- Email: [email protected]
ZnS core-shell QDs and their application for the Methyl orange (MO) degradation
Accepted Manuscript A novel process for synthesis of CdSe/ZnS core-shell QDs and their application for the Methyl orange (MO) degradation
M. Mehrjoo, M. Molaei, M. Karimipour PII:
S0254-0584(17)30651-X
DOI:
10.1016/j.matchemphys.2017.08.033
Reference:
MAC 19931
To appear in:
Materials Chemistry and Physics
Received Date:
09 January 2017
Revised Date:
11 July 2017
Accepted Date:
14 August 2017
Please cite this article as: M. Mehrjoo, M. Molaei, M. Karimipour, A novel process for synthesis of CdSe/ZnS core-shell QDs and their application for the Methyl orange (MO) degradation, Materials Chemistry and Physics (2017), doi: 10.1016/j.matchemphys.2017.08.033
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT A novel process for synthesis of CdSe/ZnS core-shell QDs and their application for the Methyl orange (MO) degradation M. Mehrjoo,M. Molaei*and M. Karimipour Department of Physics, Faculty of Science, Vali-e-Asr University, Rafsanjan, Iran *Corresponding
author email: [email protected]
Abstract CdSe/ZnS core-shell quantum dots (QDs) were prepared using combination of microwave irradiation and photochemical synthesis of sulfur based material. Versatile analyses of X-ray diffraction (XRD), Fourier transform-infrared (FT-IR), Transmission electron microscopy (TEM), Photoluminescence (PL) and UVVisible (UV-Vis) spectroscopy were employed to verify the formation of core-shell structure at nanoscale. Due to the growth of ZnS as shell, trap state emission of CdSe cores quenched significantly and the fundamental band gap emission of CdSe QDs with FWHM of above 36 nm was clarified as an evidence of the formation of core-shell structure. Moreover, the results revealed that CdSe/ZnS QDsshow superior photocatalytic activity rather than CdSe QDsfor photo-degradation of Methyl orange (MO). Keywords: CdSe/ZnS, Quantum dots, Photoluminescence, Photocatalytic 1. Introduction II-VI QDs are suitable materials for various applications such as LEDs [1], solar cells [2], sensors [3], biological probes [4] and photocatalyst, duo to quantum confinement effects and high surface to volume ratio. NCs possess high surface to volume ratio as more than about 80% of atoms are on the surface [2], therefore surface undergoesvarious properties of NCs significantly [5]. In NCs, numerous surface dangling bondscreate trap states thatinfluence different properties of QDs, especially their optical properties [6]. Therefore, controlling these surface traps and their passivation has been a focus of researchers either by organic capping 1
ACCEPTED MANUSCRIPT agent molecules or inorganic capping materials as shell [7]. Although organic materials have been efficient for the control of the growth of QDs but so far they have not been efficient for the passivation of surface defects [8]. Inorganic materials with higher band gap in comparison to the core can be used to the better passivation of the surface states of the cores and also it has an important effect against oxidization of the QDs [7,8]. As an II-VIcompound,CdSe QDs have attracted great attention and they can be shelled by ZnS as a higher band gap semiconductor [9]. Up todate,CdSe and CdSe/ZnS QDs have been synthesized by different methods.Tong and co-workers prepared CdSe QDs in water using a one-pot microwave method [10].Shanying Li et al. synthesized CdSeusing an electrochemical method [11].Jin Hua Li produced CdSe by a room temperature method [12].Moghadam et al. reported synthesis of CdSe by microwave irradiation [13].Molaei et al. synthesized CdSe QDs by a microwave assisted approach using a reaction between CdSO4 and NaHSe [14]. Mathew and co-workers prepared CdSe QDs using a method at 80°C in aqueous medium and then ZnS shell has been grown on CdSe core using an IR illumination [15].Gerion et al. synthesized CdSe/ZnS core-shell QDs using silica coating[16].Gema et al. reported microwave synthesis of CdSe/ZnS core-shell QDs [17].Zuala et al. prepared CdSenano wires by a solvothermal approach using Na2SeO3 as Se source [18].Amiri et al. reported synthesis of CdSe by chemical precipitation method using Na2SeO3[19].Meiting et al. produced CdSe QDs by a chemical method at 60°Cusing Na2SeO3[20] and also there are some other reported methods.Microwave assisted route is one of the novel method and is a very rapidly developing area of research. Microwaveand photochemical are novel, rapid and interesting methods for synthesis of QDs [21-23]. In the present work, firstly, CdSe QDs were prepared by a simple microwave approach and then synthesized QDs shelled by ZnS using a simple photochemical method. SubsequentlyCdSe QDs properties and the effect of the ZnS shell on the improvement of their propertieshave been investigated by means of different characterization tools.
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ACCEPTED MANUSCRIPT 2.Experimental 2.1. Materials: 3CdSO4. 8H2O (Merck, 99%), Na2SeO3 (Merck, 99%), thioglycolic acid (TGA) (Merck, 99%), Na2S2O3 (Merck, 99%) and zinc acetate dihydrate (Zn(OAc)2)(Merck, 99%). 2.2. Microwave synthesis of CdSe QDs In this work, firstly CdSe QDshave been prepared by a microwave activated reaction between CdSO4and Na2SeO3 as precursors and TGA as capping agent molecule [23]. The related reaction is as follows: Na2SeO3 + CdSO4 +TGA + Microwave irradiation → CdSe QDs Typically,0.06 g of CdSO4 was dissolved in 30 ml D.I water and then 0.05 ml of TGA was added to this solution. The pH value of the solution was adjusted to 12 by adding proper amount of NaOH1M. Then0.86 g of Na2SeO3 was dissolved in 20 ml water. The prepared solutions were mixed together and the final solution was placed at the center of a microwave system and was irradiated at 720W for 90 s only. 2.3.Photochemical growth of CdSe/ZnS core-shell QDs ZnS shell was grown on as prepared CdSe QDs by a photosensitive reaction between Zn(OAc)2 and Na2S2O3. Based on the previously reports, Na2S2O3 is a UV-sensitive material which it will be decomposed under UVillumination [23-25]. CdSe/ZnS QDs growth reactionsare as follow: Na2S2O3 + UV- illumination → 2Na+ + S2- + SO3 CdSe(QDs) + Zn(OAc)2 +Na2S2O3 + UV- illumination → CdSe/ZnS QDs.
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ACCEPTED MANUSCRIPT To growth ZnS shell, 0.023 g of Zn(OAc)2and 0.086 g of Na2S2O3 were dissolved in 40 ml D.I water and then 10 ml of non-extractedor as-preparedCdSe QDs was added to this solution.This solution was placed under a high pressure mercury lamp and was illuminated for 90 min. The related reactionswere as follow: Fig. 1 shows a schematicgraphic of the CdSe/ZnS QDs formation process. 2.4. Characterization XRDmeasurement was carried out using an advanced d8 Bruker system. TEM imageswere recorded by a Philips EM 208 transmission electron microscope with an accelerating voltage of 100 kV. Infrared absorption data were obtained by using a Fourier-transform-infrared (FT-IR). UV–Visible (UV–Vis) and photoluminescence (PL) spectra were collected using an Avantes spectrometer (Ava Spec-2048 TEC). 3. Results and Discussion 3.1. XRD and TEManalyses Fig. 2 shows the XRD pattern of prepared core-shell and QDs. The Bragg peaks at 2Ɵ=26.7ᵒ and 45.69ᵒbelong to the (101) and (103) planes of the hexagonal wurtzite structure of CdSe. These peaks wereshifted to higher angles due to the growth of shell and shrinkage of lattice parametersof core structure for the CdSe/ZnS core-shell. Fig. 3 is the TEM image of the CdSe (a) and CdSe/ZnS (b), synthesized QDs. CdSe synthesized QDs are morphologically round with a uniform size distribution which is an advantage of the microwave based methods. The mean particle size is about 5 nm. For CdSe/ZnS particles, the prepared image indicates shelling of CdSe cores by a uniform ZnS shell.
3.2. FT-IR Fig.4 describes the FT-IR spectra of TGA capped CdSe and CdSe/ZnS core-shell QDs. An absorption band due to S ‒ H vibration at 2560 cm-1 is absent, indicating the formation of S ‒ Cd bonds between TGA and 4
ACCEPTED MANUSCRIPT CdSe cores. The shift of the asymmetric vibration of carboxylic group of TGA from 1720 cm-1 to 1560 cm-1, indicates that ‒ COOH in TGA turns into its anion which also results in the appearance of symmetric vibration of carboxylic anion at 1385 cm-1 [26]. It is seen that for CdSe/ZnS (S-2 capping) QDs, the bands at 2851 and 2922 cm-1 corresponding to C ‒ H stretching in the organic ligand have been weakened, demonstrating the high efficiency of ligand exchange using S-2 ions [27]. 3.3.UV-Vis and PL analysis Fig.5depictsthe absorption and PL spectrum of CdSe core QDs. The band gap of the bulk CdSe at room temperature is about 1.74 eV, but it is estimated about 2.9 eV for QDs which is the indication of quantum confinement effect.PL spectrumindicates a broad band white emission with a peak at about 600 nm (2.06 eV). There are competing pathways for electron-hole recombination, band edge and surface trap states recombination (Fig. 6)which are responsible for the emission of CdSe QDs[14]. Based on the presented model, the PL peak is related to the surface trap states indicating the high density of the surface defects for the TGA capped CdSe QDs. It is worthwhile to notice that with microwave irradiation timeevolution, one can readily tune the QD’s size and their optical band gap. Fig. 7 apparently confirms this statement by showing the red shift of absorption spectra (a), Tauc plot for band gap estimation (b) in the course of irradiation time and size-band gap comparison (c). The apparent decrease of band gap from 3.15 to 2.7 eV during 90 s of irradiation accompanied with the increase of QD size from 1.6 to 1.95 nm which were obtained from Effective Mass Approximation (EMA) formula) [26].In consistent with the decrease of the band gap and growth of the QDs, the PL peak indicated a red shift by spanning synthesis time (Fig. 8). For the CdSe/ZnS core-shell QDs the absorption band is located around 485 nm. Their PL emission is quite different with the CdSe core QDs (Fig. 9). It consists of two main peaks: the first one is a narrow and intense peak located at about 490 nm with a FWHM of about 36 nm and the other one is a broad and weak peak 5
ACCEPTED MANUSCRIPT at about 600 nm. Based on Fig. 6, the first peak is a near band edge emission related to the free exciton recombination from the CdSe band edge and the second one can be attributed to the recombination from the deep levels. The high quality growth of ZnS shell around CdSe cores passivatesnon radiative trapping centers leading to the enhancement of band edge emission and vanishing of deep levels emission. The FWHM of the band edge emission (36 nm) is lower than the obtained results by the other groups [16, 17, 28, 29], indicating the advantage of the synthesis approach compared to the other ones. The last but not the least interesting capability of the present work isthat one can tune the ZnS shell thickness only by UV-illumination time. Fig.10 indicates PL spectra of the CdSe/ZnS QDs for different photochemical treatment time of ZnS shell. With the increase of photochemical treatment time, trap states emission decreased gradually and band edge emission increased and the optimum value of shell growth was obtained for 90 min of UV illumination. 3.4. Photocatalytic performance Due to the high photosensitivity, QDs can be used as photocatalyst. The photocatalytic activity of CdSe and CdSe/ZnS core-shell QDs was investigated by choosing methyl orange (MO) as pollutant. Synthesized QDs was extracted from the solution by centrifugation of the solution at 5000 rpm for 10 min.5mg of CdSe and 𝑔
CdSe/ZnS extracted QDs were added into the 25 ml of a 0.01𝐿 MO solution. The finally prepared solution was placed under a UV-Lamp for different intervals.The behavior of the characteristic absorption peak of MO (495 nm) was monitored during the catalytic degradation process. Fig. 11 describes the photocatalytic degradation of MO by CdSe (a) and CdSe/ZnS (b) core-shell QDs. The intensity of the absorption peak of MO decreased gradually by extending UV-illumination time, confirming degradation of MO. Fig.12 indicates the degradation rate of MO at different intervals by CdSe (a) and CdSe/ZnS (b) coreshell QDs. The degradation percentile of CdSe was about 50% which was promoted to the 70% for the CdSe/ZnS core-shell QDs. Fig. 13 is a diagram presenting the photo-degradation mechanism of CdSe 6
ACCEPTED MANUSCRIPT andCdSe/ZnS core-shell QDs.ZnS shell enhanced the photocatalytic activity of CdSe cores, because it has a higher and lower conduction and valance bands, respectively compare to the ones belonging to CdSe, therefore ZnS decreases the required energy for transferring carriers to vacuum states or it can be said the redox potential decreases and can enhance the photo –activation of carriers and consequently enhances the photocatalyst activity.
4. Conclusion CdSe/ZnS core-shell QDs were synthesized using a combination of microwave and photochemical approaches. CdSe QDs exhibited a broad band emission related to the surface trap states which it was quenched considerably after ZnS shell growth and a narrow band edge emission with FWHM of about 36 nm was appeared. The ZnS shell thickness was tunable by UV-illumination timeevolution as the optimum value of the shell was obtained for 90 min. Synthesized QDs indicated strong photocatalytic activity. The activity of the CdSe/ZnS QDs for photo-degradation of Methyl orange (MO) was stronger than that for CdSe QDs.
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References [1] M. Molaei and S. Pourjafari, Bull. Mater. Sci. 2014, 37, 9. [2] M. Karimipour, S. Mashhon, M. Molaei, M. Molaei, N. Taghavinia, Electron. Mater. Lett. 11, (2015), 625.
[3] H. Mattoussi, G. Palui and H. B. Na, Adv. Drug Deliv. Rev. 2012, 64,138. [4] M. J. Bowers, J. R. McBride and S. J. Rosenthal, J. Am. Chem. Soc. 2005, 127, 15378. [5] L. Dworak, V.V. Matylitsky, V.V. Breus, M. Braun, T. Basche, J. Wachtveitl, J. Phys. Chem. 2011, 115, 3949 [6] M. Zubair, M. Mustafa, K. Lee, C. Yoon, Y. H. Doh and K. Hyun,Chem, Eng. J. 2014, 253, 325. [7] M. Molaei, H. Hasheminejad and M. Karimipour, Electr. Mater. Lette. 2015, 11, 7. [8] Sh. Pan, Sh. Ebrahim, M. Soliman and Q. Qiao, Springer Science+Business Media Dordrecht. 2013, 15, 1420. [9]G. M. Duran, A. M. Contento and A. Rios, Analytica. Chimica. Acta. 2013,801, 84. [10] T. Xuan, X. Wang, G. Zhu, H. Li, L. Pan and Zh. Sun,J. Alloys Compounds. 2013, 558, 105. [11] L. Shanying, Zh. Haipeng and T. Damin,Mater. Sci. 2013, 16, 149. [12] J. H. Li, C. L. Ren, X. Y. Liu, Z. D. Hu and D. S. Xue, Mater. Sci. Eng. 2007, 458, 319. [13] M. M. Baghbanzadeh, A. Keilbach and C. Kappe, Chem. 2012, 23, 7432. [14] M. Molaei, F. Salaribardsiri, A. R.Bahador and M. Karimipour, Mod. Phys. Lett. B. 2015, 29, 1650074. [15] S. Mathew, B. S. Bhardwaj, A. D. Saran, P. Radhakrishnan, V.P.N. Nampoori, C.P.G. Vallabhan, J. R. Bellare, Opt. Mater. 2015, 39, 46. [16] D. Gerion, F. Pinaud, Sh. C. Williams, W. J. Parak, D. Zanchet, Sh. Weiss and A. Paul Alivisatos, J. Phys. Chem. B. 2001, 105, 8861. [17] G. M. Duran, M. R. Plata, M. Zougagh, A. M. Contento and A. Rlos, J. Colloid Inter. Sci. 2014, 428, 235.
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ACCEPTED MANUSCRIPT [18] L. Zuala, R. Madaka and P. Agarwal, Hindawi Publishing Corportion. 2013, 10, 257359. [19] Gh. R. Amiri, S. Fatahian and S. Mahmoudi, Scientific Research. 2013, 4, 134. [20] M. Dong, J. Xu, Sh. Li, Y. Zhou and Ch. Huang, Luminescence. 2014, 29, 818. [21] M. Molaei, F. Karimimaskon, A. Lotfiani, M. karimipour and M. Khanzadeh,J. Semiconductor Tech. andSci. 2013, 143, 649. [22] M. Molaei, A. R. Khezripour and M. Karimipour, Appl. Surf. Sci. 2014, 317, 236. [23] S. Abbasi, M. Molaei, M. Karimipour, Luminescence, DOI: 10.1002/bio.3300 [24] A. R. Bahador, M. Molaei, M. Karimipour, J. Lumin, 166 (2015) 101. [25] M. Molaei, F. Sarhani, F. Salaribadsiri and M. Karimipour, Mod. Phys. Lett. B, 2015, 29, 1650093. [26] Y. Wang, J. P. Lu and Zh. Fa. Tong, bull. Mater. sci. 2010, 33, 543. [27]S. Das, S. Banerjee, A. Dutta, T. Sinha, Materials Science in Semiconductor Processing. 2015, 40, 412. [28]M. R. Karim, Z. Ferdous, and H. Ünlü, Journal of Innovation and Scientific Research, 2014, 5, 11. [29] A. G. Bakanov, N. A. Toropov, and T. A. Vartanyan, Optics and Spectroscopy, 2015, 3, 118.
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ACCEPTED MANUSCRIPT Figures captions Fig. 1. A schematic picture of synthesizing CdSe/ZnS QDs process.
Fig. 2. XRD patterns of synthesized CdSe and CdSe/ZnS QDs.
Fig. 3. TEM images of CdSe (a) and CdSe/ZnS (b) core-shell QDs.
Fig. 4. FTIR spectra of CdSe and CdSe/ZnS core-shell QDs.
Fig. 5. Absorption and PL spectra of CdSe QDs for 90 S of microwave irradiation. (excitation wavelength was 365 nm) Fig. 6. A schematic diagram describing CdSe emission mechanism
Fig. 7. Absorption spectra (a) Tauc plot of absorption spectra (b) and size-band gap (c) of the CdSe QDs synthesized at different microwave irradiation intervals
Fig. 8. PL spectra of the CdSe QDs synthesized at different microwave irradiation intervals
Fig. 9. Absorption and PL spectra of CdSe/ZnS core-shell
Fig. 10. PL spectra of CdSe/ZnS core-shell QDs with different thicknesses of ZnS shell Fig. 11. Absorption spectra demonstrating the photocatalytic degradation of MO by CdSe (a) and CdSe/ZnS (b) core–shell QDs Fig. 12. Comparison of photocatalytic activities of CdSe and CdSe/CdS QDs. Fig. 13. Proposed mechanism for the MO photo degradation by CdSe (a) and CdSe/ZnS (b) core–shell QDs
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ACCEPTED MANUSCRIPT
•CdSe/ZnS QDs were synthesized by a combination of microwave and photochemical
approaches• Trap state emission of CdSe cores quenched significantly after ZnS shell growth• CdSe/ZnS QDs show superior photocatalytic activity rather than CdSe QDs
M. Mehrjoo, M. Molaei, M. Karimipour PII:
S0254-0584(17)30651-X
DOI:
10.1016/j.matchemphys.2017.08.033
Reference:
MAC 19931
To appear in:
Materials Chemistry and Physics
Received Date:
09 January 2017
Revised Date:
11 July 2017
Accepted Date:
14 August 2017
Please cite this article as: M. Mehrjoo, M. Molaei, M. Karimipour, A novel process for synthesis of CdSe/ZnS core-shell QDs and their application for the Methyl orange (MO) degradation, Materials Chemistry and Physics (2017), doi: 10.1016/j.matchemphys.2017.08.033
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT A novel process for synthesis of CdSe/ZnS core-shell QDs and their application for the Methyl orange (MO) degradation M. Mehrjoo,M. Molaei*and M. Karimipour Department of Physics, Faculty of Science, Vali-e-Asr University, Rafsanjan, Iran *Corresponding
author email: [email protected]
Abstract CdSe/ZnS core-shell quantum dots (QDs) were prepared using combination of microwave irradiation and photochemical synthesis of sulfur based material. Versatile analyses of X-ray diffraction (XRD), Fourier transform-infrared (FT-IR), Transmission electron microscopy (TEM), Photoluminescence (PL) and UVVisible (UV-Vis) spectroscopy were employed to verify the formation of core-shell structure at nanoscale. Due to the growth of ZnS as shell, trap state emission of CdSe cores quenched significantly and the fundamental band gap emission of CdSe QDs with FWHM of above 36 nm was clarified as an evidence of the formation of core-shell structure. Moreover, the results revealed that CdSe/ZnS QDsshow superior photocatalytic activity rather than CdSe QDsfor photo-degradation of Methyl orange (MO). Keywords: CdSe/ZnS, Quantum dots, Photoluminescence, Photocatalytic 1. Introduction II-VI QDs are suitable materials for various applications such as LEDs [1], solar cells [2], sensors [3], biological probes [4] and photocatalyst, duo to quantum confinement effects and high surface to volume ratio. NCs possess high surface to volume ratio as more than about 80% of atoms are on the surface [2], therefore surface undergoesvarious properties of NCs significantly [5]. In NCs, numerous surface dangling bondscreate trap states thatinfluence different properties of QDs, especially their optical properties [6]. Therefore, controlling these surface traps and their passivation has been a focus of researchers either by organic capping 1
ACCEPTED MANUSCRIPT agent molecules or inorganic capping materials as shell [7]. Although organic materials have been efficient for the control of the growth of QDs but so far they have not been efficient for the passivation of surface defects [8]. Inorganic materials with higher band gap in comparison to the core can be used to the better passivation of the surface states of the cores and also it has an important effect against oxidization of the QDs [7,8]. As an II-VIcompound,CdSe QDs have attracted great attention and they can be shelled by ZnS as a higher band gap semiconductor [9]. Up todate,CdSe and CdSe/ZnS QDs have been synthesized by different methods.Tong and co-workers prepared CdSe QDs in water using a one-pot microwave method [10].Shanying Li et al. synthesized CdSeusing an electrochemical method [11].Jin Hua Li produced CdSe by a room temperature method [12].Moghadam et al. reported synthesis of CdSe by microwave irradiation [13].Molaei et al. synthesized CdSe QDs by a microwave assisted approach using a reaction between CdSO4 and NaHSe [14]. Mathew and co-workers prepared CdSe QDs using a method at 80°C in aqueous medium and then ZnS shell has been grown on CdSe core using an IR illumination [15].Gerion et al. synthesized CdSe/ZnS core-shell QDs using silica coating[16].Gema et al. reported microwave synthesis of CdSe/ZnS core-shell QDs [17].Zuala et al. prepared CdSenano wires by a solvothermal approach using Na2SeO3 as Se source [18].Amiri et al. reported synthesis of CdSe by chemical precipitation method using Na2SeO3[19].Meiting et al. produced CdSe QDs by a chemical method at 60°Cusing Na2SeO3[20] and also there are some other reported methods.Microwave assisted route is one of the novel method and is a very rapidly developing area of research. Microwaveand photochemical are novel, rapid and interesting methods for synthesis of QDs [21-23]. In the present work, firstly, CdSe QDs were prepared by a simple microwave approach and then synthesized QDs shelled by ZnS using a simple photochemical method. SubsequentlyCdSe QDs properties and the effect of the ZnS shell on the improvement of their propertieshave been investigated by means of different characterization tools.
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ACCEPTED MANUSCRIPT 2.Experimental 2.1. Materials: 3CdSO4. 8H2O (Merck, 99%), Na2SeO3 (Merck, 99%), thioglycolic acid (TGA) (Merck, 99%), Na2S2O3 (Merck, 99%) and zinc acetate dihydrate (Zn(OAc)2)(Merck, 99%). 2.2. Microwave synthesis of CdSe QDs In this work, firstly CdSe QDshave been prepared by a microwave activated reaction between CdSO4and Na2SeO3 as precursors and TGA as capping agent molecule [23]. The related reaction is as follows: Na2SeO3 + CdSO4 +TGA + Microwave irradiation → CdSe QDs Typically,0.06 g of CdSO4 was dissolved in 30 ml D.I water and then 0.05 ml of TGA was added to this solution. The pH value of the solution was adjusted to 12 by adding proper amount of NaOH1M. Then0.86 g of Na2SeO3 was dissolved in 20 ml water. The prepared solutions were mixed together and the final solution was placed at the center of a microwave system and was irradiated at 720W for 90 s only. 2.3.Photochemical growth of CdSe/ZnS core-shell QDs ZnS shell was grown on as prepared CdSe QDs by a photosensitive reaction between Zn(OAc)2 and Na2S2O3. Based on the previously reports, Na2S2O3 is a UV-sensitive material which it will be decomposed under UVillumination [23-25]. CdSe/ZnS QDs growth reactionsare as follow: Na2S2O3 + UV- illumination → 2Na+ + S2- + SO3 CdSe(QDs) + Zn(OAc)2 +Na2S2O3 + UV- illumination → CdSe/ZnS QDs.
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ACCEPTED MANUSCRIPT To growth ZnS shell, 0.023 g of Zn(OAc)2and 0.086 g of Na2S2O3 were dissolved in 40 ml D.I water and then 10 ml of non-extractedor as-preparedCdSe QDs was added to this solution.This solution was placed under a high pressure mercury lamp and was illuminated for 90 min. The related reactionswere as follow: Fig. 1 shows a schematicgraphic of the CdSe/ZnS QDs formation process. 2.4. Characterization XRDmeasurement was carried out using an advanced d8 Bruker system. TEM imageswere recorded by a Philips EM 208 transmission electron microscope with an accelerating voltage of 100 kV. Infrared absorption data were obtained by using a Fourier-transform-infrared (FT-IR). UV–Visible (UV–Vis) and photoluminescence (PL) spectra were collected using an Avantes spectrometer (Ava Spec-2048 TEC). 3. Results and Discussion 3.1. XRD and TEManalyses Fig. 2 shows the XRD pattern of prepared core-shell and QDs. The Bragg peaks at 2Ɵ=26.7ᵒ and 45.69ᵒbelong to the (101) and (103) planes of the hexagonal wurtzite structure of CdSe. These peaks wereshifted to higher angles due to the growth of shell and shrinkage of lattice parametersof core structure for the CdSe/ZnS core-shell. Fig. 3 is the TEM image of the CdSe (a) and CdSe/ZnS (b), synthesized QDs. CdSe synthesized QDs are morphologically round with a uniform size distribution which is an advantage of the microwave based methods. The mean particle size is about 5 nm. For CdSe/ZnS particles, the prepared image indicates shelling of CdSe cores by a uniform ZnS shell.
3.2. FT-IR Fig.4 describes the FT-IR spectra of TGA capped CdSe and CdSe/ZnS core-shell QDs. An absorption band due to S ‒ H vibration at 2560 cm-1 is absent, indicating the formation of S ‒ Cd bonds between TGA and 4
ACCEPTED MANUSCRIPT CdSe cores. The shift of the asymmetric vibration of carboxylic group of TGA from 1720 cm-1 to 1560 cm-1, indicates that ‒ COOH in TGA turns into its anion which also results in the appearance of symmetric vibration of carboxylic anion at 1385 cm-1 [26]. It is seen that for CdSe/ZnS (S-2 capping) QDs, the bands at 2851 and 2922 cm-1 corresponding to C ‒ H stretching in the organic ligand have been weakened, demonstrating the high efficiency of ligand exchange using S-2 ions [27]. 3.3.UV-Vis and PL analysis Fig.5depictsthe absorption and PL spectrum of CdSe core QDs. The band gap of the bulk CdSe at room temperature is about 1.74 eV, but it is estimated about 2.9 eV for QDs which is the indication of quantum confinement effect.PL spectrumindicates a broad band white emission with a peak at about 600 nm (2.06 eV). There are competing pathways for electron-hole recombination, band edge and surface trap states recombination (Fig. 6)which are responsible for the emission of CdSe QDs[14]. Based on the presented model, the PL peak is related to the surface trap states indicating the high density of the surface defects for the TGA capped CdSe QDs. It is worthwhile to notice that with microwave irradiation timeevolution, one can readily tune the QD’s size and their optical band gap. Fig. 7 apparently confirms this statement by showing the red shift of absorption spectra (a), Tauc plot for band gap estimation (b) in the course of irradiation time and size-band gap comparison (c). The apparent decrease of band gap from 3.15 to 2.7 eV during 90 s of irradiation accompanied with the increase of QD size from 1.6 to 1.95 nm which were obtained from Effective Mass Approximation (EMA) formula) [26].In consistent with the decrease of the band gap and growth of the QDs, the PL peak indicated a red shift by spanning synthesis time (Fig. 8). For the CdSe/ZnS core-shell QDs the absorption band is located around 485 nm. Their PL emission is quite different with the CdSe core QDs (Fig. 9). It consists of two main peaks: the first one is a narrow and intense peak located at about 490 nm with a FWHM of about 36 nm and the other one is a broad and weak peak 5
ACCEPTED MANUSCRIPT at about 600 nm. Based on Fig. 6, the first peak is a near band edge emission related to the free exciton recombination from the CdSe band edge and the second one can be attributed to the recombination from the deep levels. The high quality growth of ZnS shell around CdSe cores passivatesnon radiative trapping centers leading to the enhancement of band edge emission and vanishing of deep levels emission. The FWHM of the band edge emission (36 nm) is lower than the obtained results by the other groups [16, 17, 28, 29], indicating the advantage of the synthesis approach compared to the other ones. The last but not the least interesting capability of the present work isthat one can tune the ZnS shell thickness only by UV-illumination time. Fig.10 indicates PL spectra of the CdSe/ZnS QDs for different photochemical treatment time of ZnS shell. With the increase of photochemical treatment time, trap states emission decreased gradually and band edge emission increased and the optimum value of shell growth was obtained for 90 min of UV illumination. 3.4. Photocatalytic performance Due to the high photosensitivity, QDs can be used as photocatalyst. The photocatalytic activity of CdSe and CdSe/ZnS core-shell QDs was investigated by choosing methyl orange (MO) as pollutant. Synthesized QDs was extracted from the solution by centrifugation of the solution at 5000 rpm for 10 min.5mg of CdSe and 𝑔
CdSe/ZnS extracted QDs were added into the 25 ml of a 0.01𝐿 MO solution. The finally prepared solution was placed under a UV-Lamp for different intervals.The behavior of the characteristic absorption peak of MO (495 nm) was monitored during the catalytic degradation process. Fig. 11 describes the photocatalytic degradation of MO by CdSe (a) and CdSe/ZnS (b) core-shell QDs. The intensity of the absorption peak of MO decreased gradually by extending UV-illumination time, confirming degradation of MO. Fig.12 indicates the degradation rate of MO at different intervals by CdSe (a) and CdSe/ZnS (b) coreshell QDs. The degradation percentile of CdSe was about 50% which was promoted to the 70% for the CdSe/ZnS core-shell QDs. Fig. 13 is a diagram presenting the photo-degradation mechanism of CdSe 6
ACCEPTED MANUSCRIPT andCdSe/ZnS core-shell QDs.ZnS shell enhanced the photocatalytic activity of CdSe cores, because it has a higher and lower conduction and valance bands, respectively compare to the ones belonging to CdSe, therefore ZnS decreases the required energy for transferring carriers to vacuum states or it can be said the redox potential decreases and can enhance the photo –activation of carriers and consequently enhances the photocatalyst activity.
4. Conclusion CdSe/ZnS core-shell QDs were synthesized using a combination of microwave and photochemical approaches. CdSe QDs exhibited a broad band emission related to the surface trap states which it was quenched considerably after ZnS shell growth and a narrow band edge emission with FWHM of about 36 nm was appeared. The ZnS shell thickness was tunable by UV-illumination timeevolution as the optimum value of the shell was obtained for 90 min. Synthesized QDs indicated strong photocatalytic activity. The activity of the CdSe/ZnS QDs for photo-degradation of Methyl orange (MO) was stronger than that for CdSe QDs.
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ACCEPTED MANUSCRIPT [18] L. Zuala, R. Madaka and P. Agarwal, Hindawi Publishing Corportion. 2013, 10, 257359. [19] Gh. R. Amiri, S. Fatahian and S. Mahmoudi, Scientific Research. 2013, 4, 134. [20] M. Dong, J. Xu, Sh. Li, Y. Zhou and Ch. Huang, Luminescence. 2014, 29, 818. [21] M. Molaei, F. Karimimaskon, A. Lotfiani, M. karimipour and M. Khanzadeh,J. Semiconductor Tech. andSci. 2013, 143, 649. [22] M. Molaei, A. R. Khezripour and M. Karimipour, Appl. Surf. Sci. 2014, 317, 236. [23] S. Abbasi, M. Molaei, M. Karimipour, Luminescence, DOI: 10.1002/bio.3300 [24] A. R. Bahador, M. Molaei, M. Karimipour, J. Lumin, 166 (2015) 101. [25] M. Molaei, F. Sarhani, F. Salaribadsiri and M. Karimipour, Mod. Phys. Lett. B, 2015, 29, 1650093. [26] Y. Wang, J. P. Lu and Zh. Fa. Tong, bull. Mater. sci. 2010, 33, 543. [27]S. Das, S. Banerjee, A. Dutta, T. Sinha, Materials Science in Semiconductor Processing. 2015, 40, 412. [28]M. R. Karim, Z. Ferdous, and H. Ünlü, Journal of Innovation and Scientific Research, 2014, 5, 11. [29] A. G. Bakanov, N. A. Toropov, and T. A. Vartanyan, Optics and Spectroscopy, 2015, 3, 118.
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ACCEPTED MANUSCRIPT Figures captions Fig. 1. A schematic picture of synthesizing CdSe/ZnS QDs process.
Fig. 2. XRD patterns of synthesized CdSe and CdSe/ZnS QDs.
Fig. 3. TEM images of CdSe (a) and CdSe/ZnS (b) core-shell QDs.
Fig. 4. FTIR spectra of CdSe and CdSe/ZnS core-shell QDs.
Fig. 5. Absorption and PL spectra of CdSe QDs for 90 S of microwave irradiation. (excitation wavelength was 365 nm) Fig. 6. A schematic diagram describing CdSe emission mechanism
Fig. 7. Absorption spectra (a) Tauc plot of absorption spectra (b) and size-band gap (c) of the CdSe QDs synthesized at different microwave irradiation intervals
Fig. 8. PL spectra of the CdSe QDs synthesized at different microwave irradiation intervals
Fig. 9. Absorption and PL spectra of CdSe/ZnS core-shell
Fig. 10. PL spectra of CdSe/ZnS core-shell QDs with different thicknesses of ZnS shell Fig. 11. Absorption spectra demonstrating the photocatalytic degradation of MO by CdSe (a) and CdSe/ZnS (b) core–shell QDs Fig. 12. Comparison of photocatalytic activities of CdSe and CdSe/CdS QDs. Fig. 13. Proposed mechanism for the MO photo degradation by CdSe (a) and CdSe/ZnS (b) core–shell QDs
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•CdSe/ZnS QDs were synthesized by a combination of microwave and photochemical
approaches• Trap state emission of CdSe cores quenched significantly after ZnS shell growth• CdSe/ZnS QDs show superior photocatalytic activity rather than CdSe QDs