Synthesis of tin nanodendrites via galvanic replacement reaction and their thermal conversion to nanodendritic tin oxide for ultrasensitive electrochemical sensing

Abstract

Tin nanodendrites have been fabricated using galvanic replacement reaction between stannous chloride (SnCl₂) and thermal evaporated zinc film composed of Zn nanodiscs. The replacement reaction is completed in a few seconds, and the produced Sn nanodendrites possess a single crystalline structure. The growth process of Sn nanodendrites is studied by changing the concentration of SnCl₂ in the replacement reaction, and the growth direction is identified by electron diffraction analysis. The as-prepared Sn nanodendrites are further oxidized by rheotaxial growth and thermal oxidation (RGTO) process to generate tin oxide nanodendrites. The electrochemical properties of SnO₂ nanodendrites are demonstrated in the electrocatalytic reaction of hydrogen peroxide (H₂O₂), and the reduction of H₂O₂ is tremendously enhanced in terms of the onset potential and the magnitude of reduction current. The amperometric H₂O₂ sensor, using the as-prepared SnO₂ nanodendrites as the electrochemical catalyst, shows a wide linear range, a good limit of detection, a high sensitivity, and good reproducibility. This study provides a promising route to the facile synthesis of functional metals/metal oxides with unusual nanostructures, which have great potential in sensing applications.

---

1. Introduction

The synthesis of nanostructured metals and metal oxides has been receiving intense interest because of their unique and/or superior catalytic, electronic, photonic, and magnetic properties.^1,2 In the past decade, nanomaterials with various shapes (e.g.nanowires, nanotubes, nanodiscs, etc.) have been successfully synthesized, and their chemical and physical properties are found to be strongly dependent on the size and shape of nanomaterials. Additionally, nanomaterials with several new geometric configurations have also been reported, such as nanohelixes,³ zigzag nanobelts,⁴ and more complicated 3D structures. Up to date, a number of techniques have been developed for preparing nanostructured metals/metal oxides with controllable size and shape. For instance, the chemical reduction of a metal precursor by a reducing agent in the solution phase, with or without the aid of a surfactant and/or a stabilizer, is extensively applied to control the morphology of the nanostructured metals;^5,6 electrochemical reduction of metal precursors is another widely used method in this regard. The synthesis of metal oxide nanostructures can be obtained through the thermal evaporation of corresponding metal sources with an oxygen supply.⁷

Recently, a facile method, galvanic replacement, has been used as an amazingly simple and effective alternative to prepare noble metal or metal alloy nanostructures based on the electrical potential difference between two metals.⁸ Generally, metal nanostructures with higher reactivity are synthesized first, and serve as sacrificial templates. Metals with lower reactivity are then replaced from their salt precursors by the active metal templates. The degree of the replacement reaction depends on the size of metal templates, the amount of metal salts, and reaction time.^9,10 For instance, silver (Ag) nanoparticles with a 13 nm diameter can be completely transformed into gold or palladium if the precursors of gold (Au) or palladium (Pd) are sufficient, otherwise alloys are formed.^11,12Ag nanowires were used to displace Pt or PtPd to form the corresponding nanotubes, which demonstrated good catalytic activity towards oxygen reduction reactions.¹³Pb nanowires were obtained by the replacement reaction of Pb ions on zinc foils.¹⁴ Furthermore, it has been found that thiourea can decrease the standard potentials of metals, which has made the replacement process of copper by tin possible.¹⁵ However, it is unclear whether the replacement reaction with the involvement of highly active metals could form novel nanostructures.

In this paper, we address these queries and report on the synthesis of Sn nanodendrites by employing zinc nanodiscs for the replacement reaction. Zn can be spontaneously replaced by Sn in a normal aqueous solution according to their standard potentials:

Sn²⁺ + 2 e⁻ = Sn E⁰ = −0.140 V (1)

Zn²⁺ + 2 e⁻ = Zn E⁰ = −0.7618 V (2)

The as-prepared Sn nanodendrites are characterized and the growth process is proposed. Furthermore, nanodendritic Sn can be converted to nanodentritic tin oxide by thermal treatment. As tin oxide is an n-type semiconductor with a wide band gap energy (3.6 eV), it has a higher conductivity compared to those of other common metal oxides such as TiO₂ and SiO₂, and possesses good electrocatalytic properties. It has been widely used in electrochemical sensors.^16–19 Therefore, a further application of the as-prepared SnO₂ nanodentrites for enhanced electrochemical reduction of hydrogen peroxide as well as its sensitive detection was also demonstrated. These results indicate that the novel nanodendritic tin and tin oxide may have promising applications in electrochemical applications.

2. Experimental

Synthesis of Zn films and Sn nanodendrites: A thin layer of zinc was coated onto a glass substrate using a thermal evaporator (NTE-3000) of zinc (99.993%, Alfa Aesar) at the vacuum level of 10⁻⁶ torr. The deposition rate was controlled at 1 Å per second and the final thickness of the zinc layer was 65 nm. The as-prepared metallic films were directly used for the replacement reaction. Stannous chloride (SnCl₂, Fisher) was dissolved in water to prepare aqueous solution right before use. Glass substrate with Zn film was immersed into the SnCl₂ solution for several seconds to complete the replacement reaction. The substrates were then carefully rinsed with DI water.

Thermal conversion of Sn nanodentrites to tin oxide nanodendrites: To oxidize metallic films, the substrates were heated in a muffle furnace, or a tube furnace with an oxygen supply. The heating process follows a two-step procedure: 2 h at 200 °C and then 4 h at 700 °C.

Material characterization: A JEOL 6335F field emission scanning electron microscope (FESEM) was employed to examine the morphology of the as-prepared samples. The crystal structures of the samples were characterized by Oxford diffraction XcaliburTM PX Ultra with ONYX detector. A Tecnai T12 TEM was used for morphological imaging. HRTEM, selected-area electron diffraction (SAED) and micro-area electron diffraction were carried out with a JEOL 2010 FasTEM.

Detection of hydrogen peroxide by tin oxide nanodendrites: A glassy carbon electrode (GCE, dia. 3 mm) was first polished with alumina slurries (1 μm and 0.05 μm) and then cleaned by sonication in ethanol, acetone, and water successively. The SnO₂ nanodendrites were carefully scratched off from substrate and suspended in ethanol. The concentration of SnO₂ nanodendrites was determined using quartz crystal microbalance. 5 μL of SnO₂ nanodentrite suspension was then dropped onto GCE. After ethanol was completely evaporated, 5 μL of 0.5 wt% Nafion was used to entrap the nanodendrites. The electrodes are denoted as SnO₂/Nafion/GCE. The Nafion coated GCE (Nafion/GCE) was also prepared as a control electrode. All electrochemical experiments were performed using a standard three-electrode configuration consisting of a working electrode, an Ag/AgCl reference electrode, and a platinum disc counter electrode. The cyclic voltammetry (CV) was performed in a 0.1 M phosphate buffer (pH 7.4) in the absence and presence of 2 mM H₂O₂ utilizing an electrochemical workstation (CHI-601) at a scan rate of 50 mV/s. For amperometric detection, all measurements were performed by applying an appropriate potential (vs.Ag/AgCl reference) to the working electrode and allowing the transient background current to decay to a steady-state value, prior to the addition of H₂O₂. The current response resulting from the addition of H₂O₂ was recorded. A stirred solution was employed to provide convective transport.

3. Results and discussion

3.1. Tin nanodendrites

Due to its low standard potential, Zn was chosen as the more active metal in the replacement reaction. Zn film was prepared by vacuum thermal evaporation due to its low melting point. A typical SEM image of Zn film is shown in Fig. 1A. It can be seen that the zinc film is composed of well-faceted hexagonal Zn nanodiscs with a diameter of 650 nm. A higher magnification SEM image of a twinned Zn nanodisc is shown in Fig. 1B. It can be found that the Zn nanodisc has a smooth surface, and is packed with several layers of Zn platelets with a typical thickness around tens of nanometres. This result is consistent with the observation from Guo, et al.²⁰Fig. 1C shows a typical TEM image of two overlaid Zn nanodiscs, which clearly shows the layered structure of Zn nanodiscs from the edge contrast. The SAED pattern (Fig. 1D) indicates that the Zn nanodisc is single-crystalline dominated by (0001) crystal facets, which is the most stable facet intrinsically.

Fig. 1 A typical FESEM image of Zn nanodiscs; B. A high magnification side-view of a twinned Zn nanodisc; C. A TEM image of Zn nanodiscs; D. The SAED pattern of Zn nanodiscs.

Zn is an active material in the activity series of metals, therefore it can displace many metals which are lower in the series. For example, tin, which is slightly less active than Zn, can be replaced by Zn. In this study, stannous chloride (SnCl₂) is used as the source of tin ions. When the replacement reaction occurred, the color of the Zn film immediately changed from silvery gray to a darker grey in a few seconds, indicating the formation of Sn. XRD was carried out to reveal the crystal structure of the replacement product. Comparing XRD pattern a with b in Fig. 2, it was found that all the peaks after the replacement reaction can be assigned to tetragonal Sn (JCPDS 4-673), while no peak correlated with Zn (JCPDS 4-831) was observed, indicating that this displacement process was completed within several seconds.

Fig. 2 XRD patterns of Zn nanodiscs (a) and Sn nanodendrites (b).

The SEM image in Fig. 3A shows the typical morphology of the as-synthesized Sn. Interestingly, the products display a dendrite-like structure. The main stem of the dendrite is typically 350–450 nm in diameter, and the branches are about 150–250 nm, which grow almost perpendicular to the main stem. The Sn rods of both main stem and branches have a faceted and sectional structure (Fig. 3A inset). Fig. 3B shows a TEM image of a Sn nanodendrite and the corresponding SAED pattern from the edge of a branch, which reveals that the architectures are solid and the dendrite Sn is single-crystalline. Further investigation was performed by micro-area electron diffraction analysis in TEM to find out the growth direction. Analysis was conducted on both sides of branches and the main stem, and the patterns obtained were the same for all the selected regions (Fig. S1), indicating the single crystalline dendrite with growth direction along [220]. By controlling reaction conditions, no tin nanodiscs or nanodiscs with a hollow interior could be formed. One possible reason is that during the replacement reaction, Zn is replaced by Sn and Sn grows along its own preferential direction.

Fig. 3 A. FESEM image of Sn nanodentrites. The inset is a high magnification SEM image of the branches; B. TEM image of Sn nanodentrite. The inset is the SAED pattern.

A dendritic structure has been reported previously from the replacement reaction between Ag⁺ and Zn plates.²¹ Basically, the formation of dendrites is usually a diffusion control process, in which the particles are more easily captured by the tip parts. In our case, since the Zn nanodiscs are very thin, the longitudinal diffusion (perpendicular to the film surface) is negligible, while transverse walking (parallel to the surface) of Sn²⁺ should be dominant. In order to have a clearer view of the growth process, diluted SnCl₂ solutions were used to react with the Zn film, given that this replacement reaction is too fast (less than several seconds) to study the evolution of Sn nanodendrites. A series of SnCl₂ concentrations ranging from 0.01 M to 1 M were examined. When the concentration of Sn precursor was 0.01 M, no obvious geometry could be observed, and only nanoparticles with irregular morphology were generated (Fig. 4A). When the concentration of SnCl₂ increased to 0.1 M, the majority of the products had a faceted structure, while only few of them showed shorter dendritic structure (Fig. 4B and inset). With a further increase of SnCl₂ concentration to 0.5 M, dendritic Sn formed (Fig. 4C), but the amount was lower than those observed with 1 M SnCl₂ (Fig. 3A). In addition, it is clearly observed from Fig. 4C that some sub-branches also grew from the branches to form a more complicated structure. Theoretically, material growth has large consequences in regards to its own properties with a minimal surface energy or lower energy barrier, and the shape produced by faster growth along energetically favorable crystallographic directions.²² For this reaction, (220) type low-index surfaces of Sn are likely to have the lowest energy, and the high concentration of Sn²⁺ would possibly induce the anisotropic growth along [220] to form dendrites. Additionally, single-crystalline Zn may also direct the formation and growth of Sn dendrites.²³ As growth proceeds, branches are initiated with sufficient amount of Sn source from the stem to form a dendrite structure. Compared with other replacement reactions, the fast reaction rate is probably due to the reasons that both Sn and Zn are active metals and possess quite different standard potentials (driving force).¹³

Fig. 4 FESEM images of Sn nanostructures prepared through replacement reaction between Zn nanodiscs and SnCl₂ with different concentration: A. 10 mM SnCl₂. B. 100 mM SnCl₂. C. 500 mM SnCl₂; all scale bars are 1 μm.

3.2. Tin oxide nanodendrites

Thermal oxidation is one of the most commonly employed approaches to synthesize novel nanostructured metal oxides from metals.²⁴ Since tin dioxide is an important semiconductor with unique optical and electrical properties, there are a number of reports regarding the thermal oxidation of Sn to SnO₂. As Sn has a low melting temperature, the rheotaxial growth and thermal oxidation (RGTO) process was developed with the aim of retaining the morphology of the original Sn after its conversion, which has been widely used to prepare SnO₂ nanowire, nanodiscs or films.²⁵ In this study, the as-prepared nanodendritic Sn was first heated to 200 °C (lower than 232 °C, the melting temperature of bulk tin), and maintained at this temperature for 2 h to oxidize the surface layers as a supporting sheath to retain the structures, followed by heating up to 700 °C to completely oxidize the samples. XRD analysis was conducted to examine the oxidation process. As shown in Fig. 5, different phases of tin oxides are observed at different temperatures. With the increase of temperature, it was found that the film of Sn changed to brownish, and from XRD data, the extra peaks mainly from tetragonal SnO (JCPDS 06-0395, a = b = 0.3802 nm, c = 0.4836 nm) first appeared along those from Sn. With further increase of temperature, the reflections associated with SnO increased in intensity, while Sn diffraction peaks disappeared and a complete transformation of Sn to SnO was observed when maintaining the temperature at 350 °C (Fig. 5b). When the temperature went up to 500 °C, the sample became white color, demonstrating that another phase of oxides was formed, and SnO₂ was observed and coexisted with SnO in the XRD pattern. When the temperature reached 700 °C, the XRD study shows that the sample was completely oxidized to form SnO₂. However, surprisingly, two forms of SnO₂, tetragonal (JCPDS 29-1484) and orthorhombic (JCPDS 41-1445), were observed in the final products (Fig. 5c). To rule out the oxygen effect, thermal treatments were also conducted with either air or just pure O₂ flow throughout the whole oxidation process (Fig. 5d). Same patterns of the product were found, so that the difference of oxygen concentration and oxidation time may not be the crucial factors leading to the formation of two crystalline structures of SnO₂. The reasons are still unclear, but similar phenomena has been observed by other groups.²⁶

Fig. 5 XRD patterns for the oxidized products obtained at different temperature: Sn nanodendrites (a) were heated to 350 °C in air (b), 700 °C in air (c), and 700 °C in O₂ (d).

As shown in Fig. 6A, the products obtained after thermal treatment at 700 °C maintain the dendritic structures of Sn, but the surface became granular. The branches slightly deform probably due to the molten low-melting-point of Sn when the temperature is higher than 232 °C. TEM was further employed to study the oxidized samples. Since the dendrites adhered strongly to the substrate after thermal treatment, the samples were carefully scratched off from substrate to prepare the TEM samples, so that the structures may be destroyed to some extent (Fig. 6B). Moreover, granular structures were clearly observed on the surface of the SnO₂ nanodendrites. The thermal oxidation process may be described as follows: in an oxygen environment and at an elevated temperature lower than the melting temperature of Sn, the oxidation starts at the surface of the Sn particles, and SnO phases are nucleated and formed as dispersed clusters on the surface of Sn. SnO would decomposed into Sn₃O₄ and Sn during heating treatment when oxygen is controlled by diffusion control.²⁷ With further increase of temperature, the Sn₃O₄ decomposes into SnO₂ and Sn. The SnO₂ keeps on growing from the surface to the core while the Sn is continuously oxidized when oxygen is diffused into the inside of dendrites.²⁸ Finally, SnO₂ nanodentrites were formed with the same morphology of the Sn precursor.

Fig. 6 A typical SEM image (A) and a typical TEM image (B) of tin oxide nanodendrites; inset of Figure A is a high magnification SEM image of branches with a scale bar of 200 nm.

3.3. Tin oxide nanodendrites for electrochemical sensors

Tin and tin oxide are important materials with promising applications in many fields.^29,30 The nanodendrites generated by this method have highly branched structures and high surface area, which are suitable for catalysis. The electrocatalytic activity of the as-prepared SnO₂ architectures was thus evaluated by cyclic voltammogram (CV). H₂O₂ was selected as a model compound because it is a product of enzymatic reactions catalyzed by a large number of oxidases, and is practically important in the field of biosensor development. Fig. 7 displays the CVs in the absence and presence of 2 mM H₂O₂ as recorded with SnO₂/Nafion/GCE and a Nafion/GCE, respectively. The CV of the Nafion/GCE only shows a very small background current in buffer, while a dramatic increase of current signal towards the negative ends of the potential range was observed when the SnO₂/Nafion/GCE was used in buffer, which may be attributed to the large surface area and enhanced electron transfer of the as-synthesized SnO₂ nanodendrites. Upon the addition of 2 mM H₂O₂, one can see that no obvious redox response to H₂O₂ is observed over most of the potential range for Nafion/GCE. However, the reduction currents were greatly enhanced due to the strong electrocatalytic reaction of H₂O₂ for SnO₂/Nafion/GCE, whereas there is no significant enhancement towards the oxidation of H₂O₂ in the examined region. The reduction process starts at ca. 0 V (vs.Ag/AgCl). Compared to Nafion/GCE, SnO₂ nanodendrite-modified electrode offers a positive shift in the cathodic overpotential for H₂O₂ reduction, which may be owing to a substantial increase in the electron transfer rate for the reduction of H₂O₂ probably resulting from the enhanced kinetic and transport effect of H₂O₂ at the SnO₂/Nafion/GCE interface. In addition, tin oxide has been well studied for its surface oxygen depletion property.³¹ At a negative potential, the tin oxide nanodendrites tend to be in a more reduced state, which may facilitate the absorption of oxygen atoms in H₂O₂, and thus enhance the electron transfer to reduce H₂O₂.

Fig. 7 Cyclic voltammograms of the Nafion/GCE and the SnO₂/Nafion/GCE electrode in 0.1 M pH 7.4 phosphate buffer in the absence and presence of 2 mM H₂O₂, respectively. Scan rate = 50 mV s⁻¹.

In order to study the effect of applied potentials on electrocatalytic reduction of H₂O₂ at the SnO₂ nanodendrites-modified electrode, the amperometric responses to successive additions of H₂O₂ were recorded with respect to different applied potentials (Fig. 8). As the reduction of H₂O₂ for SnO₂/Nafion/GCE starts at 0 V and increases significantly towards the negative ends, stepwise current responses to H₂O₂ at −0.1V were observed as expected (Fig. 8A). In addition, the reduction currents increase sharply with further decrease of applied potentials (Fig. 8B and C). For all the applied potentials, the SnO₂/Nafion/GCE responds rapidly to the changes in H₂O₂ concentration, producing steady-state signals within 10 s. The amperometric results match the CVs results well. The corresponding calibration curves are presented in Fig. 8D and the analytical characteristics such as the linear range, sensitivity and the limit of detection (LOD) were summarized in Table 1. One can see that the highest sensitivity (265.6 μA mM⁻¹ cm⁻²) and the widest linear range (up to 2.7 mM) were obtained at an applied potential of −0.3 V. However, the best limit of detection (59 nM) at a signal-to-noise of 3 was achieved at an applied potential of −0.1 V because of the lowest noise level at −0.1 V. This LOD is very impressive and significantly better than recently reported H₂O₂ sensors.^32–36 For a higher concentration of H₂O₂, the plot deviates from linearity, which may be attributed to the saturation of the electrocatalytic activity of SnO₂ nanodendrites (Fig. 8D). The repeatability was examined by recording the responses from 30 μM of H₂O₂ injection. The standard deviations were 6.37%, 8.79%, and 9.59% at the applied potential of −0.1, −0.2, and −0.3 V, respectively, indicating the good repeatability of the H₂O₂ detection. It can be seen that the performance of the developed sensor is superior compared with most of H₂O₂ sensors in literature. Specifically, the sensitive detection of H₂O₂ at a negative applied potential can greatly minimize the intervention from interferences such as acetaminophen, uric acid and ascorbic acid, which are not electroactive at such low applied potential, and thus has the potential to enhance the selectivity in the (bio)sensor development.

Fig. 8 Amperometric response of the SnO₂/Nafion/GCE with successive additions of H₂O₂ to 0.1 M pH 7.4 phosphate buffer at an applied potential of −0.1 V (A), −0.2 V (B), and −0.3 V (C), respectively; D. The corresponding calibration curves. Insets show the amperometric response in low concentration range.

Table 1 Analytical characteristics of the SnO₂/Nafion/GCE towards H₂O₂ detection at different applied potentials

Applied potential (V vs.Ag/AgCl)	Linear range up to (mM)	Sensitivity (μA mM^−1cm^−2)	Detection limit (μM)
−0.1	1.2	14.44	0.059
−0.2	1.7	90	0.36
−0.3	2.7	265.6	0.60

---

Conclusions

In summary, single-crystalline Sn nanodendrites have been synthesized by displacement reaction with thermally evaporated Zn nanodiscs. The Sn nanodendrites grow along [002] directions. Through a RTGO process, the as-prepared Sn nanodendrites are further oxidized to form oxides which retain nanodendritic structures and consist of tetragonal and orthorhombic SnO₂. The SnO₂ nanodendrites have exhibited enhanced catalytic property toward hydrogen peroxide reduction. Rapid, sensitive and repeatable amperometric detection of H₂O₂ has been achieved at the SnO₂ nanodendrite modified electrode. This study provides a simple way to synthesize unusual nanostructures which have great potential in various applications ranging from catalysts to sensors.

References

1. J. Y. Liu, T. Luo, T. S. Mouli, F. L. Meng, B. Sun, M. Q. Li and J. H. Liu, Chem. Commun., 2010, 46, 472.
2. W. Z. Jia, L. Su, Y. Ding, A. Schempf, Y. Wang and Y. Lei, J. Phys. Chem. C, 2009, 113, 16402.
3. R. S. Yang, Y. Ding and Z. L. Wang, Nano Lett., 2004, 4, 1309.
4. J. H. Duan, S. G. Yang, H. W. Liu, J. F. Gong, H. B. Huang, X. N. Zhao, R. Zhang and Y. W. Du, J. Am. Chem. Soc., 2005, 127, 6180.
5. J. Y. Chen, B. J. Wiley and Y. N. Xia, Langmuir, 2007, 23, 4120.
6. S. H. Sun, D. Q. Yang, D. Villers, G. X. Zhang, E. Sacher and J. P. Dodelet, Adv. Mater., 2008, 20, 571.
7. J. Zhou, Y. Ding, S. Z. Deng, L. Gong, N. S. Xu and Z. L. Wang, Adv. Mater., 2005, 17, 2107.
8. X. M. Lu, J. Y. Chen, S. E. Skrabalak and Y. N. Xia, Proc. Inst. Mech. Eng., Part N, 2007, 221, 1.
9. X. W. Teng, Q. Wang, P. Liu, W. Han, A. Frenkel, W. Wen, N. Marinkovic, J. C. Hanson and J. A. Rodriguez, J. Am. Chem. Soc., 2008, 130, 1093.
10. Y. G. Sun, B. Wiley, Z. Y. Li and Y. N. Xia, J. Am. Chem. Soc., 2004, 126, 9399.
11. Q. B. Zhang, J. P. Xie, J. H. Yang and J. Y. Lee, ACS Nano, 2009, 3, 139.
12. Q. B. Zhang, J. P. Xie, J. Liang and J. Y. Lee, Adv. Funct. Mater., 2009, 19, 1387.
13. Z. W. Chen, M. Waje, W. Z. Li and Y. S. Yan, Angew. Chem., Int. Ed., 2007, 46, 4060.
14. C. Y. Wang, M. Y. Lu, H. C. Chen and L. J. Chen, J. Phys. Chem. C, 2007, 111, 6215.
15. G. F. Cui, X. Ke, H. Liu, J. W. Zhao, S. Q. Song and P. K. Shen, J. Phys. Chem. C, 2008, 112, 13546.
16. N. Q. Jia, Q. Zhou, L. Liu, M. M. Yan and Z. Y. Jiang, J. Electroanal. Chem., 2005, 580, 213.
17. E. Topoglidis, Y. Astuti, F. Duriaux, M. Gratzel and J. R. Durrant, Langmuir, 2003, 19, 6894.
18. L. Shi, Y. Xu, S. Hark, Y. Liu, S. Wang, L. Peng, K. Wong and Q. Li, Nano Lett., 2007, 7, 3559.
19. J. H. He, T. H. Wu, C. L. Hsin, K. M. Li, L. J. Chen, Y. L. Chueh, L. J. Chou and Z. L. Wang, Small, 2006, 2, 116.
20. C. F. Guo, Y. S. Wang, P. Jiang, S. H. Cao, J. J. Miao, Z. W. Zhang and Q. Liu, Nanotechnology, 2008, 19, 445710.
21. H. J. You, J. X. Fang, F. Chen, M. Shi, X. P. Song and B. J. Ding, J. Phys. Chem. C, 2008, 112, 16301.
22. H. Brune, C. Romainczyk, H. Roder and K. Kern, Nature, 1994, 369, 469.
23. Y. Ding, P. X. Gao and Z. L. Wang, J. Am. Chem. Soc., 2004, 126, 2066.
24. Z. R. Dai, Z. W. Pan and Z. L. Wang, Adv. Funct. Mater., 2003, 13, 9.
25. A. Kolmakov, Y. X. Zhang and M. Moskovits, Nano Lett., 2003, 3, 1125.
26. M. O. Orlandi, A. J. Ramirez, E. R. Leite and E. Longo, Cryst. Growth Des., 2008, 8, 1067.
27. K. M. Li, Y. J. Li, M. Y. Lu, C. I. Kuo and L. J. Chen, Adv. Funct. Mater., 2009, 19, 2453.
28. N. M. A. Hadia, S. V. Ryabtsev, P. V. Seredin and E. P. Domashevskaya, Phys. B, 2010, 405, 313.
29. W. M. Zhang, J. S. Hu, Y. G. Guo, S. F. Zheng, L. S. Zhong, W. G. Song and L. J. Wan, Adv. Mater., 2008, 20, 1160.
30. H. T. Chen, S. J. Xiong, X. L. Wu, J. Zhu, J. C. Shen and P. K. Chu, Nano Lett., 2009, 9, 1926.
31. M. Batzill, K. Katsiev, J. M. Burst, U. Diebold, A. M. Chaka and B. Delley, Phys. Rev. B, 2005, 72, 165414.
32. J. S. Huang, D. W. Wang, H. Q. Hou and T. Y. You, Adv. Funct. Mater., 2008, 18, 441.
33. S. J. Guo, S. J. Dong and E. K. Wang, Small, 2009, 5, 1869.
34. C.X. Guo, X.T. Zheng, S.R. Ng, Y.C. Lai, Y. Lei and C.M. Li, Chem. Commun., 2011, 47, 2652.
35. J.P. Liu, C.X. Guo, C.M. Li, Y.Y. Li, Q.B. Chi, X.T. Huang, L. Liao and T. Yu, Electrochem. Commun., 2009, 11, 202.
36. C.X. Guo, F.P. Hu, C.M. Li and P.K. Shen, Biosens. Bioelectron., 2008, 24, 819.

---

Footnote

† Electronic Supplementary Information (ESI) available: Micro-XRD patterns for the corresponding regions in Sn nanodentrites. See DOI: 10.1039/c1ra00433f/

---