Metal–Organic Frameworks as Active Materials in Electronic Sensor Devices

In the past decade, advances in electrically conductive metal–organic frameworks (MOFs) and MOF-based electronic devices have created new opportunities for the development of next-generation sensors. Here we review this rapidly-growing field, with a focus on the different types of device configurations that have allowed for the use of MOFs as active components of electronic sensor devices

Cation exchange at the secondary building units of metal–organic frameworks

Cation exchange is an emerging synthetic route for modifying the secondary building units (SBUs) of metal–organic frameworks (MOFs). This technique has been used extensively to enhance the properties of nanocrystals and molecules, but the extent of its applications for MOFs is still expanding. To harness cation exchange as a rational tool, we need to elucidate its governing factors. Not nearly enough experimental observations exist for drawing these conclusions, so we provide a conceptual framework for approaching this task. We address which SBUs undergo exchange, why certain ions replace others, how the framework influences the process, the role of the solvent, and current applications. Using these guidelines, certain trends emerge from the available data and missing experiments become obvious. If future studies follow this framework, then a more comprehensive body of observations will furnish a deeper understanding of cation exchange and inspire future applications.United States. Dept. of Energy. Office of Basic Energy Sciences (Award DE-SC0006937)3M CompanyAlfred P. Sloan FoundationResearch Corporation for Science AdvancementNational Science Foundation (U.S.). Graduate Research Fellowship (Grant 1122374)MISTI (Hayashi Seed Fund

Selective formation of biphasic thin films of metal–organic frameworks by potential-controlled cathodic electrodeposition

Cathodic electrodeposition lends itself to the formation of biphasic metal–organic framework thin films at room temperature from single deposition baths using potential bias as the main user input. Depending on the applied potential, we selectively deposit two different phases as either bulk mixtures or bilayer films.United States. Dept. of Energy. Office of Basic Energy Sciences (Award DE-SC0006937)3M CompanyMISTI (Hayashi Seed Fund)National Science Foundation (U.S.). Graduate Research Fellowship (Grant 1122374

Measuring and Reporting Electrical Conductivity in Metal–Organic Frameworks: Cd

Electrically conductive metal–organic frameworks (MOFs) are emerging as a subclass of porous materials that can have a transformative effect on electronic and renewable energy devices. Systematic advances in these materials depend critically on the accurate and reproducible characterization of their electrical properties. This is made difficult by the numerous techniques available for electrical measurements and the dependence of metrics on device architecture and numerous external variables. These challenges, common to all types of electronic materials and devices, are especially acute for porous materials, whose high surface area make them even more susceptible to interactions with contaminants in the environment. Here, we use the anisotropic semiconducting framework Cd₂(TTFTB) (TTFTB⁴⁻ = tetrathiafulvalene tetrabenzoate) to benchmark several common methods available for measuring electrical properties in MOFs. We show that factors such as temperature, chemical environment (atmosphere), and illumination conditions affect the quality of the data obtained from these techniques. Consistent results emerge only when these factors are strictly controlled and the morphology and anisotropy of the Cd2(TTFTB) single-crystal devices are taken into account. Most importantly, we show that depending on the technique, device construction, and/or the environment, a variance of 1 or even 2 orders of magnitude is not uncommon for even just one material if external factors are not controlled consistently. Differences in conductivity values of even 2 orders of magnitude should therefore be interpreted with caution, especially between different research groups comparing different compounds. These results allow us to propose a reliable protocol for collecting and reporting electrical properties of MOFs, which should help improve the consistency and comparability of reported electrical properties for this important new class of crystalline porous conductors.United States. Department of Energy. Office of Basic Energy Sciences (Award DE-SC0006937)National Science Foundation (U.S.) (Award 1122374

Moisture Farming with Metal-Organic Frameworks

In a recent issue of Science, Kim et al. describe a device that captures water vapor from the atmosphere at low relative humidity by using a metal-organic framework as the active sorbent. This first-of-its-kind water harvester can be powered by only solar thermal energy

Chemiresistive Sensor Arrays from Conductive 2D Metal–Organic Frameworks

Applications of porous metal–organic frameworks (MOFs) in electronic devices are rare, owing in large part to a lack of MOFs that display electrical conductivity. Here, we describe the use of conductive two-dimensional (2D) MOFs as a new class of materials for chemiresistive sensing of volatile organic compounds (VOCs). We demonstrate that a family of structurally analogous 2D MOFs can be used to construct a cross-reactive sensor array that allows for clear discrimination between different categories of VOCs. Experimental data show that multiple sensing mechanisms are operative with high degrees of orthogonality, establishing that the 2D MOFs used here are mechanistically unique and offer advantages relative to other known chemiresistor materials.Camille & Henry Dreyfus Foundation. Postdoctoral Program in Environmental ChemistrAlfred P. Sloan FoundationResearch Corporation for Science Advancement3M CompanyNational Science Foundation (U.S.). Graduate Research Fellowship Program (Grant 1122374)Massachusetts Institute of Technology. Institute for Soldier Nanotechnologie

Dynamic Structural Flexibility of Fe-MOF-5 Evidenced by ⁵⁷Fe Mössbauer Spectroscopy

Temperature-dependent ⁵⁷Fe Mössbauer spectra were collected on Fe[subscript x]Zn[subscript 4−x](1,4-benzenedicarboxylate)₃ (Fe-MOF-5). When measured under an Ar atmosphere, the data at higher temperatures reveal thermal population of the lowest-lying electronic excited state, as expected for low symmetry tetrahedral ferrous ions. In the presence of N₂, however, the temperature dependence becomes exaggerated and the spectra cannot be fitted to a single species. A fluctuating electric field gradient at the Fe nuclei best explains these data and suggests dynamic structural distortions induced by weak interactions with N₂. This direct evidence of dynamic behaviour at MOF open metal sites is relevant for the use of MOF SBUs in catalysis, gas separation, and other applications that invoke similar phenomena

New talent: Americas

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Selective Dimerization of Ethylene to 1-Butene with a Porous Catalyst

Current heterogeneous catalysts lack the fine steric and electronic tuning required for catalyzing the selective dimerization of ethylene to 1-butene, which remains one of the largest industrial processes still catalyzed by homogeneous catalysts. Here, we report that a metal–organic framework catalyzes ethylene dimerization with a combination of activity and selectivity for 1-butene that is premier among heterogeneous catalysts. The capacity for mild cation exchange in the material MFU-4l (MFU-4l = Zn[subscript 5]Cl[subscript 4](BTDD)[subscript 3], H[subscript 2]BTDD = bis(1H-1,2,3-triazolo[4,5-b],[4′,5′-i])dibenzo[1,4]dioxin) was leveraged to create a well-defined and site-isolated Ni(II) active site bearing close structural homology to molecular tris-pyrazolylborate complexes. In the presence of ethylene and methylaluminoxane, the material consumes ethylene at a rate of 41,500 mol per mole of Ni per hour with a selectivity for 1-butene of up to 96.2%, exceeding the selectivity reported for the current industrial dimerization process.Saudi AramcoAmerican Society for Engineering Education. National Defense Science and Engineering Graduate Fellowship3M CompanyResearch Corporation for Science Advancement. Cottrell Scholars ProgramAlfred P. Sloan Foundatio

Electrochemical oxygen reduction catalysed by Ni3(hexaiminotriphenylene)2

Control over the architectural and electronic properties of heterogeneous catalysts poses a major obstacle in the targeted design of active and stable non-platinum group metal electrocatalysts for the oxygen reduction reaction. Here we introduce Ni[SUBSCRIPT 3](HITP)[SUBSCRIPT 2] (HITP=2, 3, 6, 7, 10, 11-hexaiminotriphenylene) as an intrinsically conductive metal-organic framework which functions as a well-defined, tunable oxygen reduction electrocatalyst in alkaline solution. Ni[SUBSCRIPT 3](HITP)[SUBSCRIPT 2] exhibits oxygen reduction activity competitive with the most active non-platinum group metal electrocatalysts and stability during extended polarization. The square planar Ni-N[SUBSCRIPT 4] sites are structurally reminiscent of the highly active and widely studied non-platinum group metal electrocatalysts containing M-N[SUBSCRIPT 4] units. Ni[SUBSCRIPT 3](HITP)[SUBSCRIPT 2] and analogues thereof combine the high crystallinity of metal-organic frameworks, the physical durability and electrical conductivity of graphitic materials, and the diverse yet well-controlled synthetic accessibility of molecular species. Such properties may enable the targeted synthesis and systematic optimization of oxygen reduction electrocatalysts as components of fuel cells and electrolysers for renewable energy applications.United States. Department of Energy. Office of Basic Energy Sciences (Award DESC0006937