Development of high-performance gas sensors integrated on wearable substrates with two-dimensional nanostructure materials

Abstract

In this dissertation, much effort has been dedicated to developing high-performance gas sensors utilizing two-dimensional (2D) nanostructure materials integrated onto wearable substrates. Low-temperature and non-corrosive manufacture processes were explored to adapt wearable textile as a sensor platform, and solution process was selected to deposit sensing nanomaterials. The focus was placed on the control of wettability of the sensing solution by adjusting the ratio between two types of solvents, ethanol and water. Improved adhesion between sensing film and hydrophilic fabric was obtained by tailoring wetting properties of solution where the surface tension of the solution and capillary force of spatial hole in the woven fabric were balanced, resulting in uniformly stacked structure and high resistance of bending. Room-temperature sensing properties of two-dimensional (2D) nanomaterials were studied using two representative 2D nanomaterials, graphene oxide (GO) and molybdenum disulfide (MoS2). An attempt was made to develop an understanding of sensing properties of 2D nanomaterials by examining their material properties. In particular, the effect of surface dangling bonds of 2D nanomaterials on sensing properties was mainly investigated. Analysis of the trend in the sensing behavior of 2D nanomaterials in terms of surface functionality provides guidance for material selection with desired sensing properties. Newly emerged 2D MXene was introduced as a promising room-temperature sensing material with their intriguing surface chemistry. The capability of titanium carbide (Ti3C2Tx) to sense an array of VOC gases was demonstrated. The possible sensing mechanism of the MXene sensor was proposed in terms of the interaction between sensing species and the oxygen terminated surface of MXene. Another MXene material, vanadium carbide (V2CTx), was also investigated. 2D V2CTx gas sensors showed outstanding gas sensing performance and sensitivity toward non-polar gases such as hydrogen and methane. Compared to Ti3C2Tx sensor, enhanced sensing response of V2CTx was observed to the tested VOC gases, and selectivity to hydrogen was shown. Transformation of ordered structure and constituent elements of MXenes play an important role in interaction between analyte and MXenes showing outstanding selectivity and LoD to non-polar gases. To enhance sensing performance of 2D nanomaterials, the incorporation of 2D materials and metal oxides was considered. The gas sensing performance of the GO/TiO2 hybrid was enhanced due to titanium dioxide (TiO2). Detection of various reducing gases was enabled with higher response and reduced recovery time than GO sensor. To extend long-term stability of the sensor, the GO/TiO2 solution was irradiated by ultraviolet. Due to the photocatalysis of TiO2, the gas sensing behavior of the hybridized sensor was converted from n-type to p-type, which confirms that the dominant sensing material of the composite is GO, and TiO2 acts as a catalyst. The function to identify target gas was maintained over one month, showing strong resistance to humidity due to photo reduced GO. The emphasis places on more specific elaboration of each materials’ role in sensing mechanism by tailoring charge transfer under the same hybrid structure to provide a framework for effective design of new hybrids and ultimately advance their sensing performances.