Low-Dose Electron Crystallography: Structure Solution and Refinement

Two-Dimensional Metal–Organic Frameworks with Unique Oriented Layers for Oxygen Reduction Reaction: Tailoring the Activity through Exposed Crystal Facets

Towards modeling spatiotemporal processes in metal–organic frameworks

Porous materials have become a burgeoning research interest in materials science because of their intrinsic porous characteristics, versatile chemical compositions, and abundant functionalities. Re...

Metal-organic frameworks (MOFs) are characterized by metal nodes (inorganic building units, IBUs) linked by organic molecules (linker) resulting in a 3D network with pores that allow them to easily interact, absorb and release liquid and gas phases such as moisture, solvents... [1].These materials are very challenging to investigate due to their tendency to crystallize as microcrystalline powders.Therefore, methods such as 3-dimensional electron diffraction (3DED) and XRPD are excellent tools to achieve structure determination [2].In this research, we report the use of Transmission Electron Microscopy (TEM) to attain the structure solution of two new MOFs (Cu-and Zn-DPC), built by a new linker molecule.Both compounds were initially characterized using XRPD with the aim of determining their crystal structure.However, this approach was not successful.Therefore, 3DED was used to achieve the crystal structure through direct methods.One of the challenges of the study of these DPCs using 3DED is their beam and vacuum sensitivity.Since they contain guest molecules, upon introducing them inside the TEM, their structure starts to change due to water and/or solvent loss.These alterations result in a decrease in crystallinity, variation of length of the crystallographic axes, shearing of the cell, loss of volume and coordination change of the metallic cations.Therefore, in order to enable an ab initio structure solution, fast data acquisition is essential.In this project, using Fast Automated Electron Diffraction Tomography (Fast-ADT) we were able to achieve structure solution of such complex samples with room temperature measurements in an FEI TECNAI-F30 STWIN transmission electron microscope equipped with a ULTRASCAN4000 CCD camera.While the structures of these compounds feature similar building blocks, there are significant differences in their resulting networks; CuDPC can be described as a layered 2D coordination polymer, while ZnDPC features a layered IBU, but the interconnection through the linker molecules results in a 3D network.Both materials change within the TEM environment as a function of the time: the CuDPC shows a reduction of the distance between the IBUs accompanied with a shrinking of the lattice parameter b, and overall cell volume reduction upon removal of H 2 O from the IBU and the coordination environment of the copper ions (which changes from square pyramidal to square planar), while ZnDPC also shows lattice shrinking and volume reduction to a smaller extent, with -angle changes being the more evident variation in the structure.

The last few years have seen a revolution in the field of 3D electron diffraction or diffraction tomography. We have moved from only acquiring a few low index zone axis patterns to full tomographic data sets recording all accessible areas of reciprocal space. These new larger data sets have made it easier for structure solution techniques such as direct methods from the x-ray world to be applied to the electron diffraction data for structure solution. While structure solution with tomographic electron diffraction is non trivial when compared to the x-ray case it is significantly easier than it was a few years ago. Mugnaioli et al. We are now in a situation where the most difficult and time consuming step can be the assignment of the space group to a data set. Electron diffraction has many advantages over the x-ray case in terms of the manner in which we can manipulate the electron beam. This allows the collection to convergent beam diffraction (CBD) or large angle convergent beam diffraction (LACBED) patterns, via the recently developed technique by Beanland et al. These techniques can make the assignment of space group significantly easier affair, and the path to structure solution a lot smoother. We will present the combination of data from tomographic, selected area (SA) and nano-beam (NBD) datasets, with diffraction from tomographic LACBED experiments where using the strengths of each technique can be leveraged for a much quicker route to structure solution.

Porous materials are of high importance, a.o. in the field of catalysis. A typical example of relevant and well-known crystalline, microporous materials is zeolites. These stable aluminosilicates possess tuneable Lewis and Brønsted acidity, in combination with microporous channels and/or cages allowing for size and shape selectivity. As a result, these zeolites are used for many catalytic applications, including large industrial processes such as fluid catalytic cracking (FCC). Another class of highly crystalline porous solids, namely metal-organic frameworks (MOFs), have gained a lot of attention over the last few decades. These materials, built-up of metal nodes and organic linkers, are interesting due to their huge versatility: material properties, such as pore size and Lewis acidity, can be tuned to its specific application by simply tailoring its building blocks. Comparable to zeolites, MOFs can be used for a wide range of applications, such as catalysis, separation and in electronic (sensing) devices. Of both zeolites and MOFs, a large number of framework structures are known. Most of these compounds have been developed through the process of trial-and-error and/or educated guesses. Ultimately, one wishes to apply true rational design to the development of catalysts for novel applications. The required fundamental understanding of crystal growth needed for such purposes is unfortunately still lacking in literature. To study nucleation and growth behaviour, a model system apt for near-field characterisation is needed. As a result, 2D thin-film structures have been developed to facilitate high control over, and easy analysis of, crystal growth. Additionally, by converting these porous materials into thin-films, the gate-way to (membrane) specific applications is opened. To gain fundamental insights into catalytic thin-films, and to establish structure-performance correlations, we apply multiple (in-situ) characterization techniques. Initially, hetero-epitaxial growth processes were probed to uncover crystal nucleation and growth mechanisms. This real-time in-situ research was performed on HKUST-1 surface-mounted MOFs, or SURMOFs. During a one-pot synthesis between metal-ions and linkers, the surface topology was continually measured using atomic force microscopy (AFM). On a single (10 x 10 µm2) AFM spot we could follow the formation of MOF islands over multiple timeframes. Using a tailor-made script, these (growing) islands were labelled and tracked over all timeframes. From this, a (temperature-dependent) growth rate could be established. Next to structural information, the chemical nature of the formed MOF grains was uncovered using infra-red reflection absorption spectroscopy (IRRAS) at grazing angles, a highly sensitive technique providing data from monolayer coverage and up. Additionally, a novel photo-induced force microscopy (PiFM, or AFM-IR) technique was applied to these SURMOFs, which is able in distinguishing grain specific chemical nature of the nm-sized MOF islands. This knowledge of nucleation and growth behaviour can be applied to the production of high-quality SURMOFs. Whereas the direct synthesis provides a facile medium for in-situ AFM investigations, a layer-by-layer synthesis offers high control over SURMOF fabrication quality. This synthesis, in which the growth phases of the inorganic and organic compounds are separated, is a perfect model system for the systematic study of growth parameters and their effect of resulting SURMOF quality. Using IRRAS, we can fully identify peaks in the HKUST-1 SURMOF spectrum, including precursor species, which were previously unknown in literature. Additionally, in contrast to previously posed growth mechanisms, our (AFM-)IR studies show growth to occur through a solution-mediated mechanism. Moreover, AFM-IR can be used to directly probe defect formation on the nm-scale, allowing for catalytic structure-performance correlations. In accordance with the found mechanism, existing synthesis procedures can be optimized in terms of quality and speed. For zeolite ZSM-5, an innovative approach to enhance catalytic performance is to prepare homo-epitaxially grown zeolite ZSM-5 as continuous, oriented films with accessibility to only the straight zeolite channels. Applying advanced characterization techniques, such as tip-enhanced Raman spectroscopy (TERS), single-molecule fluorescence microscopy as well as grazing incidence characterization methods, we can explore the synthesis mechanisms, as well as study 3-D diffusion and related reactivity through a single pore orientation of zeolite ZSM-5 down to the level of a single molecule. Furthermore, we show the methanol-to-hydrocarbons reaction on ZSM-5 thin-films can be studied with AFM-IR. AFM-IR is highly sensitive to the surface chemical structure with regards to Al3+ framework incorporation as well as hydrocarbon formation, providing a more complete nanoscale picture of the structural and catalytic information. These combined case studies show the strength of microscopy and spectroscopy tools in understanding and improving catalytic materials. Figure 1

We present a controlled fabrication of selective ultrathin metal–organic framework (MOF) nanosheets as preassembling platforms, yolk–shell structured with a few-layered N-doped carbon (NC) shell-en...

SummaryAccurate prediction of the solubility of chemical substances in solvents remains a challenge. The sparsity of high-quality solubility data is recognized as the biggest hurdle in the development of robust data-driven methods for practical use. Nonetheless, the effects of the quality and quantity of data on aqueous solubility predictions have not yet been scrutinized. In this study, the roles of the size and the quality of data sets on the performances of the solubility prediction models are unraveled, and the concepts of actual and observed performances are introduced. In an effort to curtail the gap between actual and observed performances, a quality-oriented data selection method, which evaluates the quality of data and extracts the most accurate part of it through statistical validation, is designed. Applying this method on the largest publicly available solubility database and using a consensus machine learning approach, a top-performing solubility prediction model is achieved.

NaN3, as a kind of rich-nitrogen energetic material, engaged in the synthesis of hierarchical transition metal (Co/Ni) organic framework materials (MOFs) through a facile hydrothermal reaction, during which aqueous N3 − function groups being as an unique ligand in preparation of novel coordination polymer with the existence of Co2+ (or Ni2+) salts and dicyandiamide. The obtained compounds (Co-eMOF, Ni-eMOF, NiCo-eMOF) were characterized by structure and composition analysis, specific surface area, thermogravimetric analysis (TGA) and crystal X-ray diffraction. The prepared products regardless of the metal cautions (Co, Ni, or coexist Co and Ni), perform a well three-dimensions (3D) nanostructures with spherical shaped monoliths, known as organic framework materials. The as-prepared products with developed porosity demonstrate a high surface area in range of 80∼130 m2/g. XRD patterns for these three examples show similar lattice crystalline, which to an extent matches well with the simulated MOF-74 materials, suggesting profound formation of oriented crystal growth. XPS reveals the compounds all equipped with high nitrogen content (54∼60 at%), which is largely hinging on the contribution from N3 − function groups, suggesting the superiority of NaN3 serving as the rich nitrogen precursor. Accounting for extensive application, we will apply this as-prepared (Co/Ni) MOF materials as one kind of nitrogen-rich precursor and conduct them for further calcination treatment for preparation of graphite-based transition metal carbides (TMC), and it is highly anticipated to achieve efficient activity towards electrocatalysis in our follow-up actions.

Understanding excited-stateprocesses is essential fordesigningnew functional organic materials. Modeling excited states in organiccrystals is challenging due to the need to balance localized and delocalizedprocesses and the competition between intramolecular and intermolecularinteractions. Cluster models have proven highly effective for describingweakly interacting organic crystals; however, nonperiodic calculationson periodic systems must account for mechanical and electrostaticcoupling to the crystal lattice, particularly in cases of extendedcoordination where covalent bonds are severed, such as in organicpolymers and metal–organic frameworks (MOFs). Point chargeembedding is a low-cost method for incorporating long-range electrostatics,enabling the consideration of long-range interactions using Ewaldembedding. Small clusters have been effective for modeling excited-stateprocesses in MOFs, yet embedding has rarely been included in suchstudies. In this work, we examine some of the challenges in describingexcited states in covalently connected organic crystals using ONIOM­(QM:QM’)embedding techniques across systems with increasing coordination:diC4–BTBT (an organic molecular crystal), polythiophene(an organic polymer), and two MOFs (QMOF-d29cec2 and MOF-5). We analyzethe effects of using different electronic structure methods, includingTDHF, TDDFT, ADC(2), and CC(2). One of the main challenges is thatembedded cluster models are susceptible to overpolarization near theQM:QM’ boundary. To address this, we assess the impact of differentcharge redistribution schemes (Z-N (N = 0, 3), RC, and RCD) and implement them in fromage. Additionally,we compare cluster and periodic models. We find that localized modelseffectively reproduce excited states in both nonconnected systems(diC4–BTBT) and fully connected MOFs, whereas polythiopheneremains the most challenging due to band conduction. The accuracyof vertical excitations, oscillator strengths, and simulated spectrais strongly influenced by model size, boundary charges, redistributionschemes, and level of theory. We further analyze the effect of vibrationalbroadening using the nuclear ensemble approach to predict the absorptionand emission spectra of MOF-5. Our results provide a heuristic guidefor nonperiodic studies of crystalline excited states, highlightingthe remarkable relationship between molecular crystals and MOFs, whichwill be explored in the future work.

During the last decade, the synthesis and application of metal-organic framework (MOF) nanosheets has received growing interest, showing unique performances for different technological applications. Despite the potential of this type of nanolamellar materials, the synthetic routes developed so far are restricted to MOFs possessing layered structures, limiting further development in this field. Here, a bottom-up surfactant-assisted synthetic approach is presented for the fabrication of nanosheets of various nonlayered MOFs, broadening the scope of MOF nanosheets application. Surfactant-assisted preorganization of the metallic precursor prior to MOF synthesis enables the manufacture of nonlayered Al-containing MOF lamellae. These MOF nanosheets are shown to exhibit a superior performance over other crystal morphologies for both chemical sensing and gas separation. As revealed by electron microscopy and diffraction, this superior performance arises from the shorter diffusion pathway in the MOF nanosheets, whose 1D channels are oriented along the shortest particle dimension.

Achieving high battery performance from low‐cost, easily synthesisable electrode materials is crucial for advancing energy storage technologies. Metal–organic frameworks (MOFs) combining inexpensive transition metals and organic ligands are promising candidates for high‐capacity cathodes. Iron‐chloranilate‐water frameworks are herein reported to be produced in aqueous media under mild conditions. Removal of reticular water from known [Fe2(CAN)3(H2O)4] ⋅ 4H2O yields a new supramolecular metal–organic framework (SMOF), [Fe2(CAN)3(H2O)4]. Removing coordination water, a new 2D honeycomb‐like MOF forms, Fe2(CAN)3, stable without counterions and solvent. This MOF adopts the unusual ABC layer‐stacking, as determined using a combination of ab initio random structure searching, electron diffraction, and Rietveld refinement of powder XRD data. Magnetometry, Mossbauer and Raman spectroscopy confirm that all three [Fe2(CAN)3(H2O)x]⋅yH2O phases contain HS‐Fe3+ and CAN2−, with magnetic ordering temperatures increasing (5→20 K) as the Fe−CAN connectivity increases. The SMOF and MOF show reversible (de)insertion of >4Li+/f.u. at average 2,59 V and 2,76 V vs Li+/Li, respectively. [Fe2(CAN)3] achieves 146 mAh/g at 1 C, thus specific energy (563 Wh/kg) and power (446 W/kg) in Li half‐cells competitive with conventional LiFePO4 (~580 Wh/kg and ~450 W/kg). Beyond Li, [Fe2(CAN)3] delivers 394 Wh/kg and 421 Wh/kg, for Na and K half‐cells respectively, becoming a competitive cathode for sustainable batteries.

The concept of crystal engineering is making crystals with a purpose and it is an indisputable fact that multicomponent molecular solids such as co-crystals, metal-organic frameworks (MOFs), and solid solutions are at the forefront of crystal engineering research. Multicomponent crystalline systems are important targets in the quest for novel solid forms that can show improvement of physical and chemical properties (solubility, intrinsic dissolution rate, morphology, thermal and hydration stability, etc.) compared to those of the parent components. As a result, multicomponent solids can find application in a variety of industrial fields.
\nThe research activity performed in my PhD was mainly dedicated to the investigations of multicomponent crystalline assemblies, namely molecular and ionic co-crystals and molecular solid solutions. The project can be subdivided into the following topics:
\n1.\tInvestigation of solid solutions consisting of organic molecules.
\n2.\tCo-crystallization as a tool to modify physicochemical properties of interest.
\n3.\tThe effect of co-crystallization on chirality and its possible application for chiral resolution.
\nThe first two parts of the project were focused on the search for novel multicomponent crystalline assemblies capable of modifying the physicochemical properties of interest: thermostability for the obtained molecular solid solutions and enhancement of the solubility and intrinsic dissolution rate characteristics for the ionic co-crystals. The final chapter of this project was dedicated to the investigation of the influence of co-crystallization of chiral molecules of pharmaceutical interest with inorganic salts.

Focused ion beam milling and MicroED structure determination of metal-organic framework crystals

Several studies suggest that metal ordering within metal-organic frameworks (MOFs) is important for understanding how MOFs behave in relevant applications; however, these siting trends can be difficult to determine experimentally. To garner insight into the energetic driving forces that may lead to nonrandom ordering within heterometallic MOFs, we employ density functional theory (DFT) calculations on several bimetallic metal-organic crystals composed of Nd and Yb metal atoms. We also investigate the metal siting trends for a newly synthesized MOF. Our DFT-based energy of mixing results suggest that Nd will likely occupy sites with greater access to electronegative atoms and that local homometallic domains within a mixed-metal Nd-Yb system are favored. We also explore the use of less computationally extensive methods such as classical force fields and cluster expansion models to understand their feasibility for large system sizes. This study highlights the impact of metal ordering on the energetic stability of heterometallic MOFs and crystal structures.