To What Extent is Gold (Au) Coating Effective in Improving the Ethanol Gas Sensing Capabilities of Zinc Oxide (ZnO) Nanowires to Lower Their Operating Temperatures?

Abstract:

Nanowires are less than 100 nanometers in diameter. In recent years, nanowires are largely investigated due to their extensive applications. Among these nanostructures, zinc oxide nanowires (ZnO NWs) are desirable on account of their unique properties as semiconductors. Easily accessible and fabricated, ZnO NWs are piezoelectric, biocompatible and photosensitive, which allows them to solve environmental, electronic, and energy-related problems. For example, they are strong candidates for solar cells, UV lasers, and toxic gas sensors. One of ZnO NW’s promising attributes lies in chemical sensing. Given current environmental concerns such as climate change, chemical gas sensing has attracted more attention. Specifically, ethanol gas sensing abilities are highly appealing due to ethanol’s role in energy generation. Ethanol is a renewable fuel, which makes cleaner than gasoline, crude oil, and diesel fuel. ZnO NWs sense based on changes in their electrical conductivity when reacting with a substance - in this case ethanol. As a reductive gas, ethanol gas releases electrons to the ZnO NW conduction band. As a result, concentration of electrons increases, leading to higher electrical conductivity. However, ZnO NW sensors can only operate under high temperatures, typically around 300˚C, which limits daily usage. Although extensive research pertaining to ZnO NW ethanol sensors has been conducted, few report possible solutions to the aforementioned problem. Methods such as UV-irradiation, heterojunction, and doping are all available to improve ZnO NW sensitivity to ethanol, but Au coating is the most feasible under the context of a limited high-school laboratory. Au enhances sensitivity by increasing the width of the NW surface depletion layer. Simple Au sputtering will be employed to disperse Au nanoparticles upon the surfaces of ZnO NWs. The Au-coated ZnO will be placed in a sealed plastic container, where it will be exposed to ethanol gas. Crocodile clips and copper wires will connect the ZnO NW to a multimeter that measures resistance. This investigation will first use the thermal oxidation method to grow ZnO NWs, then evaluate the ethanol sensing capabilities of Au-coated ZnO NWs at 150˚C or lower. Finally, it will conclude whether Au coating is a viable method to to lower the operating temperature of ZnO NW sensors.