Investigation of volatile gas sensors integrated on flexible and wearable substrates with functional nanomaterials

Abstract

In this dissertation, development of gas sensors on wearable substrates was studied to overcome the limitations, e.g., high-temperature process for fabricating sensing material, high sensing temperature, insufficient sensitivity and selectivity. The approach to overcome these limitations was focused on developing coating methods on fabrics, catalyst doping, and investigation of alternative sensing materials. A representative metal oxide with good semiconducting properties, ZnO (3.37 eV band gap), was explored as a sensing material. Thin films of the ZnO material were coated on sensor platforms by different deposition techniques: sputtering, sol-gel, dip-pad-cure method, and electrophoretic deposition and their properties were studied. Additionally, ZnO nanorod structure was grown by thermolysis-assisted chemical solution deposition on ZnO seed layer to enhance the sensing performance by using nanostructure. Among these deposition methodologies, dip-pad cure method was found to be suitable to coat ZnO at low temperature on fabric substrates.

To lower sensing temperature from several hundred Celsius (i.e. 300 °C) to room temperature in semiconductor oxide sensors, the effects of catalyst on gas sensing mechanism and performance were studied. Additionally, composition gradient effects on gas sensing behavior to improve gas selectivity were explored by adapting different metal elements. The combination of two metallic catalysts by varying their concentrations in a sensor platform was investigated: platinum and palladium combination and platinum and titanium combination. Since the amount and island size of catalysts strongly influence structural modification of metal oxide materials, the sensing properties can be changed. Therfore, the fundamental study of catalyst elements and their effects on change in concentration were conducted to realize chemical gas sensor module with improved selectivity and sensitivity under multiple gas combination.

Another approach to reduce sensing temperature of semiconductor oxides is to utilize non-traditional oxide materials. As a new sensing material, graphene oxide was used due to its good semiconducting and chemical properties. Graphene oxide presents conducting or insulating properties according to its reduction condition. Therefore, reduction of graphene oxide was studied to achieve room temperature gas detection reported in the literature. However, exploration of non-treated graphene oxide, i.e. pristine graphene oxide, was rarely conducted. This study investigated the use of non-treated graphene oxide for gas sensors, and its reduction behavior on electrical properties such as N-P transition was studied.

Finally, the combinatorial or composite structure was investigated with SnO2 nanostructure and graphene oxide to enhance gas selectivity. The ratio of graphene oxide to SnO2 was changed as the amount of SnO2 was added. To explore how they interact to improve the gas sensing behavior, the combined structure was also modified, for examples bimorph structure and multilayer structure.