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Metal deposition by electroless plating on polydopamine functionalized micro- and nanoparticles
Journal of Colloid and Interface Science 411 (2013) 187–193
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Metal deposition by electroless plating on polydopamine functionalized micro- and nanoparticles Giovanni Mondin, Florian M. Wisser, Annika Leifert, Nasser Mohamed-Noriega, Julia Grothe, Susanne Dörfler ⇑, Stefan Kaskel Department of Inorganic Chemistry, Dresden University of Technology, Bergstrasse 66, D-01069 Dresden, Germany
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Article history: Received 4 July 2013 Accepted 11 August 2013 Available online 28 August 2013 Keywords: Electroless plating Polydopamine Nanoparticles WC Al2O3 Copper Silver Metal matrix composites
a b s t r a c t A novel approach for the fabrication of metal coated micro- and nanoparticles by functionalization with a thin polydopamine layer followed by electroless plating is reported. The particles are initially coated with polydopamine via self-polymerization. The resulting polydopamine coated particles have a surface rich in catechols and amino groups, resulting in a high affinity toward metal ions. Thus, they provide an effective platform for selective electroless metal deposition without further activation and sensitization steps. The combination of a polydopamine-based functionalization with electroless plating ensures a simple, scalable, and cost-effective metal coating strategy. Silver-plated tungsten carbide microparticles, copper-plated tungsten carbide microparticles, and copper-plated alumina nanoparticles were successfully fabricated, showing also the high versatility of the method, since the polymerization of dopamine leads to the formation of an adherent polydopamine layer on the surface of particles of any material and size. The metal coated particles produced with this process are particularly well suited for the production of metal matrix composites, since the metal coating increases the wettability of the particles by the metal, promoting their integration within the matrix. Such composite materials are used in a variety of applications including electrical contacts, components for the automotive industries, magnets, and electromagnetic interference shielding. Ó 2013 Elsevier Inc. All rights reserved.
1. Introduction Metal coated ceramic particles are traditionally of great interest for their optical properties and find many applications in the fields of biochemistry, catalysis, photonics, electronics, and photonics [1]. Metal coated nanoparticles and microparticles are also gaining attention as potential candidates in the fabrication of higher quality metal matrix composites (MMCs) [2–4]. MMCs are composite materials consisting of a metal or alloy matrix containing ceramic particles, usually carbides, nitrides, and oxides. The combination of the specific strength and stiffness of the ceramic material with the ductility and toughness of the metal leads to many remarkable improvements in the MMCs over monolithic metals, such as better chemical and erosion resistance, higher strength, higher wear resistance, and better high-temperature properties [5,6]. As a result of their superior physical and mechanical properties, MMCs are the materials of choice for numerous applications in the ⇑ Corresponding author. Fax: +49 (0) 351 463 37287. E-mail addresses: [email protected] (G. Mondin), florian. [email protected] (F.M. Wisser), [email protected] (A. Leifert), [email protected] (N. Mohamed-Noriega), [email protected] (J. Grothe), susanne.doerfl[email protected] (S. Dörfler), [email protected] (S. Kaskel). 0021-9797/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2013.08.028
automotive and aerospace industries, thermal management, and electronics [2–6]. The simplest way to produce MMCs is the incorporation of ceramic particles in a molten metal, followed by stirring and casting [5]. A homogeneous distribution of the ceramic particles in the molten metal is crucial for the fabrication of a good composite material, but difficult to realize. Ceramic particles are not well wetted by the molten metal due to different surface energies and hence aggregate very easily, especially nanoparticles because of their high surface area. An elegant and simple solution to obtain a uniform particle distribution in the metallic melt is to coat the ceramic particles before the mixing process, in order to minimize the surface energy difference between the particles and the molten metal [4]. A second well established method for the production of MMCs is the infiltration of a liquid metal in a ceramic body. Also in this case, a metal coating of the ceramic body improves the wettability by the metal, promoting a uniform infiltration process [7]. Another problem is the high reactivity of some metals used as matrix materials with the ceramic particles, e.g., aluminum with silicon carbide, which results in the formation of undesired phases and the subsequent deterioration of the mechanical properties [8]. This can be avoided by coating the ceramic particles with an inert metal layer.
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Metal coated ceramic particles can be fabricated by many processes, such as metal vapor deposition [9], sputtering [10], seed growth [11], and electroless plating (EP) [12–14]. Especially, metal vapor deposition and sputtering processes suffer from inhomogeneous coatings and a cost-intensive scaling up. In contrast, electroless plating is the most widespread method because of its versatility, fast deposition rate, simplicity, and high potential for mass production. EP is a controlled chemical reduction of metal ions on a substrate, which has to be properly activated in order to deposit the metal selectively on the substrate surface [15,16]. The traditional activation process is performed by immersion of the substrate in Sn-based and Pd-based solutions. This method, although substrate-independent, is very expensive and time-consuming and involves the use of heavy metals [17,18]. Recently, several organic-based activation methods have been developed in an effort to overcome the shortcomings of the Sn/Pd-based treatments [19–21]. These new methods are usually cost-effective and relatively simple, but on the other hand, they are substrate-dependent and effective only with a limited amount of materials, usually oxides, and ineffective with materials such as carbides and nitrides, which are very attractive for MMCs. In this work, we use polydopamine coating as a new and substrate-independent activation process for the electroless metallization of micro- and nanoparticles. Any substrate, regardless of its geometry and material, can be covered with a polydopamine layer with a very simple and fast reaction. Moreover, the catechols and amino groups in the polydopamine coating have a high affinity toward metal ions and can therefore be used to promote metal deposition on the particles surface during electroless metallization. Lee et al. were the first group developing a facile approach to generate polydopamine coatings on several objects, inspired by the adhesive proteins found in mussels [22]. By simply immersing a substrate in an aqueous solution of dopamine buffered at pH 8.5, they observed the deposition of a surface-adherent polydopamine coating. Several oxides, noble metals, polymers, and ceramics were successfully tested showing the substrate-independence of this technique. The wide range of secondary reactions that can be carried out using the functional groups of polydopamine, such as grafting of various organic molecules, is particularly interesting. Several other research groups used polydopamine coatings for different applications, such as functionalization of carbon nanotubes [23,24], separators for high-power Li-Ion batteries [25], capsules for drug delivery [26,27], surface wetting control [28,29], and as substrate for growing noble metal nanoparticles for antibacterial and SERS applications [30,31]. Also, several reports on the mechanism of dopamine polymerization and the structure of polydopamine followed the influential work by Lee et al. [32–34]. Although the growing interest in polydopamine and catecholic chemistry, only few works deal with the polydopamine functionalization of particles. Si et al. and Zhou et al. successfully fabricated
polydopamine coated Fe3O4 nanoparticles [35,36], while in the work by Li et al., polydopamine coated Fe3O4 particles are also decorated with silver [37]. The work by Wang et al. explores the use of polydopamine for the fabrication of core-shell metalized particles, albeit only the coating of 25 lm silica spheres with silver is investigated [38]. In this paper, we follow the approach of using dopamine to create a functional polydopamine surface coating, using silver plating as a model system and extending the idea on new materials and smaller particles by plating copper on WC microparticles (1 lm average size) and Al2O3 nanoparticles (13 nm average size), both of which are of particular interest in the fabrication of composite materials [3–5]. 2. Experimental section 2.1. Materials Aluminum oxide nanoparticles with an average primary particle size of 13 nm were purchased from Sigma–Aldrich. Tungsten carbide powder with approx. 1 lm average particle size was obtained from H.C. Starck GmbH. Before use, the WC powder was washed in a mixture of 37% HCl and ethanol (1:1, v/v). Dopamine hydrochloride, copper sulfate pentahydrate (P98.0%), ethylenediaminetetraacetic acid (EDTA, P99.0%), hydrazine hydrate solution (78–82%), 3,5-diiodo-L-tyrosine dehydrate, and potassium sodium tartrate (99.0%) were all purchased from Sigma–Aldrich. Silver nitrate (99.5%) was purchased from Grüssing GmbH. Ethylenediamine (99.0%) was purchased from VWR International GmbH. 2.2. Polydopamine synthesis and polydopamine coating of particles 3 g of WC powder was suspended in 100 mL of a Tris–HCl buffer solution (10 mM, pH = 8.5) containing 2 g/L dopamine, sonicated for 5 min and then left in solution for 25 h with strong stirring to avoid aggregation. A polydopamine layer was formed on the particles and on the reaction vessel. A similar procedure was followed with alumina nanoparticles: 0.2 g of Al2O3 nanoparticles was suspended in 100 mL of a Tris–HCl buffer solution (10 mM, pH = 8.5) containing 2 g/L dopamine, sonicated for 15 min, and then left in solution with strong stirring. Because of their much smaller size, they were left in solution for only 1 h, as opposed to 25 h polymerization time of the WC particles. The polydopamine coated particles were washed via consecutive centrifugation cycles with deionized water and ethanol, until a clear solution is obtained, and then vacuum dried with a rotary evaporator at 40 °C. Pure polydopamine used as reference in the FTIR analysis was synthesized in the same conditions as described above (2 g/L dopamine, 10 mM Tris–HCl buffer solution at pH = 8.5, 25 h polymerization time), without sonication and addition of particles.
Fig. 1. Schematic illustration of the fabrication process. The particles are first coated with a thin polydopamine layer via self-polymerization of dopamine. The high affinity toward metal ions of the catechols and amino groups of the polydopamine layer promotes a selective metal deposition on the coated particles during the subsequent electroless plating process.
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Fig. 2. (a) Possible dopamine polymerization mechanism and polydopamine structure modified after [22,39,40]. (b) Proposed dopamine polymerization mechanism and structure of polydopamine by Dreyer et al. [41].
2.3. Electroless metallization of polydopamine coated particles Copper electroless plating of alumina nanoparticles coated with polydopamine was performed in an aqueous solution containing 13.5 g/L CuSO45H2O as copper source, 10 g/L EDTA as complexing
agent, and 3 g/L Al2O3 nanoparticles. The solution was kept at room temperature and sonicated for 15 min prior to the addition of 33 mL/L hydrazine hydrate solution as reducing agent, which triggers the electroless deposition process. Copper electroless plating of the polydopamine coated WC particles was performed in an
Fig. 3. SEM images of the WC microparticles (a and b) before the polydopamine coating process and (c and d) after the polydopamine coating process.
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Fig. 5. Transmission FTIR spectra of (a) WC microparticles and (b) Al2O3 nanoparticles, both before and after the polydopamine coating process. Also, the FTIR spectrum of pure polydopamine is reported as reference.
Fig. 4. TEM images of the Al2O3 nanoparticles (a) before the polydopamine coating process and (b) after the polydopamine coating process.
aqueous solution containing 7.5 g/L CuSO45H2O, 5.5 g/L EDTA, and 10 g/L WC particles. The solution was kept at room temperature and sonicated for 5 min prior to the addition of 18.5 mL/L hydrazine hydrate solution. Silver plating on the WC powder was performed using 10 g/L WC particles and a plating bath containing 4.5 g/L AgNO3, 13.5 mL/L ethylenediamine, 0.3 g/L 3,5-diiodo-Ltyrosine dehydrate, and 12.0 g/L potassium sodium tartrate as described elsewhere [19]. After the plating process, the particles were washed via consecutive centrifugation cycles with deionized water and ethanol, until a clear solution is obtained, and then vacuumdried with a rotary evaporator at 40 °C. 2.4. Characterization X-ray diffractograms were obtained using a Panalytical X’Pert Pro diffractometer in Bragg-Brentano geometry with Cu Ka1 radiation. Scanning electron microscopy (SEM) images were acquired with a DSM-982 Gemini (Zeiss) at an acceleration voltage of 8 kV. Transmission electron microscopy (TEM) images were taken with a JEOL 2010 operated at an acceleration voltage of 80 kV. Thermogravimetric analysis (TGA) was performed with a STA 409 PC/PG-Luxx analyzer (Netzsch) in synthetic air atmosphere and using a heating rate of 5 °C/min. Fourier transform infrared (FTIR)
Fig. 6. TGA curves of pristine WC particles, polydopamine coated WC particles (Pdop@WC), pristine Al2O3 nanoparticles, and polydopamine coated Al2O3 nanoparticles (Pdop@Al2O3).
spectra were recorded with a Bruker Vertex 70 (32 scans, 2 cm resolution).
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3. Results and discussion 3.1. Polydopamine coated particles In Fig. 1, the general fabrication process is reported. The first step is the polydopamine (Pdop) coating of particles via auto-oxidative polymerization of dopamine. The polymerization process and the polydopamine structure are still not perfectly understood. Lee et al. suggest a mechanism that involves the oxidation of the
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Fig. 7. SEM images of silver coated Pdop@WC microparticles.
Fig. 8. SEM images of copper coated Pdop@WC microparticles.
Fig. 9. SEM images of (a) pristine Al2O3 nanoparticles, (b) copper plated Pdop@Al2O3 nanoparticles.
catechols to quinine and a subsequent melanin-like polymerization as shown in Fig. 2a [22]. Similar mechanisms were also proposed by others [39,40]. A study from Dreyer et al. shows that polydopamine is not made of covalently bonded units, but of monomers which interact with each other through noncovalent forces like hydrogen bonding, p-stacking and charge transfer, as shown in Fig. 2b, resulting nonetheless in a highly stable material [41]. Polydopamine coated WC particles were investigated with SEM. Fig. 3a and b shows the WC particles before the coating process, characterized by sharp edges and flat surfaces. Fig. 3c and d shows the WC powder after the polydopamine coating process. The particles have smoothened corners and edges, and the surface shows a fine granularity, typical of polydopamine coatings [28,38]. According to Lee et al., 25 h of dopamine polymerization corresponds to a polydopamine layer thickness of approximately 50 nm [22]. Polydopamine coated alumina nanoparticles were investigated with TEM. Fig. 4a and b shows Al2O3 nanoparticles before and after polydopamine coating, respectively. In Fig. 4b, a thin but conformal polydopamine layer is observed on the surface of the crystalline alumina nanoparticles. This polydopamine layer has a thickness of approx. 2 nm after 1 h polymerization, in good agreement with the expected thickness from Lee et al. FTIR analysis was used to investigate the chemical particlepolydopamine interaction. FTIR spectra of the WC particles and
Al2O3 nanoparticles are shown in Fig. 5a and b, respectively. In the WC FTIR spectrum, several peaks appear between 1000 cm 1 and 1600 cm 1 after the polydopamine polymerization process, as in the polydopamine reference spectrum, indicating a successful coating process. Dopamine has several strong IR absorption peaks at wavelengths higher than 1600 cm 1, but none are observed in Fig. 5a, indicating that all dopamine molecules were effectively polymerized or removed [23,41]. In particular, the spectrum shows three characteristic peaks at 1078 cm 1, 1336 cm 1 and 1518 cm 1 which are respectively assigned to the stretching vibrations of C– OH groups, aromatic rings absorption, and N–H shearing vibrations [23,42,43]. In the FTIR spectrum of the alumina nanoparticles in Fig. 5b, several weak absorption peaks appear between 1200 cm 1 and 1600 cm 1, as in the polydopamine coated WC powder and in the polydopamine reference. The strong peak at 1500 cm 1 is assigned to the N-H groups shearing vibrations, while other peaks are covered by the broad absorption bands of alumina. In this case, a small shift of the absorption peaks to smaller wavelength is observed, indicating a stronger interaction of the polydopamine coating with the Al2O3 particles surface as compared to WC particles, probably due to the higher reactivity of the oxide and of the nanoparticles themselves. Thermogravimetric analysis was performed to confirm the polymerization of dopamine on the particles surface (Fig. 6). Uncoated WC particles are stable up
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Fig. 10. (a) TEM image of the copper plated Pdop@Al2O3 nanoparticles. (b) Particle size distribution histogram (total particle population = 40).
to 400 °C, when the oxidation reaction of WC to WO3 starts and an 18% weight gain is observed [44]. Polydopamine coated WC particles show an initial mass loss due to the decomposition of the organic phase. This mass loss reaches a value of 3% at 450 °C, when the WC oxidation becomes dominant and an 18% weight increase takes place. As received Al2O3 nanoparticles show a 6% mass loss due to the water molecules bound on Al-OH groups [45,46], while the polydopamine coated Al2O3 nanoparticles show a more substantial weight loss due to the decomposition of the organic layer, about 25%, at first at a high rate followed by a slower rate up to 600 °C, a trend reported also in other works [23,24]. The higher mass loss of the polydopamine coated alumina nanoparticles in comparison with the WC particles can be explained by the greater polydopamine shell thickness to core material radius ratio in the nanoparticles, where 13 nm particles have a 2 nm thick polydopamine shell, while 1 lm WC particles have a 50 nm polydopamine shell.
coated Pdop@WC particles. In all diffractograms, the WC (JCPDS 25-1047) diffraction peaks at 31.5°, 35.7°, 48.4°, 64.0°, 65.8°, 73.0°, 75.5°, and 77.0° can be seen. The diffraction patterns of pristine WC particles and of Pdop@WC particles are identical, indicating that the polydopamine layer is amorphous and has no effect on the WC particles structure. The silver coated Pdop@WC diffractogram shows the WC diffraction peaks along with two clearly visible peaks at 38.2° and 44.3°, and two other peaks at 64.5° and 77.5° partially covered by the WC diffraction peaks, corresponding to the (1 1 1), (2 0 0), (2 2 0), and (3 1 1) lattice planes of silver (JCPDS 4-783). In the copper coated Pdop@WC diffraction pattern, the typical WC diffraction peaks can be seen, along with three peaks at 43.3°, 50.4°, and 74.1° corresponding to the (1 1 1), (2 0 0), and
3.2. Metal plated particles The polydopamine coated particles were coated by a metal layer via electroless plating due to the high affinity to metal ions of the catechols and amino groups of the polymer coating. The use of a reducing agent to reduce metal ions to metal on the particles surface is necessary with copper, while silver ions can be reduced to silver by polydopamine alone (see additional supporting information). However, using a reducing agent, a silver coating is deposited in much less time and shows a better quality. Figs. 7 and 8 show SEM images of the silver plated WC powder and the copper plated WC powder, respectively. The surface of the metal coated particles is much rougher than the as received WC particles (Fig. 3a) and the polydopamine coated WC particles (Fig. 4b and c). The particles look completely covered with a coarse metal layer with a grainy morphology consisting of coalesced metal grains, characteristic of metals deposited by electroless plating, and other solution-based processes [47–51]. Fig. 9a and b shows SEM images of the as received Al2O3 nanoparticles and the same nanoparticles after the copper electroless plating process, respectively. An increase in size due to the polydopamine layer and the copper coating is clearly visible. The particles are uniformly coated and have an average size of 30–40 nm. Fig. 10 shows a TEM image of the copper plated Pdop@Al2O3 nanoparticles with the respective particle size distribution histogram for a population of 40 nanoparticles. The particles have a slightly irregular shape and sizes in the 20– 40 nm range, in good agreement with the particles size observed in Fig. 9b. Fig. 11a shows the XRD patterns of the pristine WC particles, Pdop@WC particles, copper coated Pdop@WC particles, and silver
Fig. 11. (a) XRD patterns of pristine WC particles, polydopamine coated WC particles (Pdop@WC), copper coated Pdop@WC, and silver coated Pdop@WC. (b) XRD patterns of pristine Al2O3 nanoparticles, polydopamine coated Al2O3 nanoparticles (Pdop@Al2O3), and copper coated Pdop@Al2O3 nanoparticles.
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(2 2 0) lattice planes of copper (JCPDS 4-836). No other peak due to impurities or undesired byproducts like oxides is observed. In Fig. 11b, the diffraction patters of pristine Al2O3 nanoparticles, Pdop@Al2O3, and copper plated Pdop@Al2O3 are shown. In the diffraction patterns of pristine Al2O3 nanoparticles and Pdop@Al2O3, only the broad peaks of d-alumina (JCPDS 4-877) nanoparticles can be observed, while in the copper plated Pdop@Al2O3, diffraction pattern also the copper diffraction peaks at 43.3°, 50.4°, and 74.1° are present. 4. Conclusions A general and versatile process for the synthesis of metal coated particles using polydopamine-assisted electroless plating was reported. The process consists of the fabrication of a polydopamine layer on the particles surface via dopamine oxidative polymerization, followed by a controlled metal deposition via electroless plating on the polydopamine-functionalized particles. This process was used to fabricate copper coated WC microparticles and Al2O3 nanoparticles, as well as silver coated WC microparticles, with a conformal and compact metal shell. The use of both nanoparticles and microparticles, as well as an oxide and a carbide as particle materials, shows the versatility and substrate-independence of this technique. This method offers a cost-effective, scalable, and generic route for the manufacture of metal coated particles without the conventional Pd/Sn-based sensitization steps, especially for applications in the fabrication of metal matrix composites. Moreover, because of the high reactivity and versatility of the polydopamine layer, the dopamine polymerization process introduced in this work can also be used as a particle activation strategy in other applications such as in biotechnology, optics, and drug delivery. Acknowledgments
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This work was fully funded by the European Regional Development Fund (Europäischer Fonds für regionale Entwicklung – EFRE) and the Free State of Saxony (Sächsische Aufbaubank – SAB Project No. 100112077). The authors gratefully thank the Leipniz Institute for Solid State and Materials Research Dresden, especially Nicole Geißler and Dr. Sandeep Gorantla for technical assistance with TEM, Prof. Dr. Büchner and Dr. Silke Hampel (Institute for Solid State Research) for project cooperation in the European Centre for Emerging Materials and Processes ECEMP subproject C1 ‘‘NanoWearJoin’’. Appendix A. Supplementary material
[38] [39] [40] [41] [42] [43] [44] [45] [46] [47]
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2013.08.028.
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Metal deposition by electroless plating on polydopamine functionalized micro- and nanoparticles Giovanni Mondin, Florian M. Wisser, Annika Leifert, Nasser Mohamed-Noriega, Julia Grothe, Susanne Dörfler ⇑, Stefan Kaskel Department of Inorganic Chemistry, Dresden University of Technology, Bergstrasse 66, D-01069 Dresden, Germany
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Article history: Received 4 July 2013 Accepted 11 August 2013 Available online 28 August 2013 Keywords: Electroless plating Polydopamine Nanoparticles WC Al2O3 Copper Silver Metal matrix composites
a b s t r a c t A novel approach for the fabrication of metal coated micro- and nanoparticles by functionalization with a thin polydopamine layer followed by electroless plating is reported. The particles are initially coated with polydopamine via self-polymerization. The resulting polydopamine coated particles have a surface rich in catechols and amino groups, resulting in a high affinity toward metal ions. Thus, they provide an effective platform for selective electroless metal deposition without further activation and sensitization steps. The combination of a polydopamine-based functionalization with electroless plating ensures a simple, scalable, and cost-effective metal coating strategy. Silver-plated tungsten carbide microparticles, copper-plated tungsten carbide microparticles, and copper-plated alumina nanoparticles were successfully fabricated, showing also the high versatility of the method, since the polymerization of dopamine leads to the formation of an adherent polydopamine layer on the surface of particles of any material and size. The metal coated particles produced with this process are particularly well suited for the production of metal matrix composites, since the metal coating increases the wettability of the particles by the metal, promoting their integration within the matrix. Such composite materials are used in a variety of applications including electrical contacts, components for the automotive industries, magnets, and electromagnetic interference shielding. Ó 2013 Elsevier Inc. All rights reserved.
1. Introduction Metal coated ceramic particles are traditionally of great interest for their optical properties and find many applications in the fields of biochemistry, catalysis, photonics, electronics, and photonics [1]. Metal coated nanoparticles and microparticles are also gaining attention as potential candidates in the fabrication of higher quality metal matrix composites (MMCs) [2–4]. MMCs are composite materials consisting of a metal or alloy matrix containing ceramic particles, usually carbides, nitrides, and oxides. The combination of the specific strength and stiffness of the ceramic material with the ductility and toughness of the metal leads to many remarkable improvements in the MMCs over monolithic metals, such as better chemical and erosion resistance, higher strength, higher wear resistance, and better high-temperature properties [5,6]. As a result of their superior physical and mechanical properties, MMCs are the materials of choice for numerous applications in the ⇑ Corresponding author. Fax: +49 (0) 351 463 37287. E-mail addresses: [email protected] (G. Mondin), florian. [email protected] (F.M. Wisser), [email protected] (A. Leifert), [email protected] (N. Mohamed-Noriega), [email protected] (J. Grothe), susanne.doerfl[email protected] (S. Dörfler), [email protected] (S. Kaskel). 0021-9797/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2013.08.028
automotive and aerospace industries, thermal management, and electronics [2–6]. The simplest way to produce MMCs is the incorporation of ceramic particles in a molten metal, followed by stirring and casting [5]. A homogeneous distribution of the ceramic particles in the molten metal is crucial for the fabrication of a good composite material, but difficult to realize. Ceramic particles are not well wetted by the molten metal due to different surface energies and hence aggregate very easily, especially nanoparticles because of their high surface area. An elegant and simple solution to obtain a uniform particle distribution in the metallic melt is to coat the ceramic particles before the mixing process, in order to minimize the surface energy difference between the particles and the molten metal [4]. A second well established method for the production of MMCs is the infiltration of a liquid metal in a ceramic body. Also in this case, a metal coating of the ceramic body improves the wettability by the metal, promoting a uniform infiltration process [7]. Another problem is the high reactivity of some metals used as matrix materials with the ceramic particles, e.g., aluminum with silicon carbide, which results in the formation of undesired phases and the subsequent deterioration of the mechanical properties [8]. This can be avoided by coating the ceramic particles with an inert metal layer.
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Metal coated ceramic particles can be fabricated by many processes, such as metal vapor deposition [9], sputtering [10], seed growth [11], and electroless plating (EP) [12–14]. Especially, metal vapor deposition and sputtering processes suffer from inhomogeneous coatings and a cost-intensive scaling up. In contrast, electroless plating is the most widespread method because of its versatility, fast deposition rate, simplicity, and high potential for mass production. EP is a controlled chemical reduction of metal ions on a substrate, which has to be properly activated in order to deposit the metal selectively on the substrate surface [15,16]. The traditional activation process is performed by immersion of the substrate in Sn-based and Pd-based solutions. This method, although substrate-independent, is very expensive and time-consuming and involves the use of heavy metals [17,18]. Recently, several organic-based activation methods have been developed in an effort to overcome the shortcomings of the Sn/Pd-based treatments [19–21]. These new methods are usually cost-effective and relatively simple, but on the other hand, they are substrate-dependent and effective only with a limited amount of materials, usually oxides, and ineffective with materials such as carbides and nitrides, which are very attractive for MMCs. In this work, we use polydopamine coating as a new and substrate-independent activation process for the electroless metallization of micro- and nanoparticles. Any substrate, regardless of its geometry and material, can be covered with a polydopamine layer with a very simple and fast reaction. Moreover, the catechols and amino groups in the polydopamine coating have a high affinity toward metal ions and can therefore be used to promote metal deposition on the particles surface during electroless metallization. Lee et al. were the first group developing a facile approach to generate polydopamine coatings on several objects, inspired by the adhesive proteins found in mussels [22]. By simply immersing a substrate in an aqueous solution of dopamine buffered at pH 8.5, they observed the deposition of a surface-adherent polydopamine coating. Several oxides, noble metals, polymers, and ceramics were successfully tested showing the substrate-independence of this technique. The wide range of secondary reactions that can be carried out using the functional groups of polydopamine, such as grafting of various organic molecules, is particularly interesting. Several other research groups used polydopamine coatings for different applications, such as functionalization of carbon nanotubes [23,24], separators for high-power Li-Ion batteries [25], capsules for drug delivery [26,27], surface wetting control [28,29], and as substrate for growing noble metal nanoparticles for antibacterial and SERS applications [30,31]. Also, several reports on the mechanism of dopamine polymerization and the structure of polydopamine followed the influential work by Lee et al. [32–34]. Although the growing interest in polydopamine and catecholic chemistry, only few works deal with the polydopamine functionalization of particles. Si et al. and Zhou et al. successfully fabricated
polydopamine coated Fe3O4 nanoparticles [35,36], while in the work by Li et al., polydopamine coated Fe3O4 particles are also decorated with silver [37]. The work by Wang et al. explores the use of polydopamine for the fabrication of core-shell metalized particles, albeit only the coating of 25 lm silica spheres with silver is investigated [38]. In this paper, we follow the approach of using dopamine to create a functional polydopamine surface coating, using silver plating as a model system and extending the idea on new materials and smaller particles by plating copper on WC microparticles (1 lm average size) and Al2O3 nanoparticles (13 nm average size), both of which are of particular interest in the fabrication of composite materials [3–5]. 2. Experimental section 2.1. Materials Aluminum oxide nanoparticles with an average primary particle size of 13 nm were purchased from Sigma–Aldrich. Tungsten carbide powder with approx. 1 lm average particle size was obtained from H.C. Starck GmbH. Before use, the WC powder was washed in a mixture of 37% HCl and ethanol (1:1, v/v). Dopamine hydrochloride, copper sulfate pentahydrate (P98.0%), ethylenediaminetetraacetic acid (EDTA, P99.0%), hydrazine hydrate solution (78–82%), 3,5-diiodo-L-tyrosine dehydrate, and potassium sodium tartrate (99.0%) were all purchased from Sigma–Aldrich. Silver nitrate (99.5%) was purchased from Grüssing GmbH. Ethylenediamine (99.0%) was purchased from VWR International GmbH. 2.2. Polydopamine synthesis and polydopamine coating of particles 3 g of WC powder was suspended in 100 mL of a Tris–HCl buffer solution (10 mM, pH = 8.5) containing 2 g/L dopamine, sonicated for 5 min and then left in solution for 25 h with strong stirring to avoid aggregation. A polydopamine layer was formed on the particles and on the reaction vessel. A similar procedure was followed with alumina nanoparticles: 0.2 g of Al2O3 nanoparticles was suspended in 100 mL of a Tris–HCl buffer solution (10 mM, pH = 8.5) containing 2 g/L dopamine, sonicated for 15 min, and then left in solution with strong stirring. Because of their much smaller size, they were left in solution for only 1 h, as opposed to 25 h polymerization time of the WC particles. The polydopamine coated particles were washed via consecutive centrifugation cycles with deionized water and ethanol, until a clear solution is obtained, and then vacuum dried with a rotary evaporator at 40 °C. Pure polydopamine used as reference in the FTIR analysis was synthesized in the same conditions as described above (2 g/L dopamine, 10 mM Tris–HCl buffer solution at pH = 8.5, 25 h polymerization time), without sonication and addition of particles.
Fig. 1. Schematic illustration of the fabrication process. The particles are first coated with a thin polydopamine layer via self-polymerization of dopamine. The high affinity toward metal ions of the catechols and amino groups of the polydopamine layer promotes a selective metal deposition on the coated particles during the subsequent electroless plating process.
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Fig. 2. (a) Possible dopamine polymerization mechanism and polydopamine structure modified after [22,39,40]. (b) Proposed dopamine polymerization mechanism and structure of polydopamine by Dreyer et al. [41].
2.3. Electroless metallization of polydopamine coated particles Copper electroless plating of alumina nanoparticles coated with polydopamine was performed in an aqueous solution containing 13.5 g/L CuSO45H2O as copper source, 10 g/L EDTA as complexing
agent, and 3 g/L Al2O3 nanoparticles. The solution was kept at room temperature and sonicated for 15 min prior to the addition of 33 mL/L hydrazine hydrate solution as reducing agent, which triggers the electroless deposition process. Copper electroless plating of the polydopamine coated WC particles was performed in an
Fig. 3. SEM images of the WC microparticles (a and b) before the polydopamine coating process and (c and d) after the polydopamine coating process.
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Fig. 5. Transmission FTIR spectra of (a) WC microparticles and (b) Al2O3 nanoparticles, both before and after the polydopamine coating process. Also, the FTIR spectrum of pure polydopamine is reported as reference.
Fig. 4. TEM images of the Al2O3 nanoparticles (a) before the polydopamine coating process and (b) after the polydopamine coating process.
aqueous solution containing 7.5 g/L CuSO45H2O, 5.5 g/L EDTA, and 10 g/L WC particles. The solution was kept at room temperature and sonicated for 5 min prior to the addition of 18.5 mL/L hydrazine hydrate solution. Silver plating on the WC powder was performed using 10 g/L WC particles and a plating bath containing 4.5 g/L AgNO3, 13.5 mL/L ethylenediamine, 0.3 g/L 3,5-diiodo-Ltyrosine dehydrate, and 12.0 g/L potassium sodium tartrate as described elsewhere [19]. After the plating process, the particles were washed via consecutive centrifugation cycles with deionized water and ethanol, until a clear solution is obtained, and then vacuumdried with a rotary evaporator at 40 °C. 2.4. Characterization X-ray diffractograms were obtained using a Panalytical X’Pert Pro diffractometer in Bragg-Brentano geometry with Cu Ka1 radiation. Scanning electron microscopy (SEM) images were acquired with a DSM-982 Gemini (Zeiss) at an acceleration voltage of 8 kV. Transmission electron microscopy (TEM) images were taken with a JEOL 2010 operated at an acceleration voltage of 80 kV. Thermogravimetric analysis (TGA) was performed with a STA 409 PC/PG-Luxx analyzer (Netzsch) in synthetic air atmosphere and using a heating rate of 5 °C/min. Fourier transform infrared (FTIR)
Fig. 6. TGA curves of pristine WC particles, polydopamine coated WC particles (Pdop@WC), pristine Al2O3 nanoparticles, and polydopamine coated Al2O3 nanoparticles (Pdop@Al2O3).
spectra were recorded with a Bruker Vertex 70 (32 scans, 2 cm resolution).
1
3. Results and discussion 3.1. Polydopamine coated particles In Fig. 1, the general fabrication process is reported. The first step is the polydopamine (Pdop) coating of particles via auto-oxidative polymerization of dopamine. The polymerization process and the polydopamine structure are still not perfectly understood. Lee et al. suggest a mechanism that involves the oxidation of the
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Fig. 7. SEM images of silver coated Pdop@WC microparticles.
Fig. 8. SEM images of copper coated Pdop@WC microparticles.
Fig. 9. SEM images of (a) pristine Al2O3 nanoparticles, (b) copper plated Pdop@Al2O3 nanoparticles.
catechols to quinine and a subsequent melanin-like polymerization as shown in Fig. 2a [22]. Similar mechanisms were also proposed by others [39,40]. A study from Dreyer et al. shows that polydopamine is not made of covalently bonded units, but of monomers which interact with each other through noncovalent forces like hydrogen bonding, p-stacking and charge transfer, as shown in Fig. 2b, resulting nonetheless in a highly stable material [41]. Polydopamine coated WC particles were investigated with SEM. Fig. 3a and b shows the WC particles before the coating process, characterized by sharp edges and flat surfaces. Fig. 3c and d shows the WC powder after the polydopamine coating process. The particles have smoothened corners and edges, and the surface shows a fine granularity, typical of polydopamine coatings [28,38]. According to Lee et al., 25 h of dopamine polymerization corresponds to a polydopamine layer thickness of approximately 50 nm [22]. Polydopamine coated alumina nanoparticles were investigated with TEM. Fig. 4a and b shows Al2O3 nanoparticles before and after polydopamine coating, respectively. In Fig. 4b, a thin but conformal polydopamine layer is observed on the surface of the crystalline alumina nanoparticles. This polydopamine layer has a thickness of approx. 2 nm after 1 h polymerization, in good agreement with the expected thickness from Lee et al. FTIR analysis was used to investigate the chemical particlepolydopamine interaction. FTIR spectra of the WC particles and
Al2O3 nanoparticles are shown in Fig. 5a and b, respectively. In the WC FTIR spectrum, several peaks appear between 1000 cm 1 and 1600 cm 1 after the polydopamine polymerization process, as in the polydopamine reference spectrum, indicating a successful coating process. Dopamine has several strong IR absorption peaks at wavelengths higher than 1600 cm 1, but none are observed in Fig. 5a, indicating that all dopamine molecules were effectively polymerized or removed [23,41]. In particular, the spectrum shows three characteristic peaks at 1078 cm 1, 1336 cm 1 and 1518 cm 1 which are respectively assigned to the stretching vibrations of C– OH groups, aromatic rings absorption, and N–H shearing vibrations [23,42,43]. In the FTIR spectrum of the alumina nanoparticles in Fig. 5b, several weak absorption peaks appear between 1200 cm 1 and 1600 cm 1, as in the polydopamine coated WC powder and in the polydopamine reference. The strong peak at 1500 cm 1 is assigned to the N-H groups shearing vibrations, while other peaks are covered by the broad absorption bands of alumina. In this case, a small shift of the absorption peaks to smaller wavelength is observed, indicating a stronger interaction of the polydopamine coating with the Al2O3 particles surface as compared to WC particles, probably due to the higher reactivity of the oxide and of the nanoparticles themselves. Thermogravimetric analysis was performed to confirm the polymerization of dopamine on the particles surface (Fig. 6). Uncoated WC particles are stable up
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Fig. 10. (a) TEM image of the copper plated Pdop@Al2O3 nanoparticles. (b) Particle size distribution histogram (total particle population = 40).
to 400 °C, when the oxidation reaction of WC to WO3 starts and an 18% weight gain is observed [44]. Polydopamine coated WC particles show an initial mass loss due to the decomposition of the organic phase. This mass loss reaches a value of 3% at 450 °C, when the WC oxidation becomes dominant and an 18% weight increase takes place. As received Al2O3 nanoparticles show a 6% mass loss due to the water molecules bound on Al-OH groups [45,46], while the polydopamine coated Al2O3 nanoparticles show a more substantial weight loss due to the decomposition of the organic layer, about 25%, at first at a high rate followed by a slower rate up to 600 °C, a trend reported also in other works [23,24]. The higher mass loss of the polydopamine coated alumina nanoparticles in comparison with the WC particles can be explained by the greater polydopamine shell thickness to core material radius ratio in the nanoparticles, where 13 nm particles have a 2 nm thick polydopamine shell, while 1 lm WC particles have a 50 nm polydopamine shell.
coated Pdop@WC particles. In all diffractograms, the WC (JCPDS 25-1047) diffraction peaks at 31.5°, 35.7°, 48.4°, 64.0°, 65.8°, 73.0°, 75.5°, and 77.0° can be seen. The diffraction patterns of pristine WC particles and of Pdop@WC particles are identical, indicating that the polydopamine layer is amorphous and has no effect on the WC particles structure. The silver coated Pdop@WC diffractogram shows the WC diffraction peaks along with two clearly visible peaks at 38.2° and 44.3°, and two other peaks at 64.5° and 77.5° partially covered by the WC diffraction peaks, corresponding to the (1 1 1), (2 0 0), (2 2 0), and (3 1 1) lattice planes of silver (JCPDS 4-783). In the copper coated Pdop@WC diffraction pattern, the typical WC diffraction peaks can be seen, along with three peaks at 43.3°, 50.4°, and 74.1° corresponding to the (1 1 1), (2 0 0), and
3.2. Metal plated particles The polydopamine coated particles were coated by a metal layer via electroless plating due to the high affinity to metal ions of the catechols and amino groups of the polymer coating. The use of a reducing agent to reduce metal ions to metal on the particles surface is necessary with copper, while silver ions can be reduced to silver by polydopamine alone (see additional supporting information). However, using a reducing agent, a silver coating is deposited in much less time and shows a better quality. Figs. 7 and 8 show SEM images of the silver plated WC powder and the copper plated WC powder, respectively. The surface of the metal coated particles is much rougher than the as received WC particles (Fig. 3a) and the polydopamine coated WC particles (Fig. 4b and c). The particles look completely covered with a coarse metal layer with a grainy morphology consisting of coalesced metal grains, characteristic of metals deposited by electroless plating, and other solution-based processes [47–51]. Fig. 9a and b shows SEM images of the as received Al2O3 nanoparticles and the same nanoparticles after the copper electroless plating process, respectively. An increase in size due to the polydopamine layer and the copper coating is clearly visible. The particles are uniformly coated and have an average size of 30–40 nm. Fig. 10 shows a TEM image of the copper plated Pdop@Al2O3 nanoparticles with the respective particle size distribution histogram for a population of 40 nanoparticles. The particles have a slightly irregular shape and sizes in the 20– 40 nm range, in good agreement with the particles size observed in Fig. 9b. Fig. 11a shows the XRD patterns of the pristine WC particles, Pdop@WC particles, copper coated Pdop@WC particles, and silver
Fig. 11. (a) XRD patterns of pristine WC particles, polydopamine coated WC particles (Pdop@WC), copper coated Pdop@WC, and silver coated Pdop@WC. (b) XRD patterns of pristine Al2O3 nanoparticles, polydopamine coated Al2O3 nanoparticles (Pdop@Al2O3), and copper coated Pdop@Al2O3 nanoparticles.
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(2 2 0) lattice planes of copper (JCPDS 4-836). No other peak due to impurities or undesired byproducts like oxides is observed. In Fig. 11b, the diffraction patters of pristine Al2O3 nanoparticles, Pdop@Al2O3, and copper plated Pdop@Al2O3 are shown. In the diffraction patterns of pristine Al2O3 nanoparticles and Pdop@Al2O3, only the broad peaks of d-alumina (JCPDS 4-877) nanoparticles can be observed, while in the copper plated Pdop@Al2O3, diffraction pattern also the copper diffraction peaks at 43.3°, 50.4°, and 74.1° are present. 4. Conclusions A general and versatile process for the synthesis of metal coated particles using polydopamine-assisted electroless plating was reported. The process consists of the fabrication of a polydopamine layer on the particles surface via dopamine oxidative polymerization, followed by a controlled metal deposition via electroless plating on the polydopamine-functionalized particles. This process was used to fabricate copper coated WC microparticles and Al2O3 nanoparticles, as well as silver coated WC microparticles, with a conformal and compact metal shell. The use of both nanoparticles and microparticles, as well as an oxide and a carbide as particle materials, shows the versatility and substrate-independence of this technique. This method offers a cost-effective, scalable, and generic route for the manufacture of metal coated particles without the conventional Pd/Sn-based sensitization steps, especially for applications in the fabrication of metal matrix composites. Moreover, because of the high reactivity and versatility of the polydopamine layer, the dopamine polymerization process introduced in this work can also be used as a particle activation strategy in other applications such as in biotechnology, optics, and drug delivery. Acknowledgments
[3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37]
This work was fully funded by the European Regional Development Fund (Europäischer Fonds für regionale Entwicklung – EFRE) and the Free State of Saxony (Sächsische Aufbaubank – SAB Project No. 100112077). The authors gratefully thank the Leipniz Institute for Solid State and Materials Research Dresden, especially Nicole Geißler and Dr. Sandeep Gorantla for technical assistance with TEM, Prof. Dr. Büchner and Dr. Silke Hampel (Institute for Solid State Research) for project cooperation in the European Centre for Emerging Materials and Processes ECEMP subproject C1 ‘‘NanoWearJoin’’. Appendix A. Supplementary material
[38] [39] [40] [41] [42] [43] [44] [45] [46] [47]
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2013.08.028.
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