6550
Langmuir 1998, 14, 6550-6555
Fabrication of Hybrid Layered Films of MoS2 and an Amphiphilic Ammonium Cation Using the Langmuir-Blodgett Technique Yoshiaki Taguchi,† Ryota Kimura,† Reiko Azumi,‡ Hiroaki Tachibana,‡ Naoto Koshizaki,‡ Masaki Shimomura,‡ Nobuyuki Momozawa,†,§ Hideki Sakai,†,§ Masahiko Abe,†,§ and Mutsuyoshi Matsumoto*,‡ Faculty of Science and Technology, Science University of Tokyo, 2641 Yamazaki, Noda, Chiba 278-8510, Japan, National Institute of Materials and Chemical Research, Higashi, Tsukuba, Ibaraki 305-8565, Japan, and Institute of Colloid and Surface Science, Science University of Tokyo, 2641 Yamazaki, Noda, Chiba 278-8510, Japan Received May 11, 1998. In Final Form: August 25, 1998 Inorganic/organic hybrid ultrathin films of MoS2 and a cationic amphiphile, dihexadecyldimethylammonium bromide (DHA+Br-), were prepared using the Langmuir-Blodgett (LB) technique. The surface pressure-area isotherms of DHA+Br- changed by introducing exfoliated MoS2 particles into the subphase. On the other hand, the isotherms of icosanoic acid in the presence and absence of MoS2 particles in the subphase were essentially the same. The UV/vis reflection spectra of DHA+(Br-) monolayers on MoS2 suspension showed a broad absorption band assignable to MoS2, whereas that of icosanoic acid did not show any appreciable band. These results suggest the formation of hybrid monolayers consisting of DHA+ and MoS2. The hybrid monolayers were transferred successfully onto substrates using a horizontal lifting method to form LB films. That was confirmed by the infrared and UV/vis absorption spectroscopies and AFM, although the transfer ratios were not determined. The X-ray diffraction patterns of the LB films showed the layered structure in which organic and inorganic sheets are stacked alternately. The AFM image of a single-layer LB film of DHA+ and MoS2 showed flat, plate-like particles with diameters in the submicrometer region. The structure of the hybrid LB films depended strongly on the MoS2 concentration in the subphase for the preparation of the hybrid monolayers. The number of DHA+ molecules per unit area of the LB film increased with increasing MoS2 concentration in the subphase while the amount of MoS2 in the LB film remained unchanged. Further, the tilt angle of the hydrocarbons of DHA+ was large and the interlayer spacing of the LB film was small when the MoS2 concentration in the subphase was large. A model of the formation of the hybrid film was proposed.
Introduction Inorganic/organic hybrid molecular materials have recently attracted considerable interest since they provide an opportunity to combine the functionalities associated with each component. In crystalline forms, perovskitetype layered compounds present inorganic/organic alternate layered structures,1-7 in which monovalent cations are replaced by organic cations. Confinement of excitons,1,2 highly efficient electroluminescence,3,4 and the tuning of the electrical conductivities5,6 have been realized, owing to the presence of the organic layers in the structures. It should be noted that excitons, electroluminescence, and conductivities are associated with the inorganic components. On the other hand, the orientation * To whom correspondence should be addressed. † Faculty of Science and Technology, Science University of Tokyo. ‡ National Institute of Materials and Chemical Research. § Institute of Colloid and Surface Science, Science University of Tokyo. (1) Ishihara, T.; Takahashi, J.; Goto, T. Solid State Commun. 1989, 9, 933. (2) Kataoka, T.; Kondo, T.; Ito, R. Phy. Rev. B. 1993, 47, 2010. (3) Era, M.; Morimoto, S.; Tsutsui, T.; Saito, S. Synth. Met. 1995, 71, 2013. (4) Hattori, T.; Taira, T.; Era, M.; Morimoto, S.; Tsutsui, T.; Saito, S. Chem. Phys. Lett. 1996, 254, 103. (5) Mitzi, D. B.; Wang, S.; Feild, C. A.; Guloy, A. M. Science 1995, 267, 1473. (6) Mitzi, D. B.; Feild, C. A.; Harrison, W. T. A.; Guloy, A. M. Nature 1994, 369, 467. (7) Azumi, R.; Honda, K.; Goto, M.; Akimoto, J.; Oosawa, Y.; Tachibana, H.; Tanaka, M.; Matsumoto, M. Mol. Cryst. Liq. Cryst. 1996, 276, 237.
of organic cations has been controlled by using the perovskite lattice.7 To obtain hybrid molecular materials as ultrathin films, the LB method is promising since the arrangement of the components can be controlled in well-defined manners. In many cases, the electrostatic interaction between the organic and inorganic components is the main driving force of the formation of the hybrid films. Inorganic semiconductor clusters have been prepared between organic layers using floating monolayers or LB films as the interfaces for crystal growth.8,9 Colloidal semiconductor clusters stabilized by organic molecules8 and monolayers of clay minerals after being rendered amphiphilic through the reactions with organic molecules10,11 have been transferred onto solid substrates using the LB technique. LB films of cationic amphiphile/polyoxometalates have been formed, and the magnetic properties have been investigated.12 Bilayer films have been utilized as the templates for the growth of inorganic materials.13-15 The alternatives to fabricate ultrathin films of inorganic/ (8) Fendler, J. H.; Meldrum, F. C. Adv. Mater. 1995, 7, 607. (9) Moriguchi, I.; Hosoi, K.; Nagaoka, H.; Tanaka, I.; Teraoka, Y.; Kagawa, S. J. Chem. Soc., Faraday Trans. 1994, 90, 349. (10) Inukai, K.; Hotta, Y.; Taniguchi, M.; Tomura, S.; Yamagishi, A. J. Chem. Soc., Chem. Commun. 1994, 959. (11) Kotov, N. A.; Meldrum, F. C.; Fendler, J. H. Langmuir 1994, 10, 3797. (12) Clemente-Leon, M.; Mingotaud, C.; Angicole, B.; Gomez-Garcia, C. J.; Coronado, E.; Delhaes, P. Angew. Chem., Int. Ed. Engl. 1997, 36, 1114. (13) Kimizuka, N.; Handa, T.; Ichinose, I.; Kunitake, T. Angew. Chem., Int. Ed. Engl. 1994, 33, 2483.
10.1021/la980551r CCC: $15.00 © 1998 American Chemical Society Published on Web 10/09/1998
Films of MoS2 and an Ammonium Cation
organic hybrid molecular materials are a layer-by-layer accumulation method by the complexation of multivalent cations with organic phosphates16-19 and an alternate adsorption method.20-23 The electrostatic interaction between the organic and inorganic components plays an important role in the formation of the films. In the former case, the usage of multivalent cations yields highly ordered films and the orientation of the organic component is controlled. The drawback of this method is the limitation to the inorganic species. Superlattices of layered inorganic sheets sandwiched by organic polycations have been prepared using the latter method. However, it may be difficult to control the orientation of the organic components as long as the polycations are used. MoS2 is a promising material for use as an inorganic component of inorganic/organic hybrid films. This material can be exfoliated into single-layer platelets by intercalating lithium into the layer, followed by reaction with water.24 The exfoliated MoS2 particles possess net negative charges due to surface hydration,25 which offers the possibility to employ the polyion complex method26-29 to fabricate hybrid LB films with cationic amphiphiles. In this report, we will demonstrate that the superstructures consisting of monolayer sheets of inorganic semiconductor and organic amphiphiles can be obtained using the LB method. We will then show that the structure of the LB films depends strongly on the concentration of MoS2 in the subphase. Experimental Details Materials. Commercially available dihexadecyldimethylammonium bromide (DHA+Br-, Sogo Pharmaceutical Co., Ltd.) and icosanoic acid (Acros Organics Co., Ltd.) were used to form monolayers without purification. MoS2 powder (purity > 99.9%) was purchased from High Purity Chemicals. Exfoliation of MoS2. MoS2 was exfoliated in a manner similar to that described in the literature.24 MoS2 (3.0 g) was stirred under nitrogen in ca. 45 mL of 1.6 M n-hexane solution of n-butyllithium for 48 h at room temperature. After filtration, lithium-intercalated MoS2 was washed with n-hexane to remove excess n-butyllithium. Lithium-intercalated MoS2 was introduced into distilled water, and an opaque suspension formed after a H2 gas evolution (Figure 1). Exfoliated MoS2 was filtered, redispersed in ca. 200 mL of water, and dialyzed with flowing water until the pH value of the water became less than 6.8. (14) Ichinose, I.; Kimizuka, N.; Kunitake, T. J. Chem. Phys. 1995, 99, 3736. (15) Kimizuka, N.; Kunitake, T. Adv. Mater. 1996, 8, 89. (16) Lee, H.; Kepley, L.; Hong, H.-G.; Mallouk, T. E. J. Am. Chem. Soc. 1988, 110, 618. (17) Katz, H. E.; Scheller, G.; Putvinski, T. M.; Schilling, M. L.; Wilson, W. L.; Chidsey, C. E. D. Science 1991, 254, 1485. (18) Kepley, L.; Sackett, D. D.; Bell, C. M.; Mallouk, T. E. Thin Solid Films 1992, 208, 132. (19) Hanken, D. G.; Corn, R. M. Anal. Chem. 1995, 67, 3767. (20) Kleinfeld, E. R.; Ferguson, G. S. Science 1994, 265, 370. (21) Keller, S. W.; Kim, H.-N.; Mallouk, T. E. J. Am. Chem. Soc. 1994, 116, 8817. (22) Ingersoll, D.; Kulesza, P. J.; Faulkner, L. R. J. Electrochem. Soc. 1994, 141, 140. (23) Lvov, Y.; Ariga, K.; Ichinose, I.; Kunitake, T. Langmuir 1996, 12, 3038. (24) Joensen, P.; Frindt, R. F.; Morrison, S. R. Mater. Res. Bull. 1986, 21, 457. (25) Divigalpotiya, W. M. R.; Frindt, R. F.; Morrison, S. R. Thin Solid Films 1990, 186, 177. (26) Shimomura, M.; Kunitake, T. Thin Solid Films 1985, 132, 243. (27) Nishiyama, K.; Kurihara, M.; Fujihira, M. Thin Solid Films 1989, 179, 477. (28) Tachibana, H.; Azumi, R.; Tanaka, M.; Matsumoto, M.; Sako, S.; Sakai, H.; Abe, M.; Kondo, Y.; Yoshino, N. Thin Solid Films 1996, 284, 73. (29) Matsumoto, M.; Miyazaki, D.; Tanaka, M.; Azumi, R.; Manda, E.; Kondo, Y.; Yoshino, N.; Tachibana, H. J. Am. Chem. Soc. 1998, 120, 1479.
Langmuir, Vol. 14, No. 22, 1998 6551
Figure 1. Scheme of the preparation of the hybrid monolayers of DHA+ and MoS2 at the air-water interface. Monolayer Measurements and LB Transfer. Monolayer measurements were done on a Lauda film balance at 290 K. Suspension of exfoliated MoS2 in water was diluted for use as the subphase. The exact concentration of MoS2 in the subphase was determined from the weight of residue after drying a part of the suspension. DHA+Br- or icosanoic acid was spread to form a monolayer, and the monolayer was transferred using a horizontal lifting method onto various substrates at a surface pressure of 30 mN m-1. Quartz plates hydrophobized with 1,1,1,3,3,3-hexamethyldisilazane were used as substrates for the measurements of UV/vis absorption spectra and infrared transmission spectra. For infrared reflection-absorption (RA) measurements, the monolayers were transferred on vacuumdeposited Au mirror. Glass plates were also hydrophobized and used for X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) measurements. HOPG (highly oriented pyrolytic graphite) was used for atomic force microscopy (AFM) observations. Characterization. UV/vis reflection spectra of the monolayers at the air-water interface were measured on an Otsuka Electronics IMUC-700 spectrometer. UV/vis absorption measurements of the LB films were made on a Shimadzu UV-265FS spectrophotometer. Infrared spectra of the LB films were measured using a Perkin-Elmer Spectrum 2000 FTIR. The angle of incidence was set at 80° for RA measurements. DHA+Br- in deuterated dimethyl sulfoxide was also subjected to IR measurements. XRD measurements were performed on a Philips PW1800 system, using monochromated Cu KR radiation. XPS measurements were made on a PHI 5600ci system, using monochromated Al KR radiation. AFM images were taken on a Seiko SPA 300 with an SPI 3700 probe station using noncontact mode (dynamic force mode) at 27 kHz. Commercially available Si cantilevers with a force constant of 1.5 N/m were used.
Results and Discussion Monolayer Formation at the Air-Water Interface. Figure 2 shows the surface pressure-area isotherms of DHA+Br- and icosanoic acid on water and on suspension of single-layer platelets of MoS2 (0.30 g L-1). It is clear that the presence of MoS2 platelets in the subphase gives rise to a change in the isotherm of DHA+Br-. On the other hand, the isotherms of icosanoic acid were not affected by MoS2 platelets. Area per alkyl chain of DHA+Br- in the condensed region is larger than that of icosanoic acid. This indicates that the DHA+ molecules
6552 Langmuir, Vol. 14, No. 22, 1998
Figure 2. Surface pressure-area isotherms of the monolayers of DHA+Br- (a) on pure water and (b) on the MoS2 suspension (0.30 g L-1), and of icosanoic acid (c) on pure water and (d) on the MoS2 suspension (0.30 g L-1).
Taguchi et al.
Figure 4. IR absorption spectra of DHA+/MoS2 (0.30) LB films.
Figure 5. X-ray diffraction patterns of (a) a 10-layer DHA+/ MoS2 (0.30) LB film and (b) unexfoliated MoS2 powder cast on glass. Figure 3. UV/vis absorption spectra of DHA+/MoS2 (0.30) LB films.
are loosely packed in the monolayer films. This suggests that the alkyl chains of DHA+Br- molecules are tilted with respect to the surface normal. UV/vis reflection spectra of the monolayers were measured. In the spectrum of DHA+ monolayer on MoS2 suspension (0.30 g L-1), a reflection band due to MoS2 at around 250 nm was observed. However, no reflection bands were observed when DHA+Br- monolayer was formed on pure water. Further, no reflection bands were observed for icosanoic acid monolayer on pure water and MoS2 suspension. These results suggest the electrostatic interaction, possibly a complexation, between DHA+ cations and negatively charged MoS2, as schematically shown in Figure 1. The absence of significant interaction between icosanoic acid monolayer and MoS2 particles is understandable since MoS2 particles are negatively charged. Fabrication of LB Films. Monolayers of DHA+Brwere transferred from MoS2 suspension (0.30 g L-1) onto various substrates. Transfer ratios were not determined because a horizontal lifting method was used. The LB films of DHA+ from MoS2 suspension will be abbreviated as DHA+/MoS2 (x) LB films hereafter, where x (in grams per liter, g L-1) represents the concentration of MoS2 in the subphase. Figure 3 shows the UV/vis absorption spectra of DHA+/MoS2 (0.30) LB films. The intensity of the broad band around 250 nm, which is attributed to MoS2, was proportional to the number of layers up to 20 layers. This indicates that a constant amount of MoS2 is incorporated in each monolayer. No absorption band due to MoS2 was observed for the LB films of icosanoic acid transferred from the MoS2 suspension. Figure 4 shows the CH stretching region in the IR transmission spectra of DHA+/MoS2 (0.30) LB films. The absorption bands at ca. 2920 and 2850 cm-1 are assigned to CH2 antisymmetric and symmetric stretching modes, respectively. The intensities of these bands were also proportional to the number of layers up to 20 layers. This suggests that a constant amount of DHA+ molecules were transferred for each stroke. Since a constant amount of
MoS2 was also transferred for each stroke, each monolayer in DHA+/MoS2 LB films has the same composition. It is reasonable to assume that each layer is similar in structure. Since the monolayers were transferred layer by layer, organic and inorganic sheets should be arranged alternately in the DHA+/MoS2 LB films unless there is a drastic reorganization of the layer structure. IR RA spectra were also measured. The CH2 rocking band at 720 cm-1 in the RA spectra was found to be singlet, indicating that the hydrocarbon chains of DHA+ are not in orthorhombic packing as in the case of the LB films of cadmium icosanoate.30 The observation of this band in RA spectra also shows that the hydrocarbons are not oriented normal to the film surface. Characterization of the LB Films. Figure 5 shows the XRD patterns of a 10-layer DHA+/MoS2 (0.30) LB film and the unexfoliated MoS2 powder. In the diffraction pattern of the LB film, several broad peaks were observed. These peaks can be interpreted as (00l) reflections of a layered structure with interlayer spacing of 1.85 nm. This value is smaller than the sum of the MoS2 single-layer thickness of 0.62 nm and the length of the long axis of DHA+ molecule (ca. 2.0 nm) in a stretched form. This suggests that DHA+ molecules are tilted with respect to the film normal. Figure 6 shows the AFM image of a single-layer DHA+/ MoS2 (0.30) LB film on HOPG. The LB film is composed of flat, plate-like particles of submicrometer dimension. The particles cover almost all the surface, which suggests that the monolayer at the air-water interface has been successfully transferred on HOPG. The typical step height in the AFM images is ca. 2.0 nm, corresponding roughly to the interlayer spacing determined by the XRD measurements. This suggests that the particles consist not only of MoS2 single-layer particles but also of DHA+ molecules. These results indicate the fabrication of well-ordered structures (superstructures) in the hybrid DHA+/MoS2 (30) Rabolt, J. F.; Burns, F. C.; Schlotter, N. E.; Swalen, J. D. J. Chem. Phys. 1983, 78, 946.
Films of MoS2 and an Ammonium Cation
Langmuir, Vol. 14, No. 22, 1998 6553
Figure 8. Absorbance at 250 nm of UV/vis absorption spectra of 5-layer DHA+/MoS2 (x) LB films as a function of x. The line is to guide the eye.
DHA+/MoS
Figure 6. AFM image of a single-layer 2 (0.30) LB film on HOPG (a). The cross-sectional height profile along the horizontal line in a is shown in b. The height difference between the two points indicated by the two vertical lines in b is ca. 2.0 nm.
Figure 7. Surface pressure-area isotherms of the monolayers of DHA+Br- for various concentrations of MoS2 in the subphase: (a) 0.30, (b) 0.61, and (c) 1.02 g L-1.
LB films in which organic and inorganic components are stacked alternately. Structural Change of the LB Films by Varying the Concentration of MoS2 in the Subphase. The surface pressure-area isotherms of DHA+Br- on MoS2 subphases of various concentrations x are shown in Figure 7. With an increase in MoS2 concentration x, the molecular area of DHA+ increases, i.e., the concentration of DHA+ in the monolayer decreases. This suggests that the tilt angle of the alkyl chains of DHA+ increases with increasing x. UV/vis absorption measurements of the LB films prepared from the subphase of various values of x (0.1 e x e 1.2) showed that the spectral shape is essentially the same. Figure 8 shows the intensity of the 250-nm band of 5-layer LB films as a function of x. The intensity does not depend on x, indicating that the amount of MoS2 incorporated in the LB films is constant. This means that the number of DHA+ molecules per unit area of MoS2
Figure 9. Illustration of the space coordinates for expressing the orientation of DHA+.
particle depends on x. These results will not be reconciled if we assume that the electrostatic interaction between DHA+ and MoS2 at the air-water interface is the only driving force of the formation of the hybrid monolayers and LB films. We should take into account other interactions as well. IR spectra of the LB films were also measured. In both RA and transmission spectra, the intensities of the CH2 antisymmetric and symmetric stretching bands at ca. 2920 and 2850 cm-1, respectively, decreased with increasing x. This shows that the amount of DHA+ molecules decreased with increasing x, which is consistent with the results of surface pressure-area isotherms. The orientation of the main chains of DHA+ molecules in the LB films was investigated by comparing the infrared transmission and RA spectra. We focus on the absorption band intensities of the CH2 symmetric and antisymmetric stretching modes. The transition moments of these bands are perpendicular to each other and also perpendicular to the chain axis of the hydrocarbon. It should be noted that the following discussion is based on the assumption that all the hydrocarbon chains have the same orientation in the LB films. In Figure 9, we introduce Cartesian coordinates (X, Y, Z). X-Y plane is chosen as the surface of the substrate and Z axis is perpendicular to the substrate. The vector P indicates the direction of the chain axis of the hydrocarbon of DHA+ and has a polar angle of θ. Space coordinates (X′, Y′, Z′) was also introduced to express the orientation of DHA+ in the LB films. Y′ axis is parallel to the X-Y plane and Z′ axis has the same direction with P. M1 and M2 are the transition moments of the CH2 symmetric and antisymmetric stretching modes, respectively. M1 and M2 are parallel and perpendicular to the C-C plane of the hydrocarbon, respectively, when the hydrocarbon is in all-trans conformation. M1 has an angle φ with the coordinate X′. Is/Ias ) M12/M22 where Is and Ias are absorbance of the CH2 symmetric and antisymmetric
6554 Langmuir, Vol. 14, No. 22, 1998
Taguchi et al.
Figure 10. XPS survey spectrum of a 10-layer DHA+/MoS2 (0.26) LB film on glass. Table 1. Orientation of DHA+ Molecules in DHA+/MoS2 (x) LB Films as a Function of x x, g L-1
IT,s/IT,as
IR,s/IR,as
θ, deg
φ, deg
0.12 0.33 0.68
1.12 1.69 1.55
0.123 0.100 0.122
53 64 63
66 68 66
Figure 11. Atomic ratio of C/Mo in the LB films as a function of x. The curve is to guide the eye.
stretching bands in the IR spectrum of unoriented DHA+Br- in solution, respectively. In the coordinates (X′, Y′, Z′), the elements of M1 and M2 are expressed as follows:
M1 ) (M1 cos φ, M1 sin φ, 0)
Figure 12. The interlayer distance of the LB films as a function of x. The curve is to guide the eye.
M2 ) (-M2 sin φ, M2 cos φ, 0) The unit vectors X′, Y′, and Z′ are represented in the coordinates (X, Y, Z) as follows:
X′ ) (cos θ cos φ, cos θ sin φ, -sin θ) Y′ ) (-sin φ, cos φ, 0) Z′ ) (sin θ cos φ, sin θ sin φ, cos θ) The ratio of IR,s/IR,as can be expressed as
IR,s/IR,as ) (M1/M2)2(1/tan2 φ) ) (Is/Ias)(1/tan2 φ) where IR,s and IR,as are absorbance of the CH2 symmetric and antisymmetric stretching bands in the RA spectra of the LB films. The ratio of IT,s/IT,as is represented as
IT,s/IT,as) (M1/M2)2(cos2 θ + tan2 φ)/(cos2 θ tan2 φ + 1) ) (Is/Ias)(cos2 θ + tan2 φ)/(cos2 θ tan2 φ + 1) where IT,s and IT,as are absorbance of the CH2 symmetric and antisymmetric stretching bands in the transmission spectra of the LB films. To keep the errors minimal, we obtained the values of IR,s, IR,as, IT,s, and IT,as per monolayer from the slopes in the plots of absorbance as a function of layer number. Calculated θ and φ are summarized in Table 1. We cannot discuss the details of the results quantitatively because the angle θ varies greatly by small changes in IR,s, IR,as, IT,s, and IT,as. However, it is clear that θ, i.e., tilt angle of alkyl chains, increases with increasing x whereas φ remains constant. This is consistent with the results of surface pressure-area isotherms. The LB films were also characterized using XPS. Figure 10 shows the survey scan of a 10-layer DHA+/MoS2 (0.26) LB film. Similar results were obtained for DHA+/MoS2 (x) LB films with all the values of x investigated. The
bands due to Mo, S, and C are observed, whereas Li and Br are not detected. The absence of the Li band indicates that the Li ions intercalated into MoS2 layers by the reaction with n-butyllithium have been removed during the exfoliation. The absence of Br band shows that most of the counteranions of DHA+ have been replaced by MoS2 at the air-water interface. The small intensity of the band of nitrogen should be due to the fact that only one nitrogen atom is present in a DHA+ molecule. It should be noted that the peak positions of Mo 3d5/2 and Mo 3d3/2 in the LB films were similar to those of MoS2 before exfoliation. This indicates that the charge of Mo is +4 in the LB films.31 Figure 11 shows the atomic ratio C/Mo in DHA+/MoS2 (x) LB films determined by the XPS measurements as a function of x. It is clear that this ratio decreases with increasing x. The results are consistent with those of surface pressure-area isotherms, IR spectra and UV/vis absorption spectra since the surface pressure-area isotherms and IR spectra show the decreasing concentration of DHA+ molecules in the LB films with an increase in x while the UV/vis absorption spectra indicate that the concentration of MoS2 in the LB films is constant. The interlayer spacings of the LB films were measured by XRD. Each of the LB films showed (00l) reflections characteristic to the film. Figure 12 shows the interlayer spacing of the LB films as a function of x. The interlayer spacing decreases with increasing x. The results are consistent with those of IR measurements which show that the tilt angle of the hydrocarbons becomes larger with increasing x. The morphology of single-layer hybrid DHA+/MoS2 (x) LB films with various values of x was observed using AFM. The images were almost the same as shown in Figure 6 except that the typical step height tends to be smaller when the value of x is larger. Plausible Model of the Formation of Inorganic/ Organic Hybrid Films. The results mentioned above (31) Patterson, T. A.; Carver, J. C.; Leyden, D. E.; Hercules, D. M. J. Phys. Chem. 1976, 80, 1700.
Films of MoS2 and an Ammonium Cation
Figure 13. Schematic illustration of the structure of the hybrid monolayer at the air-water interface (a) at low concentration of MoS2 in the subphase before compression, (b) at high concentration of MoS2 in the subphase before compression, (c) at low concentration of MoS2 in the subphase after compression, and (d) at high concentration of MoS2 in the subphase after compression. The rectangles represent single-layer MoS2 platelets.
provide information on the mechanism of the formation of hybrid monolayers at the air-water interface. It should be noted that the electrostatic interaction between the amphiphile and MoS2 is prerequisite to the formation of the hybrid monolayers since we did not see any change in the surface pressure-area isotherm or reflection spectrum of the floating monolayer at the air-water interface when icosanoic acid was used instead of DHA+Br-. This suggests that the polyion complex monolayers are formed at the air-water interface in the case of DHA+Br- through the replacement of the counterions (Br-) of the amphiphiles by MoS2 particles (polyanions) present in the subphase. This is supported by the results of XPS measurements which show that Br- was absent in the LB films. To explain all the experimental results, we assume that all the DHA+ molecules have interaction with MoS2 particles at the air-water interface. This is a reasonable assumption since AFM showed that almost all the LB film surface is covered with platelets consisting of both MoS2 and DHA+. The structures of the materials at the interface before compression are illustrated in Figure 13, parts a and b. When x is small (Figure 13a), the number of DHA+ molecules per unit area of MoS2 is large because each of the MoS2 particles has interaction with a large number of DHA+ molecules. This gives rise to a small tilt angle of the alkyl chains and a large interlayer spacing of the LB film. On the other hand, when x is large (Figure 13b), the number of DHA+ molecules per unit area of MoS2 is small, the tilt angle of the alkyl chains is large, and the interlayer spacing of the LB film is small. The constant amount of MoS2 in the monolayers and the dense packing of the particles in the LB films suggest
Langmuir, Vol. 14, No. 22, 1998 6555
that the monolayers are formed when the surface of the water is fully covered by the particles. This means that the surface area of the floating monolayers at the airwater interface is governed by MoS2 particles and not by DHA+ molecules. The structures of the monolayers are shown in Figure 13, parts c and d, when x is small and large, respectively. If the monolayers are transferred onto solid substrates as they are, all the present results will be explained. The above model explains phenomenologically why the number of DHA+ molecule per unit area of MoS2 depends on the value of x. This model means that the electronic interaction between DHA+ and MoS2 is not the only determining factor of the formation of the hybrid monolayers. The changing density of DHA+ molecules on MoS2 particles will be related to the fact that the negative charges of MoS2 particles arise from the hydroxyl ions adsorbed on the particles. It has been proposed that the hydroxyl ions adsorbed on the basal plane of MoS2 particles can be replaced by water-immiscible organic molecules.25 Further, the presence of both polar and nonpolar adsorption sites is also suggested. In the present study, when DHA+ molecules approach the MoS2 particles (and vice versa), the electrostatic interaction should be very important in the initial stage. DHA+ molecules are brought into contact with the MoS2 particles and all the counterions (Br-) are replaced by the MoS2 particles. This gives rise to efficient electrostatic interaction between DHA+ and MoS2. The surplus hydroxyl ions on the basal plane of MoS2 particles should then be released as proposed in the literature,25 which enables the neutralization of the particles and the different densities of DHA+ molecules on the particles. Conclusions In this study, we have successfully fabricated hybrid LB films of inorganic semiconductor, MoS2, and a cationic amphiphile, DHA+. The results of surface pressure-area isotherms, UV/vis reflection and absorption spectroscopies, IR spectroscopies, XPS, XRD, and AFM all indicate the formation of the hybrid LB films. One of the interesting aspects of this material is that the structure of the LB films depends strongly on the MoS2 concentration in the subphase. This provides us with the opportunity to control the structure of the hybrid LB films. Since MoS2 is electroactive, modification of electronic properties of MoS2 by hybridizing with various cationic amphiphiles should be of great interest. This methodology will be effective to various compounds and offers the possibility of fabricating materials of both fundamental and practical interest. Acknowledgment. We are very grateful to Dr. H. Niino of NIMC for his assistance in the XPS measurements. LA980551R
