full papers Capacitors
Self-Assembled α-Fe2O3 Mesocrystals/Graphene Nanohybrid for Enhanced Electrochemical Capacitors Shuhua Yang, Xuefeng Song,* Peng Zhang, Jing Sun, and Lian Gao*
Self-assembled α-Fe2O3 mesocrystals/graphene nanohybrids have been successfully synthesized and have a unique mesocrystal porous structure, a large specific surface area, and high conductivity. Mesocrystal structures have recently attracted unparalleled attention owing to their promising application in energy storage as electrochemical capacitors. However, mesocrystal/graphene nanohybrids and their growth mechanism have not been clearly investigated. Here we show a facile fabrication of short rod-like α-Fe2O3 mesocrystals/graphene nanohybrids by self-assembly of FeOOH nanorods as the primary building blocks on graphene under hydrothermal conditions, accompanied and promoted by concomitant phase transition from FeOOH to α-Fe2O3. A systematic study of the formation mechanism is also presented. The galvanostatic charge/discharge curve shows a superior specific capacitance of the as-prepared α-Fe2O3 mesocrystals/graphene nanohybrid (based on total mass of active materials), which is 306.9 F g−1 at 3 A g−1 in the aqueous electrolyte under voltage ranges of up to 1 V. The nanohybrid with unique sufficient porous structure and high electrical conductivity allows for effective ion and charge transport in the whole electrode. Even at a high discharge current density of 10 A g−1, the enhanced ion and charge transport still yields a higher capacitance (98.2 F g−1), exhibiting enhanced rate capability. The α-Fe2O3 mesocrystal/graphene nanohybrid electrode also demonstrates excellent cyclic performance, which is superior to previously reported graphene-based hematite electrode, suggesting it is highly stable as an electrochemical capacitor.
1. Introduction Electrochemical capacitors (ECs), which store energy through either ion adsorption (electric double layer
S. H. Yang, Dr. X. F. Song, Prof. P. Zhang, Prof. L. Gao State Key Laboratory for Metallic Matrix Composite Materials School of Materials Science and Engineering Shanghai Jiao Tong University Shanghai 200240, China E-mail: [email protected]; [email protected] Prof. J. Sun State Key Laboratory of High Performance Ceramics and Superfine Microstructure Shanghai Institute of Ceramics Chinese Academy of Sciences Shanghai 200050, China DOI: 10.1002/smll.201303922
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capacitance) or fast surface redox reactions (pseudocapacitance), are advanced energy storage devices.[1–3] Owing to their high power densities, rapid charge/discharge rates and long cycling lives, ECs are currently at the forefront of various emerging energy applications, such as high power electronic devices, electric vehicles, or hybrid electric vehicles. Hematite (α-Fe2O3) has attracted tremendous interest for ECs owing to its natural abundance, low cost, and environmental friendliness.[4–6] For instance, Wang and co-workers reported that hematite nanostructures synthesized via a morphological-conserved route exhibited a high specific capacitance of 116.25 F g−1 at the current density of 0.75 A g−1 in 1 m Li2SO4 solution.[4] Although great efforts have been carried out to improve their electrochemical performances, developing a well-designed architecture of hematite based electrode materials with large capacitance, better rate capability, and long cycling lives still remains a great challenge in view of complexity of synthesis and efficient functionalization for hematite nanostructures. It is well-established that
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Self-Assembled α-Fe2O3 Mesocrystals/Graphene Nanohybrid for Enhanced Electrochemical Capacitors
the electrochemical capability of electrode material is mainly determined by their surface properties, porosity, and electrical conductivity, which are crucial for achieving a high level of charge transfer at the interface.[7–9] High specific surface area and porosity are desirable for high-performance electrode materials, which not only facilitate ion transport by shortening diffusion pathway, but also enhance the electroactive surface area by intercalating ions in a wide range of sites.[10–12] Mesocrystals with affluent porous structures owing to either non-uniform nanocrystals or to non-close packing of nanocrystals are promising as charge storage materials.[13–15] Mesocrystals can provide extra contribution for ECs, benefiting from a mesoporous crystalline structure, which facilitates the access of electrolyte to the electrode materials and shortens the ion diffusion time.[16–18] Hematite mesocrystals with tunable architectures offer significantly enhanced chemical and physical properties in application of energy storage and conversion.[19–22] Unfortunately, the synthesis of hematite mesocrystals seems to be plagued by polymer additives, which temporarily stabilize the primary nanoparticles, causing critical challenges, such as different additives sensitive to synthesis approaches, unfavorable decrease of surface active sites after post treatment (e.g., high temperature calcinations).[23,24] Therefore, it would be highly desirable to develop a facile and additive-free synthesis of hematite mesocrystals with sufficient pores. Excellent electrical conductivity is another critical factor in achieving high electrochemical performance. To cater for the demands of high-performance supercapacitors, intensive research efforts have been made to optimize the electrochemical performance of hematite.[25–28] Previous research has demonstrated that modifying hematite with graphene is an effective strategy to improve its electrochemical properties owing to the enhanced electrical conductivity and short ion diffusion length.[6,27] For example, Lee et al. reported a novel nanocomposite with α-Fe2O3 nanotubes anchored on reduced graphene oxide displaying a high specific capacitance of 216 F g−1 at 2.5 mV s−1 in 1 M Na2SO4 solution, which was attributed to short ion diffusion length and improved electrical conductivity.[6] As α-Fe2O3 was combined with graphene, its electrochemical performance was significantly improved owing to the synergetic effect of graphene and hematite. Yet the simple combination of α-Fe2O3 with graphene sheets by multiple steps indeed constructs weak interaction between α-Fe2O3 and graphene, which can lead to severe aggregation of α-Fe2O3, subsequent degradation of electrochemical performance, and complexity of preparation. Moreover, there seems still to be a lack of control over the nanoscale assembly of hematite or their precursors as building units grown on reduced graphene oxide, although the considerable progress of hematite/graphene composites. In particular, research on hematite mesocrystals on reduced graphene oxide is considerably rare. In this paper, a green and facile route has been successfully developed to prepare unique α-Fe2O3 mesocrystals/ graphene nanohybrid at relatively low temperature (160 °C) without the introduction of any porogen or polymer additive. The assembly process of the α-Fe2O3 mesocrystals with quasi single-crystal structure on graphene sheets through epitaxial small 2014, 10, No. 11, 2270–2279
aggregation of FeOOH nanorods as the primary building units on graphene sheets is proposed. This unique architecture offers the following advantages: first, the mesoporous nature of α-Fe2O3 mesocrystals, resulted from self-assembly process, reduces the diffusion length of ions within the pseudocapacitive materials ensuring an efficient utilization of the active materials; second, the crumpled graphene sheets with numerous wrinkles and folds in hybrid electrode provide extremely high surface areas facilitating effective access of electrolyte ions to the electrode surfaces, and substantially overcome the poor electrical conductivity of α-Fe2O3; third, the synthetic method is facile and economical in contrast with the most approaches for self-assembly of nanostructured metal oxides on graphene by using amphiphilic polymer or surfactant to direct the self-assembly of nanostructured metal oxides;[17,29,30] and finally, the as-prepared hybrid is more controllable by adjusting the experimental parameters (reaction temperature, time, precursor, etc.). Furthermore, the hybrid electrodes exhibit an enhanced specific capacitance of 306.9 F g−1, to the best of our knowledge, which is the highest for α-Fe2O3 based active materials for ECs in a mild aqueous electrolyte (Table S1, Supporting Information (SI)). Enhanced rate capability and excellently cycling stability (≈92% retention after 2,000 cycles) are also seen, which are superior to those of other α-Fe2O3[4–6,31] and iron oxide nanostructures previously reported (Table S2, SI).
2. Results and Discussion 2.1. Morphology and Characterization The self-assembled α-Fe2O3 mesocrystals/graphene nanohybrid was fabricated by a one-pot hydrothermal method. FeSO4·7H2O was used as a reducing agent for the transformation from graphene oxide (GO) to reduced graphene oxide (called “graphene” for clarity of discussion in this paper); meanwhile, graphene was utilized as scaffold for the growth of α-Fe2O3 mesocrystals. The as-formed α-Fe2O3 mesocrystals/graphene nanohybrid was characterized by X-ray power diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) (Figure 1). Figure 1a shows the XRD pattern of α-Fe2O3 mesocrystal/graphene nanohybrid. All peaks can be indexed to the rhombohedral phase of hematite (α-Fe2O3, JCPDS No. 33–0664), which confirms the formation of α-Fe2O3 on graphene sheets. Based on the Scherer equation, an average domain size of 11 nm for the (110) reflection was obtained, indicating that the α-Fe2O3 are composed of nanocrystal subunits.[13] No peaks of graphene sheets are detected, suggesting that graphene sheets are highly disordered stacking with a low degree of graphitization because the α-Fe2O3 mesocrystals on the graphene sheets prevented the graphene sheets from restacking.[32] As shown in Figure 1b–d and Figure S1 in SI, short rod-like the α-Fe2O3 is homogeneously dispersed on the crumpled graphene sheets. The crumpled graphene with numerous wrinkles and folds (as indicated by the red arrow in Figure 1c) endows the α-Fe2O3 mesocrystals/graphene nanohybrid with more reactive sites and functionalities for tuning the reaction
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Figure 1. a) XRD pattern, b) SEM image, c,d) Low- and high-magnification TEM images of short rod-like α-Fe2O3 mesocrystals/graphene nanohybrid, e) HRTEM image of a single α-Fe2O3 mesocrystal, f) FFT pattern, and g) an enlarged HRTEM image of the marked area in (e).
barrier and reaction energetics of graphene.[33,34] The high magnification TEM image (Figure 1d) reveals that the short rod-like α-Fe2O3 mesocrystals have a width of around 40 nm and a length up to about 80 nm while possessing abundant mesoscopic pores ranging from 3 to 28 nm, which derive from the non-close packing or non-uniform forming of the primary FeOOH nanorods during the self-assembly process via oriented attachment.[35] High-resolution TEM (HRTEM) images (Figure 1e) and the FFT pattern of the marked area in Figure 1e (Figure 1f) for a typical α-Fe2O3 mesocrystal on graphene sheet reveal their single-crystal-like nature, which is also confirmed by Figure S2 in SI. Meanwhile, the presence of structural defects in these pseudo-single crystals indicates that the mesocrystal structure is likely to be formed through
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the assembly of nanoscale building units.[36–39] This imperfect structure with sufficient crystallized nanodomains has been reckoned to possess excellent electrochemical properties.[40] Moreover, the HRTEM image (Figure 1g) shows the interplanar spacing between lattice fringes is 0.25 nm, which can be indexed as the (110) plane of α-Fe2O3, consistent with the result of XRD characterization. In order to obtain the surface area and pore size distribution of the as-prepared α-Fe2O3 mesocrystals/graphene nanohybrid, the liquid nitrogen cryosorption measurements were conducted, and their isotherms are shown in Figure S3 (SI). The specific surface area of α-Fe2O3 mesocrystals/graphene nanohybrid is obtained to be 89.1 m2 g−1 through a Brunauer–Emmmett–Teller (BET) analysis of nitrogen
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Figure 2. a) Raman spectra, b) XPS survey spectra, c) the narrow spectra of Fe 2p, and d) the narrow spectra of O 1s for the self-assembled α-Fe2O3 mesocrystals/graphene nanohybrid.
adsorption/desorption isotherms, which is higher than that of previously reported α-Fe2O3 materials and their derivatives, such as α-Fe2O3 hollow spheres (41.1 m2 g−1),[41]α-Fe2O3/graphene aerogel (77 m2 g−1).[42] Additionally, the corresponding Barrett–Joyner–Halenda (BJH) pore size distribution curve (Figure S3, inset, SI) shows that the pore sizes are ascribed to the mesopores (6.8 nm) in α-Fe2O3 mesocrystals and the pores (16.2 nm) constructed by the graphene sheets in the hybrid. It has been demonstrated that the pore size ranging from 2 to 50 nm is most conducive to the total capacitance due to the mesoscale pore-induced high electrochemical utilization of active materials.[43–45] The synergy of short rodlike α-Fe2O3 mesocrystals with proper pores and crumpled graphene is regarded to substantially reduce the diffusion path of ions, significantly accommodate more electroactive species, and provide excellent electrical conductivity, which are conducive to improve the electrochemical properties of ECs.[7,33,43] Raman spectra of α-Fe2O3 mesocrystals/graphene nanohybrid and GO are shown in Figure 2a. The characteristic D and G band of GO are observed at 1338 and 1585 cm−1, respectively. For α-Fe2O3 mesocrystals/graphene nanohybrid, the other peaks at 220, 287, 401, 495, and 603 cm−1 from α-Fe2O3 can be observed besides D and G peaks from graphene.[46] The higher D peak intensity of α-Fe2O3 mesocrystals/graphene nanohybrid than that of GO indicates that the reduction of GO leads to more disordered layers, as well as decrease of the number of graphene layer.[47] More information about the chemical bonding states in α-Fe2O3 mesocrystals/graphene nanohybrid, as well as the chemical small 2014, 10, No. 11, 2270–2279
element valence, was obtained by typical X-ray photoelectron spectroscopy (XPS) measurements (Figure 2b, c, and d). The survey XPS spectrum demonstrates that α-Fe2O3 mesocrystals/graphene nanohybrid consists of iron, oxygen, and carbon (Figure 2b). In the high-resolution Fe 2p spectrum (Figure 2c), two major peaks at binding energies of ≈711.2 and ≈724.5 eV are ascribed to Fe 2p3/2 and Fe 2p1/2, while a shake-up satellite at ≈719.4 eV is characteristic of Fe3+ in α-Fe2O3.[48,49] Core-level high-resolution XPS spectra in the O 1s binding energy range was obtained in α-Fe2O3 mesocrystals/graphene nanohybrid. As shown in Figure 2d, the O 1s XPS spectrum can be deconvoluted into four peaks. The peak located at 530.2 is attributed to O-Fe bonding configuration in α-Fe2O3, consistent with reported data.[49] The peak at 531.8 eV is related to C = O bonding configuration while the peak at 533.4 eV is due to the C-OH and/or C-O-C bonding configuration.[50,51] Another peak centered at 530.8 eV originates from the possible formation of a Fe-O-C bond, exhibiting that α-Fe2O3 was anchored on the graphene sheets by a Fe-O-C bond.[50,52] In comparison with the peaks at binding energies of 531.6 and 532.7 eV in O 1s XPS spectrum of the graphene (Figure S4, SI), the intensities of the O 1s peaks associated with C = O group and C-OH and/or C-O-C group in α-Fe2O3 mesocrystals/graphene nanohybrid decreased dramatically, indicating that the oxygen-containing functional groups on graphene have been substituted by iron ion in α-Fe2O3, forming the Fe-O-C bonds. It is also noted that α-Fe2O3 mesocrystals do not separate from graphene sheets even under long-time ultrasonication treatment, confirming that α-Fe2O3 mesocrystals are strongly attached on graphene
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sheets. The Fe-O-C bond between graphene and α-Fe2O3 demonstrated in XPS is further corroborated by Fourier transform infrared (FTIR) spectroscopy (Figure S5, SI). The content of α-Fe2O3 in the nanohybrid was determined by thermogravimetric analysis (TGA). As demonstrated from Figure S6 in the SI, the weight loss of self-assembled α-Fe2O3 mesocrystals/graphene nanohybrid is 18.4 wt% due to the removal of the adsorbed water, the residual oxygen groups on the surface of graphene, and the burning of the carbon sketch of graphene at about 450–500 °C,[30,53] which suggests that the α-Fe2O3 accounts for 81.6% of the total mass.
2.2. Morphology Evolution and Formation Mechanism To understand the formation process of α-Fe2O3 mesocrystals/graphene nanohybrid, their growth process was followed by investigating the intermediates harvested at different interval of reaction time. As shown in Figure 3, the obvious evolutionary stages can be clearly observed. Upon reaching the tropopause of 160 °C, a large number of spherical precursor with a lot of protuberances on its surface was obtained (Figure 3a). Prolonging the reaction time to 1 min, the bunchy product was generated due to many high-energy sites provided by protuberances on the spherical precursor surfaces. In contrast, the spherical precursor particles have dissolved (Figure 3b), which indicates that the growth process is dynamically fast. When the reaction time was increased to 1h, numerous nanorod agglomerates on graphene could be observed obviously except for some thin needle-like products, suggesting that the agglomerate should be composed of many needle-like subunits marked by dotted red lines (Figure 3c, and d). The inset in Figure 3d is the HRTEM of single needlelike subunit, which shows the lattice fringes with crystal interplanar distance of 0.41 nm, corresponding to the (100) plane of FeOOH. The related XRD pattern exhibits that all diffraction peaks were exclusively ascribed to FeOOH crystals with the orthorhombic goethite phase (JCPDS No. 29–0713, space group Pnma, a = 9.95 Å, b = 3.01 Å, c = 4.62 Å) (Figure S7b, SI), further confirming the formation of primary FeOOH nanorods. This process is described as a dissolution-recrystallization mechanism, in which the spherical precursor particles gradually dissolved into the solution and then the FeOOH nuclei species originated with consumption of the precursor particles.[54,55] When increasing the reaction time to 24 h, the α-Fe2O3 mesocrystals were finally formed on graphene sheets (Figure 3e). The XRD pattern and TEM image of the products reveals the formation of short rod-like α-Fe2O3 mesocrystals on graphene sheets with an crystalline domain size of 11 nm estimated by Scherer equation (Figure S7d, SI) and sufficient structural defects indicated by the dotted circles in the single α-Fe2O3 mesocrystals (Figure 3f and Figure S2a, b in SI). It can be found that the edges of α-Fe2O3 isooriented mesocrystals become rounded due to the Ostwald ripening mechanism.[56,57] Additionally, the coexistence of both FeOOH and α-Fe2O3, confirming that the transfer reaction from FeOOH precursor to α-Fe2O3 was incomplete, was also observed at the reaction time of 12 h. A HRTEM image
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taken from two adjacent areas in a mesocrystal is presented in Figure S8c in the SI. The lattice fringes with interplanar distances of 0.41 and 0.25 nm correspond to the (110) plane of FeOOH and the (110) plane of α-Fe2O3,[58] respectively, confirming the α-Fe2O3 mesocrystals were formed by the oriented attachment of primary FeOOH nanorod agglomerates. This is consistent with the result of corresponding XRD pattern (Figure S7c, SI). On the basis of the time-dependent experiments, a plausible growth mechanism of α-Fe2O3 mesocrystals/graphene nanohybrid was outlined in Scheme 1. At the very beginning, GO sheets in aqueous solution were negatively charged due to the abundant oxygen-containing groups on their basal planes and edges, which make them favorably bind with Fe2+ cations via electrostatic interactions.[9] When the redox reaction was carried out in the hydrothermal system, the spherical precursor with numerous protuberances on scaffold was formed in the solution through a homogeneous nucleation process. Over time, reaction system was converted to the acidic hydrothermal conditions due to retention of hydrogen ion or loss of hydroxyl ion,[58] which might make the precursor dissolve and recrystallize to form renascent FeOOH bunches, and the FeOOH nanorod agglomerates were subsequently formed on the graphene sheets due to the continuous growth of the FeOOH bunches. Finally, the FeOOH nanorods are epitaxially fused together and converted to α-Fe2O3 mesocrystals by oriented attachment and Ostwald ripening mechanism on the graphene sheets. This result is analogous with our previous findings (the oriented aggregation-mediated growth),[56,59] which leads to the single-crystal-like nature of these α-Fe2O3 mesocrystals with affluently imperfect crystallized microdomains (Figure S2, SI). Therefore, α-Fe2O3 mesocrystals on graphene sheets were formed via the synergic effect of dissolution-recrystallization, oriented attachment, and Ostwald ripening mechanism. In order to discover the effect of reaction temperature on the formation of α-Fe2O3 mesocrystals/graphene nanohybrid, we studied the evolution process of morphology and phase associated with the as-formed products under different reaction temperature while keeping other experimental conditions unchanged. It can be seen that no transformation from the FeOOH nanorods to α-Fe2O3 mesocrystals occurs below 160 °C. A typical process at 140 °C was selected and shown (Figure S9, SI). As shown in Figure S9, the FeOOH mesocrystals were formed through the oriented attachment of FeOOH nanorods, but no α-Fe2O3 phase was synthesized. Furthermore, the transition of an iso-oriented α-Fe2O3 mesocrystals via oriented attachment may be mediated by face selective interaction of the generated SO42− radicals in the solvent. This result was confirmed by controlled experiments using FeCl2 or Fe(NO3)2 instead of FeSO4 with the same of other experimental conditions. The typical TEM images of α-Fe2O3 synthesized in the presence of Fe(NO3)2 or FeCl2 are shown in Figure S10 in SI. It can be seen that only α-Fe2O3 nanoplates or nanospheres were formed in the presence of NO3− or Cl−, respectively, which confirms that SO42− from FeSO4 is necessary for the formation of iso-oriented α-Fe2O3 mesocrystals.
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Self-Assembled α-Fe2O3 Mesocrystals/Graphene Nanohybrid for Enhanced Electrochemical Capacitors
Figure 3. TEM images of products obtained under 160 °C at a) 0 min, at b) 1 min, at c,d) 1 h , and at e,f) 24 h. The inset in (d) is a HRTEM image. The circles in (f) show structural defects in a α-Fe2O3 mesocrystal.
2.3. Electrochemical Properties To evaluate the capacitive performance of α-Fe2O3 mesocrystals/graphene nanohybrid, cyclic voltammetry (CV) and galvanostatic charge–discharge (CD) measurements were carried out in 1 M Na2SO4. Figure 4a shows the CV curves of α-Fe2O3 mesocrystals/graphene nanohybrid at different scan rates ranging from 5 to 200 mV s−1 in 1 M Na2SO4 aqueous small 2014, 10, No. 11, 2270–2279
solution within a potential window from −0.2 to −1.2 V (vs. Ag/AgCl). The roughly rectangular and symmetric CV curves with small redox peaks show the typical behaviors of combination of electric double layer capacitance from graphene and pseudocapacitance from the redox reaction of α-Fe2O3. The galvanostatic charge/discharge curves of α-Fe2O3 mesocrystals/graphene nanohybrid at different current densities are shown in Figure 4b. The specific capacitance is calculated
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Scheme 1. Schematic representation of the formation mechanism of α-Fe2O3 mesocrystals and their shape-evolution processes on graphene sheets.
according to the discharge curves. The nanohybrid exhibits quite good specific capacitances, which are 306.9, 284.9, 214.8, 171.8, and 145.6 F g−1 at 3, 4, 5, 6, and 7 A g−1, respectively. Even at a high discharge current density of 10 A g−1, the higher capacitance (98.2 F g−1) is still obtained. Furthermore, the specific capacitance is superior to that of both α-Fe2O3 nanoplates/graphene and α-Fe2O3 nanospheres/ graphene obtained in the presence of Fe(NO3)2 and FeCl2 (Figure S11, SI). The specific capacitance observed on this nanohybrid is also higher than the values of other α-Fe2O3 nanostructures or nanocomposites reported in the previous literature, such as mesoporous hematite nanostructures (116 F g−1),[4] and nanoscale iron oxide-carbon nanoarchitectures (84 F g−1).[60] The higher specific capacitance of α-Fe2O3 mesocrystals/graphene nanohybrid can be attributed to synergetic effect of higher conductivity of crumpled graphene and mesoscopic porous structure of short rod-like α-Fe2O3 mesocrystals. The electrochemical impedance spectroscopy (EIS) for this nanohybrid was shown in Figure 4c. According to analysis of Nyquist plots, the nanohybrid displays a lower charge-transfer resistance and an excellent conductivity, which afford facile ion and charge transfer in the electrode materials. The improvement of the electrode conductivity can be attributed to the crumpled graphene sheets, which is consistent with the previous finding.[34,61] Moreover, the larger specific surface area (89.1 m2 g−1 as shown in Figure S3, SI) and proper pore size of α-Fe2O3 mesocrystals/graphene nanohybrid have been typically considered to be
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favorable for access of electrolyte solutions.[45] The Coulombic efficiency of the supercapacitor, η, is estimated by:[62] η = Δtd/Δtc ×100%, where Δtd and Δtc represent the discharge and charge time, respectively. The Coulombic efficiency of self-assembled α-Fe2O3 mesocrystals/graphene nanohybrid nearly exceeded 90% as calculated from the charge/ discharge curves (Figure 4d), suggesting good charge/discharge reversibility of the nanohybrid. Moreover, the cycling stability of the α-Fe2O3 mesocrystals/graphene nanohybrid electrode was investigated by charge/discharge cycling at a high current density of 5 A g−1 in the potential window ranging from −0.2 to −1.2 V. As shown in Figure 4d, the specific capacitance decreases ≈8% (from 214.8 to 196.7 F g−1) after 50 cycles while it is basically identical from cycle 50 to cycle 2000, demonstrating excellent electrochemical stability and a high reversibility of self-assembled α-Fe2O3 mesocrystals/graphene nanohybrid electrode. The significantly improved electrochemical performance of the nanohybrid can be attributed to the following factors. First, the unique short rod-like α-Fe2O3 mesocrystals/ graphene nanohybrid possesses a considerably large specific surface area (89.1 m2 g−1) and a narrow distribution in the mesopore range (3–28 nm), which enables electrolyte to access to the electrochemical sites and offers short diffusion path lengths for adsorbing ions and accelerating electron transfer. Second, the crumpled graphene with numerous wrinkles and folds in the nanohybrid provides high electronic conductivity and more reactive sites, and thus facilitate ion
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Figure 4. a) CV curves of self-assembled α-Fe2O3 mesocrystals/graphene nanohybrid at different scan rates, b) CD curves of self-assembled α-Fe2O3 mesocrystals/graphene nanohybrid at different current densities, c) EIS of self-assembled α-Fe2O3 mesocrystals/graphene nanohybrid (the inset shows the enlarged EIS at the low frequency region), d) cycle life of self-assembled α-Fe2O3 mesocrystals/graphene nanohybrid at 5 A g−1 in 1 M Na2SO4 solution.
and charge transport during the charge/discharge process, favoring the long-term electrochemical stability and the outstanding rate capability.
3. Conclusion In summary, we have successfully synthesized self-assembled α-Fe2O3 mesocrystals/graphene nanohybrid using a facile hydrothermal process without using any additives. The asprepared nanohybrid shows a mesocrystal porous structure, a large specific surface area, and high electrical conductivity, which result from the short rod-like α-Fe2O3 mesocrystals derived from loosely packed FeOOH nanorods and the crumpled graphene with numerous wrinkles and folds. It was shown that short rod-like α-Fe2O3 mesocrystals on graphene sheets were formed by the synergic effect of dissolution-recrystallization, oriented attachment, and Ostwald ripening mechanism, accompanied and promoted by concomitant phase transformation from FeOOH to α-Fe2O3. Such intriguing architecture was used for the first time as electrode material in electrochemical capacitor, which delivered an enhanced capacitance, while maintaining better rate capability and superior cycling stability. This is ascribed to the intrinsic characteristics of the mesoscopic structure of short rod-like α-Fe2O3 mesocrystals and crumpled graphene, which facilitate ion and charge transport at the interface. This material will have potential applications in other high performance energy storage system. small 2014, 10, No. 11, 2270–2279
4. Experimental Section Material Synthesis: The self-assembled α-Fe2O3 mesocrystals/ graphene nanohybrid was fabricated by a facile hydrothermal process. All chemical reagents used in this study are of analytical grade, and used without further purification. Graphite oxide (GO) was synthesized using natural graphite (Alfa Aesar, 325 mesh, 99.8%) according to the method we reported previously.[63] In a typical synthesis, 1.39 g of FeSO4·7H2O was added into 35 mL of Milli-Q water, followed by stirring for 30 min at room temperature. Then 45 mL of homogeneous GO solution (1 mg mL−1) was added to the suspension. After vigorous agitation for another 30 min, the mixture solution was transferred into a 100 mL Teflon-lined stainless steel autoclave and heated at 160 °C for 24 h. Then, the autoclave was cooled instantly to room temperature by a cold water bath. The self-assembled α-Fe2O3 mesocrystals/graphene nanohybrid was obtained through centrifugation washing with Milli-Q water for several times, and then air-drying at 60 °C for 24 h. Characterization: The powder X-ray diffraction (XRD) measurements were performed using a Goniometer Ultima IV (185 mm) diffractometer with Cu Kα radiation (λ = 1.5418 Å) at a step of 0.01° per second. The microstructural characteristics and morphologies of as-obtained products were studied using a field emission scanning electron microscopy (FESEM, FEI Sirion 200). Transmission electron microscopy (TEM) images were achieved on a JEOL JEM-2010F transmission electron microscope operated at an acceleration voltage of 200 kV. Raman spectra were taken on a DXR Raman Microscope with an excitation length of 532 nm. X-ray photoelectron spectroscopy (XPS) measurements were performed on a Kratos AXIS Ultra DLD spectrometer with a monochromatic
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AlKa X-ray source. Fourier transform infrared (FTIR) spectra were recorded using a Nicolet 6700 FTIR spectrometer. Thermogravimetric analyses (TGA) were run on a SDT Q600 V20.9 Build 20 thermogravimetric analyzer at a heating rate of 10 °C min−1 from 50 to 850 °C in air. The specific surface areas and pore size distributions of the as-prepared samples were investigated by an ASAP 2010 M+C surface area and porosimetry analyzer at 77 K. Electrochemical Measurements: The working electrodes were fabricated as follows. Briefly, the as-prepared materials, acetylene black and polyvinylidene difluoride (PVDF) were mixed in a mass ratio of 80:15:5 and dispersed in N-methyl pyrrolidone (NMP) solvent. Then the resulting slurry was coated onto the nickel foam substrate (1 cm × 1 cm) with a spatula, followed by drying at 60 °C for 12 h. Finally, the electrode was pressed under a pressure of 10 MPa. Average mass loadings are 7.2 mg cm−2 of active material (self-assembled α-Fe2O3 mesocrystals/graphene nanohybrid) on current collector (nickel foam). All electrochemical measurements were carried out by a three-electrode experimental setup (VMP3 multi-functional electrochemical analysis instrument, BioLogic, France) with a platinum foil (1 cm×1 cm) auxiliary electrode and the Ag/AgCl reference electrode. The used electrolyte was 1 M aqueous Na2SO4 solution. From the charge and discharge curves, the specific capacitance (F g−1) of electrode material was calculated using formula C = IΔt/ΔVm, where Δt is the discharge time (seconds), I is the discharge current (A), ΔV is the operating potential window (V) during the discharge, and m is the mass (g) of active materials (self-assembled α-Fe2O3 mesocrystals/graphene nanohybrid) in as-prepared electrode.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements The authors greatly acknowledge the financial support by the National Natural Science Foundation of China (51302169, 51172142), the Shanghai Municipal Natural Science Foundation (12ZR1414300), the Starting Foundation for New Teacher of Shanghai Jiao Tong University (12×100040119), the Scientific Research Foundation for Returned Overseas Chinese Scholars, State Education Ministry, and the Third Phase of 211 Project for Advanced Materials Science (WS3116205007).
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Received: December 26, 2013 Published online: February 28, 2014
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Self-Assembled α-Fe2O3 Mesocrystals/Graphene Nanohybrid for Enhanced Electrochemical Capacitors Shuhua Yang, Xuefeng Song,* Peng Zhang, Jing Sun, and Lian Gao*
Self-assembled α-Fe2O3 mesocrystals/graphene nanohybrids have been successfully synthesized and have a unique mesocrystal porous structure, a large specific surface area, and high conductivity. Mesocrystal structures have recently attracted unparalleled attention owing to their promising application in energy storage as electrochemical capacitors. However, mesocrystal/graphene nanohybrids and their growth mechanism have not been clearly investigated. Here we show a facile fabrication of short rod-like α-Fe2O3 mesocrystals/graphene nanohybrids by self-assembly of FeOOH nanorods as the primary building blocks on graphene under hydrothermal conditions, accompanied and promoted by concomitant phase transition from FeOOH to α-Fe2O3. A systematic study of the formation mechanism is also presented. The galvanostatic charge/discharge curve shows a superior specific capacitance of the as-prepared α-Fe2O3 mesocrystals/graphene nanohybrid (based on total mass of active materials), which is 306.9 F g−1 at 3 A g−1 in the aqueous electrolyte under voltage ranges of up to 1 V. The nanohybrid with unique sufficient porous structure and high electrical conductivity allows for effective ion and charge transport in the whole electrode. Even at a high discharge current density of 10 A g−1, the enhanced ion and charge transport still yields a higher capacitance (98.2 F g−1), exhibiting enhanced rate capability. The α-Fe2O3 mesocrystal/graphene nanohybrid electrode also demonstrates excellent cyclic performance, which is superior to previously reported graphene-based hematite electrode, suggesting it is highly stable as an electrochemical capacitor.
1. Introduction Electrochemical capacitors (ECs), which store energy through either ion adsorption (electric double layer
S. H. Yang, Dr. X. F. Song, Prof. P. Zhang, Prof. L. Gao State Key Laboratory for Metallic Matrix Composite Materials School of Materials Science and Engineering Shanghai Jiao Tong University Shanghai 200240, China E-mail: [email protected]; [email protected] Prof. J. Sun State Key Laboratory of High Performance Ceramics and Superfine Microstructure Shanghai Institute of Ceramics Chinese Academy of Sciences Shanghai 200050, China DOI: 10.1002/smll.201303922
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capacitance) or fast surface redox reactions (pseudocapacitance), are advanced energy storage devices.[1–3] Owing to their high power densities, rapid charge/discharge rates and long cycling lives, ECs are currently at the forefront of various emerging energy applications, such as high power electronic devices, electric vehicles, or hybrid electric vehicles. Hematite (α-Fe2O3) has attracted tremendous interest for ECs owing to its natural abundance, low cost, and environmental friendliness.[4–6] For instance, Wang and co-workers reported that hematite nanostructures synthesized via a morphological-conserved route exhibited a high specific capacitance of 116.25 F g−1 at the current density of 0.75 A g−1 in 1 m Li2SO4 solution.[4] Although great efforts have been carried out to improve their electrochemical performances, developing a well-designed architecture of hematite based electrode materials with large capacitance, better rate capability, and long cycling lives still remains a great challenge in view of complexity of synthesis and efficient functionalization for hematite nanostructures. It is well-established that
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the electrochemical capability of electrode material is mainly determined by their surface properties, porosity, and electrical conductivity, which are crucial for achieving a high level of charge transfer at the interface.[7–9] High specific surface area and porosity are desirable for high-performance electrode materials, which not only facilitate ion transport by shortening diffusion pathway, but also enhance the electroactive surface area by intercalating ions in a wide range of sites.[10–12] Mesocrystals with affluent porous structures owing to either non-uniform nanocrystals or to non-close packing of nanocrystals are promising as charge storage materials.[13–15] Mesocrystals can provide extra contribution for ECs, benefiting from a mesoporous crystalline structure, which facilitates the access of electrolyte to the electrode materials and shortens the ion diffusion time.[16–18] Hematite mesocrystals with tunable architectures offer significantly enhanced chemical and physical properties in application of energy storage and conversion.[19–22] Unfortunately, the synthesis of hematite mesocrystals seems to be plagued by polymer additives, which temporarily stabilize the primary nanoparticles, causing critical challenges, such as different additives sensitive to synthesis approaches, unfavorable decrease of surface active sites after post treatment (e.g., high temperature calcinations).[23,24] Therefore, it would be highly desirable to develop a facile and additive-free synthesis of hematite mesocrystals with sufficient pores. Excellent electrical conductivity is another critical factor in achieving high electrochemical performance. To cater for the demands of high-performance supercapacitors, intensive research efforts have been made to optimize the electrochemical performance of hematite.[25–28] Previous research has demonstrated that modifying hematite with graphene is an effective strategy to improve its electrochemical properties owing to the enhanced electrical conductivity and short ion diffusion length.[6,27] For example, Lee et al. reported a novel nanocomposite with α-Fe2O3 nanotubes anchored on reduced graphene oxide displaying a high specific capacitance of 216 F g−1 at 2.5 mV s−1 in 1 M Na2SO4 solution, which was attributed to short ion diffusion length and improved electrical conductivity.[6] As α-Fe2O3 was combined with graphene, its electrochemical performance was significantly improved owing to the synergetic effect of graphene and hematite. Yet the simple combination of α-Fe2O3 with graphene sheets by multiple steps indeed constructs weak interaction between α-Fe2O3 and graphene, which can lead to severe aggregation of α-Fe2O3, subsequent degradation of electrochemical performance, and complexity of preparation. Moreover, there seems still to be a lack of control over the nanoscale assembly of hematite or their precursors as building units grown on reduced graphene oxide, although the considerable progress of hematite/graphene composites. In particular, research on hematite mesocrystals on reduced graphene oxide is considerably rare. In this paper, a green and facile route has been successfully developed to prepare unique α-Fe2O3 mesocrystals/ graphene nanohybrid at relatively low temperature (160 °C) without the introduction of any porogen or polymer additive. The assembly process of the α-Fe2O3 mesocrystals with quasi single-crystal structure on graphene sheets through epitaxial small 2014, 10, No. 11, 2270–2279
aggregation of FeOOH nanorods as the primary building units on graphene sheets is proposed. This unique architecture offers the following advantages: first, the mesoporous nature of α-Fe2O3 mesocrystals, resulted from self-assembly process, reduces the diffusion length of ions within the pseudocapacitive materials ensuring an efficient utilization of the active materials; second, the crumpled graphene sheets with numerous wrinkles and folds in hybrid electrode provide extremely high surface areas facilitating effective access of electrolyte ions to the electrode surfaces, and substantially overcome the poor electrical conductivity of α-Fe2O3; third, the synthetic method is facile and economical in contrast with the most approaches for self-assembly of nanostructured metal oxides on graphene by using amphiphilic polymer or surfactant to direct the self-assembly of nanostructured metal oxides;[17,29,30] and finally, the as-prepared hybrid is more controllable by adjusting the experimental parameters (reaction temperature, time, precursor, etc.). Furthermore, the hybrid electrodes exhibit an enhanced specific capacitance of 306.9 F g−1, to the best of our knowledge, which is the highest for α-Fe2O3 based active materials for ECs in a mild aqueous electrolyte (Table S1, Supporting Information (SI)). Enhanced rate capability and excellently cycling stability (≈92% retention after 2,000 cycles) are also seen, which are superior to those of other α-Fe2O3[4–6,31] and iron oxide nanostructures previously reported (Table S2, SI).
2. Results and Discussion 2.1. Morphology and Characterization The self-assembled α-Fe2O3 mesocrystals/graphene nanohybrid was fabricated by a one-pot hydrothermal method. FeSO4·7H2O was used as a reducing agent for the transformation from graphene oxide (GO) to reduced graphene oxide (called “graphene” for clarity of discussion in this paper); meanwhile, graphene was utilized as scaffold for the growth of α-Fe2O3 mesocrystals. The as-formed α-Fe2O3 mesocrystals/graphene nanohybrid was characterized by X-ray power diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) (Figure 1). Figure 1a shows the XRD pattern of α-Fe2O3 mesocrystal/graphene nanohybrid. All peaks can be indexed to the rhombohedral phase of hematite (α-Fe2O3, JCPDS No. 33–0664), which confirms the formation of α-Fe2O3 on graphene sheets. Based on the Scherer equation, an average domain size of 11 nm for the (110) reflection was obtained, indicating that the α-Fe2O3 are composed of nanocrystal subunits.[13] No peaks of graphene sheets are detected, suggesting that graphene sheets are highly disordered stacking with a low degree of graphitization because the α-Fe2O3 mesocrystals on the graphene sheets prevented the graphene sheets from restacking.[32] As shown in Figure 1b–d and Figure S1 in SI, short rod-like the α-Fe2O3 is homogeneously dispersed on the crumpled graphene sheets. The crumpled graphene with numerous wrinkles and folds (as indicated by the red arrow in Figure 1c) endows the α-Fe2O3 mesocrystals/graphene nanohybrid with more reactive sites and functionalities for tuning the reaction
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Figure 1. a) XRD pattern, b) SEM image, c,d) Low- and high-magnification TEM images of short rod-like α-Fe2O3 mesocrystals/graphene nanohybrid, e) HRTEM image of a single α-Fe2O3 mesocrystal, f) FFT pattern, and g) an enlarged HRTEM image of the marked area in (e).
barrier and reaction energetics of graphene.[33,34] The high magnification TEM image (Figure 1d) reveals that the short rod-like α-Fe2O3 mesocrystals have a width of around 40 nm and a length up to about 80 nm while possessing abundant mesoscopic pores ranging from 3 to 28 nm, which derive from the non-close packing or non-uniform forming of the primary FeOOH nanorods during the self-assembly process via oriented attachment.[35] High-resolution TEM (HRTEM) images (Figure 1e) and the FFT pattern of the marked area in Figure 1e (Figure 1f) for a typical α-Fe2O3 mesocrystal on graphene sheet reveal their single-crystal-like nature, which is also confirmed by Figure S2 in SI. Meanwhile, the presence of structural defects in these pseudo-single crystals indicates that the mesocrystal structure is likely to be formed through
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the assembly of nanoscale building units.[36–39] This imperfect structure with sufficient crystallized nanodomains has been reckoned to possess excellent electrochemical properties.[40] Moreover, the HRTEM image (Figure 1g) shows the interplanar spacing between lattice fringes is 0.25 nm, which can be indexed as the (110) plane of α-Fe2O3, consistent with the result of XRD characterization. In order to obtain the surface area and pore size distribution of the as-prepared α-Fe2O3 mesocrystals/graphene nanohybrid, the liquid nitrogen cryosorption measurements were conducted, and their isotherms are shown in Figure S3 (SI). The specific surface area of α-Fe2O3 mesocrystals/graphene nanohybrid is obtained to be 89.1 m2 g−1 through a Brunauer–Emmmett–Teller (BET) analysis of nitrogen
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Figure 2. a) Raman spectra, b) XPS survey spectra, c) the narrow spectra of Fe 2p, and d) the narrow spectra of O 1s for the self-assembled α-Fe2O3 mesocrystals/graphene nanohybrid.
adsorption/desorption isotherms, which is higher than that of previously reported α-Fe2O3 materials and their derivatives, such as α-Fe2O3 hollow spheres (41.1 m2 g−1),[41]α-Fe2O3/graphene aerogel (77 m2 g−1).[42] Additionally, the corresponding Barrett–Joyner–Halenda (BJH) pore size distribution curve (Figure S3, inset, SI) shows that the pore sizes are ascribed to the mesopores (6.8 nm) in α-Fe2O3 mesocrystals and the pores (16.2 nm) constructed by the graphene sheets in the hybrid. It has been demonstrated that the pore size ranging from 2 to 50 nm is most conducive to the total capacitance due to the mesoscale pore-induced high electrochemical utilization of active materials.[43–45] The synergy of short rodlike α-Fe2O3 mesocrystals with proper pores and crumpled graphene is regarded to substantially reduce the diffusion path of ions, significantly accommodate more electroactive species, and provide excellent electrical conductivity, which are conducive to improve the electrochemical properties of ECs.[7,33,43] Raman spectra of α-Fe2O3 mesocrystals/graphene nanohybrid and GO are shown in Figure 2a. The characteristic D and G band of GO are observed at 1338 and 1585 cm−1, respectively. For α-Fe2O3 mesocrystals/graphene nanohybrid, the other peaks at 220, 287, 401, 495, and 603 cm−1 from α-Fe2O3 can be observed besides D and G peaks from graphene.[46] The higher D peak intensity of α-Fe2O3 mesocrystals/graphene nanohybrid than that of GO indicates that the reduction of GO leads to more disordered layers, as well as decrease of the number of graphene layer.[47] More information about the chemical bonding states in α-Fe2O3 mesocrystals/graphene nanohybrid, as well as the chemical small 2014, 10, No. 11, 2270–2279
element valence, was obtained by typical X-ray photoelectron spectroscopy (XPS) measurements (Figure 2b, c, and d). The survey XPS spectrum demonstrates that α-Fe2O3 mesocrystals/graphene nanohybrid consists of iron, oxygen, and carbon (Figure 2b). In the high-resolution Fe 2p spectrum (Figure 2c), two major peaks at binding energies of ≈711.2 and ≈724.5 eV are ascribed to Fe 2p3/2 and Fe 2p1/2, while a shake-up satellite at ≈719.4 eV is characteristic of Fe3+ in α-Fe2O3.[48,49] Core-level high-resolution XPS spectra in the O 1s binding energy range was obtained in α-Fe2O3 mesocrystals/graphene nanohybrid. As shown in Figure 2d, the O 1s XPS spectrum can be deconvoluted into four peaks. The peak located at 530.2 is attributed to O-Fe bonding configuration in α-Fe2O3, consistent with reported data.[49] The peak at 531.8 eV is related to C = O bonding configuration while the peak at 533.4 eV is due to the C-OH and/or C-O-C bonding configuration.[50,51] Another peak centered at 530.8 eV originates from the possible formation of a Fe-O-C bond, exhibiting that α-Fe2O3 was anchored on the graphene sheets by a Fe-O-C bond.[50,52] In comparison with the peaks at binding energies of 531.6 and 532.7 eV in O 1s XPS spectrum of the graphene (Figure S4, SI), the intensities of the O 1s peaks associated with C = O group and C-OH and/or C-O-C group in α-Fe2O3 mesocrystals/graphene nanohybrid decreased dramatically, indicating that the oxygen-containing functional groups on graphene have been substituted by iron ion in α-Fe2O3, forming the Fe-O-C bonds. It is also noted that α-Fe2O3 mesocrystals do not separate from graphene sheets even under long-time ultrasonication treatment, confirming that α-Fe2O3 mesocrystals are strongly attached on graphene
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sheets. The Fe-O-C bond between graphene and α-Fe2O3 demonstrated in XPS is further corroborated by Fourier transform infrared (FTIR) spectroscopy (Figure S5, SI). The content of α-Fe2O3 in the nanohybrid was determined by thermogravimetric analysis (TGA). As demonstrated from Figure S6 in the SI, the weight loss of self-assembled α-Fe2O3 mesocrystals/graphene nanohybrid is 18.4 wt% due to the removal of the adsorbed water, the residual oxygen groups on the surface of graphene, and the burning of the carbon sketch of graphene at about 450–500 °C,[30,53] which suggests that the α-Fe2O3 accounts for 81.6% of the total mass.
2.2. Morphology Evolution and Formation Mechanism To understand the formation process of α-Fe2O3 mesocrystals/graphene nanohybrid, their growth process was followed by investigating the intermediates harvested at different interval of reaction time. As shown in Figure 3, the obvious evolutionary stages can be clearly observed. Upon reaching the tropopause of 160 °C, a large number of spherical precursor with a lot of protuberances on its surface was obtained (Figure 3a). Prolonging the reaction time to 1 min, the bunchy product was generated due to many high-energy sites provided by protuberances on the spherical precursor surfaces. In contrast, the spherical precursor particles have dissolved (Figure 3b), which indicates that the growth process is dynamically fast. When the reaction time was increased to 1h, numerous nanorod agglomerates on graphene could be observed obviously except for some thin needle-like products, suggesting that the agglomerate should be composed of many needle-like subunits marked by dotted red lines (Figure 3c, and d). The inset in Figure 3d is the HRTEM of single needlelike subunit, which shows the lattice fringes with crystal interplanar distance of 0.41 nm, corresponding to the (100) plane of FeOOH. The related XRD pattern exhibits that all diffraction peaks were exclusively ascribed to FeOOH crystals with the orthorhombic goethite phase (JCPDS No. 29–0713, space group Pnma, a = 9.95 Å, b = 3.01 Å, c = 4.62 Å) (Figure S7b, SI), further confirming the formation of primary FeOOH nanorods. This process is described as a dissolution-recrystallization mechanism, in which the spherical precursor particles gradually dissolved into the solution and then the FeOOH nuclei species originated with consumption of the precursor particles.[54,55] When increasing the reaction time to 24 h, the α-Fe2O3 mesocrystals were finally formed on graphene sheets (Figure 3e). The XRD pattern and TEM image of the products reveals the formation of short rod-like α-Fe2O3 mesocrystals on graphene sheets with an crystalline domain size of 11 nm estimated by Scherer equation (Figure S7d, SI) and sufficient structural defects indicated by the dotted circles in the single α-Fe2O3 mesocrystals (Figure 3f and Figure S2a, b in SI). It can be found that the edges of α-Fe2O3 isooriented mesocrystals become rounded due to the Ostwald ripening mechanism.[56,57] Additionally, the coexistence of both FeOOH and α-Fe2O3, confirming that the transfer reaction from FeOOH precursor to α-Fe2O3 was incomplete, was also observed at the reaction time of 12 h. A HRTEM image
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taken from two adjacent areas in a mesocrystal is presented in Figure S8c in the SI. The lattice fringes with interplanar distances of 0.41 and 0.25 nm correspond to the (110) plane of FeOOH and the (110) plane of α-Fe2O3,[58] respectively, confirming the α-Fe2O3 mesocrystals were formed by the oriented attachment of primary FeOOH nanorod agglomerates. This is consistent with the result of corresponding XRD pattern (Figure S7c, SI). On the basis of the time-dependent experiments, a plausible growth mechanism of α-Fe2O3 mesocrystals/graphene nanohybrid was outlined in Scheme 1. At the very beginning, GO sheets in aqueous solution were negatively charged due to the abundant oxygen-containing groups on their basal planes and edges, which make them favorably bind with Fe2+ cations via electrostatic interactions.[9] When the redox reaction was carried out in the hydrothermal system, the spherical precursor with numerous protuberances on scaffold was formed in the solution through a homogeneous nucleation process. Over time, reaction system was converted to the acidic hydrothermal conditions due to retention of hydrogen ion or loss of hydroxyl ion,[58] which might make the precursor dissolve and recrystallize to form renascent FeOOH bunches, and the FeOOH nanorod agglomerates were subsequently formed on the graphene sheets due to the continuous growth of the FeOOH bunches. Finally, the FeOOH nanorods are epitaxially fused together and converted to α-Fe2O3 mesocrystals by oriented attachment and Ostwald ripening mechanism on the graphene sheets. This result is analogous with our previous findings (the oriented aggregation-mediated growth),[56,59] which leads to the single-crystal-like nature of these α-Fe2O3 mesocrystals with affluently imperfect crystallized microdomains (Figure S2, SI). Therefore, α-Fe2O3 mesocrystals on graphene sheets were formed via the synergic effect of dissolution-recrystallization, oriented attachment, and Ostwald ripening mechanism. In order to discover the effect of reaction temperature on the formation of α-Fe2O3 mesocrystals/graphene nanohybrid, we studied the evolution process of morphology and phase associated with the as-formed products under different reaction temperature while keeping other experimental conditions unchanged. It can be seen that no transformation from the FeOOH nanorods to α-Fe2O3 mesocrystals occurs below 160 °C. A typical process at 140 °C was selected and shown (Figure S9, SI). As shown in Figure S9, the FeOOH mesocrystals were formed through the oriented attachment of FeOOH nanorods, but no α-Fe2O3 phase was synthesized. Furthermore, the transition of an iso-oriented α-Fe2O3 mesocrystals via oriented attachment may be mediated by face selective interaction of the generated SO42− radicals in the solvent. This result was confirmed by controlled experiments using FeCl2 or Fe(NO3)2 instead of FeSO4 with the same of other experimental conditions. The typical TEM images of α-Fe2O3 synthesized in the presence of Fe(NO3)2 or FeCl2 are shown in Figure S10 in SI. It can be seen that only α-Fe2O3 nanoplates or nanospheres were formed in the presence of NO3− or Cl−, respectively, which confirms that SO42− from FeSO4 is necessary for the formation of iso-oriented α-Fe2O3 mesocrystals.
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Figure 3. TEM images of products obtained under 160 °C at a) 0 min, at b) 1 min, at c,d) 1 h , and at e,f) 24 h. The inset in (d) is a HRTEM image. The circles in (f) show structural defects in a α-Fe2O3 mesocrystal.
2.3. Electrochemical Properties To evaluate the capacitive performance of α-Fe2O3 mesocrystals/graphene nanohybrid, cyclic voltammetry (CV) and galvanostatic charge–discharge (CD) measurements were carried out in 1 M Na2SO4. Figure 4a shows the CV curves of α-Fe2O3 mesocrystals/graphene nanohybrid at different scan rates ranging from 5 to 200 mV s−1 in 1 M Na2SO4 aqueous small 2014, 10, No. 11, 2270–2279
solution within a potential window from −0.2 to −1.2 V (vs. Ag/AgCl). The roughly rectangular and symmetric CV curves with small redox peaks show the typical behaviors of combination of electric double layer capacitance from graphene and pseudocapacitance from the redox reaction of α-Fe2O3. The galvanostatic charge/discharge curves of α-Fe2O3 mesocrystals/graphene nanohybrid at different current densities are shown in Figure 4b. The specific capacitance is calculated
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Scheme 1. Schematic representation of the formation mechanism of α-Fe2O3 mesocrystals and their shape-evolution processes on graphene sheets.
according to the discharge curves. The nanohybrid exhibits quite good specific capacitances, which are 306.9, 284.9, 214.8, 171.8, and 145.6 F g−1 at 3, 4, 5, 6, and 7 A g−1, respectively. Even at a high discharge current density of 10 A g−1, the higher capacitance (98.2 F g−1) is still obtained. Furthermore, the specific capacitance is superior to that of both α-Fe2O3 nanoplates/graphene and α-Fe2O3 nanospheres/ graphene obtained in the presence of Fe(NO3)2 and FeCl2 (Figure S11, SI). The specific capacitance observed on this nanohybrid is also higher than the values of other α-Fe2O3 nanostructures or nanocomposites reported in the previous literature, such as mesoporous hematite nanostructures (116 F g−1),[4] and nanoscale iron oxide-carbon nanoarchitectures (84 F g−1).[60] The higher specific capacitance of α-Fe2O3 mesocrystals/graphene nanohybrid can be attributed to synergetic effect of higher conductivity of crumpled graphene and mesoscopic porous structure of short rod-like α-Fe2O3 mesocrystals. The electrochemical impedance spectroscopy (EIS) for this nanohybrid was shown in Figure 4c. According to analysis of Nyquist plots, the nanohybrid displays a lower charge-transfer resistance and an excellent conductivity, which afford facile ion and charge transfer in the electrode materials. The improvement of the electrode conductivity can be attributed to the crumpled graphene sheets, which is consistent with the previous finding.[34,61] Moreover, the larger specific surface area (89.1 m2 g−1 as shown in Figure S3, SI) and proper pore size of α-Fe2O3 mesocrystals/graphene nanohybrid have been typically considered to be
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favorable for access of electrolyte solutions.[45] The Coulombic efficiency of the supercapacitor, η, is estimated by:[62] η = Δtd/Δtc ×100%, where Δtd and Δtc represent the discharge and charge time, respectively. The Coulombic efficiency of self-assembled α-Fe2O3 mesocrystals/graphene nanohybrid nearly exceeded 90% as calculated from the charge/ discharge curves (Figure 4d), suggesting good charge/discharge reversibility of the nanohybrid. Moreover, the cycling stability of the α-Fe2O3 mesocrystals/graphene nanohybrid electrode was investigated by charge/discharge cycling at a high current density of 5 A g−1 in the potential window ranging from −0.2 to −1.2 V. As shown in Figure 4d, the specific capacitance decreases ≈8% (from 214.8 to 196.7 F g−1) after 50 cycles while it is basically identical from cycle 50 to cycle 2000, demonstrating excellent electrochemical stability and a high reversibility of self-assembled α-Fe2O3 mesocrystals/graphene nanohybrid electrode. The significantly improved electrochemical performance of the nanohybrid can be attributed to the following factors. First, the unique short rod-like α-Fe2O3 mesocrystals/ graphene nanohybrid possesses a considerably large specific surface area (89.1 m2 g−1) and a narrow distribution in the mesopore range (3–28 nm), which enables electrolyte to access to the electrochemical sites and offers short diffusion path lengths for adsorbing ions and accelerating electron transfer. Second, the crumpled graphene with numerous wrinkles and folds in the nanohybrid provides high electronic conductivity and more reactive sites, and thus facilitate ion
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Figure 4. a) CV curves of self-assembled α-Fe2O3 mesocrystals/graphene nanohybrid at different scan rates, b) CD curves of self-assembled α-Fe2O3 mesocrystals/graphene nanohybrid at different current densities, c) EIS of self-assembled α-Fe2O3 mesocrystals/graphene nanohybrid (the inset shows the enlarged EIS at the low frequency region), d) cycle life of self-assembled α-Fe2O3 mesocrystals/graphene nanohybrid at 5 A g−1 in 1 M Na2SO4 solution.
and charge transport during the charge/discharge process, favoring the long-term electrochemical stability and the outstanding rate capability.
3. Conclusion In summary, we have successfully synthesized self-assembled α-Fe2O3 mesocrystals/graphene nanohybrid using a facile hydrothermal process without using any additives. The asprepared nanohybrid shows a mesocrystal porous structure, a large specific surface area, and high electrical conductivity, which result from the short rod-like α-Fe2O3 mesocrystals derived from loosely packed FeOOH nanorods and the crumpled graphene with numerous wrinkles and folds. It was shown that short rod-like α-Fe2O3 mesocrystals on graphene sheets were formed by the synergic effect of dissolution-recrystallization, oriented attachment, and Ostwald ripening mechanism, accompanied and promoted by concomitant phase transformation from FeOOH to α-Fe2O3. Such intriguing architecture was used for the first time as electrode material in electrochemical capacitor, which delivered an enhanced capacitance, while maintaining better rate capability and superior cycling stability. This is ascribed to the intrinsic characteristics of the mesoscopic structure of short rod-like α-Fe2O3 mesocrystals and crumpled graphene, which facilitate ion and charge transport at the interface. This material will have potential applications in other high performance energy storage system. small 2014, 10, No. 11, 2270–2279
4. Experimental Section Material Synthesis: The self-assembled α-Fe2O3 mesocrystals/ graphene nanohybrid was fabricated by a facile hydrothermal process. All chemical reagents used in this study are of analytical grade, and used without further purification. Graphite oxide (GO) was synthesized using natural graphite (Alfa Aesar, 325 mesh, 99.8%) according to the method we reported previously.[63] In a typical synthesis, 1.39 g of FeSO4·7H2O was added into 35 mL of Milli-Q water, followed by stirring for 30 min at room temperature. Then 45 mL of homogeneous GO solution (1 mg mL−1) was added to the suspension. After vigorous agitation for another 30 min, the mixture solution was transferred into a 100 mL Teflon-lined stainless steel autoclave and heated at 160 °C for 24 h. Then, the autoclave was cooled instantly to room temperature by a cold water bath. The self-assembled α-Fe2O3 mesocrystals/graphene nanohybrid was obtained through centrifugation washing with Milli-Q water for several times, and then air-drying at 60 °C for 24 h. Characterization: The powder X-ray diffraction (XRD) measurements were performed using a Goniometer Ultima IV (185 mm) diffractometer with Cu Kα radiation (λ = 1.5418 Å) at a step of 0.01° per second. The microstructural characteristics and morphologies of as-obtained products were studied using a field emission scanning electron microscopy (FESEM, FEI Sirion 200). Transmission electron microscopy (TEM) images were achieved on a JEOL JEM-2010F transmission electron microscope operated at an acceleration voltage of 200 kV. Raman spectra were taken on a DXR Raman Microscope with an excitation length of 532 nm. X-ray photoelectron spectroscopy (XPS) measurements were performed on a Kratos AXIS Ultra DLD spectrometer with a monochromatic
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AlKa X-ray source. Fourier transform infrared (FTIR) spectra were recorded using a Nicolet 6700 FTIR spectrometer. Thermogravimetric analyses (TGA) were run on a SDT Q600 V20.9 Build 20 thermogravimetric analyzer at a heating rate of 10 °C min−1 from 50 to 850 °C in air. The specific surface areas and pore size distributions of the as-prepared samples were investigated by an ASAP 2010 M+C surface area and porosimetry analyzer at 77 K. Electrochemical Measurements: The working electrodes were fabricated as follows. Briefly, the as-prepared materials, acetylene black and polyvinylidene difluoride (PVDF) were mixed in a mass ratio of 80:15:5 and dispersed in N-methyl pyrrolidone (NMP) solvent. Then the resulting slurry was coated onto the nickel foam substrate (1 cm × 1 cm) with a spatula, followed by drying at 60 °C for 12 h. Finally, the electrode was pressed under a pressure of 10 MPa. Average mass loadings are 7.2 mg cm−2 of active material (self-assembled α-Fe2O3 mesocrystals/graphene nanohybrid) on current collector (nickel foam). All electrochemical measurements were carried out by a three-electrode experimental setup (VMP3 multi-functional electrochemical analysis instrument, BioLogic, France) with a platinum foil (1 cm×1 cm) auxiliary electrode and the Ag/AgCl reference electrode. The used electrolyte was 1 M aqueous Na2SO4 solution. From the charge and discharge curves, the specific capacitance (F g−1) of electrode material was calculated using formula C = IΔt/ΔVm, where Δt is the discharge time (seconds), I is the discharge current (A), ΔV is the operating potential window (V) during the discharge, and m is the mass (g) of active materials (self-assembled α-Fe2O3 mesocrystals/graphene nanohybrid) in as-prepared electrode.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements The authors greatly acknowledge the financial support by the National Natural Science Foundation of China (51302169, 51172142), the Shanghai Municipal Natural Science Foundation (12ZR1414300), the Starting Foundation for New Teacher of Shanghai Jiao Tong University (12×100040119), the Scientific Research Foundation for Returned Overseas Chinese Scholars, State Education Ministry, and the Third Phase of 211 Project for Advanced Materials Science (WS3116205007).
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Received: December 26, 2013 Published online: February 28, 2014
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