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A General Approach to One-Pot


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A General Approach to One-Pot Fabrication of Crumpled Graphene-Based Nanohybrids for Energy Applications
Shun Mao,?,§ Zhenhai Wen,?,§ Haejune Kim,? Ganhua Lu,? Patrick

Hurley,? and Junhong Chen?,* Department of Mechanical Engineering, University of Wisconsin;Milwaukee, 3200 North Cramer Street, Milwaukee, Wisconsin 53211, United States and ?Global Technology and Innovation, Power Solutions, Johnson Controls Inc., 5757 North Green Bay Avenue, Milwaukee, Wisconsin 53209, United States. §These authors contributed equally to this work. raphene oxide (GO) is a promising precursor for the creation of carbonbased nanostructures because it can be synthesized in large quantities by oxidizing inexpensive graphite powders and can be reduced through di?erent methods to obtain tailored properties by controlling reduction conditions.1?4 Due to the unique properties and potential applications of GO/ graphene,5?7 there is a great demand in the fabrication and assembly of designed architectures based on GO/graphene patterning, stacking, folding, and scrolling.8?14 It was recently reported that aqueous GO sheets can be converted into crumpled graphene (CG) balls by capillary force associated with rapid solvent loss and physical deformation through an aerosolization process, from which GO is transformed from a twodimensional (2D) sheet to a three-dimensional (3D) structure.15,16 The as-produced CG is of great interest for fundamental scienti?c research and practical technological applications. First of all, the CG structure exhibits signi?cantly enhanced speci?c surface area and possesses excellent stability under redispersion or other material processing treatments. Additionally, the synthetic method is facile and suitable for scalable continuous manufacturing. Finally, the controlled deformation of 2D graphene nanostructures can markedly increase reactive sites and can be further used for tuning the reaction barrier and reaction energetics of graphene, which opens up new opportunities for various applications as was demonstrated in Nature by R. Ruo?.17 The performance of a CG ball could be further enhanced through decoration or modi?cation with various functional nanostructures. Such CG-based nanohybrids are envisaged to synergistically o?er unique properties of both crumpled graphene and
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ABSTRACT

Crumpled graphene oxide (GO)/graphene is a new type of carbon nanostructure that has drawn growing attention due to its three-dimensional open structure and excellent stability in an aqueous solution. Here we report a general and one-step approach to produce crumpled graphene (CG)?nanocrystal hybrids, which are produced by direct aerosolization of a GO suspension mixed with precursor ions. Nanocrystals spontaneously grow from precursor ions and assemble on both external and internal surfaces of CG balls during the solvent evaporation and GO crumpling process. More importantly, CG?nanocrystal hybrids can be directly deposited onto various current-collecting substrates, enabling their tremendous potential for energy applications. As a proof of concept, we demonstrate the use of hybrid electrodes of CG?Mn3O4 and CG?SnO2 in an electrochemical supercapacitor and a lithium-ion battery, respectively. The performance of the resulting capacitor/battery is attractive and outperforms conventional ?at graphene-based hybrid devices. This study provides a new and facile route to fabricating high-performance hybrid CG?nanocrystal electrodes for various energy systems.
KEYWORDS: crumpled graphene . nanocrystal . nanohybrid . aerosolization . electrochemical supercapacitor . lithium-ion battery

functional nanostructures, providing tremendous opportunities for improving and expanding applications.18 For instance, it was reported that cargo-?lled graphene nanosacks could be manufactured by aerosolizing GO suspension and nanoparticle mixtures.19 However, research into CG is still in its infant stage, and methods for controlled functionalization of CG are yet to be developed. In fact, the lack of an e?cient and general synthetic method represents a huge challenge to implement such CGbased hybrids for practical applications.
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* Address correspondence to jhchen@uwm.edu. Received for review June 25, 2012 and accepted July 28, 2012. Published online July 29, 2012 10.1021/nn302818j
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We herein report a general approach for one-step synthesis of nanohybrids of CG decorated with various nanocrystals of precious metals (e.g., Pt, Ag) and metal oxides (e.g., SnO2, Mn3O4). The novelty in the present method is that nanocrystals are locally crystallized and grown in situ on the surface of CG sheets during the solvent evaporation and GO crumpling process, which is quite di?erent from previously reported methods that mix presynthesized nanoparticles with GO suspension before the aerosolization process. In a typical synthesis, precursor ions (e.g., Mn2?, Sn4?, Ag?, and [PtCl6]2?) were dissolved in a GO suspension, which is then aerosolized and dried through a tube furnace by using an ultrasonic nebulizer. CG ball-based hybrids with di?erent nanocrystals can be fabricated during this process. Of more practical signi?cance, the product CG-based nanohybrids can be collected onto various types of substrates, enabling a direct route to fabricate binder-free electrodes for various energy storage and conversion devices, such as supercapacitors, lithium-ion batteries, and solar cells. RESULTS AND DISCUSSION CG?Nanocrystal Hybrids from Aerosolization of GO Suspension Mixed with Precursor Ions. Figure 1 shows the experimental setup for producing CG?nanocrystal hybrids using an aerosolization system. An aqueous dispersion of GO sheets with precursor ions was nebulized to generate aerosol droplets that flowed through a tube furnace preheated at 600?700 °C by an Ar/H2 carrying gas. Because the average size of generated droplets was around 1.7 μm due to the high working frequency of the ultrasonic nebulizer (2.4 MHz), the solvents rapidly evaporated in the tube furnace, leading to the shrinkage of GO sheets and the subsequent compression of GO sheets into crumpled balls with a submicrometer size. Simultaneously, precursor ions adsorbed on the surface of GO sheets could be decomposed or reduced during the GO sheets' evaporation and crumpling process, leading to the loading of nanocrystals on the CG surface. Because of the high temperature, GO sheets were also partially reduced, as indicated by the color change from brown to black. To investigate the morphology of the prepared CG?nanocrystal hybrids, SEM imaging was carried out and results are shown in Figure 2. Without the precursor, bare CG balls can be produced by this method, which is consistent with previous reports.15,16 SEM images (Figure 2a,b) indicate that each of the CG balls is formed by a single piece of GO sheet, and in general, a larger GO sheet led to a CG ball with a larger diameter. The GO used in this study had a wide size distribution ranging from tens of nanometers to several micrometers, and the CG balls show a size distribution from 50 to 600 nm with a peak size at 200 nm (Figure 2c, red curve). On the basis of experimental results, the size of the CG balls was mainly dependent on the size of the initial GO sheets as
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well as the number of layers of the GO or GO thickness. This observation agrees well with previous reports (Supporting Information, Figure S1).16 To create a hybrid nanostructure of GO?nanocrystals, precursor ions were intentionally introduced as the source of nanocrystals, and the original idea of the design was to use the high temperature during the GO evaporation and crumpling to promote the nanocrystal growth on the surface of GO. Because precursor ions were adsorbed on the surface of the GO sheet, the GO sheet worked as a substrate for the localized crystallization or thermal decomposition. The product hybrid structures suggested that nanocrystals successfully grew on the surface of GO sheets during the GO evaporation and crumpling even though the GO ?ight time was as short as 1?2 s in the tube furnace. SEM images (Figure 2e?l) reveal that various nanocrystals, including Mn3O4, SnO2, Ag, and Pt nanocrystals, are uniformly deposited on the surface of CG balls, indicating the local growth of nanoparticles during the GO aerosolization and crumpling process. It should be noted that, at the macroscopic level, the CG?nanocrystal ball surface is smoother than that of the bare CG ball. This change in the surface smoothness of the CG ball can be attributed to the ?lling of nanocrystals in the innerspace of folded CG and the interspace between the folded CG sheet, as con?rmed by TEM images (Figure 3). The CG?nanocrystal hybrids have a wide size distribution from 50 to 500 nm with a peak size at 150 nm (Figure 2c, blue curve). The nanocrystal size ranges are also plotted in Figure 2d. It was demonstrated that the nanocrystal size was variable depending on chemical properties of the ion source, the reaction temperature, and the precursor concentration. On the basis of measurements from SEM and TEM images, the sizes of nanocrystals on CG are 20 ( 10, 3 ( 1, 60 ( 30, and 18 ( 5 nm for Mn3O4, SnO2, Ag, and Pt, respectively. It is proposed that di?erent precursors can result in di?erent reactions, as shown in reactions (1)?(3). For transition metal oxides, the metal ions initially adsorbed on the GO surface via electrostatic interactions, and the subsequent drying and annealing treatments resulted in the formation of transition metal oxides on CG. For precious metals, the metal ions can be reduced or decomposed in situ on the surface of CG. GO ? Ma? f GO=Ma? f G ? Mx Oy (M ? transition metal) GO ? Ag? ? H2 f G ? Ag GO ? [PtCl6 ]2 ? f G ? Pt (1) (2) (3)

For chemical synthesis of inorganic nanocrystals, a surface surfactant is commonly used to control the size and morphology of the nanocrystal. Therefore, to investigate the surfactant function in this aerosolization
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Figure 1. Preparation of CG?nanocrystal hybrids by rapid compression of GO sheets in evaporating aerosol droplets and simultaneous chemical reactions for growth of nanocrystals on the CG surface. Schematic illustration of the experimental setup and the aerosolization/high-temperature-induced GO crumpling and nanocrystal growth process. Figure 2. SEM images of bare CG and CG?nanocrystal hybrids: (a,b) bare CG balls; (e,f) CG?Mn3O4 nanocrystal hybrids; (g,h) CG?Ag nanocrystal hybrids; (i,j) CG?SnO2 nanocrystal hybrids; (k,l) CG?Pt nanocrystal hybrids. (c) Size distributions of bare CG and CG?nanocrystal (NC) hybrids. (d) Measured size ranges of nanocrystals from SEM and TEM images.

process, polyvinylpyrrolidone (PVP) was used in the CG?Pt hybrid synthesis. The product Pt nanocrystals with PVP had a much smaller size as well as narrower size distribution when compared with Pt nanocrystals synthesized without the PVP surface surfactant (Supporting Information, Figure S2). On the basis of results of CG?Pt nanohybrids produced with and without PVP, it is concluded that surfactants can be used to control the size and morphology of nanocrystals in this aerosolization process, similar to its function for chemical reactions in an aqueous phase. To investigate the crystalline structure of nanocrystals and also the interior structure of CG balls, TEM characterization (Figure 3) was performed on bare CG balls, CG?Mn3O4, and CG?SnO2 hybrids. Figure 3a shows a typical CG ball structure, which is a threedimensional structure with a folded graphene sheet. The formation of CG balls is driven by the capillary force
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associated with rapid solvent loss; the physical deformation naturally creates folds on the graphene surface. The TEM image clearly shows the folded structure, evidenced from the interlaced carbon lattices from the HRTEM image (Figure 3b). Figure 3c,d and Figure 3e,f show TEM and HRTEM images of CG?Mn3O4 and CG?SnO2 hybrids, respectively. The images clearly show that both CG?Mn3O4 and CG?SnO2 hybrids have a hollow structure with uniform nanocrystal decoration on both external and internal surfaces of the CG balls, which can be well understood since precursor ions initially adsorbed on both sides of the GO sheets before aerosolization. HRTEM images and SAED patterns con?rm that the Mn3O4 and SnO2 nanocrystals have a good crystalline structure. It was revealed that the lattice spacings of Mn3O4 nanocrystals are 0.49, 0.47, and 0.25 nm, which match well with the lattice spacings of tetragonal Mn3O4(111), (002), and (311), respectively.
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Figure 3. (a,b) TEM and HRTEM images of bare CG balls; (c,d) TEM and HRTEM images of CG?Mn3O4 nanocrystal hybrids; (e,f) TEM and HRTEM images of CG?SnO2 nanocrystal hybrids.

The lattice spacings of SnO2 nanocrystals are around 0.33 nm, which corresponds with the lattice spacing of rutile SnO2(110). On the basis of HRTEM images, the size of the nanocrystals is determined as 20 ( 10 and 3 ( 1 nm for Mn3O4 and SnO2, respectively. To further con?rm the successful synthesis of CG?nanocrystal hybrids, element mapping and XRD measurements were carried out and the results are shown in Supporting Information, Figures S3 and S4. According to the structure characterization of CG?nanocrystal hybrids, it is convincing that the aerosolization of GO suspension mixed with various precursors could serve as a general method to produce CG?nanocrystal hybrids. The present synthetic strategy shows great promise owing to the following advantages. First of all, the as-developed method can be carried out in one step and can be used for fabricating CG-based hybrids by loading various nanocrystals, including metals and metal oxides. Second, the size and morphology of the CG balls could be tuned by the size and thickness of initial GO sheets; and the size and the morphology of nanocrystals could be tuned by various parameters, such as the precursor concentration, aerosolization temperature, as well as the precursor source, which enables a deliberate synthetic design based on requirements of a speci?c application. Third, the CG balls are remarkably stable
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because such a crumpled ball is stabilized by locally folded, π?π stacked ridges as a result of plastic deformation, which can withstand redispersion in an aqueous solvent without unfolding or collapsing the structure.15 Fourth, because the crumpled structure of CG balls bears a higher speci?c surface area than commonly used ?at GO/graphene papers, and a hollow/ open structure facilitates the nanocrystal assembly on both external and internal surfaces of the CG balls, it is believed that this unique hybrid structure can be potentially useful for various applications. Last but not least, such nanohybrids can be directly deposited on the surface of a speci?ed substrate without binder assistance and additional processing steps. For instance, in our preliminary experiments, ITO, copper foil, nickel foam, and carbon cloth were used as substrates to prepare nanohybrid electrodes for supercapacitors and lithium-ion batteries. CG?Mn3O4 Hybrids for an Electrochemical Supercapacitor. As a proof of concept, we first investigated electrochemical properties of CG?Mn3O4 hybrids collected onto an ITO substrate and explored its potential application as the active electrode in electrochemical capacitors (ECs). Depending on the charge storage mechanism as well as the active materials used, ECs can be categorized into three types: electric double-layer capacitors (EDLCs), pseudocapacitors, and hybrid electrochemical capacitors.20 On one hand, EDLCs store charges electrostatically via reversible ion adsorption at the electrode/ electrolyte interface; the crumpled graphene is therefore expected to show attractive EDLC properties due to its high specific surface area. On the other hand, manganese oxides (MnOx) is one of the transition metal oxides that is typically used as active pseudocapacitive materials because of its high theoretical specific capacitance (Csp ? 1370 F 3 g?1).21 By combining these two types of materials, it is anticipated that a high-performance capacitor can be obtained as a result of both EDLC and pseudocapacitor mechanisms. The electrochemical properties of the as-prepared CG?Mn3O4 hybrid were studied by performing cyclic voltammetry (CV) using a three-electrode cell system, as shown in the schematic of Figure 4a. The CG?Mn3O4 hybrid ?lm on an ITO glass, which shows a brown color mainly resulting from the tetragonal Mn3O4 nanocrystals (Figure 4a), was used as the working electrode, and the Pt electrode and Ag/AgCl electrode were used as the counter electrode and the reference electrode, respectively. Figure 4b shows CVs for a CG?Mn3O4 electrode over a range of scan rates of 5?100 mV 3 s?1 with a potential window range of ?0.6?1.2 V versus the Ag/ AgCl reference electrode. The voltammetric curves show two pairs of peaks that correspond to the series of electrochemical redox reactions of Mn ions. The ?rst pair of peaks in the range of ?0.25?0.18 V could be attributed to the electrochemical conversion between the low valence state manganese ions (Mn2? and Mn3?), while
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Figure 4. (a) Schematic of the capacitor test for CG?Mn3O4 nanocrystal hybrid electrode in a 1.0 M Na2SO4 aqueous electrolyte; digital pictures show the ITO glass before and after coating CG?Mn3O4 nanocrystal hybrids. (b) Cyclic voltammograms for CG?Mn3O4 at di?erent scan rates (CV curve for ITO-only at a scan rate of 50 mV 3 s?1 is included for comparison). (c) Galvanostatic charging/discharging curves measured with di?erent current densities for the assembled hybrid EC. The inset curve shows the calculated speci?c capacitance at di?erent current densities. (d) Cycling performance of hybrid ECs at a current density of 20 A 3 g?1.

the coupled peaks appearing at 0.75?1.10 V could be attributed to the redox reaction of high valence state manganese ions (Mn5? and Mn7?). On the basis of anodic and cathodic peaks, it is expected that the manganese ions undergo reversible oxidation and reduction reactions in the electrolyte during CV measurements, and detailed reactions may need further characterizations to indentify.21?25 As the scan rate increases from 5 to 100 mV 3 s?1, the peak current increases accordingly and the shape of the CV curves shows a little distortion accompanied by the shift of the anodic and cathodic peaks' position, which can be ascribed to the relatively slow kinetics of the reaction on the Mn-based oxides.20 It should be noted that, at the same scan rate (50 mV 3 s?1), the ITO-only electrode showed no peak in the scan windows and presented a much lower current density than that of the CG?Mn3O4, indicating that the modi?cation of the high-surface-area CG balls with Mn3O4 did signi?cantly enhance the speci?c capacity of EDLCs and pseudocapacitors. Galvanostatic charge?discharge measurements were also carried out on the CG?Mn3O4 at di?erent current densities, and the results are shown in Figure 4c. It can be seen that the charging curves are basically symmetrical with their corresponding discharge counterparts, and the results are in good agreement with that of the CV characterization. The speci?c capacitance is calculated from the discharge curves (see Supporting Information for more details). At a low current density of 5 A 3 g?1, the speci?c
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capacitance can reach as high as 1027 F 3 g?1, which is superior to or comparable with other high-performance GO/graphene/CNT-MnOx hybrid nanostructure-based chemical capacitors (308?1250 F 3 g?1).20,21,26?29 The inset of Figure 4c shows the speci?c capacitance as a function of current density for the CG?Mn3O4 hybrids. The results indicate that CG?Mn3O4 hybrids not only deliver a high speci?c capacitance but also maintain a good rate capability at a high current density. Actually, a speci?c capacitance of 404 F 3 g?1 (40% retention) was retained at a current density as high as 40 A 3 g?1. The electrochemical stability of the CG?Mn3O4 was also evaluated using the galvanostatic charge?discharge technique at a current density of 20 A g?1. It was demonstrated that, based on the cycling test, the CG?Mn3O4 maintained ?78% capacitance retention over 1000 cycles at a current density of 20 A 3 g?1, which is comparable with those reported in previous work (typically 69?79% retention over 1000 cycles).30,31 The loss of the capacity mainly results from dissolution of Mn3O4 or mechanical stripping of CG?Mn3O4 during ion adsorption/desorption. It should be noted that the cyclability of the crumpled CG?Mn3O4 could be further improved through annealing treatment or adjusting the loading of Mn3O4 on CG. CG?SnO2 Hybrids for Lithium-Ion Battery. As mentioned above, the proposed route can be extended to directly fabricate various electrodes based on CG?nanocrystal hybrids for energy devices, such as lithium-ion batteries (LIBs). Sn-based materials show appealing
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Figure 5. (a) Schematic of the LIB anode test for CG?SnO2 nanocrystal hybrid on a copper foil. The initial two charge (red) and discharge (black) curves at a current density of 50 mA 3 g?1 for (b) CG?SnO2 and (c) ?at graphene?SnO2. (d) Capacity retention of CG?SnO2 and ?at graphene?SnO2 at various current densities.

properties of large capacity as anode materials in LIBs, and graphene-supporting Sn-based composites with various structures have been extensively investigated.32,33 Here, CG?SnO2 nanohybrids were directly deposited on a copper current collector (copper foil) without adding any binder and conductive carbon (Figure 5a). For comparison, the conventional flat graphene?SnO2 nanohybrids were synthesized to fabricate LIB electrodes through minor modifications of a previous route.34 A galvanostatic charge/discharge method was carried out to compare electrochemical properties of CG?SnO2 nanohybrids and flat GO?SnO2 as anodes in LIBs. Figure 5b,c displays the initial two charge?discharge curves for CG?SnO2 and flat GO?SnO2 nanohybrids, respectively. There are two observable plateaus ranging from 1.0 to 0 V in the first discharging process for both electrodes; these plateaus could be attributed to various reactions, including the transformation of SnO2 and Li? to Sn and Li2O, the formation of a series of Li?Sn alloys, and the Li insertion in graphene nanosheets (reactions (4)?(6)). 2x Li? ? 2xe ? ? SnOx f x Li2 O ? Sn y Li? ? ye ? ? Sn T Liy Sn (0 e y e 4:4) z Li? ? ze ? ? G T Liz G (4) (5) (6)

Obviously, there is a remarkable change between the first and the second discharge curve for the flat GO?SnO2, indicating a large capacity loss in the initial two discharging processes. Such a result was somehow consistent
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with previous Sn-based anode materials ascribed to the irreversible reaction, volume expansion or pulverization, and the formation of a solid electrolyte interface (SEI) layer. Notably, the CG?SnO2 nanohybrids display a much smaller change, signifying a much smaller capacity loss in the initial two cycles. Figure 5d compares the cycling performance and the rate capability of CG?SnO2 nanohybrids and conventional flat graphene?SnO2. Interestingly, the CG?SnO2 nanohybrids exhibit a remarkable improvement in various performance parameters, including the specific capacity, rate capability, and stability. At first, the CG?SnO2 nanohybrids display a capacity of 925.0 mAh 3 g?1 at a current density of 50 mA 3 g?1 in the first discharge process, which is much higher than that of flat graphene?SnO2 (828.2 mAh 3 g?1). Additionally, the initial capacity loss is about 15.2% in the CG?SnO2 nanohybrids, while the flat graphene?SnO2 exhibits a distinct fading of discharge capacity in the initial two cycles (around 33.5% loss). The reversible capacity decreased slowly with increasing current density. Even at a high rate of 200 mA 3 g?1, CG?SnO2 nanohybrids still delivered a specific capacity of about 380 mAh 3 g?1, with a strong contrast to the flat graphene?SnO2 that only has a corresponding capacity of 185 mAh 3 g?1. Although the discharge capacity gradually decreased in the subsequent cycling, the CG?SnO2 nanohybrids discharged a capacity of 545.96 mAh 3 g?1 and maintained this capacity in the following cycles when the current density was switched back to 50 mA 3 g?1. However, it only delivered a capacity of 417.7 mAh 3 g?1 after 60 cycles for the flat graphene?SnO2. Actually, the as-developed CG?SnO2
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nanohybrids also outperformed other graphene/CNT SnO2-based LIB anodes.35,36 The stable open structure in CG?SnO2 nanohybrids could be a key factor responsible for the improved performance because such a unique nanostructure can greatly alleviate the volume change issue associated with SnO2 anodes. Furthermore, several favorable properties of CG, such as good electrical conductivity and excellent mechanical properties, also can contribute to the enhanced performance. There are several advantages arising from the GO aerosolization method to fabricate CG?nanocrystal electrodes for energy storage and conversion systems. At ?rst, the electrode fabrication is completed in one single step, greatly decreasing the burden of electrode preparation. Second, the amount of active materials can be readily controlled through adjusting the concentration and aerosolization time of the suspension mixture, which provides a reliable strategy to optimize the corresponding performance for speci?c applications. Third, CG balls have an excellent stable structure in aqueous electrolyte and high accessible surface areas, which could be bene?cial for rapid mass transfer of ions due to their open structure. Fourth, in this route, there are no binder and additional conductive agents needed to prepare the electrode. Moreover, because of the nature of the materials, the aerosolization method could be applied to assemble CG?nanocrystal hybrids onto various current colleting substrates, such as ITO glass, Cu foil, Ni foam, and carbon cloth (Figure S5, Supporting Information). As proved in our preliminary experiments, the CG?Mn3O4 and CG?SnO2 hybrids showed excellent performance in applications of electrochemical supercapacitors and lithium-ion batteries. It is believed that the demonstrated devices with CG?nanocrystal hybrids can be further optimized by tuning the reduction level of GO, the size and distribution of the nanocrystals, and the loading of nanocrystals. This novel aerosolization method is also promising for large-scale fabrication of electrodes using graphene?

nanocrystal hybrids since it is a continuous manufacturing method and the material synthesis and assembly could be completed in one step. It should be noted that many types of inorganic nanomaterials could be prepared using this method and subsequently used for a broad range of applications including batteries, capacitors, catalytic systems, and sensors. For instance, CG?Pt hybrids could be used as a catalyst in fuel cell systems to catalyze methanol and formaldehyde oxidation reactions or oxygen reduction reactions, while CG?Ag hybrids are potentially useful in gas sensors. In addition, this method is expected to be extendable to GO/graphene?polymer hybrids with appropriate choice of precursor organics based on solubility and the drying temperature (organics can easily burn at high temperatures). A previous study showed that the CG structure could be achieved at low temperatures and even at room temperature.19 Therefore, there is a huge potential in producing various crumpled graphene-based hybrids with di?erent functionalities using this method, which o?ers another promising direction to develop graphene-based functional devices. CONCLUSIONS In summary, we have developed a general method to produce CG?nanocrystal hybrids through the aerosolization of GO suspension and precursor mixtures. Various nanocrystals including Mn3O4, SnO2, Ag, and Pt were synthesized and decorated on the surface of CG balls during GO sheets evaporation and crumpling process. This method represents a general approach to one-pot fabrication of CG-based nanohybrid electrodes. As a demonstration, CG?Mn3O4 and CG?SnO2 hybrid electrodes are fabricated and showed excellent performance in electrochemical supercapacitors and lithium-ion batteries. The method reported here o?ers huge opportunities in fabrication and applications of hybrid graphene?nanocrystal structures and opens up a new avenue for designing crumpled GO/graphenebased functional devices.

METHODS
Synthesis of CG?Nanocrystal Hybrids. Two types of GO were used in this study: monolayer GO ordered from Nanoinnova Technologies SL and multilayer GO produced from natural graphite powder using the modified Hummers method reported previously.37 The monolayer GO sheet has a smaller lateral size ranging from tens of nanometers to two or three micrometers, and the multilayer GO has a larger lateral size in the range of tens of nanometers to more than ten micrometers. Monolayer GO was used in the hybrid nanostructure synthesis, and the multilayer GO was used as a comparison to investigate the difference in the resulting CG structure under the same experimental parameters. The GO suspension (0.2 mg GO/1 mL DI water) was prepared by dispersing the GO powder in DI water with the aid of ultrasonication. Mn(NO3)2 3 4H2O (8.0 mM), SnCl4 3 5H2O (8.0 mM), AgNO3 (8.0 mM), and H2PtCl6 (5.0 mM) were added into the GO suspension as precursors for the synthesis of CG?nanocrystal hybrids. To control the morphology and size of the Pt nanocrystal, polyvinylpyrrolidone (PVP) (2.0 g 3 L?1) was added and used as

the surfactant. The mixed solutions were nebulized by an ultrasonic nebulizer (2.4 MHz, 241T, Sonaer) to form aerosol particles, which were carried by Ar/H2 gas at a flow rate of 0.5 L 3 min?1 through a horizontal tube furnace preheated at a desired temperature (600?700 °C). An Ar and H2 mixture (9:1) was used as the carrier gas for GO and AgNO3; pure Ar gas was used for the other mixtures. The GO sheets were quickly dried and crumpled during flight in the tube furnace, and thermal decomposition or chemical reduction of precursors led to the formation of nanocrystals on the surface of GO sheets and resulted in a hybrid CG?nanocrystal structure. Si wafer, ITO glass (1 ? 1 in.), Cu foil (0.25 in. diameter disk), Ni foam, and carbon cloth were used as substrates to collect the aerosol hybrid nanomaterials downstream of the tube. The typical collection time was 1?2 h. For CG?Mn3O4 and CG?SnO2 hybrids, thermal annealing was carried out at 350 °C in Ar gas at a flow rate of 2 L 3 min?1 for 1 h to improve the crystallinity of nanocrystals. CG?Nanocrystal Hybrid Characterization. The morphology of asprepared samples, the selected area electron diffraction (SAED)

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pattern, the energy-dispersive X-ray spectrum (EDS), and the elemental mapping were obtained using a Hitachi (H 9000 NAR) transmission electron microscope (TEM) and a Hitachi (S-4800) scanning electron microscope (SEM) equipped with an energydispersive spectroscopy analyzer. The size distributions of CG hybrids and nanocrystals were obtained by measuring and counting each hybrid structure in SEM and TEM images. Powder X-ray diffraction (XRD) was performed on a Scintag XDS 2000 X-ray powder diffractometer with monochromatized Cu KR radiation (λ = 1.5418 ?); the data were collected between scattering angles (2θ) of 10 and 70°. Electrochemical Supercapacitor and Lithium-Ion Battery Measurements. The electrochemical characterization was carried out using a CHI 600 electrochemical workstation (CHI Inc., USA). The threeelectrode cell consisted of a Ag/AgCl electrode as the reference electrode, Pt as the counter-electrode, and the CG?Mn3O4 on ITO glass as the working electrode. A 1.0 M Na2SO4 solution served as the electrolyte in the three-electrode cell at room temperature. Cyclic voltammograms (CV) were carried out and recorded from ?0.6 to 1.2 V at a scan rate of 5?100 mV 3 s?1. Galvanostatic charge?discharge measurements were performed with constant current densities from 5 to 50 A 3 g?1. Coin cells were assembled in an argon-filled glovebox for a lithium-ion battery. Lithium pellets were used for both the counter and the reference electrodes, and the CG?SnO2 collected on a copper foil was directly used as a working electrode. The electrolyte solution was 1.0 M LiPF6 dissolved in a mixture of ethylene carbonate (EC), dimethyl carbonate (DMC), and diethyl carbonate (DEC) with a volume ratio of EC/DMC/DEC = 1:1:1. Discharge?charge curves were recorded from 2.0 to 0.01 V at various current densities. Galvanostatic cycling experiments were conducted using a Land Battery Tester. Con?ict of Interest: The authors declare no competing ?nancial interest. Acknowledgment. Financial support for this work was provided by the U.S. National Science Foundation (CMMI-0900509) and the U.S. Department of Energy (DE-EE0003208). The authors thank Prof. M. Gajdardziska-Josifovska for TEM access at the HRTEM Laboratory of UWM, Dr. H. A. Owen for technical support with SEM analyses, and Dr. S. E. Hardcastle for technical support with XRD analyses. The SEM imaging was conducted at the Electron Microscope Laboratory of UWM. The XRD was conducted at the Advanced Analysis Facility of UWM. Supporting Information Available: Characterization of CG produced from di?erent GO sources, characterization of CG?Pt hybrids produced with and without PVP, element mapping data of CG?nanocrystal hybrids, XRD data of CG?Mn3O4 hybrids, and characterization of CG deposited on various substrates. This material is available free of charge via the Internet at http://pubs.acs.org.

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