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Electroless nickel deposition on amino-functionalized silica spheres
Surface & Coatings Technology 200 (2005) 2249 – 2252 www.elsevier.com/locate/surfcoat
Electroless nickel deposition on amino-functionalized silica spheres Jining Gao, Fangqiong Tang*, Jun Ren Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100101, People’s Republic of China Received 30 April 2004; accepted in revised form 8 July 2004 Available online 12 August 2004
Abstract A simple method for elctroless nickel deposition on amino-functionalized monodisperse silica spheres is described. This method is based on the palladium species that were chemiadsorbed on self-assembled monolayers (SAMs) of 3-aminopropyltrimethoxysilane on silica particle surfaces. The transmission electron microcopy (TEM) images of the products show that nickel is deposited on the silica surface as a thin layer. Energy-dispersive X-ray (EDX) analysis of the nickel coated silica spheres show the presence of Ni, P, O, and Si elements. TEM and EDX analysis suggest the successful nickel deposition on the surfaces of silica particles by the present electroless deposition (ELD) process. D 2004 Elsevier B.V. All rights reserved. Keywords: Electroless deposition; Nickel; Silicon oxide; TEM
1. Introduction There is currently a great deal of interest in the fabrication of core-shell particles with a dielectric core and a metallic layer because of their wide applications in optics, photonics, catalysis and biochemistry [1–3]. Several routes have been explored to fabricate such core-shell particles, including chemical reduction [4], sonochemical deposition [5], self-assembly [1,6], electroless deposition [7,8], etc. Electroless deposition (ELD) is a convenient method to obtain thin metallic films on various insulating substrates at a low cost [9]. This method uses metastable solutions that contain a reducing agent and complexed metal ions as the source of the metal. The presence of complex-forming molecules inhibits the spontaneous reduction of the metal ions in solution until a substrate activated with the proper catalyst is immersed into the ELD bath. ELD initiates at the catalytic sites of the surface and proceeds in an autocatalytic manner. A variety of metals, including Cu, Ag, Au, Co, Ni, and some alloys of these metals, can be electrolessdeposited from solution.
* Corresponding author. Tel.: +86 10 64888064; fax: +86 10 64879375. E-mail address: [email protected] (F.Q. Tang). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.07.001
Although the technology for electroless deposition on rather large substrates is well established, coating on very small substrates, such as submicron-sized spheres, still remains a technical challenge. Catalytic sites should be created on the surfaces for initiating electroless deposition on non-metallic species. The most widely used methods were the btwo-stepQ process. In this method, the substrate surface was immersed successively in SnCl2 and then in PdCl2 solution. Shukla et al. [10] have deposited copper on the surface of fly-ash cenosphere particles with a diameter of tens to hundreds of micrometers, the deposited copper particles are about 200–300 nm in diameter. However, the silica or polyethylene spheres have low chemical reactivity, large surface curvature, smooth surfaces and small diameters, and the so deposited catalytic nanoparticles have poor adhesion to these spheres, it is difficult to make a continuous electroless plating layer for these particles. A series of ways have been used to increase the number of activated sites and improve the metal coating, a roughness step usually needed before the deposition step [8]. By these electroless deposition methods, the degree of surface coverage is low and the metallic coating is nonuniform [7]. Self-assembled monolayers (SAMs) have attracted considerable scientific interest because they provide a method for creating well-defined surfaces with controllable chem-
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ical functionality. SAMs have been successfully used in the electroless deposition of metals to achieve the selective adhension of catalyst species [11,12]. However, the adhesion was apparently prohibited on free Si–OH groups, such as silica glass, or silicon surfaces. In the present work, we describe a simple method to deposit nickel nanoparticles on silica spheres based on amino group bounded palladium species, which has been widely used for deposition of metals on polymer substrates recently, but has not been implemented on spherical colloids until now.
2. Experimental details Monodisperse silica spheres with a diameter of 240 nm were prepared using the well-known Stober method [13]. In a typical experiment, a 5 ml ethanol solution containing 2.4 M tetraethyl orthosilicate was added to a 35 ml ethanol solution of water and ammonia. The 40 ml mixture containing 0.3 M tetraethyl orthosilicate, 2.3 M H2O, and 1.0 M NH3 was stirred at 30 8C for 5 h. The resulting silica spheres were centrifugally separated from the suspension, ultrasonically washed with ethanol three times and dispersed in ethanol. The scheme of the electroless nickel deposition procedure is shown in Fig. 1. An excess of 3-aminopropyltrimethoxysilane (APTMS, 0.1 ml) was added to a 50 ml portion of the silica spheres while vigorously stirring and allowed to react for 2 h at room temperature. The APTMSfunctionalized silica spheres were purified by repeated centrifuging and redispersing in ethanol. The functionalized silica spheres were transferred to a 50 ml ethanol solution containing 0.01 g PdCl2 and stirred for 2 h. This activate step allow PdCl2 chemiadsorbed onto the functionalized silica spheres. The activated particles were washed by repeated centrifuging and redispersing using ethanol and deionized water. The washed particles were dispersed in 30 ml de-ionized water and then 10 ml of the electroless deposition solution contains 0.126 M NiCl2d 6H2O, 0.094 M NaH2PO2d H2O, 0.93 M NH4Cl, 0.17 M Na3C6H5O7d 2H2O was added. The pH value of the electroless deposition solution was adjusted to 8.25F0.05 with NH3d H2O at 25 8C. The reaction was allowed to
continue for 1 h at 65 8C. The nickel-coated particles were centrifuged and washed with de-ionized water, and then dried at room temperature. The chemicals used in this experiment are all analyzed reagent products. Absolute ethanol (99.7%) and doubly distilled water were used for the process. The overall morphologies of silica and nickel coated silica particles were obtained by transmission electron microscopy using a JEM-2000 FX electron microscope. Samples for the transmission electron microscopic measurements were obtained by placing a drop of sample suspension on a Formvar copper grid, followed by air drying to remove the solvent. The energy-dispersive spectrometry (EDS) spectra were obtained using a JSM-6301 F scanning electron microscopy. The samples were placed on Al foils for the EDS analysis.
3. Results and discussion Fig. 2 shows the transmission electron microcopy (TEM) images of the silica spheres, silica spheres after the activation step using PdCl2 solution and nickel coated silica spheres. From Fig. 2c and d, we can see that unlike the original silica spheres with smooth surfaces, a uniform layer was formed on silica microspheres after the electroless nickel deposition, but there is no significant change after the activation step as indicated in Fig. 2b. The diameter of the coated particles increase to about 260 nm compared with the original silica particles with a diameter of 240 nm. Pd(II) is noticed to chemically bind ligands containing nitrogen, sulfur and phosphorous donor atoms. Recently a new catalysis approach was pioneered for electroless metal deposition by using organosilane ultrathin films that possess ligating amine groups in conjunction with tin-free, aqueous Pd(II) catalysts [11,12]. Xu et al. [11] extensively investigated the deposition of metals employing SAMs and raised an assumption that covalent metal–ligand bond formation could serve as an effective mechanism for selectively anchoring Pd(II) colloids to an appropriate surface. The functionalized process provides an amine moiety coating for the exterior of the silica particles. After the activate step, the color of the silica dispersions turn into
Fig. 1. Scheme for electroless nickel deposition on silica spheres (APTMS represents 3-aminopropyltrimethoxysilane, H2NCH2CH2CH2Si (OCH3)3).
J. Gao et al. / Surface & Coatings Technology 200 (2005) 2249–2252
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Fig. 2. Transmission electron microscopy (TEM) images of the (a) silica spheres, (b) silica spheres after activation using PdCl2, (c) silica spheres after electroless nickel deposition, (d) magnified image of the coated silica spheres.
yellow. This indicates the Pd(II) was adsorbed onto the silica spheres. After these activated silica spheres were transferred to the electroless nickel deposition bath, the Pd(II) species on the silica spheres were reduced to Pd by
the H2PO2 in the ELD bath. And the electroless nickel deposition was initiated, the color of the dispersion turned to black within several minutes, there were bubbles in the dispersion.
Fig. 3. Energy-dispersive spectrometry (EDS) spectra of silica particles before (A) and after (B) the electroless nickel deposition process.
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J. Gao et al. / Surface & Coatings Technology 200 (2005) 2249–2252
PdCl2 is difficult to dissolve in water but it can be dissolved in ethanol, HCl and their mixtures, and PdCl2 can be reduced to metallic Pd in ethanol and water mixtures. We have used ethanol solution and HCl–water mixtures solution of PdCl2 to activate the silica particles. But the electroless nickel deposition cannot be initiated after being activated using HCl–water mixtures solution of PdCl2 and washing with water, probably because the H+ in the PdCl2 solution is easy to ligand with NH2 groups on the silica surfaces than Pd2+, there is not enough catalytic sites to initiate the reaction. So we use ethanol as a solvent in this process, other than aqueous solution of PdCl2. We think that it is Pd(II) species other than Pd(II) colloids on the silica surfaces after the activation step. Both the functionalization of the silica particles with APTMS and the activation using Pd(II) solution are critical in this electroless deposition procedure. When we use silica spheres without functionalization, the electroless nickel deposition does not occur after the activation, this indicates the amine group enhanced the Pd(II) adsorption through the metal–ligand bond. When we use the functionalized particles without activation, the electroless deposition also does not occur, since there are no catalytic sites on these silica sphere surfaces, the electroless deposition cannot be initiated. Fig. 3A and B are the energy-dispersive spectrometry (EDS) spectra of silica particles before and after electroless nickel deposition, respectively. EDS analysis detected the presence of Ni, Pd and P, along with Si and O of the silica particles after the electroless nickel deposition (Fig. 3A), but there is only Si, O and Pd after the activation step (Fig. 3B), thus confirming successful nickel deposition on the silica particles surfaces using the present electroless deposition technique. From Fig. 3A we can see the presence of Pd after the activation step. The presence of Al peak is because of the Al foil used in the sample preparation. Since after the activation step there are catalytic sites on the silica spheres, we are sure that this method can also be used to electroless deposition of other metals on the functionalized silica spheres, such as copper, cobalt, silver, etc. And this method can also be applied to any particles with NH2 groups on the surfaces.
4. Conclusions In summary, nickel has been deposited on monodisperse silica spheres with smooth surfaces using a novel electroless nickel deposition method. It avoids former roughening condition and stannous chloride sensitization. The functionalization of silica spheres with APTMS is an important step in this procedure. The resultant nickel coated silica microspheres are of interest in the photonic, magnetic, and catalytic applications.
Acknowledgments We appreciate the financial support of the National 863 Project of China (No.AA2002302108).
References [1] F. Caruso, Adv. Mater. 13 (2001) 11. [2] C. Graf, A. van Blaaderen, Langmuir 18 (2002) 524. [3] S.J. Oldenburg, R.D. Averitt, S.L. Westcott, N.J. Halas, Chem. Phys. Lett. 288 (1998) 243. [4] J. Ding, X.L. Ren, F.Q. Tang, Chin. J. Inorg. Chem. 19 (2003) 993. [5] S. Ramesh, Y. Koltypin, R. Prozorov, A. Gedanken, Chem. Mater. 9 (1997) 546. [6] S.L. Westcott, S.J. Oldenburg, T.R. Lee, N.J. Halas, Langmuir 14 (1998) 5396. [7] Y. Kobayashi, V. Salgueirino-Maceira, L.M. Liz-Marzan, Chem. Mater. 13 (2001) 1630. [8] Z. Chen, P. Zhan, J.H. Zhang, Z.L. Wang, W.Y. Zhang, N.B. Min, Chin. Phys. Lett. 20 (2003) 1369. [9] X.X. Jiang, W. Shen (Eds.), The Fundamentals and Practice of Electroless Plating, National Defence Industry Press, Beijing, China, 2000, p. 12. [10] S. Shukla, S. Seal, J. Akesson, R. Oder, R. Carter, Z. Rahaman, Appl. Surf. Sci. 181 (2001) 35. [11] L.N. Xu, J.H. Liao, L. Huang, D.L. Ou, Z.R. Guo, H.Q. Zhang, C.W. Ge, N. Gu, J.Z. Liu, Thin Solid Films 434 (2003) 121. [12] W.J. Dressick, C.S. Dulcey, J.H. Georger Jr., G.S. Calabrese, J.M. Calvert, J. Electrochem. Soc. 141 (1994) 210. [13] W. Stober, A. Fink, E. Bohn, J. Colloid Interface Sci. 26 (1968) 62.
Electroless nickel deposition on amino-functionalized silica spheres Jining Gao, Fangqiong Tang*, Jun Ren Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100101, People’s Republic of China Received 30 April 2004; accepted in revised form 8 July 2004 Available online 12 August 2004
Abstract A simple method for elctroless nickel deposition on amino-functionalized monodisperse silica spheres is described. This method is based on the palladium species that were chemiadsorbed on self-assembled monolayers (SAMs) of 3-aminopropyltrimethoxysilane on silica particle surfaces. The transmission electron microcopy (TEM) images of the products show that nickel is deposited on the silica surface as a thin layer. Energy-dispersive X-ray (EDX) analysis of the nickel coated silica spheres show the presence of Ni, P, O, and Si elements. TEM and EDX analysis suggest the successful nickel deposition on the surfaces of silica particles by the present electroless deposition (ELD) process. D 2004 Elsevier B.V. All rights reserved. Keywords: Electroless deposition; Nickel; Silicon oxide; TEM
1. Introduction There is currently a great deal of interest in the fabrication of core-shell particles with a dielectric core and a metallic layer because of their wide applications in optics, photonics, catalysis and biochemistry [1–3]. Several routes have been explored to fabricate such core-shell particles, including chemical reduction [4], sonochemical deposition [5], self-assembly [1,6], electroless deposition [7,8], etc. Electroless deposition (ELD) is a convenient method to obtain thin metallic films on various insulating substrates at a low cost [9]. This method uses metastable solutions that contain a reducing agent and complexed metal ions as the source of the metal. The presence of complex-forming molecules inhibits the spontaneous reduction of the metal ions in solution until a substrate activated with the proper catalyst is immersed into the ELD bath. ELD initiates at the catalytic sites of the surface and proceeds in an autocatalytic manner. A variety of metals, including Cu, Ag, Au, Co, Ni, and some alloys of these metals, can be electrolessdeposited from solution.
* Corresponding author. Tel.: +86 10 64888064; fax: +86 10 64879375. E-mail address: [email protected] (F.Q. Tang). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.07.001
Although the technology for electroless deposition on rather large substrates is well established, coating on very small substrates, such as submicron-sized spheres, still remains a technical challenge. Catalytic sites should be created on the surfaces for initiating electroless deposition on non-metallic species. The most widely used methods were the btwo-stepQ process. In this method, the substrate surface was immersed successively in SnCl2 and then in PdCl2 solution. Shukla et al. [10] have deposited copper on the surface of fly-ash cenosphere particles with a diameter of tens to hundreds of micrometers, the deposited copper particles are about 200–300 nm in diameter. However, the silica or polyethylene spheres have low chemical reactivity, large surface curvature, smooth surfaces and small diameters, and the so deposited catalytic nanoparticles have poor adhesion to these spheres, it is difficult to make a continuous electroless plating layer for these particles. A series of ways have been used to increase the number of activated sites and improve the metal coating, a roughness step usually needed before the deposition step [8]. By these electroless deposition methods, the degree of surface coverage is low and the metallic coating is nonuniform [7]. Self-assembled monolayers (SAMs) have attracted considerable scientific interest because they provide a method for creating well-defined surfaces with controllable chem-
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ical functionality. SAMs have been successfully used in the electroless deposition of metals to achieve the selective adhension of catalyst species [11,12]. However, the adhesion was apparently prohibited on free Si–OH groups, such as silica glass, or silicon surfaces. In the present work, we describe a simple method to deposit nickel nanoparticles on silica spheres based on amino group bounded palladium species, which has been widely used for deposition of metals on polymer substrates recently, but has not been implemented on spherical colloids until now.
2. Experimental details Monodisperse silica spheres with a diameter of 240 nm were prepared using the well-known Stober method [13]. In a typical experiment, a 5 ml ethanol solution containing 2.4 M tetraethyl orthosilicate was added to a 35 ml ethanol solution of water and ammonia. The 40 ml mixture containing 0.3 M tetraethyl orthosilicate, 2.3 M H2O, and 1.0 M NH3 was stirred at 30 8C for 5 h. The resulting silica spheres were centrifugally separated from the suspension, ultrasonically washed with ethanol three times and dispersed in ethanol. The scheme of the electroless nickel deposition procedure is shown in Fig. 1. An excess of 3-aminopropyltrimethoxysilane (APTMS, 0.1 ml) was added to a 50 ml portion of the silica spheres while vigorously stirring and allowed to react for 2 h at room temperature. The APTMSfunctionalized silica spheres were purified by repeated centrifuging and redispersing in ethanol. The functionalized silica spheres were transferred to a 50 ml ethanol solution containing 0.01 g PdCl2 and stirred for 2 h. This activate step allow PdCl2 chemiadsorbed onto the functionalized silica spheres. The activated particles were washed by repeated centrifuging and redispersing using ethanol and deionized water. The washed particles were dispersed in 30 ml de-ionized water and then 10 ml of the electroless deposition solution contains 0.126 M NiCl2d 6H2O, 0.094 M NaH2PO2d H2O, 0.93 M NH4Cl, 0.17 M Na3C6H5O7d 2H2O was added. The pH value of the electroless deposition solution was adjusted to 8.25F0.05 with NH3d H2O at 25 8C. The reaction was allowed to
continue for 1 h at 65 8C. The nickel-coated particles were centrifuged and washed with de-ionized water, and then dried at room temperature. The chemicals used in this experiment are all analyzed reagent products. Absolute ethanol (99.7%) and doubly distilled water were used for the process. The overall morphologies of silica and nickel coated silica particles were obtained by transmission electron microscopy using a JEM-2000 FX electron microscope. Samples for the transmission electron microscopic measurements were obtained by placing a drop of sample suspension on a Formvar copper grid, followed by air drying to remove the solvent. The energy-dispersive spectrometry (EDS) spectra were obtained using a JSM-6301 F scanning electron microscopy. The samples were placed on Al foils for the EDS analysis.
3. Results and discussion Fig. 2 shows the transmission electron microcopy (TEM) images of the silica spheres, silica spheres after the activation step using PdCl2 solution and nickel coated silica spheres. From Fig. 2c and d, we can see that unlike the original silica spheres with smooth surfaces, a uniform layer was formed on silica microspheres after the electroless nickel deposition, but there is no significant change after the activation step as indicated in Fig. 2b. The diameter of the coated particles increase to about 260 nm compared with the original silica particles with a diameter of 240 nm. Pd(II) is noticed to chemically bind ligands containing nitrogen, sulfur and phosphorous donor atoms. Recently a new catalysis approach was pioneered for electroless metal deposition by using organosilane ultrathin films that possess ligating amine groups in conjunction with tin-free, aqueous Pd(II) catalysts [11,12]. Xu et al. [11] extensively investigated the deposition of metals employing SAMs and raised an assumption that covalent metal–ligand bond formation could serve as an effective mechanism for selectively anchoring Pd(II) colloids to an appropriate surface. The functionalized process provides an amine moiety coating for the exterior of the silica particles. After the activate step, the color of the silica dispersions turn into
Fig. 1. Scheme for electroless nickel deposition on silica spheres (APTMS represents 3-aminopropyltrimethoxysilane, H2NCH2CH2CH2Si (OCH3)3).
J. Gao et al. / Surface & Coatings Technology 200 (2005) 2249–2252
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Fig. 2. Transmission electron microscopy (TEM) images of the (a) silica spheres, (b) silica spheres after activation using PdCl2, (c) silica spheres after electroless nickel deposition, (d) magnified image of the coated silica spheres.
yellow. This indicates the Pd(II) was adsorbed onto the silica spheres. After these activated silica spheres were transferred to the electroless nickel deposition bath, the Pd(II) species on the silica spheres were reduced to Pd by
the H2PO2 in the ELD bath. And the electroless nickel deposition was initiated, the color of the dispersion turned to black within several minutes, there were bubbles in the dispersion.
Fig. 3. Energy-dispersive spectrometry (EDS) spectra of silica particles before (A) and after (B) the electroless nickel deposition process.
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J. Gao et al. / Surface & Coatings Technology 200 (2005) 2249–2252
PdCl2 is difficult to dissolve in water but it can be dissolved in ethanol, HCl and their mixtures, and PdCl2 can be reduced to metallic Pd in ethanol and water mixtures. We have used ethanol solution and HCl–water mixtures solution of PdCl2 to activate the silica particles. But the electroless nickel deposition cannot be initiated after being activated using HCl–water mixtures solution of PdCl2 and washing with water, probably because the H+ in the PdCl2 solution is easy to ligand with NH2 groups on the silica surfaces than Pd2+, there is not enough catalytic sites to initiate the reaction. So we use ethanol as a solvent in this process, other than aqueous solution of PdCl2. We think that it is Pd(II) species other than Pd(II) colloids on the silica surfaces after the activation step. Both the functionalization of the silica particles with APTMS and the activation using Pd(II) solution are critical in this electroless deposition procedure. When we use silica spheres without functionalization, the electroless nickel deposition does not occur after the activation, this indicates the amine group enhanced the Pd(II) adsorption through the metal–ligand bond. When we use the functionalized particles without activation, the electroless deposition also does not occur, since there are no catalytic sites on these silica sphere surfaces, the electroless deposition cannot be initiated. Fig. 3A and B are the energy-dispersive spectrometry (EDS) spectra of silica particles before and after electroless nickel deposition, respectively. EDS analysis detected the presence of Ni, Pd and P, along with Si and O of the silica particles after the electroless nickel deposition (Fig. 3A), but there is only Si, O and Pd after the activation step (Fig. 3B), thus confirming successful nickel deposition on the silica particles surfaces using the present electroless deposition technique. From Fig. 3A we can see the presence of Pd after the activation step. The presence of Al peak is because of the Al foil used in the sample preparation. Since after the activation step there are catalytic sites on the silica spheres, we are sure that this method can also be used to electroless deposition of other metals on the functionalized silica spheres, such as copper, cobalt, silver, etc. And this method can also be applied to any particles with NH2 groups on the surfaces.
4. Conclusions In summary, nickel has been deposited on monodisperse silica spheres with smooth surfaces using a novel electroless nickel deposition method. It avoids former roughening condition and stannous chloride sensitization. The functionalization of silica spheres with APTMS is an important step in this procedure. The resultant nickel coated silica microspheres are of interest in the photonic, magnetic, and catalytic applications.
Acknowledgments We appreciate the financial support of the National 863 Project of China (No.AA2002302108).
References [1] F. Caruso, Adv. Mater. 13 (2001) 11. [2] C. Graf, A. van Blaaderen, Langmuir 18 (2002) 524. [3] S.J. Oldenburg, R.D. Averitt, S.L. Westcott, N.J. Halas, Chem. Phys. Lett. 288 (1998) 243. [4] J. Ding, X.L. Ren, F.Q. Tang, Chin. J. Inorg. Chem. 19 (2003) 993. [5] S. Ramesh, Y. Koltypin, R. Prozorov, A. Gedanken, Chem. Mater. 9 (1997) 546. [6] S.L. Westcott, S.J. Oldenburg, T.R. Lee, N.J. Halas, Langmuir 14 (1998) 5396. [7] Y. Kobayashi, V. Salgueirino-Maceira, L.M. Liz-Marzan, Chem. Mater. 13 (2001) 1630. [8] Z. Chen, P. Zhan, J.H. Zhang, Z.L. Wang, W.Y. Zhang, N.B. Min, Chin. Phys. Lett. 20 (2003) 1369. [9] X.X. Jiang, W. Shen (Eds.), The Fundamentals and Practice of Electroless Plating, National Defence Industry Press, Beijing, China, 2000, p. 12. [10] S. Shukla, S. Seal, J. Akesson, R. Oder, R. Carter, Z. Rahaman, Appl. Surf. Sci. 181 (2001) 35. [11] L.N. Xu, J.H. Liao, L. Huang, D.L. Ou, Z.R. Guo, H.Q. Zhang, C.W. Ge, N. Gu, J.Z. Liu, Thin Solid Films 434 (2003) 121. [12] W.J. Dressick, C.S. Dulcey, J.H. Georger Jr., G.S. Calabrese, J.M. Calvert, J. Electrochem. Soc. 141 (1994) 210. [13] W. Stober, A. Fink, E. Bohn, J. Colloid Interface Sci. 26 (1968) 62.