Free-Standing, Ordered Mesoporous Few-Layer Graphene

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Free-Standing, Ordered Mesoporous Few-Layer Graphene Framework Films Derived from Nanocrystal Superlattices Self-Assembled at the Solid- or Liquid-Air Interface Li Ji, Guannan Guo, Hongyuan Sheng, Shanli Qin, Biwei Wang, Dandan Han, Tongtao Li, Dong Yang, and Angang Dong Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b00870 • Publication Date (Web): 12 May 2016 Downloaded from http://pubs.acs.org on May 16, 2016

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Chemistry of Materials

Free-Standing, Ordered Mesoporous Few-Layer Graphene Framework Films Derived from Nanocrystal Superlattices Self-Assembled at the Solid- or Liquid-Air Interface Li Ji,† Guannan Guo,†,‡ Hongyuan Sheng,† Shanli Qin,† Biwei Wang,† Dandan Han,† Tongtao Li,† Dong Yang,*‡ and Angang Dong*† †

Collaborative Innovation Center of Chemistry for Energy Materials, Shanghai Key

Laboratory of Molecular Catalysis and Innovative Materials, and Department of Chemistry, Fudan University, Shanghai 200433, China. ‡

State Key Laboratory of Molecular Engineering of Polymers and Department of

Macromolecular Science, Fudan University, Shanghai 200433, China.

*To whom correspondence should be addressed: [email protected]; [email protected] (A.D.)

1

ACS Paragon Plus Environment


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ABSTRACT Three-dimensional (3D) porous graphene frameworks, which combine the advantages of both porous materials and graphene, have recently attracted enormous attention for electrochemical energy storage. Despite the tremendous progress, it remains a grand challenge to synthesize graphene frameworks with ordered porosity and well-defined macroscopic morphologies. Herein, we report the design and synthesis of centimeter-scale, free-standing thin films of ordered mesoporous graphene frameworks (MGFs) from 2D nanocrystal superlattices self-assembled at the solid- or liquid-air interface. The resultant MGF films possess uniform thicknesses tunable in the range from a few hundred nanometers to several tens of micrometers, highly ordered and interconnected mesoporosity, ultrathin pore walls comprising few-layer graphene, and high surface areas. To demonstrate their potential applications in energy storage, MGF films are used as electrode materials to build supercapacitors, which exhibit high specific capacitances with excellent cycling stabilities in both aqueous and organic electrolytes, with the capacitive performance comparable to or higher than that of most graphene-based materials developed previously.

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INTRODUCTION As an important class of carbonaceous materials, ordered mesoporous carbons (OMCs) have found applications in diverse areas such as adsorption, catalysis, and energy storage and conversion,1 owing to their ordered porosity, tunable pore sizes in the range of 2-50 nm, and high surface areas. Since the pioneering work by Ryoo et al.2 and Heyon et al.,3 many groups have reported the synthesis of OMCs with properly designed porous structures using various templates such as mesoporous silica,2,3 silica colloids,4-6 and block-copolymer micelles.7,8 In addition to the internal porosity, controlling the macroscopic morphology of OMCs is equally important for exploiting their potentials for practical applications.1 Thanks to the recent synthetic advances, OMCs with desired morphologies such as spheres,9,10 fibers,11 films,7 and monoliths12 are now routinely achievable by controlling reaction conditions. Among them, two-dimensional (2D), free-standing thin films of OMCs are particularly suitable for use in chemical separation, sensing, and flexible energy storage and conversion devices,13-16 owing to their unique 2D geometry in combination with the enhanced mass and charge transport properties.13 Going a step further, one can reasonably expect that replacing the amorphous carbon frameworks with graphene or few-layer graphene to afford mesoporous graphene thin films will greatly extend the applications of OMCs, due to the substantially enhanced electrical conductivity and chemical stability.17 Unfortunately, the strategies previously developed for synthesizing OMCs cannot be directly applied to the growth of their graphene counterparts.18,19 3



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Because of the grand challenges associated with the direct synthesis of ordered mesoporous graphene,20 much effort has been devoted to developing alternative strategies over the past few years.21-28 Various templates such as polymer spheres24 and nickel foams25 have been employed to prepare porous graphene materials. The self-assembly of graphene (oxide) sheets represents another efficient approach for creating hierarchically porous graphene macrostructures including thin films.26-29 The 3D porous graphene materials obtained show great potential for use as electrode materials in supercapacitors.29 However, most porous graphene frameworks reported to date possess a disordered porous structure with pore sizes ranging from a few nanometers to several tens of micrometers,30 which will inevitably influence mass transport and therefore device performance. In a previous study, we demonstrated that self-assembled, long-range ordered arrays of colloidal nanocrystals (NCs), also known as NC superlattices,31 could be used to construct ordered mesoporous graphene.32 The resultant graphene frameworks, which are derived from the long-chain organic ligands originally stabilizing colloidal NCs, possess ultrathin pore walls comprising few-layer graphene and tunable pore sizes in the range of 9-16 nm. To the best of our knowledge, this is the first report on the growth of ordered mesoporous graphene.33 Despite the highly ordered mesoporosity, our method suffers from an inability to control the macroscopic morphology of graphene frameworks, as the previous drying-mediated assembly process involved typically produces 3D NC superlattices in the form of isolated islands with ill-defined morphologies.34 Therefore, the synthesis of 2D mesoporous graphene thin films with well-defined porosity and 4


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precisely-controlled pore sizes has not yet been realized to date. In this work, we for the first time report the bottom-up construction of centimeter-scale, free-standing mesoporous graphene framework (MGF) thin films. Our approach builds upon recent progress in the growth of large-area, 2D NC superlattices by solid-air and liquid-air interfacial assembly techniques.31,35 MGF thin films are obtained based on the transformation of 2D NC superlattices by ligand carbonization, NC etching, and framework graphitization. Although the approach presented here appears to be time consuming, it enables the growth of large-area thin films of highly ordered mesoporous few-layer graphene frameworks that could not be realized by previously reported methods. Moreover, the resulting MGF films possess uniform thicknesses tunable in the range from a few hundred nanometers to several tens of micrometers, precisely controlled pore sizes, and high surface areas. These desirable structural characteristics combined with the high electrical conductivity and excellent mechanical strength of graphene frameworks render MGF thin films highly promising for supercapacitor applications. Electrochemical measurements show that MGF films exhibit high specific capacitances with excellent cycling stabilities in both aqueous and organic electrolytes. EXPERIMENTAL SECTION Materials. Oleic acid (OA, 90%), oleylamine (OAm, 70%), 1-octadecene (ODE, 90%), benzyl ether (98%), and poly(methyl methacrylate) (PMMA, M w = 120000) were purchased from Aldrich. Sodium oleate was obtained from TCI. Iron chloride hexahydrate

(FeCl3·6H2O),

tetraethylammonium

tetrafluoroborate

5


(TEABF4),


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propylene carbonate (PC), and diethylene glycol (DEG) were purchased from Aladdin. Iron acetylacetonate (Fe(acac)3) and cobalt acetylacetonate (Co(acac)2) were purchased from Alfa Aesar. All chemicals were used as received without further purification. Synthesis of Fe3O4 NCs. Monodisperse Fe3O4 NCs with OA as the surface-capping ligands were synthesized by the thermal decomposition of iron oleate, which was synthesized by reaction of sodium oleate and FeCl3·6H2O. In a typical procedure to synthesize 13 nm Fe3O4 NCs, 72 g of iron oleate and 17.2 g of OA were dissolved in 300 g of ODE in a 1 L three-neck flask. The solution was heated at 320 o

C under N2 for 1 h. After the solution was cooled down to room temperature, ethanol

and isopropanol were added to precipitate Fe3O4 NCs, and the precipitated NCs were redispersed in toluene or hexane to form stable colloid solutions with a concentration of 40 mg/mL. The NC size could be tuned by varying the reaction temperature and/or the amount of OA. Synthesis of CoFe2O4 NCs. Monodisperse CoFe2O4 NC seeds with OA and OAm as the surface-capping ligands were synthesized by a seed-mediated growth method. In a typical procedure to synthesize 5 nm CoFe2O4 NC seeds, 11.2 g of Fe(acac)3, 4 g of Co(acac)2, 9 g of OA, and 42 g of OAm were dissolved in 50 mL of benzyl ether in a 200 mL three-neck flask. The solution was heated at 200 oC under N2 for 1.5 h and was further heated at 295 oC under N2 for 1 h. After the solution was cooled down to room temperature, ethanol was added to precipitate CoFe2O4 NC seeds, and the precipitated NC seeds were redispersed in 45 mL of hexane to form 6


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stable colloid solutions. In a typical procedure to synthesize 8 nm CoFe2O4 NCs, 5.6 g of Fe(acac)3, 2 g of Co(acac)2, 4.5 g of OA and 21 g of OAm were dissolved in 80 mL of benzyl ether in a 200 mL three-neck flask. 20 mL of solution of 5 nm CoFe2O4 NC seeds in hexane was injected into the solution at 110 oC. After reaction at 110 oC under N2 for 30 min, the solution was heated at 200 oC for 1 h and was further heated at 295 o

C for 40 min. After the solution was cooled down to room temperature, ethanol was

added to precipitate CoFe2O4 NCs, and the precipitated NCs were redispersed in hexane to form stable colloid solutions with a concentration of 40 mg/mL. Self-Assembly of 2D Fe3O4 NC Superlattice Films. Fe3O4 NC superlattice films were grown by either solid-air or liquid-air interfacial assembly techniques. In a typical procedure to grow NC films with a thickness of 5 µm by solid-air interfacial assembly, an aluminum (Al) foil (~ 1.5 × 1.5 cm2, ~ 20 µm in thickness) was placed in a porcelain combustion boat, into which 2 mL of Fe3O4 NC solution in toluene was added to completely wet the Al foil. The solvent was then allowed to evaporate under ambient conditions, resulting into an oily Fe3O4 NC superlattice film covering the entire Al foil after the complete evaporation of toluene. Thinner NC superlattice films with thicknesses less than 1 µm were obtained by the liquid-air interfacial assembly technique using DEG as the subphase.35 In a typical procedure to grow NC membranes with thickness of 900 nm, 1 mL of Fe3O4 NC solution in hexane was drop-casted onto the surface of DEG confined in a Teflon well. The well was then covered by a glass slide, and the solvent was allowed to evaporate under ambient conditions. The complete evaporation of hexane yielded a solid membrane, which 7



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could be easily transferred to an Al foil from the DEG surface. In both cases, the thickness of NC superlattice films could be tuned by controlling the amount of NC solutions added. Fabrication of Free-Standing MGF Films. The as-assembled of Fe3O4 NC superlattice films supported on Al foils were heated in a quartz tube furnace at 500 oC in Ar for 2 h with a heating rate of 2 oC/min, converting the surface-coating OA ligands into amorphous carbon shells. After cooling down to room temperature, a thin layer (~ 20 nm in thickness) of PMMA was deposited onto the surface of NC films by spin-coating. The PMMA solution was prepared by dissolving PMMA powders in toluene with a concentration of 2 wt%. The PMMA-coated NC films were then treated in a dilute HCl solution to remove Fe3O4 NCs and Al foils, resulting into thin films of mesoporous carbon frameworks, which could be readily transferred to a porcelain combustion boat from the water surface. After drying, the mesoporous carbon films were heated at 1000 oC in Ar for 2 h, converting the amorphous carbon frameworks into highly graphitic ones while largely retaining the film morphology and integrity. The resulting free-standing MGF films could be easily manipulated by a tweezer. Hydrophilic Treatment of MGF Films. To improve the wetting properties of graphene frameworks in aqueous electrolytes for supercapacitor measurements, the MGF films were treated in a mixed acid consisting of concentrated H2SO4 and HNO3 with a volume ratio of 3:1 at room temperature. After 6 h of acid treatment, the MGF films were washed repeatedly with deionized water until a pH value of 7 was reached. 8


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With the preservation of the highly ordered mesoporosity, the acid-treated MGF films became hydrophilic due to the introduction of hydroxyl and carboxyl groups during acid treatment. Instrumentation. Scanning electron microscopy (SEM) and high-resolution SEM (HRSEM) images were recorded on a Zeiss Ultra-55 microscope operated at 5 kV. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were obtained by using a Tecnai G2 20 TWIN microscope operated at 200 kV. Small angle X-ray scattering (SAXS) was performed on a Nanostar U small angle X-ray

scattering

system

using

Cu-Kα

radiation

(40

kV,

35

mA).

N2

adsorption–desorption isotherms and pore size distributions were recorded on a Tristar 3000 instrument. The pore size distribution curve was determined by using the Barrett-Joyner-Halenda (BJH) model. Before measurements, the samples were degassed at 300 °C for 12 h. Raman spectra were collected at room temperature on an XploRA Raman system. Thermogravimetric analysis (TGA) was performed on a Mettler Toledo TGA instrument. Fourier transform infrared (FTIR) spectroscopy was performed on a PerkinElmer Spectrum Two FT-IR Spectrometer. X-ray photoelectron spectroscopy (XPS) was performed on a PerkinElmer PHI 5000C ESCA system. Electrical conductivity measurements were performed on a SANFENG SB120 four-point probe measurement system connected to a SANFENG SB118 DC power supply. Electrochemical Measurements of Supercapacitor Performance. Cyclic voltammetry (CV), galvanostatic charge/discharge, and electrochemical impedance 9



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spectroscopy (EIS) measurements were conducted on a CHI 606E electrochemical analyzer system. The cyclic voltammetry measurements were carried out at different scan rates ranging from 5 to 200 mV/s. The galvanostatic charge-discharge measurements were carried out at different current densities ranging from 0.5 to 20 A/g. The electrochemical impedance spectroscopy measurements were carried out at a frequency range from 1 MHz to 0.1 Hz. Cycling stability tests were carried out on a LANHE CT 2001A electrochemical analyzer system. To prepare the working electrodes, MGF films (~ 2.5 µm in thickness), acetylene black, and polytetrafluoroethylene (PTFE) binder with a mass ratio of 8:1:1 were homogeneously mixed in ethanol to form a slurry, which was then rolled into a uniform film onto a stainless steel mesh. The mass loading of MGF films on each working electrode was approximately 2.0 mg with a coating area of 1.5 cm × 1.5 cm. The capacitance performance measurements were conducted with a three-electrode setup, with a Pt foil and Ag/AgCl (Ag/Ag+ for organic electrolytes) as the counter and reference electrodes, respectively. The aqueous electrolyte was 1 M H2SO4, while 1 M TEABF4 dissolved in PC was used as the organic electrolyte. The specific capacitances (C) were calculated according to C = I (Δt)/m(ΔV), where I is the applied current (A), Δt is the discharge time (s), m is the total mass of active materials (g), ΔV is the voltage window during the discharge (V). A two-electrode setup was adopted to determine the energy and power densities of MGF films with two nearly identical electrodes in 1 M H2SO4 or 1 M TEABF4/PC. The energy density (E) and power density (P) were calculated according to the equations E = (1/8)C/∆V2 and P = E/∆t, 10


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respectively, where C is the specific capacitance of a single electrode. RESULTS AND DISCUSSION Two approaches involving the controlled assembly of colloidal NCs at the solid-air and liquid-air interfaces, respectively, are employed in the present study to grow 2D NC superlattice films, as depicted in Figure 1. In both cases, monodisperse Fe3O4 NCs capped by OA are used as building blocks for assembling 2D NC superlattices. Specifically, thicker NC superlattice films with a thickness in the range of 1-20 µm are directly assembled on the surface of an Al foil by a solvent-evaporation-driven assembly process (Figure 1a), while thinner NC superlattice films with a thickness less than 1 µm can be realized by the drying-mediated assembly process occurring on the surface of DEG (Figure 1b). In the latter case, the as-assembled NC superlattice membranes floating on the DEG surface are transferred to Al foils before ligand carbonization. We choose Al foils as substrates in both cases, mainly because Al foils can be readily etched away along with Fe3O4 NCs during acid treatment. To fabricate MGF films, the Al foils with the supported NC films resulted from both pathways are heated at 500 oC to carbonize the surface-coating OA ligands. Prior to the removal of Fe3O4 NCs, a thin layer of PMMA is deposited onto NC films by spin-coating to minimize the influence of acid etching on the film integrity. The subsequent removal of Fe3O4 NCs yields thin films of mesoporous carbon, the frameworks of which are converted into few-layer graphene by heat treatment at 1000 oC, resulting into MGF films with pore sizes tunable by varying the diameter of Fe3O4 NCs. 11



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In a typical procedure to grow thick NC superlattice films, Al foils with a thickness of ~ 20 µm were immersed in a Fe3O4 NC solution in toluene in a porcelain combustion boat, and the solvent was then allowed to evaporate under ambient conditions. It was found that the use of concentrated NC solutions having a concentration higher than 40 mg/mL was crucial for the growth of large-area NC superlattice films with uniform and controllable thicknesses, as NC superlattice films assembled from diluted NC solutions were prone to be separated into smaller domains (Figure S1). The resultant NC films were thermally treated at 500 oC in Ar for 2 h to carbonize the OA ligands attached at the NC surface. 500 oC was the optimal heating temperature based on our trials, as the OA ligands were not thoroughly carbonized when NC films were heated at 400 oC while NCs tended to sinter and the long-range ordered superlattice structure was no longer maintained when heated at 600 oC (Figure S2). Figure 2a shows a photograph of a carbonized NC film having a lateral size of ~ 1.5 × 1.5 cm2, which was self-assembled from 13 nm Fe3O4 NCs (Figure S3). TGA indicated that the carbon content in carbonized NC films was ~ 7.8% (Figure S4). Low-magnification SEM indicated that the carbonized NC film was uniform in thickness with smooth surfaces without cracks, consisting of a mosaic of superlattice domains with different lattice projections and/or orientations (Figure 2b). HRSEM revealed that individual domains were composed of close-packed NCs with a high degree of NC ordering (Figure 2c). SEM focusing on the edge of NC films confirmed the superlattice structure across the entire film thickness (Figure S5). Typical lateral dimensions of superlattice domains were on the order of a few to several tens of 12


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micrometers (Figure 2b). We speculate that the large domain size, which was attributed to the high monodispersity of Fe3O4 NC building blocks (~ 5% in size distribution, Figure S3), helped to minimize the crack generation during the carbonization process. SAXS was employed to evaluate the degree of NC ordering across large areas. At least seven well-resolved scattering peaks indexed to a face-centered cubic (fcc) symmetry were observable in SAXS patterns (Figure 2d), thus confirming the long-range NC ordering, consistent with SEM observations. The thickness of NC superlattice films, which was determined from the cross-sectional SEM images, could be tuned in the range of 1-20 µm by changing the amount of NC solutions used for self-assembly. Figure 2e shows a representative NC film with a thickness of ~ 5 µm. Note that our attempt to grow NC films with thicknesses below 1 µm was not successful, as discontinuous films with non-uniform thicknesses would be formed upon further decreasing the amount of NC solutions. To grow thinner NC superlattice films, the liquid-air interfacial assembly technique developed by our group35 was employed (Figure 1b). In a typical procedure, an appropriate amount of hexane solution containing Fe3O4 NCs was spread on the surface of DEG confined in a Teflon well. The subsequent hexane evaporation yielded a floating NC superlattice membrane, the thickness of which could be controlled in the range of 0.2 to 1 µm by varying the amount of NC solutions. The NC superlattice membrane was transferred to an Al foil prior to ligand carbonization. Figure 2f shows a carbonized NC superlattice membrane with a thickness of ~ 900 nm, corresponding to ~ 60 layers of close-packed NCs. As was the case of thicker NC films, NC 13



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superlattice membranes were also continuous without apparent cracks over large areas, consisting of superlattice domains with lateral dimensions ranging from several hundred nanometers to a few micrometers (Figure S6). Prior to acid treatment, a thin layer of PMMA (~ 20 nm in thickness) was deposited onto the surface of NC superlattice films as a support layer to preserve the film integrity, as NC films, regardless thickness, will disintegrate into small pieces during the etching process without the pre-deposition of PMMA. Both Fe3O4 NCs and Al foils were etched away during HCl treatment, yielding mesoporous carbon films floating on the water surface (Figure 3a). To enable MGF films, the mesoporous carbon films were transferred to a porcelain combustion boat and were then subjected to graphitization at 1000 oC, during which the amorphous carbon frameworks were converted into few-layer graphene while largely retaining the film morphology and integrity. The resulting MGF films could be manipulated by a tweezer (Figure 3b), in suggestive of their free-standing nature with considerable mechanical strength. Apparently, the thickness of MGF films could be widely tuned by controlling that of NC superlattice films, as shown in Figure 3c and d, in which the two MGF films with a thickness of ~ 2.5 µm and 570 nm were derived from NC films self-assembled at the solid-air and liquid-air interfaces, respectively. HRSEM (Figure 3e) and TEM (Figure 3f) established that MGF films possessed a 3D ordered, interconnected mesoporous structure with a pore size of ~ 10 nm. The slightly reduced pore size as compared with the initial diameter of Fe3O4 NCs could be attributed to the framework contraction caused by heat treatment. Tilted experiments in TEM allowed the observation of MGF 14


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films from different directions and the identification of lattice projections such as (111) and (110) suggested that MGF films possessed the same fcc symmetry inherited from NC superlattice films (Figure 3g and h). Detailed structural and textural features of MGF films were examined using various characterization techniques. HRTEM revealed that the pore walls of MGF films were composed of 3-5 layers of stacking graphene (Figure 4a). The highly graphitic nature was also consistent with Raman spectroscopy (Figure 4b), in which MGF films exhibited the characteristic G and 2D bands of graphene at 1585 and 2696 cm-1, respectively.23 It is noteworthy that the sharp D band at 1352 cm-1, which corresponded to disordered carbon, could be attributed to the structural defects arising from the spherical curvature of the pore walls.32 Raman spectroscopy also reflected the influence of the calcination temperature on the graphitization degree of MGF films. As shown in Figure S7, performing calcination at 800 oC was insufficient to fully graphitize carbon frameworks as indicated by the largely overlapped D and G bands, implying that the transition from amorphous to highly graphitic frameworks primarily occurred at around 1000 oC. It should be mentioned that heat treatment at temperatures over 1200 oC typically led to fractured MGF films, although the ordered mesoporous structure of MGFs could be maintained up to 1600 oC. The sharp scattering peaks in SAXS confirmed the long-range ordered porous structure of MGF films with fcc symmetry (Figure 4c), consistent with TEM observations (Figure 3g and h). The N2 adsorption-desorption isotherms of MGF films exhibited a type-IV curve with a large H2-type hysteresis loop (Figure 4d), characteristic of uniform 15



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cage-like mesopores.36 The Brunauer–Emmett–Teller (BET) surface area and pore volume of MGF films were determined to be 1160 m2/g and 1.8 m3/g, respectively. In accordance with the previously reported results based on 3D graphene frameworks,32 MGF films also exhibited a bimodal porous structure as indicated by the pore size distribution (Figure 4d, inset). The large pores at ~ 10 nm corresponded to the removed Fe3O4 NCs, whereas the small pores in the range of 2-5 nm were ascribed to the interconnected windows between neighboring pores. Taking advantage of the combined advantageous structural and textural features including 2D geometry, highly ordered and interconnected porosity, high surface area, and highly graphitic nature, the MGF thin films obtained were investigated for their potential as electrode materials for supercapacitors. The capacitance performance was evaluated in both aqueous and organic electrolytes. Figure 5a shows the CV curves of MGF films tested in 1 M H2SO4 at various scan rates. All CV curves exhibited a nearly rectangular shape even at a high scan rate of 200 mV/s, indicating a typical electrical double-layer capacitance (EDLC) behavior,29 which was also corroborated by the ideal triangular-shaped galvanostatic charge/discharge curves (Figure S8a). The specific capacitances determined from the discharge curves were 186 and 167 F/g at a current density of 0.5 and 1 A/g, respectively, which were comparable to or higher than those of the previously reported carbonaceous materials, including OMCs,5 graphene sheets,37 carbon nano-onions,38 and carbon nanotubes.39 The capacitive performance of MGF films could be further enhanced by modifying the surface wetting properties through acid treatment with H2SO4 and 16


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HNO3, which rendered the originally hydrophobic graphene frameworks hydrophilic due to the introduction of oxygen-containing groups (i.e., hydroxyl and carboxyl groups). As shown in the FTIR spectra (Figure S9), a much stronger peak corresponding to OH stretching (broad peak beyond 3,000 cm-1) was detected and the characteristic peak of carbonyl (C=O) stretching (~ 1716 cm-1) was observed after the acid treatment, indicating the presence of a large portion of hydroxyl (-OH) and carboxyl (-COOH) groups on the graphene framework surface after acid treatment. As shown in the XPS spectra (Figure S10), the oxygen atomic percentage of MGF films and the acid-treated MGF films was 9.51% and 19.24%, respectively, further confirming an increment in the oxygen-containing groups on the graphene framework after acid treatment. The effect of acid treatment on the surface wetting properties of MGFs could also be reflected by dispersity test (Figure S11). After dispersed in water by ultrasonication, the untreated MGF films tended to refloat on the water surface due to their hydrophobicity, while the acid-treated MGFs films could stay in the suspension for a long time before precipitation, indicative of a greater hydrophilicity than the untreated MGFs films. EIS measurements were conducted to evaluate the resistance of MGF films before and after acid treatment. Both MGF films and the acid-treated MGF films displayed a nearly vertical line at the low frequency region (Figure S12), indicating a good capacitance behavior. The slightly larger high frequency loop and longer Warburg curve (the 45o portion of the curve) exhibited by the acid-treated MGF films than the pristine MGF films (Figure S12, inset) indicated an increased charge-transfer 17



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resistance and diffusion resistance brought by the introduction of oxygen-containing surface groups, which correlated to the inferior electrical conductivity of the acid-treated MGF films (~ 0.9 S/m) compared to that of the pristine MGF films (~ 20 S/m) as revealed by electrical conductivity measurements. Despite the increased charge-transfer resistance and diffusion resistance, the CV curves of the acid-treated MGF films largely retained the rectangular shape (Figure 5b). The additional pair of redox peaks compared with the untreated MGF films was associated with the presence of the oxygen-containing groups. Impressively, the specific capacitances of the acid-treated MGF films were calculated to be 310 and 262 F/g at a current density of 0.5 and 1 A/g, respectively (Figure S8b), which outperformed the untreated MGF films

and

most

3D

graphene

materials

reported

previously,

including

macroporous/mesoporous graphene frameworks,21 mesoporous carbon/graphene composites,22 graphene/activated carbon composites,40 non-stacked reduced graphene oxide,41 and porous graphene spheres.42,43 The enhanced electrochemical performance was due in part to the improved wetting properties, which facilitated the access of electrolyte

ions

to

the

graphene

framework

surface.

Additionally,

the

oxygen-containing groups introduced during acid treatment might also contribute to the improved capacitance by providing additional pseudocapacitance. The specific capacitance of the acid-treated MGF films remained at 203 F/g when the current density was increased to 20 A/g (Figure 5c), indicative of excellent rate capability. Moreover, the acid-treated MGF films exhibited superior long-term cycling stabilities, as indicated by the nearly constant capacitance (98.3%) for up to 20000 cycles at a 18


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current density of 20 A/g (Figure 5d). All these results corroborated the excellent capacitance performance of the acid-treated MGF films in nearly all aspects. To evaluate the electrochemical performance of MGF films in organic electrolytes, further measurements were conducted in 1 M TEABF4/PC in a voltage window of 2.5 V. The nearly rectangular CV curves at various scan rates of 5-200 mV/s were consistent with the EDLC behavior without apparent contribution from pseudocapacitance (Figure 5e). The specific capacitances at a current density of 0.5 and 1 A/g were determined to be 122 and 111 F/g, respectively (Figure S13). Despite the reduced capacitance values as compared with aqueous electrolytes, the energy density of MGF films could reach 34.2 Wh/kg due to the wider operating voltage window, much higher than that achievable in 1 M H2SO4 (Figure 5f). As was the case of aqueous electrolytes, MGF films also exhibited excellent cycling stabilities in organic electrolytes (Figure S14a). Interestingly, the capacitance of MGF films gradually increased and reached ~ 112% of the initial capacitance after 3000 cycles, maintaining ~ 105% of the initial capacitance after 10000 cycles. Presumably, the enhanced capacitance was attributed to the activation of the electrode materials41 and/or the improved ion accessibility during the cycling process.21 The highly ordered mesoporosity of MGF films was well retained after long-term cycling as revealed by ex situ TEM (Figure S14b), which helped to explain their superior cycling stability. As MGF films are expected be highly conductive due to their high graphitization degree, the additional acetylene black (10 wt%) might not be necessary during the preparation of the working electrodes. To address this issue, control experiments 19



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without acetylene black were carried out (Figure S15-S17), in which the working electrodes were prepared by mixing MGF films and PTFE binder with a mass ratio of 9:1. As expected, the capacitive performance of MGF films without acetylene black performed almost identically with those with acetylene black in both aqueous (1 M H2SO4, Figure S15) and organic (1M TEABF4/PC, Figure S16) electrolytes. In contrast, as the electrical conductivity of the acid-treated MGF films was inferior to that of the pristine MGF films (Figure S12), the addition of acetylene black could greatly reduce the charge-transfer resistance of the acid-treated MGF films and accordingly enhance their capacitive performance by approximately 5% in 1 M H2SO4 (Figure S17). To further investigate the effect of NC size on the capacitive performance of the resultant MGF films, we used a series of monodisperse spinel oxide NCs (i.e., 8 nm CoFe2O4 NCs, 13 nm Fe3O4 NCs, and 18 nm Fe3O4 NCs) to derive MGF films with different mesopore sizes (Figure S18). MGF films derived from larger Fe3O4 NCs (13 nm and 18 nm), possessing similar BET surface areas (1160 m2/g and 1164 m2/g), performed almost identically and outperformed MGFs films derived from smaller CoFe2O4 NCs (8 nm) (Figure S19). The major reason was that the synthesis of MGF films from smaller NCs was not as easy to control as that from larger NCs. As smaller NCs possessed a greater surface-to-volume ratio than larger NCs, they were more easily sintered when NC superlattice films were heated at 500 oC, such that the resultant MGF films were partially disordered or collapsed (Figure S18c), which presumably decreased their specific surface areas and suppressed their capacitive 20


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performance. CONCLUSION In summary, we have demonstrated that large-area, free-standing few-layer graphene framework thin films with controllable thicknesses and highly ordered mesopores could be realized by transforming of 2D NC superlattices self-assembled at the solid- or liquid-air interface. The resulting mesoporous thin films were constructed from interconnected few-layer graphene frameworks, possessing a specific surface area as high as 1160 m2/g. When used as the electrode materials for supercapacitors, MGF films exhibited excellent capacitance performance, including high specific capacitance and superior cycling stability in both aqueous and organic electrolytes. The superior electrochemical performance of MGF thin films is attributable to their unique and advantageous structural and textural properties. In particular, the highly ordered mesoporosity combined with the thin thickness of MGF films is expected to be beneficial for the rapid diffusion of electrolyte ions. Considering the gap between the capacitive performance of our MGF films and state-of-the-art values, we are working on creating micropores on the ultrathin pore walls of MGF films while retaining their highly ordered mesoporosity and superior electrical conductivity, which will significantly enhance their specific surface areas and hopefully boost their performance on capacitive energy storage. Ongoing efforts are also being made to fabricate thicker (> 50 µm) MGF films for constructing binder-free and flexible supercapacitors. ASSOCIATED CONTENT 21



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Supporting Information. SEM images, TEM images, TGA scan, Raman spectra,

galvanostatic charge/discharge curves, FTIR spectra, wetting property test, Nyquist plots, and cycling stability. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (A.D.); [email protected] (Y.D.) Notes The authors declare no competing financial interests. ACKNOWLEDGMENTS A.D. acknowledges the financial support from National Basic Research Program of China (973 program: 2014CB845602), Natural National Science Foundation of China (21373052), Shanghai International Science and Technology Cooperation Project (15520720100), and the “1000 Youth Talents” Plan. D.Y. is grateful for financial support from Natural National Science Foundation of China (51103026, 51373035, and 51373040), the Shanghai Scientific and Technological Innovation Project (11JC1400600 and 124119a2400), and International Science and Technology Cooperation Program of China (2014DFE40130).

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Performance Supercapacitors. Adv. Mater. 2013, 25, 4437-4444. 42. Wang, C.; Wang, Y.; Graser, J.; Zhao, R.; Gao, F.; O'Connell, M. J. Solution-Based Carbohydrate Synthesis of Individual Solid, Hollow, and Porous Carbon Nanospheres Using Spray Pyrolysis. ACS Nano 2013, 7, 11156-11165. 43. Lee, J.-S.; Kim, S.-I.; Yoon, J.-C.; Jang, J.-H. Chemical Vapor Deposition of Mesoporous Graphene Nanoballs for Supercapacitor. ACS Nano 2013, 7, 6047-6055.

27



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Figure 1. Schematic illustration of the synthesis of MGF films from 2D Fe3O4 NC superlattices grown by solid-air (a) and liquid-air (b) interfacial assembly, respectively. To enable MGF films, the 2D NC superlattice films resulted from the two self-assembly pathways were subjected to ligand carbonization, acid etching, and framework graphitization.

28


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Figure 2. (a) Photograph of a centimeter-scale Fe3O4 NC superlattice film supported on an Al foil after ligand carbonization. (b) Low-magnification, top-view SEM image of carbonized NC films. The dashed lines indicated the grain boundaries between adjacent superlattice domains. (c) HRSEM image of the region indicated in (b), showing the high degree of NC ordering. (d) Representative SAXS pattern of carbonized Fe3O4 NC films, showing the long-range ordered superlattice structure with an fcc symmetry. (e, f) Cross-sectional SEM images of carbonized Fe3O4 NC superlattice films grown by solid-air and liquid-air interfacial assembly, respectively.

29



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Figure 3. (a) Photograph of a mesoporous carbon film floating on the water surface after acid treatment. (b) Photograph of a free-standing MGF film upon framework graphitization. (c, d) Cross-sectional SEM images of MGF films derived from NC superlattices grown by solid-air and liquid-air interfacial assembly, respectively. (e) Representative top-view HRSEM image of MGF films. (f) Low-magnification TEM image of MGF films viewed from the (111) direction. (g) High-magnification TEM image of the region indicated in (f). (h) TEM image of MGF films viewed along the (100) direction. Insets in (g) and (h) are the corresponding fast Fourier transforms (FFTs).

30


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Figure 4. (a) HRTEM images of MGF films viewed along the (111) direction, showing the pore walls of MGF films were composed of 3-5 layers of stacking graphene. (b) Representative Raman spectrum of MGF films. (c) Typical SAXS pattern of MGF films, showing the long-range ordered porous structure with an fcc symmetry. The SAXS pattern of NC superlattice films was also included for comparison. (d) N2 adsorption-desorption isotherms and the corresponding pore size distribution (inset) of MGF films, respectively.

31



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Figure 5. (a) CV curves of MGF films at different scan rates in 1 M H2SO4. (b) CV curves of the acid-treated MGF films at different scan rates in 1 M H2SO4. (c) Specific capacitance as a function of current densities for the acid-treated MGF films. The capacitance change of MGF films before acid treatment was also included for comparison. (d) Cycling stability of the acid-treated MGF films at a current density of 20 A/g in 1 M H2SO4. (e) CV curves of MGF films at different scan rates in 1 M TEABF4/PC. (f) Energy density at different power density for MGF films in 1 M TEABF4/PC. The Ragone plot of the acid-treated MGF films in 1 M H2SO4 was also included for comparison.

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TOC graphic

33


Free-Standing, Ordered Mesoporous Few-Layer Graphene

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