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Sonochemical preparation of multifunctional rGO-ZnS-TiO2 ternary nanocomposite and its application for CV dye removal
Optik - International Journal for Light and Electron Optics 208 (2020) 164532
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Optik journal homepage: www.elsevier.com/locate/ijleo
Original research article
Sonochemical preparation of multifunctional rGO-ZnS-TiO2 ternary nanocomposite and its application for CV dye removal
T
Devyani P. Kalea, Sayali P. Deshmukha, Sachin R. Shirsathb, Bharat A. Bhanvasea,* a Department of Chemical Engineering, Laxminarayan Institute of Technology, Rashtrasant Tukadoji Maharaj Nagpur University, Nagpur, 440033, MS, India b Chemical Engineering Department, Sinhgad College of Engineering, Pune, 411041, MS, India
A R T IC LE I N F O
ABS TRA CT
Keywords: Ultrasound rGO-ZnS-TiO2 nanocomposite TEM Photocatalyst CV dye
This work deals with the conventional and sonochemical preparation of reduced graphene oxideZnS-TiO2 (rGO-ZnS-TiO2) ternary multifunctional nanocomposite and its application for crystal violet dye (CV) dye removal. The prepared nanocomposite samples have been thoroughly characterized using various techniques and confirms its successful formation. The efficacy of the synthesized multifunctional nanocomposite was investigated as an adsorbent as well as photocatalyst for the removal of crystal violet dye from wastewater. The experimental results indicated that the nanocomposites exhibited superior adsorption compared to its photocatalytic activity. Almost 97 % CV dye was removed by adsorption and photodegradation at temperature of 35 °C with initial dye concentration of 50 ppm and nanocomposite dose of 0.4 g/L.
1. Introduction The sectors like dyeing, textile, tannery and the paint industries generate wastewater streams containing hazardous dyes. Dyes can lead to hazardous by-products when they are subjected to processes like oxidation, hydrolysis etc. in the wastewater phase [1]. These dye molecules and formed by-products are extremely poisonous, carcinogenic, mutagenic and allergenic to human being and other animals [2]. Thus, it becomes imperative to treat dye containing wastewater prior to it being released into the receiving body of water. Different treatment processes like adsorption [3], and photocatalytic degradation [4] are extensively used for removing dyes from wastewater. Photocatalytic degradation using TiO2 has received lot of attention of numerous investigators due to its exceptional properties however; it is suffering from objectionable recombination of electrons and holes and having lower effectiveness under visible light for waste water remediation [5]. Numerous approaches have been used to resolve these issues and one of the methods is doping of TiO2 to extend its absorption threshold. Transition metal sulphides like ZnS and CdS widely studied for their photocatalytic capabilities are comparable to TiO2 [6]. Similar to TiO2, ZnS nanoparticles also suffer from the disadvantage of rapid recombination of photo-generated electron and hole pairs and the aggregation of nanoparticles that significantly limits its outstanding photoelectric properties. Also, compared with the performance of individual TiO2 or ZnS, the photocatalytic activity of ZnS/TiO2composite can be substantially enhanced due to the quantum confinement effects [7]. The combination of ZnS with TiO2 supported on a conductive support like graphene (which has larger specific surface area, outstanding electronic transport property and higher chemical stability) can prove to be effective photocatalyst for the treatment of wastewater [8]. For the preparation of nanostructures such as transition metals having higher surface area, oxides and colloids, the use of ⁎
Corresponding author. E-mail address: [email protected] (B.A. Bhanvase).
https://doi.org/10.1016/j.ijleo.2020.164532 Received 29 December 2019; Received in revised form 8 March 2020; Accepted 8 March 2020 0030-4026/ © 2020 Elsevier GmbH. All rights reserved.
Optik - International Journal for Light and Electron Optics 208 (2020) 164532
D.P. Kale, et al.
ultrasound have significant role in uniform dispersion of nano-sized particles, reduction in the particle size and avoiding the restacking of nanoparticles to form agglomerates [9,10]. Due to ultrasonic irradiations, the cavitation takes place in the liquid which is the formation, growth and implosive collapse of cavities. This collapse of the cavities generates localized high temperature which is greater than 10,000 K and pressure of about 1000 atm as well as very high cooling rates (> 1010K/s). These extreme conditions can encourage the formation of self-assembly nanostructures and crystallization of nanostructured materials [9]. Therefore, the present work attempts the preparation of the ternary rGO-ZnS-TiO2 nanocomposite with the aid of ultrasound. The detailed characterization of the prepared nanocomposites was accomplished in order to study the properties. The synthesized nanocomposites were tested for their multifunctional capability for CV dye adsorption as well as photocatalytic degradation. 2. Experimental 2.1. Materials Graphite powder, sodium nitrate (NaNO3), hydrochloric acid, concentrated sulphuric acid (98 %), H2O2 (30 %), KMnO4 and sodium hydroxide were procured from Loba Chemie Pvt. Ltd., Mumbai, India. The obtained chemicals were of analytical grade, which were used for the preparation of GO using ultrasonically modified Hummers method [11,12]. Zinc chloride (ZnCl2, AR), isopropanol and titanium isopopoxide were procured from Sisco Research Laboratories, Mumbai, India. Sodium sulphide (AR) flakes was obtained from Molychem, Mumbai, India. The obtained chemicals were used as received. Distilled water was used during the experimentation. 2.2. Preparation of GO and rGO-ZnS-TiO2 composite by sonochemical route Modified Hummers method [13] assisted with ultrasound was used for the synthesis of GO from graphite powder. The detailed procedure for preparation of GO is reported by Barai et al. [13]. The obtained GO product was used for the preparation of rGO-TiO2ZnS nanocomposite. In order to carry out the preparation of rGO-TiO2-ZnS nanocomposite particles in the presence of ultrasound (Dakshin, 22 KHz, 240 W, Probe diameter =20 mm), initially 0.1 mol (15.43 g) ZnCl2 was dissolved in 30 mL distilled water. To this prepared ZnCl2 solution, 0.2 g GO was added andsubjected to sonication for 5 min. Further, 0.1 mol (9.61 g) Na2S was added to the prepared GO and ZnCl2 solution. Then the obtained mixture was subjected to sonication for 1 h and the subsequent product was separated by filtration and washed using distilled water three times and dried for 24 h at 100 °C in an oven, which is graphene oxide-ZnS nanocomposite. Further, TiO2 nanoparticle deposition on the prepared graphene oxide-ZnS nanocomposite particles was carried out using precursors of TiO2. For the same, initially the addition of 1.25 g of graphene oxide-ZnS nanocomposite particles to 50 mL of iso-propanol was accomplished and to this formed suspension 5 mL of titanium (IV)-isopropoxide was added and subjected to the sonication for 5 min. The NaOH solution was prepared by adding 2.71 g of NaOH in 50 mL distilled water and its gradual addition was accomplished at the rate of 10 mL/min in presence of sonication within 30 min. Finally, the obtained suspension was sonicated further for 30 min. The resulting rGO-TiO2-ZnS nanocomposite particles were washed with water, separated by filtration and dried at 150 °C in an oven. Further heat treatment was given to this product at 500 °C in furnace to get rGO-ZnS-TiO2 nanocomposite. The formation mechanism of rGO-ZnS-TiO2 nanocomposite is depicted in Fig. 1. Similar process is repeated for the preparation of rGO-TiO2-ZnS nanocomposite particles by conventional method. 2.3. Characterization of rGO-ZnS-TiO2 nanocomposite UV/Vis spectrum of rGO-ZnS-TiO2 nanocomposite prepared with the aid of ultrasonication was recorded on UV/VIS Spectrophotometer (LABINDIA, Model: UV3200). X-ray diffraction pattern was recorded with the use of powder Rigaku Mini-Flex Xray diffractometer. FTIR spectrum was obtained using Fourier Transform Infrared Spectrophotometer (Shimadzu-IR Affinity-1, Japan). Raman spectrum was obtained on STR-500 Confocal Micro Raman Spectrometer. Elemental Map images and EDAX analysis was carried out with the help of Transmission Electron Microscopy (Tecnai G2 20, FEI Company). SEM Images were obtained using Scanning Electron Microscopy (Make: Carl Zeiss). TEM images of the rGO-ZnS-TiO2 nanocomposite were obtained from a Transmission Electron Microscope (Tecnai G2 20, FEI Company). XPS analysis was carried out with the use of an Omicron ESCA (Electron Spectroscope for Chemical Analysis), Germany. The concentration of Crystal Violet (CV) dye in the aqueous solution was determined with the help of UV–vis spectrophotometer (LABINDIA, Model: UV3200). 2.4. Photocatalytic degradation of CV dye using prepared nanocomposite The application of ultrasonically prepared rGO-ZnS-TiO2 nanocomposite photocatalysts was tested for the photocatalytic degradation of CV dye in a batch mode. The experiments were performed with 100 mL of CV dye solution and suitable quantity of rGOZnS-TiO2 nanocomposite photocatalyst. The mixture of CV dye and ultrasonically prepared rGO-ZnS-TiO2 nanocomposite particles was stirred in a photocatalytic reactor for 30 min in darkness to attain the equilibrium and the concentration of CV dye was measured. Then the CV dye degradation experiments were carried out in the presence of UV irradiations. The samples were centrifuged and analysed by UV–vis spectrophotometer for the estimation of the concentration of CV dye. The influence of initial CV dye concentration (50–100 ppm) and catalyst loading (0.3 to 0.5 g/L) on the photocatalytic degradation of CV dye was investigated at 35 2
Optik - International Journal for Light and Electron Optics 208 (2020) 164532
D.P. Kale, et al.
Fig. 1. Formation mechanism of rGO-ZnS-TiO2 nanocomposite photocatalyst with aid of ultrasonic irradiations.
°C with batch volume equal to 100 mL. The % dye removal was obtained from Eq. (1).
%Dye Removal =
C0 − C × 100 C0
(1)
where, C0 = initial concentration, C = Concentration of CV dye at time t. 3. Results and discussion 3.1. UV/Vis analysis UV/vis spectra of ultrasonically prepared GO-ZnS nanocomposite, GO-TiO2 and rGO-ZnS-TiO2 nanocomposite and conventionally prepared rGO-ZnS-TiO2 nanocomposite are depicted in Fig. 2. The obtained spectra show an absorption edge in UV region for all the samples prepared by both methods. It has been observed that the absorption edge of ultrasonically prepared rGO-ZnS-TiO2 nanocomposite shows red shift (reduction in the band gap) compared with other prepared nanomaterials, which is attributed to the reduction in the particle size of both the nanomaterials TiO2 and ZnS which are deposited on rGO nanosheets. The absorption around 330 nm in case of GO-ZnS, for conventionally and ultrasonically prepared rGO-ZnS-TiO2 nanocomposite is due to excitonic transition and is attributed to quantum confinement effect [14]. In conclusion, the improvement in the optical properties of ultrasonically prepared rGO-ZnS-TiO2 nanocomposite would be advantageous for photocatalytic applications. 3.2. XRD analysis of rGO-ZnS-TiO2 nanocomposite Fig. 2 depicts the XRD patterns of the rGO-ZnS-TiO2 nanocomposite photocatalyst prepared by conventional and ultrasound assisted method. For both the preparation approaches, the XRD patterns are showing the characteristic peaks around 2θ = 28.26, 47.34 and 56.42° which are corresponding to (1 1 1), (2 2 0), and (3 1 1) planes confirming successful formation of ZnS nanoparticles that are deposited on the rGO nanosheets (JCPDS NO.77-2100) [15]. Moreover, the peaks around 2θ = 35.91, 37.65, 39.80, 45.10, 51.83 and 62.33° are indexed to the anatase phase of TiO2 nanoparticles deposited on rGO nanosheets [16]. Further the characteristic peaks at 29.50 and 41.02° are attributed to the presence of rutile phase of TiO2 and also the peak at 31.42° is indexed to brookite phase of TiO2. However, anatase phase of TiO2 was observed to be dominating in the case of ultrasound assisted method of preparation compared to that of conventional method. Ultrasound plays an important role in the reduction of the particle size of both 3
Optik - International Journal for Light and Electron Optics 208 (2020) 164532
D.P. Kale, et al.
Fig. 2. UV/Vis spectra of the different nanocomposite photocatalysts and XRD pattern, FTIR spectrum and SEM images of rGO-ZnS-TiO2 nanocomposite photocatalyst prepared by (A) ultrasound assisted and (B) conventional method.
ZnS and TiO2 nanoparticles deposited on rGO nanosheets. Further, reduction of the GO to rGO is confirmed from the presence of the characteristic peak at 26.80°. However, this peak is observed prominently in case of sonochemical method which is attributed to enhanced reduction and exfoliation to form final rGO on which ZnS and TiO2 nanoparticles are deposited uniformly. 3.3. FTIR analysis of rGO-ZnS-TiO2 nanocomposite Fig. 2 depicts the FTIR spectra of rGO-ZnS-TiO2 nanocomposite in the range of 4000–400 cm−1. The deposition of TiO2 and ZnS nanoparticles and the reduction of GO to rGO was confirmed with recorded FTIR spectra. In both the methods of synthesis, the weak characteristic peak at 1738 cm−1, due to CO] stretching vibration of COOH groups, confirms the substantial reduction of GO into rGO during its formaiton. The characteristic peaks at 2921 and 2848 cm−1 are attributed to the CH2 asymmetric and symmetric stretching, respectively. Also the peaks at 1594 and 1544 cm−1 are attributed to the C]O stretching. Further, peaks at 1047 and 1092 cm−1 are assigned to the unsaturated and saturated CeOH group in rGO. The loading of TiO2 nanoparticles on rGO nanosheets is confirmed by the presence of characteristic peak at 834 cm-1 that are assigned to Ti–O–C bonding. Moreover, the deposition of ZnS nanoparticles on rGO is confirmed with the presence of peaks in the range of 400−680 cm-1 which indicates the formation of Zn-S bond and this region is considered as the fingerprint region for metal sulfide bond. Thus peaks at 465, 576 and 672 cm−1 are attributed to presence of ZnS in the formed rGO-ZnS-TiO2 nanocomposite. 4
Optik - International Journal for Light and Electron Optics 208 (2020) 164532
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Fig. 3. TEM images of rGO-ZnS-TiO2 nanocomposite photocatalyst prepared by (A to C) ultrasound assisted method and (D) conventional method.
3.4. SEM and TEM analysis of rGO-ZnS-TiO2 nanocomposite Fig. 2 depicts the SEM images of rGO-ZnS-TiO2 nanocomposite particles prepared by both methods. In both the cases the roughly wrinkled surface morphology of rGO was observed and the wrinkled 2-D microstructures of rGO nanosheets can be clearly seen. The 2-D structure of rGO nanosheets shows regular wrinkles in the case of sonochemical method which is assumed to assist the uniform growth and dispersion of ZnS and TiO2 nanoparticles on its surface attributed to the intense environment generated by ultrasonication. Also another possible reason is the presence of shearing, turbulence and micromixing in the reaction medium due to physical effects of the ultrasonic irradiations. Simultaneously, deposition of ZnS and TiO2 nanoparticles on rGO helps to prevent the aggregation and restacking of the graphene sheets. TEM analysis of the rGO-ZnS-TiO2 nanocomposites was also performed and obtained images are depicted in Fig. 3. At higher magnification, it has been clearly observed that both the nanoparticles are homogeneously adhered to rGO nanosheets. Further, well defined spherical nature of ZnS nanoparticles have been observed. In the case of ultrasound assisted method (Fig. 3A and B) no agglomeration of the nanoparticles was recorded and distinct nanoparticles were observed to be deposited on rGO uniformly (Fig. 3B). Also the size of TiO2 nanoparticles was observed to be in the range of 20−40 nm for the ultrasonic method, whereas for the conventional method of synthesis, it is around 70−80 nm. This reduction and uniform distribution of the nanoparticles on the rGO is attributed to the ultrasonic irradiations. Further, SAED image for the rGO-ZnSTiO2 nanocomposite prepared by ultrasound assisted method is depicted in Fig. 3C. In the case of conventional method, the TEM image (Fig. 3D) evidently depicts the substantial amount of agglomeration in the formed nanocomposite. 3.5. Elemental analysis, EDAX and map images of rGO-ZnS-TiO2 nanocomposite Fig. 4 shows elemental contents of rGO-ZnS-TiO2 nanocomposite photocatalyst prepared with the aid of ultrasonic irradiations. The elemental map images showed that the elemental contents of rGO-ZnS-TiO2 nanocomposite contained 86.62, 5.10, 4.47, 2.59 and 1.22 wt % of carbon, zinc, oxygen, sulphur and Ti, respectively. The elemental analysis also indicated that the formed nanocomposite material is of highest purity. 5
Optik - International Journal for Light and Electron Optics 208 (2020) 164532
D.P. Kale, et al.
Fig. 4. Elemental Map images for C, Zn, Ti, O and S of rGO-ZnS-TiO2 nanocomposite photocatalyst prepared by ultrasound assisted method and EDAX of rGO-ZnS-TiO2 nanocomposite photocatalyst prepared by (A) ultrasound assisted method and (B) conventional method.
Fig. 4(A) and (B) shows EDAX of rGO-ZnS-TiO2 nanocomposite photocatalysts synthesized with the aid of ultrasonic irradiations and conventional stirring, respectively. The signal for C mainly originates from the graphene sheets, while those for O and Ti are from the TiO2 and Zn and S are from ZnS nanoparticles. As seen in Fig. 4, Kα and Kβ characteristic peaks from Ti appear at 4.51 and 4.92 keV for both the synthesis methods. Further, a moderate Kα peak from O appears at 0.52 keV. Similarly, Kα peak for carbon is at 0.277 keV, Lα peak for Zinc is at 1.012 keV and Kα and Kβ peaks from Sulfur appear at 2.309 and 2.465 keV reconfirming the presence of those elements in the nanocomposite.
3.6. XPS analysis of ultrasonically prepared rGO-ZnS-TiO2 nanocomposite Fig. 5 shows the overall XPS spectra of rGO-ZnS-TiO2 nanocomposite prepared with the aid of ultrasonic irradiations. The deconvoluted XPS spectra of the individual components are also depicted in the Fig. 5, which shows the peaks of C 1s, O 1s, Ti 2p, Zn 2p and S 2p clearly. The XPS spectra of C 1s is depicted in Fig. 5. The peak location at 284.89 eV is attributed to the sp2 carbon species which is for CCe bonds. Further, the peaks detected at binding energies between 286 and 290 eV are allocated to epoxy, hydroxyl and carbonyl species which contains oxygen functionalities located on the surface of GO. In the present study weaker peak at 289.48 eV is attributed to carboxyl carbon (O=C–O), which confirms the substantial reduction of GO to rGO. The O 1s region is shown in figure which shows the characteristic peak at 532.17 eV. This is due to the oxygen in ultrasonically prepared rGO-ZnS-TiO2 nanocomposite which is coming from rGO and TiO2 and other peak at 536.49 eV is attributed to O = CeOHe bonding. In Ti XPS spectrum, peaks centered at 463.95 eV (Ti 2p1/2) and 458.01 eV (Ti 2p3/2) are allocated to the spin orbital-splitting photoelectrons in Ti4+ state [17]. From Zn XPS spectrum, the peaks observed at 1021.68 and 1044.63 eV can be allocated to Zn 2p3/2 and 2p1/2, respectively, which shows the separation between these two observed peaks is about 23.0 eV as expected for ZnS [18]. For sulfur XPS spectrum, the binding energies at 161.27 eV and 163.45 eV are assigned to S 2p3/2 and S 2p1/2, respectively. In the overall XPS spectra of rGO-ZnSTiO2 nanocomposite prepared with the aid of ultrasonic irradiations, apart from the peaks corresponding to S, C, Ti, O and Zn, 6
Optik - International Journal for Light and Electron Optics 208 (2020) 164532
D.P. Kale, et al.
Fig. 5. XPS survey spectrum of ultrasonically prepared rGO-ZnS-TiO2 nanocomposite and resolved fitting signal of C 1s, O 1s, Ti 2p, Zn 2p and S 2p.
additional peak is observed for Na 1 s at 1071.38 eV. The probable reason for this may that during the synthesis of graphene oxide, sodium nitrate and sodium hydroxide were used hence minor amounts of unreacted sodium may be detected in the nanocomposite.
3.7. Effect of initial CV dye concentration The effect of various concentrations of CV dye was investigated for three different concentrations whilst keeping the other conditions constant. During the adsorption equilibrium studies it was found that for 30 min of equilibration time maximum amount of dye was adsorbed by the rGO-ZnS-TiO2 nanocomposite. The percent removal of CV dye due to adsorption only was observed to be 91.15, 60.53 and 63.29 % for 50, 70 and 100 ppm CV dye concentrations, respectively. Surprisingly it was found that the rGO-ZnS7
Optik - International Journal for Light and Electron Optics 208 (2020) 164532
D.P. Kale, et al.
Fig. 6. Effect of Initial Concentration of CV dye on its degradation using rGO-ZnS-TiO2 nanocomposite photocatalyst prepared by ultrasound assisted method (Batch volume =100 mL, Temperature =35 °C, Catalyst loading =0.4 g).
TiO2 nanocomposite prepared with the aid of ultrasonic irradiations is a very effective adsorbent as well. It may be because reduced graphene oxide intensifies the specific surface area of the prepared nanocomposite thereby providing substantially higher number of active sites for adsorption of CV dye ions. Further photocatalytic degradation studies showed that overall 97.02, 96.34 and 96.14 % degradation of CV dye was achieved for 50, 70 and 100 ppm concentrations respectively (Fig. 6). From the results it can be said that as the CV dye concentration increases, the percent degradation of the selected dye decreases. This may be because of the fact that as the dye concentration increases the quantum of dye molecules adsorbed on the surface of the photocatalyst gets enhanced that results in the decrease in the path length of photons entering the CV dye solution. Also a considerable amount of UV light may be absorbed by the dye molecules instead of nanocomposite particles thereby reducing the efficiency of the photocatalytic reaction [5]. Moreover, the photocatalytic effect is not seen prominently for 50 ppm concentration, however for other two CV dye concentrations higher amount of degradation was observed. This may be because in case of 50 ppm concentration already 91.15 % has been removed by adsorption process and hence there is not much availability of CV dye molecules for the photocatalytic degradation process.
3.8. Effect of rGO-ZnS-TiO2 nanocomposite photocatalyst loading In order to decide the optimal amount of rGO-ZnS-TiO2 nanocomposite prepared with the aid of ultrasonic irradiations, experiments were performed at varied loading of catalyst. During the adsorption equilibrium studies, it was observed that for 30 min of equilibration time, the removal of CV dye was about 89.70, 91.15 and 92.44 % for 0.3, 0.4 and 0.5 g/L of rGO-ZnS-TiO2 nanocomposite, respectively. Again the adsorption was observed to be predominant for all the three cases and the possible explanations are deliberated in the previous section. Further, the results of the photocatalytic degradation are presented in Fig. 7. The photocatalytic degradation studies indicated that overall 94.83, 97.02 and 96.30 % degradation was achieved for 0.3, 0.4 and 0.5 g/L of rGO-ZnS-TiO2 nanocomposite, respectively. The results suggest that as the amount of nanocomposite increased from 0.3 to 0.4 g/L the degradation of CV dye was found to be increased from 94.83–97.02%, respectively and further increase in the loading of rGO-ZnSTiO2 nanocomposite (0.5 g/L) showed marginal decrease in the degradation, which was observed to be 96.30 %. As discussed earlier the nanosized rGO-ZnS-TiO2 nanocomposite particles showed strong adsorption capabilities and the excessive adsorption reduces the photocatalytic activity, which is due to mutual screens among rGO-ZnS-TiO2 nanocomposite particles. In addition, the dispersion of excessively loaded rGO-ZnS-TiO2 nanocomposite particles will hinder the UV light irradiation and that restricts the efficient usage of light leading to decrease in the photocatalytic degradation of CV dye [4,5]. Thus the obtained results suggest an optimal dose of 0.4 g/L of rGO-ZnS-TiO2 nanocomposite prepared with the aid of ultrasonic irradiations to achieve effective degradation of CV dye. Nanocomposite prepared with the aid of ultrasonic irradiations showed excellent multi-functionality for the application of
Fig. 7. Effect of catalyst loading on degradation of CV dye using rGO-ZnS-TiO2 nanocomposite photocatalyst prepared by ultrasound assisted method (Batch volume =100 mL, Temperature =35 °C, Initial dye concentration =50 ppm). 8
Optik - International Journal for Light and Electron Optics 208 (2020) 164532
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adsorption as well as photocatalytic degradation of selected CV dye. It can be said that it is a versatile nanocomposite which can be considered for these two applications for wastewater treatment. 4. Conclusions A ternary multifunctional rGO-ZnS-TiO2 nanocomposite photocatalyst was successfully synthesized by conventional and ultrasound assisted approach which was confirmed from various characterization techniques. SEM images revealed non-uniform and distorted morphology of rGO-ZnS-TiO2 nanocomposite for conventional method as against the ultrasound assisted method. The TEM analysis revealed that the size of TiO2 nanoparticles for ultrasonic method is in the range of 20−40 nm and that for conventional method it is around 70−80 nm. The photocatalytic degradation capability of synthesized rGO-ZnS-TiO2 nanocomposite with the aid of ultrasonic irradiations was investigated for CV dye degradation. The studies established that the synthesized rGO-ZnS-TiO2 nanocomposite exhibited superior adsorption and photocatalytic activity and this can be considered as a multifunctional ternary nanocomposite material for adsorption and photocatalytic degradation of CV dye Declaration of Competing Interest The authors declare that there are no conflicts of interest. Acknowledgments This work was supported by the Rashtrasant Tukadoji Maharaj Nagpur University, Nagpur under Innovative Research Activities [Sanction order no. vikas/javak/2277 dated 7/10/2015]. References [1] F. Guzman-Duque, C. Pétrier, C. Pulgarin, G. Peñuela, R.A. Torres-Palma, Effects of sonochemical parameters and inorganic ions during the sonochemical degradation of crystal violet in water, Ultrason. Sonochem. 18 (2011) 440–446. [2] R.Y. Lin, B.S. Chen, G.L. Chen, J.Y. Wu, H.C. Chiu, S.Y. Suen, Preparation of porous PMMA/Na+-montmorillonite cation-exchange membranes for cationic dye adsorption, J. Memb. 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Optik journal homepage: www.elsevier.com/locate/ijleo
Original research article
Sonochemical preparation of multifunctional rGO-ZnS-TiO2 ternary nanocomposite and its application for CV dye removal
T
Devyani P. Kalea, Sayali P. Deshmukha, Sachin R. Shirsathb, Bharat A. Bhanvasea,* a Department of Chemical Engineering, Laxminarayan Institute of Technology, Rashtrasant Tukadoji Maharaj Nagpur University, Nagpur, 440033, MS, India b Chemical Engineering Department, Sinhgad College of Engineering, Pune, 411041, MS, India
A R T IC LE I N F O
ABS TRA CT
Keywords: Ultrasound rGO-ZnS-TiO2 nanocomposite TEM Photocatalyst CV dye
This work deals with the conventional and sonochemical preparation of reduced graphene oxideZnS-TiO2 (rGO-ZnS-TiO2) ternary multifunctional nanocomposite and its application for crystal violet dye (CV) dye removal. The prepared nanocomposite samples have been thoroughly characterized using various techniques and confirms its successful formation. The efficacy of the synthesized multifunctional nanocomposite was investigated as an adsorbent as well as photocatalyst for the removal of crystal violet dye from wastewater. The experimental results indicated that the nanocomposites exhibited superior adsorption compared to its photocatalytic activity. Almost 97 % CV dye was removed by adsorption and photodegradation at temperature of 35 °C with initial dye concentration of 50 ppm and nanocomposite dose of 0.4 g/L.
1. Introduction The sectors like dyeing, textile, tannery and the paint industries generate wastewater streams containing hazardous dyes. Dyes can lead to hazardous by-products when they are subjected to processes like oxidation, hydrolysis etc. in the wastewater phase [1]. These dye molecules and formed by-products are extremely poisonous, carcinogenic, mutagenic and allergenic to human being and other animals [2]. Thus, it becomes imperative to treat dye containing wastewater prior to it being released into the receiving body of water. Different treatment processes like adsorption [3], and photocatalytic degradation [4] are extensively used for removing dyes from wastewater. Photocatalytic degradation using TiO2 has received lot of attention of numerous investigators due to its exceptional properties however; it is suffering from objectionable recombination of electrons and holes and having lower effectiveness under visible light for waste water remediation [5]. Numerous approaches have been used to resolve these issues and one of the methods is doping of TiO2 to extend its absorption threshold. Transition metal sulphides like ZnS and CdS widely studied for their photocatalytic capabilities are comparable to TiO2 [6]. Similar to TiO2, ZnS nanoparticles also suffer from the disadvantage of rapid recombination of photo-generated electron and hole pairs and the aggregation of nanoparticles that significantly limits its outstanding photoelectric properties. Also, compared with the performance of individual TiO2 or ZnS, the photocatalytic activity of ZnS/TiO2composite can be substantially enhanced due to the quantum confinement effects [7]. The combination of ZnS with TiO2 supported on a conductive support like graphene (which has larger specific surface area, outstanding electronic transport property and higher chemical stability) can prove to be effective photocatalyst for the treatment of wastewater [8]. For the preparation of nanostructures such as transition metals having higher surface area, oxides and colloids, the use of ⁎
Corresponding author. E-mail address: [email protected] (B.A. Bhanvase).
https://doi.org/10.1016/j.ijleo.2020.164532 Received 29 December 2019; Received in revised form 8 March 2020; Accepted 8 March 2020 0030-4026/ © 2020 Elsevier GmbH. All rights reserved.
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ultrasound have significant role in uniform dispersion of nano-sized particles, reduction in the particle size and avoiding the restacking of nanoparticles to form agglomerates [9,10]. Due to ultrasonic irradiations, the cavitation takes place in the liquid which is the formation, growth and implosive collapse of cavities. This collapse of the cavities generates localized high temperature which is greater than 10,000 K and pressure of about 1000 atm as well as very high cooling rates (> 1010K/s). These extreme conditions can encourage the formation of self-assembly nanostructures and crystallization of nanostructured materials [9]. Therefore, the present work attempts the preparation of the ternary rGO-ZnS-TiO2 nanocomposite with the aid of ultrasound. The detailed characterization of the prepared nanocomposites was accomplished in order to study the properties. The synthesized nanocomposites were tested for their multifunctional capability for CV dye adsorption as well as photocatalytic degradation. 2. Experimental 2.1. Materials Graphite powder, sodium nitrate (NaNO3), hydrochloric acid, concentrated sulphuric acid (98 %), H2O2 (30 %), KMnO4 and sodium hydroxide were procured from Loba Chemie Pvt. Ltd., Mumbai, India. The obtained chemicals were of analytical grade, which were used for the preparation of GO using ultrasonically modified Hummers method [11,12]. Zinc chloride (ZnCl2, AR), isopropanol and titanium isopopoxide were procured from Sisco Research Laboratories, Mumbai, India. Sodium sulphide (AR) flakes was obtained from Molychem, Mumbai, India. The obtained chemicals were used as received. Distilled water was used during the experimentation. 2.2. Preparation of GO and rGO-ZnS-TiO2 composite by sonochemical route Modified Hummers method [13] assisted with ultrasound was used for the synthesis of GO from graphite powder. The detailed procedure for preparation of GO is reported by Barai et al. [13]. The obtained GO product was used for the preparation of rGO-TiO2ZnS nanocomposite. In order to carry out the preparation of rGO-TiO2-ZnS nanocomposite particles in the presence of ultrasound (Dakshin, 22 KHz, 240 W, Probe diameter =20 mm), initially 0.1 mol (15.43 g) ZnCl2 was dissolved in 30 mL distilled water. To this prepared ZnCl2 solution, 0.2 g GO was added andsubjected to sonication for 5 min. Further, 0.1 mol (9.61 g) Na2S was added to the prepared GO and ZnCl2 solution. Then the obtained mixture was subjected to sonication for 1 h and the subsequent product was separated by filtration and washed using distilled water three times and dried for 24 h at 100 °C in an oven, which is graphene oxide-ZnS nanocomposite. Further, TiO2 nanoparticle deposition on the prepared graphene oxide-ZnS nanocomposite particles was carried out using precursors of TiO2. For the same, initially the addition of 1.25 g of graphene oxide-ZnS nanocomposite particles to 50 mL of iso-propanol was accomplished and to this formed suspension 5 mL of titanium (IV)-isopropoxide was added and subjected to the sonication for 5 min. The NaOH solution was prepared by adding 2.71 g of NaOH in 50 mL distilled water and its gradual addition was accomplished at the rate of 10 mL/min in presence of sonication within 30 min. Finally, the obtained suspension was sonicated further for 30 min. The resulting rGO-TiO2-ZnS nanocomposite particles were washed with water, separated by filtration and dried at 150 °C in an oven. Further heat treatment was given to this product at 500 °C in furnace to get rGO-ZnS-TiO2 nanocomposite. The formation mechanism of rGO-ZnS-TiO2 nanocomposite is depicted in Fig. 1. Similar process is repeated for the preparation of rGO-TiO2-ZnS nanocomposite particles by conventional method. 2.3. Characterization of rGO-ZnS-TiO2 nanocomposite UV/Vis spectrum of rGO-ZnS-TiO2 nanocomposite prepared with the aid of ultrasonication was recorded on UV/VIS Spectrophotometer (LABINDIA, Model: UV3200). X-ray diffraction pattern was recorded with the use of powder Rigaku Mini-Flex Xray diffractometer. FTIR spectrum was obtained using Fourier Transform Infrared Spectrophotometer (Shimadzu-IR Affinity-1, Japan). Raman spectrum was obtained on STR-500 Confocal Micro Raman Spectrometer. Elemental Map images and EDAX analysis was carried out with the help of Transmission Electron Microscopy (Tecnai G2 20, FEI Company). SEM Images were obtained using Scanning Electron Microscopy (Make: Carl Zeiss). TEM images of the rGO-ZnS-TiO2 nanocomposite were obtained from a Transmission Electron Microscope (Tecnai G2 20, FEI Company). XPS analysis was carried out with the use of an Omicron ESCA (Electron Spectroscope for Chemical Analysis), Germany. The concentration of Crystal Violet (CV) dye in the aqueous solution was determined with the help of UV–vis spectrophotometer (LABINDIA, Model: UV3200). 2.4. Photocatalytic degradation of CV dye using prepared nanocomposite The application of ultrasonically prepared rGO-ZnS-TiO2 nanocomposite photocatalysts was tested for the photocatalytic degradation of CV dye in a batch mode. The experiments were performed with 100 mL of CV dye solution and suitable quantity of rGOZnS-TiO2 nanocomposite photocatalyst. The mixture of CV dye and ultrasonically prepared rGO-ZnS-TiO2 nanocomposite particles was stirred in a photocatalytic reactor for 30 min in darkness to attain the equilibrium and the concentration of CV dye was measured. Then the CV dye degradation experiments were carried out in the presence of UV irradiations. The samples were centrifuged and analysed by UV–vis spectrophotometer for the estimation of the concentration of CV dye. The influence of initial CV dye concentration (50–100 ppm) and catalyst loading (0.3 to 0.5 g/L) on the photocatalytic degradation of CV dye was investigated at 35 2
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Fig. 1. Formation mechanism of rGO-ZnS-TiO2 nanocomposite photocatalyst with aid of ultrasonic irradiations.
°C with batch volume equal to 100 mL. The % dye removal was obtained from Eq. (1).
%Dye Removal =
C0 − C × 100 C0
(1)
where, C0 = initial concentration, C = Concentration of CV dye at time t. 3. Results and discussion 3.1. UV/Vis analysis UV/vis spectra of ultrasonically prepared GO-ZnS nanocomposite, GO-TiO2 and rGO-ZnS-TiO2 nanocomposite and conventionally prepared rGO-ZnS-TiO2 nanocomposite are depicted in Fig. 2. The obtained spectra show an absorption edge in UV region for all the samples prepared by both methods. It has been observed that the absorption edge of ultrasonically prepared rGO-ZnS-TiO2 nanocomposite shows red shift (reduction in the band gap) compared with other prepared nanomaterials, which is attributed to the reduction in the particle size of both the nanomaterials TiO2 and ZnS which are deposited on rGO nanosheets. The absorption around 330 nm in case of GO-ZnS, for conventionally and ultrasonically prepared rGO-ZnS-TiO2 nanocomposite is due to excitonic transition and is attributed to quantum confinement effect [14]. In conclusion, the improvement in the optical properties of ultrasonically prepared rGO-ZnS-TiO2 nanocomposite would be advantageous for photocatalytic applications. 3.2. XRD analysis of rGO-ZnS-TiO2 nanocomposite Fig. 2 depicts the XRD patterns of the rGO-ZnS-TiO2 nanocomposite photocatalyst prepared by conventional and ultrasound assisted method. For both the preparation approaches, the XRD patterns are showing the characteristic peaks around 2θ = 28.26, 47.34 and 56.42° which are corresponding to (1 1 1), (2 2 0), and (3 1 1) planes confirming successful formation of ZnS nanoparticles that are deposited on the rGO nanosheets (JCPDS NO.77-2100) [15]. Moreover, the peaks around 2θ = 35.91, 37.65, 39.80, 45.10, 51.83 and 62.33° are indexed to the anatase phase of TiO2 nanoparticles deposited on rGO nanosheets [16]. Further the characteristic peaks at 29.50 and 41.02° are attributed to the presence of rutile phase of TiO2 and also the peak at 31.42° is indexed to brookite phase of TiO2. However, anatase phase of TiO2 was observed to be dominating in the case of ultrasound assisted method of preparation compared to that of conventional method. Ultrasound plays an important role in the reduction of the particle size of both 3
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Fig. 2. UV/Vis spectra of the different nanocomposite photocatalysts and XRD pattern, FTIR spectrum and SEM images of rGO-ZnS-TiO2 nanocomposite photocatalyst prepared by (A) ultrasound assisted and (B) conventional method.
ZnS and TiO2 nanoparticles deposited on rGO nanosheets. Further, reduction of the GO to rGO is confirmed from the presence of the characteristic peak at 26.80°. However, this peak is observed prominently in case of sonochemical method which is attributed to enhanced reduction and exfoliation to form final rGO on which ZnS and TiO2 nanoparticles are deposited uniformly. 3.3. FTIR analysis of rGO-ZnS-TiO2 nanocomposite Fig. 2 depicts the FTIR spectra of rGO-ZnS-TiO2 nanocomposite in the range of 4000–400 cm−1. The deposition of TiO2 and ZnS nanoparticles and the reduction of GO to rGO was confirmed with recorded FTIR spectra. In both the methods of synthesis, the weak characteristic peak at 1738 cm−1, due to CO] stretching vibration of COOH groups, confirms the substantial reduction of GO into rGO during its formaiton. The characteristic peaks at 2921 and 2848 cm−1 are attributed to the CH2 asymmetric and symmetric stretching, respectively. Also the peaks at 1594 and 1544 cm−1 are attributed to the C]O stretching. Further, peaks at 1047 and 1092 cm−1 are assigned to the unsaturated and saturated CeOH group in rGO. The loading of TiO2 nanoparticles on rGO nanosheets is confirmed by the presence of characteristic peak at 834 cm-1 that are assigned to Ti–O–C bonding. Moreover, the deposition of ZnS nanoparticles on rGO is confirmed with the presence of peaks in the range of 400−680 cm-1 which indicates the formation of Zn-S bond and this region is considered as the fingerprint region for metal sulfide bond. Thus peaks at 465, 576 and 672 cm−1 are attributed to presence of ZnS in the formed rGO-ZnS-TiO2 nanocomposite. 4
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Fig. 3. TEM images of rGO-ZnS-TiO2 nanocomposite photocatalyst prepared by (A to C) ultrasound assisted method and (D) conventional method.
3.4. SEM and TEM analysis of rGO-ZnS-TiO2 nanocomposite Fig. 2 depicts the SEM images of rGO-ZnS-TiO2 nanocomposite particles prepared by both methods. In both the cases the roughly wrinkled surface morphology of rGO was observed and the wrinkled 2-D microstructures of rGO nanosheets can be clearly seen. The 2-D structure of rGO nanosheets shows regular wrinkles in the case of sonochemical method which is assumed to assist the uniform growth and dispersion of ZnS and TiO2 nanoparticles on its surface attributed to the intense environment generated by ultrasonication. Also another possible reason is the presence of shearing, turbulence and micromixing in the reaction medium due to physical effects of the ultrasonic irradiations. Simultaneously, deposition of ZnS and TiO2 nanoparticles on rGO helps to prevent the aggregation and restacking of the graphene sheets. TEM analysis of the rGO-ZnS-TiO2 nanocomposites was also performed and obtained images are depicted in Fig. 3. At higher magnification, it has been clearly observed that both the nanoparticles are homogeneously adhered to rGO nanosheets. Further, well defined spherical nature of ZnS nanoparticles have been observed. In the case of ultrasound assisted method (Fig. 3A and B) no agglomeration of the nanoparticles was recorded and distinct nanoparticles were observed to be deposited on rGO uniformly (Fig. 3B). Also the size of TiO2 nanoparticles was observed to be in the range of 20−40 nm for the ultrasonic method, whereas for the conventional method of synthesis, it is around 70−80 nm. This reduction and uniform distribution of the nanoparticles on the rGO is attributed to the ultrasonic irradiations. Further, SAED image for the rGO-ZnSTiO2 nanocomposite prepared by ultrasound assisted method is depicted in Fig. 3C. In the case of conventional method, the TEM image (Fig. 3D) evidently depicts the substantial amount of agglomeration in the formed nanocomposite. 3.5. Elemental analysis, EDAX and map images of rGO-ZnS-TiO2 nanocomposite Fig. 4 shows elemental contents of rGO-ZnS-TiO2 nanocomposite photocatalyst prepared with the aid of ultrasonic irradiations. The elemental map images showed that the elemental contents of rGO-ZnS-TiO2 nanocomposite contained 86.62, 5.10, 4.47, 2.59 and 1.22 wt % of carbon, zinc, oxygen, sulphur and Ti, respectively. The elemental analysis also indicated that the formed nanocomposite material is of highest purity. 5
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Fig. 4. Elemental Map images for C, Zn, Ti, O and S of rGO-ZnS-TiO2 nanocomposite photocatalyst prepared by ultrasound assisted method and EDAX of rGO-ZnS-TiO2 nanocomposite photocatalyst prepared by (A) ultrasound assisted method and (B) conventional method.
Fig. 4(A) and (B) shows EDAX of rGO-ZnS-TiO2 nanocomposite photocatalysts synthesized with the aid of ultrasonic irradiations and conventional stirring, respectively. The signal for C mainly originates from the graphene sheets, while those for O and Ti are from the TiO2 and Zn and S are from ZnS nanoparticles. As seen in Fig. 4, Kα and Kβ characteristic peaks from Ti appear at 4.51 and 4.92 keV for both the synthesis methods. Further, a moderate Kα peak from O appears at 0.52 keV. Similarly, Kα peak for carbon is at 0.277 keV, Lα peak for Zinc is at 1.012 keV and Kα and Kβ peaks from Sulfur appear at 2.309 and 2.465 keV reconfirming the presence of those elements in the nanocomposite.
3.6. XPS analysis of ultrasonically prepared rGO-ZnS-TiO2 nanocomposite Fig. 5 shows the overall XPS spectra of rGO-ZnS-TiO2 nanocomposite prepared with the aid of ultrasonic irradiations. The deconvoluted XPS spectra of the individual components are also depicted in the Fig. 5, which shows the peaks of C 1s, O 1s, Ti 2p, Zn 2p and S 2p clearly. The XPS spectra of C 1s is depicted in Fig. 5. The peak location at 284.89 eV is attributed to the sp2 carbon species which is for CCe bonds. Further, the peaks detected at binding energies between 286 and 290 eV are allocated to epoxy, hydroxyl and carbonyl species which contains oxygen functionalities located on the surface of GO. In the present study weaker peak at 289.48 eV is attributed to carboxyl carbon (O=C–O), which confirms the substantial reduction of GO to rGO. The O 1s region is shown in figure which shows the characteristic peak at 532.17 eV. This is due to the oxygen in ultrasonically prepared rGO-ZnS-TiO2 nanocomposite which is coming from rGO and TiO2 and other peak at 536.49 eV is attributed to O = CeOHe bonding. In Ti XPS spectrum, peaks centered at 463.95 eV (Ti 2p1/2) and 458.01 eV (Ti 2p3/2) are allocated to the spin orbital-splitting photoelectrons in Ti4+ state [17]. From Zn XPS spectrum, the peaks observed at 1021.68 and 1044.63 eV can be allocated to Zn 2p3/2 and 2p1/2, respectively, which shows the separation between these two observed peaks is about 23.0 eV as expected for ZnS [18]. For sulfur XPS spectrum, the binding energies at 161.27 eV and 163.45 eV are assigned to S 2p3/2 and S 2p1/2, respectively. In the overall XPS spectra of rGO-ZnSTiO2 nanocomposite prepared with the aid of ultrasonic irradiations, apart from the peaks corresponding to S, C, Ti, O and Zn, 6
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Fig. 5. XPS survey spectrum of ultrasonically prepared rGO-ZnS-TiO2 nanocomposite and resolved fitting signal of C 1s, O 1s, Ti 2p, Zn 2p and S 2p.
additional peak is observed for Na 1 s at 1071.38 eV. The probable reason for this may that during the synthesis of graphene oxide, sodium nitrate and sodium hydroxide were used hence minor amounts of unreacted sodium may be detected in the nanocomposite.
3.7. Effect of initial CV dye concentration The effect of various concentrations of CV dye was investigated for three different concentrations whilst keeping the other conditions constant. During the adsorption equilibrium studies it was found that for 30 min of equilibration time maximum amount of dye was adsorbed by the rGO-ZnS-TiO2 nanocomposite. The percent removal of CV dye due to adsorption only was observed to be 91.15, 60.53 and 63.29 % for 50, 70 and 100 ppm CV dye concentrations, respectively. Surprisingly it was found that the rGO-ZnS7
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Fig. 6. Effect of Initial Concentration of CV dye on its degradation using rGO-ZnS-TiO2 nanocomposite photocatalyst prepared by ultrasound assisted method (Batch volume =100 mL, Temperature =35 °C, Catalyst loading =0.4 g).
TiO2 nanocomposite prepared with the aid of ultrasonic irradiations is a very effective adsorbent as well. It may be because reduced graphene oxide intensifies the specific surface area of the prepared nanocomposite thereby providing substantially higher number of active sites for adsorption of CV dye ions. Further photocatalytic degradation studies showed that overall 97.02, 96.34 and 96.14 % degradation of CV dye was achieved for 50, 70 and 100 ppm concentrations respectively (Fig. 6). From the results it can be said that as the CV dye concentration increases, the percent degradation of the selected dye decreases. This may be because of the fact that as the dye concentration increases the quantum of dye molecules adsorbed on the surface of the photocatalyst gets enhanced that results in the decrease in the path length of photons entering the CV dye solution. Also a considerable amount of UV light may be absorbed by the dye molecules instead of nanocomposite particles thereby reducing the efficiency of the photocatalytic reaction [5]. Moreover, the photocatalytic effect is not seen prominently for 50 ppm concentration, however for other two CV dye concentrations higher amount of degradation was observed. This may be because in case of 50 ppm concentration already 91.15 % has been removed by adsorption process and hence there is not much availability of CV dye molecules for the photocatalytic degradation process.
3.8. Effect of rGO-ZnS-TiO2 nanocomposite photocatalyst loading In order to decide the optimal amount of rGO-ZnS-TiO2 nanocomposite prepared with the aid of ultrasonic irradiations, experiments were performed at varied loading of catalyst. During the adsorption equilibrium studies, it was observed that for 30 min of equilibration time, the removal of CV dye was about 89.70, 91.15 and 92.44 % for 0.3, 0.4 and 0.5 g/L of rGO-ZnS-TiO2 nanocomposite, respectively. Again the adsorption was observed to be predominant for all the three cases and the possible explanations are deliberated in the previous section. Further, the results of the photocatalytic degradation are presented in Fig. 7. The photocatalytic degradation studies indicated that overall 94.83, 97.02 and 96.30 % degradation was achieved for 0.3, 0.4 and 0.5 g/L of rGO-ZnS-TiO2 nanocomposite, respectively. The results suggest that as the amount of nanocomposite increased from 0.3 to 0.4 g/L the degradation of CV dye was found to be increased from 94.83–97.02%, respectively and further increase in the loading of rGO-ZnSTiO2 nanocomposite (0.5 g/L) showed marginal decrease in the degradation, which was observed to be 96.30 %. As discussed earlier the nanosized rGO-ZnS-TiO2 nanocomposite particles showed strong adsorption capabilities and the excessive adsorption reduces the photocatalytic activity, which is due to mutual screens among rGO-ZnS-TiO2 nanocomposite particles. In addition, the dispersion of excessively loaded rGO-ZnS-TiO2 nanocomposite particles will hinder the UV light irradiation and that restricts the efficient usage of light leading to decrease in the photocatalytic degradation of CV dye [4,5]. Thus the obtained results suggest an optimal dose of 0.4 g/L of rGO-ZnS-TiO2 nanocomposite prepared with the aid of ultrasonic irradiations to achieve effective degradation of CV dye. Nanocomposite prepared with the aid of ultrasonic irradiations showed excellent multi-functionality for the application of
Fig. 7. Effect of catalyst loading on degradation of CV dye using rGO-ZnS-TiO2 nanocomposite photocatalyst prepared by ultrasound assisted method (Batch volume =100 mL, Temperature =35 °C, Initial dye concentration =50 ppm). 8
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adsorption as well as photocatalytic degradation of selected CV dye. It can be said that it is a versatile nanocomposite which can be considered for these two applications for wastewater treatment. 4. Conclusions A ternary multifunctional rGO-ZnS-TiO2 nanocomposite photocatalyst was successfully synthesized by conventional and ultrasound assisted approach which was confirmed from various characterization techniques. SEM images revealed non-uniform and distorted morphology of rGO-ZnS-TiO2 nanocomposite for conventional method as against the ultrasound assisted method. The TEM analysis revealed that the size of TiO2 nanoparticles for ultrasonic method is in the range of 20−40 nm and that for conventional method it is around 70−80 nm. The photocatalytic degradation capability of synthesized rGO-ZnS-TiO2 nanocomposite with the aid of ultrasonic irradiations was investigated for CV dye degradation. The studies established that the synthesized rGO-ZnS-TiO2 nanocomposite exhibited superior adsorption and photocatalytic activity and this can be considered as a multifunctional ternary nanocomposite material for adsorption and photocatalytic degradation of CV dye Declaration of Competing Interest The authors declare that there are no conflicts of interest. Acknowledgments This work was supported by the Rashtrasant Tukadoji Maharaj Nagpur University, Nagpur under Innovative Research Activities [Sanction order no. vikas/javak/2277 dated 7/10/2015]. References [1] F. Guzman-Duque, C. Pétrier, C. Pulgarin, G. Peñuela, R.A. Torres-Palma, Effects of sonochemical parameters and inorganic ions during the sonochemical degradation of crystal violet in water, Ultrason. Sonochem. 18 (2011) 440–446. [2] R.Y. Lin, B.S. Chen, G.L. Chen, J.Y. Wu, H.C. Chiu, S.Y. Suen, Preparation of porous PMMA/Na+-montmorillonite cation-exchange membranes for cationic dye adsorption, J. Memb. 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