A Novel Approach for the Preparation of AgBr Nanoparticles from

Vinod Kumar N. Gupta , Anurag Mehra , Rochish Thaokar. Colloids and Surfaces A: Physicochemical and Engineering Aspects 2012 393, 73-80 ...
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A Novel Approach for the Preparation of AgBr Nanoparticles from Their Bulk Solid Precursor Using CTAB Microemulsions Maen M. Husein,*,† Eva Rodil,‡,§ and Juan H. Vera‡ Department of Chemical and Petroleum Engineering, UniVersity of Calgary, Calgary, Canada, T2N 1N4, Department of Chemical Engineering, McGill UniVersity, Montreal, Canada, H3A 2B2, and Department of Chemical Engineering, UniVersity of Santiago de Compostela, E-15782 Santiago, Spain ReceiVed September 26, 2005 Microemulsions are suitable reaction media to prepare a wide variety of nanoparticles and provide control over their sizes. However, as typically used, microemulsions limit rates of rapid reactions and suffer from low reactant solubilization capacity. This work presents a new application of a novel approach aimed at minimizing these limitations. This approach, which was previously applied for AgCl nanoparticle preparation, involves solubilization of a bulk silver halide in the form of higher halides, by means of reaction with the surfactant counterion of a microemulsion, and the reprecipitation of silver halide nanoparticles in the water pools of individual reverse micelles. CTAB microemulsions were employed because they possess a reactive counterion and are known to have a high solubilization capacity for ionic reactants. Despite their high solubilization capacity, CTAB microemulsions achieved lower nanoparticles uptake (molar concentration of the colloidal nanoparticles) for the same surfactant concentration when compared to our previous study. The effect of the following variables on the nanoparticle uptake and the particle size was investigated: (1) operation variables, including rate of mixing and temperature; and (2) microemulsion variables, including CTAB and n-butanol concentrations, and water-to-surfactant mole ratio, R. These variables provide a comprehensive test to the proposed mechanism and expose the role of the surfactant layer rigidity. The nanoparticle uptake increased as the rate of mixing, temperature, and CTAB concentration increased, and decreased as n-butanol concentration and R increased. High n-butanol concentration and R values reduced the effective surfactant concentration and contributed to less surfactant layer rigidity and to particle aggregation.

Introduction Water-in-oil (w/o) microemulsions, or reverse micelles, are widely used for the preparation of a variety of nanoparticles due to their ability to efficiently mix reactants in their nanosize water pools, providing a highly homogeneous product with specified sizes stabilized by a surfactant layer. Precipitates of nanoparticles, including those of silver bromide, are typically prepared by carrying out a reaction upon mixing two identical microemulsions, each containing one of the reactants forming the precipitate.1-10 Once mixed, reverse micelles containing the two reactants migrate due to mixing and Brownian motion, collide, and their surfactant surface layer opens allowing the reaction to commence. The resulting precipitate gets stabilized in the water pools by the surrounding surfactant layer upon decoalescence of the reverse micelles. Rapid reactions, including precipitation reactions, * Corresponding author. E-mail: [email protected]. † University of Calgary. ‡ McGill University. § University of Santiago de Compostela. (1) Curri, M. L.; Agostiano, A.; Manna, L.; Monica, M. D.; Catalano, M.; Chiavarone, L.; Spagnolo, V.; Lugara, M. J. Phys. Chem. B 2000, 104, 83918397. (2) Debuigne, F.; Jeunieau, L.; Wiame, M.; Nagy, J. B. Langmuir 2000, 16, 7605-7611. (3) Jeunieau, L.; Verbouwe, W.; Rousseau, E.; Van der Auweraer, M.; Nagy, J. B. Langmuir 2000, 16, 1602-1611. (4) Ohde, H.; Ye, X.-R.; Wai, C. M.; Rodriguez, J. M. Chem. Commun. 2000, 23, 2353-2354. (5) Jeunieau, L.; Nagy, J. B. Colloids Surf., A 1999, 151, 419-434. (6) Liu, C.-Y.; Zhang, Z.-Y.; Wang, C.-Y. J. Imaging Sci. Technol. 1999, 43, 492-497. (7) Palla, B. J.; Shah, D. O.; Garcia-Casillas, P.; Matutes-Aquino, J. J. Nanoparticle Research 1999, 1, 215-221. (8) Schmidt, J.; Guesdon, C.; Schoma¨cker, R. J. Nanopart. Res. 1999, 1, 267276. (9) Taleb, A.; Petit, C.; Pileni, M. P. Chem. Mater. 1997, 9, 950-959. (10) Monnoyer, P.; Fonseca, A.; Nagy, J. B. Colloids Surf., A 1995, 100, 233-243.

present a challenge when carried out using the mixing of two microemulsions scheme because their kinetics is often limited by the rate of opening of the surfactant layer.11-13 Rigid surfactant layer opens slowly causing simultaneous nucleation and aggregation, which in turn result in the formation of large particles with broad size distribution.14 Research effort devoted to speeding up the opening of the surfactant layer by reducing its rigidity through manipulating certain microemulsion variables was detailed elsewhere.11,12,14 A less rigid surfactant protective layer, on the other hand, provides less protection against nanoparticle aggregation, especially at high concentration of the colloidal nanoparticles.15 In addition to the above limitation, microemulsion techniques generally suffer from low reactant solubilization capacity.16 Microemulsions formed with hexadecyltrimethylammonium bromide, CTAB, are favored as reaction media because they allow for high ionic reactants loading.17 In this work, we present a novel approach for the preparation of AgBr precipitate of nanoparticles using bulk AgBr powder as a precursor. This approach is aimed at minimizing the impact of the surfactant surface layer opening, and the limits on ionic reactant solubilization capacities. The same method was previously applied by our group for the preparation of AgCl nanoparticles starting from bulk AgCl powder.15 The new method utilizes an interaction between the halide in the form of cationic (11) Bagwe, R. P.; Khilar, K. C. Langmuir 2000, 16, 905-910. (12) Bagwe, R. P.; Khilar, K. C. Langmuir 1997, 13, 6432-38. (13) Bommarius, A. S.; Holzwarth, J. F.; Wang, D. I. C.; Hatton, T. A. J. Phys. Chem. 1990, 94, 7232-9. (14) Chew, C. H.; Gan, L. M.; Shah, D. O. J. Dispersion Sci. Technol. 1990, 11, 593-609. (15) Husein, M. M.; Rodil, E.; Vera, J. H. J. Colloid Interface Sci. 2005, 288, 457-467. (16) Husein, M. M.; Weber, M. E.; Vera, J. H. Langmuir 2000, 16, 91599167. (17) Fang, X.; Yang, C. J. Colloid Interface Sci. 1999, 212, 242-251.

10.1021/la052618y CCC: $33.50 © 2006 American Chemical Society Published on Web 01/26/2006

Approach for the Preparation of AgBr Nanoparticles

surfactant counterion and the bulk silver halide powder. The following mechanism was proposed to explain the type of interaction and the steps leading to silver halide nanoparticle formation. The surfactant molecules together with their water of hydration get adsorbed at the surface of the powder. Because the amount of water in this environment is very small, the very high concentration of the halide surfactant counterion induces the formation of soluble higher halides.15,18 Surfactant molecules at the solid surface and the reverse micellar surface exchange by virtue of the dynamic nature of microemulsion systems, leading to the migration of the higher halides to the interfacial area of the reverse micelles. Finally, the higher halides diffuse to the bulk water pool, where they precipitate as silver halide monomers due to the abundance of water. Nuclei form within individual reverse micelles once the critical nucleation concentration is exceeded. Particle growth follows by adding more of the silver halide monomers. This mechanism of nucleation and growth dominates under conditions of rigid surfactant layer. The surfactant protective layer limits the size of the nanoparticles and protects them from further aggregation. Nucleation and growth within individual reverse micelles minimize the role of intermicellar exchange of solubilizate and the opening of the surfactant layer.19-21 The current work considers the formation of AgBr precipitate of nanoparticles in CTAB/n-butanol/water/ isooctane microemulsions. The choice of CTAB as a surfactant couples the advantage of an active halide counterion and the ability of forming stable microemulsions with high water content and high concentrations of ions in the water pools.17 The effect of the following variables on the nanoparticle uptake, that is, the molar concentration of the colloidal AgBr nanoparticles, and the particle size distribution was investigated: (1) operation variables; including rate of mixing and temperature; and (2) microemulsion variables; including CTAB and n-butanol concentration, and water to surfactant mole ratio, R. These variables provide a comprehensive test to the proposed mechanism and expose the role of the surfactant layer rigidity. Nanoparticles of silver bromide serve as model material to study the quantum confinement effects of indirect gap semiconductors,22-24 sensitive photographic material for high-speed photography,22,25 and efficient photocatalyst for hydrogen generation from a solution of methanol and water.26 Experimental Procedure The types of surfactant, cosurfactant, and organic phase were kept unchanged throughout the experiments. The microemulsions were prepared by mixing certain amounts of hexadecyltrimethylammonium bromide, CTAB, 99% pure (Sigma, Oakville, ON), n-butanol, 99.4% pure (Fisher Scientific, Montreal, QC), isooctane, 99% pure (Fisher Scientific, Montreal, QC), and deionized water in a volumetric flask until a clear phase was obtained. A 10 mL volume of the microemulsions was brought in contact with bulk AgBr powder, 99% pure (Acros Organics, NJ), at specified conditions of mixing rate and temperature in 45 mL centrifuge tubes. When studying the effect of mixing, a 40 mL volume of the microemulsions was used to allow for sufficient volume for analysis. Excess amount of the (18) Ottewill, R. H.; Woodbridge, R. F. J. Colloid Sci. 1961, 16, 581-594. (19) Husein, M. M.; Rodil, E.; Vera, J. H. J. Nanopart. Res., submitted. (20) Husein, M. M.; Rodil, E.; Vera, J. H. J. Colloid Interface Sci. 2004, 273, 426-434. (21) Husein, M. M.; Rodil, E.; Vera, J. H. Langmuir 2003, 19, 8467-8474. (22) Zhang, H.; Mostafavi, M. J. Phys. Chem. B 1997, 101, 8443-8448. (23) Chen, W.; McLendon, G.; Marchetti, A.; Rehm, J. M.; Freedhoff, M. I.; Myers, C. J. Am. Chem. Soc. 1994, 116, 1585-1586. (24) Johansson, K.; Marchetti, A.; McLendon, G. J. Phys. Chem. 1992, 96, 2873-2879. (25) Belloni, J.; Mostafavi, M.; Marignier, J.; Amblard, J. J. Imaging Sci. 1991, 35, 68-74. (26) Kakuta, N.; Goto, N.; Ohkita, H.; Mizushima, T. J. Phys. Chem. B 1999, 103, 5917-5919.

Langmuir, Vol. 22, No. 5, 2006 2265 AgBr powder was used in all of the experiments to ensure equilibrium conditions. The AgBr powder mean particle diameter was 0.75 mm. The nanoparticle uptake, that is, the molar concentration of the colloidal AgBr nanoparticles in the microemulsion, was measured using atomic absorption spectroscopy (model SH11, Thermo Jarrel Ash, Waltham, MA). The detection range for silver was from 1 to 20 ppm. Standards containing 2, 3, and 5 ppm Ag were prepared by diluting a 396 ppm Ag standard in the form of colloidal AgBr nanoparticles. The colloidal AgBr nanoparticle standard was prepared by adding 30 µL of 66 000 ppm Ag in the form of AgNO3 aqueous solution to a single microemulsion containing 0.2 M CTAB, 1.6 M n-butanol, and water in isooctane to bring the final value of the water to surfactant mole ratio, R, to 7.19 The dilution step in the standards preparation was achieved using microemulsions containing the same concentrations of CTAB and n-butanol, and at R ) 7. Volumes of 4 mL were collected from the experimental samples at specified times and filtered out using Fisher Brand 45 µm microfilters (Fisher Scientific, Montreal, QC). After filtration, all samples were clear. Volumes of the filtered samples were diluted to concentrations between 2 and 5 ppm Ag with microemulsions identical to the ones used to prepare the experimental samples, to avoid any precipitation of the AgBr nanoparticles. Three replicates were prepared for some of the samples, and the 95% confidence intervals are shown in the figures. The UV absorption of the colloidal AgBr nanoparticles was taken directly after the filtration step using a Cary Varian 1/3 UV/ visible spectrophotometer (Varian Techtron Pty Ltd., Australia). A range of wavelengths between 190 and 500 nm was covered. Reference solutions of microemulsions identical to the ones used to prepare the experimental samples were used for the UV measurements. The nanoparticles were recovered for their size analysis by addition of thiophenol capping agent (Aldrich, Milwaukee, WI) to the filtered samples in a volume ratio of 1.0 µL thiophenol/1.0 mL. The microemulsions were then mixed for 1 min in a vortex mixer, placed in an ultrasonic bath for 30 min, and then left to stand for 24 h at room temperature. The nanoparticle-capping agent precipitate was collected after the 24-h period by centrifuging the samples at 1000 rpm for 5 min. It should be noted that the addition of thiophenol to a microemulsion containing no silver bromide nanoparticles did not result in any precipitation or in any observable change. Ten milliliters of methanol, 99.8% pure (Fisher Scientific, Montreal, QC), was added to the precipitate and mixed for 1 min using a vortex mixer. The methanol system was placed in the ultrasonic bath for 1 h. One drop of the methanol suspension was placed on a copper grid covered with Formvar and was left to evaporate for 24 h. The copper grid was manually shaken for a few seconds so that only a thin layer of the methanol suspension was present on the grid, hence minimizing the coagulation during evaporation. Photographs of the nanoparticles were taken at different degrees of magnification from different areas on the grid using a JEOL 2000FX transmission electron microscope, TEM (JEOL USA Inc., MA), equipped with a Gatan 792 Bioscan 1k × 1k wide angle multiscan CCD camera. Histograms showing the particle size distribution were constructed on the basis of the different photographs. An average of 1000 particles was used per histogram.

Results and Discussion All of the microemulsions employed in this study were stable at the temperatures considered, and in the presence or in the absence of the AgBr powder. No UV peaks could be found around 400 nm, which leads to the conclusion that the reduction of Ag+ to Ag was insignificant.11,26 1. Effect of Mixing. Interphase mass transport often limits reaction rates between reactants occurring in different phases. Mixing increases the rate of heterogeneous reactions by increasing the area of contact between the reactants, as a result of dispersing the phases, and by increasing the rate of mass transfer to and from the reaction zone. Fixing the surface area between the phases

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Figure 1. Nanoparticles uptake as a function of time obtained when mixing AgBr powder at 0 and 300 rpm and 22 °C with CTAB microemulsion: 0.2 M CTAB, 1.6 M butanol, R ) 7.

Figure 3. TEM photographs and the corresponding particle size distribution histograms obtained when mixing AgBr powder at (a) 0 rpm, (b) 300 rpm, and 22 °C with CTAB microemulsion: 0.2 M CTAB, 1.6 M butanol, R ) 7.

Figure 2. Absorption spectra of the colloidal AgBr nanoparticles obtained when mixing AgBr powder for 1, 6, and 24 h at 22 °C with CTAB microemulsion: 0.2 M CTAB, 1.6 M butanol, R ) 7, (a) 0 rpm, (b) 300 rpm. Reference cell: 1.6 M n-butanol, R ) 7, 0.2 M CTAB.

while mixing would expose the role of mass transfer.27 For the purposes of the current work, investigating the role of each factor separately was not considered essential, and the two factors were left to vary simultaneously. The effect of mixing was studied by collecting data on nanoparticle uptake and size distribution from a sample that was kept mixing for 24 h at 300 rpm and another one kept without mixing for the same period of time. The microemulsions consisted of 0.2 M CTAB, 1.6 M n-butanol, and a mole ratio of water to surfactant, R, of 7. The temperature of the two samples was fixed at 22 °C. Figure 1 is a plot of the nanoparticle uptake as a function of time. Figure 2 shows the UV absorption spectra of the colloidal AgBr nanoparticles after 1, 6, and 24 h runs. Figure 3 shows the TEM photographs and the corresponding particle size distribution histograms for the samples at 0 and 300 rpm after the 6 h run. Figure 1 shows that at 300 rpm the nanoparticle uptake increased with time and leveled off after around 4 h of mixing, while without mixing the nanoparticle uptake was still increasing with time. Therefore, for the experiments to follow, a 6 h mixing time at 300 rpm was considered sufficient to achieve equilibrium. The nanoparticle uptake after 24 h was 1.9 and 0.4 mM for the (27) Chan, A. F.; Evans, D. F.; Cussler, E. L. AIChE J. 1976, 22, 1006-1012.

Figure 4. Nanoparticle uptake as a function of temperature obtained when mixing AgBr powder for 6 h at 300 rpm with CTAB microemulsion: 0.2 M CTAB, 1.6 M butanol, R ) 7.

samples with and without mixing, respectively. Figure 2 shows UV absorption spectra specific to colloidal AgBr nanoparticles.19,20,28,29 These spectra increased continuously with time for the sample without mixing and remained essentially unchanged between 6 and 24 h for the sample with mixing. For the same surfactant type and concentration, a time-independent UV absorption spectrum reveals constant nanoparticle uptake and limited nanoparticle aggregation.30-32 Hence, an insignificant change in UV peaks of the samples between 6 h and longer mixing times was used as a quick check of attaining equilibrium. Figure 2a shows small peaks at around 285 nm. These peaks (28) Correa, N. M.; Zhang, H.; Schelly, Z. A. J. Am. Chem. Soc. 2000, 122, 6432-6434. (29) Zhang, H.; Schelly, Z. A.; Marynick, D. S. J. Phys. Chem. A 2000, 104, 6287-6294. (30) Tanaka, T.; Saijo, H.; Matsubara, T. J. of Photogr. Sci. 1979, 27, 60-65. (31) Hirai, T.; Sato, H.; Komasawa, I. Ind. Eng. Chem. Res. 1994, 33, 32623266. (32) Hirai, T.; Sato, H.; Komasawa, I. Ind. Eng. Chem. Res. 1993, 32, 301419.

Approach for the Preparation of AgBr Nanoparticles

Figure 5. Absorption spectra of the colloidal AgBr nanoparticles obtained when mixing AgBr powder for 6 h at 300 rpm with CTAB microemulsion: 0.2 M CTAB, 1.6 M butanol, R ) 7, T ) (a) 22 °C, (b) 30 °C, (c) 45 °C. Reference cell: 1.6 M n-butanol, R ) 7, 0.2 M CTAB.

indicate the presence of particles with large diameter at small nanoparticle uptake. The TEM photographs shown in Figure 3 indicate that mixing had a small effect on the particle size and the particle size distribution. This observation is confirmed by the insignificant red shift in the UV absorption spectra for the samples under mixing conditions. Similar results were obtained in our previous study.15 Mixing increases the rate of the interfacial reaction and results in the formation of more nuclei. Growth that takes place on more nuclei contributes toward the formation of smaller nanoparticles. Mixing, on the other hand, increases the rate of collision between nanoparticle-populated reverse micelles

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and the possibility of particle aggregation. The fact that mixing had a limited effect on the particle size suggests the existence of a rigid surfactant protective layer. 2. Effect of Temperature. The effect of temperature on the equilibrium nanoparticle uptake was studied by mixing AgBr powder at 300 rpm for 6 h with microemulsions consisting of 0.2 M CTAB, 1.6 M n-butanol, and a water-to-surfactant mole ratio, R, of 7, at temperatures ranging between 22 and 45 °C. Higher temperatures were avoided to ensure stable microemulsions. The equilibrium nanoparticle uptake is plotted in Figure 4 as a function of temperature. The UV absorption of the colloidal AgBr nanoparticles for three selected temperatures is shown in Figure 5, and the TEM photographs with the corresponding particle size distribution histograms for the same temperatures are shown in Figure 6a-c. Figure 4 shows that the equilibrium nanoparticle uptake increased almost linearly with the temperature. This increase in the uptake is also captured by the increase in the size of the UV absorption peaks in Figure 5. In addition, Figure 5 shows broader absorption ranges and a red shift in the absorption threshold, indicating an increase in the particle size and the particle size distribution as temperature increased. This increase in the particle size and the particle size distribution is captured by the TEM results of Figure 6a-c. Despite the fact that the UV absorption peaks consistently increased in size as the nanoparticle uptake increased, the size of these peaks does not serve as an exact measurement of the nanoparticle uptake, because, in addition to the nanoparticle concentration, it varies with the nanoparticle size, excess ion in the lattice, surfactant concentration, and surfactant-nanoparticle interaction.19,30 The increase in the equilibrium nanoparticle uptake can be thought of as analogous to the increase in the equilibrium concentration of a product of an endothermic equilibrium reaction. However, the analogy is not totally true because, in addition to temperature, the equilibrium nanoparticle uptake also depends on variables that control the

Figure 6. TEM photographs and the corresponding particle size distribution histograms obtained when mixing AgBr powder for 6 h at 300 rpm with CTAB microemulsion: 0.2 M CTAB, 1.6 M butanol, R ) 7, T ) (a) 22 °C, (b) 35 °C, (c) 45 °C.

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Figure 7. Nanoparticle uptake as a function of CTAB concentration obtained when mixing AgBr powder for 6 h at 300 rpm and 22 °C with CTAB microemulsion: 1.6 M butanol, R ) 7.

particle size, and it decreases for very large or very small particles due to particle stabilization limitations.15 The increase in the particle size with the temperature suggests the formation of larger reverse micelles.17 Another factor that might have contributed to larger nanoparticles is particle aggregation. As the temperature increased, the higher nanoparticle uptake and the higher kinetic energy of the reverse micelles increased the probability of collision between the nanoparticle-populated reverse micelles. It seems likely that these conditions were accompanied by a decrease in the rigidity of the CTAB layer, and hence aggregation commenced. Contrary to the above results, the dioctyldimethylammonium chloride microemulsion system showed a maximum AgCl uptake as a function of temperature, and a monotonic decrease in the AgCl nanoparticle size. It is believed that the small AgCl nanoparticles have been protected from aggregation at the high-temperature conditions by a rigid dioctyldimethylammonium chloride layer.15 3. Effect of CTAB Concentration. According to the proposed mechanism for the nanoparticles formation, an essential step is the solubilization of AgBr at the powder surface. Solubilization is achieved by the formation of higher silver halides, for example, AgBr2-, AgBr32-, etc.,18 as a result of a reaction between AgBr and CTAB counterion at the surface. The surfactant molecules carry the higher halides back to the reverse micelles, where they eventually precipitate in the water pools. Consequently, an increase in CTAB concentration should increase the nanoparticle uptake. The effect of increasing CTAB concentration from 0.1 to 0.3 M at 1.6 M n-butanol and R ) 7 was studied by mixing microemulsions at different CTAB concentrations and AgBr powder at 300 rpm and 22 °C for 6 h. The nanoparticle uptake, expressed as mM of AgBr, is presented as a function of CTAB concentration in Figure 7. The UV absorption of the colloidal AgBr nanoparticles for the samples at 0.1, 0.2, and 0.3 M CTAB is shown in Figure 8. The TEM images and the corresponding particle size distribution histograms for the same samples are shown in Figure 9a-c. Figure 7 shows a linear increase in the nanoparticle uptake as CTAB concentration increased. A similar trend of AgCl nanoparticle uptake was reported for the dioctyldimethylammonium chloride microemulsion system.15 The increase in the uptake with the increase in the CTAB concentration supports the

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Figure 8. Absorption spectra of the colloidal AgBr nanoparticles obtained when mixing AgBr powder for 6 h at 300 rpm and 22 °C with CTAB microemulsion: 1.6 M butanol, R ) 7, (a) 0.1 M, (b) 0.2 M, (c) 0.3 M CTAB. Reference cell: 1.6 M n-butanol, R ) 7, (a) 0.1 M, (b) 0.2 M, (c) 0.3 M CTAB.

proposed mechanism. The inverse of the slope of the line is approximately 1.7 × 102 mol of CTAB/mol of AgBr nanoparticles, indicating that 1.7 × 102 mol of CTAB is required to stabilize 1 mol of the colloidal AgBr nanoparticles, at constant values of all of the other variables. This ratio is very high as compared to that of the dioctyldimethylammonium chloride system.15 Hence, dioctyldimethylammonium chloride is more suitable for forming silver halides from their solid powder because it could form stable microemulsions at conditions that favored high nanoparticle uptake. However, applying dioctyldimethylammonium chloride to a powder of AgBr will result in the formation of AgCl nanoparticles due to the nature of its counterion. Results on AgBr nanoparticle preparation using dioctyldimethylammonium bromide will be communicated shortly. The larger UV absorption peaks for higher CTAB concentration in Figure 8 indicate an increase in the nanoparticle uptake. The TEM results presented in Figure 9a-c show that the size of the nanoparticles increased and the size distribution became broader as the surfactant concentration increased, indicating probable aggregation. Higher nanoparticle uptake increases the probability of collision between the nanoparticle-populated reverse micelles and promotes nanoparticle aggregation when accompanied by a decrease in the rigidity of the surfactant protective layer.14,15,19,20 It seems that increasing CTAB concentration at constant R and n-butanol concentration have resulted in less rigid surfactant layer. A similar trend in the nanoparticle size with the surfactant concentration was reported in our previous study.15 Another factor that contributes toward the formation of larger particles is the increase in the size of the reverse micelles as CTAB concentration increased at constant R.17 The number of moles of surfactant molecules required to stabilize a mole of the colloidal AgBr nanoparticles was independent of the particle size, probably because of the large excess of surfactant molecules. In terms of the equilibrium reaction analogy, increasing the surfactant concentration is equivalent to increasing a reactant concentration, which results in higher product concentration, that is, in a higher nanoparticle uptake. 4. Effect of n-Butanol Concentration. Most surfactants are unable to form w/o microemulsions without the assistance of a cosurfactant. The water uptake of Winsor type II microemulsions

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Figure 9. TEM photographs and the corresponding particle size distribution histograms obtained when mixing AgBr powder for 6 h at 300 rpm and 22 °C with CTAB microemulsion: 1.6 M butanol, R ) 7, (a) 0.1 M, (b) 0.2 M, (c) 0.3 M CTAB.

Figure 10. Nanoparticle uptake as a function of butanol concentration obtained when mixing AgBr powder for 6 h at 300 rpm and 25 °C with CTAB microemulsion: 0.2 M CTAB, R ) 7.

is highly dependent on the type and concentration of the cosurfactant.33 The AgCl nanoparticle uptake by dioctyldimethylammonium chloride microemulsions depended heavily on the concentration of the n-decanol cosurfactant.15 The effect of n-butanol on the nanoparticle uptake by CTAB microemulsion system was evaluated by increasing the concentration of n-butanol from 1.6 to 3.5 M at [CTAB] ) 0.2 M and R ) 7.0. The samples were left to mix for 6 h at 300 rpm and 25 °C. The nanoparticle uptake versus n-butanol concentration is plotted in Figure 10. The UV absorption of the colloidal AgBr nanoparticles for selected samples is shown in Figure 11, and the TEM photographs and the corresponding particle size distribution histograms for the same samples are shown in Figure 12a-c. Figure 10 indicates that the nanoparticle uptake decreased as the concentration of n-butanol increased. The UV absorption size in Figure 11 decreased as the cosurfactant concentration (33) Wang, W.; Weber, M. E.; Vera, J. H. Colloid Interface Sci. 1994, 168, 422-427.

Figure 11. Absorption spectra of the colloidal AgBr nanoparticles obtained when mixing AgBr powder for 6 h at 300 rpm and 25 °C with CTAB microemulsion: 0.2 M CTAB, R ) 7, (a) 1.6 M, (b) 2.5 M, (c) 3.5 M butanol. Reference cell: 0.2 M CTAB, R ) 7, (a) 1.6 M, (b) 2.5 M, (c) 3.5 M n-butanol.

decreased. Figure 12a-c shows an increase in the particle size with the increase in the cosurfactant concentration. The decrease in the nanoparticle uptake and the increase in the particle size with increasing n-butanol concentration are attributed to lower effective CTAB concentration, and to nanoparticle aggregation.15,19,20 At high concentrations, the cosurfactant tends to stay in the bulk oil-phase and acts as a cosolvent. Consequently, the oil phase becomes more polar and more accommodating to the

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Figure 12. TEM photographs and the corresponding particle size distribution histograms obtained when mixing AgBr powder for 6 h at 300 rpm and 25 °C with CTAB microemulsion: 0.2 M CTAB, R ) 7, (a) 1.6 M, (b) 2.5 M, (c) 3.5 M butanol.

Figure 13. Nanoparticle uptake as a function of water to surfactant mole ratio, R, obtained when mixing AgBr powder for 6 h at 300 rpm and 22 °C with CTAB microemulsion: 0.2 M CTAB, 1.6 M butanol.

nondissociated surfactant molecules.1,33 Fewer number of CTAB molecules at the powder surface shifts the equilibrium toward lower colloidal nanoparticle concentration. Fewer number of surfactant molecules at the reverse micellar protective layer leads to nanoparticle aggregation. Contrary to what is expected, Figure 11 shows no significant shift in the UV absorption threshold. The expected red shift might have been overshadowed by the decrease in the effective surfactant concentration as n-butanol increased. A smaller number of surfactant molecules capping the nanoparticle decreases its size as measured by UVspectroscopy.1,9 5. Effect of Water to Surfactant Mole Ratio, R. The water to surfactant mole ratio dictates the rigidity of the surfactant protective layer and the interaction between the colloidal

Figure 14. Absorption spectra of the colloidal AgBr nanoparticles obtained when mixing AgBr powder for 6 h at 300 rpm and 22 °C with CTAB microemulsion: 0.2 M CTAB, 1.6 M butanol, R ) (a) 7, (b) 10, (c) 15. Reference cell: 0.2 M CTAB, 1.6 M n-butanol, R ) (a) 7, (b) 10, (c) 15.

nanoparticles and the surfactant protective layer.14 The water to surfactant mole ratio was increased from 7 to 15 at CTAB and n-butanol concentrations of 0.2 and 1.6 M, respectively. The samples were mixed at 300 rpm and 22 °C for 6 h. A mole ratio of water to surfactant of 7 was the lowest possible mole ratio to achieve a stable microemulsion. The AgBr nanoparticle uptake for five different values of R, the UV absorption of the colloidal AgBr nanoparticles for selected samples, and the TEM photographs and their corresponding particle size distribution histograms are depicted in Figures 13, 14, and 15a-c, respectively.

Approach for the Preparation of AgBr Nanoparticles

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Figure 15. TEM photographs and the corresponding particle size distribution histograms obtained when mixing AgBr powder for 6 h at 300 rpm and 22 °C with CTAB microemulsion: 0.2 M CTAB, 1.6 M butanol, (a) R ) 7, (b) R ) 10, (c) R ) 15.

As shown in the figures, the nanoparticle uptake decreased, the particle size increased, and the particle size distribution became broader as the water content of the microemulsion increased. Figure 13 shows about 50% reduction in the nanoparticle uptake as R increased from 7 to 15. Figure 14 shows an insignificant shift in the UV absorption threshold and a drop in the size of the UV absorption peaks. This drop in the UV absorption peaks is attributed mainly to the drop in the colloidal nanoparticle concentration, because, as Figure 15a-c shows, the nanoparticle size did not vary much within the range of R covered in this study. In terms of an equilibrium reaction analogy, high water content is equivalent to lower effective CTAB concentration and reactant concentration, and shifts the equilibrium toward lower nanoparticle uptake and lower product concentration. Higher water content reduces the rigidity of the surfactant protective layer and the interaction with the nanoparticles and promotes aggregation.19 In addition, a higher water content may result in the formation of larger reverse micelles.17 Larger reverse micelles contribute toward the formation of larger nanoparticle. The small blue shift in the UV absorption threshold as water content increased is probably due to the decrease in the number of surfactant molecules capping the nanoparticle.1,9

Conclusions A novel approach for nanoparticle preparation starting from their solid powder was presented. This approach does not require microemulsion-soluble reactants and, for rigid surfactant layer, reduces the role of intermicellar exchange of solubilizate by promoting nucleation within individual reverse micelles. The formation of the nanoparticles is based on a reaction between the surfactant counterion and the solid powder, which results in the solubilization of the solid, followed by reprecipitation of the soluble species in the water pools as nanoparticles. A reaction mechanism that describes the formation of the nanoparticles was outlined. CTAB microemulsions were stable at values of

microemulsion variables that favored less nanoparticle uptake when compared to dioctyldimethylammonium chloride microemulsions. The effect of several parameters, including operation and microemulsion parameters, was evaluated to provide a test for the proposed mechanism and explore the role of the surfactant surface layer rigidity on the phenomenon. Mixing had a significant effect on the nanoparticle uptake, and only 20% of the equilibrium uptake was achieved after 24 h without mixing. The significant role of mixing is in accordance with the interfacial nature of the phenomenon as proposed by the mechanism. Mixing, on the other hand, had an insignificant effect on the particle size, suggesting the existence of a rigid surfactant protective layer. The increase in temperature increased the nanoparticle uptake and the particle size. The increase in the nanoparticle uptake is explained by an endothermic equilibrium reaction analogy, while the increase in the particle size and size distribution is attributed to a decrease in the rigidity of the surfactant layer and/or the formation of larger reverse micelles at higher temperatures. The nanoparticle uptake increased as the surfactant concentration increased, which is in agreement with the active role of the surfactant in the proposed mechanism. The increase in the nanoparticle mean size and the particle size distribution with the increase in the surfactant concentration is attributed to aggregation as a result of the higher rate of collision between nanoparticlepopulated reverse micelles and lesser surfactant layer rigidity. The increase in the n-butanol concentration and the water content decreased the effective CTAB concentration, and hence shifted the equilibrium toward lower nanoparticle uptake. High n-butanol concentration made the oil phase more accommodating to the nondissociated surfactant molecules. Higher water content, on the other hand, consumed more of the surfactant molecules in the microemulsion stabilization. The increase in the nanoparticle size as n-butanol concentration or water content increased is attributed to particle aggregation.

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The UV absorption spectra generally showed a red shift in the absorption threshold as the nanoparticle size increased. The size of the absorption peak increased as the nanoparticle uptake increased. However, the size of the absorption peak is not a good measure of the nanoparticle uptake, because it is also a function of the mean particle size and the interaction between the CTAB headgroups and the AgBr nanoparticles.

Husein et al.

Acknowledgment. We are grateful to the Natural Sciences and Engineering Research Council of Canada and to the Ministerio de Ciencia y Tecnologia of Spain (project 2004/PC059) for financial support.

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