- Email: [email protected]
shell nanocomposites
Journal of Molecular Structure 1013 (2012) 156–162
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc
Dependence of structural, vibrational spectroscopy and optical properties on the particle sizes of CdS/polyaniline core/shell nanocomposites R. Seoudi a,b,⇑, A.A. Shabaka a, M. Kamal c, E.M. Abdelrazek c, Wael.H. Eisa a a
Spectroscopy Department, Physics Division, National Research Center, Dokki, Cairo, Egypt Department of Physics, Faculty of Science, Umm Al-Qura University, Makkah, Saudi Arabia c Physics Department, Faculty of Science, Mansoura University, Mansoura, Egypt b
a r t i c l e
i n f o
Article history: Received 4 August 2011 Received in revised form 9 January 2012 Accepted 17 January 2012 Available online 28 January 2012 Keywords: CdS/PANI nanocomposite TEM XRD FTIR UV–visible spectroscopy
a b s t r a c t Cadmium sulfide/polyaniline (CdS/PANI) nanocomposites were prepared by polymerization of aniline on the CdS nanoparticles using facile synthetic steps. Transmission Electron Microscope (TEM) confirmed that CdS/PANI nanocomposites were synthesized in the form of core/shell structure. According to the patterns of X-ray diffraction (XRD), the particle sizes of the cored CdS were changed with the change of CdCl2 to Na2S molar ratio. Fourier transform infrared (FTIR) spectra revealed that, CdCl2 to Na2S was used to control the polymerization process of aniline. The oxidation degree of PANI was increased with increasing CdCl2 to Na2S. The UV–visible spectra of CdS/PANI core/shell nanocomposite was contained the absorption of the PANI shell as well as the CdS nanoparticles core at certain ratios of CdCl2 and Na2S. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Semiconductor capped polymers nanocomposites have been widely studied due to their wide range of possible applications [1,2]. Absorption and emission energy was affected by particle size due to the quantum confinement effect and it gives properties between molecule and bulk [3–5]. CdS nanoparticles were the most widely studied materials because of their many exciting technological applications (e.g., optical, magnetic, catalytic and electrical) [6]. Polymer is one of appropriate choice as a matrix material; it enables an easy processing of nanocomposites and can be used to control the particle size and shape of the nanoparticles [7]. Conducting polyaniline is used as organic polymer and has been intensively studied because of its low cost, high conductivity, ease of preparation and good environment stability [8–11]. It has been used in a large variety of applications such as light-emitting, electronic devices and chemical sensors. There unique chemical and physical properties were controlled by the oxidation and protonation state [12–16]. The development of inorganic/organic composite has grown rapidly because of their wide range of potential [17,18]. The reactivity of such stabilizers in semiconductor photoinitiated reaction is generally unknown. Illuminated semiconductor suspensions have been shown to be suitable initiator of the polymerization of methyl methacrylate, styrene and 1-vinylpyrene [19,20].
Different particle sizes of semiconductors (i.e. PbS, ZnS, CdSe, ZnSe, TiO2 and CdS) colloids in heterogeneous photochemical studies has several advantages over suspended powder or solid electrode. Hoffman et al. [21] synthesized the CdS with different particle sizes and used it as a photoinitiator to polymerize of vinylic monomers. Many synthesized methods of the composites between polyaniline and inorganic nanoparticles have been reported. Lu et al. [22,23] have prepared PANI/CdS composite micro-wires by introducing hydrogen bond or electrostatic interaction between the carboxyl groups capped CdS nanoparticles and the polyaniline molecules. In this work, different particle sizes of CdS/PANI core/shell nanocomposites were prepared using facile synthetic steps. CdS nanoparticles were used to initiate the polymerization process under laboratory lightning due to their interesting optical and photocatalytic properties. The photopolymerization determine the formation of PANI at the outer surface of CdS nanoparticle. The present synthetic route is simple and it does not need expensive oxidizing agents, surfactants, templates or complicated apparatus. Effects of the CdS nanoparticle sizes with and without PANI shell on the structural, vibrational and optical properties were studied using TEM, X-ray diffraction, FTIR and UV–VIS spectroscopic techniques. 2. Experimental 2.1. Synthesis of CdS/PANI nanocomposites with different sizes of CdS
⇑ Corresponding author at: Spectroscopy Department, Physics Division, National Research Center, Dokki, Cairo, Egypt. Tel.: +20 233308157; fax: +20 233370931. E-mail address: [email protected] (R. Seoudi). 0022-2860/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2012.01.016
Dried cadmium chloride (CdCl2) and sodium sulfide (Na2S) were purchased from (S.d. Fine-Chem. Ltd.) and aniline from Fluka.
R. Seoudi et al. / Journal of Molecular Structure 1013 (2012) 156–162
All chemicals were analytical grade and used without further purification. In our previous work [24], it could be succeeded to prepare CdS/PANI nanocomposite at one molar ratio of CdCl2 and Na2S with one particle size of CdS (5.3 nm). In this work, CdS/PANI nanocomposite was prepared at different molar ratio of CdCl2 to Na2S to change and control the particle sizes of CdS nanoparticles capped by PANI. To form that, a 0.917 g of CdCl2 was dissolved into 50 mL of degassed and distilled water to get an aqueous solution of concentration 0.1 M. A 50 mL of Na2S with different molarities (1.6, 0.8, 0.4, 0.1, 0.025, 0.0125 and 0.00625) were added to the prepared CdCl2 solutions under argon gas flow with vigorous stirring to achieve CdCl2 to Na2S molar ratios of (1:16), (1:8), (1:4), (1:1), (4:1), (8:1) and (16:1) respectively. The aqueous medium was stirring again and it is converted to yellow color immediately due to the formation of CdS. The stirring was continued further for a specific time in order to facilitate complete nanoparticle precipitation. The precipitate was separated by high speed centrifuge and washed repeatedly with water and ethanol to get rid of unreacted species and byproduct. The sample was dried at 40 °C for 6 h. To prepare CdS/PANI core/shell nanocomposites, typical amount of each CdS nanoparticles samples was added to 1 mL of aniline monomer and dissolved in 100 mL of methanol. The solution was bubbled with argon for 20 min to remove oxygen and then sealed. After sufficient ultrasonic treatment for approximately 1 h, the mixture was magnetically stirred for 48 h at ambient condition and exposed to laboratory lighting (neon lamp). Finally, the greenish-yellow precipitate was separated using centrifuge and washed repeatedly with water, ethanol and then dried at 40 °C for 6 h. 2.2. Measurements The morphology and particle size distribution were studied using Transmission Electron Microscope (JEOL JEM 2010, Japan) operated at 200 kV accelerating voltage. The powder samples were suspended in distilled water using ultrasonic water path. The suspension was centrifuged to collimate the large size particles. Then a drop of the suspension was put into the carbon grid and until drying. The crystalline structure was determined from X-Ray Diffractometer Philips (PW 13900, Netherlands); using Cu Ka k = 1.54 A. The vibrational spectra of the prepared samples were studied using Fourier transform infrared (FTIR) spectrometer (Jasco 6100, Japan). The samples were measured in far-IR range using the CsI disc technique whereas they were measured in mid-IR range using KBr method. The UV–visible spectra were measured in the range of 1000–200 nm using Jasco 570 UV–VIS–NIR spectrophotometer using the suspended samples. 3. Results and discussion 3.1. Transmission Electron Microscope (TEM) of CdS/PANI nanocomposite The TEM images of CdS/PANI core shell nanocomposite at the molar ratios of CdCl2 to Na2S were (16:1), (8:1), (4:1), (1:1), (1:4), (1:8), and (1:16) are shown in Fig. 1. It can be seen that the regularly spherical shape and well-dispersed nanocomposites of a novel core/shell structure were formed. The inner dark core may be CdS nanoparticle and the gray color shell is a polyaniline. This is due to the electron density of CdS is so much greater than polyaniline [25,26]. It can confirm that the PANI takes place only at the outer surface of the cadmium sulfide nanoparticles. It can be noted that, TEM images of CdS nanoparticle prepared at the last three molar ratios (1:4), (1:8) and (1:16) is a rarely observe any PANI around the nanoparticle. This indicated that the aniline monomer does not polymerize with increasing S2.
157
3.2. X-Ray diffraction (XRD) of CdS/PANI nanocomposite The X-ray diffraction patterns of the CdS/PANI core/shell nanocomposite were shown in Fig. 2. It can be noticed that, three peaks were observed at 2h° = 26, 43 and 51°. These peaks can be indexed to (1 1 1), (2 2 0), and (3 1 1) planes of CdS [27]. The position and the relative intensities of these peaks indicate to the CdS nanoparticles were formed with cubic structure and these results are agreed with the result in the Card JCPDS No. 80-0019. By comparing the diffraction patterns of CdS nanoparticles prepared at the different CdCl2 to Na2S ratios, it can be concluded that the CdS samples were crystalline due to the broad XRD features at three prominent lattice planes. The CdS amorphous have only a very broad single nearest neighbor peak near the (1 1 1) line [28]. The XRD peaks of the cubic CdS were broad in accordance with their small grain size and the low degree of crystallinity [29]. The particle size was calculated using Scherer formula [30].
D¼
Kk b cos h
where D is the particle diameter, K is a constant and equals 0.9, k is the X-ray wavelength; Cu Ka k = 1.54 A and h is the diffraction angle. It was found that the particle sizes were 6.8, 6.2, 5.9, 5.3, 4.85, 4.7 and 3.8 nm of the CdCl2 to Na2S molar ratio at (1:16), (1:8), (1:4), (1:1), (4:1), (8:1) and (16:1) respectively. The X-ray pattern obtained that, CdS structure cannot be changed by organic PANI shell layer localized at the particle surface [31,32]. Only surface state of the particle could be affected in case of possible interactions between the phases. The TEM and X-ray results indicated that the molar ratio of CdCl2 to Na2S plays an important role in the controlling of the particle sizes for CdS. 3.3. FTIR Spectroscopic Results of CdS/PANI nanocomposite Infrared absorption spectra of CdS/PANI core/shell nanocomposites with different molar ratios of CdCl2 to Na2S were shown in Fig. 3. The PANI does not exhibit any absorption bands in the range from 340 to 190 cm1. The broad band centered at 250 cm1 could be assign to Cd–S stretching vibration [33]. The position and shape of this band does not change with the changing of the molar ratios due to the unaffected of the particle size of the vibration in this range. Fig. 4 shows the spectra of CdS nanoparticles prepared at (1:16), (1:8) and (1:4) molar ratios. The peak appeared at 3400 cm1 assign to the stretching vibration of OAH group of water adsorbed. The week band at 1635 cm1 attribute to adsorbed CO2 on the surface of the particles. No other bands is observed in this figure indicating to there is no chemical or electrostatic interaction was occurred between aniline monomer and CdS nanoparticle at the excess of sulfur ions. Fig. 5 is the infrared spectra of CdS/PANI core/shell nanocomposites prepared at different molar ratio of CdCl2 to Na2S (1:1), (4:1), (8:1), and (16:1) in the range from 4000 to 400 cm1. The band dominates about 3500 cm1 was thought due to adsorbed water. The bands appeared at 3248 and 3310 cm1 assign to symmetric and asymmetric NAH stretching vibrations. The aromatic CAH stretching vibration was identifiable above 3000 cm1 as a weak peak at 3040 cm1. The peak at 1618 cm1 may be ascribed to the NH scissoring vibration overlapping with C@N stretching vibration and combination with C@C [34,35]. The frequency vibration at 1565 cm1 has a major contribution from the quinoid rings (Q) and may be assigned as N@Q@N stretching vibration and at 1495 cm1 indicates the presence of benzenoid ring (B) units. The last peak is reasonable to assign the NABAN stretching vibration. The appearances of these two bands reflect that, the polyaniline contains both the quinoid and benzenoid ring structure. Last three bands were slightly shifted to the lower frequency side with
158
R. Seoudi et al. / Journal of Molecular Structure 1013 (2012) 156–162
Fig. 1. (a–g) TEM image of CdS/PANI core/shell nanocomposite prepared with CdCl2 to Na2S molar ratio of: (a) (16:1), (b) (8:1), (c) (4:1) and (d) (1:1), (e) (1:4), (f) (1:8) and (g) (1:16) after reaction with aniline monomer in methanol solution for 48 h.
decreasing the particle size of CdS core [36]. The intensity ratio of (I1565/I1495) indicates the degree of oxidation of PANI. This ratio was given in Table 1 and it increase from 1.2 to 1.42 with decrease the particle size of CdS core indicating to the higher quinoid to benzenoid ratio. This can be ascribed to increase in oxidation degree of polyaniline around CdS [37]. Two peaks appeared at 1460 and 1385 cm1 for the sample prepared at the molar ratio (1:1) may be related to stretching vibration of the benzene ring and CAN+ stretching in QBtQ units (Bt denotes trans-benzenoid units) [38–40]. By increasing the CdCl2 to Na2S ratio (decreasing the particle size of CdS) the absorption peak at 1385 cm1 become
narrower, sharper and the band at 1460 cm1 was shifted to 1400 cm1. The rapid decay of the peak at 1460 cm1 may be attributed to the conversion of benzenoid units to quinonoid units as the size of the CdS core was decreased reflecting the efficiency of the catalytic activity of CdS nanoparticle. Two bands observed at 1272 and 1232 cm1 attribute to the CAN stretching vibration in the quinoid and benzenoid imine units respectively [36,41]. The absorption peak at 1175 cm1 may be regarded as a pure CAH bending vibration in the quinoid ring (vibrational mode of B-NH+ = Q). This band assigned to electronic-like band and measures the degree of delocalization of electrons on PANI [42,43].
R. Seoudi et al. / Journal of Molecular Structure 1013 (2012) 156–162
159
Fig. 4. (a–c) Mid-FTIR spectra of CdS with CdCl2 to Na2S molar ratio of: (a) (1:16), (b) (1:8), and (c) (1:4) after reaction with aniline monomer.
monomers in the PANI exhibit a head-to-tail polymerization mechanism. 3.4. UV–visible absorption spectroscopic of CdS/PANI nanocomposite
Fig. 2. (a–g) X-ray diffraction patterns of different CdS/PANI core/shell nanocomposites prepared with CdCl2 to Na2S molar ratio of: (a) (16:1), (b) (8:1), (c) (4:1), (d) (1:1), (e) (1:4), (f) (1:8), and (g) (1:16).
Fig. 3. (a–g) Far-IR spectra of CdS/PANI core/shell nanocomposites prepared with CdCl2 to Na2S molar ratio of (a) (16:1), (b) (8:1), (c) (4:1), (d) (1:1), (e) (1:4), (f) (1:8) and (g) (1:16).
The presence of this band reflects the increase in the electron delocalization and decrease of CdS particle size. The absorption bands noted at 1118 and 840 cm1 correlate to 1, 2, 4-ring substitution whereas those at 1018, 1032, 802, 746, 690 and 500 cm1 is 1, 4ring substitution [38]. The intensity of the band at 1118 cm1 was decreased as the size of CdS core decreases whereas the band at 1018 cm1 and the shoulder at 1050 cm1 behave in opposite manner. In addition, the weak signals at 802, 746, 690 and 500 cm1 continuously grow as the CdS particle size decreases. The presence of 1, 4-ring substitution absorption peaks at the expense of that of 1, 2, 4-ring substitution indicates that the coupling rings produce preferentially linear elements of chain and/or the
Fig. 6 shows the ultraviolet visible spectra of CdS prepared at different concentration of CdCl2 to Na2S [(1:16), (1:8) and (1:4)]. The shoulder appeared at about 250 nm may be assign to the optical transition of the first excitonic state. The broad and low intensity band at about 490 nm of CdS prepared at the ratio (1:16) is due to the allowed transition between the electronic state in the conduction band and the hole state in the valence band. It was blue shifted gradually from 490 to 473 nm as the molar ratio of the CdCl2 to Na2S changed from (1:16) to (1:4). These results indicating that, the particle size of CdS decreases with increasing the Cd ions [44,45]. There is no characteristic peak of PANI and/or aniline monomer could not detect in these spectra. This observation is agreement with the results of FTIR. Fig. 7 shows the electronic spectra of aniline and CdS/PANI nanocomposite prepared at the CdCl2 to Na2S ratio [(1:1), (4:1), (8:1) and (16:1)]. There is no absorption band in the wavelength range from 1000 to 300 nm of aniline and the two bands at 276 and 228 nm can be assigned to p p⁄ and locally excited n p⁄ transition between energy levels respectively. By comparing the aniline spectrum with CdS/PANI nanocomposites spectra it seen that, these two bands slightly red shifted in CdS/PANI nanocomposites. This shift was increased with increasing the Cd ions. A new absorption peak was observed at 348 nm for the sample of CdCl2 to Na2S ratio (1:1) assign to p p⁄ transition. It is related to the extent of conjugation between adjacent phenyl rings in the PANI chain [41]. This band was red shifted and become broad; in the samples of CdCl2 to Na2S ratios of (4:1) and (8:1). This is may be due to convoluted with the absorption peak of the CdS nanoparticles that appear in the range 460– 400 nm. It is appeared and become clear again at 420 nm for the ratio of CdCl2 to Na2S (16:1). The red shift occur in the above mentioned absorption peaks can be interpreted in terms of the extent of conjugation of the double bond between phenyl rings in the PANI chain. With increasing the conjugation length, the stabilization energy is increased. This means that, the energy level of the antibonding orbital decrease and consequently the energy required for the transition of p and n electrons to p⁄ level decreases. The spectra in Fig. 7 contain the absorption bands of CdS nanoparticle [46–50]. It is not display typical PANI bands due to the PANI formed even with an oxidant differs from the typical PANI [51]
160
R. Seoudi et al. / Journal of Molecular Structure 1013 (2012) 156–162
Fig. 5. (a–d) FTIR spectra of CdS/PANI core/shell with CdCl2 to Na2S molar ratio of: (a) (1:1), (b) (4:1), (c) (8:1), and (d) (16:1). The bands at 1272, 1232 and 1176 cm1 have been deconvoluted into Gaussian peaks.
Table 1 The intensity ratio of infrared absorption bands at 1565–1495 cm1 registered for different CdS/PANI core/shell. Cd:S ratio
1:1 4:1 8:1 16:1
Intensity
Ratio N@Q@N/NABAN
N@Q@N Peak at 1565
NABAN Peak at 1495
I1565/I1495
0.808 1.212 1.300 1.649
0.679 0.989 1.015 1.250
1.19 1.23 1.28 1.32
3.5. Reaction mechanism of CdS/PANI nanocomposite The UV–visible spectra confirmed that, the absorption energy was increased with increasing the concentration of Cd2+ ions during the preparation process of CdS nanoparticles due to the forming of small particles. Since the particle sizes are extremely small, and the particle diameters are much less than the excitation wavelength, they exhibit negligible light scattering, thus the accurate determination of quantum yields in heterogeneous photochemistry is possible. The surface energy of the small particles is very high and the excess of metal ions chemisorbed on the surface during synthesis and generate a cloud of oppositely charged counter ions. This cloud is called electric double layer. These layers are resulted from a Columbic repulsion between two particles which in turn stabilizing the formed nanoparticles and prevent particle agglomeration as shown in Fig. 8. Due to high surface energy, surface is passivated with cations (Cd2+), which in turn attract anions (Cl) towards the surface of the particle. Increase in the sulfur ions S2 increases the particles sizes of CdS and this is evident in the UV–visible spectra. Also, when the Na2S ratio increases, the number of Na+ was increased and this leads to increase of the ionic strength for the reaction medium. According to the Debye–Hückel-theory [52] for electrolytes the thickness of the double layer is defined by:
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi er eo kB T ¼ 2NA e2 I j 1
Fig. 6. (a–c) UV–visible spectra of CdS nanoparticles with CdCl2 to Na2S molar ratios of (a) (1:4), (b) (1:8), and (c) (1:16) after reaction with aniline monomer.
or perhaps the thickness of the PANI shell around CdS is very small and their concentrations is low during UV–visible measurements.
with er and eo being the electric permittivity of the medium and the vacuum, kB Boltzmann’s constants, NA Avogadro’s constant, T the absolute temperature, e the elementary charge and I the ionic strength. The thickness of the double layer depends strongly on the ionic strength in the solution. This can cause the electrostatically stabilized in solution can be coagulated if the ionic strength of the dispersing medium is increased and this leads to the compression of the double layer and shortens the range of the repulsion [53]. Reducing the Columbic repulsions, causing the particles to collide with each other frequently and aggregate to form larger particles, which settle down due to their weight. The size of the particles is directly related to the absorption wavelength, the particle size growth recognize as a continuous shift of the excitonic absorption
161
R. Seoudi et al. / Journal of Molecular Structure 1013 (2012) 156–162
Fig. 8. Simplified model of CdS particle nucleating due to reaction between Cd2+ and S2 growing subsequently to form seeds.
Fig. 9. Schematic nanocomposite.
illustration
of
the
formation
of
CdS/PANI
core/shell
the surface of the CdS nanoparticles, resulting in the considerably higher local concentration of aniline around the CdS particles. With the absorption of visible light the photogenerated hole react with adsorbed aniline molecule to form resonance-stabilized aniline radical cations and initiated the polymerization reaction. During the oxidative polymerization, PANI was initially formed on the part of the CdS surface, which may act as a point of nucleation, and then it preferentially grew around the CdS particles as a core/shell formation (see Fig. 9) [56]. 4. Conclusion
Fig. 7. (a–e) UV–visible absorption spectra of: (a) aniline monomer and different CdS/PANI core/shell with CdCl2 to Na2S molar ratios of (b) (1:1), (c) (4:1), (d) (8:1), and (e) (16:1).
band to longer wavelengths. To check the degree of homogeneous polymerization, aniline monomer was added to methanol in the presence of CdS nanoparticles and stirring for 48 h under dark. The UV–visible absorption spectrum not changed and this indicating that CdS nanoparticles unable to oxidize the aniline in the polymerization process without absorption of light. The band gap energy of CdS nanoparticles can be easily activated with visible light (1.65–3.26 eV) [54]. Small nanopartcile have higher surface area to volume ratios thus enhancing their catalytic activity and these properties are combining to give high polymerization efficiencies in the increase in effective band gap energy with decreasing particle sizes. Illumination of the CdS with light of energy greater than the band gap leads to band gap excitation resulting in creation of electron–hole pairs; holes in the valence band and electrons in the conduction band. þ
CdS þ ht ! hVB þ eCB Since the recombination of electron–hole pairs in CdS is so rapid; it occurring in a picoseconds and it can be used as an effective photocatalysis [55]. Therefore when CdS of higher CdCl2 to Na2S molar ratios (1:1, 4:1, 8:1 and 16:1) it immersed into the aniline solution, aniline monomers appear to be physically adsorbed (through hydrogen bonding and/or electrostatic interaction) onto
A series of CdS/PANI core/shell nanocomposites were successfully prepared at ambient conditions. X-ray and TEM results confirmed that, the CdCl2 to Na2S molar ratio is a determining factor for controlling the particle size of CdS/PAN core shell nanocomposites. The CdS nanoparticles prepared at the ratios (1:4), (1:8) and (1:16) appeared without PANI shell. As the particle becomes smaller, there is a larger surface area-to-volume ratio enhances light absorption, increasing the probability of radical formation at the surface. Smaller volume, meanwhile, increases the probability that the electron–hole pair will reach the surface to participate in redox reactions with the medium. References [1] M. Molaei, M. Marandi, E. Saievar-Iranizad, N. Taghavinia, B. Liu, H.D. Sun, X.W. Sun, J. Lumin. 132 (2012) 467. [2] K. He, M. Li, L. Guo, Int. J. Hydrogen Energy 37 (2012) 755. [3] C. Wang, D. Chen, X. Jiao, Sci. Technol. Adv. Mater. 10 (2009) 23001. [4] W. Yao, S. Yu, Adv. Funct. Mater. 18 (2008) 3357. [5] G. Yi, C. Wang, W. Park, Semicond. Sci. Technol. 20 (2005) 22. [6] M. Xiao, L. Wang, Y. Wu, X. Huang, Z. Dang, Nanotechnology 19 (2008) 15706. [7] S.S. Ray, M. Okamoto, Prog. Polym. Sci. 28 (2003) 1539. [8] J. Li, J.Z. Zhang, Coord. Chem. Rev. 253 (2009) 3015. [9] D. Wang, Semiconductor Nanocrystal Quantum Dots, Springer-Vienna, 2008. pp. 171–196. [10] B. Chitara, S.V. Bhat, S.R.C. Vivekchand, A. Gomathi, C.N.R. Rao, Solid State Commun. 147 (2008) 409. [11] N. Parvatikar, S. Jain, S. Khasim, M. Revansiddapp, S.V. Bhoraskar, M.V.N. Prasad, Sens. Actuators, B 114 (2006) 599. [12] J.S. Miller, Adv. Mater. 5 (2004) 587. [13] Q. Sun, M. Park, Y. Deng, Mater. Chem. Phys. 110 (2008) 276. [14] X. Luo, A.J. Killard, A. Morrin, M.R. Smyth, Anal. Chim. Acta 575 (2006) 39. [15] G. Cui, J.S. Lee, S.J. Kim, H. Nam, G.S. Cha, H.D. Kim, Analyst 123 (1998) 1855.
162
R. Seoudi et al. / Journal of Molecular Structure 1013 (2012) 156–162
[16] A.A. Karyakin, L.V. Lukachora, E.E. Karyakina, A.V. Orlov, G.P. Kappachora, Anal. Commun. 36 (1999) 153. [17] J.X. Huang, J.A. Moore, J.H. Acquaye, R.B. Kaner, Macromolecules 38 (2005) 317. [18] R.K. Naqarale, G.S. Gohil, V.K. Shahi, R. Ranqarajan, J. Colloid Interface Sci. 287 (2005) 198. [19] Y. Nosaka, N. Ohta, H. Miyama, J. Phys. Chem. 94 (1990) 3752. [20] J. Rabani, J. Phys. Chem. 93 (1989) 7707. [21] A.J. Hoffman, G. Mills, H. Yee, M.R. Hofmann, J. Phys. Chem. 96 (1992) 5546. [22] X.F. Lu, Y.H. Yu, L. Chen, H.P. Mao, W.J. Zhang, Y. Wei, Chem. Commun. 13 (2004) 1522. [23] X.F. Lu, H. Gao, J.Y. Chen, D.M. Chao, W.J. Zhang, Y. Wei, Nanotechnology 16 (2005) 113. [24] R. Seoudi, M. Kamal, A.A. Shabaka, E.M. Abdelrazek, W. Eisa, Synth. Met. 160 (2010) 479. [25] J.L. Boone, G. Cantwell, M.D. Shaw, J. Appl. Phys. 58 (1985) 2296. [26] L. Zhang, H. Peng, P.A. Kilmarti, C. Soeller, R. Tilley, J. Travas-Sejdic, Macromoecul. Rapid Commun. 29 (2008) 598. [27] L. Lutterotti, S. Matthies, H.-R. Wenk, in: Proceeding of the Twelfth International Conference on Textures of Materials (ICOTOM-12), vol. 1, 1999, p. 1599. [28] C.B. Murray, D.J. Norris, M.G. Bawendi, J. Am. Chem. Soc. 115 (1993) 8706. [29] H.C. Warad, S.C. Ghosh, B. Hemtanon, C. Thanachayanont, J. Dutta, Sci. Technol. Adv. Mater. 6 (2005) 296. [30] U. Holzwarth, N. Gibson, Nat. Nanotechnol. 6 (2011) 534. [31] C. Danielle, S. Michelle, A. Ivo, Z. Aldo, Chem. Mater. 15 (2003) 4658. [32] S. Min, F. Wang, Y. Han, J. Mater. Sci. 42 (2007) 9966. [33] R.A. Nyquist, R.O. Kagel, Infrared Spectra of Inorganic Compounds, Academic Press, New York, 1971. [34] M. Trchova, I. Sedenkova, E.N. Konyushenko, J. Stejskal, P. Holler, G. CiricMarjanovic, J. Phys. Chem. B 110 (2006) 9461.
[35] M. Trchováà, J. Stejskal, Pure Appl. Chem. 83 (2011) 1803. [36] O.P. Dimitriev, N.A. Ogurtsov, A.A. Pud, P.S. Smertenko, Yu.P. Piryatinski, Yu.V. Noskov, A.S. Kutsenko, G.S. Shapoval, J. Phys. Chem. C 112 (2008) 14745. [37] A. Parsa, S.A. Ghani, Polymer 49 (2008) 3702. [38] J. Tang, X. Jing, B. Wang, F. Wang, Synth. Metal. 24 (1988) 231. [39] S. Quillard, G. Louarn, S. Lefrant, A.G. Macdiarmid, Phys. Rev. B 50 (1994) 12496. [40] I. Šedeˇnková, J. Prokeš, M. Trchová, J. Stejskal, Polym. Degrad. Stab. 93 (2008) 428. [41] M.M. Ayad, N.A. Salahuddin, A.K. Abou-Seif, M.O. Alghaysh, Eur. Polym. J. 44 (2008) 426. [42] W.R. Salaneck, B. Liedberg, O. Ingan, R. Erlandsson, I. Lundstrsm, A.G. MacDiarmid, M. Halpern, N.L.D. Somasiri, Mol. Cryst. Liq. Cryst. 121 (1985) 19. [43] J.P. Buisson, S. Lefrant, J. Phys. Chem. 100 (1996) 6998. [44] W. Wang, I. Germanenko, M.S. El-Shall, Chem. Mater. 14 (2002) 3028. [45] N. Herron, Y. Wang, H. Eckert, J. Am. Chem. Soc. 112 (1990) 1322. [46] S.V. Bhat, S.R.C. Vivekchand, Chem. Phys. Lett. 433 (2006) 154. [47] N. Singh, M.V. Kulkarni, S.P. Lonkar, A.K. Viswanath, P.K. Khanna, Synth. React. Inorg. Metal-Org. Nano-Metal. Chem. 37 (2007) 153. [48] S. Chen, L. Lin, Macromolecules 28 (1995) 1239. [49] Y. Xi, J. Zhou, H. Guo, C. Cai, Z. Lin, Chem. Phys. Lett. 412 (2005) 60. [50] P.K. Khannaa, Sunil P. Lonkar, V.V.V.S. Subbarao, Ki-Won Jun, Mater. Chem. Phys. 87 (2004) 49. [51] I. Sapurina, J. Stejskal, Polym. Int. 57 (2008) 1295. [52] A.V. Dobrynin, M. Rubinstein, J.F. Joanny, J. Chem. Phys. 109 (1998) 9172. [53] G. Schmid, Clusters and Colloids, Wiley VCH, New York, 1994. [54] C. Karunakaran, S. Senthilvelan, Sol. Energy 79 (2005) 505–512. [55] A.L. Linsebigler, G. Lu, J.T. Yates, Chem. Rev. 95 (1995) 735–758. [56] M.R. Nabid, R. Sedghi, P.R. Jamaat, N. Safari, A.A. Entezami, Appl. Catal., A: Gen. 328 (2007) 52–57.
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc
Dependence of structural, vibrational spectroscopy and optical properties on the particle sizes of CdS/polyaniline core/shell nanocomposites R. Seoudi a,b,⇑, A.A. Shabaka a, M. Kamal c, E.M. Abdelrazek c, Wael.H. Eisa a a
Spectroscopy Department, Physics Division, National Research Center, Dokki, Cairo, Egypt Department of Physics, Faculty of Science, Umm Al-Qura University, Makkah, Saudi Arabia c Physics Department, Faculty of Science, Mansoura University, Mansoura, Egypt b
a r t i c l e
i n f o
Article history: Received 4 August 2011 Received in revised form 9 January 2012 Accepted 17 January 2012 Available online 28 January 2012 Keywords: CdS/PANI nanocomposite TEM XRD FTIR UV–visible spectroscopy
a b s t r a c t Cadmium sulfide/polyaniline (CdS/PANI) nanocomposites were prepared by polymerization of aniline on the CdS nanoparticles using facile synthetic steps. Transmission Electron Microscope (TEM) confirmed that CdS/PANI nanocomposites were synthesized in the form of core/shell structure. According to the patterns of X-ray diffraction (XRD), the particle sizes of the cored CdS were changed with the change of CdCl2 to Na2S molar ratio. Fourier transform infrared (FTIR) spectra revealed that, CdCl2 to Na2S was used to control the polymerization process of aniline. The oxidation degree of PANI was increased with increasing CdCl2 to Na2S. The UV–visible spectra of CdS/PANI core/shell nanocomposite was contained the absorption of the PANI shell as well as the CdS nanoparticles core at certain ratios of CdCl2 and Na2S. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Semiconductor capped polymers nanocomposites have been widely studied due to their wide range of possible applications [1,2]. Absorption and emission energy was affected by particle size due to the quantum confinement effect and it gives properties between molecule and bulk [3–5]. CdS nanoparticles were the most widely studied materials because of their many exciting technological applications (e.g., optical, magnetic, catalytic and electrical) [6]. Polymer is one of appropriate choice as a matrix material; it enables an easy processing of nanocomposites and can be used to control the particle size and shape of the nanoparticles [7]. Conducting polyaniline is used as organic polymer and has been intensively studied because of its low cost, high conductivity, ease of preparation and good environment stability [8–11]. It has been used in a large variety of applications such as light-emitting, electronic devices and chemical sensors. There unique chemical and physical properties were controlled by the oxidation and protonation state [12–16]. The development of inorganic/organic composite has grown rapidly because of their wide range of potential [17,18]. The reactivity of such stabilizers in semiconductor photoinitiated reaction is generally unknown. Illuminated semiconductor suspensions have been shown to be suitable initiator of the polymerization of methyl methacrylate, styrene and 1-vinylpyrene [19,20].
Different particle sizes of semiconductors (i.e. PbS, ZnS, CdSe, ZnSe, TiO2 and CdS) colloids in heterogeneous photochemical studies has several advantages over suspended powder or solid electrode. Hoffman et al. [21] synthesized the CdS with different particle sizes and used it as a photoinitiator to polymerize of vinylic monomers. Many synthesized methods of the composites between polyaniline and inorganic nanoparticles have been reported. Lu et al. [22,23] have prepared PANI/CdS composite micro-wires by introducing hydrogen bond or electrostatic interaction between the carboxyl groups capped CdS nanoparticles and the polyaniline molecules. In this work, different particle sizes of CdS/PANI core/shell nanocomposites were prepared using facile synthetic steps. CdS nanoparticles were used to initiate the polymerization process under laboratory lightning due to their interesting optical and photocatalytic properties. The photopolymerization determine the formation of PANI at the outer surface of CdS nanoparticle. The present synthetic route is simple and it does not need expensive oxidizing agents, surfactants, templates or complicated apparatus. Effects of the CdS nanoparticle sizes with and without PANI shell on the structural, vibrational and optical properties were studied using TEM, X-ray diffraction, FTIR and UV–VIS spectroscopic techniques. 2. Experimental 2.1. Synthesis of CdS/PANI nanocomposites with different sizes of CdS
⇑ Corresponding author at: Spectroscopy Department, Physics Division, National Research Center, Dokki, Cairo, Egypt. Tel.: +20 233308157; fax: +20 233370931. E-mail address: [email protected] (R. Seoudi). 0022-2860/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2012.01.016
Dried cadmium chloride (CdCl2) and sodium sulfide (Na2S) were purchased from (S.d. Fine-Chem. Ltd.) and aniline from Fluka.
R. Seoudi et al. / Journal of Molecular Structure 1013 (2012) 156–162
All chemicals were analytical grade and used without further purification. In our previous work [24], it could be succeeded to prepare CdS/PANI nanocomposite at one molar ratio of CdCl2 and Na2S with one particle size of CdS (5.3 nm). In this work, CdS/PANI nanocomposite was prepared at different molar ratio of CdCl2 to Na2S to change and control the particle sizes of CdS nanoparticles capped by PANI. To form that, a 0.917 g of CdCl2 was dissolved into 50 mL of degassed and distilled water to get an aqueous solution of concentration 0.1 M. A 50 mL of Na2S with different molarities (1.6, 0.8, 0.4, 0.1, 0.025, 0.0125 and 0.00625) were added to the prepared CdCl2 solutions under argon gas flow with vigorous stirring to achieve CdCl2 to Na2S molar ratios of (1:16), (1:8), (1:4), (1:1), (4:1), (8:1) and (16:1) respectively. The aqueous medium was stirring again and it is converted to yellow color immediately due to the formation of CdS. The stirring was continued further for a specific time in order to facilitate complete nanoparticle precipitation. The precipitate was separated by high speed centrifuge and washed repeatedly with water and ethanol to get rid of unreacted species and byproduct. The sample was dried at 40 °C for 6 h. To prepare CdS/PANI core/shell nanocomposites, typical amount of each CdS nanoparticles samples was added to 1 mL of aniline monomer and dissolved in 100 mL of methanol. The solution was bubbled with argon for 20 min to remove oxygen and then sealed. After sufficient ultrasonic treatment for approximately 1 h, the mixture was magnetically stirred for 48 h at ambient condition and exposed to laboratory lighting (neon lamp). Finally, the greenish-yellow precipitate was separated using centrifuge and washed repeatedly with water, ethanol and then dried at 40 °C for 6 h. 2.2. Measurements The morphology and particle size distribution were studied using Transmission Electron Microscope (JEOL JEM 2010, Japan) operated at 200 kV accelerating voltage. The powder samples were suspended in distilled water using ultrasonic water path. The suspension was centrifuged to collimate the large size particles. Then a drop of the suspension was put into the carbon grid and until drying. The crystalline structure was determined from X-Ray Diffractometer Philips (PW 13900, Netherlands); using Cu Ka k = 1.54 A. The vibrational spectra of the prepared samples were studied using Fourier transform infrared (FTIR) spectrometer (Jasco 6100, Japan). The samples were measured in far-IR range using the CsI disc technique whereas they were measured in mid-IR range using KBr method. The UV–visible spectra were measured in the range of 1000–200 nm using Jasco 570 UV–VIS–NIR spectrophotometer using the suspended samples. 3. Results and discussion 3.1. Transmission Electron Microscope (TEM) of CdS/PANI nanocomposite The TEM images of CdS/PANI core shell nanocomposite at the molar ratios of CdCl2 to Na2S were (16:1), (8:1), (4:1), (1:1), (1:4), (1:8), and (1:16) are shown in Fig. 1. It can be seen that the regularly spherical shape and well-dispersed nanocomposites of a novel core/shell structure were formed. The inner dark core may be CdS nanoparticle and the gray color shell is a polyaniline. This is due to the electron density of CdS is so much greater than polyaniline [25,26]. It can confirm that the PANI takes place only at the outer surface of the cadmium sulfide nanoparticles. It can be noted that, TEM images of CdS nanoparticle prepared at the last three molar ratios (1:4), (1:8) and (1:16) is a rarely observe any PANI around the nanoparticle. This indicated that the aniline monomer does not polymerize with increasing S2.
157
3.2. X-Ray diffraction (XRD) of CdS/PANI nanocomposite The X-ray diffraction patterns of the CdS/PANI core/shell nanocomposite were shown in Fig. 2. It can be noticed that, three peaks were observed at 2h° = 26, 43 and 51°. These peaks can be indexed to (1 1 1), (2 2 0), and (3 1 1) planes of CdS [27]. The position and the relative intensities of these peaks indicate to the CdS nanoparticles were formed with cubic structure and these results are agreed with the result in the Card JCPDS No. 80-0019. By comparing the diffraction patterns of CdS nanoparticles prepared at the different CdCl2 to Na2S ratios, it can be concluded that the CdS samples were crystalline due to the broad XRD features at three prominent lattice planes. The CdS amorphous have only a very broad single nearest neighbor peak near the (1 1 1) line [28]. The XRD peaks of the cubic CdS were broad in accordance with their small grain size and the low degree of crystallinity [29]. The particle size was calculated using Scherer formula [30].
D¼
Kk b cos h
where D is the particle diameter, K is a constant and equals 0.9, k is the X-ray wavelength; Cu Ka k = 1.54 A and h is the diffraction angle. It was found that the particle sizes were 6.8, 6.2, 5.9, 5.3, 4.85, 4.7 and 3.8 nm of the CdCl2 to Na2S molar ratio at (1:16), (1:8), (1:4), (1:1), (4:1), (8:1) and (16:1) respectively. The X-ray pattern obtained that, CdS structure cannot be changed by organic PANI shell layer localized at the particle surface [31,32]. Only surface state of the particle could be affected in case of possible interactions between the phases. The TEM and X-ray results indicated that the molar ratio of CdCl2 to Na2S plays an important role in the controlling of the particle sizes for CdS. 3.3. FTIR Spectroscopic Results of CdS/PANI nanocomposite Infrared absorption spectra of CdS/PANI core/shell nanocomposites with different molar ratios of CdCl2 to Na2S were shown in Fig. 3. The PANI does not exhibit any absorption bands in the range from 340 to 190 cm1. The broad band centered at 250 cm1 could be assign to Cd–S stretching vibration [33]. The position and shape of this band does not change with the changing of the molar ratios due to the unaffected of the particle size of the vibration in this range. Fig. 4 shows the spectra of CdS nanoparticles prepared at (1:16), (1:8) and (1:4) molar ratios. The peak appeared at 3400 cm1 assign to the stretching vibration of OAH group of water adsorbed. The week band at 1635 cm1 attribute to adsorbed CO2 on the surface of the particles. No other bands is observed in this figure indicating to there is no chemical or electrostatic interaction was occurred between aniline monomer and CdS nanoparticle at the excess of sulfur ions. Fig. 5 is the infrared spectra of CdS/PANI core/shell nanocomposites prepared at different molar ratio of CdCl2 to Na2S (1:1), (4:1), (8:1), and (16:1) in the range from 4000 to 400 cm1. The band dominates about 3500 cm1 was thought due to adsorbed water. The bands appeared at 3248 and 3310 cm1 assign to symmetric and asymmetric NAH stretching vibrations. The aromatic CAH stretching vibration was identifiable above 3000 cm1 as a weak peak at 3040 cm1. The peak at 1618 cm1 may be ascribed to the NH scissoring vibration overlapping with C@N stretching vibration and combination with C@C [34,35]. The frequency vibration at 1565 cm1 has a major contribution from the quinoid rings (Q) and may be assigned as N@Q@N stretching vibration and at 1495 cm1 indicates the presence of benzenoid ring (B) units. The last peak is reasonable to assign the NABAN stretching vibration. The appearances of these two bands reflect that, the polyaniline contains both the quinoid and benzenoid ring structure. Last three bands were slightly shifted to the lower frequency side with
158
R. Seoudi et al. / Journal of Molecular Structure 1013 (2012) 156–162
Fig. 1. (a–g) TEM image of CdS/PANI core/shell nanocomposite prepared with CdCl2 to Na2S molar ratio of: (a) (16:1), (b) (8:1), (c) (4:1) and (d) (1:1), (e) (1:4), (f) (1:8) and (g) (1:16) after reaction with aniline monomer in methanol solution for 48 h.
decreasing the particle size of CdS core [36]. The intensity ratio of (I1565/I1495) indicates the degree of oxidation of PANI. This ratio was given in Table 1 and it increase from 1.2 to 1.42 with decrease the particle size of CdS core indicating to the higher quinoid to benzenoid ratio. This can be ascribed to increase in oxidation degree of polyaniline around CdS [37]. Two peaks appeared at 1460 and 1385 cm1 for the sample prepared at the molar ratio (1:1) may be related to stretching vibration of the benzene ring and CAN+ stretching in QBtQ units (Bt denotes trans-benzenoid units) [38–40]. By increasing the CdCl2 to Na2S ratio (decreasing the particle size of CdS) the absorption peak at 1385 cm1 become
narrower, sharper and the band at 1460 cm1 was shifted to 1400 cm1. The rapid decay of the peak at 1460 cm1 may be attributed to the conversion of benzenoid units to quinonoid units as the size of the CdS core was decreased reflecting the efficiency of the catalytic activity of CdS nanoparticle. Two bands observed at 1272 and 1232 cm1 attribute to the CAN stretching vibration in the quinoid and benzenoid imine units respectively [36,41]. The absorption peak at 1175 cm1 may be regarded as a pure CAH bending vibration in the quinoid ring (vibrational mode of B-NH+ = Q). This band assigned to electronic-like band and measures the degree of delocalization of electrons on PANI [42,43].
R. Seoudi et al. / Journal of Molecular Structure 1013 (2012) 156–162
159
Fig. 4. (a–c) Mid-FTIR spectra of CdS with CdCl2 to Na2S molar ratio of: (a) (1:16), (b) (1:8), and (c) (1:4) after reaction with aniline monomer.
monomers in the PANI exhibit a head-to-tail polymerization mechanism. 3.4. UV–visible absorption spectroscopic of CdS/PANI nanocomposite
Fig. 2. (a–g) X-ray diffraction patterns of different CdS/PANI core/shell nanocomposites prepared with CdCl2 to Na2S molar ratio of: (a) (16:1), (b) (8:1), (c) (4:1), (d) (1:1), (e) (1:4), (f) (1:8), and (g) (1:16).
Fig. 3. (a–g) Far-IR spectra of CdS/PANI core/shell nanocomposites prepared with CdCl2 to Na2S molar ratio of (a) (16:1), (b) (8:1), (c) (4:1), (d) (1:1), (e) (1:4), (f) (1:8) and (g) (1:16).
The presence of this band reflects the increase in the electron delocalization and decrease of CdS particle size. The absorption bands noted at 1118 and 840 cm1 correlate to 1, 2, 4-ring substitution whereas those at 1018, 1032, 802, 746, 690 and 500 cm1 is 1, 4ring substitution [38]. The intensity of the band at 1118 cm1 was decreased as the size of CdS core decreases whereas the band at 1018 cm1 and the shoulder at 1050 cm1 behave in opposite manner. In addition, the weak signals at 802, 746, 690 and 500 cm1 continuously grow as the CdS particle size decreases. The presence of 1, 4-ring substitution absorption peaks at the expense of that of 1, 2, 4-ring substitution indicates that the coupling rings produce preferentially linear elements of chain and/or the
Fig. 6 shows the ultraviolet visible spectra of CdS prepared at different concentration of CdCl2 to Na2S [(1:16), (1:8) and (1:4)]. The shoulder appeared at about 250 nm may be assign to the optical transition of the first excitonic state. The broad and low intensity band at about 490 nm of CdS prepared at the ratio (1:16) is due to the allowed transition between the electronic state in the conduction band and the hole state in the valence band. It was blue shifted gradually from 490 to 473 nm as the molar ratio of the CdCl2 to Na2S changed from (1:16) to (1:4). These results indicating that, the particle size of CdS decreases with increasing the Cd ions [44,45]. There is no characteristic peak of PANI and/or aniline monomer could not detect in these spectra. This observation is agreement with the results of FTIR. Fig. 7 shows the electronic spectra of aniline and CdS/PANI nanocomposite prepared at the CdCl2 to Na2S ratio [(1:1), (4:1), (8:1) and (16:1)]. There is no absorption band in the wavelength range from 1000 to 300 nm of aniline and the two bands at 276 and 228 nm can be assigned to p p⁄ and locally excited n p⁄ transition between energy levels respectively. By comparing the aniline spectrum with CdS/PANI nanocomposites spectra it seen that, these two bands slightly red shifted in CdS/PANI nanocomposites. This shift was increased with increasing the Cd ions. A new absorption peak was observed at 348 nm for the sample of CdCl2 to Na2S ratio (1:1) assign to p p⁄ transition. It is related to the extent of conjugation between adjacent phenyl rings in the PANI chain [41]. This band was red shifted and become broad; in the samples of CdCl2 to Na2S ratios of (4:1) and (8:1). This is may be due to convoluted with the absorption peak of the CdS nanoparticles that appear in the range 460– 400 nm. It is appeared and become clear again at 420 nm for the ratio of CdCl2 to Na2S (16:1). The red shift occur in the above mentioned absorption peaks can be interpreted in terms of the extent of conjugation of the double bond between phenyl rings in the PANI chain. With increasing the conjugation length, the stabilization energy is increased. This means that, the energy level of the antibonding orbital decrease and consequently the energy required for the transition of p and n electrons to p⁄ level decreases. The spectra in Fig. 7 contain the absorption bands of CdS nanoparticle [46–50]. It is not display typical PANI bands due to the PANI formed even with an oxidant differs from the typical PANI [51]
160
R. Seoudi et al. / Journal of Molecular Structure 1013 (2012) 156–162
Fig. 5. (a–d) FTIR spectra of CdS/PANI core/shell with CdCl2 to Na2S molar ratio of: (a) (1:1), (b) (4:1), (c) (8:1), and (d) (16:1). The bands at 1272, 1232 and 1176 cm1 have been deconvoluted into Gaussian peaks.
Table 1 The intensity ratio of infrared absorption bands at 1565–1495 cm1 registered for different CdS/PANI core/shell. Cd:S ratio
1:1 4:1 8:1 16:1
Intensity
Ratio N@Q@N/NABAN
N@Q@N Peak at 1565
NABAN Peak at 1495
I1565/I1495
0.808 1.212 1.300 1.649
0.679 0.989 1.015 1.250
1.19 1.23 1.28 1.32
3.5. Reaction mechanism of CdS/PANI nanocomposite The UV–visible spectra confirmed that, the absorption energy was increased with increasing the concentration of Cd2+ ions during the preparation process of CdS nanoparticles due to the forming of small particles. Since the particle sizes are extremely small, and the particle diameters are much less than the excitation wavelength, they exhibit negligible light scattering, thus the accurate determination of quantum yields in heterogeneous photochemistry is possible. The surface energy of the small particles is very high and the excess of metal ions chemisorbed on the surface during synthesis and generate a cloud of oppositely charged counter ions. This cloud is called electric double layer. These layers are resulted from a Columbic repulsion between two particles which in turn stabilizing the formed nanoparticles and prevent particle agglomeration as shown in Fig. 8. Due to high surface energy, surface is passivated with cations (Cd2+), which in turn attract anions (Cl) towards the surface of the particle. Increase in the sulfur ions S2 increases the particles sizes of CdS and this is evident in the UV–visible spectra. Also, when the Na2S ratio increases, the number of Na+ was increased and this leads to increase of the ionic strength for the reaction medium. According to the Debye–Hückel-theory [52] for electrolytes the thickness of the double layer is defined by:
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi er eo kB T ¼ 2NA e2 I j 1
Fig. 6. (a–c) UV–visible spectra of CdS nanoparticles with CdCl2 to Na2S molar ratios of (a) (1:4), (b) (1:8), and (c) (1:16) after reaction with aniline monomer.
or perhaps the thickness of the PANI shell around CdS is very small and their concentrations is low during UV–visible measurements.
with er and eo being the electric permittivity of the medium and the vacuum, kB Boltzmann’s constants, NA Avogadro’s constant, T the absolute temperature, e the elementary charge and I the ionic strength. The thickness of the double layer depends strongly on the ionic strength in the solution. This can cause the electrostatically stabilized in solution can be coagulated if the ionic strength of the dispersing medium is increased and this leads to the compression of the double layer and shortens the range of the repulsion [53]. Reducing the Columbic repulsions, causing the particles to collide with each other frequently and aggregate to form larger particles, which settle down due to their weight. The size of the particles is directly related to the absorption wavelength, the particle size growth recognize as a continuous shift of the excitonic absorption
161
R. Seoudi et al. / Journal of Molecular Structure 1013 (2012) 156–162
Fig. 8. Simplified model of CdS particle nucleating due to reaction between Cd2+ and S2 growing subsequently to form seeds.
Fig. 9. Schematic nanocomposite.
illustration
of
the
formation
of
CdS/PANI
core/shell
the surface of the CdS nanoparticles, resulting in the considerably higher local concentration of aniline around the CdS particles. With the absorption of visible light the photogenerated hole react with adsorbed aniline molecule to form resonance-stabilized aniline radical cations and initiated the polymerization reaction. During the oxidative polymerization, PANI was initially formed on the part of the CdS surface, which may act as a point of nucleation, and then it preferentially grew around the CdS particles as a core/shell formation (see Fig. 9) [56]. 4. Conclusion
Fig. 7. (a–e) UV–visible absorption spectra of: (a) aniline monomer and different CdS/PANI core/shell with CdCl2 to Na2S molar ratios of (b) (1:1), (c) (4:1), (d) (8:1), and (e) (16:1).
band to longer wavelengths. To check the degree of homogeneous polymerization, aniline monomer was added to methanol in the presence of CdS nanoparticles and stirring for 48 h under dark. The UV–visible absorption spectrum not changed and this indicating that CdS nanoparticles unable to oxidize the aniline in the polymerization process without absorption of light. The band gap energy of CdS nanoparticles can be easily activated with visible light (1.65–3.26 eV) [54]. Small nanopartcile have higher surface area to volume ratios thus enhancing their catalytic activity and these properties are combining to give high polymerization efficiencies in the increase in effective band gap energy with decreasing particle sizes. Illumination of the CdS with light of energy greater than the band gap leads to band gap excitation resulting in creation of electron–hole pairs; holes in the valence band and electrons in the conduction band. þ
CdS þ ht ! hVB þ eCB Since the recombination of electron–hole pairs in CdS is so rapid; it occurring in a picoseconds and it can be used as an effective photocatalysis [55]. Therefore when CdS of higher CdCl2 to Na2S molar ratios (1:1, 4:1, 8:1 and 16:1) it immersed into the aniline solution, aniline monomers appear to be physically adsorbed (through hydrogen bonding and/or electrostatic interaction) onto
A series of CdS/PANI core/shell nanocomposites were successfully prepared at ambient conditions. X-ray and TEM results confirmed that, the CdCl2 to Na2S molar ratio is a determining factor for controlling the particle size of CdS/PAN core shell nanocomposites. The CdS nanoparticles prepared at the ratios (1:4), (1:8) and (1:16) appeared without PANI shell. As the particle becomes smaller, there is a larger surface area-to-volume ratio enhances light absorption, increasing the probability of radical formation at the surface. Smaller volume, meanwhile, increases the probability that the electron–hole pair will reach the surface to participate in redox reactions with the medium. References [1] M. Molaei, M. Marandi, E. Saievar-Iranizad, N. Taghavinia, B. Liu, H.D. Sun, X.W. Sun, J. Lumin. 132 (2012) 467. [2] K. He, M. Li, L. Guo, Int. J. Hydrogen Energy 37 (2012) 755. [3] C. Wang, D. Chen, X. Jiao, Sci. Technol. Adv. Mater. 10 (2009) 23001. [4] W. Yao, S. Yu, Adv. Funct. Mater. 18 (2008) 3357. [5] G. Yi, C. Wang, W. Park, Semicond. Sci. Technol. 20 (2005) 22. [6] M. Xiao, L. Wang, Y. Wu, X. Huang, Z. Dang, Nanotechnology 19 (2008) 15706. [7] S.S. Ray, M. Okamoto, Prog. Polym. Sci. 28 (2003) 1539. [8] J. Li, J.Z. Zhang, Coord. Chem. Rev. 253 (2009) 3015. [9] D. Wang, Semiconductor Nanocrystal Quantum Dots, Springer-Vienna, 2008. pp. 171–196. [10] B. Chitara, S.V. Bhat, S.R.C. Vivekchand, A. Gomathi, C.N.R. Rao, Solid State Commun. 147 (2008) 409. [11] N. Parvatikar, S. Jain, S. Khasim, M. Revansiddapp, S.V. Bhoraskar, M.V.N. Prasad, Sens. Actuators, B 114 (2006) 599. [12] J.S. Miller, Adv. Mater. 5 (2004) 587. [13] Q. Sun, M. Park, Y. Deng, Mater. Chem. Phys. 110 (2008) 276. [14] X. Luo, A.J. Killard, A. Morrin, M.R. Smyth, Anal. Chim. Acta 575 (2006) 39. [15] G. Cui, J.S. Lee, S.J. Kim, H. Nam, G.S. Cha, H.D. Kim, Analyst 123 (1998) 1855.
162
R. Seoudi et al. / Journal of Molecular Structure 1013 (2012) 156–162
[16] A.A. Karyakin, L.V. Lukachora, E.E. Karyakina, A.V. Orlov, G.P. Kappachora, Anal. Commun. 36 (1999) 153. [17] J.X. Huang, J.A. Moore, J.H. Acquaye, R.B. Kaner, Macromolecules 38 (2005) 317. [18] R.K. Naqarale, G.S. Gohil, V.K. Shahi, R. Ranqarajan, J. Colloid Interface Sci. 287 (2005) 198. [19] Y. Nosaka, N. Ohta, H. Miyama, J. Phys. Chem. 94 (1990) 3752. [20] J. Rabani, J. Phys. Chem. 93 (1989) 7707. [21] A.J. Hoffman, G. Mills, H. Yee, M.R. Hofmann, J. Phys. Chem. 96 (1992) 5546. [22] X.F. Lu, Y.H. Yu, L. Chen, H.P. Mao, W.J. Zhang, Y. Wei, Chem. Commun. 13 (2004) 1522. [23] X.F. Lu, H. Gao, J.Y. Chen, D.M. Chao, W.J. Zhang, Y. Wei, Nanotechnology 16 (2005) 113. [24] R. Seoudi, M. Kamal, A.A. Shabaka, E.M. Abdelrazek, W. Eisa, Synth. Met. 160 (2010) 479. [25] J.L. Boone, G. Cantwell, M.D. Shaw, J. Appl. Phys. 58 (1985) 2296. [26] L. Zhang, H. Peng, P.A. Kilmarti, C. Soeller, R. Tilley, J. Travas-Sejdic, Macromoecul. Rapid Commun. 29 (2008) 598. [27] L. Lutterotti, S. Matthies, H.-R. Wenk, in: Proceeding of the Twelfth International Conference on Textures of Materials (ICOTOM-12), vol. 1, 1999, p. 1599. [28] C.B. Murray, D.J. Norris, M.G. Bawendi, J. Am. Chem. Soc. 115 (1993) 8706. [29] H.C. Warad, S.C. Ghosh, B. Hemtanon, C. Thanachayanont, J. Dutta, Sci. Technol. Adv. Mater. 6 (2005) 296. [30] U. Holzwarth, N. Gibson, Nat. Nanotechnol. 6 (2011) 534. [31] C. Danielle, S. Michelle, A. Ivo, Z. Aldo, Chem. Mater. 15 (2003) 4658. [32] S. Min, F. Wang, Y. Han, J. Mater. Sci. 42 (2007) 9966. [33] R.A. Nyquist, R.O. Kagel, Infrared Spectra of Inorganic Compounds, Academic Press, New York, 1971. [34] M. Trchova, I. Sedenkova, E.N. Konyushenko, J. Stejskal, P. Holler, G. CiricMarjanovic, J. Phys. Chem. B 110 (2006) 9461.
[35] M. Trchováà, J. Stejskal, Pure Appl. Chem. 83 (2011) 1803. [36] O.P. Dimitriev, N.A. Ogurtsov, A.A. Pud, P.S. Smertenko, Yu.P. Piryatinski, Yu.V. Noskov, A.S. Kutsenko, G.S. Shapoval, J. Phys. Chem. C 112 (2008) 14745. [37] A. Parsa, S.A. Ghani, Polymer 49 (2008) 3702. [38] J. Tang, X. Jing, B. Wang, F. Wang, Synth. Metal. 24 (1988) 231. [39] S. Quillard, G. Louarn, S. Lefrant, A.G. Macdiarmid, Phys. Rev. B 50 (1994) 12496. [40] I. Šedeˇnková, J. Prokeš, M. Trchová, J. Stejskal, Polym. Degrad. Stab. 93 (2008) 428. [41] M.M. Ayad, N.A. Salahuddin, A.K. Abou-Seif, M.O. Alghaysh, Eur. Polym. J. 44 (2008) 426. [42] W.R. Salaneck, B. Liedberg, O. Ingan, R. Erlandsson, I. Lundstrsm, A.G. MacDiarmid, M. Halpern, N.L.D. Somasiri, Mol. Cryst. Liq. Cryst. 121 (1985) 19. [43] J.P. Buisson, S. Lefrant, J. Phys. Chem. 100 (1996) 6998. [44] W. Wang, I. Germanenko, M.S. El-Shall, Chem. Mater. 14 (2002) 3028. [45] N. Herron, Y. Wang, H. Eckert, J. Am. Chem. Soc. 112 (1990) 1322. [46] S.V. Bhat, S.R.C. Vivekchand, Chem. Phys. Lett. 433 (2006) 154. [47] N. Singh, M.V. Kulkarni, S.P. Lonkar, A.K. Viswanath, P.K. Khanna, Synth. React. Inorg. Metal-Org. Nano-Metal. Chem. 37 (2007) 153. [48] S. Chen, L. Lin, Macromolecules 28 (1995) 1239. [49] Y. Xi, J. Zhou, H. Guo, C. Cai, Z. Lin, Chem. Phys. Lett. 412 (2005) 60. [50] P.K. Khannaa, Sunil P. Lonkar, V.V.V.S. Subbarao, Ki-Won Jun, Mater. Chem. Phys. 87 (2004) 49. [51] I. Sapurina, J. Stejskal, Polym. Int. 57 (2008) 1295. [52] A.V. Dobrynin, M. Rubinstein, J.F. Joanny, J. Chem. Phys. 109 (1998) 9172. [53] G. Schmid, Clusters and Colloids, Wiley VCH, New York, 1994. [54] C. Karunakaran, S. Senthilvelan, Sol. Energy 79 (2005) 505–512. [55] A.L. Linsebigler, G. Lu, J.T. Yates, Chem. Rev. 95 (1995) 735–758. [56] M.R. Nabid, R. Sedghi, P.R. Jamaat, N. Safari, A.A. Entezami, Appl. Catal., A: Gen. 328 (2007) 52–57.