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Effects of CdS quantum dot in polymer nanocomposites: In terms of luminescence, optic, and thermal results
Radiation Physics and Chemistry 156 (2019) 137–143
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Effects of CdS quantum dot in polymer nanocomposites: In terms of luminescence, optic, and thermal results
T
⁎
İlker Çetin Keskina, , Murat Türemişb, Mehmet İsmail Katıc, Rana Kibara, Ahmet Çetina a
Department of Physics, Faculty of Art and Science, Manisa Celal Bayar University, Manisa, Turkey Department of Physics, Faculty of Engineering and Natural Sciences, Bursa Technical University, Bursa, Turkey c Experimental Science Applications and Research Center, Manisa Celal Bayar University, Manisa, Turkey b
A R T I C LE I N FO
A B S T R A C T
Keywords: Quantum dots Radioluminescence (RL) Low-Density Polyethylene (LDPE) Optical absorption Structural characterization Thermal analysis
CdS quantum dots (QDs) which have unique luminescence efficiency were synthesized by two-phase method using oleic acid (OA) as a surfactant. The nanocomposites have been obtained by blended CdS quantum dots with low-density polyethylene (LDPE) in different ratios. Primarily, radioluminescence (RL) properties were investigated as well as their structural (FT-IR, XRD), morphological (TEM, SEM), thermal (TG-DTA) and absorption (UV–Vis) properties of these nanocomposites. The RL peaks of OA-capped CdS QD were observed at 456 nm and 655 nm. As a consequence of the nanocomposites being doped with powder CdS QD, a significant blue shift was observed in the absorption bands. The optical band gap of CdS was calculated as ̴ 2.3 eV. The nanocomposites blended with CdS QD, this value increased to ̴ 2.7 eV. It has also been observed that nanoparticles cause nanocomposites to have lower melting temperatures.
1. Introduction The quantum dots (QDs) as a semiconductor material are one of the most attractive and commonly used nanomaterials because of their optical and electronic properties. Thanks to their broad absorption range, spectral purity, photochemical stability and also ease of bandgap tunability properties, these materials gain numerous application areas (Mansur, 2010; Mansur et al., 1999; Carrillo-Carrión et al., 2009; Wei et al., 2016). Especially due to its controllable dimensions, from medicine and biotechnological applications according to their emission behavior, to imaging and hybrid solar batteries, has become widely used in many fields (Dai et al., 2015; Jokerst et al., 2009). Nanocrystals show unique properties owing to the quantum size effect and the presence of a large number of unsaturated surface atomsYang et al. (2014). Another interesting material that has recently been widely used is nanocomposites. The small volume of nano-sized additive materials means that they have a very large surface area. Thus, the superficial properties of the nanocomposite material directly influence the physical and chemical properties of the matrix (Lei et al., 2016). The low-density polyethylene based nanocomposites have attracted considerable interest in the industrial and technological field owing to their remarkable morphological, mechanical, chemical, thermal, optical and other significant changes (Hussain et al., 2006). Polyethylene (PE) is the mostly used thermoplastics among polymers due to its rigidity, ⁎
no-affected by moisture, perfect chemical inertness, flexibility, and extensibility (Türemiş et al., 2018). The LDPE was chosen for this study because of its ease of processing and its extraordinary electrical properties. The interaction of polymer and QD plays an important role in the luminescence and optical changes of nanocomposites. The interaction between polymer and Cd ions can be either intra- or interchain or both. The intrachain is prevalent for a numerous group of polymer-metal chelates showing comparatively high chemical and thermal stability. The factors that can affect the polymer-metal ion interactions are intrinsic to the polymer: nature of atoms in the chain, nature of the functional groups, molecular weight and polydispersity, and the distance between functional groups. Outer factors for the polymer; ionic strength, nature, and charge of the metal ion, temperature, or nature of the counterion of the metal ion (Rivas et al., 2008). The CdS quantum dot prepared by the two-phase method was doped into low-density polyethylene (LDPE) and polymer nanocomposites were prepared by solution blending method. The optical absorption spectra of composites which are consisted of three polymer nanocomposites prepared with CdS adding and a pure LDPE composite were obtained, and the band gap energy values of samples were calculated from these spectra. The previous studies mainly focus on the photoluminescence (PL) or fluorescence spectroscopies. It is because of the availability and easy to use of the systems. Although the RL provides greatly enhanced spectral
Corresponding author. E-mail address: [email protected] (İ.Ç. Keskin).
https://doi.org/10.1016/j.radphyschem.2018.11.006 Received 19 February 2018; Received in revised form 27 September 2018; Accepted 6 November 2018 Available online 09 November 2018 0969-806X/ © 2018 Elsevier Ltd. All rights reserved.
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data by the volumetric excitation of the sample throughout, the setup of the system and data collection is relatively challenging. However, in the fluorescence system, which is frequently used in the quantitative analysis of quantum dots, the material can only be examined in the colloid structure. If the sample is solid, it should be dissolved in certain molarities with solvent. We specifically used RL system mainly for three reasons; measurements can be taken from a pure sample without using any solvent, the spectrum can be obtained in any phase, and the detected emission intensity is significantly higher than other techniques The latter, the PL and fluorescence spectroscopy, which are used to determine the emission characteristics of QDs, is usually induced by low energy sources compared with the ionizing radiation. The RL system, in which high-energy X-rays is used as excitation source enables the detection of sputtered electrons by inducing electrons in deep traps in semiconductor nanocrystals. This is a significant advantage of the RL system (Kati et al., 2012; Arslanlar Tuncer et al., 2013).
(111)
CdS QD
Intensity (a.u)
1680
1260
(220) (311)
840 (331)
420
0 10
20
30
40
50
60
70
80
2θ (degree)
Fig. 1. XRD pattern of powder CdS QD.
gun, under accelerating voltage in the range of 20–120 kV. The powder XRD analysis of the CdS was recorded using a Panalytical X-ray diffractometer with CuKα radiation source, equipped with a graphite monochromator. XRD pattern was obtained by step scanning from 10° to 80° 2θ in steps of 0.0501° with account of 2 s per step, i.e.fast XRD profiles of 30 min. The FTIR spectra were recorded for absorbance in the region 650–4000 cm−1 using an Agilent Technologies Cary 660 Spectrometer with room temperature. The thermal analysis (TG-DTA) of samples was performed Hitachi SII Exstar 7300 thermal analyzer. The thermal behavior of nanocomposites was studied in the temperature range of 25–575 °C at a heating rate of 10° min−1 in the air atmosphere. SEM was used to examine the morphology of nanocomposites by using a Philips XL-30S FEG e SEM.
2. Materials and method 2.1. Synthesis of CdS nanocrystal CdS nanocrystals were synthesized by the two-phase method (Wang et al., 2005, 2006). 0.4 g of cadmium myristate as a cadmium source, 2 g of oleic acid as a surfactant is dissolved in 80 ml of toluene at a temperature of 80 °C and kept for later use in the process. In a further step, 80 ml of distilled water is heated to 100 °C under nitrogen and thiourea as a sulfur source is added to the reaction flask. The temperature was kept constant at 100 °C. Nitrogen saturation was provided, and the toluene phase solution was added to the water phase with the aid of a stirrer. The growth and optical properties of nanocrystal were controlled with UV–Vis by taking samples at different intervals during the process. When the desired size is obtained according to the absorption spectra, the reaction is stopped. The biphasic mixture in the reaction flask is separated using a separatory funnel.
3. Results and discussion 3.1. XRD pattern of CdS QD The XRD pattern of the OA capped CdS QD shown in Fig. 1. The broad peaks refer that the grain size of nanopowder (NP) is very small. The XRD pattern also confirms the crystalline cubic structure of CdS. Diffraction peak shown in this pattern corresponds to the (1 1 1), (2 2 0), (3 1 1) and (3 3 1) plane reflections of cubic CdS. There are three main diffraction peaks at 26,7°, 44.1°, 52,1°. The lattice parameter a = 5.83 Å is in correlation with the values reported in the literature (Dai et al., 2007; Li et al., 2014).
2.2. Preparation of QD added polymer nanocomposites In the heater with the temperature stabilized at 130 °C, the weighed LDPE granules were heated to the required softness in the borosilicate glass beaker. The weighed powder CdS QD was completely dissolved by stirring in 20 ml of toluene. This solution is incorporated into the molten polyethylene. Also, toluene is a proper solvent for polyethylene material. 20 ml of toluene in the solution provides even better dissolution of the polyethylene (Li et al., 2006). The mixture was stirred at 130 °C. for 10 min to remove toluene from it. Then, the mixture was cooled down to room temperature and allowed to stand for at least 48 h. In a very small amount of toluene residues that may be present have been removed from the acceptable level. 0.30 g of the dried mixture was weighed and again softened by heating at 110 °C for 15 min at the melting point of polyethylene. Thus, more homogeneous and smoother composites are obtained by pressurizing. The sample is cooled down to room temperature under a pressure of 10 MPa. 1.5 mm thickness and 16 mm diameter smooth surface nanocomposites were obtained. For the RL measurements of the CdS QD and nanocomposites, excitation was made with a Machlett OEG-50A X-ray tube operated at a maximum experimental level of 30 kV and 15 mA. Luminescence detection system is conducted with a Yobin Yvon spectrometer, coupled to a liquid nitrogen cooled CCD detector. The fluorescence spectra of CdS QD were recorded by Agilent Technologies Cary Eclipse Spectrophotometer at 290 nm excitation wavelength. Optical absorption spectra of QD and nanocomposites were recorded at room temperature in the wavelength region of 200–2000 nm using Perkin-Elmer Lambda 950 spectrophotometer. TEM images were obtained using a FEI Tecnai G2 -Biotw Spirit High Contrast Transmission Electron Microscopy and operated with Lanthanum hexaboride (LaB6) electron
3.2. Size determination of CdS QD The TEM image and the histogram of the particle size distribution of the OA capped CdS QD is shown in Fig. 2 and Fig. 3. The distribution of particle size for CdS QDs are nearly monodispersed and spherical in
Fig. 2. The TEM image of OA capped CdS QD. 138
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3.4. SEM Images of nanocomposites The images reveal that the grains of QD are spherical in shape and uniformly distributed all over the surface of the blends. The distribution with increasing the contribution ratio is seen to take a more orderly state. LDPE has covered up the quantum dots, so this situation has led to slightly decrease in RL intensity. When (c) and (d) images compared in Fig. 5, it's seen that nanocomposite which has higher contribution rate is more homogeneous. This condition corresponds to broader absorption bands in the absorption spectra. As shown in Fig. 5, the 100 µm magnification image of 5% QD doped sample was not enough to detect quantum dot structures. However, it appears that the surface of the nanocomposite was quite smooth. Fig. 3. The particle size histogram of CdS.
3.5. Optical properties shape. The TEM image of CdS QD revealed a distinctly spherical shape pattern of the particle distribution and also showed that the particles were highly monodispersed and homogenous. The average size distribution of the CdS QD was measured ̴ 2.6 nm.
The radioluminescence measurements of CdS QD and the prepared nanocomposites were taken with the RL system created by Jobin Yvon monochromatic integrated X-ray unit. Fig. 6 shows that the powder CdS and RL spectra of quantum dot added polymer nanocomposites. In Fig. 6, the main emission peak of CdS QD is located at 655 nm and there is a weak peak at 456 nm. The recombination of excitons and trapped electron-hole pairs at shallow traps that cause the band edge luminescence between 450 and 500 nm. Under X-ray excitation, the RL emission of CdS QDs at 465 nm indicates that the photo generated electron-hole pair was trapped with upon their recombination (Dhage et al., 2011). The wide peak which is extending from ̴ 520 nm to ̴ 900 nm was observed around the red emission band centered at 655 nm. This broad emission is attributed to deep-level emission which was due to crystalline surface defects (Dai et al., 2007; Husham et al., 2015; Shen et al., 2013). These defects could originate from, VS (S vacancy), VCd (Cd gap), S (S cracks), Cd (Cd cracks) (Ma et al., 2002; Talakonda, 2016). The quenching of emission intensity is quite high for CdS QD. The spectrum of the powder CdS QD is reduced 20 times so that the nanocomposites can be displayed together with the RL spectra. The spectra of nanocomposites showed blue shift compared with the quantum dot spectrum, and it's appear that LDPE reduced the RL intensity of the QD. In the RL spectrum especially main peak of powder QD at 655 nm obviously shifted blue region at around 625 nm for QD added LDPE matrix. So, with the reduction of the contribution rate in the nanocomposites, the samples were seen to have emission peaks at higher energy levels with minor shifts. This phenomenon, which is expressed as the blue shift in the RL spectrum, is also an indication that the material has become more insulator due to the band theory. As a matter of fact, the CdS QD known as semiconductors has become more insulating when blended with the polymer matrix, and about 30 nm shift in the RL spectrum confirms that. Of course, the absorption spectra of nanocomposites have also supported this view. The decrease in RL intensity upon addition of CdS QD could be attributed to the charge transfer from polymer to the nanocrystals. This charge transfer results from the formation of separated electron-hole pair that recombines non-radiatively which leads to quenching of emission intensity of nanocomposites (Fang et al., 2010; Keskin et al., 2017). In Fig. 7, the low intensity of the emission peak of CdS, which has a broad fluorescence spectrum, is thought to be caused by surface defects that may occur during the synthesis process or surfactant material (Sehgal and Narula, 2015; Wang and Hsiao, 2009). CdS have fluorescence emission around 430 nm. The optical band gap values obtained by using the Eg = hc/λonset equation, where λ is the onset wavelength which can be determined by the intersection of two tangents on the absorption edges. Also, λonset represents the electronic transition start wavelength (Townsend and Kelly, 1973). From the Fig. 8, the optical band gap of CdS was calculated as 2.29 eV (Kaur and Tripathi, 2015; Nieto-Zepeda et al., 2017).
3.3. FT-IR spectra of CdS QD and nanocomposites Fig. 4 shows the FT-IR spectra of CdS QD and doped nanocomposites. In Fig. (4-b), it was determined that the absorbance intensity of pure LDPE showed a decrease with the contribution. The absorbance decrease means that as an inorganic structure CdS QD was successfully doped inside LDPE matrix which has an organic structure (Yu et al., 2003; Mohan and Oluwafemi, 2016). However, in the inset spectra at 900–1800 cm−1 it is observed that especially the 5% added sample gives characteristic peaks by increasing the intensity, especially at 1543 cm−1 differs from LDPE spectrum. As it known in the literature the OA has a strong band at 1711 cm−1 which is derived from –C˭O stretching in carboxyl group. This band can be seen in the spectrum of OA capped CdS at 1543 cm−1 for –C-O stretching. It means that OA ligands are capped on the CdS QD surface, forming a carboxylate (-COO-) structure (Kim et al., 2015; Ayele, 2016; Zhang et al., 2006). In the FT-IR spectrum of OA capped CdS QD (Fig. 4-a), there are main peaks at 2848 cm−1 and 2915 cm−1 which are due to the –CH2 symmetric and asymmetric stretching. The peak at 1421 cm−1 is attributed to the OH bending from OA. The peaks at 1409 cm−1 may be attributed to sulphate group S˭O (Chandra et al., 2014) The C˭O bonds disappeared in the spectrum of the CdS NCs at 1290 cm−1 (Teng et al., 2008), the band at 1285 cm−1 exhibited the presence of the C–O stretch (Saravanan et al., 2011). The peaks at 1469 cm−1 for -CH2 scissoring and 719 cm cm−1 for -(CH2)n-vibration (Kim et al., 2015).
Absorbance (a.u)
(a)
(b)
Wavenumber (cm )
Fig. 4. FT-IR spectrum of CdS QD (a) and CdS doped polymer nanocomposites (b). 139
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Fig. 5. The SEM images taken for sample LDPE (a), OA capped powder CdS QD (b), 0.2 wt% CdS nanocomposite (c), 5 wt% CdS nanocomposite (d).
The absorption spectrum of LDPE (Deiting et al., 2016) showed a significant shift to the red region with the CdS QD contribution. It's seen that with increasing of contribution rate, it is seen that optical band gap values can be reduced from 2.71 eV to 2.58 eV (Table 1). At the 0.2 wt %, CdS added sample, even though the additive ratio was very low, is reduced dramatically the band gap value of LDPE. The band gap of LDPE at about 4.6 eV has decreased to 2.7 eV. According to the values given in Table 1, it is seen that the band gap values obtained from both methods are very close to each other.
1
RL Intensity (a.u)
/20
3.6. Thermal analysis of nanocomposites The thermal stability of LDPE and CdS QD added nanocomposites have been investigated. Thermogravimetric curves are given in Fig. 9. Although the thermogravimetric curves of nanocomposite samples showed no significant differences taking into account compared with the LDPE, The change became obvious by the QD amount increasing in the polymer matrix. Liu et al. (2013). At the TG analysis, the effect of the nanocrystals added in LDPE on the melting rate of nanocomposites was observed. As can be seen in Table 2, at low addition ratios (0.2% and 1%), nanocomposites show close thermal stability to LDPE. In spite of that, the sample with 5% CdS addition showed faster degradation than the other nanocomposites (Ma et al., 2013; Li et al., 2015). In Fig. 10, CdS QD also contributed to the thermal conductivity of the polymer matrix, with the most active being dominant in the 5% CdS doped sample. The melting point of pure polyethylene decreased from 110 °C (Şirin et al., 2009; Suthan et al., 2011) to 106 °C, due to the quantum dot addition (Fig. 11). Nano-sized additive materials have lower melting temperature and melting time compared to micron-sized materials. However, they require higher melt torque. Cowie (2007) Taking into account the contribution of the additive materials used in composites to the heat transfer, it is evident in Fig. 11 that the quantum dot additives cause a significant change in the thermal conductivity properties when added into the polymer matrix. In Fig. 8, it is seen that the conductivity characteristic of CdS is much higher than that of the polymer in the graph of optical absorption. Compared with pure polyethylene, it is
Wavelength (nm)
Fig. 6. RL spectra of nanocomposites in different ratios (0.2, 1, 5 wt%.CdS) and CdS QD.
Fig. 7. The fluorescence spectrum of OA capped CdS QD and images. The inset images of the nanocomposites under UV lamp (λex= 365 nm) and daylight, which is apparent in the 5 wt% (a), 1 wt% (b), 0.2 wt% (c) CdS added nanocomposites, and pure LDPE (d) The nanocomposites are seen violet under UV light while they are orange-yellow in daylight. The powder CdS in eppendorf is also the same.
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Fig. 8. The optical spectra of pure LDPE, OA capped CdS QD powder, and CdS QD added polymer nanocomposites. The absorption spectra (a) and the Tauc plot (b) to determine the bandgap of the samples.
0.2 wt%
1 wt%
5 wt%
CdS QD
267 4,64 4.52
457 2.71 2.72
459 2.70 2.65
481 2.58 2.60
540 2.29 2.27
Weight (%)
Wavelength (nm) Band Gap (eV) Band Gap (eV) (Tauc Plot)
LDPE
M icrovolt Exo Down (μ V)
Table 1 The calculated optical band gap (Eg) and onset wavelength values of CdS QD, LDPE and, CdS added nanocomposites.
Temperature ( C)
Microvolt Exo Down ( μ V)
Fig. 10. DTA curves of the nanocomposites.
Temperature (oC)
Fig. 9. TG curves of CdS doped polymer nanocomposites. Table 2 Thermal degradation values of nanocomposites. Degradation Step (°C)
Residue Weight Percentage
Sample
Tstart
Tmax
Tend
430 °C
460 °C
490 °C
LDPE 0.2 wt% 1 wt% 5 wt%
184 277 333 215
475 476 475 473
496 496 497 496
16.70 4.80 4.48 8.46
24.41 20.72 21.33 25.37
89.39 89.22 89.47 91.51
Temperature ( C)
Fig. 11. The changing in the melting point of LDPE with QD additive.
clear that the thermal conductivity of the nanocomposites formed by the incorporation of CdS into the polymer increases in proportion to the percentage of the additive. This effect is most apparent in the 5 wt% CdS added sample. While the melting point of pure polyethylene is 110 °C, the melting point of this sample has reduced to 106 °C. In the TG graph of the same sample in Fig. 9, the effect of CdS on the polymer is noteworthy. The nanocrystals have caused the nanocomposite to have a lower melting temperature.
4. Conclusions The lattice defects, impurity defects, structural defects and their distribution in the structure are responsible for the luminescence emission that is commonly observed in semiconductor materials during X-ray excitation. In this respect, in this study aimed to show the radioluminescence behavior of QD structure and nanocomposites blended with QD. The polymer nanocomposites have been prepared with CdS 141
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quantum dots by considering that quantum dots may be an ideal nanocomposite additive due to their high quantum yield and the fact that energy band gap and dimensions can be controlled. The optical, structural, morphological, and thermal properties of LDPE matrix nanocomposites blended with QD at different ratios have been investigated. The nanocomposites showed the same luminescence characteristics as the powder QD. It has been observed that these nanocomposites with QD additive have significant advantages in experimental studies in terms of being easy to shape, being flexible, being able to prepare in desired quantity and size and having more suitable storage conditions. The radioluminescence properties of CdS QD and nanocomposites were investigated. Two different peaks were obtained at 456 nm and 655 nm. The nanocomposites showed blue shift compared with the CdS RL spectrum, and the LDPE matrix has reduced the intensity of the CdS QD. OA as a surfactant is a significant factor for RL emission and intensity. Also, the decrease in RL intensity could be referred to the various atom vacancies, surface defects and charge transfer from polymer to the nanocrystals. The CdS QD show a broad fluorescence emission band located at ∼430 nm under UV excitation (λ = 290 nm). The band gap of OA capped CdS is determined to be 2.3 eV. It was observed that the bandgap of CdS added nanocomposites increase with the addition ratio of the LDPE. By increasing the contribution rate, it has been seen that optical band gap value of LDPE can be reduced from ∼4.6 eV to ∼2.6 eV. The melting point of LDPE decreased from 110 °C to 106 °C, due to the quantum dot addition.
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Acknowledge We would like to thank Prof. Dr. Mahmut Kuş and Sümeyra Büyükçelebi for the synthesis of the quantum dot. References Arslanlar Tuncer, Y., Kibar, R., Çetin, A., Canımoğlu, A., 2013. Radioluminescence properties of copper- and terbium-implanted strontium titanate radioluminescence properties of copper- and terbium-implanted strontium titanate. Lett. Spectrosc. 364–366. https://doi.org/10.1080/00387010.2012.738278. Ayele, D.W., 2016. A facile one-pot synthesis and characterization of Ag2Se nanoparticles at low temperature. Egypt. J. Basic Appl. Sci. 3, 149–154. https://doi.org/10.1016/j. ejbas.2016.01.002. Carrillo-Carrión, C., Cárdenas, S., Simonet, B.M., Valcárcel, M., 2009. Quantum dots luminescence enhancement due to illumination with UV/Vis light. Chem. Commun. (Camb.). 5214–5226. https://doi.org/10.1039/b904381k. Chandra, P.P., Mukherjee, A., Mitra, P., 2014. Synthesis of nanocrystalline CdS by SILAR and their characterization. J. Mater. 2014, 1–6. https://doi.org/10.1155/2014/ 138163. Cowie, J.M., 2007. Polymers: Chemistry & Physics of Modern Materials, 2nd edition. CRC Pres, Newyork. Dai, G., Gou, G., Wu, Z., Chen, Y., Li, H., Wan, Q., Zou, B., 2015. Fabrication and microphotoluminescence property of CdSe/CdS core/shell nanowires. Appl. Phys. A 119, 343–349. https://doi.org/10.1007/s00339-014-8973-3. Dai, Z., Zhang, J., Bao, J., Huang, X., Mo, X., 2007. Facile synthesis of high-quality nanosized CdS hollow spheres and their application in electrogenerated chemiluminescence sensing. J. Mater. Chem. 17, 1087. https://doi.org/10.1039/b614203f. Deiting, D., Börno, F., Hanning, S., Kreyenschmidt, M., Seidl, T., Otto, M., 2016. Investigation on the suitability of ablated carbon as an internal standard in laser ablation ICP-MS of polymers. J. Anal. At. Spectrom. 31, 1605–1611. https://doi.org/ 10.1039/C6JA00020G. Dhage, S.R., Colorado, H.A., Hahn, T., 2011. Morphological variations in cadmium sulfide nanocrystals without phase transformation. Nanoscale Res. Lett. 6, 1–5. https://doi. org/10.1186/1556-276X-6-420. Fang, Y., Chen, L.I., Wang, C., Chen, S.U., 2010. Facile synthesis of fluorescent quantum dot-polymer nanocomposites via frontal polymerization. J. Polym. Sci. Polym. Chem. 48, 2170–2177. https://doi.org/10.1002/pola.23986. Husham, M., Hassan, Z., Selman, A.M., Allam, N.K., 2015. Microwave-assisted chemical bath deposition of nanocrystalline CdS thin films with superior photodetection characteristics. Sens. Actuators, A Phys. 230, 9–16. https://doi.org/10.1016/j.sna. 2015.04.010. Hussain, F., Hojjati, M., Okamoto, M., Gorga, R.E., 2006. J. Compos. Mater. https://doi. org/10.1177/0021998306067321. Jokerst, J.V., Raamanathan, A., Christodoulides, N., Floriano, P.N., Pollard, A.A., Simmons, G.W., Wong, J., Gage, C., Furmaga, W.B., Redding, S.W., McDevitt, J.T., 2009. Nano-bio-chips for high performance multiplexed protein detection: determinations of cancer biomarkers in serum and saliva using quantum dot bioconjugate
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Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem
Effects of CdS quantum dot in polymer nanocomposites: In terms of luminescence, optic, and thermal results
T
⁎
İlker Çetin Keskina, , Murat Türemişb, Mehmet İsmail Katıc, Rana Kibara, Ahmet Çetina a
Department of Physics, Faculty of Art and Science, Manisa Celal Bayar University, Manisa, Turkey Department of Physics, Faculty of Engineering and Natural Sciences, Bursa Technical University, Bursa, Turkey c Experimental Science Applications and Research Center, Manisa Celal Bayar University, Manisa, Turkey b
A R T I C LE I N FO
A B S T R A C T
Keywords: Quantum dots Radioluminescence (RL) Low-Density Polyethylene (LDPE) Optical absorption Structural characterization Thermal analysis
CdS quantum dots (QDs) which have unique luminescence efficiency were synthesized by two-phase method using oleic acid (OA) as a surfactant. The nanocomposites have been obtained by blended CdS quantum dots with low-density polyethylene (LDPE) in different ratios. Primarily, radioluminescence (RL) properties were investigated as well as their structural (FT-IR, XRD), morphological (TEM, SEM), thermal (TG-DTA) and absorption (UV–Vis) properties of these nanocomposites. The RL peaks of OA-capped CdS QD were observed at 456 nm and 655 nm. As a consequence of the nanocomposites being doped with powder CdS QD, a significant blue shift was observed in the absorption bands. The optical band gap of CdS was calculated as ̴ 2.3 eV. The nanocomposites blended with CdS QD, this value increased to ̴ 2.7 eV. It has also been observed that nanoparticles cause nanocomposites to have lower melting temperatures.
1. Introduction The quantum dots (QDs) as a semiconductor material are one of the most attractive and commonly used nanomaterials because of their optical and electronic properties. Thanks to their broad absorption range, spectral purity, photochemical stability and also ease of bandgap tunability properties, these materials gain numerous application areas (Mansur, 2010; Mansur et al., 1999; Carrillo-Carrión et al., 2009; Wei et al., 2016). Especially due to its controllable dimensions, from medicine and biotechnological applications according to their emission behavior, to imaging and hybrid solar batteries, has become widely used in many fields (Dai et al., 2015; Jokerst et al., 2009). Nanocrystals show unique properties owing to the quantum size effect and the presence of a large number of unsaturated surface atomsYang et al. (2014). Another interesting material that has recently been widely used is nanocomposites. The small volume of nano-sized additive materials means that they have a very large surface area. Thus, the superficial properties of the nanocomposite material directly influence the physical and chemical properties of the matrix (Lei et al., 2016). The low-density polyethylene based nanocomposites have attracted considerable interest in the industrial and technological field owing to their remarkable morphological, mechanical, chemical, thermal, optical and other significant changes (Hussain et al., 2006). Polyethylene (PE) is the mostly used thermoplastics among polymers due to its rigidity, ⁎
no-affected by moisture, perfect chemical inertness, flexibility, and extensibility (Türemiş et al., 2018). The LDPE was chosen for this study because of its ease of processing and its extraordinary electrical properties. The interaction of polymer and QD plays an important role in the luminescence and optical changes of nanocomposites. The interaction between polymer and Cd ions can be either intra- or interchain or both. The intrachain is prevalent for a numerous group of polymer-metal chelates showing comparatively high chemical and thermal stability. The factors that can affect the polymer-metal ion interactions are intrinsic to the polymer: nature of atoms in the chain, nature of the functional groups, molecular weight and polydispersity, and the distance between functional groups. Outer factors for the polymer; ionic strength, nature, and charge of the metal ion, temperature, or nature of the counterion of the metal ion (Rivas et al., 2008). The CdS quantum dot prepared by the two-phase method was doped into low-density polyethylene (LDPE) and polymer nanocomposites were prepared by solution blending method. The optical absorption spectra of composites which are consisted of three polymer nanocomposites prepared with CdS adding and a pure LDPE composite were obtained, and the band gap energy values of samples were calculated from these spectra. The previous studies mainly focus on the photoluminescence (PL) or fluorescence spectroscopies. It is because of the availability and easy to use of the systems. Although the RL provides greatly enhanced spectral
Corresponding author. E-mail address: [email protected] (İ.Ç. Keskin).
https://doi.org/10.1016/j.radphyschem.2018.11.006 Received 19 February 2018; Received in revised form 27 September 2018; Accepted 6 November 2018 Available online 09 November 2018 0969-806X/ © 2018 Elsevier Ltd. All rights reserved.
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data by the volumetric excitation of the sample throughout, the setup of the system and data collection is relatively challenging. However, in the fluorescence system, which is frequently used in the quantitative analysis of quantum dots, the material can only be examined in the colloid structure. If the sample is solid, it should be dissolved in certain molarities with solvent. We specifically used RL system mainly for three reasons; measurements can be taken from a pure sample without using any solvent, the spectrum can be obtained in any phase, and the detected emission intensity is significantly higher than other techniques The latter, the PL and fluorescence spectroscopy, which are used to determine the emission characteristics of QDs, is usually induced by low energy sources compared with the ionizing radiation. The RL system, in which high-energy X-rays is used as excitation source enables the detection of sputtered electrons by inducing electrons in deep traps in semiconductor nanocrystals. This is a significant advantage of the RL system (Kati et al., 2012; Arslanlar Tuncer et al., 2013).
(111)
CdS QD
Intensity (a.u)
1680
1260
(220) (311)
840 (331)
420
0 10
20
30
40
50
60
70
80
2θ (degree)
Fig. 1. XRD pattern of powder CdS QD.
gun, under accelerating voltage in the range of 20–120 kV. The powder XRD analysis of the CdS was recorded using a Panalytical X-ray diffractometer with CuKα radiation source, equipped with a graphite monochromator. XRD pattern was obtained by step scanning from 10° to 80° 2θ in steps of 0.0501° with account of 2 s per step, i.e.fast XRD profiles of 30 min. The FTIR spectra were recorded for absorbance in the region 650–4000 cm−1 using an Agilent Technologies Cary 660 Spectrometer with room temperature. The thermal analysis (TG-DTA) of samples was performed Hitachi SII Exstar 7300 thermal analyzer. The thermal behavior of nanocomposites was studied in the temperature range of 25–575 °C at a heating rate of 10° min−1 in the air atmosphere. SEM was used to examine the morphology of nanocomposites by using a Philips XL-30S FEG e SEM.
2. Materials and method 2.1. Synthesis of CdS nanocrystal CdS nanocrystals were synthesized by the two-phase method (Wang et al., 2005, 2006). 0.4 g of cadmium myristate as a cadmium source, 2 g of oleic acid as a surfactant is dissolved in 80 ml of toluene at a temperature of 80 °C and kept for later use in the process. In a further step, 80 ml of distilled water is heated to 100 °C under nitrogen and thiourea as a sulfur source is added to the reaction flask. The temperature was kept constant at 100 °C. Nitrogen saturation was provided, and the toluene phase solution was added to the water phase with the aid of a stirrer. The growth and optical properties of nanocrystal were controlled with UV–Vis by taking samples at different intervals during the process. When the desired size is obtained according to the absorption spectra, the reaction is stopped. The biphasic mixture in the reaction flask is separated using a separatory funnel.
3. Results and discussion 3.1. XRD pattern of CdS QD The XRD pattern of the OA capped CdS QD shown in Fig. 1. The broad peaks refer that the grain size of nanopowder (NP) is very small. The XRD pattern also confirms the crystalline cubic structure of CdS. Diffraction peak shown in this pattern corresponds to the (1 1 1), (2 2 0), (3 1 1) and (3 3 1) plane reflections of cubic CdS. There are three main diffraction peaks at 26,7°, 44.1°, 52,1°. The lattice parameter a = 5.83 Å is in correlation with the values reported in the literature (Dai et al., 2007; Li et al., 2014).
2.2. Preparation of QD added polymer nanocomposites In the heater with the temperature stabilized at 130 °C, the weighed LDPE granules were heated to the required softness in the borosilicate glass beaker. The weighed powder CdS QD was completely dissolved by stirring in 20 ml of toluene. This solution is incorporated into the molten polyethylene. Also, toluene is a proper solvent for polyethylene material. 20 ml of toluene in the solution provides even better dissolution of the polyethylene (Li et al., 2006). The mixture was stirred at 130 °C. for 10 min to remove toluene from it. Then, the mixture was cooled down to room temperature and allowed to stand for at least 48 h. In a very small amount of toluene residues that may be present have been removed from the acceptable level. 0.30 g of the dried mixture was weighed and again softened by heating at 110 °C for 15 min at the melting point of polyethylene. Thus, more homogeneous and smoother composites are obtained by pressurizing. The sample is cooled down to room temperature under a pressure of 10 MPa. 1.5 mm thickness and 16 mm diameter smooth surface nanocomposites were obtained. For the RL measurements of the CdS QD and nanocomposites, excitation was made with a Machlett OEG-50A X-ray tube operated at a maximum experimental level of 30 kV and 15 mA. Luminescence detection system is conducted with a Yobin Yvon spectrometer, coupled to a liquid nitrogen cooled CCD detector. The fluorescence spectra of CdS QD were recorded by Agilent Technologies Cary Eclipse Spectrophotometer at 290 nm excitation wavelength. Optical absorption spectra of QD and nanocomposites were recorded at room temperature in the wavelength region of 200–2000 nm using Perkin-Elmer Lambda 950 spectrophotometer. TEM images were obtained using a FEI Tecnai G2 -Biotw Spirit High Contrast Transmission Electron Microscopy and operated with Lanthanum hexaboride (LaB6) electron
3.2. Size determination of CdS QD The TEM image and the histogram of the particle size distribution of the OA capped CdS QD is shown in Fig. 2 and Fig. 3. The distribution of particle size for CdS QDs are nearly monodispersed and spherical in
Fig. 2. The TEM image of OA capped CdS QD. 138
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3.4. SEM Images of nanocomposites The images reveal that the grains of QD are spherical in shape and uniformly distributed all over the surface of the blends. The distribution with increasing the contribution ratio is seen to take a more orderly state. LDPE has covered up the quantum dots, so this situation has led to slightly decrease in RL intensity. When (c) and (d) images compared in Fig. 5, it's seen that nanocomposite which has higher contribution rate is more homogeneous. This condition corresponds to broader absorption bands in the absorption spectra. As shown in Fig. 5, the 100 µm magnification image of 5% QD doped sample was not enough to detect quantum dot structures. However, it appears that the surface of the nanocomposite was quite smooth. Fig. 3. The particle size histogram of CdS.
3.5. Optical properties shape. The TEM image of CdS QD revealed a distinctly spherical shape pattern of the particle distribution and also showed that the particles were highly monodispersed and homogenous. The average size distribution of the CdS QD was measured ̴ 2.6 nm.
The radioluminescence measurements of CdS QD and the prepared nanocomposites were taken with the RL system created by Jobin Yvon monochromatic integrated X-ray unit. Fig. 6 shows that the powder CdS and RL spectra of quantum dot added polymer nanocomposites. In Fig. 6, the main emission peak of CdS QD is located at 655 nm and there is a weak peak at 456 nm. The recombination of excitons and trapped electron-hole pairs at shallow traps that cause the band edge luminescence between 450 and 500 nm. Under X-ray excitation, the RL emission of CdS QDs at 465 nm indicates that the photo generated electron-hole pair was trapped with upon their recombination (Dhage et al., 2011). The wide peak which is extending from ̴ 520 nm to ̴ 900 nm was observed around the red emission band centered at 655 nm. This broad emission is attributed to deep-level emission which was due to crystalline surface defects (Dai et al., 2007; Husham et al., 2015; Shen et al., 2013). These defects could originate from, VS (S vacancy), VCd (Cd gap), S (S cracks), Cd (Cd cracks) (Ma et al., 2002; Talakonda, 2016). The quenching of emission intensity is quite high for CdS QD. The spectrum of the powder CdS QD is reduced 20 times so that the nanocomposites can be displayed together with the RL spectra. The spectra of nanocomposites showed blue shift compared with the quantum dot spectrum, and it's appear that LDPE reduced the RL intensity of the QD. In the RL spectrum especially main peak of powder QD at 655 nm obviously shifted blue region at around 625 nm for QD added LDPE matrix. So, with the reduction of the contribution rate in the nanocomposites, the samples were seen to have emission peaks at higher energy levels with minor shifts. This phenomenon, which is expressed as the blue shift in the RL spectrum, is also an indication that the material has become more insulator due to the band theory. As a matter of fact, the CdS QD known as semiconductors has become more insulating when blended with the polymer matrix, and about 30 nm shift in the RL spectrum confirms that. Of course, the absorption spectra of nanocomposites have also supported this view. The decrease in RL intensity upon addition of CdS QD could be attributed to the charge transfer from polymer to the nanocrystals. This charge transfer results from the formation of separated electron-hole pair that recombines non-radiatively which leads to quenching of emission intensity of nanocomposites (Fang et al., 2010; Keskin et al., 2017). In Fig. 7, the low intensity of the emission peak of CdS, which has a broad fluorescence spectrum, is thought to be caused by surface defects that may occur during the synthesis process or surfactant material (Sehgal and Narula, 2015; Wang and Hsiao, 2009). CdS have fluorescence emission around 430 nm. The optical band gap values obtained by using the Eg = hc/λonset equation, where λ is the onset wavelength which can be determined by the intersection of two tangents on the absorption edges. Also, λonset represents the electronic transition start wavelength (Townsend and Kelly, 1973). From the Fig. 8, the optical band gap of CdS was calculated as 2.29 eV (Kaur and Tripathi, 2015; Nieto-Zepeda et al., 2017).
3.3. FT-IR spectra of CdS QD and nanocomposites Fig. 4 shows the FT-IR spectra of CdS QD and doped nanocomposites. In Fig. (4-b), it was determined that the absorbance intensity of pure LDPE showed a decrease with the contribution. The absorbance decrease means that as an inorganic structure CdS QD was successfully doped inside LDPE matrix which has an organic structure (Yu et al., 2003; Mohan and Oluwafemi, 2016). However, in the inset spectra at 900–1800 cm−1 it is observed that especially the 5% added sample gives characteristic peaks by increasing the intensity, especially at 1543 cm−1 differs from LDPE spectrum. As it known in the literature the OA has a strong band at 1711 cm−1 which is derived from –C˭O stretching in carboxyl group. This band can be seen in the spectrum of OA capped CdS at 1543 cm−1 for –C-O stretching. It means that OA ligands are capped on the CdS QD surface, forming a carboxylate (-COO-) structure (Kim et al., 2015; Ayele, 2016; Zhang et al., 2006). In the FT-IR spectrum of OA capped CdS QD (Fig. 4-a), there are main peaks at 2848 cm−1 and 2915 cm−1 which are due to the –CH2 symmetric and asymmetric stretching. The peak at 1421 cm−1 is attributed to the OH bending from OA. The peaks at 1409 cm−1 may be attributed to sulphate group S˭O (Chandra et al., 2014) The C˭O bonds disappeared in the spectrum of the CdS NCs at 1290 cm−1 (Teng et al., 2008), the band at 1285 cm−1 exhibited the presence of the C–O stretch (Saravanan et al., 2011). The peaks at 1469 cm−1 for -CH2 scissoring and 719 cm cm−1 for -(CH2)n-vibration (Kim et al., 2015).
Absorbance (a.u)
(a)
(b)
Wavenumber (cm )
Fig. 4. FT-IR spectrum of CdS QD (a) and CdS doped polymer nanocomposites (b). 139
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Fig. 5. The SEM images taken for sample LDPE (a), OA capped powder CdS QD (b), 0.2 wt% CdS nanocomposite (c), 5 wt% CdS nanocomposite (d).
The absorption spectrum of LDPE (Deiting et al., 2016) showed a significant shift to the red region with the CdS QD contribution. It's seen that with increasing of contribution rate, it is seen that optical band gap values can be reduced from 2.71 eV to 2.58 eV (Table 1). At the 0.2 wt %, CdS added sample, even though the additive ratio was very low, is reduced dramatically the band gap value of LDPE. The band gap of LDPE at about 4.6 eV has decreased to 2.7 eV. According to the values given in Table 1, it is seen that the band gap values obtained from both methods are very close to each other.
1
RL Intensity (a.u)
/20
3.6. Thermal analysis of nanocomposites The thermal stability of LDPE and CdS QD added nanocomposites have been investigated. Thermogravimetric curves are given in Fig. 9. Although the thermogravimetric curves of nanocomposite samples showed no significant differences taking into account compared with the LDPE, The change became obvious by the QD amount increasing in the polymer matrix. Liu et al. (2013). At the TG analysis, the effect of the nanocrystals added in LDPE on the melting rate of nanocomposites was observed. As can be seen in Table 2, at low addition ratios (0.2% and 1%), nanocomposites show close thermal stability to LDPE. In spite of that, the sample with 5% CdS addition showed faster degradation than the other nanocomposites (Ma et al., 2013; Li et al., 2015). In Fig. 10, CdS QD also contributed to the thermal conductivity of the polymer matrix, with the most active being dominant in the 5% CdS doped sample. The melting point of pure polyethylene decreased from 110 °C (Şirin et al., 2009; Suthan et al., 2011) to 106 °C, due to the quantum dot addition (Fig. 11). Nano-sized additive materials have lower melting temperature and melting time compared to micron-sized materials. However, they require higher melt torque. Cowie (2007) Taking into account the contribution of the additive materials used in composites to the heat transfer, it is evident in Fig. 11 that the quantum dot additives cause a significant change in the thermal conductivity properties when added into the polymer matrix. In Fig. 8, it is seen that the conductivity characteristic of CdS is much higher than that of the polymer in the graph of optical absorption. Compared with pure polyethylene, it is
Wavelength (nm)
Fig. 6. RL spectra of nanocomposites in different ratios (0.2, 1, 5 wt%.CdS) and CdS QD.
Fig. 7. The fluorescence spectrum of OA capped CdS QD and images. The inset images of the nanocomposites under UV lamp (λex= 365 nm) and daylight, which is apparent in the 5 wt% (a), 1 wt% (b), 0.2 wt% (c) CdS added nanocomposites, and pure LDPE (d) The nanocomposites are seen violet under UV light while they are orange-yellow in daylight. The powder CdS in eppendorf is also the same.
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Fig. 8. The optical spectra of pure LDPE, OA capped CdS QD powder, and CdS QD added polymer nanocomposites. The absorption spectra (a) and the Tauc plot (b) to determine the bandgap of the samples.
0.2 wt%
1 wt%
5 wt%
CdS QD
267 4,64 4.52
457 2.71 2.72
459 2.70 2.65
481 2.58 2.60
540 2.29 2.27
Weight (%)
Wavelength (nm) Band Gap (eV) Band Gap (eV) (Tauc Plot)
LDPE
M icrovolt Exo Down (μ V)
Table 1 The calculated optical band gap (Eg) and onset wavelength values of CdS QD, LDPE and, CdS added nanocomposites.
Temperature ( C)
Microvolt Exo Down ( μ V)
Fig. 10. DTA curves of the nanocomposites.
Temperature (oC)
Fig. 9. TG curves of CdS doped polymer nanocomposites. Table 2 Thermal degradation values of nanocomposites. Degradation Step (°C)
Residue Weight Percentage
Sample
Tstart
Tmax
Tend
430 °C
460 °C
490 °C
LDPE 0.2 wt% 1 wt% 5 wt%
184 277 333 215
475 476 475 473
496 496 497 496
16.70 4.80 4.48 8.46
24.41 20.72 21.33 25.37
89.39 89.22 89.47 91.51
Temperature ( C)
Fig. 11. The changing in the melting point of LDPE with QD additive.
clear that the thermal conductivity of the nanocomposites formed by the incorporation of CdS into the polymer increases in proportion to the percentage of the additive. This effect is most apparent in the 5 wt% CdS added sample. While the melting point of pure polyethylene is 110 °C, the melting point of this sample has reduced to 106 °C. In the TG graph of the same sample in Fig. 9, the effect of CdS on the polymer is noteworthy. The nanocrystals have caused the nanocomposite to have a lower melting temperature.
4. Conclusions The lattice defects, impurity defects, structural defects and their distribution in the structure are responsible for the luminescence emission that is commonly observed in semiconductor materials during X-ray excitation. In this respect, in this study aimed to show the radioluminescence behavior of QD structure and nanocomposites blended with QD. The polymer nanocomposites have been prepared with CdS 141
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quantum dots by considering that quantum dots may be an ideal nanocomposite additive due to their high quantum yield and the fact that energy band gap and dimensions can be controlled. The optical, structural, morphological, and thermal properties of LDPE matrix nanocomposites blended with QD at different ratios have been investigated. The nanocomposites showed the same luminescence characteristics as the powder QD. It has been observed that these nanocomposites with QD additive have significant advantages in experimental studies in terms of being easy to shape, being flexible, being able to prepare in desired quantity and size and having more suitable storage conditions. The radioluminescence properties of CdS QD and nanocomposites were investigated. Two different peaks were obtained at 456 nm and 655 nm. The nanocomposites showed blue shift compared with the CdS RL spectrum, and the LDPE matrix has reduced the intensity of the CdS QD. OA as a surfactant is a significant factor for RL emission and intensity. Also, the decrease in RL intensity could be referred to the various atom vacancies, surface defects and charge transfer from polymer to the nanocrystals. The CdS QD show a broad fluorescence emission band located at ∼430 nm under UV excitation (λ = 290 nm). The band gap of OA capped CdS is determined to be 2.3 eV. It was observed that the bandgap of CdS added nanocomposites increase with the addition ratio of the LDPE. By increasing the contribution rate, it has been seen that optical band gap value of LDPE can be reduced from ∼4.6 eV to ∼2.6 eV. The melting point of LDPE decreased from 110 °C to 106 °C, due to the quantum dot addition.
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