Low Hydrothermal Temperature Research Articles

Controllable growth of ZnO nanowhiskers by a simple solution route

One-dimensional (1D) needle-like ZnO nanowhiskers have been grown directly from aqueous solution containing Zn(OH) 4 2− ions which is produced by zinc chloride and sodium hydroxide, in the presence of sodium dodecyl sulfate (SDS). Through the improved novel approach, uniform ZnO nanowhiskers with diameters ranging from 80 to 120 nm and an average length of about 4 μm (aspect ratio ∼40:1) were successfully prepared at a low hydrothermal temperature (85 °C) with a short reaction time (5 h). The nanowhiskers exhibit a perfect single crystalline wurtzite structure with [0 0 0 1] as the growth orientation. The growth mechanism, as well as the controllable factors on the crystal size and shape, is also discussed in detail.

The use of CTAB to improve the crystallinity and dispersibility of ultrafine magnesium hydroxide by hydrothermal route

The high-dispersed lamellar ultrafine magnesium hydroxide was obtained at relatively low hydrothermal temperature in the presence of cationic surfactant, cetyl trimethyl ammonium bromide (CTAB). The procedure described in this study is attractive since proper amount of CTAB could promote the dissolution and precipitation of magnesium hydroxide in hydrothermal system, resulting in the well-defined morphology, narrow size distribution and good crystallinity of ultrafine particles. The resultant magnesium hydroxide was characterized by X-ray powder diffraction (XRD), thermogravimetric analysis-differential scanning calorimetry (TGA-DSC), field-emission scanning electron microscopy (FESEM) and laser diffusion analysis. The method led to the production of particles with mean size of 400 nm and a thickness of 60 nm. The optimal conditions of preparation were hydrothermal treatment at 150 °C for 6 h at pH 11 with Mg 2+/CTAB molar ratio of 80.

Controllable synthesis, characterization, and electrochemical properties of manganese oxide nanoarchitectures

Flowerlike manganese oxide microspheres and cryptomelane-type manganese oxide nanobelts were selectively synthesized by a simple decomposition of KMnO4 under mild hydrothermal conditions without using template or cross-linking reagents. The effect of varying the hydrothermal times and temperatures on the nanostructure, morphology, compositional, and electrochemical properties of the obtained manganese oxides was investigated. X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) studies showed that the flowerlike manganese oxide microspheres could be obtained at relatively low hydrothermal temperatures, while high hydrothermal temperatures were favorable for the formation of cryptomelane-type manganese oxide nanobelts. A morphology and crystalline evolution of the nanostructures was observed as the hydrothermal temperature was increased from 180 to 240 °C. On the basis of changing the temperatures and hydrothermal reaction times, the formation mechanism of cryptomelane-type manganese oxide nanobelts is discussed. Cyclic voltammetry (CV) was used to evaluate the electrochemical properties of the obtained manganese oxide nanostructures, and the results show that the electrochemical properties depend on their shape and crystalline structure. This easily controllable, template-free, and environmentally friendly method has the potential for being used in syntheses of manganese oxide nanomaterials with uniform morphologies and crystal structures.

Fabrication of zeolite L thin films with different orientations using anisotropic growth of seed crystals by secondary growth method

Zeolite L thin films were fabricated by applying fine zeolite L seed crystals on glass substrates and then hydrothermally treating the seeded substrates in reaction sols. The influence of anisotropic growth of the seed crystals on the orientation of the films was examined. The reaction sols were prepared by mixing KOH, Al, colloidal silica, and water. With increasing the amount of the KOH in the sols or with decreasing the hydrothermal temperature, the zeolite L crystals obtained from the sols changed from a rod-like shape to a clam-like one, indicating a decrease in the relative rate of the growth along the c-axis of zeolite L with a hexagonal lattice to one normal to the c-axis. For the film fabricated at a small amount of KOH or at a high hydrothermal temperature, the c-axis of the zeolite crystals was oriented normal to the substrate. The fabrication at a large amount of KOH or at a low hydrothermal temperature led to films consisting of zeolite crystals whose c-axis is parallel to the substrate. When the aspect ratio of the precipitated crystals decreased to about one, the c-axis of the zeolite crystals in the films became parallel to the substrate.

Characterization and optical properties of ZnSe prepared by hydrothermal method

ZnSe powders were prepared at 125–200 °C by hydrothermal method using Zn and Se as source materials. The powders were cubic zinc blende structure ZnSe, and some ZnSeO 4·H 2O found in ZnSe prepared at 125 and 150 °C due to insufficient reducing atmosphere at low hydrothermal temperature. High hydrothermal temperature was helpful to completely consume the source materials and to synthesize well-crystallized and stoichiometric ZnSe. The ZnSe showed steep optical absorption at wavelength below 467 nm, and absorption wavelength shift was observed in the ZnSe with different grain size and post-annealing process.

Selective catalytic reduction of NO over Co/beta-zeolite: effects of synthesis condition of beta-zeolites, Co precursor, Co loading method and reductant

The effects of preparation method for beta-zeolite, Co-precursor, loading method and different type of reductants on the performance of Co/beta-zeolite catalyst for HC-selective catalytic reduction (SCR) of NO have been investigated. The catalyst properties and nature of active sites have been characterized by N2 adsorption–desorption isotherms, X-ray diffraction (XRD), thermogravimetric analysis (TGA), TPR, X-ray photoelectron spectroscopy (XPS) and NMR. The catalyst activity is not affected by the type of cobalt precursor but is significantly influenced by the cobalt loading methods. The ion-exchange method results in a much higher activity than impregnation method. Although there is little influence of the silica source on the catalyst properties, the preparation condition of support beta-zeolite is found to substantially affect the activity of the resulting catalyst. The low hydrothermal synthesis temperature causes the presence of amorphous mesoporous phase, making the resulting catalyst poorly resistant to steam. The catalytic performance is substantially affected by different reductants used, with propane performing as a proper reductant for SCR of NO.

Hydrothermal synthesis of high-purity BaTiO 3 powders: control of powder phase and size, sintering density, and dielectric properties

High-purity BaTiO 3 fine powders have been synthesized hydrothermally from BaCl 2 and TiCl 4 under moderate conditions, and the tetragonal content and particle size of as-prepared powders have been characterized quantificationally. Eighty-nanometer and 40% tetragonal content BaTiO 3 powders can be obtained by adjusting the hydrothermal temperature and the initial concentration of TiCl 4 to a value of 240 °C and 1.00 mol/l. At a lower hydrothermal temperature, 80-nm cubic BaTiO 3 powders result; and at a lower initial concentration of TiCl 4, 150- and 500-nm BaTiO 3 powders with 40% tetragonal phase content result. Because of the successful preparation of these powders, the effect of powder phase and size on the sintering density and dielectric properties of these powders is more clearly investigated. The effect of tetragonal content on the dielectric constant is much more significant than the effect of the particle size. The powder with 80 nm average size and 40% tetragonal content has the highest sintering density (5.83 g/cm 3 and 96.5% of the theoretical density) and the highest dielectric constant (6200) and the lowest dielectric loss.

Calcium aluminate cements in fly ash/calcium aluminate blend phosphate cement systems: Their role in inhibiting carbonation and acid corrosion at a low hydrothermal temperature of 90°C

Study was focused upon formulating sodium polyphosphate-modified fly ash/calcium aluminate blend (SFCB) geothermal well cements with advanced anti-carbonation and anti-acid corrosive properties. At a low hydrothermal temperature of 90°C, to improve these properties, we investigated the effectiveness of various calcium aluminate cement (CAC) reactants in minimizing the rate of carbonation and in abating the attack of H2SO4 (pH ∼ 1.6). We found that the most effective CAC had two major phases, monocalcium aluminate (CA) and calcium bialuminate (CA2), and a moderate CaO/Al2O3 ratio of 0.4. The reaction between sodium polyphosphate (NaP) and CA or CA2 at room temperature led to the formation of amorphous dibasic calcium phosphate hydrate and anionic aluminum hydroxide caused by the decalcification of CA and CA2. When SFCB cement made with this CAC was exposed to 4% NaHCO3-laden water at 90°C, some carbonation of the cement occurred, forming calcite that was susceptible to the reaction with H2SO4. This reaction resulted in the deposition of gypsum gel scales as the acid corrosion product on the cement surfaces. The scale layer clinging to the cement protected it from further corrosion. Under such protection, the amorphous dibasic calcium phosphate hydrate → crystal hydroxyapatite and anionic aluminum hydroxide → crystal boehmite phase transitions were completed in acid solution. Meanwhile, the further chemical and hydration reactions of NaP with fly ash led to the formation of additional crystalline Na-P type zeolite phases. Thus, we propose that passivation of the surface of the cement by deposition of gypsum, following the formation of these reaction products, which are relatively inert to acid, are the acid corrosion-inhibiting mechanisms of the SFCB cements.

Phase Transformation Behavior at Low Temperature in Hydrothermal Treatment of Stable and Unstable Titania Sol

Nanosize titania sol was prepared from titanium tetraisopropoxide (TTIP) and conditions for the formation of stable sol were identified. As the H+/TTIP mole ratio decreased and H2O/TTIP mole ratio increased, stable sol was likely to be formed. The size and crystallinity remained unchanged after hydrothermal treatment of the stable sol at between 160 and 240°C. However, hydrothermal treatment of unstable sol produced rod-like particles and crystallinity of particles was changed from anatase to rutile. This difference in phase transformation at low hydrothermal treatment temperature was likely to be caused by the fact that stable sol remained to be stable even after hydrothermal treatment, while unstable sol had a tendency to be aggregated.