Mechanism of YF3 Nanoparticle Formation in Reverse Micelles

ARTICLE pubs.acs.org/Langmuir

Mechanism of YF3 Nanoparticle Formation in Reverse Micelles Jean-Luc Lemyre, Sebastien Lamarre, Ariane Beaupre, and Anna M. Ritcey* Departement de Chimie and CERMA, Universite Laval, Pavillon Alexandre-Vachon 1045, Avenue de la Medecine, Quebec G1V 0A6, Canada

bS Supporting Information ABSTRACT: This article reports an investigation of the mechanism of YF3 nanoparticle formation in two variants of the reverse microemulsion precipitation method. These two variants involve the addition of F
, either as a microemulsion or directly as an aqueous solution, to Y3+ dispersed in nonionic reverse micelles. The two methods yield amorphous and single-crystal nanoparticles, respectively. The kinetics of reagent mixing are studied by 19F NMR and colorimetric model reactions, and the particle growth is monitored by TEM. Mixing and nucleation are shown to occur within seconds to minutes whereas particle growth continues for 4 to 48 h, depending on the particle type. Moreover, the growth rate remains constant during most of the growth period, indicating that Ostwald ripening is the most probable growth mechanism. The single-emulsion method also produces a minority amorphous population that exhibits significantly different growth kinetics, attributed to a coagulation mechanism. Secondary growth experiments, involving the addition of precursor ions to mature particles, have been conducted to evaluate the relative importance of nucleation and the competitive growth of existing particle populations. The key differences between the two methods reside in the nucleation step. In the case of the classical method, nucleation occurs upon intermicellar collisions and under conditions of comparable concentrations of Y3+ and F
. This method generates more numerous stable nuclei and smaller particles. In the singlemicroemulsion method, nucleation occurs in the presence of excess F
through the interaction of Y3+-containing micelles with microdroplets of aqueous F
. These conditions lead to the formation of crystalline particles and a wider size distribution of unstable nuclei.

I. INTRODUCTION Nanoparticles occupy an important place in many fields of modern science. They are of interest in chemistry, physics, medicine, and engineering and in both fundamental and applied science. The past decade has seen an ever-increasing number of publications reporting the synthesis, characterization, properties, and applications of nanoparticles covering a wide range of chemical compositions. Despite the vast existing literature, challenges remain. In general, the achievement of size and shape control represents an important objective because these characteristics typically influence particle properties. This article focuses on nanoparticle synthesis via the reverse microemulsion method. This method has been employed for the preparation of nanoparticles from a diverse variety of materials, including metals,1
5 silica6,7 and other oxides,8
12 semiconductors,13
18 polymers,19
21 and even superconductors.22
24 It has also been used to obtain particles with core
shell architectures.25
29 The reverse microemulsion method can be applied to a wide range of materials because it is based on simple aqueousphase reactions. In principle, any reaction between water-soluble reagents that does not destabilize the microemulsion can be carried out in the reverse micellar environment. Within this environment, reactions that produce a solid phase typically yield nanoparticles. The reverse micelle approach generally offers exceptional control of particle size, which can be conveniently modulated through the r 2011 American Chemical Society

variation of simple system parameters such as the relative quantities of solvent, surfactant, and reagents. Furthermore, essentially monodisperse size distributions are obtained. Although this preparation method has been frequently used in the past 20 years for various types of nanoparticles, its success remains largely dependent on the trial-and-error determination of optimal composition conditions. Furthermore, the processes that lead to the characteristic monodispersity and size control are not fully understood. Unfortunately, the study of this type of synthesis is difficult for several reasons. Even though the processes involved are dynamic, very few system parameters can be monitored in real time. For example, while static characteristics such as the micelle size and aggregation number can be found, more interesting dynamic parameters, such as the rate and efficiency of material exchange between micelles, are much more difficult to quantify. The dynamics of mixing have been investigated by fluorescence quenching experiments for several systems.30,31 However, because the efficiency of intermicellar exchange is dependent on the flexibility of the surfactant layer,32 the results obtained for a given surfactant
organic solvent combination cannot be generalized to other systems. Furthermore, comparative studies, based on Received: June 23, 2011 Revised: August 11, 2011 Published: August 15, 2011 11824

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Langmuir the systematic variation of synthesis conditions, are complex to design because of the impossibility of changing only one parameter at a time. For example, the micelle size cannot be varied by simply changing the volume of water in the system without simultaneously changing either the concentration or the total quantity of reactants. It then becomes unclear if any resulting variation in particle size arises from a change in micelle size induced by the modified volume of water or by the differences in the quantity or concentration of the reagents. This article involves the synthesis of YF3 nanoparticles within the micellar system composed of surfactant igepal CO520, water, and cyclohexane. As previously reported,33 two variations of the preparation method yield significantly different particle populations. The aim of this article is to gain a better understanding of the fundamental mechanisms of YF3 nanoparticle formation in this microemulsion system. Experiments are furthermore designed to identify the key differences between the two, seemingly similar, synthesis methods. In addition to the fundamental importance of understanding the mechanism of particle formation and growth, the results presented here may be of practical interest. The main disadvantage of the preparation of nanoparticles in reverse microemulsions is the small quantity of material obtained for a given reaction volume, even in the case of quantitative yields. This is because the method necessitates an important, inactive volume of supporting fluid. A clearer understanding of the mechanisms of particle nucleation and growth could lead to the design of new synthesis methods, which are not necessarily limited to the microemulsion environment and are amenable to the production of larger quantities of material. Furthermore, the identification of the appropriate experimental conditions for the suppression of nucleation during secondary growth is crucial to the preparation of particles with controlled core
shell architectures.

II. EXPERIMENTAL SECTION Nanoparticles were prepared by the two methods outlined below. All chemicals were supplied by Aldrich and used as received without further purification. Classical Reverse Microemulsion Method. Amorphous yttrium fluoride nanoparticles were prepared using the previously described microemulsion method.33 Reverse microemulsions (r-μE) were first prepared by mixing 0.5 mL of a 0.04 M aqueous solution, containing either yttrium chloride (YCl3) or ammonium hydrogen difluoride (NH4HF2), with 2 g of the commercial polyoxyethylene nonylphenol surfactant igepal CO520 (4-(C9H19)C6H4(OCH2CH2)5OH) and 15 mL of cyclohexane. Homogeneous reverse microemulsions were obtained by mixing with a magnetic stirrer, followed by 10 min in an ultrasonic bath. Equal quantities of the two separate microemulsions containing YCl3 and NH4HF2 were then mixed together, with agitation under ambient conditions, to produce the YF3 particles. The resulting suspension was clear to the eye. Single Reverse Microemulsion Method. Single-crystal yttrium fluoride nanoparticles were prepared using the previously described microemulsion method.33 A reverse microemulsion was first prepared by mixing 0.5 mL of 0.04 M aqueous YCl3 and 2 g of igepal CO520 in 15 mL of cyclohexane. As above, the microemulsion was mixed with a magnetic stirrer followed by 10 min in an ultrasonic bath. With vigorous stirring under ambient conditions, 0.5 mL of aqueous 0.04 M NH4HF2 was then added directly to the microemulsion. The resulting suspension of YF3 nanocrystals appeared clear. Secondary Growth Experiments. These experiments involve the addition of precursor reagents (Y3+ and F
) to a pre-existing population of mature particles, still within their original microemulsion environment.

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The various experiments differ in the exact way in which the additional ions are introduced. All experiments in this section employ the same initial YF3 sample, obtained via the single-microemulsion method. This initial population consists of a mixture of 35 nm octahedral and 75 nm amorphous spherical particles. Additional reagents for secondary growth were added in four different ways. Yttrium was added first, incorporated either in reverse micelles at the same concentration and water/surfactant ratio as employed in the original synthesis or in a small volume of a concentrated aqueous solution. A relatively concentrated solution (0.4 M instead of 0.04 M) was employed to minimize the amount of additional water (0.05 mL instead of 0.5 mL). The microemulsion was homogenized with stirring for 10 min before the addition of the second reagent. Fluoride was then added, either as a reverse microemulsion or an aqueous solution, in both cases at a concentration equivalent to that of the original synthesis. Final particle populations were sampled more than 60 h after this addition to ensure sufficient time for complete growth. Because the amount of material added is equivalent to that employed in the original synthesis, its equal distribution among existing particles, without the formation new particles, would result in a doubling of the particle volume. Colorimetric Model Reactions. Two model reactions leading to color change upon mixing have been carried out in the micellar system. The first is the protonation of methyl red by NH4HF2. The second involves the precipitation of Prussian blue from FeCl3 and K4Fe(CN)6. The mixing of reagents was achieved as in the classical and singlemicroemulsion methods, with identical microemulsion compositions and reagent concentrations as employed for the YF3 nanoparticle synthesis. In the case of the single-emulsion method, both possible orders of reagent addition were tested. Transmission Electron Microscopy. Transmission electronic microscopy (TEM) images and electron diffraction patterns were recorded with a JEOL JEM-1230 at an accelerating voltage of 80 kV. Samples were prepared by allowing a drop of the YF3 suspensions, as obtained by the syntheses described above, to dry directly on a carboncoated nickel microscope grid. Size measurements were made from 100 to 300 randomly selected particles on TEM images with Scion Image software. TEM provides 2D projection images, and the octahedral crystals appear as hexagons. The distance between two opposite corners of a hexagon, however, cannot be used to measure the crystal size because these two points do not lie in the same plane and the length thus measured would be smaller than the actual distance. The particle size was rather measured by the distance between second adjacent corners of the hexagonal projection, which corresponds to the actual length of an octahedron edge. The octahedral particle sizes mentioned in this article refer to the octahedron edge, with one exception. In the case of the kinetics experiments, the crystalline particle shape is ill-defined, at times shorter than 2 h, and the size is measured as the diameter of a sphere. The particle volume was calculated according to the particle shape. In the case of octahedral particles, the calculated volume corresponds to that of a solid octahedron with the measured edge length. Amorphous particles, as well as very small ill-defined crystalline particles observed at short times in the kinetic experiments, were approximated as spheres, and their volume was calculated as such from the measured diameters. 19 F Nuclear Magnetic Resonance. 19F nuclear magnetic resonance measurements were performed at 25 °C on a 400 MHz Varian Inova spectrometer. Samples were prepared using cyclohexane-d12.

III. RESULTS AND DISCUSSION YF3 Nanoparticles from the Classical Microemulsion Method. TEM images of particles obtained by the mixing of

two microemulsions containing YCl3 and NH4HF2 are shown in Figure 1a. This method yields amorphous YF3 particles with an undefined, somewhat spherical shape. The size of the particles is 11825

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Table 1. Comparison between Nanoparticles Obtained via Two Similar Microemulsion Synthesis Schemes with the Same Initial Microemulsion Compositiona classical method

single μe method

amorphous

single crystal and amorphous

undefined shape

octahedron, 42 nm

16 nm

triangular prism, 72 nm spherical, 76 nm

monodisperse a

Figure 1. TEM image of YF3 nanoparticles prepared with (A) the classical microemulsion method and with (B, C) the single-microemulsion method.

found to increase with the water content of the microemulsion, as is generally expected for this method.34 Particles ranging in size from 6 to 47 nm can be obtained, with the maximum attainable size being limited by the range of microemulsion stability with respect to water content at room temperature. Electron diffraction measurements reveal only diffuse halos, indicating the amorphous nature of the particles. YF3 Nanoparticles from the Single-Microemulsion Method. This method consists of the addition of NH4HF2, as an aqueous solution, to a reverse microemulsion containing YCl3. This synthesis produces a mixture of octahedral single crystals

monodisperse

2 g of surfactant/0.5 mL of water/15 mL of cyclohexane.

and spherical amorphous particles, as shown in Figure 1b,c. Octahedral particles from 20 to 350 nm have been obtained by varying the surfactant, water, and reagent concentrations. The characteristics of the different YF3 particles produced by the two synthesis methods are summarized in Table 1. Particle Growth Kinetics. The kinetics of nanoparticle formation was monitored by the TEM observation of samples isolated at various times during the growth process. Samples were prepared by rapidly drying a few drops of the microemulsion mixture on a TEM grid, thus preventing further growth of the particles. Representative TEM images are shown in Figure 2 and illustrate the evolution of YF3 particle size and shape for both the two-emulsion and single-emulsion methods. The singleemulsion method yields a mixture of crystalline and amorphous particles, and representative images for the two particle types are provided in the middle and bottom rows, respectively. For both methods, particles, although very small, are visible in samples isolated soon after the mixing of reagents and reach a size comparable to that of the micelles in less than 5 min. After 30 min, the morphological differences between amorphous and crystalline particles can be distinguished. After 4 h, the octahedral shape of the crystalline particles is clear. The evolution of particle size, as evaluated from TEM images, as a function of time is plotted in Figure 3. Interestingly, particles formed by the classical method reach their final size after 4 h, whereas the octahedral and amorphous particles obtained by the single-microemulsion method continue to grow for 24 h and 2 days, respectively. Furthermore, no additional nuclei appear during the growth process. It is also significant to note that although the same quantities of precursor reagents are employed, smaller particles are obtained with the two-emulsion method, implying the formation of a larger number of stable nuclei. The rate of particle growth, expressed in terms of the amount of material added to the growing particles, is better represented by a plot of particle volume as a function of time, as shown in Figure 4. Significant differences are observed among the three populations. The growth rate of the amorphous particles prepared by the single-emulsion method (hereafter referred to as amorphous type I) is about 5 times faster than that of the octahedral particles growing within the same sample. Furthermore, from the inset, one can see that the initial growth rate of the amorphous particles prepared by the classical method (hereafter denoted type II) is very similar to that of the crystalline particles obtained from a single emulsion. Presumably, particles reach their final size sooner for the classical method because more nuclei are formed and the reactants are thus consumed faster. Importantly, Figure 4 shows that constant growth rates are maintained for most of the time span of particle formation. Constant growth rates are characteristic of zeroth-order kinetics, indicating that 11826

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Figure 2. TEM images of YF3 nanoparticles isolated at different times during the growth process. Particles produced by the classical two-microemulsion method are shown on the top row. The single-microemulsion method produces a mixture of crystalline and amorphous particles, and representative images for each of these morphologies, obtained from a single synthesis, are shown in the middle and bottom rows, respectively. Note that crystalline particles are also visible on the bottom row.

Figure 3. Size of YF3 nanoparticles, as evaluated from TEM images, as a function of time for amorphous (II) particles prepared by the classical microemulsion method (1) and crystalline, octahedral particles (0) and amorphous (I) particles (b) obtained from the single-microemulsion method. Dotted lines have been added to guide the eye. Particle size standard deviations of 1
3, 1
2, and 2
6 nm are found for the amorphous (II), crystalline, and amorphous (I) particles, respectively.

the growth rate is independent of the reagent concentration. This could be due to surface-restricted growth, presumably caused by the adsorption of surfactant, or to a limited feeding rate of reagents that are slowly released by the dissolution of smaller particles. These possibilities will be revisited in the general discussion section. Secondary Growth Experiments. These experiments consist of the addition of Y3+ and F
to emulsions containing an existing population of mature particles in order to reinitiate growth. The results from these experiments are summarized in Figure 5 and Table 2 and present TEM images and quantitative results from the secondary growth experiments, respectively. The table shows particle size before and after each experiment for each particle population. The volume increase is also included. Finally, the percentage of added reagents consumed by each particle population is presented in order to evaluate the relative importance of

Figure 4. Volume of YF3 nanoparticles as a function of time for amorphous (II) particles prepared by the classical microemulsion method (1) and crystalline octahedral particles (0) and amorphous (I) particles (b) obtained from the single-microemulsion method. The inset shows an enlargement of the first 2 h.

competing processes. As indicated in the first column of Table 2, the various experiments differ in the way in which the precursor reagents are added. The results have been analyzed by estimating the relative importance of competing processes that consume the additional ions, namely, secondary nucleation followed by the growth of new nuclei and the growth of pre-existing particles. It must be noted that the mode of ion addition influences the final overall composition of the system. The water to surfactant ratio is particularly important because it determines the size and number of reverse micelles present in the system. The composition of the various samples is thus provided as Supporting Information. The same original sample of preformed particles was employed for experiments 1 to 4. The starting particle population was prepared by the single-emulsion method and thus contains both crystalline and amorphous (I) particles, as illustrated by the central micrograph of Figure 5. Although the relative populations of the two types of particles cannot be quantified reliably by TEM because of the partial sampling nature of microscopy, it is clear 11827

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22 ( 2 5000 small occurrence 39 ( 2 30 000 46 ( 3 50 000 20 30 60 40 200 400 35 ( 2 37 ( 2 38 ( 2 41 ( 2 39 ( 2 52 ( 2 58 ( 2 microemulsion microemulsion aqueous 40 mM aqueous 40 mM aqueous 80 mM aqueous 120 mM

a

original particlesd 1 microemulsion 2 aqueous 400 mM 3 microemulsion 4 aqueous 400 mM 4b 2 aqueous 400 mM 4c 3 aqueous 400 mM

All experiments have been duplicated, and differences in size were less than 1 nm for all octahedral particles and less than 3 nm for most amorphous particles. Errors in size reported here are the standard deviations of 100
300 measurements per particle population. b Estimated values. c Percentage of the added reagents consumed by a particle population. In the case of the original particles, it is the percentage of reagents in the original synthesis. d Octahedral/amorphous: 97:3 by number, 70:30 by volume.

80 40 20 negligible 16 ( 3 2000 20 ( 3 4000 54 ( 5 80 000 small occurrence 64 ( 9 100 000 69 ( 9 200 000 30 10 40 40 70 80 000 300 000 300 000 600 000 400 000 400 000 40 100 100 300 200 200 75 ( 5 83 ( 5 100 ( 7 99 ( 4 115 ( 12 108 ( 6 109 ( 6 70 10 20 40 30

(nm3) (%) (nm3) (%) experiment

volume increase

particle edge (nm) fluoride addition yttrium addition

that the crystalline particles are much more numerous than the amorphous ones. It is, however, possible to estimate the ratio of the two populations from experiment 4. As discussed below, no significant secondary nucleation occurs in this experiment. Because no new particles are formed, all additional reagent is consumed by particle growth, without any change in the population ratio. It is thus possible to calculate the population ratio by solving simple equations because the volume of both particle types and the total volume of YF3 present are known, both before and after the second addition. Further details are provided as Supporting Information. The calculation indicates that 97% of the particles are crystalline whereas 3% are amorphous. This corresponds to a 70:30 mass ratio because of the larger size of the amorphous particles. It is important to note that this calculation is based on an increase in particle size that is comparable to the experimental error. The relative amounts of reagent consumed for each of the competing processes reported in Table 2 should therefore be considered to be only rough estimates.

octahedral

Table 2. Quantitative Results from Second Growth Experiments 1 to 4a

b

Figure 5. TEM images of the YF3 particles produced by the singlemicroemulsion method before and after the regrowth experiments. The central micrograph shows the original particle populations. The numbered arrows refer to the four different experiments described in the text. For each experiment, the order of ion addition is indicated by the numbers in parentheses. The abbreviations “aq” and “μE” signify the addition of reagents as either an aqueous solution or a microemulsion, respectively.

4000 6000 10 000 8000 50 000 70 000

reagents consumed (%)b,c particle volume (nm3)b reagents consumed (%)b,c

particle diameter (nm)

volume increase

amorphous

b

reagents consumed (%)b,c

particle edge (nm)

particle volume (nm3)b

particle diameter (nm)

amorphous octahedral

second nucleation

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Langmuir Yttrium Added as a Microemulsion, Followed by the Addition of Fluoride as a Microemulsion. In the first experiment, both YCl3 and NH4HF2 are added in the form of reverse microemulsions, identical to those employed in the classical microemulsion method. The resulting particles are shown in Figure 5-1. Three different populations of nanoparticles are clearly present. The initial crystalline particles do not appear to have undergone any significant change in size, although a small increase, comparable to the standard deviation of the measurement, may have occurred. More significant growth is observed for the amorphous (I) particles, which is consistent with the higher growth rate illustrated in Figure 4 for this population. Most of the added material (80%), however, served to form new particles arising from a second nucleation. This new population consists of small (16 nm), roughly spherical particles similar to those prepared by the classical microemulsion method. Because the global consumption of the added ions depends on the relative number of particles, the volume change associated with the growth of individual particles is also included in Table 2. The results indicate that although 80% of the added ions serve to produce new particles, similar amounts of material are required for the observed 2 nm increase in the diameter of a 35 nm crystalline particle as for the formation of a new 16 nm particle. This suggests that these two populations have similar growth mechanisms and comparable access to the added reagents. The high global consumption of the added ions by the new amorphous (II) population simply reflects the relatively large number of this type of particle. The addition of the second reagent (F
) as a microemulsion, as in the classical method, favors the formation of more numerous nuclei. Finally, the data in Table 2 show that the volume increase of the pre-existing amorphous (I) particles exceeds that of the other two particle types by more than an order of magnitude. As was found in the kinetics experiments, the amorphous (I) particles produced as a minor component by the single-emulsion synthesis appear to grow via a mechanism different from that of both the crystalline particles and the amorphous particles (II) produced by the classical method. Yttrium Added as an Aqueous Solution, Followed by the Addition of Fluoride as a Microemulsion. In the second experiment, YCl3 is added as an aqueous solution. The amount of YCl3 added is equivalent to that employed in the original synthesis, but the concentration of the solution has been increased by an order of magnitude to minimize the effect of additional water on the microemulsion. After adequate mixing to re-establish a clear, stable system, an equivalent molar quantity of NH4HF2 is introduced in the form of a microemulsion. The resulting particles are shown in Figure 5-2. As in the previous experiment, three populations of nanoparticles are present. The initial crystalline particles are still present and have undergone a 30% increase in volume. The preexisting amorphous (I) particles have also grown, more than doubling their initial volume. Around 40% of the added reagent has served to form a new particle population that consists of 20 nm roughly spherical particles, similar to those prepared by the classical microemulsion method. In comparison with experiment 1, the new population of amorphous (II) particles consumed less of the added reagents and grew to a larger size. This observation implies that fewer nuclei have been produced, notwithstanding the lower overall water content for the same number of ions. However, the probability of nucleation depends not only on the ion concentration but also on the number of micelles. The addition of either of the reagents as an aqueous solution, rather than as a microemulsion, results in an increase in the water to surfactant ratio. At the compositions employed for the majority of the experiments, the micelle size remains relatively

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constant (Supporting Information), and an increase in the water to surfactant ratio results in a corresponding increase in the number of micelles. An increase in the number of micelles, for a given number of ions, will in turn decrease the probability of nucleation. Because F
is added in the form of a microemulsion, secondary nucleation occurs under conditions analogous to those of the classical method, and as expected, amorphous particles are obtained. Yttrium Added as a Microemulsion, Followed by the Addition of Fluoride as an Aqueous Solution. In the third experiment, YCl3 is added as a reverse microemulsion to the existing particles. Fluoride is then introduced as an aqueous solution under vigorous stirring, as in the single-microemulsion method. This experiment results in a mixture of four particle populations, as shown in Figure 5-3. Both populations of initial particles grow significantly, with increases in volume of 60 and 100% for the octahedral and amorphous (I) particles, respectively. This growth accounts for the consumption of about 80% of the added material. Secondary nucleation also occurs, producing both crystalline octahedral and amorphous (I) particles, using only 20% of the added reagents. The two new populations appear similar to those obtained by the single-microemulsion synthesis, that is, octahedral single crystals mixed with larger amorphous spherical particles. Yttrium Added as an Aqueous Solution, Followed by the Addition of Fluoride as an Aqueous Solution. In the fourth experiment, YCl3 is added as a concentrated aqueous solution as in experiment 2. NH4HF2 is then added directly as a solution, as in the single-microemulsion method. The resulting particles are presented in Figure 5-4. In this case, the number of particles arising from secondary nucleation is negligible. Pre-existing crystalline particles have undergone a modest size increase (40% volume) using a quarter of the available material. The minority amorphous (I) particles consumed most of the reagents and quadrupled their volume. Experiments 4b and 4c were conducted in a similar manner except that a larger volume of YCl3 and more concentrated NH4HF2 were added in order to double and triple the reagent loading. The increase in the quantity of added ions promotes the nucleation of new particles. It also enhances the relative growth of crystalline particles with respect to the amorphous (I) ones. The overall observations of the regrowth experiments indicate that the nature of the particles arising from secondary nucleation is determined by the manner in which the second reagent (NH4HF2) is added. When NH4HF2 is added in the form of a reverse microemulsion (experiments 1 and 2), amorphous (II) particles are formed, as in the classical two-emulsion method. When fluoride is added as an aqueous solution (experiments 3, 4b, and 4c), secondary nucleation results in a mixture of amorphous (I) and single-crystal particles analogous to those formed by the single-emulsion method. The manner in which NH4HF2 is added also influences the relative importance of secondary nucleation with respect to the growth of pre-existing particles, with nucleation being favored by the addition of fluoride as a microemulsion. This result is consistent with the observation that the classical method produces smaller particles, as noted in Table 1. The manner in which the first reagent (YCl3) is added also affects the resulting particle populations. Whereas the addition of YCl3 as either a microemulsion or an aqueous solution does not modify the nature of the particles obtained, it does influence the relative importance of particle growth with respect to secondary 11829

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nucleation, with secondary nucleation being more important when YCl3 is added as a microemulsion. Because the delay between the additions of the two reagents is sufficient for the complete incorporation of YCl3 within the micelles, this observation is probably due to the difference in the final composition. When YCl3 is added as a solution, rather than as a microemulsion, the final water to surfactant ratio is higher because no surfactant is added. As mentioned above, these conditions increase the number of micelles and favor the growth of existing particles over the nucleation of a new population. Furthermore, it is primarily the amorphous (I) particles that grow. With the same mode of addition for the second reagent (experiments 1 vs 2 and 3 vs 4), the original amorphous particles grow more under conditions of a higher water to surfactant ratio. This may be an indication that the amorphous (I) particles benefit from a different growth mechanism and that this mechanism is more efficient with a more flexible surfactant layer. Addition of Mature Particles to a Growing Population. In this series of experiments, a microemulsion containing preformed, mature particles is added at variable times to an immature population, initiated by the single-microemulsion method. Particles from this series of experiments are presented in Figure 6, and their size characteristics are summarized in Table 3. The

composition of the reaction mixture is identical to that in experiment 3, but the order of addition of the components (mature particles vs free ions) is reversed. In the early stages of this experiment, free ions can thus nucleate new particles in the absence of mature particles. Very similar final particle populations are obtained for delay times of between 2 and 30 min. In all cases, new nucleation leads to 19 nm crystalline octahedral particles. Furthermore, both populations of pre-existing particles grow significantly. Octahedral particles increase their volume by 60
80% whereas amorphous (I) particles triple in volume. The observation that the final particle sizes are independent of the delay before the addition of mature particles suggests that nucleation is complete within the first 2 min. If this were not the case, then one would predict an increase in the number of new particles for longer delays. An increase in the number of particles would result in smaller final sizes for all populations because less material would be available for the growth of both pre-existing particles and each new nucleus. Interestingly, very few small (∼50 nm) amorphous (I) particles are observed in TEM images of samples 5a
5d. This is surprising because small amorphous particles are present in significant numbers in samples isolated after 30 min of reaction but before the addition of preformed particles. Moreover, the size increase of the added preformed amorphous (I) particles is more important in experiment 5 than in experiment 3. These observations suggest that the newly forming amorphous (I) population is consumed by the growth of the added pre-existing amorphous (I) particles. We also examined the aging of particle mixtures. Microemulsions containing different particle populations have been mixed and sealed for 2 years. Two sizes of amorphous (II) particles were prepared by the classical microemulsion method (14 and 46 nm). Two sizes of octahedral particles were prepared by the singlemicroemulsion method (33 and 48 nm). These also contained larger spherical amorphous (I) particles (53 and 98 nm, respectively). All possible binary mixtures of these four samples were prepared. After 2 years, all particle populations were present and exhibited no significant changes in size. This demonstrates the stability of all mature particle types and the absence of a mechanism for interparticle conversion. 19 F NMR. 19F NMR spectra were recorded in an effort to identify the fluoride species present in the microemulsion mixtures during the synthesis, which serves to feed the particle growth. Unfortunately, because of the very low 19F concentration in such samples, long acquisition times are required and the kinetics of

Figure 6. TEM images of YF3 nanoparticles from experiment 5, obtained after the addition of mature particles to a growing population initiated by the single-microemulsion method. Mature particles were added after delays of 2, 5, 10, and 30 min.

Table 3. Quantitative Results from Experiment 5a octahedral

amorphous

volume increaseb

second nucleation

volume increaseb

octahedral

time of

particle

particle

particle

addition (min)

edge (nm)

(%)

(nm3)

diameter (nm)

(%)

(nm3)

edge (nm)

volumeb (nm3)

amorphous

original particles 5a 2

35 ( 2 41 ( 2

60

10 000

65 ( 5 95 ( 6

200

300 000

19 ( 2

3000

small occurrence

5b

5

43 ( 2

90

20 000

95 ( 5

200

300 000

20 ( 2

4000

small occurrence

5c

10

42 ( 2

70

10 000

93 ( 5

200

300 000

19 ( 2

3000

small occurrence

5d

30

42 ( 2

70

10 000

100 ( 5

300

400 000

19 ( 2

3000

small occurrence

experiment

c

particle

a

All experiments have been duplicated, and differences in size were less than 1 nm for all octahedral particles and less than 3 nm for most amorphous particles. Errors in size reported here are the standard deviations of 100
300 measurements per particle population. b Estimated values. c Original particles are from a different batch from experiments 1
4. The population ratio is unknown. 11830

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Table 4. Poisson Distribution of Ions within the Reverse Micelles, Calculated for the Dispersion of 0.5 mL of YCl3(aq) and 0.5 mL of NH4HF2(aq), Both 0.04 M, in 15 mL of Cyclohexanea percentage of micelles containing the specified number of ions F


Y3+

number of ions 0

98.2

96.5

1

1.8

3.5

2

0.016

0.06

3

0.0001

0.0008

4

0.0000004

0.000007

5

0.000000002

0.00000005

a

The water content of the micelles was estimated from DLS size measurements.

fast processes cannot be studied. Nevertheless, after 40 min of acquisition, a single resonance at
120 ppm, characteristic of the presence of free aqueous fluoride, is evident in the spectrum of a microemulsion containing NH4HF2. This signal is immediately lost, however, upon mixing with a microemulsion of YCl3. Similarly, no 19F NMR signal can be detected immediately after the addition of F
to a microemulsion containing Y3+. This observation indicates that, for both methods of particle synthesis, most of the free fluoride is rapidly consumed upon mixing. The TEM images of Figure 2 show, however, that nanoparticle formation is far from complete at such early stages. At this point, we can only speculate about the state of the fluoride between the beginning of the reaction and the complete formation of YF3 nanoparticles. Presumably, the fluoride is stored as undetectable, unstable, solid aggregates that are able to redissolve and provide ions for the growth of stable nuclei. The concentration of soluble fluoride in equilibrium with these aggregates is too low to be detected by NMR and too low to initiate further nucleation. Kinetics of Mixing by Colorimetric Model Reactions. Model reactions leading to rapidly detectible color changes were performed in order to characterize the kinetics of mixing in the Igepal micellar system. Indicator Dye. The first experiment involves monitoring the color change of an indicator dye (methyl red) dissolved within reverse micelles upon the addition of acidic NH4HF2, either as a second microemulsion or directly as an aqueous phase. In both cases, the color change occurs almost instantaneously. This experiment demonstrates that the mixing of reagents in the micellar system is very rapid relative to crystal growth and can be excluded as a kinetically limiting step in particle synthesis. These results are in agreement with the rapid timescale of mixing (microseconds to milliseconds) reported for other systems.31 Prussian Blue. This second experiment involves the precipitation of Prussian blue particles within the micellar system. The reaction proceeds by mixing pale-yellow FeCl3(aq) with colorless K4Fe(CN)6(aq) to produce a dark-blue pigment, Prussian blue, Fe4[Fe(CN)6]3 3 xH2O(s) (x = 14
16).35,36 Because this reaction requires the formation of solid nuclei, it is a better model for particle formation. For both mixing methods, the color change occurs within 2 to 3 s. This again demonstrates that, within the limited timescale accessible with these simple experiments, the difference between the two methods of particle synthesis cannot be attributed to differences in the kinetics of mixing.

Mechanism of Particle Formation. Nucleation appears to be the crucial step in determining the crystalline or amorphous nature of the YF3 nanoparticles. Both types of particles are stable, and no interconversion between mature particles occurs. In considering the nucleation step, it is important to keep in mind the low rate of occupancy of the reverse micelles by the precursor ions. The occupancy of the micelles for a typical synthesis, based on the assumption of the Poisson distribution37
39 for the reagents, is presented in Table 4. This simple calculation indicates that less than 2% of the micelles contain yttrium. Nucleation requires the simultaneous presence, within a given micelle, of a sufficient number of precursor ions to reach local saturation and overcome the surface energy cost of forming a solid nucleus. For the nucleation to be effective, there must be enough material for the nuclei to reach a critical size. Below this size, nuclei will be unstable and redissolve. Only nuclei reaching the critical size can spontaneously grow to become particles. The critical size of the nuclei depends on the reagent concentration and the surface energy of the solid
liquid interface, according to eq 1,41

ΔGnucleation ¼ Sγ þ V ΔGcrystallization

ð1Þ

where S and V are the surface and volume of the nucleus, γ is the interfacial energy, and ΔGcrystallization is the free energy of crystallization. ΔGcrystallization is a function of the reagent concentration and becomes negative as saturation is reached. Because of the very small size of the micelles, local concentrations can be very high, even for a few ions, favoring nucleation. Moreover, the interfacial energy is lowered by the adsorption of surfactant to the nuclei, also promoting nucleation. These considerations would suggest that reverse micelles constitute environments in which the critical nucleus size is smaller than that associated with precipitation in simple aqueous solutions. Within reverse micelles, stable nuclei composed of as few as two to four metal ions have been proposed for Ni2B39 and CdS,42 respectively. For a given amount of material, systems that facilitate nucleation will yield smaller particles. The colorimetric experiments indicate that mixing within the micellar system occurs rapidly, within a few seconds. Ionic reactions are known to be very fast, and YF3 should precipitate as soon as the precursor ions meet. Although not sensitive enough to follow fluoride concentrations in early stages of particle formation, 19F NMR indicates the absence of free solvated fluoride 40 min after mixing. Because the nanoparticles require a considerably longer time to reach their final size, fluoride must be present in a transitory state, other than as free ions, during the synthesis. It can be reasonably assumed that the primary reagents (Y3+ and F
) react quickly to form monomers, unstable nuclei, and a few stable nuclei. The single-microemulsion method yields two types of particles: amorphous and octahedral crystalline, both present after 30 min. Although it is tempting to speculate that the amorphous particles result from the incorporation of F
within the micelles, followed by intermicellar collisions under conditions analogous to those of the two-emulsion method, the differences in the growth kinetics of the two types of amorphous particles refute this hypothesis. At the compositions considered here, the reverse micelles have a diameter of 4 nm, as measured by DLS.40 Nuclei, presumably contained within the micelles during early stages, grow to exceed the micelle size within the first 5 min. It is reasonable to assume that the particles remain surrounded by a layer of water and surfactant during growth and can exchange material with other reverse micelles in the system. The regular distance of separation 11831

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Langmuir and the absence of aggregation, as seen in Figures 1 and 2, support the presence of a surfactant layer. The presence of water within the surfactant layer is thermodynamically favored for this system and has been shown by NMR for AOT-coated AgBr particles.39 A water layer is also necessary for the exchange of ions and the growth of particles. Also relevant to the discussion is the observation that the outcome of the single-microemulsion method depends on the order in which the precursor ions are introduced. The synthesis described in this article proceeds by the addition of an aqueous fluoride solution to a yttrium-containing microemulsion and results in a mixture of octahedral and spherical particles. The reverse order, that is, the addition of an aqueous yttrium solution to a fluoride-containing microemulsion, produces amorphous particles of undefined form, similar to those obtained by the classical method. In the single reverse microemulsion method, the added solution will initially disperse in the organic phase in the form of droplets that are certainly larger than the water reservoirs within the micelles. The initial mixing of precursor ions through the interaction of these droplets with a micelle will therefore occur under conditions of a significant molar excess of the second reagent, as illustrated in Figure 7b. The influence of the order of ion addition emphasizes how crucial the few first seconds of mixing are in determining the nature of the resulting particles. Apparently, the local fluoride excess during the nucleation step favors the formation of crystalline YF3 nanoparticles, whereas an excess of yttrium leads to amorphous particles. Excess fluoride is not present during nucleation in the classical twoemulsion method, which might explain the different nanoparticles obtained. These observations could be explained by the electrostatic interactions involved during ion assembly. Under conditions of excess fluoride, the initial organization of three anions around a single Y3+ ion and the subsequent addition of a second metal ion are electrostatically favorable. Ordered nuclei of the correct stoichiometry could thus be produced, leading to crystalline particles. In contrast, in the case of excess Y3+ at the moment of nucleation, the probability of forming a neutral YF3 entity is greatly reduced. One can speculate that under these conditions intermediate species such as YF+2 and YF2+ are initially formed and their subsequent aggregation and the addition of fluoride lead to less-ordered particles. It should also be noted that preliminary scale-up experiments indicate that for the single-microemulsion method the final particle size decreases slightly with an increase in absolute volume, even though the concentrations of all constituents are held constant. This is another hint that nucleation occurs in the first moments of contact between the added aqueous phase and the reverse micelles because the size of this interface is the only parameter that is modified in the scale-up process. Experiment 5 also confirms that the nucleation process is complete within the first 2 min of mixing. In the case two microemulsions, the mixing of reagents occurs via intermicellar exchanges largely described in the literature.24,39,43
45 Briefly, a fraction of micellar collisions results in the formation of short-lived, nonstable dimers, permitting the intermicellar exchange of surfactant, water, and ions. In the case of the addition of an aqueous phase to a reverse microemulsion, the mixing process is less clear. Obviously, the addition of water must trigger a reorganization of the system. According to our previous studies of this micellar system,40 the reverse micelle size does not significantly change under these conditions because of a large excess of surfactant. Although the micellar composition is probably

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Figure 7. Schematic illustrating the nucleation conditions for (A) the classical microemulsion method and (B) the single-microemulsion method.

slightly different,46 it can be reasonably assumed that the primary effect of the addition of an aqueous solution is an increase in the number of micelles. The exact mechanism of micellar reorganization and the distribution of the added reagent, however, remain unknown. However, because of the rapidity of nucleation, it can be expected that it occurs before the micellar system reorganizes to its final state, upon the collision of Y3+-containing micelles with microdroplets of the fluoride solution. In fact, if micellar reorganization occurred before nucleation, then the two methods would lead to identical systems and identical particles. A schematic highlighting the essential differences between the presumed nucleation conditions for the two methods is provided in Figure 7. Given that the precursor ions are equally accessible to all populations, particles should grow at the same rate and thus reach a similar final size. However, the TEM results presented in Figures 2
4 reveal that the growth of amorphous (I) particles is 5 times faster and persists over a longer time period than that of the two other populations. In fact, an amorphous (I) particle consumes 10 times more material than an octahedral particle growing within the same sample. The higher growth rate for the amorphous (I) particles can be explained by either a different growth mechanism or by a limitation of the growth of the crystalline particles. All particle populations grow by the addition of ions acquired through collisions with reverse micelles. Y3+(aq) and F
(aq) are consumed in the early stage of the synthesis but remain available at low concentrations in equilibrium with unstable nuclei. Reagents are therefore stored in less-stable nuclei, and the particle growth rate is limited by their availability, which is held constant at the solubility. In other words, particles grow by Ostwald ripening from smaller unstable nuclei, as previously described47
49 for AgCl particles prepared by a similar method. The Ostwald ripening process is supported by the constant growth rate of both particle populations and the considerably long time necessary to complete the process.49 Even if Ostwald ripening is responsible for nanoparticle growth, once the particles attain a certain size, the solubility difference between populations is insufficient for material exchange, as indicated by the aging experiments. In addition to Oswald ripening, amorphous (I) particles may benefit from a second growth mechanism, namely, coagulation. 11832

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Langmuir This process, which involves the coagulation of two nucleated particles to form a larger one, has been previously demonstrated for the growth of CdS particles in reverse micellar systems.15,50
52 Such intermicellar particle exchange is limited to small particles because the opening of the surfactant layer becomes increasingly energetically unfavorable with increasing size.53 Nevertheless, up to a certain size, it is possible that nuclei are added directly to larger growing amorphous particles without passing though the dissolution step. Furthermore, the coagulation mechanism allows for the addition of more material per collision than does the Ostwald ripening mechanism, thus providing a faster growth rate. Higher water to surfactant ratios lead to more a flexible surfactant layer that facilitates the transfer of nuclei between micelles. Indeed, the regrowth experiments show that systems with higher water concentrations promote the relative growth of the amorphous (I) particle population. Moreover, experiment 4, for which the water/surfactant ratio is the highest during particle growth, also presents the greatest increase in size of the amorphous (I) particles. The persistence of particle growth over a longer time period also supports coagulation as the growth mechanism for amorphous (I) particles. After 24 h, the octahedral particles stop growing, most likely because of a lack of available reagents. The remaining nuclei may be slightly larger and less soluble, thus preventing Ostwald ripening, but continue to feed the growth of the amorphous (I) particles by coagulation. Although these experimental observations suggest that the amorphous (I) particles result from coagulation, this mechanism should not exhibit a constant growth rate because the decrease in the nuclei concentration would lead to a decrease in the growth rate. Indeed, the growth rate does slow down somewhat for this population during the second half of the growth period (Figure 4). It should also be noted that the appearance of these particles, as visualized by electronic microscopy, suggests that coagulation is involved because they resemble a fused collection of smaller particles. An alternative explanation for the different growth rates of the two particle populations is the possibility of a more difficult surface integration of the ions by the crystalline particles.41 The crystalline surface may be less accessible to reagents because of a significant surface charge or a stronger interaction with the surfactant than with the amorphous particles. This explanation, however, does not account for the differences between the amorphous (I) and amorphous (II) particles. A consideration of all of the results presented here thus supports Ostwald ripening as the growth mechanism for both the crystalline and amorphous (II) particles and coagulation as the primary growth mechanism for the amorphous (I) particles. These two mechanisms differ in the size of the nuclei that feed the growth. Ostwald ripening relies primarily on small nuclei that, because of their non-negligible surface energy, redissolve to maintain a sufficient concentration of free ions. Because solubility decreases with increasing size, larger nuclei will contribute less to this mechanism. Once all of the smaller nuclei have been consumed, growth by Ostwald ripening effectively ceases. The amorphous (I) particles continue to grow beyond this point because the remaining larger nuclei can support growth by coagulation. The observed differences between the two methods could thus be explained by the size distribution of the nuclei formed upon mixing. According to this interpretation, only relatively small nuclei are formed upon the mixing of two microemulsions. These nuclei redissolve and support the growth of amorphous (II) particles. After 4 h, all nuclei are consumed and the growth stops abruptly. In the case of the single-emulsion method, nuclei with a wider size distribution

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are formed. As in the classical method, the smaller nuclei feed Oswald ripening in the early stages of particle growth. Because the small nuclei are preferentially consumed, the average nucleus size increases with time and the free ion concentration consequently decreases. Eventually, the ion concentrations become insufficient to support significant Oswald ripening and the growth of the crystalline particles stops. The cessation of particle growth is, however, less abrupt in this case because of the larger size distribution of the source nuclei. This distribution presumably includes large aggregates that are essentially insoluble but able to contribute to the growth of the amorphous (I) particles by the coagulation mechanism. It should be noted that, although consistent with the principal results, this interpretation remains speculative. It also fails to explain all of the details of the more complex secondary growth experiments.

IV. CONCLUSIONS The results presented in this article provide new insight into the mechanisms of nanoparticle nucleation and growth in the microemulsion environment. More specifically, important differences between the classical two-emulsion and single-emulsion methods have been explained. In both methods, reagent mixing occurs within a few seconds. Within a few seconds to minutes, all free ions are consumed by nucleation, but only a small fraction of the nuclei grow to become mature particles. Precursor ions (Y3+ and F
) are stored in unstable solid particles and are available to feed the growth of stable nuclei through either Ostwald ripening or coagulation. The important differences between particles obtained by the two methods can be primarily attributed to differences in the nucleation conditions. In the classical microemulsion method, mixing occurs via intermicellar collisions and thus nucleation takes place at near-stoichiometric ratios of a relatively small number of precursor ions. These conditions lead to the formation of numerous nuclei that yield relatively small amorphous particles. In the single-microemulsion method, nucleation presumably occurs when reverse micelles containing Y3+ collide with a microdroplet of the added fluoride solution. These conditions result in a large molar excess of F
, which appears to be necessary for the nucleation of crystalline particles. Crystalline particles and amorphous (II) particles obtained by a classical method grow at the same rate. Moreover, the growth rate remains constant over most of the growth period. It is therefore concluded that these particles grow by Ostwald ripening, fed by the dissolution of small, unstable nuclei. Amorphous (I) particles, formed by the single-microemulsion method, show significantly faster growth that is attributed to a coagulation mechanism. This mechanism relies on the presence of larger, less soluble nuclei, which are formed only in the single-emulsion method. Finally, conditions permitting reinitiated growth in the quasiabsence of secondary nucleation have been identified (experiment 4). Such conditions are essential to the eventual preparation of particles with core
shell architectures. ’ ASSOCIATED CONTENT

bS

Supporting Information. Method of determination of the particle population ratio from experiment 4, microemulsion compositions for the secondary growth experiments, and a plot of reverse micelle size as a function of water content, as determined

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Langmuir by DLS. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Phone: 418-656-2368. Fax: 418-656-7916. E-mail: anna.ritcey@ chm.ulaval.ca.

’ ACKNOWLEDGMENT We acknowledge NanoQuebec, Le Fonds Quebecois de la Recherche sur la Nature et les Technologies (FQRNT), and the National Sciences and Engineering Research Council of Canada (NSERC) for their financial support. ’ REFERENCES (1) Capek, I. Adv. Colloid Interface Sci. 2004, 110, 49. (2) Yadav, O. P.; Palmqvist, A.; Cruise, N.; Holmberg, K. Colloids Surf., A 2003, 221, 131. (3) Chen, F.; Xu, G.-Q.; Hor, T. S. A. Mater. Lett. 2003, 57, 3282. (4) Barnickel, P.; Wokaun, A.; Sager, W.; Eicke, H. F. J. Colloid Interface Sci. 1992, 148, 80. (5) Wang, C.-C.; Chen, D.-H.; Huang, T.-C. Colloids Surf., A 2001, 189, 145. (6) Arriagada, F. J.; Osseo-Asare, K. J. Colloid Interface Sci. 1999, 211, 210. (7) Osseo-Asare, K.; Arriagada, F. J. Colloids Surf. 1990, 50, 321. (8) Nassar, N.; Husein, M. Phys. Status Solidi A 2006, 203, 1324. (9) Fang, X.; Yang, C. J. Colloid Interface Sci. 1999, 212, 242. (10) Zarur, A. J.; Ying, J. Y. Nature 2000, 403, 65. (11) Giannakas, A. E.; Vaimakis, T. C.; Ladavos, A. K.; Trikalitis, P. N.; Pomonis, P. J. J. Colloid Interface Sci. 2003, 259, 244. (12) Sun, L.; Zhang, Y.; Zhang, J.; Yan, C.; Liao, C.; Lu, Y. Solid State Commun. 2002, 124, 35. (13) Pinna, N.; Weiss, K.; Sack-Kongehl, H.; Vogel, W.; Urban, J.; Pileni, M. P. Langmuir 2001, 17, 7982. (14) Ethayaraja, M.; Bandyopadhyaya, R. J. Am. Chem. Soc. 2006, 128, 17102. (15) Hirai, T.; Sato, H.; Komasawa, I. Ind. Eng. Chem. Res. 1994, 33, 3262. (16) Agostiano, A.; Catalano, M.; Curri, M. L.; Della Monica, M.; Manna, L.; Vasanelli, L. Micron 2000, 31, 253. (17) Ward, A. J. I.; O’Sullivan, E. C.; Rang, J.-C.; Nedeljkovic, J.; Patel, R. C. J. Colloid Interface Sci. 1993, 161, 316. (18) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706. (19) Hammouda, A.; Gulik, T.; Pileni, M. P. Langmuir 1995, 11, 3656. (20) Summers, M.; Eastoe, J.; Davis, S. Langmuir 2002, 18, 5023. (21) Fang, J.; Stokes, K. L.; Wiemann, J.; Zhou, W. Mater. Lett. 2000, 42, 113. (22) Ayyub, P.; Multani, M. S. Mater. Lett. 1991, 10, 431. (23) Li, F.; Vipulanandan, C. IEEE Trans. Appl. Supercond. 2003, 13, 3196. (24) Pillai, V.; Kumar, P.; Hou, M. J.; Ayyub, P.; Shah, D. O. Adv. Colloid Interface Sci. 1995, 55, 241. (25) Viger, M.; Live, L.; Therrien, O.; Boudreau, D. Plasmonics 2008, 3, 33. (26) Bae, D.-S.; Han, K.-S.; Adair, J. H. J. Mater. Chem. 2002, 12, 3117. (27) Lin, J.; Zhou, W.; Kumbhar, A.; Wiemann, J.; Fang, J.; Carpenter, E. E.; O’Connor, C. J. J. Solid State Chem. 2001, 159, 26. (28) Jain, R.; Shukla, D.; Mehra, A. Ind. Eng. Chem. Res. 2006, 45, 2249. (29) Shukla, D.; Mehra, A. Langmuir 2006, 22, 9500. (30) Jada, A.; Lang, J.; Zana, R.; Makhloufi, R.; Hirsch, E.; Candau, S. J. J. Phys. Chem. 1990, 94, 387.

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