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Study of Poly(N,N-dimethylacrylamide)/CdS Nanocomposite Organic/Inorganic Gels Vlasoula Bekiari,† Konstantinos Pagonis,‡ Georgios Bokias,‡ and Panagiotis Lianos*,† Engineering Science Department, University of Patras, GR-26500 Patras, Greece, and Department of Chemistry, University of Patras, GR-26500 Patras, Greece Received May 7, 2004 CdS nanoparticles have been synthesized and stabilized in poly(N,N-dimethylacrylamide) hydrogels. The properties of the composite material have been characterized by UV-vis spectroscopy, scanning electron microscopy, X-ray diffraction, thermogravimetric analysis, and steady-state and time-resolved luminescence spectroscopy. This material can be obtained in three different states: swollen, shrunk, and freeze-dried. The swollen and the freeze-dried states correspond to a nanocomposite organic/inorganic (wet or dry) gel containing CdS nanoparticles of ∼50 nm diameter while the shrunk state is a two-phase system containing CdS crystals, which precipitate forming interesting geometrical shapes.
Introduction Synthesis of semiconductor nanoparticles in geometrically restricted environments has been a popular and wellstudied procedure. Several types of restricted geometries have been used, among them surfactant reverse micelles and microemulsions,1-5 block copolymer micelles,6-9 organic or composite organic-inorganic matrices,10-19 etc. Recent works study hydrogels as nanoreactors for producing semiconductor or metal nanoparticles20-24 since †
Engineering Science Department. Department of Chemistry. * To whom correspondence should be addressed: Tel 30 2610 997587, Fax 30 2610 997803, e-mail
[email protected]. ‡
(1) Lianos, P.; Thomas, J. K. J. Colloid Interface Sci. 1987, 117, 505. (2) Modes, S.; Lianos, P. J. Phys. Chem. 1989, 93, 5854. (3) Simmons, B. A.; Sichu, L.; Vijay, J. T.; McPherson, G. L.; Bose, A.; Zhou, W.; He, J. Nano Lett. 2002, 2, 263. (4) Pinna, N.; Weiss, K.; Urban, J.; Pileni, M.-P. Adv. Mater. 2001, 13, 261. (5) Agostiano, A.; Catalano, M.; Curri, M. L.; Della Monica, M.; Manna, L.; Vasanelli, L. Micron 2000, 31, 253. (6) Zhao, H.; Douglas, E. P.; Harrison, B. S.; Schanze, K. S. Langmuir 2001, 17, 8428. (7) Yeh, S.-W.; Wei, K.-H.; Sun, Y.-S.; Jeng, U.-S.; Liang, K. S. Macromolecules 2003, 36, 7903. (8) Yang, C.-H.; Awschalom, D. D.; Stucky, G. D. Chem. Mater. 2002, 14, 1277. (9) Kane, R. S.; Cohen, R. E.; Silbey, R. Chem. Mater. 1999, 11, 90. (10) Gerion, D.; Pinaud, F.; Williams, S. C.; Parak, W. J.; Zanchet, D.; Weiis, S.; Alivisatos, P. A. J. Phys. Chem. B 2001, 105, 8861. (11) Lee, J.; Sundar, V. C.; Heine, J. R.; Bawendi, M. G.; Jensen, K. F. Adv. Mater. 2000, 12, 1102. (12) Huang, J.; Lianos, P.; Yang, Y.; Shen, J. Langmuir 1998, 14, 4342. (13) Farmer, S. C.; Patten, T. E. Chem. Mater. 2001, 13, 3920. (14) Monteiro, O. C.; Esteves, A. C. C.; Trindade, T. Chem. Mater. 2002, 14, 2900. (15) Bhattacharjee, B.; Ganguli, D.; Chaudhuri, S. J. Fluoresc. 2002, 12, 369. (16) Zelner, M.; Reisfeild, R.; Cohen, H.; Tenne, R. Chem. Mater. 1997, 9, 2541. (17) Lakowicz, J. R.; Gryczynski, I.; Gryczynski, Z.; Murphy, C. J. J. Phys. Chem. B 1999, 103, 7613. (18) Bekiari, V.; Lianos, P. Langmuir 2000, 16, 3561. (19) Zhang, J.; Coombs, N.; Kumacheva, E.; Lin, Y.; Sargent, E. H. Adv. Mater. 2002, 14, 1756. (20) Pardo-Yissar, V.; Bourenko, T.; Wasserman, J.; Willner, I. Adv. Mater. 2002, 14, 670. (21) Gattas-Asfura K. M.; Zheng, Y.; Micic, M.; Snedaker, M. J.; Ji, X.; Sui, G.; Orbulescu, J.; Andreopoulos, F. M.; Pham, S. M.; Wang, C.; Leblanc, R. M. J. Phys. Chem. B 2003, 107, 10464. (22) Pardo-Yissar, V.; Gabai, R.; Shipway, A. N.; Bourenko, T.; Willner, I. Adv. Mater. 2001, 13, 1320.
new interesting applications are envisaged by employing these nanocomposite materials. The properties of such materials are determined by the swollen/shrunk state of the hydrogel. By choosing the suitable polymer, this swollen/shrunk state can be effectively controlled by various external stimuli, like temperature,24 electric field,25 pH,26 solvent,27 etc. The incorporation of nanoparticles into the hydrogel is usually based on the “breathing in” technique, where the dry hydrogel is swollen with an aqueous solution containing the already preformed nanoparticles.20,22 An alternative approach is the in situ formation of the nanoparticles from the appropriate ionic precursors. This method has been successfully employed to form Ag or CdS nanoparticles on the surface of poly(methyl methacrylate-comethacrylic acid) latexes19 or in the interior of poly(Nisopropylacrylamide-co-acrylic acid) microgels.28 Because of the acrylate anions, these gels are negatively charged, and the introduction of the precursors of the nanoparticles is performed via ion exchange of the latex or microgel counterions with Ag+ or Cd2+. Furthermore, the formation of the nanoparticles is localized around the acrylate anions. In the present work, we apply the second procedure for the in situ preparation of CdS nanoparticles in a nonionic hydrogel, based on poly(N,N-dimethylacrylamide), PDMAM. This is a hydrophilic, nonionic, synthetic polymer proposed for a variety of applications, including two-phase catalysts,29 hydrogels for drug-delivery purposes,30 or polymer supports for protein synthesis.31 Contrary to the aforementioned negatively charged mi(23) Jones, C. D.; Serpe, M. J.; Schroeder, L.; Lyon, A. J. Am. Chem. Soc. 2003, 125, 5292. (24) Xulu, P. M.; Filipsei, G.; Zrinyl, M. Macromolecules 2000, 33, 1716. (25) Tanaka, T.; Nishio, I.; Shun, S.-T.; Ueno-Nishio, S. Science 1982, 218, 467. (26) Tanaka, T.; Fillmore, D.; Sun, S.-T.; Nishio, I. Phys. Rev. Lett. 1980, 45, 1636. (27) Pagonis, K.; Bokias, G. Polymer 2004, 45, 2149. (28) Xu, S.; Zhang, J.; Paquet, S.; Lin, Y.; Kumacheva, E. Adv. Funct. Mater. 2003, 13, 468. (29) Kondo, S.; Nakashima, N.; Hado, H.; Tsuoa, K. J. Polym. Sci., Polym. Chem. 1990, 28, 2229. (30) Yeh, P. H.; Kopeckova, P.; Kopecek, J. J. Polym. Sci., Polym. Chem. 1994, 32, 1627. (31) Atherton, E.; Clive, D. L. J.; Sheppard, R. C. J. Am. Chem. Soc. 1975, 97, 6584.
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crogels, the distribution of CdS nanoparticles in the PDMAM hydrogel is expected to be homogeneous throughout the whole hydrogel volume. The synthesis and characterization of these hybrid materials are presented here. Our attention is especially focused toward the fate of the semiconductor nanoparticles as a function of the degree of swelling of the hydrogel. This will directly reflect on the luminescent properties of CdS nanoparticles. CdS nanoparticles associated with a hydrogel is an important luminescent material. This property can be employed to probe the structure of the nanocomposite material, and it can serve as a luminescent probe of other macromolecules, particularly those of biological importance.21 Materials and Methods All reagents were from Aldrich except ammonium persulfate (Serva). High-purity Millipore water was used in all experiments. Poly(N,N-dimethylacrylamide) (PDMAM) was synthesized by the following procedure: 5.0 g of N,N-dimethylacrylamide and 0.25 g (2 mol % over the monomer) of methylene bis(acrylamide) (cross-linker) were dissolved in 50 mL of water under stirring at room temperature. The solution was deoxygenated, and then 0.1 g of ammonium persulfate and 0.1 g of N,N,N′,N′-tetramethylenediamine were added. The formation of the gel was almost immediate. The reaction was left to proceed for 24 h, and then the gel was immersed in pure water for 1 week. Water was renewed daily. Finally, the swollen gel was cut into 2 g pieces and subjected to freeze-drying. Hydrogels with incorporated CdS nanocrystallites were synthesized as follows: 0.1 g of dry polymer was immersed into a beaker containing 10 mL of a 5 mM aqueous solution of Cd(NO3)2‚5H2O. The gel was left in solution for more than 3 days, allowing sufficient time to reach equilibrium swelling. Then it was copiously washed with pure water. Finally, it was immersed into a 10 mL aqueous solution of 5 mM Na2S‚9H2O, and it was left there for 3 days, even though shorter time (∼1 day) suffices to complete interaction. CdS was then formed, and this was obvious by coloring, which spread all over the hydrogel (not the solution). The thus-prepared materials consist of swollen hydrogel containing CdS nanocrystallites as will be discussed below. These hydrogels were dried either by exposure to ambient conditions for 25 days or by freeze-drying at liquid nitrogen temperature. Freeze-drying preserves the initial volume of the gel, while ambient drying is accompanied by shrinkage. Dried gel loses about 90% of its original water content. The results are discussed in terms of the swelling ratio, S, defined as
S ) W/W0
(1)
where W0 and W are the weights of the dry polymer and the swollen gel, respectively. Fluorescence probes and quenchers were introduced at premeasured concentrations according to the following procedures: tris(2,2′-bipyridine)ruthenous trichloride hexahydrate, [Ru(bpy)32+], and methyl viologen (MV2+) were solubilized in the original aqueous solution used for the swelling of the gel. Pyrene was first dissolved in ethanol, and the dry gel was let to swell with this solution for 2 days. Then it was taken out, washed with ethanol, and dried by exposure to ambient conditions. Then, the gel with incorporated pyrene was immersed in pure water, and the system was treated in the same way as for the samples containing the hydrophilic molecules. UV-vis absorption measurements were made with a Cary 1E spectrophotometer and steady-state fluorescence measurements with a spectrofluorometer that consists of ORIEL components in a standard configuration: 150 W xenon lamp, excitation monochromator (25 cm), thermostated sample holder, emission monochromator (25 cm) at right angle, and computer-driven detection system. Spectra were corrected for both lamp and photomultiplier spectral response profile. Time-resolved luminescence decay profiles were recorded with the single-photoncounting technique using an IBH nanosecond hydrogen flash lamp and ORTEC electronics. X-ray diffraction measurements
Figure 1. Absorption spectrum of a swollen (S ) 20) (1) and a shrunk (S ) 1.5) (2) CdS/PDMAM system. were made with a Philips PW 1840 diffractometer. SEM images were obtained with a JEOL JSM 6300 microscope equipped with an EDS detector for elemental analysis (Oxford). TGA measurements were performed at a DuPont 990 thermal analyzer coupled to a DuPont 951 TGA accessory. The heating rate was 20 °C/min. Fluorescence decay profiles were analyzed by a model of stretched exponentials that gives the value of the noninteger exponent f, which is a measure of restrictions imposed by hosting material on probe mobility:32
I(t) ) I0 exp(-t/τ0) exp[-C1(t/τ0)f + C2(t/τ0)2f]
(2)
where 0 < f < 1, C1 and C2 are fitted parameters, and τ0 is the decay time of the luminophore in the absence of quencher. f is larger in less restricted domains, and it theoretically takes values g0.67 at and above the percolation threshold.32
Results and Discussion As described in the previous section, the polymer was first immersed in an aqueous solution of Cd(NO3)2 where it adsorbs cadmium ions during swelling. Then the swollen polymer was taken out of the cadmium-containing solution, and it was immersed in the Na2S solution. The interaction between the Cd2+ and S2- was immediate, and it was obvious by the appearance of a yellow color, which is associated only with the hydrogel and does not extend over the solution. Indeed, when the gel was pulled out of the solution, the solution contained no CdS traces, at least to a quantity detectable by spectroscopy. UV-vis absorption spectra of both swollen and shrunk gels confirmed formation of CdS, as can be seen in Figure 1. Curve 1 shows the absorption spectrum right after the hydrogel was taken out of the solution and washed (S ) 20). Curve 2 shows absorption after 25 days slow drying and 90% weight loss (S ) 1.5). Weight (i.e., water) loss in this case is accompanied by shrinkage, i.e., volume decrease; therefore, CdS was then more concentrated and optical density increased. Curve 2 was recorded with a lot of background noise since the geometry of the dry gel is not convenient for UV-vis absorption spectroscopy. It does, nevertheless, show that shrunk gels contain large CdS particles, since absorption abruptly increases at the absorption onset at ∼520 nm. Indeed, SEM image of the sample with S ) 1.5, presented in Figure 2a, revealed that drying resulted in precipitation of CdS crystals with very interesting shapes. The precipitation of large CdS crystals during gel shrinking can be avoided by keeping (32) Lianos, P. J. Fluoresc. 2004, 14, 11. (b) Lianos, P. Hetercycl. Chem. Rev. 1996, 3, 53.
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Figure 3. XRD spectrum of freeze-dried CdS/PDMAM.
Figure 2. Scanning electron microscope images of the CdS/ PDMAM system: (A) shrunk gel (S ) 1.5); (b) freeze-dried gel. The scale bars correspond to 20 µm (A) and 200 nm (B).
the hydrogel at liquid nitrogen temperature by performing freeze-drying. Freeze-drying leads to a nanocomposite inorganic/organic CdS/PDMAM dry material whose SEM image is shown in Figure 2b. Nanocrystals with small dimensions (∼50 nm) are clearly observed. However, clustering of these nanocrystals to larger structures is also evidenced in Figure 2b. The nature of the crystals was verified both by in situ elemental analysis and by separate XRD measurements. XRD analysis of the freezedried swollen CdS/PDMAM sample gave the spectrum of Figure 3, which contains CdS peaks. By employing the Sherrer formula,33 the size of the nanocrystals was found around 50 nm, in perfect agreement with the observations of Figure 2a. These results are qualitatively in agreement with the pore size of the hydrogel under high swelling conditions. TGA study of pure PDMAM and CdS/PDMAM revealed the behavior seen in Figure 4. It is noted that when freezedried, CdS/PDMAM preserves the behavior of the pure polymer, but slowly dried CdS/PDMAM demonstrates important deviation from pure polymer behavior apparently because in the case of slow drying a two-phase material is created while in the case of freeze-drying a nanocomposite material is created with similar behavior as the pure polymer. Excitation of the CdS/PDMAM at various swelling ratios resulted in relatively strong luminescence emission. (33) Bouras, P.; Stathatos, Lianos, P.; Tsakiroglou, C. Appl. Catal., B 2004, 51, 273.
Figure 4. TGA data on CdS/PDMAM system: (1) pure polymer without CdS, freeze-dried; (2) in the presence of CdS, freezedried; and (3) in the presence of CdS, slow ambient drying (S ) 1.5).
Figure 5. Luminescence spectra of the CdS/PDMAM system: (1) swollen gel (S ) 20); (2) slow drying, after 75% weight loss, (S ) 7); and (3) slow dried final gel (S ) 1.5).
Representative spectra are presented in Figure 5. The luminescence maximum was at 450 nm for the highly swollen sample, S ) 20, and 650 nm for the most shrunk sample, S ) 1.5. The freeze-dried highly swollen gel emitted at short wavelength as the corresponding wet
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Table 1. Variation of the Exponent f of Eq 1 during Drying of PDMAM Hydrogels state of hydrogel
f value case of Ru(bpy)32+-MV2+
f value case of pyrene
wet gel, S ) 20 after 75% weight loss, S ) 7 after 90% weight loss, S ) 1.5 after freeze-drying
0.74 0.50 0.37 0.44
0.39 0.45 0.60 0.41
gel. Obviously, short and long wavelength emission can be associated with small and large CdS crystals. Curve 1 shows that wet gels do contain some large particles, but in slowly dried gels emission is produced only by grown species. The variation of the size during the drying period reflects on Curve 3 where emission from both small and large particles is simultaneously registered for the sample with an intermediate swelling ratio, S ) 7. It is known that small CdS particles emitting at short wavelength have a shorter excited-state lifetime.34 This fact was also verified in the present case. Indeed, the decay time of luminescence monitored at 450 nm was only 15 ns, but it was 560 ns when monitored at 650 nm. The CdS/PDMAM hydrogel then makes an interesting photophysical system. A popular indirect method to characterize a gel is by employing fluorescence probing.32a Thus, PDMAM hydrogels have been characterized by both steady-state and time-resolved fluorescence quenching techniques. The luminescence decay profiles were analyzed by the model of stretched exponentials given by eq 1. First, the gel was studied by incorporating the hydrophilic luminophore-quencher pair Ru(bpy)32+-MV2+, which probes hydrophilic domains.35 The noninteger exponent f was calculated, and its values are tabulated in Table 1. f evolved with decreasing S. For S ) 20, f was large, obtaining a value above the percolation threshold (i.e., f > 0.67). This was expected since these gels retain a lot of water. As water evaporated (S ) 1.5), f dropped, and when the gel lost most of water, f took a very low value, revealing that the hydrophilic domain became then restricted and compartmentalized far below percolation threshold. This holds true also for freeze-dried samples where f value was practically equivalent. The same gels were also studied by employing pyrene, a hydrophobic probe, where quenching is obtained by excimer formation.35 Pyrene behavior, as revealed by the corresponding f values (Table 1), showed that the hydrophobic domain of the gel was very restricted in swollen gels but substantially expanded in more shrunk gels. On the contrary, freeze-dried gels had equally restricted hydrophobic domains as wet gels, indicating that the polymer is not structurally modified when freeze-dried. The extent of excimer pyrene formation can be represented by the (34) Fojtik, A.; Weller, H.; Koch, V.; Henglein, A. Ber. Bunsen-Ges. Phys. Chem. 1984, 88, 969. (35) Bekiari, V.; Ferrer, M.-L.; Lianos, P. J. Phys. Chem. B 1999, 103, 9085.
Table 2. Variation of Pyrene Excimer Formation during Drying of PDMAM Hydrogels state of hydrogel
value of IE/IM
wet gel, S ) 20 after 75% weight loss, S ) 7 after 90% weight loss, S ) 1,5 after freeze-drying
1.91 0.74 0.31 0.71
ratio IE/IM of the maximum intensities of excimer and monomer emission. The higher the ratio, the larger the extent of excimer formation. As seen in Table 2, excimer formation was extensive in swollen gels, but it was very limited in shrunk gels and less limited in freeze-dried gels. In combination with the corresponding f values, this means that in swollen gels, where the hydrophobic domain is very restricted, there is a high probability for an excited pyrene molecule to find itself together with a nonexcited molecule and form an excimer. When the gel is slowly dried and shrunk, the hydrophobic domain becomes less restricted so that pyrene molecules are more dispersed, and the probability of excimer formation becomes lower. These results, albeit indirect, depict a clear picture of the structural variations of the polymer and the effect these variations have on CdS particles. CdS is expected to be accommodated in the hydrophilic domain. As the polymer is slowly dried, the hydrogel shrinks and the hydrophobic domain expands at the expense of the hydrophilic domain. The originally small and well-dispersed CdS nanocrystals are forced to come close together, and this encourages them to grow. When, however, drying is made by the freeze-drying method, the polymer suffers only small, if any, structural variations. For this reason no shrinkage is then observed. Consequently, CdS nanoparticles remain dispersed and associated with the polymer network generating a nanocomposite inorganic/organic material. This material is luminescent, mainly emitting at wavelengths below 500 nm. Conclusions The CdS/PDMAM system can be found at three principal states: the swollen hydrogel steadily retaining dispersed CdS nanocrystals, the freeze-dried gel preserving a structure similar to the swollen hydrogel, and the slowly dried gel which shrinks and gives a two-phase system involving large CdS crystals. Swollen and freeze-dried gel are nanocomposite materials, one containing water and the other dry but similar as far as CdS nanoparticles are concerned. CdS is luminescent with a standard behavior, i.e., short wavelength luminescence and small decay time for nanoparticles and long wavelength luminescence and long decay time for crystals. Acknowledgment. We are deeply indebted to Prof. P. Koutsoukos of the Chemical Engineering Department, University of Patras, Greece, for the XRD measurements. LA048866W
