Synthesis of W18O49 Nanorod via Ammonium Tungsten Oxide and Its

ARTICLE pubs.acs.org/Langmuir

Synthesis of W18O49 Nanorod via Ammonium Tungsten Oxide and Its Interesting Optical Properties Chongshen Guo,* Shu Yin, Yunfang Huang, Qiang Dong, and Tsugio Sato Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, 980-8577, Japan

bS Supporting Information ABSTRACT: W18O49 nanorods were synthesized by pyrolyzing (NH4)xWO3+x/2 nanorods precursors, which were prepared by a hydrothermal reaction using sulfate as a structure-directing agent, in a reductive atmosphere of H2(5 vol %)/N2 at 500 °C for 1 h. W18O49 nanorods showed high transmittance in the visible region as well as excellent shielding properties of NIR lights. A simulated experiment revealed that excellent heat insulating performance can be realized by applying a 70% visible light transparent W18O49 coating on a quartz glass. Meanwhile, the W18O49 nanorods also showed strong absorption of NIR light and instantaneous conversion of photoenergy to heat. In a word, W18O49 nanorods hold interesting optical properties and are a promising material in a wide range of applications.

1. INTRODUCTION In the past decades, tungsten oxide related compounds have attracted much attention for their applications in gas sensors, electrochromic devices, photocatalysts, field emission displays, etc.1
7 Among them, a significant fraction of the work has been devoted to fabrication of one-dimensional (1D) tungsten oxides (WO3-x, x g 0) due to their unique applications. In particular, monoclinic W18O49 (or WO2.72) is of great interests owing to its unusual defect structure and promising properties, such as chromism, photocatalysis, and sensing capabilities. Until now, several methods have been reported to synthesize one-dimensional reduced type tungsten oxide. Growth of quasi-aligned W18O49 nanotubes and nanowires have been developed in previous works.8,9 Wang et al. synthesized W18O49 nanowires by annealing WCx films followed by oxidation.10 In addition, colloidal W18O49 nanorods have been synthesized by pyrolyzing W(CO)6 at 250
270 °C in the presence of Me3NO 3 2H2O and oleylamine.11 It is highly desirable to develop a facile synthesis process of morphology controlled W18O49 nanoparticles with low-cost precursors and without special equipments, harsh experimental requirements, or poisonous reagents. We found that the tungsten bronze type compounds (MxWO3, where M = Cs, Rb, K, Na, etc.) consisting of mixed valence tungsten ion(such as W6+ and W5+), showed excellent NIR shielding properties when dispersed as nanosized particles.12
16 The transparent coating with infrared shielding ability has been used to bring a saving of air conditioning energy of rooms by cutting off the heat rays of sunlight in summer, and preventing the release of indoor heat in winter.17,18 For the application as a heat ray shielding material, excellent shielding ability of NIR rays as well as high visible light transparency are r 2011 American Chemical Society

required.12,13 Since W18O49 consists of mixed valence tungsten ions, it is expected to show NIR shielding ability. In this work, W18O49 nanorods were synthesized by reductive pyrolysis of rod-like (NH4)xWO3+x/2 precursors, which were prepared by a hydrothermal reaction using sulfate ions as a structure-directing agent. In addition, the optical properties of W18O49 nanorods, such as the NIR absorption and instant conversion of photoenergy to thermal energy, were also discussed.

2. EXPERIMENTAL SECTION All of the reagents were of analytical grade and used as received without further purification. In a typical procedure, the ammonium tungsten oxide was synthesized as follows. First, Na2WO4 and (NH4)2SO4 were dissolved in 50 mL of distilled water under stirring, where the concentrations of (NH4)2SO4 and Na2WO4 were adjusted to 0.2 and 0.1 M, respectively. (The optimized concentrations of SO42- and WO42- as well as their molar ratio in preparing hexagonal tungsten oxides have been confirmed in our previous works.14,15) Then, the pH of the mixed solution was acidified to 1.5 by adding 3 M HCl. The resultant solution was transferred into a Teflon-lined autoclave and heated at 200 °C for 24 h. After the hydrothermal reaction, the autoclave was cooled to room temperature naturally. The product was collected by centrifugation and washed repeatedly with water and ethanol followed by vacuum drying at 60 °C overnight. The W18O49 was prepared by reducing the as-obtained (NH4)xWO3+x/2 in an H2(5 vol %)/N2 atmosphere at 500 °C for 1 h. In order to evaluate its NIR shielding characteristics, the W18O49 powder was Received: July 3, 2011 Revised: August 27, 2011 Published: August 29, 2011 12172

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Figure 2. EDS profile of as-synthesized (NH4)xWO3+x/2 by hydrothermal reaction at 200 °C and pH 1.5 for 24 h using Na2WO4 and (NH4)2SO4 as starting materials.

Figure 1. XRD patterns of (a) as-synthesized (NH4)xWO3+x/2 nanorods by hydrothermal reaction at 200 °C and pH 1.5 for 24 h using Na2WO4 and (NH4)2SO4 as starting materials and (b) W18O49 nanorods prepared by reducing the as-obtained (NH4)xWO3+x/2 in an H2(5 vol %)/N2 atmosphere at 500 °C for 1 h. dispersed in a collodion-ethanol mixed solution at a mass ratio of ethanol: collodion: W18O49 = 1.0: 0.93: 0.15. Then, the coating solution was painted on quartz glass by an applicator with a concave shape and a depth of 12.5 μm. 2.1. Characterization. The phase compositions of the samples were determined by X-ray diffraction analysis (XRD, Shimadzu XD-1) using graphite-monochromized CuKα radiation. The size and shape of the nanoparticles were observed by a transmission electron microscope (TEM, JEOLJEM-2010). HRTEM images and SAED images were obtained on a ZEISS LEO 922 with an accelerating voltage of 200 kV. The optical response of the coating was measured using a spectrophotometer (JASCO V-670), giving an output of transmittance in the UV, visible, and infrared ranges (200
2700 nm). Energy-dispersive X-ray spectroscopy (EDS) was employed for approximate elemental analyses. The thermogravimetric and differential thermal analyses (TGDTA, Rigaku, TG8101D) were performed for the samples from room temperature to 900 °C with a heating rate of 10 °C/min in air. Thermographic measurements were recorded by a thermographicmeter (FLIR System i7)

3. RESULTS AND DISCUSSION Figure 1a shows the XRD pattern of as-prepared ammonium tungsten oxide by the hydrothermal process. The reflections could be well indexed as the hexagonal (NH4)xWO3+x/2 phase (JCPDS file No.42
0452). No XRD peak corresponding to impurities, such as WO3 or NaxWO3 was detected. After structure refinement, the cell constants were determined as a = 7.387 Å, c = 7.517 Å which are almost the same as the published ones.19,20 Comparing the intensities of the (0 0 2) and (2 0 0) peaks of the sample with those in the standard powder diffraction card, it was found that the relative intensity of (0 0 2) was greatly increased, implying that preferential growth occurred along the c-axis direction. Figure 1b shows the XRD pattern of the calcined sample. Since the sample was identified as W18O49 (JCPDS card No: 05
0392), it was confirmed that (NH4)xWO3+x/2 was converted to W18O49 by

calcination under a reductive atmosphere of H2 (5 vol.%)/N2 at 500 °C for 1 h. The chemical reaction may be expressed by eq 1. The product showed a stronger intensity of the (0 1 0) peak in comparison to the bulk one. Similar behaviors have been reported for W18O49 nanowires, which show a preferential growth along the (0 1 0) direction.8,11,21 18ðNH4 Þx WO3þx=2 þ 5H2 ¼ 18xNH3 þ ð5 þ 9xÞH2 O þ W 18 O49

ð1Þ

The EDS technique was employed for approximate elemental analyses for the as-synthesized sample, and the profile is presented in Figure 2. Peaks corresponding to the elements of O and W can be clearly observed, and no impurities other than copper which came from the copper grid could be observed in the spectra. What is notable, the Na+ and NH4+ coexisted in the starting solution, but there is no Na+ in the final product. It is known that bigger alkaline ions, such as Cs+ and NH4+, are more easily inserted into the hexagonal tunnel of WO3 than smaller ones, such as Na+ and K+, under similar hydrothermal reaction conditions. This is because the smaller cations cannot be held firmly in the hexagonal tunnel of WO3.22,23 Therefore, the larger NH4+ is more easily incorporated in than smaller Na+. Due to the limitation of EDS for detecting the elemental N, the amount of N was determined by a chemical combustion method. The results revealed that there was 1.7 wt.% N in the as-prepared sample ((NH4)xWO3+x/2), whereas no N element was detected in the reduced sample (W18O49). According to this result, the x value of (NH4)xWO3+x/2 was calculated as 0.29. The size and morphology of the as-prepared sample were observed by transmission electron microscope (TEM). Figure 3a shows the typical TEM image of as-synthesized (NH4)xWO3+x/2. The sample exhibited one-dimensional rodlike morphology with diameters in the range of 30
80 nm and the length of several micrometers. The (NH4)xWO3+x/2 with the one-dimensional hexagonal channel structure may be preferential to grow along the c-axis, and this supposition could be confirmed by the selected area of ED analysis (shown in Figure 3b). This is also in agreement with the XRD results shown in Figure 1. Regular diffraction spots also imply that the nanorod was single crystals with good crystallinity. After the reductive calcination, the (NH4)xWO3+x/2 was converted to W18O49 without loss of the one-dimensional 12173

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Figure 4. TG plots of (a) (NH4)xWO3+x/2 and (b) W18O49 recorded in air with a heating rate of 10 °C/min.

Figure 3. (a) TEM and (b) selected area ED profiles of (NH4)xWO3+x/2 nanorods; (c) TEM, (d) selected area ED, and (e)
(f) HR-TEM images of W18O49 nanorods. Figure 5. W4f core-level XPS spectra of W18O49 nanorods.

rod-like morphology, as shown in Figure 3c. More details about the crystal structure were studied by HR-TEM (Figure 3e,f). Clear lattice fringes could be seen for the W18O49 single crystal nanorods. The crystalline lattice constant perpendicular to the direction of the nanorod was calculated as 0.413 nm, and the crystal panel was identified as (0 0 4). Fabrications of one-dimensional metal oxide nanocrystals using SO42- as structure-directing agent were intensively reported in other works.24
28 It has been considered that, selectively absorbing of the SO42- ions to specific crystallographic facets slows down the growth rate along a particular crystallographic direction and results in a preferential growth along another direction as the growth rate of crystallographic facets varies exponentially with their surface energy. In close related cases of synthesizing hexagonal potassium tungsten oxides by employing similar hydrothermal synthetic approach, it was reported that SO42- may act as a capping agent, which is preferentially adsorbed on the faces parallel to the c-axis of the hexagonal tungsten oxide nanocrystals, leading to preferential growth along the c-axis.14,15,28 In this work, it is reasonable to suggest that the formation of 1D nanostructures of (NH4)xWO3+x/2 may be closely attributed to the directional absorption of SO42- and intrinsic anisotropic property of hexagonal ammonium tungsten oxide. Subsequently, the (NH4)xWO3+x/2 nanorods undoubtedly provide structural embryo for fabricating 1D W18O49 nanocrystals in further stepwise. The thermal behavior of the samples was studied by thermogravimetry analyses in an air atmosphere with a heating rate of

10 °C/min. The TG plots of the hexagonal (NH4)xWO3+x/2 and W18O49 were given in Figure 4. The weight loss up to 200 °C might be assigned to the desorption of water, including the loss of surface adsorbed water (∼90 °C) and structural water elimination (∼200 °C) for both of the two samples. Combining the TGMS results in ref 19 with Figure 4a of this work, it is logical to conclude that, the obvious weight loss in the temperature range of 220
400 °C could be well attributed to the departure of NH3 from the surface as a result of (NH4)xWO3+x/2 decomposition. On the basis of the above discussion, the proposed overall reaction of eq 1 could be divided into two elementary reactions with an intermediate of HxWO3+x/2(or WO3 3 x/2H2O). In a temperature range of 250
450 °C, a slight weight gain occurred for W18O49 (Figure 4b). This effect is related to the oxidation process of the W18O49 by air oxygen, above a certain temperature, as described by eq 4. The XRD analysis confirmed that the resultant powders after TG measurement were WO3. ðNH4 Þx WO3þx=2 ¼ xNH3 þ Hx WO3þx=2

ð2Þ

18Hx WO3þx=2 þ 5H2 ¼ W 18 O49 þ ð5 þ 9xÞH2 O

ð3Þ

2W 18 O49 þ 5O2 ¼ 36WO3

ð4Þ

The XPS photoelectron spectroscopy (XPS) provides important information about the surface electronic structure and 12174

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Figure 6. (a) Photograph of W18O49 film coated on quartz glass; (b) 2D and (c) 3D AFM images of the film, and (d) cross section analysis.

elemental valence of the product (Figure 5). The curves could be fitted as two spin
orbit doublets, W4f7/2 and W4f5/2, for the interval of about 2.1 eV. The peaks at 34.4 and 36.5 eV, and 35.4 and 37.5 eV were attributed to W5+ and W6+, respectively, which agrees with the reported values.13 Although the XPS peak could be well fitted as a mixture of W5+ and W6+, we still cannot eliminate the presence of trace amount of W4+. Therefore, we suggest that the chemical valence of W may be in the range of +4 to +6. For the optical test, the W18O49 sample was coated on a quartz substrate using an applicator after mixing the as-obtained powder with Vis-NIR light transparent binder. Figure 6a shows a typical photograph of a W18O49 film. The film was deep blue originating from the W18O49 nanorods. AFM images of the W18O49 film coated on a quartz glass (Figure 6a) were observed to provide detailed information about the surface morphology and the homogeneity of the coated films (see Figure 6). The surface layer of the W18O49 film was uniform and smooth as shown in Figure 6b,c, with a mean interface roughness (Ra) of 24.2 nm. The maximum peak-to-valley distance based on the cross sectional analysis was determined to be about 159 nm (Figure 6d). In addition, the thickness of the W18O49 tinted film determined on SURFCOM130A was about 1 μm.

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Figure 7 shows the transmittance spectra of the samples. Tungsten trioxide (WO3) possessed a wide band gap of 2.62 eV and was transparent in visible and NIR light ranges (Figure 7b). Due to the lack of free electrons, the ammonium tungsten oxide ((NH4)xWO3+x/2) also showed no NIR shielding performance in the near-infrared region (Figure 7a). In contrast, the W18O49 nanorods of this work showed high transmittance in the visible region as well as excellence shielding properties in the NIR region, indicating its promising performance as a NIR shielding material. For comparison, we tested the optical response of bulk W18O49 and the results are shown in Figure 7c. The bulk W18O49 showed a certain NIR shielding ability, however, the NIR shielding performance and visible light transparency were much lower, probably due to the difference in the microstructure consisting of much larger inhomogeneous particles. As an effective solar filter material, small size and special morphology of the particles are highly required.12,13 The ITO glasses are widely used and well-known as effective infrared-ray cutoff material.29
31 As shown in Figure 7d, it showed excellent visible light transparency, but the shielding ability of NIR of wavelengths of less than 1400 nm was modest. The value of 100
T(%) should be the sum of absorption (A%) and reflectance (R%) of light, where T(%) is transmittance. It can be seen that the reflectance of W18O49 nanorod sample (R-e) was quite limited in all wavelength ranges, indicating that the shielding of NIR light by W18O49 was mainly caused by the absorption of light. In contrast, ITO glass showed strong light reflectance (R %) in the NIR region, indicating that the NIR shielding by ITO glass is mostly caused by the reflectance, instead of absorption of light.29
31 It is widely accepted that the optical and electrical properties of functional materials are strongly influenced by their nanostructures, including morphologies, sizes, orientation, and so on. Previous works have found that, in the case of employing reduce type tungsten oxides for the applications as NIR shielding filers, a sharp size and/or morphology dependent optical response was observed,12
16 owing to a fact that the nanosized particles often show unique surface electronic structures and crystallographic defects. For example, the characteristic extinction in NIR region can be realized when the particle size was reduced below certain region. However, it is expected that the nanorods may exhibit transverse and longitudinal surface plasmon resonances that correspond to electron oscillations perpendicular and parallel to the rod length direction, respectively. Therefore, it is the fact that nanosized one-dimensional W18O49 nanocrystal showed excellent and unique characteristics as compared to its bulk counterparts. Moreover, the strength and width of absorption peaks were distinctly enhanced as decreasing of particles size were also reported for another type of NIR absorbent of LaB6.32 In this work, the potential applications as solar filter were tested in the form of film which containing W18O49 nanocrystals at a low concentration, and it therefore could realize high transmittance in the visible region. Although the original mechanism of NIR absorption on W18O49 nanorods is not fully known, it is thought to be closely related to the small polarons and oxygen deficiency. On the one side, the origin of this pronounced absorption of NIR, even in presence of limited amount of W18O49, was considered as being due to small polarons, as discussed by Salje and Guttle.33 On the other side, the oxygen deficiency in tungsten suboxides (WO3-x) leads to a complexordered structure known as the Magneli structure.34 It is wellknown that significant optical properties of WO3-x are not only governed by the structure of material but also by the faults or 12175

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Figure 7. Transmittance spectra of (a) (NH4)xWO3+x/2 nanorods, (b) WO3, (c) bulk W18O49, (d) ITO glass(5Ω/0) and (e) W18O49 nanorods; (R-d) and (R-e) show the reflectance spectra of ITO glass and W18O49 nanorods, respectively. The gray background shows the relative energy wavelength distribution of the solar spectrum at the sea level.

Figure 8. (A) Schematic illustration of the simulated experiment; sealed boxes with a facet covered by quartz glass, W18O49 coated quartz glass and ITO glass, respectively, were irradiated by a 50 W halogen lamp and the temperature changes dependent on time were recorded by an electronic thermometer; (B) Temperature dependence on irradiation time.

oxygen deficiency in the materials. In a previous work, it has been reported that the monoclinic W18O49 (i.e., WO2.72) with the largest amount of oxygen deficiency in the WO2.7
WO3 region exhibited superior typical absorption of near-infrared light to other type tungsten suboxides.34 The one-dimensional W18O49, possessing larger surface-to-volume ratio and high aspect ratio as compare to their bulk counterpart, has been considered with more oxygen deficiencies and may be appear to different defects level,35 which resultantly accounts for the pronounced absorption of light in both UV and NIR regions. Similar results that rodlike nanocrytals showed stronger absorbing of UV and NIR light were also found for the tungsten bronze compounds of CsxWO3 and KxWO3.12,15 As a transparent coating possessing an infrared-ray cutoff function that can selectively transmit the visible light and strongly absorb infrared light as well, W18O49 nanorods contained film may be a promising candidate as solar filter. It may be applicable as a transparent coating to realize a remarkable saving of energy of air-conditioning of rooms in summer and winter. In order to test the applied properties of W18O49 nanorods as a solar heat-filter, a simulated experiment was carried out by irradiating three sealed boxes covered by quartz glass, W18O49 coated quartz glass and ITO glass, respectively, then, the temperature

changes upon irradiation time were monitored. The experiment was performed at a room temperature of 19 °C and the schematic illustration of the simulated experiment is presented in Figure 8A. A 50W halogen lamp was used as a light source. It can be seen that the inner temperature of the quartz glass set box increased significantly with irradiation time and reached a maximum of about 34 °C after exposure for 1 h (Figure 8B(a)). Although the temperature increment could be depressed by substituting quartz glass with commercial ITO glass, prolonged irradiation still caused the temperature to increase remarkably, due to its incompleteness of NIR shielding ability for wavelengths of less than 1400 nm. In contrast, excellent heat insulating performance was realized by applying a 70% visible light transparent W18O49 coating on the quartz glass. After being irradiated by halogen lamp for 1 h, the inner temperature was just 25.8 °C, which was much lower than that covered by the quartz glass or ITO glass. In order to investigate the photothermal conversion properties of W18O49 material, the powder sample was put on a paper, and then radiated by a 50W halogen lamp for 4
6 s in ambience. The temperature distribution was recorded by thermographic analysis. Figure 9b shows the images of powder on the paper. The measurement was performed at an ambient temperature of 19 °C. 12176

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nanorod in this work is a promising material for practical applications in optical related areas.

’ ASSOCIATED CONTENT

bS

Supporting Information. Thermographic images of bulk W18O49 and bulk WO3 and room-temperature PL spectra of samples. This material is available free of charge via the Internet at http://pubs.acs.org. Figure 9. (a) Thermographic images of W18O49 powder on the paper. The samples were irradiated by 50W halogen lamp for 4
6 s; (b) The shape of W18O49 powder on the paper.

Within only 4
6 s, the temperature of the powder applied areas increased from 19 °C to 32
35 °C, whereas the temperatures of paper without the sample were almost the same as the ambient temperature (Figure 9a). Despite the fact that it could not quantitatively measure the photothermal conversion efficiency as lack of basic parameters such as specific heat capacity of W18O49, coefficient of heat conductivity and so on, however, we can also conclude that our sample has high conversion efficiency by comparing it with bulk W18O49 or WO3, which exhibited little temperature increment upon the irradiation (Supporting Information). The distinguishable temperature difference indicates the quick conversion of absorbed NIR light energy to local heating energy on the W18O49 nanorods. To investigate the chemical stabilities of W18O49 nanorods upon the irradiation, we have repeated 10 cycles of photoheating and then cooling down. The conversion of W18O49 to WO3 upon the irradiation was not observed, even after prolonged irradiation of 30 min, where the temperature balance was built at about 105 °C. It is thus reasonable to suggest that the W18O49 nanorod is chemical stable below the oxidation temperature of about 200 °C. The interesting optical properties of W18O49 nanorod endow it to be a promising material in a wide area of applications, such as photothermal therapeutic drug delivery (The nearinfrared (NIR) light used in this manner provides deep-tissue penetration with high spatial precision without damaging normal biological tissues due to the low-energy absorption of NIR light by normal tissues),36 optical fibers, sun visors, ink for preventing money-substituting, cash cards from being forged, optical screens, and so on.17

4. CONCLUSIONS In summary, crystalline (NH4)xWO3+x/2 nanorods with a uniform size and morphology were successfully synthesized by a facile wet-chemical method using conventional sulfate as a capping agent. After reduction treatment in an atmosphere of H2(5 vol %)/N2 at 500 °C, the (NH4)xWO3+x/2 converted to W18O49 without loss of its one-dimensional morphology. The thin film of W18O49 nanorods was coated on a quartz glass for the optical test as a solar filter. It was found that W18O49 nanorods could strongly cut off the NIR light by absorption, while showing a high transparency of visible light. For evaluating the feasibility as a solar filter, a simulated experiment was performed and the results further confirmed the excellent heat insulating function of W18O49 nanorods, even compared with commercially used ITO glass. Due to the high efficiency of absorption of NIR light, powder of W18O49 nanorods exhibited instantaneous opt-thermal conversion upon NIR irradiation. In a word, the W18O49

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This research was supported in part by the Management Expenses Grants for National Universities Corporations from the Ministry of Education, Culture, Sports, Science for Technology of Japan (MEXT), and by the Adaptable and Seamless Technology transfer Program through target-driven R&D, JST, and Grant-in-Aid for Science Research (No. 23241025). ’ REFERENCES (1) Li, X. L.; Lou, T. J.; Sun, X. M.; Li, Y. D. Inorg. Chem. 2004, 43, 5442. (2) Kwak, G.; Lee, M.; Yong, K. Langmuir 2010, 26, 9964. (3) Santato, C.; Odziemkowski, M.; Ulmann, M.; Augustynski, J. J. Am. Chem. Soc. 2001, 123, 10639. (4) Sayama, K.; Mukasa, K.; Abe, R.; Abe, Y.; Arakawa, H. Chem. Commun. 2001, 23, 2416. (5) Zhou, J.; Gong, L.; Deng, S. Z.; Chen, J.; She, J. C.; Xu, N. S.; Yang, R.; Wang, Z. L. Appl. Phys. Lett. 2005, 87, 223108. (6) Wang, J. M.; Khoo, E.; Lee, P. S.; Ma, J. J. Phys. Chem. C. 2008, 112, 14312. (7) Zhao, Z. G.; Miyauchi, M. J. Phys. Chem. C. 2009, 113, 6539. (8) Li, Y.; Bando, Y.; Goldberg, D. Adv. Mater. 2003, 15, 1294. (9) Zhou, J.; Ding, Y.; Deng, S. Z.; Gong, L.; Xu, N. S.; Wang, Z. L. Adv. Mater. 2005, 17, 2107. (10) Wang, S. J.; Chen, C. H.; Ko, R. M.; Kuo, Y. C.; Wong, C. H.; Wu, C. H.; Uang, K. M.; Chen, T. M.; Liou, B. W. Appl. Phys. Lett. 2005, 86, 263103. (11) Lee, K.; Seo, W. S.; Park, J. T. J. Am. Chem. Soc. 2003, 125, 3409. (12) Guo, C. S.; Yin, S.; Zhang, P. L.; Yan, M.; Adachi, K.; Chonan, T.; Sato, T. J. Mater. Chem. 2010, 20, 8227. (13) Guo, C. S.; Yin, S.; Yan, M.; Sato, T. J. Mater. Chem. 2011, 21, 5099. (14) Guo, C. S.; Yin, S.; Sato, T. Nanosci. Nanotechnol. Lett. 2011, 3, 413. (15) Guo, C. S.; Yin, S.; Huang, L. J.; Sato, T. ACS Appl. Mater. Interfaces 2011, 3, 2794. (16) Guo, C. S.; Yin, S.; Huang, L. J.; Yang, L.; Sato, T. Chem. Commun. 2011, 47, 8853. (17) Nishihara, A.; Hayashi, T.; Sekiguchi, M. US. Pat. 1996, No. 5518810. (18) Okada, M.; Yamada, Y.; Jin, P.; Tazawa, M.; Yoshimura, K. Thin Solid Films 2003, 442, 217. (19) Huo, L. H.; Zhao, H.; Mauvy, F.; Fourcade, S.; Labrugere, C.; Pouchard, M.; Grenier, J. C. Solid State Sci. 2004, 6, 679. (20) Zhan, J. H.; Yang, X. G.; Xie, Y.; Li, B. F.; Qain, Y. T.; Jia, Y. B. Solid State Ionics 1999, 126, 373. (21) Lou, X. W.; Zeng, H. C. Inorg. Chem. 2003, 42, 6169. (22) Fan, R.; Chen, X. H.; Gui, Z.; Sun, Z.; Li, S. Y.; Chen, Z. Y. J. Phys. Chem. Solids 2000, 61, 2029. 12177

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