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, prepa red via the sol-gel route, pure and Mo6+-added. The materials were characte rized as powders or thick films using a variety of techniques. The morpholo gy 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 divided by 50 nm) particles, even after thermal treatments at 850 degrees C 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 th at Mo lowers the intensity of the light scattered by free electrons and the intensity of a broad absorption, previously assigned to the photoionizatio n of V-o(+) [V-o(+) + h nu --> V-o(2) + e(-) (c.b.)]. Accordingly, electric al data show that molybdenum markedly lowers (of about 2 orders of magnitud e) the conductance of the films in air. Electrical measurements show that M o lowers the response of tin oxide towards CO, but leaves almost unaltered or enhances its ability to sense NO2, depending on the thermal pretreatment s. Both pure and Mo-added materials treated at 650 degrees C show the same response to NO2. However, fur the pure material treated at 850 degrees C th e response to NO2 is halved, while it is almost unaffected by the thermal t reatment on the Mo-added materials. The sensing temperature of maximum resp onse is in My case 150 degrees C. FT-IR spectroscopy was also employed to o btain information on the Ma 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 int eraction were carefully investigated. (C) 1999 Elsevier Science S.A. All ri ghts reserved.