Graphene Nanosheets Decorated with Silver Nanoparticles
onto Polyurethane Nanofibers for preparing Flexible
Transparent Conductive Thin Films
Hsi-Wen Tien,Yuan-Li Huang, Sheng-Tsung Hsiao, Wei-Hao Liao, Shin-Yi Yang, Chen-Chi M. Ma*
Department of Chemical Engineering, National Tsing-Hua University, Hsin-Chu 30013, Taiwan
摘要於 GNS 表面沉積上導電性極高的奈米銀金屬顆粒(AgNps-GNS)，由於 GNS 具有極佳的機械彎
折度，對於沉積在表面的 AgNps 也有優異的接著力，因此，AgNps-GNS 同時具備了 GNS 的可撓
性與 AgNps 的高導電性。PET 軟性透明基板經過 120 秒的 PU 電紡纖維沉積，其 PU 奈米纖維可建
構二維網狀之連續結構，再浸泡於 0.050 wt% AgNps-GNS (5:1) 的溶液，即可得到具連續的導電通
路且留有光通路之透明導電膜。熱熔解 PU 奈米纖維後，其透明導電薄膜在 85%的光穿透度下，表
面阻抗為 150 Ω/□。另外，經由抗折性質測試可得知，當 AgNps:GNS 比例為 3:1 時，薄膜具有最佳
的電學性能及穩定度，當彎折角度達 900 時，相較於 AgNps-GNS (5:1)薄膜的表面阻抗值上升了兩
個數量級，而 AgNps-GNS (3:1)薄膜表面阻抗僅上升了一個數量級。
關鍵詞：石墨烯、靜電紡絲、透明導電膜
AbstractA simple method of integrating hybrid thin filmsof graphene nanosheets(GNS) and silver
nanoparticles (AgNps) byin situ chemical reductionto prepare transparent conductive films (TCFs) is
studied. The surface functional groups of graphite oxide (GO) serve as nucleation sites of silver ions for
adsorption of AgNps. To fabricate conductive films with high transmittance, polyurethane (PU)
nanofibers are introduced to help construct two-dimensional (2D) conductive networks consisting of
AgNps and GNS (AgNps-GNS). This method requires only a low percentage of conducting AgNps-GNS
material covering the transparent substrate, thereby improving the transmittance and maintaining a
percolating conductive network. The flexible GNS serve as nanoscale bridges between conductive AgNps
and PU nanofibers, resulting in a highly flexible TCF. The optical transmittance can be further increased
after melting the PU nanofibers at 100 °C. A fused film obtained after electrospinning a PU solution for
120 s and immersion in 0.05 wt.% AgNps-GNS (5:1) solution has a surface resistance of 150 Ω/sq and 85
% light transmittance. Mechanical testing shows that AgNps-GNS on flexible substrates yields excellent
robustness when subjected to bending. Thus, TCFs with a 3:1 ratio of AgNps:GNS have high
conductivity, good mechanical durability, and barely one order of magnitude increase of surface
resistance due to 900blending.
Keywords: graphene nanosheets, electrospinning, transparent conductive films
INTRODUCTION
We present a direct and effective way to synthesize large silver nanoparticles-graphene nanosheets
(AgNps-GNS) films on polyurethane (PU) nanofiber flexible substrate by self-assembly. Nanofibers are
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introduced to guide the construction of 2D conductive networks without compromising transmittance.
Electrospinning (ES) is currently the most powerful technique to fabricate continuous ultra-long
nanofibers due to its versatility, ease of use, ability to align structures and control fiber diameters [1].The
main advantage of this technique, however, is that the conductivecomponent does not form a uniform
coating
but
a
percolating
network,
permitting
100%
optical
transmission
throughthe
“holes”.Polyurethane(PU) was used to fabricate the nanofibers in this study. Its structure includesthe
repeating N–H groups which can be hydrogen-bonded tothe carbonyl oxygen groups on
Polyvinylpyrrolidone (PVP). PVP was used as a modifier for adsorption that could attach itself via the
pyrrolidone rings close to both surfaces of the GO plane through strong π-π interactions[2]. PVP chains on
graphene sheets can improve the adsorption of GNS on the surface of electrospun nanofibers by taking
advantage of the binding capability of PVP carbonyl oxygen groups that provide hydrogen bonding with
neighboring nanofibers. Hence, based on the above discussions, we examine a simple procedure as shown
in Figure 1 for the preparation of flexible transparent conductive thin films.
EXPERIMENTAL SECTION
(1) Reduction and preparation of AgNps, GNS and AgNps-GNS composites
Typically, GO (50 mg, was prepared by modified Hummers method), AgNO3 (50 mg), PVP (200
mg), DI water (100 mL) and EG (100 mL) were mixed in a 500 mL four-necked flask under a nitrogen
atmosphere. NaBH4 (50 mL, 13 mmol) was slowly added, and the reaction mixture was stirred at 100 °C
for 24 h. The solid AgNps-GNS (1:1) product was isolated by repeated centrifugation (10,000 rpm) ten
times. This process was repeated for the three different weight ratios of AgNO3 to GO at 1:1, 3:1, 5:1 in
the reaction solution.
(2) Electrospinning process
The PU solution was fed into a 20 ml syringe with a stainless steel needle (18 gauge needle). The
flow rate of the solution was controlled using a syringe pump and kept constant at 0.05 mm/min. A
voltage of 20 kV was applied directly to the stainless steel needle. The distance between the needle tip
and the collector was maintained at ~20 cm. A set of six PET slides (30×60 mm2) was taped on an
aluminum foil, which was used as the collecting electrode. The nanofiber-coated PET slides were then
used as a substrate for deposition.
(3) Preparation of transparent conductive thin film on PET substrate
To study TCFs properties, a PET slide coated with the PU nanofibers was immersed into AgNps (0.5
wt.%), GNS (0.05 wt.%) and AgNps-GNS (0.05 wt.%) solution for 10 min, rinsed briefly (60 s) in
deionized water, and dried at 50 °C to obtain self-assembled AgNps/PU, GNS/PU and AgNps-GNS/PU
films.
RESULTS and DISCUSSION
(1) XRD analysis
Metallic silver particles can be obtained easily by reducing silver ions in silver nitrate solution [3-4].
Figure 2shows the existence of AgNps in the products, confirmed by X-raydiffraction (XRD). In cuves2(a)
to2(d),the peaks at approximately 38.20, 44.30, and 64.40can be indexed to the three strongest reflection
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peaks, (111), (200) and (220), of the face-centered cubic silver structure. We used the Scherrer
equation[5]to estimate the average sizes of the AgNps as 45 nm and ~13 nm with AgNps and AgNps-GNS,
respectively. The size of the AgNps can be decreased by the presence of GO during reduction.A broad
peak at approximately 26.60 represents the hexagonal structure of graphene, Fig.2(e).Curves 2(b) to 2(d)
showno obvious diffraction peaks attributed to graphite, which suggests that AgNps was keeping the
stacking of the AgNps-GNS sheets disordered; therefore, the characteristic diffraction peaks of the
layered structure disappeared.Previous study also suggests that the silver nanoparticles are decorated on
GNS, which is not arranged in uniform, regular stacks [6].
(2) TEM analysis
Fig. 3(a) shows the TEM image of thin GNS with some corrugations, suggesting a flexible structure
of the GNS. Fig. 3(b) reveals TEM images of a GNS decorated with AgNps. Numerous individual AgNps
with spherical shape are found to attach and distribute on the surface of GNS. Fig. 3(c) shows coverage of
AgNps on GNS of approximately 82 % in the product with a ratio of AgNps to GNS of 3:1, which is
larger than the 1:1 shown in Fig. 3(b). According to Fig. 3(d) to 3(f), the density of AgNps on the GNS
surface increases when the ratio of AgNO3 to GNS is increased from 1:1 to 5:1. Fig. 3(f) shows that the
almost transparent GNS sheets are thickly coated by AgNps for AgNps-GNS (at a ratio of 5:1).
(3) SEM analysis
Fig. 4 shows the morphology of the as-deposited film obtained by electrospinning the 20 wt.% PU
solution with an ES time of 180 s. The PU nanofibers formed a network when they were coated on the
PET substrate. The nanofibers coated with GNS which interacts strongly with PU due to PVP wrapped on
GNS surface. Urea groups on PU nanofibers can form hydrogen bonding with neighboring carbonyl
oxygen groups on PVP, providing strong interactions between GNS and PU nanofibers.
(4) Surface resistance and transmittance analysis
Sheet resistance and transmittance are two important properties of thin films for application in TCFs.
Figure 5 shows these two properties for AgNps, GNS and AgNps-GNS self-assembled onto nanofibers
plotted as a function of electrospun (ES) time. The surface resistance gradually decreases while the
density of PU nanofibers increases with increasing ES time. Fig. 5(a) shows that the lowest surface
resistance obtained is 35 Ω/sq by dipping PET in AgNps-GNS (5:1) solution electrospun for 180 s.
Meanwhile, the transmittance decreases as the ES time increases as shown in Figure 5(b). The
transmittance of pristine PU mats is decreased to 71% with ES time of 180 s arising from the formation of
a dense nanofibers network. The AgNps-GNS (3:1)/ PU film with ES time of 180 s gives a transmittance
of 68%, while the neat PU sample without self-assembled AgNps-GNS has a transmittance of 71%.
CONCLUSION
We prepared TCFs from GNS-decorated AgNps. Interconnected networks of AgNps with an average
size ~13 nm effectively enhanced the electrical conductivity of GNS. Additionally, we introduced
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nanofibers as a guide to build up 2D conductive networks of AgNps-GNS-based sheets. The AgNps-GNS
did not form a uniform coating but a percolated network aided by PU nanofibers, allowing optical
transmission through the “holes” in the network.This is important since the large conductivity of AgNps
can be retained with little compromise on transmittance. The AgNps-GNS(5:1)/PU thin film exhibits a
substantial increase in transmittance after melting treatment, producing a thin film with surface resistance
of 150Ω/sq and transmittance of 85 % at 550 nm.The resulting M-AgNps-GNS/PU films are flexible
without anysignificant change in sheet resistanceeven bent more than 600.We conclude, however, that a
suitable ratio of AgNps to GNS is 3:1, which has an optimal combination of mechanical flexibilityand
electrical property. This finding provides the physical basis for development of hybrid metal particle/GNS
films for future large-area transparent and flexible electrode applications.
Figure 1.Procedure for decorating AgNps on GNS surface and self-assembly of AgNps-GNS onto the
surface of PU nanofibers.
Intensity (A.U.)
(e) GNS
(d) AgNps-GNS (1:1) 11.2 nm
(c) AgNps-GNS (3:1) 12.5 nm
(b) AgNps-GNS (5:1) 13.1 nm
(a) AgNps 45 nm
111
10
20
30
40
200
220
50
60
70
2 Theat (degree)
Figure 2.XRD patterns of AgNps, AgNps-GNS (1:1), AgNps-GNS (3:1), AgNps-GNS (5:1) and GNS.
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Figure 3.TEM images of different ratio of AgNps to GNS: (a) 0:1, (b) 1:1 and (c) 3:1.
High-magnification TEM images of different ratio of AgNps to GNS: (d) 1:1, (e) 3:1 and (f) 5:1.
Figure 4. High-resolution SEM images of PU nanofiber mats. (a) Electrospun for 180 s. PU nanofiber
selectrospun for 180 s dipped with(b) AgNps, (c) GNS, (d)AgNps-GNS (5:1) and
(e)AgNps-GNS (3:1) after melting. (f) shows SEM micrograph of (e) under low
magnification.
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12
10
AgNps
GNS
AgNps-GNS (1:1)
AgNps-GNS (3:1)
AgNps-GNS (5:1)
(a)
11
10
Surface Resistance (/sq)
10
10
9
10
8
10
7
10
6
10
5
10
4
10
3
10
2
10
1
10
60
90
120
150
180
Electrospinning time (s)
Transmittance at 550 nm (%)
100
(b)
PU nanofiber
AgNps-GNS (1:1)
AgNps-GNS (3:1)
AgNps-GNS (5:1)
95
90
85
80
75
70
65
60
60
90
120
150
180
Electrospinning time (s)
Figure 5. (a) Surface resistance and (b) transmittance of PU nanofiber thin films plotted as a function of
electrospinning time and dipping with different solution.
ACKNOWLEDGMENTS
We thank the Taiwan Textile Research Institute (100A0136J2) and National Science Council, Taiwan for
financially supporting this research (Graduate Program for Studying in Australia/ New Zealand). YLH
and YKY are Visiting Scholars to the CAMT at the University of Sydney. YWM also acknowledges the
Australian Research Council for supporting his research projects on polymer nanocomposites.
Note: This manuscript has been submitted to “CARBON” and accepted.
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