Fast Process To Decorate Silver Nanoparticles on Carbon

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A Fast Process to Decorate Silver Nanoparticles on Carbon Nanomaterials for Preparing High Performance Flexible Transparent Conductive Films Yu-An Li, Yin-Ju Chen, and Nyan-Hwa Tai Langmuir, Just Accepted Manuscript • DOI: 10.1021/la401662d • Publication Date (Web): 11 Jun 2013 Downloaded from http://pubs.acs.org on June 14, 2013

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A Fast Process to Decorate Silver Nanoparticles on Carbon Nanomaterials for Preparing High Performance Flexible Transparent Conductive Films Yu-An Li, Yin-Ju Chen, and Nyan-Hwa Tai* Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan

KEYWORDS optoelectronics, flexible transparent conductive film, carbon nanotubes, graphene nanosheets, silver nanoparticle decoration

ACS Paragon Plus Environment

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ABSTRACT This work demonstrates a fast process to decorate silver (Ag) nanoparticles onto the functionalized few-walled carbon nanotubes (f-FWCNTs) and graphene nanosheets (f-GNs). The Ag-coated carbon nanomaterials were used as fillers which mixed with poly (3,4ethylenedioxythiophene)-poly(4-stryensulfonate)

(PEDOT:PSS)

for

preparing

high

optoelectronic performances of flexible transparent conductive films (TCFs). The Ag nanoparticles with a particle size of approximate 5 nm were uniformly distributed on the surfaces of the f-FWCNTs (Ag@f-FWCNTs) and the f-GNs (Ag@f-GNs). The Ag ions play the role of electron acceptors during the reduction process, which increases the hole concentrations in PEDOT:PSS, f-FWCNTs, and f-GNs, therefore enhancing the electrical conductivity of the TCFs. Additionally, the Schottky barrier was decreased due to the increase of work functions of the carbon fillers caused by Ag decoration. The X-ray diffraction spectrum of Ag@f-GNs depicts the formations of the face-centered cubic Ag nanoparticles, and the peak of (002) graphene plane slightly shift to the lower frequency indicating that f-GN interlayer was intercalated with Ag ions or Ag nanoparticles. When the mixture of 2.0 wt% of Ag@f-FWCNTs and 8.0 wt% of Ag@f-GNs containing PEDOT:PSS dispersant was coated onto a PET substrate, outstanding optoelectronic properties with a sheet resistance of 50.3 ohm/sq and a transmittance of 79.73% at a wavelength of 550 nm were achieved.


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INTRODUCTION Indium tin oxide (ITO) is by far the most popular material used for producing transparent conductive electrodes which have been widely utilized in liquid crystal displays,1 organic lightemitting diodes,2 touch panels,3 and solar cells.4 However, the worldwide supplies of indium are gradually decreases, thereby result in ITO applications with restriction.5 In addition, the ITO film is inherently inflexible, experiencing an increase in electrical resistance when subjected to bending; therefore, its applications in flexible electronic devices are also limited.6 Furthermore, the ITO layer is usually fabricated in an expensive and low-throughput vacuum sputter system, which significantly increases the cost of the ITO-based products significantly. To solve the drawbacks of using ITO layer as a conductive film, this work adopted the Agnanoparticle-decorated functionalized few-walled carbon nanotubes (Ag@f-FWCNTs) and graphene nanosheets (Ag@f-GNs) as conductive fillers and mixed with poly (3,4ethylenedioxythiophene)-poly (4-stryrenesulfonate) (PEDOT:PSS) for fabricating flexible transparent conductive films (TCFs). The purpose of using PEDOT:PSS as the dispersant is due to its electrical conducting property. Few-walled carbon nanotubes (FWCNTs) possess a high aspect ratio, good chemical stability, superior electron transportation, and excellent mechanical flexibility.7 The reason we used FWCNTs in this study is that FWCNTs possess superior properties than those of single-walled (SWCNTs) and multi-walled CNTs (MWCNTs). SWCNTs have very narrow diameter which benefits the optical transmittance, however, SWCNTs show one-third in metallic and two-thirds in semiconducting depending on their chirality. Therefore, the presence of the semiconducting SWCNTs may increase the contact electrical resistance. MWCNTs possess good electrical conductivity due to their more conductive π channels; however, the larger diameter decreases the optical transnittance.8 FWCNTs perform


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moderate conducting and transmittance properties with a more balance result, which is suitable for preparing transparent conductive thin film.9 Graphene nanosheets (GNs) with sheet-like structure have a high specific surface area, excellent electrical conductivity, and good chemical stability.10,11 The selection of FWCNTs as fillers instead of single-walled carbon nanotubes (SWCNTs) or multi-walled carbon nanotubes (MWCNTs) arises from the fact that FWCNTs have excellent conductive and transmittance properties attributing to narrow diameters, which are the characteristic properties of SWCNTs; in addition, FWCNTs possess good electrical conductivity which is one of the characteristic properties of MWCNTs. To further increase the pathways for electron transfer, one of the strategies is decorating metal nanoparticles on the surfaces of conductive fillers. In our previous study, we successfully decorated palladium (Pd) nanoparticles onto the surfaces of functionalized FWCNTs (fFWCNTs) and significantly enhanced the electrical conductivity of CNT-based TCFs.9 However, the Pd decorating process was complicated and time consuming. In this work, we chose Ag as the decorating metal, and developed an easy and fast process to decorate Ag nanoparticles onto the surfaces of f-FWCNTs and f-GNs; the process has the advantages of reducing the decorating process time and enhancing the electrical conductivity of the fabricated TCFs. Many published articles revealed the decoration of Ag nanoparticles on the surfaces of CNTs and GNs. Some studies dealt with using the electroless plating method to decorate Ag nanoparticles on CNTs.12-14 Before the plating of Ag nanoparticles onto a substrate, the substrate has to be sensitized and activated. Both these two processes are pH-sensitive reactions which needed to be precisely controlled. The use of high-energy radiation beams such as plasma or gamma ray irradiation to decorate Ag nanoparticles on CNTs has also been reported.15-17 However, the energy may destroy the CNT or GN structures resulting in reduction of the


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electrical conductivity. Using bridging materials such as benzyl mercaptan and cysteamine can effectively bind Ag nanoparticles with the thiol groups on CNTs.18-20 However, the bridging materials are inherent electrical insulators, which therefore increases the electrical resistance of the synthesized product. Although the bridging materials can be removed using ultraviolet light or ozone;21 the harsh conditions may damage the CNTs. A far better method for decorating Ag nanoparticles onto the surfaces of GNs is to first exfoliate graphite to form graphite oxides (GO), and immerse the GO in a silver precursor solution, followed by a reduction reaction using hydrazine monohydrate (N2H4•H2O) or sodium borohydride (NaBH4) as the reduction reagent, which causes both the Ag ions and the GO to be reduced simultaneously.22,23 However, the processes involved were complicated and difficult to control. In this work, we propose an easy and fast process to decorate Ag nanoparticles on the surfaces of f-FWCNTs and f-GNs. The process is performed via the reaction of f-FWCNTs and f-GNs with Ag ions in silver nitrate dissolved in an ethanol solution, and the Ag decorating reaction can be completed within 1 h, faster than other previous work which needed to take several hours.12-23 The electrostatic attraction between the carboxyl groups on the f-FWCNTs and the f-GNs causes the migration of Ag ions to the surfaces of the f-FWCNTs and the f-GNs. In addition, ethanol plays dual roles as a solvent and as a weak reagent for reducing Ag ions to Ag nanoparticles.24 Using this method, the electrical conductivity of TCFs can be effectively increased when Ag nanoparticles are deposited on the surfaces of the f-FWCNTs and the f-GNs.


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EXPERIMENTAL SECTION Processing of Purified FWCNTs The commercial pristine FWCNTs (XinNano Materials, XNM-HP-12050) have an average diameter of 4.0 nm and a length of approximate 10.0 μm with the purity of 86%. They were oxidized in air at a temperature of 600°C for 1 h followed a process by immersing them in an 8.0 M solution of hydrochloric acid under stirring for 1 h and then subjected to a filtration process to remove the catalysts. The purified FWCNTs, denoted as p-FWCNTs, were subsequently washed with copious amounts of 10 MΩ DI water until the wash water reached a pH of 7.0.25,26 Based on the Raman spectrum (figure not shown), we found that the as-purchased FWCNTs contained a small amount of SWCNTs. Fabrication of GNs Natural graphite flakes (10 mesh, 99.9 % metals basis, Johnson Matthey) were used as starting materials for preparing GNs. These graphite flakes were mixed with a mixture containing 80 vol% sulfuric acid (98%, ECHO Chemical) and 20 vol% of nitric acid (70%, ECHO Chemical) and stirred at room temperature for 16 h. After acid treatment, the residual carbon materials were filtered and washed with DI water until the wash water reached a pH of 6.0. The wet cake-like mixture was then dried at 70°C for 2 days. After drying, the powders were heated in a 900 W microwave oven (JMO23888A, YJEPROUD) for 30 s, forming worm-like expanded graphite (EG). The GNs were obtained by adding the synthesized EG to ethanol (99.99%, ECHO Chemical) and then sonicated for 16 h followed by the filtration process and subsequently drying at 70°C for 2 days.10


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Processing of Functionalized FWCNTs and GNs The wet p-FWCNTs and the dried GNs were individually immersed into a 3:1 v/v mixture of concentrated H2SO4 and HNO3 and subsequently sonicated for 1 h.27 Both the products were then collected by vacuum filtration and washed with copious amounts of DI water until the wash water reached a pH of 7.0. The products were assigned as f-FWCNTs and f-GNs for functionalized FWCNTs and functionalized GNs, respectively. One-Step Process for Silver Decorating The f-FWCNTs were uniformly dispersed in ethanol and sonicated for 1 h. In another beaker, an ethanol solution containing 10.0 mM silver nitrate was prepared and sonicated for 1 h. These two solutions were mixed and sonicated for another 1 h. The mixture was then filtered and dried at 150°C for 24 h for obtaining Ag@f-FWCNTs. The Ag@f-GNs were fabricated using the same method. Ink Processing Concentration of the as-purchased PEDOT:PSS (HC Starck, PH500) is approximate 10 mg mL-1, which was used as surfactant for dispersing carbon nanomaterials. The ink with the amount of 20 mL, containing 18 mL of ethanol and 2 mL of PEDOT:PSS polymer, was mixed with fFWCNTs and/or f-GNs with a total amount of 2 mg. The composition was so designed that the filler concentration was adjusted to be approximate 10.0 wt%, thus the content of the PEDOT:PSS in the solution was approximate 20 mg. After all of these materials were mixed and sonicated for 1 h, the solution was centrifuged at 10000 rpm in a Kubota 7780 centrifuge for another 1 h. A dark blue ink was obtained by collecting the upper layer supernatant of the


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centrifuged solution. Different ink compositions are shown in Table S1 in Supporting Information. Processing of TCFs The wire-wound (Industry Tech, No. 9) coating method for preparing 22 µm thick layer was utilized to fabricate the TCFs on a poly(ethylene terephthalate) (PET) film with a 91% transmittance.28 The sheet resistance and the transparency of the film were controlled by coating different number of layers of ink on PET film. The wet TCFs were then dried at 120°C for 60 s. Characterization of TCFs The optical transmittance of the TCFs was measured by a Hitachi u-3410 UV/Vis spectrometer at wavelengths ranging from 350 nm to 750 nm, and the sheet resistance of the TCFs was measured using a conventional instrument (Mitsubishi Chemical MCP-T600). The optical performance of the TCFs in this work was evaluated by subtracting the light shielding of the PET film. The transmittance obtained at a wavelength of 550 nm was used as the optical transmittance of the TCFs. The TCFs made from different composition are denoted by the notations explained in Table S1, which are expressed as CXG10-X where C and G stand for carbon nanotubes and graphene nanosheets, respectively, and X represents the ratio of carbon nanotubes. The optoelectronic performance of the TCFs containing different number of layers and ratios of f-FWCNTs/f-GNs analyzed at a wavelength of 550 under the total loading of 10 wt% are shown in Figure S1 in Supporting Information. It is evident based on the results shown in Figure S1 that the PEDOT:PSS matrix containing 2.0 wt% of f-FWCNTs and 8.0 wt% of f-GNs (f-C2G8) possesses the best performance showing a sheet resistance and an optical transmittance of 92 ohm/sq and 78.87%, respectively. We used the same ratio as the f-FWCNT/f-GN sample to


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prepare the p-FWCNTs/GNs (C2G8) and the Ag@ f-FWCNTs/Ag@f-GNs (Ag@f-C2G8) ink for fabricating TCFs.

RESULTS AND DISCUSSION The microstructures of fillers were examined using transmission electron microscopy (TEM) as shown in Figure 1. Figure 1a and 1b show the p-FWCNTs with low and high magnification, respectively. The p-FWCNTs are seen to exhibit hair-like structures with diameters ranging from 3.0 to 5.0 nm, which corresponds to 5 to 10 graphitic layers. Figure 1c and 1d depict the TEM images of GNs at low and high magnification, respectively. The GNs used in this study were several micrometers in lateral dimension and several nanometers in thickness. Figure 1d shows a typical GN thickness of 4.0 nm which is corresponding to approximate 10 graphitic layers. After Ag decoration, it is observed that homogeneous Ag nanoparticles with an average diameter of 5.0 nm are uniformly distributed on the surfaces of f-FWCNTs and f-GNs, as shown in Figure 1e through 1h. In addition, more Ag nanoparticles adhered at the edges than on the surface of the fGNs suggesting that the edges may contain more defects or sp3 structures, which provides more active sites for Ag nanoparticles to undergo nucleation and growth. Figure 2 shows the X-ray diffraction (XRD) patterns of GNs, f-GNs and Ag@f-GNs. The peaks at 38.10° and 44.28° correspond to the (111) and the (200) planes, respectively, of the facecentered cubic Ag nanoparticles.29 The peaks at 10-18° represent the presence of oxygenated structures. The peak intensity of the f-GNs is stronger than that of the GNs indicating more carboxyl and carbonyl groups were generated on the f-GNs after the functionalization process. The major peaks at 24.74° for GNs, 23.92° for f-GNs, and 23.16° for Ag@f-GNs represent the hexagonal (002) graphite plane corresponding to interlayer distances of 0.359, 0.372, and 0.384


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nm, calculated based on the Bragg’s formula, for GNs, f-GNs, and Ag@f-GNs, respectively. The shift of the diffraction peak by a magnitude of 1.58° from 24.74° for GNs to 23.16° for Ag@fGNs illustrates the intercalation of Ag ions and/or Ag nanoparticles to the interlayer of the f-GNs resulting in expansion of the graphitic interlayer after the Ag ions were reduced. The intercalations of Ag ions and/or Ag nanoparticles can produce more conductive pathways between the interlayer of GNs for electron transfer. Based on the X-ray photoelectron spectroscopy (XPS) results depicted in Figure 3, the electronic states of C2G8, f-C2G8, and Ag@f-C2G8 can be identified. As shown in Figure 3a, only C1s can be detected in the C2G8 filler and no O1s peak can be found indicating high purity of the C2G8 filler. On the other hand, the presence of O1s peak in the f-C2G8 filler is due to grafts of carboxyl and carbonyl groups on the carbon material during the functionalization process. After Ag decoration, the Ag3p and Ag3d signals can be observed in the Ag@f-C2G8 filler. Figure 3b shows the XPS spectrum of Ag3d peak of the Ag@f-C2G8 filler. In the Ag3d spectrum, the peaks at 368.1 and 374.2 eV correspond to the chemical states of 3d5/2 and 3d3/2, respectively. These two peaks correspond well to oxide-free Ag metallic nanoparticles.30 Furthermore, the Ag3d5/2 peak can be resolved into three component peaks, located at 367.3, 367.8, and 368.3 eV, corresponding to AgO, Ag2O, and Ag metallic states, respectively.31 It is clear that the Ag metallic state dominates the Ag3d5/2 peak of the Ag@f-C2G8 filler indicating that the majority of the nanoparticles decorated on the carbon material surfaces were metallic Ag. AgO and Ag2O are the intermediate products of the Ag decoration process via the reaction of Ag ions and carboxyl groups. In addition, the slight shifts of these three fitted peaks to a higher binding energy by 0.1-0.3 eV, as compared with the results reported for the Ag oxides and


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metal,31 are attributed to the presence of more electronegative oxygen atoms from the functional groups on the carbon surfaces.32 Ultraviolet photoelectron spectrometry (UPS) provides useful information for better understanding the variation in the electronic structure of films containing different fillers. As shown in curve (a) in Figure 4, the work functions of the carbon fillers, measured by UPS, are 4.86, 5.01, and 5.09 eV for C2G8, f-C2G8, and Ag@f-C2G8, respectively. The Ag ion has higher positive redox potential (Ag+/Ag, E0 = +0.799 V with the standard hydrogen electrode (SHE) as the reference electrode) than those of the CNTs (E0 = +0.500 V with the SHE) and the GNs (E0 = +0.380 V with the SHE);33-35 therefore, Ag ions have stronger potential to accept electrons and behave like p-dopants.36,37 The electrons transferred from the carbon fillers to Ag ions leaves numerous holes in the FWCNTs and the GNs, which results in the enhancement of electrical conductivity. In addition, the increase in the work function of the filler reduces the Schottky barrier height (SBH). During the reduction process for the decoration of Ag nanoparticles on the f-FWCNT and f-GN surfaces, Ag ions accept electrons from semiconducting SWCNTs included in the FWCNTs, which resulted in a decrease of the Fermi level of the f-FWCNTs because of the p-type doping effect.38 SBH is usually adopted to evaluate the interface properties between metal particles, Ag nanoparticles and metallic CNTs/GNs, and semiconducting SWCNTs. There are two types of SBH, n-type and p-type SBH (p-SBH), and the Ag-CNT/GN contact is considered as a p-SBH which is defined as the difference between the work function of Ag and the top of the valence band of the semiconducting CNTs. The p-SBH influences the contact resistance between the Ag nanoparticles and the semiconducting CNTs.39 As a result, the contact resistance was reduced because the increase of the work function induces the decrease of p-SBH and the downward shift of the Fermi level.9 The curve (b) shown in Figure 4 depicts an increase in the


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work function of the films from 4.98 eV for the Blank (without containing carbon fillers) film to 5.13 eV for the Ag@f-C2G8 film. When the PEDOT:PSS matrix contained the FWCNTs and the GNs, the increase in work functions of the films implies that the electrons transfer from the PEDOT:PSS polymer to carbon nanomaterials produce holes in the PEDOT:PSS resulted from p-type doping effect, consequently reducing the sheet resistance. The work function was increased to 5.13 eV for the Ag@f- C2G8 film indicating that the Ag nanoparticles further enhanced the p-type doping effect which accepted electrons from adjacent materials and left more holes in them. The increase of work function implies the decrease of contact barrier between these materials, as a result, reduced the contact resistance. Decoration of Ag nanoparticles on the f-GN and the f-FWCNT surfaces can be explained by the direct redox reaction between carbon materials and metal ions. The redox potentials of Ag+/Ag, CNTs, and GNs, as mentioned previously, are +0.799, +0.500, and +0.380 V, respectively; thus there will be a spontaneous transfer of electrons from f-FWCNTs and f-GNs to Ag ions which converts Ag+ to Ag nanoparticles on the filler surfaces. Additionally, Ag ions can also be reduced by ethanol under basic condition and deposited onto the surfaces of f-FWCNTs and f-GNs. Ethanol is known to be a weak reducing agent,24 and the ethanol with a pH value of 9.0 was used in this work. The Ag ions, supplied from the silver nitrate dissolved in the ethanol solution; diffused onto the surfaces of f-FWCNTs and f-GNs and reacted with grafted OHgroups to form Ag2O nanoparticles which were subsequently reduced by ethanol and deposited Ag nanoparticles on the surfaces of carbon fillers. The processes can be expressed by the following steps: 24,40

2Ag+ + 2OH-ads → Ag2Oads + H2O

E0 = +0.459 V (1)


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Ag2Oads + CH3CH2OH → CH3CHO + 2Agads + H2O

E0 = +1.367 V (2)

Ag2Oads + CH3CHO → CH3COO- + 2Agads + H+

E0 = +1.750 V (3)

H++ OH-ads → H2O

E0 = +0.829 V (4)

The overall reaction can be written as:

4Ag+ + 5OH-ads + CH3CH2OH → CH3COO- + 4Agads + 4H2O

E0 = +4.405 V (5)

where OH-ads, Ag2Oads, and Agads refer to the OH- groups, the Ag2O intermediates, and the Ag nanoparticles that are adsorbed onto the surfaces of f-FWCNTs or f-GNs. The reaction rates of Eqs. 2, 3, and 4 were relatively fast because Ag2O is a short-lived intermediate in the reaction process.40 In addition, the Ag decoration reaction is spontaneous because the redox potential of the overall reaction in Eq. 5 is +4.405 V, which was estimated based on redox half equations, listed in Eqs. 6-8, obtained from a standard redox potential table.

Ag2O + 2H+ + 2e- → 2Ag + H2O

E0 = +1.170 V (6)

CH3CH2OH → CH3CHO + 2H+ + 2e-

E0 = +0.197 V (7)

CH3COO- + 3H+ + 2e- →CH3CHO + H2O

E0 = -0.580 V (8)


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Figure 5 shows the optoelectronic properties of the film prepared by using various fillers. The films with high sheet resistance (the data circled in Figure 5, with the sheet resistance over 106 ohm/sq), containing C2G8, f-C2G8, or Ag@f-C2G8 have lower transmittance than the Blank film, it is because, on the one hand, the presence of carbon fillers darkens the films; on the other hand, the filler amount in the film is too small to form electrically conductive percolation, causing high sheet resistance. However, for the transmittance lower than 95%, the sheet resistance of the Blank film is kept in the range of 102 to 103 ohm/sq, and it is found that using hybrid carbon nanomaterials as fillers reduced the electrical sheet resistance of the TCFs significantly. The fC2G8 film exhibits lower electrical sheet resistance than the C2G8 film because the p-dopant effect of the f-C2G8 fillers was generated in the functionalization process, which decreases the overall electrical resistivity of the film.41,42 In addition, the C2G8 film including a lot of pores also lowers the electrical conductivity (The FE-SEM surface morphology images of the PEDOT:PSSbased TCFs are shown in Figure S2 in Supporting Information). The film containing Ag@f-C2G8 possesses a lowest sheet resistance of 50.3 ohm/sq with a moderate transmittance of 79.73%. The sheet resistance of the Ag@f-C2G8 film is only 15% of that for the Blank film which has the sheet resistance and the transmittance of 339 ohm/sq and 78.25%, respectively; it is because that the Ag nanoparticles generated more conductive pathways to lower the electrical resistance of the Ag@f-C2G8 film. TCFs are widely used in various electronic products, such as touch screens and flexible solar cells. A photograph shown in Figure 6 demonstrates an application of TCFs using the Ag@fC2G8 as filler. Ten light-emitting diode (LED) lights were connected in series and adhered to the TCF surface using Ag paste as binder. The power consumption of each LED light is approximate


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0.07 W, and the applied voltage to the demonstration kit shown in Figure 6 was approximate 30 V. Figure 6 demonstrates that the TCFs, coated with the mixture of Ag@f-C2G8 fillers and PEDOT:PSS, possesses superior performances as an electrode with high transparency and flexibility.

CONCLUSIONS This work demonstrates an enhancement in the electrical conductivity of TCFs using PEDOT:PSS as dispersant and Ag@f-C2G8 as fillers at a loading of 10.0 wt%. The Ag@fFWCNTs and and Ag@f-GNs were fabricated with a fast Ag decorating process. The introduction of Ag@f-FWCNTs and Ag@f-GNs into the PEDOT:PSS matrix not only formed a three-dimensional network but also increased the contact points between the Ag nanoparticles and the fillers, resulting in increase in the number of electrical conductive pathways. In addition, the reduction of Ag ions to Ag nanoparticles increased the concentration of holes in both the fillers and the PEDOT:PSS polymer, leading to a reduction in the contact resistance. After Ag decoration, the homogenous Ag nanoparticles with an average size of 5.0 nm were found to be distributed uniformly on the surfaces of f-FWCNTs and f-GNs. In addition, the Ag ions and/or Ag nanoparticles may intercalate into the GN interlayer and expand the spacing between graphitic layers. Ethanol was used both as a solvent and as an electron donor to dissolve and to reduce the Ag ions. When 2.0 wt% of Ag@f-FWCNTs and 8.0 wt% of Ag@f-GNs were used as hybrid fillers, the TCFs with an extremely low sheet resistance of 50.3 ohm/sq and a high transmittance of 79.73% at a wavelength of 550 nm were achieved.


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(a)

(b)

50 nm

2 nm

(c)

(d)

100 nm

5 nm

(e)

(f)

50 nm

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5 nm

To be continued


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(g)

50 nm

(h)

5 nm

Figure 1. TEM images of p-FWCNTs, GNs, Ag@f-FWCNTs and Ag@f-GNs at low (a), (c), (e), (g) and high (b), (d), (f), (h) magnifications, respectively. The arrows in (b) indicate the pFWCNTs with the graphitic layer of 5-10 layers and the diameter of 3.0-5.0 nm. The arrows in (d) represent a GN has a thickness of approximate 4.0 nm corresponding to 10 graphitic layers.


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Ag(111)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Ag(200)

(002)

Ag@f-GNs f-GNs

(002)

23.16

o

23.92

o

(002) 24.74

GNs

10

20

o

30

40

50

2 (degree)

Figure 2. The XRD patterns of GNs, f-GNs, and Ag@f-GNs. The GNs and the f-GNs exhibit two major peaks on the XRD spectrum. The peaks appeared in the range of 10-18° indicate the presence of carboxyl and carbonyl functional groups, while the other peaks at 24.74° and 23.92° corresponds to the hexagonal structure of the (002) graphene plane for GNs and f-GNs, respectively. The Ag@f-GNs shows four major peaks. The peaks at 10-18° and at 23.16° are fGN signals, while the other two peaks at 38.10° and 44.28° indicate the presence of the (111) and the (200) planes of the Ag nanoparticles, respectively.30


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(a) C1s Ag3d

O1s

Ag3p

Ag@f-C2G8 Intensity (a.u.)

f-C2G8

C2G8 0

100

200

300

400

500

600

Binding Energy (eV)

(b) Ag3d3/2: 374.2 Ag3d5/2: 368.1

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Ag3d

374.5

368.3 373.9

367.8

373.1

367.3

365

370

375

380

Binding Energy (eV)

Figure 3. XPS spectra of (a) the spectral region from 0 to 650 eV, and (b) the Ag3d region. In panel (b), the Ag3d5/2 was fitted to three peaks: 367.3 eV for AgO, 367.8 eV for Ag2O, and 368.3 for metallic Ag.33 The binding energy was calibrated using the C1s photoelectron peak at 284.6 eV as reference.


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5.20

Carbon fillers TCFs with carbon fillers

5.15

Work Function (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Ag@f-C2G8: 5.13

5.10

f-C2G8: 5.07 C2G8: 5.04

5.05 5.00 4.95

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Ag@f-C2G8: 5.09

(b)

Blank: 4.98

f-C2G8: 5.01

4.90

(a) 4.85

C2G8: 4.86

Figure 4. Variations of the work function of C2G8 fillers subjected to different treatments (curve a) and TCFs comprising of PEDOT:PSS matrix and C2G8 fillers subjected to different treatments (curve b).


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10

8

10

7

10

6

10

5

10

4

10

3

10

2

10

1

Blank

Sheet Resistance (ohm/sq)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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C2G8 f-C2G8 Ag@f-C2G8

80

85

90

95

100

Transmittance at a wavelength of 550 nm (%)

Figure 5. The relationship between the electrical sheet resistances and the optical transmittances of the TCFs prepared by adding different fillers. The weight ratios of the f-FWCNTs and the fGNs in the conductive polymer were 2.0 wt% and 8.0 wt%, respectively. Variations of sheet resistances and transmittances of the TCFs were due to different thicknesses of the coated film. The film containing Ag@f-C2G8 fillers possesses the best performance with an average sheet resistance of 50.3 ohm/sq and an optical transmittance of 79.73%.


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(a)

(b)

Figure 6. (a) A photograph of 10 LED lights adhering to flexible TCFs comprising of PEDOT:PSS and Ag@f-C2G8 in a series connection. The flexible TCF used in this photograph has a sheet resistance of 53.2 ohm/sq and a transmittance of 80.03% under an applied voltage of 30 V. (b) The schematic circuitry of (a).


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ASSOCIATED CONTENT Supporting Information. List of different ink recipe for getting optimum composition of f-FWCNTs and f-GNs. Comparison of the electrical sheet resistance and the optical transmittance of the films containing different thicknesses and amounts of f-FWCNTs and f-GNs. FESEM surface morphology images of the PEDOT:PSS-based TCFs. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

ACKNOWLEDGMENT This work was supported by the National Science Council under Grant No. 98-2221-E-007-045MY3. The authors would like to thank Prof. Yu-Tai Tao at Academia Sinica, Taiwan, for his support on the UPS measurement.

REFERENCES (1) Suzuki, Y.; Niino, F.; Katoh, K. Low-resistivity ITO films by dc arc discharge ion plating for high duty LCDs. J. Non-Cryst. Solids 1997, 218, 30-34.


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TOC GRAPHIC


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Fast Process To Decorate Silver Nanoparticles on Carbon

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