Unconventional Gas Sensing Mechanism in Phase‐Separated n‐Type Mixed Tungsten Oxide 2D‐Nanosheets Compared against Tungsten (VI) Oxide

ABSTRACT Unconventional gas sensing underlies a mixed phase composition of 2D tungsten oxide nanosheets with multiple charge states, which are otherwise inaccessible, generating select catalytic surfaces. Here, we report an unconventional gas‐sensing in oxygen‐deficient WO _3−x . These materials are n‐type phase‐separated metal oxide semiconductors comprised of distinct poorly insulating WO _2.9 domains embedded in the hard insulating WO ₂ host, consisting of spatially separated competing phases. Our study highlights the fundamental differences between phase‐separated WO _3−x and WO ₃ systems, where WO _3−x exhibits unconventional sensitivity towards NO ₂ (−125% at 10 ppm) and CO (94% at 10 ppm), unlike pristine WO ₃ (685% at 10 ppm NO ₂ and −84% at 10 ppm CO). The WO _3−x sensor shows excellent cross‐sensitivity towards NO ₂ and CO, dominated by their chemisorption onto different surface(s) of 2D WO _2.9 and WO ₂ nanosheets, validated by theory. The pristine WO ₃ sensor exhibits high sensitivity with fast response/recovery times at room temperature, setting records among existing conventional sensors. These results suggest an unknown, possibly novel mechanism is required to explain the unconventional sensing in WO _3−x . Unconventional gas sensing, coupled with cross‐sensitivity and a combination of conventional sensing across a wide temperature range, is desirable for real‐world applications for sensors operating in mixed gas environments.