A Facile Route to Oriented Nickel Hydroxide Nanocolumns and

The green precipitate was isolated by centrifugation using distilled water and absolute ethanol and were dried at 50 °C for 4 h. Porous NiO nanocolum...
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J. Phys. Chem. C 2007, 111, 5622-5627

A Facile Route to Oriented Nickel Hydroxide Nanocolumns and Porous Nickel Oxide Jixin Zhu,† Zhou Gui,*,† Yanyan Ding,† Zhengzhou Wang,† Yuan Hu,† and Mingqiang Zou‡ State Key Lab of Fire Science, UniVersity of Science and Technology of China, Hefei, 230027, People’s Republic of China, and Institute of Inspection and Quarantine and Equipment DeVelopment, Chinese Academy of Inspection and Quarantine, Beijing 100025, People’s Republic of China ReceiVed: December 16, 2006; In Final Form: February 7, 2007

A facile route has been developed to synthesize nickel hydroxide nanocolumns at 60 °C. The characterized results show that the nanocolumns are formed by stacking of oriented Ni(OH)2 nanosheets. The porous NiO nanocolumns and nanosheets are obtained by the Ni(OH)2 nanocolumns annealed at 500 °C for 3 h. The NiO nanocolumn structure is unstable and has a tendency to exfoliate to the individual nanosheets. The factors on the formation of the nanocolumns were investigated and the catalytic property of the porous NiO nanostructures for CO oxidation is reported.

1. Introduction The nanostructures of nickel hydroxide and nickel oxide possess unique physical and chemical properties arousing increasing attention. Ni(OH)2 is well-known as the important active material in the battery positive electrode for its high power density, high energy, low toxicity, and so forth properties.1 NiO is a valuable semiconductor and catalytic and antiferromagnetic material used in different fields, for instance, catalysis,2,3 gas sensors,4 and magnetic materials.5 Many results show that the electrochemical utilization and practical capacity of nickel electrodes are directly affected by the morphology and size of the active materials, and nanoscale Ni(OH)2 particles can improve the electrochemical performance of the electrodes.6-8 Numerous works have been developed to synthesize different morphologies of nickel hydroxide and nickel oxide. Especially, β-Ni(OH)2 crystallizes with a hexagonal brucite-type structure and Ni(OH)2 layers stack along the c-axis with an interlayer distance of 4.6 Å. It is often selected as the discharge-state material in the electrodes because of the stability in strong alkaline electrolyte and good reversibility when charged to β-NiOOH.9 Meyer et al.10 synthesized plateletlike nanoparticles of nickel hydroxide and studied their surface property. Coudun et al.11 obtained nickel hydroxide by the coupled effect of ammonia and template agent. Chen and Gao12 reported the synthesis of different morphologies of nickel hydroxide through using ethanol as growth media. Liang et al.13 accepted β-Ni(OH)2 nanosheets at 200 °C using nickel acetate as the nickel source and aqueous ammonia as the alkaline and complexing reagent. Li et al.14 prepared nanoflakes of nickel hydroxide using metal oxalate conversion method at 160 °C. Wang et al.15 synthesized NiO nanorings and studied their catalytic property for CO oxidation. However, some of the above-mentioned methods need high temperature, and the others need templates, which complicate the synthetic processes. Therefore, exploring new synthesis methods for novel Ni(OH)2 and NiO nanostructures will be useful to find new applications or to improve existing performances. * To whom correspondence should be addressed. E-mail: zgui@ ustc.edu.cn. † University of Science and Technology of China. ‡ Chinese Academy of Inspection and Quarantine.

In this paper, we obtained nickel hydroxide nanocolumns at low temperature (60 °C), and each of the nanocolumns was formed by ordered stacking of oriented nickel hydroxide singlecrystal nanosheets. The porous nickel oxide nanocolumns and nanosheets were successfully gained by Ni(OH)2 nanocolumns annealed at 500 °C for 3 h. The porous nanosheet nanostructure is self-assembled by textured NiO nanocrystals. It is interesting to find that the NiO nanocolumn structure is unstable and has a tendency to exfoliate to the individual nanosheets under hightemperature annealing. We have further investigated the factors on the formation of the β-Ni(OH)2 nanocolumns and have compared the catalytic activities for CO oxidation with different porous NiO nanostructures synthesized. 2. Experimental Section All reactants were analytically pure, and no further purification was needed. Nickel hydroxide nanocolumns were synthesized by adding 50 mL 0.075 M hydrazine (N2H4‚H2O) aqueous solution into 50 mL 0.05 M NiCl2‚6H2O aqueous solution and by reacting at 60 °C for 24 h. The green precipitate was isolated by centrifugation using distilled water and absolute ethanol and were dried at 50 °C for 4 h. Porous NiO nanocolumns and nanosheets (sample 1) were gained by Ni(OH)2 nanocolumns annealed at 500 °C for 3 h in muffle furnace in air condition and the porous nanosheets (sample 2) were obtained upon further annealing sample 1 NiO at 700 °C for another 3 h. X-ray powder diffraction (XRD) analysis was performed using a Rigaku D/Max X-ray diffractometer with graphite monochromated Cu KR radiation (λ ) 1.5418 Å). Thermogravimetric (TG) analyses of the precursor samples were carried out using a Netszch STA 409C in a flowing air atmosphere in a temperature range from room temperature to 700 °C. Scanning electron microscopy (SEM) images were recorded on a JSM6700F scanning electron microscope, working at 100 KeV acceleration voltages. Transmission electron microscopy (TEM) images, high-resolution transmission electron microscope (HRTEM) images, and selected area electron diffraction (SAED) patterns were performed on a JEOL2010 at 200 KeV. The BET (Brunauer-Emmett-Teller) surface area of the samples was measured by N2 adsorption using QuadraSorb Station 4 surface area analyzer.

10.1021/jp0686588 CCC: $37.00 © 2007 American Chemical Society Published on Web 03/24/2007

Facile Route to Nickel Hydroxide Nanocolumns

Figure 1. XRD pattern of (a) the sample synthesized by adding 50 mL 0.075 M N2H4‚H2O aqueous solution into 50 mL 0.05 M NiCl2‚ 6H2O aqueous solution and by reacting at 60 °C for 24 h and (b) the nickel hydroxide synthesized by using Ni(CH3COO)2‚4H2O as the nickel source under invariable condition.

The catalytic activities for CO oxidation were evaluated in a fixed-bed quartz tubular reactor and 0.1 g of catalyst was placed in the reactor. The reactant gases (1.0% CO, 99% dried air) went through the reactor at a rate of 20 mL min-1. The composition of the gas exiting the reactor was monitored by gas chromatography (GC-14C). 3. Results and Discussion Characterization of Nickel Hydroxide Nanocolumns. Figure 1a shows the XRD pattern of the obtained nickel hydroxide product. All the diffraction peaks can be readily indexed to a pure single phase of β-Ni(OH)2 with the hexagonal structure according to the JCPDS 14-0117. The diffraction peak of (001) is the strongest in all the diffraction peaks shown in the XRD pattern, which indicates that an unusual nanostructure and preferential growth could exist. The morphology of the β-Ni(OH)2 product was characterized by SEM in Figure 2. From the images of Figure 2a and Figure 2b, corresponding to the sample of Figure 1a, it is obviously observed that the as-synthesized products are dominated (above 90%) by nanocolumns which are formed by nanosheets as building blocks with layer-by-layer stacking structure. The diameters and the thicknesses of the Ni(OH)2 nanosheets are 200-250 nm and 25 ( 5 nm, respectively. Coudun et al.11 prepared the nickel precursor Ni(DS)2 using sodium dodecyl sulfate (SDS) as the template agent in the first step and then adding ammonia to prepare the Ni(OH)2. However, we synthesized the Ni(OH)2 in the facile one-step and template-free synthetic process. Figure 1b, Figure 2c, and 2d, corresponding to the sample obtained with the different nickel reactants, will be discussed later. The structure of Ni(OH)2 was further examined by TEM shown in Figure 3. The nanocolumns can be clearly observed, which further testified to the morphology of Ni(OH)2 nanocolumns with nanosheet stacked structure, as shown in Figure 3a and Figure 3b. Figure 3c shows a typical individual nanosheet with obviously hexagonal shape and the angles of adjacent edges of 120° as the description. The insert of Figure 3c is the corresponding SAED pattern, taking on the single sheet. The perfect diffraction dots present hexagonal dot array, which indicates that the Ni(OH)2 nanosheet as the basic building block to the nanocolumn is single crystal. It can be easily indexed

J. Phys. Chem. C, Vol. 111, No. 15, 2007 5623 according to the SAED pattern with the incident electron beam along the [001] direction. Figure 3d shows the HRTEM image of a typical single Ni(OH)2 nanocolumn from the side view. The lattice fringe is clearly observed and consists of several individual lattice fringes of Ni(OH)2 nanosheet. The individual lattice fringes further confirm that the Ni(OH)2 nanosheet is formed with a single-crystalline structure. The interplanar spacing is 0.23 nm (2.3 Å), which is in good agreement with the d value of (002) plane of the hexagonal Ni(OH)2. The lattice fringes with the same spacing of different nanosheets suggest that the Ni(OH)2 nanosheet stacking direction is along [00l]. The above characterization results indicate that the β-Ni(OH)2 nanocolumns we synthesized are self-assembled by singlecrystal nanosheets stacking with the same crystallographic direction. Characterization of Nickel Oxide Nanocolumns and Nanosheets. The XRD pattern of Figure 4 shows that Ni(OH)2 has been transformed into NiO completely after annealing at 500 °C for 3 h (sample 1), in which all the diffraction peaks can be perfectly indexed to the pure products of NiO with the face-centered cubic phase (JCPDS 78-0643) and no characteristic peaks from other crystalline forms are detected. The insert of Figure 4 is the TG curve for the decomposition of Ni(OH)2. From room temperature to 280 °C, the weight loss could be caused by desorption of water on the surface of Ni(OH)2. The major weight loss happens rapidly from ∼280 °C to ∼340 °C by the dehydration of Ni(OH)2 to NiO. The morphology of the as-prepared NiO (sample 1) was investigated by TEM and HRTEM. Figure 5a shows that some NiO (about 70%) maintained the nanocolumn shape of Ni(OH)2, but the quantity (about 30%) of the individual NiO nanosheets is obviously increased as compared with that of Ni(OH)2. It implies that the NiO stacking structure is thermodynamically unstable and easy to exfoliate to the individual nanosheets, which is the more stable structure for NiO. To prove that, we further heated the as-prepared NiO sample 1 again at 700 °C for 3 h (sample 2) and observed that (shown in Figure 5b) the quantity of nanocolumn decreases to only about 10%, and most of the sample is in the dispersed nanosheet structure. Figure 5c shows the TEM image of the typical individual nanosheet of the as-prepared NiO (sample 1). It reveals that the nanosheet is composed of a large number of individual nanocrystals. These NiO nanoparticles are self-assembled to form a closely packed nanocrystal sheet. The average size of the nanocrytalline particles is about 13 nm, which is consistent with the results calculated by Scherrer’s formula. There are gaps/pores between the nanocrystallites which indicate that the nanosheets display a porous structure because of the removal of water molecules.16 The SAED pattern of many nanoparticles on the nanosheet is shown in the inset in Figure 5c, when the electron irradiation area was adjusted to cover dozens of nanoparticles. The SAED pattern obtained shows the clear spots just like the nature of a bulk single crystal, which implies that all the particles on the sheet adopted high-orientation alignment.17,18 The stacking structure of NiO sample 1 is further examined by HRTEM shown in Figure 5d, which is carried out on a typical single nanocolumn from the side view. The calculated interplanar spacing is 0.24 nm (2.4 Å), which is in good agreement with the d spacing value of (111) plane of the cubic NiO. The lattice fringes shown in Figure 5d indicate that the different nanosheets on the nanocolumn are oriented stacking along the [111] direction. As compared with Figure 3d, the space distance between the NiO nanosheets on the nanocolumn shown in Figure 5d is larger than that of Ni(OH)2 nanocolumns. It also proves

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Figure 2. (a, b) The SEM images of nanocolumns Ni(OH)2, corresponding to the sample of Figure 1a; (c, d) the SEM image of the stacking sheets with large size Ni(OH)2, corresponding to the sample of Figure 1b.

that the oriented stacking structure of Ni(OH)2 is more stable than that of NiO. We have synthesized the porous NiO nanocolumns and nanosheets just by annealing the Ni(OH)2 nanocolumns in an elevated temperature. The porous nanosheet nanostructure is self-assembled by textured NiO nanocrystals. The NiO nanocolumn is self-assembled by porous nanosheets stacking with the same crystallographic direction, but the stacking structure of NiO is unstable and has the tendency to exfoliate to the individual nanosheets. Formation of Nickel Hydroxide. It is time to investigate the factors on the formation of the Ni(OH)2 nanocolumns. There are many factors that influence the dimension and morphology of the as-synthesized Ni(OH)2 nanocrystals. The intrinsic crystal structure is the essential factor for the morphology control. It is well-known that the β-Ni(OH)2 possesses a brucite-like structure with an interlayer spacing of 4.60 Å and a Ni‚‚‚Ni distance within the layer of 3.12 Å. The oxygen atoms of the hydroxide ions in the brucite structure are located at the apexes of octahedrons and bridge three nickel ions.19 Strong covalentionic bonds hold the atoms within the layers. The hexagonal sheetlike nanocrystals along the c-axis are thermodynamically stable and can be easily formed. This is similar to the formation of the cobalt hydroxides, which prefer to grow into nanoplatelets because of their intrinsic lamellar structure.20 The external environment can remarkably influence the growth of nanocrystals, for example, the kinetics of nucleation and crystal growth. In this synthetic process, the formation of Ni(OH)2 nanocolums indicated that the nucleation and growth

were well controlled. It is well-known that Cd2+, Ni2+, and Mg2+ could form tetradentate complex with N,N′-bidentate ligand, for example, hydrazine hydrate21 and ethylenediamine.22 Solvent hydrazine hydrate molecule has basicity and coordination properties. In the lower molar ratio of hydrazine/Ni2+,23 it could act as a complexing reagent for Ni2+ ions to form the [Ni(N2H4)2]2+ complex at once, when both of the reactants’ aqueous solutions were mixed in a conical flask. The complex is stable in alkaline systems at ambient temperature, but with increasing temperature the stability of the complex decreases.21 At relatively high temperature, OH- could react with the complex, causing the bonds between the metal ion Ni2+ and the N atoms of hydrazine to become weaker and the bonds between Ni2+ and OH- to form little by little. Finally, nanocolumn Ni(OH)2 was obtained in the precipitate. To verify the action of the bidentate ligand of hydrazine in the selfassembly of Ni(OH)2 nanoplatelets, we used ammonia instead of hydrazine under the same conditions. The obtained product was confirmed to be a pure single phase of β-Ni(OH)2 by XRD analysis (Supporting Information S1). From the TEM images (Supporting Information S2), it was clearly observed that the particles are dispersive individual nanosheets, and no nanocolumn structure is found. The bidentate ligand of hydrazine used here may play a key role in the formation of nanocolumn morphology, but the detailed mechanism still needs to be studied in future work. To investigate the influence of the different anions to the morphologies of the Ni(OH)2 products, we designed another experiment by using Ni(CH3COO)2‚4H2O instead of NiCl2‚6H2O

Facile Route to Nickel Hydroxide Nanocolumns

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Figure 3. TEM images of the Ni(OH)2 nanocrystals. (a, b) The naonocolumns; (c) a typical individual nanosheet with hexagonal shape, the insert is the SEAD pattern; (d) a typical nanocolumn performed from side view.

Figure 4. . XRD pattern of the product obtained by thermal decomposition of β-Ni(OH)2 nanocolumns at 500 °C for 3 h and the insert is TG curve.

under invariable condition. Figure 1b is the XRD pattern and Figure 2c and 2d is the SEM images of the sample. From Figure 1b, we can obviously observe that the diffraction peak of (001) is extremely strong, the diffraction peaks of (101), (102), and (110) are greatly weak, and the diffraction peaks of (100) and (111) almost disappear.24 Figure 1b indicates that the assynthesized Ni(OH)2 is oriented along [001] direction. The difference in XRD patterns between the two samples indicates the difference in microstructure and morphology, which is

further confirmed by SEM images. The sample by using Ni(CH3COO)2‚4H2O as the nickel source exhibits the same stacking structure, which may be due to the hydrazine added, but the sheets have larger sizes than that of nanocolumns. The CH3COO-1 anions could influence the process of the formation of the complex,25,26 which acts as the precursor of the polycondensation process and leads to a probably smaller nucleation rate, decreasing the number of nuclei and inducing the formation of larger sheets, resulting in the variation of the morphologies. It implies that we can control different sizes and morphologies of the nickel hydroxide stacking sheets through changing the experimental conditions. It was observed in our experiments that the green transparent NiCl2‚6H2O aqueous solution was turned into blue transparent solution without any precipitate when the hydrazine hydrate aqueous solution was added. As the mixture kept on heating at 60 °C, the green precipitate gradually appeared and a pure Ni(OH)2 product was obtained eventually after post-treatments. We propose the following reaction scheme for the formation of Ni(OH)2:

Ni2+ + 2N2H4 ) [Ni(N2H4)2]2+

(1)

N2H4 + H2O ) N2H5+ + OH-

(2)

[Ni(N2H4)2]2+ + 2OH- ) Ni(OH)2

(3)

The hydrazine can also play a reducing agent in some reactions, but the reducing ability depends on reaction temper-

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Figure 5. (a) TEM images of the NiO nanocolumns and nanosheets (sample 1) obtained by the Ni(OH)2 nanocolumns annealed at 500 °C for 3 h; (b) TEM images of porous NiO nanosheets (sample 2); (c) TEM image of a typical individual NiO nanosheet (sample 1); the insert is an SEAD pattern of NiO nanoparticles revealing the orientation of nanoparticles on the sheet; (d) HRTEM image of a typical NiO nanocolumn (sample 1) performed from side view.

ature and pH value of solution. Pure nickel particles could be obtained when the pH of the reaction mixture was over 10.0 and the temperature was higher than 85 °C.19,27 In our experiment, the starting pH value of the complex solution was only 8.3 and the temperature was kept at 60 °C. The alkalinity was not strong enough and the temperature was not high enough to allow reduction reaction to go further than hydroxide precipitation. Porous Nanosheets and Catalytic Measurements. When the precursors of Ni(OH)2 nanocolumns were annealed at 500 °C for 3 h, they were decomposed into the most thermodynamically stable cubic NiO. In this process, the precursor nanocolumns underwent dehydration and small water molecules escaped and lots of pores on the nanosheet (Figure 5b) formed simultaneously. The removal of binding water molecules does not damage the regular arrangement of Ni and O atoms, and heating may provide the energy to make the NiO nanoparticles self-assemble with high orientation (insert of Figure 5b).28 The reason that the products of NiO still keep nanosheet morphology without damage to dispersive nanoparticles is because of nanocontact between each nanoparticle, which stabilizes the nanosheet structure mechanically against collapse or fracture during the removal of the binding water molecules.18

Figure 6 shows the catalytic activities of different porous NiO nanostructures for CO oxidation. Generally, nanocatalytic materials with a high surface area provide the materials with better catalytic activity.29,30 However, surface area is not the only key factor determining the activity of the catalyst.15 Sample 2 porous NiO (Figure 6b) exhibits a higher catalytic activity than that of sample 1 (Figure 6a) for CO oxidation, as the temperature (150 °C) of catalytic activity is lower than that of sample 1 (250 °C), despite its surface area (18.6 gm-2) which is lower than that of sample 1 NiO nanocloumns (42.3 gm-2). The porous nanosheets with a higher catalytic activity may be because they are completely exposed and the dispersive single NiO porous nanosheet possesses more atoms on edges and corners than that of the NiO porous nanocolumns,15,31,32 which are conventionally considered as active sites for adsorption of reactants. Another possible reason is that the dispersive porous NiO nanosheets possess more reactive planes than that of the porous NiO nanocolumns. 4. Conclusion In this study, we have developed a facile and low-cost route to synthesize high-yield single-crystalline Ni(OH)2 nanocolumns

Facile Route to Nickel Hydroxide Nanocolumns

Figure 6. Percentage conversion versus temperature plots for the oxidation of CO over porous NiO we synthesized: (a) sample 1 synthesized by calcining Ni(OH)2 nanocolumns at 500 °C for 3 h; (b) sample 2 porous NiO by further annealing sample 1 at 700 °C for another 3 h.

at 60 °C with convenient disposal. Hydrazine used here may play a key role in the formation of nanocolumn morphology. We can control different sizes and morphologies of the nickel hydroxide stacking sheets by changing the experimental conditions. NiO nanocolumns and nanosheets obtained after Ni(OH)2 nanocolumns annealed at designed temperature for a certain amount of time. The dispersive individual NiO porous nanosheet exhibits a higher catalytic activity for CO oxidation. Acknowledgment. This work was supported by the National Natural Foundation of China (No. 20571070) and National Science & Technology Supporting Program of China (2006BAK10B). Supporting Information Available: XRD pattern and TEM images of Ni(OH)2. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Chen, J.; Bradhurst, D. H.; Dou, S. X.; Liu, H. K. J. Electrochem. Soc. 1999, 146, 3606.

J. Phys. Chem. C, Vol. 111, No. 15, 2007 5627 (2) Dooley, K. M.; Chem, S. Y.; Ross, J. R. H. J. Catal. 1994, 145, 402. (3) Levin, D.; Ying, J. Y. Stud. Surf. Sci. Catal. 1997, 110, 367. (4) Kumagai, H.; Matsumoto, M.; Toyoda, K.; Obara, M. J. Mater. Sci. Lett. 1996, 15, 1081. (5) Felic, A. C.; Lama, F.; Piacentini, M. J. Appl. Phys. 1997, 80, 3678. (6) Guangli, C.; Brinda, B. L.; Ellen, R. F.; Charles, R. M. Nature 1998, 393, 346. (7) Han, X.; Xie, X.; Xu, C.; Zhou, D.; Ma, Y. Opt. Mater. 2003, 23, 465. (8) Cai, F. S.; Zhang, G. Y.; Chen, J.; Gou, X. L.; Liu, H. K.; Dou, S. X. Angew. Chem., In. Ed. 2004, 43, 4212. (9) Dai, J. X.; Li, S. F. Y.; Xiao, T. D.; Wang, D. M.; Reisner, D. E. J. Power Sources 2000, 89, 40. (10) Meyer, M.; Be´e, A.; Talbot, D.; Cabuil, V.; Boyer, J. M.; Re´petti, B.; Garrigos, R. J. Colloid Interface Sci. 2004, 277, 309. (11) Coudun, C.; Hochepied, J. F. J. Phys. Chem. B 2005, 109, 6069. (12) Chen, D. L.; Gao, L. Chem. Phys. Lett. 2005, 405, 159. (13) Liang, Z. H.; Zhu, Y. J.; Hu, X. L. J. Phys. Chem. B 2004, 108, 3488. (14) Li, X. L.; Liu, J. F.; Li, Y. D. Mater. Chem. Phys. 2003, 80, 222. (15) Wang, D. S.; Xu, R.; Wang, X.; Li, Y. D. Nanotechnology 2006, 17, 979. (16) Gao, P. X.; Wang, Z. L. J. Am. Chem. Soc. 2003, 125, 11299. (17) Greene, L. E.; Law, M.; Tan, D. H.; Montano, M.; Goldberger, J.; Somorjai, G.; Yang, P. D. Nano Lett. 2005, 5, 1231. (18) Gui, Z.; Fan, R.; Mo, W. Q.; Chen, X. H.; Yang, L.; Zhang, S. Y.; Hu, Y.; Wang, Z. Z.; Fan, W. C. Chem. Mater. 2002, 14, 5053. (19) Chakroune, N.; Viau, G.; Ammar, S.; Jouini, N.; Gredin, P.; Vaulay, M. J.; Fie’vet, F. New J. Chem. 2005, 29, 355. (20) Hou, Y. L.; Kondoh, H.; Shimojo, M.; Kogure, T.; Ohta, T. J. Phys. Chem. B 2005, 109, 19094. (21) Li, Y. D.; Li, L. Q.; Liao, H. W.; Wang, H. R. J. Mater. Chem. 1999, 9, 2675. (22) Li, Y. D.; Sui, M.; Ding, Y.; Zhang, G. H.; Zhuang, J.; Wang, C. AdV. Mater. 2000, 12, 818. (23) Park, J. W.; Chae, E. H.; Kim, S. H.; Lee, J. H.; Kim, J. W.; Yoon, S. M.; Choi, J. Y. Mater. Chem. Phys. 2006, 97, 371. (24) Kurmoo, M.; Day, P.; Derory, A.; Estourne’s, C.; Poinsot, R.; Stead, M. J.; Kepert, C. J. J. Solid State Chem. 1999, 145, 452. (25) Jouini, N.; Poul, L.; Fie’vet, F.; Robert, F. Eur. J. Solid State Inorg. Chem. 1995, 32, 1129. (26) Poul, L.; Jouini, N.; Fie’vet, F.; Herson, P. Z. Kristallogr. 1998, 213, 416. (27) Li, Y. D.; Li, C. W.; Wang, H. R.; Li, L. Q; Qian, Y. T. Mater. Chem. Phys. 1999, 59, 88. (28) Gui, Z.; Liu, J.; Wang, Z. Z.; Song, L.; Hu, Y.; Fan, W. C.; Chen, D. Y. J. Phys. Chem. B 2005, 109, 1113. (29) Ying, J. Y.; Mehnert, C. P.; Wong, M. S. Angew. Chem., Int. Ed. 1999, 38, 56. (30) Boschloo, G.; Hagfeldt, A. J. Phys. Chem. B 2001, 105, 3039. (31) Zhou, K. B.; Wang, X.; Sun, X. M.; Peng, Q.; Li, Y. D. J. Catal. 2005, 229, 206. (32) Fuertes, A. B. J. Phys. Chem. Solids 2005, 66, 741.