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polyimide hybrid films: Processing, morphology and properties
Materials Chemistry and Physics 138 (2013) 350e357
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Preparation of surface-silvered graphene-CNTs/polyimide hybrid films: Processing, morphology and properties Zhixiang Zheng a, b, Zaihua Wang a, Qingliang Feng a, Fengyuan zhang a, Yongling Du a, Chunming Wang a, * a b
College of Chemistry and Chemical Engineering, Lanzhou University, 730000 Lanzhou, China School of Basic Medical Sciences, Ningxia Medical University, 750004 Yinchuan, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
< Graphene-carbon nanotubes (GrCNTs) nanocomposite has been in situ synthesized. < The Gr-CNTs nanocomposite was used as a filler to synthesize GrCNTs/polyimide (PI) hybrid films. < The conductivity of Gr-CNTs/PI can be regulated by regulating the content of Gr-CNTs in PI matrix. < Surface-silvered Gr-CNT/PI was prepared by ion exchange and electrochemical reduction process. < The surface-silvered Gr-CNT/PI hybrid film can improve the conductivity of this hybrid films.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 July 2012 Received in revised form 20 November 2012 Accepted 24 November 2012
Silver nanoparticles modified graphene-carbon nanotubes/polyimide (Gr-CNTs/PI) films have been prepared by electrochemical reduction of silver nitrate on potassium hydroxide hydroxylated of Gr-CNTs/ PI films surface. The as-prepared nanocomposites were characterized by transmission electron microscopy, scanning electron microscopy, X-ray diffraction analyzer and semiconductor characterization system. The lower content of Gr-CNTs (10 wt. %) doping in PI matrix can improve the conductivity of PI films more clearly than pure CNTs. The conductivity can be regulated by controlling Gr-CNTs content in PI matrix. These silver nanoparticles into Gr-CNTs/PI films presented here can act as deposition seeds which can initiate subsequent electroless silver or copper or electrodeposition other metal. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Polymers Composite materials Thin films Electrical conductivity
1. Introduction The flexible polyimide (PI) film has been widely used in soft electronics and microelectronics fields. The application of PI film is motivated because of its numerous outstanding properties, such as
* Corresponding author. Tel.: þ86 9318911895; fax: þ86 9318912582. E-mail address: [email protected] (C. Wang). 0254-0584/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2012.11.067
superior mechanical properties, excellent thermal stability, high glass transition temperature, and good resistance to solvents [1]. The PI film is an attractive matrix for composite materials of future device generation to which chemical and thermal stability is required. In recent years, considerable attention has been paid to the modification of PI film with organic/inorganic hybrid materials. It was found that the electrical, mechanical, thermal properties of PI films were significantly improved by the doping of a small amount of nano-fillers [2,3].
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The polymer nanocomposite using carbon nanotubes (CNTs) as fillers was first reported in 1994 by Ajayan et al. [4]. Recently, much attention has been given to the use of CNTs in composite materials. Polymer nanocomposite based on CNTs have been used for improving mechanical, thermal, electrical properties of polymers [5] because the CNTs possessing high flexibility [6], low mass density [7], and large aspect ratio. Principally, for obtaining conductive CNTs-polymer composites (such as CNTs-PI), the highly electrical conductive CNTs fillers are dispersed into the polymer matrix. However, the bottleneck for using CNTs as fillers for polymer-based composites is that the as-produced CNTs are held together in bundles by very strong van der Waals interactions [8], and the poor dissolubility in various solvent. The challenge is thus to incorporate exfoliated individual CNTs, or at least relatively thin CNTs bundles, inside a polymer matrix and improves the solvability in various solvents. The key is dispersing the CNTs into the polymer matrix as well as the quality of the interface fillerematrix and the efficient translation of nanotube properties both into the polymer matrix and between nanotubes. So, fabrication methods have overwhelmingly focused on improving nanotube dispersion because better nanotube dispersion in the polymer matrices has been found to improve properties. In addition, the discovery of graphene with its combination of extraordinary physical properties (such as high electrical conductivity, mechanical strength and optical absorption properties) and ability to be dispersed in various polymer matrices has created a new class of polymer composite [9e11]. There have few literature on the preparation of graphene (Gr)-polymer [12e15]. And all these composites containing Gr were prepared by the mechanical mixing of the as-prepared Gr with polymers, which are not beneficial for the uniform dispersion of Gr in the polymer matrix. Simultaneously, in order to broaden the application of PI films, a surface-silvered or coppered PI films was prepared in previous work [16e21]. The surface modification approach can be applied to the fabrication of thin uniform composite layers containing silver nanoparticles. To realize the batch production of silver-decorated polyimide, a facile and low-cost route is more desired. Recently, some literature [16e22] have been reported the two-layer mode or monolayer mode (metal/PI): that modified by using alkaline hydrolysis to open imide rings, followed by ion exchange with metal ions and the reduction of metal ions on PI. In the reduction of metal ions step, the methods for reducing metal involve a chemical reaction (sodium borohydride as reductants), a thermally-induced hydrogen reaction, and an ultraviolet (UV)-light-induced photochemical reaction (TiO2 as photocatalysts) [17e19]. On hydrogeninduced process, high temperature treatment raises problems of diffusion and oxidation of the metal nanoparticles [18,19]. In ultraviolet (UV)-light-induced process and chemical reductive process, the elimination of TiO2 and the difficulty of controlling interfacial structure between metal thin films and the underlying PI because of the ease of diffusion of metal in the PI [17]. The purpose of this work is to use a facile and green method to situ synthesizes Gr-CNTs hybrid films, and use Gr-CNTs as a filler to synthesize Gr-CNTs/PI hybrid films. Compared with CNTs/PI [23], the electric conduction ability of this hybrid films can be improved more clearly by doping the lower content of Gr-CNTs (10 wt. %) in PI. And the conductibility of Gr-CNTs/PI hybrid films can be regulated by controlling over the content of Gr-CNTs nanocomposite in PI matrix. At the same time, the work used a novel and efficient route for the preparation of flexible and high conductivity surfacesilvered Gr-CNTs/PI hybrid films. This technique involves a simple alkali treatment-based surface modification of the Gr-CNTs/PI films and an ion exchange reaction, and electrochemical reduction process. The present method exploits the homogeneous distribution of silver nanoparticles in the modified layer. This new-style
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hybrid material is hopefully used in electronic, solar cells and biosensors fields on a large scale. 2. Experimental section 2.1. Chemical reagents 4,40 -Oxydianiline (ODA), 4,40 -oxydiphthalic anhydride (ODPA) (Alfa Aesar) were respectively dried at 100 and 150 C under vacuum prior to use. NMP was distilled over calcium hydride before use. Other chemicals were of analytical grade and used without further purification. High purity nitrogen was used in the reimidization process. 2.2. Preparation of graphene-carbon nanotubes nanocomposite The treatment of CNTs’ surface is according to the previous report [24]. In brief, CNTs were refluxed in 60% HNO3 at 60 C for 6 h to remove the metal impurities. After the purification process, the surface oxidation of the CNTs was carried out by refluxing CNTs in 1:1 conc. H2SO4 and conc. HNO3 at 60 C for 24 h. Gr nanosheet aqueous dispersion was prepared by the chemical reduction method [10]. Briefly, 5 mg graphite oxide, prepared from natural graphite by a modified Hummers method [25,26], was dispersed in 5 mL water to form colloids through ultrasonication for 30 min. Then, 5.0 mL of hydrazine solution (35 wt. % in water) and 35.0 mL of ammonia solution (28 wt. % in water) were added into the colloids under stirring. A few minutes later, the colloids was put in a water bath (w95 C) for 1 h and the graphene colloids were obtained which was used for the sequential reaction of a self-assembly technique with CNTs forming hybrid Gr-CNTs nanocomplex [27]. The proportions of CNTs to Gr sheets were 1:1, 1:2 and 2:1. The aqueous colloidal suspensions of Gr nanosheets were poured into the CNTs conglomerations and the mixture was sonicated using an ultrasonic bath cleaner (150 W, KQ-300GDV Jiangsu Kunshan) for 1 h. This suspension was centrifuged for 15 min at 3000 rpm (1710g) to remove the unstabled CNTs groups, thus giving a suspension of the Gr-CNTs hybrids and the excess Gr sheets in the supernatant. Next, the Gr-CNTs hybrids were separated from the excess Gr sheets by repeating centrifugation (10,000 rpm (19,000g), 20 min) and water washing steps. 2.3. Fabrication of the Gr-CNTs/PI hybrid films The solution of Gr-CNTs in anhydrous NMP was sonicated for 48 h in an ultrasonic bath (150 W, KQ-300GDV Jiangsu Kunshan) and was then transferred to a three neck round bottom flask equipped with a magnetism stirrer, nitrogen gas inlet and drying tube outlet filled with anhydrous calcium chloride (CaCl2). Predried 4-aminophenyl ether was added to the solution, and the mixture was stirred for 30 min. And then the mixture was stirred overnight under nitrogen after dried dianhydride (ODPA) was added. The contents of Gr-CNTs against polyamic acid (PAA) was 10 wt. %. The Gr-CNTs/PAA dispersions were casted onto a clean ITO glass, and evaporated at 60 C for 4 h followed by stepwise baking at each temperature of 80, 150, 240 C for 1 h, and 280 C for 0.5 h, respectively and solvent-free Gr-CNTs (1:1, 1:2 and 2:1)/PI hybrid film with about 23 mm thicknesses was obtained. 2.4. Preparation of the surface-silverized Gr-CNTs/PI hybrid films Silver nanoparticles modified PI hybrid films were fabricated according to previous work [20,28]. In this work, the preparation was completed by hydrolysis of PI resin using an alkali solution, subsequent incorporation of metal ions through ion exchange,
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solution at the potential 0.50 V for some minutes and Ag nanoparticle modified Gr-CNTs/PI hybrid films was obtained. After rinsing, the modified layers are again transformed to polyimide through a thermally induced dehydration reaction (re-imidization) at a certain temperature (250 C) in a nitrogen atmosphere in a quartz tube for 30 min. 2.5. Characterization and electrochemical measurement The surface morphology was carried out on Scanning electron microscopy (SEM, J4800 Japan) and Transmission electron microscope (TEM, Tecnai G2 F30, FEI, USA). The phase structures were measured by X-ray diffraction (XRD) experiments on an X-ray diffraction analyzer (XRD, Rigaku D/max-2400, Cu Ka radiation, l ¼ 0.15406 nm). The electric conductance properties were determined by Keithley 4200-SCS semiconductor characterization system. The electrochemical experiments were carried out on an electrochemical cell with a three-electrode configuration controlled by a CHI 660A electrochemical workstation (CHI., USA). Ag nanoparticle/Gr-CNTs/PI hybrid film supported on ITO glassy was used as working electrode. The Pt wire and saturated calomel electrode (SCE) served as the counter electrode and reference electrode. 3. Results and discussion 3.1. Morphology analysis of Gr-CNTs/PI hybrid films
Scheme 1. The fabrication of Ag/Gr-CNTs/PI hybrid films.
followed by electrochemical reduction of ions (Scheme 1). The preparation of silver-coated conductive film was carried out according to the schematic diagram in Fig. 2. In a typical procedure, the ODPA-ODA type PI films were initially immersed in a 4 M potassium hydroxide aqueous solution at 60 C for 10 min [29], followed by thorough rinsing with distilled water. Prior to exchange ions, the Gr-CNTs/PI substrate was rinsed with acetone and distilled water and then cleaned with distilled water in an ultrasonic for 5 min. The formed carboxylic acid groups can be used as ion exchange sites for subsequent doping of metal ions using 0.01 M AgNO3 solution at 60 C for 0.5 h, followed by rinsing with deionized water, thus generating the ion doped precursor layers (The content of Ag nanoparticles in Ag/Gr-CNTs/PI hybrid film was measured by ICP (SPS7700, Seiko Instruments) and ion loading: 4.2 0.30 mmol cm2). The Agþ adsorbed on the surface were reduced by using chronoamperometric technique in 0.1 M HClO4
Fig. 1 shows the SEM image of Gr nanosheets and Gr-CNTs nanocomposite and layer structured graphene nanosheets (Fig. 1a) were obtained and CNTs dispersed uniformly distributed on the surface of Gr nanosheets (Fig. 1b). The SEM image of Gr-CNTs (Fig. 1b) clearly illustrates that highly dispersed nanotubes are uniformly distributed on the surface of Gr nanosheets. Fig. 2 shows the schematic diagram for the fabrication of GrCNTs nanocomposite, Gr-CNTs/PI and Ag/Gr-CNTs/PI hybrid films. The inset is the digital pictures of Gr-CNTs/PI hybrid film obtained by immersing the film-substrate into hot water and the TEM image of the Gr-CNTs nanocomposite, indicating that layer-structured Gr nanosheets and the Gr-CNTs nanocomplex was synthesized and the nanocomposite possess both the intrinsic feature of CNTs and Gr. The CNTs were inlaid among the Gr nanosheets by the p-conjugated multiple aromatic regions. The tubular structure of CNTs could effectively prevent the irreversible aggregation of Gr. Remarkably, the CNTs were inlaid in the surface of Gr can form incorporate thoroughfare of electron transfer and thereby improve the rate of electron transfer in Gr-CNTs/PI hybrid films. Simultaneously, the electron-rich Gr-CNTs nanocomposite can effectively avoid the loss of metal particles from the surface of hybrid films.
Fig. 1. SEM images of Gr nanosheets (a) and Gr-CNTs nanocomposite (b).
Z. Zheng et al. / Materials Chemistry and Physics 138 (2013) 350e357
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Fig. 2. Schematic diagram of the structure formation of Gr-CNTs nanocomposite, Gr-CNTs/PI hybrid films and Ag/Gr-CNTs/PI hybrid films, inset is the TEM images of the Gr-CNTs nanocomplex.
After direct ion exchange and electrochemical reduction process, the Gr-CNTs/PI hybrid films self-metallized and come into being uniformity Ag nanoparticles in modified layer, leading to high conductivity of the film.
To investigate the dispersion and morphology of nanotubes and nanosheets in fully imidized Gr-CNTs/PI hybrid films, SEM measurements were characterized after dipped into liquid nitrogen and then broken. As was shown in Fig. 3, Gr-CNTs are dispersed
Fig. 3. SEM micrographs of the fractured surface of Gr-CNTs (1:2)/PI (a and b), Gr-CNTs (1:1)/PI (c and d), and Gr-CNTs (2:1)/PI (e and f) hybrid films with different magnification times, respectively. The CNTs marked by red arrows. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 4. FTIR spectrum of Gr-CNTs/PI resin (a) and after KOH treatment (c), and after subsequent ion exchange (d) followed by electro-reduction and heat treatment in a nitrogen atmosphere at 250 C for 30 min (b). The FT-IR spectrum is in 1100e 1850 cm1 region.
homogeneously throughout the PI films without any obvious aggregates. The layer structure, bright dots and lines (as marked by the red arrows in Fig. 3b, d and f) indicated that the Gr nanosheets and CNTs were doping in PI matrix. 3.2. The morphology analysis and X-ray diffraction of Ag/Gr-CNTs/ PI hybrid films Fig. 4 shows the FT-IR spectra of bare Gr-CNTs/PI hybrid film and treated film. We focus our attention on the carbonyl stretching band in an imide ring vibration because KOH treatment gives rise to the cleavage of imide rings [30], with associated bands at 1776 and 1715 cm1 arising from symmetric and antisymmetric carbonyl stretching vibrations, respectively [31]. The KOH treatment results in a significant change in the FT-IR spectra as can be seen in Fig. 4c. The band assigned to the carbonyl stretching of imide rings disappears completely. The bands at (1500e1600 cm1) arise from the superposition of the ionized carboxyl vibration mode of carboxyl groups complexed with Kþ ions. The bands at 1650 cm1 and 1560 cm1 should be attributed to carbonyl stretching and Ne
H bending of amide [32,33]. The FT-IR results demonstrate that KOH treatment yields poly(amic acid) through a hydration reaction, which is usually utilized as a precursor for polyimide resin. After ion exchange, although the spectrum is mostly similar in form (Fig. 4d) as compared to that of the KOH-treated film (Fig. 4c), a slight decrease in intensity at 1650 cm1 and complex features around 1500e1600 cm1 suggest that a number of coordination states coexist within the Agþ complexes. Fig. 5 shows the typical SEM images of after alkali treatments in the first step (Fig. 5a) and silver-adsorbed modified Gr-CNTs/PI film after electro-reduced in 0.1 M HClO4 solution at the potential 0.50 V for 600 s and heat treatment (re-imidization) process (Fig. 5b). It can distinctly be seen that the Gr sheets and CNTs uniformly dispersed in PI matrix marked in red arrowhead in Fig. 5a, and Ag nanoparticles uniformly dispersed in the modified layer of Gr-CNTs/PI hybrid film (Fig. 5b). Most of the Ag nanoparticles were arranged linearly (which shows the same structure with the schematic diagram in Fig. 2) which attributes to the electron-rich and the sandwich lamination structure of Gr-CNTs nanocomposite. The electron-rich of Gr-CNTs can effectively prevent the loss of metal ions. Additionally, the mean size of Ag nanoparticles could be controlled effectively by tuning reduction potential. Fig. 6 shows the XRD patterns of the Ag/Gr-CNTs/PI hybrid films (red line), Gr-CNTs/PI hybrid films (black line), Gr-CNTs (green line) and pure PI films (blue line). Three diffractions (200), (220), and (311) indicated that face-centered cubic structured silver formed after thermally induced dehydration reaction (re-imidization) for 30 min at 250 C [20]. The X-ray diffraction patterns suggest that different degrees of silver aggregation occurred on the film surface. Fig. 7 shows the SEM micrographs of Ag/Gr-CNTs/PI hybrid films prepared on different reduction potential (a) 1.0 V, (b) 0.5 V, (c) 0.3 V and (d) 0.0 V, respectively. It was shown that the particle diameter of Ag nanoparticles increased with the increasing of reduction potential (from 1.0 V to 0.0 V), obviously. However, the reduction potential makes no difference to the nanoparticles uniform distribution in the modified layer. Compared with previous report [20,28], the particle diameter of Ag nanoparticles could be availably controlled by tuning the reduction potential. Thus, the present composite is defined as Ag/Gr-CNTs/PI hybrid films, with monodispersed silver nanoparticles homogeneously dispersed in modified layer of high-performance Gr-CNTs/PI matrix. The additional advantage of the uniform incorporation of silver nanoparticles into Gr-CNTs/PI films presented here is that they can initiate subsequent electroless deposition of silver or copper plating
Fig. 5. SEM images of Gr-CNTs/PI hybrid films after alkali treatments process (a) and silver-adsorbed modified Gr-CNTs/PI hybrid films after electro-reduced and heat treatment (reimidization) process (b). The surface Agþ ions were reduced and loaded Ag nanoparticles on the surface of Gr-CNTs/PI films through chronoamperometric technique in 0.1 M HClO4 solution at the potential 0.50 V for 600 s.
Z. Zheng et al. / Materials Chemistry and Physics 138 (2013) 350e357
Fig. 6. XRD patterns of the Ag/Gr-CNTs/PI hybrid films (red line) and Gr-CNTs/PI hybrid film (black line) and Gr-CNTs nanocomposite (green line) and pure PI films (blue line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
or electrodeposition other metal. In this process, the silver nanoparticles can act as deposition seeds as well as adhesion promoters through a nanoscale mechanical interlocking effect, which may be suitable for further miniaturization of electronics devices to achieve large-scale integration. 3.3. Thermal properties of the Gr-CNTs/PI hybrid films Thermal stability is a critical aspect of PI-based hybrid films as they are potentially used as high performance engineering plastics. The thermal properties of PI are usually improved by the addition of inorganic additives [14]. Fig. 8 shows the thermogravimetric profiles of the pure polyimide, Gr-CNTs (1:2)/PI, Gr-CNTs (1:1)/PI,
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and Gr-CNTs (2:1)/PI hybrid films and the corresponding thermogravimetric analysis (TGA) of those samples were shown in Table 1. The thermal stability of polyimide was improved slightly by the incorporation of Gr-CNTs (The TGA curves for the pure polyimide and the hybrid films were obtained at a heating rate of 10 C min1 under N2.). According to TGA analysis we know that the thermal stability of the hybrid films decreased with the increasing of graphene content in dopant (Gr-CNTs). As shown in Fig. 8 and Table 1, The 5 wt. % decomposition temperature (Td) decreased from 492 C (pure PI) to 290 C (Gr-CNTs (2:1)). The thermal stability reduces of the hybrid films can be attributed to the decarboxylate reaction of Gr-CNTs composite [14]. And the 10 wt. % decomposition temperature (Td10) also decreased with the content of Gr in dopant (Gr-CNTs) increasing from 0 to 50 wt. %, but increased while the content of Gr in dopant is greater than 50 wt. %. When the content of Gr in dopant increased to 50 wt. %, the thermal stability of the hybrid films began to enhance which should be caused by the unique thermal property of the Gr. Although the carboxyl or hydroxyl groups introduced into the Gr sheets and CNTs are easily decomposed at high temperature and may serve as catalysts for the degradation of PI and thus deteriorate the thermal stability of PI matrix, the thermal stability of the hybrid films was no worse than that of pure PI [14]. Hence, in situ heat treatment after incorporating Gr-CNTs into the PAA matrix is a facile method to fabricate high performance Gr-CNTs/PI hybrid films. The significant improvement of properties of polyimide can be attributed to the fine dispersion of high aspect ratio Gr-CNTs, and strong interfacial adhesion and interlocking structure between the graphene, CNTs and the matrix, arising from the wrinkled morphology of Gr and sandwich structure morphology of Gr-CNTs. 3.4. Electric conductance properties of the Ag/Gr-CNTs/PI hybrid films Fig. 9 shows the correlation of the conductance and the ratio of Gr in fillers (Gr-CNTs) composite. For doping Gr-CNTs into PI matrix,
Fig. 7. SEM micrographs of Ag/Gr-CNTs/PI hybrid films that prepared on different reduction potential (a) 1.0 V, (b) 0.5 V, (c) 0.3 V and (d) 0.0 V, respectively. The content of fillers (Gr þ CNTs) in PI matrix is 10 wt. %.
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effectively and the pure Gr can easily come into being the irreversible aggregation. Therefore, Gr cannot be dispersed in PI matrix effectively. After surface-silvered, electro-reduction and heat treatment (reimidization) process, the conductance of the Gr-CNTs/PI hybrid films increased obviously (the conductance of Ag/Gr-CNTs (2:1)/PI is 2.19 104 S). The conductance of Ag/Gr-CNTs (2:1)/PI is about twenty times as larger as Gr-CNTs (2:1)/PI which should be attributed to that the surface of hybrid films was modified by Ag nanoparticles. Therefore, the finely dispersed Gr-CNTs throughout the PI matrix are responsible for the significant reinforcement of the electronic, mechanical and thermal properties of the Gr-CNTs/ PI hybrid films. 4. Conclusions
Fig. 8. TGA curves of the pure PI, the Gr-CNTs (1:2)/PI, the Gr-CNTs (1:1)/PI, and the GrCNTs (2:1)/PI hybrid films.
Table 1 Thermogravimetric analysis of pure PI, Gr-CNT (1:2)/PI, Gr-CNT (1:1)/PI, and Gr-CNT (2:1)/PI hybrid films.
PI Gr-CNTs (1:2)/PI Gr-CNTs (1:1)/PI Gr-CNTs (2:1)/PI
Td (5%) ( C)
Td (10%) ( C)
Td (15%) ( C)
Td (20%) ( C)
492.73 421.43 385.59 290.33
534.75 528.17 523.83 544.78
559.92 552.51 552.40 569.78
576.89 570.96 570.79 587.23
In this work, a new methodology for preparing silver nanoparticles based Gr-CNTs/PI hybrid film was reported. The films microstructure, for example, particle size, volume fraction, and films thickness could be systematically controlled. The presence of Gr-CNTs dramatically affects the films microstructure and the conductivity of Gr-CNTs/PI hybrid films. Besides, due to the potential applications of this hybrid films in many devices, the description of a novel kind of material presents obvious technological relevance. Some applications of the films such as sensors, electrochemical materials are currently under investigation in our group. The strategy is quite general and extendable to other noble metals and transition metal alloys in the PI that it composited with the organic/inorganic hybrid materials. Acknowledgment
the conductance of Gr-CNTs/PI hybrid films enhanced obviously and the conductance (Siemens) increased with the increase of Gr content in filler (Gr-CNTs). Compared with CNTs/PI, the electric conductance ability of Gr-CNTs (2:1)/PI (the conductance is 1.101 105 S) is much larger than CNTs/PI (the conductance is 1.723 1010 S). The higher conductance of Gr-CNTs/PI hybrid film is attributed to that Gr-CNTs filler in PI matrix can bind together effectively and form the route way of electron (Fig. 2). However, those pure CNTs filler in PI matrix cannot form the route way of electron transfer
Fig. 9. The curve is the linear relationship of the conductance and the ratio of Gr in dopant (Gr-CNTs) composite.
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Preparation of surface-silvered graphene-CNTs/polyimide hybrid films: Processing, morphology and properties Zhixiang Zheng a, b, Zaihua Wang a, Qingliang Feng a, Fengyuan zhang a, Yongling Du a, Chunming Wang a, * a b
College of Chemistry and Chemical Engineering, Lanzhou University, 730000 Lanzhou, China School of Basic Medical Sciences, Ningxia Medical University, 750004 Yinchuan, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
< Graphene-carbon nanotubes (GrCNTs) nanocomposite has been in situ synthesized. < The Gr-CNTs nanocomposite was used as a filler to synthesize GrCNTs/polyimide (PI) hybrid films. < The conductivity of Gr-CNTs/PI can be regulated by regulating the content of Gr-CNTs in PI matrix. < Surface-silvered Gr-CNT/PI was prepared by ion exchange and electrochemical reduction process. < The surface-silvered Gr-CNT/PI hybrid film can improve the conductivity of this hybrid films.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 July 2012 Received in revised form 20 November 2012 Accepted 24 November 2012
Silver nanoparticles modified graphene-carbon nanotubes/polyimide (Gr-CNTs/PI) films have been prepared by electrochemical reduction of silver nitrate on potassium hydroxide hydroxylated of Gr-CNTs/ PI films surface. The as-prepared nanocomposites were characterized by transmission electron microscopy, scanning electron microscopy, X-ray diffraction analyzer and semiconductor characterization system. The lower content of Gr-CNTs (10 wt. %) doping in PI matrix can improve the conductivity of PI films more clearly than pure CNTs. The conductivity can be regulated by controlling Gr-CNTs content in PI matrix. These silver nanoparticles into Gr-CNTs/PI films presented here can act as deposition seeds which can initiate subsequent electroless silver or copper or electrodeposition other metal. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Polymers Composite materials Thin films Electrical conductivity
1. Introduction The flexible polyimide (PI) film has been widely used in soft electronics and microelectronics fields. The application of PI film is motivated because of its numerous outstanding properties, such as
* Corresponding author. Tel.: þ86 9318911895; fax: þ86 9318912582. E-mail address: [email protected] (C. Wang). 0254-0584/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2012.11.067
superior mechanical properties, excellent thermal stability, high glass transition temperature, and good resistance to solvents [1]. The PI film is an attractive matrix for composite materials of future device generation to which chemical and thermal stability is required. In recent years, considerable attention has been paid to the modification of PI film with organic/inorganic hybrid materials. It was found that the electrical, mechanical, thermal properties of PI films were significantly improved by the doping of a small amount of nano-fillers [2,3].
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The polymer nanocomposite using carbon nanotubes (CNTs) as fillers was first reported in 1994 by Ajayan et al. [4]. Recently, much attention has been given to the use of CNTs in composite materials. Polymer nanocomposite based on CNTs have been used for improving mechanical, thermal, electrical properties of polymers [5] because the CNTs possessing high flexibility [6], low mass density [7], and large aspect ratio. Principally, for obtaining conductive CNTs-polymer composites (such as CNTs-PI), the highly electrical conductive CNTs fillers are dispersed into the polymer matrix. However, the bottleneck for using CNTs as fillers for polymer-based composites is that the as-produced CNTs are held together in bundles by very strong van der Waals interactions [8], and the poor dissolubility in various solvent. The challenge is thus to incorporate exfoliated individual CNTs, or at least relatively thin CNTs bundles, inside a polymer matrix and improves the solvability in various solvents. The key is dispersing the CNTs into the polymer matrix as well as the quality of the interface fillerematrix and the efficient translation of nanotube properties both into the polymer matrix and between nanotubes. So, fabrication methods have overwhelmingly focused on improving nanotube dispersion because better nanotube dispersion in the polymer matrices has been found to improve properties. In addition, the discovery of graphene with its combination of extraordinary physical properties (such as high electrical conductivity, mechanical strength and optical absorption properties) and ability to be dispersed in various polymer matrices has created a new class of polymer composite [9e11]. There have few literature on the preparation of graphene (Gr)-polymer [12e15]. And all these composites containing Gr were prepared by the mechanical mixing of the as-prepared Gr with polymers, which are not beneficial for the uniform dispersion of Gr in the polymer matrix. Simultaneously, in order to broaden the application of PI films, a surface-silvered or coppered PI films was prepared in previous work [16e21]. The surface modification approach can be applied to the fabrication of thin uniform composite layers containing silver nanoparticles. To realize the batch production of silver-decorated polyimide, a facile and low-cost route is more desired. Recently, some literature [16e22] have been reported the two-layer mode or monolayer mode (metal/PI): that modified by using alkaline hydrolysis to open imide rings, followed by ion exchange with metal ions and the reduction of metal ions on PI. In the reduction of metal ions step, the methods for reducing metal involve a chemical reaction (sodium borohydride as reductants), a thermally-induced hydrogen reaction, and an ultraviolet (UV)-light-induced photochemical reaction (TiO2 as photocatalysts) [17e19]. On hydrogeninduced process, high temperature treatment raises problems of diffusion and oxidation of the metal nanoparticles [18,19]. In ultraviolet (UV)-light-induced process and chemical reductive process, the elimination of TiO2 and the difficulty of controlling interfacial structure between metal thin films and the underlying PI because of the ease of diffusion of metal in the PI [17]. The purpose of this work is to use a facile and green method to situ synthesizes Gr-CNTs hybrid films, and use Gr-CNTs as a filler to synthesize Gr-CNTs/PI hybrid films. Compared with CNTs/PI [23], the electric conduction ability of this hybrid films can be improved more clearly by doping the lower content of Gr-CNTs (10 wt. %) in PI. And the conductibility of Gr-CNTs/PI hybrid films can be regulated by controlling over the content of Gr-CNTs nanocomposite in PI matrix. At the same time, the work used a novel and efficient route for the preparation of flexible and high conductivity surfacesilvered Gr-CNTs/PI hybrid films. This technique involves a simple alkali treatment-based surface modification of the Gr-CNTs/PI films and an ion exchange reaction, and electrochemical reduction process. The present method exploits the homogeneous distribution of silver nanoparticles in the modified layer. This new-style
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hybrid material is hopefully used in electronic, solar cells and biosensors fields on a large scale. 2. Experimental section 2.1. Chemical reagents 4,40 -Oxydianiline (ODA), 4,40 -oxydiphthalic anhydride (ODPA) (Alfa Aesar) were respectively dried at 100 and 150 C under vacuum prior to use. NMP was distilled over calcium hydride before use. Other chemicals were of analytical grade and used without further purification. High purity nitrogen was used in the reimidization process. 2.2. Preparation of graphene-carbon nanotubes nanocomposite The treatment of CNTs’ surface is according to the previous report [24]. In brief, CNTs were refluxed in 60% HNO3 at 60 C for 6 h to remove the metal impurities. After the purification process, the surface oxidation of the CNTs was carried out by refluxing CNTs in 1:1 conc. H2SO4 and conc. HNO3 at 60 C for 24 h. Gr nanosheet aqueous dispersion was prepared by the chemical reduction method [10]. Briefly, 5 mg graphite oxide, prepared from natural graphite by a modified Hummers method [25,26], was dispersed in 5 mL water to form colloids through ultrasonication for 30 min. Then, 5.0 mL of hydrazine solution (35 wt. % in water) and 35.0 mL of ammonia solution (28 wt. % in water) were added into the colloids under stirring. A few minutes later, the colloids was put in a water bath (w95 C) for 1 h and the graphene colloids were obtained which was used for the sequential reaction of a self-assembly technique with CNTs forming hybrid Gr-CNTs nanocomplex [27]. The proportions of CNTs to Gr sheets were 1:1, 1:2 and 2:1. The aqueous colloidal suspensions of Gr nanosheets were poured into the CNTs conglomerations and the mixture was sonicated using an ultrasonic bath cleaner (150 W, KQ-300GDV Jiangsu Kunshan) for 1 h. This suspension was centrifuged for 15 min at 3000 rpm (1710g) to remove the unstabled CNTs groups, thus giving a suspension of the Gr-CNTs hybrids and the excess Gr sheets in the supernatant. Next, the Gr-CNTs hybrids were separated from the excess Gr sheets by repeating centrifugation (10,000 rpm (19,000g), 20 min) and water washing steps. 2.3. Fabrication of the Gr-CNTs/PI hybrid films The solution of Gr-CNTs in anhydrous NMP was sonicated for 48 h in an ultrasonic bath (150 W, KQ-300GDV Jiangsu Kunshan) and was then transferred to a three neck round bottom flask equipped with a magnetism stirrer, nitrogen gas inlet and drying tube outlet filled with anhydrous calcium chloride (CaCl2). Predried 4-aminophenyl ether was added to the solution, and the mixture was stirred for 30 min. And then the mixture was stirred overnight under nitrogen after dried dianhydride (ODPA) was added. The contents of Gr-CNTs against polyamic acid (PAA) was 10 wt. %. The Gr-CNTs/PAA dispersions were casted onto a clean ITO glass, and evaporated at 60 C for 4 h followed by stepwise baking at each temperature of 80, 150, 240 C for 1 h, and 280 C for 0.5 h, respectively and solvent-free Gr-CNTs (1:1, 1:2 and 2:1)/PI hybrid film with about 23 mm thicknesses was obtained. 2.4. Preparation of the surface-silverized Gr-CNTs/PI hybrid films Silver nanoparticles modified PI hybrid films were fabricated according to previous work [20,28]. In this work, the preparation was completed by hydrolysis of PI resin using an alkali solution, subsequent incorporation of metal ions through ion exchange,
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solution at the potential 0.50 V for some minutes and Ag nanoparticle modified Gr-CNTs/PI hybrid films was obtained. After rinsing, the modified layers are again transformed to polyimide through a thermally induced dehydration reaction (re-imidization) at a certain temperature (250 C) in a nitrogen atmosphere in a quartz tube for 30 min. 2.5. Characterization and electrochemical measurement The surface morphology was carried out on Scanning electron microscopy (SEM, J4800 Japan) and Transmission electron microscope (TEM, Tecnai G2 F30, FEI, USA). The phase structures were measured by X-ray diffraction (XRD) experiments on an X-ray diffraction analyzer (XRD, Rigaku D/max-2400, Cu Ka radiation, l ¼ 0.15406 nm). The electric conductance properties were determined by Keithley 4200-SCS semiconductor characterization system. The electrochemical experiments were carried out on an electrochemical cell with a three-electrode configuration controlled by a CHI 660A electrochemical workstation (CHI., USA). Ag nanoparticle/Gr-CNTs/PI hybrid film supported on ITO glassy was used as working electrode. The Pt wire and saturated calomel electrode (SCE) served as the counter electrode and reference electrode. 3. Results and discussion 3.1. Morphology analysis of Gr-CNTs/PI hybrid films
Scheme 1. The fabrication of Ag/Gr-CNTs/PI hybrid films.
followed by electrochemical reduction of ions (Scheme 1). The preparation of silver-coated conductive film was carried out according to the schematic diagram in Fig. 2. In a typical procedure, the ODPA-ODA type PI films were initially immersed in a 4 M potassium hydroxide aqueous solution at 60 C for 10 min [29], followed by thorough rinsing with distilled water. Prior to exchange ions, the Gr-CNTs/PI substrate was rinsed with acetone and distilled water and then cleaned with distilled water in an ultrasonic for 5 min. The formed carboxylic acid groups can be used as ion exchange sites for subsequent doping of metal ions using 0.01 M AgNO3 solution at 60 C for 0.5 h, followed by rinsing with deionized water, thus generating the ion doped precursor layers (The content of Ag nanoparticles in Ag/Gr-CNTs/PI hybrid film was measured by ICP (SPS7700, Seiko Instruments) and ion loading: 4.2 0.30 mmol cm2). The Agþ adsorbed on the surface were reduced by using chronoamperometric technique in 0.1 M HClO4
Fig. 1 shows the SEM image of Gr nanosheets and Gr-CNTs nanocomposite and layer structured graphene nanosheets (Fig. 1a) were obtained and CNTs dispersed uniformly distributed on the surface of Gr nanosheets (Fig. 1b). The SEM image of Gr-CNTs (Fig. 1b) clearly illustrates that highly dispersed nanotubes are uniformly distributed on the surface of Gr nanosheets. Fig. 2 shows the schematic diagram for the fabrication of GrCNTs nanocomposite, Gr-CNTs/PI and Ag/Gr-CNTs/PI hybrid films. The inset is the digital pictures of Gr-CNTs/PI hybrid film obtained by immersing the film-substrate into hot water and the TEM image of the Gr-CNTs nanocomposite, indicating that layer-structured Gr nanosheets and the Gr-CNTs nanocomplex was synthesized and the nanocomposite possess both the intrinsic feature of CNTs and Gr. The CNTs were inlaid among the Gr nanosheets by the p-conjugated multiple aromatic regions. The tubular structure of CNTs could effectively prevent the irreversible aggregation of Gr. Remarkably, the CNTs were inlaid in the surface of Gr can form incorporate thoroughfare of electron transfer and thereby improve the rate of electron transfer in Gr-CNTs/PI hybrid films. Simultaneously, the electron-rich Gr-CNTs nanocomposite can effectively avoid the loss of metal particles from the surface of hybrid films.
Fig. 1. SEM images of Gr nanosheets (a) and Gr-CNTs nanocomposite (b).
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Fig. 2. Schematic diagram of the structure formation of Gr-CNTs nanocomposite, Gr-CNTs/PI hybrid films and Ag/Gr-CNTs/PI hybrid films, inset is the TEM images of the Gr-CNTs nanocomplex.
After direct ion exchange and electrochemical reduction process, the Gr-CNTs/PI hybrid films self-metallized and come into being uniformity Ag nanoparticles in modified layer, leading to high conductivity of the film.
To investigate the dispersion and morphology of nanotubes and nanosheets in fully imidized Gr-CNTs/PI hybrid films, SEM measurements were characterized after dipped into liquid nitrogen and then broken. As was shown in Fig. 3, Gr-CNTs are dispersed
Fig. 3. SEM micrographs of the fractured surface of Gr-CNTs (1:2)/PI (a and b), Gr-CNTs (1:1)/PI (c and d), and Gr-CNTs (2:1)/PI (e and f) hybrid films with different magnification times, respectively. The CNTs marked by red arrows. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 4. FTIR spectrum of Gr-CNTs/PI resin (a) and after KOH treatment (c), and after subsequent ion exchange (d) followed by electro-reduction and heat treatment in a nitrogen atmosphere at 250 C for 30 min (b). The FT-IR spectrum is in 1100e 1850 cm1 region.
homogeneously throughout the PI films without any obvious aggregates. The layer structure, bright dots and lines (as marked by the red arrows in Fig. 3b, d and f) indicated that the Gr nanosheets and CNTs were doping in PI matrix. 3.2. The morphology analysis and X-ray diffraction of Ag/Gr-CNTs/ PI hybrid films Fig. 4 shows the FT-IR spectra of bare Gr-CNTs/PI hybrid film and treated film. We focus our attention on the carbonyl stretching band in an imide ring vibration because KOH treatment gives rise to the cleavage of imide rings [30], with associated bands at 1776 and 1715 cm1 arising from symmetric and antisymmetric carbonyl stretching vibrations, respectively [31]. The KOH treatment results in a significant change in the FT-IR spectra as can be seen in Fig. 4c. The band assigned to the carbonyl stretching of imide rings disappears completely. The bands at (1500e1600 cm1) arise from the superposition of the ionized carboxyl vibration mode of carboxyl groups complexed with Kþ ions. The bands at 1650 cm1 and 1560 cm1 should be attributed to carbonyl stretching and Ne
H bending of amide [32,33]. The FT-IR results demonstrate that KOH treatment yields poly(amic acid) through a hydration reaction, which is usually utilized as a precursor for polyimide resin. After ion exchange, although the spectrum is mostly similar in form (Fig. 4d) as compared to that of the KOH-treated film (Fig. 4c), a slight decrease in intensity at 1650 cm1 and complex features around 1500e1600 cm1 suggest that a number of coordination states coexist within the Agþ complexes. Fig. 5 shows the typical SEM images of after alkali treatments in the first step (Fig. 5a) and silver-adsorbed modified Gr-CNTs/PI film after electro-reduced in 0.1 M HClO4 solution at the potential 0.50 V for 600 s and heat treatment (re-imidization) process (Fig. 5b). It can distinctly be seen that the Gr sheets and CNTs uniformly dispersed in PI matrix marked in red arrowhead in Fig. 5a, and Ag nanoparticles uniformly dispersed in the modified layer of Gr-CNTs/PI hybrid film (Fig. 5b). Most of the Ag nanoparticles were arranged linearly (which shows the same structure with the schematic diagram in Fig. 2) which attributes to the electron-rich and the sandwich lamination structure of Gr-CNTs nanocomposite. The electron-rich of Gr-CNTs can effectively prevent the loss of metal ions. Additionally, the mean size of Ag nanoparticles could be controlled effectively by tuning reduction potential. Fig. 6 shows the XRD patterns of the Ag/Gr-CNTs/PI hybrid films (red line), Gr-CNTs/PI hybrid films (black line), Gr-CNTs (green line) and pure PI films (blue line). Three diffractions (200), (220), and (311) indicated that face-centered cubic structured silver formed after thermally induced dehydration reaction (re-imidization) for 30 min at 250 C [20]. The X-ray diffraction patterns suggest that different degrees of silver aggregation occurred on the film surface. Fig. 7 shows the SEM micrographs of Ag/Gr-CNTs/PI hybrid films prepared on different reduction potential (a) 1.0 V, (b) 0.5 V, (c) 0.3 V and (d) 0.0 V, respectively. It was shown that the particle diameter of Ag nanoparticles increased with the increasing of reduction potential (from 1.0 V to 0.0 V), obviously. However, the reduction potential makes no difference to the nanoparticles uniform distribution in the modified layer. Compared with previous report [20,28], the particle diameter of Ag nanoparticles could be availably controlled by tuning the reduction potential. Thus, the present composite is defined as Ag/Gr-CNTs/PI hybrid films, with monodispersed silver nanoparticles homogeneously dispersed in modified layer of high-performance Gr-CNTs/PI matrix. The additional advantage of the uniform incorporation of silver nanoparticles into Gr-CNTs/PI films presented here is that they can initiate subsequent electroless deposition of silver or copper plating
Fig. 5. SEM images of Gr-CNTs/PI hybrid films after alkali treatments process (a) and silver-adsorbed modified Gr-CNTs/PI hybrid films after electro-reduced and heat treatment (reimidization) process (b). The surface Agþ ions were reduced and loaded Ag nanoparticles on the surface of Gr-CNTs/PI films through chronoamperometric technique in 0.1 M HClO4 solution at the potential 0.50 V for 600 s.
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Fig. 6. XRD patterns of the Ag/Gr-CNTs/PI hybrid films (red line) and Gr-CNTs/PI hybrid film (black line) and Gr-CNTs nanocomposite (green line) and pure PI films (blue line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
or electrodeposition other metal. In this process, the silver nanoparticles can act as deposition seeds as well as adhesion promoters through a nanoscale mechanical interlocking effect, which may be suitable for further miniaturization of electronics devices to achieve large-scale integration. 3.3. Thermal properties of the Gr-CNTs/PI hybrid films Thermal stability is a critical aspect of PI-based hybrid films as they are potentially used as high performance engineering plastics. The thermal properties of PI are usually improved by the addition of inorganic additives [14]. Fig. 8 shows the thermogravimetric profiles of the pure polyimide, Gr-CNTs (1:2)/PI, Gr-CNTs (1:1)/PI,
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and Gr-CNTs (2:1)/PI hybrid films and the corresponding thermogravimetric analysis (TGA) of those samples were shown in Table 1. The thermal stability of polyimide was improved slightly by the incorporation of Gr-CNTs (The TGA curves for the pure polyimide and the hybrid films were obtained at a heating rate of 10 C min1 under N2.). According to TGA analysis we know that the thermal stability of the hybrid films decreased with the increasing of graphene content in dopant (Gr-CNTs). As shown in Fig. 8 and Table 1, The 5 wt. % decomposition temperature (Td) decreased from 492 C (pure PI) to 290 C (Gr-CNTs (2:1)). The thermal stability reduces of the hybrid films can be attributed to the decarboxylate reaction of Gr-CNTs composite [14]. And the 10 wt. % decomposition temperature (Td10) also decreased with the content of Gr in dopant (Gr-CNTs) increasing from 0 to 50 wt. %, but increased while the content of Gr in dopant is greater than 50 wt. %. When the content of Gr in dopant increased to 50 wt. %, the thermal stability of the hybrid films began to enhance which should be caused by the unique thermal property of the Gr. Although the carboxyl or hydroxyl groups introduced into the Gr sheets and CNTs are easily decomposed at high temperature and may serve as catalysts for the degradation of PI and thus deteriorate the thermal stability of PI matrix, the thermal stability of the hybrid films was no worse than that of pure PI [14]. Hence, in situ heat treatment after incorporating Gr-CNTs into the PAA matrix is a facile method to fabricate high performance Gr-CNTs/PI hybrid films. The significant improvement of properties of polyimide can be attributed to the fine dispersion of high aspect ratio Gr-CNTs, and strong interfacial adhesion and interlocking structure between the graphene, CNTs and the matrix, arising from the wrinkled morphology of Gr and sandwich structure morphology of Gr-CNTs. 3.4. Electric conductance properties of the Ag/Gr-CNTs/PI hybrid films Fig. 9 shows the correlation of the conductance and the ratio of Gr in fillers (Gr-CNTs) composite. For doping Gr-CNTs into PI matrix,
Fig. 7. SEM micrographs of Ag/Gr-CNTs/PI hybrid films that prepared on different reduction potential (a) 1.0 V, (b) 0.5 V, (c) 0.3 V and (d) 0.0 V, respectively. The content of fillers (Gr þ CNTs) in PI matrix is 10 wt. %.
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effectively and the pure Gr can easily come into being the irreversible aggregation. Therefore, Gr cannot be dispersed in PI matrix effectively. After surface-silvered, electro-reduction and heat treatment (reimidization) process, the conductance of the Gr-CNTs/PI hybrid films increased obviously (the conductance of Ag/Gr-CNTs (2:1)/PI is 2.19 104 S). The conductance of Ag/Gr-CNTs (2:1)/PI is about twenty times as larger as Gr-CNTs (2:1)/PI which should be attributed to that the surface of hybrid films was modified by Ag nanoparticles. Therefore, the finely dispersed Gr-CNTs throughout the PI matrix are responsible for the significant reinforcement of the electronic, mechanical and thermal properties of the Gr-CNTs/ PI hybrid films. 4. Conclusions
Fig. 8. TGA curves of the pure PI, the Gr-CNTs (1:2)/PI, the Gr-CNTs (1:1)/PI, and the GrCNTs (2:1)/PI hybrid films.
Table 1 Thermogravimetric analysis of pure PI, Gr-CNT (1:2)/PI, Gr-CNT (1:1)/PI, and Gr-CNT (2:1)/PI hybrid films.
PI Gr-CNTs (1:2)/PI Gr-CNTs (1:1)/PI Gr-CNTs (2:1)/PI
Td (5%) ( C)
Td (10%) ( C)
Td (15%) ( C)
Td (20%) ( C)
492.73 421.43 385.59 290.33
534.75 528.17 523.83 544.78
559.92 552.51 552.40 569.78
576.89 570.96 570.79 587.23
In this work, a new methodology for preparing silver nanoparticles based Gr-CNTs/PI hybrid film was reported. The films microstructure, for example, particle size, volume fraction, and films thickness could be systematically controlled. The presence of Gr-CNTs dramatically affects the films microstructure and the conductivity of Gr-CNTs/PI hybrid films. Besides, due to the potential applications of this hybrid films in many devices, the description of a novel kind of material presents obvious technological relevance. Some applications of the films such as sensors, electrochemical materials are currently under investigation in our group. The strategy is quite general and extendable to other noble metals and transition metal alloys in the PI that it composited with the organic/inorganic hybrid materials. Acknowledgment
the conductance of Gr-CNTs/PI hybrid films enhanced obviously and the conductance (Siemens) increased with the increase of Gr content in filler (Gr-CNTs). Compared with CNTs/PI, the electric conductance ability of Gr-CNTs (2:1)/PI (the conductance is 1.101 105 S) is much larger than CNTs/PI (the conductance is 1.723 1010 S). The higher conductance of Gr-CNTs/PI hybrid film is attributed to that Gr-CNTs filler in PI matrix can bind together effectively and form the route way of electron (Fig. 2). However, those pure CNTs filler in PI matrix cannot form the route way of electron transfer
Fig. 9. The curve is the linear relationship of the conductance and the ratio of Gr in dopant (Gr-CNTs) composite.
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