Organic guest molecule induced ultrafast breathing of an epitaxially grown metal–organic framework on a self-assembled monolayer

Purna Chandra Rao , Prabu Mani , Younghu Son , Jiyun Kim and Minyoung Yoon *
Department of Chemistry and Green-Nano Materials Research Center, Kyungpook National University, Daegu 41566, Republic of Korea. E-mail: myyoon@knu.ac.kr

Received 10th July 2021 , Accepted 1st September 2021

First published on 1st September 2021


Abstract

We report epitaxially grown new two-dimensional metal–organic framework (MOF) thin films on a self-assembled monolayer (SAM). We fabricated these epitaxial thin-films using stepwise layer-by-layer seeding followed by solvothermal treatment. The MOF thin films exhibit ultrafast structural flexibility (through breathing) compared to their bulk samples upon uptake of organic guest molecules.


Surface chemistry-driven nanostructured materials on surfaces assist tremendous technologies such as catalysis, sensing, magnetism, energy conversion, and devices to develop modern industrialization.1 The fabrication of thin films using functionalized nanostructural materials has played a pivotal role in various device applications, like chemical sensors, electrodes, membranes, and so on.2,3 Remarkably, the well-defined, porous structures of zeolites,4 covalent–organic frameworks (COFs),5,6 and their associated materials have shown substantial interest in the fabrication of thin films in comparison with organic polymers, metal-oxides, and various activated carbon materials. Due to the limited control of chemical functionalities in zeolites, the crystalline and porous coordination polymers or the so-called metal–organic frameworks (MOFs) comprising tailored structures and tunable functionalities have been studied extensively; also, this could stimulate fascinating properties on the surfaces for precise applications.7–10 Recently, the downsizing of MOF crystallites to the nanoscale regime (i.e., nanocrystals, thin films) has received much interest due to their outperforming properties and new fields of applications.11–13 Upon downsizing, the physiochemical properties of nanoscale materials change compared to the bulk scale; this occurs because of the confinement effect.14 Notably, it is necessary to maintain or improve the bulk scale properties (crystallinity, porosity, flexibility, etc.) of MOFs upon downsizing for specific applications. In the case of thin films, similar properties cannot be easily achieved using conventional synthetic methods such as crystal growth or coating on the substrate, spray techniques, and deposition methods, etc.15 Therefore, new synthetic approaches have been developed for making the epitaxial (or highly oriented) MOF thin films. Among all, the stepwise liquid-phase epitaxial (LPE) method has been widely used for the synthesis of various homogeneous epitaxial MOF thin films.16,17 However, this method has the drawback of performing either at low temperatures or having long time-consuming cycles.18–20 Considering these factors, new approaches for developing epitaxial MOF thin films at ambient temperature are highly pressing to rule out the drawbacks earlier raised.

In MOFs, the structural phase transitions through the breathing phenomenon are the key advantages of MOFs for specific recognition of guest molecules in various research fields. The flexibility-induced breathing has been widely studied in the bulk state of MOFs;21 however, downsizing the bulk-states into nanometer-size generates different intrinsic properties and its breathing behavior is rarely explored.22–24 For example, Cu2(bdc)2(bipy) and DUT-8 (Ni) MOF compounds exhibit different structural transformations as compared to bulk and nanocrystalline forms, upon guest sorption.25,26 Since MOF thin films have practical importance, adapting the structural modification through the breathing effect in thin films could give more impact on guest-driven applications;27 however, this implementation is still in its infancy. Bein and co-workers,28 reported MOF thin film's flexibility upon ethanol sorption. Kitagawa and co-workers,29,30 and Fischer and co-workers31 reported how film thickness affected MOF thin film's structural response upon sorption of guest molecules like methanol, and water molecules, and found different structural changes as compared to bulk samples. With this attention, we focused on a new approach to prepare an epitaxial thin film by a simple synthetic procedure (LBL seeding followed by solvothermal treatment) and then targeted to bring the similar bulk-state breathing effect into thin films for developing chemical-sensor-based devices.

In this study, we synthesized a two-dimensional (2D) metal–organic framework denoted as Zn2(aip)2(bpy)·2DMF (1) (aip = 5-aminoisophthalic acid, bpy = 4,4′-bipyridine, DMF = N,N-dimethylformamide) and fabricated epixaial 2D MOF thin films on Au-coated self-assembled monolayers (SAMs). Unlike the conventional epitaxial MOF thin film synthetic method, LPE, we demonstrated that the epitaxial thin films were easily prepared by 5 times layer by layer (LBL) seeding followed by a solvothermal reaction. The film crystallinity and morphology were characterized using synchrotron X-ray diffraction (XRD) studies and scanning electron microscopy (SEM) analysis, respectively. In addition, we investigated the breathing behavior of the fabricated thin films upon the incorporation of DMF guest molecules (Scheme 1). Also, we conducted the micro-contact printing (μCP) pattern using PDMS stamps.


image file: d1cc03721h-s1.tif
Scheme 1 Schematic illustration of breathing behavior in bulk state and metal–organic framework thin films.

The single crystals of compound 1 were synthesized using the solvothermal method at 100 °C for 24 h (see the ESI for more details). X-ray structural analysis confirms the 2D-layered-pillar-layered structure of 1 (Fig. S1, ESI), in which the aip linker connectivity with the zinc metal centers forms the layered structure; these layers were interconnected by bpy linkers. Flexibility-induced breathing behavior of 1 and 1′ and their structure in the bulk form was recently reported (Fig. S2, ESI).32 The exceptional breathing phenomenon and the layered pillared architecture motivated us to extend this study to the thin film level. MOF thin films were fabricated using the stepwise LPE method, which consists of three consecutive steps: (i) preparation of SAMs on Au, (ii) MOF seeding on the SAM (layer by layer (LBL) MOF growth), (iii) MOF growth using a solvothermal method (Scheme S2) (see the ESI for the detailed procedure). Herein, we have chosen three different functionalized alkyl thiols [namely, aminoethanethiol (AET), aminodecanethiol (ADT), and 11-mercapto-undecanoic acid pyridin-4-ylamide (MUAP)] to investigate the surface properties of SAMs for MOF thin films fabrication denoted as film-AET, film-ADT, and film-MUAP.

Initially, our focus was toward characterization of epitaxially grown MOF thin films on three Au-SAMs. To understand the surface effect of Au-SAMs for thin film growth, we prepared SAMs on a gold substrate at two-time intervals (8 h and 1 week). Because of the low electron density and limited thickness of the thin film, we have chosen a powerful synchrotron XRD technique for the analysis of MOF thin films in preference to conventional XRD. To determine the crystallinity and ordered orientation of these three MOF thin films, we have performed synchrotron XRD (in-plane: θ fixed at 2θ and out-of-plane: θ–2θ at 298 K) experiments. The out-of-plane XRD analysis of these three thin-films showed less intense and relatively strong diffraction peaks along 0k0 planes at 8 h and 1 week, respectively (Fig. S3, ESI). Thus, it is understood that less reaction time (8 h) can minimize the reaction rate of thiol–gold (S–Au) bond formation, thus reducing the seed grown on the surface of SAMs. The film-AET possesses a strong diffraction peak at 010 and three weak diffraction peaks along the 020, 030 and 040 directions. In a similar fashion, the film-ADT and film-MUAP exhibited diffraction peaks along their corresponding planes, as observed in the film-AET (Fig. 1).


image file: d1cc03721h-f1.tif
Fig. 1 Out-of-plane (θ–2θ mode) synchrotron X-ray diffraction (XRD) patterns of the bare gold substrate, thin films (film-AET, film-ADT, film-MUAP), and simulated diffraction pattern of activated compound 1′. The inset figure represents the scattering geometries of the XRD study and structural orientation along the 010 and 020 planes.

In contrast, the in-plane XRD analysis of these thin films at 1 week exhibited several diffraction peaks corresponding to the hkl planes, which is quite different from the out-of-plane data (Fig. 2). Hence, it is evidenced that these thin films are epitaxially grown or formed in preferred orientations. Moreover, the out-of-plane and in-plane XRD patterns are consistent with simulated patterns, as calculated from the single-crystal X-ray structure of activated compound 1′. The reason for the inconsistency of both XRD patterns with the as-synthesized bulk-compound 1 (simulated data) is because of the fast-structural response of thin films after synthesis (i.e., ethanol washing and dried under N2 stream). Therefore, synchrotron XRD analysis suggested that epitaxially grown MOF thin films on three different Au-SAMs exhibited high crystallinity with preferred crystal orientation.


image file: d1cc03721h-f2.tif
Fig. 2 In-plane (θ fixed at 2θ) synchrotron X-ray diffraction (XRD) patterns of the thin films (film-AET, film-ADT, film-MUAP), and simulated diffraction pattern of activated compound 1′. The inset figure represents the scattering geometries of the XRD study and structural orientation along the indexed hkl planes.

In addition, we conducted rocking curve experiments (θ-scan) for further confirming the uniform orientation of three MOF thin films grown on Au-SAMs at 1 week. Based on the out-of-plane synchrotron XRD studies, it was confirmed that the fabricated thin films revealed preferential crystal growth in the 010 direction. In this regard, the rocking measurements of the three thin films were fixed at the 010 peak position in the out-of-plane geometry. The film-AET possesses a sharp peak at 2.58° with full-width at half-maximum (FWHM) of 0.77°, whereas film-ADT and film-MUAP show a little broad peak, around 2.59° and 2.39° with FWHM of 1.90° and 1.35°, respectively (Fig. S4, ESI). These results also indicated a well-ordered orientation of these MOF thin films.

Our next investigation was finding out the crystal domain size (i.e., thickness) of thin films, which was calculated from out-of-plane XRD patterns using the Debye–Scherrer equation. The crystallite domain sizes of ∼47, 39 and 56 nm for film-AET, film-ADT and film-MUAP were calculated from the diffraction peak along the 020 direction, respectively. The difference in domain size mismatch between these thin films may be attributed to the formation of various surface environments by longer (ADT), shortened (AET) spacer and pyridyl-functionality (MUAP) of alkyl thiols. The SEM micrographs confirmed that these thin films possess perfectly aligned crystallites with sizes ranging from 50 to 100 nm (Fig. 3, and Fig. S5, ESI). Also, the SEM image with the observed burn surface hit by the electron beam clarified the presence of MOF nanocrystallites on the Au surface (Fig. S6, ESI). Furthermore, these morphology studies provided the mechanism of epitaxially grown MOF thin film formation on Au-SAMs (Fig. S7, ESI). We carried out the ellipsometry analysis to measure the thickness of MOF thin films. By considering the chain lengths of SAMs, we focused on film-AET and film-MUAP. The AET Au-SAM and MUAP Au-SAM possess thicknesses of 0.7–1.2 nm and 2.6–4 nm, respectively and the film-AET and film-MUAP have a thickness of 2.5–4.8 nm and 9.4–15 nm, respectively.


image file: d1cc03721h-f3.tif
Fig. 3 The morphology studies of (a) film-AET, and (b) film-ADT analysed through the scanning-electron microscopy (SEM) technique.

Additionally, we performed two control experiments to determine the significance of our approach. Firstly, the LBL-MOF growth on Au-SAMs (5 layers seeding) without following the solvothermal reaction reveals no diffraction peaks (Fig. S8, ESI). The latter, only solvothermal synthesis without LBL-growth, resulted in forming individual crystal islands instead of ordered MOF growth as verified by SEM analysis (Fig. S9, ESI). Therefore, all three consecutive steps are necessary to fabricate the preferred orientation of MOF thin films.

Based on our previous studies, the breathing behavior of the bulk sample (1) shows structural flexibility by the shrinkage and expansion of the void/pores upon inclusion and exclusion of guest DMF molecules and vice versa. With this motivation, we are insisting that similar flexibility through the breathing phenomenon in MOF thin films may provide practical utility and have a massive impact on chemical-based sensors. The breathing behavior of the fabricated thin films was analysed using a conventional powder XRD diffractometer. The PXRD patterns of film-AET, film-ADT and film-MUAP indicated a similar breathing effect as studied in the bulk-state compound 1 (Fig. 4 and Fig. S10, ESI). To understand the actual time scale of framework breathing, we performed time dependent PXRD analysis on the bulk, nanocrystals by mechanical grinding and thin film of the compound 1. In particular, the bulk and nanocrystal MOFs were activated at 100 °C under dynamic vacuum, whereas the MOF thin films were activated by purging with N2 gas. This resulted in the bulk and nanocrystalline MOFs needing 12 h and 6 h, respectively for structural transformation (Fig. S11 and S12, ESI). However, the MOF thin films require only 2 min for complete transformation (Fig. S13, ESI). Furthermore, we soaked these activated compounds in the guest molecule (DMF) to determine the time-course of structural reversibility. The results showed that the bulk and nanocrystalline MOF require 48 h and 24 h, respectively for changing to the pristine state, whereas the thin film requires 60 min (Fig. S11–S13, ESI). In comparison to the bulk and nanocrystal states, the ultra-fast breathing in these thin films may be attributed to controlled thickness, which can minimize the activation barrier for guest sorption and increase the speed of guest molecule sorption through neighboring layers. Therefore, the as-synthesized thin films demonstrated ultrafast structural flexibility upon addition and removal of DMF guest molecules, which may be employed in chemical-sensor-based devices.


image file: d1cc03721h-f4.tif
Fig. 4 Powder X-ray diffraction patterns of breathing assisted structural flexibility in film-MUAP upon soaking and removal of DMF guest molecules.

To develop thin-film materials for real-time devices, it is necessary to make a pattern with the desired shape or size on the surface. To demonstrate this, we have performed the μCP pattern using different PDMS stamps, by following a similar protocol of film fabrication. Amine-terminal SAM was patterned by μCP and the remaining area was filled with alkyl-terminal SAM, which may allow the formation of MOF thin film on amine-terminal SAM patterns (Scheme S3, ESI). Simple linear and square shaped MOF patterns were easily prepared. The SEM images confirmed the expected size and shape of the patterned surface (Fig. S14, ESI). In addition, a more complex pattern that is similar to an electrical circuit pattern was also prepared by the same method. We carried out two subsequent studies to understand the thickness of MOF seed growth and overall morphology of the pattern using atomic force microscopy (AFM) and SEM analysis, respectively. The SEM and AFM image reveal few-layered seed growth with film thickness ∼10 nm on Au-SAMs (Fig. S15, ESI).

In conclusion, we have fabricated epitaxial 2D MOF thin films on Au-SAMs and they were characterized using various instrumental techniques. For the preparation of epitaxial MOF thin films, the LPE method is commonly used, which requires many complicated steps. However, this work presents only 5 steps LBE seeding followed by a solvothermal treatment, a conventional MOF synthetic method can produce epitaxial thin films. The synchrotron XRD and SEM analyses confirm the ordered orientation of MOF thin films on three different Au-SAMs. Furthermore, we investigated the breathing behavior of the fabricated thin films for DMF guest molecules; the results convey the ultrafast breathing behavior at the thin film level compared to the bulk state. We demonstrated the MOF patterning using the PDMS stamping method; this will have real-time applications for manufacturing devices in various fields.

This work was supported by the National Research Foundation of Korea (NRF) (NRF-2019H1D3A1A01102895 and NRF-2021R1A4A5030513). The X-ray experiments at the PAL, 2D-SMC and 5A MS-XRS beamlines were supported by the Ministry of Science and ICT. We acknolwedge Prof. J. Lee for ellipsometry experiments.

Conflicts of interest

There are no conflicts to declare.

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Footnotes

Electronic supplementary information (ESI) available. See DOI: 10.1039/d1cc03721h
These authors contributed equally to this work.

This journal is © The Royal Society of Chemistry 2021