Microsphere Organization of Nanorods Directed by PEG Linear

We demonstrate the sphere organization of ZnO, Bi2S3, MnO2, and La(OH)3 ..... Site-Specific Nucleation and Growth Kinetics in Hierarchical Nanosynthes...
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Langmuir 2006, 22, 1383-1387

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Microsphere Organization of Nanorods Directed by PEG Linear Polymer Xingfu Zhou, Shuyi Chen, Danyu Zhang, Xuefeng Guo, Weiping Ding,* and Yi Chen Lab of Mesoscopic Chemistry, Department of Chemistry, Nanjing UniVersity, Nanjing 210093, China ReceiVed August 2, 2005. In Final Form: December 13, 2005 We demonstrate the sphere organization of ZnO, Bi2S3, MnO2, and La(OH)3 nanorods directed by PEG linear polymer. Our study shows that zinc, bismuth, manganese, or lanthanum species added to PEG solutions, in which PEG molecules are well dissolved in a coil state, convert the polymer coils to aggregate structures, which further aggregate into micrometer-sized Mn+-PEG globules. The concentration of metallic species is higher in the globules than in bulk solutions. The surfaces of the globules act as soft templates for the initial nucleation and thereafter the growth of the nanorods. Finally, echinus-type assemblies of single-crystalline nanorods form by the metallic species hydrolyzing or reacting with deposition agents. This approach opens the possibility of using polymers as soft templates to control the organization of nano building units into designed structures.

Introduction The development of a rational route to the multidimensional assembly of nano building blocks into desired structures is a significant challenge in the design of advanced nanodevices.1 Self-assembly driven by various interactions is an effective strategy for forming versatile soft nanocrystal assembly motifs.2 Understanding factors and mechanisms governing the formation of nanocrystal assemblies would allow the design of desired nanostructures for optical, microelectronic, chemical, and biological applications.3 Such a capability is attractive to scientists not only because of its importance in understanding the concept of self-organization with artificial building blocks but also for its great application potential.4 As a result of rapid advancements in synthetic strategies, highly organized building blocks of metals,5 semiconductors,6 copolymers,7 organic-inorganic hybrid materials,8 and biominerals9 have been synthesized using various methods. However, the controlled organization of rodlike building blocks and the growth of nanorods in ordered aggregative structures are still important research topics. Wet chemical methods with so-called soft templates have been widely used in * To whom correspondence should be addressed. E-mail: dingwp@ nju.edu.cn. (1) Huang, Y.; Duan, X. f.; Wei Q. Q.; Lieber, C. M. Science 2001, 291, 630. (2) (a) Puntes, V. F.; Krishnan, K. M.; Alivisatos, A. P. Science 2001, 291, 2115. (b) Whitesides, G. M.; Grzybowski, B. Science 2002, 295, 2418. (c) Peng, X. G.; Manna, L.; Yang, W. D.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P. Nature 2000, 404, 59. (3) Shipway, A. N.; Katz, E.; Willner, I. ChemPhysChem 2000, 1, 18. (4) (a) Noble, P. F.; Cayre, O. J.; Alargova, R. G.; Velev, O. D.; Paunov, V. N. J. Am. Chem. Soc. 2004, 126, 8092. (b) Hosono, E.; Fujihara, S.; Kakiuchi, K.; Imai, H. J. Am. Chem. Soc. 2004, 126, 7790. (5) (a) Garcia-Santamaria, F.; Salgueirino-Maceira, V.; Lopez, C.; Liz-Marzan, L. M. Langmuir 2002, 18, 4519. (b) Correa-Duarte, M. A.; Perez-Juste, J.; SanchezIglesias, A.; Giersig, M.; Liz-Marzan, L. M. Angew. Chem., Int. Ed. 2005, 44, 4375. (c) Kaltenpoth, G.; Himmelhaus, M.; Slansky, L.; Caruso, F.; Grunze, M. AdV. Mater. 2003, 15, 1113. (d) Zeng, H.; Li, J.; Liu, J.;. Wang, Z.; Sun, S. Nature 2002, 420, 395. (6) (a) Yuan, J.; Laubernds, K.; Zhang, Q.; Suib, S. L. J. Am. Chem. Soc. 2003, 125, 4966. (b) Yada, M.; Taniguchi, C.; Torikai, T.; Watari, T.; Furuta, S.; Katsuki, H.; AdV. Mater. 2004, 16, 1448. (c) Fan, H.; Yang, K.; Boye, D. M.; Sigmon, T.; Malloy, K. J.; Xu, H.; Lopez, G. P.; Brinker, C. J. Science 2004, 304, 567. (d) Gao, P.; Wang, Z. J. Am. Chem. Soc. 2003, 125, 11299. (e) Hu, J.; Ren, L.; Guo, Y.; Liang, H.; Cao, A.; Wan, L.; Bai, C. Angew. Chem., Int. Ed. 2005, 44, 1269. (7) (a) Jenekhe, S. A.; Chen, X. L. Science 1998, 279, 1903. (b) Ikkala, O.; Brinke, G. T. Science 2002, 295, 2407. (c) Duan, H.; Kuang, M.; Wang, J.; Chen, D.; Jiang, M. J. Phys. Chem. B 2004, 108, 550. (8) Du, J.; Chen, Y. Angew. Chem., Int. Ed. 2004, 43, 5084. Du, J.; Chen, Y. Angew. Chem., Int. Ed. 2004, 43, 5194. (9) Shenton, W.; Pum, D.; Sleytr, U. B.; Mann, S. Nature 1997, 389, 585.

this area and have encouraged continuous insightful research work directed toward understanding mechanisms to be carried out.10 The alignment of gold nanorods directed by carbon nanotubes is an interesting example reported recently by CorreaDuarte et al.5b Regarding the generation of curved architectures from prefabricated components, Dinsmore et al. have developed the “colloidosomes” method, in which emulsified droplets act as soft templates for the self-assembly of colloidal particles (e.g., polymeric beads) into elastic spherical shells,11 and Park et al. have recently reported the organization of 1D organic-inorganic hybrid rods into various curved single-layer superstructures consisting of bundles, tubes, or sheets.12 Micrometer urchin-type assembly of various compounds with nano building blocks has been reported by some research groups.13-15 The exact role of the polymer in the organizing process, however, has long been an issue of debate. Extensive research is underway toward its clarification.13 Recently, Cao and co-workers have reported the hollow sphere assembly of V2O5 nanorods using PVP as a template.14b They concluded that the introduction of PVP is crucial in obtaining uniform hollow microspheres because of the template effect of the polymer. Mo et al. also thought that a spheric template was important in a similar synthesis.15 To date, little is known about the structures of metallic species and polymers in the solution at the beginning of the hydrothermal process,16 and the template effects of polymers on the assembly of nanorods are also still a concern. The reversible coil-globule transition of a single long-chain polymer has been well investigated in theory and experiment in the past two decades.17 The transition reflects the competition (10) (a) Kim, F.; Kwan, S.; Akana, J.; Yang, P. J. Am. Chem. Soc. 2001, 123, 4360. (b) Love, J. C.; Urbach, A. R.; Prentiss, M. G.; Whitesides, G. M. J. Am. Chem. Soc. 2003, 125, 12696. Morales, A. M.; Lieber, C. M., Science 1998, 279, 208. (c) Huang, M. H.; Mao, S.; Feick, H.; Yan, H.; Wu, Y.; Kind, H.; Webber, E.; Russo, R.; Yang, P. Science 2001, 292, 1897. (11) Dinsmore, A. D.; Hsu, M. F.; Nikolaides, M. G.; Marquez, M.; Bausch, A. R.; Weitz, D. A. Science 2002, 298, 1006. (12) Park, S.; Lim, J.-H.; Chung, S. W.; Mirkin, C. A. Science 2004, 303, 348. (13) (a) Chen, J.; Herricks, T.; Geissler, M.; Xia, Y. J. Am. Chem. Soc. 2004, 126, 10854.(b) Wang, Y.; Jiang, X; Xia, Y. J. Am. Chem. Soc. 2003, 125, 16176. (14) (a) Liu, B.; Zeng, H. C. J. Am. Chem. Soc. 2004, 126, 8124. (b) Cao, A. M.; Hu, J. S.; Liang, P. L.; Wan, L. J. Angew. Chem., Int. Ed. 2005, 44, 4391. (15) Mo, M. S.; Yu, J. C.; Zhang, L. Z.; Li, S. A. AdV. Mater. 2005, 17, 756. (16) Xu, F.; Xie, Y.; Zhang, X.; Wu, C Z.; Tian, X. B. New J. Chem. 2003, 27, 1331. (17) (a) Nishio, I.; Sun, S. T.; Swislow, G. Nature 1979, 281, 208. (b) Nakata, M., Phys. ReV. 1995, E51, 5770. (c) Zhang, G. Z.; Wu, C. Phys. ReV. Lett. 2001, 86, 822. (d) Zhang, G. Z.; Wu, C. J. Am. Chem. Soc. 2001, 123, 1376. (e) Yoshikawa, K.; Yoshikawa, Y.; Koyama, Y.; Toshio, K. J. Am. Chem. Soc. 1997, 119, 6473.

10.1021/la052105r CCC: $33.50 © 2006 American Chemical Society Published on Web 01/13/2006

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of two molecular forces (i.e., the attraction between solvent molecules and polymer chains and the interaction among sections of the polymer chain itself). Linear poly(ethylene glycol) (PEG) has been used in the synthesis of a series of nanoparticulates and 1D materials in solution.18 Here we describe its role in the synthesis of hollow microsphere assemblies of ZnO, Bi2S3, MnO2, and La(OH)3 nanorods and emphasize that the metallic species (i.e., zinc, lanthanum, bismuth, and manganese cations) increase the attraction among the polymer chains by coordination and cause the aggregation of multiple PEG chains to micrometer spheres. The spheres act as a soft template for the initial nucleation and thereafter the directional growth of nanorods. Experimental Section Chemicals. All chemicals are analytical-grade reagents and were purchased from Shanghai Chemical Reagent Corp. Poly(ethylene glycol) (PEG, MW 20 000), ethylene glycol (EG), ethanol, Zn(NO3)2‚6H2O, Bi(NO3)3‚5H2O, MnSO4‚H2O, La(NO3)3‚4H2O, NaOH, thiourea, H2O2 (30%), and ammonia (25%) were used as reactants without further purification, and doubly deionized water was used throughout. The key step in the aggregation of nanorods described above is the nucleation of nanorods on the PEG globules, at which the concentration of transition-metal species is higher than in bulk solution. For the four different compounds, the nucleation conditions were optimized in the experiment and are described in the following paragraphs. The solvent used for the different metallic compounds was selected according to dynamic light scattering (DLS) measurements. If the signals of particle response from the solution were stable and repeatable by dissolving a metallic compound in a PEGcontaining solvent (i.e., water, ethanol, ethylene glycol or their mixture), then the solvent is adequate for the assembly of nanorods. Microsphere Organization of ZnO Nanorods. PEG (MW 20 000, 2 g) and 0.003 mole of Zn(NO3)2‚6H2O were dissolved in 5 mL of doubly deionized water and 50 mL of anhydrous ethanol. The mixture experienced 10 min. of supersonic (20 kHz) agitation in a pulverizer at a power of 400 W to ensure that all of the reagents were dispersed homogeneously in the solution and was left to rest at room temperature for 3 h. Then 0.06 mole of NaOH diluted with 10 mL of deionized water was added to the above solution, and 80 mL of the transparent solution was hydrothermally treated at 180 °C for 20 h in a Teflon-lined autoclave. After the reactions, white crystalline products were harvested by centrifugation. The nature of the reaction of ZnO formation is the hydrolysis of sodium zincate. Microsphere of Bi2S3 Nanorods. PEG (MW 20 000, 2 g) and 4.8507 g of Bi(NO3)3‚5H2O (0.01 mole) were dissolved in 30 mL of EG. The mixture experienced 10 min. of supersonic (20 kHz) agitation in a pulverizer at a power of 400 W to ensure that all of the reagents were dispersed homogeneously in the solution and left to rest at room temperature for 3 h. Then 1.5224 g (0.02 mole) of thiourea was added to the above solution, which was then hydrothermally treated at 180 °C for 20 h in a Teflon-lined autoclave. Microsphere of La(OH)3 Nanorods. PEG, (MW 20 000, 2 g) and 0.01 mole of La(NO3)3 were dissolved in 30 mL of deionized water in a flask. The mixture experienced 10 min. of supersonic (20 kHz) agitation in a pulverizer at a power of 400 W to ensure that all of the reagents were dispersed homogeneously in the solution and left to rest at room temperature for 3 h. Then the pH of the solution was adjusted to 6.0 with aqueous ammonia, and the mixture was hydrothermally treated in a Teflon-lined autoclave at 180 °C for 20 h. Microsphere of MnO2 Nanorods. PEG (MW 20 000, 2 g) and 0.01 mole of MnSO4‚H2O were dissolved in 50 mL of deionized water, The mixture experienced 10 min. of supersonic (20 kHz) (18) (a) Bognitzki, M.; Hou, H. Q.; Ishaque, M.; Frese, T.; Hellwig, M.; Schwarte, C.; Schaper, A.; Wendorff, J. H.; Greiner, A. AdV. Mater. 2000, 12, 637. (b) Cao, M. H.; Wang, Y. H.; Guo, C. X. J. Nanosci. Nanotech. 2004, 4, 824. (c) Li, Z. Q.; Xiong, Y. J.; Xie, Y. Inorg. Chem. 2003, 42, 8105. (d) Hammond, P. T. AdV. Mater. 2004, 16, 1271.

Letters agitation in a pulverizer at a power of 400 W to ensure that all of the reagents were dispersed homogeneously in the solution and was left to rest at room temperature for 3 h, to which 0.015 mole of H2O2 was added. Finally, the mixture was hydrothermally treated at 160 °C for 15 h. All products were collected and cleaned with hot water and absolute alcohol three times to remove the remaining PEG and then dried at 60 °C. Characterization. Powder XRD measurements were performed on a Philips X’Pert MPD Pro X-ray diffractometer with graphite monochromatized high-intensity Cu KR radiation at 50 kV. Raman spectra (λ ) 514 nm) were collected for detection of the organic compound contained in the samples. Scanning electron microscopy (SEM) images were taken on a JSM-5900 instrument. The average diameters of the complex sphere of the transition-metal compoundPEG species were determined by DLS on Brookhaven BI-9000AT and Zeta Plus instruments (Brookhaven Instruments Corporation); all measurements were repeated three times. The transmission electron microscopy (TEM) images and electronic diffraction (ED) patterns were taken on a JEOL JEM-200CX instrument at an acceleration voltage of 200 kV. The high-resolution transmission electron microscopy (HRTEM) observations and energy-dispersive X-ray analysis (EDX) were performed on a JEOL JEM-2010 instrument at an acceleration voltage of 200 kV.

Results and Discussion Morphology and Structure. As depicted in Figure 1, all four samples show morphology similar to that of the micrometer sphere, which is the secondary structure made from an aggregate of nanorods. In the Figure, the top right insets show overviews of the samples. The spheres appear to be uniform and several micrometers in diameter, but they are different in size among samples. Further analysis of the structure of ZnO nanorods shows that the direction of 〈0001h〉 points out of the sphere, which is the slower-growth end.19 This means that the faster-growth end 〈0001〉 of the nanorods is on the surface of the PEG cores. The TEM image of a single ZnO nanorod and corresponding selected-area electron diffraction pattern (SAED) are shown in the left inset of the ZnO SEM images, indicating single crystallinity and the 〈0001〉 direction in the long axis of the nanorod. Figure 2 shows XRD patterns of the spherical organization of the four samples that elucidate their crystalline structure. They are phase-pure wurtzite-type ZnO (a ) 3.25 Å and c ) 5.21 Å, JCPDS 89-1397), Bi2S3 (a )11.149 Å, b ) 11.304 Å, and c ) 3.971 Å, JCPDS 17-320), MnO2 (a ) 9.266 Å, b ) 2.860 Å, and c ) 4.512 Å, JCPDS 43-1455), and La(OH)3 (a ) 6.528 Å and c ) 3.858 Å, JCPDS 41-4019), respectively. Mechanism of the Sphere Organization of Nanorods. Interactions among metallic species and PEG chains are important in the current investigation. To describe the skeleton of the interaction of PEG and metallic species, TEM and DLS methods are used to clarify the size of the aggregates of metallic species and the polymer template. Taking the ZnO synthesis as an example, 3 mL of stock solution is poured into the terrarium for the light scattering measurement, and a drop of this solution is also used for TEM observation. As shown in Figure 3a, PEG dissolved in ethanol without the addition of zinc nitrate shows a coil-like state, which is composed of bundles of PEG chains. It is noteworthy that the aggregation of PEG chains produces a tube-like shape when zinc nitrate is added to the solution (Figure 3b). The interaction among metallic species and PEG chains makes PEG chains wrap with each other with zinc species in between.20 Figure 3c shows the tube-like structures further (19) Li, W. J.; Shi, E. W.; Zhong, W. Z.; Yin, Z. W. J. Cryst. Growth 1999, 203, 186 (20) Michael, J. B.; George, R. A. J. Am. Soc. Mass Spectrom. 2002, 13, 177.

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Figure 1. SEM images of microsphere assemblies of ZnO, Bi2S3, γ-MnO2, and La(OH)3 nanorods. Scale bars are 200 nm, 1 µm, 1 µm, and 300 nm, respectively. The top right insets show overviews of the as-prepared samples. The left inset of ZnO depicts the TEM image of a single nanorod of ZnO and corresponding electron diffraction patterns.

The nature of metallic species (e.g., coordination ability and molecular volume in solvated M-PEG complexes) has a profound effect on the size of the assembled globules.22 Figure 3d shows the effective diameters of Zn-PEG, Mn-PEG, La-PEG, and Bi-PEG, measured by the light scattering method. The effective diameters are about 480, 543, 1024, and 1834 nm for the four M-PEG complexes. The globules act as a source of metallic ions for nucleation and direct the growth of the nanorods to form the microspheres. In this study, the introduction of PEG is crucial to obtaining a uniform spherical organization of nanorods; no spherical organization of nanorods is obtained in the absence of PEG under the current synthetic conditions.

Figure 2. XRD patterns of the samples, which are indexed to (a) ZnO (JCPDS 89-1397), (b) Bi2S3 (JCPDS 17-320), (c) MnO2 (JCPDS 43-1455), and (d) La(OH)3 (JCPDS 41-4019).

aggregated to globules after 3 h of solution aging. TEM shows that the further aggregated globule is about 500 nm in diameter, consistent with the results of DLS (∼480 nm, Figure 3d). Some fine structures with a tube shape can still be seen on the surface of the globules. Interestingly, the globule falls to pieces when bombarded by high-energy electrons in the TEM chamber, and the shape of the pieces is similar to the original tube-like structures (right inset of Figure 3c), reflecting the fact that the globules consist of tube-like structures via weak van De Waals forces.7 The minimization of the total surface energy is the key factor in the transition of the tube-like structures to globules.12,21 The left inset of Figure 3c shows EDX results for the globule. The ratio of Zn/C is about 0.19 as measured at the globules, which much larger than 0.03, the value for the homogeneous distribution of zinc and PEG. Thus, the concentration of zinc species is much higher in the globules than in bulk solution. (21) Pileni, M. P. Nat. Mater. 2003, 2, 145.

Figure 4a shows a broken sphere of ZnO nanorods, a granule of size ∼520 nm indicated by a square in the Figure, that matches the central cavity of the broken sphere and could be thought of as the PEG polymer core. The inset of Figure 4a depicts a model of the ZnO rod in basic solution.23 The +c end with a plane of (0001) at the end of the prismatic cone can be found in the interior of the broken sphere. It is known that the -c end with the (0001h) plane is the slowest growth direction whereas the +c end with a plane of (0001) is the fastest growth direction.19 All ZnO nanorods point out with flat (0001h) planes, as shown in Figure 1, implying that the nanorods nucleate and grow on the surface of the Zn-PEG polymer template. PEG nuclei encapsulated by the nanorods in the center of the assembled sphere give rise to a spectroscopy response. The upper inset of Figure 4b shows the Raman spectrum of the ZnO sphere assembled nanorods after a thorough washing. The peaks at 330, 429, and 575 cm-1 are attributed to ZnO,25 whereas those peaks at 1046, 1138, and 2930 cm-1 belong to the contribution of organic PEG. (22) Robi, D. G.; Zhang, J. H.; Cary, B. B. J. Alloys Compd. 1997, 249, 41. (23) Wang, H. H.; Xie, C. S.; Zeng, D. W. J. Cryst. Growth 2005, 277, 372. (24) Zheng, M. J.; Zhang, L. D.; Li, G. H.; Shen, W. Z. Chem. Phys. Lett. 2002, 363, 123. (25) Damen, T. C.; Porto, S. P. S.; Tell, B. Phys. ReV. 1966, 142, 570.

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Figure 3. (a) TEM micrographs of PEG chains. (b) Addition of zinc nitrate to the above solution leads to unordered tube-like structures; the marked area shows a small globule connected with the tube-like structures. (c) Tube-like assembly further aggregated to globules with fine tube-like structures at the surface. The top inset shows that the globule falls to pieces under long-time bombardment by high-energy electrons in the TEM chamber. The bottom inset shows EDX results obtained for the globules. (d) Effective diameters for solvated Zn-PEG, Bi-PEG, La-PEG, and Mn-PEG globules measured by the light scattering method.

Figure 4. (a) SEM images of a broken sphere; the marked dashed square shows the organic nuclei matched to the central cavity of the broken sphere; the inset shows a structural model of a ZnO nanrod. (b) Correlation between the results of SEM and light scattering measurements. The top inset shows the Raman spectrum of the sphere of ZnO nanorods, and the lower inset depicts a model of a ZnO nanorod assembled sphere.

The diameter of the sphere organization of ZnO single-crystal nanorods is ∼3.1 µm (Figure 1a), and the nanorods have a uniform length of 1.3 µm. Then the size of the central PEG nucleus can be deduced as ∼0.5 µm in diameter, which matches the results of TEM and light scattering. This is also the case for the other three samples, albeit their sizes of sphere organization and the polymer nuclei are different from each other. Figure 4b shows a correlation between the sizes of the polymer nuclei measured by SEM and light scattering methods; they are in good agreement. The lower inset of Figure 4b shows a geometrical model for the sphere organization of ZnO nanorods. ZnO nanorods with a length of 1.3 µm and a diameter of ∼100 nm symmetrically encircle the ∼500 nm PEG globule. Physical geometric limits, longrange electrostatic interactions, and even the strong chemical

bonding between the contacting lateral surfaces at the inner ends of the rods maintain the stability of the sphere. A general mechanism is proposed and shown in Figure 5. The addition of metallic species to linear-PEG-containing ethanol solution, in which PEG is well dissolved in a coil state, led to the aggregation of the polymer coils to unordered tube-like structures, which further aggregated the Mn+-PEG globules. The metallic species wrapped in the globules will preferentially hydrolyze to corresponding solid metallic compound nuclei at the globule surface by the reaction of concentrated metallic species with deposition reagent at the interface. As soon as the nuclei form, the metallic species may diffuse to the nuclei across the interface for growth because of the asymmetric distribution of metallic species in the vicinity of the interface. The concentration gradient of metallic species pointed out from the inner globule to the bulk solution. Maybe this is also a mechanism similar to “VLS” in solution. The anisotropic nature of the crystal structure of the products is also important to the anisotropic growth to nanorods. The current mechanism for the growth of nanorods on PEG globules is different from what Peterson et al. have reported for ZnO nanorods grown on solid glassy substrates.26 In their research, the adhesion of ZnO species in solution directly onto the solid glassy substrate is very difficult because of the great structural mismatch and the weak interaction between the slippery glassy substrate and crystalline ZnO. These factors lead to the need for a sputtered ZnO film on the glassy substrates before growth in solution. Insight into such an organization processes is important in understanding the mechanism of the fabrication of a new category of materials. It could therefore offer opportunities for further fundamental research on the full use of chemical assembly as (26) Peterson, R. B.; Fields, C. L.; Gregg, B. A. Langmuir 2004, 20, 5114.

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Figure 5. Schematic of the process of sphere organization of nanorods. (a) Mixed solution of PEG and metallic ions. (b) PEG coils and metallic species assemble to unordered tube-like structure. (c) Unordered tube structures aggregate to globules. (d) Nanorods-assembled sphere.

well as for technological applications in curved 3D or planar organization of electronic and optically active nano or meso building blocks.

Conclusions The sphere organization of ZnO, Bi2S3, MnO2, and La(OH)3 nanorods directed by complexes of metallic species and PEG chains has been elucidated. The metallic species added to linearPEG-containing ethanol solution convert the polymer coils and metallic species to tube-like structures, which further aggregate into globules. The globules contain more concentrated metallic

species than bulk solutions. The globules act as a soft template for the initial nucleation and thereafter the growth of the nanorods. Finally, single-crystalline nanorods of inorganic compounds grow and aggregate into echinus-type assemblies by the metallic species hydrolyzing or reacting with deposition agents. The unveiled mechanism will be useful for the organization of nanorods on flat and curved surfaces with featured structures. Acknowledgment. We are thankful for financial support from the MOST of China under contract no. 2003CB615804. LA052105R