Nano- and Microsized Phases in the WO3-ZnO-Nd2O3-Al2O3 System for Applications in Environmental Monitoring

WO3 and WO3-based materials in the form of thin films, either as bulk or as nanostructures, have been widely used for the detection of a variety of gases, such as NO2, H2, NH3 and Cl2. The purpose of this study was to prepare materials from the WO3-ZnO-Nd2O3-Al2O3 system containing high amount of WO3 for applications in environmental monitoring by applying the melt quenching method. Homogenized batches of the starting oxides were melted for 20 min at 1,240 °C in platinum crucibles in air atmosphere. The melts were quenched by pouring and pressing between two copper plates (cooling rates 101–102 K/s). Glassy-crystalline samples containing the monophase AlW12O37.5 was produced by quenching a melt with the composition of 80WO3⋅10Nd2O3⋅10Al2O3. Glassy-crystalline sample containing nanosized W18O49 (~60 nm) was obtained after cooling of a melt with the composition 76WO3⋅9.5ZnO⋅9.5Nd2O3-5Al2O3. A mixture of WO3 and AlW12O37.5 in the amorphous matrix was prepared by crystallization of a quenched sample with the composition 75WO3⋅10Nd2O3⋅5ZnO⋅10Al2O3. Samples were prepared by the screen printing technique as gas sensor s; thereafter dc resistance measurements were performed in the presence of relative humidity and ammonia. It was established that the glassy-crystalline sample containing the monophase AlW12O37.5 exhibits a significant sensitivity towards ammonia and relative humidity, while other phases show a good sensitivity only towards relative humidity.