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Purification of Copper Nanowires To Prepare Flexible Transparent Conductive Films with High Performance Chenxia Kang, Sanjun Yang, Min Tan, Chenhuinan Wei, Qiming Liu, Ju Fang, and Gang Liu ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00326 • Publication Date (Web): 15 Jun 2018 Downloaded from http://pubs.acs.org on June 19, 2018

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ACS Applied Nano Materials

Purification of Copper Nanowires To Prepare Flexible Transparent Conductive Films with High Performance Chenxia Kang, Sanjun Yang, Min Tan, Chenhuinan Wei, Qiming Liu*, Ju Fang, Gang Liu Key Laboratory of Ariticial Micro- and Nano-structures of Ministry of Education, School of Physics and Technology, Wuhan University, Wuhan 430072, China *Email: [email protected]

KEYWORDS copper nanowires, water-hydrophobic system, purification, high performance, flexible transparent conductive films ABSTRACT Copper nanowire (Cu NW) films have been widely studied due to their potential applications in optoelectronic devices, but there are rare reports of high-performance NW films without nanoparticles (NPs). In this paper, a water-hydrophobic organic solvent system was used to efficiently separate NPs from NWs. As a result, the transmittance of the purified Cu NW/PET films improved approximately 3% compared with the unpurified Cu NW/PET films at a given sheet resistance. And the high-quality Cu NW/PET films have a transmittance of 97.61% at 102.9 Ω sq-1. A detailed mechanism of purification is also proposed. This facile purification method is highly efficient and widely applicable to improve the performance of flexible transparent conductive films, which will meet the need of commercial applications of NW films with excellent optoelectronic performance.

1. INTRODUCTION Transparent conducting films (TCFs) are one kind of important components of optoelectronic devices such as displays,1-3 touch-screens,4 organic photovoltaics (OPVs),5-6 light emitting diodes (LEDs),7-8 sensors,9-12 and so on. Currently, tin-doped indium oxide (ITO) is the most widely used material in the field of transparent conductors because of its outstanding conductivity (Rs = 50 Ω sq-1) and optical transmittance (T = 95%).8 However, there are some disadvantages that limit the further development of ITO, including the scarcity of indium, its inherent ceramic brittleness and the tedious sputtering process. Thus, many researchers have committed to develop novel alternative materials which are economical and produced with solution-phase coating processes.

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Recently, several materials have been proved that their optoelectronic properties are equal to or even better than those of ITO, such as carbon nanotubes,13-15 graphene,16-17 conducting polymers,18-19 and metal nanowires.8,

20-26

However, metal nanowires films with excellent

optoelectronic and flexible properties are particularly promising because of their economical solution-phase synthesis and simple manufacturing process. Although solution synthesis is a popular method to synthesize metal nanowires, some by-products such as nanoparticles and low-aspect-ratio nanorods are inevitable to be introduced in the production process. These unwanted morphologies are usually formed on account of the non-instantaneous nucleation and diffusion-limited growth, which give rise to the growth of particles along multiple pathways.27-28 The presence of these side products can seriously impact the performance of nanowire networks. For example, the optoelectronic properties of metal nanowires flexible transparent conductive films are greatly damaged by these nanoparticle impurities because they not only have no contributions to the electrical conductivity but also decrease the film transparency due to their fairly strong light scattering properties.28-30 So, it is urgently needed to find out a simple and feasible purification method to prepare high-performance transparent electrodes. There are several separation techniques to remove nanoparticle impurities, including centrifugation,31 filtration,28 selective precipitation,32 and so on. For example, Liu et al. reported a facial centrifugation method to separate Au nanowires (NWs) and nanoparticles (NPs) in a single organic phase,31 which is very useful to purify metal nanomaterials with different shapes. Pradel et al. published a cross-flow filtration method to purify nanowires using hollow-fiber membranes,28 and the effect of separation is mainly determined by the size of the nanowires and the fiber membrane. Recently, Wiley et al. developed a selective precipitation method to purify Ag NWs.32 In this method, Ag NWs were coated by PVP which is insoluble in acetone. Thus, the nanowires could aggregate and deposit to the bottom of the flask to achieve the purpose of purification. Despite the usefulness of the above purification methods, a facile, widely applicable separation method and also the understanding of the operation mechanism behind it are still in great need. Moreover, these reports are mostly focused on the purification of gold and silver nanowires, but rarely reported on copper nanowires. Copper with nearly equal electrical conductivity to silver is 100 times cheaper and 1000 times more abundant than silver.26, 33 The flexible transparent conductive films based on Cu NWs


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have been widely studied due to their excellent performances and low price. For example, Rathmell et al.21, 33 reported the growth mechanism of nanowires with spherical particles and prepared the Cu NW films. The spherical particles at one end of the NWs are difficult to be removed, which will reduce the optoelectronic properties of the film. Zhao et al.30 developed a facile method to prepare stable r-GO/Cu NW hybrid TCFs under mild conditions, but the optoelectronic performances (T = 82%, Rs = 50 Ω sq-1) compared with ITO are not yet satisfactory. Gou et al.34 demonstrate a method with nickel ions to synthesis high aspect ratio Cu NWs and prepared the Cu NW film (T = 93%, Rs = 51 Ω sq-1) with Cu NPs. Although the Cu NW films have been extensively investigated, there are rare reports of high-performance NW films without nanoparticles (NPs). Herein, we demonstrate an efficient and low-toxicity separation method to purify Cu NWs synthesized by a modified hydrothermal reaction using octadecylamine (ODA) as a capping agent. We also proposed a possible separation mechanism based on the different crystal structures of Cu NPs and Cu NWs and their different adsorption capacities for capping agent. Based on a water-hydrophobic organic solvent (e.g. n-hexane) system, this purification method can facilely produce large quantities of high-purity Cu NWs, which were subsequently dispersed in isopropanol and then transferred to a polyethylene terephthalate (PET) substrate by vacuum filtration. The as-prepared Cu NW/PET films display excellent electro-optical performances (e.g. T = 97.61%, Rs = 102.9 Ω sq-1) after glacial acetic acid treatment. The purification of Cu NWs increases the transmittance by 3% under a given sheet resistance.

2. EXPERIMENTAL 2.1 Synthesis of Cu NWs Copper (II) chloride dihydrate (CuCl2·2H2O), glucose, octadecylamine (ODA), glacial acetic acid (GAA), n-hexane and isopropanol (IPA) were purchased from the Sinopharm Chemical Reagent Co., Ltd. All of the analytical grade chemicals were used without further purification. Cu NWs were synthesized using a simple hydrothermal method as previous reports.35-38 Typically, ODA (1.294 g) was slowly added to the 60 ml mixed aqueous solution of glucose (0.1585 g) and CuCl2·2H2O (0.1364 g) under magnetic stirring and then vigorously stirred for 6 h at room temperature until the solution forms a uniform blue emulsion. The formed emulsion was placed in



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a Teflon-lined autoclave of 100 ml capacity and kept for 24 h at 120 ℃. After the reaction was finished, cooling down the autoclave to room temperature naturally and then the solution with a reddish-brown color was decanted into a vessel. 2.2 Separation of Cu NWs and Cu NPs After synthesis, the Cu NWs were purified by adding a non-polar organic solvent (e.g. n-hexane) to the synthesized solution. Firstly, the products were obtained by centrifugation at 2.5 K rpm for 5 min and washed with deionized water for three times. The above supernatant containing excess ODA was carefully pipetted away and the precipitate below was re-dispersed into deionized water. Subsequently, adding n-hexane to the above solution and then vortexing for about 10 seconds. After vortexing, it was obviously observed that the n-hexane and water separated with a distinct interface. The Cu NWs could completely settle to the bottom of the n-hexane within 20 min. Then the above n-hexane containing lots of organics was discarded and the Cu NWs at the bottom of n-hexane were transferred to another 50 mL of centrifugal tube along with about 10 mL deionized water. Then, adding n-hexane to above centrifugal tube and vortexing again. The purified Cu NWs could be obtained after three cycles of this purification process. Finally, the Cu NWs were washed three times with isopropanol and kept in isopropanol for further use. The purification result compared with other purification methods is shown in table 1. Table 1. Comparison of the characteristic and purification results of different purification methods. Purification methods

Characteristic

Purification results

water-hydrophobic

simple, scalable, no size

high-purify,

system

requirement,

good dispersion

centrifugation

time-consuming

Examples our work

low-purity, poor

Ag,28 Au31

dispersion, size selectivity, poorly

low purity, poor

scalable

re-dispersion

selective precipitation

simple, scalable

high-purify

Ag32

gel electrophoresis

need pretreatment

great precision

-

filtration


Ag28

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dialysis

not suitable for nanowire

molecules et

purification

al.

small substances

2.3 Fabrication of Cu NW TCFs First of all, the Cu NWs were sufficiently dispersed into isopropanol with ultrasonic vibration for 30 seconds and then vacuum filtrated onto a Teflon filter (SterliTech, 0.2 µm pore size, 47 mm diameter). After the filtration, the Cu NWs on the filter membrane were kept in air for about 30 min at room temperature for drying. Then, a PET substrate, which was treated with alcohol for 10 min under ultrasonic vibration and dried naturally in air, was placed on the side of the filter membrane with captured Cu NWs and then pressed against the other side of the filter membrane by using a tablet press with 15 M pa pressure for 5 min. Subsequently, the Teflon filter was carefully removed from the PET substrate. The prepared Cu NW/PET TCFs were immersed in dilute glacial acetic acid (GAA: IPA = 1:50) for 2-4 min to remove copper oxides and residual organics, and then immersed the films in pure isopropanol for about 1min to remove the residual acid. Finally, the treated films were dried in air for 10 min at room temperature. 2.4 Characterization X-ray powder diffraction (XRD) result was explored by using an (PANalytical, X’Pert Pro) X-ray diffractometer with Cu Kα radiation (λ =1.5406 Å, 40 kV at 40 mA). The morphologies of Cu NWs and Cu NPs were examined by a FEI Sirion FEG field-emission scanning electron microscopy (SEM) operating at 20 kV and an Olympus BX51 optical microscope. The HR-TEM images and energy dispersive X-ray spectroscopy (EDS) were obtained on a JEM-2010FEF microscope working at 200 kV. JEM-2010HT microscope operating at 200 kV was used to obtain selected-area electron diffraction (SAED) patterns. Transmittance spectra and sheet resistances of Cu NW films were measured with a UV-visible spectrophotometer (Jena, Germany 210PLUS) and a four-point probe (RTS-9) respectively. The haze was measured by a haze meter (CS-700). The thermogravimetric analysis (TGA) curves were acquired using a NETZSCH STA 449C Jupiter with a heated speed at 10 K min-1 in the nitrogen atmosphere.

3. RESULTS AND DISCUSSION As shown in Figure 1a, the color of the solution changed from blue to reddish-brown. It is



indicated that Cu (II) ions were reduced to Cu atoms by heating CuCl2 with ODA and glucose at 120 ℃ for 24 h. After the reaction, the effective separation of Cu NWs and Cu NPs was achieved by using a water-hydrophobic organic solvent (e.g. n-hexane) system. This facile separation process involved adding non-polar organic solvents to the suspended aqueous solution of Cu NWs. Figure 1b and 1c demonstrate the separation schematic diagrams and the corresponding separation procedure pictures respectively. As shown in Figure 1c, initially there are a lot of organics in the reddish-brown original solution. However, the color of the suspension becomes brighter after washing with water, which illustrates that some of the organics have been removed. When adding the n-hexane to the suspended aqueous solution and vortexing, the Cu NWs and Cu NPs are evenly distributed in the mixture of n-hexane and water. Subsequently, settling for about 30 min, a distinct interface of n-hexane and water appeared in the centrifuge tube, and Cu NWs precipitated to the bottom of the n-hexane and Cu NPs settled to the bottom of the water.

Figure 1. (a) Photographs of the solution before and after the reaction. (b) Scheme illustrating the purification process. (c) Pictures displaying the purification steps. Figure 2 shows the SEM images of original Cu nanoproducts, purified Cu NPs and Cu NWs after three cycles of separation. Before purification, the Cu NWs suspension was highly contaminated by a number of Cu NPs (Figure 2a). The unavoidable production of Cu NPs is attributed to the non-instantaneous nucleation which is difficult to control and causes the nanoparticles to grow along multiple pathways.27 In addition, the unwanted NPs have more percentage content than previous report that was performed at 30 ml scale, which is perhaps because the increased scale (60 ml in this work) makes it more difficult to control the nucleation and crystal growth. Figure 2b and 2c display the Cu nanomaterials from water and n-hexane


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independently, demonstrating the high-efficiency of purifying Cu NWs. The curves in Figure 2d show the XRD patterns of both purified and unpurified Cu NWs dropped on a glass slid respectively. They both exhibit two diffraction peaks at 2θ = 43.3° and 50.4°, which correspond to the {111} and {200} planes separately (JCPDS #04-0836). Also, the XRD patterns suggest that the Cu products are of face-centered cubic phase with no impurities (CuO or Cu2O). Moreover, the intensity ratio of (111) and (200) peak in purified Cu NWs is significantly higher than that in unpurified Cu NWs, which is attributed to the presence of a large number of Cu nanocubes in the unpurified Cu NWs (Figure 2a). Cu nanocubes are known to be bound by six {100} facets and the intensity of (200) peak is higher than (111).39-41 Prior to purification, large amounts of Cu nanocubes present in the Cu NWs significantly increase the intensity of (200) peak. After purification, the intensity of the (200) peak is significantly reduced, and the intensity ratio of (111) and (200) peak is significantly increased. These results indicate the effective removal of Cu NPs and the enrichment of {111} facets of the Cu NWs. In addition, a typical EDS spectrum in the top right inset of Figure 2d depicts only the presence of Cu, demonstrating the pure Cu NWs formed. The Ni signal as well as C signal are attributed to Ni net coated with carbon film used in the measurement. Figure S1 shows the UV-visible absorption spectrum of purified Cu NWs dispersed in isopropanol. An absorption peak could be observed at 567 nm, which is consistent with previous reports.34, 42 This peak may result from the plasma excitation in Cu NWs.34, 43

Figure 2. SEM images of (a) Cu NWs before purification, (b) Cu NPs separated from Cu nanowires, and (c) Cu NWs after purification. (d) XRD patterns of Cu NWs before and after purification, top right inset is the EDS spectrum of a Cu NW. The three insets of picture a, b and c



are the enlarged parts of their corresponding pictures. To further investigate the structure of Cu NWs and Cu NPs, the TEM analysis was used. Figure 3 shows TEM images of Cu nanoproducts and corresponding selected area electron diffraction (SAED) patterns. As shown in Figure 3a, the Cu NPs are mainly nanotetrahedrons and nanocubes. The TEM (Figure S2c) and SEM images (inset of Figure 2b) of Cu nanotetrahedrons demonstrated that they were bound by four {111} facets.44 In addition, the SAED pattern of a Cu nanocube as shown in Figure 3b, viewed along the [110] zone axis, reveals that the Cu nanocubes are single crystals with six identical {100} facets.44 Figure 3c shows the schematic models of the Cu nanotetrahedron and the Cu nanocube respectively. The TEM images of Cu NWs, shown in Figure 3d and its inset, confirm that the Cu NWs have five side planes forming a pentagonal pyramid tip. Figure 3e displays the SAED pattern of the Cu NW taken along the [110] zone axis, which is a superimposition of two face-centered cubic (fcc) patterns. This observation confirms that Cu NWs are not single but multiple-twinned crystals, which is consistent with the previous reports.40, 45-48 Figure 3f shows the schematic model of a Cu NW with five {100} side planes and pentagonal pyramid tips covered by five {111} facets. The different crystal structures of Cu NWs and Cu NPs are attributed to that they grown from different crystal seeds. Cu NPs without lattice defects are stemmed from the single crystal seeds.40, 49 Moreover, Cu NWs are derived from five twinned decahedral nanoparticles, which can be regarded as a combination of five tetrahedrons.40, 49

The theoretical angle is 70.5° between two {111} facets of a tetrahedron, leaving a gap of 7.5°

on the top surface of the decahedral seed. Therefore, in this case, the adjacent atoms between the five twin-boundaries will be separated to compensate the gap, which caused internal lattice strain and dislocation.48, 50 Since the energy of the lattice defect zones in twin-boundaries is higher than that of the single crystal regions, the capping agents are more likely to be adsorbed in these defect zones49 and the adsorption is stronger. In fact, as shown in Figure S2a, an obvious organic layer exists on the surfaces of Cu NWs and Cu NPs before the separation. However, after the separation, the amount of organics on the surfaces of Cu NWs and Cu NPs is significantly different. There is still an obvious organic layer on the surface of Cu NWs while almost no organics on the surface of Cu NPs shown in Figure S2(b-d) and 3d. Additionally, as shown in Figure S3, the EDS spectrum of Cu NW shows the C and N signal come from the residual organic matter. However, there is no


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N signal only a weak C signal in the EDS spectrum of Cu NP. The Ni signal is attributed to pure Ni net used in the measurement. These results show that the amount of capping agent on the surface of Cu NW is more than that of Cu NP after the separation. Moreover, it is noteworthy that NWs tend to form a net mixed with more organics while NPs are monodispersed, which may also explain the easy removal of the organic materials on the surface of NPs in the process of separation. This result is similar to the previous report of Ding et al, who proved the clear existence of residual ODA on the surface of NWs.35

Figure 3. The TEM images, corresponding SAED patterns and schematic models of (a-c) Cu NPs (nanotetrahedrons and nanocubes) and (d-f) Cu NWs. The inset of picture d is one of the end of Cu NW. The parts in the white boxes from the insets of picture b and e correspond with their SAED patterns respectively. The amount of residual organic materials on the surface of Cu NWs and Cu NPs can be further explored by thermogravimetric analysis (TGA) (Figure 4a). Cu NWs and Cu NPs are obtained directly after separation from n-hexane and water respectively and both dried at 60 ℃ for 24 h in a vacuum drying oven. Then the samples were heated from room temperature to 600 ℃ at 10 K min-1 under nitrogen atmosphere during the thermogravimetric analysis. As shown in Figure 4a, Cu NPs exhibit a good thermal stability below 250 ℃，and have a low weight loss nearly 8 wt. % ranging from 250 ℃ to 460 ℃. For the Cu NWs, the mass loss is about 18 wt. % between 160 ℃



and 270 ℃ and 28 wt. % from 270 ℃ to 460 ℃. However, pure ODA, as a contrastive experiment, shows a rapid mass loss at around 160 ℃ and is almost exhausted at a temperature of 270 ℃, which is in agreement with previous reports.51 The reason for the abrupt mass loss of pure ODA is the thermal decomposition and carbonization of ODA.51-52 In addition, the mass loss of the Cu NWs from 160 ℃ to 270 ℃ was similar to that of pure ODA, indicating the decomposition of the physically adsorbed ODA molecules between nanowires.52 The SEM image of Cu NWs (Figure 2c) can confirm the presence of physically adsorbed ODA molecules between nanowires. Figure S4, obtained after the thermogravimetric analysis, shows that there is no residual ODA on the surface of Cu NWs and the shape of Cu NWs has been broken compared with Figure 3d. So, at higher temperatures, the weight loss is due to the chemically bonded ODA molecules on the surfaces of the Cu NWs and Cu NWs themselves.53-54 In the temperature ranging from 270 ℃ to 460 ℃, the mass loss of Cu NWs is as much as 20% higher than that of Cu NPs, demonstrating that the residual organic materials on the surface of NWs is more than that of NPs after the separation, which is consistent with the results of TEM analysis. This difference in mass loss between NWs and NPs is attributed to the lattice distortions existing in the Cu NWs which lead to the stronger adsorption of organics to the lattice defect zones and the formation of the nanocrystal-polymer conjugates.41 Therefore, it is more difficult for the polymer to escape from twin-boundaries compared with Cu NPs. After the temperature of 460 ℃, the mass loss may result from the evaporation of Cu itself. Firstly, due to the atoms on the surface and interface of the nanocrystal have incompletely coordinated dangling bonds, the melting points of NWs and NPs are different from that of conventional bulk materials and reduced with decreasing size.55-56 Secondly, the different evaporation capacity of Cu NWs and Cu NPs is ascribed to the lattice defects. For Cu NWs, Cu atoms are easy to escape from the surface at high temperatures because of the lattice defects with higher surface energy.40, 49 On the contrary, the surface atoms of Cu NPs without lattice defects are more stable and difficult to be removed when they are heated at high temperatures. Figure 4b and 4c display the morphologies of Cu NWs and Cu NPs respectively after thermogravimetric analysis (TGA). As shown in the inset of Figure 4b, the shape of Cu NWs has been broken and they become thinner and flocculent due to the escape of Cu atoms at high temperatures. Figure 4c shows the melted Cu NPs with irregular shape. The inset of Figure 4c displays that the surface of the nanoparticles are still smooth, demonstrating that fewer Cu atoms


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escaped from the surface of Cu NPs, which gives rise to the slight mass loss of the Cu NP sample (Figure 4a).

Figure 4. (a) TGA curves of Cu NPs, Cu NWs and pure ODA. Optical images of (b) purified Cu NWs and (c) separated Cu NPs after TGA. The two insets of picture b and c are the SEM images of their corresponding pictures. To better understand the separation process of NWs and NPs, we propose the following possible mechanism. Figure 5a shows the schematic models of Cu NWs and Cu NPs as well as the structural formula of an ODA molecule. It has been reported that the ODA molecule is amphiphilic, which has a long hydrophobic alkyl tail and a hydrophilic amino head.51, 53. In the Figure 5a, the orange ball represents the hydrophilic head of an ODA molecule, and gray curve represents its hydrophobic tail. In the process of synthesis, the hydrophilic heads adsorbed on the surface of Cu nanomaterials, while the hydrophobic tails were exposed to the solution, forming a hydrophobic and single molecule membrane.57 In the separation process, the strong attraction between hydrophobic tails and n-hexane molecules makes the Cu nanostructures covered by hydrophobic molecule membrane incline to stay in n-hexane. At first, the n-hexane and water are phase-separated for their immiscibility and the n-hexane is at the top of the centrifuge tube due to its lower density (Figure 5(b1)). Then when sufficiently vortexing the mixture of n-hexane and water containing Cu nanoproducts, the Cu nanoproducts tend to stay in n-hexane as shown in Figure 5(b2). This phenomenon results from the interaction between the hydrophobic tails of ODA and the n-hexane as well as water molecules. Figure 5(b3) shows that only a few of organics on the surface of Cu NWs dissolved into the n-hexane when shaking the solution. This is attributed to the existence of twin defects in NWs and the formed NWs meshes. The former factor leads to the stronger adsorption of twin boundaries to the capping agents

49

and the latter factor prevent the

residual organics from contacting with the n-hexane directly. On the contrary, due to the weak adsorption of NPs to organic materials and their monodispersity that makes them contact fully



with organic solvents, the residual organic materials on the surface of Cu NPs are almost completely removed by n-hexane when shaking the solution. This is consistent with the TEM images (Figure S2) of Cu nanoproducts, which clearly show the concentration change of residual ODA on the surface of Cu NWs and Cu NPs before and after the addition of n-hexane. Next, when n-hexane reassembled above water, the force between n-hexane and residual organics drives the NWs to overcome gravity into the n-hexane (Figure 5(b4)). This result is consisted with the previous report of Li et al, who displayed the different solubility of GO and GO-ODA in CHCl3/H2O mixture.53 For Cu NPs, the oxygen atoms of the water molecules bond with the bare Cu atoms on the surface of NPs to form an aqueous film. The NPs surrounded by water are repulsed by n-hexane and consequently located in water. Settling for about 30min, NWs and NPs deposited at the bottom of n-hexane and water respectively because of gravity (Figure 5(b5)). This purification process is mainly based on the different surface properties of NWs and NPs, and the separation result is highly repetitive. This separation mechanism is different from that proposed by Qian et al.,58 who reported that the separation of NWs and NPs depends on the distinguished facet difference and the total surface area.

Figure 5. Schematic models (a) and the proposed separation mechanism (b1 - b5) of Cu NWs and Cu NPs. To verify the universality of the proposed mechanism, we conducted experiments using other hydrophobic (e.g. toluene and dichloromethane) organic solvents. As shown in the insets of Figure 6a and 6c, toluene and dichloromethane are immiscible with water to form phase separation. The ideal separation effect of Cu NWs and Cu NPs was achieved in toluene (Figure 6a and 6b) and dichloromethane (Figure 6c and 6d). For toluene, it is similar with n-hexane that the Cu NWs are completely located in the upper toluene and the Cu NPs are completely dispersed in the lower water when purifying the Cu NWs (the inset of Figure 6a). Unlike n-hexane and toluene, dichloromethane is under the water due to its higher density. However, the similar separation effect of Cu NWs and Cu NPs is realized. As shown in the inset of Figure 6c, Cu NWs are


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observed at the lower dichloromethane while the Cu NPs appear in the upper water. This is attributed to the attraction between the dichloromethane molecules and the hydrophobic ends of ODA molecules on the surface of Cu NWs and the repulsion between the dichloromethane and water molecules. The optical images of Cu NPs and Cu NWs separated by dichloromethane are shown in Figure 6c and 6d, demonstrating that the dichloromethane produces equally high-quality Cu NWs compared with n-hexane and toluene. These phenomena further illustrate that the separation of Cu NWs and Cu NPs is not due to their possible difference of density but the molecule polarity of organics. Results indicate that n-hexane is not the only solvent to purify Cu NWs, and other commonly hydrophobic organic solvents can also achieve the same separation effect, demonstrating the universality of this separation method.

Figure 6. Optical images of Cu NWs and Cu NPs separated by toluene (a, b) and dichloromethane (c, d) respectively. The two insets of picture a and c are the photographs of purifying Cu NWs using toluene and dichloromethane respectively. Purified Cu NWs were well-dispersed in isopropanol and vacuum filtrated on the Teflon filter (0.2 µm pore size), followed by transferring to the PET substrate at room temperature. Since Teflon has low surface energy, the Cu NWs can be easily transferred on the target substrate.59 The as-prepared Cu NW films possess poor conductivity because some residual organics and copper oxides on the surface of Cu NWs impede the transfer of electrons between the NWs. Traditionally, high temperature treatment under reducing or inert atmosphere can remove both chemisorbed organics and surface oxides, improving the conductivity of the films. But for most flexible substrates, poor heat resistance is a fatal weakness, and thin nanowires tend to melt at



temperatures exceed 250 °C33. So we used a simple chemical treatment method to remove the residual organics and surface oxides effectively without damaging the underlying Cu NWs.37, 60-62 The conductivity of Cu NW films had a large improvement after treating them with diluted glacial acetic acid (GAA).

Figure 7. Typical SEM images of films prepared by (a) purified and (b) unpurified Cu NWs. The two insets of picture a and b are the enlarged parts of their corresponding pictures. (c) Plot of optical transmittance (at 550 nm) vs sheet resistance for purified and unpurified Cu NW films, as well as other representative transparent conductive materials (Ag NWs,63 ITO,8 Cu NWs,64 CNT,65 PEDOT，66 graphene17 and Au NWs67-68). The error bars reveal the standard deviation of six measurements. Figure 7a and 7b show the SEM images of purified and unpurified Cu NW films prepared by a vacuum filtration method. From Figure 7b, it is clear that many nanoparticles exist in the Cu NW network. Figure 7c displays a plot of the transmittance (%T) at λ = 550 nm versus sheet resistance (Rs) for acetic acid-treated films of Cu NWs before and after purification, as compared to some representative results of previous literatures for solution-coated transparent conducting films. The optical transmittance of purified Cu NW films was about 3% higher than that of non-purified Cu NWs films under the same resistance. This is chiefly because the nanoparticles presenting in nanowire network can act as the center of light scattering to decrease light transmittance, and they have no contributions to the electrical conductivity due to their smaller size with poor connection on the existing nanowire network.28-29, 44 Moreover, in Figure 7c, it can be clearly seen that the performance of films made of purified Cu NWs is better than that of other transparent conductive films at sheet resistances >45 Ω/sq. As shown in Figure S5a and S5b, the purified Cu NW film not only has a higher transmittance at the same resistance but also has a lower haze at the same transmittance compared with non-purified Cu NW film. The haze factor is used to quantify the light scattering efficiency of transparent conducting films and strongly


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depends on the network density.37, 69 Due to NPs have a stronger light scattering effect, NPs contribute more to the haze factor. Furthermore, the figure of merit (FOM),8, 70 generally used to assess the optoelectronic performance of TCFs, is investigated as shown in Figure S5c. The FOM of purified Cu NW films is higher than that of non-purified Cu NW films at the given transmittance, which is also attributed to that the NPs do not contribute to conductivity but also cause light scattering. These results further suggest that the purification method used in this work is beneficial to effectively improve the optoelectronic performance of transparent conductive films.

Figure 8. (a) A plot of sheet resistance vs bending radius shows no change after curving along different radius. (b) A plot of sheet resistance vs number of bending cycle displays excellent stability of the Cu NW films after 1000 bends. The two insets of picture a and b are the schematic diagram that the Cu NW films bend along cylinders with different radius and a particular cylinder (R = 16 mm) respectively. (c) Plots of sheet resistance and transmittance vs time in days suggests the stability of the Cu NW films in air at room temperature. (d) Picture of an electrical circuit with an electrical and flexible Cu NW film, the inset shows a Cu NW/PET film on a paper with “Copper Nanowire Films”. Additionally, we investigated the flexible performance of purified Cu NW/PET films by the bending test. Figure 8a shows the sheet resistance evolution with the bending radius by curving the Cu NW film (T = 83.99%, λ= 550 nm) along cylinders with different radius from 0° to 180°. The sheet resistance of the film has no obvious change after curving along different radius. In



addition, when curving the film along a particular cylinder (R = 16 mm), the sheet resistance remained remarkably stable even after 1000 bending cycles as shown in Figure 8b. To explore the anti-oxidation performance of the Cu NW films, we measured the sheet resistance and transmittance of the films placed in air at room temperature over 24 days. As shown in Figure 8c, the sheet resistance of the Cu NW films increased by only 6.5 Ω/sq (from 12.9 Ω/sq to 19.4 Ω/sq), keeping superior stability over this period of time. And the transmittance of the Cu NW films has almost no change over 24 days. These results suggest that the Cu NW films in air are not so easily oxidized. Moreover, we also investigated the electrical and flexible performance by assembling an electrical circuit with a LED. As shown in Figure 8d, when the voltage was applied, the LED light can be lit up with the film under bending conditions, suggesting the excellent bending properties of the prepared transparent conductive films. The inset of Figure 8d shows the highly optical transmittance of the purified Cu NW/PET films. These results indicate that Cu NW/PET films have good optoelectronic and flexible performances which can meet the requirements of commercial applications.

4.

CONCLUSIONS

In summary, we synthesized high aspect ratio Cu NWs by a simple hydrothermal method and achieved the highly efficient separation of Cu NWs and Cu NPs by using a water-hydrophobic organic solvent system. This purification method is simple and scalable with no requirement on the size and morphology of NPs. The separation process is analyzed in detail and attributed to the different crystal structures of Cu NWs and Cu NPs. Other hydrophobic organic solvents are used in this work to demonstrate the general applicability of this purification method. A vacuum filtration method was used to make purified Cu NW flexible transparent conducting films on PET with a transmittance of 97.61% at 102.9 Ω/sq-1. At a given sheet resistance, the transmittance of the purified Cu NW films improved about 3% compared with unpurified Cu NW films. The purification process mentioned in this report can be likely applied to other nanowire films with high optoelectronic performance for practical applications.

ASSOCIATED CONTENT Supporting Information


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In this supporting information, additional TEM images, EDS spectrums, haze factor, and FOM are introduced.

AUTHOR INFORMATION Corresponding Author *Email: [email protected] ORCID Qiming Liu: 0000-0001-6470-1656

Funding The National Nature Science Foundation of China (51572202) and the National Nature Science Foundation of Jiangsu Province (BK20171234).

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This research work was financially supported by the National Nature Science Foundation of China (51572202) and the National Nature Science Foundation of Jiangsu Province (BK20171234).

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