Size-Dependent Mechanical Properties of a MetalâOrganic Framework

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Size-Dependent Mechanical Properties of a Metal-Organic Framework: Increase in Flexibility of ZIF-8 by Crystal Downsizing Al A Tiba, Alexei V Tivanski, and Leonard R. MacGillivray Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.9b02125 • Publication Date (Web): 21 Aug 2019 Downloaded from pubs.acs.org on August 22, 2019

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Size-Dependent Mechanical Properties of a MetalOrganic Framework: Increase in Flexibility of ZIF-8 by Crystal Downsizing Al A. Tiba, Alexei V. Tivanski,* and Leonard R. MacGillivray*

Department of Chemistry, University of Iowa, Iowa City, IA, 52242-1294 USA.

*Corresponding authors: [email protected]; [email protected]

ABSTRACT

Size engineering is an emerging strategy to modulate the mechanical properties of crystalline materials. Herein, micro- and nano-dimensional single crystals of the prototypical metal-organic framework (MOF) ZIF-8 are generated using solvothermal and solution methods, respectively. Atomic force microscopy-based nanoindentation

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technique was used to measure the Young’s modulus values of micro- and nanodimensional individual ZIF-8 crystals. We demonstrate that crystal downsizing to nanoscale dimensions results in a 40% reduction in crystal stiffness. The change is attributed to a greater contribution of surface effects to the physical properties of nanocrystalline ZIF-8. The observed change in the mechanical properties may be used to explain reported size-dependent changes in gas adsorption of ZIF-8, thought to be a result of differences in framework flexibility at the nanoscale. Our work provides an important example on how downsizing of crystalline metal-organic materials can give rise to specific and tunable physical properties.

Keywords: Metal-organic frameworks, atomic force microscopy nanoindentation, ZIF-8, mechanical properties, size-dependence, flexibility

Metal-organic frameworks (MOFs) offer promising applications for gas storage,1 separation,2 catalysis,3 and electronics,4 but there can be challenges associated with


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maintaining the mechanical integrity of MOFs when deployed commercially.5,6 A fundamental understanding of mechanical properties of porous MOFs, particularly in the context of dependence on crystal size at the nanoscale, is lacking.7 Recently, crystalline materials that span macro- to nano-dimensions have been shown to exhibit properties based on crystal size.8 Crystal downsizing of porous coordination polymers (PCPs), for example, strongly affects adsorption kinetics and thermodynamics of gaseous guests into host pores.9 Higher, or lower, applied pressures can be required for guest molecules to efficiently adsorb onto nano- or meso-crystals than micro-crystals, particularly when structural transformations (e.g. breathing) may be necessary for gas uptake.9-12 Moreover, a molecular-scale shape-memory effect in which an adsorption stress deforms the original shape of the nanopore, only to be recovered by thermal treatment of the material, was observed by crystal downsizing of a PCP and the shape-memory effect became more prevalent as the crystal dimensions decreased to nanoscale.9 These phenomena are tentatively attributed to differences in the intrinsic flexibilities and elasticities between micro- and nano-dimensional solids.9,10,11 In our own work, we have demonstrated organic crystals of nanoscale dimensions to be either stiffer or more


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compliant than micro-dimensional counterparts.13,14,15 The prediction of size-dependent properties of solid-state molecular materials can be difficult,16 while experimental characterization can provide insight to design materials with size-specific properties. Nano-dimensional materials exhibit an extremely high surface-to-volume ratio compared to microscopic solids, which results in an exceedingly dominant contribution of surface energy towards total free energy of a nano-sized solid.17 The issue is particularly relevant for MOFs where crystal size can alter particle shape and gas adsorption properties.9,18,19

Zeolitic imidazolate framework-8 (ZIF-8) is one of the most studied MOFs owing to its facile synthesis, low-cost raw starting materials, and remarkably high chemical and thermal stability.20 The framework is defined by a series of tetrahedrally-coordinated Zn2+ ions linked and bridged by methylimidazolate ligands. ZIF-8 contains flexible pore openings that swing open by reorientation of the imidazolate linkers enforced by guest adsorption once a required threshold pressure is reached. Specifically, by applying a pressure of 1.47 GPa to ZIF-8, the framework structure will undergo a single-crystal reversible phase transition wherein the imidazolate linkers twist, increasing the accessible


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pore size, which in turn provides unique and highly efficient uptake of different gases.21 Moreover, ZIF-8 exhibits size-dependent gas adsorption kinetics where an increase in pressure is required to induce linker reorientation with decreasing crystal size.10 We propose that these differences may in fact stem from differences in the mechanical properties between micro- and nano-dimensional materials, as we show below.


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Herein, we report the first study on mechanical properties of single-crystal nanodimensional MOFs. We show that the reduction in size of the micro-sized ZIF-8 to ~100

Figure 1. Optical and AFM images for micro- and nano-dimensional ZIF-8: (a) optical microscope image of micro-sized crystal showing rhombic dodecahedron morphology with solid blue lines shown for clarity, (b) AFM amplitude image of a ~100 μm sized crystal with a relatively smooth surface, (c) AFM 3D height image showing three representative individual nanocrystals with prism-like morphologies and heights 150-200 nm, (d) AFM 3D height image of a representative nanocrystal showing prism-like morphology withresults height of nm and base width ~250 nm. in stiffness (Young’s modulus) from ca. 3.7 nm in ~130 an approximate 40%ofdecrease

GPa to 2.3 GPa as determined by atomic force microscopy (AFM) nanoindentation.

Micro-dimensional single crystals of ZIF-8 were synthesized solvothermally in dimethylformamide (DMF) as reported (Fig. 1a,b).20 The formation of the MOF was


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confirmed by powder X-ray diffraction (PXRD) (Fig. S1). The single crystals exhibited rhombic dodecahedron morphologies with base sizes and heights on the order of 50-150 μm (Fig. 1b). Young’s modulus values of micro-sized ZIF-8 are reported from 2.9-3.3 GPa depending on the crystallographic plane, measured using a nanoindenter.22 A previous AFM nanoindentation study on micron-sized crystals (~2 µm) of ZIF-8 determined an average Young’s modulus for all crystallographic faces to be 3.90 ± 1.02 GPa.23 Differences in Young’s moduli values pertaining to faces will be ascribed to effects of lowmoderate elastic anisotropy.22 We next synthesized nano-dimensional single crystals of ZIF-8 in aqueous solution under ambient conditions, as reported (Fig. 1c,d).24 PXRD analysis revealed structurally pure crystals, in agreement with the calculated powder pattern (Fig. S1). Thermogravimetric analyses (TGA) data for both micro- and nanodimensional samples showed comparable weight losses in the range of 12-14% (Fig. S3). AFM imaging of ca. 10 nanocrystals with prism morphologies revealed crystal heights of 100-200 nm and 200-300 nm in diameter (Fig. 1d).


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For

the

AFM

nanoindentation

experiments,

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repeated

force-indentation

measurements (Fig. 2) were recorded on samples of both micro- (blue) and nanodimensional (green) individual crystals of ZIF-8. Repeated force-indentation curves were collected on four sample positions on three different micro-dimensional single crystals for a total of 72 force curves (Fig. 2a, blue). Young’s modulus values were determined by fitting the data to the Johnson-Kendall-Roberts (JKR) contact model to determine the stiffness (See ESI).25 The data were combined into a histogram (Fig. 2b, blue), yielding average Young’s modulus and standard deviation values to be 3.7 ± 0.3 GPa. The value is consistent with results previously obtained from AFM nanoindentation on micron-sized ZIF-8 crystals.23 Similar to micro-dimensional crystals, repeated force-indentation curves (Fig. 2a, green) were collected on four individual nanocrystals at four different positions at the approximate nanocrystal center for a total of 72 force curves. Average Young’s modulus values for the different nanocrystals were statistically similar and within one standard deviation, thus enabling us to combine the values from all individual nanocrystals into a single histogram (Fig. 2b, green). We note, since for nanocrystals we cannot unambiguously determine which crystallographic plane is probed by AFM


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nanoindentation, the Young’s modulus results for nanocrystals are expected to yield an average response reflective of all planes of the micro-dimensional solids, as we have described previously.13,22 The average Young’s modulus and standard deviation values were 2.3 ± 0.4 GPa. Thus, the nano-dimensional ZIF-8 crystals displayed a nearly 40% (a)

(b)

Figure 2. AFM data for micro- (blue) and nano-dimensional (green) ZIF-8: (a) representative forceindentation plots of the approach to the crystal surface. Symbols represent data and solid line is the fit to the JKR contact model, (b) histograms of Young’s moduli for the micro- and nano-dimensional ZIF-8 crystals (Gaussian fits in dashed and solid black lines).

reduction in the Young’s modulus compared to the micro-dimensional solids (3.7 ± 0.3 GPa).

We note the size-dependent Young’s modulus values reported here were not documented in a previous AFM nanoindentation study on ZIF-8, likely due to inherent differences in sample sizes and morphologies.23 The nanocrystals reported by Tan


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comprised a polycrystalline thin film of ~2 μm thickness containing many individual nanocrystals of 300-500 nm in size. Probing a thin film with an AFM tip can create a significant creep effect wherein sliding or slippage of adjacent polycrystalline aggregates on an individual force plot measurement basis could overestimate Young’s modulus values by an order of 900%.23 The AFM nanoindentation methodology conducted here alleviates the issue by directly probing an individual nano-dimensional ZIF-8 crystal with less than 10 nm of indentation depth. Within the limited size range of the nanocrystals studied here (heights of 200-300 nm), no apparent size dependency on the Young’s modulus with respect to nanocrystals height was observed. A decrease in Young’s modulus is, nevertheless, realized via our crystal downsizing to sub-micron and nanoscale dimensions.

The increase in flexibility of ZIF-8 with decrease in crystal size towards nanoscale dimensions is remarkable, although the origin of this behavior is unknown. We note surface-sensitive X-ray photoelectron spectroscopy study has found a Zn-rich surface relative to the methylimidazolate linkers.10 Tan has shown that the bulk mechanical


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properties of ZIFs are dependent more on the imidazole rather than the metal ion.26 Crystal downsizing leads to an increased surface-to-volume ratio thus could decrease the number of methylimidazolates which contribute to the rigidity of the framework. This phenomenon may help to provide insight into recently reported size-dependent guest adsorption phenomena of ZIF-8. Kumari has recently ascribed the ability of nanoscale

versus microscale ZIF-8 (~100 nm) to adsorb more CO2, N2, and CH4 gases to, in part, a difference in framework flexibility.19 An explanation based on accessibility to pores in relation to swinging of the molecular imidazolates was given. Likewise, Tanaka has ascribed the ability of nanoscale ZIF-8 to exhibit faster uptakes of butanol to effects of changes in mechanical properties at the surface.10 Kitagawa has also attributed a molecular-scale shape-memory effect of a PCP to changes in framework flexibility at the nanoscale.9

We also note that a comprehensive study on structure-mechanical property relationships of micro-sized ZIFs in general using, in contrast to our work, a nanoindenter showed that Young’s moduli are inversely correlated to internal pore diameter defined as


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the solvent accessible volume (SAV).26 All highly porous ZIFs with SAV approaching values of 50% (e.g. ZIF-8, -20, and -68) consistently displayed lower stiffnesses, where an enhanced flexibility of certain imidazolate linkers allowed for a greater capacity of guest encapsulation. The further decrease in stiffness of nano-dimensional ZIF-8 that we report here may explain the improvement in gas adsorption efficiency and a potential increase of SAV as crystal size decreases.

In summary, we have demonstrated that single crystals of ZIF-8 exhibit sizedependent mechanical properties wherein a reduction in crystal size from micro- to nanodimensions results in a significant increase in flexibility. The reduction in stiffness at the nanoscale can be attributed to differences in the terminating groups at the surface of ZIF-8 and a concurrent increase in the surface-to-volume ratio. The mechanical properties at the nanoscale may account for variability in gas adsorption behavior of ZIF-8 crystals. We are expanding our studies to other relevant porous ZIFs, and related PCPs, to enable the development of general design strategies that allow the construction of materials with targeted mechanical properties. We expect the strategy of crystal


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downsizing to provide a means to understand the properties of structural integrity and robustness, which are undoubtedly integral to performance and applications.

ASSOCIATED CONTENT Supporting Information. Full experimental details including materials, methods, synthesis, and analysis along with characterization data from atomic force microscopy, optical microscopy, thermal analysis, and powder X-ray diffraction.

AUTHOR INFORMATION

Corresponding Author:

[email protected]; [email protected]

Author Contributions


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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT We thank T. Lansakara for assistance with the AFM measurements. We also thank Prof. D. E. Wurster and Z. O. Assaf for assistance with collecting TGA data.

ABBREVIATIONS MOF, metal-organic framework; ZIF, zeolitic imidazolate framework; AFM, atomic force microscopy; PCP, porous coordination polymer.

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