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J. Phys. Chem. C 2008, 112, 106-111
Controllable Growth of ZnO Nanostructures by a Simple Solvothermal Process Baomei Wen,† Yizhong Huang,‡ and John J. Boland*,† School of Chemistry and Center for Research on AdaptiVe Nanostructures and NanodeVices (CRANN), Trinity College, Dublin 2, Ireland, and Department of Materials, UniVersity of Oxford, Oxford, OX1 3PH, United Kingdom ReceiVed: August 24, 2007; In Final Form: October 17, 2007
A solvothermal method for growing ZnO nanostructures with different morphologies is presented. Nanowires, nanorods, nanocorns, nanoshuttles, nanoworms, and nanoflowers with excellent crystallinity have been successfully fabricated by combining two different coordination agents and adjusting the ratio of solvents and the reaction time. The nanowires and nanorods are of high aspect ratio (>500 and 50, respectively) and exhibit a perfect single crystalline wurtzite structure. The growth process responsible for the variation in morphology is discussed.
Introduction Recent progress in the synthesis and characterization of nanostructures has been driven by the need to develop strategies to tailor the properties of these materials for specific applications. Control of the morphology and dimensionality has attracted widespread attention due to its role in determining the electrical and thermal transport in addition to the optical and mechanical properties in nanoscale systems.1,2 This is particularly important in the case of one-dimensional structures, whose large surfaceto-volume ratio and dimensionality make them especially sensitive to surface-mediated interactions that, in turn, impact the properties and performance of these materials. It is therefore critically important to develop synthetic strategies that control both size and shape while at the same time yielding materials with well-defined compositions and structural morphologies. ZnO is an important semiconductor material with extensive applications in electronics, photoelectronics, sensors, and optical devices.3-6 It is well-known for its wide band gap (3.37 eV), large exciton binding energy at room temperature (60 meV), and excellent chemical and thermal stability. Consequently, designing ZnO nanostructures with different morphologies and sizes is of significant importance from the standpoint of both basic fundamental research and the development of novel devices.7 To date, various synthetic approaches have been developed to fabricate ZnO nanostructures, which can be classified into two categories, vapor-phase and solution-phase synthesis. Vapor-phase processes such as vapor-liquid-solid growth (VLS),8 chemical vapor deposition (CVD),9 thermal decomposition,10 and thermal evaporation11 are favored for their simplicity and high-quality products, but these gas-phase methods generally require high temperatures and expensive equipment, which may limit potential applications, particularly those requiring large-scale production. Moreover, even a weak temperature gradient in the CVD deposition region can often result in a remarkable change in both the shape and size of these materials.12,13 For these reasons, there is a significant need to develop a low-temperature, large-scale, versatile route to ZnO synthesis. * To whom correspondence should be addressed. E-mail:
[email protected]. † Trinity College. ‡ University of Oxford.
Solution-phase routes are appealing due to their low growth temperature, low cost, high efficiency, and potential for scaleup. So far, microemulsion, hydrothermal self-assembly and template-assisted sol-gel processes have been employed to synthesize ZnO nanowires and nanorods. For example, Vayssieres et al. developed a simple low-temperature solution process for preparing highly oriented ZnO microrods and nanorods at 95 °C.14-16 Liu et al. used a seeded growth process to synthesize helical ZnO nanorods and nanocolumns at a similar temperature.17 However, most of these methods yield arrays of onedimensional ZnO crystals whose morphologies are limited to rod-like, needle-like, or wire-like structures. In addition, most wet chemical methods18-22 fail to produce small-diameter ZnO nanowires with a narrow size distribution and large aspect ratio. Thus, it is necessary to find an effective and controllable method to fabricate ZnO nanostructures with different morphologies, including small-diameter ZnO nanowires. Herein, we expand on these synthetic methods to fabricate ZnO nanostructures that include a series of new morphologies such as corn-like, flower-like, shuttle-like, worm-like, and long rod-like structures, in addition to nanowires with typical diameters of 20 nm. This method involves a mild solvothermal route using a thin nucleation layer of nanostructural ZnO deposited by room-temperature direct current (DC) magnetron reactive sputtering. All ZnO nanostructures obtained exhibit excellent crystallinity. The novel aspect of this work is the unique combination of three key experimental parameters. (1) Solvent composition: ethanol or mixture of ethanol and H2O was used as a solvent for reaction media instead of H2O. Ethanol has a lower boiling point and higher vapor pressure than H2O, which allows the relatively low temperature required for crystallization of ZnO to be achieved. (2) Surfactant: cetyltrimethylammoniumbromide (CTAB) was used as the modifying and controlling agent for the shape in a traditional hexamethylenetetramine (HMTA) system. This novel approach of combining two different coordination agents to control the morphology of ZnO nanostructures has led to these unique and interesting nanostructures. (3) Seed layer: a 40 nm thick ZnO layer was used as growth seeds, which effectively lowered the interfacial energy between the crystal nuclei and the substrate and decreased the nucleation barrier, thus facilitating the growth
10.1021/jp076789i CCC: $40.75 © 2008 American Chemical Society Published on Web 12/13/2007
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TABLE 1: Summary of Experimental Conditions and the Morphology of Different ZnO Nanostructures H2O EtOH CTAB Zn2+/C6H12N4 reaction time morphology (mL) (mL) (mol/L) (mol ratio) (h) wire-like rod-like corn-like shuttle-like worm-like flower-like
0 0 0 20 20 20
100 100 100 80 80 80
0.000 0.001 0.005 0.005 0.005 0.010
1:1 1:1 1:1 1:1 1:1 1:1
24 24 48 24 48 24
of the ZnO nanowires. Here, we show that by tuning these parameters, it is possible to control the morphology of ZnO nanostructures. Experimental Section All of the reagents used in the experiments were analytical grade and utilized without further purification. To grow ZnO nanostructures, first, a 40 nm thick ZnO seed layer was formed by direct current (DC) magnetron reactive sputtering on thermally oxidized Si substrates under an Ar and O2 atmosphere. All deposition was carried out at room temperature with a base pressure < 10-8 Torr. Subsequently, solvothermal growth was carried out by suspending the Si substrates with a ZnO seed layer in a sealed autoclave filled with a reaction solution. In a typical experiment, the reaction solution was prepared by the following procedure: Equimolar ethanol solutions (0.001 M) of zinc nitrate hexahydrate (Zn(NO3)2‚6H2O) and hexamethylenetetramine (C6H12N4, HMTA) were prepared in a vessel under constant stirring, and then, a specified amount (see Table 1) of CTAB (0-0.01 M) was added. The mixed solution was vigorously stirred for 2 h until it became clear and then was transferred into a sealed autoclave and heated at 80 °C. The substrates were removed from the growth solution, dipped in deionized water, and dried. Table 1 provides a summary of the reaction conditions. The as-prepared samples on the silicon substrates were directly characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The XRD patterns were recorded by a Philips X’Pert Pro X-ray diffractometer with the Cu KR radiation (λ )1.54056 Å). SEM images were obtained on a Carl Zeiss EVO 50 microscope. Microstructural analysis of the nanostructured ZnO was conducted using a JEOL 3000F highresolution transmission electron microscopy (HRTEM) at an acceleration voltage of 300 kV. Results and Discussion Structure and Morphology. All of the nanostructures presented were grown on the Si substrate covered with a 40 nm ZnO seed layer. Typical SEM images for ZnO nanowires and nanorods are shown in Figure 1. The nanowires are of high aspect ratio (>500), with the diameter of 20 nm and lengths of up to 10 µm. These nanowires have uniform structure and straight morphologies. Such characteristics are essential for fabricating nanowire devices or for measuring electromechanical properties.23-25 Moreover, the length of the nanowires can be experimentally controlled by adjusting the growth time. During extended growth, nanowires become intertwined like “noodles” (Figure 1a). The observed entanglement of the nanowires may be also promoted post-facto by the capillarity action of the solvent,26 that is, by the surface tension of the water, during drying in the oven. Although the use of zinc nitrate and HMTA in an aqueous system for the growth of the ZnO nanorod/wire is well studied14,27-30 and represents an effective way to achieve high-quality ZnO nanowire arrays, it lacks the ability to produce
Figure 1. SEM images of ZnO nanowires and nanorods: (a,c) lowmagnification image, (b,d) high-magnification image.
wires with high aspect ratios (>50). As previous reports show,14,31 decreasing the concentration of the reactants reduces the diameter of the nanowires, but typically, a reduction in length is also observed. To obtain high aspect ratio wires, the samples must be repeatedly introduced into fresh solution baths every few hours.32 In ethanol media, we demonstrate that high aspect ratio (>500) nanowires can be easily synthesized under appropriate EtOH conditions in the presence of seeds. Moreover, by introducing a low concentration of CTAB in ethanol solvent,
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Figure 3. XRD patterns of the substrate and various ZnO nanostructures: (a) Si substrate with a ZnO seed layer, (b) wire-like, (c) rodlike, (d) corn-like, (e) shuttle-like, (f) worm-like, (g) flower-like. Figure 2. SEM images of ZnO nanostructures with different morphologies: (a,b) corn-like, (c,d) shuttle-like, (e,f) worm-like, (g,h) flower-like.
ZnO nanorods with diameters in the range from 100 to 300 nm are obtained (Figure 1c,d). Nanorods do not form the wellaligned arrays reported in the literature.14,15 In addition, we would like to point out that the nanorods prepared in this work are much longer than those obtained from an aqueous solution of zinc nitrate and HMTA, which are limited to between 1.5 and 2 µm.27,32 Different ZnO growth morphologies (corn-like, shuttle-like, worm-like, and flower-like) are possible by tailoring the experimental growth conditions. Figure 2 shows SEM images of the complete range of ZnO nanostructures. The morphology of ZnO nanostructures strongly depends on the solvent, the concentration of the surfactant CTAB, and the reaction time. Novel corn-like nanostructures are obtained at a longer reaction time and a higher concentration of CTAB (Figure 2a,b). It is worth noting that bundles of small nanowires extend from the ends of corn-like nanostructures, while the middle exhibits the expected hexagonal shape. Adding water to ethanol solvent produces shuttle-like ZnO nanostructures with lengths of up to 2 µm (Figure 2c,d). The shuttle-like ZnO nanostructure acts as a seed for epitaxial plate-like growth, and secondary growth follows, resulting in the nanoworm morphology (Figure 2e,f) at prolonged reaction time. Further increasing of the concentration of CTAB results in the formation of flower-like ZnO nanostructures. As shown in Figure 2g,h, SEM images reveal that the flowers consist of nanoneedles with lengths of about
1.5 µm, and each flower has a center as a common growth point for all of the constituent nanoneedles. X-ray diffraction (XRD) patterns shown in Figure 3 were recorded to examine the crystal phase of ZnO nanostructures of different morphologies. All products exhibit the wurtzite structure, with diffraction peaks that can be indexed to a hexagonal phase with lattice constants of a ) 0.325 nm and c ) 0.521 nm (JCPDS card No. 36-1451). No characteristic peaks from impurities are detected within experimental error, and the sharp shape and line widths of the diffraction peaks indicate that all of the ZnO nanostructures have excellent crystallinity. Further structural characterization of the nanowire and the nanorod was performed by TEM. Figure 4 shows typical TEM images of ZnO nanowires and nanorods. The nanowires appear reasonably uniform at low magnification but are found to have rough surfaces when viewed at high magnification. As an example, Figure 4b shows a bright-field TEM image of a single nanowire, which is decorated with bumps over the surface. In contrast to nanowires, ZnO nanorods have a fairly smooth surface, as shown in Figure 4d. Using a selected-area electron diffraction (SAED) technique, both nanowires and nanorods are revealed to be perfectly single crystalline and to have a wurtzite structure. By indexing the diffraction pattern (inset in Figure 4b,e), the main axis of the nanowire and the nanorod, which is equivalent to the growth direction, is determined to be along [0001]. High-resolution TEM images shown in Figure 4c,f further confirm the crystal phase structure and perfection of the atomic arrangement. ZnO Seed Layer. In the present scheme, the initial deposition of the ZnO seed layer plays a critical role in the formation of
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Figure 4. TEM images of ZnO nanowires and nanorods: (a,d) typical low-magnification images, (b,e) a single nanowire or nanorod and the corresponding SAED patterns (inset), (c,f) high-resolution TEM image and two-dimensional atom crystal lattice image (insert).
small-diameter nanowires. Otherwise, microrods instead of nanowires are produced, which is similar to what has been reported earlier16,27 and where water was used as the solvent. As an example, Figure 5a shows a typical SEM image with a fragmentary ZnO seed layer following a growth time of 12 h. The nanowires selectively grow on the area with the ZnO seed layer. This is demonstrated by ZnO nanowire growth on a “scratched” substrate, where part of the ZnO seed layer was intentionally removed. It can be clearly seen that the nanowires failed to grow on the silicon substrate without the ZnO seed layer (shown in Figure 5b). In the area where the seed layer is nonuniform, there is obvious bundling of the nanowires along with the presence of irregularly shaped crystals, as indicated by an arrow in Figure 5c. The important role of seed nuclei has been alluded to in the literature. Zeng et al. observed that the ultrasonic pretreatment of the solution mixture is an important step prior to the hydrothermal reaction, which may generate a suitable quantity of ZnO cluster nuclei for the subsequent hydrothermal growth. Without the ultrasonic pretreatment, much larger and shorter ZnO rods and particles have been obtained.21 Introducing the seed layer effectively lowers the interfacial energy between the crystal nuclei and the substrate, hence decreasing the nucleation barrier and facilitating the growth of the ZnO nanowires. Moreover, the seed layer also increases the surface roughness, which results in more vacancies that promote the growth of the ZnO.
Figure 5. SEM images of ZnO nanowires grown on irregular seeded silicon substrates.
Precursor Salt. Ammonia or ammonium salts were introduced into the reaction system because the lone-pair electrons on the nitrogen can coordinate with the empty orbitals of metal ions, generating metal-ammonium complexes. These complexes are known to stabilize structural units that modify, promote, and even direct the formation of versatile nanostructures.33 HMTA can hydrolyze slowly to provide a gradual supply of ammonia, which can form ammonium hydroxide, as well as to complex with zinc(II) to form Zn(NH3)42+ and control the growth of ZnO nanostructures. Additionally, HMTA is also a tetradentate ligand and tends to bind to metal ions in different coordination modes, which can kinetically control species in solution. Controlling the shape and the size of the nanostructures by the surfactants is well established.34 The assistance of surfactants that are adsorbed preferentially on specific planes of inorganic crystals is known to drive the production of anisotropic structures. For example, Zeng et al.21 have used ethylenediamine (EDA) as chelating ligands to the Zn2+ cations to inhibit the radial enlargement of the rods. This is consistent with the present study. CTAB is a cationic surfactant with an ammonium ion. Experimentally, the presence of CTAB has a profound effect on the final morphology of ZnO nanostructures,
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Figure 6. (a) Illustration of a zincite crystal with various planes marked; (b) growth schematic diagram of ZnO nanostructures in different experimental conditions.
and the concentration of CTAB is a key parameter in controlling the morphology of ZnO nanostructures. Reaction Mechanism. The overall reaction in the system can be expressed by the following equation
Zn2+ + CH3(CH2)15N(Br)(CH3)3/C6H12N4 f Zn2+amino complex f Zn(OH)2 f ZnO (1) In an ammonium rich environment, most of the zinc ions form the amine complex. When the temperature of the solution is increased, the amine complex reacts with the reaction media and produces ZnO crystals, and heterogeneous nucleation will take place preferentially on the ZnO seed surface. In general, crystal growth is determined by a combination of internal, structurally related factors (intermolecular bonding preferences or dislocations) and external factors (supersaturation, solvents, time, and additives).35 ZnO is a polar crystal, and O2ions are in a hexagonal closes-packed (HCP) arrangement, with each Zn2+ lying within a tetrahedral group of four oxygen ions. Zn and oxygen atoms are arranged alternatively along the c axis, exhibiting a positive polar plane that is rich in Zn2+ and a negative polar plane that is rich in O2-. The inherent asymmetry along the c axis leads to the anisotropic growth of 1-D ZnO crystallites. The most stable crystal of ZnO is a wurtzite structure consisting of polar (0001) and (0001h) planes and nonpolar (1000) planes. Figure 6a is an illustration of a ZnO crystal structure. The nonpolar faces have lower surface energy, while the polar faces have a relatively higher surface energy, which tend to rearrange themselves to minimize total surface energy.36 The formation of ZnO nanostructures is attributed to the difference in the growth rate of the various crystal facets. The velocities for growth under hydrothermal conditions are reported to be (0001) > (0110) > (1000). In addition, HMTA being a nonpolar chelating agent would preferentially attach to the nonpolar facets of ZnO nanostructures, thereby exposing the (0001) plane for epitaxial growth. All of these factors will promote the growth along the [0001] direction and drive the formation of the ZnO nanowires under the current conditions. However, when a small amount of CTAB is added to the reaction solution, as a cationic surfactant, CTAB adsorbs on the positive polar plane that is rich in Zn, thus retarding the growth rate of the (0001) plane. Under the low CTAB condition, longitudinal growth is still dominant, resulting in the formation of relatively low aspect ratio nanorods. With a slightly higher concentration of CTAB, the growth rate of the (0001) plane is further impeded and the epitaxial growth of the side facets will lead to the formation of the nanocorns, especially at long
Wen et al. reaction times (24-48 h). A schematic growth diagram of ZnO nanostructures in different experimental conditions is shown in Figure 6b. The solvent also plays a role in determining the different growth behavior of ZnO nanostructures. The interface-solvent interactions depend on surface energies of different crystal faces and solvent properties such as polarity, viscosity, and saturated vapor pressure. Indeed, Praserthdam et al.37 have recently reported that the dielectric property of the solvents is strongly correlated with the shape of ZnO nanostructures. Wang et al.38 have found that different solvents lead to different ZnO nanostructure morphologies. Liu et al.39 have suggested that the concentration of water is of critical importance to ZnO growth behavior, and the usage of an alcoholic environment is key to the growth of high aspect ratio ZnO nanowires. Here, we observed that when the polar solvent H2O was added to the reaction solution, the higher dielectric constant of H2O further stabilized the polar surface and impeded c axis growth, which resulted in the formation of the ZnO nanoshuttles. Nanoworms are the products of secondary platelet growth on the ZnO nanoshuttles during extended reaction times. When the concentration of CTAB in the reaction solution is further increased, the nanoflower morphology is obtained. Evidently, the increased CTAB concentration leads to a decrease in selectivity such that the secondary growth bears no crystallographic relationship to the original primary growth structure. This is consistent with SAED analysis, which shows that individual needles of the nanoflowers are single crystals. Conclusions In summary, ZnO nanostructures with a diversity of welldefined morphologies, such as high aspect ratio wire-, rod-, corn-, shuttle-, worm-, and flower-like, have been successfully fabricated by a simple low-temperature solvothermal route. The morphologies of ZnO nanostructures can be controlled conveniently by varying the solvent and surfactant. The controllable growth of ZnO nanostructures described here may open up the possibility of exploring novel applications in the areas of electronics and optoelectronic nanodevices, in addition to highvolume applications. Acknowledgment. This work was supported by Science Foundation Ireland under Grant 00/PI.1/C077A References and Notes (1) Alivisatos, A. P. J. Phys. Chem. 1996, 100, 13226. (2) Duan, X.; Huang, Y.; Cui, Y.; Wang, J.; Lieber, C. M. Nature 2001, 409, 66. (3) Xia, Y. N.; Yang, P. D.; Sun, Y. G.; Wu, Y. Y.; Mayers, B.; Gates, B.; Yin, Y. D.; Kim, F.; Yan, H. Q. AdV. Mater. 2003, 15, 353. (4) Gao, P. X.; Wang, Z. L. J. Am. Chem. Soc. 2003, 125, 11299. (5) Wang, X. D.; Summers, C. J.; Wang, Z. L. Nano Lett. 2004, 4, 423. (6) Shen, G. Z.; Bando, Y.; Lee, C. J. J. Phys. Chem. B 2005, 109, 10578. (7) Huang, M.; Mao, S.; Feick, H.; Yan, H.; Wu, Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. Science 2001, 292, 1897. (8) Yang, P.; Yan, H.; Mao, S.; Russo, R.; Johnson, J.; Saykally, R.; Morris, N.; Pham, J.; He, R.; Choi, H. J. AdV. Funct. Mater. 2002, 12, 323. (9) Park, W. I.; Kim, D. H.; Jung, S. W.; Yi, G. C. Appl. Phys. Lett. 2002, 80, 4232. (10) Xu, C. K.; Xu, G. D.; Liu, Y. K.; Wang, G. H. Solid State Commun. 2002, 122, 175. (11) Yao, B. D.; Chan, Y. F.; Wang, N. Appl. Phys. Lett. 2002, 81, 757. (12) Kong, X.; Ding, Y.; Wang, Z. J. Phys. Chem. B 2004, 108, 570. (13) Han, X.; Wang, G.; Jie, J.; Choy, W.; Luo, Y.; Yuk, T.; Hou, J. J. Phys. Chem. B 2005, 109, 2733. (14) Vayssieres, L. AdV. Mater. 2003, 15, 464.
Controllable Growth of ZnO Nanostructures (15) Vayssieres, L.; Keis, K.; Lindquist, S. E.; Hagfeldt, A. J. Phys. Chem. B 2001, 105, 3350. (16) Vayssieres, L.; Keis, K.; Hagfeldt, A.; Lindquist, S. E. Chem. Mater. 2001, 13, 4395. (17) Tian, Z. R.; Voigt, J. A.; Liu, J.; Mckenzie, B.; Mcdermott, M. J. J. Am. Chem. Soc. 2002, 124, 12954. (18) Wu, J. J.; Liu, S. C. AdV. Mater. 2002, 14, 215. (19) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Science 2001, 291, 1947. (20) Guo, L.; Ji, Y. L.; Xu, H.; Simon, P.; Wu, Z. J. Am. Chem. Soc. 2002, 124, 14864. (21) Liu, B.; Zeng, H. C. J. Am. Chem. Soc. 2003, 125, 4430. (22) Sun, X. M.; Chen, X.; Deng, Z. X.; Li, Y. D. Mater. Chem. Phys. 2002, 78, 99. (23) Lin, Y. R.; Yang, S. S.; Tsai, S. Y.; Hsu, H. C.; Wu, S. T.; Chen, I. C. Cryst. Growth Des. 2006, 6, 1951. (24) Greene, L. E.; Law, M.; Goldberger, J.; Kim, F.; Johnson, J. C.; Zhang, Y. F.; Saykally, R. J.; Yang, P. D. Angew. Chem., Int. Ed. 2003, 42, 3031. (25) Guo, M.; Diao, P.; Cai, S. M. J. Solid State Chem. 2005, 178, 1864. (26) Verges, M. A.; Mifsud, A.; Serna, C. J. J. Chem. Soc., Faraday Trans. 1990, 86, 959. (27) Music, S.; Popovic, S.; Maljkovic, M.; Dragcevic, E. J. Alloys Compd. 2002, 347, 324.
J. Phys. Chem. C, Vol. 112, No. 1, 2008 111 (28) Hung, C. H.; Whang, W. T. Mater. Chem. Phys. 2003, 82, 705. (29) Greene, L. E.; Yuhas, B. D.; Law, M.; Zitoun, D.; Yang, P. D. Inorg. Chem. 2006, 45, 7535. (30) Wu, B.; Heidelberg, A.; Boland, J. J. Nat. Mater. 2005, 4, 525. (31) Kubota, N. Cryst. Res. Technol. 2001, 36, 749. (32) Na Ayudhya, S. K.; Tonto, P.; Mekasuwandumrong, O.; Pavarajarn, V.; Praserthdam, P. Cryst. Growth Des. 2006, 6, 2446. (33) Mo, M. S.; Yu, J. C.; Zhang, L. Z.; Li, S. A. AdV. Mater. 2005, 17, 756. (34) Zhang, Z.; Sun, H.; Shao, X.; Li, D.; Yu, H.; Han, M. AdV. Mater. 2005, 17, 42. (35) Tong, Y.; Liu, Y.; Dong, L.; Zhao, D.; Zhang, J.; Lu, Y.; Shen, D.; Fan, X. J. Phys. Chem. B 2006, 110, 20263. (36) Wang, D.; Song, C. J. Phys. Chem. B 2005, 109, 12697. (37) Li, F.; Ding, Y.; Gao, P.; Xin, X.; Wang, Z. L. Angew. Chem. 2004, 116, 5350. (38) Wang, C.; Shen, E.; Wang, E.; Gao, L.; Kang, Z.; Tian, C.; Lan, Y.; Zhang, C. Mater. Lett. 2005, 59, 2867. (39) Cheng, J. P.; Zhang, X. B.; Tao, X. Y.; Lu, H. M.; Luo, Z. Q.; Liu, F. J. Phys. Chem. B 2006, 110, 10348.
