Electrochemical Deposition of Eu3+-Doped CeO2 Nanobelts with

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Electrochemical Deposition of Eu3+-Doped CeO2 Nanobelts with Enhanced Optical Properties Zi-Long Wang, Gao-Ren Li,* Yan-Nan Ou, Zhan-Ping Feng, Dun-Lin Qu, and Ye-Xiang Tong* MOE of Key Laboratory of Bioinorganic and Synthetic Chemistry/School of Chemistry and Chemical Engineering/Institute of Optoelectronic and Functional Composite Materials, Sun Yat-sen UniVersity, Guangzhou 510275, People’s Republic of China ReceiVed: July 29, 2010; ReVised Manuscript ReceiVed: October 15, 2010

Recently, rare earth (RE) ion-doped CeO2 has attracted much attention for special optical, magnetic, and catalytic properties. First reported in this paper is a facile electrochemical synthesis of Eu3+-doped CeO2 nanobelts with greatly improved optical properties. The synthesized Eu3+-doped CeO2 nanobelts were characterized by energydispersive spectrometry, scanning electron microscopy, transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The results of XRD and TEM indicate that Eu3+-doped CeO2 nanobelts were well crystallized and have cubic crystal structures. The XPS results show Eu3+ and Ce4+ coexist in these prepared nanobelts and indicate Eu3+-doped CeO2 nanobelts were successfully prepared. The formation mechanisms of Eu3+-doped CeO2 nanobelts were preliminarily investigated. The correlation between the band gap energies and the morphologies of these samples was studied by UV-vis absorption spectrum. The results indicate Eu3+-doped CeO2 nanobelts had an excellent response in the visible region of solar spectrum and showed potential application for solar cells. The photoluminescent properties of Eu3+-doped CeO2 nanobelts were investigated, and the remarkable enhancement of luminescence can be clearly observed because of the rapid increase of oxygen vacancies and their special morphology. 1. Introduction It is well-known that 1D nanobelts may exhibit unique electrical, optical, magnetic, mechanical, and thermal properties that are obviously different from those of bulk materials, and they have been the focus of intense research owing to their fascinating physical and chemical properties and promising applications in nanoscale electronic or optoelectronic devices, biomedicine, sensing, or catalysis.1-5 Nanostructured rare-earth (RE) oxides have attracted great interest because of their wide applications in luminescent devices, magnets, biological labeling, and catalysts based on the electronic, optical, and chemical characteristics arising from their 4f electrons.6-25 CeO2 nanostructures as important functional materials have been widely investigated compared with the other RE oxide nanostructures.26-35 Recently, increasing interest focused on RE ion doped CeO2 nanostructures for enhancing their optical, magnetic, or catalytic properties or increasing its temperature stability and ability to store and release oxygen.36-51 For example, Gd3+-doped CeO2 nanomaterials have shown the enhanced optical and catalytic properties and potential applications in the next generation of compact solid oxide fuel cells for their enhancement in oxygenexchange processes;36,37 Sm3+-doped CeO2 nanomaterials were found to have unusually strong orange photoluminescence due to a dominant magnetic-dipole 4G5/2 f 6H5/2 transition.38 Recently Eu3+ ion doped CeO2 nanostructures have attracted much attention because of their special optical properties with respect to the CeO2 nanoparticles due to the increase of oxygen vacancy in the CeO2 nanoparticles.39-44 At present, Eu3+ ion doped CeO2 has been prepared by various methods such as solid-state reaction,52 sol-gel process,53 solution precipitation,54 and nonhy* To whom correspondence should be addressed, ligaoren@ mail.sysu.edu.cn and [email protected].

drolytic solution route.39-44 However, high temperature, high pressure, or surface capping agent is often relied in the above methods. It is well-known that CeO2 has isotropic structure, and it is difficult to prepare 1D CeO2-based anisotropic nanoparticles, such as nanotubes, nanobelts, or nanowires. In this paper, we first investigated the synthesis of Eu3+-doped CeO2 nanobelts by electrochemical deposition at room temperature and usual atmospheric pressure without any templates. The electrochemical deposition route has shown a facile and low-cost method for the preparation of nanomaterials.55 The growth rate can easily be well controlled by changing electrochemical deposition parameters, such as current density and salt concentration. As we all know, the nanobelts as a family of 1D nanostructures with unique properties have attracted much attention since the first report of ZnO nanobelts in 2001,56 and have been regarded as the ideal system for understanding the spectacular transport process of light, heat, and electricity in 1D nanomaterials.57 The one-dimensionality provides a transport channel and sufficient space for charge separation. RE3+ ion doping is a widely applied technological process in materials science that involves incorporating atoms or ions of appropriate elements into host lattices to yield hybrid materials with desirable properties and functions.58 Therefore, such Eu3+-doped CeO2 nanobelts are expected to offer enhanced optical properties over sheet-shaped particles with a diameter on the same scale. In addition, these prepared Eu3+-doped CeO2 nanobelts will provide a marvelous opportunity for exploring the novel optical properties of RE3+ ion-doped CeO2. 2. Experimental Section The electrochemical deposition of Eu3+-doped CeO2 was carried out in solution of 0.01 M Ce(NO3)3 + 0.001 M Eu(NO3)3 + 0.1 M NH4NO3 by galvanostatic electrolysis. The current

10.1021/jp1070924  2011 American Chemical Society Published on Web 12/27/2010

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Figure 2. The typical EDS pattern of Eu3+-doped CeO2 deposits (5 atom % Eu3+).

Figure 1. SEM images of (a) low magnification, (b) high magnification, HRTEM image (c), and SAED pattern (inset) of Eu3+-doped CeO2 nanobelts prepared in solution of 0.01 M Ce(NO3)3 + 0.001 M Eu(NO3)3 + 0.1 M NH4NO3 with current density of 2.0 mA/cm2.

density is chosen in the range of 0.5-2.0 mA/cm2. In this experiment a simple three-electrode cell was used in our experiments. A highly pure Pt foil (99.99 wt %, 0.25 cm2) was used as the auxiliary electrode. A saturated calomel electrode (SCE) was used as the reference electrode that was connected to the cell with a double salt bridge system. All the electrochemical deposition experiments were carried out in a configured glass cell at room temperature, in which a Cu plate (99.99 wt %, 0.5 cm2) served as the substrate. Before electrodeposition, Cu substrate was cleaned ultrasonically in 0.1 M HCl, distilled water, and acetone and then rinsed in distilled water again. The products were characterized by X-ray diffractometry (D/MAX 2200 VPC with Cu KR radiation). An Oxford Instrument’s INCA energy-dispersive spectrometer (EDS) was employed to analyze chemical composition. Microstructures of the deposits were characterized by field emission scanning electron microscopy (FE-SEM; JSM-6330F). The X-ray photoelectron spectroscopy (XPS, ESCALAB 250) was used to assess the chemical state and surface composition of the deposits. The UV-vis spectra of the samples were recorded on a UV-vis-NIR spectrophotometer (UV-3150). The photoluminescence (PL) spectra were carried out by spectrofluorophotometer (RF-5301PC) at room temperature, and the excitation wavelength used in PL measurement was 290 nm. 3. Results and Discussion Electrochemical deposition was first carried out in a solution of 0.01 M Ce(NO3)3 + 0.001 M Eu(NO3)3 + 0.1 M NH4NO3 with current density of 2.0 mA/cm2. Figure 1a shows a typical low-magnification FE-SEM image of Eu3+-doped CeO2 deposits, and it reveals that the deposits consist of a large quantity of nanobelts with length of about 2 µm. The high-magnification FE-SEM image of Eu3+-doped CeO2 deposits is shown in Figure 1b, and it shows that the width of a Eu3+-doped CeO2 nanobelt is about 200-400 nm, and the thickness is about 35 nm. The microstructural details of Eu3+-doped CeO2 nanobelts were further investigated by TEM. The high-resolution TEM (HRTEM) image of a Eu3+-doped CeO2 nanobelt is shown in Figure 1c, and it displays lattice fringes for nanobelts, indicating these prepared Eu3+-doped CeO2 nanobelts possess polycrystalline structure. The interplanar spacings of the nanocrystals are about

Figure 3. Ce 3d (1), O 1s (2), and Eu 3d (3) XPS spectra for Eu3+doped CeO2 deposits (XPS spectrum of Ce 3d in CeO2 is shown in (1(b))).

0.32 and 0.28 nm, which are identical with the (111) and (200) facet distance of the CeO2 phase, repectively. The selected area electron diffraction (SAED) pattern also shows a polycrystalline structure and is consistent with a cubic structure of CeO2 with strong ring patterns due to (111), (200), (220), and (311) planes. EDS measurements were also carried out for Eu3+-doped CeO2 deposits. Figure 2 shows the representative EDS pattern of Eu3+-doped CeO2 nanobelts. An oxygen peak at about 0.5 keV and Ce signals at about 4.8 and 5.2 keV were observed. The Eu peaks at 0.9, 4.9, 5.5, and 6.0 keV can be observed. Therefore, the elements of O, Ce, and Eu were demonstrated to be electrodeposited. The composition analysis showed the contents of Eu in Eu3+-doped CeO2 nanobelts were about 5.8 atom %. The XPS analyses of Eu3+-doped CeO2 nanobelts were carried out, and Figure 3 shows the typical results of XPS analysis. The XPS spectrum in Figure 3-1(a) shows the presence of Ce4+ and Ce3+ ions in the CeO2 nanoparticles. It is wellknown that there always coexisted with a small amount of Ce3+ at the surface of CeO2.39-44 The peaks centered at 917.9 and 899.8 eV in Figure 3-1(a) can be attributed to the Ce4+ contribution. The peaks centered at 902 and 883 eV can be attributed to the Ce3+ contribution. When Eu3+ was doped into CeO2, the XPS result of Ce 3d in Figure 3-1(a) shows that the peaks of Ce4+ 3d3/2 and Ce4+ 3d5/2 become stronger than that of the undoped CeO2 nanocrystals, while those of Ce3+ 3d3/2 and Ce3+ 3d5/2 are relatively weak, indicating the decrease of the Ce3+ fraction on the surface of CeO2. This can be attributed to the replacement of Ce3+ by trivalent Eu3+ ions. The O 1s scan was shown in Figure 3-2, and the peak centered at 529.1

Eu3+-Doped CeO2 Nanobelts

Figure 4. XRD pattern of Eu3+-doped CeO2 nanobelts.

Figure 5. SEM image of Eu3+-doped CeO2 nanosheets prepared in solution of 0.01 M Ce(NO3)3 + 0.001 M Eu(NO3)3 + 0.1 M NH4NO3 with current density of 0.5 mA/cm2.

eV can be attributed to the O2- contribution. The peaks centered at 1127.1, 1136.9, 1156.6, and 1166.9 eV in Figure 3-3 can be attributed to the Eu3+ contribution. Therefore, the XPS results indicated these obtained nanobelts were Eu3+-doped CeO2 with a small quantity of impurity of Ce3+ oxidation state. The X-ray diffraction (XRD) pattern of Eu3+-doped CeO2 nanobelts is shown in Figure 4. The peaks of CeO2 (111), (200), (220), and (311) planes were observed, indicating a facecentered cubic phase (JCPDS 34-0394). However, the diffraction peaks of Eu, Eu2O3, or Eu(OH)3 were not observed, which indicates the Eu3+ ions have entered into CeO2 lattices. The calculated cell parameter (a) is equal to 0.5402 nm, a little smaller than that of bulk CeO2 (0.5411 nm). This may be due to the lattice constriction effect resulting from Eu3+ ions. The broadening of the reflections of Eu3+-doped CeO2 nanobelts in Figure 4 is ascribed to the small nanocrystallines. The average crystallite sizes of the Eu3+ ion doped CeO2 samples were calculated from X-ray line broadening of the reflections of (111) using Scherrer’s equation (i.e., D ) Kλ/(β cos θ), where λ is the wavelength of the X-ray radiation, K is a constant, θ is the diffraction angle, and β is the full width at half-maximum). The average crystallite size of Eu3+ ion doped CeO2 nanobelts was determined to be about 5 nm. The electrochemical formation mechanism of the Eu3+-doped CeO2 nanobelt is illustrated as follows. When the current density was decreased to 0.5 mA/cm2, Eu3+-doped CeO2 porous nanostructures consisting of nanosheets were obtained. Figure 5 shows the SEM image of the prepared Eu3+-doped CeO2 sheets, and their thickness is about 20 nm. As we all know, the final shapes of nanostructures were largely affected by the

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Figure 6. XRD pattern of the fresh deposits deposited with current density of 2.0 mA/cm2 before drying.

growth habit of crystals.59-63 The materials with anisotropic structures can easily grow into anisotropic nanoparticles, such as nanobelts, nanowires, and nanorods. However, for the materials with isotropic structures, such as CeO2, they will difficultly grow into anisotropic nanoparticles. In order to obtain anisotropic nanoparticles, the capping reagents or templates are widely employed. For example, the various templating methods were developed for the synthesis of CeO2 nanorods or nanowires.64-68 In this study, Eu3+-doped CeO2 nanobelts were successfully obtained without using any templates or capping reagents. Interestingly, Eu3+-doped CeO2 nanobelts were yielded at a high current density of electrodeposition, whereas a low current density of electrodeposition led to the formation of Eu3+doped CeO2 nanosheets. The reasons for the formation of Eu3+doped CeO2 nanosheets and nanobelts were described as follows. During electrodeposition, the NO3- ions are reduced to produce the OH- ions (eq 1), and the anisotropic Ce(OH)3 nuclei as an intermediate product will be formed as soon as Ce3+ ions encounter the produced OH- ions (eq 2).69 When the electrodeposition was carried out with a lower current density, the concentration of the produced OH- ions is low in solution. So the dissolution/recrystallization rate of Ce(OH)3 was slow, and there might exist inadequately high chemical potential for driving the anisotropic growth of the Ce(OH)3 nuclei.69 Under this condition, these formed Ce(OH)3 will be immediately transformed to CeO2 with isotropic structures via eq 2. In addition, at the same time, Eu3+ ions will also react with OHions to form Eu2O3 (eq 3). During electrodeposition, mixed CeO2 and Eu2O3 at atomic level were always obtained, and accordingly the homogeneous Eu3+-doped CeO2 were prepared (eq 4). Since the Eu3+ ions were doped into CeO2 lattices, the crystal structures of the doped deposits were determined by CeO2 phase. As CeO2 has isotropic structures, the isotropic growths of crystals will happen, leading to the formation of Eu3+-doped CeO2 nanosheets. However, when the electrodeposition was carried out with a high current density, the produced OH- ion concentration will be high enough, and the dissolution/recrystallization rate was considerably promoted so as to drive the Ce(OH)3 nuclei to grow anisotropically,69 which finally leads to the formation of Eu3+-doped Ce(OH)3 nanobelts. With the instability of Ce(OH)3, it will be finally oxidized into CeO2 without shape changes,69 leading to the formation of Eu3+-doped CeO2 nanobelts. During the experiments we found that the color of the obtained deposits changed from white to yellow before and after drying, indicating that a different substances were obtained from the fresh products. The XRD pattern of the fresh products before drying is shown in Figure 6, which proves the existence of Ce(OH)3 at a higher current density. The formation

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Figure 7. Schematic diagram for the shape-selective synthesis of RE ion-doped CeO2 nanosheets and nanobelts.

Figure 8. The normalized UV-vis absorption spectra of (a) CeO2 nanosheets, (b) Eu3+-doped CeO2 nanosheets, and (c) Eu3+-doped CeO2 nanobelts.

of Eu3+-doped CeO2 nanosheets and nanobelts is schematically described in Figure 7. The effect of NH4NO3 during the formation of nanocrystals is discussed as follows. First, NO3ion in NH4NO3 is favored for the formation of OH- ion via reaction 1. The OH- ion is necessary for the synthesis of CeO2. Second, NH4+ ions in NH4NO3 can balance the basicity of deposition solution by reacting with OH- ions, and accordingly the deposit rate of Eu3+-doped CeO2 can be well adjusted.

NO3- + H2O + 2e f NO2- + 2OH-

(1)

+O2

4Ce3+ + 12OH- f 4Ce(OH)3 98 4CeO2 + 6H2O

Figure 9. PL spectra of (a) CeO2 nanosheets, (b) Eu3+-doped CeO2 nanosheets, and (c) Eu3+-doped CeO2 nanobelts.

CeO2 nanoparticles compared with bulk CeO2. When Eu3+ was doped into CeO2, the Ce3+ ions may be replaced by trivalent Eu3+ ions. Accordingly, the change from Ce4+ to Ce3+ ions will decrease evidently when Eu3+ ions are doped into CeO2 nanoparticles, and the contribution of blue shifting arising from this valence change will become small. Therefore, the red shifting occurs in the absorption spectrum of Eu3+-doped CeO2 nanosheets compared with CeO2 nanosheets. In addition, it can be clearly seen that the absorption spectrum of Eu3+-doped CeO2 nanobelts obviously exhibited a red shift and a stronger absorption compared with those of Eu3+-doped CeO2 nanosheets, and this may be attributed to the effect of nanobelts. According to eq 5, the optical band gaps (Eg) of CeO2 nanosheets, Eu3+doped CeO2 nanosheets, and Eu3+-doped CeO2 nanobelts can be estimated by using the data for the absorption spectra:

(2) 3+

2Eu

-

+ 6OH f Eu2O3 + 3H2O

Eu2O3 + CeO2 f Ce1-xEuxO2-δ

Rhw ) C(hυ - Eg)n

(5)

(3) (4)

The correlation among the band gap energies, shapes, and the doping of the samples were studied. Spectra a-c of Figure 8 show UV-vis absorption spectra of CeO2 nanosheets, Eu3+doped CeO2 nanosheets (5 atom % Eu), and Eu3+-doped CeO2 nanobelts (5 atom % Eu), respectively. Eu3+-doped CeO2 nanosheets and Eu3+-doped CeO2 nanobelts both exhibit stronger absorption bands in the UV range compared with CeO2 nanosheets. Furthermore, the onsets of their absorption spectra both exhibited a red shift compared with that of CeO2 nanosheets. This red shift can be explained as follows. It is wellknown that CeO2 as an ultraviolet blocking material has strong absorption properties in the ultraviolet range,39-44 which is due to the charge-transfer transition from O2- (2p) to Ce4+ (4f) orbitals in CeO2.70 However, for CeO2 nanostructures, there always coexisted a small amount of Ce3+ on the surface of CeO2,16 and the valence change from Ce4+ to Ce3+ might have some additional contributions to the absorption of CeO2 nanoparticles, which led to the blue shift of absorption spectrum for

(where hυ is photon energy, R is the absorption coefficient, C is the constant, n ) 2 for an indirectly allowed transition, and n ) 1/2 for a directly allowed transition).71 The optical band gap values can be estimated to be about 2.91 eV for CeO2 nanosheets, 2.50 eV for Eu3+-doped CeO2 nanosheets, and 2.38 eV for Eu3+-doped CeO2 nanobelts, respectively. Therefore, the band gap values of Eu3+-doped CeO2 nanobelts is narrower than those of Eu3+-doped CeO2 nanosheets and CeO2 nanosheets, which indicates a better response in the visible region of the solar spectrum and shows the potential application as solar cells.72 The photoluminescent (PL) properties of CeO2 nanosheets, Eu3+-doped CeO2 nanosheets (5 atom % Eu), and Eu3+-doped CeO2 nanobelts (5 atom % Eu) were examined under the identical instrumental conditions, and their room-temperature PL spectra are shown in Figure 9. For CeO2 nanosheets, only a very weak broad band emission with maxima at 440 nm was observed (this emission spectrum was enlarged by 10 times) (Figure 9a). However, the band emission can be remarkably enhanced by Eu3+ doping (Figure 9b). The Eu3+-doped CeO2 nanosheets show much stronger emission bands (more than 20 times than that of CeO2 nanosheets) with maxima at 422 nm.

Eu3+-Doped CeO2 Nanobelts When Eu3+ ions are doped into CeO2 nanosheets, the extrinsic oxygen vacancies will be formed. So the oxygen vacancy concentration in Eu3+-doped CeO2 nanoparticles will be higher than those in the undoped CeO2 nanoparticles. The higher oxygen vacancy concentration in Eu3+-doped CeO2 nanoparticles will lead to the stronger emission band.39-44 Furthermore, the band emission can be further enhanced when Eu3+-doped CeO2 nanosheets were changed to Eu3+-doped CeO2 nanobelts. Figure 9c shows Eu3+-doped CeO2 nanobelts have a stronger emission band than that of Eu3+-doped CeO2 nanosheets, and this can be attributed to the effect of special nanostructure. Conclusions Here a facile electrochemical deposition route was developed for the synthesis of Eu3+ ion doped CeO2 nanobelts. Ce(OH)3 as the intermediate is crucial for the growth of Eu3+-doped CeO2 nanobelts. TEM results show these synthesized nanobelts have a polycrystalline structure. XPS results indicate these obtained deposits are Eu3+-doped CeO2 with a small quantity of impurity of Ce3+ oxidation state. The results of XRD indicate Eu3+-doped CeO2 nanobelts are well-crystallized and have a cubic structure. It can be clearly observed that the absorption spectrum of Eu3+doped CeO2 nanobelts obviously exhibited a red shift and a stronger absorption compared with those of Eu3+-doped CeO2 nanosheets and CeO2 nanosheets. The PL spectrum of Eu3+doped CeO2 nanobelts shows remarkable enhancement compared with Eu3+-doped CeO2 nanosheets and CeO2 nanosheets because of the rapidly increasing oxygen vacancies and their special morphologies. Importantly, this cardinal principle of shape and doping enhanced optical properties of Eu3+-doped CeO2 nanobelts is expected to apply to other RE3+-doped CeO2 systems. Acknowledgment. This work was supported by NSFC (21073240, 20603048, 20873184, and 90923008), Guangdong Province (2008B010600040 and 9251027501000002), and the Fundamental Research Funds for the Central Universities (09lgpy17). Supporting Information Available: SEM images of Eu3+doped CeO2 samples prepared at different deposition conditions. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Liu, J.; Li, Y.; Huang, X.; Li, Z.; Li, G.; Zeng, H. Chem. Mater. 2008, 20, 250–257. (2) Li, G.-R.; Feng, Z.-P.; Zhong, J.-H.; Wang, Z.-L.; Tong, Y.-X. Macromolecules 2010, 43, 2178–2183. (3) Wang, J.; Tafen, D. N.; Lewis, J. P.; Hong, Z.; Manivannan, A.; Zhi, M.; Li, M.; Wu, N. J. Am. Chem. Soc. 2009, 131, 12290–12297. (4) Ma, Y.; Qi, L.; Shen, W.; Ma, J. Langmuir 2005, 21, 6161–6164. (5) Yang, X.; Gao, X.; Gan, Y.; Gao, C.; Zhang, X.; Ting, K.; Wu, B. M.; Gou, Z. J. Phys. Chem. C 2010, 114, 6265–6271. (6) Cao, C.-Y.; Cui, Z.-M.; Chen, C.-Q.; Song, W.-G.; Cai, W. J. Phys. Chem. C 2010, 114, 9865–9870. (7) Liu, Y.; Wen, C.; Guo, Y.; Lu, G.; Wang, Y. J. Phys. Chem. C 2010, 114, 9889–9897. (8) Han, W.-Q.; Wen, W.; Hanson, J. C.; Teng, X.; Marinkovic, N.; Rodriguez, J. A. J. Phys. Chem. C 2009, 113, 21949–21955. (9) Reddy, B. M.; Thrimurthulu, G.; Katta, L.; Yamada, Y.; Park, S.E. J. Phys. Chem. C 2009, 113, 5882–15890. (10) Pati, R. K.; Lee, I. C.; Hou, S.; Akhuemonkhan, O.; Gaskell, K. J.; Wang, Q.; Frenkel, A. I.; Chu, D.; Salamanca-Riba, L. G.; Ehrman, S. H. ACS Appl. Mater. Interfaces 2009, 1, 2624–2635. (11) Kim, S.; Lee, J. S.; Mitterbauer, C.; Ramasse, Q. M.; Sarahan, M. C.; Browning, N. D.; Park, H. J. Chem. Mater. 2009, 21, 1182–1186. (12) Pati, R. K.; Lee, I. C.; Gaskell, K. J.; Ehrman, S. H. Langmuir 2009, 25, 67–70.

J. Phys. Chem. C, Vol. 115, No. 2, 2011 355 (13) Avila-Paredes, H. J.; Jain, P.; Sen, S.; Kim, S. Chem. Mater. 2010, 22, 893–897. (14) Li, L.; Yang, H. K.; Moon, B. K.; Fu, Z.; Guo, C.; Jeong, J. H.; Yi, S. S.; Jang, K.; Lee, H. S. J. Phys. Chem. C 2009, 113, 610–617. (15) Liu, X.; Zhou, K.; Wang, L.; Wang, B.; Li, Y. J. Am. Chem. Soc. 2009, 131, 3140–3141. (16) Zhou, K.; Yang, Z.; Yang, S. Chem. Mater. 2007, 19, 1215–1217. (17) Yang, D.; Wang, L.; Sun, Y.; Zhou, K. J. Phys. Chem. C 2010, 114, 8926–8932. (18) (a) Cao, Y. C. J. Am. Chem. Soc. 2004, 126, 7456. (19) Liang, X.; Wang, X.; Zhuang, Y.; Xu, B.; Kuang, Li, S. Y. J. Am. Chem. Soc. 2008, 130, 2736–2737. (20) Feldmann, C. AdV. Funct. Mater. 2003, 13, 101. (21) Yan, L.; Yu, R.; Chen, J.; Xing, X. Crys. Growth Des. 2008, 8, 1474–1477. (22) Wu, G. S.; Zhang, L. D.; Cheng, B. C.; Xie, T.; Yuan, X. Y. J. Am. Chem. Soc. 2004, 126, 5976. (23) Yu, M.; Lin, J.; Fu, J.; Zhang, H. J.; Han, Y. C. J. Mater. Chem 2003, 13, 1413. (24) Zhang, Y. W.; Si, R.; Liao, C. S.; Yan, C. H.; Xiao, C. X.; Kou, Y. J. Phys. Chem. B 2003, 107, 10159. (25) Si, R.; Zhang, Y. W.; You, L. P.; Yan, C. H. Angew. Chem., Int. Ed. 2005, 44, 3256. (26) Zhou, Y.; Perket, J. M.; Zhou, J. J. Phys. Chem. C 2010, 114, 11853–11860. (27) Mlondo, S. N.; Thomas, P. J.; O’Brien, P. J. Am. Chem. Soc. 2009, 131, 6072–6073. (28) Kamada, K.; Higashikawa, K.; Inada, M.; Enomoto, N.; Hojo, J. J. Phys. Chem. C 2007, 111, 14508–14513. (29) Wu, Z.; Li, M.; Howe, J.; Meyer, H. M.; Overbury, S. H. Langmuir 2010, 26, 16595–16606. (30) Barreca, D.; Gasparotto, A.; Maccato, C.; Maragno, C.; Tondello, E. Langmuir 2006, 22, 8639–8641. (31) Matolin, V.; Matolinova´, I.; Va´clavu¨, M.; Khalakhan, I.; Vorokhta, M.; Fiala, R.; Pisˇ, I.; Sofer, Z.; Poltierova´-Vejpravova´, J.; Mori, T.; Potin, V.; Yoshikawa, H.; Ueda, S.; Kobayashi, K. Langmuir 2010, 26, 12824– 12831. (32) Brezesinski, T.; Wang, J.; Senter, R.; Brezesinski, K.; Dunn, B.; Tolbert, S. H. ACS Nano 2010, 4, 967–977. (33) Cui, R.; Lu, W.; Zhang, L.; Yue, B.; Shen, S. J. Phys. Chem. C 2009, 113, 21520–21525. (34) Balle´e, E.; Ringuede´, A.; Cassir, M.; Putkonen, M.; Niinisto´, L. Chem. Mater. 2009, 21, 4614–4619. (35) Xiao, H.; Ai, Z.; Zhang, L. J. Phys. Chem. C 2009, 113, 16625– 16630. (36) Li, G.-R.; Qu, D.-L.; Arurault, L.; Tong, Y.-X. J. Phys. Chem. C 2009, 113, 1235–1241. (37) Perez-Coll, D.; Nunez, P.; Ruiz-Morales, J. C.; Pena-Martinez, J.; Frade, J. R. Electrochim. Acta 2007, 52, 2001. (38) Fujihara, S.; Oikawa, M. J. Appl. Phys. 2004, 95, 8002–8006. (39) Yashima, M.; Takizawa, T. J. Phys. Chem. C 2010, 114, 10670. (40) Yashima, M.; Tomoya, T. J. Phys. Chem. C 2010, 114, 2385–2392. (41) Reddy, B. M.; Thrimurthulu, G.; Katta, L.; Yamada, Y.; Park, S.E. J. Phys. Chem. C 2009, 113, 15882–15890. (42) Reddy, B. M.; Saikia, P.; Bharali, P.; Park, S.-E.; Muhler, M.; Gru¨nert, W. J. Phys. Chem. C 2009, 113, 2452–2462. (43) Reddy, B. M.; Saikia, P.; Bharali, P.; Yamada, Y.; Kobayashi, T.; Muhler, M.; Gru¨nert, W. J. Phys. Chem. C 2008, 112, 16393–16399. (44) Wang, Z.; Quan, Z.; Lin, J. Inorg. Chem. 2007, 46, 5237–5242. (45) Zhou, G.; Gorte, R. J. J. Phys. Chem. B 2008, 112, 9869–9875. (46) Maher, R. C.; Cohen, L. F.; Lohsoontorn, P.; Brett, D. J. L.; Brandon, N. P. J. Phys. Chem. A 2008, 112, 1497–1501. (47) Go, Y. B.; Jacobson, A. J. Chem. Mater. 2007, 19, 4702–4709. (48) Rupp, J. L. M.; Drobek, T.; Rossi, A.; Gauckler, L. J. Chem. Mater. 2007, 19, 1134–1142. (49) Reddy, B. M.; Bharali, P.; Saikia, P.; Khan, A.; Loridant, S.; Muhler, M.; Gru¨nert, W. J. Phys. Chem. C 2007, 111, 1878–1881. (50) Zhang, G.; Shen, Z.; Liu, M.; Guo, C.; Sun, P.; Yuan, Z.; Li, B.; Ding, D.; Chen, T. J. Phys. Chem. B 2006, 110, 25782–25790. (51) Beckers, J.; Rothenberg, G. Dalton Trans. 2008, 37, 6573–6578. (52) Liu, X.; Chen, S.; Wang, X. J. Lumin. 2007, 127, 650–654. (53) Guo, H.; Qiao, Y. Appl. Surf. Sci. 2008, 254, 1961–1965. (54) Babu, S.; Schulte, A.; Seal, S. Appl. Phys. Lett. 2008, 92, 123112. (55) Choi, K.-S. Dalton Trans. 2008, 37, 5432–5438. (56) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Science 2001, 291, 1947. (57) Sun, T.; Qiu, J.; Liang, C. J. Phys. Chem. C 2008, 112, 715–721. (58) Wang, F.; Han, Y.; Lim, C. S.; Lu, Y.; Wang, J.; Xu, J.; Chen, H.; Zhang, C.; Hong, M.; Liu, X. Nature 2010, 463, 1061–1065. (59) Xu, L. F.; Guo, Y.; Liao, Q.; Zhang, J. P.; Xu, D. S. J. Phys. Chem. B 2005, 109, 13519. (60) Zhang, Y. W.; Yan, Z. G.; You, L. P.; Si, R.; Yan, C. H. Eur. J. Inorg. Chem. 2003, 22, 4099.

356

J. Phys. Chem. C, Vol. 115, No. 2, 2011

(61) Partzke, G. R.; Krumeich, F.; Nesper, R. Angew. Chem., Int. Ed. 2002, 41, 2446. (62) Zhang, J.; Sun, L. D.; Yin, J. L.; Su, H. L.; Liao, C. S.; Yan, C. H. Chem. Mater. 2002, 14, 4172. (63) (a) Wang, X.; Li, Y. D. Angew. Chem., Int. Ed. 2002, 41, 4790. (b) Liang, X.; Wang, X.; Zhuang, Y.; Xu, B.; Kuang, S. M.; Li, Y. D. J. Am. Chem. Soc. 2008, 130, 2736–2737. (64) Vantomme, A.; Yuan, Z. Y.; Du, G. H.; Su, B. L. Langmuir 2005, 21, 1132. (65) Kuiry, S. C.; Patil, S. D.; Deshpande, S.; Seal, S. J. Phys. Chem. B 2005, 109, 6936. (66) Tang, B.; Zhuo, L. H.; Ge, J. C.; Wang, G. L.; Shi, Z. Q.; Niu, J. Y. Chem. Commun. 2005, 3565.

Wang et al. (67) Yada, M.; Sakai, S.; Torikai, T.; Watari, T.; Furuta, S.; Katsuki, H. AdV. Mater. 2004, 16, 1222. (68) Sun, C. W.; Li, H.; Wang, Z. X.; Chen, L. Q.; Huang, X. J. Chem. Lett. 2004, 33, 662. (69) Mai, H.-X.; Sun, L.-D.; Zhang, Y.-W.; Si, R.; Feng, W.; Zhang, H.-P.; Liu, H.-C.; Yan, C.-H. J. Phys. Chem. B 2005, 109, 24380–24385. (70) Zhang, Y. W.; Si, R.; Liao, C. S.; Yan, C. H.; Xiao, C. X.; Kou, Y. J. Phys. Chem. B 2003, 107, 10159. (71) Weber, W. H.; Hass, K. C.; McBride, J. R. Phys. ReV. B 1993, 48, 178. (72) Corma, A.; Atienzar, P.; Garcı´a, H.; Chane-Ching, J.-Y. Nat. Mater. 2004, 3, 394–397.

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