DFT investigation and Polypyrrole sensitization mechanism of a selective NO2 sensor for room temperature application based on MXene@PPy heterojunction

Assembling a heterojunction-based composite nanomaterial to simultaneously achieve higher response and superior selectivity towards NO2 under room-temperature conditions remains a great challenge. In this work, the MXene@Polypyrrole heterojunction was synthesized via a facile in-suit chemical polymerization approach. The distribution of sphere-like PPy nanoparticles was confirmed via various characterization techniques. The response value of the 0.5MXP sensor toward 50 ppm NO2 is significantly higher than that of the pristine MXene based sensor, entailing greater response (6.5-fold). Nearly 98.11% recovery to the base resistance was observed in each sensing cycle. Noteworthy, the Schottky barriers formed between MXene and PPy contact surface, leading to the ideal selectivity and a lower limit of detection. Additionally, the sensor was tested for a 31-days-period with minimal deviation, predicting outstanding reproducibility and durability. The detailed polypyrrole sensitization mechanism containing selectivity mechanism, underlying heterojunction mechanism as well as the Density Function Theory (DFT) calculation were also revealed, offering new ideas for the development of room-temperature nitrogen dioxide sensors. Simultaneously, DFT calculation (21 models, 42 images) revealed the adsorption energy, electron transfer, and charge density differences between the NO2 molecules and the substrate (MXene, PPy and MXene@PPy). In summary, the outstanding MXene surface functionalization has been exploited by decorating with polypyrrole (PPy) nanoparticles.