Perspective pubs.acs.org/cm
Engineering Crystal Modifiers: Bridging Classical and Nonclassical Crystallization† Katy N. Olafson,‡ Rui Li,‡ Bryan G. Alamani,‡,§ and Jeffrey D. Rimer*,‡ ‡
Department of Chemical and Biomolecular Engineering, University of Houston, Houston, Texas 77204, United States Department of Chemical Engineering, University of the Philippines, Diliman, Quezon City 1101, Philippines
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ABSTRACT: The use of growth modifiers in natural, biological, and synthetic crystallization is a ubiquitous strategy for controlling growth and achieving desired physicochemical properties. For crystals that grow classically (i.e., monomer-by-monomer addition), theories of crystallization are well established and the field of growth modification is rather mature, although many questions remain regarding the molecular driving forces of modifier−crystal interactions. A new frontier in crystallization is the application of classical methods to tailor materials that grow nonclassically (i.e., growth by the addition of species more complex than monomers). A recent surge of interest and activity in this field has been driven by mounting evidence of both inorganic and organic materials that grow via nonclassical pathways. In these systems, the challenge of elucidating the mechanism(s) of crystallization is underscored by a diversity of growth units that far outnumber those available for classical routes. In this Perspective, we discuss growth modification in nonclassical crystallization, including examples in the literature, the challenges associated with elucidating the modes of modifier action, and to what degree classical theories can be applied to these complex problems as a means of establishing versatile blueprints for crystal engineering.
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INTRODUCTION
many open-ended questions regarding design criteria to identify modifiers with desired specificity. The multifaceted nature of modifier engineering is embodied within a quote by R. B. Fuller, who describes a designer as “an emerging synthesis of artist, inventor, mechanic, objective economist, and evolutionary strategist”. Indeed, when designing modifiers for practical applications, such as an additive in industrial synthesis or a newly developed drug, there are multiple elements to consider that are important to ensure the concepts extend beyond the benchtop: economics, for example, is one of the more critical aspects. Even understanding how modifiers function in complex natural processes, such as biomineralization,12−14 relies upon the ability to think beyond conventional wisdom. The advent of nonclassical crystallization, with an expanding list of materials15 either shown or postulated to grow by this mechanism, is calling into question classical models and theories that have molded our thinking for more than one century. Looking forward, one topic receiving disproportionate attention is the application of classical models and approaches of crystal growth modification to nonclassical phenomena. Few studies in the literature explicitly address the effect(s) of growth modification in nonclassical crystallization. In the case of biominerals, several materials (e.g., CaCO3)16 have been
Growth modification is a highly efficient and facile method for mediating the growth rate of crystalline materials (or hierarchical structures) with tailored physicochemical properties. Crystals are present in everyday life and have a broad range of applications, some desired (e.g., industrial applications)2,3 and others undesired (e.g., pathological diseases).4 To this end, the need to either control crystal growth or to suppress it, respectively, can be accomplished through the use of crystal growth modifiers (alternatively referred to as inhibitors or imposters). As we will discuss, modifiers substantially vary in size, chemical composition, and structure. Their basic mode of action is to mediate the addition of solute to growing crystals, often through specific binding to crystallographic faces, which physically blocks solute incorporation into active growth sites.5 In some instances, such as ice modification by antifreeze proteins,6 modifiers bind to all crystal surfaces and completely suppress growth. Similar methodologies have been applied in the design of therapeutics to treat human diseases, such as kidney stones5,6 and malaria,11 and to prevent scaling (mineral buildup) in oil and gas recovery.7 Alternatively, modifiers are essential in many biological systems as natural regulators of crystallization. Examples include mollusk shells,8 bone matrix,9 sea urchin teeth,10 and coccolith exoskeletons.11 These systems have inspired biomimetic approaches to achieve similar outcomes in synthetic crystallization, which is a subject with
Received: August 23, 2016 Revised: September 26, 2016
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This Perspective is part of the Up-and-Coming series. © XXXX American Chemical Society
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Chemistry of Materials Scheme 1. Physicochemical Properties of Three Classes of Crystal Growth Modifiersa
a
(i) Protein structure taken from the RCSB PDB.1
in crystal habit (e.g., tuning cuprous oxide crystal shapes with chloride ions).22 Molecular modifiers provide increased flexibility to tune their specificity for different crystallographic faces, which leads to new morphologies that differ from the progenitor crystal. Macromolecules, on the other hand, tend to be the most effective modifiers owing to their multiple binding moieties that can inhibit (or fully block) solute attachment at multiple sites on the crystal surface. Scheme 1 provides an overview of the various physicochemical factors of modifiers that dictate their recognition for binding to specific crystal faces. Ions have two primary effects on crystallization. The first is a thermodynamic effect through the alteration of solution ionic strength, which can either increase or decrease solubility, leading to a reduction or an increase in the rate of crystal growth, respectively. The second is a kinetic effect whereby ions directly interact with crystal surfaces and inhibit their rate of growth. Prior studies have shown that relative trends in effectiveness of ions, such as alkali metals, are not always straightforward. For example, we have shown that the effects of alkali metals on calcium oxalate monohydrate (COM) crystallization23 exhibit the following trend (from most to least effective growth inhibitor): Li+ > Na+ ≈ Cs+ > K+. This ordering does not follow trends based on their physical properties, such as hydrated radius, and also does not abide by the well-known Hofmeister series,24 which describes the ability of ions to destabilize colloidal dispersions. It was hypothesized that the ordering could possibly be related to Collins’ law of matching water affinity that predicts the attraction between two oppositely charged ions by considering the relative hydration energies of each ion pair (illustrated in Scheme 1). Ions are categorized as one of two types: high charge density ions (kosmotropes, K) and low charge density ions (chaotropes, C) that bind water molecules strongly and weakly, respectively. Collins’ law predicts that the dissociation of ion pairs is more energetically favorable when one ion (either the cation or anion) is a kosmotrope and the oppositely
shown to form via nonclassical pathways; however, investigation of growth modification has largely focused on single crystals, which appear to grow classically (although a recent study suggests polynuclear complexes play a role in layer advancement).17 We have published studies of growth modification in syntheses of zeolites,18 which are an exemplary class of materials that grow by nonclassical pathways involving a vast number of growth units.19,20 Herein we predominantly focus on examples of zeolite growth modifiers and discuss the complexity of elucidating their modes of action at a molecular level with implications for expanding this knowledge to a broader class of crystalline materials. In the following sections of this Perspective, we discuss the wide variety of modifiers that have been employed in crystallization, including their physicochemical properties and traditional theories to describe their mechanisms of action. We will differentiate the basic principles of classical and nonclassical crystallization, outlining where knowledge gaps exist in models to describe the latter. We will conclude with future outlook on the continued growth of this field, including the challenges that lie ahead for developing rational approaches to design and implement growth modifiers in a variety of applications spanning from energy to medicine.
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PROPERTIES OF MODIFIERS Modifiers influence crystal growth in a multitude of ways that include (but are not limited to) the inhibition/promotion of growth by physically blocking solute attachment (i.e., preferential binding to crystal faces), altering solubility by forming complexes with solute, and disrupting local environments surrounding crystal−solution interfaces (e.g., solvent ordering). Modifiers can be subdivided into three general categories: ions, molecules, and macromolecules. At the most basic level, ions can be highly effective modifiers that have a range of effects: from minor changes in surface topography (e.g., altered roughness of ammonium dihydrogen phosphate crystal surfaces by Fe3+ or Al3+ ions),21 to more notable changes B
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Chemistry of Materials charged ion is a chaotrope (i.e., C−K or K−C combinations).25 Our proposed hypothesis is that the pairing of an inhibitor and solute with similar water affinity (i.e., C−C or K−K) could enable counterion adsorption on the crystal surface; but questions remain as this phenomenon has not been validated for ion−crystal interactions. Moreover, Collins’ law has only been applied to pairings of monovalent ions; therefore, it remains to be determined if this theory can be extended to multivalent ions. Molecules are a second class of modifiers with virtually unlimited potential for tailored specificity owing to a wide variety of design parameters. The nature of molecule−crystal interactions can involve van der Waals, electrostatic, hydrogen bonding, and π−π stacking, among other types of bond formation. Solvent can play a significant factor in modifier adsorption, such as hydrophobic interactions where entropy contributions to modifier−crystal binding energy can be significant.26 The structure of modifiers, including stereochemistry,27 is a critical design parameter, as is the wide range of functional moieties (Ri groups). The spatial distribution of Ri moieties throughout the molecule can give rise to proximal binding groups that are capable of interacting cooperatively with crystal interfaces.28 Understanding how these factors give rise to the molecule’s recognition for specific crystallographic faces is difficult to predict a priori. There are numerous examples where the insertion or removal of a single functional group can lead to dramatic changes in modifier efficacy and/or specificity.29 As an illustrative example in Scheme 1, placement of two functional groups (R1 and R2) at adjacent carbons (e.g., 3−4 positions of an aliphatic molecule) may impose steric limitations that prevent both groups from interacting with a crystal surface as opposed to increased interstitial spacing (e.g., 2−4 positioning) that may be more optimal for cooperative binding. Macromolecules, a third class of modifiers, are highly effective and frequently studied in the literature. Examples include polymers,30 proteins,31,32 peptides (or peptoids),2,33 and various biomolecules (e.g., DNA, polysaccharides, etc.).34 The sequencing of functional moieties (or side groups) is one the most important design parameters for this class of modifiers. In the case of proteins, the primary amino acid sequence and post-translational modifications are important factors.35,36 For synthetic polymers, one can select from a variety of homo-, diblock, or triblock polymer motifs. Secondary or tertiary structure may be an important factor, but is not a prerequisite for macromolecules to be effective modifiers. For example, it has been shown that highly ordered proteins (e.g., β-helix structure)37 and globular proteins38 can be equally effective. One of the questions often posed is whether macromolecules either adopt structure or lose structure upon adsorption at crystal interfaces. As such, the macromolecule’s conformation in solution may not be representative of its structure at the solid interface. Moreover, the size of the macromolecule is an important design parameter. In general, polymers are more effective modifiers than their corresponding monomers due to the proximal binding groups on the former that cooperatively bind to multiple surface sites.39 In many instances, functional groups located in close proximity to each other within the primary sequence can act in tandem to affect crystal growth. These groups may be composed of different types of binding moieties (e.g., hydrogen bonding and hydrophobic residues)26 or zwitterionic segments35 within the macromolecule.
Table 1 lists examples of various geological, synthetic, and biological crystals that can be tailored by different classes of Table 1. Examples of Crystals Tailored with Different Classes of Modifiers22,32,42−56 class 1: ions alkali metals (e.g., Li, K, Na,...) alkaline earth metals (e.g., Mg, Ca, Sr,...) halides (e.g., F, CI. Br,...) transition elements (e.g., Fe, Zn,...) others (e.g., Al, B, La,...) class 2: molecules amino acids (e.g., Asp, Glu,...) ionic liquids inorganics (e.g., pyrophosphate,...) surfactants (cationic. anionic, nonionic) organics (e.g., citrate,) class 3: macromolecules proteins (e.g., osteopontin,...) biomolecules (e.g., DNA, sugars,...) polymers/polyelectrolytes peptides peptoids
class 1 examples calcium phosphates, ref 42 nitratine (NaNO3), ref 43 barite (BaSO4), ref 44 metal organic frameworks, ref 45
class 2 examples hematin, ref 46 calcium carbonates, ref 47 metals (e.g.,Ag, Pt,...), ref 48, 49 metal oxides (e.g., Cu2O, TiO2,...), ref 22, 50 proteins (e.g., lysozyme,...), ref 51 class 3 examples calcium oxalates, ref 32 pharmaceuticals (e.g., paracetamol,...), ref 52 hydrates, ref 53 gypsum (CaSO4), ref 54 aluminosilicates (zeolites, zeotypes), ref 55, 56
modifiers. The list is far from complete, but gives an idea of the types of crystals and modifiers reported in the literature. In many cases, such as biominerals, all three classes of modifiers have proven to be effective (here we list each crystalline material only once). The types of crystalline materials are diverse, ranging from metals, ionic crystals, and minerals to proteins, pharmaceuticals, and other organic compounds. The list of examples is continually expanding as it is increasingly evident that growth modifiers are a highly efficient method of tailoring crystalline materials with desired properties, such as size, morphology, and structure (e.g., polymorphs).
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MECHANISMS OF CRYSTAL GROWTH Classical Mechanisms of Crystal Growth. Crystal growth through classical routes occurs by the addition of solute (that is, monomer) from solution to a crystal surface. A Kossel crystal (Figure 1a) is a model system40 that illustrates the three most common sites on crystal faces for monomer and/or modifier attachment: kinks, steps, and terraces (Figure 1b).41 Monomer incorporation into a kink site is the most energetically favorable due to the formation of three monomer−crystal bonds (compared to two bonds for steps and a single bond for terraces). Moreover, monomer attachment to a kink regenerates the kink, and does not change the surface free energy of the crystal. During crystal growth, the driving force for monomer attachment is the chemical potential that is governed by the degree of supersaturation σ surrounding the crystal interface. In other words, the rate of growth is proportional to the supersaturation, whereas the largest surface area(s) that determine the bulk crystal habit correspond to the slowest growth directions. The final crystal size and habit are C
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parameters can be quantified by varying the temperature or pressure (thus directly changing the degree of supersaturation σ), or by altering the growth medium (e.g., ionic strength, pH, solvent, etc.). Fundamental studies of crystal growth mechanisms and modifier−crystal interactions with in situ AFM allows for the quantification of the step velocities, rate of layer generation, and percent inhibition/promotion of these two processes by the addition of modifiers. Classical crystal growth typically involves a 2-dimensional (2D) layered mechanism where new layers nucleate and steps advance across the surface by the addition of monomers.64 In some instances, layers nucleate as islands, as observed in ice crystallization (Figure 2a).65 A more commonly observed
Figure 1. (a) Classical crystallization occurs through monomer-bymonomer addition (monomers are illustrated as blocks). (b) Kossel crystal where monomers attach to surface sites, either by their direct incorporation into kink/step edges or by their adsorption onto terraces followed by possible surface diffusion and incorporation into kink/step sites. (c) Judicious selection of growth conditions can influence crystal habit. The parameters listed here are merely a subset of factors that affect crystal growth.
determined by a combination of kinetic and thermodynamic driving forces,57 which are influenced by the selection of growth conditions (Figure 1c). Modifiers, in particular, substantially alter the anisotropic growth rate(s) of crystals with concomitant effects on crystal habit. Monomer attachment to a crystal surface occurs by either direct incorporation or surface diffusion (Figure 1b). Both pathways involve dynamic events at sites presented along the solid−liquid interface. Direct incorporation occurs when a monomer in solution attaches to a kink, step edge, or terrace site. Alternatively, monomers can add to a kink/step site after first adsorbing onto a crystal terrace, then diffusing along the surfaces and attaching to the growth site.58,59 The lifetime of a monomer on a crystal surface is governed by desorption, adsorption, and surface “hopping” that lead to a dynamic sequence of events. The immediate layer of solvent surrounding the crystal surface can impose barriers for monomer (or modifier) adsorption. In aqueous solutions, water structuring can occur for up to three layers from the liquid−solid interface.60 Adsorbate interaction with crystal sites invariably requires the displacement of solvent molecules, an energetic barrier predominantly governed by entropy. Direct observation of crystal growth can be accomplished by tracking single crystals using in situ techniques such as atomic force microscopy (AFM),61 interferometry,62 or advanced optical microscopy.63 These noninvasive techniques permit the analysis of crystallization under realistic conditions to assess the effects of growth modifiers. Thermodynamic and kinetic
Figure 2. Examples of crystals that grow classically. (a) Ice crystallization via 2D nucleation events, as observed by optical microscopy.65 (Reprinted from ref 65 with permission.) Atomic force microscopy (AFM) images of crystals grown from dislocations are shown for: (b) L-cystine (001) surface where steps far from the screw dislocation have a height equal to the unit cell6 (From ref 6. Reprinted with permission from AAAS.); (c) calcium oxalate monohydrate (010) surface29 (From ref 29. Reprinted with permission from AAAS.); (d) calcite {1014} surface68 (Reprinted from ref 68 with permission.); (e) insulin (100) surface69 (Reprinted with permission from ref 69.); (f) ferritin (111) surface showing 2D nucleation of islands64 (Reprinted with permission from ref 64.).
phenomenon is the formation of growth hillocks emanating from a dislocation source (Figure 2b−e),9 or a combination of both pathways (Figure 2f).64 One factor dictating the predominant growth mechanism is the degree of supersaturation σ. At low σ (near equilibrium), crystal surfaces are often composed of hillocks.66,67 Layers may be a single step (Figure 2c,e) with height equal to the unit cell, or steps can travel in pairs (Figure 2d). More complex examples include Lcystine (Figure 2b), which is a hexagonal crystal that grows by screw dislocations. On L-cystine (001) surfaces, steps near the D
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Chemistry of Materials dislocation source are generated with height = c/6 (length of a single molecule).6 As these layers advance across the surface, they merge to form a step with height = c (six molecules; one unit cell). Collectively, the examples presented in Figure 2 highlight different surface layers and growth sites that are available for modifier interaction during classical crystal growth. Nonclassical Mechanism(s) of Crystal Growth. Nonclassical pathways differ from classical routes based on the physical state of the solute in solution, which gives rise to a wide variety of precursors that potentially serve as growth units for crystallization. Precursors vary with respect to their microstructure (i.e., amorphous or crystalline),70 size and shape, and state of matter (i.e., solid or liquid-like).14,71 Furthermore, the physicochemical properties of precursors can change (or evolve) over the course of crystal nucleation and growth,72,73 which adds another level of complexity when trying to elucidate the pathways of crystallization. Figure 3a provides an overview of different growth units that can participate in crystallization. The classical pathway is
or they can be covalently bound to form amorphous primary particles or bulk solid aggregates. These particles can act as growth units (Figure 3a, 2a) via a process referred to as “crystallization by particle attachment” (or CPA).15 For zeolites, it has been shown that primary particles evolve (Figure 3a, 2b) in both size and microstructure at elevated temperature and/or longer synthesis time.73,74 It does not appear that these primary particles become fully crystalline, but it is also misleading to describe them as amorphous. Indeed, evolving precursors exhibit some local ordering, which is evidenced by time-resolved spectroscopy75 and calorimetry76 experiments; however, it is not well understood how this degree of ordering contributes to CPA processes. Prior models posit that energetic barriers for precursor addition to crystal surfaces are dependent upon the degree of precursor evolution.77 It is also evident from in situ AFM20 that a disorder-to-order transformation postattachment is required in order for precursors to rearrange and integrate structurally into the underlying crystal lattice (Figure 3a, 2c). Alternatively, there is evidence in the literature78,79 showing that nanocrystallites can be growth units in CPA processes involving oriented attachment (Figure 3a, 3a). Figure 4 provides examples of nonclassical crystallization. In zeolite synthesis, there are a wide range of (alumino)silicate precursors that assemble during the induction period and are present throughout crystallization, serving as putative growth units in CPA and/or reservoirs of nutrient (solute) released over time. It is common to observe colloidal particles and large aggregates (Figure 4a), often termed worm-like particles.72 The first studies of zeolite precursors published in the 1990s focused largely on the growth of silicalite-1, which involves the selfassembly of nanosized primary particles (1−6 nm).82 It is more common, however, to observe the formation of larger spheroidal particles (20−100 nm) in zeolite synthesis (e.g., Figure 4b inset).83 Some studies posit that these particles are gels84 whereas other studies indicate the presence of solid particles with heterogeneous composition (i.e., siliceous interior and aluminous exterior). There is consensus within the zeolite community that bulk amorphous phases facilitate heterogeneous nucleation; however, the mechanism of nucleation is highly debated.83 Our group83 and others85 have provided evidence that nucleation originates on the exterior surfaces of precursors (Figure 4b), which is counter to alternative hypotheses86 suggesting nucleation occurs within the interior of gel particles. Despite differing opinions of precursor participation in nucleation, their direct role in crystallization is a ubiquitous observation. For instance, ex situ electron microscopy has been used to capture intermediate stages of zeolite crystallization (Figure 4c), revealing growing crystals covered in deposits that resemble the size and shape of precursors (indicative of CPA). Similar observations are reported for other minerals, such as magnetite (Figure 4d), that grow by the nonoriented attachment of amorphous primary particles to crystal surfaces. The amorphous colloidal particles (or gels) discussed thus far constitute the majority of precursors encountered in literature; however, there are examples of alternative precursors, such as liquid-like droplets or crystalline particulates. For instance, several groups16,87,88 have suggested that amorphous calcium carbonate (ACC) precursors in biomineral systems are liquidlike with respect to the association of hydrated calcium and carbonate ions. In situ electron microscopy studies of ACC provide evidence of this liquid-like property, and have captured
Figure 3. Pathways of crystallization. (a) Solute addition to a crystal interface can occur through a variety of pathways. Classical routes involve monomer attachment to the crystal surface, 1a. Nonclassical pathways (1b−3a) can involve the formation of oligomeric species, 1b, that directly add to the crystal surface, 1c. Monomers and/or oligomers can form amorphous, 2a, or nanocrystalline, 3a, particles that either directly attach to the crystal surface or undergo structural “evolution” prior to addition, 2b. The latter involves changes in size (e.g., Oswald ripening) or structural rearrangements (e.g., development of local ordering). Precursors undergo a disorder-to-order transition postattachment, 2c. (b) Unfinished layers on crystal surfaces can grow via 2-dimensional (2D) growth through monomer addition to kinks/steps, which advance the layer forward with time. CPA processes result in “deposits” that exhibit 3-dimensional (3D) growth wherein monomers and/or precursors continue to add to the deposits.
represented by the addition of monomers (Figure 3a, 1a). It is difficult to envision a scenario where monomer addition is absent, even if this route is overshadowed by more dominant pathways. Monomers can form oligomeric structures (e.g., dimers, trimers, etc.) based on speciation reactions (Figure 3a, 1b). Any growth unit exceeding the size of a monomer is categorized as a precursor, and the direct attachment of precursors to crystal surfaces constitutes growth via a nonclassical pathway. The mechanism of oligomer addition to crystals (Figure 3a, 1c) closely resembles that of the classical route. Alternatively, monomers and/or oligomers can assemble into particles or aggregates thereof. The microstructure of these species is often not well characterized. Monomers or oligomers can be loosely connected to form gels or liquid-like aggregates, E
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function of time. The disorder-to-order transition of amorphous particles (postattachment) occurs by ripening and/or solid state rearrangement. In parallel with either of these processes, the concerted addition of monomer and primary particles results in 3D growth (Figure 3b, right). To this end, classical and nonclassical pathways occur simultaneously. Evidence of dual mechanisms in zeolite growth is gleaned from AFM images of crystal surfaces at various stages of crystallization (Figure 5). Under conditions of high
Figure 4. Examples of nonclassical systems with labeled precursor (P) and crystal (C) phases. (a) SEM image of an aluminosilicate worm-like particle (WLP)80 precursor of zeolite LTL. (b) TEM darkfield image of zeolite FAU crystallites (bright spots) nucleating on the exterior surfaces of colloidal precursors. Inset: corresponding brightfield image showing the aggregated particles (scale bar equals 50 nm). (c) Zeolite CHA crystal extracted during an intermediate stage of growth is rough due to the attachment of amorphous precursor particles. (d) TEM image of the nonoriented attachment of amorphous primary particles to the surface of a magnetite (Fe3O4) crystal.81 (Reprinted with permission from ref 81. Copyright 2013 Nature Publishing Group.) (e) In situ TEM image of an amorphous calcium carbonate (ACC) precursor with liquid-like properties feeding solute to two vaterite crystals.16 (Reprinted with permission from ref 16. Copyright 2014 American Association for the Advancement of Science.) (f) In situ TEM image taken moments prior to the oriented attachment of a single crystalline iron oxyhydroxide precursor to a crystal surface. (Image provided by J.J. De Yoreo.)
Figure 5. Evidence of zeolite growth by dual mechanistic pathways: nonclassical (left column; a and c) and classical (right column; b and d). Examples are shown for the following zeolite framework types: silicalite-1, or MFI type (a and b)20 (From ref 20. Reprinted with permission from American Association for the Advancement of Science.) and SSZ-13 or CHA type (c and d)90 (Reprinted from ref 90 with permission. Copyright 2015 American Chemical Society.). In situ AFM measurement of a silicalite-1 surface in (a) revealed the concurrent addition of monomers and amorphous primary particles. Scale bars equal 500 nm.
supersaturation, surfaces of zeolite crystals appear rough owing to the deposition of primary particles (Figure 5a) that evolve into 3D layers composed of macrosteps (Figure 5c). When crystals are removed from a growth solution after days of hydrothermal treatment, their surfaces often contain layers (or hillocks) with step heights comparable to unit cell dimensions (Figure 5b,d).20,90 This suggests that the final stage of zeolite crystallization, which occurs when solutions are near equilibrium and when the concentration of precursors is low, is dominated by classical 2D layer growth. However, in situ measurements of zeolite growth (e.g., silicalite-1 in Figure 5a) have shown that both classical and nonclassical pathways occur simultaneously.20 In most cases, the contribution of each pathway to the overall rate of zeolite formation is not well understood, and remains a subject of ongoing investigation.
time-resolved stages of CaCO3 crystallization (Figure 4e) wherein the ACC particles directly transfer solute to growing crystallites.16 Similar techniques have been used to track the growth of another biomineral, iron oxyhydroxide, and showed that nanocrystal precursors (Figure 4f) directly incorporate into a crystal by oriented attachment.89 These two examples, along with others shown in Figure 4, illustrate the diversity of precursors and pathways that are encountered in nonclassical crystallization that must be taken into consideration when elucidating the mechanism(s) of growth modification. Focusing now on the surface of crystals that grow by CPA processes, Figure 3b illustrates the general modes of layer growth that can occur by either 2- or 3-dimensional mechanisms. Classical crystal growth via monomer-by-monomer attachment to kink/step sites leads to 2D layer nucleation and spreading (Figure 3b, left). Conversely, the attachment of amorphous precursor particles generates 3D nuclei that undergo further molecular rearrangement and growth as a
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GROWTH MODIFIERS Classical Modes of Action. As outlined in Scheme 1, crystal growth modifiers can take a variety of forms and have a wide range of effects on the kinetics of crystallization that include promotion and inhibition (the latter is a more common phenomenon). Inhibitors alter anisotropic growth rates with F
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postulate that roughened surfaces are created through competitive dissolution/attachment events, generating more kink sites that lead to nonuniform step advancement and the disappearance of distinct layers. Mechanisms of crystal growth inhibition based on modifier binding to active sites on crystal surfaces are kinetically driven. Modifiers adsorbed on crystal surfaces impede the addition of solute, leading to a reduction in the anisotropic rate(s) of growth. Conversely, thermodynamic effects on the rate of crystallization can arise during the addition of modifiers.92 For example, favorable interaction between soluble monomer and modifier can lead to the formation of a monomer−modifier complex in solution. If sufficient complexation occurs (i.e., on the order of solute concentration), this phenomenon can reduce the relative supersaturation to a level that either partially or fully inhibits crystallization. Moreover, it is possible that complexes are the species that bind to crystal surface sites and impede layer growth. In this way, both kinetic and thermodynamic factors can play a role in crystal growth inhibition. Nonclassical Growth Inhibition: The New Frontier. Compared to processes described in the previous section, defining surface sites in nonclassical crystal growth is more complex. For example, how do we define kink sites in 3D islands that begin as an amorphous deposit, structurally transition into crystalline domains, and continue to grow 3dimensinally by a complex sequence of dynamic events? The surfaces of crystals that grow via nonclassical routes are continually changing. This presents a challenge to identify “active” sites for modifier adsorption. Given the multiple pathways of crystal growth presented in Figure 3a, modifiers can act by various mechanisms. They can, for example, alter monomer, oligomer, or precursor attachment (Figure 3a: 1a, 1c, and 2a, respectively), speciation reactions (Figure 3a, 1b), the self-assembly of amorphous particles, and the structural evolution of precursors during crystal growth (Figure 3a, 2b). Modifiers may also influence the disorder-to-order transition of precursors (postattachment) on crystal surfaces (Figure 3a, 2c). Lastly, modifiers can potentially interact with species in solution to form complexes with monomers/oligomers (i.e., a thermodynamic pathway), or stabilize precursors to prevent them from aggregating and/or attaching to crystal surfaces (i.e., colloidal stabilization). Indeed, the number of routes by which a modifier can inhibit nonclassical crystal growth far outnumbers the options for classical growth illustrated in Figure 6. There are relatively few examples of modifiers in nonclassical crystallization owing to the recent advent of this field of research.15 Our group has published several articles on the subject of zeolite growth modifiers (or ZGMs). Zeolites were one of the first materials suggested to grow by CPA processes.83 To date, we have applied ZGMs to four different framework types: MFI (silicalite-1),20 LTL (zeolite L),72 MOR (mordenite),56 and CHA (SSZ-13).90 Here we highlight the results of LTL and CHA modification with ZGMs that seemingly operate by different mechanisms. Growth solutions used to prepare both of these zeolites are composed of bulk amorphous precursors (LTL, Figure 4a; CHA, Figure 8a). LTL crystals exhibit cylindrical habit (Figure 7a) with pores oriented axially in the [001] direction. The most effective ZGMs identified for LTL synthesis were alcohols (e.g., diols and triols) and amines. The addition of most alcohols and primary amines to LTL growth solutions reduced the crystal aspect ratio, resulting in the formation of thin cylindrical disks (Figure 7b).
concomitant changes in crystal size and habit, which are factors that influence their properties for various applications. Understanding the fundamental mechanisms by which modifiers alter solute attachment to crystals allows us to select modifiers with desired specificity. Modifiers can bind to kinks, steps, and/or terraces. A common example of modifier inhibition is step pinning3 wherein modifiers adsorb onto crystal terraces and impede the advancement of unfinished layers (Figure 6a). As an advancing layer encounters an
Figure 6. Illustrations of classical modifier−crystal interactions. (a) Step pinning occurs when modifiers adsorb onto a crystal surface (terrace sites), separated by distance Δx, and impede step velocity v when the radius of curvature R is less than a critical radius Rcrit. (b) Kink blocking occurs when modifier adsorption to a kink site results in suppression of layer growth on a crystal surface. (c) A third mechanism involves an induced strain on the crystal lattice upon adsorption of modifiers to step edges, which under certain conditions can lead to the dissolution of layers.91 Modifiers attach to the growing step edges present on crystal surfaces to impede step advancement. (d) Illustration of the strain induced by the attachment of modifiers to step edges that can lead to dissolution under certain conditions.29 (Adapted from ref 29.)
adsorbed modifier on terrace sites, its step velocity v is reduced due to the layer being pinned. This reduction in velocity is imposed due to the strain from the step curvature. If two or more modifiers are spaced close enough (distance Δx) on the terrace, the step can be completely arrested due to the step curvature being less than the critical radius Rcrit. Modifiers can also bind to kink sites through a mode of action referred to as “kink blocking” (Figure 6b)57 where the adsorbed modifier frustrates layer advancement by occupying a location for monomer attachment, thereby preventing the growth of the crystal at this location. If enough modifiers bind to kinks, the crystal surface will become highly strained (again due to the curvature imposed by bound modifiers) and step advancement will either be fully arrested or substantially reduced. A third mechanism, as illustrated in Figure 6c, is modifier binding to step edges, which in many ways operates similar to kink blocking; however, recent studies in our group91 have shown that adsorbed modifiers can induce localized strain that dissolves the crystal under certain conditions, as illustrated in Figure 6d. The exact mechanism for this mode of action has yet to be fully reconciled, as the phenomenon has only been observed under a specific range of modifier concentration. For example, if the quantity of modifier is too low we observe behavior similar to kink blocking. If the quantity of modifier (i.e., modifier coverage on crystal surfaces) is too high, we G
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Perspective
Chemistry of Materials
such, their disorder-to-order transformation is not as simple as the schematic depicted in Figure 3 (path 2c). ZGMs that alter LTL growth most likely impact the release of solute (silica or alumina) from precursors to growing crystals. This is qualitatively consistent with observations that LTL precursors are highly siliceous (e.g., Si/Al > 100)72 and require exchange with the Al-rich solution to form an aluminosilicate crystal with composition Si/Al ≈ 3. There are several misconceptions of ZGMs in the literature. The most common is labeling modifiers as “solvents” that alter the surface free energy of crystals. This thermodynamic argument has also been raised to describe the effect of modifiers on a variety of crystalline materials. For many reported cases, this putative mode of action is based on false premises because the concentration of modifier required to alter crystal growth is relatively lowso low, in fact, that crystal surfaces are predominantly surrounded by water. In zeolite synthesis, less than 3 wt % ZGM is sufficient to have a notable effect on crystal size. Under such dilute conditions, high modifier concentration at the crystal surface is brought about by adsorption, often leading to Langmuir-like behavior wherein modifier efficacy plateaus at higher modifier concentration, suggesting the adsorbed modifier has reached full coverage on the crystal surface. Our study of LTL provided clear evidence of this effect, illustrating that ZGM action is a kinetic phenomenon where modifier adsorption to particle surfaces (precursor or crystal) leads to a reduction in the growth rate. The effect of ZGMs on SSZ-13 (CHA) appears to operate by a mechanism that differs from that of LTL. SSZ-13 crystals grow nonclassically from solutions composed of 100 nm spheroidal precursors (Figure 8a).90 High resolution TEM images of the very first crystals detected in SSZ-13 synthesis (Figure 8b) reveal interfaces laden with what appears to be precursors and smaller particulates (
