Assessing the synergy effect of additive and matrix on single-crystal growth: Morphological revolution resulted from gel-mediated enhancement on CIT-calcite interaction

CIT additives are active in crystal morphology modification at relatively high concentration. Once agarose gel is simultaneously introduced, the curved morphological feature of calcite crystal emerges at a much lower concentration of CIT additives.     

Functionalizing Single Crystals: Incorporation of Nanoparticles Inside Gel‐Grown Calcite Crystals

Synthetic single crystals are usually homogeneous solids. Biogenic single crystals, however, can incorporate biomacromolecules and become inhomogeneous solids so that their properties are also extrinsically regulated by the incorporated materials. The discrepancy between the properties of synthetic and biogenic single crystals leads to the idea to modify the internal structure of synthetic crystals to achieve nonintrinsic properties by incorporation of foreign material. Intrinsically colorless and diamagnetic calcite single crystals are turned into colored and paramagnetic solids, through incorporation of Au and Fe₃O₄ nanoparticles without significantly disrupting the crystalline lattice of calcite. The crystals incorporate the nanoparticles and gel fibers when grown in agarose gel media containing the nanoparticles, whereas the solution‐grown crystals do not. As such, our work extends the long‐history gel method for crystallization into a platform to functionalize single‐crystalline materials.     

Crystallization on surfaces of well-defined topography

Single crystals of calcite with regular patterned surfaces comprising close-packed arrays of hemispherical cavities or domes were produced by crystallization on colloidal monolayers or PDMS replicas of these monolayers, respectively. Perfect replication of the substrate topography was achieved for all colloidal particles, irrespective of their size and surface chemistry when the substrate geometry permitted unrestricted ion flow to the growing crystal. This work demonstrates that crystallization within a mould provides a very general route to producing single crystals with curved surfaces and unusual morphologies and that such patterning can be applied from the micro- to the nanoscale.     

Well-defined star-shaped calcite crystals formed in agarose gels

Single crystals of calcite exhibiting a morphology of well-defined 8-armed stars, which evolved from original rhombohedral calcite crystals with their 8 points extending radially into eight arms, were produced by crystallization of CaCO3 in agarose gels.     

The Effect of Additives on the Early Stages of Growth of Calcite Single Crystals

As crystallization processes are often rapid, it can be difficult to monitor their growth mechanisms. In this study, we made use of the fact that crystallization proceeds more slowly in small volumes than in bulk solution to investigate the effects of the soluble additives Mg²⁺ and poly(styrene sulfonate) (PSS) on the early stages of growth of calcite crystals. Using a “Crystal Hotel” microfluidic device to provide well‐defined, nanoliter volumes, we observed that calcite crystals form via an amorphous precursor phase. Surprisingly, the first calcite crystals formed are perfect rhombohedra, and the soluble additives have no influence on the morphology until the crystals reach sizes of 0.1–0.5 μm for Mg²⁺ and 1–2 μm for PSS. The crystals then continue to grow to develop morphologies characteristic of these additives. These results can be rationalized by considering additive binding to kink sites, which is consistent with crystal growth by a classical mechanism.     

Visualizing the toughening origins of gel-grown calcite single-crystal composites

The mechanical properties of gel-grown calcite single-crystal composites can be modified by incorporation of guest materials including agarose gel fiber, MWCNTs and GO. The in-situ observation of the crystal deformation suggests that the guest incorporation toughens the single-crystal host by the shielding effect of nanofiber on crack-bridging at nanoscale.     

P(St-co-MAA)乳液体系对CaCO3结晶的调控

大量研究表明, 有机/无机界面上的相互作用[1]是控制无机结晶的晶型、形貌、粒径等特征的决定因素. 本文利用乳液聚合方法合成了在诱导无机矿化后依然保持较为刚性界面的、能与无机离子作用的微球, 并在乳液中进行碳酸钙结晶实验, 用XRD, FTIR和SEM等手段对结晶进行了表征.     

Surface and bulk morphology of cold-crystallized poly(ethylene terephthalate)

The morphology/habit of crystals of cold-crystallized poly(ethylene terephthalate) (PET) has been evaluated using scanning and transmission electron microscopy and using atomic force microscopy. The combination of different preparation and analysis techniques allowed assessing the structure at the nanometer scale of films of PET at both the surface and the bulk. It is found that crystals formed on heating the amorphous glass to a temperature higher than the glass transition temperature are of lamellar shape in the bulk and almost isometric habit at the surface. This finding is explained by different rates of nucleation/crystallization in the bulk and at the surface, being supported by the observation of nanometer-scale surface heterogeneities after quenching PET to ambient temperature before crystallization was initiated by heating.     

Effect of inorganic anions on the morphology and structure of magnesium calcite

Calcium carbonate was precipitated from calcium hydroxide and carbonic acid solutions at 25 degrees C, with and without addition of different magnesium (MgSO(4), Mg(NO(3))(2) and MgCl(2)) and sodium salts (Na(2)SO(4), NaNO(3) and NaCl) of identical anions, in order to study the mode of incorporation of magnesium and inorganic anions and their effect on the morphology of calcite crystals over a range of initial reactant concentrations and limited c(i)(Mg(2+))/c(i)(Ca(2+)) molar ratios. The morphology, crystal size distribution, composition, structure, and specific surface area of the precipitated crystals, as well as the mode of cation and anion incorporation into the calcite crystal lattice, were studied by a combination of optical and scanning electron microscopy (SEM), electronic counting, a multiple BET method, thermogravimetry, FT-IR spectroscopy, X-ray diffraction (XRD), and electron paramagnetic resonance (EPR) spectroscopy. In the systems of high initial relative supersaturation, precipitation of an amorphous precursor phase preceded the formation of calcite, whereas in those of lower supersaturation calcite was the first and only polymorphic modification of calcium carbonate that appeared in the system. The magnesium content in calcite increased with the magnesium concentration in solution and was correlated with the type of magnesium salt used. Mg incorporation caused the formation of crystals elongated along the calcite c axis and, in some cases, the appearance of new [011] faces. Polycrystalline aggregates were formed when the c(i)(Mg(2+))/c(i)(Ca(2+)) molar ratios in solution were increased. Addition of sulfate ions, alone, caused formation of spherical calcite polycrystalline aggregates.     

Effects of Carboxylic Acids on Calcite Formation in the Presence of Mg2+ Ions

The effects of seven carboxylic acids on calcite formation in the presence of Mg2+ ions, whose molar concentration ratio Mg2+/Ca2+ = 0.5 exclusively induced aragonite precipitation in the absence of carboxylic acids, were studied using a double diffusion technique. The presence of carboxylic acids, acrylic acid, maleic acid, tartaric acid, malonic acid, malic acid, succinic acid, and citric acid in the gel medium favored the formation of magnesian calcite relative to the amount of the additives. Induction time and the positions of the first precipitation were measured to analyze the behavior of crystallization based on the equivalency rule. The formation of magnesian calcite was also studied with the help of Avrami's equation (solid-state model for transformation). The results of applying this equation suggested that aragonite transformed into calcite through a solid-to-solid process. The formation of magnesian calcite was interpreted as the following process: aragonite nuclei, formed owing to Mg2+ ions at the initial stage of CaCO3 crystallization, transformed into calcite nuclei through a solid-to-solid process while their growth was inhibited by the adsorption of carboxylic acids. The magnesian calcite crystals grew on crystal seeds of calcite formed from aragonite nuclei. Copyright 1999 Academic Press.     

Crystallization behavior of polylactide on highly oriented polyethylene thin films

The crystalline structure and morphology of the PLA crystallized isothermally from the glassy state on highly oriented PE substrates at 130℃were investigated by means of optical microscopy,AFM and X-ray diffraction.The results indicate that the PE substrate influences the crystallization behavior of PLA remarkably,which leads to the growth of PLA crystals on PE substrate always in edge-on form rather than the twisted lamellar crystals from edge-on to flat-on when crystallizing the PLA on glass surface under the same condition.The edge-on PLA lamellae on the PE substrate are preferentially arranged with their long axes in the chain direction of the PE substrate crystals.It is further demonstrated that except for the different crystal orientation,the PE does not influence the crystalline modification and crystallinity of the PLA.     

Atomic force microscopy investigation of the mechanism of calcite microcrystal growth under Kitano conditions

A combined atomic force microscopy (AFM)-inverted optical microscopy technique has been used to image the surface of calcite single microcrystals, with dimensions of 10-20 microm, at high resolution. The microcrystals were grown on a glass substrate using the Kitano method, a process that involves the outgassing of carbon dioxide from a saturated solution of calcium carbonate. The resulting increase in the supersaturation of the solution, with respect to calcium carbonate, induces crystallization. It is demonstrated, for the first time, that calcite microcrystals formed in this way exhibit a single spiral growth hillock on the (104) surface, as evidenced by a spiral step pattern, indicating that growth occurs at steps arising from an individual screw dislocation. The subsequent reactivity of these crystals under Kitano conditions has been followed in situ using AFM imaging.     

An organic hydrogel as a matrix for the growth of calcite crystals

The growth of calcite in an aqueous gel of was studied and the appearance of the crystals was found to change over time. Crystals removed from the gel at progressively longer times showed severely affected surfaces resulting from dissolution. If crystals were removed from the gel after 3.5 hours, at which point there were no etch pits, and then placed in either buffer or pure water, etch pits, similar to those observed on crystals that are left in the gel, were observed. Control calcite crystals exposed to similar conditions (water or buffer) show no significant dissolution after equivalent times. A probable cause of the altered dissolution is the non-specific occlusion of gelator aggregates at sites of imperfection. The gel appears to provide a microenvironment in which the molecules that form the matrix also participate in the crystallization. This system allows the study of the unique properties of a gel for influencing the nucleation and growth of inorganic crystals, some of which may be important for better understanding biomineralization.     

Hydrogels coupled with self-assembled monolayers: an in vitro matrix to study calcite biomineralization

This paper describes the control of the nucleation and growth of calcite crystals by a matrix composed of an agarose hydrogel on top of a carboxylate-terminated self-assembled monolayer (SAM). The design of this matrix is based upon examples from biomineralization in which hydrogels are coupled with functionalized, organic surfaces to control, simultaneously, crystal morphology and orientation. In the synthetic system, calcite crystals nucleate from the (012) plane (the same plane that is observed in solution growth). The aspect ratio (length/width) of the crystals decreases from 2.1 +/- 0.22 in solution to 1.2 +/- 0.04 in a 3 w/v % agarose gel. One possible explanation for the change in morphology is the incorporation of gel fibers inside of the crystals during the growth process. Etching of the gel-grown crystals with deionized water reveals an interpenetrating network of gel fibers and crystalline material. This work begins to provide insight into why organisms use hydrogels to control the growth of crystals.     

Crystallization of calcium carbonate with the filtration of aqueous solutions through a microporous membrane

It is established that the filtration of water through a microporous membrane does not change the hardness of the water; it does, however, reduce the amount of scale deposit, due to the crystallization of salts in water in the form of aragonite. The effect is consistently observed in water with a hardness of more than 7.0 H, a content of hydrocarbonate ions of more than 500 mg/L, and a pH ≥ 7.3. It is shown that introducing the seeds of calcite crystals into a filtrate results in the precipitation of calcite rather than aragonite. It is concluded that quasi-softening in the case of hard water microfiltration is caused by the removal of calcite micronuclei, and thus by conditions being created for the crystallization of aragonite as a thermodynamically less stable form.     

Crystallization of CaCO3 in the presence of sulfate and additives: experimental and molecular dynamics simulation studies

The effects of sulfate and BHTPMP (Bis (hexamethylene) triaminepentakis (methylene phosphonic acid)) on the crystallization rate, phase composition and morphology of calcium carbonate have been studied. It was observed that sulfate reduces the nucleation rate and favors the formation of aragonite form in the calcium carbonate precipitate. Moreover, in the presence of sulfate the rhombohedral morphology of the calcite crystals is modified, and during the formation of calcite, the development of {104} faces are more significantly prohibited than {110} faces. In the presence of sulfate together with BHTPMP, the crystallization process is inhibited and the modified morphology and the dominant calcite form are observed in the solid. The results from molecular dynamics simulations show the more strong combination of sulfate with calcite surface, in particular the {104} face, in comparison with the aragonite surface. The strong interaction of BHTPMP with sulfate and the aragonite surface favors the formation of the dominant calcite phase in the precipitate.     

Self-organization of rotaxane thin films into spatially correlated nanostructures: morphological and structural aspects

The self-organization of rotaxane thin films into spatially correlated nanostructures is shown to occur upon a thermal stimulus. The mechanism of formation of nanostructures and their organization has been investigated using atomic force microscopy, bright field transmission electron microscopy, selected area electron diffraction, and molecular mechanics simulations. The evolution of the nanostructures follows a complex pathway, where a rotaxane thin film first dewets from the substrate to form nanosized droplets. Droplets coalesce by ripening, generating spatially correlated motifs. In a later stage, the larger droplets change shape, nucleate, and coalesce to yield crystallites that grow into larger crystals by incorporating the surrounding droplets. The results show the following: (i) the nanostructures represent a metastable state of a crystallization process; (ii) spatial correlations emerge during ripening, but they are destroyed as stable nuclei are formed and crystallization proceeds to completion; iii) crystallization, either on graphite or amorphous carbon films, leads to a precise basal plane, viz. (010), which has minimum surface energy. The inherent degrees of freedom permitted in the rotaxane architecture favors the re-organization and nucleation of the film in the solid state. Low-energy trajectories leading to crystallites with stable surfaces and minimum energy contact plane are found to occur via concerted, small amplitude, internal motions without disruption of packing and intermolecular contacts.     

Agarose template for the fabrication of macroporous metal oxide structures

Agarose gels have been applied as templates for the formation of macroporous metal oxide structures. The preparation of the agarose template is extremely simple, and with variation of the agarose content, control over morphology is demonstrated: The average pore size decreases from 180 to 55 nm and the surface area increases from 238 to 271 m2 g(-1) with increasing agarose content in the gel. The gelling temperature was also found to influence the final template morphology. Conducting sol-gel chemistry within the template structure followed by removal of the template by heating to 450 degrees C gives porous inorganic oxides. The technique has been demonstrated for the oxides of titanium, zirconium, niobium, and tin. The final morphology of the metal oxide is homogeneous and results from a coating of the agarose structure. The pore diameter decreased and the specific surface area of the titanium dioxide materials increased from 28 to 66 m2 g(-1) as the agarose content in the template is increased from 0.5 to 5.0 wt%. The overall pore size and surface area are lower than the original gel due to shrinkage occurring with the sol-gel process, as well as crystallization and a loss of microporosity in the final material.     

A novel electromagnetism‐assisted imprinting technology to replicate microstructures onto a large‐area curved surface using a flexible magnetic mold

Replication of microstructures from a mold onto a curved surface is difficult. The conformal contact between the mold and the substrate has to be ensured. The present study proposes an innovative mechanism, which employs an electromagnetic disk to provide magnetic force and a PDMS flexible mold with a layer compounded magnetic powder. This mechanism provides not only the gradual contact from center to edge to avoid air entrapment but also conformal contact between the mold and the substrate during the imprinting operation. A system based on this electromagnetic soft imprinting technology has been implemented, and imprinting to replicate microstructures from the mold onto a curved surface has been carried out. The results reveal that the PDMS magnetic mold and the electromagnetic disk‐controlled magnetic force can successfully perform the imprinting and accurately replicate the microstructures onto the large‐area, curved surface glass. The PDMS flexible magnetic mold incorporated with the magnetic disk can be employed to achieve the conformal contact between the mold and the substrate. In addition, due to the low surface free energy of the PDMS, the de‐molding without sticking can be easily accomplished. Copyright © 2008 John Wiley & Sons, Ltd.     

Spatially resolved product formation in the reaction of formic acid with calcium carbonate (1014): the role of step density and adsorbed water-assisted ion mobility

The reaction of calcium carbonate (1014) single-crystal surfaces with formic acid (HCOOH) vapor was investigated using atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). AFM images indicate the reaction produces rather well-defined crystallites, preferentially at step edges and at distinct angles to one another and mirroring the rhombohedral structure of the calcite surface, while exposing unreacted carbonate surface. The size and surface density of the crystallites depend upon substrate step density, exposure time, and relative humidity. XPS data confirmed the crystallite composition as the expected calcium formate product. The AFM images show erosion and pit formation of the calcite surface in the vicinity of the product crystallites, clearly providing the spatially resolved characterization of the source of Ca ions. AFM experiments exploring the effects of water vapor on the reacted surface show that the calcium formate crystallites are mobile under conditions of high relative humidity, combining to form larger crystallites and nanometer-sized crystals with an orthorhombohedral habit consistent with the alpha form, as confirmed by X-ray diffraction. The implications for the reactions described here are discussed.