L. Sangaletti

Synthesis and optical properties of nanosized powders: lanthanide-doped Y2O3

Abstract Y1.8Nd0.2O3 and Y1.8Er0.2O3 powder samples were prepared by combustion synthesis. X-ray diffraction showed that the materials are nanostructured, with average coherence length 5–10 nm. The visible and infrared luminescence spectra of nanometric cubic Y1.8Nd0.2O3 and Y1.8Er0.2O3 show some important differences from those of the bulk materials. In the case of Y1.8Er0.2O3, faster relaxation of some excited states occurs, leading to changes in the relative intensities of several optical transitions. As far as nanostructured Y1.8Nd0.2O3 is concerned, the 4 F 3/2 → 4 I 9/2 transition is characterized by a perturbed intensity pattern of the Stark components and by an anomalous integrated intensity. Possible explanations for this behaviour are proposed.

Sub-ppm NO2 sensors based on nanosized thin films of titanium-tungsten oxides

Abstract We describe the preparation of thin films of TiWO 3 obtained by means of r.f. sputtering deposition followed by a thermal oxidation. The sensing characteristics of these thin films were obtained by measuring the response towards NO 2 in the interval 0.5–20 ppm and to other interfering gases like ethyl alcohol, CH 4 and CO; the influence of water vapour to the response towards NO 2 was also investigated. The material seems to be a promising detector of NO 2 for environmental monitoring.

FINE STRUCTURES IN THE X-RAY PHOTOEMISSION SPECTRA OF MNO, FEO, COO, AND NIO SINGLE CRYSTALS

Abstract The main achievements in the study of X-ray photoemission of MnO, FeO, CoO, and NiO, single crystals are discussed. For these compounds the oxygen 1s, the cation 2s, 2p, and 3s core line spectra and the one-electron removal valence band spectra are reported. The unresolved problems in the understanding of the fine structure present in the X-ray photoemission spectra are evidenced.

Preparation and micro-structural characterization of nanosized thin film of TiO2WO3 as a novel material with high sensitivity towards NO2

WO 3 -TlO 2 thin films were prepared by r.f. reactive sputtering from a Ti 0.1 -W 0.9 target. Annealing in synthetic air-flow of the initial W-Ti-O deposited compound results in layers with different features, depending on the temperature at which the annealing is being carried out. Morphological and structural characteristics of the thin films have been studied through X-ray diffraction, electron microscopy and Raman techniques. After annealing at 600°C, a polycrystalline thin film isostructural to WO 3 is achieved. This layer is capable of detecting concentrations of NO 2 lower than 1 ppm. Annealing of the film at 800°C results in a nanosized structurally stable TiO 2 -anatase layer. This layer can detect a few ppm of NO 2 in dry air even at as high a temperature as 800°C.

Oxidation of Sn Thin Films to SnO 2 . Micro-Raman Mapping and X-ray Diffraction Studies

The oxidation of tin layers deposited onto alumina substrates is investigated with the aim to identify the different steps of the process and obtain information on the sample homogeneity, phase segregation, and degree of oxidation. It is shown that at least three phases coexist at 450 °C, Sn, SnO, and SnO2, and remarkable inhomogeneities, already visible at an optical inspection, are found in the thin film. A micro-Raman mapping of the layer shows that these inhomogeneities are related to the presence of different Sn oxidation states, as evidenced by the inhomogeneous distribution of SnO and SnOx Raman bands. The thin film becomes homogeneous after annealing treatments above 550 °C, where only the SnO2 cassiterite phase is detected.

Microstructure and morphology of tin dioxide multilayer thin film gas sensors

Abstract Structural, morphological, and electrical measurements have been carried out on SnO 2 multilayer thin film grown by the rheotaxial growth and thermal oxidation method on Al 2 O 3 substrates. The analysis of X-ray and electron diffraction patterns shows that, in addition to the SnO 2 cassiterite phase, a contribution from another SnO 2 phase is present, which can be related to cassiterite by introducing microtwinning effects. The electrical measurements show that these thin films have a higher sensitivity towards CO with respect to the conventional single layer SnO 2 sensors.

Morphology and microstructural properties of TiO2 nanopowders doped with trivalent Al and Ga cations

The effects of doping on the morphological and microstructural properties of TiO2 nanopowders produced by laser pyrolysis were investigated mainly by x-ray diffraction (XRD) and electron microscopy. Samples of TiO2 powders were prepared by doping with different trivalent cations (Al and Ga). The powders were calcined at different temperatures in the range 400–1000 °C for 18 h, as well as at constant T = 700 °C up to 160 h. After each thermal treatment, XRD patterns were collected. The analysis of XRD patterns allowed us to estimate the microstrains and average crystallite size and to observe the evolution of the microstructural parameters with temperature. Both Al and Ga inhibited the crystallite growth of TiO2 anatase and the rutile phases, this effect being larger in the Al-doped powders.

Niobium-titanium oxide powders obtained by laser-induced synthesis: Microstructure and structure evolution from diffraction data

The influence of the niobium content on the anatase-to-rutile phase transition in nanopowders of Nb–Ti oxides was studied and the changes in the particle size and microstrain distribution obtained at different temperatures were analyzed. A correlation is found between the initial microstructure in the Ti 1 – x Nb x O 2 ( x = 0.03, 0.2) powder and the niobium content. The presence of Nb was found to inhibit the growth of both the anatase and the rutile phases.

ELECTRICAL AND STRUCTURAL PROPERTIES OF RGTO-IN2O3 SENSORS FOR OZONE DETECTION

Abstract Bare and Au-doped indium oxide thin films have been characterized as ozone sensors. Active layers have been obtained by oxidation of a metallic In thin film through the RGTO technique. Structural characterization showed that only the In2O3 phase is present for oxidation temperatures greater than 250°C. At the operating temperature of 400°C, response to ozone was greater, stable and reproducible. Au introduction as a catalyst increased the overall response and reduced to about 300°C the temperature where ozone/surface reactions are enhanced.

Temperature effects on the size of anatase crystallites in MoTiO2 and WTiO2 powders

Abstract The substitution effects of Ti by Mo and W ions on the stability of anatase in commercially available TiO 2 powders have been studied by X-ray diffraction, transmission electron microscopy, and surface area measurements in the 300–1000 K temperature range. The powder containing W shows smaller microstrains compared to those of pure anatase phase and those containing Mo. The stabilization effect of vacancies in the anatase phase is discussed and the anatase to rutile transformation caused by annealing is proposed to be related to the disappearance of Ti 3+ ions in the anatase structure. Finally, a possible redox mechanism Ti 3+ ⇔ Ti 4+ + e − is proposed to play a fundamental role in the catalytic and sensor properties of this material.