Synthesis of calcium carbonate polymorphs in the presence of polyacrylic acid

Calcium carbonate precipitates are prepared from a solution of CaCl₂ and K₂CO₃ in the presence of polyacrilic acid. The effect of polyacrilic acid incorporation in the [25–80 °C] temperature range on crystal morphologies and CaCO₃ precipitated polymorph concentrations are investigated using scanning electron microscopy and X-ray diffraction quantitative microstructural and phase analysis. Large changes in morphology and phase proportions are observed in the presence of polyacrylic acid, which strongly depend on the solution temperature. While crystallization of vaterite is favoured in the presence of polyacrilic acid up to 50 °C, it is largely destabilized at higher temperatures. Our process also enables the elaboration of particles in the range 10–20 nm.     

Growth of nanofibrous barium carbonate on calcium carbonate seeds

Fibrous barium carbonate (BaCO₃/witherite) crystals 50–100 nm in diameter and several microns in length were grown on calcium carbonate (CaCO₃) seeds at temperatures as low as 4 °C. The BaCO₃ fibers were deposited onto calcite rhombs or CaCO₃ films using the polymer-induced liquid-precursor (PILP) process, which was induced with the sodium salt of polyacrylic acid (PAA). The structure and morphology of the resultant fibers were investigated using scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected-area electron diffraction (SAED), and polarized light microscopy (PLM). Fibers were successfully grown on calcite seeds of various morphologies, with a range of barium concentrations, and PAA molecular weight and concentration. Two categories of fibers were grown: straight and twisted. Both types of fibers displayed single-crystalline SAED diffraction patterns, but after examining high-resolution TEM lattice images, it was revealed that the fibers were in fact made up of nanocrystalline domains. We postulate that these nanocrystalline domains are well aligned due to a singular nucleation event (i.e., each fiber propagates from a single nucleation event on the seed crystal) with the nanocrystalline domains resulting from stresses caused by dehydration during crystallization of the highly hydrated precursor phase. These BaCO₃ fibers grown on calcite substrates further illustrate the robustness and non-specificity of the PILP process.     

Preparation and structure of calcium peroxide‐templated porous calcium carbonate crystals

Crystalline calcium carbonate with randomly dispersed porous structure was prepared through co‐ crystallization with calcium peroxide and the following template elimination by a post heating treatment and washing with water. The artificial CaCO₃ possess abundant macro‐mesopores structures and high surface area. This approach may open a new general route for the preparation of crystals with high porosity and structure specialty. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis of Si-MCM-48 membrane by solvent extraction of the surfactant template

Mesoporous MCM-48 silica membranes have been synthesized on porous supports under hydrothermal conditions, using tetraethyl orthosilicate (TEOS) as silica source, cetyltrimethylammonium bromide (CTAB) as template surfactant. And then the templates were removed by solvent extraction instead of calcination. The results of the TG and FT-IR showed that the solvent of 1 M HCl/EtOH solution was feasible to extract the templates in the MCM-48 materials and over 90% of the templates were extracted at room temperature for 24 h. The results of XRD, SEM indicated that the MCM-48 membranes were prepared on the porous supports and extraction did not destroy the mesostructure of the MCM-48. The compactness of the extracted MCM-48 membrane was evaluated by the permeation of single gas (N₂ and H₂) with transmembrane pressure of 60-220 kPa. The permeance of N₂ was independent of the transmembrane pressure and the ideal separation factor was about 3.45. All the results demonstrate that the method, solvent extraction to remove surfactant template, is more effective to synthesize high quality MCM-48 membranes than calcination.     

Formation of GaN nanodots on Si (1 1 1) by droplet nitridation

GaN nanodots (NDs) are obtained by Ga metallic droplet formation on Si (1 1 1) substrates followed by their nitridation. The size and density of Ga droplets and GaN NDs can be controlled by varying the growth temperature within the range 514–640 °C. Atomic force microscopy (AFM) investigation of Ga droplets shows an increase in the average diameter with temperature. The average diameter of GaN NDs increases with growth temperature while their density decreases more than one order of magnitude. In addition, the formation of a GaN crystallite rough layer on Si, in-between NDs, indicates that a spreading mechanism takes place during the nitridation process. High-resolution transmission electron microscopy (HRTEM) is used for the investigation of shape, crystalline quality and surface distribution of GaN dots. X-ray photoelectron spectroscopy (XPS) results confirm that Ga droplets that are transformed into GaN NDs spread over the sample surface during nitridation.     

Preparation of Sb2S3 nanomaterials with different morphologies via a refluxing approach

Single-crystalline antimony trisulfide (Sb₂S₃) nanomaterials with flower-like and rod-like morphologies were successfully synthesized under refluxing conditions by the reaction of antimony trichloride (SbCl₃) and thiourea with PEG400 and OP-10 as the surfactants. X-ray diffraction (XRD) indicates that the obtained sample is orthorhombic-phase Sb₂S₃ with calculated lattice parameters a=1.124 nm, b=1.134 nm and c=0.382 nm. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images show that the flower-like Sb₂S₃ is 9–10 μm in size, which is composed of thin leaves with thickness of 0.05–0.2 μm, width of 0.8–2.2 μm and length of 2.5–3 μm, and the rod-like Sb₂S₃ is 45–360 nm in diameter and 0.7–4 μm in length, respectively. UV–Vis analysis indicates that the band gap of Sb₂S₃ nanorods is 1.52 eV, suitable for photovoltaic conversion. A possible mechanism of formation was proposed. The effects of reaction time and surfactants on the growth of nanomaterials with different morphologies were also investigated.     

The onset of spherulitic growth in crystallization of calcium carbonate

Batch and semi-batch crystallization experiments were performed in aqueous solutions for the three anhydrous polymorphs of calcium carbonate vaterite, aragonite and calcite to investigate the effect of crystallization parameters on the onset of spherulitic growth and particle morphology.     

Low-temperature deposition of crystalline silicon nitride nanoparticles by hot-wire chemical vapor deposition

The nanocrystalline alpha silicon nitride (α-Si₃N₄) was deposited on a silicon substrate by hot-wire chemical vapor deposition at the substrate temperature of 700 °C under 4 and 40 Torr at the wire temperatures of 1430 and 1730 °C, with a gas mixture of SiH₄ and NH₃. The size and density of crystalline nanoparticles on the substrate increased with increasing wire temperature. With increasing reactor pressure, the crystallinity of α-Si₃N₄ nanoparticles increased, but the deposition rate decreased.     

Silica-zirconia mixed oxide samples by an hybrid materials based innovative preparation procedure: Influence of preparation procedure and composition on active sites

In this work, the interaction of amorphous silica-zirconia mixed oxide samples obtained from inorganic-organic silica-based hybrid materials with pyridine and CO₂ was studied to investigate their acid/base character. Several silica-zirconia mixed oxide powders characterized by different [Zr/Si] atomic ratios were prepared and treated at increasing temperatures both in a conventional muffle and with microwave technology. The powder samples were characterized with Diffuse Reflectance Infrared Fourier Transform (DRIFT) and X-ray Photoelectron (XP) spectroscopies. The surface acidic and basic active sites were investigated (with DRIFT spectroscopy) by chemisorbing probe molecules (pyridine, carbon dioxide). The obtained results revealed the presence of both Lewis and Brønsted acidic sites on the amorphous silica-zirconia mixed oxide powder surfaces. Several acidic sites characterized by different strength were observed; the acidic sites distribution is markedly influenced by the sample composition and by the heat treatment: more numerous acidic sites form on the surface of the samples treated with microwaves with respect to the muffle treated ones; the increment of the temperature and the decrease of the zirconium content cause a significant decrement of the acidic sites. No basic sites were revealed.     

Modification of montmorillonite by quaternary polyesters

In this paper, we report on the preparation of organically modified montmorillonite (OMM) and its properties, using quaternary polyesters. The polyester was synthesized from N-methyldiethanolamine and succinic acid anhydride, converted to the quaternary polyester using benzyl bromide, and characterized by nuclear magnetic resonance (NMR), size exclusion chromatography (SEC), differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). Quaternary polyesters with a quaternization degree of 0.25 and 0.5 were used to modify montmorillonite (MMT). The intercalation was determined by X-ray diffraction. Interlayer spacing increased with decreased MMT/polyester ratio and with increased degree of quaternization. The thermal stability of polyester bound to MMT greatly increased.     

Vertically aligned N-doped carbon nanotubes by spray pyrolysis of turpentine oil and pyridine derivative with dissolved ferrocene

Vertically aligned nitrogen-doped carbon nanotubes were synthesized from the pyrolysis of a mixture of turpentine oil, 4-tert-butylpyridine (C₉H₁₃N) and ferrocene on silicon and quartz substrate in nitrogen atmosphere at 700 °C by simple spray pyrolysis technique. SEM, TEM, TGA/DTA, Raman spectroscopy, XPS and electron probe micro analysis (EPMA) techniques were used to characterize the structural analysis and composition of the as-grown N-doped carbon nanotubes. Morphology of the films was greatly affected by the nature of the substrate. From the XPS and EPMA data, it was found that nitrogen content of the nanotubes were 1.6 at.% and 2 at.% on silicon and quartz substrate, respectively. Our studies show that two different types of N atoms can be present in these materials. These are ‘pyridinic’ and ‘graphitic’ nitrogen with binding energies of 398.2 eV and 400.4 eV, respectively. Raman spectroscopy reveals that graphitization of carbon nanotubes grown on silicon is better than nanotubes grown on quartz substrate. Thermogravimetric analysis showed that the thermal stability of as-prepared nanotubes grown on silicon substrate is higher than the nanotubes deposited on quartz substrate.     

Influence of polyvinylpyrrolidone on the precipitation of calcium carbonate and on the transformation of vaterite to calcite

The precipitation process of calcium carbonate (CaCO₃) in the absence and presence of poly (N-vinyl-2-pyrrolidone) (PVP) was investigated using scanning electron microscopy, powder X-ray diffraction, and Fourier transform infrared spectroscopy methods at room temperature. The results indicate that PVP does not affect the polymorphy, but has influence on their morphology and size of CaCO₃ crystals. With the addition of PVP, the amorphous CaCO₃ could aggregate into bigger amorphous spherulites before transforming to crystalline state. Also, the transformation from the thermodynamically unstable vaterite to the stable calcite was investigated in the presence of PVP. As a particle-stabilizing agent, PVP molecules inhibit the formation of vaterite, however, promote the formation of calcite as well as the rate of the solvent-mediated transformation from vaterite to calcite.     

Growth of aragonite calcium carbonate nanorods in the biomimetic anodic aluminum oxide template

In this study, a biomimetic template was prepared and applied for growing calcium carbonate (CaCO₃) nanorods whose shape and polymorphism were controlled. A biomimetic template was prepared by adsorbing catalytic dipeptides into the pores of an anodic aluminum oxide (AAO) membrane. Using this peptide-adsorbed template, mineralization and aggregation of CaCO₃ was carried out to form large nanorods in the pores. The nanorods were aragonite and had a structure similar to nanoneedle assembly. This aragonite nanorod formation was driven by both the AAO template and catalytic function of dipeptides. The AAO membrane pores promoted generation of aragonite polymorph and guided nanorod formation by guiding the nanorod growth. The catalytic dipeptides promoted the aggregation and further dehydration of calcium species to form large nanorods. Functions of the AAO template and catalytic dipeptides were verified through several control experiments. This biomimetic approach makes possible the production of functional inorganic materials with controlled shapes and crystalline structures.     

Facile preparation of diversified patterns of calcium carbonate in the presence of DTAB

Nano- and micro-sized calcium carbonate (CaCO₃) with various morphologies including multi-petal-flower-shaped, multi-step-cube-shaped, coral-shaped, dendrite-shaped and multi-antenna-shaped was successfully prepared using dodecyltrimethylammonium bromide (DTAB) micellar vevulsant. The effects of temperature, pH and the concentration of DTAB micellar solution on the morphology and crystalline form of CaCO₃ were systematically investigated. The prepared CaCO₃ was characterized by Scanning Electron Microscope (SEM), Transmission Electron Microscopy (TEM), X-ray Diffraction (XRD) and Fourier Transform Infrared Spectroscopy (FTIR). The concentration of DTAB micelle, pH and reaction temperature are found to play crucial roles in the morphology, size and crystalline form of the final products. On the base of the characterizations, a possible self-assembled mechanism was proposed. The novel multi-petal-flower-shaped and multi-antenna-shaped CaCO₃ may have some unique properties and potential applications in the future.     

Formation of Ge nanoislands before the completion of a wetting layer in the Ge/Si(1 1 1) system

The formation of Ge nanoislands directly on Si(1 1 1) surface before the completion of a wetting layer was studied by scanning tunneling microscopy and Raman scattering spectroscopy. The mechanism of the wetting layer formation in the Ge/Si(1 1 1) system depends on the rate of Ge deposition. Within the temperature range 350–500 °C, with Ge deposition rates of the order of 10⁻³ bilayers/min, the Ge wetting layer is formed by the multilayer growth mechanism. Therefore, the arrays of Ge islands with the densities of 10⁹–10¹² cm⁻², depending on the rate of Ge deposition, appear directly on the Si surface during the evolution of the wetting layer. The height of Ge islands is limited by 3 bilayers. The lateral dimensions depend on the coverage of Ge and on the growth temperature. A series of lines related to the quantization of the phonon spectrum along the growth direction [1 1 1] was observed in the spectra of Raman scattering by optical phonons of Ge nanoislands.     

In-situ size control and structural characterization of barium titanate nanocrystal prepared by crystallization of amorphous phase

We examined the influence of nanocrystalline particle size on the cubic-to-tetragonal phase transition of barium titanate (BaTiO₃). In-situ X-ray diffraction (XRD) and differential scanning calorimetry (DSC) were used in combination with electron microscopy to study the evolution of lattice structure and phase-transformation behavior with heat treatment and particle growth for BaTiO₃. The lattice constants were obtained for various polycrystal orientations in the diameter size range 10–90 nm. In-situ XRD results during the crystallization of amorphous 4BaTiO₃-SiO₂ revealed a critical size of ∼25 nm for the cubic-to-tetragonal phase transition of BaTiO₃ at room temperature.     

Study on crystallization of calcium carbonate from calcium sulfate and ammonium carbonate in the presence of magnesium ions

Batch reactive crystallization of calcium carbonate (CaCO₃) from ammonium carbonate ((NH₄)₂CO₃) and calcium sulfate (CaSO₄) was investigated in the presence of magnesium (Mg²⁺) ions. It was observed that Mg²⁺ ions partly inhibited the conversion of CaSO₄ into CaCO₃. When the content of Mg²⁺ was less than 2%, the reduction in conversion rate of CaSO₄ was less than 2%, and the effect of Mg²⁺ ions could be ignored. Effect of impurity on crystallization kinetics of CaCO₃, including the growth rate and nucleation rate, was investigated. The results revealed that when Mg²⁺ ions content was less than 1%, Mg²⁺ could promote the growth of CaCO₃ and inhibit the nucleation process, which was favorable for the filtration of CaCO₃.When the content of Mg²⁺ ions was greater than 1%, Mg²⁺ inhibited the growth of CaCO₃, which resulted in explosion nucleation and led to a large number of particles in the solution, which was unfavorable for the filtration of CaCO₃. Based on the Bransom model, the particle size distribution equations of CaCO₃ were established. X‐ray diffraction patterns and scanning electron microscopy images exhibited the existence of spherical vaterite of CaCO₃ due to the reaction of CaSO₄ with (NH₄)₂CO₃ under the effect of Mg²⁺ ions, which was inconsistent with the results reported in the literatures.     

Growth of calcium carbonate in polyacrylamide hydrogel: Investigation of the influence of polymer content

The growth of calcium carbonate crystals has attracted growing attention as a model system for biomineralisation processes. Organic molecules and gelatinous matrices are known to play an essential role in the formation of hard tissues. For the investigation of the function of specific influence factors, a model experiment is necessary. Several hydrogels were previously tested as growth matrices for calcium carbonate.

For laboratory experiments, a double diffusion set-up for the growth of crystals in gels was established earlier. Calcium carbonate crystals were grown in polyacrylamide hydrogels.

Here the influence of the polymer content in the hydrogels on the crystallisation behaviour is reported. Time-resolved and spatially resolved crystallisation experiments were conducted. The collected calcium carbonate precipitates were analysed by light microscopy, scanning electron microscopy and X-ray diffraction.

The morphology of the developing crystals was found to be dependent on the polymer content of the hydrogels.     

Crystallization habit of calcium carbonate in presence of sodium dodecyl sulfate and/or polypyrrolidone

The synthesis of calcium carbonate (CaCO₃) crystals from aqueous solutions containing sodium dodecyl sulfate (SDS), poly(N-vinyl-1-pyrrolidone) (PVP) or SDS/PVP complexes has been performed through a slow titration method. It was found that aragonite and calcite coexisted in the prepared crystals. The formation of aragonite in the precipitation systems without magnesium ions indicates that at ambient temperature ca. 26.0°C, initially formed amorphous CaCO₃ could also transfer into aragonite in the sedimentary phase, which indicates the controlling factor of organic additives in the nucleation and growth process of CaCO₃ crystals. The appearance of hexagonal crystals in the suspensible phase confirmed the hexagonal crystallization cell of vaterite, and revealed the colloidal-dispersion function of the SDS/PVP complex in the crystallization process of CaCO₃.     

Polymorphism and morphology of calcium carbonate precipitated in mixed solvents of ethylene glycol and water

The influence of (mono) ethylene glycol (MEG) on polymorphism and the resulting morphology of calcium carbonate have been studied for activity-based supersaturation ratios in the range of 3–10 and temperatures from 25–80 °C in mixed solvents of ethylene glycol and water at ratios of 0–90 wt%. The presence of a co-solvent in the solution affects the supersaturation, because of changes in the activity coefficients and the solubility of the salt, a fact that is usually not accounted for in similar studies in the literature. In the present study, the effect of the solvent was isolated from the accompanying change in the supersaturation. MEG was found to affect both the polymorphic abundance in the precipitates, the morphology of the particles and the transformation rates. High concentrations of MEG favoured the precipitation of vaterite and higher temperatures promoted the formation of aragonite. The particle size was reduced in experiments with high MEG concentrations at supersaturations ratios comparable to water solutions, illustrating that the nucleation rate is affected by the co-solvent. The morphology of the calcium carbonate particles was changed at various conditions of MEG concentrations and temperatures from cubes of calcite to spherical, flower-like and dumbbell particles of vaterite and aragonite needles. MEG prolongs the transformation time of metastable polymorphs and the effect was shown to be caused by the solvent itself, probably as a result of kinetic stabilization by delaying the growth rate of the more stable polymorphs.