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Enhancement of Nanocrystalline Zinc Oxide based Electronic Gas Sensor by Surface Modification

Hou, Yue
http://rave.ohiolink.edu/etdc/view?acc_num=toledo1396609072

Abstract Details

2014, Doctor of Philosophy, University of Toledo, College of Engineering.
The increasing concerns of industrial safety, chemical control and environmental pollution are spurring demand for high performance gas sensors. Growing use of gas sensors is making gas sensors on demand. After decades of research and development activities, semiconductor based gas sensors are now used in a variety of applications. However, challenges still remain in the area of sensitivity, selectivity, response and recovery speeds and power consumption. Therefore, improvement of metal oxide gas sensors by the incorporation of different technology is important. In this research, modification of metal oxide semiconductor based gas sensor by impurity doping, laser irradiation, and plasma treatment was investigated. Zinc oxide (ZnO) is an n-type semiconductor with a wide direct band gap (~3.3 eV) and large binding energy (~60 meV). Due to its superior electrical properties and chemical stability, ZnO has been considered one of the most promising materials for gas sensor applications. ZnO thin films have been fabricated by different techniques, such as rf magnetron sputtering, pulsed laser deposition, molecular beam epitaxy, and sol-gel. Sol-gel is a powerful alternative for vacuum deposition. iii The purpose of this research was to enhance properties and gas sensor performance of nanocrystalline so-gel derived ZnO thin films via surface modification techniques. The effects of process conditions, impurity doping, laser irradiation, laser doping and plasma treatment on properties and gas sensor performance were investigated. The gas sensor performance of ZnO thin films was investigated at different operating temperatures for various reducing and oxidizing gases such as H2, NH3, CH4 and NOx. Al-doped ZnO thin films were prepared using the sol-gel process by changing the Al concentration from 0 to 5.0 at% using two different Zn precursors. It was found that 3.0 at% Al-doped ZnO films had optimum properties such as high electrical conductivity, crystallinity, high sensing response and short response time for ZnO films derived with both Zn precursors. Ga-doped ZnO thin films were also presented by changing the concentration of Ga from 0.1 to 1.0 at%. The gas sensing behavior was investigated at an operating temperature of 130oC. It was found that the 0.3 at% Ga-doped ZnO thin film sensor had more than a 40% higher sensing response and a shorter response time than the sensors made with as-deposited films. Laser irradiation was utilized as a novel heat treatment method in this dissertation. A pulsed laser system with a wavelength of 532 nm was used as the irradiation source. Laser irradiation produced two kinds of Al-doped ZnO films depending on the laser energy level. The impact of laser irradiation was also varied according to the film thickness. The Al-doped ZnO sensors exhibited enhanced sensor performance with optimum laser fluence compared with that of as-deposited sensors. The results suggested that the crystallinity of the Al-doped ZnO thin films was essential to achieve an optimum iv gas detection capability. A laser doping process using a pre-deposited Al precursor layer for ZnO thin films was also investigated. Plasma treatment was utilized in the research with the intention to adjust the number of intrinsic defects in ZnO films. Both O2 and H2 plasmas were carried out with the treatment time varying from 3 to 15 min. The gas sensor performance was investigated for NH3 and NOx at various concentrations. An improvement of more than 50% in sensing response was observed with the optimum treatment time for NH3 detection. The impact of H2 plasma treatment of ZnO sensors on its gas sensor performance was also studied. The selectivity was studied for a likelihood mixture of reducing gases. The results indicated that ZnO sensors had a low detection limit towards NH3. It was found that the selectivity of these reducing gases was in the order of NH3 > H2 > CH4. However, unless high concentration of NH3 exposure is a factor, ZnO sensors are selective respond to H2. Furthermore, the ZnO sensors are also capable of discriminating different gases such as NH3, H2, CH4 and NOx. This research involves different techniques, which have been employed on the ZnO thin film based gas sensors in order to enhance the gas sensing characteristics such as sensing response and response time. As expected, the ZnO sensors exhibited high response, short response time and decent selectivity toward target gases with optimum treatment conditions. The results also suggested that the ZnO sensors were capable of detecting ppm level gases at relatively low operating temperature range (100 ~ 200oC), which is beneficial in many applications. Therefore, these techniques can be utilized to manufacture gas sensors using metal oxide semiconductors.
Ahalapitiya Jayatissa (Committee Chair)
Sanjay Khare (Committee Member)
Lesley Berhan (Committee Member)
Mehdi Pourazady (Committee Member)
Sorin Cioc (Committee Member)
Ambalangodage Jayasuriya (Committee Member)
173 p.
Materials Science; Mechanical Engineering
Zinc Oxide; Thin Film; Doping; Sol-gel; Gas Sensor; Surface Modification; Laser Irradiation; Plasma

Recommended Citations

Citations

  • Hou, Y. (2014). Enhancement of Nanocrystalline Zinc Oxide based Electronic Gas Sensor by Surface Modification [Doctoral dissertation, University of Toledo]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=toledo1396609072

    APA Style (7th edition)

  • Hou, Yue. Enhancement of Nanocrystalline Zinc Oxide based Electronic Gas Sensor by Surface Modification. 2014. University of Toledo, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=toledo1396609072.

    MLA Style (8th edition)

  • Hou, Yue. "Enhancement of Nanocrystalline Zinc Oxide based Electronic Gas Sensor by Surface Modification." Doctoral dissertation, University of Toledo, 2014. http://rave.ohiolink.edu/etdc/view?acc_num=toledo1396609072

    Chicago Manual of Style (17th edition)

toledo1396609072
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This open access ETD is published by University of Toledo and OhioLINK.