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CHAPTER 9 CONCLUSION AND FUTURE WORK 9.1 CONCLUSION Pure and doped Zinc oxide (ZnO) nanostructures were synthesized using simple sol–gel chemical route and were annealed at 500C for 2h. The effect of doping on the structural, morphological and optical properties was studied for an optimum concentration. Structural and morphological analysis were carried out using XRD, EDX, SEM, TEM and AFM, while optical properties were explored using UV-Visible, Photoluminescence, FTIR and μRaman spectroscopy. The antibacterial activities of the as-synthesized pure and doped ZnO nanostructures were carried out against both gram positive and gram negative bacteria by using well diffusion method. Furthermore, the photocatalytic activities of these samples are also studied by degrading methylene blue (MB) in water under the irradiation of UV light. On the basis of the results obtained during the course of this study, the following key conclusions are summarized as: The influences of annealing effects have been explored on the crystallinity, morphology, optical and magnetic properties of Ag-ZnO nanorods. The synthesized Ag-ZnO nanorods are found to have hexagonal wurtzite crystal structures and their grain size increases while lattice strain decreases on annealing. Due to annealing effect, Ag doped ZnO shows higher saturation magnetization at room temperature. Photoluminescence emission spectra pointed up additional peaks due to annealing effect of AgZnO. These supplementary emission bands point towards promising applications of the annealed Ag doped ZnO samples, especially bearing in mind their low cost and availability. UV-driven white light emission generation in luminescent lamps, flexible displays, and down-shifting of solar spectrum 145 for enhancement of efficiency of solar cells are some of their device applications. The magnetization increases with annealing indicating that annealed Ag doping ZnO might be an effective way to obtain more prominent RT-FM in ZnO-based DMS nanoparticles. The Ag-ZnO nanocomposites were synthesized to investigate the bacterial sensitivity against Gram positive and Gram negative bacteria, comparatively by using well diffusion method. Ag/ZnO nanorods were synthesized by the simple wet chemical sol-gel method. The grain size was controlled by using polyvinyl pyridine as capping agent. Nanoparticle crystallinity, quality of the samples, chemical composition, and the optical properties were investigated by XRD, μ-RS, FTIR, PL spectrometer and HRTEM. The enhanced bioactivity was demonstrated by studying the antibacterial activity of ZnO and Ag/ZnO samples. These improved bioactivities of smaller particles were attributed to the higher surface to volume ratio. The smaller particles need more particles to cover a bacterial colony, which results in the generation of active oxygen species, which will kill bacteria more effectively. Therefore, Ag/ZnO nanorods were found to be more effective for killing the bacteria and thus, contribute to the greater mechanical damage for all the functions of bacteria. Hence, synthesized Ag/ZnO nanorods showed potential applications in photodegradation of organic dye pollutants and destruction bacteria. XRD analysis revealed that the as-synthesized Cr doped ZnO nanostructures are well-crystalline and possessing hexagonal wurtzite structure in nano regime. SEM images confirmed that the synthesized nanostrucures were grown in high density with less agglomeration. The different types of morphologies were also observed after doping in ZnO nanostructures. Optical absorption measurements confirmed that doping enhances the excitonic oscillator strength and showed the blue shift in the band-gap which is also supported by Raman effect and PL. Doped nanostructures were found to be more effective than undoped ZnO and thus 146 contribute to the greater mechanical damage to all functions of bacteria and enhanced the bactericidal impact of Cr doped ZnO nanostructures. ZnO and Al-doped ZnO nanopowders were successfully synthesized via simple sol-gel chemical technique. The influences of Al doping on the microstructure, morphology, optical and antibacterial properties of ZnO and its photocatalytic activity are investigated. It is interesting to observe Al doped ZnO nanoparticles proved to be more lethal to the bacteria than Cr-doped ZnO nanostructures. Also same scenario was observed in the degradation efficiency of the organic pollutant, M.B dye. The reason may be due to the excellent substitution of Al nanoparticles in ZnO which is also confirmed by EDX. This enhancement is possible because ionic radius of Al (0.54nm) is less than that of Cr (0.63nm). Finally, the investigations of optical, structural, antibacterial and photocatalytic properties of sol-gel synthesized dual doped (Al-Cr) - ZnO nanostructures were presented. XRD results showed that undoped and dualdoped ZnO nanostructures have crystalline nature. The morphology was found as honey-comb like structure for dual-doping sample. Optical absorption measurements showed that when the dopants Al and Cr were incorporated into ZnO, there is a difference in the optical absorption towards the edge which is blue shifted from 3.32 to 3.41 eV after dual-doping. The enhanced bioactivity was demonstrated by studying the antibacterial activity of undoped and dual doped ZnO samples against two bacteria Microccus leutus (M. leutus) and Vibrio cholera (V. cholera). The enhanced bioactivity of smaller particles is attributed to the higher surface area to volume ratio. The smaller particles need more particles to cover a bacterial colony which results in the generation of active oxygen species which will kill bacteria more effectively. After doping the experimental diameter values of inhibition zones for M. leutus and V. cholera bacteria estimated are 20 and 24 mm. It indicates doped ZnO were found to be more effective towards V. cholera than M. leutus, thus contribute to the greater mechanical damage to all 147 functions of bacteria and enhanced bactericidal impact of dual doped ZnO nanoparticles. 9.2 FUTURE WORK ZnO is a very promising candidate for a number of applications like sensing, photovoltaic, antibacterial and biomedical, gas sensing, optoelectronic device and dilute magnetic semiconductors when doped with transition metals. Therefore, in future this work can be extended in the following different ways as given below: 1. ZnO nanostructures showed enhanced antibacterial activities after doping against both gram positive and gram negative bacteria. The next step is to check their toxic behavior for biomedical applications. 2. In the present investigations, doped ZnO nanostructures showed good luminescence properties. Attempts can be made to understand the non-linear optical (NLO) property for optoelectronics applications. 3. Thin films can also be prepared and the properties can be compared. The film parameters are to be optimized suitable for biomedical and photocatalytic applications. 148