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Application of the Langmuir-Blodgett Technique to Prepare a Clay-Metal Complex Hybrid Film Kenji Tamura, Hitoshi Setsuda, Masahiro Taniguchi,† and Akihiko Yamagishi* Division of Biological Sciences, Graduate School of Science, Hokkaido University, Sapporo 060, Japan Received November 3, 1998. In Final Form: May 20, 1999 A clay-metal complex multilayer film was prepared by the Langmuir-Blodgett technique. A monolayer of an amphiphilic ruthenium(II) complex, [Ru(phen)2(dC18bpy)](ClO4)2 (phen ) 1,10-phenanthroline, dC18bpy ) 4,4′-dioctadecyl-2,2′-bipyridyl), was formed on an aqueous suspension of hectorite clay (Li-HT). The monolayer acted as a template for adsorption of a clay particle. A hybrid monolayer of a clay and a metal complex was transferred onto a hydrophilic glass plate to form a Z-type multilayer film. X-ray diffraction, UV-visible and FT-IR absorption spectra, and Brewster angle and atomic force microscopic (AFM) measurements confirmed the formation of an ordered film of [Ru(phen)2(dC18bpy)]2+/hectorite with the layer thickness of 5.4 nm.
Introduction Construction of organic/inorganic nanostructured materials is an important target of modern materials research. The research has been motivated by the purpose of developing functional materials such as sensors, electrode-modifiers, nonlinear optical devices, and pyroelectric materials.1-6 Of particular interest is the preparation of a highly ordered organic/inorganic ultrathin film in which an organic small molecule or a macromolecule is intercalated between inorganic layers. An inorganic layer is incorporated to reinforce the lamellar structure of a film. One method of preparing such a hybrid film is to bond chemically an inorganic sheet to an organic anchor which is self-assembled on a solid surface. Multilayer formation is accomplished by bridging neighboring layers with a bifunctional agent.7 Another method is to deposit a polyelectrolyte and an inorganic layer alternatively onto a solid substrate by immersing the substrate into their solutions.8 A layer-by-layer structure is maintained by electrostatic interaction between oppositely charged layers. Although the classical Langmuir-Blodgett (LB) technique is a powerful approach to construct multilayer molecular assemblies,9 the method unfortunately suffers from critical drawbacks such as mechanical weakness and * Corresponding author. Fax: (+81)11-706-4909. E-mail:
[email protected]. † Present address: Central Research Laboratory, Kawasaki, Showa Denko K. K., 3-2, Chidori-cho, Kawasaki-ku, Kawasaki, Kanagawa 210-0865, Japan. (1) Lvov, Y.; Ariga, Y. K.; Ichinose, I.; Kunitake, T. Langmuir 1996, 12, 3038. (2) Kotov, N. A.; Haraszi, T.; Turi, L.; Zavala, G.; Geer, R. E.; De`ka`ny, I.; Fendler, J. H. J. Am. Chem. Soc. 1997, 119, 6821. (3) Ichinose, I.; Kimizuka, N.; Kunitake, T. J. Phys. Chem. 1995, 99, 3736. (4) Tsutsumi, N.; Sakata, K.; Kunitake, T. Chem. Lett. 1992, 1465. (5) Swalen, J. D.; Allara, D. L.; Andrade, J. D.; Chandross, E. A.; Garof, F. S.; Israelachvili, J.; McCarthy, T. J.; Murray, R.; Pease, R. F.; Rabolt, J. F.; Wynne, K. J. Langmuir 1987, 3, 932. (6) Kotov, N. A. ; De`ka`ny, I.; Fendler, J. H. J. Phys. Chem. 1995, 99, 13065. (7) Rong, D.; Kim, Y.; Mallouk, T. E. Inorg. Chem. 1990, 29, 1531. (8) Kleinfeld, E. R.; Ferguson, G. S. Science 1994, 265, 370. (9) Gaines, G. L., Jr. Insoluble Monolayers at Liquid-Gas Interfaces; Interscience Publishers: New York, 1996.
sensitivity to contaminants. Recently, we have been applying the LB method to prepare thin films of inorganic materials and metal complexes.10 As an extension of the work, we have shown that an ultrathin hybrid film of a clay mineral and an amphiphilic metal complex can be built by using the LB technique. According to the method, a monolayer of an amphiphilic Ru(II) complex was formed onto an aqueous dispersion of a clay. The monolayer was used as a template for adsorption of a clay particle. A ruthenium(II) complex, [Ru(phen)2(dC18bpy)]2+ (phen ) 1,10-phenanthroline, dC18bpy ) 4,4′-dioctadecyl-2,2′bipyridyl) (Figure 1), was used as an amphiphilic compound. The optical isomers of the compound were obtained in a highly purified form. Ruthenium(II) polypyridyl complexes are known to be an efficient converter of photon energy to chemical energy.11 Such a new hybrid film could be an important step toward the creation of organized layered compounds for controlling the deposition sequence and orientation of intercalated molecules. Experimental Section Materials. [Ru(phen)2(dC18bpy)]2+ was prepared by refluxing [Ru(phen)2Cl2] (50 mg) and dC18bpy (70 mg) with AgNO3 (32 mg) in 20 mL of an ethanol solution for 20 min. An orange precipitate was formed by adding NaClO4 to the reactant mixture. Resolution of [Ru(phen)2(dC18bpy)]2+ was performed by adding 0.1 g of sodium antimonyl tartrate salt to 20 mL of a 1:1 waterethanol (v/v) solution of the racemic mixture. ∆-[Ru(phen)2dC18bpy]2+ was filtered off as an insoluble antimonyl tartrate salt. The anion of the salt was converted to a perchlorate by use of an anion-exchange resin. Λ-[Ru(phen)2(dC18bpy)](ClO4)2 was obtained by adding an excess amount of sodium perchlorate to the filtrate. The optical purities of both enantiomers were estimated to be higher than 95%. The CD spectra of ∆- and Λ-enantiomers gave ∆ ) 324 and -330, respectively, at 281 nm. (10) (a) Inukai, K.; Hotta, Y.; Taniguchi, M.; Tomura, S.; Yamagishi, A. J. Chem. Soc., Chem. Commun. 1994, 959. (b) Yamagishi, A.; Goto, Y.; Taniguchi, M. J. Phys. Chem. 1994, 100, 1827. (c) Yamagishi, A.; Sasa, N.; Taniguchi, M. Langmuir 1997, 13, 1689. (d) Aramata, K.; Kamachi, M.; Taniguchi, M.; Yamagishi, A. Langmuir 1997, 13, 5161. (e) Hotta, Y.; Inukai, K.; Taniguchi, M.; Yamgishi, A. J. Electroanal. Chem. 1997, 429, 107. (f) Kashiwara, S.; Taniguchi, M.; Nakata, M.; Taniguchi, M.; Yamagishi, A. J. Mater. Chem. 1998, 8, 2253. (11) Rotzinger, F. P.; Munavalli, S.; Comte, P.; Hurst, J. K.; Gratzel, M.; Pern, F.; Franck, A. J. J. Am. Chem. Soc. 1987, 109, 6619.
10.1021/la981396j CCC: $15.00 © 1999 American Chemical Society Published on Web 07/30/1999
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Figure 2. Surface pressure-molecular area (π-A) curves at 25 °C when (a) 100, (b) 200, and (c) 300 µL of 2 × 10-4 M chloroform solutions of [Ru(phen)2(dC18bpy)](ClO4)2 were spread on an aqueous 0.1 M NaClO4 solution or (d) 75, (e) 100, and (f) 125 µL of 8.4 × 10-5 M chloroform solutions of the same complex were spread on an aqueous suspension of Li-HT (0.008 g/L).
Figure 1. Molecular model of [Ru(phen)2(dC18bpy)]2+. The clay used was a synthetic lithium hectorite (Li-HT, Topy Industries Co., Ltd) with the composition Li+0.60(Mg1.40Li0.60)Si4O10F2. The cation-exchange capacity (CEC) was determined to be 140 mequiv/100 g by the Ca2+ adsorption method. It was dispersed by stirring for 1 day in distilled water and adjusted to 80 mg/L. Instruments. Surface pressure-area (π-A) curves were measured with a Langmuir trough (U. S. I., Japan). A π-A curve was recorded 30 min after a chloroform solution of a ruthenium complex was spread onto an aqueous suspension of Li-HT. The temperature of the subphase was maintained at 20 °C. The trough surface was compressed at the compression rate 10 cm2/min. A monolayer was transferred by the vertical dipping method onto a hydrophilic glass plate (for the measurements of X-ray diffraction, UV-visible spectra, and AFM images) or a silicon wafer (for the measurements of IR spectra). Deposition was carried out at the dipping rate 10 mm/min and the surface pressure 20 mN/m. Brewster angle microscopy (BAM) experiments were performed with an instrument constructed by Dr. H. Yokoyama (Electotechnical Laboratory, Japan). Reflected light passed through a lens to a CCD camera, and the resulting video signal was fed to a video system. Atomic force microscopic images were recorded at room temperature under air with a Nanoscope III scanning probe microscope (DI Instruments, USA). Nanoprobe integral cantilevers (Si3N4) were used, having tips with a spring contact of 0.06 nm-1 (Park Scientific). The AFM images were obtained in the tapping mode with filters off. The d scan head was used with the maximum scan range 12 µm × 12 µm × 4.4 µm.
Results and Discussion Monolayer Behaviors. Curves a-c in Figure 2 show the surface pressure versus molecular area (π-A) isotherms
when 100-300 µL of a 6.1 × 10-5 M chloroform solution of racemic [Ru(phen)2(dC18bpy)] (ClO4)2 was spread on an aqueous 0.1 M NaClO4 solution. Surface pressure rose from zero at 1.2 nm2/molecule and increased steeply until it attained a plateau above 50 mN/m. The compressibility of a monolayer was calculated to be 0.011 m/mN at 0.75 nm2/molecule. The lift-off area was nearly equal to the cross section of a head group (1.4 nm2/molecule). The results indicated that the molecule formed a closely packed monolayer and collapsed at surface pressures above 50 mN/m. Comparing the isotherms for racemic and enantiomeric [Ru(phen)2(dC18bpy)]2+, it was concluded that the packing density was a little higher in the heterochiral monolayer than in the homochiral one.10b When the measurements were performed on a subphase of aqueous 0.1 M NaClO4, the same results were obtained for both the racemic and enantiomeric complexes. Curves d-f in Figure 2 are isotherms when 75-125 µL of a 8.4 × 10-5 M chloroform solution of [Ru(phen)2(dC18bpy)](ClO4)2 was spread on an aqueous suspension of Li-HT (0.008 g/L). These curves were displaced 1 nm2 toward larger molecular area than curves a-c, supporting the interaction of a clay particle with a cationic monolayer. The surface pressure rose from zero at 1.9 ( 0.1 nm2/ molecule and attained a plateau at the molecular area 1.2 nm2. The compressibility of a monolayer was calculated to be 0.0088 m/mN at 1.50 nm2/molecule. It was assumed that a clay particle formed a single-layer film at the liftoff area (1.9 nm2/molecule). According to the elemental composition of Li-TH (given in the Experimental Section), one side of a clay layer is estimated to carry a negative charge of -2.0 e for an area of 1.9 nm2. In other words, the surface negative charge was nearly compensated by the charge of an adsorbed metal complex. The results indicated that adsorption took place according to the ionexchange mechanism. Interaction of a clay with a floating monolayer was studied in situ with a Brewster angle microscope. Figure 3A is the observation of aqueous 0.1 M NaClO4. When racemic [Ru(phen)2(dC18bpy)](ClO4)2 was spread on an aqueous 0.1 M NaClO4 solution, a bright region with a size of a few thousand micrometers was observed to float on a water surface. This implied that the complex formed a monolayer island even at zero surface pressure. The whole surface was uniformly bright at 30 mN/m, indicating
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Figure 4. XRD patterns for (a) [Ru]/[Li-HT] (7 layers) and (b) [Ru]/[NaClO4] (10 layers) transferred onto hydrophilic quartz at 20 mN/m.
Figure 3. Brewster angle microscope images of a trough surface: (A) a surface of an aqueous NaClO4 solution; (B) the same surface compressed to 20 mN/m after a chloroform solution of [Ru(phen)2(dC18bpy)](ClO4)2 was spread; (C) the same surface compressed over 50 mN/m; (D) a surface when a chloroform solution of [Ru(phen)2(dC18bpy)](ClO4)2 was spread onto an aqueous dispersion of Li-HT (0.008 g/L); (E) the same surface compressed to 20 mN/m; (F) the same surface compressed over 40 mN/m.
that the surface was completely covered with a monolayer (Figure 3B). When surface pressure exceeded 50 mN/m, there appeared an intensely bright straight line that extended over a few thousand micrometers (Figure 3C). This indicated the collapse of the monolayer to result in the formation of a multilayer or a microcrystal. When the same metal complex was spread on an aqueous suspension of Li-HT (0.008 g/L), bright spots with the radii 10-100 µm were seen at zero surface pressure (Figure 3D). These spots were undoubtedly due to clay particles adsorbed to the monolayer islands of a metal complex. The density of spots increased on compressing a surface until the whole surface was covered with spots at 30 mN/m (Figure 3E). When the surface pressure exceeded 40 mN/m, bright lines appeared in a zigzag way, indicating the collapse of a film (Figure 3F). Multilayer Properties. Monolayers could be efficiently deposited onto a solid substrate such as a hydrophilic quartz plate and a silicon wafer. Deposition took place only in the upward direction (Z-type deposition). A film is denoted as [Ru]/[NaClO4] or [Ru]/[Li-HT] when a monolayer of racemic [Ru(phen)2(dC18bpy)]2+ is deposited from an aqueous NaClO4 solution or an aqueous Li-HT dispersion, respectively. Thin (a) and bold (b) curves in Figure 4 show XRD patterns for [Ru]/[NaClO4] (10 layers) and [Ru]/[Li-HT] (7 layers), respectively, transferred onto hydrophilic quartz at 20 mN/m. The basal spacing was determined to be 4.2 and 5.3 nm for [Ru]/[NaClO4] and [Ru]/[Li-HT], respectively. For the former, the basal spacing was nearly equal to the sum of the molecular
Figure 5. Infrared absorption spectra for (a) [Ru]/[NaClO4] (10 layers) and (b) [Ru]/[Li-HT] (7 layers) transferred onto a silicon wafer.
height of [Ru(phen)2(dC18bpy)]2+ (3.6 nm) and the diameter of ClO4- (0.5 nm)12 or 4.1 nm. For the latter, the basal spacing was 0.7 nm larger than the sum of the molecular height of [Ru(phen)2(dC18bpy)]2+ (3.6 nm) and the thickness of a single clay layer (1.0 nm)13 or 4.6 nm. Thus the film was likely to include either a double clay layer or other kinds of countercations adsorbed on the opposite side of a clay layer. Dotted (a) and solid (b) curves in Figure 5 show infrared absorption spectra for [Ru]/[NaClO4] (10 layers) and [Ru]/ [Li-HT] (7 layers), respectively, transferred onto a silicon wafer. Peaks at 2923 and 2852 cm-1 or the asymmetric and symmetric stretching vibrations of methylene groups of dC18bpy ligands, respectively, were seen in both samples, while peaks at 1004 and 467 cm-1, which were assigned to the Si-O-Si stretching bands in Li-HT, were seen only for [Ru]/[Li-HT].14 These experiments confirmed the inclusion of a clay layer within the LB films and the well-defined lamellar structure of the built-up films. To determine the orientation of the alkyl chain of a ruthenium complex in a film, the polarized infrared spectrum was recorded on a film of [Ru]/[Li-HT]. Figure 6 shows the results when transversely (a) and perpendicularly (b) polarized lights, respectively, were incident at an incident angle of 45°. The dichroic ratio A|/A⊥ was obtained to be 0.8 ( 0.1 for both the symmetric and asymmetric stretching bands of methylene groups. Since the transition (12) Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements; Butterworth- Heinemann: Oxford, 1984; p 1016. (13) Brindley, G. W.; Brown, G. Crystal Structures of Clay Minerals and their X-ray Identification; Mineralogical Society, London, 1980; p 6. (14) Sutubican, V.; Roy, R. J. Am. Ceram. Soc. 1961, 44, 625.
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Figure 7. Polarized electronic absorption spectra for [Ru]/ [Li-HT] (5 layers) deposited on a hydrophilic quartz substrate, when (a) transversely and (b) perpendicularly polarized lights were incident on the sample at an angle of 45°.
Figure 6. Polarized infrared spectra for [Ru]/[Li-HT] (7 layers) deposited on a silicon wafer, when (a) transversely and (b) perpendicularly polarized lights were incident on the sample at an angle of 45°.
moments of both bands are perpendicular to the alkyl chain, A⊥/A| is related to the orientation of the alkyl chain by the following equation:15
A⊥/A| ) {2 - 2/n2 sin2 R + (3/n2 sin2 R - 1) sin2 γ}/ (2 - sin2 γ) (1) in which n, R and γ denote the refractive index of a film, the incident angle, and the angle of the alkyl chain with respect to the normal direction of a film, respectively. Inserting n ) 1.45 and R ) 45°, γ was calculated to be 15° ( 5°. On the basis of the molecular model of the metal complex (Figure 1), the results are consistent with the situation that the metal complex orients the molecular plane of dC18bpy (or 4,4′-dioctadecyl-2,2′-bipyridyl) normal to a surface. Polarized electronic absorption spectra were measured for [Ru]/[Li-HT] (5 layers) deposited on a hydrophilic quartz substrate. Parts a and b of Figure 7 show the spectra when transversely and perpendicularly polarized lights, respectively, were incident on the sample at an incident angle of 45°. The absorption band at 460-430 nm was assigned to be the metal-to-ligand charge transfer (MLCT) band.16 The dichroic ratio at 450 nm, A⊥/A|, was obtained to be 0.75 ( 0.05. The MLCT band is the composite of three transitions from a central Ru(II) ion to three ligands. Since all of the transition moments are perpendicular to the pseudo three-fold symmetry axis (C3′), A⊥/A| is expressed by the same equation as eq 1, in which γ denotes the angle of C3′ with respect to the normal direction of a film. γ was calculated to be 18° ( 3°. The results are also (15) Akutsu, H.; Kyougoku, Y.; Nakahara, H.; Fuhuda, K. Chem. Phys. Lipids 1975, 15, 222. (16) Riesen, H.; Wallace, L.; Krausz, F. J. Phys. Chem. 1996, 100, 17138.
consistent with the situation that the molecular plane of dC18bpy is normal to a film surface. The deposition state of a clay sheet was investigated with an atomic force microscope. Figure 8A shows the AFM image of a floating film deposited onto a hydrophilic glass plate in the upward direction under the conditions of Figure 3E. A surface was covered with planar particles (denoted by a). There was a lower part (denoted by b) among the particles. The surface of a was estimated to be 1.5 ( 0.5 nm higher than that of b. The value was comparable to the difference of basal spacing (1.1 nm) between [Ru]/[NaClO4] (4.2 nm) and [Ru]/[Li-HT] (5.3 nm), as determined by X-ray measurements. In the image of a different part (Figure 8B), particles were partially overlapped to form a double layer, as indicated by c. The thickness of the upper layer was estimated to be 5.0 ( 1.0 nm. The value was comparable to the basal spacing of [Ru]/[Li-HT] (5.3 nm). The above AFM results were consistent with a view that the regions of a, b, and c were a monolayer of [Ru(phen)2(dC18bpy)]2+/hectorite, a monolayer of [Ru(phen)2(dC18bpy)](ClO4)2, and a double layer of [Ru(phen)2(dC18bpy)]2+/hectorite, respectively, as schematically shown in Figure 9. Notably the double layer regions (c) were connected smoothly with single-layer regions (a), implying that a clay layer was so flexible as to change its shape according to the up-and-down of an underneath surface.17 More interestingly, there were two facing particles whose rough edges seemed to compensate each other (indicated by two arrows in Figure 8A). This implied that these were originally parts of the same layer but that they were torn into two fragments during deposition. Figure 8C shows the image of a larger crystal. There were aggregated regions observed near an edge. This indicated that some metal complexes were not fixed on a clay surface but moved to form an aggregate. It is not clear what kind of property was responsible for the mobility of a metal complex on a clay surface. For comparison, an aqueous suspension of Li-HT (0.008 g/L) was cast on a glass plate and dried. AFM of the surface indicated that most of clay particles were aggregated to form large domains a few (17) Nakazawa, H.; Yamada, H.; Fujita, T. Appl. Clay Sci. 1992, 6, 395.
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Figure 8. (A) AFM image (3.9 µm × 3.9 µm) of a film deposited onto a hydrophilic glass plate under the conditions of Figure 3E. (B) AFM image (1.8 µm × 1.8 µm) of the same sample, showing the overlap of two layers. (C) AFM image (2.1 µm × 2.1 µm) of a larger crystal in a different sample, showing the aggregation of metal complexes at an edge region. (D) AFM image (6.4 µm × 6.4 µm) when an aqueous suspension of Li-HT was cast on a glass plate.
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as shown in Figure 8D. The thickness of a particle was estimated to be 1.5 ( 0.5 nm, indicating that it was either a single or double layer. We have used the adsorption of a clay layer on a positively charged monolayer of [Ru(phen)2(dC18bpy)]2+ and the Langmuir-Blodgett technique to construct new organic/inorganic nanostructured films. Currently, we are investigating the effect of the size and charge density of a clay layer for various kinds of natural and synthetic clay minerals. The preliminary results indicate that the method described in this paper can be extended to all types of clay minerals to give rise to new lamellar organized materials. Appropriate choice of clay minerals and amphiphilic compounds allows LB films having particular optical and electrochemical properties to be constructed.18
Figure 9. Schematic drawing of a hybrid film of a clay and a metal complex. Parts a, b, and c, correspond to those of the AFM images (Figure 8).
hundred nanometers in size and tens of nanometers in height. In rare cases, an isolated clay particle was observed
Acknowledgment. We wish to thank Dr. Hiroshi Yokoyama (Electrotechnical Laboratory, Japan) for constructing a BAM instrument. LA981396J (18) Taguchi, Y.; Kimura, R.; Azumi, R.; Tachibana, H.; Koshizaki, N.; Shimomura, M.; Momozawa, N.; Sakai, H.; Masahiko, A.; Matsuyoshi, M. Langmuir 1998, 14, 6550.
