Silver Nanoparticle Formation in Microemulsions Acting Both as

A novel method of making silver nanoparticles in water-in-oil microemulsions using the surfactants as both the reducing agent and as the structure-dir...
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Langmuir 2005, 21, 11387-11396

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Silver Nanoparticle Formation in Microemulsions Acting Both as Template and Reducing Agent Martin Andersson,*,† Jan Skov Pedersen,‡ and Anders E. C. Palmqvist†,§ Department of Materials and Surface Chemistry, Competence Centre for Catalysis, Chalmers University of Technology, SE-412 96 Go¨ teborg, Sweden, and Department of Chemistry and iNANO Interdisciplinary Nanoscience Center, University of Aarhus, DK-8000 Aarhus, Denmark Received April 8, 2005. In Final Form: September 14, 2005 A novel method of making silver nanoparticles in water-in-oil microemulsions using the surfactants as both the reducing agent and as the structure-directing agent is presented. Since no external strong reducing agent is used the kinetics of the formation is slow, which makes it possible to study the silver nanoparticle formation in situ. The microemulsions used were based on either the nonionic surfactant Brij30 (C12E4), which reduces the silver ion to metallic silver and is thereby partly oxidized, or mixtures of Brij30 and AOT (sodium bis(2-ethylhexyl) sulfosuccinate, where the latter does not reduce the silver ions. The influences of silver ion and nonionic surfactant concentrations on the formation kinetics of the nanoparticles were followed in situ using UV-vis spectroscopy, and both parameters were found to have a big influence. The microemulsion droplet’s size, size distribution, and shape were examined by small-angle X-ray scattering (SAXS), and the formed silver nanoparticles were studied using both transmission electron microscopy and SAXS. The SAXS measurements showed that the presence of silver nitrate does not affect the microemulsion systems noticeably and that the droplet’s size and shape are retained during the particle formation. It is shown that the size and morphology of the particles do not directly follow the shape and size of the microemulsion droplets even though there is a relation between the droplet size and the radii of the formed particles.

Introduction During the past decades, the formation of nanoparticles has gained considerable attention. This is partly due to the high specific surface area of nanoparticles, which is important, for example, when utilizing them as catalytic materials or sensors. In addition, if the size of the particle is reduced to the same length scale (a few nanometers) as that of the mean-free path of the electrons in the material they exhibit properties that differ from those of the bulk materials of the same composition.1 This so-called quantum size effect has an influence on their optic, electronic, magnetic, catalytic, and photochemical properties,2-5 and hence, new materials with new properties can be envisioned. It is not only the size of the particles that influences the material’s characteristics but also the morphology,1,6 and also the relative spatial arrangement of the nanoparticles7-10 may be of great importance. To exploit these effects calls for new methods for the formation of nanoparticles and the ability to tailor make their size and shape. * Corresponding author. Phone: +46(0)31-7725611. Fax: +46(0)31-160062. E-mail: [email protected]. † Department of Materials and Surface Chemistry, Chalmers University of Technology. ‡ University of Aarhus, DK-8000 Aarhus, Denmark. § Competence Centre for Catalysis, Chalmers University of Technology. (1) El-Sayed, M. A. Acc. Chem. Res. 2001, 34, 257. (2) Maier, S. A. Adv. Mater. 2001, 13, 1501. (3) Kamat, P. V. J. Phys. Chem. B 2002, 106, 7729. (4) Valden, M.; Lai, X.; Goodman, D. W. Science 1998, 281, 1647. (5) Binns, C. Surf. Sci. Rep. 2001, 44, 1. (6) Wang, Z. L. J. Phys. Chem. B 2000, 104, 1153. (7) Sampaio, J. F.; Beverly, K. C.; Heath, J. R. J. Phys. Chem. B 2001, 105, 8797. (8) Henrichs, S.; Collier, C.; Saykally, R.; Shen, Y.; Heath, J. R. J. Am. Chem. Soc. 2000, 122, 4077. (9) Qi, L.; Gao, Y.; Ma, J. Colloids Surf., A 1999, 157, 285. (10) Taleb, A.; Petit, C.; Pileni, M. P. J. Phys. Chem. B 1998, 102, 2214.

Silver nanoparticles are of great interest because of their use in applications such as surface-enhanced Raman spectroscopy11,12 and catalysis.13,14 Different approaches have been utilized for the formation of silver nanoparticles including formation in microemulsions,15,16 thermal decomposition,17 Langmuir-Blodgett films,18,19 biosynthesis using biological microorganisms,20 and the use of liquid crystalline phases made of surfactant aggregates,9,21 etc. A microemulsion is a thermodynamically stable mixture of oil, water, and surfactant (sometimes also a cosurfactant). It is a macroscopically homogeneous mixture, but microscopically consisting of water or oil aggregates surrounded by a monolayer of surfactants.22 Pileni and co-workers16,23 have used the water-in-oil microemulsion technique for the formation of silver nanoparticles and for inducing their self-organization into 2D lattices. They showed that the particle radius is directly proportional to the size of the microemulsion water droplets, which in turn is determined by the molar ratio of water-to(11) Tian, Z. Q.; Ren, B.; Wu, D. Y. J. Phys. Chem. B 2002, 106, 9463. (12) Champion, A.; Kambhampati, P. Chem. Soc. Rev. 1998, 27, 241. (13) Pradhan, N.; Pal, A.; Pal, T. Colloids Surf., A 2002, 196, 247. (14) Nakatsuji, H.; Nakai, H.; Ikeda, K.; Yamamoto, Y. Surf. Sci. 1997, 384, 315. (15) Zhang, Z. Q.; Patel, R. C.; Kothari, R.; Johnson, C. P.; Friberg, S. E.; Aikens, P. A. J. Phys. Chem. B 2000, 104, 1176. (16) Petit, C.; Lixon, P.; Pileni, M. P. J. Phys. Chem. B 1993, 97, 12974. (17) Xue, B.; Chen, P.; Hong, O.; Lin, J. Y.; Tan, K. L. J. Mater. Chem. 2001, 11, 2378. (18) Manna, A.; Imae, T.; Iida, M.; Hisamatsu, N. Langmuir 2001, 17, 6000. (19) Henrichs, S. E.; Sample, J. L.; Shiang, J. J.; Heath, J. R. J. Phys. Chem. B 1999, 103, 3524. (20) Ahmad, A.; Mukherjee, P.; Senapati, S.; Mandal, D.; Khan, M. I.; Kumar, R.; Sastry, M. Colloids Surf., B 2003, 28, 313. (21) Andersson, M.; Alfredsson, V.; Kjellin, P.; Palmqvist, A. Nano Lett. 2002, 2, 1403. (22) Jo¨nsson, B.; Lindman, B.; Holmberg, K.; Kronberg, B. Surfactants and Polymers in Aqueous Solution; Wiley: 1998. (23) Taleb, A.; Petit, C.; Pileni, M. P. Chem. Mater. 1997, 9, 950.

10.1021/la050937j CCC: $30.25 © 2005 American Chemical Society Published on Web 10/20/2005

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surfactant (W ) [H2O]/[surfactant]). Their approach has been to ion exchange Na(AOT) (sodium bis(2-ethylhexyl) sulfosuccinate) to Ag(AOT) and prepare microemulsions using Ag(AOT) and subsequently reduce the silver ions by addition of either NaBH4 or N2H4 to form metallic silver. Basically, their procedure is to form two identical waterin-oil microemulsions with respect to water, oil, and surfactant concentrations, one of them containing the silver ion exchanged surfactant (Ag(AOT)) and one containing the reducing agent in a Na(AOT)-based microemulsion. Upon mixing of the two microemulsions, crystalline nanoparticles of metallic silver are formed. The formation is influenced by the high dynamics in the microemulsion, which results in the exchange of matter and the exposure of silver ions to the reducing agent. This method produces spherical silver nanoparticles with a particle size between 2 and 6 nm in diameter depending on W. Even though the formation of nanoparticles with the use of water-in-oil microemulsions has been utilized for the formation of a large number of materials, the mechanism of the particle formation is still not understood in detail. The fact that the droplet size has an effect on the formed particle size indicates that the water droplet acts as a template, which hinders the growth of the nanoparticles beyond the surfactant boundary. Interestingly, the actual size of the water droplets is not the same as those of the formed particles, and the particle size differs depending on, e.g., the substance formed and on the concentration of the inorganic component.24 Further, it is uncertain if the shape of the droplets influences the shape of the formed particles. Recently, Filankembo et al.25 published a paper on the influence of anions on the morphology of nanoparticles formed in microemulsions. Their method of forming the particles is based on the reduction with hydrazine, which is added to a Cu(AOT) microemulsion, and the Cu particles form rapidly due to the high exchange rate between the droplets. They have been able to make nanoparticles with shapes of nanorods and cubes with the use of the same water-in-oil microemulsions but with different salts and salt concentrations added. They reached the conclusion that small amounts of anions such as chloride and bromide influence the particle shape of copper nanoparticles and that the cause for the formation of different particle morphologies probably is due to impurities inside the colloidal systems and not the actual droplet shape. The study presented by Zhang et al.15 suggests that the size and size distribution of silver nanoparticles made by the microemulsion technique are dependent on the formation and aggregation kinetics of small silver clusters. The authors showed that the mixing order is of great importance for the formation of silver nanoparticles, i.e., how the blending of the silver ion and reducing agent is achieved. Recently, Lo´pez-Quintela published a nice review on the formation mechanisms and growth control of nanomaterials synthesized in microemulsions.26 Here he points out the importance of the dynamics of the microemulsion systems and emphasizes that most of the reactions studied for the formation of nanoparticles have been done on systems where the kinetics of the chemical reactions that are carried out are much faster than the droplet’s material interchange rate. In these cases the particle formation is controlled by mass transport, and the formation is hence (24) Pileni, M. P. Nat. Mater. 2003, 2, 145. (25) Filankembo, A, G. S.; Lisiecki, I.; Pileni, M. P. J. Phys. Chem. B 2003, 107, 7492. (26) Lo´pez-Quintela, M. A. Curr. Opin. Colloid. Inteface Sci. 2003, 8, 137.

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governed by the droplet collision and coalescence properties of the microemulsion. If instead the particle formation is slower than the microemulsion droplet interchange dynamics information regarding the reaction kinetics and the influence of the microemulsion droplet size and shape are obtained when studying the particle formation. Many fewer studies have been performed according to this latter case, and they often concern more complicated reactions such as, e.g., the hydrolysis and condensation reactions of alkoxides during the formation of silica particles.27 Liz-Marza´n et al. showed that Ag+ ions could be reduced in ethanol when certain surfactants were present and that the most efficient surfactant for the reduction was a nonionic ethoxylated surfactant.28 It was further shown that the reduction rate of silver ions using this technique is dependent on the silver concentration in a first-order fashion.28 The reduction of the silver ions is associated with an oxidation of the hydrophilic oxyethylene groups of the surfactant mainly into aldehydes.21,29 In a recent study the use of nonionic surfactants for the reduction of silver ions in microemulsions as well as in liquid crystalline phases was examined with a focus on how the surfactants are affected by the presence of silver ions.30 Here we present a microemulsion-based method for the formation of silver nanoparticles, where we utilize the dual functionality of a nonionic surfactant as both the template and the reducing agent. Due to its slow kinetics of forming the nanoparticles this method is suitable for studying the particle formation inside the water droplets in situ by UV-vis spectroscopy. Since the reducing agent used is the surfactant in the microemulsion the silver particle formation is not influenced by the dynamics of the microemulsion, which makes it possible to study, specifically, the droplet’s templating effect on the particles and their formation kinetics. The UV-vis spectroscopy provides information on the formation kinetics as well as an indirect measure of the size and size distribution of the silver nanoparticles. With the use of different ratios of nonionic surfactant and the ionic surfactant AOT (which does not give rise to silver reduction), the influence of the surfactant concentration is examined. The SAXS technique is used for determining the droplet size, size distribution, shape, and the dependence on the ratio of the two surfactants. In addition, the silver nanoparticle size is determined. TEM is also used for studying the size of the final silver particles and their morphology. Experimental Section Materials. AgNO3 (99.995% purity) and n-heptane (99%, HPLC grade) were purchased from Aldrich. Brij30 (polyoxyethylene 4 lauryl ether) (in short termed C12E4) and AOT (sodium bis(2-ethylhexyl) sulfosuccinate, 99%) were purchased from Sigma. All chemicals were used as received, and the water used was double distilled. Particle Synthesis. The silver particles were produced in a water-in-oil microemulsion consisting of n-heptane as the continuous oil phase and Brij30 or mixtures of Brij30 and Na(AOT) as the surfactant. The compositions used were 20 wt % surfactant or mixture of surfactants, 5 wt % water solution, and 75 wt % n-heptane. Microemulsions containing different weight ratios of Brij30 to AOT were studied, and mixtures with [Brij30]/ ([Brij30] + [AOT]) ) 1, 0.75, 0.5, 0.25, and 0 were prepared. Stirring the surfactant and adding the AgNO3 solution and the (27) Osseo-Asare, K.; Arriagada, F. J. J. Colloid Interface Sci. 1999, 218, 68. (28) Liz-Marzan, L. M.; Lado-Tourino, T. I. Langmuir 1996, 12, 3585. (29) Bodin, A.; Shao, L. P.; Nilsson, J. L. G.; Karlberg, A. T. Contact Dermatitis 2001, 44, 207. (30) Currie, F.; Andersson, M.; Holmberg, K. Langmuir Lett. 2004, 20, 3835.

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n-heptane resulted in the transparent microemulsions. The molar weight of Brij30 is approximately 362 g/mol, and the molar weight of AOT approximately 444.6 g/mol, which gives a W (water-tosurfactant molar ratio) of 5 if only Brij 30 is used and a W of 6 if only AOT is used. The aqueous solution contained 1, 2, or 3 wt % AgNO3. All reactions were carried out at room temperature and in darkness. Experimental Techniques. UV-vis absorption spectra of silver-containing water-in-oil microemulsions were measured using a GBC 920 UV-vis double-beam spectrometer. The reference used was a microemulsion with the same composition as the sample but without silver added. No dilution of the samples was necessary, and the measurements were performed in a 1 mm quartz cuvette at room temperature. The SAXS measurements were performed on the pinhole SAXS instrument at the University of Aarhus.31 The instrument is a commercially available small-angle X-ray camera (NanoStar, Bruker AXS), which is modified and optimized for solution scattering and adapted to a rotating anode X-ray source (Cu KR). The instrument has an integrated vacuum in order to reduce background. The instrument covers a range of scattering vectors q between 0.008 and 0.35 Å-1 (q ) (4π/λ) sin θ, where 2θ is the scattering angle and λ ) 1.542 Å, the wavelength). The silver-particle-containing microemulsions were placed in a 1.8 mm diameter quartz glass capillary, and the data were collected for 1/2 h using a positionsensitive gas detector (HiSTAR). All measurements were made at room temperature. The SAXS data were azimuthally averaged, corrected for variations in detector efficiency and for spatial distortions. A measurement on pure water was used as background and was subtracted from the data from the microemulsions. Finally, the data were converted to absolute scale using the scattering from pure water as a primary standard.32 To investigate the final size and size distribution of the synthesized Ag nanoparticles, the microemulsion components have to be removed. Therefore, the solutions were diluted by a factor of about 10 with ethanol and left for at least a day to let the silver particles sediment. The liquid was subsequently carefully removed. This procedure was repeated 4 times. The final solutions with Ag particles in a small amount of ethanol were dried so that only the silver Ag particles were left. The SAXS measurements were performed on these dried powders supported on Scotch tape. A background measurement on a piece of tape was subtracted from the data on the powders. TEM was performed on a Philips CM120 Bio TWIN and on a conventional JEOL 1200EXII microscope both operated at an accelerating voltage of 120 kV. A droplet of the microemulsion after the reaction was put on a holey carbon copper grid and dried at room temperature before insertion into the microscope.

Results and Discussion UV-Vis Spectroscopy. The optical properties of colloidal silver particles require the application of Mie theory.33 If the diameter of the particles is less than 20 nm the absorption in the UV range is only dependent on the dipole term in the Mie summation.34 For a dispersion of N particles per unit volume the absorption (A) can be expressed as

A ) CNl/2.303 where C is the absorption cross section and l is the optical path length. The absorption cross section can be written as

C ) 18πV(ω)2/λ{(1(ω) + 2m)2 + 2(ω)2} where V is the spherical particle volume, λ is the incident wavelength, and ω is the corresponding frequency. The (31) Pedersen, J. S. J. Appl. Crystallogr. 2004, 37, 369. (32) Orthaber, D.; Bergmann, A.; Glatter, O. J. Appl. Crystallogr. 2000, 33, 218. (33) Mie, G. Ann. Phys. 1908, 25, 377. (34) Greighton, J. A.; Eaton, D. G. J. Chem. Soc., Faraday Trans. 1991, 87, 3881.

Figure 1. Absorption spectra of silver nanoparticles synthesized in situ in a water-in-oil microemulsion containing 75% heptane, 20% Brij30, and 5% water solution with the AgNO3 concentrations of (a) 2%, (b) 3%, and (c) 4%. A spectrum was collected every hour.

complex relative permittivity of the metal is (ω) ) 1(ω) + i2(ω), which has a maximum in absorption at 1(ω) ) -2m(ω). The wavelength of the resonance is given by the wavelength dependence of 1(ω), and the width and the intensity is given by the 2(ω). In the case of metallic particles 1(ω) is a linear function of the frequency. The bandwidth of the absorption peak and the inverse of the peak height are proportional to 2(ω). When the diameter of the silver particles is smaller than the electron meanfree path (52 nm for silver) the 2(ω) is dependent on the particle size.15,16 In this region an increase in particle size will result in a more narrow absorption peak and higher peak intensity. In fact, there is a linear relationship between the half-width of the absorption peak and the reciprocal of the particle diameter. Further, the position and the number of absorption peaks give information of the particle’s morphology. For example, a spherical particle

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Figure 2. Absorption at 420 nm as a function of time for silver nanoparticle formation followed in situ in a water-in-oil microemulsion containing 75% heptane, 20% Brij30, and 5% water solution with the AgNO3 concentration being 2%, 3%, and 4%. The solid lines are fits by a first-order reaction, and the given k values are the first-order rate constants.

will give rise to one peak, whereas an ellipsoidal particle will give rise to two peaks.34 These optical properties make UV spectroscopy a suitable method for following the nanoparticle formation as it gives information of both the kinetics and the morphology of the formed particles. In our case the kinetics is relatively slow (on the order of hours), so it is possible to study the reaction in situ without the need to use stop flow or similar techniques. Figure 1a-c shows the UV absorption spectra for the silver formation as it occurs in the Brij30 microemulsion and its dependence on the AgNO3 concentration. The observed peak at approximately 420 nm corresponds well with that of silver nanoparticles.15,16 A scan was made every hour. From these it can be seen that no significant difference between the absorption spectra shapes exists at the final stage, hence, no difference in the morphology and size of the formed silver particles was observed. The figure shows an increase in intensity and that the peak position shifts slightly to lower wavelengths as the reduction proceeds, which has been observed in similar studies.16 Figure 2 shows the absorption at the absorption maximum (420 nm) as a function of time illuminating the difference in kinetics as the AgNO3 concentration is varied. From these data it can be seen that if the concentration is increased the growth rate of the silver particles is more rapid. For a 2 wt % AgNO3 solution it takes approximately 24 h to complete the formation, and for a 4 wt % AgNO3 solution it takes about 12 h. The solid lines in the figure are fits to the first-order rate equation, and the obtained rate constants (k) are given in the figure. These constants are on the same order as those reported by Liz-Marza´n et al., who studied the silver reduction rate using surfactants as reducing agents when dissolved in ethanol.28 This observation reveals that the reduction of silver ions is not effected by the presence of the microemulsion but is a direct result of the reducing agent, i.e., the surfactant and its concentration. To study the influence of the surfactant (Brij30) concentration microemulsions consisting of 75/25 and 50/50 mixtures of Brij30 and AOT surfactants, respectively, with the same composition of oil and water were prepared. Figure 3a,b shows the absorption spectra of the mixed surfactant microemulsion containing 2 wt % AgNO3, with a scan collected every 24 h for the two different mixtures of surfactants, respectively.

Figure 3. Absorption spectra of a water-in-oil microemulsion containing 75% heptane, 20% surfactant mixtures, and 5% water (2% AgNO3); (a) shows the spectra for the 25% AOT and 75% Brij30 mixture and (b) shows the spectra for the 50% AOT and 50% Brij30 mixture. A spectrum was collected every 24 h with the first scan collected after 24 h from the time that the microemulsions were prepared.

Figure 4. Absorption maximum vs time for microemulsions containing different amounts of silver nitrate and Brij30 surfactant.

When compared with the Brij30-only surfactant-containing microemulsion system (Figure 1a) it can be deduced that the formed particles in the mixed surfactant system are smaller in size (broader peak) and that the kinetics is drastically slowed. For the case of the 50/50 surfactant mixture it takes approximately 20 days for completion. In a corresponding system with only AOT no silver reduction is observed during this time. Figure 4 shows the wavelength for the maximum absorption as a function of time for only Brij30-containing microemulsions with different amounts of silver as well as for one microemulsion containing the 50/50 Brij30/

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Figure 5. SAXS data (points) for microemulsions containing 20 wt % Brij30, 75 wt % heptane, and 5 wt % water solution containing (a) 4 wt % AgNO3, (b) 2 wt % AgNO3 solution, or (c) no AgNO3 compared to the SAXS data of microemulsions where different amounts of Brij30 are substituted by AOT: (d) 25 wt %, (e) 50 wt %, (f) 75 wt %, and (g) 100 wt % AOT. The fits (curves) are from a polydisperse hard-sphere model with ellipsoidal core-shell particles (droplets).

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Table 1. Results from the Fit of the Polydisperse Hard-Sphere Model with Core-Shell Ellipsoidal Particlesa 〈R〉 Z  s Frel Vdroplet Rsca1

〈R〉 Z  s Frel Vdroplet Rsca1

[Å] [Å]

[Å] [Å]

25% AOT

50% AOT

75% AOT

100% AOT

27.8 ( 0.34 39.0 ( 1 1.79 ( 0.05 4.40 ( 0.05 0.691 ( 0.02 0.0815 ( 0.006 1.22 ( 0.02

21.84 ( 0.13 20.8 ( 0.9 1.57 ( 0.04 4.67 ( 0.07 0.60 ( 0.01 0.0752 ( 0.0005 1.22 ( 0.02

20.39 ( 0.3 24.5 ( 1.0 1.92 ( 0.07 4.03 ( 0.07 0.59 ( 0.02 0.101 ( 0.007 1.09 ( 0.01

17.4 ( 2.1 31.3 ( 19 1.60 ( 0.5 3.1 ( 0.6 0.439 ( 0.28 0.056 ( 0.08 0.96 ( 0.2

4 wt % AgNO3

2 wt % AgNO3

0 wt % AgNO3 100% Brij30)

35.1 ( 0.7 39.0 ( 2.4 1.90 ( 0.10 5.54 ( 0.12 0.98 ( 0.02 0.0795 ( 0.009 1.19 ( 0.03

35.0 ( 0.8 38.54 ( 2.2 1.94 ( 0.11 5.57 ( 0.12 1.10 ( 0.06 0.0786 ( 0.01 1.17 ( 0.03

35.0 ( 0.6 38.00 ( 1.8 1.88 ( 0.08 5.60 ( 0.10 0.96 ( 0.03 0.0851 ( 0.008 1.18 ( 0.02

a 〈R〉 is the average particle size, Z is the polydispersity parameter of the Schultz distribution,  is the axis ratio of the prolate ellipsoids, Vdroplet is the volume fraction of water and surfactant head group, and Rsca1 is the scale factor between particle radius and hard-sphere interaction radius.

Figure 6. Number size distribution of the droplets as a function of radii (R) for a water-in-oil microemulsion consisting of 20 wt % Brij30, 75 wt % heptane, and 5 wt % aqueous solution of either a 2 wt % AgNO3 solution or a 4 wt % AgNO3 solution showing no difference in size between these droplet solutions. Also, the number size distributions of microemulsions where different amounts of Brij30 are substituted by AOT are shown for 25 wt %, 50 wt %, 75 wt %, and 100 wt % AOT. The distributions are given for equivalent water-core radii of the ellipsoidal core-shell particles of a polydisperse hard-sphere model fitted to the SAXS data. Full curve: pure Brij30 droplets without AgNO3. Long dashes: pure Brij30 droplets with 2 wt % AgNO3. Medium dashes: pure Brij30 droplets with 4 wt % AgNO3. (The curves for pure Brij30 droplets are almost coinciding). Short dashes: droplets with 25 wt % AOT. Dotted curve: droplets with 50 wt % AOT. Dashed-dotted curve: droplets with 75 wt % AOT. Dashed-double-dotted curve: droplets with 100 wt % AOT.

AOT surfactant mixture. Here it is seen that all the different compositions of microemulsions give rise to a blue shift and that the final wavelength for the absorption maximum is dependent on the initial silver ion concentration. This phenomenon is believed to come from the fact that the particles are not entirely spherical and that not only the particle size but also the particle size distribution change during the particle formation process. An interesting result is that the microemulsion containing 50% AOT has its maximum absorption almost at the same final absorption maximum as that of the 100% Brij30, when containing the same amount of silver (2 wt %).

Small-Angle X-ray Scattering. Microemulsions. SAXS measurements were carried out on a series of microemulsions with different AOT/Brij30 ratios. The results shown in Figure 5 are similar to those found in previous studies of microemulsions for systems with droplets, which are polydisperse in size.35 To extract structural information from the data, a model has to be fitted to the data. The relatively high concentration of water and surfactant (5 wt % water and 20 wt % surfactant) makes it necessary to include interparticle interaction effects in the model for the SAXS data. To describe these effects, the polydisperse hard-sphere model in which the scattering is calculated within the Percus-Yevick approximation was employed. The expressions were taken from the work of Vrij.36 The model is the same as that used by Arleth and Pedersen35 in an extensive contrast variation study of AOT microemulsions. In agreement with this work, it turned out that the data could only be modeled satisfactorily if the droplets were assumed to be anisotropic. Interparticle interference effects influence the low-q part of the recorded data, and in order to model this part satisfactorily, the size distribution has to be relatively narrow. However, the high-q part of the data cannot be modeled for the same size distribution as the data are more smeared than predicted by the model. Smearing of the form factor minima of the droplets can be the result of either polydispersity or deviation from the spherical shape. Therefore, an ellipsoidal model was used for the droplets form factor. We note that shape fluctuations of the droplets are quite fast compared to the translational diffusion of the droplets, and therefore the interparticle interference is only influenced by the average droplets shape. In the model34 the droplets are taken as ellipsoids of revolutions, all with the same axis ratio . In addition, the droplets were modeled as core-shell particles for which the water core and the headgroup shell is included, whereas the surfactant tail region is disregarded. The excess scattering length density is proportional to the electron density difference between the component of the microemulsion and the surrounding oil phase. The electron density of the water is higher than that of the oil resulting in a positive excess scattering length density in the water core region. The headgroup region of the surfactant (shell) has an even higher electron density, whereas the surfactant tails have an electron density very close to that of the oil. The ratio (35) Arleth, L.; Pedersen, J. S. Phys. Rev. E 2001, 63. (36) Vrij, A. J. Chem. Phys. 1979, 71, 3267.

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Figure 7. SAXS data of silver nanoparticles made by microemulsions with different amounts of AOT (a) 0 wt %, (b) 25 wt %, (c) 50 wt %, and (d) 75 wt % AOT. The model uses a Schultz distribution of spheres with an additional q-4 contribution and a constant background. Table 2. Radius of Gyration as Determined from the SAXS Measurement on the Formed Ag Nanoparticles Made from Microemulsions Consisting of Different Amounts of AOT Surfactants

Figure 8. Number size distribution as a function of radii (R) for the formed Ag particles made from microemulsions consisting of different ratios of AOT to Brij30. The particle size increases as the amount of Brij30 is increased.

of the scattering length density of the core relative to that in the shell is given in Table 1 as Frel. The headgroup shell thickness was fixed at δhead ) 5 Å. A finite width of the interface, s, in the droplets was introduced by multiplying the form factor by a factor exp(-s2q2/2). The size distribution was taken as a Schultz distribution. The R values are equivalent radii of spheres with the same volume as those of the ellipsoids. The excess scattering length densities are not known accurately, and therefore the data were not fitted on an absolute scale. The interaction radii of the droplets were taken as a constant Rscal times the equivalent sphere radius R. Since the tails of the surfactants are not included in the structural model the volume fraction Vdroplet

AOT concn %

Rgyration (Å)

0 25 50 75

62.0 51.1 32.4 27.6

determined in the fit is the volume fraction of only the water and the headgroup of the surfactant molecules. However, the presence of the tails is reflected in the interaction radius, and thus Rscal is larger than unity. The following model parameters were fitted: for describing the particle structure, the average radius 〈R〉, the polydispersity factor Z of the Schultz distribution, the interface width s, the axis ratio of the particles , and the ratio of the scattering length density in the core relative to that in the headgroup region Frel; for describing the interaction effects, the volume fraction of the droplets Vdroplet and the scale factor between the radius and the hard-sphere interaction radius Rscal. The latter influences the curve at low q, whereas the structural parameters influence the curve at intermediate and high q. The optimum values of the parameters were well defined in the fits demonstrating that the number of fit parameters is reasonable. The fits are excellent (Figure 5a-g) except for the 100% AOT sample (discussed below). The results (Table 1) are consistent and vary systematically with the composition. The data are thus in agreement with a prolate shape with an axis ratio of about 1.8 ( 0.1, which is fairly independent of the AOT content. In a previous detailed contrast

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Figure 9. TEM micrographs of the larger population of silver. The particles were formed by microemulsions containing (a) 0 wt %, (b) 25 wt %, (c) 50 wt %, and (d) 75 wt % AOT of the total surfactant concentration.

variation study of pure AOT microemulsions,34 an axis ratio of 1.6 ( 0.1 was determined; however, these droplets had a much higher water-to-surfactant ratio (W ) 38) compared to the ratio of W ) 5-6 for the present droplets. W ) 38 is close to emulsification failure, so those droplets have a maximum swelling, which is not the case for W ) 5-6. It is therefore quite reasonable that the axis ratio is larger for the lower values of W. Figure 6 displays the number size distribution as a function of droplet radii, and from this it is easily seen that the droplet size decreases as the AOT amount is increased. The average radius of the pure Brij droplets is 39 Å, whereas it decreases to about 17 Å for the pure AOT micelles (see Table 1). The decrease in droplet size with increasing AOT fraction is in agreement with earlier studies made by other techniques.37,38 The relative width of the size distribution is σ ) 1/(Z + 1)1/2. For the droplets without AOT, σ ) 0.16, whereas it is slightly larger (σ ≈ 0.22) for the droplets with AOT. For pure AOT droplets with W ) 38 a value of σ ≈ 0.16 has previously been determined, and the present values are in good agreement with this. The interface of the droplets has a width of about 5 Å independent of AOT content, which is slightly larger than that found previously (about 3 Å) for pure AOT droplets.34 The volume fraction Vdroplet is about 8%, which is quite reasonable, since it corresponds to the volume fraction of both the water (5 wt (37) Naza´rio, L. M. M.; Crespo, J. P. S. G.; Holzwarth, J. F.; Hatton, T. A. Langmuir 2000, 16, 5892. (38) Naza´rio, L. M. M.; Hatton, T. A.; Crespo, J. P. S. G. Langmuir 1996, 12, 6326.

%) and the surfactant headgroups. The interaction radius is found to be about 1.2 times the droplet radius (of the water and headgroup region), in agreement with the observation that the hydrocarbon tails of the surfactant molecules have to be included when considering the interaction radius. From the table it can finally be deduced that the silver concentration (2% or 4%) does not influence the size and shape of the water droplets and that it is the same as that without any salt added. The prolate ellipsoidal model does not fit well the data for the pure AOT microemulsion (Figure 5g). The increase in the data at low q indicates that the particles most likely are real cylinders. There are several indications in the literature that a change in morphology takes place at low values of W. For example, the surfactant aggregation number39 follows a different scaling below W ) 20 and approaches a constant value below W ) 10, and the percolation volume fraction for conduction and the viscosity display an anomaly40 around W ) 6-8. These observations can be explained by the droplets becoming elongated at low W values. In addition SAXS41 and SANS42 studies of AOT microemulsions with, respectively, brine instead of water and Co counterions support our conclusion that the droplets might be cylindrical in the vicinity of W ) 6-8. (39) Cabos, C.; Delord, P. J. Appl. Crystallogr. 1979, 12, 502. (40) Peyrelasse, J.; Moha-Ouchane, M.; Boned, C. Phys. Rev. A 1988, 38, 4155. (41) Hirai, M.; Kawai-Hirai, R.; Yabuki, A.; Takizawa, T.; Hirai, T.; Kobayashi, K.; Amemiya, Y.; Oya, M. J. Phys. Chem. B 1996, 99, 6652. (42) Eastoe, J.; Steytler, D. C.; Robinson, B. H.; Heenan, R. K.; North, N. A.; Dore, C. J. J. Chem. Soc., Faraday Trans. 1994, 90, 2497.

Silver Nanoparticle Formation in Microemulsions

Figure 10. TEM micrographs of the smaller population of silver nanoparticles. The particles were formed by microemulsions containing (a) 0 wt %, (b) 25 wt %, (c) 50 wt %, and (d) 75 wt % AOT of the total surfactant concentration.

Ag Particles. The SAXS data for the formed silver nanoparticles are shown in Figure 7. In the log-log representation, it can be observed that the data nearly follows a q-4 behavior. However, there are small bumps on the data, which carry information on the size of the particles. The bump moved to higher q values for increasing amounts of AOT, suggesting that the particles are smaller for higher AOT content. The q-4 behavior at low q can have two origins: either it is due to an additional population of quite large particles with R > 300 Å or it is due to aggregation of the smaller particles into larger aggregates also with R > 300 Å. The TEM micrographs (Figures 9 and 10) show that there are in fact two populations of quite different sizes in the samples. We expect the smaller particles, which give rise to the bump, to be polydisperse in size, and this is included in the model using a Schultz distribution for the size. The smaller particles can have a quite high concentration in the powder, and therefore correlation effects can be present in the SAXS data. However, the presence of the q-4 contribution at low q makes it impossible to observe these effects, and they were therefore not included in the model. In addition to the distribution of spheres, the model contained a q-4 contribution and a constant to describe

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residual background in the data. The fits are shown in Figure 7 and follow the measured data very well. The number size distributions are plotted in Figure 8 as a function of particle radii, and a large decrease in the particle size is observed as the AOT amount is increased. The large polydispersity makes it difficult to compare the size distributions with the size distributions of the microemulsions. Therefore, the gyration radius was calculated using the particle size distributions according to Rg2 ) 3〈R8〉/(5〈R6〉), where 〈Rn〉 is the nth moment of the size distributions. In Table 2 the gyration radii of the particles are shown, and they confirm that the particles decrease in size as the AOT concentration is increased. It should also be noted that the Rg values are much larger than the radii of microemulsion droplets. However, note that the change in Ag particle size from pure Brij droplets to pure AOT droplets is about a factor of 2, which corresponds very well to the change in average droplet’s size for the same microemulsions. Transmission Electron Microscopy. Figure 9a-d and Figure 10a-d show micrographs of formed silver nanoparticles made with the use of microemulsions consisting of different ratios of Brij30 and AOT. No pronounced difference could be established between the different microemulsions with respect to the morphology of the formed particles. All samples consist of particles having two different populations of particle sizes, which both have relatively broad particle size distributions in agreement with the SAXS results. In all samples, there are many more of the smaller particles. Figure 9 shows the larger particles (10-30 nm in diameter) where some of them are facetted crystals. No pronounced difference between the different samples, with respect to size and shape, is seen on the larger particles. However, looking at the smaller particles, which are predominantly spherical (Figure 10) and which dominate the samples, there is a pronounced difference regarding the particle size between the samples. The micrographs show that the particles made from the microemulsion containing only Brij30 surfactant (Figure 10a) have a diameter of about 6-7 nm, and those made from the one containing the mixture of 25/75 Brij30 and AOT (Figure 10d) have a particle diameter of about 3-4 nm in diameter. The trend and the magnitudes are in good agreement with the SAXS results (Table 2). Conclusion A novel method of making silver nanoparticles in waterin-oil microemulsions using the surfactants as both the reducing agent and as the structure-directing agent has been developed. Since no external strong reducing agent is used the kinetics of the silver particle formation is slow, which makes it possible to study the process in situ. The microemulsions used were based on the nonionic surfactant Brij30, which is oxidized upon reducing the silver ions to metallic silver. In addition, mixtures of Brij30 and AOT (which does not reduce the silver) were evaluated, and it was thereby possible to study the silver nanoparticle formation dependency on both the silver nitrate concentration as well as the reducing agent concentration without the influence of the microemulsion dynamics. It can be concluded from UV-vis measurements that the silver and surfactant (Brij30) concentrations have a big influence on the particle formation, which follows a first-order reaction kinetics. No major impact of these parameters was, however, found on the morphology of the formed particles. The size of the nanoparticles was found to increase with increasing Brij30 concentration in the

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microemulsions. The particle formation kinetics followed first-order reaction kinetics. TEM confirmed that there is no significant difference in the morphology of the formed nanoparticles when formed in the different systems. Two populations of particles were observed: one consisting of larger particles (10-30 nm in diameter), which were about the same in all samples, and one consisting of smaller particles (3-7 nm in diameter), depending on the ratio of Brij30 to AOT. An increase in the particle size of the latter population was observed as the Brij30 concentration was increased. SAXS measurements showed that the presence of silver nitrate does not affect the microemulsion systems, i.e., the droplet’s size and shape are retained during the particle formation. The SAXS analysis made on microemulsions with different ratios of Brij30/AOT shows that the droplet size decreases with increasing amount of AOT present and that at 100% AOT the droplets have a cylindrical shape rather than a spherical one. Also from the SAXS analysis made on the formed Ag nanoparticles a general trend toward bigger particle size was found when utilizing the more Brij30-containing microemulsions. These results support the conclusion that bigger microemulsion droplets have a tendency to give larger particles.

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The study shows that the droplet size influences the nanoparticle size even though the actual size is not the same. The shape of the microemulsion droplets, however, does not appear to influence the morphology of the formed silver nanoparticles. Acknowledgment. The authors would like to thank Gunnel Karlsson, Lund University, for part of the TEM imaging. This work was financially supported by the Swedish Foundation for Strategic Research (SSF) through its Colloid and Interface Technology program. A.E.C.P. acknowledges support from the Swedish Research Council and the Competence Centre for Catalysis, which is financially supported by the Swedish National Energy Administration and the member companies: AB Volvo, Johnson Matthey CSD, Saab Automobile AB, Perstorp AB, Albemarle Catalyst BV, MTC AB, and the Swedish Space Corporation. The assistance of Dr. Kurt Erlacher with some of the SAXS measurements and the financial support from the Danish Natural Science Research Council are gratefully acknowledged. LA050937J