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Alternate Multilayer Deposition from Ammonium Amphiphiles and Titanium Dioxide Crystalline Nanosheets Using the Langmuir-Blodgett Technique Tetsuya Yamaki*,† and Keisuke Asai‡ Department of Materials Development, Takasaki Radiation Chemistry Research Establishment, Japan Atomic Energy Research Institute (JAERI), 1233 Watanuki, Takasaki, Gunma 370-1292, Japan, and Department of Quantum Engineering and Systems Science, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan Received November 27, 2000. In Final Form: February 8, 2001 A new type of TiO2 nanosheet-organic complex multilayer film was successfully prepared by the Langmuir-Blodgett technique. Surface pressure-area (π-A) curves demonstrated that a stable monolayer of dioctadecyldimethylammonium bromide (DODAB) was formed on an aqueous suspension of TiO2 nanosheets derived from an exfoliated layered titanate, HxTi2-x/40x/4O4‚H2O (x ∼ 0.7; 0, vacancy). The hybrid monolayer of the ammonium amphiphiles and the TiO2 nanosheet was quantitatively transferred onto a hydrophobic quartz plate to form a multilayer film. X-ray diffraction measurements confirmed the formation of an ordered structure of DODAB/TiO2 with an interlayer distance of 3.4 nm.
Introduction The construction of organic/inorganic nanocomposite films has been an important target of functional materials research. Colloid chemical methodologies provide a viable approach to the preparation of such hybrid films.1,2 Of particular interest are formation of nanosized semiconductor crystallites under Langmuir monolayers and their layer-by-layer transfer onto solid substrates by the Langmuir-Blodgett (LB) technique.3,4 Ordered multilayers of CdS, ZnS, and PbSe nanoparticle layers “sandwiched” by organic amphiphilic layers have been produced using these processes.5,6 Evidence is presented in this Letter for the layer-bylayer transfer of ordered nanostructured films composed of alternating layers of cationic dioctadecyldimethylammonium bromide (DODAB) and negatively charged titanium dioxide (TiO2) nanosheets onto solid substrates. TiO2 crystallites in the nanometer range are currently intriguing because of their excellent optoelectronic properties for solar cells7 and photocatalysts.8 However, there have been only a few reports on the deposition of organic/TiO2 multilayers by the LB method, which allows the rational control of the ultrathin semiconducting film structure.3,9 * To whom correspondence should be addressed. † Japan Atomic Energy Research Institute. ‡ The University of Tokyo. (1) Fendler, J. H. Membrane-Mimetic Approach to Advanced Materials; Springer-Verlag: Berlin, 1994; Advances in Polymer Science Series, Vol. 113. (2) Kotov, N. A.; Meldrum, F. C. Adv. Mater. 1995, 7, 607. (3) Kotov, N. A.; Meldrum, F. C.; Wu, C.; Fendler, J. H. J. Phys. Chem. 1994, 98, 2735. (4) Kotov, N. A.; Meldrum, F. C.; Fendler, J. H. J. Phys. Chem. 1994, 98, 8827. (5) Fendler, J. H. In Thin Films, v.20. Organic thin films and surfaces: directions for the nineties; Ulman, A., Eds.; Academic Press: San Diego, New York, Boston, 1995; pp 11-40. (6) Ozin, G. Adv. Mater. 1993, 4, 612. (7) Tan, M. X.; Laibinis, P. E.; Nguyen, S. T.; Kesselman, J. M.; Stanton, C. E.; Lewis, N. S. Principles and applications of semiconductor photochemistry; Tan, M. X., Laibinis, P. E., Nguyen, S. T., Kesselman, J. M., Stanton, C. E., Lewis, N. S., Eds.; John Wiley & Sons: New York, 1994; Vol. 41, pp 21-144. (8) Kamat, P. V. Native and Surface Modified Semiconductor Nanoclusters; Kamat, P. V., Ed.; John Wiley & Sons: New York, 1997; Vol. 44, pp 273-343.
Almost all the efforts in this field include applications of spherical TiO2 nanoparticles in aqueous or organic sols prepared by the hydrolysis of titanium salts. In such a preparation, it is very difficult to control the particle size and its distribution. In the present study, exfoliated single sheets of a titanate with a lepidocrocite-like layered structure10,11 were used as the TiO2-nanocrystalline source for the preparation of the organic/TiO2 nanostructures. The TiO2 nanosheets, exfoliated elementary fragments of the wellcrystallized layered compound synthesized at high temperatures, should have a well-defined composition, structure, and uniform ultrathin thickness. Such novel anisotropic morphology has not yet been accomplished in the system of TiO2. Therefore, nanostructures constructed from them are of interest for both fundamental studies and a host of applications. This study could also be a significant step toward the creation of future organic/TiO2 composites based on the so-called membrane-mimetic approach.1 Experimental Section Exfoliated TiO2 nanosheets were prepared by the established procedure10,11 as follows. The layered Cs compound, CsxTi2-x/40x/4O4 (x ∼ 0.7; 0, vacancy), was obtained by twice repeating the calcination (800 °C, 20 h) of a stoichiometric mixture of Cs2CO3 and TiO2 (1:5.3 in molar ratio).12,13 To prepare a layered precursor compound for exfoliation, HxTi2-x/40x/4O4‚H2O, the interlayer Cs ions were extracted by stirring ∼25 g of CsxTi2-x/40x/4O4 in 1 mol dm-3 HCl (1 dm3) at ambient temperature.13 Vigorous shaking of HxTi2-x/40x/4O4‚H2O with an aqueous 0.0825 mol dm-3 solution of tetrabutylammonium hydroxide (TBAOH), (C4H9)4NOH, produced a translucent colloidal suspension (the solution-to-solid ratio was 250 cm3 g-1). The resulting suspension, whose concentration was adjusted to 0.2 g dm-3 (pH ) 11.5), was used (9) Li, L.; Chen, Y.; Kan, S.; Zhang, X.; Peng, X.; Liu, M.; Li, T. Thin Solid Films 1996, 284-285, 592. (10) Sasaki, T.; Watanabe, M.; Hashizume, H.; Yamada, H.; Nakazawa, H. J. Am. Chem. Soc. 1996, 118, 8329. (11) Sasaki, T.; Watanabe, M.J. Phys. Chem. B 1997, 101, 10159. (12) Grey, I. E.; Li, C.; Madsen, I. C.; Watts, J. A. J. Solid State Chem. 1987, 66, 7. (13) Sasaki, T.; Watanabe, M.; Michiue, Y.; Komatsu, Y.; Izumi, F.; Takenouchi, S. Chem. Mater. 1995, 7, 1001.
10.1021/la0016423 CCC: $20.00 © 2001 American Chemical Society Published on Web 04/03/2001
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Figure 2. π-A isotherms of DODAB monolayers spread on an aqueous solution of TBAOH (a) and on the TiO2-nanosheet sol (b). mode was carried out over an angular range of 1.5-10.0° at 0.005° intervals in 2θ.
Results and Discussion
Figure 1. Schematic illustration of the preparation process of the DODAB/TiO2 alternating film using the LB technique. as a subphase under the DODAB monolayers. The average dimensions of the TiO2 sheets are 0.75 × ∼1000 × ∼1000 nm3 according to the previous reports of Sasaki et al.10,11,13,14 The schematics of the deposition of the DODAB/TiO2 alternating film are illustrated in Figure 1. A calculated amount of a chloroform solution of DODAB (1.6 × 10-3 mol dm-3) was evenly spread at 20 °C on the surface of the aqueous TiO2 sol in the Langmuir trough using a Hamilton syringe (step 2). The spreading solvent was allowed to evaporate prior to the twodimensional compression. The compression at a velocity of 3000 mm2 min-1 was performed on a Langmuir film balance and monitored by the measurement of surface pressure versus surface area (π-A) isotherms. LB films were prepared by compressing the monolayer at a surface pressure of 30-40 mN m-1 and allowing 60 min for equilibration (step 3). The deposition was carried out by a vertical dipping technique with the film balance and a lifter (step 4). A quartz plate, precoated with trichlorosilane layers to make the surface hydrophobic, was first dipped downward through the monolayer and then raised through it at a rate of 20 mm min-1. There was a 13-min delay at the top of each dipping cycle. The UV-vis absorption spectra of the deposited films were recorded using a Hitachi U-3200 spectrophotometer in the transmission mode. The absorption data for the TiO2-nanosheet hydrosol were obtained after the original suspensions were diluted to obtain an appropriate range of absorbance. X-ray diffraction (XRD) patterns were taken with a Philips X’pert-MRD instrument using Cu KR (λ ) 1.542 Å) irradiation through a Ni filter. The measurement in a θ-2θ scan (14) In situ X-ray diffraction study showed that the layered structure was completely delaminated into single sheets.10
The π-A isotherms of the DODAB monolayers, floating on an aqueous solution of TBAOH (diluted to establish a pH of 11.5) and on a 0.2 g dm-3 TiO2-nanosheet hydrosol are shown in Figure 2. The low collapse pressure (40 mN m-1) is the manifestation of an unstable DODAB monolayer formation on the subphase without the TiO2 nanosheets. On the other hand, a stable DODAB monolayer, with a pronounced condensed phase and a collapse pressure of about 65 mN m-1, was formed on the TiO2 sol. Upon a decrease in the area of the monolayer on the TiO2-nanosheet sol, π smoothly started to rise from zero at about 2.5 nm2 molecule-1. This value was much larger than that for the DODAB monolayer without TiO2 (approximately 0.8 nm2 molecule-1 as shown in curve a). According to many reports by Yamagishi et al. regarding clay-organic complexes on a water surface,15-17 it is reasonable to consider that such an expansion in the monolayer is due to complexation of the TiO2 nanosheets with the ionized ammonium (NR4+) groups of the DODAB monolayer. Obviously, electrostatic interaction should contribute to the immobilization of the negatively charged TiO2 nanosheets. On the basis of this consideration, we can interpret the above results as follows. Each TiO2 nanosheet is completely separated at the air-water interface when a chloroform solution is spread on the surface of the sol subphase. Upon compression, the sheets approach each other and make contact. Under this condition, the surface pressure increases from zero due to the repulsive interactions between the TiO2 sheets at their edges. With further compression, π was saturated at about 25 mN m-1, showing a plateau at a molecular area of 1.2 nm2 molecule-1. After the plateau region was passed, it rapidly rose until the collapse pressure (65 mN m-1). Extrapolation of the condensed regions of the π-A curves to π ) 0 led to a molecular area of 0.56 nm2 molecule-1 for the DODAB monolayer. The observed critical surface area was close to that of the DODAB monolayers on an aqueous 1.0 × 10-3 mol dm-3 AgNO3 solution (0.65 nm2 molecule-1). (15) Inukai, K.; Hotta, Y.; Taniguchi, M.; Tomura, S.; Yamagishi, A. J. Chem. Soc., Chem. Commun. 1994, 959. (16) Tamura, K.; Setsuda, H.; Taniguchi, M.; Takahashi, M.; Yamagishi, A. Clay Sci. 1998, 10, 409. (17) Tamura, K.; Setsuda, H.; Taniguchi, M.; Yamagishi, A. Langmuir 1999, 15, 6915.
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Figure 3. UV-visible absorption spectra of the original colloidal suspension of TiO2 nanosheets (a) and the LB films (11 layers) prepared from it (b).
Thus it can be concluded that the homogeneous TiO2 layer was prepared by complex formation of the DODAB molecules and the TiO2 nanosheets and subsequent compression in the LB trough. The good reproducibility of the π-A isotherms also implies that the TiO2-sheet density spread on the surface is well controlled under the present experimental conditions. This leads to the generation of various packed structures. It is expected that such a floating film should be transferred on substrates without any change in the state on the subphase surface. In general, the efficiency of the LB film formation strongly depends on the monolayer surface pressure;18,19 the present system did not prove to be an exception. The deposition of the LB films is inefficient at surface pressures below 15 mN m-1. However, a transfer ratio of nearly unity was achieved at 40 mN m-1 using hydrophobic substrates. In this case, only the first dipping resulted in the deposition of one TiO2-nanosheet layer sandwiched between the headgroups of two DODAB monolayers, that is, the transfer as Y-type layers. Upon further dipping, however, an effective deposition occurred only when the substrate left the subphase (Z-type transfer) (see Figure 1). During each dipping cycle, the electrostatically attracted TiO2 nanosheets were transferred along with the monolayer and, thus, incorporated between the DODAB layers. Apparently, in isotherm b of Figure 2, a high concentration of the TiO2 nanosheets in the available area under the monolayer can be attained under this condition. Figure 3 shows the absorption spectrum of the deposited LB film together with the absorption data of the original TiO2-nanosheet dispersion used as the subphase. The absorption peak at 268 nm, corresponding to a band gap of 4.68 eV, for the film was significantly blue-shifted with respect to the bulk TiO2 (anatase, 3.18 eV; rutile, 3.03 eV)20 as well as to the parent layered titanate, H0.7Ti1.82500.175O4‚H2O. The large magnitude of the energy shift (>1.5 eV), the origin of which was discussed in a previous study,11 should be attributed to the molecular nature of the delaminated nanosheets and their size quantization.21 (18) Ulman, A. An Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly; Academic Press: San Diego, CA, 1991. (19) Langmuir-Blodgett Films; Roberts, G., Ed.; Plenum Press: New York, 1990. (20) Cox, P. A. Transition Metal Oxides; Oxford Science Publications: New York, 1992; p 105. (21) The fact that the thickness of the nanosheets is comparable to or below the theoretically predicted size of the exciton in TiO2 supports this attribution.11
Figure 4. XRD pattern for the ammonium/TiO2-nanosheet alternating film transferred onto the hydrophobic quartz glass (11 layers).
As can be seen, the position of the absorption peak was found to be invariant between the TiO2 sheets in the sols and in the LB films, although the baseline absorbance increased possibly due to the scattering or reflection of light. The band gap change, ∆Eg, by exciton confinement in anisotropic two-dimensional crystallites is predominantly governed by the thickness of the system, L, according to ∆Eg ∝ L-2.22-24 Thus, this observation indicates that the formation of particulate multilayers did not alter the TiO2 sheet thickness or induce crystallite coalescence. The absorption features in terms of the peak with the sharp onset were not significantly modified by the LB deposition onto the substrate. Furthermore, the peak absorbance of the LB film linearly increased with the number of depositions, substantiating a quantitative and uniform transference. XRD measurements are useful for studying the structure in the perpendicular direction to the plane of LB films. Figure 4 is the XRD pattern of the LB film, which was formed on the quartz glass substrate as a result of 11 deposition cycles, that is, possessing 11 layers of the TiO2 nanosheets between the ammonium molecules. This pattern represented (00l) XRD peaks up to l ) 3, from which the periodic long spacing (d) was calculated to be about 3.4 nm. The d spacing was nearly equal to the sum of the thickness of a single TiO2 layer (0.75 nm) and the molecular height of the ammonium cation which was roughly estimated on the basis of the molecular structure (2.5 nm), being consistent with the Z-type configuration. Conclusions The above results have shown that the hybrid monolayer composed of the nanosized sheet of TiO2 and the ammonium amphiphile molecule was formed at the air-water interface and efficiently transferred layer-by-layer onto the solid substrate using the LB technique. It should be (22) Shinada, M.; Sugano, S. J. Phys. Soc. Jpn. 1966, 21, 1936. (23) Sandroff, C. J.; Hwang, D. M.; Chung, W. M. Phys. Rev. B 1986, 33, 5943. (24) Sandroff, C. J.; Kelty S. P.; Hwang, D. M. J. Chem. Phys. 1986, 85, 2237.
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again emphasized that the TiO2 nanosheets in the film have a uniform ultrathin thickness and unique anisotropic morphology led by the exfoliation of the well-defined layered compound. The preliminary results indicate that the method described in this Letter can be extended to various types of amphiphilic molecules to produce new lamellar organized films. From the standpoint of such a
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versatile material design, further investigations are currently underway. Acknowledgment. The authors gratefully thank H. Fujii and R. Shinohara for their technical assistance. LA0016423
