J. Phys. Chem. C 2007, 111, 1113-1118
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Malate-Assisted Synthesis of ZnO Hexagonal Architectures with Porous Characteristics and Photoluminescence Properties Investigation Jianbo Liang,† Sha Bai,† Yansheng Zhang,‡ Ming Li,†,‡ Weichao Yu,† and Yitai Qian*,†,‡ Department of Chemistry, and Hefei National Laboratory for Physical Sciences at Microscale, UniVersity of Science and Technology of China, Hefei, Anhui 230026, P.R. China ReceiVed: September 25, 2006; In Final Form: October 31, 2006
ZnO 3D hierarchical architectures with hexagonal shape and uniform size have been successfully synthesized with the assistance of sodium malate by a simple hydrothermal process. Scanning electron microscopy (SEM) images show that the hexagonal particles are composed of platelet-like nanoparticles that are orderly arranged to multilayer stacks. High-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) analysis indicate that these nanoplatelets are single-crystal and grow perpendicularly to the c-axis of wurtzite ZnO. Nitrogen adsorption-desorption measurements reveal that the specific surface area of the sample can reach to 25 m2‚g-1, which is as high as that of 16 nm sized ZnO nanorods (aspect ratio of 1.6:1). The pore size distribution curve suggests the specific surface area improvement is due to the existence of small pores embedded in the 3D architectures. These small pores are attributed to the small gaps between these nanoplatelets. Malate ions have been found to play a key role in the formation of the porous 3D architectures. Room temperature photoluminescence measurements show that the porous architectures prepared in the presence of malate indeed exhibit intense ultraviolet exciton emission centered at 387 nm. The defects related yellow and green emissions have been greatly quenched, suggesting the sample is in high crystalline quality though it possesses porous characteristics.
Introduction Systematic control over the size, morphology, phase, and architectures of inorganic crystals at micro- and nanoscale levels is still a significant challenge in the modern synthetic scientific field, which is of increasing attention due to their strong influence on materials properties.1,2 Zinc oxide, a remarkable II-VI compound semiconductor with a direct wide band gap of 3.37 eV and large exciton binding energy of 60 meV at room temperature, has attracted considerable interest in the past years mainly because of the unique optical and electrical properties, as well as its potential application in optical waveguides,3 surface acoustic wave transducers,4 blue light-emitting diodes (LEDs),5 solar cells,6 chemical sensors and photocatalysts,7,8 etc. In particular, since the ultraviolet lasing from ZnO nanostructures has been demonstrated at room temperature,9 much effort has been devoted to synthesize ZnO with controlled size and architecture. Solid-vapor phase growth (SVG) and chemical vapor deposition are the major physical methods to fabricating ZnO nanostructures including nanobelts,10a nanocombs,10b nanospings, nanohelixes, 10c hierarchial nanostructures,11 nanobridges and nanonails,12 and nanotubes.13 In contrast, the solution-phase synthesis (SPS), including microemusion and hydrothermal growth, has also been proved to be effective and convenient in preparing ZnO complex nanostructures such as helical columns,14 volcano-like15a,15b and tower-like15c nanotube arrays, hexagonal disks and rings,16 rotor-like17a or pyramid-shaped nanocrystals,17b self-assembled hollow microsemispheres,18 etc. Inorganic materials with porous architectures can exhibit a wide variety of applications ranging from bioengineering, * Author to whom correspondence should be addressed, E-mail: ytqian@ ustc.edu.cn. Phone: +86-551-3603204. Fax: +86-551-3601589 † University of Science and Technology Of China. ‡ Hefei National Laboratory for Physical Sciences at Microscale.
catalysis, and environmental engineering to chemical and gas sensors for their high surface-to-volume ratio. Generally, porous materials are often synthesized via a surfactant-templating formation process.19 There have been emerging attempts to directly grow inorganic materials with porous architectures without the assistant of templates.20,21 For ZnO, it has been reported that nanorods with porous surface and mesoporous polyhedrals have been prepared by metal -assisted vapor-phase processes.10d,22 In this work, we report the synthesis of ZnO porous 3D architectures by a hydrothermal process. Sodium malate has been introduced to control the microstructure of the products. According to the nitrogen adsorption-desorption measurements, this sample possesses higher specific surface areas than the products obtained in the absence of malate salts. This is mainly attributed to the existence of small pores embedded in the 3D architectures. In addition, room temperature photoluminescence measurements show that the sample prepared in the presence of malate exhibits intense ultraviolet exciton emission. The defects related yellow and green emissions can be barely observed. The high intensity ratio between the exciton emission and defects related emission indicates that although the sample shows porous characters, it indeed possesses good crystalline quality. Experiment Section All the reagents used in the experiments were in analytical grade (purchased from Shanghai chemical Industrial Company) and used without further purification. In a typical synthesis, 2.5 mmol Zn (Ac) 2‚2H2O were dissolved in 50 mL of diluted water. White gels appeared immediately when 0.5 mL of commercial ammonia (28 wt%) was added. The mixtures were stirred from 5 min and 2.5 mmol
10.1021/jp0662808 CCC: $37.00 © 2007 American Chemical Society Published on Web 12/22/2006
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Figure 1. XRD pattern of the ZnO sample prepared in the presence of sodium malate.
sodium malate was added. The pH value of the final solution after being stirred for 15 min was measured to be 8.0. The mixtures were transferred into a 55 mL Teflon-lined autoclave, maintained at 120 °C for 8 h. The white powders collected from the bottom of the container were washed with distilled water and absolute alcohol in turns, vacuum-dried, and kept for further characterization. The powder XRD patterns were recorded with a Rigaku (Japan) D/max-γA X-ray diffractionmeter equipped with graphite monochromatized Cu KR radiation (λ ) 1.541874 Å). SEM images were obtained with a field emission scanning electron microscope (FESEM), JEOL JSM-6700F, and Sirion 200, FEI. The structures of the products were investigated by transmission electron microscopy (Hitachi H-800 with an accelerating voltage of 200 kV) and HRTEM (JEOL 2010). Thermogravimetry analysis was carried out on a Shimadzu TA-50 at a heating rate of 10 deg/min under a stream of air. The nitrogen adsorption and desorption isotherms were measured with a Micrometrics ASAP 2000 system after the sample was degassed in vacuum overnight. Optical property investigations were performed on the JY LABRAM-HR Laser MicroRaman Spectrometer: the Raman spectrum was obtained with 515 emission lines, and the photoluminescence (PL) measurements were also carried out using the 325 nm excition line of the He-Cd laser at room temperature.
Figure 2. FESEM image of the ZnO sample obtained in the presence of sodium malate: (a) panoramic image shows the abundance of micropartticles; (b) high magnification view of the samples; and (c) typical image of a selected hexagonal microarchitecture.
Results and Discussion Morphology and Structure. The synthesis of ZnO porous architectures has been achieved by thermal treatment of freshly prepared Zn-OH compounds in aqueous solution in the presence of sodium malate. The related X-ray diffraction (XRD) pattern (Figure 1) shows sharp peaks corresponding to wurtzite ZnO (Joint Committee on Powder Diffraction Strands (JCPDS) No. 05-0664), confirming the formation of pure ZnO powders. Figure 2a shows a low-magnification scanning electron microscopy (SEM) image of the ZnO products after hydrothermal treatment at 120 °C for 8 h, manifesting the formation of highly uniform hexagonal shaped microparticles abundantly. The edge length of these particles ranges from about 2 to 5 µm. An enlarged SEM image shown in Figure 2b suggests these microparticles, which exhibit well-defined hexagonal geometry with six symmetrical edges, are indeed composed of clearly distinguished platelet-like nanoparticles. Close views clearly show that these nanoplatelets, which are self-stacking one layer by one layer to form ordered arrangement, are about 30-60 nm thick. These orderly stacks are somewhat similar to the calcite mesocrystals.23 It is interesting that these nanoplatelets
Figure 3. TEM images of the ZnO hexagonal architectures: (a) a single hexagonal ZnO particle; (b) detailed image taken from the edge of the particle; and (c and d) HRTEM image of the subunit nanoplates.
always stack on one side of the microarchitecture, leaving a retuse concave on the center of this surface, whereas the other surface is relatively flat. This may result from the polarity of ZnO and different activity of the polar surfaces.10b Although small interstices can be observed from the enlarged images, these hexagonal microparticles are stable and could not be destroyed into individual nanoplates even under high-intensity ultrasonication subjections, indicating the integrating force between the adjacent nanoplates is the strong chemical bonding at the contacting surface as well as weak van der Waals’ interactions.18 The intrinsic crystalline structures of the hexagonal architectures were investigated by high-resolution transmission electron microscopy (HRTEM). Figure 3a shows a typical image of an isolated ZnO hexagonal. The center portion of the hexagonal is lighter than the edge, confirming that the existence of the retuse concave at the center. The magnified image taken
Synthesis of ZnO Hexagonal Architectures
Figure 4. Nitrogen adsorption/desorption isotherm and BarrettJoyner-Halenda (BJH) pore size distribution plot (inset) of assynthesized ZnO particles.
from one edge shows saw-tooth-like brims, manifesting the hexagonal architectures are composed of orderly arranged subunits. The angle between the two adjacent planes shown in Figure 3b is measured as 120°. The selected area electron diffraction (SAED) pattern taken from the microparticle shows hexagonally symmetric spots with a slight tilt, indicating the crystalline nature as well as the same orientations of the nanoplates. This diffraction pattern can be indexed as the 〈0001〉 crystalline zone axis. The appearance of periodic diffraction spots suggests that these nanoplate subunits self-assembled into highly oriented aggregates, and diffracted as a single crystal. The SAED investigation demonstrates that the crystallographic axes, i.e., the 〈0001〉 direction of these nanoparticles, are in a parallel line and these subunit nanoparticles are of the same orientation along this direction.10d,24 The HRTEM image of a subunit particle shows the lattice fringes with a spacing of 0.28 nm, which corresponds to the interplace of (101h0) planes of hexagonal ZnO. The hexagonally arranged image indicates that the particle is grown along the (0001) surface planes. On the basis of the HRTEM and SAED measurements, we conclude that the hexagonal architectures are constructed by platelet-like nanoparticles with parallel c-zone axes. These subunit particles expand along the planes perpendicular the [0001] direction of ZnO. Energy-disperse X-ray spectrum (EDS) and thermal gravimetric analysis (TGA) measurements further confirm asprepared samples are real ZnO. There are no malate salts incorporate in the particles (see the Supporting Information). To provide further insight into the inner structure of the hexagonal ZnO architectures, Brunauer-Emmett-Teller (BET) gas sorptometry measurements were conducted to examine the porous nature of the ZnO hexagonal architectures. Figure 4 shows the N2 adsorption/desorption isotherm and the pore-size distribution (inset) of ZnO hexagonal architectures. The isotherms are identified as type IV, which is characteristic of mesoporous materials.25 The pore-size distribution obtained from the isotherm indicates a number of pores less than 5 nm in the sample. These pores presumably arise from the interstices among the nanoplatelets within the ZnO hexagonal architectures. The large pores of around 100 nm are attributed to the spaces between the microparticles. The sharp distribution of the mesopores around 100 nm suggests that the microparticles have high monodispersity.26 The BET specific surface area of the sample is calculated from N2 isotherms at -196.6 8 °C, and is found to be as much as about 25.1 m2 g-1. Actually, the surface area of the sample prepared without addition of sodium malate
J. Phys. Chem. C, Vol. 111, No. 3, 2007 1115 is only 0.4 m2 g-1. For a comparation, we perform a theory calculation of ZnO particles with well-defined shapes.25 Calculated results show that the specific surface area of the ZnO solid hexagonals with a similar size is only 0.16 m2 g-1. And the specific surface area of our sample is compared with 16 nm sized particles with an aspect ratio c/a ) 1.6 (see the Supporting Information). Thus the large specific surface area possessed by the sample is mainly due to the existence mesopores embedded in the microarchitectures. Shape Evolution Process. For a complete view of the formation process of the ZnO hexagonal architectures and their growth mechanism, detailed time-dependent shape evolution studies were conducted at the standard synthesis conditions. White gels appeared immediately as ammonia was drip into the zinc-malate solution. The TEM image shows that the gels are composed of chain-like colloidal particles. ED investigations indicate these gels are in the amorphous state. These amorphous components are assumed to be Zn-OH compounds and they can slowly transform into ZnO seeds under hydrothermal conditions. For the reaction proceeding for 2 h, hexagonal ZnO nanodisks with an edge length of 50 nm and a height of 20 nm were observed, as viewed form the TEM image in Figure 5b. Focusing the electron beam on one particle showed hexagonal symmetry patterns that can be assigned to be the 〈0001〉 zone axis, indicating the particle is crystalline ZnO lying flatly on the (0001) surface planes. When the aging times were prolonged to 4 h, tiny branches stretch radiately from the surface of the hexagonal seeds, suggesting a secondary growth takes place on the substrate seeds. The ED pattern recorded from a crystal shows hexagonal symmetry diffraction spots with slight elongation, further indicating that the secondary particles are grown from the surface of the initially formed hexagonal nanodisk. The crystals formed at this stage may be the rudiments of the ZnO hexagonal architectures. For further aging these crystals will expand along the base plane and grow into the hexagonal architectures shown in Figure 2. On the basis of the above time-dependent morphology evolution observations, the formation process of the hexagonal architectures can be proposed as occurring by secondary particles growing epitaxially on the initially formed ZnO hexagonal seeds, during which a solid-solution-solid process may be involved. The driving force for the epitaxy growth may be the high activity of the (0001) planes of ZnO nanocrystals, which is a typical polar surface composed of alternating Zn2+ and O2- planes.10 Notably, the secondary growth takes place only on one (0001) surface, which is consistent with the polar crystal characters of ZnO.10b We find the observed shape evolution process is similar to that of the formation of vaterite hexagonal mesocrystals, which are grown through an aggregation-medicated crystallization process in the presence of the N-trimethylammonium derivative of hydroxyethyl cellulose.23b The similarity between the porous ZnO microparticles and vaterite mesocrystals in shape and the growth process further reveals that the ZnO samples obtained in the presence of malate salts show mesocrystal characteristics. Effects of Malate Ions on ZnO Growth. It has been observed that the formations of hexagonal shaped singlecrystalline ZnO nanodisks are the key step during the hierarchical structure formation. Since no hexagonal nanodisks could be observed in the absence of sodium malate, it is believed that the malate salts play a key role during the entire reaction. Malates are a typical R-hydroxy carboxylate derivative denoted as R1R2C(OH)COO- (R1 ) COOHCH2, R2 ) H for malate). In aqueous solution, R-hydroxy carboxylate can strongly
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Figure 5. TEM image and ED patterns of samples obtained at different growth stages: (a) 0, (b) 2, and (c) 4 h.
Figure 6. ZnO samples obtained in the presence of different concentrations of sodium malate: (a) 5, (b) 17, (c) 100, and (d) 250 mM.
coordinate with metal ions such as Zn2+, Cd2+, and Ag+ in the form of five-membered ring chelates.27 In our experiments, the initially formed amorphous zinc compound can dissolve into the solution, and then transfer to ZnO for the ammonia hydrolyzation under hydrothermal conditions. During this process, malate ions can complex with Zn2+ and conduce to the ZnO formation. This complex modified solid-solutionsolid (SLS) process benefits kinetically controlling the growth process and favors the size, shape, and crystalline quality improvements of the products. In addition, we observe the aspect ratio (denoted as the length to the thickness of the subunits) of the ZnO products can be greatly reduced in the presence of malate ions. Figure 6 shows the ZnO samples obtained with addition of different amounts of malate salts. Branched ZnO particles were the main products without addition of malate (see the Supporting Information). The single branch takes a rod shape and it is consistent with the ZnO growth habit in alkali solution.28 As malate salts were introduced into the solution, the branches will change their shapes from rods, to disks, and even leaves. The aspect ratios thus can be tuned from 0.1 to several tens by carefully alternating the malate concentration. This shape tuning effect has been previously observed for another R-hydroxy carboxylate derivative, citrates, (R1 ) R2 ) COOH).14,29 It has been suggested that citrate ions may bind on the (0001) surface planes and exert strong inhibiting effects on ZnO elongation. Voight et al. have reported that by using molecular modeling techniques, the geometries and binding energies of citrate ions on different ZnO surfaces are calculated. Their preliminary results confirmed citrate molecules adsorbed preferentially on the (0001) surfaces.14b We believe that the structural similarity will result in similar effects. Because malate ions can also strongly bind with zinc ions, they may also fat on the ZnO surfaces, especially selectively on the (0001) zinc terminated planes. The plane specific adhesive effects can thus
Figure 7. Room temperature photoluminescence spectrum of the ZnO particles obtained in the prescence of malate salts. Intensive ultraviolet excitons emission can be observed in the spectrum.
retard ZnO nanocrystals elongation. This shape-controlled effect is similar to that AOT16a or polyacrylamide16b exert on ZnO hexagonal plates and ring formation. Moreover, we found that with other typical R-hydroxy carboxylate derivatives such as tartrate (R1 ) CHOHCOOH, R2 ) H) and gluconate (R1 ) (CHOH)3CH2OH, R2 ) H), similar ZnO complex structures can also be obtained, and these R-hydroxy carboxylates exert similar effects on ZnO growth. Whereas disodium succinate is used instead of these R-hydroxy carboxylates, only rod-like crystals are obtained. The above experiments indicate that R-hydroxy carboxylates exert specific functions based on their structural similarity. We also note that the R-hydroxy carboxylates show shape control effects on nickel triangle formation.30 And with R-hydroxy carboxylate modification, high mechanical strength apatitic cement can be obtained.31 Here, we observed that these R-hydroxy carboxylates can show unique effects on ZnO complex structure synthesis. These R-hydroxy carboxylates may find further application in nanomaterial synthesis as functional organic molecules. Photoluminescence and Raman Measurements. Photoluminescence spectra of the porous hexagonal architectures obtained in malate solution were measured at room temperature excited with a He-Cd laser line (λ ) 325 nm). As Figure 7 shows, the PL spectrum of hexagonal particles shows sharp and intense UV emission centered at 387 nm and a merely detectable green-yellow emission around 580 nm. The UV emission is a characteristic near-band-edge transition.32 The green-yellow emissions, which are associated with surface and bulk defects,33 are barely observed in this sample. The ratio of the UV to greenyellow emission intensity (IUV:IGY) provides one versatile measurement of ZnO material quality.15a,b In the photoluminescence investigations, the intensity counts are related to several device parameters such as pump intensity, incident light size,
Synthesis of ZnO Hexagonal Architectures
J. Phys. Chem. C, Vol. 111, No. 3, 2007 1117 in the formation of the ZnO hexagonal architectures. Room temperature photoluminescence and Raman property investigation results indicate that the product’s crystalline qualities can be greatly improved with a simple addition of the malate salts. The novel geometry appearance, hierarchical architectures, and high crystalline qualities may render the sample potential application in microoptics fabrication. The reported method is simple and manipulation with facilities. In addition, with our systematical investigations, other R-hydroxylated oligocarboxylic salts including citrate, tartrate, and gluconate can also be found effective in shape control and crystalline qualities improvement of ZnO microparticles. These organic components are biocomponent and environmentally benign, which may inspire future nanosynthesis and product quality improvements.
Figure 8. Raman spectrum of the as-synthesized ZnO hexagonal particles.
and detector stations. Therefore, we can obtain more useful information from the relative intensity between the UV and yellow-green bands (IUV:IGY). In general, high IUV:IGY ratios can be obtained by high-temperature processing or a subsequent annealing by selecting different gas atmospheres. Up to now, it is still a change to get high-quality ZnO products in one step by the solution process. For this malate-mediated synthesis, the products show much more intense UV emission than that directly harvested from the alkali solution, indicating they possess high crystalline qualities and are nearly free of surface and bulk defects. The Raman spectrum of the ZnO powders was recorded to investigate the vibrational properties of the hexagonal microparticles. Wurtzite ZnO belongs to the C6V4 (P63mc) space group. There are two formula units included in the primitive cell and all atoms occupy the 2b sites of symmetry C3V. At the Γ point of the Brillouin zone, group theory predicts the existence of the following optic modes: Γopt ) A1 + 2B1 + E1 + 2E2.34,35 Among these vibrational models, A1, E1, and E2 are Raman active and B1 is Raman forbidden; in addition, A1 and E1 are infrared active and can split into longitudinal and transverse optical components. In bulk ZnO, only E2 and A1 (LO) can be observed in the unpolarized Raman spectrum according to the selection rules. In our sample, the sharp band at 432 cm-1 is attributed to the ZnO nonpolar optical phonons high E2 vibration mode, which corresponds to the characteristic band of the wurtizite phase. The band at 383 cm-1 corresponds to A1 symmetry with the TO model. The bands at 573 cm-1 are attributed to the E1 (LO) models. The second modes located at 208, 334, and 1050-1200 cm-1 can also be observed in the spectrum.34,35 In addition, a sharp and intensive band centered at 95 cm-1, which can be assigned to the E2 low vibration mode,35 can be clearly observed in the spectrum. For our sample, the low fluorescence background and the appearance of a high mode vibration band indicate the high crystalline qualities of the ZnO products. In summary, ZnO hexagonal shaped microparticles constructed with orderly arranged nanoplates were successfully prepared by a hydrothermal process with the assistance of malate salts. These subunit nanoplates, which are expanded along the ab planes of ZnO, were found to hold a parallel c-axis according to the SAED and HRTEM results. And nitrogen adsorptiondesorption measurements indicated that there is a serious number of small pores embedded in the hexagonal particles. The shape evolution process was investigated by time-dependent TEM investigations. Malate salts, which can effectively depress the ZnO nanoparticles elongating along the c-axis, play a key role
Acknowledgment. We gratefully acknowledge financial support from National Nature Science Fund of China and the 973 Project of China (No.2005CB623601). Supporting Information Available: EDS spectrum of the ZnO samples, TGA curves of sodium malate and ZnO samples, detailed calculations for ZnO particles specific surface area, and the SEM image of ZnO sample without addition of sodium malate. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Liber, C. M. Solid State Commun. 1998, 107, 607. (b) Alivisatos, A. P. Science 1996, 271, 933. (2) Mann, S. Angew. Chem., Int. Ed. 2000, 39, 3392. (3) Van De Pol, F. C. M. Ceram. Bull. 1990, 69, 1959. (4) Kadota, M. Jpn. J. Appl. Phys. Part 1 1997, 36, 3076. (5) Tsukazaki, A.; Ohtomo, A.; Omuna, T.; Ohtani, M.; Makino, T.; Suyama, M.; Ohtani, K.; Chichibu, S. F.; Fuke, S.; Segawa, Y.; Ohno, H.; Koinama, H.; Kawasaki, M. Nat. Mater. 2005, 4, 42. (6) Rensmo, H.; Keis, K.; Lindstrom, H.; Sodergren, S.; Solbrand, A.; Hagfeldt, A.; Lindquist, S. E.; Wang, L. N.; Muhammed, M. J. Phys. Chem. B 1997, 101, 2058. (7) Golego, N.; Studenikin, S. A.; Cocivera, M. J. Electrochem. Soc. 2000, 147, 1592. (8) Wilmer, H.; Kurtz, M.; Klementiev, K.; Tkachenko, V. O. P.; GrRnert, W.; Hinrichsen, O.; Birkner, A.; Rabe, S.; Merz, K.; Driess, M.; Woll, C.; Muhler, M. Phys. Chem. Chem. Phys. 2003, 5, 4736. (9) (a) Huang, M. H.; Mao, S.; Feick, H.; Yan, H.; Wu, Y.; Kimg, H.; Weber, E.; Russo, R.; Yang, P. Science 2001, 292, 1897. (b) Cao, H.; Zhao, Y. G.; Ho, S. T.; Seelig, E. W.; Wang, Q. H.; Chang, R. P. H. Phys. ReV. Lett. 1999, 82, 2278. (10) (a) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Science 2001, 291, 1947. (b) Wang, Z. L.; Kong, X. Y.; Zuo, J. M. Phys. Rew. Lett. 2003, 91, 185502. (c) Kong, X. Y.; Wang, Z. L. Nano Lett. 2003, 3, 1265. (d) Gao, P. X.; Wang, Z. L. J. Am. Chem. Soc. 2003, 125, 11299. (11) (a) Lao, J. Y.; Wen, J. G.; Ren, Z. F. Nano Lett. 2002, 2, 1287. (b) Zhan, J. H.; Bando, Y.; Hu, J. Q.; Golberg, D.; Kurashima, K. Small 2006, 2, 62. (12) Lao, J. Y.; Huang, J .Y.; Wang, D. Z.; Ren, Z. F. Nano Lett. 2003, 3, 235. (13) Zhang, X. H.; Xie, S. Y.; Jiang, Z. Y.; Zhang, X.; Tian, Z. Q.; Xie, Z. X.; Huang, R. B.; Zheng, L. S. J. Phys. Chem. B 2004, 117, 10114. (14) (a) Tian, Z. R.; Voigt, J. A.; Liu, J.; Mckenzie, B.; Mcdermott, M. J. J. Am. Chem. Soc. 2002, 124, 12954. (b) Tian, Z. R.; Voigt, J. A.; Liu, J.; McKenzie, B.; McDermott, M. J. M.; Rodriguez, A.; Konishi, H.; Xu, H. F. Nat. Mater. 2003, 2, 821. (15) (a) Sun, Y.; Fuge, G. M.; Fox, N. A.; Riley, D. J.; Ashfold, M. N. R. AdV. Mater. 2005, 17, 2477. (b) Sun, Y.; Riley, D. J.; Ashfold, M. N. R. J. Phys. Chem. B 2006, 110, 15186. (c) Wang, Z.; Qian, X. F.; Yin, J.; Zhu, Z. K. Langmuir 2004, 20, 3441. (16) (a) Li, F.; Ding, Y.; Gao, P. X.; Xin, X. Q.; Wang, Z. L. Angew. Chem., Int. Ed. 2004, 43, 5238. (b) Peng, Y.; Xu, A. W.; Deng, B.; Antonietti, M.; Colfen, H. J. Phys. Chem. B 2006, 110, 2988. (17) (a) Gao, X. P.; Zheng, Z. F.; Zhu, H. Y.; Pan, G. L.; Bao, J. L.; Wu, F.; Song, D. Y. Chem. Commun. 2004, 12, 1428. (b) Zhou, X.; Xie, Z. X.; Jang, Z. Y.; Kuang, Q.; Zhang, S. H.; Xu, T.; Huang, R. B.; Zheng, L. S. Chem. Commun. 2005, 4, 45572. (18) Mo, M. S.; Yu, J. M.; Zhang, L Z.; Li, S. K. A. AdV. Mater. 2005, 17, 756.
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