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View Article OnlineView Journal | View Issue

Constructing 2D

aMOE Laboratory of Bioinorganic and S

Functional Materials, School of Chemistr

510275, People's Republic of China. E-mail:bKey Laboratory of Polymer Composites a

Education, School of Chemistry, Sun Y

People's Republic of ChinacDepartment of Physics and Astronomy, Rut

136 Frelinghuysen Road, Piscataway, New JdDepartment of Physics, New Jersey Institute

USAeKey Laboratory for Polymeric Composite

Education, School of Materials Science an

Guangzhou 510275, People's Republic of Ch

† Electronic supplementary informa10.1039/c9ta09397d

Cite this: J. Mater. Chem. A, 2020, 8,190

Received 26th August 2019Accepted 20th November 2019

DOI: 10.1039/c9ta09397d

rsc.li/materials-a

190 | J. Mater. Chem. A, 2020, 8, 190–

MOFs from 2D LDHs: a highlyefficient and durable electrocatalyst for wateroxidation†

Mengke Cai,a Qinglin Liu,a Ziqian Xue,a Yinle Li,a Yanan Fan,a Aiping Huang,b

Man-Rong Li, a Mark Croft,c Trevor A. Tyson,d Zhuofeng Ke e

and Guangqin Li *a

Two-dimensional (2D) materials have been widely applied in electrochemical conversion technologies,

especially toward the water oxidation reaction (WOR) in metal–air batteries and water splitting. Here, we

demonstrate a facile ligand-assisted synthetic method, promoting the transformation of 2D layered

double hydroxides (LDHs) into 2D metal–organic frameworks (MOFs). The CoFe-LDH precursor acts as

an adjustable metal release source, controlling heterogeneous nucleation for 2D MOFs. Compared with

most cobalt-based electrocatalysts, the optimized CoFe 2D MOFs exhibit a superior WOR performance

on glassy-carbon electrodes (overpotential of 274 mV at 10 mA cm�2 and a Tafel slope of 46.7 mV

dec�1) and long-term stability, due to the unique 2D characteristics and coupling effect between Co and

Fe ions. More importantly, this work highlights the ability to transform 2D LDHs into 2D MOFs and

reveals the intrinsic factors for excellent performance in the WOR.

Introduction

Water splitting by electrolysis (2H2O / O2 + 2H2) providesa potential and encouraging path for the generation of efficient,clean and renewable H2 fuel to support human civilization.1–3

However, the efficiency of water electrolysis is limited by largeoverpotentials, especially for the water oxidation reaction(WOR), and the use of expensive noble electrocatalysts hindersthe long-term development for water electrolysis application.4–9

In recent years, further research has been conducted to explorecost-effective and efficient non-noble electrocatalysts for theWOR.10–12 Notably, two-dimensional (2D) materials haveattracted very signicant attention in the eld of heterogeneouselectrocatalysis due to their unique physical, chemical, and

ynthetic Chemistry, Lehn Institute of

y, Sun Yat-Sen University, Guangzhou

liguangqin@mail.sysu.edu.cn

nd Functional Materials of Ministry of

at-Sen University, Guangzhou 510275,

gers, The State University of New Jersey,

ersey 08854, USA

of Technology, Newark, New Jersey 07102,

and Functional Materials of Ministry of

d Engineering, Sun Yat-Sen University,

ina

tion (ESI) available. See DOI:

195

electronic properties. As a result, graphene,13–15 graphiticC3N4,16,17 transition metal dichalcogenides (TMDs),18–20 layereddouble hydroxides (LDHs),21–23 2D metal–organic frameworks(MOFs)24–28 and 2D covalent-organic frameworks (COFs)29,30 areconsidered as promising water splitting catalysts. Additionally,how to rationally design and synthesize 2D materials is also ofgreat interest and challenge. The synthesis of 2D materialsusually depends on chemical vapor deposition and physicalexfoliation, involving both top–down and bottom–upapproaches.31–33 Recently, 2D materials such as TMDs witha unique microstructure were obtained by means of precursortransformation, exhibiting superior catalytic propertiescompared with direct synthesis.21,26,34 Therefore, synthesizing2D materials through precursor transformation is promising tosignicantly broaden the design and synthesis of electro-catalysts for energy storage and conversion.

MOFs are constructed by coordination bonds between metalnodes/clusters and organic ligands with periodic reticularchemistry.35 Due to precise periodicity, nano-dimensionalMOFs possess denite accessible active sites and well-denedstructures and have exhibited promising performance towardswater oxidation.24–26 However, the poor conductivity, low masspermeability and stability have limited their development inelectrocatalysis. Thus, to design highly efficient electrocatalysts,construction of 2D MOFs with nanosheets could be an appro-priate strategy to satisfy the following: nanolayers to strengthenelectron transfer and mass transport;24,25,36,37 exposed activesites with large surface area, and coordinatively unsaturatedmetal sites.24,26,38–43

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http://crossmark.crossref.org/dialog/?doi=10.1039/c9ta09397d&domain=pdf&date_stamp=2019-12-14http://orcid.org/0000-0001-8424-9134http://orcid.org/0000-0001-9064-8051http://orcid.org/0000-0002-1233-5591https://doi.org/10.1039/c9ta09397dhttps://pubs.rsc.org/en/journals/journal/TAhttps://pubs.rsc.org/en/journals/journal/TA?issueid=TA008001

Fig. 1 Schematic illustration for the ligand-assisted transformation toprepare 2D-MOFs.

Fig. 2 SEM images of (a) CoFe-LDH and (b) LM-160-12. TEM imagesof (c) CoFe-LDH and (d) LM-160-12. HRTEM image (e), HAADF-STEMimage (f) and EDXSmapping images of LM-160-12 for Co (g), Fe (h) andO (i). AFM image (j) and height curves (k) of the as-prepared LM-160-12, showing measured dimensions of nanosheets.

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Here, we report CoFe bimetal 2D MOFs as electrocatalysts forthe WOR, for the rst time, prepared from the transformation ofthe precursor CoFe-LDHs as the template. So far, there are exten-sive reports on 2D-LDHs containing Ni, Co, Fe and Mn exhibitingexcellent catalytic properties in the WOR, especially for NiFe-LDHsknown as the most promising catalyst.10,12,13,20,22,23 For Ni-basedmaterials, the addition of Fe dramatically enhances WORactivity. But for Co-based materials, it is not well-documented andCoFe-based materials usually show more modest WOR activitycompared with NiFe-based materials.49–53 So it is a signicantchallenge to construct novel CoFe-based materials with betteractivity than CoFe-LDHs and explore the relevant mechanism.More importantly, LDHs have adjustable and reasonable layerspacing, allowing ligands to attack inner Co and Fe from theinterlayer, which makes them a good candidate to form 2DMOFs.Thus CoFe 2D-LDHs are chosen as precursors because of theirbimetallic Co/Fe composition and 2D layered characteristics.Interestingly, with the transformation from 2D LDHs to 2DMOFs,the obtained electrocatalyst loaded on glassy-carbon (GC) elec-trodes demonstrate a low overpotential of 274 mV at 10 mA cm�2

under alkaline conditions. During the long-term stability test, the2D MOFs loaded on nickel foam (NF) maintain highly stableactivity for 70 h at a constant overpotential of 271 mV.

Results and discussion

The transformation was conducted via a ligand-assisted proce-dure by mixing the precursor CoFe-layered double hydroxide(CoFe-LDH) and terephthalic acid in DMF at various tempera-tures and times (Fig. 1), and the correspondingly transformedspecimens were named LM-T-t (T: temperature/�C, t: time/hour). To clearly understand the process of transformation ofCoFe-LDH into bimetallic 2D MOFs, powder X-ray diffraction(PXRD) tests were carried out on all specimens. In Fig. S1a,† the(003) and (006) diffractions can be observed at 2q ¼ 11.51� and23.18� for the prepared CoFe-LDH, indexed as a bimetal ferro-cobalt hydroxide with CO3

2� (carbonate) as the interlayercounterion. Interestingly, the (003) and (006) diffractions shif-ted to a lower angle, indicating interlayer ion-exchange for theCoFe-LDH. The basal interlayer spacing was calculated to be0.77 nm for the pristine CoFe-LDH, shiing to 0.84 nm for bothLM-100-12 and LM-130-12 based on the Bragg equation(2d sin q ¼ nl). The PXRD patterns showed that the character-istic diffraction peaks of the (003) and (006) planes of CoFe-LDHdisappeared, while the (200), (201) and (�201) facets of the 2DCo-MOFs emerged and became stronger with the increase ofreaction temperature and time (Fig. S1a and b†). Finally, withexcessive transformation time, another MOF phase diffractionpeak arose in PXRD patterns, which was usually obtained by thecoordination of Fe3+ with terephthalic acid and named MIL-88B(Fe) (Fig. S1b†) (No. 1415803, space group P63/mmc, Cam-bridge Crystallographic Date Centre).44–46

Furthermore, N2-adsorption/desorption isotherms areshown in Fig. S2a and b.† The specimens LM-160-12, LM-160-24and LM-160-36 all exhibited reversible microporous regions andadsorption hysteresis loops, categorized as microporous type-IVisotherms unlike those of the precursor CoFe-LDH, LM-100-12

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and LM-130-12. Moreover, the hysteresis loop area of LM-160-12 was larger than that of the others, indicating the presenceof multilevel pores and a unique hierarchical structure. Addi-tionally, the Brunauer–Emmet–Teller (BET) surface area of LM-160-12 was calculated to be 105.1 m2 g�1 in Fig. S3,† higher thanthat of CoFe-LDH (39.2 m2 g�1), LM-100-12 (39.8 m2 g�1) andLM-130-12 (43.2 m2 g�1) and lower than that of LM-160-24(174.5 m2 g�1) and LM-160-36 (247.8 m2 g�1), which was wellconsistent with XRD results. The total pore volume of speci-mens is shown in Fig. S3;† both LM-160-12 (0.151 cm3 g�1) andLM-160-24 (0.154 cm3 g�1) possessed nearly twice the porevolume of the precursor CoFe-LDH (0.085 cm3 g�1). The poresizes of the specimens (Fig. S4 and S5†) were calculated usingnonlocal density functional theory (NLDFT), showing that themesoporous distribution of LM-160-12 was the largestcompared with other specimens. The coexistence of the unique

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Fig. 3 Identification of the transformation process by comparing theactive metal content and ratio. Measured metal content of Co (a) andFe (b) and the molar ratio of Co/Fe (c). The dissociation of cations forCoFe-LDH soaked in DMF at 160 �C (d). The schematic diagram ofstructural transformation from LDHs to 2D MOFs (e). The localunsaturated (green dashed circle) model for the metals on the surfaceof LM-160-12 (f) and four sets of non-equivalent [MO6] octahedra (g).Blue for Co, orange for Fe, red for O, grey for C, and white for H.

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microporous and mesoporous structure in LM-160-12 maymake it suitable for electrocatalysis.

The morphology was characterized by scanning eld-emission scanning electron microscopy (SEM), transmissionelectron microscopy (TEM), and atomic force microscopy(AFM). The SEM image (Fig. 2a) indicated that the CoFe-LDHnanosheets possessed a hexagonal shape with a lateral meandiameter of about 50–500 nm, consistent with reported work.21

Aer transformation via the ligand-assisted method at 160 �Cfor 12 hours (Fig. 2b), spindle-like thin nanosheets were ob-tained for LM-160-12. The TEM images (Fig. 2c and d) alsorevealed the large morphological changes and coincide wellwith Fig. 2a and b. Interestingly, upon tracking the morphologyof the transformation (Fig. S6 and S7†), it suggested that theligand-assisted transformation from 2D CoFe-LDHs to 2DMOFsresulted in obvious crystal dissociation, recrystallization, andirregular growth. Due to a lack of solution agitation and lack ofsurfactants in the solid–liquid reaction system, agglomerationand disordered growth happened in the long-term reaction,such as for LM-160-24 and LM-160-36. Encouragingly, highresolution transmission electron microscopy (HRTEM) andhigh angle annular dark eld scanning transmission electronmicroscopy (HAADF-STEM) images clearly elucidated thespindle-like shape for LM-160-12 (Fig. 2e and f), and thenanosheets possessed a homogeneous distribution of Co, Feand O elements, which was shown by energy-dispersive X-rayspectroscopy (EDXS) (Fig. 2g–i). Moreover, the AFM image ofLM-160-12 in Fig. 2j apparently showed that the nanosheetsconsisted of ultrathin sheets, as shown in the red dashed circle,and the thickness of the ultrathin sheets was determined to be4.84–7.39 nm (Fig. 2k). The thickness of a single coordinationstructural layer was calculated to be about 0.97 nm in reportedCoNi 2D MOFs,24 suggesting that LM-160-12 ultrathin nano-sheets included 5–8 coordination layers. The above results alsorevealed that LM-160-12 possessed characteristics of inherent2D structural periodicity, which may provide exposed activesurfaces for MOF-based electrocatalysts.47–49

To further identify the transformation process from LDHs to2D MOFs, the active metal content and ratio of all specimenswere measured by inductively coupled plasma-mass spectrom-etry (ICP-MS). Both Co and Fe content showed a downwardtrend with the increase of transformation temperature and time(Fig. 3a and b), because of Co and Fe ions coming only from theprecursor CoFe-LDH in the transformation process. The molarratio of Co/Fe, however, varied between 2.2 for LM-160-12 withthe most 2D characteristics to 1.5 for LM-160-36 (Fig. 3c). Thiscoincided well with the PXRD results that a high proportion ofthe iron content tends to form a microporous structure MIL-88B(Fe). The excessive doping of trivalent iron affected thecoordination environment between bivalent cobalt and ter-ephthalic acid to form 2D MOFs. Next, we carried out a disso-ciation experiment to monitor the cation release of theprecursor CoFe-LDH, soaked in DMF at 160 �C measured byICP-MS. As shown in Fig. 3d, the dissociation ratio of Co/Fe wasbetween 3.5 for the initial 6 hours and decreased to 2.9 aer 42hours, which signicantly exceeded the initial ratio of Co/Fe 2.0for the precursor CoFe-LDH. In other words, the dissociation of

192 | J. Mater. Chem. A, 2020, 8, 190–195

cobalt was faster than that of iron at the beginning, whichshowed that the terephthalic ligand was more inclined tocoordinate with cobalt in the early stage of heterogeneousnucleation. These suggested that precursor CoFe-LDH, anadjustable metal release source, played a key role in the ligand-mediated transformation process (Fig. 3e). As shown in Fig. 3e, fand S8,† the terephthalic ligand molecules separated the 2Dbimetal layers along an axis, which were composed of pseudooctahedral [CoO6] and [FeO6]. Here, the unsaturated metal sites(green dashed circle) on the surface of LM-160-12 satised thedesirable four-electron pathway for the WOR (Fig. 3f). However,there were four sets of non-equivalent octahedral [MO6], derivedfrom different metal centers and ligands containing oxygen(Fig. 3g). So it is an enormous challenge to clarify the specicreaction pathways and active centers (Co or Fe).49–53

All electrochemical measurements for the WOR were per-formed with 1.0 M KOH solutions as the electrolyte. As shown inFig. 4a, the linear sweep voltammetry (LSV) curves were recor-ded to compare each specimen's WOR catalytic activity. Itshould be noted that a rather small overpotential (274 mV) wasrequired for LM-160-12 to deliver a current density of 10 mAcm�2 (Fig. 4b), which is comparable with that of commercialRuO2 (271 mV) and superior to most cobalt-based and pristineMOF electrocatalysts (Tables S1 and S2†). Interestingly, thespecimen LM-160-24, possessing better crystallinity and specicsurface area than LM-160-12, exhibited lower WOR catalyticactivity. To understand how and why the weakly crystalline 2DMOF LM-160-12 possesses the best WOR electrocatalyticactivity, correlative Tafel slopes were calculated and are shownin Fig. 4c. LM-160-12 showed the smallest Tafel slope (46.7 mVdec�1) compared with isostructural LM-160-24 (46.9 mV dec�1)

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and the other specimens, indicating better kinetic activitypossibly due to fast mass transport and electron transfer.Electrochemical impedance spectroscopy (EIS) was also con-ducted to evaluate the electron transfer ability and understandthe reaction kinetics (Fig. 4d). The equivalent circuit model (theinset of Fig. 4d) was set to t the impedance responses, where Rsrepresents solution resistance, Rct represents electron transferresistance, Rp represents surface porosity and CPE stands forconstant phase element. The Rct and Rp values of LM-160-12(47.8 and 3.4 ohm) were smaller than those of isostructuralLM-160-24 (49.8 and 9.4 ohm), also suggesting faster electrontransfer and ion diffusion in the WOR process (Table S3†).

Furthermore, the electrochemical surface area (ECSA) wasmeasured by evaluating the electrochemical double-layercapacitance (Cdl), which was proportional to the slope ofcurrent differences plotted against scanning rates. LM-160-12possessed a Cdl value of 25.4 mF cm

�2, nearly twice that ofLM-160-24 (14.7 mF cm�2), signicantly exceeding that of otherspecimens (Fig. S9–S11†). This ECSA result showed a volcanictype trend, differing from the observed ascending type for theBET surface area. It was clear that the ECSA is not exactly equalto the BET specic surface, so the unique 2D nanosheets'morphological characteristics had a vital impact on the elec-trocatalytic process. In addition, we compared the LSV curvesnormalized by the ECSA for all samples in Fig. S12,† where LM-160-12 was superior to others in intrinsic WOR activity and theirWOR activity tendency was not entirely consistent with the LSVcurves in Fig. 4a. Interestingly, the results indicate that thedifferences in the WOR performance not only originated fromexposed active sites but also the intrinsic activity. To investigatethe long-term electrocatalytic stability, we performed chro-nopotentiometric measurements to evaluate the 2D MOF LM-160-12 loaded on both glassy carbon electrode (GCE) andnickel foam (NF). As revealed in Fig. S13,† LM-160-12 loaded onthe GCE showed 10 h stability with a slight decrease in activity,

Fig. 4 (a) LSV curves, (b) comparison of WOR overpotential at 10 and100 mA cm�2, (c) Tafel plots and (d) EIS curves (the equivalent circuitdiagram is shown in the inset, dots represent raw data and dashed linesrepresent fitted data) of various specimens. RHE denotes the reversiblehydrogen electrode.

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and it was supposed that bubble breakup resulted in catalystshedding due to fast oxygen evolution. When replaced withapplicable NF as the support, a highly stable current density of10 mA cm�2 was obtained for at least 70 h. Importantly, theHRTEM, HAADF-STEM and EDXS mapping images (Fig. S14†)of LM-160-12 nanosheets loaded on NF aer the long-termstability test were consistent with the observation before thereaction. The PXRD results (Fig. S15†) also illustrated that thediffraction peaks of 2D MOFs, (200), (201) and (�201) facets,were still retained modestly aer the long-term reaction.Moreover, the valence electron structures before and aerelectrocatalysis were further conrmed by XPS. From the XPSresults (Fig. S16†), the binding energy of Fe 2p3/2 shied to lowbinding energy, while an opposite up-shi was observed for Co2p3/2 at the same time. This suggested that long-term electro-catalysis promoted charge transfer from Fe to Co by the ligandscontaining oxygen, and deeper studies on electronic structuresshould be implemented.

Next, to further verify the bimetal interaction, XPS survey wascarried out. As shown in Fig. S17,† four elements Co, Fe, O andC were detected on the surface for all specimens. And the molarratio of Co/Fe measured by XPS was consistent with thatdetected by ICP-MS (Table S4†). Aer the transform from LDHto 2D MOFs, the O2� 1s (530.7 eV for CoFe-LDH) peak shied tohigher binding energies (531.2 eV for LM-160-12 and LM-160-24), respectively (Fig. S18†). Meanwhile, Fe3+ 2p3/2 shied tolow binding energy (Fig. 5a), and the same up-shi was ob-tained for Co2+ 2p3/2 (Fig. 5b). The above results indicated thatsome of the electrons were transferred from Co2+ to Fe3+

through the bridging oxygen, which could adjust the electrondensity of the metal sites. It is obvious that the peroxidation ofCo2+ to a higher valence state of Co was induced by the p-donation between Fe3+ and bridging O2�, because enhanced p-donation triggered more partial charge transfer from Co2+ toO2�, which may contribute to the best activity of LM-160-12.

To probe the electronic and local structural motifs, X-rayabsorption near-edge structure (XANES) was performed on thespecimens. As shown in Fig. 5c and S19a,† the X-ray absorptioncurves of both LM-160-12 and LM-160-24 show Fe K-edge shisquite close to, albeit somewhat lower, those of the referenceLa2FeVO6. This supports a formal chemical Fe valence stateclose to Fe3+ in both materials. At the Co K-edge, the XANESspectra of both specimens showed that the Co atom oxidationstate is higher than +2 (Fig. 5d and S19b†). This is consistentwith partial substitution of Fe in Co sites in MOFs, which led tocharge transfer between both atoms, in accordance witha previous report on metal doping engineering in MOFs.54 Thebroad unresolved Co–Kmain edge peak near 7725 eV in LM-160-12, compared to the sharper features in the LM-160-24 spectrum(Fig. 5d) should also be noted. This broadening is consistentwith a signicant structural distortion of the nearest neigh-bours to Co sites in the former system compared to the latter.55

In the Fe and Co pre-edge region, two pre-edge peaks at around7114.2 eV and 7709.8 eV were shown both in LM-160-12 and LM-160-24 (Fig. 5e and f). Both peaks were derived from the tran-sition, from quadrupole allowed 1s to 3d and dipole allowed 1sto 3d-4p hybrid orbitals.56 Here non-centrosymmetric

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Fig. 5 (a) Fe 2p3/2 and (b) Co 2p3/2 high resolution XPS spectra of theprecursor CoFe-LDH, LM-160-12 and LM-160-24. (c) The Fe K-edgeand (d) Co K-edge XANES spectra for LM-160-12 and LM-160-24 andstandard FeO, La2FeVO6, CoO and LaCoO3. (e and f) Expanded pre-edge peaks' comparison of LM-160-12 and LM-160-24, respectively.

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distortions enhance stronger transitions into the 3d–4p hybridorbitals, and the enhanced pre-edge feature spectral weight forthe LM-160-12 material supports a more strongly distorted andunsaturated local environment. In general, the combination ofXANES, ECSA and morphological characterization clearlyrevealed that the existence of more coordination unsaturatedmetal sites and local structural distortion on LM-160-12surfaces, compared with LM-160-24, resulted in excellentWOR activity. Hence, we propose that the excellent catalyticperformance may mainly originate from the unique 2D char-acteristics rather than the coupling effect between Co and Fe,which not only provide more coordination unsaturated metalsites but also facilitate electron transport and transfer and iondiffusion.57–59

Conclusions

In summary, a novel ligand-assisted synthetic strategy isdemonstrated to prepare bimetal 2D MOF nanosheets as highlyactive and durable electrocatalysts for the WOR, by the trans-formation from 2D LDHs. Furthermore, the precursor LDHsserve as an adjustable metal release source to control theheterogeneous nucleation process, forming layered bimetal 2DMOFs. The coupling effect between Co and Fe promotes thedelocalization of Co 3d electron and enhances the WOR activity.More importantly, the layered bimetal 2D CoFe-MOFs possessmore exposed active sites, rapid electron transfer and faster iondiffusion, which make major contributions to the superior

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activity in our work. We believe that the unique ligand-assistedsynthetic strategy will extend opportunities towards construct-ing 2D MOF-based materials for widespread applications suchas energy storage and conversion.

Conflicts of interest

The authors declare no conict of interest.

Acknowledgements

This work was supported by the 100 Talents Plan Foundation ofSun Yat-sen University, the Program for Guangdong Intro-ducing Innovative and Entrepreneurial Teams (2017ZT07C069),Guangdong Natural Science Funds for Distinguished YoungScholar (No. 2015A030306027), and NSFC projects (201875287,21821003, and 21890380). Part of this research used the QAS, 7-BM beamline at the National Synchrotron Light Source II, a U.S.Department of Energy (DOE) Office of Science User Facilityoperated for the DOE Office of Science by Brookhaven NationalLaboratory under Contract No. DE-SC0012704.

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https://doi.org/10.1039/c9ta09397d

Constructing 2D MOFs from 2D LDHs: a highly efficient and durable electrocatalyst for water oxidationElectronic supplementary information (ESI) available. See DOI: 10.1039/c9ta09397dConstructing 2D MOFs from 2D LDHs: a highly efficient and durable electrocatalyst for water oxidationElectronic supplementary information (ESI) available. See DOI: 10.1039/c9ta09397dConstructing 2D MOFs from 2D LDHs: a highly efficient and durable electrocatalyst for water oxidationElectronic supplementary information (ESI) available. See DOI: 10.1039/c9ta09397dConstructing 2D MOFs from 2D LDHs: a highly efficient and durable electrocatalyst for water oxidationElectronic supplementary information (ESI) available. See DOI: 10.1039/c9ta09397dConstructing 2D MOFs from 2D LDHs: a highly efficient and durable electrocatalyst for water oxidationElectronic supplementary information (ESI) available. See DOI: 10.1039/c9ta09397dConstructing 2D MOFs from 2D LDHs: a highly efficient and durable electrocatalyst for water oxidationElectronic supplementary information (ESI) available. See DOI: 10.1039/c9ta09397d