J. Phys. Chem. B 2006, 110, 6543-6548
6543
Controlled Synthesis of High-Quality PbS Star-Shaped Dendrites, Multipods, Truncated Nanocubes, and Nanocubes and Their Shape Evolution Process Guangjun Zhou, Mengkai Lu1 ,* Zhiliang Xiu, Shufen Wang, Haiping Zhang, Yuanyuan Zhou, and Shumei Wang State Key Laboratory of Crystal Materials, Shandong UniVersity, Jinan 250100, P.R. China ReceiVed: September 3, 2005; In Final Form: January 25, 2006
Well-defined single-crystalline PbS nano- and microstructures including dendrites, multipods, truncated nanocubes, and nanocubes were synthesized in high yield by a simple solution route. Novel star-shaped PbS dendrites with six symmetric arms along the 〈100〉 direction, each of which shows one trunk (long axis) and four branches (short axes), have been achieved using Pb(AC)2 and thioacetamide (TAA) as precursors, under the molar ratio Pb(AC)2/TAA ) 2/1, at initial reaction temperature 80 °C, refluxing for 30 min at 100 °C, in the presence of cetyltrimethylammonium bromine (CTAB). The “nanorods” in each branch are parallel to each other in the same plane and are perpendicular to the trunk. The truncated nanocubes mainly bounded by the {100} plane were prepared under a different Pb(AC)2/TAA molar ratio, at initial reaction temperature 40 °C, refluxing for 12 h at 100 °C. Based on the systematic studies on their shape evolution, a possible growth mechanism of these PbS nano- and microstructures was proposed. The shapes of PbS nanocrystals with facecentered cubic (fcc) structure are mainly determined by the ratio (R) between the growth rates along the 〈100〉 and 〈111〉 directions. The Pb(AC)2/TAA molar ratio and the initial reaction temperature influence the growth ratio R in the formation of PbS nuclei at an early stage, which results in the final morphology of PbS nanocrystals. Under the current experimental conditions, we can control the PbS shape evolution by simply tuning the molar ratio, the initial reaction temperature, and the period of reaction. Based on the systematic studies on the shape evolution, this approach is expected to be employed for the control-shaped synthesis of other fcc structural semiconductor nanomaterials. The photoluminescence properties were investigated and the prepared nano- and microstructures displayed a very strong luminescence around 600-650 nm at room temperature.
1. Introduction Nanometer-scale materials have attracted considerable interest in recent years due to their unique physical and chemical properties and potential applications in nanoscale devices.1-6 It is well-known that the shape and size of nanocrystals have much influence on their widely varying properties. The architectural control of nanosized materials with well-defined shape is important for the success of “bottom-up” approaches toward future nanodevices. In the past decades, there has been an increasing number of excellent studies on novel nanostructural materials with various shapes, such as nanorods,7-9,42 nanocubes,10-15 nanobelts,16-18 nanowires,19-22,43 nanofibers,23-26 nanotubes,27-30 and dendrites.31-34 The ability to understand and predict the final architecture of nanoscale building blocks is still limited. If we understand the growth mechanism and the shape-guiding process, it will be possible to program the system to yield the building blocks with desired shape and crystallinity. Semiconductor nanostructures have outstanding electronic and optical properties and are useful on nanodevices such as lightemitting diodes,35 single-electron transistors,36 and infrared detectors.37 As one of the semiconductor compounds, PbS is an important π-π semiconductor material with a narrow band gap energy and large excition Bohr radius.38 Moreover, quantumsize PbS has exceptional third-order nonlinear optical properties, * Corresponding author. Fax: +86-531-88565403. E-mail: mklu@ icm.sdu.edu.cn.
which means it should be useful on optical devices such as optical switches.39 PbS quantum dots with stable efficient luminescence in the near-IR spectral range has potential application in communication, biological imaging, and infrared photodetector.40 PbS nanocrystals with various morphologies has been achieved. For example, cubic-shaped PbS microcrystals and nanocrystals have been produced by the decomposition of a single-source precursor.41 Rodlike PbS nanocrystals have been obtained using a combination of surfactant and polymer matrix as a template.42 PbS nanowires as well as nanosheets have been prepared by a polymer-assisted solvothermal method.43,44 Rodbased PbS multipods were synthesized from the thermal decomposition of a molecular precursor.45 Star-shaped PbS microcrystals with eight symmetric arms along the 〈111〉 direction have been formed by an aqueous phase route.46,47 Dendrites have recently attracted much attention due to the interesting morphology, properties, and potential applications. Dendritic nanostructures of semiconductor compounds are one type of attractive supramolecular structures.33,48,49 In this paper, novel star-shaped PbS dendrites with six symmetric arms along the 〈100〉 direction, multipods, truncated nanocubes, and nanocubes were prepared in high yield via a simple surfactantassisted route in the presence of CTAB under different reaction conditions, respectively. The shapes of PbS nanocrystals evolved from metastable star-shaped multipods as a transient species to stable truncated nanocubes. The growth mechanism of the PbS nano- and microstructures was proposed. As a mode study for
10.1021/jp0549881 CCC: $33.50 © 2006 American Chemical Society Published on Web 02/18/2006
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Zhou et al.
the shape-guiding strategy, we present a rational synthetic scheme that yields novel architectures of semiconductor nanocrystals. Simultaneously, we elucidated the specific role of important growth parameters for shape determinations. These results establish that it is possible to control the morphology of semiconductor nanocrystals using solution chemistry, which has important implications for both fundamental scientific studies and future technological application. 2. Experimental Section Preparation of PbS Dendrites. All the reagents were purchased from Shanghai Chemistry Co. with analytical-grade purity and were used without further purification. The synthesis of PbS products was carried out by the thermal decomposition of thioacetamide (TAA) in aqueous solution of lead acetate at suitable initial reaction temperature and period of reflux in the presence of surfactant. In a typical experiment, 6 mmol of Pb(AC)2 and 3 g of CTAB were dissolved in 80 mL of deionized water in a three-necked flask equipped with a condenser. The mixtures were then heated to 80 °C under continuous vigorous stirring. Twenty milliliters of 3 mmol of thioacetamide (TAA) solution was added dropwise to the above solution at 80 °C (initial reaction temperature), up to the final concentrations of 0.06 mol/L Pb(AC)2 and 0.03 mol/L TAA, and the mole ratio of Pb(AC)2/TAA of 2/1. The color of the reaction mixture changed to black slowly in the dropwise procedure. When the addition of TAA was completed, the reaction mixture was heated to 100 °C and refluxed for 30 min. The resulting black products were separated by centrifugation at 3500 rpm for 20 min, washed several times with water and ethanol, and then dried at 60 °C for 5 h in a vacuum-dryer. Characterization of Samples. The X-ray diffraction (XRD) patterns were recorded using a Rigaku D/Max 2200PC diffractometer with graphite monochromatized Cu KR irradiation (λ ) 1.5418 Å). Transmission electron microscopy (TEM) images were carried out using a JEM-100CX2 transmission electron microscope. Scanning electron microscopy (SEM) images were measured on a JEOL JSM-6700f scanning electron microscope. Emission spectra were measured on an Hitachi 850 fluorescence spectrophotometer. All the measurements were carried out at room temperature. 3. Results and Discussion Morphology and Nanostructure of PbS Dendrites. The morphologies and structures of the products have been investigated by SEM and TEM. Well-defined, star-shaped PbS dendrites (Figure 1a and 1b) were synthesized as described in the Experimental Section, using Pb(AC)2 and TAA as precursors, the molar ratio being controlled at Pb(AC)2/TAA ) 2/1 and the initial reaction temperature at 80 °C, and refluxed for 30 min at 100 °C, in the presence of 3 g of CTAB/100 mL. Indeed, the yield of this structure is very high, and almost all of the crystals are six-arm star-shaped dendritic nanostructures. The typical SEM of individual PbS dendrite (insert in Figure 1b) shows a clear view of its three-dimensional (3D) structure with six symmetric arms that grow along 〈100〉 directions. Each arm has one trunk (long axis) and four branches (short axes). The “nanorods” in each branch are parallel to each other and in the same plane and are perpendicular to the trunk. Moreover, there is an interesting phenomenon that the junction of the six symmetry arms is broken by a 20 min ultrasonic wave of irradiation (45 W). All the star-shaped structures disband and the dispersed mono-arm dendrites are then present in the products as shown in Figure 2a and 2b. From
Figure 1. (a) Low- and (b) high-magnification SEM images of starshaped PbS dendrites with six arms along 〈100〉 directions synthesized under the molar ratio Pb(AC)2/TAA ) 2/1, the concentrations of Pb(AC)2 and TAA of 0.06 and 0.03 mol/L, respectively, and initial reaction temperature 80 °C, with refluxing for 30 min at 100 °C, in the presence of 3 g of CTAB/100 mL.
the higher magnification SEM image of mono-arm dendrites (Figure 2c), it can be clearly seen that the “nanorods” perpendicular to the trunk in the branches have complex delicate structures with one smaller trunk and four smaller branches. Influence of the Pb(AC)2/TAA Molar Ratio and the Initial Reaction Temperature on the PbS Morphologies. The morphology of the PbS nano- and microcrystals depends on the concentrations of reagents, surfactant, molar ratio of precursor, synthesis temperature, and time. All the parameters were found to be interdependent, thus resulting in PbS nanoand microcrystals with various morphologies. Among the various parameters, the Pb(AC)2/TAA molar ratio and the initial reaction temperature are particularly crucial for the control of both the morphology and the size of PbS nano- and microcrystals. To investigate the influence of the Pb(AC)2/TAA molar ratio and the initial reaction temperature on the PbS morphology, a series of separate experiments were carried out by changing the molar ratio from 2/1 to 1/1 and 1/3 and the initial reaction temperature from 80 °C to 35, 40, 50, 60, and 100 °C, respectively, under conditions of fixed refluxing time (30 min). The morphologies of PbS nano- and microcrystals prepared under various Pb(AC)2/TAA molar ratios at different initial reaction temperatures are shown in Table 1. It can be seen from Table 1 that all the products were dendrites when the molar ratio was 2/1 at various initial reaction temperatures. When the Pb(AC)2/TAA molar ratio was changed to 1/1, the final products were PbS dendrites only when the initial reaction temperature was above 60 °C. When the Pb(AC)2/TAA molar ratio was
Controlled Synthesis of High-Quality PbS
J. Phys. Chem. B, Vol. 110, No. 13, 2006 6545
Figure 2. (a) SEM and (b) TEM images of mono-arm dendrites prepared by 20 min ultrasonic wave of irradiation. (c) Higher magnification SEM images of the mono-arm dendrites.
TABLE 1: Influence of the Pb(AC)2/TAA Molar Ratio and the Initial Reaction Temperature on the PbS Morphologies molar ratio Pb(AC)2/TAA
35 °C
40 °C
2/1a 1/1b 1/3c
dendrites multipods multipods
dendrites multipods multipods
initial reaction temperature 50 °C 60 °C dendrites nanoparticles multipods
dendrites dendrites nanoparticles
80 °C
100 °C
dendrites dendrites dendrites
dendrites dendrites dendrites
a-c The final concentrations: a[Pb(AC)2] ) 0.06 mol/L, [TAA] ) 0.03 mol/L; b[Pb(AC)2] ) 0.03 mol/L, [TAA] ) 0.03 mol/L; c[Pb(AC)2] ) 0.03 mol/L, [TAA] ) 0.09 mol/L; refluxed for 30 min at 100 °C, in the presence of 3 g of CTAB/100 mL.
Figure 3. TEM images of the PbS nanostructures obtained under various initial reaction temperatures: (a) 35, (b) 40, (c) 50, (d) 60, (e) 80, and (f) 100 °C. The molar ratio of Pb(AC)2/TAA ) 1/1 and the concentrations of Pb(AC)2 and TAA are 0.03 and 0.03 mol/L, respectively, with refluxing for 30 min at 100 °C, in the presence of 3 g of CTAB/100 mL.
changed to 1/3, the PbS dendrites were obtained while the initial reaction temperature was above 80 °C. If the initial reaction temperature was decreased to 60 °C, only nearly spherically nanoparticles could be achieved. If the initial reaction temperature was decreased to 50, 40, and 35 °C, respectively, only star-shaped multipod nanocrystals could be obtained. Figure 3 shows the TEM images of the PbS nanostructures obtained under a different initial reaction temperature, the molar ratio Pb(AC)2/TAA ) 1/1, and refluxed for 30 min at 100 °C in the presence of 3 g of CTAB/100 mL. When the initial reaction temperature was fixed at 35 and 40 °C, respectively, the star-shaped PbS multipod nanocrystals were formed (Figure 3a and 3b). However, when the initial reaction temperature was increased to 50 °C, the nearly spherically PbS nanoparticles were formed (Figure 3c). When the initial reaction temperature was fixed at 60 °C, the dendritic structures were achieved (Figure 3d). When the initial reaction temperature was increased to 80 and 100 °C, respectively, the morphology and size of the PbS dendrites did not show significant change (Figure 3e and 3f).
Morphological Evolution Process of the PbS Nanocrystals. To investigate the morphological evolution process of the PbS nanocrystals, the molar ratio of Pb(AC)2/TAA was fixed at 1/3 and the initial reaction temperature was fixed at 40 °C under the same other reaction conditions. Refluxed time was changed from 10 min to 30 min, 2 h, 5 h, and 12 h; a variety of PbS nanocrystals including star-shaped multipods (Figure 4a and 4b), nanoparticles (Figure 4c), and truncated nanocubes (Figure 4d and 4e) were obtained. Figure 4 indicated clearly the evolution process of the PbS nanocrystals by varying the refluxed time. Figure 5a shows the SEM images of the truncated nanocubes (the same samples in Figure 4e). It is clearly seen that the large quantity of nanocrystals have a smooth surface and almost all corners and edges of these nanocubes were slightly truncated. The edge length of these truncated nanocubes are about 70-80 nm. Figure 5b shows the SEM image of individual slightly truncated nanocubes sitting on the silicon substrate against one of its square/hexagon facets, indicating that the slightly truncated nanocubes were bounded mainly by {100} facets. Figure 5c
6546 J. Phys. Chem. B, Vol. 110, No. 13, 2006
Figure 4. Morphological evolution process of the PbS nanocubes: TEM images of the samples synthesized at initial reaction temperature 40 °C, under different refluxing times of (a) 10 min, (b) 30 min, (c) 2 h, (d) 5 h, and (e) 12 h. The molar ratio of Pb(AC)2/TAA ) 1/3 and the concentrations of Pb(AC)2 and TAA are 0.03 and 0.09 mol/L, respectively, in the presence of 3 g of CTAB/100 mL.
shows the SEM image of individual truncated nanocubes sitting on the silicon substrate against one of its triangular facets, illustrating the high symmetry of these nanocubes. It is clearly seen from Figures 5b and 5c that the truncated nanocube is a single crystal, with its square/hexagon facets being indexed to {100} planes and triangular ones to {111} plane. Formation of PbS Nanocubes. To explore the influence of the different sulfur ion sources and surfactant on the morphology of PbS crystals under the same experimental conditions, the following experiments were performed. First, we employed thiourea instead of TAA as a sulfur ion source, at an initial reaction temperature of 40 °C and refluxing for 12 h at 100 °C in the presence of CTAB. PbS macrocrystals were obtained (Figure S1a). Second, we employed 3 g of the polymer poly(vinylpyrrolidone) (MW ) 30000) (PVP) instead of 3 g of
Zhou et al. CTAB, using TAA as a sulfur source. Only irregular nanoparticles were observed (Figure S1b in the Supporting Information). However, when these two parameters were varied simultaneously, using an equal mole of thiourea instead of TAA as precursor, and equal polymer PVP instead of surfactant CTAB as capping regent and under the same reaction conditions, the PbS nanocubes were exclusively formed as shown in Figure 6. In addition, the concentration of CTAB is also an important factor in the formation of PbS nanocrystals. When no surfactant was introduced, only irregular nanoparticles were formed (Figure S2a and S2c in the Supporting Information). When the concentration of CTAB was decreased to 1 g/100 mL in the reaction solution, the bigger cubes (Figure S2b in the Supporting Information) with about 200 nm edge length were obtained at the initial temperature 40 °C and refluxed for 12 h at 100 °C, and no dendrites were formed (Figure S2d in the Supporting Information) at the initial temperature 80 °C and refluxing for 30 min at 100 °C. XRD Measurements and Photoluminescence Spectra. Both the truncated nanocubes (the same samples in Figure 5a) and star-shaped dendrites (the same samples in Figure 1a) were also self-assembled on single-crystalline silicon wafers for powder XRD characterization. Figure 7a,b presents the XRD patterns recorded on these two samples, indicating that types of nanocrystals present a well-defined fcc structure with the Fm3m space group (a ) 5.936 Å). For a comparison, the relative intensities of the diffraction peaks from the standard card (JCPDS file number 05-0592) were labeled on the bottom of Figure 7 as well. In Figure 7a, it is worth noting that the ratio between the intensities of (200) and (111) diffraction peaks is higher than the conventional value (3.2 versus 1.2). This indicates that the nanocubes abound in {100} facets, which are consistent with the results from the SEM observation. The room-temperature photoluminescence spectra of the obtained PbS samples are shown in Figure 8. These spectra exhibit different features depending on their morphological variation. With the excitation wavelength of 495 nm, the emission maximums of the photoluminescence spectra of the obtained PbS samples are around 632 nm. The photoluminescence properties of nanocrystals are generally impacted by many factors, such as nanoparticle shape, size, and size distribution. In particular, the effect of lattice defect could not be ignored. It is thought that the nanocubes with higher crystallinity have low concentration of defects, which act as sites for nonradiative recombination of electron-hole pairs.50 As a result, the emission intensities of perfect nanocubes are stronger than those of truncated nanocubes and star-shaped dendrites. The emission
Figure 5. (a) SEM images of truncated PbS nanocubes synthesized at initial reaction temperature 40 °C, with refluxing for 12 h at 100 °C and the molar ratio of Pb(AC)2/TAA ) 1/3, in the presence of 3 g of CTAB/100 mL. (b) High-magnification SEM image of a slightly truncated PbS nanocube sitting on the silicon substrate against one of its square/hexagon facets. (c) High-magnification SEM image of a truncated PbS nanocube sitting on the silicon substrate against one of its triangular facets.
Controlled Synthesis of High-Quality PbS
Figure 6. TEM images of PbS nanocubes synthesized using thiourea as a precursor. The molar ratio of Pb(AC)2/thiourea ) 1/3 and the concentrations of Pb(AC)2 and thiourea are 0.03 and 0.09 mol/L, respectively, at initial reaction temperature 40 °C, with refluxing for 12 h at 100 °C, in the presence of 3 g of PVP/100 mL.
Figure 7. X-ray powder diffraction patterns of (a) truncated nanocubes and (b) star-shaped dendrites.
J. Phys. Chem. B, Vol. 110, No. 13, 2006 6547 bounded by the most stable {111} planes will be formed when R ) 1.73, and perfect cubes bounded by the less stable {100} planes will result if R is reduced to 0.58. For the truncated nanocubes illustrated in Figure 5, the ratio R should have a value close to 0.7-0.87. TAA was substituted by an equal mole of thiourea as precursor, and CTAB was substituted by PVP as capping regents. First, the growth of PbS crystals is weakened when thiourea is used as a sulfur source. Second, PVP is a nonionic surfactant and CTAB is a cation surfactant, so PVP is a weak binding ligand to the PbS crystal. Since the formation of the 2D nuclei on {111} faces has a low activation energy in weakly binding capping molecules,52 growth on the {111} faces is now favored and the R is reduced to about 0.58. Nanocubes bound by the {100} plane were formed as shown in Figure 6. When the initial reaction temperature was increased to 80 °C, the growth rates were accelerated. The difference between the enhanced growth rates on the {100} and {111} planes induced the ratio R to have a value of more than 1.73, which resulted in the formation of the star-shaped dendrites as shown in Figure 1a and 1b. Based on the systematic studies of varying growth parameter and detailed structure, we have discovered the critical factor for determining architectural features of the PbS nanocrystals. The molar ratio of Pb(AC)2/TAA and the initial reaction temperature have an influence on the growth ratio R in the formation of PbS nuclei at an early stage, which results in the final morphology of PbS nanocrystals. The molar ratio and the initial reaction temperature play a key role in the formation of PbS nanostructures. In other words, we can control the PbS shape evolution by simply tuning the molar ratio and the initial reaction temperature and the period of reaction. Not only can the stable PbS nano- and microstructures such as truncated nanocubes (Figure 4e and Figure 5) and six-arm star-shaped dendrites (Figure 1) be obtained, but also the metastable nanostructures such as star-shaped multipods nanocrystals (Figure 3a, 3b and Figure 4a, 4b) and nearly spherically nanoparticles (Figure 3c and Figure 4c) can be achieved under the current experimental conditions. Although another type of morphology of PbS nanocrystals, octahedron or truncated octahedron, was not observed this time, we believe that it could also be achieved under this experimental procedure by varying the growth parameters. This work is currently in progress. 4. Conclusion
Figure 8. Room-temperature photoluminescence spectra of PbS samples with various morphologies: (a) irregular nanoparticles prepared in the absence of surfactant, (b) star-shaped dendrites, (c) truncated nanocubes, and (d) nanocubes prepared in the presence of surfactant.
intensities of the irregular nanocrystals obtained in the absence of surfactant are weakest. Mechanism of Fabrication. It is well-known that surface energies associated with different crystallographic planes are usually different, and a general sequence may hold, γ{111} < γ{100} < γ{110}.51 The growth rates on different surface facets are dominated by the surface energy. When a particle grows, facets tend to the low-index planes to minimize the surface energy. As illustrated by Z. L.Wang,51 the shape of an fcc nanocrystal is mainly determined by the ratio (R) between the growth rates along the 〈100〉 and 〈111〉 directions. Octahedra
In summary, a simple solution process has been successfully developed to prepare PbS nano- and microstructures with various morphologies. Their shape evolution process and the influence of the growth parameters on the growth ratio R have been discussed. The desired architecture of building blocks such as star-shaped dendrites and truncated nanocubes can be consistently obtained by programming the growth parameters such as the molar ratio, the initial reaction temperature, and period of reaction. Based on the systematic studies on the shape evolution, this approach is expected to be employed for the control-shaped synthesis of other fcc structural semiconductor nanomaterials. The photoluminescence properties were investigated and the prepared structures displayed a very strong luminescence around 600-650 nm at room temperature. These properties may be promising for applications in the fabrication of photoelectric materials. Acknowledgment. This work is supported by the awarded funds of excellent state key laboratory (No. 50323006) and the Natural Science Foundation of Shandong Province (No. Y2003F08).
6548 J. Phys. Chem. B, Vol. 110, No. 13, 2006 Supporting Information Available: TEM images of the PbS nano- and macrostructures synthesized under various initial reaction temperature, precursor, and surfactant. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Alivisators, A. P. J. Phys. Chem. 1996, 100, 13226. (2) Hu, J.; Odom, T. W.; Lieber, C. M. Acc. Chem. Res. 1999, 32, 435. (3) Peng, X. G.; Manna, L.; Yang, W. D.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisators, A. P. Nature 2000, 404, 59. (4) Patzke, G. R.; Krumeich, F. K.; Nesper, R. Angew. Chem., Int. Ed. 2002, 41, 2446. (5) Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayers, B.; Gates, B.; Yin, Y.; Kim, F.; Yan, H. AdV. Mater. 2003, 15, 353. (6) Duan, X.; Huang, Y.; Cui, Y.; Wang, J.; Lieber, C. M. Nature 2001, 409, 66. (7) Manna, L.; Scher, E.; Kadavanich, A.; Alivisators, A. P. J. Am. Chem. Soc. 2000, 122, 12700. (8) Puntes, V. F.; Krishnan, K. M.; Alivisators, A. P. Science 2001, 291, 2115. (9) Mayers, B.; Gates, B.; Yin, Y.; Xia, Y. AdV. Mater. 2001, 13, 1380. (10) Ahmadi, T. S.; Wang, Z. L.; Green, T. C.; Henglein, A.; El-Sayed, M. A. Science 1996, 272, 1924. (11) Sun, Y.; Xia, Y. Science 2002, 298, 2176. (12) Wang, D.; Mo, M.; Yu, D.; Xu, L.; Li, F.; Qian, Y. Cryst. Growth Des. 2003, 3, 717. (13) Gou, L.; Murphy, C. J. Nano Lett. 2003, 3, 231. (14) Lifshitz, E.; Bashouti, M.; Kloper, V.; Kigel, A.; Eisen, M. S.; Berger, S. Nano Lett. 2003, 3, 857. (15) Feng, J.; Zeng, H. C. Chem. Mater. 2003, 15, 2829. (16) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Science 2001, 291, 1947. (17) Dai, Z. R.; Pan, Z. W.; Wang, Z. L. J. Phys. Chem. B 2002, 106, 902. (18) Liu, Z.; Liang, J.; Li, S.; Peng, S.; Qian, Y. Chem. Eur. J. 2004, 10, 634. (19) Zach, M. P.; Ng, K. H.; Penner, R. M. Science 2000, 290, 2120. (20) Murphy, C. J.; Jana, N. R. AdV. Mater. 2002, 14, 80. (21) Xiong, Y.; Xie, Y.; Li, Z.; Li, X.; Gao, S. Chem. Eur. J 2004, 10, 654. (22) Lu, Q.; Gao, F.; Zhao, D. Nano Lett. 2002, 2, 725. (23) Bognitzki, M.; Czado, W.; Frese, T.; Schaper, A.; Hellwig, M.; Steinhart, M.; Greiner, A.; Wendorff, J. H. AdV. Mater. 2001, 13, 70. (24) Yuan, Z. Y.; Zhou, W. Z.; Su, B. L. Chem. Commun. 2002, 1202. (25) Sawall, D. D.; Villahermosa, R. M.; Lipeles, R. A.; Hopkins, A. R. Chem. Mater. 2004, 16, 1606.
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