Electrolyte Effects on CdS Nanocrystal Formation in Chelate Polymer

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Langmuir 1998, 14, 595-601

595

Electrolyte Effects on CdS Nanocrystal Formation in Chelate Polymer Particles: Optical and Distribution Properties Hiroshi Yao, Yukako Takada, and Noboru Kitamura* Division of Chemistry, Graduate School of Science, Hokkaido University, Kita-ku, Sapporo 060, Japan Received May 8, 1997. In Final Form: November 14, 1997 CdS nanocrystals were prepared in chelate polymer microparticles, and characterizations of the CdS/ polymer hybrids were performed by absorption microspectroscopy, optical microscopy, and transmission electron microscope measurements. When CdS nanocrystals were prepared by the reaction of Cd2+polymer beads with a diluted HS- solution, formation of CdS was confined to the surface layer of the host polymer particle with a layer-by-layer size distribution. A preparation of CdS in the presence of NaCl (0.5 M) gave CdS with a smaller mean diameter, as compared to a sample synthesized without NaCl. The addition of NaCl also leads to a change in dispersion properties of CdS as well as in the time profile of CdS formation; the initial rate of CdS nanocrystal formation was accelerated by the addition of NaCl. The CdS nanocrystal formation rate was proportional to the square root of the reaction time, indicating that diffusion of HS- into the polymer particles controlled both optical and distribution characteristics of CdS. The results were explained in terms of a Donnan equilibrium model.

Introduction Physical and chemical properties of semiconductor nanocrystals, whose sizes are small compared to bulk exciton diameters, have attracted broad interest because they show characteristic nonlinear optical effects1,2 and photocatalytic effects.3 Preparation and characterization of semiconductor nanocrystals are worth studying to further advance basic science associated with the transition region between molecules and solids.4,5 Various sizecontrolled preparation methods have been explored and developed.6,7 In most cases, preparations of nanocrystals are conducted in homogeneous solution, although formation of nanocrystal assemblies in ordered geometries have been also investigated. For applications of nanocrystals to functional materials and/or devices, in situ preparation in zeolite,8 porous glasses,9 and organo-clay complexes10 has been demonstrated. Organic polymers have been also used as a host matrix, and CdS or PbS nanocrystals were prepared in ionic polymers such as Nafion (perfluoroethylenesulfonic acid polymer)11,12 and an ethylenemethacrylic acid copolymer.13 Among various host matrices, ion-exchange resin beads are quite interesting since (1) Wang, Y. Acc. Chem. Res. 1991, 24, 133. (2) Takagahara, T. Phys. Rev. 1987, B36, 9293. (3) Kamat, P. V. Chem. Rev. 1993, 93, 267. (4) Alivisatos, A. P. Science 1996, 271, 933. (5) Steigerwald, M. L.; Brus, L. E. Acc. Chem. Res. 1990, 23, 183. (6) Vossmeyer, T.; Katsikas, L.; Giersig, M.; Popovic, I. G.; Diesner, K.; Chemseddine, A.; Eychmu¨ller, A.; Weller, H. J. Phys. Chem. 1994, 98, 7665. (7) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706. (8) Herron, N.; Wang, Y.; Eddy, M. M.; Stucky, G. D.; Cox. D. E.; Moller, K.; Bein, T. J. Am. Chem. Soc. 1989, 111, 530. (9) Kuczynksi, J.; Thomas, J. K. J. Phys. Chem. 1985, 89, 2720. (10) De´ka´ny, I.; Turi, L.; Tomba´cz, E.; Fendler, J. H. Langmuir 1995, 11, 2285. (11) Kuczynski, J. P.; Milosavljevic, B. H.; Thomas, J. K. J. Phys. Chem. 1984, 88, 980. (12) Wang, Y.; Suna, A.; McHugh, J.; Hilinski, E. F.; Lucas, P. A.; Johnson, R. D. J. Chem. Phys. 1990, 92, 6927. (13) Wang, Y.; Suna, A.; Mahler, W.; Kasowski, R. J. Chem. Phys. 1987, 87, 7315.

they are useful for preventing crystal flocculation,14 in spite of a capability of dense loading of nanocrystals in the matrix. Furthermore, interactions between nanocrystal surfaces and the host matrix would be controlled through a choice of functional groups in the host polymers. Thus, size and dispersion properties of nanocrystals can be related to the chemical and three-dimensional structural features of the host polymers. Such studies are worth conducting for future development of nanocrystal hybrid systems. Recently, we reported size regulation and dense dispersion of CdS nanocrystals in chelate polymer microparticles having iminodiacetate ligands. The chelating groups can stabilize Cd2+ cations, and subsequent reaction of Cd2+ with HS- anions15,16 yields CdS nanocrystals in the host polymer. Also, we demonstrated that the CdS crystal size and its dispersion characteristics were dependent on the size of the host polymer particles.14 These results suggested that the flux of HS- anions, diffusing from the surrounding water phase to the polymer interior, played an essential role in the nucleation and dispersion of CdS. If diffusion of HS- anions to the polymer governs CdS formation, then the ionic strength of the solution should influence nucleation processes. In traditional synthetic methods for semiconductor nanocrystals, the presence of a foreign salt such as NaCl and KCl is known to interfere with the generation of small nanoclusters, since addition of a salt brings about a thinner electrical double layer around the surface of nanocrystals which leads to flocculation.17 In homogeneous aqueous solutions, thus, nanocrystals are not prepared in the presence of a foreign salt. In the present study, however, we found unique NaCl effects on CdS nanocrystal formation in chelate polymer microparticles. We report here that the crystal size and distribution characteristics of CdS in polymer micropar(14) Yao, H.; Kitamura, N. Bull. Chem. Soc. Jpn. 1996, 69, 1227. (15) Under the present condition, ionic species in the aqueous Na2S solution are Na+, HS-, and OH-. (16) Cotton, F. A.; Wilkinson, G.; Gaus, P. L. Basic Inorganic Chemistry; John Wiley & Sons: New York, 1987. (17) Israelachvili, J. N. Intermolecular and Surface Forces; Academic Press, London, 1985.

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ticles can be modulated by addition of an electrolyte (NaCl) and discuss the origin of the salt effects on the basis of a Donnan potential model. Experimental Section Materials. Chelate polymer microparticles (Chelex 100, 200400 mesh, Analytical Grade, Bio-Rad Laboratories), made of a styrene-divinylbenzene copolymer having iminodiacetate ligands (-CH2N(CH2COO-)2), were used as a host matrix to prepare CdS nanocrystals. The fixed charge concentration of the ion exchange group in the polymer is 2.0 mequiv/g or 0.4 mequiv/mL in the dry or wet condition, respectively.18 The polymer particles were soaked in distilled water and washed successively with methanol, aqueous HCl (2 M), aqueous NaOH solutions (2 M), and again thoroughly with distilled water. The pH of the eluent was ∼10. Careful washing of the resin was necessary for reproducible CdS/polymer hybrid preparation. After being treated with ethanol, the polymer particles were dried under vacuum and stored. Cadmium acetate (Cd(CH3COO)2‚2H2O, GR grade, Kanto Chemical), sodium sulfide (Na2S‚9H2O, GR grade, Wako Pure Chemicals), and sodium chloride (NaCl, GR grade, Kanto Chemical) were used as received. Preparation of CdS Nanocrystals/Polymer Hybrids. The polymer particles (0.107 g, dried weight) were soaked in an aqueous Cd(CH3COO)2 solution (8.4 × 10-3 M, 5.0 mL) for 10 min under sonication and stored for 2 days at room temperature. All Cd2+ ions were incorporated into the polymer particles as judged by the fact that the supernatant solution did not react with HS- ions to produce CdS. The ratio of adsorbed Cd2+ ions to the total adsorption capacity of the resin was about 0.4. The Cd2+-incorporated polymer particles were then washed with distilled water several times. CdS nanocrystals/polymer hybrids (sample a) were prepared as follows. The Cd2+-polymer particles were dispersed in 10 mL of distilled water for 10 min. A freshly-prepared dilute aqueous Na2S solution (3.8 × 10-4 M, 100 mL) was then added to the solution under vigorous stirring. The polymer particles turned pale yellow slowly owing to formation of CdS. After being stirred for 2 h, the solution was allowed to stand for 2 days at room temperature. In order to obtain a time profile of absorbance of CdS in the polymer particles, an aliquot of the solution (several milliliters) was sampled at various times after mixing. After the supernatant solution was removed, the particles were washed thoroughly with water and ethanol. The sample was then dried under vacuum. Sample b was obtained by a procedure analogous to that employed for sample a. However, the Cd2+-polymer particles were pretreated with 10 mL of an aqueous NaCl solution (0.5 M) for 10 min before addition of the Na2S solution. Cd2+ ions did not elute to the solution phase under these conditions. As the diffusion coefficient of Na+ in iminodiacetate chelate resins (Dowex A-1) is similar to that of Chelex 100 (1.2 × 10-7 cm2/s),19 10 min is considered to be enough for homogeneous diffusion of the ion into polymer particles with a diameter of about 100 µm. The polymer particles turned pale yellow slowly, but the rate was faster than that for sample a. After 2 h of being stirred, the solution was allowed to stand for 2 days at room temperature. After procedures analogous to those for sample a, the particles were dried in vacuo. Measurements. Absorption spectra of single CdS/polymer particles were measured with a microspectroscopy system consisting of an optical microscope (Nikon, Optiphoto 2) and a polychromator (Oriel, Multispec 257) multichannel photodetector (Princeton Instruments, ICCD-576E/G) set as described previously.20,21 A 150 W Xe lamp (Hamamatsu Photonics, L2273) was used as a light source for absorption measurements. The probe beam was scattered by dry CdS/polymer particles under the microscope, so the polymer particles were dispersed in water (18) Chelex 100 chelating ion exchange resin Instruction Manual. (19) Marinsky, J. A. Ion Exchange; Marcel Dekker: New York, 1966; Vol. 1. (20) Yao, H.; Inoue, Y.; Ikeda, H.; Nakatani, K.; Kim, H.-B.; Kitamura, N. J. Phys. Chem. 1996, 100, 1494. (21) Kitamura, N.; Nakatani, K.; Kim, H.-B. Pure Appl. Chem. 1995, 67, 79.

Figure 1. Absorption spectra of the CdS nanocrystals prepared with or without NaCl for reaction times of (a) 30 min and (b) 48 h. The diameter of the host chelate polymer used was ∼100 µm. to reduce light scattering. As the chelate polymer particles swelled in water, the diameter of a particle (100-110 µm) was determined for solution samples. The photographs of single CdS/polymer particles were obtained using a video printer (Mitsubishi, CP-11), whose images were measured with a CCD video camera (Sony, Model DXC-930) attached to the microscope. X-ray diffraction (XRD) patterns were measured in the 2θ range of 20 to 60° by means of a RINT 2000 (Rigaku) X-ray diffractometer with 0.154 nm Cu KR radiation. Transmission electron microscopy (TEM) was conducted using a Hitachi H-300 (for low magnification) or a JEOL JEM 2010 (for high magnification) electron microscopes.

Results and Discussion Electrolyte Effects on CdS Nanocrystal Formation in Chelate Polymer Particles. Figure 1a shows the absorption spectra of CdS for samples a ([NaCl] ) 0) and b ([NaCl] ) 0.5 M), prepared with a reaction time of 30 min (diameter of the chelate polymer particles; dp ) ∼100 µm). The absorption spectrum of CdS for a given dp and reaction time was reproducible within (5%. Therefore, the spectroscopic data can provide information on average CdS crystal size and distribution characteristics in the host polymer. Absorbance of CdS for sample a (λ < 500 nm) was considerably smaller than that for sample b. The amount of CdS produced during the first 30 min is much larger in the presence of NaCl (sample b) compared to

CdS Nanocrystal Formation

Figure 2. Time profiles of CdS formation in the chelate polymer particles.

that of sample a. Figure 1b shows absorption spectra of samples a and b at a reaction time of 48 h. A blue-shifted spectrum was observed for CdS prepared in the presence of NaCl. Besides influencing the amount of CdS produced, the addition of NaCl affects the optical and distribution properties of CdS in the host polymer. The smaller absorbance of sample a at λ < 500 nm indicates that the CdS formation rate for this sample is lower than that for sample b. In order to make a quantitative discussion, a time profile of the absorbance at 450 nm (nearly excitonic shoulder) was measured for each sample and the results and summarized in Figure 2. Both samples exhibited a fast increase in absorbance during the first 2 h; this will be discussed below. However, CdS formation finished within 2 h in the presence of NaCl (0.5 M), while formation continued gradually for up to 48 h in the absence of NaCl. The latter result indicates that the crystal growth rate of CdS in the absence of NaCl is very slow compared to that in the presence of NaCl. As discussed later in detail, this leads to a difference in the absorption onset between two samples. Differences in the average diameters of CdS nanocrystals for both samples should be reflected in their XRD patterns. A broad and intense XRD reflection peak at 2θ ) 26.5° and very weak reflection peaks at 2θ ) ∼44° and ∼52° were observed. These peaks are consistent with the diffraction pattern corresponding to the zinc blende (cubic) structure of CdS (∼26.5° for (111), ∼44° for (220), and ∼52° for (311)).6 Mean diameters were determined through a Gaussian fit22 of the reflection peak at 26.5° by the Debye-Scheller formula23 to be 3.8 and 3.1 nm for samples a and b, respectively. As expected from the results in Figure 1b, it is concluded that smaller CdS nanocrystals are prepared in chelate polymers containing NaCl. The NaCl effects on mean diameters of CdS nanocrystals would be due to differences in the diffusion rate of HS- ions into the Cd2+-polymer particle. Namely, the diffusion rate of HS- ions plays an essential role in the NaCl effects as seen from the results in Figure 2; this point will be discussed separately in the following sections. Dispersion Textures of CdS Nanocrystals in Chelate Polymer Particles. In order to investigate the distribution characteristics of CdS nanocrystals in the polymers, TEM measurements were performed. Since the chelate polymer swelled in water, elaborate efforts were necessary to make thinly sliced samples. Figure 3 (22) Langford, J. I.; Louer, D. Powder Diffr. 1986, 1, 211. (23) Bawendi, M. G.; Kortan, A. R.; Steigerwald, M. L.; Brus, L. E. J. Chem. Phys. 1989, 91, 7282.

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shows low magnification TEM images of the cross section of a CdS/polymer particle for sample a, where parts a, b, and c show the images of the surface area and regions located at about 4-5 and 8-9 µm inside from the polymer surface, respectively. Interesting and peculiar dispersion textures, showing layer-by-layer distributions of CdS, were observed. Namely, the size of the CdS crystals increases on going from the particle surface to inner regions, as recognized by the fact that the morphology changes from the surface (Figure 3a) to the inside of the polymer particle (Figure 3c). Furthermore, it is important to note that CdS crystal formation terminated at ca. 8-10 µm from the polymer particle surface, as shown in Figure 3c. No crystals were observed at the center part of the polymer. Typical high-magnification TEM images are shown in Figure 4 (a, in the vicinity of the polymer surface; b, ca. 4-5 µm inside from the polymer surface). The images indicate that no flocculation of CdS was observed in the vicinity of the polymer surface, while the nanocrystals with a diameter of ∼4-7 nm flocculate to produce larger particles (several tens of nanometers) at ∼4-5 µm away from the polymer surface. Nanocrystal size distributions obtained from the TEM images in Figure 4 are shown in Figure 5. Although a log-normal size distribution of CdS nanocrystals was observed in the vicinity of the polymer surface, a normal size distribution was found in the flocculation area (∼4 µm inside from the polymer surface). The layer structure of CdS in the polymer can be easily detected with an optical microscope. A typical example of the optical microscope image for sample a is shown in Figure 6a. A clear ring structure can be seen at around 8 µm inside from the polymer surface. Taking the TEM image in Figure 3 into account, this ring structure is concluded to correspond to the edge of the generated CdS nanocrystals layer (schematically shown in Figure 6b). Thus, the length from the polymer surface to the ring structure (L) represents the width of the CdS dispersion layer in the polymer. Detection of the ring boundary is probably due to inhomogeneous distributions of CdS in the polymer. Since the refractive index of CdS is quite larger than that of the polymer, flocculated CdS dispersion causes changes in light scattering intensities inside and outside of the structure.24 It is noteworthy that, in the presence of NaCl (0.5 M), the width of the CdS dispersion layer (L) is larger as compared to that in the absence of NaCl. The relevant TEM images of the cross sections are shown in Figure 7. No flocculation was observed in the polymer surface (Figure 7a) similar to the results for sample a (Figure 3a). At ca. 9-10 µm inside from the surface, although slight flocculation was recognized, CdS was distributed homogeneously (Figure 7b) in contrast to the results for sample a (Figure 3b). Clear flocculation of CdS, observed at ∼8-9 µm from the surface for sample a, was observed at ca. 15-16 µm from the surface for sample b. A close inspection of Figure 7c indicates that single small nanocrystals are also dispersed in addition to flocculated particles. This is an unique property of sample b. Prevention of CdS flocculation by added NaCl was thus proved directly. In an aqueous homogeneous electrolyte solution, fast flocculation of CdS is induced due to screening of the electrical double layer around the nanocrystal surfaces by an added electrolyte.17 Therefore, the present NaCl effects are characteristic of the CdS/polymer hybrid systems. Also, the results indicate that addition of a foreign electrolyte can control L, which is another unique characteristics of the present system.25 (24) Debye, P.; Bueche, A. H. J. Appl. Phys. 1949, 20, 518.

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Figure 3. TEM images of the CdS-polymer particle for sample a (low magnification). (a), (b), and (c) are the images of a region next to the surface, ca. 4-5 µm inner from the surface, and ca. 8-9 µm inside from the surface (at around the edge of the produced CdS), respectively. The lower side of the image corresponds to the inner area of the polymer particle.

Figure 4. TEM images of the CdS-polymer particle for sample a (high magnification). (a) and (b) are the images of the surface area and ca. 4-5 µm inside from the surface of the polymer.

A large number of small CdS nanocrystals (∼3 nm) are produced in the polymer surface, while relatively large CdS (∼5 nm) but in fewer numbers are generated in the inner part of the polymer. This is due to neither structural differences between the surface and inner areas of the polymer nor concentration inhomogenieties of Cd2+, since (25) When visible absorption of CdS and L were examined in the presence of other 1-1 electrolyte (LiCl, KCl, or tetramethylammonium chloride, 0.5 M), they were quite similar to those of the sample prepared in the presence of NaCl (0.5 M). Thus, we suppose that any specific electrolyte effects are not operative in the present system as long as using 1-1 electrolytes.

CdS nanocrystals were produced homogeneously in the whole polymer particle through the reaction of the Cd2+polymer with a concentrated (∼10-2 M) aqueous Na2S solution.14 Thus, the primary reason will be a difference in the degree of supersaturation between the polymer surface and interior. In the polymer particle, the concentration of HS- ions is expected to be higher on the surface as compared to that in the inner part owing to diffusion and subsequent reaction of HS- along the radial direction of the polymer. A relatively high [HS-] in the polymer surface produces many CdS nuclei due to a high

CdS Nanocrystal Formation

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Figure 5. Observed CdS crystal size distributions of sample a (histogram). (a) shows the size distribution in the vicinity of the polymer surface, and (b) shows that of ∼4 µm inside from the polymer surface. The solid curve in (a) is a fit by a lognormal distribution function with a mean crystal diameter of 2.7 nm and a standard deviation of 0.4. The dashed curve in (b) is a fit of a normal distribution function with a mean crystal diameter of 4.6 nm and a standard deviation of 1.8 nm.

degree of supersaturation, and this leads to formation of many nanocrystals with small sizes. We suppose that crystal growth is restricted at the surface layer probably due to interactions of CdS nanocrystals with the chelating ligands and to a lack of Cd2+ ions. On the other hand, since HS- is consumed mainly on the surface, [HS-] is low in the inside of the polymer. Thus, the number of CdS nuclei produced within the polymer is much lower than that on the surface. Owing to a low degree of supersaturation, crystal growth exceeds nucleation, and CdS grows to form large crystals.26 Electrolyte Effects on Nanocrystal Distributions in Chelate Polymer Particles. The variation of L with added NaCl indicates that the diffusion length of HS- in the polymer is influenced by the salt. In the presence of NaCl the color change in the polymer particles from colorless to yellow during CdS formation took place faster than that in the absence of NaCl. As seen in Figure 2, a large difference in the absorbance of CdS was noticed during the initial stage of the reaction (t < 30 min). In order to obtain a clearer picture, the time course of absorbance of CdS was measured in detail at a shorter time scale (t < 10 min) for both samples a and b. Figure 8 is a plot of absorbance at 450 nm and square root of the reaction time (t). At a given t, the absorption spectral band shapes of the two samples were almost the same, so that nanocrystal formation with similar sizes took place as t went by. We obtained good linear relationships between the absorbance and t1/2 for both samples and the slope of the plot was 4.6 × 10-3 and 8.9 × 10-3 s-1/2 for (26) Ramsden, J. J. Surf. Sci. 1985, 156, 1027.

Figure 6. An optical microscope image of a CdS-polymer particle (sample a, (a)) and its schematic illustration (b).

sample a or b, respectively. The t1/2 dependence of the absorbance indicates that HS- ions diffuse into the polymer particle in a quasi-linear fashion as expected from the relation ∆ ) (2Dt)1/2, where ∆ and D represent the diffusion length and the diffusion coefficient of HS- in the host polymer, respectively. Upon observation of the CdS/polymer particles at various reaction times with the optical microscope, it was confirmed that L is a function of t. Thus, L values can be used as a measure of the diffusion length of HS-. However, L was not measured precisely in the region of L < 4 µm since the ring boundary was hard to establish. Thus, a relation between absorbance at 450 nm and L was examined. The results are shown in Figure 9. A linear relationship between absorbance and L was observed, and the data in the presence and absence of NaCl fall on the same line. The results indicate that since L corresponds to the width of the CdS nanocrystals layer in the host polymer, the density of CdS is the same for both samples

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Figure 7. TEM images of the CdS-polymer particle for sample b (low magnification). (a), (b), and (c) show images near the surface area, ca. 9-10 µm from the surface, and ca. 15-16 µm from the surface, respectively.

Figure 8. Time dependencies of CdS absorbance at 450 nm for sample a ([NaCl] ) 0 (b)) and sample b ([NaCl] ) 0.5 M (O)).

for a given absorbance. Also, the results demonstrate that L can be estimated from the absorbance at 450 nm.27 According to Figures 8 and 9, thus, time courses of L were obtained. On analysis of the data as a linear diffusion process (i.e., L ) (2Dt)1/2), the diffusion coefficient of HSin the polymer was calculated to be 2.5 × 10-10 or 1.1 × 10-9 cm2 s-1 for sample a or b, respectively. Since diffusion of HS- in the presence of NaCl is faster than that without the electrolyte, a large number of CdS nuclei would be generated in the polymer particles for sample b. A large number of the nuclei grow to small CdS nanocrystals, and as a result, the concentrations of Cd2+ and HS- in the vicinity of the nanocrystals in sample b become lower than those in sample a. Then, further crystal growth will be suppressed even upon prolonged reaction. (27) In the initial time region, very small nanocrystals are generated at the surface of the polymer, so that absorbance at 450 nm cannot be used as a measure of the exciton absorption. In the present t and L regions, however, the absorbance can be used for quantitative discussion as described in the main text.

Figure 9. A relation between the observed absorbance at 450 nm and L.

Analyses of CdS Formation Rate in Chelate Polymer Particles by Donnan Equilibrium Model. Firstly, we consider an influence of polymer swelling on HS- diffusion. It has been reported that the degree of swelling is affected by an added electrolyte concentration. In the present case, since the osmotic pressure difference between the water and resin phases is smaller at higher electrolyte concentrations, particle swelling is suppressed in the presence of NaCl.28 Diffusion of HS- ions is supposed to be slower in smaller pore-sized gels as compared with larger ones, so that swelling effects can be neglected as a possible origin for the different diffusion rate of HS- ions in the polymer particles with NaCl. The electrolyte effects on ionic diffusion in the polymer and, thus, those on CdS distributions, are interpreted qualitatively by a Donnan equilibrium model reported for charged-membrane systems by Teorell29 and Meyer and (28) Incze´dy, J. Analytical Application of Ion Exchangers; Pergamon Press: Oxford, 1966. (29) Teorell, T. Prog. Biophys. Biophys. Chem. 1953, 3, 305.

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Sievers.30,31 According to the model, the Donnan potential difference between the water and polymer phases controls the rate of Na2S diffusion in the polymer. At the initial stage, Na+ and HS- diffuse into the polymer, and HSreacts with Cd2+ to form CdS. CdS formation proceeds from the surface to the inner volume of the particle, so that Na+ locates in the surface layer of the particle during the initial stages of the reaction. Without NaCl, thus, Na+ would distribute to the water phase to generate the Donnan potential difference, which makes the potential of the polymer surface negative. Moreover, since Na+ is unlikely to exchange with Cd2+ in the chelate polymer owing to a large selectivity difference between Cd2+ and Na+ (∼107 larger18), this also facilitates distribution of Na+ into the aqueous phase. At a certain time after mixing the Na2S and Cd2+-polymer solutions, HS- no longer diffuses into the polymer interior due to this negative surface potential, and diffusion of the ion becomes slow. This is what is observed for sample a, where L is relatively short compared to that of sample b. When enough NaCl is added in the water phase, Na+ and Cl- ions are distributed into the polymer and this makes the Donnan potential lower than that in the absence of NaCl. Thus, HS- is likely to diffuse into the polymer interior, inducing a faster diffusion of HS- in sample b, and thus, L becomes larger. Addition of an electrolyte certainly causes a change in the Donnan potential. For diffusion of a 1-1 electrolyte in a cation exchange resin with a fixed charge concentration X, the Donnan potential difference (∆φ) is given as in eq 129,30

() ()

∆φ ) φi - φo) -

Bi RT ln F Bo

)-

Ao RT ln F Ai

(1)

where φ, B, and A are a potential, counter ion (cation) concentration, and co-ion (anion) concentration, respectively, and the subscripts of i and o represent the polymer and water phases, respectively. R, T, and F represent gas constant, temperature, and Faraday constant, respectively. In the polymer phase, charge neutralization should be satisfied

B i ) Ai + X B o ) Ao ≡ C

(2)

[ x ( )]

X RT ln + F 2C

Conclusion In situ preparation of CdS nanocrystals in chelate polymer microparticles was demonstrated for the first time in the presence of an electrolyte. Unique electrolyte effects on nanocrystal distribution characteristics in the polymer were observed. The use of chelate polymer microparticles as a host matrix is the origin of the present results, by which the diffusion rate of HS- anions in the polymer and subsequent nucleation to CdS are modulated by addition of an inert salt through the change in the Donnan potential of the polymer. Besides the addition of an electrolyte, the optical and dispersion properties of CdS in the polymer can be also controlled by an injection method of HS- and the host polymer size as reported previously.14 Therefore, the properties of the CdS/polymer hybrid can be modulated in several different ways. Since semiconductor nanocrystals embedded in a dielectric medium may be applicable in functional materials and devices, the present results are important as a way to control the preparation of semiconductor nanocrystal/organic polymer hybrids. Acknowledgment. The authors thank Dr. S. Yamamoto, Mr. O. Matsuoka, and Mr. N. Kuramitsu (Mitsui Toatsu Chemicals, Inc.) for helping us to get the TEM images. The work was partly supported by a Grant-inAid on Priority-Area-Research “New Polymers and Their Nano-Organized Systems” from the Ministry of Education, Science, Sports and Culture (08246202). LA970480G

According to eqs 1 and 2, the Donnan potential difference is given by eq 330,32

∆φ ) -

As X equals 0.4 equiv/L for the polymer in a wet state,18 ∆φ is calculated to be -390 or -10 mV for [NaCl] ) 0 or 0.5 M, respectively. A fixed charge concentration X is a function of the degree of swelling of the polymer. However, if the degree of swelling in the presence of NaCl is assumed to be reduced to 1/3 of the value in the swollen state, ∆φ would be estimated to be -26 meV, which is still much smaller than the value without NaCl. The large negative Donnan potential difference is reduced by the addition of NaCl.33 Faster diffusion of HS- into the polymer in the presence of NaCl implies generation of many CdS nuclei and small CdS particles for sample b. This is proved by the blueshift of the onset wavelength (Figure 1b). In the absence of NaCl, slow diffusion of HS- resulted in slow CdS formation due to a small number of nuclei. Hence, relatively large CdS particles are produced as compared with sample b. All the results of the NaCl effects, including the salt effects on L, can be explained by the changes in a Donnan potential with an added electrolyte and subsequent HS- diffusion rate in the polymer.

1+

X 2C

2

(3)

(30) Meyer, K. H.; Siever, J. F. Helv. Chim. Acta 1936, 19, 649. (31) Meyer, K. H.; Siever, J. F. Helv. Chim. Acta 1936, 19, 665. (32) Toyoshima, Y.; Kobatake, Y.; Fujita, H. Trans. Faraday Soc. 1967, 63, 2814.

(33) Diffusion of a 1-1 electrolyte in charged membranes under the Donnan potential difference was discussed quantitatively in ref 31. Thus, we can estimate roughly the diffusion coefficient of univalent anion in a positively charged polymer with or without an external electrolyte after making some assumptions concerning diffusion coefficient (D0), the distribution coefficient (Q0), and the transport number (t0) of the anions in a noncharged polymer. According to the reference, the diffusion coefficient in the charged polymer containing NaCl becomes larger than that without NaCl due to fixed-charge screening by the electrolyte under the condition of the present X and [HS-]. However, we could not estimate the exact values because a precise evaluation of Q0 and t0 is not possible.