Hydrothermal synthesis of nano‐crystalline BaMoO4 under mild conditions using simple additive

Large‐scale high‐quality BaMoO₄ nanocrystals have been synthesized in aqueous solutions under mild conditions with citrate as a simple additive. The crystals have bone‐like, spindle‐like and wheatear‐like morphologies assembled from nanoparticles, nanofibers and have been characterized by X‐ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) techniques. The results showed that experimental parameters had great influences on the shape evolution of products. The adjustment of these parameters such as room temperature stirring time, reaction temperature and reaction time of hydrothermal reaction, can lead to obvious morphology changes of products, and the growth mechanism has been proposed. Room‐temperature photoluminescence indicated that the as‐prepared BaMoO₄ nanocrystals had a strong blue emission peak at 481.5 nm. This facile route could be employed to synthesize more promising nanomaterials with interesting self‐assembly structures. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structure and photoluminescence properties of SrWO4 3D microspheres synthesized by the surfactant‐assisted hydrothermal method

In this paper, we report large‐scale high‐quality SrWO₄ 3D microspheres synthesized in aqueous solutions under mild conditions with cetyl trimethyl ammonium bromide as a simple cationic surfactant. These crystals have been characterized by X‐ray diffraction, transmission electron microscopy and field emission scanning electron microscopy techniques. The crystal growth processes were employed to investigate the formation mechanism of SrWO₄ 3D microspheres. Room‐temperature photoluminescence indicated that the as‐prepared SrWO₄ 3D microcrystals had strong emission peaks at about 432 and 505 nm, respectively.     

Surface‐morphology evolution of ZnO nanostructures grown by hydrothermal method

Surface‐morphology evolution of ZnO nanocrystals has been observed by the hydrothermal process. The effects of stirring time and ammonia content on the morphology evolution have been discussed, respectively. Extension of stirring time of the precursor results in morphology transformation from star‐like to wire‐like ZnO nanocrystals. ZnO nuclei aggregation and uniform Zn(OH)₂ precipitation can readily explain these two morphologies, respectively. By increasing the ammonia content in the solution, the morphology of ZnO crystals is transformed from an irregular shape to hexagon sheets to nanorods, and the side length of ZnO crystals is decreased accordingly. Hollow structures are realized at the subsequent solution aging process. Variation of zinc ammonic complex and minimum surface energy can well explain the morphology evolution of ZnO nanostructures.     

Synthesis of spindle‐shaped α‐FeOOH and α‐Fe2O3 nanocrystals

Spindle‐shaped α‐FeOOH nanocrystals were facilely synthesized using a poly (vinyl pyrrolidone) (PVP)‐assisted route under hydrothermal conditions. The chemical compositions and morphol‐ogies of the as‐prepared samples were characterized in detail by X‐ray power diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscope (TEM). The experimental results reveal that these spindle‐shaped α‐FeOOH nanocrystals have self‐organized into assemblies with hierarchical nanostructures. The crucial roles of PVP in the hydrothermal synthesis of hierarchical α‐FeOOH nanostructures were discussed. The possible formation mechanism was also suggested. Moreover, the spindle‐shaped α‐Fe₂O₃ nanocrystals could be easily obtained after calcining the α‐FeOOH prepared by the PVP‐assisted hydrothermal process. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Relationship between morphology and structure of shape‐controlled CeO2 nanocrystals synthesized by microwave‐assisted hydrothermal method

The influence of synthesis conditions on morphology and structure of CeO₂ nanocrystals prepared by microwave‐assisted hydrothermal method were studied by XRD, TEM, Raman spectroscopy and photoluminescence measurements. Decisive effect of the OH^‐/Ce³⁺ molar ratio in the reaction mixture on the size and shape of CeO₂ nanocrystals was established and explained. Rise of the concentration of OH^‐ groups at the surface of growing CeO₂ nanocrystals promotes shape transformation from irregular (spheroidal) to cube and enables oriented attachment mode of particle growth. I was found that the unit cell parameter and intensity of Ce³⁺ defect related band in the luminescence and Raman spectra in ceria nanoparticles increase as follows: irregular shape nanocrystals < large cube < small primary cube shape particles. The reason is growing contribution of total subsurface volume under {100} faces, exposed by ceria nanocubes, where enhanced concentration of Ce³⁺ and oxygen vacancies occur.     

Hydrothermal synthesis of CdSe hierarchical microspheres

CdSe hierarchical microspheres have been successfully synthesized by a hydrothermal route at 120 °C for 16 h via a reaction between CdCl₂ and Na₂SeSO₃ in ionic liquid (1‐butyl‐3‐methylimidazolium bromide)‐water mixed medium. The structure and morphology of the as‐prepared products have been investigated by XRD and SEM, and the results indicate that the CdSe hierarchical microspheres have wurtzite structures and are self‐assembled by nanorods. It has been found that ionic liquid, reaction temperature, and reaction time have influence on the morphology of the products. The possible growth mechanism of CdSe with special morphology has been discussed. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Preparation and magnetic properties of CoWO4 nanocrystals

Cobalt tungstate (CoWO₄) nanocrystals with an average size of 20–50 nm were synthesized via a template‐ or surfactant‐free hydrothermal route. The crystal structure and morphology of the as‐synthesized CoWO₄ sample were characterized by X‐ray diffraction, scanning electron microscopy and transmission electron microscopy. Magnetic measurements on the as‐synthesized CoWO₄ nanocrystals indicate a Néel temperature (T_N) of ∼40 K. This lower T_N may be a result of the nanostructured particles that reduce the exchange coupling. The new synthetic route presented in this paper has potential applications to fabricate other metal tungstates (MWO₄) materials.     

Crystallization and optical properties of CaWO4 and SrWO4

Single crystals of calcium tungstate and strontium tungstate have been grown by double decomposition flux reaction technique using lithium chloride as flux. Growth conditions are optimized to synthesize well faceted crystals. Effect of primary and secondary flux density in the growth charge has been studied. Thermogravimetric study reveals that the grown crystals are highly stable in the temperature range 25 – 1000°C. Analysis of optical absorption normal to the ab‐plane in the spectral range of 200 – 800 nm reveals the true absorption edge, the nature of transition being the allowed indirect one at 4.60 eV and 4.56 eV respectively for CaWO₄ and SrWO₄. The crystals have been characterized by determining useful pertinent optical and dielectric parameters. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of CuCl2 on hydrothermal crystallization of calcium sulfate whiskers prepared from FGD gypsum

Using purified flue‐gas desulfurization (FGD) gypsum as raw material, effects of CuCl₂ on crystal morphology, phase structure, aspect ratio and crystallization of hydrothermal products prepared via hydrothermal crystallization in H₂SO₄‐H₂O solutions were investigated. The results show that dosage of CuCl₂ has a significant effect on the morphology, aspect ratio and crystallization of calcium sulfate whiskers (CSWs), but no effect on their phase transformation . At a dosage of 15 g CuCl₂/kg FGD gypsum, the produced calcium sulfate whiskers had diameters ranging from 1 to 3 μm with average aspect ratio greater than 200 . Transmission electron diffraction patterns and highly magnified surface morphology of CSWs were found different from those of self‐assembly crystals. Compared to self‐assembly crystals, the produced CSWs showed a single crystal structure and their surface was very smooth.     

On the Morphological Changes and Twinning of ZnS (Sphalerite) Crystallites under Hydrothermal Conditions

Morphological characteristics and twinning mechanism of ZnS crystals under hydrothermal conditions have been investigated in this paper. It was shown that under hydrothermal conditions the morphology of ZnS crystallites changes along the four‐fold axis directions, and the crystals are observed in a positive or negative tetrahedron, or in a combination of positive and negative tetrahedra depending on the growth conditions. The positive tetrahedral areas on the crystallites get larger with increase of the concentrations of OH^‐ and S^2‐ in solutions, whereas the twinned crystallites of ZnS taking an elliptic shape with (111) as composition plane are easily formed in weak basic solutions. It can be found that the morphologies of ZnS crystals are in accordance with the crystallization orientations of positive or negative coordination tetrahedra ([S‐Zn₄]⁶⁺, [Zn‐S₄]^6‐) in the crystal although, in some cases, the practical morphology could be greatly affected by growth conditions, and the twinning mechansim can be suggested based on the linkage of growth units of positive and negative coordination tetrahedra, which were formed in the solution. The present investigations further indicated that the crystal chemistry approach based on the linkage/incorporation of growth units previously proposed by us can be sucessfully applied to interpret the growth mechanisms of the crystals and to control a desirable morphology.     

Synthesis of SrMnO3 powders with different morphologies by hydrothermal method

By using two different reagents as the starting materials, SrMnO₃ crystallines with two different morphologies rhombus‐like and rod‐like, have been directly synthesized by hydrothermal conditions. The effects of the hydrothermal parameters on the quality of the SrMnO₃ samples were given for the fist time. And a preparative mechanism for the formation of different morphologies using different reagents during the hydrothermal reaction has been discussed. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of ferric ions on the morphology and size of magnetite nanocrystals synthesized by ultrasonic irradiation

Two‐dimensional plate‐like Fe₃O₄ nanocrystals and nanoparticles could be synthesized by a simple one‐step sonochemical method through ultrasonic irradiation in reverse co‐precipitation solution at low temperature. This technique provided a facile and rapid way to prepare Fe₃O₄nanocrystals with different morphology and size. Magnetite nanoplates were synthesized with only ferrous salt adding into alkali solution, and adding ferric ions with low molar ratio in the metal salts solution would lead to the formation of very small magnetite nanoparticles (∼10 nm). The size of as‐prepared magnetite nanoparticles increased with increasing reaction temperature and showed narrow size distribution, the standard deviation less than 2 nm. This investigation indicated that ferric ions had significant influence on the morphology of Fe₃O₄ nanocrystals. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Monodisperse CeO2 sub‐micro spherical aggregates with controllable building blocks

Monodisperse CeO₂ spherical aggregates with diameters ranging from 200 to 300 nm have been successfully synthesized through a facile hydrothermal method. The structure and morphology of the samples were characterized by powder X‐ray diffraction (XRD), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM) and field‐emission scanning electron microscopy (FE‐SEM). The building blocks (primary nanocrystals) of the spherical aggregates could be effectively tuned by adding different amount of urea. Furthermore, N₂ adsorption/desorption experiment displays a gradual increase of BET surface areas of spherical aggregates with increasing the amount of urea. Finally, the formation mechanism of CeO₂ spherical aggregates was preliminarily discussed. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Controllable one‐step synthesis of CuO,Cu2O and Cu

In the present report, CuO, Cu₂O and Cu have been successfully synthesized through a facile, one‐step hydrothermal method at a relative low temperature by controlling only the concentration of citric acid. Compared with other synthetic methods, the present method is mild, high‐efficient and nontoxic. The crystal structure and morphology of the as prepared samples were characterized by X‐ray diffraction and scanning electron microscopy, respectively. The mechanism for the crystal phase and morphological changes with different citric acid concentrations were discussed. The possible reaction process of the synthesis was also studied on the basis of the experimental results. We hope that this facile, one‐step hydrothermal method could be used in controlling synthesis of other metals and metal oxides under appropriate experimental conditions.     

Synthesis of tunable 3D ZnO architectures assemblied with nanoplates

A simple hydrothermal method has been used to synthesize 3D ZnO architectures without any surfactants. The assemblies were composed of thin nanoplates with the thickness of about 35nm. X‐ray diffraction analysis (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and photoluminescence (PL) characterized the structure, morphology and optical property of products. The thickness of plates and assembly pattern can be controlled by adjusting the experimental parameters. Furthermore, these ZnO nanoplate assemblies exhibited enhanced photocatalytic activity in the degradation of methylene blue (MB). (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth kinetics of vanadium pentoxide nanostructures under hydrothermal conditions

The work reported here involved a study of the growth kinetics of V₂O₅nH₂O nanostructures under hydrothermal conditions. The coarsening process of V₂O₅nH₂O nanoribbons was followed by subjecting the as-prepared suspensions to hydrothermal treatments at 80 °C for periods ranging from 0 to 7200 min. X-ray diffraction (XRD) confirms that the hydrothermal treatments at 80 °C caused no significant modification of the long-range order structure of samples subjected to different periods of hydrothermal treatment. Field emission scanning transmission electron microscope (FE-STEM) was used to analyze the morphology and width distribution of the nanostructures. The results indicated that the crystal growth mechanism in the [1 0 0] direction of vanadium pentoxide 1D nanostructure under hydrothermal conditions is well described by the oriented attachment (OA) mechanism. This evidence was supported by HRTEM images showing the existence of defects at the interface between nanostructures, which is characteristic of the oriented attachment (OA) mechanism.     

Structure and thermomagnetic properties of powders produced from melt spun FeNbBCu ribbons

Amorphous ribbons of Fe₇₇Nb₇B₁₅Cu₁ prepared by melt-spinning and powders produced from them by ball-milling were characterized by means of calorimetry, X-ray diffraction, scanning electron microscopy and vibrating sample magnetometry. Upon thermal treatment the amorphous alloy experiences a primary crystallization that leads to bcc-Fe nanocrystals dispersed in an amorphous matrix. Magnetic measurements indicate that this alloy in the amorphous and nanocrystalline state is a good soft magnetic material. Values of saturation magnetization and coercivity are 120 Am²/kg and 5 A/m respectively, for the alloy in the nanocrystalline state. Pre-annealing, post-relaxation and nanocrystallization as well as various milling parameters were explored and the structural and magnetic changes induced have been studied. The analysis of the particle size distribution and morphology of the powders show that the brittleness resulting from pre-annealing of the ribbons is very effective in reducing the particle’s size. Recovery of the high coercitive field induced by milling is achieved by post-annealing to an extent that depends mostly on the milling conditions.     

Capsule‐like α‐Fe2O3 nanoparticles: Synthesis,characterization, and growth mechanism

Uniform capsule‐like α‐Fe₂O₃ particles were synthesized via a simple hydrothermal method, employing FeCl₃ and CH₃COONa as the precursors and sodium dodecyl sulfate (SDS) as soft template. X‐ray powder diffraction (XRD), field emission scanning electron microscopy (FE‐SEM), transmission electron microscopy (TEM), and high‐resolution transmission electron microscopy were used to characterize the structure of synthesized products. Some factors influencing the formation of capsule‐like α‐Fe₂O₃ particles were systematically investigated, including different kinds of surfactants, the concentration of SDS, and reaction times. The investigation on the evolution formation reveals that SDS was critical to control the morphology of final products, and a possible five‐step growth mechanism was presented by tracking the structures of the products at different reaction stages.     

Synthesis and characterization of single-crystalline PrCO3OH dodecahedral microrods and its thermal conversion to Pr6O11

Single-crystalline PrCO₃OH dodecahedral microrods with an orthorhombic structure have been successfully synthesized by the hydrothermal method used urea as the precipitator. Pr₆O₁₁ dodecahedral microrods have been obtained by thermal conversion of PrCO₃OH dodecahedral microrods at 600 °C in air for 6 h. The as-synthesized products were characterized by X-ray powder diffraction, field-emission scanning electron microscope, transmission electron microscopy, high-resolution transmission electron microscopy, selected-area electron diffraction, X-ray photoelectron spectra, fourier transform infrared spectroscopy and thermogravimetry–differential thermal analysis. The effect of the reaction parameters on the morphology of the product has been investigated. The dodecahedral microrods with larger size and better crystallinity can be obtained under the higher reaction temperature. The possible formation mechanism of PrCO₃OH microrods was discussed.     

HRTEM and GIXRD studies of CdS nanocrystals embedded in Al2O3 films produced by magnetron RF-sputtering

In this paper we report on the structural properties of as-grown CdS nanoparticles embedded in Al₂O₃ films produced by a magnetron RF-sputtering technique. Grazing incidence X-ray diffraction together with high-resolution transmission electron microscopy (HRTEM) and electron diffraction were used to study the crystallinity and morphology of the CdS nanocrystals. Depending on the deposition parameters, elongated or spherical nanocrystals were grown. HRTEM shows evidence of the growth of CdS nanocrystals at room temperature with sizes in the range of 3–8 nm, and indicates that the nanocrystals formed in the cubic phase during the early stages of the deposition process. Stress-free films were formed under selected deposition conditions.