The Role of NiO Doping in Reducing the Impact of Humidity on the Performance of SnO₂‐Based Gas Sensors: Synthesis Strategies, and Phenomenological and Spectroscopic Studies

Abstract The humidity dependence of the gas‐sensing characteristics in SnO 2 ‐based sensors, one of the greatest obstacles in gas‐sensor applications, is reduced to a negligible level by NiO doping. In a dry atmosphere, undoped hierarchical SnO 2 nanostructures prepared by the self‐assembly of crystalline nanosheets show a high CO response and a rapid response speed. However, the gas response, response/recovery speeds, and resistance in air are deteriorated or changed significantly in a humid atmosphere. When hierarchical SnO 2 nanostructures are doped with 0.64–1.27 wt% NiO, all of the gas‐sensing characteristics remain similar, even after changing the atmosphere from a dry to wet one. According to diffuse‐reflectance Fourier transform IR measurements, it is found that the most of the water‐driven species are predominantly absorbed not by the SnO 2 but by the NiO, and thus the electrochemical interaction between the humidity and the SnO 2 sensor surface is totally blocked. NiO‐doped hierarchical SnO 2 sensors exhibit an exceptionally fast response speed (1.6 s), a fast recovery speed (2.8 s) and a superior gas response ( R a / R g = 2.8 at 50 ppm CO ( R a : resistance in air, R g : resistance in gas)) even in a 25% r.h. atmosphere. The doping of hierarchical SnO 2 nanostructures with NiO is a very‐promising approach to reduce the dependence of the gas‐sensing characteristics on humidity without sacrificing the high gas response, the ultrafast response and the ultrafast recovery.