Yu. D. Tret’yakov

ZnO formation under hydrothermal conditions from zinc hydroxide compounds with various chemical histories

The sequence of phase transformations of zinc hydroxide compounds with various chemical histories during hydrothermal (HT) treatment is studied by X-ray powder diffraction and thermal analysis (TG/DTA). ɛ-Zn(OH)2 (wulfingite) is the major intermediate in ZnO production by the HT process. The micromorphology and photocatalytic activity of ZnO powders are studied.

Microwave-Assisted Hydrothermal Synthesis and Photocatalytic Activity of ZnO

The effect of synthesis temperature on the morphology of fine zinc oxide powders prepared by microwave-assisted hydrothermal processing of Zn(OH)2 suspensions has been studied using x-ray diffraction and low-temperature nitrogen BET measurements. The photocatalytic activity of the powders for Methyl Orange photodegradation has been assessed as a function of synthesis temperature. The results indicate that the specific surface of ZnO powders plays a key role in determining their photocatalytic activity.

Oxygen Nonstoichiometry of Nanocrystalline Ceria

Correlation relations between oxygen nonstoichiometry and particle size in nanocrystalline CeO2 − x are revised. The ceria unit cell parameter is shown to increase from 0.5410 to 0.5453 nm as the particle size decreases from 23 to 2.3 nm. The CeO2 − x critical particle size where cerium(IV) is completely reduced to cerium(III) is calculated as 1.1–1.3 nm.

Nanocrystalline ceria: Synthesis, structure-sensitive properties, and promising applications

Directed search for further manifestations of size-dependent effects in the functional properties of nanomaterials constitutes a key line of nanotechnologies research. The major size-dependent effects were analyzed for nanocrystalline ceria. The practically significant techniques for preparation of ceria nanopowders with controllable micromorphology were analyzed. Selected promising applications of this material were described.

Synthesis and Properties of Colored Copper-Containing Apatites of Composition Ca5(PO4)3CuyOy + δ(OH)0.5 − y − δX0.5 (X = OH, F, Cl)

Ca5(PO4)3CuyOy + δ(OH)0.5 − y − δX0.5 compounds (for X = OH, y = 0.01–0.3; for X = F, y = 0.01–0.1; for X = Cl, y = 0.1) have been synthesized by heat treatment of oxide-carbonate mixtures at 1150°C in air and have been characterized by X-ray diffraction, electronic spectroscopy, and magnetic measurements. The compounds have an apatite structure in which copper atoms substitute for part of the hydroxyl hydrogen atoms in hexagonal channels. The electronic spectrum shows two main absorption bands due to d-d transitions in copper(II) linearly coordinated to two oxygen atoms, as well as extra, weaker bands, whose contribution to the overall spectrum decreases with decreasing y. The latter are assignable to copper atoms occupying other sites in the crystal lattice. The temperature dependence of the magnetic susceptibility of the compounds obeys the Curie-Weiss law with a Curie constant close to zero. The Weiss constant characterizes the copper(II) content of the compounds and correlates qualitatively with the intensity of the main absorption bands in the visible spectrum. The fraction of copper(II) in the total amount of copper in the apatites increases in the substituent order X: Cl, OH, F, as well as upon the low-temperature annealing of the compounds in air. At the same time, copper(II) fraction depends only slightly on the total copper content. As the copper(II) content increases, the color of apatite changes from pink to dark claret.

Hydroxyapatite-based ceramic materials prepared using solutions of different concentrations

Hydroxyapatite, Ca10(PO4)6(OH)2, powders with enhanced sinterability have been synthesized through precipitation from calcium nitrate and ammonium hydrogen phosphate solutions at pH 9, t= 60°C, and a Ca/P atomic ratio of 1.67, and their properties have been studied: phase composition, particle size distribution, loose density, and green density. The initial solution concentration is shown to influence the properties of the powders and the ceramics fabricated from them. Comparison of the particle size distributions in disaggregated powders and the grain size distributions in the ceramics indicates that the ceramics inherit the structure of the corresponding powders. Optimizing the synthesis conditions in order to enhance the sinterability of the powders, we obtained green compacts with the highest shrinkage rate in the range 850–950°C and shrinkage onset at 600°C, which is 100–150°C lower in comparison with powders synthesized in earlier studies from calcium nitrate and ammonium hydrogen phosphate.

Hydrothermal synthesis of efficient TiO2-based photocatalysts

We carried out a thorough study of the micromorphology, physicochemical properties, and photocatalytic activity of nanocrystalline anatase powders formed during the hydrothermal treatment of titanyl sulfate solutions of various compositions. A new procedure is proposed for the hydrothermal synthesis of nanocrystalline anatase having specific surface areas of ∼50 m2/g and photocatalytic activities exceeding the activity of the commercially available Degussa P25 formulation.

Fractal structure of ceria nanopowders

The formation of ceria nanoparticles from water-alcohol solutions of cerium(III) nitrate has been studied by UV/VIS spectroscopy, low-temperature nitrogen adsorption measurements, x-ray diffraction, transmission electron microscopy, and thermal analysis, and the effect of high-temperature annealing on the fractal structure of the CeO2 nanopowders has been examined.

Two-dimensional spatially ordered Al2O3 systems: Small-angle neutron scattering investigation

The structure of anodized aluminum oxide films has been investigated by the small-angle neutron scattering method. A theoretical solution is obtained for describing neutron scattering from the Al2O3 ordered porous structure. Analysis of the neutron-experiment data shows the possibility of obtaining porous membranes with ideally periodic hexagonal packed pores on a large area (∼0.5 cm2).

Synthesis and properties of colored copper-containing alkaline-earth phosphates with an apatite structure

Colored compounds A5(PO4)3Cu0.1OHz, where A = Ba1-xSrx (x=0,0.5,0.8), Sr1-y Cay (y=0,0.2, 0.5, 1), and Ba1/3Sr1/3Ca1/3 with an apatite structure containing copper ions in the hexagonal channels have been synthesized by the thermal treatment of the corresponding carbonates, oxides, and ammonium hydrophosphate at 1150°C. The use of the composition with A = Ba0.5Ca0.5 gives a multiphase product containing apatite (Ba1-xCax)5(PO4)3CuyOHz with x ∼ 0.2 as the major phase. The hexagonal lattice parameters of apatite depend linearly on the content of solid solution in the alkaline-earth metals. In the series of A = Ba, Ba-Sr, Sr, Sr-Ca, and Ca, the color of the samples changes smoothly from blue through violet to pink, whereas the samples with A = Ba0.5Ca0.5 and Ba1-3Sr1-3Ca1-3 are light gray. The diffuse reflectance electronic spectra exhibit two absorption bands characteristic of the copper ions in the hexagonal channels. Their positions depend on the alkaline-earth metal content of the compound. The third characteristic band appears in the spectrum of calcium apatite. Its relative intensity increases when calcium is gradually replaced by strontium and especially by barium, which results in worsening and even loss of the sample color. These spectral changes are presumably due to the transition of some copper ions from the hexagonal channels to other positions of the crystal lattice.