Akalanka B.
Ekanayake
,
Al A.
Tiba
,
Leonard R.
MacGillivray
and
Alexei V.
Tivanski
*
Department of Chemistry, University of Iowa, Iowa City, IA 52242-1294, USA. E-mail: alexei-tivanski@uiowa.edu
First published on 23rd October 2024
Crystal size engineering is an emerging strategy to modulate mechanical and gas adsorption properties of metal–organic frameworks (MOFs). Fundamental principles on how the properties of these materials change with size remain to be understood and explored. Here, supermicron-, micro-, and nano-sized single crystals of a prototypical MOF zeolitic imidazolate framework-7 (ZIF-7) were generated using a solvothermal method. Atomic force microscopy (AFM) imaging revealed that nano- and micro-sized crystals exhibited rounded and prism-like morphologies, respectively. AFM nanoindentation was used to assess the stiffness (Young's modulus) of the rounded and prism-like crystals as a function of crystal size. We demonstrate that the framework flexibility increases (Young's modulus decreases) with crystal size reduction for both morphologies, which can be attributed to a larger number of point defects (missing metal nodes and/or missing linkers) for smaller crystals. Remarkably, scanning electron microscopy (SEM) energy dispersive X-ray spectroscopy measurements on individual prism-like micro-sized crystals of various sizes revealed a decreasing trend in the Zn/C ratio with crystal size reduction. Collectively, the size-dependent SEM and AFM characterization studies suggest that smaller crystals have lower relative metal content via a larger number of missing metal node defects. Our findings highlight how the mechanical properties of MOFs can vary significantly as a function of crystal size likely due to a variable and size-dependent number of missing metal node defects. Such size-dependent behavior especially towards the nanoscale is thus important to consider for the rational design of various functional crystalline materials.
Crystal downsizing of MOFs to submicrometer or nanoscale dimensions is beneficial for various applications. Submicrometer- or nano-sized MOFs have higher surface areas and more efficient mass transport characteristics, thus offering improved properties for catalysis.12 Nano-sized switchable MOFs with controlled morphologies and sizes are critical for controlled drug delivery owing to enhanced cellular uptake, uniform drug distribution, and precise, stimuli-responsive release profiles.13,14 A significant number of as-synthesized switchable MOFs are already in the submicrometer- or nano-size range.15 However, previously reported gas adsorption studies of various switchable MOFs at different sizes showed a significant change in gate opening pressure or even complete suppression of gate opening once crystal dimensions are reduced below a certain critical size (typically in the submicrometer size range).13,16,17 While the exact origin of such size-dependent behavior is not known, it is generally assumed to be related to the change in the degree of cooperativity between neighbouring unit cells of a framework, which may change due to a varying number of point defects (e.g., missing linkers or metal nodes) as the crystal size decreases towards the nanoscale.13,18,19 Mechanical studies on various MOFs also show that framework flexibility can change as a result of crystal downsizing to the nanoscale, where the effect was also attributed to changes in the relative number of defects (e.g., missing linkers), where crystals with a larger number of defects are expected to correspond to more disorder and, thus, softer frameworks.20–24 It is worth noting that studies also demonstrate that missing metal node defects can affect the properties of MOFs, such as catalytic activity.19
Recently, we reported how the synthesis of ZIF-8 submicrometer-sized crystals using a modulator (i.e., triethylamine) resulted in a larger number of missing linker defects and a more flexible framework relative to ZIF-8 crystals of similar size generated without the modulator.23 Furthermore, it was observed that crystals with a higher relative number of point defects also exhibited a downward shift in gate opening pressure.23 Thus, defect and size engineering of a switchable MOF can modify mechanical properties and degree of cooperativity between neighbouring unit cells, and in some cases even lead to significant changes or even complete suppression of switchable behavior. Therefore, studies to develop a better understanding of the effects of size reduction of switchable MOFs towards the nanoscale are needed to facilitate the development of submicrometer- and nano-sized switchable MOFs for various applications.
A fundamental understanding of size-dependent effects on the properties of MOFs across various dimensions (e.g., the macroscale, supermicron, submicrometer, and nanoscale) is lacking.25 In this regard, while atomic force microscopy (AFM) nanoindentation can be uniquely employed to directly measure stiffness (Young's modulus) of individual crystals of various sizes, it would be advantageous to identify a method that complements AFM work and provide insight into the compositional origin of flexibilities of MOF materials.24,26–28 Herein, we report the size-dependent mechanical properties of zeolitic imidazolate framework-7 (ZIF-7). ZIF-7 was selected for several reasons. First, reports show successful facile synthesis of ZIF-7 crystals at various morphologies and sizes with high chemical and thermal stability.29,30 Second, ZIF-7 is a switchable MOF with size-dependent gas adsorption characteristics, where previous studies show that the gate opening pressure decreases as the ZIF-7 crystal size decreases towards the nanoscale from the microscale.31–33 In our current study, we show that size reduction of ZIF-7 crystals from the supermicron to nanoscale results in a significant decrease in Young's modulus. While the reduction in Young's modulus is consistent with an increase in the number of defects at the nanoscale, we now reveal using size-dependent scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) single-crystal data that the reduction in mechanical properties is related to relative metal content as crystal size decreases. The data, thus, imply that the changes in mechanical properties can be attributed to the varying number of missing metal nodes. Our use of AFM, SEM, and EDX to identify the type of point defect illustrates the importance of implementing combined techniques to gain insight into behaviours of MOF materials and underscores the importance of understanding size-dependent mechanical properties and links to a varying number of defects in MOFs and gas adsorption properties.
AFM nanoindentation experiments were performed by first recording force versus vertical piezo displacement curves (i.e., force curves) of individual substrate-deposited ZIF-7 crystals. The acquisition of force curves and corresponding data analysis was carried out as reported previously.23,24,26,27,35 The force curves were then converted into force versus indentation distance data and then the approach to crystal surface data were fit to the Johnson–Kendall–Roberts (JKR) contact model to determine Young's modulus of the crystal.23,24,26,27 The JKR model was selected due to several reasons. First, the indentation distances were limited to 5 nm (∼3 times less than the tip radius of curvature) and measurements were performed on a relatively flat crystal surface, where the typical area-equivalent diameter of the contact region between the AFM tip and the crystal surface at the maximum indentation distance is at least six times larger than the base size of the smallest crystal studied. These experimental conditions allow us to approximate the AFM nanoindentation experiment as a sphere indenting into a semi-infinite flat surface. Second, the force–indentation data in the contact region display close overlap between the approach and retract from the crystal surface data, confirming that purely elastic nanoindentation and force curves show the presence of adhesion forces between the AFM tip and crystal surface. Collectively, these are consistent with the JKR model assumptions, facilitating its application for this study. Furthermore, previous works have successfully applied this approach model to various solids of different sizes to determine Young's modulus of solids. Typically, 4–6 force curves were collected for each individual crystal at a relatively flat crystal surface region. Images of individual crystals were collected before and after force curve measurements to ensure that there are no apparent changes in the crystal surface. Noteworthily, AFM imaging of individual crystals does not provide direct information on which crystallographic plane is being probed via nanoindentation. Thus, force–indentation measurements are performed at multiple locations across the crystal surface and reported average Young's modulus value for each crystal is assumed to represent an average response from likely several different crystallographic planes.
Fig. 1 Comparison of ZIF-7 PXRD patterns: simulated pattern from single crystal X-ray diffraction data31 (red) and experimental pattern (black). |
AFM nanoindentation experiments were performed by collecting force–indentation curves on multiple individual ZIF-7 crystals of various morphologies and sizes, with representative force–indentation curves for nano-sized rounded, micro-sized prism-like, and supermicron-sized individual crystals being determined (Fig. 2d). Typically, 4–6 repeated force–indentation curves at various positions over a relatively flat region on the crystal surface were recorded per individual rounded nanocrystal (total of 16 crystals) and prism-like micro-sized crystal (total of 16 crystals). For supermicron-sized crystals, 25 repeated force–indentation curves at various positions over a typical crystal surface were collected. Nanoindentation studies on supermicron-sized crystals were performed to compare our AFM based mechanical characterization results with previously published mechanical studies on macro-sized ZIF-7 crystals.7 Each force–indentation curve was fit using the Johnson–Kendall–Roberts elastic contact model36 to determine the average Young's modulus of each crystal following a reported approach.23,24,26,27,35 Young's modulus for a typical supermicron-sized crystal (base size of ∼40 μm and height of ∼20 μm) was determined to be 5.9 ± 1.5 GPa, which closely overlaps with the indenter measurements on macro-sized ZIF-7 crystals reported in the literature (∼6.2 GPa for (101) crystallographic plane),25 confirming applicability of AFM nanoindentation to accurately determine the mechanical properties of individual ZIF-7 crystals.
The average Young's moduli of individual crystals vary as a function of corresponding crystal size (log–log axis) for nano-sized rounded (red triangles) and micro-sized prism-like (green squares) crystals (Fig. 3). Young's moduli of both nano- and micro-sized crystals exhibit a clear size-dependent behaviour, where crystals continuously become softer (lower Young's modulus) as the size decreases. In particular, the crystal size reduction (from ∼7.2 μm to ∼110 nm) results in over one order of magnitude decrease in Young's modulus (from 3.9 ± 0.8 GPa to 0.21 ± 0.02 GPa), which implies a remarkable increase in framework flexibility at the nanoscale. Error bars in Fig. 3 represent one standard deviation, calculated from Young's moduli obtained through 4–6 individual force–indentation measurements taken at several different positions on the crystal surface. The dependence of Young's modulus versus crystal size is clearly linear (Fig. 3) in the log–log axis across the entire studied crystal size range; thus, the mechanical data were fit to a power law function (shown as a black solid line in Fig. 3), yielding the following relationship between the size-dependent Young's modulus (E, in GPa) and crystal size (D, in micrometres): E = (0.90 ± 0.07 GPa)D0.61 ± 0.05, with R2 = 0.993. There is no current explanation for the apparent power law dependence and more studies over a wider range of switchable MOFs, which we believe is beyond the scope of the present work, are needed to elucidate if the observed trend is general or uniquely specific to ZIF-7. Noteworthily, the size-dependent Young's modulus variability appears to be similar for both rounded and prism-like morphologies, suggesting that the size-dependent response is largely governed by size rather than the morphological differences. While the exact origin of such a size-dependent mechanical behavior of ZIF-7 crystals is unknown, we hypothesize that it is likely due to a size-dependent number of defects, as will be discussed below.
The observed size-dependent Young's modulus response is consistent with our previous studies where we reported a similar phenomenon with ZIF-8, and crystal downsizing to the nanoscale (from ca. 100 μm to ca. 100 nm) results in smaller yet significant ∼40% reduction in Young's modulus.24 Furthermore, Tan et al. have also studied size-dependent behaviour in a related MOF system (ZIF-8), where it was shown that smaller nanocrystals formed during the early stages of MOF crystallization appear to have larger concentration of defects relative to larger crystals, similar to our observations described in the present study.21 However, we note that other works reported that smaller crystals were found to be stiffer (less flexible) and thus likely contain fewer defects, thus more studies on other MOFs are needed to gain a better understanding of the factors that control such size-dependent behaviour (e.g., assessing the influence of different metals, organic linkers, and crystal structures).37,38
Previously reported CO2 gas adsorption studies at 0 °C on rounded nano- (sizes of ∼100–200 nm) and prism-like micro- (sizes of ∼2–10 μm) ZIF-7 crystals showed ‘S-shaped’ type IV isotherms, confirming that the gate opening transitions occur at each size range.31,32 However, there were significant differences in the shape of the isotherm and gate opening pressure between these samples. Specifically, the gate opening pressure was found to decrease as crystal sizes decreased from micro- to nano-sized crystals. Additionally, the gas adsorption isotherm showed steeper and stepwise characteristics for micro-sized prism-like crystals relative to more gradual and cooperative gate-opening structural expansion for nano-sized rounded crystals.31–33 These size-dependent gas adsorption differences in similar morphologies and size-ranges of ZIF-7 crystals can be rationalized with the size-dependent mechanical properties determined here. In particular, the lower gate opening pressure for nano- versus micro-sized crystals is consistent with the lower Young's modulus at the nanoscale, which implies reduced resistance of the framework to adsorptive stress caused by rotation of organic linkers during the gate opening transition.24 Furthermore, the extent of cooperative transformations in the framework is likely lower for more flexible nanocrystals with likely a larger number of defects (e.g., missing metal nodes), which can explain more gradual gas adsorption uptake during gate-opening transition observed at the nanoscale.
To test if the relative number of defects in ZIF-7 crystals changes as a function of crystal size, single crystal SEM and EDX measurements were performed (Fig. 4). A typical SEM image of prism-like ZIF-7 crystals showed morphologies consistent with the AFM data (Fig. 4a). EDX spectra were collected for 10 different prism-like individual crystals with sizes ranging from ∼0.8 to 4.5 μm. EDX spectra for four selected crystals of various sizes showed a clear decrease in intensities of both the carbon K edge (at 0.27 keV) and the zinc L edge (at 1.02 keV) with crystal size reduction (Fig. 4b). Since the zinc L edge signal is assumed to be directly proportional to the number of metal nodes in the framework, while the carbon K edge signal is reflective of the number of organic linkers, the ratio of Zn to C (i.e., C normalized) is indicative of the relative number of metal nodes to organic linkers. Carbon normalized EDX spectra for the same four selected crystals (Fig. 4c) show that as the crystal size decreases, the zinc L edge signal normalized to C clearly decreases, indicating a lower Zn/C ratio in smaller crystals, which is confirmed for all studied crystals (Fig. 4d). The reduction in the Zn/C ratio with crystal downsizing can be attributed to either a relative decrease in the number of metal nodes (i.e., lower relative zinc L edge signal) or an increase in the relative number of organic linkers (i.e., larger carbon K edge signal). While SEM data alone cannot differentiate between these two possibilities, we note that a possible increase in the relative number of organic linkers with crystal downsizing would correspond to a lower relative number of point defects and, thus, would be expected to yield a more rigid framework (higher Young's modulus) for smaller crystals13 – contrary to the size-dependent mechanical trends that we observe. Therefore, the reduction in the Zn:C ratio and increase in the framework flexibility at the nanoscale can be attributed to an increase in the relative number of missing metal nodes as the crystal size decreases. Our findings further suggest that Young's modulus measured for individual MOFs could potentially be used to evaluate the relative number of defects in isostructural crystals of varying sizes.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ma00804a |
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