Control of Colloid Growth and Size Distribution by AdsorptionSilver

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Control of Colloid Growth and Size Distribution by AdsorptionsSilver Nanoparticles and Adsorbed Anisate D. Burshtain, L. Zeiri, and S. Efrima* Department of Chemistry, Ben Gurion University, P.O. Box 653, Beer Sheva, Israel 84105 Received May 7, 1998. In Final Form: January 14, 1999 We study the formation of silver colloids and the effect of an additive on their growth by measuring the kinetics of silver reduction in the presence of anisic acid, combined with transmission electron microscopy. The kinetics show an initial fast reduction of 1-3% of the total silver present in the solution, then a plateau region with hardly any reduction going on, and finally a region of accelerated growth and aggregation. The results are interpreted quantitatively in terms of the adsorption of the additives on the silver clusters at the early stages of the reaction and a slow-down of their subsequent growth. This is an example for the use of additives to achieve a control over the size distribution of colloidssdetermining the average size of the particles and the width of the distribution.

Introduction The size distribution of colloidal dispersions of solid particles is determined (barring aggregation and coalescence) by the rate of nucleation of the solid cores and the subsequent growth. For a given amount of material, the ratio between the rates of these two processes determines the size and number of particles. Fast nucleation tends to produce smaller and more numerous particles, while when growth is the prominent process, the colloid usually is composed of a smaller number of larger particles. In a previous study, we demonstrated one aspect of this behavior by discussing the effect of alizarin yellow on the nucleation of silver nanocolloids.1 In the present report the adsorption of anisic acid (p-methoxybenzoic acid) during the production of silver nanoparticles is shown to affect mainly the growth process. We will show that the adsorption of anisate eventually blocks the surface of the growing particles and significantly slows down the growth well below the usual diffusion controlled rate. It was shown in the past that anisic acid is important in the preparation and the surface modification and stabilization of interfacial silver colloids.2-8 In this work we measured the kinetics of the reduction of silver ions to a silver hydrosol in the presence of varying amounts of anisic acid. The kinetics are followed by titration of the residual silver ions after the reaction is stopped by acidification at various reaction times. The size distribution at each point was determined by transmission electron microscopy, TEM, carried out in parallel. This study addresses a basic issue in colloidal chemistry and contributes toward acquiring a control over the colloidal size distribution. Experimental Section Silver colloids are prepared by reduction of a (usually 0.05M) AgNO3 solution made basic by ammonia to point of resolubilization of the silver oxide (pH ∼9). The reagent solution contains * To whom correspondence should be addressed. Telephone: 9727-6472751. Fax: 972-7-6472943. E-mail: [email protected]. (1) Zhai, X.; Efrima, S. Langmuir 1997, 13, 420. (2) Yogev, D.; Efrima, S. J. Phys. Chem. 1988, 92, 5754. (3) Yogev, D.; Efrima, S. J. Phys. Chem. 1988, 92, 5761. (4) Yogev, D.; Shtutina, S.; Efrima, S. J. Phys. Chem. 1990, 94, 752. (5) Yogev, D.; Efrima, S. J. Colloid Interface Sci. 1991, 147, 88. (6) Efrima, S., CRC Crit. Rev. Surf. Sci. 1991, 1, 167. (7) Bradley, M.; Krech, J.; Efrima, S. J. Phys. Chem. 1995, 99, 292. (8) Efrima, S. Heterocycl. Chem. Rev. 1994, 1, 339.

various concentrations of (ionized) anisic acid (with pKa 4.5), typically 0%, 0.025%, 0.05% or 0.1%.(0, 1.3 × 10-3, 3.3 × 10-3, and 6.6 × 10-3 M, respectively). The reduction is carried out with a fresh 0.5% hydrazine sulfate solution added at a volume sufficient for the reduction of 32% of the total silver content (leaving 0.034 M silver nitrate in solution), according to

N2H4 + 4Ag+ + 4OH- f 4Ag0 + 4H2O + N2 These concentrations were chosen as they turn out to be convenient for monitoring the kinetics and are relevant to other studies involving interfacial colloids. Larger amounts of reductant result in copious aggregation. Lower amounts of silver are possible, and some results are reported below. At 30 s intervals, the reaction is stopped by the addition of acetic acid to reach pH ∼ 3. The colloid and most of the anisic acid are separated out by centrifugation. The supernatant is titrated in the dark for the determination of the remaining silver ions using potassium bromide in the Fajans method,9 with 3 × 10-4 M eosin as the indicator. The acetic acid helps in enhancing the color change. The typical experimental accuracy in the titration is in the range 2-6% in both the presence and the absence of anisic acid. The acidification practically stops the reaction to the extent that only slight changes in the silver ion concentration are detected after a couple of days, while the titrations are carried out within minutes of halting the reaction. The materials we use are AgNO3 AR (Aldrich, 99+%), hydrazine sulfate (BDH, >98.5%), anisic acid (Aldrich, 99%), eosin (2′,4′,5′,7′-tetrabromofluorescin, Merck, AR), glacial acetic acid (Sigma, ACS reagent), and potassium bromide (Fluka, >99%). All materials are used as obtained from the manufacturer without any further purification. Water is of ∼18 MΩ cm resistivity and was obtained from a Barnsted E-Pure purifier. TEM measurements are taken on a JEOL-JEM 2010 electron microscope equipped with an Oxford Link Pentafat X-ray analysis, model 6498. The colloids are deposited on Ted Pella, Inc. 300 mesh copper grids coated with Lacey carbon and Formvar (catalog no. 01883-F). A drop of the sol is placed on the grid which after ∼15 min is dried with the tip of a piece of filter paper. In this fashion most of the solution containing free silver nitrate is removed. Finally, the grids are allowed to dry in air, before inspecting them in the microscope.

Results The kinetics curves giving the percentage of reduction of silver ions out of the total amount of silver ions in the (9) Vogel, A. I. A Textbook of Quantitative Inorganic Chemistry, 3rd ed.; Longman: London, 1972; pp 76-81, 258-266.

10.1021/la980543a CCC: $18.00 © 1999 American Chemical Society Published on Web 04/06/1999

Control of Colloid Growth

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Figure 1. Kinetics of silver ion reduction by hydrazine sulfate with 0, 0.025, 0.05, and 0.1% anisic acid.

Figure 4. TEM taken 360 s from the initiation of the reduction of silver ions. 0.1% anisic acid. The lower edge of the picture is 939 nm long.

Figure 2. TEM taken 30 s from the initiation of the reduction of silver ions. 0.1% anisic acid. The lower edge of the picture is 57.5 nm long. Figure 5. Size distribution of the silver particles (A) 0.1% anisic acid; (B) 0.05% anisic acid, taken at 30 s.


solution (the maximum reduction can be 32% as determined by the amount of the reductant) as a function of time elapsed from the initiation of the reduction until its cessation by acidification are presented in Figure 1. In the absence of anisic acid there is a rapid reaction which is completed within our 30 s time resolution. A dark suspension appears immediately, which after a while precipitates out. With 0.025% anisic acid there is initially (within the first 30 s) a fast reaction of ∼1-2% of the silver. Then the reaction progresses very slowly, if at all, until after 90 s it picks up at an accelerating rate, which eventually slows down beyond 25% reduction (the maximum reduction being 32%). A dark, gray coloration appears mostly in association with the ascending part of the curve. A similar behavior is observed for 0.05% and 0.1% anisic acid, showing an approximate plateau of slow reaction and then an accelerated rate at ∼150 and 210 s, respectively. At these higher anisic acid concentrations it is easier to see that the plateau has a slope, indicating that a reaction is still taking place, albeit at a slow, but definite, rate. Within our uncertainty the rates in the plateau do not seem to depend significantly on the concentration of the anisic acid.

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Burshtain et al.

Figure 7. TEM taken 30 s from the initiation of the reduction of silver ions. 0.05% anisic acid. The lower edge of the picture is 188 nm long. Figure 6. X-ray emission analysis of the sample of Figure 2 taken at 30 s with 0.1% anisic acid.

In the later stages of the reaction, beyond the end of the plateau region, a gray coloration develops in the bulk of the solution, and a deposit of a silver film on the walls of the container appears. The high OD of the solution at this stage, as well as the contribution of the film on the walls prevents an effective use of extinction measurements to follow the kinetics. That is the one reason we chose the chemical titration method. Using containers treated with dichlorodimethylsilane to alter the surface properties did not have any noticeable effect on the kinetics we measured in the bulk, though the wall deposit was reduced drastically. Similarly, using plastic containers did not seem to affect the kinetics. This indicates that surface nucleation and reaction is not a major concern in this system. As a corroboration of this conclusion, we can estimate that at the plateau region with 1-3% reduction, if a considerable part of the reaction occurs on the walls then one should have obtained a highly nontransparent silver coating. Such a coating is not observed at the plateau region. Figures 2-4 show typical TEM pictures taken along the kinetic curve with 0.1% anisic acid at 30 s, at 180 s (on the plateau region), and at 360 s (at the steep rise). At 30 s, almost all the samples are composed of small, isolated particles. In some frames larger particles were also observed, but they were overwhelmingly outnumbered by the small ones. Figure 5A shows the particle size distribution at 30 s for 0.1% anisic acid. A narrow distribution around a diameter of 3.5 nm is observed. Figure 6 shows the X-ray fluorescence analysis of the sample taken at 30 s, indicating pure silver (the copper lines come from the grid). At 180 s (Figure 3) larger particles (up to ca. 20 nm diameter) become abundant alongside the small ∼5 nm particles. However, we do not see larger crystals or aggregates. At 360 s we observe only large aggregates on the scale of several hundred nm. A similar trend occurs also for 0.05% anisic acid, though the particles are larger and less uniform, and large crystals and aggregates appear at an earlier stage, compared to 0.1% anisic acid trends (Figures 7-9). As seen in Figure 5B at 30 s the particle diameters are centered around 6-7


nm and the distribution is rather broad (∼(3 nm). X-ray diffraction taken in the e-microscope (for 0.05% anisic acid at 240 s) shows in Figure 10 mixed aggregates as well as single crystals, as indicated by the residual hexagonal spottiness of the diffraction rings. Using 0.025% anisic acid, electron microscopy at 30 s shows some ∼10 nm particles, but many aggregates and crystals (Figures 11 and 12). By 150 s all the individual particles disappear. In TEM there is a possibility that the e-beam itself might reduce any excess of silver nitrate deposited on the grid or even that silver ions still in solution might be reduced during the grid sample preparation. However, we take care to siphon off the solution from the grid, together with the excess silver nitrate, and do not let the whole solution


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Figure 9. TEM taken 240 s from the initiation of the reduction of silver ions. 0.05% anisic acid. The lower edge of the picture is 18.7 µm long.


Figure 10. Electron diffraction pattern taken 240 s from the initiation of the reduction of silver ions. 0.05% anisic acid. The diffraction is from the lower object in Figure 9.

Figure 12. Electron diffraction pattern taken 60 s from the initiation of the reduction of silver ions. 0.025% anisic acid.

evaporate. Thus, only very little free silver ions, if any, are present on the grid. More importantly, to rule out these possibilities, we carried out experiments with just enough silver corresponding to the end of the plateau regions; i.e., there were no excess silver ions. We observed similar size distributions. Discussion The kinetic behavior we find for the reduction of silver ions to a colloid in the presence of anisic acid can be explained by the following simple model: anisic acid adsorbs on silver particles (and nuclei), blocks the surface, and slows down the growth stage. The small amount of the anisic acid, compared to the amount of silver ions in

solution in our system, precludes its role as affecting the redox potential of the silver ions. There simply is a large excess of silver ions that are not directly associated with the anisic acid, and they would have reacted at the same rate as in the total absence of anisic acid. Additives can influence the kinetics of colloid formation by affecting either the nucleation stage or the growth or, perhaps, both. The basic modern theoretical treatment of colloid nucleation and growth is attributed to La Mer and Dinegar,10 and an illuminating discussion of metal colloids was given by Kirkland et al.11 Our interpretation of the results presented herein suggests that the anisic acid (10) La Mer, V. K.; Dinegar, R. H. J. Am. Chem. Soc. 1950, 72, 4847. (11) Kirkland, A. I.; Edwards, P. P.; Jefferson, D. A.; Duff, D. G. Annu. Rep. Prog. Chem. C 1990, 87, 247.

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adsorbs on the silver particles and blocks their further growth (by steric hindrance or perhaps coloumbic repulsion). Such behavior is known in other systems. As an example, Teranishi and Miyake12 discussed the effect of poly(vinylpyrrolidone) on the production of palladium nanoparticles in terms of inhibition of growth and prevention of aggregation. This mechanism is in contradistinction to cases when impurities (or, more notably, the presence of surfaces or solids) predominantly affect the nucleation stage, and often actually induce nucleation. For instance, in a previous study it was shown that low concentrations of alizarin yellow accelerate the rate of nucleation of silver colloids,1 resulting in smaller (and more numerous) particles. Longenberger and Mills13 showed that certain polymers affect metal colloid formation (most probably the nucleation) by binding of metal ions in cavities and a direct reaction with groups on the polymer. Other results and discussions that touch upon the question at hand, though not directly relevant in detail, can be found in refs 14-18. Going back to our results, we first note that the appearance of the gray coloration in the reaction cell is delayed when anisic acid is used. In its absence the solution turns dark and turbid within a few seconds after adding the reductant (hydrazine). In its presence the coloration appears only after many dozens of seconds, with this delay increasing with the increase in the concentration of the anisic acid. Though this fact seems to point to a deleterious effect of anisic acid on the reaction, an accelerated nucleation stage cannot be ruled out yet. Faster nucleation involves a rapid creation of many particles, but it also means that the particles are smaller. Considering that scattering and absorbance (giving rise to the turbidity and coloration of the solution) strongly depend on the size of the particles and their state of aggregation, one can expect a decrease of the coloration even for a scenario of a more rapid nucleation. This is especially understandable in comparison to the fast appearance of copious aggregation in the absence of anisic acid. In the present experiment, we measure directly the rate of the overall reaction by titration. As discussed above the relative contribution of the reaction at the walls especially in the earlier stages of the reduction (the plateau region) is small, so we practically measure the rate of the reaction in the bulk of the solution. We find that after a very short initial stage the reaction indeed slows down considerably, and a plateau region is observed in the kinetic curves. In fact, this is the reason we followed the kinetics by tedious titrations, rather than by spectrophotometry that can be carried out in situ and in a continuous fashion, but its interpretation is ambiguous, at times. In a previous study (reduction of silver with eriochrome B)19 we found that in that case the kinetics could be followed meaningfully by measurement of the extinction only at the very early stages (when the particles were small and nonaggregated). In addition, in the present system, the extinction was very weak in the early stages of the reduction, and difficult to follow in the later stages due to the strong extinction of the solution and the coating on the cell walls. Titration (12) Teranishi, T.; Miyake, M.; Chem. Mater. 1998, 10, 594. (13) Longenberger, L.; Mills, G., J. Phys. Chem. 1995, 99, 475. (14) de Cointet, C.; Mostafavi, M.; Khatouri, J.; Belloni, J. J. Phys. Chem. B 1997, 101, 3512. (15) Henglein, A. Chem. Rev. 1989, 89, 1861. (16) Kapoor, S.; Lawless, D.; Kennenpohl, P.; Meisel, D.; Serpone, N. Langmuir 1994, 10, 3018. (17) Vijaya Sarathy, K.; Kulkarni, G. U.; Rao, C. N. R. Chem. Commun. 1997, 537. (18) Fan, C.; Jiang, L. Langmuir 1997, 13, 3059. (19) Zhai, X.; Efrima, S. J. Phys. Chem. 1996, 100, 1779.

Burshtain et al.

“sees” only processes that involve silver reduction (formation of new particles and growth of older ones) and is oblivious to the aggregation or precipitation of the particles. In this case, it has an advantage over spectrophotometry that is sensitive to all of these processes We propose that the rapid reduction we observe in the absence of anisic acid represents predominately (diffusioncontrolled) growth of a small number of nuclei formed by relatively slow nucleation. A simple calculation shows that the average distance between particles at a concentration of 1013 particles/cm3 (this is obtained from eq 4 below) is a few hundred nanometers. The diffusion of silver ions over such distances is faster than a millisecond, in line with the fast kinetics we observe (Figure 1). When anisic acid is present, it quickly adsorbs on the nuclei that form in the slow process of nucleation and retards their growth. This results in the plateau region we observe in the kinetic curves. We use here the term “nuclei” rather loosely. These “nuclei” are not necessarily the small embryo clusters in which a balance between the energy of the bulk and the surface energy of the particle exists, which are invoked in the well-known theory of critical size colloid formation.10 In fact, the TEM pictures show these species to be several nanometers large (Figures 2, 3, and 7). Nucleation continues in parallel with the retardation of growth, most probably at a rate independent of anisic acid, as indicated by the similar (gentle) slopes of the plateau region at various concentrations. During this stage the surface of every newly formed particle tends to coat with anisic acid molecules, which practically stops the growth. However, once the free anisic acid in the solution is exhausted, growth of the particles cannot be impeded anymore. This marks the end of the plateau, when once again diffusion-controlled growth and aggregation become the prominent processes. In fact, in this respect, the situation now becomes almost similar to the case when anisic acid was entirely absent. The final leveling of the kinetic curve is, of course, due to the exhaustion of the reductant. This model is supported by the TEM pictures that show the preponderance of small, nanoscale, particles until the end of the plateau. Moreover, as we show below, the TEM results showing smaller particles in the plateau region for the higher anisic acid concentrations, are also consistent with the predictions of this model. A similar behavior in the reduction of gold where a plateau region was observed followed by a swift reduction phase was reported in a very early work by Svedberg.20 They used conductivity measurements to monitor the reaction. There, too, the slow kinetics was attributed to nucleation and the fast accelerated rates to growth. This simple model has quantitative implications that are born out by experiment, as follows. Let a be the area cross section of an anisic acid anion in its adsorbed state on silver (∼0.25 nm2), nAA the number concentration of anisic acid molecules, ra the area weighted average radius of the particles, rv the volume weighted average radius of the particles, np the number of particles per unit volume, WAg the weight concentration of silver introduced into the solution, and f the fraction of silver that was reduced by the end of the plateau region. FA2 is the density of silver (10.5 gr/cm3). The weighted average radii are given by

ra2 )

∑i Piri2

(1)

(20) Svedberg, T. In Colloid Chemistry, Chemical Catalog Co.: Reinhold, NY, 1928.


rv3 )

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∑i Piri3

(2)

with Pi being the probability of having a particle of radius rp ) ri. Pi values are obtained from the distributions in the TEM pictures (as in Figure 5). The area balance at the end of the plateau region when all the anisic acid is used up for coating the total surface of the colloidal particles is given now by

np4πra2 ) nAAa

(3)

and that of the volume by

np(4/3)πrv3 ) fWAg/FAg

(4)

The ratio between these equations gives

(rv3/ra2)nAA ) 3 fWAg/(FAga) ) const

(5)

Indeed in our experiment (rv3/ra2)Caa (with Caa the weight % concentration of anisic acid) for 0.1% and 0.05% anisic acid, is approximately constant, 0.21 and 0.195 nm%, respectively. Furthermore, inserting values for the various radii into eq 5, one also finds that the fraction of the total silver that is reduced by the end of the plateau is ∼1.3% for 0.1% anisic acid and 1.2% for 0.05% anisic acid. Experimentally we found a ∼1-3% reduction at that point. We take this rough agreement as an indication that indeed the simple model we propose here captures the essential features of the system. In fact, this agreement is surprising considering the crudeness of the model and the neglect of a partition of the anisic acid between the colloid and the solution, as well as some possible deposition of silver onto the walls. We also assumed a compact adsorbed anisic acid layer (a ) 0.25 nm2) which probably compensated for the neglect of a possible partition. This treatment implies that large particles are obtained only when one tries to reduce amounts of silver larger than this ∼1-3%. If one limits the total amount of silver

to the level reduced at the plateau or below that, only small, nanometer particles will be obtained. We conducted such experiments, at lower silver concentrations, which showed that indeed predominately small, nonaggregated particles form (that accumulate at the interface and form an interfacial colloid). Unlike the cases of alizarin yellow1 and dithizone21 where the additives were taken at very small concentrations (typically at a ratio of 2000 silver atoms to 1 additive molecule), or the cases of impurities in general, anisic acid has an effect here only at much higher concentrations (silver ion/anisic acid molecular ratios of 10:1 to 30:1). Thus, it is indeed likely that in the former cases only the nucleation can be affected while in the latter the influence is on the growth stage. We could not detect any significant effect of anisic acid at concentrations below about 0.01%. In fact, highly reflective and stable silver interfacial colloids are produced at concentrations >0.025%, indicating small, uniform and well-coated particles. Typical reflectivities of the interfacial film are 60-65%.2-8 At low anisic acid concentrations, such as 0.01%, reflectivities of only 20% are observed, and they deteriorate rapidly. Summary We presented here a TEM and chemical titration study of silver colloid formation via nucleation and growth in the presence of anisic acid that adsorbs onto the surface of the particles and prevents their further growth. Only when the slow nucleation produces a sufficient number of particles, so that there is no free anisic acid left, does the (diffusion-controlled) growth pick up again, accompanied by aggregation. A simple model of nucleation, adsorption and growth describes the results in a quantitative manner. Acknowledgment. This work was supported in part by the Israel Science Foundation founded by the Israel Academy of Sciences and Humanities. LA980543A (21) Zhai, X.; Efrima, S. J. Phys. Chem. 1996, 100, 10235.

Control of Colloid Growth and Size Distribution by AdsorptionSilver

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