Rapid self-assembly of ultrathin graphene oxide film and application to silver nanowire flexible transparent electrodes

Abstract

Featuring outstanding electrical and optical properties, silver nanowires (AgNWs) have been regarded as one of the most promising candidates for ITO to manufacture transparent conductive electrodes. However, the poor long-term stability of bare AgNWs, due to sulfidation/oxidation corrosion, is an unavoidable and urgent problem in practical applications. In the present work, a large-area ultrathin and uniform graphene oxide (GO) film was freely self-assembled at the interface of pentane–water by a rapid process within only 3 minutes, and subsequently transferred onto the surface of AgNW film by a simple dip coating process, resulting in an impressive improvement in the conductive performance and stability of the AgNWs. The ultrathin GO film was formed by the evaporation driven instability effect of acetone to induce the self assembly of GO nanosheets and an assistant thermal treatment to accelerate the formation rate. The thickness of GO film could be effectively controlled by changing the amount of acetone and the self-assembly time. The sheet resistance of the GO/AgNW electrode has been decreased approximately 3–4 times, with only a 2% loss in transmittance, compared to the original AgNW electrode. A GO/AgNW electrode with a sheet resistance of 21.5 Ω sq⁻¹ at 90% transmittance has been achieved. The stability of the AgNW electrodes at room temperature and high temperature (120 °C) environments has been improved using GO as a protective film. The uniform and large-scale GO film can be transferred onto various substrates by a simple dip coating method with an arbitrary shape, which will open a new window for the protection of various metal nanowires.

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1. Introduction

Silver nanowire (AgNW)-based flexible transparent conductive films (TCFs) have attracted a lot of research interest in various applications including light emitting diodes, solar cells, touch screens, sensors, etc.^1–3 Compared to the vacuum-evaporated indium-tin-oxide (ITO) conductive film, uniformly distributed and randomly oriented AgNWs exhibit superior properties such as mechanical flexibility and solution processability,⁴ which widely expand their flexible electronic applications when deposited onto flexible substrates such as plastics, papers or even textiles.^5,6 However, AgNW TCFs suffer from several problems such as high contact resistance, low transmittance as well as poor stability, which largely restrict their applications in commercial devices. Various methods, such as thermal welding, light-sintering, and embedding AgNWs with a second component have been applied to enhance the connection between AgNWs and decrease the junction resistance.^7–12 As high temperature is usually needed during the thermal welding process, flexible substrates are limited in terms of poor thermal tolerance. Moreover, the long-term stability of AgNWs exposed to ambient air, especially hot air, is a key issue because bare AgNWs are easily corroded via sulfidation/oxidation due to a high surface-to-volume ratio and the intrinsic properties of silver.^13–15 Thus, much effort has been focused on the introduction of other materials, such as polymers and carbon-based materials, onto the surface of AgNW film to not only reduce the contact resistance but also improve the reliability of AgNWs.^4,16–20 Meanwhile, retaining the high transmittance of AgNW-based TCFs is also crucial and essential even if a second material is introduced. Hence, looking for suitable ultrathin materials and controlling the thickness are necessary to achieve high conductivity, high transmittance and high stability AgNW-based TCFs. Carbon-based materials, such as carbon nanotubes (CNTs) and graphene-based materials, have been considered as suitable materials to enhance the conductive performance and stability of AgNW TCFs.^21–24 However, the dispersion difficulty of CNTs and the expensive CVD process for graphene synthesis largely restrict their applications as a protective layer for cheap, flexible and solution-processable AgNW TCFs. Thus, we attempt to develop a convenient solution-process to form ultrathin graphene-based materials and protect AgNW TCFs.

Two dimensional graphene oxide (GO), which is usually known as a promising solution-processable precursor for the bulk production of graphene,^25,26 can be easily and cheaply prepared using a solution oxidation and exfoliation process of graphite. GO can be stably dispersed in water even at a high concentration due to the surrounding large amount of hydrophilic oxygen-related functional groups and the GO thin film transferred from the GO dispersion is stable and bendable.²⁷ Moreover, the GO thin film is more transparent than graphene film with the same number of sheet layers.^28,29 Thus, a solution processable preparation of uniform and ultrathin GO film is quite suitable for soldering AgNW junctions and improving the stability of AgNWs. Although GO film has been prepared using several techniques such as spin coating, spray coating (SC), and dip coating (DC),^30–32 it is still difficult to obtain ultrathin uniform GO films due to the crumpling and aggregation of GO nanosheets during the above fabrication processes. Recently, an interfacial self-assembly strategy was developed to form uniform GO film. It utilized the air/liquid or liquid/liquid interfaces as an effective scaffold to control and assemble uniform GO nano-films with tunable thickness and properties.^33,34 The self-assembly process of GO film usually needs a driving force, such as thermal diffusion and organic solvent evaporation, to migrate GO nanosheets from the bulk aqueous phase to the interface. Cheng et al. produced a free-standing thick GO film through a simple thermal driving self-assembly process at the liquid/air interface. The GO film was flexible and semi-transparent with a thickness range of 0.5–20 μm by adjusting the formation time.³⁵ However, such semi-transparent GO film are not suitable for AgNW TCFs because they may largely decrease the transmittance of the AgNW electrode. In order to form thin GO films, Chen et al. used the evaporation effect of ethanol to gather GO nanosheets at the pentane–water interface to form GO film with various thicknesses in tens of minutes.³⁶ Similarly, Chen et al. also accomplished acetone-induced thin GO film formation at the water–air interface in about 40 minutes.³⁷ Different from ethanol, acetone has a higher vapor pressure than ethanol and can provide a stronger driving force for the assembly of GO nanosheets. Although these reports suggest that the self assembly formation of thin GO film can be achieved, speeding up the interface assembly rate to obtain ideal ultrathin GO film with a controllable thickness is still of great importance. Moreover, the self-assembly of ultrathin GO film on the surface of solution-processable AgNW TCFs for improving the conductive performance and stability is seldom investigated until now.

In this work, a rapid self-assembled uniform GO film was developed at the pentane–water interface within only 3 minutes, which combined the evaporation-driven instability effect of acetone to form large-scale ultrathin GO film and a pre-heating step to largely accelerate the formation rate. The introduction of pentane could improve the ultra-flat spreading of the GO nanosheets at the interface and the thickness of the GO film could be freely tailored by adjusting the injection amount of acetone and assembly time. This GO film was easily transferred onto the surface of AgNW/polyethylene terephthalate (PET) to form a GO/AgNW/PET electrode. Compared to a bare AgNW/PET electrode, the sheet resistance of the GO/AgNW/PET electrode has been decreased approximate 3–4 times with only a 2% loss in transmittance. Furthermore, the GO/AgNW/PET electrode showed an enhanced long-term stability in the conductivity performance at room temperature and at 120 °C. More importantly, the solution-processable self-assembled GO film could be transferred onto various substrates, which expanded its applications into many more fields.

2. Experimental

2.1 Preparation of AgNWs

Silver nitrate (AgNO₃), polyvinylpyrrolidone (PVP, average molecular weight 360k in terms of monomeric units), ethylene glycol (EG), pentane, ethanol and acetone were obtained from Wako Chemicals (Japan). All the chemicals were used as received without further purification. Silver nanowires were prepared by a one-step polyol method reported in a previous study, with length of > 50 μm and average diameter of 100 nm.³⁸ In a typical reaction, 0.98 g PVP was firstly dissolved in 125 g EG. Then, 1.1 g AgNO₃ and 17 g FeCl₃ solution in EG with a concentration of 600 μM were added. The mixture was then reacted at 110 °C for 12 h. Afterwards, the precipitate was washed with acetone and ethanol several times. The obtained AgNWs were dispersed in ethanol with a concentration of 0.25 wt% for use.

2.2 Preparation of graphene oxide film

A low-oxidation grade GO powder was supplied by NiSiNa materials Co., Ltd. (Japan). In order to keep the sheet size of GO in a range, a two-step centrifugation process was used to obtain suitable GO nanosheets. 0.2 g GO was absolutely dispersed into 100 mL water by a mild sonication for 1 h. The as-prepared GO solution was initially centrifuged at 3000 rpm for 30 min to remove the huge precipitation and impurities. Then the supernatant was again centrifuged at 8000 rpm for 30 min. The obtained precipitate was repeatedly dispersed in 100 mL water to obtain a GO dispersion with a concentration of 0.01 mg mL⁻¹. 5 mL of as-prepared GO dispersion was added to a 6 cm diameter glass dish. 5 mL pentane was subsequently layered to create a pentane–water interface. 2 mL acetone was then directly injected into the aqueous phase. After that, the mixed solution was put onto a 110 °C heating plate for 1 minute and then left at room temperature to wait for assembly of the GO film at the pentane–water interface. After 3 minutes, a large visible floating GO film was left on the surface of the aqueous solution. The floating GO film could be easily transferred onto various substrates by a simple rapid dip coating method which will be illustrated in detail in the next paragraph.

2.3 Preparation of GO/AgNW/PET hybrid TCFs

Firstly, the PET substrate was pre-treated with nitrogen plasma for 1 min to enhance its hydrophilic property. Then, 30 μL of AgNW dispersion in ethanol was drop casted onto PET substrate pieces (15 mm × 20 mm) to obtain the AgNW/PET electrode. Due to the uniform distribution of AgNWs on PET and the strong capillary function between AgNWs and PET (Fig. S1†), the AgNW/PET electrodes could be dipped into the solution and rapidly pulled up, during which the floating GO film uniformly covered the AgNW/PET. Both the hydrophilic PET surface and the rough AgNW surface benefited from the uniform and continuous coverage with ultrathin GO film. Afterwards, the obtained GO/AgNW/PET hybrid electrode was then dried at 80 °C on a hot plate for 3–5 minutes to remove any residual water and organics. The process is shown in Scheme 1.

Scheme 1 Preparation of GO film and the GO/AgNW/PET electrode.

2.4 Characterization

The morphology of the materials were characterized by field-emission scanning electron microscopy (SEM, Hitachi SU8020, Tokyo, Japan) operated with an accelerating voltage of 5.0 kV. The surface roughness of the films was determined in the dynamic force microscope (DFM) mode of an atomic force microscope (AFM, Nanocute SII Nano Technology, Tokyo, Japan). The hydrophilic treatment of the substrate was performed on a plasma device (Sekisui AP-T02-L, Osaka, Japan). Transmittance spectra were measured using a UV-visible near infrared spectrophotometer (JascoV670, Hachioji, Tokyo, Japan). The sheet resistance was measured by the four probe method with a surface resistivity meter (Mitsubishi Lorester-GP T610, Kanagawa, Japan). The X-ray diffraction (XRD) patterns were collected using an X-ray diffractometer (RigakuSmartLab, Akishima, Tokyo, Japan) with CuKα radiation at 45 kV and 40 mA.

3. Results and discussion

The interfacial self-assembly strategy, especially oil–water interfacial self-assembly, is a simple and easily operable process for low cost and high efficiency fabrication of nano-films with various sizes, thicknesses and shapes.^39,40 In an interfacial self-assembly process, except for the excellent dispersing ability of the objects, an effective driving force is necessary.⁴¹ In the present work, acetone with a high vapor pressure was used and directly injected into the aqueous phase to give an abundant force for GO nanosheet accumulation and assembly at the pentane–water interface. Moreover, a 1 minute pre-thermal treatment of the pentane–GO–acetone solution effectively accelerated the gathering and self-assembly of the GO nanosheets into GO film in a lateral edge-to-edge manner within only 3 minutes. In addition, these gathered GO nanosheets were ultra flatly spread due to the interface tension force. With the continuous evaporation of pentane, a large visible floating GO film was stably left on the surface of the aqueous solution (Fig. S2†) and could be easily transferred onto various solid substrates by a rapid dip coating process.

Fig. 1a shows the AFM image of the GO nanosheets on a silicon wafer. These GO nanosheets were flatly distributed onto the substrates with irregular shapes and lateral dimensions of 1–3 microns (Fig. 1a and S3†). The measured thickness (inside Fig. 1a) was about 1.2 nm, which was consistent with the fully exfoliated graphene oxide reported in the previous literature,⁴² indicating the existence of single layered GO nanosheets. The structure of GO was further investigated by XRD patterns, shown in Fig. 1b. We observed peaks at 2θ values ∼26° and 42° in the XRD spectrum of GO film, which were similar to the (0002) and (1010) peaks of graphite.⁴³ The d-spacing of the GO sheets can be calculated as 0.344 nm and 0.418 nm from peaks at 26° and the secondary peak at 21°, which was larger than the 0.336 nm of graphite due to the oxygen-related group added at the sheet edge and above the sheet surface.

Fig. 1 (a) The tapping mode AFM image of GO sheets on the silicon surface and the height profile of the AFM image (inside). (b) XRD pattern of GO film.

Fig. 2 shows the SEM images of AgNW/PET and GO/AgNW/PET films. The diameter of the AgNWs was about 100 nm, and their lengths reached up to 60 μm (Fig. 2a and S4†). No other silver nanostructures, such as spheres, cubes, or rods, were observed. These AgNWs which were uniformly distributed and randomly oriented on PET formed a transparent conductive electrode. From Fig. 2a–c, we could see that the morphology of the pristine AgNWs in the SEM images is very clear. After covering with GO film (Fig. 2d–f), an obvious change from a clear to foggy morphology was observed due to the charge effect of the insulated GO film. Moreover, the continuous and ultrathin GO film assembled on the surface of the AgNWs by solvent-evaporation capillary forces could be observed, which might be related to the electrostatic attraction between the AgNWs and GO thin film. Moreover, the charge effect happening between AgNWs (Fig. 2c, shown with yellow arrows) due to the huge contact resistance between the wires, seemed to disappear due to the coverage of the GO film on the AgNWs (Fig. 2f), which implied that the GO film might improve the adhesion and junction contact between AgNWs. This phenomenon has been confirmed in many reports due to the introduction of materials onto the surface of AgNWs films.^16–20 The significantly improved conductive performance with a small transmittance decrease of the GO/AgNW/PET hybrid electrode induced by coverage with the ultrathin GO film will be discussed later.

Fig. 2 SEM images of drop-coated AgNWs (a–c) and AgNWs covered by GO film (d–f).

In order to confirm that the GO film was uniformly covered on AgNWs, we investigated the EDX mapping image of Ag, C, and O elements of GO with AgNWs on a Si substrate fabricated using the same conditions as on PET substrates (Fig. 3). Fig. 3a shows the SEM image of an approximate 10 μm × 10 μm area. The uniform, continuous distribution of C and O elements throughout the area could be seen (Fig. 3c and d), which was obviously different from that of the Ag element consistent with a wire pattern (Fig. 3b). These results indicated that a continuous and uniform GO film was formed on AgNWs, which was reasonably consistent with the results in Fig. 2.

Fig. 3 (a) SEM image of GO/AgNWs/PET film and the corresponding EDS mapping images of (b) Ag, (c) C, and (d) O elements.

An intriguing characteristic of interfacial self-assembly is that the film thickness can be controlled by easily adjusting the experimental parameters. Except for the importance of a stable GO dispersion in water, the suitable pre-heating process, the injection amount of acetone and the self-assembly time also affect the thickness of GO film. When the pentane–GO–acetone solution was directly left at room temperature, the GO film with incomplete and an inhomogeneous structure was formed with a time-consuming process over several hours. A pre-heating step could obviously speed up the formation rate of GO film. However, too high a temperature or a long heating time would cause a high instability of the mixture, which obstructed the assembly process of GO film. Thus, we found that uniform ultrathin GO film could be obtained in a process combining a pre-heating process at 110 °C for 1 minute with a subsequent standing process at room temperature for 3 minutes. On the other hand, the injection amount of acetone and self-assembly time were also crucial for the thickness control of GO film. Fig. 4 shows the AFM images of GO film on silicon wafer formed with different injection amounts, 2 mL (Fig. 4a) and 3 mL (Fig. 4b), of acetone in a 3 minute assembly process. From Fig. 4a, it is clear that the self-assembled GO film was composed of many irregular GO nanosheets and that each GO sheet mainly interacts with another in an edge-to-edge assembling geometry. Moreover, all the GO nanosheets showed a very flat and spread state, without buckling or wrinkling, due to the strong interface tension force. The height profile at the bottom of Fig. 4a shows that the thickness of GO film was about 2.4 nm with 1–2 layers of GO nanosheets. When the injection amount of acetone was increased to 3 mL, a more continuous GO film with a nearly complete coverage on the substrate was obtained (Fig. 4b). Except for several folded nanosheets, most of the GO nanosheets were flat. The multi-layer GO nanosheets formed a uniform GO film with a thickness of 12.6 nm which corresponded to over 10 layers of GO nanosheets (bottom of Fig. 4b). An increased injection amount of acetone could cause an increased surface pressure and give a larger driving force to accumulate many more GO nanosheets at the interface to form a thicker GO film. Similarly, the self-assembly time also affected the formation performance of the GO film. When the injection amount of acetone was fixed at 2 mL, the assembly time was extended to 5 minutes; an AFM image of the obtained GO film is shown in Fig. 4c. An overlapping assembly of random GO nanosheets has also been observed and the thickness of the GO film was about 3.6 nm, and was composed of 2–3 layers of GO nanosheets (bottom of Fig. 4c). This result suggested that prolonging the time could also control the thickness of the GO film.

Fig. 4 (a and b) Tapping mode AFM images and (bottom) the corresponding height profiles of GO films prepared with 2 mL and 3 mL of acetone injection in a 3 minute assembly, respectively. (c) Tapping mode AFM image and (bottom) the corresponding height profile of GO film prepared with 5 minutes of assembly time and 2 mL of acetone injection.

As mentioned above, the thickness of the GO film would affect the transmittance of the GO film. The corresponding transmittance at 550 nm (T₅₅₀) of GO films in Fig. 4a–c were 98%, 90% and 97%, respectively, which were tested by transferring the GO films onto a PET substrate. All these results greatly implied that the transmittance of GO film was determined by the thickness of GO film, which could be controlled arbitrarily.

After the ultrathin GO film had been formed, it was then assembled onto the surface of a AgNW/PET electrode to improve the conductive performance. Fig. 5 presented the sheet resistance as a function of T₅₅₀ for AgNW/PET and GO/AgNW/PET electrodes. A pristine AgNW/PET electrode with a sheet resistance (R_s) of 110 Ω sq⁻¹ at T₅₅₀ = 95% was fabricated by a simple drop coating method as mentioned in the experimental. When it was covered with GO film, the T₅₅₀ decreased to 93% and R_s largely decreased to only 32.3 Ω sq⁻¹, approximately 3.5 times lower than the original value. It should be noted that only a 2% loss in transmittance occurred which agreed with the transmittance of pure GO ultrathin film (Fig. 4a). Furthermore, a pristine AgNW/PET electrode with R_s of 87.5 Ω sq⁻¹ at T₅₅₀ = 93% was covered with GO film and a GO/AgNW/PET electrode with T₅₅₀ = 90% was obtained. The R_s decreased to 21.5 Ω sq⁻¹, approximately 4 times lower than the original value with a transmittance loss of 3%. These results indicated that the ultrathin uniform self-assembled GO film efficiently improved the conductive performance of AgNWs. The reasons may be summarized as follows: (1) the ultrathin and uniform GO film could adhere well to the AgNW/PET electrode by a strong capillary force; (2) as the conductive performance of the GO/AgNW/PET was mainly decided by the AgNW network, GO film could effectively wrap around AgNWs to enhance the junction contact between AgNWs due to its good hydrophilicity and simultaneously enhance the adhesion between AgNWs and the substrate without any other post-treatment.^16,20,31 Moreover, the performance of GO/AgNW/PET was compared with some representative results of other groups, as shown in Fig. 5b. Our results indicated that the ultrathin GO films were far more effective than other materials introduced onto the surface of AgNW film for high conductivity and high transmittance TCFs.^44–48 The interfacial process is very simple and rapid, especially when combined with the solution-processable AgNW film to improve the TCF performance.

Fig. 5 (a) The evolution of sheet resistance and transmittance before and after GO coating on AgNW/PET electrodes. (b) Plot of transmittance vs. sheet resistance for GO/AgNW TCFs in this work and ref. 44–48.

As an emerging transparent conductive film, AgNW films with a low resistance and high transmittance are not enough; high reliability is very important for practical applications. Here we test four different films, PET, AgNW/PET, GO/PET, and GO/AgNW/PET hybrid film, at room temperature and high temperature (120 °C) to observe the changes in T₅₅₀ and R_s (Fig. 6), which could further evaluate the long-term stability. From Fig. 6a, no noticeable changes in the transmittance of the four films could be observed after 28 days at room temperature. However, the R_s increased dramatically for the pristine AgNW/PET electrode after two weeks in air, while almost no change occurred for the GO/AgNW/PET electrode except measurement deviation (Fig. 6b). It implied that the stability of the AgNW/PET electrodes was improved after it was protected by the ultrathin GO film. The obvious R_s increase of the bare AgNW/PET electrodes might be related to the formation of silver oxide on the surface of AgNWs, which had been confirmed in the previous report.¹⁴ In contrast, the GO covered AgNW/PET exhibited more stability in R_s (Fig. 6b) because GO film can act as a passivation layer to isolate oxygen gas and moisture.⁴⁹ The stability of the same series electrodes at 120 °C after 6 days was also observed (Fig. 6c and d). It is found that the transmittance of PET has slightly decreased due to the degradation of the PET film (Fig. 6c). Compared with pure PET, GO/PET film showed a clear decrease in transmittance, which may be caused by partial reduction of GO under a long-term thermal environment in addition to PET degradation.⁵⁰ Moreover, the transmittance of bare AgNW/PET also clearly decreased, which may have been caused by rapid corrosion under high temperature as well as the degradation of PET film. The transmittance of GO/AgNW/PET showed a similar decrease with GO/PET, which may also be caused by the partial reduction of GO and PET degradation. However, the sheet resistance of AgNW/PET increased more than 24-fold, while the GO/AgNW/PET showed a much slower increase in sheet resistance than the pristine AgNW/PET (Fig. 6d). These results clearly indicated that the long-term stability of the AgNW/PET electrode could be greatly improved by using GO as a protective layer. However, the high temperature stability of AgNW electrodes is very complicated due to the degradation of polymer substrates and partial reduction of GO film, which needs further study.

Fig. 6 Changes in transmittance at 550 nm and sheet resistance of the different hybrid films after 28 days of room temperature exposure (a and b) and 6 days of high temperature (120 °C) exposure (c and d).

4. Conclusions

We demonstrated a rapid self-assembled route for the preparation of ultrathin GO film in only three minutes. The thickness of the GO film can be controlled by easily changing the injection amount of acetone and the assembling time. After the AgNW/PET electrode was covered with GO film, the sheet resistance of the AgNWs was dramatically decreased approximately 3–4 times with only a 2% loss in transmittance due to the small optical absorption of ultrathin GO film. The long-term stability of the AgNW/PET electrode could also be greatly improved by using GO as a protective layer. This rapid interfacial assembly of GO film is suitable for many other metal nanowire materials and can greatly broaden various applications, such as electronic devices and energy storage.

Acknowledgements

This work was supported by International Research Promotion Program (IRPR) of Osaka University, Grant-in-Aid for Scientific Research (Kaken S, 24226017), COI Stream Project. C. H. Wu acknowledges the financial support from IRPR for her research work and Prof. Masaya Nogi provided help in the AFM measurement.

References

1. D. S. Leem, A. Edwards, M. Faist, J. Nelson, D. D. C. Bradley and J. C. de Mello, Adv. Mater., 2011, 23, 4371–4375.
2. F. Xu and Y. Zhu, Adv. Mater., 2012, 24, 5117–5122.
3. C. H. Liu and X. Yu, Nanoscale Res. Lett., 2011, 6, 75.
4. Z. B. Yu, Q. W. Zhang, L. Li, Q. Chen, X. F. Niu, J. Liu and Q. B. Pei, Adv. Mater., 2011, 23, 664–668.
5. S. De, T. M. Higgins, P. E. Lyons, E. M. Doherty, P. N. Nirmalraj, W. J. Blau, J. J. Boland and J. N. Coleman, ACS Nano, 2009, 3, 1767–1774.
6. X. Y. Zeng, Q. K. Zhang, R. M. Yu and C. Z. Lu, Adv. Mater., 2010, 22, 4484–4488.
7. J. Lee, I. Lee, T. S. Kim and J. Y. Lee, Small, 2013, 9, 2887–2894.
8. Y. Q. Jiang, J. Xi, Z. X. Wu, H. Dong, Z. X. Zhao, B. Jiao and X. Hou, Langmuir, 2015, 31, 4950–4957.
9. B. Deng, P. C. Hsu, G. C. Chen, B. N. Chandrashekar, L. Liao, Z. Ayitimuda, J. X. Wu, Y. F. Guo, L. Lin, Y. Zhou, M. Aisijiang, Q. Xie, Y. Cui, Z. F. Liu and H. L. Peng, Nano Lett., 2015, 15, 4206–4213.
10. S. M. Lee, J. H. Lee, S. Bak, K. Lee, Y. Li and H. Lee, Nano Res., 2015, 8, 1882–1892.
11. S. B. Yang, H. Choi, D. S. Lee, C. G. Choi, S. Y. Choi and I. D. Kim, Small, 2015, 11, 1293–1300.
12. J. Y. Lee, S. T. Connor, Y. Cui and P. Peumans, Nano Lett., 2008, 8, 689–692.
13. J. L. Elechiguerra, L. Larios-Lopez, C. Liu, D. Garcia-Gutierrez, A. Camacho-Bragado and M. J. Yacaman, Chem. Mater., 2005, 17, 6042–6052.
14. J. Jiu, J. Wang, T. Sugahara, S. Nagao, M. Nogi, H. Koga, K. Suganuma, M. Hara, E. Nakazawa and H. Uchida, RSC Adv., 2015, 5, 27657–27664.
15. C. Mayousse, C. Celle, A. Fraczkiewicz and J. P. Simonato, Nanoscale, 2015, 7, 2107–2115.
16. R. Zhu, C. H. Chung, K. C. Cha, W. B. Yang, Y. B. Zheng, H. P. Zhou, T. B. Song, C. C. Chen, P. S. Weiss, G. Li and Y. Yang, ACS Nano, 2011, 5, 9877–9882.
17. S. P. Chen and Y. C. Liao, Phys. Chem. Chem. Phys., 2014, 16, 19856–19860.
18. L. Li, Z. B. Yu, C. H. Chang, W. L. Hu, X. F. Niu, Q. Chen and Q. B. Pei, Phys. Chem. Chem. Phys., 2012, 14, 14249–14254.
19. D. Y. Choi, H. W. Kang, H. J. Sung and S. S. Kim, Nanoscale, 2013, 5, 977–983.
20. D. Lee, H. Lee, Y. Ahn and Y. Lee, Carbon, 2015, 81, 439–446.
21. S. T. Hsiao, H. W. Tien, W. H. Liao, Y. S. Wang, S. M. Li, C. C. Mma, Y. H. Yu and W. P. Chuang, J. Mater. Chem. C, 2014, 2, 7284–7291.
22. J. Y. Woo, K. K. Kim, J. Lee, J. T. Kim and C. S. Han, Nanotechnology, 2014, 25, 285203.
23. A. J. Stapleton, R. A. Afre, A. V. Ellis, J. G. Shapter, G. G. Andersson, J. S. Quinton and D. A. Lewis, Sci. Technol. Adv. Mater., 2013, 14, 035004.
24. H. O. Choi, S. B. Yang, B. H. Min, D. W. Kim, K. M. Cho and H. T. Jung, Sci. Adv. Mater., 2014, 6, 2304–2311.
25. G. Eda and M. Chhowalla, Adv. Mater., 2010, 22, 2392–2415.
26. J. Kim, L. J. Cote and J. X. Huang, Acc. Chem. Res., 2012, 45, 1356–1364.
27. J. I. Paredes, S. Villar-Rodil, A. Martinez-Alonso and J. M. D. Tascon, Langmuir, 2008, 24, 10560–10564.
28. Y. W. Zhu, S. Murali, W. W. Cai, X. S. Li, J. W. Suk, J. R. Potts and R. S. Ruoff, Adv. Mater., 2010, 22, 3906–3924.
29. I. Jung, D. A. Dikin, R. D. Piner and R. S. Ruoff, Nano Lett., 2008, 8, 4283–4287.
30. J. Kim, L. J. Cote, F. Kim, W. Yuan, K. R. Shull and J. X. Huang, J. Am. Chem. Soc., 2010, 132, 8180–8186.
31. J. Liang, L. Li, K. Tong, Z. Ren, W. Hu, X. Niu, Y. Chen and Q. Pei, ACS Nano, 2014, 8, 1590–1600.
32. I. Jurewicz, A. Fahimi, P. E. Lyons, R. J. Smith, M. Cann, M. L. Large, M. W. Tian, J. N. Coleman and A. B. Dalton, Adv. Funct. Mater., 2014, 24, 7580–7587.
33. J. J. Shao, W. Lv and Q. H. Yang, Adv. Mater., 2014, 26, 5586–5612.
34. K. Feng, Y. W. Cao and P. Y. Wu, J. Mater. Chem., 2012, 22, 11455–11457.
35. C. M. Chen, Q. H. Yang, Y. G. Yang, W. Lv, Y. F. Wen, P. X. Hou, M. Z. Wang and H. M. Cheng, Adv. Mater., 2009, 21, 3007–3011.
36. F. M. Chen, S. B. Liu, J. M. Shen, L. Wei, A. D. Liu, M. B. Chan-Park and Y. Chen, Langmuir, 2011, 27, 9174–9181.
37. L. Wei, F. M. Chen, H. Wang, T. H. Zeng, Q. S. Wang and Y. Chen, Chem.–Asian J., 2013, 8, 437–443.
38. J. Jiu, T. Araki, J. Wang, M. Nogi, T. Sugahara, S. Nagao, H. Koga, K. Suganuma, E. Nakazawa, M. Hara, H. Uchida and K. Shinozaki, J. Mater. Chem. A, 2014, 2, 6326–6330.
39. L. F. Hu, M. Chen, X. S. Fang and L. M. Wu, Chem. Soc. Rev., 2012, 41, 1350–1362.
40. J. Shim, J. M. Yun, T. Yun, P. Kim, K. E. Lee, W. J. Lee, R. Ryoo, D. J. Pine, G. R. Yi and S. O. Kim, Nano Lett., 2014, 14, 1388–1393.
41. S. Y. Gan, L. J. Zhong, T. S. Wu, D. X. Han, J. D. Zhang, J. Ulstrup, Q. J. Chi and L. Niu, Adv. Mater., 2012, 24, 3958–3964.
42. S. Stankovich, D. A. Dikin, R. D. Piner, K. A. Kohlhaas, A. Kleinhammes, Y. Jia, Y. Wu, S. T. Nguyen and R. S. Ruoff, Carbon, 2007, 45, 1558–1565.
43. L. Y. Meng and S. J. Park, Bull. Korean Chem. Soc., 2012, 33, 209–214.
44. S. T. Hsiao, H. W. Tien, W. H. Liao, Y. S. Wang, S. M. Li, C. C. Mma, Y. H. Yu and W. P. Chuang, J. Mater. Chem. C, 2014, 2, 7284–7291.
45. Y. H. Ko, J. W. Lee, W. K. Choi and S. R. Kim, Chem. Lett., 2014, 43, 1242–1244.
46. P. Meenakshi, R. Karthick, M. Selvaraj and S. Ramu, Sol. Energy Mater. Sol. Cells, 2014, 128, 264–269.
47. H. W. Tien, S. T. Hsiao, W. H. Liao, Y. H. Yu, F. C. Lin, Y. S. Wang, S. M. Li and C. C. M. Ma, Carbon, 2013, 58, 198–207.
48. M. X. Jing, C. Han, M. Li and X. Q. Shen, Nanoscale Res. Lett., 2014, 9, 588.
49. Y. H. Yang, L. Bolling, M. A. Priolo and J. C. Grunlan, Adv. Mater., 2013, 25, 503–508.
50. W. F. Chen and L. F. Yan, Nanoscale, 2010, 2, 559–563.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra24896e

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