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Langmuir 1998, 14, 7034-7046
Formation Mechanisms and Aggregation Behavior of Borohydride Reduced Silver Particles Dirk L. Van Hyning and Charles F. Zukoski* Department of Chemical Engineering, University of Illinois, Urbana, Illinois 61801 Received March 23, 1998. In Final Form: August 4, 1998
In this work, we examine the formation mechanisms of nanoscale silver particles produced by the reduction of silver perchlorate with sodium borohydride. Evidence is presented that the reaction pathway does not follow classical nucleation and growth theory, but is dominated by colloidal interactions. Upon injection of silver into a sodium borohydride solution, a molecular species absorbing at 220 nm is produced in less than 1 s. We suggest that this species contains borohydride and small particles of reduced silver. The reaction mixture is initially dark as the result of the aggregation of the small silver particles into larger particles which have broad absorption spectra. During an “intermediate” stage, transmission electron microscopy and absorbance data show that even larger (∼6-10 nm) particles grow at the expense of the “monomeric” silver particles. Later in the reaction, electrochemical potential measurements show that the borohydride concentration suddenly decreases. Direct measurement of interparticle forces demonstrate that this change in the solution conditions drives the particle surface potential toward zero and results in increased adhesive forces. The resulting aggregation manifests itself in a darkening of the solution color. At low temperatures this corresponds to a 10 times increase in the particle size whereas, at high temperatures, the increase is minimal. This effect can be linked to the number of monomeric silver particles remaining during the final transition.
I. Introduction Many studies have attempted to relate particle size and morphology of gold and silver sols to reaction environment.1-5 However, the mechanisms of growth and origin of uniformity achieved in the reaction pathways are still largely unknown. The model typically invoked to explain the mechanism of production of monodisperse colloidal particles is that of LaMer and Dinegar6 which argues that the number of particles in a solution is fixed in a very short burst of nucleation. Particles then grow by the addition of monomers (atomic species) at the surface of the new nuclei. This model has been invoked in the classic work by Turkevich and co-workers for the formation of gold sols.7-9 However, experimental work on SiO2,10-15 TiO2,11,16,17 and platinum18 indicate that aggregation may (1) Matijevic, E. Chem. Mater. 1993, 5, 412. (2) Medendorp, N.; Bowman, K.; Trumble, K. Mater. Sci. Eng., A 1996, 212, 222. (3) Bogush, G.; Zukoski, C. J. Non-Cryst. Solids 1988, 104, 95. (4) Schneider, S.; Halbig, P.; Grau, H.; Nickel, U. Photochem. Photobiol. 1994, 60, 605. (5) Barnickel, P.; Wokaun, A.; Sager, W.; Eicke, H.-F. J. Colloid Interface Sci. 1991, 148, 80. (6) LaMer, V.; Dinegar, R. J. Am. Chem. Soc. 1950, 72, 4847. (7) Turkevich, J.; Garton, G.; Stevenson, P. J. Colloid Interface Sci. Suppl. 1954, 1, 26. (8) Turkevich, J. Gold Bull. 1985, 18 (3), 86. (9) Turkevich, J. Gold Bull. 1985, 18 (4), 125. (10) Bogush, G.; Zukoski, C. Ultrastructure Processing of Advanced Ceramics; Wiley: New York, 1988; p 477. (11) Look, J.-L.; Bogush, G.; Zukoski, C. Faraday Discuss. Chem. Soc. 1990, 90, 345. (12) Bogush, G.; Zukoski, C. J. Colloid Interface Sci. 1991, 142, 19. (13) Okubo, T.; Okada, S. J. Colloid Interface Sci. 1997, 192, 490. (14) Bailey, J. K.; Mecartney, M. L. Colloids. Surf. 1992, 63 (1-2), 151. (15) Bailey, J. K.; Macosko, C. W.; Mecartney, M. L. J. Non-Cryst. Solids 1990, 125 (3), 208. (16) Lee, K.; Look, J. L.; Harris, M. T.; McCormick, A. V. J. Colloid Interface Sci. 1997, 194, 78. (17) Santacesaria, E.; Tonello, M.; Storti, G.; Pace, R.; Carra, S. J. Colloid Interface Sci. 1986, 111, 44. (18) Duff, D.; Edwards, P.; Johnson, B. J. Phys. Chem. 1995, 99, 15934.
play an important mechanistic role in producing monodisperse colloidal particles. Modeling19 efforts have shown that continuous nucleation followed by aggregation may result in monodisperse particle size distributions. Further experimental support for this idea can be found in the production of gold sols by reduction of HAuCl4 by trisodium citrate. In this system,20-22 the initial reaction products are large clusters (∼100-200 nm) which shrink in size as the reaction proceeds until a final size of ∼15 nm is reached. Because of the scattering and absorption properties of gold particles, the shrinking of the particles is accompanied by a change in the reaction solution color from black to red. This behavior, obviously contrary to the LaMer model, is explained by a competitive adsorption between AuCl4- ions and citrate ions which impart different surface charges. Early in the reaction, AuCl4binds, imparting a low surface potential and resulting in weakly attractive interparticle forces. As a result, small particles aggregate reversibly. As the reaction proceeds, AuCl4- is consumed, citrate binds to the particle surfaces, the particle charge increases, and the pair potential becomes generally repulsive. Clusters then fall apart to produce the final particle size distribution. In the present work, we explore the formation of uniform silver particles through the reduction of silver perchlorate by sodium borohydride. The reaction follows a series of well-defined stages as observed visually. These are shown in Figure 1. Figure 1A, which was taken at ∼1 s into the reaction, shows that the solution has a darker yellowgreenish color which is produced by particles that exhibit significant long-wavelength absorbance. Figure 1, parts B and C, shows the second stage where the solution color changes from a darker yellow-green to a bright yellow (19) Kim, S.; Zukoski, C. J. Colloid Interface Sci. 1990, 139, 198. (20) Chow, M.; Zukoski, C. J. Colloid Interface Sci. 1994, 165, 97. (21) Biggs, S.; Mulvaney, P.; Zukoski, C.; Grieser, F. J. Am. Chem. Soc. 1994, 116, 9150. (22) Biggs, S.; Chow, M.; Zukoski, C.; Grieser, F. J. Colloid Interface Sci. 1993, 160, 511.
10.1021/la980325h CCC: $15.00 © 1998 American Chemical Society Published on Web 10/30/1998
Formation Mechanics of NaBH4 Reduced Ag Particles
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Figure 1. Photographs of reacting silver solutions at various times in the reaction: (A) ∼0.5 s after injection of silver perchlorate solution; (B) ∼10 s into the reaction showing bright yellow color; (C) ∼40 min into the reaction; (D) ∼45 min into the reaction showing dark final solution color. Reaction conditions are [Ag] ) 0.250 mM and [NaBH4] ) 1.50 mM.
color which persists for a significant period of time. This yellow color is due to small (5-10 nm) particles in solution which exhibit only short-wavelength (∼400 nm) absorbances. Finally, Figure 1D shows stage 3 where, late in the reaction, the solution takes on a significantly darker color due to an increase in the particle size which results in long (500-800 nm) wavelength absorption. As will be reported in following sections, the intensity of these transitions vary with solution conditions. However, for a wide range of reaction conditions, the final sols are produced through the same three qualitative stages. Many recipes are given in the literature for synthesizing these colloidal silver particles with sodium borohydride. However, the existence of the stages as shown in Figure
1 and their origin has seen no attention. Here, we argue that these stages result from a coupling of solution reactions (silver/borohydride and borohydride/water) and surface properties of silver particles. In section II we present our experimental methods followed in section III with a description of the optical and physical properties of the silver sols. These results suggest that, upon injection of silver ions into an agitated borohydride solution, small borohydride-bound silver particles are formed. These clusters then grow by a mechanism that is largely aggregative in nature, and the parameters that control the growth rates and ultimate particle sizes are those which control aggregation. In section IV we draw conclusions.
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II. Materials and Methods Production of Colloidal Silver. As stated above, the variation in synthesis “recipes” in the literature which purport to make essentially the same final particles is staggering.5,23-29 After careful investigation we found that the final particles are indeed insensitive to wide ranges of mixing conditions. As a result, in our studies, no one method of synthesis from the literature was used. The reactions were carried out in Erlenmeyer flasks which were either soaked in a strong nitric acid bath (∼6 M) overnight or washed in concentrated nitric acid. The acid wash was followed by rinsing in copious amounts of deionized water. Fresh ice-cold 0.295 M NaBH4 solutions were prepared before each set of experimental runs from the highest purity NaBH4 available (Aldrich Chemical). Stock 1-L 5 mM AgClO4 (99.999% purity, Aldrich Chemical) solutions were prepared and kept under airtight conditions in darkness to maintain uniformity between experimental runs. Silver solutions were discarded after 2 weeks to ensure minimal photodegradation effects. The Erlenmeyer flasks were filled with the appropriate amounts of deionized water and then brought to the temperature at which the reaction was to take place along with the silver perchlorate and sodium borohydride solutions. The temperature of the reaction was controlled using an ice bath for the under 5 °C runs and a Neslab RTE-5B refrigerated circulating bath for the experiments at 10 °C and above. Before the silver was injected, an appropriate volume of the sodium borohydride (SBH) solution was added to the reaction vessel under agitation (provided by a magnetic Teflon stir bar) using an Oxford adjustable micropipet. This order of reactant addition is the standard procedure in the literature26,30,31 going back to the method used by Blatchford et al.23 Five milliliters of the silver stock solution was then injected into the reaction vessel as fast as possible using the micropipet to begin the reaction. Total injection time was on the order of 0.1 s. Experiments were conducted at varying temperatures, silver concentrations, and sodium borohydride concentrations. “Standard” reaction conditions are [Ag+] ) 0.250 mM, T ) 5 °C, and R ) 6. A convenient notation used here and that will be used throughout this work will be R which will indicate the NaBH4: AgClO4 molar ratio. While the standard conditions were investigated heavily, changes in [Ag+], T, and R were investigated systematically. The value of R was always greater than 1, as a large excess of sodium borohydride is used to both reduce the ionic silver and to stabilize the growing particles. Variable silver concentration experiments were conducted at [Ag+] ) 0.125 and 0.500 mM, keeping the other parameters constant. Experiments were also conducted in the same manner for R ) 3 and 12 and for T ) 10, 15, 20, and 25 °C. Studies were conducted to ascertain what reagent proportions ([Ag+] and corresponding R values) produced particle stability. R values less than ∼1.4 resulted in complete aggregation of the particles for all silver concentrations studied. Particle instability was also observed at high silver and sodium borohydride concentrations where the total ionic concentration resulted in ionic strengths large enough to collapse the electrostatic double layer. System Analysis. The silver particle solutions were analyzed using four techniques: transmission electron microscopy (TEM), UV-vis photodiode array spectrophotometry (PDAS), solution potential measurements, and atomic force microscopy (AFM). (A) Transmission Electron Microscopy. Particle size distributions were tabulated primarily using TEM analysis on (23) Creighton, J.; Blatchford, C.; Albrecht, M. J. Chem Soc. Faraday Trans. 2 1979, 75, 790. (24) Zhai, X.; Efrima, S. J. Phys. Chem. 1996, 100, 1779. (25) Silvert, P.-Y.; Herrera-Urbina, R.; Duvauchelle, N.; Vijayakrishnan, V.; Elhsissen, K. J. Mater. Chem. 1996, 6, 573. (26) Heard, S.; Grieser, F.; Barraclough, C.; Sanders, J. J. Colloid. Interface Sci. 1983, 93, 545. (27) Jolivet, J.-P.; Gzara, M.; Mazieres, J.; Lefebvre, J. J. Colloid Interface Sci. 1985, 107, 429. (28) Faraday, M. Philos. Trans. 1857, 147, 145. (29) Carey Lea, M. Am. J. Sci. 1889, 37, 476. (30) Siiman, O.; Bumm, L.; Callaghan, R.; Blatchford, C.; Kerker, M. J. Phys. Chem. 1983, 87, 1014. (31) Lee, P.; Meisel, D. J. Phys. Chem. 1982, 86, 3391.
Van Hyning and Zukoski a Hitachi 420 electron microscope. Then 400-mesh TEM grids were purchased through SPI supplies, coated with a thin layer of Formvar resin, and carbon-coated for strength. To ensure that no secondary “on-grid” aggregation and/or coalescence occurred, any solution sample to be put on the TEM grids were brought to 0.1% (w/v) in poly(vinyl alcohol) (PVA, 31 000-50 000 MW, Aldrich Chemical) so as to provide steric stabilization. Samples were transferred from a solution to a grid by removing an aliquot of the reacting solution via a pipet, mixing with PVA, and rapidly flowing the solution through the grid onto clean filter paper. Although the addition of PVA should halt any aggregation phenomena, the grids were dried immediately to minimize any reaction of solution while on the grid. Particle size distributions were obtained by manually measuring 100-particle radii with a Mitsumo digital caliper and comparing them to Duke Scientific polystyrene-size standards. (B) Photodiode Array Spectrophotometry. A second method of ascertaining the particle size and aggregation state was through ultraviolet-visible (UV-vis) spectrophotometry. For this study, a Hewlett-Packard 8453 spectrophotometer was used to collect spectra at rates up to 10 spectra/s. For all spectra recorded, deionized water “blank” spectra were recorded in the cuvette to be used for the given experiment and were automatically subtracted from the sample spectra. For nontransient spectra, solutions were transferred via a pipet to a 1-cm path length silica cuvette (Fisher Scientific Catalog). For all kinetic runs, the solution was placed in the appropriate temperature bath and placed on a magnetic stir plate as close to the instrument as possible. To bring the sample into the light path, a Masterflex peristaltic pump was used to draw solution through a quartz flow cell (2-mm path length) and back into the solution. On the fastest pump speed, the time required to bring the solution from the reaction vessel into the light path was ∼0.2 s. Teflon tubing was used to prevent contamination of the sample. Spectra were recorded and transferred to spreadsheet form for all calculations. In many reactions, upon mixing, the absorbance increased uniformly over the entire wavelength range. The reaction of borohydride with silver and water produces hydrogen bubbles, and thus we assume that this uniform increase in absorbance is due to scattering from hydrogen bubbles. This baseline is subtracted off the absorbance spectra, allowing us to explore chemical absorption phenomena. (C) Potential Measurements. To gain some understanding of the solution electrochemical conditions, potential measurements were taken at various experimental conditions using the method of Kerker et al.32 The potential of the solution was measured between a pure silver wire electrode and a standard 0.1 M KCl calomel electrode, both dipped in a constantly stirred solution. The silver wire was washed in concentrated perchloric acid before the experiments were conducted to remove any contamination. Care was taken to prevent movement of the wire and SCE during the experiment, apart from movement due to stirring, as drastic movement seemed to slightly vary the reading. Data were recorded manually from a LCD display. (D) Atomic Force Microscopy. (1) Silver Probes. Silver probes were made using silver spheres and standard Digital Instruments silicon nitride tips. Of the four cantilevers available on each tip, the short thick cantilever was used due to excessive bending of the other cantilevers tried when the probe was attached. The attached probes were 325-mesh 99.9% silver particles obtained from Cerac and were roughly spherical. The probes were imaged on a Hitachi scanning electron microscope to observe the tip geometry and obtain an approximate radius of interaction. The probes were manufactured using a modified method of Ducker et al.33 as described previously.34 (2) Silver Flat Surfaces. The substrate upon which the silver surface was deposited was thin (∼0.5-mm thickness) silicon wafers. Prior to deposition of the silver, the silicon wafer was washed in concentrated hydrofluoric acid. Pure silver shot (99.9999% Aldrich #20436-6) was used as the source of the deposition. Once made, the silver wafers were then cut into (32) Blatchford, C.; Siiman, O.; Kerker, M. J. Phys. Chem. 1983, 87, 2503. (33) Ducker, W.; Senden, T.; Pashley, R. Nature 1991, 353, 239. (34) Van Hyning, D. Master’s Thesis, Univeristy of Illinois Urbanas Champaign 1997.
Formation Mechanics of NaBH4 Reduced Ag Particles 1-cm square pieces. Prior to use the silver squares were washed in ethanol to remove organic contamination. They were then attached via two-sided tape to a small metal disk to be mounted onto the magnetic piezoelectric AFM stage. (3) AFM Measurements. For all experiments in this work, a Nanoscope II AFM (Digital Instruments) was used. Instrument operation as well as probe and sample preparation have been described previously.34 Because irregular surfaces with obvious asparities were used for force measurement, no imaging of the surface was conducted for roughness measurements to minimize contamination on the tip. For all experimental conditions except the highest sodium borohydride concentration, at least 5 force curves of 512 data points each were taken at each experimental condition so as to present reliable averages of measured parameters for comparison. At higher NaBH4 concentrations, hydrogen gas which is evolved in borohydride hydrolysis prevented collection of 5 spectra. The AFM records values of the voltage of the photodetector (a measure of cantilever deflection) as a function of piezoelectric crystal voltage (a measure of position of the substrate). These data were converted into force as a function of tip-sample separation using a computer program written by Derek Chan at the University of Melbourne. To convert deflection to force, the spring constant of the cantilever/probe system was measured by the method of Hansma et al.35 Here, the resonant frequency of several cantilevers with and without tungsten spheres of radius R is measured and used to calculate a spring constant. This method produced a value of 0.174 ( 0.02 N/m. Because of the highly reflective nature of the substrate, interference patterns were created on the detector from the substrate and cantilever, producing a periodic baseline instead of a flat one. This baseline was removed using an Excel spreadsheet program but required that each force curve include two points that could be distinguished as being the same point in the period.
Langmuir, Vol. 14, No. 24, 1998 7037
Figure 2. Typical series of absorption spectra during the first ∼5 s of the reduction of silver [0.250 Mm] with borohydride [1.50 Mm]. Curve A is taken on the sodium borohydride solution prior to injection of the silver. Curve B is taken after 0.2 s while curve C is taken after 5 s.
III. Experimental Results and Discussion A wide variety of recipes for the precipitation of silver sols are reported in the literature.5,23-29 The majority of them indicate that the reaction produces a bright yellow solution with slightly varying particle sizes. When attempts were made to produce “standard” silver particles, we observed that the reacting solution went through a distinct series of color changes before arriving at the final color. Extreme care was taken to not deviate significantly from the literature. Glassware was cleaned by several methods, including soaking overnight in an acid bath, washing with concentrated nitric acid prior to use, and using various acids and cleaning agents. Silver nitrate and silver perchlorate were both used and found to follow the same color sequence. Varying the mixing conditions and silver solution volume to borohydride solution volume ratios were attempted. Several new containers of reagents were used to investigate the possibility of reagent fouling. In all the above cases, the phenomena were consistent. As heating has been indicated to convert all excess borohydride quickly36 and eliminate many effects of borate reactions, different rates, methods, and magnitudes of heating were attempted. In all cases, heating above ∼45 °C resulted in a complete aggregation of the particles leaving large (∼mm) macroscopic agglomerates. These studies led us to conclude that the color changes seen in Figure 1 were not of interest in previous studies where the primary focus was on using the silver particles rather than their formation mechanism. Early Reaction Times. The phenomena shown in Figure 1 occur in three distinct steps. The first of those (35) Cleveland, J. P.; Manne, S.; Bocek, D.; Hansma, P. K. Rev. Sci. Instrum. 1993, 64, 403. (36) Heard, S.; Grieser, F.; Barraclough, C.; Sanders, J. J. Phys. Chem. 1985, 89, 389.
Figure 3. Variation in 220-nm initial peak height with silver concentration. Linear fit shows that a variation in the 220-nm peak is directly proportional to the silver concentration.
is the initial darkening of the solution which then “flashes” bright yellow. This “stage”, most distinct at higher temperatures (∼25 °C), lasts only ∼1-3 s so UV-vis spectrophotometry was the only technique used to analyze this portion of the reaction. Figure 2 shows a series of absorbance spectra for [Ag+] ) 0.250 mM and R ) 6. This series of spectra is typical of the solution absorbance behavior during early reaction times. Curve A shows the “tail” of an absorbance peak at λ ) 190 nm due to aqueous sodium borohydride. Upon injection of silver, a peak centered at λ ) 220 nm (curve B) rapidly (∼0.1 s) appears. This “220-nm” peak was observed under all experimental conditions. For Ag+ ) 0.250 mM at all sodium borohydride and temperature conditions, the peak at 220 nm rose to an initial absorbance of ∼0.37 AU. Figure 3 shows initial absorbance spectra for varying silver concentrations taken at the same time into the reaction, indicating that the height of the 220-nm peak is proportional to the silver concentration. These data strongly suggest that absorbance at 220 nm is due to a species involving silver. It is also well-known that colloidal silver particles absorb light in the UV regime. The absorbance of such spheres can be characterized using Mie’s equations on the extinction of electromagnetic radiation37 where extinction is written in terms of particle scattering cross sections. Assuming spheres scatter and (37) Mie, G. Ann. Phys. 1908, 25, 4.
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Figure 4. Difference in the low wavelength absorption for early reaction products and final reaction products. Final particles exhibit an inflection point in the absorbance as the Mie theory predicts with bulk dielectric data. The Mie theory curve calculated for 10-nm particles with unaltered bulk dielectric data using Mie equations (53). Particle number density used in calculation was 1.27 × 1017 particles/mL.
absorb light independently, the absorbance, A, can be written as
A ) Nπr2(Qext)
(1)
where Qext can be separated into absorbance and scattering components:
Qext ) Qabs + Qsca
(2)
with
Qext )
Qsca )
2 x2
2 x
2
∞
∑1 (2n + 1)Re(an + bn)
(3)
∞
∑1 (2n + 1){|an|2 + |bn|2}
(4)
where x is a size parameter 2πa/λ (where a is the particle diameter) and an and bn are the well-known Mie coefficients38,39 listed in the Appendix. For all calculations in this work, only the first three coefficients were used, as the particle sizes are in the size range where higher order terms have been shown to be negligible.38-41 Figure 4 shows two absorbance spectra of silver particles in solution and one theoretical curve. The curve predicted from Mie equations for 10-nm diameter particles shows good agreement with actual solution spectra recorded from final solutions when only larger (∼10 nm) particles remain. Note that both spectra exhibit an inflection point at λ ∼ 300 nm. The differences between the calculated curve and final experimental curves near 200 nm are due to remaining borohydride in the reacting solution. In addition, the deviation in the spectral region near 375 nm is due to the fact that polydispersity in particle size exists in the reacting solution while calculations were performed only for an approximate mean size (∼10 nm). The early reaction time spectrum shows significantly different (38) Kerker, M. The Scattering of Light and Other Electromagnetic Radiation; Academic Press: New York, 1969. (39) van de Hulst, H. C. Light Scattering by Small Particles; Wiley: New York, 1957. (40) Look, D. J. Colloid Interface Sci. 1976, 56, 386. (41) Deirmendjian, D. J. Opt. Soc. Am. 1960, 51, 620.
Figure 5. Short time evolution of UV-visible absorbance for a silver solution reacted at 15 °C. The bump at ∼370 nm is due to equipment error and not to species in solution. Reaction carried out at [Ag] ) 0.250 mM, R ) 6, and T ) 15 °C. The interval between spectra is 0.1 s.
character from the other two, having no inflection point at 300 nm and significant Lorentzian character. We conclude that the 220-nm peak is not absorbance due to low-wavelength dielectric character of silver particles but rather results from another species in the solution. One possibility is a silver-sodium borohydride complex, which has been found to exist. However, these complexes thermally decompose above -30 °C and must be prepared in diethyl ether.42 Other silver(I)-borohydride complexes have been synthesized with methyldiphenylphosphine ligands, but they also are stable only at low temperatures and nonaqueous solvents due to the high reactivity of these species in water.43 Because this is an aqueous reduction which takes place at ∼0 °C and the 220-nm peak lingers under certain conditions for hours, it seems extremely unlikely that this is a simple silver-borohydride complex. The chemical identity of the species which absorbs at 220 nm remains unknown. The hypothesis adopted here is that, during the initial reduction, a large number density of very small (1-3 nm) silver particles are produced with an extremely high total surface area. Thus, the absorption maximum at 220 nm is the result of a shift in borohydride absorption to longer wavelengths (lower energy) due to association with this newly formed surface. The mechanisms responsible for the observed phenomena will now be discussed in light of this hypothesis, and a discussion of further supporting evidence will follow. After the 220-nm peak rapidly grows in, the absorbance peak associated with the surface plasmon resonance of larger colloidal silver particles appears. Figure 5 shows a typical series of initial absorbance spectra taken in 0.1-s intervals. Two features are immediately apparent. First, the location of the surface plasmon peak originally is centered at ∼375 nm and then red shifts to ∼385 nm. Second, the long-wavelength absorbance initially is high and then decreases as the surface plasmon absorbance peak narrows. The absorbance at higher wavelengths gives rise to the initially dark character of the reacting mixture when observed visually. The rapid transition from dark to bright yellow results from both the loss of long wavelength absorption and the strong absorption of violet light while the surface plasmon peak is growing. (42) Lancashire, R. Encyclopedia of Inorganic Complex Chemistry; p775. (43) Bommer, J.; Morse, K. Inorg. Chem. 1980, 19, 587.
Formation Mechanics of NaBH4 Reduced Ag Particles
Langmuir, Vol. 14, No. 24, 1998 7039
Figure 7. Calculation of surface plasmon resonance peak halfwidth and location as a function of the particle radius. Halfwidth calculations are consistent between models using dielectric data altered for size using KKE models (50) and Drude models. Peak locations differ for the two models and are shown above.
Figure 6. (Top) Absorbance at 400 and 550 nm during early reaction times at 25 °C. (Bottom) Ratio of absorbance at 400 nm to absorbance at 525 nm.
This is shown in Figure 6 where the time dependence of absorbance at 400 and 550 nm is presented. The absorbance at 550 nm is an indicator of higher wavelength absorbances which impart a darker solution color while absorbance at 400 nm is a measure of the lower wavelength absorbance which imparts a yellow color to the solution. As Figures 5 and 6 show, at times less than 0.5 s, both the 220-nm peak and the absorbance at 550 nm are still increasing. Over the next second, the surface plasmon peak narrows, resulting in an increase in absorbance at 400 nm. At the same time, absorbance at 550 nm decreases. More importantly, during this time (0.6-1.0 s), the ratio of absorbance at 400 nm to the absorbance at 550 nm increases dramatically. Therefore, although there remains a long wavelength absorbance after this initial stage, the much stronger absorbance of violet light near 400 nm is the cause of the intense yellow color. This dark-to-yellow transition is not observed at 5 °C and becomes more distinct with increasing temperature. As indicated in Figure 6, the absorbance at 550 nm passes through a maximum early in the reaction as the surface plasmon begins rapid growth. Although the longwavelength absorbance observed at 25 °C is small, it is significant, and the maximum absorbance decreases systematically with the reaction temperature to a value of 0 at 2 °C. This result indicates that the temperature is altering the kinetics of growth in a nonmonotonic manner and suggests the presence of more than one reaction, each with a different activation energy. In previous studies,20,44 the dark color of metal sols produced by long-wavelength absorbance was indicative of the presence of large particles. No evidence for such large particles was found in transmission electron micrographs of sol samples taken early in the reaction. An alternative explanation for the long-wavelength absorption can be found in the dielectric properties of very small (44) Turkevich, J.; Garton, G.; Stevenson, P. Discuss. Faraday Soc. 1951, 11, 55.
(3 nm) particles but also the 220-nm peak were removed. Therefore, it appears that the “220-nm species” is one that is electrostatically stabilized and it is highly unlikely that it is due to a chemical species. It has been suggested that oxygen may significantly affect the formation of silver particles and the surface plasmon band, even making the process reversible.71-73 To investigate the role of oxygen, a series of experiments were conducted where reactions were performed under oxygen-poor conditions using nitrogen sparging. These experiments showed that depriving the solution of oxygen drastically slowed the hydrolysis of borohydride. However, the appearance of the 220-nm peak and the subsequent stages as described above were not affected apart from the hindered borohydride consumption. (66) Henglein, A.; Ershov, B.; Janata, E. J. Phys. Chem. 1993, 97, 339. (67) Henglein, A.; Mulvaney, P.; Linnert, T. Ber. Bunsen-Ges. Phys. Chem. 1990, 94, 1449. (68) Henglein, A.; Mulvaney, P.; Linnert, T.; Weller, H. J. Am. Chem. Soc. 1990, 112, 4657. (69) Henglein, A. Isr. J. Chem. 1993, 33, 77. (70) Henglien, A. Ber. Bunsen-Ges. Phys. Chem. 1977, 82, 556. (71) Mulvaney, P.; Linnert, T.; Henglein, A. J. Phys. Chem. 1991, 95, 7843. (72) Pal, T.; Sau, T.; Jana, N. Langmuir 1997, 13, 1481. (73) Kapoor, S. Langmuir 1998, 14, 1021.
Langmuir, Vol. 14, No. 24, 1998 7045
A significant body of literature exists discussing how the surface plasmon resonance band of silver can be affected by the surface state of the particle.69,74-76 In these studies, it is not clear whether the observed effects are due to a change in free electron concentration within the particle or a change in the refractive index around the particle. However, these studies show that, in general, the adsorption of a nucleophilic molecule can red-shift and broaden the surface plasmon band and adsorption of an electrophilic reagent has the opposite effect. Although there are significant differences between these trends and what we observe, it is possible that changes in the surface state of the particle can be acting in an uncharacteristic way to affect changes in not only the surface plasmon band but also the 220-nm region. To investigate this, experiments were performed where a poly(vinyl alcohol) (PVA) was added at various reaction times. PVA is known to strongly adsorb to silver surfaces and is an excellent steric stabilizer. If the observations we have made were caused by gradual changes in the surface state of the particle, upon addition of PVA we would expect to see drastic changes in both the height and the half-width of the surface plasmon peak to a similar final spectrum regardless of the time when the PVA was added. This is not the case. When the PVA was added prior to the addition of silver, the 220-nm peak grows in and no growth of the surface plasmon peak is observed until a drastic final transition. When added during the intermediate stage, the PVA causes the surface plasmon band to redshift repeatably by ∼7 nm with no change in the height or half-width. It also follows that if the observed changes in the 220-nm peak are due to a change in the solution conditions or surface state of the particle due to borohydride consumption, the 220-nm peak could be replenished by the addition of more NaBH4-. Repeated experiments demonstrated that addition of borohydride during stage 2 did not alter the 220-nm peak. The size of this peak was only altered during stage 2 by the addition of more silver. On the basis of these experiments we conclude that the 220-nm peak is a marker for silver but may result from the optical properties of an adsorbed molecular species. Thus, we anticipate that this peak will be proportional to the surface area of silver in solution and decreases due to aggregation and coalescence of silver particles. As a consequence, we expect to affect the rate of decrement of the 220-nm absorbance by simply increasing the ionic strength and compressing the ionic double layer. A series of experiments were conducted where solutions were reacted under normal conditions (T ) 2 °C, [Ag] ) 0.250 mM, and R ) 12). Three minutes into the reaction, varying concentrations of NaClO4 were added and the absorbance spectra were recorded. The results are shown in Figure 17. The rate of change of the 220-nm and surface plasmon peaks both increase with ionic strength. This result again demonstrates that the 220-nm peak results from an absorption on electrostatically stabilized silver particles. Because of our inability to deconvolute the spectra to independently determine size and number density, we are unable to show that the 220-nm peak changes with the surface area. Finally, as stated above, when PVA is added to the solution before the addition of silver, the 220-nm peak grows in and is “frozen” for a period of hours with no growth of the surface plasmon peak. During this period of time, a series of TEM grids were prepared and particle size (74) Henglein, A. J. Phys. Chem. 1993, 97, 5457. (75) Rostalski, J.; Quinten, M. Colloid Polym. Sci. 1996, 274, 648. (76) Ung, T.; Giersig, M.; Dunstan, D.; Mulvaney, P. Langmuir 1997, 13, 1773.
7046 Langmuir, Vol. 14, No. 24, 1998
Figure 17. Absorbance at 390 and 220 nm of reacting solutions after additions of varying amounts of NaClO4 solution. Molarities at the right are NaClO4 molarities of the reacting solution after addition. Other reaction conditions are T ) 2 °C, [Ag] ) 0.250 mM, and R ) 12.
distribution was tabulated. The resulting particle size distribution was sharp with an average particle size of 2-3 nm. Although unequivocal evidence of the identity of the 220-nm species remains elusive, the above experimental data are compelling evidence supporting our hypothesis.
Van Hyning and Zukoski
all coalesced with larger particles, thus making the final transition mild. However, at low temperatures the sol is destabilized in the presence of a large quantity of the silver “monomers” as indicated by a large 220-nm peak. The result is large, ramified, and elongated particles. That this final transition occurs when the borohydride is depleted indicates that the 220-nm species are primarily composed of reduced silver. This reaction mechanism suggests that all the silver is rapidly reduced and particle growth occurs primarily by aggregation of these “monomeric” silver particles. These data allow us to speculate on the role of aggregation in silver sol growth. The rate of silver particle growth is too slow to be diffusion-limited. As a result there must be a barrier to aggregation. If we assume the primary particles are spheres with a diameter of 2 nm, a coating of 0.1-nm thick would be sufficient to reduce van der Waals attraction to a negligible level, even if we overestimate the attraction with the bulk Hamaker constant. Thus, the initial stages of silver particle growth may be limited by the rate of exchange of borohydride between the solution and the bulk. Thus, only rarely do colliding “monomeric” silver particles coalesce. As the clusters grow into particles, stability is produced by electrostatic and steric mechanisms. Loss of these stabilizing mechanisms due to consumption of borohydride late in the reaction produces aggregation. Clearly for “particles” on the order of the size of solvent molecules, classical colloidal stability models provide only qualitative insights into the forces dominating rates of aggregation and coalescence. However, armed with the concept that these small particles need not be stable against aggregation provides important insights on how particle size distributions are controlled during precipitation. Acknowledgment. This work was supported by the Materials Research Laboratory at the University of Illinois at UrbanasChampaign by the U.S. Department of Energy through Grant no. DEFG02-96 ER 45439.
IV. Summary In this study we characterize the reduction of silver perchlorate by sodium borohydride through a variety of techniques. The following picture of the particle formation mechanism emerges. Upon mixing, all of the ionic silver is reduced to form a yet unidentified silver species. On the basis of the instability of silver (I)/borohydride complexes in addition to other evidence, we conclude that this species is likely a large number density of small particles (1-3 nm) that exhibit an extremely weak and diffuse surface plasmon band. We hypothesize that the absorbance at 220 nm is due to borohydride adsorbed to the large surface area provided by these small particles. This reaction occurs in less that 2.0 s after mixing at all temperatures studied. At longer times, silver particles with dielectric properties well-described by the Drude model and bulk dielectric data grow at the expense of the silver “monomers”. As a result, the available surface area decreases, thus decreasing the absorption at 220 nm. Initially, very small (∼2-3 nm) particles display a broad absorption band imparting a dark color to the reacting solution. If the sodium borohydride concentration is high enough, the particles grow to a stable size near 6-10 nm and, during this growth process, the plasmon absorption band narrows, resulting in yellow solutions. Later in the reaction, the borohydride/water reaction depletes the particle surfaces of the stabilizing borohydride anion and particles aggregate. At high temperatures this destabilization occurs after the small “monomeric” particles have
Appendix Expression for Mie coefficients an, bn:
a1 ) 2x3‚u1
m′2 - v1 2
m′ + 2w1
b1 ) -2x3‚u1
1 - v1 1 + 2w1
m′2 - v2 1 a2 ) - x5‚u2 6 3 m′2 + w1 2 where
u1 ) 1 +
3 2 2 x - xi 10 3
v1 ) 1 +
1 2 m′2 2 x x 10 10 w 1 ) 1 + x2 -
u2 ) 1 -
5 2 x 42
v2 ) 1 +
m′2 2 x 10
1 2 m′2 2 x x 21 21 1 m′2 2 x w2 ) 1 + x 2 6 21
and
m′ ) LA980325H
n1 + in2 n ) n0 n0
