Optimizing Sputtered SnO2:Dy Thin Films for NO2 Gas Detection

Abstract

Notwithstanding the success of SnO2 as a fundamental material for gas sensing, it has often been criticized for its cross-sensitivity and high operational temperatures. Therefore, in this study, RF-sputtered SnO2 thin films were subjected to a modification process through doping with a rare earth element, dysprosium (Dy), and subsequently deposited onto two different types of substrates: alumina and glass substrates. All thin films underwent a comprehensive series of characterizations aimed at ensuring their suitability as NO2 sensors. The dysprosium doping levels ranged from 1 to 7 wt.% in increments of 2% (wt.%). X-ray patterns showed that all deposited films exhibited the tetragonal rutile structure of SnO2. The optical band gap energy (Eg) increased with Dy doping, while the Urbach energy decreased with Dy doping. Field emission scanning electron microscopy (FESEM) revealed highly compacted grainy surfaces with high roughness for alumina substrate thin films, which also exhibited higher resistivity that increased with the levels of Dy doping. Energy-dispersive X-ray spectroscopy (EDX) analyses confirmed the stoichiometry of both types of thin films. Gas sensing tests were conducted at different operating temperatures, where the highest response to nitrogen dioxide, over 42%, was recorded for the higher dopant level at 250 °C. Moreover, the sensor’s selectivity toward nitrogen dioxide traces was evaluated by introducing interfering gases at higher concentrations. However, the sensors showed also significant responses when operated at room temperature. Also, we have demonstrated that higher stability is related to the temperature of the sensors and Dy ratio. Hence, a detailed discussion of the gas-sensing mechanisms was undertaken to gain a deeper insight into the NO2 sensitivity exhibited by the Dy-doped SnO2 layer.