Langmuir 1997, 13, 2575-2577
Lamellar Multilayer Gold Cluster Films Deposited by the Langmuir-Blodgett Technique K. S. Mayya, V. Patil, and Murali Sastry* Materials Chemistry Division, National Chemical Laboratory, Pune 411 008, India Received November 29, 1996. In Final Form: February 11, 1997
The area of nanoparticle research has witnessed tremendous growth due to the unusual chemical and physical properties demonstrated by this intermediate state of matter.1 Many approaches are currently being pursued for the synthesis and organization of nanoparticles with potential applications in mind. The more exciting and experimentally simple routes are based on a two-step process involving synthesis of colloidal nanoparticles and immobilization via self-assembly,2,3 an interesting extension of which has been used to form multilayers of gold and CdS nanoparticles by Brust et al.4 Nanoparticles may also be grown in a single step by chemical synthesis under or within the ordering influence of an organic template.5-8 Fendler and others have recognized that the air-water interface can be used for the organization of surfactantstabilized nanoparticles and have shown that multilayer films of the nanoparticles can be deposited using the versatile Langmuir-Blodgett (LB) technique.9,10 In a totally different approach, the growth of multilayer nanoparticle films by the LB technique was also shown to be possible through interaction of suitably derivatized colloidal particles from the subphase with the polar group of the Langmuir monolayer.11,12 This latter approach for the deposition of lamellar particulate films has not received much attention, and in this paper we demonstrate that reasonably compact, lamellar multilayer gold colloidal particle films can be transferred to solid supports by the LB method. To the best of our knowledge, formation of metal cluster films by this method has not been demonstrated until now and has many advantages over the chemical insertion route6-8 currently receiving attention. * Author for communication. Ph: 0091-212-337044. Fax: 0091212-330233. E-mail:
[email protected]. (1) Henglein, A. Top. Curr. Chem. 1988, 143, 113. (2) Colvin, V. L.; Goldstein, A. N.; Alivisatos, A. P. J. Am. Chem. Soc. 1992, 114, 5221. (3) (a) Freeman, R. G.; Hommer, M. B.; Grabar, K. C.; Jackson, M. A.; Natan, M. J. J. Phys. Chem. 1996, 100, 718. (b) Grabar, K. C.; Allison, K. J.; Baker, B. E.; Bright, R. M.; Brown, K. R.; Freeman, R. G.; Fox, A. P.; Keating, C. D.; Musick, M. D.; Natan, M. J. Langmuir 1996, 12, 2353. (4) Brust, M.; Etchenique, R.; Calvo, E. J.; Gordillo, G. J. J. Chem. Soc., Chem. Commun. 1996, 1949. (5) (a) Yang, J.; Meldrum, F. C.; Fendler, J. H. J. Phys. Chem. 1995, 99, 5500. (b) Yi, K. C.; Horvolgyi, Z.; Fendler, J. H. J. Phys. Chem. 1994, 98, 3872. (6) Pan, Z.; Liu, J.; Peng, X.; Li, T.; Wu, Z.; Zhu, Z. Langmuir 1996, 12, 851. (7) (a) Urquhart, R. S.; Furlong, D. N.; Gegenbach, T.; Geddes, N. J.; Grieser, F. Langmuir 1995, 11, 1127. (b) Urquhart, R. S.; Hoffmann, C. L.; Furlong, D. N.; Geddes, N. J.; Rabolt, J. F.; Grieser, F. J. Phys. Chem. 1995, 99, 15987. (8) (a) Leloup, J.; Maire, P.; Ruadel-Teixier, A.; Barraud, A. J. Chim. Phys. (Paris) 1985, 82, 695. (b) Leloup, J.; Ruadel-Teixier, A.; Barraud, A. Thin Solid Films 1992, 210/211, 407. (9) (a) Meldrum, F. C.; Kotov, N. A.; Fendler, J. H. Langmuir 1994, 10, 2035. (b) Kotov, N. A.; Meldrum, F. C.; Wu, C.; Fendler, J. H. J. Phys. Chem. 1994, 98, 2735. (10) Nakaya, T.; Li, Y.; Shibata, K. J. Mater. Chem. 1996, 6, 691. (11) (a) Tian, Y.; Wu, C.; Fendler, J. H. J. Phys. Chem. 1994, 98, 4913. (b) Zhao, X. K.; Xu, S.; Fendler, J. H. J. Phys. Chem. 1990, 94, 2573. (12) Peng, X.; Zhang, Y.; Yang, Y.; Zou, B.; Xiao, L.; Li, T. J. Phys. Chem. 1992, 96, 3412.
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The strategy consists of electrostatically immobilizing negatively charged, carboxylic acid derivatized colloidal gold particles at the air-water interface using positively charged polar groups of a fatty amine Langmuir monolayer. The gold colloid was prepared a la Lee and Meisel13 by dissolving 240 mg of chloroauric acid (HAuCl4) in 500 mL of water and the solution brought to a boil, after which a solution of 1% sodium citrate (50 mL) was added with continued boiling for ∼1 h. This yielded a hydrosol with a pH of 3. After cooling the hydrosol, capping of the gold clusters was done by mixing 9 mL of the gold hydrosol with 1 mL of the 10-4 M concentrated solution of the bifunctional molecule 4-carboxythiophenol (4-CTP) in absolute ethanol. This leads to an overall Au/4-CTP stoichiometry of ca. 140:1 in the hydrosol. The process of capping may be viewed as three-dimensional self-assembly of the bifunctional molecule 4-CTP on the gold cluster surface.14 The thiol groups are known to bind strongly to gold clusters,14 yielding carboxylic acid derivatization of the gold particles. A red shift in the surface plasmon resonance from 525 to 535 nm was detected by optical absorption spectroscopy, indicative of coordination of the bifunctional molecule to the gold cluster surface. A typical low-magnification transmission electron micrograph15 of the 4-CTP-capped gold sol is shown in Figure 1A, the scale bar corresponding to 120 nm. The particle size distribution (PSD) obtained from the micrographs is shown in Figure 1B. A Gaussian fit to the PSD data yielded a mean cluster diameter of 12.6 nm and a standard deviation of 2.9 nm. After allowing the excess ethanol in the gold hydrosol solution to evaporate for 20 min, 60 µL of a chloroform solution of octadecylamine (C18 amine, chloroform solution concentration of 1.01 mg of octadecylamine in 1 mL of chloroform) was evenly spread on the gold hydrosol in a Teflon trough of 600 cm2 surface area. The hydrosol pH was adjusted to 8 using ammonia. At this pH, both the carboxylic acid and amine groups are fully ionized and oppositely charged (pKB of C18 amine ) 10.6; pKA of 4-carboxythiophenol ) 4.5), yielding maximum electrostatic interaction. The surface charge on the gold clusters leads to great stability of the cluster size distribution, which was found to remain unchanged over many weeks. Figure 2 shows the surface pressure vs area per molecule plot (π-A isotherm, measured on a Nima Model 611 Langmuir trough equipped with a Wilhelmy plate) for C18 amine monolayers on water (curve A, water pH ) 8) and on the gold hydrosol recorded after 30 min (curve B, solid line), 60 min (dotted line), and 90 min (dashed line) of spreading of the fatty amine monolayer. An expansion of the monolayer on the gold hydrosol surface with time is observed when compared with the amine monolayer on pure water, which is indicative of cluster attachment to the charged headgroups of the amine monolayer. The π-A isotherms after 60 min of equilibration of the monolayer remain unchanged. A region of fairly large incompressibility is observed for the monolayers on the gold hydrosol, which is important for transfer of compact LB monolayers. The inset shows the variation in monolayer area with time at a surface pressure of 25 mN/m. A steady decrease in area is observed which was found to stabilize after ∼4 h of compression of the monolayer. This time interval and surface pressure determine the optimum conditions for transfer of the gold cluster-C18 amine films to solid substrates. (13) Lee, P. C.; Maisel, D. J. Phys. Chem. 1982, 86, 3391. (14) Weisbecker, C. S.; Merritt, M. V.; Whitesides, G. M. Langmuir 1996, 12, 3763. (15) Transmission electron microscopy measurements were performed on a Phillips TEM 301 T instrument operating at 80 kV at a magnification of 57 000.
© 1997 American Chemical Society
2576 Langmuir, Vol. 13, No. 9, 1997
Notes
Figure 3. (A) QCM mass uptake as a function of number of gold cluster-amine monolayers with the gold hydrosol at pH ) 8. The solid line is a linear fit to the data. (B) XRD pattern of a 20 ML C18 amine-gold cluster LB film (upper curve, ×200) together with the XRD pattern of a thick Au film deposited on glass. Peak assignments are indicated in the figure.
Figure 1. (A) TEM image of 4-CTP-capped gold colloidal particles. Scale bar is 120 nm. (B) Histogram of the gold cluster size distribution obtained from electron microscopy. The solid line is a Gaussian fit to the PSD data.
Figure 2. π-A isotherms of octadecyl amine on (1) water at pH ) 8 (curve A); (2) surfactant-stabilized gold hydrosol (pH ) 8) after 30 min (curve B, solid line); 60 min (dotted line); and 90 min (dashed line) of equilibration of monolayer. Inset shows variation in monolayer area with time at a constant surface pressure of 25 mN/m.
Under the conditions arrived at above, it was found that good quality multilayer films of the gold cluster-C18 amine monolayer could be built up by sequential transfer onto Si(111) wafers and quartz substrates by the classical Langmuir-Blodgett technique. The quality of the gold cluster LB films improved considerably if a hydrophobic substrate was used. This was indicated by a monolayer transfer ratio that remained consistently in the range 0.91.0 for transfers of up to 20 monolayers (MLs). Hence all results presented below are for substrates rendered hydrophobic by deposition of 1 ML of lead arachidate prior to transfer of the gold cluster LB film. Quartz crystal microgravimetry (QCM)16 was used to check the integrity of the monolayers during the successive transfers onto the substrate surface. Figure 3A shows the cumulative mass transfer variation with number of immersion cycles of the gold-coated quartz crystal. The (16) QCM measurements were performed on an Edwards FTM5 microbalance using a 6 MHz quartz crystal. The stability and resolution of the microbalance were (1 Hz, yielding a mass sensitivity of 12.1 ng/cm2.
gold coating on the quartz crystal was hydrophobic, and hence a lead arachidate monolayer was not used for the QCM measurements. Good transfer (0.9-1.0 monolayer transfer ratio) was obtained for immersion rates of 25 mm/min with 15 min drying time between successive immersions. From Figure 3, it is seen that uniform transfer occurs consistently for a number of immersion cycles. Uniform gold cluster LB films could be deposited up to 20 MLs thickness, after which the transfer ratio deteriorated. The mass increase per bilayer determined from a linear least squares fit to the QCM data (Figure 3A, solid line) is 4500 ng/cm2. Taking a value of 130 Å from TEM measurements (Figure 1B) as the diameter of the gold clusters, this mass increase corresponds to 25% surface area coverage of the colloidal particles within the hydrophilic region of the film. This value of surface coverage is larger than the 15% obtained by Natan et al.3b for self-assembled colloidal gold particle monolayers. The rather small surface coverages obtained for such colloidal systems may be rationalized in terms of strong intercluster repulsive interactions which will certainly be operative in the negatively charged clusters studied here. The X-ray diffraction pattern using Cu KR radiation for a 20 ML C18 amine-gold cluster film grown on hydrophobized Si(111) wafers is shown in Figure 3B together with the diffraction pattern of a thick gold film deposited on glass by thermal evaporation. While the (111) and (200) peaks are observed for the thick gold film, the (200) reflection is not discernible above the noise in the LB film diffraction pattern. The size of gold clusters in the LB film was estimated from the (111) Bragg peak width to be 12 ( 2 nm, which agrees favorably with the cluster size in the primary hydrosol determined using TEM. UV-vis optical absorption measurements were performed on LB films of the gold cluster-C18 amine films of varying thickness transferred onto hydrophobized quartz substrates. Figure 4A shows the measured optical absorption spectra for films of 2-8 ML thickness, while Figure 4B shows the variation in the surface plasmon absorption maximum (at 650 nm) with film thickness. The linearity of the plot in Figure 4B is indicative of layerby-layer growth of the gold cluster film in a lamellar fashion11a by the LB technique. The optical properties of the films were found to remain unchanged on storage under ambient conditions for many weeks. No deterioration in the optical properties was found for prolonged immersion of the gold cluster LB films in water, indicating strong attachment of the clusters to the hydrophilic groups of the amine monolayers. This was corroborated by QCM measurements on films immersed in water for weeks with no detectable loss in mass. While QCM and optical absorption spectroscopy measurements do indicate lamellar growth of the LB films of
Notes
Figure 4. (A) Optical absorption spectra of several multilayer C18 amine-gold cluster LB films (thickness indicated next to the curves) deposited on hydrophobized quartz substrates. (B) Plot of the variation in optical absorbance at 650 nm with number of monolayers in the LB films shown in part 4a. The solid line is a linear fit to the data. The inset shows a plot of the variation in film thickness with number of monolayers transferred onto hydrophobized quartz substrates determined from optical interferometry. The solid line is a linear least squares fit to the data.
C18 amine-gold clusters, optical interferometry gives direct evidence for such a deposition. The inset of Figure 4B shows a plot of the LB film thickness as determined by optical interferometry vs the number of monolayers transferred onto hydrophobized glass substrates. It is clearly seen that there is a linear increase in film thickness up to 20 MLs of the C18 amine-gold cluster complex. From a linear least squares fit to the film thickness data, a thickness increment of 15 nm per bilayer (per dip) is calculated. Assuming a 5.0 nm thickness contribution per bilayer from the C18 amine molecule,17 the size of gold colloidal particles in the bilayer is determined to be 10 nm, in reasonable agreement with TEM and XPD data.
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To summarize, it has been demonstrated using QCM, optical absorption spectroscopy, and optical interferometry that lamellar films of carboxylic acid derivatized gold colloidal particles can be grown by the versatile Langmuir-Blodgett technique through electrostatic attachment of the clusters to fatty amine Langmuir monolayers. Large anionic complexes such as [PtCl6]2- and [TiO(C2O4)2]2- have been successfully incorporated into fatty amine multilayers by the Langmuir-Blodgett technique,17,18 and the incorporation of gold clusters in the manner described above may be viewed as an analogous process where the cluster behaves like a “giant anion”. The generation of nanoparticles in organic matrices as described in this paper shows promise for the deposition of well-defined superlattice structures through alternate transfers in different hydrosol subphases with different Langmuir monolayers and in the deposition of lamellar films with different clusters within the same hydrophilic layers for which no viable methodology currently exists. Future work will focus on the above problems as well as understanding the nature of complexation of the clusters at the air-water interface. Acknowledgment. Financial support from the Council of Scientific and Industrial Research (CSIR), Govt. of India, is gratefully acknowledged by K.S.M. and V.P. The authors thank Dr. P. Ganguly, Head, Materials Chemistry Division, NCL Pune, for use of the Langmuir trough; Dr. K. Vijayamohanan, Materials Chemistry Division, NCL Pune for 4-CTP; and Ms. Z. Ansari, Dept. of Physics, University of Poona, for assistance with optical interferometry measurements. LA962057Y (17) Ganguly, P.; Paranjape, D. V.; Sastry, M. J. Am. Chem. Soc. 1993, 115, 793. (18) Ganguly, P.; Paranjape, D. V.; Sastry, M. Langmuir 1993, 9, 577.
