Invited Feature Article pubs.acs.org/Langmuir
Maintaining the Structure of Templated Porous Materials for Reactive and High-Temperature Applications Stephen G. Rudisill, Zhiyong Wang, and Andreas Stein* Department of Chemistry, University of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 55455, United States S Supporting Information *
ABSTRACT: Nanoporous and nanostructured materials are becoming increasingly important for advanced applications involving, for example, bioactive materials, catalytic materials, energy storage and conversion materials, photonic crystals, membranes, and more. As such, they are exposed to a variety of harsh environments and often experience detrimental morphological changes as a result. This article highlights material limitations and recent advances in porous materialsthree-dimensionally ordered macroporous (3DOM) materials in particularunder reactive or high-temperature conditions. Examples include systems where morphological changes are desired and systems that require an increased retention of structure, surface area, and overall material integrity during synthesis and processing. Structural modifications, changes in composition, and alternate synthesis routes are explored and discussed. Improvements in thermal or structural stability have been achieved by the isolation of nanoparticles in porous structures through spatial separation, by confinement in a more thermally stable host, by the application of a protective surface or an adhesive interlayer, by alloy or solid solution formation, and by doping to induce solute drag.
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INTRODUCTION Over the last two decades, much progress has been made in synthesizing functional materials with structural features on multiple length scales ranging from the macroscopic to the nanometer domain. Structural hierarchy is a distinguishing feature in many of these materials. For instance, in the realm of porous materials, it is now possible to synthesize materials that contain well-defined micropores for the selective uptake of guest molecules, coexisting with mesopores to enhance interfacial contact and macropores to facilitate rapid mass transport. Structures with bicontinuous or even tricontinuous phases can be fabricated that allow, for example, electron transport through one phase and ion transport through another phase. Such nanoporous architectures have been demonstrated to provide benefits for a variety of technological applications. From filtration to catalysis, electrochemistry to insulation, and energy storage to conversion, materials with nanoscale porosity are becoming increasingly desirable for making industrial and consumer processes and products cleaner and more efficient. As fabrication techniques continue to improve and control over material architectures reaches smaller dimensions, new challenges arise. In many of the applications mentioned above, the materials are exposed to reactive environments or high temperatures that, in some cases, approach or exceed 1000 °C. These conditions can create problems for generating and maintaining structural features and thus the enhanced activity derived from these features. As the length scales of solid components in these porous materials are decreased, in pursuit of higher surface areas or more specific activity, it becomes more challenging to maintain the structure under extreme conditions. © XXXX American Chemical Society
This review provides examples of some of the challenges that are encountered for templated, nanostructured materials under reactive or high-temperature conditions and approaches that can be used to address these issues. High temperature in the context of this review refers to temperatures greater than ca. 550 °C, at which point an organic template is typically eliminated in air. We will focus largely on systems studied in our own research group, particularly those involving colloidal crystal and surfactant templating, but the general concepts also apply to nano- and microstructured materials fabricated by other methods. A particularly useful platform involves threedimensionally ordered macroporous (3DOM) and macroporous/mesoporous (3DOM/m) materials, which can possess structural features on multiple length scales. Therefore, they provide excellent models for examining structural evolution during synthesis, processing, and active use simultaneously on different length scales. We will briefly describe the methods of pore templating in these materials and examine the resulting structures as well as their mechanical stability. We will then discuss examples where changes in structure or composition are desirable when the material is exposed to reactive environments, followed by numerous systems where structural changes need to be avoided. We will highlight some of the approaches that have been used to mitigate the effects of chemically reactive or high-temperature environments on the morphology of the material. The review will conclude with a brief discussion of as-yet unsolved issues related to the stability of nanostructured Received: February 3, 2012 Revised: March 9, 2012
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materials. The Supporting Information provides additional helpful references.
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Invited Feature Article
STRUCTURAL ELEMENTS OF 3DOM MATERIALS
On the basis of the geometry of the parent structure, the inverse opal structure consists of 74% void space. For 3DOM materials prepared by solution or sol−gel precursors, the actual void space is typically higher because the precursor filling the interstitial space in the colloidal crystal template may be diluted and the skeletal walls shrink because of condensation and/or crystallization during pyrolysis. As a result, walls are often thinner than in a true inverse opal. The skeletal structure consists of a series of nodes consisting of two shapesoctapodal and tetrapodalthat are connected by curved struts that taper to their thinnest dimension at the midpoint. These nodes result from the octahedral (Oh) and tetrahedral (Td) holes in the parent fcc lattice, and the struts are templated by trigonal holes present at each intersection between the Oh and Td holes. For template spheres in the 300−500 nm range, the resulting nodes have dimensions on the order of 100 nm. The connecting struts are tens of nanometers thick. The pores of the material are of the same order of magnitude as the original template, though there is some shrinkage of the pores. Linear shrinkage up to about 30% occurs during calcination, when polymer spheres are used for templating, whereas silica sphere templates limit shrinkage to ca. 10%.4 The morphology of the walls varies depending on the synthesis method. Depending on the interactions between the surfaces of the template and the infiltrating precursor, the material will surface template or volume template, leading to two distinct morphologies (Figure 1). Surface-templated
PORE TEMPLATING Porous materials can be found with pore diameters on many length scales, from angstroms to millimeters, in a variety of inorganic, organic, and hybrid compositions. The length scales of interest for this review are the microporous ( 3DOM/m C > 3DOM/m C/C > 3DOM C as the resistance of the material toward deformation increased. Filling mesopores with a secondary carbon phase enhanced this resistance toward deformation, but not to the extent of nonporous macropore walls with a continuous carbon skeleton. Through optimization of the synthesis of 3DOM/m C, the mechanical stability could be improved further. A 3DOM carbon sample with cubic mesopores around the macropores showed even greater resistance to indentation than 3DOM C lacking templated mesopores in the walls.11 The ability to fracture hierarchically porous 3DOM materials has been exploited for the fabrication of uniformly sized anisotropic nanoparticles. Recalling the earlier discussion, a facecentered-cubic colloidal crystal contains Td and Oh holes. In a typical synthesis of 3DOM materials, the component occupying the Td and Oh holes is interconnected through struts resulting from trigonal holes. However, under certain conditions, especially if the bridges are weakened by additional mesopores, the structure disassembles into its individual building blocks the tetrapodal and octapodal nodes mentioned above. This can occur directly through the internal stresses that develop during calcination as the material condenses and shrinks.12 Alternatively, external forces such as mechanical stirring can break apart the 3DOM skeleton to produce tetrapods and cubes (with negative curvature on each face) as replicas of the Td and Oh holes.13 Under calcination conditions, these are only kinetically stabilized and eventually transform to smaller spheres and rounded cubes (with positive curvature), respectively, each with a narrow size distribution (Figure 3). This powerful method of forming shaped particles has been applied to materials with various compositions, including silica, carbon, and a number of transition-metal oxides/phosphates. More details are provided in a recent review.14
SiCN, and W, larger monolithic pieces with dimensions of several millimeters have been prepared.5−8 These have varying degrees of mechanical stability. Whereas 3DOM SiO2 is quite brittle, 3DOM C forms relatively robust objects that can be readily handled and used, for example, as self-supporting porous electrodes. The mechanical stability of 3DOM SiO2 and 3DOM C materials has been investigated by nanoindentation methods.9,10 For 3DOM SiO2, an indentation deformation mechanism was proposed that included a small amount of elastic deformation by cell-wall bending, followed by cell-wall fracture, pore collapse, localized densification, and compaction. An interesting observation from these studies was that pores adjacent to the contact impression from the indentation tip remained undeformed, implying that there was no strain transfer to material adjacent to the contact area with the trigonal pyramidal tip (Figure 2). The modulus/hardness ratio of
Figure 2. SEM image showing the residual indentation impression from a trigonal pyramidal tip at a peak load of 1 mN in 3DOM SiO2. Inelastic deformation occurred only in the region directly contacted by the tip. No radial cracks extended from the vertices of the imprint. Such cracks were observed in the case of fused silica. The schematic diagram on the bottom illustrates the action of the tip on the supported 3DOM SiO2 film. Reprinted with permission from ref 9.
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DESIRED MORPHOLOGICAL CHANGES: BIOACTIVE GLASSES For some applications, the increased reactivity that accompanies a high-surface-area material with nanometer-sized structural components is desirable. An example in the context of 3DOM materials relates to bioactive glasses for bone repair. Bioactive glasses consisting of typical compositions CaO−SiO2, CaO−P2O5−SiO2, and CaO−MgO−P2O5−SiO2, form biologically compatible hydroxycarbonate apatite (HCA) layers at the interface with bone in vivo and bond strongly to the bone.15 Similarly, HCA can be formed in vitro when the material is
3DOM SiO2 was significantly greater than that of dense silica.9 Hierarchically porous 3DOM/m SiO2 was compared with three types of 3DOM carbon {macropores only (3DOM C), mesoporous walls surrounding macropores (3DOM/m C), and the same mesopores filled with a graphitic phase (3DOM/m C/C)}. Porous carbon materials like these are of interest as porous electrodes, catalyst supports, and host structures for nanoparticle growth. The response of these porous carbon
Figure 3. Shaped nanoparticles prepared by the in situ disassembly of inverse opal skeletons. The schematic on the left illustrates the origin of tetrapodal/small spherical and cubic nanoparticles that replicate tetrahedral and octahedral holes in the face-centered-cubic colloidal crystal template. The TEM images in the middle show examples of a silica tetrapod and two cubes. The SEM image on the right shows a mixture of cubes and small spheres obtained by the disassembly of a 3DOM silica skeleton. Reprinted with permission from ref 12. C
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Figure 4. SEM images and schematic diagram illustrating the morphological changes that occur in a 3DOM CaO·4SiO2 bioactive glass during immersion in simulated body fluid (SBF). (Counterclockwise from top left) SEM image of the 3DOM CaO·4SiO2 bioglass calcined at 600 °C. The image shows the interconnected macropore structure that is fully accessible to SBF. During immersion in SBF, silica partially dissolves from the pore walls and repolymerizes on the surface together with Ca2+ and PO43− ions from the SBF. The SEM image of the sample after immersion in SBF for 3 h shows that at this point the macropore skeleton is decorated with small nodules of an amorphous calcium phosphate/hydroxide/carbonate phase. With a longer immersion time in SBF, the surface deposit starts to crystallize, forming hydroxy carbonate apatite (HCA). As HCA continues to crystallize, it forms an opal-like intermediate structure, and after 4 days in SBF, large polycrystalline aggregates of HCA have replaced the 3DOM bioglass structure. Adapted with permission from ref 17.
structuring techniques to convert a material with a desirable architecture to a similarly structured material with a different composition. These methods are especially useful for structuring micro- and nanomaterials with complex 3D architectures that may be readily formed in one composition (e.g., silica) but are more difficult to form in another composition.21 In practice, there may be limitations for the minimum dimensions and features that are faithfully reproduced, depending on the reaction conditions. In addition, pseudomorphic processes can allow access to compositions that cannot be directly synthesized as a nanostructured material. Because of its hierarchical architecture, the 3DOM structure is well suited to exploring systematically possible limits to the range of dimensions to which shape-preserving transformations are applicable. Such effects were studied for a transformation of 3DOM silica to titania.21 The starting material contained smooth, ca. 60-nm-thick walls of amorphous silica surrounding a periodic array of interconnected macropores with a spacing of ca. 330 nm (Figure 5a). Because of its periodicity on the length scale of visible light, the material appeared to be opalescent. The first transformation step involved a reaction of the porous solid with gaseous TiF4. At a temperature of 235 °C when TiF4 has a vapor pressure of ca. 13 kPa, the reaction was pseudomorphic on both the bulk scale and the scale of the macropores. The product remained opalescent, indicating that a periodic repeat structure was still present, as confirmed by SEM analysis (Figure 5b). However, at the length scale of the pore walls, significant changes in morphology were observed. Rather than TiO2, a TiOF2 phase had formed throughout ca. twothirds of the sample. Its cubic unit cell translated into interconnected cubic particles with edge lengths of 133 nm. The complete transformation to TiO2 (anatase) required another solid−gas reaction with moist air at 300 °C. In this step, pseudomorphism was observed on the scale of tens of micrometers, on the submicrometer macropore scale, and on the
soaked in simulated body fluid, an aqueous solution with the ion concentrations and pH of typical human blood plasma.16 The benefits of a reactive bioglass with accessible pore structure were demonstrated with 3DOM bioactive glasses, which exhibited significantly faster dissolution and more rapid growth of HCA in simulated body fluids compared to nontemplated control samples.17,18 As illustrated in Figure 4, during immersion in simulated body fluid, the relatively smooth, periodic skeletal surface is first covered with small spheres consisting of amorphous calcium phosphate, which are converted to flakelike HCA nanocrystallites that eventually grow to spherical aggregates that are several micrometers in diameter. The importance of the internal surface area was demonstrated by comparing 3DOM bioactive glasses with varying pore diameters: a decrease in macropore size accelerated the glass degradation and apatite formation processes.18 The thinner walls that accompany smaller macropores in the 3DOM structures would also have contributed to the acceleration effects. Thus, these 3DOM architectures may provide a useful tool for tuning the bone growth rate for optimal structural development.
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DESIRED MORPHOLOGICAL CHANGES: PSEUDOMORPHIC TRANSFORMATIONS Techniques have been developed that allow the pseudomorphic transformation of one material composition to another while preserving the shape of the original material. According to the Oxford dictionary, a pseudomorph is a crystal consisting of one mineral but having the form of another. In the literature, the definition of a pseudomorphic transformation has been broadened to include shape-preserving displacement reactions,19 whether or not the starting structure is crystalline. Ion exchange reactions and topotactic transformations fall within the broader scope of this definition, as do silicification and calcification reactions found abundantly in nature.20 In principle, pseudomorphic transformations can be powerful D
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Figure 5. Schematic overview and SEM images of the conversion steps involved in transforming (a) 3DOM SiO2 into (b) 3DOM TiOF2 via TiF4 vapor and then into (c) 3DOM TiO2 by steam treating the material. (d) This SEM image shows evidence of a nanoscale Kirkendall effect in the formation of TiO2 nanocubes by the pseudomorphic solid−gas transformation. Many of the cubes appear to be hollow. Some of these are outlined in the image by dashed circles. Reprinted with permission from ref 21.
of silicon alkoxides in solutions of water, alcohol, and ammonia27,28). For example, when preformed silica spheres with diameters in the range from ca. 5−50 μm are exposed to an alkaline solution containing alkyl ammonium surfactants for periods of minutes to a few hours and are then treated hydrothermally, the product contains particles with the same spherical outward appearance and particle size distribution. However, the spheres have undergone an internal reorganization process and now contain mesopores with the typical character of the mesoporous sieve, MCM-41 (hexagonal mesopore arrays). The proposed mechanism for this transformation involves the nucleation of MCM-41 at the surfaces of primary nanoparticles by progressive dissolution and reprecipitation with the incorporation of the available surfactant. This is a solution-mediated mechanism, not a true solid-state transformation, but mass transport occurs over very short distances in the immediate vicinity of the silica surface.27 The extent of silica condensation in the MCM-41 spheres is sufficiently low that a phase transition to MCM-48 with a cubic mesopore structure can occur.29 The MCM-48 phase is itself metastable toward a lamellar mesostructure upon extended synthesis times.29 The transformation depends on the degree of condensation of the parent spheres, which varies with the solvent used in the Stöber synthesis of the spheres.30 Parent spheres prepared in ethanol exhibit a higher degree of condensation that limits sphere dissolution, allowing for a largely pseudomorphic transformation. The resulting mesoporous spheres are porous throughout. However, when parent spheres are synthesized in isopropanol or butanol, they are more reactive and release more silica into solution. Porous shells with large, corrugated mesopores are then formed on the surfaces of the remaining spheres, leaving the cores solid.30 The mesoporous spheres have been demonstrated to be good candidates for stationary phases in HPLC.31
scale of the cubic particles (Figure 5c). The latter maintained the shape dictated by the cubic TiOF2 preform, even though anatase itself is noncubic. The conversion steps are controlled by both thermodynamic and kinetic parameters. The product structure depended critically on reaction temperatures and times, giving only narrow windows for successful pseudomorphic transformations. At 190 °C, no conversion took place, and above 300 °C (i.e., above the sublimation temperature of TiF4), the material was converted mostly to crystalline TiOF2 with an irregular structure. At the reaction temperature of 235 °C, hollow cubes were observed, which may be evidence of a nanoscale Kirkendall effect (Figure 5d).22 This effect is normally observed in metal transformations, resulting from different diffusion rates for the metal atoms and leading to void structures.23 Here it is a pathway toward hollow titania particles that may be of interest as photocatalytic host structures. The outcome of these transformation reactions depends strongly on the particular compositions and geometries. For silica-based systems, for example, when preforms with larger dimensions were employed (e.g., silica spheres with diameters near 600 nm), the behavior depended on the annealing history of the spheres.21 Unannealed spheres were completely lost during the transformation. Annealed spheres formed cubes on the outside but maintained an amorphous silica core. These data show that the degree of silica condensation and the object dimensions both have a significant influence on solid−gas transformation reactions. Composition is also important. A similar transformation of 3DOM tungsten oxide to tungsten carbide in a methane/hydrogen atmosphere maintained the original morphology with only a slight increase in the surface roughness of the macropore walls.24 A magnesiothermic reduction of silica diatom frustules with magnesium vapor produced silicon replicas in which features with submicrometer sizes were reproduced.25 However, a similar reaction starting with mesoporous silica (10 nm pores) produced a macroporous silicon product (ca. 200 nm pores), suggesting that in this system the lower limit for faithful reproductions is also above the mesopore length scale.26 Pseudomorphic transformations have also been applied to solution-based processes. The most common application aims at introducing pores into nonporous, amorphous silica spheres prepared by Stöber syntheses (i.e., a process that generates monodisperse silica spheres by the hydrolysis and condensation
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DESIRED MORPHOLOGICAL CHANGES: DESILICATION OF ZEOLITES The ability to maintain the external particle shape and the overall crystallinity of a material is also important during the generation of mesoporous zeolites by desilication.32,33 Desilication occurs when aluminosilicate zeolites are treated in alkaline solutions at elevated temperatures. When this is done, for example, using the commercially important zeolite ZSM-5, mesopores can be introduced in a controlled way. E
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Figure 6. (Left) Schematic illustration of the influence of the Si/Al ratio on the desilication treatment of ZSM-5 zeolites in alkaline solution and the associated mechanism of pore formation. Reprinted with permission from ref 32. (Right) An SEM showing hollow ZSM-5 zeolite particles after the desilication treatment of ordinary ZSM-5 crystals. Reprinted with permission from ref 34.
The introduction of accessible mesopores into zeolite crystals enables more efficient utilization of the material by improving intracrystalline diffusion to and away from active sites. As opposed to dealumination processes, which alter the acidic properties of a zeolite, mesoporous zeolites synthesized by desilication maintain their acidic properties. The predisposition for a zeolite to lose Si in an alkaline environment is determined by the framework Al. Zeolites containing a large fraction of framework Al's are more inert to Si extraction than those with a lower Al content, with the optimal molar Si/Al ratios being in the range from 25 to 50.32 This effect is illustrated in Figure 6. When an aluminum gradient is present in a zeolite crystal, the mesopore structure can vary accordingly through the crystal.33 Commonly, the Al content is higher near the skin of a crystal. Preferential Si extraction of the core region with a lower Al concentration therefore produces hollow particles (Figure 6, right).33,34 Treatment times and temperatures also influence the mesopore development. Importantly, under optimized conditions, the crystallinity of the zeolite is well preserved. ZSM-5 zeolite crystals with a dual mesopore structure were generated by exposing the parent zeolite particles to reaction conditions that promote both desilication and the surfactantinduced reassembly of dissolved silicate/aluminosilicate species around micelles.34 The latter process yields ca. 3 nm mesopores, and the former produces 10−30 nm mesopores under alkaline reaction conditions. Although the surface area of the zeolite products is greatly enhanced, their crystallinity is well preserved.
nanostructured materials, which depend on nanoscale features in order to retain efficacy, the focus is on limiting the extent of sintering. Sintering is a general term comprising a number of different material transport processes that occur when materials are exposed to temperatures at which atom diffusion becomes significant. In the context of this review, grain growth is the most important of these processes. Grain growth is governed by the migration of grain boundariesthe interfaces between individual crystallites in a polycrystalline materialallowing for crystallites to grow into and eventually consume their neighboring crystallites.35 Extensive grain growth results in the distortion and eventually destruction of nano/microstructures. Other effects that need to be considered include the loss of volatile components when dealing with multicomponent systems, recrystallization effects, and reduced melting temperatures associated with nanostructured materials. Here again, the 3DOM architecture represents a unique structure for investigating sintering because its structural components on multiple length scales can be simultaneously exposed to the same thermal stresses. As previously described, the structure can be considered to be an array of nodes originating from the octahedral and tetrahedral holes of the parent fcc sphere lattice. These nodes are connected by curved walls, or struts, that taper to a thin section at the midpoint between two nodes. During sintering, areas with high concentrations of grain boundaries tend to grow faster, and larger grains tend to grow at the expense of smaller ones.36 This results in the nodes and struts behaving differently at high temperature and the larger polyhedral nodes, with more grain boundaries, tending to grow at the expense of the smaller columnar struts. These effects were studied in detail for the sintering of monolithic 3DOM α-alumina at temperatures of 1000−1350 °C.37 The structure first becomes considerably more open as the width of the walls decreases, leading to an increase in the size of the windows linking pores together (Figure 7). This is similar to what occurs in other less-ordered porous materials.38 As the temperature is further increased, the struts begin to shrink longitudinally as more material is absorbed by the nodes until the nodes are entirely in contact with one another. Order may still be retained at this point, but
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EFFECTS OF HIGH TEMPERATURES ON MORPHOLOGIES Micro- and nanoarchitectures undergo sintering and grain growth when exposed to high temperatures, and these processes can greatly alter the morphology of the material. Much of this change is predictable and has been well studied in the ceramic processing literature. However, the traditional focus of ceramicists has been largely on enhancing and exploiting sintering processes to yield materials with near-theoretical density in order to maximize the mechanical strength and thermal resistance or to eliminate permeability. In contrast, for F
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the windows and pores have shrunk considerably. Further heating after this point results in a loss of order as the nodes grow into one another, leaving a disordered porous network.
Figure 7. SEM images of macroporous α-Al2O3 calcined at 1150 °C for 6 h (left) and further calcined at 1300 °C for 4 h (right). Sintering causes the nodes to grow at the expense of the struts, which become even thinner. Reprinted with permission from ref 37.
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SOLAR THERMOCHEMICAL CYCLING WITH POROUS CeO2 CeO2 is a material that is currently under study for its ability to split H2O and CO2 at high temperatures. This is accomplished by thermally dissociating oxygen from the CeO2 lattice at a high temperature (>1000 °C), producing a nonstoichiometric oxide, CeO2−δ. CeO2 is then regenerated by flowing H2O or CO2 over the nonstoichiometric oxide at a lower temperature (100 m2/g. Each material was thermochemically cycled at 800 °C using H2 in the reduction step and H2O or CO2 in the fuel production step. After six cycles, the mesoporous ceria had been densified, resulting in a specific surface area of
