Fabrication and Optical Properties of Large-Scale Nutlike ZnO

Apr 15, 2009 - Xinjuan Wang , Qinglin Zhang , Qiang Wan , Guozhang Dai , Chunjiao Zhou , and Bingsuo Zou. The Journal of Physical Chemistry C 2011 115...
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J. Phys. Chem. C 2009, 113, 8016–8022

Fabrication and Optical Properties of Large-Scale Nutlike ZnO Microcrystals via a Low-Temperature Hydrothermal Route Yi Zeng,† Tong Zhang,*,† Wuyou Fu,‡ Qingjiang Yu,‡ Guorui Wang,§ Yanyan Zhang,‡ Yongming Sui,‡ Lijie Wang,† Changlu Shao,§ Yichun Liu,§ Haibin Yang,‡ and Guangtian Zou‡ State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin UniVersity, Changchun 130012, People’s Republic of China, State Key Laboratory of Superhard Materials, Jilin UniVersity, Changchun 130012, People’s Republic of China, and Center for AdVanced Optoelectronic Functional Material Research, Northeast Normal UniVersity, Changchun 130024, People’s Republic of China ReceiVed: October 9, 2008; ReVised Manuscript ReceiVed: March 9, 2009

Large-scale uniform nutlike ZnO microcrystals are successfully synthesized via a facile hydrothermal process. Field emission scanning electron microscopy and transmission electron microscopy results reveal that the as-prepared ZnO products have average lengths of 2.2 µm and diameters of 1.8 µm, possessing a wurtzite structure. The evolution process of the nutlike ZnO microcrystals has been viewed by FESEM characterization, and the results reveal that the reaction time plays a crucial role in determining the final size and shape of the samples. It is also found that the existence of triethanolamine (TEA) is vital to the formation of the complex microcrystals formed from self-assembled nanoparticles, and a possible formation mechanism is proposed. Optical properties of the ZnO sample are also investigated by PL spectroscopy. It is found that the intensity of the visible emission shows an obvious decrease after the nutlike ZnO is annealed in air for 90 min. Moreover, the UV emission is further identified to originate from the recombination of the radiative free exciton and the donor bound excition by the temperature-dependent PL spectra. 1. Introduction ZnO is considered a versatile semiconductor material with excellent properties and extensive applications.1 It has been investigated for applications in many areas, such as electronics, photoelectronics, sensors, and catalysts.2-4 Recently, the morphologically controllable synthesis of ZnO nano/microstructures is spurred by the recent success in the observation of roomtemperature UV lasing from nanorod arrays by Yang’s research group.5 The control over the size and morphology of nanometeror micrometer-sized ZnO crystals represents a great challenge to realize the design of novel functional devices. This is due to the fact that the optical and electronic properties of ZnO crystals can be modulated by changing their size and morphology, which finally determine practical applications.6 Up to now, various techniques have been employed to synthesize ZnO nanostructures with various shapes.7-11 Most of the synthesis techniques demand stringent reaction conditions, such as vacuum techniques, high temperature, the use of catalysts and noxious gas compounds, or complicated controlling processes, which may result in poor dispersion, impurity, or the decomposition of the final product, and are unfavorable for low-cost and large-scale production. Therefore, it is of great importance and necessity to develop a technique to synthesize ZnO only in mild reaction conditions. The facile solution-based synthesis approaches become a promising choice to solve the above problems due to their facile manipulation, low cost, and potential for scale-up. It is maybe the simplest and most effective * To whom correspondence should be addressed. Phone: +86-43185168385. Fax: +86-431-85168417. E-mail: [email protected]. † State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering. ‡ State Key Laboratory of Superhard Materials. § Northeast Normal University.

way to prepare rods,12 tubes,13 disks,14 flowers,15 dumbbells,16 or other novel nanostructures by self-assembly of nanoscaled building blocks.17 Ordered self-assembly of nanoscaled building blocks, such as nanoparticles, nanorods, and so forth, into various singlecrystalline architectures has become a hot topic in recent material research fields.18 Recently, Pacholski et al. have reported the formation of high-quality single-crystalline ZnO nanorods based on coalescence of quasi-spherical ZnO nanoparticles by a facile solution method.19 Mo et al. have reported the self-assembly synthesis of nanorods and nanosheets into hollow microhemispheres and microspheres by the poly(sodium 4-styrenesulfonate) (PSS)-assisted hydrothermal method.20 Liu et al. have prepared large-scale ZnO ellipsoids formed from two types of building blocks of nanorods and nanoparticles by a poly(ethylene glycol) (PEG)-assisted solution method.18c Moreover, several groups have synthesized ZnO nanostructures by the addition of triethanolamine (TEA) in aqueous solution.21 In that regard, the use of the organic functional groups to modify the growth habit by selective adsorption and subsequent controlled removal of organic additives at interfaces represents an important route to explore the formation of complex nanostructures.22 Therefore, it is both fundamentally interesting to understand the selfassembly process and potentially important to explore a simple route for the synthesis of complex ZnO nanostructures. Herein, we report the successful fabrication of nutlike ZnO microcrystals via a simple TEA-assisted hydrothermal route at low temperature (95 °C). The as-prepared products were illuminated in terms of their crystallinity, morphology, and structure. Effects of the reaction time on the size and shapes of the ZnO products are analyzed, and the formation mechanism of nutlike ZnO microcrystals is discussed. Furthermore, the room-temperature pho-

10.1021/jp808939n CCC: $40.75  2009 American Chemical Society Published on Web 04/15/2009

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Figure 1. XRD pattern of the as-synthesized ZnO products.

toluminescence and low-temperature photoluminescence (PL) of the nutlike ZnO microcrystals are also investigated. 2. Experimental Procedures All the reagents in the experiments were analytical grade (purchased from Beijing Chemicals Co., Ltd.) and used as received without further purification. The solvent used for the reaction system is deionized water with a resistivity of 18.0 MΩ · cm. In a typical synthesis, the precursor solution was prepared by dissolving 40 mL of an aqueous solution of zinc acetate dihydrate (Zn(CH3COO)2 · 2H2O, 0.1 M) and 2.0 mL of TEA under vigorous magnetic stirring for 10 min. The white aqueous solution was adjusted to a pH value of 9.5. Then the mixed solution was transferred to a Teflon-lined stainless steel autoclave, which was sealed tightly and maintained at 95 °C for 2 h and then naturally cooled down to room temperature. Finally, the products were collected by centrifugation, washed several times with absolute ethyl alcohol and deionized water, and dried in air at 90 °C for further characterization. The obtained samples were characterized with X-ray powder diffraction (XRD, Rigaku D/max-Ra, Cu KR radiation), field emission scanning electron microscopy (FESEM, JEOL JSM6700F, 5 kV) equipped with an energy-dispersive X-ray spectrometer (EDX), transmission electron microscopy/selected area electron diffraction (TEM/SAED, Hitachi H-8100, 200 kV), high-resolution transmission electron microscopy (HRTEM, JEOL JEM-3010, 200 kV), and Raman spectroscopy (HR-800 with a backscattering configuration made by HORIBA Jobin Yvon Company, France) using the 514 nm line of an argon ion laser at room temperature. The PL measurements were performed on an HR-800 LabRam Infinity Spectrophotometer using a continuous He-Cd laser with the excitation wavelength of 325 nm as the excitation source. 3. Results and Discussion Structure and Morphology. As shown in Figure 1, the representative XRD pattern of the as-synthesized products demonstrates that all of the diffraction peaks are well assigned to the wurtzite structure of ZnO with a lattice constant of a ) 0.324874 nm and c ) 0.520391 nm (JCPDS No. 36-1451). No diffraction peaks from impurities are observed. The morphology and surface element composition of the ZnO sample are investigated using FESEM and EDX. The FESEM image shows that bulk quantities of ZnO products exhibit an approximately uniform shape and have a similar size, as shown in the Supporting Information. The enlarged image in Figure

Figure 2. (a) Low-magnification FESEM image of large-scale nutlike ZnO samples and (b) high-magnification FESEM image of an individual ZnO microcrystal.

2a clearly reveals that the obtained ZnO, with diameters of about 1.8 µm and lengths of 2.2 µm, exhibits the well-defined nutlike structure composed of asymmetrical twinned ZnO cones. The nutlike shape is also confirmed by the approximate hexagonal cross sections observed from the vertically aligned ZnO microstructures and the bipyramidal cross sections detected from the horizontally dispersed microstructures (see arrows 1 and 2 of Figure 2a). An individual ZnO microcrystal (Figure 2b) exhibits a typical asymmetrical twinned-cone structure with a rough surface. The surface element composition of the asprepared ZnO is identified by EDX measurements (see the Supporting Information). The EDX results demonstrate that no peaks of other elements except Zn and O can be clearly seen in the survey spectrum. Figure 3 shows the typical TEM and HRTEM images of the nutlike ZnO samples. The low-magnification image (Figure 3a) shows that all ZnO nuts are uniformly distributed, which is in good agreement with the above FESEM results. As shown in Figure 3b, a compact standard asymmetrical morphology of the individual ZnO nut could be observed. It is noteworthy that after long ultrasonic treatment during the preparation of the TEM specimens, the nutlike structures are sufficiently stable, which cannot be destroyed. As shown in Figure 3c, the SAED pattern taken from one-half of the nut shows diffraction spots corresponding to a single-crystal hexagonal wurtzite ZnO structure with the growth direction along the long axis of ZnO[0001] (c axis). From the SAED pattern, it is interesting and surprising that the assembled structure with micrometer-scale size exhibits an almost single-crystalline diffraction pattern. It can be concluded that the formation of the nutlike structures is not a simple aggregation of small nanoparticles but is made of two asymmetrical single-crystal cones using their (0001j) planes as a common interface. The HRTEM image shows that the (0002) plane of the wurtzite ZnO with the interplanar spacing of ∼0.26

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Figure 3. (a) Low-magnification TEM image of large-scale ZnO nuts. (b) Enlarged TEM image of a single ZnO microcrystal. (c) The SAED pattern. (d) The HRTEM image taken from the ZnO sample.

nm can be clearly identified, and the results further confirm that the ZnO cone is single-crystalline in nature and preferentially grows along the [0001] direction (Figure 3d). Morphology Evolution and Growth Mechanism. To understand the growth mechanism of the nutlike ZnO microcrystals, we have investigated the morphology evolution of the samples with respect to different reaction times. Figure 4 displays the morphology evolution of the ZnO samples without the hydrothermal process and collected after different reaction times, including 10 min, 30 min, 2 h, 6 h, and 12 h. The FESEM observation of the sample only stirred for 10 min reveals that disorderly floccules exist randomly (Figure 4a). When the hydrothermal time is 10 min, the high-magnification image (Figure 4b) shows that many agglomerates, about 400 nm in diameter, are randomly scattered in the floccules. As seen from the TEM image in the inset of Figure 8, the floccules are actually composed of ZnO nanoparticles with an average diameter of 6 nm. It can be concluded that the final products grow out from these agglomerate floccules, which is similar to the early growth process of ZnO troughs.23 As the reaction time of the hydrothermal process is prolonged to 30 min, it is observed that a bulk quantity of ZnO nanoparticles tends to aggregate in compact cones with coarse surfaces (Figure 4c). With increase of the reaction time up to 2 h, the aggregations transform into the nutlike ZnO with a twinned-cone shape (Figure 4d). The

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Figure 5. Raman spectra of the ZnO samples prepared at different reaction times: (a) 10 min, (b) 30 min, (c) 2 h, and (d) 6 h.

twinned cones are asymmetrical since one subunit is much larger than the other. The smaller cone is mainly composed of loosely aggregated particles. The detailed characteristics of the ZnO microcrystals are described in the above section. When the reaction time increases to 6 h, bigger and longer ZnO twinned cones in bulk quantity are obtained (Figure 4e). The loosely aggregated particles of the smaller subunit coalesce to form a bigger and longer cone with better crystal quality, which suggests that ZnO nanoparticles are only an intermediate and further coalesce to form cones as the reaction time increases. These ZnO microstructures show whole ellipsoidal shapes, with average center diameters of about 1.8 µm and lengths of about 4 µm. The middle of the ZnO ellipsoid exhibits the compact and smooth hexagonal shape, and the two tips are rather rough. Furthermore, the obvious grain boundary is observed at the middle part of the ZnO ellipsoid, as shown in the highmagnification inset of Figure 4e. Extending the reaction time to 12 h causes the smaller subunit of the twinning cones to evolve to be equivalent with the bigger one, and typical spindlelike ZnO microcrystals are formed (Figure 4f), in comparison with the ZnO obtained at 6 h. It can be found that the change of the surface subunit and the size of microstructures may be attributed to a dissolution-recrystallization process through Ostwald ripening under hydrothermal conditions. Raman spectroscopy is also performed to study the crystallization degree of the ZnO samples prepared at different reaction times. Figure 5 shows the room-temperature Raman spectra of

Figure 4. FESEM images of the ZnO samples prepared at different reaction times: (a) only stirring, (b) 10 min, (c) 30 min, (d) 2 h, (e) 6 h, and (f) 12 h. The inset of (e) is a single ZnO ellipsoid.

Large-Scale Nutlike ZnO Microcrystals

Figure 6. FESEM image of the aggregated flowerlike ZnO nanorods prepared without the TEA-assisted hydrothermal process.

these samples in the range of 200-800 cm-1. All the observed obvious peaks could be assigned to the wurtzite ZnO structure according to the literature values.24 The relatively high peaks at 383, 413, 438, 540, and 575 cm-1 are assigned to A1(TO), E1(TO), E2(high), A1(LO), and E1(LO) modes, respectively.25,26 It is found that the E2(high) mode of these samples becomes stronger when the reaction time increases, which may mean the improvement of the crystallization of the prepared ZnO samples. To confirm the effect of TEA on the morphology of the ZnO sample, the controlled experiment of the hydrothermal process without TEA has been carried out. Figure 6 shows the FESEM image of the ZnO sample prepared without TEA, while other conditions are kept the same. It exhibits that flowerlike ZnO nanostructures are composed of some taperlike branches consisting of plenty of tightly aggregated nanorods with an average diameter of 60 nm, but no nutlike ZnO products are obtained. Therefore, on the basis of the morphological study, it can be concluded that TEA produces a significant effect on the size and shape of the obtained ZnO samples. Further detailed information on the ZnO samples synthesized for 2, 6, and 12 h can be obtained in Figure 7a-c to understand the growth process. As shown in Figure 7a, the top view of the nutlike ZnO microcrystal exhibits well-resolved hexagonal edges and corners, and the ZnO (0001j) plane is covered with aggregated nanoparticles. It is suggested that these aggregated nanoparticles form the underdeveloped subunit of the twinned

J. Phys. Chem. C, Vol. 113, No. 19, 2009 8019 ZnO cone. When the reaction proceeds to 6 h, as shown in Figure 7b, the appearance of the smooth {101j0} side planes indicates that the twinned cones grow longer along the c axis and show better crystal perfection. When the reaction time is further prolonged to 12 h, the smaller subunit forms a bigger and longer cone to be almost equivalent with the other. The final morphology of a ZnO crystal is mainly determined by its intrinsic crystal structure and many external factors, such as solvents, basicity, temperature, time, and capping reagent. Among them, the capping molecules can tailor the surface energy according to the capping of the nuclei’s surface and, thus, control the shapes of microcrystals.16 In the solution system, growth rates of different planes are different as follows: V(0001) > V(1j011j) > V(1j010) > V(1j011) > V(0001j).27 As a result, the fastest growth along the [0001] direction spurs ZnO to grow along the c axis, and thus, the elongated rodlike shape is much more favorable. When capping molecules are added, the relative order of the growth rate can be modified and further determine the size and morphology. It can be seen from the morphological observation in the evolution of ZnO samples and the controlled experiment that the reaction time and capping molecules are the key parameters in determining of ZnO morphologies. A possible growth model for the hydrothermal synthesis with and without TEA can be simply described in Figure 7d,e. The reaction process can be mainly expressed as follows

Zn2++2OH- f Zn(OH)2

(1)

Zn(OH)2 f ZnO + H2O -

(2)

Zn(OH)2 + 2OH + TEA f [Zn(OH)4] -TEA 2-

(3)

In our experiments, when the pH value arrives at 9.5, there are enough Zn(OH)2 in the aqueous solution, which could decompose into a large quantity of ZnO nuclei when the degree of supersaturation exceeds the critical value (reactions 1 and 2). Without TEA, the abundant quantity of growth units

Figure 7. High-magnification FESEM images illustrating the particular appearance of a (a) nut, (b) ellipsoid, and (c) ZnO spindle. (d,e) Schematic growth diagram of the ZnO microcrystals by the hydrothermal process with and without TEA.

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promotes the anisotropic growth of ZnO along the c axis, resulting in rod-based ZnO flowers (Figures 6 and 7d). However, in the TEA-assisted hydrothermal process (Figure 7e), TEA, being a cationic surfactant, can have interactions with [Zn(OH)4]2- to form TEA ligands due to the Coulomb force action (reaction 3). The formed TEA ligands have the ability to selectively adsorb on some specific crystal planes and then restrain the anisotropic growth of ZnO crystallites, which can drive the ZnO nuclei to form sufficiently stable and uniform nanoparticles in the solution. Though the aggregation process to form the larger crystals is structurally and energetically favorable with the reaction time increasing, the building blocks would always randomly aggregate into disordered crystals rather than single crystals in the absence of surface-adsorbed ligands.28 In the present work, the TEA ligands also appear to play a vital role in the oriented attachment. As demonstrated by FESEM and TEM results, this typical anisotropic aggregation process should be due to different particle-aggregation potentials and driving forces in all of the spatial dimensions.16,18c,29 On the other hand, the polarity and electrostatic attraction among nanoparticles are decreased by the adsorption of TEA ligands on polar surfaces, which allows the nanoparticles to keep on colliding and rotating more easily, and the random aggregation might be gradually replaced by the ordered and oriented assembly. With further rotation of these nanoparticles to share the same 3D crystallographic orientation and subsequent coalescence process, loosely aggregated ZnO structures with a cone shape are formed. As the hydrothermal process proceeds, the compact and crystalline hexagonal cones are gradually formed through Ostwald ripening. Moreover, TEA can hydrolyze in the aqueous solution and form the N(CH2CH2OH)3-H+ complex, which would adsorb on the negative polar plane (0001j) by the Coulomb interaction, serving as the buffer layer for the growth of the smaller end of the cone (reaction 4). The subunit cones gradually grow larger and longer with the increase in the reaction time and finally form well-defined spindle-like ZnO microcrystals. During the whole process, TEA plays an important dual role, which not only serves as a complexing reagent to influence the morphology of the ZnO crystals but also releases OH- to change the alkaline environment. As to the complex growth process, more in-depth studies are necessary to further understand their growth process, which can provide important information for crystal design and morphology-controlled synthesis of ZnO and other oxides. Photoluminescence. Generally, the room-temperature PL spectrum of ZnO commonly consists of an ultraviolet (UV) emission and a broad, visible emission band, and the origination of these emissions has been widely investigated.30-32 Figure 8 shows the PL spectra of the ZnO samples prepared at different reaction times of 10 min, 30 min, 1 h, and 2 h reveal similar features. It is noteworthy that the intensity ratio of the UV emission and the visible emission is different. Compared with the UV emission (∼392 nm) of the ZnO samples with the reaction times of 30 min, 1 h, and 2 h, the UV emission peak of the ZnO sample with the reaction time of 10 min has a blue shift to higher energy and is located at ∼386 nm, which may be explained by the effect of quantum confinement.33 The TEM result indicates that this ZnO sample with the reaction time of 10 min consists of nanoparticles with an average diameter of 6 nm, as shown in the inset of Figure 8. The improved intensity ratio of the UV emission and the visible emission of all the ZnO samples indicates that the crystal quality of the ZnO samples gets better with increasing the reaction time. Distinctly, the nutlike ZnO microcrystals prepared at the reaction time of

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Figure 8. Room-temperature PL spectra of the ZnO samples prepared at different reaction times: (a) 10 min, (b) 30 min, (c) 1 h, and (d) 2 h. The inset is the TEM image of the ZnO nanoparticles with the reaction time of 10 min.

Figure 9. Room-temperature PL spectra of the nutlike ZnO microcrystals with the reaction time of 2 h before and after thermal annealing at 300 °C in air for 90 min. The inset is the FESEM image of the annealed ZnO microcrystals.

2 h have the best crystal quality. Nevertheless, in our samples, the visible emission is not negligible. The green emission broad bands of all ZnO samples exhibit a similar shape and are centered at ∼565 nm, which indicates that there are oxygen vacancies or structural defects in the ZnO samples. Furthermore, Figure 9 shows the PL spectrum of the nutlike ZnO microcrystals annealed at 300 °C in air for 90 min. It is worth mentioning that the morphology of the microcrystals after annealing does not change. The UV emission intensity of the annealed ZnO microcrystals increases dramatically, and contrarily, the intensity of the visible emission obviously decreases, indicating an improved crystal quality after annealing. This result can be attributed to the decrease in the amount of the singly ionized oxygen vacancies in the ZnO. Therefore, further work is focused on obtaining the optimal annealing time or temperature for further enhancing the UV emission while completely suppressing the visible emission of our samples, and it is now undergoing. Figure 10 shows the PL spectrum of the nutlike ZnO microcrystals measured at 81 K. It shows a dominant peak at 3.297 eV, which is known as the donor bound exciton (D0X) emission.34 The weak emission observed at ∼3.312 eV as a shoulder at the higher-energy side of the D0X peak is attributed to the free exciton (FX) emission, accompanied by the 1LO photon replica observed at 3.239 eV.

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J. Phys. Chem. C, Vol. 113, No. 19, 2009 8021 uniform nutlike ZnO microcrystals at low temperature. The nutlike ZnO microcrystals are composed of two asymmetrical hexagonal cones that possess a single-crystal wurtzite structure and grow along the [0001] direction. The experimental results demonstrate that the nutlike ZnO sample is composed of self-assembled nanoparticles, which are formed by the oriented-attachment mechanism. TEA is of great importance in the formation process. The room-temperature and low-temperature PLs have been systematically investigated, and the results show that primary UV emission is from bound and free exciton recombination. This TEA-assisted hydrothermal method can probably be expanded to produce other semiconductors with novel morphologies for various potential applications.

Figure 10. PL spectrum of the nutlike ZnO microcrystals at 81 K.

To study the origin of the UV emission peak, the temperaturedependent PL spectra of the nutlike ZnO are measured from 81 to 283 K, as shown in the inset of Figure 11. When the measurement temperature was increased, the FX and D0X show an obvious red shift, and the intensities are decreased, which is due to the thermal ionization of the bound excitons. Moreover, the intensity of the D0X emission decreases more rapidly than that of the FX emission, and the FX emission becomes dominant above 243 K. Figure 11 shows the temperature dependence of the peak energy of FX and D0X, which fits well with Varshini’s semiempirical equation35

RT2 Eχ(T) ) Eχ(0) T+β

(5)

where Eχ(0) is the peak energy at absolute zero temperature and R and β are the fitting parameters. In Figure 11, the two lines represent the fitted temperature dependences for FX and D0X emission, and the lines fit well with the experimental values. The fitting parameters of R and β are -2.75 × 10-4 ((0.47 × 10-4) eV K-1 and -481.6 ((33.89) K for the emissions. Eχ(0) equals 3.317 and 3.301 eV for FX and D0X emissions, respectively.

Figure 11. Temperature dependence of the excitonic emission energies of the nutlike ZnO microcrystals: 9 and b represent FX and D0X emissions, respectively. The inset shows the temperature-dependent PL spectra of the nutlike ZnO microcrystals.

4. Conclusions In conclusion, the present TEA-assisted hydrothermal process offers a facile method to synthesize large-scale

Acknowledgment. We gratefully acknowledge financial support from the Science and Technology Office, Jilin Province, China (Grant No. 2006528), and the Open Project of the Key Laboratory of Low Dimensional Materials & Application Technology (Xiangtan University), Ministry of Education, China (Grant No. KF0706). Supporting Information Available: (a) Low-magnification FESEM image of large-scale nutlike ZnO sample and (b) EDX results of the nutlike ZnO sample. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Wong, E. M.; Searson, P. C. Appl. Phys. Lett. 1999, 74, 2939. (2) Choi, K.-S.; Lichtenegger, H. C.; Stucky, G. D.; McFariand, E. W. J. Am. Chem. Soc. 2002, 124, 12402. (3) (a) Sberveglieri, G.; Baratto, C.; Comini, E.; Faglia, G.; Ferroni, M.; Ponzoni, A.; Vomiero, A. Sens. Actuators, B 2007, 121, 208. (b) Gao, P. X.; Wang, Z. L. J. Am. Chem. Soc. 2003, 125, 11299. (4) (a) Shen, G. Z.; Bando, Y.; Lee, C. J. J. Phys. Chem. B 2005, 109, 10578. (b) Han, X. H.; Wang, G. Z.; Jie, J. S.; Choy, W. C. H.; Luo, Y.; Yuk, T. I.; Hou, J. G. J. Phys. Chem. B 2005, 109, 2733. (c) Shen, G. Z.; Bando, Y.; Lee, C. J. J. Phys. Chem. B 2005, 109, 10779. (5) 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. (6) (a) Chan, E. M.; Mathies, R. A.; Alivisatos, A. P. Nano Lett. 2003, 3, 199. (b) Peng, W. Q.; Qu, S. C.; Cong, G. W.; Wang, Z. G. Cryst. Growth Des. 2006, 6, 1518. (c) Jiang, C. L.; Zhang, W. Q.; Zou, G. F.; Yu, W. C.; Qian, Y. T. J. Phys. Chem. B 2005, 109, 1361. (7) Park, W. I.; Yi, G. C.; Kim, M.; Pennycook, S. J. AdV. Mater. 2002, 14, 1841. (8) (a) Vayssieres, L.; Keis, K.; Hagfeldt, A.; Lindquist, S. E. Chem. Mater. 2001, 13, 4395. (b) Vayssieres, L. AdV. Mater. 2003, 15, 464. (9) Lao, J. Y.; Huang, J. Y.; Wang, D. Z.; Ren, Z. F. Nano Lett. 2003, 3, 235. (10) (a) Kong, X. Y.; Wang, Z. L. Nano Lett. 2003, 3, 1625. (b) Kong, X. Y.; Ding, Y.; Yang, R.; Wang, Z. L. Science 2004, 303, 1348. (11) Shen, G.; Bando, Y.; Lee, C. J. J. Phys. Chem. B 2005, 109, 10578. (12) Tian, Z. R.; Voigt, J. A.; Liu, J.; Mckenzie, B.; Mcdermott, M. J. J. Am. Chem. Soc. 2002, 124, 12954. (13) Wei, A.; Sun, X. W.; Xu, C. X.; Dong, Z. L.; Yu, M. B.; Huang, W. Appl. Phys. Lett. 2006, 88, 213102. (14) 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. (15) (a) Cao, J. M.; Wang, J.; Wang, B. Q.; Chang, X.; Zheng, M. B.; Wang, H. Y. Chem. Lett. 2004, 33, 1332. (b) Zhang, T.; Zeng, Y.; Fan, H. T.; Wang, L. J.; Wang, R.; Fu, W. Y.; Yang, H. B. J. Phys. D: Appl. Phys. 2009, 42, 045103. (16) Zhang, H.; Yang, D. R.; Li, D. S.; Ma, X. Y.; Li, S. Z.; Que, D. L. Cryst. Growth Des. 2005, 5, 547. (17) Mo, M. S.; Yu, J. C.; Zhang, L. Z.; Li, S. A. AdV. Mater. 2005, 17, 756. (18) (a) Zhang, Z.; Sun, H.; Shao, X.; Li, D.; Yu, H.; Han, M. AdV. Mater. 2005, 17, 42. (b) Vayssieres, L.; Graetzel, M. Angew. Chem., Int. Ed. 2004, 43, 3666. (c) Liu, J. P.; Huang, X. T.; Sulieman, K. M.; Sun, F. L.; He, X. J. Phys. Chem. B 2006, 110, 10612. (19) Pacholski, C.; Kornowski, A.; Weller, H. Angew. Chem., Int. Ed. 2002, 41, 1188. (20) Mo, M. S.; Yu, J. C.; Zhang, L. Z.; Li, S. A. AdV. Mater. 2005, 17, 756.

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