Palladium-free catalytic electroless copper deposition on bamboo fabric: Preparation, morphology and electromagnetic properties

Applied Surface Science 258 (2012) 4782–4787

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Palladium-free catalytic electroless copper deposition on bamboo fabric: Preparation, morphology and electromagnetic properties Yinxiang Lu ∗ , Qian Liang, Longlong Xue Department of Materials Science, Fudan University, Shanghai 200433, China

a r t i c l e

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Article history: Received 2 September 2011 Received in revised form 9 November 2011 Accepted 16 January 2012 Available online 24 January 2012 Keywords: Copper/bamboo fabric composite Electroless deposition Palladium-free Electromagnetic

a b s t r a c t Bamboo fabric is subjected to solvent treatment with 3-mercaptopropyltrimethoxysilane (MPTS) before metal deposition. Raman and IR analyses indicate that MPTS is successfully grafted on the fabric. Copper is deposited on the pretreated fabric by a palladium-free catalytic electroless process, and then copper/bamboo fabric (Cu/BF) composite is obtained. SEM (scanning electron microscopy) observation reveals that copper is uniformly covered on the fabric. Chemical composition and crystal structure of the composite are detected by EDX (energy-dispersive X-ray analysis), XPS (X-ray photoelectron spectroscopy) and XRD (X-ray diffraction) measurements, peaks for Cu0 are found in these patterns. The water absorption ratio for the title composite is about 162% by immersion in water, or 8.9% by putting in an environmental condition (humidity of 65 ± 2%). The Cu/BF composite is firm and can pass a Scotch® -tape peel adhesion test. The electromagnetic interference (EMI) shielding effectiveness (SE) of the composite (copper on fabric: 39 g/m2 ) is more than 48 dB at frequency ranging from 0.2 to1000 MHz. © 2012 Elsevier B.V. All rights reserved.

1. Introduction With the increasing number of applications of electronic and wireless devices, electromagnetic radiation is becoming a serious problem that disturbs the stable working conditions of the electronic appliances and probably damages the human body. There is a technological requirement for high electromagnetic interference (EMI) shielding effectiveness (SE) materials [1–4]. There are several EMI shielding options including conductive metal coatings and polymer composites. However, the high filler loading that is needed to achieve adequate level of shielding adversely affects the economic feasibility and mechanical properties of most polymer composites. Thus, conductive metal coatings are currently the most widely used EMI shielding materials [5,6]. A great deal of work has been carried out on metal-coated synthetic fibers, especially polyester [7,8], which takes the leading position in the world fiber market. Comparatively less attention has been paid to natural fibers [9,10], such as cotton, silk and bamboo fabric (BF), aiming to produce electrical conductive textiles for apparel and technical end-uses. Bamboo fabric has been growing in popularity because it has many unique properties and is more sustainable than most textile fibers. Bamboo fabric is light and strong, has excellent wicking properties, and is to some extent antibacterial [11,12]. However, bamboo fabric is not intrinsically conductive.

Therefore, electroless metal plating has to apply to the fabric. It is known that electroless plating can be initiated upon the catalyzed surface, and the catalytic process is usually carried out in SnCl2 /PdCl2 mixed solutions [13]. The biggest weakness of the conventional activation processes is that Pd compounds are expensive and can significantly increase the cost of the plating process. So, it is necessary to develop Pd-free activation process, which is low-cost and can obtain good metal coating [14]. In the present work, an efficient palladium-free process for the electroless copper plating on bamboo fabric is developed (Fig. 1). The bamboo fabric is initially modified with thiol groups in a 3mercaptopropyltrimethoxysilane (MPTS) ethanol solution. Copper is then deposited on the modified fabric by electroless deposition without catalysts, using dimethylamineborane (DMAB) as a reducing reagent. Various characterization techniques including scanning electron microscopy (SEM), X-ray diffraction (XRD), energy-dispersive X-ray analysis (EDX) and shielding effectiveness measurements have been utilized to obtain information on the sample morphology, and the quantity of Cu/bamboo fabric (Cu/BF) composite. Moreover, water sorption property of the composite is investigated, and compared it to the uncoated textile. 2. Experimental 2.1. Materials

∗ Corresponding author. Tel.: +86 21 65642871; fax: +86 21 65642871. E-mail address: [email protected] (Y. Lu). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2012.01.093

Plain weave 100% natural bamboo fabrics (108 × 58 counts/cm2 , 220 g/m2 ) with a thickness of 100 ␮m of white color were used

Y. Lu et al. / Applied Surface Science 258 (2012) 4782–4787 Table 1 Composition and operation conditions of electroless plating. Chemical

Concentration (g/L)

CuCl2 ·2H2 O H3 BO3 C10 H14 N2 Na2 O8 ·2H2 O (EDTANa2 ·2H2 O) C2 H10 BN (DMAB) pH Temperature (◦ C) Time (h)

18 12 36 14 7 60 3

as the substrates. The surface area of each specimen is 200 (20 × 10) cm2 . 3-Mercaptopropyltrimethoxysilane was purchased from Sinopharm Chemical Reagent Co. Other reagents were of analytical grade and were used without further purification unless otherwise mentioned. 2.2. MPTS modification BFs were ultrasonically cleaned in acetone and ethanol, respectively, for 5 min and dried at 60 ◦ C for an arbitrary length of time. The treatment of fabrics with MPTS (0.05 M) modification was carried out in 80/20 (v/v) ethanol/water (100 ml) for 24 h and heated at 100 ◦ C for 1 h in order to promote the actual chemical coupling [15]. After the treatment, samples were washed in an ethanol/water (80/20, v/v) solution at room temperature. 2.3. Electroless copper deposition The MPTS modified bamboo fabrics were put into a neutral aqueous solution without catalyst. The composition of electroless bath and the operating conditions were listed in Table 1. Deionized water was used to prepare the solutions. The pH was adjusted using NaOH or HCl to a final value of 7. After plating, the samples were carefully rinsed with deionized water and ethanol, and then dried in an oven for 1 h at 50 ◦ C. 2.4. Water absorption Before water absorption measurements, the samples were placed in a desiccator under vacuum with P2 O5 for at least 24 h to reduce the moisture content to the maximum under soft conditions of temperature [16]. Besides, just before loading the sorption process, to ensure the complete drying step, the treated fabrics were kept under a vacuum for sufficient time to obtain a constant dry mass. In that way, initially, the moisture content of all the treated fabrics was highly reduced and was below 1%.

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The above treated samples were immersed in deionized water, or kept in an environmental condition (65 ± 2% relative humidity) at room temperature. To evaluate the effect of absorption phenomena during the experiment, the content of water absorbed by the sample was calculated by the weight difference between the weight of the textile before and after water absorption. Weight was measured with an electronic microbalance option (ShunyuFA1104, China) to a resolution of 1 ␮g. The amount of absorbed water (Q) at the time (t) was defined as: Q = [(M − M0 )/M0 ] × 100, where M was the specimen mass (g) at the time (t) and M0 was the dry specimen mass (g). Q values were plotted vs. time (h) to give the water absorption curve. For water immersion process, the samples were performed on a centrifuge (80-1, China) for 30 s (speed: 1000 rpm) before weight detection. 2.5. Characterizations Raman spectrum was characterized by Raman spectrometer (LabRam-1B, France, JY Co., Ltd.). IR spectrum was obtained by a FT-IR spectrometer (Nicolet Nexus 470). Scanning electron micrographs were gained using Philips XL 30 electron microscope. The chemical composition of the copper deposit was determined using energy dispersive X-ray analysis attached to the SEM. X-ray photoelectron spectroscopy (XPS) measurement was performed on a PHI 5000 C ESCA system with Mg K␣ source at 14.0 KV and 25 mA. All the binding energies were referenced to the C 1 s peak at 284.6 eV of the surface adventitious carbon. X-ray diffraction pattern (2 ranges from 40◦ to 95◦ ) was recorded at room temperature with scanning speed of 0.15◦ /min using Cu K␣ radiation ( = 0.154 nm) from a 40 kV X-ray source (Rigaku D/max-␥B) and diffracted beam monochromator, operated at 100 mA. Spectrum analyzer (ATTEN AT5011, China) was used to measure the shielding effectiveness of copper-coated textiles (20 × 10 cm2 ). The coaxial transmission line method as described in ASTM D 4935-99 was used to test the electromagnetic interference shielding effectiveness of the conductive fabrics [17]. For SE reported here, at least ten sample measurements were performed and averaged. For the evaluation of adhesion, a standard Scotch® -tape test (ASTM D 3359 cross-cut tape test performed with a 3 M Scotch® -Ruban Magic Tape) was used [18]. The adhesion strength was checked by pulling the tape to assure any film was peeled off from the substrate or not. 3. Results and discussion 3.1. Raman and IR analysis Raman (a) and IR (b) spectra of MPTS treated bamboo fabric are displayed in Fig. 2. Bamboo substrate is mostly composed of

Fig. 1. Process for the preparation of copper/bamboo fabric composite.

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Fig. 2. Raman (a) and IR (b) spectra of MPTS modified bamboo fabrics.

cellulose, hemicellulose, lignin and some pectin [19]. In Fig. 2(a), silicon–oxygen bending (Si O) at 469 cm−1 , Si O Si at 778 cm−1 are assigned. Ether C O C stretch gives rise to medium peak at 835 cm−1 . C C stretch is the dominant contribution in the 900–1040 cm−1 region with the bands for the most branched celluloses [20]. Cellulose or hemicellulose has strong Raman bands from ether at 1096 cm−1 and from C C at 1169 cm−1 . The region 1200–1500 cm−1 is typical for C H bends, many of which have strong Raman intensity. In cellulose or hemicellulose, bands in this region are found at 1259, 1356 and 1462 cm−1 . Methyl and methylene groups have strong Raman bands in the region 1410–1500 cm−1 , and the large intensity of the bands around 1462 cm−1 is characteristic for the alkane parts in the natural fabric [21]. Bands in the region of 1500–1700 cm−1 have been previously observed to arise from vibrational modes of cellulose and lignin [20]. Lignin features a very strong aromatic ring stretching, which can be assigned to the bands at 1560 and 1650 cm−1 . Hydrocarbon stretching modes observed at 2920 cm−1 are characteristic of natural fabrics. The particularly sharp hydrocarbon stretching modes observed in the methylene region at 2896 cm−1 , occur in a similar position to those reported for native lignin and pectin [19–21]. The O H stretching band of hydroxyl (∼3221 cm−1 ) and the S H stretching band of thiol (∼2490 cm−1 ) are observed as weak broad peaks. The infrared spectrum (Fig. 2(b)) reveals a broad absorption band at 3491 cm−1 that can be attributed to the O H stretching associated to polar groups linked through intra- and intermolecular hydrogen bonding. Furthermore, not only the absorption band at 3491 cm−1 but also the band at 1128 cm−1 is characteristic of glycosidic groups in natural fabrics, the latter is assigned to C C and C O C stretching vibrations. Additionally, a band at 2896 cm−1 is detected and is indicative of C H stretching vibrations due to CH2 and CH3 groups. A sharp band at 1642 cm−1 is also detected, which is related to H O H stretching. A band due to CH2 stretching vibrations is observed near 1438 cm−1 . Moreover, an absorption band near 1373 cm−1 is detected and it is due to the C H bending vibration present in cellulose and hemicellulose chemical structures. Finally, a sharp band at 900 cm−1 , which is typical of glycosidic linkages in natural fabrics, is found in the anomeric region [22]. The band observed at 664 cm−1 in spectra is assigned as the Si C stretching mode mixed with Si O stretching. The nearby very weak polarized band observed at 615 cm−1 is assigned to CH2 rocking mixed with Si C stretching vibration. Particularly, S H stretching is observed in the region of 2513 cm−1 , which is overlapped by the C H stretching band. The Raman and IR spectra are shown to be

Fig. 3. SEM micrographs of Cu/BF composite at low (a) and high (b) magnification.

able to identify the main organic functions and chemical bonds present in the MPTS modified bamboo fabrics. 3.2. Morphology, composition and crystal structure analysis The surface morphology of thin metallic coatings may affect their electrical, mechanical, and optical properties. SEM micrographs with different magnifications of copper-plated bamboo fibers are presented in Fig. 3. Bamboo fibers are uniformly covered with dense copper particles that are clearly visible and the copper particles are well dispersed on fiber surfaces with the present electroless plating method. The uniform coating is also evident at high magnification (Fig. 3(b)). The enlarged image of the product indicates that the copper plating is composed of grain-shaped copper particles with diameter of sub-micrometer. The chemical composition of Cu/BF composite was analyzed by EDX. EDX results in Fig. 4 illustrate that the coated fabric contains three kinds of elements: C, Cu and N. Randomly selecting the

Fig. 4. EDX analysis of Cu/BF composite.

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Fig. 5. XPS spectra of Cu/BF composite with wide (a) and narrow (b) and (c) scan.

analytical zone of the samples, similar results are obtained, which indicates that Cu coating has successfully coated on the bamboo fabrics. In order to identify surface composition of electroless copper film, XPS measurement was employed to test the Cu/BF sample. The XPS spectra (wide (a) and narrow (b) and (c) scan) of copper plating are shown in Fig. 5. In XPS spectra, peaks at 952, 932, 124 and 78 eV are ascribed to Cu 2p1/2 , Cu 2p3/2 , Cu 3 s and Cu 3p, respectively [23]. The result clearly shows that Cu element exists as Cu0 , which is also proved by EDX spectrum. XRD analysis of the Cu-coated bamboo fabric was conducted. The obtained XRD pattern of the fabric after copper deposition is shown in Fig. 6. The peaks at 43.0, 50.1, 74.1 and 90.1◦ correspond to the crystal faces of (1 1 1), (2 0 0), (2 2 0) and (3 1 1) of copper, respectively [23]. The copper oxide phase is not detected in the deposits. The result is matched with the major peaks of Cu comparison with the standard data of JCPDS (04-0836), which reveals pure Cu layer coated on bamboo fabric is a face centered cubic (fcc) structure.

substrate. The copper coating is easily destroyed by mechanical washing during the operation. Given the coating is attached to the surface by some chemical bonds, and then the adhesion strength will be improved dramatically. It is known that, high active metal (Ag, Cu) particles can react with thiols to form metal–sulfur chemical bonds in solution [24]. So the key point is to modify the surface

3.3. Adhesion property Metal/fabric adhesion is an important consideration in the development of copper coatings for conductive fibers. The conventional weakness of Cu/fabric system is due to the low adhesion strength (weak van der Waals force) between the metal layer and

Fig. 6. XRD pattern of Cu/BF composite.

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Fig. 7. Water absorption curves for uncoated (a) and copper coated (b) bamboo fabrics.

of bamboo substrate with thiols. Thiol group (–SH) layer can be chemisorbed on a glass substrate that has hydroxyl groups (–OH) via Si O bonds by MPTS modification [15]. Bamboo fabric also has hydroxyl groups on its surface, so it can be modified by the same method. Then –SH groups are covered on the bamboo fabrics by MPTS soaking (Fig. 2). Copper thin film is deposited on the thiol grafted bamboo substrate because of the interactions between Cu and – SH groups (Fig. 1). The mechanic property of Cu/BF was evaluated with a Scotch® -tape test. It is found that the copper coatings never fail in the Scotch® -tape test, which indicates a firm metal coating. 3.4. Water absorption In order to evaluate the wetting behavior of bamboo fabrics, the water absorption test was carried out. In Fig. 7 the water absorption curves for uncoated and copper-coated bamboo fabric are reported. Comparing these two samples, the improvement of water repellence can be evaluated. The maximum Q in water immersion process is 230% and 162% for uncoated (a) and copper-coated bamboo fabrics, respectively. The corresponding data is 14.8% and 8.9% by 65 ± 2% relative humidity process. The uncoated samples absorb a great quantity of water and reach the saturation value in 8–10 h. From the apparent phenomenon, copper-coated bamboo fabrics show a comparable weaker wetting behavior than uncoated fabric. This can be explained by the copper coating attached to the fabric hindering the penetration and the absorption of water molecules into bamboo fibers. The decreasing of absorbed water is an indicator of the protective effect: the general trend is the increasing of “no wetting” behavior, which indicates a water repellence improvement [25]. 3.5. Surface resistance and shielding effectiveness It is known that the surface resistance of copper-coated fabrics is related to the weight of copper coating. The surface conductivity for the composite (copper on fabric: 39 g/m2 ) is 2.8 × 105 S/cm, measured by an electrical conductivity tester HC2512B (Henan, China), which approaches about half of the conductivity of the bulk copper [26]. According to the Schelkunoff theory [27], better conductivity leads to higher shielding effectiveness. Fig. 8 shows the results of EMI shielding effectiveness of copper-coated bamboo fabrics. It can be seen that the sample has SE of 48–59 dB at a frequency range of 0.2–1000 MHz, suggesting that above 99.99% of the external radiation or internal emission can be attenuated. The copper-coated bamboo fabrics form a network in the composite, which will induce

Fig. 8. Shielding effectiveness of Cu/BF composite.

absorbed wastage and multiple reflected wastages [28]. So the conductive fabrics can be used in advanced electronic products and national defense field.

4. Conclusions Copper/bamboo fabric composite have been prepared by a Pdfree electroless plating process. The bamboo textile is initially modified by 3-mercaptopropyltrimethoxysilane, then SH groups are grafted onto the substrate. Copper is deposited on the MPTS pretreated substrate by electroless deposition without catalysts by using dimethylamineborane as a reducing reagent. The composite has good electrical conductivity, which is about half of the bulk copper. EMI shielding effectiveness of the composite is more than 48 dB at frequency ranging from 0.2 to 1000 MHz. Water absorption for Cu/BF is about 162% by immersion in water, or 8.9% by putting in an environmental condition (humidity of 65 ± 2%), less than those uncoated fabrics, which indicates a water repellence improvement. SEM analysis shows that the copper coating is composed of grainshaped particles. The adhesion strength between the metal layer and substrate is large enough to pass the Scotch® -tape test. EDX, XPS and XRD spectra show that Cu0 is detected in the copper layer, confirming that copper film has successfully coated on the bamboo fabrics using the present electroless process.

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