Mira Ristić

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.

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.

Pulsed blue laser curing of hybrid composite resins

Clinical performance of light-curing composite restorations is greatly influenced by the quality of the curing-light. Currently used photopolymerization units have some important drawbacks, such as decreasing light output with time and distance, which results in a relatively low degree of conversion and shallow depth of cure, particularly of darker shades. Experiments with continuous argon laser polymerization showed overheating of the composite sample, as well as increased shrinkage of the material. In this study a pulsed laser, set at 468 nm (the maximum of the camphorquinone absorption coefficient), with 20-ns pulse duration, repetition rate of 10 Hz and energy of 10 mJ per pulse, was used as a light source. The aim of the study was to evaluate the effect of polymerization of light and dark shades of three different hybrid composites cured by pulsed laser at the surface and at 3.0 mm depth. The degree of conversion was measured by Fourier transform infrared spectroscopy (FTIR). Applying pulsed blue laser, significantly better results were obtained for both shades compared to standard polymerization values. Very weak dependence of the degree of conversion, between the surface measurements and those at 3.0 mm, were observed in the case of pulsed laser polymerization due to the piercing nanopulses and the monochromatic light at 468 nm.

Formation and characterization of the solid solutions (CrxFe1−x)2O3, 0⩽x⩽1

The solid solutions (CrxFe1−x)2O3, 0 ⩽ x ⩽ 1, were prepared by traditional ceramic procedures. The samples were characterized using X-ray diffraction, Mössbauer, Fourier transform infra-red (FT-IR) and optical spectroscopic measurements. In the whole concentration range two phases exist phase F, α-(CrxFe1−x)2O3, which is isostructural with α-Fe2O3 and phase C, which is closely related to Cr2O3. Phase F exists in samples heated up to 900°C, for 0 ⩽ x ≲ 0.95. Phase C exists from x≳0.27 to x=1 for samples heated up to 900°C and from x≳0.65 to x=1 for samples heated up to 1200 °C. For samples heated up to 900 °C, the solubility limits were 27.5 ± 0.5 mol% of Cr2O3 in α-Fe2O3 and 4.0 ± 0.5 mol % of α-Fe2O3 in Cr2O3. For the samples heated at 1200 °C the diffraction peaks for the F and C phases in the two phase region were severely overlapped and thus the solubility limits could not be determined accurately as for previous samples. 57Fe Mössbauer spectra of the samples heated up to 1200 °C showed significant broadening of spectral lines and a gradual decrease of the hyperfine magnetic field with increase of x up to 0.50. For x≳0.7, a paramagnetic doublet with collapsing sextet was observed. The spectra were interpreted in terms of an electronic relaxation effect; however, an agglomeration of iron ions which would contribute to the superparamagnetic effect could not be excluded. The FT-IR spectra showed transition effects in accordance with the X-ray diffraction results. The most intense absorption bands, observed for the samples heated up to 1200 °C, were located at ∼ 460 and 370 nm (22 000 and 27 000cm−1) for x⩾ 0.5, ∼500 and 360 nm for x < 0.3, and might be correlated with the strong enhancement of the pair transitions through antiferromagnetic interactions. The intensification of the 6A1 → 4T1 Fe3+ ions in all spectra and the development of the absorption at 13000 cm−1 due to a metal-metal charge transfer (Cr3+ → Fe3+) transition, might be explained by exchange coupling which has been observed in some spinel compounds.

Formation of oxide phases in the system Eu(2)O(3)-Fe(2)O(3)

Mixed metal oxides in the system Fe2O3-NiO were prepared by coprecipitation of Fe(OH)3/Ni(OH)2 and the thermal treatment of hydroxide coprecipitates up to 800 or 1100°C. X-ray diffraction showed the presence of α-Fe2O3, NiO and NiFe2O4 in samples prepared at 800°C. The oxide phases α-Fe2O3, NiO, NiFe2O4 and a phase with structure similar to NiFe2O4 were found in samples prepared at 1100°C. Fourier transform-infrared spectra of oxide phases formed in the system Fe2O3-NiO are discussed. Two very strong infrared bands at 553 and 475 cm−1, a weak intensity infrared band at 383 cm−1 and two shoulders at 626 and 441 cm−1 were observed for α-Fe2O3 prepared at 1100°C. NiFe2O4, prepared at the same temperature, showed two broad and very strong infrared bands at 602 and 411 cm−1, while NiO showed a broad infrared band at 466 cm−1. Fourier transform infrared spectroscopic results were in agreement with X-ray diffraction.

Fourier transform far-infrared spectroscopic evidence for the formation of a nickel ferrite precursor in binary Ni(II)-Fe(III) hydroxides on coprecipitation

Abstract Fourier transform infrared (FTIR) spectroscopic measurements, performed in the far-IR region (200–800 cm−1 on binary coprecipitated Ni(II)-Fe(III) hydroxides (20–60 at.% Fe ( Ni + Fe ) ), are presented and discussed. Heating the sample containing 60 at.% Fe to 600°C, monitored by differential scanning calorimetry (DSC) measurements, results in the formation of a nickel ferrite structure from its precursor present in binary hydroxides. The role of Fe-to-Ni ratio non-uniformities within the surface and bulk of the binary hydroxide crystallites, as revealed by Mossbauer measurements, Auger electron spectroscopy and depth profiling performed previously, is also discussed with regard to the formation of α-Fe2O3 and NiO impurities on thermal treatment.

Composite Photopolymerization with Diode Laser

Under clinical conditions, the time needed for the proper light curing of luting composites or the multi-incremental buildup of a large restoration with halogen curing units is quite extensive. Due to the development of high power curing devices, such as argon lasers and plasma arc lights and, in order to decrease curing time, halogen and LED devices have developed a high intensity polymerization mode. This study compared the degree of conversion using Fourier Transform Infrared Spectroscopy (FT-IR) of two composite materials: Tetric Ceram and Tetric EvoCeram polymerized with three polymerization modes (high, low and soft mode) of a Bluephase 16i LED curing unit and blue diode laser intensity of 50 mW on the output of the laser beam and 35 mW/cm2 on the resin composite sample. Descriptive statistic, t-test, ANOVA, Pearson Correlation and Tukey Post hoc tests were used for statistical analyses. The results show a higher degree of conversion for the polymerization of composite samples with all photopolymerization modes of the LED curing unit. However, there is no significant difference in the degree of conversion between the LED unit and 50-second polymerization with the blue diode laser. Tetric EvoCeram shows a lower degree of conversion regardless of the polymerization mode (or light source) used.

Characterization of the precipitates formed during the denitration of simulated HRLW

The denitration of several chemical compositions of simulated HRLW (highly radioactive liquid waste) was performed using formic acid as reducing agent. Precipitates formed during the denitration of simulated HRLW were analyzed using X-ray diffraction and57Fe Mössbauer spectroscopy. Goethite and amorphous fraction were the principal phases in these precipitates. It was found that the chemical composition of simulated HRLW and the experimental conditions of denitration have more influence on the crystallinity and the particle size than on the phase composition of the precipitates. Thermal treatment of denitrated precipitates caused the solid state transformation of goethite+amorphous fraction into hematite. The values of hyperfine magnetic field (HMF) of hematite were decreased, thus indicating the substitution of Fe3+ ions with other metal cations.

Porous Silicon Covered with Silver Nanoparticles as Surface-Enhanced Raman Scattering (SERS) Substrate for Ultra-Low Concentration Detection

Microporous and macro-mesoporous silicon templates for surface-enhanced Raman scattering (SERS) substrates were produced by anodization of low doped p-type silicon wafers. By immersion plating in AgNO3, the templates were covered with silver metallic film consisting of different silver nanostructures. Scanning electron microscopy (SEM) micrographs of these SERS substrates showed diverse morphology with significant difference in an average size and size distribution of silver nanoparticles. Ultraviolet–visible–near-infrared (UV-Vis-NIR) reflection spectroscopy showed plasmonic absorption at 398 and 469 nm, which is in accordance with the SEM findings. The activity of the SERS substrates was tested using rhodamine 6G (R6G) dye molecules and 514.5 nm laser excitation. Contrary to the microporous silicon template, the SERS substrate prepared from macro-mesoporous silicon template showed significantly broader size distribution of irregular silver nanoparticles as well as localized surface plasmon resonance closer to excitation laser wavelength. Such silver morphology has high SERS sensitivity that enables ultralow concentration detection of R6G dye molecules up to 10−15 M. To our knowledge, this is the lowest concentration detected of R6G dye molecules on porous silicon-based SERS substrates, which might even indicate possible single molecule detection.

Enhanced near-infrared response of nano- and microstructured silicon/organic hybrid photodetectors

Heterojunctions between an organic semiconductor and silicon are an attractive route to extending the response of silicon photodiodes into the near infrared (NIR) range, up to 2000 nm. Silicon-based alternatives are of interest to replace expensive low band-gap materials, like InGaAs, in telecommunications and imaging applications. Herein, we report on the significant enhancement in NIR photodetector performance afforded by nano- and microstructuring of p-doped silicon (p-Si) prior to deposition of a layer of the organic semiconductor Tyrian Purple (TyP). We show how different silicon structuring techniques, namely, electrochemically grown porous Si, metal-assisted chemical etching, and finally micropyramids produced by anisotropic chemical etching (Si μP), are effective in increasing the NIR responsivity of p-Si/TyP heterojunction diodes. In all cases, the structured interfaces were found to give photodiodes with superior characteristics as compared with planar interface devices, providing up to 100-fold improvement in short-circuit photocurrent, corresponding with responsivity values of 1–5 mA/W in the range of 1.3–1.6 μm. Our measurements show this increased performance is neither correlated to optical effects, i.e., light trapping, nor simply to geometric surface area increase by micro- and nanostructuring. We conclude that the performance enhancement afforded by the structured p-Si/organic diodes is caused by a yet unresolved mechanism, possibly related to electric field enhancement near the sharp tips of the structured substrate. The observed responsivity of these devices places them closer to parity with other, well-established, Si-based NIR detection technologies.