Langmuir 1996, 12, 851-853
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Notes Formation of Cadmium Sulfide Monolayers within Stearic Acid Langmuir-Blodgett Films Zhiyu Pan and Juzheng Liu Department of Chemistry, National Lab of Molecular and Biomolecular Electronics, Southeast University, Nanjing 210096, People’s Republic of China Xiaogang Peng and Tiejin Li Department of Chemistry, Jilin University, Changchun 130023, People’s Republic of China Zhonghan Wu and Ming Zhu Physics Department, Southeast University, Nanjing 210096, People’s Republic of China Received March 10, 1995. In Final Form: September 25, 1995
Introduction The synthesis and characterization of Q-state inorganic nanoparticles represent areas of intense recent research. In this field it is a very important problem to make the Q-state inorganic nanoparticles in an orderly form and, at same time, maintain the properties of each individual nanoparticle.1-3 The Langmuir-Blodgett (LB) method is one of the methods used for the preparation of orderly assemblies of inorganic nanoparticles.4 Because the distance between the adjacent metal ions is about 0.5 nm within the fatty acid salt LB films, only the inorganic compound monolayers can be synthesized in an orderly way by the reaction of the fatty acid salt LB films with other small chemical reagents.4,5 After the preparation of lead sulfide monolayers within stearic acid (SA) LB films,5 we have tried to prepare cadmium sulfide monolayers within SA LB films. Cadmium stearate (CdSt2) LB films can be easily deposited by a method similar to that used for lead stearate (PbSt2) LB films. The transfer properties of the two stearate salt LB films are also similar to each other.6 Furthermore, the structure of the two stearate salt LB films revealed by X-ray diffraction (XRD) and infrared spectroscopy (IR) are almost the same:5,7 the two kinds of salt LB films are present as an orthorhombic crystal. On the other hand, the preparation and properties of the nanostructure of cadmium sulfide were widely studied in colloidal solution8 and orderly matrices9 including LB films.10 XRD analysis of PbS and CdS in stearate LB films and IR studies showing conversion of Cd2+ to CdS in stearate and arachidate films have been published showing results (1) Stucky, G. D.; Mac Dougall, J. E. Science 1990, 247, 669. (2) Colvin, V. L.; Goldstein, A. N.; Alivisatos, A. P. J. Am. Chem. Soc. 1992, 114, 5221. (3) Mann, S. Nature 1993, 365, 499. (4) Peng, X.; Lu, R.; Zhao, Y.; Qu, L.; Chen, H.; Li, T. J. Phys. Chem. 1994, 98, 7052 and references therein. (5) (a) Peng, X.; Wei, Q.; Jiang, Y.; Chai, X.; Li, T.; Shen, J. Thin Solid Films 1992, 210/211, 401. (b) Peng, X.; Guan, S.; Chai, X.; Jiang, Y. J. Phys. Chem. 1992, 96, 3170. (6) Peng, J. B.; Ketterson, J. B.; Dutta, P. Langmuir 1988, 4, 1198. (7) Umemura, J.; Kamata, T.; Kawai, T.; Takenaka, T. J. Phys. Chem. 1990, 93, 62. (8) For example: Brus, L. E. J. Phys. Chem. 1986, 90, 2555. (9) For example: Kimizuka, N.; Kunitake, T. Chem. Lett. 1991, 2039.
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similar to those presented by our group,11,12 but not enough attention has been paid to some interesting and detailed phenomena in these systems (the pressure dependence of the structure of the CdS in the LB films and the IR quantitative analysis of the reaction process of LB films and gas etc.). Recently, we reported that pure Y-type PbSt2 LB films can be deposited under a surface pressure of 20 mN/M by using a faster transfer speed (no less than 10 cm/min).13,14 Under the same conditions, pure Y-type CdSt2 LB films can be deposited only by using a transfer speed faster than 15 cm/min.14 However, cadmium sulfide monolayers cannot be prepared by such a reaction of the pure Y-type CdSt2 LB films with H2S. According to the references,15 under a higher surface pressure, pure Y-type LB films of fatty acid salts may be obtained by using a lower transfer speed. In our experiments, we observed that the pure Y-type CdSt2 LB films can be deposited under a surface pressure higher than 25 mN/M by using a slower transfer speed, for example, 1.0 cm/min. This fact implies that the CdSt2 LB films deposited under a higher surface pressure should possess larger interaction between the adjacent hydrocarbon chains within a monolayer. It is the larger interaction that prevents the CdSt2 molecules from overturning when the new monolayer is under the subphase.15 The different interlayer and intralayer interactions are very reasonably due to the different deposited conditions of LB films.16 We guessed that this fact implies that the LB films deposited under higher surface pressure can provide a sufficient position control for the guest cadmium sulfide monolayer. In other words, cadmium sulfide monolayers may be obtained by using the pure Y-type CdSt2 LB films deposited under higher surface pressure as the matrix and the aggregated form of cadmium sulfide in LB films can be influenced by the pressure of H2S and the surface pressure under which the LB films were deposited. Experimental Section The CdSt2 LB films were deposited by the following method. A solution of SA (1 × 10-3 mol/L in CHCl3) was spread on the surface of the subphase. The subphase was CdCl2 (2 × 10-4 mol/L) and NaHCO3 (3 × 10-4 mol/L) aqueous solution. The films were deposited onto the substrate (CaF2, Si, and glass slide) under certain surface pressures, which will be indicated in the text and figure captions. The compression speed was 7 cm2/min. The temperature was controlled to higher than 292 K. XRD and IR spectra were taken with a Rigaku D/max RA diffraction meter and a Nicolet 5DX IR spectrometer, respectively. (10) (a) Ruaudel, A. T.; Leloup, J.; Barraud, A. Mol. Cryst. Liq. Cryst. 1986, 134, 347. (b) Smotkin, E. S.; Lee, C.; Bard, A. J.; Campion, A.; Fox, M. A.; Mallouk, T. E.; Webber, S. E.; White, J. M. Chem. Phys. Lett. 1988, 152, 265. (c) Yi, K. C.; Fendler, J. H. Langmuir 1990, 6, 1519. (d) Scoberg, D. J.; Grieser, F.; Furlong, D. N. J. Chem. Soc., Chem. Commun. 1991, 515. (e) Zhao, X. K.; Fendler, J. H. J. Phys. Chem. 1991, 95, 3716. (11) Erokhin, V.; et al. Makromol. Chem., Macromol. Symp. 1991, 46, 359. (12) Moriguchi, I.; Tanaka, I.; Teraoka, Y.; Kagawa, S. J. Chem. Soc., Chem. Commun. 1991, 1401. (13) Peng, X.; Chen, H.; Kan, S.; Bai, Y.; Li, T. Thin Solid Films 1994, 242, 118. (14) Peng, X. Ph.D. Thesis, Jilin University, People’s Republic of China, 1992. (15) Binks, B. P. Adv. Colloid Interface Sci. 1991, 34, 343 and reference therein. (16) (a) Gupta, V. K.; Kornfield, J. A.; Ferencz, A.; Wegner, G. Science 1994, 265, 940. (b) Kajiyama T.; et al. Chem. Lett. 1989, 1047; Bull. Chem. Soc. Jpn. 1992, 65, 864.
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Langmuir, Vol. 12, No. 3, 1996
Figure 1. (a) X-ray diffraction pattern of a 9-layer CdSt2 LB film deposited on oxide-silicon under 25 mN/M. Top: before the reaction. Bottom: after the reaction. (b) X-ray diffraction pattern of a 9-layer CdSt2 LB film deposited on glass under 30 mN/M. Top: before the reaction. Bottom: after the reaction.
Notes
Figure 2. X-ray diffraction pattern of a 9-layer CdSt2 LB film deposited under 37.5 mN/M. Top: before the reaction. Bottom: after the reaction.
In a vacuum system, CdSt2 LB films were exposed to H2S gas at a pressure of 0.25 Torr for a certain time. This H2S pressure is smaller than that used for the preparation of lead sulfide monolayers reported previously.5,13 The measurements were carried out immediately after the reaction.
Results and Discussion We observed that the H2S pressure has an important effect on the formation of the lead sulfide monolayer within SA LB films.5,13 After we found that cadmium sulfide monolayers cannot be formed by the reaction of pure Y-type CdSt2 LB films deposited under 25 mN/M with H2S at a pressure of 1 Torr, we decreased the H2S pressure from 1 to 0.25 Torr gradually. Indeed lower H2S pressure weakened the destruction of the LB films. However, we still failed to prepare cadmium sulfide monolayers by the use of the CdSt2 LB films deposited under 25 mN/M. This forced us to consider other aspects of the LB matrix. Figure 1 illustrates the XRD patterns of pure Y-type CdSt2 LB films before and after reaction with H2S. The CdSt2 LB films (9 layers thick) were deposited under a surface pressure of 25 mN/M (Figure 1a) or 30 mN/M (Figure 1b). From Figure 1, we see that the structure of the LB films underwent a vigorous change. Before the reaction, the long spacing of the CdSt2 LB films is about 5.0 nm. After the reaction, the amplitudes of the diffraction peaks apparently decreased. Only one of the (001) Bragg peaks can be easily distinguished before the reaction. Moreover, a diffraction peak due to a new phase appears. In other words, the diffraction peaks in the pattern after the reaction correspond to two long spacing values. The first one is 5.0 nm, which equals the value for CdSt2 LB films, and the other is 4.0 nm. This value equals the long spacing of pure Y-type SA LB films.5 It may be an accidental phenomenon. Despite the second long spacing, the XRD patterns indicate that, in the direction perpendicular to the plane of the substrate, the positions of part of the metal ions within the LB films were changed.5 In other words, the cadmium sulfide was
Figure 3. Conversion ratio of CdSt2 to SA by the reaction versus reaction time. The conversion ratios were calculated from the data in IR spectra. O: data of 33-layer CdSt2 LB films. +: 21-layer CdSt2 LB films. 4: 11-layer CdSt2 LB films.
aggregated, the structure of the LB films was partly destroyed, and a new phase appeared. Parts a and b of Figure 1 show different levels of disruption, but the qualitative feature is the same. If the CdSt2 LB films were deposited under 37.5 mN/M, the result was totally different. Figure 2 is the XRD pattern of the CdSt2 LB films (9 layers thick) before and after reaction with H2S for 3 days. From Figure 3, we can find that when the reaction was finished, only about 6% of the CdSt2 was not converted to cadmium sulfide and SA. These residual CdSt2 molecules cannot be converted under these conditions. From Figure 2, we observe that the qualitative features of the XRD pattern were not changed after the reaction. From the position of the diffraction peaks, we found that the value of the long spacing of the LB films was still 5.0 nm. It is well recognized that the reflection intensity of multilayers is dependent on the electron density. For the films composed of a metal soap of a fatty acid, the films can be
Notes
viewed as pseudocrystals and the unit cell of the films as a unit cell of metal ions.17 So these facts imply that, in this case, the position of the metal ions within the polar plane of the LB films was almost not changed.5 In other words, we obtained monolayers of cadmium sulfide. The monolayer structure could also be proved by IR spectra to be just like the result that our group reported.5 All these phenomena were similar to those in the instance of lead sulfide monolayer. In comparison with the XRD pattern before the reaction, the breadths (amplitudes) of all the diffraction peaks in the pattern after the reaction increased (decreased) a little. These phenomena may be due to the change in the electron density in the polar plane of the LB films after the entrance of the sulfur element or the slight movement of the metal ions in the direction perpendicular to the substrate.5 At the same time, we also observed that the reaction speed of the LB films deposited under 37.5 mN/M depended on the layer number of the LB films. This was determined by the record of the IR spectra of the LB films reacted with H2S for a certain time just as in Figure 3. It was found that the time for completing the reaction of 11layer, 21-layer, and 31-layer CdSt2 LB films deposited under 37.5 mN/M was about 42, 78 and 140 h, respectively. It indicated that the reaction speed depended on the layer number of the LB films. It is acknowledged that defects of the surface of the LB films and the interface between LB layers could not be avoided. This seems to imply that the number of defects could be reduced enough to be neglected in the condition in which the LB films were deposited because the H2S gas must infuse into LB films from the pin holes and crack between the molecules of the LB films and react with Cd2+. If there are many defects of the LB films, the layer number dependence on the reaction speed would be difficult to observe. On the other hand, that the reaction speed depends on the layer number would be partially attributed to the formation of the CdS monolayer. Because of the formation of the monolayer, the structure of the LB films is almost not destroyed. We suggest that the 6% CdSt2 was also not converted to CdS and SA due to the formation of the monolayer. Part of the results can be found in the supporting information.
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We reported previously that the lead sulfide monolayers within SA LB films can be prepared by the reaction of PbSt2 LB films deposited under surface pressures higher than 20 mN/M of H2S gas (1 Torr).5,11 However, the above results revealed that only CdSt2 LB films deposited under a much higher surface pressure (as high as 37.5 mN/M) can be used for the formation of cadmium sulfide monolayers. We consider that this is partly due to the smaller interaction between the adjacent hydrocarbon chains within a monolayer of CdSt2 in comparison with that in PbSt2 LB films.14 In general, the sulfide compound is inclined to aggregate in order to decrease the surface energy. As the matrix of the sulfide monolayers, the sublayer of hydrocarbon chain of the LB monolayer should possess enough position control for the inserted inorganic monolayer; i.e., there should be a strong interaction between the hydrocarbon chains of the LB monolayers. Therefore, in comparison with lead stearate LB films, higher surface pressure is necessary for increasing the interaction between the hydrocarbon chain of the CdSt2 LB films. It is also the reason why, to deposit pure Y-type CdSt2 under the surface pressure of 20 mN/M, LB films need higher transfer speed than in the instance of PbSt2 as mentioned above. The fact above would imply another novel conclusion: that the LB technique is an efficient tool to synthesize Q-state inorganic nanoparticles in an orderly form. If we deposit the LB films under different conditions and make them react in different conditions, we can get different structures of inorganic nanoparticles within LB films according to our desire. It is acknowledged that most of the potential applications of LB films are based on the premise of perfect molecular layering and oriention.18 Acknowledgment. The authors thank the reviewers for very informative suggestions and Dr. Z. H. Lu for detailed discussions. Supporting Information Available: Integrated intensity as a function of time for CdO and CO2 stretching vibrations after reaction of H2S gas with CdSt2 LB films (2 pages). Ordering information is given on any current masthead page. LA9501905
(17) (a) Pomerantz, M.; Segmuller, A. Thin Solid Films 1980, 68, 33. (b) Henke, B. L. Adv. X-Ray Anal. 1964, 7, 460. (c) Hans, R. P.; Davidson, F. D. Rev. Sci. Instrum. 1965, 36, 230. (d) Yuan, C. W.; Wu, C. R.; Bai, J. J.; et al. Langmuir, in press.
(18) Viswanathan, R.; Zasadzinski, J. A.; Schwartz, D. K. Science 1993, 261, 449.
