Systematic design and functionalisation of amorphous zirconium metal-organic frameworks.

Controlling the structure and functionality of crystalline metal-organic frameworks (MOFs) using molecular building units and post-synthetic functionalisation presents challenges when extending this approach to their amorphous counterparts (aMOFs). Here, we present a new bottom-up approach for synthesising a series of Zr-based aMOFs, which involves linking metal-organic clusters with specific ligands to regulate local connectivity. In addition, we overcome the limitations of post-synthetic modifications in amorphous systems, demonstrating that homogeneous functionalisation is achievable even without regular internal voids. By altering the acidity of the side group, length, and degree of connectivity of the linker, we could control the porosity, proton conductivity, and mechanical properties of the resulting aMOFs.

Gd Metal-Organic Framework Thin Film for On-Chip Local Magnetic Refrigeration.

Dense metal-organic frameworks with high spin paramagnetic nodes are competitive materials for cryogenic magnetic refrigeration, particularly in applications for which local cooling is advantageous. We focus on obtaining thin films of gadolinium formate, which has a large volumetric magnetocaloric effect. Continuous and homogeneous deposits of gadolinium formate are successfully formed on silicon by means of aerosol jet printing, with control over the film thickness from 0.35 µm up to 2.5 µm. The excellent cooling power of the deposits is evidenced via direct measurements of the cooling of a 200 µm silicon wafer down to sub-K temperatures by a single demagnetization from 1 T and 2 K, thereby demonstrating the potential of this approach for on-chip local magnetic refrigeration.

Mind the Gap: The Role of Mass Transfer in Shaped Nanoporous Adsorbents for Carbon Dioxide Capture.

Adsorptive separations by nanoporous materials are major industrial processes. The industrial importance of solid adsorbents is only expected to grow due to the increased focus on carbon dioxide capture technology and energy-efficient separations. To evaluate the performance of an adsorbent and design a separation process, the adsorption thermodynamics and kinetics must be known. However, although diffusion kinetics determine the maximum production rate in any adsorption-based separation, this aspect has received less attention due to the challenges associated with conducting diffusion measurements. These challenges are exacerbated in the study of shaped adsorbents due to the presence of porosity at different length scales. As a result, adsorbent selection typically relies mainly on adsorption properties at equilibrium, i.e., uptake capacity, selectivity and adsorption enthalpy. In this Perspective, based on an extensive literature review on mass transfer of CO2 in nanoporous adsorbents, we discuss the importance and limitations of measuring diffusion in nanoporous materials, from the powder form to the adsorption bed, considering the nature of the process, i.e., equilibrium-based or kinetic-based separations. By highlighting the lack of and discrepancies between published diffusivity data in the context of CO2 capture, we discuss future challenges and opportunities in studying mass transfer across scales in adsorption-based separations.

Chemical Bonding and Crystal Structure Schemes in Atomic/Molecular Layer Deposited Fe-Terephthalate Thin Films.

Advanced deposition routes are vital for the growth of functional metal-organic thin films. The gas-phase atomic/molecular layer deposition (ALD/MLD) technique provides solvent-free and uniform nanoscale thin films with unprecedented thickness control and allows straightforward device integration. Most excitingly, the ALD/MLD technique can enable the in situ growth of novel crystalline metal-organic materials. An exquisite example is iron-terephthalate (Fe-BDC), which is one of the most appealing metal-organic framework (MOF) type materials and thus widely studied in bulk form owing to its attractive potential in photocatalysis, biomedicine, and beyond. Resolving the chemistry and structural features of new thin film materials requires an extended selection of characterization and modeling techniques. Here we demonstrate how the unique features of the ALD/MLD grown in situ crystalline Fe-BDC thin films, different from the bulk Fe-BDC MOFs, can be resolved through techniques such as synchrotron grazing-incidence X-ray diffraction (GIXRD), Mössbauer spectroscopy, and resonant inelastic X-ray scattering (RIXS) and crystal structure predictions. The investigations of the Fe-BDC thin films, containing both trivalent and divalent iron, converge toward a novel crystalline Fe(III)-BDC monoclinic phase with space group C2/c and an amorphous Fe(II)-BDC phase. Finally, we demonstrate the excellent thermal stability of our Fe-BDC thin films.

3D printing of ferromagnetic passive shims for field shaping in magnetic resonance imaging.

Magnetic Resonance Imaging (MRI) often encounters image quality degradation due to magnetic field inhomogeneities. Conventional passive shimming techniques involve the manual placement of discrete magnetic materials, imposing limitations on correcting complex inhomogeneities. To overcome this, we propose a novel 3D printing method utilizing binder jetting technology to enable precise deposition of a continuous range of concentrations of ferromagnetic ink. This approach grants complete control of the magnitude of the magnetic moment within the passive shim enabling tailored corrections of B0 field inhomogeneities. By optimizing the magnetic field distribution using linear programming and an in-house written Computer-Aided Design (CAD) generation software, we printed shims with promising results in generating low spherical harmonic corrections. Experimental evaluations demonstrate feasibility of these 3D printed passive shims to induce target magnetic fields corresponding to second-order spherical harmonic, as evidenced by acquired B0 maps. The electrically insulating properties of the printed shims eliminate the risk of eddy currents and heating, thus ensuring safety. The dimensional fabrication accuracy of the printed shims surpasses previous methods, enabling more precise and localized correction of subject-specific inhomogeneities. The findings highlight the potential of binder-jetted 3D printed passive shims in MRI shimming as a versatile and efficient solution for fabricating passive shims, with the potential to enhance the quality of MRI imaging while also being applicable to other types of Magnetic Resonance systems.

Polymorphism and orientation control of copper-dicarboxylate metal-organic framework thin films through vapour- and liquid-phase growth.

Precise control over the crystalline phase and crystallographic orientation within thin films of metal-organic frameworks (MOFs) is highly desirable. Here, we report a comparison of the liquid- and vapour-phase film deposition of two copper-dicarboxylate MOFs starting from an oriented metal hydroxide precursor. X-ray diffraction revealed that the vapour- or liquid-phase reaction of the linker with this precursor results in different crystalline phases, morphologies, and orientations. Pole figure analysis showed that solution-based growth of the MOFs follows the axial texture of the metal hydroxide precursor, resulting in heteroepitaxy. In contrast, the vapour-phase method results in non-epitaxial growth with uniplanar texture only.

Vapor-assisted synthesis of the MOF-74 metal-organic framework family from zinc, cobalt, and magnesium oxides.

In this work, we investigate the vapor-assisted synthesis of the metal-organic framework MOF-74 starting from three metal oxides (ZnO, CoO, and MgO). Depending on the nature of the added vapor (H2O, DMF, DMSO), the metal oxide, and the temperature, the outcome of the reaction can be directed towards the desired porous phase. Ex situ and in situ XRD measurements reveal the formation of an intermediate phase during the reaction of MgO with H4dobdc, while the MOF-74 phase forms directly for ZnO and CoO. The reduced CO2 uptake of the resulting materials compared to solvothermally prepared MOFs might be offset by the convenience of the presented route and the promise of a high space time yield.

Metal-Free Electrocatalytic Diacetoxylation of Alkenes.

1,2-Dioxygenation of alkenes leads to a structural motif ubiquitous in organic synthons, natural products and active pharmaceutical ingredients. Straightforward and green synthesis protocols starting from abundant raw materials are required for facile and sustainable access to these crucial moieties. Especially industrially abundant aliphatic alkenes have proven to be arduous substrates in sustainable 1,2-dioxygenation methods. Here, we report a highly efficient electrocatalytic diacetoxylation of alkenes under ambient conditions using a simple iodobenzene mediator and acetic acid as both the solvent and an atom-efficient reactant. This transition metal-free method is applicable to a wide range of alkenes, even challenging feedstock alkenes such as ethylene and propylene, with a broad functional group tolerance and excellent faradaic efficiencies up to 87 %. In addition, this protocol can be extrapolated to alkenoic acids, resulting in cyclization of the starting materials to valuable lactone derivatives. With aromatic alkenes, a competing mechanism of direct anodic oxidation exists which enables reaction under catalyst-free conditions. The synthetic method is extensively investigated with cyclic voltammetry.

Comparison of Thin-Film Capacitor Geometries for the Detection of Volatile Organic Compounds Using a ZIF-8 Affinity Layer.

Their chemical diversity, uniform pore sizes, and large internal surface areas make metal-organic frameworks (MOFs) highly suitable for volatile organic compound (VOC) adsorption. This work compares two geometries of capacitive VOC sensors that use the MOF material ZIF-8 as an affinity layer. When using a permeable top electrode (thickness < 25 nm), the metal-insulator-metal (MIM) sandwich configuration exhibits superior sensitivity, an improved detection limit, and a smaller footprint than the conventional interdigitated electrode layout. Moreover, the transduction of VOC adsorption in ZIF-8 via MIM capacitors is more sensitive to polar VOCs and provides better selectivity at high loadings than gravimetric and optical transductions.

Fabrication of 3D Oriented MOF Micropatterns with Anisotropic Fluorescent Properties.

Micropatterning crystalline materials with oriented pores is necessary for the fabrication of devices with anisotropic properties. Crystalline and porous metal-organic frameworks (MOFs) are ideal materials as their chemical and structural mutability enables precise tuning of functional properties for applications ranging from microelectronics to photonics. Herein, a patternable oriented MOF film is designed: by using a photomask under X-ray exposure, the MOF film decomposes in the irradiated areas, remaining intact in the unexposed regions. The MOF film acts simultaneously as a resist and as functional porous material. While the heteroepitaxial growth from aligned Cu(OH)2 nanobelts is used to deposit oriented MOF films, the sensitivity to radiation is achieved by integrating a brominated dicarboxylate ligand (Br2 BDC) into a copper-based MOF Cu2 L2 DABCO (DABCO = 1,4-diazabicyclo[2.2.2]octane; L = BDC/Br2 BDC). The lithographed samples act as diffraction gratings upon irradiation with a laser, thus confirming the quality of the extended MOF micropattern. Furthermore, the oriented MOF patterns are functionalized with fluorescent dyes. As a result, by rotating the polarization angle of the laser excitation, the alignment of the dye in the MOF is demonstrated. By controlling the functional response to light, this MOF patterning protocol can be used for the microfabrication of optical components for photonic devices.

On the gelation of humins: from transient to covalent networks.

Humins are a by-product of many acid-catalyzed biorefinery processes converting polysaccharides into platform chemicals. The valorization of humin residue to increase the profit of biorefinery operations and reduce waste is a field that is growing interest as the production of humins continues to increase. This includes their valorization in materials science. For successful processing of humin-based materials, this study aims to understand the thermal polymerization mechanisms of humins from a rheological perspective. Thermal crosslinking of raw humins leads to an increase in their molecular weight, which in turn leads to the formation of a gel. Humin's gels structure combines physical (thermally reversible) and chemical (thermally irreversible) crosslinks, and temperature plays an essential role in the crosslink density and the gel properties. High temperatures delay the formation of a gel due to the scission of physicochemical interactions, drastically decreasing their viscosity, whereas upon cooling a stronger gel is formed combining the recovered physicochemical bonds and the newly created chemical crosslinks. Thus, a transition from a supramolecular network to a covalently crosslinked network is observed, and properties such as the elasticity or reprocessability of humin gels are influenced by the stage of polymerization.

Nanoporous Metal-Organic Framework Thin Films Prepared Directly from Gaseous Precursors by Atomic and Molecular Layer Deposition: Implications for Microelectronics.

Atomic/molecular layer deposition (ALD/MLD) allows for the direct gas-phase synthesis of crystalline metal-organic framework (MOF) thin films. Here, we show for the first time using krypton and methanol physisorption measurements that ALD/MLD-fabricated copper 1,4-benzenedicarboxylate (Cu-BDC) ultrathin films possess accessible porosity matching that of the corresponding bulk MOF.

Chemical Vapor Deposition and High-Resolution Patterning of a Highly Conductive Two-Dimensional Coordination Polymer Film.

Crystalline coordination polymers with high electrical conductivities and charge carrier mobilities might open new opportunities for electronic devices. However, current solvent-based synthesis methods hinder compatibility with microfabrication standards. Here, we describe a solvent-free chemical vapor deposition method to prepare high-quality films of the two-dimensional conjugated coordination polymer Cu-BHT (BHT = benzenehexanothiolate). This approach involves the conversion of a metal oxide precursor into Cu-BHT nanofilms with a controllable thickness (20-85 nm) and low roughness (<10 nm) through exposure to the vaporized organic linker. Moreover, the restricted metal ion mobility during the vapor-solid reaction enables high-resolution patterning via both bottom-up lithography, including the fabrication of micron-sized Hall bar and electrode patterns to accurately evaluate the conductivity and mobility values of the Cu-BHT films.

Vapor-Assisted Conversion of Heterobimetallic Titanium-Organic Framework Thin Films.

Heterobimetallic Metal-Organic Frameworks (MOFs) synergically combine the properties of two metal ions, thus offering significant advantages over homometallic MOFs in gas storage, separation, and catalysis, among other applications. However, these remain centered on bulk materials, while applications that require functional coatings on solid supports are not developed. We explore for the first time the deposition of heterometallic Ti-based MOF thin films using vapor-assisted conversion on substrates functionalized with a self-assembled monolayer. Furthermore, metal-induced dynamic topological transformation allows the conversion of MUV-10(Ca) films into MUV-101(Co) and MUV-102(Cu), which is not accessible through direct synthesis, without morphologically altering the films. These nonconventional thin-film deposition techniques enable homogeneous and crystalline coatings of heterometallic titanium MOFs that also maintain their corresponding porosity.

Vapor-Assisted Powder Synthesis and Oriented MOF-CVD Thin Films of the Metal-Organic Framework HKUST-1.

A vapor-assisted synthesis method was developed for the metal-organic framework (MOF) HKUST-1 in both powder and film format. The use of a solvent template supplied from the vapor phase is essential to form the framework under these conditions. Chemical vapor deposition of HKUST-1 films (MOF-CVD) results in smooth films that show the expected adsorption behavior. Interestingly, the HKUST-1 films obtained this way show a (111) preferred crystallographic orientation.

Vapor-Phase Loading of an Ionic Liquid into a Zeolitic Imidazolate Framework.

Composites formed by a metal-organic framework (MOF) and an ionic liquid (IL) are potentially interesting materials for applications ranging from gas separation to electrochemical devices. Consequently, there is a need for robust and low-cost preparation procedures that are compatible with the desired applications. We herein report a solvent-free, one-step, and vapor-based ship-in-bottle synthesis of the IL@MOF composite 1-butyl-3-methylimidazolium bromide@ZIF-8 in powder and thin film forms. In this approach, volatile IL precursors evaporate and subsequently adsorb and react within the MOF cages to form the IL.

How Reproducible are Surface Areas Calculated from the BET Equation?

Porosity and surface area analysis play a prominent role in modern materials science. At the heart of this sits the Brunauer-Emmett-Teller (BET) theory, which has been a remarkably successful contribution to the field of materials science. The BET method was developed in the 1930s for open surfaces but is now the most widely used metric for the estimation of surface areas of micro- and mesoporous materials. Despite its widespread use, the calculation of BET surface areas causes a spread in reported areas, resulting in reproducibility problems in both academia and industry. To prove this, for this analysis, 18 already-measured raw adsorption isotherms were provided to sixty-one labs, who were asked to calculate the corresponding BET areas. This round-robin exercise resulted in a wide range of values. Here, the reproducibility of BET area determination from identical isotherms is demonstrated to be a largely ignored issue, raising critical concerns over the reliability of reported BET areas. To solve this major issue, a new computational approach to accurately and systematically determine the BET area of nanoporous materials is developed. The software, called "BET surface identification" (BETSI), expands on the well-known Rouquerol criteria and makes an unambiguous BET area assignment possible.

Vapor-Phase Processing of Metal-Organic Frameworks.

ConspectusPorous metal-organic frameworks (MOFs), formed from organic linkers and metal nodes, have attracted intense research attention. Because of their high specific surface areas, uniform and adjustable pore sizes, and versatile physicochemical properties, MOFs have shown disruptive potential in adsorption, catalysis, separation, etc. For many of these applications, MOFs are synthesized solvothermally as bulk powders and subsequently shaped as pellets or extrudates. Other applications, such as membrane separations and (opto)electronics, require the implementation of MOFs as (patterned) thin films. Most thin-film formation methods are adapted from liquid-phase synthesis protocols. Precursor transport and nucleation are difficult to control in these cases, often leading to particle formation in solution. Moreover, the use of solvents gives rise to environmental and safety challenges, incompatibility issues with some substrates, and corrosion issues in the case of dissolved metal salts. In contrast, vapor-phase processing methods have the merits of environmental friendliness, control over thickness and conformality, scalability in production, and high compatibility with other workflows.In this Account, we outline some of our efforts and related studies in the development and application of vapor-phase processing of crystalline MOF materials (MOF-VPP). We first highlight the advances and mechanisms in the vapor-phase deposition of MOFs (MOF-VPD), mainly focusing on the reactions between a linker vapor and a metal-containing precursor layer. The characteristics of the obtained MOFs (thickness, porosity, crystallographic phase, orientation, etc.) and the correlation of these properties with the deposition parameters (precursors, temperatures, humidity, post-treatments, etc.) are discussed. Some in situ characterization methods that contributed to a fundamental understanding of the involved mechanisms are included in the discussion. Second, four vapor-phase postsynthetic functionalization (PSF) methods are summarized: linker exchange, guest loading, linker grafting, and metalation. These approaches eliminate potential solubility issues and enable fast diffusion of reactants and guests as well as a high loading or degree of exchange. Vapor-phase PSF provides a platform to modify the MOF porosity or even introduce new functionalities (e.g., luminescence photoswitching and catalytic activity). Third, since vapor-phase processing methods enable the integration of MOF film deposition into a (micro)fabrication workflow, they facilitate a range of applications with improved performance (low-k dielectrics, sensors, membrane separations, etc.). Finally, we provide a discussion on the limitations, challenges, and further opportunities for MOF-VPP. Through the discussion and analysis of the vapor-phase processing strategies as well as the underlying mechanisms in this Account, we hope to contribute to the development of the controllable synthesis, functionalization, and application of MOFs and related materials.

Chemical Vapor Deposition of Ionic Liquids for the Fabrication of Ionogel Films and Patterns.

Film deposition and high-resolution patterning of ionic liquids (ILs) remain a challenge, despite a broad range of applications that would benefit from this type of processing. Here, we demonstrate for the first time the chemical vapor deposition (CVD) of ILs. The IL-CVD method is based on the formation of a non-volatile IL through the reaction of two vaporized precursors. Ionogel micropatterns can be easily obtained via the combination of IL-CVD and standard photolithography, and the resulting microdrop arrays can be used as microreactors. The IL-CVD approach will facilitate leveraging the properties of ILs in a range of applications and microfabricated devices.

Surfactant-assisted synthesis of titanium nanoMOFs for thin film fabrication.

We use dodecanoic acid as a modulator to yield titanium MOF nanoparticles with good control of size and colloid stability and minimum impact to the properties of the framework to enable direct fabrication of crystalline, porous thin films.