An Indium Tin Oxide Conductive Network for Flexible Electronics

Article pubs.acs.org/JPCC

An Indium Tin Oxide Conductive Network for Flexible Electronics Produced Using a Cotton Template Peiran Hu,† Hongzhi Wang,*,† Qinghong Zhang,† and Yaogang Li*,‡ †

State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, Donghua University, Shanghai 201620, People’s Republic of China ‡ College of Materials Science and Engineering, Donghua University, Shanghai 201620, People’s Republic of China ABSTRACT: A regular indium tin oxide (ITO) conductive network is fabricated by a simple “dipping and drying” process using cotton as a template. The flexible composite consists of an interconnected conductive network of ITO, which acts as a transport channel for charge carriers, and a poly(dimethyl siloxane) substrate. The composite shows a very high electrical conductivity of ∼5 S m−1, which is ∼12 times of magnitude higher than those of other ordinary ITO-based composites. Moreover, it exhibits superior electrical/mechanical performance when bent or twisted compared to other ITO-based composites. The unique network structure and outstanding electrical, mechanical, and optical properties of the composite possess great potential for use in flexible, foldable, and stretchable electronics and other devices.

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INTRODUCTION Recently, intense interest has arisen in flexible electronics to meet the technological demands of modern society. Flexible electronics including flat-panel displays, light-emitting devices, touch-sensitive control panels, and new classes of flexible solar cell are being developed.1−3 Stable electrical/mechanical performance of hybrid devices in different bent or twisted positions is very important,4 but many conducting nanoparticles scatter in flexible substrates with finite interconnection and show very poor electrical conductivity. More critically for flexible conductive device applications, ordinary conducting nanoparticles are rather brittle and often crack when bent.2,5,6 A regular conductive network exhibits highly stable electrical performance because it contains more “conductive junctions” than scattered conducting nanoparticles in different bent or twisted positions. Chen et al. grew a three-dimensional graphene network by chemical vapor deposition, and its composites possessed very high electrical conductivities that were ∼6 orders of magnitude larger than chemically derived graphene-based composites.7 With a simple “dipping and drying” process using singlewalled carbon nanotube (SWNT) ink, Hu et al. produced a stretchable, porous textile with conductivity of 125 S cm−1.8 However, the material was black and had a low optical transmittance. Display producers and researchers prefer to use transparent conductors.9 Indium tin oxide (ITO) is the bestknown transparent conductive oxide used in optoelectronics because of its relatively low resistivity, and high optical transmittance in the visible and near-infrared regions, which are combined with excellent environmental stability, reproducibility, and good surface morphology.10−12 The major © 2012 American Chemical Society

limitation of ITO is its inherent brittleness, which makes it difficult to use in flexible displays.1,13,14 When bent or twisted, ordinary ITO particles and structures show poor electrical conductivity because of high resistance at intersheet junction contacts. However, ITO in an ordered conductive network, in which charge carriers can move rapidly with low resistance, shows outstanding electrical properties.7 A network of free-standing ITO nanotubes was fabricated by a surface sol−gel process using cellulosic substances as a template. The ITO network showed an intrinsic electronic conductivity higher than those of other ITO nanostructures, indicating its potential for use in chemical sensors and other flexible devices.10 However, the network was thick and dense, which resulted in poor optical transmittance. In this work, we attempt to change the state of aggregation of ITO particles by preparing conductive ITO networks using a unique template method, and observe their electrical/mechanical and optical properties. The textile templates, composed of cotton or polyester, have a hierarchical structure with complicated surface morphology, and possess functional groups such as hydroxyl groups, and high porosity.8,15,16 Because of the controllable structure and high porosity of the textile templates, the resulting ITO conductive network (ICN) should exhibit considerable optical transmittance. Herein, ITO is impregnated on a cotton template to synthesize a regular conductive network, and characterize its morphology, electrical/mechanical, and optical properties. Importantly, the ICN/poly(dimethyl siloxane) (PDMS) Received: January 17, 2012 Revised: April 24, 2012 Published: April 24, 2012 10708

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composite fabricated possesses excellent electrical conductivity even under flexion.

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EXPERIMENTAL METHODS Synthesis of Conductive Network. All reagents were of analytical grade and used as obtained without further purification. ITO precursor solution was prepared using a simple hydrothermal method. Indium chloride (InCl3·4H2O, 1.1732 g) and tin chloride (SnCl4·5H2O, 0.1435 g) were added to deionized water (30 mL). NaOH or concentrated ammonia solution was then added gradually until the pH of the mixture was 8. The mixture was sealed in a Teflon-lined stainless steel autoclave and then maintained at 80 °C for 12 h. The mixture was cooled to room temperature, and then the product was rinsed with deionized water and ethanol until no chloride ions remained in the solution. Cotton (2 g) was dipped in the ITO precursor solution for 2 h under vacuum and then dried at 60 °C for 30 min. This process was repeated three times. Finally, the cotton was calcined at 350−550 °C for 4 h to give ICN. Characterization. Powder X-ray diffraction (XRD) was carried out on a Rigaku D/max 2550 V X-ray diffractometer using Cu Kα irradiation (λ = 0.15406 nm) at 40 kV and 300 mA. Fourier transform infrared (FTIR) spectra were recorded on a Nicolet NEXUS-670 spectrometer with KBr pellets in the 4000−500 cm−1 region. N2 adsorption desorption isotherms at −196 °C were measured on a Quantachrome Autosorb 1-MP analyzer. The specific surface area of the sample was determined using a multipoint Brunauer−Emmett−Teller (BET) method. The adsorption branch of nitrogen adsorption−desorption isotherms was used to determine the pore size distribution by the Barrett−Joyner−Halenda method, assuming a cylindrical pore model. The size and morphology of the asprepared products were determined at 20 kV using a Hitachi S4800 field emission scanning electron microscope (FE-SEM). Optical images of the patterns were taken by a charge-coupled device (CCD) camera mounted on a transmission-mode microscope (XPF-550, Caikon). Photographs of the patterns were taken by a CCD video camera (PowerShot G10, Canon). Electrical resistances were measured with a 4-point probe method (MCP-T360, Mitsubishi Chemical Analytech Co. Ltd.) at room temperature. The pins of the probe (MCP-TP06, Mitsubishi Chemical Analytech Co. Ltd.) were aligned parallel at a distance of 1.5 mm, and the current used was 10 mA. Measurements of mechanical properties were performed on samples of the same size (5 mm in width ×60 mm in length) using a Dejie DXLL-20000 universal tester. UV−visible (UV− vis) transmittance spectra of the composite samples were collected with a spectrophotometer (Lambda 950, PerkinElmer Co. Ltd.).

Figure 1. XRD patterns of ICN samples at various calcination temperatures: (a) precursor, (b) 300 °C, (c) 350 °C, (d) 450 °C, and (e) 550 °C.

closely with the standard data in the literature (JCPDS card No. 06−0416). The FTIR spectra displayed in Figure 2 show the progression of the reaction during the calcination process. A strong, broad

Figure 2. FTIR spectra of (a) ICN before calcination, (b) ICN after calcination, and (c) ITO nanoparticles.

band is observed at 3420 cm−1, which could be attributed to O−H stretching vibration.17 Bands at 1061, 1428, and 2901 cm−1 are attributed to organic functional groups in cotton, including CC and CO groups. Bands caused by O−H bending vibrations, epoxide groups, and skeletal ring vibrations are observed around 1635 cm−1. After calcination, these bands disappeared. Other bands appeared from 490 to 610 cm−1 that may be attributed to the vibrations of ITO. The spectra of ICN after calcination and ITO nanoparticles are almost identical. This shows that the ICN sample after calcination is pure ITO. The possible formation process of the ICN is shown in Figure 3. A cotton template is formed of fabric fibers, which contain numerous individual cotton fibrils.8 In the preparation process, the ITO precursor particles attach to the surface of the fibrils, and then become crystalline ITO particles after calcination. The cotton template caused the ITO particles to form a regular conductive network. This process was confirmed by obtaining FE-SEM images of ICN before and after

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RESULTS AND DISCUSSION The crystallinity and structure of the as-synthesized samples were confirmed by XRD. A diffraction peak at around 2θ = 31.5° in Figure 1a corresponds to the ITO precursor. Figure 1b−e shows that heat treatment above 350 °C is required to achieve a fully crystalline sample. The broad peaks observed are characteristic of nanoparticles, and are centered at peak positions consistent with the pure cubic phase of indium oxide.11 The pattern for ITO is very similar, because of the low concentration of Sn (10%) in the samples. The XRD patterns are consistent with a pure cubic bixbyite structure, and agree 10709

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including slowing down the heating rate and keeping the reaction at the critical temperature were adopted in this work. To get some flexible performance, a glass plate that was impregnated with PDMS resin (Figure 5a) was used as a substrate. PDMS is a clear, viscous, heat-curable resin.18,19 ICN with a macroscopic network structure (Figure 5b) was then placed on the resin surface. Because of its autologous gravitation, the ICN sank slightly and combined with the resin. To obtain a film of proper thickness, a heat molding technique was used. The sample was heat treated at 120 °C for 15 min to obtain a flexible, conductive composite film (Figure 5c). The size of the optimized film was approximately 2 cm. In terms of material properties, the use of an inorganic/ organic hybrid-type structure offers advantages such as thermal stability, optical transparency, and ease of functionalization. These properties originate from inorganic/organic phases that are synergistically combined to form a homogeneous hybrid network. In addition, the reinforcement provided by the ICN not only significantly reduces the thermal expansion coefficient, but also increases the modulus and flexibility remarkably.18 Stable electrical performance of hybrid devices in different bending or twisting positions is a critical challenge for flexible substrates.4 The electrical/mechanical fatigue properties of the composite at different bending positions across the junction area with bending angles from −60° to +60° (width ca. −5 to +5 mm) were measured (Figure 6). Systematic analyses of electrical/mechanical properties were performed by attaching the ICN film to a flexible PDMS resin substrate and bending the device to a specific angle and width, as shown in the inset of Figure 6. ICN and ITO nanoparticles that were fabricated by hydrothermal method with the same quantities (10 wt %) were used to prepare the composite, and electrical/mechanical fatigue properties were measured under identical conditions. The ICN/PDMS film was a laminate film comprising ICN sheet and PDMS substrate (Figure 5c), and the ITO nanoparticle/PDMS film had a similar laminate structure; only the ITO nanoparticles were decentralized and stochastic. After bending or twisting, the resistivity of the ICN/PDMS composite only increased by 300%, from 20 to 80 Ω·cm (±5%), because of its regular network structure (inset SEM images in Figure 6). Compared with the ICN/PDMS composite, the fatigue properties of the ITO nanoparticle/ PDMS composite are very poor. Its resistivity increased about 50 times, from 20 Ω·cm to over 1000 Ω·cm. The flexible ICN/ PDMS composite contains an interconnected network, which allows rapid transport of charge carriers to give a high electrical conductivity of ∼5 S m−1, which is ∼12 orders of magnitude greater than that of the ITO nanoparticle/PDMS composite.7 Only the conductive network structure possesses stable electrical performance in different bending or twisting positions. Figure 7 shows N2 adsorption−desorption isotherms and the corresponding pore size distribution curves for the ICN and ITO nanoparticle samples. All samples exhibited type IV isotherms according to IUPAC classification, which corresponds to mesoporous solids.20 The BET surface area of the samples was measured by determining the amount of N2 absorbed as a monolayer on the sample. From this value, the average particle size was calculated assuming the presence of spherical particles using the equation

Figure 3. Schematic diagram outlining the ICN preparation process: (a) cotton template, (b) ICN before calcination, and (c) ICN after calcination.

calcination. As shown in Figure 4a−c, many ITO precursor particles were attached to the fibril surface, and their particle

Figure 4. FE-SEM images of samples: (a−c) ICN before calcination and (d−f) ICN after calcination at 450 °C for 4 h.

size distribution was in the range of 30−50 nm. After calcination, ITO particles with diameters of 20−30 nm (Figure 4f) remained distributed in a conductive network (Figure 4d,e). During the heat treatment, the network structure was easily distorted if the decomposition of cotton was too violent. Thus, in order to keep the morphology of the network, two methods

D= 10710

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Figure 5. (a) Schematic illustration of the fabrication process of the ICN/PDMS composite film. The translucent upper layer is resin, and the opaque lower layer is the glass plate. (b) Optical microscope images of ICN and cotton (inset). (c) Photograph of the flexible ICN/PDMS composite. The inset shows the size of the composite.

same as that of the ITO nanoparticles (56.12 m2/g), suggesting that the average particle sizes of ITO in the ICN and ITO nanoparticle samples are nearly identical. This implies that the

Figure 6. Electrical resistivities at different calcination temperatures and bending angles. Inset: Schematic diagram of the bending directions, bending angle, and width, together with corresponding FESEM images. (a) ICN/PDMS composite; (b) ITO nanoparticle/ PDMS composite.

where D is the particle size, ρ is the theoretical density of the material, and A is the BET surface area of the sample.21 The BET surface area of ICN is 60.79 m2/g, which is almost the

Figure 7. Nitrogen adsorption−desorption isotherms and corresponding pore size distributions (inset) of (a) ICN, and (b) ITO nanoparticle samples. 10711

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nanoparticle/PDMS sample (80%). However, the ICN/PDMS composite possessed higher transmittance than the ITO nanoparticle/PDMS sample at wavelengths below 400 nm. Overall, the ICN/PDMS composite exhibits relatively high optical transmittance in the visible and ultraviolet regions.

state of aggregation in these samples is responsible for their different properties. In addition, as shown in Figure 8, the load and strain of the ICN/PDMS composite are increased by a factor of ∼1.7

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CONCLUSIONS Synthesis of a novel conductive ITO network using a cotton template and its flexible composite was demonstrated. The ITO nanoparticles in the ICN were seamlessly interconnected into a regular, flexible network. The unique network structure, stable electrical performance at different bending angles, and outstanding mechanical and optical properties of ICN and its composite should make them suitable for many applications including embedded health monitoring devices, wearable displays, sensory skins for robotics, new classes of flexible solar cell, and so on.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (H.W.); [email protected] (Y.L.). Notes

Figure 8. Mechanical properties of PDMS resin, ICN/PDMS composite and ITO nanoparticle/PDMS composite.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support by the Shanghai Municipal Education Commission (No.07SG37), the Natural Science Foundation of China (No. 51072034, 51172042), the Cultivation Fund of the Key Scientific and Technical Innovation Project (No.708039), the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning, and the Program of Introducing Talents of Discipline to Universities (No.111-204).

compared with pure PDMS resin. For example, the load increased from 7.4 N for PDMS resin to 13.1 N for the composite. Furthermore, compared to the ITO nanoparticle/ PDMS sample, the load and strain of the ICN/PDMS composite are increased about 10−15%. These results indicate that the mechanical properties of the ICN/PDMS composite are improved by introducing a conductive network.22,23 The excellent electrical and mechanical properties of the composite and the unique network structure of ICN combine the advantages of both material and structural layout, which gives the ICN/PDMS composite great potential for use as flexible and stretchable electronics.7 UV−vis transmission spectra of ICN/PDMS and ITO nanoparticle/PDMS composite samples, both with a thickness of 1 mm, are shown in Figure 9. The ICN/PDMS sample allowed more than 60% transmittance at wavelengths above 550 nm, which was a little lower than that of the ITO

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REFERENCES

(1) Rathmell, A. R.; Bergin, S. M.; Hua, Y. L.; Li, Z. Y.; Wiley, B. J. Adv. Mater. 2010, 22, 3558−3563. (2) Kylberg, W.; Castro, F. A.; Chabrecek, P.; Sonderegger, U.; Chu, B. T. -T.; Nüesch, F.; Hany, R. Adv. Mater. 2011, 23, 1015−1019. (3) Xia, X. Y.; Wang, S. S.; Jia, Y.; Bian, Z. Q.; Wu, D. H.; Zhang, L. H.; Cao, A. Y.; Huang, C. H. J. Mater. Chem. 2010, 20, 8478−8482. (4) Manekkathodi, A.; Lu, M. Y.; Wang, C. W.; Chen, L. -J. Adv. Mater. 2010, 22, 4059−4063. (5) Chen, Z.; Cotterell, B.; Wang, W.; Guenther, E.; Chua, S. -J. Thin Solid Films 2001, 394, 202−206. (6) Park, H. J.; Kang, M. -G.; Ahn, S. H.; Guo, L. J. Adv. Mater. 2010, 22, E247−E253. (7) Chen, Z. P.; Ren, W. C.; Gao, L. B.; Liu, B. L.; Pei, S. F.; Cheng, H. M. Nat. Mater. 2011, 10, 424−428. (8) Hu, L. B.; Pasta, M.; Mantia, F. L.; Cui, L. F.; Jeong, S.; Deshazer, H. D.; Choi, J. W.; Han, S. M.; Cui, Y. Nano Lett. 2010, 10, 708−714. (9) Gordon, R. G. MRS Bull. 2000, 25, 52−57. (10) Aoki, Y.; Huang, J. G.; Kunitake, T. J. Mater. Chem. 2006, 16, 292−297. (11) Gilstrap, R. A.; Capozzi, C. J.; Carson, C. G.; Gerhardt, R. A.; Summers, C. J. Adv. Mater. 2008, 20, 4163−4166. (12) Ba, J. H.; Rohlfing, D. F.; Feldhoff, A.; Brezesinski, T.; Djerdj, I.; Wark, M.; Niederberger, M. Chem. Mater. 2006, 18, 2848−2854. (13) Jeong, H. J.; Jeong, H. D.; Kim, H. Y.; Kim, J. S.; Jeong, S. Y.; Han, J. T.; Bang, D. S.; Lee, G. -W. Adv. Funct. Mater. 2011, 21, 1526− 1532. (14) Hecht, D. S.; Hu, L. B.; Irvin, G. Adv. Mater. 2011, 23, 1482− 1513.

Figure 9. UV−vis transmittance spectra of ICN/PDMS and ITO nanoparticle/PDMS composites. 10712

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(15) Tao, X. Y.; Dong, L. X.; Wang, X. N.; Zhang, W. K.; Nelson, B. J.; Li, X. D. Adv. Mater. 2010, 22, 2055−2059. (16) Liu, Y.; Li, N.; Qi, Y. P.; Dai, L.; Bryan, T. E.; Mao, J.; Pashley, D. H.; Tay, F. R. Adv. Mater. 2011, 23, 975−980. (17) Hou, C. Y.; Zhang, Q. H.; Zhu, M. F.; Li, Y. G.; Wang, H. Z. Carbon 2011, 49, 47−53. (18) Jin, J. H.; Ko, J. -H.; Yang, S. C.; Bae, B. -S. Adv. Mater. 2010, 22, 4510−4515. (19) Zeng, X. Y.; Zhang, Q. K.; Yu, R. M.; Lu, C. Z. Adv. Mater. 2010, 22, 4484−4488. (20) Zhi, Y.; Li, Y. G.; Zhang, Q. H.; Wang, H. Z. Langmuir 2010, 26, 15546−15553. (21) Juárez, R. E.; Lamas, D. G.; Lascalea, G. E.; Walsöe de Reca, N. E. J. Eur. Ceram. Soc. 2000, 20, 133−138. (22) Liu, Y.; Zhu, M. F.; Liu, X. L.; Zhang, W.; Sun, B.; Chen, Y. M.; Adler, H. -J. P. Polymer 2006, 47, 1−5. (23) Zhu, Y. W.; Murali, S.; Cai, W. W.; Li, X. S.; Suk, J. W.; Potts, J. R.; Ruoff, R. S. Adv. Mater. 2010, 22, 3906−3924.

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