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Langmuir 1997, 13, 4490-4492
Incorporation of Colloidal Metal Particles in Thermally Evaporated Fatty Amine Films via Selective Electrostatic Interactions Murali Sastry,* V. Patil, and K. S. Mayya Materials Chemistry Division, National Chemical Laboratory, Pune 411 008, India Received March 20, 1997. In Final Form: May 27, 1997
There is much research activity in the area of nanoparticles motivated to a large extent by the application potential of this “neglected dimension” of matter.1 While the synthesis of small particles is an important aspect of the research, the organization of the particles forms an equally important component, especially if device applications are to be realized. Organic matrices are being increasingly investigated for the controlled growth of small particles either at the air-water interface2,3 or by chemical insertion into Langmuir-Blodgett (LB) films.4,5 Multilayer lamellar films of clusters have also been successfully grown by the LB technique by electrostatic immobilization of the colloidal particles in Langmuir monolayers.6,7 It has been shown that thermally evaporated films of fatty acids can be spontaneously organized via selective ionic interaction of cations by immersion of the film in a suitable electrolyte.8 This leads to an organized lamellar film structure similar to c-axis-oriented Y-type LB films. Recognizing that the principle of ion exchange is quite general, this approach has been extended to the reorganization of fatty amine films as well via anion incorporation using [PtCl6]2- and [TiO(C2O4)]2- ions.9 In this communication, we demonstrate that negatively charged carboxylic acid derivatized silver colloidal particles can be incorporated into thermally evaporated fatty amine films by an analogous mechanism. While the charged carboxylic acid derivatized clusters may be viewed as giant anions, it would perhaps be more accurate to view the process of incorporation of the clusters in fatty amine films as arising through selective electrostatic binding of the clusters in the amine matrix.10 Octadecylamine films of 150, 500, and 1000 Å thickness were thermally evaporated in an Edwards E306A coating unit at a pressure of 1 × 10-7 Torr. The film thickness was monitored using a water-cooled Quartz Crystal Microbalance (QCM). The films were deposited at room temperature onto a number of quartz substrates as well as gold-coated 6 MHz AT-cut quartz crystals for QCM measurements. The silver colloidal particles were synthesized by borohydride reduction of Ag2SO4 solution as detailed by Vukovic and Nedeljkovic.11 The as-prepared * Author for communication: Phone, 0091-212-337044; fax, 0091212-337044; e-mail,
[email protected]. (1) See the editorial and related articles on clusters in a recent issue of Science 1996, 271. (2) Yi, K. C.; Horvolgyi, Z.; Fendler, J. H. J. Phys. Chem. 1994, 98, 3872. (3) Yang, J.; Meldrum, F. C.; Fendler, J. H. J. Phys. Chem. 1995, 99, 5500. (4) Urquhart, R. S.; Furlong, D. N.; Gegenbach, T.; Geddes, N. J.; Grieser, F. Langmuir 1995, 11, 1127. (5) Leloup, J.; Ruadel-Teixier, A.; Barraud, A. Thin Solid Films 1992, 210/211, 407. (6) Tian, Y.; Wu, C.; Fendler, J. H. J. Phys. Chem. 1994, 98, 4913. (7) Sastry, M.; Mayya, K. S.; Patil, V.; Paranjape, D. V. J. Phys. Chem. B 1997, 101, 4954. (8) Ganguly, P.; Pal, S.; Sastry, M.; Shashikala, M. N. Langmuir 1995, 11, 1078. (9) Pal, C. Ph.D. Thesis, University of Poona, 1996. (10) Sun, L.; Johnson, B.; Wade, T.; Crooks, R. M. J. Phys. Chem. 1990, 94, 8869. (11) Vukovic, V. V.; Nedeljkovic, J. M. Langmuir 1993, 9, 980.
S0743-7463(97)00305-3 CCC: $14.00
Figure 1. (a) Kinetics of silver cluster incorporation into thermally evaporated octadecylamine films of various thickness at a colloidal pH of 8.5. The thickness values are indicated next to the curves. (b) Kinetics of silver cluster incorporation into thermally evaporated octadecylamine films of thickness 500 Å for different pH values of the colloidal solution. The pH values are indicated next to the curves.
hydrosol had a pH ∼ 10. Transmission electron microscopy (TEM) of the hydrosol yielded an average cluster size of 70 ( 10 Å. The clusters were then capped with 4-carboxythiophenol (4-CTP) by mixing to the silver hydrosol a solution of the bifunctional molecule in absolute ethanol to yield an overall surfactant concentration of 10-5 M. The capping of the silver colloidal particles was followed by optical absorption spectroscopy which showed a shift in the surface plasmon resonance from 384 to 400 nm. Thiol groups are known to form strong covalent linkages with silver12 thus leading to carboxylic acid derivatized clusters. After the clusters were capped, the hydrosol pH was adjusted in the range 7-12.5 using ammonia or H2SO4. The amine-coated AT-cut quartz crystals and quartz substrates were immersed in the silver hydrosol adjusted to different pH values at room temperature, and the mass uptake was monitored as a function of time.13 We would like to mention here that the QCM measurements of cluster incorporation with time in the thermally evaporated amine film were made ex-situ after careful drying of the film with flowing N2. Therefore, contributions to the QCM mass loading arising from swelling of the films in solution can be neglected. As a check, thermally evaporated amine films of 500 Å thickness were immersed in water adjusted to pH values in the range 7-12.5 for 1 h and the mass change was measured ex-situ. No change in the film mass was observed at all pH values indicating negligible retention of water in the film after removal from water. Hence, it is reasonable to assume that the ex-situ mass changes observed in the amine films on immersion in the colloidal solution arise due to incorporation of the silver colloidal particles alone as will be corroborated by optical absorption measurements presented below. Figure 1a shows the kinetics of mass increase of the QCM for octadecylamine films of different thickness on immersion in a silver hydrosol at pH ) 8.5. At this pH, both the amine molecules and carboxylic acid groups on the cluster are expected to fully ionized (-NH3+ and -COO- respectively) leading to maximum electro(12) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152. (13) An Edwards FTM5 quartz crystal microbalance with a (1 Hz frequency stability and resolution was used for the measurement. This yields a mass resolution of ∼12 ng/cm2 for a 6 MHz crystal. The measured frequency change was converted to mass uptake using the Sauerbrey formula (Sauerbrey, G. Z. Phys. (Munich) 1959, 155, 206).
© 1997 American Chemical Society
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static interaction. It is seen that the initial rate of mass uptake is fairly constant for all the films with the final equilibrium value increasing with film thickness. It is also observed that the time taken for complete incorporation of the silver clusters increases with film thickness reaching close to 72 h for the 1000 Å film. The film thickness of the 500 and 1000 Å films increased to 1000 and 1800 Å, respectively, after incorporation of the clusters, the thickness being measured by optical interferometry. Assuming uniform distribution of 70 Å size clusters within the volume of the film (taking into account the final thickness of the film), the volume fraction of the carboxylic acid derivatized silver clusters in the amine films is determined from the QCM equilibrium mass loadings (Figure 1) to be 20 and 18%, respectively, for the above films. This value is quite close to the value of ∼15% surface coverage obtained by Natan et al.14 for selfassembled colloidal gold particles. The rather low volume/ surface fractions observed may be rationalized in terms of repulsive electrostatic interactions between the clusters preventing them from reaching a close-packed structure.14 Figure 1b shows the kinetics of cluster incorporation into an octadecylamine film of 500 Å thickness at three different colloidal solution pH values of 7, 8.5, and 11.5 as determined from QCM. It is observed that the equilibrium mass loading is maximum for the clusters incorporated at pH ) 8.5 while it is considerably less for the pH ) 7 and 11.5 cases. This may be explained as follows. At pH ) 7, while the amine groups are expected to be fully charged (pKA of octadecylamine ) 10.5) the carboxylic acid groups of 4-carboxythiophenol may not be completely ionized. This is a reasonable assumption given that the pKA value of benzoic acid ) 4.8 (the closest to 4-CTP) and that for self-assembled monolayers, the pKA value for acid groups can shift to higher values by up to 3 pH units accompanied by an increase in the width of the titration curve.15 At pH ) 11.5, the amine groups are not fully ionized while the carboxylic acid groups on the cluster are. At pH ) 8.5, the electrostatic attractive interaction between the COO- and NH3+ groups is maximum leading to the highest mass uptake. Another aspect to be considered is the role of variation in swelling of the amine films upon immersion in different pH colloidal solutions which could affect the rate and degree of cluster incorporation in the organic film. Detailed studies are in progress to clarify this point and will form the basis of further communications. The ability to control the cluster density in the film by altering the charge on either the amine matrix or the metal clusters and thus modulate the degree of electrostatic interaction is an important result of this investigation. Optical absorption spectroscopy measurements on a 500 Å amine film on quartz immersed in silver hydrosol at different pH values for 3 days (the time period determined from the QCM kinetics study) are shown in Figure 2. It is seen that the surface plasmon resonance intensity at ∼450 nm increases from pH ) 7 to 8.5 indicating an increase in the cluster density. Above pH ) 8.5, there is a progressive decrease in the plasmon resonance intensity indicating less cluster incorporation. This result is in agreement with the QCM results shown in Figure 1b which showed maximum cluster incorporation at pH ) 8.5. The absorption spectrum corresponding to pH ) 7 shows evidence for slight aggregation of the clusters. It is difficult to determine whether aggregation has occurred within (14) Grabar, K. C.; Smith, P. C.; Davis, J. A.; Musick, M. D.; Jackson, M. A.; Walter, D. G.; Guthrie, A. P.; Natan, M. J. J. Am. Chem. Soc. 1996, 118, 1148. (15) Lee, T. R.; Carey, R. I.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 741.
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Figure 2. Optical absorption spectra of 500 Å thick octadecylamine films with silver colloidal particles incorporated at various pH values (pH values indicated next to the curves). Inset shows (a) the variation of absorbance maximum with fatty amine film thickness (filled circles, left axis) and (b) equilibrium mass loading with film thickness (filled squares, right axis) after immersion in pH ) 8.5 colloidal solution for 3 days.
the amine matrix or in the colloidal solution but the latter appears likely given the reduced electrostatic stabilization of the clusters in solution at this pH. The inset shows the absorption maximum intensity as a function of film thickness on immersion in the colloidal solution at pH ) 8.5 (filled circles, left axis) as well as the equilibrium mass uptake with thickness determined from QCM measurements (filled squares, right axis). It is observed that the absorbance and mass uptake are fairly linear with thickness indicating incorporation of the same density of clusters in all the films for the same pH value of the colloidal solution. Fourier transform infrared (FTIR) spectroscopy16 was performed on the 1000 Å film deposited on a Si(111) wafer before and after silver cluster incorporation (maximum incorporation at pH ) 8.5) in order to understand the nature of ordering of the amine molecules. For comparison, a 21 monolayer (ML) Langmuir-Blodgett film of octadecylamine film complexed to (PtCl6)2- ions (Am-HPT) was also studied by FTIR, since this is a standard amine salt and has been reasonably well investigated in the LB form.17,18 Figure 3a shows the spectrum recorded in the region 2500-3500 cm-1 for the thermally evaporated amine film (labeled 1), amine film with Ag clusters incorporated (2), and Am-HPT film (3). The methylene antisymmetric and symmetric modes at 2920 and 2850 cm-1, respectively, are clearly seen along with the NH2 antisymmetric vibration at 3330 cm-1 for the thermally evaporated and amine with Ag cluster films (Figure 3a, curves 1 and 2). The NH2 antisymmetric vibration shifts to 3200 cm-1 on formation of the salt in agreement with other reports.18 The position of the methylene antisymmetric and symmetric stretch modes at the above mentioned frequencies is known to indicate close packing in the alkyl chains of the amine molecules.19 Therefore, incorporation of the clusters into the amine film has not (16) FTIR measurements were made in the transmission mode at a resolution of 2 cm-1 on a Unicam Genesis Spectrometer. (17) Ganguly, P.; Paranjape, D. V.; Sastry, M. J. Am. Chem. Soc. 1993, 115, 793. See this reference for experimental details of film formation. (18) Bardosova, M.; Tregold, R. H.; Ali-Adib, Z. Langmuir 1995, 11, 1273. (19) Hostetler, M. J.; Stokes, J. J.; Murray, R. W. Langmuir 1996, 12, 3604.
4492 Langmuir, Vol. 13, No. 16, 1997
Figure 3. (A) FTIR spectrum of the thermally evaporated amine film (1000 Å, curve 1); amine film with Ag cluster incorporation at pH ) 8.5 (curve 2 and 21 monolayer octadecylamine-(PtCl6)2- complex LB film (curve 3) in the region 2500-3500 cm-1. (B) FTIR spectrum of films as in (a) in the spectral region 1200-1350 cm-1.
affected the microcrystalline environment already present in the thermally evaporated films which is of course present in the Am-HPT film.17 The fact that the NH2 antisymmetric band does not shift on incorporation of Ag clusters indicates weak coupling with the amine molecules as would be expected for purely electrostatic interactions. This is to be contrasted with salt formation which leads to a large shift in the above resonance (Figure 3a, curve 3). Features observed in the spectral region 1200-1350 cm-1 are shown in Figure 3b for the above films with same labeling as in Figure 3a. A number of well-resolved peaks are seen in the thermally evaporated amine and amine with Ag cluster films (Figure 3b, curves 1 and 2) which are not clearly seen for the Am-HPT film (Figure 3b, curve 3). These features are the twisting and rocking progression bands, and their presence is a strong indicator of crystallinity and close all-trans packing of the alkyl chains.19,20 On comparison of curves 1 and 2, it is observed that the progression bands become sharper on incorpora(20) Yang, J.; Peng, X.; Zhang, Y.; Wang, H.; Li, T. J. Phys. Chem. 1993, 97, 4484.
Notes
tion of the clusters indicating that the clusters induce some order in the thermally evaporated films, possibly through formation of close packed monolayers of amine molecules on the Ag cluster surface. Another interesting observation, though not germane to this study, is the lack of the progression bands in the Am-HPT film (Figure 3b, curve 3) which is well known to be crystalline.17,18 LB films of Am-HPT are known to have an interdigitated structure17 which affects the packing of the chains. This is a tentative explanation for the absence of the progression bands in the Am-HPT film but further work is required before this aspect can be understood. Thus, it has been demonstrated that carboxylic acid derivatized silver clusters can be incorporated into thermally evaporated fatty amine films through selective electrostatic interactions. This enables variation of the cluster density in the film by simple control of the colloidal solution pH. Cluster incorporation leads to better packing of the alkyl chains of the thermally evaporated amine molecules. While cluster incorporation as shown above appears to be similar to ion incorporation in fatty acid8 and amine films,9 the interactions between the negatively charged clusters and positively charged amine molecules are purely electrostatic and therefore weak. The approach outlined in this communication opens up the exciting possibility of obtaining mixed cluster films and size selective cluster incorporation into organic matrices and can be extended to semiconductor and oxide colloidal particles as well as macroions such as “polyballs”.21 As mentioned earlier, further work is in progess to understand the role of film swelling on cluster incorporation as well as the mechanism of cluster incorporation and the spatial distribution of the clusters in the organic film. Acknowledgment. V.P. and K.S.M. wish to thank the Council for Scientific and Industrial Research (CSIR), Government of India, for research fellowships. Useful discussions with Dr. P. Ganguly, Head, Materials Chemistry Division, NCL Pune, are gratefully acknowledged. The authors thank Dr. Malvankar, Organic Chemistry Division, NCL Pune, for assistance with the FTIR measurements. LA9703050 (21) Ben-Tal, N. J. Phys. Chem. 1995, 99, 9642.
