Simone Battiston

Cathodoluminescence evaluation of oxygen vacancy population in nanostructured titania thin films for photocatalytic applications.

Room-temperature results of cathodoluminescence (CL) spectroscopy investigations are presented for nanostructured titanium dioxide (anatase) thin films (500 nm thick) deposited via RF magnetron sputtering on high-purity silica substrates. The collected CL bands of the anatase thin films, as deposited and after different annealing cycles, showed a broad morphology consisting of three Voigtian bands located at 500, 550, and 610 nm that were partially overlapping. The overall CL emission increased with increasing temperature and time of the annealing cycle as a consequence of the increased crystallinity of the thin film. A clear trend was found for the oxygen-vacancy-related band (located at 610 nm), whose relative intensity decreased, as compared with the as-grown sample, after annealing in air; the higher the annealing temperature, the lower the relative intensity. We evaluated the photoactivity of the nanostructured thin film samples by measuring their photocatalytic activity in aqueous solution toward the degradation of phenol. A relationship between the decrease in oxygen vacancy concentration as a consequence of the annealing and the increase in the photoactivity was highlighted.

Tuning the thermal diffusivity of silver based nanofluids by controlling nanoparticle aggregation

With the aim of preparing stable nanofluids for heat exchange applications and to study the effect of surfactant on the aggregation of nanoparticles and thermal diffusivity, stable silver colloids were synthesized in water by a green method, reducing AgNO₃ with fructose in the presence of poly-vinylpyrollidone (PVP) of various molecular weights. A silver nanopowder was precipitated from the colloids and re-dispersed at 4 vol% in deionized water. The Ag colloids were characterized by UV-visible spectroscopy, combined dynamic light scattering and ζ-potential measurements, and laser flash thermal diffusivity. The Ag nanopowders were characterized by scanning electron microscopy and thermal gravimetric analysis. It was found that the molecular weight of PVP strongly affects the ζ-potential and the aggregation of nanoparticles, thereby affecting the thermal diffusivity of the obtained colloids. In particular, it was observed that on increasing the molecular weight of PVP the absolute value of the ζ-potential is reduced, leading to increased aggregation of nanoparticles. A clear relation was identified between thermal diffusivity and aggregation, showing higher thermal diffusivity for nanofluids having higher aggregation. A maximum improvement of thermal diffusivity by about 12% was found for nanofluids prepared with PVP having higher molecular weight.

Structural, compositional and functional properties of Sb-doped Mg2Si synthesized in Al2O3-crucibles

Magnesium silicide (Mg2Si) is a promising candidate for thermoelectric energy conversion due to its low toxicity, the abundance of its raw constituents and its low density, allowing manufacturing of light, sustainable and relatively cheap devices. Mg2Si needs to be doped in order to increase its efficiency, making this material competitive among materials operating in the intermediate temperature range. In this work, a synthesis procedure based on melting of the raw elements in easily available and cheap Al2O3 crucibles was developed to obtain polycrystalline Sb doped Mg2Si materials in a wide range of compositions. Powders from the crushed lumps were consolidated via spark plasma sintering and then thermally annealed to obtain dense pellets of Sb:Mg2Si with Sb = 0.0, 0.1, 0.3, 0.5, 0.7, 1.0 and 1.5 at%. The effects of Sb doping and of the synthesis and sintering technique on composition, morphology and stability of n-type Mg2Si are discussed. Transport properties (Seebeck coefficient, electrical and thermal conductivity, charge carrier density) were evaluated in order to elucidate the composition–property relationship within this material system and find the optimal doping amount to optimize its thermoelectric properties.

Effective and Low-Cost Synthesis of Sulphur-Modified TiO 2 Nanopowder with Improved Photocatalytic Performances in Water Treatment Applications

In the present paper, sulphur-modified titanium dioxide (S-TiO2) is prepared as nanopowder in mixed rutile-anatase phase by an unprecedented simple, reproducible and cheap synthetic procedure, directly employing elemental sulphur powder as sulphur source. TiO2 and several reference TiO2 samples obtained in pure rutile or anatase phase were also prepared with nanometric size and compared to S-TiO2 as well as Degussa P25. The prepared samples and the reference benchmark were characterised by X-ray diffraction (XRD), scanning electron microscopy (SEM), UV-Vis spectroscopy, Fourier transform infrared spectroscopy (FT-IR), BET specific surface area and X-ray photoelectron spectroscopy (XPS) techniques, while their photoactivity was determined with respect to methyl red degradation as typical probe reaction. The results from the characterisation and photocatalytic measurements were discussed and inter-correlated, thus providing a complete and consistent analysis of the samples performances. The prepared sulphur-modified titanium dioxide appeared as a very efficient and long-lasting photocatalyst with respect to the unmodified TiO2 and to the benchmark Degussa P25 (S-TiO2 appears to be two times faster than P25) for the methyl red removal under UV lighting, also in repeated cycles.

Cathodoluminescence insights into the ionic disorder of photocatalytic anatase films

The nature of ionic disorder and the effect of structural defects on the photocatalytic function of anatase are revisited in the light of direct experimental evidence retrieved on the molecular scale by cathodoluminescence (CL) spectroscopy. CL spectra, collected on different types of photocatalytically efficient or inefficient anatase films, embodied a composite optical response of electron-compensating majority types of ionic disorder in the anatase lattice. This paper describes the dual experimental output obtained by systematically monitoring optically active off-stoichiometry sites, as follows: (i) quantitative analyses of film stoichiometry including the interactions of different lattice-defect populations; and (ii) stability/evolution of off-stoichiometry sites upon post-fabrication annealing cycles and their effects on the photocatalytic activity of the films. CL experiments provided us with direct access to the structural state of the defective anatase lattice, thus unfolding some missing detail ...