Tailoring of Gas Sensing Characteristics Using Bilayer Sensor with Oxide Semiconductor Sensing Layer and Nanoscale Au Catalytic Overlayer

Abstract

Introduction Gas sensing characteristics of oxide semiconductor chemiresistors can be enhanced or controlled by loading noble metal catalysts to the entire sensing materials. Although uniform catalyst loading generally promotes the sensing reaction between analyte gas and ionized oxygen on the surface, it may induce other adverse effects such as 1) the excessive increase of sensor resistance depending on work function of metal catalyst, 2) the gas oxidation at the upper part of sensing film, and 3) the gas reforming during the gas transport within sensing film. Excessively high sensor resistance hampers the cost-effective signal measurement using conventional electric circuits, while it is difficult to separate the gas sensing, reforming, and oxidation reactions, leading to low controllability of gas sensing characteristics. Herein, a novel bilayer design with an oxide chemiresistors sensing layer (SnO2, ZnO, and Co3O4) and nanoscale catalytic Au overlayer was suggested to solve above problems. Bilayer sensor design effectively separated the sensing and catalytic reaction, leading to the excellent control of gas sensing characteristics without affecting the sensor resistance in air. Method Sensing materials SnO2 hollow spheres were prepared using ultrasonic spray pyrolysis of mixed solution consisting of 0.1 M of Tin (II) chloride dihydrate (SnCl2•2H2O, ≥ 99.99%) and 0.025 M of citric acid monohydrate (C6H8O7•H2O, ≥ 99.0%). Droplets of this source solution were then generated by six ultrasonic transducers (resonance frequency: 1.7 MHz) and transferred into a quartz tube (inner diameter: 55 mm) in an electric furnace heated to 700 ºC in air (flow rate : 10 L/min) for pyrolysis. As-prepared precursor powders were converted into SnO2 hollow spheres via heat treatment at 600 ºC for 2 h in air. SnO2 sensing films were fabricated by screen printing onto Pt interdigitated electrode (IDE) (electrode gap: 100 mm) patterns on SiO2/Si substrates (size = 8 mm x 8 mm). Catalytic Au nanoparticles were deposited on the SnO2 thick films through e-beam evaporation of Au grains (99.998%) in a vacuum chamber (below 2×10-6 Torr) at a rate of 0.1 Å sec−1 (nominal thicknesses: 0.5, 1, and 3 nm), as calibrated by a quartz crystal microbalance, which were converted into discrete configurations of Au nanoparticles through heat treatment at 550 ºC for 2 h. Additionally, SnO2 thick films with 1-nm-thick Au overlayer were also heat treated at 550 ºC for 0.5 and 4 h to investigate the effects of Au configuration on gas sensing characteristics. Results and Conclusions The configuration of Au nanoparticle overlayer was changed by controlling the Au-coating thickness and annealing time and investigated the gas sensing characteristics of SnO2 sensors with various configurations of Au nanoparticles overlayer. The Au nanoparticle overlayer significantly enhanced the methylbenzene selectivity and response of SnO2 thick film sensor by reforming of less reactive methylbenzenes into more reactive species and suppressing the cross-responses to relatively reactive interference gases (e.g. ethanol and HCHO) through oxidative filtering. Furthermore, the methylbenzene selectivity and response of ZnO and Co3O4 were also significantly enhanced by introducing the Au nanoparticle overlayer, confirming the general validity on the role of Au nanoparticle overlayer. In all the sensing films, the gas transport as well as sensor resistance is hardly influenced by nanoscale catalytic Au overlayer, demonstrating that the bilayer sensor is promising platform to control the gas sensing characteristics by separating catalytic reaction and sensing reaction and by tuning the catalyst-induced gas reforming and oxidation reaction. These sensors can be used for various sensor applications with new functionality.