Insights in the Ionic Conduction inside Nanoporous Metal-Organic Frameworks by Using an Appropriate Equivalent Circuit

The conduction of protons and other ions in nanoporous materials, such as metal-organic frameworks (MOFs), is intensively explored with the aim of enhancing the performance of energy-related electrochemical systems. The ionic conductivity, as a key property of the material, is typically determined by using electrochemical impedance spectroscopy (EIS) in connection with a suitable equivalent circuit. Often, equivalent circuits are used where the physical meaning of each component is debatable. Here, we present an equivalent circuit for the ionic conduction of electrolytes in nanoporous, nonconducting materials between inert and impermeable electrodes without faradaic electrode reactions. We show the equivalent circuit perfectly describes the impedance spectra measured for the ion conduction in MOFs in the form of powders pressed into pellets as well as for MOF thin films. This is demonstrated for the ionic conduction of an aprotic ionic liquid, and of various protic solvents in different MOF structures. Due to the clear physical meaning of each element of the equivalent circuit, further insights into the electrical double layer forming at the MOF-electrode interface can be obtained. As a result, EIS combined with the appropriate reference circuit allows us to make statements of the quality of the MOF-substrate interface of different MOF-film samples

Metal‐Organic Framework Thin Films Grown on Functionalized Graphene as Solid‐State Ion‐Gated FETs

The unique properties of 2D-materials like graphene are exploited in various electronic devices. In sensor applications, graphene shows a very high sensitivity, but only a low specificity. This shortcoming can be mastered by using heterostructures, where graphene is combined with materials exhibiting high analyte selectivities. Herein, this study demonstrates the precise deposition of nanoporous metal-organic frameworks (MOFs) on graphene, yielding bilayers with excellent specificity while the sensitivity remains large. The key for the successful layer-by-layer deposition of the MOF films (SURMOFs) is the use of planar polyaromatic anchors. Then, the MOF pores are loaded with ionic liquid (IL). For functioning sensor devices, the IL@MOF films are grown on graphene field-effect transistors (GFETs). Adding a top-gate electrode yields an ion-gated GFET. Analysis of the transistor characteristics reveals a clear Dirac point at low gate voltages, good on-off ratios, and decent charge mobilities and densities in the graphene channel. The GFET-sensor reveals a strong and selective response. Compared to other ion-gated-FET devices, the IL@MOF material is relatively hard, allowing the manufacturing of ultrathin devices. The new MOF-anchoring strategy offers a novel approach generally applicable for the functionalization of 2D-materials, where MOF/2D-material hetero-bilayers carry a huge potential for a wide variety of applications

Programmed Molecular Assembly of Abrupt Crystalline Organic/Organic Heterointerfaces Yielding Metal‐Organic Framework Diodes with Large On‐Off Ratios

Structurally well‐defined, crystalline organic/organic heterojunctions between C$^{60-}$ and anthracene‐based semiconductors are realized via layer‐by‐layer deposition of metal‐organic framework, MOF, thin films. As demonstrated by X‐ray diffraction, perfect epitaxy is achieved by adjusting the lattice constants of the two different MOFs. Deposition of top electrodes allows to fabricate p–n as well as n–p devices. Measurements of the electrical properties reveal the presence of high‐performance diodes, with a current on/off ratio of up to 6 orders of magnitude and an ideality factor close to unity. The crystalline nature of the abrupt organic/organic heterojunction provides the basis for a rational, simulation‐based optimization and tailoring of such organic semiconductor interfaces

Conductivity measurement of ionic liquids confined in the nanopores of metal–organic frameworks: a case study for [BMIM][TFSI] in HKUST-1

Nanoporous materials like metal–organic frameworks (MOFs) attract considerable attention as porous host for electrolytes like ionic liquids (ILs). The conductivity and mobility of the ions in the pores are among the key properties and their experimental quantification is of paramount importance. Here, three different approaches for the quantification of the ion conductivity of IL@MOF via electrochemical impedance spectroscopy (EIS) are compared: the material in the form of IL-impregnated MOF powders pressed into pellets between two planar electrodes, MOF films grown on substrates with deposited electrodes loaded with IL by impregnation, and the IL-loaded MOF films where excess IL is removed. Contact-angle measurements and EIS data show that the excess IL on the outer MOF surface of the film or pellet results in apparent conductivities, larger than the intrinsic conductivity of the IL@MOF. Removing the excess IL enables the experimental quantification of the intrinsic IL@MOF conductivity

Stability and Degradation of Metal–Organic-Framework Films under Ambient Air Explored by Uptake and Diffusion Experiments

The loading with guest molecules is the key feature for most applications of metal–organic frameworks (MOFs). The limited stability of MOFs against environmental factors, like humid air, is often a severe problem which hinders real-life applications. Here, the stability of four common MOFs, UiO-66, UiO-67, HKUST-1, and ZIF-8, under long-term exposure to humid air and under exposure to water vapor is explored. Transient uptake experiments with toluene and other volatile organic compounds as probe molecules are combined with structural investigations via X-ray diffraction and infrared spectroscopy. In line with previous publications showing the structural stability of ZIF-8 and UiO-66, it is found that its uptake properties are not affected by exposure to air. On the other hand, HKUST-1 shows clear structural decomposition in air and degradation of the uptake properties. Unexpectedly, while the diffraction and spectroscopy data of UiO-67 do not suggest a corrosion of the structure upon air exposure, the guest-loading data show a strong decrease of the uptake amount and a deceleration of the uptake rate. Both features strongly indicate the formation of surface barriers for the mass transfer in UiO-67, like in HKUST-1. The study underlines the importance of transient uptake experiments for characterizing the stability of MOF materials

Sensitive Detection of a Gaseous Analyte with Low‐Power Metal–Organic Framework Functionalized Carbon Nanotube Transistors

A highly sensitive and low-power sensing platform for detecting ethanol molecules by interfacing high-purity, large-diameter semiconducting carbon nanotube transistors with a metal–organic framework layer is presented. The new devices outperform similar graphene-based metal–organic framework devices by several orders of magnitude in terms of sensitivity and power consumption, and can detect extremely low ethanol concentrations down to sub-ppb levels while consuming only picowatts of power. The exceptional sensor performance results from the nanotube transistor\u27s high on/off ratio and its sensitivity to charges, allowing for ultra-low power consumption. The platform can also compensate for shifts in threshold voltage induced by ambient conditions, making it suitable for use in humid air. This novel concept of MOF/CNTFETs could be customized for detecting various gaseous analytes, leading to a range of ultra-sensitive and ultra-low power sensors

Proton-conduction photomodulation in spiropyran-functionalized MOFs with large on–off ratio

Proton conduction in nanopores is important for applications in fuel cells, chemical sensors and information processing devices inspired by nature. Here, we present a nanoporous material, a metal–organic framework (MOF) thin film, allowing photomodulation of the aqueous and alcoholic proton conduction of the guests by almost two orders of magnitude. The MOF film possesses spiropyran groups which undergo reversible UV-light induced isomerization to the merocyanine form, a highly polar, zwitterionic molecule, where the strong binding of the guests to the merocyanine isomer efficiently suppresses the proton conduction. Such materials with photomodulated ionic conduction contribute to the development of advanced, remotecontrollable chemical sensors and to switchable devices interfacing with biological systems

Interplay of Electronic and Steric Effects to Yield Low‐Temperature CO Oxidation at Metal Single Sites in Defect‐Engineered HKUST‐1

In contrast to catalytically active metal single atoms deposited on oxide nanoparticles, the crystalline nature of metal‐organic frameworks (MOFs) allows for a thorough characterization of reaction mechanisms. Using defect‐free HKUST‐1 MOF thin films, we demonstrate that Cu$^{+}$/Cu$^{2+}$ dimer defects, created in a controlled fashion by reducing the pristine Cu2$^{+}$/Cu$^{2+}$ pairs of the intact framework, account for the high catalytic activity in low‐temperature CO oxidation. Combining advanced IR spectroscopy and density functional theory we propose a new reaction mechanism where the key intermediate is an uncharged O$_{2}$ species, weakly bound to Cu$^{+}$/Cu$^{2+}$. Our results reveal a complex interplay between electronic and steric effects at defect sites in MOFs and provide important guidelines for tailoring and exploiting the catalytic activity of single metal atom sites