Hybrid organic–inorganic perovskite (HOIP) crystals are promising optoelectronic materials, but little is known about either the thermodynamic and kinetic controls on crystal growth or the underlying growth mechanism(s). Herein, we use fluid-cell atomic force microscopy (AFM) and solution nuclear magnetic resonance (NMR) spectroscopy to investigate the growth of the model HOIP crystal CH3NH3PbBr3 (MAPbBr3) and to determine how formic acid (HCOOH) modulates the thermodynamics and kinetics of growth. The results show that growth of MAPbBr3 in dimethylformamide (DMF) proceeds through the classical pathway by the spreading of molecular crystal steps generated at screw dislocations on the {100} surface. Temperature-dependent step velocity measurements demonstrate that with increasing concentration, HCOOH decreases the solubility of MAPbBr3. From the AFM data, we also determine the apparent kinetic coefficient (β) of step movement as a function of HCOOH concentration. 1H NMR measurements indicate that HCOOH increases the lifetime of the methylammonium (MA+) ions and promotes the association of MAPbBr3, thus tuning the solubility of the perovskite. We further propose that HCOOH alters the molecular tumbling motion and bulk diffusion of the MA+ ions, possibly via H-bonding. Our findings establish a direct correlation between the mesoscale crystal growth kinetics and the molecular-scale interactions between organic additives and constituent ions, providing unprecedented insights for developing predictive syntheses of HOIP crystals with defined size, crystal habit and shape, and defect distribution.
Nanoparticles The peptoid-directed formation of five-fold twinned Au nanostars through particle attachment and facet stabilization is reported by James J. DeYoreo, Chun-Long Chen et al. in their Research Article (e202201980).
Abstract Hierarchical nucleation pathways are ubiquitous in the synthesis of minerals and materials. In the case of zeolites and metal–organic frameworks, pre‐organized multi‐ion “secondary building units” (SBUs) have been proposed as fundamental building blocks. However, detailing the progress of multi‐step reaction mechanisms from monomeric species to stable crystals and defining the structures of the SBUs remains an unmet challenge. Combining in situ nuclear magnetic resonance, small‐angle X‐ray scattering, and atomic force microscopy, we show that crystallization of the framework silicate, cyclosilicate hydrate, occurs through an assembly of cubic octameric Q 3 8 polyanions formed through cross‐linking and polymerization of smaller silicate monomers and other oligomers. These Q 3 8 are stabilized by hydrogen bonds with surrounding H 2 O and tetramethylammonium ions (TMA + ). When Q 3 8 levels reach a threshold of ≈32 % of the total silicate species, nucleation occurs. Further growth proceeds through the incorporation of [(TMA) x (Q 3 8 )⋅ n H 2 O] ( x −8) clathrate complexes into step edges on the crystals.
The unique features of metal–organic frameworks (MOFs), such as their large surface areas and diversity of structures, make them suitable for a broad range of applications including storage, separation, and sensing of gases. Among all the MOFs, Mg-MOF-74 with the highest CO2 uptake at 1 bar and 25 °C would be particularly beneficial for CO2-related applications. One of the most critical enabling technologies for implementing Mg-MOF-74 is the preparation of dense and continuous films that would maximize the sorption behaviors. However, Mg-MOF-74 thin films present significant challenges in demonstrating large-scale coatings. Herein, we demonstrate for the first time high-quality Mg-MOF-74 films synthesized via a vapor-assisted crystallization (VAC) process. The VAC process described herein provides dense and highly crystalline layers of the Mg-MOF-74 thin film with a low coefficient of variation of film thickness below 7%. By minimizing the solvent use, the VAC process is also more environmentally friendly than conventional techniques. In this work, we first optimized a precursor solution for the VAC process and then investigated the effects of synthesis temperature, time, and droplet volume on the growth, crystallinity, and thickness of VAC Mg-MOF-74 films. The porosity of the MOF film was assessed by measuring the CO2 uptake at room temperature and 1 bar. The obtained VAC Mg-MOF-74 films possess a well-defined microporosity, as deduced from CO2 adsorption studies via quartz crystal microbalance (QCM) and comparison with bulk Mg-MOF-74 reference data. Furthermore, CO2 cyclic adsorption–desorption experiments on the VAC Mg-MOF-74 films showed scaled uptakes to a wide range of CO2 concentration without showing significant variations in the baseline. We specifically demonstrate how the film’s quality of the MOF affects adsorption behavior of CO2 on VAC Mg-MOF-74 and drop-cast Mg-MOF-74 films.
Different from the conventional solution precipitation, amorphous precursor involves widely in biomineralizations. It is believed that the development of crystalline structures with a well-defined shape in biological systems is essentially facilitated by the occurrence of these transient amorphous phases. However, the previous studies have not elucidated the physicochemical factors influencing the transformation from the transient phase into the stable phase. In this study, the evolutions from the amorphous calcium phosphate to the different-shaped (hexagon and octahedron; octahedron is an unexpected morphology of the crystal with space group of R3̅c) single crystals of β-tricalcium phosphate (β-TCP) were examined. The hexagonal β-TCP crystals were formed via the phase transformation of amorphous precursor in CaCl2−Na2HPO4-ethylene glycol solution; however, the octahedral β-TCP crystals were formed in Ca(OH)2-(NH4)2HPO4-ethylene glycol solution. Because the interfacial energies between amorphous phase and crystals were much smaller than those between solutions and crystals, the crystallization of the β-TCP phase occurred directly in the amorphous substrate rather than from the solution. It was interesting that the final morphology of product was also determined by the interfacial energy between the transformed crystal and solution. The current work demonstrated that the amorphous precursor epitaxial nucleation process and morphology selection of crystals in the amorphous phase could also be understood by an interfacial energy control. This result might provide an in-depth understanding of the biomimetic synthesis of crystals via a pathway of amorphous precursors.
Generally, two or more phosphors are mixed to achieve multiplex colour in the fluorescence industry. However, such a simple mixture of fluorescent materials usually leads to colour discrepancy and may even affect the colour uniformity due to the distinct physicochemical properties of the different components. Fabrication of a core–shell structure is a novel strategy to prepare colour-tuned fluorescent materials. Here we report a core–shell structure made up from two phosphors, Y2O3:Eu and LnPO4, which can emit red and green light respectively. It is important to control the homogeneous and relatively low supersaturation of LnPO4 (Ln = La, Ce, and Tb) in the solution so that the precipitates of LnPO4 can deposit onto the Y2O3:Eu particles uniformly. This could result in an LnPO4 shell around the Y2O3:Eu core to form micron-sized complex particles. In the preparation of the core–shell structure, the slow hydrolysis of tripolyphosphate to release free phosphate ions is a key factor. It is well known that dissociation of tripolyphosphate is temperature sensitive so that this reaction can be controlled by heating the solution. Under UV excitation, both the core (Y2O3:Eu) and the newly formed shell (LnPO4) can emit their characteristic light; it is interesting that each individual core–shell complex can provide the multiplex colour homogeneously at the micron scale. By adjusting the proportion of core and shell in the complex, the fluorescence colours of the micron-sized phosphor can be tuned conveniently.
The regulation mechanism of organic additives on the crystallization of inorganic crystal is fundamentally important in biomineralization. Experimentally, it was found that the amino acids glycine (Gly) and glutamic acid (Glu) could lead to the formation of rod- and plate-like hydroxyapatite (HAP) crystallites, respectively. The detailed adsorption behavior of Gly and Glu on HAP crystal faces was studied by molecular dynamics (MD) simulation. The specific adsorption sites and patterns of Gly and Glu on the (100) and (001) faces of HAP crystals were revealed at the atomic level. The amino acids adsorbed on the HAP (001) and (100) faces with their positive amino groups occupied vacant calcium sites, and their negative carboxylate groups occupied vacant P or OH sites precisely and formed an ordered adsorption layer. The atomic force microscopy pulling simulation and free energy calculation showed that Glu was much more difficult to depart from the HAP (001) face than that from the (100) face. This result indicated that Glu preferred to adsorb strongly onto the HAP (001) face, which resulted in the formation of plate-like HAP. However, Gly did not show any significantly preferential adsorption between these two HAP faces. Thus, the habits of HAP, rod-like crystallites, were not altered during the HAP crystallization in the presence of Gly. Combined with experimental results, our study demonstrated that the MD simulation of interfacial structures could improve our understanding of biological regulation in mineralization processes at the atomic level.
Amongst the challenges for a variety of research fields are the visualization of solid-liquid interfaces and understanding how they are affected by the solution conditions such as ion concentrations, pH, ligands, and trace additives, as well as the underlying crystallography and chemistry. In this context, three-dimensional fast force mapping (3D FFM) has emerged as a promising tool for investigating solution structure at interfaces. This capability is based on atomic force microscopy (AFM) and allows the direct visualization of interfacial regions in three spatial dimensions with sub-nanometer resolution. Here we provide a detailed description of the experimental protocol for acquiring 3D FFM data. The main considerations for optimizing the operating parameters depending on the sample and application are discussed. Moreover, the basic methods for data processing and analysis are discussed, including the transformation of the measured instrument observables into tip-sample force maps that can be linked to the local solution structure. Finally, we shed light on some of the outstanding questions related to 3D FFM data interpretation and how this technique can become a central tool in the repertoire of surface science.
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