Spinel ZnMn2O4 nanoplate assemblies fabricated via “escape-by-crafty-scheme” strategy

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  • Dynamic Article LinksC

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    reported the synthesis of metal oxides by a thermal trans-

    formation method using nanoscale metalorganic frameworks

    (MOFs) as precursors.3139 Through a simple thermal treatment

    process, the metal oxides with morphology similar to the nano-

    scale MOFs could be facilely synthesized. The separated

    morphology and composition control step of this method

    provides more possibility for precisely controlling the structure

    and morphology of the metal oxides. However, the thermal

    transformation of MOFs to metal oxides has mainly focused on

    MxOy, and the preparation of mixed-metal oxides using this

    method is seldom reported.40,41 Herein, we present a general

    method for the synthesis of spinel structured mixed-metal oxides,

    such as ZnMn2O4, CoMn2O4, and NiMn2O4, with well defined

    morphology and composition, by thermal treatment of nano-

    scale mixed-metalorganic frameworks (MMOFs) via an

    escape-by-crafty-scheme strategy, as shown in Fig. 1. The key

    issue for this method is to prepare MMOFs precursors with

    homogeneous mixing of different metal ions in a crystal structure

    with well-controlled morphology. After simple thermal treat-

    ment process, the spinel structured metal oxides with

    morphology similar to the MMOFs precursors could be

    successfully prepared. The potential of ZnMn2O4 nanoplate

    assemblies as anode materials for lithium ion batteries was also

    investigated.

    Experimental

    Synthesis of ZnMn2ptcda MMOFs

    In a typical synthesis,39 Zn(OAc)2$2H2O (0.133 mmol) and

    Mn(OAc)2$4H2O (0.267 mmol) were dissolved in 22.5 mL

    deionized water, and ptcda (0.2 mmol) was dissolved in 12.5 mL

    NaOH solution (0.8 mmol NaOH). The ptcda solution was

    added dropwise to the mixture solution of metal acetates with

    stirring. The immediate formation of a precipitate was observed.

    The reaction mixture was stirred at room temperature for 30 min,

    and then transferred to a Teflon-lined stainless steel vessel

    (45 mL) and heated at 100 C for 4 h. After cooling down toroom temperature, the precipitate was collected by centrifuga-

    tion, washed with water and dried. ZnMn6ptcda MMOFs were

    prepared in a similar way to ZnMn2ptcda MMOFs except

    a Zn : Mn molar ratio of 1 : 6 was used. For other MMn2ptcda

    MMOFs, the synthesis process was similar to that described for

    ZnMn2ptcda except using M(II) acetate instead of zinc(II)

    acetate.

    Fig. 1 Illustration of the two step process for the preparation of spinel

    ZnMn2O4 using ZnMn2ptcda as a precursor by an escape-by-crafty-

    scheme strategy.

    This journal is The Royal Society of Chemistry 2012

    Formation of ZnMn2O4 nanomaterials

    ZnMn2ptcda MMOFs were thermally treated in air at 450C

    for 1 h with a ramp of 5 C min1. CoMn2O4 and NiMn2O4 wereformed from CoMn2ptcda and NiMn2ptcda respectively,

    under the same thermal treatment process.

    Characterization

    The thermogravimetric (TG) analysis was performed in air with

    a heating rate of 5 C min1 by using a NETZSCH STA-449F3thermogravimetric analyzer. The powder X-ray diffraction data

    (PXRD) were collected on a Rigaku D/Max2500 PC diffrac-

    tometer with Cu Ka radiation (l 1.5418 A) in the 2q range of370 with a scan speed of 5 min1 at room temperature.Scanning electron microscopy (SEM) was undertaken on a FEI

    QUANTA 200F scanning electron microscope operating at an

    acceleration voltage of 20 kV. The samples were sputtered with

    gold prior to imaging. Selected-area electron-diffraction (SAED)

    and high-resolution transmission electron microscopy

    (HRTEM) images were recorded on a FEI Tecnai F30 micro-

    scope with a point resolution of 0.20 nm operated at 300 kV. The

    nitrogen sorption experiments were performed at 196 C ona Micromeritics ASAP 2020 system. Prior to measurement, the

    samples were out-gassed at 120 C for 6 h. The BET (BrunauerEmmettTeller) specific surface areas were calculated using

    adsorption data in a relative pressure range of P/P0 0.050.25.The pore size distribution curve was calculated from the

    adsorption branch using the BJH (BarrettJoynerHalenda)

    method.

    Electrode fabrication and performance measurements

    The ZnMn2O4 electrode slurry was prepared by mixing 75 wt%

    active material, 15 wt% acetylene carbon black, 10 wt% poly-

    vinylidene fluoride (PVDF) binder and an adequate amount of

    N-methyl-2-pyrrolidone (NMP). The slurry was coated onto

    copper foil and dried under vacuum at 85 C overnight. Theelectrode was cut into a shape with an area of 1 cm2. The loading

    weight of the active materials in the electrode was about 2.0 mg

    cm2. The electrochemical performance of the ZnMn2O4 elec-

    trode was evaluated in a 2016 coin-type ZnMn2O4Li cell, in

    which the lithium electrode was used as the counter electrode as

    well as the reference electrode. The electrolyte was 1 M LiPF6 in

    ethylene carbonate (EC) and diethyl carbonate (DEC) (1 : 1 by

    volume). A Celgard 2400 (polypropylene) was used as the sepa-

    rator. All the cells were assembled in an argon-filled glove box

    maintaining the moisture and oxygen levels below 1.0 ppm. All

    electrochemical measurements were evaluated galvanostatically

    between 0.01 and 3.0 V using a Neware instrument (China). The

    chargedischarge measurements were tested under a current

    density of 60 mA g1. For the rate capability measurements, the

    cells were both charged and discharged at a current density of

    603600 mA g1.

    Results and discussion

    ZnMn2ptcda, containing Zn2+ and Mn2+ ions, was prepared by

    reaction of Zn2+ and Mn2+ (the molar ratio of Zn : Mn is 1 : 2)

    with perylene-3,4,9,10-tetracarboxylic dianhydride (ptcda) in

    J. Mater. Chem., 2012, 22, 1332813333 | 13329

    http://dx.doi.org/10.1039/C2JM32261G

  • Fig. 3 PXRD patterns of ZnMn2ptcda precursor and spinel ZnMn2O4prepared by escape-by-crafty-scheme strategy using ZnMn2ptcda as

    a precursor and by co-precipitation method (450 C, 1 h, air atmosphere).

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    NaOH solution under hydrothermal conditions. The scanning

    electron microscopy (SEM) image of ZnMn2ptcda clearly

    shows that the sample is composed of flower-like assemblies

    (particle size of ca. 4 mm) constructed of nanoplates with thick-

    ness of ca. 60 nm (Fig. 2a). Energy-dispersive X-ray spectroscopy

    (EDX) analysis shows that ZnMn2ptcda is composed of carbon,

    oxygen, manganese and zinc elements, and the Mn : Zn molar

    ratio is2 (Fig. S1). The EDXmappings reveal that manganeseand zinc elements are homogeneously distributed in the whole

    ZnMn2ptcda sample (Fig. 2b). Furthermore, the elemental

    distribution analysis on the selected region confirms the uniform

    distribution of Zn and Mn elements (Fig. 2d). This proves that

    both elements are distributed homogeneously in the ZnMn2

    ptcda particle. The transmission electron microscopy (TEM) and

    selected area electron diffraction (SAED) analysis of an isolated

    particle display single crystal properties with periodic diffraction

    spots (Fig. 2c). The powder X-ray diffraction (PXRD) pattern of

    ZnMn2ptcda is shown in Fig. 3. ZnMn2ptcda displays strong

    diffraction peaks, exhibiting a three-dimensional framework

    which is isostructural with Znptcda MOFs.39 The sample has

    (001) crystallographic texture, a crystalline structure with an

    orthorhombic crystal system and Pbam space group. All of the

    diffraction peaks can be well indexed, which indicates that

    ZnMn2ptcda is a pure phase complex. The above results show

    that ZnMn2ptcda is a pure MMOFs phase with homogeneous

    distribution of Zn and Mn ions. The fabrication of nanoscale

    MMOFs with mixed metal ions has not been reported previ-

    ously, although the post-synthesis ion-exchange method has been

    used to prepare nanoscale MMOFs.34,42 Our results indicate that

    nanoscale MMOFs with controllable composition, morphology

    and crystal phase can be successfully synthesized by using mixed

    metal ions in the initial synthesis mixture.

    By thermal treatment of ZnMn2ptcda in air at 450C for 1 h,

    brown powders and yellow crystals were obtained. The yellow

    crystal is perylene, formed by the decarboxylation of ptcda,

    Fig. 2 (a) SEM image, (b) EDX mapping SEM images, (c) TEM image

    of a selected particle (inset is the SAED pattern), and (d) STEM image of

    a selected region and the elemental mapping of Zn and Mn of ZnMn2

    ptcda.

    13330 | J. Mater. Chem., 2012, 22, 1332813333

    showing that the thermal treatment involves the escape-by-

    crafty-scheme of perylene as we previously reported.41 The

    brown powders can be ascertained to be tetragonal spinel struc-

    tured ZnMn2O4 with I41/amd space group (JCPDS card no. 07-

    0354) by XRD characterization (Fig. 3). All the diffraction peaks

    can be assigned to the spinel structure phase, indicating that high

    purity spinel ZnMn2O4 has been synthesized. ZnMn2O4 has

    a normal spinel structurewithZn2+ occupying the tetrahedral sites

    and Mn3+ occupying the octahedral sites, forming ZnO4 groups

    andMnO6 groups respectively. ZnMn2O4 is isostructural with the

    spinel Mn3O4. This clearly indicates that zinc is well incorporated

    into the Mn3O4 lattice without phase segregation, forming

    ZnMn2O4 with the replacement of Mn2+ by Zn2+.

    The SEM image reveals that the resulting ZnMn2O4 material is

    composed of nanoplate assemblies inherited from the ZnMn2

    ptcda precursor with flower-like morphology (Fig. 4a). The

    particle size of the resulting ZnMn2O4 is about 1.6 mm, which is

    smaller than that of the ZnMn2ptcda precursor because of the

    escape of the organic portion. The nanoplates, which assemble to

    form the flower-like morphology, are porous under observation

    of the TEM image (Fig. 4b). Furthermore, the nanoplates are

    composed of nanoparticles with a particle size of about 12 nm

    (Fig. 4c). Fig. 4d displays a representative HRTEM image of the

    ZnMn2O4 nanoparticles. They are well-crystallized in the

    tetragonal phase with the (101), (112), and (211) crystal lattices

    having interplane spacings of 0.48 nm, 0.30 nm, and 0.24 nm,

    respectively. ZnMn2O4 nanoplate assemblies show the uniform

    distribution of Zn and Mn in the particle with a Mn : Zn molar

    ratio of 2 (Fig. 4e and S1). The porous structure of theZnMn2O4 nanoplate assemblies can be further confirmed by N2sorption analysis (Fig. 4fg). The sample exhibits a typical type

    IV isotherm with a type H3 hysteresis loop, showing that this

    material has a mesoporous structure. The sample has a BET

    surface area of 42 m2 g1 and uniform distribution of the meso-

    pores with a pore diameter of 17.4 nm. It should be mentioned

    that the spinel structured ZnMn2O4 could also be formed even at

    a treatment temperature as low as 350 C. For comparison

    This journal is The Royal Society of Chemistry 2012

    http://dx.doi.org/10.1039/C2JM32261G

  • Fig. 4 (a) SEM, (b) TEM, and (cd) HRTEM images of ZnMn2O4, (e)

    STEM image of a selected region of a ZnMn2O4 particle and the

    elemental mapping of Zn andMn, (f) N2 adsorptiondesorption isotherm

    and (g) pore size distribution of ZnMn2O4.

    Fig. 5 (a) TG curve and (b) PXRD pattern of ZnMn2ptcda before and

    after thermal treatment at different temperatures for 1 h in air.

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    purposes, ZnMn2O4 was also prepared using the conventional

    co-precipitation method, which is a general method for the

    preparation of mixed-metal oxides.13 The spinel structure could

    not be formed at treatment temperatures below 450 C. TheXRD results show that ZnMn2O4 prepared by co-precipitation

    method is not as well crystallized as that prepared using ZnMn2

    ptcda as a precursor at the same thermal treatment conditions

    (Fig. 3 and S2). This is probably due to the uniform distribution

    of Zn and Mn ions in a well-defined coordination state in the

    crystalline ZnMn2ptcda precursor. This suggests the advantages

    of this transformation method using MMOFs as a precursor

    compared with the conventional co-precipitation method.

    Through varying the Zn : Mn molar ratio in the initial

    mixture, ZnMn6ptcda MMOFs (Zn : Mn molar ratio of 1 : 6)

    with a nanoplate morphology could also be prepared (Fig. S3).

    However, the nanoplates with diameter about 2 mm are

    uniformly distributed with only a limited amount of flower-like

    assemblies. The EDX result confirms the uniform distribution of

    Zn and Mn in the nanoparticles. After thermal treatment of

    This journal is The Royal Society of Chemistry 2012

    ZnMn6ptcda in a similar way to ZnMn2ptcda, off-stoichio-

    metric Zn0.43Mn2.57O4, with the nanoplate morphology inherited

    from ZnMn6ptcda, was formed. The XRD and N2 sorption

    isotherm show that Zn0.43Mn2.57O4 has a well-crystallized spinel

    structure and mesoporous textural features similar to those of

    ZnMn2O4 (Fig. S4).

    The thermal transformation process from MMOFs to spinel

    structured mixed-metal oxide was investigated by thermogravi-

    metric (TG) analysis using ZnMn2ptcda as a model (Fig. 5a).

    ZnMn2ptcda has two consecutive weight loss steps. The

    preliminary weight loss at 150 C corresponds to the release ofcoordinated water molecules. There is a well-defined weight loss

    due to the escape of perylene by decarboxylation above 345 C.Thermal treatment of ZnMn2ptcda at temperatures below

    345 C results in the obvious {00h} diffraction peaks in its XRDpattern, although loss of fine structure and a lattice contraction

    are observed. When the temperature reaches 350 C, ZnMn2ptcda is converted into spinel structured ZnMn2O4 immediately

    by perylene escaping from the MMOFs through decarboxylation

    (Fig. 5b). The structural and TG results suggest that ZnMn2

    ptcda, with its robust lamellar structure, acts as a precursor for

    the formation of ZnMn2O4 nanoparticles with similar

    morphology. Due to the complex structure of the ZnMn2ptcda

    precursor, the exact mechanism responsible for this trans-

    formation from original single crystalline ZnMn2ptcda

    MMOFs to polycrystalline ZnMn2O4 is not completely under-

    stood yet, though a template-engaged quasi-topotactic trans-

    formation process might be plausible.39

    This escape-by-crafty-scheme strategy could be extended for

    the formation of various spinel structured mixed-metal oxides

    J. Mater. Chem., 2012, 22, 1332813333 | 13331

    http://dx.doi.org/10.1039/C2JM32261G

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    using MMOFs as precursors. Fig. 6 displays the SEM images of

    CoMn2ptcda andNiMn2ptcda synthesized by a similarmethod

    to ZnMn2ptcda with a Co (or Ni) : Mn molar ratio of 1 : 2.

    CoMn2ptcda has a flower-like morphology composed of nano-

    plates similar to ZnMn2ptcda (Fig. 6a) and NiMn2ptcda is

    irregular nanolamella (Fig. 6d). EDX analyses confirmed the

    uniform distribution of Co (or Ni) and Mn in the MMOFs

    (Fig. S5). CoMn2ptcda and NiMn2ptcda were respectively

    converted into CoMn2O4 and NiMn2O4 by thermal treatment at

    450 C in air. As shown in the SEM images, the resultingmaterialsmaintain the morphology of the corresponding MMOFs precur-

    sors (Fig. 6b and e). The Co (or Ni) signals are observed in the

    EDX spectra, confirmi...

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