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Preparation of silver nanoparticles using the SPG membrane emulsification technique
Journal of Membrane Science 354 (2010) 1–5
Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Rapid communication
Preparation of silver nanoparticles using the SPG membrane emulsification technique Emiri Kakazu, Takuya Murakami, Kazuki Akamatsu ∗, Takashi Sugawara, Ryuji Kikuchi, Shin-ichi Nakao 1 Department of Chemical System Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
a r t i c l e
i n f o
Article history: Received 16 August 2009 Received in revised form 21 December 2009 Accepted 21 February 2010 Available online 1 March 2010 Keywords: Membrane emulsification Nanoparticles SPG membrane Silver Microemulsion
a b s t r a c t Silver nanoparticles with diameters around 10 nm were synthesized successfully using the SPG membrane emulsification technique for the first time. In this method, W/O microemulsions containing silver ions in the water phase were prepared using the SPG membrane emulsification technique and utilized as reaction spaces for the reduction of silver ions. 5–20 nm sized silver nanoparticles with around 15–20% CVs were obtained as a result of the small reaction space of the W/O microemulsion prepared using the SPG membrane emulsification technique. The obtained nanoparticles were characterized using DLS, UV–vis, XRD, and TEM–EDX. In addition, we investigated the effects of the concentration of the aqueous silver nitrate solution and the pore size of the SPG membrane, and the results indicated that we could control the diameter of the nanoparticles by varying such parameters. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Nanoparticles have received great attention because they are one of the key technologies that underlie nanotechnology. They serve as fundamental devices in various fields from electronics and catalysis to medicine and biotechnology. In particular, nanoparticles exhibit unique size-dependent optical and electronic properties because of their size; thus, practical uses in many fields have been proposed and demonstrated [1–6]. In addition, many researchers have proposed preparation methods for size- and shape-controlled nanoparticles [7–11]. These methods are generally classified according to the reaction phase, i.e. gas phase, liquid phase, and solid phase methods. Among these, a liquid phase method known as the reversed micelle method is often employed. In this method, particles are formed in the isolated inner water phase of W/O microemulsions. Today, one of the mainstream methods of preparing emulsions is by mechanical stirring, using a homogenizer or colloid mill. However, it is often recognized that such processes are energy-intensive and that emulsions prepared by such mechanical methods have polydisperse size distributions. Kobayashi et al. [12] used a microfluidizer to prepare W/O emulsions containing soy bean oil in which 5 wt% polyglyc-
∗ Corresponding author. Tel.: +81 3 5841 7300; fax: +81 3 5841 7300. E-mail address: [email protected] (K. Akamatsu). 1 Present address: Department of Environmental and Energy Chemistry, Faculty of Engineering, Kogakuin University, 2665-1 Nakano-machi, Hachioji-shi, Tokyo 192-0015, Japan. 0376-7388/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2010.02.056
erol condensed ricinoleate or tetraglycerol condensed ricinoleate (TGCR) were dissolved in the continuous phase and 5 wt% aqueous glucose solution as the dispersion phase. They obtained polydisperse emulsions with 0.15–0.26 m average diameter and 42–53% coefficients of variation (CVs). In addition, there are also many reports of the synthesis of silver nanoparticles using the reversed micelle method. Zhang et al. [13] synthesized silver nanoparticles of average diameter 3.39 nm and standard deviation 1.26 nm from aqueous silver nitrate solution/dodecane microemulsions using sodium bis-(2-ethylhexyl) sulfosuccinate (AOT) as a surfactant and hydrazine as reducing agent. Zhang et al. [14] also synthesized silver nanoparticles of 6.5 nm average diameter and 3.0 nm standard deviation using a mixture of isoamyl alcohol and cyclohexane as the continuous phase, and aqueous silver nitrate solution as the dispersion phase, with sodium dodecyl sulfate as the surfactant, and hydrazine as the reducing agent. Egorova and Revina [15] synthesized silver nanoparticles using n-octane or n-heptane as the continuous phase, and aqueous silver nitrate solution as the dispersion phase, with AOT as the surfactant, and quercetin (natural plant pigment from the group of flavonoids) as the reducing agent, and estimated the particle radius to be 1–1.5 nm according to Mie theory. However, there are few studies concerning continuous operation in order to meet increasing demand for nanoparticles in the near future. In one of the few examples, Kawa et al. [16] used a continuous flow reactor to prepare monodisperse CdSe nanoparticles. In this study, we propose preparation of nanoparticles using the membrane emulsification technique that enables both the preparation of monodisperse microemulsions in a continuous operation.
2
E. Kakazu et al. / Journal of Membrane Science 354 (2010) 1–5
By this method, the aqueous dispersion phase can be pressurized into the continuous phase through a porous membrane, resulting in the formation of emulsions. For example, Xu et al. [17] used mullite (3Al2 O3 ·2SiO2 ) ceramic membrane, and Yamazaki et al. [18] used PTFE membrane to obtain W/O emulsions. It should be noted that we can obtain monodisperse emulsion droplets when we use a membrane with narrow pore size distribution. There are two well-known membranes with narrow pore size distributions: anodic porous alumina membrane and Shirasu porous glass (SPG) membrane. There are reports of preparation of monodisperse emulsions using anodic porous alumina membrane by Yanagishita et al. [19], and the SPG membrane emulsification technique by Shimizu et al. [20]. The emulsions obtained were monodisperse, with sizes controlled in the range of 1–40 m. As a result of the superior strength of the SPG membrane relative to the anodic porous alumina membrane, the SPG membrane is more suitable for membrane emulsification. This is because the membrane needs pressure resistance for preparation of smaller sized emulsions. In principle, the size of the emulsions is determined by the pore size of the membrane used in these membrane emulsification methods and higher pressures are necessary to prepare smaller emulsions using membranes with smaller pore sizes. In addition, this method is also suitable for continuous production because it is a membrane operation. The SPG membrane emulsification technique, therefore, has the capability of manufacturing monodisperse nanoparticles. However, no research has been conducted on the preparation of smaller monodisperse emulsions containing salt ions with much lower concentration, probably because of the instability of such emulsions. Additionally no study of nanoparticle synthesis using the SPG membrane emulsification method has been reported. In this study, we demonstrate a novel method of preparing metal nanoparticles using the SPG membrane emulsification technique.
Silver nanoparticles were selected for demonstration. First, the W/O microemulsions containing silver aqueous solutions were prepared, using the SPG membrane emulsification. Then, after adding hydrazine, silver nanoparticles were formed in each W/O microdroplet. Here we report the size distributions of microemulsion droplets and nanoparticles obtained using dynamic laser scattering (DLS) using this method. In addition, we characterized the obtained nanoparticles using UV–vis spectroscopy and X-ray diffraction (XRD) analysis. Morphology of the nanoparticles was characterized by transmission electron microscope and energy dispersive X-ray spectroscopy (TEM–EDX). 2. Material and methods 2.1. Materials Kerosene, hydrazine, and silver nitrate were purchased from Wako Pure Chemical Industries Ltd., Japan. TGCR was kindly provided by Sakamoto Yakuhin Kogyo Co., Japan. All chemicals were used without further purification. 2.2. Preparation of W/O microemulsions by SPG membrane emulsification Monodisperse W/O microemulsions containing silver nitrate were first prepared using the SPG membrane emulsification method. Fig. 1 shows a conceptual diagram for the synthesis of monodisperse silver nanoparticles using the SPG membrane emulsification technique. An external pressure type micro kit (SPG Techno Co. Ltd., Miyazaki, Japan) was employed as the membrane emulsification apparatus. Two different hydrophobic SPG membranes (tubular type, ∅ 1 cm, length 20 mm, SPG Techno Co. Ltd.)
Fig. 1. Conceptual diagram of SPG membrane emulsification and nanoparticle preparation. (1) SPG membrane, (2) continuous phase, (3) dispersed phase, (4) N2 gas, (5) magnetic stirrer, and (6) stirring bar.
E. Kakazu et al. / Journal of Membrane Science 354 (2010) 1–5
with 200 nm and 300 nm pore sizes, respectively, were used. To prepare the continuous phase, kerosene containing 0.75 wt% of TGCR was mixed with the same amount of distilled water and left at rest for two weeks to saturate with water [21]. Before the emulsification step, an SPG membrane was dipped into the continuous phase and subjected to ultrasonic treatment in order to wet the membrane with the continuous phase. A volume of 1–2 mL aqueous silver nitrate solution with concentration of 2.1–200 mol/m3 was pressurized into a volume of 30–80 mL continuous phase using nitrogen gas. The pressure at which the emulsion formed on the surface of the SPG membrane was 0.2–0.3 MPa, and membrane emulsification began with stirring. The stirring speed was 250 rpm. The size and size distribution of the microemulsion droplets were measured by DLS (Zetasizer Nano ZS90, Malvern Instruments Ltd., UK). 2.3. Preparation and characterization of silver nanoparticles Silver nanoparticles were synthesized by dropwise addition of hydrazine to the emulsion and subsequent reduction of inner silver ions. The resulting mixture was stirred at 250 rpm for two days in order to complete the reduction reaction. The obtained nanoparticles were characterized using TEM–EDX (JEM-2010F, JEOL Ltd., Japan) and UV–vis (V630, JASCO Corporation, Japan) analysis. The crystalline structure of the samples was investigated by XRD (RINT 2400, Rigaku Corp., Japan). 3. Results and discussion 3.1. Preparation of W/O microemulsions containing silver nitrate and silver nanoparticles W/O microemulsions were prepared from kerosene containing 0.75 wt% of TGCR and aqueous silver nitrate solution, using the 200 nm SPG membrane. The concentration of silver nitrate in the emulsions was 20 mol/m3 . This result is noteworthy because there have been few reports concerning the preparation of microemulsions using such dilute aqueous solutions, and such small pore size membranes, until now. The saturation treatment was conducted to enable emulsification using such dilute ion concentrations [21]. These microemulsions were stable for at least one day. As soon as the microemulsions were prepared, hydrazine was added dropwise using a Pasteur pipette. As to the formation of the nanoparticles, at first reduction of inner silver ions was expected to be occurred by hydrazine, followed nucleation and growth of the silver nanoparticles. Upon stirring, the mixture gradually changed color to clear yellow. This change in coloration implied the formation of silver nanoparticles. To clarify the quantitative relationship between microemulsion droplet size and nanoparticle size, we measured the sizes of the microemulsion droplets and the nanoparticles using DLS. Fig. 2 shows the size distributions of microemulsion droplets and nanoparticles. The W/O microemulsions and nanoparticles had average diameters of 212 nm and 10 nm, respectively, with CV of 20% and 17%. Generally, it is said to be difficult to prepare W/O microemulsions using dilute aqueous solutions in membrane emulsification. This is because when the ion concentration in the aqueous phase is low a large capacity to dissolve the dispersion phase in the continuous phase helps to form a gel layer near the surface of the membrane [21]. In comparison with previous studies, our aqueous solution was much less concentrated. Based on our results, we have proven that small nanoparticles can be prepared using fine emulsions with dilute silver solutions. However, the CVs were rather large for SPG membrane emulsification technique. In general, the CV of the emulsions prepared by this technique is
3
Fig. 2. Size distributions of W/O microemulsion droplets (solid line) and silver nanoparticles (dotted line) measured by DLS.
reported to be in the region of 10%. Additionally in the case of the SPG membrane emulsification, it is often said that the droplet size of the emulsion is proportional to the pore size of the SPG membrane and that the proportional constant is around 3. On the other hand, the droplet size was less than twice as the pore size of the SPG membrane in our case. However it should be noted that these reports were based on the experimental data with larger pore sizes and that there are few studies on the relationship between droplet size and pore size of the SPG membrane with smaller pores than 500 nm. Furthermore, according to the few studies [21], the proportional constant became less than 2 in the case of smaller pore size of the SPG membrane. These are consistent with our results. Further studies should be conducted to discuss this issue, because there are few studies about it. Supposing that one nanoparticle was formed from one microemulsion droplet, the diameter of the nanoparticle was easily predicted by considering the mass balance of silver in a microemulsion, according to the following equation:
3
4 y 3 2
=
Dw 4 3 2
3
CM,
(1)
where is the density of solid silver, y is the diameter of the silver nanoparticle, Dw is the diameter of the W/O microemulsion droplet, C is the concentration of silver ion, and M is the molar mass of silver. Under our experimental conditions, the particle diameter was predicted to be 13 nm, while the average diameter measured by DLS was 10 nm, as mentioned above. Thus, we consider that the assumption of one nanoparticle per droplet is appropriate in such dilute systems and this enables us to design the particle size by controlling the concentration of silver ions and the pore size of the SPG membrane. 3.2. Characterization of the obtained nanoparticles It has been reported that Ag4 + clusters are formed in the early stage of Ag+ ion reduction [22,23] and that these clusters can exist for many hours in air. Therefore, there was a possibility that the obtained nanoparticles were not silver nanoparticles, but actually silver ion clusters. To identify the composition of the nanoparticles, we conducted UV–vis measurements. Fig. 3 shows the UV–vis absorption spectra. An obvious absorption peak at 420 nm was observed and attributed to the surface plasmon resonance of silver nanoparticles [24]. On the other hand, there was no peak observed at about 265 nm, where Ag4 + clusters have an absorption peak [14]. Thus, it was confirmed that the obtained particles were silver nanoparticles and not silver clusters or dust. Identification of the crystalline structure by XRD was also performed, and the XRD pattern of the obtained nanoparticles indicated that they had silver crystalline structure. Based on these results, the nanoparticles were crystalline silver.
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Fig. 3. UV–vis spectrum of silver nanoparticles with 10 nm average diameter, prepared using the SPG membrane emulsification technique.
Fig. 5. Size distribution of W/O microemulsion droplets (solid line) and silver nanoparticles (dotted line) obtained by DLS measurement for (a) sample 2 and (b) sample 3.
of organic substances, apparently the surfactant. The TGCR would adhere onto the surface of the silver nanoparticles. The existence of surfactant was also observed by FT-IR spectroscopy. Thus, we can say that silver nanoparticles were successfully prepared using the SPG membrane emulsification technique. This method may be applied to the preparation of other metal nanoparticles, such as gold. 3.3. Effect of AgNO3 concentration and pore size of SPG membrane We prepared silver nanoparticles under three different conditions, as shown in Table 1, in order to determine the effects of silver nitrate concentration and pore size of the SPG membrane. In the Table 1, Pc is the critical pressure. SPG membranes with two different pore sizes (200 nm and 300 nm) and three different concentrations (2.1 mol/m3 , 20 mol/m3 , and 200 mol/m3 ) of silver nitrate were used. Sample 1 was the same as the sample discussed in Sections 3.1 and 3.2. Fig. 5 shows the size distributions of W/O microemulsion droplets and silver nanoparticles obtained by DLS for (a) sample 2 and (b) sample 3. These results are also summarized in Table 1. As shown in Fig. 5 and Table 1, both the W/O microemulsions and the nanoparticles showed the similar CVs as the sample (1), shown in Figure 2. UV–vis and XRD measurements also characterized the particles as silver nanoparticles. We successfully synthesized silver nanoparticles under the conditions of
Fig. 4. TEM image of silver nanoparticles with 10 nm average diameter, prepared using the SPG membrane emulsification technique.
The analyses mentioned above investigated only the behavior or features of the particles as a group, and do not inform us about the morphology of the particles themselves. Therefore, we conducted TEM–EDX analysis to understand the morphology of individual nanoparticles. Fig. 4 shows a TEM image of the obtained nanoparticles. Silver nanoparticles with diameters around 10 nm were observed, consistent with the DLS measurement results shown in Fig. 2. It seemed that the obtained silver nanoparticles were polycrystalline because of the clear electron diffraction. EDX analysis was also conducted and showed that the nanoparticles contained C and O in addition to Ag. This strongly suggested the existence
Table 1 Experimental conditions for the preparation of W/O microemulsions using the SPG membrane emulsification technique and the emulsion diameters, and average silver nanoparticle diameters and coefficients of variation. Sample no.
C [mol/m3 ]
Dm [nm]
Pc [kPa]
Dw [nm]
Predicted size [nm]
Measured average diameter [nm]
CV [%]
1 2 3
20 2.1 200
200 300 300
252 263 258
212 302 377
13 8.4 48
10 11 14
17 15 20
E. Kakazu et al. / Journal of Membrane Science 354 (2010) 1–5
samples 2 and 3. In other words, we can say that it was possible to control the average diameter of the nanoparticles by changing the concentration of silver nitrate and the pore size of the SPG membrane. For samples 1 and 2, the predicted size of the nanoparticles based on Eq. (1) and the measured size were nearly the same. Therefore, the assumption mentioned in Section 3.1 was considered valid. For sample 3, the measured size was smaller than the predicted size. This was probably because several nanoparticles were formed in each microemulsion droplet. The difference among samples 1, 2, and 3 was the concentration of silver nitrate. It appeared that single nanoparticles were synthesized in each W/O microemulsion droplet only with smaller concentrations of silver nitrate, allowing us to predict the average diameter of the nanoparticles prepared by this method. We are now investigating the mechanism for nanoparticle formation in W/O microemulsions prepared using the SPG membrane emulsification technique; especially the effect of silver ion concentration. 4. Conclusions We report the first preparation of silver nanoparticles using the SPG membrane emulsification technique. In this novel preparation method, W/O microemulsions prepared by the SPG membrane emulsification technique were utilized as reaction spaces for nanoparticle synthesis. The obtained nanoparticles were around 10 nm in diameters, with 15–20% CVs, and crystalline sphere-like shape. These CV values were rather large for the SPG membrane emulsification technique, however there are few reports on preparation of microemulsions by using SPG membranes with smaller pores and this difficulty may be one of the reasons. We also synthesized nanoparticles after varying the concentration of silver nitrate and the SPG membrane pore size, the results of which indicated that we could control the particle size by changing these two parameters. This method may be applicable to the preparation of other metal nanoparticles, such as gold. Moreover, it is worthwhile to further develop this method for large scale production. Acknowledgments TGCR was kindly supplied by Sakamoto Yakuhin Kogyo Co., Japan. A part of this work, TEM–EDX, was conducted in Center for Nano Lithography & Analysis, the University of Tokyo, supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. We also acknowledge partial financial support from the Kato Foundation for Promotion of Science. References [1] M. Turner, V.B. Golovko, O.P.H. Vaughan, P. Abdulkin, A. Berenguer-Murcia, M.S. Tikhov, B.F.G. Johnson, R.M. Lambert, Selective oxidation with dioxygen by gold nanoparticle catalysts derived from 55-atom clusters, Nature 454 (2008) U981–U1031.
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[2] P. Liu, Y.L. Wu, Y.N. Li, B.S. Ong, S.P. Zhu, Enabling gate dielectric design for all solution-processed, high-performance, flexible organic thin-film transistors, J. Am. Chem. Soc. 128 (2006) 4554–4555. [3] S.J. Park, T.A. Taton, C.A. Mirkin, Array-based electrical detection of DNA with nanoparticle probes, Science 295 (2002) 1503–1506. [4] C.J. Meledandri, J.K. Stolarczyk, S. Ghosh, D.F. Brougham, Nonaqueous magnetic nanoparticle suspensions with controlled particle size and nuclear magnetic resonance properties, Langmuir 24 (2008) 14159–14165. [5] S. Bhattacharjee, D.M. Dotzauer, M.L. Bruening, Selectivity as a function of nanoparticle size in the catalytic hydrogenation of unsaturated alcohols, J. Am. Chem. Soc. 131 (2009) 3601–3610. [6] S. Maenosono, T. Suzuki, S. Soita, Superparamagnetic FePt nanoparticles as excellent MRI contrast agents, J. Magn. Mater. 320 (2008) L79–L83. [7] T. Vassilieff, A. Sutton, A.K. Kakkar, Shape control in silver metal nanoparticle construction using dumb-bell dendrimers, J. Mater. Chem. 18 (2008) 4031–4033. [8] C.H. Walker, J.V. St. John, P. Wisian-Neilson, Synthesis and size control of gold nanoparticles stabilized by poly(methylphenylphosphazene), J. Am. Chem. Soc. 123 (2001) 3846–3847. [9] P.D. Jadzinsky, G. Calero, C.J. Ackerson, D.A. Bushnell, R.D. Kornberg, Structure of a thiol monolayer-protected gold nanoparticle at 1.1 Å resolution, Science 318 (2007) 430–433. [10] G.A. Somorjai, J.Y. Park, Colloid science of metal nanoparticle catalysts in 2D and 3D structures. Challenges of nucleation, growth, composition, particle shape, size control and their influence on activity and selectivity, Top. Catal. 49 (2008) 126–135. [11] W.Z. Zhang, X.L. Qiao, J.G. Chen, Q.Y. Chen, Self-assembly and controlled synthesis of silver nanoparticles in SDS quaternary microemulsion, Mater. Lett. 62 (2008) 1689. [12] I. Kobayashi, X.F. Lou, S. Mukataka, M. Nakajima, Preparation of monodisperse water-in-oil-in-water emulsions using microfluidization and straight-through microchannel emulsification, J. Am. Oil Chem. Soc. 82 (2005) 65–71. [13] W.Z. Zhang, X.L. Qiao, J.G. Chen, H.G. Wang, Preparation of silver nanoparticles in water-in-oil AOT reverse micelle, J. Colloid Interface Sci. 302 (2006) 370–373. [14] W.Z. Zhang, X.L. Qiao, J.G. Chen, Formation of silver nanoparticles in SDS inverse microemulsions, Mater. Chem. Phys. 109 (2008) 411–416. [15] E.M. Egorova, A.A. Revina, Synthesis of metallic nanoparticles in reverse micelles in the presence of quercetin, Colloid Surface A: Physicochem. Eng. Aspects 168 (2000) 87–96. [16] M. Kawa, H. Morii, A. Ioku, S. Saita, K. Okuyama, Large-scale production of CdSe nanocrystal by a continuous flow reactor, J. Nanopart. Res. 5 (2003) 81– 85. [17] J. Wu, W.H. Jing, W.H. Xing, N.P. Xu, Preparation of W/O emulsions by membrane emulsification with a mullite ceramic membrane, Desalination 193 (2006) 381–386. [18] N. Yamazaki, K. Naganuma, M. Nagai, G.H. Ma, S. Omi, Preparation of W/O (water-in-oil) emulsions using a PTFE (polytetrafluoroethylene) membrane—a new emulsification device, J. Disper. Sci. Technol. 24 (2003) 249– 257. [19] T. Yanagishita, Y. Tomabechi, K. Nishio, H. Masuda, Preparation of monodisperse SiO2 nanoparticles by membrane emulsification using ideally ordered anodic porous alumina, Langmuir 20 (2004) 554–555. [20] T. Nakashima, M. Shimizu, M. Kukizaki, Particle control of emulsion by membrane emulsification and its applications, Adv. Drug Deliv. Rev. 45 (2000) 47–56. [21] M. Shimizu, T. Nakajima, M. Kukizaki, Preparation of W/O emulion by membrane emulsification and optimum condition for its monodispersion, Kagaku Kougaku Ronbun. 28 (2001) 310–316, in Japanese. [22] C. Petit, P. Lixon, M.P. Pileni, In-situ synthesis of silver nanocluster in AOT reverse micelles, J. Phys. Chem. 97 (1993) 12974–12983. [23] T. Linnert, P. Mulvaney, A. Henglein, H. Weller, Long-lived nonmetallic silver clusters in aqueous-solution—preparation and photolysis, J. Am. Chem. Soc. 112 (1990) 4657–4664. [24] M. Kowshik, S. Ashtaputre, S. Kharrazi, W. Vogel, J. Urban, S.K. Kulkarni, K.M. Paknikar, Extracellular synthesis of silver nanoparticles by a silver-tolerant yeast strain MKY3, Nanotechnology 14 (2003) 95–100.
Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Rapid communication
Preparation of silver nanoparticles using the SPG membrane emulsification technique Emiri Kakazu, Takuya Murakami, Kazuki Akamatsu ∗, Takashi Sugawara, Ryuji Kikuchi, Shin-ichi Nakao 1 Department of Chemical System Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
a r t i c l e
i n f o
Article history: Received 16 August 2009 Received in revised form 21 December 2009 Accepted 21 February 2010 Available online 1 March 2010 Keywords: Membrane emulsification Nanoparticles SPG membrane Silver Microemulsion
a b s t r a c t Silver nanoparticles with diameters around 10 nm were synthesized successfully using the SPG membrane emulsification technique for the first time. In this method, W/O microemulsions containing silver ions in the water phase were prepared using the SPG membrane emulsification technique and utilized as reaction spaces for the reduction of silver ions. 5–20 nm sized silver nanoparticles with around 15–20% CVs were obtained as a result of the small reaction space of the W/O microemulsion prepared using the SPG membrane emulsification technique. The obtained nanoparticles were characterized using DLS, UV–vis, XRD, and TEM–EDX. In addition, we investigated the effects of the concentration of the aqueous silver nitrate solution and the pore size of the SPG membrane, and the results indicated that we could control the diameter of the nanoparticles by varying such parameters. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Nanoparticles have received great attention because they are one of the key technologies that underlie nanotechnology. They serve as fundamental devices in various fields from electronics and catalysis to medicine and biotechnology. In particular, nanoparticles exhibit unique size-dependent optical and electronic properties because of their size; thus, practical uses in many fields have been proposed and demonstrated [1–6]. In addition, many researchers have proposed preparation methods for size- and shape-controlled nanoparticles [7–11]. These methods are generally classified according to the reaction phase, i.e. gas phase, liquid phase, and solid phase methods. Among these, a liquid phase method known as the reversed micelle method is often employed. In this method, particles are formed in the isolated inner water phase of W/O microemulsions. Today, one of the mainstream methods of preparing emulsions is by mechanical stirring, using a homogenizer or colloid mill. However, it is often recognized that such processes are energy-intensive and that emulsions prepared by such mechanical methods have polydisperse size distributions. Kobayashi et al. [12] used a microfluidizer to prepare W/O emulsions containing soy bean oil in which 5 wt% polyglyc-
∗ Corresponding author. Tel.: +81 3 5841 7300; fax: +81 3 5841 7300. E-mail address: [email protected] (K. Akamatsu). 1 Present address: Department of Environmental and Energy Chemistry, Faculty of Engineering, Kogakuin University, 2665-1 Nakano-machi, Hachioji-shi, Tokyo 192-0015, Japan. 0376-7388/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2010.02.056
erol condensed ricinoleate or tetraglycerol condensed ricinoleate (TGCR) were dissolved in the continuous phase and 5 wt% aqueous glucose solution as the dispersion phase. They obtained polydisperse emulsions with 0.15–0.26 m average diameter and 42–53% coefficients of variation (CVs). In addition, there are also many reports of the synthesis of silver nanoparticles using the reversed micelle method. Zhang et al. [13] synthesized silver nanoparticles of average diameter 3.39 nm and standard deviation 1.26 nm from aqueous silver nitrate solution/dodecane microemulsions using sodium bis-(2-ethylhexyl) sulfosuccinate (AOT) as a surfactant and hydrazine as reducing agent. Zhang et al. [14] also synthesized silver nanoparticles of 6.5 nm average diameter and 3.0 nm standard deviation using a mixture of isoamyl alcohol and cyclohexane as the continuous phase, and aqueous silver nitrate solution as the dispersion phase, with sodium dodecyl sulfate as the surfactant, and hydrazine as the reducing agent. Egorova and Revina [15] synthesized silver nanoparticles using n-octane or n-heptane as the continuous phase, and aqueous silver nitrate solution as the dispersion phase, with AOT as the surfactant, and quercetin (natural plant pigment from the group of flavonoids) as the reducing agent, and estimated the particle radius to be 1–1.5 nm according to Mie theory. However, there are few studies concerning continuous operation in order to meet increasing demand for nanoparticles in the near future. In one of the few examples, Kawa et al. [16] used a continuous flow reactor to prepare monodisperse CdSe nanoparticles. In this study, we propose preparation of nanoparticles using the membrane emulsification technique that enables both the preparation of monodisperse microemulsions in a continuous operation.
2
E. Kakazu et al. / Journal of Membrane Science 354 (2010) 1–5
By this method, the aqueous dispersion phase can be pressurized into the continuous phase through a porous membrane, resulting in the formation of emulsions. For example, Xu et al. [17] used mullite (3Al2 O3 ·2SiO2 ) ceramic membrane, and Yamazaki et al. [18] used PTFE membrane to obtain W/O emulsions. It should be noted that we can obtain monodisperse emulsion droplets when we use a membrane with narrow pore size distribution. There are two well-known membranes with narrow pore size distributions: anodic porous alumina membrane and Shirasu porous glass (SPG) membrane. There are reports of preparation of monodisperse emulsions using anodic porous alumina membrane by Yanagishita et al. [19], and the SPG membrane emulsification technique by Shimizu et al. [20]. The emulsions obtained were monodisperse, with sizes controlled in the range of 1–40 m. As a result of the superior strength of the SPG membrane relative to the anodic porous alumina membrane, the SPG membrane is more suitable for membrane emulsification. This is because the membrane needs pressure resistance for preparation of smaller sized emulsions. In principle, the size of the emulsions is determined by the pore size of the membrane used in these membrane emulsification methods and higher pressures are necessary to prepare smaller emulsions using membranes with smaller pore sizes. In addition, this method is also suitable for continuous production because it is a membrane operation. The SPG membrane emulsification technique, therefore, has the capability of manufacturing monodisperse nanoparticles. However, no research has been conducted on the preparation of smaller monodisperse emulsions containing salt ions with much lower concentration, probably because of the instability of such emulsions. Additionally no study of nanoparticle synthesis using the SPG membrane emulsification method has been reported. In this study, we demonstrate a novel method of preparing metal nanoparticles using the SPG membrane emulsification technique.
Silver nanoparticles were selected for demonstration. First, the W/O microemulsions containing silver aqueous solutions were prepared, using the SPG membrane emulsification. Then, after adding hydrazine, silver nanoparticles were formed in each W/O microdroplet. Here we report the size distributions of microemulsion droplets and nanoparticles obtained using dynamic laser scattering (DLS) using this method. In addition, we characterized the obtained nanoparticles using UV–vis spectroscopy and X-ray diffraction (XRD) analysis. Morphology of the nanoparticles was characterized by transmission electron microscope and energy dispersive X-ray spectroscopy (TEM–EDX). 2. Material and methods 2.1. Materials Kerosene, hydrazine, and silver nitrate were purchased from Wako Pure Chemical Industries Ltd., Japan. TGCR was kindly provided by Sakamoto Yakuhin Kogyo Co., Japan. All chemicals were used without further purification. 2.2. Preparation of W/O microemulsions by SPG membrane emulsification Monodisperse W/O microemulsions containing silver nitrate were first prepared using the SPG membrane emulsification method. Fig. 1 shows a conceptual diagram for the synthesis of monodisperse silver nanoparticles using the SPG membrane emulsification technique. An external pressure type micro kit (SPG Techno Co. Ltd., Miyazaki, Japan) was employed as the membrane emulsification apparatus. Two different hydrophobic SPG membranes (tubular type, ∅ 1 cm, length 20 mm, SPG Techno Co. Ltd.)
Fig. 1. Conceptual diagram of SPG membrane emulsification and nanoparticle preparation. (1) SPG membrane, (2) continuous phase, (3) dispersed phase, (4) N2 gas, (5) magnetic stirrer, and (6) stirring bar.
E. Kakazu et al. / Journal of Membrane Science 354 (2010) 1–5
with 200 nm and 300 nm pore sizes, respectively, were used. To prepare the continuous phase, kerosene containing 0.75 wt% of TGCR was mixed with the same amount of distilled water and left at rest for two weeks to saturate with water [21]. Before the emulsification step, an SPG membrane was dipped into the continuous phase and subjected to ultrasonic treatment in order to wet the membrane with the continuous phase. A volume of 1–2 mL aqueous silver nitrate solution with concentration of 2.1–200 mol/m3 was pressurized into a volume of 30–80 mL continuous phase using nitrogen gas. The pressure at which the emulsion formed on the surface of the SPG membrane was 0.2–0.3 MPa, and membrane emulsification began with stirring. The stirring speed was 250 rpm. The size and size distribution of the microemulsion droplets were measured by DLS (Zetasizer Nano ZS90, Malvern Instruments Ltd., UK). 2.3. Preparation and characterization of silver nanoparticles Silver nanoparticles were synthesized by dropwise addition of hydrazine to the emulsion and subsequent reduction of inner silver ions. The resulting mixture was stirred at 250 rpm for two days in order to complete the reduction reaction. The obtained nanoparticles were characterized using TEM–EDX (JEM-2010F, JEOL Ltd., Japan) and UV–vis (V630, JASCO Corporation, Japan) analysis. The crystalline structure of the samples was investigated by XRD (RINT 2400, Rigaku Corp., Japan). 3. Results and discussion 3.1. Preparation of W/O microemulsions containing silver nitrate and silver nanoparticles W/O microemulsions were prepared from kerosene containing 0.75 wt% of TGCR and aqueous silver nitrate solution, using the 200 nm SPG membrane. The concentration of silver nitrate in the emulsions was 20 mol/m3 . This result is noteworthy because there have been few reports concerning the preparation of microemulsions using such dilute aqueous solutions, and such small pore size membranes, until now. The saturation treatment was conducted to enable emulsification using such dilute ion concentrations [21]. These microemulsions were stable for at least one day. As soon as the microemulsions were prepared, hydrazine was added dropwise using a Pasteur pipette. As to the formation of the nanoparticles, at first reduction of inner silver ions was expected to be occurred by hydrazine, followed nucleation and growth of the silver nanoparticles. Upon stirring, the mixture gradually changed color to clear yellow. This change in coloration implied the formation of silver nanoparticles. To clarify the quantitative relationship between microemulsion droplet size and nanoparticle size, we measured the sizes of the microemulsion droplets and the nanoparticles using DLS. Fig. 2 shows the size distributions of microemulsion droplets and nanoparticles. The W/O microemulsions and nanoparticles had average diameters of 212 nm and 10 nm, respectively, with CV of 20% and 17%. Generally, it is said to be difficult to prepare W/O microemulsions using dilute aqueous solutions in membrane emulsification. This is because when the ion concentration in the aqueous phase is low a large capacity to dissolve the dispersion phase in the continuous phase helps to form a gel layer near the surface of the membrane [21]. In comparison with previous studies, our aqueous solution was much less concentrated. Based on our results, we have proven that small nanoparticles can be prepared using fine emulsions with dilute silver solutions. However, the CVs were rather large for SPG membrane emulsification technique. In general, the CV of the emulsions prepared by this technique is
3
Fig. 2. Size distributions of W/O microemulsion droplets (solid line) and silver nanoparticles (dotted line) measured by DLS.
reported to be in the region of 10%. Additionally in the case of the SPG membrane emulsification, it is often said that the droplet size of the emulsion is proportional to the pore size of the SPG membrane and that the proportional constant is around 3. On the other hand, the droplet size was less than twice as the pore size of the SPG membrane in our case. However it should be noted that these reports were based on the experimental data with larger pore sizes and that there are few studies on the relationship between droplet size and pore size of the SPG membrane with smaller pores than 500 nm. Furthermore, according to the few studies [21], the proportional constant became less than 2 in the case of smaller pore size of the SPG membrane. These are consistent with our results. Further studies should be conducted to discuss this issue, because there are few studies about it. Supposing that one nanoparticle was formed from one microemulsion droplet, the diameter of the nanoparticle was easily predicted by considering the mass balance of silver in a microemulsion, according to the following equation:
3
4 y 3 2
=
Dw 4 3 2
3
CM,
(1)
where is the density of solid silver, y is the diameter of the silver nanoparticle, Dw is the diameter of the W/O microemulsion droplet, C is the concentration of silver ion, and M is the molar mass of silver. Under our experimental conditions, the particle diameter was predicted to be 13 nm, while the average diameter measured by DLS was 10 nm, as mentioned above. Thus, we consider that the assumption of one nanoparticle per droplet is appropriate in such dilute systems and this enables us to design the particle size by controlling the concentration of silver ions and the pore size of the SPG membrane. 3.2. Characterization of the obtained nanoparticles It has been reported that Ag4 + clusters are formed in the early stage of Ag+ ion reduction [22,23] and that these clusters can exist for many hours in air. Therefore, there was a possibility that the obtained nanoparticles were not silver nanoparticles, but actually silver ion clusters. To identify the composition of the nanoparticles, we conducted UV–vis measurements. Fig. 3 shows the UV–vis absorption spectra. An obvious absorption peak at 420 nm was observed and attributed to the surface plasmon resonance of silver nanoparticles [24]. On the other hand, there was no peak observed at about 265 nm, where Ag4 + clusters have an absorption peak [14]. Thus, it was confirmed that the obtained particles were silver nanoparticles and not silver clusters or dust. Identification of the crystalline structure by XRD was also performed, and the XRD pattern of the obtained nanoparticles indicated that they had silver crystalline structure. Based on these results, the nanoparticles were crystalline silver.
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Fig. 3. UV–vis spectrum of silver nanoparticles with 10 nm average diameter, prepared using the SPG membrane emulsification technique.
Fig. 5. Size distribution of W/O microemulsion droplets (solid line) and silver nanoparticles (dotted line) obtained by DLS measurement for (a) sample 2 and (b) sample 3.
of organic substances, apparently the surfactant. The TGCR would adhere onto the surface of the silver nanoparticles. The existence of surfactant was also observed by FT-IR spectroscopy. Thus, we can say that silver nanoparticles were successfully prepared using the SPG membrane emulsification technique. This method may be applied to the preparation of other metal nanoparticles, such as gold. 3.3. Effect of AgNO3 concentration and pore size of SPG membrane We prepared silver nanoparticles under three different conditions, as shown in Table 1, in order to determine the effects of silver nitrate concentration and pore size of the SPG membrane. In the Table 1, Pc is the critical pressure. SPG membranes with two different pore sizes (200 nm and 300 nm) and three different concentrations (2.1 mol/m3 , 20 mol/m3 , and 200 mol/m3 ) of silver nitrate were used. Sample 1 was the same as the sample discussed in Sections 3.1 and 3.2. Fig. 5 shows the size distributions of W/O microemulsion droplets and silver nanoparticles obtained by DLS for (a) sample 2 and (b) sample 3. These results are also summarized in Table 1. As shown in Fig. 5 and Table 1, both the W/O microemulsions and the nanoparticles showed the similar CVs as the sample (1), shown in Figure 2. UV–vis and XRD measurements also characterized the particles as silver nanoparticles. We successfully synthesized silver nanoparticles under the conditions of
Fig. 4. TEM image of silver nanoparticles with 10 nm average diameter, prepared using the SPG membrane emulsification technique.
The analyses mentioned above investigated only the behavior or features of the particles as a group, and do not inform us about the morphology of the particles themselves. Therefore, we conducted TEM–EDX analysis to understand the morphology of individual nanoparticles. Fig. 4 shows a TEM image of the obtained nanoparticles. Silver nanoparticles with diameters around 10 nm were observed, consistent with the DLS measurement results shown in Fig. 2. It seemed that the obtained silver nanoparticles were polycrystalline because of the clear electron diffraction. EDX analysis was also conducted and showed that the nanoparticles contained C and O in addition to Ag. This strongly suggested the existence
Table 1 Experimental conditions for the preparation of W/O microemulsions using the SPG membrane emulsification technique and the emulsion diameters, and average silver nanoparticle diameters and coefficients of variation. Sample no.
C [mol/m3 ]
Dm [nm]
Pc [kPa]
Dw [nm]
Predicted size [nm]
Measured average diameter [nm]
CV [%]
1 2 3
20 2.1 200
200 300 300
252 263 258
212 302 377
13 8.4 48
10 11 14
17 15 20
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samples 2 and 3. In other words, we can say that it was possible to control the average diameter of the nanoparticles by changing the concentration of silver nitrate and the pore size of the SPG membrane. For samples 1 and 2, the predicted size of the nanoparticles based on Eq. (1) and the measured size were nearly the same. Therefore, the assumption mentioned in Section 3.1 was considered valid. For sample 3, the measured size was smaller than the predicted size. This was probably because several nanoparticles were formed in each microemulsion droplet. The difference among samples 1, 2, and 3 was the concentration of silver nitrate. It appeared that single nanoparticles were synthesized in each W/O microemulsion droplet only with smaller concentrations of silver nitrate, allowing us to predict the average diameter of the nanoparticles prepared by this method. We are now investigating the mechanism for nanoparticle formation in W/O microemulsions prepared using the SPG membrane emulsification technique; especially the effect of silver ion concentration. 4. Conclusions We report the first preparation of silver nanoparticles using the SPG membrane emulsification technique. In this novel preparation method, W/O microemulsions prepared by the SPG membrane emulsification technique were utilized as reaction spaces for nanoparticle synthesis. The obtained nanoparticles were around 10 nm in diameters, with 15–20% CVs, and crystalline sphere-like shape. These CV values were rather large for the SPG membrane emulsification technique, however there are few reports on preparation of microemulsions by using SPG membranes with smaller pores and this difficulty may be one of the reasons. We also synthesized nanoparticles after varying the concentration of silver nitrate and the SPG membrane pore size, the results of which indicated that we could control the particle size by changing these two parameters. This method may be applicable to the preparation of other metal nanoparticles, such as gold. Moreover, it is worthwhile to further develop this method for large scale production. Acknowledgments TGCR was kindly supplied by Sakamoto Yakuhin Kogyo Co., Japan. A part of this work, TEM–EDX, was conducted in Center for Nano Lithography & Analysis, the University of Tokyo, supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. We also acknowledge partial financial support from the Kato Foundation for Promotion of Science. References [1] M. Turner, V.B. Golovko, O.P.H. Vaughan, P. Abdulkin, A. Berenguer-Murcia, M.S. Tikhov, B.F.G. Johnson, R.M. Lambert, Selective oxidation with dioxygen by gold nanoparticle catalysts derived from 55-atom clusters, Nature 454 (2008) U981–U1031.
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