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Field-Directed and Confined Molecular Assembly of Mesostructured Materials: Basic Principles and New Opportunities† Jie Fan,‡ Shannon W. Boettcher, Chia-Kuang Tsung, Qihui Shi, Martin Schierhorn, and Galen D. Stucky* Department of Chemistry & Biochemistry, UniVersity of California at Santa Barbara, Santa Barbara, California 93106 ReceiVed August 16, 2007. ReVised Manuscript ReceiVed NoVember 13, 2007
Molecular assembly enables the formation of material systems with multiple compositions and functions that are structured at the mesoscale (2-50 nm) and beyond. This approach allows structure control through the competitive tuning of bulk and surface interactions to yield new mechanical, catalytic, optoelectronic, biological, and other properties. The molecular-assembly process is governed by the interactions between different components of the assembling system and with their external environment. This review summarizes the fundamental principles of molecular assembly in the synthesis of mesostructured inorganic–organic materials and focuses on recent attempts to utilize external fields (magnetic, electric, or mechanical) and dimensional confinement (in one, two, and three dimensions) to direct the molecular assembly of mesostructured organic–inorganic hybrids with astonishing complexity.
1. Introduction During the past century, chemists have primarily focused on making and breaking the strong covalent bonds formed by shared electrons between adjacent atoms. With this approach, it is possible to combine atoms into molecules and extended structures with nearly arbitrary atomic-scale configurations.1 Molecules of increased size and complexity require ever more demanding synthetic methods, and for many years, meso- or larger-scale designed configurations generated by molecular assembly had limited accessiblity. Traditionally, however, chemists have not viewed hydrogen bonds, van der Waals forces, and medium- to long-range electrostatic forces—all of which are much weaker than covalent bonds—as a chemical glue for assembling molecules into materials.2 This is in spite of the fact that nature is built on this approach: nearly all that surrounds us, from cells to trees, is knit together using weak interactions between molecules. The formation process for many of the mesostructured materials, whereby a collection of small molecules, electrolytes, polymers, and cosolvents spontaneously combine into larger, well-defined, supramolecular assemblies or aggregates due to these weak forces, has historically been termed by chemists and materials scientists as “self-assembly”, “cooperative assembly”, or “molecular assembly”. Here we will use the latter more generic term, with “molecular” being defined in the broadest sense and including, for example, solvent molecules, solvated inorganic ions, and inorganic oligomers as well as the usual organic molecules and polymers. †
Part of the “Templated Materials Special Issue”. Current address: Department of Chemistry, Zhejiang University, Hangzhou, Zhejiang Province 310027, China. * To whom correspondence should be addressed: e-mail
[email protected], phone 805-893-4872, fax 805-893-4120. ‡
Over the past two decades, researchers have made large advances in understanding the basic rules of molecular assembly as well as in developing methods to simultaneously control intermolecular interactions and reaction kinetics to create material systems with hierarchical ordering and complexity.3,4 Such methodologies, which make use of molecular assembly, have been recognized as the most promising approach for fabrication of a wide variety of nanoand mesostructured materials.1,3,5 Mesostructured inorganic–organic hybrids, and the associated porous mesostructured inorganic frameworks, are classic examples of the application of molecular assembly in material synthesis. In the early 1990s, Mobil and Japanese scientists reported the discovery of M41S and FSM materials—inorganic silicate frameworks whose structures are generated by the molecular assembly of many cationic surfactant molecules.6,7 Under basic conditions, the electrostatic interaction between solvated silicate anions and cationic surfactant assemblies (often referred to as the organic template), combined with the hydrophobic interactions of the nonpolar surfactant tails, drives the formation of mesostructured silica with hexagonal, cubic, or lamellar symmetries (e.g., MCM-41, MCM-48, and MCM-50).7 Huo et al. found that ordered mesoporous silica structures could be obtained in acidic media at or below the isolectric point of silica (pH ∼ 2–3)8 as opposed to classic basic pH approach used for the synthesis of both zeolite and the Mobil and Japanese mesoporous materials. This was the first reported example of the designated use of the isoelectric point for the synthesis of ordered silica materials and of metal oxides in general. The molecular assembly of cationic silica species with cationic quaternary ammonium species requires that the assembly takes place through bridging anions, and the manner in which anions could be used to determine the assembly of the mesostructured phase was demonstrated.8,9
10.1021/cm702328k CCC: $40.75 2008 American Chemical Society Published on Web 12/15/2007
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Utilizing silica precursors in acidic solutions with cationic surfactants and triblock copolymers consisting of poly(ethylene oxide), poly(propylene oxide), and poly(ethylene oxide) (e.g., EO20PO70EO20 or P123) as the organic template, the SBA series of mesostructured materials can be obtained.10 Some of these, such as SBA-8, a highly crystalline 2-D orthorhombic structure (cmm) made with bolaform cationic surfactants,11 had structures previously unreported for pure liquid crystal surfactant phases. After removal of the supramolecular template (surfactant), the resulting mesoporous materials show uniform, large-diameter (1.5 to ∼27 nm) pore arrays with a symmetric arrangement dictated by the surfactant assembly and ultrahigh internal surface areas (>1000 m2/g).7,10,12 This class of hybrid mesostructured materials, and the related mesoporous materials, have numerous applications in areas such as catalysis, adsorption, optics, biology and biochemistry, and the synthesis of other nanostructured materials.13–20 Since the discovery of the MCM, FSM, and SBA classes of materials, the scientific community involved in this research has been expanding rapidly. Excellent progress has been made, which can be divided into four major directions:4,21–25 (1) the development of the basic principles regarding the inorganic–organic molecular assembly process, (2) the application of these principles to synthesize new materials, (3) the design of new supramolecular assembly systems, and (4) the exploitation of mesostructured and mesoporous materials for applications in different fields. This review will cover recent advances in directions (1) and (3). We begin with a discussion of the basic principles regarding the molecular assembly of mesostructured materials in the bulk before focusing on recent attempts to design systems that utilize external fields (magnetic, electric, or mechanical) or dimensional confinement (in one, two, and three dimensions) to direct the assembly of ordered/aligned, and sometimes complicated (single-helical, core–shell doublehelical, doughnut, and peapod),26 mesostructures. We conclude the review with a discussion of future challenges in this research area. 2. Basic Principles A general definition of molecular assembly of mesostructured materials is the spontaneous co-organization of organic molecules (such as amphiphilic surfactants or block copolymers) with inorganic precursors into ordered nanostructures through noncovalent interactions. This process generally takes place in the absence of a relevant external field. However, when an external field is applied whose interaction energies with supramolecular assemblies (aggregates of surfactants and inorganic precursors) are larger than the thermal energy, such a field can have drastic effects on the final structure. Furthermore, the assembly process can be classified as occurring either in the bulk or in a spatially confined space. Systems are effectively bulk when each defining dimension is several orders of magnitude larger than the relevant molecular assembly scalar. (For mesostructured materials this is the repeat distance in the symmetric assembly, typically 2-50 nm.) For a confined system, the assembly process takes place within a space that has at least
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one dimension that is close (within ∼1 order of magnitude) to the scale of the molecular-assembly scalar. Furthermore, for confined systems the surface energy of the confining interface becomes especially important. The early synthetic recipes for mesostructured materials were performed without the influence of external fields and yielded bulk materials, e.g., hydrothermal synthesis of MCM-417 and SBA-15.10 These syntheses tend to form large crystals with highly ordered pore arrays. However, in nature the most astonishing molecular assembly systems (such as cells) are often directed by energy gradients and occur in highly confined environments. Relevant synthetic analogues to such systems are of great importance. The basic principles regarding molecular assembly for the synthesis of mesostructured materials include the interactions of different components (surfactant molecules and inorganic precursors) of the assembling system between themselves and with their external environment. These principles were developed at an early stage in the history of mesostructured materials and were adapted from liquid crystal research.8,27 In the following section we briefly review the important thermodynamic and kinetic aspects of molecular assembly for the synthesis of mesostructured materials. 2.1. Thermodynamics. For a bulk mesostructured system, the synthesis involves four main processes that contribute to the total free energy of formation, ∆Gmes, for the final organic–inorganic mesostructure: (1) ∆Ginter, the free energy of formation for the organic–inorganic interfaces, (2) ∆Ginorg, the free energy of formation for the covalent linkage of the inorganic frameworks, (3) ∆Gorg, the free energy of formation for the noncovalent molecular assembly of the organic supramolecular templates (in some cases this must also include interactions of inorganic precursor which can strongly interact with isolated surfactant molecules), and (4) ∆Gsol, the contribution from solvation and solvent coassembly.9,27 The relationship between these parameters is given by ∆Gmes ) ∆Ginter + ∆Ginorg + ∆Gorg + ∆Gsol
(1)
In most mesostructure-forming systems, either ∆Ginter or ∆Gorg dominates the molecular assembly process. ∆Ginter accounts for the weak interactions associated with the organic–inorganic interfaces, which could be hydrogen bonds, electrostatic interactions, or van der Waals interactions. In the synthesis of MCM-41,7 the silicate species are negatively charged at high pH (∼10–13) and directly interact with cationic surfactant through electrostatic forces. The free energy that binds the organic and inorganic species together (∆Ginter) is on the order of 1-5 kcal/mol (Figure 1).3,28 For MCM-41, the second most important interaction is the assembly of the silicate-cationic surfactant intermediate hybrid precursors (∆Gorg) to form the hybrid supramolecular assembly. ∆Ginorg reflects both the condensation of the inorganic frameworks to form strong covalent bonds (e.g., -O-, -S-) and the potential electrostatic interactions between noncovalently bonded inorganic clusters. Such covalent bond enthalpies are on the order of 40-200 kcal/ mol, more than an order of magnitude larger than those of noncovalent bonds. Therefore, controlling the formation kinetics of the inorganic framework is crucial for guaranteeing that ∆Ginorg will not dominate and lead to the formation
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Figure 1. Top: schematic representation of an organic amphiphile (surfactant) and its interactions with a soluble inorganic species. Bottom: thermodynamic and kinetic parameters for both the “cooperative selfassembly” and the “liquid-crystal templating” pathway relevant to the molecular assembly of mesostructured materials.
of nonstructured macroscopic precipitates. ∆Gsol is the change in the free energy of solvation of the various species during nucleation and within the assembly process. In addition to the substantial entropic role that is involved in this process, because the nature of the solution changes during the assembly process (the solution composition can change due to the addition of alcohols from alkoxide hydrolysis, and the concentration of dissolved species can change due to condensation and assembly processes), ∆Gsol is usually a complicated and time-dependent parameter that must also be considered in the context of the kinetic and diffusion processes. In the case of the hydrothermal synthesis of MCM-41 and SBA-15, the formation pathway is well-explained by a “cooperative assembly” mechanism. First, the surfactant molecules interact electrostatically with the charged silica precursors in solution. Charge-matching between organic surfactant and inorganic silicate components is necessary to tightly bind the two together. The interactions between individual inorganic-modified surfactant molecules, including hydrophobic forces and the formation of covalent -Si-O-Sibonds, lead to the final ordered mesostructures. The “cooperative” aspect of this mechanism derives its name from the observation that both inorganic and organic components are essential to achieve ordered structures. Therefore, for syntheses following this mechanism, the formation of the silica-surfactant hybrid precursor (whose formation energy is given by ∆Ginter) is critical for mesostructure synthesis. It is important to recognize that the addition of inorganic species to the organic surfactants can drastically alter the assembly behavior of the surfactant so that prediction of the mesostructure based on the purely organic liquid crystal structure, which is determined in a lower dimensional composition phase space, is no longer valid.8,29–31 Another synthetic route to mesostructured materials, termed “liquid-crystal templating”, utilizes high concentrations of the organic surfactant.24,32 In this case, the assembly of the organic molecules into liquid crystalline phases is assumed to dominate over other thermodynamic processes
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(∆Gorg > ∆Ginter), and these phases form even in the absence of the inorganic species. However, as mentioned above, the addition of inorganic species to the organic phase space adds one or more additional compositional phase dimensions to the overall system and will always affect, to a larger or smaller degree depending on the relative strengths of the various interactions described above, the nature of the final mesostructure. The ability to attain equilibrium with high concentrations of surfactant is also more problematic. Kinetic control of the inorganic condensation is again critical to prevent the formation of macroscopic inorganic precipitates. 2.2. Kinetics. There are three primary kinetic processes whose relative rates should be considered in the formation of organic–inorganic mesostructures: (1) kinter, the rate constant for the reaction of organic surfactants with inorganic precursors to form a hybrid inorganic–organic interface, (2) korg, the rate constant for the assembly of the organic molecules (or in some cases the hybrid inorganic–organic intermediate) into ordered supramolecular assemblies, and (3) kinorg, the rate constant for the reaction of two inorganic precursors to form a covalent bond.9 As mentioned above, the free energy of formation for the inorganic framework via covalent bonds (∆Ginorg) is usually much larger than the binding energy of the organic–inorganic interface (∆Ginter). Therefore, kinetic control of the inorganic condensation (covalent bond forming) reactions is the key to keep ∆Ginorg from dominating the assembly process. For the successful supramolecular assembly of mesostructured materials, the rate constants of these three processes should be ordered as kinter > korg > kinorg (2) In the synthesis of mesostructured silica, kinetic control of silicate condensation is simple and effective, which has allowed for the creation of an enormous variety of different structures.24 Generally, silica-based mesostructured powders are synthesized from tetraethyl orthosilicate (TEOS) with amphiphilic organic templates in either basic7 or acidic8–10 water-ethanol mixtures. Both basic and acidic conditions guarantee the fast hydrolysis of TEOS to yield hydrophilic silicate monomers. Compared with other metal oxide precursors under similar conditions (e.g., Al2O3, TiO2, ZrO2, Nb2O5, and VOx), silicate condensation (linkage of two monomers via an oxo bond) kinetics are usually easier to control, which allows for co-organization with the supermolecular assemblies rather than precipitation of bulk silica or disordered composites.33 For example, the highest quality ordered mesoporous silicas are synthesized in either basic media (pH ∼ 13) or acidic media (pH < 2) where molecular silicate species are stable and negatively or positively charged, respectively. In the SBA-15 synthesis, ordered products are difficult to obtain when pH > 4, unless additional sol–gel regulating reagents (such as fluoride anion) are added to the assembly system.34 Under near-neutral conditions, silica precursors (such as tetramethyl orthosilicate, TMOS) undergo slow hydrolysis but rapid condensation. The unbalanced hydrolysis and condensation rate of silica precursors leads to silicate species with unhydrolyzed methoxy groups, which weakens the electrostatic interaction at the organic–inorganic interface and results in the formation of ill-defined composites. Addition of small amounts of fluoride can greatly speed
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Figure 2. (a) Size distribution and time evolution of individual metal alkoxides (Ta and Zr) and their mixture in the AcHE sol–gel solution measured by dynamic light scattering. (b) Transmission electron microscopy (TEM) images of mesoporous metal oxides derived from the AcHE sol–gel process. Adapted from ref 12. Copyright 2006 American Chemical Society.
up the hydrolysis of the silica precursor by weakening the Si-O bonds and make successful synthesis of SBA-15 materials over a wide range of pH conditions (pH 0-9) possible.34 The control of the hydrolysis and condensation reaction kinetics of non-siliceous oxides is challenging. Non-silica metal oxide precursors (generally chlorides or alkoxides) usually undergo rapid hydrolysis and condensation reactions.35 This is problematic—if the inorganic precursors grow larger than a critical size (usually ∼2–5 nm, depending on the surfactant), they can no longer be effectively organized by the weak intermolecular forces driving the molecular assembly. Indeed, uncontrolled condensation normally yields macroscopic inorganic–organic phase segregation. Strategies to control the hydrolysis/condensation rates of transition metal oxide precursors include utilizing specific pH ranges, stabilizing ligands, nonaqueous media, preformed nanoclusters, controlled hydrolysis via limited addition of water, or some combination thereof.23 Evaporation-induced self-assembly (EISA), first described for mesostructured silica, has proven to be an extremely useful process for controlling inorganic condensation kinetics of non-silicate mesostructures by decoupling the processes governed by korg and kinorg.36,37 In EISA, the evaporation of a volatile solvent drives the coassembly of the organic surfactants. The inorganic component is activated to form covalent bonds at a later time, usually via a thermal treatment. Given the EISA method and the availability of stable soluble inorganic precursors, ordered inorganic–organic mesostructures can be synthesized fairly easily. Removing the organic component of the hybrid material, and subsequent framework crystallization, can still be quite challenging.38 Generally, it is easier to crystallize frameworks with thick walls that are assembled from inorganic precursors which do not contain a large fraction of organic stabilizers (for example, metal chelators such as trifluoroacetic acid)39 that otherwise must also be removed. Following early efforts,8,40–42 a general method to control kinter during the synthesis of mesoporous metal oxide
frameworks was realized by utilizing metal chloride precursors in “nonaqueous” alcohol solutions.43,44 The addition of metal chlorides to an alcohol generates HCl in situ yielding stable chloroalkoxy precursors, whose properties can be varied with the alcohol chain length. However, the long gelation time due to the weak interactions between the partially hydrolyzed metal chloride inorganic precursors renders this method unsuitable for the preparation of macroscopic materials such as membranes with µm to mm thicknesses or monoliths on the mm to cm scale. This method can be improved by the partial or complete replacement of highly reactive metal chlorides with lessreactive metal alkoxides followed by the controlled addition of acid and water.29,45 Sanchez and co-workers have demonstrated that this works exceptionally well to produce thin films from a large variety of mesostructured oxides and have worked out in detail the chemical and physical processes occurring throughout the synthesis.46,47 Zhao and co-workers extended this method by selecting acid–base pairs as precursors to yield multicomponent mesostructured minerals, including metal phosphates, metal borates, and mixed-valence metal oxides.48 Fan et al. reported a simple and widely applicable methodology to control kinorg in a sol–gel solution composed of acetic acid, hydrogen chloride, and ethanol (AcHE).12,35 Unlike the previous methods described above, in this sol–gel system, inorganic species form stable nanoparticles with similar sizes in the range of 1.6-8.0 nm (measured using dynamic light scattering) and normalized growth rates (from 2.8 × 10-4 to 1.2 × 10-2 nm min-1), which allows nanoparticle co-organization with amphiphilic block copolymers into ordered mesostructures (Figure 2). Most importantly, the diverse condensation kinetics of a variety of metal oxide materials can be homogenized—which allows for the fabrication of complicated and novel mesoporous multicomponent metal oxides, such as those containing high concentrations of rare earth and transition metal oxides. It should be noted that the inorganic components involved in the molecular assembly of mesostructured materials need
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Figure 3. (a) Schematic representation showing the preparation of mesoporous silica films under a high applied magnetic field and the resulting magnetically induced orientation of a 2D-hexagonal ordered mesophase. (b) Geometry model of the in-plane X-ray diffraction and angular dependence of maximum diffraction intensity as a function of incident angle (φ) of X-rays beam for mesostructured (c) CTAB-silicate and (d) P123-silicate composites. The incident X-rays at φ ) (90° are parallel to the magnetic field as indicated by arrows. Reprinted with permission from ref 57. Copyright 2006 Royal Society of Chemistry.
not be entirely “inorganic”. The successful assembly of periodic mesostructured organotitanates,39 organosilicates,49–51 and polymers52,53 suggests that the basic principles described above are generally applicable to chemical systems that can interact with the organic supramolecular templates and then be further cross-linked into three-dimensional frameworks. The discussion above has been based on the molecular assembly of mesostructured materials of bulk systems in the absence of external fields. When the assembly occurs under the influence of an external field or in a confined space, additional thermodynamic contributions need be taken into consideration. Employing external forces to direct molecular assembly opens up new opportunities for the fabrication of material systems with hierarchical mesoscopic ordering and engineered complexity. These topics will be addressed in the following sections. 3. Field-Directed Assembly Although typical mesostructured materials are well-ordered on the meso length scale, the bulk materials typically consist of a collection of “microdomains” randomly oriented with respect to each other. The application of external fields is one promising route to yield macroscopically aligned mesostructures. In the presence of such a field, the free energy of formation of the mesostructure can now be expressed as ∆Gmes ) ∆Ginter + ∆Ginorg + ∆Gorg + ∆Gsol + ∆Gext (3) where ∆Gext is the difference in free energy between a random macroscopic alignment and alignment with the external field. The hexagonal mesostructured phase is typically chosen for field alignment as there is a large difference in materials properties along or perpendicular to the cylinder/ tube axis. 3.1. Magnetic-Field-Directed Assembly. When a magnetic field is applied during the molecular assembly process, the free energy of mesostructure formation depends on the field orientation. The free energy difference between align-
ment of an tubular supramolecular assembly parallel to the field versus perpendicular to the field is given by a simple model (developed for block copolymer melts): ∆GB,ext ) -VB2∆χ ⁄ (2µ0)
(4)
V is the volume of supramolecular aggregate, B is the applied magnetic field strength, ∆χ ) χ| - χ⊥, and µ0 is the vacuum permeability. χ| and χ⊥ are the magnetic susceptibilities along the directions parallel and perpendicular to the molecular axis of the surfactant molecule, respectively.54 When ∆GB,ext for a mesostructure microdomain is larger than thermal energy (kT, 0.6 kcal/mol at 300 K), the magnetic field will exert an appreciable effect on the molecular assembly process. Firouzi et al. were the first to use magnetic fields to macroscopically align unpolymerized silicate-surfactant (CTAB) mesophases.55 Alignment at the centimeter scale was achieved by heating the mesophases above their isotropic-anisotropic transition temperature in absence of silica polymerization, followed by cooling through the transition in a strong magnetic field (11.7 T). Deuterium nuclear magnetic resonance measurements indicate that the silica-surfactant liquid crystals orient on the basis of the collective diamagnetic susceptibilities of the molecular axis. Tolbert and co-workers found that it is possible to retain long-range alignment of the hexagonal silicate-surfactant mesophase even after solidification of silicate networks by acid or thermal treatment.56 In this case, the two-dimensional mesochannels in the film orient parallel to the magnetic field direction. The degree of mesostructure alignment can be quantified using grazing incidence X-ray diffraction techniques (Figure 3b).57 Here, the geometry of the incident beam and detector are such that the scattering vector lies in the plane of the sample. The detector is aligned so that the diffracted intensity from the hexagonal (010) set of planes is maximized. Then the sample is rotated to scan the radial angle (φ). The width of the observed peaks (there are two, 180° out of phase, due
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to the equivalency of opposite directions) is directly related to the degree of sample alignment. For normal thin film samples, for which the channels do not show any preferential alignment, the diffracted intensity will be independent of φ. After silicate polymerization, silicate-CTAB mesocomposite show a Gaussian distribution of orientations centered along the magnetic field axis with a full width at halfmaximum (fwhm) of ∼85°. According to eq 4, the volume of supramolecular assembly will affect ∆GB,ext. Larger supramolecular assemblies are expected to better align with the external field. Triblock copolymers (e.g., P123, F127) are much larger than CTAB. Consequently, Yamauchi et al. found a narrow orientation distribution (fwhm ) 20°) for the silicate-P123 system (Figure 3).57,58 3.2. Electric-Field-Directed Assembly. If tubular supramolecular assemblies have a dielectric constant gradient along the surfactant molecular axis, electric fields will exert a force on the molecular assembly that tends to align it with the external field. In general, a system containing two different dielectric materials has its lowest energy when the interfaces are oriented parallel to the applied electric field. Hence, a mesostructured material with hexagonal symmetry formed in the presence of a strong electric field should align the hexagonal channels parallel to the field direction. The change in free energy between field-aligned and unaligned states, again taken from the block-copolymer melt literature, has been shown to be proportional to the dielectric constant difference (∆) between the two blocks:
inorganic precursor solution can hinder the alignment process by moving to screen the applied electric field. 3.3. Flow-Field-Directed Assembly. Macroscopic alignment of a mesostructure can also be achieved utilizing a flow device.62 Hillhouse et al. reported the application of a flow field to manipulate the orientation of tubular silicatecetyltrimethylammonium bromide (CTAB) aggregates.63 When organic–inorganic films are synthesized under a flow field and for short deposition times, the tubular mesostructures are elongated along the flow direction and acquire a tapelike morphology, which is in contrast to the disklike morphology of films grown in contact with a quiescent mixture of the same composition. Wiesner and co-workers implemented a custom-built device operating in a Buchi evaporator to generate a steady flow during EISA.64 The hybrids were prepared via the coassembly of the amphiphilic diblock copolymer, poly(isoprene-block-ethylene oxide), with organically modified ceramic precursors, (3-glycidyloxypropyl)trimethoxysilane and aluminum sec-butoxide, prehydrolyzed into sol nanoparticles. Small-angle X-ray scattering studies revealed that lamellae lay parallel to the substrate with a high order parameter. For the hexagonal mesostructure, the cylinders align with the flow direction. Besides the efficacy of this relatively simple flow technique, it is also possible to synthesize bulk polymer-inorganic hybrids with macroscopic alignment—provided that the inorganic precursors condense slowly enough to allow the supramolecular assemblies to orient before loss of fluidity.65
∆GE,ext ∝ E2∆ ⁄ 〈〉
4. Molecular Assembly under Dimensional Confinement
(5)
where E is the electric field and 〈〉 is the average dielectric constant of the two components.59,60 Trau et al. reported the application of electric fields to direct organic–inorganic mesostructured systems.61 Silicatesurfactant (CTAC, cetyltrimethylammonium chloride) selfassembly was carried out within microcapillary molds under the application of an external electric field. The mesochannels were found aligned parallel to the capillary walls. However, the electric field effect on the mesostructure alignment is not entirely clear in this study. In such microcapillary confinement, tubular supramolecular assemblies will take a configuration to minimize the number of end-caps (see below). Consequently, they will tend to arrange along the long axis of the capillary rather than truncating at capillary walls even in absence of electric fields. A partial rearrangement of mesostructure when applying external electric field was reported by Wang et al.48 The transition from two-dimensional hexagonal mesostructure to irregular elliptical mesostructure was confirmed by X-ray diffraction analysis and TEM imaging as well as by “imaging” with noble metal replication. The partial mesostructure rearrangement instead of mesochannel reorientation is explained by the low dielectric constant gradient across the organic–organic interface and the limited mobility of the silica frameworks. The difficulty in demonstrating effective microdomain orientation control for mesostructured silicates is likely due to the presence of charged inorganic precursors, soluble ions, and/or charged organic surfactants. Mobile ions in the
Up to this point, our discussion has focused on molecular assembly occurring in an essentially infinite three-dimensional space; i.e., we did not explicitly consider interactions between the components involved in the assembly and any other surfaces (such as air or substrates). However, when molecular-assembly processes occur in a space whose dimensions approach the molecular assembly repeat unit, the final morphology of the material will be significantly affected by the “confinement effect”. The manner in which confinement alters the final morphology is primarily based on three factors. (1) Commensurability. Interaction between the amphiphilic supramolecular assemblies and the confinement boundary and subsequent minimization of the interfacial energy will cause stretching or compressing of the assembly whenever the repeat unit is incommensurate with the boundary dimensions (i.e., confinement dimensions are not an integer multiple of the assembly scalar). (2) Entropy. Confinement will retard the free movement of polymer or hydrocarbon chains that compose the surfactant, which constitutes a loss in entropy of the system.66 The free energy change due to this entropy loss can be written as ∆Gconf ) k ln(∆Ω)
(6)
where k is the Boltzmann constant and ∆Ω is the difference between the number of possible conformational microstates available to the supramolecular assembly in the confined space versus in the bulk. The effect of ∆Gconf on the assembly
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process becomes significant only when dimensional limitations are within several repeat periods of the supramolecular morphology. Figure 4 schematically illustrates how the change in free energy results in a structural transition from tubular supramolecular assemblies to spherical supramolecular assembles to single molecules as the degree of confinement is increased. (3) Surface energy. Because confined structures have a large surface-to-volume ratio, surface and interface energy effects can sometimes dominate the orientation and symmetry of the final structure. (A good example of this is the observation that hexagonally packed tubes generally lie flat when synthesized as a thin film on a flat substrate.) A more detailed discussion will follow below. In order to better discuss confinement in the following sections, we have decided to classify confinement effects into several categories. First, confinement can be classified by the dimensionality of the confining field. This field can be one-, two-, or three-dimensional in nature, and we will address each case in the following three sections. We further break down the types of confinement by considering the nature of the confining field. In this review two general types of confining fields are discussed: (1) Physical confinement is the most easily visualized and occurs when the assembly process is forced to take place within a space whose confining dimensions are defined by an incompressible solid (for instance inside the pores of porous alumina membranes). (2) Surface-tension-induced confinement occurs when there is a confining field due to an air–liquid (i.e., microdroplet aerosol) or immiscible liquid–liquid boundary (i.e., microemulsion). In both cases surface energy effects will play an important role. In thin film self-assembly, both types of confinement are present. 4.1. One-Dimensional Confinement at Planar Surfaces. The one-dimensional (1D) confinement case occurs when thin films of mesostructured materials are synthesized on a surface or between two planar substrates. In the former case, air is considered the second confining interface (a legitimate assumption if the dimensions of the film are within 1 order of magnitude of the natural repeat unit of the supramolecular assembly). Mesostructured films (with repeat units between 2 and 50 nm) generally range between 10 and 300 nm in thickness and therefore fall within this category. Interface effects often dominate the assembly process for 1D-confined thin films and can therefore be used to tailor materials properties. Mesostructured materials with twodimensional (2D) hexagonal symmetry are of special technological interest because, when properly aligned, they can be used as nanoscale tubes for efficient mass transport for reactions or separations or as templates to grow other ordered materials. Because of the preferential interactions between supramolecular assemblies and the substrate, the cylindrical channels normally align parallel to the substrate. These interactions can be enhanced by utilizing treated surfaces, such as rubbed polyimides, to achieve single-crystal-like parallel alignment.67,68 As an extreme example, Kuemmel et al. have recently shown that monolayers of block copolymer micelles combined with inorganic precursors yield unique surface patterns dominated by interfacial affects.69,70 Although the parallel conformation may be practical for specific uses, many of the most interesting applications
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Figure 4. Space-confinement effect on supramolecular assemblies. Under strong confinement, entropic considerations favor the formation of smaller supramolecular assemblies.
require perpendicular alignment such that the surface of the film consists of an array of open pore ends. Consequently, there has been much effort toward inducing the perpendicular geometry—which is reviewed in the remainder of this section. Much of the initial work on one-dimensional confinement was performed on block copolymer melts in the absence of inorganic species. On the basis of theoretical calculations, the orientation of microphase-separated block copolymer mesostructures can be controlled by modifying the interfacial energies of the top and bottom surfaces. Self-consistent-field theory and Monte Carlo simulations have predicted that lamellar mesophases of diblock copolymers should align parallel to substrates that interact with a strong preference for either block.71–75 Conversely, neutral substrates, which have no preference for either block, cause the lamellae to align perpendicularly to the substrate. A recent density functional theory study predicts the same trend for the 2-D hexagonal mesophase.76 Experimental results obtained by Russell and co-workers have confirmed these predictions.77 A solvent-vapor-annealing step was used to control the air/film interface, which resulted in the formation of highly ordered and well-oriented diblock copolymer poly(styrene-b-ethylene oxide) (PS-PEO) nanostructures. Introducing inorganic precursors to the system further complicates the process of vertical alignment. The hydrolysis and condensation of traditional sol–gel inorganic precursors with the preorganized block copolymer films, such as metal alkoxides, could alter the macroalignment or microphase separation of block copolymer due to the presence of alcohols produced from the sol–gel reaction and the deposition of inorganic precursors into different polymer domains. To solve this problem, Freer et al. synthesized an oligomeric organosilicate precursor (silsesquioxane, SSQ) that is selectively miscible with the poly(ethylene oxide) block (PEO) of a polystyrene (PS) poly(ethylene oxide) block copolymer.78 The coassembly of PS-PEO and SSQ gave rise to oriented mesoporous organosilicate thin films. During the synthesis, the energies at the two interfaces are adjusted by varying the substrate chemistry and the solvent vapor used for annealing. Under a mixed chloroform/octane vapor environment, the interfacial energy of PS and PEO + SSQ domains becomes nearly equivalent at both vapor/film and film/ substrate interfaces. The use of SSQ also avoids the production of alcohols that is coincident with the metal alkoxide sol–gel process (Figure 5).
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Figure 5. (a) Schematic illustration of block copolymer (PS-b-PEO) and organosilicate (SSQ) coassembled structures. AFM images (10 nm height scale) of the porous films containing (b) a cylindrical and (c) a spherical morphology. The inset in (b) is the SAXS profile of the film. TEM cross-sectional micrographs of films with (d) cylindrical and (e) spherical morphology. Adapted with permission from ref 78. Copyright 2005 American Chemical Society.
Resorcinol-formaldehyde is another suitable precursor that can co-organize with the block copolymer while maintaining alignment.79 First, resorcinol and the diblock copolymer polystyrene-block-poly(4-vinylpyridine) (PSP4VP) were preorganized into a well-ordered mesostructured film by solvent-induced structural annealing. Second, the insitu polymerization was initiated by a formaldehyde vapor treatment to the resorcinol-formaldehyde resin. Finally, the resin was converted into carbon at high temperature, and PS-P4VP was thermodecomposed to leave behind mesoporous carbon networks. The preorganized aligned mesostructures remain throughout the process. The neutralization of the air/film interface by selective solvent-vapor annealing can be replaced by applying another chemically neutral substrate. Koganti et al. reported the alignment of PEO-PPO-PEO-templated sol–gel silica or titania films by sandwiching the films between a pair of modified substrates, which were each coated with either a random PEO-PPO copolymer or PEO-PPO-PEO triblock copolymer to render them “chemically neutral”.80,81 They
found that films thicker than 240 nm required the sandwich geometry to ensure the mesochannels aligned vertically to the substrate throughout the entire thickness. One chemically neutral surface is sufficient for films with reduced thickness.80 Tolbert and co-workers reported another approach which makes use of cubic mesoporous solids as a substrate to grow vertical mesochannels.82 The typical substrate surface pattern is a cubic self-assembled inorganic–organic composite with the (111) mesocrystal orientation exposed at the air interface. The prepatterned substrate, with an essentially 2D array of hydrophobic/hydrophilic areas, is commensurate with the repeat distance of the bulk hexagonal phase, causing preferential alignment of the cylinders normal to the substrate. 4.2. Two-Dimensional Confinement. Adding a second degree of confinement augments the morphological possibilities of materials formed by the supramolecular assembly process. Theoretical calculations on block copolymers in tubular confinement predict geometries including concentric circular channels, stacked disk structures, helical and multiple helical configurations, and peapod formations.26,83–87 The
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essential geometry-determining factors are the volume fraction of the block copolymers, the diameter of the confining tube, and the chemical interactions between the tube and the polymer. Many of the theoretically predicted structures have been confirmed in the laboratory. Porous anodic alumina (PAA) templates with tunable cylindrical channels ranging from 10 to 400 nm in diameter provide an ideal platform to experimentally study supramolecular assembly under twodimensional physical confinement. Russell and co-workers observed that symmetric polystyrene-block-polybutadiene (PS-b-PBD) block copolymers, which exhibit a lamellar morphology in the bulk, formed concentric cylinders along the nanorod axis under confinement. The number of concentric cylinders was highly dependent on the PAA channel diameter and the equilibrium repeat distance of the polymer. Under confinement, block copolymers that exhibit a 2D hexagonal configuration in the bulk underwent changes of symmetry and separation distances.88 In subsequent work it was shown that a further decrease in PAA channel diameters down to 33–45 nm caused the lamellar-phase-forming polymers to distort into stacked disks or helices.89,90 Goesele and co-workers studied91 the assembly of polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) as a function of PAA channel diameter ranging from 400 nm down to 25 nm. The resulting nanorods consisted of cylindrical lamellae with parallel orientation to the c-axis and core–shell structures for the smallest diameter. No other morphologies were observed. The assembly of mesostructured inorganic–organic supramolecular hybrids has also been studied within PAA templates. Yang et al. reported the synthesis of mesostructured silica materials prepared with F-127 in commercially available anodic alumina membranes.92 Nanotubes and nanofibers were selectively obtained by changing the surface chemistry of the alumina walls. The mesopores formed selectively parallel to the PAA channels. Mesostructured silica materials synthesized by using P123 and CTAB also resulted in pores predominantly oriented parallel to the wall of the columnar alumina channels.93,94 Such composite templates are advantageous for applications in “nanodevice” fabrication or biomacromolecule separation because they combine the structural rigidity of PAA with the ability to fine-tune the channel dimension of the silica mesostructures. Wu et al. systematically studied the confined assembly of silica-surfactant composite mesostructures within cylindrical nanochannels of varying diameters.26 The block copolymers were forced into cylindrical nanochannels with dimensions that were only several repeat periods of the supramolecular assemblies. Precursor solutions and reaction conditions that yielded SBA-15 (2D-hexagonal phase) in the bulk formed unprecedented silica mesostructures with chiral mesopores such as single- and double-helical geometries under 2D confinement. Both commensurability and imposed curvature were found to influence the morphology. On tightening the degree of confinement, the mesopore morphology changed from coiled-cylindrical to a spherical cagelike geometry. The progression of ob-
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served morphologies leads to a simple scheme that connects the structures observed in thin films with those under cylindrical confinement (Figure 6). Each structure can be geometrically constructed by coiling parallel cylinders using different rolling schemes. The ability to obtain helical silica structures with chiral mesopores is of special interest from a synthetic standpoint because no chiral molecules or subunits have been used as precursors. Self-consistent-field calculations accord well with the experimental findings. Recently, Thomas et al. investigated the influence of 2D confinement on the assembly of spherical micelles formed from block copolymers by analyzing their replicas in silica.95 Spherical micelles, apart from increasing their sizes, follow assembly patterns similar to those of “hard spheres” under cylindrical confinement wherever the channel dimensions are commensurate with the natural repeat units of the micellar assembly. Incommensurate channel diameters, however, lead to larger distortions in micellar dimensions and increased anisotropy. The density of pores in the resulting mesostructures was influenced by the concentration of polymer in the precursor solution—an even distribution could only be achieved for a certain concentration while higher and lower concentrations resulted in opposite pore-density gradients along the c-axis of the structures. It is interesting to note that mesoporous silica fibers with circular or longitudinal internal architectures, like those formed in PAA, can be synthesized entirely in solution without using direct physical confinement.96,97 The diameter of the mesoporous fibers ranges from 50 to 300 nm. It is believed that interactions between solvent and surfactant molecules with the solid fiber external surface during growth play a large role in the assembly. Increasing the synthesis temperature modifies the interface interactions and leads to a transition from “longitudinal” to “circular” internal architectures for the mesostructured fibers. Although not a physical confinement or a surface-tension-induced confinement, the utilization of organic molecules to tune various surface energies during growth is another good strategy to force a nanoscale “confined” geometry for both conventional inorganic or organic crystals and the mesostructured materials under discussion here.98 4.3. Three-Dimensional Confinement. Molecular assembly in submicron-scale liquid droplets is a well-defined example of three-dimensional (3D) surface-tensioninduced confinement of mesostructured materials. This approach was first demonstrated by Lu et al.99 A solution with amphiphilic molecules and inorganic precursors was atomized and sent into an aerosol reactor (Figure 7). The aerosol droplets were dried in the heating zone, forcing the evaporation of the solvent and driving the molecular assembly within the micron/nanosized spherical liquid droplets. The 3D curved air/solution interface dictated the final mesostructure of the organic–inorganic hybrids. In the bulk assembly of mesostructured materials, as the surfactant concentration is increased, the symmetry of the resulting mesostructured hybrid changes from disordered to cubic, hexagonal, and finally lamellar. Mesostructures formed in liquid droplets generally only exhibit disordered
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Figure 6. Different mesostructures formed under cylindrical confinements. The top panel shows the effects on a system that forms a cubic mesostructure whereas the bottom panel shows the effects on a system that forms a hexagonal mesostructure in the bulk. Electron microscopy images (SEM on the top panel, TEM on the bottom panel) of the confined mesostructures with the confining dimensions given in nm. The TEM images from the mesostructures formed from the hexagonal phase were taken of silver replicas of the void space to aid in imaging. Adapted from refs 26 and 95.
or hexagonal mesophases. At high surfactant concentrations, a unique onionlike mesostructure, which is the 3Dconfined version of lamellar structures observed in bulk systems, was observed (Figure 7). In-situ small-angle diffraction has been used to understand some of the effects of evaporation and solution chemical composition on the aerosol molecular assembly process.100 Arsenault et al. introduced a lamella-forming block copolymer into 3D void spaces of nanoscale dimensions.101 The void spaces were created in one of two ways, either by the assembly of close-packed silica microspheres or by utilizing close-packed polystyrene microspheres as a template for an inverse silica replica. Several unique morphologies were observed due to the preferential interactions of the silica
wall structure with the hydrophilic component of the block copolymer and the periodicity of the physical confinement. 5. Outlook We conclude with a discussion of several challenges facing the molecular assembly of mesostructured materials. What will be the new opportunities for mesostructured materials? How far can we extend molecular assembly for these material systems? One important challenge is the asymmetric molecular assembly of mesoporous materials with defined chirality.4 Chiral materials are ubiquitous in nature, from DNA chains to patterns of snail shells,102,103 and 65 of the 230 space groups show chiral symmetry. However, despite the potential
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Figure 7. (a) Onionlike mesostructure formed in droplet assembly system produced by (b) aerosol reactor. Reprinted with permission from ref 99. Copyright 1999 Nature Publishing Group.
of chiral mesostructures for selective catalysis and separation, the occurrence of mesoscale chirality in mesopores is rare. Recently, researchers have reported twisted hexagonal mesoporous materials with a strong left-handed chiral preference templated by an anionic surfactant with chiral hydrophilic headgroups.104,105 There are several later reports of the formation of helical mesostructured materials without obvious handedness selectivity (i.e., the product appears to be a racemic mixture of the two components).106–109 Several questions rise from these results. (1) What is the driving force for helical mesostructure formation? It seems that chiral surfactants are not necessary for the formation of helical mesostructures, and the interface interaction is more crucial in forming the structure.26,106 Yu and co-workers suggested that the reduction of organic–inorganic interface energy is responsible for the helical mesostructure. (2) The pitch of the helical mesostructure (the length of one complete helix turn) is generally in the range of 200-1000 nm in bulk molecular-assembly systems, which is about 2 orders of magnitude larger than the mesopore size. This chirality will have little effect in determining the reactivity or diffusivity of molecular enantiomers accommodated within the mesochannels. In one 2D-confined molecular assembly system,26 the pitch of the mesostructured helices can be reduced to 27 nm, only twice the block copolymer micelle diameter. Can one develop chiral inorganic porous materials in which the chirality is on a relevant molecular length scale? Furthermore, many of the chiral systems yield materials that are racemic mixtures of both left- and right-handed mesostructures. A clear challenge remains. (3) How can one prepare enantiopure helical mesostructures? The use of a chiral surfactant might not be necessary. One quite interesting possibility would be to use a geometry that drives the formation of a helical structure (such as the porous alumina membranes) and then add a small amount of a chiral additive that tips the equilibrium toward one-handedness of the resulting structure. A second challenge which has been addressed by Pinnavaia, Kaliaguine, and others is the molecular assembly of mesostructured materials whose mesoscale walls are composed of a crystalline microporous material, such as zeolite.110–113 Kinetic control of the formation of the crystalline zeolite frameworks is critical to ensure that the large ∆Ginorg will not dominate the assembly process. An alternative solution is to increase ∆Ginter and ∆Gorg. Choi et al. designed an amphiphilic organosilane surfactant to direct the synthesis of mesoporous aluminosilicates consisting of zeolite walls.114 Covalent bonds between the surfactant and inorganic species increase the free energy of formation for
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the organic–inorganic interface, which helps to counteract the influence of ∆Ginorg so that tunable mesoporosity can be retained. Compared to the ∆Ginorg for zeolite formation, the free energy of formation for metal-organic frameworks (MOFs) is low.115,116 Together with a low MOF crystallization temperature (near room temperature), the synthesis of mesostructured materials with MOF walls seems possible. The molecular assembly systems discussed in this review yield material systems structured at the mesoscale (2-50 nm). However, it clear that simultaneous or sequential assembly at many length scales—a feature that living systems demonstrate with ease—would be of inestimable value. In biology, molecules are turned into macromolecules and supramolecular assemblies that then make up the various components of cells. Cells assemble into tissues, tissues into more complicated organs, and finally into a complex living being. Nature demonstrates the possibility to control composition, spatial position, and function. Researchers have started assembling components at different lengths scales but often rely on external, physical molds117 to do so, such as microstamps and/or polystyrene spheres118 or porous alumina membrane templates.92 Using molecular assembly to construct structures with functional features spanning molecular to macroscopic length scales, without the use of external “molds”, is a grand challenge. The assembly processes of living systems are considered nonequilibrium or dynamic—complicated ordered structures only develop when the system (an organism in the biological example) is dissipating energy.5 They are also self-regulating in the sense of using the competing kinetics of different processes to control the spatial extent of large scale domain organization.119 To mimic this, energy must be introduced into molecular-assembly systems to direct the assembly behavior of each functional component and competing reactions that accelerate and inhibit the overall assembly process must be set in motion. The energy input can be via light, heat, or, most appropriately, chemical potential, while the kinetic differential necessary for selfregulation can be determined by diffusion, rates of hydrolysis, or other competitive activation energy barriers. The realization of multiscale molecular assembly will also rely heavily on the ability to combine multiple molecular assembly systems that act coherently over several length scales (e.g., microemulsions, supramolecular assemblies, and inorganic clusters) and multifunctional components120 (e.g., polymers, semiconductors, and grafted organic groups). Acknowledgment. We thank Dr. Arne Thomas and Prof. Yiying Wu for valuable contributions. S.W.B. thanks the National Science Foundation (NSF) for a Graduate Research Fellowship. This material is based upon work supported by the NSF under Award DMR-02-33728 and made use of MRL Central Facilities supported by the MRSEC Program of the NSF under Award DMR05-20415. We thank Peter Allen for graphic design.
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