Preparation And Application Of In₂O₃-based Semiconducting Gas Sensors

Gas sensors based on semiconducting metal oxides are widely applied in environmental monitoring, process control, safety alarm, medical diagnostics due to their low cost, high sensitivity, good stability, and so on. However, several disadvantages including poor selectivity, slow response and recovery rate, and high working temperature still exist in the commercial semiconducting gas sensors, and obviously limit their practical applications. In this thesis, we systematically investigate the relationship among preparation methods, morphologies, and sensing performances of the gas-sensing materials by taking typical semiconducting metal oxide, In2O3, as an example. The obtained research findings to solve the above shortcomings are as following:(1) Preparation of In2O3gas-sensing materials with various morphologies. Via selecting deposition positions, varying amount of starting material, changing reaction temperature, several morphologies including truncated octahedron strings, nanowires, nanocrystal chains, lollipop-like structures, octahedron crystals, are achieved using chemical vapor deposition (CVD) method. And the supersaturation-controlled morphology mechanism is fully explored, which would be considered as a basis for preparation of metal oxides with different morphologies. Moreover, the sensing performances of the truncated octahedron strings, nanowires, nanocrystal chains synthesized from one-pot are compared, results of which exhibit a morphology-dependent relationship.(2) In-situ fabrication of gas sensor by chemical vapor deposition of truncated octahedron strings on the ceramic tube substrate. The gas-sensing materials are stably attached onto the substrate and not easy to peel off. The as-fabricated sensor shows a significant response and a good selectivity toward formaldehyde, and can be applied as safety alarm for generated formaldehyde in interior decoration.(3) Fabrication of gas sensor array by single semiconducting metal oxide. Sensors based on semiconducting metal oxides often display a poor selectivity due to the defect of their sensing mechanism. To solve this problem, we develop a solution-cast process to prepare16sensors with different layer thickness using only one type of metal oxide (porous In2O3microtubes). The as-fabricated sensors exhibit independent response toward ethanol, thus they can be incorporated into a novel electronic nose. This electronic nose is successfully applied in distinguishing four alcohols at the same concentrations (100ppm), as well as14volatile organic compounds (VOCs) at both their immediately dangerous to life or health (IDLH) and the permissible exposure limit (PEL) concentrations. We expect that this method will expand the application of other types of semiconducting metal oxides-based electronic nose which has been largely limited by the types of metal oxides and dopants.(4) Preparation of graphene modified hierarchical In2O3-based gas sensor. Sensors on the basis of semiconducting metal oxides are always operated at a relatively high temperature, which obviously hampers their potential in practical applications. With InCl3and graphene oxide (GO) as precursors, we prepared graphene modified hierarchical In2O3using sodium dodecyl sulfate (SDS) and urea to tune product morphology after the anneal treatment. The as-fabricated sensor can be used to detect NO2at room temperature. Furthermore, the sensor exhibits an excellent selectivity toward NO2, though the concentration of other tested gases is200times that of NO2.(5) Development of an additive-free method to prepare graphene modified In2O3Since the residual additives in the composites also induce a poor sensing performance, we develop an additive-free method to avoid the drawback. The In2O3/rGO composites are synthesized by microwave-assisted hydrothermal method using InN nanowires and GO as precursors. The effect of precursor amount ratios (InN and GO) on In2O3morphologies and distribution in graphene sheets are systematically examined. The uniformly distributed In2O3cubes embedded into graphene sheets are obtained with InN and GO amount ratio of1:1as precursors. Compared to other graphene-based sensors in previous literatures, our sensor exhibits a higher sensitivity. Moreover, the sensor shows an excellent selectivity to NO2, even though other gas concentration is1,000times that of NO2.