Confined Water Cluster Formation in Water Harvesting by Metal-Organic Frameworks: CAU-10-H versus CAU-10-CH3

Several metal-organic frameworks (MOFs) excel in harvesting water from the air or as heat pumps as they show a steep increase in water uptake at 10-30 % relative humidity (RH%). A precise understanding of which structural characteristics govern such behavior is lacking. Herein, CAU-10-H and CAU-10-CH3 are studied with H, CH3 corresponding to the functions grafted to the organic linker. CAU-10-H shows a steep water uptake ≈18 RH% of interest for water harvesting, yet the subtle replacement of H by CH3 in the organic linker drastically changes the water adsorption behavior to less steep water uptake at much higher humidity values. The materials' structural deformation and water ordering during adsorption with in situ sum-frequency generation, in situ X-ray diffraction, and molecular simulations are unraveled. In CAU-10-H, an energetically favorable water cluster is formed in the hydrophobic pore, tethered via H-bonds to the framework μOH groups, while for CAU-10-CH3, such a favorable cluster cannot form. By relating the findings to the features of water adsorption isotherms of a series of MOFs, it is concluded that favorable water adsorption occurs when sites of intermediate hydrophilicity are present in a hydrophobic structure, and the formation of energetically favorable water clusters is possible.

Promoting Photocatalytic Activity of NH2-MIL-125(Ti) for H2 Evolution Reaction through Creation of TiIII- and CoI-Based Proton Reduction Sites

Titanium-based metal-organic framework, NH2-MIL-125(Ti), has been widely investigated for photocatalytic applications but has low activity in the hydrogen evolution reaction (HER). In this work, we show a one-step low-cost postmodification of NH2-MIL-125(Ti) via impregnation of Co(NO3)2. The resulting Co@NH2-MIL-125(Ti) with embedded single-site CoII species, confirmed by XPS and XAS measurements, shows enhanced activity under visible light exposure. The increased H2 production is likely triggered by the presence of active CoI transient sites detected upon collection of pump-flow-probe XANES spectra. Furthermore, both photocatalysts demonstrated a drastic increase in HER performance after consecutive reuse while maintaining their structural integrity and consistent H2 production. Via thorough characterization, we revealed two mechanisms for the formation of highly active proton reduction sites: nondestructive linker elimination resulting in coordinatively unsaturated Ti sites and restructuring of single CoII sites. Overall, this straightforward manner of confinement of CoII cocatalysts within NH2-MIL-125(Ti) offers a highly stable visible-light-responsive photocatalyst.

Competitive and Cooperative CO2-H2O Adsorption through Humidity Control in a Polyimide Covalent Organic Framework

In order to capture and separate CO₂ from the air or flue gas streams through nanoporous adsorbents, the influence of the humidity in these streams has to be taken into account as it hampers the capture process in two main ways: (1) water preferentially binds to CO₂ adsorption sites and lowers the overall capacity, and (2) water causes hydrolytic degradation and pore collapse of the porous framework. Here, we have used a water-stable polyimide covalent organic framework (COF) in N₂/CO₂/H₂O breakthrough studies and assessed its performance under varying levels of relative humidity (RH). We discovered that at limited relative humidity, the competitive binding of H₂O over CO₂ is replaced by cooperative adsorption. For some conditions, the CO₂ capacity was significantly higher under humid versus dry conditions (e.g., a 25% capacity increase at 343 K and 10% RH). These results in combination with FT-IR studies on equilibrated COFs at controlled RH values allowed us to assign the effect of cooperative adsorption to CO₂ being adsorbed on single-site adsorbed water. Additionally, once water cluster formation sets in, loss of CO₂ capacity is inevitable. Finally, the polyimide COF used in this research retained performance after a total exposure time of >75 h and temperatures up to 403 K. This research provides insight in how cooperative CO₂-H₂O can be achieved and as such provides directions for the development of CO₂ physisorbents that can function in humid streams.

Structure-Property Relationship of Piezoelectric Properties in Zeolitic Imidazolate Frameworks: A Computational Study

Metal-organic frameworks (MOFs) are a class of nanoporous crystalline materials with very high structural tunability. They possess a very low dielectric permittivity ε_r due to their porosity and hence are favorable for piezoelectric energy harvesting. Even though they have huge potential as piezoelectric materials, a detailed analysis and structure-property relationship of the piezoelectric properties in MOFs are lacking so far. This work focuses on a class of cubic non-centrosymmetric MOFs, namely, zeolitic imidazolate frameworks (ZIFs) to rationalize how the variation of different building blocks of the structure, that is, metal node and linker substituents affect the piezoelectric constants. The piezoelectric tensor for the ZIFs is computed from ab initio theoretical methods. From the calculations, we analyze the different contributions to the final piezoelectric constant d₁₄, namely, the clamped ion (e₁₄⁰) and the internal strain (e₁₄^int) contributions and the mechanical properties. For the studied ZIFs, even though e₁₄ (e₁₄⁰ + e₁₄^int) is similar for all ZIFs, the resultant piezoelectric coefficient d₁₄ calculated from piezoelectric constant e₁₄ and elastic compliance constant s₄₄ varies significantly among the different structures. It is the largest for CdIF-1 (Cd²⁺ and -CH₃ linker substituent). This is mainly due to the higher elasticity or flexibility of the framework. Interestingly, the magnitude of d₁₄ for CdIF-1 is higher than II-VI inorganic piezoelectrics and of a similar magnitude as the quintessential piezoelectric polymer polyvinylidene fluoride.

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.

Emergence of Coupled Rotor Dynamics in Metal-Organic Frameworks via Tuned Steric Interactions

The organic components in metal–organic frameworks (MOFs) are unique: they are embedded in a crystalline lattice, yet, as they are separated from each other by tunable free space, a large variety of dynamic behavior can emerge. These rotational dynamics of the organic linkers are especially important due to their influence over properties such as gas adsorption and kinetics of guest release. To fully exploit linker rotation, such as in the form of molecular machines, it is necessary to engineer correlated linker dynamics to achieve their cooperative functional motion. Here, we show that for MIL-53, a topology with closely spaced rotors, the phenylene functionalization allows researchers to tune the rotors’ steric environment, shifting linker rotation from completely static to rapid motions at frequencies above 100 MHz. For steric interactions that start to inhibit independent rotor motion, we identify for the first time the emergence of coupled rotation modes in linker dynamics. These findings pave the way for function-specific engineering of gear-like cooperative motion in MOFs.

Synthesis and Structure-Property Relationships of Polyimide Covalent Organic Frameworks for Carbon Dioxide Capture and (Aqueous) Sodium-Ion Batteries

Covalent organic frameworks (COFs) are an emerging material family having several potential applications. Their porous framework and redox-active centers enable gas/ion adsorption, allowing them to function as safe, cheap, and tunable electrode materials in next-generation batteries, as well as CO2 adsorption materials for carbon-capture applications. Herein, we develop four polyimide COFs by combining aromatic triamines with aromatic dianhydrides and provide detailed structural and electrochemical characterization. Through density functional theory (DFT) calculations and powder X-ray diffraction, we achieve a detailed structural characterization, where DFT calculations reveal that the imide bonds prefer to form at an angle with one another, breaking the 2D symmetry, which shrinks the pore width and elongates the pore walls. The eclipsed perpendicular stacking is preferable, while sliding of the COF sheets is energetically accessible in a relatively flat energy landscape with a few metastable regions. We investigate the potential use of these COFs in CO2 adsorption and electrochemical applications. The adsorption and electrochemical properties are related to the structural and chemical characteristics of each COF, giving new insights for advanced material designs. For CO2 adsorption specifically, the two best performing COFs originated from the same triamine building block, which-in combination with force-field calculations-revealed unexpected structure-property relationships. Specific geometries provide a useful framework for Na-ion intercalation with retainable capacities and stable cycle life at a relatively high working potential (>1.5 V vs Na/Na+). Although this capacity is low compared to conventional inorganic Li-ion materials, we show as a proof of principle that these COFs are especially promising for sustainable, safe, and stable Na-aqueous batteries due to the combination of their working potentials and their insoluble nature in water.

Pillared cobalt metal-organic frameworks act as chromatic polarizers

The ease with which molecular building blocks can be ordered in metal-organic frameworks is an invaluable asset for many potential applications. In this work, we exploit this inherent order to produce chromatic polarizers based on visible-light linear dichroism via cobalt paddlewheel chromophores.

Rotational Dynamics of Linkers in Metal⁻Organic Frameworks

Among the numerous fascinating properties of metal⁻organic frameworks (MOFs), their rotational dynamics is perhaps one of the most intriguing, with clear consequences for adsorption and separation of molecules, as well as for optical and mechanical properties. A closer look at the rotational mobility in MOF linkers reveals that it is not only a considerably widespread phenomenon, but also a fairly diverse one. Still, the impact of these dynamics is often understated. In this review, we address the various mechanisms of linker rotation reported in the growing collection of literature, followed by a highlight of the methods currently used in their study, and we conclude with the impacts that such dynamics have on existing and future applications.

Emergence of Nonlinear Optical Activity by Incorporation of a Linker Carrying the p-Nitroaniline Motif in MIL-53 Frameworks

p-Nitroaniline presents the typical motif of a second-order nonlinear optically (NLO) active molecule. However, because of its crystallization in an antiparallel and hence centrosymmetric structure, the NLO activity is lost. In this contribution, the p-nitroaniline motif was built successfully into the MIL-53 metal-organic framework. More precisely, MIL-53 was synthesized with 2-amino-5-nitroterephthalate as organic linker, with Al³⁺, Ga³⁺, or In³⁺ as inorganic cation. The Al and Ga structures are polar, as confirmed by second-harmonic generation microscopy, yielding stable NLO materials. Indeed, they contain a 22-36% surplus of the dipolar 2-amino-5-nitro-terephthalate oriented in a parallel fashion. The indium compound was shown to be less crystalline and centrosymmetric. Ab initio modeling of the second-order NLO response shows that the Al and Ga materials show a response comparable to typical inorganic commercial NLO materials such as KDP. As a hybrid material, capable of low-temperature synthesis and processing and the ultrafast NLO responses associated with organic materials, this material can potentially provide an interesting venue for applications with respect to traditional inorganic NLO materials.

Conformational Changes of a Surface-Tethered Polymer during Radical Growth Probed with Second-Harmonic Generation

The surface-induced polymerization of a chromophore-functionalized monomer was probed in situ for the first time using a nonlinear optical technique, second-harmonic generation. During the first hours of the polymerization reaction, dramatic changes in the tilt angle of the chromophore-functionalized side groups were observed. Following evaluation of the nonlinear optical data with those obtained from atomic force microscopy and ultraviolet-visible, we conclude that second-harmonic generation efficiently probes the polymerization reaction and the conformational changes of the surface-grafted polymer. With polymerization time, the conformation of the surface-tethered polymer changes from a conformation with the polymer backbone and its side groups flat on the surface, i.e., a "pancake" conformation, to a conformation where the polymer backbone is stretched away combined with tilted side groups or an enlarged tilt angle distribution, i.e., a "brush-type" conformation.

Contact Forces between Single Metal Oxide Nanoparticles in Gas-Phase Applications and Processes

In this work we present a comprehensive experimental study to determine the contact forces between individual metal oxide nanoparticles in the gas-phase using atomic force microscopy. In addition, we determined the amount of physisorbed water for each type of particle surface. By comparing our results with mathematical models of the interaction forces, we could demonstrate that classical continuum models of van der Waals and capillary forces alone cannot sufficiently describe the experimental findings. Rather, the discrete nature of the molecules has to be considered, which leads to ordering at the interface and the occurrence of solvation forces. We demonstrate that inclusion of solvation forces in the model leads to quantitative agreement with experimental data and that tuning of the molecular order by addition of isopropanol vapor allows us to control the interaction forces between the nanoparticles.

Electronic origins of photocatalytic activity in d0 metal organic frameworks

Metal-organic frameworks (MOFs) containing d⁰ metals such as NH₂-MIL-125(Ti), NH₂-UiO-66(Zr) and NH₂-UiO-66(Hf) are among the most studied MOFs for photocatalytic applications. Despite structural similarities, we demonstrate that the electronic properties of these MOFs are markedly different. As revealed by quantum chemistry, EPR measurements and transient absorption spectroscopy, the highest occupied and lowest unoccupied orbitals of NH₂-MIL-125(Ti) promote a long lived ligand-to-metal charge transfer upon photoexcitation, making this material suitable for photocatalytic applications. In contrast, in case of UiO materials, the d-orbitals of Zr and Hf, are too low in binding energy and thus cannot overlap with the π* orbital of the ligand, making both frontier orbitals localized at the organic linker. This electronic reconfiguration results in short exciton lifetimes and diminishes photocatalytic performance. These results highlight the importance of orbital contributions at the band edges and delineate future directions in the development of photo-active hybrid solids.

Second harmonic generation microscopy reveals hidden polar organization in fluoride doped MIL-53(Fe)

Polar metal-organic frameworks have potential applications as functional non-linear optical, piezoelectric, pyroelectric and ferroelectric materials. Using second harmonic generation microscopy we found that fluoride doping of the microporous iron(iii) terephthalate MOF MIL-53(Fe) induces a polar organization in its structure, which was not previously detected with XRD. The polar order is only observed when both fluoride and guest molecules are present, and may be related to a complex interplay between the adsorbates and the framework, leading to a modification of the positioning of fluoride in the inorganic Fe-chains. Combined polarized second harmonic generation microscopy and scanning pyroelectric microscopy show that the polar axis is unidirectional and of the same sense over the whole crystal, extending up to 100 micrometers. This finding shows how MOF materials can be endowed with useful properties by doping MOFs with fluoride.

Organic Linker Defines the Excited-State Decay of Photocatalytic MIL-125(Ti)-Type Materials

Recently, MIL-125(Ti) and NH2 -MIL-125(Ti), two titanium-based metal-organic frameworks, have attracted significant research attention in the field of photocatalysis for solar fuel generation. This work reveals that the differences between these structures are not only based on their light absorption range but also on the decay profile and topography of their excited states. In contrast to MIL-125(Ti), NH2 -MIL-125(Ti) shows markedly longer lifetimes of the charge-separated state, which improves photoconversion by the suppression of competing decay mechanisms. We used spectroelectrochemistry and ultrafast spectroscopy to demonstrate that upon photoexcitation in NH2 -MIL-125(Ti) the electron is located in the Ti-oxo clusters and the hole resides on the aminoterephthalate unit, specifically on the amino group. The results highlight the role of the amino group in NH2 -MIL-125(Ti), the electron donation of which extends the lifetime of the photoexcited state substantially.

Unraveling a two-step oxidation mechanism in electrochemical Cu-MOF synthesis

To employ the full potential of electrochemical (ec) synthesis to grow metal–organic frameworks (MOFs) in more complex organizations at the mesoscale, it is vital to understand the underlying crystallization reaction pathway.

In situ orientation-sensitive observation of molecular adsorption on a liquid/zeolite interface by second-harmonic generation

The inherently surface-specific technique of second-harmonic generation was employed to probe the adsorption of an organic molecule, a hemicyanine dye, on b-oriented silicalite-1 films in situ. Measurements were performed in a purpose-built cell for solution experiments. By measuring at two different polarization combinations of the fundamental and second-harmonic light, the orientation of the adsorbed molecules was measured continuously. It has been observed that the adsorbed molecules gradually align themselves with the straight pores of the zeolite crystallites, thus adsorbing into the pores.