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Ind. Eng. Chem. Res. 2009, 48, 10104–10109
Preparation of Silver Nanoparticles Using a Spinning Disk Reactor in a Continuous Mode Clifford Y. Tai,* Yao-Hsuan Wang, Chia-Te Tai, and Hwai-Shen Liu Department of Chemical Engineering, National Taiwan UniVersity, Number 1, Section 4, RooseVelt Road, Taipei 106, Taiwan
A spinning disk reactor (SDR) operated in a continuous mode was adopted for producing silver nanoparticles, aiming at an increase in the production rate. Two protecting agents, i.e., poly(vinylpyrrolidone) (PVP) and hydroxypropylmethyl cellulose (HPMC), were tested, which have been proven to be effective in our previous work of recycle operation. Using PVP as protecting agent, the effects of the operating variables, including the rotation speed, the feeding rate, the dosage of protecting agent, and reactant concentration, on the particle size and yield of the silver product were investigated. The experiment was further conducted using HPMC as protecting agent to compare the experimental results between the two protecting agents. The use of PVP gave smaller silver nanoparticles; however, a higher yield was obtained by using HPMC. Although the yield of continuous operation was lower than that of recycle operation, the production rate was much higher for either protecting agent, providing a great potential for commercialization. 1. Introduction A variety of methods for preparing silver nanoparticles have been reported, including mechanical milling,1 physical vapor deposition,2 spray pyrolysis,3 photochemical reduction,4,5 microemulsion,6 precipitation by direct mixing,7-9 and so on. The precipitation method, involving the mixing of two solutions, i.e., the solutions of silver salt and reducing agent, is most popular for its simplicity, low cost, and ease of manipulation. A protecting agent was also required in the preparation of nanoparticles for preventing the particles from growth or agglomeration. Although large-scale production of silver nanoparticles from solution precipitation still remains a challenge, several investigators have reported successful cases in laboratory-scale experiments.7-9 However, the applied reducing agents, such as dimethylformamide, hydrazine, and formaldehyde, are biologically toxic and environmentally hazardous in spite of their high activity. Although mild reducing agents, such as glucose10 and starch,11 have been used, the precipitation processes took a long reaction time or required an elevated temperature or pressure to achieve a high reaction conversion. Using a stirred batch reactor to achieve a high conversion and nanosized particles, the concentrations of reactants were kept low, and thus the solid content of the collected silver suspension was too low for commercial use. Also, size control in the nanometer range of the silver product was difficult. In the conventional precipitation process, poor mixing usually occurs in a stirred tank, yielding a large size of the agglomerates and a wide distribution of silver product. To overcome these problems, an efficient and cost-effective method, the highgravity precipitation technique, has been developed. The reactor with a rotating disk as the main part is called a spinning disk reactor (SDR), in which a centrifugal force is used to spread the incoming streams and produce an extremely thin film on the rotating-disk surface. Inside the thin film, intense turbulence generated by the spinning disk enhances the mixing efficiency and the frequency of collision between reacting molecules, thus shortening the reaction time. The high efficiency of micromixing in the SDR has been confirmed by Chen et al.12 Cafiero et al.13 * To whom correspondence should be addressed. E-mail: cytai@ ntu.edu.tw.
have applied the SDR to prepare barium sulfate in the range between 0.7 and 3.0 µm. They suggested that the intense mixing in the SDR enhanced homogeneous nucleation, resulting in small size and a narrow distribution of product particles. In our laboratory, several model compounds have been synthesized using the high-gravity reactive precipitation via a gas-liquid or liquid-liquid contact in the SDR, such as BaCO314 and Mg(OH)2..15 Recently, we successfully synthesized silver nanoparticles using the high-gravity precipitation technique that combined economical benefit and benign environmental impact.16 The SDR operated in a recycle mode was adopted to prepare silver nanoparticles via reactive precipitation, using glucose and starch as the reducing and protecting agent, respectively. The inexpensive and environmentally friendly materials were the same as those used by Raveendran et al.10 Also, the dosage of the protecting agent in the experiment was only one-fifth to one-tenth of that used in the conventional methods.7 However, the size of the starch-protected silver particles was larger than 10 nm. In the second report of our serial research on silver nanoparticle preparation, we substituted poly(vinylpyrrolidone) (PVP) or hydroxypropylmethyl cellulose (HPMC) for starch as the protecting agent.17 The smallest silver product with a mean size below 10 nm was obtained using PVP as the protecting agent. The experiment was further conducted at various glucose concentrations to increase the yield. When the glucose concentration was low, PVP was superior to HPMC and starch; however, HPMC was the best protecting agent at high glucose concentration, giving the silver particles below 10 nm. The experimental results clearly show that the protecting agent plays a crucial role in the preparation of silver nanoparticles. The protective mechanism of PVP has been discussed in many reports.4,7,9,18 Silvert et al.18 indicated that the presence of PVP could promote nucleation during the formation of metallic silver particles, because a PVP macromolecule in solution likely adopted a pseudorandom coil conformation, taking part in some form of association with the metal atoms. In addition, the individual unit of PVP contains an imide group, and the N and O atoms of this group probably have a strong affinity for silver ions and metallic silver on the surface of silver particles. Due to the strong affinity between PVP and silver particles, a firm protective layer of PVP forms on the particle
10.1021/ie9005645 CCC: $40.75 2009 American Chemical Society Published on Web 09/03/2009
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Figure 1. Experimental setup of the high-gravity system.
surface, preventing agglomeration or growth of primary particles by the steric hindrance effect of polymers. Zhang et al.9 compared the IR spectra between the pure PVP powder and the PVP-protected silver nanopowder, confirming that PVP formed a protective layer on the particle surface not only by weak physical adsorption but also by strong chemical adsorption. Although the protective mechanism of HPMC is less understood, it is a common stabilizer for drug particles. Yokoi et al.19 reported that HPMC would be able to adsorb on the surface of cefditoren pivoxil (CDTR-PI) particles and thus inhibit the crystal growth. In our previous reports on the preparation of silver nanoparticles using SDR, all the experiments were conducted in a recycle mode, giving a low production rate.16,17 To increase the production rate, we adopted a continuous mode in this study. Using PVP as the protecting agent and aiming at the control of particle size and an increase in the production rate of silver product, the effects of various operating variables, including the rotating speed of the disk, the feeding rate of the reactant solution, AgNO3 concentration, the PVP/AgNO3 weight ratio, and the glucose/AgNO3 concentration ratio, were investigated. A comparison of experimental results between PVP and HPMC as protecting agent was also made at various AgNO3 concentrations. Finally, the experimental results between continuous and recycle operations were compared. 2. Experimental Section The experimental setup of the high-gravity system is shown in Figure 1, which comprises a liquid feeding system, a spinning disk reactor (SDR), and a slurry collection vessel (G). The liquid feeding system consists of two tanks (A and B), a flow meter (D), and a liquid distributor (E). The main part of the highgravity system is the SDR, in which the spinning disk (F) with a diameter of 19.5 cm is made of stainless steel and driven by a variable-speed motor. It should be noted that the disk is placed vertically, instead of horizontally, so that the produced particles can easily drop out of the reactor chamber. The details of the high-gravity system have been shown in the report of Tai et al.16 Metallic silver was produced from the reduction reaction of silver nitrate as shown in eq 1, using glucose as the reducing agent. An alkali, such as sodium hydroxide, was added to neutralize the produced protons and accelerate the reaction rate. NaOH
2Ag+ + C6H12O6 + H2O 98 2Ag V +C6H11O7- + 3H+ (1)
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To produce nanoparticles, a protecting agent, such as PVP or HPMC, was required for preventing agglomeration or growth of silver particles. The SDR can be operated in two different modes, i.e., recycle mode and continuous mode. In our previous reports, all of the experiments were conducted in a recycle mode.16,17 In this experiment, we adopted the continuous mode of operation. First, we charged tank A with an aqueous solution of AgNO3 and protecting agent and tank B with an aqueous solution of glucose and NaOH. Then, the two solutions were pumped at the same feeding rate (0.8-5 L/min) onto the center of the spinning disk that rotated at a specific speed, ranging from 1000 to 4000 rpm. Due to the centrifugal force, the liquid was accelerated to spread on the disk surface and form a thin film where the reduction reaction took place. Finally, the produced slurry hit the housing of the reactor, and subsequently flowed into a collection vessel. The experimental setups between the two operation modes were basically the same except for two points. First, when a continuous mode was adopted, the produced slurry directly flowed into a collection vessel without recycling, instead of flowing back to tank B for recycling. Second, for a continuous operation, the retention time for the mixed streams on the spinning disk was too short, so that the reduction reaction might proceed further in the collection vessel. To prevent the reaction proceeding in the collection vessel, the reduction reaction was accelerated by raising the glucose/AgNO3 feeding mole ratio, because the reducing agent, glucose, is much cheaper than silver nitrate. The glucose/AgNO3 feeding mole ratio was raised to a value that is higher than the stoichiometric ratio, varying from 1 to 5. After an experimental run, the yield and the particle size of the silver colloids were checked by continuous stirring of the slurry in the collection vessel for 1 h. The yield and the particle size did not change much, indicating that the reduction reaction did not proceed further in the collection vessel. This meant that the chosen feeding mole ratio of glucose/AgNO3 resulted in a reduction reaction that was fast enough for a continuous operation. After the reactant solutions in the tanks were exhausted, the collected slurry was centrifuged to separate the silver particles from solution. Then, the particles were washed twice with a mixture of deionized water and acetone, in a volume ratio of 1-3, to remove protecting agent and other impurities adsorbed on the silver particles. At last, the centrifuged particles were air-dried to calculate the yield, which is the amount of dried product divided by the theoretical amount of a complete reaction. To determine the size distribution of the silver product, the sample of suspension was prepared by diluting the collected slurry with deionized water and subsequently analyzed by a dynamic light scattering analyzer (Malvern, 3000HSA). In addition, we measured the pH value of the outlet streams from the reactor, intending to explain the effect of AgNO3 concentration. 3. Results and Discussion 3.1. Effects of Operating Variables Using PVP as Protecting Agent. Continuous processes are widely commercialized due to their high production rate. Using SDR for producing silver nanoparticles, we adopted the continuous mode of operation, and investigated the effects of operating variables, including the rotating speed of the disk, the feeding rate of the reactant solution, the reactant concentration, and the dosage of the protecting agent, on the particle size and yield of silver product. The protecting agent, PVP, was chosen because it performed best among the protecting agents tested in the previous study.
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Figure 2. Effect of rotating speed on the yield and particle size of the silver product produced by the SDR in a continuous mode. Other operating variables kept constant were LA ) LB ) 1 L/min, [AgNO3] ) 0.01 M, [glucose] ) 0.01 M, [NaOH] ) 0.07 M, and PVP/AgNO3 ) 1 (weight ratio).
3.1.1. Effects of Physical Variables. The effect of rotating speed was discussed by adjusting the rotating speed of the disk from 1000 to 4000 rpm, while keeping the following variables constant: AgNO3 and glucose concentrations were both 0.01 M; NaOH concentration was 0.07 M; the PVP/AgNO3 weight ratio was 1; the feeding rates of the solutions from tanks A and B were both at 1 L/min. As shown in Figure 2, the volume mean size decreased with an increase in the rotating speed, and remained below 10 nm for the speed range between 1000 and 4000 rpm. The yield slightly increased from 60.7 to 69.4% when the rotating speed was varied from 1000 to 4000 rpm. The results implied that the mixing intensity at 1000 rpm was high enough to produce silver particles below 10 nm, and a faster speed above 1000 rpm gave a more intense mixing of the reactants, resulting in a higher yield. According to the investigation reported by Mohr and Newman,20 and Law et al.,21 the flow on the rotating disk is laminar when the Re, which is defended as r2Ω/ν, is less than 1.5 × 105 and is turbulent for Re being larger than 3.0 × 105. In this experiment, the flow was laminar on part of the disk surface and became turbulent before leaving the disk. The flow entered the transition region at a specific radius, which was 4.10, 2.90, 2.37, and 2.05 cm for 1000, 2000, 3000, and 4000 rpm, respectively. Apparently, the mixing was more intense at higher rotating speed. Furthermore, the feeding rate of the reactant solution was varied from 0.8 to 5 L/min to study its effect on the particle size and yield, while keeping the other variables constant as indicated in Figure 3. It should be noted that 5 L/min was the allowable maximum feeding rate for the high-gravity system used in this work. The feeding rate in the range between 0.8 and 5 L/min had less effect on the yield, varying from 65.0 to 69.4%, and the mean size, ranging from 3 to 5 nm. In Figure 3, the particle mean sizes are below the reliable limit of the particle size analyzer, which is 5 nm. Thus no conclusion can be drawn for the effect of the feeding rate. Chen et al. reported that micromixing on a spinning disk deteriorated with an increase in the feeding rate due to the thickening of the liquid film12 and would result in a silver product with a larger mean size and a broader size distribution. However, the effect of the feeding rate was not observed in our system with the feeding rate up to 5 L/min. 3.1.2. Effect of Reactant Concentration. To increase the production rate, AgNO3 concentration was increased by a factor of 10, from 0.01 to 0.10 M, while the other variables were fixed as indicated in Figure 4. It should be noted that the PVP/AgNO3 weight ratio was reduced from 1 to 1/2, because a high
Figure 3. Effect of feeding rate on the yield and particle size of the silver product produced by the SDR in a continuous mode. Other operating variables kept constant were N ) 4000 rpm, LA/LB ) 1, [AgNO3] ) 0.01 M, [glucose] ) 0.01 M, [NaOH] ) 0.07 M, and PVP/AgNO3 ) 1 (weight ratio).
Figure 4. Effect of AgNO3 concentration on the yield and particle size of the silver product produced by the SDR in a continuous mode. Other operating variables kept constant were N ) 4000 rpm, LA ) LB ) 5.0 L/min, [glucose]/[AgNO3] ) 1, [NaOH] ) 0.07M, and PVP/AgNO3 ) 1/2 (weight ratio).
concentration of PVP would greatly reduce the yield for the PVP/AgNO3 weight ratio being kept at 1. For example, the yield was 70.9 and 92.0% for the PVP/AgNO3 ratio being 1 and 1/2, respectively, at the AgNO3 concentration of 0.01 M. At a high concentration of protecting agent (PVP or HPMC), the mixing efficiency and thus the frequency of collision between reacting molecules were reduced, so that the yield decreased as indicated in our previous report.17 Increasing the AgNO3 concentration from 0.01 to 0.10 M, the variation of mean size was less significant, between 8 and 10 nm, but the yield decreased about 30.0%, from 72.0 to 43.9%, as shown in Figure 4. The decrease in yield will be explained later. It is clear that the production rate enhanced by increasing the AgNO3 concentration was traded off by the low yield. In this study, the rotating speed and the feeding rate had been fixed at the highest condition, i.e., 4000 rpm and 5 L/min, respectively. To increase the yield at high AgNO3 concentration, the remaining adjustable operating variables included PVP/AgNO3 weight ratio, glucose/AgNO3 concentration ratio, and NaOH concentration. The study regarding the effect of NaOH concentration was excluded, because Tai et al. has found that the produced silver particles would agglomerate at a NaOH concentration above 0.07 M and the reaction rate would be too slow at lower NaOH concentrations.17 Therefore, NaOH concentration was still fixed at 0.07 M in the investigation on the effects of PVP/AgNO3 and glucose/AgNO3 ratios.
Ind. Eng. Chem. Res., Vol. 48, No. 22, 2009 Table 1. Effect of PVP/AgNO3 Weight Ratio on the Yield and Particle Size of the Silver Product Produced by the SDR in a Continuous Modea particle size of silver product run no.
PVP/AgNO3 weight ratio
yield (%)
no. mean (nm)
vol. mean (nm)
080410 080331-1b
1/2 1/4
43.9 40.8
8.0 9.8
080331-2b
1/5
49.6
13.6
10.4 12.3 (93.3%) 69.5 (6.5%) 15.4 (94.2%) 91.4 (5.8%)
Other operating variables kept constant were N ) 4000 rpm, LA ) LB ) 5.0 L/min, [AgNO3] ) 0.10 M, [glucose] ) 0.10 M, and [NaOH] ) 0.07 M. b The particle size distribution had two modes. a
Table 2. Effect of [glucose]/[AgNO3] on the Yield and Particle Size of the Silver Product Produced by the SDR in a Continuous Modea particle size of silver product run no.
[glucose]/ [AgNO3]
yield (%)
no. mean (nm)
080410 080403-1b
1 3
43.9 41.8
8.0 11.7
080403-2b
5
41.3
11.0
vol. mean (nm) 10.4 13.9 (93.3%) 96.1 (6.5%) 19.4 (96.7%) 271.9 (2.9%)
a Other operating variables kept constant were N ) 4000 rpm, LA ) LB ) 5.0 L/min, [AgNO3] ) 0.10 M, [NaOH] ) 0.07 M, and PVP/ AgNO3 ) 1/2 (weight ratio). b The particle size distribution had two modes.
3.1.3. Effects of PVP/AgNO3 and Glucose/AgNO3 Ratios at High AgNO3 Concentration. As mentioned above, a high concentration of PVP would reduce the mixing efficiency, causing a low yield. Therefore, the effect of PVP/AgNO3 weight ratio was studied by varying the PVP/AgNO3 weight ratio from 1/2 to 1/5, while the other operating variables were kept constant as given in Table 1. The experimental results showed that a decrease in the PVP/AgNO3 weight ratio indeed increased the yield by several percent; however, slight agglomeration of primary particles, as judged from the two modes in the particle size distribution, took place at a weight ratio below 1/4, resulting in a larger mean size above 10 nm. This meant that the amount of PVP added in the solution was not enough to disperse all the primary particles for a weight ratio below 1/4. The effect of glucose/AgNO3 concentration ratio on the yield and mean size was also investigated at high AgNO3 concentration. Adjusting the glucose/AgNO3 concentration ratio from 1 to 5 by keeping the other variables constant as indicated in Table 2, the variation in yield between 41.3 and 43.9% was not significant. In addition, the mean size became larger due to slight agglomeration of particles for the concentration ratio above 1. 3.2. Comparison of Experimental Results between PVP and HPMC as Protecting Agent. When PVP was used as the protecting agent, the improvement in production rate was limited by the low yield at high AgNO3 concentrations. Therefore, another protecting agent, HPMC, was tested to compare the experimental results between the two systems at various AgNO3 concentrations between 0.01 and 0.10 M by keeping the other variables fixed, and the experimental results of yield and mean size were presented in Table 3, in which the production rate and the pH value of the outlet streams were also included. Two points should be noted in this table. First, the glucose/AgNO3 concentration ratio was different for the two systems. The previous report indicated that HPMC performed well under a high glucose/AgNO3 concentration ratio over 4; however, PVP performed well under a lower ratio.17 Consequently, the glucose/
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AgNO3 concentration ratio was fixed at 1 for the PVP system and 5 for the HPMC system. Second, the production rate presented in Table 3 referred to the amount of the produced silver particles based on one day’s running of a SDR and was estimated by the following equation. P(kg/day) ) LA(L/min) × 60(min/h) × 24(h/day) × CA((mol of Ag+)/L) × yield((mol of Ag)/ (mol of Ag+)) × 0.107((kg of Ag)/(mol of Ag))
(2)
where P represented the production rate, LA and CA were the feeding rate and the concentration of AgNO3 solution, respectively, and LA was at a maximum value of the system, i.e., 5 L/min. The experimental results, including the yield, mean size, and production rate, were separately compared as follows. 3.2.1. Comparison of the Yield. The yield shown in Table 3 was plotted against the AgNO3 concentration for the two systems, as shown in Figure 5. For the PVP system, the yield gradually decreased from 72.0 to 43.9% when the AgNO3 concentration was varied from 0.01 to 0.10 M. When HPMC was used as the protecting agent, the yield remained about 90.0% for the AgNO3 concentrations between 0.01 and 0.05 M, which was higher than that of the PVP system by 20.0 to 40.0%, but substantially decreased when the AgNO3 concentration was raised from 0.05 to 0.10 M. When the AgNO3 concentration was 0.10 M, the yield was almost the same (ca. 40.0%) for the two systems. The decrease in yield for both systems can be explained as follows. As known from eq 1, the proton was the byproduct of the reduction reaction, and excess protons would retard the reaction rate, thus causing a low yield. In the present case, the AgNO3 concentration was increased under the constraints of fixed glucose/AgNO3 and protecting agent/AgNO3 ratios, and a specific NaOH concentration. It meant that the concentration of the produced proton increased with a rise in AgNO3 concentration, and the concentration of NaOH would be insufficient for neutralizing the produced protons at a specified AgNO3 concentration; consequently, excess protons caused a substantial decrease in the pH value of the mixed streams somewhere in the radial direction on the disk, resulting in the decrease in yield. Another factor that should be taken into consideration was the concentration of the protecting agent. The concentration of the protecting agent was also raised with an increase in AgNO3 concentration, so a high concentration of PVP or HPMC might be another reason that resulted in the low yield at high AgNO3 concentrations. In Figure 5, it should be noted that the trend in the yield change corresponding to the concentration of AgNO3 for the PVP system was quite different from that for the HPMC system. The different behaviors of the two systems can be further explained by the pH value of the outlet streams from the reactor shown in Table 3, which was influenced by two factors, i.e., the amount of the produced proton and the alkalinity of the protecting agent itself. At AgNO3 concentration of 0.01 M, the pH value of the outlet streams was above 12.4 for both the PVP and HPMC systems, because the amount of the produced proton from the reduction reaction was not high enough to cause a sharp drop in pH. When AgNO3 concentration was increased to 0.05 M, the pH value of the outlet streams decreased to 10.0 for the HPMC system, but 6.0 for the PVP system. The difference in outlet pH might be caused by the difference in alkalinity between the two protecting agents. To compare the alkalinities between PVP and HPMC, we dissolved the two protecting agents into water separately at the same dosages as those used in this study, i.e., 0.09-0.85 wt % and then measured the pH value of the aqueous solution at 25 °C. The measured pH value was 8.1-8.4 for
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Table 3. Comparison of the Experimental Results between PVP and HPMC as Protecting Agent at Various AgNO3 Concentrations, Using the SDR in a Continuous Modea run no.
type of protecting agent
080506 080416 080410 080505 080424 080428
PVP PVP PVP HPMC HPMC HPMC
[AgNO3](M)
[glucose]/ [AgNO3]
outlet pH value
yield (%)
vol. mean size (nm)
production rate (kg/day)
0.01 0.05 0.10 0.01 0.05 0.10
1 1 1 5 5 5
12.45 6.03 4.42 12.49 10.01 4.54
72.0 49.7 43.9 90.5 88.6 42.7
8.2 ( 0.204 8.7 ( 0.091 10.5 ( 0.636 15.8 ( 0.403 31.2 ( 1.355 36.6 ( 2.239
5.5 19.1 33.8 7.0 34.1 32.9
a Other operating variables kept constant were N ) 4000 rpm, LA ) LB ) 5 L/min, [NaOH] ) 0.07 M, and protecting agent/AgNO3 ) 1/2 (weight ratio).
Figure 5. Comparison of the yield between PVP and HPMC as protecting agent at various AgNO3 concentrations, using the SDR in a continuous mode. Other operating variables kept constant were N ) 4000 rpm, LA ) LB ) 5.0 L/min, [glucose]/[AgNO3] ) 1 (PVP system), [glucose]/[AgNO3] ) 5 (HPMC system), [NaOH] ) 0.07 M, and protecting agent/AgNO3 ) 1/2 (weight ratio).
Figure 6. Comparison of the mean size between PVP and HPMC as protecting agent at various AgNO3 concentrations, using the SDR in a continuous mode. Other operating variables kept constant were N ) 4000 rpm, LA ) LB ) 5.0 L/min, [glucose]/[AgNO3] ) 1 (PVP system), [glucose]/ [AgNO3] ) 5 (HPMC system), [NaOH] ) 0.07 M, and protecting agent/ AgNO3) 1/2 (weight ratio).
HPMC and 4.1-4.8 for PVP, indicating that HPMC is more basic than PVP. At AgNO3 concentration of 0.05 M, the moles of added sodium hydroxide and produced proton were almost equal in the mixed streams. Possibly, HPMC, as a weak base, somewhat neutralized the produced protons to create a relatively basic environment on the disk, so that a faster reaction rate and thus a high yield near to 90.0% were achieved. On the other hand, the yield dropped to 50.0% using PVP, which is a weak acid, as the protecting agent. When the AgNO3 concentration was 0.10 M, the outlet pH value was 4.4 and 4.5 for the PVP and HPMC systems, respectively. It meant that no matter which protecting agent was used, the produced proton dominated the pH of the outlet streams, resulting in a sharp drop of pH and thus a similar low yield of about 43.0%. 3.2.2. Comparison of the Mean Size. The mean size presented in Table 3 was plotted against the AgNO3 concentration for the two systems, as shown in Figure 6. It should be noted that the protecting agent/AgNO3 ratio was fixed at 1/2 in order to obtain a high yield. The mean sizes of the PVP system were rather independent of the AgNO3 concentration and were below 10 nm. For the HPMC system, the volume mean size varied from 15 to 35 nm when increasing the AgNO3 concentration from 0.01 to 0.10 M. Thus HPMC is a less effective dispersant than PVP for reducing the particle size under the conditions investigated. If we had used a higher ratio of protecting agent/AgNO3, we would have obtained a smaller size for the HPMC system. The protective mechanism of PVP has been discussed in many reports,4,7,9,18 indicating that the individual unit of PVP contains an imide group, and the N and O atoms of this group probably have a strong affinity for silver ions and metallic silver on the surface of silver particles. Due to the strong affinity between PVP and silver particles, a firm
protective layer of PVP forms on the particle surface, inhibiting agglomeration or growth of primary particles by the steric hindrance effect. Although the protective mechanism of HPMC is less understood, the pH of the outlet streams, which decreased with an increase in the AgNO3 concentration, appears to have a different effect on the performance of the protecting agent between PVP and HPMC due to the different chemical nature between the two polymers; thereby, the trend in the change of the mean size corresponding to the concentration of AgNO3 was different between the two systems. 3.2.3. Comparison of the Production Rate. To compare the production rates between the two systems using PVP and HPMC as protecting agents, the yield and mean particle size should be considered. As shown in Table 3, when using PVP as the protecting agent, the silver nanoparticles with a mean size below 10 nm were produced at a rate up to 33.8 kg/day. Nonetheless, the yield was lower than 50.0% for this production rate, implying significant loss of reactants. For the HPMC system, silver nanoparticles with a larger mean size, ca. 30 nm, were obtained at a similar production rate, i.e., 34.1 kg/day; however, a much higher yield of 88.6% was obtained for this production rate. 3.3. Comparison of Experimental Results between Continuous and Recycle Modes. In the previous report, silver nanoparticles were prepared adopting a recycle mode of operation.17 Because the use of PVP as the protecting agent would give the silver product below 10 nm for either operation mode, we examined the experimental results of the PVP system to compare the yield and production rate of the silver product below 10 nm between the two modes. When the experiment was conducted in a recycle mode, the highest yields were 92.0 and 65.8% on the basis of 0.01 and 0.1 M AgNO3 solutions of
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10 L, respectively. The operation time required for 10 L AgNO3 solution in the recycle run was 13 min for either concentration. The production rate was calculated by substituting the above data into the following equation. P(kg/day) ) [CA((mol ofAg+)/L) × 10(L) × yield((mol of Ag)/(mol of Ag+)) × 0.107((kg of Ag)/ 1 1 (mol of Ag))]/ 13(min) × (h/min) × (day/h) (3) 60 24
[
]
The calculated production rate increased from 1.1 to 7.9 kg/ day when increasing the AgNO3 concentration from 0.01 to 0.10 M. On the other hand, the yield and production rate of the PVP system have been presented in Table 3. When the AgNO3 concentration varied from 0.01 to 0.10 M, the yield decreased from 72.0 to 43.9% and the production rate increased from 5.5 to 33.8 kg/day. The experimental results showed that the yield decreased by 30.0%, and the production rate increased to 5-6fold for the continuous operation. The yield decreased with an increase in the AgNO3 concentration for both modes of operation, and a recycle mode gave a higher yield than a continuous mode. The low yield of continuous mode was due to its short retention time; nonetheless, a continuous mode still gave a much higher production rate due to its shorter operation time. 4. Conclusion The SDR operated in a continuous mode was adopted to prepare silver nanoparticles in the presence of different protecting agents. Using PVP as the protecting agent, the effects of operation variables on the particle size and yield of silver product were studied, and the results are as follows: the feeding rate in the adjustable range between 0.8 and 5 L/min having less effect on the size and yield, a rotating speed of 1000 rpm being high enough to produce the silver particles below 10 nm, and a faster speed above 1000 rpm resulting in a higher yield due to the improvement in mixing intensity. To increase the production rate, the concentration of reactant was increased. The increase in AgNO3 concentration caused a significant decrease in yield due to a large amount of proton produced from the reduction reaction to inhibit the reaction rate. Further, the PVP/AgNO3 or glucose/AgNO3 ratio was varied in the experiment, attempting to increase the yield at high AgNO3 concentrations. However, the improvement in the yield was limited at high AgNO3 concentrations. In view of this, the experiment was further conducted using HPMC as protecting agent to compare the experimental results between the two systems. The trends in the changes of yield and mean size behave differently by varying the AgNO3 concentration, due to the different chemical nature between the two protecting agents. In brief, the use of PVP gave smaller silver nanoparticles; however a higher product yield was obtained by using HPMC. As a whole, the continuous mode of operation gave a lower yield as compared to the recycle mode; however, continuous mode gave a much higher production rate, showing the potential for commercialization of the high-gravity process for producing silver nanoparticles with a narrow size distribution. Acknowledgment The authors express their thanks to the Ministry of Economic Affairs of Taiwan for financial support of this work. Nomenclature CA ) concentration of AgNO3 (M) LA ) feeding rate of AgNO3 solution (L/min)
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LB ) feeding rate of glucose solution (L/min) N ) rotating speed of the spinning disk (rpm) P ) production rate (kg/day) Re ) Reynolds number (r2Ω/ν) r ) radial coordinate (cm) Ω ) rotation speed (1/s) ν ) kinematic viscosity of the solution (cm2/s)
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ReceiVed for reView April 08, 2009 ReVised manuscript receiVed August 11, 2009 Accepted August 20, 2009 IE9005645
