Direct Imaging of Co-CUK-1 Framework with H2O Guests

Flexible architectural design with metal clusters and organic ligands attributes Metal-Organic Frameworks (MOFs) one of the most talented-porous materials [1]. Host-guest interactions renders such versatility, and thermal-vibration interpretation is crucial of guest molecules encapsulated by host frames [2]. Moreover, atomic-structural definition is essential to materialize structure-engineered multifunctionality exhibition of MOFs [3]. Herein, we primally identified thermal-vibration behavior of H­2O guest molecules in M-CUK-1 (M = Metal, CUK= Cambridge University-KRICT) by in-situ heating Raman spectroscopy. The H2O guest molecules behave distinctively depending on the metal species. Then, we defined real-space atomic structure of Co-CUK-1, especially for “channels” where guests maneuver in one-dimension. Through ab initio calculations, spatial distribution of H2O guest molecules was explicated in Co-CUK-1 channels. Our experimental approach combined with in-situ heating Raman spectroscopy and atomic-scale investigation by STEM coupled with ab initio calculations guarantee cornerstone for integrated study of MOFs which is versatile though extremely vulnerable under electron beams. In-situ heating Raman spectroscopy for H2O guests. a Experimental set up for in-situ heating Raman spectroscopy. From the designed “Kit-chamber”, the Raman spectroscopy is investigated with “M-CUK-1”/”SiO2/Si” substrate heated to 120°C on “Heating plate”. Laser source wavelength of 532 nm and power of 2.5 mW. b Restricted H2O vibration (yellow winding arrows) encapsulated by honeycomb Co frame (blue shaded polygons). The color-codes are the sample as Fig. 1. c Raman spectra of Co-CUK-1/SiO2/Si of 100∼4000 cm-1 wave number. The Raman spectra was presented at RT (black), 120°C heating for 20 min (red), 40 min (blue), 60 min (green), 80 min (purple), 100 min (brown), and 120 min (cyan), respectively. Inset: Raman spectra measured at the green shaded region in c (wave number of 3300cm-1-3800 cm-1) The Raman peak (3660cm-1, H2O vapor phase) remains even heating for 120 min as denoted by red shaded region. This indicates H2O guests are tenaciously adsorbed in “vapor phase”. Note that the atomic configuration of Co-CUK-1 is very convoluted and O-H bond exist in Co-CUK-1 frames, therefore, the interpretation of Raman spectra is tremendously complicated. Unveiling atomic structure of Co-CUK-1. a Low magnification HAADF-STEM of Co-CUK-1 nano particle.) Left: Wide view (HAADF-STEM) of nano particle viewed along [101] orientation. Right: Corresponding FFT of left panel. The HAADF-STEM and FFT confirmed honeycomb frame structures formed by the Co atoms. c Top (Bottom): Magnified HAADF-(ABF-)STEM inside of Co-CUK-1 nano particle. Insets: simulated HAADF-(ABF-)STEM and atomic configuration superimposed in top(bottom) images of c. While the HAADF-STEM denotes the clear honeycomb frame by Co atoms, the weak contrast observed inside of pore structure in ABF-STEM indicates H2O guests which are captured in pore structure. The color-codes are the same as Fig. 1.