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J. Phys. Chem. B 2000, 104, 8962-8966
Preparation of Semiconductor Nanoparticle-Polymer Composites by Direct Reverse Micelle Polymerization Using Polymerizable Surfactants Takayuki Hirai,*,† Tatsufumi Watanabe,† and Isao Komasawa†,‡ Department of Chemical Science and Engineering, Graduate School of Engineering Science, and Research Center for Photoenergetics of Organic Materials, Osaka UniVersity, Toyonaka 560-8531, Japan ReceiVed: April 7, 2000; In Final Form: June 29, 2000
Metal sulfide semiconductor nanoparticles of CdS and coprecipitated CdS-ZnS, when prepared in reverse micellar systems consisting of cetyl-p-vinylbenzyldimethylammonium chloride (CVDAC), were immobilized via an in situ polymerization of CVDAC, using 2,2′-azobisisobutyronitrile or visible light irradiation. The nanoparticles were incorporated successfully into the polymerized CVDAC (PCV) and retained their size and quantum size effect. Since PCV can both retain and cover the nanoparticles effectively, the stability of nanoparticles, versus photoirradiation, is improved as compared to CdS-containing polyurea (J. Phys. Chem. B 1999, 103, 10 120), but the photocatalytic activity is suppressed. PCV is soluble in polar solvents and a CdS nanoparticle-containing transparent film can therefore be prepared by means of casting an ethanol solution of CdS-PCV onto a quartz sheet. An Au nanoparticle-containing PCV was also prepared via in situ Au formation and polymerization by visible light irradiation.
Introduction There has been much interest recently in the preparation and processing of nanoparticles, as formed from various materials such as metals, metal sulfides, and oxides, using reverse micellar systems. In previous papers,1,2 a novel method has been presented for the immobilization of nanoparticles, when prepared in reverse micellar systems, via an in situ polymerization of diisocyanates. CdS, coprecipitated CdS-ZnS (CdZnS), TiO2, AgI, and Ag nanoparticles prepared in AOT reverse micellar systems have been immobilized successfully in polyurea (PUA) formed in the micelles via the reaction of diisocyanates with water molecules. The resulting CdS-PUA and CdZnS-PUA products demonstrated photocatalytic activity for H2 generation in 2-propanol aqueous solution. The in situ diisocyanate polymerization may prove be a more universal and convenient method for the immobilization of nanoparticles, as compared to that based on the surface modification of metal sulfide nanoparticles using thiol molecules,3,4 followed by polymerization, to form CdS-polythiourethane5,6 and CdS-polyurethane.7 This alternative in situ polymerization method, utilizing polymerizable surfactants, is investigated in the present work. The polymerization of surfactant monomers (surfmers) has previously been carried out by Aoyagi et al.,8 Berg et al.,9 and Ohtani et al.10 using cetyl-p-vinylbenzyldimethylammonium chloride (CVDAC), by Frans et al.11 using N,N-didodecyl-Nmethyl-N-((2-methacryloyloxy)ethyl)ammonium chloride (DDMMEAC), and by Pileni et al.12,13 using didecyldimethylammonium methacrylate. In these studies, a radical polymerization or photopolymerization, mostly in the reverse micellar systems, has been employed but an in situ immobilization of nanoparticles, formed in reverse micelles, has not been reported. In the * To whom correspondence should be addressed. E-mail: hirai@cheng. es.osaka-u.ac.jp. Telephone: +81-6-6850-6272. Fax: +81-6-6850-6273. † Graduate School of Engineering Science. ‡ Research Center for Photoenergetics of Organic Materials.
present study, CVDAC is employed as the polymerizable surfactant, to form the reverse micellar system for subsequent nanoparticle preparation and immobilization. The composite containing nanoparticles obtained were characterized mainly via SEM and absorption spectra measurements. Experimental Section Chemicals. Chloromethylstylene and N,N-dimethyl-n-hexadecylamine were supplied by Tokyo Chemical Industry, Ltd. (TCI). Toluene was obtained from Ishizu Seiyaku Ltd. Cd(NO3)2‚4H2O, Zn(NO3)2‚6H2O, and all other chemicals originated via Wako Pure Chemical Industries, Ltd. Deionized water was single distilled and filtered, using a 0.45-µm membrane filter (Nihon Millipore Kogyo), prior to use. Preparation of Metal Sulfide Nanoparticles in a Reverse Micellar Solution. CVDAC was synthesized according to the procedure reported by Aoyagi et al.8 and dissolved in toluene to a concentration of 0.1 mol/L. The water content of this solution, Wo () [H2O + 2-propanol]/[CVDAC], normally 1.5), was controlled by the addition of a required quantity of 10 vol % 2-propanol aqueous solution of the appropriate metal ion, and the metal concentration was adjusted to 4 mmol/L for the resulting reverse micellar solution. The 2-propanol was required in order to avoid degradation of the metal sulfide particles either by radical attack and/or via a photoinduced positive hole effect. The following parameter x is defined to express the feed reactant composition in the reverse micellar solutions, for the case when the coprecipitated CdS-ZnS (denoted CdZnS) is prepared.
x ) [Zn2+]/([Cd2+] + [Zn2+])
(1)
Metal sulfide nanoparticles, CdS or CdZnS, were prepared by introducing H2S gas (4 mmol/L) into the metal ion-containing reverse micellar solution, contained in a glass vessel with stirring for 1 h. Polymerization of CVDAC. The polymerization of CVDAC was initiated by adding 2,2′-azobisisobutyronitrile (AIBN, 4
10.1021/jp001364g CCC: $19.00 © 2000 American Chemical Society Published on Web 08/17/2000
Semiconductor Nanoparticle-Polymer Composites mmol/L) to 10 mL of the reverse micellar solution. After 1 h of bubbling with Ar, on an ice bath, to purge O2, the solution was heated at 333 K for a further 1 h. The resulting gel was separated by using a centrifuge and was washed with acetone several times and dried in vacuo overnight. This polymer that was obtained is hereafter denoted as PCV. The photopolymerization of CVDAC was carried out using 20 mL of the reverse micellar solution. Following 1 h of Ar bubbling on an ice bath in order to purge dissolved O2, the solution was photoirradiated using a 2 kW xenon lamp (Ushio UXL-2003D-O) for 30 min. Irradiation light with wavelength λ < 400 nm and wavelength in the IR range was cut off by means of a 20 wt % NaNO2 aqueous solution as a water filter. The resulting gel was separated via centrifugation, washed with acetone several times, and dried in vacuo overnight. Analysis. Absorption spectra were recorded on a diode-array UV-visible spectrophotometer (Hewlett-Packard 8452A). For the PCV composites, the absorption spectra were recorded following their dissolution in ethanol. The size of the semiconductor nanoparticles was estimated from the absorption onset, according to the Brus equation,14 as shown in previous papers.3,15 1H and 13C NMR spectra for PCV were measured with a JEOL JNM-AL400. SEM and EDX measurements were carried out on an FE-SEM (Hitachi S-5000) and a SEM equipped with EDX (Hitachi S-2250N and Philips EDAX DX-4). Thermal analyses were carried out with a thermogravimeter/differential thermal analyzer (TG-DTA, Shimadzu TG-DTA 50). The metal contents of the PCV composites were determined, by dissolution of the CdS from a weighed sample of CdS-PCV in 6 mol/L HCl solution, using an inductively coupled argon plasma emission spectrometer (ICP-AES, Nippon Jarrell-Ash ICAP-575 Mark II). Photoirradiation Experiment. An approximately 20-mg sample of CdS-PCV was dispersed in 25 mL of aqueous solution containing 0.05 mol/L Na2S and 0.05 mol/L Na2SO3 using ultrasonication, in the presence of sodium hexametaphosphate (12.5 mg). In this procedure, Na2S and Na2SO3 were employed as a sacrificial electron donor for the positive hole photogenerated on the CdS nanoparticles. The photoirradiation (λ > 400 nm) was carried out in a test tube sealed with a septum, and the same procedure as used for photopolymerization. The quantity of H2 formed in the gas phase of the tube, following 18 h photoirradiation, was measured by gas chromatography (Shimadzu GC-14B) as described previously,15 and the absorption spectrum was recorded. Results and Discussion Polymerization of CVDAC and Preparation of CdS-PCV. The synthesized CVDAC is soluble in polar solvents such as methanol, ethanol, acetone, and chloroform and also in aromatic solvents such as benzene and toluene but is insoluble in paraffinic solvents. The CVDAC/toluene solution (0.1 mol/L) was found to dissolve water up to a value of Wo ) 3. The resulting PCV, thus obtained by radical polymerization using AIBN, was soluble in methanol, ethanol, and chloroform but insoluble in acetone, benzene, and toluene. During polymerization, an absorption peak at around 255 nm for CVDAC was found to decrease, whereas an absorption peak at around 230 nm increased, attributable to the loss of CdC bond. The 1H and 13C NMR spectra showed that the spectra for PCV did not contain any peaks attributable to alkene. The thermal analyses showed that the PCV was stable up to ca. 423 K and the CVDAC was stable up to 373 K. The CdS nanoparticles were able to be prepared in CVDAC reverse micellar system in a Wo range of between 1.0 and 2.5.
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Figure 1. Absorption spectra for CdS nanoparticles in reverse micelles (Wo ) 1.5, 1 h after preparation) and in CdS-PCV prepared by polymerization using AIBN.
Polymerization using AIBN was carried out successfully in the presence of CdS nanoparticles, and the resulting CdS-PCV particles were smaller than 0.1 mm in size and were of irregular shape. EDX analysis showed that the signal for Cd was obtained from the CdS-PCV particle, thus indicating the CdS nanoparticles to be incorporated into the PCV matrix. The CdS content in CdS-PCV prepared was calculated as 30.7 µmol of CdS/g of CdS-PCV. Compared to this, the theoretical value for the CdS content in PCV, calculated assuming 100% yield for CdS formation and for CVDAC polymerization, is 93.5 µmol of CdS/g of CdS-PCV. The much smaller value for the actual CdS content as compared to the theory may be caused by a lower yield for CdS formation and/or the partial dissolution of the CdS from CdS-PCV into HCl solution. The absorption spectrum for the CdS nanoparticles incorporated in PCV was measured following dissolution of CdS-PCV in ethanol. As shown in Figure 1, the effect of polymerization is to shift the onset wavelength of the spectrum slightly toward a higher value, and this corresponds to the growth in size of the CdS nanoparticles from 4.8 nm in reverse micelles to 5.5 nm in PCV. This particle growth may be caused by heat treatment at 333 K during polymerization. Despite this, the CdS nanoparticles still show a quantum size effect, as shown by the blue shift in the absorption onset, as compared to that for bulk CdS (500 nm, 2.5 eV).16 Photopolymerization of CVDAC and Preparation of CdS-PCV and CdZnS-PCV. To suppress the undesirable CdS particle growth, during heat treatment, an alternative polymerization procedure was therefore employed. Under conditions of photoirradiation (λ > 400 nm), the reverse micellar solution, in the presence of CdS nanoparticles, exhibited gelation within a time of 20 min, whereas gelation was much slower in the absence of CdS. CVDAC is known to be polymerized by UV irradiation9 but exhibits no absorption band at wavelengths of λ > 400 nm. This suggests that the CdS nanoparticles in reverse micelles may act as photocatalysts for the polymerization of CVDAC. The absorbance attributed to the CdS nanoparticles is decreased when the photopolymerization is carried out in the absence of 2-propanol and/or without the presence of Ar bubbling. This is due to the dissolution of the CdS. CdS-PCV particles of size less than 0.1 mm are obtained, as shown in Figure 2a,b, and the size and morphology are similar to those for CdS-PCV, when prepared via radical polymerization using AIBN. EDX analyses were carried out, and the Cd signal, obtained from the CdS-PCV particle, is shown in Figure 2c, thus indicating the CdS nanoparticles to be incorporated within the PCV matrix. The CdS content in the prepared CdS-PCV was calculated as 30.4 µmol of CdS/g of CdS-PCV, which is
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Figure 3. Absorption spectra for CdS nanoparticles (a) in reverse micelles (1 h after preparation) and in CdS-PCV prepared by photopolymerization. (a) Wo ) 1.5; (b) the effect of varying Wo.
Figure 4. Absorption spectra for CdZnS nanoparticles of x ) 0, 0.5, and 0.75 in CdZnS-PCV prepared by photopolymerization.
Figure 2. (a) SEM image, (b) FE-SEM image, and (c) EDX analysis with respect to Cd for CdS-PCV prepared by photopolymerization at Wo ) 1.5.
comparable to the value obtained for CdS-PCV, prepared by polymerization using AIBN. As shown in Figure 3a, the growth of the CdS nanoparticles is suppressed by employing the photopolymerization method; the size of the CdS nanoparticles in the PCV matrix being estimated as 4.9 nm, compared to 4.8 nm in the reverse micellar system. Thus, the photopolymerization method, when carried out at room temperature, is effective in incorporating the CdS nanoparticles into PCV and also in maintaining their quantum
size effect. The effect of the Wo value, of the reverse micellar system employed in the CdS preparation, on the absorption spectrum for CdS-PCV is shown in Figure 3b. As for the AOT/ isooctane/H2O reverse micellar system,15,17 the absorption onset for CdS nanoparticles is red-shifted by increasing Wo, when in the presence of the CVDAC/toluene/2-propanol aqueous solution reverse micellar system. The estimated size of the CdS nanoparticles in CdS-PCV is 4.7 nm at Wo ) 1.0, 4.9 nm at Wo ) 1.5, and 6.3 nm at Wo ) 2.5. At Wo ) 2.5, however, a characteristic exciton peak for CdS at around 430 nm disappears. This indicates the increase in polydispersity for CdS nanoparticles,18 and thus the accurate size estimation for the CdS nanoparticles may be difficult in this case. The coprecipitated CdS-ZnS, CdZnS, is also found to be incorporated in PCV, via a similar photopolymerization method. With increasing the ZnS content in the CdZnS, the absorption onset is moved to the shorter wavelength region, as shown in Figure 4. The band gap energy of the metal sulfides and thus the cutoff wavelength of CdZnS-PCV are therefore also controllable by controlling the x value. Effect of Photoirradiation to CdS-PCV. The CdS-PCV particles thus obtained via photopolymerization were used for photoirradiation experiments, and the absorption spectra obtained are shown in Figure 5. The absorption onset for CdSPCV was found to shift slightly toward the longer wavelengths,
Semiconductor Nanoparticle-Polymer Composites
Figure 5. Absorption spectra for CdS nanoparticles in CdS-PCV prepared by photopolymerization. As prepared (in ethanol), before and after 18 h of photoirradiation (λ < 400 nm), in aqueous solution containing 0.05 mol/L Na2S and 0.05 mol/L Na2SO3.
Figure 6. Absorption spectrum for CdS-PCV film prepared on a quartz sheet.
when CdS-PCV was dispersed in the aqueous solution, probably due to the slight fusion of the CdS nanoparticles adjacent to each other in the PCV matrix. Photoinduced CdS nanoparticle growth was also observed, following 18 h photoirradiation, and the resulting particle size was estimated as 5.6 nm. The undesirable particle growth of the CdS is, however, much suppressed in the CdS-PCV in this case, as compared to that observed for CdS-PUA, where the absorption onset shifts to reach that of the bulk CdS (500 nm).1,2 This difference between CdS-PCV and CdS-PUA, although there is no chemical bond between CdS and the polymer in both composites, suggests that the PCV is a more rigid matrix and can therefore hold and cover the nanoparticles more effectively. This feature for CdS-PCV may, however, be a weak point, when applied as a photocatalyst. The quantity of H2 formed, in this case, during 18-h photoirradiation, was 1.3 µmol of H2/g of CdS-PCV and thus 0.043 µmol of H2/µmol of CdS, which is smaller than the value obtained for CdS-PUA (49.7 µmol of H2/g of CdS-PUA and thus 0.130 µmol of H2/µmol of CdS).2 Preparation of CdS-PCV Film. PCV is soluble in some polar solvents. Thus, the CdS-PCV film is able to be prepared by casting an ethanol solution of CdS-PCV onto a quartz sheet, as also in the cases for CdS-PUA2 and CdS-polyurethane.7 The CdS-free PCV film is optically transparent at λ > 320 nm. Figure 6 shows the absorption spectrum for CdS nanoparticles in a PCV film of ca. 20-µm thickness and shows an almost identical onset wavelength to that obtained for CdS-PCV in ethanol (dotted line). Thus, the incorporation of CdS nanoparticles in PCV transparent film is successful in maintaining their particle size and thus the cutoff wavelength characteristic. Incorporation of Au Nanoparticles into PCV. One of the advantages of the present incorporation method employed for
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Figure 7. Absorption spectra for HAuCl4 in reverse micellar solution and Au-PCV prepared by photoreaction.
nanoparticles is that it is a universal method applicable to nanoparticles of any material formed in reverse micellar systems, as in the case using PUA.1,2 Thus, the incorporation of Au nanoparticles, via an in situ preparation of Au and PCV by photoirradiation, was investigated as a particular case study. Au nanoparticles can be prepared in reverse micellar system by laser light irradiation (353 nm) to HAuCl4, as reported by Kurihara et al.19 In the present work, a 0.1 mol/L CVDAC/toluene/H2O reverse micellar solution (Wo ) [H2O]/[CVDAC] ) 2) containing 0.09 mmol/L HAuCl4 was irradiated to visible light (λ > 400 nm) for 20 h, following 1 h Ar bubbling. As shown in Figure 7, the resulting PCV composite showed the characteristic absorption band at around 540 nm, which is attributable to the Au formation in the PCV matrix. Thus, the preparation of AuPCV is successful, and the PCV matrix is effective in stabilizing the Au nanoparticles, as for CdS nanoparticles. Conclusion The present study describes a novel immobilization method for nanoparticles via direct reverse micelle polymerization using polymerizable surfactants, cetyl-p-vinylbenzyldimethylammonium chloride (CVDAC), using 2,2′-azobisisobutyronitrile or visible light irradiation. This method is a universal method for the recovery and immobilization of nanoparticles, such as metal sulfide and pure metal, prepared in a reverse micellar systems consisting of CVDAC, without the occurrence of undesirable nanoparticle coagulation. A nanoparticle-containing transparent PCV film can also be prepared. Acknowledgment. We are grateful to Mr. Masao Kawashima of “Gas Hydrate Analyzing System (GHAS)”, Osaka University, for his help in the SEM measurement, and to the Division of Chemical Engineering for the Lend-Lease Laboratory System. We are also grateful to the financial support through a Grant-in-Aid for Scientific Research (No. 10450286 and 11650781) from the Ministry of Education, Science, Sports and Culture, Japan. References and Notes (1) Shiojiri, S.; Hirai, T.; Komasawa, I. Chem. Commun. 1998, 1439. (2) Hirai, T.; Watanabe, T.; Komasawa, I. J. Phys. Chem. B 1999, 103, 10120. (3) Shiojiri, S.; Hirai, T.; Komasawa, I. J. Chem. Eng. Jpn. 1997, 30, 86. (4) Shiojiri, S.; Hirai, T.; Komasawa, I. J. Chem. Eng. Jpn. 1998, 31, 142. (5) Shiojiri, S.; Miyamoto, M.; Hirai, T.; Komasawa, I. J. Chem. Eng. Jpn. 1998, 31, 425. (6) Hirai, T.; Miyamoto, M.; Watanabe, T.; Shiojiri, S.; Komasawa, I. J. Chem. Eng. Jpn. 1998, 31, 1003.
8966 J. Phys. Chem. B, Vol. 104, No. 38, 2000 (7) Hirai, T.; Miyamoto, M.; Komasawa, I. J. Mater. Chem. 1999, 9, 1217. (8) Aoyagi, T.; Terashima, O.; Suzuki, N.; Matsui, K.; Nagase, Y. J. Controlled Release 1990, 13, 63. (9) Berg, J. M.; Claesson, P. M. J. Colloid Interface Sci. 1994, 163, 289. (10) Ohtani, N.; Furutani, H.; Tsukada, A.; Hori, T. Kobunshi Ronbunshu 1997, 54, 163. (11) Frans, D. S.; Gilbert, V.; Carine, J.; Nadine, W.; Andre, P. Bull. Soc. Chim. Belg. 1990, 99, 1045. (12) Hammouda, A.; Gulik, Th.; Pileni, M. P. Langmuir 1995, 11, 3656.
Hirai et al. (13) Moumen, N.; Pileni, M. P.; Mackay, R. A. Colloids Surf. 1999, 151, 409. (14) Brus, L. E. J. Chem. Phys. 1984, 80, 4403. (15) Hirai, T.; Shiojiri, S.; Komasawa, I. J. Chem. Eng. Jpn. 1994, 27, 590. (16) Lippens, P. E.; Lannoo, M. Phys. ReV. B 1989, 39, 10935. (17) Pileni, M. P. Langmuir 1997, 13, 3266. (18) Weller, H.; Schmidt, H. M.; Koch, U.; Fojtik, A.; Baral, S.; Henglein, A.; Kunath, W.; Weiss, K. Chem. Phys. Lett. 1986, 124, 557. (19) Kurihara, K.; Kizling, J.; Stenius,; Fendler, J. H. J. Am. Chem. Soc. 1983, 105, 2574.
