Gas Sensor Based On Semiconducting Metal Oxide Porous Thin Film

Semiconducting oxide materials based gas sensors have been developed in varietyof applications such as industrial emission control, civil life, food security, medicaldiagnosis, explosive and toxic gases alarm and environmental monitoring, due to theirirreplaceable advantageous features, such as high sensitivity, small physical size, lowcost and simplicity in fabrication. The gas sensing characteristics such as gas sensitivity,response time, selectivity, stability and reproducibility are greatly influenced by thesurface area, donor density, agglomeration, porosity, the presence of catalysts, and thesensing temperature of the sensing oxide materials. To achieve high-performancesensors, the synthesis method is one of the most crucial parameters to control thesensing properties of the final metal oxide, especially with respect to sensitivity andlong-term stability. Surface area, crystal orientation, oxide stoichiometry,defects/impurity concentration, sinter neck and grain sizes are strictly determined by thesynthesis process. Therefore one major challenge in the improvement of gas sensors isto develop the facile approach to synthesize new functional materials, especially withwell-defined structure and properties. Porous thin film materials with largesurface-to-volume ratios are particularly promising in sensor fabrication. In addition,porous materials with defined nanostructural properties offer further advantages ascompared to conventional gas-sensing materials. The wet chemical methods have beendeveloped to synthesize the porous thin film of metal oxides which allows theproduction of tailored and stabilized nanoparticles prior to their assembly in films,resulting in high sensitivity and reasonable stability. However, reproducibility of thesensing films prepared by wet methods is generally poor because the control of thestructural parameters is difficult by conventional synthesis methods, such as sol–gelprocesses, as they are interdependent (e.g. grain size, grain interconnectivity, pore sizeand pore architecture). The conventional physical synthesis of metal oxides film such as sputtering can improve the control over both material properties and film structuralparameters but it has no feasibility to generate the pores in the film. In this thesis wereport a method to prepare well-ordered nanoporous metal oxide film by simplesputtering using a self-assembly film of polystyrene spheres as a soft template.The well-ordered porous thin films with high specific surface area have beenfabricated on a desired substrate using self-assembled monolayer of polystyrene (PS)beads as the soft templates combined with a simple physical sputtering deposition. Thedifferent semiconductor gas sensing materials such as SnO2, ZnO, In2O3, etc. have beenapplied in the fabrication of the porous nanostructured film gas sensors. The pore sizeand homogeneity of the film thickness of the porous thin film can be preciselycontrolled, resulting in the controllable sensing performance. The doping andmultilayered porous thin film, for example, Cu-doped SnO2porous thin film sensor,Cu-doped SnO2/In2O3double layer porous thin film sensor and ZnO/SnO2double layerporous thin film sensor can be easily achieved by the co-sputtering or sequent sputtering.The nanoporous thin film gas sensors fabricated by the method show a significantlyenhancement in the sensing performance including high sensitivity, selectivity,reproducibility, and fast response and recovery toward H2S and CO. For example, thesensitivity of Cu-doped SnO2porous sensor is one order higher than that of un-dopedSnO2sensor with the average response and recovery times to100ppm H2S of about10.1and42.4s, respectively, at the optimum operating temperature of180°C. Thewell-defined porous sensors fabricated by the facile method exhibit high reproducibilitydue to the accurately controlled process. The facile fabrication process can be easilyextended to other semiconductor oxide gas sensors fabrication with easy doping andmultilayer porous nanostructure for the practical applications.