Investigations on the Temperature-Dependent Interaction of Water Vapor with Tin Dioxide and Its Implications on Gas Sensing.

This work presents an operando infrared spectroscopic study of the temperature-dependent water adsorption on pristine SnO2 surfaces and discusses the possible implications on the oxygen ionosorption and gas-sensing mechanism. The impact of water on the sensor resistance, CO-sensing performance, and CO conversion was studied, and the obtained phenomenological results provide the basis for discussing the operando spectroscopic investigation findings. The provided information allows identification of three different water adsorption regimes ranging from physisorption and dominant associative adsorption to mainly dissociative water adsorption. In these regions, water has different impacts on the surface composition, sensor resistance, and gas-sensing performance.

TEXS: in-vacuum tender X-ray emission spectrometer with 11 Johansson crystal analyzers.

The design and first results of a large-solid-angle X-ray emission spectrometer that is optimized for energies between 1.5 keV and 5.5 keV are presented. The spectrometer is based on an array of 11 cylindrically bent Johansson crystal analyzers arranged in a non-dispersive Rowland circle geometry. The smallest achievable energy bandwidth is smaller than the core hole lifetime broadening of the absorption edges in this energy range. Energy scanning is achieved using an innovative design, maintaining the Rowland circle conditions for all crystals with only four motor motions. The entire spectrometer is encased in a high-vacuum chamber that allocates a liquid helium cryostat and provides sufficient space for in situ cells and operando catalysis reactors.

Current Understanding of the Fundamental Mechanisms of Doped and Loaded Semiconducting Metal-Oxide-Based Gas Sensing Materials.

Introducing additives in semiconducting metal oxides includes, besides the use of filters, dynamic operation procedures and chemometric approaches, the most common way of tuning the sensitivity, selectivity, and stability of chemoresitsive gas sensors. For the vast majority of commercially used gas sensing materials, the introduction of additives is essential and is one of the longest lasting topics in gas sensor research. This Review discusses the different chemical and electrical sensitization mechanisms of additives as well as the role of different structures. Based on state-of-the-art experimental findings, this Review revises and updates the concepts that are used to explain the mechanisms through which the additives influence the performance of typical gas sensing materials, i.e., oxide nanoparticles arranged in a porous layer. The first sections classify the different additive structures, namely, doped or loaded oxides as well as mixtures of oxides, and describe the basic working principle of pristine semiconducting metal oxide gas sensors. The subsequent sections discuss different chemical and/or electrical contributions to the sensitization by additive structures, their mutual influence on each other, and the way they impact the sensing properties. The presented concepts and models are essential for understanding the complex role of additives and provide the basis for a knowledge-based design of gas sensors based on semiconducting metal oxide nanoparticles, which is outlined in a separate section.

Rhodium Oxide Surface-Loaded Gas Sensors.

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.

Trends and Advances in the Characterization of Gas Sensing Materials Based on Semiconducting Oxides.

The understanding of the fundamental properties and processes of chemoresistive gas sensors based on semiconducting metal oxides is driven by the available characterization techniques and sophisticated approaches used to identify structure-function-relationships. This article summarizes trends and advances in the characterization of gas sensing materials based on semiconducting metal oxides, giving a unique overview of the state of the art methodology used in this field. The focus is set on spectroscopic techniques, but the presented concepts apply to other characterization methods, such as electronic, imaging or diffraction-based techniques. The presented concepts are relevant for academic research as well as for improving R&D approaches in industry.

Temperature-Dependent NO2 Sensing Mechanisms over Indium Oxide.

The surface species responsible for NO2 gas sensing over indium oxide was studied by operando DRIFTS coupled to a multivariate spectral analysis. It revealed the important roles of surface nitrites on the temperature-dependent gas sensing mechanism and the interaction of such nitrites with surface hydroxyls. A highly hydroxylated surface with high concentration of surface adsorbed H2O is beneficial to enhance the concentration of adsorbed NO2, present as nitrites, thus explaining superior sensing response at lower operating temperatures.