Surface coating with various metals on spherical polymer particles by using barrel sputtering technique

Journal of Alloys and Compounds 441 (2007) 162–167

Surface coating with various metals on spherical polymer particles by using barrel sputtering technique Akira Taguchi a , Tomohito Kitami b , Hironari Yamamoto a , Satoshi Akamaru a , Masanori Hara a , Takayuki Abe a,∗ a

Hydrogen Isotope Research Center, University of Toyama, Gofuku 3190, Toyama 930-8555, Japan Nippon Pillar Packing Co. Ltd., 541-1 Utsuba, Shimo Uchigami, Sanda, Hyogo 669-1333, Japan

b

Received 17 January 2006; accepted 24 July 2006 Available online 2 November 2006

Abstract Micron-sized spherical polymer particles were homogeneously coated with Au, Ag, Pd, Cu, and Ni by the barrel sputtering technique. Surface coating was successfully performed independent of the sputtered metal and the diameter of the polymer particles used. Optical microscope observations revealed a mirror-like appearance with a color that depended on the coated metal. SEM and EDX studies revealed that the surface of metal-coated samples was very smooth and the sputtered metal was homogeneously distributed over the entire sample surface. The amount of coated metal was evaluated using TG–DTA and ICP-AES measurements. The thickness of the metal film was within several 10 nm. © 2006 Elsevier B.V. All rights reserved. Keywords: Barrel sputtering system; Surface coating; Powdery materials

1. Introduction Considerable attention has been focused on the surface coating of organic and inorganic particles in different industrial fields such as the manufacture of pharmaceutical products, catalysts, ceramics, electronics, bioanalytical and biomedical applications, and so on [1–12]. With regard to surface modification, the sputtering technique offers the following advantages such as synthesis of thin films with minimal impurity, easily controllable process parameters, and flexibility in synthesizing metals, alloys, metal oxides, nitrides, or carbides [13,14]. Therefore, sputtering deposition is a powerful and appropriate technique for surface modification. Due to these advantages, sputtering deposition would have many practical applications in various industrial fields. However, due to the difficulty of the instrumental set-ups, there are only several reports applying sputtering technique for surface coating on particle materials [15–21]. Takeshima et al. developed a DC sputtering system equipped with a cylindrical barrel for the modification of small particles

∗

Corresponding author. Tel.: +81 76 445 6933; fax: +81 76 445 6931. E-mail address: [email protected] (T. Abe).

0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.07.131

[15,16]. The system appears to be applicable to industrial purposes and can be potentially used for the surface modification of particles. However, few scientific studies have been conducted by using this system. Recently, Fernandes et al. developed an instrument equipped with the sputtering system for particle coating [18]. The compaction behavior of a WC particle improved due to a stainless steel coating on its surface: No pressing binder is required to obtain green compacts. More recently, Veith et al. reported the one-step preparation of a CO oxidation catalyst [19]. For this reaction, activity of gold nanoparticles supported on ␥-Al2 O3 , prepared by magnetron sputtering deposition, is comparable to that of catalysts prepared by traditional methods such as chemical vapor deposition or impregnation techniques. We have extensively studied the surface coating of particles by using our “barrel sputtering” system [17,20,21,27,28]. Our results revealed that one of the important factors for surface modification by the sputtering technique is the use of a hexagonal barrel instead of a cylindrical one. This leads to efficient mixing of base materials, thereby resulting in a homogeneous coating over individual particles [20,21]. In addition, by coating Pt on Al2 O3 or polymer materials (thickness of the Pt film deposited on the polymer was within 50 nm), it was found that the behavior of the Pt-coated particles was similar to that of pure bulk Pt in electrochemical measurements. These results clearly

A. Taguchi et al. / Journal of Alloys and Compounds 441 (2007) 162–167

demonstrate the potential capabilities of giving new functions to mother particles by surface coating. However, since not only Pt but also other metals can impart functions to particles, it is important to investigate the feasibility of surface coating with various metals. For example, group XI metals in the periodic table exhibit excellent conductivity. Pd and Ni are important materials that can be used as catalysts, underlayers, or decorations for electro- and electroless plating [22]. In this paper, we demonstrate the surface coating with Au, Ag, Pd, Cu, and Ni on spherical polymer particles. 2. Experimental The spherical PMMA polymer particles (Techpolymer MB30X-15SS and XX-450Z) used in this study were kindly supplied by Sekisui Plastics Co. Ltd. The mean particle diameters were 15 and 5 ␮m, respectively. These particles were used as received. Surface modification was performed by using our newly developed barrel sputtering system. Details of the set-up of the instrumental system are described in Refs. [20,21]. Metal modification of the particle surface was performed as follows. To the hexagonal barrel, ca. 3.00 g of polymer particles was loaded, and then, the vacuum chamber equipped with the hexagonal barrel was evacuated to 8.0 × 10−4 Pa. Highly pure argon gas (99.995%) was gradually introduced into the evacuated chamber up to a pressure of 2.0 Pa, and this pressure was maintained during the sputtering process. The target metals used in this study were Au (99.99%), Ag (99.99%), Pd (99.95%), Cu (99.99%), and Ni (99.9%). Typical RF (13.56 MHz) magnetron sputtering conditions were as follows; sputtering power, 100 W; and sputtering time, 1 h. During the sputtering process, a swinging motion with ±75◦ at the speed corresponding to 4.0 rpm was given to the hexagonal barrel. The metal–polymer composites thus prepared are denoted as M(x), where M and x denote the sputtered transition metal and the diameter of the polymer particles in ␮m, respectively. For a Ni coating (Ni(15)), the modification was performed at 200 W for a rather long duration (8 h) because the magnetic property of the Ni target often reduced the sputtering efficiency [23]. The definition of the metal-coated samples and their preparation conditions are summarized in Table 1. At least three samples of Au(15), Au(5), Ag(15), and Cu(15) were prepared at the desired conditions in order to investigate the reproducibility of the barrel sputtering system. X-ray diffraction (XRD) study was conducted using the PW1825/00 (Philips) system with Cu K␣ radiation. The diffraction patterns were recorded using a variable slit. Thermal analysis was performed by TG–DTA (Shimadzu DTG-50H). Several milligrams of the sample were heated to 600 ◦ C in a Pt pan at a ramp rate of 20 ◦ C/min and then maintained at this temperature for 0.5 h. The ICP-AES (Perkin-Elmer, Optima 3300-XL) analysis was performed on selected samples. The Au(15), Ag(15), Cu(15), and Pd(15) samples were dissolved in aqua regia or nitric acid, and then diluted to ca. 5 ppm of the respective metals. An optical microscope (KEYENCE, VHX-200) and a scanning electron microscope, SEM (JEOL, JSM-5600LV) equipped with EDX (JEOL, JED-2140) were used to investigate the surface of the metal-coated polymer. For microscopic measurements, the metal-coated sample was first dispersed in ethanol or toluene under ultrasonic radiation. One or two drops of the homogeneously dispersed

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solution were placed on the observation grid and used for measurements after drying at ambient temperature.

3. Results and discussion Fig. 1 shows the photograph of the prepared samples in the glass bottles. The colors of Au-, Ag-, Pd-, Cu-, and Ni-coated samples are dark brown, light gray, light black, reddish brown, and dark gray, respectively. These colors are completely different from that of the uncoated sample, suggesting that the particle surface is coated with metal. The metal-coated samples are first characterized by XRD measurements. Typical XRD patterns of these samples with a diameter of 15 ␮m and the uncoated sample are shown in Fig. 2. While the uncoated sample exhibits three broad halo patterns centered at 2θ = ca. 15◦ , 30◦ , and 42◦ (Fig. 2(F)), the metalcoated samples exhibit several diffraction signals attributed to the modified metal. In the XRD pattern of Au(15) shown in Fig. 2(A), the diffraction signals are observed at 2θ = 38.3◦ , 44.3◦ , 64.7◦ , 77.7◦ , and 81.8◦ . These five diffraction signals can be assigned to the (1 1 1), (2 0 0), (2 2 0), (3 1 1), and (2 2 2) peaks of metallic gold (cubic Fm3m, JCPDS 04-0784). No other diffraction signals were observed, indicating high purity of deposited Au. Similarly, the Ag(15), Pd(15), Cu(15), and Ni(15) samples are given diffraction signals corresponding to the respective coated metal as the Miller index of the cubic Fm3m symmetry is shown in Fig. 2(B)–(E). The full width at half maximum (FWHM) values for the (1 1 1) peak of Au(15), Ag(15), Pd(15), Cu(15), and Ni(15) were 0.63◦ , 0.57◦ , 1.62◦ , 0.90◦ , and 1.71◦ , respectively. These FWHM values were larger than those of the respective bulk metals. In particular, those of Pd(15) and Ni(15) were significantly larger than those of the other samples. In our previous study, the FWHM value of Pt-coated polymer particles was reported as 2.0–2.4◦ . This large value was attributed to the defects and/or distortions in the Pt layer based on XRD and careful TEM observations of a cross-sectional area of the Pt film [21]. Thornton and

Table 1 Summary of the preparation conditions of metal-coated polymer particles Sample definition

Metal sputtered

Polymer diameter (␮m)

Sputtering conditions

Au(15) Au(5)

Au

15 5

100 W, 1 h 100 W, 1 h

Ag(15) Ag(5)

Ag

15 5

100 W, 1 h 100 W, 1 h

Pd(15) Cu(15) Ni(15)

Pd Cu Ni

15 15 15

100 W, 1 h 100 W, 1 h 200 W, 8 h

Fig. 1. Photographs of Au-, Ag-, Pd-, Cu-, and Ni-coated samples and the uncoated sample (φ = 15 ␮m). From left to right: Au(15), Ag(15), Pd(15), Cu(15), Ni(15), and the uncoated sample.

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Fig. 2. XRD patterns of metal-coated polymer particles: (A) Au(15), (B) Ag(15), (C) Pd(15), (D) Cu(15), (E) Ni(15), and (F) uncoated PMMA polymer (φ = 15 ␮m). The numbers in parentheses indicate the Miller indexes of the respective metals in Fm3m symmetry.

Hoffman reported that stress in a metal film coated was accumulated with an increase in the melting point of the metal used [24]. The melting points of Au, Ag, Pd, Cu, and Ni, which are used in this study, are 1064, 961, 1555, 1084, and 1455 ◦ C, respectively, [25]. It is evident that the melting points of Pd and Ni are higher than those of the others. In addition, the melting point of Pt is 1769 ◦ C, and is higher than those of the metals used in this study. Thus, it can be concluded that line broadening observed in the coated samples is probably due to the presence of defects and/or distortions in the coated layers. It can be observed that the signal intensities associated with each sample were different. However, since the sputtering rate and atomic scattering factors depended on the metal used, no useful information could be obtained in the present study. Fig. 3 shows the optical microscope images of the metalcoated samples and the uncoated sample. As shown in Fig. 3(A), four Au(15) samples appeared as dark yellow circles with a diameter of ca. 15 ␮m. In addition, a bright white spot with a diameter of ca. 8 ␮m was observed at the top of each sample, which was attributed to the scattering of light from the

optical light source. Moreover, as indicated by red arrows in the photograph, pale spots were observed on the surface of the coated sample. Evidently, these observations differed from that of the uncoated sample shown in Fig. 3(G). The uncoated sample was colorless and transparent, and light from the source was reflected slightly. Thus, it can be assumed that the surface of the Au-coated sample has a smooth mirror-like appearance. Similar results were obtained for the Au-coated sample with a diameter of 5 ␮m (denoted as Au(5)), as shown in Fig. 3(B). This implied that Au can be successfully coated on a 5 ␮m polymer particle by using our barrel sputtering system. It should be noted that the present authors have reported that a Pt coating on the polymer particle with a diameter of ca. 1 ␮m can be achieved by using this system [21]. For the samples coated with Ag, Pd, Cu, and Ni, as shown in Fig. 3(C)–(F), their color differed from that of the Au-coated samples, which depended on the respective metal coated. Light reflection was observed in these samples as well as the Au(15). In order to examine the surface of the samples, further investigation was conducted using the SEM equipped with EDX (Fig. 4), where (A)–(G) shows the SEM images of the coated samples and the uncoated one, and (A )–(F ) shows the mapping images of each element corresponding to the coated metal, which were obtained by EDX measurements. As shown in Fig. 4(G), the surface of the uncoated sample with a diameter of 15 ␮m exhibits some bulges, however, it is generally smooth. It should be noted that since no metallic element was detected by the EDX measurements of the uncoated sample, no mapping image was shown in Fig. 4(G). This indicates that the metal-coated sample was uniformly covered with the metal film. On the other hand, the surface of the coated samples was also smooth and almost the same as that of the uncoated sample. Even the surface of Au(5) was smooth, as shown in Fig. 4(B), and there was no apparent difference between the Au(15) and Au(5) samples. In the element mapping images of all the coated samples, each sputtered element was homogeneously distributed over the entire sample surface. These results indicate that the polymer particle could be coated or covered with a uniform metal film. Thus, the reflection of light from the surface of the coated samples, as observed in the optical microscope experiment, can be attributed to the smoothness of the surface. It should be noted that no exfoliation of the coated films was observed after ultrasonic treatment with ethanol or toluene, indicating that the films strongly adhered to the polymer particle surface. The amount of noble and transition metals coated is evaluated from the weight loss on the thermal analysis. A TG–DTA curve of the uncoated sample (φ = 15 ␮m) is shown in Fig. 5(A). A drastic weight loss was observed from 250 to 400 ◦ C with the appearance of a small endothermic DTA signal followed by a large exothermic one. Although a negligibly small weight loss (less than 1 wt.%) was observed with the appearance of the exothermic DTA signal at 540 ◦ C, the total weight loss at 600 ◦ C reached 100 wt.%, indicating that the polymer particle was completely burned off. For the TG–DTA curve of Au(15) shown in Fig. 5(B), the polymer support was combusted at around 300 ◦ C. The total weight loss at 600 ◦ C was ca. 85 wt.%. Therefore, the amount of metal in Au(15) sample is ca. 15 wt.%.

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Fig. 3. Optical microscope images of surface-coated polymer particles: (A) Au(15), (B) Au(5), (C) Ag(15), (D) Pd(15), (E) Cu(15), (F) Ni(15), and (G) uncoated polymer (φ = 15 ␮m).

Fig. 4. SEM and corresponding EDX color mapping images of coated and uncoated polymer particles: (A) Au(15), (B) Au (5), (C) Ag(15), (D) Pd(15), (E) Cu(15), (F) Ni(15), and (G) uncoated polymer (φ = 15 ␮m).

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Fig. 5. TG–DTA curves of metal-coated and uncoated polymers: (A) uncoated polymer, (B) Au(15), and (C) Cu(15).

The thermal analysis was performed for four different Au(15) samples prepared under the same sputtering conditions. The average amount of Au was calculated to be 15.20 ± 1.38 wt.%, as listed in Table 2. Fig. 5(C) shows the TG–DTA curve of the Cu(15) sample. Although a large weight loss was also observed from 250 to 400 ◦ C, a large endothermic peak was observed at around 330 ◦ C, which was different from the phenomenon of the Au(15) sample. The remain of the Cu(15) sample after calcination at Table 2 Amount of metal in coated samples determined by TG–DTA and ICP and the thickness of the metal film Sample

Au(15) Au(5) Ag(15) Ag(5) Pd(15) Cu(15) Ni(15)

Film thicknessa (nm)

Amount of coated metal TG–DTA (wt.%)

ICP (wt.%)

15.20 ± 1.38 16.07 ± 0.65 10.79 ± 0.63 10.84 5.71b 5.55 ± 0.13b 13.71b

12.9 ± 1.6 8.5 ± 0.1 5.2 4.0 ± 0.2

27.7 ± 2.8 9.9 ± 0.5 34.3 ± 3.8 11.5 13.7 15.6 ± 0.4 41.8

Fig. 6. XRD patterns of the sample after calcination at 600 ◦ C for 1 h: (A) Cu(15), (B) Pd(15), and (C) Ni(15).

600 ◦ C was analyzed by XRD (Fig. 6(A)), resulting that many signals assignable to CuO was observed. It should be noted that no signals that could be assigned to Cu metal were measured. This indicates that Cu coated on the surface of the polymer particle was completely oxidized during the combustion of the polymer. Similarly, in the XRD patterns of the remains of the Pd(15) and Ni(15) samples after calcination (Fig. 6(B) and (C)), the signals of PdO or NiO were observed, while no signals of Pd or Ni metals were observed. From these results, the amounts of Cu, Pd, and Ni coated on the polymer particle were calculated from the formula weight of CuO, PdO, and NiO, respectively, and are listed in Table 2. Further, the ICP-AES measurement was performed for the Au(15), Ag(15), Pd(15), and Cu(15) samples in order to verify the amount of each metal determined from the TG–DTA measurements. The amount of metal coated was determined as 12.9 ± 1.6, 8.5 ± 0.1, 5.2, and 4.0 ± 0.2 wt.%, respectively, as summarized in Table 2. These values were in good agreement with those obtained by TG–DTA. Based on the result of the TG–DTA measurements, the average thicknesses of each metal film (tm , in nm) coated on the polymer particle were calculated using the following equation:

a

Calculated from TG–DTA data. Calculated from the weight of corresponding oxides (CuO, PdO, and NiO, see Fig. 6) formed at 600 ◦ C. b

tm = 1000 ×

WTG d ρpoly 6 ρmetal 100 − WTG

A. Taguchi et al. / Journal of Alloys and Compounds 441 (2007) 162–167

Here, the symbols d, ρpoly , ρmetal , and WTG denote the diameter of the polymer particle (in ␮m), density of the PMMA polymer (1.19 g/cm3 ) [26], density of the bulk metal [25], and amount of coated metal (in wt.%), respectively. It was assumed that the surface area was equal to the geometrical area (πd2 ) of the particle that was used as the support. The calculated thickness of each metal film is shown in Table 2; it was found within several 10 nm. 4. Conclusions Surface coating with Au, Ag, Pd, Cu, and Ni on spherical polymer particles was performed by using the barrel sputtering system. The optical microscope and SEM measurements revealed that the surface of the metal-coated samples was very smooth with a mirror-like appearance. The SEM and EDX analyses indicated a homogeneous distribution of each sputtered element over the entire sample surface. In addition, the metal film exhibited good adhesion to the polymer particle. The amount of metal coated was determined by TG–DTA and ICP-AES measurements, and the thickness of the metal film was estimated to be within several 10 nm. Based on these results, it was concluded that a thin film can be coated on the particle surface by the sputtering technique. These data demonstrate the distinct advantage of the barrel sputtering system for the surface modification of particles. Surface coated particles are widely used in the practical applications of electronic devices and ceramics. Since sputtering permits the use of a significantly wider selection of coating materials including metals, metal oxides, and alloys, the barrel sputtering technique has the potential for becoming a novel surface modification technique [27,28]. Acknowledgements Authors thank Ms. Miyoko Kitanaka and Mr. Susumu Hoshi (Nippon Pillar Packing Co. Ltd.) for their help in thermal analysis. Authors also thank to Mr. Noriaki Sekiguchi (Toyama Industrial Technology Center, Toyama, Japan) for his help in SEM measurements.

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