such as morphology, porosity, mechanical stability, and degrad-ability were investigated [111]. To produce PCL and poly(dl-lactide) with greatercontrol of porosity, polymer solutions were prepared by dissolving in 1,4-dioxanewith the addition of 92 wt% sugar and salt particles [112]. Then the freeze-dryingprocess was combined with leaching of the sugar and salt particles. Zhang et al.explored the possibility of using compressed CO2 solution for the directional freez-ing process (Figure 9.9) [113]. A sugar acetate, 1,2,3,4,6-pentaacetyl-β-d-galactose,was dissolved in liquid CO2. The resulting solution was then directionally frozenin liquid nitrogen, and the frozen sample left in a fume cupboard with the valveopen to slowly release CO2. This led to the formation of a well-defined alignedporous structure. This process involved no organic solvent and also avoided thefreeze-drying step.
The combination of emulsion templating and freeze-drying to prepare porousmaterials has several advantages. For example, emulsion stability in general is not aproblem because the emulsion structure is locked in by the rapid freezing process.The percentage of the droplet phase in the emulsion can be varied over a widerange of 10–95%, thus providing great control on the pore structure and porosityof the resulting porous materials. In one study, cyclohexane was emulsified inaqueous SCMC solutions containing SDS as a surfactant at volume ratios of 0 : 100,20 : 80, 40 : 60, 60 : 40, and 75 : 25. Freeze-drying of the emulsions produced porousmaterials with both emulsion-templated and ice-templated pores (Figure 9.10)[114]. These porous polymeric materials were further used as templates to prepareporous zirconia with systematically controlled pore morphology and pore volume.
Solid CO2+ BGAL(aligned)
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Figure 9.9 Preparation of an aligned poroussugar acetate material by unidirectional freez-ing of a solution of 1,2,3,4,6-pentaacetylβ-D-galactose (BGAL) in liquid CO2. The ar-row represents the freezing direction. Thesolid CO2 is removed subsequently by directsublimation to yield a porous, solvent-free
structure with no additional purificationsteps. Aligned porous BGAL produced by di-rectional freezing of a liquid CO2 solution.The arrow represents the approximate direc-tion of freezing. Reproduced from Ref. [113]by permission of the American Chemical So-ciety.
282 9 Feature Synthesis of Hierarchically Porous Materials Based on Green Easy-Leaching Concept
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Figure 9.10 Porous SCMC materials prepared by freeze-drying the emulsions with differentemulsion ratios. From (a) to (e), the volume percentage of oil phase is 0, 20, 40, 60, and75%. The circle indicates one of the emulsion-templated pores. Reproduced from Ref. [114]by permission of the Royal Society of Chemistry.
A 3D interconnected fibrous network of PLA could be prepared by a procedure in-volving thermally induced gelation, solvent exchange, and freeze-drying. Variablessuch as polymer concentration, thermal annealing, solvent exchange, and freezingtemperature could affect the size of the formed nanofibers [115, 116]. Very recently,it was found that diluted aqueous polymer solutions could be frozen in liquidnitrogen and then freeze-dried to produce polymeric nanofibers with diameters inthe range of 200–600 nm [117]. A range of hydrophilic nanofibers, including PVA,SCMC, and alginate, was successfully obtained. The polymeric nanofibers couldthen be used as templates to prepare hollow crystalline titania microtubes andFe2O3 nanofibers.
Spray-freezing into liquid (SFL) (Figure 9.11) can produce microparticles[118]and has been used to prepare microparticles containing poorly water-soluble drugs(e.g., danazol and carbamazepine) either alone or with pharmaceutical excipi-ents [118–120]. For example, hollow biodegradable PLA particles with porousshell walls have been prepared both by emulsification/freeze-drying and byspray/freeze-drying.
In summary, the ice-templating process described here is self-assembly in nature.This phenomenon is critical for various applications, such as the cryopreservationof biological cell suspensions and the purification of pollutants. This combination ofhierarchy and functionality opens up the possibility of application of these ‘‘green’’
Figure 9.11 Schematic representation of an SFL process using liquid nitrogen as the cryo-genic medium. Reproduced from Ref. [118] by permission of Elsevier B.V.
materials to a variety of novel applications in fields such as biotechnology (e.g.,biosensors and biocatalytic systems for organic synthesis and fuel cell technologies)and biomedicine.
9.4Hierarchically Structured Porous Materials Synthesized by Easy Selective-LeachingMethod
Selective leaching of one phase from a composite has been used to synthesizea variety of porous inorganic materials. Some physical mixing method suchas sintering can be used to form the composite with two immiscible phases.Immersion in an appropriate solution will dissolve out the sacrificial phase, leavingbehind a porous monolith of the desired phase. A composite of rock salt NiO andwurtzite ZnO, for example, is helpful in understanding this process [121]. First, thecomposite obtained by uniaxial pressing method has two immiscible phases (NiOand ZnO phases), and the two phases are in intimate contact with one another.Second, after selective leaching, the macroporous NiO (Figure 9.12) can be easilyobtained, since ZnO is soluble in alkali solution while NiO is insoluble.
Such selective leaching allows adjusting the porosity and pore size by changingthe starting phase ratio, particle diameter, and heat treatment. Furthermore,solid-state reactions have been developed to form an intimately mixed two-phasecomposite, one of which is sacrificial. The porosity and the pore size may betherefore tuned across a wide range. Adjusting the initial volume fraction of thesacrificial phase has a corresponding effect on the porosity. The average porediameters between 500 nm and 5 mm can be synthesized by the leaching method.First, nearly identical grain sizes for the two phases are usually preferred. This
284 9 Feature Synthesis of Hierarchically Porous Materials Based on Green Easy-Leaching Concept
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Figure 9.12 Scanning electron microscopic (SEM) images of (a) dense composite of NiOand ZnO phases and (b) the resulting macroporous NiO that remains after alkali leaching.Reproduced from Ref. [121] by permission of the American Chemical Society.
characteristic is important if a pore structure with open connectivity is desired. Amorphology composed of large grains of one phase surrounded by small grains ofthe other phase can lead to undesired effects on the monolith on leaching. If thegrains of the sacrificial phase are much larger than those of the desired phase, theresulting pores will be large and isolated. Conversely, if the grains of the desiredphase are much larger than the sacrificial phase, no connectivity between the grainsof the desired phase will exist, and on leaching, the monolith will turn to a powder.Second, the leaching must have no effect on the desired phase, and the connectivityof the desired grains is maintained. As the pores are formed via leaching, theymust be connected and do indeed appear continuous.
Selective leaching has a long history, for example, the depletion gilding byselective leaching of Cu from Au–Cu alloys [122]. This is observed in technologicallyimportant alloy systems. For example, Erlebacher et al. have reported that thisprocess results in the formation of a chemically tailored nanoporous gold made bydealloying Ag–Au [123]. Raney has also developed a technology to form the porousmetal known as Raney nickel by dealloying aluminum from an aluminum–nickelalloy [124]. This leaching process has been also extended to other systems. Forexample, Suzuki et al. have reported the ceramic CaZrO3 with macropores byreleasing of carbon dioxide during the decomposition of dolomite [125]. Singhet al. have synthesized a macropore network of titania by controlled evaporation oftitanium sol [126].
With the development of solid reactions, such as eutectic cooling to crossingphase boundaries, selective leaching can lead to other hierarchically structuredmaterials. As an example, porous YSZ samples have been synthesized by leachingthe nickel out of the NiO/YSZ composites with 2.2 M HNO3 at 353 K [127].Porosities >75% could be achieved without structural collapse of the YSZ phase.The range of pore size of macroporous sheets of YSZ is around 1–5 mm in diameter(Figure 9.13). Finally, the method was applied to the fabrication of a solid oxide fuelcell with a copper-based anode operating on H2 and n-butane. Such a method hasalso been used to form porous zirconia, after the removal of the magnesia phasefrom a composite of magnesia and zirconia [128]. Similarly, porous Vycor glass hasbeen obtained through acid leaching of the borosilicate-rich phase [129].
Figure 9.13 SEM images of 50 : 50 NiO/YSZcomposite after acid leaching. Plots of poresize distribution of 100% YSZ and NiO/YSZcermet with ratio of 1 : 1 after reduction byH2 and leaching in nitric acid. Broken line
(- - -) is 100% YSZ cermet, and solid line( – ) is NiO/YSZ cermet with ratio of 1 : 1.Reproduced from Ref. [127] by permission ofJohn Wiley & Sons, Inc.
Recently, a general strategy described by Seshadri et al. [130] to leach out thesacrificial phase in solution (Figure 9.14a) resulted in a macroporous monolithof the desired phase. Thus far, several procedures for modifying the resultingmacroporous material have also been described. The first case shows a metal withthe macropore structure being retained after reduction of a macroporous oxide(Figure 9.14b). The second case shows the macropore walls with a conformalcoating of a second phase by a process of reactive dip coating (Figure 9.14c).The third case shows the formation of hierarchically porous monoliths with ameso-macroporous structure by subsequent leaching of a sacrificial element fromthe macropore walls (Figure 9.14d).
To prepare macroporous monoliths with hierarchical structure by selectiveleaching, there are some typical methods or reactivities. First, metatheses (AB +CD → AC + BD) are amenable to selective leaching, because the soluble saltobtained as a second phase (such as the sulfate) is water soluble. Here, AC could bethe desired phase and BD could be the sacrificial phase that is removed by leaching.The resulting material could be rendered macroporous. PbZrO3 composites withother inorganic materials, as well as PbZrO3 particles, could be obtained bymetathetic reactions in the solid state [131]. A variant of solid-state metathesis(so-called ‘‘assisted metathesis’’) is used to prepare the very important perovskiteoxides PbTiO3 and La1−xSrxMnO3(x = 0.0 and 0.3) at reduced temperatures. Inaddition, Kaner et al. have also made the technique particularly well known bycombining metathesis with self-propagation, forming powders ranging from ZrNand GaN to MoS2 [132, 133]. Concurrently with these work, the use of metatheticroutes to produce a soluble salt as a second phase has been wildly developed tosynthesize macroporous perovskites oxides and LaMnO3 [134, 135].
Next, a variety of subsequent reactions have been carried out to hierarchicalstructure using liquid- or gas-phase reactants, while a porous monolith formed. For
286 9 Feature Synthesis of Hierarchically Porous Materials Based on Green Easy-Leaching Concept
(a)
(b)
(c)
(d)
Figure 9.14 Scheme showing (a) formation of a macroporous ceramic through selectiveleaching of a two-phase composite. The resulting material may then be (b) turned into aporous metal, (c) decorated with a conformal coating, or (d) rendered hierarchically porousthrough vapor-phase leaching. Reproduced from Ref. [130] by permission of the Royal Soci-ety of Chemistry.
example, reactive dip coating can be used to form conformal coatings of a secondphase along the inner pore walls of NiO [121]. Gas-phase reactions can also be usedto alter the chemistry of the porous material. A flowing 5% H2/N2 atmosphere hasbeen used to reduce porous monoliths of NiO and ZnFe2O4 to their metallic state[121]. The pore structure was maintained, and the faceted nature of the originalNiO structure smoothed out. Porous oxides appear to fracture at grain boundaries(Figure 9.15).
In the previous cases, a significant volume loss occurred on reduction of themetal, resulting in smaller pores penetrating into the pore walls, although themesopores are rather transient [136]. On the basis of this phenomenon, Seshadriet al. have developed a novel strategy for forming hierarchically porous materialsusing volume loss and involving temperatures at which the pores that form arenot rapidly closed. They focused on synthesis of oxides, since oxides do not sinteras rapidly. In the reduction of Mn3O4 to MnO, the volume loss is significant,and the process occurs at reasonably low temperatures in a flowing 5% H2/N2
atmosphere. Minimal densification of pellets of macroporous Mn3O4 is formed bysimply pressing and firing [137], which have some grain connectivity, but occurred(Figure 9.16a). Mn3O4 pellets reduced to MnO maintain the macropore networkand grain connectivity (Figure 9.16b). Additionally, the volume loss inherent inthe phase change results in mesopores penetrating into the macropore walls
Figure 9.15 Scheme describing the preparation of macroporous monoliths. A single-phasemetal–organic precursor is formed in solution (a). The precipitated precursor is decom-posed in air, and the resulting powder is pressed into a pellet and sintered in air (b). Theresulting oxide composite is leached in base to form a macroporous oxide (c). The oxidecan be reduced in hydrogen to form a porous metal (d) or may be subjected to reactive dipcoating (e). Reproduced from Ref. [120] by permission of the American Chemical Society.
(Figure 9.16c). The macropores are 1 mm in diameter, and the mesopores are50 nm across. The resulting pores are rectangular and aligned across a grain,suggesting that crystallographic relations control pore orientation (Figure 9.16d).
Interestingly, reoxidation of hierarchically porous MnO to Mn2O3 closes themesopores without altering the macropore network. Reduction of Mn2O3 back toMnO reforms the mesopores in the macropore walls. As the pore-forming processrelies solely on strain due to a phase change with corresponding oxygen loss,the process may be cycled. After several redox cycles, the MnO sample developspores on two different length scales. Some broadening of the mesopore sizedistribution is observed. This is a good example of morphological regenerationof a porous material [137]. Such mesopore regeneration may prove to be usefulin high-temperature applications. In a way similar to the leaching of Zn fromZn0.3Fe0.7 and ZnO from a dense composite of ZnO and ZnMn2O4, macroporousZnMn2O4 can be obtained. Then, reduction to rock salt and vapor-phase leaching ofthe zinc in a flowing 5% H2/N2 atmosphere produces mesopores in the macroporewalls. The resulting hierarchically porous monolith of MnO is composed of aligned
288 9 Feature Synthesis of Hierarchically Porous Materials Based on Green Easy-Leaching Concept
(a) (b)
(c) (d)
1 μm 1 μm
200 nm 100 nm
Figure 9.16 (a) SEM image of macroporous Mn3O4. (b) On reduction in a 5% H2/N2
atmosphere to MnO, the macropores are maintained and an additional level of porosity hasbeen induced. The mesoporous fracture surface on the right side of image (c) is shown athigher magnification in micrograph (d). Reproduced from Ref. [137] by permission of theAmerican Chemical Society.
rectangular pores. The pore morphology is similar to that obtained from thereduction of Mn3O4, with an average diameter of 50 nm.
It is important to note that the hierarchically porous structures are composedof macropore walls that are made of mesoporous single crystals. A hierarchicallyporous pellet of MnO was backfilled with epoxy and polished down, and a lamellawas formed with a focused ion beam (Figure 9.17) [138]. The contrast variationsacross each grain are caused by density variations due to the pores. Using aselected area aperture and aligning on a zone axis, it was possible to determinethat the samples were composed of crystalline grains 300–1000 nm in diameter.From this, it can be concluded that pore morphology is correlated with pore wallcrystallography and pore walls are made up of {100} crystal faces, which are thelowest energy faces of the rock salt lattice. These porous crystals are reminiscentof those found in sea urchin spines and in their synthetic replicas. Porous singlecrystals are extremely unusual and show potential for improved performanceover their polycrystalline analogs because of the absence of grain boundaries. Forhigh-temperature applications, porous single crystals are significantly more stable
Figure 9.17 Scheme showing the formation of hierarchically porous monoliths. Startingfrom a dense two-phase composite (a) of ZnO and ZnMn2O4, alkali leaching removes theZnO phase, leaving macroporous ZnMn2O4 (b). Vapor-phase leaching of Zn in flowing hy-drogen forms mesopores in the macropore walls of (b), giving a hierarchically porous mate-rial (c). (d) TEM micrograph of MnO/epoxy lamella formed with a focused ion beam. Whenseen at higher magnification, (e) the lamella has density variations due to the presence ofpores. The region on the left has been tilted into the [100] zone axis (inset). Reproducedfrom Ref. [138] by permission of the American Chemical Society.
because of lowered surface energies and the elimination of grain boundaries asthe primary means of mass transport. Electrical transport is improved becauseof the absence of grain boundary scattering. In nature, sea urchins form spinesof porous calcite single crystals [139]. Prior synthetic efforts to form poroussingle crystals have either used controlled nucleation within a porous template[140, 141] or etching of a bulk single crystal [142]. In contrast, pores can beinduced in crystalline grains through controlled phase changes that result in avolume loss.
In this process, the selective-leaching processes can be controlled to give rise tounusual hierarchically porous architectures. The resulting porous monoliths haveled to conformal coatings, porous metals, and hierarchically porous structures. Itis worthy to note that the formation of mesoporous single crystals through phase
290 9 Feature Synthesis of Hierarchically Porous Materials Based on Green Easy-Leaching Concept
changes and vapor-phase leaching of a sacrificial element is extremely intriguing.As these approaches are quite general, it is desired that this easy-leaching processinspires further investigations into the formation of porous materials throughselective-leaching methods.
9.5Other Easy-Leaching Concepts in the Synthesis of Hierarchically Structured PorousMaterials
Here we show other easy-leaching concepts, such as salt, gas produced by chemicalreaction, and chemical erodibility (or chemical etching). These methods partlycontain the easy-leaching concepts, although all the templates used are not greeneasy-leaching.
These materials have been synthesized by a multiphase process of acid-catalyzedsilica sol–gel chemistry in the presence of inorganic salts and self-assemblingblock copolymers [4]. Inorganic salts play an important role in the formation ofthe meso–macro silica structures that are grown at the interface of inorganicsalt solution droplets. The meso–macrostructured silica network can be varied(Figure 9.18), depending on the electrolyte strength of the inorganic salts and theamphiphilic block copolymer structure-directing agents. The macropore dimen-sions are established by the sizes of the salt solution droplets, such as NaCl, LiCl,KCl, NH4Cl, or NiSO4, which can be adjusted by regulating the evaporation rateof the solvent. At the interstices separating the electrolyte droplets, amphiphilicblock copolymer species assemble in the presence of silica to form well-orderedcomposite mesophases. The morphology of the silica membrane can be modifiedby changing the concentration of the inorganic salt, although inorganic salt crystalswere inevitably co-grown with the silica membrane.
9.5.2Biomodal Mesoporous Silicas by Dilute Electrolytes
By adding dilute electrolytes in the gel mixture, nonionically templated [Si]-MSU-Xmesoporous silicas with bimodal pore systems in the pore size range 3.0–9.0 nmwere synthesized [143]. The results obtained indicate that electrolytes can ex-ert considerable influence over all of the template and silica assembly andcondensation processes. The presence of two different mesophases (lamellarand hexagonal) in the network is the origin of the two pore-size distributions(Figure 9.19). It is hypothesized that weakly ionic templating systems mightprovide insights into the interactive roles between weakly ionic biological fluids
9.5 Other Easy-Leaching Concepts in the Synthesis of Hierarchically Structured Porous Materials 291
(a) (b)
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Figure 9.18 (a and b) Scanning electron microscopic (SEM) images at different magnifi-cations of as-synthesized meso-macrostructured silica membranes prepared by using P123(EO20PO70EO20) block copolymer species in NaCl solution after washing with deionizedwater. (c) SEM image showing smaller macropores as compared to (a) in a silica mem-brane prepared with a small amount of ethylene glycol under otherwise identical conditions.SEM images of (d) an acicular NaCl single crystal and (e) inorganic salt NaCl crystalsco-grown with the silica membrane. The SEM images were obtained on a JEOL 6300-F mi-croscope. Transmission electron micrographs (TEM) of (f) the mesostructured silica strutsin the calcined silica membrane of (a) prepared using the block copolymer P123 in NaClsolution and (g), calcined silica membrane of (c) prepared with a small amount of ethyleneglycol. The TEM images were acquired on a 2000 JEOL electron microscope operating at200 kV. For the TEM measurements, the samples were prepared by dispersing the powderedproducts as a slurry in acetone, after which they were deposited and dried on a holey car-bon film on a Cu grid. Reproduced from Ref. [4] by permission of the American ChemicalSociety.
and hydrogen-bonding templates such as polysaccharides in biomineralizationsystems. Notably, cations exert structure-directing effects on a proposed flexiblePEO/water/silicate ternary complex, leading to modified micelle packing and sub-sequently modified pore symmetries of materials formed in neutral solutions.Anions, on the other hand, modify the rate and extent of TEOS hydrolysis andcondensation through the formation of five-coordinate intermediates of varying
292 9 Feature Synthesis of Hierarchically Porous Materials Based on Green Easy-Leaching Concept
(a)
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82 nm
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Figure 9.19 TEM images of hydrother-mally treated and calcined Si-MSU-Xmesoporous silicas prepared from di-lute electrolyte solutions: (a) NaClsolution, (b) NH4Cl solution, and(c) HF solution. Reproduced fromRef. [143] by permission of the RoyalSociety of Chemistry.
9.5 Other Easy-Leaching Concepts in the Synthesis of Hierarchically Structured Porous Materials 293
strengths. Fluoride is a special case in that it causes not only greatly increased
TEOS condensation and smaller particle size of spherical morphology but also
increased pore diameters. Thus, by adding dilute electrolytes, the sol–gel pro-
cesses can be modified and controlled to realize the synthesis of bimodal porous
silicas.
9.5.3Hierarchical Bioactive Porous Silica Gels by Gas Templating
Mann et al. have reported a gas-templating route to a new class of bioactive silica
gels with hierarchical pores [144]. This gas-template is totally different from the
above-mentioned bubbling process, which produces air–liquid foam [58–60]. The
gas (CO2 bubbles) is generated by in situ decomposition of a mixture of sodium
bicarbonate and sodium hydrogen pyrophosphate, which results in the formation
of the large macropores while the reaction is carried out within the silica bioglass
during gelation (Figure 9.20). Finely divided calcium carbonate is also included in
the reaction mixture as a space filler, which is removed after gelation by acid washing
to produce additional pores with dimensions usually less than a micrometer. A
closed approach has also been reported for the preparation of osseous calcium
phosphate cements [145]. The method consists in adding NaHCO3 to the starting
cement powder (Biocement D) and using two different liquids, first, a basic liquid
to form the paste and later, an acid liquid to obtain CO2 bubbles, which finally
create macropores in calcium phosphate cements. This strategy extends to both
colloidal and sol–gel methods of formation of silica gel (or organofunctionalized
silica gels) with high porosity. The porous silica gels can be strengthened by organic
polymerization to induce the nucleation of calcium phosphate from simulated body
fluid and to act as slow-release vectors for the anti-inflammatory drug ibuprofen.
(a) (b)
Figure 9.20 Optical photographs showing highly porous silica monoliths: (a)low-magnification image of shaped bioglass and (b) higher magnification image showingmacroporous texture. Scale bar in (b) is 0.5 cm. Reproduced from Ref. [144] by permissionof the Royal Society of Chemistry.
294 9 Feature Synthesis of Hierarchically Porous Materials Based on Green Easy-Leaching Concept
9.5.4Hierarchically Porous Materials by Chemical Etching
Chemical etching has also been successfully exploited in the preparation ofhierarchically porous materials [11]. For example, mesoporous silica was primarilytreated in a solution of NH4OH; this chemical etching process resulted in theformation of bimodal mesoporous materials (Figure 9.21). The pore structurecharacteristics were affected by the treatment temperature and time. Ammoniashould play an important role in such a pore expansion process. Owing to theirvolatility, ammonia molecules penetrate inside the nanochannels more easily thanwater, and thus the swelled channels lead to pore size expansion from 2.3 to about4 nm. Since the thermal stability of surfactant CPCl is low (melting point of 86 ◦C),part of the surfactant species in the mesochannels may start to decompose afterhydrothermal treatment for three days. Some neutral solubilizing species, such aspyridium, may be formed, which may result in degradation of part of the channels.Also, since some surfactant molecules would be leached out during hydrothermaltreatment, the silica walls would tend to collapse inward, making the mesochannelsinterconnected. The porosity of these two types of mesopores can be controlled byvarying the treatment time as well as the concentration of ammonia.
9.5.5Hierarchically Porous Materials by Sublimation
Liu et al. demonstrated a sublimation method (an alternative selective-leachingmethod without chemical etching or reduction) to prepare highly porous
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Figure 9.21 (a) Nitrogen adsorption–desorption isotherms of the hydrotreated andcalcined sample and its pore size distribu-tion plot (inset). (b) Typical TEM image ofhydrotreated samples showing the existence
of double-mesopores. A model of pore sys-tems is schematically shown in the insert.Reproduced from Ref. [11] by permission ofthe Royal Society of Chemistry.
9.5 Other Easy-Leaching Concepts in the Synthesis of Hierarchically Structured Porous Materials 295
(a) (b)
(c) (d)
5.0 μm 5.0 μm
5.0 μm 5.0 μm
Figure 9.22 SEM images of pellet prepared with 40 vol% SnO2 – 60 vol% CeO2
nano-powders made by combustion CVD from a single precursor solution. (a) Surfaceand (b) cross-sectional views of pellet sintered at 1450 ◦C × 5 h. (c) Surface and (d)cross-sectional views of pellet after reduction at 727 ◦C × 2 h. Reproduced from Ref. [146]by permission of John Wiley & Sons, Inc.
CeO2 (Figure 9.22) [146]. After preparation of intimately distributed 40 vol%SnO2 – 60 vol% CeO2 composite nanopowder using combustion CVD, the SnO2
phase can be removed by sublimation during the firing process, since the meltingpoint of SnO2 (1630 ◦C) is near the typical firing temperatures of CeO2 (usually1350–1500 ◦C). Furthermore, the high sublimation temperature of SnO2 willprevent distortion or collapse of the host material during firing, which is apotential problem when low-temperature ‘‘pore formers’’ are used. This methodprovides a simple way to introduce additional porosity into ceramic materials andcan be directly incorporated into ceramic production routes without introducingadditional procedures, although the sintering temperate is very high.
296 9 Feature Synthesis of Hierarchically Porous Materials Based on Green Easy-Leaching Concept
9.6Summary
The green synthesis of hierarchical structures is focused on easy-leaching templatetechniques. The green advantage is that it is greatly decreases waste, cost, andenvironment concern of templates. Applications of these methods to the synthesisof hierarchically porous materials have been extensively discussed. It is evidentthat, with all the easy-leaching template techniques, novel modifications andimprovements that render these methods more efficient and less environmentallyharmful are constantly being reported in the literature. One can only hope for thewidespread dissemination as well as for the universal adoption of this growing andincreasingly significant trend in synthetic technology of hierarchically structuredmaterials.
Acknowledgments
This work was realized in the framework INANOMAT (Interuniversity AttractionPole IAP-P6/17), a Belgian federal government project, and ‘‘Redugaz,’’ an InterregIV (France-Wallonia) project funded by the European Union and the WalloonCommunity. B.L. Su acknowledges the Chinese Central Government for an ‘‘Expertof the State’’ position in the program of ‘‘Thousands Talents’’ and the ChineseMinistry of Education for a ‘‘Changjiang Scholar’’ position at the Wuhan Universityof Technology. X.Y. Yang thanks the Fonds National de la Recherche Scientifique(FNRS) for their Charge de Recherches positions.
