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Continuous Increase in the Framework Flexibility of ZIF-90 with Crystal-Size Reduction to the Nanoscale: Evidence for Missing Metal Nodes Using Single-Crystal Infrared Spectroscopy
Journal article   Open access   Peer reviewed

Continuous Increase in the Framework Flexibility of ZIF-90 with Crystal-Size Reduction to the Nanoscale: Evidence for Missing Metal Nodes Using Single-Crystal Infrared Spectroscopy

Akalanka B. Ekanayake, Collin S. Hill, Thumini R. Dias, Leonard R. MacGillivray and Alexei V. Tivanski
Langmuir, Vol.42(37), pp.27342-27348
09/10/2026
DOI: 10.1021/acs.langmuir.6c03586
Appears in  UI Libraries Support Open Access
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https://doi.org/10.1021/acs.langmuir.6c03586View
Published (Version of record) Open Access

Abstract

Here we investigate the effects of crystal size on the mechanical properties (Young’s modulus, framework flexibility) of a switchable metal–organic framework (MOF): zeolitic imidazolate framework-90 (ZIF-90) individual crystals with sizes ranging from micro- to nanodimensions. Atomic force microscopy (AFM) nanoindentation measurements over individual crystals reveal a significant reduction in Young’s modulus with crystal-size reduction that follows a continuous power-law relationship, where Young’s modulus decreases by nearly 1 order of magnitude as crystal base size decreases from ∼5 to 0.4 μm. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) analyses coupled with the AFM results reveal that the origin of the size-dependent framework flexibility is primarily attributed to an increased concentration of missing metal node defects for smaller crystals. The presence of missing metal node defects in ZIF-90 crystals was further confirmed by single-crystal optical photothermal infrared spectroscopy measurements. Significantly, the value of the power-law exponent in Young’s modulus versus crystal-size data was related to the magnitude of the relative increase in the number of missing metal node defects, indicating that the extent of the framework flexibility change with crystal size can in part be attributed to the rate of defect accumulation with crystal downsizing at the nanoscale. These findings indicate that the power-law scaling of mechanical properties may serve as an approach to assess the size-dependent extent of point-defect evolution in switchable MOFs, potentially enabling a rational design of solids with controlled functional properties.
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