Oriented Nanoscale Films of Metal–Organic Frameworks By Room‐Temperature Gel‐Layer Synthesis

Abstract

Metal-organic frameworks (MOFs) assembled from organic building blocks and metal-based connectors have attracted much interest owing to their large pores and their enormous structural diversity. Particularly, the generation of homogeneous thin films of MOFs is highly desirable in view of potential applications including chemical sensors, catalysts, and also optical devices. Several elegant examples of MOF film growth have recently been reported (see below), but a general and convenient method for the synthesis of homogeneous oriented thin films is not known. Herein we present a novel strategy for the growth of highly oriented thin films of MOFs based on the storage of one reaction partner for framework synthesis in a polymer gel layer, followed by diffusion of the other reaction partner into the gel layer and to a nucleation interface provided by a functionalized selfassembled monolayer (Scheme 1).The unique structural and chemical features of metalorganic frameworks (MOFs) [1] make them attractive hosts for gas storage, [2] gas purification, [3] catalysis, [4] separation, [5] and chemical sensing.[6] MOFs consist of molecular linkers and vertices that are typically occupied by molecular complexes or cluster entities (secondary building units; SBUs).[7] MOFs can be either rigid or flexible, depending on the linker component, the free space within the structure, and on different host-guest interactions. The extreme sensitivity and selectivity of flexible MOFs towards guest molecules is a promising feature regarding possible applications such as chemical sensors.[8] For chemical sensing, membrane separations, optical devices, and for the assembly of complex nanoscale structures, it would be highly desirable to obtain thin oriented MOF films. Two different methods for the synthesis of MOF thin films have been recently developed: direct growth from solvothermally pretreated solutions, [9][10][11] and step-by-step growth from molecular precursors. [12] Thin-film synthesis of MOFs from solvothermally pretreated solutions was recently realized in the cases of [Zn 4 O(BDC) 3 ] (BDC 2À = 1,4-benzenedicarboxylate; MOF-5), [9] [Cu 3 (BTC) 2 ] (BTC 3À = 1,3,5-benzenetricarboxylate; HKUST-1), [10] and flexible [Fe 3 O(BDC) 3 (OAc)] (Fe-MIL-88B).[11] Depending on the synthetic procedure, the films have more-or-less densely packed oriented crystals or islands of crystals. An alternative step-by-step growth concept based on alternating exposure to the molecular linkers and metal ions was also developed, thus generating very thin and homogeneous layers of HKUST-1.[10] To the best of our knowledge, the step-by-step approach has only been applied to rigid MOF structures containing an [M 2 (CO 2 ) 4 ] (M = Zn 2+ , Cu 2+) paddlewheel motif as the SBU.Based on the insights obtained from the recent work on MOF film synthesis, we aimed at developing an alternative, more general approach that would allow us to supply the reactants at high concentrations into a thin layer close to the nucleating surface of the functionalized self-assemb...

Tuning optical properties of MOF-based thin films by changing the ligands of MOFs

“…The R a of MIL-101(Cr), NH 2 -MIL-101(Cr), and NO 2 -MIL-101(Cr) thin films are only 3.4, 3.0, and 4.2 nm, respectively. The R q of them are 4.4, 3.8, and 5.5 nm, which are smaller than those of MOF-based thin films in previous reports [18,34,35]. For instance, Demessence et al [18] reported that the root-mean-square roughness of MIL-101(Cr) optical thin films was calculated to be around 12 nm (nearly half of the MOF NP diameter).…”

“…For instance, Demessence et al [18] reported that the root-mean-square roughness of MIL-101(Cr) optical thin films was calculated to be around 12 nm (nearly half of the MOF NP diameter). The root-mean-square surface roughness of gel-layer synthesized Fe-MIL-88B_NH 2 thin film was 5-9 nm (from atomic force microscope images) [34]. These results suggest that the fabricated MOF optical thin films are of high quality.…”

Synthetic routes toward MOF nanomorphologies

“…The surfactant N-ethyl perfluorooctylsulfonamide (N-EtFOSA) forms microemulsions in a solvent mixture of the IL 1,1,3,3-tetramethylguanidinium acetate (TMGA) and SCCO 2 , as the interactions between CO 2 and the fluorocarbon tails of the surfactants are strong. 53,54 In a standard synthesis procedure, Zn(NO 3 ) 2 , 1,4-benzenedicarboxylic acid (H 2 bdc) and N-EtFOSA were added to TMGA and heated in a high-pressure cell under 16.8 MPa CO 2 pressure at 80 C for 48 h. Transmission electron microscopy (TEM) images (Fig. 2) show nanoparticles of roughly 80 nm diameter with a uniform size distribution and a well ordered system of mesopores.…”

Metal‐Organic Frameworks: Coordination Polymer Nanoparticles and Macrostructures

“…Schoedel et al used a thin poly(ethylene oxide) gel layer deposited on a gold slide as a storage medium to confine a high concentration of metal ions near to a nucleating surface functionalized with self-assembled monolayers. 85 Very homogeneous PCP thin films were obtained in unique crystal orientation on COOH-functionalized SAMs after synthesis in the presence of the organic linker. Other groups reported the deposition of [Cu 3 (btc) 2 ] monodisperse crystals in patterns down to the single-crystallite level by the use of soft lithographic 86 and inkjet printing techniques.…”