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Langmuir 2001, 17, 8428-8433
Preparation of CdS Nanoparticles in Salt-Induced Block Copolymer Micelles Hanying Zhao,† Elliot P. Douglas,*,† Benjamin S. Harrison,‡ and Kirk S. Schanze‡ Department of Materials Science and Engineering, University of Florida, Gainesville, Florida 32611, and Department of Chemistry, University of Florida, Gainesville, Florida 32611 Received August 23, 2001. In Final Form: October 15, 2001 A novel preparation method of CdS nanoparticles with controllable size and stability is presented. Poly(styrene-block-2-vinylpyridine) (PS-b-P2VP) and cadmium ions form aggregates of single micelles called compound micelles, upon addition of the cadmium acetate salt into a solution of the block copolymer in tetrahydrofuran. The growth of CdS nanoparticles is confined to the core of single micelles after introduction of hydrogen sulfide gas into the solution. UV-visible spectroscopy, fluorescence spectroscopy, and transmission electron microscopy were employed to characterize the prepared nanoparticles. UVvisible absorption spectra show that larger nanoparticles are produced at lower 2VP:Cd2+ molar ratio. UV-visible absorption spectra and fluorescence spectra both indicate that with a decrease of block copolymer concentration in tetrahydrofuran, the size of the CdS nanoparticles decreases. The stability of the CdS nanoparticles and micelles was also investigated. UV-visible results show that the prepared CdS nanoparticles have good colloidal stability.
I. Introduction The synthesis of semiconductor nanoparticles has attracted much interest due to their size-dependent properties and great potential for many applications, especially as nonlinear optical materials.1-5 Nanoparticles exhibit unique properties owing to quantum size effects and the presence of a large number of unsaturated surface atoms. During the past decade, many synthetic methods based on polymer materials have been developed, among which synthesis of nanoparticles in a polymer matrix is prominent.6-13 Besides acting as a stabilizer, the use of a polymeric matrix as a medium for nanoparticle formation provides for processability, solubility, and control of nanoparticle growth.6,7 Most of the methods are based on compartmentalizing the metals to prepare nanoparticles using in situ reactions. In Cohen’s group, semiconductor clusters were prepared within microphase-separated diblock copolymer films.8-11 The microdomains distributed throughout block copolymer * To whom all correspondence should be addressed. Tel: 352846-2836. FAX: 352-392-3771. E-mail:
[email protected]. † Department of Materials Science and Engineering. ‡ Department of Chemistry. (1) Henglein, A. Chem. Rev. 1989, 89, 1861. (2) Spanhel, L.; Hoasse, M.; Weller, H. J.; Henglein, A. J. Am. Chem. Soc. 1987, 109, 5649. (3) Henglein, A.; Gutierrez, M. Ber. Bunsen-Ges. Phys. Chem. 1983, 87, 852. (4) Rossetti, R.; Hull. R.; Gibson, J. M.; Brus, L. E. J. Chem. Phys. 1985, 82, 552. (5) Sun, Y.; Riggs, J. E. Int. Rev. Phys. Chem. 1999, 18, 43. (6) Caseri, W. Macromol. Rapid Commun. 2000, 21, 705. (7) Bronstein, L. M.; Mirzoeva, E. S.; Valetsky, P. M.; Solodovnokov, S. A.; Register, R. A. J. Mater. Chem. 1995, 5, 1197. (8) Sankaran, V.; Cummins, L. L.; Schrock, R. R.; Cohen, R. E.; Silbey, R. J. J. Am. Chem. Soc. 1990, 12, 6858. (9) Kane, R. S.; Cohen, R. E.; Silbey, R. Chem. Mater. 1996, 8, 1919. (10) Cummins, C. C.; Schrock, R. R.; Cohen, R. E. Chem. Mater. 1992, 4, 27. (11) Yue, J.; Sankaran, V.; Cohen, R. E.; Schrock, R. R. J. Am. Chem. Soc. 1993, 115, 4409. (12) Moffitt, M.; Eisenberg, A. Chem. Mater. 1995, 7, 1178. (13) Moffitt, M.; McMahon, L.; Pessel, V,; Eisenberg, A. Chem. Mater. 1995, 7, 1185.
films can be thought of as nanoreactors within which nanoclusters are synthesized. Moffitt and Eisenberg et al. prepared CdS nanoclusters using small ionic aggregates present in styrene-based random ionomers12 and spherical ionic microdomains of cadmium-neutralized styrene-based diblock ionomers.13 The ionic aggregates in random ionomers or the ionic microdomains in diblock ionomers both can be viewed as nanoreactors. Recently, the starburst dendrimer poly(amidoamine) (PAMAM) has been used as a stabilizer and template for inorganic nanoclusters in solution.14-17 The size of the dendrimer polymer chains is in the range of 1-15 nm. In this approach, precursor ions are accumulated within the dendrimer molecules due to electrostatic interaction, coordination to the amine groups, or chemical reactions. Recently several groups have prepared nanoparticles in polymer micelles.18-23 Block copolymer chains form micelles in a selective solvent. These micelles have a welldefined size and architecture. A micelle core can be regarded as a nanosized reaction vessel, and the growth of colloidal particles can be limited to the micelle core. Because micelles are very stable in the selective solvent, concentrated solutions of nanoparticles can be achieved. By use of amphiphilic diblock copolymers in aqueous/polar (14) Lakowicz, J. R.; Gryczynski, I.; Gryczynski, Z.; Murphy, C. J. Phys. Chem. 1999, 103, 7613. (15) Sooklal, K.; Hanus, L. H.; Ploehn, H. J.; Murphy, C. J. Adv. Mater. 1998, 10, 1083. (16) Grohn, F.; Kim, G.; Bauer, B. J.; Amis, E. J. Macromolecules 2001, 34, 2179. (17) Grohn, F.; Bauer, B. J.; Akpalu, Y. A.; Jackson, C. L.; Amis, E. J. Macromolecules 2000, 33, 6042. (18) Platonova, O. A.; Bronstein, L. M.; Solodovnikov, S. P.; Yanovskaya, I. M.; Obolonkova, E. S.; Valetsky, P. M.; Wenz, E.; Antonietti, M. Colloid Polym. Sci. 1997, 275, 426. (19) Mossmer, S.; Spatz, J. P.; Moller, M.; Aberle, T.; Schmidt, J.; Burchard, W. Macromolecules 2000, 33, 4791. (20) Underhill, R. S.; Liu, G. Chem. Mater. 2000, 12, 2082. (21) Antonietti, M.; Wenz, E.; Bronstein, L.; Seregina, M. Adv. Mater. 1995, 7, 1000. (22) Antonietti, M.; Forster, S.; Hartmann, J.; Oestreich, S. Macromolecules 1996, 29, 3800. (23) Underhill, R. S.; Liu, G. Chem. Mater. 2000, 12, 3633.
10.1021/la011348q CCC: $20.00 © 2001 American Chemical Society Published on Web 11/29/2001
Preparation of CdS Nanoparticles
media, the metal nanoparticles can also be isolated in the corona region and in the vicinity of the hydrophobic micelle core owing to the poor interaction of the metal ions with a hydrophobic block, such as polystyrene.24,25 The aim of this study is to develop a new method to prepare semiconductor nanoparticles in salt-induced micelles. The polymer used in this study is poly(styreneblock-2-vinylpyridine) (PS-b-P2VP). Upon addition of cadmium ions into a solution of the polymer in THF, a solvent for both of the blocks, micelles form due to complexation of Cd2+ with the 2-vinylpyridine (2VP) units. The micelle core is composed of 2VP/Cd2+ complexes, and the shell is composed of polystyrene blocks. Therefore precursor ions will be confined in the core of the micelles, resulting in a nanoreactor. Concomitant with introduction of H2S gas, CdS nanoparticles are produced. The resulting nanoparticles are stabilized by the interaction between 2VP units with the surface of the CdS nanoparticles. Compared with other micelle approaches for the synthesis of CdS nanoparticles, the method described herein provides a simple means to prepare CdS nanoparticles with controllable size. To our knowledge, this is the first report on the preparation of CdS nanoparticles within saltinduced micelles. We believe this method is suitable not only for the preparation of CdS nanoparticles but also for other nanoparticles, especially those that can be prepared from the reduction of an appropriate metal ion-2VP complex.
Langmuir, Vol. 17, No. 26, 2001 8429
Figure 1. Transmission infrared spectra of poly(styrene-block2-vinylpyridine) block copolymer and its complex with cadmium acetate. The molar ratio of 2-vinylpyridine to cadmium ions is 1:0.5.
II. Experimental Section Synthesis of PS-b-P2VP Diblock Copolymer. The synthesis of the block copolymer was performed using sequential anionic polymerization. Initiated by sec-butyllithium, the polymerization of PS block was carried out in THF at -75 °C. 2-Vinylpyridine monomer was added dropwise after the PS block was capped with 1,1-diphenylethylene. The molecular weight of the block copolymer is 25.5 × 103 g/mol (Mn) with a polydispersity of 1.30, measured by gel permeation charomatography (GPC) relative to PS standards. The content of 2-vinylpyridine units in the block copolymer is about 35 mol % as measured by 1H NMR. Synthesis of CdS Nanoparticles. PS-b-P2VP was dissolved in THF at varying concentration under vigorous stirring for 1 h. Cadmium acetate dihydrate in a mixture of methanol and THF (1:2 V/V) was added to the polymer solution under stirring at room temperature, and a light-blue solution was achieved, indicating that polymer micelles were formed in the solution. CdS nanoparticles were prepared by bubbling H2S gas through the micelle solution for 10 min. The resulting yellow stable CdS colloidal solutions were characterized after 24 h of aging because the CdS nanoparticle dispersion exhibits a minor change in size within the first 24 h, after which it is stable for a long time.26,27 Infrared Spectra. Infrared spectra were recorded on a Nicolet MaGNA-IR 760 spectrometer at a resolution of 2 cm-1. The films for IR measurements were prepared by casting solutions onto the surface of Teflon films. Before the measurements, the films were dried in a vacuum oven at 70 °C for 1 day. UV-Visible Absorption Spectroscopy. UV-visible absorption spectra were recorded on a Shimadzu UV-vis spectrophotometer (UV-2401PC), by scanning the CdS-containing solution in a 1-cm quartz cell. The scanning range was from 190 to 700 nm. Absorption from the solvent was subtracted from each spectrum. Fluorescence Spectroscopy. Steady-state fluorescence spectra were obtained on an SPEX Fluorolog spectrophotometer equipped with a 450-W Xe arc lamp and a PMT detector. The excitation wavelength was 350 nm and the bandwidths were 10 (24) Mayer, A. B. R Mater. Sci. Eng., C 1998, 6, 155. (25) Mayer, A. B. R. Polym. Adv. Technol. 2001, 12, 96. (26) Qi, L.; Colfen, H.; Antonietti, M. Nano Lett. 2001, 1, 61. (27) Huang, J.; Lianos, P.; Yiang, Y.; Shen, J. Langmuir 1998, 14, 4342.
Figure 2. Transmission electron micrograph of Cd(Ac)2 salt induced micelles. The molar ratio of 2VP to Cd2+ is 1:0.5. The concentration of the block copolymer in THF is 26.6 g/L. The scale bar in the image represents 100 nm. nm for excitation and emission. Fluorescence measurements were obtained on the colloidal suspensions contained in quartz cuvettes. Transmission Electron Microscopy. Electron microscopy was performed on a Zeiss EM 10A transmission electron microscope operated at 100 kV. Transmission electron microscopy (TEM) samples were prepared by depositing a droplet of micelle solution or CdS-containing colloidal solution (
