Subscriber access provided by UNIV OF LOUISIANA
Article
Constructing NiCo/Fe3O4 Heteroparticles within MOF-74 for Efficient Oxygen Evolution Reactions Xiaolu Wang, Hai Xiao, Ang Li, Zhi Li, Shoujie Liu, Qinghua Zhang, Yue Gong, Lirong Zheng, Youqi Zhu, Chen Chen, Dingsheng Wang, Qing Peng, Lin Gu, Xiaodong Han, Jun Li, and Yadong Li J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 23 Oct 2018 Downloaded from http://pubs.acs.org on October 23, 2018
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 7 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Journal of the American Chemical Society
Constructing NiCo/Fe3O4 Heteroparticles within MOF-74 for Efficient Oxygen Evolution Reactions Xiaolu Wang†, Hai Xiao†, Ang Li‡, Zhi Li*†, Shoujie Liu§, Qinghua Zhang||, Yue Gong||, Lirong Zheng ⊥ , Youqi Zhu†, Chen Chen†, Dingsheng Wang†, Qing Peng†, Lin Gu||, Xiaodong Han‡, Jun Li†, Yadong Li*† †Department ‡
of Chemistry, Tsinghua University, Beijing 100084, China.
Beijing Key Laboratory of Microstructure and Property of Advanced Materials, Beijing University of
Technology, Beijing 100024, China. §College
of Chemistry and Materials Science, AnHui Normal University, WuHu 241000, China
||Beijing
National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of
Sciences, Beijing 100190, China. ⊥Beijing
Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of
Sciences, Beijing 100049, China. KEYWORDS: MOF-74, MOF derivatives, Partial pyrolysis, In-situ ETEM, Oxygen evolution reaction (OER) ABSTRACT: Metal–organic frameworks (MOF) have recently emerged as versatile precursors to fabricate functional MOF derivatives for oxygen evolution reactions (OER). Herein, we developed a controlled partial pyrolysis strategy to construct robust NiCo/Fe3O4 heteroparticles within MOF-74 for efficient OER using trimetallic NiCoFe-MOF-74 as precursor. The partial pyrolysis method preserves the framework structure of MOF for effective substrates diffusion while producing highly active nanoparticles. The as-prepared NiCo/Fe3O4/MOF-74 delivered remarkably stable OER current with an overpotential as low as 238 mV at 10.0 mA cm−2 and an Tafel slop of 29 mV/dec, outperforming those of pristine NiCoFe-MOF-74, totally decomposed MOF derivatives, and most reported non-noble metal based electrocatalysts. The key for the formation of NiCo/Fe3O4/MOF-74 nanostructures is that the metals can be decomposed from NiCoFe-MOF-74 in the order of Ni, Co, and Fe under controlled heat treatment. Density functional theory calculations reveals that the underlying NiCo promotes the OER activity of Fe3O4 through exchange stabilization of active oxygen species. 4e- in base) at the anode retard the improvement and optimization of water splitting efficiency. Metal-organic frameworks (MOF), constructed by the coordination of metal ions with organic ligands, have emerged as a class of versatile porous materials for a wide range of applications.7-8 In particular, the ultrahigh surface area, unique pore/channel structures, and abundant accessible metal sites make MOF ideal candidate materials for electrocatalysis.9-11 Actually,
INTRODUCTION Developing sustainable energy conversion and storage technologies are crucial to alleviate the environmental pollutions and foreseeable energy shortage. Water splitting (H2O → H2 +½O2) to produce H2 has emerged as an important strategy to convert renewable light/electrical energy into chemical fuels.1-6 However, the sluggish kinetics of the 4etransfer oxygen evolution reaction (OER: 4OH- → O2 + 2H2O + 1
ACS Paragon Plus Environment
Journal of the American Chemical Society 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
MOF themselves have been reported catalytic active for OER, whose reactive centers were regarded as the metal sites at the anodes of MOF.12-14 However, the stability issues of MOF could hinder their widespread applications and long-term use.15
thermal crystallization (see the Supporting Information for experimental details). The X-ray diffraction (XRD) pattern of NiCoFe-MOF-74 showed well-defined diffraction peaks that were consistent with simulated MOF-74 crystalline structure (Figure S1). High-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) image and corresponding energy dispersive spectroscopy (EDS) mapping images indicated the as-prepared NiCoFe-MOF-74 showed a unique rugby ball shape with Ni, Co, Fe elements homogeneously distributed in the nanostructures (Figure S2).
On the other hand, MOF can be used as precursors to fabricate varies functional MOF derivatives, such as metals and metal oxides,16-23 metal sulfides,24-25 metal phosphides,26-29 and porous carbons30, by means of thermal treatment under specific atmosphere, which are generally more stable than MOF for OER.30-31 However, currently reported direct pyrolysis of MOF crystals to functional derived materials seems straightforward and facile but is actually not optimized for electrocatalysis.32 The total decomposition of MOF usually leaded to the dramatic decrease of surface area and the wreck of well-defined MOF pore/channel structures, which embedded considerable quantity of active metal sites in the bulk phase and impeded the reactants from accessing reactive centers. To address this issue, we conceived it would be favorable to construct functional nanoparticles within MOF, which is expected to inherit the advantages of both pristine MOF and MOF derivatives. Thermal treatment can be an effective methods for the synthesis, nevertheless, the heating conditions should be strictly controlled to prevent the dramatic decrease of MOF surface area and the total deconstruction of MOF pore structures.33 MOF-74 are a family of MOF constructed by the coordination of divalent transition metals and 2,5-dioxidoterephthalate with a formula of M2(DOT)(H2O)2 (M = Mg, Mn, Fe, Co, Ni, and Zn; DOT = 2,5-dihydroxyterephthalate).34-35 Characteristic features of MOF-74 are 1) the composition of the divalent metals nodes (M = Mg, Mn, Fe, Co, Ni, Zn, etc.) can be widely selected and modified.36 2) MOF-74 possessed one-dimensional channels with aperture diameters of about 1 nm, which are large enough for the diffusion of OER substrates. 3) divalent transition metals in MOF-74 have different bonding strengths with DOT ligands. Through controlled thermal treatment, mixed metals could be sequentially degraded from MOF substrate to obtain highly active heteroparticles which are difficult to construct by conventional methods. These features make MOF-74 an ideal precursor to synthesize composite catalysts with extensively tunable properties.
Figure 1. (a) Scheme of the fabrication of trimetal NiCoFe-MOF-74 and the partial pyrolysis of NiCoFe-MOF-74 to NiCo/Fe3O4/MOF-74. (b) Frames from Movie S1 acquired at various temperatures and times of the pyrolysis of NiCoFe-MOF-74 with ETEM under an N2 atmosphere from room temperature to 800oC. (c) XRD patterns of NiCoFe-MOF-74 and NiCoFe-MOF-74-400oC-1h (NiCo/Fe3O4/MOF-74). (d) N2 adsorption-desorption isotherm and pore size distributions (inset) of NiCoFe-MOF-74 and NiCoFe-MOF-74-400oC -1h (NiCo/Fe3O4/MOF-74) at 77K.
Herein, we reported a controlled pyrolysis strategy to synthesize highly active and durable semi-MOF derivatives for OER from tri-metal NiCoFe-MOF-74. The partial pyrolysis strategy retained the skeleton structures of the MOF for efficient substrate diffusion meanwhile creating highly active and stable nanoparticles. The obtained NiCo/Fe3O4/MOF-74 electrocatalyst exhibited an ultralow overpotential of only 238 mV to reach 10 mA/cm2 current density for OER on glass carbon electrode, which surpassed those of pristine NiCoFe-MOF-74, commercial RuO2 and most reported non-precious metal catalysts (Table S1). Moreover, the NiCo/Fe3O4/MOF-74 was much stable than pristine NiCoFe-MOF-74 and commercial RuO2 for long-term OER test.
The pyrolysis behavior of obtained NiCoFe-MOF-74 was studied by the in-situ environmental transmission electron microscope (ETEM) under N2 atmosphere. As can be seen from the captured video (Movie S1) and representative movie images acquired at different temperatures (Figure 1b), the MOF was stable below 400oC, maintaining the rugby shape with smooth surface. After 40s annealing at 400oC, the surface of the MOF became rough and ultra-fine nanoparticles started to emerge from the MOF substrates (Figure 1b-400oC-40s). When the heating time was prolonged to 110s, the number of the nanoparticles within MOF significantly increased (indicated by white arrows in Figure 1b-400oC-110s). As the temperature was increased to ~600-800oC, the nanoparticles grew larger, and eventually the MOF was totally decomposed.
RESULTS AND DISCUSSION Synthesis and Characterization of NiCoFe-MOF-74 and NiCo/Fe3O4/MOF-74. As schemed in Figure 1, NiCoFe-MOF-74 was firstly synthesized by mixing divalent metals (Ni2+, Co2+, Fe2+) with DOT in dimethyl formamide/ethanol/water solution, followed by 24 h solvent 2
ACS Paragon Plus Environment
Page 2 of 7
Page 3 of 7 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Journal of the American Chemical Society green (e), red (f), brown (g) rectangles in (c). Simulated atomic models are overlapped on the experimental images. (f, and g).
From the in-situ ETEM experiments, it was confirmed that ultra-small nanoparticles started to emerge at 400oC and MOF can be decomposed at temperature higher than 600oC. Therefore, we calcined the NiCoFe-MOF-74 in a tube furnace under N2 atmosphere at 400oC for 1h to obtain the partial pyrolyzed sample (NiCoFe-MOF-74-400oC-1h). It can be seen that the XRD diffraction peaks attributed to MOF-74 remained in the calcined sample, indicating the crystalline MOF structure survived under controlled annealing condition (Figure 1c). Notably, the diffraction peaks attributed to metal/metal oxide were not observed in XRD pattern due to the small particles grain size and low diffraction peak intensity. Brunauer–Emmett–Teller (BET) analysis showed the calcined sample (558m2/g) retained 68% of the specific surface area of the NiCoFe-MOF-74 precursor (820m2/g). And the micropores structures of MOF-74 remained after controlled pyrolysis. Contrarily, if the NiCoFe-MOF-74 was calcined at 600oC under N2 for 2h (NiCoFe-MOF-74-600oC-2h), the diffraction peaks attributed to MOF-74 completely disappeared meanwhile the peaks belonged to NiCo alloy, CoFe alloy, and Fe3O4 emerged (Figure S3). Along with deconstruction of MOF structure, the BET specific surface area and the pore volumes of NiCoFe-MOF-74-600oC-2h (Figure S4) dramatically decreased, which were only 24% and 21% those of pristine NiCoFe-MOF-74, respectively (Table S2).
Electron microscopy characterizations were conducted to study the nanostructure of NiCoFe-MOF-400oC-1h. As shown in the transmission electron microscopy (TEM) images (Figure S5) and HAADF-STEM image (Figure 2a, 2b and Figure S6), numerous homo-dispersed nanoparticles emerged after 1h pyrolysis at 400oC. The nanoparticles did not show characteristic surface-enriched distribution features (as indicated by the red dotted line in Figure S5a), suggesting the partial decomposition occurred at both the surface and the internal part of MOF-74. We then increase the magnification of electron microscopy to observe the individual nanoparticle. As can be seen from the high resolution HAADF-STEM image (Figure 2c), the individual nanoparticle possessed several distinct crystal structures. Corresponding scanning transmission electron microscope electron energy loss spectroscopy (STEM-EELS) elemental mapping (Figure) indicated the Ni, Co, Fe elements were hierarchically distributed in the nanoparticle, where Ni and Co enriched in the core and Fe distributed on the skin coat of the nanoparticle. Notably, the distribution of O perfectly matched that of Fe, suggesting a metallic NiCo alloy core and Fe oxides thin shell in the nanoparticle. Figures 2e−2g display the enlarged images from the selected areas marked by the rectangles in Figure 2. As shown in Figure 2e, the core nanocrystal gave clear crystal lattice fringes with a d-spacing of 0.177 nm, which corresponded to the (002) plane of the NiCo alloy. The shells showed spinel structured Fe3O4 on the surface of the fcc NiCo alloy (Figures 2f and 2g). The d-spacing of 0.481 nm in Figure 2f and 0.415 nm in Figure 2g corresponded to the (11-1) and (002) plane of Fe3O4. Image simulation and atomic model of Fe3O4 along the [110] and [123] directions were overlapped on the experimental images. Through comparing the high resolution HAADF-STEM images with Fe3O4 structure models, we found perfectly match between them (also see Figure S7 and S8), which indicated the successful fabrication of NiCo/Fe3O4 heterostructure within MOF-74 by partially pyrolyzing NiCoFe-MOF-74 at 400oC for 1h under inert atmosphere. This unusual elemental distribution indicated the Ni, Co, Fe were dissociated with DOT ligands in the order of Ni, Co, and Fe under heating. As evidenced in thermogravimetric analysis (TGA) of mono metal (Ni, Co, Fe) MOF-74 (Figure S9), the decomposition temperatures increased in the order of Ni, Co, and Fe, which reflected the bonding strengths of these three metals with the O atoms of organic ligands (Ni-O
