Size Fractionation of Graphene Oxide Nanosheets via Controlled

Aug 25, 2017 - GO nanosheets with a narrow size distribution can be obtained by controlling the growth rate of the freezing front. This interesting ph...
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Size Fractionation of Graphene Oxide Nanosheets via Controlled Directional Freezing Hongya Geng,† Bowen Yao,‡ Jiajia Zhou,§,∥ Kai Liu,† Guoying Bai,† Wenbo Li,† Yanlin Song,† Gaoquan Shi,‡ Masao Doi,∥ and Jianjun Wang*,†,⊥ †

Key Laboratory of Green Printing, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China Key Laboratory of Bioorganic Phosphorus Chemistry & Chemical Biology, Department of Chemistry, Tsinghua University, Beijing 100084, China § Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of Education, School of Chemistry, and ∥ Center of Soft Matter Physics and Its Applications, Beihang University, Beijing 100191, China ‡

S Supporting Information *

ABSTRACT: The properties and functions of graphene oxide (GO)-based materials strongly depend on the lateral size and size distribution of GO nanosheets; therefore, GO and its derivatives with narrow size distributions are highly desired. Here we report the size fractionation of GO nanosheets by controlled directional freezing of GO aqueous dispersions. GO nanosheets with a narrow size distribution can be obtained by controlling the growth rate of the freezing front. This interesting phenomenon can be explained by the adsorption of GO nanosheets on the ice crystal surface in combination with the stratification of GO nanosheets at the ice growth front. Such a convenient size fractionation approach will be essential for practical applications of chemically modified graphene, including GO, reduced GO, and their assemblies or composites.

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INTRODUCTION Graphene and its derivatives have enjoyed significant attention in numerous intriguing applications because of their unique two-dimensional (2D) arrangement of carbon atoms and excellent electronic,1,2 electrical,3,4 optical,5,6 and mechanical properties.7,8 The functionality and properties of graphene oxide (GO)-based materials are determined by the lateral size and size distributions of GO nanosheets.9−11 For example, GO with smaller lateral sizes exhibits an enhanced antimicrobial activity.12 GO of large lateral size is highly desirable for the preparation of conductive monoliths13 and films.14−16 However, GO nanosheets usually have a broad size distribution due to the random chemical exfoliation process that is the most widely used method for the mass production of GO.17−19 Therefore, continuous efforts have been made to fractionate asprepared GO nanosheets so that GO of narrow size distributions can be obtained.20−30 Herein, we report for the first time the size fractionation of GO nanosheets by directionally freezing a GO aqueous dispersion at controlled rates. We found that GO nanosheets with narrow size distributions can be obtained when we directionally froze a GO dispersion with a concentration of 0.10 mg mL−1 (the freezing rate was tuned from 0.2 to 45.0 μm s−1). In this rate regime, the size of GO nanosheets trapped in ice crystals increased from several nanometers to tens of micrometers as the freezing rate decreased. The mechanism for the size fractionation of GO nanosheets is based on the © 2017 American Chemical Society

following facts: (1) GO nanosheets are adsorbed onto the surface of ice crystals due to the unique arrangement of hydroxyl groups on the basal plane of GO nanosheets, facilitating the preferred formation of hydrogen bonds between GO nanosheets and ice crystals,31 and (2) stratification of GO nanosheets occurs at the ice growth front because of the force exerted by the growing ice together with the Brownian motion of GO nanosheets as well as their intersheet interactions.32 We further demonstrated that GO nanosheets of narrowed size distribution can be directly utilized as inks with desired rheological behaviors, and three-dimensional (3D) porous structures with tunable pore sizes can be printed by utilizing these inks.

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EXPERIMENTAL SECTION

Preparation of GO. GO was prepared by exfoliating natural graphite via the Hummers method33 followed by dialysis for at least 1 week. The as-prepared GO dispersion was freeze-dried and dispersed in Milli-Q water by mild stirring to avoid possible mechanical damage resulting from sonication. Size Fractionation of GO. Twenty milliliters of GO aqueous dispersion prepared by Hummers’s method was transferred into a cylindrical glass bottle (the height is 4 cm; the diameter is 1 cm). Sterilized plastic Petri dishes full of liquid nitrogen (N2) were placed atop the glass bottle. For a given GO concentration (e.g., 0.10 mg Received: May 27, 2017 Published: August 25, 2017 12517

DOI: 10.1021/jacs.7b05490 J. Am. Chem. Soc. 2017, 139, 12517−12523

Article

Journal of the American Chemical Society

Figure 1. (a) Illustration of the size fractionation of GO via the controlled directional freezing. The GO aqueous dispersion was frozen from the top at a controlled freezing rate. GO nanosheets with smaller sizes were obtained from the ice melt, and larger sheets were left in the residual liquid. (b) AFM images of the GO nanosheets obtained from the GO dispersion before fractionation, the ice melt, and the residual liquid (from top to bottom). Insets are the corresponding digital photos. (c) UV−vis spectra of GO dispersions. The inset is the concentrations of GO dispersions before and after size fractionation obtained by measuring the solid content via evaporating the water of the disperion. (d) Statistics of ID/IG of different GOs, that is, GO before fractionation, GO from the residual liquid, and GO from the ice melt. mL−1), the freezing rate (the growth rate of the freezing front) was controlled by controlling the distance between the cooling center and the interface of air/GO dispersions. We positioned the opening of the bottle downward for the easy obtainment of the ice [Figure S1a, Supporting Information (SI)]. Direct Printing of GO Inks. GO inks were prepared by dispersing freeze-dried fractionated GO nanosheets into 10 mL of deionized water under mild stirring for 24 h. The GO inks (20 mg mL−1) with GO of various lateral sizes were injected into a syringe (3 cm3 barrel, EFD Inc.) attached by a Luer-Lok to a micronozzle (200 μm inner diameter). The patterns were then printed via a multiaxis dispensing system (2400, EFD Inc.) equipped with an air-powered fluid dispenser (Ultimus I, EFD Inc.). The printed GO 3D macrolattices and patterns were dried in a vacuum oven at 60 °C for 12 h and then were transferred into a sealed ground glass bottle. Several drops of a HI aqueous solution (57 wt %) were added into the sealed ground-glass bottle. After being kept away from light at room temperature overnight, the printed GO 3D macrolattices and patterns were chemically reduced. Finally, they were washed with deionized water and vacuum-dried at 60 °C for 12 h. Characterizations. The elemental analysis and the energy of the valence band were determined by X-ray photoelectron spectroscopy (ESCALab250-XL, VG). The size fractionation was confirmed by a Bruker Multimode 8 atomic force microscope (AFM) (Bruker), JEOL4800 field-emission scanning electron microscope (JEOL Ltd.), and Nikon Eclipse LV100ND optical microscope with 50× and 100× objective (Tokyo, Japan). Microscope images of GO nanosheets were taken by placing GO nanosheets on a Si surface with a 300 nm SiO2 thin layer. UV−vis adsorption spectra of GO aqueous dispersions were recorded by using a Shimadzu UV-2550 UV−vis spectrophotometer in quartz cuvettes (Shimadzu). The X-ray diffraction (XRD) data was collected with an Empyrean diffractometer using monochromatic Cu Kα1 radiation (λ = 1.5406 Å) at 40 kV (Rigaku). Raman spectra were

performed on a LabRAM ARAMIS spectrometer equipped with a 532 nm laser (HORIBA Jobin Yvon). Two-dimensional (2D) Raman spectra of GO nanosheets were taken on a Horiba HR800 Raman system with 532 nm excitation. A 10× objective was used to focus the laser beam. The photoluminescence properties of graphene oxide were characterized using a fluorescence spectrometer (Hitachi F-4500 FL).

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RESULTS AND DISCUSSION Figure 1a schematically illustrates a GO dispersion (0.10 mg mL−1) frozen from the top at a rate of 45.0 μm s−1. AFM investigations were carried out to analyze the size distributions of GO nanosheets in the crude GO dispersion before fractionation (CGO), the ice melt, and the residual liquid, respectively (Figure 1b). As confirmed by AFM imaging, the height of these GO nanosheets is of ∼0.8 nm, which is typical for single-layer GO nanosheets (Figure S1b, SI).34 The lateral size of the CGO nanosheets ranges from several nanometers to tens of micrometers (Figure S2a,b, SI), as is typical for the GO nanosheets prepared by the Hummers method. Interestingly, the GO nanosheets collected from the ice melt exhibit a narrow size distribution with an average lateral size around 5.0 nm. The narrow size distribution of these GO nanosheets can also be verified by dynamic light scattering (Figure S2c, SI). In the residual liquid, the GO nanosheets exhibit much larger lateral sizes. Meanwhile, the insets of Figure 1b display that the appearance of the GO dispersions changed greatly, i.e., the ice melt of the GO dispersion became lighter and the residual liquid of the GO dispersion turned much darker in comparison with the CGO dispersion. Light transmittance measurements of these three GO dispersions reveal qualitatively that the 12518

DOI: 10.1021/jacs.7b05490 J. Am. Chem. Soc. 2017, 139, 12517−12523

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Figure 2. (a) SEM images of GO nanosheets in the ice melt collected at various freezing rates; the scale bar is 20 μm. (b) Plot of the GO nanosheets size in the ice phase versus the growth rate of the freezing front. When the growth rate lies between 0.2 and 45.0 μm s−1, the size of the GO nanosheets in the ice melt decreases with the increase of the growth rate of the freezing front. The solid curve corresponds to the best fit of the experimental data. The equation fitted to the experimental data is y = 53.64e−x/0.5 + 12.9e−x/7.87 + 2.12 (R2 = 0.997). (c−e) Schematic illustrations of the mechanism for size fractionation. GO nanosheets with various sizes are represented by orange lines with different lengths. Arrows show the freezing direction.

between 0.2 and 45.0 μm s−1, the GO nanosheets trapped in ice crystals have narrow size distributions. With the increase of Vfr, the average size of the GO nanosheets trapped in ice crystals decreases gradually (panels II−V of Figure 2a,b). When Vfr further increases beyond 45.0 μm s−1, GO nanosheets trapped in ice crystals exhibit a broad size distribution again (Figure 2a, panel VI). Figure 2c−e illustrates the mechanism for the size fractionation. When Vfr < 0.2 μm s−1, the moving ice front shows no obvious effect on the distribution of GO nanosheets in the dispersion. In this case, the Brownian motion of GO nanosheets is more rapid than the growth rate of the freezing front. Consequently, GO nanosheets with various sizes diffuse randomly to the ice/liquid interface and can contact with the surfaces of ice crystals (Figure 2c).39 The randomly diffused GO nanosheets then are adsorbed on the surfaces of ice crystals via the formation of hydrogen bonds between GO nanosheets and ice crystals.31 On the other hand, when Vfr > 45.0 μm s−1, GO nanosheets are accumulated ahead of the freezing front, leading to the depression of the freezing temperature, a phenomenon termed constitutional supercooling.29 In this case, the ice/liquid interface at the freezing front is unstable and GO nanosheets are all trapped in ice crystals of high growth rate as schematically illustrated in Figure 2e.40 A similar phenomenon was also observed by other groups, i.e., colloidal particles were all trapped in ice when the ice/liquid interface advancing rate is above a critical value.41 When Vfr is between 0.2 and 45.0 μm s−1, the synergetic effect of the growing ice on GO nanosheets, the Brownian motion of GO nanosheets, and the cross-interaction of GO nanosheets of different sizes lead to the stratification of GO nanosheets at the ice growth front, as illustrated in Figure 2c [a detailed theoretical analysis of the stratification is provided in Figures S5−S7 (SI), which give a qualitative explanation for the

concentration of GO nanosheets in the ice melt is lower than that of the dispersion before the fractionation procedure, while that of the residual liquid is much higher (Figure 1c). In order to compare quantitatively the concentrations of GO dispersions, we measured the solid content by evaporating the water of the dispersion. As shown in Figure 1c, the concentration of the ice melt decreases to ∼0.03 mg mL−1 and that of the residual liquid increases to ∼0.17 mg mL−1 (inset of Figure 1c). To study the difference in the physicochemical properties of the size-fractionated GO nanosheets, the ratio of the disorderinduced character of the D band at 1350 cm−1 and the doubly degenerate phonon mode G band at 1590 cm−1 was measured (ID/IG).35 To ensure the statistical significance of the Raman data, more than 150 spectra for each sample were obtained by employing 2D Raman mapping technology [Figures S3 (SI) and 1d]. The average ID/IG of GO nanosheets in ice melt, before fractionation, and in residual liquid was found to be 1.25, 1.01, and 0.92. The higher ID/IG of GO nanosheets indicates that GO nanosheets in the ice melt have smaller graphite domains and more structure defects. This result is consistent with the data in Figure 1b; GO nanosheets of larger sizes possess a smaller edge-to-area ratio and thus have a lower oxidation degree and more ordered graphite domains.36 Additionally, it is shown that the 2D band of the separated GO nanosheets is centered at 2683 cm−1, which is typical for the single-layer GO nanosheets (Figure S4, SI).37,38 These distinct differences consolidate the successful size fractionation of GO nanosheets by controlled directional freezing. We then systematically investigated the effect of the growth rate of the freezing front (Vfr) on the size of GO nanosheets trapped in ice crystals (Figure 2a). When Vfr < 0.2 μm s−1, almost a full-size range of GO nanosheets was trapped in ice, as shown in the panel I of Figure 2a,b. As Vfr is in the range 12519

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Figure 3. (a−d) Schematic illustration of the size fractionation process: a CGO dispersion was frozen with the liquid N2 being placed atop and the GO nanosheets in the ice melt were taken out for further characterization. For each cycle, about 5 mL of ice melt was obtained. The procedure was repeated to obtain GO nanosheets with different size distributions. (e) Optical microscopy images and corresponding size distributions of chemically reduced fractionated GO nanosheets on 300 nm SiO2/Si substrate imaged with white light; the scale bar is 50 μm.

Figure 4. (a) Storage and loss moduli of GO dispersions as a function of shear stress and (b) log−log plots of the viscosity of GO dispersions as a function of the shear rate: P1 (black), P2 (red), CGO (green), P3 (blue), and P4 (cyan). The four portions (P1, P2, P3, and P4) of GO dispersions are concentrated to 20.0 mg mL−1. (c) Photograph of reduced 3D graphene macrolattices printed with various GO inks under same printing parameters (pressure = 15 psi; speed = 5 mm s−1). (d) Top view optical microscopic images of the 3D printed graphene macrolattice of a higher magnification. Scale bar, 1 mm. (e) The cross-sectional SEM images show typical porous structures of printed 3D macrolattices. Scale bar, 20 μm.

mL−1, size fractionation is almost not possible any more with our crude experimental setup. This observation can be explained as follows: When the concentration of GO at the ice growth front reaches a critical value and the diffusion of GO nanosheets is significantly inhibited, constitutional freezing occurs and GO nanosheets are nonselectively trapped in ice crystals.42 Therefore, a low concentration of GO dispersions allows a larger window of the freezing rate, within which size fractionation can be achieved.43 To demonstrate the practical usefulness of this facile size fractionation approach, we simply repeated the size fractiona-

stratification].32 Note that, in this specific rate range, smaller GO nanosheets are in closer proximity to the freezing front with the increase of the growth rate. As GO binds to the surface of ice crystals, the size fractionation of GO nanosheets can be realized by controlling the ice growth rate between 0.2 and 45.0 μm s−1. The effect of the GO concentration on the size fractionation is also investigated, as shown in Figures S8 and S9 (SI). One can see that as the concentration of GO increases, the range of the freezing rate narrows, between which size fractionation can be achieved, and when the GO concentration is above 1.0 mg 12520

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attributed to the enhanced π−π interaction between larger GO nanosheets.49 It is obvious that the fractionated GO dispersions exhibit appropriate rheological behaviors for printing, i.e., the shear-thinning behavior as well as the fact that the storage modulus is higher than the loss modulus.50,51 We further utilized fractionated GO inks for 3D printing. Figure 4c shows a printed 3D macrolattice with GO aqueous dispersions as the ink. One can see that the 3D macrolattices with P1 and P2 as the inks collapse, possibly due to the low apparent viscosity of P1 and P2 GO inks, while the 3D macrolattice with P4 as the ink has a sharp edge, suggesting that the P4 GO ink is more effective in generating printed patterns with a higher spatial resolution.52 The distinct 3D structures printed with different GO inks can also be seen in Figure 4d. The wall thickness of the structure printed by the P4 GO ink is about 200 μm, which is comparable to the inner diameter of the micronozzle (200 μm). This can be attributed to the high apparent viscosity of the P4 GO ink, which avoids the spread of the inks.53 Moreover, the higher storage modulus of the P4 GO ink than the loss modulus prevents the collapse of the freshly printed 3D structure.54 Further investigation with scanning electron microscopy (SEM) reveals that the printed 3D structures are porous and the pore size can be well adjusted by using the fractionated GO inks. The pore size of the printed 3D structure increases with the size of GO nanosheets. Note that porous graphene materials with tunable pore size have broad applications, such as catalysis scaffold, oil/water fractionation, and water cleaning.49,55

tion procedure to fractionate one GO dispersion into four portions, as schematically shown in Figure 3a−d. Briefly, a CGO dispersion (20 mL of 0.10 mg mL−1) in a cylindrical glass bottle was frozen from the top by placing a Petri dish full of liquid N2 atop the bottle. Each time, around 5 mL of the GO dispersion was frozen and taken out. As the distance from the liquid nitrogen increases, the growth rate of freezing front decreases. Additionally, this is a stepwise size fractionation process. The fractionation process was repeated for the residual dispersion. After each step, the concentration of the residual liquid increased slightly from the original 0.10 mg mL−1 to ∼0.11, ∼0.12, and ∼0.14 for P1, P2, P3, and P4, respectively. As such, we obtained four portions of GO nanosheets from the ice melt (P1, P2, P3, and P4). The size of GO nanosheets trapped in ice crystals was characterized by optical microscopy. As reported by Geim and Novoselov,44 single graphene sheets have adequate interference color with a maximized contrast, when placed atop the silicon (Si) wafer surface with a silicon dioxide (SiO2) layer of 300 nm in thickness.45 Figure 3e shows the optical microscopy images of GO nanosheets on this Si/ SiO2 wafer. The average size of GO nanosheets in P1 is ∼0.9 μm, much smaller than that of GO nanosheets (∼9 μm) in P2. Note that the freezing rate decreases with the increase of the distance between the liquid N2 and the surface of the GO dispersion. As the freezing rate further decreases (the distance between the surface the GO dispersion increases), the average size of GO nanosheets in P3 or P4 increases to ∼20 or ∼30 μm. XPS survey, Raman spectrum, and XRD investigations also show that the oxidation degree of GO nanosheets decreases in the sequence of P1, P2, P3, and P4, while the prefect graphite domains increases (Figures S10 and S11, SI), consistent with the conclusion that smaller GO nanosheets are more oxidized than the larger ones.46 The low-energy XPS spectra of the fractionated chemically reduced GO nanosheets were further studied, indicating the change of the energy of the valence band of the GO nanosheets with the lateral sizes (Figure S12a, SI).47 Photoluminescence investigation also displayed the sizedependent emission of the fractionated GO nanosheets in the visible range (Figure S12b, SI). Moreover, the conductivity of the chemically reduced P4 is more than 2 times higher than that of the chemically reduced CGO, mainly due to their much lower intersheet contact resistance (Figure S13, SI).48 GO nanosheets of P1, P2, P3, and P4 were further investigated for their application as printable inks. In previous reports, GO dispersions of high concentrations or with other additives were usually required to achieve the desired rheological behavior for printing.32 In strong contrast, the rheological property of fractionated GO inks can be adjusted simply by changing the lateral size of GO nanosheets without any additives. Figure 4a demonstrates the storage and loss moduli of GO dispersions of various lateral sizes (20 mg mL−1). The CGO ink exhibits a constant storage modulus of ∼600 Pa before it yields at ∼40 Pa. Interestingly, the storage modulus of fractionated GO inks from P1, P2, P3, and P4 cover the range ∼94, ∼279, ∼1266, and ∼8153 Pa, and the yield stress can be adjusted as ∼5, ∼11, ∼56, and ∼110 Pa, respectively. With the increase of the GO lateral size, higher storage modulus and yield stress are observed. Figure 4b shows that GO inks exhibit a shear-thinning behavior, and the apparent viscosity ranges from ∼103 to ∼105 at a low shear rate of 10−1 s−1 by merely tuning the lateral size of GO nanosheets. The increase of the appearance viscosity with the lateral size of GO can be

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CONCLUSION

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ASSOCIATED CONTENT

In summary, we report for the first time the size fractionation of GO nanosheets via controlled directional freezing of GO disperisons. When the growth rate of the freezing front is appropriate, stratification at the ice growth front occurs. As such, GO nanosheets with a narrow size distribution are adsorbed on the ice crystal surface because of the formation of hydrogen bonds between GO nanosheets and ice crystals. We show that size fractionated GO dispersions with a narrow size distribution can be used as inks to print 3D architectures with tunable pore sizes, as the rheological behaviors of the GO dispersions can be tuned by adjusting the lateral size of GO nanosheets in the dispersion.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b05490. Schematic illustration of the size fractionation, histogram of the height of GO nanosheets, characterization of the separated GO nanosheets and GO films prepared by cast drying, microscopy images of patterns printed with GO inks, Raman spectra of GO nanosheets, theortetic considerations, volume fraction profiles at different times, plot of the GO concentration versus the freezing rate range, optical microscopy images of GO nanosheets, XPS surveys of GO nanosheets, optical image of the printing process, low-energy XPS spectra of GO nanosheets, and XRD patterns of the printed GO films before and after chemical reduction (PDF) 12521

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AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Jiajia Zhou: 0000-0002-2258-6757 Yanlin Song: 0000-0002-0267-3917 Jianjun Wang: 0000-0002-1704-9922 Present Address ⊥

J.W.: Institute of Chemistry, Chinese Academy of Sciences, Zhongguancun North First Street 2, 100190 Beijing, China. Author Contributions

The manuscript was written through contributions of all authors. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for the financial support from the 973 Program (2013CB933004), the Key Special Project of Nanotechnology of China (2016YFA0200200), the Chinese National Nature Science Foundation (51436004, 21421061, 51433005), and the Strategic Priority Research Program of Chinese Academy of Sciences (XDA09020000).

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REFERENCES

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DOI: 10.1021/jacs.7b05490 J. Am. Chem. Soc. 2017, 139, 12517−12523

Article

Journal of the American Chemical Society (53) Naficy, S.; Jalili, R.; Aboutalebi, S. H.; Gorkin Iii, R. A.; Konstantinov, K.; Innis, P. C.; Spinks, G. M.; Poulin, P.; Wallace, G. G. Mater. Horiz. 2014, 1, 326−331. (54) Sun, X.; Liu, Z.; Welsher, K.; Robinson, J. T.; Goodwin, A.; Zaric, S.; Dai, H. Nano Res. 2008, 1, 203−212. (55) Nallan, H. C.; Sadie, J. A.; Kitsomboonloha, R.; Volkman, S. K.; Subramanian, V. Langmuir 2014, 30, 13470−13477.

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DOI: 10.1021/jacs.7b05490 J. Am. Chem. Soc. 2017, 139, 12517−12523