Mechanical Properties of Microcrystalline MetalâOrganic Frameworks

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Mechanical Properties of Microcrystalline Metal−Organic Frameworks (MOFs) Measured by Bimodal Amplitude ModulatedFrequency Modulated Atomic Force Microscopy Yao Sun,† Zhigang Hu,‡ Dan Zhao,*,‡ and Kaiyang Zeng*,† †

Department of Mechanical Engineering, National University of Singapore, 9 Engineering Drive 1, 117576 Singapore Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, 117585 Singapore

‡

S Supporting Information *

ABSTRACT: Direct measurement of the mechanical properties of microcrystalline metal−organic framework (MOF) nanoparticles is challenging and rarely explored. In this work, we apply an effective method to realize elastic modulus mapping of a series of isostructural single MOF nanoparticles (100−500 nm) via bimodal amplitude modulated-frequency modulated atomic force microscopy. By probing five types of zirconium (Zr) and hafnium (Hf) isostructural UiO-66-type MOFs, we experimentally found that UiO-66(Hf)-type MOFs possess the higher elastic modulus (46−104 GPa) than that of UiO-66(Zr)-type MOFs (34−100 GPa), both of which are higher than that of reported zinc/copper based MOFs (3−10 GPa). We also experimentally demonstrate that the mechanical properties of MOFs can be tuned by adjusting the chemical functionalities of the ligands or using different metal nodes. In detail, the sterically bulky functional groups increase the mechanical properties of the resultant UiO-66-type MOFs, possibly due to the increased atomic density. These results pave a way to the direct measurement of mechanical properties of MOFs crystalline particles and provide an incisive perspective to the design of MOFs with high mechanical properties. KEYWORDS: bimodal amplitude modulated-frequency modulated atomic force microscopy, mechanical properties, metal−organic frameworks, elastic modulus mapping

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INTRODUCTION Metal−organic frameworks (MOFs), known as a next generation of adsorbent materials, have attracted wide attention during the past two decades.1−5 Many studies have been conducted for the synthesis6−13 and applications of MOFs,5 while the study of their mechanical properties remains scarce and far from incisive.14,15 In fact, the mechanical properties of these materials play an important role in determining their macroscopic performance under practical working conditions, such as in the presence of mechanical stress, shear flow, and others.16,17 Furthermore, deeper understanding of the microstructures and mechanical properties is also helpful in designing and synthesizing mechanically rigid or soft MOF materials that can survive from structural distortion (as adsorbents) or respond to external mechanical stress (as mechanic sensing devices).14 Different from measuring mechanical properties of bulk materials using conventional mechanical testing, the evaluation of mechanical properties of MOFs is quite complex because most of the reported MOFs are discrete particles, ranging from tens of nanometers to hundreds of micrometers.7 The traditional nanoindentation technique has been proven as an effective method to obtain mechanical properties of MOFs.18 For example, the early measurement of mechanical properties © 2017 American Chemical Society

of MOFs was conducted on MOF-5 single crystals with crystal sizes larger than 100 μm.19,20 The measured elastic modulus of MOF-5 is only 3 GPa, which is much lower than the calculated values of ∼20 GPa by density functional theory.19,21 Cheetham and his co-workers have systematically studied the structure− mechanical property relationships of ZIFs (Zeolitic Imidazolate Frameworks, a subgroup of MOFs) via this technique.22,23 They have found that the mechanical properties of ZIFs were closely correlated to the rigidity and functionalities of organic building blocks as well as the density of frameworks. Moreover, the measured elastic moduli of ZIFs range from 3 to 10 GPa, which are slightly higher than that of MOF-5.23 Since then, many studies have been devoted to the measurements of mechanical properties of MOF single crystals24−34 or MOFbased thin films35,36 using the nanoindentation techniques. However, a majority of MOFs samples synthesized with known recipes are all fine crystalline particles, which are generally less than 1 μm, and single crystals large enough for traditional measurements such as nanoindentation are difficult to prepare.15 Received: May 15, 2017 Accepted: August 29, 2017 Published: August 29, 2017 32202

DOI: 10.1021/acsami.7b06809 ACS Appl. Mater. Interfaces 2017, 9, 32202−32210

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ACS Applied Materials & Interfaces

Figure 1. (a) Chemical structures of the ligands. (b) Crystal structures of UiO-66-type MOFs.

Figure 2. Schematic illustration of AM-FM mode: the first eigenmode amplitude is controlled to create the topography of the sample, while the second eigenmode contact resonance frequency shift is used to calculate tip−sample stiffness and elasticity.

mode (elasticity by FM). The calculated elastic moduli (E) are in the range of 3−6 GPa. The AM-FM technique used in their work is a new technique to characterize mechanical properties of nanomaterials and has been developed only recently.48 However, the reference material (Matrimid5218) with an established elastic modulus of 4 GPa adopted in their work is not very suitable for MOFs, and this technique needs to be further improved to ensure the accuracy of determination of the elastic modulus. The bimodal (multifrequency) AFM refers to the simultaneous excitation of two (or more) eigenmodes of the probe cantilever. Excitation frequency is typically selected at resonance frequency of each eigenmode. Through simultaneous excitation of two or more bending vibration modes, or simultaneous excitation of higher harmonic of the AFM cantilever, it can achieve fast imaging of morphology as well as elastic property of materials.49 Since the probe cantilever has different resonance frequencies, elastic constants, and quality factor at each eigenmode, adopting different eigenmode vibrations can provide different properties with different sensitivity. 48 According to the selection of amplitude modulation or frequency modulation, bimodal AFM can be divided into AM-AM mode, AM-FM mode, and FM-FM mode. In this work, we therefore aim to study the microstructures and mechanical properties of isostructural UiO-66-type MOFs

Generally speaking, most of the hydrothermally and chemically stable MOFs, which are highly promising for practical applications, are in the forms of micro- or even nanoparticles (20 nm to 1 μm), exemplified by UiO-66(Zr)type or UiO-66(Hf)-type MOFs,37−43 MILs,44−46 etc. The measurements of mechanical properties of these MOF particles using traditional experimental methods are very challenging; for example, only theoretical calculations have been attempted on UiO-66(Zr) and UiO-66(Hf) types of MOFs.15,47 Comparing to the Atomic Force Microscopy (AFM) based technique, nanoindentation makes much larger and deeper indentations. This can decrease lateral spatial resolution and limit applicability to thin films or small particles. Whereas the AMFM technique works like a tapping mode AFM so that it can be applied to the materials with sizes only to several nanometers, which beyond the limit of the nanoindentation technique and the load applied in AFM is very small, usually in the order of nano-Newtons (nN) or even smaller so that it is more preferable than nanoindentation to characterize soft materials. Recently, Tan and his co-workers46 conducted experiments to measure the elastic modulus of HKUST-1 thin films via the bimodal amplitude modulated-frequency modulated (AM-FM) approach using multifrequency atomic force microscopy (AFM) with high-resolution amplitude tapping mode (topography by AM) and high-sensitivity frequency modulation 32203


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where d, FN, and R are the deformation, applied force, and tip radius, respectively. If a single reference material is adopted, the relationship between tip−sample contact stiffness, tip− reference material contact stiffness, tip−sample equivalent elastic modulus, and tip−reference material equivalent elastic modulus could be obtained from eq 5

microcrystalline nanoparticles (Figure 1) with 10 different ligands and metals via the novel AM-FM approach. Their elastic moduli are quantitatively determined and compared with previously reported values of MOFs and other inorganic materials.

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RESULTS AND DISCUSSION Principles of Bimodal AM-FM Study. In this study, we apply AM-FM mode on MOF nanoparticles. The AM-FM technique uses first eigenmode amplitude as feedback to perform topography imaging and second eigenmode contact resonance frequency shift to calculate elastic modulus. Figure 2 shows the working principle of the AM-FM technique. The AM-FM imaging requires two signal generators and two lock-in amplifiers. Excitation frequencies are usually the first two eigenmode resonance frequencies. The amplitude of higher eigenmode (10 mV) is generally much smaller than the amplitude of the first eigenmode (2 V). The much smaller drive amplitude for the second eigenmode is chosen to avoid the perturbation to the first eigenmode cantilever oscillation. Two excitation signals are then superimposed and input into the piezoelectric ceramic fixed on the probe. The two generated excitation signals are also input into the two lock-in amplifiers as reference signals to separate two amplitudes and phases response of the probe at the same time. In this technique, the relationship of force gradient between tip and sample can be expressed as50 kts ≈ 2k 2Δf2 /f 20

⎛ k * /k ⎞3/2 s−tip c * * ⎟ Es−tip = Eref −tip⎜⎜ ⎟ * ⎝ kref −tip/kc ⎠

where E*s−tip, E*ref−tip, k*s−tip, k*ref−tip, and kc indicate the tip−sample equivalent elastic modulus, tip−reference material equivalent elastic modulus, tip−sample contact stiffness, tip−reference material contact stiffness, and cantilever spring constant, respectively. It should be noted the item inside the parentheses in eq 5 becomes nondimensional. From the Hertz contact model, Es−tip * can be written as Es*− tip =

−1

* −tip/kc)/(ks*− tip/kc)]3/2 −1]/M t } + [[(kref

(1)

(2)

(3)

Because of the uncertainty of contact radius, reference material with a known elastic modulus is usually adopted to calculate the elastic modulus of the sample. If the elastic modulus of the reference material is proximate to the sample, the contact radius of the tip−reference material is postulated to be the same with the tip−sample contact radius. The constant coefficient C could be determined by replicating the same experiment and obtaining a relationship between the equivalent elastic modulus and second eigenmode contact resonance frequency shift on the reference material. For the Hertz contact model, the contact stiffness between tip and sample is a first derivative of applied force to deformation, which can be written as δFN = δd

3

6E*2RFN

(7)

The detailed synthesis and characterizations of UiO-66(Zr/ Hf)-type MOF nanoparticles are presented in the Supporting Information (Figures S1−S7 and Table S1). We have used similar synthetic conditions as those in the literature51−53 and make sure the synthesized MOFs are of high quality (high crystallinity and surface area) in order to make a fair comparison. Because of the nature of microsized particles in these MOFs, when preparing the sample for AM-FM measurements, around 10 mg of MOF powder was mildly dispersed into 20 mL of ethanol, and 100 μL of the resultant colloidal solution was pipetted onto a clean silicon wafer substrate. The silicon wafer substrate was then left dry at 80 °C for 2 h, followed by the AFM measurements (MFP-3D, Asylum Research, CA, USA). Topography and Elastic Modulus Mapping of Single MOF Nanoparticle. The mechanical properties of UiO-66 MOFs have been predicted via the first-principles density functional theory (DFT), and results showed that the bulk modulus (41 GPa) of UiO-66(Zr) was 10 times higher than those of other benchmark MOFs (e.g., MOF-5, ZIF-8, HKUST-1).15,47,54,55 Here, we experimentally measure the elastic moduli of 10 UiO-66-type MOFs (Figure 1) utilizing the novel AM-FM technique directly. Because of the high degree of framework connection, UiO-66(Zr) is often cited as a benchmark MOF of high chemical and thermal stability.56 However, by introducing different chemical functionalities, the thermal stability of these UiO-66-type MOFs varies from one to another (Figures S4 and S5). This also prompts us to study how the chemical functionalities would affect the mechanical properties. During our measurements, the contact stiffness mapping containing 256 × 256 test points was obtained synchronously with in situ topography mapping (Figure S8). The elastic modulus mapping was further transformed from the calculated results. The calibrated silicon probe (AC160TS, Asylum

Here, ac is the contact radius between tip and the sample and E* is the tip−sample equivalent elastic modulus. According to the eqs 1 and 2 above, E* is a linear function of Δf 2, with a constant C.50

k* =

(6)

* −tip/kc)/(ks*−tip/kc)]3/2 /Mref Ms = {[(kref

where and Δf 2 denote the force gradient between tip and sample, second eigenmode elastic constant of the probe, second eigenmode free vibration resonance frequency of the probe, and tip−sample second eigenmode contact resonance frequency shift due to the changes of the surface topography and tip movements, respectively. By assuming that the tip− sample contact is a perfect Hertz contact, the relationship between the tip−sample force gradient (kts) and tip−sample equivalent elastic modulus can be written as50

E* = k 2Δf2 /acf 20 = C Δf2

1 1 + Mt Ms

where Mt and Ms are elastic modulus of the tip and sample. Substituting eq 6 into eq 5, the elastic modulus of the sample can be obtained:

kts, k2, f 02,

kts = 2acE*

(5)

(4) 32204


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Figure 3. Topography mapping of (a) UiO-66(Zr); (b) UiO-66(Zr)-(OH)2; (c) UiO-66(Zr)-NH2; (d) UiO-66(Zr)-(COOH)2; (e) UiO-66(Zr)(F)4. Elastic modulus mapping of (f) UiO-66(Zr); (g) UiO-66(Zr)-(OH)2; (h) UiO-66(Zr)-NH2; (i) UiO-66(Zr)-(COOH)2; (j) UiO-66(Zr)-(F)4.

Figure 4. Topography mapping of (a) UiO-66(Hf); (b) UiO-66(Hf)-(OH)2; (c) UiO-66(Hf)-NH2; (d) UiO-66(Hf)-(COOH)2; (e) UiO-66(Hf)(F)4. Elastic modulus mapping of (f) UiO-66(Hf); (g) UiO-66(Hf)-(OH)2; (h) UiO-66(Hf)-NH2; (i) UiO-66(Hf)-(COOH)2; (j) UiO-66(Hf)(F)4.

MOF samples, the same measurement is conducted on the reference material with the same tip and experimental parameters. The topography and calculated elastic modulus mappings of the UiO-66(Zr)-type MOFs and UiO-66(Hf)-type MOFs are shown in Figures 3 and 4, respectively. From topography mappings (Figures 3a−e and 4a−e), the diameters of single particles for both UiO-66(Hf)-type MOFs and UiO-66(Zr)-type MOFs are approximately 100−500 nm, which is in good agreement with the crystal size reported in the literature.41,57,58 Similar to the Field Emission Scanning Electron Microscopy (FESEM) image (Figure S3), both UiO66(Hf) and UiO-66(Zr) particles have an octahedral shape (reflected by the regular quasi-diamond shape in 2D images, Figures 3a and 4a, while the others have the spherical shape. For both UiO-66(Hf)-type and UiO-66(Zr)-type MOFs particles, UiO-66-(COOH)2 or UiO-66-(F)4 has larger particle sizes than that of the UiO-66-(OH)2 as well as UiO-66-NH2. The height differences of UiO-66(Hf)-type MOFs are in the range of several nanometers to hundreds of nanometers based on stack level. Surface roughness (Ra) values (determined by Igor Pro 6.3.7, WaveMetrics, OR, USA) are 2.443 and 37.472 nm in maximum for UiO-66(Zr)-type and UiO-66(Hf) type MOFs. According to Figure 3(f−j), the elastic moduli of UiO66(Zr), UiO-66(Zr)-(OH)2, UiO-66(Zr)-NH2, UiO-66(Zr)(COOH)2, and UiO-66(Zr)-(F)4 are approximately 22−45

Research, Oxford Instruments, CA, USA) with a spring constant around 35 N m−1 was used in our AM-FM experiments. The probe specifications are seen in Table S2. It is noted that the reference material should be carefully selected when using AM-FM mode to accurately measure the elastic modulus of the materials. One key factor is that the elastic constant of reference materials should be very similar to that of the measured sample. To have a better understanding of the influence of reference material on modulus results in the AMFM technique, we adopted wafers of pure magnesium (metal Mg, E ∼ 40 GPa from nanoindentation experiments), pure tin (metal Sn, E ∼ 50 GPa from nanoindentation experiments), and glass (E ∼ 71 GPa from nanoindentation experiments) to calculate the elastic modulus mappings from stiffness mappings of UiO-66 MOFs, respectively, as shown in Figure S9. It can be seen that the calculated elastic modulus of MOFs increases as the elastic constant of the reference material increases. Therefore, the quantitative characterization of mechanical property by AM-FM is closely relied on the reference material. Since the theoretically computed bulk modulus value of UiO-66 MOFs is 41 GPa using DFT calculations, this theoretical value can serve as the foundation of this study. In this work, pure Mg was chosen as the reference material to calculate the elastic modulus of the UiO-66, which can reduce the calculation errors contributed from the reference material. For every test on the 32205


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Table 1. Summary of Calculated and Experimental Elastic Modulus of Common MOFs (Numbers 1−5) and UiO-66-Type MOFs in This Study (Numbers 6−15)a number

MOF

calculated (GPa)

experimental (GPa)

experimental range (GPa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

HKUST-1 (AM-FM) HKUST-1 (nanoindentation) MOF-5 (nanoindentation) ZIF (nanoindentation) UiO-67(Zr) (nanoindentation) UiO-66(Zr) UiO-66(Hf) UiO-66(Zr)-NH2 UiO-66(Hf)-NH2 UiO-66(Zr)-(OH)2 UiO-66(Hf)-(OH)2 UiO-66(Zr)-(COOH)2 UiO-66(Hf)-(COOH)2 UiO-66(Zr)-(F)4 UiO-66(Hf)-(F)4

10−3563 10−3563 21.619 6.526,65 21.555 27−4163 39.515 N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A.

N.A. N.A. N.A. N.A. 20.255 34 46 40 48 48 59 60 65 100 104

3−664 9.335 2.719 3−823 N.A. 22−45 30−60 7−70 17−70 17−70 26−80 7−90 5−130 69−140 28−160

a

The literatures for calculated modulus values for MOFs 1−7 are included. The literatures for experimental modulus values for MOFs 1−4 are also included. The experimental modulus values (modulus values of the peaks in Figure 5c,d) and modulus value ranges for MOFs 6−15 are determined from this study. N.A.: not available.

Figure 5. Distribution curves of AM-FM elastic modulus mappings of (a) UiO-66(Hf/Zr). (b) CDF curves of the distribution curves presented in (a). (c) Distribution curves of UiO-66(Hf)-type MOFs. (d) Distribution curves of UiO-66(Zr)-type MOFs. (Note: The percent label on the y axis of distribution curve represents the percentage of data points located in each modulus value (x axis).)

and UiO-66(Hf)-type MOFs determined from the AM-FM modulus mappings are summarized in Table 1 (5th column). Disclosing Structure and Property Relationship. We further analyzed the distribution curves from the elastic modulus mappings shown in Figures 3f and 4f using a commercial software (SPIP 6.5.2, Image Metrology A/S, Denmark). The distribution curves indicate the percentage each modulus value occupies among all the data points in a modulus mapping. For example, there are 1000 data points with modulus value 46 GPa; then the percentage of this modulus value (46 GPa) is 1000/256/256. As can be seen in Figure 5a, the modulus distribution curves of UiO-66(Hf) and UiO66(Zr) show peaks at ∼46 GPa and ∼34 GPa in the horizontal

GPa, 17−70 GPa, 7−70 GPa, 7−90 GPa, and 69−140 GPa. From Figure 4f−j, the elastic moduli of UiO-66(Hf), UiO66(Hf)-(OH)2, UiO-66(Hf)-NH2, UiO-66(Hf)-(COOH)2, and UiO-66(Hf)-(F)4 are approximately 30−60 GPa, 26−80 GPa, 17−70 GPa, 5−130 GPa, and 28−160 GPa. The large range of the elastic modulus values in this measurement is most likely due to the structural anisotropy associated with the nanoparticles of the MOF materials. The elastic modulus of the UiO-66-type MOFs measured in this work intersects with those of ceramics, metals, and polymers, and is very similar to the large elastic modulus ranges simulated for MIL-140 MOF.54 The value ranges of elastic modulus of the UiO-66(Zr)-type 32206


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ACS Applied Materials & Interfaces axis with percentage ∼ 3.2% and ∼ 1.6% in the vertical axis, respectively. In Figure 5b, the CDF (cumulative distribution function) curve of UiO-66(Zr) is totally located at the left side of UiO-66(Hf), indicating that the average modulus of UiO66(Zr) is lower than that of the UiO-66(Hf). The K-S test of CDF curves shows UiO-66(Hf) is obviously stiffer than UiO66(Zr) (p < 0.001). In addition, around 80% of the tested UiO66(Hf) particles show the moduli are within the range of 40− 50 GPa, while the moduli of 80% of the UiO-66(Zr) particles are within the range of 30−38 GPa. Figure 5c,d shows the distribution curves of the elastic modulus of UiO-66(Hf)-type and UiO-66(Zr)-type MOFs. It can be seen that the modulus distribution curves of UiO-66(Hf)-type MOFs with dangling side groups are much wider than that of the UiO-66(Hf) MOF (Figure 5c), which suggests that attaching functional groups may lead to large variation in the elastic moduli of UiO-66(Hf)type particles. Similarly, the modulus distributions of UiO66(Zr)-type MOFs with dangling side groups are also much wider than that of the UiO-66(Zr) MOF (Figure 5d), demonstrating incorporating side groups can enlarge the modulus variation for UiO-66-type MOFs. In addition, it could be observed that the distribution curves show peaks at 46, 48, 59, 65, and 104 GPa for UiO-66(Hf), UiO-66(Hf)-NH 2 , UiO-66(Hf)-(OH) 2 , UiO-66(Hf)(COOH)2, and UiO-66(Hf)-(F)4, respectively (Figure 5c), whereas the peaks for UiO-66(Zr), UiO-66(Zr)-NH2, UiO66(Zr)-(OH)2, UiO-66(Zr)-(COOH)2, and UiO-66(Zr)-(F)4 are at 34, 40, 48, 60, and 100 GPa, respectively (Figure 5d). All the modulus values of the peaks are concluded in Table 1 (4th column). It is found that the BDC-type ligands with larger molecular mass groups could possibly help to enlarge the elastic modulus. For example, the moduli of UiO-66(Hf/Zr)-NH2, UiO-66(Hf/Zr)-(OH)2, and UiO-66(Hf/Zr)-(COOH)2 are in ascending order. Meanwhile, the molecular weights (MWs) of −NH2, −(OH)2, and −(COOH)2 are 16, 34, and 90. This finding agrees well with the results by Tan et al., who demonstrated that sterically bulky linkers increased the elastic modulus of zeolitic imidazolate frameworks (ZIFs) due to increased short-range interlinker interactions.23 This observed correlation can help us to understand the insights of the structure−mechanical property relationship that can be helpful for future design of new materials. Another interesting thing worthy of noting is that the modulus of UiO-66(Hf/Zr)-(F)4 (MW F4 = 78) is larger than that of the UiO-66(Hf/Zr)(COOH)2, which can be ascribed to its more rigid and compact crystal structure as well as the strong hydrogen bonding between F and μ3-OH in the clusters in UiO-66(Hf/Zr)(F)4.59−61 Generally speaking, the elastic modulus of UiO-66(Zr)-type MOFs is generally lower than that of the UiO-66(Hf)-type MOFs, which can be explained by the higher dissociation energy of Hf−O bonds (802 kJ mol−1) than that of Zr−O bonds (776 kJ mol−1).62 UiO-66(Hf)-type MOFs possess the highest elastic modulus (46−104 GPa) by incorporating different ligands of different functionalities and metal nodes, which is higher than the value of 3−20 GPa for other reported MOFs (Table 1). The outstanding mechanical properties of UiO-66(Hf)-type MOFs suggest that using Hf as the metal nodes in MOF synthesis can enhance the elastic modulus of the resultant MOFs. In order to further confirm the mechanical properties of UiO-66-type MOFs, we have summarized the elastic modulus of reported MOFs and other representative inorganic materials

in Figure 6. The elastic moduli of UiO-66-type MOFs (modulus values of the peaks in Figure 5c,d) scatter among

Figure 6. Elastic modulus contrast of different MOFs and MOF resembling materials. (Note: The elastic moduli of MOF-5, ZIF-8 are from nanoindentation experiments.)

the MTN, Silicalite-1, ZSM-5 and occupy the place larger than ZSM-5, lying in the range of 34−104 GPa. It can be obviously seen that the UiO-66(Hf)-(F)4 has the potentially highest elastic modulus of 104 GPa, followed by its UiO-66-type counterpart MOFs, comparable to those of calculated ZrMOF materials (11−140 GPa),15,54,55 all of which are higher than other reported experimentally measured values of the MOFs, usually within the range of 2−10 GPa, for example: ZIF-8 (3.2 GPa), MOF-5 (2.7 GPa), ZIF-zni (∼9.2 GPa), etc.14 Moreover, all the UiO-66-type MOFs possess a higher elastic modulus than that of the benchmark inorganic materials, such as Montmorillonite (15 GPa).14 Finally, the elastic modulus of UiO-66(Hf)-type MOFs is comparable to that of the zeolite materials, such as Silicalite-1 (40 GPa), Zeolite Socony Mobil-5 (57 GPa), and so on.14

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CONCLUSIONS In summary, we have adopted an effective method to systematically measure the elastic modulus values of single MOF nanoparticles based on the emerging bimodal amplitude modulated-frequency modulated AFM technique. By using this novel technique, the topography and stiffness mappings of single MOF nanoparticles can be obtained simultaneously. With a proper reference material and mathematical transformations, their elastic modulus mappings can be derived from the stiffness mappings. We have characterized the microstructures and mechanical properties over a wide range of metal ions and chemical functionalities. The UiO-66(Hf) MOFs tend to show higher elastic modulus than that of the UiO-66(Zr)type MOFs. Meanwhile, all UiO-66-type MOFs show higher elastic modulus than that of the other reported MOFs (3−10 GPa). In addition, the sterically bulky functional groups are demonstrated to have significant influence on the mechanical properties. One important thing worthy of noting is that the elastic modulus range of the UiO-66 intersects with those of ceramics, metals, and polymers. Therefore, MOFs well connect 32207


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the essentially rigid inorganic solids and relatively soft polymers. From the practical point of view, given that UiO-66-type MOFs are expected to be used as adsorbents, which are usually operated under shear flow or mechanical stress, the fundamental understanding of their mechanical properties is quite imperative. Our results will complement the extensive research already done on their synthesis and properties as well as the mechanical properties of the other single-crystalline MOF materials. The AM-FM approach in this work can overcome the limitations of the nanoindentation technique. We believe this study will open a new realm of experimentally measuring the mechanical properties of MOFs crystalline powders, which can further guide the accurate and rational design and synthesis of novel MOF materials with high mechanical strength for various emerging and practical applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b06809. Details of experiments and methods, PXRD, TGA, N2 sorption data (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (K.Z.). *E-mail: [email protected] (D.Z.). ORCID

Yao Sun: 0000-0001-8802-0580 Zhigang Hu: 0000-0003-1916-6484 Dan Zhao: 0000-0002-4427-2150 Kaiyang Zeng: 0000-0002-3348-0018 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the Ministry of Education (MOE) Singapore through the National University of Singapore under MOE AcRF grant (R-265-000-495-112) for Dr. Zeng and MOE AcRF grant (R-279-000-429-112) for Dr. Zhao. Y.S. also thanks the National University of Singapore for the postgraduate scholarship.

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REFERENCES

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