Article pubs.acs.org/IECR
In Situ Synthesis of CdS Quantum Dot−Partially Sulfonated Polystyrene Composite: Characterization and Optical Properties Jolly Vakayil Antony,†,‡,§ Philip Kurian,† Nampoori Parameswaran Narayanan Vadakkedathu,∥ and George Elias Kochimoolayil*,†,⊥ †
Department of Polymer Science and Rubber Technology, Cochin University of Science and Technology, Cochin, Kerala, India 682022 ‡ Department of Chemistry, Government Brennen College, Thalassery, Kerala, India 670106 ∥ International School of Photonics, Cochin University of Science and Technology, Cochin, Kerala, India 682022 S Supporting Information *
ABSTRACT: A simple, direct method for generating a nanohybrid of CdS quantum dots (QDs) in functionalized waste polymer shows good optical properties combined with photocatalytic activities. Homogeneous sulfonation of expanded polystyrene waste (EPS) forms partially sulfonated polystyrene (PSS). Hydrogelation of the PSS facilitates the attachment of CdS QDs through sulfonic acid groups by ion-exchange mechanism, followed by trapping within the polymer network, ultimately generating an organic/inoganic nanohybrid. X-ray diffraction verifies the existence of the CdS nanocrystals and defines their cubic crystalline structure and crystallite size. High resolution transmission electron microscopy shows the formation of QDs in the nanohybrid. A blue shift in ultraviolet−visible absorption and photoluminescence confirm the QDs formation. The polymer chains effectively passivate the surface of CdS nanocrystals that exhibit exciton emission (455−471 nm) with less surface defect state emission (540 nm). In addition, the photocatalytic activity of the composite in degrading an organic dye is demonstrated.
1. INTRODUCTION Controlled growth of semiconductor nanocrystals attached to polymers has emerged as being important in designing materials with optical and electronic properties.1−3 Quantum confinement of electron−hole within the semiconductor nanoparticle occurs as the size of the particle approaches its exciton Bohr radius. Because of energy level quantization, these particles, called quantum dots (QDs), have size dependent physical and chemical properties.4 Quantum size effects exhibited by the QDs were evaluated by observing the blue shift in the optical absorption spectrum.5 The nature of surface passivation also affects the optical properties of the QDs.6 The potential applications such as optical switching,7 nonlinear optics8 and biomedical labeling9 of the composite depend upon the interfacial interaction between QDs and polymer. It is wellknown that photocurrent generation and photocatalytic and photosynthetic reactions at semiconductor particles provide the possibility of utilization of solar energy.10−12 Researchers have had a keen interest in synthesizing nanocomposites of CdS nanoparticles, and a wide range of chemical methods have been developed, which include (i) ex situ method,13 (ii) in situ method,14 and (iii) postpolymerization method.15 A direct attachment between the polymer and the QDs makes the last two methods more attractive. The challenge is to design the polymers or the monomers to control the particle aggregation during the composite formation. Various papers report the generation of CdS QDs through the ionic functional groups of the polymer without the use of capping molecules. Du et al.16 and Wang et al.17 synthesized nanocomposites of CdS QDs in sulfonated polystyrene and sulfonated poly(N-vinylcarbazole), respectively. QDs are © 2014 American Chemical Society
stabilized by the polymer in dimethylformamide (DMF) solution, followed by precipitation of the composite. The solution based synthesis was also reported using polymers containing coordinating groups such as thiols.18 Recently several researchers have successfully synthesized CdS QDs stabilized using polymers such as poly(acrylic acid), poly(ethyleneimine), and so on in aqueous solution, suitable for biological labeling.19 Template assisted CdS QD synthesis has been extensively studied.20 Zeolite, silk fibroin, polymer blend, and nafion film were used as templates for the generation of CdS QDs.21,8 Moffitt and Eisenberg synthesized CdS QDs on a carboxyl functionalized polymer.22 In a number of studies amphiphilic block copolymers and dendrimers have been used as a template for the synthesis of CdS nanoparticles.14 The photostability, photocatalytic activity, and thermal properties of CdS QDs by incorporating them in glass matrix have been reported.23,24 Dantas et al. have studied the magnetic properties of a composite containing a glass matrix and Mn2+ doped ZnTe QDs.25 Electron−hole formation in the CdS particles of the nanocomposite by the absorption of visible light has made the nanohybrid suitable for photocatalysis.26 Nanocomposite formation using polymer hydrogel27,28 and commercial ionexchange resin29 was also reported. However, reports on the preparation of semiconductor nanoparticles over a modified waste polymer are hardly reported. Functionalization of waste polymer for a useful product has been adopted as one of the Received: Revised: Accepted: Published: 2261
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The swelled PSS in each beaker was washed several times with water under magnetic stirring to remove unabsorbed ions. The PSS hydrogel soaked with Cd2+ ions was dipped in freshly prepared N2 purged Na2S aqueous solution, and then a bright yellow composite was formed which was dried in a vacuum oven at 60 °C. 2.4. Characterization. 1H NMR measurements were performed with a Bruker 400 spectrometer (400 MHz). PSSs synthesized under different conditions were dissolved in deuterated dimethyl sulfoxide (DMSO-d6), while EPS was dissolved in deuterated chloroform (CDCl3). Relative areas of peaks corresponding to aliphatic protons and three types of aromatic protons were obtained from the Bruker spectrometer. The sufonate content in the four PSSs was determined using its NMR spectra as previously reported.31 Elemental analysis was carried out in Elementar vario EL III instrument and was used to find the sulfonation level ( f) of the PSS. “f ” of each PSS was obtained from the above two methods and compared. Differential scanning calorimetry (DSC) analysis of different PSSs was done on a universal TA Instruments Q 200, under nitrogen atmosphere. Samples of about 6 mg taken in the pan were heated to 250 °C at 20 °C/min and cooled to 40 °C at 20 °C/min in nitrogen atmosphere. After keeping the sample under isothermal condition for 5 min, the second run of heating and cooling was done as in the first run. Sulfonic acid vibration bands in PSS and its change during the attachment of Cd2+ ions and CdS formation was studied using a Thermo Nicolet Avatar 370 FT-IR spectrometer. The KBr pellet was prepared using the cracked pieces of the sample film and scanned in the range 400−4000 cm−1. Thirty two scans were taken with a resolution of 4 cm−1. Thermal stability of PSS and PSS−CdS were compared using TGA/differential thermal analysis (DTA) from a universal TA Instruments Q50. The samples were heated from 25 to 700 °C at 10 °C/min in nitrogen atmosphere. Morphological characterization of the composite was done by FE-SEM. Small pieces of the composite were directly coated on the carbon film to take the FE-SEM image in a Hitachi SU6600 instrument. Samples were subjected to gold sputtering prior to electron microscopy to give the necessary conductivity. An HR-TEM FEI model TECNAI G2 F30 instrument operated at an accelerating voltage of 300 kV (Cs = 0.6 mm; resolution, 1.7 Å) was used to analyze the size of nanocrystals in the hybrid. Powdered composite was dispersed in DMF, and a drop of the solution was placed on a 200 mesh copper grid coated with carbon film (ICON Analytical). This was subsequently dried under vacuum overnight and then loaded in the electron microscope chamber. The powder X-ray diffraction pattern of the nanocomposite in the 2θ range from 10° to 70° was taken using a Bruker AXS D8 Advance diffractometer equipped with graphite monochromated Cu Kα radiation (λ = 1.5406 Å).The full width at half-maximum (fwhm) of the prominent diffraction peak and its diffraction angle were used to obtain the crystallite size. 2.5. Optical Properties. Optical properties of the composite were studied using UV−vis absorption and PL measurements. The absorption spectrum of the composite in DMF solvent was measured using a Shimadzu UV−visible spectrophotometer. The absorption onset and absorption maximum were obtained by drawing tangents on the exciton peak. The Henglein empirical equation was used to calculate the size range of CdS particles and was confirmed by TEM image. Diffuse-reflectance absorption spectrum of the composite powder was compared with bulk CdS powder in a Varian
methods to reduce the volume of polymer waste. Sulfonated expanded polystyrene waste has been used earlier in water purification and for the removal of hazardous metal ions.30 The sulfonation of expanded polystyrene waste (EPS) to a partially sulfonated polystyrene (PSS) for the synthesis of PSS−CdS QD hybrid is the topic of this work. Homogeneous sulfonation of EPS is proposed to be carried out to obtain the PSS with a sulfonation level of around 30%. Sulfuric acid and silver sulfate are used instead of acetyl sulfate as reagents in 1,2dichloroethane (DCE) solvent for partial sulfonation of EPS. Proton nuclear magnetic resonance (1H NMR) spectroscopy is used to determine the extent of sulfonation and compared with elemental analysis (EA). The partially sulfonated polystyrene is likely to behave as hydrogel and act as a solid template for the attachment of CdS. The resultant nanocomposite is expected to display optical properties corresponding to CdS QDs. The nanocomposite is to be characterized by Fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), and high resolution transmission electron microscopy (HR-TEM). The method of thermogravimetric analysis (TGA) could be used not only to find the thermal stability of the composite but also to find the inorganic content. Optical properties in terms of UV−vis absorption and luminescence spectra of the synthesized composites are proposed to be used to ascertain its application as photocatalyst and biomolecule labeling. The influence of the PSS sulfonation level in nanocomposite formation and photocatalytic activity of the composite in organic dye degradation is also proposed to be investigated.
2. EXPERIMENTAL SECTION 2.1. Materials. Expanded polystyrene, a waste collected from packing cartons, was used for the synthesis of partially sulfonated polystyrene. Analytical reagent grade sulfuric acid (98%), silver sulfate, Cd(CH3COO)2·2H2O, Na2S·9H2O, methanol, toluene, and 1,2-dichloroethane were purchased from Merck. PSS was dissolved in tetrahydrofuran (THF; Loba Chemie) containing little water, and PSS−CdS composite was dissolved in DMF (Merck). Dialysis tubing (Sigma Aldrich) with cutoff of MW 12,000−14,000 was used for the purification of PSS solution. 2.2. Synthesis of Partially Sulfonated Polystyrene. Sulfuric acid containing 1% Ag2SO4 was added dropwise to a homogeneous solution of EPS in DCE under ice cold condition. The temperature gradually increased, and the reaction was carried out at 50 and 60 °C under stirring for 3 h. When the solution became milky, water was added to precipitate the PSS hydrogel. Filtered out PSS was dissolved in THF (1% water). Purification was achieved by dialyzing the PSS solution against distilled water for a week. The solvent from the purified PSS solution was removed by keeping, under a fume chamber, films designated as PSS2 and PSS4 respectively at the two temperatures. Partial sulfonation of EPS was also conducted as above using sulfuric acid (98%) alone at 50 and 60 °C, represented as PSS1 and PSS3.The PSS films obtained under four different conditions were used for PSS−CdS nanocomposite formation 2.3. Nanohybrid Preparation. The PSS films with four different sulfonation levels were immersed in 1 M aqueous solution of cadmium acetate dihydrate in separate beakers. The solutions were stirred slowly for 2 h at 70 °C and kept overnight. The solution in the beakers was replaced with fresh 1 M cadmium acetate solution and left to stand for 2 more days. 2262
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Cary 5000 UV−vis spectrophotometer. The steady state photoluminescence spectrum of the composite in DMF was acquired in the wavelength range of 300−800 nm at room temperature with different excitation light from a He−Cd laser. Influence of the excitation wavelength in the PL of the composites was studied. The luminescence of the composite powder was also obtained using a FluoroMax-3 spectrofluorimeter. 2.6. Photodegradation of Organic Dye. The efficiency of the composite in photocatalytic degradation of organic pollutants on exposure to sunlight was verified. Methylene blue (MB) was dissolved in water (15 ppm) to simulate an aqueous system containing organic pollutants. To the dye solution was added the PSS2−CdS composite (35 mg), stirred overnight in dark to attain adsorption−desorption equilibrium of dye molecules over the catalyst. The solution was stirred under sunlight during summer. UV−vis absorption spectra of the solution prior to sunlight irradiation and at different time intervals of exposure were recorded using a Shimadzu spectrophotometer. The absorption maximum wavelengths of MB at 291 and 663 nm were used to establish the degradation and decolorization of the dye, respectively.32
3. RESULTS AND DISCUSSION 3.1. Synthesis of PSS. The reagents H2SO4/Ag2SO4 have been used successfully for the partial sulfonation of EPS in homogeneous solution at 50 and 60 °C. In previous reports, this reagent in excess has been used for the heterogeneous sulfonation of polystyrene33 while homogeneous sulfonation has been carried out using acetyl sulfate.31 Seeking to overcome the difficulty in preparing acetyl sulfate using acetic anhydride and H2SO4, sulfuric acid containing 1% Ag2SO4 was used. It is found that H2SO4 (98%) alone is also quite capable of sulfonating a homogeneous solution of EPS at 60 °C but fails at 50 °C. Purified PSS films are free from ionic impurities and residual acid content. The swelling capacity of PSS in water gives the first indication about the sulfonation level. PSS1 is not able to swell in water, while moderately sulfonated PSS2 and PSS3 swell in water and highly sulfonated PSS4 almost dissolves in water. This difference in behavior toward water is due to the difference in ionic content. 1 H NMR differentiates the synthesized PSSs under different conditions. NMR spectra of the four PSSs and EPS are shown in Figure 1, which is used to find the extent of sulfonic acid functional groups in PSS. EPS and the four PSSs give the band (δ = 1−2 ppm) corresponding to three aliphatic protons of the repeating unit in the polymer backbone. Three aromatic peaks at δ = 6.5 ppm, δ = 7.1 ppm, and δ = 7.35 ppm correspond to ortho protons, indistinguishable meta, para protons of unsulfonated rings, and meta protons of sulfonated rings, respectively, in the polymer. The most downfield shifted peak at δ = 7.35 ppm is an additional peak in PSS compared to EPS, which confirms the sulfonation. The relative areas of peaks at δ = 7.1 ppm and δ = 7.35 ppm were used to calculate the extent of sulfonation (f) and compared with that calculated from elemental analysis.31 Table 1 gives the sulfonation conditions and the sulfonation level of four different PSSs. The aromatic peak ratio of indistinguishable meta and para protons to ortho protons is 3:2 for EPS, which agrees well with that of pure polystyrene. Sulfonate content in PSS1 is very low to detect using NMR spectroscopy because of the absence of a peak at δ = 7.35 ppm. However a less aromatic peak ratio (2.8:2) of the PSS1 compared to EPS (3:2) indicates a minor
Figure 1. 1H NMR spectra of (a) EPS, (b) PSS1, (c) PSS2, (d) PSS3, and (e) PSS4.
Table 1. Comparison of Four Synthesized Partially Sulfonated Polystyrenes (PSSs) PSS
reagent/catalyst
temp, °C
xa,b
xa,c
PSS1 PSS2 PSS3 PSS4
H2SO4 H2SO4/Ag2SO4 H2SO4 H2SO4/Ag2SO4
50 50 60 60
d 0.27 0.36 0.76
d 0.31 0.33 0.73
a c
x is the degree of sulfonation. bObtained from 1H NMR spectra. Obtained from elemental analysis. dToo low to be measured.
level of sulfonation. Temperature and silver sulfate catalyst increase the extent of sulfonation reaction in homogeneous media. PSS2 and PSS3 having sulfonation levels of 27% and 35%, respectively, are water swelling ionomers, while PSS4, which is 76% sulfonated, behaves as a water-soluble polyelectrolyte. NMR spectra of the four PSSs prepared using H2SO4/Ag2SO4 are similar to the work of Baigl et al.,31 where the sulfonating reagent was acetyl sulfate. The glass transition temperature (Tg) of PSS is higher than that of EPS because of restrictions in polymeric chain movement by ionic interactions (see the Supporting Information). 3.2. Synthesis of Nanocomposite. PSS2 and PSS3 films have the capacity to swell in aqueous media containing cadmium ions and act as templates for the formation of CdS particles. PSS−CdS composite formation was triggered by the Cd2+ ions aggregation on polymer ionic core followed by CdS generation. Hydrogelation of PSS opens the pores of the 2263
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polymer network for the diffusion of the ions into the interior and form CdS throughout. As soon as the CdS has formed, it may be detached from the ionic functional groups because of the high affinity of sulfide ions toward cadmium ions. However, the particles are trapped within the polymeric network. Yellow nanocomposite was formed on drying, which is insoluble in water, methanol, or ethanol, even though partially soluble in DMF. Scheme 1 shows the illustration for the formation of the Scheme 1. Illustration of PSS Template Assisted Generation of CdS QDs in Aqueous Solution
Figure 2. FTIR spectra of (a) PSS2, (b) Cd2+ attached PSS2, and (c) PSS2−CdS.
slight change in the position and intensity of the CC stretching vibration at 1619 cm−1 is attributed to the coordination between cadmium ions and phenyl groups. Also, the decrease in band intensity at 840, 750, and 699 cm−1 of phenyl groups supports metal−ring interaction. Attachment of cadmium ions to the ionic core slightly decreases the O−S−O vibration peaks at 1170 and 1120 cm−1, but Cd−S bond formation detaches the metal from the ionic functional and regenerates the peak. However, the CdS particles are retained within the polymer network as there is interaction with phenyl groups. The ionic core deprived of cadmium ions is occupied by sodium ions, and hence there is no change in infrared absorption at 3430 cm−1. Inorganic content in the composite separates the polymer chains, which enhances the -CH2vibrations at 2920 and 2862 cm−1. The XRD pattern of the yellow PSS2−CdS nanocomposite powder is shown in Figure 3, which reveals three diffraction peaks at 2θ values of 26.9°, 44°, and 51.9°. These peaks correspond to the reflections from (111), (220), and (311)
nanocomposite. In earlier works, CdS nanoparticles were embedded in a sulfonated polystyrene matrix16 in solution phase while here PSS is present as a hydrogel template in aqueous media. The sulfonation of EPS to a moderate extent forms a hydrogel and makes it possible to trap the CdS. The less sulfonated polymer (PSS1) could not swell in water, and it lacks the ability to form CdS within its framework. The highly sulfonated polymer (PSS4) behaves as a water-soluble polyelectrolyte and is not able to act as a template for CdS synthesis in aqueous media. The sulfonation levels of PSS2 and PSS3 are suitable enough to form the composite by the hydrogel template method. Sahiner et al. have reported a hydrogel synthesis for the template assisted CdS composite formation,34 while Zhang et al. have reported the advantage of polymer gel template nanoparticle synthesis over other template assisted syntheses.28 The present method has the advantage of using the sulfonated waste polymer for the hydrogel template synthesis of CdS QD nanocomposite. The synthesized hydrogel may be used for the removal of Cd2+ ions from industrial effluents as reported recently35 for the subsequent formation of CdS QD nanohybrid. This method of hybrid synthesis can be extended to ZnS−PSS nanocomposite, which also has potential application in UV light induced processes. 3.3. Characterization of Nanohybrid. Proof for the sulfonic acid functional in PSS and a change in the polymer structure during the inclusion of CdS is provided by FTIR spectra, as referred to in Figure 2. PSS shows vibration bands at 1030, 1120, and 1170 cm−1 corresponding to O−S−O stretching.36 The characteristic bands of polystyrene at 1619, 840, 750, and 699 cm−1 are also observed in PSS, but with less intensity. These bands are due to the vibrations of CC stretching in benzene ring and C−H bending in para disubstituted and monosubstituted benzene rings.37 Cadmium attachment to the -SO 2−OH groups by ion-exchange mechanism is confirmed by the considerable decrease in -OH vibration at 3430 cm−1. The existence of this band even after the cadmium attachment is due to the presence of water. A
Figure 3. Powder XRD of PSS2−CdS composite. Inset: TGA curves of (a) PSS2 and (b) PSS2−CdS. 2264
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Figure 4. (a) FESEM and (b, c) TEM images. Inset: SAED pattern of the PSS2−CdS composite.
planes of cubic CdS. XRD peaks from the composite powder are broadened relative to bulk CdS, because of the presence of fine CdS crystallites.38 Crystallites of size 4.2 nm are obtained by substituting fwhm of the (111) diffraction peak in the Debye−Scherrer formula.39 TGA curves in Figure 3 (inset) compare the thermal stability of the polymer and its composite. An initial weight loss of 10% around 100 °C is due to the removal of water. The major weight loss of 60% is observed for PSS2 starting from 390 °C to an onset of 430 °C. PSS2−CdS composite shows a similar weight loss, but in less quantity at a higher temperature between 440 and 500 °C. This weight loss in the two thermograms has been attributed to the thermal degradation of the polymer, and the increased thermal stability of the composite is due to the effective incorporation of CdS nanocrystals within the polymer chains. Pronounced thermal stability of the composite is a consequence of the large surface area of CdS that are in intimate contact with the PSS2. Similar results were obtained earlier for polymer−semiconductor nanocomposites.1 The weight of end products in the PSS2 and its composite remain steady up to 700 °C, as indicated by the plateau in the thermogram. Residual weights of the polymer and its composite are 22% and 45%, respectively, whose difference corresponds to the inorganic content in the composite. This is used for the approximate calculation of the CdS content in the nanohybrid.29,34,46 Thus, the PSS2− CdS composite consists of 29% by weight of CdS. The residual weights of the PSS3 and its CdS QD composite are 16% and 42%, respectively. This shows the existence of 31% by weight of CdS in PSS3−CdS. The appearance of CdS particles in the composite is presented in Figure 4, in which FE-SEM (Figure 4a) displays the morphology of the composite. The equipment is not sensitive enough to find the particle size, which can be determined using HR-TEM (Figure 4b,c). The images with selected area electron diffraction (SAED) demonstrate that the CdS particles are QDs of size less than 6 nm, the size of an exciton in the bulk material.40 3.4. Optical Properties of Composites. UV−vis absorption and PL of PSS2 and its composite are shown in Figure 5. PSS2 absorbs below 400 nm, while the composite absorption starts around 500 nm.The absorption spectrum of the composite has characteristics of CdS QDs with an absorption shoulder and absorption edge.41 PSS2, obtained from the EPS, exhibits a luminescence peak at 410 nm because of the fluorescence impurities or chromophoric groups. The commercial polystyrene samples (EPS) exhibit blue emission at
Figure 5. UV−vis absorption and photoluminescence (365 nm excitation) of PSS2 (···) and PSS2-CdS composite (−) in DMF.
about 410 nm that is caused by strongly fluorescent impurities present at low concentration and forming part of the macromolecule. Fluorescence in the blue region may also be due to chromophores formed by conjugated double bonds and phenyl groups.42 Even after the sulfonation reaction on EPS, the existence of chromophoric centers in PSS2 is responsible for its blue emission at about 410 nm. PSS2−CdS composite also shows the emission at 410 nm along with the characteristic CdS QD band edge emission in the region of 455−471 nm. Thus, the composite formation has not affected the emission wavelength of PSS2 except for a decrease in the intensity. This is due to the large fluorescence quantum efficiency of CdS QDs as compared to that of PSS2. The blue shifts in the absorption onset and luminescence peak of the composite from 515 nm for bulk CdS underline the quantum-confined effect in the CdS particles.40 Absorption and PL of PSS3−CdS also give similar exciton shoulder and emission peaks, respectively, except for its blue shifting and surface defects state emission are represented in Figure 6. The absorption edges (λe) for the PSS2−CdS composite and PSS3−CdS composite have been obtained at 481 and 497 nm, which correspond to CdS particle sizes of 4.8 and 5.8 nm in the respective composites.39 The exciton maxima (λm) of the two composites have occurred at 437 and 460 nm, which give the lowest sizes of CdS particles present. The lower end and higher end of the particle distribution in the composites have been obtained from λm and λe, respectively, 2265
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reduced in favor of the alternative deexcitation pathway involving surface defects. Dependence of the PL spectra on excitation wavelength can be verified from Figure 6 (inset). The positions of band edge emission peaks are almost the same for the two composites irrespective of the excitation. In PSS2, less intense surface defect state emission is at a longer wavelength (540 nm), which is observed only by 400 nm excitation, while the PSS3 composite shows intense defect state emission at the expense of exciton recombination. These are attributed to the difference in the density of surface trap levels in the quantum dots of the two composites. The surface area of nanoparticles and its surface passivation by stabilizing molecules are the two factors that decide the density of surface defect levels.48 Here, in the composites, the difference in defect state emission can be explained on the basis of surface passivation rather than surface area. Comparatively, small sized CdS particles are detached from the ionic core of PSS2 gel and efficiently trapped inside the polymer chains than the PSS3 gel. This results in effective surface passivation of QDs and less surface defects in the PSS2−CdS nanocomposite. Zhao et al. have observed fewer surface defects in the solution of CdS nanoparticles synthesized in less concentrated block copolymer micelles.49 The high density of surface trap states in CdS of PSS3 enhances the probability of crossover to these traps, and the composite shows high intense defect state emission with a corresponding decrease in band edge emission.The luminescence of the synthesized composite is in order with various systems reported earlier for fluorescence imaging.45−52 Premachandran et al.18 and Zhuang et al.19 have obtained luminescence peaks of CdS QDs at 470 and 540 nm due to excitonic emission and defect state emission, respectively. Similar emissions have been given by a fluorescent nanohybrid synthesized by Mansur and Mansur51 using functionalized poly(vinyl alcohol) for detecting biological species. Although the luminescence properties of the synthesized composites establish the possibility of using these as fluorescent biolabels, its insolubility in water limits its application in biolabeling. The water-insoluble composites have the potential to be used as photocatalyst in the degradation of organic pollutants in water .53 We have evaluated UV−vis diffuse-reflectance absorbance and luminescence of PSS2−CdS powder. Blue shifted absorption with high absorbance of the composite relative to the bulk CdS is observed. This accounts for the existence of quantized CdS particles in the composite (see the Supporting Information). Similar diffuse-reflectance absorptions were reported earlier to differentiate the CdS nanocomposite from the bulk.29 The gradual increase of absorption in bulk CdS is because of the localized energy levels in the forbidden gap, while the nanocomposite gives more or less sharp absorption with a definite band gap. Luminescence of powdered PSS2 and PSS2−CdS are compared in Figure 7. While the polymer gives an emission at 435 nm, its composite gives an additional protruding shoulder at 460 nm because of the exciton recombination at CdS QDs.21 The coincidence of composite emission in solid state and in DMF solution is attributed to stabilization of CdS particles and their strong interaction with the polymer. Polymer hydrogels were synthesized earlier for the preparation, stabilization of metal, and semiconductor nanoparticles that find application in catalysis.1 We have modified a polymer waste to a hydrogel, which is used for CdS QDs formation.
Figure 6. UV−vis absorption and photoluminescence (365 nm excitation) of PSS3−CdS in DMF. Inset: photoluminescence of PSS2−CdS (−) and PSS3−CdS (···) at (a) 365 and (b) 400 nm.
using Henglein’s empirical curve, which is a relation connecting λm or λe with CdS particle diameter (2R).14,22 The Brus equation and the Peng equation, also relating the particles diameter and wavelength of absorption, have been used by various researchers in calculating the CdS particle size.43 The PSS2−CdS composite consists of particles in the range of 3.2− 4.8 nm, while, in the PSS3−CdS composite, particles are between 3.9 and 5.8 nm. A similar calculation for the particle size distribution (d1/2) in polymer−semiconductor composite was used earlier.44 As expected PSS2, having a low sulfonic acid content (small nanoreactor), precipitates CdS of lower size in the polymer network. The high sulfonic acid content in PSS3 creates large ionic aggregates of -SO3− in the polymer template. These ionic aggregates attract more Cd2+ ions and facilitate the growth of large nanocrystals within the gel compared to PSS2.22 PSS2−CdS composite gives broad band edge emission in the blue region (455−471 nm), while PSS3−CdS composite exhibits remarkable Stokes shifted green emission at 540 nm in addition to band edge emission at 490 nm.45 Emission spectra of the composites give peaks corresponding to the polymer and the CdS QDs. This shows that the incorporation of CdS does not make any change in the polymeric structure and also rules out the charge transfer mechanism between the polymer and particle. Emission at 471 nm near the band edge is attributed to recombination of excited state excitons in the nanocrystals. Blue shift in the band edge emission compared with the emission of bulk CdS (515−520 nm) is due to quantum confinement of charge carriers within the particles.46,47 Broad band edge emission of the PSS2 composite with peaks at 455 and 471 nm corresponds to the two ends of distributed particles. Comparatively large size particles in the PSS3 composite are again confirmed by the shift in band edge emission toward longer wavelengths (490 nm). Band edge emission given by the two composites are well correlated with the size distribution of CdS QDs that were calculated using UV−vis absorption. The strong blue shift in the absorption and emission of strongly confined particles has been observed in the PSS2 nanocomposite relative to the PSS3 nanocomposite. PL band edge emission in the PSS3 composite is substantially 2266
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considerable decrease in the absorbance at 663 nm of the dye solution and corresponding decrease at 291 nm. The degradation of MB solution was 54% in 180 min by the action of the composite in the presence of sunlight. The percentage degradation is comparable to the reported works, where TiO2 and UV light have been used.32,54 Kale et al. have achieved 80% MB degradation in 100 min using visible light in the presence of CdS/glass photocatalyst.24 The present study has well established the organic dye degradation using the synthesized composite and sunlight, the ultimate energy resource. Optimization of the catalyst amount and degradation conditions such as pH, temperature, and effect of catalyst type, viz., PSS2 and PSS3 composites with and without H2O2 reagent, etc. are being investigated.
4. CONCLUSION A simple method for the formation of CdS QDs on PSS in aqueous solution is proposed. The advantage of the present synthetic route is that it utilizes a waste polymer for nanocomposite formation with good optical properties. Partial sulfonation of expanded polystyrene waste has been carried out using sulfuric acid and silver sulfate in DCE solvent. PSS2 and PSS3 behave as hydrogel to facilitate the template synthesis of CdS QDs from aqueous solution. Inclusion of CdS within the polymer has been followed using FTIR. HR-TEM imaging gives the CdS particles of size below 6 nm that are distributed in the composite. High thermal stability of the composite compared to the polymer is observed from TGA curves. The blue shifts of the absorption peak and exciton emission peak have been observed in the composite, which confirms the formation of QDs. The sulfonation level of the PSS template has an influence over the UV−vis absorption and luminescence of its composite and hence the CdS particle size. PSS2−CdS has achieved more blue shift in the exciton shoulder, absorption edge, and luminescence emission compared to the PSS3−CdS composite. Ionic aggregate size in PSS3 is more than that of PSS2, and the nanocrystal precipitated in PSS3 is larger in size. Further, the composite is also found to be efficient in degrading an organic dye under sunlight.
Figure 7. Photoluminescence of powdered PSS2−CdS composite. Inset: Photoluminescence of PSS2 powder.
3.5. Organic Dye Degradation. Organic dye degradation using the photocatalyst PSS2−CdS composite under sunlight has been investigated. The decrease in the absorption maximum at wavelengths of 663 and 291 nm in the UV−vis absorption spectra (Figure 8) of MB solution by the action of
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ASSOCIATED CONTENT
S Supporting Information *
Text describing the sulfonation level (f) of different PSS is calculated from the area of 1H NMR peaks corresponding to different aromatic protons and figures showing differential scanning calorimetry (DSC) curves of EPS, PSS1, and PSS2 and diffuse-reflectance absorbance spectra of bulk CdS and PSS2−CdS composite. This material is available free of charge via the Internet at http://pubs.acs.org.
Figure 8. UV−vis absorption spectra of MB solution during photodegradation.
the composite in the presence of sunlight are ascribed to the decolorization and degradation of MB, respectively. This can be correlated with the photocatalytic activity of the composite in removing organic pollutants. Before catalysis the color of the dye solution has decreased considerably, because of the adsorption of dye on the catalyst. The photocatalytic degradation has started on the interface between the CdS particles and polymer on exposure to sunlight. Details of the degradation mechanism in a hybrid has been reported elsewhere.26,54 Initially, since adsorbed dye molecules are involved in degradation, the absorbance at 291 nm decreases but there is no appreciable decolorization of dye solution. As adsorbed dye molecules have degraded, more molecules diffuse to the interface and undergo degradation. In 120 min there is a
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Present Address
⊥ Albertian Institute of Science and Technology, Cochin University P.O., Cochin, Kerala, India 682022.
Notes
The authors declare no competing financial interest. § Addtional contact information. E-mail (J.V.A.): jollyvakayil@ cusat.ac.in. 2267
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ACKNOWLEDGMENTS We gratefully thank the Physics Department, CUSAT, for taking photoluminescence measurement of powdered composite. We also thank the University Grants Commission (UGC), India for granting permission to do this research work in Cochin University of Science and Technology, Kerala and for financial support.
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ABBREVIATIONS
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REFERENCES
CDCl3 = deuterated chloroform DCE = 1,2-dichloroethane DMF = N,N- dimethylformamide DMSO-d6 = deuterated dimethyl sulfoxide DSC = differential scanning calorimetry DTA = differential thermal analysis EA = elemental analysis EPS = expanded polystyrene waste FE-SEM = field emission scanning electron microscopy fwhm = full width at the half-maximum 1 H NMR = proton nuclear magnetic resonance spectroscopy HR-TEM = high resolution transmission electron microscopy MB = methylene blue PL = photoluminescence PSS = partially sulfonated polystyrene QDs = quantum dots SAED = selected area electron diffraction TGA = thermogravimetric analysis THF = tetrahydrofuran UV−vis = ultraviolet−visible XRD = X-ray diffraction
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