Synthesis of core–shell structured alumina/Cu microspheres using activation by silver nanoparticles deposited on polydopamine-coated surfaces

Abstract

A facile approach was developed to synthesize core–shell structured alumina/Cu microspheres by means of activation by silver nanoparticles in situ deposited on the polydopamine-coated alumina microsphere surface. This approach takes advantage of both the sufficient reductive capacity of polydopamine towards silver ions and the high activation of silver nanoparticles towards copper deposition. The chemical compositions of composite microspheres were characterized by energy dispersive X-ray spectroscopy (EDS) and Fourier transform infrared spectroscopy (FTIR). The surface morphologies and crystalline structures of the coated alumina were studied by field-emission scanning electron microscopy (FESEM) and X-ray diffraction (XRD), respectively. The adhesion strength between the copper layer and the substrate was evaluated by ultrasonic exposure and thermal shock test. The results showed that silver nanoparticles effectively catalyzed the copper deposition reaction on the polydopamine-coated surface, which led to a faster and more reliable plating process compared with the process without activation. The compact and continuous copper layer obtained showed strong adhesion on the substrate surface.

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1. Introduction

Composite particles with core–shell structures have received increasing attention owing to their unique and enhanced physicochemical properties and potential applications in various fields such as photonics, electronics, pharmacology and catalysis.^1–4 Surface metallization, involving electroplating,⁵ electroless plating,^6,7 chemical vapor deposition (CVD)⁸ or sputtering deposition,⁹ is one of the effective strategies for building tailored particles with metal shells to improve the thermal, electrical or electromagnetic properties of the particles. Although these methods have been commonly used and success has been achieved, the tedious steps, environment-polluting solvents involved in the processes and the weak adhesion strength of the deposited metals have limited their widespread application.

Inspired by the protein-based adhesives excreted by marine mussels, Lee and co workers¹⁰ found that a thin and adherent polydopamine (PDA) layer could be formed on virtually all types of material surfaces via spontaneous oxidative self-polymerization of dopamine in an aqueous solution. The polydopamine layer could serve as an extremely versatile platform for secondary reaction to produce tailored functional layers, such as metal layers, self-assembled monolayers and grafted polymer coatings. It has been demonstrated that polydopamine possesses the ability to bind various metal ions and reduce some of them such as Au³⁺, Ag⁺, Pt³⁺ into their corresponding metals under basic conditions.¹¹ In such a case, exogenous reducing agent is not necessary, but its addition could help the further deposition process to obtain a thicker and more compact metal layer. Up to now, electroless silver or gold plating using polydopamine as a functionalized layer has been performed on various kind of surfaces, including particles,^12–14 fibers^15–17 and films.^18–20 Zhang and co workers²¹ obtained a compact and continuous silver layer on polydopamine-coated hollow carbon microspheres with glucose as a reducing agent. Their results showed that the size, compactness and continuity of the silver nanoparticles increased with the increase of AgNO₃ concentration. The electrical conductivities of copper and silver are very close, 1.678 × 10⁻⁸ Ω m and 1.586 × 10⁻⁸ Ω m, respectively, while the cost of electroless silver plating is much higher than that of electroless copper plating (Ag: $0.708 per gram, Cu: $0.007 per gram),²² and thus electroless copper plating is more economical for practical applications.

For polydopamine-assisted copper deposition, the addition of an exogenous reducing agent is essential owing to the insufficient reductive capacity of polydopamine. Lee and co workers¹⁰ firstly reported electroless copper deposition on polydopamine-coated nitrocellulose film and three-dimensional plastic objects using dimethylamine borane (DMAB) as an exogenous reducing agent. However, when Lee's approach was applied on powders, it was found that the deposition rate was very slow at room temperature. More importantly, the copper deposition reaction happened not only on the surface of the microspheres, but also in the bulk solution and on the inner wall of the vessels. The leading cause of the undesired deposition is the subtle difference in the reactivity between cupric ions on the microsphere surface and in the bulk solution. From the viewpoint of electroless plating, it is necessary to activate the surface to avoid undesired deposition before performing the electroless plating on non-catalytic surfaces.

Alumina-supported copper has been used as catalyst in many industrial applications, such as dehydrogenation of alcohol, methanol synthesis and water-gas shift reaction. Copper-coated alumina is also used as reinforcing material for Cu-based composites owing to its high strength, high Young's modulus, excellent thermal stability and great hardness. The wetting property and interface strength between copper matrix and alumina could be improved by coating a layer of copper onto the alumina surface. Herein is reported a facile and efficient approach to depositing copper layers on alumina microspheres employing the activation of silver nanoparticles that are in situ deposited on polydopamine-coated surfaces. The interface interaction between alumina and copper was enhanced by the presence of PDA. The plating rate was increased by catalysis by the silver nanoparticles. Polydopamine-coated alumina microspheres were simply prepared by immersing the microspheres in dopamine aqueous solution for a certain period of time. Silver nanoparticles, which were in situ deposited on the surface via the reduction of silver ions by polydopamine, acted as catalytic seeds and successfully accelerated the electroless copper deposition on the polydopamine-coated microsphere surface. Finally, the copper layer was protected by self-assembled monolayers of octadecanethiol to slow down corrosion, including oxidation and acid corrosion. The surface chemistry of the modified alumina microspheres was studied by Fourier transform infrared spectroscopy (FTIR) and energy dispersive X-ray spectroscopy (EDS). Scanning electron microscopy (SEM) was used to investigate the morphologies of the samples. The crystalline structures were studied by X-ray diffraction (XRD). The adhesion strength between the copper layer and the substrate was evaluated by ultrasonic treatment and thermal shock test. The binding of self-assembled monolayer was qualitatively measured by FTIR and the wettability with water.

2. Experiment

2.1. Materials

Alumina microspheres with a mean diameter of about 15 μm were obtained from Xuancheng Jingrui New Material Co., Ltd, Anhui Province, P. R. China. The alumina microspheres were cleaned in ethanol and deionized (DI) water separately by ultrasound for 20 min and dried at 80 °C for 6 h in an oven before use. Dopamine hydrochloride, tris(hydroxymethyl)aminomethane (Tris), dimethylamine borane (DMAB) and octadecanethiol were purchased from Aladdin Industrial Corporation. Silver nitrate, copper chloride, ethylene diamine tetraacetic acid disodium salt (EDTA·2Na) and boric acid were obtained from Guangzhou Jinhuada Chemical Co., Ltd, P. R. China. All reagents were of commercial analytical grade and used as received.

2.2. Preparation of copper-coated alumina microspheres

Polydopamine-coated alumina microspheres were prepared following the methods reported in ref. 10. Alumina microspheres were dispersed in the aqueous dopamine solution with magnetic stirring at room temperature for 6–24 h. Dopamine solution (2 g L⁻¹) was prepared by dissolving dopamine. HCl in Tris–HCl buffer solution (10 mM, pH = 8.5). The color of the dopamine solution changed from colourless to light pink and then to dark brown, which indicated the self-polymerization of dopamine may involve melanin formation. Finally, the alumina microspheres were separated by centrifugation and rinsed thoroughly with DI water. The product was denoted Al₂O₃/PDA.

The as-prepared Al₂O₃/PDA microspheres were added to silver nitrate solution (50 mM), and stirred for 30 min to give enough time for the adsorption and the reduction of silver ions by the polydopamine sublayer. The samples were filtered and washed thoroughly with DI water. The product was denoted Al₂O₃/PDA/Ag_NP.

The as-received Al₂O₃/PDA/Ag_NP microspheres were placed into electroless copper plating solution, which contained 50 mM copper chloride as copper source, 50 mM EDTA·2Na as complexing agent, 0.1 M boric acid as stabilizer, and 0.1 M DMAB serving as reducing agent. The solution was stirred for 30 min. Then the samples were separated by filtration and washed thoroughly with DI water and ethanol successively. The product was denoted Al₂O₃/PDA/Ag_NP/Cu.

The Al₂O₃/PDA/Ag_NP/Cu microspheres were dispersed into 1-octadecanethiol solution (with a concentration of 10 mM in anhydrous ethanol), and stirred for 30 min. The resulting samples were filtered and washed with anhydrous ethanol, and then dried in a vacuum oven at 40 °C for 1 h. The whole process is schematically showed in Scheme 1.

Scheme 1 Schematic illustration of the procedure for electroless copper plating on polydopamine-coated alumina microsphere activated by silver nanoparticles.

2.3. Characterization

SEM investigation. The morphology and surface chemistry of the modified alumina surfaces were investigated by field-emission scanning electron microscopy (FESEM) (Nova NanoSEM 430, FEI, USA) equipped with an EDS detector (X-Max^N, Oxford, UK). The tested surfaces were sputtered with a thin layer of platinum prior to microscopy.

Infrared spectra measurement. Small amounts of samples were mixed with potassium bromide and pressed into thin disks. FTIR spectra were collected from 4000 cm⁻¹ to 400 cm⁻¹ with a resolution of 4 cm⁻¹ for 32 scans (Vertex 70, Bruker, Germany).

Thermogravimetric analysis. About 5–10 mg of the samples was heated from 30 °C to 800 °C at a rate of 20 K min⁻¹ under N₂ atmosphere in a Netzsch TG 209 F1 analyzer.

X-ray diffraction test. X-ray diffraction (XRD) (X'Pert PRO, PANalytical, The Netherlands) was used to test the crystalline structures of the samples using Cu K_α radiation with a wavelength of 1.5418 Å, and the diffraction patterns were recorded in the 2θ range of 10–90°.

Adhesion strength. The adhesion strength of the copper layer was evaluated by ultrasonic exposure test and thermal shock test. The ultrasonic exposure test was performed in water at 20 °C for 30 min with a Branson 3210 ultrasonicator (40 kHz, 130 W). The thermal shock test was carried out following the thermal cycle: −15 °C for 1 h in a cryogenic tank; room temperature (about 25 °C) for 0.5 h; 85 °C for 1 h in a constant temperature oven; then room temperature for another 0.5 h. The thermal cycle was repeated three times.

Water contact angle. The as-prepared Al₂O₃/PDA/Ag_NP/Cu powders before and after treatment with octadecanethiol were spread on glass slides. The powder was compressed lightly by another glass slide to form an even and continuous powder sheet. Then a droplet was dripped onto the sheet surface. The contact angle was measured with a Kruss contact angle goniometer (DSA100, Germany).

3. Results and discussion

3.1. Preparation of the Al₂O₃/PDA microspheres

Generally, the deposition of dopamine is an oxidative self-polymerization process which can be accelerated by performing in oxygen atmosphere²³ or adding oxidants.^24,25 In this work, we found that the morphology of modified surfaces obtained in pure oxygen atmosphere was much rougher than that obtained under ambient air atmosphere. So in this study the polydopamine deposition was carried out under ambient air conditions.

The successful coating of polydopamine on the surface of alumina microspheres was confirmed by FTIR, TGA and SEM. Pure polydopamine was synthesized as a control under the same reaction conditions as Al₂O₃/PDA. Two peaks that appeared at 1620 cm⁻¹ and 1509 cm⁻¹ in the FTIR spectrum of Al₂O₃/PDA microspheres (Fig. 1b) were assigned to phenylic C=C stretching vibrations and N–H shearing vibrations of polydopamine, repectively. A broad absorption peak at 3243 cm⁻¹ was ascribed to the stretching vibrations of O–H and N–H. TGA curves (Fig. 2) showed nearly no weight loss of the pristine alumina upon heating to 800 °C owing to its excellent thermal stability; weight losses of 3.4% and 42.3% of Al₂O₃/PDA microspheres and pure PDA, respectively, were observed within the same temperature range, resulting from decomposition of the organic phase. Therefore, approximately 8.0 wt% polydopamine coating was found on the surfaces of alumina microspheres based on the TGA data.

Fig. 1 FTIR spectra of (a) pristine alumina microspheres, (b) Al₂O₃/PDA microspheres, and (c) PDA.

Fig. 2 TGA curves of (a) pristine alumina microspheres, (b) Al₂O₃/PDA microspheres, and (c) PDA.

Many distinct wrinkles and grooves can be observed on the surface of pristine alumina microspheres in the SEM images (Fig. 3a), which might result from the preparation process of spherical alumina. A mass of tiny particles randomly scattered on the alumina surface can be seen when the oxidation reaction time was 6 h (Fig. 3b). As the reaction time went on, the number and the size of the particles increased (Fig. 3c). The particles touched each other and merged. Finally, the coalescent particles formed an integrated coating on the surface of alumina microspheres with some observable raised particles embedded (Fig. 3d). Wang and co workers¹⁴ got a similar rough surface after the aluminum microspheres were coated by polydopamine.

Fig. 3 SEM images of (a) pristine alumina microspheres and Al₂O₃/PDA microspheres with reaction times of (b) 6 h, (c) 12 h and (d) 24 h.

3.2. Preparation of Al₂O₃/PDA/Ag_NP/Cu microspheres

Catechol groups in polydopamine molecules are generally accepted to have a central role in the polydopamine-assisted metallization process. Silver ions can be adsorbed on the polydopamine surface and strongly chelated with the ortho-phenolic hydroxyls of catechol groups. During the oxidation process of dopamine into benzoquinone, catechol groups release electrons and protons.²³ The chelated silver ions catch the electrons and are in situ reduced into silver nanoparticles and deposited on the polydopamine surface. Ball and co workers²⁶ found that the reduction of silver ions had no effect upon the electron spin resonance spectroscopy (ESR) signal of polydopamine film while the X-ray photoelectron spectroscopy (XPS) results showed a small increase in the density of quinone groups and a small decrease of catechol groups on the polydopamine surface, which strongly supports the above mechanism.

In this work, discontinuous and randomly scattered silver nanoparticles with an average diameter of about 60 nm on the microsphere surfaces (Fig. 4a) were expected to serve as catalytic seeds to activate the surfaces. In order to evaluate the surface activity of pristine alumina, Al₂O₃/PDA and Al₂O₃/PDA/Ag_NP microspheres, the same amount of each of these three kinds of microspheres was dispersed in a plating bath with the same constituents for 30 min. It was found that no copper particles were observed on the surfaces of pristine alumina and Al₂O₃/PDA microspheres (Fig. 4b and c), while the entire surfaces of Al₂O₃/PDA/Ag_NP microspheres were covered by copper layers (Fig. 4d). The copper deposition was difficult to trigger without catalytic seeds on the surfaces of pristine alumina and Al₂O₃/PDA microspheres. As for the silver-activated process, cupric ions around silver nanoparticles were catalytically reduced by exogenous reducing agent and deposited on the surface with silver nanoparticles as nucleation centers. Subsequently, the already deposited copper particles self-catalyzed further deposition; thus a thin continuous and compact copper layer could be formed. The copper layers could also be deposited on the Al₂O₃/PDA microspheres by raising the bath temperature or prolonging reaction time. However, undesired copper particles were then also found in the bulk solution and on the inner wall of the beaker at the same time (inset of Fig. 4c). Therefore, the silver-activated process was proven to be more efficient and easier.

Fig. 4 SEM images of (a) Al₂O₃/PDA/Ag_NP, (b) Al₂O₃, (c) Al₂O₃/PDA and (d) Al₂O₃/PDA/Ag_NP microspheres treated by electroless copper plating under the same conditions. The inset of (c) shows the plating bath after raising the bath temperature or prolonging reaction time.

The adhesive strength between the copper layer and the substrate was evaluated by ultrasonic treatment and thermal shock test. As shown in the SEM images, after ultrasonic treatment for 30 min (Fig. 5a and b) and thermal shock testing for 3 cycles (Fig. 5c and d), the copper layers still firmly adhered to the substrates, which indicated good adhesion between the copper layers and alumina surfaces. The copper-coated microspheres treated ultrasonically showed a smoother surface because the weakly attached particles had been removed by vibration. Such strong adhesion is a striking advantage of polydopamine; however, the exact adhesion mechanism remains elusive. Reviewing the previous findings,^27–30 the interactions between polydopamine and substrates vary depending on the surface properties of the substrates. For metal or metal oxide surface, coordination bonding and chelating bonding interaction may play central roles in adhesion.

Fig. 5 SEM images of Al₂O₃/PDA/Ag_NP/Cu microspheres after (a and b) ultrasonic treatment for 30 min and (c and d) three cycles of thermal shock.

The chemical composition of microspheres obtained in each step was confirmed by EDS data. For the pristine alumina particles, the atom ratio of O/Al was 1.42 (Fig. 6a), close to the theoretical value 1.5, and no carbon element was found. After the deposition of polydopamine, carbon could be detected at 0.28 keV with a content of 20.03 wt% (Fig. 6b), and the content of Al decreased by almost 18.02 wt%, which confirmed the presence of polydopamine on the alumina surface. Ag L_α and Ag L_β were found at 2.98 keV and 3.15 keV, respectively, with a total content of 17.65 wt% on Al₂O₃/PDA/Ag_NP microsphere surfaces (Fig. 6c). Two distinct peaks of Cu K and Cu L could be seen at 8.02 keV and 0.94 keV (Fig. 6d), respectively, with a total amount of 60.55 wt%. Signals of C, O, Al and Ag were also observed in the spectrum because the thicknesses of deposited layers are less than the detection depth of EDS.

Fig. 6 EDS spectra and element contents of (a) pristine alumina microspheres, (b) Al₂O₃/PDA, (c) Al₂O₃/PDA/Ag_NP, and (d) Al₂O₃/PDA/Ag_NP/Cu (the peak around 2.1 keV is attributed to platinum).

The crystalline structure of the alumina microspheres and modified core–shell structured microspheres was detected by XRD. The diffraction patterns of the microspheres are presented in Fig. 7. Many sharp diffraction peaks seen in Fig. 7a were ascribed to the crystalline structure of alumina (JCPDS 46-1212 and JCPDS 46-1215). Polydopamine had no effect on the XRD pattern (Fig. 7b). This result is similar to that of Fu et al.,¹⁶ who found that polydopamine layers on silica nanofibers did not change the diffraction pattern. Two weak diffraction peaks appearing at 2θ = 38.1° and 44.3° in Fig. 7c were assigned to the diffraction peaks of Ag (111) and Ag (200), respectively (JCPDS, 04-0783). Other weaker peaks (2θ value at 64°, 77° and 82°) of the silver diffraction pattern were not detected owing to the low content of silver nanoparticles. After the electroless copper plating, three new diffraction peaks corresponding to the (111), (200) and (220) lattice planes of copper (JCPDS, 04-0836) appeared at 2θ values of 43.4°, 50.6°and 74.2°, respectively. No diffraction peaks belonging to cupric oxide or cuprous oxide could be observed, which means that copper particles in metallic form were plated on the alumina surface (Fig. 7d).

Fig. 7 XRD patterns of (a) pristine alumina microspheres, (b) Al₂O₃/PDA, (c) Al₂O₃/PDA/Ag_NP, and (d) Al₂O₃/PDA/Ag_NP/Cu.

3.3. Self-assembled monolayers of octadecanethiol on the surface of Al₂O₃/PDA/Ag_NP/Cu microspheres

Copper can be readily corroded in humid oxygen-containing atmospheres. One of the effective ways to minimize this problem is the self-assembly of n-alkanethiols system. Alkanethiols are easy to adsorb onto the surfaces of face centered cubic (fcc) metals, such as gold, silver, copper and nickel, to form self-assembled monolayers.³¹ The densely packed monolayers provide a barrier against water and oxygen penetration.³² It is difficult to collect the test information to confirm the existence of a single molecular layer on the surface owing to weak signals. In the FTIR spectrum of Al₂O₃/PDA/Ag_NP/Cu microspheres after the adsorption of octadecanethiol as shown in Fig. 8b, two weak peaks corresponding to the asymmetric and symmetric stretching vibrations of methylene could be discerned at 2920 cm⁻¹ and 2850 cm⁻¹, respectively. A peak at 1384 cm⁻¹ was assigned to the symmetric deformation vibration of methyl of octadecanethiol. The inset pictures in Fig. 8 show an obvious difference in the wettability of composite microspheres before and after treatment with octadecanethiol. Water droplets could be easily absorbed into the sheet made of Al₂O₃/PDA/Ag_NP/Cu microspheres owing to capillarity (Fig. 8c). By contrast, after the microspheres were treated with octadecanethiol, the water droplet kept a nearly spherical shape on the sheet surface (Fig. 8d) because of the super-hydrophobicity of the long alkyl chains. The results shown in Fig. 8 qualitatively confirmed the presence of octadecanethiol on the surface of Al₂O₃/PDA/Ag_NP/Cu microspheres.

Fig. 8 FTIR spectra of Al₂O₃/PDA/Ag_NP/Cu microspheres (a) before and (b) after treatment with octadecanethiol. The inset pictures show water droplets on a sheet made of Al₂O₃/PDA/Ag_NP/Cu microspheres (c) before and (d) after treatment with octadecanethiol.

4. Conclusion

A facile and efficient approach to synthesizing core–shell structured alumina/Cu microspheres catalyzed by silver nanoparticles on polydopamine-coated alumina microspheres is proposed. In this work, the silver nanoparticles in situ reduced by polydopamine served as catalytic seeds to subsequently catalyze the electroless copper plating. Continuous and compact copper layers were obtained on the activated surface and the adhesion between the copper layer and substrate was strong enough to endure ultrasonic treatment and thermal shock test. This approach takes advantage of both the sufficient reducing capacity of polydopamine towards silver ions and the high capacity of silver nanoparticles to activate copper deposition. It leads to a fast and feasible electroless copper plating process, which strongly extends the application of polydopamine-assisted electroless metallization. The material-independent deposition of polydopamine makes this approach possible to apply to a wide range of substrates to metallize their surfaces. The core–shell structured alumina/Cu composite particles can be used as reinforcing material of Cu-based composites, functional fillers of electrically and thermally conductive polymer-based composites and electromagnetic interference (EMI) shielding coatings, as well as catalyst for many important industrial reactions.

Acknowledgements

The authors sincerely appreciate the financial support from the National Natural Science Foundation of China (Grant No. 21073066) and the Science and Technology Program of Guangzhou, China (Grant No. 1561000293).

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