Transparent and highly conductive liquid-phase exfoliated graphite films treated with low-temperature air-annealing

Materials Chemistry and Physics 143 (2013) 85e92

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Transparent and highly conductive liquid-phase exfoliated graphite films treated with low-temperature air-annealing Xiaoying Tong a, Cuncun Xie a, Ling Si b, Jianfei Che a, **, Yinghong Xiao b, * a Key Laboratory of Soft Chemistry and Functional Materials, College of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, People’s Republic of China b Jiangsu Collaborative Innovation Center of Biomedical Functional Materials, College of Chemistry and Materials Science, Nanjing Normal University, Nanjing 210097, People’s Republic of China

h i g h l i g h t s
Highly stable graphene dispersion was prepared by exfoliation with the aid of Nafion.
Low-temperature annealing was employed to treat graphene films for the first time.
Graphene films possess sheet resistance of 2.86 KU sq1 and light transmittance over 84%.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 26 March 2013 Received in revised form 11 July 2013 Accepted 16 August 2013

This article presents a novel and simple method of liquid-phase exfoliation to fabricate graphene films that possess high conductivity and good light transparency. Graphite was exfoliated in watereethanol mixture, with the aid of Nafion, to give highly stable graphene dispersion. Transparent graphene thin films were easily deposited by vacuum filtration from the Nafion-stabilized graphene dispersion. More important, low-temperature air-annealing (at 250  C for 2 h) was employed to treat freshly-prepared graphene films for the first time. It demonstrates that the technique is advantageous and quite efficient for the fabrication of exfoliated graphite films with defect-free structure and high purity, confirmed by TEM, SEM, FTIR, XPS, and Raman spectra. The resulting graphene films possess a sheet resistance lower than 2.86 kU sq1 and optical transmittance over 84% at a typical wavelength of 550 nm. Ó 2013 Elsevier B.V. All rights reserved.

Keywords: Fullerenes Thin films Electrical conductivity Optical properties

1. Introduction Transparent conductive films (TCFs),1 with their unique combination of electronic conductivity and transparency in the visible region of the spectrum [1], play a critical role in many current and emerging optoelectronic devices, such as solar cells [2,3], lightemitting diodes [4], flat-panel displays [5], antistatic and antiglare coatings [6], and low-emissivity windows [6,7]. The traditional conducting and transparent metal oxides, such as indium tin oxide (ITO) and fluorine tin oxide (FTO), still remain some challenges in practical use due to their disadvantages in high manufacture cost, chemical instability, brittle nature and limited near-infrared transparency [8,9].

* Corresponding author. Tel./fax: þ86 25 8589 1536. ** Corresponding author. Tel./fax: þ86 25 8431 5949. E-mail addresses: [email protected] (J. Che), [email protected] (Y. Xiao). 1 Transparent conductive films (TCFs); graphene oxide (GO); chemically reduced graphenes (CRGs). 0254-0584/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2013.08.017

Recently graphene has attracted tremendous attention as a novel optoelectronic material to replace the traditional transparent conductors. Graphene is the first example of truly two-dimensional materials. The researches yield graphene’s opacity of only 2.3  0.1% and negligible reflectance (<0.1%) due to its one-atomthick structure, implying its good optically transparent properties [10,11]. Furthermore, charge carriers in an individual graphene sheet delocalize over the entire sheet and can travel thousands of interatomic distances without scattering. As a zero-gap semiconductor, an individual graphene sheet exhibits very high in-plane conductivity with carrier mobility up to 200,000 cm2 V1 s1 [12]. Its high transparency and low resistivity make this two dimensional crystal ideally suitable for the fabrication of transparent conductor. Together with its high chemical stability and mechanical strength, graphene should improve the durability of the optoelectronic devices. Thus far, two traditional types of TCF fabrication methods based on graphene have enjoyed reliable success: (i) Fabrication from graphene oxide (GO) followed by chemical reduction. It is

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well known that GO can be easily exfoliated to give monolayer with high yield and efficiency [13]. Therefore most of the recent efforts on graphene film investigation were based on GO. Through a liquid-phase reduction, the conductivity of the insulating GO films can be partially recovered by reducing agents, as typical as hydrazine. However, the reduction is not sufficient, for the resulting films often display resistivity higher than 104 U sq1 at ca. 80% transmittance [14e17]. High-temperature thermal treatment is able to further restore the sp2 structure of graphene and the resistivity can be decreased to ca. 103 U sq1 [18,19], but it is inapplicable with flexible substrate such as PET due to the high temperature; and (ii) Fabrication from chemical vapor deposition (CVD) graphene followed by transference. At present, CVD is the most promising and almost impeccable method for the preparation of graphene. The typical CVD process includes hydrocarbon pyrolysis of carbon species and graphene forms on the surface of the catalyst from the dissociated carbon species [20]. By using CVD method, some researchers have successfully synthesized graphene on nickel foil [21] or copper foil [22], which opens an avenue to the controllable synthesis of large area, high conductivity and uniform graphene. However, the preparation of graphene transparent conductive films from CVD is restricted to equipments and size, and its nondestructive transfer technology also has certain difficulties, thus it cannot simultaneously realize mass production and low cost manufacture. Very recently, a novel and facile route for the fabrication of graphene TCFs from liquid-phase exfoliated graphite is becoming an increasing interest. Exfoliation of graphite in liquid-phase was firstly found to give oxide-free graphene monolayer with high quality and yield by Coleman and his co-workers [23,24]. The approach is direct, simple and benign with no need of complicated oxidationereduction process. Normally the oxidatione reduction process is instinctively necessary for the production of chemically reduced graphenes (CRGs), in which the oxidation causes extensive damage of pep conjugated bonds in the carbon skeleton and the distortion of the structure is very difficult to be restored during reduction. The exfoliated graphite with low oxygen content can prevent electrode failing caused by the adsorption of the oxide thus have potential applications in electro-analytical NADH sensors, facilitating the exceptionally stable and sensitive detection [25]. Furthermore, the well protected sp2 structure during liquid-phase exfoliation guarantees high conductivity of the resulting graphene sheets, which attract the attentions of researchers to fabricate high-performance graphene TCFs from liquid-phase exfoliated graphite [26e28]. Kang et al. reported graphene TCFs fabricated by the top-down method with sheet resistance of 0.3 kU sq1 and 73% transmittance at 550 nm, which is the lowest sheet resistance for graphene TCFs [28]. Herein we report a new method to prepare highly conductive and transparent graphene films from Nafion-assisted liquid-phase exfoliated graphite. In particular, graphite powder was exfoliated in watereethanol mixture with the aid of Nafion to give a graphene dispersion. From the dispersion graphene films were deposited by vacuum filtration. Instead of the traditional high-temperature thermal annealing, a low-temperature air-annealing technique for graphene TCFs was employed in our approach. The annealing purpose here is only to get rid of impurities between the adjacent graphene sheets rather than reconstruct the conjugated system thanks to the low defect content for liquid-phase exfoliated graphite in comparison with that of CRGs. After low-temperature annealing treatment, films with a sheet resistance of 2.68 kU sq1 and light transparency of 84% were obtained. Our method has obvious advantages of being eco-friendly by using acid-free water

dispersion and energy-cheap by using low-temperature airannealing. 2. Experimental 2.1. Graphene dispersion preparation Graphite powders (particle size of 1e44 mm, Sinopharm Chemical Reagent Co., Ltd.) were dispersed in a watereethanol mixture solution (volume ratio 1:1) of perfluorosulfonated cation-exchange polymer (Nafion, Jiangsu Huayuan Hydrogen Technology Development Co., Ltd) by sonication for 1 h, using a tip sonication instrument (Scientz-IID, Ningbo Scientz Biotechnology Co., Ltd.). The resulting dispersion was left to stand overnight and then centrifuged at 5000 rpm for 30 min (TGL-16G, Shanghai Yuefeng Instruments & Meters Co., Ltd). After centrifugation, the supernatant was decanted and collected. The centrifuged dispersion was subjected to a second centrifugation at 7000 rpm for 30 min and the supernatant was decanted and retained for use. 2.2. Fabrication of graphene films The resulting dispersions after two-time centrifugation were first diluted 100-fold with deionized water and then vacuumfiltered with mixed cellulose ester membranes (0.22 mm poresize, 47 mm diameter). After that the membranes with captured graphene sheets were filtered by large quantity of deionized water to remove Nafion. To transfer graphene films, the membranes were cut into certain shape and size, wetted with deionized water, and pasted onto glass slide substrates with the graphene film surface against the substrate surface. A dead weight of 1 kg was applied overnight to enhance adhesion and drive out any entrapped air bubbles from the interface. Then the membranes were dissolved in an acetone bath and graphene thin films were left on the glass slides. The resulting graphene films were dried and further thermally annealed at 250  C in vacuum or air for 2 h, respectively. 2.3. Characterization The concentration and stability of graphene suspension were determined using a UVevis spectrophotometer (UV-2100, Beijing Rayleigh Analytical Instrument Corp.) based on the Lamberte Beer law. The UV recording was set at 660 nm as the reference absorption of graphene, with the solvent as blank. The measurements of electrical properties were made before and after annealing using four-probe technique (SDY-4 meter, Guangzhou Institute of Semiconductor Materials). Fourier transform infrared (FTIR) spectrum was recorded using a Shimadzu FTIR-8400S spectrometer with pure KBr as background. Raman spectra were taken on a Renishaw RM2000 confocal microspectrometer using a 100 objective lens with a 514.5 nm laser excitation. Thermo-gravimetric analysis (TGA) was carried out on a thermal analyzer Shimadzu DTG-60 with a heating rate of 10  C min1 and an air flow rate of 20 mL min1. Transmission electron microscope (TEM) images were taken with a Jeol 2100, operated at 200 kV. Surface morphologies of graphene films were determined by an optical microscope (BM-11, Shanghai Optical Instrument Factory NO. 6) and Hitachi S-4800 II field emission scanning electron microscope (FESEM). X-ray photoelectron spectroscopy (XPS) was performed in a VG ESCALAB MK II spectrometer equipped with a monochromatic Mg Ka (hy ¼ 1253.6 eV) source at a power of 240 W (12 kV  20 mA).

X. Tong et al. / Materials Chemistry and Physics 143 (2013) 85e92

The carbon peak at 285.0 eV was used as a reference to correct for charging effects. 3. Results and discussion 3.1. Graphene dispersion Graphite powder was exfoliated into flakes in a watereethanol solution, using Nafion as the stabilizing agent. The cartoon in Scheme 1 schematically illustrates the proposed process. The graphene dispersion before centrifugation is dark-black. After centrifugation, a gray dispersion is obtained with concentration as high as 0.067 mg mL1 which can remain stable for a month as shown in Fig. 1A. The high stability of the dispersion is from the efficient dispersing capacity of Nafion. Nafion is a fluorine-containing surfactant which has been evidenced to be an effective dispersant for carbon nanotubes [29] and graphene [30]. Perfluoroalkyl backbone of Nafion is more hydrophobic than that of the traditional hydrocarbon-based surfactants, which imparts stronger interaction with graphene surface. In watere ethanol mixture solution, the adsorbed Nafion layers provide steric and electrostatic repulsion between graphene sheets, preventing the sheets from forming irreversible agglomerates or even restacking to form graphite through Van der Waals interactions. Sedimentation experiment of the dispersion was carried out and the result is shown in Fig. 1B. The measured points can be fitted to a bi-exponential function which divides this process into two sedimentary phases and one stable phase.

87

According to the fitting parameters, 0.67% of the flakes precipitate in a short period of 20.9 h and 5.56% precipitates over a longer period (ca. 206 h). The rest 93.7% of the sample is stable over the time frame of 32 days. As the sedimentation time is related to the dimensions of the sediments [31], these stable flakes are supposed to be successfully exfoliated into thin graphene sheets. The successful exfoliation is also confirmed by TEM. It is observed that nearly all sheets are present in monolayer or a few-layer structure (Fig. 1C and D), indicating a high exfoliation degree. It is reported that oxygen functional groups and defects generally have adverse effects on the excellent performance of graphene, including but not limited to conductivity. The liquidphase exfoliation method avoids the complicated oxidatione reduction process, in which the oxidation will destroy the graphene structure while the reduction cannot reconstruct the extensive p-conjugated orbital system thoroughly. The highquality and defect-free structure of the exfoliated graphene is confirmed by FTIR and XPS, as shown in Fig. 2. Unlike traditional CRGs, the FTIR spectrum of the as-prepared graphene (Fig. 2A) shows no significant absorption peaks from 1715 to 1740 cm1, the characteristic features for C]O, eCOOH and CeOH groups [32]. The peaks from 974 to 1260 cm1 are attributed to the characteristic absorptions of residual Nafion, including vibration absorptions of CeOeC, eSO3e and CeF. XPS in Fig. 2B shows no indication of graphene to be oxidized. A series of weak peaks can be observed in the deconvoluted XPS C1s spectra, which are attributed to carbon atoms in different functional

Scheme 1. Schematic illustration of liquid-phase exfoliation of graphite with the aid of Nafion.

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Fig. 1. (A) Photograph of a graphene dispersion in Nafion solution before (left) and after (right) centrifugation. (B) Absorbance (l ¼ 660 nm) as a function of time for a centrifuged graphene dispersion (CG ¼ 0.067 mg mL1). (C) and (D) TEM images of graphene sheets. (C) A few-layer graphene sheet. (D) A single graphene sheet.

groups: aromatic or conjugated C (284.5 eV), C in CeOeC bonds (286.7 eV), C in eCSO3e bonds (289.4 eV), and C in FeCeF bonds (292.3 eV). In spite of the conjugated C atoms, the ratio of the calculated relative percentages of the remaining groups is similar to that of Nafion, implying the groups are assigned to the residual Nafion instead of oxidized graphene in the exfoliated graphene. 3.2. Surface morphology of graphene films The graphene-rich, Nafion-stabilized dispersions were diluted and vacuum filtered to obtain thin graphene films as shown in Fig. 3A. These films, observed by optical microscopy (Fig. 3B), are found to be continuous, uniform and dense in comparison with the blank region on the left. FESEM observation was conducted to further investigate the structure and morphology of the films. As shown in Fig. 3C and D, graphene flakes with various shape and size (200e500 nm) randomly stack on each other. These interlaced flakes form an enormous electrical network, in which carriers travel around, providing graphene films with excellent electrical conductivity. It also can be seen that the flakes are not smooth with small grains on the surface before thermal annealing. These impurities are associated with amorphous carbon particles which are supposed to result from the sonication procedure. These particles can hinder the contact between graphene sheets, resulting in poor conductivity of graphene films. After thermal annealing, the film looks rather smooth and the graphene flakes are clean and tightly laminated on the substrate. Therefore further treatments such as thermal annealing are often employed to improve the film performance.

3.3. Effect of thermal annealing Up to date, thermal annealing is a popular way to increase the conductivity of TCFs based on CRGs [33]. There are two reasons for conductivity increasing with thermal annealing: the extensive pconjugated orbital system is partially reconstructed and the impurities are removed. The conductivity increases with annealing temperature. In particular, the acceptable properties are often obtained after high-temperature thermal treatment (generally 800e 1000  C). But in the applications of flexible TCFs, thin polymer films often serve as the substrates. The high temperature of thermal annealing is greatly limited for even the best ovenproof polyimide film which can be used for long time only below 310  C. In our approach, a low-temperature thermal annealing technique for graphene TCFs was employed. The main purpose of thermal annealing here is to get rid of impurities rather than reconstruct the conjugated system due to the low defect content of the liquidphase exfoliated graphene in comparison with CRGs. Thus it is an advantageous strategy to perform a low-temperature thermal annealing treatment. Fig. 4A shows the weight loss of graphite and exfoliated graphene in air with increasing temperature. The curve for graphite powder is almost flat, indicating its stability over the whole range from 30 to 600  C. As for exfoliated graphene, three stages of weight loss can be clearly seen. The slope before 100  C is due to the evaporation of solvent molecules remaining in the sample. After 400  C, the sample weight starts to decrease drastically, demonstrating the oxidation and decomposition of the graphene sheets, which is in agreement with the reported thermal properties of

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89

edges are formed causing ID/IG of the as-prepared graphene is higher than that of graphite powder. After vacuum annealing, the ID/IG of graphene films does not increase compared to that of asprepared films, implying no structure change occurs. After airannealing, a little increase of ID/IG is observed, which is expected to be caused by partial oxidation of graphene sheets. However, compared to CRGs, D and G bands of the air-annealed graphene film are much narrower and ID/IG is much lower. It demonstrates that after air-annealing treatment, the content of defects in our samples is still lower than that in CRGs. The structural integrality of graphene sheets is well preserved after liquid-phase exfoliation. It is worth noting that a 2D band blue-shift is observed from 2705 cm1 for the as-prepared films to 2713 cm1 for vacuumannealed films and 2715 cm1 for air-annealed films, respectively. It has been reported that an increase of distance between two adjacent layers of graphite could cause a red-shift of 2D band, due to weakened interlayer interactions [36]. That is to say that the blue-shift implies a decrease of interlayer distance. It is reasonable since after vacuum annealing, residual solvent molecules are driven out from the as-prepared films, resulting in a decrease in distance between graphene flakes. Air-annealing not only gets rid of the solvent molecules, but also more effectively removes most of the impurities between the flakes. Therefore the upper graphene flakes collapse down to the lower ones, leading to a further decrease in interlayer distance and better contact between the graphene flakes, which is corroborated by the above SEM observation. 3.4. Transparency and sheet resistance of graphene films

Fig. 2. (A) FTIR spectra of graphene (a) and graphite powder (b). (B) Deconvoluted XPS C1s spectra of graphene.

graphene [32]. Interestingly, the appearance of slope in the shade region from 200 to 280  C is quite unexpected compared to the reported data of graphene. It is expected to be caused by the oxidation of impurities such as amorphous carbon, small-size graphene fragments and residual Nafion, which are more susceptible to be decomposed by oxygen than graphene sheets. The content of these impurities is about 3 wt%. Fig. 4B shows the weight loss of graphene at a constant temperature of 250  C. It can be seen that in the whole heating process, sample weight falls, owing to elimination of residual water and impurities, and the weight loss stops at about 25 min. After that, only a little loss of sample weight is observed in the following 3 h. It is evidenced that graphene sheets are more stable than impurities and thermal annealing at 250  C is safe and effective in eliminating the impurities. Raman spectroscopy is used to evaluate the effects of thermal annealing on the structure of graphene films, which is shown in Fig. 5 with the spectra of graphite powder and CRGs as controls. Three prominent bands can be seen: D band, G band and 2D band appearing around 1350, 1580, and 2700 cm1, respectively. The bands’ frequencies and intensity ratio of D and G bands are listed in Table 1. It is well-known that the intensity ratio of D (ID) to G (IG) band can be used as an indication of defect quantity [34]. The less is the defect content, the lower is the value of ID/IG [35]. In sonication, graphite sheets are cut into smaller pieces and a number of new

Transparency and sheet resistance are two important properties of TCFs in practical applications. Fig. 6 presents UVevis spectra of graphene films with different treatments. The spectra are recorded from 380 to 800 nm. Since the thickness of the film is hard to measure, the graphene mass per square centimeter is used as a function of the thickness. It can be seen that film transmittance decreases with the increase of graphene mass (Fig. 6A). After vacuum thermal annealing, the optical transmittance of the films slightly changes (Fig. 6B). In comparison, air thermal annealing seems more efficient in improving transmittance, with an increment of 8e10% (Fig. 6C). Thermal annealing causes a removal of residual solvent molecules and a rearrangement of graphene sheets, making the films more homogeneous. Meanwhile, when heated in air, most of the amorphous carbon, small-size graphene fragments and residual Nafion are supposed to be oxidized and removed. The simultaneous effects of removing solvent, decreasing carbon impurities and structure rearranging make the films thin and uniform, and thus elevate their optical transmittance further. It is worth to note that in the research of graphene films prepared from CRGs the darkening of the reduced material has been observed during thermal annealing, a phenomenon which is attributed to partial restoration of p-electron system in GO [17]. However, in this report, low-temperature thermal annealing does not result in any structure reconstruction (as discussed in Raman analysis). Therefore transparency increase instead of decrease is observed for our thermal annealed films. Sheet resistance is also measured for the graphene films with different treatments, plotted as a function of graphene mass density and transmittance at 550 nm, respectively (Fig. 7). The sheet resistance falls off as the thickness of films increases, shown in Fig. 7A. It can be seen that vacuum thermal annealing decreases the sheet resistance by approximately two orders of magnitude. Meanwhile, air thermal annealing reduces the sheet resistance by as high as four orders of magnitude. It indicates that air thermal annealing is a much more efficient way to enhance the conductivity

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Fig. 3. (A) Photograph of a graphene film on a glass slide (graphene mass density is 2.9 ug cm2), (B) Optical micrograph of an as-prepared graphene film. FESEM images of graphene films before (C) and after (D) thermal annealing.

of graphene films. Again, according to the above analyses air thermal annealing treatment eliminates not only solvent molecules, but also most of impurities on the surface of graphene sheets. It results in a compact structure and better contact between adjacent graphene sheets, and thus formation of more electrical pathways. Fig. 7B shows the relationship between sheet resistance and optical transmittance. As the optical transmittance goes up, the sheet resistance increases. Eventually, films with a sheet resistance of 2.86 kU sq1 and corresponding optical transmittance of 84% at a typical wavelength of 550 nm are achieved.

110

A

Weight (%)

100

a b

90 80 70

4. Conclusions

60

a graphite b graphene

50 0

Graphite was exfoliated in watereethanol mixture, with the aid of Nafion, to give a stable graphene dispersion with concentration

100 200 300 400 500 600 Temperature

300

B a

Weight (%)

250 200

90 b

150

80

100 a temperature b weight

70 60

0

50

100 150 Time (min)

200

Temperature

100

Air-annealed

Intensity (au)

110

Vaccum-annealed As-prepared

CRG G

50

D

Grahpite powder

2D

0 1000

1500

2000

2500

3000

-1

Raman shift (cm ) Fig. 4. (A) Weight loss of graphite powder (a) and graphene (b) as a function of temperature. (B) Graphene weight loss as a function of time at a constant temperature of 250  C for 3 h.

Fig. 5. Raman spectra (514 nm) of graphite powder, CRG and exfoliated graphene films before and after thermal annealing at 250  C in vacuum or air for 2 h.

X. Tong et al. / Materials Chemistry and Physics 143 (2013) 85e92 8

Table 1 Frequency of D, G and 2D bands & intensity ratio of D and G bands.

uD/cm1

uG/cm1

u2D/cm1

ID/IG

Graphite powder CRG As-prepared Vacuum-annealed Air-annealed

1357 1345 1352 1354 1351

1579 1581 1576 1576 1579

2722 2683, 2925 2705 2713 2715

0.08 1.28 0.41 0.45 0.68

up to 0.067 mg mL1. The characterization of FTIR, XPS and Raman spectrum confirmed that liquid-phase exfoliation method avoids complicated oxidationereduction process and can be applied to obtain graphene films with high-quality and defect-free structure.

Sheet resistance (ohm/sq)

10

Sample

A 6

10

4

10

2

10

As-prepared Vacuum-annealed Air-annealed

0

10

2

100

A

8

40 20 As-prepared

2.9 microgram 4.5 microgram 5.8 microgram 8.7 microgram 10.1 microgram

500 600 700 Wavelength (nm)

800

Sheet resistance (ohm/sq)

Transmittance (%)

60

400

4 6 8 10 -2 Graphene mass (microgram cm )

10

80

0

91

100

B 6

10

4

10

2

10

As-prepared Vacuum-annealed Air-annealed

0

10

30

40

50

60

70

80

90 100

Transmittance (%)

Transmittance (%) 80

B Fig. 7. Sheet resistance plotted as a function of a graphene mass density (A) and transmittance at 550 nm (B) for As-prepared, Vacuum-annealed and Air-annealed films.

60 40 20 Vacuum-annealed 250 , 2h 0

400

500 600 700 Wavelength ( nm)

800

100 Transmittance (%)

C

Acknowledgments

80

This work was supported by Specialized Research Fund for the Doctoral Program of Higher Education of China (20123219110010), Natural Science Foundation of Jiangsu Province of China (Grant No. BK2012845) and Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

60 40 20

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Air-annealed 250 , 2h 0

From Nafion-stabilized graphene dispersion, TCFs were easily deposited by vacuum filtration. For the first time, low-temperature air thermal annealing was employed for post-treatment of graphene films. The thermal annealing eliminates not only solvent molecules, but also most impurities on the surface of graphene sheets. The post-treated graphene films have a compact structure and better contact between adjacent graphene sheets, and thus possess more electrical pathways. After treatment, films with a sheet resistance of 2.86 kU sq1 and corresponding optical transmittance of 84% at a typical wavelength of 550 nm are achieved.

400

500 600 700 Wavelength (nm)

800

Fig. 6. Transmittance spectra of graphene films obtained with different treatments. (A) As-prepared, (B) vacuum-annealed (250  C, 2 h) and (C) air-annealed (250  C, 2 h).

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