Direct reduction of graphene oxide films into highly conductive and flexible graphene films by hydrohalic acids

Transcription

1 CARBON 48 (2010) available at journal homepage: Direct reduction of graphene oxide films into highly conductive and flexible graphene films by hydrohalic acids Songfeng Pei, Jinping Zhao, Jinhong Du *, Wencai Ren, Hui-Ming Cheng ** Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang , People s Republic of China ARTICLE INFO ABSTRACT Article history: Received 3 August 2010 Accepted 6 August 2010 Available online 10 August 2010 We report a simple but highly-effective hydrohalic acid reducing method to reduce graphene oxide (GO) films into highly conductive graphene films without destroying their integrity and flexibility at low temperature based on the nucleophilic substitution reaction. GO films reduced for 1 h at 100 C in 55% hydroiodic (HI) acid have an electrical conductivity as high as 298 S/cm and a C/O ratio above 12, both of which are much higher than films reduced by other chemical methods. The reduction maintains good integrity and flexibility, and even improves the strength and ductility, of the original GO films. Based on this reducing method, a flexible graphene-based transparent conductive film with a sheet resistance of 1.6 kx/sq and 85% transparency was obtained, further verifying the advantage of HI acid reduction. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Graphene-based conductive films have recently attracted considerable attention due to their potential application in transparent conductors [1 4], gas sensors [5], supercapacitors [6], etc. In particular, the graphene-based transparent conductive films (TCFs) are far more flexible than traditional indium doped tin oxide films and have promising applications in future bendable electronic and optoelectronic devices. The largest obstacle, however, is the achievement of a large-area graphene film that is both highly conductive and highly flexible. Until now, large-area graphene films prepared using graphene oxide (GO) obtained by chemical exfoliation are still of great value, since they are suitable for large-scale production, have exceptionally low cost and can easily assemble into films by simple solution processes such as filtration [6,7], spray coating [2] and dip-coating [1,3]. In contrast, graphene obtained by other methods, including mechanical exfoliation [8,9], epitaxial growth [10] and chemical vapor deposition [11], is of high quality but limited quantity and is difficult to assemble into films. However, oxygen-containing functional groups attached to GO make the assembled film almost insulating [12,13]. Therefore, an effective deoxygenating process must be carried out to make the as-assembled insulating GO films conductive [14 16]. Generally, the deoxygenating processes of GO involve high temperature thermal annealing [1,3] or low temperature chemical reduction [2,17,18]. The former is highly effective, but usually needs a temperature above 1000 C; while the latter can be realized at a temperature lower than 100 C, which is extremely important for practical applications, since graphene films are commonly supported on substrates such as plastics that cannot stand high temperature. Strong alkali agents, hydrazine [1,18] and sodium borohydride (NaBH 4 ) [2,19], have been well accepted as effective chemical reducing agents for the de-oxygenation, but the electrical conductivity of the reduced GO (r-go) film is low. However, both hydrazine and NaBH 4 are not suitable for the reduction of GO films, especially for those needing high flexibility for applications in flexible devices, due to the stiffening and disintegration * Corresponding author. ** Corresponding author: Fax: addresses: jhdu@imr.ac.cn (J. Du), cheng@imr.ac.cn, carbon@imr.ac.cn (H.-M. Cheng) /$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi: /j.carbon

2 CARBON 48 (2010) of the films during reduction. Becerril et al. [1] reported that immersing GO films in a hot aqueous hydrazine solution results in film fragmentation and delamination even on substrates with a c-aminopropyltriethoxysilane adhesion layer. We observed the same phenomena even at room temperature, and found that hydrazine vapor reduction caused the r-go film to be rigid and, similarly, NaBH 4 solution reduction caused the r-go film to be fragile due to H 2 bubbles bursting. Therefore, a more effective route must be developed to reduce GO films into highly conductive graphene films without destroying their original high flexibility and integrity. We have used an acidic agent, hydroiodic (HI) acid, to efficiently reduce GO films. The obtained r-go film can have a much higher electrical conductivity and C/O atomic ratio than those of films reduced by other chemical methods, while maintaining very good flexibility with a much higher tensile strength than that of the original GO film. The effective reduction of GO film by HI acid can also provide valuable implications for understanding the reducing mechanism of GO, which accordingly may facilitate the de-oxygenation and application of GO and other carbon materials with oxygencontaining groups. 2. Experimental 2.1. Materials and method The 55% HI acid was purchased from Aladdin-reagent Inc., and used as received. GO was prepared from natural graphite flake by a modified Hummers method [20]. The fabrication of free-standing GO films and TCFs assembled on a polyethylene terephthalate (PET) substrate was carried out following the unique assembly process at a liquid/air interface reported by Chen et al. [21]. Typically, GO hydrosol was heated to 353 K for 10 min to 5 h in a water bath, during which a smooth and condensed film was formed at the liquid/air interface; the film thickness could be controlled by varying the heating time. The thick film was transferred to a polytetrafluoroethene substrate to form a free-standing GO film after drying; and the thin film was transferred to the PET substrate to form pre-reduced TCFs. The reduction was carried out by immersing GO films into a HI acid solution in a sealed cuvette that was placed in a thermostatted oil bath. The reduction of GO films by 85% N 2 H 4 ÆH 2 O solution and 50 mm NaBH 4 aqueous solution was carried out by liquid immersion at room temperature. The reduction of GO films by hydrazine vapor was carried out by vaporizing N 2 H 4 ÆH 2 Oat40 C according to [1] Characterization of GO and r-go films Elemental composition analysis was carried out using X-ray photoelectron spectroscopy (XPS, ESCALAB 250) using focused monochromatized Al Ka radiation ( ev) and the XPS spectra were fitted using the XPS peak 4.1 software in which a Shirley background was assumed. The microstructure was characterized by X-ray diffraction (XRD, D/Max-2400) with Cu Ka radiation (k = Å) and Raman spectroscopy (JY Labram HR 800 spectrometer) using nm wavelength laser. The sheet resistance (R s, X/sq) of the r-go films was measured by a four-probe method and the corresponding volume conductivity (r, S/cm) was calculated using the formula: r = 1/(R s t), where t (unit: cm) is film thickness. The tensile tests were carried out with a Hounsfield H5K-S materials tester. 3. Results and discussion 3.1. Possible reducing mechanism It is known that epoxy and hydroxyl groups are the main oxygen-containing functional groups attached to GO [12,19,22,23]. Therefore, how to effectively remove epoxy and hydroxyl groups is the key for the reduction of GO. Halogenation agents, including concentrated hydrobromic (HBr) acid and HI acid, are acidic reducing agents that can catalyze the ring-opening reaction of epoxy groups and convert them into hydroxyl groups [24]. Previous studies show that the halogenation reaction of alcohol can be realized by moderate heating (6100 C), resulting in the substitution of hydroxyl groups by halogen atoms. Recently, Elias et al. [25] reported the reversible hydrogenation of graphene in which hydrogen atoms can be attached and removed from graphene without destroying the carbon lattice. As shown in Table S1 [Supplementary information (SI)], the binding energy between a carbon and a halogen atom (Br or I) is lower than that of the C H bond, so that halogen may be more easily removed than hydrogen from the carbon basal plane. Therefore, a highlyeffective reduction of GO films by hydrohalic acids is expected. The possible reaction mechanism is presented in Fig. 1: (a) the ring-opening reaction of an epoxy group and (b) the substitution reaction of a hydroxyl group by a halogen atom. The substituted halogen atoms are expected to be easily eliminated from the carbon lattice and produce graphene as shown in the final step (b). As a representative, HI acid was investigated in detail for the reduction of the GO film since the C I binding energy is the lowest among the carbon halogen bonds HI acid reduction in comparison with other chemical reducing agents Fig. 2 shows optical photographs of the reducing process by immersing GO films into different reducing agents for different times. As shown in Fig. 2a, before GO film immersion, numerous H 2 bubbles are observed in the NaBH 4 aqueous solution, while this is not so in N 2 H 4 ÆH 2 O and HI acid solutions. Therefore, as soon as a GO film is immersed in a NaBH 4 aqueous solution, the GO film starts to break up. After 10 s reduction (Fig. 2b), a GO film immersed in N 2 H 4 ÆH 2 Oiscovered by many fine bubbles (Fig. 2f) similar to the GO film immersed in a NaBH 4 solution (Fig. 2e), which means that hydrazine can react with GO and produce gases. However, we can find hardly any bubbles around the GO film immersed in a HI acid solution (Fig. 2g). As shown in Fig. 2e g, both GO films immersed in the NaBH 4 and N 2 H 4 ÆH 2 O solution prefer to float on the liquid surface and are covered by bubbles, while the GO film immersed in HI acid soon drops to the bottom of the cuvette and no bubbles are observed. Subsequently, the GO film

3 4468 CARBON 48 (2010) Fig. 1 Possible reaction mechanism of GO reduction by hydrohalic acids. (a) Ring-opening reaction of an epoxy group. (b) Halogenation substitution reaction of a hydroxyl group. The substituted halogen atoms are expected to be easily eliminated from the carbon lattice. (X = iodine or bromine). Fig. 2 Optical photographs of the reducing process by immersing a GO film into different reducing agents for different times at room temperature. (a) Three liquid reducing agents: 50 mm NaBH 4 aqueous solution (NaBH 4 ), 85% N 2 H 4 ÆH 2 O solution (N 2 H 4 ), and 55% HI acid solution (HI). (b d) The GO films reduced by the three agents for 10 s, 10 min and 16 h. (e g) Enlarged views from (b) which show that the phenomenon occurred after 10 s immersion of GO films to the three liquid reducing agents. immersed in the N 2 H 4 ÆH 2 O solution also begins to break up, as shown in Fig. 2c. After 16 h reaction, the films immersed both in the N 2 H 4 ÆH 2 O and NaBH 4 aqueous solution are broken down to small graphene debris, but the film in the HI acid solution maintains its integrity very well (Fig. 2d). It should be noted that these parallel experiments were carried out at room temperature under the same treatment conditions reported for N 2 H 4 ÆH 2 O (85%) [18] and NaBH 4 solutions (50 mm) [2]. We found that N 2 H 4 ÆH 2 O and NaBH 4 solutions at higher temperature can make GO films break up much easier and quicker. Although 55% HI acid can reduce GO films well at room temperature, higher temperatures make the reduction

4 CARBON 48 (2010) Fig. 3 Optical photographs and mechanical properties of the GO films reduced by different chemical agents. (a) Asassembled GO film. (b) 1 h HI acid-reduced GO film at 100 C. (c) Stress strain curve of the GO film and HI acid-reduced GO film. (d f) Hydrazine vapor-, N 2 H 4 ÆH 2 O- and NaBH 4 -reduced GO films. The scale bar in (d f) is 5 mm. of GO films more effective; as a result, the subsequent reduction of GO films by HI acid was carried out at 100 C. The optical photographs of the GO films reduced by different chemical agents are shown in Fig. 3. As shown in Fig. 3a, the original GO film appears a deep yellow 1 color. After 1 h reduction in HI acid at 100 C, the r-go film (Fig. 3b) shows a shining metallic luster, due to the increase of electrical conductivity and reflectivity of visible light. A pleasant surprise is that the r-go film produced by 1 h HI acid reduction at 100 C has a mean volume electrical conductivity of 298 S/ cm, much higher than that of GO films reduced by other chemical routes reported at present (Table S2), and this is also higher than for the sample reduced by hydrazine vapor (2.7 S/cm) in this work. Moreover, the HI acid-reduced GO film maintains good flexibility and can be rolled randomly (inset in Fig. 3b); and shows a 21% increase in tensile strength and a substantial increase in ductility (Fig. 3c), whereas the GO films reduced by N 2 H 4 ÆH 2 O and NaBH 4 solutions become broken up, as shown in Fig. 3e and f, respectively. The hydrazine vapor-reduced GO film (Fig. 3d) maintains the original integrity, but has obvious distortions and becomes too rigid to be rolled. The cross-sectional view of the HI acid-reduced film (Fig. 4a and b) shows a shrinkage of the film thickness from about 5 to 2.5 lm due to the removal of the oxygen-containing functional groups, but that of the hydrazine vapor-reduced film (Fig. 4c and Fig. S1d) shows more than 10 times an expansion in thickness, caused by gas release. According to these comparative studies, only the HI acid-reduced GO film maintains the original high flexibility and integrity, and even has improved ductility and strength. High resolution C1s peaks in the XPS of the r-go films prove that oxygen-containing groups have been removed. The C1s spectrum of the original GO film (Fig. 5a) reveals that it consists of two main components arising from C O (hydroxyl and epoxy, ev) and C@C/C C (284.6 ev) groups and two minor components from C@O (carbonyl, ev) and O C@O (carboxyl, ev) groups [2,18]. After HI acid reduction, the hydroxyl and epoxy groups, that are the majority of oxygen-containing groups in GO film, are almost all removed and the C C bonds become dominant, as shown by the one single peak with a small tail in the higher binding energy region in Fig. 5b. XRD spectrum shows that the interlayer distance of the r-go film (Fig. 5c) is decreased to 3.57 Å (2h = 24.4 ) from 8.10 Å (2h = 10.9 ) for the original GO film, and the thickness of the r-go film shrinks from 5 to 2.5 lm (Fig. 4a and b) due to the elimination of the oxygen-containing groups on the graphene sheets. The surface C/O atomic ratio detected by XPS is 2.1 for the original GO film and is increased to above 12 after HI acid reduction. This value is higher than those achieved by other chemical routes and similar to that of a high temperature treated GO film (Table S2), indicative of the high effectiveness of our method for removing oxygen-containing groups. The 2D peak (2647 cm 1 ) in the Raman spectrum (Fig. 5d) is obviously increased after HI acid reduction, which has never been observed for other chemical reducing methods [2,15,18,19], strongly suggesting the restoration of sp 2 carbon in r-go films [26]. Because of the high percentage of sp 3 carbon in GO films, the observed increase in the intensity ratio of the D peak to the G peak (I D /I G ) further proves the increase of sp 2 carbon in r-go films [27]. 1 For interpretation of the references to color in this figure, the reader is referred to the web version of this article.

5 4470 CARBON 48 (2010) Fig. 4 Cross-sectional SEM images of GO and r-go films. (a) Original GO film. (b) HI acid-reduced film. (c) Hydrazine vaporreduced film. The arrows mark the position where the film thickness was measured. Fig. 5 Structural characteristics of the GO films before and after 1 h HI acid reduction at 100 C. (a and b) XPS C1s peak, (c) XRD spectra, and (d) Raman spectra. Due to the restoration of defects in the GO sheets and the decrease of interlayer distance, the interaction among the r-go sheets is increased; therefore, the tensile strength and electrical conductivity of the r-go film are greatly improved. Good mechanical strength and electrical conductivity are important to graphene-based films in applications

6 CARBON 48 (2010) as flexible conductors and energy-storage devices, etc. [6,28]. The above results prove that HI acid is a more effective chemical agent than hydrazine and NaBH 4 in the reduction of GO films Dynamic process of GO film reduction by HI acid We investigated the dynamic reducing process of GO films by measuring the volume electrical conductivity and chemical structure for different immersion times. It is interesting to find that the volume electrical conductivity of the GO films is not monotonically increased, but follows a three-stage evolution as shown in Fig. 6a: (I) In the first 2 min of treatment, the insulating GO film rapidly becomes conductive and attains an electrical conductivity around 206 S/cm. (II) Between 2 and 8 min treatment, the film conductivity decreases quickly to 56 S/cm and remains stable for about 5 min. (III) After 8 min treatment the film conductivity steadily increases to its highest mean value of 298 S/cm, and remains stable afterwards. To explain such behavior, the full spectra of XPS, Raman and XRD of the as-reduced GO films for different times were measured and are shown in Figs. S2 S4. Based on the analysis to these spectra, three remarkable characteristics of the reducing process are identified. Fig. 6 Properties and structure variation of the GO films with time as a result of 55% HI acid reduction at 100 C. (a) Electrical conductivity, (b and d) C/O and I/C atomic ratio detected by XPS, (c) Peak intensity ratio of the GO to the r-go films (I GO /I r-go ) obtained from XRD spectra, and (e) Peak intensity ratio of the I 5 to the G mode (I I/I G ) obtained from Raman spectra. The electrical conductivity variation with time follows three stages: (I) 0 2 min; (II) 2 8 min; (III) after 8 min as indicated by vertical dotted lines Diffusion controlled outer-to-inner reduction of GO film As important evidence of GO reduction, the increase of C/O atomic ratio (Fig. 6b from XPS in Fig. S2) with increasing reducing time is steady. During the first 2 min of treatment, the C/O atomic ratio of GO film rapidly increases from 2.1 to 8.2, indicating the highly efficient reduction of the GO film by HI acid; after 30 min treatment, the C/O ratio is stable at 12. Because the detection depth of XPS is only several nanometers, the variation of C/O ratio reveals only the change of film surface. Fig. S3 shows the XRD spectra of the as-reduced GO films for different times, we can see that the GO film has only one intense peak centered at 10.9 (GO peak), while there appears to be another intense peak centered around 24.2 (r- GO peak) after HI acid reduction because the reduced part has a different interlayer distance from the unreduced part. With the increase of reducing time, the GO peak constantly decreases while the r-go peak steadily increases. So we can also use the intensity ratio of the GO peak to the r-go peak (I GO /I r-go ) to describe the reducing state of the GO film. As shown in Fig. 6c, during the first 8 min of reduction, I GO /I r-go drops fast and then changes slowly to zero at 45 min. Because the chemical reactions need direct contact between the reactants, the reducing process should be a gradual outer-to-inner process controlled by the diffusion of reactants (HI) within the GO film. Since XPS results only present the change of GO sheets on the very surface and XRD can reflect the structure inside the film better, a much longer time is necessary to achieve the stable state of I GO /I r-go ratio for a 5 lm thick GO film. Combined with the change of electrical conductivity, we believe that the full reduction of a 5 lm thick GO film needs min. We also reduced a thin GO film with thickness around 100 nm, as shown in a movie. The reducing process needs no more than 1 min. These results show that the reducing reaction between GO and HI acid is very fast, while the complete reduction of a thick GO film is controlled by diffusion Chemical doping of an as-reduced GO film by iodine According to the steady variation of C/O and the I GO /I r-go ratio with reducing time, the reducing effect cannot be the only factor resulting in the non-monotonic change of the film electrical conductivity during stages I and II. Interestingly, XPS has revealed that the GO reduction by HI acid leaves some iodine within the film. We use the atomic ratio of iodine to carbon (I/C) to describe the quantity of residual iodine within the r-go film. As shown in Fig. 6d, during stages I and II, the I/C atomic ratio increases to 0.05 after 2 min immersion and decreases sharply afterwards. This trend is coincident with the electrical conductivity change. Since it has been well reported that chemical doping can severely change the electrical properties of carbon materials [29 36], the non-monotonic electrical conductivity change may be closely related with the iodine doping effect. Raman spectroscopy is usually used to characterize the doping state of carbon materials. As shown in Fig. S4, after HI acid reduction, a new intense peak appears at 165 cm 1, which is reported to be the resonantly enhanced Raman band of polyiodides (I 5, composed of I and excess neutral I 2 ) [29]. Indeed, I 5 is widely present in iodine-doped carbon nanotubes (CNTs) [30 32] and low-dimensional organic polymers

7 4472 CARBON 48 (2010) [33,34]. In those cases, electron transfer from CNTs and polymer hosts to polyiodide chains creates a large number of mobile hole carriers, resulting in a sharp increase in electrical conductivity. Therefore, the presence of I 5 can be typical of the chemical doping of graphene by iodine. We use the intensity ratio of the I 5 peak to the G peak (1590 cm 1 )(I I /I G ) to describe the doping degree of the film and the I I /I G variation with immersion time is shown in Fig. 6e. The changing trend coincides with that of the electrical conductivity. As a result, the sharp increase and then sharp decrease of the film volume electrical conductivity during stages I and II can be attributed to the simultaneous reduction and iodine doping effect, and the latter may play a more important role Spontaneous elimination of iodine during reduction From Fig. 6d and e, we can see that the amount of residual iodine and the doping effect constantly decrease after 2 min treatment. After 1 h treatment, the I/C atom ratio is less than These results show that the iodine participating in the reducing reaction does not accumulate in the film but separates from the reduced film. The movie also shows that an obvious reddish-brown substance, which is mainly composed of I and neutral I 2, is released from the r-go film immediately after being immersed in HI acid. Since there is no additional treatment carried out during the reducing process, the elimination of most iodine should be spontaneous. We used XPS to investigate the chemical states of the residual iodine in the r-go film after both 2 min and 1 h HI acid treatment, and the I 3d 5/2 peak for each film is shown in Fig. 7. The residual iodine is mainly present in two states: neutral iodine (I 2, 620 ev) and the iodine anion (I, 619 ev). After 2 min reduction, the iodine anion is the main contributor (78.9%); while after 1 h, the residual iodine exists mainly as neutral iodine (60.3%). According to these results, we can propose that, in the whole reducing process, the iodine anions are the main reducing agent which donates electrons to achieve the restoration of the conjugated structure of graphene. These anions are oxidized to neutral iodine atoms during the redox reaction, and the iodine atoms further assemble to form polyiodides (I 3 or I 5 ), which are the stable Fig. 7 High resolution XPS I 3d 5/2 peaks of a GO film after 2 min (left) and 1 h (right) 55% HI acid treatment. The peak split indicates that the residual iodine in the r-go film after HI acid treatment is mainly composed of neutral iodine (I 2, 620 ev) and iodine anions (I, 619 ev). states of iodine atoms in solution. These polyiodides are expected to have low interaction with graphene; so the iodine can separate spontaneously from the r-go film in the form of a polyiodide solution. According to the above discussion, we can explain the change of electrical conductivity of the GO films during HI acid reduction as follows. The high electrical conductivity achieved in stage I is attributed to simultaneous reduction and iodine doping, and the latter may play a more important role. Subsequently, in stage II, the doping effect becomes weak with the elimination of iodine but the inner part of the thick film is not fully reduced; thus the electrical conductivity of the film decreases. In stage III, the reducing effect becomes dominant and the electrical conductivity increases gradually till the film is completely reduced after min. Though most iodine can be spontaneously eliminated from the r-go film, there is still a small amount of residual iodine in the completely reduced film, so we cannot exclude a contribution of iodine chemical doping to the final high electrical conductivity of the r-go film. While distinct from the common approaches to doping graphite and CNTs by iodine [31,35], which usually result in unstable compounds, the iodine in the r-go films is rather stable because it cannot be completely removed from the r-go film even by a 2 h thermal annealing at 400 C. Furthermore, we found no obvious change in the electrical conductivity, mechanical strength and elemental composition after several-months storage and even by further thermal treatment at 150 C for 24 h. Therefore, the electrical conductivity of the r-go films reduced by HI acid is stable enough for practical applications Applications of the HI acid reducing method To further verify the advantage of HI acid, we used it to reduce a thin GO film for application as a flexible TCF, which is one of the important potential applications of graphene. A GO film with a thickness about 10 nm on a flexible PET substrate was reduced by only 30 s immersion in 55% HI acid at 100 C and a r-go based TCF with surface resistance 1.6 kx/sq and 85% transparency at 550 nm was obtained. The performance is better than other reported results achieved by chemical reducing methods and at the same level as that achieved by high temperature annealing (Table S3). More importantly, the HI acid reduction is more time-saving than any other reducing methods. Since hydrazine is intrinsically an alkali agent and a NaBH 4 aqueous solution can produce NaOH during the reduction, it is believed that the chemical reduction of GO must use alkaline agents [17]. However, our proposal is to use an acidic agent for the GO reduction. We found that 46% HBr acid can also reduce GO films effectively, and the highest surface C/O atomic ratio achieved was 10.8 and the highest volume electrical conductivity achieved was 156 S/cm, though the reduction using HBr acid is not as fast as that of HI acid. However, another halogenation agent, SOCl 2, showed a relatively weak reducing effect with the highest surface C/O atomic ratio of 4.3 and only marginally increased film conductivity. These results are shown in Fig. S5; and they are the indirect proof that the reduction of GO films by halogenation agents may be based on halogenation reactions, that is, a nucleo-

8 CARBON 48 (2010) philic substitution reaction. Since the nucleophilic substitution reaction can be carried out with both acidic and alkaline surroundings, we consider that the nucleophilic substitution process may be the basic process of the GO reduction by chemical routes. 4. Conclusion As a representative of halogenation agents, HI acid is a highly effective low temperature reducing agent for GO. The HI acidreduced GO film can achieve a C/O atomic ratio above 12 and an electrical conductivity as high as 298 S/cm, much higher than those of the GO films reduced by hydrazine and NaBH 4. Meanwhile, this reducing process can maintain good integrity and flexibility and even improve the strength and ductility of the original GO films. Highly-effective reduction combined with good mechanical properties of the r-go films can not only facilitate the large-scale production and application of large-area graphene-based conductive films, but is also a valuable method for the large-scale production of graphene by reducing GO. Based on this reducing method, a flexible graphene-based TCF with a sheet resistance of 1.6 kx/sq and 85% transparency at 550 nm wavelength was achieved, further verifying the advantage of HI acid reduction. A nucleophilic substitution reaction is presumed to be the basic process of GO reduction by chemical routes, which is important to understand the mechanism of chemical reduction of GO. Acknowledgements S.F. Pei and J.P. Zhao are equal main authors. This work was supported by the National Natural Science Foundation of China (Nos and ), the Key Research Program of Ministry of Science and Technology, China (No. 2006CB932703) and by Chinese Academy Sciences (KGCX2- YW-231). 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