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Research on a new surface activation process for electroless plating on ABS plastic
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Materials Letters 62 (2008) 1089 – 1091 www.elsevier.com/locate/matlet
Research on a new surface activation process for electroless plating on ABS plastic Xuejiao Tang ⁎, Meng Cao, Chengliang Bi, Lijuan Yan, Baogui Zhang College of Environmental Science & Engineering, Nankai University, Tianjin, 300071, PR China Received 17 June 2007; accepted 26 July 2007 Available online 2 August 2007
Abstract A newly low-cost and environment-friendly process of surface activation on ABS plastic was carried out by employing a kind of biopolymer to fix catalyst metal on the substrate by chemical sorption. The ABS foils after each step of pretreatment were investigated by XPS to understand the reaction mechanisms. It is confirmed that the new method enhanced the adhesive strength of plating layer and substrate by chemical sorption, instead of the physical sorption in the conventional sensitizing-activation method. Nickel deposition on the activated ABS surface started as soon as it was dipped into an electroless nickel plating solution. A tight, dense and continuous structure of Ni–P plating player was obtained. The microstructure appearances of the activated surface and Ni–P plating layer were characterized by SEM. © 2007 Elsevier B.V. All rights reserved. Keywords: Surface activation; Nickel deposition; Chitosan thin film; X-ray techniques; Electron microscopy
1. Introduction Metallized ABS with both outstanding properties of engineering plastic and metal can be used widely in electronic industry, petrolic industry and national defense field. Many studies on the activation pretreatment method for nonconducted substrates have been carried out [1–6]. Activation is to employ the catalyst metal (e.g. palladium) sites on the substrate to initiate oxidation of a reducing agent in an electroless plating solution and is one of the pivotal steps because of its direct effect on the quality of plating layer. A classical and conventional activation method as we know is a sensitizing-activation method of dipping the substrate into a stannous chloride - palladium chloride bath to employ the catalyst palladium cluster [7,8]. But this method involves numerous problems, such as the use of highly toxic Sn, waste of noble metal and failure in adhesion of plating player, which are still major problems in commercialization. Other methods [9– 12] have recently been reported that the substrate whereon was spin-coated a thin metal-organic precursor film was irradiated by excimer radiation to decompose the precursor to be metal ⁎ Corresponding author. Tel.: +86 2223503592; fax: +86 2223508807. E-mail address: [email protected] (X. Tang). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.07.055
clusters. But expensive complex equipments and/or dangerous chemicals are usually required. A kind of surface activation process for ABS had been studied on wherein the surface was coated with a liquid containing at least chitosan or a chitosan derivative before catalyst fixation [13–15]. However, in order to enhance the adhesion of the film on substrate, the surface of substrate had to be roughened in a chromic acid etching solution and a paint which had good physical adherence to the substrate was added into the abovemetioned treatment liquid, which contained a butyl acetate/ethyl acetate/n-butanol/toluene/butyl cellosolve mixed solvent. Besides the pollution of chromium, the solution was complex and no-go, not suitable for the commercial manufacturing. In the present work, a faintly etching step without chromium is performed to make the surface hydrophilic. The adhesive strength between the film and substrate can be greatly enhanced and the pollution of chromium can be avoided. Then an easy process is studied on wherein is employed a simple chitosan film to fix catalyst metal (palladium) on the substrate by chemical sorption instead of physical sorption in the conventional method. This process can also avoid the toxicity of Sn and reduce the cost to be adapted for large-scale commercial manufacturing. The chemical compositions on the substrate surface after each treatment step are characterized by X-ray photoelectron spectroscopy (XPS) and the
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Table 1 XPS data of Pd, PdCl2, CTS, ABS, ABS–CTS and ABS–CTS–Pd (eV) Samples
C1s
N1s
O1s
Pd 3d5/2
Pd 3d3/2
Pd PdCl2 CTS ABS ABS–CTS ABS–CTS–Pd
– – 284.6 286.2 286.7 284.6 285.7 284.6 286.2 287.7 284.6 286.1 310.0
– – 399.2 399.3 399.2 400.9 399.2 401.2
– – 532.1 531.9 531.0 532.6 531.0 532.6
335.8 [16] 338.0 [16] – – – 335.8 338.0
341.1 [16] 343.3 [16] – – – 341.3 343.3
reaction mechanisms are discussed. The appearances of the activated surface and Ni–P plating layer on ABS are characterized by Scanning Electron Microscopy (SEM). 2. Experimental 2.1. Etching and activating The faintly etching was performed on the surface of degreased ABS foils by dipping in a mix solution of 40 ml hydrogen peroxide (30%) and 160 ml sulfuric acid (98%) at room temperature for 5 min. After rinse, the etched foils were dipped into 1% acetic acid solution containing 1% chitosan (CTS for short) for 5 min at room temperature, and then dried at 60 °C for 30 min. Subsequently, the foils (ABS–CTS) was immersed in a solution of palladium chloride (PdCl2 · 2H2O: 0.2 g/l, HCl: 2 g/l) at 40 °C for 5 min, rinsed, and then reduced in sodium hypophosphite solution at 40 °C for 2 min, namely ABS–CTS–Pd. Nickel deposition was achieved by dipping the activated substrates (ABS–CTS–Pd) into an electroless nickel plating solution containing sodium citrate as complexing agent and sodium hypophosphite as reducing agent at 40 °C for 20 min. 2.2. Surface characterization The chemical compositions and reactions on the substrate surface in each treatment step were characterized by XPS. XPS spectra were recorded using a Kratos Axis Ultra DLD spectrometer (UK) and employing a monochromated Al–Ka X-ray source (hv = 1486.6 eV), hybrid (magnetic/electrostatic) optics and a multi-channel plate and delay line detector. The appearances were characterized by SEM. Electron micrographs were taken by a SHIMADZU SS-550 scanning electron microscope (JP).
After the CTS film on the surface of ABS being dried at 60 °C for 30 min, the functional groups − OH (O1s absorption peak appeared at binding energy of 532.1 eV in Table 1 for CTS) and − NH3(N1s at 399.2 eV for CTS) had combined chemically with the hydrophilic functional groups (−OH and/or −COOH) on the surface of ABS to form new chemically-stable functional group − C(O)–NH–, −C(O)– O–, −C–O–C– (corresponding to C1s at 287.7, 287.7 and 286.2 eV for ABS–CTS, respectively), so that the adhesive strength of CTS film and ABS substrate surface had been enhanced largely. The O1s spectrum absorption peak of ABS–CTS (532.6 eV) shifted 0.7 eV to a higher binding energy comparing to that of ABS (531.9 eV). Chemical-shift of binding energy in XPS is a kind of displacement effect of spectrum absorption peak, which is resulted from the changes of the electron binding energy of the atoms in different chemical conditions [17]. So the chemical reactions between the functional groups −OH, −NH3 and the hydrophilic functional groups were confirmedly supported again. The other O1s spectrum absorption peak of ABS–CTS appeared at binding energy of 531.0 eV, lower than that of CTS (532.1 eV). Protons breaking away from hydroxide groups could result in the transfer of electrons around the oxygen atoms. So the result indicates that it was the hydroxide groups of CTS that lost the protons, which induced the thickness of the electron clouds around oxygen atoms increased and binding energy became lower. The N1s spectrum absorption peak of ABS–CTS (400.9 eV) shifted 1.7 eV to a higher binding energy comparing to that of CTS (399.2 eV), which demonstrates the reaction mechanism between group − COOH of ABS and −NH3 of CTS to form new group − C(O)–NH–. The other N1s spectrum absorption peak in ABS–CTS (399.2 eV) corresponded to the N1s position of −CN in ABS and −NH3 in CTS. The N1s spectrum absorption peak of ABS–CTS–Pd (401.2 eV) shifted 2.0 eV to a higher binding energy comparing to that of −NH3 in CTS (399.2 eV). This result indicates that palladium had chelated with the
3. Results and discussion The XPS investigations of CTS, ABS, ABS–CTS and ABS–CTS– Pd were undertaken. The XPS data are listed in Table 1. As showed in Table 1, three C1s photoelectron spectrum peaks of CTS appeared at binding energies of 286.7, 286.2 and 284.6 eV, which correspond to the peaks position of − C–N, −C–O and − C–O–C–, respectively. The formation of a new hydrophilic functional group (−OH and/or −COOH) on the ABS substrate surface after slightly etching could be supported by the fact that the O1s absorption peak was detected out at 531.9 eV in Table 1 for ABS.
Fig. 1. SEM photograph of ABS–CTS–Pd (×2000).
X. Tang et al. / Materials Letters 62 (2008) 1089–1091
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sorption instead of the physical sorption in the conventional sensitizing-activation method. And the faintly etching solution without chromium was employed to modify the surface to be hydrophilic from the view point of an environment-friendly concern. A tight, dense and continuous structure of Ni–P plating player was obtained from an electroless nickel plating using this activation method. This is a low-cost and environment-friendly surface activation process which can be applied into large-scale commercial manufacturing. Acknowledgements Fig. 2. SEM photograph of Ni–P plating layer with a novel method of activation (×8000).
nitrogen atom which had a couple of isolated electrons, which induce the thickness of the electron clouds around nitrogen atom decreased and consequently binding energy became higher. So the palladium was fixed in CTS film by chemical-sorption, with higher adhesive strength than physical-sorption in conventional surface activation method. In Table 1, two Pd species double peaks of Pd3d5/2 and Pd3d3/2 appeared in XPS data for ABS–CTS–Pd, right double peaks at binding energies of 338.0 and 343.3 eV (corresponding to Pd3d peaks of PdCl2) and left ones (shoulder peak) at binding energies of 335.8 and 341.1 eV (corresponding to Pd3d peaks of Pd), respectively. The results indicate that Pd (0) had successfully formed in ABS–CTS–Pd after being reduced by sodium hypophosphite solution, which could be sufficiently used as catalyst to start the following electroless plating. The SEM photograph of ABS–CTS–Pd was showed in Fig. 1. The grains of Pd clusters having average size of 0.5–1.0 μm were fine and uniform, which were well-suited for catalysis to initiate the electroless plating. Nickel deposition started immediately after dipping ABS– CTS–Pd into the electroless solution. The achieving plating player was glossy and smooth by optical observation. In Fig. 2 was shown Ni–P plating layer on ABS electrolessly produced with the described novel method of activation. As showed in Fig. 2, the nickel deposition was successfully achieved and a tight, dense and continuous structure of Ni–P plating player was obtained.
4. Conclusion A new surface activation process for electroless plating on ABS by employing biopolymer chitosan had been brought forward. It is confirmed that the new method enhanced the adhesive strength of plating layer and substrate by chemical
The authors gratefully acknowledge financial support by the program (05YFSYSF030) from Tianjin municipal science and technology committee. References [1] B. Haba, K. Sugai, Y. Morishige, S. Kishida, Appl. Surf. Sci. 79 (1994) 381–384. [2] Y.D. Chen, A. Reisman, I. Turlik, D. Temple, J. Electrochem. Soc. 142 (1995) 3911–3913. [3] Z. Toth, T. Szörenyi, A.L. Toth, Appl. Surf. Sci. 69 (1993) 317–320. [4] M.H. Bernier, R. Izquierdo, M. Meunier, SPIE Proc. Ser. 2045 (1994) 330–337. [5] H. Esrom, R. Seeböck, M. Charbommier, M. Romand, Surf. Coat. Technol. 125 (2000) 19–24. [6] R. Seeböck, H. Esrom, M. Charbonnier, M. Romand, U. Kogelschatz, Surf. Coat. Technol. 142 (2001) 455–459. [7] M.G. Shipley, Sherborn, US Patent 3,874,882.1995. [8] M.G. Shipley, A. William, J. Conan, US Patent 3,904,792.1975. [9] J.Y. Zhang, H. Esrom, I.W. Boyd, Appl. Surf. Sci. 96 (1996) 399–404. [10] G.J. Fisanick, M.E. Gross, J.B. Hopkins, M.D. Fennell, K.J. Schnoes, A. Katzir, J. Appl. Phys. 57 (1985) 1139–1142. [11] M.E. Gross, A. Appelbaum, P.K. Gallagher, J. Appl. Phys. 61 (1987) 1628–1632. [12] A. Gupta, R. Jagannathan, Appl. Phys. Lett. 51 (1987) 2254–2256. [13] Y. Omura, US Patent 5,660,883.1996. [14] Y. Omura, Y. Nakagawa, T. Murakami, Adv. Chitin Sci. 2 (1998) 902–907. [15] Y. Omura, E. Renbutsu, M. Morimoto, H. Saimoto, Y. Shigemasa, Polym. Adv. Technol. 14 (2003) 35–39. [16] A. Tressaud, S. Khairoun, H. Touhara, N. Watanabe, Z. Anorg. Allg. Chem. 540 (1986) 291–299. [17] C.X. Cheng, Surf. Phys. Chem. (1995) 676.
Materials Letters 62 (2008) 1089 – 1091 www.elsevier.com/locate/matlet
Research on a new surface activation process for electroless plating on ABS plastic Xuejiao Tang ⁎, Meng Cao, Chengliang Bi, Lijuan Yan, Baogui Zhang College of Environmental Science & Engineering, Nankai University, Tianjin, 300071, PR China Received 17 June 2007; accepted 26 July 2007 Available online 2 August 2007
Abstract A newly low-cost and environment-friendly process of surface activation on ABS plastic was carried out by employing a kind of biopolymer to fix catalyst metal on the substrate by chemical sorption. The ABS foils after each step of pretreatment were investigated by XPS to understand the reaction mechanisms. It is confirmed that the new method enhanced the adhesive strength of plating layer and substrate by chemical sorption, instead of the physical sorption in the conventional sensitizing-activation method. Nickel deposition on the activated ABS surface started as soon as it was dipped into an electroless nickel plating solution. A tight, dense and continuous structure of Ni–P plating player was obtained. The microstructure appearances of the activated surface and Ni–P plating layer were characterized by SEM. © 2007 Elsevier B.V. All rights reserved. Keywords: Surface activation; Nickel deposition; Chitosan thin film; X-ray techniques; Electron microscopy
1. Introduction Metallized ABS with both outstanding properties of engineering plastic and metal can be used widely in electronic industry, petrolic industry and national defense field. Many studies on the activation pretreatment method for nonconducted substrates have been carried out [1–6]. Activation is to employ the catalyst metal (e.g. palladium) sites on the substrate to initiate oxidation of a reducing agent in an electroless plating solution and is one of the pivotal steps because of its direct effect on the quality of plating layer. A classical and conventional activation method as we know is a sensitizing-activation method of dipping the substrate into a stannous chloride - palladium chloride bath to employ the catalyst palladium cluster [7,8]. But this method involves numerous problems, such as the use of highly toxic Sn, waste of noble metal and failure in adhesion of plating player, which are still major problems in commercialization. Other methods [9– 12] have recently been reported that the substrate whereon was spin-coated a thin metal-organic precursor film was irradiated by excimer radiation to decompose the precursor to be metal ⁎ Corresponding author. Tel.: +86 2223503592; fax: +86 2223508807. E-mail address: [email protected] (X. Tang). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.07.055
clusters. But expensive complex equipments and/or dangerous chemicals are usually required. A kind of surface activation process for ABS had been studied on wherein the surface was coated with a liquid containing at least chitosan or a chitosan derivative before catalyst fixation [13–15]. However, in order to enhance the adhesion of the film on substrate, the surface of substrate had to be roughened in a chromic acid etching solution and a paint which had good physical adherence to the substrate was added into the abovemetioned treatment liquid, which contained a butyl acetate/ethyl acetate/n-butanol/toluene/butyl cellosolve mixed solvent. Besides the pollution of chromium, the solution was complex and no-go, not suitable for the commercial manufacturing. In the present work, a faintly etching step without chromium is performed to make the surface hydrophilic. The adhesive strength between the film and substrate can be greatly enhanced and the pollution of chromium can be avoided. Then an easy process is studied on wherein is employed a simple chitosan film to fix catalyst metal (palladium) on the substrate by chemical sorption instead of physical sorption in the conventional method. This process can also avoid the toxicity of Sn and reduce the cost to be adapted for large-scale commercial manufacturing. The chemical compositions on the substrate surface after each treatment step are characterized by X-ray photoelectron spectroscopy (XPS) and the
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Table 1 XPS data of Pd, PdCl2, CTS, ABS, ABS–CTS and ABS–CTS–Pd (eV) Samples
C1s
N1s
O1s
Pd 3d5/2
Pd 3d3/2
Pd PdCl2 CTS ABS ABS–CTS ABS–CTS–Pd
– – 284.6 286.2 286.7 284.6 285.7 284.6 286.2 287.7 284.6 286.1 310.0
– – 399.2 399.3 399.2 400.9 399.2 401.2
– – 532.1 531.9 531.0 532.6 531.0 532.6
335.8 [16] 338.0 [16] – – – 335.8 338.0
341.1 [16] 343.3 [16] – – – 341.3 343.3
reaction mechanisms are discussed. The appearances of the activated surface and Ni–P plating layer on ABS are characterized by Scanning Electron Microscopy (SEM). 2. Experimental 2.1. Etching and activating The faintly etching was performed on the surface of degreased ABS foils by dipping in a mix solution of 40 ml hydrogen peroxide (30%) and 160 ml sulfuric acid (98%) at room temperature for 5 min. After rinse, the etched foils were dipped into 1% acetic acid solution containing 1% chitosan (CTS for short) for 5 min at room temperature, and then dried at 60 °C for 30 min. Subsequently, the foils (ABS–CTS) was immersed in a solution of palladium chloride (PdCl2 · 2H2O: 0.2 g/l, HCl: 2 g/l) at 40 °C for 5 min, rinsed, and then reduced in sodium hypophosphite solution at 40 °C for 2 min, namely ABS–CTS–Pd. Nickel deposition was achieved by dipping the activated substrates (ABS–CTS–Pd) into an electroless nickel plating solution containing sodium citrate as complexing agent and sodium hypophosphite as reducing agent at 40 °C for 20 min. 2.2. Surface characterization The chemical compositions and reactions on the substrate surface in each treatment step were characterized by XPS. XPS spectra were recorded using a Kratos Axis Ultra DLD spectrometer (UK) and employing a monochromated Al–Ka X-ray source (hv = 1486.6 eV), hybrid (magnetic/electrostatic) optics and a multi-channel plate and delay line detector. The appearances were characterized by SEM. Electron micrographs were taken by a SHIMADZU SS-550 scanning electron microscope (JP).
After the CTS film on the surface of ABS being dried at 60 °C for 30 min, the functional groups − OH (O1s absorption peak appeared at binding energy of 532.1 eV in Table 1 for CTS) and − NH3(N1s at 399.2 eV for CTS) had combined chemically with the hydrophilic functional groups (−OH and/or −COOH) on the surface of ABS to form new chemically-stable functional group − C(O)–NH–, −C(O)– O–, −C–O–C– (corresponding to C1s at 287.7, 287.7 and 286.2 eV for ABS–CTS, respectively), so that the adhesive strength of CTS film and ABS substrate surface had been enhanced largely. The O1s spectrum absorption peak of ABS–CTS (532.6 eV) shifted 0.7 eV to a higher binding energy comparing to that of ABS (531.9 eV). Chemical-shift of binding energy in XPS is a kind of displacement effect of spectrum absorption peak, which is resulted from the changes of the electron binding energy of the atoms in different chemical conditions [17]. So the chemical reactions between the functional groups −OH, −NH3 and the hydrophilic functional groups were confirmedly supported again. The other O1s spectrum absorption peak of ABS–CTS appeared at binding energy of 531.0 eV, lower than that of CTS (532.1 eV). Protons breaking away from hydroxide groups could result in the transfer of electrons around the oxygen atoms. So the result indicates that it was the hydroxide groups of CTS that lost the protons, which induced the thickness of the electron clouds around oxygen atoms increased and binding energy became lower. The N1s spectrum absorption peak of ABS–CTS (400.9 eV) shifted 1.7 eV to a higher binding energy comparing to that of CTS (399.2 eV), which demonstrates the reaction mechanism between group − COOH of ABS and −NH3 of CTS to form new group − C(O)–NH–. The other N1s spectrum absorption peak in ABS–CTS (399.2 eV) corresponded to the N1s position of −CN in ABS and −NH3 in CTS. The N1s spectrum absorption peak of ABS–CTS–Pd (401.2 eV) shifted 2.0 eV to a higher binding energy comparing to that of −NH3 in CTS (399.2 eV). This result indicates that palladium had chelated with the
3. Results and discussion The XPS investigations of CTS, ABS, ABS–CTS and ABS–CTS– Pd were undertaken. The XPS data are listed in Table 1. As showed in Table 1, three C1s photoelectron spectrum peaks of CTS appeared at binding energies of 286.7, 286.2 and 284.6 eV, which correspond to the peaks position of − C–N, −C–O and − C–O–C–, respectively. The formation of a new hydrophilic functional group (−OH and/or −COOH) on the ABS substrate surface after slightly etching could be supported by the fact that the O1s absorption peak was detected out at 531.9 eV in Table 1 for ABS.
Fig. 1. SEM photograph of ABS–CTS–Pd (×2000).
X. Tang et al. / Materials Letters 62 (2008) 1089–1091
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sorption instead of the physical sorption in the conventional sensitizing-activation method. And the faintly etching solution without chromium was employed to modify the surface to be hydrophilic from the view point of an environment-friendly concern. A tight, dense and continuous structure of Ni–P plating player was obtained from an electroless nickel plating using this activation method. This is a low-cost and environment-friendly surface activation process which can be applied into large-scale commercial manufacturing. Acknowledgements Fig. 2. SEM photograph of Ni–P plating layer with a novel method of activation (×8000).
nitrogen atom which had a couple of isolated electrons, which induce the thickness of the electron clouds around nitrogen atom decreased and consequently binding energy became higher. So the palladium was fixed in CTS film by chemical-sorption, with higher adhesive strength than physical-sorption in conventional surface activation method. In Table 1, two Pd species double peaks of Pd3d5/2 and Pd3d3/2 appeared in XPS data for ABS–CTS–Pd, right double peaks at binding energies of 338.0 and 343.3 eV (corresponding to Pd3d peaks of PdCl2) and left ones (shoulder peak) at binding energies of 335.8 and 341.1 eV (corresponding to Pd3d peaks of Pd), respectively. The results indicate that Pd (0) had successfully formed in ABS–CTS–Pd after being reduced by sodium hypophosphite solution, which could be sufficiently used as catalyst to start the following electroless plating. The SEM photograph of ABS–CTS–Pd was showed in Fig. 1. The grains of Pd clusters having average size of 0.5–1.0 μm were fine and uniform, which were well-suited for catalysis to initiate the electroless plating. Nickel deposition started immediately after dipping ABS– CTS–Pd into the electroless solution. The achieving plating player was glossy and smooth by optical observation. In Fig. 2 was shown Ni–P plating layer on ABS electrolessly produced with the described novel method of activation. As showed in Fig. 2, the nickel deposition was successfully achieved and a tight, dense and continuous structure of Ni–P plating player was obtained.
4. Conclusion A new surface activation process for electroless plating on ABS by employing biopolymer chitosan had been brought forward. It is confirmed that the new method enhanced the adhesive strength of plating layer and substrate by chemical
The authors gratefully acknowledge financial support by the program (05YFSYSF030) from Tianjin municipal science and technology committee. References [1] B. Haba, K. Sugai, Y. Morishige, S. Kishida, Appl. Surf. Sci. 79 (1994) 381–384. [2] Y.D. Chen, A. Reisman, I. Turlik, D. Temple, J. Electrochem. Soc. 142 (1995) 3911–3913. [3] Z. Toth, T. Szörenyi, A.L. Toth, Appl. Surf. Sci. 69 (1993) 317–320. [4] M.H. Bernier, R. Izquierdo, M. Meunier, SPIE Proc. Ser. 2045 (1994) 330–337. [5] H. Esrom, R. Seeböck, M. Charbommier, M. Romand, Surf. Coat. Technol. 125 (2000) 19–24. [6] R. Seeböck, H. Esrom, M. Charbonnier, M. Romand, U. Kogelschatz, Surf. Coat. Technol. 142 (2001) 455–459. [7] M.G. Shipley, Sherborn, US Patent 3,874,882.1995. [8] M.G. Shipley, A. William, J. Conan, US Patent 3,904,792.1975. [9] J.Y. Zhang, H. Esrom, I.W. Boyd, Appl. Surf. Sci. 96 (1996) 399–404. [10] G.J. Fisanick, M.E. Gross, J.B. Hopkins, M.D. Fennell, K.J. Schnoes, A. Katzir, J. Appl. Phys. 57 (1985) 1139–1142. [11] M.E. Gross, A. Appelbaum, P.K. Gallagher, J. Appl. Phys. 61 (1987) 1628–1632. [12] A. Gupta, R. Jagannathan, Appl. Phys. Lett. 51 (1987) 2254–2256. [13] Y. Omura, US Patent 5,660,883.1996. [14] Y. Omura, Y. Nakagawa, T. Murakami, Adv. Chitin Sci. 2 (1998) 902–907. [15] Y. Omura, E. Renbutsu, M. Morimoto, H. Saimoto, Y. Shigemasa, Polym. Adv. Technol. 14 (2003) 35–39. [16] A. Tressaud, S. Khairoun, H. Touhara, N. Watanabe, Z. Anorg. Allg. Chem. 540 (1986) 291–299. [17] C.X. Cheng, Surf. Phys. Chem. (1995) 676.