A Highly Stable Two-Dimensional Copper(II) Organic Framework for Proton Conduction and Ammonia Impedance Sensing

This work reports the design and fabrication of a proton conductive 2D metal-organic framework (MOF), [Cu(p-IPhHIDC)]_n (1) (p-IPhH₃ IDC=2-(p-N-imidazol-1-yl)-phenyl-1 H-imidazole-4,5-dicarboxylic acid) as an advanced ammonia impedance sensor at room temperature and 68-98 % relative humidity (RH). MOF 1 shows the optimized proton conductivity value of 1.51×10^-3  S cm^-1 at 100 °C and 98 % RH. Its temperature-dependent and humidity-dependent proton conduction properties have been explored. The large amount of uncoordinated carboxylate groups between the layers plays a vital role in the resultant conductivity. Distinctly, the fabricated MOF-based sensor displays the required stability toward NH₃ , enhanced sensitivity, and notable selectivity for NH₃ gas. At room temperature and 68 % RH, it gives a remarkable gas response of 8620 % to 130 ppm NH₃ gas and lower detection limit of 2 ppm towards NH₃ gas. It is also found that the gas response of the ammonia sensor increases linearly with the increase of NH₃ gas concentration under 68-98 % RH and room temperature. Moreover, the sensor indicates excellent reversibility and selectivity toward NH₃ versus N₂ , H₂ , O₂ , CO, CO₂ , benzene, and MeOH. Based on structural analyses, activation energy calculations, water and NH₃ vapor absorptions, and PXRD determinations, proton conduction and NH₃ sensing mechanisms are suggested.