NANO LETTERS
Water-in-CO2 Microemulsions as Nanoreactors for Synthesizing CdS and ZnS Nanoparticles in Supercritical CO2
2002 Vol. 2, No. 7 721-724
Hiroyuki Ohde, Mariko Ohde, Franklin Bailey, Hakwon Kim, and Chien M. Wai* Department of Chemistry, UniVersity of Idaho, Moscow, Idaho 83844 Received October 31, 2001; Revised Manuscript Received April 28, 2002
ABSTRACT Semiconductor nanopaparticles of CdS and ZnS were synthesized by mixing two water-in-CO2 microemulsions with one containing S2- ions and the other containing Cd2+ or Zn2+ ions in the water core. Nanoparticle formation was monitored in situ by measuring their absorption spectra in the UV−vis range using a high-pressure fiber-optic system. The size of the nanoparticles formed in the microemulsion was found to depend on the water-to-surfactant ratio (W). At W ) 6 and 12, the band gaps of CdS were calculated to be 3.50 and 3.15 eV, corresponding to particle radii of 1.4 and 1.7 nm, respectively. The ZnS particles synthesized at W ) 12 showed a band gap of 4.26 eV, corresponding to a mean particle radius of 1.6 nm. This microemulsion-plus-microemulsion approach offers a simple method for synthesizing various nanoparticles in supercritical CO2 using water-soluble reagents as starting materials.
Introduction. There has been much interest recently in synthesizing nanometer-sized semiconductor particles because such particles exhibit size dependent optical and electrical properties.1-3 Water-in-oil microemulsions with nanometer-sized water core are well suited for this purpose. The particle growth occurs within the water core of the microemulsion and the precursors of the semiconductor particles are water-soluble ions. Since the radii of the water cores are determined by the water-to-surfactant ratio (W), the size of the semiconductor particles synthesized in the microemulsion can also be controlled by the W value. Using water-in-oil microemulsions, semiconductor nanoparticles with narrow size distributions including those of cadmium sulfide (CdS)4 and zinc sulfide (ZnS)5 have been synthesized. One problem of using water-in-oil microemulsions for nanoparticle synthesis is separation and removal of solvent from products. In recent years, supercritical carbon dioxide (SF CO2) has been extensively studied as a solvent for chemical synthesis. The advantages of using supercritical CO2 for chemical reactions and synthesis are described in a number of recent publications.6-8 Water-in-CO2 microemulsions have been investigated recently as a reaction system for synthesizing inorganic nanoparticles. Ji et al. first reported the synthesis of silver nanoparticles in a water-in-CO2 microemulsion by chemical reduction of silver ions in the water core with a reducing agent sodium triacetoxyborohydride.9 Other CO2soluble reducing agents such as sodium cyanoborohydride and N,N,N′,N′-tetramethyl-p-phenylenediamine were later * Corresponding author. E-mail address:
[email protected]. 10.1021/nl010084p CCC: $22.00 Published on Web 05/10/2002
© 2002 American Chemical Society
used by Ohde et al. for the synthesis of silver and copper nanoparticles in a water-in-CO2 microemulsion.10 In another report, these authors showed that silver halide nanoparticles could be synthesized by mixing water-in-CO2 microemulsions containing silver and halide ions separately in the water core.11 The CO2 microemulsions used in our previous studies were made by a mixture of two surfactants, sodium bis(2ethylhexyl) sulfosuccinate (AOT) and a perfluorinated polyether phosphate (PFPE-PO4). Since AOT is not soluble in supercritical CO2, the PFPE-PO4 probably acts like a cosurfactant making the microemulsion soluble in the fluid phase. Recent reports show that CO2-soluble microemulsions can be formed by using a fluorinated AOT.12 Other groups have also used fluorinated surfactants to make microemulsions for nanomaterial synthesis. For example, Holmes et al. used a PFPE-COONH4 (ammonium carboxylate perfluoropolyether) stabilized water-in-CO2 microemulsion containing Cd2+ in the water core for the synthesis of cadmium sulfide Q particles.13 In that study, an aqueous solution of Na2S was injected directly into the CO2 phase to cause formation of CdS particles in the water core. Using this method, the W value of the microemulsion was changing as the Na2S solution was introduced into the CO2 phase. In this letter, we report the synthesis of CdS and ZnS nanoparticles by mixing two water-in-CO2 microemulsions of the same W value but containing S2- ions and Cd2+ or Zn2+ ions separately in the water core. The microemulsions were stabilized using a fluorinated AOT synthesized in our laboratory. By collision, exchange of ions between the microemulsions takes place, leading to the formation of CdS
or ZnS in the water core. The microemulsions thus act like nanoreactors for the synthesis of these metal sulfide nanoparticles. This microemulsion-plus-microemulsion approach appears to offer a new method for synthesizing a variety of nanoparticles in supercritical CO2 using water-soluble reagents as starting materials. Synthesizing semiconductor nanoparticles in supercritical CO2 offers several advantages over the conventional water-in-oil microemulsion approach including fast reaction speed, rapid separation and easy removal of solvent from nanoparticles. Since the stability of the CO2 microemulsion depends on density, one can change temperature and pressure of the system to break down the microemulsion for direct deposition of the nanoparticles in small structures on substrates placed in the supercritical fluid system. Experimental Section. Cadmium nitrate, zinc nitrate and sodium sulfide were purchased from Aldrich. The fluorinated AOT, sodium bis(2,2,3,3,4,4,5,5-octafluoro-1-pentyl)-2-sulfosuccinate, was synthesized by modification of the procedures given by Yoshino,14 Erkey,15 and Eastoe.16,17 The synthesis is outlined below. A mixture of 2,2,3,3,4,4,5,5octafluoro-1-pentanol, maleic anhydride and p-toluenesulfonic acid monohydrate as the catalyst in toluene was refluxed and the liberated water was removed azeotropically. Evaporation of solvent, washing with hot water, and purification by vacuum distillation gave a pure corresponding diester, bis(2,2,3,3,4,4,5,5-octafluoro-1-pentyl)maleate (62% yield). This diester was then dissolved in 1:1 mixture of ethanol/water and refluxed with 1.1 equivalent of sodium metabisulfite to form the sodium salt of bis(2,2,3,3,4,4,5,5octafluoro-1-pentyl)-2-sulfosuccinate, which was purified by Soxhlet extraction with ethyl acetate (97% yield). The first step (esterification) of our synthetic procedure followed that reported by Yoshino14 and Erkey.15 The second step (sulfonation) was adopted from the procedure reported by Eastoe.16,17 A combination of these reported procedures gave a better yield and reproducibility according to our experiments. The liquid CO2 was metered with an ISCO syringe pump (model 260 D) and pump controller (series D). The liquid CO2 was introduced to the high-pressure reactors via stainless steel tubing (1/16 in o.d. × 0.03 in i.d.). The pressure vessels were heated with a thermal block and the temperature was kept constant and controlled with a J type sensor and an Omega digital controller. Two homemade high-pressure vessels were utilized. One vessel (9.5 mL volume) was equipped with a fiber-optic system (3 mm path length) connected to a CCD array UV-vis spectrometer.17 The other one was a 20 mL high-pressure view cell with sapphire windows.2 The high-pressure vessels were connected to each other via 1/16 in. stainless steel tubing. Each system was isolated from other by Valco high-pressure valves. A schematic diagram of the system is shown in Figure 1. The reverse micellar solutions were prepared by utilizing the fluorinated AOT (F-AOT, 15 mM) at W ) [H2O]/[FAOT] ) 6, 9, 12, and 15. The amounts of Cd(NO3)2 or Zn(NO3)2 and NaS were always 1.6 × 10-5 mol and 3.2 × 10-5 mol, respectively for all W values. A known amount 722
Figure 1. Schematic diagram of the supercritical CO2 microemulsion reaction system.
of the aqueous solution and F-AOT were placed in the highpressure reactor, and liquid CO2 was introduced. The aqueous solution containing Cd(NO3)2 or Zn(NO3)2 was always placed in the 9.5 mL fiber-optic cell, and the aqueous solution containing Na2S was placed in the 20 mL view-cell. Each solution was stirred for 1 h in a pressurized vessel to ensure the formation of a homogeneous optically transparent microemulsion. The vessels were both pressurized to 100 atm first for making the CO2 microemulsions. At the end of 1 h, the 20 mL vessel was pressurized to 200 atm and the microemulsion containing the S2- solution was pushed into the fiber-optic cell containing Cd2+ or Zn2+ by opening an interconnecting valve and the spectra were recorded. Results and Discussion. The UV-vis absorption spectra of CdS nanoparticles formed in the water-in-CO2 microemulsions with various W values are shown in Figure 2a. The spectra were recorded 3 min after the mixing of the two microemulsions containing Cd(NO3)2 and Na2S, separately. The spectrum of CdS nanoparticles prepared in the microemulsion with W ) 12 (curve 2) shows a broad band in the approximate range between 330 and 400 nm that is almost consistent with that of CdS nanoparticles prepared in a waterin-oil microemulsion with W ) 10.4 The absorption of the surfactant, which occurs below 230 nm, does not interfere with the characterization of the nanoparticles. The nanoparticles are light yellow in color based on visual observation through the view-cell windows. A theoretical study for the semiconductor particles has shown that the threshold wavelength of the UV-vis absorption spectrum provides a reasonable estimate of the size of the semiconductor nanoparticles produced by experiments.4,5 With reduction in particle size, the band gap of the semiconductor becomes larger and there is a concomitant blue shift in the absorption spectrum. This is called the quantum size effect of the semiconductor particle. As shown in Figure 2a (curves 1, 2, 3, and 4), with reduction in the W value, a blue shift in the UV-vis absorption spectrum was observed. The results indicate that the mean particle size of the semiconductor nanoparticles can be controlled by the W value of the waterin-CO2 microemulsion. Nano Lett., Vol. 2, No. 7, 2002
Figure 3. TEM image of the CdS nanoparticles (W ) 12) collected in acetone using the RESS process. (a) Aggregates of the CdS nanoparticles; scale ) 100 nm, (b) Inside of the aggregate; scale ) 20 nm, (c) A high-resolution TEM image of a single CdS nanoparticle; scale ) 5 nm.
Figure 2. (a) UV-vis spectra of the CdS nanoparticles in the water-in-CO2 microemulsions with W ) 6, 9, 12, and 15. (b) Change of the absorbance at 350 nm with time for the experiment with W ) 12 (taken at 4.5-second intervals).
Wang et al. used eq 1 to calculate the band gap energies of the CdS nanoparticles prepared in a water-in-oil microemulsion,19 σhν ) A(hν - Eg)1/2
(1)
where A is the proportional factor, hν is the photon energy, and Eg is the band gap energy. The molar absorption coefficient of the CdS nanoaprticle, σ, was determined as follows. Based on Lambert-Beer’s law, σ can be calculated from absorbance of CdS nanoparticles, concentration of CdS nanoparticles, and optical path length. The concentration of CdS nanoparticles was calculated by assuming the amount of CdS nanoparticles formed in CO2 to be equal to the amount of Na2S injected into the fiber optic cell. With W ) 6, 9, 12, and 15, the calculated Eg values are 3.50, 3.41, 3.15, and 2.85 eV, respectively. The diameters of the nanoparticles can be estimated from the band gap energies using Brus’s equation,1,2 Eg ) Eg,bulk + (h2/2dp2){(1/me) + (1/mh)} - (3.6e2/4πdp) (2) where Eg,bulk is the bulk band gap, h is the Planck constant, dp is the particle diameter, me is the effective mass of an electron, mh is the effective mass of a hole, e is the charge of an electron, and is the dielectric constant of the semiconductor. Using the band gap energies calculated using Nano Lett., Vol. 2, No. 7, 2002
eq 1 and the following parameters for CdS: Eg,bulk ) 2.5 eV, me/m0 ) 0.19, mh/m0 ) 0.8 and /o ) 5.7, the particle diameters of the CdS nanoparticles prepared in the waterin-CO2 microemulsion with W ) 6, 9, 12, and 15 are estimated to be 2.7, 2.8, 3.3, and 4.2 nm, respectively. The size of the CO2 microemulsion is known to depend on the W value and the larger water cores tend to produce larger CdS nanoparticles as indicated by the absorption spectra and the calculations. Brus’s equation tends to overestimate the nanoparticle size.20,21 Nevertheless, it provides theoretical support and a qualitative description of the relative sizes of the nanoparticles formed in the CO2 microemulsion system. Figure 2b shows the rate of formation of the CdS nanoparticles in the water-in-CO2 microemulsion with W ) 12. The spectra of CdS nanoparticles were measured at 4.5-second intervals, and the absorbances at 350 nm were plotted with time. Since the surfactant F-AOT absorbs below 230 nm, it has a negligible contribution to the absorbance at 350 nm. The two microemulsions were mixed at the second data point. The absorption peak of CdS nanoparticles increased very rapidly and reached a saturation value in about 10 s. By collision, exchange of contents between two microemulsions and the agglomeration of CdS to a nanometer-sized particle apparently took place effectively. The mixing process between the two solutions in the fiber-optic cell and the view cell might be the rate-determining step for the CdS nanoparticle synthesis using microemulsion-plus-microemulsion approach. The CdS nanoparticles in the water-in-CO2 microemulsion with W ) 12 were collected in an acetone solution by the rapid expansion of supercritical solution (RESS) method.22 The CdS nanoparticles collected by this method tend to agglomerate and form an aggregates that are 50-100 nm in size (Figure 3a). Each aggregate consists of hundreds of CdS nanoparticles of 5-10 nm in size (Figure 3b). The particle sizes shown in Figure 3b are larger than the CdS nanoparticle size calculated from eq 2. The RESS method for collection apparently can cause agglomeration of the CdS nanoparticles. Methods for preventing particle agglomeration during nanoparticle collection are currently under investigation in our laboratory. One approach may be to use a metal stabilizer to stabilize the nanoparticles. High-resolution TEM pictures were also taken for the CdS nanoparticles. The image in Figure 3c shows the lattice fringe indicating CdS nanoparticles with crystalline cores. 723
Figure 5. TEM image of the ZnS nanoparticles (W ) 12) collected in acetone using the RESS process. (a) Aggregates of ZnS nanoparticles; scale ) 200 nm, (b) Inside of the aggregate; scale ) 50 nm.
This study demonstrates that water-in-CO2 microemulsions can be used as nanoreactors for preparation of CdS and ZnS semiconductor nanoparticles in supercritical fluid CO2 by mixing microemulsions containing appropriate ionic species in the water core. The microemulsion-plus-microemulsion approach described in this paper offers a simple method for synthesis of a variety of nanomaterials in supercritical CO2 using ions or water-soluble compounds as starting materials.
Figure 4. (a) UV-vis spectrum of the ZnS nanoparticles synthesized in the water-in-CO2 microemulsions with W ) 12. (b) Change of the absorbance at 270 nm with time (taken at 4.5-second intervals).
Figure 4a shows the absorption spectrum of ZnS nanoparticle in the water-in-CO2 microemulsion with W ) 12. The ZnS nanoparticles were synthesized by mixing two microemulsions containing Zn(NO3)2 and Na2S, separately. The spectrum of ZnS nanoparticles shows an absorption band in the approximate range between 250 and 300 nm and is also consistent with a previous report.5 Based on the threshold wavelength of the spectrum, the band gap energy of the ZnS nanoparticles prepared in the microemulsion with W ) 12 was calculated to be 4.26 eV. Using the band gap energy and the following parameters for ZnS (Eg,bulk ) 3.7 eV, me/m0 ) 0.25, mh/m0 ) 0.59, and /o ) 5.2), the particle diameter at W ) 12 was calculated to be 3.1 nm. Figure 4b shows the change in absorbance of the 270 nm peak with time. The spectra were taken at 4.5 s intervals and the mixing of two microemulsions took place at the third data point. The absorbance of ZnS nanoparticles also increased very rapidly and reached a saturation value in about 10 s, similar to that observed in the CdS case. Figure 5 shows a TEM image of the collected ZnS nanoparticles in acetone. The images show that the ZnS nanoparticles also tend to agglomerate and form aggregates of approximately 50-100 nm in diameter (Figure 5a). The sizes of the ZnS nanoparticles in the aggregate are in the range of 5-15 nm in diameter (Figure 5b). 724
Acknowledgment. This work was supported by a DEPSCoR grant (DAAD19-01-1-0458) from the Army Research Office (ARO). H.K. thanks Kyung Hee University, South Korea for a one-year sabbatical leave at the University of Idaho. References (1) Brus, L. E. J. Chem. Phys. 1984, 80(9), 4403. (2) Brus, L. E. J. Phys. Chem. 1986, 90, 2555. (3) Wang, Y.; Suna, A.; Mahler, W.; Kasowski, R. J. Chem. Phys. 1987, 87(12), 7315. (4) Towey, T. F.; K-Lodhi, A.; Robinson, B. H. J. Chem. Soc., Faraday Trans. 1990, 86(22), 3757. (5) Hirai, T.; Sato, H.; Komasawa, I. Ind. Eng. Chem. Res. 1994, 33, 3262. (6) Smart, N. G.; Wai, C. M.; Phelps, C. L. Chem. Br. 1998, 34(8), 34. (7) Lin, Y.; Wai, C. M. Anal. Chem. 1994, 66, 1971. (8) Wai, C. M.; Hunt, F.; Ji, M.; Chen, X. J. Chem. Educ. 1998, 75, 1641. (9) Ji, M.; Chen, X.; Wai, C. M.; Fulton, J. L. J. Am. Chem. Soc. 1999, 121, 2631. (10) Ohde, H.; Hunt, F.; Wai, C. M. Chem. Mater. 2001, 13(11), 4130. (11) Ohde, H.; Rodriguez, J. M.; Ye, X.-R.; Wai, C. M. Chem. Commun. 2000, 2353. (12) Eastoe, J.; Cazelles, B. M. H.; Steytler, D. C.; Holmes, J. D.; Pitt, A. R.; Wear, T. J.; Heenan, R. K. Langmuir 1997, 13, 6980. (13) Holmes, J. D.; Bhargava, P. A.; Korgel, B. A.; Johnston, K. P. Langmuir 1999, 15, 6613. (14) Yoshino, N.; Komine, N.; Suzuki, J.; Arima, Y.; Hirai, H. Bull. Chem. Soc. Jpn. 1991, 64, 3262. (15) Liu, Z.; Erkey, C. Langmuir 2001, 17, 274. (16) Downer, A.; Eastoe, J.; Pitt, A. R.; Simister, E. A.; Penfold, J. Langmuir 1999, 15, 7591. (17) Nave, S.; Eastoe, J.; Penfold, J. Langmuir 2000, 16, 8733. (18) Hunt, F.; Ohde, H.; Wai, C. M. ReV. Sci. Instr. 1999, 70, 4661. (19) Wang, Y.; Suna, A.; Mahler, W.; Kasowski, R. J. Chem. Phys. 1987, 87(12), 73. (20) Lippens, P. E.; Lannoo, M. Phys. ReV. B. 1989, 39, 10935. (21) Rama Krishna, M. V.; Friesner, R. A. J. Chem. Phys. 1991, 95(11), 8309. (22) Matson, D. W.; Fulton, J. L.; Petersen, R. C.; Smith, R. D. Ind. Eng. Chem. Res. 1987, 26, 2298.
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Nano Lett., Vol. 2, No. 7, 2002
