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Mar 27, 2014 - We have employed small-angle X-ray scattering (SAXS), dynamic light ... Erin Finley , Andrew S. Paterson , Angelica Cobb , Richard C. W...
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Spherical-to-Cylindrical Transformation of Reverse Micelles and Their Templating Effect on the Growth of Nanostructures Soma Sharma† and Ashok K. Ganguli*,†,‡ †

Department of Chemistry, Indian Institute of Technology, Hauz Khas, New Delhi 110016, India Institute of Nano Science & Technology, Phase X, Mohali, Punjab 160062, India

‡

ABSTRACT: We discuss a complete mechanistic study on the anisotropic growth of zinc oxalate nanostructures within reverse micelles. We have employed small-angle X-ray scattering (SAXS), dynamic light scattering (DLS), and transmission electron microscopy (TEM) to understand the detailed growth of the nanostructures. We have been able to observe the generation of nuclei and their aggregation to a critical size beyond which they form nanostructures of higher dimensions in self-assembled templates. One of our aims was to find a correlation between size and shape of microemulsion droplets (MDs) and that of the resulting nanostructures of zinc oxalate (ZO) which grow within the MDs. Combination of SAXS and DLS show in situ growth of nanoparticles in the individual droplets which consume the water-insoluble product formed and undergo exchange coalescence with other droplets. The structural transition of the MDs is captured by observing the change in shape anisotropy, together with a detailed structural analysis of micelles in which the nanostructures grow as a function of time. Importantly, once the reaction is triggered, the nucleation of the droplets start instantly, and a very short period is noticed where MDs become cylindrical with approximate aspect ratio of 4:1 in which nanostructures grow anisotropically and achieve an average critical size of 55 nm (elongated nanoparticles) signifying the existence of short nucleation-dominant particle growth period, beyond which a transition from elongated nanostructures to small rods is observed. The critical size for the elongated droplets is 80 nm in length and 18 nm in diameter, and these critical dimensions at the point of transition are a new finding about an asymmetric particle before the rods begin to start self-assembling. Once the shape of microemulsions turns cylindrical, the dynamical exchange with other microemulsions is very fast at both ends, resulting in the formation of nanorods of zinc oxalate and an increase in the aspect ratio of these rods. This growth process can be viewed as a morphologically templated nucleation process, and the droplets act as shaping vesicles for the formation of ZO nanorods. This study is significant since it attempts to correlate the size and shape of the reverse micellar (microemulsion) droplets with the newborn product nanoparticles inside the droplets and the subsequent growth of the nanoparticles within the droplets.

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INTRODUCTION Over the past decade, development of the processes to obtain nanomaterials with controlled size and morphology has been the center of attention due to their important applications based on optical,1,2 electrical,3 magnetic,4 and mechanical properties and on therapeutics5−7 and energy conversion and storage.8 For the preparation of nanoparticles, micelles and emulsion droplets are often used as templates.9−13 Reverse micelles (RMs) are formed in a microemulsion which is an isotropic, thermodynamically stable dispersion of water droplets in a continuous oil phase facilitated by a surfactant and sometimes also a cosurfactant. The amphiphilic nature of surfactants makes them prone to their spontaneous aggregation and self-organization in a variety of supramolecular forms such as micelles, reverse micelles (RMs), vesicles, emulsions, and liquid crystals. The shape of the micelle or reverse micelle can be controlled by several factors including temperature, concentration, additives, and ionic strength, and variation of these parameters may result in modification of micellar structure. Theoretical approaches and simulations have provided a wealth of information on the structural and © 2014 American Chemical Society

dynamical properties of the nascent micellar droplets and exchange taking place during reaction which are difficult to be explored experimentally.14−16 Very little is known from direct microscopic or spectroscopic observations on the droplet coalescence and structural changes accompanying the growth of particles from nascent microemulsion droplets (MDs).17−20 Our understanding is not yet perfect regarding the shape of the micellar droplet, its transition from a spherical to cylindrical aggregate, and the formation of nanostructures inside the micelles and their growth process. The uncertainty of the actual process has prevented us to decipher the understanding of the temporal evolution of the templates (reverse micelles) and their shape transitions. It has been demonstrated by theoretical studies (with very few experimental studies) that micelles can adopt various shapesspheres, disks, ellipsoids, or rodsand that at low concentration of nonionic surfactants the micelles formed are close to being spherical.21−23 Recently, Velinova et Received: January 21, 2014 Revised: March 26, 2014 Published: March 27, 2014 4122

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mechanism of the growth of nanostructures and helps to develop a self-consistent description of the entire process. There have been some studies on the nucleation and growth kinetics of nanomaterials in reverse micelles.52 Optimization of experimental parameters reveals the temporal evolution and formation mechanism of different nanomaterials. In our recent report, we have studied the kinetic steps of nanorod growth by following the intermicellar exchange of CTAB-MDs over eight consecutive days using fluorescence correlation spectroscopy (FCS), DLS, and TEM;17 however, the structural evolution of MDs and their shape transitions could not be figured out. Also, most of these earlier studies21,22,51,52 have been carried out on spherical micelles without any anisotropy, though the growth of anisotropic nanostructures is of significance due to their applications. A detailed structural study on RMs with different morphologies is lacking,21 and no definite understanding on the structural origin of these anisotropic particles has emerged so far. Herein, we aimed to understand the structural evolution of CTAB reverse micellar droplets and the product synthesized in it by following the formation of ZO nanorods inside the polar core of CTAB MDs over 15 h. SAXS is employed to monitor these transitions at droplet coalescence level, the process which initiates the reaction and allows the subsequent growth of nanostructures. DLS and TEM are used to follow the nanoparticle/nanorod growth, which complement the SAXS results. By analyzing SAXS data with a suitable fitting model, we were able to obtain the shape and size distribution of the MDs and that of the droplets containing product during the entire span of the reaction (15 h). The approach reported herein may prove of great value in establishing a direct relation between the shape and size of template with that of the nanostructures grown in it, a key topic toward the design of nanostructures with well-controlled architectures, and can be readily extended to the study of other systems. Subsequently, we have decomposed the ZO synthesized at different time intervals to the corresponding oxide and investigated their size, shape, and growth behavior. This study is an attempt to understand the role of shape and size of the microemulsion droplets in controlling the size, shape, and growth of nanocrystalline ZO and the oxide nanoparticles obtained subsequently. An understanding of the factors that affect the growth kinetics of anisotropic nanostructures is desirable from both synthetic and mechanistic standpoints which would ultimately lead to a microscopic understanding of how to control nanostructures. Herein, we present a systematic account of the kinetics of the growth of ZO nanorods in CTAB reverse micelles.

al. have demonstrated sphere-to-rod transitions of nonionicsurfactant-based micelles modeled by molecular dynamics simulations.22 Also, Zhang et al.23 and Magno et al.24 have used SAXS along with other techniques to understand the structural changes of the RMs while synthesizing Ag and Pt nanoparticles. In spite of availability of a large body of literature, there does not exist a systematic investigation of the kinetics of the nanostructure formation in reverse micellar system. The ultimate aim is to precisely determine the template structure responsible for a specific morphology so that we can synthesize products (nanostructures) with desired shape and properties. Cetyltrimethylammonium bromide (CTAB) is a cationic surfactant which is extensively used in biochemical investigations, as a detergent, solubilizer, emulsifier, and a template for the production of inorganic nanomaterials with controlled size, shape, assembly, and aspect ratio. We have used the CTAB-based microemulsion system to synthesize different metal oxalate nanorods of copper,25 nickel,26 and zinc27 which follow fast kinetics (reaction is over in 15 h) and iron oxalate17,28 which follows slow kinetics (reaction takes 7−8 days). CTAB-based RMs have been investigated by using dynamic light scattering (DLS), pulse field gradient NMR, small-angle neutron scattering (SANS), near-IR spectroscopy, and conductivity.29−39 An important issue about inorganic nanoparticle formation using the microemulsion route is the shape and size control of the nanoparticles. It has been argued in several studies that the inorganic particle size increases linearly with the water-to surfactant molar ratio (W0) at low water content,40−45 while for larger water droplets, where the overall mass fraction of the aqueous phase exceeds a certain threshold, the inorganic particle radius levels off.46−48 More recently, it has been suggested that the interplay between the chemical reaction rate and the material interdroplet exchange plays an important role in the mechanism of nanoparticle formation in heterogeneous media such as microemulsions.49 The fact that the water droplet size/shape has an effect on the resulting particle size does not imply that the droplets act as templates for the particle formation since, interestingly, a direct correlation between the actual size/shape of the water droplets and that of the formed particles has not yet been found.50,51 Nevertheless, most reactions take place in solution, so the information about this correlation in size/shape is a requisite for chemists to precisely design, control, and grow the desired nanostructures. To address this issue, we believe that there needs to be a paradigm shift in the way reactions within microemulsion is studied. This paper aims at investigating systematically the influence of the shape and size of the MDs containing zinc oxalate (ZO) on the final product (ZO nanorods) which grow with time, by using SAXS, DLS, and TEM. The characterization of the w/o microemulsions containing the metal ions and the oxalate ions as well as the time evolution of the particle formation and growth in w/o microemulsion was studied by the above methods. A combination of characterization methods is essential to reveal sufficient information on droplets which would enable us to understand the nanoparticle formation process. Individually, DLS, SAXS, and TEM are incapable to follow all the above parameters through the initial process of nucleation and growth. Hence, they do not provide sufficient information on the shape of the droplets (DLS), introduce artifacts (TEM), or miss the larger aggregates (SAXS). The combination of DLS, TEM, and SAXS reveals a complete

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MATERIALS AND METHODS Commercially available cetyltrimethylammonium bromide (CTAB, Sigma, 99%), 1-butanol (Spectrochem, 99%), isooctane (Spectrochem, 99%), chloroform (SRL 99.5%), ethanol (Merck, 99.9%), and methanol (Qualigens, 99.8%) were used in the synthesis. Zinc nitrate (Qualikems, 98%) and ammonium oxalate (CDH, 99%) were used in the synthesis. All the solvents were dried before use. Zinc oxalate precursor was prepared using the reverse micellar route. Microemulsions with CTAB as the surfactant, 1butanol as the cosurfactant, isooctane as the nonpolar phase, and 0.1 M aqueous solution of Zn2+ and C2O42− were prepared. Microemulsion I contained 0.1 M zinc nitrate solution while microemulsion II contained 0.1 M ammonium oxalate solution. The weight fraction of various constituents in the microemulsion was 16.76% of CTAB, 13.9% of n-butanol, 59.29% of 4123

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size distribution, and shape of droplets to understand the nanoparticle formation process. The SAXS technique is very useful for studying the size and shape of discrete RM and their interactions. Here, SAXS was used in order to determine the size and shape of the water-inoil (w/o) microemulsion droplets in which ZO nanostructures grow. This technique is used to study the correlation between shape of the template and the product synthesized in it. Our data confirm that two types of droplets are observed in the microemulsion system. One which is spherical in shape having size approximately 4 nm, and the second one is representative of the growing nanodroplets whose shape and size change with the reaction time. After analyzing the data with a suitable model, it was observed that the particles increase their size from smaller to larger aggregates and the shape changes from spherical to elliptical, which later on transforms to nanorods which further grows to larger rods with time. As mentioned in our earlier report,25 we have explained how CTAB molecules carrying positive head groups arranged themselves around metal oxalate ions and form elongated reverse micelles. Nikoobakht et al.53 have also explained that CTAB can form a bilayer around the growing Au nanostructures and assist the growth of nanorods. Here in this study it is confirmed from SAXS and DLS studies that along with the spherical droplets of smaller size which contain solubilized ions bigger droplets of elliptical shape are observed which were formed after the exchange of micelles. There have been some reports where it is mentioned that complete exchange of the micelles takes place prior to the growth of nanostructures, but contrary to the above in our study the exchange of the reactants in the transient dimer takes place at a faster rate and then exchange and growth take place simultaneously.54 After complete exchange of the reactants the smaller droplets that were present in the system are fed into the growing nanostructures and result into nanorods of higher dimensions. Size and Shape of Templating Microemulsion Droplets. SAXS Data. Figure 1 shows small-angle X-ray

isooctane, and 10.05% of aqueous phase. The two microemulsions were slowly mixed and stirred overnight on a magnetic stirrer. The precursor was separated from the surfactant and nonpolar phase by centrifugation at an interval of every 2 h of reaction. The precursor was then washed with a 1:1 chloroform/methanol mixture to remove the surfactant and dried at room temperature. These ZO nanorods synthesized at different time intervals were then calcined at 300 °C for 6 h (by keeping heating rate constant in all cases) to obtain the resulting zinc oxide (ZnO) nanostructures. To carry out the in situ DLS and SAXS measurements, an aliquot of the reaction mixture was taken out every half an hour initially and then after every 1 h at the later stage of the reaction from the reaction mixture. To obtain the redispersed product, a part of reaction mixture was centrifuged at different time intervals and washed with a mixture of chloroform and methanol and dried at room temperature. DLS and TEM measurements were carried out on these ZO nanostructures. The polycrystalline nanoparticles were studied by powder X-ray diffraction (PXRD) using a Bruker D8 Advance diffractometer with Ni-filtered Cu Kα radiation. TEM and high-resolution TEM (HRTEM) studies were carried out on an FEI Technai G2 20 electron microscope operated at 200 kV. TEM samples were prepared by putting a drop of ultrasonically dispersed samples in reverse micelles and ethanol on a carbon-coated copper grid and dried at room temperature. The size of these nanoparticles/nanorods was also measured by DLS using a Malvern Zeta Sizer ZS 90 fitted with a 633 nm laser. SAXS measurements were done on Rigaku SmartLab X-ray diffractometer operating at 9 kW (200 mA and 45 kV) using Cu Kα (λ = 1.5406 Å) radiation. Small-angle X-ray scattering is commonly used to determine the shape and size of particles in colloidal solution. The measured scattering signal originates from the difference in scattering between the growing nanoparticles and the solvent. The scattering intensity I(q), as a function of the scattering vector q (defined in eq 1), was corrected for counting time and for sample absorption. q = (4π /λ) sin(θ /2)

(1)

where θ and λ are the scattering angle and the wavelength, respectively. The background scattering (solvent-filled capillary) was measured separately and subtracted from the scattering curve. Data analysis was based on fitting the curve to an appropriate model using a least-squares procedure.

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RESULTS AND DISCUSSION

In our earlier studies,27 homogeneous and monodispersed nanorods of zinc oxalate (ZO) were synthesized with CTAB as the surfactant, 1-butanol as the cosurfactant, and isooctane as the oil phase,27 and also, homogeneous zinc oxide nanoparticles (55 nm) were obtained by the decomposition of the oxalate precursor. Here, our aim is to understand the factors affecting the growth kinetics of anisotropic nanocrystals within reverse micelles. In this study in situ SAXS and DLS are used for the determination of size and structure of CTAB reverse micelles with or without any inorganic material present in it at room temperature. Also, DLS and TEM measurements were carried out on the ZO nanostructures as synthesized in these droplets and their corresponding decomposed product to determine their size, morphology, assembly, and aspect ratio. A combination of characterization methods is essential to obtain sufficient information, including mean size, number density,

Figure 1. Scattering intensity I(q) as a function of the scattering vector q from MDs of a CTAB/isooctane/1-butanol/water containing ZO nanostructures (red line) recorded after 30 min of reaction. The smooth line (blue) indicates the corresponding fit.

scattering raw data recorded during the synthesis of ZO nanostructures in CTAB reverse micellar systems at different time during the reaction. Since typical measurement time required for obtaining the scattering data with conventional laboratory equipment are in the range of several minutes, hence initially the measurements were carried out at an interval of 30 min and later this interval was increased to 1 h and so on. The resulting isotropic scattering data were azimuthally averaged to obtain I(q), which is the intensity scattered by the sample as a function of the scattering vector q. The raw data along with the corresponding fit (Figure 1) confirm the quality of the obtained 4124

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microemulsions (very fast) at both ends of the droplet.55 The droplet interaction kinetics and their shape transitions could be followed with SAXS, and the data were analyzed by using a suitable fitting model. The rapid formation of ZO nanostructures become evident from Figure 4, indicating that after half an hour numerous

data. The evaluated SAXS data plotted vs time shows the size distribution of smaller droplets (Figure 2) and that of

Figure 2. Size distributions of spherical droplets (obtained by SAXS) of CTAB reverse micellar system which contain reactant species or solublized ions obtained by SAXS. Figure 4. Aspect ratio obtained for the rod-like micelles obtained by analyzing SAXS data with a suitable fitting model. Rod-like micelles with highest aspect ratio obtained after 2.5 h of reaction suggest short nucleation dominant growth period of nanostructures. SAXS could not detect the presence of larger aggregates after 5 h of reactions due to limited resolution of the instrument.

Figure 3. Long time evolution of ZO nanostructures present in the reverse micellar droplets. Radius of the reverse micellar droplets in which ZO nanostructures are growing obtained from SAXS data as a function of reaction time. Inset shows the expansion of sizes and size distributions of the droplets and rod-like micelles (note: maximum size was obtained after 4 h of reaction).

Figure 5. In situ size distribution of nanostructures in microemulsion solution obtained from DLS at different time intervals. Three types of regions in the droplet sizes and their distribution which correspond to reactants/byproducts, aggregates where nucleation dominant growth takes place, and the region where growth of the nanorods take place.

aggregates (Figure 3 and inset). At the start of the reaction, mixing of equimolar microemulsions containing both the reactants, the droplets start colliding with each other and form an intermediate encounter complex that coalesces to form the transient dimer, where exchange of reactants take place. The lifetime of this transient dimer is very small, and we could not observe this dimer by using SAXS for which faster time scale techniques are required. These transient dimers result into spherical and cylindrical aggregates in which ZO nanostructures grow which finally convert to rod-like micelles, and the product acquires the shape of the micelles (Scheme 1). The cylindrical microemulsion droplet may dynamically exchange with other

particles have been formed. The evaluated SAXS data suggest the formation of nanostructures with different shapes and size distributions within half an hour of the reaction. Two kinds of droplets and their aggregates were observed: one with smaller dimensions, approximately 3−4 nm (confirmed also by DLS also, Figures 5 and 6) and a second due to larger aggregates/ droplets in which reaction and particle formation are going on. Subsequently, particles and droplets further grow, accompanied by a shape transformation of the droplets. The radius vs time

Scheme 1. Schematic Representation Showing the Mechanism of Change in Shape of the Droplets (MDs) and Formation of Nanorods

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which zinc oxalate nanostructures grow and acquire the shape of the template (surfactant aggregate). This shift toward larger size is observed until 4 h, beyond which it decreases. The variation of droplet sizes with time (Figure 3) distinctly shows nanoparticles grow up to nanorods with a maximum size up to 4 h, and after that only nanorods are observed. In fact, it is found that beyond the critical size most of the particles switch to anisotropic nanorods as observed by DLS studies (see below). This phenomenon is also supported by the TEM data as we start seeing the formation of small nanorods after 4 h (discussed below). It should be noted here that SAXS could not detect the MDs containing large nanorods/nanoparticles because of its limited resolution, but these droplets and nanostructures can be seen in DLS and TEM studies. In Situ DLS Data. Although SAXS unfolds the trend of nanostructure growth, to further confirm these results, DLS was employed to characterize the system at the ensemble level (an aliquot of the sample was taken out from the reaction mixture for in situ SAXS and DLS measurements). The hydrodynamic size (d) of droplets containing solubilized ions is found to remain almost constant for initial reaction time (2−3 h, 4 nm), similar to that obtained by SAXS (4 nm). DLS studies were also carried out to observe the in situ growth kinetics of ZO nanostructures in microemulsion solution. We found that in situ DLS could detect large nanostructures (possibly the nanorods) after 2 h of reaction. In an ensemble it is possible that many MDs containing rods (and particles) associate to form large aggregates which are mainly detected rather than the single nanorods in the in situ DLS measurements. In the initial hours of reaction (up to 2 h) along with the small droplets (blue region), in situ DLS measurements could detect the very short nucleation-dominant nanoparticle growth of nanostructures present in the droplets (red region) (Figure 6). Beyond 2 h it could not detect the smaller droplets, which could be due to the availability of very few MDs containing the nucleated particles which have very low (total) scattered intensity to be detected by DLS. Hence, using DLS, one can easily follow the growth of these nanorods. It is important to note here that the simultaneous growth of nanorods and nanoparticles in the initial phase of the reaction (as confirmed from our TEM and DLS data) supports the prediction of Edgar et al.,59 which suggests that particles and rods can grow simultaneously in solution. If few isotropic nanoparticles break their symmetry to grow anisotropically to form nanorods, the rest of the particles that do not undergo the symmetry breaking can grow normally. In the case of gold it is proposed that if particles of less than 5 nm break their symmetry, they can grow into nanorods. A similar argument might be applicable for the present ZO nanostructures, as they also show similar bifurcation near the critical particle size of 50−60 nm. However, nanorods of smaller dimensions are observed in both SAXS and TEM measurements in the early stage of the reaction which supports the prediction of Edgar et al.59 This suggests that the ZO nanoparticles nucleate and grow until they become elongated and reach their critical size of 50− 60 nm, and beyond this size only nanorods can grow. The DLS plot (Figure 5) clearly shows three distinct regions of droplets with different size distributions in which ZO nanorods are formed: droplets that contain reactants or solubilized ions which are observed in the initial period of the reaction, droplets containing ZO nanostructures which are growing with time, and the droplets with the ZO nanorods which are growing to larger aspect ratio with time. The initial nucleation dominant

Figure 6. Change in the peak size of the three well-defined regions of sizes. Plot shows different growth periods of ZO nanostructures. Red circle indicates maximum transition in the size is observed after 2 h of the reaction, and up to this point of reaction most of the particles grow to a particular size which is critical for the breaking their symmetry and further they grow anisotropically.

plot from the SAXS data of smaller droplets having size of approximately 4 nm is symmetric in shape and confirms the spherical shape of the droplets (Figure 2). As these droplets are present until the end of the reaction, it suggests that they contain solubilized ions or byproducts of the reaction. The size (d) curve for bigger droplets or aggregates contains ZO nanostructures which are asymmetrical in shape (Figure 3 and inset), with a pronounced peak on the lower-d side and an extended long tail on the higher-d side, which is characteristic of rod-like or cylindrical micelles.56−58 Initially, these droplets have smaller extended tails whose length increases as the reaction proceeds and results in a broad particle size distribution suggesting the transformation of cylindrical droplets to rod-like droplets. The inset in Figure 3 confirms the change in the sphericity of the droplets to cylindrical and then to rod-like micelles, and peak d value increases with reaction time. This asymmetry in the size curve demonstrates that the reverse micelles which contain growing ZO nanostructures are ellipsoidal. Hence, SAXS confirms the presence of spherical as well as cylindrical reverse micelles. It was found that the peak d and the tail correspond to the diameter and length of the rods, respectively, which was observed to increase with reaction time. We have calculated the aspect ratio of these rod-shaped reverse micelles, and it was found that droplets with highest aspect ratio were obtained after 2 h of the reaction (Figure 4). At this point the maximum number of cylindrical droplets were observed in solution (in which the reaction was going on) and achieved a critical size with length and diameter approximately 80 and 18 nm, respectively. These MDs influences and control the nanostructures growing in them. This is the transition point where growth of the nanorods start and elongated particles with an approximate size of 50−60 nm are obtained (TEM images see below) from these elongated droplets. The gradual increase in size in initial time also suggests the existence of very short nucleation-dominant nanoparticle growth through droplet (elongated) coalescence which leads to elongated nanoparticles until a critical size of 50−60 nm, beyond which they grow anisotropically to form smaller nanorods and further into larger nanorods. Also, after 4 h of reaction droplets with highest dimension are obtained and the full width at half-maxima (FWHM) is found to be maximum, which suggests the transition of maximum spherical droplets to rod-like droplets in 4126

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growth period (2 h) of droplets leads to elongated particles of size of 50−60 nm, and then later they switch to nanorods (transition from elongated particles to rods) with higher dimensions (Figure 6). The particles grow to a critical size (∼55 nm) beyond which there is a break in their symmetrical growth and subsequently leads to anisotropic growth of the nanostructures. In the growth process of zinc oxalate nanorods, a very short nucleation-dominant nanoparticle growth period is noticed, then a short period where elongated nanoparticles break their symmetry and start forming small nanorods, and finally a period when nanorods grow to longer dimensions and their aspect ratio increases. In situ DLS data show that until 2 h of reaction MDs containing ZO nanostructures of approximate size 50−60 nm are present in the solution. The hydrodynamic radius of purely spherical particles can be calculated by using the Stokes− Einstein equation. For asymmetric particles, the Stokes− Einstein equation is not applicable, and in that case Stick hydrodynamic boundary condition can be applied to modify it.60,61 According to Stick theory for rod-like particles, the diffusion coefficient parallel to the major axis (D∥) and that of the minor axis (D⊥)62 are expressed as eqs 2 and 3 D =

kBT ln(L /d) 2πηL

(2)

D⊥ =

kBT ln(L /d) 4πηL

(3)

Figure 7. DLS measurements of ZO nanostructures redispersed in ethanol. The peak Rh shifted to higher values with time, showing the continuous growth of nanostructures. Up to 4 h of reaction both the particles and rods were observed to grow, but beyond that only nanorod growth takes place at the expense of the smaller nanostructures.

observed that until 4 h from start of reaction both particles and rods grow simultaneously, but beyond that only nanorods grow at the expense of the smaller nanostructures. At this point (∼4 h after reaction is initiated) of time a large number of droplets associate to shift the equilibrium from particles to rods. TEM Studies. Figure 8 shows the evolution of nanostructures with time as observed by TEM. We have carried out TEM measurements on ZO nanostructures formed in the reaction mixture at different time intervals to investigate the growth mechanism and formation of nanorods starting from the nucleated nanoparticles. In these images we see the growth of uniform nanoparticles in the initial period (2 h) of reaction. In this nucleation-dominant period we can see the alignment of particles in one direction and the nanoparticles increase their size relatively faster, and in the period of 2−4 h both elongated particles and small rods are observed, and after 4 h of reaction they start growing into nanorods with higher dimension. At 0 h (just after the solutions are mixed) a mixture of very small particles (∼7 nm) and bigger particles of average size of 50 nm is obtained in which particles are observed to align in one direction (see Figure 8), and after 1 h of reaction the smaller particles grow from 7 to 10 nm and the bigger particles of critical size (50−60 nm) align to form larger nanostructures (supported by DLS, see Figure 7). After 2 h of reaction a similar mixture of nanorods and nanoparticles with size of 50− 60 nm is observed. However, at this point a large number of elongated nanoparticles are obtained which further grow into nanorods after 4 h of reaction. Interestingly, the TEM images of the ZO nanostructures (up to 4 h) show a mixture of nanorods and elongated nanoparticles, indicating the coexistence of both the morphologies. After 4 h, a few nanorods with smaller dimension were also detected, though the equilibrium shifts toward the formation of nanorods with much larger dimensions. Beyond 6 h of reaction ZO nanorods grow further with increase in aspect ratio and nanorods with highest aspect ratio were obtained after completion of reaction (15 h). By combination of all the three techniques, it was observed that a very short nucleation-dominant nanoparticle growth period (until 2 h of reaction) followed by a period (2−4 h) where elongated nanoparticles switch over to anisotropic growth to form nanorods and finally the period (beyond 4 h) where

where η is the viscosity of the medium, T is the temperature, kB is the Boltzmann constant, L is length, and d is diameter of the rods. The total diffusion coefficient of the particle can then be calculated using eq 4. DT = (D + 2D⊥)/3

(4)

We have applied the above procedure to predict the asymmetry of the droplets observed in in situ DLS. The distribution in the lengths of the rods was calculated for the critical size achieved at 2 h. In this calculation, the diameter (d) of the elongated droplets was determined by SAXS. The length obtained from this method for the droplets of an average size of 55 nm (in situ DLS) is calculated as 80 nm, which matches exactly with the dimension of the elongated droplets present in the system and observed in SAXS data. This indicates that the critical size for the elongated droplets is 80 nm in length and 18 nm in diameter, and these critical dimensions are a new finding about an asymmetric particle before the rods begin to start selfassembling. Shape and Size of Resulting Nanostructures. DLS of Redispersed Product. Interestingly, however, when DLS measurements were performed on ZO nanostructures extracted from the reaction mixture and redispersed in ethanol, we observed an increasing size of nanostructures with time which matches with the SAXS and TEM results (see Figure 7). A mixture of small and larger nanostructures is obtained in the initial 3 h of reaction. The nanostructures are slightly larger than 55 nm (the critical size of droplets) observed by SAXS and in situ DLS, which might be due to agglomeration of nanostructures. The growth of nanostructures is found to start at this point of reaction and continue beyond 4 h. This indicates that unlike SAXS, DLS could detect the larger nanoparticles beyond 4 h of reaction if the particles were extracted from the mixture and redispersed in ethanol. It is also 4127

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Figure 8. TEM images for the growing nanocrystals (zinc oxalate) as obtained at different time intervals of reaction. At the start of reaction the nucleation-dominant period is observed, where the particles are aligning in one direction and the nanoparticles increase their size relatively faster, in the period of 2 h both particles and rods are observed, and after 4 h of reaction, nanorods grow in length very fast. Fully grown nanorods are seen after 15 h of reaction.

Figure 9. TEM micrographs of zinc oxide nanostructures obtained by the thermal decomposition of ZO obtained after different reaction times.

precursor after longer duration of reaction gives nanorods with high aspect ratio. The morphology of ZO nanostructures can be retained in the decomposed product, which also shows the variation in the morphology of zinc oxide nanostructures with time. Mechanism of Particle Formation. Using small-angle X-ray (SAXS), we observe the presence of maximum number of droplets with anisotropy after 4 h of reaction (maximum FWHM), which suggests that at this stage the smaller nanorods rapidly grow and form the anisotropic nanorods with higher dimensions, and this is also seen in the TEM images. The aspect ratio of the nanorod increases with reaction time from 2:1 (2 h) to 14:1 (15 h). This implies that there is a more attractive interaction between the particles along the longer axis compared to the lateral axis of the rods that leads to the increase in aspect ratio. Such an increase originates from the fact that CTAB decreases the surface charge density of the ionic micelles and thereby promotes the formation of low-curvature micellar structures like nanorods.63 It has been earlier

elongation of nanorods is observed, and they further grow in dimension to nanorods. In this study, the nanodroplets achieve a critical size in a very short period, which makes the reaction kinetics very fast and allows them to change their symmetry and grow fast into nanorods. While in our earlier report on iron oxalate nanorods,17 where we have studied the intermicellar exchange, the kinetics was very slow which could be due to the long nucleation dominant growth period. Hence, initial nucleation dominant growth period and achievement of critical size are the key steps in deciding the rate kinetics of the reaction. On the basis of TGA studies, the different zinc oxalate nanostructures were further decomposed to zinc oxide nanostructures. TEM studies (Figure 9) show formation of spherical particles (with an average size of 10 and 50 nm) of zinc oxide from ZO precursor (obtained very close to the start of reaction and after 1 h of reaction). ZO obtained after 2 and 4 h also decomposed, and small nanorods of zinc oxide were obtained having aspect ratio of 2:1 and 4:1, respectively. The decomposed product (ZnO) obtained from the oxalate 4128

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Figure 10. Schematic diagram showing plausible mechanism for the growth of ZO nanoparticles and nanorods inside the polar core of MDs. Droplets containing reactants interact to first produce an intermediate encounter complex in which micellar exchange takes place and product formation starts. Initially both spherical and anisotropic reverse micellar droplets are present in which nanoparticles and nanorods both are growing simultaneously, but later on after achieving a critical size all the particles switch their symmetry to anisotropic droplets and nanorods grow in length very fast.

suggested64 that, due to positive surface charges, these cationic surfactants assemble on the surface of the growing nanostructure (which have negative zeta potential) and subsequently allow the growth along the longer axis, leading to the formation of nanorods. Zhao et al. found cylindrical-to-spherical shape transformation of lecithin reverse micelles induced by CO2 by using SAXS.65 They have suggested that the main reason for the micellar shape transformation could be due to the reduction of the degree of hydrogen bonding between surfactant headgroups and water with added CO2. Here in our case the anisotropy is due to formation of assembly of cationic surfactant molecules on the growing ZO nanostructures with negative zeta potential. As mentioned in our earlier report,25 we have explained how CTAB molecules carrying positive head groups arranged themselves around metal oxalate ions and form elongated reverse micelles. Nikoobakht et al.53 have also explained that CTAB can form a bilayer around the growing Au nanostructures and assist the growth of nanorods. In our study also it is confirmed both by SAXS and by DLS that along with the spherical droplets of smaller size which contains solubilized ions, bigger droplets of elliptical shape are observed which are formed after the exchange of micelles. Thus, it is suggested that cationic surfactants lead to an assembly of surfactant molecules on the surface of the growing.26 Hence, the growth would be restricted along the sides leading to the formation of nanorods (Figure 10). It is probable that the surfactant molecules (CTAB) do not associate with the ZO nuclei on the waterenriched domains as shown in Figure 10. Because of the cylindrical shape of microemulsions the dynamical exchange

with other microemulsions is very fast at both ends, resulting in the formation of nanorods.55 Figure 10 depicts a possible mechanism of how MDs interact to coalesce and exchange reactants with time leading to the growth of the nanostructures. From the start of the reaction until about 2 h, SAXS, DLS, and TEM observed similar sizes of the growing nanoparticles, signifying the existence of short nucleation-dominant particle growth. During 2−4 h of reaction, the particles undergo symmetry breaking and start forming nanorods which has been confirmed by all the three techniques. After 4 h of reaction, SAXS observes only the smaller particles due to its limited resolution, and DLS and TEM start showing only nanorods. In TEM, the growth in nanorod length and aspect ratio changes drastically for the oxalate precursor beyond 4 h of reaction (Figure 8). This observation is supported by SAXS results which show that, after 4 h of reaction, the rate of droplet coalescence reaches its maximum, indicating the highest rate of smaller droplets being fed into the growing nanorod system. Under these conditions it is observed that morphology of ZO nanostructures obtained by cationic surfactants could be maintained in the decomposition product (zinc oxide).

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CONCLUSIONS A mechanistic study on the growth of zinc oxalate nanostructures in CTAB reverse micelles, focusing on the correlation between shape/size of MDs containing nanostructures with that of the product growing with time, has been performed using in situ SAXS, DLS, and TEM measurements. The morphology of zinc oxalate (ZO) nanostructures depends 4129

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critically on structure of reverse micellar droplets. Spherical droplets are formed after the initial mixing of the droplets. These grow in size with reaction time and then deviate from sphericity and become elongated. Finally, these elongated nanoparticles align to form nanorods. An increase in size beyond a critical average size of 55 nm causing a elongated particle-to-rod transition is observed. The reverse micelles with the critical size are stable in isooctane and are present until all the smaller droplets grow into dimension with anisotropy and achieve highest aspect ratio with time. The growth of these droplets and transition in shape from sphere-to-rod-like micelles can be correlated with that of the final product as synthesized in the droplets. Hence, initial nucleation dominant growth period and achievement of critical size is the key step in deciding the rate of the reaction. These droplets act as shaping vesicles for the formation of ZO nanorods, and the shape of the droplets controls the shape of the product synthesized in it. This growth process can be viewed as a morphologically templated nucleation process. A combination of characterization methods is necessary to reveal the sufficient information including size, size distribution, and shape of the droplets to understand the growth kinetics. Morphology of the precursor can be retained in the product (oxide), and the influence of the growth pattern of the precursor can be observed on the morphology of the product zinc oxide. For the first time this study has provided an understanding of the structural evolution of microemulsion droplets containing nanostructures and a definitive proof of the surfactant aggregate (reverse micelle) dictating the shape of the nanostructures.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected], [email protected]; Tel +91-11-26591511, +91-172-2210073/75; Fax +91-1722211074, +91 11 2658 1102 (A.K.G.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by DST (Nano Mission) (SR/NM/ NF95/2010), DeitY (12(4)/2007-PDD), Govt. of India. We thank Dr. De and his student Anuradha for helping us in SAXS experiments. Soma Sharma thanks CSIR for providing a fellowship.

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