Role of Particle Size in Individual and Competitive Diffusion of

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Role of Particle Size in Individual and Competitive Diffusion of Carboxylic Acid Derivatized Colloidal Gold Particles in Thermally Evaporated Fatty Amine Films Vijaya Patil,† R. B. Malvankar,‡ and Murali Sastry*,† Materials Chemistry and Organic Chemistry (Technology) Divisions, National Chemical Laboratory, Pune 411 008, India Received February 17, 1999. In Final Form: June 14, 1999 We have recently shown that nanocomposites of colloidal particles in a fatty lipid matrix can be grown via a diffusion process controlled by selective electrostatic interactions. In this paper, a detailed investigation of the diffusion of carboxylic acid derivatized gold colloidal particles of different sizes into thermally evaporated octadecylamine films using quartz crystal microgravimetry (QCM), transmission electron microscopy, and UV-vis absorption and Fourier transform infrared (FTIR) spectroscopies is described. The QCM kinetics of gold cluster incorporation has been analyzed in terms of a one-dimensional Fickiantype diffusion model, and it is found that the cluster diffusivity increases with decreasing cluster size. The pH at which maximum cluster incorporation in the amine occurs was found to be dependent on the cluster size as well. FTIR spectroscopy of the fatty amine-gold particle composites indicated weak coupling of the clusters to the protonated amine groups as well as interesting cluster size dependent changes in the amine and methylene antisymmetric deformations as well as the methylene scissoring bands. In a competitive diffusion process of large and small gold particles, it was observed that bigger gold particles were preferentially incorporated into the amine matrix even though the cluster diffusivity is higher for the smaller gold particles.

Introduction An important topical problem in the area of nanoscale materials is the organization of nanoparticles in thin film form. One of the routes being seriously pursued uses colloid chemistry for synthesis of the nanoparticles followed by organization in thin film form by a variety of methods such as self-assembly of the clusters1 and organization at the air-water interface by hydrophobic stabilization of capped clusters.2 In this laboratory, we have developed an approach for the formation of thin films of nanocomposites of surface-modified colloidal particles with fatty lipid molecules via selective electrostatic interaction both in Langmuir monolayers3 and by diffusion in thermally evaporated films.4 The versatility of this method is that the density of clusters in the fatty lipid films can be controlled quite easily through variation of the colloidal solution pH, which modulates simultaneously the charge * To whom correspondence should be addressed. Ph: 0091-205893044. Fax: 0091-20-5893044. E-mail: [email protected]. † Materials Chemistry Division. ‡ Organic Chemistry Division. (1) (a) Colvin, V. L.; Goldstein, A. N.; Alivisatos, A. P. J. Am. Chem. Soc. 1992, 114, 5221. (b) Doron, A.; Katz, E.; Willner, I. Langmuir 1995, 11, 1313. (c) Chumanov, G.; Sokolov, K.; Gregory, B. W.; Cotton, T. M. J. Phys. Chem. 1995, 99, 9466. (d) Freeman, R. G.; Hommer, M. B.; Grabar, K. C.; Jackson, M. A.; Natan, M. J. J. Phys. Chem. 1996, 100, 718. (e) Bandyopadhyay, K.; Patil, V.; Vijayamohanan, K.; Sastry, M. Langmuir 1997, 13, 5244. (2) (a) Meldrum, F. C.; Kotov, N. A.; Fendler, J. H. J. Phys. Chem. 1994, 98, 4506. (b) Kotov, N. A.; Meldrum, F. C.; Wu, C.; Fendler, J. H. J. Phys. Chem. 1994, 98, 2735. (c) Sastry, M.; Patil, V.; Mayya, K. S.; Paranjape, D. V.; Singh, P.; Sainkar, S. R. Thin Solid Films 1998, 324, 239. (3) (a) Mayya, K. S.; Patil, V.; Sastry, M. Langmuir 1997, 13, 2575. (b) Sastry, M.; Mayya, K. S.; Patil, V.; Paranjape, D. V.; Hegde, S. G. J. Phys. Chem. B. 1997, 101, 4954. (c) Patil, V.; Mayya, K. S.; Pradhan, S. D.; Sastry, M. J. Am. Chem. Soc. 1997, 119, 9281. (d) Mayya, K. S.; Sastry, M. J. Phys. Chem. B 1997, 101, 9790. (e) Mayya, K. S.; Patil, V.; Sastry, M. J. Chem. Soc., Faraday Trans. 1997, 93, 3377. (f) Mayya, K. S.; Sastry, M. Langmuir 1998, 14, 74. (g) Mayya, K. S.; Patil, V.; Kumar, M.; Sastry, M. Thin Solid Films 1998, 312, 308. (h) Sastry, M.; Mayya, K. S.; Patil, V. Langmuir 1998, 14, 5198.

on the functional groups in the film matrix as well as on the colloidal particle surface.3,4 In this paper, we address the problem of electrostatically controlled diffusion of carboxylic acid derivatized gold colloidal particles in thermally evaporated fatty amine films in greater detail. More specifically, we have investigated the diffusion of carboxylic acid derivatized gold colloidal particles in thermally evaporated octadecylamine (ODA) films using quartz crystal microgravimetry (QCM), transmission electron microscopy (TEM), and UV-vis absorption and Fourier transform infrared (FTIR) spectroscopies. The QCM kinetics of cluster diffusion has been analyzed in terms of a one-dimensional diffusion model,4d and the cluster diffusivities were calculated as a function of the cluster size, hydrosol pH, and amine film thickness. The gold cluster diffusivities were found to increase as the cluster size decreased. More interestingly, in a competitive diffusion of large (130 Å) and small (20 Å) gold clusters, it was observed by UV-vis absorption and FTIR spectroscopies that the larger clusters were preferentially incorporated despite their lower diffusivities. Another interesting observation from this investigation was the size dependence of the colloidal solution pH at which maximum cluster incorporation into the ODA matrix occurred. This indicates that the energetics of ionization of the carboxylic acid groups on the colloidal particle surface is strongly affected by the nanoscale curvature of the particles. Presented below are details of the investigation. Experimental Details Gold clusters of 130 ( 30, 35 ( 7, and 20 ( 5 Å diameter (hydrosols 1, 2, and 3, respectively) were synthesized and capped with 4-carboxythiophenol (4-CTP) as described below. (4) (a) Sastry, M.; Patil, V.; Mayya, K. S. Langmuir 1997, 13, 4490. (b) Patil, V.; Sastry, M. J. Chem. Soc., Faraday Trans. 1997, 93, 4347. (c) Patil, V.; Sastry, M. Langmuir 1997, 13, 5511. (d) Sastry, M.; Patil, V.; Sainkar, S. R. J. Phys. Chem. B 1998, 102, 1404. (e) Patil, V.; Sastry, M. Langmuir 1998, 14, 2707.

10.1021/la990170t CCC: $18.00 © 1999 American Chemical Society Published on Web 09/18/1999

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(a) Hydrosol 1. The gold colloidal particles of 130 ( 30 Å size were synthesized by citrate reduction of HauCl4 as described elesewhere.3a This yielded a clear, deep violet hydrosol at a pH of ca. 3. After the hydrosol was cooled, the hydrosol pH was increased to 8.5 using ammonia and the gold colloidal particles were capped with 4-CTP by mixing 9 mL of the gold sol and 1 mL of 4-CTP in absolute ethanol. The concentration of the 4-CTP in ethanol was adjusted to yield a capping concentration of 10-5 M in the hydrosol. Thiol groups are known to bind strongly to gold clusters5 thereby yielding carboxylic acid derivatization of the gold particles with 4-CTP. The capping of the colloidal particles was followed by UV-vis absorption spectroscopy recorded on a Hewlett-Packard 8542 diode array spectrophotometer operated at a resolution of 2 nm. A shift in the surface plasmon resonance from 524 nm for the bare gold colloid to 530 nm on capping was observed indicative of coordination of the bifunctional molecule to the gold cluster surface.5d (b) Hydrosol 2. Clusters smaller than those obtained by citrate reduction of chloroauric acid described above could be routinely prepared by borohydride reduction of the gold salt. More specifically, a solution of chloroauric acid prepared from 1.5 × 10-3 g of HAuCl4 dissolved in 100 mL of water was reduced using 0.01 g of NaBH4. A clear, red solution at pH ) 8.5 was obtained. These clusters were then capped with 4-CTP as described above for the citrate-reduced gold colloid. A red shift in surface plasmon resonance from 510 nm for the bare gold sol to 514 nm on capping with 4-CTP was detected. TEM measurements carried out on the 4-CTP capped sol using a Philips TEM 301 T instrument (operating voltage ) 80 kV; magnification ) 57 000) yielded a cluster diameter of 35 Å and a standard deviation of 7 Å (Supporting Information, Figure 1). (c) Hydrosol 3. The smallest clusters of the series could be reproducibly obtained by reducing 100 mL of chloroauric acid solution (prepared as in the case of hydrosol 2) with 0.01 g of NaBH4 in the presence of 10-5 M of 4-CTP. The surface plasmon resonance for this gold colloid was observed at 502 nm. TEM measurements of the sol yielded a cluster diameter of 20 Å and a standard deviation of 5 Å (Supporting Information, Figure 2). The presence of 4-CTP during the metal salt reduction process limits the size of the colloidal gold particles. Control over the gold colloidal particle size by simple variation of the relative surfactant concentration in the salt solution during reduction has been studied in detail for alkanethiols by Leff et al.6 They observed that as the AuCl4-/thiol ratio increases, the gold cluster size increases and that this behavior could be explained in terms of a statistical thermodynamic formulation.6 We point out that while the reduction of the chloroaurate ions was done in an organic solvent (toluene) by Leff et al,6 in this study the reduction was carried out in an aqueous medium. The ramifications of control over the colloidal particle size in an aqueous medium where hydrophobic interactions would play a major role are being investigated.7 Thin films of ODA of thickness 250, 500, and 1000 Å were thermally evaporated onto AT-cut quartz crystals, Si(111) wafers, TEM grids, and quartz substrates in an Edwards E 306A coating system equipped with a liquid nitrogen trap. The pressure during film deposition was better than 1 × 10-7 Torr and the deposition rate was ca. 1 Å/min. The film thickness/deposition rate was monitored in situ using a water-cooled QCM. After deposition of the ODA films, the kinetics of cluster incorporation was followed by immersing the amine film coated quartz crystals in the different 4-CTP capped 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 frequency counter that 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 (5) (a) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (b) Brust, M.; Fink, J.; Bethell, D.; Schiffrin, D. J.; Kiely, C. J. Chem. Soc., Chem. Commun. 1995, 1655. (c) Weisbecker, C. S.; Merritt, M. V.; Whitesides, G. M. Langmuir 1996, 12, 3763. (d) Mayya, K. S.; Patil, V.; Sastry, M. Langmuir 1997, 13, 3944. (6) Leff, D. V.; Ohara, P. C.; Heath, J. R.; Gelbart, W. M. J. Phys. Chem. 1995, 99, 7036. (7) Gole, A.; Sastry, M. Manuscript in preparation.

Patil et al. ng/cm2. The frequency change was converted to a mass loading using the Sauerbrey formula.8 The kinetics of gold cluster diffusion were studied using QCM for the following experimental conditions: (1) amine films of 500 Å thickness immersed in hydrosol 1 at pH values of 5.2, 6, 7, and 11.2, the pH of the sols was adjusted after capping with 4-CTP as described earlier using dilute H2SO4 and ammonia solutions; (2) amine films of 250, 500, and 1000 Å thickness immersed in hydrosol 1 at a pH ) 7; (3) amine films of 500 Å thickness immersed in hydrosols 1-3 maintained at pH values 7, 8.5, and 8.5, respectively. FTIR measurements of 500 Å thick ODA films on Si(111) substrates as a function of cluster diffusion time were carried out ex situ after immersion of the films in the hydrosols 1 and 3 kept at pH values 7 and 8.5, respectively. Thorough washing and drying of the films was done prior to FTIR measurement. The measurements were made in the absorption mode on a Unicam Genesis FTIR spectrometer operated at a resolution of 2 cm-1. The kinetics of cluster diffusion into the amine films was also studied using UV-vis absorption spectroscopy. ODA films (500 Å thick) on quartz substrates were immersed in the 4-CTP capped gold hydrosols 1, 2, and 3 maintained at pH values 7, 8.5, and 8.5, respectively, and the UV-vis absorption spectra were recorded both in situ and ex situ for different times of immersion of the films in the respective hydrosols.9 To study the role of cluster size in a competitive diffusion process, we prepared a hydrosol containing big (130 Å) and small gold clusters (20 Å) by mixing equal volumes of the individual hydosols 1 and 3. We would like to mention here that the volume ratio of the individual sol used in 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. 260 times as many small clusters per unit volume as the large gold clusters. The kinetics of cluster incorporation was followed by immersing 500 Å thick amine films coated on AT-cut quartz crystals, quartz, and Si(111) substrates in the 1:1 mixed hydrosol (pH adjusted to 7.7) and measuring the QCM mass uptake, optical absorption, and the FTIR spectra at different time intervals after thorough washing and drying of the films. A 500 Å thick amine film deposited on a carbon-coated TEM grid was also immersed in the mixed hydrosol and TEM measurements were made on the film at the end of 120 h of immersion. The TEM micrograph was analyzed to determine the particle size distribution (PSD) of the colloids in the competitive diffusion process.

Results and Discussion 1. Cluster-Size-Dependent Contributions to the Diffusion Process. Figure 1 shows the optical absorption spectra recorded for hydrosols 1, 2, and 3 after capping with 4-CTP. With decreasing particle size, the optical absorption spectra show a systematic blue shift and broadening of surface plasmon resonance. The inset of Figure 1 (left axis) shows a plot of the variation in the plasmon resonance maximum (λmax) with cluster size for 500 Å thick ODA films immersed in sols 1-3 for 120 h, the spectra being measured in air. This will be discussed at a later point in the paper. The damping and broadening of the surface plasmon resonance with reducing cluster size may be explained in terms of scattering of the free electrons with the particle surface.10,11 The blue shift in the plasmon resonance wavelength with reducing cluster size is in agreement with the observation of others.11c What (8) Sauerbrey, G. Z. Phys. (Munich) 1959, 155, 206. (9) The quartz substrates for optical absorption measurements were cut to fit in a snug, vertical position in the spectrophotometer quartz cuvette. Spectra of the ODA films were recorded during immersion in the hydrosol and in air after evacuating the cuvette and thoroughly washing and drying the film prior to measurement. (10) (a) Henglein, A. J. Phys. Chem. 1993, 97, 5457. (b) Mulvaney, P. Langmuir 1996, 12, 788. (11) (a) Kreibig, U.; Genzel, L. Surf. Sci. 1985, 156, 678. (b) Alvarez, M. M.; Khoury, J. T.; Schaaff, G. T.; Shafigullin, M. N.; Vezmar, I.; Whetten, R. L. J. Phys. Chem. B 1997, 101, 3706. (c) Logunov, S. L.; Ahmadi, T. S.; El-Sayed, M. A.; Khoury, J. T.; Whetten, R. L. J. Phys. Chem. B 1997, 101, 3713.

Role of Particle Size in Diffusion

Figure 1. Optical absorption spectra recorded from 4-CTP capped sols 1, 2, and 3 (see text for details). The inset shows a plot of the plasmon resonance wavelength, λmax, with cluster size (left axis) for 500 Å thick ODA films immersed in sols 1-3 for 120 h at pH values 7, 8.5, and 8.5, respectively, measured in air. The inset also shows a plot of the shift in the plasmon resonance wavelength, ∆(λ), for the above films measured in air and during immersion in the respective hydrosols (right axis, see text for details).

is germane to this investigation is that the optical absorption signatures of the three hydrosols is quite distinct and can therefore be used to identify the size of the clusters during diffusion in the amine films. As will be shown later, this is important in understanding the competitive diffusion of large and small gold colloidal particles in the amine films. Before the kinetics of mass uptake in the amine films during immersion in sols 1-3 are studied, a simple QCM experiment was performed to investigate the pH dependence of the equilibrium mass uptake in 500 Å thick ODA films. In all the cases, the mass uptake measurements were performed after immersion of the films in the respective sols for 120 h, which, based on the QCM kinetics measurements to be discussed later, ensures complete cluster incorporation in the amine matrix. The mass uptake data converted to an equilibrium percentage volume fraction in the film is plotted as a function of the hydrosol pH and is shown in Figure 2. The data have been fit to a Lorentzian profile, and the peak positions for sols 1-3 (squares, circles, and triangles, respectively) were found to be at pH values 7.2 (130 Å), 8.35 (35 Å), and 8.45 (20 Å), respectively. The inset of Figure 2 shows a plot of the pH at which maximum cluster incorporation occurs (right axis, pHmax) as well as the full width at halfmaximum (fwhm) of the Lorentzians (left axis) with cluster size. It is observed that the width of the curve decreases with decreasing cluster size. The pH-dependent electrostatic interaction between the carboxylic acid derivatized gold particles and the amine matrix will be determined by the titration behavior of both the amine film and the carboxylic acid groups in the 4-CTP three-dimensional self-assembled monolayers (3-D SAMs). The curves plotted in Figure 2 may be viewed as convolutions of the titration profiles from the ODA matrix and the carboxylic acid groups in the 3-D SAMs. The titration curve for the amine film is not expected to change with the colloidal particle size, but it is probable that the surface curvature of the colloidal particles could affect the pKa (as well as the width of the titration curve) of the carboxylic acid groups in the 3-D SAMs. It is well-known that lateral interactions between carboxylic acid groups can affect the pKa values

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Figure 2. Equilibrium percentage volume fraction of clusters as determined from QCM for ODA films of 500 Å thickness after immersion for 120 h in sol 1 (squares), sol 2 (circles), and sol 3 (triangles) as a function of the pH of the sol. The solid lines are fits to the data with Lorentzian curves. The inset shows a plot of the pH at which maximum cluster incorporation occurs (right axis) and width of the Lorentzian curves (left axis) plotted as a function of cluster size for the data shown in the main part of the figure.

as well as the width of the titration curve in planar SAMs.12 It is possible that increasing surface curvature could lead to reduced electrostatic interaction between carboxylic acid groups on the colloidal particle surface via a purely geometrical effect. We would like to mention here that the pH at which maximum cluster incorporation occurs for 4-CTP-capped 70 Å silver particles in ODA films was found to be ca. 8.5.8a,9 This indicates that in addition to the nanoscale curvature of the colloidal particle surface, the chemical nature of the colloidal particle also plays an important role in the ionization of the carboxylic acid groups. This aspect of the work requires further study and is outside the scope of this investigation. The kinetics of mass uptake determined using QCM for a 500 Å thick ODA film immersed in gold sols at pH ) 5.2, 6, 7, and 11.2 are shown in Figure 3. The solid lines in the figure are based on a 1-D diffusion model calculation, which has been described in detail elsewhere.4d For the analysis, the mass of the gold clusters was taken to be 2.3 × 10-8 ng (mass of a 130 Å diameter gold cluster). It is observed from Figure 3 that the mass uptake and, consequently, the gold cluster density in the film is a maximum at pH ) 7 and falls at lower and higher pH values of the sol in agreement with the data shown in Figure 2. Thus, by simple alteration of the colloidal solution pH, the density of clusters in the film can be controlled. The parameters obtained from analysis of the QCM data of Figure 3 in terms of a 1-D diffusion model are listed in Table 1 (section A).13 It is seen from the table that the even though the variation in cluster diffusivity with solution pH is not very large, maximum mass uptake occurs for the films immersed in the pH ) 7 hydrosol where the cluster (12) (a) Bain, C. D.; Whitesides, G. M. Langmuir 1989, 5, 1370. (b) Lee, T. R.; Carey, R. I.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 741. (13) The choice of the film thickness in fitting the QCM mass uptake data is important in obtaining reliable diffusivities, as has been discussed in detail in ref 4d. In keeping with the logic that nearly 60% of the cluster incorporation occurs within the first few hours of immersion, the final thickness of the films (determined by optical interferometry) was used in the fits to the 1-D diffusion equation. These thickness values are indicated in Table 1 within parentheses.

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Figure 3. QCM mass uptake data as a function of time of cluster incorporation for 500 Å thick ODA films immersed in sol 1 maintained at different pH values (values indicated next to the respective curves). The solid lines are fits to the data based on a 1-D diffusion model (see text for details).

Figure 4. QCM mass uptake data as a function of time of cluster incorporation for ODA films of different thickness (thickness indicated next to the respective curves) immersed in sol 1 (pH ) 7). The solid lines are fits to the data based on a 1-D diffusion model (see text for details).

Table 1. Parameters Based on a 1-D Diffusion Model Obtained from Fits to the QCM Mass Uptake Data of Gold Colloidal Particles in Thermally Evaporated Octadecylamine Films C0 × 1018 (clusters/cm3)

D × 104 (Å2/h)

A. Film thickness ) 500 Å (950 Å),a Sol 1 (130 Å size Au) pH ) 5.2 0.08 1.3 pH ) 6.0 0.15 1.2 pH ) 7.0 0.21 1.2 pH ) 11.2 0.07 0.9 B. Hydrosol pH ) 7.0, Sol 1 (130 Å size Au) thickness ) 250 Å (520 Å)a 0.25 thickness ) 500 Å (950 Å)a 0.21 thickness ) 1000 Å (1900 Å)a 0.20

0.5 1.2 1.8

C. Film Thickness ) 500 Å sol 1 (950 Å, pH ) 7)a 0.21 sol 2 (1100 Å, pH ) 8.5)a 10.37 sol 3 (1300 Å, pH ) 8.5)a 400

1.2 2.5 6.4

a

The numbers in the parentheses are the values of the film thickness used in the 1-D diffusion calculations.

concentration at the film-hydrosol interface (C0) is maximum. This indicates that the equilibrium mass uptake in the amine films is influenced to a larger extent by C0 than the cluster diffusivity and is an important conclusion of this study. Figure 4 shows the QCM mass uptake with time for the ODA films of 250, 500, and 1000 Å thickness on immersion in sol 1 at pH ) 7. The choice of the hydrosol pH for this experiment was dictated by the pH at which maximum cluster incorporation occurs (Figure 2; Table 1, section A). The solid lines are 1-D diffusion model fits to the data, and the parameters obtained from the fits are listed in Table 1, section B. It is seen from Table 1 that while the cluster diffusivity increases with increasing film thickness, the cluster concentration at the film/hydrosol interface decreases. In our earlier studies on the diffusion of carboxylic acid derivatized CdS and silver colloidal particles in amine films, a similar increase in the cluster diffusivity with film thickness was observed.4b,d However, the cluster density at the film surface was largely independent of film thickness and this is an aspect not understood at present. Another point to be noted is that for high amine film thickness (1000 Å and above), a pronounced “step” appears in the mass uptake curve after

Figure 5. QCM mass uptake data as a function of time of cluster incorporation for 500 Å thick ODA films during immersion in sols 1-3 (triangles, squares, and circles, respectively) at pH values 7, 8.5, and 8.5, respectively. The solid lines are fits to the data based on a 1-D diffusion model (see text for details). The inset shows a plot of the time required for equilibration of cluster density (teq) in the ODA films as a function of cluster size.

ca. 45% of clusters are incorporated into the amine matrix (Figure 4, arrow). Such steps have not been observed in our studies of the diffusion of CdS4b and silver4d colloidal particles in amine films. Figure 5 shows the QCM mass uptake measured ex situ in 500 Å amine films during immersion in hydrosols 1 (triangles, sol pH ) 7), 2 (squares, sol pH ) 8.5), and 3 (circles, sol pH ) 8.5). Please note that the pH of sols 1-3 was taken to be that at which maximum cluster incorporation was observed in the earlier experiment (Figure 2). The solid lines in the figure are least-squares fits to the data based on the 1-D diffusion model, and the parameters obtained from the fits are listed in Table 1, section C. The masses of the gold colloidal particles in hydrosols 2 and 3 were taken to be 4.5 × 10-10 and 0.86 × 1010 ng, respectively, in the analysis. It is observed that the diffusivity of the clusters into the film is a strong function of the colloidal particle size increasing from 1.2 × 104 Å2/h for the 130 Å diameter clusters to 6.4 × 104

Role of Particle Size in Diffusion

Figure 6. (A) UV-vis spectra recorded in air for 500 Å thick ODA films as a function of time of immersion of the films in sol 1 (pH ) 7). The time of immersion is indicated next to the respective curves. (B) UV-vis spectra recorded in the hydrosol for 500 Å thick ODA films during immersion of the films in sol 1 (pH ) 7). The time of immersion is indicated next to the respective curves. The inset of this figure shows a plot of the intensity of the surface plasmon resonance with time of immersion of the ODA film in sol 1 measured in air.

Å2/h for the 20 Å particles. The time taken for the cluster density in the film to equilibrate to its maximum value (teq) is also is found to decrease with decreasing cluster size and is shown in the inset of Figure 5. However, it is important to notice here that the equilibrium mass loading of the colloidal particles in the film is nearly identical and independent of the particle size. Another important observation from Table 1, section C, is that along with an increase in diffusivity with reducing cluster size there is an increase in the concentration of the colloidal particles at the film/solution interface. While the increase in diffusivity with reducing cluster size appears physically meaningful, the increase in cluster concentration is contrary to what is expected from purely electrostatic considerations.14 Clearly, a simple electrostatic picture of the diffusion process is not applicable in the present case. We remark here that recent work on X-ray reflectivity of large multivalent ions immobilized at charged Langmuir monolayers has shown that the Poisson-BoltzmannStern formalism cannot adequately explain the high counterion density observed near the charged lipid surface.15 For a more comprehensive understanding of the difference in cluster size dependent diffusivity and the probable mechanism of cluster diffusion, we have measured the UV-vis absorption spectra of the films as a function of time both in situ and ex situ. UV-vis absorption spectra of a 500 Å thick amine film on quartz measured in air and during immersion in hydrosol 1 (pH ) 7) as a function of time are shown in Figure 6A and Figure 6B, respectively. The time of immersion is indicated next to the curves. The spectrum of the amine film in the hydrosol measured at time t ) 0 has been subtracted from all the spectra recorded for the immersed films to account for absorption from the colloidal particles in the sol. Fresh sol was substituted at regular time intervals to maintain a uniform colloidal particle concentration in the sol and thus faithfully reproduce a constant absorption background used in the subtraction procedure outlined above. It is observed from Figure 6 that the surface plasmon resonance intensity increases steadily with time for the (14) Israelachvili, J. N. Intermolecular and surface forces, 2nd ed.; Academic Press: San Diego, CA, 1991; Chapter 12. (15) Cuvillier, N.; Rondelez, F. Thin Solid Films 1998, 327-329, 19.

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Figure 7. (A) UV-vis spectra recorded in air for 500 Å thick ODA films as a function of time of immersion of the films in sol 2 (pH ) 8.5). The time of immersion is indicated next to the respective curves. (B) UV-vis spectra recorded in the hydrosol for 500 Å thick ODA films during immersion of the films in sol 2 (pH ) 8.5). The time of immersion is indicated next to the respective curves. The inset of this figure shows a plot of the intensity of the surface plasmon resonance with time of immersion of the ODA film in sol 2 measured in air.

Figure 8. (A) UV-vis spectra recorded in air for 500 Å thick ODA films during immersion of the films in sol 3 (pH ) 8.5). The time of immersion is indicated next to the respective curves. (B) UV-vis spectra recorded in the hydrosol for 500 Å thick ODA films during immersion of the films in sol 3 (pH ) 8.5). The time of immersion is indicated next to the respective curves. The inset of this figure shows a plot of the intensity of the surface plasmon resonance with time of immersion of the ODA film in sol 3 measured in air.

films in solution and the corresponding films measured in air. The inset of Figure 6B shows a plot of the variation of the plasmon resonance intensity with time for the films measured in air. The time taken for the cluster density to stabilize in the film (ca. 100 h) agrees quite well with the QCM mass uptake kinetics data shown in Figure 3. A comparison of parts A and B of Figure 6 reveals an interesting shift in the surface plasmon resonance from 610 nm for the immersed films to 620 nm for the films measured in air. Similar cluster diffusion kinetics into 500 Å thick amine films on quartz have been studied for films immersed in hydrosols 2 and 3 (pH for both sols ) 8.5) and are shown in Figures 7 and 8, respectively. The shift in the surface plasmon resonance for films measured in air and in solution are found from Figures 7 and 8 to be 16 and 34 nm for hydrosols 2 and 3, respectively. The shift in the surface plasmon resonance for films measured in air and in the hydrosol is plotted as a function of cluster size in the inset of Figure 1 (right axis) and clearly shows that the shift increases with decreasing cluster size. The

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increase in the plasmon resonance wavelength on going from the film immersed in the hydrosol to the films measured in air as well as the dependence of this shift on the gold particle size can be rationalized in terms of the effective medium Mie theory.10 Since the carboxylic acid derivatized colloidal particle surface as well as the amine groups are hydrophilic, they are expected to be highly solvated on immersion of the composites in the hydrosol thereby leading to swelling of the film. As a consequence, the effective refractive index of the matrix in which the clusters are embedded would decrease during immersion in the hydrosol (refractive index (H2O) ) 1.33; refractive index (film) ) 1.55). A lowering of the ambient refractive index can be shown to lead to a shift to lower absorption wavelengths, as is observed (Figures 6-8). As mentioned earlier, the equilibrium mass uptake in the amine films is nearly a constant for the three different cluster sizes (Figure 5). This indicates that the density of clusters is much higher for the composites formed from sol 3 in comparison to those formed from sol 1, and as a consequence, the exposed surface area of the colloid would be higher for the smaller cluster composites. The degree of water uptake, which is expected to be in direct proportion to the exposed hydrophilic areas, would be higher for the composites containing smaller clusters and would thereby lead to a larger reduction in the effective dielectric constant of the amine matrix. This line of reasoning explains the larger shift in the surface resonance observed for the films grown from sol 3 measured in air and in the hydrosol when compared to the films grown from sols 1 and 2. The time variation of the plasmon resonance intensity measured in air for the films immersed in hydrosols 2 and 3 are shown as insets in Figures 7B and 8B, respectively. The time taken for the cluster density in the films to equilibrate is ca. 50 and 20 h for sols 2 and 3, respectively, and is in excellent agreement with the QCM data (Figure 5, inset). Thus, the optical absorption spectra also show a strong dependence of the cluster diffusion process on the particle size. The kinetics of the cluster diffusion process has also been followed using FTIR spectroscopy. Figures 9A and 10A show the FTIR spectra for 500 Å thick ODA films measured as a function of time of immersion in sol 1 (130 Å size Au clusters, pH ) 7) and sol 3 (20 Å size Au clusters, pH ) 8.5), respectively, in the range 2775-3025 cm-1. The inset shows the corresponding spectra recorded in the range 3275-3375 cm-1. The dotted lines in Figures 9 and 10 (both parts A and B) correspond to the as-deposited amine films recorded prior to immersion in the respective sols. The peak at 3330 cm-1 (inset, Figures 9A and 10A) corresponds to the -NH2 antisymmetric vibration and is known to shift to ca. 3200 cm-1 on salt formation with different anions.16 The fact that negligible shift in this resonance occurs on cluster diffusion into the film indicates weak coupling of the gold clusters with the amine molecules in the matrix. This is not surprising since the interaction between the amine group and clusters is expected to be primarily electrostatic. However, a reduction in intensity of this vibrational mode with time of cluster diffusion is clearly seen, the 3330 cm-1 band reducing in intensity much more rapidly for the ODA film immersed in the 20 Å gold sol. This is clearly brought out in Figure 11A, which shows a plot of the time variation of the -NH2 stretching intensity for films immersed in (16) (a) Ganguly, P.; Pal, S.; Sastry, M.; Shashikala, M. N. Langmuir 1995, 11, 1078. (b) Bardosova, M.; Tregold, R. H.; Ali-Adib, Z. Langmuir 1995, 11, 1273.

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Figure 9. (A) FTIR spectra recorded from a 500 Å thick ODA film as a function of time of immersion in sol 1 (pH ) 7) in the spectral range 2775-3025 cm-1. The time of immersion is given in the box. The inset shows a plot of the FTIR spectra recorded in the range 3275-3375 cm-1 and correspond sequentially (moving bottom to top) to the curves shown in the main part of the figure. (B) FTIR spectra recorded from a 500 Å thick ODA film as a function of time of immersion in sol 1 (pH ) 7) in the spectral range 1450-1500 cm-1. The time of immersion corresponds to the labels given in the box of Figure 9A.

Figure 10. (A) FTIR spectra recorded from a 500 Å thick ODA film as a function of time of immersion in sol 3 (pH ) 8.5) in the spectral range 2775-3025 cm-1. The time of immersion is given in the box. The inset shows a plot of the FTIR spectra recorded in the range 3275-3375 cm-1 and corresponding sequentially (moving bottom to top) to the curves shown in the main part of the figure. B) FTIR spectra recorded from a 500 Å thick ODA film as a function of time of immersion in sol 3 (pH ) 8.5) in the spectral range 1450-1500 cm-1. The time of immersion corresponds to the labels given in the box of Figure 10A.

sol 1 (curve 1, circles) and sol 3 (curve 2, squares). Please note that the curves in Figures 11A-C are an aid to the eye and have no physical significance. Two strong bands which occur at 2850 and 2920 cm-1 correspond to the methylene symmetric and antisymmetric stretching vibrations, respectively (Figures 9A and 10A).17 It is observed from the figure that the intensity of the methylene symmetric/antisymmetric stretching vibrations decreases with cluster incorporation, without change in the peak positions. The time variation of the methylene antisymmetric stretch band intensity during cluster incorporation is shown in Figure 11B for the ODA film immersed in sol (17) Hostetler, M. J.; Stokes, J. J.; Murray, R. W. Langmuir 1996, 12, 3604.

Role of Particle Size in Diffusion

Figure 11. (A) Variation in the FTIR intensity with time for the -NH2 stretching band (3330 cm-1) for 500 Å thick ODA films immersed in sol 1 (curve 1, circles), sol 3 (curve 2, squares), and the 1:1 mixture of sol 1 and sol 3 (pH ) 7.7, curve 3). The data have been normalized to the value at time t ) 0. (B) Variation in the FTIR intensity with time for the -CH2 antisymmetric stretching band (2920 cm-1) for 500 Å thick ODA films immersed in sol 1 (curve 1, circles), sol 3 (curve 2, squares), and the 1:1 mixture of sol 1 and sol 3 (pH ) 7.7, curve 3). The data have been normalized to the value at time t ) 0. (C) Variation in the FTIR intensity with time for the -CH2 scissoring band (1465 cm-1) for 500 Å thick ODA films immersed in sol 1 (curve 1, circles), sol 3 (curve 2, squares), and the 1:1 mixture of sol 1 and sol 3 (pH ) 7.7, curve 3). The data have been normalized to the value at time t ) 0.

1 (curve 1, circles) and sol 3 (curve 2, squares). In this case as well, the intensity of the band reduces much more rapidly for the film immersed in sol 3 in comparison to the film immersed in sol 1. The above observations suggest the following changes occurring in the amine matrix during cluster incorporation. It is known that the position of the methylene antisymmetric and symmetric vibration modes are a sensitive indicator of the presence of gauche defects in the hydrocarbon chains.17 The fact that the peak positions remain fixed with time at 2920 and 2850 cm-1 for the -CH2 antisymmetric and symmetric vibrations, respectively, and do not shift to higher wavenumbers indicates that the hydrocarbon chains are relatively defectfree even after complete cluster incorporation. The reduction in intensity with time implies that the orientation of the hydrocarbon chains is changing during cluster incorporation. To maximize the electrostatic interaction between the amine molecules and the charged colloidal particle surface, the amine layers would have to form spherical sheaths around the clusters. This can be done without significantly affecting the packing of the hydrocarbon chains but would lead to a randomization of the orientation of the hydrocarbon chains. Since the measurements were made in the transmission mode at normal incidence, randomization of the hydrocarbon chain orientation would lead to an effective loss of the absorption cross section for excitation of the stretch modes parallel to the film surface. A similar argument may also be advanced for the loss in intensity of the -NH2 stretching vibrations with time of cluster incorporation. The observation that the intensity decrease for both the -NH2 and -CH2 stretch modes is much more rapid during incorporation of the smaller 20 Å size gold clusters (Figures 9, 10, 11A, and 11B) indicates that the smaller clusters diffuse into the amine film much more rapidly. This is in agreement with the QCM findings which yielded a higher diffusivity for the 20 Å cluster in comparison with the 130 Å size gold clusters (Table 1, section C).

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Figures 9B and 10B show the time-dependent FTIR spectra for the ODA films immersed in sols 1 and 3, respectively, in the range 1450-1550 cm-1. In this region there are primarily two main peaks occurring at 1465 and 1472 cm-1. The peaks around 1470 cm-1 are assigned to the -CH2 scissoring motions, the splitting of which is dependent on the crystal field.18 The splitting of this band is therefore a measure of the crystallinity of the film. In this case as well, the methylene scissoring bands disappear much more rapidly for the ODA films immersed in sol 3 than for films immersed in sol 1. This is clearly seen in Figure 11C, which shows a plot of the time variation of the 1465 cm-1 peak for the films in the two sols (curve 1, sol 1, circles; curve 2, sol 3, squares). The intensity reduction of the scissoring bands may also be explained in terms of a randomization of the hydrocarbon chains in the ODA matrix. It may be mentioned here that the positions of the two peaks do not change with time indicating that the film crystallinity, and as we have seen earlier (Figures 9A and 10A), the packing of the hydrocarbon chains does not change significantly during cluster incorporation. The time required for the methylene antisymmetric vibration intensity to stabilize for the large (130 Å) and small (20 Å) size gold particles are seen to be ca. 100 and 20 h, respectively (Figure 11B, curves 1 and 2), in good agreement with the QCM observations presented earlier (Figure 5, inset). It is possible that the loss in intensity observed in the methylene antisymmetric and symmetric stretch vibrations as well as the methylene scissoring modes during incorporation of the colloidal particles described above could be a consequence of loss of ODA molecules from the film. Prolonged immersion of the ODA films in aqueous solution containing uncoordinated 4-CTP molecules could lead to dissolution of the ODA molecules via complexation with the solution phase 4-CTP molecules. The increase in mass observed by QCM measurements during cluster incorporation could obscure the loss of ODA from the film. To test this possibility, the following simple experiments were performed. A 1000 Å thick ODA film on quartz of surface area 2 cm2 was immersed in 50 mL of sol 1, and the surface of the sol was cleaned using a water pump. It may be remarked here that this is standard procedure for cleaning the surface of subphases prior to spreading and measuring the pressure-area isotherms for surfactant Langmuir monolayers.19 The surface tension of the gold sol containing the ODA film was then measured after 96 h of immersion, the time of immersion being chosen to be close to that observed for maximum cluster incorporation from QCM measurements (Figure 4). The surface tension of this system was measured to be 70.2 dyn/cm, fairly close to that of pure water. If the ODA molecules had indeed gone into solution, these surfactant molecules would have subsequently formed a monolayer on the surface of the sol due to the large hydrophobic component of the alkyl tails of the molecule and, thus, should have led to a measurable reduction in the surface tension, which is not observed. In another experiment, ethanolic solutions with water of 10-5 M 4-CTP and ODA molecules were prepared, the concentrations of ODA chosen to mimic weight loss of 5 and 10% from 1000 Å thick ODA films (area 2 cm2 as before). After complete evaporation of ethanol, the surface tension of the above two solutions was measured and found (18) Rabolt, J. F.; Burns, F. C.; Schlotter, N. E.; Swalen, J. D. J. Chem. Phys. 1983, 78, 946. (19) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly; Academic Press: San Diego, CA, 1991.

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Figure 12. QCM mass uptake data as a function of time of cluster incorporation for 500 Å thick ODA films during immersion in sols 1 (pH ) 7), 3 (pH ) 8.5), and a 1:1 mixture of sols 1 and 3 (pH ) 7.7, sol indicated next to the respective curves). The solid lines are fits to the data based on a 1-D diffusion model (see text for details).

to be 66.4 and 62.3 dyn/cm, respectively. The above experiments indicate that the loss in intensity observed in the FTIR measurements of the methylene vibrational modes is not likely to be a consequence of loss of ODA molecules from the film but due to randomization in orientation of the ODA molecules during cluster incorporation. 2. Simultaneous Diffusion of Small and Large Clusters into ODA Films. A hydrosol containing both small (20 Å) and large (130 Å) gold colloidal particles was prepared by mixing sol 1 and sol 3 in equal volumes. Assuming complete reduction of the respective salt solutions, the ratio of the number density of small and large clusters in solution can be easily shown to be 260:1. Figure 12 shows a plot of the QCM mass uptake for a 500 Å thick ODA film during immersion in sol 1 (curve A, sol pH ) 7), in sol 3 (curve B, sol pH ) 8.5), and in an equivolume mixture of sols 1 and 3 (curve C, sol pH ) 7.7). The pH of the mixed sol was taken to be the mean of the pH at which maximum cluster incorporation occurs for the individual large and small gold clusters. This was done in order to avoid bias in the diffusion of any one cluster species. It can be seen that the mass uptake behavior for the ODA film in the mixture is quite different from that recorded from the individual sols (Figure 12). A pronounced step is observed in the data for the mixed sols, and consequently, we did not attempt to fit the data to a 1-D diffusion model. However, it is observed that the equilibrium mass uptake for the ODA film immersed in the mixed sol is almost identical to that obtained for films immersed in the individual sols. The pH-dependent variation in the equilibrium cluster concentration in 500 Å thick ODA films immersed in the mixed sol for 120 h is shown in Figure 13 (curve C). The data corresponding to the individual sols 1 (curve A) and 3 (curve B) are also shown for comparison in Figure 13. It can be seen that the maximum cluster uptake occurs at a pH value (pH ) 7.7) between that of the extremes defined by the 130 Å (pH ) 7.1) and 20 Å (pH ) 8.45) gold clusters. The spread in the data is also larger for the mixed sol than for the individual sols (fwhm ) 3). The QCM measurements described above therefore indicate that the mass uptake behavior of the mixed sols is quite different from the individual sols.

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Figure 13. Equilibrium percentage volume fraction of clusters as determined from QCM for ODA films of 500 Å thickness after immersion for 120 h in sol 1 (curve A), sol 3 (curve B), and the 1:1 mixture of sols 1 and 3 (curve C) as a function of the pH of the sol. The solid lines are fits to the data with Lorentzian curves.

Figure 14. UV-vis spectra recorded in air for 500 Å thick ODA films as a function of time of immersion of the films in 1:1 mixture of sols 1 and 3 (pH ) 7.7). The time of immersion is indicated next to the respective curves. The inset shows a plot of the variation in intensity of the surface plasmon resonance with time for the above film.

Figure 14 shows the UV-visible spectra recorded ex situ from an ODA film of thickness 500 Å as a function of time of immersion in the mixed sol (pH ) 7.7). The position of the plasmon resonance for this film (620 nm) is very close to that observed for an ODA film immersed in sol 1 (Figure 6A, resonance at 622 nm) indicating that the spectrum is dominated by the bigger clusters diffusing into the film. FTIR measurements carried out on an ODA film immersed in the mixture (sol pH ) 7.7) in the spectral ranges 2775-3025 cm-1 and 1450-1550 cm-1 are shown in parts A and B of Figure 15, respectively. The inset of Figure 15A shows a plot of the time variation of the -NH2 stretch vibration for the amine film immersed in the mixture. Curves 3 in Figure 11 show in a concise form the intensity variation with time for the -NH2, methylene antisymmetric and methylene scissoring vibrations, respectively, for the above film. A comparison of curves 1 to 3 in Figure 11 indicates that the changes in the vibrational modes are dominated by the larger gold clusters. This is in agreement with the optical absorption

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Figure 15. (A) FTIR spectra recorded from a 500 Å thick ODA film as a function of time of immersion in the 1:1 mixture of sols 1 and 3 (pH ) 7.7) in the spectral range 2775-3025 cm-1. The time of immersion is given in the box. The inset shows a plot of the FTIR spectra recorded in the range 3275-3375 cm-1 and corresponding sequentially (moving bottom to top) to the curves shown in the main part of the figure. (B) FTIR spectra recorded from a 500 Å thick ODA film as a function of time of immersion in the 1:1 mixture of sols 1 and 3 (pH ) 7.7) in the spectral range 1450-1500 cm-1. The time of immersion corresponds to the labels given in the box of Figure 15A.

data shown in Figure 14. The above observation that the larger clusters dominate the UV-vis and FTIR spectra is surprising considering that the diffusivity of the smaller particles is much higher than that of the bigger gold particles (Table 1, section C) While the QCM, UV-vis, and FTIR data provide some information on what happens during a competitive diffusion process of large and small gold colloidal particles into the ODA matrix, a method such as TEM would enable direct estimation of the relative concentration of the colloidal particles in the film. Figure 16A shows a TEM picture recorded for a 500 Å thick ODA film deposited on a carbon-coated electron microscope grid after immersion in the hydrosol mixture (pH ) 7.7) for 120 h. It can be seen that the film comprises a mixture of both small and large gold particles. Figure 16B shows a plot of the particle size distribution (PSD) determined from the above micrograph. It can be clearly seen that the distribution is bimodal with peaks at ca. 22 and 122 Å. These two peaks represent the small and large colloidal particles in the film. The PSD also shows that the 20 Å clusters occur 10 times more frequently than the 130 Å size gold particles, and therefore, the cluster population in the films obtained from immersion of the ODA film in the mixture should be in fact dominated by the small gold particles. However, if one considers that the ratio of the surface areas of the large and small particles is ca. 42 (ratio of the squares of the radii), accounting for the 10-fold excess of the smaller clusters still leaves the mixed distribution dominated by the larger clusters in phenomena where surface effects are important. This clearly explains why the UV-vis (which yield the surface plasmon excitations) and the FTIR (which yield information on the packing of the hydrocarbon chains coordinated to the particle surface) measurements showed that the large gold particles dominated the diffusion process. Even though the concentration of the small gold particles in solution is ca. 260 times greater than that of the large gold particles, the concentration ratio is considerably reduced in the films indicating preferential absorption of the larger gold particles. This

Figure 16. (A) TEM picture of a 500 Å thick ODA film deposited on carbon-coated grids after 120 h of immersion in the 1:1 mixture of sols 1 and 3 (pH ) 7.7). The scale bar corresponds to 1700 Å. (B) The particle size distribution histogram for the TEM picture shown Figure 16A. The peaks occur at 22 and 122 Å.

process may be rationalized in terms of the energy required to distort the packing of the hydrocarbon chains in the ODA matrix. It is clear that the hydrocarbon sheath surrounding the colloidal particle surface would be required to be much more distorted around the smaller colloidal particles and thus would be energetically less favorable than when coordinated to the large gold particles. We had proposed a similar line of reasoning in our earlier study of the competitive diffusion of silver and gold colloidal particles in ODA films.4c In conclusion, it has been shown that colloidal gold particles can be incorporated in ODA films by simple immersion of the organic films in the colloidal solution. The diffusion of the colloidal particles into the organic matrix has been modeled in terms of a 1-D Fickian type diffusion, and it is observed that the cluster diffusivity increases as the cluster size is reduced. The pH at which maximum cluster incorporation occurs is strongly dependent on the cluster size indicating that the nanoscale curvature influences strongly the ionization of the carboxylic acid groups in the monolayer surrounding the particles. FTIR spectroscopy of the ODA films during cluster incorporation indicates weak coupling of the particles to the amine matrix and a time-dependent randomization of the orientation of the amine molecules, the rate of randomization being greater for films incorporating small colloidal gold particles in comparison to large particles. This is in agreement with UV-vis measurements of ODA films performed both in situ and ex situ. In a competitive diffusion process of both big (130 Å) and small (20 Å) gold particles, larger gold particles

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are preferentially incorporated although the diffusivity is greater for the smaller particles. This has been rationalized in terms of the energy cost arising from distortion of the amine molecular packing, the distortion being less for the larger particles Acknowledgment. V.P. thanks the Department of Science and Technology (DST, Government of India) for a research fellowship. This work was supported by a

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research grant from the Council for Scientific and Industrial Research, Government of India. Supporting Information Available: Transmission electron micrographs together with the particle size distributions for sols 2 and 3. This material is available free of charge via the Internet at http://pubs.acs.org. LA990170T