Kinetically controlled synthesis of wurtzite ZnS nanorods through mild thermolysis of a covalent organic-inorganic network

The high-temperature (over 1020 degrees C) polymorph of ZnS, wurtzite ZnS, has been successfully prepared through a low-temperature (180 degrees C) hydrothermal synthesis route in the presence of ethylenediamine (en). The effects of en concentrations, reactant concentrations, reaction temperatures, and reaction times on crystal structures and shapes of ZnS have been investigated. We have demonstrated that the wurtzite ZnS showing rodlike morphology can be kinetically stabilized in the presence of en, especially at a high reactant concentration under appropriate hydrothermal conditions. Besides phase evolution of ZnS from hexagonal to cubic, morphological transformation from nanorods to nanograins has also been observed in the present investigation. Nanograins of phase-pure cubic ZnS, the thermodynamically stable polymorph, are easily prepared, and no hexagonal ZnS nanorods are detected in "pure" water, i.e., in the absence of en molecules. The above investigations indicate that the controlled fabrication of wurtzite ZnS nanorods is due to a mediated generation of the lamellar phase, ZnS.0.5en, a covalent organic-inorganic network based on ZnS slabs, and to its subsequent thermolysis in aqueous solution. The controlled growth of wurtzite ZnS nanorods and sphalerite ZnS nanograins provides us an opportunity to structurally modulate physical properties. These wurtzite ZnS nanorods display narrower and stronger blue emission than sphalerite ZnS nanograins.     

Synthesis and structural metastability of CdTe nanowires

A new organometallic preparation method is described for CdTe nanowires with a high aspect ratio and a predominantly metastable wurtzite phase. The optical and morphological properties of the resulting nanowires were studied, as well as the influence of elevated temperatures on the crystallographic properties. A phase transition from wurtzite to sphalerite was observed at about 500 degrees C. The results show that the wurtzite phase is stabilized by the synthetic method and the surfactants.     

Wurtzite-to-rocksalt structural transformation in nanocrystalline CoO

Hexagonal CoO nanocrystals are coarsened under hydrothermal conditions to investigate the effect of particle size on phase transformation and stability property. Structural stability and phase transformation of the hexagonal CoO phase have been investigated by X-ray powder diffraction with Rietveld refinement, transmission electron microscopy, X-ray absorption fine structure, and differential scanning calorimeter. It is found that the hexagonal CoO phase is a metastable phase, which increases its grain size from 50 to 250 nm for refluxing times from 1 to 6 h at 200 degrees C. After 12 h, cubic-structured CoO grains with an average grain size of 20 nm are observed, which spread around big hexagonal CoO grains. After about 24 h, only the cubic CoO phase with an average grain size of 25 nm is detected. The onset temperature of hexagonal-to-cubic phase transformation in CoO is estimated to be 378 degrees C by DSC, using a heating rate of 20 deg/min. The results obtained indicate that the hexagonal-to-cubic phase transformation in nanocrystalline CoO is by nucleation and growth mechanism, starting from the surface to the center of the hexagonal grains.     

A molecular dynamics study of structural relaxation in tetrahedrally coordinated nanocrystals

The reorganisation of nanocrystals in order to reduce their surface energies has been examined in computer simulations. The relaxation takes a qualitatively different path for sphalerite- and wurtzite-structured particles. The surfaces of the sphalerite particles reconstruct into hexagonal nets, but the interior remains identifiable as sphalerite-like, whereas wurtzite particles form facetted, hexagonal nanorods by virtue of a reorganisation of the whole particle which involves the creation of a low energy internal interface between oppositely oriented domains. Despite the reorganisation, the diffraction patterns remain compatible with a wurtzite structure with some internal strain. The dipole moments of thermalized wurtzite particles are compared with experimental results for CdSe.     

Effects of Dissipation on Contact Angle Measurements Using a Dynamic Method

We report studies of the effect of hydrothermal treatment on physical properties such as crystalline phase, size, and morphology of nanosized cadmium sulfide (CdS) particles. CdS precipitates have been synthesized by the reaction of Cd(NO(3))(2) with Na(2)S at room temperature. These CdS precipitates have been hydrothermally treated in the range 120-240 degrees C with variation of the treatment time. The effects of acid catalysts and other additives were also investigated. The particles prepared were characterized by XRD, TEM, and BET methods. With increased hydrothermal treatment temperature and time, crystallization from amorphous to crystalline form, cubic or hexagonal, and an increase of particle size occurred. CdS particles of well-developed hexagonal form were obtained at a hydrothermal treatment temperature of 240 degrees C; the primary hexagonal grain size was on the order of 20-30 nm. The addition of an acid catalyst, HCl, or of Cd(NO(3))(2) into the precipitate sol promoted crystal growth and phase transformation during the hydrothermal treatment, but another additive, Na(2)S, showed the opposite trend. It appears that hydrothermal treatment combined with proper additives could be an effective method for preparation of nanosize crystalline CdS particles. Copyright 2001 Academic Press.     

Phase transformation of boron nitride under hypothermal conditions

Phase transformation among different boron nitride (BN) phases in hydrothermal solution was investigated. It was found that hexagonal boron nitride (hBN) firstly formed in the solution at relatively low temperature (i.e., 220 °C). After that, a spot of hBN began to transform into wurtzite boron nitride (wBN) and cubic boron nitride (cBN) at 230 °C. More and more hBN converted into wBN and cBN with the increase in temperature, and this transformation process completed at 300 °C. In this paper, we have explained the mechanism of the above phase transformation by using a reported “puckering mechanism”.     

Hydrothermal routes to prepare nanocrystalline mesoporous SnO2 having high thermal stability

We report simple hydrothermal routes to prepare thermally stable SnO2 particles having high specific surface areas and mesoporosity. The preparation method includes a new combination of synthetic processes: hydrolysis of tin(IV) chloride at 95 degrees C in the absence of alkaline solutions (aqueous NH3 or NaOH), formation of nanocrystalline SnO2, and subsequent hydrothermal treatments at temperatures between 100 and 200 degrees C. After annealing treatments of the hydrothermally treated SnO2 particles at 400 or 500 degrees C, their crystallite sizes remained smaller than 7.7 nm and their specific surface areas were still higher than 110 m2/g, indicative of the high thermal stability against particle growth and sintering. Furthermore, mesoporosity evolved with a relatively narrow pore size distribution typically in the range of 3.0-4.3 nm. The effects of the hydrothermal treatment were explained by uniformization of the particle size that was beneficial to the suppression of particle growth.     

Unraveling the crystallization mechanism of CoAPO-5 molecular sieves under hydrothermal conditions

The hydrothermal crystallization of CoAPO-5 molecular sieves has been studied using time-resolved in-situ SAXS/WAXS, UV-vis, Raman, and XAS. Data collected during heating to 180 degrees C allowed the observation of different steps occurring during the transformation of the amorphous gel into a crystalline material from a macroscopic and atomic perspective. Raman spectroscopy detected the initial formation of Al-O-P bonds, whereas SAXS showed that these gel particles had a broad size distribution ranging from ca. 7 to 20 nm before crystallization began. WAXS showed that this crystallization was sharp and occurred at around 160 degrees C. Analysis of the crystallization kinetics suggested a one-dimensional growth process. XAS showed that Co(2+) transformed via a two-stage process during heating involving (i) a gradual transformation of octahedral coordination into tetrahedral coordination before the appearance of Bragg peaks corresponding to AFI, suggesting progressive incorporation of Co(2+) into the poorly ordered Al-O-P network up to ca. 150 degrees C, and (ii) a rapid transformation of remaining octahedral Co(2+) at the onset of crystallization. Co(2+) was observed to retard crystallization of AFI but provided valuable information regarding the synthesis process by acting as an internal probe. A three-stage, one-dimensional crystallization mechanism is proposed: (i) an initial reaction between aluminum and phosphate units forming a primary amorphous phase, (ii) progressive condensation of linear Al-O-P chains forming a poorly ordered structure separated by template molecules up to ca. 155 degrees C, and (iii) rapid internal reorganization of the aluminophosphate network leading to crystallization of the AFI crystal structure.     

Formation of Ag nanostrings induced by lyotropic liquid-crystalline phospholipid multilayer

Morphological variation of the Ag nanoparticles embedded in a lyotropic phospholipid (1,2-dioleoyl-sn-glycero-3-phosphoethanolamine, DOPE) membrane during hydration was investigated. Hydration at 5 °C resulted in transformation of the Ag nanoparticles into a bundle of Ag nanostrings as the Ag nanoparticles conformed to the H(II) phase of the DOPE molecules. Above 30 °C, the nanoparticles quickly coarsened into large polygonal-shaped particles since high mobility of the lipid molecules overwhelmed the tendency for the Ag nanoparticles to order. The result provided an insight into the long-term stability of nanoparticles trapped in different lipid membranes depending on the structural ordering of the molecules.     

Relative brookite and anatase content in sol-gel-synthesized titanium dioxide nanoparticles

Sol-gel synthesis of titania typically produces a mixture of brookite and anatase. Rietveld refinements were used to systematically track the brookite content and particle size as functions of synthetic variables. Results demonstrate that brookite content and anatase particle size decrease with decreasing Ti/H(2)O ratios. In syntheses at pH 3, the addition of HCl resulted in increased amorphous content compared to samples synthesized using HNO(3). Similar amorphous contents were observed for particles prepared at pH 6-9. Hydrothermal aging for 4 h at 200 degrees C of sol-gel products containing substantial amorphous titania resulted in higher brookite content than did hydrothermal aging of sol-gel products containing little to no amorphous titania. Finally, dialysis prior to hydrothermal aging appeared to inhibit phase transformation from brookite to anatase in aged materials. Results presented demonstrate that considerable control over the relative anatase and brookite contents can be achieved through control of synthetic variables.     

Structural,optical and magnetic characterization of ZnO nanorods synthesized using hydrothermal technique at low temperature

Pure ZnO nanorods were grown from aqueous solutions at low temperature (90 °C) by hydrothermal growth technique on sapphire (0001) substrate coated with ZnO thin film. X-ray diffraction results show that these nanorods crystallize in the wurtzite structure having space group P6₃mc and that they are oriented along the c-axis. Raman and photo-luminescence studies show the presence of oxygen vacancies in the ZnO nanorods. The ZnO nanorods show room temperature ferromagnetism.     

Size- and shape-dependent phase transformations in wurtzite ZnS nanostructures

This paper describes the equilibrium morphologies of zinc sulfide nanoparticles in the wurtzite phase as a function of size, determined using ab initio Density Functional Theory (DFT) simulations and a shape-dependent thermodynamic model predicting the Gibbs free energy of a nanoparticle. We investigate the relative stabilities of a variety of nanoparticle shapes based on the wurtzite structure and show how the aspect ratio of wurtzite nanorods moderates the size-dependent phase transformation to the zinc blende phase. We find that while wurtzite nanoparticles are thermodynamically unstable with respect to the low energy rhombic dodecahedron morphology in the zinc blende phase at all sizes, shape- and size-dependent phase transformations occur when other zinc blende morphologies are present. Despite popular synthesis of zinc sulphide nanoparticles in the wurtzite phase, an in-depth thermodynamic study relating to the relative stability of wurtzite shapes and comparison with the zinc blende phase does not exist. Therefore this is the first thermodynamic study describing how shape can determine the solid phase of zinc sulfide nanostructures, which will be of critical importance to experimental applications of nanostructured zinc sulfide, where phase and shape determines properties.     

A study of cadmium sulfide nanocrystalline films by grazing incidence X-ray diffraction

Thin cadmium sulfide films were prepared on a monocrystalline-crystal silicon substrate by chemical deposition from aqueous solutions. Grazing incidence X-ray diffraction revealed that the cadmium sulfide films are comprised of nanocrystal particles, with 80% of the particles having a size of 5 ± 1 nm. Some nanocrystals have a wurtzite structure, while others, a sphalerite one. The presence of cubic phase in the films is indicative of a nonequilibrium state of the nanocrystalline films. Thirty minutes after the onset of the formation of cadmium sulfide, the size and crystal structure of the constituent particles of the film become independent of the deposition time—only the film thickness increases. In addition, the initial stage of the formation of the cadmium sulfide film is accompanied by the deposition of cadmium hydroxide Cd(OH)₂.     

Specifics of the thermal transformations of the wurtzite phase of boron nitride

The kinetics of the transformation of the BN wurtzite phase to the graphite modification was studied at normal pressure and 600–970°C. At these temperatures and certain thermal treatment durations, along with the formation of the graphite phase, the reverse transition from g-BN to w-BN occurs, a behavior indicative of a higher thermodynamics stability of the wurtzite phase.     

Nanosized gismondine grown in colloidal precursor solutions

A colloidal molecular sieve with GIS-type structure was prepared from aged aluminosilicate precursor solutions containing tetramethylammonium (TMA) hydroxide under hydrothermal treatment at 100 degrees C. The nucleation and the development of the GIS zeolite structure were studied by dynamic light scattering, scanning electron microscopy, X-ray diffraction, Raman and infrared spectroscopies, and liquid-state NMR spectroscopy. It is shown that the aging at room temperature leads to the formation of subcolloidal particles that incorporate TMA cations and form larger aggregates. After an extended heating of 13 days, a complete transformation from amorphous precursor material to crystalline GIS-type colloidal particles is observed. The mean hydrodynamic radius of the crystalline GIS particles is in the range of 30-50 nm. The specific template-framework interactions influence the spectral features of the TMA cations incorporated in the zeolite structure, thus making possible the use of the corresponding Raman spectra and 13C NMR data for the examination of the crystallinity of GIS-type colloidal particles stabilized in water.     

Rapid synthesis of MFI zeolite nanocrystals

A rapid synthesis procedure for nonagglomerated silicalite nanocrystals has been developed. This was achieved by concentrating the precursor sol before 10-12 h of aging at 80 degrees C, followed by hydrothermal synthesis at 175 degrees C for 90 min. The high silica concentration in the concentrated sol accelerated the aggregation of primary units that were present early in the system. Thus, little silica nutrients were left for growth when the secondary particles were converted to zeolite during hydrothermal reaction. As a result, fully dispersible nanocrystals were obtained within a day instead of weeks as reported previously. The aggregation of primary units during the 80 degrees C aging process as well as the conversion of these aggregates into zeolite has been followed by DLS, XRD, and FTIR. In light of the new results, the nucleation and growth mechanisms of MFI zeolite that have been under debate in the literature were reexamined.     

Chemical Approach to a New Crystal Structure: Phase Control of Manganese Oxide on a Carbon Sphere Template

The stabilization and growth of a non‐native structure, hexagonal wurtzite MnO (h‐MnO), is explored via kinetic control of manganese precursor on a carbon sphere template. MnO is most stable in the cubic rock‐salt structure (c‐MnO), and a number of studies have focused on the synthesis and properties of this rock‐salt phase. However, h‐MnO has not been fully characterized before our work. Prolonged heating at a relatively low temperature yields c‐MnO, whereas rapid heating of the reaction mixture at reflux produces h‐MnO in the presence of carbon spheres. The effect of benzyl amine concentration on the formation of two different oxidation states (c‐MnO and t‐Mn₃O₄) was examined as well. Moreover, the structural stability of the manganese oxides and phase transition of MnO in terms of the wurtzite to rock‐salt structural transformation have been investigated.     

Der Chemische Transport von Mischkristallen im System FeS/MnS/ZnS

Chemical Vapor Transport of Solid Solutions. 7. Chemical Vapor Transport of FeS/MnS/ZnS Mixed Crystals By means of chemical vapor transport using iodine as transport agent (900 → 800 °C) it is possible to prepare in the quasiternary system FeS/MnS/ZnS the mixed crystals (Fe,Mn,Zn)S (sphalerite and wurtzite type), (Fe,Mn)S(ZnS) (NaCl type) and FeS(MnS,ZnS) (NiAs type) in form of single crystals. Based on the composition of these phases the phase diagram for the system FeS/MnS/ZnS at 800 °C was drawn up. The incongruent transport process leads to the accumulation of ZnS in the crystallization zone.     

Temperature-reversible ultrathin films of N-isopropylacrylamide terpolymer adsorbed at the solid-liquid interface

This article describes the stability and reversibility of ultrathin films of N-isopropylacrylamide (NIPA)-vinylimidazole (VI)-poly(ethylene glycol) (PEG) graft terpolymer adsorbed at the solid-liquid interface upon temperature cycling from below to above its phase transition temperature. The coil-to-globule and globule-to-coil phase transitions were captured by in situ fluid tapping atomic force microscopy (AFM). The film thickness of 1 nm was determined by AFM, X-ray photoelectron spectroscopy, and X-ray reflectivity. The concentration required to reach full coverage was found to be higher when adsorption occurred below the phase transition temperature. From 23 to 42 degrees C, the adsorbed NIPA terpolymer film was observed to be molecularly smooth, corresponding to the close-packed structure of flexible polymer coils. Particles containing between one and a few globules appeared abruptly at the interface at 42-43 degrees C, the same temperature as the solution phase transition temperature, which was determined by dynamic light scattering. The size of the particles did not change with temperature, whereas the number of particles increased with increasing temperature up to 60 degrees C. The particles correspond to the collapsed and associated state of the globules. The film morphological changes were found to be reversible upon temperature cycling. Subtle differences were observed between dip-coated and spin-coated films that are consistent with a higher degree of molecular freedom for spin-coated films. The study contributes to the fundamental understanding and applications of smart ultrathin films and coatings.     

NaOH concentration effect on the oriented attachment growth kinetics of ZnS

In this work, the crystal growth kinetics of ZnS nanoparticles coarsened under 100 degrees C with NaOH concentration from 2 to 8 M was investigated, aiming to study the role of NaOH concentration on the oriented attachment growth kinetics. It reveals that 2 M NaOH is sufficient to lead to two-stage growth kinetics of ZnS nanoparticles, resulting in pure and multistep oriented attachment growth characteristics in the first stage. When the NaOH concentration increases, the rate of crystal growth by oriented attachment mechanism increases, while the time period for crystal growth at the pure oriented attachment stage was similar. We suggest that the concentration of solute is critical to enhance the oriented attachment growth rate and achieve exclusively oriented attachment growth of nanoparticles at a large size scale.