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Formation of hybrid monolayers and Langmuir–Blodgett-type multilayers from ammonium cations and TiO2 crystalline nanosheets
Thin Solid Films 393 Ž2001. 154᎐160
Formation of hybrid monolayers and Langmuir᎐Blodgett-type multilayers from ammonium cations and TiO 2 crystalline nanosheets T. Yamaki a,U , R. Shinoharab , K. Asai b a
Department of Materials De¨ elopment, Takasaki Radiation Chemistry Research Establishment, Japan Atomic Energy Research Institute (JAERI), 1233 Watanuki, Takasaki, Gunma 370-1292, Japan b Department of Quantum Engineering and Systems Science, Graduate School of Engineering, The Uni¨ ersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
Abstract A novel hybrid monolayer composed of ammonium cations and TiO 2 crystalline nanosheets was formed at an air᎐water interface. We employed, as the subphase under the monolayer, stable colloidal suspensions obtained by exfoliating a lepidocrocite-type layered titanate, H xTi 2 ᎐ x r4 I x r4 O4 ⭈ H 2 O Ž x; 0.7; I, vacancy. into elementary host layers. Surface pressure-area isotherms demonstrated that spreading molecules of dioctadecyldimethylammonium bromide ŽDODAB. on the suspensions resulted in complexation of DODAB with the exfoliated single layers of quasi-TiO 2 . The hybrid monolayer was quantitatively transferred onto solid substrates by the Langmuir᎐Blodgett ŽLB. technique to form a multilayer film. UV-Visible absorption spectra and X-ray diffraction measurements confirmed that the film possessed a DODABrTiO2 alternating structure with a repeating unit of 3.4 nm. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Langmuir᎐Blodgett films ŽLB films.; Monolayers; Multilayers; Nanostructures; Quantum effects; Titanium oxide
1. Introduction The well-ordered arrangement of semiconductor fine crystallites is an essential criterion in the construction of nanostructured thin films. Currently, available technologies are based on ‘wet’ colloid chemical approaches, which have been partly accomplished by the use of membrane mimetic templates w1,2x. Fendler et al. w3x and Ozin w4x reported the generation and characterization of monoparticulate layers of CdS, ZnS and PbSe nanoparticles on aqueous solution surfaces in a Langmuir film balance. Furthermore, these monoparticulate nanocrystalline semiconducting films could be transferred, layer-by-layer, onto solid substrates by the U
Corresponding author. Tel.: q81-27-346-9422; fax: q81-27-3469690. E-mail address: [email protected] ŽT. Yamaki..
Langmuir᎐Blodgett ŽLB. technique. Their approach is different from forming thin films by the incorporation of oxide precursors between the headgroups of LB films and subsequently, destroying the surfactants Žin the LB film. by heat treatment w5,6x. Among a diverse range of semiconductors, titanium dioxide ŽTiO 2 . is attractive because of its high photovoltaic activities and chemical stability. In particular, small TiO 2 particles in the range of nanometers have been intensively studied for device applications with a low-cost, high-efficiency solar cell w7x. However, there have been only a few reports on the deposition of organicrTiO 2 multilayers by the LB method, allowing a rational control of the ultra-thin film structure w8,9x. This is largely due to difficulties in the preparation of high-quality TiO 2 particles in aqueous or organic sols, which are supposed to be used as the nanocrystalline source. Most of the TiO 2 crystallites are synthesized by
0040-6090r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 1 . 0 1 0 6 3 - X
T. Yamaki et al. r Thin Solid Films 393 (2001) 154᎐160
growth from molecular precursors, e.g. the hydrolysis of titanium salts. In such preparations of small particles, it is not easy to control the chemical composition, crystal structure and crystallinity as well as the particle shape and dimensions. The products are often poorly crystalline and contain a substantial amount of amorphous components. They are generally of spherical particles where the distribution of their size is inevitably large. The formation of novel hybrid monolayers composed of TiO 2 nanocrystallites and cationic dioctadecyldimethylammonium bromide ŽDODAB. at an air᎐water interface and their layer-by-layer transfer onto solid supports resulting in ordered nanostructured films are the subjects of the present paper. The threestep process is schematically shown in Fig. 1. Stable colloidal suspensions Žhydrosols. were synthesized by exfoliating a titanate with a lepidocrocite-like layered structure Žstep 1. and were then employed as a subphase on which the DODAB monolayer was spread Žsteps 2 and 3.. The colloids should be regarded as semiconductor nanosheets of quasi-TiO 2 . The TiO 2 nanosheets, exfoliated elementary fragments of the well-crystallized layered compound synthesized at high temperatures, have a well-defined composition, structure and uniform ultra-thin thickness w10᎐12x. Such anisotropic morphology has not yet been accomplished in the TiO 2 system. Therefore, nanostructures constructed from them are of interest for both fundamental studies and a host of applications.
155
2. Experimental details 2.1. Materials Titanium dioxide Žrutile form. and cesium carbonate Ž99.99% purity, Furuuchi Kagaku, Co., Ltd.. were used as supplied. All the other chemicals were of reagent grade. Milli-Q filtered water ŽMillipore Co., ) 18 M⍀ ⭈ cmy1 . was used throughout the experiments. 2.2. Synthesis of colloidal single-sheet suspensions (step 1 in Fig. 1) A cesium titanate of composition Cs xTi 2 ᎐ x r4 I x r4 O4 Ž x; 0.7, I, vacancy. was prepared by a conventional solid-state procedure w12,13x. An intimate mixture of Cs 2 CO 3 and TiO 2 Ž1:5.3 in molar ratio. was placed in a Pt crucible with a lid and was heated at 1073 K for 1 h to be decarbonated. The mixture was ground and then subjected to two cycles of calcination Ž1073 K, 20 h. with grinding at intervals. A polycrystalline powder of the Cs titanate was converted into the protonic form by being stirred in 1 mol dmy3 HCl solution at ambient temperature w12x. The solution-to-solid ratio was 40 cm3 gy1 . The solution was replaced every 24 h, which was repeated four times. Then acid residue was removed by washing the solid product with water until the pH value of the super-
Fig. 1. Schematic illustration of the fabrication process of the DODABrTiO2 hybrid monolayer and its multilayer film.
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T. Yamaki et al. r Thin Solid Films 393 (2001) 154᎐160
natant exceeded 5. The resulting protonic titanate, H xTi 2 ᎐ x r4 I x r4 O4 ⭈ H 2 O, was dried over a saturated NaCl solution to a constant weight. A weighed amount Ž4 g. of H xTi 2 ᎐ x r4 I x r4O4 ⭈ H 2 O was interacted with an aqueous 0.0825 mol dmy3 solution Ž1 dm 3 . of tetrabutylammonium hydroxide ŽTBAOH., ŽC 4 H 9 .4 NOH. The mixture was vigorously shaken at approximately 298 K for more than 2 weeks, which produced a translucent colloidal suspension. The amount of TBAOH corresponds to a five-fold excess re la tive to th e e xch a n ge a b le proton s in H xTi 2 ᎐ x r4 I x r4 O4 ⭈ H 2 O. The resulting dispersion, whose concentration was adjusted to 0.01᎐0.2 g dmy3 , was used as the subphase under the DODAB monolayers. In situ X-ray diffraction study in the previous reports w10᎐12x showed that the layered structure was completely delaminated into single sheets and that their average dimensions were 0.75= ; 1000 = ; 1000 nm3.
˚ . irradiation at 40 kV and 30 mA. The graded s 1.542 A parabolic X-ray mirror in the system can convert the divergent radiation to a quasi-parallel beam. The sizes of the divergence and receiving slits were 0.5⬚ and 0.05 mm, respectively. The measurement in a -2 scan mode was carried out over an angular range of 1.5᎐10.0⬚ at 0.005⬚ intervals in 2. To estimate the lattice distortion of the present films, the integral breadth, , was obtained without considering geometrical factors and instrumental broadening. Such an approximation would be valid when it is used to analyze the data for the poorly ordered films where the fluctuation of the layer structure is very large Žsee Section 3.. Note that the unit of  was easily converted from radians to reciprocal nanometers by the relation:
2.3. Preparation of hybrid monolayers at an air᎐water interface (step 2)
3. Results and discussion
A calculated amount of a chloroform solution of DODAB Ž1.6= 10y3 mol dmy3 . was spread at 293 K on the surface of an aqueous TBAOH solution or the TiO 2 hydrosol in the Langmuir trough using a Hamilton syringe. The spreading solvent was allowed to evaporate prior to the two-dimensional compression. The compression at a velocity of 3000 mm2 miny1 was performed on a Langmuir film balance and monitored by the measurement of surface-pressure vs. surface area Ž -A. isotherms. 2.4. LB film deposition (step 3) LB films were prepared by compressing the monolayer at a surface pressure of 30᎐40 mN my1 and allowing 60 min for equilibration. The deposition was carried out by a vertical dipping technique with the film balance and a lifter. A quartz plate, pre-coated with trichlorosilane layers to make the surface hydrophobic, was immersed and subsequently extracted from the subphase at a rate of 20 mm miny1 . There was a 9᎐13-min delay at the top of each dipping cycle.
 Ž rad . s  Ž nmy1 . =
. cos
Ž1.
3.1. Monolayer beha¨ ior at the air᎐water interface monitored by -A isotherms The curve Ža. in Fig. 2 shows the -A isotherm of the DODAB monolayers, floating on an aqueous solution of TBAOH without the TiO 2 nanosheets. The low collapse pressure Ž40 mN my1 . is a manifestation of an unstable DODAB monolayer formation on the subphase without the TiO 2 nanosheets. On the other hand, as shown in curve Žb., a stable DODAB monolayer, with a pronounced condensed phase and a collapse pressure of approximately 65 mN my1 , was formed on a 0.2 g dmy3 TiO 2-nanosheet sol. Upon a decrease in the area of the monolayer on the TiO 2-nanosheet sol, smoothly started to rise from zero at approximately 2.5 nm2 moleculey1 . This value
2.5. Instrumental analysis The UV-visible absorption spectra of the films were recorded using a Hitachi U-3200 spectrophotometer in the transmission mode. The absorption data for the TiO 2-nanosheet colloidal solution were also obtained after the original suspensions were diluted to obtain an appropriate range of absorbance. X-Ray diffraction ŽXRD. patterns were measured by a Philips X’pertMRD instrument with Ni-monochromatized Cu K ␣ Ž
Fig. 2. Surface pressure-area Ž -A. isotherms at 293 K when a chloroform solution of DODAB was spread on Ža. an aqueous solution of TBAOH and Žb. the TiO 2-nanosheet sol.
T. Yamaki et al. r Thin Solid Films 393 (2001) 154᎐160
was much larger than that for the DODAB monolayer without TiO 2 wapprox. 0.8 nm2 moleculey1 as shown in curve Ža.x. According to previous studies by Yamagishi et al. regarding clay-organic complexes on a water surface w14᎐16x, such an expansion in the monolayer supports complexation of the TiO 2 nanosheets with the . groups of the DODAB ionized ammonium ŽNRq 4 monolayer. Obviously, electrostatic interaction should contribute to the immobilization of the negatively charged TiO 2 nanosheets. Based on this consideration, we can interpret the above results as follows. Each TiO 2 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 due to the repulsive interactions between the TiO 2 sheets at their edges. With further compression, was saturated at approximately 25 mN my1 , showing a plateau at a molecular area of 1.2 nm2 moleculey1 . After passing the plateau region, rapidly rose up to the collapse pressure Ž65 mN my1 .. Extrapolation of the condensed regions of the -A curves to s 0 led to a molecular area of 0.56 nm2 moleculey1 for the DODAB monolayer. The observed critical surface area was close to that of the DODAB monolayers on an aqueous 1.0= 10y3 mol dmy3 AgNO3 solution Ž0.65 nm2 moleculey1 .. Thus, it can be concluded that the homogeneous TiO 2 layer was prepared by complex formation of the DODAB molecules and the TiO 2 nanosheets and subsequent compression in the LB trough. The good reproducibility of the -A isotherms also implies that the TiO 2-sheet density on the surface is well controlled under the present experimental conditions. This leads to the generation of various packed structures.
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however, an effective deposition occurred only when the substrate left the subphase Ž Z-type transfer . Žsee Fig. 1.. During each dipping cycle, the electrostatically attracted TiO 2 nanosheets were transferred along with the monolayer and were thus incorporated between the DODAB layers. Apparently, in the isotherm of Fig. 2b, a high concentration of the TiO 2 nanosheets in the available area under the monolayer can be attained under this condition. Fig. 3 depicts the room-temperature absorption spectrum of the deposited LB film together with the absorption data of the TiO 2-nanosheet dispersion used as the subphase. Both the data are characterized by a well-developed peak near the absorption onset and a pronounced spectral blue shift. The former feature, i.e. the absorption maximum combined with the steep rise of the threshold is in contrast to the data reported for small-sized TiO 2 semiconductors, which are featureless with a rather gentle onset. The absorption peak at 268 nm, corresponding to a photon energy of 4.68 eV, for the film was significantly blue-shifted with respect to the bulk TiO 2 Žanatase, 3.18 eV, rutile, 3.03 eV. w19x as well as to the parent layered titanate, H 0.7 Ti 1.825 I 0.175 O4 ⭈ H 2 O. The energy shift is very large in comparison with values for the small particles of TiO 2 w20,21x. The origin of such a spectral change, as discussed in a previous study w11x, should be attributed to the size-quantization effect. The fact that the thickness of the nanosheets is comparable to or below the theoretically predicted size of the exciton in TiO 2 Ž0.75᎐1.90 nm. w22x supports this attribution. The band-gap change, ⌬ Eg , by quantum confinement in anisotropic two-dimensional crystallites is given as follows w23᎐25x:
3.2. Properties of multilayers prepared by the LB technique
It is expected that such a floating film could 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 w17,18x, the present system did not prove to be an exception. The deposition of the LB films is inefficient at surface pressures below 15 mN my1 . However, a transfer ratio of near unity was achieved at 40 mN my1 using hydrophobic substrates. In this case, only the first dipping resulted in the deposition of one TiO 2-nanosheet layer sandwiched between the headgroups of two DODAB monolayers, i.e. the transfer as Y-type layers. Upon further dipping,
Fig. 3. UV-visible absorption spectra of Ža. the original colloidal suspension of TiO 2 nanosheets and Žb. the LB films Ž11 layers. prepared from it on a quartz substrate. The data obtained for the protonic layered titanate H 0.7 Ti 1.825 I 0.175 O4 ⭈ H 2 O in a diffuse reflectance mode is denoted by a dashed line.
T. Yamaki et al. r Thin Solid Films 393 (2001) 154᎐160
158
⌬ Eg s
h2 8 x y
ž
1 1 h2 q q L2x L2y 8 z L2z
/
Ž2.
where h is Plank’s constant, x y and z are the reduced mass of electron-hole pairs, and L x , L y and L z are crystalline dimensions. In Eq. Ž2., the suffixes x, y and z express parallel and perpendicular directions with respect to the sheet, respectively. Because L x , L y 4 L z for the present nanosheet system Ž L z ; 0.75 nm., the first term can be ignored. Consequently, the blue shift in the spectra is predominantly governed by the sheet thickness. As can be seen in Fig. 3, the position of the absorption peak was found to be invariant between the TiO 2 sheets in the sols and in the LB films, although the baseline absorbance increased possibly due to the scattering or reflection of light. This observation confirms that the formation of particulate multilayers did not alter the TiO 2 sheet thickness or induce crystallite coalescence. The absorption features in terms of the peak with the sharp onset, possibly due to the molecular nature of the delaminated nanosheets, were not significantly modified by the LB deposition onto the substrate. Fig. 4 shows the absorption spectra of the LB film with a different number of layers on both sides of the quartz substrate. The peak absorbance of the LB film linearly increased with the number of depositions while the peak-top energy was unchanged. This substantiates a quantitative and uniform transference. XRD measurements are useful for studying the structure in the perpendicular direction to the plane of LB films. Fig. 5 is the XRD pattern of the LB film, which was formed on the quartz glass substrate as a result of 11 deposition cycles, i.e. possessing 11 layers
Fig. 4. Absorption spectra of the LB films possessing Ža. 2, Žb. 4 and Žc. 11 layers of TiO 2 nanosheets between the ammonium molecules. The inset shows the peak absorbance at 268 nm as a function of the number of the TiO 2 -nanosheet layers in the film.
Fig. 5. XRD pattern for the ammoniumrTiO2 -nanosheet alternating film transferred onto the hydrophobic quartz glass Ž11 layers..
of the TiO 2 nanosheets between the ammonium molecules. In this pattern, we recognized Ž00 l . XRD peaks up to l s 3. The periodic long spacing Ž d . was calculated to be approximately 3.4 nm from the position of these peaks. The d spacing was nearly equal to the sum of the thickness of a single TiO 2 layer Ž0.75 nm. and the molecular height of the ammonium cation which was estimated on the basis of the molecular structure Ž2.5 nm., consistent with the Z-type configuration. The XRD pattern of Fig. 5, exhibiting only lowerorder reflections, is markedly different from that of LB films of popular fatty acid salts. Such a decay in the peak intensity implies a large fluctuation of the average distance between the adjacent layers. The lattice distortion of the obtained multilayer is then roughly evaluated by the theoretical model in the following. When, as in the present case, the contribution of the geometrical factors and instrumental resolution to peak broadening is relatively small enough to be ignored, only the crystalline size and lattice distortion effects are considered to be convoluted into a diffraction peak profile w26,27x. According to the microstrain model w28,29x, line breadths of the reflections were separated into broadening due to each effect. If the profiles are Cauthy, the integral breadth  Žconverted to reciprocal nanometer units using Eq. Ž1.. can be given as:
s
Ž 2 ² 2 :. 1 q L001 d0
1r2
l
Ž3.
T. Yamaki et al. r Thin Solid Films 393 (2001) 154᎐160
159
where L001 is the crystalline size normal to the Ž001. plane, ² 2 : denotes the mean-square lattice distortion and d 0 is the length of the repeat unit in the w00 l x direction. The first term represents the broadening originating from the crystalline size, which is given by the familiar Scherrer equation. The second term is expressed to relate broadening to distortion parameters regarding 00 l reflections Žcalled ‘microstrain broadening’.. As clearly shown in Eq. Ž3., because the distortion broadening depends on the order of the reflection in contrast to the size broadening, it becomes possible to distinguish between crystalline size and lattice distortion effects. It should be noted here that the crystalline size L001 can also be calculated by:
monium alternating film with a repeating unit of 3.4 nm, although the lattice distortion in the film was found to be large in comparison to LB films of popular fatty acid salts. Emphasis should be given to the fact that the TiO 2 nanosheets in the film have a uniform ultra-thin thickness and unique anisotropic morphology resulting from the exfoliation of the layered compound H xTi 2 ᎐ x r4 I x r4 O4 ⭈ H 2 O. To the best of our knowledge, the present paper represents the first description of monoparticulate layer and multilayer preparation from such the well-defined TiO 2 nanocrystallites.
L001 s Nd 0
w1x J.H. Fendler, Springer-Verlag, BerlinAdvances in Polymer Science Series, Membrane-Mimetic Approach to Advanced Materials, 113, 1994. w2x N.A. Kotov, F.C. Meldrum, Adv. Mater. 7 Ž1995. 607. w3x J.H. Fendler, in: A. Ulman ŽEd.., Thin Films, Organic Thin Films and Surfaces: Directions for the Nineties, 20, Academic Press, San Diego, New York, Boston, 1995. w4x G. Ozin, Adv. Mater. 4 Ž1993. 612. w5x D.V. Paranjape, M. Sastry, P. Ganguly, Appl. Phys. Lett. 63 Ž1993. 18. w6x D.T. Amm, D.J. Johnson, T. Laursen, S.K. Gupta, Appl. Phys. Lett. 61 Ž1992. 522. w7x A. Kay, R.H. Baker, M. Gratzel, J. Phys. Chem. 97 Ž1993. ¨ 10773. w8x N.A. Kotov, F.C. Meldrum, C. Wu, J.H. Fendler, J. Phys. Chem. 98 Ž1994. 2735. w9x L. Li, Y. Chen, S. Kan, X. Zhang, X. Peng, M. Liu, T. Li, Thin Solid Films 284r285 Ž1996. 592. w10x T. Sasaki, M. Watanabe, H. Hashizume, H. Yamada, H. Nakazawa, J. Am. Chem. Soc. 118 Ž1996. 8329. w11x T. Sasaki, M. Watanabe, J. Phys. Chem. B 101 Ž1997. 10159. w12x T. Sasaki, M. Watanabe, Y. Michiue, Y. Komatsu, F. Izumi, S. Takenouchi, Chem. Mater. 7 Ž1995. 1001. w13x I.E. Grey, C. Li, I.C. Madsen, J.A. Watts, J. Solid State Chem. 66 Ž1987. 7. w14x K. Inukai, Y. Hotta, M. Taniguchi, S. Tomura, A. Yamagishi, J. Chem. Soc. Chem. Commun., Ž1994. 959. w15x K. Tamura, H. Setsuda, M. Taniguchi, M. Takahashi, A. Yamagishi, Clay Sci. 10 Ž1998. 409. w16x K. Tamura, H. Setsuda, M. Taniguchi, A. Yamagishi, Langmuir 15 Ž1999. 6915. w17x A. Ulman, An Introduction to Ultrathin Organic Films from Langmuir᎐Blodgett to Self-Assembly, Academic Press, San Diego, 1991. w18x G. Roberts ŽEd.., Langmuir᎐Blodgett Films, Plenum Press, New York, 1990. w19x P.A. Cox, Transition Metal Oxides, Oxford Science Publications, New York, 1992. w20x C. Kormann, D.W. Bahnemann, M.R. Hoffmann, J. Phys. Chem. 92 Ž1988. 5196. w21x L. Kavan, T. Stoto, M. Gratzel, D. Fitzmaurice, V. Shklover, J. ¨ Phys. Chem. 97 Ž1993. 9493. w22x C. Kormann, D.W. Bahnemann, M.R. Hoffmann, J. Phys. Chem. 92 Ž1988. 5196. w23x M. Shinada, S. Sugano, J. Phys. Soc. Jpn. 21 Ž1966. 1936. w24x C.J. Sandroff, D.M. Hwang, W.M. Chung, Phys. Rev. B 33 Ž1986. 5943.
Ž4.
where N is the number of unit cells perpendicular to the Ž001. plane. For LB films, N can be exactly determined from the number of dipping movements or of layers in a unit cell. The value of the observed integral breadth for Ž001. reflection at 2 s 2.7⬚ is 0.049 nmy1 . By inserting N s 12 Žconsidering the first dipping as a Y-type transfer ., d 0 s 3.4 nm and l s 1 into Eqs. Ž3. and Ž4., the value of ² 2 :1r2 can then be calculated to be 3.4%. Sasanuma et al. w30x in a study of quantitative estimation of the lattice distortion of the LB multilayers, estimated ² 2 :1r2 to be 0.09᎐0.90% for the 25-layer LB films of lead stearate on a glass substrate Ž Y-type layers. using the same model and equations. As they also pointed out, the distortion of the film represented by ² 2 : was very sensitive to the conditions of sample preparation, which determined the monolayer structure on the subphase surface and the texture of the built-up multilayers. Thus, the large lattice distortion in the present samples possibly results from the dense packing of the TiO 2 nanosheets between the organic ammonium layers. The method described in this paper can be extended to various types of amphiphilic molecules to produce new lamellar organized films. It is expected, therefore, that appropriate choice of amphiphilic compounds allows the orderly structure of LB films to be improved. 4. Conclusions A hybrid monolayer composed of a nanosized sheet of quasi-TiO 2 and an ammonium amphiphile molecule was prepared at an air᎐water interface and transferred layer-by-layer onto a solid substrate using the LB technique. UV-Visible absorption spectra and XRD measurements clarified the formation of a TiO 2ram-
References
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w25x C.J. Sandroff, S.P. Kelty, D.M. Hwang, J. Chem. Phys. 85 Ž1986. 2237. w26x L.E. Alexander, X-Ray Diffraction Methods in Polymer Science, Wiley, New York, 1969. w27x H.P. Klug, L.E. Alexander, X-Ray Diffraction Procedures for Polycrystalline and Amorphous Materials, Wiley, New York, 1974.
w28x B.E. Warren, B.L. Aberbach, J. Appl. Phys. 21 Ž1950. 595. w29x B.E. Warren, Prog. Met. Phys. 8 Ž1959. 147. w30x Y. Sasanuma, Y. Kitano, A. Ishitani, H. Nakahara, K. Fukuda, Thin Solid Films 190 Ž1990. 325.
Formation of hybrid monolayers and Langmuir᎐Blodgett-type multilayers from ammonium cations and TiO 2 crystalline nanosheets T. Yamaki a,U , R. Shinoharab , K. Asai b a
Department of Materials De¨ elopment, Takasaki Radiation Chemistry Research Establishment, Japan Atomic Energy Research Institute (JAERI), 1233 Watanuki, Takasaki, Gunma 370-1292, Japan b Department of Quantum Engineering and Systems Science, Graduate School of Engineering, The Uni¨ ersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
Abstract A novel hybrid monolayer composed of ammonium cations and TiO 2 crystalline nanosheets was formed at an air᎐water interface. We employed, as the subphase under the monolayer, stable colloidal suspensions obtained by exfoliating a lepidocrocite-type layered titanate, H xTi 2 ᎐ x r4 I x r4 O4 ⭈ H 2 O Ž x; 0.7; I, vacancy. into elementary host layers. Surface pressure-area isotherms demonstrated that spreading molecules of dioctadecyldimethylammonium bromide ŽDODAB. on the suspensions resulted in complexation of DODAB with the exfoliated single layers of quasi-TiO 2 . The hybrid monolayer was quantitatively transferred onto solid substrates by the Langmuir᎐Blodgett ŽLB. technique to form a multilayer film. UV-Visible absorption spectra and X-ray diffraction measurements confirmed that the film possessed a DODABrTiO2 alternating structure with a repeating unit of 3.4 nm. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Langmuir᎐Blodgett films ŽLB films.; Monolayers; Multilayers; Nanostructures; Quantum effects; Titanium oxide
1. Introduction The well-ordered arrangement of semiconductor fine crystallites is an essential criterion in the construction of nanostructured thin films. Currently, available technologies are based on ‘wet’ colloid chemical approaches, which have been partly accomplished by the use of membrane mimetic templates w1,2x. Fendler et al. w3x and Ozin w4x reported the generation and characterization of monoparticulate layers of CdS, ZnS and PbSe nanoparticles on aqueous solution surfaces in a Langmuir film balance. Furthermore, these monoparticulate nanocrystalline semiconducting films could be transferred, layer-by-layer, onto solid substrates by the U
Corresponding author. Tel.: q81-27-346-9422; fax: q81-27-3469690. E-mail address: [email protected] ŽT. Yamaki..
Langmuir᎐Blodgett ŽLB. technique. Their approach is different from forming thin films by the incorporation of oxide precursors between the headgroups of LB films and subsequently, destroying the surfactants Žin the LB film. by heat treatment w5,6x. Among a diverse range of semiconductors, titanium dioxide ŽTiO 2 . is attractive because of its high photovoltaic activities and chemical stability. In particular, small TiO 2 particles in the range of nanometers have been intensively studied for device applications with a low-cost, high-efficiency solar cell w7x. However, there have been only a few reports on the deposition of organicrTiO 2 multilayers by the LB method, allowing a rational control of the ultra-thin film structure w8,9x. This is largely due to difficulties in the preparation of high-quality TiO 2 particles in aqueous or organic sols, which are supposed to be used as the nanocrystalline source. Most of the TiO 2 crystallites are synthesized by
0040-6090r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 1 . 0 1 0 6 3 - X
T. Yamaki et al. r Thin Solid Films 393 (2001) 154᎐160
growth from molecular precursors, e.g. the hydrolysis of titanium salts. In such preparations of small particles, it is not easy to control the chemical composition, crystal structure and crystallinity as well as the particle shape and dimensions. The products are often poorly crystalline and contain a substantial amount of amorphous components. They are generally of spherical particles where the distribution of their size is inevitably large. The formation of novel hybrid monolayers composed of TiO 2 nanocrystallites and cationic dioctadecyldimethylammonium bromide ŽDODAB. at an air᎐water interface and their layer-by-layer transfer onto solid supports resulting in ordered nanostructured films are the subjects of the present paper. The threestep process is schematically shown in Fig. 1. Stable colloidal suspensions Žhydrosols. were synthesized by exfoliating a titanate with a lepidocrocite-like layered structure Žstep 1. and were then employed as a subphase on which the DODAB monolayer was spread Žsteps 2 and 3.. The colloids should be regarded as semiconductor nanosheets of quasi-TiO 2 . The TiO 2 nanosheets, exfoliated elementary fragments of the well-crystallized layered compound synthesized at high temperatures, have a well-defined composition, structure and uniform ultra-thin thickness w10᎐12x. Such anisotropic morphology has not yet been accomplished in the TiO 2 system. Therefore, nanostructures constructed from them are of interest for both fundamental studies and a host of applications.
155
2. Experimental details 2.1. Materials Titanium dioxide Žrutile form. and cesium carbonate Ž99.99% purity, Furuuchi Kagaku, Co., Ltd.. were used as supplied. All the other chemicals were of reagent grade. Milli-Q filtered water ŽMillipore Co., ) 18 M⍀ ⭈ cmy1 . was used throughout the experiments. 2.2. Synthesis of colloidal single-sheet suspensions (step 1 in Fig. 1) A cesium titanate of composition Cs xTi 2 ᎐ x r4 I x r4 O4 Ž x; 0.7, I, vacancy. was prepared by a conventional solid-state procedure w12,13x. An intimate mixture of Cs 2 CO 3 and TiO 2 Ž1:5.3 in molar ratio. was placed in a Pt crucible with a lid and was heated at 1073 K for 1 h to be decarbonated. The mixture was ground and then subjected to two cycles of calcination Ž1073 K, 20 h. with grinding at intervals. A polycrystalline powder of the Cs titanate was converted into the protonic form by being stirred in 1 mol dmy3 HCl solution at ambient temperature w12x. The solution-to-solid ratio was 40 cm3 gy1 . The solution was replaced every 24 h, which was repeated four times. Then acid residue was removed by washing the solid product with water until the pH value of the super-
Fig. 1. Schematic illustration of the fabrication process of the DODABrTiO2 hybrid monolayer and its multilayer film.
156
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natant exceeded 5. The resulting protonic titanate, H xTi 2 ᎐ x r4 I x r4 O4 ⭈ H 2 O, was dried over a saturated NaCl solution to a constant weight. A weighed amount Ž4 g. of H xTi 2 ᎐ x r4 I x r4O4 ⭈ H 2 O was interacted with an aqueous 0.0825 mol dmy3 solution Ž1 dm 3 . of tetrabutylammonium hydroxide ŽTBAOH., ŽC 4 H 9 .4 NOH. The mixture was vigorously shaken at approximately 298 K for more than 2 weeks, which produced a translucent colloidal suspension. The amount of TBAOH corresponds to a five-fold excess re la tive to th e e xch a n ge a b le proton s in H xTi 2 ᎐ x r4 I x r4 O4 ⭈ H 2 O. The resulting dispersion, whose concentration was adjusted to 0.01᎐0.2 g dmy3 , was used as the subphase under the DODAB monolayers. In situ X-ray diffraction study in the previous reports w10᎐12x showed that the layered structure was completely delaminated into single sheets and that their average dimensions were 0.75= ; 1000 = ; 1000 nm3.
˚ . irradiation at 40 kV and 30 mA. The graded s 1.542 A parabolic X-ray mirror in the system can convert the divergent radiation to a quasi-parallel beam. The sizes of the divergence and receiving slits were 0.5⬚ and 0.05 mm, respectively. The measurement in a -2 scan mode was carried out over an angular range of 1.5᎐10.0⬚ at 0.005⬚ intervals in 2. To estimate the lattice distortion of the present films, the integral breadth, , was obtained without considering geometrical factors and instrumental broadening. Such an approximation would be valid when it is used to analyze the data for the poorly ordered films where the fluctuation of the layer structure is very large Žsee Section 3.. Note that the unit of  was easily converted from radians to reciprocal nanometers by the relation:
2.3. Preparation of hybrid monolayers at an air᎐water interface (step 2)
3. Results and discussion
A calculated amount of a chloroform solution of DODAB Ž1.6= 10y3 mol dmy3 . was spread at 293 K on the surface of an aqueous TBAOH solution or the TiO 2 hydrosol in the Langmuir trough using a Hamilton syringe. The spreading solvent was allowed to evaporate prior to the two-dimensional compression. The compression at a velocity of 3000 mm2 miny1 was performed on a Langmuir film balance and monitored by the measurement of surface-pressure vs. surface area Ž -A. isotherms. 2.4. LB film deposition (step 3) LB films were prepared by compressing the monolayer at a surface pressure of 30᎐40 mN my1 and allowing 60 min for equilibration. The deposition was carried out by a vertical dipping technique with the film balance and a lifter. A quartz plate, pre-coated with trichlorosilane layers to make the surface hydrophobic, was immersed and subsequently extracted from the subphase at a rate of 20 mm miny1 . There was a 9᎐13-min delay at the top of each dipping cycle.
 Ž rad . s  Ž nmy1 . =
. cos
Ž1.
3.1. Monolayer beha¨ ior at the air᎐water interface monitored by -A isotherms The curve Ža. in Fig. 2 shows the -A isotherm of the DODAB monolayers, floating on an aqueous solution of TBAOH without the TiO 2 nanosheets. The low collapse pressure Ž40 mN my1 . is a manifestation of an unstable DODAB monolayer formation on the subphase without the TiO 2 nanosheets. On the other hand, as shown in curve Žb., a stable DODAB monolayer, with a pronounced condensed phase and a collapse pressure of approximately 65 mN my1 , was formed on a 0.2 g dmy3 TiO 2-nanosheet sol. Upon a decrease in the area of the monolayer on the TiO 2-nanosheet sol, smoothly started to rise from zero at approximately 2.5 nm2 moleculey1 . This value
2.5. Instrumental analysis The UV-visible absorption spectra of the films were recorded using a Hitachi U-3200 spectrophotometer in the transmission mode. The absorption data for the TiO 2-nanosheet colloidal solution were also obtained after the original suspensions were diluted to obtain an appropriate range of absorbance. X-Ray diffraction ŽXRD. patterns were measured by a Philips X’pertMRD instrument with Ni-monochromatized Cu K ␣ Ž
Fig. 2. Surface pressure-area Ž -A. isotherms at 293 K when a chloroform solution of DODAB was spread on Ža. an aqueous solution of TBAOH and Žb. the TiO 2-nanosheet sol.
T. Yamaki et al. r Thin Solid Films 393 (2001) 154᎐160
was much larger than that for the DODAB monolayer without TiO 2 wapprox. 0.8 nm2 moleculey1 as shown in curve Ža.x. According to previous studies by Yamagishi et al. regarding clay-organic complexes on a water surface w14᎐16x, such an expansion in the monolayer supports complexation of the TiO 2 nanosheets with the . groups of the DODAB ionized ammonium ŽNRq 4 monolayer. Obviously, electrostatic interaction should contribute to the immobilization of the negatively charged TiO 2 nanosheets. Based on this consideration, we can interpret the above results as follows. Each TiO 2 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 due to the repulsive interactions between the TiO 2 sheets at their edges. With further compression, was saturated at approximately 25 mN my1 , showing a plateau at a molecular area of 1.2 nm2 moleculey1 . After passing the plateau region, rapidly rose up to the collapse pressure Ž65 mN my1 .. Extrapolation of the condensed regions of the -A curves to s 0 led to a molecular area of 0.56 nm2 moleculey1 for the DODAB monolayer. The observed critical surface area was close to that of the DODAB monolayers on an aqueous 1.0= 10y3 mol dmy3 AgNO3 solution Ž0.65 nm2 moleculey1 .. Thus, it can be concluded that the homogeneous TiO 2 layer was prepared by complex formation of the DODAB molecules and the TiO 2 nanosheets and subsequent compression in the LB trough. The good reproducibility of the -A isotherms also implies that the TiO 2-sheet density on the surface is well controlled under the present experimental conditions. This leads to the generation of various packed structures.
157
however, an effective deposition occurred only when the substrate left the subphase Ž Z-type transfer . Žsee Fig. 1.. During each dipping cycle, the electrostatically attracted TiO 2 nanosheets were transferred along with the monolayer and were thus incorporated between the DODAB layers. Apparently, in the isotherm of Fig. 2b, a high concentration of the TiO 2 nanosheets in the available area under the monolayer can be attained under this condition. Fig. 3 depicts the room-temperature absorption spectrum of the deposited LB film together with the absorption data of the TiO 2-nanosheet dispersion used as the subphase. Both the data are characterized by a well-developed peak near the absorption onset and a pronounced spectral blue shift. The former feature, i.e. the absorption maximum combined with the steep rise of the threshold is in contrast to the data reported for small-sized TiO 2 semiconductors, which are featureless with a rather gentle onset. The absorption peak at 268 nm, corresponding to a photon energy of 4.68 eV, for the film was significantly blue-shifted with respect to the bulk TiO 2 Žanatase, 3.18 eV, rutile, 3.03 eV. w19x as well as to the parent layered titanate, H 0.7 Ti 1.825 I 0.175 O4 ⭈ H 2 O. The energy shift is very large in comparison with values for the small particles of TiO 2 w20,21x. The origin of such a spectral change, as discussed in a previous study w11x, should be attributed to the size-quantization effect. The fact that the thickness of the nanosheets is comparable to or below the theoretically predicted size of the exciton in TiO 2 Ž0.75᎐1.90 nm. w22x supports this attribution. The band-gap change, ⌬ Eg , by quantum confinement in anisotropic two-dimensional crystallites is given as follows w23᎐25x:
3.2. Properties of multilayers prepared by the LB technique
It is expected that such a floating film could 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 w17,18x, the present system did not prove to be an exception. The deposition of the LB films is inefficient at surface pressures below 15 mN my1 . However, a transfer ratio of near unity was achieved at 40 mN my1 using hydrophobic substrates. In this case, only the first dipping resulted in the deposition of one TiO 2-nanosheet layer sandwiched between the headgroups of two DODAB monolayers, i.e. the transfer as Y-type layers. Upon further dipping,
Fig. 3. UV-visible absorption spectra of Ža. the original colloidal suspension of TiO 2 nanosheets and Žb. the LB films Ž11 layers. prepared from it on a quartz substrate. The data obtained for the protonic layered titanate H 0.7 Ti 1.825 I 0.175 O4 ⭈ H 2 O in a diffuse reflectance mode is denoted by a dashed line.
T. Yamaki et al. r Thin Solid Films 393 (2001) 154᎐160
158
⌬ Eg s
h2 8 x y
ž
1 1 h2 q q L2x L2y 8 z L2z
/
Ž2.
where h is Plank’s constant, x y and z are the reduced mass of electron-hole pairs, and L x , L y and L z are crystalline dimensions. In Eq. Ž2., the suffixes x, y and z express parallel and perpendicular directions with respect to the sheet, respectively. Because L x , L y 4 L z for the present nanosheet system Ž L z ; 0.75 nm., the first term can be ignored. Consequently, the blue shift in the spectra is predominantly governed by the sheet thickness. As can be seen in Fig. 3, the position of the absorption peak was found to be invariant between the TiO 2 sheets in the sols and in the LB films, although the baseline absorbance increased possibly due to the scattering or reflection of light. This observation confirms that the formation of particulate multilayers did not alter the TiO 2 sheet thickness or induce crystallite coalescence. The absorption features in terms of the peak with the sharp onset, possibly due to the molecular nature of the delaminated nanosheets, were not significantly modified by the LB deposition onto the substrate. Fig. 4 shows the absorption spectra of the LB film with a different number of layers on both sides of the quartz substrate. The peak absorbance of the LB film linearly increased with the number of depositions while the peak-top energy was unchanged. This substantiates a quantitative and uniform transference. XRD measurements are useful for studying the structure in the perpendicular direction to the plane of LB films. Fig. 5 is the XRD pattern of the LB film, which was formed on the quartz glass substrate as a result of 11 deposition cycles, i.e. possessing 11 layers
Fig. 4. Absorption spectra of the LB films possessing Ža. 2, Žb. 4 and Žc. 11 layers of TiO 2 nanosheets between the ammonium molecules. The inset shows the peak absorbance at 268 nm as a function of the number of the TiO 2 -nanosheet layers in the film.
Fig. 5. XRD pattern for the ammoniumrTiO2 -nanosheet alternating film transferred onto the hydrophobic quartz glass Ž11 layers..
of the TiO 2 nanosheets between the ammonium molecules. In this pattern, we recognized Ž00 l . XRD peaks up to l s 3. The periodic long spacing Ž d . was calculated to be approximately 3.4 nm from the position of these peaks. The d spacing was nearly equal to the sum of the thickness of a single TiO 2 layer Ž0.75 nm. and the molecular height of the ammonium cation which was estimated on the basis of the molecular structure Ž2.5 nm., consistent with the Z-type configuration. The XRD pattern of Fig. 5, exhibiting only lowerorder reflections, is markedly different from that of LB films of popular fatty acid salts. Such a decay in the peak intensity implies a large fluctuation of the average distance between the adjacent layers. The lattice distortion of the obtained multilayer is then roughly evaluated by the theoretical model in the following. When, as in the present case, the contribution of the geometrical factors and instrumental resolution to peak broadening is relatively small enough to be ignored, only the crystalline size and lattice distortion effects are considered to be convoluted into a diffraction peak profile w26,27x. According to the microstrain model w28,29x, line breadths of the reflections were separated into broadening due to each effect. If the profiles are Cauthy, the integral breadth  Žconverted to reciprocal nanometer units using Eq. Ž1.. can be given as:
s
Ž 2 ² 2 :. 1 q L001 d0
1r2
l
Ž3.
T. Yamaki et al. r Thin Solid Films 393 (2001) 154᎐160
159
where L001 is the crystalline size normal to the Ž001. plane, ² 2 : denotes the mean-square lattice distortion and d 0 is the length of the repeat unit in the w00 l x direction. The first term represents the broadening originating from the crystalline size, which is given by the familiar Scherrer equation. The second term is expressed to relate broadening to distortion parameters regarding 00 l reflections Žcalled ‘microstrain broadening’.. As clearly shown in Eq. Ž3., because the distortion broadening depends on the order of the reflection in contrast to the size broadening, it becomes possible to distinguish between crystalline size and lattice distortion effects. It should be noted here that the crystalline size L001 can also be calculated by:
monium alternating film with a repeating unit of 3.4 nm, although the lattice distortion in the film was found to be large in comparison to LB films of popular fatty acid salts. Emphasis should be given to the fact that the TiO 2 nanosheets in the film have a uniform ultra-thin thickness and unique anisotropic morphology resulting from the exfoliation of the layered compound H xTi 2 ᎐ x r4 I x r4 O4 ⭈ H 2 O. To the best of our knowledge, the present paper represents the first description of monoparticulate layer and multilayer preparation from such the well-defined TiO 2 nanocrystallites.
L001 s Nd 0
w1x J.H. Fendler, Springer-Verlag, BerlinAdvances in Polymer Science Series, Membrane-Mimetic Approach to Advanced Materials, 113, 1994. w2x N.A. Kotov, F.C. Meldrum, Adv. Mater. 7 Ž1995. 607. w3x J.H. Fendler, in: A. Ulman ŽEd.., Thin Films, Organic Thin Films and Surfaces: Directions for the Nineties, 20, Academic Press, San Diego, New York, Boston, 1995. w4x G. Ozin, Adv. Mater. 4 Ž1993. 612. w5x D.V. Paranjape, M. Sastry, P. Ganguly, Appl. Phys. Lett. 63 Ž1993. 18. w6x D.T. Amm, D.J. Johnson, T. Laursen, S.K. Gupta, Appl. Phys. Lett. 61 Ž1992. 522. w7x A. Kay, R.H. Baker, M. Gratzel, J. Phys. Chem. 97 Ž1993. ¨ 10773. w8x N.A. Kotov, F.C. Meldrum, C. Wu, J.H. Fendler, J. Phys. Chem. 98 Ž1994. 2735. w9x L. Li, Y. Chen, S. Kan, X. Zhang, X. Peng, M. Liu, T. Li, Thin Solid Films 284r285 Ž1996. 592. w10x T. Sasaki, M. Watanabe, H. Hashizume, H. Yamada, H. Nakazawa, J. Am. Chem. Soc. 118 Ž1996. 8329. w11x T. Sasaki, M. Watanabe, J. Phys. Chem. B 101 Ž1997. 10159. w12x T. Sasaki, M. Watanabe, Y. Michiue, Y. Komatsu, F. Izumi, S. Takenouchi, Chem. Mater. 7 Ž1995. 1001. w13x I.E. Grey, C. Li, I.C. Madsen, J.A. Watts, J. Solid State Chem. 66 Ž1987. 7. w14x K. Inukai, Y. Hotta, M. Taniguchi, S. Tomura, A. Yamagishi, J. Chem. Soc. Chem. Commun., Ž1994. 959. w15x K. Tamura, H. Setsuda, M. Taniguchi, M. Takahashi, A. Yamagishi, Clay Sci. 10 Ž1998. 409. w16x K. Tamura, H. Setsuda, M. Taniguchi, A. Yamagishi, Langmuir 15 Ž1999. 6915. w17x A. Ulman, An Introduction to Ultrathin Organic Films from Langmuir᎐Blodgett to Self-Assembly, Academic Press, San Diego, 1991. w18x G. Roberts ŽEd.., Langmuir᎐Blodgett Films, Plenum Press, New York, 1990. w19x P.A. Cox, Transition Metal Oxides, Oxford Science Publications, New York, 1992. w20x C. Kormann, D.W. Bahnemann, M.R. Hoffmann, J. Phys. Chem. 92 Ž1988. 5196. w21x L. Kavan, T. Stoto, M. Gratzel, D. Fitzmaurice, V. Shklover, J. ¨ Phys. Chem. 97 Ž1993. 9493. w22x C. Kormann, D.W. Bahnemann, M.R. Hoffmann, J. Phys. Chem. 92 Ž1988. 5196. w23x M. Shinada, S. Sugano, J. Phys. Soc. Jpn. 21 Ž1966. 1936. w24x C.J. Sandroff, D.M. Hwang, W.M. Chung, Phys. Rev. B 33 Ž1986. 5943.
Ž4.
where N is the number of unit cells perpendicular to the Ž001. plane. For LB films, N can be exactly determined from the number of dipping movements or of layers in a unit cell. The value of the observed integral breadth for Ž001. reflection at 2 s 2.7⬚ is 0.049 nmy1 . By inserting N s 12 Žconsidering the first dipping as a Y-type transfer ., d 0 s 3.4 nm and l s 1 into Eqs. Ž3. and Ž4., the value of ² 2 :1r2 can then be calculated to be 3.4%. Sasanuma et al. w30x in a study of quantitative estimation of the lattice distortion of the LB multilayers, estimated ² 2 :1r2 to be 0.09᎐0.90% for the 25-layer LB films of lead stearate on a glass substrate Ž Y-type layers. using the same model and equations. As they also pointed out, the distortion of the film represented by ² 2 : was very sensitive to the conditions of sample preparation, which determined the monolayer structure on the subphase surface and the texture of the built-up multilayers. Thus, the large lattice distortion in the present samples possibly results from the dense packing of the TiO 2 nanosheets between the organic ammonium layers. The method described in this paper can be extended to various types of amphiphilic molecules to produce new lamellar organized films. It is expected, therefore, that appropriate choice of amphiphilic compounds allows the orderly structure of LB films to be improved. 4. Conclusions A hybrid monolayer composed of a nanosized sheet of quasi-TiO 2 and an ammonium amphiphile molecule was prepared at an air᎐water interface and transferred layer-by-layer onto a solid substrate using the LB technique. UV-Visible absorption spectra and XRD measurements clarified the formation of a TiO 2ram-
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