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Simple and eco-friendly preparation of silver films coated on copper surface by replacement reaction
Applied Surface Science 258 (2012) 7430–7434
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Simple and eco-friendly preparation of silver films coated on copper surface by replacement reaction Jun Zhao, Dongming Zhang ∗ , Xingjuan Song State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China
a r t i c l e
i n f o
Article history: Received 4 February 2012 Received in revised form 5 April 2012 Accepted 9 April 2012 Available online 21 April 2012 Keywords: Cu Ag Coating Core–shell Antioxidation
a b s t r a c t Copper powders were coated by columnar nanostructured silver films by replacement reaction in aqueous system with PVP and citric acid at room temperature. The dense and uniform silver coating layer was obtained by strictly controlling Ag/Cu ratio. With different Ag/Cu ratio, different content of Ag in the composite can be obtained. The mechanism of composite powders formation and their characteristics were discussed in detail. It is noted that citric acid and PVP play different roles in preparing high quality Cu–Ag composite powders. © 2012 Elsevier B.V. All rights reserved.
1. Introduction
2. Experiment
Ag and Cu have good electrical conductivity, so they are widely used in electronic industry. However, oxidation of copper and high cost of silver are two important problems to be solved. Formation of an anti-oxidation Ag layer on Cu particles is a desirable option, which can solve the above two problems [1]. With the development of technology, there are many ways to get composite powders such as electroplating [2], electroless plating [3–5], vacuum process [6] and, etc. Electroplating and vacuum processes have low productivity effect, so these ways are hard to be applied in industry. Chemical replacement method has been regarded as the most suitable process for the synthesis of bimetallic system due to the advantages of its simplicity and high efficiency [7]. Many researchers had studied system of (Pt, Pd, Cu, Ag, Au)/(Co, Ni, Cu) [8,9], Ag/Au [10–12], Co/Pt [13] and Co/Cu [14] prepared by Chemical replacement method using different surfactant. Also, Hai [15] has prepared silver–copper composite powders with low Ag weight percentage (<8%) by electroless plating in aqueous system at room temperature. The antioxidation of copper is limited because of its low content of silver. In this manuscript, a complex agent of citric acid and dispersing agent of PVP are selected to obtain high content of silver up to 94.87% by chemical replacement reaction. And the effects of the two agents were studied. There is no any reported literature concerning these matters until now.
Copper powders with particle size from 4 to 10 m were prepared in our lab. For preparation of coating solution, 0.1 g of citric acid, 2 g of PVP and proper amount of AgNO3 were dissolved in 100 mL of water. The solution was stirred with a magnetic stirrer for 10 min to ensure the matters completely dissolved. The 10 mL absolute ethyl alcohol dispersed with Cu powders was poured into the prepared solution at room temperature by constant stirring. After 10 min the solution with Cu particles became black. The detailed preparation conditions are listed in Table 1.
∗ Corresponding author. E-mail address: [email protected] (D. Zhang). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.04.056
3. Results and discussions 3.1. Effect of Cu/Ag ratio on Cu–Ag core–shell composite Fig. 1 shows the XRD patterns of the synthesized powders with different Ag/Cu ratio. Fig. 1 indicates that both copper peaks of (1 1 1), (2 0 0), (2 2 0) and silver peaks of (1 1 1), (2 0 0), (2 2 0), (3 1 1) can be found in the four samples. And oxidized Cu peaks does not been found. With the decrease of Ag/Cu molar ratio, the relative intensity of silver to copper is weaker. Fig. 2 shows the SEM images of different samples. The original Cu powders, shown in Fig. 2a, are polyhedral with smooth surfaces and size range of 5–10 m. Fig. 2b–e indicates, that more and more free Ag fines deposited with the increase of Ag/Cu ratio. In the solution, Ag+ combined with citric acid and lose H+ to become a stable solution [16]. After copper particles are added, H+ remove the oxide layer of the copper surface. Then silver nuclei
J. Zhao et al. / Applied Surface Science 258 (2012) 7430–7434
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formed on the surface of copper particles by replacement reaction as Eq. (2), where silver nuclei act as the active sites for the further deposition of silver [17]. There H+ plays an important role because it cleansed and activated the particle surfaces so that the displacement reaction take place rapidly [16]. Indeed, if there are enough free Ag+ , silver nuclei formed on the copper particle surface by the reaction (Eq. (2)) is also large enough, the reduced Ag will contributed to the growth of the original nuclei to form a density layer on the surface of copper particles as shown in Fig. 2b and c. With the decrease of free Ag+ , silver nuclei formed on the copper particle surface by the reaction is little. As is known the replacement reaction is fast, the few silver nuclei sites are not enough for the growth of silver layer which results in the formation of free silvers as shown in Fig. 2d and e. If Ag/Cu is below 1:1, the content of silver decreases rapidly as shown in Fig. 2f. The result illustrates that uniform silver coatings can be formed with Ag/Cu molar ratio more than 1:1. Fig. 1. XRD patterns of coated Cu samples obtained with different Ag/Cu ratio.
3Ag+ + H3 C6 H5 O7 → Ag3 C6 H5 O7 + 3H+ Cu + 2Ag → Cu
2+
+ 2Ag
(1) (2)
In order to clarify the composite structure, prepared particles prepare with Ag/Cu = 1 are solidified in epoxy. After polished,
Fig. 2. SEM of coated Cu samples obtained with different Ag/Cu ratio (a) original Cu; (b) 2:1; (c) 1:1; (d) 1:2; (e) 1:4; (f) is the curve of Ag/Cu ratio and content of Ag and Cu in the composite.
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J. Zhao et al. / Applied Surface Science 258 (2012) 7430–7434
Table 1 The composition and ratios in preparing copper/silver composite bimetallic powders. Specimens
Cu/g
AgNO3 /g
Ag/Cu molar ratio
PVP /g
C6 H8 O7 ·H2 O/g
1 2 3 4
0.32 0.32 0.32 0.32
1.7 0.85 0.425 0.2125
2:1 1:1 1:2 1:4
2 2 2 2
0.1 0.1 0.1 0.1
brightness 1 zone and 2 zone shows a clear core–shell structure, as shown in Fig. 3a and b .The inner layer is copper and outer layer is mainly silver by EDS analysis in Fig. 3(2). It can be found from Fig. 3b that the thickness of outside silver layer is increase with the decrease of core copper size. The reason is that smaller core copper particles have larger specific surface area. So the small Cu particles have higher activity in the replacement to form more silver nuclei which result in the rapid growth of thick silver. 3.2. Effect of PVP and citric acid in the preparing process In order to clarify the effect of PVP and citric acid, they were added to solutions respectively with Ag/Cu = 1. When there is no PVP and citric acid as shown in Fig. 4a, many loose small particles like aggregates are formed and EDS shows the Ag content is 98.11%. In this condition, replacement reaction between Cu and Ag is heavily because original copper powder is disappeared nearly. As for there is PVP only in the solution, as shown in Fig. 4b, copper
particles change greatly and there are a lot of nano particles, which are mainly silver grains identified by EDS as Fig. 4f. While there is citric acid only in the solution as shown in Fig. 4c, there are not free fine grains and large Cu particles are wrapped with a silver layer. But the silver layer is loose and some site is not coated. It can be found that the coating Ag grow into rod-like crystal in the present of citrate as shown in Fig. 4d,which is in accordance with Djokic´ reported [16]. In comparison in Fig. 4d, the particles have a compact and smooth surface Combined with Fig. 4h shows formation of a good silver layer on copper particles. XRD of Fig. 5 shows composition of samples prepared with different additives to illustrate different effects of them. When there is no citric and PVP, the copper (1 1 1) at 43.40◦ and silver (2 0 0) at 44.29◦ are strong and separated, as shown in Fig. 5A.When there is PVP only, the XRD pattern of Fig. 5B has a similar tendency as Fig. 5A. While Fig. 5C and D have a similar tendency, Ag (2 0 0) overlap with Cu (1 1 1) and weaker enough because of the formation of Ag coatings on Cu particles. Combined with SEM of Fig. 2, it can be concluded that if there is no citric acid, Cu particles are mainly as
Fig. 3. BSEM and EDS of sample (Ag/Cu = 1) solidified in epoxy.
J. Zhao et al. / Applied Surface Science 258 (2012) 7430–7434
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Fig. 4. SEM of coated Cu samples obtained width Ag/Cu 1:1 (a) no PVP and citric acid; (b) only with PVP; (c) only with citric acid; (d) with PVP andcitric acid and (e–h) are the corresponding EDS respectively.
reducing agent because the replacement reaction is fast. As for PVP, it plays a role of protection silver particles from gathering while citric acid control the replacement reaction rate to obtain uniform silver layer by formation silver citrate.
3.3. Effect of resistance to oxidation temperature Composited powders prepared by the above method were analyzed by TG/DTA in an air flow with a heating rate of 10 ◦ C min.
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J. Zhao et al. / Applied Surface Science 258 (2012) 7430–7434
683.7 ◦ C because the oxidation of copper is finished. From the final weight gain in Fig. 6a, the oxidation product is CuO. As is known, the copper is oxidized to Cu2 O (Eq. (3)) with about 12.5% weight gain while to CuO (Eq. (4)) with 25% weight gain. The final weight gain in Fig. 6a is close to 25%, so it is determined the end oxidation is CuO. In Fig. 6b, the TG curve displayed there is no weight gain but a little decrease because of volatilization of absorbed water. It is illustrated that the oxidation resistance of copper is improved greatly. 4Cu + O2 → 2Cu2 O
(3)
2Cu + O2 → 2CuO
(4)
4. Conclusions
Fig. 5. XRD spectrums of different kind of additives.
Copper–silver core–shell composite was prepared with ecofriendly agents PVP and citric acid. The composite prepared with this simple and efficiency method has a high Ag content up to 94.87 wt%. The free silver crystal is increase with the decrease of Ag/Cu molar ratio. When Ag/Cu value is above 1, the copper particles can get a uniform and dense silver layer. And the silver crystal is rodlike in the present of citric acid. It was found that the PVP play the role of protecting silver nanoparticles from gathering and citric acid combined Ag to control the reaction rate. The oxidation resistance of coated copper is improved greatly compared with uncoated. These coated copper can be used in electronic paste to save cost. Acknowledgement This work was supported by the National Natural Science Foundation of China (51171134 and A3 Foresight Program50821140308). References
Fig. 6. Thermal analysis of (a) Cu and (b) Cu–Ag composite (Ag/Cu = 1).
According to TG curve Fig. 6a for copper particles, the increase in weight began at about 244.5 ◦ C. But there is not an exothermic peak in DTA curve because the larger Cu particles are oxidized slowly with small specific surface area. The TG curve is level off after
[1] H.T. Hai, J.G. Ahn, D.J. Kim, et al., Surface and Coatings Technology 201 (2006) 3788–3792. [2] E. Takeshima, K. Takatsu, Y. Kojima, T. Fujji, US Patent 235 (1990) 954. [3] S. Djokic, M. Dubois, R.H. Lepard, US Patent 158 (1999) 945. [4] T. Hayashi, Shimonoseki, US Patent 178 (1993) 909. [5] Xinrui Xu, Xiaojun Luo, Hanrui Zhuang, Wenlan Li, Baolin Zhang, Materials Letters 57 (2003) 3987–3991. [6] M. Cazayous, C. Langlois, T. Oikawa, C. Ricolleau, A. Sacuto, Physical Review B 73 (2006) 113402–113405. [7] Jun Zhao, DongmingZhang, Jie Zhao, Journal of Solid State Chemistry 184 (2011) 2339–2344. [8] Songrui Wang, Jian’an He, Jinglin Xie, Yuexiang Zhu, Youchang Xie, J.G. Chen, Applied Surface Science 254 (2008) 2102–2109. [9] Songrui Wang, Wei Lin, Yuexiang Zhu, Youchang Xie, J.R. McCormick, Wei Huang, J.G. Chen, Catalysis Letters 114 (2007) 169–173. [10] J. Yang, Jim Yang Lee, Heng-Phon Too, Journal of Physical Chemistry B 109 (2005) 19208–19212. [11] Y.G. Sun, Y.N. Xia, Science 298 (2002) 2176–2179. [12] S.E. Hunyadi, C.J. Murphy, Journal of Materials Chemistry 16 (2006) 3929–3935. [13] J.-I. Park, J. Cheon, Journal of the American Chemical Society 123 (2001) 5743–5746. [14] Z. Guo, C.S.S.R. Kumar, L.L. Henry, E.E. Doomes, J. Hormes, E.J. Podlaha, Journal of the Electrochemical Society 152 (2005) D1–D5. [15] H.T. Hai, J.G. Ahn, D.J. Kim, J.R. Lee, H.S. Chung, C.O. Kim, Surface & Coatings Technology 201 (2006) 3788–3792. ´ Bioinorganic Chemistry and Applications, vol. 2008, 2008, [16] S. Djokic, http://dx.doi.org/10.1155/2008/436458, Article ID 436458, 7 p. [17] K.L. Yeung, S. Christiansen, A. Varma, Journal of Membrane Science 159 (1999) 107–122.
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Simple and eco-friendly preparation of silver films coated on copper surface by replacement reaction Jun Zhao, Dongming Zhang ∗ , Xingjuan Song State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China
a r t i c l e
i n f o
Article history: Received 4 February 2012 Received in revised form 5 April 2012 Accepted 9 April 2012 Available online 21 April 2012 Keywords: Cu Ag Coating Core–shell Antioxidation
a b s t r a c t Copper powders were coated by columnar nanostructured silver films by replacement reaction in aqueous system with PVP and citric acid at room temperature. The dense and uniform silver coating layer was obtained by strictly controlling Ag/Cu ratio. With different Ag/Cu ratio, different content of Ag in the composite can be obtained. The mechanism of composite powders formation and their characteristics were discussed in detail. It is noted that citric acid and PVP play different roles in preparing high quality Cu–Ag composite powders. © 2012 Elsevier B.V. All rights reserved.
1. Introduction
2. Experiment
Ag and Cu have good electrical conductivity, so they are widely used in electronic industry. However, oxidation of copper and high cost of silver are two important problems to be solved. Formation of an anti-oxidation Ag layer on Cu particles is a desirable option, which can solve the above two problems [1]. With the development of technology, there are many ways to get composite powders such as electroplating [2], electroless plating [3–5], vacuum process [6] and, etc. Electroplating and vacuum processes have low productivity effect, so these ways are hard to be applied in industry. Chemical replacement method has been regarded as the most suitable process for the synthesis of bimetallic system due to the advantages of its simplicity and high efficiency [7]. Many researchers had studied system of (Pt, Pd, Cu, Ag, Au)/(Co, Ni, Cu) [8,9], Ag/Au [10–12], Co/Pt [13] and Co/Cu [14] prepared by Chemical replacement method using different surfactant. Also, Hai [15] has prepared silver–copper composite powders with low Ag weight percentage (<8%) by electroless plating in aqueous system at room temperature. The antioxidation of copper is limited because of its low content of silver. In this manuscript, a complex agent of citric acid and dispersing agent of PVP are selected to obtain high content of silver up to 94.87% by chemical replacement reaction. And the effects of the two agents were studied. There is no any reported literature concerning these matters until now.
Copper powders with particle size from 4 to 10 m were prepared in our lab. For preparation of coating solution, 0.1 g of citric acid, 2 g of PVP and proper amount of AgNO3 were dissolved in 100 mL of water. The solution was stirred with a magnetic stirrer for 10 min to ensure the matters completely dissolved. The 10 mL absolute ethyl alcohol dispersed with Cu powders was poured into the prepared solution at room temperature by constant stirring. After 10 min the solution with Cu particles became black. The detailed preparation conditions are listed in Table 1.
∗ Corresponding author. E-mail address: [email protected] (D. Zhang). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.04.056
3. Results and discussions 3.1. Effect of Cu/Ag ratio on Cu–Ag core–shell composite Fig. 1 shows the XRD patterns of the synthesized powders with different Ag/Cu ratio. Fig. 1 indicates that both copper peaks of (1 1 1), (2 0 0), (2 2 0) and silver peaks of (1 1 1), (2 0 0), (2 2 0), (3 1 1) can be found in the four samples. And oxidized Cu peaks does not been found. With the decrease of Ag/Cu molar ratio, the relative intensity of silver to copper is weaker. Fig. 2 shows the SEM images of different samples. The original Cu powders, shown in Fig. 2a, are polyhedral with smooth surfaces and size range of 5–10 m. Fig. 2b–e indicates, that more and more free Ag fines deposited with the increase of Ag/Cu ratio. In the solution, Ag+ combined with citric acid and lose H+ to become a stable solution [16]. After copper particles are added, H+ remove the oxide layer of the copper surface. Then silver nuclei
J. Zhao et al. / Applied Surface Science 258 (2012) 7430–7434
7431
formed on the surface of copper particles by replacement reaction as Eq. (2), where silver nuclei act as the active sites for the further deposition of silver [17]. There H+ plays an important role because it cleansed and activated the particle surfaces so that the displacement reaction take place rapidly [16]. Indeed, if there are enough free Ag+ , silver nuclei formed on the copper particle surface by the reaction (Eq. (2)) is also large enough, the reduced Ag will contributed to the growth of the original nuclei to form a density layer on the surface of copper particles as shown in Fig. 2b and c. With the decrease of free Ag+ , silver nuclei formed on the copper particle surface by the reaction is little. As is known the replacement reaction is fast, the few silver nuclei sites are not enough for the growth of silver layer which results in the formation of free silvers as shown in Fig. 2d and e. If Ag/Cu is below 1:1, the content of silver decreases rapidly as shown in Fig. 2f. The result illustrates that uniform silver coatings can be formed with Ag/Cu molar ratio more than 1:1. Fig. 1. XRD patterns of coated Cu samples obtained with different Ag/Cu ratio.
3Ag+ + H3 C6 H5 O7 → Ag3 C6 H5 O7 + 3H+ Cu + 2Ag → Cu
2+
+ 2Ag
(1) (2)
In order to clarify the composite structure, prepared particles prepare with Ag/Cu = 1 are solidified in epoxy. After polished,
Fig. 2. SEM of coated Cu samples obtained with different Ag/Cu ratio (a) original Cu; (b) 2:1; (c) 1:1; (d) 1:2; (e) 1:4; (f) is the curve of Ag/Cu ratio and content of Ag and Cu in the composite.
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J. Zhao et al. / Applied Surface Science 258 (2012) 7430–7434
Table 1 The composition and ratios in preparing copper/silver composite bimetallic powders. Specimens
Cu/g
AgNO3 /g
Ag/Cu molar ratio
PVP /g
C6 H8 O7 ·H2 O/g
1 2 3 4
0.32 0.32 0.32 0.32
1.7 0.85 0.425 0.2125
2:1 1:1 1:2 1:4
2 2 2 2
0.1 0.1 0.1 0.1
brightness 1 zone and 2 zone shows a clear core–shell structure, as shown in Fig. 3a and b .The inner layer is copper and outer layer is mainly silver by EDS analysis in Fig. 3(2). It can be found from Fig. 3b that the thickness of outside silver layer is increase with the decrease of core copper size. The reason is that smaller core copper particles have larger specific surface area. So the small Cu particles have higher activity in the replacement to form more silver nuclei which result in the rapid growth of thick silver. 3.2. Effect of PVP and citric acid in the preparing process In order to clarify the effect of PVP and citric acid, they were added to solutions respectively with Ag/Cu = 1. When there is no PVP and citric acid as shown in Fig. 4a, many loose small particles like aggregates are formed and EDS shows the Ag content is 98.11%. In this condition, replacement reaction between Cu and Ag is heavily because original copper powder is disappeared nearly. As for there is PVP only in the solution, as shown in Fig. 4b, copper
particles change greatly and there are a lot of nano particles, which are mainly silver grains identified by EDS as Fig. 4f. While there is citric acid only in the solution as shown in Fig. 4c, there are not free fine grains and large Cu particles are wrapped with a silver layer. But the silver layer is loose and some site is not coated. It can be found that the coating Ag grow into rod-like crystal in the present of citrate as shown in Fig. 4d,which is in accordance with Djokic´ reported [16]. In comparison in Fig. 4d, the particles have a compact and smooth surface Combined with Fig. 4h shows formation of a good silver layer on copper particles. XRD of Fig. 5 shows composition of samples prepared with different additives to illustrate different effects of them. When there is no citric and PVP, the copper (1 1 1) at 43.40◦ and silver (2 0 0) at 44.29◦ are strong and separated, as shown in Fig. 5A.When there is PVP only, the XRD pattern of Fig. 5B has a similar tendency as Fig. 5A. While Fig. 5C and D have a similar tendency, Ag (2 0 0) overlap with Cu (1 1 1) and weaker enough because of the formation of Ag coatings on Cu particles. Combined with SEM of Fig. 2, it can be concluded that if there is no citric acid, Cu particles are mainly as
Fig. 3. BSEM and EDS of sample (Ag/Cu = 1) solidified in epoxy.
J. Zhao et al. / Applied Surface Science 258 (2012) 7430–7434
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Fig. 4. SEM of coated Cu samples obtained width Ag/Cu 1:1 (a) no PVP and citric acid; (b) only with PVP; (c) only with citric acid; (d) with PVP andcitric acid and (e–h) are the corresponding EDS respectively.
reducing agent because the replacement reaction is fast. As for PVP, it plays a role of protection silver particles from gathering while citric acid control the replacement reaction rate to obtain uniform silver layer by formation silver citrate.
3.3. Effect of resistance to oxidation temperature Composited powders prepared by the above method were analyzed by TG/DTA in an air flow with a heating rate of 10 ◦ C min.
7434
J. Zhao et al. / Applied Surface Science 258 (2012) 7430–7434
683.7 ◦ C because the oxidation of copper is finished. From the final weight gain in Fig. 6a, the oxidation product is CuO. As is known, the copper is oxidized to Cu2 O (Eq. (3)) with about 12.5% weight gain while to CuO (Eq. (4)) with 25% weight gain. The final weight gain in Fig. 6a is close to 25%, so it is determined the end oxidation is CuO. In Fig. 6b, the TG curve displayed there is no weight gain but a little decrease because of volatilization of absorbed water. It is illustrated that the oxidation resistance of copper is improved greatly. 4Cu + O2 → 2Cu2 O
(3)
2Cu + O2 → 2CuO
(4)
4. Conclusions
Fig. 5. XRD spectrums of different kind of additives.
Copper–silver core–shell composite was prepared with ecofriendly agents PVP and citric acid. The composite prepared with this simple and efficiency method has a high Ag content up to 94.87 wt%. The free silver crystal is increase with the decrease of Ag/Cu molar ratio. When Ag/Cu value is above 1, the copper particles can get a uniform and dense silver layer. And the silver crystal is rodlike in the present of citric acid. It was found that the PVP play the role of protecting silver nanoparticles from gathering and citric acid combined Ag to control the reaction rate. The oxidation resistance of coated copper is improved greatly compared with uncoated. These coated copper can be used in electronic paste to save cost. Acknowledgement This work was supported by the National Natural Science Foundation of China (51171134 and A3 Foresight Program50821140308). References
Fig. 6. Thermal analysis of (a) Cu and (b) Cu–Ag composite (Ag/Cu = 1).
According to TG curve Fig. 6a for copper particles, the increase in weight began at about 244.5 ◦ C. But there is not an exothermic peak in DTA curve because the larger Cu particles are oxidized slowly with small specific surface area. The TG curve is level off after
[1] H.T. Hai, J.G. Ahn, D.J. Kim, et al., Surface and Coatings Technology 201 (2006) 3788–3792. [2] E. Takeshima, K. Takatsu, Y. Kojima, T. Fujji, US Patent 235 (1990) 954. [3] S. Djokic, M. Dubois, R.H. Lepard, US Patent 158 (1999) 945. [4] T. Hayashi, Shimonoseki, US Patent 178 (1993) 909. [5] Xinrui Xu, Xiaojun Luo, Hanrui Zhuang, Wenlan Li, Baolin Zhang, Materials Letters 57 (2003) 3987–3991. [6] M. Cazayous, C. Langlois, T. Oikawa, C. Ricolleau, A. Sacuto, Physical Review B 73 (2006) 113402–113405. [7] Jun Zhao, DongmingZhang, Jie Zhao, Journal of Solid State Chemistry 184 (2011) 2339–2344. [8] Songrui Wang, Jian’an He, Jinglin Xie, Yuexiang Zhu, Youchang Xie, J.G. Chen, Applied Surface Science 254 (2008) 2102–2109. [9] Songrui Wang, Wei Lin, Yuexiang Zhu, Youchang Xie, J.R. McCormick, Wei Huang, J.G. Chen, Catalysis Letters 114 (2007) 169–173. [10] J. Yang, Jim Yang Lee, Heng-Phon Too, Journal of Physical Chemistry B 109 (2005) 19208–19212. [11] Y.G. Sun, Y.N. Xia, Science 298 (2002) 2176–2179. [12] S.E. Hunyadi, C.J. Murphy, Journal of Materials Chemistry 16 (2006) 3929–3935. [13] J.-I. Park, J. Cheon, Journal of the American Chemical Society 123 (2001) 5743–5746. [14] Z. Guo, C.S.S.R. Kumar, L.L. Henry, E.E. Doomes, J. Hormes, E.J. Podlaha, Journal of the Electrochemical Society 152 (2005) D1–D5. [15] H.T. Hai, J.G. Ahn, D.J. Kim, J.R. Lee, H.S. Chung, C.O. Kim, Surface & Coatings Technology 201 (2006) 3788–3792. ´ Bioinorganic Chemistry and Applications, vol. 2008, 2008, [16] S. Djokic, http://dx.doi.org/10.1155/2008/436458, Article ID 436458, 7 p. [17] K.L. Yeung, S. Christiansen, A. Varma, Journal of Membrane Science 159 (1999) 107–122.