Exfoliation of Graphite Oxide in Water without Sonication: Bridging

Apr 29, 2014 - Dispersed graphene oxide (GO) nanosheets in water were synthesized via a new approach involving sonication-free exfoliation, as promote...
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Exfoliation of Graphite Oxide in Water without Sonication: Bridging Length Scales from Nanosheets to Macroscopic Materials Isao Ogino,* Yuya Yokoyama, Shinichiro Iwamura, and Shin R. Mukai Division of Chemical Process Engineering, Graduate School of Engineering, Hokkaido University, N13W8, Kita-ku, Sapporo 060-8628, Japan S Supporting Information *

ABSTRACT: Dispersed graphene oxide (GO) nanosheets in water were synthesized via a new approach involving sonicationfree exfoliation, as promoted by repetitive simple freeze−thaw cycles. These cycles consist of rapid freezing of an aqueous solution containing graphite oxide and subsequent thawing of the resultant solid. This approach is effective for exfoliation of graphite oxide and yields approximately 80% GO after six repetitive freeze−thaw cycles. The GO synthesized by the new method experiences minimal fragmentation during the exfoliation process and has a lateral size at least 3-fold larger than that of GO prepared by using sonication, as evidenced by atomic force microscopy and dynamic light scattering. We also demonstrate use of the resulting exfoliated GO that is synthesized using this approach as a building block for the synthesis of a low-density (≈0.02 g cm−3) macroporous monolithic material, using directional freezing, which forms ice rods within the solution that served as the template to direct the assembly of the solution-dispersed GO nanosheets into a honeycomb-like morphology with a 10 μm macropore opening and sheet-like walls. Such a unique morphology of the synthesized monolith has broad applicability for advanced functional materials that allow extremely high throughput with minimal pressure drop as well as electronic and energy storage materials.

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INTRODUCTION Exfoliation (delamination) of layered materials and reassembly of the resultant nanosheets into designed architectures with new functionality make up a promising route for obtaining advanced materials for a broad variety of applications.1−11 Material synthesis via this route aims to bridge unique properties of nanosheets with macroscopic functionality of the reassembled materials in which mesoscale science clearly plays an important role. Examples of novel materials synthesized via the exfoliation−reassembly route include thin film type energy storage materials composed of randomly stacked graphene-like nanosheets,2,3 separation media fabricated with graphene oxide (GO) or zeolite nanosheets,4,5 and high-performance catalysts composed of zeolite nanosheets.8−11 Generally, exfoliation of layered materials is accomplished by treating the materials in agent(s) that can stabilize the surface energy12 and often requires costly surfactants for this purpose. To achieve efficient exfoliation of layered materials, sonication is often also employed.13,14 However, sonication tends to damage and fragment nanosheets,13 which is undesirable from the standpoint of preserving inherent properties in the final assembled nanosheet material. In this work, we report a new sonication-free exfoliation method for the direct synthesis of GO from graphite oxide. We also demonstrate reassembly of the resultant GO into a macroporous monolith having a honeycomb-like morphology. The uniqueness of our method is that it uses only a phase change of water to achieve efficient exfoliation as well as controlled reassembly of the resultant GO. © XXXX American Chemical Society

GO has attracted significant attention in various applications such as catalysts,15 catalyst supports16 and composite materials,17,18 separation media,4 and energy storage materials.2,3 GO is typically synthesized by oxidizing graphite by using either Hummers’ method or Brodie’s method,19,20 followed by subsequent exfoliation of the resultant graphite oxide in polar solvents such as water.21 After oxidation, graphite is functionalized with various oxygen-containing groups such as epoxide and hydroxyl groups on the basal plane and carbonyl and carboxyl groups at the edge.22,23 Consequently, the graphite oxide becomes highly hydrophilic and readily accommodates polar molecules such as water and alcohols into the interlayer region. When graphite oxide is just soaked in water, layer exfoliation occurs extremely slowly. Sonication has often been used to accelerate the exfoliation process.13 However, as in the case of exfoliation of other types of layered materials, sonication fragments graphite oxide layers into graphene sheets with a relatively smaller lateral size and creates grain boundaries. Thus, if the goal is to minimize fragmentation and prepare relatively large GO, which is suitable for the preparation of monolithic materials because of minimal grain boundaries, a new milder exfoliation method is required. Our graphite oxide exfoliation approach is shown in Scheme 1 and involves a rapid phase change of water to ice and thawing. Received: April 11, 2014 Revised: April 28, 2014

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Scheme 1. Exfoliation of Graphite Oxide via Rapid Freezing of Hydrated Graphite Oxide Flakes in Water and Subsequent Thawing of the Resultant Solida

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Rapid freezing and subsequent thawing cycles are hypothesized to cause fast structural breathing (rapid changes in the interlayer spacing between layers), which leads to the efficient exfoliation of graphite oxide layers and the formation of dispersed nanosheets.

Scheme 2. Synthesis of a Honeycomb Type Monolith by the Directional Freezing of an Aqueous Solution Containing Dispersed Nanosheets, Followed by Freeze-Drying of the Resultant Solida

The directional freezing forms ice rods that are approximately 10 μm in diameter within the solution, which serves as the template to form a honeycomb-like morphology. Nanosheets initially dispersed in the solution are reassembled around the ice rods, which ultimately form sheet-like walls.

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This approach effectively exfoliates the layered structure of graphite oxide into GO that is dispersed in water. Talyzin et al. reported stepwise contraction of (00L) d spacing of a graphite oxide in water by approximately 25% while solvent water was cooled from room temperature to 230 K at a rate of 40 °C/h.24 They also reported that stepwise contraction was observed at different temperatures when bulk water was solidified for slower cooling rates. They attributed the stepwise contraction to the partial withdrawal of water molecules from the interlayer region of their graphite oxide. When the frozen sample was thawed, the original interlayer distance was restored. The contraction and expansion behavior is reversible under their experimental conditions. Although this phenomenon was reported to occur only for the graphite oxide prepared by Brodie’s method,24 we hypothesized that a similar phenomenon (changes in the number of water molecules between layers upon the phase change of the bulk water) occurs for graphite oxide prepared by Hummers’ method. Moreover, we hypothesized that a much faster phase change (≫40 °C/h) of bulk water could sufficiently perturb the hydrated graphite oxide irreversibly, leading to the efficient exfoliation of graphite oxide (Scheme 1). In other words, we aimed to achieve the efficient exfoliation by transforming the reversible structural breathing process into an irreversible exfoliation process via a rapid phase change. Because of the ease with which Hummers’ method can be safely practiced, we chose it to oxidize graphite in this work. Our multiple characterization methods demonstrate that fast

freezing and subsequent thawing readily exfoliate graphite oxide and form GO, while virtually retaining the original size of the graphene sheets. We also demonstrate that directional freezing of the resultant solution reassembles the nanosheets into a three-dimensional lightweight monolith that has a honeycomb-like morphology (Scheme 2). The preparation of honeycomb-like monoliths by directional freezing of precursor hydrogels has been reported for various materials such as silica,25−27 polymers,28 and carbons.29−35 Directional freezing of the precursor hydrogels under controlled conditions leads to the formation of ice rods in the micrometer range within the hydrogels, which acts as a template for forming straight macropores (Scheme 2). Straight macropores in these monoliths are connected with micro- and/ or mesopores present within the honeycomb walls. The straight macropores cause low resistance to fluid flow compared to that of a packed column of particles as demonstrated by our previous work.30 Thus, these monoliths have prospects in applications dealing with exfoliation where high throughput in flow is essential. In this work, we demonstrate that the GO prepared by the new method can be used as the building unit to form a honeycomb-like monolith.

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RESULTS AND DISCUSSION Exfoliation of Graphite Oxide. When a propylene tube containing the prepared graphite oxide in water (Figure 1A) was gently shaken and then the mixture frozen rapidly by

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shoulder at approximately 300 nm can be assigned to π → π* transitions of aromatic C−C bonds and n → π* transitions of CO bonds, respectively,21 which indicates an increased concentration of dispersed GO. We found that the GO yield reaches approximately 80% after six repeated freeze−thaw cycles. In comparison, a solution containing graphite oxide from the same batch was sonicated for 5 min and centrifuged. This sonication−centrifugation cycle was also repeated a total of six times as had been done for the freeze−thaw cycle. An aliquot of the supernatant solution after each cycle was diluted with distilled water by the same dilution factor and characterized by UV−vis spectroscopy. The result shows an increase in absorbance similar to that obtained for the sample obtained by the freeze−thaw method (Figure S2 of the Supporting Information). These comparisons suggest that the new approach exfoliates graphite oxide to a degree similar to that seen with sonication. Atomic Force Microscopy (AFM) Characterization of GO. To obtain direct proof of graphite oxide exfoliation by the new approach, a drop of the supernatant solution collected after six cycles of the freeze−thaw−centrifugation process was dispersed and dried on a mica substrate and characterized by AFM. Figure 3 shows an AFM image characterizing the

Figure 1. Photographs of various stages of exfoliation of graphite oxide: (A) graphite oxide soaked in water, (B) after the mixture shown in panel A had been frozen, (C) after the frozen mixture in panel B had been thawed and subsequently centrifuged, and (D−F) after the freeze−thaw−centrifugation cycle had been repeated two, three, and six times, respectively, showing gradually darker supernatant solutions.

soaking the tube in a liquid nitrogen bath, a brown solid was formed (Figure 1B). After this solid had been placed in a thermostat bath and subsequently centrifuged, the supernatant solution became slightly brown (Figure 1C), suggesting that part of the graphite oxide was exfoliated and dispersed in the water. When this freeze−thaw−centrifugation cycle was repeated a total of six times, the supernatant solution became gradually darker (panels D−F of Figure 1 for solutions after two, three, and six cycles, respectively), indicating further exfoliation of graphite oxide layers and an increased concentration of dispersed GO. On the other hand, when graphite oxide was simply soaked in water for 9 days, the supernatant solution remained faint brown (Figure S1 of the Supporting Information), suggesting that exfoliation did not proceed efficiently. UV−Vis Spectroscopy of Dispersed GO. To obtain quantitative information about the extent of exfoliation after each freeze−thaw cycle, the slurry was centrifuged and an aliquot of the resultant supernatant solution was diluted by a factor of 100 and characterized by UV−vis spectroscopy. The result shows increases in the absorbance for peaks centered at approximately 231 and 300 nm as the freeze−thaw cycle was repeated (Figure 2). The peak at 231 nm and the

Figure 3. AFM image (top) and corresponding height profile (bottom) characterizing GO prepared by the freeze−thaw method. The sample was obtained from the supernatant solution collected after six freeze−thaw−centrifugation cycles.

material obtained after this freeze−thaw process. The data show nanosheets approximately 4 μm in lateral size and 1 nm in thickness. Although a graphene sheet has a van der Waals thickness of ≈0.34 nm, a graphene oxide sheet is thicker because of the presence of surface oxygen functional groups as well as some displacement of sp3-hybridized carbon atoms above and below the graphene layer.22,23,36,37 Thus, we infer that these sheets consist of a single layer of oxidized graphene, which indicates the successful exfoliation of graphite oxide by the new method. The AFM data also show that the lateral sizes of the single sheets appear to be similar to the plate sizes of the original graphite flakes that were characterized by scanning electron microscopy (SEM) (Figure S3 of the Supporting

Figure 2. UV−vis spectra characterizing the supernatant solutions collected before (black) and after each freeze−thaw−centrifugation cycle: navy for one cycle, green for two cycles, brown for three cycles, magenta for four cycles, blue for five cycles, and red for six cycles. Each solution was diluted with distilled water by a factor of 100. The dotted lines at 231 and 300 nm can be assigned to π → π* transitions of aromatic C−C bonds and n → π* transitions of CO bonds of dispersed GO. The arrow indicates the progress of exfoliation of graphite oxide. C

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Information). Thus, the data indicate that these GO sheets had simply been peeled off from the graphite oxide particles with minimal fragmentation. On the other hand, the AFM data characterizing the GO prepared using sonication show much smaller sheets (Figure S4 of the Supporting Information). Measurement of Nanosheet Size by DLS. To further support the AFM data, the average nanosheet size of the GO was determined by dynamic light scattering (DLS). Although particle size determination by DLS for nonspherical nanoparticles such as nanosheets poses some complications, it has been reported that DLS data have a well-defined correlation with nanosheet sizes as measured via transmission electron microscopy (TEM). Coleman et al. used DLS and TEM to determine average nanosheet sizes for a series of nanosheets, such as graphene and metal sulfides, and have reported a correlation between the peak of the particle size distribution (aDLS in nanometers) obtained by DLS and the lateral nanosheet size (⟨L⟩ in nanometers) determined directly by TEM.38 We used their correlation (aDLS = 5.9⟨L⟩0.66) to calculate the average nanosheet size of the GO obtained by the new exfoliation method. For comparison, we performed exfoliation of the same batch of graphite oxide using sonication and determined the average nanosheet size by the same method. The DLS data (Figure 4) show GO nanosheets

Figure 5. SEM images characterizing the cross-sectional area of the monolith synthesized via the directional freezing of an aqueous solution containing the dispersed GO. The insets show the side view of aligned macropores surrounded by sheet-like walls and a photograph of the monolith.

area of the monolith (Figure 5) show that the synthesized monolith has a honeycomb-like morphology with a 10 μm pore opening. These macropores are nearly straight and surrounded by sheetlike walls that were formed by assembly of the GO. The monolith can be reduced by removing oxygen-containing functional groups present on its surface via thermal treatment at 1273 K in a flow of dry nitrogen. The resultant material retains the original honeycomb structure (Figure S5 of the Supporting Information) but shows a black color and an electronic conductivity (5 S m−1) 6-fold higher than that found before the heat treatment (6 × 10−6 S m−1), consistent with the formation of graphene-like sheets via the removal of a large fraction of oxygen functional groups. 39 Like 3D carbon aerogels synthesized using GO or graphene as the building block,6,40,41 our GO monolith has future prospects for applications in energy storage, electronics, catalysis, etc.

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Figure 4. Particle size distributions characterizing GO in water: red line for the freeze−thaw method and blue line for sonication.

CONCLUSIONS Graphite oxide layers can be efficiently exfoliated in water to form dispersed GO by a new sonication-free approach. This approach involves multiple cycles of rapid freezing of an aqueous solution containing graphite oxide flakes and subsequent thawing of the resultant solid. The simple method can achieve a high GO yield (≈80% after six cycles of the freeze−thaw process) and virtually retains the original nanosheet size. The GO synthesized by the new exfoliation approach can be successfully reconstructed into a macroporous monolith with honeycomb-like morphology via the directional freezing of a GO-containing aqueous solution. Such a monolith has nearly straight macropores that are surrounded by sheet-like walls derived from the reassembly of nanosheets. The simple, effective, and scalable exfoliation method has the potential to be applied to the large scale production of GO. The synthesis strategy for a new material through similar exfoliation and reassembly using the phase change of water may be general and applicable to other types of layered materials, which exhibit suitable interfacial interactions with water.

obtained by the new method that were much larger than those obtained by sonication (at least 3-fold larger). Using the correlation, we found that the GO obtained by our approach has an average lateral size of 3.6 μm, which is similar to the nominal particle size of 4 μm reported for the original graphite used in this work. Thus, the DLS data support the AFM data, showing that the new method is significantly milder than the conventional sonication method. In summary, our new exfoliation approach is simple and is as efficient as the conventional sonication method, but significantly milder than the sonication method. Preparation of a Three-Dimensional (3D) Monolith from GO by Directional Freezing. We also reconstructed the exfoliated nanosheets into a 3D carbon monolithic macroscopic material, using ice crystals as a template. Our goal was to demonstrate that exfoliated GO synthesized by our freeze−thaw approach could be successfully reassembled into a new macroporous material. A solution containing GO in a polypropylene tube was frozen unidirectionally and subsequently dried. The obtained monolith exhibits a dark gold color (Figure 5, inset) and has a low density (approximately 0.02 g cm−3) consistent with the successful exfoliation of graphite oxide by the new approach. SEM images of the cross-sectional

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EXPERIMENTAL SECTION

Synthesis of Graphite Oxide. Oxidation of graphite flakes was achieved by Hummers’ method.19 Typically, 0.30 g of NaNO3 (Wako Pure Chemical Industries Ltd., 98%) and 0.60 g of natural graphite (Ito Graphite Industries, z-5F) were charged to a 100 mL Erlenmeyer D

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flask with a Teflon-coated stir bar with a length of 30 mm. To this mixture was added 13.8 mL of H2SO4 (Wako Pure Chemical Industries Ltd., 95%), and the mixture was cooled to 273 K using an ice−water bath. To the slurry was slowly added 1.8 g of KMnO4 (Wako Pure Chemical Industries Ltd., 99.3%) to keep the temperature of the mixture below 293 K. After the addition of KMnO4, the temperature of the mixture was increased to 308 K in a thermostated bath and held for 30 min while the mixture was vigorously stirred. Deionized water (28 mL) was added dropwise to this mixture, and the temperature of the mixture was increased to 371 K and held for 30 min. The mixture was cooled using an ice−water bath for 10 min. Then, 0.60 mL of 30% H2O2 (Wako Pure Chemical Industries Ltd.) and 84 mL of deionized water were added to the mixture. After cooling at ambient temperature, the mixture was centrifuged at 3000 rpm for 5 min, and the supernatant solution was discarded. The remaining solid was washed with approximately 170 mL of deionized water a total of five times. The solid was collected by centrifugation and dried in vacuum at ambient temperature overnight. The graphite flakes used in this work have a nominal particle size of 4 μm according to the supplier’s data. SEM image characterization of the graphite shows particles 4 μm in diameter in Figure S3 of the Supporting Information. The PXRD pattern characterizing the graphite (Figure S6 of the Supporting Information) shows a sharp diffraction peak at a d spacing of 3.4 Å, consistent with the interlayer spacing of graphite. After oxidation of these graphite flakes by Hummers’ method, the resultant material exhibits a lighter color, consistent with loss of electronic conjugation upon oxidation. PXRD characterization of the oxidized material (Figure S6 of the Supporting Information) shows the disappearance of the peak at a d spacing of 3.4 Å and the appearance of a new broad peak at a lower 2θ (d spacing of 7.6 Å), indicating expansion of the interlayer distance. This d spacing matches a typical value for the interlayer spacing of graphite oxide,29 showing the successful oxidation of the graphite flakes. Exfoliation of Graphite Oxide To Synthesize Graphene Oxide (GO) by the Freeze−Thaw Method. Oxidized graphite was gently ground using a mortar and pestle. To a propylene tube (13 mm inside diameter, 128 mm length) were charged 0.01 g of the ground graphite oxide and 5 mL of deionized water. The tube was soaked in a liquid nitrogen bath and left there for approximately 30 s until the water froze. After the mixture was completely frozen, the tube was soaked in a thermostated bath set at 333 K for 10−20 min to thaw the solid. Then, the mixture was centrifuged at 3000 rpm for 5 min, and an aliquot of the supernatant solution was withdrawn for UV−vis characterization. This freeze−thaw cycle was repeated several times. In total, this exfoliation process takes