ZnO Nanostructures with Different Morphologies and Their Combinatorial Optimization with Graphene Oxide for Gas Sensor Properties

Consumer concerns on food safety and quality are increasing worldwide. Without opening the package of food products, collecting information about freshness of the product is desirable. Therefore, an electronic nose device consisting of multiple gas sensors can be utilized for rapid monitoring of food quality and freshness at low cost. Such multiple gas sensors, i.e array structure can provide selective detection and identification of various compounds by linking gas responses to signal process. Gas sensors based on chemical reaction on the surface of semiconducting metal oxides are readily available commercially. Common material of the metal oxide sensor is tin oxide doped with a small amount of a catalytic metal such as palladium or platinum. The improvement of gas sensing performance such as low operating temperature as well as selective and sensitive detection can be obtained by investigating potential oxides with controlled morphologies at the nanoscale and their composite structures.

Among various semiconducting materials, ZnO is an excellent candidate for gas sensor applications. In addition, many methods can be utilized to fabricate various ZnO nanostructures. Solution process can be simple and effective bottom-up path to construct hierarchical ZnO nanostructures on various substrates. Therefore, we investigated a solution method as controlling the morphology of ZnO nanostructures by applying DC and AC bias and by selecting different precursors. ZnO nanorods structure was synthesized using zinc nitrate hexahydrate and HMT (hexamethylenetetramine) in aqueous solution. The use of zinc chloride instead of zinc nitrate hexahydrate for zinc precursor, nanoflake structure was successfully fabricated where zinc chloride serves as a capping ion to prevent isotropic growth. With applying external voltage during solution growth, ZnO nanorods became agglomerate structures and the degree of agglomerates varies depending on the range of external bias.

Various ZnO nanostructures have been analyzed with SEM, XRD and UV-visible. Figure 1 shows one example of morphological control of ZnO nanostructures according to applying bias and precursors. The change of morphologies showed different degree of orientation of the structures by XRD result. Gas sensing response was measured to extract characteristic feature at different ZnO morphologies. Elevated operating temperature approaching 200 – 300°C from ZnO gas sensor can be lowered by combining graphene oxide (GO). Various combinatorial structures from ZnO and GO were explored. Detailed discussion on gas sensing properties and their mechanisms will be given.