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

Abstract

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.