Preparation and characterization of SnO2 and MoOx-SnO2 nanosized powders for thick film gas sensors

This work gives results about the characterization of SnO2 materials, prepared via the sol-gel route, pure and Mo6+-added. The materials were characterized as powders or thick films using a variety of techniques. The morphology of the powders was analyzed by XRD, SEM, TEM and HRTEM, their texture by volumetric measurements. The morphology of the thick films was analyzed by SEM. The goal of obtaining powders and films made by regularly shaped and nanosized (30&unknown;50 nm) particles, even after thermal treatments at 850oC is attained. FT-IR spectroscopic and electrical measurements were employed on powders and films, respectively, to obtain information on the electronic effect due to the molybdenum addition. FT-IR results show that Mo lowers the intensity of the light scattered by free electrons and the intensity of a broad absorption, previously assigned to the photoionization of VO+[VO++h->VO2++e- (c.b.)]. Accordingly, electrical data show that molybdenum markedly lowers (of about 2 orders of magnitude) the conductance of the films in air. Electrical measurements show that Mo lowers the response of tin oxide towards CO, but leaves almost unaltered or enhances its ability to sense NO2, depending on the thermal pretreatments. Both pure and Mo-added materials treated at 650oC show the same response to NO2. However, for the pure material treated at 850oC the response to NO2 is halved, while it is almost unaffected by the thermal treatment on the Mo-added materials. The sensing temperature of maximum response is in any case 150oC. FT-IR spectroscopy was also employed to obtain information on the Mo species present on the surface of the materials after treatments in oxygen and on how they are affected in the presence of the different testing gases. Furthermore surface species formed by NO2 interaction were carefully investigated.