ARTICLE pubs.acs.org/JPCC
Graphene Dispersion and Exfoliation in Low Boiling Point Solvents Arlene O’Neill,† Umar Khan,† Peter N. Nirmalraj,‡ John Boland,‡ and Jonathan N Coleman*,† † ‡
School of Physics and CRANN, Trinity College Dublin, Dublin 2, Ireland School of Chemistry and CRANN, Trinity College Dublin, Dublin 2, Ireland
bS Supporting Information ABSTRACT: One of the problems with solvent exfoliation of graphene is that the best solvents tend to have high boiling points and so are difficult to remove and can present problems for flake deposition and composite formation. Here, we demonstrate the exfoliation of graphene at relatively high concentration in low boiling point solvents such as chloroform and isopropanol. It is possible to achieve concentrations of up to 0.5 mg/mL, just under half that which can be achieved with high boiling point solvents such as N-methyl-pyrrolidone. These dispersions consist of graphene flakes of ∼1 μm length and with a thickness of less than 10 layers (e5 layers for isopropanol). For both solvents, >75% of the graphene remains dispersed indefinitely. Raman spectroscopy shows the flakes to be relatively defectfree. A significant advantage of low boiling point solvents is that they allow individual flakes to be spray cast onto substrates. Deposited densities of >10 flakes with length >1 μm per 10 μm 10 μm square have been controllably achieved. While some onsubstrate aggregation is observed, this is much less prevalent than when spraying from high boiling point solvents.
’ INTRODUCTION Recent developments in liquid phase exfoliation of graphite to give graphene mean it is now possible to prepare large quantities of dispersed graphene cheaply and easily. There are two main methods to exfoliate graphene: using high surface tension solvents16 or in water using surfactants711 or polymers12 as stabilizers. These methods have been very successful and have led to a number of advances including the preparation of polymer graphene composites,12 facile production methods for transparent conductors,13 and sorting of graphene by layer number.7 However, these methods still face some problems. For example, when processing graphene from surfactant dispersions, it can be difficult to remove residual surfactant.8 Alternatively, the best solvents tend to be nonvolatile,4 causing problems with processing. This is because the energetic cost of exfoliation falls as the solvent’s Hildebrand solubility parameter approaches 23 MPa1/2 (equivalent to saying the surface tension approaches 40 mJ/m2).4,5,14 As Trouton’s rule links the solubility parameter to the boiling point (BP) through the enthalpy of vaporization, this means the best solvents have high boiling points. This can make it difficult to remove solvents when processing graphene into films or composites.5 In particular, it is virtually impossible to deposit individual flakes from solvent exfoliated graphene, as aggregation tends to occur during the slow solvent evaporation.5 While graphene dispersions in high boiling point solvents have been transferred into low boiling point solvents by solvent exchange,15 it would be preferable to have a method which allows direct exfoliation of graphite to give stable dispersions of graphene in low boiling point solvents. Such a method would greatly simplify graphene exfoliation and significantly expand the number of applications of liquid exfoliated graphene. r 2011 American Chemical Society
If good solvents such as N-methyl-pyrrolidone have solubility parameters around 23 MPa1/2 and boiling points close to 200 C, the logical corollary of this is that solvents with lower solubility parameters and lower boiling points will probably be relatively poor. The cost of using poor solvents as dispersants is generally low concentration and possibly poor exfoliation quality and stability. However, it is possible that such issues can be addressed by optimization of processing procedures to give good quality, high concentration, stable dispersions in low boiling point solvents. The aim of this work is to identify such low boiling point solvents and optimize the dispersion procedures. The resulting suspensions must be of high concentration, be stable for a reasonable period, and should predominately contain graphene flakes with less than ∼5 layers. It has previously been demonstrated that graphene can be dispersed in a wide range of solvents.4,5 From this data, we identified three solvents with mediocre dispersion capabilities but with low boiling points: acetone, isopropanol, and chloroform. These solvents are compared to three better solvents; cyclohexanone, N-methyl pyrrolidone, and dimethylformamide. In this paper, we demonstrate a dispersion procedure which results in graphene concentrations of up to 0.5 mg/mL in the low boiling point solvents. The graphene is well exfoliated and reasonably stable. In addition, the processing procedure does not generate defects in the basal plane and allows deposition of thin multilayers on substrates. Received: November 16, 2010 Revised: February 18, 2011 Published: March 14, 2011 5422
dx.doi.org/10.1021/jp110942e | J. Phys. Chem. C 2011, 115, 5422–5428
The Journal of Physical Chemistry C
’ EXPERIMENTAL METHODS All solvents used were of Chromosolv grade and were purchased from Sigma Aldrich: acetone (product number 34850), chloroform (product number 366919), isopropanol (CAS 67-63-0), cyclohexanone (product number 398241), N-methyl pyrrolidone (product number 69120), and dimethylformamide (product number 15440). The concentration was measured by UVvis-IR absorbance spectroscopy. In all cases, the UVvis-IR absorbance spectra of the dipsersions were flat and featureless. The absorbance per cell length, A/l, of these dispersions was measured at 660 nm. The concentration remaining after centrifugation, C, was determined from A/l, taking R to be 3620 mg/mL/m (A/l = RC).6 Thin films were prepared by filtering the dispersion onto porous polyvinylidene fluoride (PVDF) membranes with a nominal pore size of 0.22 μm. Individual flakes were deposited onto SiO2 using an airbrush spray gun (Evolution Airbrush, www.graphicsdirect.co.uk). The gun was held approximately 2 cm from the substrate, and the graphene flakes were carried to the substrate by an injection of nitrogen at 2 bar pressure. The spraying was pulsed (5 s intervals) to allow solvent evaporation. No additional heat or silanization was required. A palladium grid reference (>20 nm) was sputtered onto freshly cleaved SiO2 (300 nm thermally grown oxide), allowing us to record flake positions for subsequent characterization. The substrate was also cleaned for 10 min in an oxygen plasma (Diener barrel asher) before spraying. All sonication was performed in the same low power sonic bath (Branson 1510E-MT) with a nominal power output of ∼16 W. The samples were centrifuged in a Hettich Mikro 22R centrifuge. UVvis-IR absorption spectroscopy was carried out using a Varian Cary 6000i with optiglass 1 mm cuvettes. Sedimentation measurements were carried out in a home-built apparatus with four pulsed lasers and photodiodes.16 TEM samples were prepared by drop casting a few drops of the dispersion onto holey carbon grids. Bright field TEM imaging was performed using a Jeol 2100. Another holey carbon grid was prepared by spraying 1 mL of graphene/IPA dispersion. The spray method was similar to that already mentioned, with the grid held in place by tweezers. The grid was then dried in a vacuum oven at 60 C for 24 h. Selected area electron diffraction patterns and high resolution TEM of the flakes that were sprayed onto the grid were taken using an FEI Titan(TM) 80-300 S/TEM operated at 300 kV. Raman spectra (633 nm) of the films was performed using a Horiba Jobin Yvon LabRAM-HR. Scanning Raman measurements (633 nm) were carried out using an NT_MDT NTEGRA platform incorporating a Renishaw Raman Spectroscope. Tapping mode AFM analysis was performed using a Veeco Dimension V. A silicon nitride tip with a typical resonant frequency of 300 kHz and radius of less than 10 nm was used. Gwyddion software was used to measure height profiles. ’ RESULTS AND DISCUSSION Dispersion of Graphene in Low Boiling Point Solvents. Previous work in our group has demonstrated that chloroform, isopropanol (IPA), and acetone can disperse graphene at low concentration (3.4, 3.1, and 1.2 μg/mL, respectively).4 While these concentrations are far too low for most applications, such solvents have the significant advantage of relatively low boiling points (61, 82, and 56 C, respectively). In a separate paper, we showed that the concentration of graphene dispersed in N-methyl-pyrrolidone (NMP) can be increased dramatically by sonicating at low power for very long times.6 Thus, in order to attempt to produce high
ARTICLE
Figure 1. (A) Concentration of dispersed graphene as a function of centrifugation rate for three high boiling point and three low boiling point solvents. The boiling points are given at the top of the figure. (B) Graphene concentration (5000 rpm) as a function of dispersive Hansen parameter for the data shown in part A. The dashed line illustrates the behavior found by Hernandez et al. rescaled to match the data for cyclohexanone.
concentrations of exfoliated graphene in these low BP solvents, we sonicated graphite powder (Branwell natural graphite RFL 99.5, initial concentration 3.3 mg/mL) for 48 h in a low power sonic bath (∼16 W). Each dispersion was then split into three and each portion centrifuged at three different rates: 500, 2000, and 5000 rpm. The top 6 mL (out of 10 mL) was then removed by pipette. In addition, graphite powder was dispersed using identical procedures in three high BP solvents—NMP, DMF, and cyclohexanone—all of which are known to exfoliate graphite.4 In all cases, the dispersed concentration after centrifugation (CF) was determined using UVvis-IR spectroscopy. The concentration of dispersed graphene for all six solvents, each prepared at three rotation rates, is shown in Figure 1A. Previous work has shown that higher rotation rates result in the retention of smaller flakes.6,9 The data for the three high BP solvents is similar with concentrations of ∼1, ∼0.5, and ∼0.2 mg/mL obtained for CF rates of 500, 2000, and 5000 rpm. These concentrations are much higher than those originally reported for good solvents (typically ∼10 μg/mL)4,5 and compare well to recent reports of high concentrations of graphene dispersed in NMP.6 Likewise, the data for isopropanol and chloroform are similar with surprisingly high concentrations observed: ∼0.5, ∼0.2, and ∼0.07 mg/mL for CF rates of 500, 2000, and 5000 rpm. Acetone gave poorer results: ∼0.08, ∼0.025, and ∼0.01 mg/mL for CF rates of 500, 2000, and 5000 rpm. All three solvents show significant improvement on previous results which gave concentrations of graphene in chloroform, IPA, and acetone of 3.4, 3.1, and 1.2 μg/mL, respectively (all obtained with much shorter sonication time, ∼30 min, and lower initial graphene concentration, ∼0.1 mg/mL).4 It is important to check if these relatively high concentrations are compatible with the postulated dispersion mechanism. Previously, we suggested that good solvents are those whose Hansen solubility parameters match reasonably well to those suggested 5423
dx.doi.org/10.1021/jp110942e |J. Phys. Chem. C 2011, 115, 5422–5428
The Journal of Physical Chemistry C
ARTICLE
Table 1. Dispersive, Polar, and Hydrogen Bonding Hansen Parameters for the Solvents Used in This Worka solvent
δD (MPa)1/2
δP (MPa)1/2
δH (MPa)1/2
DMF
17.4
13.7
11.3
NMP cyclohexanone
18 17.8
12.3 8.4
7.2 5.1
IPA
15.8
6.1
16.4
chloroform
17.8
3.1
5.7
acetone
15.5
10.4
graphene
18 (1521)
9.3 (317)
7 7.7 (218)
a
The Hansen parameters of graphene are also shown. The bracketed figures are the approximate range of acceptable values for graphene exfoliating solvents.
for graphene.4,14 These parameters are related to the dispersive, polar, and hydrogen bonding contributions to the cohesive energy density of the material and measure the energy required to disperse one phase in another.17,18 For both carbon nanotubes19 and graphene4 dispersed in solvents, dispersion quality is particularly sensitive to the dispersive Hansen parameter, δD; successful dispersions are only achieved for solvents with 15 MPa1/2 < δD < 21 MPa1/2.4 Conversely, reasonable dispersions can be achieved for a much wider range of polar, δP, and H-bonding, δH, Hansen parameters (from 23 MPa1/2 to 1718 MPa1/2 in each case).4 This suggests that the concentrations achieved in the present study should be controlled primarily by δD. In Figure 1B, we plot the graphene concentration for the 5000 rpm dispersions against the solvent δD. The dashed curve represents the envelope of the data reported previously for graphene, dispersed at low concentration in a wide range of solvents.4 We note that, with the exception of chloroform, all solvents match very well to the dashed line. We suggest that the chloroform data is significantly below the dashed line because its δP value of 3.1 MPa1/2 is at the very edge of the allowed range (23 MPa1/2 to 1718 MPa1/2).4 This results in the reduction of the concentration of graphene in chloroform, below what would be expected by analysis of δD alone. By contrast, the δP and δH values of the other solvents are much closer to the center of the accepted ranges (Table 1). We can measure the stability of these dispersions by monitoring the dispersion concentration optically as any unstable material sediments out (NB this is performed after centrifugation).16,20 When dispersed in good solvents such as NMP, graphene is particularly stable.4,6 However, when dispersed in mediocre solvents, one expects some degree of sedimentation. Sedimentation measurements were carried out for a number of graphene/IPA dispersions (7 independent dispersions) and graphene/chloroform dispersions (4 independent dispersions). Typical sedimentation curves for graphene dispersed in IPA and chloroform are shown in Figure 2A (48 h of sonication, CF 2000 rpm for 45 min). For chloroform, an initially rapid sedimentation saturated after ∼100 h with ∼75% of the graphene stable. In the case of IPA, a slow, steady sedimentation was observed (see curve IPA-1). This tended to saturate after >200 h with 96% of the graphene stably dispersed. However, we note that occasionally (2 out of 7) sedimentation curves like that labeled IPA-2 are observed for graphene dispersed in IPA. In such cases, we observe a steady decrease in concentration with no saturation observed after hundreds of hours. Such behavior seems to be associated with wet or aged IPA and can be avoided by using fresh solvent.
Figure 2. (A) Sedimentation curves for graphene dispersed in isopropanol and chloroform after centrifugation. C/CT is the concentration normalized to the total initial concentration. The dashed lines are fits to the inset equation (eq 1). (B) Concentration of graphene dispersed in isopropanol and chloroform as a function of sonication time (CF rate = √ 2000 rpm, 45 min). The dashed lines illustrate t behavior.
It has been shown that, during sedimentation, the concentration remaining dispersed can be approximated by CðtÞ ¼ C0 þ ðCT C0 Þet=τ
ð1Þ
for a system with one stable phase (concentration C0) and one sedimenting phase (concentration CT C0, where CT is the total initial concentration).16 Equation 1 fits extremely well to all data sets (dashed lines in Figure 2A). The chloroform dispersions were characterized by values of C0/CT in the range 0.670.96 (mean 0.80) and values of τ between 11 and 19 h with an outlying value of 88 h (overall mean 34 h). The stable isopropanol dispersions were characterized by values of C0/CT in the range 0.930.97 (with one outlying value of 0.69, overall mean 0.92) and values of τ between 45 and 221 h (overall mean 126 h). In addition, we note that the unstable IPA dispersions (IPA-2) were characterized by C0 = 0 and τ ∼ 880 h. Discounting the unstable dispersions which we attribute to impure solvent, the C0/CT values tell us that these dispersions are extremely stable with the vast majority of graphene remaining dispersed over very long time scales. The main difference between solvents is in the time constants. These were tens of hours for chloroform and hundreds of hours for IPA. Short time constants suggest the sedimenting material to be relatively large.16 This suggests the unstable phase in the chloroform dispersions to be large graphitic flakes which were not fully removed after centrifugation. Such large objects would tend to sediment with a short time constant. However, for IPA, the time constants are much longer, suggesting smaller sedimenting objects. Here, we suggest the sedimenting phase to be small graphene flakes. We can test these hypotheses by performing 5424
dx.doi.org/10.1021/jp110942e |J. Phys. Chem. C 2011, 115, 5422–5428
The Journal of Physical Chemistry C TEM on the sediment that formed after the sedimentation experiments (∼360 h). These data are shown in the Supporting Information and confirm that relatively large graphitic flakes sediment out of chloroform, while the unstable objects in the isopropanol dispersions are smaller multilayer graphene flakes. Previously, we showed that the concentration of dispersed graphene can be achieved by increasing the sonication time, t.6,9 Longer sonication times result in smaller flakes which can be dispersed at higher concentrations.6 We prepared dispersions of graphene in IPA and chloroform (initial graphite concentration of 3.3 mg/mL, 250 mL, round bottomed flask, sonic bath) for a range of sonication times followed by centrifugation (2000 rpm for 45 min) and measurement of concentration. For both solvents, the concentration increased significantly, reaching ∼0.4 and ∼0.5 mg/mL for chloroform and IPA, respectively (Figure 2B). As shown √ by the dashed lines, this increase is consistent with the t behavior previously observed for NMP dispersions.6 This suggests that the concentration is partly controlled by flake size which is in turn controlled by sonication time.6 We note that a concentration of 0.5 mg/mL is relatively high, comparing favorably with the highest concentrations observed for graphene in NMP and surfactants (1.2 and 0.3 mg/ mL, respectively).6,9 Dispersion Quality. It is critical to assess the quality of the dispersion. We have carried out TEM analysis on dispersions of graphene in both chloroform and IPA (sonicated for 48 h and centrifuged at 2000 rpm for 45 min). Shown in Figure 3A is a TEM image of a large number of graphene flakes deposited on a TEM grid. A higher magnification image of what is probably a folded monolayer with some smaller flakes attached is shown in Figure 3B. Figure 3C shows a HRTEM image of a flake, with well-defined edges, that also appears to be a monolayer. We can use electron diffraction analysis as shown in Figure 3D to test this. It is clear that the inner set of spots is more intense than the secondary set, confirming that this flake consists of a single graphene layer.5 We note that all the observed flakes have reasonable well-defined edges and appear to be of good quality with no observable holes or other damage. We can estimate both the aggregation state and the flake size distribution by careful analysis of the TEM images.46,9 By analysis of the flake edges, we have estimated the number of layers per flake, N, for ∼80 flakes, to give flake thickness histograms as shown in Figure 3E and F. Although very few monolayers are observed in either solvent, all the flakes had 7 or less layers, demonstrating the quality of exfoliation (ÆNæ = 3.2 in both solvents). We can also measure the flake length, L, and width, w, with histograms of these quantities shown in Figure 3GJ for both chloroform and IPA. In chloroform, the mean flake length and width were ÆLæ = 0.84 μm and Æwæ = 0.36 μm, respectively. Similarly, in IPA, the mean flake length and width were ÆLæ = 1.1 μm and Æwæ = 0.35 μm, respectively. These sizes are typical of exfoliated graphene.46,9 In addition, the flakes were asymmetric with an average aspect ratio of ÆL/wæ = 2.6 and 3.4 for chloroform and IPA, respectively. It is also important to check for the formation of basal plane defects during sonication. We do this by Raman spectroscopy, measured on thin films prepared by vacuum filtration of both IPA and chloroform based dispersions after 22 and 70 h of sonication. Averaged Raman spectra are shown in Figure 4 with the graphite powder spectrum shown for comparison. The defect content is generally characterized by the ratio of the intensity of the defect band (D band, ∼1300 cm1) to that of the G band (∼1300 cm1), ID/IG. This ratio increases with sonication time. While such behavior is
ARTICLE
Figure 3. TEM analysis of graphene dispersed in IPA and chloroform (48 h of sonication, CF at 2000 rpm for 45 min). (A) A TEM image of a grid coated with a large number of few-layer graphene flakes exfoliated in isopropanol. (B) A TEM image of a very thin flake, possibly a folded graphene. (C) A HRTEM image of a monolayer as confirmed by the electron diffraction pattern in part D. (EJ) Statistical analysis of TEM images showing histograms of the number of layers per flake (E and F), flake length (G and H), and flake width (I and J) for graphene dispersed in both IPA and chloroform.
indicative of defect formation, such defects can be in the basal plane or can represent new edges formed as the average flake size is reduced by continuing sonication. We have suggested that the latter case can be identified by the time dependence of the difference between ID/IG for exfoliated graphene versus graphite powder.6,9 We denote this difference as ΔID/IG(t) = ID/IG(t) (ID/IG)powder. We measured (ID/IG)powder to be 0.14 for the material used in this study. The formation of edges is characterized by ΔID/IG(t) (t)1/2.6 That this is approximately the case is illustrated in the inset of Figure 4. We can check this by noting that previous work has shown that for solvent 5425
dx.doi.org/10.1021/jp110942e |J. Phys. Chem. C 2011, 115, 5422–5428
The Journal of Physical Chemistry C
ARTICLE
Figure 4. (A) Raman spectra of starting powder and of films of graphene (IPA) deposited on PVDF membranes for sonication times of 22 and 70 h (CF 2000 rpm 45 min). Inset: The increase in ID/IG compared to that for the graphite powder, ΔID/IG, as a function of the square root of sonication time for both IPA (squares) and chloroform (triangles) dispersions. A straight line indicates that the defects being formed are associated with new edges. (B) TEM images of aggregated graphene flakes spray cast on a TEM grid. The inset shows an electron diffraction pattern taken from the region in the white circle.
dispersed graphene the relationship between ID/IG and flake size is known: ΔID/IG = 0.065[ÆLæ1 þ Æwæ1], where the dimensions are measured in micrometers.6 From the TEM analysis, we know that ÆL/wæ ∼ 3, allowing us to approximate ΔID/IG ≈ 0.26/ÆLæ. Combining this with the fit in the inset of Figure 4 gives ÆLæ = 8.7/(t)1/2, where L is in μm and t in h (this is reasonably close to the known expression for NMP dispersions: ÆLæ = 14.6/(t)1/2).6 This allows us to estimate the flake length for the 48 h sample to be 1.25 μm, consistent with the measured value of ÆLæ = 1.0 μm. Thus, the Raman data strongly suggest that the newly created defects are new flake edges rather than body defects. This implies that the exfoliation process is not overly destructive and yields good quality flakes. In addition, we note that the shape of the Raman 2D band (∼2600 cm1) is different from that of graphite and is consistent with few-layer graphene.21 This indicates that, while flakes must aggregate during film formation, the resultant stacking is not ordered AB (Bernal) stacking but random.22 We can test this by simulating film formation by spraying graphene flakes from an IPA dispersion onto a TEM grid. The deposited flakes aggregate on the grid, mirroring the processes in the early stages of film formation. Shown in Figure 4B is a TEM image of such an aggregate. In the inset is an electron diffraction pattern taken from the selected region marked by the circle. This pattern clearly consists of a number of distinct hexagonally symmetric patterns, each rotated with respect to each other. This is a clear indication of random restacking and supports the Raman data that aggregation during film formation does not result in Bernal stacking.
Figure 5. (A) Wide angle AFM scan of graphene flakes spray deposited onto SiO2 from IPA. (B) A zoom into a portion of this image. (C) A Raman map of the region in part B. This map plots the integral of the Raman scan between 1200 and 2800 cm1 plotted on a point by point basis.
Deposition of Graphene onto Substrates. We note that one of the biggest problems with graphene exfoliation in high boiling point solvents is the difficulty associated with deposition of individual flakes. The reason for this is that, during the slow evaporation of the solvents, there is plenty of time for flake aggregation. This work addresses this by exploiting the ease of solvent removal from graphene exfoliated in volatile systems such as IPA. The samples were prepared by spraying the concentrated dispersion directly onto oxygen plasma cleaned SiO2 with an airbrush spray gun. We analyzed the substrate after deposition by AFM. A widefield image of the substrate is shown in Figure 5A. This image shows large numbers of flakes distributed reasonably uniformly over the substrate. Under these conditions, >10 flakes of ∼1 μm length are deposited per 10 μm 10 μm square. Figure 5B shows a higher magnification image of this substrate. The flakes are well-defined but show some evidence of aggregation, probably during the spraying process. Analysis of the flake heights showed them to be typically between 2 and 10 nm, corresponding to multiple layers (see Figure S2 in the Supporting Information). That these heights are significantly greater than the 5426
dx.doi.org/10.1021/jp110942e |J. Phys. Chem. C 2011, 115, 5422–5428
The Journal of Physical Chemistry C
ARTICLE
behavior is indicative of sonication induced scission and suggests that the concentration is controlled by flake size. The exfoliation state is good; isopropanol dispersions display only flakes with less than 5 layers. Raman measurements show the flakes to be relatively free of basal plane defects. Dispersion in volatile solvents facilitates the deposition of graphene flakes into substrates by spraying. While some aggregation is observed, the deposited flakes tend to be more numerous and thinner than for flakes deposited with high boiling point solvents. This work will make solvent exfoliation of graphene viable in a much wider variety of situations.
’ ASSOCIATED CONTENT Figure 6. ΔID/IG for individual flakes deposited on SiO2 from IPA plotted as a function of ÆLæ1 þ Æwæ1. A straight line on this plot indicates that the defects responsible for the D band are predominately edge defects. We note that this data is consistent with our previous data (measured on graphene films) as indicated by the solid line. The scatter of the data above the line is indicative of aggregation.
thicknesses suggested by the TEM analysis confirms that, as indicated by the AFM images, aggregation occurs on deposition. However, we note that this aggregation is much less than we observe when depositing from NMP or other high boiling point solvents. We can confirm that these flakes are few-layer graphene by Raman mapping. We recorded Raman spectra from spots (∼1 μm in diameter) arranged in a grid with a separation of 0.5 μm. Analysis of individual Raman spectra showed them to be compatible with graphitic material (Figure S2C, Supporting Information). By integrating between 1200 and 2800 cm1 and plotting the integrated value as a function of position, we can generate a Raman map, as shown in Figure 5C. This area corresponds to that measured by AFM in Figure 5B. The agreement between parts B and C of Figure 5 is extremely good, confirming that the objects analyzed by AFM are indeed graphene. Finally, we can use the data contained in the Raman map to further study the correlation between flake size and Raman D:G ratio. For each in a set of 23 flakes contained in a region 10 μm μm in size, we measured the flake size and the Raman D:G ratio. We plotted this data as ΔID/IG versus L1 þ w1, as shown in Figure 6. Although there is considerable scatter, it is clear that ΔID/IG does indeed scale with L1 þ w1. As described above, previous measurements on graphene films showed that ΔID/IG = 0.065[ÆLæ1 þ Æwæ1]. This curve has been superimposed on the graph and is entirely consistent with the data. We note that much of the scatter comes from data with values of ΔID/IG which are considerably higher than expected. This can be explained by aggregation effects. In many cases, Figure 5B shows small flakes adsorbed on top of larger flakes. However, while we measure L and w for the overall aggregate size, the Raman signal includes contributions from all flakes in the aggregate. The contribution from the smaller adsorbed flakes will increase the D band intensity, meaning that ΔID/IG will be larger than expected for large individual flakes alone.
’ CONCLUSION To conclude, we have demonstrated the dispersion of graphene in the volatile solvents acetone, chloroform, and isopropanol. At moderate centrifugation rates, we can achieve concentrations as high as 0.5 mg/mL. These dispersions are reasonably stable with >80% of the dispersed phase remaining after 100 h. The dispersed concentration increases with the square root of sonication time. This
bS
Supporting Information. TEM images of the sedimenting phase. Scanning Raman map including individual flake spectra and thickness profiles. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT We acknowledgement financial support from Science Foundation Ireland through the principle investigator program (grant 07/IN.1/I1772). A.O’N. thanks IRCSET for financial support. We also acknowledge Shishir Kumar for help with substrate preparation. ’ REFERENCES (1) Blake, P.; Brimicombe, P. D.; Nair, R. R.; Booth, T. J.; Jiang, D.; Schedin, F.; Ponomarenko, L. A.; Morozov, S. V.; Gleeson, H. F.; Hill, E. W.; Geim, A. K.; Novoselov, K. S. Nano Lett. 2008, 8, 1704. (2) Bourlinos, A. B.; Georgakilas, V.; Zboril, R.; Steriotis, T. A.; Stubos, A. K. Small 2009, 5, 1841–1845. (3) Hamilton, C. E.; Lomeda, J. R.; Sun, Z. Z.; Tour, J. M.; Barron, A. R. Nano Lett. 2009, 9, 3460. (4) Hernandez, Y.; Lotya, M.; Rickard, D.; Bergin, S. D.; Coleman, J. N. Langmuir 2010, 26, 3208. (5) Hernandez, Y.; Nicolosi, V.; Lotya, M.; Blighe, F. M.; Sun, Z. Y.; De, S.; McGovern, I. T.; Holland, B.; Byrne, M.; Gun’ko, Y. K.; Boland, J. J.; Niraj, P.; Duesberg, G.; Krishnamurthy, S.; Goodhue, R.; Hutchison, J.; Scardaci, V.; Ferrari, A. C.; Coleman, J. N. Nat. Nanotechnol. 2008, 3, 563. (6) Khan, U.; O’Neill, A.; Lotya, M.; De, S.; Coleman, J. N. Small 2010, 6, 864. (7) Green, A. A.; Hersam, M. C. Nano Lett. 2009, 9, 4031. (8) Lotya, M.; Hernandez, Y.; King, P. J.; Smith, R. J.; Nicolosi, V.; Karlsson, L. S.; Blighe, F. M.; De, S.; Wang, Z. M.; McGovern, I. T.; Duesberg, G. S.; Coleman, J. N. J. Am. Chem. Soc. 2009, 131, 3611. (9) Lotya, M.; King, P. J.; Khan, U.; De, S.; Coleman, J. N. ACS Nano 2010, 4, 3155–3162. (10) Vadukumpully, S.; Paul, J.; Valiyaveettil, S. Carbon 2009, 47, 3288. (11) Li, F. H.; Bao, Y.; Chai, J.; Zhang, Q. X.; Han, D. X.; Niu, L. Langmuir 2010, 26, 12314. (12) Bourlinos, A. B.; Georgakilas, V.; Zboril, R.; Steriotis, T. A.; Stubos, A. K.; Trapalis, C. Solid State Commun. 2009, 149, 2172. (13) De, S.; King, P. J.; Lotya, M.; O’Neill, A.; Doherty, E. M.; Hernandez, Y.; Duesberg, G. S.; Coleman, J. N. Small 2009, 6, 458. (14) Coleman, J. N. Adv. Funct. Mater. 2009, 19, 3680. (15) Zhang, X. Y.; Coleman, A. C.; Katsonis, N.; Browne, W. R.; van Wees, B. J.; Feringa, B. L. Chem. Commun. 2010, 46, 7539. 5427
dx.doi.org/10.1021/jp110942e |J. Phys. Chem. C 2011, 115, 5422–5428
The Journal of Physical Chemistry C
ARTICLE
(16) Nicolosi, V.; Vrbanic, D.; Mrzel, A.; McCauley, J.; O’Flaherty, S.; McGuinness, C.; Compagnini, G.; Mihailovic, D.; Blau, W. J.; Coleman, J. N. J. Phys. Chem. B 2005, 109, 7124. (17) Barton, A. F. M. CRC Handbook of Solubility Parameters and other Cohesion Parameters; CRC Press: Boca Raton, FL, 1983. (18) Hansen, C. M. Hansen Solubility Parameters - A User’s Handbook; CRC Press: Boca Raton, FL, 2007. (19) Bergin, S. D.; Sun, Z. Y.; Rickard, D.; Streich, P. V.; Hamilton, J. P.; Coleman, J. N. ACS Nano 2009, 3, 2340. (20) Nicolosi, V.; Vrbanic, D.; Mrzel, A.; McCauley, J.; O’Flaherty, S.; Mihailovic, D.; Blau, W. J.; Coleman, J. N. Chem. Phys. Lett. 2005, 401, 13. (21) Ferrari, A. C.; Meyer, J. C.; Scardaci, V.; Casiraghi, C.; Lazzeri, M.; Mauri, F.; Piscanec, S.; Jiang, D.; Novoselov, K. S.; Roth, S.; Geim, A. K. Phys. Rev. Lett. 2006, 97. (22) Malard, L. M.; Pimenta, M. A.; Dresselhaus, G.; Dresselhaus, M. S. Phys. Rep.-Rev. Sect. Phys. Lett. 2009, 473, 51.
5428
dx.doi.org/10.1021/jp110942e |J. Phys. Chem. C 2011, 115, 5422–5428
