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Size tailoring of ZnS nanoparticles synthesized in reverse micelles and recovered by compressed CO2
J. of Supercritical Fluids 30 (2004) 89–95
Size tailoring of ZnS nanoparticles synthesized in reverse micelles and recovered by compressed CO2 Jianling Zhang, Buxing Han∗ , Juncheng Liu, Xiaogang Zhang, Guanying Yang, Huaizhou Zhao Center for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China Received 3 September 2002; received in revised form 2 April 2003; accepted 22 May 2003
Abstract Nanosized ZnS particles were synthesized in bis(2-ethylhexyl) sulfosuccinate (AOT) reverse micelles and then recovered by dissolving compressed CO2 into the micellar solution. The size of the obtained ZnS nanoparticles can be tailored in the range of 1–100 nm by varying the experimental conditions. The particles’ size decreases with increasing pressure of CO2 . The size of the particles recovered at 308.2 K is smaller than that of the particles recovered at 298.2 K. Stirring of the solution in the recovery process can reduce the particle size effectively. Increase in molar ratio of water to AOT (w) results in an increase in the particle size. However, the effect of the concentration of AOT in the micellar solution on the size of the products is very limited. © 2003 Elsevier B.V. All rights reserved. Keywords: Reverse micelles; ZnS; Nanoparticles; Compressed CO2
1. Introduction There is a great deal of interest in the preparation and applications of nanometer-sized materials due to their novel properties [1–7]. The characteristics of nanoparticles depend directly on their size and shape. Therefore, the synthesis of controlled shape and size of nanoparticles is of great importance to their applications. ZnS nanocrystallites have been widely studied for their application in many ranges, such as electroluminescent devices, solar cells and phosphors [8,9]. Among the methods to produce nanoparticles, the use of reverse micelles as a novel environment has ∗ Corresponding author. Tel.: +86-10-6256-2821; fax: +86-10-6255-9373. E-mail address: [email protected] (B. Han).
attracted much interest for its potential advantages compared with the traditional methods [10–15]. The reverse micelles are able to host in their hydrophilic core solid nanoparticles obtained in situ by suitable reactions. In a certain range, the micellar core enlarges with the increase of water content, which is characterized by the water-surfactant molar ratio (w). For the reason that the reaction is restricted in the water core, the growth of the obtained product particles can be controlled by the size of polar core. Bis(2-ethylhexyl) sulfosuccinate (AOT) is a commonly used surfactant for the formation of reverse micelles in organic solvents, which can solubilize considerable amounts of water without using any cosurfactant [16–19]. To prepare nanoparticles using reverse micelles, the recovery methods and conditions are crucial to the particle size, size distribution and dispersion. The
0896-8446/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0896-8446(03)00113-X
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traditional recovery method is by flocculation [7], evaporation to dryness [20] or adding certain chemical reagent to cause phase separation [21], which precipitate the surfactant simultaneously. The post-process is troublesome because the products contain large amount of surfactant. It is known that compressed gaseous CO2 can dissolve in many organic solvents, which results in considerable change in the solvent power of the solvents. The solubility of CO2 in the solvents is a function of pressure. Therefore, the solvency of the solvents can be tuned continuously by pressure. The diffusivity of the pressurized gas is much larger and the viscosity is much smaller than that of liquids. Thus, the transport properties can be improved by the dissolved gas. Moreover, the dissolved gas can be removed easily by pressure reduction. These features of the CO2 -expanded solvents are advantageous for many applications [22–26]. Recently, we proposed a new method to recover the nanoparticles synthesized in reverse micelles using compressed CO2 [27]. To test this method, the ZnS particles synthesized in AOT reverse micelles (w = 15, [AOT] = 50 mmol/l) were recovered at 308.2 K and 5.50 MPa. It was found that almost all the ZnS particles could be recovered by compressed CO2 , while the AOT remained in the isooctane-continuous phase. By this technique, small and well dispersed ZnS nanoparticles with narrow size distribution were obtained. Through the control of stirring rate from 100 to 400 rpm, the particle size decreased from 3–8 to 1–3 nm. In this work, we study the effect of various factors, such as temperature, pressure, w, concentration of AOT [AOT] in the micellar solution, on the size of the product particles.
2. Experimental 2.1. Materials CO2 (>99.995% purity) was provided by Beijing Analysis Instrument Factory. The surfactant AOT was purchased from Sigma with purity of 99%. The isooctane, Na2 S and ZnSO4 supplied by Beijing Chemical Plant were all AR grade. Double-distilled water was used.
2.2. Apparatus and procedures 2.2.1. Synthesis of ZnS nanoparticles in reverse micelles The procedures to synthesize ZnS nanoparticles in the reverse micelles were similar to that reported by other authors [9]. The solution of AOT in isooctane was first prepared, then reverse micellar solutions containing respectively aqueous solutions of Na2 S and ZnSO4 were prepared by adding corresponding aqueous salt solution to the surfactant solution. Then the two micellar solutions containing Na2 S and ZnSO4 , respectively were mixed and ZnS nanoparticles were formed in the reverse micelles due to the water pools can exchange their contents by a collision process. 2.2.2. Precipitation of ZnS from reverse micelles A UV-vis spectrophotometer (Model TU-1201, produced by Beijing General Instrument Co.) was used to monitor the precipitation of ZnS particles from the reverse micelles at different CO2 pressures. The temperature-controlled high-pressure sample cell and experimental procedures were the same as those used to study the UV spectra of protein in AOT reverse micelles [28]. The cell composed mainly of stainless steel body and two quartz windows, which was thermostated to ± 0.1 ◦ C of the desired temperature by a electric heater and a temperature controlling system. The optical path length and the volume of the sample cell were 1.32 cm and 1.74 cm3 , respectively. There was a magnetic stirrer in the sample cell to provide rapid mixing of CO2 and reverse micellar solutions. In a typical experiment, a suitable amount of micellar solution with synthesized ZnS particles was loaded into the sample cell by a syringe. After the thermal equilibrium had been reached, CO2 was charged into the sample cell by the high-pressure pump until the cell was full. The UV spectrum of the solution was recorded every 10 min until it was unchanged, which was an indication of equilibrium condition. In each experiment, the loaded solution contained 0.383 mg ZnS and 0.039 g AOT. 2.2.3. Recovery of nanometer-sized ZnS from the reverse micelles A known amount of micellar solution with the synthesized ZnS nanoparticles were loaded into the cylinder-shaped autoclave of 32.60 ml. CO2 was
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charged into the autoclave by a high-pressure pump until the desired pressure was reached and the solution was stirred for 1 h. The stirring was stopped and the ZnS particles were precipitated from the solution and deposited at the bottom of the autoclave. The solution and CO2 were released slowly. At the experimental conditions, the surfactants were not precipitated from the solution because all the experimental pressures were lower than the cloud point pressure. Thus, most of the surfactants remained in the removed solution. The precipitated ZnS particles at the bottom of the autoclave were collected and washed by water and ethanol. The by-products of Na2 SO4 and the small amount of surfactants (left in the solution absorbed on the product) were removed by the washing because they are readily soluble in water or ethanol. The products were dried under vacuum at 303.2 K for 4 h. 2.2.4. Characterization The morphology of the ZnS particles were examined by transmission electron microscopy (TEM) with a HITACHI H-600A electron microscope. Particles were dispersed in ethanol and then directly deposited on the copper grid. After the evaporation of the solvent, the TEM analysis was conducted.
3. Results and discussion 3.1. Precipitation of ZnS from reverse micelles The UV-vis spectroscopy can be used to monitor ZnS nanoparticles [8,9]. The ZnS nanoparticles formed in the reverse micelles do not precipitate in the absence of CO2 . However, the ZnS particles can be precipitated by dissolution of CO2 in the micellar solution, which is known by our UV study. As examples, Fig. 1 illustrates the UV spectra of reverse micellar solutions (w = 15 and [AOT] = 50 mmol/l) containing ZnS at 298.2 K and some typical CO2 pressures. For all the experiments, the concentration of ZnS synthesized in the reverse micellar solutions after expansion should be 0.22 mg/ml if the ZnS is not precipitated. The absorption band at ≈267 nm is attributed to the absorption of ZnS in the reverse micelles and that at 210–230 nm is assigned to the surfactant AOT [28]. As can be seen, the intensity of the absorption band of ZnS decreases with increasing pressure, while that
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Fig. 1. UV spectra of reverse micelles ([AOT] = 50 mmol/l, w = 15) containing ZnS at different CO2 pressures and 298.2 K.
for AOT remains unchanged in a certain pressure range. This indicates that the ZnS particles can be precipitated from the reverse micelles and the surfactant remains in the organic continuous phase. The micellar solution became completely cloudy as the pressure exceeded a certain value (usually defined as the cloud point pressure), which could be observed from the optical windows of the cell. It is easy to understand because AOT begin to precipitate at cloud point pressure. The cloud point pressure of the reverse micelles ([AOT] = 50 mmol/l, w = 15) at 298.2 and 308.2 K were determined to be 4.80 and 5.61 MPa, respectively. 3.1.1. Recovery of ZnS nanoparticles In order to study the effect of the operating conditions on the particle size of the products, we conducted the synthesis and recovery of ZnS particles at various experimental conditions by changing w, [AOT], CO2 pressure, temperature and stirring rate in the recovery process. 3.1.2. Recovery at 308.2 K The TEM photographs of ZnS nanoparticles recovered from the reverse micelles at different conditions are shown in Fig. 2(a–i). The particle size and size distribution are obtained by measuring the diameters of the particles in the micrographs. Fig. 2(a, b) were reported in our previous work [27]. Fig. 2(a–c) illustrate that stirring rate in the recovery process influence the size and the size distribu-
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Fig. 3. TEM photographs of ZnS obtained from the reverse micelle at 298.2 K (a) at 4.50 MPa, rpm = 400, [AOT] = 50 mmol/l, w = 15; (b) at 4.50 MPa, rpm = 0; [AOT] = 50 mmol/l, w = 15; (c) at 2.50 MPa, rpm = 0; [AOT] = 50 mmol/l, w = 15.
tion of the products. The particle sizes are ≈3–13, 3–8 and 1–3 nm as the stirring rates are 0 rpm (without stirring), 100 and 400 rpm. It means that the size of the ZnS particles is smaller under rapid stirring and the size distribution is narrower. In the process of precipitation, the nanoparticles can interact with each other and coalescence occurs. Under rapid stirring, the degree of molecular mobility can be enhanced, which may reduce the coalescence of the nanoparticles. Hence, smaller particles were obtained under higher stirring speed. Fig. 2(a, d, e) illustrate the TEM photographs of ZnS recovered from the reverse micelles at different CO2 pressures. The stirring rate is 400 rpm. It is obvious that an increase in recovery pressure results in a decrease in the particle size. The size varies from 4–25 to 1–3 nm upon increasing the pressure from 2.51 to 5.50 MPa. The physical coalescence of nanoparticles may be responsible for this phenomenon. As the pressure is increased, the solution is less viscous and dif-
fusivity is larger, which reduces the contacting time of the particles as they collide. This may reduce the physical coalescence of nanoparticles. Therefore, smaller particles are obtained at the higher pressures. It can be seen from Fig. 2(c, f, g) that at w = 5, w = 10, and w = 15, the particles sizes are 2–6, 3–8 and 3–13 nm, respectively, i.e. the particles recovered from the reverse micelles with smaller w are smaller. This can be attributed to the fact that the water cores in the reverse micelles are smaller at smaller w, which restricts the growth of particles in the reverse micelles. We also studied the effect of [AOT] in the micellar solution on the particle size. [AOT] changes from 25 to 100 mmol/l. The photographs are shown in Fig. 2(d, h, i). As we can see, the sizes of particles obtained from reverse micelles with different [AOT] are similar. Thus, it can be concluded that the concentration of the surfactant does not affect the particle size considerably.
Fig. 2. TEM photographs of ZnS obtained from the reverse micelle at 308.2 K (a) at 5.50 MPa, rpm = 400, [AOT] = 50 mmol/l, w = 15; (b) at 5.50 MPa, rpm = 100, [AOT] = 50 mmol/l, w = 15; (c) at 5.50 MPa, rpm = 0; [AOT] = 50 mmol/l, w = 15; (d) at 4.50 MPa, rpm = 400, [AOT] = 50 mmol/l, w = 15; (e) at 2.51 MPa, rpm = 400, [AOT] = 50 mmol/l, w = 15; (f) at 5.50 MPa, rpm = 0; [AOT] = 50 mmol/L, w = 10 (g) at 5.50 MPa, rpm = 0; [AOT] = 50 mmol/l, w = 5; (h) at 4.50 MPa, rpm = 400, [AOT] = 100 mmol/l, w = 15; (i) at 4.50 MPa, rpm = 400, [AOT] = 25 mmol/l, w = 15.
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3.2. Recovery at 298.2 K
References
All the results discussed above were obtained at 308.2 K. In order to study the influence of temperature on the particle size and size distribution, we also conducted the recovery process at 298.2 K. The TEM photographs of the obtained ZnS particles at different conditions are shown in Fig. 3(a–c). Comparing with the results at 308.2 K, the general trend is that the size of the particles recovered at the lower temperature is larger. One of the reasons may be that the solution at the lower temperature is more viscous and diffusivity is smaller. Thus, the contacting time of the particles is longer as they collide in the solution and thus they agglomerate into larger particles. Fig. 3(a, b) indicate that, as expected, the stirring can reduce the size of the particles and the sizes of the particles recovered with and without stirring are of the order of 15–30 and 60 nm, respectively. The particles size increases to ≈100 nm upon decreasing CO2 pressure to 2.50 MPa, as can be seen by comparing Fig. 3(b, c). This trend is similar to that at 308.2 K. Furthermore, the particles obtained at the lower temperature are more uniform than those obtained at the higher temperature. A reasonable explanation is that the viscosity of the solution is high at the lower temperature. Therefore, particles precipitate slowly and become bigger due to the collision of the particles. The particles precipitate quickly as they are large enough. Therefore, the particles with narrow size distribution can be obtained.
[1] R.B. Thompson, V.V. Ginzburg, M.W. Matsen, A.C. Balazs, Predicting the mesophases of copolymer-nanoparticle composites, Science 292 (2001) 2469. [2] P.D. Szuromi, Science, in: H.S. Nalwa (Ed.), Handbook of Nanostructured Materials and Nanotechnology, vol. 288, 2000 p. 1596. [3] V.I. Klimov, A.A. Mikhailovsky, S. Xu, A. Malko, J.A. Hollingsworth, C.A. Leatherdale, H.J. Eisler, M.G. Bawendi, Optical gain and stimulated emission in nanocrystal quantum dots, Science 290 (2000) 314. [4] L. Motte, F. Billoudet, E. Lacaze, M.P. Pileni, Self-organization of size-selected nanoparticles into three-dimensional superlattices, Adv. Mater. 8 (1996) 1018. [5] N. Pinna, K. Weiss, J. Urban, M.P. Pileni, Triangular CdS nanocrystals: structural and optical studies, Adv. Mater. 13 (2001) 261. [6] M.P. Pileni, T. Gulik-Krzywicki, J. Tanori, A. Filankembo, J.C. Dedieu, Template design of microreactors with colloidal assemblies: control the growth of copper metal rods, Langmuir 14 (1998) 7359. [7] P.K. Dutta, M.K. Jakupca, S. Reddy, L. Salvati, Controlled growth of microporous crystals nucleated in reverse micelles, Nature 374 (1995) 44. [8] Y. Nakaoka, Y. Nosaka, Electron spin resonance study of radicals produced by photoirradiation on quantized and bulk ZnS particles, Langmuir 13 (1997) 708. [9] P. Calandra, M. Goffredi, V.T. Liveri, Study of the growth of ZnS nanoparticles in water/AOT/n-heptane microemulsions by UV-absorption spectroscopy, Colloids Surf. 160 (1999) 9. [10] D.B. Zhang, H.M. Cheng, J.M. Ma, Y.P. Wang, X.Z. Gai, Synthesis of silver-coated silica nanoparticles in nonionic reverse micelles, J. Mater. Sci. Lett. 20 (2001) 439. [11] F.T. Quinlin, J. Kuther, W. Tremel, W. Knoll, S. Risbud, P. Stroeve, Reverse micelle synthesis and characterization of ZnSe nanoparticles, Langmuir 16 (2000) 4049. [12] F. Agnoli, W.L. Zhou, C.J. O’Connor, Synthesis of cubic antiferromagnetic KMnF3 nanoparticles using reverse micelles and their self-assembly, Adv. Mater. 13 (2001) 1697. [13] R. Bsndyopadhyaya, R. Kumar, K.S. Gandhi, Simulation of precipitation reactions in reverse micelles, Langmuir 16 (2000) 7139. [14] C. Petit, A. Taleb, M.P. Pileni, Self-organization of magnetic nanosized cobalt particles, Adv. Mater. 10 (1998) 259. [15] E. Stathatos, P. Lianos, F.D. Monte, D. Levy, D. Tsiourvas, Formation of TiO2 nanoparticles in reverse micelles and their deposition as thin films on glass substrates, Langmuir 13 (1997) 4295. [16] C.D. Borsarelli, S.E. Braslavsky, Nature of the water structure inside the pools of reverse micelles sensed by laser-induced optoacoustic spectroscopy, J. Phys. Chem. B 101 (1997) 6036. [17] R.E. Riter, E.P. Undiks, N.E. Levinger, Impact of counterion on water motion in aerosol OT reverse micelles, J. Am. Chem. Soc. 120 (1998) 6062.
4. Conclusion The zinc sulfide synthesized in AOT reverse micelles are recovered by compressed CO2 . Small and well dispersed nanoparticles can be obtained and the size of spherical ZnS particles can be tailored in the wide range of 1–100 nm by changing molar ratio of water to surfactant, pressure of CO2 , temperature and stirring rate.
Acknowledgements The authors are grateful to the National Natural Science Foundation of China (20133030).
J. Zhang et al. / J. of Supercritical Fluids 30 (2004) 89–95 [18] P. Bartsotas, L.H. Poppenborg, D.C. Stuckey, Emulsion formation and stability during reversed micelle extraction, J. Chem. Technol. Biot. 75 (2000) 738. [19] R.D. Falcone, C.N. Morrea, B.M. Aiasutti, J.J. Juana, Properties of AOT aqueous and nonaqueous microemulsions sensed by optical molecular probes, Langmuir 16 (2000) 3070. [20] M.L. Steigerwald, A.P. Alivisatos, J.M. Gibson, T.D. Harris, R. Kortan, A.J. Muller, A.M. Thayer, T.M. Duncan, D.C. Douglass, L.E. Brus, Surface derivatization and isolation of semiconductor cluster molecules, J. Am. Chem. Soc. 110 (1988) 3046. [21] D.H. Chen, S.H. Wu, Synthesis of nickel nanoparticles in water-in-oil microemulsions, Chem. Mater. 12 (2000) 1354. [22] E. Reverchon, Supercritical-assisted atomization to produce micro- and/or nanoparticles of controlled size and distribution, Ind. Eng. Chem. Res. 41 (2002) 2405. [23] E. Reverchon, Supercritical antisolvent precipitation of microand nano-particles, J. Supercrit. Fluids 15 (1999) 1.
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[24] E. Reverehon, P.G. Della, M.G. Falivene, Process parameters and morphology in amoxicillin micro and submicro particles generation by supercritical antisolvent precipitation, J. Supercrit. Fluids 17 (2000) 239. [25] K.J. Heater, D.L. Tomasko, Processing of epoxy resins using carbon dioxide as an antisolvent, J. Supercrit. Fluids 14 (1998) 55. [26] D. Li, B.X. Han, Z.M. Liu, J. Lu, Z.X. Ghang, S.G. Wang, X.F. Zhang, Effect of gas antisolvent on conformation of polystyrene in toluene: viscosity and small-angle X-ray scattering study, Mocromolecules 34 (2001) 2195. [27] J.L. Zhang, B.X. Han, J.C. Liu, X.G. Zhang, J. He, Z.M. Liu, A new method to recover the nanoparticles from reverse micelles: recovery of ZnS nanoparticles synthesized in reverse micelles by compressed CO2 , Chem. Commun. 24 (2001) 2724. [28] H.F. Zhang, B.X. Han, G.Y. Yang, H.K. Yan, Effect of CO2 and CHF3 on the solubilization of protein in reverse micelles, J. Colloid Interface Sci. 232 (2000) 269.
Size tailoring of ZnS nanoparticles synthesized in reverse micelles and recovered by compressed CO2 Jianling Zhang, Buxing Han∗ , Juncheng Liu, Xiaogang Zhang, Guanying Yang, Huaizhou Zhao Center for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China Received 3 September 2002; received in revised form 2 April 2003; accepted 22 May 2003
Abstract Nanosized ZnS particles were synthesized in bis(2-ethylhexyl) sulfosuccinate (AOT) reverse micelles and then recovered by dissolving compressed CO2 into the micellar solution. The size of the obtained ZnS nanoparticles can be tailored in the range of 1–100 nm by varying the experimental conditions. The particles’ size decreases with increasing pressure of CO2 . The size of the particles recovered at 308.2 K is smaller than that of the particles recovered at 298.2 K. Stirring of the solution in the recovery process can reduce the particle size effectively. Increase in molar ratio of water to AOT (w) results in an increase in the particle size. However, the effect of the concentration of AOT in the micellar solution on the size of the products is very limited. © 2003 Elsevier B.V. All rights reserved. Keywords: Reverse micelles; ZnS; Nanoparticles; Compressed CO2
1. Introduction There is a great deal of interest in the preparation and applications of nanometer-sized materials due to their novel properties [1–7]. The characteristics of nanoparticles depend directly on their size and shape. Therefore, the synthesis of controlled shape and size of nanoparticles is of great importance to their applications. ZnS nanocrystallites have been widely studied for their application in many ranges, such as electroluminescent devices, solar cells and phosphors [8,9]. Among the methods to produce nanoparticles, the use of reverse micelles as a novel environment has ∗ Corresponding author. Tel.: +86-10-6256-2821; fax: +86-10-6255-9373. E-mail address: [email protected] (B. Han).
attracted much interest for its potential advantages compared with the traditional methods [10–15]. The reverse micelles are able to host in their hydrophilic core solid nanoparticles obtained in situ by suitable reactions. In a certain range, the micellar core enlarges with the increase of water content, which is characterized by the water-surfactant molar ratio (w). For the reason that the reaction is restricted in the water core, the growth of the obtained product particles can be controlled by the size of polar core. Bis(2-ethylhexyl) sulfosuccinate (AOT) is a commonly used surfactant for the formation of reverse micelles in organic solvents, which can solubilize considerable amounts of water without using any cosurfactant [16–19]. To prepare nanoparticles using reverse micelles, the recovery methods and conditions are crucial to the particle size, size distribution and dispersion. The
0896-8446/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0896-8446(03)00113-X
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traditional recovery method is by flocculation [7], evaporation to dryness [20] or adding certain chemical reagent to cause phase separation [21], which precipitate the surfactant simultaneously. The post-process is troublesome because the products contain large amount of surfactant. It is known that compressed gaseous CO2 can dissolve in many organic solvents, which results in considerable change in the solvent power of the solvents. The solubility of CO2 in the solvents is a function of pressure. Therefore, the solvency of the solvents can be tuned continuously by pressure. The diffusivity of the pressurized gas is much larger and the viscosity is much smaller than that of liquids. Thus, the transport properties can be improved by the dissolved gas. Moreover, the dissolved gas can be removed easily by pressure reduction. These features of the CO2 -expanded solvents are advantageous for many applications [22–26]. Recently, we proposed a new method to recover the nanoparticles synthesized in reverse micelles using compressed CO2 [27]. To test this method, the ZnS particles synthesized in AOT reverse micelles (w = 15, [AOT] = 50 mmol/l) were recovered at 308.2 K and 5.50 MPa. It was found that almost all the ZnS particles could be recovered by compressed CO2 , while the AOT remained in the isooctane-continuous phase. By this technique, small and well dispersed ZnS nanoparticles with narrow size distribution were obtained. Through the control of stirring rate from 100 to 400 rpm, the particle size decreased from 3–8 to 1–3 nm. In this work, we study the effect of various factors, such as temperature, pressure, w, concentration of AOT [AOT] in the micellar solution, on the size of the product particles.
2. Experimental 2.1. Materials CO2 (>99.995% purity) was provided by Beijing Analysis Instrument Factory. The surfactant AOT was purchased from Sigma with purity of 99%. The isooctane, Na2 S and ZnSO4 supplied by Beijing Chemical Plant were all AR grade. Double-distilled water was used.
2.2. Apparatus and procedures 2.2.1. Synthesis of ZnS nanoparticles in reverse micelles The procedures to synthesize ZnS nanoparticles in the reverse micelles were similar to that reported by other authors [9]. The solution of AOT in isooctane was first prepared, then reverse micellar solutions containing respectively aqueous solutions of Na2 S and ZnSO4 were prepared by adding corresponding aqueous salt solution to the surfactant solution. Then the two micellar solutions containing Na2 S and ZnSO4 , respectively were mixed and ZnS nanoparticles were formed in the reverse micelles due to the water pools can exchange their contents by a collision process. 2.2.2. Precipitation of ZnS from reverse micelles A UV-vis spectrophotometer (Model TU-1201, produced by Beijing General Instrument Co.) was used to monitor the precipitation of ZnS particles from the reverse micelles at different CO2 pressures. The temperature-controlled high-pressure sample cell and experimental procedures were the same as those used to study the UV spectra of protein in AOT reverse micelles [28]. The cell composed mainly of stainless steel body and two quartz windows, which was thermostated to ± 0.1 ◦ C of the desired temperature by a electric heater and a temperature controlling system. The optical path length and the volume of the sample cell were 1.32 cm and 1.74 cm3 , respectively. There was a magnetic stirrer in the sample cell to provide rapid mixing of CO2 and reverse micellar solutions. In a typical experiment, a suitable amount of micellar solution with synthesized ZnS particles was loaded into the sample cell by a syringe. After the thermal equilibrium had been reached, CO2 was charged into the sample cell by the high-pressure pump until the cell was full. The UV spectrum of the solution was recorded every 10 min until it was unchanged, which was an indication of equilibrium condition. In each experiment, the loaded solution contained 0.383 mg ZnS and 0.039 g AOT. 2.2.3. Recovery of nanometer-sized ZnS from the reverse micelles A known amount of micellar solution with the synthesized ZnS nanoparticles were loaded into the cylinder-shaped autoclave of 32.60 ml. CO2 was
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charged into the autoclave by a high-pressure pump until the desired pressure was reached and the solution was stirred for 1 h. The stirring was stopped and the ZnS particles were precipitated from the solution and deposited at the bottom of the autoclave. The solution and CO2 were released slowly. At the experimental conditions, the surfactants were not precipitated from the solution because all the experimental pressures were lower than the cloud point pressure. Thus, most of the surfactants remained in the removed solution. The precipitated ZnS particles at the bottom of the autoclave were collected and washed by water and ethanol. The by-products of Na2 SO4 and the small amount of surfactants (left in the solution absorbed on the product) were removed by the washing because they are readily soluble in water or ethanol. The products were dried under vacuum at 303.2 K for 4 h. 2.2.4. Characterization The morphology of the ZnS particles were examined by transmission electron microscopy (TEM) with a HITACHI H-600A electron microscope. Particles were dispersed in ethanol and then directly deposited on the copper grid. After the evaporation of the solvent, the TEM analysis was conducted.
3. Results and discussion 3.1. Precipitation of ZnS from reverse micelles The UV-vis spectroscopy can be used to monitor ZnS nanoparticles [8,9]. The ZnS nanoparticles formed in the reverse micelles do not precipitate in the absence of CO2 . However, the ZnS particles can be precipitated by dissolution of CO2 in the micellar solution, which is known by our UV study. As examples, Fig. 1 illustrates the UV spectra of reverse micellar solutions (w = 15 and [AOT] = 50 mmol/l) containing ZnS at 298.2 K and some typical CO2 pressures. For all the experiments, the concentration of ZnS synthesized in the reverse micellar solutions after expansion should be 0.22 mg/ml if the ZnS is not precipitated. The absorption band at ≈267 nm is attributed to the absorption of ZnS in the reverse micelles and that at 210–230 nm is assigned to the surfactant AOT [28]. As can be seen, the intensity of the absorption band of ZnS decreases with increasing pressure, while that
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Fig. 1. UV spectra of reverse micelles ([AOT] = 50 mmol/l, w = 15) containing ZnS at different CO2 pressures and 298.2 K.
for AOT remains unchanged in a certain pressure range. This indicates that the ZnS particles can be precipitated from the reverse micelles and the surfactant remains in the organic continuous phase. The micellar solution became completely cloudy as the pressure exceeded a certain value (usually defined as the cloud point pressure), which could be observed from the optical windows of the cell. It is easy to understand because AOT begin to precipitate at cloud point pressure. The cloud point pressure of the reverse micelles ([AOT] = 50 mmol/l, w = 15) at 298.2 and 308.2 K were determined to be 4.80 and 5.61 MPa, respectively. 3.1.1. Recovery of ZnS nanoparticles In order to study the effect of the operating conditions on the particle size of the products, we conducted the synthesis and recovery of ZnS particles at various experimental conditions by changing w, [AOT], CO2 pressure, temperature and stirring rate in the recovery process. 3.1.2. Recovery at 308.2 K The TEM photographs of ZnS nanoparticles recovered from the reverse micelles at different conditions are shown in Fig. 2(a–i). The particle size and size distribution are obtained by measuring the diameters of the particles in the micrographs. Fig. 2(a, b) were reported in our previous work [27]. Fig. 2(a–c) illustrate that stirring rate in the recovery process influence the size and the size distribu-
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Fig. 3. TEM photographs of ZnS obtained from the reverse micelle at 298.2 K (a) at 4.50 MPa, rpm = 400, [AOT] = 50 mmol/l, w = 15; (b) at 4.50 MPa, rpm = 0; [AOT] = 50 mmol/l, w = 15; (c) at 2.50 MPa, rpm = 0; [AOT] = 50 mmol/l, w = 15.
tion of the products. The particle sizes are ≈3–13, 3–8 and 1–3 nm as the stirring rates are 0 rpm (without stirring), 100 and 400 rpm. It means that the size of the ZnS particles is smaller under rapid stirring and the size distribution is narrower. In the process of precipitation, the nanoparticles can interact with each other and coalescence occurs. Under rapid stirring, the degree of molecular mobility can be enhanced, which may reduce the coalescence of the nanoparticles. Hence, smaller particles were obtained under higher stirring speed. Fig. 2(a, d, e) illustrate the TEM photographs of ZnS recovered from the reverse micelles at different CO2 pressures. The stirring rate is 400 rpm. It is obvious that an increase in recovery pressure results in a decrease in the particle size. The size varies from 4–25 to 1–3 nm upon increasing the pressure from 2.51 to 5.50 MPa. The physical coalescence of nanoparticles may be responsible for this phenomenon. As the pressure is increased, the solution is less viscous and dif-
fusivity is larger, which reduces the contacting time of the particles as they collide. This may reduce the physical coalescence of nanoparticles. Therefore, smaller particles are obtained at the higher pressures. It can be seen from Fig. 2(c, f, g) that at w = 5, w = 10, and w = 15, the particles sizes are 2–6, 3–8 and 3–13 nm, respectively, i.e. the particles recovered from the reverse micelles with smaller w are smaller. This can be attributed to the fact that the water cores in the reverse micelles are smaller at smaller w, which restricts the growth of particles in the reverse micelles. We also studied the effect of [AOT] in the micellar solution on the particle size. [AOT] changes from 25 to 100 mmol/l. The photographs are shown in Fig. 2(d, h, i). As we can see, the sizes of particles obtained from reverse micelles with different [AOT] are similar. Thus, it can be concluded that the concentration of the surfactant does not affect the particle size considerably.
Fig. 2. TEM photographs of ZnS obtained from the reverse micelle at 308.2 K (a) at 5.50 MPa, rpm = 400, [AOT] = 50 mmol/l, w = 15; (b) at 5.50 MPa, rpm = 100, [AOT] = 50 mmol/l, w = 15; (c) at 5.50 MPa, rpm = 0; [AOT] = 50 mmol/l, w = 15; (d) at 4.50 MPa, rpm = 400, [AOT] = 50 mmol/l, w = 15; (e) at 2.51 MPa, rpm = 400, [AOT] = 50 mmol/l, w = 15; (f) at 5.50 MPa, rpm = 0; [AOT] = 50 mmol/L, w = 10 (g) at 5.50 MPa, rpm = 0; [AOT] = 50 mmol/l, w = 5; (h) at 4.50 MPa, rpm = 400, [AOT] = 100 mmol/l, w = 15; (i) at 4.50 MPa, rpm = 400, [AOT] = 25 mmol/l, w = 15.
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3.2. Recovery at 298.2 K
References
All the results discussed above were obtained at 308.2 K. In order to study the influence of temperature on the particle size and size distribution, we also conducted the recovery process at 298.2 K. The TEM photographs of the obtained ZnS particles at different conditions are shown in Fig. 3(a–c). Comparing with the results at 308.2 K, the general trend is that the size of the particles recovered at the lower temperature is larger. One of the reasons may be that the solution at the lower temperature is more viscous and diffusivity is smaller. Thus, the contacting time of the particles is longer as they collide in the solution and thus they agglomerate into larger particles. Fig. 3(a, b) indicate that, as expected, the stirring can reduce the size of the particles and the sizes of the particles recovered with and without stirring are of the order of 15–30 and 60 nm, respectively. The particles size increases to ≈100 nm upon decreasing CO2 pressure to 2.50 MPa, as can be seen by comparing Fig. 3(b, c). This trend is similar to that at 308.2 K. Furthermore, the particles obtained at the lower temperature are more uniform than those obtained at the higher temperature. A reasonable explanation is that the viscosity of the solution is high at the lower temperature. Therefore, particles precipitate slowly and become bigger due to the collision of the particles. The particles precipitate quickly as they are large enough. Therefore, the particles with narrow size distribution can be obtained.
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4. Conclusion The zinc sulfide synthesized in AOT reverse micelles are recovered by compressed CO2 . Small and well dispersed nanoparticles can be obtained and the size of spherical ZnS particles can be tailored in the wide range of 1–100 nm by changing molar ratio of water to surfactant, pressure of CO2 , temperature and stirring rate.
Acknowledgements The authors are grateful to the National Natural Science Foundation of China (20133030).
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