Unconventional Gas Sensing Mechanism in Phase-Separated n-Type Mixed Tungsten Oxide 2D-Nanosheets Compared against...

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 WO₂ host, consisting of spatially separated competing phases. Our study highlights the fundamental differences between phase-separated WO_3-x and WO₃ systems, where WO_3-x exhibits unconventional sensitivity towards NO₂ (-125% at 10 ppm) and CO (94% at 10 ppm), unlike pristine WO₃ (685% at 10 ppm NO₂ and -84% at 10 ppm CO). The WO_3-x sensor shows excellent cross-sensitivity towards NO₂ and CO, dominated by their chemisorption onto different surface(s) of 2D WO_2.9 and WO₂ nanosheets, validated by theory. The pristine WO₃ 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.