In Situ Fabrication and Optical Properties of a Novel Polystyrene

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In Situ Fabrication and Optical Properties of a Novel Polystyrene/Semiconductor Nanocomposite Embedded with CdS Nanowires by a Soft Solution Processing Route Shu-Hong Yu, Masahiro Yoshimura,* Jose Maria Calderon Moreno, Takeshi Fujiwara, Takahiro Fujino, and Ryo Teranishi Center for Materials Design, Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226, Japan Received June 30, 2000. In Final Form: November 9, 2000 A novel one-step soft solution-processing route called the solvothermal-copolymerization technique was successfully developed for in situ fabrication of polystyrene (PS)/CdS nanocomposites embedded with CdS nanowires in ethylenediamine media at lower temperatures (80-140 °C). In this route, the polymerization of the monomers and the formation of the CdS nanocrystallites occur simultaneously in a certain temperature range. The results of X-ray powder diffraction, transmission electron microscopy, and high-resolution transmission electron microscopy confirmed that the embedded CdS nanowires, with diameters of 4-15 nm and lengths up to several micrometers, have [001] preferential orientation. Both temperature and solvent were found to play a key role in the synthesis of the nanocomposites. The produced novel hybrid nanocomposites display obvious quantum size effects and interesting fluorescence features. The spectroscopic properties of the PS/CdS nanowire nanocomposites were found to be sensitive to synthetic conditions, including the concentrations of Cd2+ or the monomer, temperature, and reaction time.

Introduction Inorganic-organic nanocomposites are currently intensively investigated because of their potential applications as high-technology materials.1-3 Especially, nanocomposite structures provide a new method to improve the processability and stability of materials with interesting properties. These hybrid nanocomposites inherit some of the properties of both the polymer and the inorganic materials,1-3 such as the mechanical characteristic performances, catalytic, optical, and electronic features of inorganic colloids of polymers. They have promising new applications in many fields such as mechanics, optics, electronics, catalysis, and biology. Among these, semiconductor/polymer nanocomposites have attracted much recent attention because of their many advantages over the single-phase particle systems. It is well-known that photocatalytic and photosynthetic reactions at semiconductor particles provide the possibility of the utilization of solar energy for the promotion of useful chemical reactions.4,5 A number of studies on the utilization of semiconductor particles (e.g., TiO2, CdS), frequently treated with appropriate catalysts to carry out photocatalytic and photosynthetic processes, have been described.6 These particle systems have several disadvantages. First, because they are dispersed in the solvent system they are not convenient to use in continuous-flow systems7-9 and the particles also tend to flocculate and settle out with time. Second, the influence of the nano* To whom correspondence should be addressed. E-mail: [email protected]. Tel: +81 45 924 5323. Fax: +81 45 924 5358. (1) Antonietti, M.; Go¨ltner, C. Angew. Chem., Int. Ed. Engl. 1997, 36, 910. Fo¨rster, S.; Antonietti, M. Adv. Mater. 1998, 10, 195. (2) Beecroft, L. L.; Ober, C. K. Chem. Mater. 1997, 9, 1302. (3) Wozniak, M. E.; Sen, A. Chem. Mater. 1992, 4, 754. (4) Bard, A. J. J. Phys. Chem. 1979, 10, 59. Bard, A. J. J. Phys. Chem. 1982, 86, 172. (5) Gratzel, A. Acc. Chem. Res. 1981, 14, 376. (6) See, for example: Frank, S. N.; Bard, A. J. Phys. Chem. 1977, 81, 1484. Kraeutler, B.; Bard, A. J. Am. Chem. Soc. 1978, 100, 5985.

particles’ surface on their optical and electrical properties is very obvious because of their surface state, which traps electrons or holes and degrades electrical and optical properties. Investigation of excitonic properties with previously reported CdS or CdSe samples has not been satisfactory because of the weak band-edge emission at the absorption edge and the intense red-shifted emission from the deep-trapped states. Consequently, passivation of their surface by chemical processes has been widely employed.10 To organize the semiconductor nanoparticles in an orderly fashion in a matrix may provide a potential application of their special properties. Theory and experiments have showed that the properties of nanostructured materials, such as optical, electric, magnetic, adsorptive, catalytic, and other characteristics, strongly vary with the size and shape of the particles, even though they may have the same composition or molecular structure.11 The encapsulation of semiconductor particles within a bulk structure has several advantages over solution-based particle synthesis. Solid matrixes retain their shapes and do not spill or leak. Polymeric materials can be processed or manipulated into bulk structures. Polymers can be stretched and oriented, which may result in the physical alignment of the polymer chains into crystalline domains, and this could allow the construction of ordered arrays of the incorporated particles. Such structures would have a number of applications.12 Therefore, control over the particle sizes and morphologies in the composite is of great importance. (7) Krishnan, M.; White, J. R.; Fox, M. A.; Bard, A. J. Am. Chem. Soc. 1983, 105, 7002. Kakuta, N.; White, J. M.; Campion, A.; Bard, A. J.; Fox, M. A.; Webber, S. E. J. Phys. Chem. 1985, 89, 48. (8) Mau, A. W. H.; Huang, C. B.; Kakuta, N.; Bard, A. J.; Campion, A.; Fox, M. A.; White, J. M.; Webber, S. E. J. Am. Chem. Soc. 1984, 106, 6537. (9) Kuczynski, J. P.; Milosavljievic, B. H.; Thomas, J. K. J. Phys. Chem. 1984, 88, 980. (10) Alivisatos, A. P. Science 1996, 271, 933. (11) Stucky, G. D.; MacDougall, J. E. Science 1990, 247, 669. (12) Faist, J.; Capasso, F.; Sivco, D. L.; Sirtori, C. S.; Hutchinson, A. L.; Cho, A. Y. Science 1994, 264, 553. Hkouwenhoven, L. Science 1995, 268, 1440.

10.1021/la000941p CCC: $20.00 © 2001 American Chemical Society Published on Web 01/09/2001

Polystyrene/Semiconductor Nanocomposite

Previously, the semiconductor/polymer composites were used for their catalytic properties rather than their optical properties.13 In recent years, semiconductor/polymer nanocomposites have found promising applications in optical devices. The first group to formally recognize semiconductor/polymer nanocomposites such as these in terms of engineered optical media was Akimov et al. in 1992.14 The first work to use polymer blends as matrix materials was reported by Yuan et al.15,16 Various kinds of polymer- or surfactant-modified CdS17-27 or CdSe28-32 nanocomposites have been reported in recent years. Among them, polymer/inorganic nanocrystal composites are particularly interesting materials in the study of electrical transport31 and fabrication of nanocrystal-based light-emitting diodes (LEDs) prepared with polymers and CdSe nanocrystals.10,28,29,32 Especially, charge separation at the interface between organic molecules and nanocrystals is currently of great interest because of the report of the efficient photovoltaic devices fabricated by the absorption of organic dyes on TiO2 nanocrystalline films by O’Regan and Gra¨tzel.33 A CdS/ PVK (poly(N-vinylcarbazole)) polymer composite was found to be a photoconductive material by Wang and Herron.17 Dabbousi et al. reported the electroluminescence in blends of CdSe/poly(N-vinylcarbazole).34 Greenham et al.31 reported the photoluminescence and photoconductivity of composite materials by mixing of MEH-PPV and CdSe or CdS. They found that rapid charge separation occurred at the polymer/nanocrystal interface, which was confirmed by strong photoluminescence quenching. Prior to this work, Golden et al.35 reported a novel method for fabrication of (Mo3Se3)n/poly(vinylene carbonate) nanocomposites containing mono- and multiwire cables, which involves the preparation of solutions con(13) Meissner, D.; Memming, R.; Kastening, B. Chem. Phys. Lett. 1983, 96, 34. (14) Akimov, I. A.; Denisyuk, I. Y.; Meshkov, A. M. Opt. Spectrosc. 1992, 72, 558. (15) Yuan, Y.; Cabasso, I.; Fender, J. Macromolecules 1990, 23, 3198. (16) Yuan, Y.; Fender, J.; Cabasso, I.; Chem. Mater. 1992, 4, 312. (17) Wang, Y.; Herron, N. Chem. Phys. Lett. 1992, 200, 71. (18) Roescher, A.; Mo¨ller, M. Adv. Mater. 1995, 7, 151. Spatz, J. P.; Mo¨ssmer, S.; Mo¨ller, M. Chem.sEur. J. 1996, 2, 1552. Spatz, J. P.; Roescher, A.; Mo¨ller, M. Adv. Mater. 1996, 8, 337. Kane, R. S.; Cohen, R. E.; Silbey, R. Chem. Mater. 1996, 8, 1919. Moffitt, M.; Eisenberg, A. Chem. Mater. 1995, 7, 1178. Schneider, T.; Haase, M.; Kornowski, A.; Naused, S.; Weller, H.; Fo¨rster, S.; Antonietti, M. Ber. Bunsen-Ges. Phys. Chem. 1997, 101, 1654. (19) Woggon, U.; Bogdanov, S. V.; Wind, O.; Schlaad, K. H.; Pier, H.; Klingshirn, C.; Chatziagorastou, P.; Fritz, H. P. Phys. Rev. B 1993, 48, 11979. (20) Noglik, H.; Pietro, W. J. Chem. Mater. 1994, 6, 1593. (21) Premachandran, R.; Banerjee, S.; John, V. T.; McPherson, G. L. Chem. Mater. 1997, 9, 1342. (22) Olshavsky, M. A.; Allcock, H. R. Chem. Mater. 1997, 9, 1367. (23) Sun, Y. P.; Rollins, H. W. Chem. Phys. Lett. 1998, 288, 585. (24) Sookal, K.; Hanus, L. H.; Ploehn, H. J.; Murphy, C. J. Adv. Mater. 1998, 10, 1083. (25) Lakowicz, J. R.; Gryczynski, I.; Gryczynski, Z.; Murphy, C. J. J. Phys. Chem. B 1999, 103, 7613. (26) Shiojiri, S.; Hirai, T.; Komasawa, I. Chem. Commun. 1998, 1439. (27) Winiarz, J.; Zhang, L. M.; Lal, M.; Friend, C. S.; Prasad, P. N. J. Am. Chem. Soc. 1999, 121, 5287. (28) Colvin, V. L.; Schlamp, M. C.; Alivisatos, A. P. Nature 1994, 370, 354. (29) Covin, V. L.; Alivisatos, A. P. J. Chem. Phys. 1992, 97, 730. (30) Dabbousi, B. O.; Bawendi, M. G.; Onitsuka, O.; Rubner, M. F. Appl. Phys. Lett. 1995, 66, 1316. (31) Greenham, N. C.; Peng, X. G.; Alivisatos, A. P. Phys. Rev. B 1996, 54, 17628. (32) Huynh, W.; Peng, X. G.; Alivisatos, A. P. Adv. Mater. 1999, 11, 923. (33) O’Regan, G.; Gra¨tzel, M. Nature 1991, 353, 737. (34) Dabbousi, O.; Bawendi, M. G.; Onitsuka, O.; Rubner, M. F. Appl. Phys. Lett. 1995, 66, 1316. (35) Golden, J. H.; DiSalvo, F. J.; Fre´chet, J. M.; Silcox, J.; Thomas, M.; Elman, J. Science 1996, 273, 782.

Langmuir, Vol. 17, No. 5, 2001 1701 Scheme 1. Illustration of a Soft Solution-Processing Route Called the Solvothermal-Copolymerization Technique for Fabrication of Polystyrene/CdS Nanowire Nanocomposites

sisting of inorganic salts dissolved in an organic monomer that are bulk-polymerized in the presence of a cross-linking agent by choosing soluble (LiMo3Se3)n and the polar monomer vinylene carbonate as starting materials in the presence of the free-radical initiator. A first work on the deposition of one-dimensional CdS nanowires in a solid silica matrix by using porous Vycor glass for stabilizing CdS nanoparticles was reported by Thomas et al.36 In this paper, we report a new strategy for the fabrication of polystyrene (PS)/CdS nanowire nanocomposites by a soft solution-processing (SSP) route called the solvothermalcopolymerization technique. The organic solvent ethylenediamine was chosen as both the shape controller of CdS nanocrystals and the reaction medium. The spectroscopic and optical properties of the PS/CdS nanowire nanocomposites were investigated. Experimental Section Chemical Reagents and Reactors. All the chemicals used in this study were reagent grade without further purification. All the chemicals were provided by Wako Pure Chemical Industries, Ltd., Osaka, Japan. Cd(NO3)2‚4H2O and thiourea (Tu) were used as the Cd source and S source, respectively. 2,2′Azobisisobutyronitrile (AIBN) was used as a radical initiator of the polymerization reaction. Ethylenediamine was chosen as the shape controller and reaction medium. Commercial Teflon-lined autoclaves of 40 mL capacity were used as the reactors, which are made by SANPLATEC Company, Japan. Preparation Procedures. All the experiments were accomplished in a Teflon-lined autoclave of 40 mL capacity, which was filled with 36 mL of ethylenediamine (en) up to 90% of the total volume. In a typical preparation procedure of the PS/CdS nanocomposite, 0.001 mol of analytical grade Cd(NO3)2‚4H2O and 0.002 mol of thiourea (Tu) were dissolved in 36 mL of ethylenediamine. A mixture of 0.025 mol of styrene (C6H5CHd CH2, St) monomer and 0.014 g of AIBN was put into a Teflon inner stainless steel autoclave with a capacity of 40 mL. The autoclave was heated in an oven at 80-140 °C for 4-12 h. The product obtained was washed with absolute alcohol and distilled water. The samples were dried at 60 °C for 2 h and then ground into powders for characterization. The preparation strategy can be illustrated as Scheme 1. Characterization. The products were characterized by X-ray powder diffraction (XRD) patterns employing a scanning rate of 0.02° s-1 in the 2θ range from 10° to 70°, using a MAC Science MXP-3VA diffractometer equipped with graphite monochromated Cu KR radiation (λ ) 1.5405 Å) operated at 40 mA and 40 kV. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) observation was conducted on a Hitachi Model H-900 transmission electron microscope, using an accelerating voltage of 300 kV. Fourier transform infrared (FTIR) transmission spectra were taken on a JEOL JIR-7000 spectrometer, which operated from 4000 to 500 cm-1 at room temperature. The samples were mixed with (36) Kuczynski, J.; Thomas, J. K. J. Phys. Chem. 1985, 89, 2720.

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KBr powder for measurement, and the background correction was made using a reference blank KBr pellet. Room-temperature UV-vis measurements were made with a Shimadzu UV-3100PC scanning spectrophotometer. Room-temperature fluorescence spectra were recorded on a Hitachi F-4500 fluorescence spectrophotometer.

Results and Discussion XRD Measurements. XRD patterns obtained under different conditions are shown in Figures 1-4. XRD patterns of the products obtained at different temperatures are shown in Figure 1. Figure 1a shows the presence of an amorphous phase with a wider peak at 2θ ) 19.4°, corresponding to the pure PS phase. In contrast, XRD patterns for the nanocomposites in Figure 1b-e consist of two obvious phases; one is the amorphous polymer phase which has a wider peak at 2θ ) 19.4°, and the other is the hexagonal CdS phase. Parts c and d of Figure 1 show that the 002 peak is sharper than the other peaks and has preferential [001] orientation, indicating that the CdS nanocrystallites grow along this direction. This result is consistent with those by TEM and HRTEM observations, which we will discuss later. The calculated cell constants for the CdS nanowires in Figure 1d are a ) 4.140 Å and c ) 6.71 Å, which are close to the reported data for CdS (JSPDS Card File No. 41-1049). Formation of PS/CdS Nanocomposites. It is wellknown that the radical initiator AIBN is decomposed at 45-65 °C and releases free radicals as described in eq 1. The produced radicals will initiate the polymerization of the monomers and lead to producing PS.

The formation process of CdS nanocrystallites is that thiourea decomposes in basic media to release S2-, which bonds with the complex ion [Cd(en)3]2+ in the solution.37 Then, the CdS nanowires form after losing the volatile ethylenediamine (en) molecules at certain temperatures. The processes can be formulated as eqs 2 and 3:

(NH2)2CS + 2OH- S CH2N2 + H2O + S2-

(2)

Cd(en)32+ + S2- w CdS(en)m S CdS(en)m-n + n(en) (3) Previous results have demonstrated that polyamines such as ethylenediamine play a key role in the formation of group II-VI nanowires.38 Both the temperature and the solvent were found to play key roles in the synthesis of the nanocomposites. The results show that the polymerization of the monomers and the formation of the CdS nanocrystallites could occur simultaneously in a certain temperature range (80-140 °C) for 4-12 h. Effects of Temperature on Synthesis of PS/CdS Nanocomposites. Figure 1b,e shows the XRD patterns for the products obtained at different temperatures from 90 to 140 °C. Figure 1 shows that the diffraction peaks for the CdS phase increased with increasing temperature, whereas the peak intensity for the PS phase decreased with increasing temperature. We found that the polymerization reaction deteriorated and the amount of PS (37) Yang, J.; Zeng, J. H.; Yu, S. H.; Yang, L.; Zhou, G. E.; Qian, Y. T. Chem. Mater. 2000, 12, 3259. (38) Yu, S. H.; Wu, Y. S.; Yang, J.; Han, Z. H.; Xie, Y.; Qian, Y. T.; Liu, X. M. Chem. Mater. 1998, 10, 2309. Yu, S. H.; Yang, J.; Han, Z. H.; Zhou, Y.; Yang, R. Y.; Qian, Y. T.; Zhang, Y. H. J. Mater. Chem. 1999, 9, 1283. Yu, S. H.; Yang, J.; Han, Z. H.; Yang, R. Y.; Qian, Y. T.; Zhang, Y. H. J. Solid State Chem. 1999, 147, 637.

Figure 1. XRD patterns of the pure PS and the PS/CdS nanocomposites prepared at different temperatures by the present route. (a) PS obtained at 90 °C for 12 h and (b)-(e) the PS/CdS obtained at different temperatures for 12 h (Cd2+ ) 5.0 × 10-4 mol, Cd2+/Tu ) 1:2, St ) 0.025 mol, AIBN ) 0.014 g): (b) 90 °C, (c) 100 °C, (d) 120 °C, and (e) 140 °C.

decreased with increasing temperature (the same phenomena occurred in the absence of Cd2+ and thiourea), because of the rapid burst of free radicals by the higher rate of AIBN decomposition at higher temperatures, leading to PS of lower molecular weight. In contrast, the amount of CdS in the products increased. These results indicated that both the CdS nanocrystallite sizes and the CdS contents in the composites increase with increasing temperature. When the temperature is lower than 70 °C, only the polymerization reaction occurred in the system but no CdS phase formed. The uniform PS/CdS nanocomposites can be obtained in the temperature range of 80-100 °C. When the temperature is higher than 100 °C, phase segregation will occur and the produced CdS nanoparticles tend to aggregate together. Effects of Solvents on Polymerization, Shape of CdS Nanocrystals, and Formation of PS/CdS Nanocomposites. The results show that solvent plays a key role in the formation of uniform PS/CdS nanocomposites. When water was chosen as the solvent, the product was found to be composed of two totally separated phases, that is, polymer phase and CdS phase. Also, the polymerization reaction occurs so fast that the produced polymer becomes very hard and cannot even be ground into powders. When the mixed solvent en/H2O (v/v) ) 1:1 was used, the same phenomenon occurs. In contrast, only the pure CdS phase was obtained if pyridine was chosen as the solvent instead of ethylenediamine. If ethanol was used as the solvent, the product was composed of the CdS phase and very viscous liquid, but no solidified polymer was obtained. In addition, only spherical particles can be obtained in ethanol, water, and pyridine,38 in which there is no coordination or weaker coordination with Cd2+. Ethylenediamine plays a key role in the formation of CdS nanowires. Our previous work on the synthesis of single-phase CdS nanoparticles demonstrated that the shape of CdS nanocrystals can be well controlled by using different solvents.38 Compared with pyridine, ethanol, and water, ethylenediamine has the strongest coordination ability with Cd2+ and is a strong Lewis base. The special shape forming of CdS nanowires may be related to the strong N-chelating effect of bidentate ligand ethylenediamine with Cd2+ and its stronger hydrogen bonding action effect of the protophilic properties among molecules. Recent work on the detailed formation process of CdS nanowires in ethylenediamine media confirmed that an

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Figure 2. TEM images of the PS/CdS nanocomposites prepared by the present route at different temperatures for 12 h (Cd2+ ) 5.0 × 10-4 mol, Cd2+/Tu ) 1:2, St ) 0.025 mol, AIBN ) 0.014 g): (a) 90 °C, (b) 100 °C, (c) 120 °C, and (d) 140 °C.

accordion-like folding process was found during the formation of CdS nanowires.37 Further studies indicate that the dissociation of ethylenediamine molecules adsorbed on the surface of CdS results in the formation of CdS nanowires.37 In the present work, ethylenediamine is found to be the best medium and shape controller for the fabrication of PS/CdS nanowire nanocomposites. In addition, other factors, such as the concentration of Cd2+ or styrene monomer and reaction time, also have effects on the polymerization, the formation of CdS nanocrystals and their crystallinity, and the phase composition in the composites. TEM and HRTEM Observations. TEM images for the samples obtained at different temperatures (90-140 °C) for 12 h were shown in Figure 2. The TEM images in Figure 2a-d show that all the CdS nanocrystallites are homogeneously nanowires with widths of 4-15 nm and lengths up to several micrometers. In addition, TEM observations confirmed that the sizes of the nanowires increase with increasing temperature, which is consistent with the results by XRD analysis and UV-vis absorption spectra. The TEM images in Figure 2c,d show that the CdS nanowires tend to aggregate together. Some phase segregation occurred when the temperature was higher

than 100 °C and the reaction time was longer than 6 h. This phase segregation has a significant effect on the spectroscopic properties of the nanocomposites, which we will discuss later. The HRTEM image in Figure 3 shows a typical 13 nm diameter CdS nanowire displaying wellresolved (002) lattice planes. The lattice spacing perpendicular to the nanowire axis, 3.3 ( 0.1 Å, is in good agreement with the 3.36 Å spacing of (002) planes in bulk CdS. This observation directly confirmed that the produced CdS nanoparticles are wirelike structures with [001] orientation, which is consistent with the results of XRD analysis. The selected area diffraction pattern (SAED) for the nanowire is shown as the inset in Figure 3 for the sample obtained at 100 °C, demonstrating that the CdS nanocrystallites are polycrystalline with a wurtzite structure; the diffraction rings correspond to 100, 002, 101, 102, 110, 103, and 102 planes. FTIR Transmission Spectra. FTIR transmittance spectra for the dried pure PS sample, the PS/CdS nanocomposite sample, and the pure CdS samples are shown in Figure 4. Parts a and b of Figure 4 display the same features, and the CdS phase does not contribute absorption in this range as shown in Figure 4c. Figure 4a shows the spectrum of the pure PS sample obtained in the

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Figure 3. High-resolution TEM image and electron diffraction pattern (inset) for the PS/CdS nanocomposites prepared by the present route at 100 °C for 12 h (Cd2+ ) 5.0 × 10-4 mol, Cd2+/Tu ) 1:2, St ) 0.025 mol, AIBN ) 0.014 g), indicating a typical CdS nanowire with a diameter of ca. 13 nm and well-resolved (002) lattice planes. The insetted SAED pattern shows the CdS nanowires were of a polycrystalline nature and indexed to the wurtzite structure.

Figure 4. FTIR transmittance spectra for the pure PS sample (a), the PS/CdS nanocomposite sample (b), and the pure CdS samples (c) by the present route. The synthetic conditions are (a) 140 °C, 4 h, St ) 0.025 mol, AIBN ) 0.014 g; (b) 120 °C, 12 h, Cd2+ ) 5.0 × 10-4 mol, Cd2+/Tu ) 1:2, St ) 0.025 mol, AIBN ) 0.014 g; (c) 140 °C, 12 h, Cd2+ ) 1.0 × 10-3 mol, Cd2+/Tu ) 1:2.

absence of Cd2+ and a sulfur source by the present route, which is typical for PS. Figure 4a clearly revealed the C-H stretching vibration bands of the phenyl ring at 3083, 3063, 3027, and 3000 cm-1. The bands at 2922 and 2850

cm-1 are due to the methyl C-H stretching vibration. The bands at 1600, 1582, 1492, and 1450 cm-1 were the stretching vibrations from the CdC bond in the phenyl ring. The bands at 900-650 cm-1 were the out-of-plane mode of the C-H and CdC bend. The band at 699 cm-1 was the CdC bend in the phenyl ring. It has been reported that the out-of-plane band at 600-500 cm-1 was very sensitive to the conformation of the aliphatic chain. The general nature of the transmittance spectra for the PS and the PS/CdS nanocomposites in this range is the presence of an obvious band at 540 cm-1 and a wider band at 570-550 cm-1. According to the literature,39-41 it is possible to conclude that the wavenumber of the vibration is stable at 540 cm-1 when a sequence of four or more trans-conformations of carbon-carbon bonds occurs. When gauche-conformations are present, the vibration of phenyl rings in the neighborhood of the gauche-links is shifted toward higher frequencies and observed around 550 cm-1. In our case, there is a clear shoulder near 550 cm-1 in both the pure PS sample and the PS/CdS samples as indicated by the arrow in Figure 4a,b, which we attributed to the presence of the gauche-conformation in the PS chain. (39) Jasse, B.; Lety, A.; Monnerie, L. J. Mol. Struct. 1973, 18, 413. (40) Jasse, B.; Monnerie, L. J. Mol. Struct. 1977, 39, 165. (41) Jasse, B.; Monnerie, L. J. Phys. D: Appl. Phys. 1975, 8, 863.

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Figure 5. Room-temperature UV-visible absorption spectra of the pure PS and the PS/CdS nanocomposites prepared at different temperatures by the present route. (a) PS obtained at 90 °C for 12 h and (b)-(e) the PS/CdS obtained at different temperatures for 12 h (Cd2+ ) 5.0 × 10-4 mol, Cd2+/Tu ) 1:2, St ) 0.025 mol, AIBN ) 0.014 g): (b) 90 °C, (c) 100 °C, (d) 120 °C, and (e) 140 °C.

UV-Vis Absorption Spectroscopy and Fluorescence Spectra. The optical properties of the PS/CdS nanowire nanocomposites are both interesting and remarkable. UV-vis absorption and fluorescence spectra of commercial PS are very complex, and their interpretation has attracted some controversy.42 Figure 5a showed that the pure PS, produced in the absence of Cd2+ and thiourea by the present method, absorbs light at a wavelength range of 200-300 nm and has a weak absorbance tail from 300 to 400 nm. The stronger absorption in the range of 200-300 nm is due to the π-π* transition of the phenyl group, and the weaker absorption tail from 300 to 400 nm is attributed to π-π* interband transition of the polymer main chain. The absorption peak at 269 nm is attributed to the alkylsubstituted phenyl group of PS, and the absorption shoulder at 290 nm is due to the phenyl group absorption.43 UV absorption spectra of the commercial PS and the laboratory products showed that the phenyl groups of PS do not contribute absorbance at wavelengths above 300 nm.43 In addition, it has been reported that residues of initiators, head to head links, and oligomers, the main impurities of styrene, should not contribute to absorbance above 290 nm also. Klo¨pffer suggested that some chromophores (R ) phenyl) were present as in-chain impurities in PS, such as -HCdCHR, -H2C-CRdCR-CH2-, -H2C-CRdCR-CHdCR-CH2-, and -H2C-CdOR, which could account for the UV absorbance at 290 nm and above.43 The weak oxygen complexes of benzene derivatives have also been suggested to be partly responsible for the weak PS absorption tail above 290 nm.43-45 UV-vis absorption spectra for the PS/CdS nanocomposites obtained at different temperatures are shown in parts b and e of Figure 5, which display the same general features. All the spectra are simply the sum of the absorption spectra of the constituent parts of the composite powders, that is, PS and CdS phases. We believe that the main absorption characteristics beyond 300 nm should be attributed to the CdS nanocrystals. To determine the band gap, we have fitted the absorption data to the direct transition eq 4 by extrapolating the linear portions of the curves to absorption equal to zero.46

R hν ) A(hν - Eg)1/2

(4)

where R is the absorption coefficient, hν is the photo energy,

Eg is the direct band gap, and A is a constant. The obtained band gap for the pure PS in Figure 5a is about 4.1 eV. The band gaps for the PS/CdS nanocomposites in Figure 5b-e are 2.72, 2.67, 2.53, and 2.46 eV, respectively. The absorption edges for the PS/CdS samples obtained at different temperatures showed a clear systematic blue shift compared with that of bulk CdS, 512 nm (2.42 eV): (b) 90 °C, 456 nm; (c) 100 °C, 464 nm; (d) 120 °C, 490 nm; (e) 140 °C, 504 nm. The most remarkable feature for as-prepared PS/CdS nanowire nanocomposites is that the second well-defined peak at 372 nm (3.33 eV) appears for the nanocomposite samples obtained at 90-120 °C. It was reported that only the presence of a narrow cluster size distribution allows the observation of this spectral feature.47-50 We attribute it to the presence of a second excitonic peak of CdS nanowires in the nanocomposites. These observations are sometimes taken to suggest a highly monodispersed sample of quantum dots in which evidence for excitonic recombination is observed for the first (close to band edge) and second higher energy excitons in relatively strongly quantized CdS clusters.47-50 Another feature is that the absorption intensity increased with increasing reaction temperature. Both the particle sizes and the relative CdS content in the nanocomposites are increasing with the increasing temperature, which will contribute to the increasing of absorption intensity and the shift of the absorption edge to low energy. The fluorescence properties of the nanocomposites are found to be sensitive to synthetic conditions such as reaction temperature, reaction time, and the concentration of Cd2+ or the monomer. The PS/CdS nanocomposites prepared by the present method have more interesting optical features than those obtained from other methods. The excitation spectrum for the pure polystyrene obtained at 90 °C for 12 h displays a structureless characteristic as shown in Figure 6a. However, the excitation spectra in Figure 6b,e for the PS/CdS nanocomposite samples produced at different temperatures show a similar pronounced peak at about 340 nm. The fluorescence spectra using an excitation wavelength of 340 nm are shown in Figure 7. The fluorescence spectrum in Figure 7a for the pure PS obtained at 90 °C consists of a broad band with two broad peaks centered at 411 and 491 nm in the range of 350-580 nm. Further study shows that the emission range and peak positions of the PS samples at maximum depend on the excitation wavelength. This observation is consistent with that reported in the literature.43 It is believed that the blue emission at about 410 nm is caused by strongly fluorescent impurities, present at low concentration and forming part of the macromolecules.43 Fluorescence in the blue region may be due to chromophores formed by conjugated double bonds and phenyl groups.43 In fact, the chromophores have been found to be present in all commercial samples at (42) Beddard, G. S.; Allen, N. S. Emission Spectroscopy. Comprehensive polymer science: The synthesis, characterization, reactions and applications of polymers; Booth, C., Price, C., Eds.; Pergamon Press: 1989; pp 449-516. (43) Klo¨pffer, W. Eur. Polym. J. 1975, 11, 203. (44) Chien, J. C. W. J. Phys. Chem. 1965, 69, 4317. (45) Nowakowska, N.; Najbar, J.; Waligora, B. Eur. Polym. J. 1976, 12, 387. (46) Wang, Y.; Suna, A.; Mahler, W.; Kasowski, R. J. Chem. Phys. 1987, 87, 7315. (47) Modes, S.; Lianos, P. J. Phys. Chem. 1989, 93, 5834. (48) Brus, L. E.; Rosetti, R.; Nakahara, S. J. Chem. Phys. 1983, 79, 1086. (49) Wang, Y.; Suna, A.; McHugh, J.; Hilinski, E. F.; Lucas, P. A.; Johnson, R. J. Chem. Phys. 1990, 92, 6927. (50) Spanhel, S.; Anderson, M. A. J. Am. Chem. Soc. 1993, 112, 2278.

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Figure 6. Room-temperature excitation spectra of the pure PS and the PS/CdS nanocomposites prepared at different temperatures by the present route. (a) PS obtained at 90 °C for 12 h and (b)-(e) the PS/CdS obtained at different temperatures for 12 h (Cd2+ ) 5.0 × 10-4 mol, Cd2+/Tu ) 1:2, St ) 0.025 mol, AIBN ) 0.014 g): (b) 90 °C, (c) 100 °C, (d) 120 °C, and (e) 140 °C. The monitoring emission wavelength is 440 nm.

Figure 7. Room-temperature fluorescence spectra of the pure PS and the PS/CdS nanocomposites prepared at different temperatures by the present route. (a) PS obtained at 90 °C for 12 h and (b)-(e) the PS/CdS obtained at different temperatures for 12 h (Cd2+ ) 5.0 × 10-4 mol, Cd2+/Tu ) 1:2, St ) 0.025 mol, AIBN ) 0.014 g): (b) 90 °C, (c) 100 °C, (d) 120 °C, and (e) 140 °C. The excitation wavelength was 340 nm.

smaller concentrations or in the laboratory. Rather extended conjugated π-electron systems or large Stokes shifts would be required to account for the long wavelength fluorescence bands. Fluorescence emissions near 400 nm and strong Stokes shifts have been reported for cis-stilbene and similar compounds as solids and in solid solutions.51 Furthermore, recent reports about the spectroscopic data of various polymers based on stilbene units have demonstrated that both broad structureless red-shifted emission52 and red-shifted structured fluorescence were observed.53 In contrast, the emission spectra for the PS/CdS nanocomposites show more interesting features than that of pure PS. A stronger and narrower blue emission band is centered at 400 nm in the range of 350-490 nm for the PS/CdS nanocomposite sample obtained under the same (51) Fischer, G.; Fischer, E.; Stegemeyer, H. Ber. Bunsen-Ges. Phys. Chem. 1973, 77, 685.

Yu et al.

conditions as that for the pure PS. The main emission peak for the nanocomposites has a blue shift of about 11 nm compared with that of the pure PS. Similar features were observed in the samples obtained at higher temperatures (100-140 °C), except for the weaker but wider emission peak at different positions in the range of 450570 nm. The most striking feature is that the emission intensity for the PS/CdS nanocomposites is stronger and the emission range is narrower than that for the pure PS samples as shown in Figure 7. As shown in Figure 7b, only one well-defined emission peak at 400 nm was observed for the sample obtained at 90 °C; however, obvious emission peaks or shoulders at longer wavelengths and their red shift were observed for the samples obtained at 100, 120, and 140 °C, which were located at 447, 514, and 527 nm, respectively, as shown in Figure 7c-e. We believed that this red-shift characteristic would be related to the increasing of the particle sizes of embedded CdS nanowires, which is consistent with the results from UVvis absorption spectra. In fact, a very weak but measurable green emission band centered at about 530 nm was observed for the pure CdS nanowires obtained at 120 °C in the absence of styrene monomer by the present route. However, this assignment needs to be confirmed further because these weaker emission bands may also be the emission residues of the incomplete quenching of the PS fluorescence. As shown in Figures 6 and 7, the samples obtained at lower temperatures (90-100 °C) show stronger excitation and fluorescence peaks than those obtained at higher temperatures (120-140 °C). Our observations confirmed that phase segregation between the CdS phase and the PS phase occurred if the reaction temperature was higher than 100 °C and reaction time was longer than 6 h, because of the increasing concentration of CdS nanocrystals in the composites and van der Waals interaction between the nanocrystals. TEM images also show that the CdS nanowires tend to aggregate together and phase segregation occurs while the reaction temperature increases or reaction time is prolonged. The aggregation of the nanocrystals and the phase segregation will result in the incomplete emission quenching of the composites as shown in Figure 7, which is similar to that reported by Greenham et al.31 Although a higher concentration of CdS nanocrystals is present in the composites obtained at higher temperatures (120-140 °C), both the blue emission and green emission were still not completely quenched. Similarly, the initial Cd2+ concentration and reaction time also have a significant influence on the optical properties of the nanocomposites. The strong emission quenching effect was also observed if the initial Cd2+ concentration increased or the reaction time was prolonged. With an increase of the initial concentration of Cd2+, both the blue emission and green emission were much quenched. In addition, the weaker blue-green emission band for the samples obtained at 100 °C blueshifted from 481 to 475 nm when the initial Cd2+ concentration increased from 5.0 × 10-4 to 1.0 × 10-3 mol. Similarly, the weaker green band for the samples obtained (52) Catala´n, J.; Zima´nyi, L.; Saltiel, J. J. Am. Chem. Soc. 2000, 122, 2377. Brocklehurst, B.; Bull, D.; Evans, M.; Scott, P.; Stanney, G. J. Am. Chem. Soc. 1975, 97, 2977. Anger, I.; Sandros, K.; Sundahl, M.; Wennestrom, O. J. Phys. Chem. 1993, 97, 1929. Letsinger, R. L.; Wu, T. J. Am. Chem. Soc. 1994, 116, 811. (53) Oldham, W. J., Jr.; Miao, Y.-J.; Lachicotte, R. J.; Bazan, G. C. J. Am. Chem. Soc. 1998, 120, 419. Bazan, G. C.; Oldham, W. J., Jr.; Lachicotte, R. J.; Tretiak, S.; Chemyak, V.; Mukamel, S. J. Am. Chem. Soc. 1994, 116, 811. Song, X.; Geiger, C.; Furman, I.; Whitten, D. G. J. Am. Chem. Soc. 1994, 116, 4103.

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at 120 °C blue-shifted from 516 to 489 nm when the initial Cd2+ concentration increased from 2.5 × 10-4 to 5.0 × 10-4 mol. The absorption edge for the nanocomposite obtained at 120 °C shifted from 468 nm (2.65 eV) to 490 nm (2.53 eV) when the reaction time was prolonged from 4 to 12 h. The strong fluorescence quenching of the blue emission and incomplete quenching of the green emission also occurred when the reaction time was prolonged. The interesting emission quenching effect observed in the present samples needs to be investigated further.

posites were sensitive to synthetic conditions such as temperature, reaction time, and the concentration of Cd2+ or the monomer. Especially, as-prepared hybrid nanocomposites display intense bright blue emissions and weaker but tunable green emissions by controlling the reaction conditions. Interestingly, a strong emission quenching effect was observed for this novel hybrid material. These interesting features suggest that the produced PS/CdS nanocomposites may find applications in the fabrication of novel optical and electronic devices.

Summary and Conclusions

Acknowledgment. This work has been supported by the Research for the Future Program of the Japan Society for Promotion of Science (JSPS) (JSPS-RFTF-96R06901). We thank Professor Masato Kakihana of the Tokyo Institute of Technology for use of the UV-vis spectrometer and PL measurements.

We demonstrated a novel one-step soft solutionprocessing (SSP) route called the solvothermal-copolymerization technique for in situ fabrication of polystyrene(PS)/CdS nanocomposites embedded with CdS nanowires at lower temperatures (80-140 °C) in ethylenediamine media. In this route, the polymerization of the monomers and the formation of the CdS nanocrystallites could occur simultaneously in a certain temperature range. Both the temperatures and the solvent were found to play a key role in the synthesis of the nanocomposites. The spectroscopic properties of the PS/CdS nanowire nanocom-

Note Added after ASAP Posting. This article was released ASAP on 1/9/2001 with changes in the author byline and in the acknowledgment. The correct version was posted on 2/16/2001. LA000941P