Langmuir 1994,10, 3213-3216
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Structure of Cobalt Stearate and Cobalt Sulfide-Stearic Acid Langmuir-Blodgett Films Xuzhong Luo, Zhiqiang Zhang, and Yingqiu Liang* Department of Chemistry, Nanjing University, Nanjing 210093, China Received January 13, 1994. I n Final Form: June 10, 1994@ The local microstructure of 25-layer Langmuir-Blodgett (LB)film of cobalt(I1)stearate (Cost21has been studied by small-angle X-ray diffraction, Fourier transform infrared transmission (FTIR-T)and reflectionabsorption (FTIR-RA)spectra. The hydrocarbon chains in the Cost2 LB film are packed in a hexagonal fashion with all-trans (zi zag! conformation, and the chain axes are aligned normal to the film plane, the After the reaction of the Cost2 LB film with H2S, FTIR and X-ray diffraction long spacing being 50.0 measurements showed that two-dimensional COSmonolayers are formed in the LB matrix with Co2+ substituted by H+. Remarkable changes in the structure of the LB film were observed after the insertion of COSmonolayers. The hydrocarbon chains change into orthorhombic subcell packing and tilt on the substrate, and the long spacing becomes 40.0 A.
1.
Introduction Q-state, low-dimensional layer or cluster structural inorganic substances can be obtained by chemical reactions in the microenvironment of LB films. It was reported that the Q-state thin layers or clusters of Ag,1,2HgS,3-5 PbS,6 and Fe2037inserted in the LB matrixes showed different properties from the bulk substances. The semiconductive characteristics of CuS layers in the stearic acid (SA) LB film hgve been testified by X-ray photoelectron spectroscopy (XPS),8 which shows the potential perspective of applications for the preparation of lowdimensional inorganic substances in the LB matrixes. So further studies on the properties and constructions of inorganic substances inserted into LB films and the preparation of functional materials in this kind of special microenvironment are of great significance. In this paper, the preparation of a 25-layer Cost2 LB film and synthesis of COSin the polar planes of SA LB film are reported, and small-angle X-ray diffraction, Fourier transform infrared transmission (FTIR-T), and reflection-absorption (FTIR-RA) spectra have been recorded toptudy their structural characteristics comprehensively. It has been showed that the LB films before and after the insertion of COS both maintain ordered assembly structures, but the hydrocarbon chain packing fashion, the molecular orientation, and the long spacing change greatly after the insertion, which differs from the results of earlier research on other inorganic substances in LB matrixes. The inserted COSis distributed as twodimensional monolayers in the polar planes of the stable SA LB film. Experimental Section Stearic acid was A.R. grade and recrystallized from ethanol (A.R.grade). CoClz6Hz0, acetone,and chloroform were all A.R. * Author to whom correspondenceshould be addressed. Abstract published inAdvanceACSAbstracts, August 1,1994. (1)Leloup, J.;Mairc, P.; Ruaudel-Teixter,A.; Barraud, A. J . Chem. @
Phys. 1985,82,695. (2) Belbeoch, B.; Roulliay, M.; Tournaric, M. J . Chem. Phys. 1985, 82,701. (3) Ruaudel-Teixter, A.; Leloup, J.;Barraud, A.Mol. Cryst. Lq.Cryst. 1986. ~. . 134. -, 347. (4)Barraud, A.;Leloup, J.;Mairc, P.;Ruaudel-Teixter, A. Thin Solid Films 1985,133, 133. ( 5 ) Zylberajch, C.; Ruaudel-Teixter, A.;Barraud, A. Thin Solid Films 1989,179, 9. (6) Peng, X. G.; Guan, S. Q.; Chai, X. D.; Jiang, Y. S.; Li, T. J.J . Phys. Chem. 1992,96, 3170. (7) Peng, X. G.; Zhang, Y.; Yang, J.; Zou, B. S.; Xiao, L. Z.; Li, T. J. J . Phys. Chem. 1992, 96, 3412. (8) Chen, H. J.;Chai,X. D.; Wei, Q.; Jiang, Y. S.;Li, T. J. Thin Solid Films 1989,178,535. ~
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A , n I2 Figure 1. X-ray diffraction patterns of the 25-layer CoStz LB film. O
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grade. Ion-free water was purified by a double distiller before experiments. The 25-layer LB film of CoStz was built up with a LB d e v i ~ e . ~ The solution of SA (in chloroform, 1.0 x 10-3 M) was spread on a water subphase containing CoClz in a concentration of 1.0 x M with pH adjusted in the range of 5.8-6.2. The film was deposited(Y-type)onto a substrate (CaFz,germanium,or quartz) at a constant surface pressure (25mN/m)and at the temperature of 20 "C. The prepared CoStz LB film was placed into a desiccator with P205 for 24 h before the measurements ofX-ray diffraction and infrared spectra. In a vacuum system, CoStz LB film was exposed to H&g) at a pressure of 5 Torr for 6 h. FTIR and X-ray diffraction experiments were carried out immediately after the reaction. Infrared spectra were measured on a Nicolet 170SX FTIR spectrophotometer equipped with a TGS dector. For the RA measurements, a Barnes Analytical reflection attachment was used at the angle of incidence of 45". X-ray diffraction patterns were obtained using a Rigaku D/max X-ray diffractionmeter. The substrates were quartz, CaFz, and Ge for X-ray diffraction, FTIR-T, and RA measurements, respectively. Before experimentation, CaFz and Ge substrates were cleaned by successive ultrasonication in acetone, chloroform, acetone,and water for 5 min each, and quartz substrate was soaked in hot cleaning solutionfor 24 h and then washed by doubly distilledwater several times.
Results and Discussion 1. Structure of Multilayer Cost2 LB Film. X-ray Diffraction from the Cost2 LB Film. Figure 1shows
the X-ray diffraction patterns of 25-layer Cost2 LB film. A series of equidistant diffraction peaks are displayed in (9) Xu,W. Q.; Tian, Y. C.; Zhang, D. Z.; Liang, Y. Q.Chem. J . Chin. Uniu. 1990,11, 434.
0 1994 American Chemical Society
Luo et al.
3214 Langmuir, Vol. 10, No. 9, 1994 the figure, demonstrating the periodic structure of the film. The peak intensity decreases with increasing 28 angle, and the 4-and 6-order weakening phenomenon appears due to the well-ordered assembly of the hydrocarbon chains, which is considered to be a typical feature of the X-ray diffraction patterns of LB films.1° The long spacing D of LB films is related with a do01value of the corresponding peak as
D = I do,,
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By substituting dool value into eq 1, the long spacing of Cost2 LB film is obtained as 50.0 f 0.1 According to the chain length of Cost2 monomolecule, the repeating unit in the film is bilayer, which conforms to the features of Y-type deposited LB films and also coincides with the D value of other MSt2 LB films (M= Ca, Cd, Pd).6J1J2 Compared with the long spacing 39.6 of multilayer SA LB film,13J4the value of Cost2 undergoes an increase of about 10 A. It was observed that multilayer SA LB film has a very similar structure to that of C-form crystal of SA with its hydrocarbon chains packed in an orthorhombic unit cell. The orientation angle 4 of the chain axis from the normal direction of the film surface in multilayer SA LB film is about 30°.15 But as for 1-layerSALB film the hydrocarbon chain axis is perpendicular to film surface, packed in a hexagonal unit cell. On the other hand, systematic studies on the X-ray diffraction patterns of cadmium stearate LB films have been madell and the long spacing, D (in relates to the number of carbon atoms n as
A.
A
A),
D = 2.5n
+ 5.3
(2)
The orientation angle of the hydrocarbon chain axis in the CdSt2 LB films is evaluated to be about 10" by eq 2 and by other mothods16J7as well. But the result of a RHEED experimentla indicated that the chain axis is oriented highly perpendicular to the film surface. If the roughness of solid substrates is taken into c~nsideration,'~ it can be said that the hydrocarbon chain axis is virtually normal to the microsurface of the substrates in the CdSt2 LB films, and also in the multilayer Cost2 LB film, whose long spacing is the same as the former. So it is concluded that the lon spacing of multilayer stearate salts LB films of ca. 50 indicates the vertical orientation of chain axis to film surface, and if the long spacing was smaller than 50 A,a tilt orientation would be implied which is similar to that of multilayer SA LB film. In other words, the X-ray diffractionmeasurements can be applied to not only study the ordered assembly structure and the long spacing of MStz LB films but also evaluate the orientation of hydrocarbon chain axis. FTIR Spectra of the Cost2LB Film. Figure 2a is the FTIR-T spectrum of 25-layer CoSh LB film. The strong peaks at ca. 2917 and 2850 cm-l are assigned to the antisymmetric and symmetric CH2 stretching vibrations of the Cost2 hydrocarbon chains, r e s p e c t i ~ e l y . ' ~It, ~has ~
x
(10) Pomerantz, M.; Segmuller, A. Thin Solid Films 1980, 68, 33. (ll)Matsuda, A.; Sugi, M.; Fukui, T.; Iizima, S.; Miyahara, M.; Otsubo, Y. J.Appl. Phys. 1977,48, 771. (12) Sugi, M.; Fukui, T.; Iizima, S.; Iriyama, K. Bull. Electrotech. Lab. 1979,43, 825. (13) Clark, G. L.; Leppla, P. W. J.Am. Chem. SOC.1936,58, 2199. (14) Malta, V. J . Chem. SOC.B 1971, 548. (15) Higashiyma, T.; Nogami, K. Bull. Chem. SOC.Jpn. 1972, 45, 2367. (16) Umemura, J.; Kamata, T.; Kawai, T.; Takenaka, T. J. Phys. Chem. 1990,94, 62. (17) Duschl, C.; Knoll, W. J. Chem. Phys. 1988, 88, 4062. (18)Russell, G. J.; Petty, M. C.; Peterson, I. R.; Roberts, G. G.; Lloyd, J. P.; Kan, K. K. J . Mater. Sci. 1984, 3, 25. (19)Snyder, R. G. J. Chem. Phys. 1967, 47, 1316.
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Figure 2. FTIR-Tspectrum (a) and RA spectrum (b) of the 25-layer Cost2 LB film.
been known that the frequencies of CH2 stretching bands are sensitive to the conformation of methylene chain^,^^^^^ and the values of 2917 and 2850 cm-' are indicative of the all-trans (zigzag)conformation of the coagel state of the methylene chains. The band progression in the 14001150 cm-l region due to the CH2 wagging modes of the long (CH&, sequence21,22 confirms in another aspect the results obtained by the analyses of the CH2 stretching bands, indicating the two-dimensional crystalline construction of the Cost2 LB film. The strong coupling bands at ca. 1557 and 1541 cm-l are assigned to antisymmetric COO- stretching vibrations and the broadening in the bands suggests the formation of a dimer structure.23The CH2 scissoring vibration provides structural information about the hydrocarbon chain packing state. This band appears as a single peak near 1468 cm-l, characteristic of the hexagonal packing f a s h i ~ n . This ~ ~ , result ~ ~ agrees well with the single peak of the CH2 scissoring band and subcell packing fashion of 1-layerSA LB film but is greatly different from that of LB films of other stearate salts (Ca, Cd, Pb) in which the CH2 scissoring band splits into two peaks (1463 and 1472 cm-l) caused by the crystal field, characterizing an orthorhombic packing fashion of the hydrocarbon chains in the films. The FTIR-RA spectrum of a 25-layer Cost2 LB film is shown in Figure 2b. Compared with the FTIR-T spectra, the band intensities of CH2 stretching vibrations (ca. 2917 and 2850 cm-l) decrease apparently, and the strong peaks (20)Kawai, T.; Umemura, J.;Takenaka, T.; Kodama, N.; Seki, S. J. Colloid Interface Sci. 1985, 103, 56. (21) Kimura, F.; Umemura, J.; Takenaka, T. Langmuir 1986,2,96. (22) Liang, Y. Q.; Jiang, Y. T.; Tain, Y. C. Acta Phys.-Chim. Sin. 1991, 7, 72. (23) Vogel, C.; Corset, J.;Dupeyrat, M. J.Chem. Phys. 1979,76,903. (24) Kawai, T.; Umemura, J.; Takenaka, T. Bull. Inst. Chem. Res. Kyoto Univ. 1983, 61, 314. (25) Cameron, D. G.; Gudgin, E. F.; Mantsch, H. H. Biochim. Biophys. Acta 1980, 596, 483.
Structure of Cost2 LB Films of the antisymmetric COO- stretching band (ca. 1557 and 1541 cm-'1 and the CHz scissoring band (1468 cm-') disappear in the RA spectra. But the symmetric COOstretching vibration band at ca. 1456 cm-' displays itself in Figure 2b which is not present in the transmission spectra (Figure 2a). The mutual exclusion of these two kinds of spectra is due to the different orientational relations between the electric vectors of incident light and the corresponding transition moments. The electric vector of incidence in the transmission spectra is parallel to the substrate, while in the RA spectra it is vertical. If the transition moment is parallel to the electric vector, the absorption is the maximum, and it is zero when the vector is perpendicular to the transition moment.16p22Comparing the intensity changes in the two spectra, it is evident that the transition moments of the antisymmetric COOstretching and the CHZscissoring bands lie almost parallel to the substrate because they give strong absorption in the transmission spectrum and nearly do not absorb in the RA measurement. As for the symmetric COOstretching band, it strongly appears in the RA spectrum but vanishes in the transmission one, so its transition moment is almost normal to the substrate. Because the transition moment of the CH2 scissoring band is perpendicular to the hydrocarbon chain axis, and the peaks of the CH2 stretching vibrations in RA spectrum are weaker than in transmission one, it can be concluded that the hydrocarbon chains in Cost2 LB film take vertical orientation to the substrate. It should be emphasized in particular that the hexagonal close-packed structure of the hydrocarbon chains in the Cost2 LB film deduced by the single peak of the CHZscissoring band gives powerful evidence for the perpendicular orientation of the chain axis, which is the same as that in 1-layer SA LB film, and is in good aggrement with the results of the X-ray diffraction measurements discussed above. This is the distinct feature of the CoStz LB film diverse from other stearate salt LB films. In other words, the hydrocarbon chains in the Cost2 LB film are packed parallel to each other in a hexagonal unit cell with all-trans conformation, and the chain axes are aligned perpendicular to the film surface. The long spacing of the film is 50 A, and its local microstructure is schematically illustrated in Figure 3. 2. Structure of the COS-SA LB Film. X-ray Diffraction from the COS-SA LB Film. Shown in Figure 4 is the X-ray diffraction pattern of the 25-layer LB film of Cost2 after the reaction with H2S for 6 h. It can be seen that equidistant Bragg peaks remain in the diffraction patterns, but only three peaks are observed due to the weak diffraction intensity. The even-order weakening phenomenon can still be distinguished as in Figure 1, suggesting the maintenance of the periodic arrangement of hydrocarbon chains. The long spacing is obtained by applying eq 1to be 40.0 f0.5 8, that is to say, the lon spacing changes from ca. 50 A (CoStz LB film) to ca. 40 after being treated with H2S. This value is very close to the long spacing of multilayer SA LB film. Furthermore, their FTIR spectra almost coincidewith each other (vide infra), showing that after the treatment Co2+ is substituted by H+ and COSformed in the reaction is inserted into stable SA LB film (designated as COS-SA LB film). Because the electron density of sulfur atoms introduced by the reaction is higher than that of carbon, hydrogen, and oxygen atoms, the electron density distribution in the periodic structural unit of the film is altered and the scattering effect of the atoms is enhanced. This is why only three diffraction peaks are measured (Figure 4). After the reaction the film still retains the well-ordered periodic structure, indicating that the COS
R
Langmuir, Vol. 10, No. 9, 1994 3215
a
Figure 3. Local microstructure of the Cost2 LB film. 0 designate cobalt and oxygen atoms, respectively.
and
P Y \
e
Figure 4. X-ray diffraction patterns of the 25-layer COS-SA LB film, Le., the Cost2 LB film after the reaction with H2S.
produced by the reaction forms two-dimensional monolayers in the stable SA LB film, and no three-dimensional microcrystallites or clusters are generated, for the formation ofthem will inevitably destroy the well-ordered arrays of hydrocarbon chains in the LB film.6,26 Further investigations on Figure 4 show that there are no new phase diffraction peaks. Because Co atoms possess higher electron density than other kinds of atoms in the film, the long spacing obtained by X-ray diffraction (26) Miyoshi, H.; Yamashika, M.; Yoneyama, H.; Mod, H. J . Chem. SOC.,Faraday Trans. 1989,86,815.
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3216 Langmuir, Vol.10,No.9,1994
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Figure 6. Local microstructure of the COS-SA LB film. 0 and 0 designate cobalt and oxygen atoms, respectively. 3000
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Figure 5. FTIR-Tspectra of the COS-SA LB film (a) and the stable multilayer SA LB film (b), respectively. measurements can be regarded as that of the unit cell of Co2+ ions (40 A),27and the cobalt sulfide can thus be thought to spread in the polar planes of the SA LB film. But the long spacing of Co2+subcell is 50 in the CoStp LB film before the reaction. Therefore the great change in D value after the reaction is indicative of the structural variation of the film. It is quite unlike the result of an earlier research on the PbStz LB film prior and subsequent to the treatment with H2S6 Both metal sulfides (COS and PbS) are proved to be inserted in the polar planes of the films as two-dimensional monolayers, but in the case of PbS-SA LB film the long spacing does not change from that of PbStz LB film. This reveal that the structure of the LB film as matrix for the formation of PbS is not the same as the stable SA LB film deposited in a general manner. In fact, PbS are in the metastable SA LB film, which differs from the case of COS-SA LB film. FTIR Spectra of the COS-SA LB Film. Figure 5a is the FTIR-T spectrum of 25-layer COS-SA LB film, i.e., the Cost2 LB film after the reaction upon HpS. And Figure 5b is that of the multilayer SA LB film deposited in a general manner. In these two spectra, the corresponding band frequencies, relative intensities, and the crystal field
A
(27) Yoshioka, Y.; Nakahara, H.; Fukuda, K. Thin Solid Films 1986, 133, 11.
splitting are almost the same as each other. Furthermore, in the spectrum of Figure 5a, the lack of any carbonyl band in the free C=O region near 1745 cm-' and in the sideways dimers which absorb near 1720-1730 cm-l, and also the absence of the antisymmetric COO- stretching bands near 1557 and 1541 cm-l strongly imply that after the reaction all the cobalt(I1) ions are substituted by H' and stable SA LB film containing COS is formed. By comparing the infrared spectrum of Cost2 LB film (Figure 2a) with that of COS-SA LB film, remarkble changes on the CH2scissoring vibrational band are observed. Before the reaction it appears as a single peak near 1468 cm-', and after the reaction it splits into two peaks at 1462 and 1473 cm-l due to the crystal field, demonstrating the transformation of the packing state of the chains from hexagonal fashion to orthorhombic one, just the same as that in stable multilayer SA LB film. It serves as further evidence for the stableness of the SA LB film as a matrix containing COSmonolayers. To sum up, the multilayer COS-SA LB film has the matrix of stable SA LB film whose structural features have been mentioned above, with the long spacing of ca. 40 In the film Co2+ions are limited in the polar planes by the ring structure of carboxylic groups arising from hydrogen bonds, forming two-dimensional monolayers by the connection of sulfur atoms. The local microstructure of the COS-SA LB film is depicted in Figure 6. Further investigation reveals that the properties of the cobalt sulfide described above are different from those of bulk COSwhich will be reported in another paper. Acknowledgment. This work was financially supported by a grant for major research project from the State Science and Technology Commission of China.
A.
