Exceptional NH₃ Detection by Cu(cyhdc) MOF Sensor Due to H₂O Coadsorption

Metal-organic framework (MOF)-based electrical impedance sensors are a growing class of sensors that show utility in the detection of environmentally toxic gases. Detection of trace NH₃ has been particularly difficult to design due to the low electrical response of NH₃. However, this has been circumvented by judiciously selecting a MOF that has enhanced electrical response to NH₃ due to coadsorption with predominant atmospheric gases such as water. Herein, an MOF Cu(cyhdc) based sensor has been successfully demonstrated for the enhanced detection of environmentally toxic NH₃ gas. The sensor shows a change in impedance when it is exposed to 5 ppm of NH₃. At 20 °C, >30% relative humidity (RH) is necessary to elicit a change, with the response increasing with RH and reaching 3170× at 92% RH, producing the largest published response to NH₃ for a MOF direct electrical sensor. In the absence of water, no change is observed toward 5 ppm of NH₃ over 20-50 °C. This electrical response is largely driven by huge decreases in the imaginary component of the impedance, attributed to increased capacitance at the surface of the MOF crystallites upon NH₃ and H₂O coadsorption. Complementary structural and microstructural characterization proves that the Cu(cyhdc) crystalline structure and morphology remain intact under trace NH₃ and/or H₂O adsorption. Despite extremely low NH₃, loadings seen in TGA, XPS, and FT-IR confirm NH₃ and H₂O adsorption, and changes to the metal-carboxylate IR peak positions are observed upon NH₃ adsorption. Concentrated primarily at the outer surfaces of the MOF crystallites, this NH₃ and H₂O coadsorption effectively increases the surface capacitance across the Cu(cyhdc) powder and enables direct electrical detection of trace NH₃. Together, these results demonstrate how coadsorption of specific molecules (H₂O) can be used to enable the electrical detection of trace toxic gases that would otherwise not have produced a MOF sensor response.