Lidia Armelao

Cerium-based conversion layers on aluminum alloys

Abstract Mixed layers of Al and Ce oxides have been obtained in conversion coatings on two aluminum alloys (AA6061 and AA2618). The microstructure and the chemical composition of the protective films have been examined by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS). The corrosion resistance in NaCl solution was investigated and a comparison with a traditional chromate conversion coating on the same alloys is given. The coatings obtained by immersion in boiling solution of Ce(III) salts cover uniformly the alloys surface with a protective layer, whereas the H 2 O 2 –cerium conversion coating process produces a thick, but cracked layer with “dry-mud” morphology, which preferentially covers the intermetallic particles on the samples surface. The boiled cerium conversion coating treatments are as effective in corrosion resistance as the traditional chromating process, but the interest for industrial purposes is limited because of long application times. The H 2 O 2 –cerium conversion coating process produces effective corrosion resistance layers, thinner than the ones obtained by traditional processing, but very interesting for industrial applications due to the limited environmental impact.

Photocatalytic and antibacterial activity of TiO2 and Au/TiO2 nanosystems

This work focuses on the photocatalytic performances and antibacterial activity of TiO2 and Au/TiO2 nanosystems. While the former are obtained by a sol?gel route, the latter are synthesized by an innovative hybrid RF-sputtering/sol?gel approach, followed by ex situ annealing in air up to 600??C. Important information on nanoparticle size, shape and distribution is obtained by the combined use of glancing incidence x-ray diffraction (GIXRD) and field emission-scanning electron microscopy (FE-SEM). Subsequently, the photocatalytic performances of the obtained nanosystems in the decomposition of the azo-dye Plasmocorinth B and their antibacterial activity in the elimination of Bacillus subtilis are illustrated and discussed in comparison with films obtained from standard Degussa P25 powders. The obtained results show a significant dependence of the functional performances on the systems compositional, structural and morphological properties. In particular, the dispersion of gold nanoparticles on the TiO2 matrix has a beneficial influence on the azo-dye photodegradation, whereas the antimicrobial activity of Au/TiO2 films is retarded with respect to pure TiO2.

Macroporous WO3 Thin Films Active in NH3 Sensing: Role of the Hosted Cr Isolated Centers and Pt Nanoclusters

Macroporous WO(3) films with inverted opal structure were synthesized by one-step procedure, which involves the self-assembly of the spherical templating agents and the simultaneous sol-gel condensation of the semiconductor alkoxide precursor. Transition metal doping, aimed to enhance the WO(3) electrical response, was carried out by including Cr(III) and Pt(IV) centers in the oxide matrix. It turned out that Cr remains as homogeneously dispersed Cr(III) centers inside the WO(3) host, while Pt undergoes reduction and aggregation to form nanoclusters located at the oxide surface. Upon interaction with NH(3), the electrical conductivity of transition metal doped-WO(3) increases, especially in the presence of Pt dopant, resulting in outstanding sensing properties (S = 110 ± 15 at T = 225 °C and [NH(3)] = 74 ppm). A mechanism was suggested to explain the excellent electrical response of Pt-doped films with respect to the Cr-doped ones. This associates the easy chemisorption of ammonia on the WO(3) nanocrystals, promoted by the inverted opal structure, with the catalytic action exerted by the surface Pt nanoclusters on the N-H bond dissociation. The overall results indicate that in Pt-doped WO(3) films the effects of the macroporosity positively combine with the electrical sensitization promoted by the metal nanoclusters, thus providing very lightweight materials which display high functionality even at relatively low temperatures. We expect that this synergistic effect can be exploited to realize other functional hierarchical metal oxide structures to be used as gas sensors or catalysts.

Oxygen and ozone evolution at fluoride modified lead dioxide electrodes

This work examines the behaviour of fluorine modified β-PbO2 electrodes in the processes of O2 and O3 evolution in sulphuric acid. The electrochemical kinetic analyses of these processes are based on quasi-steady-state polarisation and impedance data. The good agreement between the two sets of measurements allows some basic conclusions to be drawn. In particular, the O2 evolution process is always inhibited at F-doped PbO2 electrodes, and impedance results suggest possible changes in the mechanism, with electrodesorption of intermediates becoming more important as the concentration of the doping element increases. The interpretation of the data for the less positive potentials region invokes the specific adsorption of SO42− as a factor influencing the kinetics of O2 evolution. The current efficiency for O3 formation as a function of the amount of NaF added to the PbO2 growth solution reaches a maximum for a concentration of 0.01 mol dm−3. A plausible cause for the decrease on the higher concentration side is the discharge of adsorbed SO42− (or HSO4−) eventually yielding persulphate. This reaction is known to be favoured in the presence of a relatively high amount of fluoride in the electrolyte. An analysis of the results of modified neglect of diatomic differential overlap (MNDO) calculations on Pb cluster models and of X-ray photoelectron spectroscopy (XPS) data suggests that the coverage by weakly adsorbed oxygen species (OH and H2O) is an important parameter that is influenced by F-doping.

A sol–gel approach to nanophasic copper oxide thin films

Abstract Nanostructured copper oxide films were prepared via sol–gel starting from ethanolic solutions of copper (II) acetate [Cu(CH 3 COO) 2 ·H 2 O]. Films were obtained by dip-coating at room temperature in air and were subsequently heat-treated at different temperatures (100–900 °C) in oxidizing (air), inert (N 2 ) or reducing (4% H 2 in N 2 ) atmospheres. The evolution of the oxide coatings under thermal treatment was studied by glancing incidence X-ray diffraction, X-ray photoelectron spectroscopy and X-ray excited Auger electron spectroscopy. Different crystalline phases were observed as a function of the annealing conditions. Depending on both temperature and atmosphere, the film composition resulted single- or multi-phasic. All the layers were nanostructured with an average crystallite size lower than 20 nm. The most relevant results concerning sample composition and microstructure as well as their mutual relations with the synthesis conditions are presented and discussed.

Sol–gel synthesis and characterisation of ZnO-based nanosystems

Semiconductor nanoclusters embedded in thin coatings of transparent and homogeneous silica glasses are suitable materials for the development of optical devices. The optical properties of such systems are strongly dependent on the elemental composition and the morphology of the coatings as well as on the nanocluster size distribution, their mutual interactions and the interactions with the host matrix. To achieve a good control over film composition and morphology the sol–gel route was used. The sol–gel synthesis of ZnO nanocrystals embedded in silica has been faced by the study of the gel-derived binary system ZnO–SiO2. The dip-coating procedure from alcoholic solutions containing tetraethoxysilane [Si(OC2H5)4, TEOS] and zinc acetate [Zn(CH3COO)2] was adopted. Zinc oxide nanograins have been generated in silica by hydrolysis of TEOS and the zinc salt and subsequent thermal annealing of the coatings. The system evolution under thermal treatment was studied by X-ray photoelectron spectroscopy, X-ray diffraction, transmission electron microscopy, UV-Vis absorption spectroscopy and secondary-ion mass spectrometry.

Synthesis and characterization of crystalline tin oxide nanoparticles

Monodisperse non-aggregated spheroidal nanoparticles of SnO2 cassiterite are prepared through hydrolysis of tin isopropoxide in the presence of acetylacetone and p-toluenesulfonic acid, followed by ageing at 60 °C. The obtained sols remain stable for several months at 4 °C. The nanoparticles have been characterized in the solid state (xerosol) by powder X-ray diffraction, TEM, FTIR, TGA-DTA, and 119Sn MAS and 13C CP-MAS NMR. The mean size of the cassiterite oxide core is about 1–2 nm. These particles do not aggregate in suspension because their surfaces are protected by an hybrid organic–inorganic layer containing acetylacetonate ligands, acetylacetone, p-toluenesulfonates and water.

Optical and surface properties of inorganic and hybrid organic-inorganic silica-titania sol-gel planar waveguides

Inorganic and hybrid organic–inorganic silica (SiO2)–titania (TiO2) planar waveguides, with a relative molar composition 70–30, have been fabricated by sol–gel dip-coating. The composition and the density of the film samples have been measured by Rutherford backscattering spectrometry, nuclear reaction analysis and elastic recoil detection analysis. The measured compositions have been found to agree with the nominal composition of the sols. The film density of the inorganic samples increased with the firing temperature in the range 1.80–2.58. A root mean square roughness (rms) <1 nm was measured on the surface of the samples by atomic force microscopy. A refractive index in the range 1.61–1.54 was measured (λ=632.8 nm). The inorganic waveguides had smaller losses (∼0.3 dB cm−1) compared to the hybrid waveguides (∼1 dB cm−1).

Bismuth oxychloride nanoflakes: Interplay between composition-structure and optical properties

Strongly (001) oriented BiOCl nanoflakes have been prepared at room temperature by the controlled hydrolysis of bismuth chloride in the presence of acetylacetone. The nanoflakes thermally treated in air up to 600 °C have been studied by X-Ray Photoelectron Spectroscopy (XPS), X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM). Composition, structure and morphology of the nanoflakes have been correlated to their electronic absorption and luminescence properties. Irrespective of the thermal treatments, the samples are characterized by transmittance higher than 98% in the near-infrared region. In the mildly annealed specimen (≤ 200 °C), the absorption bands in the ultraviolet can be effectively exploited for the selective excitation of the blue (394 nm) and green (520 nm) photoluminescence, the latter being visible only for λ(exc) > 310 nm. Conversely, at higher temperature only the blue emission is observed which, on the basis of the observed trend, can be assigned to emitting centres located in the oxide sheet of the Bi-O-Cl stacks.

Rational Design of Ag/TiO2 Nanosystems by a Combined RF-Sputtering/Sol-Gel Approach

The present work is devoted to the preparation of Ag/TiO(2) nanosystems by an original synthetic strategy, based on the radio-frequency (RF) sputtering of silver particles on titania-based xerogels prepared by the sol-gel (SG) route. This approach takes advantage of the synergy between the microporous xerogel structure and the infiltration power characterizing RF-sputtering, whose combination enables the obtainment of a tailored dispersion of Ag-containing particles into the titania matrix. In addition, the systems chemico-physical features can be tuned further through proper ex situ thermal treatments in air at 400 and 600 degrees C. The synthesized composites are extensively characterized by the joint use of complementary techniques, that is, X-ray photoelectron and X-ray excited Auger electron spectroscopies (XPS, XE-AES), secondary ion mass spectrometry (SIMS), glancing incidence X-ray diffraction (GIXRD), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), electron diffraction (ED), high-angle annular dark field scanning TEM (HAADF-STEM), energy-filtered TEM (EF-TEM) and optical absorption spectroscopy. Finally, the photocatalytic performances of selected samples in the decomposition of the azo-dye Plasmocorinth B are preliminarily investigated. The obtained results highlight the possibility of tailoring the system characteristics over a broad range, directly influencing their eventual functional properties.