Synthesis of small diameter silver nanowires via a magnetic-ionic-liquid-assisted polyol process

Abstract

Silver nanowires (Ag NWs) with a small diameter of 20 nm and an aspect ratio of 1000 were successfully synthesized using the polyol method in the presence of a magnetic ionic liquid (MIL). The MIL used was 1-butyl-3-methylimidazolium tetrachloroferrate (bmim[FeCl₄]), which served as a soft template and ionic salt. Wire formation was highly dependent on the content of bmim[FeCl₄], and the diameters of Ag NWs produced in this process can be effectively controlled by adjusting the concentration of 1-butyl-3-methylimidazolium bromide. Self-assembled local structures in MILs are likely to act as soft-templates for highly organized Ag nanostructures. Selective precipitation was used to purify the Ag NWs obtained. The transparent electrode films exhibited the highest optoelectronic performance, with a transmittance of 91%, low haze value of 1.5%, and low sheet resistance of 15 Ω sq⁻¹. These results reveal that the produced Ag NWs performed better than conventional indium tin oxide film.

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Introduction

One-dimensional (1D) nanostructures have recently attracted considerable interest because of their unique electrical, optical, and thermal properties that make them ideal for use as a transparent electrode material for flexible electronics, photonic devices, and sensors.^1–6 In particular, silver nanowires (Ag NWs) with a small diameter and high aspect ratio, exhibit excellent optical and electrical properties, thus making them a highly suitable candidate for use as transparent electrodes.

Many synthesis methods have been investigated to control the growth of Ag in 1D nanostructures using various hard templates such as alumina membranes and block copolymers,^7,8 organic soft templates,⁹ and polymer-based capping reagent^10–13 techniques with the polyol process. These studies mainly focused on controlling the diameters and lengths, crystal structure, and optoelectrical properties of wires produced. In particular, Xia et al.^10–13 demonstrated a salt-mediated polyol process using NaCl and PtCl₂ to prepare large quantities of Ag NWs with diameters of 25–50 nm using a poly(vinylpyrrolidone) (PVP)-based capping reagent. Ag NWs produced were determined to have a pentagonal-twinned structure with the surface being bound by the [100] facets and end surfaces being bound by the [111] facets. They suggested that Ag NWs grow from multiply twinned nanoparticles and that the passivation of the more active [100] surfaces by the adsorption of PVP led to the 1D growth of wires by the [111] facets.¹⁰ Only recently, Ag NWs with a diameter of 20 nm and aspect ratio of up to 1000 were reported to have been synthesized via the polyol process using NaCl/NaBr salts¹⁴ and via by a high-pressure polyol method.¹⁵ These NWs were used to produce 2D films, formed by a network of 20 nm-diameter NWs, which could produce a sufficiently transparent electrode film suitable for flexible electronic devices, showing greater performance than conventionally used indium tin oxide (ITO). However, although it is possible to create the thinner Ag NWs that are necessary for device applications, other factors must be considered. The synthesis of these Ag NWs has not yet also been fully demonstrated via the polyol process. That is, as in the previous studies,^7–13 the growth mechanisms of a Ag NWs using the inorganic Cl⁻/Br⁻ salts or the ionic liquid such as ammonium Cl⁻/Br⁻ salts have been described. In this case, the size of the nanowires has been reported to be generally synthesized in the diameter of 25–50 nm range except for special synthesis conditions.^14,15 For the purpose of effective size control of the Ag NWs, although the various research to control the growth of the nanowires carried also out using several types of assisted-soft templates,^10–13 Ag NWs of very fine structure having an aspect ratio of more than 1000 with a small diameter of less than 20 nm is also not been reported so far.

Currently, Ag NWs with small diameters and high aspect ratios have attracted much interest because of their good optical properties, which lead to a low degree of light scattering, making them excellent candidates for use as transparent electrodes. There is a need to develop processes that are more efficient for synthesizing Ag NWs with small-sized diameters and a continuous range of lengths up to at least 20 μm to make effective network junctions between wires.

In this study, we newly present the controlled polyol synthesis of Ag NWs with diameter of 20 nm and aspect ratios of up to 1000 using a magnetic ionic liquid (MIL), 1-butyl-3-methylimidazolium tetrachloroferrate (bmim[FeCl₄]), which acts as a soft template and ionic salt. MIL shows bmim[FeCl₄], which is a type of ionic liquids; it is an organic salt composed of heterocyclic cations (imidazolium) and FeCl₄⁻ anions. A key feature of MILs is that their chemical and physical properties can be changed just by varying their cations, substituents, and anions as desired. MILs have low melting points (below −80 °C)¹⁶ and exhibit unique properties such as high polarity, non-flammability, high ionic conductivity, and thermal stability above 400 °C. Their use has previously been reported in catalysis,¹⁷ as inert solvents in electrochemistry,¹⁸ and as reaction media or templates for nanomaterial synthesis.^18–20 ILs that exhibit a strong response to magnetic fields, e.g., bmim[FeCl₄], are termed as MILs.²¹ In a previous study, we demonstrated the use of bmim[FeCl₄] as a template for the synthesis of small-sized conductive polypyrrole spheres and tubes with a diameter of approximately 60 nm.²⁰ Moreover, bmim[FeCl₄] can act as a soft template as well as a synthetic catalyst for organized polymers. Shapes and sizes of synthesized polypyrrole polymers are affected by local self-assembled structures that are formed by bmim-based ILs in the presence of a magnetic field. In this work, these bmim[FeCl₄] that this structural specificity as the soft template using the polyol reaction could be experimentally verified that acts efficiently in the synthesis of Ag NWs than when used alone PVP as a assisting reagent and/or used a conventional IL of different structures as assisted-materials.

Herein, we demonstrate that Ag NWs with a small diameter of 20 nm and aspect ratio of 1000 were synthesized through the polyol method in the presence of a bmim[FeCl₄] as shown in Fig. 1.

Fig. 1 Molecular structure of the MIL (1-butyl-3-methylimidazolium tetrachloroferrate: bmim[FeCl₄]) and ionic liquid (1-butyl-3-methylimidazolium bromide: bmim[Br]).

Experimental section

Ag NWs were synthesized through a modified polyol process via the chemical reduction of AgNO₃ (GR, Aldrich) using bmim[FeCl₄] (GR, TCI) with a PVP reducing agent (average molecular weight, M_w = 1 300 000, Aldrich) in the presence of 1-butyl-3-methyl-imidazolium bromide (bmim[Br], TCI) as a control reagent.

First, four stock solutions were prepared: (A) 300 mM PVP, (B) 3 mM bmim[FeCl₄], (C) 15 mM bmim[Br], and (D) 300 mM AgNO₃ in ethylene glycol (EG). Next, 220 mL of EG, 35 mL of solution A, 2 mL of solution B, and 15 mL of solution C were added into a 1000 mL round-bottom flask, which was placed in an oil bath at room temperature. The solutions were then mixed for 20 min by vigorous stirring. Next, 25 mL of solution D was quickly added into the flask, and the mixture was then stirred for an hour at 170 °C. After synthesis, the products were cooled to room temperature, and final products, Ag NWs, were washed with DI water. To precipitate the products, acetone was slowly added to the mixture of the reaction solution and DI water. Because Ag NWs were capped with PVP and PVP is not soluble in acetone, Ag NWs aggregated and settled at the bottom of the reaction vessel. The precipitates were then re-dispersed in DI water and re-precipitated with acetone five times to produce highly pure Ag NWs.

For the preparation of transparent electrode film, the purified Ag NWs, with approximately 20–23 nm diameter having an aspect ratio of more than 1000, were dispersed in DI water for an optimized density of 0.5 mg mL⁻¹ and directly coated on an O₂ plasma-treated polyester (PET) substrate film via a conventionally available wet coating method using the Mayer bar coating technique to fabricate the transparent electrode film. The density of deposited Ag NWs layer was controlled by the volume of the Ag NW solution, determined by the mesh size of the Mayer bar.

The morphology and molecular structure of dispersed Ag NWs were observed using a field emission scanning electron microscope (FE-SEM, JEOL-JSM5410) and transmission electron microscopy (TEM, JEOL-JEM2100F). Optical property and surface plasmon resonance (SPR) spectra were obtained using an ultraviolet spectrophotometer (UV-Vis, Shimadzu UV-3150) and a haze meter (NDH 7000). Electrical properties of Ag NW transparent electrode films were determined using a standard four-point probe technique (Laresta GP, MCP-T60).

Result and discussion

The polyol synthesis is a simple method to obtained Ag wires via reduction of AgNO₃ in liquid polyol condition. Several kind of polyol methods have been demonstrated to control the growth as a one-dimensional metal using various soft-templates. In this work, we newly synthesized Ag NWs by reducing AgNO₃ within the bmim[FeCl₄] also known as a magnetic ion liquid, which also acted as ionic salts or assisted-template agent, with PVP as a capping agent. The synthesis mechanism has been reported that typically occur in the two step reaction as illustrated in Fig. 2: an initial seed grain formation stage, which formed octahedral-structured Ag seeds, and a growth stage where pentagonal-structured Ag wires were formed. In the seeding stage, several kinds of Ag seed particles depending on reaction conditions different environment are formed by nucleation of AgNO₃ in EG solvent. In the growth stage, these Ag seeds grow into wires in the presence of Ag⁺ and PVP as a surface-capping reagent, which kinetically controls the growth rate of the Ag surfaces and subsequently induces 1D growth along the [110] direction. Here, bmim[FeCl₄] was used as a soft template to effectively control the Ag particle seeds and the growth of the subsequent wires. According to the previous study,¹⁰ PVP as a surface-capping reagent has been reported to play a role as the passivation of the more active [100] surfaces by the adsorption of PVP led to the 1D growth of wires by the [111] facets, and there molecular weight is a particularly large impact in the longitudinal direction in the synthesis process, a fact that has been proved experimentally also. In this sense, the PVP of average molecular weight 1 300 000 were used to obtain the Ag NWs of the possible long structured-wire in this experiment.

Fig. 2 Reaction scheme of Ag NWs produced using bmim[FeCl₄] as a soft template with Ag NO₃ in the polyol process.

The synthesis shown in Fig. 2, reaction scheme A, when the mol ratio of the bmim[FeCl₄] and Ag NO₃ was low concentration of down to 0.0003, spherical shaped Ag seeds of approximately 30–80 nm size are formed, and these seed particles are finally grown to Ag particles of 50–100 nm size, but it did not grow to the wires. Fig. 2, reaction scheme B, details the case when the ratio of bmim[FeCl₄] and AgNO₃ was in the range of 0.0003 to 0.003. Ag seed crystals of pentagonal-twin structure (octahedral) are mainly formed at the initial stage together with some other crystal structures such as pentagonal and tetrahedral. These octahedral structured Ag seeds are then grown to pentagonal-shaped Ag wires during the second step. In this case, according to previous paper,¹⁰ Ag wires has been described to be growing only from pentagonal-twin structured crystals because the protection of the more active [100] surfaces by the adsorption of PVP led to the 1D growth of wires by the [111] facets. However, in these concentration ranges, the relatively small pentagonal-twin seed particles are formed and the diameter of the final Ag NWs become thinner and elongated as the concentration of bmim[FeCl₄] is increased. When the ratio of bmim[FeCl₄] and AgNO₃ increased to 0.003, several kinds of Ag seed particles were produced alongside large quantities of iron oxide particles in the first step before some shorter, thicker Ag MWs were created with clusters of iron oxide particles, as shown in Fig. 2, reaction scheme C. It should be noted that other variables, such as reaction temperature, time, and mixing ratios also have some limited effect on the Ag NW shapes generated. However, wire formation yield and sizes have previously been shown to be largely dependent on the content of bmim[FeCl₄] in the reaction vessel. Notably, bmim[FeCl₄] is an isotropic organic liquid composed entirely of ions, but have also been known to serve as anisotropic structure due to its self-organizing abilities, which can be altered by external influences such as magnetic field, and temperature. However, the microscopic orientation of self-organized domains in bmim[FeCl₄] appears to play a key role when bmim[FeCl₄] is used as a template for the synthesis because the boundary in randomly oriented poly-domains highly disturbs anisotropic transition of charges and ions, and the boundary of this isotropic liquid domain is increased with increasing temperature. Gordon et al.²² reported that bmim [PF₆] appears to exhibit anisotropic behavior at high temperatures, and Pringle et al.²³ used bmim[PF₆] as a template for the synthesis of conducting polymer. We previously reported the use of bmim[FeCl₄] as a template for polymer synthesis to create 60 nm nanoparticles and nanotubes of polypyrroles, with the structures of bmim[FeCl₄] self-organized domains being significantly affected by a magnetic field.²⁰

In this synthesis, bmim[FeCl₄] is an ionic conductor comprising imidazolium-based cationic hydrocarbon (bmim⁺) and anionic FeCl₄⁻ as shown in Fig. 1, and chloride ions of FeCl₄⁻ species actually at as an etching agent for Ag in the polyol synthesis of Ag NWs. As the results, the concentration of bmim[FeCl₄] could be changed independently from the concentration of chloride. Thus, chloride ions to the AgNO₃ precursor act as the groups in controlling the growth of Ag nuclei, and the nucleation rate of Ag⁺ has been also known to be in close relationship with the product size. Here, the combination of Ag⁺ and chloride ions reduced the concentration of Ag⁺, and thus effectively increased the percentage of twinned seed particles required for growth of a wire. In addition, bmim[Br], imidazolium-based ILs composed of bmim⁺ and Br⁻, also added to the synthesis as a control reagent because, especially, the bromide ion was being known that a large influence on the NW diameter. Thus, when bromide ions are introduced during the synthesis of Ag NWs, insoluble salts of AgBr are formed. This delayed the production of Ag ions and inhibits the growth of these ions in the radial direction of Ag NWs because Br⁻ adsorbed on the surface of these NWs reduced the rate of atomic addition to the NWs.^14,24 In this experiment, we added to bmim[Br] in the polyol synthesis to observe the effect of bromide ions on the diameter of the Ag NWs. As shown in Fig. 3(A), when a small amount of bmim[Br] (0.03 mol) was added in the polyol synthesis, Ag NWs were produced with an average diameter of 42 nm. The addition of 0.05 mM bmim[Br] reduced the diameter of Ag NWs to 25 nm (Fig. 3(B)), and the addition of 0.12 mM bmim[Br] reduced the diameter of Ag NWs to 22 nm (Fig. 3(C)). The smallest Ag NWs (18 nm) were produced by further increasing the concentration of bmim[Br] to 0.25 mM (Fig. 3(E)), but this resulted in the formation of many nanoparticles, thus reducing the wire yield to 50% or less. These experimental results are presented in Table 1 along with the SPR spectrum data of each sample. However, the diameter of Ag NWs was more effectively controlled by the presence of mixed salts (bmim[FeCl₄] and bmim[Br]) in the polyol process. Here, the MIL, bmim[FeCl₄], significantly contributed to the synthesis by acting as a support template for wire growth and/or seed crystal formation. In particular, it can be determined that the wire diameter was affected more than the wire length. MIL solutions appeared to act more effectively as a size-controllable template within the polyol process. Note that when ammonium-based ILs were used as soft templates in previous research,⁹ wires with an average diameter of 30 nm have been reported, which are thicker than wires reported in this study (20 nm).

Fig. 3 (A) SPR absorption characteristics of synthesized 42 nm Ag NWs (a), 25 nm samples (b), 22 nm samples (c), 20 nm samples (d), and 18 nm samples (e). (B) High-resolution SEM images of synthesized Ag NW products obtained from different concentrations of bmim[Br]: (a) 0.03, (b) 0.05, (c) 0.12, and (e) 0.25 mM. Concentrations of AgNO₃, PVP, and bmim[FeCl₄] in each of these reactions were 300 mM (25 mL), 300 mM (35 mL), and 3 mM (2 mL), respectively, and the reaction temperature was 170 °C.

Table 1 Summary of experimental results: average diameters and SPR peak positions of Ag NW products obtained from different concentrations of bmim[Br]. Samples a, b, c, d, and e are 0.03, 0.05, 0.12, 0.18, and 0.25 mM bmim[Br], respectively

Sample	bmim[Br] (mM)	Average diameter (nm)	SPR peak (nm)
a	0.03	42	351, 375
b	0.05	25	352, 368
c	0.12	22	354, 366
d	0.18	20	354, 365
e	0.25	18	361

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Fig. 4 shows an SEM image of as-synthesized Ag NWs with an average diameter of 22 nm and an aspect ratio of up to 1000. Fig. 4(D) and (E) show the distributions of the diameter and length of synthesized wires, respectively. SEM images show that the diameter of each individual wire is uniform along its length, with a narrow size distribution of 15–30 nm diameter (average diameter = 22 nm) and a length of 15–35 μm (average length = 23 μm). The SPR spectrum, which was used to identify optical characteristics of synthesized wires, is shown in Fig. 4(C). The SPR data demonstrate that Ag NWs exhibit various optical phenomena, such as absorption and light scattering, specifically related to SPR, with characteristics that are dependent on the geometry and size of metal nanostructures.^25,26 In the present study, in the SPR band of as-synthesized Ag NWs, two characteristic peaks were observed at wavelengths of 354 and 366 nm, indicative of small-diameter wires (average 22 nm). These SPR peek positions became blue shifted to 365 nm and 361 nm when the diameter of Ag NWs reduced to 20 nm and 18 nm, respectively, as shown in Fig. 3 and Table 1. Accordingly an SPR data can be used to identify the size and shape of Ag NWs by examining bands that appear in a particular wavelength. Thus, these SPR peaks can be attributed to the transverse modes of 1D Ag NWs with pentagonal cross-sections, which correspond to out-of-plane quadrupole and dipole resonance modes. Smaller Ag NWs, as well as to provide a blue-shift in the peaks may also reduce the amount of light scattering. In transparent electrode film, the optical property due to the less scattering light may be advantageously used making excellent transparent conductor. Fig. 5 displays the (B) HRTEM images of the tip of an individual pentagonal Ag NW with a diameter of 19 nm capped with a PVP layer less than 1.8 nm thick, in which the wires were determined to grow along the [110] direction and (C) SAED pattern of the Ag NW with a twinned structure. The X-ray diffraction pattern taken from the sample prepared in TPA indicates that the crystal structures of these wires were face-centered cubic (fcc) in Fig. 5(A), and it is seen that all diffraction peaks can be indexed according to the fcc phase of Ag. It is worth noting that the intensity ratio of the reflection at [111] and [200] exhibits relatively high values, indicating the preferred [111] orientation of the wires. The longitudinal axis was oriented along the [110] direction. In particular, as shown in TEM image, the thickness of PVP as the surface-capping reagents of Ag NWs has been controlled at 1.8 nm or less in order to obtain an optimum thickness that can ensure stability at the same time while maintaining the electrical conductivity of 19 nm diameter Ag NWs.

Fig. 4 (A) SEM images at low magnification (×2000) and high magnification (×70 000) of Ag NWs with an average diameter of 22 nm and length of 23 μm directly coated on an O₂-plasma-treated polyester substrate using a Meyer bar. (B) Ag NW sample solution dispersed in water and (C) it SPR spectrum. (D) Diameter distribution of as-synthesized Ag NWs and (E) length distribution of the same samples.

Fig. 5 (A) XRD pattern of the Ag NWs. (B) TEM images of the tip of an individual pentagonal Ag NW with a diameter of 19 nm capped with a PVP layer less than 1.8 nm thick and (C) SAED pattern of the Ag NW with a twinned structure.

As-purified Ag NWs, which were purified as previously detailed, with an average diameter of 22 nm and average length of 23 μm were suspended in DI water at an optimized density of 0.5 mg mL⁻¹ using ethyl cellulose. These NWs were then directly coated on an O₂-plasma-treated polyester (PET) substrate film using a Mayer bar to fabricate a transparent electrode film. The density of deposited Ag NW was controlled by the volume of coated solution, determined by the speed of the spin coater or by the mesh size (# 3, 5, 9, and 16) of the Mayer bar. The mesh size of the Mayer bar was determined depending on the thickness of the required coating film layer. Fig. 4(A), a SEM surface image of an Ag NW film coated with a Meyer bar (#3) using the Ag NW suspended solution (Fig. 4(B)) are shown. Ag NWs networked electrode films were obtained without any post-treatment.

Fig. 6(A) shows the light transmittance spectrum of Ag NWs networked films measured from the sample having a surface resistance value of 30 Ω sq⁻¹. As shown in Fig. 6(A), this 2D coated film exhibited excellent light transmittance of up to 95% (PET film based). These electrical and optical data almost match the data of ITO on glass (i.e., a sheet resistance as low as 30 Ω sq⁻¹, transmittance of 95%, and haze value as low as 1.0, from literature data²⁷). Fig. 6(B) and (C) show the plots of specular transmittance (%T, λ = 550 nm) and haze, respectively, versus sheet resistance in the range of 5–100 Ω sq⁻¹ for Ag NW films, in addition to data presented in literature results^9,27 for transparent conductors.

Fig. 6 (A) UV transmittance spectrum of Ag NW film with average diameters of 20–23 nm obtained at 30 Ω sq⁻¹. (B) Plot of specular transmittance (λ = 550 nm) vs. sheet resistance for Ag NW films. Error bars show one standard deviation for 5–10 measurements. The performances of ITO²⁷ and 30 nm-diameter Ag NWs⁹ are shown for comparison.

As shown in Fig. 6(B) and (C), the Ag NW film exhibited a high light transmittance of up to 95% and a low haze value of ≤0.6 (PET film base), for samples with a sheet resistance of more than 30 Ω sq⁻¹. Even at an ultra-low resistance of 15 Ω sq⁻¹, the excellent optoelectronic performance (transmittance of 91% and haze of 1.5%) is shown. The light transmittance of the optical property data was superior by 5–10% to that of 30 nm Ag NW samples,⁹ as shown in the curve of Fig. 6. Such an improvement in transmittance and haze is attributed to the effects of NW diameter. Overall, it is observed that the network films comprising small-diameter NWs exhibited superior optical properties compared with those comprising larger-diameter NWs. As a result, 2D Ag films formed by a network of approximately 20 nm-sized NWs can act as sufficiently transparent electrode films for electronic devices, owing to the low intensity of scattered light, compared with ITO.

Conclusion

The introduction of bmim[FeCl₄] into the polyol synthesis enables the production of Ag NWs with an aspect ratio of up to 1000 and diameter of 20 nm. Here, bmim[FeCl₄] acted as a soft template for synthesized Ag seed particles and wires; bmim[FeCl₄] also acted as an ionic salt. Dimensions of Ag NWs, particularly the diameter, are largely affected by the concentration of bmim[FeCl₄]. The diameter of Ag NWs produced by a process can be effectively controlled by adjusting the concentration of bmim[Br]. Moreover, the results reported here support the belief that self-assembled local structures exist in imidazolium-based ILs, bmim[FeCl₄]. In addition, bmim[FeCl₄] can be a promising new reaction medium in the synthesis of metal NWs when using the polyol process. When transparent electrode films were coated with an ethyl-cellulose-based ink solution to form Ag NW networked casting films, these electrode films exhibited a transmittance of 91% and haze value of 1.5% at a sheet resistance of 15 Ω sq⁻¹, sufficiently comparable with ITO.

Acknowledgements

This work was financially supported in part by the Converging Research Center Program through the Ministry of Science (ICT and Future Planning) (2013K000201), and the Industrial Core Technology Development Project through the Ministry of Knowledge and Commerce (10035644 and 10035648).

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