Synthesis of ZnO nanostructures for gas sensing applications
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Research on nanotechnology has become increasingly popular because of their unique physical, chemical, optical and catalytic properties compared to their bulk counterparts. Nanotechnology revolutionize many technology and industry sectors such as energy, food safety, environmental sciences, medical sciences and medical instrumentation, homeland security, and many others. Among II-VI semiconductor materials, ZnO is a unique and distinguish material with wide band-gap (3.37 eV) and the large exciton binding energy (60 meV) at room temperature. ZnO nanostructures have attracted great attention among the researchers due to its peculiar properties such as high electron mobility, high thermal conductivity, good transparency, and easiness to fabricate different type of nanostructures. The objective of this research is to synthesize different ZnO nanostructures using low-cost sol-gel spin coating method for gas sensing applications. A thin seed layer of ZnO was deposited to provide the nucleation sites for the growth of ZnO nanostructures. The surface morphology, structural, optical and electrical properties of the synthesized ZnO nanostructures was characterized using SEM, XRD, PL, Raman, XPS and sourcemeter. Gold and silver interdigitated electrodes were thermally evaporated on the surface of ZnO nanostructures using stainless steel shadow mask. The current research successfully demonstrated the application of sol-gel method to synthesize different nanostructures. Moreover, the fabricated ZnO based nano-sensors have also been applied for gas sensing applications. In this thesis, we have studied the effect of precursor molar concentrations (0.0125- 0.075 M) on the morphology of ZnO nanostructures. It was observed that the ZnO nanorods tend to grow along with the ZnO nanoflakes and the density of ZnO nanorods increases as the molar concentrations increased from 0.05 M to 0.075 M. The PL and Raman spectra also redshifted and the redshifting were attributed due to local heating. The effect of different solvent for the seed solution preparation was also studied. It was observed that different seed solution greatly affects the growth direction of ZnO nanorods as well the conductivity of the device. The detailed study was conducted to investigate the effect of UV exposure time on the device current stability and it was observed that after 60 min of UV exposure a stable current flow was observed. The determination of stable current after UV exposure is important for UV-based gas sensing and optoelectronic applications. Finally, we fabricated the Pd-doped ZnO nanorods, and pure ZnO nanorods for gas sensing applications. The fabricated sensors were successfully tested for a wide range of hydrogen and acetone concentrations 40-360 ppm, and 0.05- 457 ppm, respectively. It was observed that the sensor was at least 25 fold more sensitive over the literature documented Pd-doped ZnO nanorods in detecting hydrogen at room ambient temperature. The fabricated sensor displayed the decrement in grain boundary resistance from 11.95 to 3.765 KΩ when the hydrogen concentration was increased from 40 to 360 ppm. The acetone sensor based on ZnO nanorods exhibited excellent acetone sensing over a wide range of acetone concentrations (0.05- 457 ppm). The sensor exhibited the response and recovery times of 8 s and 428 s, respectively, for 183 ppm of acetone. The sensor also displayed good repeatability under the exposure of 183 ppm of acetone.