Article pubs.acs.org/IECR
Studies on the Continuous Precipitation of Silver Nanoparticles Kan-Sen Chou,*,† Yu-Chun Chang,† and Lien-Hua Chiu‡ †
Department of Chemical Engineering, National Tsing Hua University, Hsinchu, Taiwan 30043 Taiwan Textile Research Institute, New Taipei City, Taiwan 23674
‡
ABSTRACT: The continuous precipitation of silver nanoparticles was performed at room temperature using sodium borohydride (NaBH4) as the reducing agent. The effects of the concentration of the silver precursor (i.e., silver nitrate), the quantity of dispersing polymer [poly(vinyl pyrrolidone), PVP], and the flow rates of the reacting solutions on the resulting particle size and its distribution were investigated and are reported herein. Particles with average sizes ranging from 13 to 130 nm were obtained. Our results show that there is an optimal flow rate for each precursor concentration to obtain a uniform size distribution of the silver nanoparticles. This phenomenon can be explained in terms of the sequence of mixing, reaction, supersaturation generation and subsequent nucleation, growth, and agglomeration steps. Size control was mainly achieved through the use of PVP/AgNO3 weight ratios ranging from 0.05 to 1.5. Other properties such as residual PVP quantity and boron and sodium contamination are also reported here for several representative samples.
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INTRODUCTION Because of their unique structures, nanosized materials often exhibit quite different chemical and physical characteristics than their corresponding bulk materials. These characteristics have become a popular subject of research in recent years. To prepare nanosized metal colloids, chemical reduction is frequently chosen because of its simplicity and ease of processing to obtain the desired products. In a typical process, one can choose various types of metal salt precursors, reducing agents, and reaction conditions to synthesize nanosized metal colloids with the desired size and purity characteristics. In the chemical reduction of silver ions to obtain nanosized silver colloids, common reducing agents include sodium borohydride (NaBH4),1 formaldehyde (HCHO), 2,3 and glucose,4 to name a few. Sodium borohydride is capable of reducing many metal ions because of its strong reducing power. In the case of silver, the reducing reaction is very intense, and a protective dispersing agent would not have sufficient time to coat each particle, so that the resulting particle sizes would increase as a result of agglomeration effects. One would then have to limit the precursor concentration to avoid the associated agglomeration effects. Formaldehyde is also capable of reducing silver ion to silver at room temperature under alkaline conditions. In this case, the adjustment of the solution pH is of critical importance in the control of the final size distribution. By contrast, glucose is a very mild reducing agent, and hence, its reaction is slow. To increase the reaction rate, the solution must often be heated to 75 °C.5 Most previous research on silver nanoparticles was carried out batchwise in the laboratory. It thus suffered the disadvantages of using small quantities, being time-consuming, and giving poor reproducibility. It is thus desirable to study continuous processes for synthesizing nanosized metal colloids. For silver nanoparticles, the potential demand as an additive in the textile industry6,7 is quite large, and hence, there is a need to investigate continuous precipitation processes. Relatively few studies on continuous precipitation processes can be found in the literature. Tai et al.8 used a spinning disk reactor and © 2012 American Chemical Society
glucose as a reducing agent to synthesize nanosilver colloids. They showed that the size decreased from about 10 to 4 nm when the spinning rate was increased from 1000 to 4000 rpm. The conversion, however, was only 70%. Hartlieb et al.9 used a narrow channel reactor and hydrogen as the reducing agent to obtain silver colloids. They studied the effect of channel size and demonstrated the possibility of long-term continuous operation to obtain silver nanoparticles 3.5 nm in size. Huang et al.10 used a simple T-shaped mixer plus a tubular reactor and lixivium of sundried Cinnamoum camphora leaf as the reducing agent to obtain silver colloids of around 5−40 nm in size. Because of the weak reducing power of lixivium, the reaction was carried out at 90 °C. He et al.11 synthesized nanosilver in organic solvent using silver acetate as the precursor. They used a capillary microflow reactor in a high-temperature oil bath at 170 °C. Recently, Tseng et al.12 applied an electrochemical discharge method using citrate ion to cap the surface of the silver nanoparticles and to increase the zeta potential for particle stability. As the above literature summary shows, a systematic study on the continuous precipitation of nanosilver using a common reducing agent such as NaBH4 at room temperature has not yet been reported. It is therefore the objective of this work to investigate the effects of several operating variables on the resulting properties of nanosilver colloids in a continuous process. The size uniformity of the products was observed and is reported herein.
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EXPERIMENTAL SECTION Reagent-grade silver nitrate (Next Chimica, Centurion, South Africa) and sodium borohydride (Alfa Aesar, Ward Hill, MA) were used in this work to synthesize nanosilver colloids. The concentration of the precursor solution was a variable to be Received: Revised: Accepted: Published: 4905
September 6, 2011 March 11, 2012 March 22, 2012 March 26, 2012 dx.doi.org/10.1021/ie202032h | Ind. Eng. Chem. Res. 2012, 51, 4905−4910
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reactor was adopted from that of Wagner and Kohler,13 who used their reactor for the continuous synthesis of gold nanoparticles. The width and depth of the microchannel were both 3 mm. A plastic tube of 5 mm in diameter was connected to the outlet of the mixer to complete the reaction. Here, the flow rates of the peristaltic pump were changed in the range from 0.83 to 3.57 mL/s to study the effect of flow rate on the particle size and distribution. For characterization, the particle morphology was observed by field-effect scanning electron microscopy (FE-SEM; S-4700, Hitachi, Tokyo, Japan), and the particle size and standard deviation were measured mainly from SEM images with the help of image analyzer software (Image J, National Institute of Health). About 100 ± 25 particles were counted in each case. The mean size and standard deviation were calculated using Excel. The crystal structure was characterized by X-ray diffraction (XRD 500, Rigaku, Tokyo, Japan). Atomic analysis (AA; 5100 PC, Perkin-Elmer, Wellesley, MA) was also performed to obtain data on the conversion of the process, as well as the purity of products in terms of residual sodium and boron contamination. Finally, thermal gravimetric analysis (TGA; SSC5000, Seiko, Chiba, Japan) was carried out to determine the residual quantity of PVP in the colloids.
studied, but the molar ratio of NaBH4 to AgNO3 was kept at 0.7 throughout this work. The precipitation reaction occurs as follows 2Ag+ + 2BH 4− + 6H2O → 2Ag + 7H2 + 2B(OH)3
To help with the dispersion of silver colloids, poly(vinyl pyrrolidone) (PVP; Sigma, St. Louis, MO; molecular weight = 40000) was added to the silver precursor at a weight ratio to AgNO3 of 1 in most experiments. However, to vary the particle size of silver colloids, this weight ratio of PVP to AgNO3 was systematically changed from 0.05 to 1.5 (the corresponding molar ratio varied from 0.076 to 2.283 based on the weight of a PVP unit of 111) while the concentration of AgNO3 was kept at 0.1 N in these experiments. After the reaction, the solution was thoroughly mixed with acetone (volume ratio of 2.5) to settle out the nanosilver colloids, which were then redispersed in deionized water for further analysis and applications. The synthesis reaction was carried out in a simple microchannel reactor (Figure 1) made of acrylic resin. This
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RESULTS AND DISCUSSION 1. Size Uniformity. One of the focuses of this work was to determine how the process variables affect the particle size and the uniformity of the nanosilver colloids. Figure 2 shows the effect of the flow rate on the resulting particle size and its standard deviation (number-average). Clearly, there exists an optimal flow rate for this precursor concentration that will result in small, uniform silver colloids. The size distribution will be wider for either lower or higher flow rates. Similar experiments were then carried out for different precursor concentrations between 0.01 and 0.05 M. The number-average size at each precursor concentration and under the optimal flow rate is summarized in Figure 3. A roughly linear relationship can be observed among the variables of particle size, precursor concentration, and optimal flow rate. Higher flow rates must be used when the precursor concentration is higher to obtain
Figure 1. Photograph of the microchannel reactor used in this work (channel size = 3 × 3 mm).
Figure 2. Effect of flow rate on the number-average mean particle size and its standard deviation (PVP/AgNO3 weight ratio = 1.0, [AgNO3] = 0.1 M). 4906
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At the same time, PVP molecules will adsorb onto the surface of the silver colloids to avoid agglomeration, which increases the particle size and widens its distribution. Because the reducing agent (i.e., NaBH4) is a very strong one, it is capable of quickly reducing silver ion to silver when the two species collide with each other. Thus, the degree of mixing by changing flow rates determines how fast these two reacting species make contact with each other.14 As reported in Table 1, basic hydrodynamic quantities such as the flow rate (q), average velocity (u = q/A, where A is the cross-sectional area for flow), Reynolds number (Re = duρ/μ, where d is the equivalent diameter of the flow channel and ρ and μ are the density and viscosity of the solution, respectively), mean residence time (tr = L/u, where L is the length of the flow channel), and mixing time (tm)15 were calculated or estimated to provide a general description of the flow conditions in the reactor. The results show that the flow in the reactor was laminar flow and that the estimated mixing times were much shorter than the mean residence times. At the optimal flow rate, the degree of mixing is sufficiently strong, meaning that the rate of generation of supersaturation is high. As a result, nucleation will occur within a short time, and the size distribution of the colloids will be narrow. When the flow rate is lower, the extent of mixing will be weaker, and the supersaturation will be generated with a wider time distribution. Hence, the subsequently formed colloids will have a wider size distribution. At the same time, when the flow rate is higher, the rate of PVP adsorption on the silver colloid to prevent agglomeration can become the limiting factor in controlling the size distribution of the products. It is worth noting that, at the precursor concentration of 0.1 M, agglomeration might be unavoidable. A slightly wider size distribution of final colloids might then result. 2. Size Control. In this work, we controlled the particle size simply by changing the weight ratio of PVP to AgNO3 from 0.05 to 1.5. As a result, the average size varied from about 130 to 13 nm, as shown in Figure 4. The effect was quite significant when the PVP amount was low and became less significant when the weight ratio between the PVP and AgNO3 exceeded
Figure 3. Effect of precursor concentration and optimal flow rate on the mean particle size.
Table 1. Some Representative Quantities of the Flow in the Microchannel Reactor flow rate (mL/s)
average velocity (m/s)
Re
mean residence time, tr (s)
mixing time, tm (ms)
0.83 1.54 2.5
0.184 0.342 0.556
554 1026 1666
1.3 0.7 0.4
∼25 ∼20 ∼8
uniform colloids. The average size varied from 16 to 26 nm when the precursor concentration was changed from 0.01 to 0.1 M. In general, when a silver ion meets a reducing agent, it will be reduced to silver, and the silver concentration, expressed as supersaturation, will increase accordingly. At some point, the silver colloids will nucleate to consume much of the supersaturation. Finally, these nuclei will continue to grow to consume the rest of the supersaturation to reach equilibrium.
Figure 4. Effect of relative quantity of PVP/AgNO3 on mean particle size and its standard deviation (concentration of AgNO3 = 0.1 M, flow rate = 2.5 mL/s). 4907
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dissolved separately and mixed prior to continuous precipitation to ensure uniform nucleation and growth of silver nanoparticles. Even though PVP was very useful in controlling the size of the silver nanoparticles, it would be an obstacle to electron transfer in conductive applications. Extra work on washing to remove as much PVP as possible or some type of thermal treatment to decompose and remove these molecules from the interfacial region between particles would be needed for them to be used in electronic devices. In Figure 5c, there are a few obscure regions. We believe that these were primarily caused by the loss of PVP molecules because of the incomplete washing procedure. Therefore, researchers would have to balance the benefits of achieving a small particle size by using more PVP against the costs of removing the PVP afterward. 3. Particle Characterization. Figure 6 displays the TGA data of two representative samples for PVP/AgNO3 ratios of 0.1 and 1. The data on weight loss due to PVP removal are quite obvious and consistent with each other in temperature range. Kao16 showed that the quantity of a fully adsorbed PVP layer is about 5.0 mg/m2. We could then calculate the theoretical PVP quantity fully adsorbed on each particle with a known size and compared the results with the data (actually adsorbed) obtained from TGA. The calculated results for these two samples are summarized in Table 2. Clearly, only a small fraction (less than 10%) of the added PVP molecules could actually adsorb onto the silver nanoparticles. This was very likely due to the very short period of reaction time (or contact time), only about 1 s or less, for this continuous precipitation process. In a typical batch precipitation, the reaction time usually lasts for more than 10 min. This fact also suggests that the rate at which PVP molecules adsorb onto Ag nanoparticles is limited. As a result, the quantities of PVP adsorbed represented only 44−92% of full coverage. Figure 7 shows the XRD pattern of the product. These nanoparticles were crystalline. Even in such a short reaction time, the silver atoms were still able to pack easily into an ordered structure. The purity of the nanoparticles was checked for Na and B contamination by atomic analysis. The results were 8.1 ppm Na and 4.8 ppm B, indicating a high purity of the products.
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CONCLUSIONS The continuous precipitation of silver nanoparticles was investigated here using AgNO3, NaBH4, and PVP in a microchannel reactor. The effects of the AgNO3 concentration (0.01−0.1 M), quantity of PVP relative to AgNO3, and solution flow rates on the particle size and size distribution were studied. Our results show that there exists an optimal flow rate for each precursor concentration to obtain nanoparticles with a narrow standard deviation. Because the flow rate affects the degree of mixing, it therefore affected the particle size distribution. On the other hand, the average size could be varied from 130 to 13 nm by simply changing the PVP/AgNO3 ratio from 0.05 to 1.5. PVP was found to play the role of a protective agent, and it prevented particle growth by limiting the rate of adsorption on the particle surface to avoid agglomeration. Because of the very short reaction time in this continuous precipitation process, the percentage of PVP molecules successful adsorbed onto silver nanoparticles was low and less than 10%. The nanoparticles obtained from this process were high in purity and showed a good crystalline structure. With a production rate as high as 180 g/h, this continuous precipitation process of silver nano-
Figure 5. SEM images of different sized silver colloids: (a) PVP/ AgNO3 = 0.05, size = 130 ± 34 nm; (b) PVP/AgNO3 = 0.2, size = 63 ± 13 nm; (c) PVP/AgNO3 = 1.0, size = 23.5 ± 4 nm.
1.0. Size variations (in terms of standard deviation) also decreased slightly, from 3.0 to 1.5 nm, when more PVP was added (i.e., for a ratio of 1.0 versus 1.5) to the reacting system. A few representative SEM images are shown in Figure 5. As can be easily seen, the resulting silver nanoparticles were relatively uniform. Because the PVP molecules are also weak reducing agents toward silver and it usually took 20−30 min to complete the dissolution of the PVP, a small amount of silver nanoparticles would inevitably be generated when the PVP and AgNO3 were dissolved together. These small amounts of silver particles would then lead to a wide distribution in size. It is therefore recommended that the PVP and AgNO3 be 4908
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Figure 6. Two representative TGA plots to show weight loss due to removal of adsorbed PVP molecules.
Table 2. Summary of Different PVP Quantities for Each Sample
a
average size (nm)
added
actually adsorbed
fully adsorbeda
0.1 1
91.8 23.3
0.1565 1.565
0.0132 (44%) 0.1127 (92%)
0.0347 0.1485
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REFERENCES
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Based on 5.0 mg/m2 adsorption of PVP on silver nanoparticles16
Figure 7. XRD pattern of silver nanoparticles, showing the crystalline structure of the product.
particles can probably meet the demand for industrial applications in the future.
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ACKNOWLEDGMENTS
The authors thank both Taiwan Textile Research Institute (TTRI) and National Science Council (NSC, Taiwan NSC-992221-E007-095) for the financial support of this research.
PVP [g/(g of Ag)] PVP/AgNO3 (w/w)
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AUTHOR INFORMATION
Corresponding Author
*Tel.: 886-03-5713691. Fax: 886-03-5715408. E-mail: kschou@ che.nthu.edu.tw. Notes
The authors declare no competing financial interest. 4909
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