Selectively Expanding Graphene Oxide Paper for Creating

Article pubs.acs.org/JPCC

Selectively Expanding Graphene Oxide Paper for Creating Multifunctional Carbon Materials Jinmei Zhu,† Lianwen Zhu,† Zhufeng Lu,‡ Li Gu,*,§ Shulong Cao,† and Xuebo Cao*,† †

Key Lab of Organic Synthesis of Jiangsu Province and Department of Chemistry, Soochow University, Suzhou, Jiangsu 215123, China ‡ School of Biology and Chemical Engineering, Jiaxing University, Jiaxing, Zhejiang 314001, China § School of Materials and Textile Engineering, Jiaxing University, Jiaxing, Zhejiang 314001, China

ABSTRACT: Flexible, paper-based graphene and graphene oxides are now emerging as a new class of materials with a variety of applications in electrochemical devices, novel composites, antibacterial agents, separation membranes, and so on. But the tight layer and the poor permeability limit their applications in the fields requiring permeable layers and tunable layer spacing. We demonstrate that the temperature-dependent decomposition reaction of ammonium nitrate can be utilized to modulate the layer spacing of graphene oxide paper and modify its permeability. Unlike the commonly used intercalation method, our strategy enables the layer spacing of the paper to be expanded over large range (123%−20 000%) and avoids the occupation of layer room by guest molecules. Dependent on the expansion amplitude, the papers exhibit a variety of interesting applications, including the highly efficient exclusion of small organic molecules, the separation of ultrathin nanoparticles, and the loading of polar and nonpolar guest molecules. Further, owing to the recovery of the electrical conductivity, the modified papers with large permeability should be also potential materials for flexible, paper-based electrochemical devices.

1. INTRODUCTION Flexible paper-based graphene and graphene oxides (GO) are now emerging as a new class of materials with a variety of applications in electrochemical devices,1−4 novel composites,5,6 antibacterial agents,7 separation membranes,8,9 and so on. Like graphite, high-quality graphene and GO papers made by the flow-directed assembly method have layered, planar structures where single GO and graphene platelets are stacked through both intralayer and interlayer cross-links.10−13 Owing to the tight layer and the steric hindrance from oxygen-containing groups remained on basal planes and edges of GO or chemically converted graphene, the permeation of molecules and ions into the dried, pristine papers is quite difficult. This adversely affects the application of the papers in the fields that require permeable layers and tunable layer spacing. For instance, GO and graphene papers used as electrode materials of electrochemical devices will require the greatest large layer spacing,1−5 so that the paths for the transport of electrolyte ions are unimpeded. In contrast, papers used for nanofiltration and gas storage will require smaller but precisely controlled layer spacing,8−10 so that the papers are selectively permeable and only allow the passage of gas- or liquid-phase molecules with comparable sizes. Generally, the modulation of the layer spacing of GO and graphene papers can be realized through three main approaches. The first one is to intercalate small molecules © 2012 American Chemical Society

into the layer of the paper via physical adsorption or chemical bonding. For instance, Yildirim et al. report the expansion of GO layer and the uptake of hydrogen through selective interlinkage of GO with diboronic acid.14 Nguyen et al. tune the layer spacing of GO paper by intercalating primary alkylamine with different carbon chain lengths.15 Although the intercalation method is convenient, it can only modulate the layer spacing of the papers in a small amplitude (∼1 nm). Further, it leaves behind small molecules in the papers, which is usually undesired because they occupy the layers and reduce the available rooms. The second approach is developed by Li and co-workers.9 They take advantage of the intrinsic corrugations on graphene and modulate the layer spacing of the fabricated papers through the control over the amplitude of the corrugation. This method is also limited in the small-range modulation of the layer spacing of the paper. The last one is to use the traditional freeze-drying method,16,17 which enables the formation of rich pores in the papers. But this method is known to require special equipment. Ammonium nitrate (AN), a low-cost inorganic compound widely applied in agriculture and industry, shows three modes of decomposition over the temperature range of 140−400 °C Received: July 24, 2012 Revised: August 27, 2012 Published: October 14, 2012 23075

dx.doi.org/10.1021/jp307296u | J. Phys. Chem. C 2012, 116, 23075−23082

The Journal of Physical Chemistry C

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and leaves no residues (eqs 1−3).18 At 140 °C, it decomposes into ammonia and nitric acid slowly (eq 1). At 200 °C, the decomposition reaction is accelerated and 1 mol of AN can produce 3 mol of gases (eq 2). At 400 °C, AN decomposes suddenly, and 1 mol of AN can produce 3.5 mol of gases (eq 3). We are thus inspired that if AN were interacted into GO paper and heated, gases evolved will push GO platelets apart (Scheme 1). As a result, the layer spacing is increased and its

paper and does not require special equipments. Dependent on the expansion amplitude, the paper shows a variety of applications. The slightly expanded papers can be utilized as highly efficient nanofiltration platforms for separating small organic molecules or ultrathin nanoparticles (e.g., quantum dots of CdTe and 13 nm Au particles). The extremely expanded papers can serve as amphiphilic carriers to load polar or nonpolar molecules in a high capacity. Furthermore, although there is no additional reduction treatment, the expanded GO papers are found to restore the electrical conductivity, which may make them potential materials for fabricating flexible, paper-based electrochemical devices.

Scheme 1. Scheme Showing the Route to the Selective Expansion of GO Papera

2. EXPERIMENTAL SECTION 2.1. Preparation of GO Paper, Intercalation of AN, and Thermal Expansion. Aqueous GO solution (concentration: 0.5 mg/mL) was prepared following the description in the literature.19 GO paper was made by vacuum filtration of 20 mL of GO solution through a mixed cellulose−ester membrane (50 mm in diameter, 0.2 μm pore size). Once the filtration was finished, the wet GO paper together with the cellulose membrane was immediately immersed into 40 wt % of AN solution. After the impregnation overnight, the GO paper was taken out, dried at room temperature, and peeled from the cellulose membrane. The GO paper was then heated at the temperature of 140, 200, or 400 °C, corresponding to a time of 120 min, 120 min, and 5 s, respectively. 2.2. Synthesis of Green-Emitting CdTe Quantum Dots (QDs) and 13 nm Au Nanoparticles. CdTe QDs stabilized by thioglycollic acid (TGA) were synthesized using the method reported in the literature.20 Briefly, 0.228 g of CdCl2·2.5H2O and 0.11 g of TGA were dissolved in 50 mL of distilled water, and the pH of the solution was adjusted to pH 11.0 with 1 M

a

Step i: AN is intercalated into GO paper via the simple immersion method. Step ii: the paper is heated at a given temperature and gases evolved push the GO platelets apart.

expansion amplitude can be manipulated by selecting the decomposition mode of AN. NH4NO3 → NH3 + HNO3

(1)

NH4NO3 → N2O + 2H 2O

(2)

2NH4NO3 → 2N2 + O2 + 4H 2O

(3)

Herein, by taking advantage of the temperature-dependent thermal decomposition reaction of AN, we successfully expand GO paper in a controllable manner. This strategy is facile and enables the layer spacing to be tuned in a wide range (123%− 20 000%), while it does not occupy the interior space of the

Figure 1. (a−d) Photographs of GO paper, 140 °C-paper, 200 °C-paper, and 400 °C-paper, respectively. (e−h) SEM images of the surfaces of GO paper, 140 °C-paper, 200 °C-paper, and 400 °C-paper, respectively. (i−l) SEM images of the cross sections of GO paper, 140 °C-paper, 200 °Cpaper, and 400 °C-paper, respectively. 23076



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NaOH solution. After nitrogen deaeration for 30 min the solution was added by a certain volume of fresh oxygen-free NaHTe solution prepared by the reaction of tellurium powder with NaBH4. The resulting faint yellow color solution was refluxed 2 h to form green-emitting QDs. 13 nm gold nanoparticles were synthesized using the citratebased method.21 Typically, 2 mL of HAuCl4 (concentration: 25 mM) was diluted by 170 mL of deionized (DI) water and heated up to boil. Then the solution was added by 6 mL of sodium citrate solution (concentration: 34 mM) under vigorous stirring. After further boiling for 10 min, the winered color solution was cooled down to room temperature. 2.3. Characterizations. The morphology observations of the papers were carried out on a Hitachi S-4800 scanning electron microscope (SEM). Small-angle X-ray diffraction (SAXRD) measurements were performed on an X′Pert PRO SUPER rA rotation anode X-ray diffractometer with Ni-filtered Cu Kα radiation. The BET surface area was measured by using a QuadraSorb SI surface area analyzer. UV−vis absorption spectra were recorded on a Shimadzu 3150 UV−vis−near− infrared spectrophotometer. The Raman spectrum was recorded on a HR800 Raman microspectrometer under the excitation of a 514 nm line out of a 20 mW He−Cd laser. Fourier transform infrared spectra were recorded on a Bio-Rad FTS 575C instrument. The electrical conductivities of the papers were measured by the two-probe method. The electrodes were fabricated by sandwiching the edges of the paper between two copper foils. Then, the electrodes were a CHI660c potentiostat−galvanostat (CH Instruments Inc.) for conductivity measurements. 2.4. Nanofiltration Experiments. In the experiments, 50 mL of dye solution was suction-filtered through the 140 °Cpaper (50 mm in diameter). A total of five dyes (methyl orange, methylene blue, rhodamine B, fluorescein, and Sudan III) dissolved in DI water, ethanol, or toluene had been investigated. The concentrations of all the dyes were controlled at 20 μM. To avoid the fracture of the paper and the rising of errors, the filtration was operated under a low pressure (2 kPa). When 10 mL of solution had permeated through the paper, the filtration was stopped. The filtrate was collected for subsequent analysis by UV−vis absorption spectroscopy. The filtration rate is calculated by using the equation

F = V /ts

Figure 2. SAXRD patterns of various papers: (a) the pristine GO paper, (b) GO paper intercalated by AN, (c) 140 °C-paper, and (d) 200 °C-paper.

Figure 3. (a) FT-IR spectra for the pristine GO paper, 140 °C-paper, 200 °C-paper, and 400 °C-paper. (b) Corresponding Raman spectra.

(4)

where V is the total volume of MO solution, t is the filtration time, and s is the surface area of the paper. The filtration of the QDs or Au nanoparticle solution with the 200 °C-paper was the same as the above description. 2.5. Adsorption Experiments. A piece of the 400 °Cpaper of precisely known mass was brought about to contact the liquid of acetone, ethanol, cyclohexane, toluene, petroleum ether, dimethylformamide, chlorobenzene, mineral oil, or chloroform. After saturated adsorption, the paper was taken out and its mass was measured to determine the loading capacity.

Figure 4. Panoramic SEM image of the cross section of the 400 °Cpaper.

Table 1. Thickness and Specific Surface Area for Various Papers GO paper thickness (μm) specific surface area (m2/g)

3. RESULTS AND DISCUSSION Figure 1a shows a representative GO paper made by the flowdirected assembly method (diameter: 50 mm; thickness: 5.6 ± 0.3 μm). SEM observations reveal that the surface of the paper is quite smooth (Figure 1e), and the cross section is arranged by numerous near-parallel GO platelets (Figure 1i). Prior to the intercalation of AN, the paper supported by the filter should

5.6 ± 0.3

140 °Cpaper

200 °Cpaper

400 °C-paper

6.9 ± 0.2 3.1

7.5 ± 0.2 13.2

1106 ± 28.5 72.6

keep wet. Otherwise, along with the dehydration of GO paper, GO platelets will stack tightly and the permeation of AN inside the paper will be difficult.9 Relative to that of the pristine GO 23077



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According to IR spectra (Figure 3a), the characteristic peaks for GO appear at 3500 cm−1 (C−OH), 1736 cm−1 (CO), 1634 cm−1 (aromatic CC), 1380 cm−1 (carboxyl C−O), and 1120 cm−1 (C−O).24,25 Except the peak around 1380 cm−1, other peaks for oxygen functional groups are weakened after GO paper was heated at 140 °C, indicating that oxygen functional groups are partially removed in the process of heating. The noticeable intensity of the peak around 1380 cm−1 is because carboxyl groups are more difficult to be removed relative to hydroxyl and epoxy groups.24,25 Raman data (Figure 3b) are consistent with the IR results. The ratios between the intensities of D band and G band (ID/IG) for the pristine GO paper and the 140 °C-paper are 0.93 and 1.09, respectively. For GO, it is accepted that the increase of the ratio of ID/IG is correlated with the deoxidization and the reduction of GO.26 For the heating temperature of 200 °C, the paper is found to present metallic luster (Figure 1c), and its physical thickness is further increased to 7.5 ± 0.2 μm (expansion ratio: 134%) (Figure 1k). Like the 140 °C-paper, the 200 °C-paper also features a distinguishable layered structure. However, the longrange order of the layer is poor; thereby, no diffraction peak is observed in the SAXRD pattern (Figure 2d). Evidently, the accelerated decomposition of AN at 200 °C increases the layer spacing but damages the periodical arrangement of the platelets. Despite this, we can quantitatively deduce the average layer spacing of the 200 °C-paper to be ca. 3.0 nm on the basis of the expansion ratio and the layer spacing for the pristine GO paper. Interestingly, SEM characterization of the surface of the 200 °C-paper reveals that the edges of some platelets are lifted up by the gases evolved (Figure 1g), which is different from the smooth surfaces of the pristine GO paper and 140 °C-paper (Figures 1e and 1f). This result also implies that the decomposition of AN at 200 °C proceeds violently relative to that at 140 °C. In addition, GO within the 200 °C-paper is also reduced, as evidenced by IR and Raman data (Figure 3). By comparison with the ratio of ID/IG (1.13 vs 1.09), it can be found that the reduction degree of the 200 °C-paper is higher than that of the 140 °C-paper. This is understandable because the reduction mechanisms for the two papers are not identical. For the 140 °C-paper, the reduction of GO is caused by the thermal effect.27 For the 200 °C-paper, besides the thermal reduction, chemical reduction also plays an important role in the conversion of GO to graphene. A strongly reducing gas of N2O is released from the decomposition of AN at 200 °C, which can interact with the oxidizing GO. Consequently, the 200 °C-paper is deeply reduced and presents metallic luster. When heated at 400 °C, the thermal decomposition of AN proceeds more violently, and a loose, elastic paper was formed within 3−5 s (Figure 1d). From SEM image of the cross section of the paper, it can be seen that the platelets are pushed away

Figure 5. Tests of the separation performance of 140 °C-paper. (a) Cartoon images describe the change of the permeability of GO paper before and after expansion. (b) UV−vis absorption spectra of methyl orange solution (red curve), the filtrate (blue curve), and DI water (dotted curve). (c) Photograph showing the feed solution and the filtrate.

paper, the mass of GO paper impregnated is increased by 0.01 g, indicative of the intercalation of AN. Further evidence of the intercalation of AN is provided by SAXRD measurements. As shown in Figures 2a and 2b, the layer periodicities for GO paper before and after the impregnation are 2.3 and 2.7 nm, respectively. The increased layer spacing (4 Å) is approximately twice the ionic radius of NO3− (ca. 2.3 Å),22 which implies that each layer of GO paper is inserted by monolayered AN molecules. To promote the decomposition of AN, the modified GO paper was heated at a given temperature (140, 200, or 400 °C). After heating at 140 °C, the GO paper changes its color from translucent brown to opaque black (Figure 1b). The decrease of the transmittance and the color change are caused by two factors: the increase of the physical thickness of the paper and the thermal conversion of GO to graphene which has a larger extinction coefficient.23 SEM observation on the cross section of the 140 °C-paper demonstrates that its thickness is increased from the initial 5.6 ± 0.3 μm to 6.9 ± 0.2 μm (expansion ratio: 123%) (Figure 1j). However, the near-parallel arrangement of the platelets is not altered, which is attributed to the smooth decomposition of AN at 140 °C. SAXRD characterization confirms the well-ordered structure of the 140 °C-paper (Figure 2c). From the position of the peak (2θ = 3.6°), the periodicity of the layer is estimated to be 2.5 nm. That is, the interlayer distance of the 140 °C-paper is increased by 1.1 times relative to that for GO paper, close to the expansion ratio revealed by SEM. Thermal conversion of GO to graphene within the 140 °Cpaper is verified by Raman and IR spectroscopy (Figure 3).

Table 2. Summary of the Parameters of Dyes and the Separation Performance of the 140 °C-Paper dyes

solvents

concentration (μM)

molecular size (Å)

filtration rate (L h−1 m−2)

reject yield (%)

methylene blue methyl orange rhodamine B fluorescein fluorescein Sudan III Sudan III

DI water DI water DI water DI water ethanol ethanol toluene

20 20 20 20 20 20 20

6 8 8 11 11 7 7

240 205 216 235 227 173 146

∼100 ∼100 ∼100 ∼90 ∼100 ∼100 ∼85

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Figure 6. 140 °C-paper for filtering a variety of dye solutions: (a) aqueous methylene blue solution; (b) aqueous rhodamine B solution; (c) aqueous fluorescein solution; (d) ethanol solution of fluorescein; (e) ethanol solution of Sudan III. (f) Toluene solution of Sudan III. In each UV−vis absorption spectrum, the dotted curve, blue curve, and colored curve correspond to the absorption spectra of pure solvent, the filtrate, and the feed solution, respectively. Insets in the spectra show the photographs of the corresponding feed solution and the filtrate.

based on the N2 adsorption principle may reflect the change of the permeability (Table 1). For GO paper, its specific surface area is negligible because the tight layers prevent the permeation and adsorption of N2. The 140 °C-paper and the 200 °C-paper show a measurable specific surface area of 3.1 and 13.2 m2 g−1, respectively, which demonstrate that the papers become permeable and thus provide more adsorption sites for N2. Since the 400 °C-paper is rich of pores, its specific surface area is significantly increased to 72.6 m2 g−1, comparable to that of expanded graphite (EG).28 However, EG needs to be expanded under the condition of much higher temperature (>800 °C). On the basis of the expansion amplitudes of the papers, we demonstrate their several potential applications for nanofiltration and adsorption. As indicated by BET surface area tests, the 140 °C-paper and the 200 °C-paper become permeable to small molecules such as nitrogen, in contrast to the pristine GO paper which is difficult for the passage of small molecules. However, the average layer spacing of the 140 °Cpaper and the 200 °C-paper is only little larger than that for the pristine GO paper (2.5 and 3.0 nm vs 2.3 nm). Recently, carbon nanotube membranes with pore size near 2 nm have been demonstrated to be excellent in the transport of water molecules.29−32 Especially, when the tips of the nanotubes are

by the gases and large amounts of pores are formed (Figure 1l). Accordingly, the physical thickness of the 400 °C-paper is significantly increased to 1.1 mm (Figure 4), about 200 times greater than that for the pristine GO paper. The violent decomposition of AN also leads to the development of large amounts of cracks within the paper (Figure 1h). The cracks are helpful in the connection of the pores and thus increase the effective surface area of the paper, although they may be disadvantageous to the mechanical properties. In addition, although the heating time for preparing the 400 °C-paper is only several seconds, the high temperature is sufficient to remove oxygen functional groups from GO instantly (Figure 3). In a word, by means of the temperature-dependent decomposition of AN, we can expand GO paper slightly or extremely. As a consequence of the increase of the layer spacing of the papers, the van der Waals interactions between the neighboring layers should be weakened. Besides, since the heating process had removed parts of oxygen-containing groups from the basal plane of GO platelet, the steric hindrance that limits the permeability of the paper is decreased. Both the factors will favor the permeation and transport of gas- or liquidphase molecules within the modified papers. The Brunauer− Emmett−Teller (BET) specific surface area measurements 23079



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Figure 7. 200 °C-paper for separating ultrathin particles from solution. (a) UV−vis absorption spectra of CTe QDs solution (green curve), the filtrate (blue curve), and DI water (dot curve). (b) Photographs of QDs solution and the filtrate under the excitation of UV light. (c) SEM image shows that QDs appear as filter cake on top of the paper. (d) UV−vis absorption spectra of 13 nm Au particle solution (wine-red curve), the filtrate (blue curve), and DI water (dotted curve). (e) Photographs of 13 nm Au particle solution and the filtrate. (f) SEM image of Au nanoparticles retained by the paper.

Figure 8. 400 °C-paper for loading polar and nonpolar guest molecules.

Figure 9. Room temperature I−V curves for the 200 °C-paper (red), 400 °C-paper (blue), and the pristine GO paper (green). The inset shows the I−V measurement geometry.

functionalized with charged functional groups, the nanotube membranes will behave like protein channels in biological systems and exhibit significant ion exclusion ability. Herein, the 140 °C-paper and the 200 °C-paper share some common structural features with the carbon nanotube membranes: the average layer spacing (2.5 nm) is close to the tube diameter, and there are also charged carboxyl groups (−COO−) remaining on the GO platelets. Consequently, it is rational to expect the two papers to exhibit highly efficient nanofiltration performance. Further, GO paper is known to be sensitive to water, but the 140 °C-paper and 200 °C-paper are both stable in water and organic solvents because parts of hydrophilic oxygen functional groups have been removed. For the 140 °C-paper, a series of dilute dye solutions (20 μM) were used as the test liquids, which are the common models for the characterization of nanofiltration membranes. As a typical example, Figure 5 shows the filtration of aqueous

solution of methyl orange, an anionic dye with molecular size of ca. 8 Å.33 It can be seen that water molecules freely pass through the 140 °C-paper while methyl orange is completely excluded, as learned from the decolorization of the solution and the disappearance of the absorption band in the UV−vis absorption spectra. The calculated filtration rate F is 205 L h−1 m−2. Further investigations into other dye solutions (e.g., methylene blue, Rhodamine B, fluorescein, and Sudan III) confirm the high separation efficiency of the 140 °C-paper. The separation performance is summarized in Table 2, and the spectral and visual evidence are shown in Figure 6. In all cases, solvent such as water, ethanol, or toluene can freely permeate the paper, whereas the relatively large dye molecules, despite their shapes and polarity, are effectively excluded by the paper. Generally, the rejection mechanism of pores in the 2 nm regime is either through electrostatic interactions or through steric 23080



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papers ideal electrode materials for flexible, paper-based electrochemical devices such as supercapacitors and Li-ion batteries.41

effects.29 Herein the 140 °C-paper has an average layer spacing of 2.5 nm and affords charged carboxyl groups. Thus, we believe that both the electrostatic interactions and steric effects govern the excellent separation efficiency of the paper. For the 200 °C-paper, it is unable to exclude dye molecules completely when tested by the above dye solutions. A part of dye molecules are found to pass through it, together with solvent molecules. This is because the 200 °C-paper has a larger layer spacing relative to the 140 °C-paper. However, it shows a high efficiency in excluding ultrathin particles. Taking greenemitting CdTe quantum dots (QDs) (size: ∼4 nm) as example,34 they were completely excluded and appeared as a filter cake on top of the paper (Figure 7a−c). Consequently, the filtrate does not exhibit the distinct fluorescence of QDs when excited by the UV light. Further experiments on the filtration of 13 nm Au nanoparticle solution gives consistent results (Figure 7d−f), confirming the high efficiency of 200 °Cpaper in separating ultrathin particles from the solution. It is worth noting that, if GO papers that experienced thermal treatment at 140 or 200 °C (without the intercalation of AN) were used as the filtration membranes to perform the above experiments, no flux is detected. In recent years, owing to the increasing demand of water purification, nanofiltration as a cost and energy effective technique has received intense attention.35,36 As shown, the excellent nanofiltration performance may make the slightly expanded GO paper an alternative class of separation membranes. The 400 °C-paper is featured by rich pores, large specific surface area, and light weight. Structurally, it is composed of hydrophobic planar aromatic networks and hydrophilic oxygencontaining groups remained on the platelets, as learned from the IR peaks around 1380 cm−1 (carboxyl C−O) and 1736 cm−1 (CO) (Figure 3a). Therefore, 400 °C-paper is essentially a type of amphiphilic carbonaceous materials (ACM) that are valuable in catalysis, adsorption, and enrichment of biomolecules.37,38 In this study we have tried to use it to carry polar and nonpolar guest molecules. The results demonstrate that the 400 °C-paper can load them in extremely large capacities (Figure 8). Specifically, the uptake capacity is up to 215 times the weight of the paper in the case of chloroform. By comparison with a recently reported porous carbonaceous material,39 it can be found that the uptake capacity of our paper is much larger. By the way, it is also possible to make the paper load nonpolar or polar molecules solely through the surface modification to form a low-energy or high-energy surface.39 Besides the above applications, another interesting feature is that the 200 °C-paper and 400 °C-paper restore the electrical conductivity (Figure 9), in contrast to the insulating property of GO paper. The 140 °C-paper is likewise nonconductive, which can be attributed to its low reduction degree. The bulk conductivities calculated for the 400 °C paper and the 200 °C paper are 0.12 and 150 S/m, respectively, sufficient for many electrical applications.40 As discussed before, the 200 °C-paper undergoes not only the thermal reduction but also the chemical reduction. Consequently, it is not surprising that it shows much larger bulk conductivity than the 400 °C-paper, although its heating temperature is lower. In addition, the extremely high expansion amplitude of 400 °C-paper, which will lead to an insufficient 3D conductive network, might be another key factor for the lower conductivity of the 400 °C-paper. Anyway, the electrical conductivity of the 200 °C-paper and 400 °Cpaper, together with the large permeability, will make the

4. CONCLUSION In summary, we have demonstrated that the temperaturedependent decomposition reaction of AN can be utilized to tune the layer spacing of GO paper and modify its permeability. Unlike the commonly used intercalation method, our strategy enables the layer spacing of the paper to be expanded over large size ranges and avoids the layers to be occupied by guest molecules. Performance studies suggest that the slightly expanded papers may serve as highly efficient nanofiltration membranes for the highly efficient exclusion of small organic molecules and the separation of ultrathin nanoparticles. The extremely expanded paper is amphiphilic carrier and shows a high capacity in loading polar and nonpolar molecules. Further, since the modified papers combine the permeability and the electrical conductivity, they should be also potential materials for fabricating flexible, paper-based electrochemical devices such as supercapacitors and Li-ion batteries.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (X.C.); [email protected] (L.G.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support from the National Natural Science Foundation of China (21107032), the Natural Science Foundation of Jiangsu Province, China (BK2010210), and the Project Funded by the Priority Academic Program Development (PAPD)of Jiangsu Higher Education Institutions.

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REFERENCES

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