Assessing the Mechanism of NH3 Sensing in Flexible Carbon-Nanotube-Based Chemoresistive Sensors via In Situ Photoemission Spectroscopy

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 (NH3)-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 similar to 0.4% ppm-1. Under UHV, a nominal NH3 concentration of similar to 8 ppm produces a smaller relative response of similar to 0.5%, comparable to that obtained at 3 ppm in dry air. In situ micro-focused XPS reveals reversible (approximate to 1 eV) shifts in the C 1s core-level binding energy during NH3 exposure, confirming that NH3 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.