Spatio‐Chemical Heterogeneity of Defect‐Engineered Metal–Organic Framework Crystals Revealed by Full‐Field Tomographic X‐ray Absorption Spectroscopy

Abstract

The introduction of structural defects in metalorganic frameworks (MOFs), often achieved through the fractional use of defective linkers,i se merging as am eans to refine the properties of existing MOFs.T hese linkers,m issing coordination fragments,c reate unsaturated framework nodes that may alter the properties of the MOF.Aproperty-targeted utilization of this approach demands an understanding of the structure of the defect-engineered MOF.W ed emonstrate that full-field X-raya bsorption near-edge structure computed tomography can help to improve our understanding.T his was demonstrated by visualizing the chemical heterogeneity found in defect-engineered HKUST-1 MOF crystals.Anonuniform incorporation and zonation of the defective linker was discovered, leading to the presence of clusters of as econd coordination polymer within HKUST-1. The former is suggested to be responsible,i np art, for altered MOF properties; thereby, advocating for aspatio-chemically resolved characterization of MOFs.

Visualizing defects and pore connectivity within metal–organic frameworks by X-ray transmission tomography

“…This higher intensity variance on the surface is due to the presence of less intense (porous) voxels as well as high intensity outlier regions (see Section 13 of the ESI †). Our hypothesis is that these high intensity regions correspond to vanadium clusters that precipitate during crystallization which would be in line with the ndings of Ferreira-Sanchez et al 56 It is worth noting that a strong variation in the concentration of elements (atomic number, Z) could also cause such variation as the X-ray absorption coefficient (note that m f Z 4 ), although the purity of the V precursor used was >99%. Hence, it seems unlikely the widespread presence of lighter or heavier elements throughout the crystal.…”

“…Recently, a rst X-ray 3D study of HKUST-1 MOFS was published by Ferreira-Sanchez et al, addressing issues such as spatially-resolved metal speciation and by-product phase formation 56 However, this work focused on chemical heterogeneities in the crystal and not on porosity. Despite the tremendous importance of this property in the applications in which MOFs are typically used, to the best of our knowledge, no detailed studies focusing on porosity have been reported yet.…”

Confinement of Luminescent Guests in Metal–Organic Frameworks: Understanding Pathways from Synthesis and Multimodal Characterization to Potential Applications of LG@MOF Systems

“…Related to the above is scanning tunnelling X-ray microscopy (STXM), a spectro-microscopy technique resembling SEM/TEM imaging, but enables one to perform nanoscale chemical imaging using X-ray photons . Another 3-D imaging technique demonstrated was full-field tomographic XAS, where a defect-engineered HKUST-1 single crystal (∼50 μm) as studied to visualize its spatio-chemical heterogeneities and identify secondary phases at a resolution of ∼2 μm. It will be exciting to explore the use of synchrotron 3-D imaging tools to interrogate LG@MOF systems; for example, to visualize micro/nanostructural defects and to quantify chemical heterogeneities of NCs@MOF and QDs@MOF composite crystals, as well as to interrogate the sample performance under in situ / operando conditions.…”

Monochromatic computed tomography using laboratory-scale setup

“…This method, known as X-ray absorption near-edge spectroscopy (XANES), offers a non-destructive tool for the analysis of the chemistry of a given element, most importantly its oxidation state and local atomic coordination 4 . It has shown success in being utilized as a contrast method for tomography in numerous materials research applications such as investigating nano- and mesoscale chemical compositions and phase transitions in battery materials 5 – 7 , degradation and inactivation of catalyst materials 8 , 9 , and heterogeneity of defect-engineered metal-organic framework crystals 10 . It has also been demonstrated that a similar idea can be applied to inelastic X-ray scattering (X-ray Raman spectroscopy) to obtain tomographic data on the chemical state of low-Z elements to e.g.…”