Imparting hydrophobicity to a MOF on layered MXene for the selective, rapid, and ppb level humidity-independent detection of NH₃ at room temperature

The sensitivity of chemiresistive sensors is inherently compromised by ambient humidity and trace level detection of toxic gases has potential challenges at room temperature. Herein, we designed a metal-organic framework (MOF) on a layered MXene hybrid by tagging a ZIF-67-based MOF on layered Ti₃C₂T_x MXene and following this with a surface ligand exchange process to design a highly sensitive, humidity tolerant chemiresistive sensor for ultra-low ppb level (200 ppb) NH₃ sensing. The gas selectivity of MXenes was influenced by surface tagging with the MOF, which creates high surface-active features that promote the interaction and selectivity of NH₃ on the MXene surface. In addition, a passive shell ligand exchange reaction provides not only a hydrophobic surface and environmental stability to the hybridized surface but also contributes to the sensing performances. The hybridized H-MOF/MXene-based sensor exhibited a superior NH₃ sensing response (ΔR/R_g = 6.9, 1 ppm) at room temperature with high selectivity and reliability and a theoretical detection limit of 12.8 ppb. Passive ligand exchange had a significant effect on the sensing response at room temperature but improved humidity resistance and long-term durability. The H-MOF/MXene response to NH₃ was only reduced by 0.22% and 0.27% at relative humidities of 76% and 93%, which represented 1.2 and 8.3-fold improvements in the sensing response versus MOF₆/MXene and bare MXene at an NH₃ concentration of 10 ppm. Furthermore, the sensing mechanism involved electronic interactions and charge transfer through a Schottky junction between the MOF and MXenes and the synergistic promotion of the sensing response on the hybridized H-MOF/MXene platform. This work provides a means of designing a surface functionalized MOF on MXene heterostructures that enables the production of sensors tailored to diverse environmental conditions.