Hierarchical bimetallic metal-organic frameworks with controllable assembling sub-units and interior architectures for enhanced ammonia detection

A series of hierarchical three-dimensional (3D) manganese-cobalt benzenetricarboxylic acid (Mn-Co BTC) metal–organic frameworks (MOFs) with controllable sub-units and interior architectures have been synthesized at room temperature without templates, etchants, or high-pressure conditions. The assembling sub-units of the hierarchical 3D Mn-Co BTC MOFs can be controlled by tuning the Mn/Co ratio, with higher Mn content favoring one-dimensional (1D) sub-units, while higher Co content favors two-dimensional (2D) sub-units. The hierarchical hollow Mn-Co BTC spheres exhibit a frequency change (ΔF) of 383.7 Hz to 69.5 ppm of NH3 which is 1.7-, 2.1-, 4.7-, and 6.9-times higher than the ΔFs of hierarchical flower-like Mn-Co BTC, hierarchical Mn-Co BTC nanorods, Mn-BTC nanospheres, and Co-BTC nanoplates, respectively. In addition, they exhibit low limit of detection (1.12 ppm), high selectivity (41 %) and excellent stability for NH3 sensing with a minor change of 3.1 % in the ΔF value after 5 months of testing. The superior NH3 sensing performance on the hierarchical hollow Mn-Co BTC spheres may be attributed to the hollow cavities which can enhance the diffusion of NH3 molecules and enable them to interact with the less accessible metallic active sites on the inner cores of this MOF. The possible sensing mechanisms include the chemisorption of NH3 molecules on unsaturated metal (Mn and Co) centers as well as the hydrogen bonding between the hydroxyl and carboxyl groups with NH3 to boost the adsorption rate.