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Small‐angle X‐ray scattering as a multifaceted tool for structural characterization of covalent organic frameworks
Author(s) -
Maia Renata Avena,
Carneiro Leonardo Simões de Abreu,
Cifuentes Jhonny Mauricio Cerón,
Buarque Camilla Djenne,
Esteves Pierre Mothé,
Percebom Ana Maria
Publication year - 2020
Publication title -
journal of applied crystallography
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.429
H-Index - 162
ISSN - 1600-5767
DOI - 10.1107/s1600576720011553
Subject(s) - small angle x ray scattering , microporous material , materials science , scattering , mesoporous material , porosity , characterization (materials science) , small angle scattering , nanostructure , crystallography , chemical engineering , nanotechnology , composite material , chemistry , optics , organic chemistry , physics , engineering , catalysis
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 N 2 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.