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. 2020 Jan 22;6(2):247–253. doi: 10.1021/acscentsci.9b01205

Tracking and Visualization of Functional Domains in Stratified Metal–Organic Frameworks Using Gold Nanoparticles

Xuedi Qin †,‡,§, Sanfeng He †, Jiasheng Wu †, Yaqi Fan †, Fang Wang †, Songwei Zhang †, Siqi Li †, Lianshun Luo †, Yanhang Ma †, Yongjin Lee †, Tao Li †,*
PMCID: PMC7047430  PMID: 32123743

Abstract

graphic file with name oc9b01205_0005.jpg

We report here a new technique for the identification and visualization of functional domains in stratified metal–organic frameworks (MOFs). The technique, namely, gold diffusion enabled domain identification, utilizes the diffusion of Au nanoparticles within MOF cavities to track and selectively stain the more Au-philic domain in an MOF particle thereby allowing direct observation of domains, determination of domain sequences, and, in certain cases, domain boundaries under transmission electron microscopy. This method is an excellent tool for studying MOF materials with complex domain hierarchy.

Short abstract

We report a new technique for the identification and visualization of functional domains in stratified metal−organic frameworks.

Introduction

Through two decades of efforts, metal–organic frameworks (MOFs) have been established as an exciting class of functional porous materials due to their structural complexity and variability.1−8 Such complexity can be realized through three approaches: (1) periodic organization of multiple molecular building blocks (MBBs) through topological design;9−15 (2) random mixing of MBBs (multivariate MOFs or MTV MOFs);16−18 and (3) engineering domain building blocks (DBBs) within mesoscopic or macroscopic size regime.19 In recent years, the third approach has attracted increasing attention because manipulating material hierarchy at mesoscopic scale is a powerful tool to unlock unique properties absent in a single material. Notable examples of MOFs with mesoscopic hierarchy include but are not limited to stratified MOFs,20−25 yolk–shell MOFs,26,27 gradient MOFs,28,29 hollow MOFs,26,30,31 Janus MOFs32 etc.

On the other hand, the escalating complexity of MOF materials comes with the need for advanced characterization techniques to unambiguously elucidate and support their structural claims. Unfortunately, material characterization is a nontrivial task and often appears to be the bottleneck for materials design. Taking stratified MOF as an example, when the MOF particles are larger than a few micrometers, their strata can be identified and visualized using optical microscopy through fluorescent labeling.33−37 When the MOF particles fall below a submicron size regime, electron density or elemental contrast between strata is needed for the identification of domains using electron microscopic (EM) techniques. For those materials lacking these features, strata within an MOF become indistinguishable from each other using direct imaging methods. For MTV MOFs with complex domain distribution, their structural elucidation has to rely upon a combination of sophisticated and indirect characterization methods such as solid-state NMR and molecular dynamic simulation.38

While metal nanoparticles embedded in MOF matrices have been widely explored for advanced catalysis,39−44 they have not been used as staining agents to study the structural hierarchy of MOF materials. Here, for the first time, we utilize the diffusion of Au nanoparticles (Au NPs) during gas phase reduction to track and visualize the functional domains within stratified MOFs. This method relies upon the diffusion and preferential docking of Au NPs in the more Au-philic domain in a stratified MOF to enhance its domain contrast under an electron beam.45−47 Therefore, it is named as gold diffusion enabled domain identification (GDEDI). GDEDI comprises four steps: (1) HAuCl4 is loaded into a stratified MOF through wet impregnation; (2) Au(III) is reduced to Au(0) in a reductive gas atmosphere (H2); (3) Au(0) species diffuse within the MOF cavities and preferentially dock at the more Au-philic stratum; and (4) Au NPs grow larger through Ostwald ripening. The high electron density contrast of the resulting Au NPs allows easy identification of functional domains within a stratified MOF using bright field imaging under regular transmission electron microscopy (TEM). Because of the sharp contrast of Au NPs, the domain boundaries can be outlined with high spatial resolution. The feasibility of the GDEDI has been demonstrated using nonepitaxially and epitaxially grown stratified MOFs with various functional groups. In particular, for UiO-66-NO2@UiO-66-NH2, a core–shell MOF that shows neither electron density contrast nor elemental difference between two strata, the domain boundaries can be outlined effortlessly using GDEDI, which is otherwise difficult to characterize using any existing characterization tools.

Results and Discussion

To demonstrate, we selected a Zr-based core–shell MOF, MOF-801@UiO-66 (801@66), to study the nucleation and growth behavior of Au NPs. 801@66 comprises a denser MOF-801 core and a more porous polycrystalline UiO-66 shell.48,49 This density difference allows the core/shell boundary to be visualized under TEM for the verification of our method. MOF-801 particles with an average size of 195 ± 80 nm were synthesized according to a reported method (Figure S1). Polycrystalline UiO-66 shell was then uniformly grown on the surface of MOF-801 through a rapid nucleation approach developed previously by our group (Figures 1A and S1).25 The crystallinity of both MOFs was confirmed by the powder X-ray diffraction (PXRD) patterns (Figure S2). The thickness of the shell was measured to be 36 ± 2 nm based on the TEM image (Figures 1A and S1).

Figure 1.

Figure 1

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(A) TEM image of MOF-801@UiO-66. (B) STEM image, line scans and EDS elemental mapping of Au(III)-801@66. (C) Schematic illustration, XPS spectra and corresponding TEM images of Au(III)-801@ 66 after 0, 1, and 10 min of reduction. XPS peaks, 90.8 and 87.3 eV, corresponding to the 4f orbital electrons of Au(III) shifted to 89.3 and 85.8 eV, which corresponds to 4f orbital electrons of Au(I). Peaks at 88.6 and 85 eV can be assigned to Au(0).50−52 The peaks of XPS are corrected by C 1s 284.8 eV as reference. TEM images of (D) UiO-66@MOF-801 , (E) MOF-801@UiO-66-NH2, and (F) MIL-101(Cr)@UiO-66 after performing GDEDI, samples in D and E were ultra-microtomed to 30 nm sections.

Next, HAuCl4 was loaded into the pores of 801@66 through a double solvent impregnation method affording Au(III)-801@66. This process has been demonstrated by Qiu et al. to be an effective way to achieve even distribution of metal ions in MOFs.53 Indeed, the energy-dispersive X-ray spectroscopy (EDS) results confirmed that Au distributed evenly across the core–shell particle (Figure 1B). The gas phase reduction was carried out at 200 °C under a dynamic H2 flow. To monitor the reduction process of Au(III), we performed X-ray photoelectron spectroscopy (XPS) and TEM analysis on samples at 0, 1, and 10 min after reduction (Figure 1C). Before reduction, the XPS of Au(III)-801@66 shows that Au existed as a mixture of Au(III) and Au(0). However, Au NPs were not yet observable under regular TEM at this stage. After 1 min reduction, we observed the disappearance of Au(III), enhancement of the Au(0) signal, and the emergence of Au(I) under XPS. Meanwhile, Au NPs started to appear in the MOF-801 core (Figure 1C). After 10 min of reduction, the XPS signal of Au dramatically diminished to near background level indicating an inward migration of Au species. The TEM image clearly shows that all the Au NPs are located in the core with their outer boundary aligned well with the observable core–shell boundary. When the core–shell sequence was inversed (UiO-66@MOF-801, Figure S1d), the diffusion directionality of Au species was also inversed resulting in selective staining of the MOF-801 shell (Figure 1D). Interestingly, when UiO-66 was replaced by UiO-66-NH2 (MOF-801@UiO-66-NH2, Figure S2c), MOF-801 was no longer the favored domain. Instead, UiO-66-NH2 was selectively stained (Figure 1E). If UiO-66 was kept as a shell but the core was replaced by MIL-101(Cr) and MIL-53(Cr) (MIL-101(Cr)@UiO-66 and MIL-53(Cr)@UiO-66),54,55 Au NPs preferentially grew in the Cr-MOF cores (Figure 1F, Figure S2). The fact that Au NPs nucleated and grew within a preferred domain is likely because of the different Au-philicity in MOFs. An Au-philic MOF provides cavities that can better stabilize the surface of the resulting Au NPs during growth. These examples demonstrate the feasibility of using Au NPs to track and differentiate MOF domains.

To rule out the structural factor from the equation, we sought to investigate the functionality dependence of Au-philicity in a variety of UiO-66 analogues. Ten core–shell MOFs involving four functional groups, -NH2, -NO2, -H, and -Br2, were synthesized through epitaxial growth (Figures S3–S9, Table S1). Taking UiO-66-NH2@UiO-66-Br2 (NH2@Br2) and UiO-66-Br2@UiO-66-NH2 (Br2@NH2) as examples, due to similar electron density between core and shell, it is challenging to observe their domain boundaries with bright field TEM imaging. Although EDS elemental mapping can be used to identify the sequence of the core–shell based on the spatial distribution of Br and N, the core–shell boundary cannot be clearly outlined due to limited spatial resolution (Figure 2A). In contrast, GDEDI allows the UiO-66-NH2 domain to be selectively stained and visualized under TEM. More impressively, the even distribution of Au NPs contours the core–shell phase boundaries enabling the shell thickness to be precisely measured as 16 ± 2 nm (for NH2@Br2) and 21 ± 1 nm (for Br2@NH2) (Figure 2B,E). Note that a few Au nanoparticles were found in the bottom part of the inner domain outlined by the red line (Figure 2E). This is due to sample smearing under shear force during cutting. This phenomenon can be confirmed by the correlation between cutting direction and smearing direction (Figure S10). 3-D TEM tomography of Br2@NH2 again confirms that all the Au NPs are located in the shell (Figures S11–S12, Video S1).56 The shell thickness of NH2@Br2 was calculated to be 16 nm based on 1H NMR data, which is in excellent agreement with the GDEDI approach (detailed calculation can be found in Figure S13). To definitively prove that the GDEDI determined phase boundary matches with the real phase boundary of the core–shell MOF, we replaced Zr core with heavier Hf core, affording Hf-UiO-66-NH2@UiO-66-Br2 (Hf-NH2@Zr-Br2) and Hf-UiO-66-Br2@UiO-66-NH2 (Hf-Br2@Zr-NH2). Figure 2C,F indeed confirmed that the inner boundary of Au NPs is in good agreement with observed core–shell phase boundary through mass–thickness contrast. Through careful measurement, the shell thickness outlined by Au NPs is 35.4 ± 4.1 nm, very close to the real observable shell thickness (36.7 ± 2.9 nm) (Figure S14, Tables S2–S3).

Figure 2.

Figure 2

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STEM image, line scans and EDS elemental mapping of (A) UiO-66-NH2@UiO-66-Br2 and (D) UiO-66-Br2@UiO-66-NH2. TEM images of (B) UiO-66-NH2@UiO-66-Br2, (C) Hf-UiO-66-NH2@UiO-66-Br2, (E) UiO-66-Br2@UiO-66-NH2, and (F) Hf-UiO-66-Br2@UiO-66-NH2, after performing GDEDI. Samples in C, E, and F were ultra-microtomed to 30–50 nm sections. The domain boundaries are outlined by red dotted lines.

Extending this strategy to other core–shell MOFs led to the conclusion that the Au-philicity in UiO-66-X analogues follows the order of -NH2 > -NO2 > -H > -Br2 (Figure 3A–E). It is worth noting that for UiO-66-NO2@UiO-66-NH2 and UiO-66-NH2@UiO-66-NO2, EDS mapping is incapable of differentiating the domains because both strata show identical elemental information and similar electron density. However, GDEDI yet again allows the UiO-66-NH2 domain to be selectively labeled by Au NPs with sharp contrast at the phase boundaries in both cases. Through imaging processing, the phase boundary within UiO-66-NO2@UiO-66-NH2 can be determined by a plot profile based on the average mass–thickness contrast extracted from a TEM tomography image (Figure S15). As control experiments to the core–shell examples, Au NPs were found to be evenly distributed in single phase MOFs regardless of functionality (Figure S16).57 Further extending this method to MTV MOFs with various ratios of -NH2 and -Br2 or -NH2 and -NO2 has led to the same conclusion (Figure S17). More impressively, GDEDI can also be applied to more complicated scenarios such as identifying the middle layer domain in a triple-layer stratified MOF, UiO-66-Br2@UiO-66-NH2@UiO-66-Br2 (Figure 3F).

Figure 3.

Figure 3

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TEM images of (A) UiO-66-NH2@UiO-66-NO2, (B) UiO-66-NO2@UiO-66-Br2, (C) UiO-66-NO2@UiO-66, (D) UiO-66-NO2@UiO-66-NH2, (E) UiO-66-Br2@UiO-66, and (F) UiO-66-Br2@UiO-66-NH2@UiO-66-Br2 after performing GDEDI. (G) Binding energy between functional groups and Au. Samples in D–F were ultra-microtomed to 30–50 nm sections. Red dotted lines represent the domain boundaries in each sample.

To understand the nature of Au-philicity in MOF pores, the binding energy of four functional groups, -NH2, -NO2, -H, and -Br, on the (111) facet of Au was calculated using density functional theory (DFT). The results show that -NH2 exhibits the highest binding energy at 15.2 kJ/mol followed by -NO2 (8.5 kJ/mol), -H (6.7 kJ/mol), and -Br (6.1 kJ/mol) (Figure 3G). This order is consistent with our experimental observation which suggests that the origin of the preferential docking behavior of Au NPs in specific MOF domains is a result of Au surface stabilization by surrounding functional groups. It is both surprising and encouraging that a tiny difference of 0.6 kJ/mol between -H and -Br can be easily recognized by Au NPs.

We next turned to understand the growth mechanism of Au NPs in the MOF; we selected Au(III)-NH2@Br2 and monitored the growth process of Au NPs using TEM. We found that Au NPs formed within the outer layer of the MOF particles in less than 1 min but gradually disintegrated and completely migrated to the core after 3 min (Figure S18). Surprisingly, after treating Au NPs-containing NH2@Br2 with 1 M HCl, localized etching around each Au NPs was observed in the MOF suggesting the growth of Au NPs resulted in the collapse of the neighboring MOF structure leaving it prone to chemical etching (Figure S19). This evidence suggested that the size of Au NPs is not limited by the MOF cavity size.

Lastly, to showcase the ability of GDEDI method in handling complex samples, we purposefully mixed three MOF particles: UiO-66-Br2, UiO-66-NH2, and NH2@Br2 in one solution. Because of their similarity in morphology and size, distinguishing them is no easy task. Performing GDEDI on this mixture allows rapid particle identification. As shown in Figure 4, four particles on the left exhibit uniform Au NPs distribution, suggesting them to be UiO-66-NH2. The particle on the right top of the image exhibits no Au NPs inclusion indicating that it is UiO-66-Br2. The remaining three crystals exhibit a distribution of Au NPs only in the core, suggesting them to be NH2@Br2 (more examples can be found in Figure S20).

Figure 4.

Figure 4

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TEM image of the mixed of UiO-66-Br2, UiO-66-NH2, and NH2@Br2 after performing GDEDI. The domain boundaries are outlined by red dotted lines.

Conclusion

In summary, we reported a new method, namely, GDEDI, for the characterization of functional domains in stratified MOFs. This method differentiates MOF domains based on Au-philicity and locates them through Au NP diffusion. The high electron density of Au NPs then allows visualization of MOF domains under TEM. In some cases where small Au NPs were evenly distributed throughout a Au-philic domain, it is even possible to identify domain boundaries through imaging processing. In the context of the rapid growing MOF field, we believe this method can be a useful complementary method to other routine characterization techniques for the study of MOF materials with domain complexity.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (Grant No. 21701110) and the start-up funding from ShanghaiTech University for T. L. This work was partially supported by the Commission for Science and Technology of Shanghai Municipality (17ZR1418600) and the National Natural Science Foundation of China (Grant No. 21875140) for Y. M. This work is partially supported by CℏEM, SPST, ShanghaiTech under Grant No. EM02161943. This work made use of the resources of the Instrumental Analysis Center of SPST at ShanghaiTech University. We also thank the high-performance computing platform of ShanghaiTech University and the Shanghai Supercomputer Center for the use of their computing resources.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acscentsci.9b01205.

  • Materials, synthesis, characterization, supplemental figures, tables, and references (PDF)

  • 3-D TEM tomography tilt series (MP4)

The authors declare no competing financial interest.

Supplementary Material

oc9b01205_si_001.pdf (7.9MB, pdf)
oc9b01205_si_002.mp4 (10.8MB, mp4)

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