The use of semiconducting metal-oxide (MOX) based gas sensors in demanding applications such as climate and environmental research as well as industrial applications is currently hindered by their poor reproducibility, selectivity, and sensitivity. This is mainly due to the sensing mechanism which relies on the change of conductivity of the metal-oxide layer. To be of use for advanced applications metal-oxide (MOX) gas sensors need to be carefully prepared and characterized in laboratory environments prior to deployment. This paper describes the working principle, design, and use of a new apparatus that can emulate real-world conditions in the laboratory and characterize the MOX gas sensor signal in tailor-made atmospheres. In particular, this includes the control of trace gas concentrations and the control of oxygen and humidity levels which are important for the surface chemistry of metal-oxide based sensors. Furthermore, the sensor temperature can be precisely controlled, which is a key parameter of semiconducting, sensitive layers, and their response to particular gas compositions. The setup also allows to determine the power consumption of each device individually which may be used for performance benchmarking or monitoring changes of the temperature of the gas composition. Both, the working principle and the capabilities of the gas measurement chamber are presented in this paper employing tin dioxide (SnO2) based micro sensors as exemplary devices.

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In order to take full advantage of novel functional materials in the next generation of sensorial devices scalable processes for their fabrication and utilization are of great importance. Also understanding the processes lending the properties to those materials is essential. Among the most sought-after sensor applications are low-cost, highly sensitive and selective metal oxide based gas sensors. Yet, the surface reactions responsible for provoking a change in the electrical behavior of gas sensitive layers are insufficiently comprehended. Here, we have used near-edge x-ray absorption fine structure spectroscopy in combination with x-ray microscopy (NEXAFS-TXM) for ex-situ measurements, in order to reveal the hydrogen sulfide induced processes at the surface of copper oxide nanoparticles, which are ultimately responsible for triggering a percolation phase transition. For the first time these measurements allow the imaging of trace gas induced reactions and the effect they have on the chemical composition of the metal oxide surface and bulk. This makes the new technique suitable for elucidating adsorption processes in-situ and under real operating conditions.

This paper describes a novel micromachined platform serving as an interface between nanosized, gas sensitive metal oxide particles, and the macroscopic world. Through a combination of ink-jet printing and microelectromechanical systems technologies, it thus becomes possible to quickly test and characterize new nanosized metal oxide particles with respect to their gas sensitivity. Within the framework of this report, we describe the design considerations, thermal finite-element method simulations, processing, characterization, and utilization of the platform. Due to the low-power consumption, the hotplate provides an experimental platform to test nanoparticle-based metal oxide gas sensors for mobile systems.[2014-0215]

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Here, we present results on the investigation of the percolation phase transition in copper(II)oxide (CuO) and show how it may be used to determine trace gas concentrations. This approach provides a highly selective sensing mechanism for the detection of hydrogen sulfide even in oxygen depleted atmospheres. In real-world applications, this scenario is encountered in biogas plants and natural gas facilities, where reliable H2S sensing and filtering are important because of the destructive effects H2S has on machinery. As opposed to gas detection via standard metal-oxide reaction routes, the percolation dynamics are demonstrated to be independent of the surface morphology in accordance with the universality of phase transitions. The sensing behavior of ink-jet printed CuO layers was tested for a large set of parameters including layer temperature, hydrogen sulfide (H2S) and oxygen concentration, as well as the sensitivity towards other gas species. The electrical percolation of the sensing layer is heralded by a dramatic drop in the overall resistivity of the CuO layer for temperatures below 200 °C. The observed percolation phenomena in this temperature regime are unique to H2S even in comparison with related volatile thio-compounds making the sensing mechanism highly selective. At elevated temperatures above 300 °C, the phase transition does not occur. This enables two distinct operational modes which are tunable via the sensor temperature and also allows for resetting the sensing layer after an electrical breakthrough.

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This manuscript describes the design considerations, implementation, and laboratory validation of an odor sensing module whose purpose is to support people that suffer from incontinence. Because of the requirements expressed by the affected end-users the odor sensing unit is realized as a portable accessory that may be connected to any pre-existing smart device. We have opted for a low-cost, low-power consuming metal oxide based gas detection approach to highlight the viability of developing an inexpensive yet helpful odor recognition technology. The system consists of a hotplate employing, inkjet-printed p-type semiconducting layers of copper(II) oxide, and chromium titanium oxide. Both functional layers are characterized with respect to their gas-sensitive behavior towards humidity, ammonia, methylmercaptan, and dimethylsulfide and we demonstrate detection limits in the parts-per-billion range for the two latter gases. Employing a temperature variation scheme that reads out the layer’s resistivity in a steady-state, we use each sensor chip as a virtual array. With this setup, we demonstrate the feasibility of detecting odors associated with incontinence.