Preparation of Semiconductor Nanoparticleâ Polyurea Composites

10120

J. Phys. Chem. B 1999, 103, 10120-10126

Preparation of Semiconductor Nanoparticle-Polyurea Composites Using Reverse Micellar Systems via an in Situ Diisocyanate Polymerization Takayuki Hirai,*,† Tatsufumi Watanabe,† and Isao Komasawa†,‡ Department of Chemical Science and Engineering, Graduate School of Engineering Science, Osaka UniVersity, Toyonaka 560-8531, Japan, and Research Center for Photoenergetics of Organic Materials, Osaka UniVersity, Toyonaka 560-8531, Japan ReceiVed: August 4, 1999

The polymerization of diisocyanates, such as hexamethylene diisocyanate, via reaction with water to form polyurea (PUA), has been utilized to immobilize nanoparticles, when formed in reverse micelles. Metal sulfide nanoparticles (CdS, ZnS, coprecipitated CdS-ZnS, and ZnS-coated CdS), as well as TiO2, AgI, and Ag nanoparticles, were immobilized successfully without undesirable particle coagulation, by the simple addition of diisocyanate to the micellar solution. The resulting PUA composites consist of powder of 0.01-0.1 mm in size and where the immobilized nanoparticles are more or less exposed to the external surface, such that they are utilizable as photocatalysts for the generation of H2 from 2-propanol aqueous solution. With tolylene 2,4-diisocyanate used as monomer, the resulting PUA composite is soluble in both DMF and DMSO, and a nanoparticle-containing transparent film can therefore be prepared by casting the resulting DMSO solution onto a quartz sheet.

Introduction There has been much recent interest in the preparation and processing of nanoparticles formed from various materials, including metals, metal sulfides, and oxides, when using reverse micellar systems. In previous papers,1,2 the thiol-mediated immobilization of CdS or coprecipitated CdS-ZnS nanoparticles into polythiourethane (PTU) have been investigated. The nanoparticles, when formed in reverse micelles, may be recovered into toluene or DMSO via surface modification with thiol molecules,3,4 and the dispersed particles then immobilized into PTU via the polyaddition of dithiol and hexamethylene diisocyanate (HDI). The resulting composite PTU particles are then utilizable as photocatalysts for the reduction of H2O and the generation of H2. Binding of the thiol molecules may inevitably, however, affect the surface structure of the sulfide particles and passivate the surface sulfur vacancies. Further, metal or metal oxide nanoparticles cannot be immobilized by this method, since the surface modification of such particles with thiol is difficult. The above problems may be overcome by a method of in situ preparation of a silica support for the metal nanoparticles in a reverse micellar solution.5 This method is attractive because it can prevent the undesirable aggregation of the nanoparticles during the immobilization stage, and it can be used both for the immobilization of metal oxide and metal sulfide particles, as well as for metal particles. Silica is also ideal as the support of the nanoparticle catalysts for some chemical reactions. If such nanoparticle immobilization, however, could be carried out using in situ synthesized polymers, the properties of the support could then be varied much more freely than with silica. The preparation of polymer particles in a reverse micellar system has been reported by Premachandran et al.,6 utilizing * Corresponding author. E-mail: [email protected]. Tel: +81-6-6850-6272. Fax: +81-6-6850-6273. † Department of Chemical Science and Engineering. ‡ Research Center for Photoenergetics of Organic Materials.

an enzyme (horseradish peroxidase) solubilized in the micelles as catalyst for the polymerization of 4-ethylthiophenol and 4-hydroxythiophenol. They attached CdS nanoparticles formed in separate micellar solution to the resulting thiol-containing polymer, via sulfhydryl groups. Synthesis of the polymer is impossible in the micellar solution containing CdS particles, because the enzyme is deactivated in the presence of Cd2+. Thus, this particular polymer is not suitable for the in situ immobilization of nanoparticles in micelles. The present work describes a novel immobilization method for nanoparticles, when formed in reverse micellar systems, into polymer particles synthesized in situ. Diisocyanates are reported to react with water to form polyurea (PUA), via the hydrolysis of an isocyanate group to an amino group, followed by polymerization to form an urea bond.7 This reaction thus enables the immobilization of nanoparticles, in reverse micelles, via reaction with the water in the micelles.8 The immobilized nanoparticles thus formed have been characterized by both absorption and diffuse reflectance spectrophotometry. The effects of the diisocyanates on the preparation of the PUA and on the stability of the immobilized nanoparticles versus photoirradiation have been investigated, with both effects especially concerning the applicability of the composite to applied photocatalytic reaction. Experimental Section Chemicals. Sodium bis(2-ethylhexyl) sulfosuccinate (Aerosol OT; AOT), hexamethylene diisocyanate (HDI), tolylene 2,4diisocyanate (TDI), and m-xylylene diisocyanate (XDI) were supplied by Tokyo Chemical Industry, Ltd. (TCI). Isooctane (2,2,4-trimethylpentane) was obtained from Ishizu Seiyaku Ltd. Cd(NO3)2‚4H2O, Zn(NO3)2‚6H2O, Na2S‚9H2O, and all other chemicals originated via Wako Pure Chemical Industries, Ltd. All reagents were used without further purification. The preparation and filtration of the reverse micellar solutions was carried out in the same way, as described in a previous paper.3,9

10.1021/jp9927653 CCC: $18.00 © 1999 American Chemical Society Published on Web 10/28/1999

Semiconductor Nanoparticle-Polyurea Composites

J. Phys. Chem. B, Vol. 103, No. 46, 1999 10121

Preparation of Metal Sulfide Nanoparticles in a Reverse Micellar Solution. The following parameters x and y are defined to express the feed reactant compositions in the reverse micellar solutions.

x ) [Zn2+]/([Cd2+] + [Zn2+])

(1)

y ) [S2-]/([Cd2+] + [Zn2+])

(2)

The molar ratio of water to AOT ()[H2O]/[AOT]; water content) is denoted as Wo. Metal sulfide nanoparticles were prepared as reported previously.9 A micellar solution (0.1 mol/L AOT/isooctane, Wo ) 6, 100 mL), containing Cd(NO3)2 for CdS preparation, Zn(NO3)2 for ZnS preparation, and both Cd(NO3)2 and Zn(NO3)2 for coprecipitated CdS-ZnS (denoted CdZnS) preparation, was added rapidly to a second micellar solution containing Na2S (100 mL) of equal Wo and stirred vigorously by magnetic stirrer at 298 K in a glass vessel. ZnS-coated CdS nanoparticles, denoted ZnS(CdS), were prepared by precipitating ZnS onto CdS nanoparticles formed in reverse micelles. ZnS precipitation was carried out by the slow and alternate addition of Zn(NO3)2 micellar solution and Na2S micellar solution to the micellar solution containing the core CdS nanoparticles, 1 h after their formation, since the formation of CdS in reverse micelles at Wo ) 6 almost finished in 1 h.9 Immobilization of Nanoparticles into Polyurea. A weighed quantity of undiluted diisocyanate (HDI, TDI, or XDI) was added rapidly to the nanoparticle-containing reverse micellar solution 2 min to 2 h after nanoparticle formation, where the time period required was a function of the particular diisocyanate employed. After stirring for a required period, for example, 18 h for the case using HDI, the formed polymer powder was collected by centrifugation, washed with n-hexane and diethyl ether, and then dried in vacuo overnight. The CdS, ZnS, CdZnS, and ZnS(CdS) nanoparticle-PUA composites are denoted further in the text as CdS-PUA, ZnS-PUA, CdZnS-PUA, and ZnS(CdS)-PUA, respectively, or more generally as PUA composite. Analysis. The water content of the reverse micellar solution (Wo) was determined by a Karl Fischer moisture meter (Kyoto Electronics MKS-1). Absorption spectra for the semiconductor nanoparticles in the micellar solutions were recorded using a diode-array UV-visible spectrophotometer (Hewlett-Packard 8452A). Diffuse reflectance spectra for nanoparticles immobilized in PUA were recorded, following dispersion of the PUA composites into the aqueous solution used for the photoirradiation experiment, on a UV-vis spectrophotometer (Japan Spectroscopy V-550) equipped with an integrating sphere attachment (ISV-469). The diameter of the semiconductor nanoparticles (dp) was estimated from the absorption onset according to the Brus equation,10 as shown in the previous paper.3,9 IR absorption spectra for PUA were recorded on an FTIR spectrophotometer (Japan Spectroscopy FT/IR-610). SEM and EDX measurements were carried out using an FE-SEM (Hitachi S-5000) and a SEM equipped with EDX (Hitachi S-2250N and Philips EDAX DX-4). As was also the case for polythiourethane (PTU) composites,1,2 PUA is soluble in concentrated H2SO4 solution but insoluble in 6 mol/L HCl solution and 1% H2O2 solution. The metal contents of the PUA composites (total analysis) were thus determined following dissolution of a weighed PUA composite in a concentrated H2SO4 solution,1 by using an inductively coupled argon plasma emission spectrometer (ICP-AES, Nippon Jarrell-Ash ICAP575 Mark II). On the other hand, when PUA composite is contacted with either HCl or H2O2 solution, only a fraction of

the CdS nanoparticles, exposed to the external surface, are dissolved. Thus, the ratio of the partial dissolved quantity of Cd obtained via contact with 6 mol/L HCl or 1% H2O2 to the total quantity of Cd obtained via H2SO4 decomposition was calculated as the particle exposed fraction F.1,8 Photoirradiation Experiment. A 1-2 mg sample of PUA composite was dispersed in 25 mL of a 10 vol % 2-propanol aqueous solution by ultrasonication with sodium hexametaphosphate (12.5 mg). In this procedure, the 2-propanol was employed as a sacrificial electron donor for the positive hole, photogenerated in the semiconductor nanoparticles. A 20 mL aliquot of this mixture was purged in a test tube with argon for 1 h, sealed with a septum, and then photoirradiated with a 2 kW xenon lamp (Ushio UXL-2003D-O). Irradiation light with wavelength λ < 300 nm and that in the IR range were cut off by the Pyrex glass tubing and by the water filter, respectively. When wavelengths only greater than 400 nm were required, 15 wt % NaNO2 aqueous solution was used as a water filter. The quantity of H2 formed in the gas phase of the tube was measured by gas chromatography (Shimadzu GC-14B), as described previously.9 Results and Discussion Preparation of Polyurea (PUA) in a Reverse Micellar Solution Using Hexamethylene Diisocyanate (HDI). Iwakura et al. have investigated the reaction of diisocyanates with water to form polyurea (PUA). This proceeds via the formation of intermediates having both isocyanate and amino groups, as shown below for hexamethylene diisocyanate (HDI).7

OdCdN(CH2)6NdCdO + H2O f OdCdN(CH2)6NH2 + CO2 (3) nOdCdN(CH2)6NH2 f -[-CONH(CH2)6NH-]n- (4) The authors reported that such intermediates may form cyclic urea compounds via an intramolecular reaction, when the concentration of the diisocyanate is low or its methylene chain is short, as, for example, with tetramethylene diisocyanate. When the diisocyanate is added to a reverse micellar system, however, its reaction with water occurs near the micellar interface, which suggests that the reactants (monomers) are much more concentrated, as compared with the bulk solution and that, in the present study, polymerization may be predominant. Figure 1 exhibits FE-SEM images for PUA(HDI) formation, under differing sets of experimental conditions. In the water (1.08 mL)-acetone (20 mL) homogeneous system, reaction with HDI (6 mmol), following stirring for 3 days, produces irregularly shaped PUA particles of ca. 1 µm in diameter, as shown in Figure 1a. In the water (30 mL)-isooctane (10 mL) two-phase system, reaction with HDI (3.12 mmol) occurs at the interface, when the solution is not stirred, and produces a thin film of PUA (Figure 1b) after 11 days. Following the removal of the PUA film from the interface, the solution was then stirred for 3 days. The resulting PUA morphology consists of fused rods of less than a hundred nanometers in diameter (Figure 1c). PUA formation, produced by adding 2.4 mmol of HDI in a reverse micellar solution (200 mL, Wo ) 6), gives a rather ordered morphology consisting of twisted rods, as shown in Figure 1d. This morphology is likely to be caused by polymerization in the reverse micellar systems and probably at the micellar interfaces. Karayigitoglu et al. have found that the microstructure of the reverse micelles can control the morphology of the phenolic polymer particles, enzymatically synthesized in situ.11

10122 J. Phys. Chem. B, Vol. 103, No. 46, 1999

Hirai et al.

Figure 1. FE-SEM images for PUA(HDI), when formed (a) in a water-acetone homogeneous system, (b) at the interface of a water-isooctane two-phase system without stirring, (c) in a water-isooctane two-phase system with stirring, (d) in a reverse micellar system (Wo ) 6), and (e) in a reverse micellar system (Wo ) 6) containing CdS nanoparticles.

Polymerization in the presence of CdS nanoparticles produces fused rodlike aggregates, as shown in Figure 1e. Thus, the presence of nanoparticles may also influence the polymer morphology. Curve a in Figure 2 shows the IR absorption spectrum for nanoparticle-free PUA, formed from HDI in the reverse micellar system. Three peaks at ca. 1260, 1550, and 3300 cm-1 are attributable to NsH bonds and the peak at ca. 1650 cm-1 to that of CdO bonds. The characteristic IR absorption peaks and the result of elemental analysis8 confirm the formation of PUA. The effect of the water pH in the reverse micellar system on

the polymerization of HDI was investigated. “Alkaline”, “neutral”, and “acidic” reverse micelles (Wo ) 6) were prepared by dissolving 0.01 mol/L HNO3, distilled water, and 0.01 mol/L NaOH, respectively, into AOT/isooctane solution. The polymerization rate, as evaluated from the increase in the turbidity of the solution, due to the formation of PUA, was found to be increased with increasing pH. Immobilization of CdS and ZnS Nanoparticles into PUA Using HDI. The resulting CdS-PUA formed was irregularly coagulated and had a particle size in the range 0.01-0.1 mm. EDX analyses were carried out before and after immobilizing


Figure 2. IR absorption spectrum for nanoparticle-free PUA formed from (a) HDI, (b) XDI, and (c) TDI in a reverse micellar system (Wo ) 6).

Figure 3. EDX analyses for Cd in CdS-PUA(HDI) (a) before and (b) after immobilizing the CdS-PUA particles in epoxy resin and slicing this with a rotary microtome.

the CdS-PUA particles in epoxy resin and slicing this with a rotary microtome (Microm HM 325). The signal for Cd was obtained both from the surface (Figure 3a) and from the sliced cross section (Figure 3b) of the CdS-PUA, thus indicating the CdS nanoparticles to be incorporated into the PUA matrix. However, the value of the particle exposed fraction F obtained was almost unity.8 Thus, almost all the CdS nanoparticles are more or less exposed to the external surface microscopically. The absorption spectra for CdS and ZnS nanoparticles in a reverse micellar solution are shown in Figure 4. Although these


Figure 4. Absorption spectra for (a and c) CdS and (b and d) ZnS nanoparticles in a reverse micellar solution, (a and b) 1 min and (c and d) 18 h after formation. Diffuse reflectance spectra for (e) CdS and (f) ZnS nanoparticles immobilized in PUA following 18 h of polymerization. Wo ) 6, [Cd2+] ) [Zn2+] ) [S2-] ) 0.6 mmol/L, and [HDI] ) 12 mmol/L.

sulfide particles are allowed to grow in the micelles, during the 18 h stirring procedure, they still show a quantum size effect, as shown by the blue shift for the absorption onset, as compared to that for bulk CdS (500 nm, 2.5 eV) and ZnS (335 nm, 3.7 eV).12 The diffuse reflectance spectra for CdS and ZnS nanoparticles immobilized in PUA (collected following 18 h of polymerization) are similar to those obtained in a reverse micellar solution at 18 h, thus indicating that the nanoparticles are successfully immobilized and retaining their size. The particle size was estimated as 4.70 and 4.37 nm for CdS and ZnS, respectively. The immobilization ratio for CdS and ZnS nanoparticles was determined from the decrease in absorbance for the CdS or ZnS in a reverse micellar solution, following immobilization and, as shown in Figure 5a, as a function of the quantity of HDI added. Figure 5b shows the quantity of CdS and ZnS immobilized in the PUA, measured via H2SO4 decomposition of the PUA composites and followed by ICP-AES measurement. By increasing the quantity of the HDI, the immobilization ratio thus increases up to approximately 90%, although the actual quantity of the nanoparticles, immobilized per milligram of PUA composite, decreases. ZnS nanoparticles require a greater quantity of HDI than CdS to achieve the 90% immobilization. Preparation of CdS-PUA Using Tolylene 2,4-Diisocyanate (TDI) and m-Xylylene Diisocyanate (XDI). In the cases of the use of TDI or XDI in place of HDI as diisocyanate, the corresponding PUA was obtained in a shorter reaction time than for HDI; i.e., 1 h was needed for TDI and 2 h for XDI, whereas more than 15 h was needed for the polymerization of HDI. The resulting PUAs formed in reverse micellar systems showed the characteristic absorption peaks in accordance with NsH and CdO bonds in the IR absorption spectra, as curves b and c in Figure 2. Although the respective PUAs were obtained as a white powder, they were also optically transparent, with an absorption onset at ca. 260 nm for PUA(HDI), 310 nm for PUA(TDI), and 280 nm for PUA(XDI). TG-DTA analyses showed that thermal decomposition of the PUA occurred at temperatures over 473 K for PUA(HDI), 453 K for PUA(TDI), and 493 K for PUA(XDI). All the PUAs were soluble in concentrated H2SO4, but only PUA(TDI) was soluble in DMF and DMSO. The immobilization of CdS nanoparticles formed in a reverse micellar solution into PUA via in situ polymerization of TDI and XDI was carried out successfully. The resulting CdS-


Hirai et al.

Figure 5. (a) Immobilization ratio for CdS and ZnS nanoparticles and (b) the quantity of CdS and ZnS immobilized in PTU, as a function of the quantity of HDI added. Wo ) 6, [Cd2+] ) [Zn2+] ) [S2-] ) 0.1 mmol/L.

Figure 6. Absorption spectra for (a) CdS nanoparticles prepared at Wo ) 3, 6, and 10, [Cd2+] ) [S2-] ) 0.1 mmol/L, and (b) CdZnS (x ) 0.5) and ZnS(CdS) nanoparticles prepared at Wo ) 6, and incorporated in PUA(TDI) films. [TDI] ) 6 mmol/L.

PUA(TDI) and CdS-PUA(XDI) have morphologies similar to that obtained for HDI (Figure 1e). Using TDI, a CdS-PUA transparent film was able to be formed by casting a DMSO solution of CdS-PUA onto a quartz sheet, as was also the case for CdS-polyurethane (CdS-PU).13 Figure 6a shows the absorption spectra for CdS nanoparticles in a PUA film of 6080 µm thickness. The values of band gap Eg and particle size dp are estimated from the absorption onsets as 3.08 eV and 3.40 nm for the CdS nanoparticles prepared at Wo ) 3, 2.88 eV and 4.02 nm for Wo ) 6, and 2.76 eV and 4.63 nm for Wo ) 10. Thus, the particle size of the CdS and the cutoff wavelength of the film are easily controllable by changing the Wo value of the reverse micellar system. Immobilization of Coprecipitated CdS-ZnS and ZnSCoated CdS Nanoparticles into PUA Using HDI. Coprecipitated CdS-ZnS (CdZnS) nanoparticles were prepared for feed values of x ) 0.25, 0.5, and 0.75. The resulting CdZnS-PUA, prepared using HDI, was dissolved in concentrated H2SO4, and the Cd and Zn concentrations were measured using ICP-AES. The actual molar composition for CdZnS in the PUA composite, x′ ) [Zn2+]/([Cd2+] + [Zn2+]), was thus calculated. The relative quantities of Cd and Zn per milligram of PUA composite are shown in Table 1, together with the feed and actual molar compositions, x and x′. The value of x′ is smaller than the corresponding x value, owing to the lower solubility of CdS as compared to ZnS.9,14 Diffuse reflectance spectra for the CdZnS nanoparticles, immobilized in PUA, are shown in Figure 7a and demonstrate that the absorption onset wavelength varies according to the value of x. The absorption onset, however, shifts toward the longer wavelength following the photoirradiation in 10 vol % 2-propanol aqueous solution, as shown in Figure 7b, except for the case of x ) 1 (ZnS-PUA) which remains constant. Thus, the photoinduced growth for CdS and CdZnS nanoparticles

occurs probably via the fusion of adjacent particles within the PUA composite. This particle growth is also observed where CdS-PUA and CdZnS-PUA are irradiated to visible light (λ > 400 nm). SEM measurements however showed that no appreciable change in morphology occurred by photoirradiation of the PUA composites. ZnS-coated CdS nanoparticles were prepared at a value of y ) 2 for CdS formation and at y ) 0.5 for ZnS deposition on the CdS nanoparticles. An increase in absorbance, owing to ZnS precipitation, was observed for wavelengths less than 340 nm, as shown in Figure 8.9 The resulting ZnS(CdS) nanoparticles were incorporated into PUA, by using HDI, and then photoirradiated. As shown in Figure 8, the absorption onset is retained at ca. 430 nm, showing that the nanoparticles are protected from photoinduced growth by the ZnS coating. This is consistent with the result that, as shown in Figure 7, ZnS nanoparticles immobilized in PUA are stable for photoirradiation. Also as shown in Figure 6b, PUA(TDI) transparent film containing ZnS(CdS) nanoparticles and CdZnS coprecipitated nanoparticles, are also capable of being prepared. Photocatalytic Properties of PUA Composites. As compared to semiconductor nanoparticles in reverse micelles9 and also CdS nanoparticles surface-modified with thiols,3 PUA and also PTU composites1,2 are attractive for use in photocatalysts, since the composites are fairly stable to photoirradiation and can also be easily separated from solution following photocatalytic reaction. The relative quantities of H2 formed for differing composites, during 18 h of photoirradiation, are shown in Table 1. For irradiated light of λ > 300 nm (runs 1-5), the quantity of H2 formed is increased with an increase in x′ value. This is because ZnS can be photoexcited and thus may have greater H2 production ability owing to the larger band gap, as compared to CdS. In contrast, for light irradiation of λ > 400 nm (runs 6-10), no appreciable H2 production on ZnS-PUA is observed,



TABLE 1: Metal Contents in the Resulting PUA Composites Prepared Using HDI and the Results of Photocatalytic H2 Generationa H2 formed during18 h of photoirradiationb

Metal content (µmol/mg of composite) run

PUA composite

x (-)

Cd

Zn

x′ (-)

(µmol/mg of composite)

(µmol/µmol of metal)

1 2 3 4 5 6 7 8 9 10 11

CdS-PUA CdZnS-PUA CdZnS-PUA CdZnS-PUA ZnS-PUA CdS-PUA CdZnS-PUA CdZnS-PUA CdZnS-PUA ZnS-PUA ZnS(CdS)-PUA

0 0.25 0.5 0.75 1 0 0.25 0.5 0.75 1

0.472 0.495 0.446 0.220 0 0.381 0.312 0.446 0.192 0 0.121

0 0.078 0.203 0.315 0.494 0 0.045 0.203 0.296 0.466 0.823

0 0.136 0.312 0.589 1 0 0.126 0.312 0.607 1

0.206 0.231 0.653 0.578 1.20 0.0497 0.100 0.231 0.249 0 0.0831

0.436 0.403 1.01 1.08 2.43 0.130 0.280 0.356 0.510 0 0.687c

a The CdS, CdZnS, and ZnS nanoparticles were prepared at W ) 6, [Cd2+] + [Zn2+] ) [S2-] ) 0.1 mmol/L (y ) 1), and [HDI] ) 6 mmol/L o for runs 1-10. The ZnS(CdS) nanoparticles were prepared at Wo ) 6, [Cd2+] ) 0.05 mmol/L, and [S2-] ) 0.1 mmol/L (y ) 2) for CdS formation, [Zn2+] ) 0.4 mmol/L and [S2-] ) 0.2 mmol/L (y ) 0.5) for ZnS precipitation onto CdS nanoparticles, and [HDI] ) 6 mmol/L for run 11. b Photoirradiation at λ > 300 nm for runs 1-5 and λ > 400 nm for runs 6-11 was carried out in 10 vol % 2-propanol aqueous solution. c µmol/ µmol of Cd.

Figure 8. Absorption spectra for (a) CdS and (b) ZnS(CdS) nanoparticles in a reverse micellar solution (Wo ) 6) and diffuse reflectance spectra for ZnS(CdS) nanoparticles immobilized in PUA (c) before and (d) after 18 h of photoirradiation in 10 vol % 2-propanol aqueous solution (λ < 400 nm). CdS formation: Wo ) 6, [Cd2+] ) 0.05 mmol/ L, and [S2-] ) 0.1 mmol/L (y ) 2). ZnS precipitation onto CdS nanoparticles: [Zn2+] ) 0.4 mmol/L and [S2-] ) 0.2 mmol/L (y ) 0.5). [HDI] ) 6 mmol/L.

Figure 7. Diffuse reflectance spectra for CdZnS nanoparticles incorporated in PUA (a) before and (b) after 18 h of photoirradiation (λ < 300 nm) in 10 vol % 2-propanol aqueous solution. Wo ) 6, [Cd2+] + [Zn2+] ) [S2-] ) 0.1 mmol/L, and [HDI] ) 6 mmol/L.

as shown in Table 1 and Figure 9, since ZnS is hardly photoexcited by visible light. The production of H2 gives a peak at around x ) 0.75 (Table 1). Although the initial absorption onset for CdZnS (x ) 0.75) is ca. 400 nm (Figure 7a), this moves to the longer wavelength region by photoirradiation in the 2-propanol aqueous solution (Figure 7b), and thus it is shown that the PUA composite can produce H2. The photocatalytic property of CdZnS-PUA, regarding the effect of the x value, is generally similar to that obtained for coprecipitated CdSZnS nanoparticles when photoirradiated in a reverse micellar system9 and also in PTU.1 The photocatalytic activity of CdZnS-PUA is greater than that for CdS-PUA as shown in Figure 9, which is also consistent with the results reported previously.14-18

Figure 9. Quantities of H2 formed from 10 vol % 2-propanol aqueous solution by photoirradiation (λ < 400 nm) of dispersed CdS-PUA, CdZnS-PUA (x ) 0.5), and ZnS-PUA. Preparation conditions for PUA composites are the same as described in Table 1.

For ZnS(CdS)-PUA (run 11), the apparent quantity of H2 generated per milligram of PUA composite is less than that for CdZnS-PUA. However, the calculated quantity of H2 per micromole of Cd, since ZnS is not able to absorb light of λ > 400 nm, is greater than that of CdZnS-PUA. In addition, when


Hirai et al. was added. The diffuse reflectance spectrum for TiO2 immobilized in PUA, as shown in Figure 10, resembles that for TiO2 nanoparticles in reverse micelles, thus indicating that the nanoparticles are successfully incorporated into PUA. In the same way, it was found that AgI21 and Ag nanoparticles could be immobilized into PUA. Conclusion

Figure 10. (a) Absorption spectrum for TiO2 nanoparticles in a reverse micellar solution and (b) diffuse reflectance spectrum for TiO2 nanoparticles immobilized in PUA. Wo ) 2, [TTB] ) 2.5 mmol/L, and [HDI] ) 40 mmol/L.

the fluorescent spectrum was measured for CdS-containing reverse micellar solution, an emission peak at ca. 530 nm was much enhanced by ZnS coating. These findings indicate that ZnS deposition onto CdS may remove the surface defect sites of CdS and thus enhance the reducing ability of the photoexcited electrons.14,18 In previous work,8 6-fold greater ion concentrations for the CdS and ZnS formation ([S2-] ) [Cd2+] or [Zn2+] ) 0.6 mmol/ L) and a 2-fold greater quantity of HDI ([HDI] ) 12 mmol/L) were employed, and greater quantities of H2 were generated on CdS-PUA (1.59 µmol/µmol of Cd) and on ZnS-PUA (6.34 µmol/µmol of Zn) by photoirradiation at λ > 300 nm. In such cases, where greater quantities of Cd2+ and S2- or Zn2+ and S2- are fed into a reverse micellar system at constant Wo value, the quantities of the CdS or ZnS particles are increased, although the particle size remains almost independent of ion concentration.9 The number of nanoparticles per unit volume of PUA is, therefore, in this case, greater, and thus the effective surface area for photocatalytic reaction is increased. The quantity of diisocyanate is, therefore, an important factor for controlling the photocatalytic property of PUA composite, since this affects the metal content in the composite significantly, as shown in Figure 5b. On the other hand, when the CdS particles having greater size, prepared at Wo ) 20 and [S2-] ) [Cd2+] ) 0.6 mmol/L, were immobilized into PUA and photoirradiated, a smaller quantity (0.426 µmol/µmol of Cd) of H2 generation was observed, as occurred also in the case of CdS-PTU.2 Thus, the immobilization into PUA of smaller-size nanoparticles, prepared at smaller Wo, is effective for photocatalytic reaction, probably due to their greater surface area, although photoinduced growth of the nanoparticles occurs during photoirradiation. The photocatalytic properties of CdS-PUA(TDI) and CdSPUA(XDI) were also examined, for CdS nanoparticles formed at [Cd2+] ) [S2-] ) 0.6 mmol/L and [diisocyanate] ) 12 mmol/ L. The quantities of H2 formed, during 18 h of photoirradiation, were 0.86 and 0.75 µmol/µmol of Cd, respectively; these values being smaller than those obtained for CdS-PUA(HDI). Photoinduced growth of the CdS particles was also observed for these PUA composites. Immobilization of TiO2 Nanoparticles into PUA. One of the advantages of the present immobilization method for nanoparticles is that it is a universal method applicable to nanoparticles of any material formed in reverse micellar systems. Thus, TiO2 nanoparticles were also prepared via the hydrolysis of titanium tetrabutoxide (TTB)19,20 at [TTB] ) 2.5 mmol/L in Wo ) 2 reverse micellar solution, to which HDI of 40 mmol/L

The immobilization of CdS, ZnS, and composite CdS-ZnS nanoparticles formed in reverse micelles into polyurea (PUA) via an in situ polymerization of diisocyanate was investigated. This method is found to be a universal method for the recovery and immobilization of nanoparticles, such as metal sulfide, metal oxide, metal halide, and pure metal, from micelles without the occurrence undesirable particle coagulation. The resulting PUA composites can be utilized as photocatalysts, and also nanoparticle-containing transparent PUA film can be prepared. Acknowledgment. We are grateful to the Division of Chemical Engineering, Department of Chemical Science and Engineering, Osaka University, for the scientific support of the Gas-Hydrate Analyzing System (GHAS) constructed by a supplementary budget for 1995 and the financial support by a Grant-in-Aid for Scientific Research (No. 10450286) from the Ministry of Education, Science, Sports, and Culture, Japan. References and Notes (1) Shiojiri, S.; Miyamoto, M.; Hirai, T.; Komasawa, I. J. Chem. Eng. Jpn. 1998, 31, 425. (2) Hirai, T.; Miyamoto, M.; Watanabe, T.; Shiojiri, S.; Komasawa, I. J. Chem. Eng. Jpn. 1998, 31, 1003. (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) Kishida, M.; Fujita, T.; Umakoshi, K.; Ishiyama, J.; Nagata, H.; Wakabayashi, K. J. Chem. Soc., Chem. Commun. 1995, 763. (6) Premachandran, R.; Banerjee, S.; John, V. T.; McPherson, G. L.; Akkara, J. A.; Kaplan, D. L. Chem. Mater. 1997, 9, 1342. (7) Iwakura, Y.; Uno, K.; Hamatani, K. Nippon Kagaku Zasshi 1957, 78, 1416. (8) Shiojiri, S.; Hirai, T.; Komasawa, I. Chem. Commun. 1998, 1439. (9) Hirai, T.; Shiojiri, S.; Komasawa, I. J. Chem. Eng. Jpn. 1994, 27, 590. (10) Brus, L. E. J. Chem. Phys. 1984, 80, 4403. (11) Karayigitoglu, C. F.; Kommareddi, N.; Gonzalez, R. D.; John, V. T.; McPherson, G. L.; Akkara, J. A.; Kaplan, D. L. Mater. Sci. Eng. 1995, C2, 165. (12) Lippens, P. E.; Lannoo, M. Phys. ReV. B 1989, 39, 10935. (13) Hirai, T.; Miyamoto, M.; Komasawa, I. J. Mater. Chem. 1999, 9, 1217. (14) Youn, H.-C.; Baral, S.; Fendler, J. H. J. Phys. Chem. 1988, 92, 6320. (15) Kakuta, N.; Park, K. H.; Finlayson, M. F.; Ueno, A.; Bard, A. J.; Campion, A.; Fox, M. A.; Webber, S. E.; White, J. M. J. Phys. Chem. 1985, 89, 732. (16) Ueno, A.; Kakuta, N.; Park, K. H.; Finlayson, M. F.; Bard, A. J.; Campion, A.; Fox, M. A.; Webber, S. E.; White, J. M. J. Phys. Chem. 1985, 89, 3828. (17) Kobayashi, J.; Kitaguchi, K.; Tsuiki, H.; Ueno, A.; Kotera, Y. Chem. Lett. 1985, 627. (18) Kobayashi, J.; Kitaguchi, K.; Tanaka, H.; Tsuiki, H.; Ueno, A. J. Chem. Soc., Faraday Trans. 1 1987, 83, 1395. (19) Hirai, T.; Imamura, E.; Matsumoto, T.; Kuboi, R.; Komasawa, I. Kagaku Kogaku Ronbunshu 1992, 18, 296. (20) Hirai, T.; Sato, H.; Komasawa, I. Ind. Eng. Chem. Res. 1993, 32, 3014. (21) Sato, H.; Hirai, T.; Komasawa, I. J. Chem. Eng. Jpn. 1996, 29, 501.

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