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The Aerosol OT + n-Butanol + n-Heptane + Water System: Phase Behavior, Structure Characterization, and Application to Pt70Fe30 Nanoparticle Synthesis Arthur R. Malheiro, Laudemir C. Varanda,† Joelma Perez, and H. Mercedes Villullas* Departamento de Fı´sico Quı´mica, Instituto de Quı´mica, UniVersidade Estadual Paulista (UNESP), C. P. 355, 14801-970 Araraquara (SP), Brazil ReceiVed July 17, 2007. In Final Form: August 7, 2007 A phase diagram of the pseudo-ternary Aerosol OT (AOT) + n-butanol/n-heptane/water system, at a mass ratio of AOT/n-butanol ) 2, is presented. Conductivity measurements showed that within the vast one-phase microemulsion region observed, the structural transition from water-in-oil to oil-in-water microemulsion occurs continuously without phase separation. This pseudo-ternary system was applied to the synthesis of carbon-supported Pt70Fe30 nanoparticles, and it was found that nanoparticles prepared in microemulsions containing n-butanol have more Fe than those prepared in ternary microemulsions of AOT/n-heptane/water under similar conditions. It was verified that introducing n-butanol as a cosurfactant into the AOT/n-heptane/water system lead to complete reduction of the Fe ions that allowed obtaining alloyed PtFe nanoparticles with the desired composition, without the need of preparing functionalized surfactants and/or the use of inert atmosphere.
1. Introduction Nanostructured materials and nanometer-sized particles are of increasing importance in different areas of science and technology because of the large available surface areas. Among other processes, the conversion of chemical energy into electricity in proton exchange membrane fuel cells (PEMFC) requires the development of better electrocatalysts in order to improve the cell performance. One of the key factors affecting the PEMFC performance is the slow kinetics of the oxygen reduction reaction, which is responsible for overpotential losses of about 0.30.4 V under typical conditions of operation.1 Several studies have shown that bimetallic Pt-M catalysts (M ) first-row transition metal, such as Fe, Co, or Ni) show activities for oxygen reduction about 1.5-3 times higher than that of pure Pt.2-7 However, controlling the composition of bimetallic nanoparticles is usually difficult. Since Boutonnet et al. successfully synthesized metal nanoparticles (Pt, Pd, Rh, and Ir) by using water-in-oil (w/o) microemulsions,8 the interest in preparing nanoparticles using this technique has increased considerably. The microemulsion method has been used to prepare nanoparticles of different materials, such as metal, oxides, sulfides and halides.8-16 Bimetallic nanoparticles, the preparation of which involves * To whom correspondence should be addressed. E-mail: mercedes@ iq.unesp.br. † Present address: Instituto de Quı´mica de Sa ˜ o Carlos, USP, C.P. 780, 13560-970 Sa˜o Carlos (SP), Brazil. E-mail:
[email protected]. (1) Gasteiger, H. A.; Gu, W.; Makharia, R.; Mathias, M. F.; Sompalli, B. In Handbook of Fuel Cells: Fundamentals, Technology, and Applications; Vielstich, W., Lamm, A., Gasteiger, H. A., Eds.; Wiley: New York, 2003; Vol. 3, p 593. (2) Antolini, E.; Salgado, J. R. C.; Gonzalez, E. R. J. Power Sources 2006, 160, 957-968. (3) Antolini, E.; Salgado, J. R. C.; Gonzalez, E. R. Appl. Catal. B: EnViron. 2006, 63, 137-149. (4) Antolini, E. Mater. Chem. Phys. 2003, 78, 563-573. (5) Santiago, E. I.; Varanda, L. C.; Villullas, H. M. J. Phys. Chem. C 2007, 111, 3146-3151. (6) Shukla, A. K.; Raman, R. K.; Choudhury, N. A.; Priolkar, K. R.; Sarode, P. R.; Emura, S.; Kumashiro, R. J. Electroanal. Chem. 2004, 563, 181-190. (7) Xiong, L.; Manthiram, A. Electrochim. Acta 2005, 50, 2323-2329. (8) Boutonnet, M.; Kizling, J.; Stenius, P. Colloid Surf. 1982, 5, 209-225. (9) Nagy, J. B. In Handbook of Microemulsion Science and Technology; Kumar, P., Mittal, K. L., Eds.; Marcel Dekker: New York, 1999; p 499.
difficulties in controlling the composition, have also been prepared in microemulsion systems.7,17-22 Microemulsions are constituted by at least one immiscible liquid dispersed in another, such as water and an organic solvent (oil), in the form of nanometric droplets stabilized by a layer of surfactant molecules. They are thermodynamically stable, optically transparent, and isotropic systems. While microemulsions appear homogeneous on a macroscopic scale, microscopically they are heterogeneous media that exhibit distinct microphases or domains. The microscopic structure of microemulsions depends on their composition, with both the nature and the amount of each component being important. Three main types of structures have been identified: (i) water droplets in an oil continuous phase (w/o microemulsions) that usually occur in systems with high oil content; (ii) oil droplets in a water continuous phase (oil-in-water, o/w), typically occurring in water-rich microemulsions; and (iii) bicontinuous structures, whose aqueous and oil domains are randomly interconnected in the form of spongelike microstructures, and where both oil and water act as continuous phases while remaining distinct from one another. (10) Osseo-Asare, K. In Handbook of Microemulsion Science and Technology; Kumar, P., Mittal, K. L., Eds.; Marcel Dekker: New York, 1999; p 549. (11) Ganguli, D. E.; Ganguli, M. Inorganic Particle Synthesis Via Macro- and Microemusions: A Micrometer to Nanometer Landscape; Kluwer: New York, 2003; Chapter 5. (12) Capek, I. AdV. Colloid Interface Sci. 2004, 110, 49-74. (13) Eriksson, S.; Nylen, U.; Rojas, S.; Boutonnet, M. Appl. Catal. A: Gen. 2004, 265, 207-219. (14) Eastoe, J.; Hollamby, M. J.; Hudson, L. AdV. Colloid Interface Sci. 2006, 128-130, 5-15. (15) Lopez-Quintela, M. A.; Tojo, C.; Blanco, M. C.; Rio, L. G.; Leis, J. R. Curr. Opin. Colloid Interface Sci. 2004, 9, 264-278. (16) Lopez-Quintela, M. A. Curr. Opin. Colloid Interface Sci. 2003, 8, 137144. (17) Liu, Z. L.; Lee, J. Y.; Han, M.; Chen, W. X.; Gan, L. M. J. Mater. Chem. 2002, 12, 2453-2458. (18) Solla-Gullon, J.; Rodes, A.; Montiel, V.; Aldaz, A.; Clavilier, J. J. Electroanal. Chem. 2003, 554, 273-284. (19) Xiong, L.; Manthiram, A. Solid State Ionics 2005, 176, 385-392. (20) Rojas, S.; Garcia-Garcia, F. J.; Jaras, S.; Martinez-Huerta, M. V.; Fierro, J. L. G.; Boutonnet, M. Appl. Catal. A: Gen. 2005, 285, 24-35. (21) Santos, L. G. R. A.; Oliveira, C. H. F.; Moraes, I. R.; Ticianelli, E. A. J. Electroanal. Chem. 2006, 596, 141-148. (22) Godoi, D. R. M.; Perez, J.; Villullas, H. M. J. Electrochem. Soc. 2007, 154, B474-B479.
10.1021/la702146q CCC: $37.00 © 2007 American Chemical Society Published on Web 09/19/2007
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Bicontinuous phases, which are not useful for nanoparticle synthesis purposes, may exist in the intermediate phase region between o/w and w/o microemulsions. In the case of w/o microemulsions, also called reverse micelles, the surfactant molecules form aggregates with their polar groups concentrated in the interior and their hydrophobic moieties extending into the bulk oil phase. Within this “polar core”, water can solubilize, forming water pools or droplets delimited by the surfactant, which have been used by many workers as “nanoreactors” for particle synthesis.8-20 Generally, the size of these water pools is mainly characterized by the water-to-surfactant molar ratio (w ) [H2O]/[surfactant]).23,24 One of the surfactants most widely employed to form microemulsions is sodium bis(2-ethylhexyl)sulfosuccinate, usually called Aerosol OT or AOT. Depending on the temperature and on the nonpolar organic solvent used, AOT reverse micelles can uptake large amounts of water, being able to reach w values as large as 60.25 This capability of the AOT reverse micelles to solubilize large amounts of water usually results in extensive phase areas of w/o microemulsions in the phase diagrams, thus making the use of AOT very frequent in particle synthesis.9-11 Even though the ease of forming microemulsions without the need of a cosurfactant is usually pointed out as one of the advantages of using AOT, the introduction of a fourth component might bring about some benefits since it can produce changes in the micelle properties.26 The fourth component can be a salt, a cosurfactant (typically a medium-chain alcohol), or other additives, such as a polymer. Alcohols can be used to prevent the formation of rigid structures, such as gels and liquid crystals, and the effect of linear and branched alcohols in AOT microemulsions has been studied.27-30 In this work, a phase diagram of the pseudo-ternary AOT + n-butanol/n-heptane/water system, at a mass ratio of AOT/nbutanol ) 2, is presented. Conductivity measurements showed that, within the one-phase microemulsion region, the structural transition from w/o to o/w microemulsion occurs continuously without phase separation. This pseudo-ternary system was applied to the synthesis of carbon-supported Pt70Fe30 nanoparticles. It was verified that nanoparticles prepared in microemulsions with n-butanol contain more Fe than those prepared in ternary microemulsions of AOT/n-heptane/water under similar conditions. The use of the pseudo-ternary microemulsion containing n-butanol allows better control of the particle composition.
added to a fixed amount of EM determines the initial oil mass fraction (R) of the system. The content of each component was derived from precise mass measurements. Water was used as titration component and was added in small volumes under permanent stirring. The phase boundaries were established by observation of turbidity-to-transparency (or of transparency-to-turbidity) transitions. Measurements were done in an identical manner for different values of the initial oil fraction to find the boundaries of the microemulsion domain. All measurements were carried out at 25 °C. All reagents (Aldrich) were used as received. For comparison purposes, a phase diagram for the ternary system AOT/n-heptane/water was also constructed adopting the same procedure. Conductivity measurements were carried out for each water addition with a GPL-32 Crison conductimeter. For these, a small conductivity cell with Pt electrodes was kept inside the tightly closed container during the titration experiments. Calibration of the conductivity cell was done with a 0.01 mol L-1 KCl solution (conductivity ) 1412 µS cm2). PtFe nanoparticles of nominal composition 70:30 (in atoms) were prepared by adding an aqueous solution of the precursors (H2PtCl6 and FeCl2) to a mixture of n-heptane and EM (13 wt %) under constant stirring. The desired Pt/Fe atomic ratio was controlled by the amounts of each metal precursor in the aqueous phase. Dilute solutions were employed in order to have a 0.5 wt % metal content in the aqueous phase. The water/AOT molar ratio (w) was 8. The reducing agent (NaBH4) was added to the microemulsion as a solid in a molar ratio of 10:1 to metals, and the mixture was kept under constant stirring for 2 h. After that, to obtain carbon-supported nanoparticles (metal load 20% w/w), an appropriate amount of highsurface-area carbon (Vulcan XC-72, Cabot) was added into the system kept in an ultrasonic bath. The suspension was then stirred overnight. The resulting material was filtered, washed copiously with ethanol, acetone, and ultrapure water, and dried. For comparison, the same procedure was adopted to prepare carbon-supported PtFe nanoparticles (PtFe/C) using a ternary microemulsion AOT/n-heptane/water system. The PtFe/C samples were examined by X-ray diffraction (XRD) using a Rigaku, model D Max 2500 PC diffractometer. The X-ray diffractograms were obtained with a scan rate of 0.5° min-1 and an incident wavelength of 0.15406 nm (Cu KR). Transmission electron microscopy (TEM) studies were carried out with a Philips CM200 instrument operating at 200 kV, and equipped with EDX systems for energy dispersive X-ray analysis.
2. Experimental Section
3.1. Phase Behavior. Phase diagrams are useful for purposes of synthesis. The so-called “titration method” was used for controlled addition of small volumes of water into mixtures of EM (AOT + n-butanol) and n-heptane of different proportions, that is, different initial mass fractions of the hydrocarbon component. The pseudo-ternary phase diagram obtained for the AOT + n-butanol/n-heptane/water for F ) mass ratio AOT/nbutanol ) 2 and at 25 °C is shown in Figure 1a. For almost all the initial values of R, turbidity was observed for small contents of water. Further addition of water made the system reach and pass through a compositional region of permanent transparency. The appearance of permanent turbidity indicated that the system reached the maximum amount of water that can be solubilized. The region of continuous single-phase microemulsion was thus determined as being the one where permanent transparency was observed, and is indicated in the diagram as I. Below and above region I, permanent turbidity was observed. For the sake of comparison, the phase behavior of the ternary system AOT/nheptane/water was also studied using the same titration method. Figure 1b shows the phase diagram obtained for the ternary
The pseudo-ternary phase diagram for the system AOT + n-butanol/n-heptane/water was constructed considering water, nheptane, and the emulsifier (EM) as the three components of the system. The EM was a mixture of AOT and n-butanol. The mass ratio of AOT/n-butanol (F) was kept fixed and was equal to 2. A simple titration technique was used to construct the phase diagram. The EM was first prepared by mixing appropriate amounts of AOT and n-butanol, and then an adequate amount of n-heptane was added. The mixture was kept under stirring for a few minutes in a tightly closed bottle, to ensure complete dissolution of the EM into the hydrocarbon and to avoid evaporation. The amount of n-heptane (23) Pileni, M. P. J. Phys. Chem. 1993, 97, 6961-6973. (24) Wines, T. H.; Dukhin, A. S.; Somasundaran, P. J. Colloid Interface Sci. 1999, 216, 303-308. (25) Petit, C.; Lixon, P.; Pileni, M. P. Langmuir 1991, 7, 2620-2625. (26) Silber, J. J.; Biasutti, A.; Abuin, E.; Lissi, E. AdV. Colloid Interface Sci. 1999, 82, 189-252. (27) Hou, M. J.; Shah, D. O. Langmuir 1987, 3, 1086-1096. (28) Lissi, E. A.; Engel, D. Langmuir 1992, 8, 452-455. (29) Caillet, C.; Hebrant, M.; Tondre, C. Langmuir 1998, 14, 4378-4385. (30) Perez-Casas, S.; Castillo, R.; Costas, M. J. Phys. Chem. B 1997, 101, 7043-7054.
3. Results and Discussion
The AOT + n-Butanol/n-Heptane/Water System
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Figure 1. Phase diagrams at 25 °C. (a) Pseudo-ternary AOT + n-butanol/n-heptane/water system (F ) 2). Formation of microemulsion occurs in region I (shaded area), where bicontinuous (dark gray) and o/w (light gray) structures occur only at high n-heptane contents (see text). (b) AOT/heptane/water system.
system, which is in excellent agreement with those published for the same system.31 3.2. Microemulsion Structure. Electrical conductivity measurements have often been used to study structural changes in macro- and microemulsions.32-38 In order to determine the compositional domains of w/o and o/w microemulsions as well as the region of bicontinuous structure formation, electrical conductivity was measured after each water addition of the titration procedure. These measurements showed the occurrence of three different behaviors of the specific conductivity (κ) as a function of the amount of water, as shown in Figure 2. The conductivity curve in Figure 2a shows very low values of conductivity for small amounts of added water. As soon as the amount of water in the system reached the percolation threshold, Φc, the specific conductivity increased linearly with the amount of water added. This type of behavior is typical of w/o structures for which, below the percolation threshold, conduction would be given solely by droplet diffusion, and, above the percolation threshold, the conductivity might result from an increasing aqueous droplet interlinking process.39 This percolative conduc(31) Rouviere, J.; Couret, J. M.; Lindheimer, M.; Dejardin, J. L.; Marrony, R. J. Chim. Phys. Phys.-Chim. Biol. 1979, 76, 289-296. (32) Lagues, M.; Sauterey, C. J. Phys. Chem. 1980, 84, 3503-3508. (33) Moha-Ouchane, M.; Peyrelasse, J.; Boned, C. Phys. ReV. A 1987, 35, 3027-3032. (34) Boned, C.; Clausse, M.; Lagourette, B.; Peyrelasse, J.; McClean, V. E. R.; Sheppard, R. J. J. Phys. Chem. 1980, 84, 1520-1525. (35) Peyrelasse, J.; Boned, C. Phys. ReV. A 1990, 41, 938-953. (36) Mo, C. S.; Zhong, M. H.; Zhong, Q. J. Electroanal. Chem. 2000, 493, 100-107. (37) Chakraborty, I.; Moulik, S. P. J. Colloid Interface Sci. 2005, 289, 530541. (38) Alexandridis, P.; Holzwarth, J. F.; Hatton, T. A. J. Phys. Chem. 1995, 99, 8222-8232. (39) Lagourette, B.; Peyrelasse, J.; Boned, C.; Clausse, M. Nature 1979, 281, 60-62.
Figure 2. Dependence of the specific conductivity of the pseudoternary AOT + n-butanol/n-heptane/water system on the amount of added water, for different initial n-heptane contents: (a) R ) 0.74; (b) R ) 0.82; (c) R ) 0.87.
tion phenomenon observed in some w/o microemulsions can be explained in terms of “sticky droplet collisions”, as proposed by Fletcher and Robinson.40 Thus, above the percolation threshold, the “sticky collisions” between droplets would result in the formation of water microchannels through which ions could move producing a sharp increase in the conductivity. For initial contents of n-heptane up to ca. R ) 0.75, all conductivity curves looked like that presented in Figure 2a. The conductivity increased with water content until the microemulsion lost stability and transformed into a turbid system. Percolation was also observed for initial hydrocarbon contents above ca. 0.75. As shown in Figure 2b, for an initial n-heptane content of R ) 0.82, a region of very low conductivity at small amounts of added water is followed by a sharp and linear increase above the percolation threshold. When a water content Φb is reached, the curve exhibits a change in slope, and the conductivity tends toward a constant value. Further increasing the volume of added water makes the system (40) Fletcher, P. D. I.; Robinson, B. H. Ber. Bunsen-Ges. Phys. Chem. Chem. Phys. 1981, 85, 863-867.
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Figure 4. (a) XRD patterns of PtFe/C samples prepared in AOT + n-butanol/n-heptane/water microemulsion (I) and AOT/n-heptane/ water microemulsion (II). (b) Pseudo-Voigt fitting of [220] diffraction peaks. The line indicates the [220] peak position of the Pt/C sample.
Figure 3. Dependence of the specific conductivity of the AOT/ n-heptane/water system on the amount of added water, for different initial n-heptane contents: (a) R ) 0.40; (b) R ) 0.87.
enter a composition range (beginning at Φm) where the conductivity increases with increasing water content. This kind of behavior typically indicates that, in the region between Φb and Φm, the microemulsion presents a bicontinuous structure. The conductivity behavior above Φb can thus be interpreted as being due to a progressively increasing interconnection of water microchannels that leads to a coarse sponge-like structure where both oil and water coexist as continuous phases while remaining distinct from one another, defining a bicontinuous state. Above Φm, the aqueous phase becomes continuous, and the conductivity increases until the limit of stability of the microemulsion phase is reached and the system passes to a state of permanent turbidity. Curves as that shown in Figure 2c were observed for initial n-heptane contents above R ) 0.87. The variation of conductivity with the amount of water added reveals that a w/o structure initially present changes to a bicontinuous structure near the conductivity maximum (around Φb). Above Φm, the system shows a decrease in conductivity because of the dilution of the dispersed phase by added water while the microemulsion structure is gradually switching from bicontinuous to o/w. The system compositions Φb and Φm were used to define the regions of w/o, bicontinuous, and o/w structures within the microemulsion region of the phase diagram (see Figure 1). Similar conductivity curves were obtained for the ternary system AOT/n-heptane/water, as shown in Figure 3a,b for initial n-heptane contents R ) 0.40 and 0.87, respectively. 3.3. Synthesis and Characterization of Pt70Fe30 Nanoparticles. Carbon-supported PtFe nanoparticles of nominal composition Pt/Fe 70:30 (in atoms) were prepared in AOT + n-butanol/n-heptane/water microemulsions and, for comparison purposes, in the ternary system AOT/n-heptane/water. It was observed that, after the formation of the nanoparticles by the addition of the reducing agent (NaBH4), the supernatant was
completely colorless in the case of synthesis carried out in the microemulsion containing n-butanol, while it remained slightly yellowish for the ternary microemulsion (without n-butanol). Upon the addition of concentrated NH4SCN solution, the supernatant remained colorless for the former, while it turned slightly reddish for the latter, indicating the presence of small amounts of unreduced iron. The physical properties of the nanoparticles obtained in the AOT + n-butanol/n-heptane/water microemulsions were compared with those of samples obtained employing the ternary system AOT/n-heptane/water. The X-ray diffractograms for the Pt70Fe30/C samples are shown in Figure 4. Generally, XRD showed diffraction patterns associated with the face-centered cubic (fcc) Pt structure (PDF 4-802), in agreement with the results published by Toda et al.41 for PtFe alloys prepared by radio frequency magnetron sputtering. For both samples prepared in this work, diffraction peaks are broad and slightly shifted toward higher 2θ values compared to those of pure Pt, as shown in Figure 4b for the [220] diffraction peaks. This shift in peak position can be taken as an indication of alloy formation because of partial substitution of Pt by Fe in the fcc structure, producing a contraction of the lattice. From the XRD data, the lattice parameter
a)
x2λ sen θ
(1)
x2 a 2
(2)
and the Pt-Pt distance
dfcc )
were also calculated for the PtFe/C samples prepared with and without n-butanol in the microemulsion.42 The values of the lattice constant and the Pt-Pt distance obtained for the PtFe/C sample prepared in the AOT + n-butanol/n-heptane/water were smaller than those of the sample synthesized in the microemulsion system without n-butanol, indicating that the presence of the cosurfactant in the microemulsion favors the incorporation of Fe into an alloyed phase, which leads to a lattice contraction. Results published by Toda et al.41 for PtFe alloys revealed that, up to ca. 75% iron content, the formation of the chemically disordered fcc structure (solid solution) occurs. Moreover, their data showed a nearly linear variation of the lattice constant with the alloy Fe content. Figure 5 shows a comparison of the data taken from the paper by Toda et al.41 and the values predicted (41) Toda, T.; Igarashi, H.; Hiroyuki Uchida, H.; Watanabe, M. J. Electrochem. Soc. 1999, 146, 3750-3756.
The AOT + n-Butanol/n-Heptane/Water System
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Figure 5. Lattice parameter of the chemically disordered fcc structure of PtFe alloys as a function of Fe content. The symbols correspond to data taken from ref 41, and the line represents Vegard’s equation. Table 1. XRD-Derived Structural Characteristics of the Pt70Fe30/C Samples Obtained in Microemulsion Systems with and without n-Butanol as Cosurfactant
C4H9OH
crystallite size (nm)
lattice constant (Å)
Pt-Pt distance (nm)
x(Fe)
Pt/Fe (alloyed phase)
with without
2.5 2.7
3.892 3.902
2.752 2.759
0.605 0.342
70:30 83:17
by Vegard’s law assuming that the variation of lattice parameter with Fe content follows
x(Fe) )
a - ao aalloy - ao
(3)
where a is the lattice parameter of PtFe materials that depends on composition, aalloy is the lattice parameter for a PtFe solid solution taken here as 3.877 Å (PDF 29-717), and ao is the lattice parameter of carbon-supported Pt (3.915 Å) that was determined for Pt/C samples, the value of which was found to be in good agreement with literature data.43 On these bases, it seems quite reasonable to assume that Vegard’s law is also valid for the samples prepared in this work and allows estimating the fraction of alloyed Fe in the Pt70Fe30/C samples. The values of x(Fe) obtained indicate that the alloyed phase of the PtFe nanoparticles prepared in the AOT + n-butanol/n-heptane/water system has a Pt/Fe composition of 70:30. In contrast with that result, an alloyed phase composition of 83:17 was estimated for the PtFe sample prepared in the absence of n-butanol. The results obtained from XRD data indicate that larger amounts of Fe are incorporated into the PtFe alloy when the synthesis is carried out in a microemulsion containing n-butanol as cosurfactant. The average crystallite size (D) was calculated using Scherrer’s equation:42
D)
0.9λ ω cos θ
(4)
where λ is the wavelength of the X-ray radiation (1.54056 Å), ω is the full-width half-maximum intensity of the diffraction peak (in radians), and θ is the angle of the considered Bragg reflection. For that, the [220] peak of the Pt fcc structure at 2θ ∼ 67° was used (because the broad carbon peak does not interfere in this region). The average crystallite size was found to be almost independent of the microemulsion system used (with or without n-butanol). XRD results are summarized in Table 1. (42) Cullity, B. C. Elements of X-ray Diffraction, 2nd ed.; Addison-Wesley Publishing Company, Inc.: London, 1978; p 555. (43) Antolini, E.; Salgado, J. R. C.; Gonzalez, E. R. J. Electroanal. Chem. 2005, 580, 145-154.
Figure 6. TEM images and particle size histograms for PtFe/C samples prepared in microemulsions: (a) AOT + n-butanol/nheptane/water; (b) AOT/n-heptane/water. Table 2. EDX and TEM Results for the Pt70Fe30/C Samples Obtained in Microemulsion Systems with and without n-Butanol as Cosurfactant C4H9OH
EDX Pt/Fe ratio
TEM particle size (nm)
SD (nm)
σ (%)
with without
73:27 88:12
3.0 2.8
0.46 0.38
15 14
The compositions of the alloyed phases derived from XRD data were also found to be in good agreement with the atomic ratios obtained from EDX analysis. It was found that the EDX composition of PtFe nanoparticles prepared in the microemulsion containing n-butanol was very near the nominal value, while a lower than nominal amount of Fe was found for the PtFe sample prepared in the microemulsion without n-butanol. The good agreement between EDX Pt/Fe ratios (Table 2) and alloy compositions estimated from Vegard’s law appears to indicate that Fe is predominately in metallic form and alloyed with Pt in both samples. TEM images showed that PtFe/C samples synthesized with and without n-butanol in the microemulsion have similar particle size, as expected from the fact that both samples were prepared using the same water/surfactant molar ratio (w ) 8). Typical TEM images and particle size histograms are shown in Figure 6. The mean particle sizes were 3.0 and 2.8 nm for PtFe nanoparticles prepared with and without n-butanol, respectively, in good agreement with the XRD average crystallite diameter. It can also be observed that both samples show a very narrow particle size distribution and are nearly monodispersed systems (σ ) 15 and 14%, respectively).44 In summary, the results of the present study clearly show that the presence of n-butanol in the microemulsion produces PtFe (44) Hunter, R. J. Foundations of Colloid Science; Clarendon Press: Oxford, 1989; Vol. 1, p 104.
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nanoparticles containing larger amounts of Fe than those prepared in microemulsions without the cosurfactant, without modifying their size and polydispersity. Studies of the incorporation of n-alkanols into the reverse micelles in the AOT/n-heptane/water system carried out by Lissi and Engel28 evidenced that partitioning of alcohols between the aqueous and n-heptane phases occurs. The results were interpreted in terms of an interfacial localization of the polar head of the alcohol in the micellar interface with the alkyl chain extended toward the bulk solvent. These changes of the molecular structure of the interfacial film would also explain the variations observed in properties such as conductance percolation and rate of water uptake.29,30 The effect of nonionic cosurfactants on the AOT reversed micellar interface was also studied by Naza´rio et al.45 For linearchained alkyl alcohols, they found that the apparent hydrodynamic radius determined by dynamic light scattering was only slightly dependent on the concentration and chain size of the alcohol. By studying the dependence of percolation on temperature, they concluded that linear-chained alkyl alcohols increase the rigidity of the interface. Because the fluidity of the micellar interface is connected with the solubilization site of the cosurfactant, they concluded that the observed increase in rigidity of the interface would only be possible if the alcohol solubilizes in the surfactant tail region and so pushes the surfactant head groups together. On this ground, the rigidity of the micellar interface is likely to be affected by the addition of n-butanol into the AOT/n-heptane/ water system and this, in turn, would decrease the percolation of the Fe ions solubilized into the aqueous droplets toward the oil phase, resulting in the larger incorporation of Fe into the nanoparticles observed. This effect would also explain the observations of colorless or yellowish supernatants described above. Besides the changes in the rigidity of the micellar interface, Naza´rio et al.45 also observed that, for low water contents (w below ca. 20), all the plots of apparent hydrodynamic radius against w fall practically together, indicating that there was no significant effect of either alcohol chain length or concentration on the apparent hydrodynamic radius of the reversed micelles. Because in the present work the water content was rather low (w ) 8) and identical for both microemulsions (with and without n-butanol), it is reasonable to conclude that the addition of n-butanol to the AOT/n-heptane/water system would not provoke significant changes in the size of the water droplets. This would
fully explain why the presence of n-butanol as a cosurfactant does not alter the size and polydispersity of the PtFe/C nanoparticles. Studies of synthesis in AOT + n-butanol/n-heptane/water microemulsions of PtFe/C materials of compositions other than 70:30 are under way and indicate that the results presented here also apply to other catalysts compositions. Thus, introducing n-butanol as a cosurfactant into the AOT/n-heptane/water system seems a promising alternative for obtaining PtFe alloy nanoparticles with the desired composition, without the need of preparing functionalized surfactants and/or the use of inert atmosphere.46
(45) Naza´rio, L. M. M.; Hatton, T. A.; Crespo, J. P. S. G. Langmuir 1996, 12, 6326-6335.
(46) Duxin, N.; Stephan, O.; Petit, C.; Bonville, P.; Colliex, C.; Pileni, M. P. Chem. Mater. 1997, 9, 2096-2100.
Conclusions The phase diagram of the pseudo-ternary AOT + n-butanol/ n-heptane/water system, at a mass ratio of AOT/n-butanol ) 2, presents a vast one-phase microemulsion region in which the structural transition from a w/o to o/w microemulsion occurs continuously without phase separation. Carbon-supported Pt70Fe30 nanoparticles prepared in either pseudo-ternary or AOT/ n-heptane/water microemulsions exhibit similar average crystallite diameter and mean particle size. And in both cases, the PtFe nanoparticles obtained were nearly monodispersed. The composition, however, was influenced by the presence of n-butanol in the microemulsion, which lead to complete reduction of the Fe ions that allowed for obtaining alloyed PtFe nanoparticles with the desired composition, without the need of preparing functionalized surfactants and/or the use of inert atmosphere. The larger incorporation of Fe and the unchanged sizes and polydispersities observed for nanoparticles prepared in the presence of n-butanol as cosurfactant can be interpreted in terms of an increase of the rigidity of the micellar interface, which is associated with the solubilization of the alcohol in the surfactant tail region that pushes the surfactant head groups together, without significantly altering the size of the water droplets. Acknowledgment. The authors acknowledge the financial support of the Brazilian Agencies Fundac¸ a˜o de Amparo a Pesquisa do Estado de Sa˜o Paulo (FAPESP, 04/15570-8; 04/06762-0), Conselho Nacional de Desenvolvimento Cientı´fico e Tecnolo´gico (CNPq, 550153/2005-5), and Financiadora de Estudos e Projetos (FINEP, Rede PEM - 01.06.0939.00). A.R.M and L.C.V. acknowledge the fellowships granted by FAPESP. LA702146Q
