Methods for Expanding the Diversity in the Response of Metal Oxide Based Gas Sensors

Abstract

As all aspects of life become more automated and interconnected, sensors will be needed in various applications. In particular, gas sensors will find widespread use, in e.g. indoor air quality monitoring and breath analysis. Versus other detection methods, semiconducting metal oxide (SMOX) based sensors are more compact, sensitive, robust and inexpensive. Their major drawback is their inherent lack of selectivity. This limitation could be addressed by using arrays of SMOX materials with complementary sensing behavior. Today as a result of the historical development, despite the decades of research, most commercially available sensors are still based on SnO2. The work here examines three different options for creating complementary sensors: using a different n-type base metal oxide (WO3), noble surface loading and the creation of metal-oxide-metal-oxide mixtures. Based on a literature review, WO3 appeared promising and here its complementarity was verified. It was identified that the sensing behavior of WO3 is robust against changes in synthesis. Using operando diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, it was possible to identify why the resistance of WO3 increases with humidity. From this finding it became apparent why the response to oxidizing gases strongly decreases in the presence of atmospheric humidity. In order to tune the sensing behavior, surface loading with metal oxides is commonly used. Although two mechanisms, chemical sensitization and Fermi level pinning, were already suggested in the 1980s, experimental evidence was limited. Here, the effect of rhodium, palladium and platinum loading on WO3 was examined. Using operando DRIFT spectroscopy, in situ transmission electron microscopy (TEM) and X-ray absorption spectroscopy (XAS) it was shown that the Fermi level pinning mechanism dominates. As a result, Rh-loading reduces the complementarity of WO3 and SnO2 based sensors. Finally, sensors based on SnO2 and Cr2O3 mixtures were examined. Reports of gas sensors based on combinations of metal oxides, in particular mixtures of n- and p-type materials, are common in literature. These mixed materials are usually created using sophisticated and expensive methods, like the electrospinning of nanofibers. Here sensors based on nanofibers were compared to those based on randomly dispersed particles. By breaking apart the nanofibers using soft mechanical grinding, it was possible, for the first time, to clearly separate the effects of the secondary structure from the coupling between the materials. It was identified that the junctions between the materials are largely responsible for the changed sensing. Furthermore, it was shown that by varying the ratio of the metal oxides, the sensor response can be tuned, i.e. shows a p- or n- type response, and in some cases no response. In total it has been shown that other n-type materials should be considered for integration into arrays with SnO2. It has been found that the applicability of noble metal oxide surface loadings to increase the complementarity of materials is limited. It has been shown that metal-oxide-metal-oxide mixtures can be used to tune the sensing behavior and that the mechanical mixing of materials is a sufficient preparation method to attain the desired results.