Small-angle X-ray scattering as a multifaceted tool for structural characterization of covalent organic frameworks

Abstract

Small-angle X-ray scattering (SAXS) is an accurate nondestructive method that requires a minimum of sample preparation and is employed to study porosity, morphology and hierarchical structures. Zeolites and silica are among the porous materials that are widely investigated by SAXS. However, studies of covalent organic frameworks (COFs) are still scarce. In the present study, SAXS was employed to investigate meso- and microporous COFs, affording insightful information about their nanostructure textural properties. SAXS is especially useful when combined with other characterization techniques, such as powder X-ray diffraction and N2 adsorption isotherms, emerging as an efficient tool to further characterize COFs. For microporous COFs, SAXS was used mainly to obtain quantitative values of surface roughness as a function of fractal parameters, in all cases indicating surface fractals of the large-scale scattering object, namely the `grain'. Mesoporous COF studies allowed elucidation of their hexagonal structure on the basis of their structure peaks; however, the main result lies in the distinction between the pore and the grain, which are described as a hierarchical structure by the Beaucage model and evaluated according to their fractality. These COFs generally exhibit pores with mass fractal features and grains with surface fractal features when they are submitted to post-functionalization, which may be due to the poor diffusivity of the functionalizing agents into the pores. In addition, a proposed aggregation description of the porous scattering objects was envisioned, based on small-angle scattering premises, which was confirmed for a microporous COF by high-resolution transmission electron microscopy.

The order–disorder conundrum: a trade-off between crystalline and amorphous porous organic polymers for task-specific applications

“…In general, the spatial arrangement of the functional building units in the 3D macromolecular architectures from nano to macroscale governs the application potential of POPs. In this regard, the advanced structural and morphological characterisation tools, such as solid-state 2D NMR, 277,278 small- and wide-angle X-ray diffraction, 47,279 high-resolution electron microscopy, 280 and X-ray tomography 123 techniques, can shed light on the underlying configurational and supramolecular arrangements of the building units in the resultant framework or network (Fig. 31).…”

25 Jahre retikuläre Chemie

“…In dieser Hinsicht wurden dynamische Lichtstreuung (DLS) und Fluoreszenzkorrelationsspektroskopie (FCS) angewendet, um die Grçßenverteilung von in flüssigen Medien suspendierten Partikeln abzuleiten, [140] während Informationen über die Formanisotropie durch Kleinwinkel-und Weitwinkel-Rçntgenstreuung (SAXS/WAXS,small/wide-angle X-ray scattering) gewonnen wurden. [141] Insbesondere die Verwendung von WAXS,o ft verallgemeinert als PXRD (powder X-ray diffraction), ermçglicht es,d ie Grçßen von Partikeln von denen der Kristalldomänen zu unterscheiden. Obwohl diese manchmal übereinstimmen, wachsen MOFs und COFs oft in polykristallinen Aggregaten, die sich in DLS-und FCS-Experimenten wie einzelne Partikel verhalten und deren einkristalline Eigenschaften selten durch Mikroskopie beurteilt werden kçnnen.…”

Pore morphology and topology of zeolite imidazolate framework ZIF‐67 revealed by small‐angle X‐ray scattering

“…However, the disadvantages of scattering method are requirement of the appropriate modeling to the scattering data and the pre‐knowledge of material characteristics analyzed. The SAXS pore characterization of similar highly porous systems 25,26 such as MOFs and covalent organic frameworks (COFs), as the model of interpretation could play an important role, which is still very few. The present study presents the pore characterization of ZIF‐67 structure as model system using SAXS analysis for the first time.…”