Hierarchical Fe2O3@C@MnO2@C Multi-shell Nanocomposites for

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Hierarchical Fe2O3@C@MnO2@C Multi-shell Nanocomposites for High Performance Lithium-ion Battery and Catalyst Yu Zhang, Qing Li, Jiwei Liu, Wenbin You, Fang Fang, Min Wang, and Renchao Che Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00356 • Publication Date (Web): 17 Apr 2018 Downloaded from http://pubs.acs.org on April 18, 2018

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Article type: Full Paper

Hierarchical Fe2O3@C@MnO2@C Multi-shell Nanocomposites for High Performance Lithium-ion Battery and Catalyst Yu Zhang,1 Qing Li,1 Jiwei Liu,2 Wenbin You,1 Fang Fang,1 Min Wang*,1 and Renchao Che*1 1

Laboratory of Advanced Materials, Department of Materials Science, Collaborative

Innovation Center of Chemistry for Energy Materials (iChEM), Fudan University, 220 Handan Road, Shanghai 200433, China 2

School of Materials Science and Engineering, Changzhou University, Changzhou,

Jiangsu 213164, China.

ABSTRACT The Fe2O3@C@MnO2@C (FCMC) nanocomposites containing spindle-like Fe2O3 as a core and MnO2 nanoflakes as a sandwiched shell and double carbon layers have been successfully prepared by a facile method. As anode materials of lithium ion batteries (LIBs), the cycling stability, rate performance and conductivity of the prepared FCMC are far beyond that of the carbon-free Fe2O3@MnO2 (FM) nanocomposites. The hierarchical structure with double layers of carbon effectively enhances the ion conductivity and electrochemical performance of transitional metal oxides, indicating that carbon in FCMC played an important role during lithium ion storage. The initial discharge/charge capacity of FCMC electrode reaches as high as 1240.2/1215.9 mAh g-1, and the discharge capacity is over 1000 mAh g-1 at 500 mA

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g-1 after 50 cycles. Additionally, the unique hierarchical structural characteristic and double layers green carbon with high degree graphitization make FCMC an excellent catalyst in removing methylene blue (MB) dye from solution with H2O2 under a slight heating with the degradation time as short as 10 min. Our work presents a new perspective on carbon modified multilayer core-shell oxide structure, which can be applied to many fields such as energy storage and catalyst. Keywords: FCMC nanocomposites, double carbon layers, lithium ion battery, hierarchical structure, catalytic performance 1. INTRODUCTION New energy material and catalyst material have been attracting extensive interests recently. As one of the most promising energy storage devices, Lithium-ion batteries (LIBs) possess advantages of high energy density, long cycle life, and environmental friendly, and have been widely used in portable electronic devices. However, it still remains a great challenge to develop one candidate material to exhibit both outstanding storage capability and strong catalysis performance. Transition metal oxide materials show broad applications in many fields of adsorption,1 catalysis,2 wave-absorption,3 energy storage4,5 and so on. Compared to graphite, the most common anode for commercial LIBs with a theoretical capability of 372 mAh g-1, transition metal oxides can be used as potential anode materials because of their high theoretical specific capacities and shortened diffusion lengths for ions. For instance, a designed hierarchical TiO2@Fe2O3 hollow nanostructures, 6 ZnO/MnO2 urchin-like sleeve array,7 branched α-Fe2O3/SnO2 nano-heterostructures8

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and hierarchical CuO@MnO2 core-shell nanosheet arrays9 were all used as anode materials in LIBs. Nevertheless, problems of the electron conductivity, capacity fade and short cycle life make it difficult for the practical applications of transition metal oxide anodes. Briefly, an effective way to combine carbon with oxide anode is urgently requested. Among the abundant transition metal oxide, Fe2O3 and MnO2 are promising candidates to replace graphite as anode materials due to their large-scale reserves, low processing cost, easily fabricating, nontoxic, high corrosion resistance and high theoretical specific capacities (1007 and 1230 mAh g-1, respectively). Gu et al.10 synthesized hierarchical α-Fe2O3 nanotubes with a specific capacity of 764.2 mAh g-1 at a current density of 0.5 A g-1. Liu et al.11 reported a kind of nanostructured MnO2 for lithium ion batteries. However, they still suffer some problems in practical use including the poor electrical conductivity and extreme volume expansion during the extracting-inserting process of lithium ions. Especially, the MnO2 even suffers 96% volume augment during the extraction process of lithium ions. To solve these issues, two main strategies have been carried out. One is to tune appropriate shapes, while the other is to add with either conductive or active materials. Various forms of transition metal oxide, such as hematite multi-shelled hollow spheres, 12 hierarchical TiO2 tubular structure,13 α-Fe2O3 nanorods,14 and hollow metal oxide nanostructures,15 have been prepared to increase the specific surface area so as to enhance the contact conductivity between anode materials with electrolyte. Moreover, different kinds of carbon/oxide composites, such as Fe2O3/Graphene composite nanosheets,16 nanoflaky

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MnO2/carbon nanotube nanocomposites, 17 and hollow core-shell MnO2/C 18 have been synthesized to improve their poor conductivity and accelerate Li ion and electron transportion. For most of these materials, however, it is still difficult to maintain structural stability after long cycling because of volume expansion. To solve these issues as mentioned above, a handful of groups have do some great jobs recently. For example, Xie et al. 19 synthesized PPy/MnO2-rGO-CNTs composite with high specific capacity, excellent cycling stability and good rate capability. Mai et al. have prepared MnO2/C yolk-shell nanorods20 with a long-life performance, carbon-coated SiOx nanowires21 for high reversible specific capacities with self-sacrificed synthetic method, SnO2-PPy nanofilm22 with high-rate capability and cycling stability, Zn3V2O7(OH)2·2H2O microflowers23 as a high-performance anode for lithium-ion batteries. In this work, we prepared the core-shell structure Fe2O3@C@MnO2@C nanocomposite with double conductive carbon layers. The solid Fe2O3 core provides effective support and avoids of structural collapse during cycling process. The inner carbon layer coated on the surface of Fe2O3 core mainly serves as a substrate material for the formation of MnO2 nanoflakes, while the outer carbon layer coated on the surface of MnO2 can prevent huge volume change during the long-term cycling process caused by the insertion/extraction of lithium ion. Additionally, the carbon shells can effectively improve the conductivity of transition metal oxide based on the N-doped content and the high degree of graphitization. The MnO2 nanoflakes can provide a high specific capacity and excellent cycling performance and it is a kind of active catalyst or photocatalyst, which used in water

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purification filed to remove dye,24 heavy metal material25 and other organics26 from water. FCMC nanocomposites were used as catalyst for removing MB dye from solution. Double layers of carbon have a similar function either in electrochemical process or catalytic degradation of MB solution. Carbon could absorb MB molecule on the surface of MnO2 nanoflakes as much as possible, once adding of H2O2, the reaction happened instantaneously. The special hierarchical core-shell structure FCMC nanocomposites, not only present excellent rate and cycling performances as candidates for LIBs, but also act as a catalyst for degradation of dye molecules. Our findings indicated core-shell structure might stimulate inspiration to design and fabricate bifunctional material. 2. EXPERIMENTAL SECTION 2.1. Materials. Iron chloride hexahydrate (FeCl3·6H2O), ethanol (C2H5OH),

sodium dihydrogen phosphate dihydrate (NaH2PO4·2H2O), potassium permanganate (KMnO4), hydrogen peroxide (30%, H2O2) and hydrochloric acid (37%, HCl) were provided

by

Shanghai

Chemical

Reagent

Co.

Ltd.

Tris(hydroxymethyl)

aminomethane (C4H11NO3), methylene blue (MB) and dopamine·hydrochloride (C8H11NO2·HCl) were all purchased from Aladdin Industrial Inc. All reagents were of analytical grade without further purification. 2.2. Synthesis. 2.2.1. Synthesis of spindle-like α-Fe2O3@PDA nanocomposites.

The spindle-like α-Fe2O3 particles were synthesized via a facile hydrothermal method as reported before27 and with some modification.28 Typically, 60mg as-synthesized α-Fe2O3 was dispersed in 200 ml trisbuffer solution (PH=8.5) and the solution was sonicated with stirring for 10 min. Afterward, 60 mg C8H11NO2·HCl was 5


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homodispersed into the solution above, followed by magnetically stirring for 1 h at room temperature. During this process, dopamine molecule assembled on the surface of α-Fe2O3 nanocomposites and a layer of polymer was formed by self polymerization of dopamine. The resulting brown powder α-Fe2O3@PDA was obtained after centrifugation and dried in a vacuum oven for at least 8h. 2.2.2. Synthesis of α-Fe2O3@PDA@MnO2 nanocomposites. The as-synthesized α-Fe2O3@PDA nanocomposites (60 mg) and KMnO4 (160 mg) were added into 80 ml DI water. The mixed solution was sonicated for 15 min, and then transferred into a Teflon-lined autoclave with a capacity of 100 ml. After sitting for 1 h, the autoclave was heated to 160 oC and maintained for 6 h, and then allowed to cool to room temperature. The resulting deep brown powder α-Fe2O3@PDA@MnO2 was obtained after centrifugation and dried in a vacuum oven for at least 8h. 2.2.3. Synthesis of α-Fe2O3@PDA@MnO2@PDA nanocomposites. Using the obtained product of α-Fe2O3@PDA@MnO2 and C8H11NO2·HCl as reactants, and following the similar procedure of synthesizing α-Fe2O3@PDA, we can get α-Fe2O3@PDA@MnO2@PDA nanocomposites. 2.2.4. Synthesis of α-Fe2O3@MnO2 nanocomposites. The as-synthesized α-Fe2O3 (30 mg) and KMnO4 (200 mg) were dispersed in 10 ml and 25 ml DI water respectively, and the α-Fe2O3 mixed solution was added into the KMnO4 solution drop by drop, followed by adding 70 ul 37% HCl. Afterward, the mixture was diverted to Teflon-lined autoclave (50ml), and after standing for 1 h, the autoclave was heated to 110 oC and maintained for 6 h. At last, the brownish black products was rinsed and

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dried overnight, and the α-Fe2O3@MnO2 was obtained. 2.2.5.

Final

products

of

black

α-Fe2O3@C@MnO2@C

(FCMC),

α-Fe2O3@C@MnO2 (FCM), α-Fe2O3@C (FC) and α-Fe2O3@MnO2 (FM) were obtained

by

heating

α-Fe2O3@PDA@MnO2@PDA,

α-Fe2O3@PDA@MnO2,

α-Fe2O3@PDA and α-Fe2O3@MnO2 in a furnace under N2 flowing atmosphere respectively. The specific temperature program is from room temperature to 350 oC for 2 h, and then up to 600 oC for 6 h. 2.3. Characterization. Powder X-ray diffraction (XRD) measurements were carried

out using a Bruker D8 X-ray diffractometer (Germany) equipped with Ni-filtered Cu-Kα radiation (40 kV, 40 mA) in the 2θ range of 10o-90o. The information of morphology, crystal form and size of products were obtained by a Field-emission scanning electron microscope (FE-SEM, Hitachi S-4800, Japan) operated at the acceleration voltage of 1.0 kV and a JEM-2100F transmission electron microscope equipped with a JEOL Dual SDD system at the acceleration voltage of 300 kV. TGA was measured on an Pyris 1 Thermo Gravimetric Analyzer under a flow of air in the temperature range of 30-800 oC with a heating rate of 5 oCmin-1. XPS measurements were performed on an ESCALABMKLL photoelectron spectrometer with an Al-Kα source. Raman measurement was recorded using a Laser Micro-Raman spectrometer (Jobin Yvon, T64000) with a laser excitation at 514.5 nm. 2.4. Electrochemical Measurements. The electrochemical performances of the

samples were tested in the model test cell system. To prepare the electrode, 60 wt.% active material, 30 wt.% Super P carbon black and 10 wt.% polyvinylidene fluoride

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(PVDF) were homodispersed in N-pyrrolidinone (NMP) and the obtained slurry was coated on Cu foil using the doctor-blade technique. After drying in a vacuum oven at 80oC overnight, the electrode were punched into disks with diameter of 12 mm. The electrolyte was l M LiPF6 and its solvent was solution of ethylene carbonate and diethyl carbonate, whose volume ratio was 1:1. Lithium foils were used as both counter and reference electrodes and the coin-type test cells were fabricated in an Ar-filled dry glove box. The charge/discharge cycling tests were performed at a series of current densities and the cut-off voltage from 0.01 to 3.00 V (vs. Li/Li+). The CHI 600A electrochemical workstation was used to test cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). 2.5. Catalytic degradation processes. Catalytic activity testing on the degradation of

methylene blue (MB) was carried out in a 100 ml three-necked flask containing 50 ml of 100 mgL-1 MB and 20 mg of catalyst under magnetically stirring with oil bath at the constant temperature of 30 oC, 40 oC, 50 oC and 60 oC, respectively. After coming up to adsorption equilibrium at corresponding temperature for 0.5 h, 10 ml 30% H2O2 was injected to the mixed solution immediately. Remove 3ml of the mixture from the flask and centrifuge at regular intervals to remove the catalyst completely. How much degradation of MB can be analyzed with UV/Vis spectrophotometer (SHIMADZU, UV3600). When reaching the adsorption/desorption equilibrium, the original concentration of MB is marked as C0. After the catalytic reaction, the finally concentration of MB is recorded as C. The C/C0 stands for the percentage of the degradation.

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3. RESULTS AND DISCUSSION Scheme 1 shows the stepwise preparation of α-Fe2O3@C@MnO2@C with a hierarchical core-shell spindle shape. Firstly, spindle-like α-Fe2O3 was prepared via a facile hydrothermal method. Then, polydopamine with a thickness of about 5 nm is uniformly coated on the surface of α-Fe2O3 nanoparticles based on polycondensation reaction. The essence reaction of α-Fe2O3@PDA with KMnO4 is a redox reaction of carbon with KMnO4: C+2KMnO4=K2MnO4+MnO2+CO2↑ After that, the polydopamine with the thickness of about 20 nm is coated on the surface of MnO2 by polycondensation. Finally, the α-Fe2O3@C@MnO2@C is obtained by annealing under N2 atmosphere at 600 oC. The prepared FCMC shows a special core-shell structure with double carbon layers contacting with the core and the shell closely. The structure of well-crystallized α-Fe2O3 and MnO2 is reflected by the XRD pattern of FCMC (Figure. 1). No additional diffraction peaks of impurities can be detected, which confirms the high purity of the synthesized products. The structure and morphology of the as-synthesized FCMC and FM are characterized by SEM and TEM images shown in Figure 2d, 2h and Figure S1. The FCMC naoparticles are ellipsoid and remarkably uniform, with the typical long axis of 450 nm and minor axis of about 100 nm. TEM images reveal the core-shell structure with an uniform carbonization shell of 20 nm covering on the surface of MnO2 sheet. Because dopamine has the advantage of succession of template shape, the stepwise and final products maintain the spindle shape of Fe2O3 nanoparticles, as shown in Figure 2a-g. 9


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As demonstrated in high magnification TEM images (Figure 2f, 2g), the Fe2O3 particles can be easily coated with a PDA layer of ~3.5 nm in thickness, and MnO2 layer with the thickness of ~43 nm covered on the surface of Fe2O3@PDA nanocomposites. Moreover, the HRTEM images (Figure 3a, b) of FCMC displays lattice fringes with d-spacing of 0.487nm and 0.242nm, corresponding to the (200) and (211) planes of α-MnO2 shown in XRD pattern. This can illustrate that the MnO2 nanoflakes are the outermost shell except carbon. The chemical identity and circumstance of FCMC nanocomposites are confirmed by X-ray photoelectron spectroscopy (XPS) spectra as shown in Figure 4a-e. The chemical binding energies of Fe 2p, Mn 2p, C 1s, N 1s and O 1s suggest the presence of the elements Fe, Mn, C, N and O (Figure 4a). The high-resolution XPS spectrum of Fe 2p shows that the binding energies of Fe 2p3/2 and Fe 2p1/2 (Figure 4b) were located at 710.8 eV and 724.6 eV respectively, with the satellite peak at 713.5eV and 726.9eV corresponding to Fe3+ in Fe2O3, which matches with the typical binding energy value of Fe2O3. The binding energies situated around 641.7 eV and 653.4 eV (Figure 4c) correspond to the Mn 2p3/2 and Mn 2p1/2 of MnO2. Several peaks of C 1s are observed: the peak at a binding energy of 284.7 eV can be assigned to C=C and C-C, while the peak at a binding energy of 286.0 eV corresponds to C-O, C-N and C=N. The peaks at 287.6 eV and 239.3 eV can be assigned to C=O and O-C=O, respectively. In Figure 4e, the binding energy of 533.6 eV is ascribed to C-O-C, which demonstrates that PDA molecules are partially cross-linked via C-O-C bonds during carbonization. The carbon content of FCMC can be estimated to be about 22.2 %

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according to the weight decrement between 300 oC and 400 oC in TGA (Figure S2). No weight loss under 300 oC or over 400 oC indicates ideal thermostability of the FCMC materials. The typical core-shell structure of FCMC is further confirmed by the line scan profile shown in Figure 5b, c and elementary mapping images shown in Figure 5a, d-f. The elementary mappings demonstrates that the presence of elements of Mn, Fe and O, and all elements uniformly distributed in the core and shells. Figure 5b, c illustrates that Mn signal shows slight change along the lines, while the O and Fe signals shows strong change when the lines are in the core areas. This is a favorable evidence that confirms the core-shell structure. The Raman spectrum of FCMC shows two remarkable peaks at 1346.84 and 1589.46 cm-1 (Figure 6), corresponding to the D band and G band. The G band represents the degree of graphitization, while D band provides information about graphite structure defects and presence of sp2-hybridized domain. N element in PDA molecules can cause the graphite structure defects in FCMC carbon layers during carbonization. The carbon in FCMC shows high degree of graphitization (ID/IG=0.92) similar with the pristine graphene (ID/IG=0.88),29 which may be attributed to the enhanced electrical conductivity of transition metal oxide in FCMC and good electrochemical performance of LIBs. The electrochemical properties of hierarchical core-shell FCMC as anode material in LIBs have been investigated by galvanostatic discharge/charge cycling and CV experiments. The unique hierarchical structure, graphene-like carbon coating and

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the closer packed ultrathin MnO2 nanoflakes in FCMC nanocomposites contribute to the enhanced electrochemical performance. Figure 7a shows the cycle voltammogram of FCMC and FM at a scan rate of 0.1 mV s-1 in the range of 0.01 V to 3.00 V. In the first cycle, an apparent reduction peak and a weak reduction peak are observed around 0.5 V and 0.8 V, corresponding to the initial insertion of lithium ion to form lithiated manganese dioxide and iron oxide.30,31 Specifically, the peak at ~0.5V and ~0.8V are attributive to the reduction of Mn4+ to Mn and Fe3+ to Fe2+, and formation of SEI layer according to the following reactions: MnO2 + 4Li+ + 4e-↔2Li2O + Mn Fe2O3 + 6Li+ + 6e-↔2Fe + 3Li2O

(1)32 (2)33,34

To the contrary, the peaks at around 1.3 V, 2.2 V, 1.61 V and 1.85 V correspond to the extraction of lithium ion from lithiation manganese dioxide and iron oxide, indicating that the oxidation reaction of Mn0 to Mn4+ and Fe2+ to Fe3+ may proceed by two steps. The weak peaks at 1.61 V, 1.85 V or 0.8 V illuminate that Fe2O3 in FCMC hardly take part in the redox reaction with Li ion, while the Fe2O3 in FM nanocomposites has participate in the redox reaction (Figure S5). Because the Fe2O3 core locates innermost of FCMC nanocomposites, the long Li ion transferring path to Fe2O3 makes the electrochemical reaction difficult. During the following cycles, the reduction and oxidation peaks shift due to the SEI layer induced electrode polarization.35All the reduction and oxidation peaks overlap after the first cycle, which illustrates that the electrode material has an excellent electrochemical reversibility and structure stability.

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The first two and the 10th charge/discharge curves of FCMC electrode (Figure 7b) are measured with a current density of 500 mA g-1 between 0.01 to 3 V. FCMC shows an initial charge capacity of 1240 mAh g-1and a reversible capacity of 1216 mAh g-1. The formation of SEI layer probably results in the irreversible capacity of 24.3 mAh g-1 in the first cycle. To further explore the influence of structure among the FCMC and its similar nanocomposites, the lithium storage performances of all samples have been evaluated by cycling test. A comparison of the cycling performance and specific capacity about the FCMC, FCM, FC and FM are provided by Figure 7c. Unsurprisingly, FCMC obtains the highest the 50th discharge specific capacity of 1089 mAh g-1 at a current density of 500 mA g-1, whereas FCM, FC and FM obtain lower discharge capacities of 523, 339 and 473 mAh g-1, respectively. FCMC electrodes display no obvious fading of capacity until the 50th cycle and maintain high capacity of about 1000 mAh g-1 even after 50 cycles. While the specific capacities of FCM, FC and FM faded to around 300 mAh g-1 gradually after 50 cycles. The different structures and components of FCMC, FCM, FC and FM are the main reasons for the difference in capacity and cycle life among these electrodes. Benefiting from the characteristics of hybrid structure, the double carbon layers in hierarchical structure can not only keep the core-shell FCMC from collapsing during the lithium insertion/extraction, but also can improve the electric conductivity by the carbon-base anode.36 On the contrary, the naked MnO2 nanoflakes of FCM and FM lead to damage of structures during cycling process, and single component of FC could not offer the specific capacities as

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high as FCMC. In addition, FCMC anode shows excellent rate performance compared to comparative electrodes FCM, FC and FM. The rate performances of FCMC, FCM, FC and FM at the voltage between 0.01 to 3V are displayed in Figure 7d. The FCMC presents a discharge capacity of over 1000 mAh g-1 at current density of 200 mA g-1, 975 mAh g-1 at 500 mAh g-1, 884 mAh g-1 at 1000 mA g-1, 696 mAh g-1 at 2000 mA g-1, 450 mAh g-1 at 4000 mA g-1, 1051 mAh g-1 at 200 mA g-1, much better than those of FCM, FC and FM nanocomposites and previous reports.37 The inferior electrical conductivity of FM have the lowest specific capacities at the large current. The improved rate performance of FCMC can be ascribed to its hierarchical structure, the small size of nanocomposites, as well as the two graphene carbon layers which enhance the electric conductivity notably. For the sake of the structural stability of those electrodes materials, we studied the electrodes of FCMC, FCM, FC and FM after cycled 50 times employed with SEM, which shown in Figure S6. As we might expected, the structure of FCMC kept integrity, while the MnO2 on the surface of FCM (Figure S6c-d) and FM (Figure S6g-h) was destroyed during lithium insertion/extraction and its flake structure disappeared. From the Figure S6e-f, the obvious cracks of FC could be seen. We can surmise that the structure of FC will be completely damaged with the number of cycling. The main reason is that the large volume expansion of Fe2O3 during lithium insertion/extraction and the only several nanometers thick of carbon have difficulty resisting the large volume expansion. Electrochemical impedance spectroscopy (EIS) measurements are performed to

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further study the electric conductivity of FCMC. The Nyquist plots of both FCMC and FM in the charged state (shown in Figure 8) consist of a straight line in the low frequency region and a semicircle in the high-medium frequency region. The semicircle is related to charge-transfer resistance between electrodes and electrolyte, the slope line relates to the lithium-ion diffusion within the electrode.38 By comparing the Nyquist plots of two electrodes, it can be inferred that the charge-transfer resistance decrease greatly with the existence of carbon layers. Therefore, the carbon layers in transition metal oxides hierarchical core-shell structure enhance electron transfer greatly during electrochemical reaction and induce higher lithium-ion conductivity in electrode material. According to previous reports,39 MnO2 can be used as catalyst to remove dye molecules from waste water. The as-prepared FCMC is used as active catalyst to remove organic pollution (MB) from waste water. In order to study catalytic activity of FCMC, series of catalytic experiments have been carried out to note the degradation of MB with H2O2 solution at different temperature, as show in Figure 9a-d. During the degradation process, the intensity of major characteristic peak at 664 nm in absorption spectra of MB solution reduced along with time. Furthermore, the absorption peaks at 664 nm diminish gradually in the presence of FCMC with the increasing temperature. Meanwhile, the color of the catalytic system changes from blue to gray gradually, indicating the decomposition of MB dye. The FCMC materials show unsatisfactory catalytic performance at room temperature of 30 oC even when the reaction time extends to 26 min, but the situation is quite different when rising the

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temperature. It is noteworthy that the MB degradation time reduces dramatically from 40 min, 12 min to 8 min when the temperature rises from 40 oC, 50 oC to 60 oC. As shown in Figure 8e, only 30 % MB can be degraded at room temperature in 35 min but when the temperature rises to 40 oC, the degradation rate reached to almost 100%. With the temperature of 50 oC or 60 oC, the time for completing degradation reduces to at most 10 min. Hence, it is obvious that temperature is one of the most important factors for MB degradation process in FCMC catalyst system. In addition, FCMC demonstrates good cycle catalytic performance for degradation of MB dye in the waste water, and the catalytic efficiency maintains above 90% after five cycles (Figure 9f). The control experiments proved the synergy effect results from various components (Figure S3, S4). FCMC is the most active catalyst for MB degradation in the presence of H2O2 superior to the condition of pure MnO2 or Fe2O3 in the presence of H2O2, H2O2 without catalyst, only heating and FCMC without H2O2. In FCMC nanocomposites, the MnO2 component plays the dominant role in catalytic process, rather than the Fe2O3 component. The existence of H2O2 in catalyst system is essential for FCMC to perform catalytic ability. Therefore, the catalyst system with FCMC and H2O2 exhibits the optimized performance for the degradation of MB under proper high temperature,. Moreover, the degradation efficiency of FCMC is much higher even than most of catalysts reported previously and the detailed information could be referred in Table S1. At the basis of the above results, we consider that the two carbon layers in

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FCMC nanocomposites enhance the adsorption capability for MB dye, and increase the number of MB molecules on the surface of MnO2 sheets. Simultaneously, the carbon layers can facilitate fast transfer on the catalyst surface for H2O2 and sorts of free radical species, including ·OH, HOO·and O2·-. 40 Once H2O2 molecule is adsorbed on the surface of MnO2 sheets through the carbon layers, it is decomposed into several free radical species under the effect of the active catalyst and heat. The destructive oxidation of MB dye occurs effectively due to high oxidizing ability of these

free

radical

species.

Therefore,

the

hierarchical

core-shell

FCMC

nanocomposites can serve as novel and remarkable active catalyst in waste water treatment field. 4. CONCLUSION In summary, we have developed a combined method of hydrothermal treatment and follow-up calcination to prepare a core-shell structure FCMC with a spindle-like Fe2O3 as a core. The shells have three layers: (i) the inner carbon layer connecting Fe2O3 and MnO2; (ii) MnO2 nanoflakes trapped inside carbon shells; (iii) the external carbon layer coated onside the of MnO2 nanoflakes surface. The sizes of FCMC spindle structures can be controlled facilely at ~450nm in long axis and ~100nm in short axis. Two layers of carbon shell assembled by carbon sheets with only 20 nm thickness are successfully coated onside both sides of MnO2 nanoflakes. The PDA polymer not only acted as a substrate for growing MnO2 nanocomposites, but also provided a precursor source of nitrogenous and carbon. Therefore, the double carbon layers played an important role in the application fields of both catalyst and LIB electrode. Such well-defined structures can reduce degradation time and enhance 17


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catalytic performance to remove dye from waste water. In addition, FCMC showed an excellent performance in anode materials of LIBs. The unique advantage of the double layers of carbon existed inside the hierarchical structure locates at that volume expansion and structure collapse can be effectively avoided during charge/discharge process. Our findings might shed light on the design and synthesis of novel energy materials.

ASSOCIATED CONTENT

Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] Notes The authors declare no conflict of interest.

ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (11727807, 51725101, 51672050, 61790581), Science and Technology Commission of Shanghai Municipality (16DZ2260600).

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Scheme 1. Stepwise schematic for the preparation of FCMC core-shell nanocomposites.

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Fe2O3@C@MnO2@C MnO2 PDF#44-0141 Fe2O3 PDF#33-0664

Intensity(a.u.)

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110 104 200

10

20

310 110400 202

440

30

50

40

018 521214

60

1110 220

70

80

2Theta(degree)

Figure 1. XRD pattern of as-prepared FCMC nanocomposites (black), standard XRD patterns of Fe2O3 (green) and MnO2 (red)

Figure 2. FE-SEM and TEM of as-prepared (a, e) Fe2O3, (b, f) Fe2O3@PDA, (c, g) Fe2O3@PDA@MnO2, (d, h) FCMC, respectively.

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Figure 3. (a-b) HRTEM image of FCMC with different interplanar spacing.

Figure 4. (a) XPS survey spectrum of FCMC nanocomposites, (b) deconvoluted XPS Fe 2p spectra of FCMC nanocomposites, (c) Mn 2p spectra of FCMC nanocomposites, (d) deconvoluted XPS C 1s spectra of FCMC nanocomposites, (e) deconvoluted XPS O 1s spectra of FCMC nanocomposites.

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Figure 5. (a, d-f) Elemental mapping of FCMC probed by EDS, (b, c) Line scanning profile of Fe, O and Mn along major axis and minor axis of nanocomposite.

Figure 6. Raman spectra of FCMC nanocomposites.

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Figure 7. (a) CV curves of FCMC nanocomposites for the first 4 cycles at a scan rate of 0.1 mV s-1, (b) galvanostatic charge and discharge voltage profiles of FCMC at a current density of 500 mA g-1, (c) cycling performance of FCMC, FCM, FC and FM nanocomposites at a current density of 500 mA g-1 over 100 cycles, (d) rate performance of FCMC,FCM, FC and FM at various current densities.

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80

FCMC FM

70 60 50

-Z''(Ohm)

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40 30 20 10 0 0

20

40

60

80

100

120

140

160

Z'(Ohm)

Figure 8. Electrochemical impedance spectroscopy results of FCMC and FM in the full charged state.

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Figure 9. (a-d) Absorption spectra of a solution of MB in the presence of FCMC nanocomposites with 10ml 30% H2O2 solution and different heating temperature of 30, 40, 50 and 60 oC, respectively, (e) degradation rate of MB at different temperatures, (f) degradation of the MB in 10 min as a function of reused time using the FCMC as the catalyst.

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Table of Contents graphic:

The hierarchical Fe2O3@C@MnO2@C multi-shell nanocomposites is a kind of bifunctional materials and it not merely has a excellent electrochemical performance in LIBs but also can work as a kind of thermocatalyst in removing dye from waste water.

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Hierarchical Fe2O3@C@MnO2@C Multi-shell Nanocomposites for

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