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Microwave Assisted Synthesis of Zno-Pbs Heterojuction For www.nature.com/scientificreports OPEN Microwave assisted synthesis of ZnO-PbS heterojuction for degradation of organic pollutants under visible light Ganapathy Mano1,2, Subramanian Harinee3, Sampath Sridhar4, Mahalingam Ashok3 & Alagan Viswanathan1* ZnO, PbS and ZnO-PbS heterojunction were prepared by microwave irradiation to improve the organic pollutants degradation under visible light irradiation. Hexagonal (wurtzite) and cubic crystal structure of ZnO and PbS respectively were confrmed by PXRD. Nano-plate, nano-sponge and nano-sponge imprinted over nano-sheet like morphology of ZnO, PbS and ZnO-PbS respectively were revealed through FESEM analysis. HR-TEM analysis provides the formation of heterojunction. XPS analysis shows the presence of the ZnO-PbS heterojunction. UV-Visible spectroscopy confrms the enhanced visible light response of ZnO-PbS heterojunction than the bare ZnO. The PL and EIS results indicate ZnO-PbS heterojunction exhibited lowest recombination of excitons and electron transfer resistance. Synergistic efect of ZnO-PbS heterojunction leads to efcient degradation against organic pollutants than bare ZnO and PbS. Aniline and formaldehyde were successfully degraded around 95% and 79% respectively, under solar light irradiation. As-prepared photocatalysts obeys pseudo frst order reaction kinetics. HPLC analysis also confrms the successful mineralization of organic pollutants into water and CO2. Increasing the population growth demands various energy production and consumption which derives diferent form of toxic efuents. Terefore, the removal of the toxic efuents is important to control the pollution. In recent years, semiconductor mediated heterogeneous photocatalysis becoming promising technique to eliminate or cov- ert toxic into non-toxic compounds. Further, they consist additional potential properties such as non-toxicity, photochemical stability, energy conversion and low cost1–5. In recent years diferent kinds of semiconductors are employed for photocatalytic application such as ZnO, TiO2, CdS, ZnS, WO3, SnO2, SrTiO3, YFeO3 etc. Among the semiconductor based photocatalyst, ZnO and TiO2 are widely used due to their strong oxidation capacity, chemical stability, non-toxicity, low cost etc. ZnO (3.37 eV) is one of the best alternate semiconductor material for 6–18 TiO2 (3.2 eV) as it has similar band gap energy and high quantum efciency . ZnO photocatalyst is highly active under UV region in the solar spectrum, fast recombination of photo gen- erated electron-hole pairs and photo corrosion properties19. In general, the photocatalytic efciency is afected by several factors such as electron hole efective mass, difusion length, exciton lifetime, defects, band bending, sur- face band structure, thermal stability and photocorrosion20. To overcome these drawbacks, ZnO band structure was altered by several metals, non-metals and semiconductor coupling to extend its photo-response behavior into visible light region. Among them, semiconductor coupling is an efcient method to reduce the photo corrosion and rate of recombination of excitions. Tis leads to enhance the light harvesting ability up to the visible region without afecting its intrinsic character6. Recently several attempts were made in ZnO coupling with various 21,22 21,22 narrow band gap semiconducting materials like ZnO-Fe2O3 , ZnO-WO3 , etc, to enhance the removal ef- ciency than bare ZnO under visible light irradiation. 1Department of Physics, University College of Engineering Bharathidasan Institute of Technology (BIT-Campus), Anna University, Tiruchirappalli, 620024, Tamil Nadu, India. 2Department of Environmental Engineering, National Ilan University, Yilan, Taiwan. 3Department of Physics, National Institute of Technology, Tiruchirappalli, 620015, Tamil Nadu, India. 4Department of Physics, Vel Tech Rangarajan Dr. Sagunthala R&D Institute of Science and Technology, Avadi, Chennai, 600062, Tamil Nadu, India. *email: [email protected] SCIENTIFIC REPORTS | (2020) 10:2224 | https://doi.org/10.1038/s41598-020-59066-4 1 www.nature.com/scientificreports/ www.nature.com/scientificreports Figure 1. XRD patterns of ZnO, PbS and ZnO-PbS catalysts. Lead sulfide (PbS) has broad spectral response form visible to near-IR region due to its narrow direct band gap (0.41 eV). In recent years, PbS sensitized nanomaterial’s are increased the visible light response and photo-conversion efciencies. It has unique photo physical properties such as multi exciton generation (MEG), high absorption coefcients, size dependent optical properties and optoelectronic properties23,24. Recently, several methods are adopted for the preparation of ZnO-PbS heterojunction such as chemical bath deposition25, ultrasound deposition method26, low temperature method27, successive Ionic Layer Adsorption and Desorption method28,29, Radio Frequency Sputtering30 and spin coating method31. In all the above methods, most of them have some difculties and limitations to produce ZnO-PbS heterojunction such as long reaction time and cost of the equipment. Terefore, novel techniques required to overcome this problem to prepare the semiconductor coupled ZnO-PbS photocatalyst in large-scale production without any sacrifcation in efciency. Microwave irradiation technique is a best alternative heating source for the preparation of nano materials due to its rapid chemical reaction with short span of time when compared to conventional methods. Microwave irradiation involves through dipolar polarization and ionic conduction, which helps for the preparation of nano- structured materials32. Microwave method is fast, uniform heat distribution, energy efcient and simple than other conventional methods. Te size and morphology of the material can be easily controlled by the microwave parameters such as frequency, time and operating power. Terefore, microwave method is leading technique for homogeneous nucleation and fast crystallization, which leads to the preparation of nano-structured inorganic semiconductor photocatalyst. In recent years, diferent kinds of photocatalyst were prepared and reported such 33 34 35 as InVO4 , BiVO4 , CuO-Al2O3 , by the microwave irradiation. In this study ZnO-PbS heterojunction was prepared under microwave irradiation method and their photo- catalytic activities were investigated. Te ZnO-PbS heterojunction improve the visible light absorption due to enhanced excitons life time through the charge transfer between the semiconductors interface. Te structure, morphology and optical properties are characterized by XRD, FESEM, HR-TEM, XPS, PL, UV-Visible and EIS studies. Te photocatalytic activities and mechanism of the as-prepared catalyst are studied with aniline and for- maldehyde under simulated visible light and sun light. Te degradation efciency mainly depends on the trans- port of photo generated electron-hole pairs of ZnO-PbS interface quality. Tus, ZnO-PbS catalyst was optimized with diferent processing parameters. Results and Discussion XRD analysis. Figure1 represents the XRD patterns of ZnO, PbS and ZnO-PbS photocatalyst. Te ZnO and PbS are exhibits hexagonal (wurtzite) and cubic structures matched in Joint Committee on Powder Difraction Standards (JCPDS) card numbers 75–1526 (a = 3.22 and c = 5.28) and 03-0614 (a = 5.96) respectively. Te char- acteristic peaks of ZnO (100), (002), (102) and (110) shows at the angles of 32.46°, 34.12°, 47.79° and 57.97° respectively. Also, the PbS difraction peaks (111), (200), (311), (222), (400), (331) and (420) are observed at the angles of 25.55°, 29.61°, 50.57°, 52.98°, 62.13°, 68.40° and 70.51° respectively. Te XRD spectrum of coupled ZnO- PbS exhibits both ZnO and PbS peaks. Among them, PbS shows high intensity and more peaks than ZnO due to strong X-ray difraction by PbS. It may be due to the PbS imprint over ZnO in accordance with FESEM and HR-TEM analysis. Tere is no other additional peaks were observed, which confrms the only presence of ZnO and PbS. Te difraction pattern also shows that the prepared photocatalyst are polycrystalline in nature. Te average crystallite size of the ZnO, PbS and ZnO-PbS were calculated from XRD pattern using Debye-Scherrer formula Eq. (1)36 D/= αλ βθcos (1) where D is the average crystallite size, α is the scherrer constant (0.9), λ is the wavelength of the X-ray (0.154 nm), β is the full width at half maximum (FWHM) and θ is the Bragg angle of difraction. Table (1) shows the average crystallite size calculated for the prominent peaks of ZnO, PbS and ZnO-PbS. FE-SEM analysis. Figure 2 reveals the morphology of ZnO, PbS and ZnO-PbS photocatalyst using FESEM at diferent magnifcations. Te morphological characteristics of ZnO reveals nano-plate like structures oriented SCIENTIFIC REPORTS | (2020) 10:2224 | https://doi.org/10.1038/s41598-020-59066-4 2 www.nature.com/scientificreports/ www.nature.com/scientificreports S.No ZnO PbS ZnO-PbS 1 2θ β D (nm) 2θ β D (nm) 2θ β D (nm) 2 32.46 0.003433 42.04 25.55 0.003433 41.39 25.5 0.00515 27.59 3 33.89 0.010299 14.06 29.61 0.003433 41.75 29.61 0.004291 33.40 4 47.79 0.017165 8.83 50.57 0.004291 35.71 32.46 0.010299 14.01 5 57.79 0.003433 46.10 52.98 0.00515 30.06 33.89 0.010299 14.06 6 62.13 0.010299 15.70 50.57 0.00515 29.76 7 68.4 0.006866 24.40 52.98 0.00515 30.06 8 70.51 0.00515 32.95 57.97 0.027465 5.76 9 62.13 0.006866 23.56 10 68.4 0.006008 27.88 11 70.51 0.00515 32.95 Average 28 32 24 Table 1. Average crystallite size of ZnO, PbS and ZnO-PbS photocatalysts. Figure 2. FESEM analysis of ZnO (a,b), PbS (c,d), ZnO-PbS (e,f) and EDS (g) analysis. with diferent angles and size. Te measured nano-plate diameter and thickness were ~75 nm and ~15 nm respec- tively (Fig. 2(a,b)). PbS shows sponge like morphology formed by the agglomeration of small particles with the size of ~50 nm (Fig. 2(c,d)). Figure 2(e,f) shows the ZnO-PbS heterojunction nanostructure morphology. It exhibits the formation of smooth interlinked sheets with apparent spaces between the sheets and small particles are imprinted on nano- sheet.
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