Nanostructured semiconducting metal oxides and applications as gas sensors : synthesis, characterization and evaluation

• Κύριος συγγραφέας: Κουγιανός, Χρίστος
• Άλλοι συγγραφείς: Βιτωράτος, Ευάγγελος
• Μορφή: Thesis
• Γλώσσα: English
• Έκδοση: 2017
• Θέματα: Αισθητήρες αερίων; Ημιαγωγοί; Gas sensors; Semiconductors; 681.2

Gas sensors based on metal oxide semiconductors (MOS), such as SnO2, ZnO, In2O3, etc, grown in a large variety of morphologies (nanoparticles, nanowires, etc.) have emerged over the last decade as the most promising gas sensing materials. The operating mechanism relies on the change of their resistance caused by charge transfer, when the target gas molecules react with the chemisorbed surface oxygen. The process is thermally activated and usually requires high temperature operation. MOS can detect a large variety of different gases; although, selectivity is still an open issue. Carbon monoxide (CO) is a highly toxic gas, which is termed a toxic asphyxiant even at low concentrations, being at the same time colourless, odourless and tasteless, which makes its detection difficult. The main target of the current work is to advance novel nanohybrid materials for CO gas sensing. Among the various semiconducting metal oxides, ZnO nanowire arrays (NWs) have attracted considerable attention since they offer large surface area for the reactions with the target gas. ZnO NWs with high aspect ratio and surface area have been prepared, in a controllable manner, by carefully adjusting the growth parameters and have been decorated with SnO2 nanoparticles. Nanowire synthesis is based on the chemical bath deposition method on seeded substrates. SEM images were taken in order to examine the morphology of the sensing layer and the orientation of the nanowires. Electrodes are fabricated on the conducting nanostructured surface in order to measure the signal in the external circuit. After the devices were ready, they were inserted in a furnace, where temperature was controlled with a thermocouple. Devices for detecting carbon monoxide (CO) were evaluated for different concentrations over the temperature range 25 – 350 oC. By optimizing the SnO2 loading on ZnO NWs we have been able to prepare nanostructures that operate at low detection temperature, even for concentrations as low as 70 ppm. As a result promising devices operating reliably at room temperature have already been tested. Additionally, the presence of humidity and its influence in the sensitivity of the samples was tested. Finally, a different SnO2 deposition method was prepared, with the thermal oxidation of tin and the formation of stannic oxide on the top of the films.