Investigating the influence of Al-doping and background humidity on NO₂ sensing characteristics of magnetron-sputtered SnO₂ sensors

Abstract. Elevated temperatures and humidity contents affect response, lifetime and stability of metal-oxide gas sensors. Remarkable efforts are being made to improve the sensing characteristics of metal-oxide-based sensors operating under such conditions. Having versatile semiconducting properties, SnO₂ is prominently used for gas sensing applications. The aim of the present work is to demonstrate the capability of the Al-doped SnO₂ layer as NO₂ selective gas sensor working at high temperatures under the presence of humidity. Undoped SnO₂ and Al-doped SnO₂ (3 at. % Al) layers were prepared by the radio frequency (r.f.) reactive magnetron sputtering technique, having an average thickness of 2.5 μm. The sensor response of Al-doped SnO₂ samples was reduced in the presence of background synthetic air. Moreover, under dry argon conditions, Al doping contributes to obtain a stable signal and to lower cross-sensitivity to CO in the gas mixtures of CO + NO₂ at temperatures of 500 and 600 °C. The Al-doped SnO₂ sensors exhibit excellent chemical stability and sensitivity towards NO₂ gas at the temperature range of 400–600 °C under a humid environment. The sensors also showed satisfactory response (τ_res = 1.73 min) and recovery (τ_rec = 2.7 min) towards 50 ppm NO₂ in the presence of 10 % RH at 600 °C.