Thin and long silver nanowires self-assembled in ionic liquids as a soft template: electrical and optical properties
© Chang et al.; licensee Springer. 2014
Received: 22 April 2014
Accepted: 19 June 2014
Published: 3 July 2014
Thin and long silver nanowires were successfully synthesized using the polyvinylpyrrolidone (PVP)-assisted polyol method in the presence of ionic liquids, tetrapropylammonium chloride and tetrapropylammonium bromide, which served as soft template salts. The first step involved the formation of Ag nanoparticles with a diameter of 40 to 50 nm through the reduction of silver nitrate. At the growing stage, the Ag nanoparticles were converted into thin and long one-dimensional wires, with uniform diameters of 30 ± 3 nm and lengths of up to 50 μm. These Ag nanowires showed an electrical conductivity of 0.3 × 105 S/cm, while the sheet resistance of a two-dimensional percolating Ag nanowire network exhibited a value of 20 Ω/sq with an optical transmittance of 93% and a low haze value.
One-dimensional (1-D) metallic nanostructures, namely silver nanowires (Ag NWs), have recently attracted a great deal of attention for their unique electrical, optical, magnetic, and thermal properties as a promising alternative to indium tin oxide (ITO) as an electrode material used in the fabrication of devices such as electronic displays, photonics, and sensors [1–10]. Ag NWs with well-defined shapes such as lengths and diameters are particularly interesting, as they have superior optical and electrical properties, thus making them excellent candidates for transparent electrodes. However, in order to implement the optical and electrical features required for transparent electrodes, there is still a need to develop more effective processes for synthesizing Ag NWs with controllable shapes and sizes, which can be grown continuously up to at least 30 μm in length with 30-nm diameter. Several chemical approaches have been actively explored and developed in order to process Ag into 1-D nanostructures using various physical templates and surface-capping reagents (organic polymers or surfactants) in conjunction with the solution-phase polyol process [11–14]. These studies largely focused on controlling the size, shape, crystal structure, and optical/electrical properties of the Ag NWs. For example, Sun and co-workers  developed a solution-based polyol process to prepare single-crystal Ag NWs using polyvinylpyrrolidone (PVP) as a surface-capping reagent. The capping reagents were then evaluated in order to kinetically control the growth rates of the metal surfaces and subsequently induce 1-D growth leading to the formation of NWs. Based on the PVP-assisted polyol method, Xia and co-workers [15, 16] also demonstrated a salt-mediated polyol process, using NaCl, CuCl2, PtCl2, or CuCl, to prepare Ag NWs of 30 to 60 nm in diameter in large quantities. Murphy et al.  first reported the preparation of Ag NWs with uniform diameters using the seed-mediated growth approach with a rodlike micelle template, cetyltrimethylammonium bromide (CTA-B), as the capping reagent. First, Ag seed nanoparticles with an average diameter of 4 nm were prepared by reducing AgNO3 with NaBH4 in the presence of trisodium citrate. The Ag seed particles were then grown into 1-D structures with a twinned crystal arrangement in the presence of the CTA-B capping reagent. Here, the capping reagent regulates this process by confining the growth of the lateral surface and including the expansion of the surface of the wire, leading to the formation of wires with a high aspect ratio. However, continuous Ag NWs of up to 40 μm in length with a small diameter of 30 nm have yet been synthesized via the polyol method.
Thin and uniform Ag NWs were synthesized through the chemical reduction of AgNO3 (Aldrich, St. Louis, MO, USA) with PVP (average molecular weight, Mw = 1,200,000) as a capping agent in the presence of a solution containing TPA-C and TPA-B. Approximately 35 mL (0.35 M in EG) of PVP, 15 mL (0.006 M in EG) of TPA-C, and 15 mL (0.003 M in EG) of TPA-B were simultaneously added to 170 mL of EG while being stirred at 120°C. Seventy milliliters (0.1 M in EG) of AgNO3 dissolved in 70 mL of EG was then added to the reaction mixture and stirred for 40 min. The reaction was carried out within an autoclave reactor. The reaction mixture was heated at 170°C for an additional 30 min during the wire growth stage. The final products, Ag NWs, were washed with acetone several times to remove the solvent (EG), PVP, and other impurities. After washing, the precipitate was re-dispersed in H2O.
The morphology and molecular structures of the resulting dispersed Ag NWs were observed by field emission scanning electron microscopy (FE-SEM; JEOL JSM-5410, Tokyo, Japan) and transmission electron microscopy (TEM; JEOL JEM-2100 F). The optical and surface plasmon resonance (SPR) spectra were measured using ultraviolet spectroscopy (UV/vis, SHIMADZU UV-3150, Tokyo, Japan). Conductivity was measured using the standard four-point probe technique.
Results and discussion
By utilizing the experimental method mentioned above, we fabricated self-organized Ag NWs by reducing AgNO3 within the micelles of TPA salt templates, which are ammonium-based IL. This did not need any additional ions required to control the crystal growth of silvers and utilized PVP as the surface capping reagent. Surprisingly, during the first synthetic step in the building of the Ag nanostructures, Ag nanoparticles with a diameter of approximately tens of nanometers were found to exist and were subsequently converted into well-defined long wire structures. In this procedure, the diameter of the Ag nanoparticles and the Ag NWs is largely dependent on the type and amount of the ILs present in the reaction mixture. For example, the diameters of the Ag NWs produced from IL solutions of TPA-C and TPA-B mixture, TPA-C, and tetrahexylammonium chloride (THA-C) were 25 to 35 nm, 30 to 50 nm, and 35 to 55 nm, respectively, and their dispersions were also relatively wide, as shown in Figure 2II. These results confirm that there is a correlation between the sizes of the pore, micelle, and ILs employed as the soft template. In order to obtain finer and more uniform nanostructures, TPA-C was mixed with TPA-B in a ratio of 2:1 and subsequently utilized as soft template salts. The Ag nanostructures then formed Ag nanoparticles with a diameter of 30 to 40 nm during the initial reaction step and were subsequently converted into well-defined Ag NWs with a narrow and uniform diameter dispersion in the range of 27 to 33 nm and long length of up to 50 μm, as shown in Figure 2. Figure 2I displays an SEM image of the thin and long Ag NWs synthesized using the TPA-C and TPA-B mixture, while Figure 2II,III displays the distributions of the diameter and length, respectively, of the synthesized wires. Therefore, we determined that the diameter of the wires was affected more significantly than the length of the wire when the type and components of the ILs were varied. Then, the IL solutions appear to act as a size-controllable template salt within the liquid phase. In particular, the diameters of the Ag NWs were influenced by the type and components of the ILs, and their sizes could be effectively controlled within a diameter range of 20 to 50 nm according to the components of ILs.
The present work demonstrates that thin and uniform Ag NWs can be synthesized using ILs (a mixture of TPAC and TPAB) as a soft template salt when employing the PVP-assisted polyol process. Pentagonal structures twinned along the  plane are subsequently produced, and the nanowire dimensions, particularly the diameters, can be controlled by the composition of the ILs. Ag can be directly grown into thin nanowires with diameters of 30 ± 3 nm and long lengths of approximately 50 μm. Additionally, the characteristic SPR of thin Ag NWs was observed at 372 nm in the absorbance spectra, which is evidence of the formation of NWs. Furthermore, these thin and long Ag NWs were determined to possess an electrical conductivity of approximately 0.3 × 105 S/cm, and the sheet resistance of a 2-D percolating Ag network was found to be 20 Ω/sq with an optical transmittance of 93%. The light scattering intensity was largely reduced and thus improved the optical properties. It is obvious that these transparent conducting Ag NWs have the potential to outperform conventional ITO thin films, especially when used in flexible OLED devices as a possible electrode layer.
This work was financially supported in part by the Converging Research Center Program through the Ministry of Science, ICT and Future Planning (2013 K000201) and the Industrial Core Technology Development Project through the Ministry of Knowledge and Commerce (10035644).
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