Solvent-Exfoliated Graphene at Extremely High Concentration

LETTER pubs.acs.org/Langmuir

Solvent-Exfoliated Graphene at Extremely High Concentration Umar Khan,† Harshit Porwal,† Arlene O’Neill,† Khalid Nawaz,‡ Peter May,† and Jonathan N. Coleman*,† † ‡

School of Physics and CRANN, Trinity College Dublin, Dublin 2, Ireland School of Chemical and Materials Engineering, NUST, H-12 Islamabad, Pakistan

bS Supporting Information ABSTRACT: We describe three related methods to disperse graphene in solvents with concentrations from 2 to 63 mg/mL. Simply sonicating graphite in N-methyl-2-pyrrolidinone, followed by centrifugation, gives dispersed graphene at concentrations of up to 2 mg/mL. Filtration of a sonicated but uncentrifuged dispersion gives a partially exfoliated powder that can be redispersed at concentrations of up to 20 mg/mL. However, this process can be significantly improved by removing any unexfolaited graphite from the starting dispersion by centrifugation. The centrifuged dispersion can be filtered to give a powder of exfoliated few-layer graphene. This powder can be redispersed at concentrations of at least 63 mg/mL. The dispersed flakes are ∼1 μm long and ∼3 to 4 layers thick on average. Although some sedimentation occurs, ∼26 28 mg/mL of the dispersed graphene appears to be indefinitely stable.

’ INTRODUCTION It has been known for some years that graphite can be exfoliated in the liquid phase to give graphene.1 There are two main ways to do this: oxidization of graphite followed by exfoliation in water to give graphene oxide1 7 and exfoliation of graphite in solvents or surfactant solutions to give dispersed pristine graphene.8 25 Although advances in this field have been rapid, a number of outstanding problems remain. Of these, probably the most important is the relatively low concentration of dispersed graphene that can be achieved. For example, graphene oxide has been dispersed in some organic solvents at concentrations of up to 1 mg/mL6,26 28 and in water at concentrations of up to 7 mg/mL.4 Similarly, graphene was initially dispersed in solvents at extremely low concentrations of ∼10 2 mg/mL.16,17 Recently, it was shown that this could be increased to ∼1 mg/mL.18 In contrast, surfactant-dispersed graphene has not been achieved at concentrations above 1 mg/mL.20,29,30 Although these concentrations are now in the appropriate range for a number of applications, they are not high enough for many others. For example, solution-phase polymer/graphene composite formation1,30,31 would be much simpler if wellexfoliated graphene dispersions were available at high concentrations. In addition, the deposition of thin films by vacuum filtration followed by membrane dissolution12 requires dilution with large quantities of water before filtration. For this to work properly, the initial graphene concentration must be high. In addition, graphene flakes can be selected by size or thickness by chromatography or density gradient centrifugation.13 In both cases, the amount separated is limited by the starting concentration. In these and many other areas, a significant barrier to progress is the lack of high-concentration dispersions. In this article, we describe two related methods to produce very high concentration dispersions of graphene in the solvent r 2011 American Chemical Society

N-methyl-2-pyrrolidinone (NMP). The first method gives concentrations of ∼17 mg/mL at a yield (percentage of graphite exfoliated as few-layer graphene) of 17%. The second method gives metastable dispersions with a concentration of up to 63 mg/mL at a yield of 19%.

’ EXPERIMENTAL PROCEDURE Graphite powder and NMP were purchased from Sigma-Aldrich and used as supplied. Sonication was performed using a sonic tip (GEX600, flat head probe) running at 25% of maximum power (i.e., 25% of 600 W) and a (Branson 1510E-MT) sonic bath. Centrifugation (CF) was performed using a Hettich Mikro 22R typically at 500 rpm (∼25 g) for 45 min. After centrifugation, the top 80% of the supernatant was removed by pipet. Absorbance measurements were made using a Varian Cary 6000i with 1 mm cuvettes. TEM was performed using a Jeol 2100 and holey carbon grids (400 mesh). We note that for extremely high concentration samples the dispersion was diluted before depositing it onto TEM grids. To perform Raman measurements, thin films were first made by vacuum filtration of the dispersion through a nylon 0.45 μm pore size membrane (http://www.sterlitech.com/). Raman measurements (633 nm, Horiba Jobin Yvon LabRAM- HR) were performed on 10 different positions of the resultant films. These spectra were normalized and averaged.

’ RESULTS AND DISCUSSION Researchers attempting to exfoliate graphene in liquids generally face two main problems: incomplete exfoliation (∼3 layers per flake on average)18,20,22 and relatively low concentration (up to a few mg/mL).18 Although high-end applications such as Received: May 13, 2011 Revised: June 13, 2011 Published: June 15, 2011 9077

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Figure 1. (A) Dispersed concentration as a function of sonication time for graphene dispersed by tip sonicating graphite powder. (B) After 10 h of sonication, the dispersion in A was filtered through a Buchner funnel. The cake was collected from the filter, added to 100 mL of NMP, and tip sonicated again. The dispersed concentration as a function of the second sonication time is shown in B. In both cases, the dashed line represents square root behavior.

single-monolayer transistors require complete exfoliation, many applications such as the formation of composite or thin conducting films are less sensitive to the exfoliation state. However, increasing the achievable concentration will broaden the potential application space significantly. Thus, in this work we focus on increasing the achievable concentration while maintaining the exfoliation state at an acceptable level. The exfoliation of graphite is often achieved by sonicating graphite powder in the required solvent (or alternatively a polymer or surfactant solution) using either a sonic tip or bath. The concentration can usually be increased by increasing the sonication time.18,20 To provide a baseline for this work, we added 10 g of graphite powder to each of two beakers containing 100 mL of NMP. One was sonicated in a sonic bath, and the other was sonicated using a sonic tip. At various times, t, small aliquots of the dispersion were removed and centrifuged at 500 rpm for 45 min, and the supernatant was separated by pipet. In all cases, the concentration was measured by recording the absorbance at 660 nm and transforming this into the concentration using A/l = RC with R = 3620 mL/mg/m.18 The concentration of the tipsonicated graphene increased with sonication time up to t = 6 h, where a maximum value of C = 2 mg/mL was reached (Figure 1a). For longer sonication times, the concentration tended to fall off, probably because of oxidative degradation of the NMP.32 In the initial phase of increasing concentration, the data fit well to C = kt1/2 (fitting gives k = 0.8 mgmL 1h 1/2), suggesting that the concentration is controlled by flake size, which is in turn controlled by sonication-induced cutting.18,33 For bath-sonicated graphene, similar behavior was observed (Figure S1A). However, the concentration peaked at 0.6 mg/mL after 100 h, and fitting the initial portion of the curve gave k = 0.05 mgmL 1h 1/2. In both cases, these concentrations are typical of what is generally found.9,14,16 18,21 We performed TEM analysis on flakes deposited from the tipsonicated samples after various sonication times (Figure 2A).

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Figure 2. TEM images of graphene flakes deposited from the dispersions described in Figure 1. (A) TEM image of flakes deposited from a dispersion after 6 h of normal tip sonication. (B) TEM image of flakes deposited from the dispersion in Figure 1B after a second tip sonication (28 h).

At all times, the flakes appeared to be of good quality with lateral dimensions of ∼1 μm and thickness, as measured by edge counting,18 of ∼3 layers. In addition, no large graphitic crystallites were observed. It is worth noting that no differences were detected for flakes prepared with 6 or 9 h of sonication (i.e., the dip in concentration observed in Figure 1a did not correlate with any observable change in flake properties). The dispersions were then filtered through porous membranes to form thin films for characterization by Raman spectroscopy (Figures S2 and S3). The observed spectra were similar to those generally observed for liquid exfoliated graphene18,20 and had Raman D/G ratios of 100 h). That the time constants are so high indicates that the sedimenting objects are small flakes rather than larger graphitic crystallites.21,37 In addition, that the C0 values are similar for each sample suggest that 26 28 mg/mL might 9079

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Figure 3. Data for redispersed exfoliated graphene. (A) Dispersed concentration as a function of sedimentation time for a sample prepared by redispersing 600 mg of exfoliated, filtered graphene in 8 mL of NMP (no centrifugation after the second sonication). The dashed line is a fit to a monoexponential decay, appropriate for a single sedimenting phase. The fit constants are given in the Figure. (B) Dispersed concentration as a function of the mass of added graphene divided by the solvent volume. This was measured immediately after sonication. In addition, a separate sample was centrifuged immediately after sonication and measured. Also shown is the concentration of the stable fraction as estimated from sedimentation measurements. (C) Mean number of graphene layers per flakes (measured by TEM) as a function of dispersed concentration for the dispersions in B. These were measured immediately after sonication (no CF) and after ∼200 h. Approximately 80 flakes were counted for these statistics.

represent an upper limit for stably dispersed graphene. We note that these fits imply that 26 28 mg/mL of graphene will remain dispersed indefinitely. However, such an extrapolation to infinity is probably unwise. We suggest that these fits can be used to predict stability over reasonable time frames but may not be reliable for timescales that are very long compared to the measurement time. We can summarize the behavior of all four samples in Figure 3B. Here, we plot the concentration dispersed shortly after sonication (filled circles), the concentration remaining after centrifuging a portion of the dispersion immediately after sonication (open triangles), and the stably dispersed concentration (C0, closed stars). In general, ∼80% of the added graphene was initially dispersed. Centrifugation of the dispersion shortly after sonication results in ∼70% retention. However, the stably dispersed concentration remains constant at 26 28 mg/mL, again suggesting the presence of a maximum dispersed concentration. To explore the quality of the dispersion, we performed TEM analysis of the dispersions immediately after sonication and after ∼200 h of sedimentation. In each case, large quantities of very

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Figure 4. TEM images of graphene flakes deposited from the sample described in Figure 3 with an initial concentration of 63 mg/mL images (A) immediately after sonication, (B) 192 h after sonication, and (C) 312 h after sonication.

thin flakes were observed (Figure 4). We measured the flake dimensions (length, width, and number of layers18) for each of the four dispersions (Figures 3C and S5). Although the flake length (∼1 μm) and width (∼0.5 μm) did not vary significantly with the initial concentration, we found that the number of layers per flake (N) clearly increased with concentration from ∼2.5 to ∼4.5 (Figure 3C), suggesting the presence of some reaggregation. We checked the quality of the flakes for the 63 mg/mL sample by Raman spectroscopy (Figure S2). We observed goodquality spectra with a D/G ratio of ∼0.2. The small value of this ratio suggests that the defects are associated with flake edges rather than point defects in the basal plane. Even though some degree of reaggregation and sedimentation certainly occurs, it is worth noting that it occurs slowly. This means that dispersions with extremely high concentrations (at least 63 mg/mL) are available for some hours after preparation. The dispersion state of these flakes is surprisingly good with ÆNæ ≈ 4.5. We note that this dispersion is an order of magnitude more concentrated than any other reported in the literature. Although the concentration falls over time, after 200 h it is still extremely high (33 mg/mL). In addition, the aggregation state is still extremely good (ÆNæ ≈ 3.5) presumably because large aggregates sediment out. We examined the highest-concentration 9080

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Langmuir sample once more with TEM after 312 h. As shown in Figure 4C, even after 13 days, the flakes remain well-exfoliated.

’ CONCLUSIONS Dispersion and exfoliation of graphite in solvents to give graphene result in good-quality dispersions but with concentration limited to a few mg/mL. However, this dispersion procedure can be used as a pretreatment to produce partially exfoliated graphitic powder by filtration. The recovered powder can then be used as a starting material for solvent exfoliation, giving dispersions with concentrations of up to 20 mg/mL. This procedure can be improved by centrifuging the pretreated dispersion to remove any unexfoliated graphite. The remaining dispersed graphene can be collected by filtration. Redispersion of this exfoliated material results in good-quality dispersions with concentrations of at least 63 mg/mL. Although these dispersions are unstable, sedimentation occurs over hundreds of hours, allowing an accessible window of usage. After 200 h of sedimentation, the concentration remaining can be as high as ∼35 mg/mL with the dispersion consisting of good-quality flakes with on average three layers and lateral sizes of ∼1 μm  0.5 μm. These highconcentration dispersions will facilitate applications such as thin film and composite formation and experimental studies in areas such as rheology and liquid crystallinity. We note that this method can easily be extended to other solvents, surfactant-stabilized graphene, or even dispersions of other layered compounds.38 ’ ASSOCIATED CONTENT

bS

Supporting Information. Complete concentration versus sonication time data, Raman spectra, TEM and Raman analyses, complete sedimentation data, and flake size data for high-concentration dispersions. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

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