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 Nb₂CT_x 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 Nb₂CT_x 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 Nb₂CT_x nanosheets has been also clarified, and the humidity sensitivity has been optimized to −2.3 × 10⁴ with a relative humidity of 53%. Compared with the sensor prepared from as-prepared few-layered Nb₂CT_x nanosheets, the humidity sensing selectivity from oxidized few-layered Nb₂CT_x nanosheets has been improved, contributed to by the transformation of the sensing mechanism from electronic conduction to ionic conduction. Finally, the oxidized few-layered Nb₂CT_x 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.