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Growth and characterization of free-standing single crystalline tin and tin oxide nanobelts
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Materials Letters 62 (2008) 1969 – 1972 www.elsevier.com/locate/matlet
Growth and characterization of free-standing single crystalline tin and tin oxide nanobelts Xinsheng Peng, Guosheng Wu, Peter Holt-Hindle, Aicheng Chen⁎ Department of Chemistry, Lakehead University, Thunder Bay, Ontario, Canada P7B 5E1 Received 28 September 2007; accepted 29 October 2007 Available online 1 November 2007
Abstract Free-standing single crystalline tin nanobelts have been synthesized from tin foil by a novel selective etching process in NaOH solutions under hydrothermal conditions at low temperatures. The process requires neither catalysts nor templates to yield single crystalline tin nanobelts which are oriented and aligned on tin substrate. The crystal structure and morphology of these nanobelts were characterized by X-ray diffraction, scanning electron microscopy and transmission electron microscopy, showing that oriented, single crystalline tin nanobelts were formed and grew along [101] crystal direction. These single crystalline tin nanobelts were easily converted into single crystalline tin oxide nanobelts by a shapepreserving oxidation process. © 2007 Elsevier B.V. All rights reserved. Keywords: Sn; SnO2 nanobelts; Hydrothermal; Nanomaterials; Semiconductors
1. Introduction One- or quasi-one-dimensional metal nanostructures have been attracting a great deal of attention due to their potential as interconnectors in nanodevices and their promising applications in photoluminescence and electron emitters [1,2]. To understand the fundamental physical and chemical properties of onedimensional metal systems, the synthesis of 1D metal nanostructures is very important. Various physical and chemical methods have been proposed for the fabrication of quasi-onedimensional metal nanostructures [3–7]. Most of these methods, however, require removal of templates to produce free-standing metal nanowires with polycrystalline structure. 1D single crystal Sn nanostructures with diameters smaller than 200 nm are ideal for investigating the size confinement effect as bulk Sn has the relatively long coherence length of ξ(0) ∼ 200 nm. However, there are few methods developed for synthesizing single crystal tin nanowires except from the critical template electrochemical deposition process [8]. Only sparse
⁎ Corresponding author. Tel.: +1 807 345 4615; fax: +1 807 3467775. E-mail address: [email protected] (A. Chen). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.10.054
(104 per cm2) Sn whiskers of a few micrometers in diameter and a few millimeters in length are produced from electrodeposited tin [9]. To the best of our knowledge, there is no report of the synthesis of single crystal tin nanobelts. Here we report on a seedless and templateless growth of single crystalline tin nanobelts by a novel selective etching process from tin foil under hydrothermal conditions at low temperatures. Being a smart semiconductor, SnO2 has attracted much attention due to its broad applications in many fields, for instance, surface coatings, selective gas sensors and lithium secondary batteries [10–13]. Our study shows that these Sn nanobelts can be easily converted into SnO2 nanobelts by a shape-preserving oxidation process [14]. 2. Experimental The experimental process for the synthesis of Sn nanobelts is as follows. Briefly, after being rinsed with acetone and nanopure water (18.2 MΩ cm), a piece of high pure tin foil (Aldrich, 20 × 20 × 0.33 mm3) was put into an autoclave containing 30 mL of NaOH solution. Several samples were synthesized in four different NaOH concentrations: 0.5 M, 2.5 M, 5 M and 10 M at 150 °C for 10 h. After cooling to room temperature, the samples were washed with pure water several times and dried in a
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Fig. 1. (a) SEM image of the sample synthesized in 5.0 M NaOH solution at 150 °C for 10 h; XRD patterns of (b) tin foil; (c) the sample as shown in (a); and (d) the sample after ultrasonic treatment.
Fig. 2. SEM images of the tin foil after hydrothermal treatment at 150 °C for 10 h in various NaOH solutions: (a) 0.0 M; (b) 0.5 M; (c) 2.5 M; (d) & (e) 5.0 M; and (f) 10.0 M.
X. Peng et al. / Materials Letters 62 (2008) 1969–1972
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shown in Fig. 1d. The very strong peak at (112) in Fig. 1d shows that the surface is composed of (112) oriented crystal plane after the ultrasonic treatment. All these results indicate that the nanobelts were formed along b101N direction on the (112) orientated crystal plane.
Fig. 3. (a) and (b) TEM images of tin nanobelts synthesized in 5 M NaOH solution; and (c) the corresponding HRTEM image and SAED (the inset).
vacuum oven at 40 °C for 2 h. The thermal oxidation process approximating rheotaxial growth and thermal oxidation (RGTO) as we have described before was used to convert the Sn nanobelts to SnO2 nanobelts [14]. The synthesized Sn and SnO2 nanobelts were characterized by X-ray diffraction (XRD) (Philips PW 1050-3710 Diffractometer with Cu Kα radiation), scanning electron microscopy (SEM) (JEOL JSM 5900LV), transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) (JEOL 2010F). For SEM observation, the samples were directly pasted on an Al substrate using carbon conducting paste. Specimens for the TEM and HRTEM investigation were prepared by ultra-sonicating the sample in ethanol for 20 min, then a droplet of the suspension was placed on a holey copper grid covered with porous carbon film. 3. Results and discussion Fig. 1a presents a typical SEM image of the Sn foil after being hydrothermally treated in a 5.0 M NaOH solution at 150 °C for 10 h. Dense tin nanobelts are uniformly formed on the Sn foil. Some nanobelts bunch together due to their long length. The nanobelts can be detached from the tin substrate by ultrasonic treatment in ethanol. Fig. 1b–d presents the XRD patterns of a tin foil before the hydrothermal treatment (b) and the samples synthesized in 5.0 M NaOH under hydrothermal treatment at 150 °C for 10 h before (c) and after the ultrasonic treatment (d). As shown in Fig. 1b, the tin substrate is composed of β-phase polycrystalline tin grains without any preferred orientation. There are only two broad peaks (101) and (112) shown in Fig. 1c, indicating that the nanobelts are oriented with (101) and (112) plane. To further investigate orientation, the sample shown in Fig. 1a was ultra-sonicated in ethanol, and its XRD pattern was recorded as
Fig. 4. (a) SEM image of the SnO2 nanobelts; (b) and (c) the TEM and HRTEM images of SnO2 nanobelts. The insets in (b) and (c) are the EDS and SAED.
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The lower intensity in Fig. 1c as compared to Fig. 1d is due to the Sn nanobelt layer shadowing and scattering. In order to determine whether the tin nanobelts formed by outward growth starting from the tin surface (e.g., by diffusion or recrystallization in an epitaxial manner onto the underlying tin grains) or by the inward growth of cavities (e.g., by selective NaOH etching of the tin along preferred crystallographic directions), we further studied the effect of NaOH concentration on the formation of the tin nanostructures. Fig. 2 presents the SEM images of the tin foil substrates after hydrothermal treatment at 150 °C for 10 h in various NaOH concentrations. As shown in Fig. 2a, there is no fiber or whisker formed on the tin surface in the absence of NaOH. Fig. 2b shows that only some cavities but without any fiber-like structures or favorably orientated crystal surfaces were formed in a 0.5 M NaOH solution. Increasing the NaOH concentration to 2.5 M, sparse, short (about 6 μm in length) and thick tin belt (300–500 nm in thickness) arrays were formed on textured orientated crystal surfaces (Fig. 2c). When increasing the NaOH concentration to 5.0 M, as shown in Fig. 2d and e, thinner (40 nm in thickness) and longer (10 μm in length) tin nanobelts with high density were formed. Fig. 2f reveals that tin fibers with sizes ranging from tens to hundreds of nanometers, even to several micrometers, were formed in 10.0 M NaOH. The above study indicates that the tin nanobelts result from a selective etching process rather than from a deposition and recrystallization process and that the etching rate increases with the increment of NaOH concentration from 0.5 M to 10.0 M. Fig. 3a is a low magnification TEM image of several tin nanobelts taken from the sample synthesized in 5.0 M NaOH. Fig. 3b, a higher magnification TEM image of an individual tin nanobelt, shows that this nanobelt is uniform and straight with a rectangular cross section (40 nm thick, 120 nm wide). Fig. 3c presents the corresponding HRTEM image and selected area electron diffraction (the inset in Fig. 3c) of the nanobelt marked by a square in Fig. 3b, indicating that the tin nanobelts are defect-free single crystal and grow in the [101] orientation with a highly oriented surface (101). Further EDS spectra (not shown) recorded from the circled parts marked in Fig 3b demonstrate that the nanobelt is composed of tin without detectable oxygen. The single crystal nature of tin nanobelts, the common crystallographic orientation of the nanobelts axes, and the alignment of the nanobelts formed from a given tin foil are all consistent with a mechanism based on the selective (crystallographic) etching of β-tin. Although the kinetic mechanism for this crystallographic etching is unclear, it is well known that the (101) or (011) face and equivalents are the closest-packed, thermodynamically stable faces [15]. The etching rate of such closest-packed surfaces would be slower than other looser packed crystal surfaces. Our study further demonstrates that the Sn nanobelts can be converted into SnO2 nanobelts through a shape-preserving oxidation process. The morphology of SnO2 nanobelts observed in Fig. 4a is similar to that of the Sn nanobelts seen in Fig. 1a. The corresponding TEM and HRTEM, SAED and EDS shown in Fig. 4b and c were recorded from these oxide nanobelts. The oxide nanobelts were detached by ultrasonic treatment and supported on a copper grid covered with a carbon film. The TEM observation shown in Fig. 4b demonstrates that the oxidized products possess a belt-like structure. The SAED pattern and HTEM image presented in Fig. 4c reveal that the SnO2 nanobelt is a single crystal without any defects. The SAED pattern recorded perpendicular to the nanobelt long axis can be indexed for the b10–2N zone axis of single crystalline cassiterite SnO2,
indicating that the nanobelt grows along the b101N direction. Fig. 4c also shows a lattice-resolved image of the nanobelt with interspacing of 0.471 nm, in agreement with the d value of the (010) plane of the cassiterite SnO2 crystal. EDS analysis (the inset in Fig. 4c) reveals that the nanobelt is composed of Sn 32.4 at.% and O 67.6 at.%, close to the ratio of 1:2 of the bulk SnO2. All these results indicate that all the single crystal β-tin nanobelts were completely converted into single crystal cassiterite phase SnO2 nanobelts.
4. Conclusions In summary, free-standing single crystalline tin nanobelts have been formed on tin foil surfaces along the [101] crystal direction by selective etching tin foil with respect to the crystallography of β-tin in NaOH solutions under hydrothermal conditions. The etching rate and the morphology of the yielded tin nanostructures strongly depend on the temperature, etching time and the NaOH concentration. The approach described in this study can be readily scaled up to fabricate large quantities of single crystal tin or other metal nanostructures. Furthermore, these tin nanobelts were converted into tin oxide nanobelts through a simple shape-preserving oxidation process. Acknowledgements This work was supported by a Discovery Grant from the Natural Sciences and Engineering Research Council of Canada (NSERC). A. Chen acknowledges NSERC and the Canada Foundation of Innovation (CFI) for the Canada Research Chair Award in Material and Environmental Chemistry. References [1] H.W. Seo, C.S. Han, S.O. Hwang, J. Park, Nanotechnology 17 (2006) 3388–3393. [2] S.H. Luo, J.Y. Fan, W.L. Liu, M. Zhang, Z.T. Song, C.L. Lin, et al., Nanotechnology 17 (2006) 1695–1699. [3] A. Kolmakov, Y.X. Zhang, M. Moskovits, Nano Lett. 3 (2003) 1125–1129. [4] Y.T. Tian, G.W. Meng, S. Biswas, P.M. Ajayan, S.H. Sun, L.D. Zhang, Appl. Phys. Lett. 85 (2004) 967–969. [5] M. Nishizava, V.P. Menon, C.R. Martin, Science 268 (1995) 700–702. [6] X. Jiang, B. Mayers, Y. Wang, B. Cattle, Y.N. Xia, Chem. Phys. Lett. 385 (2004) 472–476. [7] M. Zach, K. Ng, R. Penner, Science 290 (2000) 2120–2123. [8] M.L. Tian, J.G. Wang, J. Snyder, J. Kurtz, Y. Liu, P. Schiffer, et al., Appl. Phys. Lett. 83 (2003) 1620–1622. [9] K.N. Tu, Phys. Rev., B 49 (1994) 2030–2034. [10] C.S. Rout, M. Hegde, A. Govindaraj, C.N.R. Rao, Nanotechnology 18 (2007) 205504–205513. [11] E. Comini, V. Guidi, C. Malagù, G. Martinelli, Z. Pan, G. Sberveglieri, Z.L. Wang, J. Phys. Chem., B 108 (2004) 1882–1887. [12] K. Dutta, S.K. De, Mater. Lett. 61 (2007) 4967–4971. [13] Y. Liang, J. Fan, X.H. Xia, Z.J. Jia, Mater. Lett. 61 (2007) 4370–4373. [14] A. Chen, X.P. Peng, K. Koczkur, B. Miller, Chem. Commun. (2004) 1964–1965. [15] S. Nakanishi, K. Fukami, T. Tada, Y. Nakato, J. Am. Chem. Soc. 126 (2004) 9556–9557.
Materials Letters 62 (2008) 1969 – 1972 www.elsevier.com/locate/matlet
Growth and characterization of free-standing single crystalline tin and tin oxide nanobelts Xinsheng Peng, Guosheng Wu, Peter Holt-Hindle, Aicheng Chen⁎ Department of Chemistry, Lakehead University, Thunder Bay, Ontario, Canada P7B 5E1 Received 28 September 2007; accepted 29 October 2007 Available online 1 November 2007
Abstract Free-standing single crystalline tin nanobelts have been synthesized from tin foil by a novel selective etching process in NaOH solutions under hydrothermal conditions at low temperatures. The process requires neither catalysts nor templates to yield single crystalline tin nanobelts which are oriented and aligned on tin substrate. The crystal structure and morphology of these nanobelts were characterized by X-ray diffraction, scanning electron microscopy and transmission electron microscopy, showing that oriented, single crystalline tin nanobelts were formed and grew along [101] crystal direction. These single crystalline tin nanobelts were easily converted into single crystalline tin oxide nanobelts by a shapepreserving oxidation process. © 2007 Elsevier B.V. All rights reserved. Keywords: Sn; SnO2 nanobelts; Hydrothermal; Nanomaterials; Semiconductors
1. Introduction One- or quasi-one-dimensional metal nanostructures have been attracting a great deal of attention due to their potential as interconnectors in nanodevices and their promising applications in photoluminescence and electron emitters [1,2]. To understand the fundamental physical and chemical properties of onedimensional metal systems, the synthesis of 1D metal nanostructures is very important. Various physical and chemical methods have been proposed for the fabrication of quasi-onedimensional metal nanostructures [3–7]. Most of these methods, however, require removal of templates to produce free-standing metal nanowires with polycrystalline structure. 1D single crystal Sn nanostructures with diameters smaller than 200 nm are ideal for investigating the size confinement effect as bulk Sn has the relatively long coherence length of ξ(0) ∼ 200 nm. However, there are few methods developed for synthesizing single crystal tin nanowires except from the critical template electrochemical deposition process [8]. Only sparse
⁎ Corresponding author. Tel.: +1 807 345 4615; fax: +1 807 3467775. E-mail address: [email protected] (A. Chen). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.10.054
(104 per cm2) Sn whiskers of a few micrometers in diameter and a few millimeters in length are produced from electrodeposited tin [9]. To the best of our knowledge, there is no report of the synthesis of single crystal tin nanobelts. Here we report on a seedless and templateless growth of single crystalline tin nanobelts by a novel selective etching process from tin foil under hydrothermal conditions at low temperatures. Being a smart semiconductor, SnO2 has attracted much attention due to its broad applications in many fields, for instance, surface coatings, selective gas sensors and lithium secondary batteries [10–13]. Our study shows that these Sn nanobelts can be easily converted into SnO2 nanobelts by a shape-preserving oxidation process [14]. 2. Experimental The experimental process for the synthesis of Sn nanobelts is as follows. Briefly, after being rinsed with acetone and nanopure water (18.2 MΩ cm), a piece of high pure tin foil (Aldrich, 20 × 20 × 0.33 mm3) was put into an autoclave containing 30 mL of NaOH solution. Several samples were synthesized in four different NaOH concentrations: 0.5 M, 2.5 M, 5 M and 10 M at 150 °C for 10 h. After cooling to room temperature, the samples were washed with pure water several times and dried in a
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Fig. 1. (a) SEM image of the sample synthesized in 5.0 M NaOH solution at 150 °C for 10 h; XRD patterns of (b) tin foil; (c) the sample as shown in (a); and (d) the sample after ultrasonic treatment.
Fig. 2. SEM images of the tin foil after hydrothermal treatment at 150 °C for 10 h in various NaOH solutions: (a) 0.0 M; (b) 0.5 M; (c) 2.5 M; (d) & (e) 5.0 M; and (f) 10.0 M.
X. Peng et al. / Materials Letters 62 (2008) 1969–1972
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shown in Fig. 1d. The very strong peak at (112) in Fig. 1d shows that the surface is composed of (112) oriented crystal plane after the ultrasonic treatment. All these results indicate that the nanobelts were formed along b101N direction on the (112) orientated crystal plane.
Fig. 3. (a) and (b) TEM images of tin nanobelts synthesized in 5 M NaOH solution; and (c) the corresponding HRTEM image and SAED (the inset).
vacuum oven at 40 °C for 2 h. The thermal oxidation process approximating rheotaxial growth and thermal oxidation (RGTO) as we have described before was used to convert the Sn nanobelts to SnO2 nanobelts [14]. The synthesized Sn and SnO2 nanobelts were characterized by X-ray diffraction (XRD) (Philips PW 1050-3710 Diffractometer with Cu Kα radiation), scanning electron microscopy (SEM) (JEOL JSM 5900LV), transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) (JEOL 2010F). For SEM observation, the samples were directly pasted on an Al substrate using carbon conducting paste. Specimens for the TEM and HRTEM investigation were prepared by ultra-sonicating the sample in ethanol for 20 min, then a droplet of the suspension was placed on a holey copper grid covered with porous carbon film. 3. Results and discussion Fig. 1a presents a typical SEM image of the Sn foil after being hydrothermally treated in a 5.0 M NaOH solution at 150 °C for 10 h. Dense tin nanobelts are uniformly formed on the Sn foil. Some nanobelts bunch together due to their long length. The nanobelts can be detached from the tin substrate by ultrasonic treatment in ethanol. Fig. 1b–d presents the XRD patterns of a tin foil before the hydrothermal treatment (b) and the samples synthesized in 5.0 M NaOH under hydrothermal treatment at 150 °C for 10 h before (c) and after the ultrasonic treatment (d). As shown in Fig. 1b, the tin substrate is composed of β-phase polycrystalline tin grains without any preferred orientation. There are only two broad peaks (101) and (112) shown in Fig. 1c, indicating that the nanobelts are oriented with (101) and (112) plane. To further investigate orientation, the sample shown in Fig. 1a was ultra-sonicated in ethanol, and its XRD pattern was recorded as
Fig. 4. (a) SEM image of the SnO2 nanobelts; (b) and (c) the TEM and HRTEM images of SnO2 nanobelts. The insets in (b) and (c) are the EDS and SAED.
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The lower intensity in Fig. 1c as compared to Fig. 1d is due to the Sn nanobelt layer shadowing and scattering. In order to determine whether the tin nanobelts formed by outward growth starting from the tin surface (e.g., by diffusion or recrystallization in an epitaxial manner onto the underlying tin grains) or by the inward growth of cavities (e.g., by selective NaOH etching of the tin along preferred crystallographic directions), we further studied the effect of NaOH concentration on the formation of the tin nanostructures. Fig. 2 presents the SEM images of the tin foil substrates after hydrothermal treatment at 150 °C for 10 h in various NaOH concentrations. As shown in Fig. 2a, there is no fiber or whisker formed on the tin surface in the absence of NaOH. Fig. 2b shows that only some cavities but without any fiber-like structures or favorably orientated crystal surfaces were formed in a 0.5 M NaOH solution. Increasing the NaOH concentration to 2.5 M, sparse, short (about 6 μm in length) and thick tin belt (300–500 nm in thickness) arrays were formed on textured orientated crystal surfaces (Fig. 2c). When increasing the NaOH concentration to 5.0 M, as shown in Fig. 2d and e, thinner (40 nm in thickness) and longer (10 μm in length) tin nanobelts with high density were formed. Fig. 2f reveals that tin fibers with sizes ranging from tens to hundreds of nanometers, even to several micrometers, were formed in 10.0 M NaOH. The above study indicates that the tin nanobelts result from a selective etching process rather than from a deposition and recrystallization process and that the etching rate increases with the increment of NaOH concentration from 0.5 M to 10.0 M. Fig. 3a is a low magnification TEM image of several tin nanobelts taken from the sample synthesized in 5.0 M NaOH. Fig. 3b, a higher magnification TEM image of an individual tin nanobelt, shows that this nanobelt is uniform and straight with a rectangular cross section (40 nm thick, 120 nm wide). Fig. 3c presents the corresponding HRTEM image and selected area electron diffraction (the inset in Fig. 3c) of the nanobelt marked by a square in Fig. 3b, indicating that the tin nanobelts are defect-free single crystal and grow in the [101] orientation with a highly oriented surface (101). Further EDS spectra (not shown) recorded from the circled parts marked in Fig 3b demonstrate that the nanobelt is composed of tin without detectable oxygen. The single crystal nature of tin nanobelts, the common crystallographic orientation of the nanobelts axes, and the alignment of the nanobelts formed from a given tin foil are all consistent with a mechanism based on the selective (crystallographic) etching of β-tin. Although the kinetic mechanism for this crystallographic etching is unclear, it is well known that the (101) or (011) face and equivalents are the closest-packed, thermodynamically stable faces [15]. The etching rate of such closest-packed surfaces would be slower than other looser packed crystal surfaces. Our study further demonstrates that the Sn nanobelts can be converted into SnO2 nanobelts through a shape-preserving oxidation process. The morphology of SnO2 nanobelts observed in Fig. 4a is similar to that of the Sn nanobelts seen in Fig. 1a. The corresponding TEM and HRTEM, SAED and EDS shown in Fig. 4b and c were recorded from these oxide nanobelts. The oxide nanobelts were detached by ultrasonic treatment and supported on a copper grid covered with a carbon film. The TEM observation shown in Fig. 4b demonstrates that the oxidized products possess a belt-like structure. The SAED pattern and HTEM image presented in Fig. 4c reveal that the SnO2 nanobelt is a single crystal without any defects. The SAED pattern recorded perpendicular to the nanobelt long axis can be indexed for the b10–2N zone axis of single crystalline cassiterite SnO2,
indicating that the nanobelt grows along the b101N direction. Fig. 4c also shows a lattice-resolved image of the nanobelt with interspacing of 0.471 nm, in agreement with the d value of the (010) plane of the cassiterite SnO2 crystal. EDS analysis (the inset in Fig. 4c) reveals that the nanobelt is composed of Sn 32.4 at.% and O 67.6 at.%, close to the ratio of 1:2 of the bulk SnO2. All these results indicate that all the single crystal β-tin nanobelts were completely converted into single crystal cassiterite phase SnO2 nanobelts.
4. Conclusions In summary, free-standing single crystalline tin nanobelts have been formed on tin foil surfaces along the [101] crystal direction by selective etching tin foil with respect to the crystallography of β-tin in NaOH solutions under hydrothermal conditions. The etching rate and the morphology of the yielded tin nanostructures strongly depend on the temperature, etching time and the NaOH concentration. The approach described in this study can be readily scaled up to fabricate large quantities of single crystal tin or other metal nanostructures. Furthermore, these tin nanobelts were converted into tin oxide nanobelts through a simple shape-preserving oxidation process. Acknowledgements This work was supported by a Discovery Grant from the Natural Sciences and Engineering Research Council of Canada (NSERC). A. Chen acknowledges NSERC and the Canada Foundation of Innovation (CFI) for the Canada Research Chair Award in Material and Environmental Chemistry. References [1] H.W. Seo, C.S. Han, S.O. Hwang, J. Park, Nanotechnology 17 (2006) 3388–3393. [2] S.H. Luo, J.Y. Fan, W.L. Liu, M. Zhang, Z.T. Song, C.L. Lin, et al., Nanotechnology 17 (2006) 1695–1699. [3] A. Kolmakov, Y.X. Zhang, M. Moskovits, Nano Lett. 3 (2003) 1125–1129. [4] Y.T. Tian, G.W. Meng, S. Biswas, P.M. Ajayan, S.H. Sun, L.D. Zhang, Appl. Phys. Lett. 85 (2004) 967–969. [5] M. Nishizava, V.P. Menon, C.R. Martin, Science 268 (1995) 700–702. [6] X. Jiang, B. Mayers, Y. Wang, B. Cattle, Y.N. Xia, Chem. Phys. Lett. 385 (2004) 472–476. [7] M. Zach, K. Ng, R. Penner, Science 290 (2000) 2120–2123. [8] M.L. Tian, J.G. Wang, J. Snyder, J. Kurtz, Y. Liu, P. Schiffer, et al., Appl. Phys. Lett. 83 (2003) 1620–1622. [9] K.N. Tu, Phys. Rev., B 49 (1994) 2030–2034. [10] C.S. Rout, M. Hegde, A. Govindaraj, C.N.R. Rao, Nanotechnology 18 (2007) 205504–205513. [11] E. Comini, V. Guidi, C. Malagù, G. Martinelli, Z. Pan, G. Sberveglieri, Z.L. Wang, J. Phys. Chem., B 108 (2004) 1882–1887. [12] K. Dutta, S.K. De, Mater. Lett. 61 (2007) 4967–4971. [13] Y. Liang, J. Fan, X.H. Xia, Z.J. Jia, Mater. Lett. 61 (2007) 4370–4373. [14] A. Chen, X.P. Peng, K. Koczkur, B. Miller, Chem. Commun. (2004) 1964–1965. [15] S. Nakanishi, K. Fukami, T. Tada, Y. Nakato, J. Am. Chem. Soc. 126 (2004) 9556–9557.