Exfoliation of metal–organic frameworks into efficient single-layer metal–organic nanosheet electrocatalysts by the synergistic action of host–guest interactions and sonication

Wei Pang , Bing Shao , Xiao-Qiong Tan , Cong Tang , Zhong Zhang * and Jin Huang *
State Key Laboratory for the Chemistry and Molecular Engineering of Medicinal Resources, School of Chemistry and Pharmacy, Guangxi Normal University, Guilin 541004, China. E-mail: zhangzhong@mailbox.gxnu.edu.cn; huangjin@mailbox.gxnu.edu.cn

Received 15th November 2019 , Accepted 20th January 2020

First published on 20th January 2020


Abstract

Single-layer two-dimensional metal–organic framework (MOF) nanosheets combine the characteristic of highly ordered structures of MOFs and unique physical and chemical properties of two-dimensional (2D) materials, which is beneficial for developing high-performance electrocatalysts and studying the structure–property relationships. However, the instability of coordination bonds during exfoliation results in great difficulties in their preparation and characterization. This work takes advantage of the anisotropy of coordination bonds in three-dimensional (3D) pillared-layer MOFs and selectively breaks the interlayer bonding through substitution of the pillared ligands by capping solvent molecules synergized with sonication in a solvent over a relatively short time course (30 min), thus leading to single-layer metal–organic nanosheets, which retain the 2D layered structure of the original crystalline counterpart. The as-synthesized single-layer metal–organic nanosheets are efficient electrocatalysts for the oxygen evolution reaction (OER) with a turnover frequency as high as 0.144 s−1 and 0.294 s−1 at overpotentials of 400 mV and 500 mV in neutral electrolyte media, respectively, which are higher than other heterogeneous catalysts.


Two-dimensional (2D) nanomaterials with atomic/molecular thickness have received great research attention because of their unique dimension and thickness-related physical and chemical properties, exhibiting promising applications in energy storage and conversion, separation, catalysis, and transparent electronic/optoelectronic devices.1–3 Recently, 2D metal–organic framework (MOF) or porous coordination polymer (PCP) nanosheets have emerged as a new class of 2D nanomaterials which exhibit superior electrocatalytic performance when applied to electrocatalysis.4–8 These single-layer 2D metal–organic nanosheets possess the following merits: atomic thickness to afford high mass permeability and superior electron conductivity,9–11 extremely high percentage of exposed active metal sites to ensure high catalytic activity,12–14 and highly ordered arrangement of atoms formed via coordination bonding that can be easily identified and precisely controlled.15,16 Thus, single-layer MOF nanosheets offer an ideal model system to explore the precise structure–property relationship for rational design of electrocatalysts at the atomic/molecular level.14,17,18

Generally, 2D MOF nanomaterials have been synthesized by two kinds of methods, i.e., the top–down method and the bottom–up method.19 The former usually requires the exfoliation of bulk intrinsic layered MOFs, and the latter involves the direct synthesis of 2D MOF nanosheets from metal and organic precursors. Although there are multitudinous 2D MOF structures, there are only a few reports on ultrathin 2D MOF nanosheets due to the poor stability and the limitations of the preparation methods.10,20–22 The major obstacles for preparing 2D MOF materials by the top–down method are the relatively similar intralayer and interlayer interactions. Since the intralayer coordination bonds of the layered 2D MOFs are not always stronger than the interlayer interactions (hydrogen bonds, π–π stacking, multiple interactions between the wrinkle surface, etc.), it leads to difficulties in the exfoliation of some 2D MOF materials by the methods similar to the traditional 2D materials.20,23–25 Different from the top–down exfoliation method, the key of the bottom–up method is to selectively control the growth direction of the layered MOF. In order to restrict the growth in the specific directions, interfacial synthesis,21,26 the surfactant assisted method,27–29 and the coordination end sealing method22,30,31 were utilized. The limitation of these methods is the low yield and difficulties in removing the surfactant molecules or end ligands from the surface of nanosheets.19

In 3D pillared-layer MOFs, the pillar ligands can be replaced by the original or post syntheses, which indicates that the 2D coordination layers in this structure are stable enough.32,33 In particular, Zhang et al. used redox pillared ligands to construct 3D pillared-layer MOFs, in which the pillars can be oxidized to lower their coordination ability and subsequently be selectively removed, while the 2D layers remain intact and become exposed to more OMS as catalytically active centers. They also explained multistep evolution from a bulk metal–organic framework to ultrathin nanosheets through single-crystal X-ray diffraction (SXRD).34,35 This shows that 3D pillared-layer MOFs can be used as the precursors for preparing single-layer or few-layer metal–organic nanosheets. Excitingly, many 3D pillared-layer MOFs have been reported32,33,36 to provide a large number of raw material sources for the preparation of 2D MOF nanomaterials. Although a few 3D pillared-layer MOFs have been exfoliated into single-layer MOF nanosheets through the replacement of the pillar ligands by capping-end ligands, the metal sites of these single-layer nanosheets are generally coordinated by the incoming ligands, limiting their application in catalysis.19,31 Currently, the pillared ligands can be replaced by some capping-end small molecules (such as H2O, MeOH or EtOH, and so on) through the host–guest action in the 3D MOFs, leading to the transformation into 2D MOFs followed by removing the capping-end small molecules via heating or high vacuum to generate OMS for various applications.37–40 Because of the considerable interlayer forces between the created 2D metal–organic coordination layers, 3D pillared-layer MOFs are difficult to transform from block to single-layer MOF nanosheets with OMS.32,41 However, the above results indicate that the coordination bonding in the 3D pillared-layer structure has obvious anisotropy, and it is possible to realize the orientation-controllable structural transformation through crystal engineering synergized with or followed by mechanical force to produce ultrathin nanosheets.

Herein, we report a novel strategy for the preparation of single-layer metal–organic nanosheets from 3D pillared-layer MOFs relying on the anisotropy of coordination bonds. The overall fabrication process is schematically illustrated in Fig. S1. To obtain 2D MOF nanosheets with OMS for electrocatalysis, a family of pillared-layer MOFs ([M2(bdc)(dabco)]·guest (M = Zn, Co, Ni, H2bdc = 1,4-benzenedicarboxylic acid, dabco = 1,4-diazabicyclo-[2.2.2]octane)) were used as the exfoliation precursors. [M2(bdc)] is the 2D metal–organic coordination layer and dabco is the pillared ligand. H2O was used as a capping-end guest molecule for exchanging the pillared ligands and then destructing the pillar structure in the 3D pillared-layer MOFs. Simultaneously, ultrasonication prevented the resulting 2D metal–organic layers from restacking. Consequently, the original MOFs can be easily and rapidly exfoliated into single-layer nanosheets (∼0.9 nm) in high yield (∼94%) by sonication exfoliation in H2O. Furthermore, we demonstrated that the as-prepared 2D materials exhibit very high OER performance in heterogeneous electro-catalysis (Fig. 1).


image file: c9nr09742b-f1.tif
Fig. 1 Schematic illustration of the process developed to produce 2D MOF nanosheets via the synergistic action of host–guest interactions and sonication.

H2bdc and dabco are extensively used organic linkers to construct a variety of 3D pillared-layer MOFs.32,36,39 Thus, a known MOF, [Zn2(bdc)2(dabco)]·4DMF·0.5H2O (DMF = N,N-dimethylformamide) (denoted as 3D-Zn), containing a [Zn2(bdc)] layer composed of dinuclear paddle-wheel secondary building units (SBUs) was used in this work (Fig. 2).36 Although many MOFs have been constructed by using a paddle-wheel type dinuclear metal carboxylate cluster (M2(CH3COO)4(H2O)2, M = transition metal), they usually reveal poor stability in water.42–47 Attractively, Chen et al. reported that the single-crystals of 3D-Zn can transform to the single-crystals of a 2D MOF [Zn2(bdc)2(H2O)2]·3DMF (denoted as 2D-Zn) after being exposed to air for 3 days. During the transformation, the pillared ligands were replaced by water molecules, and then the structure changed from 3D to 2D.39 Powder X-ray diffraction (PXRD) showed that 2D-Zn can remain stable in water for a long time (Fig. S1 and S2). In this paper, we prepared rectangular-shaped microcrystals of 3D-Zn, which transformed to the layer-shaped microcrystals of 2D-Zn after being exposed to air for 3 days (Fig. 3a and Fig. S2, S3). Based on the characteristics of their structure and morphology, 3D-Zn and 2D-Zn were used as model structures to investigate the synergistic action of host–guest interactions and external forces in the exfoliation of bulk MOF materials.


image file: c9nr09742b-f2.tif
Fig. 2 (a) The 3D pillared-layer structure of 3D-Zn. (b) Viewing of the 2D [Zn2(bdc)2] layer.

image file: c9nr09742b-f3.tif
Fig. 3 Scanning electron microscopy (SEM) images of (a) 3D-Zn and (b) 2D-Zn-few-layer. (c) Atomic force microscopy (AFM) images of the 2D-Zn-few-layer, showing the height profile of the 2D MOF nanosheet along the green lines. (d) Transmission electron microscopy (TEM) images of the 2D-Zn-single-layer. (e) TEM-energy dispersive spectrometer (TEM-EDS) mapping images of the 2D-Zn-single-layer. (f) AFM images of the 2D-Zn-single-layer, showing the measured dimensions of the individual flakes. (g) Proposed mechanisms for the transformation of a 3D pillared-layer MOF to ultrathin 2D MOF nanosheets.

Typically, sonication exfoliation of a bulk 2D structure is a simple and effective method for the synthesis of single-layer or ultrathin 2D nanomaterials. Hence, the microcrystal phase of 2D-Zn was dispersed in water with ultrasound for 30 min followed by PXRD characterization, which manifested the structural consistence of the samples before and after sonication (Fig. S4). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) revealed that the morphology of the product is a sheet-like layer, seemingly thicker than the original 2D-Zn (Fig. S5 and S6). Atomic force microscopy (AFM) imaging showed that the thickness of 2D-Zn is greater than ∼12 nm (Fig. S7). Though various solvents other than water were taken as the dispersed media, sonication exfoliation as the traditional top–down method could not provide sufficient mechanical force to thoroughly break the interlayer interaction and subsequently exfoliate the microcrystalline 2D-Zn into ultrathin or single-layer MOF nanosheets with the thickness of less than 5 nm.48

Since conventional sonication exfoliation cannot exfoliate 2D-Zn into ultrathin/single-layer nanosheets, we attempted to use 3D-Zn as the exfoliated precursor. To illustrate the effect of various solvents on the dimension reduction of 3D-Zn to a 2D layered structure and whether further exfoliation might occur for such a 2D layered system, crystalline 3D-Zn as the precursor was merely immersed in H2O without any mechanical action. It is rapidly converted to 2D-Zn within 30 min accompanied by the crystal morphology varying from a rectangular block to aggregated sheets, and the created morphology remained unchanged upon immersing in H2O for further 12 hours and 48 hours, respectively (Fig. 3a and Fig. S4, S8). The Raman spectrum analysis revealed that the aggregated sheet-like material does not present the peaks due to dabco pillars but the Raman scattering peaks from the [M2(bdc)] layer remain (Fig. S9), confirming that dabco was selectively removed from the initial 3D framework to give the layered 2D-Zn. When the aggregated sheet-like product was dispersed in H2O again with the assistance of sonication for 30 min, few-layer nanosheets were obtained (Fig. 3b and Fig. S10). Their Raman spectrum does not show any obvious difference from that of the aggregated sheets,49 and the mass spectrum of the ultrasonic solution shows a signal at m/z = 113 corresponding to monoprotonated dabco (Fig. S9 and S11).50 The thickness of few-layer nanosheets is ∼5 nm, corresponding to the sextuple [Zn2(bdc)(H2O)2]·guest layer, hereafter named 2D-Zn-few-layer (Fig. 3c and Fig. S12). Key observations made during the exposure of bulk 3D-Zn in H2O allowed us to elucidate the plausible host–guest interaction mechanisms, as depicted in Fig. 3g. H2O as the capping ligand substitutes pillared dabco to coordinate with the Zn2+ center to produce a single [Zn2(bdc)2(H2O)2] layer, following which vast solvent molecules are intercalated to expand the interlayer space and decrease the interlayer interaction, benefiting the generation of single- or few-layer MOF nanosheets with ultrathin thickness. However, lots of the created nanosheets reaggregate rapidly along the direction perpendicular to the layer, which make it impossible to separate single-layer nanosheets without the assistance of external forces.

To avoid the rapid agglomeration of the generated 2D [Zn2(bdc)2] layers, 3D-Zn was directly sonicated in water for 30 min, and the highly crystalline 3D-Zn transformed to an amorphous phase (Fig. S4). SEM and TEM showed that the bulk microcrystals of 3D-Zn transformed to sheet-like products with wrinkled morphology (Fig. 3d and Fig. S13, S14). Energy dispersive spectrometer (EDS) mapping confirmed the uniform distribution of C, O and Zn throughout the entire surface of the as-prepared nanosheets while the N element from dabco could not be detected (Fig. 3e and Fig. S15). The AFM height image showed that the thickness of the nanosheet is ∼0.9 nm (Fig. 3f and Fig. S16), close to the thickness of the single [Zn2(bdc)2(H2O)2] layer (Fig. S1b), hereafter named 2D-Zn-single-layer. In addition, using MeOH or EtOH as the dispersed solvent, the morphology of both products is the same as 2D-Zn-single-layer, respectively (Fig. S13). Furthermore, Raman spectra showed that in 2D-Zn and 2D-Zn-single-layer the peaks from dabco disappeared but the peaks from bdc2− remained (Fig. S9), confirming the removal of dabco, and the mass spectrum measurement indicated that the ultrasonic solution contains only dabco (m/z = 113 (M + H)+) (Fig. S17). All the above results verified that the 2D-Zn-single-layer is the single-layer nanosheet originating from the [Zn2(bdc)2] layer of 3D-Zn. The possible exfoliation mechanism is illustrated in Fig. 3g, the dabco pillars in 3D-Zn were selectively removed through solvent substitution and capping to yield essentially 2D layered sheets for which synergetic ultrasound was applied to reinforce the exfoliation effect and prevent the well-ordered restacking of the resultant single layer nanosheet.

Usually, ultrathin 2D nanosheets composed of redox-active metals often exhibit superior electrocatalytic performance owing to more exposed catalytically active sites.51 MOFs containing mixed metal SBUs (such as Co, Ni and Fe) are widely used for the OER because of their high electrochemical activity stemming from the synergistic effect of the active metal sites.15,52–56 Referring to the synthesis method for 3D-Zn in this work, Co(NO3)2·6H2O, Ni(NO3)2·6H2O or the mixture of Co(NO3)2·6H2O and Ni(NO3)2·6H2O was used to react with the mixed ligands of H2bdc and dabco in DMF at 120 °C, which afforded isostructural 3D pillared-layer MOFs [Co2(bdc)2(dabco)]·guest (denoted as 3D-Co), [Ni2(bdc)2(dabco)]·guest (denoted as 3D-Ni) and [CoNi(bdc)2(dabco)]·guest (denoted as 3D-CoNi), respectively, and powder X-ray diffraction (PXRD) demonstrated the phase purity of these products (Fig. S18). Similarly, 3D-Co, 3D-Ni and 3D-CoNi were exposed to air for 3 days, which afforded isostructural 2D MOFs 2D-Co, 2D-Ni and 2D-CoNi, respectively. All these 2D MOFs maintain their structural integrity even after being dipped in 0.2 M PB solution for 72 h (Fig. S19). After the sonication exfoliation of the aforementioned 3D pillared-layer MOFs in water for 30 min, the exfoliated products were collected and subjected to SEM and TEM analyses, which showed that their morphology is similar to that of the 2D-Zn-single-layer (Fig. 4a, b and Fig. S20–S22), hereafter named 2D-Co-single-layer, 2D-Ni-single-layer and 2D-CoNi-single-layer, respectively. TEM-EDS mappings of the 2D-CoNi-single-layer demonstrated the highly uniform distribution of C, O, Co and Ni elements and the molar ratio of Co/Ni is observed to be 1[thin space (1/6-em)]:[thin space (1/6-em)]0.97 (28.42 wt% of Co and 26.94 wt% of Ni) (Fig. 4c and Fig. S23, S24). AFM showed that the thickness of all the exfoliated products containing Co, Ni and Co/Ni is ∼0.9 nm, corresponding to the thickness of the single 2D coordination layer (Fig. 4e, f and Fig. S25).


image file: c9nr09742b-f4.tif
Fig. 4 (a) SEM images of the 2D-CoNi-single-layer. (b) TEM images of the 2D-CoNi-single-layer. (c) and (d) TEM-EDS mapping images of the 2D-CoNi-single-layer. (e) AFM images of the 2D-CoNi-single-layer, and (f) the height profile of the nanosheet along the green lines.

The electrode for OER performance evaluation is prepared by homogeneously depositing 0.2 mg cm−2 of the as-synthesized single-layer nanosheets onto a carbon cloth supporting electrode. The initial measurements are carried out in a standard three-electrode cell containing an O2-saturated 0.2 M PB electrolyte (pH = 7.0). Fig. 5a shows that linear-sweep voltammograms (LSV) were performed at 5 mV s−1 for evaluating the performance of all electrodes (with 100% iR correction57). As is observed, bare carbon cloth and the 2D-Zn-single-layer exhibited almost no OER activity. In comparison, the 2D-CoNi-single-layer revealed the highest OER activity which requires an overpotential of 344 mV to reach a current density of 1 mA cm−2 (η1), considerably smaller than those of the 2D-Co-single-layer (η1 = 425 mV) and 2D-Ni-single-layer (η1 = 581 mV). Similarly, the onset potential of 2D-the CoNi-single-layer for the OER was only 314 mV, while the 2D-Co-single-layer and 2D-Ni-single-layer need 495 mV and 520 mV, respectively (Fig. S26). As depicted in Fig. S27a, the electron transfer kinetics of the 2D-CoNi-single-layer was found to be faster than those of the 2D-Co-single-layer and 2D-Ni-single-layer signified by the lowest Tafel slope of 171 mV dec−1versus 175 mV dec−1 for the 2D-Co-single-layer and 182 mV dec−1 for the 2D-Ni-single-layer. After a long-term electrolysis of 16 h at a voltage of 1.8 V versus the reversible hydrogen electrode (RHE), the current density of the 2D-CoNi-single-layer is almost constant (Fig. 5b). The TEM images collected after the OER testing for the 2D-CoNi-single-layer showed that the morphology of the original nanosheets is well preserved (Fig. S28), and the IR and Raman spectra showed that its structure and composition also remain constant (Fig. S29), showing the good electrochemical stability of the 2D material.


image file: c9nr09742b-f5.tif
Fig. 5 (a) LSV measurements of the 2D-Zn-single-layer, 2D-Co-single-layer, 2D-Ni-single-layer, 2D-CoNi-single-layer, and CC in the O2-saturated 0.2 M PB electrolyte (the loading mass of all the samples was 0.2 mg cm−2 except for blank CC). (b) Chronoamperometry curve of the 2D-CoNi-single-layer at 1.8 V (vs. RHE) in the O2-saturated 0.2 M PB electrolyte. (c) LSV measurements of the 2D-CoNi-single-layer on CC with different loading masses. (d) Comparison of the TOF values at different overpotentials with the representative active OER catalysts in PB solution.

In catalysis, the best figure of merit used to evaluate the activities among the different catalyst materials is their turnover frequency (TOF).58,59 For TOF normalization, we calculated the TOF per surface-exposed metal site to avoid difficulties in distinguishing the activity of different OMS present in MOFs like those within other metal-based OER catalysts.60–62 The TOF values of the 2D-CoNi-single-layer at the overpotentials of 400 mV and 500 mV can be calculated as 0.0052 s−1 and 0.021 s−1, respectively (for calculation details, see the ESI). However, it is difficult to measure this value for a heterocatalyst because only a small portion of the active sites is used for the catalytic reaction.55,63–66 Because the 2D material 2D-CoNi-single-layer is made up of single-layer nanosheets which could expose all open metal sites and be easily dispersed in solution, we reduced its loading on the electrode to avoid overlapping of the catalytically active sites (with loadings of 1 μg, 2 μg, 5 μg, 10 μg, and 20 μg cm−2 for the 2D-CoNi-single-layer supported on conductive carbon cloth). The LSV and Tafel slope for various loadings of the catalyst in 0.2 M PB solution showed that the OER performance increases with increasing loading mass (Fig. 5c and Fig. S27b). As expected, the electric current density was enhanced with the increasing loading mass and overpotential (Fig. S30a), but the TOF values were reduced under the same testing conditions (Fig. S30b). For the lowest loading mass, the TOF of 2D-CoNi-NS-AS at the overpotentials of 400 mV and 500 mV can be calculated as 0.144 s−1 and 0.294 s−1, respectively, which is higher than other reported metal-based heterogeneous catalysts operating in neutral electrolyte, as shown in Fig. 5d and Table S1.

Conclusions

In summary, the inherent anisotropy of coordination bonding in some 3D pillared-layer MOFs offers us the opportunities to synthesize attractive MOF nanosheets through selectively destroying the interlayer coordination bonds related to the pillars and retaining the integrity of the intralayer coordination bonds. Herein, the dabco pillars in a series of well-known 3D pillared-layer MOFs were completely eliminated by substitution with terminal capping solvent molecules (such as H2O, MeOH, or EtOH and so on) affording wrinkled nanosheets of the intrinsic 2D layered MOFs. Meanwhile, the synchronously applied sonication provides sufficient force to prevent the generated layer-like nanosheets from reaggregation in an orderly manner, thus resulting in single-layer nanosheets with ultrathin thickness. Importantly, the obtained single-layer 2D MOF nanosheets showed exceptionally high OER activities manifested by particularly high TOF values. The novel strategy developed in this study may open up a new avenue to adopt 3D pillared-layer MOFs as the appropriate precursors achieving the controllable synthesis of MOF-based 2D ultrathin materials through the synergistic action of host–guest interactions and external forces.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the NSFC (Grant No. 21901051 and 21861003), the Guangxi Scientific, Technological Innovation Base and Personnel Project of China (Grant No. GUIKE2018AD19297), and the Guangxi Natural Science Foundation of China (Grant No. 2017GXNSFAA198125).

Notes and references

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Footnote

Electronic supplementary information (ESI) available: The syntheses and characterization of the catalyst, ESI figures and tables. See DOI: 10.1039/c9nr09742b

This journal is © The Royal Society of Chemistry 2020