Characterization of materials for gas sensors. Surface chemistry of SnO2 and MoOX-SnO2 nano-sized powders and electrical responses of the related thick films

This work gives further results about the properties of SnO2 nano-sized materials, prepared via a sol-gel route, pure or added with Mo at two different Mo loading (1.8 and 4.7 Mo atoms%). FT-IR spectroscopic and electrical measurements are employed on powders and films, respectively, to obtain information on the electronic effects due to the molybdenum addition. FT-IR spectra in air of the powders show that Mo lowers the concentration of the oxygen vacancies. Accordingly, electrical data show that molybdenum lowers the conductance of the films in air. Electrical measurements show that Mo in amount of 1.8 atoms% lowers the ability to sense NO2 of films fired at 650oC and leaves almost unaltered those of films fired at 850oC. At variance Mo in amount of 4.7 atoms% leaves almost unaltered the ability to sense NO2 of films fired at 650oC and enhances the ability to sense NO2 of films fired at 850oC. The sensing temperature of maximum response for all materials is in any case 150oC. FT-IR spectroscopy is not able to distinguish the different ability of the pure and Mo-added materials to sense NO2, although for some samples an electronic response is evident. At variance the spectroscopic technique has been employed to carefully investigate the nature of surface species formed by NO2/O2 interaction with the three materials and their stability at the working temperature.