Metal Oxide Gas Sensors with Au Nanocluster Catalytic Overlayer: Toward Tuning Gas Selectivity and Response Using a Novel Bilayer Sensor Design.

Noble metals or oxide catalysts have traditionally been loaded or doped to enhance the gas sensing properties of oxide semiconductor chemiresistors. However, the selective detection of various chemicals for a wide range of new applications remains a challenging problem. In this paper, we propose a novel bilayer design with an oxide chemiresistor sensing layer and nanoscale catalytic Au overlayer to provide high controllability for gas sensing characteristics. The Au nanocluster overlayer significantly enhances the methylbenzene response of a SnO thick film sensor by reforming gases into more reactive species and suppresses the responses to reactive interference gases through oxidative filtering, leading to excellent selectivity to methylbenzene. Gas sensing characteristics can be tuned by controlling the morphology, amount, and number density of Au nanoclusters through the variation in the Au coating thickness (0.5-3 nm) and thermal annealing conditions (0.5-4 h at 550 °C). Furthermore, the general validity of the proposed Au-coated bilayer sensor design was confirmed through the enhancement of response and selectivity toward methylbenzenes by coating Au nanoclusters onto ZnO and CoO sensors. The sensing mechanism, advantages, and potential applications of bilayer sensors are discussed from the perspective of the separation of sensing and catalytic reactions, as well as the reforming and oxidation of analyte gases in association with the configuration of the sensing layer and Au catalytic overlayer.