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Controlling the Size, Morphology, and Aspect Ratio of Nanostructures Using Reverse Micelles: A Case Study of Copper Oxalate Monohydrate Rajeev Ranjan, Sonalika Vaidya, Pallavi Thaplyal, Mohd. Qamar, Jahangeer Ahmed, and Ashok K. Ganguli* Department of Chemistry, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India Received January 8, 2009. Revised Manuscript Received February 17, 2009 This study focuses on understanding the growth and control of nanostructures using reverse micelles. It has been earlier realized that parameters like surfactant, cosurfactant, and aqueous content influence the size and shape of the nanostructures obtained using reverse micelles. However, a concerted effort to understand the role of these factors on the growth of a specific nanomaterial is missing. In this study we have focused on one nanomaterial (copper oxalate monohydrate) and determined how the above-mentioned factors control the size, shape, aspect ratio, and growth of these nanostructures. Our results show that cationic surfactants (CTAB, TTAB, and CPB) favor the formation of nanorods of copper oxalate. The aspect ratio of these rods could be controlled to obtain nanocubes (∼80-100 nm) and nanoparticles (∼8-10 nm) in the CTAB system using longer chain cosurfactants like 1-octanol and 1-decanol, respectively. Nanocubes of ∼50-60 and ∼60-80 nm were obtained using nonionic surfactants Triton X-100 and Tergitol, respectively. The size of the nanostructures could also be controlled by varying the molar ratio of water to surfactant (W0) by using a nonionic (Triton X-100)-based reverse micellar system. The study espouses the versatility of the microemulsion method to realize a variety of nanostructures of copper oxalate monohydrate. Our results will be of use in extending these ideas to other nanomaterials.
Introduction Nanomaterials have been a subject of intense research because of the marked transition in electrical,1 optical,2,3 and magnetic4 properties from their bulk counterpart and with the variation in their size and shape.5,6 Controlling the size and shape of nanostructures is hence of utmost importance and remains a challenging problem. Though various methods have been utilized to synthesize nanomaterials such as sol-gel, chemical vapor deposition, reverse micellar method, and coprecipitation method, however, among them the reverse micellar method allows the flexibility to control the size and shape along with monodispersity of nanostructures. Reverse micelles 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 a cosurfactant. The cosurfactant used is normally a short chain alcohol or an amine. The aqueous core of a reverse micelle acts as a reactor of nanoscale. When two microemulsions, one containing the metal ion and the other a precipitating agent both in aqueous solutions, are uniformly mixed, the reaction occurs in the aqueous region of the reverse micelle, resulting in the formation of homogeneous and monodispersed nanoparticles. Because of the involvement of several variables (surfactant, cosurfactant, solvent, amount of water, temperature), it is possible to fine-tune the composition of the microemulsion and hence *Author for correspondence: e-mail
[email protected]; Tel 9111-26591511; Fax 91-11-2685471. (1) Kim, S.; Jun, M. C.; Hwang, S. C. J. Am. Ceram. Soc. 1999, 82, 289–296. (2) Protasenko, V.; Bacinello, D.; Kuno, M. J. Phys. Chem. B 2006, 110, 25322– 25331. (3) Ghosh, P.; Patra, A. J. Phys. Chem. C 2007, 111, 7004–7010. (4) Kodama, R. H.; Makhlouf, S. A.; Berkowitz, A. E. Phys. Rev. Lett. 1997, 79, 1393–1396. (5) Zhou, W. P.; Lewera, A.; Larsen, R.; Masel, R. I.; Bagus, P. S.; Wieckowski, A. J. Phys. Chem. B 2006, 110, 13393–13398. (6) Kolar, M.; Mestankova, H.; Jirkovsky, J.; Heyrovsky, M.; Subrt, J. Langmuir 2006, 22, 598–604.
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the size and structure of the reverse micelle. The factors affecting the size and shape of nanostructured materials obtained by microemulsion synthesis have been studied earlier. Khilar et al. have investigated the role of aqueous content on the size of AgCl nanoparticles.7 They observed an increase in particle size with W0 of 5-10, but at W0 of 15 the size of particles decreased. Cason et al. have reported an increase in the growth rate with increasing W0 for copper nanoparticles.8 Addition of cosurfactants increases the surfactant film flexibility9 and coalescence rate, which results in the formation of larger sized particles. Charinpanitkul et al. have studied the size and shape of ZnS nanoparticles10 and obtained quantum dots, nanorods, and nanotubes with variation in the chain length of cosurfactant along with W0. The elastic constant of the surfactant film,7,11 intermicellar exchange rate,12,13 size, shape, and charge of polar headgroup of surfactant,8,9 surfactant hydrophobic chain length,14 presence of additives like alcohols, electrolytes, and block copolymers,8 nature of continuous oil phase,7,11,12,15 and reactant concentration16,17 are the other factors which affect the growth of particles in reverse micelles. We have already reported the synthesis of copper oxalate using CTAB as the surfactant, 1-butanol as the cosurfactant, and (7) Bagwe, R. P.; Khilar, K. C. Langmuir 1997, 13, 6432–6438. (8) Cason, J. P.; Miller, M. E.; Thompson, J. B.; Roberts, C. B. J. Phys. Chem. B 2001, 105, 2297–2302. (9) Quintela, M. A. Curr. Opin. Colloid Interface Sci. 2003, 8, 137–144. (10) Charinpanitkul, T.; Chanagul, A.; Dutta, J.; Rungsardthong, U.; Tanthapanichakoon, W. Sci. Technol. Adv. Mater. 2005, 6, 266–271. (11) Eastoe, J.; Hollamby, M. J.; Hudson, L. Adv. Colloid Interface Sci. 2006, 128, 5–15. (12) Eastoe, J.; Sharpe, D. Langmuir 1997, 13, 3289–3294. (13) Ahmad, T.; Chopra, R.; Ramanujachary, K. V.; Lofland, S. E.; Ganguli, A. K. Solid State Sci. 2005, 7, 891–895. (14) Capek, I. Adv. Colloid Interface Sci. 2004, 110, 49–74. (15) Vaidya, S.; Rastogi, P.; Agarwal, S.; Gupta, S. K.; Ahmad, T.; Antonelli, A.; Ramanujachary, K. V.; Lofland, S. E.; Ganguli, A. K. J. Phys. Chem. C 2008, 112, 12610–12615. (16) Filankembo, A.; Giorgio, S.; Lisiecki, I.; Pileni, M. P. J. Phys. Chem. B 2003, 107, 7492–7500. (17) Filankembo, A.; Pileni, M. P. J. Phys. Chem. B 2000, 104, 5865–5868.
Published on Web 3/20/2009
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Table 1. Summary of Copper Oxalate Nanostructures Synthesized Using Reverse Micellar Route with Different Surfactants (W0 = 12) surfactant 18
CTAB TTAB CPB CTAB Tergitol TX-100
cosurfactant 1-butanol 1-butanol 1-butanol 1-butanol 1-butanol 1-butanol
solvent
W0
av size and morphology aspect ratio (AR)
isooctane isooctane isooctane cyclohexane cyclohexane cyclohexane
12 12 12 12 12 12
nanorods ∼ (480 nm:130 nm); AR 3.69:1 nanorods ∼ (400 nm:150 nm); AR 2.66:1 nanorods (750 nm:500 nm); AR 1.5:1 nanorods ∼ (600 nm:200 nm); AR 3:1 nanocubes ∼50-60 nm nanocubes ∼60-80 nm
Figure 1. PXRD pattern of copper oxalate monohydrate synthesized using the CTAB/1-hexanol/isooctane system.
isooctane as the oil phase.18 In order to understand the role of surfactant, cosurfactant, and the aqueous content on the size and morphology of the nanocrystalline materials in a more meaningful way, an extensive study of the synthesis of copper oxalate nanostructures by varying surfactant, cosurfactant, and aqueous content was carried out. This is the first focused study attempting to understand the major factors which control the growth of nanoparticles by concentrating our study on one particular material, namely copper oxalate monohydrate (COM). Metal carboxylates are efficient precursors for several metal oxides having applications as sensors, catalysts, dielectric insulators, etc. Among them, first row transition metal oxides (and ZnO) are already in use in several device applications. Thus, it is important to develop suitable precursors to obtain pure metal oxide nanoparticles for bulk and thin film applications. We have embarked on this project to obtain precursors which can lead to such metal oxide nanoparticles with controlled size and homogeneity. Since copper oxide nanoparticles are of special interest because of their efficiency as nanofluids (4% of CuO improves the thermal conductivity of water by 20%) in heat-transfer applications, we chose to develop the process conditions for the controlled growth of copper oxalate nanorods which act as a precursor of CuO nanoparticles. Our studies would hopefully provide a general scheme for controlling the size and shape of the large number of metal carboxylates which may be important to obtain their corresponding metal oxides.
Experimental Procedure COM was synthesized using the reverse micellar method with variation in the surfactant, cosurfactant, and aqueous content. (18) Ahmad, T.; Chopra, R.; Ramanujachary, K. V.; Lofland, S. E.; Ganguli, A. K. J. Nanosci. Nanotechnol. 2005, 5, 1840–1845.
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Figure 2. TG/DTA plot of copper oxalate monohydrate synthesized using CTAB/1-hexanol/isooctane. Five different surfactants were employed (details later). To study the effect of cosurfactant, alcohols with varying carbon chain length, viz. 1-pentanol, 1-hexanol, 1-heptanol, 1-octanol, and 1-decanol, were used. All the other parameters were kept constant. The specific systems studied contained CTAB as the surfactant and isooctane as the oil phase. W0 was kept constant at 12 for all the systems. The aqueous content was varied using a nonionic surfactant system: the specific system studied was Triton X-100/cyclohexane/1-hexanol/aqueous phase with varying W0 values of 11, 14, and 16, keeping all the other parameters constant. The effect of varying the chain length and size of polar headgroup of cationic surfactants like CTAB (cetyltrimethylammonium bromide), CPB (cetylpyridinium bromide), and TTAB (tetradecyltrimethylammonium bromide) was also studied. Langmuir 2009, 25(11), 6469–6475
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Figure 3. IR spectra of copper oxalate monohydrate synthesized using CTAB/1-hexanol/isooctane. Table 2. Copper Oxalate Synthesized Using Reverse Micellar Route with Variation of W0 in Triton X-100/Cyclohexane/1-Hexanol System W0
av size and morphology of product
11 14 16
nanocubes of size ∼ 35-50 nm nanocubes of size ∼50-60 nm nanocubes of size ∼80-100 nm
Microemulsions using neutral surfactants varying in structure (Tergitol and Triton X-100) were also investigated. The specific systems studied were (a) CPB/isooctane/1-butanol/ aqueous phase, (b) TTAB/isooctane/1-butanol/aqueous phase, (c) CTAB/cyclohexane/1-butanol/aqueous phase (d), Tergitol/ cyclohexane/1-butanol/ aqueous phase, and (e) Triton X-100/ cyclohexane/1-butanol/aqueous phase. For all these above systems two microemulsions were prepared: one containing Cu2+ ions (0.1 M Cu(NO3)2) and the other containing oxalate ions (0.1 M (NH4)2C2O4). Both the microemulsions were mixed and stirred on a magnetic stirrer for ∼20 h. The nanoparticles obtained from solutions containing CTAB CPB and TTAB (W0 = 12) were centrifuged and then washed with a 1:1 mixture of chloroform and methanol to obtain a light blue precipitate of copper oxalate. Nanoparticles obtained from Triton X-100-based microemulsion after stirring for ∼20 h were heated slowly to reduce the volume to half. Subsequently, these nanoparticles were centrifuged and washed with methanol. For obtaining the particles from the Tergitol system (W0 = 14), acetone was added after reducing the volume to half and this solution was heated slowly to further reduce the volume to half before centrifugation, and then the product was washed with acetone. Powder X-ray diffraction studies (PXRD) were carried out on a Bruker D8 Advance diffractometer using Ni-filtered Cu KR radiation. TGA/DTA experiments were carried out on PerkinElmer Pyris Diamond TGA/DTA system on well-ground samples in a flowing nitrogen atmosphere with a heating rate of 10 °C/min. IR spectra were recorded on disks obtained after mixing with KBr and loaded on a Nicolet Protege 460 (FTIR) spectrometer operating in the range of 400-4000 cm-1. Transmission electron microscopy (TEM) studies were carried out on FEI Technai G2 20 electron microscope operated at 200 kV. Langmuir 2009, 25(11), 6469–6475
Results and Discussion Effect of Surfactant. To understand the effect of surfactant on the growth, size, and shape of nanoparticles of copper oxalate, we have studied the effect using different surfactants. Five different surfactants were employed which can be divided according to the charge of the polar headgroup as follows. (1) Cationic surfactants: (a) cetyltrimethylammonium bromide (CTAB); (b) cetylpyridinium bromide (CPB); (c) tetradecyltrimethylammonium bromide (TTAB). (2) Nonionic surfactants: (d) Tergitol and (e) Triton X-100. These surfactants vary in their charge, size of polar headgroup, and hydrophobic chain length. The other parameters, namely aqueous content, oil phase, and cosurfactant, were kept constant. The product obtained after centrifugation from the different reverse micellar systems were analyzed by PXRD and were found to be monophasic (Figure 1) corresponding to orthorhombic CuC2O4 3 H2O (JCPDS 21-0297). Thermal analysis in flowing nitrogen of nanostructures of copper oxalate (obtained using different microemulsion systems) shows a weight loss (in a single step) corresponding to one water molecule and two molecules of carbon dioxide (Figure 2). The final decomposed product was analyzed as copper. A similar behavior was observed for copper oxalate synthesized by changing surfactant/solvent/cosurfactant/W0 in the reverse micelles. The copper oxalate was also decomposed in air at 450 °C to obtain copper (II) oxide nanoparticles. Figure 3 shows a typical IR spectrum of copper oxalate monohydrate formed using the microemulsion CTAB/1-hexanol/isooctane, with the characteristic band at 19,20 and a broad band at 3441 cm-1 1648 cm-1 for C2O24 corresponding to O-H stretching. Cu-O stretching was observed around 507 cm-1, C-CdO bending mode was observed around 824 cm-1, COO- stretching at 1320 cm-1, and C-O stretching at 1362 cm-1. (19) Belevtsev, B. I.; Naugle, D. G.; Rathnayaka, K. D. D.; Parasiris, A.; Finowicki, J. F. Physica B 2005, 355, 341–351. (20) Vazquez, C. V.; Blanco, M. C.; Lopez-Quintela, M. A.; Sanchez, R. D.; Rivas, J.; Oseroff, S. B. J. Mater. Chem. 1998, 8, 991–1000.
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Figure 4. TEM micrographs for copper oxalate monohydrate synthesized using (a) TTAB/1-butanol/isooctane, (b) CPB/1-butanol/ isooctane, (c) CTAB/1-butanol/cylcohexane, (d) Tergitol/1-butanol/cylcohexane, and (e) Triton X-100/1-butanol/cylcohexane.
In our earlier study we had synthesized copper oxalate monohydrate18 nanorods (480 nm (length):130 nm (diameter)) with CTAB as surfactant, 1-butanol as cosurfactant, and isooctane as the oil phase. To extend this study, we have investigated the effect of surfactant chain length on the size and shape of copper oxalate nanoparticles by using TTAB (C-14) as compared to CTAB (C-16). The length and diameter of the rods were found to be lower for the system with TTAB as the surfactant (Figure 4a). The length of the surfactant chain influences the surfactant packing parameter.21 The lower area adopted by TTAB surfactant due to decrease in hydrophobic chain length leads to higher packing efficiency,11 and more rigid films are expected which would explain the formation of smaller rods in case of the TTAB system. (21) Pileni, M. P. J. Phys. Chem. 1993, 97, 6961–6973.
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We have also investigated the variation of the polar headgroup of the surfactant. Polar headgroups play an important part in controlling the size and morphology of nanostructured materials synthesized in the reverse micellar system.8,15 The charge of the polar headgroup of surfactant is an important factor in determining the isotropic or anisotropic growth of a particle. CPB has pyridinium ion as the polar headgroup as compared to ammonium ion in the case of CTAB. The restricted orientation of pyridinium ion due to its larger size gives a rigid surfactant layer, leading to the formation of nanorods with the dimensions of 750 nm:500 nm and a lower aspect ratio (1.5:1) (Figure 4b). Zeta potential studies on copper oxalate synthesized in the absence of surfactant (CTAB), by simply mixing equal volumes of the two aqueous solutions of copper nitrate and ammonium oxalate, shows a negative value (-14.3 mV) which suggest Langmuir 2009, 25(11), 6469–6475
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Figure 6. Electron diffraction of copper oxalate monohydrate synthesized using Tergitol/1-butanol/cylcohexane.
Figure 5. Schematic diagram showing plausible mechanism for the formation of rod.
negative surface charge on copper oxalate particles. In the presence of surfactant, a positive value for zeta potential (1.8 mV) was observed, which suggests that the particles are well capped by the cationic surfactant. Thus, it is suggested that cationic surfactants lead to an assembly of surfactant molecules on the surface of the growing nanorods (due to the negative surface charge or zeta potential).15 Hence, the growth would be restricted along the sides leading to the formation of nanorods (Figure 5). It is probable that the surfactant molecules (CTAB) do not associate with the copper oxalate nuclei on the water-enriched domains as shown in Figure 5. The dynamical exchange with other microemulsions is very fast at both ends, resulting in the formation of nanorods. It has been known that copper oxalate crystallizes in an orthorhombic structure formed by the stacking of Cu(C2O4)Cu(C2O4) ribbons22 with the ribbon axis corresponding to [001] direction. With nonionic surfactants like Triton X-100 (TX 100) and Tergitol, surfactant molecules are associated on all the domains (both water-rich and those having no water on the sides), which results in a uniform growth in all directions leading to spherical or cubic particles. The above point has been investigated in our comparison of nonionic surfactants (Tergitol and TX-100) with a cationic surfactant (CTAB). All the other parameters, namely cosurfactant (1-butanol), W0 (=12), and oil phase (cyclohexane), were kept the same in all the systems. In case of the synthesis using CTAB as the surfactant we obtained rods with aspect ratio 3:1 (length ∼600 nm, diameter∼200 nm) (Figure 4c). Both microemulsion systems using nonionic surfactants employing Tergitol and Triton X-100 led to the formation of nanocubes (∼50-60 nm with Tergitol (Figure 4d) and ∼60-80 nm (Figure 4e) with TX-100). Electron diffraction (Figure 6) studies were carried out on copper oxalate monohydrate synthesized using Tergitol which was indexed on the basis of orthorhombic copper oxalate monohydrate. This study shows the isotropic (22) Jongen, N.; Bowen, P.; Lema^itre, J.; Valmalette, J. C.; Hofmann, H. J. Colloid Interface Sci. 2000, 226, 189–198.
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Figure 7. TEM micrographs for copper oxalate monohydrate synthesized using Triton X-100/1-hexanol/cylcohexane at W0 = 14.
growth of nanoparticles in case of nonionic surfactants. It further corroborates our previous study, where we have obtained similar variation in size for nickel oxalate nanostructures synthesized in nonionic surfactant systems.15 Effect of Aqueous Content, W0. The dependence of the size and shape of the copper oxalate nanostructures on the aqueous content of the system was also investigated in same detail. Copper oxalate nanostructures were synthesized for the microemulsion system with a neutral surfactant, Triton X-100. All the other parameters were kept constant, and only the W0 of varying values of 11, 14, and 16 was used to synthesize copper oxalate monohydrate. TEM studies show the formation of nanocubes in the range of 30-50 nm at W0 of 11. Nanocubes of size ∼50-60 and ∼80-100 nm were obtained at W0 of 14 (Figure 7) and 16, respectively, showing an increase in the particle size with increase in W0 value for the Triton X-100 system. This could be explained on the basis of increase in flexibility of the surfactant film with increasing aqueous content, which results in a higher intermicellar exchange rate. With the increase in the intermicellar exchange rate we could observe an increase in the size. Effect of Cosurfactant. The effect of cosurfactant was studied by varying the chain length of the cosurfactants, keeping the other parameters constant. The surfactant used was CTAB. The following cosurfactants were used in our studies;(a) 1-butanol, (b) 1-pentanol, (c) 1-hexanol, (d) 1-heptanol, DOI: 10.1021/la900063q
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Figure 8. TEM micrographs for copper oxalate monohydrate synthesized using (a) 1-pentanol, (b) 1-hexanol, (c) 1-heptanol, (d) 1-octanol, and (e) 1-decanol as cosurfactant in the CTAB/isooctane system.
(e) 1-octanol, and (f) 1-decanol;which allowed us to study the role of cosurfactant chain length on the surfactant film flexibility which finally determines the growth of particles inside the reverse micelle. Copper oxalate monohydrate was obtained as a pure phase in all the above systems, which was confirmed by PXRD. TEM studies show the increase in the aspect ratio of submicron rods (Figure 8a,b) with an increase in the carbon chain length of the cosurfactant from 1-butanol to 1-hexanol, in agreement with prior studies.11 Surfactant film rigidity decreases with increasing the chain length of cosurfactant,10 leading to higher growth rate which results in an increase in the aspect ratio of copper oxalate submicron rods. The aspect ratio observed for these rods is summarized in Table 3. Nanorods with aspect ratio of 6:1 were obtained in 1-heptanol (Figure 8c), which is slightly lower than that obtained in 6474
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Table 3. Copper Oxalate Synthesized Using Reverse Micellar Route with Variation of Cosurfactant Chain Length in CTAB/Isooctane/ Cosurfactant System (W0 = 12) cosurfactant 18
1-butanol 1-pentanol 1-hexanol 1-heptanol 1-octanol 1-decanol
av size and morphology of product aspect ratio (AR) nanorods ∼ (480 nm:130 nm); AR 3.69:1 nanorods ∼ (750 nm:175 nm); AR 4.3:1 nanorods ∼ (950 nm:150 nm); AR 6.33:1 nanorods ∼ (1200 nm:200 nm); AR 6:1 nanocubes of ∼80-100 nm nanoparticles of ∼8-10 nm assemble to form nanorods
1-hexanol. The length and diameter of the nanorods formed with 1-hexanol are higher, though the overall aspect ratio is lesser. We observed a decrease in the particle size and change in morphology from nanorods to nanocubes and nanoparticles as the chain length of the cosurfactant increases. Nanocubes of size Langmuir 2009, 25(11), 6469–6475
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Conclusions
Figure 9. TEM micrographs for copper oxalate monohydrate synthesized using (a) CTAB/1-decanol/isooctane at higher magnification showing alignment of particles inside the nanorods and by a different microemulsion using (b) CTAB/1-butanol/cyclohexane at nearly the same magnification as (a).
80-100 nm were obtained with 1-octanol (Figure 8d). We obtained nanoparticles of size ∼8-10 nm with 1-decanol (Figure 8e), which seem to assemble to form nonuniform nanorods (Figure 9a). These particles are not the result of copper oxalate reduction on exposure to electron beam which was further confirmed by carrying out TEM studies (at nearly the same magnification) (Figure 9b) on copper oxalate nanorods synthesized by another microemulsion using CTAB/1-butanol/ cyclohexane which do not show any sign of such assembly of nanoparticles. Cason et al.8 also reported a decrease in particle growth of copper metal when the amount of 1-octanol is increased. Beyond C6 alcohols (1-hexanol), the decrease in the size of nanostructures with an increase in the cosurfactant chain length may be due to the effective interaction of the alkyl chains of the cosurfactant with the surfactant tails at the micellar interface producing a more rigid structure which results in a reduced coalescence rate and hence leading to particles of smaller dimensions.
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Copper oxalate nanostructures varying in size and morphology could be obtained by appropriately choosing the microemulsion variables. The dependence of size and morphology of the synthesized nanostructures on the aqueous content, chain length of cosurfactant, size of the polar headgroup, and charge and hydrophobic chain length of surfactant was determined. We observed a proportional increase in the particle size with increasing W0 which can be attributed to the increase in film flexibility and thus higher intermicellar exchange in the microemulsion, with nonionic surfactants. The aspect ratio of copper oxalate nanorods increased with an increase in the carbon chain length of the cosurfactants due to increase in the flexibility of the film, which increases the growth rate. However, as the carbon chain length increases beyond C6, the aspect ratio decreases, and in microemulsions with higher chain length alcohols, nanocubes and nanoparticles could be obtained (with smaller size) possibly due to effective interaction of cosurfactant chains with the hydrophobic surfactant tails. Nonionic surfactants favor the formation of smaller nanostructures with cubelike morphology due to isotropic growth of particles as compared to cationic surfactants which favor the formation of anisotropic rodlike morphology. Increasing the surfactant film rigidity by decreasing the chain length of hydrophobic group of surfactant favors nanoparticles of smaller size as observed in the TTAB system. Strained and smaller polar headgroup in the CPB system favor the formation of smaller nanoparticles due to increased rigidity of surfactant film. Thus, it is possible to fine-tune the nanostructures using a proper choice of the microemulsion system. Acknowledgment. A.K.G. thanks CSIR and DST, Government of India, for financial assistance. S.V. thanks CSIR for a fellowship.
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