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Controlled 3D Assembly of Graphene Sheets to Build Conductive, Chemically Selective and Shape-Responsive Materials

Steven J. Woltornist, Deepthi Varghese, Daniel Massucci, Zhen Cao, Andrey V. Dobrynin,* and Douglas H. Adamson*

Dr. S. J. Woltornist, D. Varghese, D. Massucci, Prof. D. H. AdamsonDepartment of Chemistry and Institute of Materials Science Polymer ProgramUniversity of ConnecticutStorrs, CT 06269, USAE-mail: adamson@uconn.eduZ. Cao, Prof. A. V. DobryninDepartment of Polymer ScienceUniversity of AkronAkron, OH 44573, USAE-mail: adobrynin@akron.edu

DOI: 10.1002/adma.201604947

(see Figure 1A,B). With the correct balance of solvents and graphite, this dispersion, or emulsion, attains a densely-packed-spheres configuration, with graphitic shells coming in contact with one another (see Figure 1A), creating a percolating net-work of graphene sheets, spanning the entire sample.

Choosing a monomer as the oil phase, such graphene-stabilized emulsions, pro-vides a scaffold for templated polymeri-zation. Polymerization of the continuous phase that fills the voids between the densely-packed, graphene-coated water droplets (butyl acrylate in the investiga-tions presented here) fixes the emul-sion structure, with the graphene sheets forming a shell-like layer at the polymer/water interface. As a shell of overlapping graphene sheets surrounds each spherical water droplet, the graphene sheets touch

at the point where the spheres touch. This results in a con-tinuous, percolating network of overlapping graphene sheets throughout the material. Removal of water produces a polymer/graphene-composite foam material, with overlapping graphene sheets covering the surface of the empty (formerly water filled) spherical cavities, as shown schematically in Figure 1C. This composite foam has selective solvent absorption properties and displays electrical conductivity that is sensitive to deformation. A unique feature of this foam is that the amount of graphene required to produce this foam and achieve percolation is found experimentally to be less than 0.15% by mass. Furthermore, the graphitic shells are adaptive and can reversibly deform through sliding of the individual graphene sheets. Potential applications of such foams include electrochemical sensing, catalyst sup-port, and capacitive deionization.

Previous reports of graphene foams or aerogels do not show the unique combination of solvent selectivity, electrical prop-erties, and low cost that we report here. The materials most closely resembling our foams are made by CVD growth of graphene on sacrificial metal surfaces.[13,14] Foams produced by this method are available commercially from Graphene Supermarket, headquartered in Calverton, NY, and are adver-tised to sell for $250 per 13 mg of material. A related mate-rial, produced from graphene oxide, sells for $175 per 0.5 g (see the Supporting Information for details). These materials have

Exfoliating graphite into its component graphene layers requires work against thermodynamics. However, the affinity of graphene to high-energy oil/water interfaces provides a path for exfoliation, as the spreading of graphene sheets at the inter-face lowers the total free energy of the system.[1] This approach overcomes graphenes lack of solubility, while avoiding the use of graphene oxide (GO),[2–5] difficult to remove solvents,[6] extended sonication times,[7] or the chemical reduction treat-ments required for reduced graphene oxide (rGO).[8–10] Tap-ping into graphene/graphites surfactant-like properties,[1,11,12] a dispersion of water droplets, each coated with overlapping graphene sheets (graphitic shell), is created in the oil phase

Driven by the surface activity of graphene, electrically conductive elastomeric foams have been synthesized by the controlled reassembly of graphene sheets; from their initial stacked morphology, as found in graphite, to a percolating network of exfoliated sheets, defining hollow spheres. This network creates a template for the formation of composite foams, whose swelling behavior is sensitive to the composition of the solvent, and whose electrical resistance is sensitive to physical deformation. The self-assembly of graphene sheets is driven thermodynamically, as graphite is found to act as a 2D surfactant and is spread at high-energy interfaces. This spreading, or exfoliation, of graphite at an oil/water interface stabilizes water-in-oil emulsions, without the need for added surfactants or chemical modification of the graphene. Using a monomer such as butyl acrylate for the emulsion’s oil phase, elastomeric foams are cre-ated by polymerizing the continuous oil phase. Removal of the aqueous phase then results in robust, conductive, porous, and inexpensive composites, with potential applications in energy storage, filtration, and sensing.

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oil absorption up to 130 times their graphene mass,[15] but are unlikely candidates for solvent or oil absorption applications due their cost. Other foams based on graphene oxide have been prepared by a “leavening” process,[16] or from coal-tar pitch, and have targeted applications, such as low cost solar thermal col-lectors.[17,18] Table S1 (Supporting Information) summarizes the absorbance properties of different types of graphene and carbon nanotube (CNT)-based, sponge-like materials, previ-ously reported in literature. These materials have been shown to adsorb impressive amounts of solvent, as compared to the initial mass of material. However, this is strongly correlated with the density of the material. A more relevant parameter

for oil recovery applications—the volume of oil adsorbed relative to the initial volume of the material (swelling factor)—rarely exceeds 1, and is often significantly less than 1, as the materials typically do not swell. Our gra-phene based foams, in addition to being oil selective, undergo significant swelling, with a swelling factor greater than 4, as illustrated in Table S2 (Supporting Information).

The use of butyl acrylate monomer as the oil phase in a graphene-stabilized emulsion, illustrates the concept of the emulsion-templated polymerization for creation of responsive graphene/polymer-composite foams. Polymerization of the butyl acrylate monomers, using a common thermal ini-tiator, azobisisobutyronitrile (AIBN), and divinylbenzene as a cross-linker, results in a network of poly(butyl acrylate) filling the interstitial region between the dispersed water droplets. After removal of water, the material has the shape of the reaction vessel in which it was made, as shown in Figure 1D. Foams produced by this technique have a density directly related to the water content of the emulsion, with common densities being around 0.3 g mL−1, and graphene content typically 1.2% by mass. Figure 1E,F shows SEM images of the internal structure of our composite material. In Figure 1E, one can clearly see the foam-like morphology con-sisting of spheres, or cells, ranging in size from roughly 100 to 300 µm in diameter, with the size of the spheres being a function of the flake size of the graphite.[19] The cross- sectional surface is composed of dimples, much like a golf ball. These dimples are a result of evaporation of water from the graphene-coated droplets that constituted the dispersed phase. Where the graphene-coated water droplets touch, only overlap-ping graphene sheets separate the dispersed water phase, and these areas are easily torn in the drying process, as shown in Figure 1F, allowing water to move from one cell to another and providing a pathway for removal of the aqueous phase. The composite-

graphene/poly(butyl acrylate) foam is highly deformable (see stress–strain curves in Figure S9, Supporting Information) with a typical compressive modulus at 3.2% strain of ≈ 0.323 MPa.

The openings between the spheres created by removal of the dispersed water phase also provide pathways for other liq-uids to enter the foam. A high surface-area, combined with significant capillary suction, enables the foam to absorb a wide range of organic solvents. Despite the role of water in the formation of the emulsion template, the foam is highly hydrophobic. As seen in Figure 2A, placing water on the foam results in no swelling, with the foam retaining its initial shape. The result is notably different with organic solvents:

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Figure 1. A) Schematic representation of the precursor, graphene-stabilized, water-in-oil emul-sion, in which graphene sheets are shown in blue, oil (or monomer) molecules are in red, and water molecules encapsulated inside graphitic shells are transparent. The front layer of the oil is removed to show the structure of the graphitic shells. Darker areas are shading to enhance the perception of depth. B) Schematic representation of the structure of the graphitic shell, consisting of overlapping graphene sheets residing at the oil/water interface. Water molecules inside the graphitic shell are shown in dark blue. C) Schematic representation of the structure of the polymer/graphene-composite after water evaporation. D) Composite foam of cross-linked poly(butyl acrylate). SEM images of graphene based poly(butyl acrylate) foam, showing E) a cross-section of the structure composed of packed spheres, and F) openings, or “win-dows”, between spheres that allow for the passage of liquids in and out.

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when tetrahydrofuran (THF) is added, as shown in Figure 2B, the foam immediately swells as it adsorbs the THF. As the solvent evaporates, the composite foam returns to its original shape.

The swelling and absorption of organic solvents by the foam takes place quickly, normally within a few seconds. However, the foam is chemically selective, so different solvents result in unequal swelling. To investigate the factors affecting absorp-tion, swell tests are performed for 17 different solvents, both hydrophilic and hydrophobic. The foams are placed in a par-ticular solvent and allowed to swell to their fullest extent. The extent of swelling for various solvents is shown in Figure 2C, as a histogram of the increment of volume increase upon foam swelling, normalized by the original volume. The foams clearly favor hydrophobic solvents, as the more hydrophilic solvents cause little to no swelling. The amount of absorbed solvent indicates that the solvent resides in the cells of the foam and is not simply swelling the polymer matrix. The change of foam volume is found to correlate with the solubility parameter of the solvent in the polymer matrix, as shown in the inset to Figure 2C. The closer the solubility parameter of the solvent is to that of the poly(butyl acrylate), the greater the extent of

swelling. This indicates that the affinity between the poly(butyl acrylate) and solvent controls the extent of foam swelling.

To demonstrate that the degree of swelling depends on the mechanical properties of the poly(butyl acrylate) network, we studied the effect of the extent of cross-linking on the foam volume expansion, as shown in Figure 2D. The more extensive the cross-linking, as defined by the molar ratio of cross-linker to monomer, the less the foams swell. This should be expected, since network modulus increases with increasing cross-linking density. The power law observed for swelling of the foam volume by acetone is close to what is expected for swelling of the polymeric network, from a dry state in θ-solvent conditions; V/V0 ∝ (ρc)−3/8 (see the Supporting Information).[20] Thus, the swelling mechanism of the foam is similar to that of a poly-meric network in a selective solvent.

The solvent can be easily removed by squeezing the foam, and shows that swelling does not destroy the foam structure. This confirms that the shell of graphene sheets coating the foam cells can easily expand in size, to accommodate polymer matrix swelling through sliding of the graphene sheets. It is interesting to point out that expansion of the cells reduces the bending energy of the graphene sheets, which, in turn,

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Figure 2. A) Water droplet beaded at the surface of the composite-graphene/poly(butyl acrylate) foam, indicating the materials hydrophobicity, and B) partially swollen composite-graphene/poly(butyl acrylate) foam after addition of THF. C) Comparison of the percent volume expansion of the com-posite-graphene/poly(butyl acrylate) foams in different solvents. Inset shows dependence of the volume ratio on the solubility parameter. D) Expansion of the foam in acetone as a function of cross-link density. E) Change in mass of the foam as a function of time, comparing single and mixed solvent systems.

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promotes foam swelling. The dependence of the composite foam swelling on the difference in the graphene and solvent solubility parameters is nonmonotonic, demonstrating an increase in foam swelling for small and intermediate values of the difference in solubility parameters (see the Supporting Information for details). This could mean that the graphitic skin covering the foam cells plays a passive role in foam swelling. One of the plausible explanations of this is that thin graphene layers are “transparent” to interactions between the solvent and the surrounding polymeric matrix of the graphitic shells. The situation is similar to that observed for interactions with substrates coated by a thin layer of graphene.[21] Neverthe-less, the open-cell foam structure of graphene-sheet-lined cells creates the necessary framework for fast solvent penetration of the foam, and maintains capillary suction until all cells are filled with the solvent. An evaluation of the importance of dif-ferent factors playing a role in foam swelling is discussed in the Supporting Information.

The chemical selectivity of the foam can be seen when mix-tures of solvents are used for foam swelling. When the foam is placed in a container containing immiscible solvents, such as water and heptane, it selectively absorbs the oil phase. The results are shown in Figure 2E, where the uptake of solvent is shown as a function of time. It is thus possible to selectively remove, or absorb, oil in the presence of water. (Movie S1 (see the Supporting Information) shows selective separation of oil from water using a graphene/poly(butyl acrylate) foam.) When the immiscible solvents are both organic, the foam swells to the extent that it would if only the less swelling organic solvent was present. In miscible, nonaqueous, one-phase systems, the foam gains mass to a degree that is an average of the mass that would be absorbed by the individual solvents. When placed in an aqueous, one-phase system, such as water with acetone or THF, the composite gains mass, although to a significantly reduced extent compared to when placed in a pure organic solvent.

The composite foams electrical conductivity is a direct result of the self-assembled percolating network of graphene sheets. As stated previously, graphene is found lining the cavi-ties of the foam. Any distortion in this arrangement leads to a change in the conducting network, producing a measurable change in the foams electrical properties. For example, addition of acetone to the foam causes an immediate increase in resist-ance, in addition to an increase in the mass of the sample, as shown in Figure S8 in the Supporting Information. Swelling of the foam by a solvent results in deformation of the shell of overlapping graphene sheets, decreasing the overlap area between sheets, increasing the length of the conductive path-ways, and decreasing the conductance of the foam. The foam resistance is proportional to the resistance of the individual graphene shells, with a proportionality numerical coefficient, Cfoam, determined by the number of percolation paths con-necting two opposite surfaces of the foam, Rfoam ≈ CfoamRshell. The resistance of the thin spherical shells, with conductivity (σ), initial radius (R0), and thickness (h0), is estimated as Rshell ≈ (h0σ) −1 (see the Supporting Information for derivation details). Swelling of the foam from its initial volume (V0), to volume (V), results in an increase in the size of the graphitic shells, by a factor of (V/V0)1/3. This leads to a decrease in the shell

thickness to h0/(V/V0)2/3. Therefore, upon swelling, the resist-ance of the swollen foam (Rsw) increases, due to a decrease of the thickness of graphitic shells, as Rsw ≈ Rfoam(V/V0)2/3. After solvent evaporation, the structure of the foam composite com-pletely recovers, returning the graphene sheets to their original arrangement, which, in turn, results in the sample returning to its original resistance.

It follows that the loss of mass through evaporation is ini-tially faster than the decrease in resistance, but return of the original resistance occurs before complete evaporation of the solvent (see Figure S8, Supporting Information). Addi-tionally, compared to the loss of mass, the decrease in resist-ance is delayed; it begins only after ≈ 20% of the solvent has evaporated, but returns to its original value, while ≈ 5% of the solvent remains. This result is shown in the Figure S8 (Sup-porting Information), and provides evidence that the change in size of the foam cells is responsible for the change in electrical resistance.

To further study the resistance change as a function of defor-mation, we measure the change in resistance as the foam is compressed with no solvents. In Figure 3A, the foam is com-pressed to ≈ 50% of its original volume, and maintained at this volume as the resistance is recorded. The resistance sharply increases to over three times its original value, followed by a slow decay. While the resistance continues to decrease with time, the rate of decrease slows and, on the time scale of the experiment, does not quite return to the original resistance. The initial increase in the foam resistance is due to elastic deformation of the polymer matrix and graphitic shells. The slow decrease of the composite foam resistance is a manifesta-tion of the foam viscoelasticity. This is associated with the rear-rangement of the graphene sheets forming the graphitic shells, and shell shape relaxation. The time dependence of the foam resistance undergoing instantaneous compression at t = tdef, can be described by a sum of exponential functions:

( ) exp /1

4

def eqR t R t t Ri

i

i∑ τ( )( )= − − +=

(1)

where Ri is the amplitude of the resistance of the ith mode, τi is the ith mode relaxation time, and Req is the equilibrium resist-ance of the foam after deformation at t → ∞. Fitting results of the data shown in Figure 3A, at the time interval t > tdef, and considering Req, Ri, and τι as fitting parameters, are presented in Figure S10 (Supporting Information). This procedure results in the two shortest foam relaxation times, equal to 0.6 and 3.27 min. These time scales correspond to sample shape relaxa-tion, and describe relaxation processes happening in the PBA network. The rearrangement of the graphitic shell structure occurs at much longer time scales, with the longest composite foam relaxation time being 197.8 min. These graphitic shell rearrangements occur without sample shape deformations.

When compression is released, the foam returns to its original shape, but, as observed with compression, the resist-ance spikes, then decays slowly to the original value, as shown in Figure 3B. This is also in agreement with the viscoelastic response of the composite foam to external loading. Note that, the data shown in Figure 3B can also be fitted to the function given by Equation (1), with the value of Req being set to the

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value of the resistance of the undeformed foam. This effect is repeatable, and Figure 3C shows a series of more than 100 com-pression and release cycles, with no apparent loss of response.

The surface activity of graphene enables the synthesis of com-posite-poly(butyl acrylate) foams with a continuous network of percolating graphene shells. The resulting material is a flexible, porous electrode that is lightweight, robust, selectively swells in organic and aqueous solvents, and has shape sensitive electrical resistance. The composite foam has applications in selective oil removal, oil sensing, and variable pressure sensing. The mate-rial is inexpensive and scalable, composed of only monomer, water, and graphite, with no oxidation or additional surfactants required. It represents a fundamental shift in the way graphene composites are made, with the rearrangement of the stacked graphene sheets driven by thermodynamics, rather than by the input of large amounts of energy. As such, it is environmentally friendly and opens new possibilities as a functional material.

Experimental SectionPreparation of Composite Sample: A 250 mL Erlenmeyer flask was

loaded with 0.88 g graphite (Asbury Carbons, grade Nano 24), 120 mL DI water, 80 mL butyl acrylate (Acros Organics, 99%), 500 µL divinylbenzene (Sigma-Aldrich, 80%), 0.24 g 2,2′-azobis(2-methylpropionitrile) (Sigma-Aldrich, 98%), and a stir bar. The contents were then mixed for about 1 min on a stir plate. The stir bar was then removed, and the contents were mixed at ≈ 10 000 rpm for 1 min, using a Silverson L5M-A high shear blender with a 0.75 inch tubular blending head, with a square hole, high shear screen. After mixing, the contents were poured gently into a 240 mL glass jar. The jar was then sealed and placed into a convection oven (Blue M, Stabil-Therm) at 65 °C for 24 h to react. The jar was then broken to remove the composite sample, which was then placed in the same oven for several days (≈ 3), until dry.

Electron Microscopy: Samples were first prepared as described above. To prepare composite samples for the electron microscope, they were first cut with a razor blade. The slices were then mounted on aluminum stubs and coated with Au/Pd in a sputter coater (Polaron Unit E5100). The samples were characterized with a JEOL 6330 field emission scanning electron microscope, with a 10 kV accelerating voltage.

Swelling and Resistance/Mass Change: A typical sample was first prepared as described above. It was then cut into a rectangular prism by first submerging it in liquid nitrogen and then cutting it with a band saw. The final dimensions were ≈ 1 cm × 1 cm × 3 cm. Two sides of the prism were then painted with colloidal silver paste (Ted Pella, Pelco). The sample was placed in the oven overnight to cure the paste. After removal from the oven, copper tape (3 m, 0.25 inch) was placed on top of both of the ends covered with the silver coating. The tape was then painted again with silver paste to ensure it was bound to the composite. The resistance was measured using a Keithley 2420 sourcemeter. The instrument was set to start taking measurements, and then 0.5 mL of acetone (Fisher, 99.5%) was pipetted onto the top of the sample, directly in between the faces covered with silver paste. This was to ensure the sample did not swell at the location of the silver paste. 300 000 data points were taken at a sampling rate of 600 ms per point, and at 0.01 V. The experiment was repeated several times until there was no hysteresis. After the resistance tests were completed, the sample was placed on an analytical balance (with the silver paste and copper tape). 0.5 mL of acetone was then placed on the sample in the same place as before. The mass data were taken at 5 s, 10 s, 20 s, 30 s, 1 min, 2 min, 3 min, 5 min, 10 min, 20 min, 30 min, 40 min, 50 min, and 60 min.

Effect of Compression on Electrical Resistance: A typical composite sample 4.5 cm in diameter was first prepared, as described above, then submerged in liquid nitrogen for 3 s, removed, and immediately cut using a band saw, to remove the top and bottom (≈ 0.5 cm). The flat top and bottom of the sample were then painted with colloidal silver paste (Ted Pella, Pelco). The sample was placed in the oven overnight to cure the paste. After removal from the oven, copper tape (3M, 1/4 inch) was placed on top of both of the ends covered with the silver coating. Before the sample was placed into the Instron 1350 for testing, the instrument was prepared by covering the compression plates with paper and tape, to prevent electrical conductivity. Once the sample was placed between the plates, it was compressed 100 times to ≈ 50% of its original height. The resistance was measured before, during, and after the compression cycles, using a Keithley 2420 sourcemeter, taking 100 000 points, 140 ms apart, with a voltage sweep from 0.001 to 0.1 V. This measurement was repeated several times (≈ 3).

Supporting InformationSupporting Information is available from the Wiley Online Library or from the author.

Figure 3. Evolution of the resistance of poly(butyl acrylate)-composite foam during compression. A) Relaxation of the foam resistance under constant compression. B) Relaxation of the foam resistance after release of compression. C) Reproducibility of the foam electrical properties under compres-sion/recovery loading protocol.

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AcknowledgementsThis work was supported by the NSF DMREF program through Grant No. DMR1535412.

Received: September 13, 2016Revised: December 22, 2016

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