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Rhodium Oxide Surface-Loaded Gas Sensors

1
Institute of Physical and Theoretical Chemistry (IPTC), University of Tuebingen, Auf der Morgenstelle 15, D-72076 Tuebingen, Germany
2
European Synchrotron Radiation Facility (ESRF), 71 Avenue des Martyrs, 38043 Grenoble, France
3
Institut de Physique et Chimie des Matériaux de Strasbourg (IPCMS), UMR 7504 CNRS-Université de Strasbourg, 23 rue du Lœss, F-67034 Strasbourg cedex 2, France
4
Institute of Catalysis Research and Technology (IKFT) and Institute for Chemical Technology and Polymer Chemistry (ITCP), Karlsruhe Institute of Technology, Kaiserstr. 12, 76131 Karlsruhe, Germany
*
Author to whom correspondence should be addressed.
Nanomaterials 2018, 8(11), 892; https://doi.org/10.3390/nano8110892
Received: 4 October 2018 / Revised: 24 October 2018 / Accepted: 25 October 2018 / Published: 1 November 2018
(This article belongs to the Special Issue Development of Semiconductor Nanomaterials for Gas Sensors)
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Abstract

In order to increase their stability and tune-sensing characteristics, metal oxides are often surface-loaded with noble metals. Although a great deal of empirical work shows that surface-loading with noble metals drastically changes sensing characteristics, little information exists on the mechanism. Here, a systematic study of sensors based on rhodium-loaded WO3, SnO2, and In2O3—examined using X-ray diffraction, high-resolution scanning transmission electron microscopy, direct current (DC) resistance measurements, operando diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, and operando X-ray absorption spectroscopy—is presented. Under normal sensing conditions, the rhodium clusters were oxidized. Significant evidence is provided that, in this case, the sensing is dominated by a Fermi-level pinning mechanism, i.e., the reaction with the target gas takes place on the noble-metal cluster, changing its oxidation state. As a result, the heterojunction between the oxidized rhodium clusters and the base metal oxide was altered and a change in the resistance was detected. Through measurements done in low-oxygen background, it was possible to induce a mechanism switch by reducing the clusters to their metallic state. At this point, there was a significant drop in the overall resistance, and the reaction between the target gas and the base material was again visible. For decades, noble metal loading was used to change the characteristics of metal-oxide-based sensors. The study presented here is an attempt to clarify the mechanism responsible for the change. Generalities are shown between the sensing mechanisms of different supporting materials loaded with rhodium, and sample-specific aspects that must be considered are identified. View Full-Text
Keywords: gas sensors; surface-loading; DRIFT spectroscopy; X-ray absorption spectroscopy; Fermi-level pinning gas sensors; surface-loading; DRIFT spectroscopy; X-ray absorption spectroscopy; Fermi-level pinning
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Staerz, A.; Boehme, I.; Degler, D.; Bahri, M.; Doronkin, D.E.; Zimina, A.; Brinkmann, H.; Herrmann, S.; Junker, B.; Ersen, O.; Grunwaldt, J.-D.; Weimar, U.; Barsan, N. Rhodium Oxide Surface-Loaded Gas Sensors. Nanomaterials 2018, 8, 892.

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