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Today semiconducting metal oxide (SMOX) based gas sensors are used in a wide array of applications. Dopants, e.g., rhodium, are often used to change the sensor response of SMOXs. The adjustment of sensing characteristics with dopants is usually done empirically, and there is a knowledge gap surrounding how the presence of dopants alters the chemistry of sensing. Here using X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), dc resistance measurements, and operando diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, it was understood how surface loading with Rh2O3changes sensing with WO3. As a result of uniform surface loading, reactions between the Rh2O3clusters and the analyte gas dominate the reception. Changes in the p–n heterojunction between Rh2O3and WO3are responsible for the transduction. These results in combination with existing literature indicate that, through controlled surface doping, it is possible to intentionally tune the sensor characteristics of SMOXs.

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 SnO2thick 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 Co3O4sensors. 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.