Low-Temperature Assembly of Ultrathin Amorphous MnO2

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Low-Temperature Assembly of Ultrathin Amorphous MnO2 Nanosheets over Fe2O3 Spindles for Enhanced Lithium Storage Chen Zeng, Wei Weng, Teng Lv, and Wei Xiao* School of Resource and Environmental Sciences, Hubei International Scientific and Technological Cooperation Base of Sustainable Resource and Energy, Wuhan University, Wuhan 430072, P. R. China

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S Supporting Information *

ABSTRACT: Carbon coating is an effective method to enhance the lithium storage of metal oxides, which, however, suffers from harsh conditions in high-temperature hydrolysis of organic mass at inert atmosphere and compromised capacity due to the presence of lowcapacity carbon. We herein report a direct assembly of ultrathin amorphous MnO2 nanosheets with thickness less than 3 nm over Fe2O3 nanospindle backbones at 95 °C as a mild-condition, shortprocess, and upscalable alternative to the classic carbon-coating method. The assembly of the amorphous MnO2 nanosheets significantly increases the electrical conductivity of Fe2O3 nanospindles. When evaluated as an anode for lithium-ion batteries, the Fe2O3@amorphous MnO2 electrode shows enhanced capacity retention compared to that of the Fe2O3 nanospindle electrode. In situ transmission electron microscopy and in situ X-ray diffraction observations of the electrochemically driven lithiation/delithiation of the Fe2O3@amorphous MnO2 electrode indicate that the assembled amorphous MnO2 nanosheets are in situ transformed into a Fe−Mn−O protection layer for better electrical conductivity, uncompromised Li+ penetration, and enhanced structural integration. The Fe2O3@amorphous MnO2 electrode therefore has a reversible capacity of 555 mAh g−1 after 100 galvanostatic charge/discharge cycles at 1000 mA g−1, comparable with that of the Fe3O4@C electrode derived via the classic carbon-coating route. KEYWORDS: lithium-ion battery, anode, Fe2O3, MnO2, in situ transmission electron microscope, in situ X-ray diffraction

1. INTRODUCTION Hematite, iron oxide (α-Fe2O3), is a redox-active, environmentally benign, cost-affordable, and resource-abundant material with widespread applications in lithium-ion batteries (LIBs),1−8 supercapacitors,9−13 water treatment,14−16 sensors,3 and catalysis.17,18 As an anode of LIBs, α-Fe2O3 shows a theoretical specific capacity as high as 1005 mAh g−1 via a conversion-reaction mechanism as shown in reaction 1, much higher than that of the state-of-the-art graphite (372 mAh g−1) anode. In comparison with the graphite anode, Fe2O3 anode experiences much larger volume change upon lithium uptake/ removal (96% of Fe2O3 vs 10% of graphite)2 and inferior electrical conductivity. The resulting inadequate cycle stability and high-rate performance significantly retard the practical implementation of Fe2O3 anode.

Elaboration and rational design of the composition and microstructure are key enabling factors for enhanced lithium storage of Fe2O3 anode. Carbon coating is an effective and welldocumented protocol to improve the lithium storage of Fe2O3 anode, in which coated carbon functions as a depolarizer for better electrical conductivity and serves as a protection or cushion for maintaining oxide integration.19−22 The carboncoating strategy involves high-temperature hydrolysis of organic mass at inert atmosphere, challenging its scale-up application. The introduced carbon has a lower specific capacity than that of oxides, and the transition from Fe2O3 to Fe3O4 with a lower specific capacity (reaction 2) also occurs in the carbon-coating process.20−22 Therefore, the carbon-coating method might decrease the overall specific capacity. The introduction of highly redox-active components other than carbon therefore comes into consideration for enhanced cycle stability and uncompromised capacity.23−25 Such an approach has been proven effective for enhanced lithium storage in Fe2O3−Ni3Se4,26 Fe2O3−SnO2,27 and Fe2O3−SnSe28 because of the synergetic interaction between Fe2O3 and the

Fe2O3 + 6Li+ + 6e− ↔ 3Li 2O + 2Fe 1005 mAh g −1 (1)

Fe3O4 + 8Li+ + 8e− ↔ 4Li 2O + 3Fe 926 mAh g −1

(2)

Received: July 15, 2018 Accepted: August 17, 2018

MnO2 + 4Li+ + 4e− ↔ 2Li 2O + Mn 1232 mAh g −1 (3) © XXXX American Chemical Society

A

DOI: 10.1021/acsami.8b11794 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

FeCl3 was dissolved in 500 mL of deionized (DI) water, followed by the addition of 27 mg of NaH2PO4. After room-temperature stirring for 1 h, the mixture was sealed in a blue-capped vial. After heating at 105 °C for 48 h, Fe2O3 nanospindles were collected by centrifugation in DI water and ethanol several times and vacuum-drying at 60 °C. 200 mg of Fe2O3 nanospindles was added into a 40 mL of aqueous solution of glucose (0.3 M) under ultrasonication. The suspension was sealed in a Teflonlined autoclave and hydrothermally treated at 180 °C for 4 h. Fe3O4@C nanospindles were then obtained after annealing the hydrothermally derived powder at 700 °C for 1 h in Ar. 2.2. Synthesis of Fe2O3@MnO2 Nanospindles. The obtained Fe2O3 nanospindles (90 mg) were added in 25 mL of aqueous solution containing 0.25 g of KMnO4 and 1 mmol HCl. The mixture was sealed and hydrothermally treated at 95 or 140 °C for different durations.35−37 The samples are denoted Fe2O3@MnO2-95 and Fe2O3@MnO2-140, respectively. 2.3. Synthesis of Layered MnO2 Nanosheets. 40 mL of aqueous solution containing 0.45 g of KMnO4 and 1.0 mL of HCl (37 wt %) was hydrothermally treated at 100 °C for 10 h.35,37 2.4. Material Characterizations. Crystallographic configuration was investigated by powder X-ray diffraction (XRD) on Rigaku Miniflex600 at a scan rate of 4° min−1 with Ni-filtered Cu Kα radiation (λ = 1.5406 Å). Microstructures were observed using field-emission scanning electron microscopy (FESEM, Zeiss SIGMA) and transmission electron microscopy (TEM, Titan G 2 60-300). Composition and spatial elementary distribution were determined using an energydispersive X-ray spectroscope (EDX, attached to the SEM) and an electron energy loss spectroscope (appended to the TEM). Surface bond information was analyzed by X-ray photoelectron spectroscopy (XPS, ESCALAB250Xi from Thermo Fisher Scientific) with C 1s at 284.8 eV as the internal reference. Electrical conductivity of the samples was tested over the pellets (10 mm in diameter) prepared from diepressing the corresponding powders in 8 MPa using the four-probe method (Suzhou JG, SZT-B). 2.5. Electrochemical Characterizations. The active material, poly(vinylidene fluoride) (PVDF), and carbon black with a weight ratio of 80:10:10 were mixed in N-methyl-2-pyrrolidone (NMP). The formed slurry was coated on a copper foil to fabricate working electrodes. The working electrodes were then subjected to vacuumdrying at 120 °C for 12 h. The mass loading of active materials was controlled to be around 2.5 mg cm−2. CR 2025 cointype cells were assembled in an Ar-filled glovebox (H2O < 0.1 ppm, O2 < 0.1 ppm) using a Li foil as the counter electrode, 1 M LiPF6 in a mixture of ethylenecarbonate (EC) and diethylcarbonate (DEC) (by weight, 1:1) as the electrolyte, and polyethylene as the separator. Galvanostatic charge/discharge cycling at 1000 mA g−1 was tested on a LAND battery tester in a voltage range of 0.01−3.0 V (vs Li/Li+). Cyclic voltammetry (CV) was recorded at 0.2 mV s−1 using a CHI660D electrochemical workstation (Chenhua). After five cycle CV scans, the coin cells were disassembled. The active-material-loaded Cu films were then thoroughly rinsed in EC and NMP to eliminate electrolyte and PVDF. The resulting films or powder was then subjected to SEM or XPS observations. 2.6. In Situ Environmental Transmission Electron Microscope (ETEM). Environmental Transmission Electron Microscope (ETEM, Hitachi H9500, 300 kV) analysis of an all-solid battery using the PI 95 H1H holder (Hysitron) was used to observe the electrochemically driven lithiation/delithiation of the Fe2O3@MnO295 electrode.38 Li metal was used as the counter electrode, while the naturally occurring Li2O over the Li metal served as the solid electrolyte. The Fe2O3@MnO2-95 electrode was lithiated at −5 V for 1.5 min and delithiated at 5 V for 1.5 min. The volume variation and selected area electron diffraction (SAED) of the nanospindle were analyzed in situ. 2.7. In Situ XRD Investigation. XRD was carried out using an Xray powder diffractometer (D8 ADVANCE, Bruker AXS GmbH Co., Ltd). The special battery was assembled using a tailor-made module with a Be window for X-ray penetration and a pure Li foil as the counter electrode. The test material mixed with carbon black and PVDF in a weight ratio of 8:1:1 was casted onto a carbon foam (14 mm in

second highly redox-active phase. Like Fe2O3, MnO2 is also an intriguing energy-storage material because of low toxicity, low cost, and excellent redox capability. MnO2 has a theoretical lithium storage capacity of 1232 mAh g−1 (reaction 3),29 even higher than that of Fe2O3. The incorporation of MnO2 with Fe2O3 therefore promises increased specific capacity. Comparable ionic radii between Fe and Mn cations facilitate a strong interaction between MnO2 and Fe2O3.30 Such an interaction promotes the substitution of Fe with Mn (vice versa) for doping Fe2O3 with manganese species for enhanced lithium storage,30,31 catalysis,32 capacitance,11 and water treatment.16 The forenamed interaction also facilitates the tunable and controllable formation of FeOx−MnOx core−shell or shell−core hybrids with a well-defined microstructure.9−18,31−34 FeOx−MnOx core−shell hybrids for lithium storage were very recently reported.31,28 However, the structure−activity relationship for enhanced lithium storage is yet to be addressed, mainly because of challenges in facile/controllable construction of FeOx−MnOx and in situ observation of the interaction between FeOx and MnOx. We herein report a low-temperature assembly of ultrathin MnO2 nanosheets with thickness less than 3 nm over Fe2O3 nanospindle backbones (Figure 1). Amorphous MnO2 nano-

Figure 1. Schematic representation of the low-temperature assembly of ultrathin amorphous MnO2 nanosheets over Fe2O3 nanospindles for enhanced lithium storage.

sheets are assembled at 95 °C, with significantly increased electrical conductivity occurring in the resulting Fe2O3@ amorphous MnO2 electrode. When evaluated as an anode for LIBs, the Fe2O3@amorphous MnO2 electrode shows enhanced capacity retention compared to that of the Fe2O3 nanospindle electrode. In situ transmission electron microscopy and in situ X-ray diffraction (XRD) observations of the electrochemically driven lithiation/delithiation of the Fe2O3@amorphous MnO2 electrode indicate that the assembled amorphous MnO2 nanosheets are in situ transformed into a Fe−Mn−O protection layer for better electrical conductivity, uncompromised Li+ penetration, and enhanced structural integration (Figure 1). The Fe2O3@amorphous MnO2 electrode therefore has a reversible capacity of 555 mAh g−1 after 100 galvanostatic charge/discharge cycles at 1000 mA g−1. The present study highlights the merits of amorphous ultrathin MnO2 nanosheets for better lithium storage and provides hints on rationally designing affordable and high-performance materials for energy storage.

2. EXPERIMENTAL DETAILS 2.1. Synthesis of Fe2O3 and Fe3O4@C Nanospindles. The preparation was carried out according to a previous report.22 2.7 g of B

DOI: 10.1021/acsami.8b11794 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 2. (a) SEM images of the Fe2O3 nanospindles, and SEM images of the derived samples obtained via the treatment in aqueous KMnO4 and HCl at 95 °C for (b) 5 h, (c) 8 h, and (d) 14 h. All scale bars are 100 nm.

Figure 3. (a) TEM images of the Fe2O3 nanospindles, and TEM images of the derived samples obtained via the treatment in aqueous KMnO4 and HCl at 95 °C for (b) 8 h and (c1−c7) 14 h. The panels (c1) and (c2) are the typical TEM images of the samples obtained via the treatment in aqueous KMnO4 and HCl at 95 °C for 14 h. The panels (c3) and (c4) illustrate Fe and Mn, respectively, being highlighted as red on the basis of (c2). The panels (c5)−(c7) represent the spatial EDX mapping of Fe, Mn, and O, respectively, on the basis of (c2). All scale bars are 100 nm. diameter) as the working electrode. A solution of 1.0 M LiPF6 in EC− DEC (1:1, by weight) was used as the electrolyte. The average mass loading on the electrodes is around 2.5 mg cm−2. The galvanostatic charge/discharge cycling between 0.01 and 3.0 V was performed using a LAND battery tester at a current density of 100 mA g−1. The X-ray diffraction signals were consecutively recorded using a two-dimensional X-ray detector. The XRD patterns of the polarized electrodes were in

situ collected, with a full XRD pattern being collected for every 10 min under dynamic voltages.

3. RESULTS AND DISCUSSION Similar ionic radii between Fe3+ and Mn3+ facilitate the heterogeneous nucleation of MnOx over Fe2O3 nanospinC

DOI: 10.1021/acsami.8b11794 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 4. Cyclic voltammetric (CV) curves of the (a) Fe2O3, (b) MnO2, (c) Fe2O3@MnO2-95, and (d) Fe2O3@MnO2-140 electrodes recorded at 0.2 mV s−1.

dles.30,31 Upon hydrothermal treatment at 95 °C, the previously bare surface of the Fe2O3 nanospindle (Figure 2a) gets branched after 5 h (Figure 2b) because of the heterogeneous nucleation of Mn species over the Fe2O3 nanospindle. By prolonging the reaction time, highly oriented nanosheets perpendicularly grow on the Fe2O3 nanospindle. Figure S1 (Supporting Information) shows the XRD patterns of the samples. All of the patterns are well-indexed to α-Fe2O3 (Hematite, JCPDS No. 72-0469, rhombohedral, R3̅c). No other peaks can be detected, indicating the formation of amorphous nanosheets. Compared to those of the intrinsic Fe2O3 nanospindles, the characteristic peaks of αFe2O3 in the two hybrids become disintensified, with smaller intensities occurring at longer reaction times. This is due to the shielding effect of the amorphous nanosheet shell on the αFe2O3 core. Figure S2 presents the EDX spectra of the samples. As can be seen, Mn occurs in the hydrothermally treated samples. The molar ratios between Mn and Fe are 7 and 12% for the 8 and 14 h samples, respectively, indicating the assembly of manganese oxide nanosheets over Fe2O3 nanospindles. K is absent in the samples. Figures 3, S3, and S4 depict TEM images and composition information of the 95 °C treated samples. Figure 3 clearly shows the generation of ultrathin nanosheets over the Fe 2 O 3 nanospindles. The EDX mapping verifies Mn in the shell and Fe in the core. In detail, the Mn mapping shows discontinuous distribution along the nanospindle, suggesting the direct assembly of MnOx nanosheets over Fe2O3 nanospindles without any interlayers. The as-obtained Fe2O3@MnO2 electrode without any buffer layer or full coverage of MnOx nanosheets over Fe2O3 nanospindles differs from the previously reported Fe2O3@MnO2 electrode,9,13 in that the latter’s MnOx shells completely cover the Fe2O3 nanospindles. Figure S3 shows that the assembled nanosheets upon an 8 h reaction have a thickness of 2 nm. No specific lattice fringes can be observed from the nanosheet regions in the two high resolution TEM (HRTEM)

images, indicating the assembly of amorphous nanosheets. The EDX spectrum mainly based on nanosheets exhibits the presence of Mn, Fe, and O, with the absence of K. EDX analysis displays a predominant Mn element. The minor Fe element comes from the Fe2O3 backbones. The above results confirm the formation of amorphous manganese oxide nanosheets over the precursory Fe2O3 spindles. By prolonging the reaction time to 14 h (Figure S4), the thickness of the assembled nanosheets remains identical, in spite of the slight curling of the nanosheets due to the increased lateral length. The SAED patterns verify that the Fe2O3 nanospindles and directly assembled MnOx nanosheets are crystalline and amorphous, respectively. Experimental runs at 70 and 80 °C for 10 h are also carried out. The results show that MnO2 nanosheets cannot be assembled at 70 and 80 °C. It is believed that a temperature higher than 80 °C is essential to assemble MnO2 nanosheets over Fe2O3 nanospindles. By simply varying the assembly temperature, the crystallinity of the coated shell can be tailored. After the hydrothermal treatment at 140 °C for 2 h, crystalline MnO2 nanosheets can be assembled. Figure S5 shows the XRD pattern of the 140 °C treated sample. Besides the peaks ascribed to α-Fe2O3, characteristic peaks from crystalline layered MnO2 nanosheets (Birnessite, JCPDS No. 80-1098, monoclinic, C2/m) are also detected (indicated by the diamonds), confirming the assembly of layered MnO2 nanosheets instead of amorphous MnO2 nanosheets over the Fe2O3 core at 140 °C. The corresponding TEM images shown in Figure S6 confirm the formation of ultrathin nanosheets over the Fe2O3 spindles. The assembled nanosheets are of a thickness less than 5 nm. As can be seen from the panel (c), d-spacing of 0.60 nm appears in the nanosheets, corresponding to the (001) d-spacing of birnessite-type layered MnO2 nanosheets. The formation of layered MnO2 nanosheets is also reflected by the existence of K in the nanosheets, which is due to the intercalation of K cations between interlayer gaps of D

DOI: 10.1021/acsami.8b11794 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 5. (a) Galvanostatic charge/discharge curves of the Fe2O3@MnO2-95 electrode at 1000 mA g−1. (b) Cycle performance of different electrodes at 1000 mA g−1. In panel (b), the empty symbols represent capacity for Li removal (discharge) and the filled symbols denote capacity for Li uptake (charge). (c) Specific capacity of different electrodes in varying C rates, with 1C being defined as 1000 mA g−1.

MnO2-95 (14 h), and Fe2O3@MnO2-140 (2 h) samples were then evaluated as anodes for LIBs. The Fe2O3@MnO2-95 (14 h) and Fe2O3@MnO2-140 (2 h) samples have comparable Mn/Fe molar ratio, that is, 10−12%. This is the reason why the two samples are compared in the present study. Figure 4 shows the cyclic voltammetric (CV) curves of the four electrodes. In Figure 4a, the peak at around 1.5 V in the first cathodic scan is for lithium intercalation in α-Fe2O3 with the generation of LixFe2O3. The following cathodic peak (c1) at 0.75 V represents the electrochemical reduction of Fe3+ to Fe0 governed by reaction 1, and the reversible process renders the formation of an anodic shoulder.1−4 In Figure 4b, the cathodic peak c2 refers to the conversion reaction of MnO2, as illustrated by reaction 3.29,36 For the two hybrids, both c1 and c2 occur in the cathodic scans. Such CV curves are consistent with the charge/discharge profiles of the hybrid that is shown in Figure 5a. In the CV curves of the Fe2O3@MnO2-95 electrode (Figure 4c), charges ascribed to Fe2O3 (c1) are much higher than those to MnO2 (c2), indicating that the assembled amorphous MnO2 nanosheets hardly retard the lithium storage of the Fe2O3 core. The c2 peaks ascribed to amorphous MnO2 nanosheets remain unchanged, suggesting the formation of a stable Mn-rich shell upon scans. Dissimilarly, the c2 peak is much higher than that of c1 in the CV curves of the Fe2O3@MnO2-140 electrode (Figure 4d). The vast majority of lithium storage capability of the Fe2O3@MnO2-140 electrode comes from the layered MnO2 shell. This phenomenon indicates that the assembled layered MnO2 nanosheets significantly hinder Li+ penetration into the Fe2O3 cores. In addition, the c2 peak gets intensified upon scans, indicating the absence of stable Mn-rich shells. Such an assumption agrees well with the SEM images of the electrode after CV scans. As shown in Figure S11, the precursory nanospindle morphology nearly disappears in the Fe2O3@ MnO2-140 electrode after five cycle scans. Alternatively, fragmental nanosheets become predominant, suggesting dis-

layered MnO2 nanosheets. During the hydrothermal treatment at 140 °C in aqueous acidic KMnO4, K+ and hydrates are chemically bonded within interlayer gaps of layered MnO2 nanosheets.36,37 This is evidenced by the presence of K in the 140 °C sample and the absence of K in the 95 °C sample (Figures S2 vs S6). Figures S7−S9 show the XPS spectra of the Fe2O3, Fe2O3@ MnO2-95 (14 h), and Fe2O3@MnO2-140 (2 h) samples. K appears only in the Fe2O3@MnO2-140 sample, arising from intercalated K+ between interlayer gaps of layered MnO2 nanosheets. The absence of K in the Fe2O3@MnO2-95 sample is in good agreement with the formation of amorphous MnO2 nanosheets at 95 °C. The three samples show similar Fe 2p highresolution XPS spectra,39 suggesting the absence of any binary oxides and no change of the Fe2O3 core after MnO2 nanosheets’ assembly. The peaks at 710.8 and 724.2 eV correspond to Fe 2p3/2 and Fe 2p1/2, respectively, between which a satellite peak assigned to α-Fe2O3 appears at 718.9 eV. The Fe 2p3/2 peaks are deconvolved into a doublet at 710.1 and 712.0 eV, ascribed to Fe located at octahedral and tetragonal sites of α-Fe2O3, respectively. The Mn 2p spectra of the Fe2O3@MnO2-95 and Fe2O3@MnO2-140 samples consist of Mn 2p3/2 and Mn 2p1/2 peaks at 642.6 and 653.4 eV, respectively, consistent with that of MnO2.35 In the three O 1s spectra, the peaks at 529.6, 531.4, and 533.2 eV are due to lattice oxygen (OL), surface hydroxyl (OH), and physically absorbed hydrates (OW), respectively.31 Table S1 compares the three O 1s spectra, highlighting the highest content of lattice oxygen (OL) in the Fe2O3@MnO2-140 sample. This evidence supports the existence of chemically bonded water in the Fe2O3@MnO2-140 sample. The K 2p spectrum of the Fe2O3@MnO2-140 sample (Figure S9) presents K 2p3/2 and K 2p1/2 peaks at 292.4 and 295.3 eV, agreeing well with the interlayer intercalated K+.40 Fe2O3, layered MnO2 (derived from the hydrothermal treatment at 140 °C without Fe2O3; see Figure S10), Fe2O3@ E

DOI: 10.1021/acsami.8b11794 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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electrode shows enhanced high-rate capability and cycle performance than that of the Fe2O3@MnO2-140 electrode. The results reconfirm that the assembly of amorphous MnO2 nanosheets over Fe2O3 is an effective method for enhanced lithium storage of Fe2O3. In comparison with previously reported Fe2O3/MnO2 composites,31 the lithium storage capability of the Fe2O3@MnO2-95 electrode is inferior. The Fe2O3@MnO2-95 electrode, however, shows better lithium storage capability than that of the reported urchinlike Fe2O3/ MnO2 hybrids.34 The two previously reported preparation methods both require high-temperature treatment and multiplestep procedures.31,34 The cycle performance at 0.2C, that is, 200 mA g−1, is shown in Figure 5c. As can be seen, the Fe2O3@ MnO2-95 electrode shows enhanced cycle stability compared to that of the pristine Fe 2 O 3 electrode, highlighting the effectiveness of assembled amorphous MnO2 nanosheets for better lithium storage of Fe2O3. The Fe2O3@MnO2-95 electrode exhibits a reversible capacity as high as 555 mAh g−1 after 100 cycles. Carbon coating was previously confirmed as an effective approach to enhance the lithium storage of Fe2O3 nanospindles.22 During the carboncoating process, the pristine Fe2O3 electrode is transformed to Fe3O4@C electrode (Figure S12). The coated carbon and the generated Fe3O4 both have lower theoretical capacity than that of Fe2O3. Therefore, the Fe3O4@C electrode has the lowest initial capacity (only 600 mAh g−1) among all of the samples. The coated carbon effectively enhances the transport of electrons and maintains the integration of the electrode, as evidenced by the capacity retention as high as 67.2% after 100 cycles. The capacity retention of the Fe2O3@MnO2-95 electrode is 56% after 100 cycles, only slightly lower than that of the Fe3O4@C electrode. The above results confirm that the assembly of amorphous MnO2 nanosheets is an alternative to the classic carbon-coating method for enhanced lithium storage of Fe2O3. The carboncoating method generally requires high-temperature hydrolysis of organic mass in inert atmosphere, tending to compromise capacity because of the generation of low-capacity Fe3O4 and carbon. In this sense, the herein reported low-temperature (95 °C) direct assembly of amorphous MnO2 nanosheets with merits of being a mild-condition and short process is intriguing for upscaling and therefore for practical applications. Figure S13 shows the high-resolution XPS spectra of the Fe2O3@MnO2-95 electrode after CV scans. It is shown that Mn2+ and Fe2+ species occur in the polarized electrode. The difference between binding energy of Mn 2p1/2 and Mn 2p3/2 is 11.7 eV, suggesting the existence of Mn2+.41 In the Mn 3s XPS spectrum, the energy separation between the two peaks is 3.9 eV, deviated from standard MnO (6.0 eV), Mn2O3 (5.3 eV), and MnO2 (4.7 eV).41,42 Such a deviation from single manganese oxides suggests the formation of binary Fe−Mn mixed oxides. Because of the generation of surface Fe−Mn mixed oxides, the Fe 2p spectrum gets deviated from the pristine Fe2O3 electrode, with lower intensity of the satellite peak from α-Fe2O3 and occurrence of Fe2+. Cation intermixing in Fe2O3−MnO2 hybrids was previously reported and confirmed as an effective process for enhanced energy storage.30,31 Figure S11 shows the SEM image of the Fe2O3@MnO2-95 electrode after CV scans. The nanospindle morphology remains, while the nanosheets disappear. During electrochemical polarizations, the assembled ultrathin amorphous MnO2 nanosheets are in situ converted to a Fe−Mn mixed oxide layer over the Fe2O3 nanospindles (as schematically illustrated in Figure 1).

connection between the Fe2O3 nanospindles and layered MnO2 nanosheets. Such a scenario might be due to the negative effect of chemically bonded hydrates in the layered MnO2 nanosheets. Conversion-reaction electrodes generally show large polarization between charge and discharge processes. Such a polarization can be compared on the basis of the CV results. The main component of the Fe2O3, Fe2O3@MnO2-95, and Fe2O3@MnO2-140 electrodes is α-Fe2O3, and the polarization of the three electrodes is therefore compared on the basis of the potential difference between the locations of the cathodic peak and the corresponding anodic peak in the second CV scans. The polarization voltages of the Fe2O3, Fe2O3@MnO2-95, and Fe2O3@MnO2-140 electrodes are 0.98, 0.89, and 0.91 V. Therefore, the assembly of MnO2 nanosheets could decrease the polarization. Figure 5b shows cycle performance of the electrodes at 1000 mA g−1, with the information being summarized in Table 1. The Table 1. Comparison of Lithium Storage Capability upon Cycles at 1000 mA g−1

samples

initial discharge capacity (mAh g−1)

initial Coulombic efficiency (%)

discharge capacity at the 100th cycle (mAh g−1)

capacity retention (%)

Fe2O3 Fe2O3@MnO2-140 Fe2O3@MnO2-95 Fe3O4@C

1043 984 992 600

79.3 69.5 69.3 55.9

162 330 555 403

15.6 33.5 56.0 67.2

pristine Fe2O3 electrode shows the highest initial discharge capacity but the worst capacity retention. A capacity as low as 162 mAh g−1 occurs after 100 cycles, retaining only 15.6% of the initial capacity. The poor performance of the pristine Fe2O3 electrode is due to poor electrical conductivity and electrode disintegration upon cycling. With the assembly of amorphous and layered MnO2 nanosheets, cycling performance of both Fe2O3@MnO2 electrodes is improved. The increased electrical conductivity of the two Fe2O3@MnO2 samples contributes to the observed enhancement. As shown in Table S2, the two Fe2O3@MnO2 samples show resistance 1 order of magnitude lower than that of the pristine Fe2O3 electrode. In comparison with the Fe2O3@MnO2-95 electrode, the Fe2O3@MnO2-140 electrode exhibits an inferior cycle stability. Figure 5b shows that discharge capacity higher than the corresponding charge capacity appears in the initial cycles of the Fe2O3@MnO2-140 electrode (empty triangles higher than the filled triangles). Such an anomaly, again, is an indication of the negative effect of the chemically bonded hydrates in the layered MnO2 nanosheets. The hydrates in the lattice of layered MnO2 nanosheets could react with Li, Li2O, and electrolyte in the cell, causing anomalously high discharge capacity and electrode failures. Such a harm might become more manifest in full-cell test because of the irreversible consumption of Li from the cathode. In the present half-cell test, excess Li exists in the cell, which, to some extent, compensates the loss of reversible capacity from the negative effect of the chemically bonded hydrates. Rate capabilities of the Fe2O3, Fe2O3@MnO2-95, and Fe2O3@MnO2-140 electrodes are shown in Figure 5c. As can be seen, the assembly of either amorphous or layered MnO2 nanosheets over Fe2O3 electrode enhances high-rate capability of Fe2O3 electrode. In comparison, the Fe2O3@MnO2-95 F

DOI: 10.1021/acsami.8b11794 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 6. In situ XRD patterns of the (a, b) Fe2O3@MnO2-95 and (c, d) Fe2O3@MnO2-140 electrodes during galvanostatic charge/discharge at 100 mA g−1.

Figure 7. In situ ETEM observation of the morphological evolution of the Fe2O3@MnO2-95 electrode during lithiation/delithiation process. (a) Schematic illustration of the all-solid nanobattery setup. (b) Calculated volume expansion of the Fe2O3 matrix on the basis of the areas indicated with dashed yellow lines in (c1−c3); still frames show the volume evolution of a single Fe2O3@MnO2 particle in the (c1) pristine, (c2) lithiated, and (c3) delithiated states.

The formation of Fe−Mn mixed oxides and the occurrence of Fe2+/Mn2+ are also detected by the in situ XRD analysis. The operando XRD patterns are measured during galvanostatic charge/discharge cycling. Each full XRD scan requires 10 min, and the next scan starts without any interruptions. Therefore, each XRD data reflects crystallographic information in the

voltage range of 10 min. As can be seen in Figure 6, Fe−Mn−O peak (Fe0.798Mn0.202O, JCPDS No. 77-2357, cubic, Fm3̅m, a = b = c = 4.3420 Å) is observed in both polarized electrodes. In the Fe−Mn−O peak, the valencies of both Fe and Mn are 2+, agreeing well with the XPS data (Figure S13). The Fe−Mn−O peaks of the Fe2O3@MnO2-95 electrode are much higher than G

DOI: 10.1021/acsami.8b11794 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces those of the Fe2O3@MnO2-140 electrode, while the Fe2O3 peaks are much higher in the Fe2O3@MnO2-140 electrode. The intensity of Fe2O3 in the Fe2O3@MnO2-95 electrode significantly decreases upon cycling, with an increase in the intensity of the Fe−Mn−O peak. The above results suggest the formation of Fe−Mn−O shell over Fe2O3 in the Fe2O3@MnO2-95 electrode (Figure 1). Such a Fe−Mn−O shell effectively maintains the integration of the electrode upon cycling (Figure S11), resulting in an enhanced cycle stability. In the Fe2O3@MnO2-140 electrode, such a Fe−Mn−O shell is minor and the spatial separation of Fe−O and Mn−O phases occurs, causing disintegration of the electrode upon cycling (Figure S11). Therefore, the assembly of amorphous MnO2 nanosheets over Fe2O3 is an effective method for enhanced lithium storage of Fe2O3, in which the amorphous MnO2 nanosheets are in situ transformed into a Fe−Mn−O shell over the Fe2O3 electrode for better integration of the electrode. An all-solid nanobattery employing Li metal and Li2O layer as the counter electrode and solid electrolyte, respectively, was then setup for in situ ETEM observation of electrochemically driven lithiation/delithiation of the Fe2O3@MnO2-95 nanospindle.38 Figure 7 and Video S1 give the lithiation (−5 V for 1.5 min)/delithiation (5 V for 1.5 min) results of a single Fe2O3@ MnO2-95 nanospindle. As can be seen, swelling of the Fe2O3 matrix is observed after lithiation, with a volume expansion of 94% occurring in the lithiated matrix. Such a value is exactly in line with the theoretical volume change of α-Fe2O3 during lithium insertion/deinsertion.2 Upon delithiation, the lithiated Fe2O3 matrix gets shrunk, with the volume shrinking from 194% for the lithiated state to 130% for the delithiated state. This is an evidence that the in situ generated Fe−Mn mixed oxide layer is Li+ permeable. Figure S14 shows the SAED patterns of the pristine and lithiated Fe2O3@MnO2-95 nanospindles. The crystalline Fe2O3@MnO2-95 nanospindle becomes poorly crystalline after lithiation, indicating lithiation of the Fe2O3 core. Such an in situ generated Fe−Mn mixed oxide layer has better electrical conductivity, uncompromised Li+ penetration, and enhanced structural integration, resulting in enhanced lithium storage of the Fe2O3@MnO2-95 nanospindles.

has better electrical conductivity, uncompromised Li+ penetration, and enhanced structural integration, resulting in enhanced lithium storage of the Fe2O3@MnO2-95 nanospindles. The present study highlights the merits of the lowtemperature assembly of amorphous MnO2 nanosheets as a mild-condition, short-process, and upscalable alternative to the classic carbon-coating method.

4. CONCLUSIONS In summary, a controllable assembly of MnO2 ultrathin nanosheets with thickness less than 3 nm over Fe 2 O 3 nanospindles has been reported. By simply varying the assembly temperature, the assembly of amorphous MnO2 nanosheets at 95 °C and crystalline layered MnO2 nanosheets at 140 °C can be tailored. The assembly of both the amorphous MnO2 and layered MnO2 nanosheets significantly enhances the electrical conductivity of the resulting Fe2O3@MnO2 electrode. When evaluated as an anode for lithium-ion batteries, both the Fe2O3@ MnO2-95 and Fe2O3@MnO2-140 electrodes exhibit improved cycling performance than that of the pristine Fe2O3 electrode. In comparison with the Fe2O3@MnO2-95 electrode, the Fe2O3@ MnO2-140 electrode conveys inferior capacity retention upon cycling due to the negative effect of the chemically bonded hydrates between interlayer gaps of layered MnO2 nanosheets. The Fe2O3@MnO2-95 electrode has a reversible capacity of 555 mAh g−1 after 100 galvanostatic charge/discharge cycles at 1000 mA g−1, comparable with that of the Fe3O4@C electrode derived via the classic carbon-coating route. The assembled amorphous MnO2 nanosheets are in situ transformed into a Fe−Mn mixed oxide thin layer over the Fe2O3 nanospindles during lithiation/ delithiation. Such an in situ generated Fe−Mn mixed oxide layer

(1) Reddy, M. V.; Yu, T.; Sow, C. H.; Shen, Z. X.; Lim, C. T.; Rao, G. V. S.; Chowdari, B. V. R. Alpha-Fe2O3 Nanoflakes as an Anode Material for Li-ion Batteries. Adv. Funct. Mater. 2007, 17, 2792−2799. (2) Lin, Y. M.; Abel, P. R.; Heller, A.; Mullins, C. B. Alpha-Fe2O3 Nanorods as Anode Material for Lithium Ion Batteries. J. Phys. Chem. Lett. 2011, 2, 2885−2891. (3) Chen, J.; Xu, L. N.; Li, W. Y.; Gou, X. L. Alpha-Fe2O3 Nanotubes in Gas Sensor and Lithium-ion Battery Applications. Adv. Mater. 2005, 17, 582−586. (4) Wang, Z.; Luan, D. Y.; Madhavi, S.; Li, C. M.; Lou, X. W. AlphaFe2O3 Nanotubes with Superior Lithium Storage Capability. Chem. Commun. 2011, 47, 8061−8063. (5) Zhang, L.; Wu, H. B.; Lou, X. W. Iron-Oxide-Based Advanced Anode Materials for Lithium-Ion Batteries. Adv. Energy Mater. 2014, 4, No. 1300958. (6) Yu, S.; Hong Ng, V. M.; Wang, F.; Xiao, Z.; Li, C.; Kong, L. B.; Que, W.; Zhou, K. Synthesis and Application of Iron-based Nanomaterials as Anodes of Lithium-ion Batteries and Supercapacitors. J. Mater. Chem. A 2018, 6, 9332−9367. (7) Zhang, S. L.; Guan, B. Y.; Wu, H. B.; Lou, X. W. Metal−Organic Framework-Assisted Synthesis of Compact Fe2O3 Nanotubes in Co3O4 Host with Enhanced Lithium Storage Properties. Nano−Micro Lett. 2018, 10, 44. (8) Gao, G.; Yu, L.; Wu, H. B.; Lou, X. W. Hierarchical Tubular Structures Constructed by Carbon-coated α-Fe2O3 Nanorods for Highly Reversible Lithium Storage. Small 2014, 10, 1741−1745.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b11794. Powder XRD patterns, EDX spectra, SEM images, HRTEM images, SAED patterns, high-resolution XPS spectra, and thermogravimetric analysis of the samples before and after electrochemical tests (PDF) TEM observation illustrates the morphology evolution upon lithiation and delithiation of the Fe2O3@MnO2-95 sample (AVI)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Wei Xiao: 0000-0001-5244-797X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS W.X. acknowledges the funding support from the National Natural Science Foundation of China (51722404 and 51674177), the Fundamental Research Funds for the Central Universities (2042017kf0167 and 2042017kf0200), and the Science & Technology Project of the General Administration of Quality Supervision, Inspection and Quarantine of China (2017IK055).

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REFERENCES

DOI: 10.1021/acsami.8b11794 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces (9) Zhu, L.; Chang, Z.; Wang, Y.; Chen, B.; Zhu, Y.; Tang, W.; Wu, Y. Core-shell MnO2@Fe2O3 Nanospindles as a Positive Electrode for Aqueous Supercapacitors. J. Mater. Chem. A 2015, 3, 22066−22072. (10) Nie, G.; Lu, X.; Chi, M.; Zhu, Y.; Yang, Z.; Song, N.; Wang, C. Hierarchical α-Fe2O3@MnO2 Core-shell Nanotubes as Electrode Materials for High-performance Supercapacitors. Electrochim. Acta 2017, 231, 36−43. (11) Zhu, J.; Tang, S.; Xie, H.; Dai, Y.; Meng, X. Hierarchically Porous MnO2 Microspheres Doped with Homogeneously Distributed Fe3O4 Nanoparticles for Supercapacitors. ACS Appl. Mater. Interfaces 2014, 6, 17637−17646. (12) Sarkar, D.; Khan, G. G.; Singh, A. K.; Mandal, K. HighPerformance Pseudocapacitor Electrodes Based on alpha-Fe2O3/ MnO2 Core-Shell Nanowire Heterostructure Arrays. J. Phys. Chem. C 2013, 117, 15523−15531. (13) Sarkar, D.; Pal, S.; Mandal, S.; Shukla, A.; Sarma, D. D. α-Fe2O3Based Core-Shell-Nanorod−Structured Positive and Negative Electrodes for a High-Performance α-Fe2O3/C//α-Fe2O3/MnOx Asymmetric Supercapacitor. J. Electrochem. Soc. 2017, 164, A2707−A2715. (14) Zhai, Y. M.; Zhai, J. F.; Zhou, M.; Dong, S. J. Ordered Magnetic Core-Manganese Oxide Shell Nanostructures and their Application in Water Treatment. J. Mater. Chem. 2009, 19, 7030−7035. (15) Zhao, Z.; Liu, J.; Cui, F.; Feng, H.; Zhang, L. One Pot Synthesis of Tunable Fe3O4-MnO2 Core-shell Nanoplates and their Applications for Water Purification. J. Mater. Chem. 2012, 22, 9052−9057. (16) Ge, X.; Song, X.; Ma, Y.; Zhou, H.; Wang, G.; Zhang, H.; Zhang, Y.; Zhao, H.; Wong, P. K. Fabrication of Hierarchical Iron-containing MnO2 Hollow Microspheres Assembled by Thickness-tunable Nanosheets for Efficient Phosphate Removal. J. Mater. Chem. A 2016, 4, 14814−14826. (17) Rui, Q.; Wang, L.; Zhang, Y.; Feng, C.; Zhang, B.; Fu, S.; Guo, H.; Hu, H.; Bi, Y. Synergistic Effects of P-doping and a MnO2 Cocatalyst on Fe2O3 Nanorod Photoanodes for Efficient Solar Water Splitting. J. Mater. Chem. A 2018, 6, 7021−7026. (18) Zhang, Y.; Ma, L.; Wang, T.; Li, X. MnO2 Coated Fe2O3 Spindles Designed for Production of C5+ Hydrocarbons in Fischer−Tropsch Synthesis. Fuel 2016, 177, 197−205. (19) Wang, Y.; Guo, X. M.; Wang, Z. K.; Lu, M. F.; Wu, B.; Wang, Y.; Yan, C.; Yuan, A. H.; Yang, H. X. Controlled Pyrolysis of MIL-88A to Fe2O3@C Nanocomposites with Varied Morphologies and Phases for Advanced Lithium Storage. J. Mater. Chem. A 2017, 5, 25562−25573. (20) Wang, Y.; Zhang, L.; Wu, Y.; Zhong, Y.; Hu, Y.; Lou, X. W. Carbon-coated Fe3O4 microspheres with a Porous Multideck-cage Structure for Highly Reversible Lithium Storage. Chem. Commun. 2015, 51, 6921−6924. (21) Li, M.; Du, H. R.; Kuai, L.; Huang, K. F.; Xia, Y. Y.; Geng, B. Y. Scalable Dry Production Process of a Superior 3D Net-Like CarbonBased Iron Oxide Anode Material for Lithium-Ion Batteries. Angew. Chem., Int. Ed. 2017, 56, 12649−12653. (22) Zhang, W. M.; Wu, X. L.; Hu, J. S.; Guo, Y. G.; Wan, L. J. Carbon Coated Fe3O4 Nanospindles as a Superior Anode Material for LithiumIon Batteries. Adv. Funct. Mater. 2008, 18, 3941−3946. (23) Li, D.; Wang, H.; Zhou, T.; Zhang, W.; Liu, H. K.; Guo, Z. Unique Structural Design and Strategies for Germanium-Based Anode Materials Toward Enhanced Lithium Storage. Adv. Energy Mater. 2017, 7, No. 1700488. (24) Zheng, Y.; Zhou, T.; Zhang, C.; Mao, J.; Liu, H.; Guo, Z. Boosted Charge Transfer in SnS/SnO2 Heterostructures: Toward High Rate Capability for Sodium-Ion Batteries. Angew. Chem., Int. Ed. 2016, 55, 3408−3413. (25) Zhou, T.; Pang, W. K.; Zhang, C.; Yang, J.; Chen, Z.; Liu, H. K.; Guo, Z. Enhanced Sodium-Ion Battery Performance by Structural Phase Transition from Two-Dimensional Hexagonal-SnS2 to Orthorhombic-SnS. ACS Nano 2014, 8, 8323−8333. (26) Zheng, P.; Zhang, Y.; Dai, Z. F.; Zheng, Y.; Dinh, K. N.; Yang, J.; Dangol, R.; Liu, X. B.; Yan, Q. Y. Constructing Multifunctional Heterostructure of Fe2O3@Ni3Se4 Nanotubes. Small 2018, 14, No. 1704065.

(27) Zeng, Y.; Luo, J. Z.; Wang, Y. Z.; Qiao, L.; Zou, B.; Zheng, W. T. Controllable Formation of Multi-layered SnO2@Fe2O3 Sandwich Cubes as a High-performance Anode for Li-Ion Batteries. Nanoscale 2017, 9, 17576−17584. (28) Zhang, Y.; Yang, J.; Zhang, Y. Z.; Li, C. C.; Huang, W.; Yan, Q. Y.; Dong, X. C. Fe2O3/SnSSe Hexagonal Nanoplates as Lithium-Ion Batteries Anode. ACS Appl. Mater. Interfaces 2018, 10, 12722−12730. (29) Xiao, W.; Chen, J. S.; Lu, Q.; Lou, X. W. Porous Spheres Assembled from Polythiophene (PTh)-Coated Ultrathin MnO2 Nanosheets with Enhanced Lithium Storage Capabilities. J. Phys. Chem. C 2010, 114, 12048−12051. (30) Ma, Y.; Fang, C. L.; Ding, B.; Ji, G.; Lee, J. Y. Fe-Doped MnxOy with Hierarchical Porosity as a High-Performance Lithium-Ion Battery Anode. Adv. Mater. 2013, 25, 4646−4652. (31) Hao, S.; Zhang, B.; Feng, J.; Liu, Y.; Ball, S.; Pan, J.; Srinivasan, M.; Huang, Y. Nanoscale Ion Intermixing Induced Activation of Fe2O3/ MnO2 Composites for Application in Lithium Ion Batteries. J. Mater. Chem. A 2017, 5, 8510−8518. (32) Zhan, S.; Qiu, M.; Yang, S.; Zhu, D.; Yu, H.; Li, Y. Facile Preparation of MnO2 Doped Fe2O3 Hollow Nanofibers for Low Temperature SCR of NO with NH3. J. Mater. Chem. A 2014, 2, 20486− 20493. (33) Santhoshkumar, P.; Prasanna, K.; Sivagami, I. N.; Jo, Y. N.; Kang, S. H.; Lee, C. W. Time-efficient Synthesis of MnO2 Encapsulated αFe2O3 Ellipsoids for Lithium Ion Battery Applications. J. Alloys Compd. 2017, 720, 300−308. (34) Wang, D.; Wang, Y.; Li, Q.; Guo, W.; Zhang, F.; Niu, S. Urchinlike α-Fe2O3/MnO2 Hierarchical Hollow Composite Microspheres as Lithium-ion Battery Anodes. J. Power Sources 2018, 393, 186−192. (35) Zhou, J.; Qin, L. F.; Xiao, W.; Zeng, C.; Li, N.; Lv, T.; Zhu, H. Oriented Growth of Layered-MnO2 Nanosheets over Alpha-MnO2 Nanotubes for Enhanced Room-temperature HCHO Oxidation. Appl. Catal., B 2017, 207, 233−243. (36) Xiao, W.; Chen, J. S.; Lou, X. W. Synthesis of Octahedral Mn3O4 Crystals and their Derived Mn3O4-MnO2 Heterostructures via Oriented Growth. CrystEngComm 2011, 13, 5685−5687. (37) Xiao, W.; Wang, D.; Lou, X. W. Shape-Controlled Synthesis of MnO2 Nanostructures with Enhanced Electrocatalytic Activity for Oxygen Reduction. J. Phys. Chem. C 2010, 114, 1694−1700. (38) Yang, L.; Dai, T.; Wang, Y. C.; Xie, D. G.; Narayan, R. L.; Li, J.; Ning, X. H. Chestnut-like SnO2/C Nanocomposites with Enhanced Lithium Ion Storage Properties. Nano Energy 2016, 30, 885−891. (39) Grosvenor, A. P.; Kobe, B. A.; Biesinger, M. C.; McIntyre, N. S. Investigation of Multiplet Splitting of Fe 2p XPS Spectra and Bonding in Iron Compounds. Surf. Interface Anal. 2004, 36, 1564−1574. (40) Wang, Y.; Zhao, S.; Zhang, Y.; Fang, J.; Zhou, Y.; Yuan, S.; Zhang, C.; Chen, W. One-pot Synthesis of K-doped g-C3N4 Nanosheets with Enhanced Photocatalytic Hydrogen Production Under Visible-light Irradiation. Appl. Surf. Sci. 2018, 440, 258−265. (41) Chigane, M.; Ishikawa, M. Manganese Oxide Thin Film Preparation by Potentiostatic Electrolyses and Electrochromism. J. Electrochem. Soc. 2000, 147, 2246−2251. (42) Xiao, W.; Xia, H.; Fuh, J. Y. H.; Lu, L. Electrochemical Synthesis and Supercapacitive Properties of Epsilon-MnO2 with Porous/ Nanoflaky Hierarchical Architectures. J. Electrochem. Soc. 2009, 156, A627−A633.

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DOI: 10.1021/acsami.8b11794 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX