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Glutathione capped inverted core-shell quantum dots as an efficient photocatalyst for degradation of organic dyes
Materials Science in Semiconductor Processing 106 (2020) 104760
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Materials Science in Semiconductor Processing journal homepage: http://www.elsevier.com/locate/mssp
Glutathione capped inverted core-shell quantum dots as an efficient photocatalyst for degradation of organic dyes K. Bhuvaneswari a, V. Vaitheeswari a, G. Palanisamy a, T. Maiyalagan b, T. Pazhanivel a, * a b
Smart Materials Interface Laboratory, Department of Physics, Periyar University, Salem, 11, Tamilnadu, India Department of Chemistry, SRM Institute of Science and Technology, Kattankulathur, 603203, Tamil Nadu, India
A R T I C L E I N F O
A B S T R A C T
Keywords: Inverted core-shell QDs Photocatalytic activity Scavenging activity Reusability
Herein, we focused on the preparation of Glutathione (GHS) capped ZnS, CdS and inverted type-I ZnS@CdS coreshell quantum dots (QDs) through reflux condensation method. The XRD and HR-TEM analysis confirms the formation of core-shell QDs with corresponding crystal structures. The optical properties and band energy of the as prepared samples were evaluated from optical absorption study. The blue shift of absorption was observed in the core-shell QDs system which may be due to the quantum size effect. The EDX and elemental mapping study confirm the existence of Zn, Cd and S elements in the as-prepared material. When compared to ZnS and CdS QDs, the as prepared ZnS@CdS QDs shows improved photocatalytic activity toward the MB dye degradation due to the effective separation of the photogenerated electron-hole pair. The inverted ZnS@CdS core-shell structure not only enhances the light absorption also to reduce the charge recombination of the photocatalysts for the effective charge and energy transfer between the components during degradation. Finally, the scavenging activity and reusability of the ZnS@CdS core-shell QDs samples were examined under UV–Vis. Light irradiation.
1. Introduction Organic dyes are used in various material production industries like paper, cosmetics, food processing, textile, etc. [1] and these water contaminants pose serious environmental hazards to human, animals, and microorganisms. Nowadays, numerous efforts have been devoted and followed to eliminating the organic dyes from wastewaters, for example, filtration, sedimentation, adsorption, photodegradation, pre cipitation, biodegradation, electrolytic chemical treatment, and membrane-based technology [2–4], among the many approaches, pho tocatalytic degradation is a significant technique for the removal of dyes in the aqueous medium [5]. Semiconductors (SCs) materials, such as TiO2, SnO2, CdS, ZnS, ZnO, SiO2 and WO3, have been widely used as an efficient catalyst for the photodegradation of dye [6–12]. However, the photocatalytic dye degradation efficiency was restricted by several factors, such as limiting light absorption (only ultraviolet (UV) light) and rapid recombination of electron and hole [13]. The organic con taminants can be efficiently degraded through the generation of photo-induced electron-holes in photo-catalysts under visible light irradiation. In practical applications of SCs, materials were limited by its basic defects, such as low quantum efficiency and low electronic
properties of nanoparticles [14,15]. Hence, the alteration of SCs by a narrow band-gap material can increase the performance of photo catalytic activity owing to solar light harvesting nature, stimulating electron transfer and dropping the electron-hole recombination [16–20]. On the other hand, sulfide- based SCs photocatalysts have involved much consideration in recent decades because of its excellent Vis-light responses and excellent catalytic behavior [21,22]. In this way, various metal-sulfide based photocatalysts consume considerable attention for water splitting. QDs was used as a photocatalyst in order to solve the abovementioned problems. In recent days, semiconductor based QDs have great attention due to their admirable optical and electronic properties [23–25]. QDs are nanocrystals and their exciton are confined in all three dimensions and they exhibit quantum confinement effects. For these reasons QDs have long fluorescence lifetimes, narrow band emission, and high quantum yield which make as an efficient and applicable for light emitting diodes, luminescent based solar concentrators and bio-analysis etc., [26,27]. The use of smaller particles was probable to result in higher photocatalytic activity [28,29]. However, the efficiency of these photocatalysts was very low and practical application also limited. Most of the researchers are trying to improve the efficiency of
* Corresponding author. E-mail address: [email protected] (T. Pazhanivel). https://doi.org/10.1016/j.mssp.2019.104760 Received 15 March 2019; Received in revised form 19 July 2019; Accepted 30 September 2019 Available online 11 October 2019 1369-8001/© 2019 Elsevier Ltd. All rights reserved.
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Fig. 3. UV–Vis. spectra of (a) ZnS QDs (b) CdS QDs and (c) ZnS@CdS QDs.
photocatalysts. The as-prepared inverted type-I glutathione capped ZnS@CdS core-shell quantum dot exhibit enhanced UV–Vis-light pho tocatalytic activity for the decolorization of MB dye compared with ZnS QDs and CdS QDs, which is cost-efficient and convenient compared to the conventional approach. Fig. 1. XRD patterns of (a) ZnS QDs (b) CdS QDs and (c) ZnS@CdS coreshell QDs.
2. Experimental section 2.1. Reagents Reagents used for experiments are zinc chloride [ZnCl2], sodium sulfide [Na2S], glutathione (GHS) and cadmium chloride [CdCl2] which were purchased from Hi-Media. Methylene blue (MB) was purchased from Merck and all the reagents are used without further purification process. Double distilled water (DDW) was used as a solvent throughout the experiments. 2.2. ZnS@CdS core-shell DQs The ZnS and CdS QDs were prepared by simple precipitation method [31]. The ZnS@CdS QDs was prepared by precipitation coming reflux condensation method. In a typical experimental procedure, GHS capped ZnS solution was mixed with CdS solution. The mixed solution was refluxed at 4 h and pH of the solution was maintained at 12 by using 0.1 mol of NaOH. Then the obtained product was centrifuged and washed with DDW and ethanol to remove residual ions. Then the product was dried at 60 � C for overnight.
Fig. 2. FTIR spectra of (a) ZnS QDs (b) CdS QDs and (c) ZnS@CdS QDs.
2.3. Materials characterization
the photocatalyst by forming hybrid systems [30]. Which offers novel prospects for controlling the transformation of photogenerated charge carriers since one element to other. ZnS is a moral material for photo catalytic dye degradation but the high band gap energy restricts the light absorption hence we are focusing to prepare a visible light sulfide-rich photocatalyst for the degradation of organic dye [31–33]. Since the identical crystal structures of ZnS and CdS, considerable attention has been intent on the preparation of hybrid formation to enhance the photocatalytic activity [34–37]. Here, we reported an efficient UV–Vis. Light photocatalyst based on the composition of an inverted type-I core-shell QDs through reflux condensation method. We tried to explore the potential use of inverted core-shell structure employing wide bandgap core and narrow bandgap shell. Most importantly, glutathione (GHS) linked prepared material sustaining the present environmental requirements of eco-friendly
The crystal structure and phase purity of as-prepared ZnS, CdS and ZnS@CdS QDs were characterized by XRD analysis through Riguku MiniFlux-II X-ray diffractometer (10� � 2θ � 70� ) using Cu Kα radiation (1.54046 Å). To understand the chemical environment and investigate the functional groups present on the surface of the as-prepared samples were subjected in to FTIR spectrum by using a Bruker model Tensor 27. The morphology of the samples were investigated by FE-SEM using Zeiss SUPRA-25 and the EDX and mapping (JEOL Model JED 2300) analysis were used to the estimation of the elementals. To understand the optical absorption abilities of as-prepared CdS, ZnS and ZnS@CdS QDs samples were examined by UV–Vis. analysis from 200 to 800 nm wavelength range using a SHIMADZU 3600 UV–Vis–NIR spectrophotometer. Inter nal morphology of the as-prepared photocatalyst was analyzed by HRTEM - Jeol/JEM 2100, with a LaB6 source. Emission spectrum of the as prepared samples were recorded by using Horiba Jobin Yvon Spectro 2
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Materials Science in Semiconductor Processing 106 (2020) 104760
Fig. 4. Tauc’s plots of (a) ZnS QDs (b) CdS QDs and (c) ZnS@CdS QDs.
entire reaction, the reactor was maintained at room temperature by circulating running water. At 20 min of time intervals, 3 ml of dye so lution was taken and then centrifuged to eliminate the catalyst particles. The absorption of the supernatant solution was studied by a Shimadzu UV3000 UV–Vis spectrophotometer and the dye absorption band maximum was measured at ~664 nm. For the reusability purpose, after the photocatalytic reaction the as-prepared photocatalyst collected by centrifuging washed with DDW and then dried at 60 ̊� C. 2.5. Radical trapping experiment The radical trapping investigation was used to find the photo catalytic reaction mechanism of ZnS@CdS DQs on MB dye. The scav enging activity hþ, OH and O-2 radicals are trapped by EDTA, 2-propanol and benzoquinone were performed separately. The trapping experiment was carried out with the accumulation of different scavenger into the catalytic reaction. The reaction samples were taken from the photo catalytic reactor to record their UV–Vis absorption spectra on the UV–Vis spectrophotometer.
Fig. 5. PL spectra of (a) ZnS QDs (b) CdS QDs and (c) ZnS@CdS QDs.
Fluromax 4.
3. Results and discussion The X-ray diffraction (XRD) patterns of as-prepared ZnS QDs, CdS QDs, and ZnS@CdS DQs were shown in Fig. 1. (a-c). In the case of ZnS QDs, all the observed diffraction peaks reveals that the presence of polycrystalline natured ZnS QDs which is well matched with the cor responding JCPDS card no. 05–0566. The peaks at 28.5� , 47.5� and 56.2� due to the influence of the lattice planes (111), (220) and (311) of the cubic nanocrystals respectively. The broad diffraction peaks at an angle (2θ degree) 20–35 region represents the formation of the cubic ZnS QDs [31]. For pure CdS QDs, the diffraction peaks at 26.85� , 30.9� ,
2.4. Photodegradation The photodegradation performance was evaluated by the photo degradation of MB dye under UV–Vis. Light irradiation. A 500 W xenon lamp with a light intensity of 4 W/cm2 was used as the light source [38, 39]. In this typical photocatalyst process, 20 mgL-1 of catalyst was introduced into the 100 ml of dye solutions under continuous stirring, and the obtained solution was kept in a darkroom for 20 min to attain equilibrium adsorption/desorption of dyes on the catalyst surface. In the 3
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Fig. 6. (a) SEM with EDX spectrum and (b) Mapping analysis of ZnS@CdS QDs respectively.
Fig. 7. TEM image of (a) ZnS QDs and (b–c) different magnification of ZnS@CdS QDs and (d) SEAD pattern of ZnS@CdS QDs.
43.15� and 52� due to the contribution of the lattice planes (111), (200), (220) and (311) are perfectly matched with cubic CdS (JCPDS no.89–0440) [40]. The ZnS and CdS diffraction patterns show broad ening phenomena because it’s smaller sizes. Fig. 1. (c) shows the
diffraction pattern of ZnS@CdS formation when related to ZnS and CdS QDs, the diffraction patterns shift to higher angles owing to the growth of CdS shells on ZnS QDs core. Moreover, the diffraction peaks positions of ZnS@CdS QDs are closer to the bulk ZnS than the bulk CdS it conforms 4
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Fig. 8. (A) Rate of degradation and (B) Pseudo-first-order kinetics linear simulation curves of MB degradation over (a) ZnS QDs (b) CdS QDs and (c) ZnS@CdS QDs.
Fig. 9. (a) Histogram for photocatalytic degradation efficiency of as prepared samples and (b) Histogram for effect of MB degradation over ZnS@CdS QDs in the presence of various scavengers.
small band at around 2980 cm-1 corresponds to C–O stretching vibra tion. The specific peak at 1550 and 1559 cm-1 represents the interaction of glutathione molecule [42]. Similarly the characteristic sharp peaks were observed near 1410 to 1395 cm-1 due to the symmetric stretching of carboxyl group corresponds to thiol capping [41]. The characteristic peaks observed below 800 cm-1 can be attributed to metal-S stretching vibrations [43]. Usually, hydroxyl (OH) and carboxylic (COOH) func tional groups are present due to the presence of biomolecules which confirms the as-prepared QDs have compatibility to bind with bio-molecule [44]. Fig. 3 shows the UV–Vis. spectra of as-prepared (a) ZnS QDs, (b) CdS QDs and (c) ZnS@CdS QDs. The photoabsorption maximum of ZnS and CdS QDs are 218 & 345 nm. The growth of CdS shell around ZnS core was changes the optical properties of the QDs. The absorption edge of as prepared ZnS@CdS QDs was redshifted when compared to ZnS (core) and blue shifted compares to CdS (shell). In general core-shell materials, optical properties are mainly based on the core material. The redshift of the absorption indicates that ZnS@CdS QDs exhibit quantum confine ment effects [45]. This could be expounded as the complete result of the size effect and the potential-well effect when a CdS shell is formed on the surface of the ZnS core finally, the size is higher than the core [46]. This allows an additional delocalization of the electronic wave function and forms a red-shift of the absorption band edge transition (size effect). Another reason is the conduction (CB) and the valence band (VB) of CdS were situated in the energy gap of the ZnS, the type-I electron-hole pairs tend to restrict in SC with a lower band gap, i.e., CdS, which offer the lowest energy positions for both charge carrier [47]. The electronic and optical properties of these nanomaterials show a significant change from
Fig. 10. Schematic illustration of possible photocatalytic reaction mechanism of ZnS@CdS QDs on MB.
the successful formation of ZnS@CdS core-shell structure. According to the Scherer equation, the average crystallite size was calculated and the obtained values are 2.8, 3.49 and 5.3 nm for ZnS, CdS and ZnS@CdS QDs respectively. When compared to pure QDs, the size of ZnS@CdS QDs size is high due to the formation of a CdS layer on ZnS core. Fig. 2 shows the Fourier transform Infra-Red (FTIR) spectra of (a) ZnS QDs, (b) CdS QDs and (c) ZnS@CdS QDs. The broad absorption band was observed between 3700-3300 cm-1 ascribed to O–H stretching of surface absorbed water molecules [41]. The small hump at 2931, 2912 and 2924 cm-1 assigned to C–H asymmetric stretching vibration and a 5
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Table 1 Comparison table of UV–Vis. Light assisted photocatalyst with degradation efficiency (%) of reported nanomaterials. S. No
Photocatalyst
Dye
Light source with Power
Photocatalyst Amount (mg/L)
Irradiation time (Min)
Efficiency (%)
Ref.
1.
Green colloidal ZnS QDs
UV light 6 W
30
120
87
[32]
2. 3. 4. 7. 8.
Mn–ZnS QDs ZnS QD-TiO2 CdS QD-GeO2 ZnS@CdS nanoparticles PVP capped ZnS@CdS nanoparticles ZnS@CdS core-shell QDs
MB and MO MO MB MB MB MB
UV light 8 W UV light 12 W Visible light 450 W Visible light 500 W Visible light 500 W
100 10 50 100 10
120 420 120 360 90
95.4 90 80.5 63 65
[27] [53] [54] [55] [56]
MB
Visible light 500 W (Intensity-4 Wcm-2)
10
120
93
Present works
9.
Fig. 11. (a) Catalyst reusability of ZnS@CdS QDs in cyclic photodegradation over MB dye for four cycles and (b) XRD patterns of ZnS@CdS QDs before and after photocatalytic reaction.
their corresponding bulk properties, which are called quantum size ef fects. The measured band gap values are 3.82, 3.7 and 2.6 eV for ZnS QDs, CdS QDs, and ZnS@CdS QDs respectively. Quantum confinement of both the hole and electron in three dimensions leads to an enhancing the effective band gap of the SC material with reducing crystallite size [48] shown in Fig. 4. The Photoluminescence (PL) spectra orginates from the rediative or non radiative recombination of self trapped exitons [49]. In this approach, the possible photocatalytic reaction mechanism of the as synthesized samples were analyzed by PL analysis. The PL emission spectrum can be generates from the recombination of charge carriers and used to express the efficiency of charge carrier transformation, trapping, separation of the photogenerated electrons and holes [50]. Fig. 5 shows the PL spectra of as ZnS, CdS and ZnS@CdS QDs were obtained by the excitation wavelength of 320 nm. The strong emission peak of ZnS QDs and CdS QDs were observed around 430 and 440 nm respectively. After the core/shell formation the PL intensity of the pre pared photocatalyst was decreased due to the low recombination rate of electron hole pair. Howerver the PL peak intensity of ZnS@CdS QDs becomes much lower than that of ZnS and CdS QDS, which indicating a lower recombination rate of photogenerated electron–hole pairs after core\shell formation. The low PL emission peak position usually con tributes high photocatalytic activity which clearly shows that, the core/shell structure offers in the higher photocatalytic activity than ZnS and CdS QDS. The decreasing PL spectra intensity clearly conform the formation of ZnS@CdS which will actively enhance the photocatalytic activity of the prepared photocatalyst. The different chemical composition of ZnS@CdS QDs was investi gated by EDX analysis which shown in Fig. 6. (a) shows the SEM image of as prepared ZnS@CdS sample with corresponding EDX spectrum. From the EDX pattern the presence of Zn, Cd, and S was clearly confirmed. Fig. 6(b). Shows, the element mapping images of ZnS@CdS QDs shows a bright point in the elemental map designated a higher
concentration of the equivalent element in that area. From this study Zn, S and Cd elements are coexistent presented in the prepared material. HR-TEM provides a higher resolution and may be better suitable for the analysis of particle size in the nano as well as sub nano-region. The morphology and particle size of ZnS QDs and ZnS@CdS QDs were observed by HR-TEM analysis which were shown in Fig. 7. (a-c). Ac cording to the HR-TEM measurements, the average size of ZnS@CdS QDs was observed around 7 � 1 nm with a core size of 2.5 �0 .4 nm. The interplanar d-spacing of ZnS@CdS QDs is 0.26 nm and the selected area electron diffraction (SAED) pattern is obtained in the reverse space of the lattice planes. Fig. 7 (d) shows the SAED configuration of the sample ZnS@CdS QDs from this image some bright spots were observed and also the concentric ring formation reveals that the particles are in the lack of crystalline in nature. 3.1. Photocatalytic dye degradation reaction In recent times, heterogeneous photocatalysis has a significant role in the photodegradation, which leads to the total mineralization of organic pollutants, especially synthetic or organic dyes. The photo catalytic activity of all the samples was evaluated by the photo decol orization of MB dye under UV–Vis. Light irradiation. The photocatalytic degradation activity of the as-prepared samples were investigate the photodegradation of MB aqueous solution under UV–Vis-light irradia tion. Self-degradation of the MB dye was examined without adding any catalysts and MB dye was observed for 120 min under UV–Vis. Light irradiation and there was no significant change in MB concentration which confirms that MB does not decomposition by itself (Fig. 8(A)). The maximum absorptions for MB at the wavelength of 664 nm and through the whole photodegradation period maximum absorption occurred almost at the same wavelength after the adding of prepared photocatalyst with increasing irradiation time the absorbance of MB dye spectra rapidly declined which confirm the prepared photocatalyst 6
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support the photocatalytic reaction. To compare the catalytic activity of as-prepared samples the first-order kinetic equation was used to fit the experimental data of MB degradation, from this result ZnS QDs shows very low photocatalytic action due to its limited Visible light absorption (Fig. 8 (B)). When compared to ZnS QDs and CdS QDs the ZnS@CdS QDs offers more attractive result for the photodegradation of MB dye mole cules because of its lesser band gap of CdS material can enhance the UV–Vis. Light absorption. The photodegradation efficiency of asprepared materials were examined and drawn among absorbance con centration of MB dye against UV–Vis. Light irradiation time shown in Fig. 9(a). Basically, the photocatalytic activity of ZnS@CdS QDs was improved due to decreased electron-hole recombination rate by sepa ration of charge by the formation of the heterojunction. The improved photocatalytic activity obtained by the formation of sulfide-rich heter ostructure which helps the charge transfer from conduction band to valence band. Fig. 10 shows the possible mechanism of the photo catalytic reaction. Table-1 contains the comparison of our experimental results with the reported values, which show the improved photo catalytic performance of ZnS@CdS QDs.
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3.2. Scavenging and stability experiment In order to explore the basic photocatalytic mechanism of ZnS@CdS QDs, radical trapping experiment was evaluated as shown in Fig. 9 (b). In this investigation, OH, hþ and O-2 radical were trapped by the addition of 2-propanol, EDTA and benzoquinone, respectively [51,52]. After the addition of 2-propanol and benzoquinone the photocatalytic activity was inhibited the MB degradation up to 15–20% and in existence of hþ radicals with the adding of EDTA, hinder nearly 50% reaction. The above results confirm the surface hþ was the main active species for inverse ZnS@CdS QDs, which played a significant role in the MB dye photodegradation. In practical applications, the reusability of photo catalysts is a key role, for this motive the reusability of the ZnS@CdS QDs photocatalyst was examined after the degradation of MB under UV–Vis. Light irradiation. In every cycle, fresh MB dye solution was taken to find the stability of the ZnS@CdS QDs photocatalyst. The experiment was repeated followed by centrifugation and then washed with DDW, and drying at 60 � C for overnight. After four consecutive series, we detected only an insignificant change in the photocatalytic dye degradation efficiency when compared to the fresh photocatalyst shown in Fig. 11 (a). The scavenging and reusability study reveals that the O-2 was the main active species of the photocatalyst and the prepared ZnS@CdS QDs sample posses durability towards MB dye degradation. In further, the stability of the as prepared photocatalyst were confirmed by the XRD analysis and the result shown in Fig. 11 (b). After 4 consecutive cycles, there no significant phase changes can be detected. Which clearly conforms the ZnS@CdS QDs have high stability. 4. Conclusion In summary, we successfully synthesized inverted type-I ZnS@CdS core-shell QDs by reflux condensation method and investigate their photocatalytic activity. ZnS and CdS are typical IIB–VIA group SCs photocatalyst which individually exhibits very low photocatalytic ac tivity. The incorporation of inorganic and organic materials was one of the effective modes to improve photocatalytic activity of prepared ma terial under UV–Vis. Light irradiation. After the formation of ZnS@CdS QDs structure can be enhanced photocatalytic activity due to its good UV–Vis. Light response. Radical trapping test proposes that photogenerated hþ radicals dominant the photocatalytic reaction. This investigation will be support for the forthcoming photocatalyst by using an inverse type-I core-shell based materials.
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Materials Science in Semiconductor Processing journal homepage: http://www.elsevier.com/locate/mssp
Glutathione capped inverted core-shell quantum dots as an efficient photocatalyst for degradation of organic dyes K. Bhuvaneswari a, V. Vaitheeswari a, G. Palanisamy a, T. Maiyalagan b, T. Pazhanivel a, * a b
Smart Materials Interface Laboratory, Department of Physics, Periyar University, Salem, 11, Tamilnadu, India Department of Chemistry, SRM Institute of Science and Technology, Kattankulathur, 603203, Tamil Nadu, India
A R T I C L E I N F O
A B S T R A C T
Keywords: Inverted core-shell QDs Photocatalytic activity Scavenging activity Reusability
Herein, we focused on the preparation of Glutathione (GHS) capped ZnS, CdS and inverted type-I ZnS@CdS coreshell quantum dots (QDs) through reflux condensation method. The XRD and HR-TEM analysis confirms the formation of core-shell QDs with corresponding crystal structures. The optical properties and band energy of the as prepared samples were evaluated from optical absorption study. The blue shift of absorption was observed in the core-shell QDs system which may be due to the quantum size effect. The EDX and elemental mapping study confirm the existence of Zn, Cd and S elements in the as-prepared material. When compared to ZnS and CdS QDs, the as prepared ZnS@CdS QDs shows improved photocatalytic activity toward the MB dye degradation due to the effective separation of the photogenerated electron-hole pair. The inverted ZnS@CdS core-shell structure not only enhances the light absorption also to reduce the charge recombination of the photocatalysts for the effective charge and energy transfer between the components during degradation. Finally, the scavenging activity and reusability of the ZnS@CdS core-shell QDs samples were examined under UV–Vis. Light irradiation.
1. Introduction Organic dyes are used in various material production industries like paper, cosmetics, food processing, textile, etc. [1] and these water contaminants pose serious environmental hazards to human, animals, and microorganisms. Nowadays, numerous efforts have been devoted and followed to eliminating the organic dyes from wastewaters, for example, filtration, sedimentation, adsorption, photodegradation, pre cipitation, biodegradation, electrolytic chemical treatment, and membrane-based technology [2–4], among the many approaches, pho tocatalytic degradation is a significant technique for the removal of dyes in the aqueous medium [5]. Semiconductors (SCs) materials, such as TiO2, SnO2, CdS, ZnS, ZnO, SiO2 and WO3, have been widely used as an efficient catalyst for the photodegradation of dye [6–12]. However, the photocatalytic dye degradation efficiency was restricted by several factors, such as limiting light absorption (only ultraviolet (UV) light) and rapid recombination of electron and hole [13]. The organic con taminants can be efficiently degraded through the generation of photo-induced electron-holes in photo-catalysts under visible light irradiation. In practical applications of SCs, materials were limited by its basic defects, such as low quantum efficiency and low electronic
properties of nanoparticles [14,15]. Hence, the alteration of SCs by a narrow band-gap material can increase the performance of photo catalytic activity owing to solar light harvesting nature, stimulating electron transfer and dropping the electron-hole recombination [16–20]. On the other hand, sulfide- based SCs photocatalysts have involved much consideration in recent decades because of its excellent Vis-light responses and excellent catalytic behavior [21,22]. In this way, various metal-sulfide based photocatalysts consume considerable attention for water splitting. QDs was used as a photocatalyst in order to solve the abovementioned problems. In recent days, semiconductor based QDs have great attention due to their admirable optical and electronic properties [23–25]. QDs are nanocrystals and their exciton are confined in all three dimensions and they exhibit quantum confinement effects. For these reasons QDs have long fluorescence lifetimes, narrow band emission, and high quantum yield which make as an efficient and applicable for light emitting diodes, luminescent based solar concentrators and bio-analysis etc., [26,27]. The use of smaller particles was probable to result in higher photocatalytic activity [28,29]. However, the efficiency of these photocatalysts was very low and practical application also limited. Most of the researchers are trying to improve the efficiency of
* Corresponding author. E-mail address: [email protected] (T. Pazhanivel). https://doi.org/10.1016/j.mssp.2019.104760 Received 15 March 2019; Received in revised form 19 July 2019; Accepted 30 September 2019 Available online 11 October 2019 1369-8001/© 2019 Elsevier Ltd. All rights reserved.
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Fig. 3. UV–Vis. spectra of (a) ZnS QDs (b) CdS QDs and (c) ZnS@CdS QDs.
photocatalysts. The as-prepared inverted type-I glutathione capped ZnS@CdS core-shell quantum dot exhibit enhanced UV–Vis-light pho tocatalytic activity for the decolorization of MB dye compared with ZnS QDs and CdS QDs, which is cost-efficient and convenient compared to the conventional approach. Fig. 1. XRD patterns of (a) ZnS QDs (b) CdS QDs and (c) ZnS@CdS coreshell QDs.
2. Experimental section 2.1. Reagents Reagents used for experiments are zinc chloride [ZnCl2], sodium sulfide [Na2S], glutathione (GHS) and cadmium chloride [CdCl2] which were purchased from Hi-Media. Methylene blue (MB) was purchased from Merck and all the reagents are used without further purification process. Double distilled water (DDW) was used as a solvent throughout the experiments. 2.2. ZnS@CdS core-shell DQs The ZnS and CdS QDs were prepared by simple precipitation method [31]. The ZnS@CdS QDs was prepared by precipitation coming reflux condensation method. In a typical experimental procedure, GHS capped ZnS solution was mixed with CdS solution. The mixed solution was refluxed at 4 h and pH of the solution was maintained at 12 by using 0.1 mol of NaOH. Then the obtained product was centrifuged and washed with DDW and ethanol to remove residual ions. Then the product was dried at 60 � C for overnight.
Fig. 2. FTIR spectra of (a) ZnS QDs (b) CdS QDs and (c) ZnS@CdS QDs.
2.3. Materials characterization
the photocatalyst by forming hybrid systems [30]. Which offers novel prospects for controlling the transformation of photogenerated charge carriers since one element to other. ZnS is a moral material for photo catalytic dye degradation but the high band gap energy restricts the light absorption hence we are focusing to prepare a visible light sulfide-rich photocatalyst for the degradation of organic dye [31–33]. Since the identical crystal structures of ZnS and CdS, considerable attention has been intent on the preparation of hybrid formation to enhance the photocatalytic activity [34–37]. Here, we reported an efficient UV–Vis. Light photocatalyst based on the composition of an inverted type-I core-shell QDs through reflux condensation method. We tried to explore the potential use of inverted core-shell structure employing wide bandgap core and narrow bandgap shell. Most importantly, glutathione (GHS) linked prepared material sustaining the present environmental requirements of eco-friendly
The crystal structure and phase purity of as-prepared ZnS, CdS and ZnS@CdS QDs were characterized by XRD analysis through Riguku MiniFlux-II X-ray diffractometer (10� � 2θ � 70� ) using Cu Kα radiation (1.54046 Å). To understand the chemical environment and investigate the functional groups present on the surface of the as-prepared samples were subjected in to FTIR spectrum by using a Bruker model Tensor 27. The morphology of the samples were investigated by FE-SEM using Zeiss SUPRA-25 and the EDX and mapping (JEOL Model JED 2300) analysis were used to the estimation of the elementals. To understand the optical absorption abilities of as-prepared CdS, ZnS and ZnS@CdS QDs samples were examined by UV–Vis. analysis from 200 to 800 nm wavelength range using a SHIMADZU 3600 UV–Vis–NIR spectrophotometer. Inter nal morphology of the as-prepared photocatalyst was analyzed by HRTEM - Jeol/JEM 2100, with a LaB6 source. Emission spectrum of the as prepared samples were recorded by using Horiba Jobin Yvon Spectro 2
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Fig. 4. Tauc’s plots of (a) ZnS QDs (b) CdS QDs and (c) ZnS@CdS QDs.
entire reaction, the reactor was maintained at room temperature by circulating running water. At 20 min of time intervals, 3 ml of dye so lution was taken and then centrifuged to eliminate the catalyst particles. The absorption of the supernatant solution was studied by a Shimadzu UV3000 UV–Vis spectrophotometer and the dye absorption band maximum was measured at ~664 nm. For the reusability purpose, after the photocatalytic reaction the as-prepared photocatalyst collected by centrifuging washed with DDW and then dried at 60 ̊� C. 2.5. Radical trapping experiment The radical trapping investigation was used to find the photo catalytic reaction mechanism of ZnS@CdS DQs on MB dye. The scav enging activity hþ, OH and O-2 radicals are trapped by EDTA, 2-propanol and benzoquinone were performed separately. The trapping experiment was carried out with the accumulation of different scavenger into the catalytic reaction. The reaction samples were taken from the photo catalytic reactor to record their UV–Vis absorption spectra on the UV–Vis spectrophotometer.
Fig. 5. PL spectra of (a) ZnS QDs (b) CdS QDs and (c) ZnS@CdS QDs.
Fluromax 4.
3. Results and discussion The X-ray diffraction (XRD) patterns of as-prepared ZnS QDs, CdS QDs, and ZnS@CdS DQs were shown in Fig. 1. (a-c). In the case of ZnS QDs, all the observed diffraction peaks reveals that the presence of polycrystalline natured ZnS QDs which is well matched with the cor responding JCPDS card no. 05–0566. The peaks at 28.5� , 47.5� and 56.2� due to the influence of the lattice planes (111), (220) and (311) of the cubic nanocrystals respectively. The broad diffraction peaks at an angle (2θ degree) 20–35 region represents the formation of the cubic ZnS QDs [31]. For pure CdS QDs, the diffraction peaks at 26.85� , 30.9� ,
2.4. Photodegradation The photodegradation performance was evaluated by the photo degradation of MB dye under UV–Vis. Light irradiation. A 500 W xenon lamp with a light intensity of 4 W/cm2 was used as the light source [38, 39]. In this typical photocatalyst process, 20 mgL-1 of catalyst was introduced into the 100 ml of dye solutions under continuous stirring, and the obtained solution was kept in a darkroom for 20 min to attain equilibrium adsorption/desorption of dyes on the catalyst surface. In the 3
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Fig. 6. (a) SEM with EDX spectrum and (b) Mapping analysis of ZnS@CdS QDs respectively.
Fig. 7. TEM image of (a) ZnS QDs and (b–c) different magnification of ZnS@CdS QDs and (d) SEAD pattern of ZnS@CdS QDs.
43.15� and 52� due to the contribution of the lattice planes (111), (200), (220) and (311) are perfectly matched with cubic CdS (JCPDS no.89–0440) [40]. The ZnS and CdS diffraction patterns show broad ening phenomena because it’s smaller sizes. Fig. 1. (c) shows the
diffraction pattern of ZnS@CdS formation when related to ZnS and CdS QDs, the diffraction patterns shift to higher angles owing to the growth of CdS shells on ZnS QDs core. Moreover, the diffraction peaks positions of ZnS@CdS QDs are closer to the bulk ZnS than the bulk CdS it conforms 4
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Fig. 8. (A) Rate of degradation and (B) Pseudo-first-order kinetics linear simulation curves of MB degradation over (a) ZnS QDs (b) CdS QDs and (c) ZnS@CdS QDs.
Fig. 9. (a) Histogram for photocatalytic degradation efficiency of as prepared samples and (b) Histogram for effect of MB degradation over ZnS@CdS QDs in the presence of various scavengers.
small band at around 2980 cm-1 corresponds to C–O stretching vibra tion. The specific peak at 1550 and 1559 cm-1 represents the interaction of glutathione molecule [42]. Similarly the characteristic sharp peaks were observed near 1410 to 1395 cm-1 due to the symmetric stretching of carboxyl group corresponds to thiol capping [41]. The characteristic peaks observed below 800 cm-1 can be attributed to metal-S stretching vibrations [43]. Usually, hydroxyl (OH) and carboxylic (COOH) func tional groups are present due to the presence of biomolecules which confirms the as-prepared QDs have compatibility to bind with bio-molecule [44]. Fig. 3 shows the UV–Vis. spectra of as-prepared (a) ZnS QDs, (b) CdS QDs and (c) ZnS@CdS QDs. The photoabsorption maximum of ZnS and CdS QDs are 218 & 345 nm. The growth of CdS shell around ZnS core was changes the optical properties of the QDs. The absorption edge of as prepared ZnS@CdS QDs was redshifted when compared to ZnS (core) and blue shifted compares to CdS (shell). In general core-shell materials, optical properties are mainly based on the core material. The redshift of the absorption indicates that ZnS@CdS QDs exhibit quantum confine ment effects [45]. This could be expounded as the complete result of the size effect and the potential-well effect when a CdS shell is formed on the surface of the ZnS core finally, the size is higher than the core [46]. This allows an additional delocalization of the electronic wave function and forms a red-shift of the absorption band edge transition (size effect). Another reason is the conduction (CB) and the valence band (VB) of CdS were situated in the energy gap of the ZnS, the type-I electron-hole pairs tend to restrict in SC with a lower band gap, i.e., CdS, which offer the lowest energy positions for both charge carrier [47]. The electronic and optical properties of these nanomaterials show a significant change from
Fig. 10. Schematic illustration of possible photocatalytic reaction mechanism of ZnS@CdS QDs on MB.
the successful formation of ZnS@CdS core-shell structure. According to the Scherer equation, the average crystallite size was calculated and the obtained values are 2.8, 3.49 and 5.3 nm for ZnS, CdS and ZnS@CdS QDs respectively. When compared to pure QDs, the size of ZnS@CdS QDs size is high due to the formation of a CdS layer on ZnS core. Fig. 2 shows the Fourier transform Infra-Red (FTIR) spectra of (a) ZnS QDs, (b) CdS QDs and (c) ZnS@CdS QDs. The broad absorption band was observed between 3700-3300 cm-1 ascribed to O–H stretching of surface absorbed water molecules [41]. The small hump at 2931, 2912 and 2924 cm-1 assigned to C–H asymmetric stretching vibration and a 5
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Table 1 Comparison table of UV–Vis. Light assisted photocatalyst with degradation efficiency (%) of reported nanomaterials. S. No
Photocatalyst
Dye
Light source with Power
Photocatalyst Amount (mg/L)
Irradiation time (Min)
Efficiency (%)
Ref.
1.
Green colloidal ZnS QDs
UV light 6 W
30
120
87
[32]
2. 3. 4. 7. 8.
Mn–ZnS QDs ZnS QD-TiO2 CdS QD-GeO2 ZnS@CdS nanoparticles PVP capped ZnS@CdS nanoparticles ZnS@CdS core-shell QDs
MB and MO MO MB MB MB MB
UV light 8 W UV light 12 W Visible light 450 W Visible light 500 W Visible light 500 W
100 10 50 100 10
120 420 120 360 90
95.4 90 80.5 63 65
[27] [53] [54] [55] [56]
MB
Visible light 500 W (Intensity-4 Wcm-2)
10
120
93
Present works
9.
Fig. 11. (a) Catalyst reusability of ZnS@CdS QDs in cyclic photodegradation over MB dye for four cycles and (b) XRD patterns of ZnS@CdS QDs before and after photocatalytic reaction.
their corresponding bulk properties, which are called quantum size ef fects. The measured band gap values are 3.82, 3.7 and 2.6 eV for ZnS QDs, CdS QDs, and ZnS@CdS QDs respectively. Quantum confinement of both the hole and electron in three dimensions leads to an enhancing the effective band gap of the SC material with reducing crystallite size [48] shown in Fig. 4. The Photoluminescence (PL) spectra orginates from the rediative or non radiative recombination of self trapped exitons [49]. In this approach, the possible photocatalytic reaction mechanism of the as synthesized samples were analyzed by PL analysis. The PL emission spectrum can be generates from the recombination of charge carriers and used to express the efficiency of charge carrier transformation, trapping, separation of the photogenerated electrons and holes [50]. Fig. 5 shows the PL spectra of as ZnS, CdS and ZnS@CdS QDs were obtained by the excitation wavelength of 320 nm. The strong emission peak of ZnS QDs and CdS QDs were observed around 430 and 440 nm respectively. After the core/shell formation the PL intensity of the pre pared photocatalyst was decreased due to the low recombination rate of electron hole pair. Howerver the PL peak intensity of ZnS@CdS QDs becomes much lower than that of ZnS and CdS QDS, which indicating a lower recombination rate of photogenerated electron–hole pairs after core\shell formation. The low PL emission peak position usually con tributes high photocatalytic activity which clearly shows that, the core/shell structure offers in the higher photocatalytic activity than ZnS and CdS QDS. The decreasing PL spectra intensity clearly conform the formation of ZnS@CdS which will actively enhance the photocatalytic activity of the prepared photocatalyst. The different chemical composition of ZnS@CdS QDs was investi gated by EDX analysis which shown in Fig. 6. (a) shows the SEM image of as prepared ZnS@CdS sample with corresponding EDX spectrum. From the EDX pattern the presence of Zn, Cd, and S was clearly confirmed. Fig. 6(b). Shows, the element mapping images of ZnS@CdS QDs shows a bright point in the elemental map designated a higher
concentration of the equivalent element in that area. From this study Zn, S and Cd elements are coexistent presented in the prepared material. HR-TEM provides a higher resolution and may be better suitable for the analysis of particle size in the nano as well as sub nano-region. The morphology and particle size of ZnS QDs and ZnS@CdS QDs were observed by HR-TEM analysis which were shown in Fig. 7. (a-c). Ac cording to the HR-TEM measurements, the average size of ZnS@CdS QDs was observed around 7 � 1 nm with a core size of 2.5 �0 .4 nm. The interplanar d-spacing of ZnS@CdS QDs is 0.26 nm and the selected area electron diffraction (SAED) pattern is obtained in the reverse space of the lattice planes. Fig. 7 (d) shows the SAED configuration of the sample ZnS@CdS QDs from this image some bright spots were observed and also the concentric ring formation reveals that the particles are in the lack of crystalline in nature. 3.1. Photocatalytic dye degradation reaction In recent times, heterogeneous photocatalysis has a significant role in the photodegradation, which leads to the total mineralization of organic pollutants, especially synthetic or organic dyes. The photo catalytic activity of all the samples was evaluated by the photo decol orization of MB dye under UV–Vis. Light irradiation. The photocatalytic degradation activity of the as-prepared samples were investigate the photodegradation of MB aqueous solution under UV–Vis-light irradia tion. Self-degradation of the MB dye was examined without adding any catalysts and MB dye was observed for 120 min under UV–Vis. Light irradiation and there was no significant change in MB concentration which confirms that MB does not decomposition by itself (Fig. 8(A)). The maximum absorptions for MB at the wavelength of 664 nm and through the whole photodegradation period maximum absorption occurred almost at the same wavelength after the adding of prepared photocatalyst with increasing irradiation time the absorbance of MB dye spectra rapidly declined which confirm the prepared photocatalyst 6
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support the photocatalytic reaction. To compare the catalytic activity of as-prepared samples the first-order kinetic equation was used to fit the experimental data of MB degradation, from this result ZnS QDs shows very low photocatalytic action due to its limited Visible light absorption (Fig. 8 (B)). When compared to ZnS QDs and CdS QDs the ZnS@CdS QDs offers more attractive result for the photodegradation of MB dye mole cules because of its lesser band gap of CdS material can enhance the UV–Vis. Light absorption. The photodegradation efficiency of asprepared materials were examined and drawn among absorbance con centration of MB dye against UV–Vis. Light irradiation time shown in Fig. 9(a). Basically, the photocatalytic activity of ZnS@CdS QDs was improved due to decreased electron-hole recombination rate by sepa ration of charge by the formation of the heterojunction. The improved photocatalytic activity obtained by the formation of sulfide-rich heter ostructure which helps the charge transfer from conduction band to valence band. Fig. 10 shows the possible mechanism of the photo catalytic reaction. Table-1 contains the comparison of our experimental results with the reported values, which show the improved photo catalytic performance of ZnS@CdS QDs.
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3.2. Scavenging and stability experiment In order to explore the basic photocatalytic mechanism of ZnS@CdS QDs, radical trapping experiment was evaluated as shown in Fig. 9 (b). In this investigation, OH, hþ and O-2 radical were trapped by the addition of 2-propanol, EDTA and benzoquinone, respectively [51,52]. After the addition of 2-propanol and benzoquinone the photocatalytic activity was inhibited the MB degradation up to 15–20% and in existence of hþ radicals with the adding of EDTA, hinder nearly 50% reaction. The above results confirm the surface hþ was the main active species for inverse ZnS@CdS QDs, which played a significant role in the MB dye photodegradation. In practical applications, the reusability of photo catalysts is a key role, for this motive the reusability of the ZnS@CdS QDs photocatalyst was examined after the degradation of MB under UV–Vis. Light irradiation. In every cycle, fresh MB dye solution was taken to find the stability of the ZnS@CdS QDs photocatalyst. The experiment was repeated followed by centrifugation and then washed with DDW, and drying at 60 � C for overnight. After four consecutive series, we detected only an insignificant change in the photocatalytic dye degradation efficiency when compared to the fresh photocatalyst shown in Fig. 11 (a). The scavenging and reusability study reveals that the O-2 was the main active species of the photocatalyst and the prepared ZnS@CdS QDs sample posses durability towards MB dye degradation. In further, the stability of the as prepared photocatalyst were confirmed by the XRD analysis and the result shown in Fig. 11 (b). After 4 consecutive cycles, there no significant phase changes can be detected. Which clearly conforms the ZnS@CdS QDs have high stability. 4. Conclusion In summary, we successfully synthesized inverted type-I ZnS@CdS core-shell QDs by reflux condensation method and investigate their photocatalytic activity. ZnS and CdS are typical IIB–VIA group SCs photocatalyst which individually exhibits very low photocatalytic ac tivity. The incorporation of inorganic and organic materials was one of the effective modes to improve photocatalytic activity of prepared ma terial under UV–Vis. Light irradiation. After the formation of ZnS@CdS QDs structure can be enhanced photocatalytic activity due to its good UV–Vis. Light response. Radical trapping test proposes that photogenerated hþ radicals dominant the photocatalytic reaction. This investigation will be support for the forthcoming photocatalyst by using an inverse type-I core-shell based materials.
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