PVP-Mediated Galvanic Replacement Synthesis of Smart Elliptic Cu

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Functional Nanostructured Materials (including low-D carbon)

PVP-Mediated Galvanic Replacement Synthesis of Smart Elliptic Cu-Ag Nanoflakes for Electrically Conductive Pastes Yu Zhang, Pengli Zhu, Gang Li, Zhen Cui, Chengqiang Cui, Kai Zhang, Jian Gao, Xin Chen, Guoqi Zhang, Rong Sun, and Chingping Wong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b16135 • Publication Date (Web): 06 Feb 2019 Downloaded from http://pubs.acs.org on February 10, 2019

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ACS Applied Materials & Interfaces

PVP-Mediated Galvanic Replacement Synthesis of Smart Elliptic Cu-Ag Nanoflakes for Electrically Conductive Pastes Yu Zhang,† Pengli Zhu,*,‡ Gang Li,‡ Zhen Cui,§ Chengqiang Cui,*,† Kai Zhang,† Jian Gao,† Xin Chen,† Guoqi Zhang,§ Rong Sun,*,‡ and Chingping Wong,‡,∥ †

State Key Laboratory of Precision Electronic Manufacturing Technology and Equipment, and

Key Laboratory of Precision Microelectronic Manufacturing Technology & Equipment of Ministry of Education, Guangdong University of Technology, Guangzhou 510006, China ‡

Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences, Shenzhen

518055, China §

Department of Microelectronics, Delft University of Technology, Delft 2628 CD, Netherlands

∥

School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA

30332, United States *Corresponding author, email: [email protected] [email protected] [email protected] KEYWORDS: Alloy nanoflakes, Cu-Ag, Elliptic, Structure-directing, Conductive paste,

ACS Paragon Plus Environment

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ABSTRACT: Elliptic Cu-Ag nanoflakes were syntheszied via a facile in-situ galvanic replacement between prepared Cu particles and Ag ions. The alloy nanoflakes with high purity and uniformity present a size of 700 × 500 nm, with a thinness of 30 nm. Nontoxic and low-cost polyvinyl pyrrolidone was used as dispersant and structure-directing agent, promoting the formation of the remarkable structure. The synthesized nanoflakes were utilized as a filler for conductive paste in an epoxy resin matrix. Conductive patterns on flexible substrates with a resistivity of 3.75 × 10-5 Ω·cm could be achieved after curing at 150°C for 2 h. Compared with traditional silver microflakes, the smart alloy nanoflakes provide much improved conductive interconnection, whose advantage could be attributed to their nanoscale thicknesses. It is also noteworthy that the conductive patterns are able to tolerate multiple bending at different angles, having good conductivity even after 200 repeated bendings. Therefore, the alloy nanoflakes could be a promising candidate conductive filler for flexible printing electronics, electronic packaging and other conductive applications.


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1. Introduction Nano- or micro-sized metallic materials have been widely used as conductive fillers in different applications, such as spherical nanoparticles for conductive ink,1−4 elliptic microflakes for conductive pastes,5−10 and so on. They can be easily printed onto the required substrates, forming conductive patterns for circuits, sensors, LEDs, and radio frequency components.11−16 In particular, electrically conductive pastes have become promising alternatives to conventional interconnect materials in semiconductor packaging, since the use of conductive paste is recognized as more environmentally friendly, easier operation, and more flexible compared with the conventional electroplating technology, also with lower operating temperature, finer pitch capacity compared with solder paste.17−18 Conductive pastes generally are mainly composed of resin and conductive filler. The resin matrix provides mechanical and adhesive strength, while the filler forms conductive path. The most commonly used conductive filler are probably micron-sized silver flakes, in which they could conduct electricity through simple physical contact between each other. Conductive pastes tend to the higher conductivity by increasing the solid content of metallic fillers, but largely limiting the application of conductive pastes in the field of higher electrical conductivity. To improve the connection situation, combination of Ag nanoparticles into the traditional conductive paste has been attempted. According to literature reports, Ag nanoparticles could melt at low temperature and provide metallurgical connections between Ag microflakes.19−21 Wong’s group proved that the physical contact between silver nanoparticles helps the paste form a conductive path because of the resin shrinkage after the curing process.19 Furthermore, they added Ag nanoparticles into the conventional Ag microflakes based conductive paste to improve


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the conductivity. However, this may generating too many interfaces inside the paste, resulting in the conductive effect was not increased obviously.20 In fact, the improvement can be well explained by the generally accepted conductive mechanism of conductive paste.22−25 The first one is macro percolation theory, refers to the fact that the volume resistance of conductive paste can be changed dramatically if the filler content reaches the certain value called percolation threshold.26 The second one is micro tunneling theory, which is believed that electrons can be activated by thermal vibrations to passing through the barrier of the polymer interface layer and jump to the adjacent conductive particles. 27−29 Considering the usually micro-flaky filler in traditional conductive pastes, the bulk conductive path can only be formed via physical contact once the filler content achieves the percolation threshold. However, if certain nano-sized fillers are added into the fillers, atoms could diffuse at lower temperature and thus form metallurgical bonds between micron flakes, greatly reducing the percolation threshold. On the other hand, the more active surface atomic state also makes the tunneling effect easier to be achieved. But, the added particles with nanosize will greatly increase the contact resistance between fillers because of the large specific surface area, thus hindering the improvement of electrical conductivity. Therefore we infer that flakes with nanosize might combine the low melting point of the nanoparticles with the high conductivity of the microflakes, which could form the interconnection network at low metallic filler content. Ag is the most common metallic filler since the low resistivity, however, the drawbacks of Ag are its fatal electromigration and relatively high cost.30−31 Hence, alternatives such as Cu and Ni are used to replace Ag as metallic fillers to fabricate low-cost and high-performance pastes. Cu could be a particularly good prospective filler because of the high conductivity, low cost, especially the better electromigration properties.32−34 Unfortunately, Cu is prone to be oxidized,


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resulting in the decreasing of conductivity and reliability. To supersede these limitations, Agcoated Cu powders and Cu-Ag alloy have been applied as conductive fillers.35−40 Nishikawa et al. reported Ag-coated Cu as the conductive filler for conductive pastes and investigated its electrical resistivity after reliability testing; the paste filled with Ag-coated Cu shown higher electrical conductivity with better stability than Cu based paste.35 Cui et al. developed Cu@Ag filled conductive pastes with good conductivity and shear strength, also low cost compared with conventional pastes.36 Moreover, Zhu et al. verified that the electrochemical migration resistance of silver-plated copper based paste was dramatically higher than that of Ag based paste, which greatly improved the reliability of the electrically conductive adhesives.37 Even, Kim et al. demonstrated the bimetallic Ag-Cu nanoparticles are less easy to be oxidized which could be ascribed to the electron transfer from copper to silver within and between particles.38 Therefore, it is reasonable to believe that Cu-Ag alloy would bring better comprehensive performance in the application of conductive pastes. Herein, a simple, inexpensive, and eco-friendly synthesis method of Cu-Ag nanoflakes has been developed. Elliptic alloy nanoflakes with high purity were prepared by in-situ replacement of Ag source with home-made Cu particles. Assisting by PVP, the reaction took place under moderate circumstance, without any other complex organics, thus side reactions and the need for tedious post-processing were eliminated. Next, the prepared nanoflakes with high purity and uniformity were employed to manufacture conductive pastes. The resistivity was studied via screen printing onto different substrates. The operation process is uncomplicated, cost-efficient, poisonless, and furthermore, is a post-treatment technology.


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2. Experimental 2.1 Materials Cu hydroxide (Cu(OH)2) was purchased from Aladdin Reagent Co., Ltd. (China). L-ascorbic acid (C6H8O6), ethylene glycol (HOCH2CH2OH, EG), ethanol (CH3CH2OH, EtOH), polyvinyl pyrrolidone (PVP, K30), silver nitrate (AgNO3), and other conductive paste solvents were purchased from Sinopharm Chemical Reagent Co., Ltd. (China). Bisphenol-F epoxy resin (EPON 862) and aliphatic epoxy resin (ERL 4221) were purchased from Shell Co., Ltd. (China) and Nantong Xingchen Synthetic Material Co., Ltd. (China), respectively. Methyl hexahydrophthalic anhydride (MHPA) and 2-ethyl-4-methylimidazole (2E4MZ) were both purchased from Shanghai Lingfeng Chemical Co., Ltd. (China). Commercial Ag microflakes from Northwest Rare Metal Materials Research Institute Ningxia Co., Ltd. (China) were used as comparison. 2.2 Preparation of Cu-Ag Nanoflakes and Their Conductive Pastes Cu particles were synthesized according to a previously reported liquid-phase reduction method.41 Then, the prepared Cu powders and PVP evenly dispersed in EtOH were heated to 50 °C with magnetic stirring. AgNO3 dissolved in H2O was one-time added into the Cu particles dispersion. The resulting reaction was maintained at 50 °C for a further 2 h, and the color gradually changed from light red-brown to black, then to silver-gray. The solution was centrifuged and washed with ethanol and deionized water. Finally, 0.4 g alloy nanoflakes could be obtained in 50 ml reaction mixture with a yield of 97%. The conductive pastes were composed of conductive filler, epoxy resin, curing agent and catalyst. The epoxy resin was a mixture of EPON 862 and ERL 4221. MHPA and 2E4MZ were


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used as the curing agent and catalyst, respectively. The nanoflake-based pastes were fabricated through high-speed planetary mixing in vacuum for 2 min, ensuring the particles could be evenly dispersed in the resin without bubbles. Using this Cu-Ag nanoflake-based paste, various required patterns were formed on polyethylene terephthalate/polyimide (PET/PI) substrates using silkscreen printing. Commercial Ag microflakes with a size of ca. 3 μm, treated under the same mixing and printing process conditions, were used as a comparison. 2.3 Characterization The morphology and size of prepared samples were observed using a field-emission scanning electron microscope (FE-SEM, FEI Nova Nano SEM 450, USA) and a transmission electron microscope (TEM, FEI Tecnai G2F20S-TWIN, USA). The size distributions of Cu particles were tested using a laser particle analyzer (Malvern Zetasizer Nano ZS90, UK). UV-Vis absorption spectra were recorded using an UV-Vis-NIR spectrometer (Shimadzu UV-3600, Japan) over the wavelength range of 300–800 nm. The powder X-ray diffraction patterns were measured with an X-ray diffractometer (Rigaku D/Max 2500, Japan) using monochromated Cu K-α radiation (λ = 1.541874 Ǻ). X-ray photoelectron spectroscopy (XPS) spectra were recorded with a PHI-5702 multifunctional spectrometer using an Al K-α X-ray excitation source. Elemental analysis of the nanoflakes was tested using inductively coupled plasma-atomic emission spectrometer (ICP-AES, PerkinElmer OPTIMA 7000DV, USA) and energy dispersive spectrometer (EDS, FEI Nova Nano SEM 450, USA). Regarding the ICP-AES test, samples were weighed and dissolved in nitric acid with different concentrations at elevated temperature. The average values of Ag and Cu metal compositions were evaluated by means of six measurements. For conductivity property analysis, the electrical resistances were tested with a Keithley 2410 SourceMeter, and 5 specimens were measured for each kind of sample.


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3. Results and Discussion 3.1 Characterization of Prepared Cu-Ag Nanoflakes Cu particles were first prepared by the modified polyol method described in our previous work.41 Figure 1a and 1b present the SEM image and size distribution, respectively, of the synthesized Cu materials, revealing their quite high degree of sphericity as well as good dispersibility and monodispersity. They have a relatively narrow size distribution of 390 ± 30 nm. Whereas, after reacting with Ag source, the shape of the particles were transformed from spherical into elliptic flake-shaped structures (Figure 1c). Overall, the synthesized nanoflakes present relatively uniform size distributions, and no other morphology was shown in the SEM images. It turns out that all the particles were transformed into nanoflakes. The elliptic flakes shown in Figure 1d present ca. 700 nm long and 500 nm wide, with a smooth and flat surface. Moreover, the flakes are relatively thin, and almost transparent, with a thickness of ca. 30 nm.

Figure 1. (a) SEM image and (b) particle size distribution of the prepared Cu particles; (c) and (d), SEM images of the prepared Cu-Ag nanoflakes.


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As the result of a further investigation, TEM images of the synthesized elliptic flakes are reproduced in Figure 2. The size and morphology are basically consistent with the previous SEM images. The same shallow contrast throughout similarly reflects the uniform thinness of the synthesized nanoflakes. The HRTEM image (Figure 2b) presents the surface morphology of the synthesized nanoflakes in more detail, showing that they are composed of several crystalline grains with varying sizes. It is obvious that the small crystalline grains of different orientations staggered grow together. Furthermore, lattice fringes are clearly apparent, and lattice d-spacings were 0.2353 nm and 0.2102 nm. They are in good agreement with the interplanar distance values of (111) planes in fcc cubic Ag and Cu crystals, respectively. The TEM mapping images of one prepared nanoflakes were shown in Figure 2(d-f). As we can see, Cu element and Ag element are

Figure 2. (a)–(c): TEM images of the prepared Cu-Ag nanoflakes; (d)–(f): TEM mapping images of the prepared Cu-Ag nanoflake.


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evenly distributed on the nanoflake, which is consistent with the HRTEM image. Thus, we conclude that the staggered grains on the surfaces of the nanoflakes are Cu and Ag crystal grains. This result proves that through the galvanic replacement reaction, the synthesized uniform elliptic nanoflakes are composed of an alloy of Ag and Cu. What's more, the elliptic alloy nanoflakes with high purity reveal excellent dispersibility and monodispersity, rendering them suitable for various conductive applications. The UV-Vis absorption spectra of prepared alloyed nanoflakes are exhibited in Figure 3a. The peak at about 415 nm, is assigned to Ag and the other peak around 550 nm could be response from Cu.42−43 To further verify the composition of the synthesized nanoflakes, the precursor Cu particles, as well as the alloyed nanoflakes, were measured via XRD. Figure 3b-A presents the XRD pattern of prepared particles, showing the sharp and strong 2θ peaks located at 43.47°, 50.67°and 74.68°. Therefore, the synthesized particles are verified to be pure Cu crystallites with high orientation. Then the XRD pattern of transformed elliptic flakes is presented in Figure 3b-B. Besides the response of Cu, typical peaks of Ag at 38.38°, 44.58°, 64.78°, 77.48°also show up, corresponding to (111), (200), (220), and (311) planes respectively. To further explore the composition of the nanoflakes, XPS was also measured. Figure 3c clearly presents the peak of Cu spectra at 932.1 eV (2p3/2), and an extremely weak peak of CuO at 934.0 eV. Combined with the XRD results, we can roughly consider that the prepared alloy flakes are almost free of copper oxide. Meanwhile, the characteristic Ag peaks appear at 368.2 eV (3d5/2) and 374.2 eV (3d3/2) in Figure 3d. These characterizations intuitively and systematically prove that the prepared elliptic nanoflakes are composed of copper and silver alloy.


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Figure 3. (a) UV-Vis absorption spectrum of synthesized Cu-Ag nanoflakes; (b) XRD patterns of (A) prepared Cu particles and (B) Cu-Ag nanoflakes; (c) and (d), XPS results of prepared Cu-Ag nanoflakes.

Figure 4. XRD and XPS results of freshly prepared (a, d), 15 days stored (b, e), and 30 days stored (c, f) nanoflakes.


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The anti-oxidation of Cu based nanomaterials is really crucial to the properties and successful applications. Therefore the antioxidant tests after 15 days and 30 days storage have been done by XRD and XPS (Figure 4). From the result we can roughly think the samples still can maintain the composition of pure Cu and Ag after 15 days. While after placed for 30 days, one peak of CuO appears on the XRD pattern and also a small peak of CuO presents on the XPS spectrum, which means Cu in the alloy nanoflakes was slightly oxided. Fortunately, it is possible to remove the O during the heat-treatment within 5% H2-containing gas, which would not seriously affect the comprehensive performance such as conductivity and mechanical strength. To verify the elementary composition of prepared elliptic nanoflakes, the overall chemical composition was obtained by means of various elemental analysis methods. Table 1 displays the original added value, theoretical value after reaction, the ICP-AES result, and the EDS result of Cu/Ag molar ratio, respectively. Among them, the theoretical value refers to two limiting values, that is, the molar ratio of Cu/Ag when the Ag ions are completely reduced by PVP (4.4270:1), or the ratio for the case that the Ag ions are completely replaced by elemental Cu (3.9271:1). Combining these values with the EDS result, it could be seen that the added Ag ions almost all replaced elemental Cu, consequently forming the elliptic alloyed nanoflakes. Table 1. Metal composition analysis of the synthesized elliptic nanoflakes. Synthesized nanoflakes

Cu/Ag molar ratio

Original added value

4.4270 : 1

Theoretical value after reaction

(4.4270 : 1) ~ (3.9271 : 1)

ICP result

3.9625 : 1

EDS result

4.0607 : 1


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3.2 Mechanism of the Formation of Prepared Cu-Ag Alloy Nanoflakes For purpose of understanding the reaction mechanism, the process has been further studied from two aspects. One is the commonly used PVP, acting as dispersant and structure-directing agent in the formation of the elliptic nanoflakes. It could significantly improve Cu particles disperse uniformity in the ethanol solution. More importantly, it might selectively adsorb on the (100) surface of Cu grains, then the other crystal planes have more chance to be in contact with, and react with Ag+.44 Afterwards, PVP would also adsorb on the (100) surface of the generated Ag grains, inhibiting its perpendicular growth rate, and ultimately forming the Cu-Ag alloy nanoflakes.45 Therefore, PVP functions as a structure-inducing agent, which mediates Cu and Ag grains. The structural guidance of PVP in the synthesis of varied metal, oxides, and alloy nanomaterials also has been reported by Koczkur et al.46 They found that the adoption of PVP could kinetically control particles growth model through adjusting solubility in various solvents, and even preferential surface stabilization. It is worth mentioning that the concentration of PVP is also very important. Too much or too little both lead to irregular and heterogeneous alloy products, as shown in Figure 5a-d ([PVP] represents the optimal concentration of PVP). Therefore, nonpoisonous and low-cost PVP was employed as dispersant and structure-directing agent. There isn’t any additional complex organics appended, thus preventing the occurrence of side reactions and the need for tedious post-processing. The second factor in the formation mechanism of the elliptic Cu-Ag nanoflakes is the regulation of Ag precursor concentration. Actually, the essence of a galvanic replacement reaction is the REDOX reaction. According to thermodynamic theory, the concentration of oxidant and reductant will influence the electrode potential of REDOX reaction.47 It means the concentration of Ag+ would affect the electrode potential of this reaction (eq 1), thus affecting


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Figure 5. SEM images (a: half [PVP]; c: twice [PVP]; e: half [Ag+]; g: twice [Ag+]) and XRD patterns (b: half [PVP]; d: twice [PVP]; f: half [Ag+]; h: twice [Ag+]) of the synthesized Cu-Ag alloys.

(eq 1)

(eq 2)


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the Gibbs free energy of the system (eq 2), and probably affect the morphology and size of the Cu-Ag alloy. Eeq is the equilibrium potential, E0 is the standard electrode potential versus a standard electrode, R is the gas constant, T is the absolute temperature, n is the number of electrons transferred in the reaction, F is the Faraday constant, a is the equilibrium activity of the oxidized and reduced forms, and ΔG is the free energy change. Before the reaction starts, the concentration of Cu2+ is almost zero, so

tends to be minus infinity. When the

galvanic replacement begins, the concentration of Cu2+ increases gradually from 0, while that of Ag+ decreases gradually due to reduction. In this process, the value of ΔG goes from minus infinity to zero. Therefore, the concentration of silver ions largely determines the value of ΔG. We all know that the more negative the ΔG value, the faster the reaction rate, and the more rapidly the silver ions in the solution are reduced. Then a large number of Ag atoms will instantly nucleate on the each crystal plane surface of the exposed Cu particles, eventually generating the Cu@Ag core-shell structure. Liu also reported the effect of silver ion concentration on the replacement reaction.48 They demonstrated that the size and morphology of the formed silver nanostructure could be adjusted by systematically changing the identity and concentration of the starting Ag (I) species for the galvanic displacement reaction. In this rapid replacing process, PVP may not be able to play the role of structural guidance, so it is not conducive to the formation of nanoflake structure. We have further verified this process through experiments. In the case of other conditions being unchanged, when half the optimal concentration of Ag+ ([Ag+]) was used, spherical particles and irregular flakes were obtained (Figure 5e and f). Instead, Cu@Ag particles with core-shell structure were easily formed if increasing the concentration of the Ag precursor (Figure 5g and h). Summarizing, the assisting


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agent PVP and displacing agent AgNO3 jointly play critical and indispensable roles in the synthesis of Cu-Ag alloy nanoflakes. On this basis, a possible growth mechanism is proposed. At the beginning, Cu particles adsorbed by PVP on some crystalline surfaces were evenly dispersed in the ethanol solution. Then the added Ag ions are selectively adsorbed on one or more crystal planes of the rest Cu crystal grains (Figure 6). The galvanic replacement reaction will take place between Ag ions and elemental Cu due to the electric potential difference, resulting in some hollows on the surface of Cu particles. Almost immediately PVP would be adsorbed on the (100) surface of the generated Ag grains, inhibiting their perpendicular growth rate, and the remaining Ag ions are able to access the hollows to replace more Cu grains. Consequently, Cu and Ag crystal grains grow together in a staggered formation to compose the elliptic alloyed nanoflakes, under the synergistic effect of cooperation of PVP and Ag salt.

Figure 6. The reaction mechanism of Cu-Ag alloyed nanoflakes.


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3.3 Properties of Cu-Ag Nanoflake-Based Conductive Pastes Due to their perfect two-dimensional structure, the smart elliptic Cu-Ag nanoflakes were dispersed into epoxy resin to prepare conductive pastes with a loading content of 80 wt.%. In order to lower the viscosity for the mixing process, two kinds of epoxy resins (bisphenol F epoxy resin and cycloaliphatic epoxy resin), anhydride curing agent, and imidazole catalyst were added into the Cu-Ag flakes filled paste. Commercially available Ag microflakes (Figure 7, size of ca. 3 μm, and thickness of ca. 250 nm) were used to make conductive pastes with the same content. After commixture of the pastes in a blender mixer, homogeneous samples were screen-printed onto flexible substrates (PI and PET). Finally, the fabrication of the conductive films was

Figure 7. SEM images of the commercial Ag microflakes (a: ×5000; b: ×10000).

Figure 8. SEM images of the cured Cu-Ag alloy paste (a: ×5000; b: ×20000).


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completed by curing at 150 °C for 2 h under nitrogen. Figure 8 shows SEM images of the cured paste with 80 wt.% load content. As seen in Figure 8, the nanoflakes were evenly dispersed in epoxy resin. After curing, the edges of the flakes became indistinct and fuzzy, compared with the original morphology (Figure 1 and 2). This is probably due to the strong activity of the thinner flakes, so the atoms on the surface of nanoflakes could be easily rearranged under the curing at 150 °C for 2 h, which also promotes the diffusion of atoms between the flakes and forms a threedimensional interconnection network inside of the paste. After the thermal curing, bulk resistances of the samples was tested to be 3.75 × 10-5 Ω·cm, which is better than that of the commercially available conductive pastes (5.26 × 10-4 Ω·cm) and most of the literature reports. Yao39 reported a chemical treatment method of Ag-coated-Cu flakes to increase the conductivity of prepared ECAs to 1.28×10-4 Ω·cm. Another filler surface treatment method was also proved to achieve the electrical resistivity of 7.6×10-5 Ω·cm.9 A new dilute agent of 4-(tert-butyl) cyclohexyl acetate was developed for micron-sized Ag paste with an electrical resistivity of Ag patterns of 3×10-6 Ω·cm after cured at 280℃ for 30 min.8 It is suggested that the nanoscale size of synthesized alloy flakes might contribute a dominant effect to the enhanced conductivity. As discussed in the introduction, the excellent conductivity of the synthesized nanoflakes could be analyzed from the perspective of two general conductive theories. The percolation theory holds that fillers can form a conductive channel only when they’re in contact or less than 1 nm apart. Compared with the commonly used microflakes, the prepared nanoflakes with smaller density could fill more volume within the paste under the same loading content, thus the percolation threshold is lowered. As for the tunnel effect, it assumes that electrons can be activated by thermal vibrations to passing through the barrier of the interface layer and jump to the adjacent conductive particles if the insulation layer thickness is


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less than 10 nm, facilitating electrical conduction. Fortunately, as a filler, nanoflakes not only have two-dimensional morphology, but also possess higher specific surface area. After conventional curing at a particular temperature, the more active surface could act to accelerate the electron tunnelling and atomic diffusion. The contact resistance per unit linkage between the flakes would thus be strongly reduced, and the fillers are able to reach the percolation threshold at a lower volume, as a result of the increased likelihood of forming a conductive path. Based on the fact that the nanoflake filler is more likely to form a conductive path at the same load content and curing temperature, a lower resistivity is achieved. Continuous circuits with 1-mm conductor width and uniform surfaces were manufactured (Figure 9). As can be seen from Figure 9a and 9b, the conductive patterns have good flexibility and bending resistance on PI and PET substrates. Figure 9c-e are photos of the conductive pattern printed on a PI substrate inside beakers of different volumes. For 100-mL, 25-mL, and 10-mL beakers, the curvatures are 40.0 m-1, 62.5 m-1, and 90.9 m-1, respectively, meaning the conductive patterns can withstand bending at different angles, having good flexibility and applicability. In fact, conductive patterns on PI and PET substrates were bent repeatedly for 200 times, by inserting them into the 10-mL beaker. The resistivity of the samples was tested during these continuous bending experiments. As shown in Figure 10a, when the sample was bent 10 times, the resistivity was 3.94 × 10-5 Ω·cm, increased by 5.1%; then it gradually stabilized at 4.02 × 10-5 Ω·cm as the number of repeated bendings increased further. While for the commercial available Ag microflakes (Figure 10b), the resistivity started at 5.26 × 10-4 Ω·cm, and then eventually stabilized around 5.38 × 10-4 Ω·cm after a small increase. Compared with this, the resistivity of our sample was slightly increased in the initial bending process, which may be due to the little bit weak wettability between the nanoflakes and resin matrix. However, these


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patterns still have good conductivity even after 200 repeated bendings, probably because of the thinness of the smart nanoflakes, which greatly improves flexibility and bending resistance. Also due to the thinness of the nanoflakes, their based conductive paste is endowed with excellent bending resistance. Compared with spherical particles, micronflakes, and other kinds of

Figure 9. Photos of (a) Bent printed pattern on PI substrate; (b) Bent printed pattern on PET substrate; (c) Bent printed pattern on PI in a 100-mL beaker; (d) Bent printed pattern on PI in a 25-mL beaker; (e) Bent printed pattern on PI in a 10-mL beaker; (f) Printed pattern connected to LED on PI substrate after 200 repeated bendings; (g) Printed pattern connected with LED on PET substrate after 200 repeated bendings.


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conductive fillers, the nanoflakes with a thickness of 30 nm have excellent flexibility and toughness, which enables the conductive patterns to resist hundreds of bends. Moreover, alloy nanoflakes based conductive pastes dramatically reduce manufacturing costs by using mostly Cu instead of Ag while maintaining low resistivity. The high conductivity strongly confirmed our conjecture that the synthesized nanoflake-based conductive pastes have much superior electrical conductivity than microflakes.

Figure 10. The resistivity changes of conductive patterns with different filler (a: Alloy nanoflakes, b: Ag microflakes) after different bending times.

To verify the electrical conductive reliability, LEDs were integrated in circuits with the printed Cu-Ag patterns on PI and PET substrates, including the samples that had undergone 200 repeated bendings into the 10-mL beaker (Figure 9f and g). Constant voltage source supplied power to the circuits. The lighted LED convincingly proved that the Cu-Ag patterns on PI and PET all worked well. Therefore, the prepared nanoflake-based conductive paste possesses good


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performance and low-cost, which means it could be employed in flexible printed electronics, such as OLED, RFID, sensors, and so on.

4. Conclusions With the assistant of dispersing and structure-directing agents PVP, elliptic Cu-Ag nanoflakes were prepared via a facile, convenient, and cost-efficient in-situ galvanic replacement. The highpurified and uniform alloy nanoflakes presenting ca. 700 nm long, 500 nm wide, and 30 nm thin were employed to manufacture paste. And conductive patterns were formed through silk-screen printing onto PET/PI substrates. Upon curing at 150 °C for 2 h, the resistivity achieved 3.75 × 10-5 Ω·cm, which is much lower than that of commercial Ag micron flakes. Moreover, conductive patterns on PET/PI substrates retain good conductivity even after hundreds of repeated bendings at different angles. It is suggested that the nanoscale size of flakes might contribute a dominant effect to the enhanced conductivity, no matter from percolation theory or tunnel effect. As a filler, nanoflakes not only have two-dimensional morphology, but also possess higher specific surface area, which could act to accelerate the electron tunnelling and atomic diffusion. Based on these results, we believed that the convenient, cost-effective, and non-toxic alloy nanoflakes could be a smart filler applying to conductive pastes. ASSOCIATED CONTENT AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]


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Notes The authors declare no competing financial interest. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT This work was partially supported by the National Natural Science Foundation of China (61704033, U1601202, 61874155), the Foundation for Distinguished Young Talents in Higher Education of Guangdong (2016KQNCX046), and the Fund of Guangdong R&D Science and Technology (2017A050501053, 2017A010106005).

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PVP-Mediated Galvanic Replacement Synthesis of Smart Elliptic Cu

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