Silver and gold nanoparticles were synthesized by the sol-gel process in SiO 2 , TiO 2 , and ZrO 2 thin films. A versatile method, based on the use of coordination chemistry, is presented for stabilizing Ag ؉ and Au 3؉ ions in sol-gel systems. Various ligands of the metal ions were tested, and for each system it was possible to find a suitable ligand capable of stabilizing the metal ions and preventing gold precipitation onto the film surface. Thin films were prepared by spin-coating onto glass or fused silica substrates and then heat-treated at various temperatures in air or H 2 atmosphere for nucleating the metal nanoparticles. The Ag particle size was about 10 nm after heating the SiO 2 film at 600°C and the TiO 2 and ZrO 2 films at 500°C. After heat treatment at 500°C, the Au particle size was 13 and 17 nm in the TiO 2 and ZrO 2 films, respectively. The films were characterized by UV-vis optical absorption spectroscopy and X-ray diffraction, for studying the nucleation and the growth of the metal nanoparticles. The results are discussed with regard to the embedding matrix, the temperature, and the atmosphere of the heat treatment, and it is concluded that crystallization of TiO 2 and ZrO 2 films may hinder the growth of Ag and Au particles.

SnO 2 nanocrystals were prepared by injecting a hydrolyzed methanol solution of SnCl 4 into a tetradecene solution of dodecylamine. The resulting materials were annealed at 500 °C, providing 6-8 nm nanocrystals. The latter were used for fabricating NO 2 gas sensing devices, which displayed remarkable electrical responses to as low as 100 ppb NO 2 concentration. The nanocrystals were characterized by conductometric measurements, X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR), and cathodoluminescence (CL) spectroscopy. The results, interpreted by means of molecular modeling in the frame of the density functional theory (DFT), indicated that the nanocrystals contain topographically well-defined surface oxygen vacancies. The chemisorption properties of these vacancies, studied by DFT modeling of the NO 2 /SnO 2 interaction, suggested that the in-plane vacancies facilitate the NO 2 adsorption at low operating temperatures, while the bridging vacancies, generated by heat treatment at 500 °C, enhance the charge transfer from the surface to the adsorbate. The behavior of the oxygen vacancies in the adsorption properties revealed a gas response mechanism in oxide nanocrystals more complex than the size dependence alone. In particular, the nanocrystals surface must be characterized by enhanced transducing properties for obtaining relevant gas responses.