Nucleation: what happens at the initial stage?

Formation of prenucleation clusters and transformation to ZnSe quantum dots and magic-size clusters

Prenucleation clusters (PNCs) play an important role in the synthesis of colloidal semiconductor quantum dots (QDs) and magic-size clusters (MSCs) of binary II-VI metal chalcogenide (ME). Here, we explore the formation and transformation of ZnSe PNCs, using a reaction of zinc oleate (Zn(OA)2), tri-n-octylphosphine selenide (SeTOP), and diphenylphosphine (HPPh2, DPP). The PNC forms above 120 °C, and relatively high feed concentrations and Zn-to-Se molar ratios favor the formation. When a prenucleation-stage sample (160 °C/30 min) is dispersed in a mixture of cyclohexane (CH) and octylamine (OTA) at 25 °C, MSC-299 (displaying optical absorption peaking at 299 nm) develops from the PNC isomerization. The PNC is the precursor compound (PC-299) of MSC-299 and is relatively transparent. When the reaction is heated at higher temperatures (such as 220 °C for 15 min), the PNC fragments to monomers (Mos) and the nucleation and growth (N/G) of QDs occur (via the one-by-one addition of Mos). In a mixture of CH and CH3OH at 25 °C, MSC-299 transforms to MSC-320 and MSC-340. The present study provides a deeper understanding of the formation and transformation of ZnSe PNCs.

Understanding the nanoscale phenomena of nucleation and crystal growth in electrodeposition

Electrodeposition is used at the industrial scale to make coatings, membranes, and composites. With better understanding of the nanoscale phenomena associated with the early stage of the process, electrodeposition has potential to be adopted by manufacturers of energy storage devices, advanced electrode materials, fuel cells, carbon dioxide capturing technologies, and advanced sensing electronics. The ability to conduct precise electrochemical measurements using cyclic voltammetry, chronoamperometry, and chronopotentiometry in addition to control of precursor composition and concentration makes electrocrystallization an attractive method to investigate nucleation and early-stage crystal growth. In this article, we review recent findings of nucleation and crystal growth behaviors at the nanoscale, paying close attention to those that deviate from the classical theories in various electrodeposition systems. The review affirms electrodeposition as a valuable method both for gaining new insights into nucleation and crystallization on surfaces and as a low-cost scalable technology for the manufacturing of advanced materials and devices.

Thermally-Induced Isomerization of Prenucleation Clusters During the Prenucleation Stage of CdTe Quantum Dots

The evolution of prenucleation clusters in the prenucleation stage of colloidal semiconductor quantum dots (QDs) has remained unexplored. With CdTe as a model system, we show that substances form and isomerize prior to the nucleation and growth of QDs. Called precursor compounds (PCs), the prenucleation clusters are relatively optically transparent and can transform to absorbing magic-size clusters (MSCs). When a prenucleation-stage sample at 25, 45, or 80 °C is dispersed in a mixture of cyclohexane (CH) and octylamine (OTA) at room temperature, either MSC-371, MSC-417, or MSC-448 evolves with absorption peaking at 371, 417, or 448 nm, respectively. We propose that PC-371 forms at 25 °C, and isomerizes to PC-417 at 45 °C and to PC-448 at 80 °C. The PCs and MSCs are quasi isomers. Relatively large and small amounts of OTA favor PC-371 and PC-448 in dispersion, respectively. The present findings suggest the existence of PC-to-PC isomerization in the QD prenucleation stage.

Nucleation and Growth of an Ensemble of Crystals during the Intermediate Stage of a Phase Transition in Metastable Liquids

In this paper, an analytical method of solving the integro-differential system of kinetic and balance equations describing the evolution of an ensemble of crystals during the intermediate phase of the bulk crystallization process is described. The theory is developed for kinetic equations of the first- and second order corresponding to the absence and presence of fluctuations in particle growth rates. The crystal-size distribution function as well as the dynamics of metastability reduction in a supercooled melt (supersaturated solution) are analytically found using the saddle-point and the Laplace transform methods. The theory enables us to obtain the crystal-size distribution function that establishes in a supercooled (supersaturated) liquid at the beginning of the final stage of a phase transformation process when Ostwald ripening, coagulation and fragmentation of crystals are able to occur.

Crystallisation in a two-dimensional granular system at constant temperature

We study the crystallisation processes occurring in a nonvibrating two-dimensional magnetic granular system at various fixed values of the effective temperature. In this system, the energy loss due to dissipative effects is compensated by the continuous energy input coming into the system from a sinusoidal magnetic field. When this balance leads to high values of the effective temperature, no aggregates are formed, because particles’ kinetic energy prevents them from aggregating. For lower effective temperatures, formation of small aggregates is observed. The smaller the values of the applied field’s amplitude, the larger the number of these disordered aggregates. One also observes that when clusters form at a given effective temperature, the average effective diffusion coefficient decreases as time increases. For medium values of the effective temperature, formation of small crystals is observed. We find that the sixth bond-orientational order parameter and the number of bonds, when considering more than two, are very sensitive for exhibiting the order in the system, even when crystals are still very small.

Anomalous structure transition in undercooled melt regulates polymorphic selection in barium titanate crystallization

The crystallization processes of titanates are central to the fabrication of optical and electrical crystals and glasses, but their rich polymorphism is not fully understood. Here, we show when and how polymorphic selection occurs during the crystallization of barium titanate (BaTiO3, BT) using in situ high energy synchrotron X-ray diffraction and ab initio molecular dynamic simulation. An anomalous structure transition is found in molten BT during cooling across the cubic-hexagonal transition temperature, which enables nucleation selection of BT by manipulating the undercooling: a cubic phase is preferred if nucleation is triggered at large undercooling, whereas a hexagonal phase is promoted at small undercooling. We further reveal that the nucleation selection between the cubic and the hexagonal phase is regulated by the intrinsic structure property of the melt, in particular, the degree of polymerization between Ti-O polyhedra. These findings provide an innovative perspective to link the polymorphic crystallization to the non-isomorphic structure transition of the melt beyond the conventional cognition of structural heredity.

Reversible disorder-order transitions in atomic crystal nucleation

Nucleation in atomic crystallization remains poorly understood, despite advances in classical nucleation theory. The nucleation process has been described to involve a nonclassical mechanism that includes a spontaneous transition from disordered to crystalline states, but a detailed understanding of dynamics requires further investigation. In situ electron microscopy of heterogeneous nucleation of individual gold nanocrystals with millisecond temporal resolution shows that the early stage of atomic crystallization proceeds through dynamic structural fluctuations between disordered and crystalline states, rather than through a single irreversible transition. Our experimental and theoretical analyses support the idea that structural fluctuations originate from size-dependent thermodynamic stability of the two states in atomic clusters. These findings, based on dynamics in a real atomic system, reshape and improve our understanding of nucleation mechanisms in atomic crystallization.

Pattern detection in colloidal assembly: A mosaic of analysis techniques

Characterization of the morphology, identification of patterns and quantification of order encountered in colloidal assemblies is essential for several reasons. First of all, it is useful to compare different self-assembly methods and assess the influence of different process parameters on the final colloidal pattern. In addition, casting light on the structures formed by colloidal particles can help to get better insight into colloidal interactions and understand phase transitions. Finally, the growing interest in colloidal assemblies in materials science for practical applications going from optoelectronics to biosensing imposes a thorough characterization of the morphology of colloidal assemblies because of the intimate relationship between morphology and physical properties (e.g. optical and mechanical) of a material. Several image analysis techniques developed to investigate images (acquired via scanning electron microscopy, digital video microscopy and other imaging methods) provide variegated and complementary information on the colloidal structures under scrutiny. However, understanding how to use such image analysis tools to get information on the characteristics of the colloidal assemblies may represent a non-trivial task, because it requires the combination of approaches drawn from diverse disciplines such as image processing, computational geometry and computational topology and their application to a primarily physico-chemical process. Moreover, the lack of a systematic description of such analysis tools makes it difficult to select the ones more suitable for the features of the colloidal assembly under examination. In this review we provide a methodical and extensive description of real-space image analysis tools by explaining their principles and their application to the investigation of two-dimensional colloidal assemblies with different morphological characteristics.

Microcanonical Nucleation Theory for Anisotropic Materials Validated on Alumina Clusters

Nucleation kinetics in gas phase remains an open issue with no general model. The derivation of the reaction constants assuming a canonical ensemble fails to describe anisotropic materials such as oxides. We have developed a general and versatile model using activated complex kinetics with a microcanonical approach. This approach handles the kinetics issue in cluster growth when the transient nature of the processes hinders the use of the canonical ensemble. The model efficiently reproduces experimental size distributions of alumina clusters formed by laser ablation with different buffer gas densities, including magic numbers. We show that the thermodynamic equilibrium is not reached during the growth. The bounding energy measured is 10 times lower than the one deduced from DFT calculation, but also the one expected from the bulk cohesive energy.

Dust Effects on Ir(0)n Nanoparticle Formation Nucleation and Growth Kinetics and Particle Size-Distributions: Analysis by and Insights from Mechanism-Enabled Population Balance Modeling

The effects of microfiltration removal of filterable dust on nanoparticle formation kinetics and particle-size distribution, in a polyoxometalate polyanion (P₂W₁₅Nb₃O₆₂^9-)-stabilized Ir(0)_n nanoparticle formation system, are analyzed by the newly developed method of Mechanism-Enabled Population Balance Modeling (ME-PBM). The [(Bu₄N)₅Na₃(1,5-COD)Ir·P₂W₁₅Nb₃O₆₂] precatalyst system produces on average Ir(0)_∼200 nanoparticles of 1.74 ± 0.33 nm and hence a particle-size distribution (PSD) of ±19% dispersion when the precatalyst is reduced under H₂ in unfiltered propylene carbonate solvent. But if the precatalyst is reduced in microfiltered solvent and microfiltered reagent solutions (where the filtered solvent is then also used to rinse dust from the glassware), then larger Ir(0)_∼300 1.96 ± 0.16 nm nanoparticles are produced with a remarkable, 2.4-fold lowered ±8% dispersion. The results and effects of the microfiltration reduction of dust are analyzed by the newly developed method of ME-PBM. More specifically, the studies reported herein address eight outstanding questions that are listed in the Introduction. Those questions include: how easy or difficult it is to fit PSD data? What is the ability of the recently discovered alternative termolecular nucleation and two size-dependent growth steps mechanism to account for the effects of dust on the PSD? What types and amount of PSD kinetics data are needed to deconvolute the PSD into the parameters of the ME-PBM? What is the reliability of the resulting rate constants? Additional questions addressed include: if the ME-PBM results offer insights into the remarkable 2.4-fold narrowing of the PSD post simple microfiltration lowering of the dust, and if the results are likely to be more general? The Summary and Conclusions section lists nine specific insights that include comments on needed future studies.

Homogeneous Nucleation Mechanism of NaCl in Aqueous Solutions

In this study, molecular dynamic simulations are employed to investigate the homogeneous nucleation mechanism of NaCl crystal in solutions. According to the simulations, the dissolved behaviors of NaCl in water are dependent on ion concentrations. With increasing NaCl concentrations, the dissolved Na+ and Cl- ions tend to be aggregated in solutions. In combination with our recent studies, the aggregate of dissolved solutes is mainly ascribed to the hydrophobic interactions. Different from the two-step mechanism, no barrier is needed to overcome the formation of the aggregate. In comparison with the classical nucleation theory (CNT), because of the formation of solute aggregate, this lowers the barrier height of nucleation and affects the nucleation mechanism of NaCl crystal in water.

Recent experimental explorations of non-classical nucleation

This work provides a clearer picture for non-classical nucleation by revealing the presence of various intermediates using advanced characterization techniques.

Polymorphic transition and nucleation pathway of barium dititanate (BaTi2O5) during crystallization from undercooled liquid

The nucleation pathway plays an important role in vitrification, preparation of glass-ceramic composites and synthesis of metastable materials. In this paper, we studied the nucleation pathway of a novel ferroelectric BaTi₂O₅ (BT2) during crystallization from undercooled liquid by aerodynamic levitation (ADL) containerless processing and structural analysis. An interesting polymorphic transition of BT2 regulated by the undercooling was observed during the crystallization process: the ferroelectric monoclinic phase (γ-BT2) was fabricated at low undercoolings and the paraelectric orthorhombic metastable phase (β-BT2) was obtained from hypercooled liquid. This polymorphic transition phenomenon corresponds to a non-classical nucleation pathway: metastable β-BT2 preferentially nucleates from undercooled melt and γ-BT2 is generated from β phase by solid-state phase transition. The two-step nucleation pathway stems from the structural heredity between the undercooled liquid and crystals. A stronger structural homology exists between the undercooled melt and β-BT2 than γ-BT2 based on diffraction data and atomic configurations analysis. This structural homology coupled with nucleation barrier calculation was used to elucidate the non-classical nucleation pathway of BT2 crystallization: the similarity of the structural unit (Ti-O polyhedra) between the undercooled liquid and the metastable β-BT2 reduces the nucleation barrier and contributes to the preferential precipitation of β-like clusters. This work reveals the formation route of BT2 from cooling melt, which not only benefits the synthesis and application of this novel functional material but also provides a guideline of the crystallization process of titanates from melt at atomic level.

Can the pathway of stepwise nucleation be predicted and controlled?

Predicting the critical nucleus size and the nucleation barrier is of central importance in controlling the dynamics of nucleation. However, as the nucleation of a crystal involves intermediate states, the prediction becomes inaccessible with currently available models. Here, we show that based on single-particle level observations, the properties of crystal nuclei in a microscopic stepwise nucleation (MSN) can be well-quantified by incorporating the size and structure order parameter into the formula of free energy without prior knowledge of interfacial tension. The quantified free energy reveals that the intermediate structures arise from thermodynamics rather than kinetics. Precritical and postcritical nuclei are distinct not only in structure but also in the mechanism of crystalline ordering. The relative stability of intermediate structures and the pathway of nucleation can be well-controlled by supercooling. Our studies offer a successful approach to quantify MSN and shed new light on resolving the long-standing discrepancies between simulations and experiments.

Molecular dynamics investigation of the local structure in iron melts and its role in crystal nucleation during rapid solidification

Nucleation process of a bcc crystal after the formation of an MRO cluster.

Electric-field assisted nucleation processes of croconic acid films

An external electric field can affect the growth dynamics of croconic acid (CA) films, especially the heterogeneous nucleation process, as evidenced by the shift of temperature dependence of the nucleation rate in the electric field.

Tailored Robust Hydrogel Composite Membranes for Continuous Protein Crystallization with Ultrahigh Morphology Selectivity

The tailored and robust hydrogel composite membranes (HCMs) with diverse ion adsorption and interfacial nucleation property are prepared and successfully used in the continuous lysozyme crystallization. Beyond the heterogeneous supporter, the HCMs functioning as an interface ion concentration controller and nucleation generator are demonstrated. By constructing accurately controlled nucleation and growth circumstances in the HCM-equipped membrane crystallizer, the target desired morphology (hexagon cube) and brand-new morphology (multiple flower shape) that differ from the ones created in the conventional crystallizer are continuously and repetitively generated with ultrahigh morphology selectivity. These tailored robust HCMs show great potential for improving current approaches to continuous protein crystallization with specific crystal targets from laboratorial research to actual engineering applications.

Nucleation of hematite nanocrystals revealed by a single nanosecond laser pulse method

Full understanding about how nanocrystals nucleate is mainly inhibited by two factors: (1) lack of a comprehensive theoretical framework which can explain and predict the occurrence of a nonclassical indirect nucleation pathway, and (2) lack of a high temporal and spatial resolution observation method to study the nucleation process. Here we introduce a single nanosecond laser pulse method with TEM observation to reveal the nucleation mechanism of hematite nanocrystals. The single pulse experimental design aims at revealing the phenomenon at the earliest stage of the crystallization process. The short nanosecond heating duration prevents the aging process which might obscure the precise nucleation information for the TEM study. By this method, it is shown that the nucleation pathway of hematite nanocrystals can be tuned by the energy input of the laser pulse. Hematite crystals, amorphous materials or both are nucleated depending on the energy of the first laser pulse. We believe that the short laser pulse method is a valuable method to study other nucleation phenomena at the nanoscale and will contribute to deepening our understanding about how different material structures form at the very early stage.

Enhanced thermoelectric performance of Cu3SbS4 flower-like hierarchical architectures composed of Cl doped nanoflakes via an in situ generated CuS template

In this work, Cu₃SbS₄ hierarchical flower-like microspheres composed of chlorine (Cl^-)-doped Cu₃SbS₄ nanoflakes are realized via a one pot solvothermal ion exchange reaction. The kinetic factors including the duration time, the ratio of source materials, and the KOH concentration, are systematically investigated. Using a suite of analytical techniques, including SEM, XRD and FTIR, the mechanism of the two stage in situ chemical transformation of CuS flower-like microspheres consisting of nanoflake intermediates to the target product Cu₃SbS₄ is elucidated. The difference in solubility between reactants and products (Ksp(CuS) > Ksp(CuSbS_x)) determines that the ion-exchange reaction from transition binary to ternary metal chalcogenides is favorable under the impetus of a thermodynamic driving force. In addition, the optical and enhanced thermoelectric transport properties are investigated. The results revealed that Cl-doped Cu₃SbS₄ exhibited an improved power factor, which was 8 times higher than that of undoped Cu₃SbS₄ at 500 K. The current study not only provides a facile and economical way to synthesize high-quality Cl-doped Cu-Sb-S three dimensional (3D) hierarchical nanostructures, but also opens up a new route for preparation of other I-V-VI multicomponent chalcogenide NCs, such as Cu-Bi-S and Cu-Pb-S systems, which would be difficult to obtain otherwise.

Nonclassical nucleation pathways in protein crystallization

Classical nucleation theory (CNT), which was established about 90 years ago, has been very successful in many research fields, and continues to be the most commonly used theory in describing the nucleation process. For a fluid-to-solid phase transition, CNT states that the solute molecules in a supersaturated solution reversibly form small clusters. Once the cluster size reaches a critical value, it becomes thermodynamically stable and favored for further growth. One of the most important assumptions of CNT is that the nucleation process is described by one reaction coordinate and all order parameters proceed simultaneously. Recent studies in experiments, computer simulations and theory have revealed nonclassical features in the early stage of nucleation. In particular, the decoupling of order parameters involved during a fluid-to-solid transition leads to the so-called two-step nucleation mechanism, in which a metastable intermediate phase (MIP) exists between the initial supersaturated solution and the final crystals. Depending on the exact free energy landscapes, the MIPs can be a high density liquid phase, mesoscopic clusters, or a pre-ordered state. In this review, we focus on the studies of nonclassical pathways in protein crystallization and discuss the applications of the various scenarios of two-step nucleation theory. In particular, we focus on protein solutions in the presence of multivalent salts, which serve as a model protein system to study the nucleation pathways. We wish to point out the unique features of proteins as model systems for further studies.

Approaches to self-assembly of colloidal monolayers: A guide for nanotechnologists

Self-assembly of quasi-spherical colloidal particles in two-dimensional (2D) arrangements is essential for a wide range of applications from optoelectronics to surface engineering, from chemical and biological sensing to light harvesting and environmental remediation. Several self-assembly approaches have flourished throughout the years, with specific features in terms of complexity of the implementation, sensitivity to process parameters, characteristics of the final colloidal assembly. Selecting the proper method for a given application amidst the vast literature in this field can be a challenging task. In this review, we present an extensive classification and comparison of the different techniques adopted for 2D self-assembly in order to provide useful guidelines for scientists approaching this field. After an overview of the main applications of 2D colloidal assemblies, we describe the main mechanisms underlying their formation and introduce the mathematical tools commonly used to analyse their final morphology. Subsequently, we examine in detail each class of self-assembly techniques, with an explanation of the physical processes intervening in crystallization and a thorough investigation of the technical peculiarities of the different practical implementations. We point out the specific characteristics of the set-ups and apparatuses developed for self-assembly in terms of complexity, requirements, reproducibility, robustness, sensitivity to process parameters and morphology of the final colloidal pattern. Such an analysis will help the reader to individuate more easily the approach more suitable for a given application and will draw the attention towards the importance of the details of each implementation for the final results.

Dust Effects on Nucleation Kinetics and Nanoparticle Product Size Distributions: Illustrative Case Study of a Prototype Ir(0)n Transition-Metal Nanoparticle Formation System

The question is addressed if dust is kinetically important in the nucleation and growth of Ir(0)_n nanoparticles formed from [Bu₄N]₅Na₃(1,5-COD)Ir^I·P₂W₁₅Nb₃O₆₂ (hereafter [(COD)Ir·POM]^8-), reduced by H₂ in propylene carbonate solvent. Following a concise review of the (often-neglected) literature addressing dust in nucleation phenomena dating back to the late 1800s, the nucleation and growth kinetics of the [(COD)Ir·POM]^8- precatalyst system are examined for the effects of 0.2 μm microfiltration of the solvent and precatalyst solution, of rinsing the glassware with that microfiltered solvent, of silanizing the glass reaction vessel, for the addition of <0.2 μm γ-Al₂O₃ (inorganic) dust, for the addition of flame-made carbon-based (organic) dust, and as a function of the starting, microfiltered [(COD)Ir·POM^8-] concentration. Efforts to detect dust and its removal by dynamic light scattering and by optical microscopy are also reported. The results yield a list of eight important conclusions, the four most noteworthy of which are (i) that the nucleation apparent rate "constant" k_1obs(bimol) is shown to be slowed by a factor of ∼5 to ∼7.6, depending on the precise experiment and its conditions, just by the filtration of the precatalyst solution using a 0.20 μm filter and rinsing the glassware surface with 0.20 μm filtered propylene carbonate solvent; (ii) that simply employing a 0.20 μm filtration step narrows the size distribution of the resulting Ir(0)_n nanoparticles by a factor of 2.4 from ±19 to ±8%, a remarkable result; (iii) that the narrower size distribution can be accounted for by the slowed nucleation rate constant, k_1obs(bimol), and by the unchanged autocatalytic growth rate constant, k_2obs(bimol), that is, by the increased ratio of k_2obs(bimol)/k_1obs(bimol) that further separates nucleation from growth in time for filtered vs unfiltered solutions; and (iv) that five lines of evidence indicate that the filterable component of the solution, which has nucleation rate-enhancing and size-dispersion broadening effects, is dust.

Mechanics at the glass-to-gel transition of thermoresponsive microgel suspensions

We study the rheology of systems of thermoresponsive microgels which can transition between a repulsive glass and an attractive gel state. We find marked differences between these two colloidal solids, within the same experimental system, due to the different origins for their dynamic arrest. While the rigidity of the repulsive systems depends solely on particle volume fraction, we find that the change in linear elasticity upon introducing attractive bonds in the system scales linearly with the adhesive bond strength which can be tuned with the temperature in our experiments. And while the glasses yield reversibly and with a rate-dependent energy dissipation, bond-reorganisation in the gels is suppressed so that their rupture is irreversible and accompanied by a high, but rate-independent, dissipation. These results highlight how colloids with responsive interactions can be employed to shed new light onto solid-solid transitions.

Thermodynamics and Kinetics of Prenucleation Clusters, Classical and Non-Classical Nucleation

Recent observations of prenucleation species and multi-stage crystal nucleation processes challenge the long-established view on the thermodynamics of crystal formation. Here, we review and generalize extensions to classical nucleation theory. Going beyond the conventional implementation as has been used for more than a century now, nucleation inhibitors, precursor clusters and non-classical nucleation processes are rationalized as well by analogous concepts based on competing interface and bulk energy terms. This is illustrated by recent examples of species formed prior to/instead of crystal nucleation and multi-step nucleation processes. Much of the discussed insights were obtained from molecular simulation using advanced sampling techniques, briefly summarized herein for both nucleation-controlled and diffusion-controlled aggregate formation.

Polymeric Flower-Like Microparticles from Self-Assembled Cellulose Stearoyl Esters

Flower-like particles (FLPs) with hierarchical surface architectures have recently attracted considerable attention due to their potentially wide application range. Hitherto, nearly all FLPs were fabricated using inorganic compounds, while versatile organic polymers have not received sufficient attention yet. Herein, we show the construction of novel organic, polymeric FLPs with diameters of 2.5-5 μm using cellulose stearoyl esters (CSEs) by means of the crystallization of side chains. CSEs with degrees of substitution of approximately 3 were transformed into FLPs during the gradual precipitation of polymer chains from the mixture of their solutions in dichloromethane (DCM) and a nonsolvent, which is driven by the evaporation of DCM. Ordered petal-like nanostructures were formed on the particle surface through the crystallization of side chains. Finally, partially crystalline FLPs containing lamellar structures were obtained. Moreover, the formation process was strongly affected by the molecular weight of CSE, concentrations of CSE solutions and the volume ratio between DCM and nonsolvents.

Correlation between hierarchical structure of crystal networks and macroscopic performance of mesoscopic soft materials and engineering principles

The performance of soft materials is correlated with the hierarchical crystal network structure by topology, correlation length, symmetry/ordering, and strength.

Hybrid crystals comprising metal-organic frameworks and functional particles: synthesis and applications

Hybrid crystals containing encapsulated functional species exhibit promising novel physical and chemical properties. The realization of many properties critically depends on the selection of suitable functional species for incorporation, the rational control of the crystallinity of the host materials, and the manipulation of the distribution of the encapsulated species; only a few hybrid crystals achieve this. Here, a novel synthetic method enables the encapsulation of functional species within crystalline metal-organic frameworks (MOFs). Various kinds of single-crystalline MOFs with incorporated particles are presented. The encapsulated particles can be distributed in a controllable manner, and the hybrid crystals are applied to the heterogeneous catalysis of the reduction of nitroarenes. These findings suggest a general approach for the construction of MOF materials with potential applications; by combining species and MOFs with suitable functionalities, new properties--not possible by other means--may arise.

Tracking Amorphous Precursor Formation and Transformation during Induction Stages of Nucleation

Hydroxyapatite (HAP) participates in vertebral bone and tooth formation by a nonclassical hitherto unknown nucleation mechanism, in which amorphous precursors form and transform during long induction periods. Elucidation of the mechanism by which amorphous precursors assemble and transform is essential to understanding how hard tissues form in vivo and will advance the design and fabrication of new biomaterials. The combination of conductance and potentiometric techniques to monitor Ca-P mineral formation has given new insight into the mechanism of nucleation. Differences detected in the dehydration rates of calcium and phosphate ions indicate the formation of nonequilibrium calcium-deficient clusters. The aggregation of these clusters forms a calcium-deficient amorphous phase I [Ca-(HPO₄)_1+x ·nH₂O]^2x-) early in the induction period, which slowly transforms to amorphous phase II [Ca-(HPO₄)·mH₂O] by dehydration. Precritical nuclei form within amorphous phase II later in the induction period, leading to mineral formation.