The humidity dependence of the gas‐sensing characteristics in SnO2‐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 SnO2 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 SnO2 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 SnO2 but by the NiO, and thus the electrochemical interaction between the humidity and the SnO2 sensor surface is totally blocked. NiO‐doped hierarchical SnO2 sensors exhibit an exceptionally fast response speed (1.6 s), a fast recovery speed (2.8 s) and a superior gas response (Ra/Rg = 2.8 at 50 ppm CO (Ra: resistance in air, Rg: resistance in gas)) even in a 25% r.h. atmosphere. The doping of hierarchical SnO2 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.

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We report the synthesis of neodymium hydroxide nanoparticles via a nonaqueous and surfactant-free sol−gel process and their subsequent thermal transformation into neodymium dioxide carbonate, which can be applied as a sensing layer for resistive-readout CO2 sensing. The sensors show an increase in resistance when exposed to CO2 in both dry and humid air in the operation temperature range of 250−400 °C, with a maximum sensor signal of 4 in humid air at 350 °C in 1000 ppm CO2. Another important feature of the sensor is the fact that exposure to water vapor leads to a pronounced decrease in resistance (opposite of the CO2 effect), which indicates different charge-transfer mechanisms. The CO2 gas-sensing mechanism was studied via the Operando approach, by performing direct-current (DC) resistance and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements simultaneously under operation conditions. This combination enables the correlation of any concentration changes of specific surface species with electrical effects. The correlation found between the concentration of surface-adsorbed OH and carbonate species and the electrical conductivity suggests that the reaction between CO2 and water-related surface species is responsible for the gas-sensing effect.

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