| AVS 64th International Symposium & Exhibition | |
| Surface Science Division | Monday Sessions |
| Session SS+AS+HC-MoA |
| Session: | Surface Science for Energy and the Environment |
| Presenter: | Kui Tan, the University of Texas at Dallas |
| Authors: | K. Tan, the University of Texas at Dallas S. Jensen, Wake Forest University S. Zuluaga, Wake Forest University E. Fuentes, University of Texas at Dallas E. Mattson, University of Texas at Dallas J.-F. Veyan, University of Texas at Dallas H. Wang, Rutgers University J. Li, Rutgers University T. Thonhauser, Wake Forest University Y.J. Chabal, University of Texas at Dallas |
| Correspondent: | Click to Email |
Enhancing gas adsorption in porous materials such as metal organic frameworks (MOFs) is critical to many technologies such as energy storage and gas capture and separation. In contrast to the previous efforts focusing on chemically modifying the entire MOFs structure to increase the affinity to gas molecules, in our recent work [Nature Communications, 2016, 7, 13871], we demonstrated an entirely new approach to trap small molecules by depositing a layer of ethylenediamine (EDA) on the external surface without penetrating inside the bulk of crystals in the gas loading process, and forms a monolayer cap that can effectively trap weakly adsorbed molecules (CO, CO2, SO2, C2H4, NO) or prevent their penetration. A combination of surface sensitive techniques in-situ infrared spectroscopy, X-ray photoelectron spectroscopy with gas cluster sputtering, low energy ion spectroscopy, and ab initio calculations demonstrate that the EDA layer is less than 1nm thick and arranges in a manner that increases CO diffusion barrier by ~24 times. Motivated by this finding, we further explore other capping media to store molecules inside MOFs. The latest experimental results suggest at least two ways: one is the deposition of a thin layer of aluminum oxide exclusively onto the external surface of certain MOFs with a narrower pore size (< 1.2 nm) by using common atomic layer disposition (ALD) precursor trimethylaluminum (TMA). Aluminum oxide is not able to grow inside MOFs since the reactivity of TMA with adsorbed H2O is significantly reduced due to nano-confinement effects; another convenient way is post-exposure of gas-loaded MOF-74 to NH3. NH3 is preferentially adsorbed on the outer layer of MOFs crystals during the controlled exposure time (<10 min), as a result of which, the diffusion barrier of small gas molecules such as CO, CO2, SO2 in NH3 decorated MOFs channels dramatically increases by the hydrogen bonding interaction. The ability to create surface barrier layers on MOFs external surfaces constitutes a new paradigm for trapping small gas molecules within nanoporous materials in general, and in MOFs in particular.