Development and Optical Properties of Cadmium Sulfide and

Development and Optical Properties of Cadmium Sulfide and Cadmium Selenide Nanoparticles in Amphiphilic Block Copolymer Micellar-like Aggregates...
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J. Phys. Chem. C 2007, 111, 15201-15209

15201

Development and Optical Properties of Cadmium Sulfide and Cadmium Selenide Nanoparticles in Amphiphilic Block Copolymer Micellar-like Aggregates K. D. Gatsouli, S. Pispas,* and E. I. Kamitsos Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation, 48 Vassileos Constantinou AVenue, 11635 Athens, Greece ReceiVed: March 1, 2007; In Final Form: July 25, 2007

CdS and CdSe nanoparticles were prepared in organic solvents using as templates inverse amphiphilic block copolymer micellar aggregates, which were induced by cadmium complexation. Amphiphilic block copolymers utilized in this work include poly(sulfonated styrene-block-tert-butylstyrene) (SPS-PtBS) and poly(styreneblock-sulfonated isoprene) (PS-SPI). Copolymers with different compositions and molecular weights were synthesized by anionic polymerization high-vacuum techniques and postpolymerization selective sulfonation reactions. Cadmium acetate was used as the cadmium precursor and thioacetamide or H2Se as the sulfur or selenium source, respectively. The nanostructured organic-inorganic hybrid materials were characterized in solution and in thin films by light scattering, UV-vis and fluorescence spectroscopy, infrared and Raman vibrational spectroscopy, and transmission electron microscopy (TEM). The influence of composition and molecular weight of block copolymers on the properties of nanoparticles was investigated at all stages of nanoparticle formation. It was shown that well-defined CdS and CdSe semiconductor nanoparticles with controllable sizes and optical properties can be formed by judicious choice of the experimental conditions following the suggested preparation protocol.

1. Introduction Today’s materials science focuses on nanostructures with characteristic dimensions between 1 and 100 nm, because of their potential applications including solar cells,1 LEDs,2 and biomedical labeling.3 To this end, inorganic nanoparticles have monopolized the interest of many research groups all over the world in recent years.4-6 The development of semiconductor nanoparticles attracts particular attention due to their sizedependent optical and electronic properties.5-7 Desired properties can be obtained by tuning the particle size, and this controls fundamental materials properties such as the band gap and the exciton energy of the system. Therefore, synthetic protocols that can control nanoparticle size are extensively investigated. Furthermore, spatial control of semiconductor nanoparticles placement in specific nanostructured morphologies may lead to many useful applications.8 In parallel, polymers are very often used as host matrices in structured hybrid materials and devices, taking advantage of their physical and mechanical properties and long-term stability.8-10 In particular, block copolymers self-organize under the appropriate compositions and conditions into various nanostructures, as a result of microphase separation between incompatible blocks.11 Due to that particular ability block copolymers have been utilized as templates for the preparation of various nanoparticles.12,13 So far, preformed block copolymer micelles in selective solvents14 and microphase separated solid films15 were utilized in nanoparticle templating protocols. On the other hand, anionic polymerization combined with selective postpolymerization functionalization reactions is a powerful tool for the controlled manipulation of block copolymer macromolecular architecture, and for the synthesis of a wide variety of * To whom correspondence should be addressed. Phone: +30-210 7273824. Fax: +30-210 7273794. E-mail: [email protected].

block copolymers, with a high degree of molecular and compositional homogeneity,11b,16 thus enabling the choice of the appropriate functional matrices. For these reasons, there is a continuous interest toward developing hybrid systems involving block copolymers and semiconductor nanoparticles and elucidating the dependence of nanoparticles’ size and properties on the chemical nature and nanostructure of block copolymers and their preparation protocols. We present here an alternative route for the in situ preparation of semiconductor clusters of CdS and CdSe in Cd2+-induced nanostructures of poly(sulfonated styrene-block-tert-butylstyrene) (SPS-PtBS) and poly(styrene-block-sulfonated isoprene) (PS-SPI) copolymers, which are soluble in nonselective solvents. The sulfonate groups act as binding sites for Cd2+ cations, and this induces micelle formation in solution17 through Cd2+/SO3- ion clustering involving many copolymer chains. Naturally, this process directs the localization of Cd2+/SO3- ion clusters in the sulfonated microphase in the solid state. Preliminary results on such systems were reported in a conference paper.18 In the previous work we were mainly concerned with SPS-PtBS copolymers and the effects of solvent thermodynamic quality toward the two blocks regarding the synthesis of semiconductor nanoparticles. The effect of thermal treatment during CdS formation was also investigated, and some comparisons with highly sulfonated PS homopolymer were made. It is further shown in the present systematic study that CdS and CdSe nanoparticle formation and properties can be controlled by the Cd2+ ion concentration, i.e., the Cd2+/SO3H ratio, and to a lesser extend by the size of the sulfonated block, by using in addition PS-SPI copolymers. This is evidenced by employing a range of physicochemical characterization techniques to probe all stages of the preparation process.

10.1021/jp071681o CCC: $37.00 © 2007 American Chemical Society Published on Web 10/03/2007

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TABLE 1: Molecular Characteristics of the Prepared Sulfonated Block Copolymers and Indicative Sizes of CdS Nanoparticles polymer

Mw

Mw/Mn

n/mb

SPS-PtBS-1 SPS-PtBS-3 SPS-PtBS-4 PS-SPI-1 PS-SPI-2

154 900 393 800 29 400 163 300 495 300

1.03 1.04 1.08 1.02 1.29

433/481 1538/756 26/154 173/1304 581/3841

a

b

SCHEME 1: Chemical Structures of the Prepared Block Copolymers

level of sulfonation composition (2RCdS) (mol %)c (nm)e (wt %)d 95 90 98 98 98

93 89

96

46% SPS 65% SPS 16% SPS 16% SPI 19% SPI

4.5 2.6 2.4 3.2 3.7

a

Calculated for the final copolymer based on the degree of sulfonation. b Values for the precursor block copolymers. c In respect to the styrene and isoprene segments of the PS and PI blocks of the precursors and from acidimetric titrations (first column) and elemental analysis (second column). d Based on the degree of sulfonation. e Indicative sizes of CdS nanoparticles formed in different copolymer samples with Cd/SO3 ) 1:2 as estimated from UV-vis spectroscopy.

2. Experimental Section 2.1. Materials. 2.1.1. Synthesis of Block Copolymers. A number of poly(styrene-b-tert-butylstyrene) (PS-PtBS) and poly(styrene-b-isoprene) (PS-PI) block copolymers, with different ratios of each block and overall molecular weights, were synthesized by anionic polymerization high-vacuum techniques.19 Purification of all reagents (solvents, monomers, etc.) was accomplished according to well-established procedures. The block copolymers were prepared by polymerizing first the styrene, using sec-butyllithium as initiator in benzene, followed by the addition of tert-butylstyrene or isoprene, accordingly, to the living polystyryllithium to give the second poly(tertbutylstyrene) (PtBS) or poly(isoprene) (PI) block. The polymerization was then terminated by the addition of degassed methanol to give the desired precursor block copolymers, which were subsequently isolated by precipitation in methanol and dried under vacuum before further use. The molecular weight and molecular weight distribution data for the precursor polymers were obtained by size exclusion chromatography, using a Waters system composed of a Waters 1515 isocratic pump, a set of three µ-Styragel mixed-bed columns, with a porosity range of 102 to 106 Å, and a Waters 2414 refractive index detector. The entire system was controlled through Breeze software. Tetrahydrofuran was used as the mobile phase at a flow rate of 1.0 mL/min at 40 °C. Composition of the copolymers and PI microstructure were determined by 1H NMR spectroscopy in CDCl3 at 30 °C, using a Bruker AC 300 instrument. The PI blocks had the expected high 1,4-microstructure. 2.1.2. Synthesis of Poly(sulfonated styrene-b-tert-butylstyrene) Copolymers. Conversion of the polystyrene block of the PSPtBS block copolymers to sulfonated polystyrene (SPS) was done via selective sulfonation of the styrene portion of the copolymers.20 The general procedure was to place 0.2 equiv of triethyl phosphate (TEP), dissolved in 1,2-dichloroethane (DCE) (2 g of TEP/100 mL of DCE), into a dry reactor fitted with a mechanical stirrer, two dropping funnels, and a thermometer. The solution was cooled to 0 °C, and 1.0 equiv of polymer dissolved in DCE (5 g of polymer/100 mL of DCE) was placed in one dropping funnel and 1.1 equiv of SO3 in DCE in the other dropping funnel. The SO3 solution, followed by polymer solution, was added alternatively in small portions, while the temperature of the reactor was maintained at 0 °C. The sulfonated copolymers precipitated from solution, followed by washing with hexane. Table 1 presents analytical data of SPSPtBS copolymers calculated on the basis of precursor data, while Scheme 1 shows their molecular structure.

2.1.3. Synthesis of Poly(styrene-b-sulfonated isoprene) Copolymers. A solution of sulfur trioxide/1,4-dioxane complex was prepared at first by adding anhydrous sulfuric acid (30% SO3) to distilled 1,4-dioxane upon stirring. The sulfur source was introduced at a stoichiometry of 1.1 mol per mol of double bonds of isoprene. A ratio of 1,4 dioxane/sulfuric acid of 10:1 was used. The reaction mixture was kept at room temperature and stirred for 2 h. A predetermined solution of sulfur trioxide/1,4 dioxane complex (10% molar excess over the double bonds) was added dropwise to a 5 wt % solution of PS-PI in dioxane, while the solution was stirred. The temperature of the solution was kept below 25 °C. After 2 h of stirring, water of the same amount with dioxane (v/v) was added to the solution, and it was stirred for half an hour at 80 °C. Water leads to the cleavage of sultone and, thus, to formation of the sulfonated products.21b Polymers were freeze-dried under vacuum before use. Sulfonation of the homopolymer poly(styrene) following the above synthetic scheme cannot be achieved, whereas sulfonated poly(isoprene) is formed in quantitative yield. The sulfonic units, as probed through infrared spectra of the sulfonated PS-PI diblocks, are those linked to the isoprene units in accordance to previous work.21 Different polymers of the two classes were prepared in order to investigate the possible influence of their molecular characteristics when utilized as matrices for the preparation of CdS and CdSe nanoparticles. SPS-PtBS-1, SPS-PtBS-3, and SPSPtBS-4 have the same nature of blocks in different ratios. The same applies to PS-SPI-1 and PS-SPI-2. The infrared spectra of sulfonated samples have shown the presence of characteristic vibration modes of the sulfonic groups at 584, 615, 1033, and 1220 cm-1 (see below), confirming the successful incorporation of the sulfonic groups. Sulfonation was found to be nearly quantitative.21,22 Acidimetric titrations showed that the levels of sulfonation were in the range of 90-98 mol % in the respective blocks (Table 1). Elemental analysis on selective samples confirmed the titration results. The molecular characteristics of the sulfonated PS-SPI copolymers are presented in Table 1, and their tentative molecular structures are shown in Scheme 1. 2.1.4. Preparation of CdS/Block Copolymer Hybrid Micelles and Composites. For each preparation about 0.1 g of the block copolymer (SPS-PtBS or PS-SPI) was dissolved in 2 mL of dimethylformamide (DMF) (a nonselective solvent). A weighed amount of cadmium acetate, Cd(Ac)2, was dissolved in 1 mL of DMF and added to the polymer solution under stirring for at least 1.5 h. The amount of Cd(Ac)2 was calculated according to the desired Cd2+/SO3H ratio in the polymer. The solution was subsequently heated at approximately 80 °C in a water bath, and thioacetamide in DMF was slowly added dropwise (S/Cd molar ratio was kept at 2). During heating, H2S is formed from the thermal decomposition of thioacetamide and serves as the sulfur source. After 1.5 h the solution turned yellowish and was

CdS and CdSe Nanoparticles in Block Copolymers

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SCHEME 2: Protocol for CdS and CdSe Nanoparticle Formation in Block Copolymer Matrices

cooled to room temperature.23 Experiments with longer heating times showed no change in properties of the systems (i.e., increase on the size of nanoparticles) as will be discussed later. From these solutions, films were deposited on silicon wafers or glass substrates by casting and were used for subsequent characterization of the polymer-inorganic hybrids in the solid state. 2.1.5. Preparation of CdSe/Block Copolymer Hybrid Micelles and Composites. A similar procedure was used for the preparation of the CdSe-containing hybrids. In this case, H2Se was formed in situ in a separate flask by reaction of Se with NaBH4 and addition of acetic acid. Without heat treatment, the H2Se was bubbled through the polymer solution until the solution turned deep yellow or brownish orange depending on the Cd2+ ion concentration. 2.2. Characterization Methods. Solid films were formed by casting the prepared solutions onto silicon wafers for infrared spectroscopic measurements. Infrared spectra were recorded in transmission on Fourier transform instruments (Bruker Optics, Equinox 55 and Bruker IFS 113v) by averaging 100 scans at 4 cm-1 resolution. UV-vis and the fluorescence spectra were obtained in solution as well as in films cast on glass slides. The UV-vis absorption spectra were recorded on a PerkinElmer spectrophotometer (Lamda 19), and the photoluminescence spectra were recorded on a double-grating excitation and a single-grating emission spectrofluorometer (Fluorolog-3, model FL3-21, Jobin Yvon-Spex). The excitation wavelength varied according to the sample investigated. Raman spectra were measured on a Jobin Yvon spectrometer (Ramanor HG 2S) with a 5 cm-1 resolution and a 90° scattering geometry. The 514.5 nm line of an argon ion laser was used for excitation at 50 mW. All spectral measurements were performed at room temperature. Light-scattering measurements were performed on solutions of the amphiphilic block copolymers in the presence of Cd2+ ions, using an ALV/CGS-3 compact goniometer system (ALV GmbH, Germany), equipped with a JDS Uniphase 22mW HeNe laser, operating at 632.8 nm, interfaced with a ALV-5000/ EPP multi-tau digital correlator with 288 channels and an ALV/ LSE-5003 light-scattering electronics unit for stepper motor drive and limit switch control. Measurements were made at a scattering angle of 90° and at 25 °C, while the average lightscattering intensity from solutions was also recorded. Autocorrelation functions were analyzed by the cumulants method and the CONTIN routine. Apparent hydrodynamic radii, Rh,90, were calculated at different Cd2+/SO3H ratios by the Stokes-Einstein

equation, Rh,90 ) kT/6πηoDapp, where k is the Boltzmann constant, T is the absolute temperature, ηo is the solvent viscosity, and Dapp is the diffusion coefficient calculated from the analysis of the correlation function at the particular polymer concentration. Polydispersities were evaluated from cumulants analysis, through the second cumulant and are given as values of the ratio µ2/Γ2, where µ2 is the second cumulant and Γ is the decay rate of the correlation function. Transmission electron microscopy (TEM) images were taken on a JEOL model JEM100C electron microscope, operated at 80 kV accelerating voltage and under bright-field conditions. Samples for TEM imaging were prepared by depositing a drop of the hybrid micellar solutions onto carbon-coated EM copper grids. Approximately 5 min after the deposition, the excess of DMF solution was blotted away with a strip of filter paper. No staining was necessary due to the large mass contrast between the organic polymer matrix and the cadmium-containing nanoparticles. 3. Results and Discussion 3.1. Formation of CdS and CdSe Nanoparticles in Block Copolymer Templates. Functionalization of the PS or PI blocks of the precursor copolymers with sulfonate groups was done in order to produce anionic binding sites and, thus, facilitate the selective incorporation of Cd2+ ions into these blocks of the copolymer. In previous works, 14a,b,f,g preformed block copolymer micelles containing carboxylic acid groups were utilized for this purpose with the neutral nonpolar/hydrophobic matrix consisting of a polystyrene phase. In the present investigation, sulfonated blocks were utilized in an effort to extend the library of functional amphiphilic block copolymers used for the in situ synthesis of semiconductor nanoparticles. Especially, the use of PS-SPI copolymers presents additional advantages since the PS-PI block copolymer precursors are also available commercially at large quantities and a variety of compositions. Additionally, the Cd2+-induced aggregate formation scheme17 employed in this study presents higher flexibility and it has been rarely utilized for the preparation of semiconductor nanoparticles in organic solvents.17c Also, the utilization of the two particular families of diblock copolymers allows for variations in glass transition temperature (Tg) of the nonpolar/hydrophobic matrix (Tg for PS and PtBS is around 100 and 130 °C, respectively), and this is expected to allow control of thin film formation and overall mechanical properties of the hybrid materials in the solid state.

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Figure 1. Light-scattering data showing micelle formation from block copolymer PS-PSPI-2 in DMF as a function of Cd2+/SO3H ratio: (a) light-scattering intensity at 90°, I90, (b) hydrodynamic radius of dissolved species, Rh,90, and, (c) size polydispersity. Error bars in (a) and (b) are of the size of the symbols.

3.1.1. Light-Scattering Results. A schematic representation of the preparation protocol followed in this investigation is given in Scheme 2. The complexation of Cd2+ ions with the sulfonated PS or the sulfonated PI blocks was carried out in solution through ion exchange with the H+ ions of the sulfonic acid in a nonselective solvent environment. Thus, divalent Cd2+ ions are expected to act as cross-links between different sulfonate groups belonging to the same or different block copolymer chains. The latter possibility would cause aggregation of the block copolymer chains, leading to formation of micellar-like aggregates with the sulfonated blocks in the core and the PtBS or PS blocks in the corona. Light-scattering data support this expectation as shown for sample PS-SPI-2 in Figure 1a, where the measured intensity of light scattered at 90°, I90, is found to increase upon increasing Cd2+/SO3H ratio in solution. This increase of I90 seems to start at a ratio of 1:2, whereas for ratios above 1:1 it remains constant. We note that the effect of scattering contrast variation, dn/dc, due to the introduction of Cd2+ cations in the systems is expected to be small, since this effect should depend linearly on the Cd2+ concentration and no appreciable change in I90 is observed at low Cd2+/SO3H ratios. Therefore, the mass of the aggregates should increase in the same manner as I90 and it should acquire a constant value for Cd2+/SO3H above 1:1. In parallel, the apparent hydrodynamic radius of the aggregates, Rh,90, increases from a value around 15 nm in the absence of Cd2+ ions, which should be associated with the existence of nonaggregated copolymer chains in solution, to a value of around 40 nm for Cd2+/SO3H ratios above 1:1 (Figure 1b). This transition is found to be independent of copolymer concentration for the concentration range investigated, i.e., 5 × 10-3 to 1 × 10-2 g/mL. Thus, it is found that I90 and Rh,90 show the same dependence on the Cd2+/SO3H ratio. The polydispersity of the aggregates (Figure 1c), as obtained from cumulant analysis, seems to increase with Cd2+/SO3H ratio from values lower than 0.1 (indicative of nonassociated block copolymer chains) to values above 0.2, which indicate clearly the formation of polydisperse aggregates.

Gatsouli et al.

Figure 2. Mid-infrared spectra of thin films of SPS-PtBS-1 (a), SPSPtBS-1/Cd2+ with Cd2+/SO3H ) 1:1 (b), and SPS-PtBS-1/CdS (c). Spectra were scaled on the 832 cm-1 band and offset to facilitate comparison.

Light-scattering measurements on the rest of the samples studied in this work showed trends similar to those in Figure 1 for PS-SPI-2, revealing the central role of Cd2+ ions in inducing formation of block copolymer aggregates. These experiments indicate also the need for using Cd2+/SO3H ratios above 1:2 in order to take advantage of the templating properties of block copolymer-induced micelles. In any case, it was found that CdS/ CdSe nanoparticle formation can be effected after introduction of S2-/Se2- ions in block copolymer solutions despite the value of the Cd2+/SO3H ratio for the range of ratios explored in this study. This may suggest a transition from an intramolecular to a multimolecular formation mechanism as far as the participation of the block copolymer chains is concerned. It is noted that light-scattering measurements on the solutions after nanoparticle formation show a large increase in the scattering intensity, and this can be attributed to the increase of the scattering contrast due to semiconductor nanoparticle formation. However, a transition-like behavior of I90 is observable around Cd2+/SO3H ) 1:1. Analysis of the correlation functions indicates that the size of the aggregates increases by a factor of 10-20%. In some cases a second peak is resolved at smaller sizes, with Rh values close to the size of the isolated block copolymer chains. This observation can be rationalized by assuming a detachment of some polymer chains from the aggregates, since electrostatic interactions with Cd2+ ions are now less possible due to CdS/CdSe formation, and the remaining chains adopt a more open conformation due the smaller number of contacts with the surface of nanoparticles. 3.1.2. Infrared Spectroscopy Results. The effect of complexation of Cd2+ ions in the sulfonated phase and the subsequent formation of CdS/CdSe nanoparticle were probed also by infrared spectroscopy on solid films cast from solutions. Characteristic infrared spectra recorded at different reaction steps toward CdS nanoparticle formation in SPS-PtBS-1 (Cd2+/SO3H ) 1:1) are shown in Figures 2 and 3, which depict spectral regions sensitive to the applied chemical processes. To facilitate comparison, the 832 cm-1 band was used for scaling the spectra. Absorption at 832 cm-1 is due to the C-H out-of-plane bending vibration of para-disubstituted benzene rings,24 and thus, this band should be relatively unaffected by the presence of Cd2+

CdS and CdSe Nanoparticles in Block Copolymers

Figure 3. Infrared spectra of thin films of SPS-PtBS-1 (a), SPSPtBS-1/Cd2+ with Cd2+/SO3H ) 1:1 (b), and SPS-PtBS-1/CdS (c), measured in (A) the overtone region of νas(SO3-) and (B) the far-infrared region. Spectra were scaled on the 832 cm-1 band (see Figure 2) and offset to facilitate comparison.

ions and/or CdS nanoparticles. On the other hand, replacement of H+ ions of the SO3H groups by Cd2+ would affect mainly vibrations of the sulfonate anions, SO3-, which form the coordination environment of Cd2+ cations. As shown in Figure 2a, the SO3- anions in pure block copolymer SPS-PtBS-1 exhibit four characteristic bands at 584, 615, 1033, and 1220 cm-1 which can be attributed to symmetric and asymmetric bending and stretching vibration modes as follows: δs(SO3-) ) 584 cm-1, δas(SO3-) ) 615 cm-1, νs(SO3-) ) 1033 cm-1, and νas(SO3-) ) 1220 cm-1.25 In addition, absorption bands due to the block copolymer backbone are observed including characteristic vibration of para-disubstituted benzene rings such as in-plane skeletal vibrations (1122, 1170 cm-1), in-plane and out-of-plane C-H bending vibrations at 1009 and 832 cm-1, respectively, and out-of-plane skeleton bending at 698 cm-1.24 Also, the formation of C-S bonding is manifested by the presence of the 673 cm-1 band, which is due to ν(C-S).25 For “isolated” sulfonate anions, with C3V local symmetry, the asymmetric vibrations would be doubly degenerate.25 Coordinating oxygen atoms of SO3- anions with Cd2+ cations in a nonsymmetric way would remove degeneracy and, thus, cause splitting of the asymmetric vibration modes. Such a case is suggested by Figure 2b corresponding to the infrared spectrum of SPS-PtBS-1/Cd2+. The symmetric vibration modes of SO3anions show a small frequency upshift upon coordination with Cd2+ cations, with δs(SO3-) shifting to ca. 588 cm-1 and νs(SO3-) peaking at 1040 cm-1. The corresponding effect on the asymmetric vibration modes of SO3- is more pronounced as manifested by drastic intensity reductions. This finding may suggest the splitting into two bands having lower and higher frequency than the degenerate mode. The appearance of a new band at 657 cm-1 and the broadening of the 588 cm-1 band in Figure 2b are in favor of the splitting of the δas(SO3-) mode. However, a similar effect is difficult to be resolved for the νas(SO3-) mode due to the presence of strongly overlapping skeletal copolymer bands. Nevertheless, the overtone 2νas(SO3-) of this mode demonstrates clearly the effect of coordinating SO3- with Cd2+ as shown in Figure 3A. Although 2νas(SO3-) appears for pure SPS-PtBS-1 at 2480 cm-1 (Figure 3A,

J. Phys. Chem. C, Vol. 111, No. 42, 2007 15205 curve a), the band loses intensity completely in SPS-PtBS-1/ Cd2+ (Figure 3A, curve b). Introduction of S2- ions into SPSPtBS-1/Cd2+ leads to formation of CdS, and the previously coordinated sulfonate anions resume their “isolated” state, as suggested by the spectra shown in Figures 2c and 3A, curve c. Thus, δs(SO3-) and δas(SO3-) regain to a large extent their relative intensity and frequency as in pure SPS-PtBS-1, νs(SO3-) shifts back to 1033 cm-1, and νas(SO3-) and 2νas(SO3-) develop again their intensity at 1220 and 2480 cm-1, respectively. Complementary information regarding the interactions of Cd2+ ions with the sulfonate sites, and their final transformation to CdS nanoparticles, can be extracted from far-infrared spectra (Figure 3B). Thus, while a broad band appears for the neat copolymer at ca. 190 cm-1, it vanishes in SPS-PtBS-1/Cd2+ and then reappears as a very broad and asymmetric band when CdS is formed. Although the origin of the 190 cm-1 band is not certain, its intensity variation is consistent with a tentative assignment to the torsion around the C-C bond connecting the sulfonated benzene ring with the polymer backbone.26 Coordination of Cd2+ with sulfonate anions, SO3-, could effectively hinder such a torsion mode, which can be activated again when the Cd2+ ions are removed to form CdS. The motion of Cd2+ cations in their sulfonate sites is expected to be active also in the far-infrared region. The far-infrared spectra of poly(styrene sulfonic acid) (PSSA) ionomers containing alkali and alkaline earth cations show characteristic bands arising from cation motion in the potential field of SO3anions,27 and the frequency of such bands (denoted as ν(MO)) was found to depend on the mass, mMn+, and charge (n+) of cation Mn+. In particular, it was found that, to a first approximation, ν(M-O) varies linearly with mMn+-1/2 and that alkaline earth cations exhibit higher ν(M-O) frequencies than alkali cations of the same mass.27 The mass of Cd2+ ion is between those of Ba2+ and Sr2+ ions, for which the ν(M-O) frequencies in PSSA ionomers were measured in the ranges of 145-170 cm-1 and 170-180 cm-1, respectively, depending on the degree of PSSA sulfonation.27 If the nature and strength of the Cd2+-sulfonate site interactions were between those for Ba2+ and Sr2+ ions, then the value of ν(Cd-O) could be expected in the region of ca. 145180 cm-1. Inspection of Figure 3B, curve b, shows the strongest far-infrared feature at 268 cm-1, which is well above the suggested frequency range if Cd2+ were behaving as a group IIA cation. Since Cd2+ belongs to group IIB, the Cd-O interactions should have acquired a considerable degree of covalency in comparison to the primarily ionic M-O interactions for group IIA cations. As shown in previously,28 covalent contributions to the M-O interactions have a 2-fold influence on the cation motion; although the strengthening of M-O bonding leads to higher ν(M-O) frequency, the corresponding reduction in effective ionic charges is manifested by a lower intensity of the far-infrared band. On this basis, the 268 cm-1 band could designate the formation of Cd2+-sulfonate sites with considerably covalent Cd-O interactions, as compared to the Coulombic nature of the alkali- and alkaline earth-oxygen bonding. Formation of CdS does not seem to eliminate absorption at ca. 270 cm-1 (Figure 3B, curve c), even though Cd2+ ions have been screened from their immediate interactions with the sulfonate sites. Such an effect can be explained by the CdS nanoparticles exhibiting infrared activity in the same frequency range. Indeed, the infrared lattice mode of crystalline CdS is measured at ca. 265 cm-1.29

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Figure 4. (A) UV-vis spectra of the hybrid SPS-PtBS-1/CdS in solution for Cd2+/SO3H ) 1:12 (a), Cd2+/SO3H ) 1:4 (b), and Cd2+/ SO3H ) 1:2 (c) and (B) UV-vis spectra of the hybrid SPS-PtBS-1/ CdSe in solution for Cd2+/SO3H ) 1:6 (a) and Cd2+/SO3H ) 1:4 (b).

3.2. Optical Properties of CdS and CdSe Nanoparticles in Block Copolymers. The presence of CdS and CdSe nanoparticles in solutions and films of the block copolymers developed here was probed by UV-vis spectroscopy. Representative spectra of solutions are shown in Figure 4, parts A and B, for CdS- and CdSe-containing block copolymers, respectively. Pure copolymers give no absorption in this spectral range, and thus, the observed absorption in terms of characteristic shoulders or well-resolved peaks should manifest the presence of CdS/CdSe nanoparticles. An important parameter of CdS/CdSe nanoparticles grown in the presence of the SPS-PtBS and PS-SPI copolymers is their size, which affects directly their optical response. The diameter of CdS/CdSe nanoparticle can be obtained from UVvis spectra in solutions or thin film forms of their composites with block copolymers. Literature data for CdS nanoparticles14b,30 suggest two empirical equations that correlate their diameter (2R) with features of their spectroscopic signature in the UVvis range. They are based on the use of the spectral absorption edge λe (in nm) and the first excitonic absorption peak λm (in nm), both shown in Figure 4. Thus, the size of CdS nanoparticle in nm can be obtained from the following equations:

2RCdS(λe) ) 0.1/(0.1338 - 0.0002345λe)

(1)

2RCdS(λm) ) (-6.6521 × 10-8)λm3 + (1.9557 × 10-4)λm2 - (9.2352 × 10-2)λm + 13.29 (2) For CdSe nanoparticles the equation based on λm is written

2RCdSe(λm) ) (1.6122 × 10-9)λm4 - (2.6575 × 10-6)λm3 + (1.6242 × 10-3)λm2 - 0.4277λm + 41.57 (3) Since the excitonic peak maximum λm is not easy to obtain from purely resolved excitonic peaks, eq 1 was used mainly for calculating the CdS diameter. However, for CdSe nanoparticles eq 3 was used. The determined nanoparticle size was found to depend strongly on the Cd2+/SO3H ratio, with a noticeable increase in nanoparticle diameter as this ratio increases. This can be attributed to an enhanced formation of larger Cd2+/SO3aggregates14a in block copolymer micelles, which are subse-

Figure 5. Size of CdX nanoparticles in SPS-PtBS-1 copolymer as a function of Cd2+/SO3H ratio for X ) S (A), and X ) Se (B).

quently transformed into CdS/CdSe particles upon reaction with S2-/Se2- anions. Representative plots of the determined nanoparticle diameter as a function of Cd2+/SO3H ratio are given in Figure 5, parts A and B, for CdS and CdSe particles, respectively, and show that particles in the range of 1.5-4.5 nm can be developed. It is interesting to note that for the same Cd2+/SO3H ratio the sizes of CdS nanoparticles are larger than those of CdSe nanoparticles. This result may originate from differences in the way S2- and Se2- ions were introduced to copolymers solutions. In the first case, S2- ions resulted from a slow thermal decomposition of thioacetamide, a process giving enough time for the CdS particles to grow in size. In the case of CdSe, H2Se gas was employed providing different reaction opportunities. This finding suggests additional means of controlling nanoparticle sizes. Besides their size, the distribution of CdS particle sizes can be estimated from the UV-vis absorption spectra making use of the following equations:

RPI ) (σ2R/2RCdS(λm))2 + 1

(4)

d1/2 ) 2RCdS(λe) - 2RCdS(λm)

(5)

where RPI is the radius polydispersity index and σ2R is the standard deviation, which for a Gaussian distribution can be taken as σ2R ) 0.5d1/2.14a Values of RPI in the range of 1.011.04 are calculated denoting a narrow size distribution for CdS particles. Complementary results on particle sizes can be obtained from TEM images of the block copolymers incorporating CdS or CdSe. Representative photographs are shown in Figure 6, parts a and b. The sizes of particles vary from 3 to 6 nm, which are within the range of size estimated from UV-vis measurements. Also, it is found that CdS nanoparticles form aggregates to a greater extent than CdSe. In cases that individual nanoparticles are resolved, they seem to have a spherical shape and a relatively narrow size distribution in accordance with results from the UV-vis absorption spectra. The location and size of nanoparticle aggregates suggest their formation within single block copolymer micelles, with diameter of the order of 100 nm in the case of sample SPS-PtBS-1/CdS (Figure 6a). This so-called “raspberry” morphology14d has been observed in other metal and semiconductor nanoparticle formation protocols involving

CdS and CdSe Nanoparticles in Block Copolymers

J. Phys. Chem. C, Vol. 111, No. 42, 2007 15207

Figure 7. UV-vis and photoluminescence (PL) spectra of the hybrids: (A) SPS-PtBS-3/CdS, Cd2+/SO3H ) 1:1, PL excitation at λex ) 450 nm, and (B) SPS-PtBS-1/CdSe, Cd2+/SO3H ) 1:1, PL excitation at λex ) 530 nm. Figure 6. TEM images of SPS-PtBS-1/CdX hybrids (Cd2+/SO3H ) 1:2) for X ) S (a) and X ) Se (b). Sizes of nanoparticles for the shown samples as estimated from UV-vis spectroscopy are 4.5 and 2.5 nm, respectively.

block copolymers,14d,17 and it is usually associated with closely located multiple nucleation sites for nanoparticle growth.14d Presumably, such nucleation sites originate from Cd2+/SO3clusters formed in the first step of the preparation protocol (Scheme 2). The nanoparticle sizes achieved in this investigation are in the same range as in previous works involving block ionomers carrying carboxylate groups.14a,b,f,g Additionally, a small influence of the length of the sulfonated block was observed in accordance with previous observations. Thus, nanoparticles formed in the presence of SPS-PtBS-1 were found larger than those formed in SPS-PtBS-4 using the same Cd2+/SO3H ratio. CdS nanoparticles with diameters up to about 4 nm could be prepared in the first case, whereas nanoparticles up to 2.4 nm were prepared in the second case. This effect should be related to the larger core formed in the case of SPS-PtBS-1, which possesses longer sulfonated PS block and larger content of SO3H groups (Table 1). The higher polarity of DMF solvent and the induced aggregation scheme employed in this study are expected to favor micelles with more swollen cores than those formed in carboxylate block ionomers in hydrocarbon solvents. However, the similar nanoparticle sizes indicate that coordination phenomena between polymeric anionic sites (-COO- or -SO3-) and Cd2+ cations play a leading role in nanoparticle formation. The larger acidity of SO3H groups employed in the present work may also contribute toward more stable coordinations for Cd2+ cations. Comparison of the results between SPS-PtBS and PS-SPI block copolymers shows that the nature of the sulfonated (i.e., SPS vs SPI) and of the solvating block (PS vs PtBS) has no influence on the size of nanoparticles. Thus, variations in the Cd2+/SO3H ratio appear to play a key role in comparison to a weaker influence from the length of the sulfonated block. The state of CdS and CdSe nanoparticles is reflected also on their photoluminescence (PL) spectra. Examples of PL spectra are presented in Figure 7, parts A and B, in comparison to corresponding UV absorption spectra of nanoparticles prepared

Figure 8. Raman spectrum of SPS-PtBS-1/CdSe hybrid (Cd2+/SO3H ) 1:1, excitation at 514.5 nm with 50 mW).

in situ in SPS-PtBS and PS-SPI copolymers. The PL spectra of CdS composites exhibit rather broad bands due to free excitons. The lack of clear excitonic bands may result from the thermal treatment employed during formation of CdS particles.7,23 In comparison to CdS, CdSe nanoparticles were prepared without thermal treatment and their PL spectra show better defined bands. However, thermal treatment of CdSe nanoparticles leads to broadening of their PL band toward longer wavelengths, as was observed for CdS/block copolymer hybrids. The spectral ranges of PL measured in this study are in agreement with those of previous reports on CdS and CdSe nanoparticles of comparable sizes even though they were prepared under different protocols in the absence of block copolymers.7 Besides PL spectroscopy, Raman spectroscopy was also employed to characterize CdSe nanoparticles. In the representative spectrum shown in Figure 8 a well-resolved vibrational band at about 209 cm-1 is observed, despite the presence of a broad fluorescence background. It is noted that the first lattice optical mode (1LO) of bulk CdSe is reported at 210 cm-1,30,31 indicating that the 209 cm-1 band in Figure 8 should be associated with the lattice vibration of CdSe nanoparticles. The weak and poorly resolved Raman peak at around 420 cm-1 should correspond to the first overtone of the 209 cm-1 band (2LO), and this provides further evidence for CdSe formation in the block

15208 J. Phys. Chem. C, Vol. 111, No. 42, 2007 copolymer matrix. Raman measurements on hybrids containing CdSe nanoparticles of different size showed no detectable frequency shifts for the range of sizes achieved in this work. The CdS/CdSe-copolymer composites prepared in this study showed the same spectral behavior in solutions and in thin film forms. This finding is of particular importance since it provides strong evidence for the role of block copolymers in processing hybrid nanocomposites without loss of their optical response. Thus, after solvent evaporation the CdS or CdSe nanoparticles should continue being located within the microphase of the sulfonated block (SPS or SPI). The block copolymers are expected to be microphase separated due to the large incompatibility of the sulfonated blocks and the hydrocarbon ones. Therefore, the solid hybrid materials should have a specific nanostructure, which depends on the morphology of the block copolymer (Scheme 2). Additional evidence for the stabilizing effect of block copolymers was provided by the observation that solution aging at 70 °C for as long as 24 h induces no changes on the optical spectra of CdS nanoparticles.18 Furthermore, the colloidal stability of the block copolymerbased solutions was higher compared to that of hybrid solutions based on highly sulfonated PS homopolymer. SPS-PtBS and PS-SPI solutions containing CdS and CdSe nanoparticles were stable for months, whereas those of a SPS homopolymer showed precipitation within a few days from preparation.18 Du et al.23 utilized sulfonated PS ionomers for CdS nanoparticle formation, and they achieved nanoparticle sizes in the range found also in this and our previous18 work. Due to the low content in sulfonate groups their preparations were also stable for months. However, by utilizing amphiphilic block copolymers a higher content in semiconductor nanoparticles can be achieved in the dispersions and in the final solid composite material (i.e., after solvent evaporation). It can be calculated, for example, that solid hybrid materials of the PS-SPI-1/CdS type with an initial Cd2+/SO3H ratio of 1:1 can contain around 12 wt % CdS, whereas those of the SPS-PtBS-3/CdS type, with the same Cd2+/SO3H ratio, can contain as much as 30 wt % CdS. At the same Cd2+/SO3H ratio, sulfonated PS ionomers contain at most 9 wt % CdS.23 Obviously, the nanoparticle content can be increased in block copolymer-based composites if higher Cd2+/SO3H ratios are used. In addition, in block copolymer-based hybrid materials the noncoordinating blocks (PtBS and PS in this case) can play a crucial role in determining the mechanical properties of the films formed from solutions. Also, it controls the blending properties of the hybrid when its addition to a different polymer matrix is desirable. These particular characteristics of block copolymer-based hybrids with semiconductor nanoparticles extend the possibilities for practical applications of such materials. 4. Conclusions CdS and CdSe nanoparticles were successfully prepared in SPS-PtBS and PS-SPI block copolymer matrices and were characterized by spectroscopy and electron microscopy. Increasing the Cd2+/SO3H ratio was found to result in nanoparticles with increasing size. Also, larger semiconductor nanoparticles were formed within block copolymers having longer sulfonated blocks, i.e., higher content of sulfonate groups. The use of the same block copolymer matrix and identical Cd2+/SO3H ratios was found to lead to CdSe nanoparticle sizes smaller than those of CdS. This was attributed to differences in the introduction process of chalcogene anions in the block copolymer solutions.

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