Mussel-Inspired Polydopamine-Functionalized Graphene as a

May 3, 2016 - between the conductive films and the substrates is essential. ... GO (PFGO) film adhered to the substrate much more easily and more unif...
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Mussel-Inspired Polydopamine-Functionalized Graphene as a Conductive Adhesion Promoter and Protective Layer for Silver Nanowire Transparent Electrodes Jinlei Miao, Haihui Liu, Wei Li, and Xingxiang Zhang* State Key Laboratory of Separation Membranes and Membrane Processes, Tianjin Municipal Key Lab of Advanced Fiber and Energy Storage, School of Material Science and Engineering, Tianjin Polytechnic University, Tianjin 300387, China S Supporting Information *

ABSTRACT: For the scalable fabrication of transparent electrodes and optoelectronic devices, excellent adhesion between the conductive films and the substrates is essential. In this work, a novel mussel-inspired polydopamine-functionalized graphene/silver nanowire hybrid nanomaterial for transparent electrodes was fabricated in a facile manner. Graphene oxide (GO) was functionalized and reduced by polydopamine while remaining stable in water without precipitation. It is shown that the polydopamine-functionalized GO (PFGO) film adhered to the substrate much more easily and more uniformly than the GO film. The PFGO film had a sheet resistance of ∼3.46 × 108 Ω/sq and a transparency of 78.2%, with excellent thermal and chemical stability; these characteristics are appropriate for antistatic coatings. Further reduced PFGO (RPFGO) as a conductive adhesion promoter and protective layer for the Ag nanowire (AgNW) significantly enhanced the adhesion force between AgNW networks and the substrate. The RPFGO-AgNW electrode was found to have a sheet resistance of 63 Ω/sq and a transparency of 70.5%. Moreover, the long-term stability of the RPFGO-AgNW electrode was greatly enhanced via the effective protection of the AgNW by RPFGO. These solution-processed antistatic coatings and electrodes have tremendous potential in the applications of optoelectronic devices as a result of their low production cost and facile processing.

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INTRODUCTION Transparent conductive films (TCF) are essentially required in the applications of optoelectronic devices, such as solar cells, electrochromic devices, organic light-emitting diodes (OLED), and touch screens.1−4 Recently, flexible optoelectronic devices have received increasing interest.5 However, conventional indium tin oxide (ITO) cannot fulfill the requirements of flexible optoelectronic devices because of its brittleness and the scarcity of indium.6−8 Therefore, it is urgently required to find alternative materials for ITO; toward this end, several substrates have emerged, including metallic nanowires,9,10 conducting polymers,11,12 carbon nanotubes,13,14 and graphene.15,16 Among these materials, one-dimensional (1D) Ag nanowires (AgNWs) and 2D graphene-based films have attracted considerable attention because of their good optoelectronic characteristics and outstanding mechanical flexibility.17 However, a few challenges remain in the practical application of AgNW and graphene-based TCF. AgNW-based TCF encounters issues of weak adhesion to substrates and low stability in harsh environments.18−20 The drawbacks of graphene-based TCF include the relatively high sheet resistance of reduced graphene oxide (RGO) and the topological defects of chemical vapor deposition-grown graphene.21−23 These disadvantages of single AgNW or graphene-based TCF can be overcome by using them together. Tien et al. © 2016 American Chemical Society

argued that AgNW not only inhibited the aggregation of graphene but also increased the electrical conductivity among graphene sheets.24 Zhang et al. reported that graphene could be an effective protective layer for AgNW; the hybrid TCF showed a high optoelectronic property with great oxidation resistance.25 Kwan et al. found that the composite electrodes of AgNW and RGO exhibited increased robustness and stability; the electrodes showed a sheet resistance of 5.3 Ω/sq and an optical transparency of 64.9%.26 Recent studies of the graphene-AgNW hybrid TCF demonstrated that the fabrication of such hybrid films is a promising approach to complement the disadvantages of the single AgNW or graphene-based TCF.27,28 The graphene-AgNW hybrid TCF could be fabricated via several techniques, such as spray coating,29 spin coating,30 dip coating,31 and vacuum filtration.25 Vacuum filtration is a widely used method because of its facile processing. However, transfer of the hybrid film from a filter membrane to the substrate is required. The film tends to break during the transfer process as a result of the weak adhesion force between the hybrid film and the substrate. Excellent adhesion between the TCF and the substrate is essential for the scalable fabrication of transparent Received: March 1, 2016 Revised: April 19, 2016 Published: May 3, 2016 5365

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Figure 1. Schematic illustration of the preparation process of RPFGO-AgNW hybrid TCF. mg of GO and 80 mg of dopamine hydrochloride were dispersed in 400 mL of 10 mM Tris-HCl (pH 8.5) buffer and then sonicated for 30 min. Afterward, the mixture was stirred at 60 °C for 24 h. The asprepared PFGO was filtered through a 0.22 μm membrane and then washed with distilled water. Finally, the PFGO was dispersed into water at a concentration of ∼0.008 mg/mL. Fabrication of AgNW. The 1D AgNWs were synthesized using a solvothermal method.25 First, 3.53 g of PVP and 11.3 mg of NaCl were dissolved in 60 mL of EG at 120 °C for 30 min. Next, the mixture was added to a AgNO3/EG (0.01 M, 90 mL) solution slowly under vigorous stirring. The combined solution was then transferred into three 100 mL autoclaves. The autoclaves were kept at 160 °C for 7 h to yield AgNW. The as-prepared AgNWs were washed repeatedly with acetone and ethanol via centrifugation at 5000 rpm. The obtained AgNWs were dispersed in water at a concentration of ∼0.009 mg/mL. Fabrication of PFGO Antistatic Coatings. The glass slides used as substrates for antistatic coatings were washed in fresh piranha solution (3:7 v/v, 30% H2O2 and concentrated H2SO4) and then rinsed with deionized water. (Caution! Piranha solution is a strong oxidant and may detonate upon contact with organic material.) PFGO films were prepared on cellulose acetate (CA) filter membranes with a pore size of 0.22 μm via the vacuum filtration method. The thicknesses of the PFGO antistatic coatings were controlled by varying the filtration volumes at 8, 10, 12, 14, and 16 mL. The membranes with captured PFGO films were then cut to the desired size when they were still wet, and the cut pieces were transferred onto glass slides. To remove the air trapped between the PFGO film and the substrate, the membrane was pressed with a glass rod and moved from one side to the other side slowly several times with increasing pressure. Then, a glass slide of the same size was placed on the CA and pressed for 30 min under 0.4 kPa pressure. Next, the glass/CA/PFGO/glass was dried in an oven at 100 °C for several minutes. To verify the adhesion of PFGO films, the CA filter membrane was peeled off of the glass slides slowly instead of using the conventional approach of dissolution using acetone. Finally, the PFGO films were dried in a vacuum oven at 100 °C for 60 min. As a comparison, GO films not functionalized by PDA were also fabricated. Fabrication of RPFGO-AgNW TCF. For RPFGO-AgNW TCF, different amounts of AgNW solution (2−14 mL) were mixed with the PFGO solution (4.5 mL). The glass slides were also used as substrates for TCF, and the PFGO-AgNW films were transferred to the substrates using nearly the same process as described above. Because the films are somewhat damaged when the CA is peeled off of the substrate, for high-performance hybrid TCF the CA was dissolved in acetone. Next, the PFGO-AgNW films were further reduced by hydrazine vapor at 100 °C for 5 h. Finally, the hybrid films were held at 200 °C for 30 min to form connecting junctions between AgNWs, as the junction resistance between AgNWs is very high.

electrodes and optoelectronic devices, which would guarantee that the TCF is undamaged during the post-treatment process.32 To date, few studies have been focused on the study of adhesion issues.33 Dopamine contains catechol and amine groups, which are the crucial functionalities for mimicking the adhesive proteins of mussels.34 Under weak alkaline pH conditions, dopamine undergoes self-polymerization to produce adherent polydopamine (PDA). PDA can adhere to virtually all types of inorganic and organic surfaces, regardless of the substrate’s chemistry.35−37 Recently, Akter et al. fabricated a polydopamine layer on the elastomeric substrate to facilitate the subsequent spraying of AgNW; the obtained AgNW TCF demonstrated excellent adhesion to the substrate.38 Jin et al. reported that the PDA and alginic acid as a nonconductive binder induced tighter-contacting AgNW networks and provided good adhesion to the substrate.39 However, graphene and PDA in previous studies act merely as a conducting protective layer or an adhesive layer for AgNW TCF, respectively.25,39 They could not overcome the problematic issues of weak adhesion and low stability simultaneously. Recent studies showed that dopamine could reduce GO during its polymerization.40,41 Herein, a novel design of reduced PDAfunctionalized graphene oxide (RPFGO) as a conductive adhesion promoter and protective layer for AgNW TCF is developed by incorporating mussel-inspired PDA. The procedure for preparing RPFGO-AgNW hybrid TCF is illustrated in Figure 1. GO was reduced via a two-step process; it was reduced and functionalized by PDA first and then further reduced by hydrazine vapor. To the best of our knowledge, RPFGO as a conductive adherent promoter and protective layer for AgNW TCF has not been described before.

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EXPERIMENTAL SECTION

Materials. Natural graphite powders (NGP) (325 mesh) were kindly provided by Qingdao Laixi Graphite Co., Ltd. Sulfuric acid (H2SO4, 98%), potassium permanganate (KMnO4, 99.3%), hydrogen peroxide (H2O2, 30%), hydrochloric acid (HCl, 37%), hydrazine hydrate (N 2H4, 80%), silver nitrate (AgNO 3, 99.8%), poly(vinylpyrrolidone) (PVP), sodium chloride (NaCl), and ethylene glycol (EG) were provided by Tianjin Guangfu Fine Chemical Research Institute and used as received. Dopamine hydrochloride (98%) and tris(hydroxymethyl)aminomethane (Tris) (99.9%) were purchased from Aladdin and used as received. Fabrication of GO and PFGO. GO powders were prepared from natural graphite powders via a modified Hummers’ method, as reported in our previous work.42 PFGO was fabricated as follows: 160 5366

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Langmuir Characterization. The sheet resistances of PFGO and RPFGOAgNW films were measured using the four-point probe method (Keithley 2700 multimeter), and the transmittances were measured using a UV−vis spectrophotometer (TU-1901). The surface morphologies of the RPFGO-AgNW films were characterized by field-emission scanning electron microscopy (FE-SEM) (Hitachi S4800). Transmission electron microscopy (TEM) was performed using a Hitachi H-800 electron microscope operating at an accelerating voltage of 120 kV. X-ray photoelectron spectroscopy (XPS) was conducted on a 60 (Genesis Edax, U.S.A.) instrument equipped with an Al Kα radiation source (hν = 1486.4 eV). X-ray diffraction (XRD) was performed using a Rigaku D/MAX-gA diffractometer with filtered Cu Kα radiation (λ = 0.15406 nm). Linear and 2D micro-Raman mapping spectra were recorded on a Renishaw InVia Raman microscope equipped with a 532 nm laser and a Horiba Jobin Yvon Xplora confocal Raman microscope equipped with a 532 nm laser. Fourier transform infrared (FT-IR) spectroscopy (TENSOR37) was performed to detect the functional groups of GO and PFGO. Thermogravimetric analysis (TGA; Netzsch STA 409, Germany) was performed in the temperature range of room temperature to 800 °C under a nitrogen atmosphere at a heating rate of 5 °C/min. The adhesion was investigated using the tape test in which a 12-mm-wide piece of 3 M Scotch tape was attached to each of the PFGO and RPFGO-AgNW films and then peeled off of the samples.

absorption bands of oxygen functionalities in GO decreased dramatically. The XPS spectra of GO and PFGO are shown in Figure 3a. The GO spectrum shows C 1s and O 1s peaks at 286 and 534

RESULTS AND DISCUSSION PFGO was fabricated by mixing the GO aqueous suspension with dopamine hydrochloride at pH 8.5. During the selfpolymerization of dopamine, the color of the mixture changed from light brown to black (inset of Figure 2a). This color

Figure 3. Full-scale XPS spectra of GO and PFGO (a), N 1s of PFGO (b), and C 1s of GO (c) and PFGO (d).

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eV, respectively. The spectrum of PFGO exhibited an additional peak at 399 eV, which is attributed to the N 1s peak.40 The N 1s peak originated from amine groups of PDA. The C 1s XPS spectra of GO and PFGO are shown in Figure 3c,d. The XPS C 1s core-level spectrum of GO can be curve-fit into three peak components with binding energies at approximately 284.7, 286.8, and 287.9 eV, which are attributable to CC/C−C, C−O, and CO species, respectively.49,50 For PFGO, four different peaks at 284.6, 285.6, 286.7, and 287.8 eV are related to the CC/C−C, C− N, C−O, and CO groups, respectively. The appearance of the C−N peak at a binding energy of 285.6 eV is consistent with the presence of a surface-functionalized PDA layer. The intensities of all C 1s peaks of carbon binding to oxygen obviously decreased, which confirmed that GO was simultaneously reduced by PDA. The thermal stability of GO and PFGO was also examined by TGA (Figure S1); PFGO exhibited approximately 4.6 wt % loss below 100 °C due to the absorbed water and approximately 15.9 wt % loss at 200 °C due to the pyrolysis of the labile oxygen-containing functional groups. The value was much lower than that of the GO, confirming the chemical reduction of GO by PDA.45 The electrical conductivity of PFGO films improved as the oxygen-containing functional groups were partially removed. Figure 4 shows the sheet resistance and transmittance at 550 nm for PFGO films. The performance of the TCF varied according to the volume of PFGO coated on the glass substrates. It is shown that the sheet resistance and transmittance of PFGO films decrease as the PFGO content increases. The sheet resistance of PFGO films ranged from ∼107 to 109 Ω/sq. For comparison, the same content of GO films was insulating. More interestingly, PFGO films adhered to the glass substrate much more easily and more uniformly than did GO films (Figure 5). 3M Scotch tape applied with finger pressure was also unable to detach a significant amount of PFGO from the surface (Figure S2a). This result is probably

Figure 2. UV−vis absorption (a) and FTIR (b) spectra of GO and PFGO.

change suggested the reduction of GO.40 The conversion of GO to PFGO via reduction by PDA was first monitored by UV−vis spectroscopy. As shown in Figure 2a, GO exhibits a characteristic absorption peak at 230 nm.43 As time went on, the absorbance peak disappeared and a broad peak with an absorption maximum at ∼260 nm was observed. The shift in the absorption peak of PFGO is due to the restoration of the π network in the graphene sheets.44 The reduction of GO is possibly due to the release of electrons during the oxidative polymerization of dopamine−hydrochloride.45 However, no obvious aggregate formation was observed when PFGO was dispersed in water, which is attributed to the stabilization by PDA.46 The simultaneous surface functionalization and reduction of GO were also confirmed by FTIR and XPS spectroscopy. As illustrated in Figure 2b, the GO characteristic bands are present at 1730 cm−1 (CO stretching), 1407 cm−1 (C−OH stretching), and 1055 cm−1 (C−O of epoxy stretching).47 For PFGO, new bands are observed at 2918 and 2850 cm−1 (C−H stretching of methylene groups in PDA), which confirms success in achieving PFGO.48 Moreover, the characteristic 5367

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significantly. This result demonstrates the successful reduction of PFGO. A higher degree of reduced GO provides a higher electrical conductance. Figure 7b displays the XRD patterns of AgNW, GO, PFGO, and RPFGO-AgNW. Four diffraction peaks observed from the XRD spectra of AgNW are indexed to the (111), (200), (220), and (311) planes of face-centeredcubic silver crystals, respectively.54 The XRD patterns of GO exhibit an interlayer distance of 0.80 nm corresponding to 11.1° (2θ) as a result of the introduction of oxygenated functional groups on graphene sheets.47 After GO was reduced and functionalized by PDA, the characteristic peak of GO disappeared and the interlayer spacing between graphene sheets decreased to 0.51 nm (2θ = 17.2°) owing to the oxygenated functional groups being partially removed. For RPFGO-AgNW TCF, the XRD pattern shows the characteristic peaks of AgNW, and no peaks attributed to silver oxide are observed, indicating the high quality of AgNW. The XRD peak of RPFGO changed to 20−25° (2θ). However, because of its low intensity compared to that of AgNW and the overlap with the pattern of the glass, the broad graphitelike peak is not apparent.55 Micro-Raman spectroscopy was used to investigate the structural disorder and homogeneity of PFGO with the relative intensities of the D and G bands (ID/IG) ratio before and after reduction. Figure 8a presents the μ-Raman spectra of GO, PFGO, and RPFGO-AgNW. All samples exhibit a strong D band at ∼1357 cm−1 and a G band at ∼1590 cm−1, corresponding to the structural disorder and the graphitized structure, respectively.56 Changes in the ID/IG ratio indicate the changes in the electronic conjugation state of the GO during reduction. The ID/IG ratios of GO, PFGO, and RPFGO are 0.92, 0.97, and 1.02, respectively. The increase in the ID/IG ratio indicates that sp2 domains were formed and that GO was reduced during the reduction.57 To assess the homogeneity of the material, μ-Raman mapping was performed over 20 × 20 μm2 areas in samples. Figure 8b−d illustrates the mapping of the ID/IG ratio across the different samples, which indicate the high homogeneity of the material. The RPFGO-AgNW hybrid film on the glass substrate is optically transparent, as shown in the inset of Figure 9a. As seen from the SEM image of RPFGO-AgNW TCF (Figure 9a), AgNW and the hollow spaces between AgNW networks were covered by RPFGO, as also demonstrated by the cross-

Figure 4. Sheet resistance versus optical transmittance of PFGO films as a function of PFGO content.

due to the catechol groups of PFGO serving as an anchor to enhance the adhesion force between the films and the glass substrates.51 Furthermore, the positively charged amine groups (−NH3+) of PFGO could also interact electrostatically with the hydroxyl groups of the substrate.33 For 8 mL of PFGO, the film has a sheet resistance ∼3.46 × 108 Ω/sq and a transparency of 78.2% at a typical wavelength of 550 nm. The conductivity and transparency of the PFGO coating with excellent thermal and chemical stability and a strong adhesion ability are sufficient for antistatic applications.52 Most previous research on PFGO focused on further functional studies, rarely paying attention to its optoelectronic properties.44,48 This work may lead to the practical use of graphene for a new generation of antistatic coatings. Figure 6a shows the plasmon peaks of AgNW. Two plasmon peaks at 380 and 350 nm are attributed to the transverse and longitudinal surface plasmon resonances of AgNW, respectively.53 This result confirms that the 1D AgNWs were successfully fabricated. As shown in Figure 6b,c, the length of AgNW reaches to tens of micrometers, and the diameter ranges from 70 to 110 nm. TEM images of the 1D AgNW also confirmed the results, as revealed in Figure 6d,e. High-performance RPFGO-AgNW transparent conductive electrodes were prepared using a vacuum filtration method. Figure 7a shows the XPS C 1s spectra of RPFGO, which clearly indicate that the number of oxygen-containing functional groups, such as C−O and CO in RPFGO, decreased

Figure 5. Photograph and schematic of PFGO antistatic coatings adhered to the glass substrates. 5368

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Figure 6. (a) UV−visible absorption peaks, (b) low- and (c) high-magnification SEM microimages, and (d) low- and (e) high-magnification TEM images of AgNW.

Figure 9. (a) SEM microimages of RPFGO-AgNW TCF; inset (a) is the photograph of the TCF. (b) Cross-sectional SEM image of the TCF.

Figure 7. (a) XPS C 1s spectra of RPFGO. (b) XRD patterns of Ag, GO/glass, PFGO/glass, and RPFGO-AgNW/glass.

sectional SEM of the film in Figure 9b. RPFGO is a protective layer for AgNW networks, and they can efficiently improve the oxidation resistance of AgNW. The 3M Scotch tape test applied with finger pressure was performed to monitor the adhesion of RPFGO-AgNW to the substrate; no obvious amount of RPFGO-AgNW was found to be peeled off of the surface (Figure S2b), and no significant increase in the sheet resistance of the TCF was observed after the taping test. An adhesion factor incorporating transmittance was used to quantitatively characterize the adhesion, which can be easily measured from absorption spectroscopy32 f=1−

Tn − T0 ΔT =1− 100 − T0 100 − T0

(1)

where f is the adhesion factor and Tn and T0 are the transmittance values of the detached film after trial and pristine samples, respectively. Tn = T0, f = 1, and no detached AgNWs demonstrate perfect adhesion and cohesion. Tn = 100% (base substrate without AgNW), f = 0, and no AgNWs left on the film demonstrate no adhesion. The adhesion factor of RPFGOAgNW(10 mL) and pure AgNW(10 mL) TCF are 0.91 and 0.17, respectively. The RPFGO-AgNW TCF exhibits excellent adhesion. Strong adhesion guarantees that the film is undamaged during the post-treatment process, especially for

Figure 8. (a) Raman spectra of GO, PFGO, and RPFGO-AgNW. (b− d) μ-Raman mapping of GO, PFGO, and RPFGO-AgNW showing the ID/IG ratios.

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to compare the optoelectrical property of TCF. The transmittance (T) and sheet resistance (Rs) can be linked by eq 259

the large-scale fabrication of TCF and optoelectronic devices. This result may be due to the strong electrostatic interactions between the positively charged −NH3+ on RPFGO and the negatively charged substrate surface and oxo and hydroxo groups on AgNW surfaces,33 in addition to the reason that we discussed above. As a conductive adhesion promoter and protective layer, RPFGO would be much more conductive than GO and PDA29,39 and more adhesive than RGO.25,26 XPS was used to investigate the changes in the chemical states induced by the interactions between RPFGO and AgNW (Figure S3). The XPS signature of the Ag 3d doublet for the AgNW Ag 3d5/2 and 3d3/2 peaks appeared at 368.2 and 374.2 eV, respectively. For RPFGO-AgNW, the Ag 3d5/2 and 3d3/2 peaks shifted to lower binding energies and appeared at 368.0 and 374.0 eV, respectively. A similar phenomenon was also observed in the study of Sun et al. They attributed the negative shift to the electron transfer from metallic Ag to the positively charged protonated amine groups.33 The XPS results indicate the movement of the electrons from the surfaces of the AgNW to the RPFGO.58 The optoelectronic properties of RPFGO-AgNW TCF are shown in Figure 10a. The performance of the TCF varied

σDC Z0 = −1/2 σOP 2R s(T − 1)

(2)

where Z0 = 377 Ω/sq is the impedance of free space. A high σDC/σOP ratio indicates a high transmittance, a low sheet resistance, and a high opto-electrical property of TCF. The σDC/σOP values of AgNW (10 mL) and RPFGO-AgNW(10 mL) TCF are 38.92 and 15.67, respectively. However, the σDC/ σOP of AgNW (10 mL) TCF decreased to 8.44 after being held at 100 °C for 72 h. The long-term stability of RPFGO-AgNW TCF is better, as observed in Figure 10b. The sheet resistance value of RPFGO-AgNW TCF is nearly constant after being held at 100 °C for 72 h. The good stability of RPFGO-AgNW TCF is due to RPFGO protecting the surface of AgNW from oxidation as its impermeable barrier property.60 The sheet resistance and transmittance of GO-AgNW TCF were also measured. However, the sheet resistance of GO-AgNW TCF was very large as the insulating GO between AgNW networks prevented AgNWs from fusing with one another.61

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CONCLUSIONS Solution-processable mussel-inspired RPFGO-AgNW hybrid TCFs were successfully prepared. RPFGO acted as a conductive adhesive promoter and protective layer for the AgNW transparent electrodes and significantly enhanced the adhesion force between the AgNW networks and substrate. The strong adhesion guarantees that TCF is undamaged during the post-treatment process. The long-term stability of RPFGOAgNW TCF was also improved as a result of the protection effect of the RPFGO overcoating layer. It is believed that RPFGO-AgNW hybrid TCF could be used for various optoelectronic devices, such as solar cells, electrochromic devices, OLED, and touch panels.

Figure 10. (a) Sheet resistance and optical transmittance of RPFGOAgNW TCF as a function of the AgNW content. (b) Changes in the sheet resistance of RPFGO-AgNW hybrid films kept at 100 °C for 72 h.

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ASSOCIATED CONTENT

S Supporting Information *

according to the AgNW contents. The sheet resistance and transmittance of RPFGO-AgNW TCF decreased as the AgNW content increased. The sheet resistance of RPFGO-AgNW (8 mL) is 225 Ω/sq, which is significantly lower than the sheet resistance of RPFGO-AgNW (6 mL). This reduction may be due to the AgNW content being sufficiently high to overcome the percolation threshold.55 The RPFGO-AgNW (10 mL) TCF has a sheet resistance of 63 Ω/sq and a transparency of 70.5%, which are sufficient for many applications in optoelectronic devices. The sheet resistance could reach 37 Ω/sq as the content of AgNW continues to increase. The sheet resistance and transmittance of AgNW TCF were also evaluated. The AgNW films tend to break during the transfer process as a result of the weak adhesion force between AgNW films and the substrates. For low AgNW content, such as 2, 4, and 6 mL of AgNW, the AgNW films were severely damaged. Therefore, we typically use 10 mL of AgNW TCF for comparison. The sheet resistance and transmittance of AgNW (10 mL) TCF are 31 Ω/sq and 74.8%, which are better than for RPFGO-AgNW(10 mL) TCF. However, the long-term stability of the AgNW electrode is worse. The sheet resistance of AgNW (10 mL) TCF increased from 31 to 117 Ω/sq after being held at 100 °C for 72 h (Figure S4). The increase in sheet resistance is due to the oxidation of AgNW.25 The direct current electrical conductivity to optical conductivity ratio (σDC/σOP) was used

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b00796. TGA curves of GO and PFGO, 3 M Scotch tape tests of PFGO and RPFGO-AgNW films, Ag 3d XPS spectra of AgNW and RPFGO-AgNW, and changes in the sheet resistance of AgNW (10 mL) TCF (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel/Fax: +86-22-83955054. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was financially supported by the National Natural Science Foundation of China (grant no. 51573135) and the Science and Technology Development Plan of Tianjin Municipal (grant no. 13JCZDJC32100).

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REFERENCES

(1) Bryant, D.; Greenwood, P.; Troughton, J.; Wijdekop, M.; Carnie, M.; Davies, M.; Wojciechowski, K.; Snaith, H. J.; Watson, T.; Worsley,

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