J. Phys. Chem. B 2004, 108, 13093-13099
13093
IR-Luminescent PbS-Polystyrene Nanocomposites Prepared from Random Ionomers in Solution W. P. Lim,†,‡ H. Y. Low,‡ and W. S. Chin*,† Department of Chemistry, National UniVersity of Singapore, 3 Science DriVe 3, Singapore 117543, and Institute of Materials Research and Engineering, 3 Research Link, Singapore 117602 ReceiVed: April 27, 2004; In Final Form: June 19, 2004
Hybrid composites of PbS nanoparticles embedded in sulfonated polystyrene matrixes were prepared without other added capping agent in the random ionomer. The nanocomposites were found to exhibit photoluminescence in both the near-infrared and visible regions. The infrared luminescence was tunable in the range 1100-1500 nm via quantum size effect. XRD, TEM, UV-vis, FTIR, DSC, and XPS techniques were employed to investigate the size, shape, morphology, absorption, and thermal properties as well as the formation mechanism of nanoparticles in the ionomer matrix. The unique microstructure of the sulfonated polystyrene ionomer was found to provide a confined medium for the formation of nanoparticles. The -SO3- groups acted as the coordination sites for Pb2+ ion aggregation, PbS nanoparticles were then successfully grown in situ at these sites with a supply of S2- from thioacetamide. The sizes and shapes of the PbS nanoparticles produced were controllable by varying the sulfonate content of the polymer and the initial feed concentration of Pb2+ ions. The average size of PbS nanoparticles was estimated to be 5-8 nm, with size dispersity of ∼10-12%. Some PbS nanorods, with an average diameter of 3.5 nm, were formed at higher Pb/SO3H feed ratios.
Introduction Bulk PbS is a direct band gap semiconductor with a nearinfrared band gap of 0.41 eV and an exciton Bohr radius of 18 nm.1 The small band gap and large exciton Bohr radius make PbS an interesting system since it is relatively easy to make particles of ∼18 nm size, which are expected to exhibit a strong quantum confinement effect. It was reported that the band gap of PbS can be widened to the visible region by forming nanoclusters.1 PbS nanoparticles are hence potentially useful in electroluminescent devices such as light-emitting diodes. The infrared luminescent characteristics of PbS could also be potentially useful as active optical elements at telecommunications wavelengths from 1300 to 1600 nm. PbS nanoparticles of suitable sizes for applications within such a wavelength region have been fabricated, in some cases also in the form of PbSpolymer nancomposites.2-4 In addition, quantum-sized PbS also has exceptional third-order nonlinear optical properties;3-5 it is thus a useful material for optical devices such as optical switches. However, inorganic nanoparticles usually have low solubility in most solvents and lack bulk mechanical properties and processing characteristics. Host materials are therefore needed to impart these properties. Recently, inorganic-organic nanocomposites have been intensively investigated because of their potential applications.6-8 These hybrid nanocomposites inherit some of the properties of both their organic and inorganic components, and sometimes also provide tunable magnetic,9 electrical,10 optical,2,8 and catalytic11 properties with improved stability. Among the many organic systems used, polymer matrix * Corresponding author. Telephone: 65-6874-8031. Fax: 65-6779-1691. E-mail:
[email protected]. † National University of Singapore. ‡ Institute of Materials Research and Engineering.
is considered a better host system.2,10,12-21 Besides acting as a stabilizing medium, a polymer matrix also provides for processibility, solubility, and the control of nanoparticle growth.12-17 Polymers are suitable for multifunctional applications, as they can be designed to possess many functionalities needed for more than one application. Random ionomers,12-14 amphiphilic block copolymers,7 and ion-complexing block copolymers16,17 have all been used to prepare various semiconducting nanoclusters, resulting in composite materials consisting of nanoparticles dispersed in a polymer matrix. Wang et al. first synthesized PbS nanoparticles in ionomer films.1 While this work demonstrated a straightforward route for preparing semiconductor nanoparticles, the resultant particle size distribution is wide. Eisenberg et al.14 and Yang et al.13 demonstrated the preparation of size-controlled metal sulfide nanoparticles in random copolymer ionomers, while Cohen et al.16 made use of metal-containing norbornene derivatives in a diblock copolymer to prepare semiconductor nanoclusters. All the above methods, however, involved the use of toxic H2S gas. Later, Olshavsky et al.18 combined the use of a single source precursor and polyphosphazene as the host polymer to produce semiconductor nanoparticles, but the particle size control was poor. Komasawa et al.19 used polymerizable surfactant to immobilize nanoparticles via direct reverse micelle polymerization. Although this method prevented undesirable nanoparticle coagulation, the particle size distribution was also poor. Recently, Stroeve et al.20 has reported an elegant use of multilayer thin film assemblies for the preparation of PbS nanoand microparticles. Unfortunately, a fairly broad distribution of the particle size was obtained. Therefore, it remains a challenge to produce monodispersed and size-adjustable semiconductor-polymer nanocomposites. While much effort has been directed toward the synthesis of monodispersed PbS nanoparticles, there are few reports on its
10.1021/jp048178l CCC: $27.50 © 2004 American Chemical Society Published on Web 08/07/2004
13094 J. Phys. Chem. B, Vol. 108, No. 35, 2004 photoluminescence properties, especially in the near-IR region. This paper reports a simple preparation of PbS nanoparticles in sulfonated polystyrene and investigates the luminescent properties of the nanocomposites. The clustering of ionic SO3- groups in the polystyrene network was used in this case as a confined medium for formation of the nanoparticles. The sulfonate group offers several attractive features for nanoparticle synthesis. For example, in comparison to carboxyl-containing ionomers, the sulfonic acid group remains ionized and maintains its charges in both low and high pH media, and this facilitates ionic binding. In addition, sulfonate groups also have a greater tolerance for divalent cations in solution compared to the carboxylated polymers. Polystyrene was chosen in this work because a uniform and optically transparent film can be prepared from this polymer. Moreover, it is relatively easy to sulfonate and the degree of ionic substitution is easy to control. The prepared hybrid nanocomposites were found to be readily soluble in dimethylformamide (DMF) and could be cast into a homogeneous film. The characterization of these nanocomposites by various spectroscopic and microscopic techniques, as well as the formation mechanism of nanoparticles within the polymer medium, will be discussed. Experimental Section (A) Materials and Preparation. Polystyrene (PS) was purchased from Aldrich with Mw ca. 230 000 and Mn ca. 140 000. Sulfonation with acetyl sulfate was carried out following an established procedure.22 A measured amount of the sulfonated polystyrene (SPS) was dissolved in 30 mL of DMF, and a known concentration of lead acetate in methanol was added slowly under stirring for 1.5 h. Thioacetamide (TAA) in DMF was then slowly added (with S/Pb molar ratio kept at 2) and the solution was stirred further at room temperature for 1.5 h. The solution gradually turned from colorless to orange brown without precipitation, indicating the formation of nanosized PbS. The resultant composite was precipitated and washed several times with methanol before drying under vacuum. (B) Characterization. The sulfonate content of SPS was determined by titration with standardized methanol solution of potassium hydroxide, using phenolphthalein as the indicator. The sulfonate content was also checked with elemental analysis (EA), using a Perkin-Elmer CHNS/O 2400 Analyzer Series II. The powdered X-ray diffraction (XRD) pattern of the PbS nanoparticles was obtained at a scanning rate of 0.02 deg s-1, with 2θ ranging from 10° to 90°, using a Bruker D5005 diffractometer with Cu KR radiation (λ ) 0.151 478 nm). Highresolution transmission electron microscopy (HRTEM) was performed using a Philips CM300 FEG instrument with an acceleration voltage of 300 kV. One drop of the PbS-PS sample in DMF solution was placed on a 200 mesh carbon-coated copper grid. The excess solution was removed with filter paper and the grid was dried in a vacuum. UV-vis absorption spectra were recorded on a Shimadzu UV-2450 spectrophotometer using pure DMF as a reference. Photoluminescence (PL) spectra were recorded at room temperature on an Accent RPM 2000 spectrometer using a He-Cd laser (325 nm) and a diode laserpumped solid-state laser (532 nm). Fourier transform infrared spectra (FT-IR) were recorded using a Bio-Rad FT-IR Spectrometer FTS 165 using KBr pellets. The glass transition temperature (Tg) of the nanocomposites were determined using a TA Instrument DSC 2010 differential scanning calorimeter at a heating rate of 10 K/min. X-ray photoelectron spectroscopy (XPS) was measured with a VG Scientific ESCALAB MK II, with a monochromatic Mg KR X-ray source (1253.6 eV) at 120
Lim et al. TABLE 1: Sulfonate Content and the Glass Transition Temperature (Tg) of the Sulfonated PS sulfonate content (SO3H/styrene mol %) sample code
titration
elem. anal.
Tg (°C)
SPS14 SPS11 SPS8
13.4 10.3 7.6
14.4 11.5 8.2
143 135 129
W (10 mA, 12kV). The C 1s peak at 285.0 eV was used as reference for the calibration of the energy scale. Results and Discussion (A) Preparation and General Properties of the Nanocomposites. We have earlier confirmed that monodispersed CdS nanoparticles can be obtained using sulfonated polystyrene (SPS) with a sulfonate content of ∼9 mol %.12 Thus, in this study, three batches of SPS with sulfonate content between 8 and 14 mol % were prepared by controlled sulfonation as shown in Table 1. The sulfonate content, expressed as the molar percentage of SO3H/styrene, was determined by titration and elemental analysis, and the two sets of values were in fairly good agreement with each other. The three batches of SPS ionomers in Table 1 were then used as the hosts for preparing PbS-PS nanocomposites with Pb/ SO3H feed ratios varying from 0.75 to 2.0. It is well established that ionomers show two types of behavior in solution depending on the polarity of the solvent.23 In low polarity solvents such as toluene and tetrahydrofuran, aggregation due to attraction between ion pairs is observed. In polar solvents such as DMF used here, the ionic groups are dissociated and the SPS ionomers behave more like polyelectrolytes. When ionic salts such as lead acetate are added, however, the electrostatic charges are screened and the characteristics of the polyelectrolytes are suppressed.24 Thus, aggregation of Pb2+ ions within the SPS matrix was effected in solution through coordination to the SO3- groups of the ionomers. A schematic illustration for the formation of these aggregations is shown in Figure 1. Formation of PbS nanoparticles later took place in situ with S2- ions released from the added TAA. The generation of S2- from TAA and the formation of PbS may be expressed by the equations
CH3CSNH2 f CH3CN + H2S Pb(OAc)2 + H2S f PbS + 2H(OAc) The Pb content in the prepared nanocomposites was determined by elemental analysis (Table 2) and found to be in fair agreement with the respective feed Pb contents. Hybrid composites containing 8-31 wt % PbS are prepared by varying the sulfonate content of the polymer and the Pb2+/SO3H feed ratio. In general, the average size of PbS nanoparticles increases with the Pb content in the composites (this will be further discussed in sections B and C). The resultant PbS-PS hybrid nanocomposites can be dissolved in DMF or cast into homogeneous films. They were found to be very stable without much change in the particle sizes and did not show signs of deterioration after storage in ambient for more than 3 months. From Table 1, it can be seen that the glass transition temperatures (Tg’s) of the undoped SPS samples are all higher than that of pure PS (∼108 °C) as expected, and increase with the degree of sulfonation.12 When these samples are doped with PbS nanoparticles, the Tg’s become even higher and increase with the Pb content (see Table 2). This suggests that specific interactions (possibly electrostatic in nature) occur between the
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Figure 1. Schematic illustration for formation of PbS nanoparticles in random SPS ionomers.
TABLE 2: Pb Content and Glass Transition Temperatures (Tg) of PbS-PS Nanocomposites and Average Diameters of PbS Nanoparticles Estimated from XRD and TEM Analyses
samplea
Pb content from EA (wt %)
Tg (°C)
SPS14•0.75 SPS14•1.0 SPS14•1.5 SPS14•2.0 SPS11•0.75 SPS11•1.0 SPS11•1.5 SPS11•2.0 SPS8•0.75 SPS8•1.0 SPS8•1.5 SPS8•2.0
13.0 16.8 24.5 31.2 11.4 14.2 19.5 26.3 8.6 10.7 14.6 19.1
146 155 159 162 140 146 153 160 136 139 140 142
av particle diam (nm) XRD TEM 6.1 6.6 7.0 8.0 5.1 6.1 6.6 7.9 6.6 6.9 7.2 8.1
6.0 6.5 7.3 7.8 5.7 6.2 7.0 7.6 6.5 7.1 7.5 8.1
Figure 2. Representative XRD pattern of PbS-PS hybridized nanocomposites.
a Numbers at the end denote the molar ratio of Pb to SO3H in the feed.
formed nanoparticles and the sulfonated polymer; such interactions have hindered the motion of polymer chains and increase the Tg of the overall composites. A similar increase in Tg has been observed in Pb2+-loaded P(MMA-co-MAA) samples, and such an increase has been found to be dependent on the amount of loading.13 (B) Morphology, Size, and Shape of the PbS Nanoparticles within the PS Matrix. In Figure 2, a representative XRD pattern of the nanocomposites reveals the cubic rock-salt structure of the PbS nanocrystallites. The broadened profile of the diffraction peaks originates from the small sizes of the nanoparticles, and the Debye-Scherrer equation can be used to estimate the particle size:25,26
D ) κλ/(β cos θ) Here, β ) FWHM of the diffraction peak corrected for instrumental factors, λ ) 1.540 56 Å, and κ ) 0.9. The average sizes of the PbS nanoparticles were estimated to be 5-8 nm as listed in Table 2. Generally, the sizes of the nanoparticles increase with the Pb content in the nanocomposites. A representative HRTEM image of the PbS nanoparticles in SPS matrix is shown in Figure 3. The image shows sharp lattice fringes with 3.0 and 2.2 Å spacing, corresponding to the (200) and (220) planes, respectively.27 The cubic morphology of the particles enables them to sit on its (001) face, revealing the (200) and (220) fringes.28 Most of the samples give sharp lattice fringes with no lattice defects such as stacking faults, and the lattice fringes extend to the edges of the particles, indicating good crystallinity. It was also observed that the PbS nanoparticles produced are slightly aspherical in shape. The average sizes of the nanoparticles (Table 2) were obtained by averaging the shorter and longer dimensions of approximately 100 particles that were
Figure 3. Representative HRTEM image of PbS-PS hybridized nanocomposites.
judged to be clearly separated from their neighboring particles. In Figure 4, average particle sizes obtained from XRD and TEM are plotted against Pb content of the nanocomposites from EA. For a fixed sulfonate content, it is clear that particle size increases with an increase in the Pb content. It is noted, however, that while the particle sizes for series SPS11 and SPS14 are clustering on the same line (dotted line in Figure 4 for illustration purpose), the particle sizes obtained for the SPS8 series are different and generally larger. This suggests that (i) the particle sizes depend mainly on the Pb content but not the sulfonate content when the latter exceeds a certain amount, and (ii) below this amount the sulfonate groups present may be too little to
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Figure 4. Average particle sizes plotted against Pb content of the nanocomposites.
Figure 6. (a)-(c) TEM image of SPS14•2.0 showing a mixture of rod- and spherical-shaped PbS nanoparticles. (d) and (e) PbS nanorods found in SPS11•2.0. (f) and (g) PbS nanorods found in SPS8•2.0.
TABLE 3: Elemental Ratios of S to Pb and S to C from XPS, and Pb(SO3)/Pb(PbS) Ratio Obtained from Fitted Pb 4f Peak Areas and Calculated from Stoichiometry of the Nanocomposites
sample SPS14•0.75 SPS14•1.0 SPS14•1.5 SPS14•2.0 SPS11•0.75 SPS11•1.0 SPS11•1.5 SPS11•2.0 SPS8•0.75 SPS8•1.0 SPS8•1.5 SPS8•2.0 Figure 5. Histograms showing the size distribution of the embedded PbS nanoparticles obtained from HRTEM images of the various SPS14 samples.
provide effective capping and thus larger particles are produced. This also further implies that the -SO3- groups are partly responsible for controlling the growth and size of the nanoparticles produced as will be discussed in section C. The size distribution of the PbS nanoparticles is depicted with histograms as shown in Figure 5. It is noted that the size dispersity for all the samples is within 10-12%. This preparation thus gives a significant improvement in terms of the uniformity of sizes compared to PbS nanoparticles synthesized in other polymer systems1,20 and alkanethiolates,29 which reported size distributions of 50% and 30% dispersity, respectively. Indeed, the size distribution of the PbS nanoparticles is comparable to that of the monodispersed PbS particles formed by using a bicontinuous cubic phase as a matrix.28 For samples prepared with higher Pb/SO3H feed ratios, TEM analyses also show the presence of some rod-shaped particles among the spherical ones (Figure 6). Most of these PbS nanorods have an average diameter of 3.5 nm and show sharp lattice fringes with 3.0 Å spacing. It appears that these PbS nanorods
calcd atomic ratio obtained from XPS Pb(SO3)/Pb(PbS) Pb(SO3)/Pb(PbS) from ratio from EA Stotal/Pbtotal C/Stotal fitted Pb 4f peak stoichiometry 2.3 2.3 2.2 2.2 2.3 2.3 2.3 2.2 2.3 2.3 2.3 2.2
51.2 50.4 46.8 45.3 64.1 61.7 59.6 57.7 86.2 84.6 82.0 77.0
3.4 2.7 1.6 1.2 2.6 1.9 1.5 1.2 2.0 1.9 1.4 1.0
3.2 2.7 1.5 1.2 2.7 1.9 1.4 1.2 2.2 1.8 1.4 1.0
are single crystals with fcc structure, and they sit in such a way that the (100) planes are parallel to the grid surface. It was noted that the relative number of rod-shaped over spherical-shaped particles increases with the Pb content. (C) Formation Mechanism of the PbS Nanoparticles within the PS Matrixes. To understand the local environment of PbS within the polymer matrix, XPS analysis was performed and the details are tabulated in Table 3. First, it is noted that the S to Pb elemental ratio is larger than unity and constant for all the samples. In addition, the S to C elemental ratio increases consistently with the Pb content of the nanocomposites. These results imply that, besides PbS nanoparticles, other sulfur species, i.e., SO3- groups, exist in the nanocomposites and remain in a fixed elemental ratio with Pb. In Figure 7, typical spectra for Pb 4f7/2 and 4f5/2 peaks are shown. It was observed that the peak profile shifted to lower binding energy as the Pb content was increased. Numerical peak fitting was thus performed, and good fits resulted in two consistent sets of 4f doublets. The first doublet (denoted as Pb(SO3) in Table 3) was fitted at 139.9 and 144.8 eV, while the second doublet (denoted as Pb(PbS)) was fitted at 138.9 and 143.8 eV, for the 4f7/2 and 4f5/2 peaks, respectively.
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Figure 8. Transmission FTIR spectra of (a) lead acetate, (b) neat SPS8 polymer, (c) SPS8•0.75 sample without S2- source added, (d) SPS8•0.75 sample reacted with TAA, and (e) SPS8•0.75 sample reacted with Na2S.
Figure 7. Pb 4f XPS peaks for samples (a) SPS8•0.75, (b) SPS8•1.0, (c) SPS8•1.5, and (d) SPS8•2.0.
Thus, as shown in Table 3, the relative fitted areas for Pb(SO3)/ Pb(PbS) components were found to be consistently decreasing for higher Pb content. Since particle size is known to increase
with the Pb content (i.e., the surface area-to-volume ratio decreases), Pb(PbS) was thus assigned to Pb in the nanoparticles while Pb(SO3) was assigned to the “surface” Pb2+ ions coordinated to the -SO3- groups. The Pb(PbS) component is found to have a higher binding energy compared to a reference bulk PbS sample (4f peaks at 137.5 and 142.4 eV, respectively). We propose that the nanoparticles are embedded in an “electronpoor” environment within the composite, in this case surrounded by the -SO3- groups which are coordinated to the “surface” Pb2+ ions. To further verify the validity of this peak assignment, the Pb(SO3)/Pb(PbS) ratio was also calculated based on the S(SO3)/ S(PbS) ratio obtained by comparing XPS and EA data; i.e., for a constant sulfonate content, any additional amount of S present is attributed to the formation of PbS. The deduced ratios (last column in Table 3) are found to be consistent with the fitted Pb(SO3)/Pb(PbS) ratios and hence support our assignment. We believe that, when Pb2+ is first added to the SPS ionomers, the Pb2+ ions bind strongly to the sulfonate groups and aggregation of ionic clusters occurs. When TAA was added to form PbS, a considerable amount of Pb2+ remained coordinated to the sulfonate groups and such coordination has helped to stabilize the formed nanoparticles. To further investigate this stabilization mechanism, two sets of test samples have been prepared and studied: set A, which are SPS8 samples loaded with various molar ratios of lead acetate; and set B, which are test samples in set A reacted with Na2S. In sample set A, the specific interactions between Pb2+ ions and the ionomers without the addition of any S2- source would be investigated. In sample set B, the effect of changing the S2source and probably the extent of reaction would be studied. Na2S is known to be a much stronger sulfur source compared to TAA, and it dissociates rapidly in solution.30 Indeed, when Na2S was stirred with the Pb-loaded samples, dark suspensions were obtained and precipitation appeared quickly. Figure 8 shows a typical comparison of the transmission FTIR spectrum of an SPS8 sample with lead acetate and the respective test samples within 800-1700 cm-1. All the sulfonated samples showed a broad ν(SO2) stretching mode at 1126 cm-1. Compared to the neat SPS8 polymer, the test sample loaded with lead acetate (set A) exhibited a new absorption peak at ∼1641 cm-1 (marked with an arrow in Figure 8c). The intensity of this peak attenuated slightly but the peak remained discernible after the sample was reacted with TAA. When reacted with Na2S, however, this peak became much broader and almost
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Figure 9. UV-visible absorption spectra of (a) SPS14•0.75, (b) SPS14•1.0, (c) SPS14•1.5, and (d) SPS14•2.0 samples. The dotted line indicates the spectrum of neat SPS14 polymer.
disappeared into the background. A similar observation has been made for the other series of samples with different Pb feed ratios. Thus, according to the proposed scheme in Figure 1, the new IR peak at ∼1641 cm-1 can be attributed to the Pb-SO3 interactions when Pb2+ ions are introduced into the ionomers. When TAA is added and S2- ions are supplied into the clusters, the number of such interactions reduces as some of the Pb2+ will react to form the PbS nanoparticles. The remaining Pb2+ ions coordinated to the SO3- groups will act as surface capping to stabilize the nanoparticles and also limit the growth of the particles (the amount of S2- diffusing into the cluster is another limiting factor). When Na2S is used, the reaction will occur more rapidly. The surface-capping effect is thus lost and larger PbS particles will be produced. The specific Pb-SO3 interactions within the polymer matrix have also influenced the thermal properties of the nanocomposites formed. Thus, while test samples in set A exhibited Tg within the range of 137-142 °C, the Tg of test samples in set B (reacted with Na2S) was found to vary within a narrow range of 132-133 °C. Comparing with the Tg of neat SPS8 polymer (129 °C in Table 1), it is clear that when a stronger sulfide source such as Na2S is added, almost all the Pb2+ ions are reacted to form PbS and the Tg of the matrix thus returns to almost its original value. In summary, we propose that the addition of Pb2+ ions has led to aggregation of ionic clusters within the macromolecular chains and induced a higher Tg. When a weaker sulfide source such as TAA is used, some Pb2+ ions remain coordinated to the sulfonate groups of the polymer; such Pb-SO3 interaction occurs mainly along the circumference of the ionic cluster cavity, providing a limiting effect to the growth of the nanoparticles. It is possible that the bulky SO3 groups have preferentially capped onto a particular lattice plane of the initial PbS seeds, thus directing the particles to grow into an aspherical or elongated shape. When Pb2+ ions are present in excess (high Pb/SO3H feed ratios), some of the particles have seemed to grow preferentially in one direction to form nanorods. (D) Optical and Luminescence Properties of the Nanocomposites. Next, the absorption and emission properties of the prepared hybrid nanocomposites are investigated. Figure 9 depicts typical absorption spectra within 270-800 nm of the nanocomposites prepared. A broad absorbance continuum can be seen rising from a long wavelength tail and peaks at ∼380 nm. The intensity of the absorption increases steadily with the Pb content, and this is similar to most other reports on PbS nanoparticles.1,31,32 The excitonic absorption peak reported at
Figure 10. Typical PL spectra at (a) near-IR (λex ) 532 nm) and (b) visible regions (λex ) 325 nm). Samples: 1, SPS14•0.75; 2, SPS14•1.0; 3, SPS14•1.5; 4, SPS14•2.0.
∼600 nm in some references,32,33 however, is absent in our case. It is well-known that this peak is strongly related to the surface charge separation and polarization effects, and is thus sensitive to charges on the surface molecules.34 Patel et al. has reported that the excitonic feature was observable for particles capped with PVA and DNA, but not when they were capped with PVP, PS, or PMMA.35 On the other hand, PbS nanorods prepared in a poly(vinylbutyral) film capped by the -SO3- group of AOT showed a featureless absorption spectrum at ∼600 nm.21 Hence, the absence of excitonic absorption in our case could be due to surface capping of the PbS nanoparticles by the sulfonate groups, as the strong electron-hole trapping effect of the SO3- ionic group may render the excitonic absorption peaks unobservable.21 Figure 10 shows the room temperature PL spectra of the PbS-PS nanocomposites in both the near-IR (λex ) 532 nm) and visible regions (λex ) 325 nm). The peak maximum of the near-IR emission (Figure 10a) was found to vary in the range of 1100-1500 nm for different samples. We believe this peak is due to the band-edge emission and the emission maximum is thus size-tunable. The band gap value (∆E) of small sized particles is normally calculated from the hyperbolic band model1 using the equation
∆E ) [Eg2 + 2p2Eg(π/R)2/m*]1/2 Here, Eg is the band gap of the bulk PbS (0.41 eV), m*/me ) 0.085 (m* is the actual electron mass), and R is the particle radius. We have thus set forth to calculate the expected ∆E value using the average particle sizes obtained from XRD and TEM analyses.
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TABLE 4: Calculated Band Gap Values Compared to Experimental Near-IR Emission Maxima
sample
av diam from XRD and TEM (nm)
SPS14•0.75 SPS14•1.00 SPS14•1.50 SPS14•2.00 SPS11•0.75 SPS11•1.00 SPS11•1.50 SPS11•2.00 SPS8•0.75 SPS8•1.00 SPS8•1.50 SPS8•2.00
6.1 6.6 7.2 7.9 5.4 6.2 6.8 7.8 6.6 7.0 7.4 8.1
calcd band edge eV nm 0.97 0.92 0.85 0.80 1.08 0.97 0.89 0.81 0.92 0.87 0.84 0.78
1270 1350 1440 1560 1150 1280 1390 1540 1350 1420 1480 1590
obsd near-IR emission (nm) 1190 1320 1480 1570 1309 1377 1448 1565 1437 1472 1556 1594
The calculated band gap values are tabulated in Table 4 and are found to be in good agreement with our observed near-IR emission maxima. This confirms the quantum size effect and a strong confinement compared to the bulk PbS (∼3000 nm). Previously reported PbS nanoparticles in polymers35 and solgels36 did not show detectable IR luminescence at room temperature. A similar near-IR emission of PbS has been reported recently by Bakueva et al., from a MEH-PPV/PbS sample prepared via a two-step method.3 The orange-red visible emission of the nanocomposites appears at ∼630-680 nm (Figure 10b). Similar visible luminescence has been reported in PbS-doped sol-gel silica glass37 as well as in PbS nanoparticles prepared inside the channels of mesoporous silica SBA-15.38 It is noted that a new visible PL band emerged at ∼550 nm for samples prepared with higher Pb/SO3H feed ratios. We suspect that this new PL band arises due to the increasing formation of aspherical and rod-shaped particles with smaller diameters. The physical reason for the origin of this PL band is now under further investigation. Conclusion We have reported a study of PbS nanoparticles having a fairly narrow size distribution (10-12%) prepared in situ within a sulfonated PS random ionomer. The embedded PbS nanoparticles have average particle sizes of 5-8 nm and displayed a strong blue-shift. PL analysis showed that the nanocomposites emit in both the visible and near-infrared regions, with the infrared PL size-tunable in the range of 1100-1500 nm. The nanoparticle size can be readily controlled by the sulfonate content of ionomers and the initial feed concentration of Pb in the reaction. It is proposed that, when Pb2+ ions were introduced into the ionomers, Pb-SO3 interaction occurred and thus influenced the size and shape of the nanoparticles produced. This interaction, and hence selectivity, is lost when a stronger sulfide source such as Na2S is used instead of TAA. Since a wide range of metal salts is capable of interacting with the sulfonate groups, we believe this method provides a straightforward option to synthesize various types of metal sulfidepolymer nanocomposites.
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