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Dealloying by metallic melt
Materials Letters 65 (2011) 1076–1078
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Dealloying by metallic melt Takeshi Wada ⁎, Kunio Yubuta, Akihisa Inoue, Hidemi Kato Institute for Materials Research, Tohoku University, Katahira 2-1-1, Sendai 980-8577, Japan
a r t i c l e
i n f o
Article history: Received 14 December 2010 Accepted 18 January 2011 Available online 25 January 2011 Keywords: Porous materials Metal and alloys Composite materials
a b s t r a c t Dealloying, which commonly involves corrosion processes in aqueous solutions, is a promising technique for preparing functional nanoporous metals. While this technique is ideal for preparing nanoporous noble metals such as of Au, it is not readily applicable to less-noble metals. Here, we propose a novel dealloying method employing a metallic melt, instead of an aqueous solution, as the dealloying liquid for a preparing of nanoporous metals. An atomic interaction among alloy components and metallic melt causes specific component to dissolve out from the alloy solid into the melt with self-organizing nanoporous structure by the remaining component. The dealloying method can be applied for preparation of nanoporous less-noble metal such as of Ti for the development of functional materials such as fluid filters, gas absorption media, and biomaterials. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Nanoporous metals have attracted considerable attention for their excellent functional properties including high catalytic activity, sensing capabilities, and surface-enhanced Raman scattering [1–4], which differ significantly from those of their dense, bulky counterparts. Such nanoporous metals have mainly been prepared by dealloying in aqueous solution. The selective corrosion of one or more components from a multicomponent alloy introduces threedimensionally continuous nanoporosities in metals [5–13]. Using this method, many types of relatively noble nanoporous metals such as of Au, Pt, Pd, Cu, and Ni, have been successfully produced from multicomponent crystalline alloy precursors based on Au-Ag, Pt–Cu, Pd–Co, Cu–Mn and Ni–Mn, respectively [6–13]. In addition, metallic glasses are drawing attention as the precursor of nanoporous metals because the long range structural homogeneity, the variety of constituent elements and the wider composition ranges are attractive for fabricating nanoporous metals with the tailored morphology [14,15]. Titanium is an attractive material with excellent properties such as corrosion resistance, chemical activity with gas species and biocompatibility [16]. Therefore, nanoporous Ti may provide new functional applications such as filters for gases or fluids, media for gas storage or adsorption, gas sensing, and biomaterials. However, in most of the acid solutions, Ti in Ti-based alloys is preferentially oxidized forming of passive film because of the low standard electrode potential of Ti. In the more aggressive solution such as hydrofluoric acid Ti in the Ti–Cubased alloy is selectively removed resulting in the formation of
⁎ Corresponding author. Tel.: +81 22 215 2112; fax: +81 22 215 2110. E-mail address: [email protected] (T. Wada). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.01.054
nanostructured pure Cu [17]. Accordingly, the dealloying based on corrosion phenomenon results in either the formation of Ti oxide or selective dissolution of Ti, indicating that it is hardly applicable for fabricating nanoporous metallic Ti. Here we propose a new dealloying method employing a metallic melt, instead of an aqueous solution, as a dealloying liquid for a preparation of nanoporous metals. This technique makes the formation of nanoporous structure possible even for less-noble metal such as Ti because the selective removal of components takes place not by a corrosion but by the attractive or repulsive force among the constituent elements, which is independent of the nobilities of constituent elements. We demonstrate the formation of nanoporous Ti by selective removal of components from typical Ti-based alloys in metallic melts caused by the strong attractive force between components of the alloy and melt; a process essentially similar to dealloying in aqueous solution. 2. Experimental Binary Ti–Cu alloy ingots with nominal compositions of Ti30Cu70 were prepared by arc-melting high-purity Ti (99.99 mass%) and Cu (99.99 mass%) metals. Thin ribbons of Ti–Cu alloy about 10 mm wide and 30 μm thick were prepared by the melt-spinning method. The pure liquid Mg (99.9 mass%) about 10 g was inductively heated in a carbon crucible at constant temperatures of 973 and 1223 K and the Ti–Cu ribbons were immersed in a Mg liquid for 5 s. After immersion, the samples were etched by the 3 mol/l nitric acid aqueous solution for 30 min at room temperature. The sample phases were identified using a microfocus X-ray diffractometer with Co-Kα radiation (XRD, Bruker D8 Discover with GADDS) using collimators having radius of 300 μm. Surface features and chemical composition were analysed using a scanning electron microscope equipped with an energy dispersive X-ray spectrometer (SEM-EDX, Hitachi S-4800 with EDAX
T. Wada et al. / Materials Letters 65 (2011) 1076–1078
Fig. 1. X-ray diffraction patterns of Ti30Cu70 alloy melt-spun ribbon (top) and annealed at 973 K for 1.8 ks under vacuum (bottom).
Genesis XM2). A tomography image of a sample was obtained by a focused ion beam/scanning electron microscope system (FIB-SEM, FEI Helios NanoLab 400s). The microstructure of a sample was observed by a transmission electron microscope (TEM, JEOL-2010) with an acceleration voltage of 200 kV. 3. Results and discussion Fig. 1 shows XRD patterns of as-spun Ti30Cu70 alloy and its structure after annealing at 973 K for 1.8 ks under vacuum (2× 10− 3 Pa). The diffraction pattern for the as-spun alloy is broad and has no Bragg peaks, indicating a single homogeneous glassy structure. The XRD pattern of its annealed state reveals that the Ti30Cu70 metallic glass crystallized into Cu3Ti2 and β-Cu4Ti compounds; this agrees well with the equilibrium phase diagram [18]. Fig. 2a shows a backscattered electron image of a cross-section of the Ti30Cu70 alloy ribbon after immersion in the Mg melt at 973 K. The cross-section was prepared by FIB method; the vertical lines visible in the micrograph are artefacts created by the curtaining effect during FIB etching [19]. The micrograph shows fine interconnected granules approximately several hundreds of nanometres in diameter embedded in the matrix. The corresponding XRD pattern in Fig. 2b shows crystalline sharp peaks identified as hexagonalclose-packed (hcp-) Ti and hcp-Mg. The TEM-EDX reveals that the average analytical compositions of the granules and the matrix are Ti98.30Cu1.70 and Mg95.84Ti3.13Cu1.03 (at.%), respectively. Formation of the hcp-Ti phase is not caused by thermal crystallization of Ti30Cu70 metallic glass, since the crystallized phases of Ti30Cu70 metallic glass are Cu3Ti2 and β-Cu4Ti compounds, as shown in Fig. 1. Instead, it may be due to the chemical reaction between the pure Mg melt and the Ti30Cu70 alloy.
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According to the enthalpies of mixing of the liquid metal, the Mg–Cu and Ti–Cu bonds are attractive, whereas the Mg-Ti bond is repulsive [20]. In the pure Mg melt, only the Cu atoms in the Ti30Cu70 alloy precursor are attracted by Mg atoms and migrate into the Mg melt as solutes. The Ti atoms left at the interface between the alloy precursor and the melt agglomerate to form fine solid granules with hcp lattice structure. This process is essentially similar to dealloying in aqueous solution, in which the base metal in an alloy of noble and base metals is selectively etched by an electrolyte, and the noble metal atoms left at the surface form continuous island due to surface diffusion [21]. For dealloying in aqueous solution to occur, the dominant factor is the standard electrode potential; however, in the present method, it is the chemical interaction among the elements. Strong interactions between Mg and Cu at elevated temperatures enable dealloying of Cu from the Ti–Cu alloy into the Mg melt. In acid solution, pure Ti is passive while Mg is highly active. This large difference in corrosion behavior enables selective removal of hcp-Mg from the hcp-Ti/hcp-Mg composite. Fig. 3a and b shows crosssectional and high-magnification SEM images, respectively, of the hcp-Ti/hcp-Mg composite after immersion in HNO3 aqueous solution for 30 min. The figures show that an open-cell nanoporous structure was present. The cross-section in Fig. 3a indicates that the porous structure is uniformly spread throughout the entire section. The highmagnification image in Fig. 3b indicates that Ti granules of about 200 nm are three-dimensionally connected. The tomogram (see the inset of Fig. 3b) confirms the bi-continuous porous structure. The volume fraction of porosity and the specific surface area calculated from this image are 47% and 3.9 × 103 m2 g− 1, respectively. The SEMEDX spectrum of the nanoporous sample in Fig. 3c reveals that it is composed entirely of Ti—neither Mg nor Cu were detected—indicating that the Mg matrix in the original hcp-Ti/hcp-Mg composite had perfectly interconnected structure and that no isolated Mg existed in the Ti phase. The TEM image and corresponding selected area electron diffraction (SAED) pattern in Fig. 3d confirm that the nanoporous sample retains the original hcp structure, as no oxide or other compound phase was observed. This indicates that immersion in HNO3 solution selectively removes the hcp-Mg phase without any degradation in the hcp-Ti. Fig. 3e shows the effects of the Mg melt temperature on the porous structure. The sample was prepared by immersing Ti30Cu70 ribbon precursors into the Mg melt at 1223 K for 5 s. It is clear that this sample has a much coarser structure than the one prepared at 973 K (Fig. 3a). The development of porosity in chemical dealloying is reportedly to be controlled by the diffusion of atoms at the alloy/ electrolyte interface [21], and a lower dealloying temperature suppresses the diffusion of atoms and causes the formation of finer porous structures [22]. Our results agree with this tendency, strongly suggesting that the development of nanoporous Ti in the Mg melt is controlled by diffusion at the alloy/Mg melt interface.
Fig. 2. (a) Backscattered cross-sectional scanning electron microscopy image and (b) X-ray diffraction pattern of Ti30Cu70 alloy after immersion in pure Mg melt at 973 K.
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T. Wada et al. / Materials Letters 65 (2011) 1076–1078
Fig. 3. (a) A large-area cross-sectional scanning electron microscopy image, (b) a high-magnification SEM image and its tomogram image (inset), (c) energy-dispersive X-ray spectrum and (d) the transmission electron micrograph and corresponding selected area electron diffraction pattern of nanoporous Ti prepared by etching Mg from Ti/Mg nanocomposite. (e) Scanning electron microscopy image of porous Ti prepared by the immersion in Mg-melt bath at 1223 K.
4. Conclusions The present dealloying method enables the evolution of porosity in Ti-based alloys. The dominant factor in selective removal of a component is the interaction of atoms between the precursor and the melt. Hence the porous structure can be formed even in less-noble metals, for which the formation of a nanoporous structure by the dealloying in aqueous solution has never been reported. Our discovery of a new dealloying technique for preparing less-noble nanoporous metals may lead to new metallic materials with exceptional functional properties. References [1] Erlebacher J, Seshadri R. MRS Bull 2009;34:561–8. [2] Xu C, Su J, Xu X, Liu P, Zhao H, Tian F, et al. J Am Chem Soc 2007;129:42–3. [3] Qiu H, Xue L, Ji G, Zhou G, Huang X, Qu Y, et al. Biosens Bioelectron 2009;24: 3014–8. [4] Ding Y, Chen M. MRS Bull 2009;34:569–76. [5] Forty AJ. Nature 1979;282:597–8.
[6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
[17] [18] [19] [20] [21] [22]
Forty AJ, Durkin P. Philos Mag A 1980;42:295–318. Pugh DV, Dursun A, Corcoran SG. J Mater Res 2003;18:216–21. Hakamada M, Mabuchi M. J Alloys Compd 2009;479:326–9. Min US, Li JCM. J Mater Res 1994;11:2878–83. Chen LY, Yu JS, Fujita T, Chen M. Adv Funct Mater 2009;19:1221–6. Smith AJ, Tran T, Wainwright MS. J Appl Electrochem 1999;29:1085–94. Hakamada M, Mabuchi M. J Alloy Compd 2009;485:583–7. Yu J, Ding Y, Xu C, Inoue A, Sakurai T, Chen M. Chem Mater 2008;20:4548–50. Lang XY, Chen LY, Guan PF, Fujita T, Chen MW. Appl Phys Lett 2009;94:213109. Lang XY, Guo H, Chen LY, Kudo A, Yu JS, Zhang W, et al. J Phys Chem C 2010;114: 2600–3. Pourbaix M. Atlas of Electrochemical Equilibria in Aqueous Solutions (translated from the French by J. A. Franklin). Houston: National Association of Corrosion Engineers; 1974. Abe H, Sato K, Nishikawa H, Takemoto T, Fukuhara M, Inoue A. Mater Trans 2009;50:1255–8. Okamoto H. A Desk Handbook: Phase Diagrams for Binary Alloys. Ohio: ASM International; 2000. Giannuzzi LA, Stevie FA. Introduction to Focused Ion Beams, Instrumentation, Theory, Techniques and Practice. Berlin: Springer; 2005. Takeuchi A, Inoue A. Mater Trans 2005;46:2817–29. Erlebacher J, Aziz MJ, Karma A, Dimitrov N, Sieradzki K. Nature 2001;410:450–3. Qian LH, Chen MW. Appl Phys Lett 2007;91:083105.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Dealloying by metallic melt Takeshi Wada ⁎, Kunio Yubuta, Akihisa Inoue, Hidemi Kato Institute for Materials Research, Tohoku University, Katahira 2-1-1, Sendai 980-8577, Japan
a r t i c l e
i n f o
Article history: Received 14 December 2010 Accepted 18 January 2011 Available online 25 January 2011 Keywords: Porous materials Metal and alloys Composite materials
a b s t r a c t Dealloying, which commonly involves corrosion processes in aqueous solutions, is a promising technique for preparing functional nanoporous metals. While this technique is ideal for preparing nanoporous noble metals such as of Au, it is not readily applicable to less-noble metals. Here, we propose a novel dealloying method employing a metallic melt, instead of an aqueous solution, as the dealloying liquid for a preparing of nanoporous metals. An atomic interaction among alloy components and metallic melt causes specific component to dissolve out from the alloy solid into the melt with self-organizing nanoporous structure by the remaining component. The dealloying method can be applied for preparation of nanoporous less-noble metal such as of Ti for the development of functional materials such as fluid filters, gas absorption media, and biomaterials. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Nanoporous metals have attracted considerable attention for their excellent functional properties including high catalytic activity, sensing capabilities, and surface-enhanced Raman scattering [1–4], which differ significantly from those of their dense, bulky counterparts. Such nanoporous metals have mainly been prepared by dealloying in aqueous solution. The selective corrosion of one or more components from a multicomponent alloy introduces threedimensionally continuous nanoporosities in metals [5–13]. Using this method, many types of relatively noble nanoporous metals such as of Au, Pt, Pd, Cu, and Ni, have been successfully produced from multicomponent crystalline alloy precursors based on Au-Ag, Pt–Cu, Pd–Co, Cu–Mn and Ni–Mn, respectively [6–13]. In addition, metallic glasses are drawing attention as the precursor of nanoporous metals because the long range structural homogeneity, the variety of constituent elements and the wider composition ranges are attractive for fabricating nanoporous metals with the tailored morphology [14,15]. Titanium is an attractive material with excellent properties such as corrosion resistance, chemical activity with gas species and biocompatibility [16]. Therefore, nanoporous Ti may provide new functional applications such as filters for gases or fluids, media for gas storage or adsorption, gas sensing, and biomaterials. However, in most of the acid solutions, Ti in Ti-based alloys is preferentially oxidized forming of passive film because of the low standard electrode potential of Ti. In the more aggressive solution such as hydrofluoric acid Ti in the Ti–Cubased alloy is selectively removed resulting in the formation of
⁎ Corresponding author. Tel.: +81 22 215 2112; fax: +81 22 215 2110. E-mail address: [email protected] (T. Wada). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.01.054
nanostructured pure Cu [17]. Accordingly, the dealloying based on corrosion phenomenon results in either the formation of Ti oxide or selective dissolution of Ti, indicating that it is hardly applicable for fabricating nanoporous metallic Ti. Here we propose a new dealloying method employing a metallic melt, instead of an aqueous solution, as a dealloying liquid for a preparation of nanoporous metals. This technique makes the formation of nanoporous structure possible even for less-noble metal such as Ti because the selective removal of components takes place not by a corrosion but by the attractive or repulsive force among the constituent elements, which is independent of the nobilities of constituent elements. We demonstrate the formation of nanoporous Ti by selective removal of components from typical Ti-based alloys in metallic melts caused by the strong attractive force between components of the alloy and melt; a process essentially similar to dealloying in aqueous solution. 2. Experimental Binary Ti–Cu alloy ingots with nominal compositions of Ti30Cu70 were prepared by arc-melting high-purity Ti (99.99 mass%) and Cu (99.99 mass%) metals. Thin ribbons of Ti–Cu alloy about 10 mm wide and 30 μm thick were prepared by the melt-spinning method. The pure liquid Mg (99.9 mass%) about 10 g was inductively heated in a carbon crucible at constant temperatures of 973 and 1223 K and the Ti–Cu ribbons were immersed in a Mg liquid for 5 s. After immersion, the samples were etched by the 3 mol/l nitric acid aqueous solution for 30 min at room temperature. The sample phases were identified using a microfocus X-ray diffractometer with Co-Kα radiation (XRD, Bruker D8 Discover with GADDS) using collimators having radius of 300 μm. Surface features and chemical composition were analysed using a scanning electron microscope equipped with an energy dispersive X-ray spectrometer (SEM-EDX, Hitachi S-4800 with EDAX
T. Wada et al. / Materials Letters 65 (2011) 1076–1078
Fig. 1. X-ray diffraction patterns of Ti30Cu70 alloy melt-spun ribbon (top) and annealed at 973 K for 1.8 ks under vacuum (bottom).
Genesis XM2). A tomography image of a sample was obtained by a focused ion beam/scanning electron microscope system (FIB-SEM, FEI Helios NanoLab 400s). The microstructure of a sample was observed by a transmission electron microscope (TEM, JEOL-2010) with an acceleration voltage of 200 kV. 3. Results and discussion Fig. 1 shows XRD patterns of as-spun Ti30Cu70 alloy and its structure after annealing at 973 K for 1.8 ks under vacuum (2× 10− 3 Pa). The diffraction pattern for the as-spun alloy is broad and has no Bragg peaks, indicating a single homogeneous glassy structure. The XRD pattern of its annealed state reveals that the Ti30Cu70 metallic glass crystallized into Cu3Ti2 and β-Cu4Ti compounds; this agrees well with the equilibrium phase diagram [18]. Fig. 2a shows a backscattered electron image of a cross-section of the Ti30Cu70 alloy ribbon after immersion in the Mg melt at 973 K. The cross-section was prepared by FIB method; the vertical lines visible in the micrograph are artefacts created by the curtaining effect during FIB etching [19]. The micrograph shows fine interconnected granules approximately several hundreds of nanometres in diameter embedded in the matrix. The corresponding XRD pattern in Fig. 2b shows crystalline sharp peaks identified as hexagonalclose-packed (hcp-) Ti and hcp-Mg. The TEM-EDX reveals that the average analytical compositions of the granules and the matrix are Ti98.30Cu1.70 and Mg95.84Ti3.13Cu1.03 (at.%), respectively. Formation of the hcp-Ti phase is not caused by thermal crystallization of Ti30Cu70 metallic glass, since the crystallized phases of Ti30Cu70 metallic glass are Cu3Ti2 and β-Cu4Ti compounds, as shown in Fig. 1. Instead, it may be due to the chemical reaction between the pure Mg melt and the Ti30Cu70 alloy.
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According to the enthalpies of mixing of the liquid metal, the Mg–Cu and Ti–Cu bonds are attractive, whereas the Mg-Ti bond is repulsive [20]. In the pure Mg melt, only the Cu atoms in the Ti30Cu70 alloy precursor are attracted by Mg atoms and migrate into the Mg melt as solutes. The Ti atoms left at the interface between the alloy precursor and the melt agglomerate to form fine solid granules with hcp lattice structure. This process is essentially similar to dealloying in aqueous solution, in which the base metal in an alloy of noble and base metals is selectively etched by an electrolyte, and the noble metal atoms left at the surface form continuous island due to surface diffusion [21]. For dealloying in aqueous solution to occur, the dominant factor is the standard electrode potential; however, in the present method, it is the chemical interaction among the elements. Strong interactions between Mg and Cu at elevated temperatures enable dealloying of Cu from the Ti–Cu alloy into the Mg melt. In acid solution, pure Ti is passive while Mg is highly active. This large difference in corrosion behavior enables selective removal of hcp-Mg from the hcp-Ti/hcp-Mg composite. Fig. 3a and b shows crosssectional and high-magnification SEM images, respectively, of the hcp-Ti/hcp-Mg composite after immersion in HNO3 aqueous solution for 30 min. The figures show that an open-cell nanoporous structure was present. The cross-section in Fig. 3a indicates that the porous structure is uniformly spread throughout the entire section. The highmagnification image in Fig. 3b indicates that Ti granules of about 200 nm are three-dimensionally connected. The tomogram (see the inset of Fig. 3b) confirms the bi-continuous porous structure. The volume fraction of porosity and the specific surface area calculated from this image are 47% and 3.9 × 103 m2 g− 1, respectively. The SEMEDX spectrum of the nanoporous sample in Fig. 3c reveals that it is composed entirely of Ti—neither Mg nor Cu were detected—indicating that the Mg matrix in the original hcp-Ti/hcp-Mg composite had perfectly interconnected structure and that no isolated Mg existed in the Ti phase. The TEM image and corresponding selected area electron diffraction (SAED) pattern in Fig. 3d confirm that the nanoporous sample retains the original hcp structure, as no oxide or other compound phase was observed. This indicates that immersion in HNO3 solution selectively removes the hcp-Mg phase without any degradation in the hcp-Ti. Fig. 3e shows the effects of the Mg melt temperature on the porous structure. The sample was prepared by immersing Ti30Cu70 ribbon precursors into the Mg melt at 1223 K for 5 s. It is clear that this sample has a much coarser structure than the one prepared at 973 K (Fig. 3a). The development of porosity in chemical dealloying is reportedly to be controlled by the diffusion of atoms at the alloy/ electrolyte interface [21], and a lower dealloying temperature suppresses the diffusion of atoms and causes the formation of finer porous structures [22]. Our results agree with this tendency, strongly suggesting that the development of nanoporous Ti in the Mg melt is controlled by diffusion at the alloy/Mg melt interface.
Fig. 2. (a) Backscattered cross-sectional scanning electron microscopy image and (b) X-ray diffraction pattern of Ti30Cu70 alloy after immersion in pure Mg melt at 973 K.
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T. Wada et al. / Materials Letters 65 (2011) 1076–1078
Fig. 3. (a) A large-area cross-sectional scanning electron microscopy image, (b) a high-magnification SEM image and its tomogram image (inset), (c) energy-dispersive X-ray spectrum and (d) the transmission electron micrograph and corresponding selected area electron diffraction pattern of nanoporous Ti prepared by etching Mg from Ti/Mg nanocomposite. (e) Scanning electron microscopy image of porous Ti prepared by the immersion in Mg-melt bath at 1223 K.
4. Conclusions The present dealloying method enables the evolution of porosity in Ti-based alloys. The dominant factor in selective removal of a component is the interaction of atoms between the precursor and the melt. Hence the porous structure can be formed even in less-noble metals, for which the formation of a nanoporous structure by the dealloying in aqueous solution has never been reported. Our discovery of a new dealloying technique for preparing less-noble nanoporous metals may lead to new metallic materials with exceptional functional properties. References [1] Erlebacher J, Seshadri R. MRS Bull 2009;34:561–8. [2] Xu C, Su J, Xu X, Liu P, Zhao H, Tian F, et al. J Am Chem Soc 2007;129:42–3. [3] Qiu H, Xue L, Ji G, Zhou G, Huang X, Qu Y, et al. Biosens Bioelectron 2009;24: 3014–8. [4] Ding Y, Chen M. MRS Bull 2009;34:569–76. [5] Forty AJ. Nature 1979;282:597–8.
[6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
[17] [18] [19] [20] [21] [22]
Forty AJ, Durkin P. Philos Mag A 1980;42:295–318. Pugh DV, Dursun A, Corcoran SG. J Mater Res 2003;18:216–21. Hakamada M, Mabuchi M. J Alloys Compd 2009;479:326–9. Min US, Li JCM. J Mater Res 1994;11:2878–83. Chen LY, Yu JS, Fujita T, Chen M. Adv Funct Mater 2009;19:1221–6. Smith AJ, Tran T, Wainwright MS. J Appl Electrochem 1999;29:1085–94. Hakamada M, Mabuchi M. J Alloy Compd 2009;485:583–7. Yu J, Ding Y, Xu C, Inoue A, Sakurai T, Chen M. Chem Mater 2008;20:4548–50. Lang XY, Chen LY, Guan PF, Fujita T, Chen MW. Appl Phys Lett 2009;94:213109. Lang XY, Guo H, Chen LY, Kudo A, Yu JS, Zhang W, et al. J Phys Chem C 2010;114: 2600–3. Pourbaix M. Atlas of Electrochemical Equilibria in Aqueous Solutions (translated from the French by J. A. Franklin). Houston: National Association of Corrosion Engineers; 1974. Abe H, Sato K, Nishikawa H, Takemoto T, Fukuhara M, Inoue A. Mater Trans 2009;50:1255–8. Okamoto H. A Desk Handbook: Phase Diagrams for Binary Alloys. Ohio: ASM International; 2000. Giannuzzi LA, Stevie FA. Introduction to Focused Ion Beams, Instrumentation, Theory, Techniques and Practice. Berlin: Springer; 2005. Takeuchi A, Inoue A. Mater Trans 2005;46:2817–29. Erlebacher J, Aziz MJ, Karma A, Dimitrov N, Sieradzki K. Nature 2001;410:450–3. Qian LH, Chen MW. Appl Phys Lett 2007;91:083105.