In Situ Self-Oxidation of Few-Layered Nb₂CT_x Nanosheets in Aqueous Solution for Achieving Improved Humidity Sensitivity and Selectivity

MXenes with numerous superior properties have great potential for portable and conformal humidity sensing, which can meet the ever-increasing requirements for noncontact medical diagnosis, noninvasive epidermal detection, and environmental monitoring. Due to the ambiguous mechanism, the absence of an efficient approach for improving the sensitivity and selectivity remains a big obstacle for realization of MXene-based humidity sensing. In this work, the evolution of humidity sensing mechanisms of few-layered Nb2CTx nanosheets before and after self-oxidation in aqueous solutions are clarified depending on a systematic comparison. The evolution of structure and chemical bonds in few-layered Nb2CTx nanosheets after different incubation durations has been investigated to understand the in situ self-oxidation process and establish the relationship between the oxidation degree and humidity sensing performance. Meanwhile, the humidity sensing mechanism of sensors for different few-layered Nb2CTx nanosheets has been also clarified, and the humidity sensitivity has been optimized to -2.3 x 10(4) with a relative humidity of 53%. Compared with the sensor prepared from as-prepared few-layered Nb2CTx nanosheets, the humidity sensing selectivity from oxidized fewlayered Nb2CTx nanosheets has been improved, contributed to by the transformation of the sensing mechanism from electronic conduction to ionic conduction. Finally, the oxidized few-layered Nb2CTx nanosheets have been prepared on a PI substrate to demonstrate its application in a flexible humidity sensor, providing a facile solution for ultrasensitive wearable gas sensing.