ZnS Nanocable and ZnS

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25850

J. Phys. Chem. B 2006, 110, 25850-25855

Conversion of ZnO Nanorod Arrays into ZnO/ZnS Nanocable and ZnS Nanotube Arrays via an in Situ Chemistry Strategy Chenglin Yan and Dongfeng Xue* State Key Laboratory of Fine Chemicals, Department of Materials Science and Chemical Engineering, School of Chemical Engineering, Dalian UniVersity of Technology, 158 Zhongshan Road, Dalian 116012, P. R. China ReceiVed: September 12, 2006; In Final Form: October 11, 2006

Vertically aligned ZnO nanorods with uniform diameter and length have been synthesized on a zinc foil substrate with ammonium persulfate as oxidant via a facile, larger scale production and inexpensively synthesized method without any templates or additives. SEM and XRD studies indicate that ZnO nanorods are well-oriented along the c-axis. The PL spectrum indicates that our as-synthesized ZnO nanorods with a stronger and wider green emission are promising candidates as electron nanoconductors in nano-optoelectronic devices. Furthermore, by an effective thioglycolic acid-assisted solution route, well-aligned ZnO/ZnS nanocable and ZnS nanotube arrays have been successfully synthesized. ZnS nanotubes show a perfect hexagonal and obvious tubular shape. Our present strategy shows mild growth conditions and good reproducibility.

1. Introduction Zinc oxide, as a II-VI semiconductor with the wide band gap (3.2 eV) and large exciton binding energy of 60 meV at room temperature, has been extensively studied due to its wide applications in photonics devices, gas sensors, and dye-sensitized solar cells (for its optoelectronic, electrical, and photoelectrochemical properties).1,2 The key issue of the orientation, size, and shape control in fabrication of nanomaterials has been attracting considerable attention due to the unique orientation, size, and shape-dependent variation of properties and important potential applications of these materials.3-7 The first report of ultraviolet lasing from one-dimensional (1D) ZnO nanoarrays in 2001 has motivated researchers to synthesize ZnO nanoarrays applicable to constructing electronic and optoelectronic devices.8 Numerous efforts have been made to explore various convenient and efficient approaches for the synthesis of ZnO nanoarrays, most of which involve the high-temperature synthesis using metals as the catalysts,1a template synthesis confined within the pores of aluminum oxide templates,9 and solution-based synthesis using seeds to direct the nucleation and growth.4,10 However, these methods often suffer from the disadvantage of introducing metal catalysts, external templates, or seeds, which could make the synthesis procedures more complex and also introduce impurities that influence the properties of ZnO nanoarrays. To resolve these problems, numerous efforts have been made to develop a simple, low-cost, and one-step method without any additives to synthesize high-quality ZnO nanoarrays. In the current work, we report an inexpensive synthesis of highly oriented and well-aligned ZnO nanoarrays on the zinc foil substrate via an environmentally benign route. The current method is a quite simple, low-temperature, and practical route, which utilizes only a zinc foil and ammonium persulfate solution. Developing such an environmentally benign and facile method is thus necessary, which would provide a new and promising technique for the development of an effective, lowcost fabrication process and good potential for scale-up. It should * To whom correspondence should be addressed. Fax: (+86) 411-88993623. E-mail: [email protected].

Figure 1. (A) XRD pattern of well-aligned ZnO nanorod arrays on the zinc foil substrate at 120 °C. (B and C) Standard diffraction patterns of ZnO and Zn are shown as references. The indexed diffraction peaks can be assigned as the pure hexagonal ZnO phase; unindexed peaks originated from those of the zinc substrate.

be noted that, in the present work, ZnO nanorod arrays were directly grown from the zinc foil substrate; the zinc foil substrate can be directly used as the reactant and a good substrate to support the obtained 1D ZnO nanorods. Therefore, ZnO nanorod arrays cannot be grown on the other substrates such as glass and sapphire substrates by employing our current ammonium persulfate oxidation method. Previously, we developed a thermal reaction process for the synthesis of ZnS tetrapods,11 nanobelts,12 and the oriented assemblies of ZnS 1D nanostructures.13 Hollow ZnS microspheres have also been synthesized at room temperature via a novel sacrificial template route in our recent work.5 However, ZnS nanotube arrays have rarely been reported so far, possibly due to the preparation difficulty compared to that of ZnO nanotube arrays. Therefore, developing a facile route toward ZnS nanotube arrays is of urgency for the development of electronic and optoelectronic devices. Herein, the prepared ZnO nanorod arrays are used as an ideal sacrificial template to fabricate ZnS nanotube arrays. For the growth of well-aligned ZnS nanotube arrays, first ZnO/ZnS nanocable arrays with ZnO as the inner core and ZnS as the outer shell are synthesized via a thioglycolic acid-assisted solution route. Subsequently remov-

10.1021/jp0659296 CCC: $33.50 © 2006 American Chemical Society Published on Web 11/30/2006

ZnO Nanorod Arrays into ZnO/ZnS and ZnS Arrays

J. Phys. Chem. B, Vol. 110, No. 51, 2006 25851

Figure 2. SEM images of ZnO nanorod arrays grown on the zinc foil substrate at 120 °C: (A) tilt view; (B) edge tilt view (the inset is the corresponding SAED pattern); (C) a high-magnification image of ZnO nanorod arrays; (D) crystal growth habit of wurtzite ZnO nanorods.

Figure 3. (A) A model of crystallographic ZnO nanorods. (B) ZnO crystal structure represented in a form of the ZnO4 tetrahedron along the [001] direction.

ing ZnO core leads to the formation of ZnS nanotube arrays. We have demonstrated that the improved activity of Zn2+ on ZnO nanorods can be achieved by introduction of thioglycolic acid; ZnO nanorod arrays can easily be converted into ZnS nanotube arrays. 2. Experimental Section 2.1. Synthesis of ZnO Nanorod Arrays. The growth experiment of ZnO nanorod arrays was carried out in a Teflonlined stainless-steel autoclave. Zinc foil with a purity of 99.9% was used as the substrate, on which ZnO nanorod arrays grew. The zinc foils were carefully cleaned with absolute alcohol and deionized water, respectively, in an ultrasound bath to remove surface impurities. In a typical synthesis, the starting solution was prepared by mixing 15 mL of 3.2 mol/L KOH solution and 15 mL of 0.24 mol/L ammonium persulfate solution under

stirring. The mixture solution was then transferred into Teflonlined stainless steel autoclaves, and the previously cleaned zinc foil was then immersed into the solution. The autoclave was maintained at 80-180 °C for 12 h and then cooled down to room temperature. The zinc foil was taken out of the solution, washed with ethanol, and finally air-dried for characterization. 2.2. Synthesis of ZnO/ZnS Nanocable and ZnS Nanotube Arrays. The preparation of ZnS nanotube arrays involved an initial synthesis of ZnO/ZnS nanocable composites and a subsequent removal of ZnO cores by the KOH treatment of these composites. In a typical synthesis, zinc foil (covered with ZnO nanorod arrays), 0.1 mL of HSCH2COOH, and 25 mL of deionized water were put together at room temperature; such a mixture solution was stirred and the pH value was adjusted to the desired value using HCl solution (1 M) or NH3‚H2O (25 wt %). Then 2.4 g of Na2S was added into the above mixture

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Figure 4. Room-temperature PL spectrum of ZnO nanorod arrays recorded from the well-aligned ZnO nanorods corresponding to the SEM image of Figure 2A.

solution. Finally, the mixture solution was transferred into a stainless-steel autoclave and placed into an oven maintained at 130-180 °C for 4-12 h. After this reaction, the obtained ZnO/ ZnS nanocables were collected and washed several times with deionized water and pure ethanol. When ZnO cores were removed by the KOH treatment of ZnO/ZnS nanocable composites, ZnS nanotube arrays were obtained. All final samples were dried at 80 °C for 2-5 h. 2.3. Characterization Techniques. The as-prepared samples were characterized by an X-ray diffractometer (XRD) on a Rigaku-DMax 2400 diffractometer equipped with the graphite monochromatized Cu KR radiation flux at a scanning rate of 0.02° s-1 in the 2θ range 5-80°. Scanning electron microscopy (SEM) images were taken with a JEOL-5600LV scanning electron microscope, using an accelerating voltage of 20 kV. Energy-dispersive X-ray (EDX) microanalysis of the samples was performed during SEM measurements. The selected area electron diffraction (SAED) patterns were taken on a JEM2000EX microscope operated at 200 kV. The photoluminescence spectrum was measured at room temperature in a spectral range of 350-600 nm using a Xe lamp with a wavelength of 325 nm as the excitation source. 3. Results and Discussion The crystal structure and orientation of ZnO nanorod arrays are investigated by XRD diffraction. Figure 1 shows the XRD pattern of the as-prepared ZnO nanorods covered on the zinc foil substrate. The highly enhanced (002) peak can be clearly seen as a result of the vertical orientation of ZnO nanorods. The indexed diffraction peaks show a pure hexagonal phase of wurtzite-type ZnO (space group: P63mc) with lattice constants a ) 3.249 Å and c ) 5.206 Å, which is consistent with the reported data (JCPDS, 79-0206). On the same pattern, some weak peaks, except those already indexed diffraction peaks, can be indexed as the hexagonal Zn phase originating from the zinc substrate. Furthermore, it is worth noting that, compared to the standard pattern of hexagonal phase ZnO, the relative intensity of the peaks corresponding to (002) and (004) planes is significantly enhanced in the obtained XRD pattern (Figure 1), which suggests that ZnO (001) planes are oriented parallel to the basal plane of the zinc foil substrate. The selection of zinc foil as the substrate for the growth of well-orientated ZnO arrays is due to the following reason: the lattice matching between ZnO and Zn crystals (both are in the hexagonal phase) facilitates the growth of well-aligned ZnO nanorod arrays; moreover, zinc foil is a conductive material, making it easy to utilize the aligned ZnO nanorods for electronic and optoelectronic devices.

Yan and Xue SEM images shown in Figure 2 are large arrays of oriented ZnO nanorods, which are vertically grown on a zinc foil substrate along the [001] direction. The top view (Figure 2A) shows that ZnO nanorods cover the zinc surface uniformly, smoothly, and compactly. The side view clearly shows that ZnO nanorods are aligned approximately normal to the zinc substrate with a diameter of about 200 nm and a thickness of about 4 µm. Such an alignment can be better perceived from the side view in Figure 2B and a high magnification of Figure 2C. Note that these nanorods are better aligned and have good mechanical strength, due to the fact that ZnO nanorods are indeed grown from the zinc foil instead of a random deposition of ZnO particles on the zinc foil. The SAED pattern of one ZnO nanorod shown in the inset of Figure 1B verifies the fact that these nanorod arrays are single-crystalline wurtzite-type. With respect to the structure of wurtzite ZnO, the polar growth of ZnO crystal along the [001] direction proceeds through adsorption of growth units onto the (001) plane and the crystal growth habit exhibits a basal polar plane (001h) and two types of low-index faces, a nonpolar (1h00) face and a tetrahedron corner-exposed polar (001) face, as shown in Figure 2D. Therefore, according to SEM and XRD measurements, it is concluded that ZnO nanorods have grown along the [001] direction, parallel to the zinc foil substrate normal. In previous reports concerning the oriented growth of 1D ZnO nanorod/nanowire arrays in aqueous solution,4,14-17 it has been demonstrated that choosing a suitable solution system is a crucial factor. In the present solution system, a typical dissolutioncrystallization mechanism is responsible for the growth of wellaligned ZnO nanorods. At the beginning of the hydrothermal reaction, the zinc surfaces are first oxidized by (NH4)2S2O8 into a large quantity of ZnO thin films, which serve as the nuclei and reduce the interface energy barrier for the crystal growth. At the same time, the fresh resulting ZnO thin films can be dissolved in the KOH solution as [Zn(OH)4]2- growth units into the solution. More growth units of [Zn(OH)4]2- can be successively supplied with the oxidation of zinc foil by (NH4)2S2O8. Consequently, the resulting [Zn(OH)4]2- could serve as growth units for the epitaxial growth of nuclei into 1D ZnO structures as soon as they reach a critical saturation point. With respect to the structure of wurtzite ZnO, the polar growth of ZnO crystal along the [001] direction proceeds through the adsorption of growth units of [Zn(OH)4]2- onto the (001) plane and the side surfaces are {011h} and/or {21h1h } due to their lower energies than that of (001).18 These nuclei would preferentially grow along the [001] direction to form 1D nanorods if the chemical environment constantly provides [Zn(OH)4]2- growth units, which can be realized in our reaction system. Figure 3A is the crystallographic model of ZnO; hexagonal wurtzite structure ZnO can be simply described as a number of alternating planes composed of tetrahedrally coordinated O2- and Zn2+ ions, stacked alternately along the c-axis. The relative stacking rate of the constituent tetrahedra in various crystal faces is strongly dependent on the bonding force of atoms in the tetrahedra at the interface. Figure 3B shows that each {ZnO4} tetrahedron has a corner in the [001] direction. The atom at the corner of a tetrahedron has the strongest bonding force (s ) 2 vu, valence unit is abbreviated as vu) as compared with the atoms at other positions (e.g., along the edge and face directions).19 The positively charged ZnO (001)-Zn surface is chemically active, and the negatively charged (001h)-O surface is inert. Therefore, ZnO crystal grows fast along the direction in which the tetrahedron corners point. ZnO preferentially grows along the [001] direction, which results in the formation of 1D ZnO

ZnO Nanorod Arrays into ZnO/ZnS and ZnS Arrays

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Figure 5. (A and B) SEM images of ZnO/ZnS nanocable arrays on the zinc foil substrate via a thioglycolic acid-assisted solution method. (C) EDX spectrum taken from the surface of ZnO/ZnS nanocables. (D) The geometrical model of ZnO/ZnS nanocable, with ZnO as the core and ZnS as the shell.

nanorods. The entire polar growth process of ZnO nanorod arrays is schematically displayed in Figure S1. The orientated growth ZnS tetrapod nanocrystals11 and nanowires13 have successfully been interpreted by chemical-bonding theory in previous work. Photoluminescence (PL) spectrum of synthesized ZnO nanorod arrays is measured using a Xe lamp as the excitation source with an excitation wavelength of 325 nm. Figure 4 shows the room-temperature PL spectrum of well-aligned ZnO nanorods. From PL spectrum, a much weaker band edge ultraviolet peak at ∼382 nm (3.25 eV) is observed for the nanorods resulting from the near band edge emission of the wide band gap ZnO, while the green emission at ∼499 (2.48 eV) is very strong and wider. It has been suggested that the green band emission corresponds to the singly ionized oxygen vacancy in ZnO and results from the recombination of a photogenerated hole with the single ionized charged state of the defect.20 The stronger the intensity of the green luminescence, the more singly ionized oxygen vacancies there are. In our case, the progressive increase of the green light emission intensity relative to the UV emission suggests that there is a greater fraction of oxygen vacancies in these nanorods. It has been reported that the higher density of oxygen vacancies the nanostructured materials have, the higher electrical conductivity they possess;21,22 therefore, our as-synthesized ZnO nanorods with a stronger and wider green emission are promising candidates as electron nanoconductors in nano-optoelectronic devices.

Early reports revealed that ZnS nanotube arrays obtained by a direct reaction of ZnO and H2S show poor qualities and morphologies23,24 and thus hamper their potential applications. Rectangular porous ZnO-ZnS nanocables have also been obtained by employing ZnO as the template.25 In the current work, an effective thioglycolic acid-assisted solution route from ZnO nanorod arrays to well-aligned ZnO/ZnS nanocable and further ZnS nanotube arrays has been successfully demonstrated. An evolution in particle shape from ZnO nanorod to ZnS nanotube array is due to the solubility difference between ZnO and ZnS and to the assistance of thioglycolic acid. ZnO nanorod arrays are used as a template for the fabrication of ZnO/ZnS nanocables by sulfurization of ZnO after a thioglycolic acidassisted reaction. When ZnO nanorod arrays are introduced into HSCH2COOH solution, ZnHS+ complex could be formed between the lone pair electrons of sulfur atom of HSCH2COOH molecule and the vacant d orbital of the Zn2+ ions, which results in an increase in the activity of Zn2+ ions on ZnO nanorods, and then ZnS nucleates and grows by dissolution of ZnO nanorods. After reaction of 4-12 h, ZnO/ZnS nanocables can be obtained. Figures 5A and B show SEM images of ZnO/ZnS nanocables on the zinc foil substrate, with ZnO as the core and ZnS as the shell. EDX analysis is used to determine the chemical composition of ZnO/ZnS nanocables. A typical EDX spectrum is shown in Figure 5C, which exhibits the presence of Zn, O, and S elements and confirms the composition of ZnO/ZnS nanocables. It can be clearly demonstrated that ZnO nanorods

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Yan and Xue

Figure 6. (A and B) SEM images of ZnS nanotube arrays (the inset of Figure 6A is a magnified image of ZnS nanotubes and the inset of Figure 6B is the corresponding SAED pattern). (C) EDX spectrum taken from the surface of ZnS nanotube arrays. (D) The geometrical model of ZnS nanotube.

are wrapped with a thin layer of ZnS, which is shown by the geometrical model in Figure 5D. Since ZnO has an amphoteric characteristic, KOH treatment of ZnO/ZnS nanocables leads to the dissolution of ZnO cores, and thus ZnS nanotube arrays can be successfully obtained. The well-aligned ZnS nanotube arrays are observed on the surface of zinc foil, as shown in Figure 6A. It can be seen that ZnS tubes have open ends with a uniform pore size of about 400 nm. The inset of Figure 6A reveals a high-magnification SEM image of one ZnS nanotube, showing a perfect hexagonal shape. The inset of Figure 6B is the SAED pattern of the selected ZnS nanotube, which indicates that the obtained ZnS nanotubes are in a polycrystalline state. EDX spectrum taken from one ZnS tube is used to confirm such a pure composition, as displayed in Figure 6C. This spectrum shows strong peaks of only Zn and S elements, which indicates that pure ZnS nanotubes can be successfully obtained by the KOH treatment of ZnO/ZnS nanocables. The geometrical model of ZnS nanotube arrays gives us clear evidence of tubeshaped structures, which are shown in Figure 6D. XRD pattern of the obtained ZnS nanotubes is shown in Figure 7 in which all diffraction peaks can be indexed to a typical zinc blende structured ZnS with the cell constant a ) 5.406 Å, which is consistent with the standard value for bulk ZnS (JCPDS Card No. 05-0566). XRD results give us the evidence that KOH treatment of the ZnO/ZnS nanocable structure can substantially remove its ZnO core, leading to ZnS nanotube arrays. Our work demonstrates that thioglycolic acid-assisted solution process for fabrication of ZnS nanotube arrays is an effective and facile method. Due to the limited solubility of ZnO in solution, zinc ions on ZnO nanorods, however, are not free in the solution. ZnO thus has difficulty in transforming into ZnS

Figure 7. XRD pattern of ZnS nanotube arrays prepared by the KOH treatment of ZnO/ZnS nanocable arrays.

through a simple direct reaction between ZnO and S2-. In our case, the improved activity of Zn2+ on the ZnO nanorods can be achieved by introducing HSCH2COOH into the reaction solution. Therefore, the ZnO/ZnS nanocables can be easily fabricated by the reaction of Na2S at the surface layer of ZnO with the assistance of HSCH2COOH. The involved chemical reactions for the fabrication of ZnO/ZnS nanocables can be formulated as follows:

HSCH2COOH + Zn2+ T ZnHS+ + CH2COOH+ (1) 2ZnHS+ + S2- T 2ZnS + H2S

(2)

In fact, there are lone pair electrons on sulfur atoms of each HSCH2COOH molecule, which provides the possibility that

ZnO Nanorod Arrays into ZnO/ZnS and ZnS Arrays HSCH2COOH can combine with Zn2+ ions to form ZnHS+ complexes. Therefore, when ZnO nanorods are introduced into HSCH2COOH solution, the surface layer of ZnO nanorods adsorb HSCH2COOH molecule, and ZnHS+ complexes are formed from the strong coordination interaction between Zn2+ and HSCH2COOH (eq 1). In this case, the activity of Zn2+ in ZnO nanorods is greatly enhanced. Sulfide ions released from Na2S solution at an elevated temperature into solutions can easily react with ZnHS+ complexes, which finally result in the formation of ZnS shell on the ZnO nanorod surface. In our current work, we have also obtained flower-like ZnS nanotubes (Figure S2) by employing the same thioglycolic acid-assisted method, which indicates that our method employed here is versatile and practical. In the absence of HSCH2COOH molecule, ZnO dissociates slowly in solution and has difficulty in producing free zinc ions to form ZnS shell with Na2S solution, due to the very small solubility product for ZnO (6.8 × 10-17) in solution at room temperature. ZnS nanotube arrays can also be obtained with ZnO nanorod arrays as the template without HSCH2COOH (under similar hydrothermal conditions). However, ZnS tubes do not have a better orientation and quality, and many of them collapsed under SEM observation (Figure S3), compared with those obtained in presence of thioglycolic acid. Hence, the origin of well-aligned ZnS nanotube arrays is suggested by the contribution of thioglycolic acid-assisted reaction. 4. Conclusion In this work, we report a one-step mild reaction route for the growth of well-aligned ZnO nanorod arrays on zinc foil substrate through direct oxidation of zinc foil with ammonium persulfate oxidant in the alkali solution. Such solution-based methods greatly facilitate the approach to scale up the fabricated ZnO nanoarrays with relative low cost under mild reaction conditions. SEM and XRD measurements show that these nanorods have grown along the [001] direction, parallel to the substrate normal. The growth direction of 1D ZnO nanorod arrays along [001] can be well-understood from the chemical bond viewpoint. This facile oxidizing model with ammonium persulfate as oxidant might also be applied to obtain other metal oxide nanoarrays (such as TiO2, CuO, and so on). Furthermore, an effective thioglycolic acid-assisted solution method is developed to fabricate well-aligned ZnS nanotube arrays. ZnS shells can be easily fabricated by the reaction of Na2S with the surface layer of ZnO with the assistance of HSCH2COOH, due to the fact that the improved activity of Zn2+ on ZnO nanorods can be achieved through forming ZnHS+ complex between HSCH2COOH molecule and Zn2+ ions. Acknowledgment. We gratefully acknowledge the financial support of the Program for New Century Excellent Talents in

J. Phys. Chem. B, Vol. 110, No. 51, 2006 25855 University (Grant No. NCET-05-0278), the National Natural Science Foundation of China (Grant No. 20471012), a Foundation for the Author of National Excellent Doctoral Dissertation of P. R. China (Grant No. 200322), the Research Fund for the Doctoral Program of Higher Education (Grant No. 20040141004), and the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry. Supporting Information Available: Schematic representation of the microscopic formation process of ZnO nanorod arrays, SEM images, and SAED patterns. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Huang, M. H.; Wu, Y.; Feick, H.; Tran, N.; Weber, E.; Yang, P. AdV. Mater. 2001, 13, 113. (2) Gao, P. M.; Ding, Y.; Mai, W. J.; Hughes, W. L.; Lao, C. S.; Wang, Z. L. Science 2005, 309, 5741. (3) Yin, Y.; Rioux, R. M.; Erdonmez, C. K.; Hughes, S.; Somorjai, G. A.; Alivisatos, A. P. Science 2004, 304, 711. (4) Tian, Z. R.; Voigt, J. A.; Liu, J.; Mckenzie, B.; Mcdermott, M. J.; Rodriguez, M. A.; Konishi, H.; Xu, H. Nat. Mater. 2003, 2, 821. (5) Yan, C.; Xue, D. J. Phys. Chem. B 2006, 110, 7102. (6) Yan, C.; Xue, D. J. Phys. Chem. B 2006, 110, 1581. (7) Sounart, T.; Liu, J.; Voigt, J.; Hsu, J.; Spoerke, E.; Tian, Z.; Jiang, Y. AdV. Funct. Mater. 2006, 16, 335. (8) Huang, M. H.; Mao, S.; Feick, H.; Yan, H.; Wu, Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. Science 2001, 292, 1897. (9) Liu, C.; Zapien, J. A.; Yao, Y.; Meng, X.; Lee, C. S.; Fan, S.; Lifshitz, Y. Lee, S. T. AdV. Mater. 2003, 15, 838. (10) Li, Q.; Li, V. K. Y.; Zhang, H.; Marks, T.; Chang, R. H. Chem. Mater. 2005, 17, 1001. (11) Zhu, Y.; Bando, Y.; Xue, D.; Golberg, D. J. Am. Chem. Soc. 2003, 125, 16196. (12) Zhu, Y.; Bando, Y.; Xue, D.; Golberg, D. Appl. Phys. Lett. 2003, 82, 1769. (13) Zhu, Y.; Bando, Y.; Xue, D.; Golberg, D. AdV. Mater. 2004, 16, 831. (14) Yu, H.; Zhang, Z.; Han, M.; Hao, X.; Zhu, F. J. Am. Chem. Soc. 2005, 127, 2378. (15) Gao, Y.; Nagai, M. Langmuir 2006, 22, 3936. (16) Cao, B.; Li, Y.; Duan, G.; Cai, W. Cryst. Growth Des. 2006, 6, 1091. (17) Fang, Y.; Pang, Q.; Wen, X.; Wang, B.; Yang, S. Small 2006, 2, 612. (18) Vayssieres, L.; Keis, K.; Lindquist, S.; Hagfeldt, A. J. Phys. Chem. B 2001, 105, 3350. (19) Xue, D.; Zhang, S. Chem. Phys. Lett. 1998, 287, 503. (20) Vanheusden, K.; Warren, W. L.; Seager, C. H.; Tallant, D. R.; Voigt, J. A.; Gnade, B. E. J. Appl. Phys. 1996, 79, 7983. (21) Xing, Y. J.; Xi, Z. H.; Xue, Z. Q.; Zhang, X. D.; Song, J. H.; Wang, R. M.; Xu, J.; Song, Y.; Zhang, S. L.; Yu, D. P. Appl. Phys. Lett. 2003, 83, 1689. (22) He, F.; Zhao, Y. Appl. Phys. Lett. 2006, 88, 193113. (23) Dloczik, L.; Engelhardt, R.; Emst, K.; Fiechter, S.; Sieber, I.; Konenkamp, R. Appl. Phys. Lett. 2001, 78, 3687. (24) Wang, Z.; Qian, X.; Li, Y.; Yin, J.; Zhu, Z. J. Solid State Chem. 2005, 178, 1589. (25) Wang, X.; Gao, P. X.; Li, J.; Summers, C. J.; Wang, Z. L. AdV. Mater. 2002, 14, 1732.