Stanko Popović

Chemical and micro structural properties of TiO2 synthesized by sol-gel procedure

Abstract Nanosized TiO 2 powders were prepared using the sol-gel procedure. The selected colloidal suspensions were stabilized with polyethylene glycol (PEG). This polymer prevented sintering of TiO 2 particles during the calcination of the starting material. X-ray powder diffraction (XRD) phase analysis showed that the samples, obtained up to 500°C, were a mixture of anatase and brookite. In the samples, obtained at 850°C and higher temperatures, rutile as a single phase was detected. The TGA/DTA curves were dependent on the preparation of TiO 2 samples. The samples were also characterized by Fourier transform infrared spectroscopy and laser Raman spectroscopy. A new method, based on low-frequency Raman scattering, was proposed for the size determination of nanosized TiO 2 . The size determination of nanosized TiO 2 by low-frequency Raman scattering was in a good agreement with crystallite size values obtained by XRD.

Dependence of nanocrystalline SnO2 particle size on synthesis route

Very fine SnO2 powders were produced by (a) slow and (b) forced hydrolysis of aqueous SnCl4 solutions and (c) hydrolysis of tin(IV)-isopropoxide dissolved in isopropanol (sol–gel route) and then characterized by X-ray powder diffraction, Fourier transform infrared and laser Raman spectroscopies, TEM and BET. The XRD patterns showed the presence of the cassiterite structure. As found from XRD line broadening the crystallite sizes of all powders were in the nanometric range. TEM results also showed that the sizes of SnO2 particles in all powders are in nanometric range. Very fine SnO2 powders showed different features in the FT-IR spectra, depending on the route of their synthesis. The reference Raman spectrum of SnO2 showed four bands at 773, 630, 472 and 86 (shoulder) cm−1, as predicted by group theory. Very fine SnO2 powders showed additional Raman bands, in dependence on their synthesis. The broad Raman band at 571 cm−1 was ascribed to amorphous tin(IV)-hydrous oxide. The additional Raman bands at 500, 435 and 327 cm−1 were recorded for nanosized SnO2 particles produced by forced hydrolysis of SnCl4 solutions. However, these additional Raman bands were not observed for nanosized SnO2 particles produced by slow hydrolysis of SnCl4 solution or the sol–gel route. The aggregation effects of nanosized particles were considered in the interpretation of the Raman band at 327 cm−1. The method of low frequency Raman scattering was applied for SnO2 particle size determination. On the basis of these measurements it was concluded that the size of SnO2 particles was also in the nanometric range and that, the sol–gel particles heated to 400 °C consisted of several SnO2 crystallites.

Hydrothermal crystallization of boehmite from freshly precipitated aluminium hydroxide

Abstract Hydrothermal crystallization of boehmite from freshly precipitated aluminium hydroxide was monitored using X-ray powder diffraction (XRD) and Fourier transform infrared (FT-IR) spectroscopy. Crystallite sizes and interplanar spacings for boehmite samples were determined. Changes in the crystallinity of boehmite influenced the corresponding FT-IR spectra. The maximum specific area for boehmite powder was 246 m2 g−1, as measured by the Brunauer–Emmet–Teller (BET) method. With an increase of the crystallinity of boehmite, the specific area decreased and for the best crystalline sample, was 91 m2 g−1. All boehmite samples contained particles of colloidal dimensions. For the best crystalline boehmite sample, hexagonal-like plates were observed and some of them were elongated.

Revised and new crystal data for indium selenides

A redetermination of crystal data for InSe (hexagonal), α-In2Se3 (rhombohedral), α-In2Se3 (hexagonal), β-In2Se3 (rhombohedral), γ-In2Se3 (hexagonal), and δ-In2Se3 (hexagonal) has been undertaken because of discrepancies among the published results in the literature and in the JCPDS Powder Diffraction File.

Synthesis of acicular α-FeOOH particles at a very high pH

Abstract The acicular α-FeOOH particles were synthesized from FeCl3 solutions at a very high pH using tetramethylammonium hydroxide as a precipitating agent. Samples were characterized by X-ray powder diffraction, FT-IR spectroscopy and transmission electron microscopy. α-FeOOH precipitated as a single phase from room temperature to 160 °C in a broad aging time interval. The ratio of the length to the width of α-FeOOH particles depended on the temperature and concentration of the starting FeCl3 solution. Multidomainic character of α-FeOOH particles gradually decreased with an increase in the temperature and time of aging.

Microstructure of nanosized TiO2 obtained by sol-gel synthesis

Abstract Nanosized TiO2 was prepared using a sol-gel procedure. The colloidal suspension was stabilized with hydroxypropyl cellulose (HPC). This polymer prevents sintering of TiO2 particles during the heating of the starting material in the form of a solid film. The TiO2 crystallite size increased from 5 to 12 nm with increase of temperature up to 500 °C, as determined by X-ray diffraction. XRD phase analysis showed that the studied samples were a mixture of anatase, as the dominant phase, and brookite. A new approach to size determination of nanophase TiO2, by low-frequency Raman scattering, was used

Raman investigation of nanosized TiO2

Nanosized TiO2 was prepared using the sol–gel procedure. The prepared powder was thermally treated up to 1000°C. X-ray diffraction (XRD) measurements showed that the starting powder and the samples obtained after heating this powder up to 500°C were mixtures of anatase, as the dominant phase, and brookite. The crystallite sizes of the samples were estimated using the Scherrer equation. A new approach to the size determination of nanosized TiO2, by low-wavenumber Raman scattering, was also applied. The particle sizes, determined by low-wavenumber Raman scattering, were in agreement with the crystallite sizes measured by XRD. Rutile was produced by heat treatment of the starting powder at 850°C and higher temperatures.

Influence of synthesis procedure on the YIG formation

The influence of synthesis procedure on the yttrium iron garnet (YIG; Y3Fe5O12) formation has been investigated by X-ray diffraction (XRD), Fourier transform infrared (FT-IR), Mossbauer and magnetization measurements. The samples were prepared by coprecipitation or ceramic processing using the starting molar ratio Y2O3/Fe2O3=3:5. The fractions of Y2O3, α-Fe2O3, YFeO3 and YIG present in the samples depended on the method of materials processing and the calcination temperature. XRD of the thermally treated hydroxide coprecipitate at 1173 K showed the formation of YIG as a dominant phase, and YFeO3 and Y2O3 as associated phases, whereas upon heating at 1473 K, YIG and a small amount of YFeO3 were found. The samples produced by combining ball-milling of the starting powder and ceramic processing at 1573 K contained YIG and a smaller amount of YFeO3, as found by XRD. It was shown that high-energy ball-milling with stainless steel can be substituted by milling with agate bowl and balls, thus decreasing the contamination of the oxide system due to wear. FT-IR and 57Fe Mossbauer spectroscopic measurements were in agreement with XRD; however, the smaller amount of YFeO3 produced at 1573 K could not be detected with certainty by means of FT-IR and 57Fe Mossbauer spectroscopies. The magnetization values of end-products measured at 5 K were in agreement with their phase composition.

Synthesis of tungsten trioxide hydrates and their structural properties

Abstract Tungsten trioxide hydrates were synthesized by (a) cation exchange reaction from sodium tungstate solution and (b) precipitation from sodium tungstate solution by the addition of HCl solution. The samples were analyzed by XRD, DTA/TGA, Raman and FT-IR spectroscopy. XRD showed formation of WO 3 ·H 2 O by cation exchange reaction, whereas WO 3 ·0.33H 2 O was identified by XRD as a product of the acidification of sodium tungstate solution with HCl solution. After heating at 320°C, WO 3 ·H 2 O transformed into WO 3 , whereas the WO 3 ·0.33H 2 O crystal structure remained and these results were in agreement with DTA/TGA measurements. The WO 3 ·H 2 O sample synthesized by cation exchange reaction showed a weight loss corresponding to one molecule of water in the crystal structure. However, samples WO 3 ·0.33H 2 O showed a much greater weight loss upon heating than could be expected on the basis of the WO 3 0.33H 2 O formula. The phase transition WO 3 ·H 2 O→WO 3 was also monitored by Raman and FT-IR spectroscopy. In the case of WO 3 ·0.33H 2 O samples, the basic features of Raman and FT-IR spectra did not change on heating to 320°C, thus indicating that the heating of WO 3 ·0.33H 2 O up to this temperature did not destroy the original crystal structure. Contrary to this, after heating the WO 3 ·H 2 O sample to 320°C, the Raman and FT-IR spectra showed a series of new bands caused by the phase transition WO 3 ·H 2 O→WO 3 .

Effect of divalent cations on the formation and structure of calcium carbonate polymorphs

The incorporation of divalent cations (Mg2+, Mn2+, Cu2+, Sr2+, Cd2+, Ba2+ and Pb2+) in vaterite and calcite has been studied by atomic absorption spectroscopy (AAS), optical and scanning electron microscopy (SEM), X-ray diffraction (XRD) and electron paramagnetic resonance (EPR) spectroscopy. The samples of vaterite and calcite were prepared by spontaneous precipitation from aqueous solutions under defined conditions. The particles of vaterite obtained were spheres, being aggregates of crystallites of 25–35 nm, and the crystals of calcite were well defined rhombohedra. All ions examined, except Cu2+, were found to be incorporated in vaterite probably forming solid solutions. Ba2+ caused a distortion of the vaterite crystal lattice and Mg2+ changed the shape of the particles. Mn2+ ions were incorporated into vaterite in at least two different environments, segregated into clusters or substituted for Ca2+, as detected by EPR. The environment of those Mn2+ ions substituted for Ca2+ in vaterite was of lower symmetry than that in calcite. Characteristic EPR spectra of free radicals formed by γ-irradiation of vaterite and calcite are presented. These spectra were used to detect formation of calcite in the vaterite system, in the presence of Cd2+ and Pb2+ ions confirming the results obtained by EPR of Mn2+ as a paramagnetic substitute for Ca2+. Free radicals characteristic of Cd2+ ions, were detected in both vaterite and calcite γ-irradiated under similar conditions.