Effects of pH and temperature on CaCO3 crystallization in aqueous solution with water soluble matrix of pearls

Water soluble matrix (WSM) was extracted from pearls originated from Hyriopsis cumingii in Zhuji, Zhejiang province, China. WSM was regarded as an additive in mineralization experiments in order to study the effect of WSM on CaCO₃ crystallization. The experiments were carried out at different pH and temperatures by gas diffusion method and solution titration method, respectively. Scanning electron microscopy (SEM) and Raman spectroscopy (Raman) were used as powerful techniques to analyze the co-effect of pH value, temperature and WSM on crystal growth of CaCO₃. The results showed that WSM could induce aragonite at different pH values of mineralization solution, and the pH value had remarkable influence on morphology of calcite rather than aragonite due to distinct supersaturation and ionic strength related to various pH values. At different solution temperatures, WSM had little effect on crystal growth of calcium carbonate while the solution temperature had notable effect on polymorph and morphology of CaCO₃ crystals. This work can provide some basic information for the polymorph and morphology control of calcium carbonate.     

Precipitation of calcium carbonate particles by gas–liquid reaction: Morphology and size distribution of particles in Couette-Taylor and stirred tank reactors

The morphology and size of CaCO₃ precipitated by CO₂–Ca(OH)₂ reaction in stirred tank and Couette-Taylor reactors were experimentally investigated. The Taylor vortex in CT reactor encouraged more homogeneous mixing conditions, resulting in the production of smaller particles with a uniform shape throughout the reactor. However, in the stirred tank reactor, the local non-homogeneity of the mixing intensity led to the simultaneous production of cube-like and spindle-like particles at a high reactant concentration. The agglomeration of CaCO₃ resulted in a bimodal size distribution. However, the morphology and size of a single particle were predominantly changed by the excess species in the solution. The largest mean size and cube-like particles were observed under stoichiometric reaction conditions. As the excess species concentration increased, the morphology was transformed to a spindle-like shape and the mean size decreased due to selective adsorption of the excess species on the crystal faces.     

Room-temperature template-free synthesis of dumbbell-like SrSO4 with hierarchical architecture

Novel dumbbell-like SrSO₄ with hierarchical architecture was fabricated with a facile template-free aqueous solution method at room temperature. The crystallographic morphology of SrSO₄ products depends mainly on the pH value of the reaction solution. The SrSO₄ products exhibit a dumbbell-like hierarchical architecture at pH=3 and 5, and have a tablet-like crystallographic morphology at pH=1 when keeping other reaction parameters unchanged. The dumbbell-like SrSO₄ synthesized at pH=3 has a length of 8–14 μm, and is composed of numerous well-aligned single crystalline nanoplates with an average width of 140 nm and a length of 0.7–1 μm. The Brunauer–Emmett–Teller (BET) surface area of the crystallized SrSO₄ products is about 2.8 m² g⁻¹. A formation mechanism is proposed for the evolution process of dumbbell-like SrSO₄ with hierarchical architecture.     

Nucleation-growth process of scale electrodeposition – influence of the magnesium ions

Electrodeposition of CaCO₃ was studied under the influence of magnesium ions present in a carbonically pure water. The investigation was performed using electrochemical, gravimetric and optical methods. The chronoamperometric measurements confirmed the inhibiting property of Mg²⁺ when its concentration is higher than 120 mg L⁻¹ in a solution containing 160 mg L⁻¹ of Ca²⁺. The optical technique led the nucleation-growth process to be accessed by means of an optical microscope positioned behind a transparent electrode. The increase of Mg²⁺ concentration changed drastically the calcite morphology. At 360 mg L⁻¹ of Mg²⁺, the calcite morphology was optically amorphous but the Raman spectrum confirmed its structure. The crystal growth was recorded in situ and the image analysis software characterised the nucleation process as well as the growth rate of the crystals. It allowed the influence of the Mg²⁺ ions on the crystallisation process of CaCO₃ to be quantified.     

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.     

Microwave-assisted synthesis of spheroidal vaterite CaCO3 in ethylene glycol–water mixed solvents without surfactants

Spheroidal vaterite CaCO₃ composed of irregular nanoparticals have been synthesized by a fast microwave-assisted method. The structures are fabricated by the reaction of Ca(CH₃COO)₂ with (NH₄)₂CO₃ at 90 °C in ethylene glycol–water mixed solvents without any surfactants. The diameters of the spheroidal vaterite CaCO₃ range from 1 to 2 μm, and the average size of the nanoparticals is about 70 nm. Bundle-shaped aragonite and rhombohedral calcite are also obtained by adjusting the experimental parameters. Our experiments show that the ratio of ethylene glycol to water, microwave power, reaction time, and two sources of ammonium ions and acetate anions are key parameters for the fabrication of spheroidal vaterite CaCO₃. A possible growth mechanism for the spheroidal structures has been proposed, which suggests that the spheroidal vaterite CaCO₃ is formed by an aggregation mechanism.     

Hydrothermal synthesis of Bi2Te3 nanowires through the solid-state interdiffusion of Bi and Te atoms on the surface of Te nanowires

Polycrystalline Bi₂Te₃ nanowires were prepared by a hydrothermal method that involved inducing the nucleation of Bi atoms reduced from BiCl₃ on the surface of Te nanowires, which served as sacrificial templates. A Bi–Te alloy is formed by the interdiffusion of Bi and Te atoms at the boundary between the two metals. The Bi₂Te₃ nanowires synthesized in this study had a length of 3–5 μm, which is the same length as that of the Te nanowires, and a diameter of 300–500 nm, which is greater than that of the Te nanowires. The experimental results indicated that volume expansion of the Bi₂Te₃ nanowires was a result of the interdiffusion of Bi and Te atoms when Bi was alloyed on the surface of the Te nanowires. The morphologies of Bi₂Te₃ are strongly dependent on the reaction conditions such as the temperature and the type and concentration of the reducing agent. The morphologies, crystalline structure and physical properties of the product were analyzed by X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED) and X-ray photoelectron spectroscopy (XPS).     

A room temperature solution-phase process to synthesize pure phase single-crystalline hexagonal cobalt hydroxide nanoplates

A simple room temperature solution-phase method has been developed to synthesize pure phase single-crystalline hexagonal β-Co(OH)₂ nanoplates. The chemical composition and morphology of the as-prepared β-Co(OH)₂ nanoplates were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and selected area electron diffraction (SAED). The results indicated that the as-prepared β-Co(OH)₂ nanoplates were composed of pure brucite-like β-Co(OH)₂ phase with single-crystalline feature. The effect of polyethylene glycol (PEG), NH₄OH, and NaOH on the morphology and size of β-Co(OH)₂ nanocrystals were discussed in detail. The growth mechanism of the as-synthesized nanoplates was also discussed in detail based on the experimental results.     

The effect of polyacrylamide on the crystallization of calcium carbonate: Synthesis of aragonite single-crystal nanorods and hollow vatarite hexagons

CaCO₃ nanorods were synthesized via a facile solution route by polymer-controlled crystallization in the presence of polyacrylamide (PAM). The morphology, size and crystal structure were characterized by means of scanning electron microscope (SEM), transmission electron microscope (TEM), and X-ray diffraction (XRD). The results suggest that the as-synthesized product was CaCO₃ (aragonite) nanorods with diameter ca. 50 nm and length ca. 1 μm. Selected area electron diffraction (SAED) pattern shows the single-crystal nature of CaCO₃ nanorods. The reaction time, temperature, pH and reactant concentration were systemically investigated. With the increase in the reaction time, hollow vaterite hexagonal disks can be obtained.     

Growth and photoluminescence properties of vertically aligned SnO2 nanowires

Vertically aligned SnO₂ nanowires (NWs) were grown for the first time by a vapor–liquid–solid method on c-sapphire with gold as a catalyst under Ar gas flow. Electron backscatter diffraction analysis indicated the NWs are single crystalline having the rutile structure, grow vertically along the [1 0 0] direction, and exhibit a consistent epitaxial relationship where lattice mismatch is estimated to be 0.3% along the SnO₂ [0 1 0] direction. The growth of these NWs is sensitive to many parameters, including growth duration, substrate type, source vapor concentration, and the thickness of the catalyst layer. Photoluminescence measurements at room temperature showed that the vertically aligned NWs exhibit an intense transition at 3.64 eV, a near band-edge transition which is rarely observed in SnO₂.     

Growth and shape control of orthorhombic Fe5(PO4)4(OH)3·2H2O single crystalline dendrites

Orthorhombic Fe₅(PO₄)₄(OH)₃·2H₂O single crystalline dendritic nanostructures have been synthesized by a facile and reproducible hydrothermal method without the aid of any surfactants. The influences of synthetic parameters, such as reaction time, temperature, the amount of H₂O₂ solution, pH values, and types of iron precursors, on the crystal structures and morphologies of the resulting products have been investigated. The formation process of Fe₅(PO₄)₄(OH)₃·2H₂O dendritic nanostructures is time dependent: amorphous FePO₄·nH₂O nanoparticles are formed firstly, and then Fe₅(PO₄)₄(OH)₃·2H₂O dendrites are assembled via a crystallization-orientation attachment process, accompanying a color change from yellow to green. The shapes and sizes of Fe₅(PO₄)₄(OH)₃·2H₂O products can be controlled by adjusting the amount of H₂O₂ solution, pH values, and types of iron precursors in the reaction system.     

Half opened microtubes of NaYF4:Yb,Er synthesized in reverse microemulsion under solvothermal condition

NaYF₄:Yb,Er micro/nanocrystals with different sizes and morphologies such as nanospheres, short flexural nanorods, and half opened microtubes, were synthesized in reverse microemulsion under solvothermal condition using the quaternary reverse microemulsion system, CTAB/1-butanol/cyclohexane/aqueous solution. The X-ray diffraction analysis confirmed that cubic phase NaYF₄:Yb,Er can completely transform to hexagonal phase with increasing reaction time. The scanning electron microscope and transmission electron microscope images revealed that the morphology of the product can be tailored by varying the reaction time. A possible crystalline growth process of the NaYF₄:Yb,Er micro/nanocrystals was discussed. The obtained half opened microtubes exhibited an intense green upconversion luminescence, which may be attractive in novel optoelectronic devices.     

Facile synthesis and luminescence properties of octahedral YVO4:Eu microcrystals

Uniform octahedral YVO₄:Eu³⁺ microcrystals have been successfully prepared through a designed two-step hydrothermal conversion method. One-dimensional precursor Y₄O(OH)₉NO₃ was first prepared through a simple hydrothermal process without using any surfactant, catalyst or template. Subsequently, well-defined octahedral YVO₄ was synthesized at the expense of the precursor during a hydrothermal conversion process. XRD results demonstrate that the diffraction peaks of the final product can be well indexed to the pure tetragonal phase of YVO₄. The SEM and TEM images indicate that the as-prepared YVO₄ sample has regular octahedral shape with sharp corners and well-defined edges. The as-obtained YVO₄:Eu³⁺ phosphor shows strong red emission under ultraviolet excitation or low-voltage electron beam excitation. Furthermore, this facile and general conversion method may be of much significance in the synthesis of many other lanthanide compounds with uniform morphology.     

Preparation of calcium carbonate crystals in the presence of trisodium citrate

The dumbbell‐like calcium carbonate (CaCO₃) crystals were synthesized in the presence of trisodium citrate. Different morphologies were obtained by changing the reaction temperature and the trisodium citrate concentration. The obtained samples were characterized by means of X‐ray diffraction (XRD) and scanning electron microscopy (SEM). The results showed that the morphology of CaCO₃ crystals was markedly affected by the reaction temperature and citrate anion concentration. The possible growth mechanism of CaCO₃ crystals was proposed. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Self-assembly of LiFePO4 nanodendrites in a novel system of ethylene glycol–water

In this work, a novel system of ethylene glycol/water (EG/W) was employed to synthesize LiFePO₄, in which dodecyl benzene sulphonic acid sodium (SDBS) was used as soft template to control particle morphology. The samples were characterized by X-ray diffractometer (XRD), field emission scanning electron microscopy (FE-SEM), high resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED) and energy dispersive X-ray spectroscopy (EDX). The LiFePO₄ sample obtained by the reported method displays interesting hierarchical nanostructure (i.e. nanodendrites), which was constructed by nanorods of 3–5 μm in length and ∼50 nm in diameter. The EG/W ratio, amount of SDBS added, hydrothermal temperature and duration played important roles in the assembly of the hierarchical nanostructures. A formation mechanism was proposed and experimentally verified. It is concluded that the nanodendrites were formed due to the end-to-end self-assembly of nanorods. Compared to previously reported methods, the reported approach shows obvious advantages of one-step synthesis, environmental friendliness and low cost, to name a few. The nanodendrites as a cathode material have a higher capacity, compared with the other samples.     

Anisotropic vapor phase growth of Ga2O3 crystalline nanobelts

Ga₂O₃ nanobelts were synthesized by gas reaction at high temperature in the presence of oxygen in ammonia. X-ray diffraction and chemical microanalysis revealed that the nanostructures were Ga₂O₃ with the monoclinic structure. Electron microscopy study indicated the nanobelts were single crystalline with broad (0 1 0) crystallographic planes. The nanostructures grew anisotropically with the growth direction of . Statistical analysis of the anisotropic morphology of the nanobelts and electron microscopy investigation of the nanobelt tips indicated that both vapor–solid and vapor–liquid–solid mechanisms controlled the growth process. The anisotropic nature of crystallographic morphology is explained in terms of surface energy.     

A catalyst-free method to silicon nanowires at relative low temperature

Well-crystallized straight Si nanowires (SiNWs) were successfully prepared in large scale via a facile reaction between NaN₃ and Na₂SiF₆ at 600 °C without using any catalyst. Characterization by X-ray powder diffraction and transmission electron microscopy demonstrates that the as-obtained product is pure-phase cubic SiNWs with diameters of 40 nm or so, and lengths of several micrometers. And the SiNWs with spherical tips can be obtained at a temperature as low as 300 °C. Heating temperature and holding time have crucial influence on the synthesis and morphology of the SiNWs. An oxide-assisted growth mechanism is responsible for the formation of the SiNWs.     

Optical and electrical properties of lithium doped nickel oxide films deposited by spray pyrolysis onto alumina substrates

Non-doped and lithium doped nickel oxide crystalline films have been prepared onto quartz and crystalline alumina substrates at high substrate temperature (600 °C) by the pneumatic spray pyrolysis process using nickel and lithium acetates as source materials. The structure of all the deposited films was the crystalline cubic phase related to NiO, although this crystalline structure was a little bit stressed for the films with higher lithium concentration. The grain size had values between 60 and 70 nm, almost independently of doping concentration. The non-doped and lithium doped films have an energy band gap of the order of 3.6 eV. Hot point probe results show that all deposited films have a p-type semiconductor behavior. From current–voltage measurements it was observed that the electrical resistivity decreases as the lithium concentration increases, indicating that the doping action of lithium is carried out. The electrical resistivity changed from 10⁶ Ω cm for the non-doped films up to 10² Ω cm for the films prepared with the highest doping concentration.     

Sublimation crystal growth of yttrium nitride

The sublimation–recombination crystal growth of bulk yttrium nitride crystals is reported. The YN source material was prepared by reacting yttrium metal with nitrogen at 1200 °C and 800 Torr total pressure. Crystals were produced by subliming this YN from the source zone, and recondensing it from the vapor as crystals at a lower temperature (by 50 °C). Crystals were grown from 2000 to 2100 °C and with a nitrogen pressure from 125 to 960 Torr. The highest rate was 9.64×10⁻⁵ mol/h (9.92 mg/h). The YN sublimation rate activation energy was 467.1±21.7 kJ/mol. Individual crystals up to 200 μm in dimension were prepared. X-ray diffraction confirmed that the crystals were rock salt YN, with a lattice constant of 4.88 Å. The YN crystals were unstable in air; they spontaneously converted to yttria (Y₂O₃) in 2–4 h. A small fraction of cubic yttria was detected in the XRD of a sample exposed to air for a limited time, while non-cubic yttria was detected in the Raman spectra for a sample exposed to air for more than 1 h.     

Morphology and crystal phase evolution of doctor-blade coated CuInSe2 thin films

CuInSe₂ (CIS) chalcopyrite thin films were prepared using a low-cost, non-vacuum doctor-blade coating and the thermal annealing method. An acetone-based precursor solution containing copper chloride, indium chloride, selenium chloride, and an organic binder was deposited onto a Mo-sputtered soda lime glass substrate using a doctor-blade coating method. After coating, the precursor films were annealed in a quartz tube furnace under low vacuum without the use of a Se atmosphere or reduction conditions. Evolution of the morphology, crystal structure, and thermal decomposition behavior of the films was analyzed by X-ray diffraction, scanning electron microscopy, and thermogravimetric analysis, and the film formation mechanism was suggested. The as-deposited precursor film gradually decomposed with increase in temperature and formed Cu_2−xSe and In₂Se₃ nuclei on the surface of the film. Incorporation of Cu_2−xSe with In₂Se₃ yielded a chalcopyrite CIS phase, which crystallized on annealing above 400 °C. The obtained CIS film showed low-resistive ohmic behavior with a Mo electrode and a high absorption efficiency for visible–infrared (IR) light, making it suitable for use in photovoltaic applications.