Investigating on Structural, Optical, Magnetic and Photocatalytic Activities of ZnS and Co 2+ : ZnS NPs Synthesized by Hydrothermal Process Under MeB Dye

In present work, the structural, morphological, optical and photocatalytic activities of ZnS and Co 2+ : ZnS NPs were prepared through chemical route as hydrothermal process at room temperature. ZnS and Co 2+ : ZnS were characterized by using various techniques such as, XRD, SEM-EDX, TEM-SAED, UV-visible, PL and BET. The spherical-like morphology with the average crystallite size was found to be 8 to 15 nm. Among them results, it showed that the Co 2+ atoms were incorporated into the ZnS lattice, forming cubic phase as the Co 2+ dopant concentration increases from 0 to 2 %. The band gap energy of the ZnS and Co 2+ : ZnS increases from 3.5 to 4.10 eV, which enables stronger absorption of UV region. During catalytic process, Co 2+ act as electron trapping center, which inhibits the recombination of the photo induced holes and electrons as showed higher degradation eciency for MeB.


Introduction
In semiconductor, the nanostructured materials are strongly attracted more interest in physics, chemistry, catalytic, material science and biology, due to size tuned optical and unique properties. Recent years, the semiconductor based photocatalytic technique has been attracted signi cant awareness in the elds of environmental remediation [1][2]. It showed ease separation of the catalyst from the reaction products of heterogeneous photo catalyst. During process, the electrons and holes undergo redox reactions present in the RhB dye solution, which is used to remove waste waters, involving the degradation of less toxic semiconductor materials. The various approaches have been used to enhance the charge separation with improved their e ciency of photo catalyst such as, elemental doping, coupling two semiconductors and so on [2][3]. Among them, the photocatalytic technique has a wider range advantages compared to traditional techniques in waste water treatment and it is used for industrial applications. ZnS is a II-VI semiconductor with the direct band gap of 3.68 eV. Recently, ZnS showed prominent applications like solar cells, LED, optoelectronic and so on that require an appropriate particle size, size distribution and methods. In past decades, the promising results were achieved by using various metal dopants such as Mg, Co, Mn, Ni and Cd. Among them, Co 2+ ions as dopant improve the visible light absorption capacity of catalyst owing to their 3d transition and their potential properties [6-10]. Up to now, there are several methods employed for the synthesis of various metal ions-doped ZnS nanoparticles such as, wetchemical, combustion process, microwave assisted method and so on. Among them, the hydrothermal process are uniquely provides e cient, nucleation, high purity, easy manipulation and producing small particles [11][12]. In the present paper, undoped and Co 2+ -doped ZnS nanoparticles were synthesized and its structural, morphological, optical properties were discussed. Further, the photocatalytic activities of undoped and Co 2+ -doped ZnS on MeB under UV radiation were studied. From these ndings, Co 2+ -doped ZnS ions are modifying the various properties and its photocatalytic mechanism was also investigated. continuous stirring for 2 hrs. Then, the ammonia solution was added to drop wise due to well control for chemical homogeneity under acetic pH condition. This mixed solution was taken in an autoclave and kept at 180 0 C for 12 hrs in a mu e furnace that, they were cooled to room temperature. Finally, the obtained sample was ltered and dried at 120 0 C for 6 hrs in a vacuum.

Preparation of Co 2+ : ZnS nanoparticles
Among them, the same procedure was adopted to preparation of Co 2+ : ZnS particles in the nanosized range using 0.01 M of CoAC 2 .2H 2 O as dopant (2% and 3%).

Characterizations
The crystalline structure was carried out by powder X-ray diffraction in a Bruker D8 Advance using monochromatized Cu kα radiation as λ = 0.15496 nm. SEM-EDX was performed with a S4800 eld emission SEM at an accelerating voltage of 5 kV. From TEM-SAED instrument, the morphology and particle size were analyzed using a TEM Philips model CM 20 operated at an accelerating voltage of 200 kV. UV-visible and PL were collected using a Varian Cary 5E UV-vis Spectrophotometer and Jobin Yvan Flourometer. From BET instrument, the N 2 adsorption measurements were done on a Micromerities ASAP 2010 physisorption apparatus.

Photocatalytic Experiment
The photocatalytic activity of synthesized samples was tested by the degradation of MeB under UV light irradiation of 365 nm. Present work, 40 mg photo catalyst was suspended in MeB solution. Then, the mixed solution was oscillated in darkness 6 h. after reaching adsorption equilibrium, the photo catalyst reaction was initiated by irradiation the system with a 350 w xenon lamp. At given time intervals, 4ml aliquots were collected centrifuged and then ltered to remove the catalyst particles for analysis. The ltrates were nally analyzed using a UV-visible spectrophotometer UV-2450. Hence, the degradation e ciency ( ) was calculated by using = Co − Ct Co × 100\% with the C o before illumination (concentration of the MeB) and C t after certain irradiation at a time (t) (concentration of the MeB). Figure 1 (a-c) shows the XRD patterns of ZnS and Co 2+ : ZnS NPs. The observed XRD peaks corresponding to three indexed planes such as, (111), (220) and (311) with the other phase peaks were not observed due to impurities, to con rm the formation of cubic structure (JCPDS. NO. # 05-0566) [13].

Result And Discussion
By using Debye-Scherer's, the average crystalline sizes were found to be 3.7, 3.2 and 2.9 nm, respectively [14][15]. From XRD, the obtained diffraction peaks are decreases with the increase of dopant concentration as Co 2+ , it's indicate a smaller crystallite size with low crystallinity due to lattice distortions [15][16]. Since, the ionic radius of Co 2+ (0.65 nm) is slightly less than that of the Zn 2+ (0.74 nm) ion and the lattice parameters (a = 5.386Å) of Co 2+ : ZnS sample are slightly less than the ZnS (a = 5.391 Å) [16][17]. Finally, the XRD peaks of Co 2+ : ZnS (3%) are broader than that of other samples, its showed smaller size of the particles.   It is concluded, 3% of Co 2+ : ZnS particle have higher optical band gap compared to other samples due to smaller particle size. Consequently, the increased optical band gap materials suggest that the synthesized nanoparticles in uence in the optical devices based applications, it can be tuned by adding the dopant as Co 2+ [19][20][21][22][23]. Figure 5 (a-c) shows PL spectra of ZnS and Co 2+ : ZnS NPs with a excitation wavelength of 350 nm. The emissions were observed at 445 nm due to recombination of sulfur vacancy defects as internal vacancy of zinc (holes trapped) and sulfur (donor level) atom. Since, the emission spectra were observed in blue shift compared to bulk counterparts and it is useful for optoelectronic devices [22][23][24]12].   Fig. 11 (a-c). In TOC images, Co 2+ : ZnS catalyst of TOC removal was found to be 10.24, 72.18 and 88.23 % after 120 min of irradiation. Among these results, it's indicates that the color disappearance of the MeB dye was faster than the degree of mineralization. During process, the quick disappearance was arising from the cleavage of the MeB dye bond and their minimal interaction with aliphatic chains. After 120 min, MeB dye molecules were converted to other intermediate forms, which exist in the solution irrespective of the dye de-colorization lead to complete mineralization beyond 120 min. Table 2 gives the COD values of initial and nal treated of Co 2+ : ZnS catalyst under MeB. It is used to measure the amount of organic components in terms of the total amount of O 2 required to oxidize it to CO 2 and H 2 O via dichromate re ux method. In table 2, the synthesized catalyst has high potential for the removal of MeB due to decreased COD values.
Consequently, TOC and COD results showed that the 3% of Co 2+ : ZnS catalyst has higher photocatalytic activities than the other samples. In addition, the detailed drgradation mechanism of undoped and Co 2+ : ZnS catalyst can be summerized below.
BET images of ZnS and Co 2+ : ZnS NPs with the N 2 adsorption-desorption isotherms as shown in Fig. 10 (a-c). The surface area and pore volume of synthesized samples were calculated as 97, 112 and 148 m 2 g − 1 and 0.25, 0.34 and 0.63 cm 3 g − 1 , respectively. Its corresponds to pore size distribution at ~(12-15), (9)(10)(11)(12)(13) and (5-9) nm, respectively. Since, the surface area of ZnS samples gradually increases with the increase of cobalt ions concentration due to smaller particle size. It is concluded that the synthesized samples was composed of many ne crystallites [27,30]. Figure 11 shows the magnetic properties of 3% doped Co 2+ : ZnS with the applied magnetic eld from − 20 to 20 K Oe at room temperature. The prepared ZnS nanoparticles favor the ferromagnetic behavior that re ect low magnetic moment and high coercivity value due to smaller particle size. Further, present case, the coercivity (Hc) of 223 Oe and the saturation magnetic moment of (Ms) 0.00025 (emu/g), respectively. As a result, a low value of saturation magnetic moment and higher value of coercivity were obtained for the 3% Co 2+ : ZnS sample compared with bulk ZnS [28]. In ferromagnetism, the cubic structure was not changed due to substitution of Co 2+ ions in the Zn 2+ surface and it would be very useful for spintronic devices [29,31].

Conclusions
In summary, spherical-like of ZnS and Co 2+ : ZnS NPs were successfully synthesized by hydrothermal process under different concentration of cobalt ions by hydrothermal process. XRD and TEM studies indicate that the Co 2+ ions were incorporated into the ZnS lattice, forming the cubic phase. The absorption spectra of ZnS and Co 2+ : ZnS NPs were exhibit blue shift ensure the quantum con nement effect compared to bulk ZnS due to smaller size of particles. ZnS and Co 2+ : ZnS NPs improves the photo degradation e ciency using MeB under UV light irradiation. The ferromagnetic behavior was observed from the VSM study. Therefore, the advantages of hydrothermal method for the synthesized nanoparticles permit large-scale production, easy manipulation, low cost.  (a-c) shows the XRD patterns of the zinc sul de samples. Figure 2 (a-f) shows the SEM-EDX images of the zinc sul de samples. Figure 3 (a-c) shows the TEM-SAED pattern of the zinc sul de samples.

Figure 4
Page 10/12 (a-c) shows the UV-Visible spectra of the zinc sul de samples. Figure 5 (a-c) shows the PL spectra of the zinc sul de samples.      (a-b) shows the Degradation percentage (%) of the zinc sul de samples. Figure 9 shows the Schematic diagram of catalytic process of the zinc sul de samples. Figure 10 shows the BET of zinc sul de samples. Figure 11 shows the VSM of the zinc sul de samples.

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