Control of the Surface of ZnO Nanostructures by Selective Wet

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Control of the Surface of ZnO Nanostructures by Selective Wet-Chemical Etching Xi-Guang Han, Ya-Qi Jiang, Shui-Fen Xie, Qin Kuang,* Xi Zhou, Dao-Ping Cai, Zhao-Xiong Xie,* and Lan-Sun Zheng State Key Laboratory for Physical Chemistry of Solid Surfaces & Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen UniVersity, Xiamen 361005, China ReceiVed: February 10, 2010; ReVised Manuscript ReceiVed: April 29, 2010

In this article, we demonstrate successful application of a top-down strategy based on a selective wet-chemical etching technique in fabrication of nanostructures with a special morphology. Pagoda-like and hexagonal pyramidal ZnO nanostructures with the polar (0001j) and {101j1} planes as exposed surfaces have been synthesized by refluxing ZnO columns mainly bounded with {101j0} nonpolar faces in the mixed solvent of oleic acid (OA) and 1-octylamine. Adequate evidence demonstrates that OA in the mixed solvent acts as the etchant in the process of morphology evolution, and the appropriate proportion of OA, reaction time, and reaction temperature are crucial for controlling the etching degree of ZnO. Through the selective wet-chemical etching, it has been confirmed that the dumbbell-like ZnO is of twinning morphologies growing along contrary direction on both sides of the (0001) twinning plane. Therefore, our present work provides a simple way to estimate the complicated twinning phenomena. 1. Introduction The anisotropic property of crystals always brings different crystal surfaces to exhibit different physical/chemical properties. Considering the fact that the surfaces of particles on the nanoscale become dominant, control of the surface structures of nanocrystals presents an important direction for tuning many important physical/chemistry properties, such as the catalytic, electronic, optical, and magnetic properties.1-15 In particular, recent reports demonstrated that the catalytic properties of nanocrystals can be intensively enhanced or precisely tuned by controlling the surface structure of nanocrystals.1-9 Up to date, well-defined shapes of nanocrystals, which determine the surface atomic arrangement and coordination, have been mostly synthesized via bottom-up crystal growth routes.12-15 To the best of our knowledge, top-down methods are hardly applied because of the limit in technique and economic cost. In this article, we report a simple top-down chemical etching route to control surface structures of ZnO nanocrystals. The chemical etching methods have attracted considerable attention and have been applied in the industries in the past decades. Wet-chemical etching has great advantages because of its capability of anisotropic etching. For example, the {111} facets of silicon crystal exhibited quite a slow etching rate by using hydrazine as the anisotropic etchant compared with the other facets.16-18 In addition, the etching rate of Si {110} became slower than that of Si {100} when the nonionic surfactants (such as NC-200 or Triton X-100) were added to tertramethyl ammonium hydroxide (TMAH) with an appropriate concentration.19,20 By applying such anisotropic etching behaviors, various kinds of micro components, such as microchips, cantilevers, cavities, and grooves in silicon substrates have been fabricated. Recently, selective/directional etching phenomena have been found during the growth of some nanocrystals.21,22 For example, when MnAc2 · 4H2O thermally decomposed in the presence of 1-octylamine/oleic acid (OA), the products transformed from oc* Corresponding authors. E-mail: [email protected] (Z.X.), qkuang@ xmu.edu.cn (Q.K.). Fax: +86-592-2183047. Tel: +86-592-2180627.

tahedral MnO nanocrystals exposed with {111} surface into sixhorn-like MnO nanocrystals with {100} surfaces by extending the reaction time.22 Six-horn-like MnO nanocrystals were considered to be the result of directional etching of octahedral MnO nanocrystals. This interesting result inspires us to try to control the surface structure of nanocrystals directly by the selective/directional etching process. ZnO is a wurtzite-type crystal that can be described as a number of alternating planes composed of four-fold tetrahedrally coordinated O2- and Zn2+ ions stacked alternatively along the c axis.23 High surface energy and high reactivity of the (0001) polar surface bring ZnO crystal easily to be directionally etched along the c axis (i.e., against the (0001) surface) under many common conditions.24-28 For instance, ZnO nanorods can be converted into ZnO nanotubes by using methenamine,24 ethylenediamine,25 NH3 · H2O,26 or KCl27 solution as etchants; in particular, the ZnO nanotip array, which has promising application in field emission devices, can be generated by in situ chemically etching the top of ZnO nanorods by using aqueous ammonia solution.28 However, no reports concern preferential etching of ZnO nanocrystals along other directions to date. In this article, we report the preparation of ZnO exposed with surfaces (0001j) and {101j1} polar planes by wet-chemical etching ZnO nanorods with {101j0} nonpolar surfaces in the presence of the mixed solvent of OA and 1-octylamine. It has been demonstrated that OA is the directional etchant in the etching process, and the (0001j) and {101j1} faces are stable surfaces in given growth environment, in contrast with the {101j0} nonpolar faces. In addition, by using the proposed selective etching technique, we can easily identify the polar axis of ZnO. As an example, we further demonstrate that the dumbbell-like ZnO is twinned crystal with the (0001) twinning plane simply by applying the selective etching. 2. Experimental Section 2.1. Materials. Zinc acetate (ZnAc2 · 2H2O, 99%), 1-octylamine (C8H19N, 99%), OA (90%), sodium hydroxide (NaOH, 96%), glycerin (C3H8O3, 99%), isopropanol (C3H8O, 99.7%),

10.1021/jp101284p  2010 American Chemical Society Published on Web 05/13/2010

Control of the Surface of ZnO Nanostructures hexamethylenetetramine (C6H12N4, 99%), and polyvinyl pyrrolidone (PVP, K-30) were used in our experiments. All chemicals were purchased from commercial suppliers (Alfa Aesar and Sinopharm Chemical Regent) and used as received without further purification. 2.2. Synthetic Methods. Synthesis of ZnO thick columns: 0.33 g ZnAc2 · 2H2O and 1.2 g NaOH were dissolved in 4.5 mL of distilled water. Then, 10.5 mL of glycerin was dropped in the above solution. After magnetic stirring for 15 min, the mixture was transferred to a Teflon-lined stainless-steel autoclave with a filling capacity of 20 mL and was kept at 150 °C for 24 h. The products were collected by centrifugation and washed several times with deionized water and ethanol. Synthesis of ZnO thin columns: 1.1 g ZnAc2 · 2H2O and 2 g NaOH were separately dissolved in 10 mL of distilled water and then successively added to 10 mL of an aqueous solution containing 0.05 g PVP. Then, 5 mL of the resulting mixture and 10 mL of distilled water were sonicated for several minutes and transferred to a Teflon-lined stainless-steel autoclave with a filling capacity of 20 mL. Then, the autoclave was heated to 180 °C for 13 h. After being cooled to room temperature, the product was separated by centrifugation from the solution and repeatedly rinsed with distilled water. Synthesis of ZnO dumbbells: In a typical synthetic process, 0.14 g hexamethylenetetramine and 0.22 g ZnAc2 · 2H2O were dissolved into 100 mL of the mixed solvent of distilled water and isopropanol (v/v ) 1) under ultrasonic. Then, 16 mL of the resulting solution was transferred to a Teflon-lined stainless steel autoclave with a filling capacity of 20 mL. The autoclave was heated to 180 °C and maintained for 24 h. After the autoclave was cooled to room temperature, the white precipitate was collected by centrifugation, washed several times with deionized water and ethanol, and finally dried at 60 °C in air. 2.3. Wet-Chemical Etching Process. In a typical synthesis, 0.66 mL of OA and 3.0 mL of 1-octylamine were mixed to form a transparent solvent at room temperature. Then, 0.162 g ZnO samples was added to the mixed solvent with intense ultrasonic treatment. The resulting white suspension was transferred to a glass tube with a length of 40 cm and was directly placed in a vertical tube furnace that was already preheated to 400 °C. After reaction for 30 min, the tube was taken out from the hot tube furnace and instantly added with ethanol. Finally, white products were collected by high-speed centrifugation and washed several times with deionized water and ethanol. Note that the glass tube must be long enough to meet that one-half was inside the furnace and the other half was out of the furnace. By being naturally refluxed in the glass tube, the solvent evaporation tried to be avoided. 2.4. Characterization of the Products. The composition and phase of as-prepared products were acquired by the powder X-ray diffraction (XRD) pattern using a Panalytical X-pert diffractometer with Cu KR radiation. The morphology and crystal structure of the as-prepared products were characterized by scanning electron microscopy (SEM, Hitachi S-4800) and high-resolution transmission electron microscopy (HRTEM, JEM 2100) with an acceleration voltage of 200 kV. All TEM samples were prepared from depositing a drop of diluted suspensions of the as-prepared ZnO samples in ethanol on a carbonfilm-coated copper grid. 3. Results and Discussion Figure 1A shows the XRD patterns of the as-prepared ZnO thick columns before and after etching. All diffraction peaks can be indexed as hexagonal wurtzite-type ZnO (JCPDS no.

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Figure 1. (A) XRD patterns of ZnO thick columns before and after etching. (B,C) SEM images of ZnO thick columns before etching and after etching. (D) Typical high-magnification SEM image of ZnO thick column after etching.

36-1451), indicating that no impurity is presented in the sample. The typical SEM images of ZnO thick columns before and after etching are shown in Figure 1B,C, respectively. The product prepared by the hydrothermal route from ZnAc2 · 2H2O consists of hexagonal columns having a smooth surface and a pyramidal tip. The length of the thick columns is about 30-50 µm, and the diameter is about 300 nm to 1 µm. After being refluxed in the mixed solvent of OA and 1-octylamine at 400 °C for 30 min, however, the smooth surface of ZnO thick columns becomes coarse, whereas the pyramidal tip remains the same as that before etching. From the corresponding high-magnification SEM image shown in Figure 1D, it can be clearly seen that these ZnO thick columns present a pagoda-like morphology, which looks like stacking of a set of hexagonal pyramids along the c axis. Besides, some separate hexagonal pyramids have also appeared in the product, as indicated by the dashed circle in the Figure 1D. The size of these ZnO hexagonal pyramids is in the range of 100-150 nm. More details of the crystal structure of the ZnO thick columns after etching were further provided by TEM. As shown in Figure 2A, the surface of the etched ZnO thick columns exhibits a series of sawtooth-like ridges where one surface is perpendicular to the long-axis direction of the column. In contrast, the pyramidal tip does not undergo any change and remains a smooth surface. The corresponding SAED pattern (the left inset of Figure 2A) recorded from the same column, which can be indexed as the [101j0] zone axis of hexagonal ZnO, reveals the single-crystal nature of the etched ZnO column. HRTEM images recorded from the pyramidal tip and sawtooth-like ridges are shown in Figure 2B,C, respectively. In these HETEM images, the clear lattice fringes with a spacing of 0.26 nm are observed along the long-axis direction, corresponding to (0002) crystal planes of wurtzite ZnO. It should be in particular pointed out that the angle between the vertical face and the inclined face of sawtooth-like ridges is equal to ∼58°, and the angle between two opposite edges of the pyramidal tip is ∼64°. Such geometrical shape of the pagoda-like ZnO structure agrees with the hexagonal pyramidal ZnO reported in our previous study,29 which happens to be built up with one (0001j) polar plane and

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Figure 2. (A) Typical low-magnification TEM image of a ZnO thick column after etching. The left is the corresponding SAED pattern recorded from the ZnO column and the right inset is the partial enlarged image of the area marked by the dashed rectangle. (B) HRTEM image of the pyramidal tip. (C) HRTEM image recorded from the area marked in part A. (D) Schematic model of a pagoda-like ZnO consisting of a series of hexagonal micropyramids.

six {101j1} polar planes. Therefore, it can be concluded that the vertical faces of the ridges on the column surface are the (0001j) planes, and the side faces are the {101j1} polar planes. That is to say, the pagoda-like structure obtained after etching from ZnO thick column can be described as stacking of a series of hexagonal pyramids with (0001j) polar plane as the base and {101j1} polar planes as the side surface, as shown in Figure 2D. From the viewpoint of the exposed crystal faces, the morphology evolution from the ZnO thick column to the pagoda-like structure virtually reflects the relative stability of the exposed surfaces in given growth environment. Because of the insert reactivity of O atoms, both the (0001j) polar plane and the {101j1} polar planes are always terminated with O atoms when exposed to the surface of nanocrystals. Therefore, the (0001j) and {101j1} polar faces are more stable than {101j0} nonpolar faces in the mixed solvent of OA and 1-octylamine. To investigate the roles of 1-octylamine and OA in the surface-dependent etching process, a series of confirmatory experiments were further conducted by refluxing ZnO thick columns in pure OA and pure 1-octylamine at 400 °C for 30 min. From Figure 3A,B, it can be clearly seen that the cloudy particle suspension gradually changed into a transparent solution after refluxed in pure OA. On the contrary, no obviously changes occurred in the morphology of ZnO thick columns when refluxed in pure 1-octylamine, as shown in Figure 3C,D. The results reveal that OA instead of organic amine is the real etchant in the etching process from the thick columns to the pagodalike structures. In addition, this judgment coincides with another experimental result that the lower the proportion of OA in the mixed solvent is the less severe the etching degree of ZnO thick columns is within the same reaction time. Therefore, the proportion of OA in the mixed solvent is the crucial factor to control the etching degree of ZnO thick columns. It has been mentioned that some small separate hexagonal pyramids can be found among the pagoda-like structures. With the increase in the reaction time, the amount of small hexagonal pyramids increased with the decrease in the pagoda-like structures. When the refluxing time was prolonged to 90 min from 30 min, the pagoda-like structures completely disappeared,

Han et al.

Figure 3. Optical photos of the suspension of ZnO thick columns before and after refluxed in (A,B) pure OA and (C,D) pure 1-octylamine of at 400 °C for 30 min. Insets in parts C and D are corresponding SEM images of thick columns ZnO before and after being refluxed in the pure 1-octylamine.

Figure 4. (A) Typical SEM image of the product obtained when the refluxing time was prolonged to 90 min. (B) Low-magnification TEM image of a separate ZnO hexagonal pyramid. The inset is corresponding SAED pattern. (C) HRTEM image recorded from one corner of hexagonal pyramid.

and the final product was composed of high-purity hexagonal pyramids, as shown in Figure 4A. The HRTEM image and corresponding SAED pattern (Figure 4B,C) indicate that these hexagonal pyramids are of a single crystal with a hexagonal outline, growing up along the [0001] direction. According to our previous study,29 the hexagonal pyramids are built up with one (0001j) polar face (i.e., the base surface) and six {101j1} polar faces (i.e., the side surface). In fact, the appropriate reaction temperature is likewise crucial to control the etching process of ZnO hexagonal columns. When the temperature decreased from 400 to 360 °C, the etching of ZnO hexagonal columns obviously becomes weak within the same reaction time.

Control of the Surface of ZnO Nanostructures

Figure 5. Optical photos of the ZnO thin columns-based suspension (A) before refluxed, (B) when the temperature reached 400 °C, and (C) after refluxed for 30 min in the mixed solvent of 3 mL of 1-octylamine and 0.66 mL of OA. The insets in parts A and C are corresponding SEM images of ZnO particles.

Once the reaction temperature is lower than 320 °C, the etching process is difficult to proceed. Therefore, the reaction time and reaction temperature are the two important factors to influence the etching process. It should be noted these hexagonal pyramids are 100-150 nm in size, much less than that of the original ZnO thick columns. Therefore, these hexagonal pyramids may not be the direct etched products from the ZnO thick columns, and very possibly they are the product from decomposition of the zinc oleate complex that is formed during the etching process. To demonstrate this hypothesis, we used ZnO thin columns instead of ZnO thick columns. As shown in the inset of Figure 5A, the ZnO thin columns used in this experiment are ∼100 nm in diameter. Figure 5A-C show the optical photos of the ZnO thin columns suspension at different steps of the etching process. When the temperature just reached 400 °C, the white suspension became a transparent solution (Figure 5B), indicating that ZnO thin columns were completely dissolved. According to our knowledge, the ZnO thin columns have been dissolved by OA to form the zinc oleate complex. When the reaction time lasted for 30 min, the white particles were renewedly observed in the solution. The corresponding SEM image (the inset of Figure 5C) indicates that the final product is composed of ZnO hexagonal pyramids. The above experiment demonstrates that the hexagonal pyramids should be generated by decomposition of the zinc oleate complex that results from the etching of OA against ZnO. According to the above experimental facts, a schematic diagram (Figure 6) has been proposed to illustrate the etching process of ZnO hexagonal columns. For ZnO thick columns, the dominating planes are nonpolar {101j0} planes (six side surface) that are composed of equivalent O2- and Zn2+ ions on the same planes. The OA prefers to react with Zn2+ ions on the surface, which results in the formation of zinc oleate complex, and further leads to the gradual dissolution of the ZnO columns. When the exposed surface is dominated with O-terminated planes (i.e., the {101j1} and (0001j) faces), the etching reaction has to be terminated, and the pagoda-like morphology is finally

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Figure 6. Schematic illustration for the etching process of the ZnO thick columns.

formed. For the etching process, the etching rate and etching degree can be controlled by the addition of 1-octylamine, which lowers the acidity of solution via the reaction: R-COOH + R-NH2 f R-COO- + R-NH3+.29 At the same time, the deliquescent product (i.e., zinc oleate complex) may decomposes and regrows to hexagonal pyramids exposed to the {101j1} and (0001j) polar faces in the mixed solvent of OA and 1-octylamine. Therefore, the pagoda-like and hexagonal pyramidal morphologies may coexist in the ultimate product. The proposed selective etching strategy has potential applications not only in fabricating ZnO-based nanodevices but also in judging the growth direction of some special morphologies of ZnO. Herein, dumbbell-like crystallite, an uncommon twinned morphology of ZnO, is selected as an example for illustrating the validity of our proposed etching strategy. Although dumbbell-like ZnO has been synthesized via various wet chemical routes in the past years,30-33 the researchers are still unable to comes to terms with the growth mechanism of such twinned crystallite. Wang et al. proposed that the dumbbell-like ZnO is a twinned crystallites with (0001) as the twinning plane, and every individual crystallite on both sides of the twinning plane grows along the [0001j] direction.30 However, their explanation lacks enough direct proof. Considering that the chemical etching against ZnO using OA as the etchant is selective, we believe that it is potentially feasible to uncover the growth mechanism of the dumbbell-like ZnO by means of our proposed selective chemical etching. As shown in Figure 7A, the dumbbell-like ZnO with lengths of 4-6 µm and diameters of 1.5 to 2.5 µm were synthesized under hydrothermal conditions by using ZnAc2 · 2H2O as raw material in the presence of hexamethylenetetramine and isopropanol. After these dumbbell-like ZnO were refluxed in the mixed solvent of OA and 1-octylamine at 400 °C for 30 min, the surface of the dumbbell-like ZnO has presented a scalelike structure, as shown in Figure 7B. Interestingly, the closeup view (inset of Figure 7B) shows that a grain boundary is clearly observed at the middle part of the dumbbell-like ZnO, and the growth directions of scales on the opposite sides are contrary. These scale-like structures are in fact hexagonal pyramids formed by selective chemical etching. According to the above phenomenon, it can be deduced that the twinning plane in the middle of dumbbell-like ZnO is (0001) terminated

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Figure 7. Typical SEM images of dumbbell-like ZnO (A) before and (B) after etching.

with Zn atoms and the flat end is (0001j) terminated with O atoms. From this example presented here, it has been demonstrated that the selective chemical etching provides a simple way to judge the complicated twinning phenomena of ZnO. 4. Conclusions In sum, we have demonstrated a wet selective chemical etching route to realize the morphology evolution from ZnO hexagonal column exposed with {101j0} nonpolar faces to pagoda-like ZnO exposed with {101j1} and (0001j) polar planes. In the etching process, OA as a selectively etching reagent plays the crucial role, and the etching degree can be controlled by tuning the amount of OA and reaction time. By means of this strategy, we further demonstrated that dumbbell-like ZnO is a twinned structure where the growth direction on both sides of the twinning plane is along the [0001j] direction. The present study gives a great impetus to the application of the surfacecontrolled ZnO by chemical etching in optoelectronic devices. Acknowledgment. This work was supported by the National Natural Science Foundation of China (grant nos. 20725310, 20721001, 20673085 and 20801045), Key Scientific Project of Fujian Province of China (grant no. 2009HZ0002-1), and the National Basic Research Program of China (grant nos. 2007CB815303, 2009CB939804). References and Notes (1) Lee, H.; Habas, S. E.; Kweskin, S.; Butcher, D.; Somorjai, G. A.; Yang, P. D. Angew. Chem., Int. Ed. 2006, 45, 7824. (2) Narayanan, R.; El-Sayed, M. A. Nano Lett. 2004, 4, 1343. (3) Tian, N.; Zhou, Z. Y.; Sun, S. G.; Ding, Y.; Wang, Z. L. Science 2007, 316, 732. (4) Hu, L. H.; Peng, Q.; Li, Y. D. J. Am. Chem. Soc. 2008, 130, 16136. (5) Han, X. G.; He, H. Z.; Kuang, Q.; Zhou, X.; Zhang, X. H.; Xu, T.; Xie, Z. X.; Zheng, L. S. J. Phys. Chem. C 2009, 113, 584.

Han et al. (6) Han, X. G.; Jin, M. S.; Xie, S. F.; Kuang, Q.; Jiang, Z. Y.; Jiang, Y. Q.; Xie, Z. X.; Zheng, L. S. Angew. Chem., Int. Ed. 2009, 48, 9180. (7) Chen, Y. X.; Chen, S. P.; Zhou, Z. Y.; Tian, N.; Jiang, Y. X.; Sun, S. G.; Ding, Y.; Wang, Z. L. J. Am. Chem. Soc. 2009, 131, 10860. (8) Jang, E. S.; Won, J. H.; Hwang, S. J.; Choy, J. H. AdV. Mater. 2006, 18, 3309. (9) Han, X. G.; Kuang, Q.; Jin, M. S.; Xie, Z. X.; Zheng, L. S. J. Am. Chem. Soc. 2009, 131, 3152. (10) Zhou, X.; Kuang, Q.; Jiang, Z. Y.; Xie, Z. X.; Xu, T.; Huang, R. B.; Zheng, L. S. J. Phys. Chem. C 2007, 111, 12091. (11) Lian, J. B.; Duan, X. C.; Ma, J. M.; Peng, P.; Kim, T.; Zheng, W. J. ACS Nano 2009, 3, 3749. (12) Yang, H. G.; Sun, C. H.; Qiao, S. Z.; Zou, J.; Liu, G.; Smith, S. C.; Cheng, H. M.; Lu, G. Q. Nature 2008, 453, 638. (13) Xu, T.; Zhou, X.; Jiang, Z. Y.; Kuang, Q.; Xie, Z. X.; Zheng, L. S. Cryst. Growth Des. 2009, 9, 192. (14) Ma, Y. Y.; Kuang, Q.; Jiang, Z. Y.; Xie, Z. X.; Huang, R. B.; Zheng, L. S. Angew. Chem., Int. Ed. 2008, 47, 8901. (15) Fan, D. B.; Thomas, J. P.; O’Brien, P. J. Am. Chem. Soc. 2008, 130, 10892. (16) Sawara, S.; Koh, M.; Goto, T.; Ando, Y.; Shinada, T.; Ohdomari, I. Appl. Surf. Sci. 2000, 159, 481. (17) Koh, M.; Sawara, S.; Shinada, T.; Goto, T.; Ando, Y.; Ohdomari, I. Appl. Surf. Sci. 2000, 162, 599. (18) Wang, H. Q.; Li, G. H.; Jia, L. C.; Li, L.; Wang, G. Z. Chem. Commun. 2009, 3786. (19) Pal, P.; Sato, K.; Gosalvez, M. A.; Shikida, M. J. Micromech. Microeng. 2007, 17, 2299. (20) Tang, B.; Pal, P.; Gosalvez, M. A.; Shikida, M.; Sato, K.; Amakawa, H.; Itoh, S. Sens. Actuators, A 2009, 156, 334. (21) Ould-Ely, T.; Prieto-Centurion, D.; Kumar, A.; Guo, W.; Knowles, W. V.; Asokan, S.; Wong, M. S.; Rusakova, I.; Lu¨ttge, A.; Whitmire, K. H. Chem. Mater. 2006, 18, 1821. (22) Han, X. G.; Jin, M. S.; Kuang, Q.; Zhou, X.; Xie, Z. X.; Zheng, L. S. J. Phys. Chem. C. 2009, 113, 2867. (23) Wang, Z. L.; Kong, X. Y.; Zuo, J. M. Phys. ReV. Lett. 2003, 91, 185502. (24) Vayssieres, L.; Keis, K.; Hagfeldt, A.; Lindquist, S. E. Chem. Mater. 2001, 13, 4395. (25) Xu, L. F.; Liao, Q.; Zhang, J. P.; Ai, X. C.; Xu, D. S. J. Phys. Chem. C 2007, 111, 4549. (26) Wang, H. Q.; Li, G. H.; Jia, L. C.; Wang, G. Z.; Tang, C. J. J. Phys. Chem. C 2008, 112, 11738. (27) Elias, J.; Tena-Zaera, R.; Wang, G. Y.; Le´vy-Cle´men, C. Chem. Mater. 2008, 20, 6633. (28) Wang, H. Q.; Li, G. H.; Jia, L. C.; Wang, G. Z.; Li, L. Appl. Phys. Lett. 2008, 93, 153110. (29) Zhou, X.; Xie, Z. X.; Jiang, Z. Y.; Kuang, Q.; Zhang, S. H.; Xu, T.; Huang, R. B.; Zheng, L. S. Chem. Commun. 2005, 5572. (30) Wang, B. G.; Shi, E. W.; Zhong, W. Z. Cryst. Res. Technol. 1998, 33, 937. (31) Yu, C. L.; Yu, Q. J.; Gao, C. X.; Yang, H. B.; Liu, B.; Peng, G.; Han, Y. H.; Zhang, D. M.; Cui, X. Y.; Liu, C. L.; Wang, Y.; Wu, B. J.; He, C. Y.; Huang, X. W.; Zou, G. T. J. Appl. Phys. 2008, 103, 114901. (32) Wang, R. H.; Xin, J. H.; Tao, X. M. Inorg. Chem. 2005, 44, 3926. (33) Yu, Q. J.; Yu, C. L.; Yang, H. B.; Fu, W. Y.; Chang, L. X.; Xu, J.; Wei, R. H.; Li, H. D.; Zhu, H. Y.; Li, M. H.; Zou, G. T.; Wang, G. R.; Shao, C. L.; Liu, Y. C. Inorg. Chem. 2007, 46, 6204.

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