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

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.