Li Xinghua, Wang Jianjian, Liu Xin, Liu Lingmei, Cha Dongkyu, Zheng Xinliang, Yousef Ali A, Song Kepeng, Zhu Yihan, Zhang Daliang, Han Yu
Physical Sciences and Engineering Division, Advanced Membranes and Porous Materials Center , King Abdullah University of Science and Technology (KAUST) , Thuwal 23955-6900 , Saudi Arabia.
Multi-scale Porous Materials Center, Institute of Advanced Interdisciplinary Studies , Chongqing University , Chongqing 400044 , P. R. China.
J Am Chem Soc. 2019 Jul 31;141(30):12021-12028. doi: 10.1021/jacs.9b04896. Epub 2019 Jul 19.
Metal-organic frameworks (MOFs) are often synthesized using various additives to modulate the crystallization. Here, we report the direct imaging of the crystal surface of MOF MIL-101 synthesized with different additives, using low-dose high-resolution transmission electron microscopy (HRTEM), and identify three distinct surface structures, at subunit cell resolution. We find that the mesoporous cages at the outermost surface of MIL-101 can be opened up by vacuum heating treatment at different temperatures, depending on the MIL-101 samples. We monitor the structural evolution of MIL-101 upon vacuum heating, using in situ X-ray diffraction, and find the results to be in good agreement with HRTEM observations, which leads us to speculate that additives have an influence not only on the surface structure but also on the stability of framework. In addition, we observe solid-solid phase transformation from MIL-101 to MIL-53 taking place in the sample synthesized with hydrofluoric acid.
金属有机框架材料（MOFs）通常使用各种添加剂来调控其结晶过程。在此，我们报告了利用低剂量高分辨率透射电子显微镜（HRTEM）对使用不同添加剂合成的MOF MIL-101晶体表面进行的直接成像，并在亚晶胞分辨率下识别出三种不同的表面结构。我们发现，根据MIL-101样品的不同，MIL-101最外层表面的介孔笼可以通过在不同温度下进行真空加热处理来打开。我们使用原位X射线衍射监测了MIL-101在真空加热时的结构演变，发现结果与HRTEM观察结果高度吻合，这使我们推测添加剂不仅对表面结构有影响，而且对框架的稳定性也有影响。此外，我们观察到在使用氢氟酸合成的样品中发生了从MIL-101到MIL-53的固-固相变。
