Control of structural flexibility of layered-pillared metal-organic frameworks anchored at surfaces.
ABSTRACT: Flexible metal-organic frameworks (MOFs) are structurally flexible, porous, crystalline solids that show a structural transition in response to a stimulus. If MOF-based solid-state and microelectronic devices are to be capable of leveraging such structural flexibility, then the integration of MOF thin films into a device configuration is crucial. Here we report the targeted and precise anchoring of Cu-based alkylether-functionalised layered-pillared MOF crystallites onto substrates via stepwise liquid-phase epitaxy. The structural transformation during methanol sorption is monitored by in-situ grazing incidence X-ray diffraction. Interestingly, spatially-controlled anchoring of the flexible MOFs on the surface induces a distinct structural responsiveness which is different from the bulk powder and can be systematically controlled by varying the crystallite characteristics, for instance dimensions and orientation. This fundamental understanding of thin-film flexibility is of paramount importance for the rational design of MOF-based devices utilising the structural flexibility in specific applications such as selective sensors.
Project description:Optical properties, which determine the application of optical devices in different fields, are the most significant properties of optical thin films. In recent years, Metal-organic framework (MOF)-based optical thin films have attracted increasing attention because of their novel optical properties and important potential applications in optical and photoelectric devices, especially optical thin films with tunable optical properties. This study reports the first example of tuning the optical properties of a MOF-based optical thin film via post-modification. The MOF-based optical thin film was composed of NH?-MIL-53(Al) nanorods (NRs) (MIL: Materials from Institute Lavoisier), and was constructed via a spin-coating method. Three aldehydes with different lengths of carbon chains were chosen to modify the MOF optical thin film to tune their optical properties. After post-modification, the structural color of the NH?-MIL-53(Al) thin film showed an obvious change from purple to bluish violet and cyan. The reflection spectrum and the reflectivity also altered in different degrees. The effective refractive index (<i>n</i><sub>eff</sub>) of MOFs thin film can also be tuned from 1.292 to 1.424 at a wavelength of 750 nm. The success of tuning of the optical properties of MOFs thin films through post-modification will make MOFs optical thin films meet different needs of optical properties in various optical and optoelectronic devices.
Project description:Dynamic and flexible metal-organic frameworks (MOFs) that respond to external stimuli, such as stress, light, heat, and the presence of guest molecules, hold promise for applications in chemical sensing, drug delivery, gas separations, and catalysis. A greater understanding of the relationship between flexible constituents in MOFs and gas adsorption may enable the rational design of MOFs with dynamic moieties and stimuli-responsive behavior. Here, we detail the effect of subtle structural changes upon the gas sorption behavior of two "SIFSIX" pillared square grid frameworks, namely SIFSIX-3-M (M = Ni, Fe). We observe a pronounced inflection in the Xe adsorption isotherm in the Ni variant. With evidence from X-ray diffraction studies, density functional theory, and molecular simulations, we attribute the inflection to a disordered to ordered transition of the rotational configurations of the pyrazine rings induced by sorbate-sorbent interactions. We also address the effect of cage size, temperature, and sorbate on the guest-induced ring rotation and the adsorption isotherms. The absence of an inflection in the Xe adsorption isotherm in SIFSIX-3-Fe and in the Kr, N2, and CO2 adsorption isotherms in SIFSIX-3-Ni suggest that the inflection is highly sensitive to the match between the size of the cage and the guest molecule.
Project description:Stimuli-responsive behaviors of flexible metal-organic frameworks (MOFs) make these materials promising in a wide variety of applications such as gas separation, drug delivery, and molecular sensing. Considerable efforts have been made over the last decade to understand the structural changes of flexible MOFs in response to external stimuli. Uniform pore deformation has been used as the general description. However, recent advances in synthesizing MOFs with non-uniform porous structures, i.e. with multiple types of pores which vary in size, shape, and environment, challenge the adequacy of this description. Here, we demonstrate that the CO2-adsorption-stimulated structural change of a flexible MOF, ZIF-7, is induced by CO2 migration in its non-uniform porous structure rather than by the proactive opening of one type of its guest-hosting pores. Structural dynamics induced by guest migration in non-uniform porous structures is rare among the enormous number of MOFs discovered and detailed characterization is very limited in the literature. The concept presented in this work provides new insights into MOF flexibility.
Project description:Metal-organic frameworks (MOFs) have been recognized as compelling platforms for the development of miscellaneous applications because of their structural diversity and functional tunability. Here, we propose that the electrocatalytic properties could be well modified by incorporating missing linkers into the MOF. Theoretical calculations suggest the electronic structure of MOFs can be tuned by introducing missing linkers, which improves oxygen evolution reaction (OER) performance of the MOF. Inspired by these aspects, we introduced various missing linkers into a layered-pillared MOF Co2(OH)2(C8H4O4) (termed as CoBDC) to prepare missing-linker MOFs. Transmission electron microscope and synchrotron X-ray measurements confirmed that the missing linkers in the MOF could be introduced and well controlled by our strategy. The self-supported MOF nanoarrays with missing linkers of carboxyferrocene exhibit excellent OER performance with ultralow overpotential of 241?mV at 100?mA?cm-2. This work opens a new prospect to develop efficient MOF-based electrocatalysts by introducing missing linkers.
Project description:Homogeneous metal-organic frameworks (MOFs)-based optical thin films have attracted increasing attention, since they can potentially be used as active components in optical/opt-electrical devices, and how to fabricate MOF thin films with high quality is the premise of practically using them. Herein, five fabrication methods of MOF films are systematically investigated and compared from the aspects of appearance, reflectivity, micro-morphology, surface roughness, and optical properties of the films. The famous robust Zr-based MOF, UiO-66 (UiO = University of Oslo) is chosen as a model, and the five methods are spin-coating, dip-coating, self-assembly, direct growth, and the stepwise layer by layer growth method. This study provides fundamental support for the application of MOFs in the optical field.
Project description:The engineering of metal-organic frameworks (MOFs) into membranes and films is being investigated, to transform laboratory-synthesized MOFs into industrially viable products for a range of attractive applications. However, rational design and construction of highly permeable MOF thin films, without trade-offs in terms of structural mechanical stability, remains a significant challenge. Herein, a simple, general strategy is reported to prepare thin MOF nanosheet (NS)-assembled frame film via heteroepitaxial growth from metal hydroxide film. As the thin MOF NS-assembled film significantly enhances the permeability of mass though the film, the resultant gold nanoparticle (Au NP)@MOF film exhibits much higher catalytic efficiency than the Au NP@MOF bulk film. Meanwhile, the unique framework of the MOF NS-assembled film resists torsion and collapse, so the composite catalyst exhibits long-term stability.
Project description:Among different types of polymorphism, disappearing polymorphism deals with the metastable kinetic form which can not be reproduced after its first isolation. In the world of coordination polymers (CPs) and metal-organic frameworks (MOFs), despite the fact that many types of supramolecular isomerism exist, we are unaware of disappearing supramolecular isomerism akin to disappearing polymorphism. This work reports a MOF with dia topology that could not be reproduced, but subsequent synthesis yielded another supramolecular isomer, a double-pillared-layer MOF. When perylene was added in the same reaction, the disappeared dia MOF reappeared with perylene as a guest in the channels. Interestingly, the photoluminescence of the dia MOF with a perylene guest is dominated by the emission of the guest molecule. The influence of guest molecules on the stabilization of the supramolecular isomers of a MOF opens up a strategy to access MOFs with different structures.
Project description:Pillared paddle-wheel-based metal-organic framework (MOF) materials are an attractive target as they offer a reliable method for constructing well-defined, multifunctional materials. A drawback of these materials, which has limited their application, is their tendency to form catenated frameworks with little accessible volume. To eliminate this disadvantage, it is necessary to investigate strategies for constructing non-catenated pillared paddle-wheel MOFs. Hydrogen-bonding substituents on linkers have been postulated to prevent catenation in certain frameworks and, in this work, we present a new MOF to further bolster this theory. Using 2,2'-diamino-[1,1'-biphenyl]-4,4'-dicarboxylic acid, BPDC-(NH2)2, linkers and dipyridyl glycol, DPG, pillars, we assembled a MOF with pcu topology. The new material is non-catenated, exhibiting large accessible pores and low density. To the best of our knowledge, this material constitutes the pcu framework with the largest pore volume and lowest density. We attribute the lack of catenation to the presence of H-bonding substituents on both linkers.
Project description:The layered 2D MOFs, owing to their enhanced flexibility and tunability, have recently emerged as a promising alternative to the 3D microporous MOFs in the quest for novel responsive functional materials. However, maintaining the simultaneous control over self-assembly of molecular building blocks as well as ordered stacking of MOF layers poses a significant synthetic challenge. We report on the controlled 2D MOF formation based on a case study of solvent-templated growth of a series of 2D Cu(II)-carboxylate MOFs varying in stacking modes and distances using a diffusion-controlled MOF deposition approach in various solvent mixtures. Moreover, we demonstrate the structural dynamics of the developed 2D MOFs involving both in-plane and out-of-plane movements of the individual 2D layers triggered by solvent exchange, which allowed for selective postsynthetic transformations between the developed 2D MOFs. We also investigated the gas adsorption properties of the developed MOFs, which demonstrates a remarkable crystal size effect on the N2 adsorption capacity using a model 2D MOF system.
Project description:Thin-film formation and transport properties of two copper-paddlewheel metal-organic framework (MOF) -based systems (MOF-14 and MOF-399) are investigated for their potential integration into electrochemical device architectures. Thin-film analogs of these two systems are fabricated by the sequential, alternating, solution-phase deposition of the inorganic and organic ligand precursors that result in conformal films via van der Merwe-like growth. Atomic force microscopy reveals smooth film morphologies with surface roughnesses determined by the underlying substrates and linear film growth of 1.4 and 2.2 nm per layer for the MOF-14 and MOF-399 systems, respectively. Electrochemical impedance spectroscopy is used to evaluate the electronic transport properties of the thin films, finding that the MOF-14 analog films demonstrate low electronic conductivity, while MOF-399 analog films are electronically insulating. The intrinsic porosities of these ultrathin MOF analog films are confirmed by cyclic voltammetry redox probe characterization using ferrocene. Larger peak currents are observed for MOF-399 analog films compared to MOF-14 analog films, which is consistent with the larger pores of MOF-399. The layer-by-layer deposition of these systems provides a promising route to incorporate MOFs as thin films with nanoscale thickness control and low surface roughness for electrochemical devices.