Assessing the Mechanism of NH ₃ Sensing in Flexible Carbon‐Nanotube‐Based Chemoresistive Sensors via In Situ Photoemission Spectroscopy

ABSTRACT Flexible carbon nanotube (CNT) chemoresistors offer a scalable, low‐cost platform for wearable gas detection. However, a clear understanding of their sensing mechanism remains essential to optimize sensitivity, selectivity, and overall reliability. In this work, we complement electrical response measurements in both dry air and ultra‐high vacuum (UHV) with synchrotron‐based in situ X‐ray photoelectron spectroscopy (XPS) to directly probe ammonia (NH 3 )–CNT interactions. In both environments, the response can be described by Langmuir‐type adsorption–desorption kinetics. In dry air (3–50 ppm), the devices exhibit a reproducible increase in resistance with a sensitivity of ∼0.4% ppm −1 . Under UHV, a nominal NH 3 concentration of ∼8 ppm produces a smaller relative response of ∼0.5%, comparable to that obtained at 3 ppm in dry air. In situ micro‐focused XPS reveals reversible (≈1 eV) shifts in the C 1s core‐level binding energy during NH 3 exposure, confirming that NH 3 acts as an electron donor. This spectroscopic evidence correlates quantitatively with the chemoresistive response, establishing intra‐CNT charge transfer as the dominant transduction mechanism. These findings underscore the effectiveness of coupling XPS with electrical analysis to unravel gas‐sensor transduction in nanomaterials and pave the way for the rational design of high‐performance CNT‐based sensors.