Enhanced Oxidation-Resistant Cu@Ni Core-Shell Nanoparticles for

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Enhanced Oxidation-Resistant Cu@Ni CoreShell Nanoparticles for Printed Flexible Electrodes Tae gon Kim, Hye Jin Park, Kyoohee Woo, Sunho Jeong, Youngmin Choi, and Su Yeon Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14572 • Publication Date (Web): 11 Dec 2017 Downloaded from http://pubs.acs.org on December 14, 2017

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ACS Applied Materials & Interfaces

Enhanced Oxidation-Resistant Cu@Ni Core-Shell Nanoparticles for Printed Flexible Electrodes Tae Gon Kim,1 Hye Jin Park, 1 Kyoohee Woo,2 Sunho Jeong,1,* Youngmin Choi,1,* Su Yeon Lee1,*

1

Division of Advanced Materials, Korea Research Institute of Chemical Technology

(KRICT) 141 Gajeongro, Daejeon 34114, Republic of Korea 2

Advanced Manufacturing Systems Research Division, Korea Institute of Machinery and

Materials (KIMM), 156 Gajeongbuk-Ro, Yuseong-Gu, Daejeon, 34103, Republic of Korea

KEYWORDS: Cu nanoparticle, core-shell nanoparticle, photonic sintering, flexible electrodes, flexible heater

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ABSTRACT In this work, fabrication and application of highly conductive, robust, flexible, and oxidationresistant Cu-Ni core-shell NP-based electrodes have been reported. Cu@Ni core-shell NPs with a tunable Ni shell thickness were synthesized by varying the Cu/Ni molar ratios in the precursor solution. Through continuous spray coating and flash photonic sintering without an inert atmosphere, large-area Cu@Ni NP-based conductors were fabricated on various polymer substrates. These NP-based electrodes demonstrate a low sheet resistance of 1.3 Ω/sq under an optical energy dose of 1.5 J cm-2. In addition, they exhibit highly stable sheet resistances (∆R/R0 < 1) even after 30 days of aging at 85 °C and 85 % RH. Further, a flexible heater fabricated from the Cu@Ni film is demonstrated, which show uniform heat distribution and stable temperature compared to those of a pure Cu film.


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INTRODUCTION Over the past decades, the development of efficient strategies to realize the potential applications of flexible optoelectronics in touch screens, smart phones, organic light-emitting diodes (OLEDs), solar cells, wearable electronics, and implantable medical devices has attracted great attention.1-5 Recently, the demand for cost-effective, highly conductive printable materials has significantly increased in various technical fields of printed optoelectronic applications.6-7 To meet these requirements, printable metallic electrodes with solution processability, high electrical conductivity, low-temperature processability, and environmental stability are being necessarily explored.8-10 A variety of conducting materials such as organometallic precursors, conductive polymers, and metallic nanoparticles (NPs) have been widely studied as alternative electrode materials.11-12 Among these, metallic nanoparticles satisfying the requirements of printable electrodes are especially attractive for high-throughput roll-to-roll (R2R) processes that require a subsequent sintering process.13-15 For example, electrodes made of Ag NPs with high conductivity have been prepared by the photonic sintering process with a flash irradiation of white light.16-17 However, the high cost and chemical instability of silver, though increasingly addressed, have considerably limited its practical and industrial applications. In recent years, Cu NPs and Cu NWs have been extensively investigated to replace Ag. The use of Cu NPs as an alternative to Ag NPs could remarkably reduce the material cost and facilitate the photonic sintering techniques for R2R processes. Additionally, the conductivity of Cu NPs is nearly as high as that of Ag NPs; hence, the synthesis of Cu NPs in highly pure metallic phase is important because of the higher thermodynamic stability of its oxide phase, which has a lower conductivity.18-19 Jeong et al. have synthesized surface-oxidefree Cu NPs and proposed a facile method for the fabrication of highly conductive and 3 ACS Paragon Plus Environment


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flexible Cu electrodes via photonic sintering.20 Moreover, Wiley and coworkers have prepared Cu NWs in aqueous solution and fabricated transparent and conductive films.21 However, Cu has unstable conductivity, which renders it unsuitable for practical use in transparent flexible electronics, as it is more sensitive to oxygen and moisture than the noble metals are. This problem has been addressed by exploring a number of approaches to protecting Cu from oxidation.22-27 For example, Wiley and coworkers carried out electroless plating of Ni, Zn, and other metals on as-prepared Cu NWs. While the Cu-Ni NW-based electrode showed 100 times higher resistance to oxidation than the Ag NWs did, the electrical conductivity of the electroless-plated NWs decreased by the dramatic increase of the surface scattering of electrons and high contact resistance due to the rough surfaces of the nanowires.28 On the other hand, several groups reported a solution process to synthesize oxidation-resistant Cu NWs with Ni shells.29-30 Coating the Cu NWs with Ni provides a highly crystalline structure, abrupt interface, and smooth surface, which result in high oxidation resistance. Nevertheless, in the case of stacked NWs, the peak-to-peak roughness is more than double the diameter of the wires due to their random arrangement. In addition, such Cu-Ni NWs are hardly compatible with the direct printing of NWs on plastic substrate. Recently, some progress has been made in preparing Cu-Ag alloy electrodes by lowtemperature precuring followed by rapid photonic sintering.26 However, the resistance to oxidation can be remarkably improved by coating with Ni, compared to Ag coating. Thus, facile synthesis of Cu NPs with improved oxidation resistance, which is rarely reported, and fabrication methods of printed metal electrodes remain great challenges for flexible electronic devices. Here, we report a highly effective strategy to improve the chemical resistance of a Cu NP particulate film-based electrode by coating a Ni protective shell on the surface of the Cu 4 ACS Paragon Plus Environment

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NPs without degrading their electrical conductivity. The electrical conductivity and resistance to oxidation of the Cu@Ni electrodes could be precisely controlled by adjusting the thickness of the Ni shell and modulating the light intensity during flash photonic sintering. The photosintered Cu@Ni electrodes exhibited high conductivities under optimal photonic sintering and long-term oxidation stabilities even under a humid ambient atmosphere at an elevated temperature. In addition to the excellent capability to resist oxidation, the Cu@Ni electrodes demonstrated highly stable conductivity under several bending cycles. Further, a flexible and printable heater based on the Cu@Ni NPs is demonstrated with a much broader operating temperature range (up to 200 °C) compared to that prepared with pure Cu NPs.

RESULTS AND DISCUSSION Figure 1a schematically illustrates the fabrication of large-area Cu@Ni core-shell NPbased electrodes on a polymeric substrate by a continuous spray coating technique and photonic sintering. First, a liquid suspension consisting of Cu@Ni core-shell NPs was deposited onto a heated substrate through a spray nozzle by back pressure. We had previously investigated the distribution and thickness of NPs on the substrate as a function of spray speed.20 The stage underneath the substrate moved with a speed of 0.3 mm min-1 during spray coating, which enabled the preparation of large-area electrodes. Subsequently, the sprayed films were continuously photo-sintered by irradiating with highly energetic photons for 10-3 s with few seconds of interval. The sample-loaded stage was moved at a speed of 300 mm min1

during photonic sintering to produce large-area electrodes and to ensure uniform photon

irradiation across the entire film. Thus, highly conductive Cu@Ni electrodes, with conductivity comparable to that of a pure bulk Cu film, were obtained on various plastic substrates after photonic sintering without further post-treatment. 5 ACS Paragon Plus Environment


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To synthesize the Cu@Ni core-shell NPs, first, surface-oxide-free Cu NPs were synthesized following the procedure previously reported by us.25 These Cu NPs were stored in toluene at a concentration of 20 wt%. Cu@Ni core-shell NPs were synthesized by adding the Cu NP solution to a 100 mL flask containing a solution of nickel acetylacetonate, oleic acid, and phenylhydrazine in oleylamine. The mixture was purged with nitrogen for 60 min and then heated to 240 °C for additional 60 min. In general, the shell growth in the presence of a core particle primarily take places via a heterogeneous nucleation process. The atoms or molecules of shell material are diffused onto the surface of core particles and prolong the creation of nuclei and growth on the surface itself, instead of generating new nuclei in the bulk phase. For synthesis of Cu-Ni core-shell NPs, Ni reduction takes place preferentially on surface of Cu NPs through heterogeneous nucleation. After the reaction was complete, the synthesized Cu@Ni core-shell NPs were washed with toluene. As shown in Figure 1b, the prepared Cu@Ni core-shell NP suspension, with 20 wt% concentration in toluene, is very stable without aggregation for 24 h. This would be due to organic capping molecule with an elongated linear chain structure. As shown in Figure 1c, the average particle sizes of the Cu@Ni core-shell NPs with a bimodal size distribution are 56.3 and 146.8 nm. The bimodal particle size in a particulate network is particularly advantageous since the small nanoparticles are mostly located in voids between neighboring large particles, thereby resulting in a well-packed particulate structure and increased area of the interparticular junction.26, 31-32 The thickness of the shell in the Cu@Ni core-shell NPs could be precisely controlled by changing the initial molar ratio of Cu and Ni ions from 1:1 to 5:1. Hereinafter, the Cu@Ni core-shell NPs are denoted by their Cu/Ni molar ratios (e.g., Cu@Ni (1:1)). Inductively coupled plasma atomic emission spectroscopy (ICP-AES) analysis confirmed that the molar 6 ACS Paragon Plus Environment

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ratio of Cu to Ni in Cu@Ni (2:1) core-shell NPs is 1.9 by weight. The compositions of all the Cu@Ni NPs were consistent with their respective Cu/Ni molar ratios used for synthesis. Further, the phases and structures of the synthesized NPs were characterized by X-ray diffraction (XRD) (Figure 2a). All the diffraction peaks could be readily assigned to the crystal structures of Cu and Ni. As seen in the XRD pattern, the synthesized pure Cu, pure Ni, and Cu@Ni core-shell NPs consisted of pure metal phases without any oxide or secondary phases. The 2θ angles of the characteristic (111) and (200) reflections of FCC metals were observed at 43.3° and 50.43° for Cu and 44.4° and 51.8° for Ni, according to JCPDS file 4-0836 (Cu) and 4-0850 (Ni), respectively. The peaks corresponding to the (111) planes of Cu@Ni (1:1) were observed at 43.3° and 44.4°, respectively. The peaks corresponding to the Ni (111) plane shifted to smaller 2θ angles as the Ni content of Cu@Ni core-shell NPs decreased due to surface alloying. Therefore, the ratio of Cu/Ni peak intensity increased as the Cu/Ni molar ratio in the initial precursor solution increased. Figure 2b shows a bright-field HRTEM image and the elemental maps of Cu and Ni in the NPs. Due to heterogeneous growth and surface alloying, the Cu atoms are mainly distributed in the center of the Cu@Ni (3:1) NPs. In contrast, the Ni atoms are located on the surface of the Cu atoms. In addition, as shown from composition distribution analysis (Figure S1 in Supporting Information), the thickness of the Ni layer could be successfully adjusted by varying the content of the Ni precursor, thus providing an effective strategy for precisely controlling the growth of Cu@Ni NPs. Interestingly, these two elements completely separated after the heterogeneous growth of Ni, even though Cu and Ni are completely miscible in all proportions. This was further confirmed by the energy dispersive X-ray spectroscopy (EDS) line-scanning analysis. The EDS line-scan elemental profiles show that Ni is distributed at the edge, while Cu is located at the center in the Cu@Ni NPs (3:1) (Figure S2 in Supporting



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Information). This morphology is critical for the oxidation-resistant stability of the Cu@Ni NP-based electrode and will be discussed below. To produce Cu@Ni NP-based electrodes, a photonic sintering process was adopted for the spray-deposited core-shell NP films rather than conventional thermal annealing. We had previously developed a photonic sintering technique for the ambient atmosphere processing of printed Cu electrodes, where the irradiated photons were absorbed into the metal NPs and the dissipated thermal phonons resulted in the thermal annealing of the films on the plastic substrate, without any inert gas or vacuum condition. Figure 3a shows the variation in electrical resistivity of Cu@Ni NP-based electrodes formed on a polyimide (PI) substrate through photonic sintering as a function of optical energy doses ranging from 1.18 to 2.47 J cm⁻2. With flash annealing, the Cu@Ni core-shell NPs could be completely converted into conductive layers (thickness: 800 nm) with electrically interconnected conductive pathways (Figure S3 in Supporting Information). From SEM images of photosintered electrodes, local welding between neighboring nanoparticles was clearly observed from photo-sintered Cu@Ni NPs with different Cu/Ni molar ratios. When surface-oxide-free Cu NPs were used to fabricate electrode by photonic sintering as a reference, a resistivity of ~7 µΩ·cm was obtained above the optical energy dose of 1.89 J cm⁻2, which increased to ~20 µΩ·cm for energy dose up to 1.2 J cm⁻2. For Cu@Ni (1:1) NPs, the resistivity after flash annealing at an optical energy dose of 1.59 J cm⁻2 increased to ~137 µΩ·cm due to the high intrinsic resistivity of Ni. By increasing the amount of Cu to 5:1, the electrical properties of the photo-sintered electrode at 1.59 J cm⁻2 improved, showing a resistivity of 52 µΩ·cm, compared to the Ni NP-based electrodes. In addition, the resistivity of Cu@Ni NPs depends on the processing parameters, the electrical voltage and the duration, as shown in Figure S4 of the Supporting Information. In conventional sintering, Cu and Ni are completely miscible 8 ACS Paragon Plus Environment

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with each other in all proportions. When a Cu (size ~ 100 nm) and Ni (~ 50 nm) NP mixture with same content of Cu@Ni NPs (2:1) was photo-sintered with an optical energy dose of 1.58 J cm⁻2, both Cu and Ni atoms completely mixed with each other (Figure S5 in Supporting Information). However, the EDS map in Figure 3b shows that the Cu@Ni NPs (2:1) retain their core-shell structures even after photonic sintering, which could be attributed to the unique geometrical structure of Cu@Ni NPs as well as the flash photonic sintering at a timescale of 10-3 s. Surface oxidation of pure Cu is the major reason for the performance degradation of Cu NP-based electrodes since the pure Cu phase readily oxidizes at elevated temperatures in air. To investigate the resistance of Cu@Ni NPs to oxidation, we performed a high temperature and high humidity test in 85 °C/85 % RH condition for 30 days. All samples were photo-sintered with an optical energy dose of 1.58 J cm⁻2. The change in normalized sheet resistance, ∆R/R0, was measured for comparison, where R0 is the initial sheet resistance and ∆R is the resistance change. Figure 3c reveals that the ∆R/R0 of the Cu electrodes without Ni coating dramatically increases to 5.72 until 21 days. Moreover, the resistance to oxidation increased with increasing Ni content from 5:1 to 1:1. In contrast to the result in oxidation resistance, core-shell NPs with higher Ni amounts exhibit lower conductivity because the conductivity of Ni is half that of Cu. Considering both conductivity and oxidation resistance, we selected the Cu@Ni (2:1) NPs as the optimal core-shell NPs to make printed flexible electrodes. Indeed, for Cu@Ni (2:1) NPs, the normalized sheet resistance was found to be remarkably stable over a period of 30 days, with a slight increase of 0.4. It should be noted that this value is quite small compared to that of the Cu electrode under the same condition, and would be acceptable for practical applications. Such high stability could be attributed to the preserved core-shell structure even after photonic sintering. The photo9 ACS Paragon Plus Environment


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sintered Cu@Ni NP electrodes were characterized by XPS and XRD before and after the 85 °C/85 % RH aging test (Figure S6a, S6b, S6c in Supporting Information). The Cu 2p3/2, Ni 2p peak and Cu (111) peak positions before and after the test remained unchanged, which confirms the electrodes’ superior resistances to oxygen and moisture in ambient atmosphere. In addition, the absence of peaks corresponding to copper oxides and nickel oxides after 30 days supports the excellent stability of Cu@Ni NP-based electrodes. We further demonstrate the oxidation effects of Cu@Ni NP-based electrode by connecting an LED lamp. As shown in Figure 3d, the brightness and illumination intensity of the LED lamp exhibited no considerable deterioration after the aging test. As shown in Figure 6d in Supporting Information, the core-shell structures of the metal NPs were stable under aging test and the flash photonic sintering technique significantly improved the oxidation resistance, thus rendering

the

Cu@Ni

electrodes

promising

for

application

in

flexible

device

interconnections. Further, the flexibilities of the photo-annealed Cu@Ni electrodes on PI substrates were examined under repeated bending cycles, as shown in Figure 4a and 4b. When the Cu@Ni electrodes were bent at a bending radius of 6 mm, the resistance increased slightly to less than 0.1 Ω. More significantly, the resistances remained unchanged after bending and recovered to their initial values even after 1,000 cycles. Figure 4b shows the variation in normalized resistance as a function of bending radius of a photo-sintered Cu@Ni (2:1) electrode on a PI substrate for 1,000 bending cycles. The electrode was bent to a bending radius of 10, 8, 6, and 4 mm, and the resistances were measured after the electrodes were straightened. The ∆R/R0 increased slightly (up to ~ 1) after 1,000 cycles. The resistance of the Cu@Ni electrode remained comparable to its initial value even after 1,000 bending cycles with a bending radius > 6 mm. Even when the bending radius was 4 mm, it demonstrated a relatively high conductivity corresponding to ∆R/R0 = 1. As shown in Figure 4c, the 10 ACS Paragon Plus Environment

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illumination intensity of the LED lamp shows no considerably deterioration when the Cu@Ni electrodes were bent to a bending radius of 4 mm. Furthermore, a highly conductive Cu@Ni layer was prepared on a 10-cm-long substrate after continuous flash photonic sintering (Figure S7 in Supporting Information). For the large-area Cu@Ni electrodes, the resistances did not significantly vary even after they were severely bent. Thus, in addition to enhanced oxidation resistance, the Cu@Ni NP-based electrode exhibited superior electromechanical stability, which is absolutely essential in flexible electronic devices and can be attributed to the strong adhesion between the Cu@Ni NP layer and the underlying plastic substrate. The adhesion between the film and the substrate was evaluated by the adhesive tape test method (Figure S8 in Supporting Information). After five repeated tape-peeling processes, the resistance maintained its initial value of ≈ 2 Ω. The flexibility of our fabrication method allows the formation of photon-sintered Cu@Ni particulate films on various substrates. Figure 4d shows the photographs of photo-sintered Cu@Ni films on PI, polyethersulfone (PES), polyethylene naphthalate (PEN), and paper substrates. After photonic sintering under optimized conditions, the Cu@Ni films formed a uniformly interconnected network and strongly adhered to the various substrates without delamination. The Cu@Ni films on PI substrate can serve as an electrical conductor only at optical energy doses of over 1.3 J cm⁻2, while its films on transparent PES and PEN substrates were operational below 1.3 J cm⁻2. After flash photonic sintering, the resistivities of the photo-sintered electrodes were 84 and 900 µΩ·cm for the PES and PEN substrates, respectively. The higher resistivities, compared with the resistivity of the Cu@Ni electrode on the PI substrate, were attributed to the limited supply of photon energy during photonic sintering. Similar to the photo-sintered Cu@Ni films on PI, the ∆R/R0 for a photo-sintered electrode on a PES, PEN, paper substrate increased slightly after 1,000 cycles of bending (Figure S9 in Supporting Information).



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To demonstrate the enhanced oxidation resistance of Cu@Ni electrodes and their potential application, we prepared a large-area Cu@Ni heater (1 × 8 cm2) on a PI substrate. The Joule heating characteristics of the Cu@Ni heater were examined, as shown in Figure 5a. The voltage applied to the Cu@Ni heater was provided by a DC power supply through a copper tape at the heater edge, and the temperature change was monitored using an IR camera. Under a constant DC bias, the temperature increased and saturated to a maximum temperature within 100 s. The maximum temperature of the Cu@Ni heater operating at 3 V was 53 °C. As the applied voltage was increased to 7 V, the Cu@Ni heater reached a steadystate temperature of 138 °C. Figure 5b presents the captured IR images of temperature distribution for the Cu@Ni heater under various voltages. As can be seen, uniform heating was achieved over the entire heater, which was attributed to the uniform spatial distribution of sheet resistance. To further compare the thermal stability of Cu@Ni electrodes with that of Cu electrodes, we measured the sheet resistances of both the Cu@Ni and Cu electrodes as a function of annealing temperature. The electrodes were annealed at 150 and 200 °C for 30 min in air. As shown in Figure 5c and 5d, for the Cu@Ni electrodes, the heaters retained their colors and normalized sheet resistances (R/R0) even under a harsh annealing condition. On the other hand, the color of the Cu electrode changed after annealing, similar to the oxidized films, and their normalized sheet resistance increased to 5.9 after annealing at 200 °C for 4 h. This result confirms that the Cu@Ni-based electrodes exhibit superior thermal stability against oxidation and are promising as low-cost, large-area flexible heaters.

CONCLUSIONS In conclusion, core-shell metallic NPs composed of Cu-rich core and Ni-rich shell have been successfully synthesized via a sequential solvothermal reaction with different 12 ACS Paragon Plus Environment

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Cu/Ni molar ratios in the precursor solution. With increasing Ni content, the oxidation resistance of the Cu@Ni NPs increased, whereas the conductivity of the Cu@Ni electrode decreased. In addition, both the protective Ni shell and the preserved core-shell structure after photonic sintering rendered the resulting Cu@Ni electrodes highly resistant to oxidation. Over a period of 30 days, the Cu@Ni (2:1) electrode exhibited a normalized sheet resistance change (∆R/R0) of less than 1, compared to the large resistance change in the pure Cu electrode. Under a cyclic bending test, the conductivity and brightness of the LED lamp exhibited no evident change. Furthermore, the flexible heater prepared from the Cu@Ni film demonstrated uniform heat distribution and a stable temperature at its steady-state temperature. Therefore, the high performance and high stability of our Cu@Ni-based electrodes demonstrate the possibility of employing the photonic sintering method to form protection layers on various conductive nanoparticle-based electrodes exposed to harsh environments.

Experimental Section Synthesis of Cu@Ni Core-Shell Nanoparticles. All the chemicals were used as received without further purification. Ni(II) acetylacetonate (Ni(C5H7O2)2, 95%), oleylamine (C18H35NH2, 99%), oleic acid (C18H34O2, 90%), phenylhydrazine (C6H5NHNH2, 97%), and toluene (C6H5CH3, anhydrous, 99.8%) were purchased from Aldrich. Surface-oxide-free Cu nanoparticles were synthesized via a solvothermal reduction under an inert environment, as reported in our work. The Cu@Ni core-shell nanoparticles were produced with various Cu:Ni ratios (1:1, 2:1, 3:1, 4:1, and 5:1) by varying the amount of Ni acetylacetonate. Ni acetylacetonate and as-prepared Cu nanoparticles were dissolved into oleylamine in a threeneck round-bottomed flask equipped with a reflux condenser and mechanical stirrer. The 13 ACS Paragon Plus Environment


solution was heated to 90 °C for 10 min and purged with nitrogen for 60 min at the same temperature. Then, phenylhydrazine was injected at an injection rate of 2 ml/min for 15 min. Thereafter, the flask was heated up to 240 °C and the solution was further aged at this temperature for 60 min. Finally, the synthesized Cu@Ni core-shell nanoparticles were washed with toluene three times.

Fabrication of Photo-Sintered Cu@Ni Electrodes. The synthesized Cu@Ni core-shell NPs dispersed in toluene were mixed with a dispersant and ultrasonicated for 15 min. The conductive ink containing Cu@Ni core-shell NPs was diluted with toluene to a concentration of 0.7 wt%. Spray printing was performed using a spraying machine with an X-Y moving stage and an injection nozzle, and the NP ink was spray-deposited on PI (Kapton film 300HN, Teijin DuPont Films, 75 µm), PES (Glastic SCL120, I-Components, 120 µm), and PEN (Teonex Q65HA, Teijin DuPont Films, 125 µm) substrates. Photonic sintering was performed using a xenon flash lamp (dimensions of 12×0.75 inch) (Sinteron 2010, Xenon Corp.) in which an A-type lamp with a broadband spectrum of 370–800 nm was used. The xenon flash lamp provided an optical energy density of 0.25–3.41 J cm⁻2 which is modulated by electrical voltage and time.

Characterization. Images of the Cu@Ni core-shell NPs and the microstructures of the conductive layers were taken by high-resolution transmission electron microscopy (HRTEM) (Talos F200S, FEI) and scanning electron microscopy (SEM) (JSM-6700, JEOL). The crystal structure of the Cu@Ni core-shell NPs was analyzed using an X-ray diffractometer (XRD) (D/MAX-2200V, Rigaku) in the 2θ range of 20–80° at 40 kV and 40 mA. Chemical structural analysis of the Cu@Ni cores-shell NPs was performed via X-ray photoelectron spectroscopy (XPS) (K-Alpha, Thermo Fisher Scientific). The stabilities of the Cu@Ni cores14 ACS Paragon Plus Environment

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shell electrodes under the aging condition of 85 °C and 85% relative humidity were determined in a Climate chamber (WK11-340, Amtest) over 28 days. The Joule heating characteristics of the Cu@Ni core-shell electrodes were examined with a DC power supply (E3634A, Keysight). The sheet resistance of the Cu@Ni core-shell electrodes were measured by a four-point probe system (FPP-HS8, Dasol Engineering). An automatic bending machine (PMC-1HS, Autonics) was used to test the long-term stability under repeated bending cycles, and the variation in resistances of the Cu@Ni core-shell electrodes were analyzed by using a source meter (Keithley 2450, Keithley).

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Details of experimental process, HRTEM image and compositional distribution analysis for Cu@Ni core-shell NPs, Photograph and SEM image of photo-sintered Cu@Ni electrodes, HRTEM images and compositional distribution analysis for a Cu and Ni NPs mixture, Change of XPS spectra for photo-sintered Cu@Ni electrode before and after 85 °C/85% RH accelerated test, Mechanical adhesion test of the photo-sintered Cu@Ni electrodes (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail : [email protected] *E-mail : [email protected] 15 ACS Paragon Plus Environment


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*E-mail : [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This research was supported by the Global Research Laboratory Program (NRF2015K1A1A2029679) and the Nano·Material Technology Development Program (NRF2015M3A7B4050306) of the National Research Foundation (NRF) of Korea funded by the Ministry of Science, Information and Communication Technologies and Future Planning. This research has been performed as a project No SKO1707C10 and supported by the KOREA RESEARCH INSTITUTE of CHEMICAL TECHNOLOGY (KRICT)

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Figure 1. (a) Schematic illustration of the fabrication of large-area Cu@Ni core-shell NPbased electrodes on a polymeric substrate. (b) Toluene dispersion of the synthesized Cu@Ni core-shell NPs in a glass vial. (c) SEM image of the synthesized Cu@Ni core-shell NPs. Scale bar of the inset image is 100 nm.


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Figure 2. (a) XRD patterns of Cu NPs, Cu@Ni NPs, and Ni NPs. (b) HRTEM image and elemental distribution maps of Cu@Ni core-shell NPs.



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Figure 3. (a) Resistivity evolution in Cu@Ni NP-based electrodes formed on a polyimide (PI) substrate after photonic sintering with optical energy doses ranging from 1.18 to 2.47 J cm⁻2. (b) HRTEM image and elemental distribution maps of a Cu@Ni NP-based electrode photo-sintered with an optical energy dose of 1.58 J cm⁻2. (c) Plot of ∆R/R0 versus time for various NP-based electrodes. (d) Photographs showing the stable resistance of a Cu@Ni NPbased electrode after 85 °C/85 % RH aging test.


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Figure 4. (a) Resistance change in a Cu@Ni NP-based electrode after bending. (b) Plot of the normalized resistance change in a Cu@Ni electrode over bending cycles at bending radii of 10, 8, 6, and 4 mm. (c) Photograph of the illuminated LED lamp mounted on the Cu@Ni electrodes bent at a bending radius of 4 mm. (d) Photographs of the photo-sintered Cu@Ni films on PI, PES, PEN, and paper substrates.



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Figure 5. (a) Temperature profiles as a function of time for different applied voltages. (b) IR images of the Cu@Ni heater for different applied voltages. (c) Photographs of Cu@Ni and Cu electrodes annealed in harsh conditions. (d) Plot of the normalized resistance change as a function of time of Cu@Ni and Cu electrodes annealed at 200 °C.


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Enhanced Oxidation-Resistant Cu@Ni Core-Shell Nanoparticles for

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