Polyethyleneimine/polyethylene-glycol/Ti3C2Tx composites for ultrasensitive room-temperature CO2 sensing

Bestowed with abundant surface terminations and good electrical conductivity, transition-metal carbides/nitrides (MXene) have potential applications as gas sensors. Here, Ti3C2Tx-HF and Ti3C2Tx-HCl MXenes were functionalized with polyethyleneimine/polyethylene-glycol (PEI/PEG) to form PEI/PEG/Ti3C2Tx-HF (P/P/Ti-1) and PEI/PEG/Ti3C2Tx-HCl (P/P/Ti-2) composites for gaseous CO2 detection at room temperature. The morphologies and elemental compositions indicated that Ti3C2Tx-HCl more easily combined with PEI/PEG than did Ti3C2Tx-HF. The response of the P/P/Ti-2 sensor to 2000 ppm CO2 was six-fold better than that of the P/P/Ti-1 sensor. In addition, the P/P/Ti-2 sensor exhibited higher selectivity and good stability. The enhanced P/P/Ti-2 sensing was attributed to heterojunctions formed at the interfaces between Ti3C2Tx-HCl and PEI/PEG because of the thinly and fully covered PEI/PEG layer on the Ti3C2Tx-HCl via abundant -OH functional groups. This was confirmed by current-voltage curves, and good Ti3C2Tx conductivity. A portable wireless CO2 indoor monitoring system based on the P/P/Ti-2 sensor was demonstrated. These results provide a better understanding of the physical and gas-sensing properties of Ti3C2Tx materials.