TiO2-Coated Core–Shell Ag Nanowire Networks for Robust and

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Article Cite This: ACS Appl. Nano Mater. 2019, 2, 2456−2466

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TiO2‑Coated Core−Shell Ag Nanowire Networks for Robust and Washable Flexible Transparent Electrodes Yilong Huang,† Yanhong Tian,*,† Chunjin Hang,† Yubin Liu,‡ Shang Wang,† Miaomiao Qi,† He Zhang,† and Qiqi Peng† †

State Key Laboratory of Advanced Welding and Joining and ‡State Key Laboratory of Robotics and System, Harbin Institute of Technology, Harbin 150001, China

ACS Appl. Nano Mater. 2019.2:2456-2466. Downloaded from pubs.acs.org by ALBRIGHT COLG on 05/02/19. For personal use only.

S Supporting Information *

ABSTRACT: Silver nanowires (AgNWs) are the most promising materials to fabricate flexible transparent electrodes (FTEs) that are used in next-generation electronics. However, there are several bottlenecks for AgNW-based FTEs to achieve large-scale applications, which include the thermal instability and rough surface topography of AgNWs and the poor interfacial adhesion between AgNWs and the used substrate. To simultaneously address these aforementioned issues, a robust and washable FTE is prepared based on a AgNW@TiO2 core−shell network embedded in polyimide (PI) substrate through a facile and scalable solution-based process. After being treated with TiO2 sol, an ultrathin, conformal, and continuous TiO2 shell is coated on the AgNWs, which can effectively suppress the atomic surface diffusion. In comparison with a pristine AgNW network that breaks into nanorods and nanospheres at 250 °C for 10 min, the AgNW@TiO2 core−shell network is stable at 300 °C, and its resistance just increases by a factor of 11 after being annealed at 400 °C for 1 h. Furthermore, the TiO2 shell simultaneously increases the electrical and optical properties of the AgNW network. After PI precursors are flowed, dried, and thermally cured, the AgNW@TiO2 core−shell network is embedded on the surface of the PI substrate with a surface roughness of 1.9 nm. In addition to high thermal stability, the conductivity of the AgNW@ TiO2−PI composite FTE remains almost unchanged after repeated peeling off cycles with 3M tape and mechanical bending cycles. It is also demonstrated that the AgNW@TiO2−PI composite FTE is washable, and the relative change in resistance (ΔR/R0) is ∼12% after 100 washing cycles, in which a variety of stress situations occur in combination. KEYWORDS: AgNW@TiO2 core−shell network, TiO2 sol, flexible transparent electrodes, peeling off and mechanical stabilities, thermal and washing stabilities

1. INTRODUCTION In recent years, flexible transparent electrodes (FTEs) have become a focus of considerable research activities because they are an essential component for next-generation electronics, such as flexible organic light-emitting diodes (OLEDs), deformable heaters, and wearable sensors.1−4 In this context, it is required to explore FTE materials with excellent electrical conductivity, high optical transparency, and mechanical compliance. So far, several materials, including graphene,5 carbon nanotubes,6,7 metal grids,8 copper nanowires,9,10 and silver nanowires,11 were intensively investigated to prepare FTEs. Among them, silver nanowires (AgNWs) synthesized by a modified polyol method are considered as the most promising materials in the fabrication of FTEs because the random AgNW network has desirable electrical, optical, and mechanical properties.12,13 In practice, the AgNWs were usually subjected to high processing temperature, mechanical scratching, and constant Joule heating in the applications of electronics, such as heaters and OLEDs.14−18 Thus, the poor thermal stability, rough surface topography of the AgNW network, and weak bonding © 2019 American Chemical Society

force between AgNWs and a substrate restrict the long-term and practical usage of AgNW-based FTEs.1,18,19 It has been reported that embedding a AgNW network within a polymer substrate can reduce the surface roughness below 2 nm and dramatically enhance the interfacial adhesion of AgNWs to a substrate.20−25 In a typical preparation procedure, AgNWs were coated on a smooth release glass substrate at first; then an acrylate monomer solution was infiltrated in the open spaces of the AgNW network; after being cured and lifted off, the AgNW network was inlaid on the surface of the polymer substrate.20 However, even if the AgNW network was embedded within a thermostable polymer matrix, the AgNW−polymer composite film exhibited increased resistance at a temperature above 250 °C.26−30 The increase in resistance is due to the nanoscale size effects of AgNWs themselves, causing the breakage of nanowires at a temperature much lower than the melting temperature of bulk silver material.31 Current approaches to Received: February 21, 2019 Accepted: March 28, 2019 Published: March 28, 2019 2456

DOI: 10.1021/acsanm.9b00337 ACS Appl. Nano Mater. 2019, 2, 2456−2466

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ACS Applied Nano Materials Scheme 1. Fabrication Routes of Robust and Washable FTEs

hexafluoropropane, and N,N-dimethylacetamide were provided by Aladdin. 2.2. Preparation of Robust and Washable FTEs. The fabrication routes of robust and washable FTEs are shown in Scheme 1. Initially, a commercially available AgNW solution with a concentration of 10 mg/mL was diluted by IPA to a concentration of 1 mg/mL. After being centrifuged at 6500 rpm for 10 min, the AgNW precipitates were collected, which were subsequently redispersed in IPA to a concentration of 1 mg/mL, followed by centrifugation at 6500 rpm for 10 min. After that, the washed AgNWs were dispersed in IPA at a concentration of 5 mg/mL. Before the prepared AgNW solution was used, it was vortexed at 1000 rpm for 1 min. Then, the AgNW ink was coated on cleaned glass using a Mayer rod, followed by annealing at 150 °C for 10 min. To prepare TiO2 sol, 0.03 mL of titanium isopropoxide and 0.25 mL of hydrogen peroxide were dispersed in 4.75 mL of DI water. After being stirred at 400 rpm for 2 h, the TiO2 sol became a clear yellow solution. To increase the wettability of TiO2 sol and thus rapidly spread on AgNW/glass, the TiO2 sol was diluted 8 times with IPA. Then, the diluted TiO2 sol was added dropwise on AgNW/glass, followed by spin coating at 2000 rpm for 1 min to remove the excess solution. After being annealed at 150 °C for 10 min, a TiO2 protective shell was coated on the AgNW surfaces. A mixture of 4,4′-oxydiphthalic anhydride and 2,2-bis[4-(4aminophenoxy)phenyl]hexafluoropropane at a molar ratio of 1:1 was dissolved in N,N-dimethylacetamide at a weight percentage of 10%. After being stirred at 600 rpm for 1 night, a viscous PI precursor solution was obtained. The PI precursor solution was flowed on the prepared AgNW@TiO2/glass using a doctor blade. A 0.5 mm air gap was set between the doctor blade and the glass substrate. To cure the PI precursors, it was sequentially annealed at 60 °C for 30 min, 100 °C for 30 min, 200 °C for 1 h, and 250 °C for 1 h. After being lifted off, a free-standing AgNW@TiO2−PI composite FTE was obtained. 2.3. Characterization. The sheet resistance of samples was measured by a Loresta AX MCP-T370 four-probe tester from Mitsubishi Chemical Analytech Co., Ltd. A Jinhua UV1800PC ultraviolet−visible spectrophotometer was used to measure the transmittance spectra of prepared samples, whereas the haze was tested by a Shimadzu UV-3101 UV−visible spectrophotometer with an integrating sphere setup. The viscosity of the TiO2 precursor solution was measured using a LICHEN NDJ-8S viscosity tester at 60 rpm. Thermogravimetric analysis (TGA) of the TiO2 precursor solution was tested by a METTLER TOLEDO TGA/SDTA851e. The microstructures and surface topography of the prepared samples were characterized by an FEI HELIOS Nanolab 600i scanning electron microscope, an FEI Tecnai G2 F30 transmission electron microscope, and a Bruker Dimension FastScan. X-ray diffraction (XRD) patterns were tested on a Bruker D8 Advance X-ray diffractometer. A homemade system with a Keysight B2902A precision source was used to measure the resistance variation during mechanical bending.

improving the thermal stability of a AgNW network mainly focused on coating AgNWs with more thermostable protective materials. AgNWs coated with reduced graphene oxide nanosheets, 32,33 Al 2 O 3 , 34 aluminum-doped zinc oxide (AZO),35,36 and TiO237 films were reported, which had prevented the atomic surface diffusion, but the continuous protective film over AgNWs prohibited the infiltration of polymer precursors into the open spaces of the network, leading to a high surface roughness that might cause a short circuit and failure of devices. Moreover, a thick protective film usually blocked electron transmission from AgNWs to deposited thin film devices. Atomic layer deposition (ALD) is a useful method to produce extra materials on AgNWs.38−41 To date, AgNW@ZrO2 core−shell networks,42 AgNW@ZnO core−shell networks,18,43 and AgNW@TiO2 core−shell networks44 have been fabricated using the ALD method and have exhibited an excellent thermal stability, whereas the core−shell structure does not block the infiltration of polymer precursors into the network. Thus, the process including preparation of a AgNW@protective material core−shell network and infiltration of polymer precursor solution can simultaneously resolve the aforementioned problems existing in AgNW-based FTEs. However, the vacuum system needed for the ALD method leads to increased manufacturing cost and production time. Therefore, a solution-based method to prepare a AgNW@ protective material core−shell network is still highly desirable. In this work, the TiO2 sol was used to treat AgNWs, resulting in a AgNW@TiO2 core−shell network. The TiO2 shell on the AgNW surfaces was ultrathin, conformal, and continuous, and its thickness varied in the range of 2−5 nm. In addition to simultaneously increasing the conductivity and transmittance, the TiO2 shell could significantly increase the thermal stability of AgNWs. Because the protective TiO2 shell had no effects on the open spaces of AgNW network, a polyimide (PI) precursor solution could easily infiltrate into the porous network. After being thermally cured, the AgNW@ TiO2 core−shell network was embedded on the surface of PI, which considerably mitigated the surface roughness and increased the bonding force between AgNWs and the PI substrate. Moreover, the prepared AgNW@TiO2−PI composite FTEs had superb mechanical flexibility and were washable.

2. EXPERIMENTAL SECTION 2.1. Materials. AgNWs with a diameter in the range of 25−35 nm and length in the range of 15−20 μm were produced by Zhejiang Kechuang Advanced Materials Co., Ltd. Isopropyl alcohol (IPA), titanium isoproxide, hydrogen peroxide, deionized (DI) water, 4,4′oxydiphthalic anhydride, 2,2-bis[4-(4-aminophenoxy)phenyl]2457

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ACS Applied Nano Materials μi = 2VAgγ /ri

3. RESULTS AND DISCUSSION 3.1. Thermal Instability of the AgNW Network. Although the network structure endows the AgNW film with excellent photoelectric properties, it has been widely reported that the AgNW network tends to transform into chains of nanospheres at a temperature in the range of 200−300 °C, which is much lower than the melting temperature of bulk silver material.31 For this thermal instability of the AgNW network, the substantial reason stems from the chemical potential gradient induced atomic diffusion on AgNW surfaces. To simplify the analysis, the network is divided into two parts: the AgNW junctions and the AgNWs themselves away from junctions. According to the Rayleigh instability theory, a single AgNW is presumed to have sinusoidal perturbation along the axis,19,45 as indicated in Figure 1a. At the trough of the waved

(1)

where VAg is the atomic volume of Ag, γ is surface tension, and both are positive. Based on this equation, the chemical potential is inversely proportional to the radius of curvature. In summary, two kinds of chemical potential gradients exist in the network. First, the radius perturbation of AgNWs causes a chemical potential gradient in themselves, causing atomic surface diffusion. Second, there is much larger chemical potential gradient from AgNWs to junctions. Due to the slight radius perturbation of AgNWs, the absolute value of the radius of curvature on AgNWs (|rAgNW‑trough| and |rAgNW‑crest|) is much greater than that at junctions (|rjunction|). Thus, the corresponding absolute value of chemical potential on AgNW surfaces is much lower than that at junctions. Because of the negative radius of curvature at junctions, the chemical potential gradient is from AgNWs to the junctions, causing more distinct atomic diffusion from AgNWs to junctions than the atomic diffusion on AgNWs themselves. According to the above analysis, the AgNW network has a tendency to fragment into discrete nanostructures, and the atomic diffusion tendency from AgNWs to junctions is much more severe. It should be noted that the atomic diffusion rate on AgNW surfaces is slow at 20 °C. After being stored at 20 °C, in the dark, with vacuum conditions for 3 months, breakage was found on just a few AgNW junctions (see Figure S1 in the Supporting Information), making a relative change in resistance (ΔR/ R0) of 12%. However, the high temperature environment could remarkably accelerate the atomic diffusion rate, leading to the thermal instability of a AgNW network at a short period. To clearly observe the accelerated morphological evolution of the network, crossed AgNWs were annealed at different temperatures. Figure 1 illustrates the scanning electron microscope (SEM) images of crossed AgNWs after being annealed at different temperatures for 30 min. Figure 1c exhibits the Mayer-rod-coated AgNWs, which show an intact crossover shape. In Figure 1d, after being annealed at 200 °C for 30 min, massive Ag atoms diffused to the junction and formed a bump at the junction. At a higher annealing temperature (225 °C), obvious breakage was found at the junction, but the AgNWs away from the junction were intact, verifying the previous analysis that the chemical potential gradient from AgNWs to junctions was much higher than that existing in AgNWs themselves. After being annealed at 250 °C for 30 min, the crossed AgNWs transformed into two chains of nanospheres. 3.2. Increasing the Thermal Stability of the AgNW Network by Coating with a TiO2 Protective Shell. Here, to address the thermal instability of the AgNW network, a TiO2 precursor solution was applied to treat the network. To prepare the TiO2 precursor solution, titanium isopropoxide and hydrogen peroxide were dissolved in DI water, whereas the reaction between titanium isopropoxide and hydrogen peroxide in water was divided into two steps: the Ti(OCHCH3CH3)4 hydrolyzed in water to form Ti(OH)4; then the Ti(OH)4 reacted with H2O2 to form Ti(OOH)4. After being stirred for 1 h, all of the titanium precursors were dissolved in the solution, which became a yellow solution. Interestingly, without heat or addition of any chemical, the viscosity of the solution continuously increased. As shown in Figure 2, the viscosity variation of the solution had two apparent increases and could be divided into three steps. In step I, the TiO2 precursor solution with excellent liquidity was in a liquid state, denoted as TiO2 sol. At this state, the

Figure 1. (a) Radius of curvature on a perturbed AgNW surface. (b) Radius of curvature at the AgNW junction. SEM images of (c) crossed AgNWs, (d) crossed AgNWs after being annealed at 200 °C for 30 min, (e) crossed AgNWs after being annealed at 225 °C for 30 min, and (f) crossed AgNWs after being annealed at 250 °C for 30 min.

AgNW, the arc is outside the AgNW and the radius of curvature is negative (rAgNW‑trough < 0). At the crest of the waved AgNW, the arc is inside AgNW and radius of curvature is positive (rAgNW‑crest > 0). As shown in Figure 1b, if the junction of AgNWs has a round shape, the arc is outside the AgNW due to the concave surface, resulting in a negative radius of curvature (rjunction < 0), which has been concluded in previous reports.18,46 The relation between chemical potential (μi) and radius of curvature (ri) is calculated as follows:46 2458

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Figure 2. Viscosity variation of the TiO2 precursor solution from 1 to 240 h.

Figure 3. TEM images of AgNW−TiO2 hybrid structure: (a) AgNW@TiO2 core−shell network after treatment with TiO2 sol; (b) AgNW−TiO2 composite film after treatment with TiO2 sol−gel; (c) AgNW−TiO2 composite film after treatment with TiO2 gel.

on the state of used TiO2 precursor solution. Figure 3 indicates the transmission electron microscope (TEM) images of the AgNW−TiO2 hybrid structure. The AgNW network was first treated with TiO2 sol, resulting in a AgNW@TiO2 core−shell network (Figure 3a). A key feature of the core−shell network was that an ultrathin, conformal, and continuous TiO2 shell with a thickness of 2−5 nm was wrapped on the nanowire. When treated with TiO2 sol−gel, a AgNW−TiO2 composite film was formed, and the TiO2 nanoparticles were larger compared with those treated with TiO2 sol, as shown in Figure 3b. The structure was consistent with the previous research, in which after treatment with TiO2 sol−gel the AgNW network was embedded onto a TiO2 film.37,48 Although, with proper control of the IPA dilution times, a AgNW@TiO2 core−shell network was still prepared after treatment with the AgNW network with TiO2 sol−gel, the TiO2 shell was rough and discontinuous, resulting in incomplete protection (see Figure S4 in the Supporting Information). As expected, treatment of the AgNW network with the TiO2 gel also resulted in a AgNW−TiO2 composite film. The difference was that larger sized TiO2 nanoparticles could be found. The structural evolution of the AgNW−TiO2 hybrid structure might be attributed to the transformation of the TiO2 precursor from Ti(OOH)4 to a Ti-peroxo complex. On one hand, the Tiperoxo complex could densify the solution, leaving more residual TiO2 precursors on the AgNW/glass after spin coating. On the other hand, the decomposition product size of the Ti-peroxo complex was much larger than that of the Ti(OOH)4. Because of these two reasons, the TiO2 protective coating evolved from an ultrathin shell to a continuous film. Interestingly, the thermal stability tests indicated that the AgNW@TiO2 core−shell network had the same ability to

Ti(OOH)4 was well dispersed in the solvent, and its viscosity increased from 1.3 to 2.6 mPa·s between 1 and 48 h. In step II, the viscosity of the solution increased rapidly to 71.5 mPa·s at 72 h. It appeared like a jelly state, called TiO2 sol−gel. In this state, some Ti(OOH)4 was transformed into a Ti-peroxo complex, which appeared like a strongly hydrated hydrogenated polymeric matrix due to the presence of some electronegative species coordinated to the Ti4+ ion, such as O2−, O−, and OH− groups.47 As shown in the inset of Figure 2, the TiO2 sol−gel had a faint yellow color and formed a concave liquid surface because of its high viscosity. In step III, the yellow solution at 240 h was full of Ti-peroxo complex and was transformed into TiO2 gel, which had extremely high viscosity (946.0 mPa·s). Although the vial was tilted, it was difficult for the TiO2 gel to flow. The fabrication process of the AgNW-TiO2 hybrid structure by treatment of the AgNW network with a TiO2 precursor solution at various states is the same as the first three steps in Scheme 1. Initially, the AgNW network was coated on a glass substrate using a Mayer rod. Then, the prepared TiO2 precursor solution was added dropwise on the AgNW/glass, followed by spin coating to control the quantity of residual solution. To increase the wettability of the TiO2 precursor solution and thus rapidly spread on the AgNW/glass, the solution was diluted 8 times using IPA, which had contact angle (CA) of approximately 2° on the AgNW/glass (see Figure S2 in the Supporting Information). Finally, the sample was annealed at 150 °C for 10 min. After being dried and decomposed in the annealing process, the TiO2 precursors were completely converted into TiO2 nanoparticles (see Figure S3 in the Supporting Information). Notably, the morphology of the resulting AgNW−TiO2 hybrid structure was dependent 2459

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Figure 4. (a) TEM and (b) tilted SEM images of the AgNW network. (c) TEM and (d) titled SEM images of the AgNW@TiO2 core−shell network. (e) HRTEM image of the AgNW@TiO2 core−shell network. (f) XRD of the TiO2 nanoparticles after being annealed at various temperatures for 1 h.

metallic nanowires, which was mainly driven by gravity, van der Waals forces between AgNWs, and capillary forces from solvent evaporation.49 By contrast, after TiO2 sol treatment, an ultrathin, conformal, and continuous TiO2 shell was found on the AgNW surfaces, as indicated in Figure 4c. It is also notable that the sample treated with TiO2 sol can maintain the network structure, as shown in Figure 4d. Moreover, TiO2 meniscuses were observed around the nanowire junctions, as the junctions could trap more TiO2 sol after spin coating due to the natural strong capillary force at the junctions.50−52 When the TiO2 sol was dried and decomposed, the volume shrinkage could drive the crossed AgNWs closer and result in tighter contact between AgNWs at the junctions, improving the network conductivity. The high-resolution transmission electron microscope (HRTEM) image was applied to analyze the crystal structure of the prepared AgNW@TiO2 core−shell network. As shown in Figure 4e, the AgNW exhibited a clear crystal

suppress the atomic surface diffusion at high temperatures compared with the AgNW−TiO2 composite films (see Figure S5 in the Supporting Information). However, the AgNW− TiO2 hybrid films have two limitations: (1) the brittle TiO2 film will compromise the flexibility of the AgNW network; (2) the open spaces of the AgNW network are filled with TiO2 nanoparticles, making AgNWs hard to embed within the surface of the polymer substrate because the polymer precursors cannot infiltrate the open spaces of the AgNW network. Considering these two limitations, the AgNW@TiO2 core−shell network is more suitable to prepare an FTE. Figure 4 presents the microstructures of the AgNW network and the AgNW@TiO2 core−shell network. From the TEM image in Figure 4a, the surfaces of AgNWs were smooth, and no polyvinylpyrrolidone (PVP) surfactant could be visible. From the SEM image in Figure 4b, the AgNWs formed an arch shape in their junctions due to the soft properties of the thin 2460

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Figure 5. (a) Transmittance of the AgNW network and the AgNW@TiO2 core−shell network at various sheet resistances. (b) Thermal stability of the AgNW network and the AgNW@TiO2 core−shell network after being annealed at different temperatures for 1 h. (c) Transmittance spectrum change of the AgNW network and the AgNW@TiO2 core−shell network after annealing. SEM images of the (d) AgNW network after being annealed at 250 °C for 10 min, (e) AgNW@TiO2 core−shell network after being annealed at 250 °C for 1 h, and (f) AgNW@TiO2 core−shell network after being annealed at 400 °C for 1 h.

are two discrepancy factors, thus higher areal coverage of AgNWs produces a higher conductance and a lower transmittance. As shown in Figure 5a, the transmittance at 550 nm of the AgNW network increased along with the increase of sheet resistance. After treatment with TiO2 sol, both electrical and optical properties of the AgNW network were increased. For example, the sheet resistance of the AgNW sample decreased from 43.2 to 38.0 Ω/sq, whereas its transmittance increased from 95.8 to 96.5%. The reduction of sheet resistance in the AgNW@TiO2 core−shell network originated from the shrinking force during TiO2 sol decomposition, which could further tighten the crossed AgNWs. The increase in transmittance was due to the fact that the TiO2 shell had almost no effect on the absorption of the AgNW network but could effectively lower the light scattering on AgNW surfaces in the visible wavelength range (see Figure S6b−d in the Supporting Information for details). The thermal stabilities of the AgNW network before and after TiO2 sol treatment were analyzed by annealing the samples at various temperatures for 1 h. The relative resistance variation of the AgNW and AgNW@TiO2 core−shell networks is shown in Figure 5b. After being annealed at 250 °C for 10 min, the resistance of the AgNW network was beyond the range of a VICTOR VC890D digital multimeter. The corresponding SEM observation in Figure 5d indicates that all the AgNWs break into nanorods and nanospheres during the annealing. By contrast, the resistance of the AgNW@TiO2 core−shell network remained almost unchanged after being annealed at 250 °C for 1 h (Figure 5e). Notably, the TiO2 shell could even provide reliable protection for the AgNWs at 300 °C, making the

structure and the lattice spacing value of Ag was 0.24 nm, which was indexed to the (111) planes of Ag, whereas the TiO2 shell on the nanowire showed an amorphous structure. It is noteworthy that no transition interface layer exists between the AgNW and the TiO2 shell, indicating that the coating of the thin TiO2 shell by a low-temperature solution-based method does not affect the AgNWs. The phase transformation of the TiO2 shell was confirmed by XRD studies, as shown in Figure 4f. The amorphous structure of the prepared TiO2 shell on AgNW surfaces was also confirmed by the XRD analysis. After being annealed at 150, 200, and 250 °C for 1 h, no diffraction peak was exhibited in the patterns, indicating the amorphous structure of the TiO2 shell. Upon thermal treatment at 300 °C for 1 h, the XRD pattern of TiO2 showed three clear peaks at 25, 48, and 62°, which were assigned to the diffraction of (101), (200), and (213) planes of anatase TiO2 (JCPDS card no. 21-1272), respectively. The peak characteristics of the anatase phase became stronger when the sample was annealed at 400 °C for 1 h. For a transparent sample, its specular transmittance tests rays that come out of the sample parallel to the incident rays; the diffuse transmittance is measured by detecting scattered rays that come out of the sample due to refraction at the sample/air interface, and haze is defined as the rate of the diffuse rays to the total rays which pass a sample (see Figure S6a in the Supporting Information for definition). As the specular transmittance is commonly used to evaluate the optical transparency of transparent electrodes, for brevity, the transmittance presents the specular transmittance in the work.2 The electrical and optical properties of a transparent electrode 2461

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Figure 6. (a) Preparation process of AgNW@TiO2−PI composite FTEs. The inset shows a photograph of a AgNW@TiO2−PI composite FTE. AFM images of a (b) AgNW network on a glass substrate, (c) AgNW@TiO2 core−shell network on a glass substrate, and (d) AgNW@TiO2−PI composite FTE. (e) SEM image of a AgNW@TiO2−PI composite FTE.

Figure 7. (a) Relative resistance variation as a function of number of peeling off cycles. (b) Relative resistance variation when annealed at different temperatures. (c) Relative resistance variation of AgNWs on PET as a function of number of bending cycles. The inset is a zoomed-in view of the relative resistance variation from 2500 to 2520 cycles. (d) Relative resistance variation of the AgNW@TiO2−PI composite FTE as a function of the number of bending cycles. The inset is a zoomed-in view of the relative resistance variation from 2500 to 2520 cycles.

to 750 °C.53 Thus, upon being coated with a TiO2 shell, the thermal stability of AgNWs was significantly increased due to the suppression in the atomic diffusion at the AgNWs/TiO2 shell interfaces. The structural evolution of AgNW and AgNW@TiO2 core−shell networks before and after annealing could also be verified by comparing the transmittance spectrum change. As shown in Figure 5c, the transmittance at 550 nm of the AgNW network decreased from 87.5 to 79.2% due to the morphological transformation from nanowires to nanorods and nanoparticles, which had higher optical scattering and absorbance. Additionally, the morphological transformation also caused a broadened absorption peak at 350 nm, which corresponded to a transverse plasmon resonance of

composite network stable after 1 h annealing. No significant fragmentation or coalescence of the nanowires was observed, exhibiting a protection property comparable to that of the 4.5 nm ZnO shell produced by the ALD method.18 After being annealed at 350 °C for 1 h, the resistance of the AgNW@TiO2 core−shell network doubled. After being annealed at 400 °C for 1 h, the AgNW@TiO2 core−shell network did not completely lose its conductivity, but its resistance increased by a factor of approximately 11 to the original value. From SEM observations (Figure 5f), there were some disconnections in the conducting network, but no nanorods or nanospheres were observed. The in situ TEM investigations exhibited that the TiO2 shell was stable against heating to a temperature up 2462

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Figure 8. Photographs of AgNW@TiO2−PI (a) during a washing cycle, (b) before washing, and (c) after washing. (d) Relative resistance variation as a function of the number of washing cycles.

the silver nanowires.53 By contrast, the AgNW@TiO2 core− shell network could maintain its structure, causing a negligible transparency loss. 3.3. Preparation of AgNW@TiO2−PI Composite FTEs. Figure 6a shows the preparation process of AgNW@TiO2−PI composite FTEs. By using a doctor blade, the PI precursor solution was flowed onto the AgNWs/glass and AgNW@ TiO2/glass. Because the PI precursors could infiltrate the open spaces of the network, the nanowires were easily transferred from the glass substrate to the polymer substrate after being thermally cured and lifted off. From the atomic force microscopy (AFM) image in Figure 6b, the as-coated AgNW network featured a very rough surface with a surface roughness (Ra) of 31.6 nm, whereas, after treatment with TiO2 sol, the surface roughness of the AgNW@TiO2 core−shell network decreased to 27.7 nm. The rough surface topography of AgNW-based FTEs would limit their use as a bottom electrode in thin film devices. Moreover, the poor interfacial adhesion between nanowires and used substrate made the handling of the FTEs difficult. Fortunately, the AgNW@TiO 2 −PI composite FTEs prepared by embedding nanowires on a PI substrate could address both issues. Figure 6c shows that all of the nanowires embed in the polymer substrate, with a surface roughness of 1.9 nm. The SEM observation also indicates that no nanowire protrudes out of the PI surface (Figure 6d). The extremely smooth surface could significantly reduce the short circuit risk of thin film devices. Due to the high temperature needed for curing the PI polymer, all of the AgNWs without a TiO2 protective shell broke into nanorods and nanospheres (see Figure S7 in the Supporting Information). After lift off, the transmittance of AgNW@TiO2−PI composite FTE had a slight decrease from 83.2 to 74.9% due to the optical absorption of the polymer substrate (see Figure S8 in the Supporting Information) but could maintain its original sheet resistance of 8.0 Ω/sq, exhibiting a strong bonding force

between AgNWs and the PI substrate that made the conducting network transfer from glass to the polymer substrate. Coating of AgNWs on a flexible polyethylene terephthalate (PET) substrate is a commonly used method to prepare FTEs. Here, the performances of the conventional FTEs based on AgNWs on PET and proposed AgNW@TiO2−PI composite FTEs were compared. For the sample prepared on PET, the AgNWs were easily peeled off from the substrate with 3M tape because of the poor bonding force between nanowires and the PET substrate. By comparison, the AgNWs embedded in the PI matrix could dramatically increase the interfacial adhesion of nanowires to the substrate. As shown in Figure 7a, the sample prepared on PET lost its conductivity after one peeling off cycle, but the AgNW@TiO2−PI sample maintained its resistance after 100 peeling off cycles. Additionally, the AgNW@TiO2−PI showed thermal stability superior to that of AgNWs on PET. As shown in Figure 7b, the AgNWs on PET were stable at 150 °C but quickly lost their conductivity at 200 °C for a few minutes due to the degradation of AgNWs and the PET polymer. The thermostable AgNW@TiO2 conducting media and PI matrix provided the composite FTEs a superb stability at both 200 and 250 °C. With respect to the mechanical stability, after the AgNWs were embedded in the PI substrate, the tolerance to mechanical deformation of AgNW-based FTEs was dramatically enhanced. Figure 7c,d exhibits the resistance variation of prepared FTEs during mechanical bending at a radius of 2 mm. For the AgNW/PET FTE, the AgNWs sat on the PET substrate by weak van der Waals force with line or point contacts, and the AgNWs were barely stacked at the junctions. During mechanical bending, the nanowires were easily sliding at the junctions, which would increase the contact resistance of AgNWs, therefore revealing enervated electrical properties after long-term bending cycles. In the AgNW@TiO2−PI composite FTE, the TiO2 shell could 2463

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ACS Applied Nano Materials Notes

mechanically weld the AgNWs at junctions. In addition, the strong interfacial adhesion of the AgNW@TiO2 to the PI matrix could further prevent the nanowire from sliding during mechanical deformation because the AgNW@TiO2 network was embedded into the PI matrix. Thus, the AgNW@TiO2−PI composite FTE showed a negligible increment of relative resistance after 5000 bending cycles. To demonstrate the usefulness of the AgNW@TiO2−PI composite FTEs as a part of wearable electronics, a demo with dimensions of 25 × 25 mm was successfully prepared. The resistance of the prepared FTE was measured to be 46.7 Ω. To prove the reliability of the FTE under a harsh environment that could occur in real life, it was handwashed, as shown in Figure 8a. To mimic a realistic scenario, detergent was dissolved in DI water and then the FTE was exposed to repeated washing cycles, in which the FTE needed to sequentially experience rinsing, rubbing, folding, and crumpling in the soap water. As shown in Figure 8b,c, after being washed and then rinsed, it was confirmed that the resistance of the FTE was almost constant. Before and after being washed, distinguishable cracks or damages were not observed on the FTE surface. After 100 washing cycles, the relative change in resistance (ΔR/R0) was about 12%, indicating its high reliability during washing. The proposed AgNW@TiO2−PI composite FTEs are believed to pave a new route to explore the applications of FTEs to nextgeneration wearable electronics, such as washable PM2.5 filters.54

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for financial from the National Natural Science Foundation of China (Grant No. 51522503) and support from Program for New Century Excellent Talents in University (NCET-13-0175).

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.9b00337. SEM image of stored AgNWs, CA of DI water and IPA on AgNW/glass, TGA of TiO2 precursor solution, TEM image of the AgNW@TiO2 core−shell network after treatment with diluted TiO2 sol−gel, relative resistance variation of AgNW films (AgNW network, AgNW@ TiO2 core−shell network, and AgNW−TiO2 composite films), transmission spectra and haze of the AgNW network, absorbance spectrum of TiO2 film, SEM image of AgNW−PI, transmission spectrum of PI thin film (PDF)

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ABBREVIATIONS

AgNW, silver nanowire FTE, flexible transparent electrode PI, polyimide OLED, organic light-emitting diode AZO, aluminum-doped zinc oxide ALD, atomic layer deposition IPA, isopropyl alcohol PVP, polyvinylpyrrolidone XRD, X-ray diffraction SEM, scanning electron microscope TEM, transmission electron microscope HRTEM, high-resolution transmission electron microscope CA, contact angle

4. CONCLUSIONS In this work, TiO2 sol was used to treat a AgNW network to achieve unprecedented thermal stability. It was demonstrated that, after being treated with the TiO2 sol, an ultrathin, conformal, and continuous TiO2 protective shell was coated onto the AgNWs, which could increase both electrical and optical properties of the AgNW network. After PI precursor solution was flowed, thermally cured, and lifted off, a AgNW@ TiO2−PI composite FTE was successfully fabricated, which had superb thermal and mechanical stabilities. Furthermore, the proposed composite FTE was washable, making it a promising conducting component for next-generation wearable electronics.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yanhong Tian: 0000-0002-5877-7096 2464

DOI: 10.1021/acsanm.9b00337 ACS Appl. Nano Mater. 2019, 2, 2456−2466

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DOI: 10.1021/acsanm.9b00337 ACS Appl. Nano Mater. 2019, 2, 2456−2466