Dielectric enhancement in cadmium sulfidepoly(methacrylic. dimethacrylic acid) nanocomposite through interfacial interaction

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1 University Technology Malaysia From the SelectedWorks of Hadi Nur 2011 Dielectric enhancement in cadmium sulfidepoly(methacrylic acid-ethylene glycol dimethacrylic acid) nanocomposite through interfacial interaction Hadi Nur, University Technology Malaysia Eriawan Rismana, University Technology Malaysia Salasiah Endud, University Technology Malaysia Available at:

2 Article Dielectric enhancement in cadmium sulfide poly(methacrylic acid-ethylene glycol dimethacrylic acid) nanocomposite through interfacial interaction JOURNAL OF COMPOSITE MATERIALS Journal of Composite Materials 45(19) ! The Author(s) 2010 Reprints and permissions: sagepub.co.uk/journalspermissions.nav DOI: / jcm.sagepub.com Hadi Nur 1, Eriawan Rismana 2 and Salasiah Endud 2 Abstract Dielectrical property on poly(methacrylic acid-ethylene glycol dimethacrylate) [] attached with nanosized CdS particles have been studied. The attachment of CdS nanoparticles alters the dielectric property of CdS- samples. By addition of a certain amount of CdS nanoparticles, the dielectric constant at room temperature and 100 Hz of the CdS- is enhanced by ca. 500 times, which is attributed to the interfacial interaction of CdS nanoparticles with polymer surface. The large value of dielectric constant is due to interfacial space charge polarization. Keywords poly(methacrylic acid-ethylene glycol dimethacrylate), CdS nanoparticle, nanocomposite, dielectric Introduction Semiconductor CdS nanoparticles have been widely studied and synthesized, because it has unique properties and used for broad field application. The material has interesting applications in many fields such as electronics, optical, electro-optical devices, 1 bio-sensing, 2 photoreactivity, and photo-catalytic reaction. 3 The material has been produced using various nanoreactors, such as dendrimers, 4 micelles or reverse micelles, 5 aggregation, 6 miniemulsion or microemulsion, 7 block copolymer, 8 and precipitation. 9 The properties of CdS nanoparticles are driven mainly by two factors which are the increase in the surface to volume ratio and a drastic change in the electronic structure of the material due to quantum mechanical effects with decreasing particles size. There are no systematic studies to date that have been published on the influence of CdS nanoparticles on the effective dielectric constant of the composites since most of the previous studies have utilized the conventional method of blending a high dielectric constant material into polymer, which has no real control on the interfacial interaction and distribution within the polymer matrix. 1 9 Having recognized that polymer is one of ideal hosts to form well-dispersed semiconductor CdS nanoparticles, here, poly(methacrylic acid-ethyleneglycol dimethacrylic acid) P(MAA- EDMA) is used as a support. One expects that the carboxylic acid functional groups will help to increase the interfacial interaction between CdS nanoparticles and the polymer matrix. It showed that cadmium sulfide poly(metacrylic acid-ethyleneglycol dimethacrylic acid) [CdS-] nanocomposite materials is unusual in that it has high dielectric constant, a property that determines its ability to become electrically polarized (i.e., separate positive and negative electrical charges). The preparation of occluded cadmium inside was carried out by ion exchange process since this method is commonly used to attach metal particles on supporting materials Ibnu Sina Institute for Fundamental Science Studies, Universiti Teknologi Malaysia, UTM Skudai, Johor, Malaysia. 2 Department of Chemistry, Faculty of Science, Universiti Teknologi Malaysia, UTM Skudai, Johor, Malaysia. Corresponding author: Hadi Nur, Ibnu Sina Institute for Fundamental Science Studies, Universiti Teknologi Malaysia, UTM Skudai, Johor, Malaysia hadi@kimia.fs.utm.my

3 2024 Journal of Composite Materials 45(19) Experimental Preparation of The poly(methacrylic acid-ethylene glycol dimethacrylate) polymer was synthesized by in situ polymerization in miniemulsion system. The miniemulsion system was prepared by using cetyl triammonium bromide (CTABr) as surfactant, 2-propanol as co-surfactant, water, methacrylic acid-ethylene glycol dimethacrylic acid (MAA-EDMA) monomer and n-decane as oil-phase. The preparation involved stirring and polymerizing a mixture of 0.4 g of CTABr, 2.5 ml of 2-propanol, 1.0 g of MAA monomer, 0.5 g of EDMA monomer, 2.5 ml n-decane and water. After stirring for 1 h, it was followed by in situ polymerization at C for 6 h using 2,2 0 -azobisisobutyronitrile (AIBN) as an initiator. The polymer was filtered, washed with methanol, and then dried at 100 C for 4 h. Preparation of CdS- nanocomposites CdS- nanocomposites were prepared in two steps by ion-exchange method. First, Cd 2þ was ion-exchanged from its methanol solution into by stirring a mixture of 0.5 g with various amounts of cadmium ions for 24 h at room temperature. The solution was filtered and washed with methanol in order to remove excess cadmium ion and then dried at 100 C for 4 h. In the second step, a dried sample of Cd 2þ -P(MAA- EDMA) (0.5 g) was soaked in a solution of various mmol Na 2 S.9 H 2 O in 50 : 50 H 2 O:methanol and the mixture was stirred overnight at room temperature. A solid sample was collected by centrifugation, first washed with water and then methanol and dried at 100 C for 4 h. The resulting sample was denoted as CdS- X%, in which X is the percentage of CdS in the sample analyzed by Atomic absorption spectrometer (AAS). Schematic preparation of CdS- nanocomposite is shown in Figure 1. For comparison, a mechanical mixture of CdS and was prepared by the addition of a calculated amount of CdS nanoparticle powder to with the percentage of CdS was 1.00 wt%. This sample was labeled as CdS- 1.00% (physical mixing). Besides, the CdS nanoparticles with size nm were synthesized according to the method reported by Khanna and Subbarao. 13 Cadmium acetate was dissolved in appropriate ratio of N,N -dimethylformamide in a round bottom flask. Sulfur powder was added to this solution in 1 : 1 ratio with respect to cadmium acetate. The reaction mixture was then kept under nitrogen flow to remove moisture and oxygen. The reaction mixture was stirred at 120 C for 4 h to obtain a yellow bright suspension. After 4 h of heating, the colloidal suspension was cooled and filtered, followed by washing with water and methanol. The final product was dried in a desiccator overnight. This sample was labeled as CdS nanoparticles Characterizations All samples were characterized by powder X-ray diffraction (XRD) for crystallinity and phase content of the solid materials, using a Bruker Advance D8 diffractometer with the Cu Ka ( ¼ A ) radiation as the diffracted monochromatic beam at 40 kv and 40 ma. The pattern was scanned in the 2y ranges between 10 and 70 at a step and step time 1 s. Atomic absorption spectrometer analysis (AAS) was employed for elemental composition analyses of Cd in the sample. UV Vis DR spectra were recorded under ambient conditions using a Perkin-Elmer Lambda 900 UV/VIS/ NIR spectrometer. The spectra were monitored in the range of nm. Infrared (IR) spectra of the samples were collected on a Shimadzu Fourier Transform H + Ion exchange with Cd 2+ H + Reaction with S 2 Cd 2+ CdS Figure 1. Schematic preparation of CdS- nanocomposite.

4 Nur et al Infrared (FTIR) spectrometer, with a spectral resolution of 2 cm 1, scans 10 s, and at a temperature of 20 C. The samples were also characterized by scanning electron microscope (SEM). The dielectric properties were measured using AC Impedance Analyzer with frequency response analyzer (Auto Lab POST AT 30). Pellets of 2 4 mm thick were prepared by placing sufficient amount of sample (50 mg) in a steel die measuring 13 mm in diameter, and a pressure of 5 tons was applied and held for 30 s. The dielectric constants (" r ) of the pellet prepared were calculated from the measured capacitance using A.C. impedance analyzer by the equation: C ¼ " r " o A/d, where " o is the dielectric constant of the free space ( F/m); A the area of the electrical conductor; and d the thickness of material. Results and discussion The CdS- nanocomposites with different CdS content, CdS nanoparticles, and bulk CdS were characterized by UV Vis DR spectroscopy (Figure 2). The spectra of CdS- were found significantly blue-shifted compared to CdS nanoparticle and bulk CdS. The direction of the blue shifted is in agreement with quantum confinement effect due to decreasing particle size. Considering that the absorption of CdS particles in the composites increases with the CdS content, only a rough estimate of the particle size could be made for CdS particles in nanocomposites by UV-Vis DR spectrometry. However, based on the onset wavelength value, the size of CdS in nanocomposites can still be estimated using Brus equation. 14 It is calculated that the size of CdS is about 7 10 nm. This result suggests that the ion-exchange and precipitation processes could control the growth, nucleation and formation of CdS nanoparticles on the surface of. Figure 3 shows the XRD patterns of CdS nanoparticles and CdS- nanocomposite. As shown in Figure 3, only the peak of polymer can be observed. CdS nanoparticles in CdS- nanocomposite cannot be detected by XRD due to the small amount of CdS and being well dispersed with very small crystallite size. Hence, the Scanning electron microscopy (SEM) was then used to examine the existence of CdS nanoparticles. Figure 4 shows the SEM photograph of and CdS-P(MAA- EDMA) where the amount of CdS was 1.08 wt%. It is clearly observed the presence of CdS nanoparticles attached on the surface of. This suggests that the smaller particles are CdS. The energy dispersive X-ray (EDX) analyses were also done and it confirmed the existence of the elements of cadmium and sulfur in nanocomposite. The frequency dependent dielectric constant of CdS- nanocomposites is presented in Figure 5. In all cases of CdS- nanocomposites, the dielectric constant shows steep decrease from its initial higher values. The steep decrease in dielectric constant is very high for CdS-P(MAA- EDMA) 1.08% sample. Above 10 5 Hz the dielectric constant remains nearly the same. Interestingly, at 100 Hz, the dielectric constant for the CdS-P(MAA- EDMA) 1.08% sample is about This value decreases to about 700 for the composite samples Abssorbance / a.u. e f d c Bulk CdS CdS nanoparticles b a Intensity / a.u. a b Wavelength / nm Figure 2. UV-Vis DR spectra of, CdS nanoparticles, bulk CdS: (a) CdS- 0.81%, (b) CdS- 0.90%, (c) CdS- 1.08%, (d) CdS- 1.23%, (e) CdS- 1.70%, and (f) CdS- 2.07% θ/ Figure 3. XRD patterns of: (a) CdS nanoparticles, (b) CdS- 1.08%, and (c) CdS- 2.07%. c

5 2026 Journal of Composite Materials 45(19) (a) (b) (c) CdS nanoparticle on the surface of Figure 4. SEM images of: (a) CdS-, (b) and (c) CdS- 1.08% with magnification 50,000 and 100,000, respectively. having 2.07 wt% of CdS. On the other hand, the dielectric constant value of pure and CdS at this frequency are about 10 and 100, respectively. Figure 6 shows a clearer comparison of the dielectric constant between CdS- nanocomposites with CdS nanoparticles, pure and CdS- (physical mixing) 1.00% measured at frequency of 100 Hz. The maximum values for dielectric constant was observed at the concentration of CdS was 1.08 wt%, which is higher ca. 500 times than those of pure. To better understand the above phenomenon, Fourier transform infrared (FTIR) measurements were carried out on CdS nanoparticles, pure P(MAA- EDMA), CdS- 1.00% (physical mixing) and CdS- with various concentrations of CdS, respectively. The broad feature between 3500 and 2500 cm 1 was undoubtedly due to the O H stretch of the carboxylic acid. Two bands at 2920 and 2850 cm 1, which were superimposed on the O H stretch, were attributed to the asymmetric CH 2 stretch and the symmetric CH 2 stretch, respectively. 15 The intense peak at 1710 cm 1 was derived from the existence of the C ¼ O stretch and the band at 1282 cm 1 exhibited the presence of the C O stretch. 15 As shown in Figure 7, it is worth noting that the C ¼ O stretch band of the carboxyl group, which is present at 1710 cm 1 in the IR spectrum of pure P(MAA- EDMA), is lowering in the spectra of the CdS- nanocomposites. Instead, a new band at 1350 cm 1 has appeared. This peak is characteristic of the symmetric n s (COO ) stretch. This reveals that CdS nanoparticels are chemisorbed onto carboxylate functional groups of. In more

6 Nur et al Dielectric constant = CdS % 2 = CdS % 3 = CdS % 4 = CdS % 5 = CdS % 6 = CdS % log frequency / Hz 7 Figure 5. Dielectric constant of CdS-PMAA-EDMA nanocomposites at various frequencies Dielectric constant = 2 = CdS nanoparticles 3 = CdS % (physical mixing) 4 = CdS % 5 = CdS % 6 = CdS % 7 = CdS % 8 = CdS % 9 = CdS % Figure 6. Dielectric constant of CdS- samples at frequency of 100 Hz. detail, Figure 8 shows the intensity of IR peak at 1350 cm 1 increase with decreasing the intensity of IR peak at 1710 cm 1, which can be associated with the degree of the interfacial interaction between CdS and polymer. There is no interfacial interaction in CdS- 1.00% (physical mixing) and CdS- with concentration of CdS lower than 1.08 wt% since no peak at 1350 cm 1 is observed in these samples. We now turn our attention to the origin of high dielectric constant of CdS- 1.08%. As this nanocomposite consist of CdS attached on the surface of polymer, dielectric constant may arise due to interfacial and space charge polarization at frequency from 100 to 10 6 Hz. It is strongly indicative of the occurrence of a Maxwell Wagner type polarization, associated with an inhomogeneous dielectric medium containing two layers of materials with different permittivity and conductivity. 16 A very steep decrease in dielectric constant with increasing frequency in CdS- PMAA-EDMA 1.08% sample (Figure 5) suggesting the occurrence of a strong Maxwell Wagner polarization effect in this nanocomposite. As described above, CdS- 1.08%, having highest dielectric constant, showed a high intensity of IR peak at 1350 cm 1 (Figures 7 and 8) which is attributed to the interfacial interaction between CdS nanoparticles and. Even though CdS- nanocomposites with CdS nanoparticles content higher than 1.08% showed a similar

7 2028 Journal of Composite Materials 45(19) CdS nanoparticles CdS- (physical mixing) 1.00 % Transmittance / a.u. CdS % CdS- 0.9 % CdS % CdS % CdS % CdS % Wavenumber / cm 1 Figure 7. FTIR spectra of CdS nanoparticles,, CdS- (physical mixing) 1.00% and CdS- with various concentration of CdS. degree of the interfacial interaction, their dielectric constant is very much lower than those of CdS-P(MAA- EDMA) 1.08%. The presence of multilayer of CdS nanoparticles on the surface of at a high concentration could be the reason of the low dielectric constant of CdS- nanocomposites with CdS nanoparticles content higher than 1.08%. On the basis of this consideration, we model a dielectric nanocomposite as a three-phase material, consisting of a polymer, an interfacial phase, and CdS nanoparticles, schematically shown in Figure 8. The interfacial phase is between polymer and CdS nanoparticles. It is reasonable to assume that the interfacial area has fixed surface area, since the IR peak at 1350 cm 1, attributed to the interfacial interaction between polymer and CdS nanoparticles, is similar. The interfacial

8 Nur et al CdS Intensity of IR peak at 1350 cm 1 / a.u. 1 = 2 = CdS % (physical mixing) 3 = CdS % 4 = CdS % 5 = CdS % 6 = CdS % 7 = CdS % 8 = CdS % Intensity of IR peak at 1710 cm 1 / a.u Interphase CdS Figure 8. The intensity of IR peaks at 1350 and 1710 cm 1 of samples as depicted in Figure 7 and schematic diagram of nanocomposite consisting of, CdS nanoparticles, and interfacial phase. areas in a nanocomposite could promote interfacial exchange coupling through a dipolar interface layer and lead to enhanced polarization and polarizability in polymer phase near the interface. 17,18 As a result, enhanced permittivity can be expected in the polymer phase near the interfaces. There are two kinds of interfacial interactions in the proposed model shown in Figure 8: (i) intermolecular interaction between CdS nanoparticles and P(MAA- EDMA) and (ii) intermolecular interaction between CdS nanoparticles and CdS nanoparticles. Only the first layer of CdS nanoparticles is contributed to interfacial interaction of CdS polymer and hence to the Maxwell Wagner polarization. The next layer of CdS nanoparticles lowered the Maxwell Wagner polarization caused by interaction between CdS nanoparticles and CdS nanoparticles. One explanation is that an external electric field, inducing an electric current along the material, produces mobility of charge carrier across the interface between CdS nanoparticles. Therefore, the decrease in the dielectric constant of CdS- nanocomposites with CdS content higher than 1.08% is observed. Conclusions The unusual enhancement of dielectric constant of CdS- can be considered by the following unique properties of CdS- composite, i.e., nanosize of CdS and the interfacial interaction between CdS and. Frequency-dependent dielectric constant at room temperature for different composites are due to interfacial space charge (Maxwell Wagner) polarization leading to the large value of dielectric constant. One concludes that highly dielectric constant CdS- composite may be prepared by ion-exchange of Cd 2þ and followed by reaction with sulfide on the surface of and cannot be achieved by mechanical mixing of CdS and.

9 2030 Journal of Composite Materials 45(19) Acknowledgments This research was supported by the Ministry of Science, Technology and Innovation (MOSTI), Malaysia under Science fund Grant and the Ministry of Higher Education (MOHE), Malaysia under Fundamental Research Grant Scheme (FRGS). References 1. Gomez-Romeo P. Hybrid organic-inorganic materials - in search of synergic activity. Adv Mater 2001; 13: Chen X, Wang X, Liu L, Yang D and Fan L. Functionalized semiconductor nanocrystals for ultrasensitive detection of peptides. Anal Chim Acta 2005; 542: Hirai T and Bando Y. Immobilization of CdS nanoparticles formed in reverse micelles onto aluminosilicate supports and their photocatalytic properties. J Coll Interf Sci 2005; 288: Crooks RM, Zhao M, Sun L, Chechik V and Yeung LK. Dendrimer-encapsulated metal nanoparticles: synthesis, characterization, and applications to catalysis. Acc Chem Res 2001; 34: Dutta P and Fendler JH. Preparation of cadmium sulfide nanoparticles in self-reproducing reversed micelles. J Coll Interf Sci 2002; 247: Libert S, Gorshkov V, Privman V, Goia D and Matijevic E. Formation of monodispersed cadmium sulfide particles by aggregation of nanosize precursors. Adv Coll Interf Sci 2003; : Agostiano A, Catalano M, Curri ML, Monica ND, Manna L and Vasanelli L. Synthesis and structural characterization of CdS nanoparticles prepared in a four-components water-in-oil. Microemulsion Micron 2000; 31: Dellinger TM and Braun PV. BiOCl nanoparticles synthesized in lyotropic liquid crystal nanoreactors. Scripta Mater 2001; 44: Banerjee R, Jayakrishnan R and Ayyub P. Effect of the size-induced structural transformation on the band gap in CdS nanoparticles. J Phys: Condens Matter 2000; 12: Zhang Z, Dai S, Fan X, Blom DA, Pennycook SJ and Wei Y. Controlled synthesis of CdS nanoparticles inside ordered mesoporous silica using ion-exchange reaction. J Phys Chem B 2001; 105: Mu-Li Wanga ML, Wanga CH and Wang W. Synthesis of CdS nanocomposites using macroporous ion-exchange resins. Mater Chem Phys 2007; 104: Zhang J, Coombs N and Kumacheva E. A new approach to hybrid nanocomposite materials with periodic structures. J Am Chem Soc 2002; 124: Khanna PK, Lonkar SP, Subbarao VVVS and Jun KW. Polyaniline-CdS nanocomposite from organometallic cadmium precursor. Mater Chem Phys 2004; 87: Brus LE. Electron electron and electron-hole interactions in small semiconductor crystallites: the size dependence of the lowest excited electronic state. J Chem Phys 1984; 80: Smith B. Infrared spectral interpretation: a systematic approach. Boca Raton: CRC Press, Kremer F and Scho nhals A. Broadband dielectric spectroscopy. New York: Springer, Zhang QM, Li HF, Poh M, Xia F, Cheng ZY, Xu HS, et al. An all-organic composite actuator material with a high dielectric constant. Nature (London) 2002; 419: Li JY. Exchange coupling in P(VDF-TrFE) copolymer based all-organic composites with giant electrostriction. Phys Rev Lett 2003; 90: