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Preparation of ordered porous NaCl and KCl crystals
Solid State Sciences 8 (2006) 1056–1060 www.elsevier.com/locate/ssscie
Preparation of ordered porous NaCl and KCl crystals Hailin Cong, Weixiao Cao ∗ College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, PR China Received 3 July 2005; received in revised form 11 November 2005; accepted 20 December 2005 Available online 5 June 2006
Abstract Solids of NaCl (or KCl) containing macropores (∼380 nm in diameter) ordered in the face centered cubic manner were prepared by codeposition of NaCl (or KCl) and organic colloids in water, followed by the removal of the polymer by calcination at 550 ◦ C. These pore crystals showed the characteristics of an optically gapped material. The pores worked as a template to prepare submicrometer spheres of a liquid crystal, 5 OCB, and also a wax which are difficult to form spheres in usual conditions. © 2006 Elsevier SAS. All rights reserved. Keywords: NaCl; KCl; Colloidal crystal; Macroporous material; Template
1. Introduction In the past decade, crystals made of monodisperse colloids have attracted much attention because of their potential applications as photonic materials and also as templates to prepare porous materials [1–10]. Monodisperse micro- or submicrometer colloids (organic or inorganic) are usually ordered in the face centered cubic (fcc) manner, leaving voids of ∼26 vol% in total. By infiltrating the voids with polymerizable monomers, for example, and removing the matrix thermally or chemically after polymerization, one can obtain a porous polymer [11–15]. A lot of porous materials, inorganic, organic, metallic and ceramic, have been successfully made by using this kind of techniques [16–22]. The products, which are the exact inverse replicas of the templates with pores distributed in a three-dimensionally ordered way, can show Bragg reflection of lights [23–27]. Vlasov et al. reported recently that a macroporous silicon material, fabricated by using colloidal silica crystals as template, worked as a photonic crystals, and an unity reflectance in two crystalline directions of the crystal around a wavelength of ∼1.3 micrometer was observed [14]. In this article, we report the first preparation of solids of NaCl and KCl containing macropores (∼380 nm in diameter) ordered in the face centered cubic manner. The structural * Corresponding author.
E-mail address: [email protected] (W. Cao). 1293-2558/$ – see front matter © 2006 Elsevier SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2005.12.012
and optical characteristics of these pore crystals as well as the application of these materials to the preparation of submicrometer spheres of a liquid crystal and a wax will also be reported. 2. Experimental 2.1. Preparation Monodispersed poly(styrene-methyl methacrylate-acrylic acid) (P(St-MMA-AA)) copolymer colloids with a diameter of ∼380 nm were synthesized according to a method described elsewhere [28]. Solids of NaCl (or KCl) containing macropores (∼380 nm in diameter) were prepared as follows: A silicon wafer (10 mm × 30 mm × 1 mm) was pretreated at 70–75 ◦ C in H2 SO4 /H2 O2 (7:3 v/v) mixture for 30 min to create a clean and hydrophilic surface. The wafer was then dipped vertically into a vessel containing 8 ml of mixture composed of 4 ml P(St-MMA-AA) latex (1.4 mg/ml) and 4 ml saturated aqueous solution of NaCl (or KCl). Following the evaporation of the water, the colloids are arranged orderly to form a colloidal crystal in the action of capillary force [19], and the infiltration of NaCl (or KCl) molecules into the voids of the crystal was performed synchronously. After deposition for 7–8 days (∼25 ◦ C), a hybrid colloidal crystal (∼1 cm2 on silicon wafer) consisting of P(St-MMA-AA) colloids and NaCl (or KCl) molecules was acquired. The porous NaCl (or KCl) crystals then were
H. Cong, W. Cao / Solid State Sciences 8 (2006) 1056–1060
obtained by calcination of the hybrid crystals in air at 550 ◦ C for 6 h. 2.2. Characterization SEM(AMRAY-1910FE, America) was used to observe the frameworks and morphologies of the colloidal and porous NaCl (or KCl) crystals. XRD data and diffuse-reflectance UV–vis spectra were recorded on X-ray diffractometer (Rigaku RINT2000) and Shimadzu UV-250 spectrometer respectively. 3. Results and discussion 3.1. SEM characterization SEM images of the colloidal crystals formed without NaCl (or KCl) are shown in Fig. 1. From the section profiles (Fig. 1(a), (b)), we can see that the crystal has the face centered cubic (fcc) packing as usual [11–14]. The hexagonal array of the 111 plane and the square array of the 100 plane are visualized clearly (Fig. 1(c), (d)). Figs. 2(a) and (b) show the SEM images of a NaCl/P(StMMA-AA) hybrid colloidal crystal. We can see that the voids of the crystal have been fully filled by NaCl molecules (bright parts). The porous NaCl crystals, an exact inverse replica of the template, were formed by calcination of the hybrid crystal at ∼550 ◦ C for 6 h. Figs. 3(a) and (c) show (111)-oriented frame-
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work of porous NaCl crystal. From Fig. 3(c), a triangular pattern below every hole can be visualized. It is because that along the (111) direction, each hole is just located above three holes of the subjacent layer. Along the (100)-direction a quadrangular patterns can be observed. The arrows in Figs. 3(c) and (d) show the cracks which may be formed in the calcination process due to the difference in thermal expansion coefficient between the P(St-MMA-AA) and NaCl. In the same manner, we obtain KCl porous crystals. The SEM images of them are shown in Figs. 4(a) and (b). The former shows a planar profile of the (111)-oriented framework. The latter shows a cracked region of the crystal, from which we can see the porous framework is composed of ∼4–5 layer colloids. 3.2. XRD and optical characterizations The XRD plots of the reagents of NaCl, KCl and the present porous NaCl and KCl frameworks are shown in Fig. 5. There are almost no differences except lower diffraction intensities of the porous ones. The P(St-MMA-AA) colloidal templates and the porous NaCl crystals possess ordered 3D structures formed from 380 nm colloids and corresponding pores, respectively. They show the characteristics of an optically gapped material. Ultraviolet and visible lights will be Bragg-reflected by their lattice planes. Fig. 6 shows the diffuse-reflectance UV–vis spectra of the NaCl crystal, P(St-MMA-AA) colloidal crystal, and porous
Fig. 1. SEM images of the P(St-MMA-AA) colloidal crystal formed in the absence of NaCl or KCl. (a) and (b) sectional images (inset plot showing the fcc packing); (c) planar image (111 crystal plane); (d) planar image (100 crystal plane).
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Fig. 2. SEM images of the P(St-MMA-AA) colloidal crystal formed in the presence of NaCl, the NaCl (a) (111) lattice planes; (b) (100) lattice planes.
Fig. 3. SEM images of porous NaCl crystal (a) and (c) (111)-oriented framework; (b) and (d) (100)-oriented framework.
Fig. 4. SEM images of the porous KCl crystal (a) (111)-oriented framework; (b) a cracked region.
H. Cong, W. Cao / Solid State Sciences 8 (2006) 1056–1060
Fig. 5. XRD plots of the reagent and porous KCl, NaCl.
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Fig. 6. Diffuse-reflectance UV–vis spectra of P(St-MMA-AA) colloidal crystal, NaCl, and ordered porous NaCl.
Fig. 7. Spheres made by porous NaCl as template. (a) 5-OCB; (b) wax.
NaCl crystal. The P(St-MMA-AA) template shows a minimum absorbance (maximum reflectance) peak at 420 nm, which accords well with the wavelength of light reflected from the (111) planes of the crystal calculated from the Bragg formula as reported by Sato et al. [8]. From the porous NaCl crystals, the peak appears at 610 nm, which also accords well with the wavelength from the (200) planes as reported by Stein et al. [15]. The reagent NaCl crystal does not show reflection peaks. Therefore, the ordered porous NaCl crystal shows characteristics of a band gap material. Essentially the same phenomenon is also observed for the ordered porous KCl crystals. 3.3. 5-OCB and wax submicrometer spheres The porous NaCl (or KCl) crystals being water soluble, these can be used as a template to make micro- or submicrometer spheres which are difficult to form in usual conditions. 5-OCB (4-Cyano-4 -n-pentyloxybiphenyl) is a kind of liquid crystal with a melting point of 54 ◦ C. The submicro spheres of 5-OCB can be prepared as follows: The melted 5-OCB (∼70 ◦ C) was infiltrated into the pores of the template and cooled naturally to room temperature, and then the NaCl was dissolved with water to obtain 5-OCB spheres (Fig. 7(a)). Using the same method,
the wax spheres were also prepared (Fig. 7(b)). This may provide a promising method to obtain micro or submicrometer spheres of many materials which are difficult to form spheres in usual conditions. 4. Conclusion Hybrid colloidal crystals composed of P(St-MMA-AA) colloids and NaCl (or KCl) were prepared with a co-deposition method. After removing the organic colloids by calcinations at 550 ◦ C, NaCl (or KCl) crystals with ordered pores were obtained. This might be a simple way to obtain three dimensionally ordered pore materials from water soluble salts or others. The porous NaCl (or KCl) crystals show the characteristics of an optically gapped material. Using the porous NaCl as template, the submicrometer spheres of a liquid crystal (5-OCB) and wax have been prepared. Acknowledgements This work is financial supported by NSFC (Contract No. 20274002). The authors are grateful to professor Z.N. Gu for his help in the sintering experiment.
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References [1] Y. Yin, Z. Li, Y. Xia, Langmuir 19 (2003) 622. [2] K.P. Velikov, C.G. Christova, R.P.A. Dullens, A. Blaaderen, Science 296 (2002) 106. [3] P. Jiang, J.F. Bertone, V.L. Colvin, Science 291 (2001) 453. [4] Q. Guo, C. Aronoux, R.E. Palmer, Langmuir 17 (2001) 7150. [5] F. Zeng, Z. Sun, C. Wang, B. Ren, X. Liu, Z. Tong, Langmuir 18 (2002) 9116. [6] L.M. Goldenberg, J. Wagner, J. Stumpe, B. Paulke, E. Gornitz, Langmuir 18 (2002) 3319. [7] S. Reculusa, S. Ravaine, Chem. Mater. 15 (2003) 598. [8] Z. Gu, A. Fujishima, O. Sato, Chem. Mater. 14 (2002) 760. [9] J.S. Sakamoto, B. Dunn, J. Mater. Chem. 12 (2002) 2859. [10] B. Duffel, R.H.A. Ras, F.C.D. Schryver, R.A. Schoonheydt, J. Mater. Chem. 11 (2001) 3333. [11] Z. Liang, A.S. Susha, F. Caruso, Adv. Mater. 14 (2002) 1160. [12] Y. Takeoka, M. Watanabe, Adv. Mater. 15 (2003) 199. [13] M. Bardosova, R.H. Tredgold, J. Mater. Chem. 12 (2002) 2835. [14] Y.A. Vlasov, X. Bo, J.C. Sturm, D.J. Norris, Nature 414 (2001) 289. [15] R.C. Schroden, M.A. Daous, C.F. Blanford, A. Stein, Chem. Mater. 14 (2002) 3305.
[16] A.D. Dinsmore, M.F. Hsu, M.G. Nikolaides, M. Marquez, A.R. Bausch, D.A. Weitz, Science 298 (2002) 1006. [17] D.Y. Wang, A.L. Rogach, F. Caruso, Chem. Mater. 15 (2003) 2724. [18] G. Yi, J.H. Moon, S. Yang, Chem. Mater. 13 (2001) 2613. [19] Q.B. Meng, C.H. Fu, Y. Einaga, Z.Z. Gu, A. Fujishima, O. Sato, Chem. Mater. 14 (2002) 83. [20] Z. Lei, Y. Zhang, H. Wang, Y. Ke, J. Li, F. Li, J. Xing, J. Mater. Chem. 11 (2001) 1975. [21] Z. Lei, J. Li, Y. Ke, Y. Zhang, H. Zhang, F. Li, J. Xing, J. Mater. Chem. 11 (2001) 2930. [22] V. Valtchev, J. Mater. Chem. 12 (2002) 1914. [23] J.D. Joannopoulos, Nature 414 (2001) 257. [24] A. Blanco, E. Chomski, S. Grabtchak, M. Ibisate, S. John, S.W. Leonard, C. Lopez, F. Meseguer, H. Miguez, J.P. Mondia, G.A. Ozin, O. Toader, H.M. Van Driel, Nature 405 (2000) 437. [25] R. Hillebrand, S. Senz, W. Hergert, U. Gosele, J. Appl. Phys. 94 (2003) 2758. [26] P. Harkins, D. Eustace, J. Gallagher, D.W.J. McComb, J. Mater. Chem. 12 (2002) 1247. [27] S.G. Romanov, T. Maka, C.M. Sotomayor Torres, M. Muller, R. Zentel, Synth. Metals 116 (2001) 475. [28] H. Cong, W. Cao, Langmuir 19 (2003) 8177.
Preparation of ordered porous NaCl and KCl crystals Hailin Cong, Weixiao Cao ∗ College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, PR China Received 3 July 2005; received in revised form 11 November 2005; accepted 20 December 2005 Available online 5 June 2006
Abstract Solids of NaCl (or KCl) containing macropores (∼380 nm in diameter) ordered in the face centered cubic manner were prepared by codeposition of NaCl (or KCl) and organic colloids in water, followed by the removal of the polymer by calcination at 550 ◦ C. These pore crystals showed the characteristics of an optically gapped material. The pores worked as a template to prepare submicrometer spheres of a liquid crystal, 5 OCB, and also a wax which are difficult to form spheres in usual conditions. © 2006 Elsevier SAS. All rights reserved. Keywords: NaCl; KCl; Colloidal crystal; Macroporous material; Template
1. Introduction In the past decade, crystals made of monodisperse colloids have attracted much attention because of their potential applications as photonic materials and also as templates to prepare porous materials [1–10]. Monodisperse micro- or submicrometer colloids (organic or inorganic) are usually ordered in the face centered cubic (fcc) manner, leaving voids of ∼26 vol% in total. By infiltrating the voids with polymerizable monomers, for example, and removing the matrix thermally or chemically after polymerization, one can obtain a porous polymer [11–15]. A lot of porous materials, inorganic, organic, metallic and ceramic, have been successfully made by using this kind of techniques [16–22]. The products, which are the exact inverse replicas of the templates with pores distributed in a three-dimensionally ordered way, can show Bragg reflection of lights [23–27]. Vlasov et al. reported recently that a macroporous silicon material, fabricated by using colloidal silica crystals as template, worked as a photonic crystals, and an unity reflectance in two crystalline directions of the crystal around a wavelength of ∼1.3 micrometer was observed [14]. In this article, we report the first preparation of solids of NaCl and KCl containing macropores (∼380 nm in diameter) ordered in the face centered cubic manner. The structural * Corresponding author.
E-mail address: [email protected] (W. Cao). 1293-2558/$ – see front matter © 2006 Elsevier SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2005.12.012
and optical characteristics of these pore crystals as well as the application of these materials to the preparation of submicrometer spheres of a liquid crystal and a wax will also be reported. 2. Experimental 2.1. Preparation Monodispersed poly(styrene-methyl methacrylate-acrylic acid) (P(St-MMA-AA)) copolymer colloids with a diameter of ∼380 nm were synthesized according to a method described elsewhere [28]. Solids of NaCl (or KCl) containing macropores (∼380 nm in diameter) were prepared as follows: A silicon wafer (10 mm × 30 mm × 1 mm) was pretreated at 70–75 ◦ C in H2 SO4 /H2 O2 (7:3 v/v) mixture for 30 min to create a clean and hydrophilic surface. The wafer was then dipped vertically into a vessel containing 8 ml of mixture composed of 4 ml P(St-MMA-AA) latex (1.4 mg/ml) and 4 ml saturated aqueous solution of NaCl (or KCl). Following the evaporation of the water, the colloids are arranged orderly to form a colloidal crystal in the action of capillary force [19], and the infiltration of NaCl (or KCl) molecules into the voids of the crystal was performed synchronously. After deposition for 7–8 days (∼25 ◦ C), a hybrid colloidal crystal (∼1 cm2 on silicon wafer) consisting of P(St-MMA-AA) colloids and NaCl (or KCl) molecules was acquired. The porous NaCl (or KCl) crystals then were
H. Cong, W. Cao / Solid State Sciences 8 (2006) 1056–1060
obtained by calcination of the hybrid crystals in air at 550 ◦ C for 6 h. 2.2. Characterization SEM(AMRAY-1910FE, America) was used to observe the frameworks and morphologies of the colloidal and porous NaCl (or KCl) crystals. XRD data and diffuse-reflectance UV–vis spectra were recorded on X-ray diffractometer (Rigaku RINT2000) and Shimadzu UV-250 spectrometer respectively. 3. Results and discussion 3.1. SEM characterization SEM images of the colloidal crystals formed without NaCl (or KCl) are shown in Fig. 1. From the section profiles (Fig. 1(a), (b)), we can see that the crystal has the face centered cubic (fcc) packing as usual [11–14]. The hexagonal array of the 111 plane and the square array of the 100 plane are visualized clearly (Fig. 1(c), (d)). Figs. 2(a) and (b) show the SEM images of a NaCl/P(StMMA-AA) hybrid colloidal crystal. We can see that the voids of the crystal have been fully filled by NaCl molecules (bright parts). The porous NaCl crystals, an exact inverse replica of the template, were formed by calcination of the hybrid crystal at ∼550 ◦ C for 6 h. Figs. 3(a) and (c) show (111)-oriented frame-
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work of porous NaCl crystal. From Fig. 3(c), a triangular pattern below every hole can be visualized. It is because that along the (111) direction, each hole is just located above three holes of the subjacent layer. Along the (100)-direction a quadrangular patterns can be observed. The arrows in Figs. 3(c) and (d) show the cracks which may be formed in the calcination process due to the difference in thermal expansion coefficient between the P(St-MMA-AA) and NaCl. In the same manner, we obtain KCl porous crystals. The SEM images of them are shown in Figs. 4(a) and (b). The former shows a planar profile of the (111)-oriented framework. The latter shows a cracked region of the crystal, from which we can see the porous framework is composed of ∼4–5 layer colloids. 3.2. XRD and optical characterizations The XRD plots of the reagents of NaCl, KCl and the present porous NaCl and KCl frameworks are shown in Fig. 5. There are almost no differences except lower diffraction intensities of the porous ones. The P(St-MMA-AA) colloidal templates and the porous NaCl crystals possess ordered 3D structures formed from 380 nm colloids and corresponding pores, respectively. They show the characteristics of an optically gapped material. Ultraviolet and visible lights will be Bragg-reflected by their lattice planes. Fig. 6 shows the diffuse-reflectance UV–vis spectra of the NaCl crystal, P(St-MMA-AA) colloidal crystal, and porous
Fig. 1. SEM images of the P(St-MMA-AA) colloidal crystal formed in the absence of NaCl or KCl. (a) and (b) sectional images (inset plot showing the fcc packing); (c) planar image (111 crystal plane); (d) planar image (100 crystal plane).
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Fig. 2. SEM images of the P(St-MMA-AA) colloidal crystal formed in the presence of NaCl, the NaCl (a) (111) lattice planes; (b) (100) lattice planes.
Fig. 3. SEM images of porous NaCl crystal (a) and (c) (111)-oriented framework; (b) and (d) (100)-oriented framework.
Fig. 4. SEM images of the porous KCl crystal (a) (111)-oriented framework; (b) a cracked region.
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Fig. 5. XRD plots of the reagent and porous KCl, NaCl.
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Fig. 6. Diffuse-reflectance UV–vis spectra of P(St-MMA-AA) colloidal crystal, NaCl, and ordered porous NaCl.
Fig. 7. Spheres made by porous NaCl as template. (a) 5-OCB; (b) wax.
NaCl crystal. The P(St-MMA-AA) template shows a minimum absorbance (maximum reflectance) peak at 420 nm, which accords well with the wavelength of light reflected from the (111) planes of the crystal calculated from the Bragg formula as reported by Sato et al. [8]. From the porous NaCl crystals, the peak appears at 610 nm, which also accords well with the wavelength from the (200) planes as reported by Stein et al. [15]. The reagent NaCl crystal does not show reflection peaks. Therefore, the ordered porous NaCl crystal shows characteristics of a band gap material. Essentially the same phenomenon is also observed for the ordered porous KCl crystals. 3.3. 5-OCB and wax submicrometer spheres The porous NaCl (or KCl) crystals being water soluble, these can be used as a template to make micro- or submicrometer spheres which are difficult to form in usual conditions. 5-OCB (4-Cyano-4 -n-pentyloxybiphenyl) is a kind of liquid crystal with a melting point of 54 ◦ C. The submicro spheres of 5-OCB can be prepared as follows: The melted 5-OCB (∼70 ◦ C) was infiltrated into the pores of the template and cooled naturally to room temperature, and then the NaCl was dissolved with water to obtain 5-OCB spheres (Fig. 7(a)). Using the same method,
the wax spheres were also prepared (Fig. 7(b)). This may provide a promising method to obtain micro or submicrometer spheres of many materials which are difficult to form spheres in usual conditions. 4. Conclusion Hybrid colloidal crystals composed of P(St-MMA-AA) colloids and NaCl (or KCl) were prepared with a co-deposition method. After removing the organic colloids by calcinations at 550 ◦ C, NaCl (or KCl) crystals with ordered pores were obtained. This might be a simple way to obtain three dimensionally ordered pore materials from water soluble salts or others. The porous NaCl (or KCl) crystals show the characteristics of an optically gapped material. Using the porous NaCl as template, the submicrometer spheres of a liquid crystal (5-OCB) and wax have been prepared. Acknowledgements This work is financial supported by NSFC (Contract No. 20274002). The authors are grateful to professor Z.N. Gu for his help in the sintering experiment.
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References [1] Y. Yin, Z. Li, Y. Xia, Langmuir 19 (2003) 622. [2] K.P. Velikov, C.G. Christova, R.P.A. Dullens, A. Blaaderen, Science 296 (2002) 106. [3] P. Jiang, J.F. Bertone, V.L. Colvin, Science 291 (2001) 453. [4] Q. Guo, C. Aronoux, R.E. Palmer, Langmuir 17 (2001) 7150. [5] F. Zeng, Z. Sun, C. Wang, B. Ren, X. Liu, Z. Tong, Langmuir 18 (2002) 9116. [6] L.M. Goldenberg, J. Wagner, J. Stumpe, B. Paulke, E. Gornitz, Langmuir 18 (2002) 3319. [7] S. Reculusa, S. Ravaine, Chem. Mater. 15 (2003) 598. [8] Z. Gu, A. Fujishima, O. Sato, Chem. Mater. 14 (2002) 760. [9] J.S. Sakamoto, B. Dunn, J. Mater. Chem. 12 (2002) 2859. [10] B. Duffel, R.H.A. Ras, F.C.D. Schryver, R.A. Schoonheydt, J. Mater. Chem. 11 (2001) 3333. [11] Z. Liang, A.S. Susha, F. Caruso, Adv. Mater. 14 (2002) 1160. [12] Y. Takeoka, M. Watanabe, Adv. Mater. 15 (2003) 199. [13] M. Bardosova, R.H. Tredgold, J. Mater. Chem. 12 (2002) 2835. [14] Y.A. Vlasov, X. Bo, J.C. Sturm, D.J. Norris, Nature 414 (2001) 289. [15] R.C. Schroden, M.A. Daous, C.F. Blanford, A. Stein, Chem. Mater. 14 (2002) 3305.
[16] A.D. Dinsmore, M.F. Hsu, M.G. Nikolaides, M. Marquez, A.R. Bausch, D.A. Weitz, Science 298 (2002) 1006. [17] D.Y. Wang, A.L. Rogach, F. Caruso, Chem. Mater. 15 (2003) 2724. [18] G. Yi, J.H. Moon, S. Yang, Chem. Mater. 13 (2001) 2613. [19] Q.B. Meng, C.H. Fu, Y. Einaga, Z.Z. Gu, A. Fujishima, O. Sato, Chem. Mater. 14 (2002) 83. [20] Z. Lei, Y. Zhang, H. Wang, Y. Ke, J. Li, F. Li, J. Xing, J. Mater. Chem. 11 (2001) 1975. [21] Z. Lei, J. Li, Y. Ke, Y. Zhang, H. Zhang, F. Li, J. Xing, J. Mater. Chem. 11 (2001) 2930. [22] V. Valtchev, J. Mater. Chem. 12 (2002) 1914. [23] J.D. Joannopoulos, Nature 414 (2001) 257. [24] A. Blanco, E. Chomski, S. Grabtchak, M. Ibisate, S. John, S.W. Leonard, C. Lopez, F. Meseguer, H. Miguez, J.P. Mondia, G.A. Ozin, O. Toader, H.M. Van Driel, Nature 405 (2000) 437. [25] R. Hillebrand, S. Senz, W. Hergert, U. Gosele, J. Appl. Phys. 94 (2003) 2758. [26] P. Harkins, D. Eustace, J. Gallagher, D.W.J. McComb, J. Mater. Chem. 12 (2002) 1247. [27] S.G. Romanov, T. Maka, C.M. Sotomayor Torres, M. Muller, R. Zentel, Synth. Metals 116 (2001) 475. [28] H. Cong, W. Cao, Langmuir 19 (2003) 8177.