Ce-doped borosilicate (BSG), phosphosilicate (PSG), and borophosphosilicate (BPSG) glasses (B:P:Si molar ratios 8:0:92, 0:8:92, and 8:8:84; Ce:Si molar ratio 1 x 10(-)(4) to 1 x 10(-)(2)) were prepared by the sol-gel method. High-resolution transmission electron microscopy (HRTEM), (31)P, (29)Si, and (11)B magic angle spinning nuclear magnetic resonance (MAS NMR), electron paramagnetic resonance (EPR), and UV-vis absorption investigations demonstrated that, in PSG and BPSG, Ce(3+) ions interact with phosphoryl, [O=PO(3/2)], metaphosphate, [O=PO(2/ 2)O](-), and pyrophosphate, [O=PO(1/2)O(2)](2)(-), groups, linked to a silica network. This inhibits both CeO(2) segregation and oxidation of isolated Ce(3+) ions to Ce(4+), up to Ce:Si = 5 x 10(-)(3). In BSG, neither trigonal [BO(3/2)] nor tetrahedral [BO(4/2)](-) boron units coordinate cerium; thus, Ce(3+) oxidation occurs even at Ce:Si = 1 x 10(-)(4), as in pure silica glass (SG). The homogeneous rare-earth dispersion in the host matrix and the stabilization of the Ce(3+) oxidation state enhanced the intensity of the photoluminescence emission in PSG and BPSG with respect to BSG and SG. The energy of the Ce(3+) emission band in PSG and BPSG matrixes agrees with the phosphate environment of the rare earth.

The thermal evolution of sol−gel SnO2-based thin films was explored by investigating their structural and morphological features. Nanostructured SnO2 and Pt-doped SnO2 layers were obtained using tetra(tert-butoxy)tin(IV) and Pt(II) acetlylacetonate as precursors. Films were prepared by spin coating from ethanol solutions with different viscosity. After drying at room temperature, they were annealed in air at 673 and 973 K. The surface morphology was analyzed by scanning electron microscopy, atomic force microscopy, and scanning near-field optical microscopy. The structural characterization was performed by means of glancing incidence X-ray diffraction and microdiffraction. Both drying at room temperature and thermal treatment at 673 K resulted in the formation of holes on the surface and inside the films. Their distribution and average dimension were found to depend mainly on the viscosity of the sol precursor, and on the presence of Pt in the films. After annealing at 973 K, surface segregation of PtO x phases and partial filling of the surface holes occurred. The effects of morphology on the electrical transport properties are discussed on the basis of sensitivity, S, measurements (S = R air/R CO, where R air and R CO stand for the resistance in air and CO/air, respectively).

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Tin dioxide and ruthenium(platinum)-doped tin dioxide were synthesized in the form of inverted opals, aiming to investigate the interaction of these materials with CO reducing gas. The results of electron paramagnetic resonance (EPR) investigation allowed us to conclude that CO interaction causes the formation of singly ionized oxygen vacancies located in the subsurface region. These ones transfer their electrons to transition metal centers, Ru or Pt, enhancing the SnO 2 surface reactivity toward CO. The reduction of Ru 4+ and Pt 4+ was assessed both by EPR and Mo ¨ssbauer spectroscopy. Resistance measurements showed that the materials are well-suitable for use in CO sensor devices because of their reproducible and fast electrical response; this was related to the homogeneous and high dispersion of Ru and Pt centers in the oxide matrix and to the subsurface location of the species active in the electrontransfer processes.

The mechanism of NO interaction with nanosized Ru(Pd,Pt)-doped SnO(2) was studied by electron paramagnetic resonance, Mössbauer, and electric resistance measurements. Three steps were proposed for the reaction between the semiconductor oxide and the gaseous component: (i) the formation of bielectronic oxygen vacancies (V(o)) in SnO(2); (ii) their single-ionization (V(o)(*)) with injection of electrons into the SnO(2) conduction band; (iii) the subsequent transfer of electrons from V(o)(*) to [Ru(Pd,Pt)](4+). The last process induces the formation of further oxygen vacancies which reduce the transition metal centers to lower oxidation states; the redox processes is enhanced and the electrical resistance in transition metal-doped SnO(2) is stronger modified with respect to the undoped material.

Monolithic borophosphosilicate glasses were prepared by the sol−gel route through xerogel densification. Tetramethoxysilane (Si(OCH3)4), trimethylborate (B(OCH3)3), and trimethyl phosphite (P(OCH3)3) were used as source compounds for Si, B, and P, respectively. After drying, samples underwent thermal treatment up to 700 °C with alternating flowing oxygen and reduced pressure steps, resulting in transparent, monolithic glasses. The sample chemical composition was analyzed by X-ray photoelectron spectroscopy (XPS). Densification was investigated by vibrational spectroscopies (FT-IR, micro-Raman). The boron oxygen hole centers (BOHC) and phosphorus oxygen hole centers (POHC) paramagnetic defects generated by X-ray irradiation of glasses were studied by electron paramagnetic resonance (EPR) spectroscopy. The results obtained on borophosphosilicate glasses showed that paramagnetic defects are not created by independent processes, as expected for randomly distributed noninteracting B and P doping elements. Diamagnetic precursors of BOHC and POHC are thus proposed to be spatially close together in the glasses.

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The reactivity of nanosized Ru(Pd,Pt)-doped SnO2, obtained by sol-gel synthesis, towards NO/Ar was investigated by Electron Paramagnetic Resonance (EPR), Mössbauer and Electrical Resistance measurements. A sensing mechanism was proposed that involves (i) the formation of bielectronic oxygen vacancies VO, (ii) their single-ionisation to VO•, which injects electrons to SnO2 conduction band, (iii) the transfer of VO• electrons to the transition metal centers reducing them to lower oxidation states. It was suggested that the electronic exchange between oxide and transition metal is responsible for the enhancement of the reactivity in doped SnO2 with respect to the undoped material.