Tunable coordinative defects in UHM-3 surface-mounted MOFs for gas adsorption and separation: A combined experimental and theoretical study Zhengbang Wang a , Hikmet Sezen a , Jinxuan Liu a , Chengwu Yang a , Stephanie E. Roggenbuck b , Katharina Peikert b , Michael Fr € oba b , Andreas Mavrantonakis c , Barbara Supronowicz c , Thomas Heine c , Hartmut Gliemann a , Christof W € oll a, * a Institute of Functional Interfaces, Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen, Germany b Institute of Inorganic and Applied Chemistry, Department of Chemistry, University of Hamburg, Martin-Luther-King-Platz 6, D-20146 Hamburg, Germany c Engineering and Science, Jacobs University Bremen, Campus Ring 1, 28759 Bremen, Germany article info Article history: Received 21 November 2014 Received in revised form 26 December 2014 Accepted 28 December 2014 Available online 13 January 2015 Keywords: Metaleorganic frameworks Surface mounted metaleorganic frameworks Thin films Defect Density-functional theory abstract The metal organic framework (MOF) UHM-3, constructed with Cu(II)-paddle wheel-type nodes and a new tetracarboxylic acid linker, 5,5 0 -(dimethylsilanediyl)diisophthalate, has a close-packed alignment of open Cu(II) sites which are of interest for applications in gas storage and separation. Here, we first report on the growth of oriented, homogeneous and virtually defect-free (below 1%) UHM-3 MOF thin films on a solid substrate using a room-temperature liquid phase epitaxy (LPE) method. Thermal postsynthetic treatment allowed to induce Cu(I) defect sites in a controlled fashion. The interaction of CO and CO 2 with the Cu(II) and Cu(I) sites was then studied using X-ray photoelectron spectroscopy (XPS) and IR- spectroscopy. The binding energy of these two species was determined using temperature-induced desorption. The interaction between the guest molecules and the Cu(I) and Cu(II) sites were also analyzed using density-functional theory (DFT). Surprisingly, both experiment and theory show that the binding energy of CO 2 to Cu(I) and Cu(II) sites are essentially identical, in pronounced contrast to CO, which binds much stronger to Cu(I). © 2015 Elsevier Inc. All rights reserved. 1. Introduction Metaleorganic frameworks (MOFs) represent an expanding class of hybrid porous materials, and attract considerable attention from a wider, interdisciplinary, scientific community, because their large surface areas, adjustable pore sizes, and controllable surface properties make them suitable for various and numerous applica- tions [1e5]. MOFs consist of metal or metal-oxo nodes and organic linkers. MOFs that exhibit accessible coordinative unsaturated metal sites are particularly interesting for sensing and catalytic applications [6e8]. Such active sites not only improve the adsorp- tion properties with regard to hydrogen (H 2 ) [9e11], methane (CH 4 ) [12], carbon dioxide (CO 2 ) [8], or carbon monoxide (CO) gases [7,13], but also function as Lewis acid sites in catalytic applications [14,15]. Only certain types of MOFs possess such active metal sites in their regular lattice. A prominent example is [Cu 3 (BTC) 2 ] n , also known as HKUST-1. It consists of Cu(II) dimers and 1,3,5-benzenetricarboxylic acid (BTC) ligands. The paddle wheel unit, where four carboxylate groups from four distinct BTC ligands are coordinated around a Cu(II) dimer (Fig. 1), displays open metal sites at the axial positions of the Cu(II) dimer [16]. Active sites can also be generated via the introduction of de- fects, e.g. missing linkers. Wu and coworkers [17] have reported direct structural evidence in UiO-66 materials of such active defect sites due to the absence of linkers. Such defects strongly affect the CO 2 adsorption behavior of UiO-66, as shown by a striking CO 2 uptake enhancement with increased defect concentration. More recently, Fischer and coworkers have reported a series of defect- engineered MOFs (DEMOFs) made by adding small concentra- tions of linkers with a reduced number of carboxylate groups to the reactant solution [18,19]. They have also demonstrated that the reduction of carboxylate groups coordinated to the Ru 2 paddle * Corresponding author. Tel.: þ49 721 608 2 3934; fax: þ49 721 608 2 3478. E-mail address: christof.woell@kit.edu (C. W€ oll). Contents lists available at ScienceDirect Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso http://dx.doi.org/10.1016/j.micromeso.2014.12.033 1387-1811/© 2015 Elsevier Inc. All rights reserved. Microporous and Mesoporous Materials 207 (2015) 53e60