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J. Phys. Chem. B 2008, 112, 9400–9405

Raman Scattering Properties of a Protonic Titanate HxTi2-x/40x/4O4 · H2O (0, vacancy; x ) 0.7) with Lepidocrocite-Type Layered Structure Tao Gao,* Helmer Fjellvåg, and Poul Norby Center for Materials Science and Nanotechnology, Department of Chemistry, UniVersity of Oslo, P.O. Box 1033, N-0315 Oslo, Norway ReceiVed: February 25, 2008; ReVised Manuscript ReceiVed: June 3, 2008

Raman scattering spectroscopy is employed to characterize a layered titanate HxTi2-x/40x/4O4 · H2O (0: vacancy; x ) 0.7) with lepidocrocite (γ-FeOOH)-type layered structure. Nine Raman lines corresponding to (3Ag + 3B1g + 3B3g) Raman-active modes expected from this orthorhombic structure (space group D25 2h-Immm) are recorded at 183, 270, 387, 449, 558, 658, 704, 803, and 908 cm-1, which are assigned to Ti-O lattice vibrations within the two-dimensional (2D) lepidocrocite-type TiO6 octahedral host layers. These intrinsic Raman bands present a clear signature that can be used for probing the protonic titanate HxTi2-x/40x/4O4 · H2O and the 2D titanate nanosheets, as well as their corresponding derivates. Introduction A layered titanate, HxTi2-x/40x/4O4 · H2O (0: vacancy; x ) 0.7), with lepidocrocite (γ-FeOOH)-type layered structure, has recently attracted great attention due to its interesting interlayer chemistry1 and potential applications in the synthesis of new functional nanomaterials.2 The compound is characterized by its relatively simple architecture and low negative charge density of its two-dimensional (2D) lepidocrocite-type TiO6 octahedral host layers in comparison with the other layered titanates.1 This layered material has excellent ion-exchange/intercalation reactivities to accommodate a wide variety of guest species such as inorganic and/or organic cations and surfactants.1,3 Moreover, it can undergo exfoliation/delamination upon intercalating bulky organic cations, producing its molecular single sheets (denoted as titanate nanosheets hereafter) with distinctive 2D morphology, small thickness, and interesting physical properties.4 These 2D titanate nanosheets are attractive building blocks for the assembly of artificial nanoarchitectures,2 such as hybrid multilayer films5 and microporous materials with inorganic pillars.6 We are particularly interested in Raman scattering property of layered titanates. Raman scattering spectroscopy is often a useful supplement to X-ray diffraction for structure characterizations of materials. Since it is sensitive to amorphous components and those with short-range order as well as to materials with long-range order,7 Raman scattering can yield a more reliable description of materials such as the layered titanate HxTi2-x/40x/4O4 · H2O, in which local structural disorders such as vacancies can be expected. Moreover, it may offer a possibility of online and/ or in situ analysis of interlayer chemistry of the protonic titanate HxTi2-x/40x/4O4 · H2O where local structural changes are usually involved as the ion-exchange/intercalation reactions proceed.1 Obviously, the application of Raman scattering spectroscopy in the protonic titanate HxTi2-x/40x/4O4 · H2O is of great interest. However, so far detailed information on Raman scattering properties of this layered compound is much limited. Actually, Sasaki et al. have measured the Raman scattering spectrum of the protonic titanate HxTi2-x/40x/4O4 · H2O;8 however, they did not present detailed analyses on the observed Raman bands. * Corresponding author. E-mail:[email protected].

In this paper, we report the preparation and Raman scattering properties of the protonic titanate HxTi2-x/40x/4O4 · H2O. Nine Raman lines corresponding to (3Ag + 3B1g + 3B3g) Ramanactive modes expected from this orthorhombic structure (space -1 spectral group D25 2h-Immm) are observed in the 100-1000 cm region, which are ascribed to Ti-O lattice vibrations within the 2D TiO6 octahedral host layers. These Raman bands present a clear signature that can be conveniently used for probing the protonic titanate HxTi2-x/40x/4O4 · H2O, and the 2D titanate nanosheets, as well as their corresponding derivates.1,5,6 Experimental Section Reagents and Materials. Cesium carbonate Cs2CO3 (99.9%), anatase-phased TiO2 (99.7%), and tetrabutylammonium (abbreviated as TBA hereafter) hydroxide aqueous solution (40 wt %) were purchased from Sigma-Aldrich Co. and used as received. Synthesis of Titanates. The protonic titanate HxTi2-x/40x/4O4 · H2O (0: vacancy, x ) 0.7) was prepared by ion-exchanging a cesium titanate CsxTi2-x/40x/4O4, which can be synthesized via a conventional solid-state reaction of Cs2CO3 and TiO2 at 800 °C.9 The acid-exchange was performed by dispersing 100 mg CsxTi2-x/40x/4O4 powders in 1 M HCl aqueous solution (200 mL) at room temperature for 1-3 days. The acidic solution was renewed every 12 h to promote a complete exchange. Then the solid material was filtered, rinsed with water and dried in air at room temperature. The as-prepared protonic titanate HxTi2-x/40x/4O4 · H2O was exfoliated/delaminated in an aqueous solution of TBAOH. This was performed by dispersing 100 mg HxTi2-x/40x/4O4 · H2O powders into 100 mL TBAOH aqueous solution (20 wt %) at 60 °C for 10 days under constant stirring. After the reaction, the solid product was separated from the suspension by centrifugation. A stable titanate nanosheet colloidal suspension was formed by dispersing the obtained solid product in water. Freeze-drying of the colloidal suspension yielded a voluminous white solid with a cotton-like appearance. Characterization. The as-synthesized materials were analyzed by X-ray powder diffraction (XRD, Siemens D5000 powder diffractometer with Cu KR1 radiation), scanning electron microscopy (SEM, FEI Quanta 200F), energy-dispersive X-ray

10.1021/jp801639a CCC: $40.75  2008 American Chemical Society Published on Web 07/16/2008

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Figure 1. Idealized crystalline structure of the protonic titanate HxTi2-x/40x/4O4 · H2O (0: vacancy; x ) 0.7) with lepidocrocite-type layered structure. The titanium vacancies are not shown for simplification.

Figure 2. SEM image (a) and XRD pattern (b) of the as-prepared protonic titanate HxTi2-x/40x/4O4 · H2O.

spectroscopy (EDXS, attached to SEM), and thermo-gravimetric analysis (TGA, Perkin-Elmer TGA 7 system; N2 flow; heating rate: 10 °C/min). Raman scattering spectra of the as-prepared materials were collected in a backscattering configuration. The samples were illuminated by using a 632.8-nm He-Ne laser on an Olympus BX 40 confocal microscope with a 50× objective. A cooled charge-coupled device (CCD) detector was used to collect the scattered light dispersed by a 1800 lines/mm grating. The wavenumber stability and the accuracy of the apparatus were checked by recording the Raman spectrum of silicon. A laser power of 1 mW was used as it is low enough to prevent damage to the samples but is sufficient to produce good-quality spectra in a reasonable time (exposure time: 30-60 s). Raman spectra with a resolution of about 2 cm-1 were collected at room temperature.

Figure 3. TGA curve of the as-prepared protonic titanate HxTi2-x/40x/4O4 · H2O.

Results and Discussion Structural Study. Figure 1 shows an idealized crystalline structure of the protonic titanate HxTi2-x/40x/4O4 · H2O (0: vacancy; x ) 0.7).1,9 The structure is orthorhombic (space group Immm) and consists of 2D TiO6 octahedral host layers and interlayer water molecules (H2O and/or H3O+). The undulating surface of the 2D TiO6 octahedral host layers contains 2-coordinated anions, O(2), along the ridges and 4-coordinated anions, O(1), in the troughs. Both the anion sites are occupied by oxygen, which is different from those in γ-FeOOH (lepidocrocite) or γ-AlOOH (boehmite) where the 2-coordinated anion sites are occupied by hydroxyl.10 As shown in Figure 1, the protonic titanate HxTi2-x/40x/4O4 · H2O has one water molecule in every pseudocubic cavity enclosed by eight 2-coordinated oxygens on neighboring host layers. Note that 70% of the interlayer water molecules are in the form of H3O+, which are derived from the exchangeable Cs atoms in the parent titanate CsxTi2-x/40x/4O4 and carry exchangeable protons.1 The presence of interlayer

water species was confirmed by the subsequent structural characterization. The neighboring TiO6 octahedral host layers are held together by hydrogen bonding between the interlayer water molecules and the 2-coordinated oxygens. The as-prepared HxTi2-x/40x/4O4 · H2O is microcrystalline powders of micrometer dimension, with rod- and plate-like morphologies (Figure 2a). A typical XRD pattern of the asprepared protonic titanate is shown in Figure 2b, in which all diffraction peaks can be readily indexed on the basis of a bodycentered orthorhombic structure (space group Immm) with unit cell parameters a ) 0.3780 nm, b ) 1.8687 nm, and c ) 0.2978 nm, which is in good agreement with the published data.1 On the basis of the XRD data, the interlayer distance of the protonic titanate is about 0.93 nm. Figure 3 displays the TGA data of the as-prepared HxTi2-x/40x/4O4 · H2O. There are two distinctive weight loss regions, 50-150 and 150-600 °C, of which the corresponding weight loss is about 10 and 5%, respectively. The XRD pattern

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Figure 4. XRD patterns of (a) dehydrated titanate HxTi2-x/40x/4O4 and (b) anatase. The materials are prepared by heating the protonic titanate HxTi2-x/40x/4O4 · H2O at 150 °C (for a) or 500 °C (for b) in air for 2 h. Curves are shifted vertically for clarity.

of an intermediate phase at 150 °C (Figure 4a) indicates that the crystalline structure of the protonic titanate is degraded, with a contracted interlayer distance of about 0.66 nm. Note that the XRD profile of the degraded titanate is quite similar to those of γ-FeOOH (interlayer distance: ∼0.62 nm) or γ-AlOOH (interlayer distance: ∼0.61 nm), which do not contain interlayer guests but possess similar lepidocrocite-type layered structures.10 This indicates that the protonic titanate HxTi2-x/40x/4O4 · H2O loses first interlayer H2O molecules, resulting in a dehydrated titanate phase, HxTi2-x/40x/4O4;1a the subsequent heating results in the collapse of the lepidocrocite-type layered structure and the formation of anatase, as evidenced by the corresponding XRD data shown in Figure 4b. These experimental data (Figure 3 and Figure 4) confirm that the interlayer protons of the protonic titanate HxTi2-x/40x/4O4 · H2O exist in the form of H3O+. The protonic titanate HxTi2-x/40x/4O4 · H2O can be readily exfoliated/delaminated into its molecular single sheets with distinctive 2D morphology, i.e., titanate nanosheets, by exchanging the interlayer H3O+ ions with bulky TBA+ ions (0.95-1.05 nm in diameter).3,4 As shown in Figure 5a, a typical SEM image reveals clearly the plate-like morphology of the resulting nanosheets with a lateral size of several to tens of micrometers and a thickness of 20-50 nm. It suggests that 20-50 titanate nanosheets were restacked into a TBAintercalated titanate thin flake upon freeze-drying.11 The XRD pattern of the TBA-intercalated titanate (Figure 5b) shows a series of well-defined sharp basal diffraction peaks. These reflections can be indexed as (0k0), which is indicative of a lamellar structure with a gallery height of 1.76 nm.4,11 The increased interlayer distance of 1.76 nm compared to 0.93 nm for the as-prepared protonic titanate indicates that the TBA+ ions have been intercalated into the interlayer region of the restacked titanate nanosheets. Raman Scattering Study. Figure 6a shows a typical Raman scattering spectrum of the protonic titanate HxTi2-x/40x/4O4 · H2O in the low wavenumber 100-1000 cm-1 spectral region, of which nine contributions are observed at 183, 270, 387, 449, 558, 658, 704, 803, and 908 cm-1. We wish to make it clear that Sasaki et al. has reported previously a similar Raman scattering spectrum for their protonic titanate HxTi2-x/40x/4O4 · H2O;8 however, no results related to detailed spectral analysis have been subsequently published. Owing to the large difference among the atomic masses of Ti, O and H, it is reasonable to assume that the observed phonons are almost pure modes, i.e., they mainly involve displacements of a single atomic species. Furthermore,

Figure 5. SEM image (a) and XRD pattern (b) of the TBA-intercalated titanate. Inset of (b) shows an enlargement at higher angles.

as a rule of thumb, it is generally correct that phonons involving displacements of light ions have large wavenumbers. In this regard, it is reasonable that the Raman bands at the low wavenumbers (Figure 6a) involve mainly the Ti-O vibrations and do not include a significant contribution from the interlayer water species. Observation of several weak Raman bands at 1645, 2260, 2861, 3200, and 3455 cm-1 due to the OH bending and/or stretching vibrations12 seems in harmony with this assumption. Before going into the details of the Raman analysis, it is necessary to predict theoretically the numbers of the Ramanactive modes of the protonic titanate HxTi2-x/40x/4O4 · H2O. Factor group analysis13 was applied to determine the distribution of vibrations among symmetry species of the D25 2h-Immm space group. Due to the nonintegral feature of the chemical formula and the vacancy involved in the unit cell, a simplification on the real crystalline structure of the protonic titanate HxTi2-x/40x/4O4 · H2O was performed for the application of factor group analysis in its conventional form. Let us suppose that the titanium sites in the 2D TiO6 octahedral host layers are fully occupied and the symmetry of the host layers is still preserved. The filling of the titanium vacancies in the lattice will at the same time remove the positive-charged interlayer counterparts.9 This results in a virtual composition Ti2O4 · H2O for the protonic titanate, where the 2D TiO6 octahedral host layers do not have any net charges. Since the presence of the titanium vacancies does not change the polymerization nature of the TiO6 octahedra,1,9 this hypothesis is quite reasonable. Note that the space group D25 2h-Immm has a nonprimitive unit cell, thus the number of formula units (Z) used shall be halved for the factor group analysis. Consequently, 2 Ti4+ and 4 O2- ions, lying on Wyckoff sites 4h and 4g, respectively, are to be considered to determine the

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Figure 6. Raman scattering spectra of the as-prepared protonic titanate HxTi2-x/40x/4O4 · H2O. Note that the intensity scales shown in panel a and b are different.

TABLE 1: Correlation Chart for Vibrations of 2 Ti4+ and 4 O-2 Ions in the Protonic Titanate HxTi2-x/40x/4O4 · H2O

a According to Table 14 in ref 13. infrared active.

b

Key: R, Raman active; IR,

Ti-O lattice vibrations. As shown in Table 1, the correlation between C2V site group and D2 h factor group yields the following irreducible representations (after subtracting the K ) 0 acoustic modes):

ΓTi-O)3Ag+3B1g+3B3g+2B1u+2B2u+2B3u

(1)

i.e., nine Raman and six IR active modes are expected for the 2D TiO6 octahedral host layers. In the case of the Ti-O vibrations, the expected fifteen fundamental vibrational transitions can be conveniently understood by considering the analogy with vibrations of a TiO6 octahedron. An ideal, isolated TiO6 octahedron of Oh symmetry will give six normal vibrations: V1 (A1g), V2 (Eg), V3 (F1u), V4 (F1u), V5 (F1g), and V6 (F2g).13 Among them, three modes (A1g

+ Eg + F1g) are Raman-active, two IR-active modes are of F1u symmetry, and the V6 (F2g) mode is inactive. The distortion of TiO6 coordination units, when linked in a three-dimensional lattice, will generally cause splitting of the degenerate modes. Moreover, the inactive modes, for instance the V6 (F2g), may become active as the symmetry is reduced.14 Obviously, the six IR modes (2B1u + 2B2u + 2B3u) expected from the factor group analysis (eq 1) are derived from the triply degenerate V3 (F1u) and V4 (F1u) modes; the splitting of one doubly degenerate Eg mode and two triply degenerate F1g and F2g modes will give the total nine Raman active modes (3Ag + 3B1g + 3B3g), in which one Ag mode (i.e., A1g) already exists. The number of the Raman-active modes related to the interlayer species is dependent on their sites in the crystalline structure of the protonic titanate HxTi2-x/40x/4O4 · H2O.13 In an idealized Immm structure (Figure 1), the interlayer species will be put on the inversion centers of this highly symmetric structure and will not contribute to any Raman-active modes. It is important to point out that, in the protonic titanate HxTi2-x/40x/4O4 · H2O, the interlayer species are H3O+ (or H2O) rather than H+ ions. That is to say, the three positive charged H atoms of an H3O ion would bond to the eight nearest-neighbor 2-coordinated oxygens; it must involve the distortion of the H3O tetrahedron. However, the precise arrangement of the three H atoms is too complex to be determined with the XRD data;15 it is thus difficult to determine theoretically the number of the Raman-active modes related to the interlayer water species. As shown in Figure 6b, the vibrational features of the interlayer water species normally give Raman bands at high wavenumbers, which is consistent with the Raman scattering studies on liquid water including H2O molecules12 as well as H3O+ ions.16 In this regard, the nine Raman bands observed at the 100-1000 cm-1 spectral region (Figure 6a) can be attributed to the Ti-O lattice vibrations within the 2D lepidocrocite-type TiO6 octahedral host layers. That is to say, all Raman modes (3Ag + 3B1g + 3B3g) related to the 2D TiO6 octahedral host layers predicted by group theoretical analysis (eq 1) are indeed observed. This conclusion is confirmed further by the Raman data of the dehydrated titanate HxTi2-x/40x/4O4 and the TBA-intercalated titanate. As shown in Figure 7a, a typical Raman scattering spectrum of the dehydrated titanate HxTi2-x/40x/4O4 has six main contributions at 181, 282, 389, 447, 657, and 708 cm-1, along with three very weak bands at 560, 816, and 930 cm-1. Thanks to their structural similarity, the Raman spectrum of the dehydrated titanate is similar to that of the protonic titanate HxTi2-x/40x/4O4 · H2O (Figure 6a). Note that there are spectral modifications due to the local structural evolutions. For example, as shown in Figure 7b, no obvious Raman band at high wavenumbers due to the interlayer water species is observed. It is reasonable since the Raman lines related to interlayer water species (as shown in Figure 6b) would decay and finally disappear with water loss, which is consistent with the experimental findings reported in Figure 3 and Figure 4a. Moreover, the dehydrated titanate possesses a C-centered orthorhombic structure (space group Cmcm), which is the same as that of the γ-FeOOH and γ-AlOOH. It indicates that the dehydration of the protonic titanate HxTi2-x/40x/4O4 · H2O may involve a lateral gliding of the host layers along the a and c axes.9b The interlayer protons of the dehydrated titanate are H+ ions, which coordinate to six nearest-neighbor 2-coordinated oxygens and may not occupy the 8f site as suggested in ref 10a. Details of the dehydrated titanate HxTi2-x/40x/4O4 are out of the scope of this paper and will be discussed elsewhere.

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Figure 7. Raman scattering spectra of the dehydrated titanate HxTi2-x/40x/4O4. Note that the intensity scales shown in panels a and b are different.

Gao et al. TBA+ ions. This assumption is reasonable for the following two considerations. First, as shown in Figure 5b, the XRD data reveal that the TBA-intercalated titanate does not inherit the orthorhombic structure (space group Immm) from the original protonic titanate (Figure 2b). Second, the interaction between the 2D TiO6 octahedral host layers and the TBA+ ions is much weak: titanate nanosheets colloidal suspension can be reproduced by dispersing the TBA-intercalated titanate into water. In this regard, we can conclude that the Raman bands at 181, 275, 383, 444, 558, 656, 701, 793, and 914 cm-1 (after subtracting the two Raman bands related to TBA+ ions) must correspond to the TiO6 octahedral layers themselves. The interpretation of the Raman scattering spectra can be done by either determining the symmetry properties of the Raman bands or assigning the observed frequencies to Ti-O vibrations with different structural features. Of course they do not exclude each other. As shown in Figure 6a, the Raman band at 704 cm-1 can be assigned to a totally symmetric Ti-O vibration, which belongs to the Ag symmetric modes in the D25 2h spectroscopic space group. The other vibrations of Ag symmetry, originated from the splitting of the degenerate modes of the TiO6 octahedron,13,14 are ascribed to the Raman bands at 449 and 270 cm-1. The presence of the three Ag Raman modes is indicative of a well-developed 2D lepidocrocite-type layered structure, which represents a clear signature that can be used for online and/or in situ probing the 2D titanate nanosheets4 and their derivates.5,6 The Raman bands at 803 and 908 cm-1 might be correlated to the stretching vibrations of the short Ti-O bonds that stick out into the interlayer spaces.9,17 The Raman band at 183 cm-1 could be ascribed to an external vibration that derives from the translational motion of the 2D TiO6 octahedral host layers since its intensity is correlated with the nature of the interlayer species. However, an unequivocal assignment of these and the other Raman bands in the protonic titanate HxTi2-x/40x/4O4 · H2O (Figure 6a) is still difficult at this stage. A complete mode identification via polarization measurement of single crystal samples is considered and will be pursued in our succeeding studies in this series. Conclusions

Figure 8. Raman scattering spectrum of the TBA-intercalated titanate.

Figure 8 shows the Raman scattering spectrum of the TBAintercalated titanate, of which eleven Raman bands at 181, 275, 383, 444, 558, 656, 701, 793, 828, 882, and 914 cm-1 are observed.11 The Raman spectrum of the TBA-intercalated titanate is similar to that of the protonic titanate HxTi2-x/40x/4O4 · H2O (Figure 6a) except for some spectral modifications. Note that the vibrations related to the TBA+ ions give also several intensive Raman bands in 750-3750 cm-1 spectral region (Supporting Information). It is thus difficult to analyze the Raman bands observed at 750-950 cm-1, where some Raman bands related to the Ti-O vibrations would be appeared. We suggest that the TBAintercalated titanate could be considered as a mixture/composite of 2D lepidocrocite-type TiO6 octahedral layers and interlayer

In summary, Raman scattering properties of the protonic titanate HxTi2-x/40x/4O4 · H2O (0: vacancy; x ) 0.7) with the lepidocrocite-type layered structure have been studied. As expected from the orthorhombic structure (space group Immm), nine Raman lines (3Ag + 3B1g + 3B3g) corresponding to Ti-O lattice vibrations within the 2D lepidocrocite-type TiO6 octahedral host layers have been recorded at 183, 270, 387, 449, 558, 658, 704, 803, and 908 cm-1. Three Ag symmetric modes in the D25 2h spectroscopic space group have been recognized at 270, 449, and 704 cm-1, which is indicative of a well-developed 2D lepidocrocite-type layered structure. The intrinsic Raman bands reported in this work present a clear signature that can be used for probing the protonic titanate HxTi2-x/40x/4O4 · H2O, the 2D titanate nanosheets, as well as their corresponding derivates. Acknowledgment. We thank Professor C. J. Nielsen and Dr. M. Glerup for their help on the Raman measurement. We are grateful to the valuable comments from the reviewers. The authors acknowledge the financial assistance from the Research Council of Norway through the NANOMAT program (163565431). Supporting Information Available: Figure showing Raman scattering data of the TBA-intercalated titanate at high wave-

Raman Scattering Properties numbers. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Sasaki, T.; Watanabe, M.; Michiue, Y.; Komatsu, Y.; Izumi, F.; Takenouchi, S. Chem. Mater. 1995, 7, 1001. (b) Sasaki, T.; Kooli, F.; Iida, M.; Michiue, Y.; Takenouchi, S.; Yajima, Y.; Izumi, F.; Chakoumakos, B. C.; Watanabe, M. Chem. Mater. 1998, 10, 4123. (2) Sasaki, T. J. Ceram. Soc. Jpn. 2007, 115, 9. (3) (a) Sasaki, T.; Watanabe, M. J. Am. Chem. Soc. 1998, 120, 4682. (b) Kaito, R.; Miyamoto, N.; Kuroda, K.; Ogawa, M. J. Mater. Chem 2002, 12, 3463. (c) Suzuki, N.; Hayashi, N.; Honda, C.; Endo, K.; Kanzaki, Y. Bull. Chem. Soc. Jpn. 2006, 79, 711. (4) (a) Sasaki, T.; Watanabe, M. J. Phys. Chem. B 1997, 101, 10159. (b) Sato, H.; Ono, K.; Sasaki, T.; Yamagishi, A. J. Phys. Chem. B 2003, 107, 9824. (c) Sakai, N.; Ebina, Y.; Takada, K.; Sasaki, T. J. Am. Chem. Soc. 2004, 126, 5851. (5) (a) Zhou, Y.; Ma, R.; Ebina, Y.; Takada, K.; Sasaki, T. Chem. Mater. 2006, 18, 1235. (b) Sumida, T.; Takahara, Y.; Abe, R.; Hara, M.; Kondo, J. N.; Domen, K.; Kakihana, M.; Yoshimura, M. Phys. Chem. Chem. Phys. 2001, 3, 640. (c) Paek, S. M.; Jung, H.; Lee; Y. J.; Park, M.; Hwang, S. J.; Choy, J. H. Chem. Mater. 2006, 18, 1134. (d) Umemura, Y.; Shinohara, E.; Koura, A.; Nishioka, T.; Sasaki, T. Langmuir 2006, 22, 3870. (6) (a) Choy, J. H.; Lee, H. C.; Jung, H.; Hwang; S. J, J. Mater. Chem 2001, 11, 2232. (b) Kooli, F.; Sasaki, T.; Watanabe, M. Langmuir 1999, 15, 1090.

J. Phys. Chem. B, Vol. 112, No. 31, 2008 9405 (7) Loudon, R. Proc. Phys. Soc. 1963, 82, 393. (8) Sasaki, T.; Nakano, S.; Yamauchi, S.; Watanabe, M. Chem. Mater. 1997, 9, 602. (9) (a) Grey, I. E.; Li, C.; Madsen, I. C.; Watts, J. A. J. Solid State Chem. 1987, 66, 7. (b) Grey, I. E.; Madsen, I. C.; Watts, J. A.; Bursill, L. A. J. Solid State Chem. 1985, 58, 350. (10) (a) Kiss, A. B.; Keresztury, G.; Farkas, L. Spectrochim. Acta 1980, 36A, 653. (b) Doss, C. J.; Zallen, R. Phys. ReV. B 1993, 48, 15626. (c) Ewing, F. J. J. Chem. Phys. 1935, 3, 420. (11) Ma, R.; Fukuda, K.; Sasaki, T.; Osada, M.; Bando, Y. J. Phys. Chem. B 2005, 109, 6210. (12) (a) Carey, D. M.; Korenowski, G. M. J. Chem. Phys. 1998, 108, 2669. (b) Rull, F. Pure Appl. Chem 2002, 74, 1859. (13) Fateley, W. G.; Dollish, F. R.; McDevitt, N. T.; Bentley, F. F. Infrared and Raman Selection Rules for Molecular and Lattice Vibrations: The Correlation Method. Wiley-Interscience: New York, 1972. (14) (a) Fadini, A. ; Schnepel, F. M. Vibrational Spectroscopy: Methods and Applications; Ellis Horwood Limited: London, England, 1989. (b) Szymanski, H. A. Raman Spectroscopy: Theory and Practice; Plenum Press: New York, 1967. (15) England, W. A.; Birkett, J. E.; Goodenough, J. B.; Wiseman, P. J. J. Solid State Chem. 1983, 49, 300. (16) Glguere, P. A.; Gulllot, J. G. J. Phys. Chem. 1982, 86, 3231. (17) Kudo, A.; Kondo, T. J. Mater. Chem. 1997, 7, 777.

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