Exceptional Electrical\nDetection of Trace NO<sub>2</sub> via Mixed Metal MOF-on-MOF Film-Based\nSensors

The tunability of metal–organic frameworks (MOFs)\nmakes\nthem exceptional materials for the development of highly selective,\nlow-power sensors for toxic gas detection. Herein, we demonstrate\nenhanced detection of NO<sub>2</sub> gas by a MOF-based electrical\nimpedance sensor made using a unique mixed metal MOF-on-MOF synthesis.\nA combined experimental and computational study was performed using\nthe exemplar Ni<sub><i>x</i></sub>Mg<sub>1–<i>x</i></sub>-MOF-74 to understand the fundamental structure–property\nrelationships behind metal mixing and MOF film synthesis methods on\nsensor performance. Density functional theory results indicated that\nthe presence of Ni in Mg-MOF-74 increased framework stability and\nincreased the electron density of states at lower energies near the\nHOMO, as well as enhanced the NO<sub>2</sub>–Mg adsorption\ninteraction. Impedance data of the Ni<sub><i>x</i></sub>Mg<sub>1–<i>x</i></sub>-MOF-74 films with larger\nNi contents showed greater impedance change after exposure to 1 ppm\nof NO<sub>2</sub> gas. Furthermore, when synthesized through either\na drop-cast or direct solvothermal film growth approach, the monometallic\nNi-based sensors had the best performance. However, the mixed metal\nNi<sub><i>x</i></sub>Mg<sub>1–<i>x</i></sub>-MOF-74 sensors synthesized through a MOF-on-MOF approach resulted\nin the highest impedance change, outperforming all monometallic Ni-based\nsensors. In particular, the mixed metal Ni-on-Mg-MOF-74 film was the\nbest-performing sensor with an impedance change of 309 upon trace\nNO<sub>2</sub> exposure. Change in impedance response after NO<sub>2</sub> exposure was improved by 52% compared to the best monometallic\nNi-on-Ni-MOF-74 sensor. Structural analysis of the Ni-on-Mg film showed\nthat the first Mg-MOF-74 layer acts as a structural template controlling\nthe structural features of the final film after metal exchange with\nNi. This led to improved film quality, evidenced by the greater crystallinity\nand larger MOF grain sizes, and resulted in enhanced sensor performance\nwhich was not achievable through other metal mixing methods. Altogether,\nthis study identifies structure–property relationships and\nsynthetic templating methods that inform MOF-based sensor design,\nallowing for improved detection of toxic compounds.