Chem. Mater. 2007, 19, 1895-1897
Facile Synthesis and Assembly of Ag/NiO Nanofibers with High Electrical Conductivity Hui Wu, Dandan Lin, Rui Zhang, and Wei Pan* State Key Lab of New Ceramic and Fine Processing, Department of Materials Science and Engineering, Tsinghua UniVersity, Beijing 100084, People’s Republic of China ReceiVed September 25, 2006 ReVised Manuscript ReceiVed January 25, 2007
Prompted by the inherent limitations of traditional lithography, it is desirable to develop new methods for the fabrication and assembly of conductive one-dimensional (1D) nanostructures for their potential applications as both interconnects and active components for the next generation of ultraminiaturized electronic devices.1 The requirements for interconnection in the future nanoelectronics are low resistivity, good ohmic contact to both p- and n-type semiconductors, low cost, and compatibility with the processing of Si complementary metal oxide semiconductor (CMOS) devices.2 There have been intensive efforts to synthesize 1-D metallic nanowires with high electrical conductivity using various methods such as matellization of DNA,3 electrochemical4 and photochemical5 reduction in aqueous surfactant media,6 and templating from porous alumina,7 polycarbonate membrane,8 or carbon nanotube.9 Some outstanding work has recently been performed describing the fabrication of silver nanowires with a conductivity of ∼0.8 × 105 S/cm by the solution-phase method based on capping reagents.10 Haynie et al. have also succeeded in preparation and self-assembly of Pd nanowires with a conductivity of about 0.5 × 104 S/cm.11 Nevertheless, it is still difficult to prepare free-standing conductive nanowires in high yield. Moreover, controlled assembly of these nanowires, which is particular important to nanoelectronic devices, remains a great challenge. Electrospinning has been extensively explored as a quick and facile technique to create continuous fibers with diameters on the nanometer scale.12 Nanofibers of more than 30 types of organic polymers have been successfully produced * To whom correspondence should be addressed. E-mail: tsinghua.edu.cn.
panw@
(1) Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayers, B.; Gates, B.; Yin, Y.; Kim, F.; Yan, Y. AdV. Mater. 2003, 15, 353. (2) Chen, L. J. JOM 2005, 57, 24. (3) Gu, Q.; Cheng, C.; Gonela, R.; Suryanarayanan, S.; Anabathula, S.; Dai, K.; Haynie, D. T. Nanotechnology 2006, 17, R14. (4) (a) Yu, Y. Y.; Chang, S. S.; Lee, C. L.; Wang, C. R. C. J. Phys. Chem. B 1997, 101, 6661. (b) Chang, S. S.; Shih, C. W.; Chen, C. D.; Lai, W. C.; Wang, C. R. C. Langmuir 1999, 15, 701. (5) Esumi, K.; Matsuhisa, K.; Torigoe, K. Langmuir 1995, 11, 3285. (6) Kim, F.; Song, J. H.; Yang, P. J. Am. Chem. Soc. 2002, 124, 14316. (7) Van der Zande, B. M.; Bohmer, I. M. R.; Fokkink, L. G. J.; Schonenberger, C. Langmuir 2000, 16, 451. (8) Cepak, V. M.; Martin, C. R. J. Phys. Chem. B 1998, 102, 9985. (9) (a) Govindaraj, A.; Satishkumar, B. C.; Nath, M.; Rao, C. N. R. Chem. Mater. 2000, 12, 202. (b) Fullam, S.; Cottell, D.; Rensmo, H.; Fitzmauice, D. AdV. Mater. 2000, 12, 1430. (10) (a) Sun, Y.; Gates, B.; Mayers, B.; Xia, Y. Nano Lett. 2002, 2, 165. (b) Sun, Y.; Xia, Y. AdV. Mater. 2002, 14, 833. (11) Cheng, C.; Gonela, R. K.; Gu, Q.; Haynie, D. T. Nano Lett. 2005, 5, 175.
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by electrospinning.13 More recently, the technique was extended further, to the fabrication of ceramic and composite nanofibers with various compositions and properties.14 Electrospinning of nanofibers with electrical activities has also been met with some success. Electrospun nanofiber mats of conductive polymers such as polyaniline and polypyrrole were prepared by several groups, with a conductivity ranging from 10-7 S/cm to 10-1 S/cm.15-17 Electrospun polypyrrole fiber mats fabricated by Lee et al. possess a conductivity of 0.5 S/cm.16 Ge et al. have successfully prepared multiwalled carbon nanotubes (MWNTs) and polyacrylonitrile (PAN) composite nanofiber mats with a conductivity up to 0.51.0 S/cm.17 A single nanofiber of polyaniline with a conductivity of 0.01 S/cm was also prepared by electrospinning.18 However, limited by their intrinsic conductivities, polymer-based nanofibers usually have conductivities below 1 S/cm. To achieve nanofibers with higher conductivity, a metallic component was introduced into the electrospinning system because of its high electrical conductivity in the bulk. Herein we report on the synthesis and assembly of silver/ nickel oxide composite nanofibers by electrospinning. The synthesized nanofibers have an ultrahigh conductivity of ∼0.5 × 105 S/cm. In a typical electrospinning process, a sample solution containing poly(vinyl alcohol) (PVA), silver nitrate, and nickel nitrate is pumped through a nozzle to which a high voltage is applied relative to a grounded aluminum foil (collector) to form an electrically charged jet of solution. The solution jet solidifies with accompanying evaporation of solvent and forms continuous fibers on the collector. The collected fibers were randomly distributed on the aluminum foil (Supporting Information, Figure 1). The precursor fibers were then calcined at 500 °C in air for 2 h. During the heating process, polymer composition was burned up, while silver and nickel oxide were formed by the following chemical reactions: 4
2AgNO3 98 2Ag + 2NO2 + O2 4
2Ni(NO3)2 98 2NiO + 4NO2 + O2 The scanning electron microscopy (SEM) image of the calcined samples (Supporting Information, Figure 2) shows (12) (a) Formalas, A. U.S. Patent 1,975,504, 1934. (b) Reneker, D. H.; Chun, I. Nanotechnology 1996, 7, 216. (c) Dzenis, Y. Science 2004, 304, 1917. (d) Li, D.; Xia, Y. AdV. Mater. 2004, 16, 1168. (e) Jang, S.-Y.; Seshadri, V.; Khil, M.-S.; Kumar, A.; Marquez, M.; Mather, P.; Sotzing, G. A. AdV. Mater. 2005, 17, 2177. (13) Subbiah, T.; Bhat, G. S.; Tock, R. W.; Parameswaran, S.; Ramkumar, S. S. J. Appl. Polym. Sci. 2005, 96, 557. (14) (a) Li, D.; Xia, Y. N. Nano Lett. 2003, 3, 555. (b) Ge, J. J.; Hou, H. Q.; Li, Q.; Graham, M. J.; Greiner, A.; Reneker, D. H.; Harris, F. W.; Cheng, S. Z. D. J. Am. Chem. Soc. 2004, 126, 15754. (c) Gao, J.; Yu, A.; Itkis, M. E.; Bekyarova, E.; Zhao, B.; Niyogi, S.; Haddon, R. C. J. Am. Chem. Soc. 2004, 126, 16698. (15) (a) Chronakis, I. S.; Grapenson, S.; Jakob, A. Polymer 2006, 47, 1597. (b) Zhu, Y.; Zhang, J. C.; Zheng, Y. M.; Huang, Z. B.; Feng, L.; Jiang, L. AdV. Funct. Mater. 2006, 16, 568. (16) Kang, T. S.; Lee, S. W.; Joo, J.; Lee, J. Y. Synth. Met. 2005, 153, 61. (17) Ge, J. J.; Hou, H.; Li, Q.; Graham, M. J.; Greiner, A.; Reneker, D. H.; Harris, F. W.; Cheng, S. Z. D. J. Am. Chem. Soc. 2004, 126, 15754.
10.1021/cm062286y CCC: $37.00 © 2007 American Chemical Society Published on Web 03/21/2007
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Figure 2. Current-voltage (I-V) characteristics of a single electrospun fiber of PVA/Ni(NO3)2/AgNO3 aligned across a 100 µm gap between two parallel electrodes (upper inset). After cutting the wire, the sample behaved as an insulator (lower inset).
Figure 1. (a) TEM image of a single Ag/NiO prepared by calcinations of the precursor fibers at 500 °C for 2 h. Inset: the SAED pattern. (b) HRTEM image of the same sample, indicating the polycrystalline structure of the calcined fiber. (c) A typical energy-dispersive X-ray spectrum obtained from the same sample. Cu signals, indicated by asterisks, were generated from a copper grid used in the energy-dispersive X-ray/HR-TEM measurement. (d) XRD pattern of synthesized nanofibers, showing the cubic Ag and cubic NiO mixture phase.
clearly that the prepared fibers remained continuous. Figure 1a shows a low-magnification transmission electron microscopy (TEM) image of the calcined nanofibers, indicating that the nanofibers have a uniform morphology, with a diameter of ∼80 nm. The study with the selected-area electron diffraction (SAED) pattern (inset of Figure 1a) indeed reveals the polycrystalline nature for the synthesized nanofibers. Figure 1d shows the X-ray diffraction (XRD) pattern of the product, which can be well-indexed as the mixed phase of cubic Ag and cubic NiO. The measured pattern is in agreement with the reported XRD patterns (JCPDS 4-0783 for Ag and JCPDS 4-835 for NiO) concerning both peak intensity and position. The high-resolution TEM (HR-TEM) image of the products (Figure 1b) confirms the polycrystalline feature of the nanofibers, which are comprised of tiny particles of Ag and NiO with diameter of ∼5 nm. X-ray elemental analyses also indicated that the prepared nanofibers were composed of silver and NiO (Figure 1c). (18) Kameoka, J.; Orth, R.; Yang, Y. N.; Czaplewski, D.; Mathers, R.; Coates, G. W.; Craighead, H. G. Nanotechnology 2003, 14, 1124.
Further, TEM elemental mapping of Ag, Ni, and O for the synthesized Ag/NiO nanofiber supports that silver was well-proportioned distributed in the fibers with a high concentration (Supporting Information, Figure 3). It is worth pointing out that NiO was added into the nanofibers for the sake of maintaining the continuous structure. Without the addition of Ni(NO3)2 into the electrospinning solution, precursor nanofibers of AgNO3/PVA can also be obtained (Supporting Information, Figure 4), but after calcinations, the products would be lines of separated Ag nanoparticles rather than freestanding nanofibers (Supporting Information, Figure 5). Hence, the high melting point ceramic NiO plays a very important role in maintaining the fiber-shaped AgNiO although the NiO content was not higher than 15 wt %. The ceramic component was functioning as a frame supporting the continuous structure of the nanofibers. By modifying the layout of a collector, it is possible to assemble the thin fibers as uniaxially aligned arrays.19 For electrical characteristics, oriented Ag/NiO nanofiber bridging two electrodes was fabricated following this method. Two parallel strips of conductive silicon coated with a 150 nm thick platinum layer were attached on to a glass plate with a gap of about 100 µm and used as the negative electrodes. During the electrospinning process, the electrostatic force drives the positively charged composite fibers so that they align between the two counter electrodes. With a collecting time of ∼10 s, individual fibers bridging two electrodes was deposited (upper inset of Figure 2). We have tested the electrical continuity of the nanofibers by measuring the resistance of an individual nanofiber at room temperature using the two-probe method. The current (I) versus applied voltage (V) measurements were first made on a single precursor nanofiber of PVA/AgNO3/Ni(NO3)2 bridging two electrodes ∼ 120 µm apart. The measurement showed ohmic behavior (Figure 2). The measured resistance was 2 × 109 Ω, corresponding to a conductivity of approximately 0.02 S/cm. When the nanofiber shown in Figure 2 was severed using the probe tip of a Keithley probe station, the sample behaved like an insulator (lower inset of Figure 2). Clearly the current passed through the nanofiber. (19) Li, D.; Wang, Y. L.; Xia, Y. N. Nano Lett. 2003, 3, 1.
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Figure 3. Current-voltage (I-V) characteristics of a single silver/NiO composite nanofiber aligned across a 150 µm gap between two paralleled electrodes (upper inset).
After calcinations of the aligned precursor fibers, freestanding silver/NiO nanofibers bridging the electrodes were obtained. The suspending fibers had a diameter of ∼90 nm and length of over 100 µm. The resistance of an array of nanofibers on an electrode, formed as described above, was found to be 10-200 Ω. Electrical properties of a single Ag/ NiO nanofiber aligned across a 150 µm gap between two electrodes (inset of Figure 3) were determined. Figure 3 shows clear evidence that the I-V curve of this nanofiber was ohmic at room temperature. The measured resistance was 4.0 kΩ, corresponding to a conductivity of approximately 0.5 × 105 S/cm. This value is about 1 × 106 higher than that of the precursor fiber (0.02 S/cm), strongly indicating that the composite nanofibers were still electrically continuous even though insulated composition (NiO) was introduced into the fiber system. In the nanofibers, the nanosized Ag particles formed an interconnected network, through which the electric current was conducted. The high measured electrical conductivity of the Ag/NiO nanofibers also indicates that good electrical contact between the nanofibers and the electrodes was achieved. One reason is that the surface resistance of the platinum coated silicon strips is extremely low, which is very helpful to achieve a good electric contact with the Ag rich nanofibers. Moreover, during the heat treatment process (at 500 °C for 2 h), the Ag nanoparticles in the nanofiber may weld with the platinum
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coating layer, and as a result, a good electric contact between the nanofibers and the electrodes was obtained. A persuasive experimental phenomenon is that the fiber firmly attached to the electrodes after heat treatment. The conductivity of the prepared nanofiber is similar to that of silver nanowires synthesized by solution-phase methods (∼0.8 × 105 S/cm)10 and lower than that of bulk silver (6.2 × 105 S/cm). The lower measured conductivity is attributed to several reasons: the presence of NiO in the nanofibers; contaminations around the surface and contact regions; and polycrystalline structure of the synthesized nanofibers. The finding suggests that Ag/NiO nanofibers have great potential to be used as contacts and interconnects for future nanoelectronics. In summary, synthesis of silver-based conductive nanofibers by the novel electrospinning technique has the following advantages: (i) The synthesis process is simple, lowcost, and environmentally friendly, with no templateremoving steps needed. (ii) Uniform nanofibers with length over 200 µm are readily produced. (iii) The prepared nanofibers have an ultrahigh conductivity of ∼0.5 × 105 S/cm, which is much larger than that of electrospun nanofibers (∼10-6 to 0.4 S/cm) reported before.15-18 These highly conductive nanofibers can be further explored to fabricate interconnects for future nanoelectronic devices. (iv) These electrospun fibers can be easily assembled into aligned arrays by using a modified fiber collector. An easy assembly of these conductive nanofibers is important for further device uses. (v) The diameter of Ag/NiO fibers is at the nanoscale, which dictates their superior performance as chemical sensors. Acknowledgment. This study was supported by the National Natural Science Foundation of China (Grant 50232020). Supporting Information Available: Experimental details; SEM images and TEM elemental mapping of the synthesized Ag/NiO nanofibers; and SEM images of silver nanoparticle lines obtained by calcination of electrospun AgNO3/PVA nanofibers (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. CM062286Y
