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Electrically conductive silver nanowires-filled methylcellulose composite transparent films with high mechanical properties
Materials Letters 152 (2015) 173–176
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Electrically conductive silver nanowires-filled methylcellulose composite transparent films with high mechanical properties Wei Xu a,c, Qingsong Xu a, Qijin Huang a, Ruiqin Tan b, Wenfeng Shen a,n, Weijie Song a,c,nn a
Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, PR China Faculty of Information Science and Engineering, Ningbo University, Ningbo 315211, PR China c Ningbo Key Laboratory of Silicon and Organic Thin Film Optoelectronic Technologies, PR China b
art ic l e i nf o
a b s t r a c t
Article history: Received 9 December 2014 Accepted 23 March 2015 Available online 1 April 2015
Transparent and conductive silver nanowires (AgNWs)-filled methylcellulose (MC) composite films were fabricated by blending silver nanowires (AgNW) with a methylcellulose solution. AgNW concentration showed significant influence on the optical, electrical and mechanical properties of composite films. The percolation threshold of AgNWs-filled methylcellulose composite films in this study was approximately 0.29 vol%. A three-dimensional (3D) network-like composite film with micron-scale thickness demonstrated the electrical percolation of the two-dimensional (2D) network. Methylcellulose blended with silver nanowires showed enhanced mechanical properties such as tensile strength and Young's modulus compared with pure methylcellulose matrix. & 2015 Elsevier B.V. All rights reserved.
Keywords: Silver nanowires Percolation threshold Composite materials Mechanical property Nanoparticles
1. Introduction Recently, conductive polymer composites have attracted much attention because of their large potential applications, such as transparent conductive coating, light-emitting diodes, antistatic materials, and electromagnetic shielding materials [1–4]. The electrical conductivity of the insulating polymer can be achieved by incorporation of conductive filler into a polymer matrix [5,6]. The insulator-toconductor transitions of an insulating matrix filled with conductive materials are described with percolation theories. Several studies have experimentally and theoretically indicated that the percolation threshold strongly depends on the characteristics and the aspect ratio of the filler particles [7,8]. Silver nanowires (AgNWs), a one-dimensional (1D) material, possess high aspect ratio and unique optical and electrical properties compared with nanoparticles, which could form ideal electrical conductive paths at lower percolation threshold in polymer matrix [9]. Cellulose, the most abundant natural polymer in nature, is renewable, biodegradable, nontoxic and biocompatible. Methylcellulose (MC), a derivative of cellulose, has characteristics of low cost, large availability and easy processing. More recently, MC-based nanocomposites have attracted much attention because of their exceptional
n
Corresponding author. Corresponding author at: Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, Ningbo, 315201, PR China. E-mail addresses: [email protected] (W. Shen), [email protected] (W. Song). nn
http://dx.doi.org/10.1016/j.matlet.2015.03.111 0167-577X/& 2015 Elsevier B.V. All rights reserved.
properties in science and industry [10]. MC matrix with silver nanoparticles has enhanced mechanical properties and excellent antimicrobial activity against various microorganisms [11]. However, only a few studies on the fabrication of transparent and conductive silver nanowire-cellulose have been reported. In this work, AgNWs were added into the MC matrix to prepare a conductive polymer using a simple method. The AgNWs can be dispersed homogeneously in the MC solution without other treatments. The electrical percolation of composite conductive films was studied and a low percolation threshold was achieved by blending with the high aspect ratio of AgNWs. In addition, the optical and mechanical properties of composite films with different contents of AgNWs were investigated.
2. Experimental section AgNWs were fabricated by a modified polyol reduction process [12], and AgNW solution with a concentration of 10 mg/mL in deionized water was used in this study. The MC solution (1 wt%) was prepared by dispersing the required amount of MC in deionized water with continuous stirring under 85 1C until completely solubilized. Then appropriate AgNW suspension was blended with aqueous MC solution to obtain a homogeneous solution. The mixture was subsequently applied to a glass surface. After curing at the gelation temperature of MC (50–55 1C) in an oven for 12 h, the composite film was cooled to room temperature and peeled off from glass substrate. The fabrication process of AgNW/MC composite films is summarized
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in Fig. 1. AgNW volume fraction Vf in the composite film can be derived from the weight fraction Wf according to formula: Vf ¼
Wf α 1 W f ð1 αÞ
where α is the ratio of polymer density to silver density, which is approximately 1/10.5 in this paper. The surface and cross-section morphologies of AgNWs-filled MC composite films were examined by a field emission scanning electron microscope (FE-SEM, Hitachi S-4800, Japan). The sheet resistance was measured by a four-point probe system (Napson Corp. Cresbox). The optical transmittance of the AgNWs-filled MC composite films was obtained at room temperature using a UV-vis-NIR spectrophotometer (Perkin-Elmer, Lambda 950, USA) with an integrating sphere. The tensile properties and Young's modulus of composite films of 50 10 mm2 were measured using a universal testing machine (Instron 5567, USA) under a cross-head speed of 5 mm/min.
3. Results and discussion
Fig. 1. (a) Schematic of AgNW/MC composite film fabrication. (b) Photograph of MC solution with 0.45 vol% silver nanowires. (c) Photograph of AgNW/MC composite film with 0.29 vol% silver nanowires.
The density of the AgNW networks in MC films can be controlled by adjusting the concentration of silver nanowires in the composite solution. Fig. 2 shows the SEM image of composite films with different silver nanowire contents. The AgNWs were quite sparse and appeared
Fig. 2. SEM images of composites films with different silver nanowire contents (a) 0.20 vol%, (b) 0.45 vol%, (c) 2.33 vol% and (d) 27.60 vol%. Cross-sectional SEM images of (e) MC film and (f) AgNW/MC composite film with 0.45 vol% silver nanowire content.
W. Xu et al. / Materials Letters 152 (2015) 173–176
175
Fig. 3. (a) Transmittance of AgNW/MC composite films with different silver nanowire contents. (b) The relationship between transmittance at 550 nm and AgNW content.
Fig. 4. (a) Resistivity and (b) mechanical properties of AgNW/MC composite film with various silver nanowire contents. Inset: linear fit of the experimental data to the percolation equation.
as a discontinuous network in the film when the volume fraction was 0.20%. More connections formed with increasing volume fraction of AgNWs. The continuous cross connection can be found over the whole film when the volume fraction increased to 27.60%. Cross-sectional SEM analysis was used to investigate the thickness of the films, which was determined to be 10 μm according to Fig. 2e. The transmission spectra of the composite films are shown in Fig. 3a. The transmittance of films decreased with an increase of silver nanowire volume fraction. This change is due to the reflecting and scattering light by the AgNWs. Fig. 3b shows the difference of the transmittance at a fixed wavelength (550 nm) with AgNW concentration. The transmittance of the MC film was reduced from 92.2% (for pure MC film) to 72.7% with the filler loading of 0.14 vol%. As the AgNW volume fraction increased to 0.45%, the film became semitransparent, with a transmittance of 52.7%. While AgNW content raised to 0.60 vol%, the transmittance decreased to 31.2%. The inverse linear relation between transmittance of films and the AgNW content is similar to the results discussed elsewhere [13,14]. Fig. 4a shows the electrical resistivity of the AgNW/MC composite film as a function of the nanowire content at room temperature. When the AgNW content increased from 0.19 vol% to 1.34 vol%, the resistivity of the MC film was reduced from 2.6 103 Ω cm to 9.2 10 3 Ω cm. As the AgNW volume fraction increased to 27.60%, the resistivity of the film decreased to 1.2 10 4 Ω cm slowly. The relationship between the experimentally determined resistivity ρ and the volume fraction of AgNWs in films can be expressed as follows [15]:
ρ¼
M ðV f V fc Þt
where Vf is the volume fraction of silver nanowires in composite film, Vfc is the critical volume fraction, t is the conductivity exponent, and M is a constant. The experimental data fitted well with R2 ¼0.998 and
Pearson's r¼ 0.999 as shown in the inset of Fig. 4. The critical volume fraction Vfc was 0.29%, which is in good agreement with the theoretical value (0.31%) from the Excluded Volume Model [16]. The conductivity exponent was t¼1.43, which is smaller than the theoretical value for the 3D NW network [17], but similar to the reported value for the 2D NW network [13,14,18]. This is mainly because the thickness of the composite films is only 10 μm, which is shorter than the average length of AgNWs. The mechanical properties such as Young's modulus and tensile strength of the AgNW/MC films with different silver nanowire contents were measured. As shown in Fig. 4b, Young's modulus of pure MC film is 3267.87145.7 MPa, which increases to 3911.17 153.3 MPa and 5519.9760.7 MPa with silver nanowire volume fraction of 0.45% and 2.33% respectively. Therefore, Young's modulus is increased by 19.7% and 68.9% compared with that of pure MC. Meanwhile, the tensile strength increases from 101.877.3 MPa for pure MC to 157.3711.5 MPa and 319.577.2 MPa for composite films with AgNW 0.45 vol% and 2.33 vol%, respectively. Therefore, it can be concluded that the tensile strength increased by 54.5% and 213.8%, respectively, compared with that of pure MC film. This indicates that AgNWs can improve the mechanical properties of the MC matrix, which was due to the excellent mechanical performance of silver nanowires as discussed elsewhere [19].
4. Conclusions Transparent and conductive AgNW/MC composite films were fabricated by a simple method. The effects of silver nanowire concentration on the optical, electrical and mechanical properties of composite films were investigated. The transmittance at 550 nm and the resistivity of the composite film were 63.7% and 32.4 Ω cm,
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W. Xu et al. / Materials Letters 152 (2015) 173–176
respectively, at the percolation threshold of 0.29 vol%. A 3D networklike composite film demonstrates the electrical percolation of the 2D network due to the micron-scale thin film. Silver nanowires blended with MC show enhanced mechanical properties such as tensile strength and Young's modulus compared with pure MC matrix. The AgNW/MC films described in this study are expected to find new applications as functional thin films for abundant renewable natural polymers in the future.
[3] [4] [5] [6] [7] [8] [9] [10] [11]
Acknowledgments We thank the financial support from National Natural Science Foundation of China, China (Grant no. 21203226, 21205127, and 21377063). References [1] Song J, Li J, Xu J, Zeng H. Nano Lett 2014;14:6298–305. [2] Zeng XY, Zhang QK, Yu RM, Lu CZ. Adv Mater 2010;22:4484–8.
[12] [13] [14] [15] [16] [17] [18] [19]
Byrne MT, Gun'ko YK. Adv Mater 2010;22:1672–88. Mahmoodi M, Arjmand M, Sundararaj U, Park S. Carbon 2012;50:1455–64. Park M, Park J, Jeong U. Nano Today 2014;9:244–60. Zhao PF, Luo YY, Yang JL, He DN, Kong LX, Zheng P, et al. Mater Lett 2014;121:74–7. Celzard A, McRae E, Deleuze C, Dufort M, Furdin G, Mareche JF. Phys Rev B 1996;53:6209–14. Munsonmcgee SH. Phys Rev B 1991;43:3331–6. Khanarian G, Joo J, Liu XQ, Eastman P, Werner D, O’Connell K, et al. J Appl Phys 2013;114 024302. Arora A, Padua GW. J Food Sci 2010;75:R43–9. Maity D, Mollick MMR, Mondal D, Bhowmick B, Bain MK, Bankura K, et al. Carbohyd Polym 2012;90:1818–25. Xu QS, Shen WF, Huang QJ, Yang Y, Tan RQ, Zhu K, et al. J Mater Chem C 2014;2:3750–5. Bergin SM, Chen YH, Rathmell AR, Charbonneau P, Li ZY, Wiley BJ. Nanoscale 2012;4:1996–2004. Zhu SW, Gao Y, Hu B, Li J, Su J, Fan ZY, et al. Nanotechnology 2013;24 335202. Hu L, Hecht DS, Gruener G. Chem Rev 2010;110:5790–844. Balberg I, Anderson CH, Alexander S, Wagner N. Phys Rev B 1984;30:3933–43. Balberg I, Binenbaum N. Phys Rev A 1985;31:1222–5. Li JT, Zhang SL. Phys Rev E 2010;81 021120. Jing GY, Duan HL, Sun XM, Zhang ZS, Xu J, Li YD, et al. Phys Rev B 2006;73 235409.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Electrically conductive silver nanowires-filled methylcellulose composite transparent films with high mechanical properties Wei Xu a,c, Qingsong Xu a, Qijin Huang a, Ruiqin Tan b, Wenfeng Shen a,n, Weijie Song a,c,nn a
Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, PR China Faculty of Information Science and Engineering, Ningbo University, Ningbo 315211, PR China c Ningbo Key Laboratory of Silicon and Organic Thin Film Optoelectronic Technologies, PR China b
art ic l e i nf o
a b s t r a c t
Article history: Received 9 December 2014 Accepted 23 March 2015 Available online 1 April 2015
Transparent and conductive silver nanowires (AgNWs)-filled methylcellulose (MC) composite films were fabricated by blending silver nanowires (AgNW) with a methylcellulose solution. AgNW concentration showed significant influence on the optical, electrical and mechanical properties of composite films. The percolation threshold of AgNWs-filled methylcellulose composite films in this study was approximately 0.29 vol%. A three-dimensional (3D) network-like composite film with micron-scale thickness demonstrated the electrical percolation of the two-dimensional (2D) network. Methylcellulose blended with silver nanowires showed enhanced mechanical properties such as tensile strength and Young's modulus compared with pure methylcellulose matrix. & 2015 Elsevier B.V. All rights reserved.
Keywords: Silver nanowires Percolation threshold Composite materials Mechanical property Nanoparticles
1. Introduction Recently, conductive polymer composites have attracted much attention because of their large potential applications, such as transparent conductive coating, light-emitting diodes, antistatic materials, and electromagnetic shielding materials [1–4]. The electrical conductivity of the insulating polymer can be achieved by incorporation of conductive filler into a polymer matrix [5,6]. The insulator-toconductor transitions of an insulating matrix filled with conductive materials are described with percolation theories. Several studies have experimentally and theoretically indicated that the percolation threshold strongly depends on the characteristics and the aspect ratio of the filler particles [7,8]. Silver nanowires (AgNWs), a one-dimensional (1D) material, possess high aspect ratio and unique optical and electrical properties compared with nanoparticles, which could form ideal electrical conductive paths at lower percolation threshold in polymer matrix [9]. Cellulose, the most abundant natural polymer in nature, is renewable, biodegradable, nontoxic and biocompatible. Methylcellulose (MC), a derivative of cellulose, has characteristics of low cost, large availability and easy processing. More recently, MC-based nanocomposites have attracted much attention because of their exceptional
n
Corresponding author. Corresponding author at: Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, Ningbo, 315201, PR China. E-mail addresses: [email protected] (W. Shen), [email protected] (W. Song). nn
http://dx.doi.org/10.1016/j.matlet.2015.03.111 0167-577X/& 2015 Elsevier B.V. All rights reserved.
properties in science and industry [10]. MC matrix with silver nanoparticles has enhanced mechanical properties and excellent antimicrobial activity against various microorganisms [11]. However, only a few studies on the fabrication of transparent and conductive silver nanowire-cellulose have been reported. In this work, AgNWs were added into the MC matrix to prepare a conductive polymer using a simple method. The AgNWs can be dispersed homogeneously in the MC solution without other treatments. The electrical percolation of composite conductive films was studied and a low percolation threshold was achieved by blending with the high aspect ratio of AgNWs. In addition, the optical and mechanical properties of composite films with different contents of AgNWs were investigated.
2. Experimental section AgNWs were fabricated by a modified polyol reduction process [12], and AgNW solution with a concentration of 10 mg/mL in deionized water was used in this study. The MC solution (1 wt%) was prepared by dispersing the required amount of MC in deionized water with continuous stirring under 85 1C until completely solubilized. Then appropriate AgNW suspension was blended with aqueous MC solution to obtain a homogeneous solution. The mixture was subsequently applied to a glass surface. After curing at the gelation temperature of MC (50–55 1C) in an oven for 12 h, the composite film was cooled to room temperature and peeled off from glass substrate. The fabrication process of AgNW/MC composite films is summarized
174
W. Xu et al. / Materials Letters 152 (2015) 173–176
in Fig. 1. AgNW volume fraction Vf in the composite film can be derived from the weight fraction Wf according to formula: Vf ¼
Wf α 1 W f ð1 αÞ
where α is the ratio of polymer density to silver density, which is approximately 1/10.5 in this paper. The surface and cross-section morphologies of AgNWs-filled MC composite films were examined by a field emission scanning electron microscope (FE-SEM, Hitachi S-4800, Japan). The sheet resistance was measured by a four-point probe system (Napson Corp. Cresbox). The optical transmittance of the AgNWs-filled MC composite films was obtained at room temperature using a UV-vis-NIR spectrophotometer (Perkin-Elmer, Lambda 950, USA) with an integrating sphere. The tensile properties and Young's modulus of composite films of 50 10 mm2 were measured using a universal testing machine (Instron 5567, USA) under a cross-head speed of 5 mm/min.
3. Results and discussion
Fig. 1. (a) Schematic of AgNW/MC composite film fabrication. (b) Photograph of MC solution with 0.45 vol% silver nanowires. (c) Photograph of AgNW/MC composite film with 0.29 vol% silver nanowires.
The density of the AgNW networks in MC films can be controlled by adjusting the concentration of silver nanowires in the composite solution. Fig. 2 shows the SEM image of composite films with different silver nanowire contents. The AgNWs were quite sparse and appeared
Fig. 2. SEM images of composites films with different silver nanowire contents (a) 0.20 vol%, (b) 0.45 vol%, (c) 2.33 vol% and (d) 27.60 vol%. Cross-sectional SEM images of (e) MC film and (f) AgNW/MC composite film with 0.45 vol% silver nanowire content.
W. Xu et al. / Materials Letters 152 (2015) 173–176
175
Fig. 3. (a) Transmittance of AgNW/MC composite films with different silver nanowire contents. (b) The relationship between transmittance at 550 nm and AgNW content.
Fig. 4. (a) Resistivity and (b) mechanical properties of AgNW/MC composite film with various silver nanowire contents. Inset: linear fit of the experimental data to the percolation equation.
as a discontinuous network in the film when the volume fraction was 0.20%. More connections formed with increasing volume fraction of AgNWs. The continuous cross connection can be found over the whole film when the volume fraction increased to 27.60%. Cross-sectional SEM analysis was used to investigate the thickness of the films, which was determined to be 10 μm according to Fig. 2e. The transmission spectra of the composite films are shown in Fig. 3a. The transmittance of films decreased with an increase of silver nanowire volume fraction. This change is due to the reflecting and scattering light by the AgNWs. Fig. 3b shows the difference of the transmittance at a fixed wavelength (550 nm) with AgNW concentration. The transmittance of the MC film was reduced from 92.2% (for pure MC film) to 72.7% with the filler loading of 0.14 vol%. As the AgNW volume fraction increased to 0.45%, the film became semitransparent, with a transmittance of 52.7%. While AgNW content raised to 0.60 vol%, the transmittance decreased to 31.2%. The inverse linear relation between transmittance of films and the AgNW content is similar to the results discussed elsewhere [13,14]. Fig. 4a shows the electrical resistivity of the AgNW/MC composite film as a function of the nanowire content at room temperature. When the AgNW content increased from 0.19 vol% to 1.34 vol%, the resistivity of the MC film was reduced from 2.6 103 Ω cm to 9.2 10 3 Ω cm. As the AgNW volume fraction increased to 27.60%, the resistivity of the film decreased to 1.2 10 4 Ω cm slowly. The relationship between the experimentally determined resistivity ρ and the volume fraction of AgNWs in films can be expressed as follows [15]:
ρ¼
M ðV f V fc Þt
where Vf is the volume fraction of silver nanowires in composite film, Vfc is the critical volume fraction, t is the conductivity exponent, and M is a constant. The experimental data fitted well with R2 ¼0.998 and
Pearson's r¼ 0.999 as shown in the inset of Fig. 4. The critical volume fraction Vfc was 0.29%, which is in good agreement with the theoretical value (0.31%) from the Excluded Volume Model [16]. The conductivity exponent was t¼1.43, which is smaller than the theoretical value for the 3D NW network [17], but similar to the reported value for the 2D NW network [13,14,18]. This is mainly because the thickness of the composite films is only 10 μm, which is shorter than the average length of AgNWs. The mechanical properties such as Young's modulus and tensile strength of the AgNW/MC films with different silver nanowire contents were measured. As shown in Fig. 4b, Young's modulus of pure MC film is 3267.87145.7 MPa, which increases to 3911.17 153.3 MPa and 5519.9760.7 MPa with silver nanowire volume fraction of 0.45% and 2.33% respectively. Therefore, Young's modulus is increased by 19.7% and 68.9% compared with that of pure MC. Meanwhile, the tensile strength increases from 101.877.3 MPa for pure MC to 157.3711.5 MPa and 319.577.2 MPa for composite films with AgNW 0.45 vol% and 2.33 vol%, respectively. Therefore, it can be concluded that the tensile strength increased by 54.5% and 213.8%, respectively, compared with that of pure MC film. This indicates that AgNWs can improve the mechanical properties of the MC matrix, which was due to the excellent mechanical performance of silver nanowires as discussed elsewhere [19].
4. Conclusions Transparent and conductive AgNW/MC composite films were fabricated by a simple method. The effects of silver nanowire concentration on the optical, electrical and mechanical properties of composite films were investigated. The transmittance at 550 nm and the resistivity of the composite film were 63.7% and 32.4 Ω cm,
176
W. Xu et al. / Materials Letters 152 (2015) 173–176
respectively, at the percolation threshold of 0.29 vol%. A 3D networklike composite film demonstrates the electrical percolation of the 2D network due to the micron-scale thin film. Silver nanowires blended with MC show enhanced mechanical properties such as tensile strength and Young's modulus compared with pure MC matrix. The AgNW/MC films described in this study are expected to find new applications as functional thin films for abundant renewable natural polymers in the future.
[3] [4] [5] [6] [7] [8] [9] [10] [11]
Acknowledgments We thank the financial support from National Natural Science Foundation of China, China (Grant no. 21203226, 21205127, and 21377063). References [1] Song J, Li J, Xu J, Zeng H. Nano Lett 2014;14:6298–305. [2] Zeng XY, Zhang QK, Yu RM, Lu CZ. Adv Mater 2010;22:4484–8.
[12] [13] [14] [15] [16] [17] [18] [19]
Byrne MT, Gun'ko YK. Adv Mater 2010;22:1672–88. Mahmoodi M, Arjmand M, Sundararaj U, Park S. Carbon 2012;50:1455–64. Park M, Park J, Jeong U. Nano Today 2014;9:244–60. Zhao PF, Luo YY, Yang JL, He DN, Kong LX, Zheng P, et al. Mater Lett 2014;121:74–7. Celzard A, McRae E, Deleuze C, Dufort M, Furdin G, Mareche JF. Phys Rev B 1996;53:6209–14. Munsonmcgee SH. Phys Rev B 1991;43:3331–6. Khanarian G, Joo J, Liu XQ, Eastman P, Werner D, O’Connell K, et al. J Appl Phys 2013;114 024302. Arora A, Padua GW. J Food Sci 2010;75:R43–9. Maity D, Mollick MMR, Mondal D, Bhowmick B, Bain MK, Bankura K, et al. Carbohyd Polym 2012;90:1818–25. Xu QS, Shen WF, Huang QJ, Yang Y, Tan RQ, Zhu K, et al. J Mater Chem C 2014;2:3750–5. Bergin SM, Chen YH, Rathmell AR, Charbonneau P, Li ZY, Wiley BJ. Nanoscale 2012;4:1996–2004. Zhu SW, Gao Y, Hu B, Li J, Su J, Fan ZY, et al. Nanotechnology 2013;24 335202. Hu L, Hecht DS, Gruener G. Chem Rev 2010;110:5790–844. Balberg I, Anderson CH, Alexander S, Wagner N. Phys Rev B 1984;30:3933–43. Balberg I, Binenbaum N. Phys Rev A 1985;31:1222–5. Li JT, Zhang SL. Phys Rev E 2010;81 021120. Jing GY, Duan HL, Sun XM, Zhang ZS, Xu J, Li YD, et al. Phys Rev B 2006;73 235409.