Highly Selective Chloromethanes Detection Based on Quartz Crystal Microbalance Gas Sensors with Ba-MOFs.

Three new metal-organic frameworks (MOFs), {(CH3NH3)3[Ba2(TTHA)(NO3)(H2O)2]}·2H2O (1), {(CH3NH3)4[Ba3(HTTHA)2(H2O)7]}·3H2O (2), and [Ba7(TTHA)2(NO3)2(H2O)10]·2H2O (3) (H6TTHA = 1,3,5-triazine-2,4,6-triamineh-exaacetic acid) have been synthesized and characterized. The sensing properties of 1-3 were explored with regard to volatile organic compounds (VOCs) by the quartz crystal microbalance (QCM) technique. The results indicated that 1 and 2 have a much higher selectivity and response to chloromethanes (CH2Cl2, CHCl3, and CCl4) compared with H2O, CH3OH, CH3CH2OH, CH3CN, (CH3)2CO, C6H6, C6H5CH3, C6H5CH2CH3, and C6H5Cl at room temperature. Furthermore, 1 and 2 sensing film also exhibits excellent reversibility and stability, and the response and recovery times are almost within 10 s. 3 displays a lower response and poor selectivity to the above VOCs. The significant difference may be caused by their different structural characteristics. The Ba2+ ions are all decacoordinated in 1 and 2, while Ba2+ ions have more open metal sites in 3. So, the high selectivity and response of 1 and 2 may be due to the exchange of coordination water molecules with chloromethanes and possible electrostatic effects between (CH3NH3)+ cations and chloromethanes containing more electronegative Cl atoms. DFT calculation results show that the bond energy of Ba-Cl and Ba-O is not much different, so chloromethanes at high concentrations may exchange coordination water to form weak Ba···Cl interactions and show higher response values. 3 has no obvious VOCs selectivity and higher response due to more open sites of Ba2+ ions and smaller pore size. This work develops a fast and effective method to detect chloromethanes, providing a new opportunity for designing QCM gas sensors coated with different MOF materials.