Comprehensive Stability Improvement of Silver ... - ACS Publications

Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Comprehensive Stability Improvement of Silver Nanowire Networks via Self-Assembled Mercapto Inhibitors Gui-Shi Liu,†,∥ Yuwang Xu,†,∥ Yifei Kong,‡ Li Wang,† Ji Wang,‡ Xi Xie,† Yunhan Luo,§ and Bo-Ru Yang*,† †

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State Key Laboratory of Optoelectronic Materials and Technologies, Guangdong Province Key Laboratory of Display Material and Technology, School of Electronics and Information Technology, Sun Yat-Sen University, Guangzhou 510006, China ‡ Wellman Center for Photomedicine, Massachusetts General Hospital, Harvard Medical School, Boston 02115, United States § Guangdong Provincial Key Laboratory of Optical Fiber Sensing and Communications, Jinan University, Guangzhou 510632, China S Supporting Information *

ABSTRACT: Instability of silver nanowire (AgNW) has been regarded as a major obstacle to its practical applications in optoelectrical devices as transparent electrodes. Physical barrier layers such as polymer, metal, and graphene have been generally employed to improve the stability of AgNW in previous study. Herein, we first report self-assembled organothiols as an inhibitor for AgNW to achieve an overall enhancement in antioxidation, antisulfidation, thermal stability, and anti-electromigration. The self-assembled monolayers (SAMs) of phenyl azoles, methoxy silane, and methyl alkane were formed on the surface of AgNW via Ag−S covalent bond as barrier layers which provided protective effects against corrosives (e.g., O2, H2S). In particular, the decoration of 2-mercaptobenzimidazole (MBI) offered the best resistance to H2S and excellent stability under a high-temperature and high-humidity environment (85 °C and 85 RH %) for 4 months. Moreover, different SAMs exhibited a stabilizing or destabilizing effect on Plateau−Rayleigh instability of AgNW, which realized the tunability of degradation temperature of AgNWs, for example, increasing by ≥100 °C with MBI SAM or decreasing by ∼50 °C with octadecanethiol SAM compared with pristine AgNWs. Notably, the MBIdecorated AgNWs could survive at 400 °C which is by far the highest bearing temperature for solution-processed AgNW film. As a result, a transparent heater made of the MBI-AgNWs exhibited superior heating characteristics (higher working temperature and durability), as compared with the pristine AgNW-based heater. Our findings on the organothiols decoration not only provide a new paradigm in overall stability improvement of NW of noble metals but also show the potential in morphology controllability of metal NW. KEYWORDS: silver nanowire, stability, self-assembled monolayer, mercapto functional azoles, transparent heater

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INTRODUCTION Over the past decades, transparent conducting materials (TCMs) have attracted significant attention because of their numerous applications, such as touch panels,1 organic light emitting diodes (OLEDs),2 solar cells,3 and heaters.4,5 Among the TCMs, silver nanowire (AgNW) which exhibits superior optoelectrical performance and high flexibility has been under consideration to replace indium tin oxide (ITO) in the next generation of flexible electronic devices.6−8 Pristine AgNW film generally suffers from large contact resistance, high surface roughness, poor adhesion to the substrate, and so forth. These issues have been addressed by many approaches, including mechanical pressing,9 embedding AgNWs into or blending with polymers,10,11 introducing additional conductive materials,12−15 forming metal oxide (MO) or metal matrix layer,16−18 and so forth. Despite bright prospects, the major obstacle remaining is the instability of AgNW.19,20 © XXXX American Chemical Society

In general, the instability of AgNW arises from tarnishing of silver and its high surface-to-volume ratio. Silver is susceptible to oxidation and sulfidation (aka corrosion) in aggressive environment [with moisture, oxygen, hydrogen sulfide (H2S), carbonyl sulfide (OCS), etc.]. High surface-to-volume ratio of AgNW further accelerates the corrosion and shortens its life time.19,21 It has been reported that the pristine AgNW networks fail within 10 days at 85 °C and 85 RH % or within merely tens of seconds by attack of 5 wt % Na2S/K2S solutions.22,23 On the other hand, high surface-to-volume ratio of AgNW leads to structural instability at an elevated temperature or a high current density. The small diameter of AgNW renders its melting temperature (Tm, e.g., ∼200 °C) Received: August 4, 2018 Accepted: October 9, 2018

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DOI: 10.1021/acsami.8b13329 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces much lower than the Tm of bulk silver (961.8 °C),24,25 so that AgNW could morphologically evolve from wire to discrete spheres at a relatively low temperature to minimize its surface energy [Plateau−Rayleigh (PR) instability].26 Regarding the electrical stability, AgNW networks under a normal dc bias easily fail because of electromigration and/or local heating at the contact points between the AgNWs.24 The poor thermal/ electrical stress tolerance of AgNWs has therefore limited the applications wherein high temperature and/or high current involve.27 Many researches have been carried out to address the stability issues. A promising method by embedding the NWs into the polymer matrix [polyvinyl alcohol (PVA),28 epoxy resin,10 NOA 85,29 iongel,13 etc.] has improved chemical/ thermal stability of AgNWs, but with insufficient protection due to the water absorption capability or uncompact wrapping of polymers.30 Besides, polymer coating is not competent to play a dual role: corrosion inhibitor and thermal stabilizer. Although blending AgNWs with a conductive material [such as reduced graphene oxide,31 PEDOT:PSS,30 and chemical vapor deposition (CVD) graphene12] is a good way to enhance the stability, the anticorrosion of the electrodes is still unsatisfactory as a result of incomplete coverage by rGO31 or degradation of PEDOT:PSS.30 Large-area monolayer graphene can serve as an encapsulating layer to provide AgNW with robust resistance to corrosion and heating, but the essential CVD and transfer process are too complex to be widely accepted.12 MOs or metals such as TiO2,32 ZnO,33,34 and Ni4 have been used as a protective passivation layer, exhibiting excellent thermal or electrical stability. The MO/metal layers effectively suppress the diffusion of silver atoms, so that the AgNW could survive at an elevated temperature up to 420 °C for TiO2,32 ≥375 °C for ZnO,16,33,34 and 400 °C for Ni,4 respectively. However, the deposition of MO or metal layer by sputter, atomic layer deposition, or electrodeposition is generally costly, time-consuming, and of low throughput. For this reason, a facile and effective method to improve the stability of AgNWs while maintaining good opto-electrical performance is urgently needed. Herein, we demonstrate self-assembled monolayers (SAMs) of various organothiols including azole derivatives, silane, and alkanethiolate as effective inhibitors for AgNWs. Thiol-derived SAMs have been employed as a passivating layer toward oxygen/sulphide to protect bulk metals. However, the protection of AgNW via thiol-based SAMs has not been studied yet. Recently, several studies have used the thiol compounds to modify AgNWs for uniform deposition35−37 and enhancement of mechanical properties of AgNW composite film.38 In this paper, we show that the thiol monolayers not only isolate corrosive gas (oxygen, sulphide, etc.) to offer excellent antioxidation and antisulfidation ability but also suppress or promote the surface diffusion of Ag atoms, enabling superior thermal and electrical stability of AgNW. Unlike reported methods of using physical barrier (polymer, graphene, metal, etc.), this is the first example of comprehensive stability enhancement of AgNW via chemical decoration with organothiol SAMs. The deposition process of the SAMs is inexpensive, simple, and scalable, but first and foremost offers excellent stability. Specifically, the AgNWs decorated with 2-mercaptobenzimidazole (MBI) could maintain good conductivity after a 4 month test with 85 °C and 85% relative humidity and show an extremely high-temperature toleration up to 400 °C which is by far the highest

bearing temperature for solution-based AgNW electrodes. With the MBI-decorated AgNWs, a transparent heater was fabricated and exhibited better durability compared with the pristine AgNWs under thermal and electric stress.

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EXPERIMENTAL SECTION

Preparation of the AgNW Films. The AgNWs with an average diameter of 17, 30, 45, and 120 nm were purchased from Zhejiang Kechuang Inc. If not mentioned otherwise, the diameter of the AgNW used is 30 nm. The AgNW dispersed in ethanol or isopropanol was further diluted to 2.5 and 1 mg mL−1 for spin-coating and dip-coating, respectively. 1-Phenyl-5-mercaptotetrazole (PMTA), MBI, 2-mercaptobenzoxazole (MBO), (3-mercaptopropyl) trimethoxysilane (MPTMS), and octadecanethiol (ODT) were purchased from Sigma-Aldrich and dissolved in ethanol with a concentration of 0.2 M, followed by an ultrasonication to dissolve completely. Glass substrates were sonicated successively in acetone, isopropyl, and deionized water for 30 min, and then blown with a nitrogen gun before use. AgNWs were spin-coated or dip-coated on ultraviolet− ozone-treated glass or PET to form a uniform network. As for dipcoating process, a clean glass (2 × 3.5 cm2) was dipped into the AgNW solution containing 0.01 wt % Triton-X100 (Sigma-Aldrich) with a descent speed of 1500 μm s−1, a withdrawing speed of 1000 μm s−1, and a dipping time of 15 s. The dipping process was repeated 12 times for each sample to achieve high conductivity. Finally, the AgNW films were immersed in the PMTA, MBI, MBO, MPTMS, and ODT solutions for 30 min, respectively, named PMTA-AgNWs, MBIAgNWs, MBO-AgNWs, MPTMS-AgNWs, and ODT-AgNWs, respectively. These samples were then rinsed by ethanol to remove residues of the inhibitors and blown dry using nitrogen gun. For comparison of the stability performance, various AgNW/ polymer composite films were also prepared by adding into recipes, over coating, or transferring process. The employed polymers included hydroxypropyl methylcellulose (HPMC), PVA, polyvinyl pyrrolidone (PVP), chitosan, NOA 63 (Norland optical adhesive 63), and epoxy resin (DELO KATIOBOND VE 115142). The preparation processes were described in the Supporting Information in detail. Stability Tests. For the accelerated oxidation test, all of the samples (pristine AgNWs, PMTA, MBI, MBO, MPTMS, and ODTdecorated AgNWs) were placed in an aging chamber (Yashilin GDJS225) with 85 °C and 85 RH % for 4 months. The variation of sheet resistance was tracked during the test. To test the antisulfidation ability, the AgNW samples were exposed to H2S gas for an hour in a home-made test bench. The H2S was generated using 6 mL of H2O, 1 mL of HCl (37%), and 0.1, 0.3, and 0.5 g of Na2S·9H2O, respectively. The concentrations of H2S were measured to be 5.1 × 103, 1.5 × 104, and 2.6 × 104 ppm by using a H2S detector (SGA-600-H2S). The sheet resistance was recorded before and after the corrosion test. Fabrication of Transparent Heaters. The dip-coated AgNWs on glass was used as a transparent conductor because of good uniformity. The pristine AgNWs and the decorated samples were used to fabricate heaters to compare their performance. Copper foil was attached to the opposite side of the AgNWs/glass using silver paste to ensure good electric contact. The heater was powered by a Keithley 2400 SourceMeter. Characterization. The length, diameter, and spacing of NWs/ spheres/segments were measured using a self-compiled software based on MATLAB language (Supporting Information, Figure S1). Optical transmittance was measured using a UV−vis spectrophotometer (Tsushima Evolution 220). The sheet resistance was measured by a four-point probe instrument (Mitsubishi, MCPT370). The contact angle was characterized by a contact angle analyzer (DataPhysics OCA 15EC). The AgNWs were characterized using an X-ray photoelectron spectrometer (XPS, NOVA-KRATOS) and scanning electron microscope (SEM, Carl Zeiss SUPRA 60) equipped with an energy-dispersive X-ray spectroscope. The heating characteristics and the IR images of the heaters were recorded using an infrared camera (Nippon AVIO InfReC R500), and the B


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Figure 1. (a) Schematic of self-organization of different thiol-tailed inhibitors to AgNW. (b) S 2p and (c) Ag 3d XPS spectra of the pristine and decorated AgNWs. (d) Transmittance spectra of the pristine and decorated AgNWs film. (e) RS change and water contact angle of the AgNW film with or without decoration. All of the samples have initial RS of ∼20 Ω sq−1, see Table S1.

Figure 2. (a) Temporal changes in RS of the pristine and decorated AgNWs during the 85 °C and 85% RH accelerated test, the initial RS for all samples are in the range of 20−26 Ω sq−1 (Table S3). (b) Morphology change of the AgNWs after the accelerated test for 1 month. (c) Ag 3d XPS spectra of the pristine AgNWs and the ODT-AgNWs before and after the accelerated test for 1 month. corresponding in situ resistances were monitored using a SourceMeter of Keithley 2400.

characteristic binding energies of S 2p3/2 of the chemisorbed mercapto azoles and alkanethiolates, respectively.39,40 The Ag−S bonds were further verified in the Ag 3d spectra. Both of the binding energies of Ag 3d3/2 and Ag 3d5/2 of the pristine AgNWs at 373.5 and 367.6 eV shifted to higher binding energies (0.6−1.1 eV) after the decoration (Figure 1c), which arose from the coordination of the inhibitors to the AgNWs. The selectively chemisorbed monolayers had trivial effects on the transmittance of the AgNW film but gave rise to variation in the interface wettability and conductivity (Figures 1d,e and S2). The layers of silane and methyl alkane did not cause any increase of the sheet resistance (RS) of the AgNW film, whereas the RS showed an increase of 18, 53, and 43% upon the decoration of benzene ring (PMTA), benzimidazole

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RESULTS AND DISCUSSION Decoration of the Mercapto Functional Molecules on the Surface of AgNWs. The SAMs of the five organothiols were achieved by immersing AgNWs into the inhibitor solutions, as illustrated in Figure 1a. Mercapto group (−SH) of the inhibitor molecules has a strong affinity with silver to form firm Ag−S covalent bonds.38 The Ag−S bonds on the AgNW surface were confirmed by the XPS spectra. As shown in Figure 1b, the signals arising from S 2p appear for all of the decorated samples. For example, the signal peaks at 162.4 eV for MBI-AgNWs and 161.7 eV for ODT-AgNWs are the C


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Figure 3. (a) RS change, (b) EDS spectra, and (c) SEM images of the pristine and decorated AgNW samples after the H2S corrosion test for 1 h. The initial RS for all of the samples is in the range of 41−53 Ω sq−1 (Table S5).

(MBI), and benzoxazole (MBO), respectively. Nevertheless, the opto-electrical performance of the decorated AgNWs is still comparable to that of the AgNWs in other studies or commercial ITO/PET film.15,41−43 As for wettability, the AgNWs were changed into a more hydrophobic film after the decorations of the hydrophobic terminal groups such as benzene ring and methyl. Among the five terminal groups, the benzimidazole grafted on AgNWs exhibited the best hydrophobicity with a water contact angle of up to 123°, as shown in Figures 1e and S2. The decorations slightly reduced the adhesion of AgNWs to the substrate because of more hydrophobic surfaces, but the adhesion of the decorated AgNWs could be significantly enhanced by spin-coating a binding layer of a UV-curable cellulose (Table S2 and Figures S2, S3). Antioxidation Enhancement via the SAMs of the Inhibitors. The oxidation resistance of the decorated AgNWs was tested by storing all of the samples in an aging chamber with 85 °C and 85 RH %. As shown in Figure 2, the RS of the pristine AgNWs exhibited a rapid growth during the accelerated test, and the film became nonconductive after storage for 50 days. This result agrees with that of other reports on the stability of pristine AgNWs in a harsh environment.21,44 In contrast, all of the decorated AgNWs exhibited better stability with an increase of RS by less than 10 folds, among which the MBI-AgNWs showed the lowest growth (RS/RS0 = 1.67). For comparison of antioxidation stability, four common water-soluble polymers (HPMC, PVP, PVA, and chitosan) and two UV-curable polymers (NOA 63 and epoxy resin) were used to construct AgNW/polymer composite films. These composite films together with the MBI-AgNWs were subjected to the same test of 85 °C & 85 RH %. The experimental results

indicated that the stability of the MBI-AgNWs was superior to those blended with water-soluble polymers and the AgNWsNOA 63 film (Figure S4). Also, the antioxidation performance of the MBI-AgNWs is better than the same or other composite films previously reported (Table S4), such as Ag/Ni core− shell,45 AgNWs/EPR,20 AgNWs/NOA 85,29 AgNW/PEDOT:PSS,30 AgNW/rGO,31 AgNW/epoxy acrylate, and so forth.21 SEM images in Figure 2b indicate that some of the pristine AgNWs, after the oxidation test for 1 month, broke down near junctions, whereas the morphologies of the decorated AgNWs were almost not changed. XPS spectra in Figure 2c show that the binding energy of Ag 3d for the pristine AgNWs moves to higher energy (374.4 and 368.4 eV), indicating that the surface of the AgNW was heavily oxidized, whereas no visible change in the Ag 3d signals was found for the ODT-AgNWs after the oxidation test. The antioxidation improvement can be attributed to the good affinity and strong Ag−S bonds of the inhibitors which form stable and tight barrier layer against oxygen. In addition, the AgNWs became more hydrophobic after the decorations. Lower moisture absorption is expected on a more hydrophobic surface, which has been reported to improve the thermal oxidation resistance of copper powders.46 As expected, the most hydrophobic MBI-AgNWs film had the best antioxidation ability among the decorated samples. Furthermore, the inhibitors with benzene ring terminal group (PMTA, MBI, and MBO) exhibited better protection performance than that of the methoxy silane and methyl alkane for AgNW, although the ODT-AgNW also had high hydrophobicity. This might indicate that the planar-structured benzene ring have a better passivation effect than linear alkyl chain. D


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Figure 4. (a) Variation of electrical resistance for the pristine and decorated AgNWs under thermal annealing with a rising rate of 10 °C/min, the RS of all samples are in the range of 18−28 Ω sq−1 (Table S6). (b) Normalized RS vs time for the pristine and MBI-AgNWs at 300 °C. (c) Average length of the AgNW fragments with/without the decoration after 250 °C annealing for 10 min.

Figure 5. (a) Schematic of a single pristine and decorated AgNW at an elevated temperature, respectively. (b) SEM images of a single pristine AgNW, MBI-AgNW, and ODT-AgNW after 300 °C annealing for 5 min. (c) SEM images of the corresponding AgNW network after 250 °C annealing for 10 min. (d) Statistical average spacing (λ) and diameter (D) of the fragments of the pristine AgNWs and ODT-AgNWs after 300 °C annealing for 5 min. The values of λ vs D were predicted by the dotted line according to eq 1. (e) Resistance variation of the pristine AgNWs and decorated AgNWs of different average diameters under thermal annealing with a rising rate of 10 °C/min. RS of all samples are in the range of 15− 25 Ω sq−1. (f) Degradation temperatures of the pristine and decorated AgNWs with different average diameters.

Antisulfidation Ability of the Decorated AgNWs. The resistance to sulfidation of the AgNWs was examined by exposing to H2S gas in a home-made chamber. As illustrated in Figure 3, all of the decorated AgNWs show a better antisulfidation ability compared with the pristine AgNWs, and the MBI is the best inhibitor for AgNW. The RS of the MBI-AgNWs only increased to three folds under a high H2S concentration atmosphere (2.6 × 104 ppm) for 1 h, whereas the other samples were degraded to nonconductive networks even under a lower concentration condition (5.1 × 103 ppm). Furthermore, the antisulfidation ability of the MBI-AgNWs was also superior to that of the polymer/AgNW composite films, including HPMC/AgNWs, PVA/AgNWs, chitosan/ AgNWs, NOA 63/AgNWs, and EPR/AgNWs (see Figure S5). It is known that under ambient conditions, bulk silver reacts strongly with the gaseous sulfur-containing compounds (e.g., H2S and OCS) to form a Ag2S corrosion layer. The corrosion

will be much more severe for nanostructured AgNWs because of its large surface-to-volume ratio. As shown in Figure 3c, the pristine AgNWs are severely corroded, resulting in a rough surface and even fractures of the NWs. The darker section of the AgNWs caused by electron charging indicates that Ag is vulcanized to Ag2S of low conductivity. The sulfidation was alleviated for all of the decorated samples, but significant damages were still observed except for the MBI-AgNWs. As shown in the energy-dispersive X-ray spectroscopy (EDS) spectra of Figure 3b, the signal from sulfur element of the MBIAgNWs is much lower than that of the pristine AgNWs after the H2S corrosion, indicating that the AgNWs were well protected by the MBI SAM. The outstanding passivation of MBI could be attributed to a strong affinity of −SH group and the tight alignment of MBI to the AgNWs’ surface. The −SH group of MBI can be easily dehydrogenated to form a strong Ag−S bond, and in the meantime, the Ag−N bond is also E


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Figure 6. Temperature profiles of the heaters made of (a) pristine AgNWs (RS = 13 Ω sq−1) and (b) MBI-AgNWs (RS = 23 Ω sq−1) with different dc bias, and (c) their corresponding infrared images at specific times. For the same bias voltage, three samples were tested to confirm the reproducibility.

surprisingly, the morphologies of the MBI-AgNWs remained unchanged (Figures 4c and S7). The opposite effect of the SAMs on the thermal stability can be associated with the PR instability.26 A single NW would break and aggregate into short rods or spheres to reduce the total surface energy at an elevated temperature. Furthermore, for the AgNW network, Ag atoms tend to diffuse into the junctions between AgNWs because of the large gradient of the Gibbs−Thomson potential,24,25 and thus the fractures easily occur at the junctions, forming spheroid-like particles. Figure 5b−c clearly shows such morphology changes of the pristine AgNW after annealing at 300 °C for 5 min or 250 °C for 10 min (also see Figure S8). The two evolution processes mentioned above are actually governed by the surface diffusion of silver atoms.26,33 Therefore, the thermal stability of the AgNWs can be significantly tailored by the inhibitor decoration. In analogy with the PR instability of the liquid film/polymer coating on a fiber,50,51 we assume that the SAMs on the NWs would undergo the PR instability at an elevated temperature. The difference is that the SAMs formed covalent bonds (Ag−S) with the surface atoms of AgNW. Therefore, the SAM, of which the melting temperature (Tm,SAM) is lower than the pristine AgNWs’ melting temperature (Tm,AgNW, ≤200 °C), could promote the surface diffusion of the Ag atom via Ag−S bonds, because the SAM tended to involve into periodically spaced bulges at a temperature between Tm,SAM and Tm,AgNW (Figure 5a). On the contrary, the SAM with a Tm > Tm,AgNW (≤200 °C) could suppress the surface diffusion of Ag atoms via the Ag−S bond under a temperature larger than Tm,AgNW but lower than Tm,SAM, which increase the melting temperature of the AgNW. The annealing experiments demonstrated the stabilizing and destabilizing mechanisms. As shown in Figures 5c and S7, no morphologic change was observed for the AgNW network decorated with MBI of Tm > 300 °C52 after 250 °C annealing for 10 min; the junctions

formed between the imidazole and the AgNWs. The interaction between MBI and AgNWs through the sulfur and nitrogen atom would result in a perpendicular adsorption configuration, which has been verified by an enhanced Raman spectrum of MBI on silver nano-particles.47 The perpendicular adsorption may facilitate constructing a tight passivation layer and provide a good hydrophobicity (WCA = 123°), accounting for the good passivation effect of MBI. Stabilizing and Destabilizing Effects of the SAMs on PR Instability of AgNWs. Structural stability of AgNWs is crucial for those devices working under high temperature or long time current flow such as OLEDs,25 solar cells,48 and heaters, 49 in which (Joule) heating would cause the fragmentations and discontinuities of AgNWs. Figure 4a illustrates the thermal performances of different AgNW samples. The MBI-AgNWs exhibited the best thermal resistance. It could maintain high conductivity up to 350 °C, whereas the others were totally nonconductive even at 220 °C (pristine, PMTA, MPTMS, and ODT) and at 300 °C (MBO). Further test was conducted to the MBI-AgNWs by keeping it at 300 °C for 120 min. Its RS only changed from 29 to 174 Ω sq−1, whereas the pristine AgNWs became nonconductive within 10 s at 300 °C (Figure 4b). This performance is superior to that of the common AgNW/polymer composite films and is already comparable to the AgNW/metal and AgNW/MO hybrid electrodes (see Figure S6 and Table S4). It is noteworthy that although all of the SAMs could enhance the corrosion resistance at different extents, different SAMs had opposite effects on the thermal stability of AgNWs: the SAMs of MBI and MBO retarded fragmentation of AgNWs, whereas the others promoted fractures at elevated temperature (Figures 4c and S7). For example, in the thermal treatment of 250 °C/ 10 min, the average length of AgNW fragments with ODT SAM was obviously less than that of the pristine AgNWs, and F


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Figure 7. (a) Average temperature evolution and (b) the corresponding resistance change of the heaters made of the pristine AgNWs (RS = 14.6 Ω sq−1) and MBI-AgNWs (RS = 22.5 Ω sq−1). See Figure S11 for the typical transmittance spectra and haze. (c−f) IR images and the temperature profiles along the middle line in the images of the AgNW and MBI-AgNW heaters, respectively.

Performance of the Transparent Heaters of AgNW with and without Decoration. To demonstrate the stability of the MBI-AgNWs under the thermal and electrical stress, transparent heaters (2 × 2.5 cm2) were fabricated. Thanks to the uniform AgNW distribution realized by the dip-coating, the heaters exhibited a uniform temperature distribution. For practicality, an average temperature instead of a maximum temperature was selected to examine the heater’s performance with various dc biases, as shown in Figure 6. For the pristine AgNW-based heater, the temperature dropped suddenly after reaching to 110 °C only for less than 30 s at a voltage of 9 V. The corresponding infrared image (a1 in Figure 6c) indicated that the electrical stressing already induced an inhomogeneous temperature distribution at 180 s, implying breakages of some AgNWs in the network. The continuous breakages further accelerated the nonhomogeneity of the temperature distribution, involving into a thermal crack within next 2 min. The rapid degradation manifested the poor thermal stability of raw AgNWs under Joule heating. In contrast, for the heater made of the MBI-AgNWs, a stable heating was realized at 130 °C with an applied voltage of 12 V for 600 s. The corresponding infrared images in Figure 6c demonstrated that a stable and uniform distribution of temperature was well maintained during the heating cycle, suggesting the good structural stability of the MBI-AgNWs under Joule heating. Long-term heating was also carried out to investigate the degradation behavior of the pristine AgNWs and the MBIAgNWs under electrical stress. As shown in Figure 7a, the temperature drop of the pristine AgNWs appeared near 100 °C after working for only 300 s, whereas for the MBI-AgNWs, the average temperature could remain stable at 135 °C for 20 min. Noted that the bias voltage of the MBI-AgNWs was higher. These temperature evolutions were in good accordance with the variation of in situ resistance, which followed three typical stages: optimization, degradation, and breakdown (Figure 7b), corresponding to sintering and disconnecting of AgNWs induced by long-term electrical stress. Surprisingly, the final resistance of the pristine sample was measured to be RS = 4.5RS0, suggesting that the residual AgNWs were not

between NWs were even not fused (see arrows in Figure 5c). However, for the inhibitors (PMTA, MPTMS, and ODT) with Tm < 150 °C,53−55 all of the decorated AgNWs broke down before 225 °C, whereas the pristine AgNWs still survived (Figure 4a). After annealing of 250 °C/10 min, the fragmentations of these decorated AgNWs were shorter than that of the pristine AgNWs, as shown in Figures 4c, S7, and S9. More clearly, the single ODT-AgNW completely degraded into a chain of spheres after the treatment of 300 °C for 5 min, whereas the morphology of the MBI-AgNW remained unchanged and the pristine AgNW only broke into spheres and/or cylindrical segments (Figure S10). The statistical average spacing (λ) between the spheres/fragments and diameter (D) for the raw AgNW obviously deviated the PR instability developed by Nichols and Mullins56 λ = 2π 2 D

(1)

In contrast, the λ and D values for the ODT-AgNW are in good agreement with the theory model. The smaller λ values express that the ODT-AgNW is more unstable under the annealing of 300 °C/5 min,57 verifying the destabilizing effect. To further demonstrate the stabilizing and destabilizing effect, the thermal stabilities of AgNWs with different average diameters (17, 45, 120 nm) were investigated by using the ODT and MBI decoration. Figure 5e displays the variation of in situ resistance of the raw AgNWs, ODT, and MBI-decorated AgNWs subjected to an increased annealing temperature. The thermal performances of all of the AgNW films with different diameters were improved by the MBI SAM or weakened by the ODT SAM. We defined the temperature corresponding to the point of R/R0 = 5 in Figure 5e as the degradation temperature (Td) of the AgNW network and found that the Td linearly increased with the AgNW diameter (Figure 5f). More importantly, Figure 5f indicates that the MBI SAM drove up the Td by an average of ∼100 °C for different diameters, whereas the ODT decoration lowers the Td by ∼50 °C for all of the cases, regardless of the diameter size. These results first demonstrated that the SAM decoration enabled tunability of the degradation/melting temperature for metal NWs. G


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mobilizing surface atoms of metal NW, which is unprecedented compared with other coating. We expect our findings to provide new clues for stability enhancement and morphology controllability of metal NW using the organothiol decoration.

completely destroyed. Similar behavior of electrically stressed AgNWs has also been reported by Bellet et al. recently.58 The corresponding infrared images and the temperature profiles along the middle line clearly revealed the failure behavior of the pristine AgNWs. A hot spot appeared at the center of the heater after working for 5 min. Then, it split into two hot spots at about 10 min which then gradually moved to the opposite ends of the heater along the middle line and finally disappeared, accompanied by a temperature decrease, as shown in Figure 7c,e. In contrast, no hot spot was observed for the MBI-AgNWs under an operating temperature up to 150 °C until 20 min, showing a uniform temperature distribution along the middle line, as shown in Figure 7d,f. The MBIAgNWs began to deteriorate after 20 min heating, producing a hot spot. The hot spots indicate the occurrence of local disconnections of the AgNWs, resulting in higher current in the surrounding region to aggravate the local Joule heating. This strong positive feedback led to the continuous shift of hot spots and final failure of the AgNW heater. It is noteworthy that both of the heaters of the pristine AgNWs and MBIAgNWs degraded at temperatures (100 and 150 °C) far below their melting temperatures (200 and 350 °C). This phenomenon suggests that instead of PR instability, the electromigration is dominant in the degradation of the AgNW network as a heater.24,25 The MBI decoration not only enhanced the melting temperature of AgNW (to 350 °C) but also offered effective inhibition to the electromigration. The heating performance of the MBI-AgNWs in this work is comparable to or better than those fabricated by metal/FTO,59 doped graphene,60 AgNW/PEDOT:PSS,61 and AgNW/PI composite film.62 Noted that the enhancement was realized by inexpensive and scalable solution-deposited SAM. Actually, the heating stability could be further enhanced by using AgNW of larger diameter because the AgNW network of larger diameter possessed higher degradation temperature, for example, 400 °C for 120 nm diameter (Figure 5e). However, high diameter would increase the haze of the AgNW film. The average diameter of the AgNWs for the heaters is 30 nm, providing a haze as low as 1.7% (at 550 nm, Figure S11) which is more desirable as a transparent heater.

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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/acsami.8b13329. Graphical user interface of the self-compiled software for the measurement of AgNW length, contact angles, and SEM images of the pristine AgNWs and decorated AgNWs; statistical histograms of the AgNW samples; adhesion test; comparison of stability performance, transmittance, and haze of the heaters; and tables of raw data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Gui-Shi Liu: 0000-0003-2507-4092 Li Wang: 0000-0001-5273-7387 Xi Xie: 0000-0001-7406-8444 Author Contributions ∥

G. S. Liu and Y. W. Xu contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National High Technology Research and Development Program of China “863 Program” (2015AA033408), the Science and Technology Projects of Guangdong Province (2015B090915003, 2016B010111003, 2016A010101017), the National Natural Science Foundation of China (61575084), the Science and Technology Project of Guangzhou (201803010097, 201707010500, 201807010077), and the Science and Technology Planning Project of Guangdong Province for Industrial Applications (2017B090917001).

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CONCLUSIONS In summary, we have demonstrated the first example of SAMassisted overall enhancement of AgNW’s stability via a low cost, solution-based deposition process. All these inhibitors, including PMTA, MBI, MBO, MPTMS, and ODT, offered excellent antioxidation ability under the accelerated test of 85 °C, 85 RH % for 120 days because of their hydrophobicity to repel the moisture and oxygen. Among these inhibitors, the MBI-AgNWs exhibited outstanding corrosion resistance when exposed to H2S. Moreover, the MBI decoration provided good suppression of the silver migration under high temperature and electrical stress, and thus, the MBI-AgNWs enabled a hightemperature tolerance to 400 °C, which is by far the highest bearing temperature for any solution-processed AgNW electrodes. And as such, the MBI-AgNW-based transparent heater showed superior heater characteristics compared with the pristine AgNWs. What is more, this paper revealed a new stabilizing and destabilizing phenomenon of PR instability of metal NW upon thiol SAM decoration, which enabled manipulation of degradation temperature of AgNWs with a decrease of 50 °C or an increase of 100 °C. Employing SAMs with different melting temperatures allows immobilizing or

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