Sn Nanorods Supported Ag Nanoparticles as Efficient Catalysts for

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

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Sn-Nanorod-Supported Ag Nanoparticles as Efficient Catalysts for Electroless Deposition of Cu Conductive Tracks Jin-Qi Xie,†,‡ Ya-Qiang Ji,† Da-Sha Mao,†,‡ Fu-Tao Zhang,† Xian-Zhu Fu,*,†,§ Rong Sun,*,† and Ching-Ping Wong∥,⊥ †

Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences, Shenzhen 518055, China Shenzhen College of Advanced Technology, University of Chinese Academy of Sciences, Beijing 100049, China § College of Materials Science and Engineering, Shenzhen University, Shenzhen 518055, China ∥ Department of Electronics Engineering, The Chinese University of Hong Kong, Hong Kong 999077, China ⊥ School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States ‡

S Supporting Information *

ABSTRACT: The applications of tin are extremely wide-ranging, in fields as diverse as Li-ion batteries, catalysis, and electronic packaging. It is always significant but still remains a great challenge to develop facile and efficient routes to synthesize Sn nanostructures. Herein, we report a facile chemical method to synthesize a Sn nanorod crystal at room temperature, and Ag ions are subsequently introduced to form the Sn-nanorod-supported Ag nanoparticles hybrid structure (Sn/Ag nanorods). The Sn/Ag nanorods exhibit comparable activity to the commercial Pd black in catalyzing the electroless copper deposition (ECD) reaction that is indispensable to fabricate printed circuit boards (PCBs). Furthermore, a screen printable adhesive is prepared by mixing the as-synthesized Sn/Ag nanorod powders and epoxy resin to fabricate activator patterns on epoxy laminate (EPL) and flexible substrates including polyethylene terephthalate (PET) and polytetrafluoroethylene (PTFE) fiber film. The printed areas are finally metalized by the ECD process to obtain the copper coatings with designed patterns that are confirmed to exhibit excellent electrical conductivity and flexibility. KEYWORDS: Sn nanorod, Sn/Ag nanorod, quartz crystal microbalance, electroless copper deposition, screen printing, printed electronics, flexible circuit

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INTRODUCTION Sn-based materials have been investigated for numerous applications such as next-generation rechargeable Li-ion batteries and lower-melting-point solders.1−5 Particularly, Sn is unique as a promoter in catalyst formulations to achieve high activity and selectivity.6−8 Typically, catalysts such as Pt and Pd can be modified by the addition of Sn, and thus exhibit significantly enhanced catalytic performance.9−17 Despite Sncontaining catalysts being well-documented,18−21 Sn-nanostructure-supported catalysts still have not been investigated. There are only a handful of reported articles that involve Sn nanostructures synthesized by either the physical method with special apparatus and high temperature or chemical process with tedious steps or rigorous conditions.2,3 It is challenging but desirable to develop facile routes to synthesize Sn nanostructures, which can provide a great facility to formulate Sn-based nanomaterials. Currently printed electronics (PEs) have attracted great attention due to their diverse applications in fields such as printed circuit boards (PCBs), flexible circuit boards (FCBs) and integrated circuits (ICs).22−24 Generally, printed circuits © XXXX American Chemical Society

can be achieved by fabrication of a conductive circuit on substrates using printable conductive inks or adhesives.25−27 Compared with copper tracks fabricated by a conventional resist-based lithographic process, printed circuits are featured by a concise process and relatively low cost. However, the circuits fabricated directly using conductive inks or adhesives are much more inferior in electrical conductivity to the copper tracks. For the acquisition of circuits that capture the advantages of both the high electrical conductivity of copper and the concise process of printed electronics, efforts could be concentrated on the electroless copper deposition (ECD) process accomplished directly on printed catalyst patterns, around which considerable work has been reported.28−32 Activation is an indispensable process for electroless copper deposition, in which the copper ions are reduced by the reducing agent, and the activator (catalyst) acts as the medium transferring electrons from the reducing agent to the copper Received: January 8, 2018 Accepted: March 23, 2018 Published: March 23, 2018 A

DOI: 10.1021/acsanm.8b00006 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

Article

ACS Applied Nano Materials ions.31,33 The excellent catalytic activity of Pd for the ECD reaction makes the Pd-based activation technology the most common application in the contemporary ECD industry. Nevertheless, it is the high material cost of Pd that makes the idea of exploring the cost-efficient alternatives attractive.34,35 It is well-established that, in addition to Pd, the transition metals including Au, Ag, and Cu also exhibit different degrees of catalytic activity toward the ECD reaction.36 Interestingly, some investigations indicated that Ag-based catalysts can be prepared similar to the formation of conventional Sn/Pd colloidal catalyst,37,38 and recently, copper patterns fabricated with printing technology and the ECD process that applies Ag as catalyst were also reported.29,39 Consequently, Ag as a promising alternative to Pd in catalyzing the ECD reaction is worthy of attention. However, it is necessary to improve the catalytic activity of Ag for ECD, for which it is difficult to meet the requirement in industrial production. Tin, a commonly used promoter in catalyst formulations, has been demonstrated to interact with the oxygen atom in the carbonyl group of unsaturated aldehyde (formaldehyde)16 that is commonly applied as the reducing agent for the ECD reaction. Additionally, a Sn/Ag bimetallic catalyst exhibiting high activity for CO2 reduction has been reported recently.6 Therefore, Sn could be considered as a promising candidate to enhance the activity of Ag for ECD. Furthermore, the anisotropic onedimensional metal nanorod could be beneficial to electron and mass transfer, and the one-dimensional nanocatalyst can avoid the drawbacks of nanoparticle of Ostwald ripening, accumulation, migration, and loss.40,41 Herein, we demonstrate a facile chemical route to synthesize a Sn nanorod at room temperature. In this approach, SnCl2 H2O is reduced by NaBH4 in anhydrous ethanol, using sodium oleate as a surface-confining agent. Furthermore, Sn-nanorodsupported Ag nanoparticles with excellent catalytic activity for ECD are prepared, for which the activity is comparable to that of commercial Pd black and remarkably higher than that of the as-prepared Ag nanoparticle in catalyzing the ECD reaction. Moreover, the as-synthesized Sn/Ag catalyst is further applied as the filler of an epoxy adhesive to print the designed patterns on various substrates including epoxy laminate (EPL), polyethylene terephthalate (PET), and hydrophilic polytetrafluoroethylene (PTFE) fiber film by screen printing. Through the ECD process, copper can be deposited on the areas that are loaded with catalyst to form the highly conductive copper coatings with the designed patterns, rather than requiring a lithographic process.

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Scheme 1. Schematic Illustration of Synthesis Procedure of the Sn/Ag Nanorods and Fabrication Process of Copper Patterns

ethyl alcohol were added to a hydrothermal synthesis reactor with a maximum capacity of 100 mL. After stirring for 1 h, 0.065 g of NaBH4 was added into the above solution at one time, and the reactor was sealed immediately followed by stirring for 4 h at room temperature, resulting in a gray dispersion solution. Subsequently, for preparing Sn/ Ag hybrid catalysts, 10 mL of AgNO3 ethyl alcohol solution (20 mM) was injected dropwise into the above Sn dispersion solution. After stirring for 3 h at room temperature, a black dispersion solution was formed. Additionally, for the preparation of Ag nanoparticles, 10 mL of AgNO3 ethanol solution (20 mM) was added dropwise to an ethanol solution (85 mL) containing 0.24 g of sodium oleate and 0.065 g of NaBH4 with continuous stirring for 3 h to yield a black−brown dispersion solution. All of these as-synthesized nanostructures can be isolated by centrifugation and washed alternately with ethanol and deionized water. The Sn/Ag nanoparticles can be prepared by increasing the concentration of SnCl2 H2O. Pretreatment of Substrates. The hydrophilic PTFE fiber film can be used directly as the substrate for ECD while EPL and PET should be pretreated. The pretreatment of EPL consists of two processes: swelling and etching.38 The swelling solution was prepared by mixing 2-(2-butoxyethoxy)ethanol and deionized water with an equal volume proportion; the etching bath is a water solution resolving KMnO4 (55 g/L) and NaOH (1.2 M). Typically, a piece of EPL was washed with deionized water in advance and subsequently immersed in the swelling solution at 80 °C for 10 min. After that, the EPL was washed with deionized water again and placed in the etching bath at 80 °C for 10 min. The pretreatment of PET was simply accomplished by oxygen plasma for 15 min after mechanical coarsening. Fabrication of Copper Patterns. The fabrication of the copper pattern is illustrated in Scheme 1. An adhesive was prepared by mixing well epoxy resin, curing agent, and the as-prepared Sn/Ag catalyst powder at the mass ratio of 3:3:2. Subsequently, the adhesive was used to screen print on substrates with designed patterns, and was followed by curing in an oven at 120 °C for 1 h. Afterward, the pretreated substrates were immersed into the ECD bath that comprises copper sulfate (15 g/L), potassium sodium tartrate (14 g/L), EDTA disodium (19.5 g/L), sodium hydroxide (14.5 g/L), 2,2′-bipyridyl (0.02 g/L), potassium ferrocyanide (0.01 g/L), and formaldehyde (15 mL/L) at room temperature to achieve metallization. Characterization. The morphologies and structures of the asprepared materials were characterized using field emission scanning electron microscopy (FE-SEM, FEI Nova Nano SEM 450) and transmission electron microscopy (TEM, Tecnai G2F20 FEI), respectively. X-ray diffraction patterns were collected using X-ray diffraction (XRD, Rigaku D/Max 2500) with Cu Kα radiation. The surface element composition and chemical state of the as-prepared samples were measured by a PHI-1800 X-ray photoelectron spectra spectrometer (XPS, PHI-1800). The silver contents in the assynthesized Sn/Ag nanorods were titrated by the Volhad method. The measurement of electrical resistance was conducted with a Keithley 2000 multimeter with four-point probes. The catalytic activity of catalysts for ECD was analyzed with an electrochemical quartz crystal microbalance (CHI440C, Shanghai,

EXPERIMENTAL SECTION

Chemicals and Materials. Tin(II) chloride dehydrate, ethanol, sodium oleate, copper sulfate pentahydrate, ethylenediamine tetraacetic acid disodium salt, potassium ferrocyanide, 2,2-bipyridyl, and potassium sodium tartrate were purchased from Sinopharm Chemical Regent Co., Ltd. Sodium borohydride, silver nitrate, and formaldehyde solution were obtained from Shanghai ling feng chemical reagent Co., Ltd. Epoxy resin (828) and hexahydro-4-methylphthalic anhydride (HMPA) were supplied by the Guangzhou Haoyun Chemical Industry Co., Ltd. (Guangzhou, China). Commercial Pd black was purchased from Sigma-Aldrich. All of these materials were used as received without further purification. High-purity N2 was supplied by the Shenzhen Hongzhou Industrial Gases Co., Ltd. (Shenzhen, China). Deionized Mini-Q water (18 MΩ cm) was used for all experiments. Synthesis. The synthesis procedure of the Sn/Ag nanorod is illustrated in Scheme 1. For the synthesis of the Sn nanorod precursor, 0.045 g of SnCl2 H2O, 0.24 g of sodium oleate, and 85 mL of absolute B


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ACS Applied Nano Materials

Figure 1. (a) SEM, (b) TEM, and (c, d) HRTEM images of the as-synthesized Sn nanorod: (inset in panel d) SAED pattern of the Sn nanorod. (e) SEM, (f) TEM, (g−i) HRTEM images, and (j) STEM-EELS mapping of the as-synthesized Sn/Ag nanorod.

Figure 2. (a) XRD patterns and (b−d) high-resolution XPS spectra of the as-synthesized Sn nanorod, Sn/Ag nanorod, and Ag nanoparticle. electrochemical test was accomplished by applying a three-electrode system in which the Hg/Hg2Cl2 electrode was used as the reference electrode and the platinum-plate electrode as the counter electrode, and the working electrode was prepared by successively dropping 10 μL of catalyst (1 μg/μL) and 10 μL of Nafion solution (0.1 wt %) onto a glass carbon electrode (GCE, 0.196 cm2). The adhesion cross-cut test42 was conducted to evaluate the adhesion between the electroless deposited copper coating and substrates. During the test, the deposited copper coating on the

China). The compositions of anodic bath, cathodic bath, and ECD bath are listed in Table S1, in which the anodic bath with absence of formaldehyde and the cathodic bath with absence of Cu ions were applied for the mixed potential theory (MPT) analysis, and the ECD bath was used in the quartz crystal microbalance (QCM) test. All of these baths were tested at room temperature after being bubbled with pure N2 for 30 min. In the QCM test, the working electrode was prepared by coating the catalyst (10 μg) onto the surface of the Au electrode uniformly, and all QCM curves were recorded for 500 s. The C


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Figure 3. (a) QCM curves of frequency changes vs ECD time of the commercial Pd black, the as-synthesized materials (Sn nanorod, Sn/Ag nanorod, and Ag nanoparticle), and the blank Au electrode. MPT curves of (b) the commercial Pd black, (c) the as-synthesized Sn/Ag hybrid nanorod, and (d) the Ag nanoparticle. substrate was cut into 100 square grids (1 mm × 1 mm), and these grids were subsequently covered and stuck with 3 M 600 tape. After that, the tape was quickly peeled to count the damaged grids. The flexibility of the copper coating on flexible substrates (EPL and PET) was characterized by the bending test. In detail, copper lines with 10 cm in length and 1 mm in width were fabricated on the flexible substrates (PET and PTFE) by the ECD process and were bent at different radii of curvature for 1000 cycles to record the change of electrical resistance.

molar percentage, which is close to the result from energydispersive spectroscopy (EDS) in Figure S2. Figure 2a shows the X-ray diffraction (XRD) patterns of the as-synthesized Ag nanoparticle, Sn nanorod, and Sn/Ag hybrid nanorod. The XRD pattern of the Sn nanorod confirms its crystalline structure of tetragonal β-Sn (space group I41/amd; a = 0.583 08, c = 0.318 10), which conforms to the standard data of JCPDS 65-2631 and agrees well with the results of HRTEM and SAED patterns in Figure 1d. The diffraction peaks of Ag (JCPDS 65-2871) appear in the XRD pattern of the Sn/Ag hybrid nanorod while no Sn/Ag alloy peaks are detected, indicating the presence of metallic Ag and the absence of Sn/ Ag alloy in the Sn/Ag hybrid structure. Figure 2b shows the fitted high-resolution Sn 3d spectrum of the as-synthesized Sn nanorod, where two dominant peaks at 495.5 and 486.9 eV can be ascribed to the peaks of Sn 3d3/2 and 3d5/2 for Sn(IV), and two faint peaks at 493.9 and 484.3 eV are consistent with the binding energies of Sn 3d3/2 and 3d5/2 for metallic Sn,43 which indicates that the surface of the Sn nanorod comprises an abundance of SnO2 and scarce metallic Sn. This result can be further confirmed by the TEM image of a typical Sn nanorod (shown in Figure S1a) and STEM-EELS mapping of the Sn nanorod (shown in Figure S3b−d). In detail, from Figure S1a, a distinct interface at the edge of a Sn nanorod can be observed, and from Figure S3b−d, it can be confirmed that oxygen exists abundantly in the Sn nanorod. Shown in Figure 2c,d is the comparison of XPS spectra between the pure substances (Sn nanorod and Ag nanoparticle) and the Sn/Ag hybrid nanorod. It is discovered from Figure 2c that the 3d binding energy of Sn shifts to the lower energy side with 0.3 eV after the deposition of Ag. Moreover, as can be observed in Figure 2d, the Ag 3d3/2 and the Ag 3d5/2 peaks for the Sn/Ag hybrid nanorod also exhibit lower binding energy with 0.6 and 0.7 eV, respectively, as compared to those for the Ag nanoparticle. The conspicuous difference of electron binding energy between the pure

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RESULTS AND DISCUSSION Morphologies, Structure, and Composition of the Sn/ Ag Nanorod. The SEM and TEM images in Figure 1a,b demonstrate the well-defined rod morphology of the assynthesized bare Sn material. Additionally, the high-resolution TEM (HRTEM) and the selected-area electron diffraction (SAED) images in Figure 1c,d show that the Sn nanorod tends to grow along the [001] direction with high crystallinity, and the atomic lattice fringes with d spacings of 0.29 and 0.28 nm are resolved, which match well with the lattice planes of (200) and (101) of β-Sn, respectively. As can be observed from the SEM, TEM, and STEM-EELS mapping images of the assynthesized Sn/Ag structure in Figure 1e−j, the addition of Ag does not destroy the morphology of the Sn nanorod, and the Ag nanoparticles disperse uniformly on the surface of the Sn nanorod. For a further investigation of the surface structure and composition of the Sn/Ag hybrid nanorod, typical TEM and HRTEM images are analyzed and displayed in Figure S1b−d. As shown in the local magnification TEM image of a Sn/Ag nanorod (Figure S1b), Ag nanoparticles are evident on the surface of the Sn nanorod, and most of the Ag particles are smaller than 10 nm. Furthermore, the composition of the assynthesized Sn/Ag hybrid nanorod is analyzed by Volhad titrimetric method that confirms the Ag content of 8.1% in D


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ACS Applied Nano Materials substances (Sn nanorod and Ag nanoparticle) and the Sn/Ag hybrid nanorod indicates that the Sn nanorods have an impact on the electron structure of Ag nanoparticles. Catalytic Activity for ECD. With the low detection limit for mass (10−9 g), quartz crystal microbalance (QCM) is usually used to reflect the mass variance on the quartz electrode during chemical reaction. Here the catalytic activity of the assynthesized Sn/Ag nanorod for ECD is characterized by means of QCM. Clearly, ECD is a mass-increasing deposition process on the substrate, and the increasing rate of mass can be applied to reflect the rate of the ECD reaction. As for the QCM technology, the mass change of the surface on the quartz electrode is inversely proportional to the frequency change (Δf) of quartz crystal, which could be described in the following equation: Δf =

R̅MA =

Table 1. ECD Kinetics of Catalysts Analyzed by QCM and MPT

(1)

where A is the area of Au plate, μ is the shear coefficient of quartz crystal, and f 0 is the base resonant frequency. Herein, A = 0.196 cm2; μ = 2.947 × 1011 g/(cm s2); and f 0 = 7.955 MHz. It can be easily calculated that the mass increment of 1.34 ng on the Au substrate corresponds to the Δf decrement of 1 Hz. As shown in Figure 3a, the QCM curves for the blank Au electrode, commercial Pd black, and the as-synthesized materials (Ag nanoparticles, Sn nanorod, and Sn/Ag nanorod) were recorded. It is discovered from Figure 3a that no apparent Δf decrement occurs on the blank Au electrode during the ECD process, indicating no influence of the Au electrode to kinetics of the ECD process. In addition, similar to the case of the blank Au electrode, the QCM curve for the electrode coated with the Sn nanorod is also horizontal, which means that the Sn nanorod does not react with Cu2+ in the ECD bath. This consequence is reasonable because of the existence of a thin layer of SnO2 on the surface of the Sn nanorod, which avoids the occurrence of the replacement reaction between the Sn nanorod and the Cu2+. The QCM curves for catalysts including the as-synthesized Sn/Ag nanorod, the Ag nanoparticle, and commercial Pd black all exhibit decreasing Δf with different slopes. Clearly, the QCM curve slope of the Sn/Ag nanorod is close to that of the commercial Pd black and significantly lower than that of the Ag nanoparticle, from which we can preliminarily infer that the Sn/Ag nanorod is more active for ECD than the Ag nanoparticle. To further clarify and quantitatively analyze the activity of catalysts for ECD, the mass of activity (MA), the average mass of activity (MA ), and the average copper deposition rate (R̅MA ) can be defined as follows:44

MA =

mCu mcatalyst

MA1 + MA 2 + ··· + MA n N

catalyst

Emp (V)

jdeposition (mA/cm2)

MA

R̅MA

Sn/Ag nanorod Ag nanoparticle commercial Pd black

−0.33 −0.51 −0.44

−0.42 −1.18 −0.36

2.52 1.06 2.74

1.02 × 10−2 4.24 × 10−3 1.09 × 10−2

Furthermore, the activity of the catalysts for ECD can also be characterized by electrochemical method. Mixed potential theory (MPT), a typical electrochemical method usually used to analyze kinetics for electroless deposition reactions,45−48 was applied here to further confirm the results from the QCM test. MPT is based on the two Tafel curves of anodic and cathodic reactions, and the coordinate values for the intersecting point of the two Tafel curves on the horizontal and vertical axis represent deposition current density (jdeposition) and mixed potential (Emp), respectively, at which the ECD reaction occurs steadily. Generally, the ECD reaction rate is proportional to the value of jdeposition. As can be seen from the MPT result in Figure 3b−d, the Sn/Ag nanorod shows a similar value of deposition current density to commercial Pd black [jdeposition(Sn/Ag) = −0.42 mA/cm2; jdeposition(Pd black) = −0.36 mA/cm2], while the difference of deposition current density between the Sn/Ag nanorod and the Ag nanoparticle is conspicuous [jdeposition(Ag) = −1.18 mA/cm2], which is in good agreement with the result from QCM. Both QCM and MPT analysis demonstrate that the assynthesized Sn/Ag nanorod exhibits remarkably higher catalytic activity toward ECD than the Ag nanoparticle, suggesting that the catalytic activity of the Ag nanoparticle can be significantly enhanced by the Sn nanorod supporter. This result could be associated with the XPS analysis mentioned above, in which the 3d electron binding energy of Ag in the Sn/Ag hybrid nanorod is conspicuously higher than that of the freestanding Ag nanoparticle. The Sn nanorod supporters have a strong promoting effect for Ag nanoparticle catalysis in ECD. Sn nanospheres as supporter are also prepared by changing the concentration of reactants. Figure S4 shows the catalytic activity of the Sn-nanorod-supported Ag nanoparticles with that of the Sn-nanosphere-supported Ag nanoparticles by QCM for ECD. The results indicate that the catalytic activity of the Snnanorod-supported Ag nanoparticles is better than that of the Sn-nanosphere-supported Ag nanoparticles for the ECD reaction. Despite the fact that the specific surface area of the nanosphere or nanoparticle is larger than that of the nanorod,

(2)

where mCu is the mass of deposited metallic Cu, and mcatalyst is the mass of catalyst used during the QCM test. Herein, mcatalyst = 10 μg (on the basis of the mass of Ag and Pd). MA =

(4)

where timesum is the sum of the deposition time for each MA 5000 (timesum = ∑1 0.1n = 1 250 250 s). It is easy from eqs 2−4 to conclude that both the average mass of activity (MA ) and the average copper deposition rate (R̅MA ) can be used as the criteria to quantitatively characterize the activity of catalysts for ECD. On the basis of the recorded QCM curves, the values of MA and R̅MA for each catalyst were obtained by calculation. As listed in Table 1, the values of MA and R̅MA of the Sn/Ag nanorod were, respectively, calculated to be 2.52 and 1.02 × 10−2, which are almost 2.5 times greater than those of the Ag nanoparticle.

−2f02 Δm A μρ

MA1 + MA 2 + ··· + MA n timesum

(3)

Here, N is the number of data points recorded from 0 to 500 s at the interval of 0.1 s; therefore N = 5000. MAn is the mass of activity (MA) corresponding to the n data point. E


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The two stages in the relation curves indicate the process of copper deposition from discrete to continuous state. For the first stage, the surface resistivity of the Cu coating using Sn/Ag nanorod catalysts decreased more greatly than that using the Ag nanoparticle catalyst at the same deposition time, suggesting the much higher efficiency of Sn/Ag nanorods as compared to that of the Ag nanoparticles for ECD catalysis. When the copper deposition becomes a continuous state, the two curves show a similar decrement because the catalysts (Sn/Ag and Ag) are enclosed by the deposited copper which would act as the self-catalyst for the further ECD reaction. Properties of the Fabricated Copper Tracks. Figure S5 shows the XRD patterns of the copper coating on various substrates, indicating the formation of cubic copper (a = 3.615; JCPDS 04-0836). The electrical bulk resistivity of the deposited copper can be calculated with the following equation:

the one-dimensional Sn-supported Ag nanoparticles might have unique advantages such as the following: (i) Anisotropic onedimensional nanostructured supporters might have fewer grain boundaries, which are beneficial to electron and mass transfer for the improvement of the catalytic efficiency. (ii) Onedimensional Sn supporters could avoid the drawbacks of the Ag catalyst active nanoparticle of Ostwald ripening, accumulation, migration, and loss to achieve higher catalytic activity and stability. Therefore, herein, the as-prepared one-dimensional Sn-supporter-based catalysts demonstrate superiority over nanoparticle- or nanosphere-supported catalysts for the ECD reaction. Process of Copper Coatings Deposited Using Different Catalysts. The surface resistivity curves of Cu coating vary with time by using Sn/Ag nanorods and Ag nanoparticles as catalyst for electroless deposition of Cu as shown in Figure 4.

ρ = R sd

(5)

where ρ is the electrical bulk resistivity, Rs the sheet resistance, and d the thickness of the metal layer. The thickness of the copper coating was determined by SEM (Figure S6). After 1 h of electroless copper deposition, the thickness of the deposited copper on EPL, PET, and PTFE substrates was determined to be 2.1, 1.9, and 2.3 μm, respectively, and the electrical bulk resistivities of the deposited copper on EPL, PET, and PTFE substrates are calculated, respectively, to be 2.10 × 10−6, 2.35 × 10−6, and 2.02 × 10−6 Ω cm (shown in Figure 5a), which are comparable to the electrical bulk resistivity of cubic copper29 (1.68 × 10−6 Ω cm, 20 °C). Moreover, the electrical bulk resistivity of the as-fabricated copper coating barely changed after conservation in a dry and enclosed environment for 6 months. Figure 6 displays the digital photos and SEM images of the copper patterns fabricated on various substrates. It can be observed clearly from the SEM images (Figure 6d−i) that the surfaces of deposited copper coatings are compact and continuous without a void or crack, which helps to explain

Figure 4. Surface resistivity of Cu coating varies with time by using Sn/Ag nanorods and Ag nanoparticles.

Figure 5. (a) Electrical bulk resistivity of cubic copper and the as-fabricated copper coatings on various substrates. (b) Cu/EPL integrated with an LED. (c) Cu/PET and (d) Cu/PTFE integrated with an LED and resistance instrument before and after being folded against a glass slide. F


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Figure 6. Digital photos (top) and SEM images (bottom) of the copper patterns deposited on (a, d, g) epoxy laminate, (b, e, h) PET, and (c, f, i) PTFE fiber film.

Figure 7. (a) Bending test scheme for the as-fabricated copper circuits. Curves of the normalized electrical resistance (R/R0) vs bending cycles for the as-fabricated copper circuits on (b) PET and (c) PTFE.

why the deposited copper coatings exhibit excellent electrical conductivity and antioxidant properties. It is worth noting that the surface morphologies of electroless Cu on various substrates are different in roughness and compactness (Figure 6d−f), which is mainly because of the diversity in surface structure and roughness of these substrates. As can be observed

in Figure 6d−f, compared with Cu/EPL and Cu/PET, the copper coating on the porous PTFE substrate is the most compact yet most rough, which can be attributed to the porous and rough surface structure of the PTFT. Circuits fabricated with low electrical resistance can not only obviously decrease the wastage of electrical energy but also G


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Additionally, the as-synthesized Sn/Ag catalyst powders are filled in a printable epoxy adhesive to fabricate copper tracks by means of screen printing and the ECD process. The asfabricated copper conductive tracks on various substrates (Cu/ EPL, Cu/PET, Cu/PTFE) all exhibit excellent electrical conductivity that is comparable to that of cubic copper, and the as-fabricated flexible circuits (Cu/PET, Cu/PTFE) are also achieved with good flexible performance. In particular, the hydrophilic PTFE film with porous structure shows a special advantage as the substrate of ECD, on which the copper coating can not only be deposited much thicker than those on EPL and PET, but also exhibit better flexibility than the Cu/ PET.

strongly favor the loading of high-power electronic components in electronics. Specially, the PTFE substrate used here is a porous fiber film with pore size of 2−5 μm on which the deposited copper coating could be thicker than those on airtight plastic substrates, and the thickness of the copper coating on PTFE substrate can achieve 50 μm without a quality problem of bubbling and delamination. Accordingly, the sheet resistance of the copper coating on PTFE could be as low as 4.2 × 10−4 Ω/sq. An adhesion cross-cut test, a commonly used method for characterizing the adhesion between substrates and electroless deposited copper coatings, was conducted here to test the adhesion of the Cu/EPL and the Cu/PET (Cu/substrate: copper coating fabricated on the substrate). As shown in Figure S7, the Cu/EPL and the Cu/PET were cut into 100 square grids (1 mm × 1 mm), and every grid almost remained intact after being peeled off the tape, suggesting the excellent adhesion between the deposited copper coatings and substrates (EPL and PET). For a description of the flexibility of the electroless deposited copper coatings, copper circuits on the flexible substrates (PET and PTFE) were fabricated to be integrated with an LED (3 V) and resistance instrument to observe the different performances of the coatings at the folded and the unfolded states. As displayed in Figure 7b,c, both the Cu/PET and the Cu/PTFE show an inconspicuous change of brightness of the LED and electrical resistance value after being folded against a glass slide, which indicates that the as-fabricated copper coatings exhibit superior flexibility. Nevertheless, it is also necessary to systematically analyze the flexibility of the as-fabricated copper tracks.49,50 As described in Figure 7, the flexibility can be analyzed by the curves of R/R0 versus bending cycles, where R and R0 are the electrical resistance of the flexible copper circuits after and before the bending tests, respectively. It can be observed from Figure 7b,c that the R/R0 values of both the Cu/ PET and the Cu/PTFE remain stable at large bending radius after 1000 cycles, while, when the bending radius decreased to some extent, copper coatings tended to be damaged, and the values of R/R0 increased with bending cycles. In addition, when the Cu/PET and the Cu/PTFE were bent at the same radius, the R/R0 value of the Cu/PET is greater than that of the Cu/ PTFE. This result indicates that the flexibility of the Cu/PTFE is superior to that of the Cu/PET, which could be attributed to the special structure of the PTFE that is fabricated by tiny PTFE fibers to form a porous structure (Figure S8b). It is also significant to the PCB industry to fabricate a conductive line with a narrow width. As shown in Figure S8a, copper lines with width of 200 μm can be well-achieved. If we want to obtain a narrower line (nanoscale), it is necessary to prepare smaller catalysts and improve the line fabrication process. Furthermore, a pen-writable catalytic ink was developed here by using the as-synthesized Sn/Ag catalyst. This ink can be written on commonly used paper such as office paper and qualitative filter paper, and typical examples of the ECD process based on this ink are given in Figure S8c,d.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.8b00006. SEM images, high-magnification TEM images, EELS mapping, optical images, and composition of baths (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Xian-Zhu Fu: 0000-0003-1843-8927 Rong Sun: 0000-0001-9719-3563 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (21203236), Guangdong Department of Science and Technology (2017A050501052), and Shenzhen Research Plan (JCYJ20160229195455154).

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REFERENCES

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CONCLUSIONS Sn nanorods are synthesized by a facile solvothermal method at room temperature, and Sn-nanorod-supported Ag nanoparticles are subsequently obtained on the basis of the Sn nanorod. The Sn-nanorod-supported Ag nanoparticles exhibit comparable catalytic activity for ECD to the commercial Pd black that is 2.5 times more active for ECD than the Ag nanoparticle. H


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Sn Nanorods Supported Ag Nanoparticles as Efficient Catalysts for

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