Langmuir 1997, 13, 5511-5513
Unusual Partitioning of Clusters Based on Their Size during Electrostatically Controlled Diffusion of Carboxylic Acid Derivatized Colloidal Particles in Thermally Evaporated Fatty Amine Films Vijaya Patil and Murali Sastry* Materials Chemistry Division, National Chemical Laboratory, Pune 411 008, India Received June 18, 1997. In Final Form: July 25, 1997
Introduction The organization of nanoparticles to form thin films is an important research problem of today1 motivated to a large extent by the interesting application potential of these particles in optoelectronics2 and catalysis3 among others. One approach being investigated for the organization of nanoparticles takes advantage of the versatility of colloid chemistry for the synthesis of the nanoparticles followed by organization in thin film form by a variety of methods such as self-assembly of suitably derivatized clusters4-6 or organization at the air-water interface by hydrophobic stabilization of capped clusters.7,8 In this laboratory, we have been studying the organization of carboxylic acid derivatized colloidal metal particles at the air-water interface by electrostatic immobilization with oppositely charged fatty amine Langmuir monolayers.9,10 In a different approach, we have recently demonstrated that carboxylic acid derivatized silver colloidal particles can be incorporated in thermally evaporated amine films by diffusion.11 The cluster diffusion process is controlled by the strength of the electrostatic interaction between the negatively charged colloidal particles and positively charged amine molecules, and furthermore, this interaction and consequently the cluster density in the film can be modulated by simple alteration of the colloidal solution pH.11 In this paper, we advance our study of cluster diffusion in fatty amine films further by investigating the role of cluster size on the diffusion process. The diffusion of carboxylic acid derivatized gold (130 ( 30 Å size) and silver (70 ( 12 Å size) clusters into thermally evaporated octadecylamine films from a hydrosol containing both clusters in different proportions has been studied, and the results are presented and discussed below. Experimental Details The preparation and carboxylic acid derivatization with 4-carboxythiophenol molecules (4-CTP) of colloidal particles of silver and gold have been described previously.9,10 Optical absorption spectra recorded using a UV-vis Hewlett-Packard
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8452 diode array spectrometer (2 nm resolution) showed a shift in the surface plasmon resonance to ∼405 nm (386 nm for uncapped silver) and 533 nm (525 nm for uncapped gold) for 4-CTP capped silver and gold clusters, respectively. The shift in the surface plasmon resonance as well as a reduction in the resonance intensity indicates chemisorption of the 4-CTP molecules.12 The 4-CTP molecules chemisorb by formation of a thiolate linkage with the metal surface13 leading to carboxylic acid derivatization of the cluster surface as discussed in other reports.9-11,13 Transmission electron microscopy of the capped silver and gold sols yielded cluster sizes of 70 ( 12 and 130 ( 30 Å, respectively.9,10 Hydrosols containing both silver and gold clusters were prepared by mixing the individual sols in different volume ratios to obtain varying gold:silver cluster concentrations in the solution. In this study, solutions of gold:silver ratios (by volume of the individual sols) ranging from 5:1 to 1:5 were prepared. Unless otherwise mentioned, a ratio of x:y should be taken to imply a mixture of x parts (by volume) of the gold sol with y parts of the silver sol. The mixed hydrosol pH was adjusted to 8.5 using HCl since the cluster incorporation in the amine films was found to be maximum at this pH.11 We would like to mention here that the volume ratio of the individual sols used throughout this study is not to be confused with the actual cluster concentration ratios. In fact, for the cluster sizes reported earlier and assuming complete reduction of the salt solution, a 1:1 solution would contain ca. 4 times as many silver clusters per unit volume as gold clusters. Optical absorption spectra of the mixed sols indicated that the clusters were stable over weeks with no evidence for flocculation or cross-linking of the clusters.14 Thin films of octadecylamine of thickness 500 Å were deposited on quartz substrates (film area ) 3 cm2) as well as gold-coated 6 MHz AT cut quartz crystals by thermal evaporation in an Edwards E 306A coating system equipped with a liquid nitrogen trap. The chamber pressure during film deposition was better than 1 × 10-7 Torr and the deposition rate was 1 Å/min. The film thickness/deposition rate was monitored in-situ using a water-cooled quartz crystal microbalance (QCM). After deposition of the octadecylamine films, the kinetics of cluster incorporation was followed by immersing the amine film coated quartz crystals separately in the 4-CTP capped silver and gold hydrosols and measuring the frequency change ex-situ after thorough washing and drying of the crystals. The change in the quartz crystal resonance frequency was measured using an Edwards FTM5 QCM which had a frequency resolution and stability of (1 Hz. For the 6 MHz crystal used in this study, this yields a mass resolution of 12 ng/cm2. The frequency change was converted to a mass loading using the Sauerbrey formula.15 The 500 Å thick octadecylamine films deposited on quartz were immersed for a period of 96 h in 20 mL of the various mixed hydrosols. This time interval was chosen based on the mass equilibration times obtained from QCM studies of the silver and gold sols. Optical absorption spectra of the mixed sols before and after complete cluster incorporation in the amine films as well as of the amine films after cluster incorporation were recorded. X-ray fluorescence (XRF) measurements were used to estimate the Au:Ag molar ratios in the fatty amine films after cluster diffusion.
Results and Discussion * Author for correspondence: phone, 0091-212-337044; fax, 0091212-337044/330233; e-mail,
[email protected]. (1) Alivisatos, A. P. Science 1996, 271, 933. (2) Colvin, V. L.; Schlamp, M. C.; Alivisatos, A. P. Nature 1994, 370, 354. (3) Hoffman, A. J.; Mills, G.; Yee, H.; Hoffman, M. R. J. Phys. Chem. 1992, 96, 5546. (4) Colvin, V. L.; Goldstein, A. N.; Alivisatos, A. P. J. Am. Chem. Soc. 1992, 114, 5221. (5) Doron, A.; Katz, E.; Willner, I. Langmuir 1995, 11, 1313. (6) Freeman, R. G.; Hommer, M. B.; Grabar, K. C.; Jackson, M. A.; Natan, M. J. J. Phys. Chem. 1996, 100, 718. (7) Meldrum, F. C.; Kotov, N. A.; Fendler, J. H. J. Phys. Chem. 1994, 98, 4506. (8) Kotov, N. A.; Meldrum, F. C.; Wu, C.; Fendler, J. H. J. Phys. Chem. 1994, 98, 2735. (9) Sastry, M.; Mayya, K. S.; Patil, V.; Paranjape, D. V.; Hegde, S. G. J. Phys. Chem. B 1997, 101, 4954. (10) Mayya, K. S.; Patil, V.; Sastry, M. Langmuir 1997, 13, 2575. (11) Sastry, M.; Patil, V.; Mayya, K. S. Langmuir 1997, 13, 4490.
S0743-7463(97)00650-1 CCC: $14.00
Figure 1 shows the QCM mass uptake measured exsitu in the 500 Å thick amine films with time during immersion in the individual gold (filled circles) and silver sols (filled squares). Please note that the solid lines in Figure 1 are an aid to the eye and have no physical significance. As mentioned above, the pH of the sols was adjusted to 8.5 since maximum cluster incorporation (12) (a) Henglein, A. J. Phys. Chem. 1993, 97, 5457. (b) Mulvaney, P. Langmuir 1996, 12, 788. (13) Weisbecker, C. S.; Merritt, M. V.; Whitesides, G. M. Langmuir 1996, 12, 3763. (14) The concentration of 4-CTP for capping was taken as 10-5 M since for the silver and gold clusters of this study, this leads to compact monolayer formation of 4-CTP (assuming complete reduction of the salts and an area/molecule of 25 Å2 per 4-CTP molecule). (15) Sauerbrey, G. Z. Phys. (Munich) 1959, 155, 206.
© 1997 American Chemical Society
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Figure 1. QCM mass uptake with time for 500 Å thick octadecylamine films immersed in 4-CTP capped silver (filled squares) and gold (filled circles) hydrosols, respectively. The inset is a magnification of the intitial mass uptake region. The solid lines are only to aid the eye and have no physical significance.
occurs at this pH.11 For completeness, we recollect that at this pH, both the amine and carboxylic acid groups are fully ionized (to -NH3+ and -COO-, respectively), thereby leading to maximum attractive electrostatic interaction.11 The inset shows a magnified view of the initial mass uptake kinetics. It is observed from Figure 1 that the rate of diffusion of the silver clusters in the film is faster than that for gold clusters. The time taken for the cluster density in the film to equilibrate to the maximum value is also smaller for silver than for gold. The QCM results described above indicate that the silver clusters diffuse into the amine films faster than gold colloidal particles. The diffusivity of the clusters is expected to be a function of not only the cluster size but also the total charge on the cluster. Two relevant parameters can be determined for the problem on hand: the total surface charge and charge density (surface charge to cluster volume ratio) of the clusters. While the charge on gold clusters is ca. 3.5 times the charge per silver cluster, the charge density of silver clusters is ca. 1.8 times larger than for gold clusters and may explain the larger diffusivity observed for the silver particles. The larger equilibrium mass loading for gold clusters in the amine film is due to the larger gold cluster size. The volume fraction of the clusters in the gold and silver amine films are calculated from the equilibrium mass loading of the curves shown in Figure 1 to be 18 and 20%, respectively. While QCM measurements can detect overall mass changes in the film during the cluster diffusion process, it cannot differentiate between the presence of gold and silver clusters simultaneously in the film. Optical absorption spectroscopy is an excellent tool for the study of partitioning in amine films since gold and silver colloidal particles exhibit strong (and well separated) surface plasmon resonances.12 Figure 2 shows some representative optical absorption spectra recorded from amine films on quartz after immersion for 96 h in different mixed hydrosols. The gold:silver volume ratios of the hydrosol are given by the side of the corresponding curves. The curves have been normalized at the silver plasmon resonance (440 nm, indicated by an arrow) and displaced for clarity. The peak occurring at ∼620 nm is due to the surface plasmon resonance of gold clusters in the film. It is immediately observed that even for the 1:2 mixed sol (we recollect that the actual silver cluster concentration is 8 times the gold concentration), the contribution of gold to the optical absorption spectrum is higher than that of
Notes
Figure 2. Representative optical absorption spectra of 500 Å thick octadecylamine films on quartz after immersion in the mixed hydrosols (see text for details). The relative concentrations of gold and silver in the mixed sol are indicated by the curves. The peaks indicated are the silver (440 nm) and gold (620 nm) surface plasmon resonances in the film.
Figure 3. Optical absorption spectra of the mixed sols (see text for details) measured before (solid lines) and after (dotted lines) cluster incorporation in 500 Å thick amine films. The gold:silver concentrations in the mixed sol are given next to the curves. The inset shows the individual 4-CTP capped spectra for the silver and gold sols.
silver. This fact even more striking considering that the optical absorption coefficient of gold calculated from the Mie theory12 for an expected 1:2 concentration of the clusters in the film works out to be ∼25% of that of silver. This clearly indicates an increase in the concentration of the gold clusters relative to that of silver in the films. A small hump is seen in the 1:5 spectrum as well (Figure 2), even though the gold cluster concentration in the mixed hydrosol is 20 times lower than that of silver. The Au:Ag molar ratios in the 2:1, 1:2, and 1:5 films shown in Figure 2 were determined from XRF measurements to be 9.6, 5.3, and 0.73, respectively. The complementary study in which the optical absorption spectra of the different mixed hydrosols before and after cluster incorporation in the amine films were measured is amenable to direct quantification. Figure 3 shows the optical absorption spectra of the mixed sols for the 2:1, 1:2, and 1:5 cases before (continuous lines) and after cluster incorporation (dashed lines) in 500 Å thick amine films. The spectra have been normalized to the silver plasmon resonance maximum and shifted for clarity.
Notes
Figure 4. Gold to silver intensity ratios obtained from the optical absorption spectra (see text for details of ratio estimation) before (curve a, solid circles, left axis) and after (curve b, solid squares, left axis) immersion of 500 Å thick amine films in the mixed sols plotted as a function of the gold volume ratio in the mixed sol. The Au:Ag molar ratios in the amine films obtained from XRF measurements as a function of gold volume ratio in the sol is also plotted (triangles, right axis).
It is clearly seen that there is a decrease in the resonance from gold at ∼535 nm after immersion of the amine films for all three cases illustrated indicating loss of gold colloidal particles from the sol to the film. In order to quantify the cluster loss from solution, the following procedure was adopted. We hasten to add that by quantification we mean estimation of a value proportional to changes in the relative ratios of the cluster concentration in the sol before and after cluster incorporation into the amine film not a determination of the actual cluster densities. Determination of the actual cluster densities from the optical absorption spectra are difficult due to the fact that damping and shifts associated with surfactant adsorption on cluster surfaces cannot be quantified with existing optical absorption theory.12b The inset of Figure 3 shows the individual optical absorption spectra recorded for the as-prepared 4-CTP capped silver and gold sols. The optical absorption spectra measured before and after film immersion for the different gold:silver volume ratios were fitted to a linear combination of the individual spectra shown in the inset using a nonlinear least-squares procedure. The ratio of the coefficients obtained from the fits to the optical absorption spectra measured before and after cluster incorporation in the amine film is therefore proportional to the cluster concentration in the sol, scaled by a constant factor. The fits were extremely good even for the spectra from the sols after cluster incorporation indicating negligible flocculation of the clusters over the 96 h experimental time frame. The gold:silver ratios obtained from the fits are plotted in Figure 4 as a function of the cluster concentration ratio in the as-prepared mixed sol before (curve a, circles) and after amine film immersion (curve b, squares). The Au:Ag molar ratios for the amine films determined from XRF measurements have also been plotted in Figure 4 (triangles, right axis) as a function of volume concentration of gold and can be seen to closely track the metal concentration ratios in the sol. It is clearly seen that there is a decrease in the gold cluster concentration in the sol (or gain in the film) after amine film immersion. This enhancement of the gold cluster density in the amine film relative to that of silver occurs even for small concentrations of gold clusters in the sol (20% by
Langmuir, Vol. 13, No. 20, 1997 5513
volume or 5% in terms of cluster concentration relative to silver in the sol) indicating that the larger gold clusters have been preferentially incorporated in the amine films. This is an important result of this study and is unusual given that the QCM mass uptake kinetics indicate clearly that the smaller silver clusters have larger diffusivities in the thermally evaporated amine films (Figure 1). It is evident from the above that other factors determine the “reverse” partitioning of larger clusters during a competitive diffusion process. During diffusion of the clusters into the amine film, the amine molecules are expected to coordinate to the carboxylic acid terminal groups on the clusters through a Coulombic attractive interaction. This leads to a monolayer of amine molecules in immediate contact with the clusters in the film. Such structures are known to form in Langmuir-Blodgett (LB) films with clusters chemically inserted16 as well as in stearate LB films with Fe2O3 nanoparticles attached at the air-water interface.17 From a purely electrostatics point of view, the higher number of carboxylic acid-amine molecule charged pairs due to the larger surface area of gold would lead to gold cluster incorporation being energetically favorable. Another contribution to the energetics of cluster diffusion arises from the packing of the amine hydrocarbon chains around the clusters. The larger surface curvature of the silver clusters leads to larger volumes available to the terminal groups of the amine molecules and, consequently, weaker van der Waals interaction between the chains.18 In other words, the presence of smaller size clusters distorts the packing of the hydrocarbon chains in the amine films to a larger extent than larger clusters with their smaller surface curvature. Therefore, in a competitive process where diffusion of clusters of different sizes into amine films is concerned, it would be energetically favorable for the larger clusters to be incorporated and coordinated to the amine molecules due to the smaller resulting distortion to the packing of the alkyl chains in the organic matrix. However, studies based on molecular modeling of the organic matrix chain packing in the presence of clusters are required before a full understanding of the “reverse” partitioning of colloidal particles in the manner described above can be fully understood. In conclusion, it has been shown that during a competitive diffusion process of fully charged colloidal particles differing vastly in size into thermally evaporated amine films, preferential incorporation of the larger particles occurs. Future work will be directed toward understanding the mechanism of cluster diffusion in the amine films in terms of a Poisson-Boltzmann-Stern electrostatic model. Acknowledgment. V.P. thanks the Council for Scientific and Industrial Research (CSIR), Government of India for a research fellowship. LA970650P (16) Urquhart, R. S.; Furlong, D. N.; Gegenbach, T.; Geddes, N. J.; Grieser, F. Langmuir 1995, 11, 1127. (17) Yang, J.; Peng, X.; Zhang, Y.; Wang, H.; Li, T. J. Phys. Chem. 1993, 97, 4484. (18) For a 70 Å diameter Ag cluster (R ) 35 Å), the number of 4-CTP molecules on the surface (25 Å2 area/4-CTP molecule) ) (4πR2)/25 ) 616. Assuming coordination of one amine molecule of length 25 Å to each 4-CTP molecule (of 7 Å dimension) leads to a modified cluster radius of 35 + 7 + 25 ) 67 Å. Area per terminal methyl group of the amine molecules on Ag clusters is therefore approximately (4π672 )/616 ) 92 Å2. By a similar calculation, this value can be shown to be ca. 56 Å2 for gold clusters of 65 Å radius.
