The electronic interaction of SnO2 with Ag and Pd particles dispersed on its surface was examined by means of X-ray photoelectron spectroscopy (XPS). The binding energies (BE) of Sn3d and O1s of Ag(1.5 wt%)-SnO2 and Pd(3.0 wt%)-SnO2, which were lower by 0.5–0.7 eV than those of pure SnO2 in the as-prepared state, shifted reversibly by reduction and oxidation treatments. These shifts in BE are shown to reflect the shifts of Fermi energy of SnO2 which are interacting electronically with the metal additives. The electronic interaction depended on the metal loading, being strongest at 1.5 wt% and 3.0 wt% loadings of Ag and Pb, respectively. The implication is that the electronic interaction is of primary importance to the inflammable gas detection by Ag-SnO2 sensors.

Electronic Interaction between Metal Additives and Tin Dioxide in Tin Dioxide-Based Gas Sensors : Surfaces, Interfaces and Films

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.

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

Since the first suggestion, during the 1950s, that high-surface-area metal oxides could be used as conductometric gas sensors enormous efforts have been made to enhance both the selectivity and the sensitivity of such devices, and to reduce their operational power requirements. This development has involved the exploration of response mechanisms, the selection of the most appropriate oxide compositions, the fabrication of two-phase ‘hetero-structures’, the addition of metallic catalyst particles and the optimisation of the manner in which the materials are presented to the gas—the structure and the nanostructure of the sensing elements. Far more of the scientific literature has been devoted to seeking such improvements in metal oxide gas sensors than has been directed at all other solid-state gas sensors together. Recent progress in the research and development of metal oxide gas sensor technology is surveyed in this invited review. The advances that have been made are quite spectacular and the results of individual pieces of work are drawn together here so that trends can be seen. Emerging features include: the significance of n-type/p-type switching, the enhancement of sensing performance of materials through the incorporation of secondary components and the advantages of interrogating sensors with alternating current rather than direct current.

https://iopscience.iop.org/article/10.1088/1361-6501/aa7443/pdf

Progress in the development of semiconducting metal oxide gas sensors: a review

Breath sensors can revolutionize medical diagnostics by on-demand detection and monitoring of health parameters in a noninvasive and personalized fashion. Despite extensive research for more than two decades, however, only a few breath sensors have been translated into clinical practice. Actually, most never even left the scientific laboratories. Here, we describe key challenges that currently impede realization of breath sensors and highlight strategies to overcome them. Specifically, we start with breath marker selection (with emphasis on metabolic and inflammatory markers) and breath sampling. Next, the sensitivity, stability, and selectivity requirements for breath sensors are described. Concepts are elaborated to systematically address these requirements by material design (focusing on chemoresistive metal oxides), orthogonal arrays, and filters. Finally, aspects of portable device integration, user communication, and clinical applicability are discussed.

Breath Sensors for Health Monitoring

Artificial olfaction based on gas sensor arrays aims to substitute for, support, and surpass human olfaction. Like mammalian olfaction, a larger number of sensors and more signal processing are crucial for strengthening artificial olfaction. Due to rapid progress in computing capabilities and machine-learning algorithms, on-demand high-performance artificial olfaction that can eclipse human olfaction becomes inevitable once diverse and versatile gas sensing materials are provided. Here, rational strategies to design a myriad of different semiconductor-based chemiresistors and to grow gas sensing libraries enough to identify a wide range of odors and gases are reviewed, discussed, and suggested. Key approaches include the use of p-type oxide semiconductors, multinary perovskite and spinel oxides, carbon-based materials, metal chalcogenides, their heterostructures, as well as heterocomposites as distinctive sensing materials, the utilization of bilayer sensor design, the design of robust sensing materials, and the high-throughput screening of sensing materials. In addition, the state-of-the-art and key issues in the implementation of electronic noses are discussed. Finally, a perspective on chemiresistive sensing materials for next-generation artificial olfaction is provided.

Rational Design of Semiconductor-Based Chemiresistors and their Libraries for Next-Generation Artificial Olfaction.

Semiconducting metal oxide based gas sensors are used in a wide spectrum of fields, ranging from the detection of hazardous gases within the environment to monitoring air quality. WO 3 is the second, after SnO 2 , most commonly used semiconducting metal oxide in commercial gas sensors. Despite its frequent application, the surface reactions responsible for sensing are largely unknown. Here, for the first time, a mechanism for the surface reaction between WO 3 and humidity can be concluded from experimental results. DC resistance measurements and operando diffuse reflectance infrared Fourier transform spectroscopy show an oxidation of the WO 3 lattice during humidity exposure. The filling of oxygen vacancies by water explains the effects atmospheric humidity has on WO 3 based sensors, specifically the increase in resistance, the higher sensor signals to CO and the lower sensor signals to NO 2 . These findings are a basis for understanding how sensing occurs with WO 3 based sensors.

The oxidizing effect of humidity on WO3 based sensors

Tungsten trioxide is the second most commonly used semiconducting metal oxide in gas sensors Semiconducting metal oxide (SMOX)-based sensors are small, robust, inexpensive and sensitive, making them highly attractive for handheld portable medical diagnostic detectors WO3 is reported to show high sensor responses to several biomarkers found in breath, eg, acetone, ammonia, carbon monoxide, hydrogen sulfide, toluene, and nitric oxide Modern material science allows WO3 samples to be tailored to address certain sensing needs Utilizing recent advances in breath sampling it will be possible in the future to test WO3-based sensors in application conditions and to compare the sensing results to those obtained using more expensive analytical methods

https://www.mdpi.com/1424-8220/16/11/1815/pdf

Understanding the Potential of WO3 Based Sensors for Breath Analysis

Semiconducting metal oxide-based gas sensors are an attractive option for a wide array of applications. In particular, sensors based on WO3 are promising for applications varying from indoor air qu...

WO3-Based Gas Sensors: Identifying Inherent Qualities and Understanding the Sensing Mechanism.

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 Rh2O3 changes sensing with WO3. As a result of uniform surface loading, reactions between the Rh2O3 clusters and the analyte gas dominate the reception. Changes in the p–n heterojunction between Rh2O3 and WO3 are 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 characterist...

Nanolevel Control of Gas Sensing Characteristics via p–n Heterojunction between Rh2O3 Clusters and WO3 Crystallites

Semiconducting metal oxide based gas sensors are a highly attractive option for use in a widespread array of applications as they can be miniaturized, are highly robust and inexpensive. This review focuses on SnO2, In2O3 and WO3, three commonly used metal oxides that are known to show a diversity in sensing behavior. In order to optimize the sensing behavior, it is imperative that the mechanism is understood. It is unanimously agreed upon that adsorption of oxygen and the resulting charge layer play a central role in the sensor response. Surface oxygen had proven to be illusive experimentally. Recent insights gained from operando methods, sophisticated surface science analysis tool and theoretical calculations are discussed. Additionally, these studies have revealed that the surface reactions are far more complicated than initially assumed. It is found that other adsorbates beyond oxygen can influence the sensor response. Depending on the site, the formation of adsorbates can have different effects on the sensor depending on the reaction site, e.g. hydroxyl group formation from humidity can result in the removal of negative charge or be electroneutral. The importance of understanding the effect of adsorbates becomes particularly evident when examining the chemically complicated volatile organic compounds, e.g. ethanol and acetone. In this review simplified schematics based on insights from theoretical and experimental studies are developed, so that the surface chemistry can be presented in an understandable manner despite its complexity. 

Anna Staerz

Papers

The oxidizing effect of humidity on WO3 based sensors

Understanding the Potential of WO3 Based Sensors for Breath Analysis

WO3-Based Gas Sensors: Identifying Inherent Qualities and Understanding the Sensing Mechanism.

Nanolevel Control of Gas Sensing Characteristics via p–n Heterojunction between Rh2O3 Clusters and WO3 Crystallites

Current state of knowledge on the metal oxide based gas sensing mechanism