Síntese e caracterização de sensores de gás a partir de NaxWO3+x/2.yH2O

Abstract

This study consists in the synthesis and characterization of gas sensors based on hexagonal sodium-tungsten bronze powders (NaxWO3+x / 2.yH2O), whereas the gas sensors were synthesized by two routes. Both used as acid precipitation method in a sodium tungstate dihydrate solution (Na2WO4 • 2H2O). The routes were different by heating systems and in the first route obtained by the microwave-assisted hydrothermal method (HM). The second one was a hot plate with a magnetic stirrer (HP). Both syntheses were very efficient from the point of view of energy expenditure, because we obtained the NaxWO3+x/2.yH2O in short time and low temperatures. Samples were characterized by means of Raman spectroscopy, Fourier Transform Infrared (FTIR), absorption in the ultraviolet and visible region (UV-Vis), photoluminescence and energy dispersive X-rays (EDS), X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM). The bronze sensors obtained were tested against NO2 and the samples prepared by both routes were proved to be sensitive to gas. The HM samples had a higher sensitivity (Rgas / Rar ≅ 7 for 100 ppm of gás) and response time (of 1.5 minutes), with a working temperature of 250 ° C. On the other hand, the samples obtained by the PA method had the advantage of operating at working temperature of about 150 ° C, but with lower sensitivity and with a greater response time and recovery time (≅ 10 minutes). From the analysis of the photoluminescence measurement, it was possible to observe some possible relations between the type of defect as well as the sensitivity. It indicates that the larger the contribution of shallow defects in the samples, the greater sensitivity. It is suggested, therefore, from these data, that the sensor property in NaxWO3+x/2.yH2O system can be improved by the control of the defects.