From Single-Component Nanowires to Composite Nanotubes - Crystal

ARTICLE pubs.acs.org/crystal

From Single-Component Nanowires to Composite Nanotubes Yina Zhu, Wei Chen, Caiyun Nan, Qing Peng,* Ruji Wang, and Yadong Li Department of Chemistry, Tsinghua University, Beijing, 100084, P. R. China

bS Supporting Information ABSTRACT: In this work, a facile, solution phase route using Co(OH)x(CO3)y nanowires as precursors was developed to fabricate SnO2/Co(OH)x+2ε(CO3)y
ε, and then they were transformed into SnO2/Co3O4 composite nanotubes by a simple thermal treatment process, which had great advantages in controlling their elemental ratios and the size of inner diameters. The mechanism is systematically discussed, which offers a clue that “diffusion” could be used as a driving force for the formation of nanotubes with different components, and single-component nanowires with replaceable negative ions may serve as good building blocks. TiO2/Co3O4 nanotubes were also successfully achieved to demonstrate the generality of the methodology, which is helpful for scientists to exploit novel functional materials with new properties and applications.

1. INTRODUCTION Nanocomposites with tunable composition, morphology, and organization patterns of the building blocks are expected to generate multifunctional devices due to the combination of the properties associated with different components.1 Besides, they also often exhibit better chemical and physical performance than monocomponent nanomaterials because of synergies among those components, for example, modified optoelectronic properties,2 excellent sensitivity in sensor response,3 and enhanced structural stability during the electrochemical cycle which efficiently prevents capacity loss in lithium-ion batteries.4 Over the past several decades, based on systematic theoretical prediction, the practice is routinely performed with various experimental studies upon bulk and nanoscale semiconductors, such as passivation of interfaces,5 bandgaps,6 and inherent structures.7 In recent years, great progress from many groups of researchers have been made to fabricate nanocomposites (either heteroor homo-) including introduction of impurity atoms,1d growth of metal tips,2a,8 coupling with dots,2d coaxial nanocables,9 and core/shell heterstructures.5,6b However, compared with monocomponent nanomaterials, preparation of nanocomposites, especially nanotubes, in a controllable manner is still much more difficult to achieve, and limited techniques have been developed. Since the discovery of carbon nanotubes in 1991 by Iijima,10 numerous kinds of strategies have been designed for constructing inorganic tubular nanostructures,11 but how to obtain composite nanotubes still lacks systematical methods. Traditional methods often depend on a heteroepitaxial growth manner in which composite nanotubes are formed by one-dimensional modulation of presynthesized nanotube composition and doping, and usually make only heteronanotubes.12 Schmuki reported an electrochemical anodization-based route to homo-nanotubes composed of TiO2 and WO3,13 but the amounts of doping compounds (WO3) are r 2011 American Chemical Society

very limited. Therefore, it is still a great challenge to explore a relatively general method for controllable synthesis of composite nanotubes and then to investigate their new properties and applications. Herein, we report a diffusion-based route to transform Co(OH)x(CO3)y nanowires into SnO2/Co(OH)x+2ε(CO3)y
ε composite nanotubes, in which Sn and Co almost uniformly distributed in the tubes with a large Sn/Co atomic ratio, and then they were converted into SnO2/Co3O4. A possible mechanism consisting of ion exchange, heterepitaxial growth, and substancediffusion has been proposed. Besides, on the basis of this method and by adding a certain amount of NaOH or Na2CO3, the Sn/Co atomic ratio and the interior diameters of the final products could be easily tuned in a relatively broad range. Our recent advances demonstrated that it is also effective for the fabrication of other composite nanotubes such as TiO2/Co3O4. These results indicate that nanowires with replaceable negative ions may serve as building blocks for constructing composite nanotubes, which provides a good model for scientists to explore novel functional materials and their applications.

2. EXPERIMENTAL SECTION Synthesis of Co(OH)x(CO3)y Nanowire (NW) Precursors. Co(OH)x(CO3)y NWs were synthesized based on that previously reported procedure with some modification (Supporting Information). Typically, 2 mmol of CoCl2 3 6H2O and 1.5 mmol of urea were dissolved in 40 mL of deionzed water to form a clear pink solution and heated to 30
45 °C. Next, 3 mL of oleylamine (preheated in the oven at Received: May 8, 2011 Revised: July 12, 2011 Published: August 16, 2011 4406

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Figure 1. (a) XRD pattern; (b) SEM images of overall products after reacting for 6 h; inset was obtained from mechanical milling, which was to prove the as-prepared tube-like structure; (c, d, e) TEM and HRTEM images of a single tube. 60
70 °C) was added dropwise into the solution under magnetic stirring and without further agitation once finished. The obtained bluegreen mixture was quickly transferred into a 40 mL Teflon-lined autoclave with ∼80% of the capacity then sealed and heated at 180 °C for 12 h. After the system was cooled to room temperature, the upper solid oil clot in the autoclave was discarded, and the resulting pink-colored suspension was separated by centrifuging at 4500 rpm for 3 min. The precipitate was washed with deionzed water and absolute ethanol several times and subjected to centrifugation. The solid sample was obtained by air-drying at 60 °C.

Synthesis of SnO2/Co(OH)x+2ε(CO3)y
ε Composite Nanotubes (NTs). In the typical synthesis of SnO2/Co(OH)x+2ε(CO3)y
ε NTs, Na2SnO3 aqueous (0.07 M, 10 mL), n-octane (20 mL), and ethanol (3 mL) were mixed together to form a two-phase system (a small quantity of Na2SnO3 may precipitate out in the lower water-phase because of reduced polarity of solvent by introducing ethanol). Then Co(OH)x(CO3)y NWs (0.21 g, ∼0.17 mmol, the relative molecular weight of Co(OH)x(CO3)y 3 zH2O is 127.37 g/mol, as determined by TGA) predispersed in ethanol (2 mL) were added. After being stirred for 10 min, the solution was transferred into a Teflon-lined autoclave of 40 mL capacity. The autoclave was sealed and heated at 100 °C for 6 h. After the autoclave was cooled to room temperature, the precipitate was collected at the bottom of the autoclave and treated as previously described. Synthesis of Co(OH)2 Internal Standard Sample. β-Co(OH)2 nanoplates were synthesized according to a reported method. In a typical procedure, CoCl2 3 6H2O and HMT were dissolved in 200 mL of a 9:1 mixture of deionized water and ethanol to give a final concentration of 5 and 60 mM, respectively. The solution was then heated at about 90 °C for 1 h under magnetic stirring. After the system was cooled to room temperature, the resulting pink-colored suspension was separated by centrifugation at 4500 rpm for 3 min. The precipitate was washed with deionzed water and absolute ethanol several times and

subjected to centrifugation. The solid sample was obtained by air-drying at 60 °C.

Synthesis of TiO2/Co(OH)x+2ε(CO3)y
ε Composite Nanotubes (NTs). For the synthesis of TiO2/Co(OH)x+2ε(CO3)y
ε NTs, 0.15 mL of tetrabutyl orthotitanate (Ti(OBu)4) was added into a mixture of 16 mL of tetrabutyl orthotitanate and 22 mL of ethanol. The solution was then stirred for 10
15 min before Co(OH)x(CO3)y NWs (0.012 g) predispersed in ethanol (2 mL) were introduced. After being stirred for the next 5
10 min, the solution was transferred into a 40 mL Teflon-lined autoclave with ∼80% of the capacity. The autoclave was then sealed and heated at 200 °C for 24 h. After the autoclave was cooled to room temperature, the precipitate was collected at the bottom of the autoclave and treated as previously described. Thermal Treatment. The as-obtained SnO2/Co(OH)x+2ε(CO3)y
ε and TiO2/Co(OH)x+2ε(CO3)y
ε nanotubes were calcined in air at designated temperatures for 1 h, respectively. Characterization. The phase of the product was verified by powder X-ray diffraction (XRD) on a Rigaku D/max 2500Pc X-ray powder diffractometer with Cu Kα radiation (λ = 1.5418 Å). Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high resolution transmission electron microscopy (HRTEM) images and energy-dispersive X-ray (EDX) spectra of the samples were obtained using a Leo-1530 field-emission scanning electron microscope (FESEM) working at 10 kV, a JEOL JEM-1200EX transmission electron microscope (TEM) working at 100 kV, and an FEI Tecnai G2 F20 S-Twin working at 200 kV. Fourier transform infrared (FT-IR) spectroscopy was recorded using a Nicolet 560 spectrograph.

3. RESULTS AND DISCUSSION Morphology and Structure. The morphology of the products was closely investigated by SEM and TEM. SEM images 4407

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Figure 2. (a) Dark field image of a single tubular structure; (b) compositional profile line scanning along the red line indicated in (a); (c, d) individual Sn-L and Co-K profile line extracted from (b); (e) EDS spectra of overall products, giving an average atomic ratio of tin and cobalt of 66.4%/33.6%.

(Figure 1a) exhibit a broad view of the wire-like products with ∼2 times diameters compared with those of Co(OH)x(CO3)y nanowire precursors (Figure S1a, Supporting Information). But both of the fragments (Figure 1a-inset, as obtained by grinding the products in a mortar) and the low-magnification TEM picture (Figure 1c) further disclose their hollow features (nanotubes, NTs), with slightly narrower inner radii compared with those of NWs precursors and smooth surfaces both inside and outside. HRTEM (Figure 1d,e) images demonstrate that the NTs are in fact composed of small nanocrystals (NCs) that are incompactly attached to each other. With careful observation of Figure 1e, it can be found that some amorphous species uniformly dispersed among these NCs are also building blocks of the NTs, although the boundaries between them cannot be clearly identified. Powder X-ray diffraction (XRD) pattern of the products after reacting for 6 h is shown in Figure 1b. It reveals that tetragonal SnO2 (JCPDS 41-1445) is the only crystalline phase, and no other reflection can be assigned to any kind of Cobased compound. However, considering the pink color of the products associated with the observed results in HRTEM, it can be speculated that amorphous cobalt-based compounds should also be the constituents of the products. To verify the above assumption, the products were further analyzed by energy-dispersive X-ray spectroscopy (EDS). The EDS pattern in a large area (Figure 2e) confirms that both Sn, Co exist in the products, giving an average Sn/Co atomic ratio of 66.4%/33.6% (Au and Si come from the wafers and plating, respectively). Figure 2, panels c (Sn-L) and d (Co-K) are the compositional profile lines by scanning across a single structure along the red line in Figure 2a. It suggests the uniform dispersancy of

both Co and Sn in the structures, which is very consistent with the observed results in HRTEM. The element contents present visibly higher distributions outside than those inside, further telling the tube-like structures. That is, the as-obtained composite products are homogeneous nanotubes. FT-IR spectra (Figure S2, Supporting Information) show that the constituents of amorphous cobalt-based compounds in the products are also basic cobalt carbonate, but the average OH
/ CO32
ratios are greatly larger than those of Co(OH)x(CO3)y precursors, so the as-prepared products can be assigned to SnO2/ Co(OH)x+2ε(CO3)y
ε (where ε varies from 0 to y). β-Co(OH)2 was added as the internal standard substance to determine the relative concentration of CO32
in the samples; then the small amount of OH
and/or H2O containing in the samples can be neglected. The sharp absorption peak at 3631 cm
1 is attributed to O
H stretching vibration in the internal standard substance. Comparing with the FT-IR measurement of the starting materials, after reacting for 6 h, the CO32
stretching vibration at 1346 cm
1 (symmetric) and 1509 cm
1 (asymmetric) remained, but the intensity of the peak significantly diminished, corresponding to the increased OH
/CO32
ratios in basic cobalt carbonate. In the spectrum in Figure S2-ii, the modes in the range of 3380
3510 cm
1 were the symmetric and asymmetric stretching vibrations of crystal water or O
H in the Co(OH)x(CO3)y precursors; in Figure S2-iii, as crystalline reactants transformed into amorphous products, O
H/H2O molecule association reactions took place, and a broad peak showed in this range. The mode at 1637 cm
1 was the bending vibration of H2O. Evolution of Morphology from Nanowires to Nanotubes. In a typical synthesis of SnO2/Co(OH)x+2ε(CO3)y
ε composite 4408

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Figure 3. Proposed mechanism for the shape transformation process from Co(OH)x(CO3)y nanowires to SnO2/Co(OH)x+2ε(CO3)y
ε nanotubes.

nanotubes, the rapid hydrolysis of SnO32
resulted in a greatly higher pH value in the system, which initiated the reaction. Subsequently negative ion exchange between OH
and CO32
happened, and the floating SnO2 3 H2O amorphous precipitate in the system gradually dissolved and recrystallized on the outer walls of the structures. The reaction can be described as eqs 1 and 2:
SnO2
3 þ H2 O f H2 SnO3 ðSnO2 3 H2 OÞ þ 2OH

ð1Þ

SnO2 3 H2 O þ CoðOHÞx ðCO3 Þy þ OH
f SnO2 =CoðOHÞxþ2ε ðCO3 Þy
ε þ CO2
3

ð2Þ

As reported in several previous works, epitaxial growth with different materials usually favors a Stranski
Krastanow (SK) mode, in which 3D-isolated islands form rather than keeping a 2D layer-by-layer growth at the early stage to relax epi-layer strain generated from lattice mismatch.5,14 Such a growth mode is driven by minimizing the total energy contributed by interfacial misfit dislocations and epi-layer strain. Examples are also available by introducing surfactants to improve the stability of 2D layer formation such as assembling of nanorod arrays hierarchically on the surface of the starting materials. It is obviously that our system with similar reactions and free of surfactants does not follow any of such traditional epitaxial core
shell growth modes but results in a nanotube structure with a smooth surface. In this regard, we consider that the noncrystalline surface of the starting material should exist before SnO2 growth happened, similar to a frequently reported procedure that polycrystalline SnO2 deposited on amorphous SiO2 spheres.15 But how is the noncrystalline surface generated? A former study in our group has demonstrated that reactive compounds could act as both templates and reactants to synthesize more stable ones.16 Apparently, easy conversions require a distinctive Ksp value difference. In our system, interaction between Co2+ and OH
is greatly stronger than that of Co2+ and CO32
, reflected by a large Ksp contrast (Ksp Co(OH)2 = 2  10
15, CoCO3 = 1.4  10
13). Therefore, it is reasonable to suppose that it is the fast exchange between OH
and CO32
that resulted in several amorphous layers on the surface of the original single-crystalline Co(OH)x(CO3)y nanowires and then led to the possibility of subsequent SnO2 deposition in a smooth mode.

To demonstrate the formation mechanism of composite nanotubes, a series of intermediate samples with different reaction times were prepared for characterization and analysis (8 min, 2 h, and 6 h later, respectively, from the moment that the temperature just reached 100 °C). Figure S3 (Supporting Information) presents the dark field images (left rows) and EDS compositional line profiles scanning along the red lines (right rows), respectively, providing at least three pieces of information: (1) at the primary stage, the ordered structure of single-crystal nanowire was fast destroyed (∼8 min, Figure S3 a/d, Supporting Information), whereas epitaxial growth of SnO2 has not yet apparently occurred. As there is no clear “heterolayer” found on the surface of the wirelike structure, the structural transformation from single-crystalline to amorphous state at the initial stage has been specifically discussed. TEM, HRTEM images, and their corresponding fast Fourier transform (FFT) patterns of the precursors and samples at 8 min are shown in Figure S4 (Supporting Information); (2) the light/dark contrast between the sample and background directly reflects the density of the sample. As the reaction proceeded, the brightness in the central part gradually diminished, implying the formation of the tube; the thickening of the outer wall indicates the development of SnO2 epi-growth (from Figure S3a,d to c,f, Supporting Information); (3) during the whole process, the quantity of Co remained unchanged, while Sn kept growing up in a limited length of time. If searched carefully, we could found that, despite the continuously enlarged Sn/Co atomic ratio, the amount of Sn always held a larger level outside than that in the inner part. On the contrary, the variation trend of Co inside/outside decreased until the majority of Co has migrated into the outer space, leaving a hollow core in the center (Figure 2a,b). All these results have reached the same conclusion that the formation of the nanotubes mainly came from the diffusion of Co(OH)x+2ε(CO3)y
ε. XRD patterns in Figure S5 (Supporting Information) indicate that at the first 8 min, the signals relative to SnO2 were very weak, and Co(OH)x(CO3)y was the dominating phase. Actually, coating of SnO2 was harder to achieve at this moment because of the relatively ordered surface of the NWs. When heating of the system was continued for the next 2 h, the reflections of SnO2 were greatly intensified, while the peaks of Co(OH)x(CO3)y gradually diminished, implying that amorphous layers arising from the transformation from crystalline Co(OH)x(CO3)y to noncrystalline Co(OH)x+2ε(CO3)y
ε promoted the deposition 4409

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Crystal Growth & Design of SnO2. In the end, the phase of Co(OH)x(CO3)y entirely disappeared after reacting for 6 h. According to the above results, a possible mechanism based ion exchange, heterepitaxial growth, and substance-diffusion can be speculated for the shape transformation process from Co(OH)x(CO3)y nanowires to SnO2/Co(OH)x+2ε(CO3)y
ε nanotubes, as illustrated in Figure 3. At the initial stage, the hydrolysis of Na2SnO3 brought a quantity of OH
into the system. Because of higher electrostatic force between OH
and Co2+, OH
ions quickly displaced CO32
on the surface of presynthesized Co(OH)x(CO3)y nanowires to form several amorphous layers, which significantly relieved substantial lattice mismatch and benefitted SnO2 recrystallization and deposition on NWs. As the reaction proceeded, the layer of SnO2 thickened. The polycrystalline state of SnO2 layers implied the existence of spaces between the boundaries of nanoparticles (NPs), which can serve as a framework for the insertion of other species. Meanwhile, ion exchange gradually reduced crystalline Co(OH)x-

Figure 4. XRD patterns of the products obtained after calcining at different temperatures.

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(CO3)y nanowires into amorphous Co(OH)x+2ε(CO3)y
ε and tiny crystalline Co(OH)x(CO3)y NPs with high activity and mobility. These highly reactive particles could pass through the Sn
Co boundaries and migrate into the interspaces of SnO2 NPs. The diffusion of Co(OH)x+2ε(CO3)y
ε led a low density in the central part of the structures, and then particles here bear relatively higher interfacial free-energy because they possess abundant dangling bonds. So minimization of the overall surface-energy became the driving force for the evolution of the structures in the third stage and finally induced the formation of homogeneous nanotubes. To check the rationality of the mechanism, either NaOH or Na2CO3 was added into the original system for characterization and analysis (here, the concentration was 0.1 M for both NaOH and Na2CO3, as an example). SEM in Figure S6 (Supporting Information) shows that the morphologies of the products are hardly changed, but EDX analysis, dark field images, and corresponding compositional profile lines across single structures (Figure S7, Supporting Information) supply two pieces of novel information: (1) The inner diameters of the as-obtained tubes relative to the rate of Co(OH)x+2ε(CO3)y
ε diffusion into the interspaces of SnO2 varied with the initial concentrations of CO32
and OH
in solution. For the case of CO32
, it indicated a procedure that incomplete ion exchange restrained by CO32
in solution led to relatively ordered and stable structures with smaller ε, making them harder to break into small and flexible particles and then they migrated into the outer layers, leaving tubular structures with thin hollow spaces. While if introducing NaOH into the system, a higher concentration of OH
led to a rapid collapse from ordered structures to small pieces with larger ε and that permeated into the SnO2 layer in a short length of time, the as-obtained nanotubes with slightly wider inner diameters than those of precursor NWs could often be found. (2) The average atomic ratios of Sn/Co following an ascending tendency from NaOH (1:1) to Na2CO3 (3:1) implies that the epi-growth of SnO2 on the one-dimensional structures were inhibited by NaOH while being promoted by Na2CO3, respectively. This phenomenon may be caused by the decreased percentage of SnO2 active sites on the surface of the structures,

Figure 5. SEM, dark field images, and compositional profile lines scanning across single tubes after calcining in air at (a, b, c) 350 °C and (d, e, f) 700 °C. 4410

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Crystal Growth & Design and then SnO2 coating from the solution was affected to a certain extent, because of the diffusion of Co(OH)x+2ε(CO3)y
ε onto the surface of the structures. But the detailed mechanism still needs to be investigated. Thermal Treatment. After a series of thermal treatments of SnO2/Co(OH)x+2ε(CO3)y
ε nanotubes at different temperatures, the pink products transformed into brownish black, and the colors were increasingly deep as the temperature went upward. Figure 4 shows typical XRD patterns of products after calcining in the air for 1 h. Before 500 °C, all the reflections of products can be assigned to tetragonal SnO2 (JCPDS 41-1445). It also can be found that the crystallinity of SnO2 improved but just in a very limited range. When the temperature rose to 700 °C, all the peaks became intensified and sharp, which suggests increased crystallinity of SnO2. Additionally, impure peaks were identified as crystalline Co3O4 appeared. Taking products after calcining at 350 and 700 °C, for example, Figure 5 shows SEM, dark field images, and corresponding compositional profile lines scanning across single tubes. It demonstrates that nanotubes after calcining at 350 °C still kept their original morphologes and features. However, when the temperature was increased to 700 °C, the surface of the nanotubes became coarsen and Co migrated back to the central part of the tubes. It suggests that tiny NPs of Co(OH)x+2ε(CO3)y
ε separated from the framework of SnO2 and gradually oxidized to Co3O4 because of the temperature used and aggregated together into the central part of the nanotubes, which led the enhanced crystallinity of Co3O4. Simultaneously, SnO2 NPs which were composed of the empty framework free of Co3O4 passed through the boundaries and assembled together, resulting in the improved crystallinity of SnO2.

4. CONCLUSION In summary, the controllable synthesis of SnO2/Co3O4 composite nanotubes has been achieved by using Co(OH)x(CO3)y nanowires as precursors and transforming from Co(OH)x+2ε(CO3)y
ε as an intermediate. In this system, a large Sn/Co atomic ratio could be obtained and tuned in a relatively broad range. The results indicate that ion exchange between OH
and CO32
, heterepitaxial growth of SnO2 onto the structures, and subsequent substance diffusion contributed mainly by Co(OH)x+2ε(CO3)y
ε into the outer walls together led to the formation of composite nanotubes. This methodology offers a clue that “diffusion” could be used as a driving force for the formation of nanotubes with different components. Our recent research demonstrated that it might be also a general route for the formation of other composite nanotubes such as TiO2/ Co3O4, although the detailed mechanism for the shape transformation process may vary to some extent (Figure S8, Supporting Information). This strategy provides a new route for scientists to exploit functional materials with novel properties and applications. ’ ASSOCIATED CONTENT

bS

Supporting Information. SEM/HRTEM images and XRD patterns of as-synthesized precursor nanowires; FT-IR spectra of Co(OH)2 as internal standard substance, Co(OH)x(CO3)y precursors and the final SnO2/Co(OH)x+2ε(CO3)y
ε products; XRD patterns, dark field images, and corresponding compositional profile lines scanning across individual structures

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at different stages; SEM/dark field images and EDS spectra of samples with NaOH or Na2CO3 as additives. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Fax: (86) 10 62788765. Tel: (86) 10 62772350. E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by NSFC (20921001, 90606006) and the State Key Project of Fundamental Research for Nanoscience and Nanotechnology (2006CB932300). ’ REFERENCES (1) (a) Ding, Y.; Gao, P. X.; Wang, Z. L. J. Am. Chem. Soc. 2004, 126, 2066. (b) Jung, Y.; Lee, S. H.; Jennings, A. T.; Agarwal, R. Nano Lett. 2008, 8, 2056. (c) Yang, Y.; Kim, D. S.; Qin, Y.; Berger, A.; Scholz, R.; Kim, H.; Knez, M.; G€osele, U. J. Am. Chem. Soc. 2009, 131, 13920. (d) Yuhas, B. D.; Zitoun, D. O.; Pauzauskie, P. J.; He, R. R.; Yang, P. D. Angew. Chem., Int. Ed. 2006, 45, 420. (e) Formo, E.; Lee, E.; Campbell, E.; Xia, Y. N. Nano Lett. 2008, 8, 668. (2) (a) Mokari, T.; Rothenberg, E.; Popov, I.; Costi, R.; Banin, U. Science 2004, 304, 1787. (b) Wang, F.; Han, Y.; Lim, C. S.; Lu, Y. H.; Wang, J.; Xu, J.; Chen, H. Y.; Zhang, C.; Hong, M. H.; Liu, X. G. Nature 2010, 463, 1061. (c) Wang, G. M.; Yang, X. Y.; Qian, F.; Zhang, J. Z.; Li, Y. Nano Lett. 2010, 10, 1088. (d) Leschkies, K. S.; Divakar, R.; Basu, J.; Enache-Pommer, E.; Boercker, J. E.; Carter, C. B.; Kortshagen, U. R.; Norris, D. J.; Aydil, E. S. Nano Lett. 2007, 7, 1793. (3) Thakar, R.; Chen, Y. C.; Snee, P. T. Nano Lett. 2007, 7, 3429. (4) Liu, J.; Manthiram, A. Chem. Mater. 2009, 21, 1695. (5) Lauhon, L. J.; Gudiksen, M. S.; Wang, D. L.; Lieber, C. M. Nature 2002, 420, 57. (6) (a) Schrier, J.; Demchenko, D. O.; Wang, L. W.; Alivisatos, A. P. Nano Lett. 2007, 7, 2377. (b) Yan, J.; Fang, X. S.; Zhang, L.; Bando, Y.; Gautam, U. K.; Dierre, B.; Sekiguchi, T.; Golberg, D. Nano Lett. 2008, 9, 2794. (c) Alpuche-Aviles, M. A.; Wu, Y. Y. J. Am. Chem. Soc. 2009, 131, 3216. (7) (a) Zeng, D. L.; Cabana, J.; Yoon, W. S.; Grey, C. P. Chem. Mater. 2010, 22, 1209. (b) Fisher, C. A. J.; Prieto, V. M. H.; Islam, M. S. Chem. Mater. 2008, 20, 5907. (8) Habas, S. E.; Yang, P. D.; Mokari, T. J. Am. Chem. Soc. 2008, 130, 3294. (9) (a) Wang, X. D.; Gao, X. D.; Li, J.; Summers, C. J.; Wang, Z. L. Adv. Mater. 2002, 14, 1732. (b) Chen, A.; Kamata, K.; Nakagawa, M.; Iyoda, T.; Wang, H. Q.; Li, X. Y. J. Phys. Chem. B 2005, 109, 18283. (10) Iijima, S. Nature 1991, 354, 56. (11) (a) Li, Y. D.; Li, X. L.; He, R. R.; Zhu, J.; Deng, Z. X. J. Am. Chem. Soc. 2002, 124, 1411. (b) Bae, C.; Yoo, H.; Kim, S.; Lee, K.; Kim, J.; Sung, M. M.; Shin, H. Chem. Mater. 2008, 20, 756. (c) Wu, G. S.; Zhang, L.; Cheng, B. C.; Xie, T.; Yuan, X. Y. J. Am. Chem. Soc. 2004, 126, 5976. (d) Hu, S.; Wang, X. J. Am. Chem. Soc. 2008, 130, 8126. (12) (a) Sun, W. T.; Yu, Y.; Pan, H. Y.; Gao, X. F.; Chen, Q.; Peng, L. M. J. Am. Chem. Soc. 2008, 130, 1124. (b) Zhang, D. F.; Sun, L. D.; Jia, C. J.; Yan, Z. G.; You, L. P.; Yan, C. H. J. Am. Chem. Soc. 2005, 127, 13492. (c) Peng, H. S. J. Am. Chem. Soc. 2008, 130, 42. (d) Wojdel, J. C.; Bromley, S. T. J. Phys. Chem. B 2005, 109, 1387. (13) Nah, Y. C.; Ghicov, A.; Kim, D.; Berger, S.; Schmuki, P. J. Am. Chem. Soc. 2008, 130, 16154. (14) (a) Eaglesham, D. J.; Cerullo, M. Phys. Rev. Lett. 1990, 64, 1943. (b) Mo, Y. M.; Savage, D. E.; Swartzentruber, B. S.; Lagally, M. G. Phys. Rev. Lett. 1990, 65, 1020. (c) Wang, H. L.; Upmanyu, M.; Ciobanu, C. V. Nano Lett. 2008, 8, 4305. (d) Kim, D. W.; Hwang, I. S.; Kwon, S. J.; 4411

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