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Self-assembly of Fe2O3 nanotubes on graphene as an anode material for lithium ion batteries
Journal of Alloys and Compounds 750 (2018) 871e877
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Self-assembly of Fe2O3 nanotubes on graphene as an anode material for lithium ion batteries Jingfeng Wang**, Lin Lin, Dannong He* National Engineering Research Center for Nanotechnology, 28 East Jiangchuan Road, Shanghai 200241, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 February 2018 Received in revised form 5 April 2018 Accepted 7 April 2018 Available online 9 April 2018
Fe2O3@rGO composite has been successfully prepared by a simple hydrothermal method with the assistance of polyvinylprrolidone (PVP). Under the dispersion of PVP, a-Fe2O3 nanotubes are well anchored on the surface of wrinkled graphene nanosheets, forming quasi-laminated-like architecture. Owing to the synergic effects between a-Fe2O3 hollow nanotubes and graphene, Fe2O3@rGO composite exhibits superior electrochemical properties as an anode material for lithium ion batteries with high reversible capacity and good rate capability. The initial discharge-charge capacities of Fe2O3@rGO composite can reach 1681 and 1367 mAhg1 at a current density of 50 mAg1, respectively. Even after 100 cycles, the capacity can be retained as high as 656 mAhg1 at a current density of 100 mAg1, much beyond than that of bare a-Fe2O3 nanotubes and Fe2O3/graphene reported previously. Meanwhile, it is found that graphene oxides can be reduced to reduced graphene oxides during the hydrothermal treatment. The critical role of graphene to the electrochemical performance of composite has also been elucidated in this paper. © 2018 Elsevier B.V. All rights reserved.
Keywords: Hydrothermal method Graphene a-Fe2O3 nanotubes Fe2O3@rGO composite Lithium ion batteries
1. Introduction Owing to the high safety and theoretical capacities of lithiumion batteries (LIBs), 3d transition metal oxides (MxOy, M ¼ Fe, Co and Ni) have received increasing attention since the pioneering studies by Tarascon et al., in 2000 [1]. Among them, Fe2O3 material has been considered to be promising candidates to replace commercial carbon-based anodes for LIBs due to their natural abundance, low cost, and environmental friendliness [2e4]. However, poor electrical conductivity and dramatic capacity fading caused by the volume expansion/contraction during the charge and discharge processes hinder the application of pure Fe2O3 materials for LIBs [5]. Some effective strategies have been proposed to improve the structure stability and electrochemical properties [6e8], such as designing unique configuration, controlling pore or hollow structures, etc. Various morphologies of Fe2O3 nanostructures, such as nanoparticles [9], nanotubes [10], nanorings [4], nanosheets [11], etc, have been designed and developed. The incorporation of graphene is also regarded as a feasible way
* Corresponding author., ** Corresponding author. E-mail addresses: [email protected] (J. Wang), [email protected] (D. He). https://doi.org/10.1016/j.jallcom.2018.04.079 0925-8388/© 2018 Elsevier B.V. All rights reserved.
to improve the electrochemical performance of the Fe2O3 materials [12]. Graphene, a monolayer of carbon atoms with a tight packing of honeycomb lattice, has recently attracted considerable interest for energy storage application owing to its chemical stability and superior electrical conductivity [13e15]. The introduction of graphene can not only improve the electrical conductive of the Fe2O3 materials, but also act as soft media to buffer the stress of volume expansion [14,16e18]. However, lack of incorporation method on the Fe2O3 surfaces is involved because graphene is twodimensional material. Guo et al. developed an in-situ wet chemistry method for the self-assembly of Fe2O3 and graphene nanosheets with the aid of surfactants under the atmospheric pressure [19]. Lee et al. published a report on the preparation of grapheneencapsulated irons oxide aggregates by the addition of the APS coupling agents into the nanoparticles surfaces [16]. Most of these incorporation techniques involve tedious treatments, or lots of expensive additives are introduced in the preparation processes. Therefore, it is highly desirable to combine graphene on the Fe2O3 surfaces through a feasible and cheap method. Graphene are commonly prepared by reducing graphene oxide (GO) via a chemical reduction method which eventually takes to reduced graphene oxide (rGO) [20]. For graphene oxide (GO), defect such as oxygen functional groups, Stone-Wales defects, and holes from the basal plane can act as active sites for interaction with
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molecules [21,22]. In this regard, GO could be considered as an ideal matrix material for supporting a-Fe2O3 materials. Herein, we demonstrate the design and fabrication of a novel hybrid nanocomposite by coupling graphene sheets and a-Fe2O3 hollow nanotubes (Fe2O3@graphene) via a simple polyvinypyrrolidone (PVP)-assisted hydrothermal route. PVP was introduced to act as a surfactant to help the dispersion of GO and a-Fe2O3 hollow nanotubes. Through this simple hydrothermal approach, the Fe2O3@rGO composite was obtained without further calcinations. Furthermore, during the hydrothermal treatment, it is found that the oxygen functional groups of GO can be removed and that the conjugated sp2 carbon atoms can be reconstructed, generating rGO wrapped on the surface of Fe2O3 nanotubes. The electrochemical performance of Fe2O3@rGO composite as an anode electrode material for LIBs was investigated and compared with bare Fe2O3. The results indicate that the hybrid Fe2O3@rGO composite exhibit not only high charge/discharge specific capacity but also good rate capability. 2. Experimental section 2.1. Preparation of a-Fe2O3 nanotubes
a-Fe2O3 nanotubes were prepared by hydrothermal treatment reported previously [23]. Typically, 3.2 mL of 0.5 M FeCl3 solution and 2.8 mL of 0.02 M NH4H2PO4 solution were dissolved in 80 mL of deionized water under vigorous stirring for 10 min. The mixture was then transferred into 100 mL Teflon-lined autoclaves at 220 C for 48 h. After cooling, the precipitate were separated by centrifugation and washed with deionized water and ethanol several times, then dried under vacuum at 80 C for 8 h. 2.2. Preparation of Fe2O3@rGO composite GO was synthesized from natural graphite powder by a modified Hummers method [20e22], the details of which have been described elsewhere. Fe2O3@rGO composites were fabricated by one-pot PVP-assisted hydrothermal method as follows: 0.20 g of GO was first dispersed in 100 mL of DI water-ethanol (H2O:C2H5OH ¼ 1:1, v:v) solution at room temperature by ultrasonication (300 W) for 1 h. Then other 100 mL of DI solution containing 0.80 g Fe2O3 powder and 0.12 g PVP was added to the GO suspension with stirring. After 6 h, the mixture was transferred into Teflon-lined autoclaves at 180 C for 24 h, and the resulting powder was separated by centrifugation and vacuum-dried at 80 C for 6 h. 2.3. Characterizations The powder X-ray diffraction (XRD) patterns were recorded on a Rigaku D/max-2200/PC X-ray diffractometer equipped with a Cu Ka radiation source (l ¼ 0.15418 nm) with a scanning rate of 10 min1. The morphology of the sample was elucidated by transmission electron microscopy (TEM) using a JEOL JEM-100CX microscope with an accelerating voltage of 100 kV, and high-resolution TEM using a JEOL JEM-2100 F microscope with an accelerating voltage of 200 kV. The thermogravimetric analyses (TGA) were carried out using a PerkinElmer TGA 7 thermogravimetric analyzer with a heating rate of 20 C min1 in air. Fourier Transform Infra-red Spectroscopy (FT-IR) measurements were carried out using a Nicolet MAGNA 550 spectrometer (Nicolet Instruments Corporation, USA). Raman spectroscopy was collected on the Bruker Optics Senterra R200-L (Bruker, Germany). 2.4. Electrochemical measurements Electrochemical properties of hybrid Fe2O3@rGO composite
were evaluated with CR 2032 coin cells. The working electrode was prepared on a copper foil by using a doctor-blade method with a slurry composed of 80 wt % Fe2O3@rGO composite, 10 wt % acetylene black, and 10 wt % polyvinylidene fluoride (PVDF) binder. Pure lithium foil was used as both the counter and reference electrodes. A microporous polypropylene membrane (Celgard 2500) was used as the separator. Coin cells were assembled in an argon filled glove box with both moisture and oxygen contents below 1.0 ppm. The electrolyte was 1.0 M of LiPF6 in ethylene carbonate/dimethyl carbonate (EC/DEC, 1:1 by volume). The galvanostatic charge and discharge experiment was performed with a battery tester LANDCT2001A in the voltage range of 0.05e3.0 V at room temperature. The cyclic voltammograms (CV) were obtained on a CHI 660E electrochemical workstation at a scanning rate of 0.5 mVs-1 in a potential range of 0.05e3.0 V (vs. Liþ/Li). 3. Results and discussion The preparation procedure for hybrid Fe2O3@rGO composite is illustrated in Scheme 1. Firstly, a-Fe2O3 hollow nanotubes were prepared by a coordination-assisted dissolution process in the presence of phosphase. These a-Fe2O3 hollow nanotubes were then mixed with GO dissolved together with the PVP molecules in DI solution and hydrothermally treated at 180 C for 24 h. Based on the hydrothermal treatment, a-Fe2O3 nanotubes were anchored covalently to GO nanosheets through functional groups such as carboxyl, and hydroxyl groups of GO. The dissolved GO nanosheets will be converted into rGO due to reduction. Finally, the obtained product was separated via centrifugation, washed thoroughly with water/ethanol solution many times and dried in vacuum at 80 C overnight. In our preparation procedure, PVP is very important to control the interfacial interactions between graphene and metal oxides during the formation of Fe2O3@rGO composite architecture. On the one hand, the PVP molecules can attach onto GO through the hydrophobic interactions to improve the dispersion of graphene [24,25]. On the other hand, they also interact with the metal oxide through the hydrophilic head groups to control surface charge of a-Fe2O3 nanotubes. Thus the mutual assembly can be triggered during the hydrothermal treatment under the temperature of 180 C. The XRD patterns of the as-prepared Fe2O3@rGO composite are shown in Fig. 1a. The diffraction peaks at 24.2 , 33.2 , 35.7, 40.9 , 49.5 , 54.1, 57.6 , 62.5 , 64.1, 72.0 , 75.5 are indexed to the crystalline planes of (012), (104), (110), (113), (024), (116), (018), (214), (300), (119), (220) for hematite (JCPDS No. 33-0664), and no characteristic peaks are observed for impurities, such as b-FeOOH, g-Fe2O3, and Fe3O4. Furthermore, no characteristic peaks of graphene are also observed, which might be due to the weak diffraction intensity in the composites. TG analysis is carried out from room temperature to 800 C in air (Fig. 1b). The major weight loss of about 20% is appeared between 300 C and 500 C, which are resulted from decomposition of graphene. Based on the analysis of TGA curves, we calculate the graphene content in hybrid composite is about 19.4 wt%. Raman spectra of hybrid Fe2O3@rGO composite as well as asmade bare Fe2O3 and graphene are presented in Fig. 2a. Raman peaks of bare a-Fe2O3 hollow nanotubes observed at 225 cm1, 290 cm1 and 407 cm1 are assigned to A1g and Eg modes for typical hematite [26], respectively. In Fig. 2a, the Raman spectra of Fe2O3@rGO composite exhibit typical peaks of hematite and characteristic peaks of the disordered (D) and graphitic (G) bands from graphene at around 1331 and 1596 cm1 [27], revealing the incorporation of a-Fe2O3 nanotubes and graphene nanosheets. Fig. 2b presents the FT-IR spectra of Fe2O3@rGO composite and GO. The peak intensity of some oxygen-containing groups, such as C-OH
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Scheme 1. Schematic representation of the preparation of Fe2O3@rGO composite.
Fig. 1. (a) XRD patterns of Fe2O3@rGO composite, (b) TGA curves of Fe2O3@rGO composite from room temperature to 800 C in air.
Fig. 2. (a) Raman spectra of as-prepared a-Fe2O3 nanotubes, GO and Fe2O3@rGO composite. (b) FT-IR spectra of as-prepared GO and Fe2O3@rGO composite.
(3400 cm1), C¼O (1720 cm1) and C-O-C (1200 cm1) [28e30], show obvious decreases for Fe2O3@rGO composite after hydrothermal reaction compared with that of GO, indicating the reduction of GO in great degree. The reduction of GO in composite through hydrothermal reaction has been reported by Xiao et al. [31,32], and our results are consistent with these reports demonstrated previously. XPS is also adopted to determine the reduction degree of GO (Fig. 3a and b). For GO materials, the XPS C1s spectra show the peaks centered at 284.8, 286.6, 287.8 and 289.1 eV are observed, corresponding to C-C (sp2), C-O, C¼O, and O-C¼O groups [33], respectively. The high intensity of the peaks with binding energy over 286 eV suggests that plenty of oxygenated functional groups exit in GO. But to our Fe2O3@rGO composite, the intensities of peaks
associated with oxygenated functional groups of GO are significantly decreased. As revealed by the strong peak at 284.6 eV, most of the carbon atoms are in sp2 configuration, characteristic of rGO. This result also indicates that most of GO had been reduced into rGO through the hydrothermal reaction, which is consistent with the above Raman and FT-IR results. In the high-resolution Fe 2p spectrum (Fig. 3c), two distinct peaks at binding energies of 710.9 eV for Fe 2p3/2 and 726.2 eV for Fe 2p1/2 with a shake-up satellite at 719.3 eV are observed. This is typical characteristic of Fe3þ in Fe2O3 [34,35]. The morphology and structure of the samples have been examined by transmission electron microscopy (TEM). GO nanosheets exhibit a distinct crumple and flexible structure, with a large scale beyond 2.0 mm (Fig. 4a). After the hydrothermal reaction, a-
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Fig. 3. (a) XPS C1s spectra of as-prepared GO, (b) XPS C1s and (c) Fe 2p spectra of Fe2O3@rGO composite.
Fig. 4. TEM images of (a) GO, (b) a-Fe2O3 nanotubes, (c) Fe2O3@rGO composite, and (d) HRTEM images of Fe2O3@rGO composite.
Fe2O3 nanotubes are well deposited on the surface of wrinkled graphene nanosheets, forming quasi-laminated-like architecture. The a-Fe2O3 nanotubes morphology was still preserved with the average lengths of 250e300 nm, the outer diameters of 80e100 nm, and the hollow diameters of about 60 nm (Fig. 4b and c). This hollow tubular structure may not only well accommodate the volume variation upon insertion/extraction of lithium ions, but also protect the active materials from severe aggregation. In the high-resolution TEM (HRTEM) images of our composite, the surface of a-Fe2O3 nanotubes was covered by few-layered graphene nanosheets (Fig. 4d). Clearly, the incorporation of graphene can provide a highly conductive flexible network for electron transfer, maintaining the structural integrity of the electrodes. Distinct crystal lattices with the distance of 0.27 nm are observed, which
can be ascribed to the (104) plane of hematite. The cyclic voltammetry (CV) curve of hybrid Fe2O3@rGO composite at a scan rate of 0.5 mV s1 between 0.05 and 3.0 V (vs. Liþ/Li) is shown in Fig. 5a. In the first cycle, the strong cathodic peak observed at 0.7 V and a small peak observed at 1.15 V are related with the multi-step electrochemical reaction that involves the phase transition from hexagonal LixFe2O3 to cubic Li2Fe2O3 and complete reduction to Fe0. In the anodic curve, only one broad peak recorded at around 1.85 V is related to the restoration of Fe0 to Fe3þ [36]. In the subsequent cycles, the small peak at 1.15 V is disappeared [37], and the disappearance of the small peak located at 1.15 V may be mainly attributed to irreversible processes of the phase transition from hexagonal LixFe2O3 to cubic Li2Fe2O3. Meanwhile, the dramatic decrease in the CV peak intensity with
J. Wang et al. / Journal of Alloys and Compounds 750 (2018) 871e877
Fig. 5. (a) CV curves of Fe2O3@rGO composite at a scan rate of 0.5 mV s1, (b) the initial two discharge-charge curves of Fe2O3@rGO composite at 50 mA g1.
cycling may be attributed to the poor reversibility of the conversion reaction. Fig. 5b shows the representative charge/discharge curves of Fe2O3@rGO composite at a current density of 50 mA g1 between 0.05 and 3.0 V. Two obvious plateaus (1.6 V and 0.8 V) are observed in the first discharge curve, corresponding separately to lithium insertion into crystalline structure of Fe2O3 and reduction of Fe3þ to Fe0 by metallic Li. The first discharge and charge capacity of hybrid Fe2O3@rGO composite are 1681 and 1367 mAh g1, respectively, giving a high columbic efficiency of approximately 81.3%. The specific capacities of Fe2O3@rGO composite are much higher than the theoretical value of pure Fe2O3 (1005 mAh g1) and graphene, and the columbic efficiency is also higher than that of Fe2O3/graphene reported in correlated literatures [31,32]. For comparison, the electrochemical properties of the Fe2O3-graphene hybrid material fabricated in our work and those of Fe2O3-graphene with similar architecture reported in literatures are summarized in Table S1. Compared with the values reported in the literature, the high electrochemical properties and coulombic efficiency are clearly obtained in this work. The high coulombic efficiency is ascribed to good attachment between graphene and a-Fe2O3 nanotubes as presented in TEM images. The higher specific capacity of Fe2O3@rGO composite could be not only resulted from the formation of solid electrolyte interface (SEI) layer with the degradation of the electrolyte [38e40], but also from electrochemical reduction of some remained oxygen-containing functional groups on the surface of graphene [41e43]. Fig. 6a shows the cycling performance of the pure a-Fe2O3
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nanotubes and Fe2O3@rGO composite electrodes at a current density of 100 mA g1. The discharge capacity of the bare a-Fe2O3 electrode decreases from 1480 mAh g1 to 183 mAh g1 after 50 cycles, which is only about 12.4% of the initial capacity, showing poor capacity retention. However, the discharge capacity of the hybrid Fe2O3@rGO composite electrode decreases from 1461 mAh g1 to 776 mAh g1 after 50 cycles. Even further 100 cycles, a discharge capacity of 656 mAh g1 is retained, which is still about 45.0% of the initial capacity, revealing its good cycling performance. The excellent capacity retention might be due to the stability of the composite structure and enhanced electronic conductivity. In addition to the remarkable cycleability, the Fe2O3@rGO composite can also exhibit an excellent rate performance. Fig. 6b shows the variations of the reversible capacities with current densities and discharge cycles. The electrodes were first cycled at 50 mAg1 for 5 cycles, followed by cycling at current densities increasing stepwise to as high as 1.0 Ag-1. A reversible capacity of approximately 1342 mAhg1 was achieved at a current density of 50 mAg1, 1072 mAhg1 at 100 mAg1, 906 mAhg1 at 200 mAg1, and 753 mAhg1 at 400 mAg1, respectively. Decreasing the current density back to 100 mAg1 after cycling at 1.0 Ag-1, reversible capacities of up to 1056 mAhg1 were restored, indicating its good reversibility of the Fe2O3@rGO electrode. We speculate it could be due to a well-organized flexible interleaved structure of Fe2O3@rGO electrode, whereas the bare a-Fe2O3 nanotubes anchored on graphene reduce the effect of volume expansion and alleviate the stress of a-Fe2O3 nanotubes upon lithium insertion/desertion, maintaining the structural integrity of composite. Fig. 7 displays the impendence spectra of the electrode materials of bare a-Fe2O3 nanotubes and hybrid Fe2O3@rGO composite. It can be seen that the resistance of the Fe2O3@rGO composite electrode is much smaller than that of the bare a-Fe2O3 electrode. The decreased resistance of the Fe2O3@rGO electrode would be chalked up to the excellent electrical conductivity of graphene and the fine contact among the a-Fe2O3 nanotubes anchored in graphene nanosheets. The large specific capacity, excellent cycling stability, and high rate capability are attributed to the unique structural features of the composite. In the Fe2O3@rGO composite electrode, self-assembly of a-Fe2O3 nanotubes can form a quasi-laminated architecture based on graphene matrix with the assistance of PVP (Fig. S1). First, such a well-organized flexible interleaved structure of the a-Fe2O3 nanotubes anchored on graphene uniformly can increase the interfacial area between the electrode and electrolyte and facilitate the electrolyte penetration. Second, hollow tubular structure of a-Fe2O3 can reduce the effect of volume expansion and alleviate the stress of nanotubes upon lithium insertion/desertion. Third, the introduction of graphene can prevent the detachment and agglomeration of a-Fe2O3 nanotubes during the frequent discharge/charge cycles. In addition, owing to the superior electrical conductivity, graphene also serves as an efficient conducting matrix to provide a highly conductive network for electron transfer and maintain the structural integrity of composite electrodes. 4. Conclusions Fe2O3@rGO composite was successfully synthesized by a simple hydrothermal method with assistance of PVP as an effective dispersant. After the hydrothermal treatment, it is found that GO can be reduced to rGO in a certain degree. Under the dispersion of PVP, a-Fe2O3 nanotubes are well anchored on the surface of wrinkled graphene nanosheets, forming quasi-laminated-like architecture. Fe2O3@rGO composite exhibits superior electrochemical properties as anode material for LIBs with high reversible capacity and good rate capability. The first discharge and charge capacity of
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Fig. 6. (a) Cycling performance of a-Fe2O3 nanotubes and Fe2O3@rGO composite, (b) rate capabilities of Fe2O3@rGO composite.
References
Fig. 7. Electrochemical impedance spectra of a-Fe2O3 nanotubes and Fe2O3@rGO composite.
Fe2O3@rGO composite can reach 1681 and 1367 mAh g1 at a current density of 50 mAg1, respectively. Even after 100 cycles, the capacity as high as 656 mAh g1 can be retained at the current density of 100 mAg1, much beyond than that of bare a-Fe2O3 nanotubes. Better electrochemical properties of Fe2O3@rGO composite should thank to the synergic effects between a-Fe2O3 hollow nanotubes and graphene. Hollow tubular structure of iron oxide possesses high specific capacity, and the presence of graphene obviously enhances the electronic conductivity of the electrode. The unique structure of the composite can effectively buffer the strain generated from the large volume variation during Liþ ion insertion/desertion, ensuring the long cycling stability.
Acknowledgments This work was financially supported by the Natural Science Foundation of Shanghai (No. 17ZR1420000), Shanghai talent development funding project (No. 2017077) and the Shanghai Rising-star Program (B-type) (No. 15QB1402300).
Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.jallcom.2018.04.079.
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Self-assembly of Fe2O3 nanotubes on graphene as an anode material for lithium ion batteries Jingfeng Wang**, Lin Lin, Dannong He* National Engineering Research Center for Nanotechnology, 28 East Jiangchuan Road, Shanghai 200241, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 February 2018 Received in revised form 5 April 2018 Accepted 7 April 2018 Available online 9 April 2018
Fe2O3@rGO composite has been successfully prepared by a simple hydrothermal method with the assistance of polyvinylprrolidone (PVP). Under the dispersion of PVP, a-Fe2O3 nanotubes are well anchored on the surface of wrinkled graphene nanosheets, forming quasi-laminated-like architecture. Owing to the synergic effects between a-Fe2O3 hollow nanotubes and graphene, Fe2O3@rGO composite exhibits superior electrochemical properties as an anode material for lithium ion batteries with high reversible capacity and good rate capability. The initial discharge-charge capacities of Fe2O3@rGO composite can reach 1681 and 1367 mAhg1 at a current density of 50 mAg1, respectively. Even after 100 cycles, the capacity can be retained as high as 656 mAhg1 at a current density of 100 mAg1, much beyond than that of bare a-Fe2O3 nanotubes and Fe2O3/graphene reported previously. Meanwhile, it is found that graphene oxides can be reduced to reduced graphene oxides during the hydrothermal treatment. The critical role of graphene to the electrochemical performance of composite has also been elucidated in this paper. © 2018 Elsevier B.V. All rights reserved.
Keywords: Hydrothermal method Graphene a-Fe2O3 nanotubes Fe2O3@rGO composite Lithium ion batteries
1. Introduction Owing to the high safety and theoretical capacities of lithiumion batteries (LIBs), 3d transition metal oxides (MxOy, M ¼ Fe, Co and Ni) have received increasing attention since the pioneering studies by Tarascon et al., in 2000 [1]. Among them, Fe2O3 material has been considered to be promising candidates to replace commercial carbon-based anodes for LIBs due to their natural abundance, low cost, and environmental friendliness [2e4]. However, poor electrical conductivity and dramatic capacity fading caused by the volume expansion/contraction during the charge and discharge processes hinder the application of pure Fe2O3 materials for LIBs [5]. Some effective strategies have been proposed to improve the structure stability and electrochemical properties [6e8], such as designing unique configuration, controlling pore or hollow structures, etc. Various morphologies of Fe2O3 nanostructures, such as nanoparticles [9], nanotubes [10], nanorings [4], nanosheets [11], etc, have been designed and developed. The incorporation of graphene is also regarded as a feasible way
* Corresponding author., ** Corresponding author. E-mail addresses: [email protected] (J. Wang), [email protected] (D. He). https://doi.org/10.1016/j.jallcom.2018.04.079 0925-8388/© 2018 Elsevier B.V. All rights reserved.
to improve the electrochemical performance of the Fe2O3 materials [12]. Graphene, a monolayer of carbon atoms with a tight packing of honeycomb lattice, has recently attracted considerable interest for energy storage application owing to its chemical stability and superior electrical conductivity [13e15]. The introduction of graphene can not only improve the electrical conductive of the Fe2O3 materials, but also act as soft media to buffer the stress of volume expansion [14,16e18]. However, lack of incorporation method on the Fe2O3 surfaces is involved because graphene is twodimensional material. Guo et al. developed an in-situ wet chemistry method for the self-assembly of Fe2O3 and graphene nanosheets with the aid of surfactants under the atmospheric pressure [19]. Lee et al. published a report on the preparation of grapheneencapsulated irons oxide aggregates by the addition of the APS coupling agents into the nanoparticles surfaces [16]. Most of these incorporation techniques involve tedious treatments, or lots of expensive additives are introduced in the preparation processes. Therefore, it is highly desirable to combine graphene on the Fe2O3 surfaces through a feasible and cheap method. Graphene are commonly prepared by reducing graphene oxide (GO) via a chemical reduction method which eventually takes to reduced graphene oxide (rGO) [20]. For graphene oxide (GO), defect such as oxygen functional groups, Stone-Wales defects, and holes from the basal plane can act as active sites for interaction with
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molecules [21,22]. In this regard, GO could be considered as an ideal matrix material for supporting a-Fe2O3 materials. Herein, we demonstrate the design and fabrication of a novel hybrid nanocomposite by coupling graphene sheets and a-Fe2O3 hollow nanotubes (Fe2O3@graphene) via a simple polyvinypyrrolidone (PVP)-assisted hydrothermal route. PVP was introduced to act as a surfactant to help the dispersion of GO and a-Fe2O3 hollow nanotubes. Through this simple hydrothermal approach, the Fe2O3@rGO composite was obtained without further calcinations. Furthermore, during the hydrothermal treatment, it is found that the oxygen functional groups of GO can be removed and that the conjugated sp2 carbon atoms can be reconstructed, generating rGO wrapped on the surface of Fe2O3 nanotubes. The electrochemical performance of Fe2O3@rGO composite as an anode electrode material for LIBs was investigated and compared with bare Fe2O3. The results indicate that the hybrid Fe2O3@rGO composite exhibit not only high charge/discharge specific capacity but also good rate capability. 2. Experimental section 2.1. Preparation of a-Fe2O3 nanotubes
a-Fe2O3 nanotubes were prepared by hydrothermal treatment reported previously [23]. Typically, 3.2 mL of 0.5 M FeCl3 solution and 2.8 mL of 0.02 M NH4H2PO4 solution were dissolved in 80 mL of deionized water under vigorous stirring for 10 min. The mixture was then transferred into 100 mL Teflon-lined autoclaves at 220 C for 48 h. After cooling, the precipitate were separated by centrifugation and washed with deionized water and ethanol several times, then dried under vacuum at 80 C for 8 h. 2.2. Preparation of Fe2O3@rGO composite GO was synthesized from natural graphite powder by a modified Hummers method [20e22], the details of which have been described elsewhere. Fe2O3@rGO composites were fabricated by one-pot PVP-assisted hydrothermal method as follows: 0.20 g of GO was first dispersed in 100 mL of DI water-ethanol (H2O:C2H5OH ¼ 1:1, v:v) solution at room temperature by ultrasonication (300 W) for 1 h. Then other 100 mL of DI solution containing 0.80 g Fe2O3 powder and 0.12 g PVP was added to the GO suspension with stirring. After 6 h, the mixture was transferred into Teflon-lined autoclaves at 180 C for 24 h, and the resulting powder was separated by centrifugation and vacuum-dried at 80 C for 6 h. 2.3. Characterizations The powder X-ray diffraction (XRD) patterns were recorded on a Rigaku D/max-2200/PC X-ray diffractometer equipped with a Cu Ka radiation source (l ¼ 0.15418 nm) with a scanning rate of 10 min1. The morphology of the sample was elucidated by transmission electron microscopy (TEM) using a JEOL JEM-100CX microscope with an accelerating voltage of 100 kV, and high-resolution TEM using a JEOL JEM-2100 F microscope with an accelerating voltage of 200 kV. The thermogravimetric analyses (TGA) were carried out using a PerkinElmer TGA 7 thermogravimetric analyzer with a heating rate of 20 C min1 in air. Fourier Transform Infra-red Spectroscopy (FT-IR) measurements were carried out using a Nicolet MAGNA 550 spectrometer (Nicolet Instruments Corporation, USA). Raman spectroscopy was collected on the Bruker Optics Senterra R200-L (Bruker, Germany). 2.4. Electrochemical measurements Electrochemical properties of hybrid Fe2O3@rGO composite
were evaluated with CR 2032 coin cells. The working electrode was prepared on a copper foil by using a doctor-blade method with a slurry composed of 80 wt % Fe2O3@rGO composite, 10 wt % acetylene black, and 10 wt % polyvinylidene fluoride (PVDF) binder. Pure lithium foil was used as both the counter and reference electrodes. A microporous polypropylene membrane (Celgard 2500) was used as the separator. Coin cells were assembled in an argon filled glove box with both moisture and oxygen contents below 1.0 ppm. The electrolyte was 1.0 M of LiPF6 in ethylene carbonate/dimethyl carbonate (EC/DEC, 1:1 by volume). The galvanostatic charge and discharge experiment was performed with a battery tester LANDCT2001A in the voltage range of 0.05e3.0 V at room temperature. The cyclic voltammograms (CV) were obtained on a CHI 660E electrochemical workstation at a scanning rate of 0.5 mVs-1 in a potential range of 0.05e3.0 V (vs. Liþ/Li). 3. Results and discussion The preparation procedure for hybrid Fe2O3@rGO composite is illustrated in Scheme 1. Firstly, a-Fe2O3 hollow nanotubes were prepared by a coordination-assisted dissolution process in the presence of phosphase. These a-Fe2O3 hollow nanotubes were then mixed with GO dissolved together with the PVP molecules in DI solution and hydrothermally treated at 180 C for 24 h. Based on the hydrothermal treatment, a-Fe2O3 nanotubes were anchored covalently to GO nanosheets through functional groups such as carboxyl, and hydroxyl groups of GO. The dissolved GO nanosheets will be converted into rGO due to reduction. Finally, the obtained product was separated via centrifugation, washed thoroughly with water/ethanol solution many times and dried in vacuum at 80 C overnight. In our preparation procedure, PVP is very important to control the interfacial interactions between graphene and metal oxides during the formation of Fe2O3@rGO composite architecture. On the one hand, the PVP molecules can attach onto GO through the hydrophobic interactions to improve the dispersion of graphene [24,25]. On the other hand, they also interact with the metal oxide through the hydrophilic head groups to control surface charge of a-Fe2O3 nanotubes. Thus the mutual assembly can be triggered during the hydrothermal treatment under the temperature of 180 C. The XRD patterns of the as-prepared Fe2O3@rGO composite are shown in Fig. 1a. The diffraction peaks at 24.2 , 33.2 , 35.7, 40.9 , 49.5 , 54.1, 57.6 , 62.5 , 64.1, 72.0 , 75.5 are indexed to the crystalline planes of (012), (104), (110), (113), (024), (116), (018), (214), (300), (119), (220) for hematite (JCPDS No. 33-0664), and no characteristic peaks are observed for impurities, such as b-FeOOH, g-Fe2O3, and Fe3O4. Furthermore, no characteristic peaks of graphene are also observed, which might be due to the weak diffraction intensity in the composites. TG analysis is carried out from room temperature to 800 C in air (Fig. 1b). The major weight loss of about 20% is appeared between 300 C and 500 C, which are resulted from decomposition of graphene. Based on the analysis of TGA curves, we calculate the graphene content in hybrid composite is about 19.4 wt%. Raman spectra of hybrid Fe2O3@rGO composite as well as asmade bare Fe2O3 and graphene are presented in Fig. 2a. Raman peaks of bare a-Fe2O3 hollow nanotubes observed at 225 cm1, 290 cm1 and 407 cm1 are assigned to A1g and Eg modes for typical hematite [26], respectively. In Fig. 2a, the Raman spectra of Fe2O3@rGO composite exhibit typical peaks of hematite and characteristic peaks of the disordered (D) and graphitic (G) bands from graphene at around 1331 and 1596 cm1 [27], revealing the incorporation of a-Fe2O3 nanotubes and graphene nanosheets. Fig. 2b presents the FT-IR spectra of Fe2O3@rGO composite and GO. The peak intensity of some oxygen-containing groups, such as C-OH
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Scheme 1. Schematic representation of the preparation of Fe2O3@rGO composite.
Fig. 1. (a) XRD patterns of Fe2O3@rGO composite, (b) TGA curves of Fe2O3@rGO composite from room temperature to 800 C in air.
Fig. 2. (a) Raman spectra of as-prepared a-Fe2O3 nanotubes, GO and Fe2O3@rGO composite. (b) FT-IR spectra of as-prepared GO and Fe2O3@rGO composite.
(3400 cm1), C¼O (1720 cm1) and C-O-C (1200 cm1) [28e30], show obvious decreases for Fe2O3@rGO composite after hydrothermal reaction compared with that of GO, indicating the reduction of GO in great degree. The reduction of GO in composite through hydrothermal reaction has been reported by Xiao et al. [31,32], and our results are consistent with these reports demonstrated previously. XPS is also adopted to determine the reduction degree of GO (Fig. 3a and b). For GO materials, the XPS C1s spectra show the peaks centered at 284.8, 286.6, 287.8 and 289.1 eV are observed, corresponding to C-C (sp2), C-O, C¼O, and O-C¼O groups [33], respectively. The high intensity of the peaks with binding energy over 286 eV suggests that plenty of oxygenated functional groups exit in GO. But to our Fe2O3@rGO composite, the intensities of peaks
associated with oxygenated functional groups of GO are significantly decreased. As revealed by the strong peak at 284.6 eV, most of the carbon atoms are in sp2 configuration, characteristic of rGO. This result also indicates that most of GO had been reduced into rGO through the hydrothermal reaction, which is consistent with the above Raman and FT-IR results. In the high-resolution Fe 2p spectrum (Fig. 3c), two distinct peaks at binding energies of 710.9 eV for Fe 2p3/2 and 726.2 eV for Fe 2p1/2 with a shake-up satellite at 719.3 eV are observed. This is typical characteristic of Fe3þ in Fe2O3 [34,35]. The morphology and structure of the samples have been examined by transmission electron microscopy (TEM). GO nanosheets exhibit a distinct crumple and flexible structure, with a large scale beyond 2.0 mm (Fig. 4a). After the hydrothermal reaction, a-
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Fig. 3. (a) XPS C1s spectra of as-prepared GO, (b) XPS C1s and (c) Fe 2p spectra of Fe2O3@rGO composite.
Fig. 4. TEM images of (a) GO, (b) a-Fe2O3 nanotubes, (c) Fe2O3@rGO composite, and (d) HRTEM images of Fe2O3@rGO composite.
Fe2O3 nanotubes are well deposited on the surface of wrinkled graphene nanosheets, forming quasi-laminated-like architecture. The a-Fe2O3 nanotubes morphology was still preserved with the average lengths of 250e300 nm, the outer diameters of 80e100 nm, and the hollow diameters of about 60 nm (Fig. 4b and c). This hollow tubular structure may not only well accommodate the volume variation upon insertion/extraction of lithium ions, but also protect the active materials from severe aggregation. In the high-resolution TEM (HRTEM) images of our composite, the surface of a-Fe2O3 nanotubes was covered by few-layered graphene nanosheets (Fig. 4d). Clearly, the incorporation of graphene can provide a highly conductive flexible network for electron transfer, maintaining the structural integrity of the electrodes. Distinct crystal lattices with the distance of 0.27 nm are observed, which
can be ascribed to the (104) plane of hematite. The cyclic voltammetry (CV) curve of hybrid Fe2O3@rGO composite at a scan rate of 0.5 mV s1 between 0.05 and 3.0 V (vs. Liþ/Li) is shown in Fig. 5a. In the first cycle, the strong cathodic peak observed at 0.7 V and a small peak observed at 1.15 V are related with the multi-step electrochemical reaction that involves the phase transition from hexagonal LixFe2O3 to cubic Li2Fe2O3 and complete reduction to Fe0. In the anodic curve, only one broad peak recorded at around 1.85 V is related to the restoration of Fe0 to Fe3þ [36]. In the subsequent cycles, the small peak at 1.15 V is disappeared [37], and the disappearance of the small peak located at 1.15 V may be mainly attributed to irreversible processes of the phase transition from hexagonal LixFe2O3 to cubic Li2Fe2O3. Meanwhile, the dramatic decrease in the CV peak intensity with
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Fig. 5. (a) CV curves of Fe2O3@rGO composite at a scan rate of 0.5 mV s1, (b) the initial two discharge-charge curves of Fe2O3@rGO composite at 50 mA g1.
cycling may be attributed to the poor reversibility of the conversion reaction. Fig. 5b shows the representative charge/discharge curves of Fe2O3@rGO composite at a current density of 50 mA g1 between 0.05 and 3.0 V. Two obvious plateaus (1.6 V and 0.8 V) are observed in the first discharge curve, corresponding separately to lithium insertion into crystalline structure of Fe2O3 and reduction of Fe3þ to Fe0 by metallic Li. The first discharge and charge capacity of hybrid Fe2O3@rGO composite are 1681 and 1367 mAh g1, respectively, giving a high columbic efficiency of approximately 81.3%. The specific capacities of Fe2O3@rGO composite are much higher than the theoretical value of pure Fe2O3 (1005 mAh g1) and graphene, and the columbic efficiency is also higher than that of Fe2O3/graphene reported in correlated literatures [31,32]. For comparison, the electrochemical properties of the Fe2O3-graphene hybrid material fabricated in our work and those of Fe2O3-graphene with similar architecture reported in literatures are summarized in Table S1. Compared with the values reported in the literature, the high electrochemical properties and coulombic efficiency are clearly obtained in this work. The high coulombic efficiency is ascribed to good attachment between graphene and a-Fe2O3 nanotubes as presented in TEM images. The higher specific capacity of Fe2O3@rGO composite could be not only resulted from the formation of solid electrolyte interface (SEI) layer with the degradation of the electrolyte [38e40], but also from electrochemical reduction of some remained oxygen-containing functional groups on the surface of graphene [41e43]. Fig. 6a shows the cycling performance of the pure a-Fe2O3
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nanotubes and Fe2O3@rGO composite electrodes at a current density of 100 mA g1. The discharge capacity of the bare a-Fe2O3 electrode decreases from 1480 mAh g1 to 183 mAh g1 after 50 cycles, which is only about 12.4% of the initial capacity, showing poor capacity retention. However, the discharge capacity of the hybrid Fe2O3@rGO composite electrode decreases from 1461 mAh g1 to 776 mAh g1 after 50 cycles. Even further 100 cycles, a discharge capacity of 656 mAh g1 is retained, which is still about 45.0% of the initial capacity, revealing its good cycling performance. The excellent capacity retention might be due to the stability of the composite structure and enhanced electronic conductivity. In addition to the remarkable cycleability, the Fe2O3@rGO composite can also exhibit an excellent rate performance. Fig. 6b shows the variations of the reversible capacities with current densities and discharge cycles. The electrodes were first cycled at 50 mAg1 for 5 cycles, followed by cycling at current densities increasing stepwise to as high as 1.0 Ag-1. A reversible capacity of approximately 1342 mAhg1 was achieved at a current density of 50 mAg1, 1072 mAhg1 at 100 mAg1, 906 mAhg1 at 200 mAg1, and 753 mAhg1 at 400 mAg1, respectively. Decreasing the current density back to 100 mAg1 after cycling at 1.0 Ag-1, reversible capacities of up to 1056 mAhg1 were restored, indicating its good reversibility of the Fe2O3@rGO electrode. We speculate it could be due to a well-organized flexible interleaved structure of Fe2O3@rGO electrode, whereas the bare a-Fe2O3 nanotubes anchored on graphene reduce the effect of volume expansion and alleviate the stress of a-Fe2O3 nanotubes upon lithium insertion/desertion, maintaining the structural integrity of composite. Fig. 7 displays the impendence spectra of the electrode materials of bare a-Fe2O3 nanotubes and hybrid Fe2O3@rGO composite. It can be seen that the resistance of the Fe2O3@rGO composite electrode is much smaller than that of the bare a-Fe2O3 electrode. The decreased resistance of the Fe2O3@rGO electrode would be chalked up to the excellent electrical conductivity of graphene and the fine contact among the a-Fe2O3 nanotubes anchored in graphene nanosheets. The large specific capacity, excellent cycling stability, and high rate capability are attributed to the unique structural features of the composite. In the Fe2O3@rGO composite electrode, self-assembly of a-Fe2O3 nanotubes can form a quasi-laminated architecture based on graphene matrix with the assistance of PVP (Fig. S1). First, such a well-organized flexible interleaved structure of the a-Fe2O3 nanotubes anchored on graphene uniformly can increase the interfacial area between the electrode and electrolyte and facilitate the electrolyte penetration. Second, hollow tubular structure of a-Fe2O3 can reduce the effect of volume expansion and alleviate the stress of nanotubes upon lithium insertion/desertion. Third, the introduction of graphene can prevent the detachment and agglomeration of a-Fe2O3 nanotubes during the frequent discharge/charge cycles. In addition, owing to the superior electrical conductivity, graphene also serves as an efficient conducting matrix to provide a highly conductive network for electron transfer and maintain the structural integrity of composite electrodes. 4. Conclusions Fe2O3@rGO composite was successfully synthesized by a simple hydrothermal method with assistance of PVP as an effective dispersant. After the hydrothermal treatment, it is found that GO can be reduced to rGO in a certain degree. Under the dispersion of PVP, a-Fe2O3 nanotubes are well anchored on the surface of wrinkled graphene nanosheets, forming quasi-laminated-like architecture. Fe2O3@rGO composite exhibits superior electrochemical properties as anode material for LIBs with high reversible capacity and good rate capability. The first discharge and charge capacity of
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Fig. 6. (a) Cycling performance of a-Fe2O3 nanotubes and Fe2O3@rGO composite, (b) rate capabilities of Fe2O3@rGO composite.
References
Fig. 7. Electrochemical impedance spectra of a-Fe2O3 nanotubes and Fe2O3@rGO composite.
Fe2O3@rGO composite can reach 1681 and 1367 mAh g1 at a current density of 50 mAg1, respectively. Even after 100 cycles, the capacity as high as 656 mAh g1 can be retained at the current density of 100 mAg1, much beyond than that of bare a-Fe2O3 nanotubes. Better electrochemical properties of Fe2O3@rGO composite should thank to the synergic effects between a-Fe2O3 hollow nanotubes and graphene. Hollow tubular structure of iron oxide possesses high specific capacity, and the presence of graphene obviously enhances the electronic conductivity of the electrode. The unique structure of the composite can effectively buffer the strain generated from the large volume variation during Liþ ion insertion/desertion, ensuring the long cycling stability.
Acknowledgments This work was financially supported by the Natural Science Foundation of Shanghai (No. 17ZR1420000), Shanghai talent development funding project (No. 2017077) and the Shanghai Rising-star Program (B-type) (No. 15QB1402300).
Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.jallcom.2018.04.079.
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