Metal oxide semiconductors as humidity and NOx sensors for environmental monitoring

Semiconducting metal oxide (SMO) sensors are one of the most widely studied groups of chemiresistive gas sensors due to their unique advantages such as low cost, small size, measurement simplicity, durability, ease of fabrication, and low detection limits (< ppm levels). Moreover, most SMO based sensors tend to be long-lived and somewhat resistant to poisoning. The SMO undergoes reduction or oxidation while reacting with the target gas and this process causes an exchange of electrons at a certain characteristic rate, thereby affecting the sensor’s resistance and yielding a certain signal. The aim of this PhD is to fabricate new semiconducting metal oxide sensors capable of detecting humidity and/or NOx gas. The research is subdivided into three main different parts: - The first is to prepare SMO sensors within a template (inorganic clay e.g. Sepiolite, glass): a) ZnO has been synthesized into the sepiolite structure by means of different acid treatments in order to check the sensitivity of the resulting materials toward humidity and NO2. Different samples have been prepared S1, S2, S6 and S24 for the samples acidified for 1, 2, 6 and 24 hours, respectively. Sepiolite leached for 2 h (S2) showed a significant ability to detect quite low Relative Humidity (RH) values. Also, S2 revealed a significant capability for detecting NO2 and H2 in an optimal working temperature 300°C. b) Crystallization of zinc oxide by melt quenching technique; The idea is to check the possibility to fabricate a percolated network from ZnO crystals within a glass matrix by means of crystallization process then to test the resulting material as NOx gas sensor. A melt with nominal composition 58% ZnO 33.3% B2O3 4% WO3 4% Bi2O3 has been quenched. The chosen glass sample was then subjected to the heat treatment for 15 hours at the temperature close to the established Tx (crystallization temperature). As obtained glass ceramic material with adding a ZnO sol-gel (to improve the adhesion of the ink onto alumina substrate) gives a great ability to detect NO2 at a quite low working temperature (150ºC), in conditions close to the environmental one's. This is a promising approach for glass ceramic materials in sensors application which could be used not only as humidity sensors but also for detecting other several gases with a quite good selectivity. - The second part is Li – doped iron oxide as a new material for NO2 detection: Various compositions of lithium doped hematite containing 1, 2, 5, 10, 15, and 20 atomic percent ‎were synthesized by solid state reaction. Electrochemical impedance spectroscopy ‎‎ (EIS) analyses under air and argon were used to determine the semiconducting behavior of the ‎samples (n- or p-type) and to ‎investigate the sensitivity of these materials towards NO2. Lithium doped hematite samples exhibit an n type semiconducting behavior. Lithium ferrites could work as NO2 sensors at a quite low operating temperature (200°C). Generally, sensor response of lithium ferrites towards NO2 revealed an acceptable linear evolution with gas concentration for some compositions. - The third is commercial SMO; In2O3 and ZnO with different morphologies. The objective of this work is the development of materials with hierarchical architectures for the realization of gas sensors capable of detecting low concentration of NO2 in air at low temperature. Different films have been prepared: ZnO and In2O3 thick films, ZnO thin films prepared by sol-gel technique, ZnO nanorods (NR) prepared by technology template of ZnO films obtained by Sol gel and ZnO nanowires (NW) prepared by electrospinning. ZnO thick film and ZnO thin film have the best sensitivity towards NO2 among the other samples.