Tailoring crystal facets of metal-organic frameworks to enhance sensing performance for aromatic vapors detection.

It is well known that metals and metal oxides with different crystal facets exhibit varying sensitivity in gas sensors, but this strategy is rarely used in metal-organic frameworks (MOFs). Herein, we proved for the first time that Cu metal-organic with high energy crystal facets (Cu-MOF-74-300) shows a much higher sensitivity than the low energy crystal facets (Cu-MOF-74-110), with a up to 2 times response more than Cu-MOF-74-110 and ultra-low limit of detection (LOD) of 68 ppb to toluene vapors. In addition, this strategy was further demonstrated on MOF-14 and HKUST-1, which are also Cu-centered and exhibit clear recognition effects on benzene and xylene, respectively. Furthermore, the Cu-MOF-74-300 shows high selectivity (92 %) and excellent stability, with a minor reduction in response of less than 1.9 % after 6 months. Additionally, the sensitive mechanism is explored by DFT and GCMC methods. The simulations reveal that Cu-MOF-74 includes two adsorption sites, one is Cu site with adsorption enthalpy (ΔH) of -59.04 kJ/mol, another is the benzene ring site (ΔH is -67.50 kJ/mol) in the ligand. The difference in charge densities reveal that the Cu2+ and benzene ring on the surface of Cu-MOF-74-300 enables synergistic sensing, leading to more electron transfer in toluene than that of Cu-MOF-74-110 (only benzene ring site). Finally, based on the temperature-varying experiments, the experimental adsorption energy (-51.35 kJ/mol) was obtained by calculations. Combined quasi-in-situ XPS, implying Cu is the critical role to the improvement of Cu-MOF-74-300 sensitivity to toluene.