Mile Ivanda

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.

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.

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 .

Sol-gel synthesis and characterization of V2O5 powders

Abstract V 2 O 5 particles were precipitated by the sol–gel method, then subjected to heat treatment. Samples were analyzed using XRD, TGA/DTA, FT-IR, Raman and transmission electron microscopy (TEM) techniques. XRD showed an amorphous-like structure of these particles with the beginning of their crystallization, whereas TEM showed their fibrillar morphology. In the FT-IR spectrum of the starting sample, not all bands typical of the crystalline V 2 O 5 spectrum were developed. The Raman spectrum showed all bands of V 2 O 5 , however, with a significant broadening. The fibrillar morphology of V 2 O 5 particles was disappearing with increase in the heating temperature, and at 600 °C, only big particles of irregular shape were obtained. Heating of the starting V 2 O 5 particles in argon atmosphere up to 600 °C produced the V 3 O 7 phase as associated with the predominant V 2 O 5 phase.

Nucleation of titania nanocrystals in silica titania waveguides

SiO2(1 − x)-TiO2(x) monomode waveguides at 632.8 nm, with x in the range 0.07–0.2 and thickness of about 0.4 μm, were deposited on silica substrates by a dip-coating technique. Nucleation of TiO2 nanocrystals and the growth of their size by thermal annealing up to 1300°C were studied by waveguided Raman scattering in the SiO2(0.8)-TiO2(0.2) composition. In the low frequency region (5–50 cm−1) of the VV and HV polarized Raman spectra the symmetric and quadrupolar acoustic vibrations are observed. The mean size of the titania particles are obtained from the frequencies of the Raman peaks. The results are compared with those obtained from the measure of the linewidths in the X-ray diffraction spectra. Nanocrystals with a mean size in the range 4–20 nm are obtained by thermal annealing in a corresponding range of 700–1300°C.

Microstructural properties of dc magnetron sputtered a-Si:H by IR spectroscopy

Abstract The amorphous hydrogenated silicon thin films, deposited by dc magnetron sputtering, were examined by near infrared and Fourier transformation infrared spectroscopy. It was found that, by increase of the hydrogen concentration from 6 to 32 at.%, the dielectric constant decreases and the frequency of absorption peak, corresponding to stretching vibrations of SiH bonds, increases. The results were analyzed within the framework of effective medium approach assuming medium composed of Si network and voids decorated with hydrogen atoms. It is concluded that the increase of the hydrogen content is accompanied by an increase of void fraction in the total volume, with an increase of void ‘decoration’ with hydrogen and also with increased contribution of larger voids. The obtained dependence upon hydrogen concentration agrees well with density measurements and predictions made based on multiple-quantum nuclear magnetic resonance spectroscopy.

Raman spectroscopy on SiO2 glasses sintered from nanosized particles

Abstract The forming and sintering of fumed silica powders is an interesting route for the preparation of large, pure or doped silica glasses with precise geometry. Here we have investigated the process of sintering of transparent silica glass by Raman spectroscopy. The main changes in the Raman spectra appeared in the range of defects bands D1 and D2. The process of sintering in oxygen, hydrogen, helium and water vapour as well as by crystallization and remelting has been monitored by the changes in amplitudes of defect bands. The results were analyzed in the small planar ring and the paracrystalline cluster interface models.

Tailoring of the free spectral range and geometrical cavity dispersion of a microsphere by a coating layer

The modal dispersion of a whispering gallery mode (WGM) resonator is a very important parameter for use in all nonlinear optics applications. In order to tailor the WGM modal dispersion of a microsphere, we have coated a silica microsphere with a high-refractive-index coating in order to study its effect on the WGM modal dispersion. We used Er(3+) ions as a probe for a modal dispersion assessment. We found that, by varying the coating thickness, the geometrical cavity dispersion can be used to shift overall modal dispersion in a very wide range in both the normal and anomalous dispersion regime.