Preparation and Formation Mechanism of Silver Particles with

ARTICLE pubs.acs.org/IECR

Preparation and Formation Mechanism of Silver Particles with Spherical Open Structures Roberto Irizarry,*,† Lourian Burwell,† and Madeline S. Leon-Velazquez‡ † ‡

DuPont Electronic Technologies, 14 T.W. Alexander Drive, Research Triangle Park, North Carolina 27709, United States Chemical Imaging Center, Department of Chemistry, The University of Puerto Rico at Mayaguez, Mayaguez, Puerto Rico 00680 ABSTRACT: A new type of highly dispersible silver particles has been developed consisting of two- and three-dimensional building blocks that form spherical shapes with open structures. The first morphology consists of uniform particles that are composed of anisotropic building blocks mainly in the form of a network of platelets forming a spherical, open-structured particle. These platelets are 1002000 nm in length, with a crystallite size in the range 20100 nm. The second morphology consists of isotropic components that are 100500 nm in length, with a crystallite size in the range of 20100 nm. In both cases, the silver powder was comprised of well-defined monosized particles and was highly dispersible. These powders were prepared by rapidly mixing a silver solution with an ascorbic acid/sodium citrate solution at low pH. A mechanism is proposed for the formation of these novel structures.

1. INTRODUCTION Precious-metal particles, such as silver, gold, palladium, and silverpalladium in the particle size range of 100 nm to 6 μm, are of great industrial importance in the hybrid circuit industry for the manufacture of conductor thick film pastes. In particular, silver paste compositions are used in front end metallization of photovoltaic cells (PV), actuators, plasma displays (PDP), and low-temperature cofired ceramics (LTCC), among others. Spherical silver particles made by precipitation methods15 and silver flakes made by mechanical milling6 are the two dominant morphologies used for these applications. These methods allow for the high-volume production of silver at low cost. The size and size distribution of these powders are manipulated using different synthesis routes and/or process parameters such as concentration, temperature, flow conditions, addition mode, and mechanical treatments. Surface treatments, mechanical milling, and blending of different types of particles are also common practices used to alter the performance of these pastes.7,8 Continuous academic and industrial effort has aimed to develop new routes to generate spherical silver particles15 with different properties. The synthesis used greatly influences the particle size, uniformity, crystallite size, and the residual organic material remaining on the particles. These properties have a direct impact on the printing and thermo-mechanical behavior of the final thick film paste. For curable pastes, these properties also impact the curing speed. Although powder properties are very important to paste performance, most attention has been paid to spherical and flake powders. The ability to modify the surface morphology of spherical powders and to create new morphologies may have a substantial impact on the development of highperformance pastes. Several studies on the formation of silver particles of different shapes have been published.912 Most are phenomenological studies of how morphology varies according to synthesis conditions. Reduction of silver with ascorbic acid under different conditions, specifically the ascorbic acid/silver ratio and the r 2011 American Chemical Society

addition of an acid or base, was shown to result in different morphologies.9 In this study, the changes in morphology were attributed to changes in the ascorbate anion concentration under different conditions. These powders were irregular and often agglomerated. A more complete synthesis was presented using the stabilizing agent Daxad-19,10 with which uniform and welldispersed spherical particles were prepared. Additionally, the generation of spherical/platelet blends was reported by changing the additions mode. Silver chains formed from nanoparticles were also reported. In a screening study performed by exchanging different reducing agents including ascorbic acid, hydroquinone, and sodium borohydride, as well as different stabilizing polymers including gum arabic, PVP, and Pluronic (PE and PF) polymers, the authors found that ascorbic acid could be used to form different shapes including spherical particles composed of rods (rice balls) and irregular flakes.11 The conditions determined for these shapes are very dilute silver solutions (∼102 M) and a very large amount of polymer (1:1 silver/polymer molar ratio), which leave a large amount of organic material on the particle (∼4%). Silver microflowers have also been generated using paraphenylenediamine as a reducing agent.12 In this work, we present a method to generate uniform spherical silver particles with open structures and new surface morphologies. This method differs from other methods, where the spherical particles are composed of very small 2060-nm primary units fully coalesced into a solid spherical structure. Two types of uniform spherical structures are generated. In the first morphology, the silver consists of submicrometer platelets growing radially to form uniform spherical, open structures of about 3.0 μm. In the second morphology, each silver particle consists of silver crystalline components in which the surface of Received: December 13, 2010 Accepted: May 31, 2011 Revised: May 27, 2011 Published: May 31, 2011 8023

dx.doi.org/10.1021/ie1024936 | Ind. Eng. Chem. Res. 2011, 50, 8023–8033

Industrial & Engineering Chemistry Research

Figure 1. Electron micrographs of silver particles with morphology M1 generated using half the sodium citrate of the reference experiment.

the particle resembles the surface of the rind of an orange. In both cases, these units are assembled to form spherical, open-structured particles, in which the d50 particle size is approximately 3 μm or larger. We call the first morphology M1 or spheroplate particles, and the second morphology M2 or orange-rind surface. A formation mechanism is proposed for both structures.

2. EXPERIMENTAL SECTION All materials, L-ascorbic acid (City Chemical, NJ), silver nitrate (Ames Goldsmith Corporation, Glens Falls, NY), nitric acid (JT Baker), and sodium citrate dihydrate powder (JT Baker), were used as received. The size and shape of the resulting silver particles were investigated by field-emission scanning electron microscopy (FE-SEM) with a JEOL (Tokyo, Japan) 6700 instrument, while their crystalline structure was determined by X-ray diffraction (XRD) using a Rigaku Miniflex diffractometer. The size distribution of the silver spheres was obtained by laser diffraction using a Microtrac S3500 particle size analyzer. The content of the organic matter in the silver particles was assessed by thermogravimetric analysis (TGA) using a TA Instruments Q5000IR autosampler. The UVvis dynamic data was obtained using a stopped-flow reactor (SFR), model SX.20 (Applied Photophysics Ltd., London, UK) with a micromixing time of 10 ms and a diode array that allowed full-spectrum recording from 250 to 800 nm at a rate of 1000 spectra per second. The photodiode array (PDA) used a linear 256element diode array with a diode separation of about 2.17 nm. The optical cell was illuminated by a xenon arc lamp (ozonefree). The light path length in the observation cell was 10 mm. 3. EXPERIMENTAL RESULTS In this section, the reference experiment used to form the morphologies M1 and M2 is described. Powder characterization in terms of shape, surface coating, particle size distribution, and crystallite sizes is investigated. 3.1. Particle Synthesis. A preparation method for two morphologies (M1 and M2) is presented (reference experiment). The only difference between the preparation of M1 and M2 is the process temperature. The synthesis starts with the preparation of two solutions. The reducing solution was prepared by adding and dissolving 0.26 mols ascorbic acid, 0.05 mols

ARTICLE

Figure 2. Electron micrographs of silver particles with morphology M1 generated using the conditions of the reference experiment. (inset) High-resolution micrograph.

sodium citrate, and 0.22 mols nitric acid to 750 mL deionized water. The silver solution was prepared by dissolving 0.47 mols silver nitrate in 250 mL deionized water. The temperature of both solutions was maintained at 70 °C (to form morphology M1). After both solutions were prepared, the silver solution was added to the reducing solution at a very low shear rate (preferably no agitation or keeping the agitation speed below 250 rpm) in less than 5 s. In all cases, rapid and complete reduction of the metal occurred. After 5 min, when the particles were fully formed, the reaction mixture was stirred for three more minutes. The reaction mixture with dispersed particles was then filtered, and the silver solids were collected. The silver powder was washed with deionized water, then freeze-dried for 24 h at 35 °C. The preparation method for M2 was the same as M1, but with a process temperature of 25 °C. Under acidic conditions, the ascorbic acid is a mild reducing agent favoring slow nucleation and diffusional growth that leads to large silver crystals. As demonstrated in this work, the sodium citrate changes the growth mechanisms to form the silver powders developed in this work. The particle morphology strongly depends on the sodium citrate concentration, temperature, and addition mode. For example, after halving the concentration of sodium citrate of the reference experiment, morphology M1 was produced (see Figure 1) with very open platelets and a less uniform powder, although very well dispersed with a very tight particle size distribution (d10 = 2.3, d50 = 3.3, d90 = 5.3). At the citrate concentration of the reference experiment, morphology M1 became very uniform as shown in Figure 2, with the same particle size distribution (d10 = 2.3, d50 = 3.3, d90 = 5.1). Depending on the conditions used, the d50 particle size ranged from approximately 3.3 to 4.4 μm. At a lower temperature, morphology M1 shifts to M2. Figure 3 shows the polycrystalline nature of the silver spheres with surfaces similar to the surface of an orange rind. These characteristics indicate that, at room temperature, the metallic particles were formed by rapid aggregation of large submicrometer subunits. The particle size distribution (PSD) data shown in Figure 4 indicate a well-dispersed, monosized powder, which compares favorably with the average size calculated from the electron micrograph. The level of particle aggregation was significantly lower than that of most precipitations 8024

dx.doi.org/10.1021/ie1024936 |Ind. Eng. Chem. Res. 2011, 50, 8023–8033


Figure 3. Electron micrographs of silver particles with morphology M2 generated using the conditions of the reference experiment for M2. (inset b and c) High-resolution micrographs.

Figure 4. Particle size distribution obtained by laser diffraction.

Figure 5. Process schematic.

carried out in aqueous solutions, especially those conducted in the absence of dispersants. 3.2. Processing Conditions. The process for the synthesis described in section 3.1 is carried out with a silver solution added very rapidly to the reaction vessel in the range of 46 s with no further mechanical agitation. This addition mode creates a jetinduced convective motion of the fluid in the reactor, mixing the reactants. A schematic of the reactor geometry is shown in Figure 5. The reaction can also be carried out using mechanical agitation to reduce the mixing time (standard impeller in the reactor). When using mechanical agitation, a good-quality product (morphology and size) was obtained for M1 and M2 up to 250 rpm. When the experiments were conducted above 250 rpm,

ARTICLE

Figure 6. Silver particles obtained when the process is carried out using a fed-batch process.

a small fraction of particles started to aggregate, forming doublets. These results indicate that low shear rate is a key parameter to avoid aggregation of the M1 and M2 morphologies. To study the importance of the fast addition mode for this synthesis, the reference experiment was also synthesized in a fedbatch mode. In the fed-batch process, the reaction was carried out with a slow, metered addition of the silver solution over a period of 1 h while stirring the solution at 250 rpm. With this process, the final particles form a highly crystalline powder with well-defined facets, as shown in Figure 6. This result shows the importance of fast addition mode to obtain the desired morphologies. These process conditions (fast addition and low shear rate) constitute a major departure from crystallization processes in which a fed-batch process with slow addition rate and with intensive mixing is the standard process. In this case, micromixing is a key parameter for product quality because it controls the history of supersaturation, which modulates nucleation and diffusional growth. For the case of this synthesis, the formation mechanism mainly involves the aggregation of primary and secondary particles (discussed in sections 4 and 5). To characterize the mixing conditions of this process, blending time experiments were conducted using conductivity probes (BHR Group, UK). In the mixing experiment, deionized water with a small quantity of highly conductive tracer (nitric acid) is placed in the feed tank. The reactor is filled with deionized water. The solution volumes for the feed tank and reactor vessel are the same as those used for the silver solution and reducing solution respectively. The conductivity probes are placed at the injection point (point A in Figure 5) and at the bottom near the reactor wall (point B in Figure 5). The blend time is determined by measuring how the conductivity changes as the injected liquid is dispersed around the vessel. The experiments were conducted under no mechanical agitation to test the blending times of the inlet jet. During the period of addition (inlet jet for 4 s), there is a large overshoot in the tracer concentration near the injection point, which quickly reaches a nearly homogeneous value in 9 s. This is the time for large blobs of incoming reactant to be dispersed and disintegrated by the jet-induced convective motion of the fluid in the reactor. The measurements at the bottom of the reactor (point B in Figure 5) showed a delay time of about 3 s before the 8025



ARTICLE

Figure 7. XRD pattern of the obtained powder (morphology M1).

Figure 8. XRD pattern of the obtained powder (morphology M2).

tracer concentration started to increase, then took 20 s to reach the final homogeneous value. The reactor macromixing time (t95) was estimated to be 21 s. No attempt was made to find the upper limit for a blending time that can still form a good product. Obviously, under these conditions, the micromixing process is very inefficient, but it does not seem to be important for product quality. 3.3. Powder Characterization. Figures 7 and 8 show the X-ray diffraction (XRD) patterns of the resultant silver powder

for morphologies M1 and M2, respectively. In all cases, the resultant products displayed similar XRD peaks corresponding to cubic phase Ag (JCPDS file No. 4-783). Also, no characteristic XRD peaks of possible Ag2O impurities were visible, indicating the preparation of phase-pure Ag powders.13 XRD data analysis shows that morphologies M1 and M2 have very different crystallite sizes estimated from Scherrer’s equation using the peaks corresponding to the {111} facets. The crystallite size 8026



ARTICLE

Table 1. ESCA Atomic Compositions sample

C 1s

O 1s

Cl 2p

Ag 3d

M1

21.2

9.7

0.2

68.9

M2

13.6

7.5

0.3

78.6

Table 2. C 1S and O 1S Atomic Deconvolution (atom %) C 1S

O 1S

sample C—C C—O CdO O—CdO C—O CdO O—CdO M1

13.6

2.4

2.8

1.6

2.5

5.1

1.4

M2

8.6

1.5

2.6

0.5

1.5

5.1

0.3

was estimated to be 868 Å for M1 and 350 Å for the M2 morphology. For both morphologies, although the structure is very open, the single particle density by helium pycnometry indicates that this is a very dense silver. The helium pycnometry value was in the range of 9.9910.14. This method measures the true particle density using the Archimedean principle of gas displacement to measure the volume of the powder sample. This characteristic is very interesting, since most powders that are formed by an aggregation mechanism have a large amount of trapped organic material. For example, a powder made by silver-ammine complexes has a typical pycnometry value of 9.80. Given the amount of citrate in the synthesis and the known affinity of citrate to silver surfaces, this result is unexpected. This method produces a highpurity powder, as shown by the TGA values of 99.599.8%. The ESCA surface characterization is presented in Tables 1 and 2. The analysis was performed over an area of 1400 200 μm2 and, thus, averaged over many particles. Glycolic carbon (C—O) photoelectrons are shifted to a higher binding energy compared to the C—C carbon; carbonyl and carboxylic carbons (CdO and COO) exhibit even higher binding energies. Multiple oxygen binding shifts are also observed in the oxygen 1s photoelectron spectra. A curve-fitting program was used to determine the contributions of each type of carbon or oxygen to the total photoelectron signal. The relative quantities are summarized in Table 2. Delocalization of charge over the carboxylate groups on the citrate may have resulted in a higher apparent carbonyl functionality compared to the carboxyl functionality. ESCA data obtained with sodium citrate by itself was also tested. The peak shape exhibited a C—C:C—O:CdO/COO ratio of roughly 2:1:3, which is expected for this molecule. The silver samples (morphologies M1 and M2) exhibited carbon 1s peak shapes that could be consistent with the carbonyl/carboxyl functionality of the citrate, plus an additional C—C hydrocarbon. The presence of a higher hydrocarbon (C—C) compared to the citrate control is likely the result of processing secondary products such as ascorbate after the silver reduction. This may explain the important role of ascorbic acid in the formation of anisotropic morphologies. Secondary ion mapping analyses showed the presence of acetate- and nitrate-covered or rich Ag, which may arise from secondary reaction products, since this reaction occurs in the presence of nitric acid. By comparing the total carbon to Ag atom percent concentrations from the ESCA analysis, the M1 morphology clearly exhibits a higher carbon relative to Ag (0.31) compared to the M2 morphology (0.17), a possible indication of the role played by citrate in forming these morphologies.

Figure 9. Electron micrographs of silver particles after milling: (a) silver particles with morphology M2; (b) spherical silver generated by the method described in ref 4.

Figure 10. Schematic for the mechanism of open-structure spherical particles with the surface morphology of an orange rind.

In the case of morphology M2, mechanical milling was able to break the particles into their building blocks (submicrometer units) even before flaking of the powder began (see Figure 9a), unlike other spherical silver4 in which milling can deform the particle but cannot break it into smaller units (see Figure 9b). This property is a unique feature of these powders.

4. FORMATION MECHANISM A mechanism for M2 formation is proposed based on the particle characterization, effect of synthesis parameters, kinetic experiments, and simulated dynamic optical response (SDOR) analysis at early stages. A mechanism is speculated for the formation of morphology M1. 4.1. Mechanism for the Morphology at Low Temperature. The following mechanism is proposed for the formation of spheres with orange-rind surfaces M2. Reduction of silver produces primary particles, P, of about 35 nm (based on XRD data) that aggregate to form secondary particles, S, of about 200 nm (based on FE-SEM). These secondary particles then aggregate to form the tertiary particles, T, which are the final particles with morphology M2. On the basis of kinetic data and model identification analysis, it is believed that the formation of primary particles is a relatively slow step, that the formation of secondary particles is a fast step, and that the formation of tertiary particles is also a slow step. The mechanism is shown schematically in Figure 10. The reduction of silver by ascorbic acid is given by

2Agþ þ C6 H8 O6 f 2Ag þ C6 H6 O6 þ 2Hþ

ð1aÞ

The reduced silver and the silver ions form different types of clusters. The initial clusters are dimers. One that is believed to be very abundant is Agþ þ Ag0 f Ag2 þ

ð1bÞ

In the presence of citrate, some of these clusters can be complexed, since complexes are energetically more favorable. 8027



ARTICLE

Figure 11. Schematic for the mechanism of the morphology M1.

There are many possible reactions that can lead to Ag2þcitrate. One of these reactions is Ag2 þ þ citrate h Ag2 þ citrate

ð1cÞ

These clusters grow until primary particles are formed. The formed primary particles then aggregate to form secondary particles. This aggregation mechanism is modeled using the population balance equation for aggregation.14,15 The secondary particles grow by this process until they reach a stabilized size (about 200 nm). The capping effect of citrate has been demonstrated,16 which may be the mechanism for the stabilization of secondary particles. The formation of tertiary particles, T, is achieved by a similar aggregation mechanism of the secondary particles. In the case of the final particles, coalescence is incomplete, possibly due to the capping action of citrate on the secondary particles. Incomplete coalescence is demonstrated by the ability to knock secondary particles off the surface of the final tertiary particle by mechanical treatment. This step is a departure from the two-step mechanism of dense spherical particles, where the primary units between 20 and 40 nm coalesce into a solid spherical particle.14,15 Although this mechanism is supported by the data generated in this work, more data are needed to completely understand exactly how these particles are formed and to prove or disprove the proposed mechanisms. 4.2. Mechanism for Morphology at High Temperature. Exactly how M1 is formed is not known, although the formation of M2 is better understood. The mechanism can be speculated on the basis of preferential attachment to the {111} facets. This preferential attachment has been observed in many systems to form nanoparticles of different shapes under very dilute conditions.1721 Several methods to control the shape of nanostructures using PVP, which interacts strongly with the {100} and the {111} facets, have been described.17,18,22 Notice that in our work, PVP did not produce the desired morphologies, indicating that preferential attachment is only part of the mechanism. Silver nanoplates have also been precipitated using PEG, PVP, and CTAB.19,20 Truncated triangular silver nanoparticles have been generated by reducing silver with hydrazine in the presence of sodium citrate.21 The anisotropic growth of these particles was attributed to the fact that citrate adsorbs more strongly to the Ag {111} facet, affecting selective growth. In ab initio electronic structure calculations performed to study the binding energy of citrate with silver {111} and {100} surfaces, citric acid was found to be much more likely to bind to the {111} surface.23 The preferential binding to the Ag {111} facet is due to a better symmetry and geometric agreement, which allows four ligand surface bonds on the {111} facet as opposed to two ligands on the {100} surface.

On the basis of these observations and the experimental results in this work, the following mechanism is speculated (see Figure 11). At a higher temperature, the effect of citrate adsorption on the surface is stronger, which could be explained by a difference in adsorption between low and high temperatures. The ESCA analysis showed more organic material in M1 than in M2. At a high temperature, anisotropic growth could be attributed to the fact that citrate adsorbs more strongly to the Ag {111} surface, slowing the growth of this facet. On the basis of XRD data, the main growth mechanism of these platelets is still believed to involve the preferential attachment of small primary particles and clusters. This preferential attachment has been observed in other anisotropic systems to form two-dimensional platelets as well.10 This growth mechanism will result in anisotropic nanoparticles that continue to evolve to form the M1 structure. These types of anisotropic nanoparticle seeds have been observed in the evolution of gold nanoparticles with CTAP.24 At high temperature, the attachment rate is much greater than the surface diffusion rate, which prevents the structure from densification. The effect of competition between these fundamental steps on morphology has been studied recently.25 Similar simulation work will be pursued in the future to further understand the evolution of these structures.

5. ROLE OF CITRATE To further understand the role of citrate in the formation of primary and secondary particles for morphology M2, UVvisible kinetic experiments were performed. The data was then analyzed using the model identified in ref 14 for the formation of spherical particles by aggregation of primary particles to form secondary particles. 5.1. UVvisible Kinetic Data. To study the role of citrate in the synthesis, two systems were studied side by side (with and without citrate). The first synthesis (control kinetic experiment) consisted of the reaction/precipitation of silver with a solution of ascorbic acid, nitric acid, and gum arabic as a stabilizer.4 This synthesis produces well-defined spherical particles. The second synthesis (reference kinetic experiment) was a dilute version of the synthesis introduced in this work. In the control kinetic experiment, the reducing solution was prepared with 2.0 102 M ascorbic acid, 1.8 102 M nitric acid, and 20% gum arabic/silver (w/w). The silver solution was prepared by dissolving silver nitrate in water to a concentration of 2.6 102 M. In the reference kinetic experiment, the reducing solution was prepared with 2.6 102 M ascorbic acid, 2.2 102 M nitric acid, and 3.4 103 M sodium citrate. The silver solution was prepared by dissolving silver nitrate in water to a concentration of 4.7 102 M. In the kinetic experiment, the silver nitrate solution and the reducing solution were injected in equal volumes into a T-mixer that filled an optical cell at room temperature. Full UVvis spectra were collected in the first 4 s of the reaction. Precipitates formed in the reference experiment under these dilute conditions did not reproduce the morphologies introduced here. The particles produced under these dilute conditions were standard spherical particles. Nevertheless, the kinetic experiment was able to provide a good idea of the role of citrate in the early stages of particle formation. The results in Figure 12 show the kinetic experiment for the first 4 s for the control kinetic experiments. Figure 13 shows the same kinetic experiment for the reference kinetic experiment. In both cases, a broad band was observed to develop at early stages. 8028



ARTICLE

Figure 12. Kinetic data for the control experiment (44 mM Ag for 4 s).

Figure 13. Kinetic data for the reference experiment (44 mM Ag for 4 s).

In the control experiment, the peak developed at about 440 nm, while the peak for the reference experiment was at about 525 nm. The broad band for the reference experiment was wider than that of the control experiment. The intensity of the dynamic data for the citrate experiments was also observed to be much smaller than the intensity for the control experiment. The same trend was observed at different concentrations. These results were analyzed using the simulated dynamic optical response (SDOR) method14 for model/mechanism identification from optical kinetic experiments. As shown in the next subsection, this analysis indicated that small primary particles are generated at a higher rate in the control experiment than in the reference experiment. In contrast, these primary particles aggregate to form larger secondary particles at a lower rate in the control experiment than in the reference experiment. 5.2. Mechanistic Model of Spherical Particle Formation. The kinetic data are analyzed using the model for the formation mechanism of spherical particles identified in ref 14. This model describes a mechanism consisting of (1) autocatalytic nucleation and (2) fast aggregation of small particles followed by (3) slow aggregation leading to stabilization of large particles. Notice that this model only describes the formation of secondary particles by the aggregation of primary particles, and therefore, it only considers the first two steps of the mechanism proposed in Figure 10. The mathematical model is described next. Model for the Formation of Primary Particles. When the silver is reduced with ascorbic acid, small clusters are formed. These clusters continue growing into primary particles mainly by

clustercluster aggregation. During this process, very small cluster complexes are also formed. All these steps are lumped, using a compact minimal pseudomechanism to form primary particles (P): k1

nðAg0 Þ f C CþC

k2

f

other species

k3

PþI

nðAg0 Þ þ I f C þ I

ð2aÞ ð2bÞ ð2cÞ

Two pseudospecies, C and I, are introduced in this pseudomechanism. The pseudospecies C represents the lumping of a population of silver clusters of different types and sizes into a single variable. The other pseudospecies I is another lumped variable representing the presence of metallic particles in the system and small clusters of silver with ligands. For example Ag2þcitrate in eq 1c is one of the many clusters lumped in I in addition to particles already formed. Equation 2a represents the process of reaction and cluster formation. Equation 2b represents the dynamic of cluster growth to form primary particles and the formation of intermediate clusters (I) during the process. Equation 2c represents how the intermediates clusters accelerate the cluster formation by autocatalysis. The dynamics of primary particle formation (P) is modeled using standard kinetic equations for these reactions. The reaction constants k1, k2, and k3 determine the nucleation rate and the level of autocatalysis. 8029



ARTICLE

Figure 14. Simulated dynamic optical response (SDOR-1) of (a) k3 = 2 1020 m3/s and B = 0.1, (b) k3 = 6.7 1021 m3/s and B = 0.05.

Model for the Formation of Secondary Particles. The primary particles aggregate to form secondary particles (S). The rate of particle aggregation to form secondary particles is governed by qðv, v0 Þ ¼

W1 ðv, v0 ÞqB ðv, v0 Þ WDLVO ðv, v0 Þ

ð3Þ

where q is the collision kernel, v and v0 are particle volumes, qB is the Brownian collision kernel, WDLVO is the stability factor given by the DLVO theory, and W1 is a phenomenological function to account for the fast aggregation of small particles with any other particle. The function W1 is defined as 0

W1 ðv, v0 Þ ¼ 1 þ AeBmin½v, v

ð4Þ

where the parameters A and B are model adjustable parameters. This function accelerates the aggregation of smaller particles into any particles by a factor W1; this effect becomes negligible for

larger particles. The reader is referred to ref 14 for a complete description of the model and the SDOR methodology. 5.3. Interpretation of the Optical Kinetic Data. The model described in the last section was utilized to generate simulated kinetic experiments using the simulated dynamic optical response (SDOR) method developed in ref 14. The SDOR method is a strategy for mechanism identification from UVvis data during particle formation kinetic experiments (see Appendix A). To understand the mechanistic difference between the control experiment and the reference experiment (and thus the effect of citrate on mechanism steps), the data in Figures 12 and 13 were interpreted using this model described in section 5.2 by modifying the model parameters. Figure 14a shows the SDOR for the case in which k1 = 0.006 1/s, k2 = 2 1020 m3/s, k3 = 2 1020 m3/s, A = 103, and B = 0.1, with all other parameters being the same as those used in ref 14. Figure 14b shows the SDOR for the case in which k1 = 0.006 1/s, k2 = 6.7 1021 m3/s, and B = 0.05. The SDOR in Figure 14a better represented the control 8030


Industrial & Engineering Chemistry Research experiment, while the SDOR in Figure 14b better represented the reference experiment. Comparing parameters for the reference experiment versus the control experiment, it is inferred that, in the presence of citrate, there is a lower production rate of primary particles (smaller rate constant, k3). It can also be inferred that the aggregation of primary particles into secondary particles is a fast step (smaller B parameter). This may explain a route for the formation of tertiary particles: slow production of primary particles with the fast consumption of them to form final secondary particles. This difference in rates may lead to tertiary particles by aggregation of secondary particles only. In terms of the model, the silver ligand lumped parameter, I, can be associated partly with the formation of Agncitrate. Pulse radiolysis experiments to reduce silver in the presence of citrate have demonstrated the presence of a citrateAg2 þ complex, which evolves slowly. 26 Vlachos 27 studied this complex theoretically and found that the citrateAg2þ complex is more energetically favorable than Ag2 þ clusters. Citrate also allows the production of Ag4þ2 , which is not thermodynamically favorable in the absence of citrate. The formation of a stable cluster may explain the lower kinetic parameter k3 leading to a reduced production rate of primary particles.

5. CONCLUSIONS This study presents a rapid and convenient method for the production of micrometer-sized dispersed silver particles consisting of isotropic and anisotropic building blocks that form welldefined spheres of uniform size. The system described is particularly attractive because it offers the ability to control morphology using temperature as a tuning knob, while maintaining the final particles as uniform dispersed spheres. The simplicity of the process and the high concentration of silver are two important attributes that make the described process an advantageous and cost-effective route to mass production. The properties of these particles may have applications in plasma display panels (PDP), low-temperature cofired ceramics (LTCC), multilayer ceramic capacitors (MLCC), and solar cells (PV). Using optical kinetic data and SDOR analysis, a mechanism is proposed for the spherical particles with an open structure and surface similar to an orange rind (M2). Analysis suggests that the main route for the production of these structures involves a slower rate of primary particle formation, a higher aggregation rate of secondary particles, and aggregation with incomplete coalescence of these secondary particles to form the final particles. A mechanism is also proposed for the formation of the open-structure spherical particles formed by a network of platelets (M1) based on the preferential attachment of citrate to certain planes. ’ APPENDIX A. SDOR ANALYSIS OF PLASMONIC KINETIC DATA The kinetic data give full UVvis absorption spectra as a function of time during the reactionprecipitation. These spectra are related to the interaction of light and metal nanoparticles. When an electromagnetic field interacts with silver particles, a coherent oscillation of electrons in the conduction band is induced (surface plasmon resonance). This UVvis absorption due to the plasmon resonance is a strong function of the particle shape and size. Given this correlation,

ARTICLE

Figure A1. UVvis spectra for 20-nm nanoparticles.14

Figure A2. Schematic of SDOR methodology

the surface plasmon resonance has been extensively used to characterize nanoparticles. One illustrative example is provided by the UVvis spectra of 20-nanometer silver particles shown in Figure A1.14 A well-defined peak at around 410 nm makes it possible to determine the size of these particles from the UVvis spectra. The broad band shown in Figures 12 and 13 cannot be directly resolved in individual sizes. This is a result of particles of very different sizes (nanometer to micrometer) present at the same time during the course of the kinetics experiment. Given the fact that the optical response is highly nonlinear with size, one cannot deconvolute the size distribution of the particles (see ref 14 for further discussions). This fact complicates the analysis since no direct physical data can be extracted from these bands. From the point of model identification, this will be an ill-posed problem in which multiple size distributions can generate the same spectra. To overcome this problem, a method for mechanistic model identification from these broad bands was introduced recently.14 In the SDOR, “trial” models predict the evolution of a particle population in a simulated optical cell as a function of time. The scattering equations for the simulated optical cell are solved to generate calculated UVvis absorption spectra. Then the simulated absorption spectra predicted by the model are compared with experimental spectra. The identification is achieved by testing different “trial” models and comparing the simulated extinction/scattering with the experimental values. This identification strategy is shown 8031



ARTICLE

Figure A3. Comparison between model prediction and experimental UVvis kinetic data at early stages (see ref 14 for experimental and modeling details).

Figure A4. Comparison between model prediction and experimental UVvis kinetic data at late stages.

schematically in Figure A2. In short, the method consists of the following steps: 1. Postulate a mechanism. 2. Build the population balance model of the proposed mechanism. a. Set the model parameters (just rough estimates at this point). 3. Make a simulated dynamics of the UV spectra predicted by the model. 4. If the simulated spectra reproduce the experimental spectra (movement of the peaks and shape of the spectra), then the proposed mechanism is accepted. 5. Repeat 14 until a suitable mechanism is found. 6. For a quantitative model, optimize the model parameters using the experimental data. This approach was used successfully for model identification of the formation of spherical silver particles from a silver ethylenediamine complex.14 As shown in Figure A3 and A4, the model predicted spectra and the experimental spectra are in perfect agreement. The model also predicts correctly the final particle size distribution (see Figure 14 in ref 14). The model identified in ref 14 using the SDOR strategy introduces a new phenomenological function (eq 4) to account for an accelerated aggregation of small particles. This is based on the analysis of a spherical approximation of all particles. Although the approximation of spherical particles is a very good approximation in most parts of the particle formation, at very early stages, aggregates of small numbers of primary particles may not approximated accurately using “equivalent” spherical particles. These very small clusters will also contribute to a broad absorption. If these very small clusters were the main factor for the broad band, then we should see a transition to a sharper peak as these aggregates restructure to spherical shapes. The experimental data did not show this transition; the broad band evolves

Figure A5. Aggregation mechanism. For a binary collision between two particles of sizes vsmall and vlarge, the shaded areas shows where W1 and WDLVO are dominant.

continuously throughout the process dynamics. Thus, it is hypothesized that the effective spherical particles in the PBM are a good first-order approximation at these early stages and that the main contribution to the broad band is a very fast aggregation of particles during the early stages. The accelerated aggregation at the early stages may be due to an enhanced diffusion and shorter diffusion distance to aggregation. If the autocatalytic model found is correct, new primary particles will form close to existing particles, shortening the diffusion distance. The diffusion is also enhanced at these small separations by van der Walls forces. A more accurate investigation of the aggregation rates at initial stages will require the calculation of nonspherical particles in the SDOR analysis. This is matter of future research. In summary, W1 gives an accelerated aggregation of small particles and the WDLVO gives a stabilization or slowdown of aggregation at very large particle sizes. These two effects operate at very different stages during the particle formation dynamic, as shown 8032


Industrial & Engineering Chemistry Research schematically in Figure A5. W1 is active (W1 > 1) at very early stages (very small particle size) and during late stages the WDLVO effect becomes dominant and W1 converges rapidly to 1.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT All analytical data were obtained in DuPont’s experimental station and Research Triangle Park analytical laboratories. The authors acknowledge discussions with John Wyre and Kathryn G. Lloyd on ESCA analysis. Part of the kinetic data was obtained by one coauthor (M.S.L.-V.) during her internship with DuPont. ’ REFERENCES (1) Sugimoto, T., Ed. Fine Particles, Synthesis, Characterization, Mechanism of Growth, Marcel Dekker: New York, 2000. (2) Velikov, K. P.; Zegers, G. E.; Blaaderen, A. Synthesis and characterization of large colloidal silver particles. Langmuir 2003, 19, 1384–1389. (3) Halaciuga, I.; Goia, D. V. Preparation of silver spheres by aggregation of nanosize subunits. J. Mater. Res. 2008, 23, 1776–1784. (4) Irizarry, R.; Rivera, V.; Glicksman, H. Process for making highly dispersable, spherical silver powder particles that are very high solids and highly ordered. US patent no. 7,648,557 B2, 2010. (5) Songping, W.; Shuyuan, M. Preparation of ultrafine silver powder using ascorbic acid as reducing agent and its application to MLCI. Mater. Chem. Phys. 2005, 89, 424–427. (6) Songping, W. Preparation of micron size flake silver powders for conductive thick films. J. Mater. Sci.: Mater Electron. 2007, 18, 447–452. (7) Tan, F.; Qiao, X.; Chen, J. Removal of chemisorbed lubricant on the surface of silver flakes by chemicals. Appl. Surf. Sci. 2006, 253, 703– 707. (8) Lee, H.; Chou, K.; Shih, Z. Effect of nano-sized silver particles on the resistivity of polymeric conductive adhesives. Int. J. Adhesion Adhesives 2005, 25, 437–441. (9) Fukoyo, T.; Imai, H. Morphological evolution of silver crystals produced by reduction with ascorbic acid. J. Cryst. Growth 2002, 241 (2002), 193–199. (10) Suber, L.; Sondi, I.; Matijevic, E.; Goia, D. V. Preparation and the mechanisms of formation of silver particles of different morphologies in homogeneous solutions. J. Colloid Interface Sci. 2005, 288, 489–495. (11) Widoniak, J.; Eiden-Assmann, S.; Maret, G. Silver particles tailoring of shapes and sizes. Colloids Surf., A 2005, 270271, 340–344. (12) Song, W.; Jia, H.; Cong, O.; Zhao, B. Silver microflowers and large spherical particles: Controlled preparation and their wetting properties. J. Colloid Interface Sci. 2007, 311, 456–460. (13) Wagner, Handbook of X-ray Photoelectron Spectroscopy; Perkin-Elmer Corporation: MN, 1979; p 519, 529. (14) Irizarry, R. Simulated dynamic optical response strategy for model identification of metal colloid synthesis. Ind. Eng. Chem. Res. 2010, 49, 5588–5602. (15) Privman, V. Diffusional nucleation of nanocrystals and their self-assembly into uniform colloids. J. Optoelectr. Adv. Mater. 2008, 10, 2827–2839. (16) Henglein, A.; Giersig, M. Formation of colloidal silver nanoparticles: capping action of citrate. J. Phys. Chem B. 1999, 103, 9533–9539. (17) Wiley, B.; Sun, Y.; Chen, J.; Cang, H.; Li, Z.; Li, X.; Xia, Y. Shape-controlled synthesis of silver and gold nanostructures. MRS Bull. 2005, 30, 356–361. (18) Wang, D; Song, C.; Hu, Z.; Zhou, X. Synthesis of silver nanoparticles with flake-like shapes. Mater. Lett. 2005, 59, 1760–1763.

ARTICLE

(19) Wang, A.; Yin, H.; Ren, M.; Liu, Y.; Jiang, T. Synergistic effect of silver seeds and organic modifiers on the morphology evolution mechanism of silver nanoparticles. Appl. Surf. Sci. 2008, 254, 6527–6536. (20) Liguo, Y.; Yanhua, Z. Preparation of nano-silver flake by chemical reduction method. Rare Met. Mat. Eng. 2010, 39, 401–404. (21) Mansouri, S. S.; Ghader, S. Experimental study on effect of different parameters on size and shape of triangular silver nanoparticles prepared by a simple and rapid method in aqueous solution. Arabian J. Chem. 2009, 2, 47–53. (22) Ji, X.; Song, X.; Li, J.; Bai, Y.; Yang, W.; Peng, X. Size control of gold nanocrystals in citrate reduction: the third role of citrate. J. Am. Chem. Soc. 2007, 129, 13939–13948. (23) Kilin, D. S.; Prezhdo, O. V.; Xia, Y. Shape-controlled synthesis of silver nanoparticles: Ab initio study of preferential surface coordination with citric acid. Chem. Phys. Lett. 2008, 458, 113–116. (24) Senapati, D.; Singh, A. K.; Ray, P. C. Real time monitoring of the shape evolution of branched gold nanostructure. Chem. Phys. Lett. 2010, 487, 88–91. (25) Gorshkov, V.; Zavalov, A.; Privman, V. Shape selection in diffusive growth of colloids and nanoparticles. Langmuir 2010, 25, 7940–7953. (26) Pillai, Z. S.; Kamat, P. V. What factors control the size and shape of silver nanoparticles in the citrate ion reduction method? J. Phys. Chem. B 2004, 108, 945–951. (27) Mpourmpakis, G.; Vlachos, D. G. Insights into the early stages of metal nanoparticle formation via first-principle calculations: the roles of citrate and water. Langmuir 2008, 24, 7465–7473.

8033


Preparation and Formation Mechanism of Silver Particles with

Recommend Documents