Nanosheets-Based Rhombohedral In2O3 3D Hierarchical

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J. Phys. Chem. C 2009, 113, 10511–10516

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Nanosheets-Based Rhombohedral In2O3 3D Hierarchical Microspheres: Synthesis, Growth Mechanism, and Optical Properties Hongxing Dong,† Zhanghai Chen,*,† Liaoxin Sun,† Liang Zhou,‡ Yanjing Ling,† Chengzhong Yu,‡ H. Hoe Tan,§ Chennupati Jagadish,§ and Xuechu Shen† Surface Physics Laboratory, Department of Physics, AdVanced Materials Laboratory, and Department of Chemistry, Fudan UniVersity, Shanghai 200433, China, and Department of Electronic Materials Engineering, Research School of Physics and Engineering, The Australian National UniVersity, Canberra, ACT 0200, Australia ReceiVed: March 30, 2009; ReVised Manuscript ReceiVed: April 28, 2009

In this Article, we demonstrate a new synthesis route for the construction of rhombohedral In2O3 (rh-In2O3) nanosheets-based three-dimensional flowerlike microspheres (NBFMs). In this route, InOOH NBFMs were first prepared by a convenient and controllable method based on a complex reaction, using InCl3 · 4H2O as the starting material, oxalic acid as the complexing agent, and a mixture of glycerol and water as solvent at 180 °C for 12 h. The rh-In2O3 NBFMs were then obtained by calcining InOOH NBFMs precursors at 490 °C under ambient pressure. Scanning electron microscopy, X-ray diffraction, energy-dispersive X-ray spectroscopy, infrared spectroscopy, and thermogravimetric analysis were adopted to investigate the evolution process of InOOH precursors, and the chemical reactions at each stage were identified. The Ostwald ripening and anisotropic crystal structure are proposed to account for the formation of InOOH NBFMs on the basis of the evaluation of the time-dependent morphology. Furthermore, the influence of solvent on the morphologies and the photoluminescence properties of the rh-In2O3 NBFMs were investigated. Introduction Nanocrystal-based 3D hierarchical microstructures have been intensively investigated due to their novel properties.1-5 Controlled synthesis of the shape, crystal structure, and size of 3D hierarchical structures is highly desirable for a potential vast range of applications. 3D superstructures of various materials such as ZnO,6 Fe2O3,7 PbS,8 Co,9 C,10 WO3,11 Co3O4,12 CdS,13 and CeO214 have been fabricated. However, the controlled construction of 3D architectures from nanobuilding blocks via chemical or physical methods still remains a challenge as the precise control of nucleation, crystallization, and growth process of nanomaterials is very difficult. Indium oxide, an important functional semiconductor material with remarkable electronic and optical properties, has attracted considerable interest for several decades. It has been widely used for optoelectronic devices and gas sensors.15-18 Well-defined In2O3 nanostructures with different morphologies such as zero-dimensional (0D) quantum dots, nanocubes and octahedrons,19-21 one-dimensional (1D) nanobelts,22 nanowires,23 nanotubes,24 nanocrystal chains,25 and two-dimensional (2D) In2O3 nanosheets26 have been successfully fabricated by a variety of methods. However, 3D In2O3 nanostructures composed by low-dimensional building blocks have not yet been reported. In addition, most of the abovementioned In2O3 structures are in a cubic lattice. The rh-In2O3 with corundum structure, on the other hand, has been rarely studied so far. The corundum-type In2O3 is previously described as a polymorph, which was obtained through the phase-transition method at high pressure and high temperature.27 Recently, rhIn2O3 nanocubes were fabricated via sol-gel method and hightemperature organic solvent process.28,29 Nanofibers30 and mul* Corresponding author. E-mail: [email protected]. † Department of Physics, Fudan University. ‡ Department of Chemistry, Fudan University. § The Australian National University.

Figure 1. (a,b,c) SEM images of the synthesized InOOH nanosheetsbased microspheres; and (d) XRD pattern of the as-obtained InOOH products.

tipods31 were synthesized using coupled hydrothermal and post annealing method. However, in these synthesis routes, there are still limitations in the crystal nucleation and growth control. In addition, up to now, there is no systematic study on the detailed growth mechanism and the optical properties of these structures. In this Article, we report the synthesis of novel rh-In2O3 NBFMs under ambient pressure by annealing InOOH NBFMs precursors at 490 °C. The InOOH precursors were fabricated by using a complex reaction process. The crystal nucleation and growth can be controlled through our synthesis route, which is distinctly different from the reported methods of fabrication of rh-In2O3 nanostructures in the literature.28-31 In addition, the

10.1021/jp902843p CCC: $40.75  2009 American Chemical Society Published on Web 05/21/2009

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Figure 2. Time-dependent evolution of the flowerlike crystal morphology at different growth stages: (a) 10 min, (b) 30 min, (c) 1 h, (d) 2 h, (e) 6 h, and (f) 12 h.

Dong et al. without any further purification. Deionized water was used throughout the experiments. Preparation of In3+ (0.51 M). 0.5 mL of HCl (12 M) solution was first added to 50 mL of deionized water. Next, 7.5 g of InCl3 · 4H2O was added, and this mixture was stirred for 10 min with the formation of a clear solution. Synthesis of InOOH. In a typical synthesis of InOOH NBFMs, 4 mL of aqueous solution of C2H2O4 (0.75 M), 3 mL of aqueous solution of NaOH (1 M), and 1.5 mL of aqueous solution of InCl3 · 4H2O (0.51 M) were mixed together, and then 15 mL of glycerol was added into the solution. After vigorous stirring for 15 min, the mixture was transferred into a Teflonlined stainless steel autoclave of 50 mL capacity. The autoclave was sealed and maintained at 180 °C for 12 h. The autoclave was then allowed to cool naturally to room temperature, after which white precipitated powder was obtained. The powder was then isolated by centrifugation and was washed several times with distilled water and absolute ethanol and then was dried at 50 °C for 3 h. Transformation into rh-In2O3. The InOOH NBFMs were then used as precursors to form rh-In2O3 by putting them in a quartz crucible and were annealed in atmosphere under ambient pressure in a furnace at 490 °C for 2 h. Characterization. The as-prepared products were characterized by scanning electron microscopy (SEM, JEOL 6400), transmission electron microscopy (TEM, JEOL 2010), X-ray diffraction (XRD, D/max-rB diffractometer with Cu KR radiation (λ ) 1.54 Å) at scanning speed of 8 deg/min in the range from 20° to 70°), and Raman spectroscopy (LabRam HR UV Raman spectrometer (JY-Horiba), excited by a He-Ne laser with wavelength of 632.8 nm). An energy-dispersive X-ray (EDX) spectroscopy facility attached to the SEM was employed to analyze the chemical composition. Infrared (IR) spectra in the range of 400-4000 cm-1 were measured using a Nicolet Nexus 470 Fourier transform infrared (FTIR) spectrometer. Thermogravimetric (TG) analysis was carried out on a Pyris 1 TGA thermal gravimetric analyzer at a heating rate of 10 °C/ min from 50 to 600 °C under N2 flow. The photoluminescence (PL) spectroscopic measurements were performed on a LabRam HR UV spectrometer (JY-Horiba) with He-Cd laser as the excitation source (λex ) 325 nm). Results and Discussion

Figure 3. FTIR spectra (a) and XRD patterns (b) of the products obtained after different reaction times of (a) 10 min, (b) 30 min, (c) 1 h, (d) 2 h, (e) 2 h, (f) 6 h, and (g) 12 h at 180 °C.

formation mechanism of InOOH and rh-In2O3 NBFMs was investigated, and the optical properties of rh-In2O3 NBFMs were also studied. Experimental Section Materials. All of the reagents, indium chloride (InCl3 · 4H2O, 99.99% purity), sodium hydroxide (NaOH, A.R), glycerol (CH2OH-CHOH-CH2OH, A.R), hydrochloric acid (HCl, 12 M), and oxalic acid (C2H2O4, A.R), were used as received

Morphology and Structure of InOOH NBFMs. Figure 1a shows a typical SEM image of the sample, which is composed of many uniform microspheres with diameters of approximately 3-5 µm. To further examine the surface morphology of the microspheres, high-magnification SEM images were recorded, as shown in Figure 1b and c. The structures are actually composed of numerous nanosheets that are randomly arranged to form a flower-like structure on the surface of the microspheres. The building blocks, 2D anisotropic InOOH nanosheets, are about 30 nm in thickness and hundreds of nanometers in the planar dimensions. The crystalline phase of InOOH was examined by XRD, as shown in Figure 1d. All detectable peaks in this pattern can be assigned to orthorhombic InOOH (JCPDS no. 71-2283). No impurity peaks can be detected, which indicates that the samples are pure InOOH with an orthorhombic structure. To investigate the formation mechanism of the InOOH NBFMs, their growth process was followed by examining the products collected at different aging time intervals. As shown in Figure 2a, at the early stage (10 min), the resultant products are totally irregular consisting of loosely assembled nanopar-

Rhombohedral In2O3 3D Hierarchical Microspheres

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Figure 4. EDX spectra of the products synthesized at different aging time intervals: (a) 30 min, (b) 1 h, and (c) 6 h.

Figure 5. TG analyses of indium oxalate precursors obtained after reaction times of (I) 10 min and (II) 30 min.

ticles. When the reaction time was prolonged to 30 min, the size of the microstructures decreased and their surfaces became relatively smooth, as presented in Figure 2b. After 1 h of reaction time, some hexagonal rod-like microcrystals with smooth surfaces are observed as shown in Figure 2c. As the reaction proceeded to 2 h (Figure 2d), the rod-like microcrystals become fragmented, and some spherical particles are generated as indicated by the red arrows. After 6 h of reaction time (Figure 2e), the rod-like microcrystals are decomposed completely, and microspheres with rough surface are obtained. Further increase of the reaction time to 12 h results in products that are composed entirely of the NBFMs, as shown in Figure 2f. On the basis of the results discussed above, it is believed that Ostwald ripening and anisotropic crystal structure are the main driving forces for the formation of 3D flowerlike microspheres.32 After 6 h of reaction, InOOH nanoparticles were formed in the solution. As the aging process continued, the small particles grow to large particles via the Ostwald ripening process. In addition, the anisotropic crystal structure would cause a tendency for crystals to grow along the planar direction. Driven by these two factors, the NBFMs were formed after a long reaction time. Figure 3a shows the FTIR spectra of the samples obtained at different stages of annealing times. The broad peaks centered at 3300-3600 cm-1 are mainly attributed to the symmetric and asymmetric modes of water molecules. The strong peaks at 1635, 1362, 1322, and 812 cm-1 correspond to the vibration and bending modes of C2O42-. The peak centered at 1100 cm-1 for the products synthesized after reaction time of 60 min is induced by the bending mode of µ-OH. The FTIR spectrum of the products synthesized with reaction time of 6 h shows that the peak of 3439 cm-1 belongs to asymmetric (In)O-H stretching vibrations. Other absorption peaks of oxalate and hydroxyl groups are greatly weakened or even vanished after 6 h of reaction, indicating the formation of InOOH. These absorption bands are consistent with the results previously reported.33,34 In Figure 3b, the X-ray diffraction patterns of the products fabricated for different reaction times demonstrated the transformation from the original precursors to the final InOOH. It shows implicitly that the final phase of InOOH formed after 6 h of reaction time coexists with some intermediate nanophase [In(OH)(C2O4)(H2O)2]3 · H2O; this result agrees with

the FTIR analysis. It is noteworthy that [In(OH)(C2O4)(H2O)2]3 · H2O, first reported as an open-framework three-dimensional indium oxalate in 2005,33 was obtained through a simpler method in our experiment. In addition, an EDX facility attached to the SEM was employed to analyze the chemical composition of the products collected at different reaction time intervals. The results are shown in Figure 4. Figure 4a shows a typical EDX spectrum of the products fabricated after 30 min reaction time. C, O, Na, and In peaks were observed. The EDX spectrum for the products synthesized after 1 h reaction time shows the C, O, and In signals, but the Na signal has disappeared (Figure 4b). The EDX given in Figure 4c shows that the products fabricated after 6 h mainly contain O and In elements. The EDX patterns of products obtained at different reaction times of 10 min, 2 h, and 12 h are almost the same as those of 30 min, 1 h, and 6 h, respectively, which are consistent with the results from FTIR and XRD measurements. From thermogravimetric (TG) analysis (Figure 5), the samples prepared after 10 min (product I) and 1 h (product II) of reaction have two weight loss steps from 50 to 600 °C under N2 atmosphere. The first weight loss of ∼9.8 wt % at the range of 50-160 °C (for product I) and 50-250 °C (for product II) corresponds to the loss of water molecules. The difference in the temperature interval is most likely due to the different microstructures of these two samples. The second and more abrupt weight loss of ∼34.8 wt % for product I in the range of 250-450 °C is assigned to the decomposition of oxalate related ligands. Different from product I, the weight loss of ∼33.2 wt % in the range of 250-350 °C for product II corresponds to the removal of oxalate ligands and µ-OH. Similar phenomena were also observed in the synthesis of other nanomaterials.33,35-37 On the basis of the above experimental results and corresponding analysis, we propose the following reactions for the mixture: + In3++ 2C2O24 + Na + 2H2O f NaIn(C2O4)2(H2O)2

(1) 3NaIn(C2O4)2(H2O)2+ H2O f

(2) [In(OH)(C2O4)(H2O)]3 · H2O + 3Na++ 3HC2O4 H++ HC2O4 f H2O + CO2 + CO

(3)

[In(OH)(C2O4)(H2O)]3 · H2O f 3InOOH + 3CO + 3CO2+ 4H2O (4) In the above reaction process, C2O42- acts as an appropriate complexing agent with In3+ to form the coordination compound NaIn(C2O4)2(H2O)2. Reaction 1 is crucial, because the resultant NaIn(C2O4)2(H2O)2 is used as the slow-releasing indium ion

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Figure 6. SEM (a,b,c) and TEM (d,e) micrographs of the sample obtained by using glycol as solvent.

source, which could prevent the precipitation of In(OH)3 in subsequent reaction. Because the resultant NaIn(C2O4)2(H2O)2 is unstable, it will further decompose into [In(OH)(C2O4)(H2O)]3 · H2O, which has an open-framework three-dimensional structure. Finally, the product InOOH was converted from the [In(OH)(C2O4)(H2O)]3 · H2O through a dehydration process. In our system, the pH value of the solution increased from 2 to 5 after the reactions have finished. The possible reason is that the oxalic acid decomposed to CO2 and CO at high temperature and high pressure in autoclave, which subsequently formed gas bubbles in the solution. These CO2 and CO bubbles may help to form loosely hierarchical microstructures. In comparison with the other direct reaction process, our complex reaction method can regulate the nucleation kinetics and growth of the products and efficiently control the morphology and the structure of the final products. Glycerol is used not only as a solvent but also as a surfactant, which could greatly affect the morphology and microstructure of the products. To better understand the effect of the glycerol, we replaced glycerol by either glycol or water. Figure 6a displays the SEM images of the sample obtained with the glycol as the solvent, clearly showing some types of six-point starshaped dendritic superstructures. The detailed structure of the sample could also be resolved from higher-magnification SEM and TEM images (Figure 6b-e). Many nanorods are assembled in some parallel order into bunches, and these bunches further reordered into six-point star-shaped or complex dendritic superstructures. The XRD pattern of these samples is similar to that in Figure 1d, indicating the formation of orthorhombic InOOH. If we use water as the solvent, the products obtained have irregularly shaped morphologies with sizes of tens of micrometers (not shown here). From the above data, we can draw the conclusion that glycerol also plays an important role in controlling the morphology of the NBFMs. Transformation and Characterization of rh-In2O3 NBFMs. Like other metal hydroxides, indium oxyhydroxide can dehydrate to form indium oxide with the same morphology upon heating. In2O3 NBFMs can be easily obtained by heating the InOOH at 490 °C for 2 h. During the annealing process, the InOOH first decomposed into In2O3 and H2O, and then the In2O3 nucleated and grew in situ to form rh-In2O3 NBFMs. Figure 7a demonstrated that the flowerlike microstructures can be well preserved during the calcination process. The structure of the final products obtained by annealing the InOOH was examined by XRD. The result is shown in Figure 7b. All of the peaks can be indexed to the pure rhombohedral phase of In2O3 (JCPDS no. 73-1809), indicating that the samples have completely transformed from InOOH into rh-In2O3 during the annealing process.

Figure 7. SEM image (a) and XRD pattern (b) of the rh-In2O3 products annealed from InOOH 3D microstructures.

Figure 8. (a) TEM image of the rh-In2O3 flowerlike microspheres under low magnification. (b) The magnified view of the surface of one single microsphere. (c,d) Selected area electron diffraction pattern along the [010] zone and the corresponding high-resolution TEM image obtained from the rectangular region marked in Figure 8b.

The microstructure of these rh-In2O3 NBFMs was further characterized by TEM, and the results are shown in Figure 8. The TEM image of some rh-In2O3 NBFMs is shown in Figure 8a. As can be seen, the microspheres have uniform diameters of about 2 µm. The detailed morphology of the flowerlike microstructures is shown in Figure 8b, revealing that the entire structure is built from many small nanosheets. Figure 8c shows the selected area electron diffraction (SAED) pattern taken from the rectangular region marked in Figure 8b. The rhombohedral

Rhombohedral In2O3 3D Hierarchical Microspheres

J. Phys. Chem. C, Vol. 113, No. 24, 2009 10515 annealing process.28,29 These oxygen vacancies (Vo*) caused localized states between the conduction band and the valence band, and they normally act as deep donors in semiconductors. Conclusions

Figure 9. (a) Raman spectra and (b) photoluminescence spectra of the as-prepared flowerlike rh-In2O3 microspheres at room temperature.

diffraction pattern that can be indexed to the [010] zone axis of rh-In2O3 indicates the single crystal nature of the nanosheet. Figure 8d shows the corresponding high-resolution TEM image. It can be seen that the fringe spacing of 0.39 nm corresponds to the (102j) crystal planes of rh-In2O3. These results further confirm that the microspheres are composed of many nanosheets, and the nanosheets are single crystal with rhombohedral structure. Raman Spectra and Optical Properties of rh-In2O3 NBFMs. To further study and examine the crystalline structure of the rh-In2O3 obtained from the annealing of InOOH, we performed Raman and photoluminescence measurements on the samples. According to the crystallography data, rhombohedral In2O3 belongs to the space group R3jc, D3d6. The indium atoms are octahedrally coordinated with two layers of oxygen atoms, and the octahedron is severely distorted. The site symmetry for the In atoms is C3, whereas the O atoms are on sites having C2 symmetry. The optical modes correspond to the irreducible representations as below.38

2A1g+ 2A1u+ 3A2g+ 2A2u+ 5Eg+ 4Eu

(5)

The A1g and Eg vibrations are Raman active, while the A1u, A2u, A2g, and Eu are Raman inactive. Figure 9a shows the room temperature Raman spectrum of the rh-In2O3 NBFMs, which was dispersed on a single crystal Si foil. It is noticed that there are nine obvious Raman modes. Among them, the mode at 520 cm-1 is caused by the single crystal Si foil. Two of the modes (163 and 502 cm-1) are attributed to the A1g modes, and the remaining six belong to the Eg modes.25 The Raman result actually provides additional evidence to demonstrate that the In2O3 nanosheets are indeed single crystal with rhombohedral structure. In addition, we also studied the PL characteristics of a single rh-In2O3 NBFM at room temperature. The result is shown in Figure 9b. It can be seen that two distinct peaks appear at 413 and 517 nm, respectively. The deconvolution of the strong, asymmetric, and broad peak gives four Lorentzian components with peaks at 413, 509, 564, and 653 nm. The different emission peaks that we observed may be attributed to the existence of oxygen vacancies (Vo*), which are probably induced during the

In summary, we report a novel method for the fabrication of rh-In2O3 NBFMs. In this method, InOOH NBFMs were first fabricated using a complex reaction. The InOOH precursors were calcinated at 490 °C under ambient pressure to form rhIn2O3 NBFMs. The driving forces for the formation of InOOH NBFMs are the Ostwald ripening and anisotropic crystal structure based on time-dependent experimental results. The chemical reactions at each stage were also identified. We find that the solvent used (glycerol) plays an important role in governing the morphologies of InOOH, and the reaction time is also a key parameter in the synthesis. Furthermore, Raman spectroscopy and PL studies are carried out on rh-In2O3 NBFMs to examine the crystallinity and optical properties of the samples. The rh-In2O3 NBFMs are expected to have novel properties and potential application in gas sensors and optoelectronic nanodevices. Other semiconductor metal oxide materials may also be synthesized using the present synthetic procedure. Acknowledgment. This work is funded by the NSFC, 973 projects of China (no. 2004CB619004 and no. 2006CB921506). We thank the Australian Government Department of Innovation, Industry, Science and Research for funding this collaborative research under the International Science Linkages (China) Program. References and Notes (1) Gracias, D. H.; Tien, J.; Breen, T. L.; Hsu, C.; Whitesides, G. M. Science 2000, 289, 1170. (2) Park, S.; Lim, J. H.; Chung, S. W.; Mirkin, C. A. Science 2004, 303, 348. (3) Zhang, D. F.; Sun, L. D.; Jia, C. J.; Yan, Z. G.; You, L. P.; Yan, C. H. J. Am. Chem. Soc. 2005, 127, 13492. (4) Petrova, H.; Lin, C. H.; Hu, M.; Chen, J.; Siekkinen, A. R.; Xia, Y.; Sader, J. E.; Hartland, G. V. Nano Lett. 2007, 7, 1059. (5) Weiss, P. S. ACS Nano 2008, 2, 1085. (6) Li, L.; Yang, H. Q.; Qi, G. C.; Ma, J. H.; Xie, X. L.; Zhao, H.; Gao, F. Chem. Phys. Lett. 2008, 455, 93. (7) Zhong, L. S.; Hu, J. S.; Liang, H. P.; Cao, A. M.; Song, W. G.; Wan, L. J. AdV. Mater. 2006, 18, 2426. (8) Ma, Y.; Qi, L.; Ma, J.; Cheng, H. Cryst. Growth Des. 2004, 4, 351. (9) Zhu, L. P.; Zhang, W. D.; Xiao, H. M.; Yang, Y.; Fu, S. Y. J. Phys. Chem. C 2008, 112, 10073. (10) Chatterjee, T.; Jackson, A.; Krishnamoorti, R. J. Am. Chem. Soc. 2008, 130, 6934. (11) Chen, D.; Ye, J. H. AdV. Funct. Mater. 2008, 18, 1922. (12) Zhu, J.; Zhang, J.; Ma, B. Y.; Tay, J. J. Mater. Res. 2007, 22, 2448. (13) Xie, R. G.; Sekiduchi, T.; Li, D. S.; Yang, D. R.; Jiang, M. H. J. Phys. Chem. B 2006, 110, 1107. (14) Zhong, L. S.; Hu, J. S.; Cao, A. M.; Liu, Q.; Song, W. G.; Wan, L. J. Chem. Mater. 2007, 19, 1648. (15) Liu, F.; Bao, M.; Wang, K. L. Appl. Phys. Lett. 2005, 86, 213101. (16) Li, B.; Xie, Yi.; Jing, M.; Rong, G.; Tang, Y.; Zhang, G. Langmuir 2006, 22, 9380. (17) Nguyen, P.; Ng, H. T.; Yamada, T.; Smith, M. K.; Li, J.; Han, J.; Meyyappan, M. Nano Lett. 2004, 4, 651. (18) Li, C.; Zhang, D.; Liu, X.; Han, S.; Tang, T.; Han, J.; Zhou, C. Appl. Phys. Lett. 2003, 82, 1613. (19) Lu, W. G.; Liu, Q. S.; Sun, Z. Y.; He, J. B.; Ezeolu, C.; Fang, J. Y. J. Am. Chem. Soc. 2008, 130, 6983. (20) Liu, X. H.; Zhou, L. B.; Yi, R.; Zhang, N.; Shi, R. R.; Gao, G. G.; Qiu, G. Z. J. Phys. Chem. C 2008, 112, 18426. (21) Lee, C. H.; Kim, M.; Kim, T.; Kim, A.; Paek, J.; Lee, J. W.; Choi, S.-Y.; Kim, K.; Park, J.-B.; Lee, K. J. Am. Chem. Soc. 2006, 128, 9326. (22) (a) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Science 2001, 291, 1947. (b) Chun, H. J.; Choi, Y. S.; Bae, S. Y.; Park, J. Appl. Phys. A 2005, 81, 539.

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