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The sensing properties of WO3-based gas sensors were evaluated at an operating temperature of 150 °C in the presence of different volatile organic compounds (VOCs). Simultaneously to the sensing tests, operandodiffuse-reflectance infrared Fourier transform spectra were acquired. Remarkably, no CO2generation was observed under gas exposure for any of the target VOCs. Catalytic combustion properties of the WO3toward acetone and ethanol at temperatures from 150 to 400 °C, both in dry and humid atmospheres, were additionally performed. Such measurements confirmed that no combustion occurred for ethanol (up to at least 300 °C) while it became detectable for acetone only starting from 250 °C. The reaction products are acetaldehyde and acetate in the case of ethanol exposure and acetate and formate in the case of acetone exposure. The oxidation catalytic effect of WO3was confirmed for both toluene, where benzaldehyde and benzoate modes were observed, and acetylene (where acetaldehyde and ethenol were detected). The systematic absence of CO2as a product and the identification of oxidized species are the basis of the proposed reaction mechanisms.

The combination of operando Diffuse Reflectance Infrared Fourier Transform Spectroscopy measurement and Density Functional Theory calculation reveals the counterintuitive HCHO sensing mechanism of In4Sn3O12. It is merely partial oxidation of HCHO into formate (or HCOOH) with medium activation energy (0.43–0.68 eV) and sufficient electron donation effect that is responsible for the sensor signal at the optimum temperature of 200 °C. The Sn (3a)-connected O is the active site and plays key roles in both HCHO adsorption and partial oxidation.

Heterostructures made from metal oxide semiconductors (MOS) are fundamental for the development of high-performance gas sensors. Since their importance in real applications, a thorough understanding of the transduction mechanism is vital, whether it is related to a heterojunction or simply to the shell and core materials. A better understanding of the sensing response of heterostructured nanomaterials requires the engineering of heterojunctions with well-defined core and shell layers. Here, we introduce a series of prototypes CNT-nMOS, CNT-pMOS, CNT-pMOS-nMOS, and CNT-nMOS-pMOS hierarchical core–shell heterostructures (CSHS) permitting us to directly relate the sensing response to the MOS shell or to the p–n heterojunction. The carbon nanotubes are here used as highly conductive substrates permitting operation of the devices at relatively low temperature and are not involved in the sensing response. NiO and SnO2are selected as representative p- and n-type MOS, respectively, and the response of a set of samples is studied toward hydrogen considered as model analyte. The CNT-n,pMOS CSHS exhibit response related to the n,pMOS-shell layer. On the other hand, the CNT-pMOS-nMOS and CNT-nMOS-pMOS CSHS show sensing responses, which in certain cases are governed by the heterojunctions between nMOS and pMOS and strongly depends on the thickness of the MOS layers. Due to the fundamental nature of this study, these findings are important for the development of next generation gas sensing devices.

It is well known that composite materials, consisting of at least two metal oxides, show qualities and sensing behavior very different from the single components. Recently, the preparation of core–shell nanomaterials for gas sensors has become extremely popular. Specifically, these materials have been found to show desirable sensor responses. The preparation of core–shell nanomaterials is, however, complex, limiting the commercial applicability. Composite materials can be more easily attained simply through the mechanical mixing of the various components. Although some studies exist that attempt to compare mechanically mixed composites to those prepared via a synthetic route, these examinations are often flawed, as due to varying preparation methods, the basic characteristics of the materials are not the same. Here, it was possible to separate the role of the contacts between the materials from that of the secondary core–shell structure, by using a soft method to mechanically break apart the structure. This ensures that the difference in morphology is the only change in the material characteristics. It was verified that the composite materials show a different sensing behavior from that of the pure materials. It was also found that regardless of the secondary structure, the composite materials showed very similar sensor responses. By examining materials containing different ratios of Cr2O3to SnO2, it was possible to attribute the sensor behavior changes to the contacts between the different metal oxides. It was shown that by varying the concentration of each oxide it is possible to attain either an n- or p-type response and at a certain concentration even no response. This work is significant because it identifies that the contact between the materials plays the dominant role in the sensor response and it shows the viability of mechanical mixing for composite sample preparation.

Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and first-principles calculations are performed to investigate the different ways in which water reacts with a SnO2surface and to evaluate the cross-interference of humidity on the detection of CO. Two different materials, chosen because of their very different properties, are investigated. The experimental results are interpreted with the help of theoretical modeling of two clean and defective surfaces, namely, (110) and (101). The experimental results show, and the theoretical calculations confirm, that water vapor can interfere with the CO detection in different ways depending on the active surface and the concentration of oxygen vacancies. This is related to the different ways in which the water vapor reacts with tin oxide: on the one hand, it can reduce the (101) surface; on the other hand, it can heal the oxygen vacancies of the defective (110) surface.

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 Rh2O3changes sensing with WO3. As a result of uniform surface loading, reactions between the Rh2O3clusters and the analyte gas dominate the reception. Changes in the p–n heterojunction between Rh2O3and WO3are 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 characteristics of SMOXs.

The metal oxide semiconductor (MOS) core–shell heterostructure nanofibers (NFs) have been successfully synthesized via an environmentally friendly coaxial electrospinning approach. To demonstrate the potential applications of the as-prepared samples, sensors based on MOS core–shell heterostructure NFs have been fabricated and their gas sensing properties were investigated. Results show that the sensors exhibit an advanced gas sensing property to trimethylamine (TMA) including the outstanding selectivity and rapid response/recovery processes in comparison with the sensors based on single MOS NFs. These phenomena are closely associated with the electron flow caused by the work function difference between MOS of the core and the shell. The approach proposed in this study may contribute to the realization of more sensitive MOS core–shell heterostructure sensors.

This work demonstrates that it is possible to follow the surface chemistry of oxygen on SnO2based gas sensing materials using operando diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The inherent difficulties, due to the intrinsic properties of the studied oxide and the limitations of the method, were overcome by comparing the results obtained for two different materials and by use of isotopically labeled gases together with the simultaneous measurement of the sensor signals. In spite of the differences in the surface composition and reactivity between the different materials, the experimental results show that the reactive oxygen species are similar in nature and the gas recognition takes place by the interplay of surface reduction and (re)oxidation.

Operando DRIFT spectroscopy together with resistance measurements was used to study sensitivity (here response change upon changing analyte concentration) of undoped SnO2 materials to water vapors. The materials synthesized using different protocols were found to manifest very different sensitivity to water, as well as to H2 and CO in dry and humid air. While for one of the materials (SnO2 Ac) low sensitivity to water combines with similarity between evolution of two bands in OH region and resistance, another material (SnO2 Cl), with high sensitivity to water vapors, manifests similarity with the resistance only for the broad band between 3590 and 3040 cm-1. Together with high sensitivity to water vapors the latter material shows higher sensitivity to CO in dry and humid air, while SnO2 Ac showed higher sensitivity to H2.

The humidity dependence of the gas‐sensing characteristics in SnO2‐based sensors, one of the greatest obstacles in gas‐sensor applications, is reduced to a negligible level by NiO doping. In a dry atmosphere, undoped hierarchical SnO2nanostructures prepared by the self‐assembly of crystalline nanosheets show a high CO response and a rapid response speed. However, the gas response, response/recovery speeds, and resistance in air are deteriorated or changed significantly in a humid atmosphere. When hierarchical SnO2nanostructures are doped with 0.64–1.27 wt% NiO, all of the gas‐sensing characteristics remain similar, even after changing the atmosphere from a dry to wet one. According to diffuse‐reflectance Fourier transform IR measurements, it is found that the most of the water‐driven species are predominantly absorbed not by the SnO2but by the NiO, and thus the electrochemical interaction between the humidity and the SnO2sensor surface is totally blocked. NiO‐doped hierarchical SnO2sensors exhibit an exceptionally fast response speed (1.6 s), a fast recovery speed (2.8 s) and a superior gas response (Ra/Rg= 2.8 at 50 ppm CO (Ra: resistance in air, Rg: resistance in gas)) even in a 25% r.h. atmosphere. The doping of hierarchical SnO2nanostructures with NiO is a very‐promising approach to reduce the dependence of the gas‐sensing characteristics on humidity without sacrificing the high gas response, the ultrafast response and the ultrafast recovery.

Sensor at work: The structure of platinum in a 50 micrometer thin SnO2layer is monitored by the combination of a specially designed experimental sensor setup with high‐energy‐resolution fluorescence‐detected X‐ray absorption spectroscopy and “range‐extended” EXAFS to gain new insights into the local structure of the platinum dopant.

Sensor in Aktion: Die Struktur von Platin in einer 50 μm dünnen SnO2‐Schicht wird in einem speziell konstruierten Sensoraufbau durch die Kombination hochauflösender Röntgenabsorptionsspektroskopie mit Fluoreszenzdetektion und vollständigen EXAFS‐Daten untersucht, um Einblicke in die lokale Struktur der Dotierung zu erlangen.

Widely applicable nonaqueous solution routes have been employed for the syntheses of crystalline nanostructured tungsten oxide particles from a tungsten hexachloride precursor. Here, a systematic study on the crystallization and assembly behavior of tungsten oxide products made by using the bioligand deferoxamine mesylate (DFOM) (product I), the two chelating ligands hexadecyltrimethylammoniumbromide (CTAB) (II) and poly(alkylene oxide) block copolymer (Pluronic P123) (III) is presented. The mechanistic pathways for the material synthesis are also discussed in detail. The tungsten oxide nanomaterials and reaction solutions are characterized by Fourier transform IR, 1H, and 13C NMR spectroscopies, powder X‐ray diffraction, scanning electron microscopy, transmission electron microscopy (TEM), high‐resolution TEM, and selected‐area electron diffraction. The indexing of the line pattern suggests WO3is in its monoclinic structure with a= 0.7297 nm, b= 0.7539 nm, c= 0.7688 nm, and β‐i;= 90.91 °. The nanoparticles formed have various architectures, such as chromosomal shapes (product I) and slates (II), which are quite different from the mesoporous one (III) that has internal pores or mesopores ranging from 5 to 15 nm. The nanoparticles obtained from all the synthetic procedures are in the range of 40–60 nm. The investigation of the gas‐sensing properties of these materials indicate that all the sensors have good baseline stability and the sensors fabricated from material IIIpresent very different response kinetics and different CO detection properties. The possibility of adjusting the morphology and by that tuning the gas‐sensing properties makes the preparation strategies used interesting candidates for fabricating gas‐sensing materials.

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Tin dioxide is a widely used sensitive material for gas sensors. Many research and development groups in academia and industry are contributing to the increase of (basic) knowledge/(applied) know-how. However, from a systematic point of view the knowledge gaining process seems not to be coherent. One reason is the lack of a general applicable model which combines the basic principles with measurable sensor parameters. The approach in the presented work is to provide a frame model that deals with all contributions involved in conduction within a real world sensor. For doing so, one starts with identifying the different building blocks of a sensor. Afterwards their main inputs are analyzed in combination with the gas reaction involved in sensing. At the end, the contributions are summarized together with their interactions. The work presented here is one step towards a general applicable model for real world gas sensors.

Lead sulfide (PbS) is a p-type semiconductor that is often applied in photodetectors and solar cells. One major problem that researchers are regularly confronted with is the loss of performance when operated in air. The sensitivity to the ambient, however, makes it an interesting material for the application in gas sensors. Although a lot of research has been focused on the aging of PbS, the influencing factors and the associated material changes are still a matter of debate. Resistance measurements of differently prepared PbS samples operated as sensors for the detection of NO2(a strongly oxidative gas) in combination with surface-sensitive methods (diffuse reflectance infrared Fourier transform spectroscopy and X-ray photoelectron spectroscopy) enabled the investigation of the changes associated with aging and their influence on the sensing mechanism. The aging mechanism was found to be predominantly influenced by the presence of residuals of the stabilizing agent oleic acid that was adsorbed at the surface of the colloidal quantum dots. Additionally, it was found that a byproduct of the consumption of oleic acid enhanced the detected sensor signal.

A novel MEA design solution - an "intelligent switch" to provide protection of the cathode catalyst layer from degradation caused by startup/shutdown events is proposed (1). This unique approach adapts gas sensor-type materials (2) that exhibit several orders of magnitude change in resistance depending on its gas environment (1) in PEM fuel cell anodes. The high resistance of the new layer in air environments prevents corrosion loop currents. In hydrogen environments, the layer becomes conductive and fuel cell performance is minimally hindered. In-situ fuel cell test results demonstrate a successful reduction in cathode peak potentials during startup events leading to reduced performance degradation in startup/shutdown stress tests. This strategy of preventing corrosion loop currents is compatible with various catalyst types and with different catalyst loadings. The approach enables MEA design to minimize air/air start/stop degradation and provides a pathway for the use of alternative catalyst options.

Operando DRIFT spectroscopy together with resistance measurements was used to study sensitivity (here response change upon changing analyte concentration) of undoped SnO2 materials to water vapors. The materials synthesized using different protocols were found to manifest very different sensitivity to water, as well as to H2 and CO in dry and humid air. While for one of the materials (SnO2 Ac) low sensitivity to water combines with similarity between evolution of two bands in OH region and resistance, another material (SnO2 Cl), with high sensitivity to water vapors, manifests similarity with the resistance only for the broad band between 3590 and 3040 cm-1. Together with high sensitivity to water vapors the latter material shows higher sensitivity to CO in dry and humid air, while SnO2 Ac showed higher sensitivity to H2.

The loading of NiO provides a new strategy to achieve humidity‐independent SnO2gas sensors. When NiO is loaded to hierarchical SnO2nanostructures, the humidity dependence of the gas sensing characteris‐tics, such as gas sensitivity, response speed, recovery speed, and resistance in air, are decreased to a negli‐gible level. On page 4456, Nicolae Barsan, Jong‐Heun Lee, and co‐workers confirmed that NiO plays the key role of a strong humidity absorber and protects the SnO2from the influence of water‐derived species.

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