Nanoparticle Size Control in Microemulsion Synthesis

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Nanoparticle size control in microemulsion synthesis Benoit Richard, Jean-Luc Lemyre, and Anna M. Ritcey Langmuir, Just Accepted Manuscript • Publication Date (Web): 24 Apr 2017 Downloaded from http://pubs.acs.org on April 26, 2017

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Nanoparticle size control in microemulsion synthesis Benoit Richard, Jean-Luc Lemyre and Anna M. Ritcey* Département de chimie and CERMA, Université Laval, Québec, Canada. Pavillon AlexandreVachon, 1045, avenue de la Médecine, Québec, Canada G1V 0A6

Abstract: Quasi-monodisperse populations of (H3O)Y3F10∙xH2O nanocrystals of varying size are prepared in Igepal-stabilized microemulsions. Correlations between microemulsion composition, micelle hydrodynamic radius and final nanoparticle size are established and shed light on the mechanism of particle size control. Under the conditions considered here, size control appears to be primarily governed by the number of micelles and the quantities of precursor ions. More specifically, the number of NPs formed can be successfully correlated with the number of micelles present and final NP size is, in turn, determined by the number of nuclei and the total amount of material available for nanocrystal formation. This insight into nanoparticle formation facilitates the selection of appropriate synthetic conditions for the preparation of populations of a targeted size.

I. Introduction The properties of functional nanomaterials are strongly dependant on structural details such as size and shape. For example, semi-conductor nanocrystals known as quantum dots exhibit sizedependant emission1, pore size and pore connectivity in mesoporous silica strongly influence the efficiency of these materials as catalytic supports or drug delivery systems2, and the dependence of the plasmonic extinction spectra of noble metal nanoparticles on particle size and shape is well documented.3 The development of synthetic methods that offer control of these structural characteristics is thus a crucial objective of nanoscience research. Among the various known methods of nanoparticle synthesis, microemulsion templating is unparalleled as a versatile route to nanoparticles of controlled size and structure.4,5 Water-in-oil microemulsions, or inverse micelles, are formed by the dispersion of a relatively small amount of water in a non-polar continuous phase with the aid of a surfactant.6 Typically, microemulsion nanoparticle syntheses involve a chemical reaction, within the aqueous phase, that converts soluble precursors to an insoluble product. This method has been employed to produce a wide variety of nanomaterials, including quantum dots7 and nanoparticles composed of metals8,9, silica10,11, metal oxides12 and lanthanide fluorides.13 1 ACS Paragon Plus Environment

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Although the microemulsion method is often referred to as a templating technique, the mechanism of nanoparticle size and shape control is far more complex than direct replication of micelle geometry. Multiple system parameters have be demonstrated to influence the outcome of a given syntheses14. The choice of surfactant15,16, including the possible use of a cosurfactant, and the nature of the continuous phase influence exchange dynamics between micelles by modifying the rigidity of the water-oil interface17 or by varying the collision frequency between two micelles18. Depending on the specific synthesis, these parameters can have an impact on nanoparticle growth, favoring certain shapes or crystalline phases19. Finally, microemulsion synthesis is not restricted to the inverse micelle region of the phase diagram20, and compositional changes can lead to phase modifications that yield different nanostructures for a given material21. By far the most extensively studied microemulsion parameter is the water to surfactant ratio, wo. In many systems, inverse micelle size varies linearly with wo22 and it is this parameter that is associated with the exceptional particle size control offered by the microemulsion method.23 Despite the strong correlation between micelle size and particle size, the two dimensions often differ by more than an order of magnitude. In a typical synthesis, the contents of a large number of micelles are thus required to produce a single nanoparticle. Particle formation is clearly a multi-step process involving nucleation within certain micelles, followed by growth through the exchange of material during micellar collisions7. According to this model, final particle size will be determined by the probability of nucleation, the total amount of precursor material available for particle formation and the relative importance of particle-particle coalescence to the growth process. Each of these processes, in turn, depends in a non-trivial fashion on the detailed composition of the microemulsion system, making particle size predictions difficult. Lanthanide fluoride nanoparticles have attracted considerable attention for potential applications in bioimaging and optics24,25,26,27, 28. In this regard, we have been investigating (H3O)Y3F10∙xH2O nanocrystals prepared by the microemulsion method13 as host materials for luminescence lanthanide ions29. The single-crystal nature30 of these particles, combined with the excellent size control offered by the synthetic method, makes them excellent candidates for the investigation of structure-property relationships. However, as noted above, despite the quasi-monodisperse nature of the particle populations obtained by a given synthesis, the identification of the exact conditions required to obtain a target particle size relies primarily on trial and error. In the present work, (H3O)Y3F10∙xH2O nanocrystals are synthesized in a water in cyclohexane microemulsion stabilized with a non-ionic surfactant. Various microemulsion compositional parameters are systematically varied and correlated with nanoparticle size, leading to the formulation of general conclusions about the mechanism of particle size control in this system.

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II. Experimental section

Synthesis of (H3O)Y3F10·xH2O nanoparticles All chemicals were purchased from Sigma Aldrich and employed without further purification. Nanoparticles were prepared according to a previously reported method31 but with modified compositional parameters. An inverse microemulsion is prepared by first dispersing polyoxyethylene(5) nonylphenyl ether (Igepal CO-520) and sodium bis(2ethylhexyl)sulfosuccinate (AOT) in cyclohexane with a 5 to 10 min treatment in an ultrasonic bath. An aqueous solution of yttrium chloride hexahydrate (YCl3∙6H2O) is then added and the mixture is homogenised by magnetic stirring for one hour. An aqueous solution of ammonium hydrogen difluoride (NH4F∙HF), of the same concentration and volume as the yttrium chloride solution, is added. Magnetic agitation of the resulting microemulsion is maintained for five to seven days. The specific quantities of reagents employed for each synthesis are provided in Tables 1 and 2. (H3O)Y3F10∙xH2O nanoparticles are isolated from the microemulsion by ultrafiltration with a solvent resistant stirred cell from EMD Millipore. Cyclohexane is first removed with a rotary evaporator. The recovered NP-surfactant mixture is dispersed in methanol and placed in the ultrafiltration cell equipped with a Ultracel® 100 kDa regenerated cellulose membrane purchased from EMD Millipore. The suspension is washed with methanol with a nitrogen pressure of 10 psi until the sample is free of excess surfactant. The NPs are dried for storage and can be redispersed in methanol, ethanol, cyclohexane and slightly in water.

Characterization Nanoparticles size and shape were characterized with a JEOL JEM-1230 transmission electron microscope (TEM) operated at an accelerating voltage of 80 kV. NP size was evaluated with ImageJ image processing software and is reported as octahedral edge length for the single crystal particles. For spherical, amorphous particle, sizes are reported as diameters. Typical size distributions are provided in Figure S1 of the Supporting information. Particle size statistics were obtained from measurements of a minimum of 250 individual NPs. For a given set of synthetic conditions, batch-to-batch variations in particle size do not exceed 5 nm. Reverse micelle size was measured by dynamic light scattering (DLS) with a Zetasizer Nano ZS system from Malvern. Average micelle hydrodynamic diameters were evaluated from volume weighted distributions. Examples are provided in Figure S2 of the Supporting information. Kinetic viscosities were measured with an Ubbelohde type viscometer at 25°C for each microemulsion composition. Dynamic viscosities were then calculated, as reported in Table S1 of the Supporting information. Additional data was also taken from ref 32.


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III. Results and discussion The microemulsion method for the synthesis of NPs is primarily employed for the exquisite control of particle size that it offers. For example, the TEM images of (H3O)Y3F10 NPs presented in Figure 1 illustrate both quasi-monodisperse populations and the broad range of particle size accessible with this method. However, because the exact particle size obtained from a given synthesis depends in a complex manner on a number of experimental parameters, it is difficult to target a given diameter. The specific compositions of the microemulsions employed for the synthesis of the NPs depicted in Figure 1 are provided in Table 1. The effect of changes in the concentration of individual microemulsion components on particle size can be evaluated by the pair-wise comparison of the various samples.

Figure 1.

TEM images of (H3O)Y3F10 nanoparticles prepared by the microemulsion method employing the conditions provided in Table 1.


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Table 1. Synthetic conditions employed for the preparation of the (H3O)Y3F10 nanoparticles presented in Figure 1.

Sample aIV bIV c d e

Mass Igepal CO-520 (g) 1.8 1.8 1.8 1.35 1.8

Mass AOTI (g) 0.2 0.2 0.2 0.15 0.2

Volume aqueous phase (mL)II 1 1 1 1 2

Concentration YCl3(aq) (mM)III 40 40 500 40 500

Particle size (nm) 36 49 67 79 100

I

Mass per 15 mL of cyclohexane. Total volume YCl3 (aq) and NH4F∙HF (aq) per 15 mL of cyclohexane. III Concentration of the aqueous phase used to prepare the microemulsion. The concentration of NH4F∙HF (aq) is identical. IV Samples a and b differ in absolute volume and contain 90 and 15 mL of cyclohexane, respectively. II

The dependence of NP size on precursor ion concentration is illustrated by comparison of Figure 1b and c; a more than tenfold increase in concentration results in an increase in particle size from 49 to 67 nm. This corresponds to a 2.5-fold change in particle volume, implying that the increase in precursor ion concentration increases both the number of nuclei formed and the amount of material available for subsequent particle growth. The effect of surfactant content can be inferred from comparison of samples b and d. When all other parameters are held constant, a 25 % decrease in the amount of surfactant results in an increase in particle size from 49 to 79 nm. Since the two samples (b and d) are prepared from the same quantities of precursor ions, any increase in size can be attributed to a decrease in the number of particles formed during nucleation. A decrease in the surfactant to water ratio, at constant water content, is expected to result in an increase in micelle size and a corresponding decrease in the number of micelles present. These two modifications have opposing implications for nucleation; the former can be predicted to increase the probability of nucleation by increasing the number of ions within a given micelle whereas the latter would decrease the number of nucleation sites. The observed increase in NP size therefore suggests that the primary impact of a change in surfactant content is through the modification of the number of micelles. Samples c and e differ in the water content of the microemulsion. A two-fold increase in the volume of the aqueous solutions employed results in an increase in particle size from 67 to 100 nm. This modification of the synthetic conditions is the most complex of all since it can be predicted to influence micelle size, the number of micelles and the total amount of material available for particle formation. The observed difference in particle size corresponds to a threefold increase in particle volume, which when compared with the two-fold increase in the number of precursor ions, suggests a decreases the probability of nucleation.


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Finally, comparison of samples a and b indicates that particle size depends on the absolute volume of the microemulsion. Since the concentrations of all components are identical for the two samples, the number and size of the micelles, as well as the average number of ions per micelle must be constant. The observed effect of microemulsion scale-up must therefore be attributed to differences in mixing in the crucial first moments upon the addition of the NH4F∙HF solution. It is relevant to note that differences in NP size are not the result of incomplete growth. Particles typically reach their final size after about five days of growth. Samples verified after two additional weeks of growth show no change in size or shape. In order to further explore the trends described above, NPs of (H3O)Y3F10 where prepared in microemulsion systems of various compositions. In addition, the influence of the concentration of the various components on micelle size was extensively investigated. Micelle size Micelle size in microemulsions is typically associated with the molar water to surfactant ratio, wo. In cases where the surfactant shows significant solubility in the continuous phase, the relationship between wo and micelle size is, however, nontrivial and depends on the absolute quantities of the various microemulsion components32. In these ternary (or quaternary, if one counts the co-surfactant) mixtures, wo can be systematically varied in two different ways: the addition of surfactant at constant water content or the addition of water at constant surfactant content. Measurements at constant water content The evolution of micelle hydrodynamic diameter, as determined from DLS measurements, as a function of surfactant content is presented in Figure 2. The corresponding data and exact microemulsion compositions are provided in Table 2. Data are provided for microemulsions prepared with and without AOT as a co-surfactant and from either pure water or YCl3 solutions as the aqueous phase. In all cases, the addition of surfactant at constant water content leads to the systematic reduction of micelle size. Furthermore, the presence of AOT as a co-surfactant systematically leads to a small reduction in micelle size.


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The procedure employed for the synthesis of the (H3O)Y3F10 nanocrystals involves the addition of an aliquot of an aqueous solution of NH4F∙HF to a microemulsion containing YCl3. As a result, the microemulsion composition changes abruptly at the moment of particle nucleation and it is

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without AOT, water only with AOT, water only without AOT, 400 mM with AOT, 400 mM

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0

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Mass surfactant /g

Micelle hydrodynamic diameter as a function of surfactant mass at constant water content. Results are shown for microemulsions prepared with Igepal alone and with 10% AOT as a cosurfactant, as indicated. All samples contain 15 mL of cyclohexane. Microemulsions prepared from pure water or 200 mM YCl3 contain a constant aqueous phase volume of 1 mL, whereas those prepared from 400 mM YCl3 contain 0.5 mL of aqueous solution. Data plotted as open squares are taken from reference 32. Dashed lines have been added solely to guide the eye.

impossible to reproduce these exact conditions for micelle size measurements. For this reason, measurements were performed for three different systems: microemulsions containing pure water as the aqueous phase, microemulsions containing 400 mM aqueous solutions of YCl3 that mimic compositions before the addition of NH4F∙HF, and samples containing 200 mM aqueous solutions of YCl3 to mimic microemulsion compositions after the addition of fluoride. It is relevant to note that micelle size cannot be determined after the actual addition of the fluoride solution because particle formation interferes with the DLS measurements. Table 2. Micelle hydrodynamic diameter for various microemulsion compositions, in 15 mL of cyclohexane. Volume aqueous phase (mL) 1 1

Mass Mass Igepal AOT (g) (g) 0.45 0.9

0.05 0.1

Without YCl3

σI

16.6 10.5

9.5 5.0

Micelle diameter (nm) Half water With YCl3, content with σI 200 mM YCl3, 400 mM 6.8 1.6 5.6 1.8 -

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1 1 0.5 2 3 1 1 1 1 0.5 2 3

1.8 2.7 1.8 1.8 1.8 0.5 1.0 2.0 3.0 2.0 2.0 2.0

0.2 0.3 0.2 0.2 0.2 -

3.7 2.3 3.5 5 5.3 23.9II 12.3II 4.0II 2.8II

1.5 0.6 1.1 2.2 2.3 -

2.8 1.6 2.7 2.8 2.6 9.1 7.3 2.7 1.7 2.9 3.1 -

0.8 0.5 0.8 0.9 1.1 2.7 2.7 0.9 0.6 1.1 1.1 -

3.4 2.0 4.4 9.9III 9.1III 16.0 3.5 2.0 3.4 48 42

1.2 0.5 1.1 1.5 1.5 9.3 1.3 0.7 1.1 18 17

I

Standard deviation of the micelle size, from cumulative measurements on DLS. Data from reference 32. III Measurements performed on the supernatant phase as described in the text II

Although the results presented in Figure 2a appear to suggest that the presence of precursor ions decreases micelle size, this difference can in fact be attributed to the different water contents of the two series of samples. This is illustrated by the data of Figure 2b that clearly indicate no significant difference in micelle hydrodynamic diameter between micelles containing either 1 mL of pure water or 1 mL of a 200 mM aqueous solution of YCl3. Measurements at constant surfactant content The evolution of micelle diameter upon the addition of water at constant surfactant content is illustrated in Figure 3. In this case, results are strikingly different in the presence or absence of AOT. With Igepal alone (open squares in Figure 3), initial changes in water content have little

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without AOT, water with AOT, water without AOT, 400 mM with AOT, 400 mM

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Figure 3.

Micelle hydrodynamic diameter /nm

a

Micelle hydrodynamic diameter / nm

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Micelle hydrodynamic diameter as a function of the volume of the aqueous phase at constant surfactant content of 2 g. Results are shown for microemulsions prepared with Igepal alone and with 10% AOT as a co-surfactant, as indicated. Data points represented by partially filled circles correspond to measurements performed on the supernatant ACS Paragon Pluscontain Environment phase, as described in the text. All samples 15 mL of cyclohexane. Data plotted as open squares are taken from reference 32. Dashed lines have been added solely to guide the eye.

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influence on micelle size. In this regime, a large fraction of the Igepal is present as free surfactant dissolved in the continuous phase32. The addition of water under these conditions thus leads primarily to an increase in the number of micelles rather than a significant increase in their size. Beyond a certain water content (1 mL in the specific case of 2 g of surfactant in 15 mL of cyclohexane; data for other compositions are available in ref 32), micelle size increases sharply upon further water addition. This point is presumably reached as the free surfactant concentration approaches the cμc (ref 32), prohibiting further removal of surfactant from the continuous phase for the formation of new micelles. With Igepal alone as the surfactant at the concentration considered here (2 g/ 15 mL cyclohexane), stable microemulsions cannot be prepared above water contents of 1.5 mL. In the presence of 10% AOT (filled squares in Figure 3), the situation is very different. Considerably more water can be incorporated in the microemulsion and with comparably little change in micelle size. The presence of precursor ions does not have a significant effect on micelle size at lower water contents. At higher aqueous phase volumes, however, microemulsions containing YCl3 are less stable than corresponding systems prepared from pure water. As illustrated in Figure 3, stable microemulsions are not accessible in the presence of YCl3 at total aqueous phase volumes exceeding 1.5 mL (at a fixed surfactant content of 2 g). In this concentration regime, samples are initially opaque and not amenable to DLS measurements. However, when left at rest for a 24 h period, macroscopic phase separation occurs and DLS measurements can be performed on the clear supernatant phase. Such measurements indicate a significant increase in micelle size, as shown by the partially filled symbols of Figure 3b. Water / surfactant ratio Micelle size as a function of wo is plotted in Figure 4 for the various experiments, without precursor ions. This plot emphases the point that although micelle size varies with wo, a given ratio can result in very different micelle diameters, depending on the exact composition of the system. Once again, the relatively small variation in micelle size with composition is evident for the systems prepared with constant surfactant content containing AOT (filled triangles). This observation can be attributed to the exceptional efficiency of AOT in the stabilization of microemulsions33 and surfactant synergy34. In particular, small angle neutron scattering measurements for microemulsions prepared from ionic/non-ionic surfactant mixtures reveal strong partitioning of the non-ionic surfactant to the interface, accompanied by a reduction in the concentration of free surfactant. The cµc, and thus the concentration of free surfactant in cyclohexane, is much lower for AOT than for Igepal35. It can therefore be predicted that replacing a portion of the Igepal with AOT, at approximately constant micelle size, will result in a larger number of micelles. Previous studies have also reported that the addition of an ionic surfactant to non-ionic water/oil systems36,37 typically results in a small reduction of both the aggregation number and the hydrodynamic diameter of the reverse micelles.


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Micelle hydrodynamic diameter /nm

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constant surfactant (2g) with AOT constant surfactant (2 g) no AOT constant water (1 mL) with AOT constant water (1 mL) no AOT

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Figure 4.

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Wateras / surfactant molarof ratiowater/surfactant ratio, for Micelle hydrodynamic diameter a function microemulsions of either fixed water content (circles) or fixed surfactant content (triangles). Filled symbols correspond to systems containing 10 % AOT as a cosurfactant whereas open symbols refer to microemulsions prepared with Igepal alone. Lines have been added only to guide the eye.

Nanoparticle size

Variation of NP size with microemulsion composition NPs of (H3O)Y3F10 were prepared with each of the microemulsion compositions listed in Table 3. TEM images of the resulting samples are provided in Figure 5. The first series of images (a-d) indicate that particle size decreases with increasing surfactant concentration at constant water content. A similar trend is observed for the second series of samples (e-i) which shows that particle size systematically increases with the addition of water at constant surfactant content. As discussed below, changes in water content also modify the total number of ions in the system since precursor salt concentrations are held constant. Particle size is plotted as a function of wo in Figure 6 for the two experiments. In the case of samples prepared with a constant water content, data is provided for samples prepared with and without AOT. Although particle size increases with wo in a quasi-linear manner in all cases, the slopes differ from one system to another. Clearly, the water/surfactant ratio alone cannot predict NP size. The combination of 1 mL of water and 2 g of surfactant corresponds to a water/surfactant ratio of 12.3. At higher values of wo, samples prepared with a fixed amount of surfactant contain more than 1 mL of water and, correspondingly, more precursor ions than samples prepared at constant water content. Despite this, for a given value of wo, it is the microemulsions prepared with fixed quantity of surfactant (2 g) that consistently yield the smallest particles, implying that nucleation is favored in these samples.


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Figure 5.

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Transmission electronic microscopy (TEM) images of (H3O)Y3F10 NPs synthesized in microemulsions with a constant water content (1 mL) and variable amounts of surfactant : (a) 0.5 g, (b) 1.0 g, (c) 2.0 g and (d) 3.0 g or with a constant surfactant content (2 g) and variable quantities of water : (e) 0.25 mL, (f) 0.5 mL, (g) 1.0 mL, (h) 2.0 mL and (i) 3.0 mL.

Table 3. Synthetic parameters of (H3O)Y3F10∙xH2O nanocrystals. Volume of cyclohexane is constant at 15 mL and concentration of aqueous solutions of YCl3 and NH4F∙HF constant at 400 mM.

Volume aqueous Mass Igepal phase (mL)I (g) 0.25 0.5 1

1.8 1.8 1.8

Mass AOT (g)

w0II

Micelle diameterIII (nm)

σIV (nm)

Particle size (nm)

σV (nm)

0.2 0.2 0.2

3.1 6.1 12.3

3.1 3.5 3.7

0.8 1.0 1.3

54 53 63

5 7 3 11


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24.5 36.8 49.0 36.8 24.5 12.3 8.2

5.0 5.3 16.6 17.3 10.5 3.7 2.3

2.3 1.9 9.5 5.0 3.3 1.5 0.6

86 111 161 122 100 66 60

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Total volume of aqueous phase from YCl3 and NH4F∙HF solutions. Calculated from the volume of added water without consideration of the presence of salt III Micelle size determined from equivalent microemulsions prepared with pure water IV Standard deviation of the micelle size, from cumulative measurements on DLS. V Standard deviation of the nanoparticle size, measured from 300 NP. II

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140 120 100 80 constant water (1mL) without AOT constant water (1mL) with AOT constant surfactant (2g) with AOT

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Figure 6.

Nanoparticle size as a function of water/surfactant ratio, for three series of samples obtained either by varying the concentration of surfactant, with and without AOT, (filled and open circles, respectively) or by varing the amount of water (triangles).

Experiments at a constant water content of 1 mL were attempted for w0 values lower than 8.2 by further increasing the surfactant concentration. However, NPs produced in these conditions exhibit differences in shape and crystallinity. Amorphous NPs31 exceeding 150 nm in size are primarily observed, in coexistence with a minority population of octahedral NPs measuring less than 50 nm in edge length. A TEM image of such as sample is provided as Figure S3 of the Supporting information. Well-defined single-crystal particles can, however, be prepared at w0 12 ACS Paragon Plus Environment

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values below 8.2, if the quantity of surfactant is limited to 2 g. Clearly higher concentrations in surfactant modify particle formation in a significant way, possibly because the microemulsion is no longer located within the spherical micelle region of the phase diagram. The amorphous and single-crystal nature of the two types of NP populations are illustrated by the electron diffraction patterns of Figure S4 in the Supporting information. Although NP size is strongly correlated with wo, this relationship between the two parameters cannot be attributed to micelle size. This can be most clearly illustrated by the first five entries in Table 3. For a fixed surfactant content of 2 g, NP size essentially doubles (54 to 111 nm) as the aqueous phase volume is increased from 0.25 to 3 mL. However, micelle size remains relatively constant within this composition range. As discussed above, these observations can be explained by an important increase in the number of micelles as water is added. However, in this experiment, the concentration of aqueous phase is held constant and the total ion content of the system thus also increases with water content. Final particle size will therefore be influenced both by the number of particles, determined by nucleation, and the total amount of material available for particle formation. Variation of NP size with precursor ion concentration In order to separate the effect of precursor ion quantities from the influence of microemulsion parameters, NPs were synthesised in a series of identical microemulsions, but with different aqueous phase concentrations. The results are presented in Figure 7 and Figure 8 for two different micellar systems. The first composition is one which leads to the formation of small micelles (< 5 nm) and microemulsions that are stable in the presence of precursor ions. The second system corresponds to a microemulsion composition that leads to relatively large micelles (~ 17 nm) when prepared with pure water, but exhibits phase separation in the presence of precursor ions. Despite the instability of the microemulsion at this composition, well-defined single-crystal NPs are obtained.


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Langmuir

Figure 7.

TEM images of (H3O)Y3F10 NPs synthesized in microemulsions containing 15 mL cyclohexane, 1 mL water and 2 g surfactant (Igepal : AOT. 9:1) at different precursor ion concentrations, 4 mM, 20 mM, 40 mM, 80 mM and 500 mM from a) to e) respectively.


Langmuir

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Figure 8.

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TEM images of (H3O)Y3F10 NPs synthesized in microemulsions containing 15 mL cyclohexane, 1 mL water and 0.666 g surfactant (Igepal : AOT. 9:1) at different precursor ion concentrations (25, 50, 100, 200, 300 and 400 mM respectively from a) to f)). Images d), e) and f) are of a different magnification than the three first images.

The dependence of NP size on reagent concentration is shown in Figure 9 for the two systems, with data being plotted both as particle edge length and volume. In general, particle size is found to increase with the amount of crystallizable material, suggesting that reagent concentration primarily affects particle growth rather than nucleation. Higher ion concentrations (at a fixed number and size of micelles) would be expected to increase the probability of nucleation, with a corresponding reduction in particle size. In addition, the quasilinear dependence of NP volume on precursor concentration observed in Figure 9a supports the hypothesis that that the number of particles remains relatively constant for this system. A somewhat similar trend is observed in Figure 9b for the destabilized microemulsion composition. In this case, it is important to note that an increasing number of large amorphous particles are found to coexist with the crystalline NPs at higher reagent concentrations (200 400 mM). The presence of this second population can explain the decrease in particle size at the highest concentration since the material consumed in the formation of amorphous particles is not available for nanocrystal growth. Also, because the effect of ion concentration on micelle size is not known at this composition, it is not possible to unequivocally attribute particle size differences solely to differences in the total amount of crystallization material.


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120

5e+5

180

3e+6

60

NP edge length /nm

NP edge length /nm

3e+5

120 2e+6

NP volume / nm3

a Nanoparticle size and volume at different concentrations b of reactive. Squares are for samples Figure 3. 160 100 3e+6 with small micelles (around 5 nm) and triangles for those with bigger micelles (

Nanoparticle Size Control in Microemulsion Synthesis

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