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Langmuir 1998, 14, 708-713
Photochemical Generation of Gold Nanoparticles in Langmuir-Blodgett Films Serge Ravaine,† Gail E. Fanucci,† Candace T. Seip,† James H. Adair,‡ and Daniel R. Talham*,† Department of Chemistry, P.O. Box 117200, and Department of Materials Science and Engineering, University of Florida, Gainesville, Florida 32611-7200 Received September 22, 1997. In Final Form: November 25, 1997 Gold nanoparticles were generated by ultraviolet irradiation of Langmuir-Blodgett (LB) films of octadecylamine (ODA), 4-hexadecylaniline (HDA), and benzyldimethylstearylammonium chloride monohydrate (BDSAC) deposited from aqueous HAuCl4 subphases. In contrast, no gold crystals were observed in irradiated LB films prepared from monolayers of dipalmitoyl-DL-R-phosphatidyl-L-serine (DPPS) and dipalmitoyl-L-R-phosphatidylcholine (DPPC). X-Ray photoelectron spectroscopy, UV-visible absorption spectroscopy, atomic force microscopy, and transmission electron microscopy measurements indicated the marked influence of the surfactants used to prepare the LB matrix on the shape of the gold particles. Particles formed in ODA and BDSAC LB films were grown with well-defined crystal faces, while particles generated in HDA LB films were irregular in shape.
Introduction In recent years there has been considerable interest in the production of metallic nanoparticles and nanoparticulate films due to their unique optical properties1 and potential applications2 in the fields of catalysis, electronic/ magnetic components, and electron microscopy markers. A variety of media have been employed for preparing metallic nanoparticles, including aqueous solutions,3-7 microemulsions,8 micelles,9 glasses,10 vesicles,11 tubules,12 silica gels,13 porous membranes,14 monolayers at airwater interfaces,15 and Langmuir-Blodgett (LB) films.16 Of these reaction media, LB films are particularly suited to constructing thin and transparent multilayer films of metallic species. In addition to the applications mentioned above, well-organized thin films containing metallic particles can be used to control interlayer processes,17 such as photoinduced charge separation for energy storage.7 * Author to whom correspondence should be addressed. † Department of Chemistry. ‡ Department of Materials Science and Engineering. (1) Preston, C. K.; Moskovits, M. J. J. Phys. Chem. 1993, 97, 8405. (2) Haruta, M.; Koyabashi, T.; Sano, H.; Yamada, N. Chem. Lett. 1987, 405. (3) Turkevich, J.; Stevenson, P. L.; Hillier, J. Discuss. Faraday Soc. 1951, 11, 55. (4) Esumi, K.; Sato, N.; Torigoe, K.; Meguro, K. J. Colloid Interface Sci. 1992, 149, 295. (5) Duff, D. G.; Baiker, A.; Edwards, P. P. Langmuir 1993, 9, 2301. (6) Grabar, K. C.; Smith, P. C.; Musick, M. D.; Davis, J. A.; Walter, D. G.; Jackson, M. A.; Guthrie, A. P.; Natan, M. J. J. Am. Chem. Soc. 1996, 118, 1148. (7) Schmitt, J.; Decher, G.; Dressick, W. J.; Brandow, S. L.; Geer, R. E.; Shashidhar, R.; Calvert, J. M. Adv. Mater. 1997, 9, 61. (8) Kurihara, K.; Kizling, J.; Stenius, P.; Fendler, J. H. J. Am. Chem. Soc. 1983, 105, 2574. (9) Esumi, K.; Matsuhisa, K.; Torigoe, K. Langmuir 1995, 11, 3285. (10) Hache, F.; Ricard, D.; Flytzanis, C.; Kreibig, U. Appl. Phys. 1988, A47, 347. (11) Meldrum, F. C.; Heywood, B. R.; Mann, S. J. Colloid Interface Sci. 1993, 161, 66. (12) Burkett, S. L.; Mann, S. J. Chem. Soc., Chem. Commun. 1996, 321. (13) Tanahashi, I.; Mitsuyu, T. J. Non-Cryst. Solids 1995, 181, 77. (14) Foss, C. A.; Hornyak, G. L.; Stockert, J. A.; Martin, C. R. J. Phys. Chem. 1994, 98, 2963. (15) Kotov, N. A.; Darbello Zaniquelli, M. E.; Meldrum, F. C.; Fendler, J. H. Langmuir 1993, 9, 3710. (16) Fujihira, M.; Poosittisak, S. J. Electroanal. Chem. 1986, 199, 481.
Barraud and co-workers first showed that silver clusters can be chemically inserted into LB films.18-21 Since these initial studies, multiparticulate layers of silver clusters have been prepared using the LB technique by Fendler and co-workers22 and Zhang and co-workers.23 The formation of LB films containing platinum nanocrystallites has also been reported.24 Very recently, Sastry and coworkers developed another route to LB films containing gold clusters based on electrostatic attractions between negatively charged acid derivatized gold colloids and the positively charged polar groups of an amine Langmuir monolayer.25 The well-defined LB film medium not only provides an opportunity to control the size of inorganic particles, but the layered nature of the films can also lead to control over the shape and even orientation of the particles that form. For example, we recently showed that cadmium dihalide plates and needles prepared from a cadmium arachidate LB film grew with discrete orientations determined by the packing of the organic LB template.26 The use of layered media to control particle shape can also be extended to oil/water/surfactant phases. Adair et al.27 have shown that hexagonal CdS and ZnS platelets are formed by precipitation with the metal cation trapped in the polar regions of the lamellar phase of an octadecylamine-water system. Both the lamellar phase and an affinity of the cations for the amine headgroup were (17) Wokaun, A. Mol. Phys. 1985, 56, 1. (18) Ruaudel-Texier, A.; Leloup, J.; Barraud, A. Mol. Cryst. Liq. Cryst. 1986, 134, 347. (19) Belbeoch, B.; Roulliay, M.; Tournarie, M. J. Chim. Phys. Phys.Chim. Biol. 1985, 82, 701. (20) Leloup, J.; Maire, P.; Ruaudel-Texier, A.; Barraud, A. J. Chim. Phys. Phys.-Chim. Biol. 1985, 82, 695. (21) Barraud, A.; Leloup, J.; Maire, P.; Ruaudel-Texier, A. Thin Solid Films 1985, 133, 133. (22) Meldrum, F. C.; Kotov, N. A.; Fendler, J. H. Langmuir 1994, 10, 2035. (23) Zhang, Y.; Li, Q.; Xie, Z.; Hua, B.; Mao, B.; Chen, Y.; Tian, Z. Thin Solid Films 1996, 274, 150. (24) Meldrum, F. C.; Kotov, N. A.; Fendler, J. H. Chem. Mater. 1995, 7, 1112. (25) Mayya, K. S.; Patil, V.; Sastry, M. Langmuir 1997, 13, 2575. (26) Pike, J. K.; Byrd, H.; Morrone, A. A.; Talham, D. R. J. Am. Chem. Soc. 1993, 115, 8497. (27) Adair, J. H.; Li, T.; Kido, T.; Havey, K.; Moon, J.; Mecholsky, J.; Morrone, A.; Ludwig, M. H.; Wang, L. Mat. Sci. Eng. Rev., in press.
S0743-7463(97)01058-5 CCC: $15.00 © 1998 American Chemical Society Published on Web 01/16/1998
Au Nanoparticle Photochemical Generation in LB Films
shown to be important factors in generating platelets. A further utility of LB systems is that they serve as useful models for studying specific chemical interactions that affect particle growth in layered media, and observations made on LB systems can then be used in the design of larger scale preparations such as those utilizing liquid crystal phases. We report here the synthesis of gold nanoparticles prepared by the photoreduction of AuCl4- ions in LB films prepared from amines, quaternary ammonium ions, and ammonium-functionalized phospholipids. We are interested in adapting the process of forming Au particles by photolysis to deposited LB films in order to determine if the layered medium can lead to anisotropic particles. The metallic nanoparticles have been characterized by UV-visible spectroscopy, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and transmission electron microscopy (TEM). The different LB templates lead to diferent types of Au particles. Single crystals, irregular shaped crystals, or multiply-twinned particles28,29 are formed depending on the nature of the amphiphile used to prepare the LB matrix. TEM and AFM analyses show that when discrete particles are formed, they are platelike with a high surface-to-volume ratio. Experimental Section Reagents and Materials. Hydrogen tetrachloroaurate trihydrate, octadecylamine (ODA), 4-hexadecylaniline (HDA), and benzyldimethylstearylammonium chloride monohydrate (BDSAC) were obtained from Aldrich (Milwaukee, WI). Dipalmitoyl-DLR-phosphatidyl-L-serine (DPPS) and dipalmitoyl-L-R-phosphatidylcholine (DPPC) were purchased from Sigma (St. Louis, MO). All commercial products were used as received. Single-crystal (100) silicon wafers, purchased from Semiconductor Processing Co. (Boston, MA), were used as deposition subtrates for XPS and AFM. Quartz plates (Chemglass, Vineland, NJ) were used as substrates for UV-visible measurements. Carbon coated Formvar-covered 300 Mesh titanium grids (Ted Pella Co., Redding, CA) were used for TEM analysis. The silicon and quartz substrates were cleaned using the RCA procedure30 and dried under nitrogen. Procedures. Spreading solutions were prepared from HPLC grade chloroform (Acros, Pittsburgh, PA) at concentrations of 0.5-3.5 mg/mL. An appropriate amount of the desired solution was carefully spread onto a 10-4 M HAuCl4 aqueous solution (pH ) 3.7), and the spreading solvent was allowed to evaporate for 10 min prior to compression. In the case of BDSAC, the subphase was adjusted to pH 5 using KOH in order to maximize the transfer ratios during the deposition process. The Langmuir monolayers were compressed at 23 ( 1 °C using a continuous barrier speed of ca. 7 (Å2/molecule)/min. For transferring films, target pressures were 25, 25, 28, 35, and 40 mN/m for BDSAC, HDA, ODA, DPPS, and DPPC, respectively. Built-up films were obtained by the vertical lifting method with a dipping speed of 5 mm/min. After each dipping cycle, the substrate was allowed to dry for 10 min in air. Gold nanoparticles were prepared by exposing the deposited LB films to a UV lamp (Model UVGL25 from UVP, Upland, CA) working at 254 nm. The lamp was held 5 cm above the film, delivering UV power of 750 µW/cm2. Instrumentation. The LB experiments were carried out with a KSV Instruments (Stratford, CT) 3000 system with a homemade Teflon trough modified to operate with double barriers. Surface pressure was measured with a platinum Wilhelmy plate suspended from a KSV microbalance. A Barnstead NANOpure (Boston, MA) purification system produced water with a resistivity higher than 17.5 MΩ‚m for all experiments. UV-visible absorbance measurements were performed with a Perkin-Elmer Lambda 6 spectrophotometer. In all cases, the absorption of a (28) Marks, L. D.; Smith, D. J. J. Cryst. Growth 1981, 54, 425. (29) Smith, D. J.; Marks, L. D. J. Cryst. Growth 1981, 54, 433. (30) Kern, W. J. Electrochem. Soc. 1990, 137, 1887.
Langmuir, Vol. 14, No. 3, 1998 709 clean quartz substrate was subtracted from the recorded spectra. TEM analyses were conducted on a JEOL JEM 200 CX electron microscope operating at 200 kV. AFM measurements were performed under ambient conditions with a Nanoscope III AFM (Digital Instruments, Santa Barbara, CA) in tapping mode using silicon nitride tips. XPS was performed with a Perkin-Elmer (Eden Prairie, MN) PHI 5000 Series spectrometer. All spectra were taken using the Mg KR line at 1253.6 eV. The spectrometer has a typical resolution of 2.0 eV, with an anode voltage of 15 kV and power setting of 300 W. The typical operating pressure was 5 × 10-9 atm. Survey scans were performed at a 45° take-off angle with a pass energy of 89.45 eV.
Results and Discussion Langmuir Monolayers. The surface pressure-area (π-A) isotherms of ODA, HDA, BDSAC, DPPC, and DPPS, recorded on a 10-4 M HAuCl4 aqueous solution, are shown in Figure 1. The pH of each subphase is given in the figure caption. Each isotherm is briefly discussed below. ODA. The isotherm of ODA exhibits a sharp increase in surface pressure near 30 Å2/mol, followed by a collapse at 26.0 ( 0.2 Å2/mol. This rather large molecular area suggests that the ODA monolayer is fully protonated and positively charged,31 allowing anions from the subphase to interact with the amine monolayer. This result is consistent with the pKa of ODA in monolayers (10.1).32 HDA. In contrast to the ODA case, the relatively small area at collapse (23.0 ( 0.3 Å2/mol) seen in the HDA isotherm indicates that the amine monolayer is not fully protonated on the acidic HAuCl4 aqueous subphase (pH ) 3.7). This observation is in agreement with the previous report of Zhao and co-workers,33 who found that the pKa of HDA at the air/water interface is 3.6 ( 0.2. Approximatively half of the HDA molecules are then protonated under our experimental conditions. All of the attempts we made to work at lower pH in order to get a fully ionized monolayer of HDA were unsuccessful, as the stability of the resulting Langmuir monolayer was drastically reduced. BDSAC. As already reported in the literature,34 the isotherm of BDSAC exhibits a liquid-expanded state at the air/solution interface. The isotherm of a similar molecule, benzyldimethylhexadecylammonium chloride, has been reported to show hysteresis on an acidic subphase.35 However, no hysteresis phenomenon was observed for BDSAC during compression-expansion-recompression experiments. In fact, the BDSAC monolayer is very stable under compression, until the collapse pressure of 29 mN/m. This stability is due to strong interactions between the anions in the subphase and the positively charged polar headgroups in the monolayer.31 DPPC. The isotherm of DPPC displays three distinguishable phases: the liquid-expanded (LE) region (A > ∼84 Å2/mol), the main transition (LE/LC) region (from A ) 60 to A ) 80 Å2/mol) and the liquid-condensed (LC) phase (A < ∼58 Å2/mol). This result is in good agreement with previously published isotherms.31,36-38 DPPS. As previously reported in the literature,35 DPPS forms a stable condensed film at the air/aqueous solution (31) Gaines, G. L., Jr. Insoluble Monolayers at Liquid-Gas Interfaces; Interscience Publishers: New York, 1966. (32) Betts, J. J.; Pethica, B. A. Trans. Faraday Soc. 1956, 52, 1581. (33) Zhao, X.; Subrahmanyan, S.; Eisenthal, K. B. Chem. Phys. Lett. 1990, 171, 558. (34) Blois, D. W.; Swarbrick, J. J. Colloid Interface Sci. 1971, 36, 226. (35) Minones, J.; Conde, O. Colloid Polym. Sci. 1988, 266, 353. (36) Mitchell, M. L.; Dluhy, R. A. J. Am. Chem. Soc. 1988, 110, 712. (37) Pallas, N. R.; Pethica, B. A. Langmuir 1985, 1, 509. (38) Clemente-Leon, M.; Agricole, B.; Mingotaud, C.; Gomez-Garcia, C. J.; Coronado, E.; Delhaes, P. Langmuir 1997, 13, 2340.
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Figure 2. UV-visible absorption spectra of a 13 layer ODA LB film after exposure to UV light for (a) 0, (b) 15, (c) 20, (d) 25, and (e) 30 min. The small feature between 530 and 550 nm is an instrumental artifact.
Figure 1. π-A isotherms of ODA, HDA, BDSAC, DPPC, and DPPS on a 10-4 M HAuCl4 aqueous solution at 23 ( 1 °C. The pH of the subphase is 3.7, except in the case of BDSAC (pH ) 5).
interface. The mean molecular area at collpase is 41.0 ( 0.3 Å2, indicating that the lipid molecules are in a closepacked state in the fully compressed monolayer. The behavior of both DPPC and DPPS at the surface of pure water at pH ) 3.7 has also been studied. The recorded
π-A isotherms were found to be identical to the ones described above, suggesting little or no interaction between the anionic species in the subphase and the zwitterionic (DPPC) or the negatively charged (DPPS) lipids. Transfer onto Solid Supports. ODA, HDA, and BDSAC monolayers are easily transferred onto hydrophilic substrates at pH of 3.7, 3.7, and 5, respectively. The maximum transfer ratio ranged between 0.85 and 1.0 during both upward and downward movements of the substrates. In the case of BDSAC, lowering the pH induced a drop in the transfer ratio. DPPS monolayers could only be transferred during the upward movement of the substrate. Multiple layers were deposited this way with a transfer ratio between 0.9 and 1.0. It is well-known that it is difficult to transfer more than one layer of DPPC onto solid supports. To our knowledge, only two papers have reported the successful deposition of this lipid,38,39 and all our attempts to deposit well-organized LB films with more than one DPPC layer from the 10-4 M HAuCl4 subphase were unsuccessful. UV-Visible Spectroscopy. The optical properties of colloidal gold provide an opportunity to monitor the formation of gold particles with UV-visible spectroscopy. Figure 2 shows the time evolution of the absorption spectra of a 13 layer ODA LB film under UV illumination over 30 min. It can be seen that the absorption band at 330 nm, which is assigned to the AuCl4- species, initially decreases and a strong plasmon band around 550 nm appears with increased irradiation time. At the same time, the color of the LB film changes gradually from colorless to deep purple. This result, similar to those previously obtained in other media, including aqueous solutions,40 silica gel,13 and rodlike micelles,9 indicates clearly that the photoreduction of gold ions to zerovalent gold nanoparticles occurs very efficiently. Furthermore, it has been reported that the full-width at half-maximum (fwhm) of the plasmon peak decreases with increasing gold particle size.41 The fwhm of the 550 nm plasmon band gradually decreases from 105 to 92 nm over 30 min, indicating that the gold nanoparticles become larger with further irradation. The absorption spectra of an 11 layer BDSAC LB film and an 11 layer HDA LB film after UV irradiation are shown in Figures 3 and 4, respectively. In both cases, the (39) Peng, J. B.; Prakash, M.; Macdonald, R.; Dutta, P.; Ketterson, J. B. Langmuir 1987, 3, 1096. (40) Yi, K. C.; Mendieta, V. S.; Castanares, R. L.; Meldrum, F. C.; Wu, C.; Fendler, J. H. J. Phys. Chem. 1995, 99, 9869. (41) Genzel, L.; Martin, T. P.; Kreibig, U. Z. Phys. 1975, B21, 339.
Au Nanoparticle Photochemical Generation in LB Films
Figure 3. UV-visible absorption spectra of an 11 layer BDSAC LB film after exposure to UV light for (a) 0, (b) 45, (c) 60, and (d) 120 min. The small feature between 530 and 550 nm is an instrumental artifact.
Langmuir, Vol. 14, No. 3, 1998 711
Figure 5. UV-visible absorption spectra of a 10 layer DPPS LB film after exposure to UV light for (a) 0, (b) 10, (c) 30, (d) 60, and (e) 120 min. The small feature between 530 and 550 nm is an instrumental artifact.
Figure 4. UV-visible absorption spectra of an 11 layer HDA LB film after exposure to UV light for (a) 0, (b) 60, and (c) 120 min. The small feature between 530 and 550 nm is an instrumental artifact.
gradual appearance of a plasmon band around 550 nm with a decreasing fwhm can be observed, indicating that metallic gold particles are formed and grow under UV irradiation. However, these absorption peaks are less intense than the band shown in Figure 2 for the ODA film. The reduced intensity is also obvious to the naked eye as the final purple color of the BDSAC and HDA LB films is much less pronounced than that of the ODA LB film. No change in the color of a white 10 layer DPPS LB film was observed after up to 2 h of UV illumination, and there was no change in its UV-visible spectrum over that time (see Figure 5). X-ray Photoelectron Spectroscopy. XPS analyses of the ODA, HDA, and BDSAC LB films reveal that the elements present in the films are C, O, N, Cl, and Au. The Au 4f5/2 (88 eV) and Au 4f7/2 (84 eV) spin doublet is always clearly observed, indicating that gold particles have been produced by irradiation. Studies on a one layer DPPC film, deposited by the scooping technique show the presence of C, O, N, and P, but no peaks indicating the presence of gold were detected. These results are in agreement with the UV-visible data. Surprisingly, XPS analysis of a 10 layer DPPS LB film reveals the presence of gold. We speculate that, in spite of repulsive electrostatic forces between the polar head of the lipid and the anionic species in the subphase, some AuCl4- ions are incorporated into the LB film during its deposition due to
Figure 6. (a) TEM micrograph of gold nanoparticles grown in a nine layer BDSAC LB film exposed to UV irradiation for 120 min. The arrows indicate MTPs of decahedral (D1 and D2) and icosahedral (I) morphology. (b) TEM micrograph of a triangular gold single crystal with truncated corners, grown in a nine layer BDSAC LB film after exposure to UV irradiation for 120 min.
incomplete drainage of the aqueous subphase. Photoreduction of these ions under the UV light produces few gold nanoparticles, which do not lead to any change in color of the DPPS LB film and cannot be detected by UVvisible spectroscopy. Transmission Electron Microscopy. More structural information on the gold nanoparticles is provided by TEM analysis. Figure 6a shows a micrograph of a nine layer BDSAC LB film which has been irradiated for 2 h under UV light. The gold particles, with sizes ranging from 20 to 300 nm, can be classified into two different categories. The first type includes multiply-twinned
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Figure 7. (a, b) TEM micrographs of gold nanoparticles grown in a nine layer ODA LB film irradiated for 120 min.
Figure 8. (a, b) TEM micrographs of gold nanoparticles grown in a nine layer HDA LB film irradiated for 120 min.
particles (MTPs), which are normally found in the early stages of growth of small face-centered-cubic (fcc) particles.28 Decahedral MTPs with 〈100〉 and 〈110〉 orientation (labeled D1 and D2, respectively) and icosahedral MTPs (labeled I) can be observed. Such particles are similar to those previously obtained by evaporation of bulk metal28,42 or under organic monolayers.40 A large number of a second type of gold particle, which are 〈111〉 epitaxed triangular single crystals,29 are also present. These crystals generally have truncated corners (see Figure 6b).43 Other small crystals can also been seen in Figure 6a and are believed to be representative of the first stages of triangular particle formation, before the characteristic shape has developed.43 Each type of particle can also be observed in Figure 7, which displays TEM micrographs of a nine layer ODA LB film previously irradiated for 2 h. The rather high density of uniformly distributed metallic particles, which is almost three times greater than results found in the cases of BDSAC and HDA, indicates that the ODA LB film is a very effective template for their growth, as previously suggested by the UV-visible data. The uniformity of the size of the observed crystals is certainly the consequence of a homogeneous distribution of the AuCl4- anions inside the LB matrix, which should present a high degree of organization. Gold particles produced in the HDA LB films are markedly different from those described above. As can be seen in Figure 8, large and irregular crystals with sizes ranging from 200 to 800 nm are formed among very small particles. This result is similar to that previously seen for the production of gold crystals by carbon monoxide reduction of HAuCl4 in bulk3 or under neutral octadecyl mercaptan monolayers.40 It appears that the partial neutrality of the HDA matrix may be responsible for its inability to template the growth of
oriented gold particles. A one layer DPPC film formed by the scooping technique and a ten layer DPPS LB film were also analyzed by TEM after UV irradiation for 2 h. No gold particles could be observed in either case, indicating that only a small number of gold ions, if any, were deposited in these films. Atomic Force Microscopy. AFM studies of ODA, HDA, and BDSAC LB films not only confirm the results obtained by TEM in terms of size and shape of the gold crystals but also reveal that the photoreduced metallic particles are rather thin. The surface analysis of a nine layer BDSAC LB film displayed in Figure 9 reveals a variation of the height of 15 ( 5 nm due to the presence of the gold nanoparticles. Similar values were obtained with ODA and HDA LB films. These results indicate that the gold crystals, which are 5-10 times larger than those generated by UV irradiation using other media,13 including Langmuir monolayers,40 exhibit a high surface to volume ratio. Mechanism of Particle Growth. The observation of gold particles in some of the films and not others indicates that strong attractive electrostatic interactions exist between the positively charged polar heads of ODA, HDA, and BDSAC and the AuCl4- anions present in the subphase. These electrostatic forces allow the transfer of the anionic species during the LB film deposition. As the zwitterionic polar heads of DPPS and DPPC do not induce similar attractive interactions, few gold anions are transferred onto solid substrates using these lipid as templates. Moreover, comparison of the UV-visible spectra displayed in Figures 2 and 4 indicate that, under our experimental conditions, the ODA monolayer binds the AuCl4- anions more efficiently than the HDA Langmuir film due to the incomplete protonation of the HDA monolayer. As was already pointed out, only half of the HDA molecules are ionized at our working pH of 3.7, thereby reducing the number of effective binding sites in the aniline monolayer. The observed differences between
(42) Marks, L. D.; Howie, A. Nature 1979, 282, 196. (43) Duff, D. G.; Curtis, A. C.; Edwards, P. P.; Jefferson, D. A.; Johnson, B. F. G.; Kirkland, A. I.; Logan, D. E. Angew. Chem., Int. Ed. Engl. 1987, 26, 676.
Au Nanoparticle Photochemical Generation in LB Films
(b)
Figure 9. (a) AFM image (700 × 700 nm) and (b) corresponding section analysis of a nine layer BDSAC LB film deposited onto a silicon wafer and exposed to UV light for 120 min.
the UV-visible spectra of the ODA and BDSAC LB films after irradiation cannot be explained in the same way, as both of these films are composed of amphiphilic molecules with a positively charged headgroup. However, the large size of the BDSAC molecule decreases the charge density at the monolayer interface, reducing the number of AuCl4ions that are transferred. Geometric and stereochemical factors, which have been shown to play an important role in the oriented nucleation and growth of organic44-46 and inorganic47-52 materials at film/water interfaces, may also be partially responsible for the different behavior. The (44) Landau, E. M.; Levanon, M.; Leiserowitz, L.; Lahav, M.; Sagiv, J. Nature 1985, 318, 353. (45) Weissbuch, I.; Addadi, L.; Leiserowitz, L.; Lahav, M. J. Am. Chem. Soc. 1988, 110, 561. (46) Landau, E. M.; Wolf, S. G.; Levanon, M.; Leiserowitz, L.; Lahav, M.; Sagiv, J. J. Am. Chem. Soc. 1989, 111, 1436. (47) Zhao, X. K.; Yang, J.; McCormick, L. D.; Fendler, J. H. J. Phys. Chem. 1992, 96, 9933. (48) Mann, S.; Heywood, B. R.; Rajam, S.; Birchall, J. D. Nature 1988, 334, 692. (49) Heywood, B. R.; Mann, S. J. Am. Chem. Soc. 1992, 114, 4681. (50) Heywood, B. R.; Mann, S. Langmuir 1992, 8, 1492.
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spacing between ammonium headgroups in the ODA film is much less than in the BDSAC film. Under some circumstances, the organic LB matrix can influence the morphology and orientation of growing inorganic particles, as was previously observed for CdI2 and CdBr2 particles grown by slow diffusion of the corresponding hydrohalogen gas into cadmium arachidate LB films.26 The CdI2 and CdBr2 particles prepared in this way formed with discrete orientations that could be related to the organization of the organic LB matrix.26 Similar effects have not been observed in the present study for the photochemical generation of gold particles in LB films. While the size and dispersion of the particles changes as the surfactant headgroup changes, these differences are most likely attributed to differences in the distribution of AuCl4- ions in the film. There is no indication that the ammonium ion headgroups used here direct crystal growth by interacting with specific faces of the growing gold particles. However, the platelike quality of the particles does reflect the thin film nature of the LB film templates. Once particles nucleate, their subsequent growth is regulated by diffusion of gold ions in the organic matrix. The supply of ions is limited to the thin film, and diffusion is easiest parallel to the planes favoring the growth of plates. The gold particles grow to be much thicker than a single organic layer, and the lamellar structure of the ODA, HDA, and BDSAC films is disrupted after the particles form. After photoreduction, X-ray diffraction is dominated by the Au particles and interlayer spacings due to the LB film are no longer observed. Conclusion Our study has demonstrated that gold particles are formed upon UV irradiation of LB films of positively charged amphiphiles transferred from an aqueous HAuCl4 subphase. TEM and AFM measurements show that the size and shape of the gold crystals is markedly influenced by the nature of the surfactant in the LB films. The predominantly platelike particles range in size from 20 to 800 nm across. Acknowledgment. This work was supported by NASA (Grant NAG8-1244). We thank Prof. R. S. Duran and Prof. R. S. Drago for making the atomic force microscope and the UV-visible spectrophotometer available to us, respectively. We thank the University of Florida Major Analytical Instrumentation Center for instrument time. LA9710583 (51) Rajam, S.; Heywood, B. R.; Walker, J. B. A.; Mann, S.; Davey, R. J.; Birchall, J. D. J. Chem. Soc., Faraday Trans. 1991, 87, 727. (52) Heywood, B. R.; Rajam, S.; Mann, S. J. Chem. Soc., Faraday Trans. 1991, 87, 735.
