Effect of Coordinatively Unsaturated Sites in MOF‐Derived Highly Porous CuO for Catalyst‐Free ppb‐Level Gas Sensors

Abstract Herein, the authors successfully synthesize diamond‐ and square‐type Cu‐metal–organic frameworks (MOFs) and then convert them to highly porous CuO nanostructures with maintaining their morphology. Both the CuO‐diamond (CuO‐D) and CuO‐square (CuO‐S) gas sensors can detect a tiny concentration of acetone gas up to 50 ppb. However, the gas sensitivity depends on the morphology and the CuO‐D exhibits a higher gas response ( R g / R a : 3.3) for 1 ppm of acetone as compared with CuO‐S sensors ( R g / R a : 2.5). The mainly exposed crystal facet on the surface significantly affects the sensor performance. Especially, coordinatively unsaturated metal sites in an exposed facet can allow enhancing the sensitivity. The (111) facet is the main exposed facet of CuO‐D and it includes Cu 3‐fold which is a coordinatively unsaturated metal site. These relatively unstable Cu 3‐fold sites are favorable to react with other molecules to stabilize themselves as compared with coordinatively saturated Cu 4‐fold . Thus, CuO‐D demonstrates higher sensitivity as compared to CuO‐S, which is the main reaction surface as (020) facet composed only of Cu 4‐fold .