Selective Metallization Induced by Laser Activation: Fabricating

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Selective Metallization Induced by Laser Activation: Fabricating Metallized Patterns on Polymer via Metal Oxide Composite Jihai Zhang, Tao Zhou, and Liang Wen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b15828 • Publication Date (Web): 20 Feb 2017 Downloaded from http://pubs.acs.org on February 24, 2017

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ACS Applied Materials & Interfaces

Selective Metallization Induced by Laser Activation: Fabricating Metallized Patterns on Polymer via Metal Oxide Composite Jihai Zhang, Tao Zhou,* and Liang Wen State Key Laboratory of Polymer Materials Engineering of China, Polymer Research Institute, Sichuan University, Chengdu 610065, China. *Corresponding author. Tel.: +86-28-85402601; Fax: +86-28-85402465; E-mail address: [email protected] (T. Zhou)

Keywords: Metallization; laser direct structuring; polymer; metal oxide; NIR pulsed laser.

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Abstract Recently, the metallization on polymer substrates has been paid more attention due to its outstanding properties of both plastics and metals. In this study, the metal oxide composite of copper-chromium oxide (CuO·Cr2O3) was incorporated into the polymer matrix to design a good laser direct structuring (LDS) material, and the well-defined copper pattern (thickness =10 µm) was successfully fabricated through selective metallization based on 1064 nm near-infrared (NIR) pulsed laser activation and electroless copper plating. We also prepared polymer composites incorporated with CuO and Cr2O3; however, these two polymer composites both had a very poor capacity of selective metallization, which has no practical value for LDS technology. In our work, the key reasons causing above results were systematically studied and elucidated using XPS, UV-vis-IR, optical microscopy, SEM, contact angle, ATR FTIR, and so on. The results showed that 54.0% Cu2+ in the polymer composite of CuO·Cr2O3 (the amount=5 wt.%) is reduced to Cu0 (elemental copper) after laser activation (irradiation); however, this value is only 26.8% for the polymer composite of CuO (the amount=5 wt.%). It was confirmed that to achieve a successful selective metallization after laser activation, not only the new formed Cu0 (the catalytic seeds) was the crucial factor, but also the number of generated Cu0 catalytic seeds was important. These two factors codetermined the final results of the selective metallization. The CuO·Cr2O3 is very suitable for applications of fabricating metallic patterns (e.g., metal decoration, circuit) on the inherent pure black or bright black polymer materials via LDS technology, which has a prospect of large-scale industrial applications.


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1. Introduction In the past decades, metallization of polymeric substrates has received considerable theoretical and experimental investigations due to its outstanding properties of both plastic and metal, including light weight, electrical conductivity, reflectivity, abrasion resistance, decorative effects, and so on.1-6 It is noteworthy that a proper pretreatment for the formation of catalytic surface is a crucial procedure prior to the electroless plating.7-13 Generally speaking, the traditional pretreatment was sulfuric-chromic acid etching followed by tin/palladium activation to deposit a catalyst onto the surface. Unfortunately, the most widely used chromic acid etchant is known as a human carcinogen and imposes serious environmental problems. Moreover, the noble metals are usually employed as the catalyst sites in the activation process, which increases the manufacturing costs.14 To the best of our knowledge, a series of alternative surface pretreatment approaches prior to electroless plating on polymer substrates were reported in the past decades.15-21 Teixeira et al.15 studied the surface pretreatment of acrylonitrile-butadiene-styrene copolymer (ABS) plastic using etching solutions based on sulfuric acid with hydrogen peroxide and/or nitric acid instead of chromic acid etchant. Recently, Alexandre Garcia et al.16 investigated a simple method for metallize ABS through poly(acrylic acid) (PAA) chemically grafted onto the ABS substrate surface. M. Bazzaoui et al.17 researched the direct plating process through chemical deposition of polypyrrole (PPy) on ABS surface. Luo et al.18 developed a palladium-free surface activation pretreatment method, and the granules copper were formed on the polycarbonate (PC) substrate after electroless copper plating. Zhang et al.19 reported electroless copper plating on the ABS resin surface modified by heterocyclic organosilane



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self-assembled film. Very recently, Zhai et al.20 described the process based on NaBH4 reducing the Cu2+ ions to Cu0 species, which acted as the catalyst to initiate autocatalytic deposition of copper in electroless plating bath. And Chen et al.21 introduced a layer-by-layer assembly technique to fabricate adhesion-enhanced metallic coating layers on the ABS plastic surfaces. Although the above reports were effective to a certain extent, these wet-chemical methods are more or less fussy and complex, also facing environment pollution problems and being harmful for sustainable development. Therefore, exploiting a more convenient and environmentally friendly technology for polymeric substrate metallization on a large scale will be urgent and of a great importance. Laser direct structuring (abbreviated to LDS) has a great application prospect in telecommunications, automotive electronics, wearable electronics, light-emitting displays (LEDs), computers, medical equipment, and other industries. Figure 1 illustrates the simple schematic of the LDS process, which merely comprises three primary steps: injection molding, laser activation, and metallization. For LDS technology, the selection of laser sensitizers incorporated into the polymer material is important and necessary. This is because, during laser activation, an appropriate laser sensitizer is reduced to metal seeds (the catalytic sites) in polymer composites and helps to generate the rough surface.22 That is to say, the laser activation process involved by laser sensitizer plays an important role in the success of metallization; furthermore, it has a contribution to the adhesion performance between the metallic layer and the substrate. Moreover, the obvious advantages of laser activation are elimination of wet-chemical pretreatment steps and offering an innovative method for the high-resolution selective activation.


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This study developed a convenient and efficient method for the fabrication of metallized patterns on polymeric substrates based on 1064 nm near-infrared (NIR) pulsed laser and the electroless copper plating. As is known, the 1064 nm NIR laser as a highly versatile industrial equipment has been widely applied in industry. For the main methods for metallization, the electroless plating is the most frequently used and has a largest-scale industrial application due to its simplicity and low cost. In our previous study, the copper hydroxyl phosphate [Cu2(OH)PO4] which has a strong absorption in the NIR region was incorporated into the polymer to successfully prepare an LDS material with an excellent performance.23 Importantly, we also confirmed the key mechanism of the selective metallization in LDS technology based on Cu2(OH)PO4.23 Therefore, a question is naturally put forward. Can there be other copper-containing substances with a high absorption in NIR region used as laser sensitizers in LDS materials? This arouses our great interest. In this study, the metal oxide composite of copper-chromium oxide (CuO·Cr2O3) was incorporated into the acrylonitrile-butadiene-styrene (ABS) matrix, and the feasibility of fabricating metallic patterns of this polymer composite via LDS technology was investigated. The experimental results demonstrated that ABS/CuO·Cr2O3 composite is a good LDS material, which can fabricate the excellent copper layer on the laser irradiated areas after the auto-catalytic electroless copper plating. Subsequently, the feasibility of copper oxide (CuO) and chromium oxide (Cr2O3) to prepare LDS materials was further studied. However, due to the poor or no selective metallization in the electroless plating, we confirmed that both CuO and Cr2O3 have no availability to be used to prepare the LDS material, and our study also revealed the key reason behind this phenomenon of CuO and Cr2O3. It is noted that the cost



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of CuO·Cr2O3 is only one-tenth of that of Cu2(OH)PO4, and therefore, it has a more industrial application prospect. Moreover, the color of CuO·Cr2O3 is solid black, and it presents a strong dyeing capability for polymers. So, CuO·Cr2O3 is very suitable for the application of fabricating metallic patterns (e.g., metal decoration) on the pure black or bright black plastic substrate via LDS technology. This study also provides a guideline for preparing essential black polymer composites for LDS.

Figure 1. Simple schematic illustration of the laser direct structuring (LDS) process. 2. Experimental 2.1. Materials Acrylonitrile-butadiene-styrene copolymer (ABS, PA-747, density: 1.03 g/cm3, melt flow rate: 1.2 g/10 min, 200 °C, 5 kg) was produced by Chi Mei Corporation (Taiwan). The copper oxide (CuO) and chromium oxide (Cr2O3) of analytical grade were supplied by Chengdu Kelong Reagent (China). Analytical grade copper-chromium oxide (CuO·Cr2O3) was


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purchased from Dazzling Jewelry New Materials Technology Co., Ltd. (China). The PXRD pattern, XPS spectra, TG curves of CuO, Cr2O3, and CuO·Cr2O3 used in our experiments are illustrated in Figures S1-S3 (Supporting Information). Analytical reagent grade copper sulfate (powder), ethylene diamine tetra-acetic acid (powder), potassium sodium tartrate (powder), sodium citrate (powder), methanol (liquid), formaldehyde (37 wt.% solution in water), absolute ethanol (liquid), and sodium hydroxide (powder) were bought from Chengdu Kelong Reagent (China). All the materials were used as received without any further treatment. 2.2. Preparation of polymer composite plates The composites of ABS/CuO·Cr2O3, ABS/CuO, and ABS/Cr2O3 with different amount of metal oxides were prepared by melt blending using a laboratory twin screw extruder machine at 210 °C. For ABS/CuO·Cr2O3 composites, the weight ratios of CuO·Cr2O3 to ABS were 99.8:0.2 (0.2 wt.%), 99.6:0.4 (0.4 wt.%), 99.4:0.6 (0.6 wt.%), 99.2:0.8 (0.8 wt.%), 99:1 (1 wt.%), 97:3 (3 wt.%), and 95:5 (5 wt.% ), respectively. For ABS/CuO composites, the weight ratios of CuO to ABS were 99:1 (1 wt.%), 97:3 (3 wt.%), 95:5 (5 wt.%), respectively. And, for ABS/Cr2O3 composites, the weight ratio of Cr2O3 to ABS was 95:5 (5 wt.%). Then, the ABS composite plates (78 mm×65 mm×3 mm) were molded using a laboratory injection molding machine at 220 °C. Finally, the obtained samples were placed into an oven at 60 °C for 4 hours to eliminate the inside stress. 2.3. Laser selective activation Laser activation in air atmosphere was conducted on an MK-GQ10B optical fiber pulsed laser machine system (λ=1064 nm, laser power: 0-10 W, Mike Laser Technology Co., Ltd.



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Kunshan, China) equipped with EZCAD 2.0 software. The details about the laser irradiation parameters were also described in our previous work.24 According to different laser parameters (laser scanning speed, laser power, and laser frequency), a series of square blocks corresponding to the different laser energy were designed. After laser irradiation, the as-prepared samples were cleaned with alcohol and deionized water in an ultrasonic bath for 10 min, respectively. 2.4. Electroless copper plating In our work, a homemade electroless copper plating bath was used, which contained copper sulfate (7 g/L) as a source of Cu2+ ions, potassium sodium tartrate (30 g/L), ethylene diamine tetra-acetic acid (3 g/L), sodium citrate (3 g/L) as the complexing agent, formaldehyde (12 ml/L) as the reducing agent, and methanol (150 ml/L) as the stabilizer. Sodium hydroxide was gradually added into the plating solution to provide an alkaline environment (pH=12.5). In the process of electroless copper plating, the time of electroless copper plating was 30 min, and the temperature was 50 °C. Furthermore, a continuous gas agitation was used to maintain the concentration of copper ion to be evenly distributed and to ensure the uniformity of the electroless copper layer. 2.5. Characterizations and testing The crystallographic information of CuO·Cr2O3, CuO, and Cr2O3 was recorded by powder X-ray diffraction (PXRD) using DX-1000 diffractometer with Cu Kα radiation (λ= 0.15418 nm). The experiments were operated at 35 kV and 25 mA. The samples were scanned in 2θ values between 10° and 80° at a scanning rate of 0.06° per second. The X-ray photoelectron spectroscopy (XPS) were collected on XASAM 800 spectrometer


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(Kratos analysis, UK) with an Al Kα X-ray source (1486.6 eV), and the X-ray beam was around 1 mm. The calibration of binding energy was carried out using 284.6 eV (C 1s peak contributed from CH2). The ultraviolet-visible-infrared (UV-vis-IR) spectroscopy of CuO·Cr2O3, CuO, and Cr2O3 was collected by UV-3600 UV-vis-NIR spectrophotometer (Shimadzu) in the region of 500-2000 nm at room temperature. The thermal gravimetric analysis (TGA) of CuO·Cr2O3, CuO, and Cr2O3 was measured using NETZSCHTG 209F1 thermal gravimetric analyzer in the range of 30-800 °C in a nitrogen atmosphere. The heating rate was 10 °C/min, and the sample size was 10 mg. The optical microscope (OM) images were observed by an optical microscope (PH-100, Phoenix Optics, China) with the reflection mode. Before the observation, the cross sections of the polymer composite samples were repeatedly polished using an abrasive paper with grain size of 7000 mesh. The micrographs of the scanning electron microscopy (SEM) were conducted on a field-emission scanning electron microscope (JEOL JSM-7500F) at 10 kV equipped with an energy-dispersive X-ray spectroscopy (EDX). Before measurements, a thin layer of gold was coated onto the surface of the samples to reduce charging during observations. The contact angle (CA) was recorded on a Krüss K100 contact-angle machine (Germany) using a sessile drop method with 2 µL double-distilled water droplet on the sample surface. The average CA values were calculated by measuring at least five different surface areas in the same samples. The attenuated total reflection (ATR) Fourier transform infrared (FTIR) spectroscopy of



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the sample surface in the region of 4000−400 cm-1 was recorded using Nicolet iS50 spectrometer equipped with a deuterated triglycine sulfate detector, and a smart iTR accessory was used. The ATR FTIR experiments were carried out at a spectral resolution of 4 cm-1, and 32 scans were coadded for each spectrum. The performance of mechanical adhesion between the obtained copper layer and polymer substrate was tested using a qualitative method according to ASTM D3359 (Scotch tape testing). Briefly, the test was carried out via suddenly removing the pressure-sensitive adhesive tape tightly sticking on the 1×1 mm2 crosshatched squares. The mechanical adhesion is usually classified into 6 levels based upon the percentage of the area being removed, including 0B (be removed more than 65%), 1B (be removed 35-65%), 2B (be removed 15-35%), 3B (be removed 5-15%), 4B (be removed less than 5%), 5B (none of the squares be removed). 3. Results and discussion


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Figure 2. (a) Schematic illustration of the galvanometer marking apparatus of 1064 nm NIR pulsed laser. (b) Computerized vector image of the coordinate system of solid square blocks (windows) with the different laser parameters, and the laser scanning speed is fixed at 2000 mm/s. Here, for example, in the window of (60 kHz, 80%), the laser pulse frequency is 60 kHz, and laser power is 8 W, respectively. (c) Digital photographs of ABS/CuO·Cr2O3 composite plate after NIR laser activation and 30 min electroless copper plating, respectively. (d) Optical microscope images of the surface and the cross-section of the obtained copper layer in the window of (60 kHz, 80%), scale bar: 100 µm. (e) SEM images of the surface of copper layer after 30 min electroless copper plating in the window of (60 kHz, 80%), scale bar: 200 µm; and the right is corresponding magnified SEM image, scale bar: 25 µm. (f) The process of electroless copper plating, The inserted image of the beaker is electroless copper plating solution. (g) Digital photographs of the obtained copper layer on the ABS substrate surface before and after scotch tape test, scale bar: 2 mm. (h) EDX analysis of the obtained copper layer, the inserted image is the corresponding SEM, scale bar 200 µm. As shown in Figure 2(a), the marking apparatus with galvanometers allows people to fabricate any fine images or patterns on a two-dimensional plane. In our experiments, the computerized vector image of the coordinated solid squares with different laser pulse frequency and laser power was designed. The laser scanning speed was fixed at 2000 mm/s. In Figure 2(b), the horizontal axis represents the laser pulse frequency (20, 30, ..., 90, 100 kHz), while the vertical axis represents the laser power (10, 20, ..., 90, 100%, for examples, 20%=2 W and 100%=10 W). The process of laser activation is achieved by scanning the sample surface line by line with a computer controlled 1064 nm NIR pulsed laser. The digital photographs of ABS/CuO·Cr2O3 composite plates after NIR laser activation and 30 min electroless copper plating are provided in Figure 2(c). It can be seen that the well-defined and high-resolution copper layer on the whole irradiated area is obtained on ABS/CuO·Cr2O3 composite surface, exhibiting an excellent performance of selective metallization in all the square blocks (windows) after 30 min electroless copper plating. As illustrated in Figure 2 (f), in our work, after laser activation, the polymer composite plate is directly immersed in the electroless copper plating solution. And, during the process of



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electroless copper plating, the metal ions in the solution can only be reduced to metal element and continue to deposit on the laser-activated areas. The optical microscope images of the surface and the cross-section of the obtained copper layer in the window of (60 kHz, 80%) are shown in Figure 2 (d). Apparently, from the surface image (the left of Figure 2 (d)), the un-irradiated area does not deposit any copper layer due to the absence of catalytic activity during electroless copper plating. Moreover, the cross-sectional image demonstrates that the thickness of metallized cooper is approximately at 10 µm after 30 min electroless copper plating. To further explore the morphologies of the obtained copper layer, the scanning electron microscopy (SEM) analyses were carried out. As shown in Figure 2 (e), it can be observed that, after electroless copper plating, the copper layer is generated on the surface of ABS substrate, which is composed of aggregated copper particles. In Figure 2 (h), the result of the energy dispersive X-ray (EDX) analysis also confirms the characteristic peaks of copper layer with one L peak at 0.9 keV and two K peaks at 8.0 keV and 8.9 keV, respectively. In addition, the adhesion property of the obtained copper layer on ABS substrate was evaluated by an intuitive and qualitative approach called the Scotch tape test. As shown in Figure 2 (g), the cross-hatched squared copper layers (1×1 mm2) are made by Cross Hatch cutter. It can be clearly seen that the squared copper layers are fully undamaged after suddenly removing a high-performance transparent scotch-tape, showing an excellent mechanical adhesion property of the copper layer. According to ASTM D3359 standard, the adhesion property between obtained copper layer and polymeric substrate reaches to the highest 5 B level, which is sufficient to meet the requirements of industrial applications.


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Figure 3. Digital photographs of the neat ABS resin, ABS/CuO, and ABS/Cr2O3 composites plates after NIR laser irradiation and 30 min electroless copper plating. From the viewpoint of chemical composition, CuO·Cr2O3 is made up of CuO and Cr2O3. Here, we naturally put forward a question. Whether can be CuO and Cr2O3 also used as laser sensitizers in LDS materials? This arouses our interest. Subsequently, the ABS/CuO (95:5) and ABS/Cr2O3 (95:5) polymer composites are prepared to investigate the feasibility for LDS technology. As shown in Figure 3, after NIR laser irradiation and 30 min electroless copper plating, for neat ABS resin, none of the copper layers are observed on the laser-irradiated areas. By contrast, for ABS/CuO composite, there appears a very small amount of copper layer on the laser-irradiated areas whose laser power are greater than 6 W (≥60 %); moreover, the quality of the obtained copper layer is very poor (0B level in Scotch tape test). Be similar with neat ABS, we also cannot observe any copper layer on the irradiation areas for ABS/Cr2O3 composite. That is to say, compared with ABS/CuO·Cr2O3, ABS/CuO and ABS/Cr2O3 composites are completely unavailable for LDS technology. What causes the above phenomenon? We speculate the main reason probably comes from the different laser activation effect of ABS/CuO·Cr2O3, ABS/CuO, and ABS/Cr2O3 composites, and thereby affecting the final results of electroless copper plating. Specifically, after laser activation, the laser-irradiated areas of ABS/CuO·Cr2O3 composite are possibly endowed with an excellent catalytic activity; however, the laser-irradiated areas of ABS/CuO and ABS/Cr2O3 composites 13 ACS Paragon Plus Environment


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have poor or no catalytic activity in electroless copper plating. To figure out the above problem encountered and to validate our speculation, a series of experiments and investigations are carried out. 3.1. Morphology of the laser-irradiated surface

Figure 4. Optical microscopy images and SEM morphology images of ABS composites after 1064 nm NIR pulsed laser irradiation in the window of (60 kHz, 80%). (a) The cross-section and the surface of ABS/CuO composite, scale bar: 100 µm; (b) the cross-section and the surface of ABS/Cr2O3 composite, scale bar: 100 µm; (c) the cross-section and the surface of ABS/CuO·Cr2O3 composite, scale bar: 100 µm; (d) SEM surface morphology of ABS/CuO composite, scale bar: 200 µm; (e) SEM surface morphology of ABS/Cr2O3 composite, scale bar: 200 µm; (f) SEM surface morphology of ABS/CuO·Cr2O3 composite, scale bar: 200 µm. The rights are corresponding magnified SEM images, scale bar: 30 µm. Theoretically, the surface of polymer composites occurring morphological changes is due to the thermal effect from a highly focused 1064 nm NIR pulsed laser beam. The cross-section and the surface images of ABS composites after laser irradiation in the window of (60 kHz, 80%) in Figure 2(c) and Figure 3 were observed using an optical microscopy. As shown in Figure 4 (a-c), the depths of laser radiation of ABS/CuO, ABS/Cr2O3, and


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ABS/CuO·Cr2O3 composites are 40.5 µm, 140.6 µm, and 26.9 µm, respectively. Commonly, the laser-effected depth follows the Lambert-Beer law:25 ଵ

ி

ఈ

ி೟೓

‫ = ܮ‬ln(

)

(1)

where L is the laser-effect depth, and α is an absorption coefficient of materials. The F is the fluence of incident laser, and Fth is the ablation threshold. The UV-vis-IR spectra of CuO, Cr2O3, and CuO·Cr2O3 in the region of 500-2000 nm are shown in Figure S4 (Supporting Information). It reveals that the used metal oxides in our work have the different absorption for the NIR laser, resulting in the different laser affecting depths. Here, the F and Fth in equation (1) are fixed. Therefore, the absorption coefficient α is smaller, and the laser affecting depth is deeper. In Figure S4 (Supporting Information), the absorbance of CuO, Cr2O3, and CuO·Cr2O3 at 1064 nm are 1.772, 0.067, and 4.260, respectively. According to equation (1), it perfectly explains the laser affecting depth of ABS/CuO, ABS/Cr2O3, and ABS/CuO·Cr2O3 composites observed in Figures 4 (a-c). From the optical microscopy images, it can be clearly seen that the laser-irradiated area of polymer composites appears the etched morphologies compared to the non-irradiated area. This is because a sudden rise of the local temperature contributing from the high-energy laser beam irradiation causes ablation, gasification, and carbonization of the polymer.26 To understand the laser etching on the composite surface, we performed laser scanning experiments of a single line on ABS composites surface using the different laser scanning speed, laser power, and laser pulse frequency. The corresponding optical microscopy images and discussion are provided in Figure S5 (Supporting Information). The scanning electron microscopy (SEM) analysis was also carried out to further investigate the surface



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morphology in the window of (60 kHz, 80%) in Figure 2(c) and Figure 3. Figure 4 (d-f) is the SEM surface morphology of ABS composites irradiated by NIR pulsed laser in air atmosphere. Interestingly, the surface of ABS/CuO, ABS/Cr2O3, and ABS/CuO·Cr2O3 composites appear different morphologies after laser irradiation. For ABS/CuO composite [Figure 4 (d)], a series of small cavities are formed due to the ejection of material fragments. However, merely a few cavities are observed on ABS/Cr2O3 composite surface [Figure 4 (e)], which demonstrates that the NIR laser only has a slight etching effect on the ABS/Cr2O3 composite. It is worth mentioning that a complex microscopic rough structure is achieved on ABS/CuO·Cr2O3 composite surface [Figure 4 (f)] due to a strong absorption of NIR laser of CuO·Cr2O3. This microscopic rough structure certainly benefits to improve the mechanical adhesion between ABS and the copper layer. Surface wettability tests of neat ABS and ABS composites after laser irradiation in the window of (60 kHz, 80%) in Figure 2(c) and Figure 3 were also performed. The surface contact angles of ABS composites were measured to evaluate the NIR laser effect of modification on the surface, and the results are shown in Figure S6 (Supporting Information). For the neat ABS, the contact angle is 92.5°. After laser irradiation, the contact angles of ABS/CuO, ABS/Cr2O3, and ABS/CuO·Cr2O3 are increased to 126.2°, 104.8° and 134.1°, respectively. The reason of this phenomenon is probably attributed to the enhancement of the surface hydrophobicity caused by the micro-etching structure. 3.2. X-ray photoelectron spectroscopy (XPS) analysis As is known, X-ray photoelectron spectroscopy (XPS) is a kind of surface analysis technique, which can obtain the surface chemical information of materials. Therefore, XPS


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measurements of polymer composites after laser activation (irradiation) were carried out. In addition, the XPS of metal oxides (CuO, Cr2O3, and CuO·Cr2O3) without any laser irradiation was also recorded. As shown in Figure 5(a), for pure CuO without laser irradiation, the

Figure 5. XPS patterns of metal oxides without laser irradiation and curve fitting results of



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XPS patterns of polymer composites after laser activation (irradiation) in the window of (60 kHz, 80%). (a) Cu 2p of pure CuO without laser irradiation and ABS/CuO after laser activation; (b) Cr 2p of pure Cr2O3 without laser irradiation and ABS/Cr2O3 after laser activation; (c) Cu 2p of pure CuO·Cr2O3 without laser irradiation and ABS/CuO·Cr2O3 after activation; (d) Cr 2p and Cu L3M4,5M4,5 of pure CuO·Cr2O3 without laser irradiation and ABS/CuO·Cr2O3 after laser activation. The XPS spectra are fitted using curve-fitting software (XPSPEAK v4.0). Cu 2p3/2 and Cu 2p1/2 characteristic peaks (the bottom of Figure 5(a)) appear at 933.6 eV and 953.6 eV,27 which are certainly attributed to Cu2+ in pure CuO, and their corresponding shake-up satellite peaks are also observed in the regions of 937–947 eV and 958–965 eV, respectively.27 However, for ABS/CuO composite after laser activation, the peaks of Cu 2p3/2 and Cu 2p1/2 shifts toward the low binding energy (the top of Figure 5(a)), and the satellite peaks also obviously decrease. Here, the asymmetric Lorentzian-Gaussian sum function is applied to curve fitting, and good fitting results are obtained in Figure 5 (the thin black line is the raw data; the thick gray line is the sum of fitting peaks). From the curve fitting results, it can be clearly seen that, besides the Cu 2p3/2 and Cu 2p1/2 peaks of Cu2+ at 933.6 eV and 953.6 eV, two new small peaks occur at 932.5 eV and 952.3 eV. According to the literature,28 these two new peaks are assigned to the Cu 2p3/2 and Cu 2p1/2 of Cu0. That is to say, some of CuO is reduced to Cu0 (elemental copper) after laser activation (irradiation). In Figure 5(b), the Cr 2p3/2 and Cr 2p1/2 peaks of Cr3+ in pure Cr2O3 without laser irradiation are observed at 576.6 and 586.5 eV (the bottom of Figure 5(b)).29 After laser activation, for ABS/Cr2O3 composite, the positions of Cr 2p3/2 and Cr 2p1/2 peaks (the top of Figure 5(b)) have no essential changes (576.9 and 586.7 eV), and the intensity ratio of these two peaks also remains constant, which indicates no change of chemical valence for Cr3+ in ABS/Cr2O3. In Figure 5(c), the Cu 2p3/2 and Cu 2p1/2 peaks (the bottom of Figure 5(c)) of Cu2+ in pure CuO·Cr2O3 without laser irradiation appear at 933.6 and 953.6 eV, accompanied by the 18 ACS Paragon Plus Environment

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characteristic shake-up satellite peaks in the regions of 937–947 eV and 958–965 eV.30 For ABS/CuO·Cr2O3 composite after laser activation, the peak positions of Cu 2p3/2 and Cu 2p1/2 moves to a lower binding energy (the top of Figure 5(c)). Moreover, compared with pure CuO·Cr2O3 (without laser irradiation), the satellite peaks of Cu 2p3/2 and Cu 2p1/2 also decrease, resulting in a significant change of the intensity ratio between the main peak and the satellite peak. The curve fitting results show two new big peaks appear at 932.5 eV and 952.3 eV, respectively. As mentioned above, these two peaks are attributed to the Cu 2p3/2 and Cu 2p1/2 of Cu0,31 revealing the chemical reduction of Cu2+ in CuO·Cr2O3 to Cu0 after laser activation. In Figure 5(d), one interesting feature is that the Cu L3M4,5M4,5 peak shifts from 568.2 eV (Cu2+, the bottom of Figure 5(d)) to 571.7 eV (Cu0, the top of Figure 5(d)), also showing the reduction of Cu2+ to Cu0 for ABS/CuO·Cr2O3 composite.32 As is known, the active metal element (e.g., Cu0, Ag0, Pd0) is the effective catalytic active center during electroless plating. So, combining XPS and the results of 30 min electroless copper plating of ABS/CuO, ABS/Cr2O3, and ABS/CuO·Cr2O3 composites plates, it can be concluded that the new formed Cu0 is the key reason to initiate and to achieve a selective metallization in electroless plating. But there still exists a problem puzzled us. We found that both ABS/CuO and ABS/CuO·Cr2O3 generate Cu0 on the surface after laser activation. However, only ABS/CuO·Cr2O3 composite achieves the copper layer with an excellent performance (meets industrial requirements) after electroless plating (Figure 2). By contrast, the obtained copper layer of ABS/CuO composite is very poor and has no practical application value (Figure 3). What causes this phenomenon? In Figure 5, through the fitting results of the Cu 2p3/2 peak (areas of 933.6 eV and 932.5 eV), it can be calculated that 54.0%



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Cu2+ (933.6 eV) in ABS/CuO·Cr2O3 composite is reduced to Cu0 (932.5 eV); however, this value is only 26.8% for ABS/CuO composite. This shows that the number of the generated Cu0 catalytic seed in ABS/CuO·Cr2O3 is far more than that of the formed in ABS/CuO, which well explains why ABS/CuO composite has a poor catalytic activity in the laser-irradiated areas. Also, it confirms the fact that the generation capacity of Cu0 for CuO·Cr2O3 during laser activation is much easier than CuO under the same conditions. Therefore, for copper-containing laser sensitizers, to achieve a successful selective metallization after laser activation, not only the generated Cu0 is the crucial factor, but also the number of the formed Cu0 catalytic seed is very important. These two factors codetermine the final results of the selective metallization.

Figure 6. XPS full spectra of ABS composite plates after laser activation (irradiation) in the window of (60 kHz, 80%). (a) ABS/CuO; (b) ABS/Cr2O3; (c) ABS/CuO·Cr2O3. Here, the formation reaction of Cu0 (elemental copper) is the common redox reactions of metal oxides induced by the laser irradiation, and the actual reducing agent is the generated amorphous carbon of ABS caused by laser.23 That is to say, the CuO·Cr2O3 absorbs laser energy to produce an instantaneous high temperature (usually >600 °C) on the polymer composite surface, resulting in the carbonization of ABS. At the same time, the metal oxide is reduced to the elemental metal by the generated amorphous carbon at high temperature. Figure 6 is the XPS full spectra of ABS composite plates after laser activation (irradiation) in 20 ACS Paragon Plus Environment

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the window of (60 kHz, 80%) in Figure 2(c) and Figure 3. The surface carbon contents of ABS/CuO, ABS/Cr2O3, and ABS/CuO·Cr2O3 after laser activation are calculated as 83.03%, 72.17%, and 88.03%, respectively. It is believed that a higher carbon content probably provides a more adequate reduction environment.23 This also indirectly explains why the number of the reduced Cu0 in ABS/CuO·Cr2O3 composite is more than that of in ABS/CuO. Figure S7 (Supporting Information) is XPS patterns of the pure CuO·Cr2O3 powder after laser irradiation, it can be observed that the Cu 2p3/2 and Cu 2p1/2 peaks of Cu2+ in pure CuO·Cr2O3 after laser irradiation appear at 933.6 and 953.6 eV, accompanied by the characteristic shake-up satellite peaks in the regions of 937–947 eV and 958–965 eV. Compared with pure CuO·Cr2O3 without laser irradiation (the bottom of Figure 5(a)), the intensity ratio of these peaks also remains unchanged. For pure CuO·Cr2O3, the XPS result reveals no formation of Cu0 during laser irradiation due to no reducing agent. As is known, for the reduction from the metal oxide to the elemental metal, the reducing agent and the high temperature are two main factors. Compared with ABS/CuO composite, the first reason of ABS/CuO·Cr2O3 composite showing a good performance of electroless plating is that the concentration of the generated carbon in ABS/CuO·Cr2O3 is more than that of ABS/CuO composite (Figure 6). The second reason is probably that Cr2O3 can significantly reduce the temperature of the reduction reaction for CuO·Cr2O3, resulting in that CuO·Cr2O3 is far easier to be reduced than CuO. In addition, for polymer composites, the capacity of the selective metallization with the different amount of CuO and CuO·Cr2O3 was also investigated. The results are shown in Figure S9 (Supporting Information). We can observe that the performance of selective



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metallization of ABS/CuO composite after 30 min electroless copper plating is both very poor when the amounts of CuO are 1 wt.% (ABS/CuO=99:1) and 3 wt.% (ABS/CuO=97:3). Expectedly, an excellent selective metallization of ABS/CuO·Cr2O3 composite is achieved in all the windows when the amount of CuO·Cr2O3 is 3 wt.% (ABS/CuO·Cr2O3=97:3) due to the generation of enough Cu0 catalytic seeds. Interestingly, when the amount of CuO·Cr2O3 is 1 wt.% (ABS/CuO·Cr2O3=99:1), the complete copper layers are only obtained in the windows whose laser power is ≥5 W (50%), which also indicates the generated Cu0 catalytic seeds is not enough for the effective selective metallization until the laser power is ≥5 W. These results also demonstrate that CuO·Cr2O3 is a good laser sensitizer for LDS materials. To further investigate the required critical number of the Cu0 atoms (per cm2) to initiate the copper plating, a series of ABS/CuO·Cr2O3 composites with different amount of CuO·Cr2O3 (0.2 wt.%, 0.4 wt.%, 0.6 wt.%, 0.8 wt.%) were prepared. The corresponding laser activation and electroless copper plating were also carried out, and the obtained copper layer are shown in Figure S10 (Supporting Information). Furthermore, to clearly illustrate the obtained copper layer, the large square patterns (2 cm×2 cm) were also fabricated with the parameters of (60 kHz, 80%), and the obtained copper layers are shown in Figure S11 (Supporting Information). The results demonstrate that the desired copper layer can be achieved when the amount of CuO·Cr2O3 is 0.6 wt.%. It indicates that the required critical number of the Cu0 atoms (per cm2) to initiate the copper plating can be gained when the amount of CuO·Cr2O3 is between 0.4 wt.% and 0.6 wt.%. Figure S8 (Supporting Information) is XPS results of these two ABS/CuO·Cr2O3 composites (0.4 wt.% and 0.6 wt.%). In Figure S8(a), through the fitting results of the Cu 2p3/2 peak (areas of 933.6 eV and 932.5 eV), it is calculated that


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47.4% Cu2+ (933.6 eV) is reduced to Cu0 (932.5 eV) in the polymer composite when the amount of CuO·Cr2O3 is 0.4 wt.%. Similarly, in Figure S8(b), it is calculated that 48.1% Cu2+ (933.6 eV) is reduced to Cu0 (932.5 eV) when the amount of CuO·Cr2O3 is 0.6 wt.%. Here, the critical number of Cu0 atoms (per cm2) can be conveniently obtained from the theoretical calculation combined with XPS results. For ABS/CuO·Cr2O3 composite, the critical Cu0 atoms/cm2 required for initiating electroless copper plating is estimated between 1.37×1016/cm2 and 2.08×1016/cm2 when with the parameters of laser activation is (60 kHz, 80%). The specific steps of calculations are provided in the Supporting Information. 3.3. ATR FTIR spectroscopy To further explore other surface chemical changes of ABS/CuO·Cr2O3 composite after laser irradiation, ATR FTIR spectroscopy was also employed in our work. ATR FTIR is particularly suitable for characterizing the chemical functional groups of polymers surface. Figure 7 illustrates typical ATR FTIR spectra of the surface of ABS/CuO·Cr2O3 composite plate after laser irradiation with different laser parameters. The ATR FTIR spectra from the bottom to the top are the collected spectra in the windows (square blocks) labeled by 1-10.



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Figure 7. ATR FTIR spectra of the surface of ABS/CuO·Cr2O3 composite plate after 1064 nm NIR pulsed laser irradiation with different laser parameters. The ATR FTIR spectra from the bottom to the top are the collected spectra in the windows (square blocks) labeled by 1-10. For these windows from 1 to 10, the laser scanning speed and laser pulse frequency are fixed at 2000 mm/s and 60 kHz, and the laser power is gradually increased from 1 W to 10 W. That is to say, the laser energy in windows 1-10 is gradually enhanced. In Figure 7, the band at 3028 cm-1 is attributed to the stretching vibration of aromatic C−H in repeat units of styrene in ABS.33 The bands at 2920 cm-1 and 2850 cm-1 are assigned to the C−H asymmetrical and symmetrical stretching of –CH2 groups in the main chains of ABS. The peak at 2237 cm-1 is the typical characteristic bands of −C≡N group in repeat units of acrylonitrile.33-36 The breathing vibration of benzene rings in repeat units of styrene appears at 1600 cm-1 and 1490 cm-1.34 In addition, the band at 1452 cm-1 is attributed to the bending vibration of –CH2 groups.34 It can be observed that ATR FTIR spectra of the windows from 1 to 10 have no significant changes, and we only notice a new peak around 1735 cm-1 appearing after laser irradiation. Moreover, the intensity of this new peak (1735 cm-1) is gradually increased when


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the laser power enhancing from 1 W (10%) to 10 W (100%). In general, the band around 1735 cm-1 is assigned to the C=O stretching vibration of C=O groups, which is always observed in polymer FTIR spectra after thermal oxidation.37 Therefore, this reveals that there has some oxidation inevitably taking place on ABS substrate due to the thermal effect of high-energy NIR laser. Moreover, the laser energy is higher; the oxidation of ABS caused by laser is more obvious. 3.4. Application for polymers

Figure 8. (a) The computerized image of the “Sun Bird” pattern. (b) Digital photographs of the “Sun Bird” pattern on ABS/CuO·Cr2O3 composite surface after laser irradiation and electroless copper plating. As shown in Figure 8, to demonstrate one of the potential applications of ABS/CuO·Cr2O3 composite using LDS technology, the decorative copper pattern of “Sun Bird” (has a history of over 3,000 years, from Jinsa Site Museum of China) is accurately fabricated onto ABS substrate at a desired position. Here, the weight ratio of ABS resin to CuO·Cr2O3 is 95:5. During laser activation, the laser scanning speed is 2000 mm/s, and the laser power and laser pulse frequency are 8 W and 60 kHz, respectively. After laser irradiation, the reduced Cu0 (copper element) is generated on the laser-activated area at the desired position. Subsequently, the copper pattern of “Sun Bird” is fabricated on polymer substrate via selective metallization in the electroless copper plating bath for 30 min, producing the decorative pattern with a high



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resolution. For LDS technology, the resolution limit is mainly depended upon the process of laser activation. The theoretically obtainable resolution is approximately at 60 µm depending on a single etched line caused by laser activation. However, the actually produced resolution after electrolss copper plating is around 100 µm due to the crown edges caused by laser etching and the overflow plating in electrolss copper plating. Importantly, the color of CuO·Cr2O3 is intrinsically solid black, which has a strong dyeing capability for polymers. The photograph of CuO·Cr2O3 powder is shown in Figure S12 (Supporting Information). The products with the pure black or bright black background have been become particularly popular during recent years. So, CuO·Cr2O3 is very suitable for the applications of fabricating metallic patterns on the inherent pure black or bright black polymer materials using LDS technology. 4. Conclusions This study successfully proposed an efficient method for fabricating metallized patterns on the polymer substrate using the metal oxide composite of copper-chromium oxide (CuO·Cr2O3) via LDS technology. For polymer LDS materials, the key to success lies in the effective selective metallization (in electroless copper plating) induced by the activation (irradiation) of 1064 nm NIR pulsed laser. The experimental results demonstrated that the polymer composite incorporated with CuO·Cr2O3 is a good LDS material, which always obtains the excellent copper layer (thickness=10 µm) on the laser-irradiated areas after 30 min electroless copper plating. Because CuO·Cr2O3 is composed of CuO and Cr2O3 from the viewpoint of chemical composition, we subsequently prepared polymer composites incorporated with CuO and


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Cr2O3 to explore the feasibility for LDS technology. However, polymer composites incorporated with CuO and Cr2O3 both has a very poor capacity of selective metallization in electroless copper plating, which has no practical value for LDS technology. In our work, the key reason causing above results was also systematically studied and elucidated. Optical microscopy showed that the laser-irradiated area of polymer composites appeared etched morphologies compared to the non-irradiated area. SEM revealed that the surface of polymer composites incorporated with CuO·Cr2O3, CuO, and Cr2O3 had different morphologies after laser irradiation. Especially, a more complex microscopic rough structure was achieved for the polymer composite incorporated with CuO·Cr2O3 due to a strong absorption of NIR laser. XPS confirmed the new generation of the Cu0 (elemental copper) on the polymer composite surface of both CuO·Cr2O3 and CuO after laser activation (irradiation), and no elemental metal was formed for the polymer composite of Cr2O3. Importantly, it was calculated that 54.0% Cu2+ in the polymer composite of CuO·Cr2O3 is reduced to Cu0 when the amount of CuO·Cr2O3 is 5 wt.%; however, this value is only 26.8% for the polymer composite of CuO (the amount=5 wt.%). This showed that the polymer composite of CuO·Cr2O3 can produce much more Cu0 catalytic seeds than that of CuO under the same conditions, which perfectly explains why the polymer composite of CuO has a poor catalytic activity in the laser-irradiated areas (leading to a poor capacity of selective metallization). In addition, ATR FTIR of the surface of the polymer composite incorporated with CuO·Cr2O3 after laser irradiation revealed that there had some oxidation inevitably taking place on the polymer substrate due to the thermal effect of high-energy NIR laser. In this study, we can conclude that to achieve a successful selective metallization after



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laser activation, not only the new formed Cu0 is the crucial factor, but also the number of the generated Cu0 catalytic seed is important. These two factors codetermine the final results of the selective metallization. It is noted that the cost of CuO·Cr2O3 is low, and therefore, it has a prospect of large-scale industrial applications. Our study also provides a guideline for preparing essential black polymer composites for LDS. This is because the color of CuO·Cr2O3 is solid black, and it presents a strong dyeing capability for polymers. Therefore, CuO·Cr2O3 is very suitable for applications of fabricating metallic patterns (e.g., metal decoration, circuit) on the inherent pure black or bright black polymer materials via LDS technology. Supporting Information. PXRD patterns, XPS spectra, TG curves, and UV-vis-IR spectra of CuO, Cr2O3, and CuO·Cr2O3; laser scanning experiments of a single line with different laser parameters; surface contact angles after laser irradiation; XPS patterns of CuO·Cr2O3 and ABS/CuO·Cr2O3 composites after laser irradiation; performance of selective metallization with the different amount of CuO and CuO·Cr2O3; estimation of the critical number of the Cu0 atoms (per cm2) to initiate the copper plating; photograph of CuO·Cr2O3.

Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 51473104), Outstanding Youth Foundation of Sichuan Province (2017JQ0006), and State Key Laboratory of Polymer Materials Engineering (Grant Nos. sklpme2014-3-06, sklpme2016-3-10). References 1. Basarir, F. Fabrication of Gold Patterns via Multilayer Transfer Printing and Electroless Plating. ACS Appl. Mater. Interfaces 2012, 4, 1324-1329. 28 ACS Paragon Plus Environment

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Photodegradation

of

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Selective Metallization Induced by Laser Activation: Fabricating

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