Preparation of Monodisperse CdS Nanocrystals by Size Selective

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J. Phys. Chem. 1996, 100, 13781-13785

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Preparation of Monodisperse CdS Nanocrystals by Size Selective Photocorrosion Hajime Matsumoto,† Takao Sakata,‡ Hirotaro Mori,‡ and Hiroshi Yoneyama*,† Department of Applied Chemistry, Faculty of Engineering, and Research Center for Ultra-High Voltage Electron Microscopy, Osaka UniVersity, Yamada-oka 2-1, Suita, Osaka 565, Japan ReceiVed: March 19, 1996; In Final Form: May 20, 1996X

Narrowing the size distribution of polydisperse CdS nanocrystals whose average diameter was 42 Å and standard deviation was 19 Å was achieved utilizing size selective photocorrosion with sequential irradiation with monochromatic light whose wavelength was changed step by step from 490 to 430 nm in air-saturated sodium hexametaphosphate solution. With decreasing the wavelength of irradiated light, the first exciton peak was gradually developed in the absorption spectrum of the resulting CdS colloid, and the nearly monodisperse CdS nanocrystals of 22 Å were finally obtained, which were thought to be the smallest particles that were present in the original CdS colloid. Analyses of the amount of sulfate ions produced by photocorrosion of Q-CdS colloids revealed that the number of Q-CdS particles in the colloid decreased with promotion of photocorrosion, suggesting that during the course of photocorrosion photocorroded CdS particles were agglomerated to give larger particles which were further photocorroded. The molar absorption coefficient of CdS particles at the first exciton peak was found to be independent of the particle size.

Introduction Quantized semiconductor nanocrystals (Q-particles) exhibit unique properties which are different from those of bulk crystals due to quantum size effects.1 Since chemical and physical properties of Q-particles depend on their size, it is desired to prepare monodisperse Q-particles to investigate the properties as a function of their size. So far, various techniques have been employed to achieve this. They are roughly classified into three categories. The first approach is to use a limited space of nanometer dimensions as a reaction zone for Q-particle synthesis. For example, Q-particles have been synthesized in cavities of zeolites2,3 and in interlayer spaces of clays.4 Inverse micells or vesicles have often been used with the same objectives.5-10 Though Q-particles prepared by these techniques had a size distribution more or less, relatively high monodisperse Q-CdS particles of a very small size distribution of (8% of the mean particle diameter were prepared recently by Korgel et al.,10 who used monodisperse vesicles of phosphatidylcholine. The second approach is concerned with rigid control of preparation conditions of Q-particles. The composition and concentration of reagents used in the preparation bath and temperature of the preparation bath are carefully controlled.11 This approach is certainly useful, but it is again inevitable to have a size distribution. The third approach is concerned with post treatments of prepared Q-particles. Chemically synthesized Q-particles were subjected to chromatography,12,13, capillary electrophoresis,14 and electrophoresis using poly(acrylamide) gel15 and sedimentation precipitation.16,17 The former three techniques are useful in narrowing the size of Q-particles from the original one, but the obtained Q-particles still have a size distribution in most cases. Q-particles that exhibit a sharp first exciton peak have not yet been obtained with use of these techniques. So far, the highest monodispersity seems to have been achieved with use of the sedimentation/precipitation technique, where the standard deviation of (5% of the mean diameter of Q-CdSe was obtained for particles ranging from 12 to 115 Å.16 † ‡ X

Department of Applied Chemistry. Research Center for Ultra-High Voltage Electron Microscopy. Abstract published in AdVance ACS Abstracts, July 1, 1996.

S0022-3654(96)00834-9 CCC: $12.00

Besides the size-controlled preparation of Q-particles, semiconductor clusters such as Cd17S4(SCH2CH2OH)2618 and Cd32S14(SC6H5)3619 were studied to understand ultimate physicochemical properties of Q-CdS particles of the smallest limit.18-22 However, CdS clusters greater than Cd32S14(SC6H5)36 in the molecular size have not yet been prepared. In this paper, we would like to report that a new technique of size selective photocorrosion provides a useful tool for preparation of highly monodisperse Q-CdS. It is well established that chalcogenide semiconductor particles are photodegraded in aqueous solution if light irradiation with energy high enough to cause bandgap excitation is made.11a,c Since the bandgap of Q-CdS is different depending on their size and the smaller the particle size the greater the bandgap, large Q-CdS particles alone can be photocorroded among Q-particles of different sizes present in a colloid if irradiation of the colloid is made with use of monochromatic light that photoexcites the large particle alone. If the Q-CdS particles absorb photons, it will be photocorroded until it does not allow any light absorption. By decreasing the wavelength of monochromatic light for irradiation, the particles to be photocorroded become small, resulting in narrowing the size distribution of the CdS particles, as reported recently in a rapid communication.23 In the present study, photodissolution behavior of polydisperse Q-CdS and changes in absorption spectra of photocorroded Q-CdS colloids have been investigated in detail as a function of wavelength of irradiated light. Experimental Section Preparation of Q-CdS Nanocrystals. Q-CdS nanocrystals were prepared by injecting H2S gas into a nitrogen-bubbled aqueous solution containing 2.0 × 10-4 mol dm-3 Cd(ClO4)2 and 2.0 × 10-4 mol dm-3 sodium hexametaphosphate (HMP) at pH 10.3 to give its concentration of 1.8 × 10-4 mol dm-3.11c The average diameter and standard deviation of the prepared Q-CdS particles were 42 and 19Å, respectively, as determined by observations of transmission electron micrographs obtained by a Hitachi H-9000 transmission electron microscope (TEM) operated at 300 keV. The electron diffraction patterns simultaneously obtained with the TEM measurements revealed that the prepared Q-CdS particles had a zinc-blende structure. © 1996 American Chemical Society

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Size Selective Photocorrosion of CdS Nanocrystals. A 500 W Xe lamp was used as a light source, and monochromatic light of desired wavelength was obtained using interference filters. The width at half of the intensity maximum of the monochromatic light was about 10 nm. The monochromatic light was irradiated onto 2.0 × 10-3 dm-3 of air-saturated Q-CdS colloid in a quartz cell (1 × 1 × 4 cm3). Absorption spectra of the colloid were measured intermittently during the course of irradiation using a photodiode array spectrophotometer (Hewlett-Packard, HP8452), and the irradiation was continued until no change in absorption spectra was observed. It usually took more than 10 h to attain such situation. Monochromatic light of 454 nm was obtained by equipping a dye laser system (Usho Optical Systems, DL-50) to a Nd:YAG laser (355 nm, Spectra-Physics, GCR-11, pulse width 7 ns). Commercially available 7-(diethylamino)-4-methylcoumarin (Coumarin 460) was used as the dye which was purchased from Aldrich and used without further purification. Quantitative Analysis of Photoproduced Sulfate Ions. The concentration of sulfate ions produced by the photocorrosion of Q-CdS particles was determined using high performance liquid chromatography (Tosoh, CCPE) equipped with an anion exchange column (Tosoh, TSKgel IC-Anion-PW) and an ion conductance detector (Tosoh, CM-8010). Borate buffer (1.3 mM, pH ) 9.1) was used as an eluent at a flow rate of 1.0 mL min-1. Estimation of the Particle Size. Distribution profiles of the size of photocorroded Q-CdS were determined by TEM observations, and from these the average diameter and the standard deviation were evaluated for Q-CdS colloids prepared by monochromatic irradiation down to 460 nm. The colloids prepared by irradiation with the shorter wavelength of monochromatic light were found to contain too little amount of Q-CdS particle to determine the particle size distribution. Then the size was estimated by applying the wavelength of the first exciton peak or shoulder of the absorption spectra of Q-CdS particles to theoretically derived relationships between the particle size and the bandgap energy which was derived by Nosaka using the finite depth potential well model.24 In those estimations, the effective mass of an electron and a hole of Q-CdS of 0.18 and 0.53, respectively, dielectric constant of 5.6, the depth of potential well of 3.6 (eV), and the bandgap of 2.4 (eV) were assumed. The advantage of the use of this model over another models developed so far, such as the tight-binding approximation25 and the empirical pseudopotential method,26 is that the higher transition energies (1Ph-1Pe, 1Dh1De) as well as the lowest exciton energy (1Sh-1Se) are relatively easily obtained by calculations with a personal computer. Results and Discussion Photocorrosion of Q-CdS Particles by White Light. Irradiation with a 500 W Xe lamp of 1.8 × 10-4 mol dm-3 Q-CdS colloid (pH 6.0) in the presence of dissolved air caused complete disappearance of absorption spectra of Q-CdS colloid after irradiation for a few hours. The theoretically predicted amount of SO42- ions (1.8 × 10-4 mol dm-3) was produced in that case, and the solution pH was unchanged with photocorrosion of Q-CdS particles. Then the net photocorrosion reaction is given by eq 1.11a,c,27 Though the net reaction seems to be

CdS + 2O2 f Cd2+ + SO42-

(1)

consisted of several photoanodic and photocathodic processes as discussed by Memming et al., 27 investigations on the anodic and cathodic processes are beyond the scope of the present

Figure 1. Size distribution of Q-CdS particles before photoirradiation.

Figure 2. Change in absorption spectra of Q-CdS colloids caused by 490 nm monochromatic light irradiation: (1) before irradiation, (2) after irradiation for 1 h (3), for 4.5 h (4), and for 15 h (5). Inset shows the time course of absorbance change at 490 nm.

study. When 0.1 mM methylviologen (MV2+) was added to the Q-CdS colloids as an electron scavenger, the photocorrosion rate was enhanced by 3 orders of magnitude because photogenerated electrons are quickly scavenged by methylviologen, resulting in a decrease in the recombination of photogenerated electrons and holes in the semiconductor particles, and then photogenerated holes are effectively involved in photoanodic dissolution of Q-CdS particles.11a,c Photocorrosion of Q-CdS Particles by Monochromatic Light. The size distribution of the Q-CdS nanocrystals obtained by TEM pictures before the photoirradiation is shown in Figure 1. Reflecting the polydispersion of the size of the Q-CdS particles, the absorption spectra of the Q-CdS colloids were broad and featureless, as shown by spectrum 1 of Figure 2. When photoirradiation of the original Q-CdS colloid was initiated using monochromatic light of 490 nm (2.53 eV), whose intensity was 5 mW, its absorption spectrum was gradually changed as shown by spectra 2-4 of the same figure. The absorbance of photoirradiated Q-CdS colloids at 490 nm was decreased with the irradiation time and became almost zero after 15 h as shown in the inset of Figure 2. The absorption onset was blue-shifted by the photocorrosion from 505 nm of the original colloid to 482 nm. Considering that the monochromatic light used for irradiation was 490 nm, the absorption onset of 482 nm obtained for the photocorroded Q-CdS colloid seems unreasonable. However, this discrepancy must have been brought about by broadness of the monochromatic light used. As already described, the width at half of the intensity maximum of the excitation light used was ca. 10 nm. Figure 3a,b shows the steady-state absorption spectra of Q-CdS colloids obtained after irradiation with monochromatic light of the wavelength given in the figure. It is recognized

Preparation of Monodisperse CdS Nanocrystals

Figure 3. Steady-state absorption spectra of Q-CdS colloids obtained after irradiation with monochromatic light of the wavelength given in the figure. The monochromatic light was obtained by passing light from a 500 W Xe lamp through interference filters. Arrows (V) show the 1Ph-1Pe transition.

that the onset wavelength became blue-shifted with decreasing excitation wavelength, indicating that large particles having a bandgap smaller than the light energy of irradiation were photocorroded until they became so small as not to allow absorption of irradiated light. Furthermore, well-structured spectra of Q-CdS particles begun to emerge with monochromatic light irradiation at 460 nm, suggesting that the standard deviation of the size distribution of Q-CdS particles was also decreased. For example, the shape of the steady-state spectrum obtained by 460 nm irradiation was very similar to that reported by Bawendi et al. for monodisperse CdSe nanocrystals having a standard deviation of (5% of the mean diameter of the particles.16 To our best knowledge, such well-structured spectra of Q-CdS colloids as given by curves 4-7 of Figure 3 have not yet been obtained, and this is the first time. When the Q-CdS colloids which gave the absorption spectrum given by curve 4 of Figure 3 was irradiated in the presence of 0.1 mM MV2+ with monochromatic light of 454 nm, which was emitted from a dye laser and was longer than the wavelength of the first exciton peak, the absorption onset and the first exciton peak of the spectrum were shifted to the same extent, but the shape of the spectrum itself was unchanged. If the shape of the first peak arose from a Gaussian distribution of Q-CdS particles, the peak position would be determined by Q-CdS particles of the average size, and the excitation with monochromatic light of 454 nm under such conditions would cause photodissolution of Q-CdS particles for particles whose bandgaps are smaller than the photon energy of 454 nm (2.73 eV). Then the absorption onset of the Q-CdS colloid must be blue-shifted to 454 nm while the exciton peak would not be blue-shifted to the same extent, being in disagreement with experimental results. Accordingly, it is concluded that the structured spectrum as given by curve 4 of Figure 3a is indebted mostly to monodisperse Q-CdS nanocrystals. According to TEM observations, the average diameter and its standard deviation of the Q-CdS particles obtained for a colloid which gave spectrum 4 of Figure 3a were 2.5 and 0.55 nm, respectively. If the particle size was estimated by applying the wavelength of the first exciton peak (436 nm) of curve 4 of

J. Phys. Chem., Vol. 100, No. 32, 1996 13783

Figure 4. Wavelength of exciton peak position versus particle size relations obtained by using the finite depth potential well model:24 (9) 1Sh-1Se, (2) 1Ph-1Se, (1) 1Dh-1Se, (() 1Ph-1Pe. Inset shows schematic illustration of discrete bands of Q-CdS derived from the finite depth potential well model. For the estimation of the transition energies, the followings were assumed. The effective mass of an electron and a hole is 0.18 and 0.53, respectively, the dielectric constant, the depth of potential well, and the bandgap of CdS are 5.6, 3.6 eV, and 2.4 eV, respectively.

Figure 3a to the finite depth potential well model, 2.7 nm was obtained. Though the Q-CdS colloid which gave spectrum 4 of Figure 3a is not monodispersive in a rigid sense and still contained particles of a size distribution, the contribution of the smaller and larger particles than the average ones to the absorption spectra is not so great as to deform its shape due to relatively low abundance. If the average size and the standard deviation were obtained for Q-CdS particles of the less promoted photocorrosions, 3.3 nm and 1.0 nm were obtained for monochromatic irradiation at 480 nm. In contrast, the original Q-CdS colloid contained Q-CdS particles of 4.9 nm of the average size and 1.9 nm of the standard deviation. It is evident from these results obtained by the TEM observations that the average diameter became small and the standard deviation decreased with irradiation of the shorter wavelengths. Figure 3b shows that once when near-monodispersive Q-CdS was obtained, the shape of the structured spectra was eventually unchanged by irradiation with monochromatic light of the shorter wavelength, though the wavelength of the exciton peak and that of absorption onset were blue-shifted. This means that Q-CdS particles can be made small by decreasing the wavelength of the excitation light without losing monodispersity. We attempted by TEM observations to determine the average size and the standard deviation of Q-CdS particles for the colloids which gave spectra 5-7 of Figure 3b, but it was unsuccessful to obtain size distribution profiles for these cases because of scarcity of Q-CdS particles prepared. Assignment of Exciton Peak. Figure 4 shows the magnitude of transition energy for several discrete transitions as a function of particle size of Q-CdS particles,24 estimated by using the finite depth potential well model. The excitation with the lowest energy is related to the 1Sh-1Se transition, and the first exciton peak must result from this transition. Then, the particle size was estimated for the particles which gave the absorption spectrum shown in Figure 3b. The second exciton peak due to 1Ph-1Pe transition is then given by arrows in Figure 3b. If these assignments are valid, we have another two absorption peaks between the first and second exciton peaks for the spectra shown in Figure 3b. For example, Q-CdS colloids prepared

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by irradiation at 460 nm had unknown peaks at 410 and 380 nm. The spin-orbit splitting might be one cause for appearance of these peaks, as discussed by Brus et al. for Q-ZnSe and Q-CdSe.28 However, the spin-orbit splitting energy is as small as 0.06 eV for CdS,29 while it is ca. 0.41 eV for CdSe,28c suggesting that the contribution of the spin-orbit coupling to the appearance of the unknown peaks of Q-CdS colloids would be negligible. A more likely mechanism of the appearance of the unknown peaks might be related to the forbidden transitions such as 1Ph-1Se and 1Dh-1Se transitions. The finite depth potential well model allows the estimation of these transition energies, which are shown in Figure 4, and such forbidden transitions seem to be in rough accord with the position of the unknown peaks. Changes in Total Number of Q-CdS Particles with Promotion of Photocorrosion. The total volume of Q-CdS particles present in the original colloids used in the photocorrosion studies is given by eq 2,

V0 ) [1.8 × 10-4]MCdS/F ) 5.4 × 10-3 cm3

Figure 5. Amount of the sulfate ions produced by irradiation of Q-CdS colloids with monochromatic light given in the figure. The irradiation was made successively by decreasing the wavelength of monochromatic light. The initial concentration of Q-CdS was 0.18 mM.

(2)

where 1.8 × 10-4 mol dm-3 is the concentration of original Q-CdS colloids, MCdS is the molecular weight of CdS (144 g mol-1), and F is the density of bulk CdS ()4.82 g cm-3). The total number of Q-CdS particles can also be calculated by using the relationship between V0 and the size distribution of Q-CdS particles as shown in Figure 1:

V0 ) N ∑fiVi

(3)

where N is the total number of Q-CdS particles, and fi and Vi are the frequency and volume of the fraction i in size distribution of Q-CdS particles. By applying ∑fiVi ) 5.73 × 10-20 cm3 which is obtained from Figure 1 and V0 of 5.4 × 10-3 (eq 2) to eq 3, the total number of CdS particles in the original colloid is determined to be 9.4 × 1016 particles dm-3. The absorbance of the original Q-CdS colloid was decreased with promotion of photocorrosion, as shown in Figure 3a,b. If the photocorrosion occurred in such a way that it was completed when the particles became so small as not to allow any absorption of irradiated photons, the total number of Q-CdS particles in the colloid must then be unchanged before and after the monochromatic irradiation. However, this was not the case because the absorbance at the first exciton peak decreased with decreasing irradiation wavelength as shown in Figure 3b. Then the total number of Q-CdS particles seems to have been decreased by the photocorrosion. In order to obtain information about this, the amount of sulfate ions produced by photocorrosion was determined as a function of excitation wavelength. In this experiment, 0.1 mM methylviologen was added to the Q-CdS colloids to enhance the rate of the dissolution of Q-CdS particles. As shown in Figure 5, the amount of sulfate ions determined after completion of photocorrosion increased with decreasing irradiation wavelength, and the final amount of sulfate ions obtained at 420 nm monochromatic light irradiation was 1.7 × 10-4 mol dm-3. After monochromatic light irradiation at 420 nm, the white light was irradiated to the colloid to fully dissolve the Q-CdS particles. The resulting solution contained SO42ions of 1.8 × 10-4 mol dm-3, which was in agreement with that obtained by irradiation with white light. As described above, highly monodisperse Q-CdS particles were obtained for irradiation with monochromatic light whose wavelength was shorter than 460 nm. The number of Q-CdS particles which survived after irradiation of such monochromatic light can then be estimated based on the initial concentration

Figure 6. Number of the particles present in Q-CdS colloids (9) and the absorption coefficient of the colloids ((), obtained after photocorrosion with monochromatic irradiations.

of Q-CdS particles and the amount of sulfate ions produced by the photocorrosion. In that case, the number of particles present in the unit volume of Q-CdS colloids is

n)

(1.8 × 10-4 - [SO42-])MCdS (π/6)(D × 10-8)3 × 4.82

(4)

where n is the number of particles (number per dm-3), 1.8 × 10-4 (mol dm-3) is the original concentration of Q-CdS colloid, D (Å) is the diameter of Q-CdS particles contained in the monodisperse colloid, which was obtained by applying the first exciton peak energy to the finite depth potential well model, and [SO42-] (mol dm-3) is the concentration of sulfate ions produced by photocorrosion. It was found that the number of Q-CdS particles obtained by eq 4 decreased with decreasing the particle size as shown in Figure 6. The molar absorption coefficient () of the first exciton peak was estimated by the following equation using the relation between the number of Q-CdS particles (n) and excitation wavelength given in this figure.

)

Aexciton n/6.02 × 1023

(5)

The results obtained are also included in Figure 6. Figure 6 clearly shows that  was independent of the particle size. The size independency of  obtained here agrees well with the previously reported results for Q-CdS17 or Q-CdTe11f particles.

Preparation of Monodisperse CdS Nanocrystals At this stage, it seems necessary to explain why the number of Q-CdS particles was decreased by photocorrosion with monochromatic irradiation. One possible explanation might be related to relaxing the function of stabilizing agents (HMP) in the course of photocorrosion of Q-CdS particles. When the photocorrosion takes place, HMP which adsorbs on the Q-CdS particles as a stabilizing agent might be desorbed with the release of Cd2+ ions from Q-CdS particles. If it were the case, then Q-CdS particles become unstable and tend to aggregate with each other to give bigger particles, which are then subjected to further photocorrosion. The decrease in the number of Q-CdS particles observed can be explained well in this way. Acknowledgment. This research was supported by Grantin-Aid for Scientific Research, No. 07455340, from the Ministry of Education, Science, Culture and Sports. The Research Fellowship of the Japan Society for the Promotion of Science for Young Scientists to H.M. is also gratefully acknowledged. References and Notes (1) (a) Steigerwald, M. L.; Brus, L. E. Annu. ReV. Mater. Sci. 1988, 19, 471. (b) Henglein, A. Chem. ReV. 1989, 89, 1861. (c) Wang, Y.; Herron, N. J. Phys. Chem. 1991, 95, 525. (d) Weller, H. Angew. Chem., Int. Ed. Engl. 1993, 32, 41. (2) (a) Wang, Y.; Herron, N. J. Phys. Chem. 1987, 91, 257. (b) Herron, N.; Wang, Y.; Eddy, M. M.; Stucky, G. D.; Cox, D. E.; Moller, K.; Bein, T. J. Am. Chem. Soc. 1989, 111, 530. (3) Abe, T.; Tachibana, Y.; Uematsu, T.; Iwamoto, M. J. Chem. Soc., Chem. Commun. 1995, 1617. (4) (a) Yoneyama, H.; Haga, S.; Yamanaka, S. J. Phys. Chem. 1989, 93, 4833. (b) Miyoshi, H.; Mori, H.; Yoneyama, H. Langmuir 1991, 7, 503. (5) (a) Meyer, M.; Wallberg, C.; Kurihara, K.; Fendler, J. H. J. Chem. Soc., Chem. Commun. 1984, 90. (b) Fendler, J. H. Chem. ReV. 1987, 87, 877. (6) Lianos, P.; Thomas, J. K. Chem. Phys. Lett. 1986, 125, 299. (7) (a) Petit, C.; Pileni, M. P. J. Phys. Chem. 1988, 92, 2282. (b) Motte, L.; Petit, C.; Boulanger, L.; Lixon, P.; Pileni, M. P. Langmuir 1992, 8, 1049. (8) Ogawa, S.; Fan, F.-R. F.; Bard, A. J. J. Phys. Chem. 1995, 99, 11182. (9) Kortan, A. R.; Hull, R.; Opila, R. L.; Bawendi, M. G.; Steigerwald, M. L.; Carroll, P. J.; Brus, L. E. J. Am. Chem. Soc. 1990, 112, 1327. (10) Korgel, B. A.; Monbouquette, H. G. J. Phys. Chem. 1996, 100, 346. (11) (a) Weller, H.; Koch, U.; Gutie´rrez, M.; Henglein A. Ber. BunsenGes. Phys. Chem. 1984, 88, 649. (b) Rossetti, R.; Hull, R.; Gibson, J. M.; Brus, L. E. J. Chem. Phys. 1985, 82, 552. (c) Spanhel, L.; Haase, M.; Weller, H.; Henglein, A. J. Am. Chem. Soc. 1987, 109, 5649. (d) Swayambunathan, V.; Hayes, D.; Schmidt, K. H.; Liao, Y. X.; Meisel, D. J. Am. Chem. Soc.

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