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Formation of Cadmium Sulfide Nanoparticles in Reverse Micelles: Extreme Sensitivity to Preparation Procedure Christopher E. Bunker,*,† Barbara A. Harruff,†,‡ Pankaj Pathak,‡ Andrew Payzant,§ Lawrence F. Allard,§ and Ya-Ping Sun*,‡ Air Force Research Laboratory, Propulsion Directorate, Wright-Patterson Air Force Base, Ohio 45433-7103, Howard L. Hunter Chemistry Laboratory, Department of Chemistry, Clemson University, Clemson, South Carolina 29634-0973, and High-Temperature Materials Laboratory, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6062 Received February 13, 2004. In Final Form: April 20, 2004
Nanoscale hydrophilic cavities in reverse micelles are widely used in the synthesis of semiconductor and other nanoparticles.1-8 Among the most commonly synthesized nanoscale semiconductors is cadmium sulfide (CdS),1,2,8 for which the quantum confinement effects on the band gap absorption have been discussed extensively.9-16 The effects are generally reflected by the significant blue shift of the electronic-absorption onset with decreasing CdS particle size.9,13-16 However, we have found that the absorption properties of CdS nanoparticles are also sensitive to procedures used with the reverse-micelle method, independent of particle-size differences. For example, the obviously different absorption spectra shown in Figure 1 correspond to CdS nanoparticle samples obtained under only slightly altered experimental conditions. As described in detail in the following, the samples were, in fact, prepared by following the same procedure, except for the use of different solution volumes. Two different samples of CdS nanoparticles were prepared. In the experiments,17 aqueous solutions of Cd(NO3)2 (0.2 M, 1 mL) and Na2S (0.2 M, 1 mL) were first prepared. For the first sample, an aliquot of each solution (36 µL, measured accurately) was used to prepare two †
Wright-Patterson Air Force Base. Clemson University. § Oak Ridge National Laboratory. ‡
(1) (a) Petit, C.; Lixon, P.; Pileni, M. P. J. Phys. Chem. 1990, 94, 1598. (b) Petit, C.; Jain, T. K.; Billoudet, F.; Pileni, M. P. Langmuir 1994, 10, 4446. (2) Motte, L.; Petit, C.; Boulanger, L.; Lixon, P.; Pileni, M. P. Langmuir 1992, 8, 1049. (3) Sato, H.; Hirai, T.; Komasawa, I. Ind. Eng. Chem. Res. 1995, 34, 2493. (4) Tanori, J.; Duxin, N.; Petit, C.; Veillet, P.; Pileni, M. P. Colloid Polym. Sci. 1995, 273, 886. (5) (a) Cizeron, J.; Pileni, M. P. J. Phys. Chem. 1995, 99, 17410. (b) Cizeron, J.; Pileni, M. P. J. Phys. Chem. B 1997, 101, 8887. (6) Haram, S. K.; Mahadeshwar, A. R.; Dixit, S. G. J. Phys. Chem. 1996, 100, 5868. (7) Pileni, M. P. Langmuir 1997, 13, 3266. (8) Curri, M. L.; Agostiano, A.; Manna, L.; Monica, M. D.; Catalano, M.; Chiavarone, L.; Spagnolo, V.; Lugara, M. J. Phys. Chem. B 2000, 104, 8391. (9) Brus, L. E. J. Chem. Phys. 1984, 80, 4403. (10) Nair, S. V.; Sinha, S.; Rustagi, K. C. Phys. Rev. B 1987, 35, 4098. (11) Henglein, A. Chem. Rev. 1989, 89, 1861. (12) Lippens, P. E.; Lannoo, M. Phys. Rev. B 1989, 39, 10935. (13) Wang, Y.; Herron, N. Phys. Rev. B 1990, 42, 7253. (14) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706. (15) Vossmeyer, T.; Katsikas, L.; Giersig, M.; Popovic, I. G.; Diesner, K.; Chemseddine, A.; Eychemuller, A.; Weller, H. J. Phys. Chem. 1994, 98, 7665. (16) Alivisatos, A. P. J. Phys. Chem. 1996, 100, 13226. (17) Harruff, B. A.; Bunker, C. E. Langmuir 2003, 19, 893.
Figure 1. Absorption and emission spectra of CdS nanoparticle samples obtained by mixing smaller (solid lines) and larger (dashed dotted lines) volumes of micellar solutions.
separate sodium dioctyl sulfosuccinate (AOT)-stabilized micellar solutions in heptane (5 mL). In both micellar solutions, the Cd(NO3)2 and Na2S concentrations were 0.2 mM, and the AOT concentration was 0.1 M, corresponding to a W0 value of 4 (W0 ) [H2O]/[AOT]). After equilibrating for 1 h, the two solutions were mixed and allowed to stand for 5 min to facilitate the formation of CdS nanoparticles in the reverse-micellar cavities. Under AOT protection, the aqueous suspension of the CdS nanoparticle sample appears to be stable, without any visual precipitation. The UV/vis absorption spectrum of the suspended CdS nanoparticle sample is shown in Figure 1. The second CdS nanoparticle sample was prepared using the same procedure, except that the Cd(NO3)2 and Na2S solutions employed in the mixing were 15 mL each (instead of the 5 mL for the first sample). The Cd(NO3)2 and Na2S concentrations in the premixing solutions were the same (0.2 mM) as for the first sample, as was the AOT concentration (0.1 M) and the corresponding W0 value (4). The aqueous suspension of the CdS nanoparticle sample thus obtained appeared to be similar to that of the first sample, but the UV/vis absorption spectrum is significantly shifted (Figure 1). The UV/vis absorption spectra of the first (from the mixing of the 5-mL solutions) and second (from the mixing of the 15-mL solutions) samples were both reproducible in repeated experiments. For the first sample, the spectrum exhibits a well-defined band, with a maximum at 345 nm and an absorption band gap onset at ∼385 nm. On the basis of the overall CdS content in the sample, the absorptivity at the 345-nm peak is calculated to be 2400 (molCdS/L)-1 cm-1. For the second sample, however, the spectrum is somewhat broader and red-shifted by ∼75 nm. It has a less well-defined absorption band that peaks at 380 nm and an absorptivity at the peak maximum of 2800 (molCdS/L)-1 cm-1. The absorption band gap onset is at ∼460 nm. Thus, the difference in the band gap energies between the two samples is ∼4200 cm-1 (or 0.52 eV), a dramatic difference for two samples obtained using the same procedure, with only a slight and seemingly trivial alteration in one of the experimental conditions. Contrary to the traditional quantum confinement effect,9-12 the significantly different band gap energies in
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Figure 3. X-ray powder diffraction results of CdS nanoparticle samples obtained by mixing smaller (top) and larger (bottom) volumes of micellar solutions.
Figure 2. TEM images of CdS nanoparticle samples obtained by mixing smaller (top) and larger (bottom) volumes of micellar solutions.
the two CdS nanoparticle samples are not associated with any meaningful changes in nanoparticle size. The reverse micelles used in the two preparation procedures had the same W0 values; thus, the comparable sizes in the two resulting CdS nanoparticle samples should be expected. The expectation was confirmed by the results of transmission electron microscopy (TEM, Hitachi HD-2000 TEM/ STEM system) analyses of the two samples. As shown in Figure 2, the CdS nanoparticles are indeed similar in size and appear to be mostly single crystals. The average particle sizes obtained from the TEM images are 3.8 and 4.0 nm for the first and second samples, respectively, with size distribution standard deviations of 1.1 and 0.9 nm for the first and second samples, respectively. The powder X-ray diffraction analysis (Scintag XDS 2000) was used to examine the crystal structures of the CdS nanoparticles, and the results suggest significant differences in the two samples. Prior to the analysis, each sample was washed repeatedly with ethanol and centrifuged to remove AOT. In the final step, the sample was washed with acetone and vigorously dried to yield a fine yellow powdery solid. Compared in Figure 3 are the powder X-ray results of the two samples, which were matched with the cubic (zinc blende phase) and hexagonal (wurtzite phase) CdS X-ray patterns in the JCPDS database. The first sample (resulting from the procedure of mixing smaller volumes of solutions) is predominantly cubic CdS (>50%), while the second sample is primarily hexagonal CdS (∼65%).13-15,18 The X-ray results suggest that the nanoscale CdS particles that have similar sizes but different crystal structures possess different optical-absorption properties, (18) Vogel, W.; Urban, J.; Kundu, M.; Kulkarni, S. K. Langmuir 1997, 13, 827.
with the spectrum of the cubic CdS blue-shifted substantially (75 nm or 0.52 eV, Figure 1). It is somewhat surprising that such a significant effect of CdS nanocrystal structure on the optical properties has not been widely recognized and discussed, despite the fact that absorption spectra are commonly used in the elucidation of the quantum confinement in nanoscale CdS. In a report by Wang and Herron on the particle size absorption correlation,13 the effect of crystal structure was cited as a possible explanation for the discrepancy in the correlation. However, the focus was on the crystal-lattice contraction in very small CdS nanoparticles (2 nm or less).13 The use of different solution volumes in the mixing of AOT micellar solutions is apparently a convenient way to prepare CdS nanoparticles of either cubic or hexagonal crystal structure. The effect of solution volume is probably associated with the mixing dynamics, which may impact the growth of the CdS particles. While syntheses of CdS nanoparticles of different crystal phases and different sizes using different methods have been reported,13 to the authors knowledge this is the first report of the preparation of cubic and hexagonal CdS nanoparticles of the same size in the same AOT reverse micelles. In addition to the absorption spectra, other optical properties of the CdS nanoparticles are affected by the difference in crystal structure. For example, the luminescence spectra of both samples are characteristic of trap-state emissions.17 While the spectral profiles appear to be similar (Figure 1, inset), the luminescence quantum efficiencies are very different. The luminescence yield of the first sample ΦL ∼ 0.02 (predominantly cubic CdS) is about 1 order of magnitude higher than that of the second sample (primarily hexagonal CdS). The results suggest that the nonradiative decay of the trap state is more efficient in the hexagonal CdS nanoparticles. In summary, a seemingly trivial change in experimental parameters in the preparation of CdS nanoparticles by mixing reverse-micellar solutions resulted in substantial changes in the crystal structure of the nanoparticles, predominantly cubic versus primarily hexagonal. The CdS nanoparticles of different crystal structures have significantly different optical properties, which should be taken into consideration in the study of the quantum-confinement effect using optical-absorption results. Acknowledgment. C.E.B. thanks Dr. Julian Tishkoff and the Air Force Office of Scientific Research (AFOSR)
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for continuing support of fuels and combustion research. Y.-P.S. acknowledges financial support from DOE (DEFG02-00ER45859). Research at Oak Ridge National Laboratory was sponsored by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of
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