Solution-Controlled Self-Assembly of ZnO Nanorods into Hollow

ARTICLE pubs.acs.org/crystal

Solution-Controlled Self-Assembly of ZnO Nanorods into Hollow Microspheres Peng Hu, Xing Zhang, Ning Han, Weicheng Xiang, Yuebin Cao, and Fangli Yuan* State Key Laboratory of Multi-phase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, PR China

bS Supporting Information ABSTRACT: A novel solution-controlled self-assembly process has been developed to fabricate nanorod assembled ZnO hollow spheres without adding any structure-directing agents and templates, and as-prepared hollow samples with size about 5 μm are constructed by the radially oriented single-crystalline nanorods with length and diameter of about 1.5 μm and 80 nm, respectively. The investigation on the evolution formation reveals that water was critical to control the assembly of the fresh formed nanocrystallites, and hollowing formation was proved to be the Ostwald ripening process by tracking the structures of the products at different reaction stages. Scanning electron microscopy, X-ray diffraction, transmission electron microscopy, and high-resolution transmission electron microscopy were used to characterize the structure of synthesized products, and gas-sensing properties investigation shows that the nanorod-assembled hollow spheres exhibit high gas response to formaldehyde at the optimum working temperature of 400 °C.

1. INTRODUCTION During the past several decades, intense research in the field of one-dimensional (1D) nanostructures has resulted in substantial progress in their synthesis, property characterization, and applications.18 Although enormous progresses have been made in 1D nanostructures, there is still a long way to go to realize these great expectations in nanodevices. Importantly, many of these applications are based on the complicated structures assembled by the individual one to ensure their performance in nanodevics.911 For example, a nanowire array based power generator that could convert mechanical energy into electrical energy has been reported, and the continuous output of electric signal could only be observed by means of the piezoelectric effect of zinc oxide nanowire arrays rather than that of the individual one.12,13 Thus, the achievement of multidimensional interconnection of nanoscale building blocks into desired 3D structures is a significant challenge in the realization of advanced nanodevices. Different from the zero-dimensional nanopartices, bigger mass and volume are an obstacle for 1D nanostructures to assemble with the assistance of the weak interaction forces among themselves, such as van der Waals, electrostatic, etc., and the strategy for construction of 1D nanostructure patterns with ordered structures is usually driven by macroscale force. Generally, organic molecules and solid templates were used to restrict the growth and assembly to construct the complicated structure, and these approaches can be easily implemented.1418 For example, ZnO microhemispheres assembled with nanorods were fabricated by the cross-linking function of the poly(sodium 4-styrenesulfonate) by hydrothermal synthesis.19 1D ZnO arrays have been widely developed using sapphire substrate as templates by chemical vapor deposition.2023 However, these synthesis approaches have r 2011 American Chemical Society

rigorous restriction on the physical properties and structure of the assisting templates, and removal of the templates is also a time-consuming and tedious process.24 Accordingly, the development of rational approaches to assemble nanoscale building blocks into definite structures is still desirable and significant, which would greatly promote their applications in nanodevices. In this paper, we reported the synthesis of nearly monodisperse ZnO hollow spheres assembled with nanorods through a one-pot polyol solvothermal synthesis process without adding any structure-directing agents, and water was introduced to control the assembly of the products due to the surface hydroxyls originated from the water. The spherical assemblies could be obtained because this configuration can obtain the maximum touching area with the minimum surface energy, and this method provides a new strategy to adjust the nanoscale interaction forces among the 1D building blocks by the solution-controlled selfassembly process.

2. EXPERIMENTAL SECTION 2.1. Materials and Preparation. All the chemical reagents were analytically pure and used as-received without further purification. In a typical procedure, 1 g of Zn(Ac)2 3 2H2O was dissolved in 10 mL of distilled water and then mixed with 60 mL of glycerol under magnetic stirring to form a clear solution. The result mixture was transferred to a 100 mL Teflon-lined autoclave and then maintained at 200 °C for 12 h. After the experiment, the precipitate was collected, washed with water and ethanol several times, and then dried in air at ambient temperature. Received: October 27, 2010 Revised: April 1, 2011 Published: April 04, 2011 1520

dx.doi.org/10.1021/cg101429f | Cryst. Growth Des. 2011, 11, 1520–1526

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Figure 1. (a) Low magnification and (b) high magnification SEM images; (c) enlarged SEM image of a single particle with a broken shell; and (d) XRD pattern of as-synthesized products.

Figure 2. (a) TEM image of synthesized products; (b) TEM image and (c) HRTEM image of the rodlike building unit.

2.2. Characterization. The products were characterized by X-ray diffraction (XRD) recorded on a Philips X’Pert PRO MPD X-ray diffractometer using Cu KR radiation. Transmission electron microscope (TEM) images were taken on a Hitachi H-800 transmission electron microscope. High resolution (HR) TEM images were obtained using a JEOL JEM-2011 transmission electron microscope. Scanning electron microscopy (SEM) images were taken with a JSM-6700F scanning electron microscope. Photoluminescence (PL) spectra recorded from 350 to 800 nm at room temperature were carried out using 325 nm as the excitation wavelength with a luminescence spectrometer (Perkin-Elmer, LS50B). The gas sensing property was tested by a homemade instrument as we reported earlier.25

3. RESULTS AND DISCUSSION The morphology and structure of obtained products were investigated by SEM and XRD analyses as shown in Figure 1.

Figure 1a gives the general SEM image of as-synthesized products, which exhibits that as-synthesized products are composed of nearly monodisperse spherical particles with average size of about 5 μm. The regular shape and compact surface of the spheres could be further confirmed from the magnified SEM image shown in Figure 1b, and some broken spheres observed from the SEM image confirm the existence of a hollow interior in the spheres. The detailed structure of a single particle with a broken shell is shown in Figure 1c, which indicates that the shell of the spheres is built with the radially aligned nanorods beyond a micrometer in length with their growth axis perpendicular to the center of the core, self-wrapping to form hollow interiors. The hollow interior with size about 2 μm could also be measured from the broken sphere shown in Figure 1c. The phase structure and the composition of the samples were investigated by the XRD analysis shown in Figure 1d, and from the pattern we can see that 1521

dx.doi.org/10.1021/cg101429f |Cryst. Growth Des. 2011, 11, 1520–1526


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Figure 3. Products obtained with different amounts of water: (a) 5 mL; (b) 8 mL; (c) 25 mL; (d) 55 mL.

Figure 4. SEM images of products synthesized in different polyols: (a) ethylene glycol; (b) 1,2-propylene glycol; (c) ethanol.

all of the diffraction peaks could be readily indexed to the hexagonal structured ZnO (JCPDS 80-0075) with lattice constants of a = 0.325 nm and c = 0.521 nm. No characteristic peaks from other impurities are detected, indicating the formation of pure ZnO products. The sharp and strong diffraction peaks also confirm the good crystallization of the products. The detailed structure of obtained products is characterized by TEM and HRTEM analyses as shown in Figure 2, and Figure 2a shows the typical TEM image of obtained ZnO spheres. Although the shell of the products is too thick to get intensive contrast between the margin and the center, the difference of thickness can be observed between their edge and center, which confirms the formation of well-defined hollow structures. Since the resulting ZnO hollow spheres are constructed with highly directional interaction of nanorods with one end assembled together, the structure of the building units is further investigated as shown in Figure 2b and c. As indicated, the nanorod exhibits uniform diameter along its stem, and the measured length and

diameter of the nanorod are about 1.5 μm and 80 nm, respectively. The clear two-dimensional lattice fringes observed from the HRTEM image (Figure 2c) confirm the single crystallinity of the building units, and the lattice spacing of adjacent lattice planes is about 0.26 nm, which can be indexed to the (001) plane of the ZnO crystal, indicating that the nanorods are growing along the [001] direction. In our experiment, the introduction of water is critical to the configuration of the synthesized assembly, and its influences on the structure of the final products are investigated in detail. In glycerol solution without water, not assembled but sheetlike nanostructures are obtained in the products (Supporting Information, Figure S1). Figure 3 shows the SEM images of products synthesized with different amounts of water. When 5 mL of water is added, ZnO bundles assembled with nanorods of length 500 nm appeared in the products, as shown in Figure 3a, and no particles were produced. Upon further increases of water to 8 mL, ZnO bundles are further assembled with one end point 1522



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Figure 5. SEM images of products synthesized with different reaction times: (a) 1 h; (b) 6 h; (c) 12 h; corresponding TEM images shown in parts df; the scale bar is 1 μm.

together to form hollow spheres as shown in Figure 3b, while the spheres obtained have a rougher surface and small size of about 2 μm compared to that synthesized with the presence of 10 mL of water (shown in Figures 1 and 2). A change of morphology is observed by further increasing the amount of water in the system, and Figure 3c gives the SEM image of products obtained by adding 25 mL of water. The spheres part from the center and extend along two directions to form a two-hemispheroidal structure. The uniform spheres, however, could not be obtained if we further increase the water to 55 mL, and the products mainly consist of rodlike structures as shown in Figure 3d. The obtained microrods are composed of two sections with different diameters, and this structure possibly originates from the further extension of the hemispheroidal structures. Polyol synthesis has been widely applied to fabricate metal and oxide nanoparticles in glycol solution due to its relatively high boiling point, strong reducing ability, and coordination ability with transition metal ions.26,27 In these synthetic strategies, polyol serves as the reactant and exclusive solvent to facilitate the reaction, and the products obtained are generally the simplest case of spherical aggregates with fine nanocrystallites. Although the polyol synthesis has been intensively investigated, few efforts were focused on the polyol synthesis of nanostructures in complicated binary solvent systems, and the growth mechanism is also seldom involved. In our experiments, however, water was introduced to the solution and two competing reactions occurred as follows: 3ZnðAcÞ2 þ 2C3 H8 O3 f 3ZnO þ 2CH3 COOCH2 CHðCH3 COOÞCH2 COOCH3 þ 3H2 O ð1Þ ZnðAcÞ2 þ H2 O f ZnO þ 2CH3 COOH

ð2Þ

When pure glycerol serves as solvent or just a small amount of water is added, reaction 1 is preferred to occur under our

experimental conditions, and the ZnO crystallites are generated mainly by esterification between zinc acetate and glycerol, as we reported previously.28 It should be noted that the freshly formed crystallites have the tendency to aggregate together to decrease their surface energy due to the surface hydroxyls origin from the water, and that is why the nanorod assemblies are synthesized instead of sheetlike nanostructures when a small amount of water is introduced to the solvent. In this case, water only acts as binder to promote the aggregation of the nanocrystals. Furthermore, when the surface-capped nanocrystals are used as building blocks, they are easily assembled into a low-energy configuration to decrease the surface energy of the particles. As a result, spherical structures are obtained when 10 mL of water was introduced to the solvent. When the water is further increased, however, reaction 2 is dominant rather than reaction 1 due to the low boiling point and viscosity of water. In this case, water did not act as binder but a reactant, and the reaction mechanism is radically different from the reaction between zinc acetate and polyol. As a result, the products obtained are also different due to the fast migratory and nucleation rate of the reaction species in water, and the rodlike structures are finally synthesized when adding 55 mL of water to the solvent, which is similar to the nanostructures synthesized in aqueous solution. In our experiments, polyol also plays a key role in the formation of hollow structures. Compared to the fast migratory and nucleation rate of reaction species in aqueous solution, the nanocrystals generated in polyol solution should be kinetically slower due to high boiling point and great viscosity, which favors the nanocrystals to immigrate adequately to find the low-energy state and form spherical aggregations due to the stability of spherical structure. Figure 4 presents the SEM images of the products synthesized in different alcohol/water solutions, from which we can see that the similar hollow spheres are observed in polyol solution including ethylene glycol/water (Figure 4a) and 1523



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Figure 6. Schematic illustration of the evolution process of obtained hollow spheres.

Figure 7. PL spectrum of self-assembly ZnO hollow spheres.

1,2-propylene glycol/water solutions (Figure 4b), while the prismoidal structures are synthesized in ethanol/water solutions because of the fast nucleation and growth rate of nanocrystals caused by the low boiling point and viscosity similar to aqueous solution. To further investigate the formation mechanism of ZnO hollow spheres, time-dependent experiments were performed to gain insight into the evolution process, and the products collected at different stages of the experiments are shown in Figure 5. From these images we can clearly understand the hollowing process occurred in the intermediate products. At the early stage of 1 h, a spherical particle with solid core is formed from the SEM (Figure 5a) and TEM images (Figure 5d), and the sphere is comprised of numerous small crystallites rather than nanorods. When the reaction time is prolonged to 6 h, the hollowing process takes place in inner core of the sphere and results in the creation of a central space from the SEM image shown in Figure 5b, and the shell thickness and cavity can also be easily identified by the TEM observation shown in Figure 5e. It should be noted that the nanorods instead of nanocrystallites served as building blocks to construct the shell of the spheres with directional arrangement during this stage (Supporting Information, Figure S2). An obvious hollowing effect is observed with a longer reaction time of 12 h, which is confirmed by the brightness contrast of the shell and core in the TEM images shown in Figure 5f, and the nanorods on the shell packed more compactly than those at reaction of 6 h (Figure 5c). During the hollowing process, there is no apparent increase in size for the particle from the SEM and TEM observations. On the basis of the above analyses, it is therefore proposed that Ostwald ripening process continues to play a major role in the formation of the nanorod-assembled hollow spheres. In the initial stage, driven by the minimization of the surface energy,

Figure 8. Response of ZnO hollow structure to formaldehyde at different working temperatures: (a) 200 °C; (b) 300 °C; (c) 400 °C; (d) 500 °C.

Figure 9. Response and recovery curves of ZnO hollow structure to formaldehyde at working temperatures of 400 °C.

the primary hydroxyl-coated ZnO colloids aggregated together to form a solid spherical appearance. Because the crystallites located in the inner cores, compared to those in the outer surfaces, have high surface energy and thus easily dissolved, the nanocrystallites located on the outside would serve as starting points for the subsequent crystallization process of the core, and the nanocrystallites located in the core would provide the source for the durative growth of the shell during the solid evacuation. During the ripening process, epitaxial growth from the initial colloidal crystallites along the [001] direction is dominant because of the one-dimensional growth habit of the hexagonal ZnO crystal, and the result of the following process should be the nanorods growing at the expense of the cores inside the spheres. 1524



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Table 1. Formaldehyde Gas Sensing Property Comparison of ZnO Nanorods and Hierarchical Structures structures

preparation methods

response

ref

brushlike nanostructure from nanorods

two-step hydrothermal growth

450 ppm

nanorods

hydrothermal

650 ppm

44

nanorods with different lengths

plasma-enhanced CVD

85120 to 205 ppm

43

hollow sphere from nanorods

one pot solvothermal growth

132205 ppm

this study

When the reaction time is extended, the size of the polycrystalline core was reduced gradually while the hollow volume was enlarged. Finally, the nanorod-assembled hollow spheres were formed with complete depletion of the core with a longer reaction time of 12 h. The schematic illustration of the evolution process of obtained hollow spheres is shown in Figure 6. The highly directional interaction of nanorods is driven by the surface hydroxyls of the nanorods originated from the water to decrease their surface energy, which could be well illustrated by the experiments conducted with different amounts of water. It is generally accepted that the fresh generated nanocrystals have the tendency to aggregate together to decrease their surface energy, and water acts as binder to promote the aggregation of the nanorods. When nanorods assemble in a side-by-side manner, the minimum exposed area to solution could be obtained and thus achieve the minimum surface energy, which results in the formation of stable spherical products in solution. In the symmetric ripening process, the growth direction is from the shell and toward the core because the growth first starts at the shell and symmetric mass transport is from the inner core to the outer shell. In addition, the one-dimensional growth from the initial colloidal crystallites is dominant because of the preferable [001] growth of the ZnO crystal. The oriented growth of nanorods from the shell to the core causes the orderly 3D nanorod assembled side-by-side with one end toward the core and is also observed in the synthesis of other crystals with a polar growth nature,29 while, for other crystals, the hollow spheres aggregated with colloids tend to be obtained because of their homogeneous growth.30 Figure 7 shows the PL spectrum of as obtained self-assembly ZnO hollow spheres. From the pattern we can see two obvious emission peaks at ∼390 and ∼494 nm, as well as two shoulder peaks at ∼460 and ∼530 nm. The weak UV emission of products located at ∼390 nm is attributed to free-exciton recombination at the near-band edge of the ZnO crystal,31 while the origin of the strong blue luminescence (∼494 nm) is more preferably attributed to the oxygen vacancy.32,33 Since ZnO is an intrinsic n-type semiconductor, it is common to observe these two dominant peaks originated from recombination of electrons from the conduction band (CB) and from oxygen vacancy with the photogenerated holes in the valence band (VB), respectively. But there also exist other crystal defects in the ZnO crystal, such as zinc vacancy and oxygen antisite/interstitial. From the literature, the shoulder peak at ∼460 nm might be attributed to the recombination of electrons in CB with holes in the zinc vacancy,34 and the one at ∼530 nm might be attributed to the recombination of electrons from CB with holes in the oxygen antisite,35 while the large tail luminescence at longer than 530 nm might be attributed to the oxygen interstitia.36 Hierarchically assembled nanostructures usually show high gas sensing properties because of their high surface area and the fabricated junctions between nanorods. For example, tungsten oxide microspheres fabricated with nanorods showed high response to

42

NO2,37 and ZnO nanorod aligned structures showed high response to ethanol.38 In this study, the gas sensing property of as synthesized ZnO hollow spheres was investigated using formaldehyde as the probe. The sensor response is defined as Ra/Rg, where Ra and Rg are defined as the resistances of a sensor in air and in the target organic compound vapor, respectively. Figure 8 gives the response of the ZnO hollow structure to formaldehyde gas at different working temperatures (relative humidity of 70%). From the pattern we can see that the sensor response of the gas sensor enhances with the increase in formaldehyde concentration at all operating temperatures. According to the Scott theory,39,40 the sensor response is directly determined by the partial pressure of the target gas, in which the partial pressure of target gas is proportional to the concentration of formaldehyde gas. Therefore, the sensor response rises continuously with the enhancement of formaldehyde concentration. In addition, the ZnO hollow microsphere based gas sensor exhibits the highest response at the working temperature 400 °C (132 at 205 ppm), which reveals the optimum working temperature of the products. It is well-known that the sensing mechanism of semiconductor based gas sensors is a surface controlled process, and the sensing property is determined by the equilibrium of adsorption and desorption of the reactive gas species occurring at the surface. High temperature favors the chemical adsorption and physical desorption process; accordingly, the optimum the sensing performance appeared at the temperature with the maximum adsorption and minimum desorption of the reactive gas. The comparison of the formaldehyde gas sensing property is made as shown in Table 1, and it is obvious that the ZnO hollow spheres possess a higher response to formaldehyde than other nanostructures. The enhanced gas response might originate from two aspects. First, there are a lot of microchannels between ZnO nanorods in the hollow spheres, which provide active sites for formaldehyde gas adsorption and reaction to induce a resistance change of the sensor. Second, oxygen vacancy can also account for the gas response, as reported in the literature.4143 As shown in Figure 7, the ZnO hollow spheres show high blue luminescence (∼494 nm), which is usually attributed to oxygen vacancy. Therefore, the nanorod assembled ZnO hollow spheres have excellent formaldehyde sensing property. Figure 9 shows the real-time sensing characteristics of assynthesized ZnO hollow spheres to formaldehyde gas at the optimum working temperatures of 400 °C. Different gas concentrations were tested in the sequence 32, 85, and 205 ppm, and it clearly shows that the response amplitudes of the sensors are increased with increasing gas concentration. It should be noted that the sensor response increased immediately when the target gas was injected to the system and then decreased rapidly and recovered its initial value after the target gas was released, and the response time and recovery time (defined as the time required to reach 90% of the final equilibrium value) are calculated as (1.0, 1.7), (0.9, 1.4), and (0.5, 1.4) min at the gas concentrations of 32, 1525


Crystal Growth & Design 85, and 205 ppm, respectively, revealing the well performance of gas sensor based on the ZnO hollow spheres. In addition, after many cycles between the test gas and fresh air, the resistance of the sensor could recover its initial state, which indicates that the sensor has good reversibility.

4. CONCLUSION In summary, nanorod self-assembled ZnO hollow spheres with size about 5 μm are synthesized by a one-pot solvothermal synthesis process without any templates and surfactants in the reaction system, and as synthesized products are nearly monodisperse and aggregated by the directional assembly of singlecrystalline nanorods with length and diameter of about 1.5 μm and 80 nm, respectively. On the basis of the hollowing evolution of ZnO hollow spheres observed by TEM and SEM, the formation mechanism is proposed as water oriented attachment followed by Ostwald ripening, and the combination of such two processes leads to a novel way to synthesize hollow structures with controlled morphology and dimensionality. In addition, the investigation on gas sensing properties indicated that assynthesized hollow spheres exhibit high response and sensitivity to formaldehyde gas due to their larger effective surface area of the aligned nanorods. ’ ASSOCIATED CONTENT

bS

Supporting Information. The TEM image of products synthesized in glycerol solution without water is shown in Figure S1, and Figure S2 reveals the structure evolution of the spheres from nanoparticles to nanorods. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Phone: þ86-10-82627058. Fax: þ86-10-62561822. E-mail: fl[email protected].

’ ACKNOWLEDGMENT This project is financially supported by the National Science Foundation of China (NSFC 50974111, 10905068). ’ REFERENCES (1) Wang, X. D.; Song, J. H.; Liu, J.; Wang, Z. L. Science 2007, 316, 102. (2) Jeong, M. C.; Oh, B. Y.; Ham, M. H.; Lee, S. W.; Myoung, J. M. Small 2007, 3, 568. (3) Liu, Q. S.; Yan, Z.; Henderson, N. L.; Bauer, J. C.; Goodman, D. W.; Batteas, J. D.; Schaak, R. E. J. Am. Chem. Soc. 2009, 131, 5720. (4) Seo, D.; Song, H. J. Am. Chem. Soc. 2009, 131, 18210. (5) R€uhle, S.; van Vugt, L. K.; Li, H. Y.; Keizer, N. A.; Kuipers, L.; Vanmaekelbergh, D. Nano Lett. 2008, 8, 119. (6) Liu, J.; Xue, D. F. Adv. Mater. 2008, 20, 2622. (7) Biswas, S.; Kai, S.; Santra, S.; Jompol, Y.; Arif, M.; Khondaker, S. I. J. Phys. Chem. C 2009, 113, 3617. (8) Amirav, L.; Alivisatos, A. P. J. Phys. Chem. Lett. 2010, 1, 1051. (9) Huang, M. H.; Mao, S.; Feick, H.; Yan, H. Q.; Wu, Y. Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. D. Science 2001, 292, 1897. (10) Lyvers, D. P.; Moon, J. M.; Kildishev, A. V.; Shalaev, V. M.; Wei, A. ACS Nano 2008, 2, 2569. (11) Zhang, H. M.; Liu, P. R.; Liu, X. L.; Zhang, S. Q.; Yao, X. D.; An, T. C.; Amal, R.; Zhao, H. J. Langmuir 2010, 26, 11226.

ARTICLE

(12) Wang, Z. L.; Song, J. H. Science 2006, 312, 242. (13) Wang, X. D.; Song, J. H.; Liu, J.; Wang, Z. L. Science 2007, 316, 102. (14) Wang, X.; Hu, P.; Yuan, F. L.; Yu, L. J. J. Phys. Chem. C 2007, 111, 6706. (15) Lou, X. W.; Yuan, C.; Archer, L. A. Adv. Mater. 2007, 19, 3328. (16) Aldeek, F.; Balan, L.; Medjahdi, G.; Roques-Carmes, T.; Malval, J.; Mustin, C.; Ghanbaja, J.; Schneider, R. J. Phys. Chem. C 2009, 113, 19458. (17) Xu, H.; Wang, W. Angew. Chem., Int. Ed. 2007, 46, 1489. (18) Yang, M.; Ma, J.; Zhang, C. L.; Yang, Z. Z.; Lu, Y. F. Angew. Chem., Int. Ed. 2005, 44, 6727. (19) Mo, M.; Yu, J. C.; Zhang, L.; Li, S. A. Adv. Mater. 2005, 17, 756. (20) Gao, P. X.; Wang, Z. L. Nano Lett. 2003, 3, 1315. (21) Greyson, E. C.; Bbbayan, Y.; Odom, T. W. Adv. Mater. 2004, 16, 1347. (22) Baik, J. M.; Lee, J. L. Adv. Mater. 2005, 17, 2745. (23) He, J. H.; Lao, C. S.; Chen, L. J.; Davidovic, D.; Wang, Z. L. J. Am. Chem. Soc. 2005, 127, 16376. (24) Lou, X. W.; Yuan, C.; Rhoades, E.; Zhang, Q.; Archer, L. A. Adv. Funct. Mater. 2006, 16, 1679. (25) Han, N.; Tian, Y. J.; Wu, X. F.; Chen, Y. F. Sens. Actuator B 2009, 138, 228. (26) Deng, H.; Li, X.; Peng, Q.; Wang, X.; Chen, J.; Li, Y. D. Angew. Chem., Int. Ed. 2005, 44, 2782. (27) Jia, B. P.; Gao, L. J. Phys. Chem. C 2008, 112, 666. (28) Du, H. C.; Yuan, F. L.; Huang, S. L.; Li, J. L.; Zhu, Y. F. Chem. Lett. 2004, 33, 770. (29) Li, B. X.; Rong, G. X.; Xie, Y.; Huang, L. F.; Feng, C. Q. Inorg. Chem. 2006, 45, 6404. (30) Hu, P.; Yu, L. J.; Zuo, A. H.; Guo, C. Y.; Yuan, F. L. J. Phys. Chem. C 2009, 113, 900. (31) Lee, M. K.; Tu, H. F. J. Appl. Phys. 2007, 101, 126103. (32) Cheng, W. D.; Wu, P.; Zou, X. Q.; Xiao, T. J. Appl. Phys. 2006, 100, 054311. (33) Borseth, T. M.; Svensson, B. G.; Kuznetsov, A. Y.; Klason, P.; Zhao, Q. X.; Willander, M. Appl. Phys. Lett. 2006, 89, 262112. (34) Roro, K. T.; Dangbegnon, J. K.; Sivaraya, S.; Leitch, A. W. R.; Botha, J. R. J. Appl. Phys. 2008, 103, 53516. (35) Tsai, C. H.; Wang, W. C.; Jenq, F. L.; Liu, C. C.; Hung, C. I.; Houng, M. P. J. Appl. Phys. 2008, 104, 53521. (36) Studenikin, S. A.; Golego, N.; Cocivera, M. J. Appl. Phys. 1998, 84, 2287. (37) Liu, Z.; Miyauchi, M.; Yamazaki, T.; Shen, Y. Sens. Actuators B 2009, 140, 514. (38) Yang, Z.; Li, L. M.; Wan, Q.; Liu, Q. H.; Wang, T. H. Sens. Actuators B 2008, 135, 57. (39) Wan, Q.; Li, Q. H.; Chen, Y. J.; Wang, T. H. Appl. Phys. Lett. 2004, 84, 3654. (40) Scott, R. W. J.; Yang, S. M.; Chabanis, G.; Coombus, N.; Williams, D. E.; Ozin, G. A. Adv. Mater. 2001, 13, 1468. (41) Gao, T.; Wang, T. H. Appl. Phys. A: Mater. Sci. Process. 2005, 80, 1451. (42) Zhang, Y.; Xu, J. Q.; Xiang, Q.; Li, H.; Pan, Q. Y.; Xu, P. C. J. Phys. Chem. C 2009, 113, 3430. (43) Han, N.; Hu, P.; Zuo, A. H.; Zhang, D. W.; Tian, Y. J.; Chen, Y. F. Sens. Actuators B 2010, 145, 114. (44) Xu, J.; Chen, Y.; Chen, D.; Shen, J. Sens. Actuators B 2006, 113, 526.

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Solution-Controlled Self-Assembly of ZnO Nanorods into Hollow

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